Commemorative Papers

Miniaturization of optical spectroscopes into Fresnel microspectrometers

[+] Author Affiliations
Yeonjoon Park

National Institute of Aerospace, Hampton, Virginia 23666

Sang H. Choi

NASA Langley Research Center, Hampton, Virginia 23681

J. Nanophoton. 7(1), 077599 (Jun 03, 2013). doi:10.1117/1.JNP.7.077599
History: Received November 29, 2012; Revised April 29, 2013; Accepted May 1, 2013
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* Editorial Note: This paper was invited to commemorate the promotion of Sang H. Choi to SPIE Fellow.

Abstract.  Miniaturized optical instruments have become very important in industry as smart phones and tablet PCs increase in popularity. A chronology of spectrometer development shows that a simple numerical point of view affords important insights. A tiny spectrometer, which is smaller than a few millimeters size, cannot easily rely on the conventional Fraunhofer diffraction due to its optical criterion limit. As an alternate solution to build smaller spectrometers, a Fresnel spectrometer chip with a gradient line grating is attractive. The fabricated Fresnel spectrometers have optical path volumes of about 1mm3 and spectral resolutions of 10 to 23 nm.

Figures in this Article

The first industrial optical diffraction grating was built by American astronomer David Rittenhouse in 1785.1 It was Joseph von Fraunhofer of Bavaria, Germany, who invented the first spectroscope in 1814 and developed the diffraction grating further in 1821.2 Most of today’s modern optical spectroscopes are founded upon Fraunhofer’s diffraction principle except for a few noble instruments such as the Fourier transform infra-red spectroscope, fiber Bragg grating spectroscope and Raman spectroscope.310 In Fraunhofer diffraction, the first order diffracted light has an angular dispersion with respect to the wavelength of light. Periodic linear lines, with constant gap spacing, reflect and diffract incoming photons into different angles with respect to the wavelengths. The diffracted photons are measured by a detector that is a long enough distance away to separate the wavelengths. In 1818, another form of diffraction was introduced and utilized by Augustin-Jean Fresnel to build a circular flat lens known now as a Fresnel lens.11 This second form of diffraction was named Fresnel diffraction and it becomes valid at very short distances such as inside the Fresnel regime. This is contrary to Fraunhofer diffraction, which is valid at long distances beyond the Fresnel regime.

Graphic Jump LocationF1 :

(a) Joseph von Fraunhofer demonstrating the spectroscope, a painting by Richard Wimmer (public domain media from wikimedia.org), (b) mathematical configuration diagram when light passes a spot, S(η,ξ,0) on an aperture and illuminates a point P(x,y,z) on a screen.12

Figure 1 describes a general optical situation when light passes a spot, S(η,ξ,0) at a coordinate (η,ξ,0) on an aperture and illuminates a point, P(x,y,z) at a coordinate (x,y,z) on a screen. If we have a constant uniform parallel light shining on a spot S on the aperture, the electric field from the spot S can be written as iAλeikrr, where A is an intensity coefficient for incoming light. According to Huygens’s principle,13 according to which the electric field at a far point P, is obtained by adding each point of the light’s wave-front surface, the electric field strength at point P, u(P) can be written by the integration of tiny electric fields from all spots like S in the aperture, so that Display Formula

u(P)=iAλeikrrdξdη,(1)
where λ=wavelength and k=wave number of light.

This equation is exact. However, it is very difficult to solve this integral equation in terms of the fixed instrumental distance r between the aperture and detector screen. Note that while the distance r is a temporary variable for integration, the distance r is a fixed engineering parameter of a given instrument that does not change over the integration. Instead of seeking a definite solution of the integral equation, we expand the term eikrr as an infinite series using a polynomial expansion of r in terms of r. Display Formula

r=rxξ+yηr+[(xξ+yη)22r3+ξ2+η22r]+.(2)

The two diffraction principles, Fraunhofer and Fresnel, differ in how many terms of Eq. (2) are included in the approximation. In Fraunhofer diffraction, the polynomial expansion of Eq. (1) is approximated by two terms in Eq. (2) using the instrumental distance r and angles α and β.14Display Formula

u(P)=S(ξ,η,0)exp[ik(lξ+mη)]dξdη,(3)
where rr(lξ+mη) and l=xr=cos(α); m=yr=cos(β).

In Fresnel diffraction, the third term of Eq. (2) is also included in the approximation to Eq. (1) in order to be more precise at short distances and the approximation becomes as the next equation. Display Formula

u(P)=iλzexp[ikz]S(ξ,η,0)exp[ik2z[(xξ)2+(yη)2]]dξdη.(4)

The Fraunhofer diffraction equation [Eq. (3)] is valid at long distances such that z>a2λ where a= the radius of the aperture and z= the distance between the aperture and screen along the optical axis. The Fresnel diffraction equation [Eq. (4)], on the other hand, is valid at short distances such that a<z<a2λ. It is interesting to notice that a more precise diffraction equation for shorter distances can be obtained by including higher order terms in the Taylor expansion, Eq. (2).

While the Fraunhofer diffraction has been used to build many spectroscopes since the first one was made by Joseph von Fraunhofer himself, the Fresnel equation has been used mainly to construct flat lenses (Fresnel lenses) which have strong chromatic aberration. It was not extended to a spectroscope until the 1960s. In 1965 and 1967, Stroke and Samson studied the application of a Fresnel zone plate to a holographic spectroscope.15,16 In 1970, Hirsch, Jordan, and Lesem, at IBM Research Lab, disclosed the idea to build the first electric spectroscope based on Fresnel diffraction instead of Fraunhofer diffraction.17 The idea, illustrated in the IBM Tech. Disclosure Bulletin, was for a kinoform lens (similar to a Fresnel zone plate) a few inches in size and mechanically moving on a linear track.18 The lens focuses the incoming parallel light into a passing aperture located at the focal distance. If the kinematic lens has a strong chromatic dispersion, the focal distance varies with the wavelength of light and the aperture acts as a wavelength filter. In 1972, Keating, Mueller, and Sawatari published a calculation that showed that the resolution of a zone plate spectrometer might be improved by adding an additional central stopping disk inside the zone plate.19 In the 1980s and 1990s, micro-electronics fabrication technologies have advanced rapidly and spread widely in industry and in academic research.20 In 1995, Kitaura, Ogata, and Mori fabricated a plastic-molded mini/micro-Fresnel lens, of 4.5 mm diameter and 14.83 mm focal length, and demonstrated a small spectrometer based on Fresnel diffraction with 85 percent efficiency.21 In spite of the successful demonstration of the micro-fabrication based Fresnel spectrometer with a linear movement, the theoretical spectral resolution of their spectrometer was estimated to decrease as its size got smaller. Kitaura et al. estimated the mathematical spectral resolving power of a zone plate based Fresnel spectrometer as the next equation. Display Formula

λΔλa4π(f/#),(proportional to lens size,a),(5)
where λ is the wavelength, Δλ is the wavelength difference which is to be revealed by a spectrometer, a is the aperture diameter, and f/# is the f-number of a zone plate lens. Because circular Fresnel zone plates have traditionally been used as flat lenses, it was natural to regard as a fixed property of a lens and to compare the spectral resolving power of different sizes, i.e., different aperture diameter a’s.2225 Therefore, as the size of zone plate (a) becomes smaller, the resolving power λΔλ was predicted to decrease linearly. This prediction has hindered further development of miniaturized Fresnel spectrometers.

In 2008, our team, in a new approach, analyzed the spectral resolution of a Fresnel zone plate not as a lens, but as a circular gradient grating by straightforwardly solving the integral Fresnel diffraction equation. 26 In that study, it was found that f/# is not constant upon miniaturization of a zone plate since f/# is defined as F/2a where F is the focal length and a is the aperture size. When the linear dimension of a zone plate is miniaturized by M times, the focal length F is shortened by M2 and the aperture size a becomes smaller by M. Therefore, Eq. (5) expands as the next form, Display Formula

λΔλa4π(f/#)M4π(M22M)=12π(Constant).(6)

Thus, the spectral resolving power does not depend on the size of the zone plate or the miniaturization factor M. Our team analyzed the spectral resolution and resolving power of Fresnel zone plates more deeply by applying a strict Rayleigh criterion27 to the intensity profile of two light sources of slightly different wavelengths, λ and λ+Δλ. The mathematical simulation shows that the spectral resolving power does not depend on the size of the plate, but it is linearly proportional to the number of Fresnel diffraction elements such as the number of rings in the plate. This surprising result reopened a chance to miniaturize the spectroscope below the millimeter regime.26,28,29 The full mathematical derivation and results can be found in the reference.26 Actually, this result is not extraordinary because the spectral resolving power of a conventional Fraunhofer spectrometer does not depend on the size of the grating, but it is proportional to the number of lines in the grating.14 In spite of the difference between Eqs. (3) and (4), the spectral resolving powers have the same linear dependence on the total number of diffraction elements and do not depend on the size of elements. With this new design principle, two different forms of Fresnel spectrometers were fabricated and tested.

Fabrication of Fresnel Circular Gratings

A circular Fresnel spectrometer can be constructed using a traditional zone plate.30 Two kinds of zone plates are possible. The first is a positive zone plate where the center has a transparent opening and the second is a negative zone plate where the center has an opaque disc. The focal distance and optical properties are almost identical except that the positive one collects photons of 0 deg phase with respect to the direct passing light and the negative one collects photons of 180 deg phase with respect to the same light. A negative zone plate is preferred for building a spectrometer because it has an opaque disc at the center that stops the 0th order direct beam.

As shown in Fig. 2, tiny Fresnel circular gratings (zone plates), with diameters of 750 μm, were fabricated using a gallium focused ion beam (FIB) machine, the Helios system from FEI-Beam technology Co.3135 A quartz disc was cleaned with acetone, isopropanol, and deionized water. A thin film of gold was deposited to the thickness of 0.4 μm from a tungsten boat using a thermal evaporator. A positive gallium (Ga+) ion beam was used to etch nanometer patterns with xenon difluoride (XeF2) enhanced etching gas.3644Figure 2(b) shows an array of Fresnel circular gratings with about 1 mm spacing. With this tiny size and gap distance, 10,000 Fresnel circular gratings can be fabricated within a 10×10cm2 area. Such an array can be used to accept large numbers of optical fibers within a small area.4550

Graphic Jump LocationF2 :

SEM image of (a) a single Fresnel circular grating of 750 μm diameter and (b) an array of Fresnel circular gratings.

Optical Performance Test and Measured Data

The optical performance of the fabricated Fresnel circular spectrometer was measured with pseudo-yellow light from a 533 nm diode-pumped-solid-state green laser and a 633 nm He-Ne red laser uniformly mixed by a beam combiner and neutral density filters as shown in Fig. 3(a). A spatial filter was used to make a uniform TEM00 beam which, finally, entered the Fresnel circular spectrometer.5154Figure 3(b) shows the configuration of the Fresnel micro-spectrometer with a moving aperture slit and detector. The Fresnel circular grating has 100 rings (50 transparent rings and 50 opaque rings) in its 750 μm diameter. This makes a focal distance of 2.4 mm for 533 nm light. Therefore, the volume of the optical path between the Fresnel circular grating and detector is only π(0.75(mm)2)2·2.4(mm)=1.06mm3. While ultra-compact conventional spectrometers, which are based on Fraunhofer diffraction, require more than 1cm3=1,000mm3 of optical path volume,55,56 the tiny 1mm3 volume of this Fresnel circular spectrometer is almost 1,000 times smaller than that of today’s commercial spectrometers. Figure 3(c) and 3(d) show the separation of selected wavelength photons from mixed yellow light by changing the optical distance (Z) between the grating and aperture slit. The remaining light passing through the center of the aperture slit is stopped by a small disc on the detector so that only the extracted light of the selected wavelength can enter the photo-detector. The aperture slit and detector unit were moved by piezo-driven linear actuators. Figure 3(e) shows the measured data which is the photo-current from a blue-enhanced silicon photo-diode detector. The software records the photo-current at each optical distance (Z) and calculates the wavelength from Z values and properties of the circular Fresnel grating. The converted data show two clear peaks at 533 nm and 633 nm wavelengths. The actual measurement shows some degree of jitter noise and broadening due to the mechanical movement of the aperture slit. Figure 3(f) shows the theoretical resolution limit of a circular Fresnel-spectrometer of 500 μm diameter versus the number of rings in the circular Fresnel grating. It is estimated that 1 nm of spectral resolution, competitive with commercial spectrometers, could be achieved with a 250-ring circular Fresnel grating.

Graphic Jump LocationF3 :

(a) Optical performance test with mixed yellow light from green and red lasers, (b) configuration diagram of Fresnel circular spectrometer, (c) separation of 633 nm photons (red) from incoming yellow light, (d) separation of 533 nm photons (green) from incoming yellow light, (e) detector current measured while scanning optical distance, (f) theoretical spectral resolution versus number of rings in Fresnel circular grating.

Design and Mathematical Simulation

In spite of the tiny optical path volume, the linear actuator, used to move the aperture slit and detector in a circular Fresnel spectrometer, is very large and requires separate driver electronics as well. Therefore, a circular Fresnel spectrometer, with a Fresnel zone plate and a movable aperture slit, is not suitable to build an integrated, motionless optical electronics chip. Fresnel gratings do not have to be limited to a circular geometry.57 A linear zone plate can be built with a series of lines with gradient spacing and widths and these have been used to focus an x-ray beam into a narrow line.58 A typical glass optical lens does not work with x-rays due to their ultra-short wavelengths.5961Figure 4(a) illustrates different types of Fresnel gratings, i.e., full and half circular and linear zone plates along with their focal axes or planes. The full circular and full linear gratings focus light on to the center axis, or plane, from both the top and bottom. The half circular and half linear gratings have the diffraction element above the focusing axis, or plane, only so that the light illumination is made from one side. Typically, a full Fresnel line grating, i.e., a linear zone plate, is used to focus an x-ray or optical beam into the plane through its center.6263 In our case, half linear gratings are suitable to construct micro-spectrometers because the light must enter the sensing surface of a detector from one side.

Graphic Jump LocationF4 :

(a) Various types of Fresnel gratings (zone-plates), (b) 3-D configuration of conventional Fraunhofer spectrometer and (c) linear Fresnel spectrometer.

In order to integrate the Fresnel grating into a tiny sensor array micro-chip, we configured of the Fresnel spectrometer, as shown in Fig. 4(c) by mounting a half linear Fresnel grating vertically on one end of a linear imaging sensor. While a conventional Fraunhofer spectrometer uses a periodic line grating with constant gaps and widths, as shown in Fig. 4(b), the Fresnel spectrometer uses a gradient line grating with changing gaps and widths as shown in Fig. 4(c). In a Fraunhofer spectrometer, the surface of the imaging detector and the groove surface of the grating face each other. In the Fresnel spectrometer, the angle between the imaging detector surface and the grating is 90 deg.

Results of a mathematical simulation, to calculate the vertical distribution of photons above and below the sensor surface, are shown in Fig. 5(a). Similar calculations were made with three different wavelengths (632 nm for red, 532 nm for green, and 432 nm for blue) for two different Fresnel spectrometers.The first with a 400-μm-high 100-line gradient grating, as shown in Fig. 5(b) and a second with a 500-μm-high 156- lines gradient grating as shown in Fig. 5(c). The plots in Fig. 5 show cross-sectional views of the photon distribution that comes from the gradient Fresnel line grating located on the left side at Z=0. The X and Z axes are defined in Fig. 4(c) as well as in the inset to Fig. 5. The height X=0 is the top surface of the imaging detector sensor. Because of the line symmetry of the grating, the photon distribution in the Y-direction is assumed to be constant and it only varies in the ZX plane. Figure 5(a) shows how the 532 nm wavelength light converges into one spot on the sensor surface. The oscillating photon intensities overlap and the intensity is maximized at the convergence point, i.e., focal point. Therefore, the sensor pixel at the convergence point receives the highest intensity of photons of one wavelength. Figure 5(b) shows the convergence of three different wavelengths of light on three different spots. This phenomenon, i.e., strong chromatic dispersion, was traditionally interpreted as a demerit of Fresnel lenses, but it is useful to create spectral resolving power for a spectrometer.64,65 Long-wavelength 632 nm red light converges around 2.5 mm from the vertical grating and short wavelength 432 nm blue light converges around 3.7 mm distance. Figure 5(c) shows that the photons converge in a narrower line when the number of grating lines is increased from 100 lines to 156 lines. Therefore, higher spectral resolution can be obtained as the number of lines increases. The 500-μm-high Fresnel line grating, of Fig. 5(b), with 156 lines has exactly the same 100 lines up to 400 μm height as the one in Fig. 5(c) and adds an additional 56 lines in the extra 100 μm from 400 μm height to 500 μm height. Therefore, they have exactly the same convergence lines but the one with more lines has better sharpness and higher spectral resolution. In a full linear Fresnel grating, there would be another photon-converging slope, from bottom to top, that would meet the top-to-bottom slope at the convergence line. In the half linear Fresnel grating, there is only the one photon-converging slope from top to bottom. It is also interesting to note by comparing Fig. 5(b) to 5(c), the Fresnel grating with more lines has more vertical convergence of photons than the one with fewer lines.

Graphic Jump LocationF5 :

Cross-sectional views of photon distributions (a) above and below the imaging sensor surface with 533 nm light passing through a 100-line Fresnel grating of 400 μm height located at the left side, (b) above the imaging sensor surface with light of three wavelengths, 432 nm, 532 nm and 632 nm, passing through a 100-line Fresnel grating of 400 μm height, and (c) for the same three lights (432, 532, 632 nm) passing through a 156-line Fresnel grating of 500 μm height.

The photons dispersed by from the Fresnel grating are detected at the convergence line for each wavelength. Therefore, the spectral resolving power of the Fresnel spectrometer comes from “dispersive convergence” while that of a conventional Fraunhofer spectrometer comes from “angular dispersion” as indicated by Eq. (3).

Typically, the actual measurement of photon intensities is made by digital electronics such as charge-coupled device or complementary metal-oxide-semiconductor pixels at the convergence points.6669 Because the detector only records the intensity data and pixel number, the pixel numbers have to be converted into more meaningful physical properties such as wavelength or energy of the photon. The data conversion equation, for a Fresnel spectrometer, comes from the convergence focal point (F) equation, F=K2λ where K is the size coefficient of the Fresnel grating by which the position of the nth line is determined, rn=Kn. A simulation with a sinusoidal light distribution on the sensor pixels shows the difference between a conventional Fraunhofer spectrometer and the new Fresnel spectrometer. While the sensitivity o a Fraunhofer spectrometer is linear in the wavelength scale and reciprocal in the energy scale, the sensitivity of a Fresnel spectrometer is linear in the energy scale and reciprocal in the wavelength scale. This difference, shown in Fig. 6, comes from the fact that the angular dispersion of a Fraunhofer spectrometer is proportional to wavelength (λ), while the convergence dispersion of the Fresnel spectrometer is proportional to the one over wavelength, 1/λ, and, of course, the energy of the photon (E) is given by E=hc/λ where h is Planck’s constant and c is the speed of light. This comparison shows that the mathematical foundation and data acquisition (DAQ) method for a Fresnel spectrometer are fundamentally different from those of a Fraunhofer spectrometer.

Graphic Jump LocationF6 :

Comparison of different wavelength/energy interpretations from similar sinusoidal photon distribution on sensor pixel array of Fraunhofer and Fresnel spectrometers.

Fabrication of Linear Fresnel Spectrometer on Microchip

Figure 7 shows an actual photo of the linear imaging sensor array chip (Hamamatsu S8378-256Q) that became a platform for building the first prototype linear Fresnel spectrometer. This chip has 256 active pixels in 25 μm pitch and 0.5 mm height, spectral response range of 200 to 1,000 nm, and maximum operating clock frequency of 500 kHz. Instead of building a spectrometer by putting a detector into a box, we built the spectrometer onto the detector chip itself. This ultra-compact integration became possible due to the ultra-small size of the Fresnel grating and its optical path length. The active sensor area is 6.4×0.5mm2 and the chip die size is 9.5×2.5mm2.

Graphic Jump LocationF7 :

The size of the linear imaging sensor array chip which was used to fabricate a linear Fresnel spectrometer.

Figure 8(a) and 8(b) show the 3-D structures of the integrated linear Fresnel spectrometer chips with side-light-loading and front-loading schemes. The 90-deg micro-mirror with aluminum reflector was purchased from Tower Optical Co. The grating side surface was coated with another aluminum layer and patterned with FIB milling. The Fresnel grating, with the 90-deg mirror, was mounted on a quartz protective cover above the chip packaging and lowered down just above the die surface to match the focusing plane on the sensor surface.

Graphic Jump LocationF8 :

(a) Side-light-loading scheme and the definition of variables, (b) front-light-loading scheme, (c) optical microscope image of the linear Fresnel grating, (d) SEM image of the 90-deg mirror with a gradient linear Fresnel grating.

The digital interface to the Fresnel spectrometer chip and control/DAQ software was developed with a DAQ interface card and C# language under .NET Framework.7072Figure 9 shows a screen shot of the Fresnel micro-spectrometer control program. It supports 1 MHz DAQ, a variable number of pixels from 100 to 4,096 pixels, internal/external clock synchronization within 100 nanoseconds timing, selectable trigger edges, real-time single and multiscan data display, data recording, and data and graph file input and output. It also supports both the Fresnel spectrometer mode and Fraunhofer spectrometer mode with automatic calibration functions. The upper graph, of Fig. 9, is a real-time, single scan data display and the lower graph is a two-dimensional (2-D) color map graph of multirecorded scans. In this 2-D map graph, the X-axis is the photon energy or wavelength, the Y-axis is the elapsed time, and the color represents the intensity of the photons. The linear Fresnel spectrometer chip can operate at very high speeds, down to the microsecond level, because it has no moving parts. In order to test the electronics and software performance, the data in Fig. 9 was generated by moving a point light on the imaging sensor array without installing the Fresnel grating.

Graphic Jump LocationF9 :

Screen shot of the microspectrometer control program.

New Data Acquisition and Conversion Method from the Fresnel Microspectrometer Chip
Conversion algorithm for physical properties

The software algorithm, used in the Fresnel spectrometer, to acquire spectrum data (wavelength and intensity) is very different from that of a conventional Fraunhofer spectrometer. Several conversion methods from sensor chip pixel number to wavelength/energy and the calibration methods are described here. Variables were defined in Fig. 8(a).

Let us assume that the imaging sensor has a total of N pixels, numbered 0 to N1. The optical distance Z is determined by optical distance Z= gap distance between Fresnel grating and the 0thpixel+(pixel number×pixel pitch)), where pixel number, n, is between 0 and N1 and the pixel pitch is a constant for the sensor chip as shown in Fig. 8(a). Then, the wavelength of the photon on the n’th pixel (pixel number=n) with the optical distance Z is determined by: wavelength=K2/Z, where K is a Fresnel grating size constant such that Display Formula

K=RL,(7)
where L is the number of gradient rings (circular grating) or gradient lines (linear grating) and R is the radius of a circular grating or the height of a linear grating.

Thus, the wavelengh is determined by the next equation, Display Formula

Wavelength=K2Z=K2gap+Pixel Number*Pixel Pitch=R2L×(gap+Pixel Number*Pixel Pitch).(8)
Then, the energy of the photons with wavelength λ is calculated as Energy=hc/λ.

In this first scheme, the signal is recorded versus pixel numbers initially, then the pixel numbers are converted into wavelengths, and then the wavelengths are converted to energies.

The second scheme uses the Fresnel spectrometer’s linearity in energy scale so that the conversion to energy is straight forward, Display Formula

Energy=hcLZR2=hcLR2(gap+Pixel Number*Pixel Pitch).(9)
Then, the wavelength is determined by Wavelength=hcEnergy.

Software flow chart

Figure 10 shows three software algorithms including the routines to convert from pixel number data to Fresnel spectrum data (energy or wavelength). The red blocks are the unique Fresnel spectrum conversion routines.

Graphic Jump LocationF10 :

Software algorithms including the Fresnel spectrum data acquisition/conversion routines, (a) after each single scan, (b) after all single scans are finished, and (c) after all multiscan pixel data streams are acquired.

Optical performance data of the first prototype

Figure 11(a) shows the full intensity scan data from all pixels of the Fresnel spectrometer chip illuminated by one He-Ne laser (633 nm). The pixel numbers are converted to a photon energy scale. The high intensity plateau on the left side is due to the unwanted illumination by the 0th order direct beam from the aperture on the front face. Since the diameter of the aperture in Fig. 11(c) was bigger than the 90-deg mirror, excess direct beam hit the imaging sensor die and made multiple reflections between the chip die and the front quartz cover. This gave a very high intensity of stray light on the pixels that are close to the Fresnel grating, the left side of the graph. The saturation voltage of the detector was 4.0 V and the 0th order direct beam exceeded this value. The dispersed 1st order peak shows the intensity of photons to a pixel at a given energy. The refresh timing was controlled to bring the 1st order 1.96 eV peak under the saturation level. Figure 11(b) shows measured data from two attenuated He-Ne lasers, one with 1.96 eV (633 nm) red photons and the other with 2.29 eV (543 nm). The long tail of the 0th order direct beam and the high level of noise peaks were caused by unwanted multiple reflections from the 0th order direct beam. In a conventional spectrometer, the 0th order direct beam is dumped into a beam absorber wall or block and only the 1st order and higher order diffracted beams enter the detector.7377 Therefore, it will be necessary to install a beam absorber or separator to remove the 0th order direct beam and reduce the stray light in the imaging sensor array region. Our data in Fig. 11 shows that the working principle and design of the linear Fresnel spectrometer are effective, however, additional design improvements are needed to reduce stray light in its tiny volume and achieve a better signal to noise ratio. In a conventional spectrometer, the stray lights are blocked and absorbed in a large black box between the grating and detector. However, in the tiny Fresnel spectrometer chip, the sensor die is typically made with a very reflective silicon wafer and the grating is attached on a shiny glass, consequently, the stray light from the 0th order direct beam is reflected and scattered all over. Therefore, the removal of this stray light in the tiny volume becomes a new technical challenge in order to improve the performance.

Graphic Jump LocationF11 :

Measured data from the first prototype linear Fresnel spectrometer chip: (a) data from all pixels with one laser, (b) comparison of measured spectra from two He-Ne lasers (1.96 eV and 2.29 eV) and (c) photo of the first prototype front-light-loading linear Fresnel spectrometer with a 90-deg mirror inside.

We have reviewed the progress of spectroscope development with respect to the fundamental diffraction equations. Two kinds of Fresnel spectrometers, circular and linear ones, are demonstrated in our research. Both of them have sub-750 μm-size gradient gratings and about 1mm3 of optical path volume for green light and a spectral resolution of 10 to 23 nm, thus, they truly enter the regime of microspectrometers. The Fresnel spectrometer takes a unique new approach to miniaturize a spectroscope beyond the limits of conventional spectroscopes that are built upon Fraunhofer diffraction. Although it differs in the founding diffraction equation itself, in dispersive convergence instead of angular dispersion, and in linearity in energy, not wavelength, it still follows the same fundamental governing optics rule as the conventional Fraunhofer spectrometer does which is the spectral resolving power of the spectroscope is determined not by the size of the grating but by the number of diffraction elements in it. While more precise calculation with vector field theory can describe the polarization dependence as well, the essential nature of this statement still keeps holding according to our study.7880

Once, the famous modern American physicist Richard Feynman81 gave a lecture titled “There’s Plenty of Room at the Bottom.”81 In which, he predicted manipulation of matter at an atomic scale by redesigning various tools.82 Feynman’s remark was also cited as the title of a recent review paper in Spectroscopy, by R. A. Crocombe in 2008, for qualitative general description of various miniature spectrometers.8386 Contrary to the general descriptive summary of many miniature spectrometers, this commemorative paper reviews mathematically quantitative approaches based on fundamental diffraction equations. It is interesting to notice that the diffraction equation, Eq. (1), and series expansion, Eq. (2), do not end in the Fresnel diffraction term but continue to higher order terms that are significant at even closer distances. Therefore, by exploring higher order terms in Eq. (2), one may build even smaller spectrometers, maybe nano-spectrometers that can effectively work in the near-field regime.87 We already know that atoms interact with photons at exact wavelengths via the selection rule δ(Ehc/λ). Thus, one may insist that nature already created atomic spectrometers below 1 nm in size. Maybe our lack of imagination is the current limit to building a commercial quality spectrometer from 1 mm to 1 nm size.

The historic development of spectroscopes indicates that future devices will have higher spectral resolving power in a smaller volume and weight.86 Development of portable IT devices such as smart phones, tablets and hand-held medical instruments is accelerating and they are seeking ultra-compact sensors. For example, a portable spectro-polarimeter of a few centimeters size was recently developed for iPhone.88 Currently, two kinds of wavelength-sensitive sensors are available in industry. The first is a color sensor that detects three to four colors such as R, G, B, and IR.8991 It is a very small and inexpensive electronic component with a price range of $10$50. Integrated spectrometers with multiple narrow band color sensors are also available, however, each sensor monitors different spots in space.92 The second kind is a mini-spectrometer that can detect a full spectrum with 2561,024pixels, but it is a large, expensive, system-level device with a price of a few hundred to several thousand dollars.9395 The Fresnel micro-spectrometer chip covers an intermediate range between the color sensor and the mini-spectrometer. The spectrometer chip is a small component-level device which can detect 100–1,024 colors. Although the first gradient grating for Fresnel spectrometers requires a high-resolution and high-accuracy patterning method such as FIB etching or e-beam lithography, the mass production of Fresnel gratings can easily be achieved using plastic injection molding or nanoimprint lithography.96103 Therefore, it is expected that they can fit in the market gap between color-sensors and mini-spectrometers with very reasonable cost.

This research was supported by the space act agreement SAA-15546 of NASA, USA and Gacheon University of Medicine and Science under KOSEF program by the Ministry of Science, Technology, and Education, Republic of Korea.

Purvis  T. L., Revolutionary America. , 1763–1800,  New York  (1995).
Jackson  M. W., Spectrum of Belief: Joseph von Fraunhofer and the Craft of Precision Optics. ,  MIT Press ,  Boston  (2000).
Green  M. J., Barner  B. J., Corn  R. M., “Real-time sampling electronics for double modulation experiments with fourier-transform infrared spectrometers,” Rev. Sci. Instrum.. 62, (6 ), 1426 –1430 (1991), CrossRef. 0034-6748 
Hartland  G. V. et al., “Time-resolved fourier-transform spectroscopy with 0.25  cm−1 spectral and less-than-10(-7) s time resolution in the visible region,” Rev. Sci. Instrum.. 63, (6 ), 3261 –3267 (1992), CrossRef. 0034-6748 
Feng  C. et al., “Miniaturization of step mirrors in a static Fourier transform spectrometer: theory and simulation,” J. Opt. Soc. Am. B Opt. Phys.. 28, (1 ), 128 –133 (2011), CrossRef. 0740-3224 
Davis  M. A., Kersey  A. D., “Application of a fiber Fourier-transform spectrometer to the detection of wavelength-encoded signals from Bragg grating sensors,” J. Lightw. Technol.. 13, (7 ), 1289 –1295 (1995), CrossRef. 0733-8724 
Qu  H. et al., “All photonic bandgap fiber spectroscopic system for detection of refractive index changes in aqueous analytes,” Sens. Actuat. B Chem.. 161, (1 ), 235 –243 (2012), CrossRef. 0925-4005 
Chitteboyina  M. M., Butler  D. P., “Tunable infrared microspectrometer based on Bragg grating,” IEEE J. Quant. Electron.. 44, (1–2 ), 182 –184 (2008), CrossRef. 0018-9197 
Rodrigo  J. A. et al., “Fresnel diffraction effects in Fourier-transform arrayed waveguide grating spectrometer,” Opt. Express. 15, (25 ), 16431 –16441 (2007), CrossRef. 1094-4087 
Zhao  J., McCreery  R. L., “Multichannel FT-Raman spectroscopy: noise analysis and performance assessment,” Appl. Spectrosc.. 51, (11 ), 1687 –1697 (1997), CrossRef. 0003-7028 
O’Connor  J. J., Robertson  E. F., Augustin-Jean Fresnel. MacTutor History of Mathematics archive, University of St. Andrews, http://www-history.mcs.st-andrews.ac.uk/Biographies/Fresnel.html.
Wimmer  R., Essay in Astronomy. ,  D. Appleton & Company ,  New York  (1900).
Baker  B. B., The Mathematical Theory of Huygens’ Principle. ,  Society Chelsea Publishing  (2003).
Ghatak  A. K., Thyagarayan  K., Optical Electronics. ,  Cambridge University Press ,  New York  (1989).
Stroke  G. W., “Fourier-transform spectroscopy using holographic imaging without computing and with stationary interferometers,” Phys. Lett.. 16, , 272  (1965), CrossRef. 0031-9163 
Samson  J. A. R., Techniques of Vacuem Ultraviolet Spectroscopy. ,  Wiley ,  New York  (1967).
Hirsch  P. M., Jordan  J. A., Lesem  L. B., IBM Technical Disclosure Bulletin. , Vol. 12, p. 1806  (1970).
Jordan  J. A. et al., “Kinoform lenses,” Appl. Opt.. 9, (8 ), 1883  (1970), CrossRef. 0003-6935 
Keating  P. N., Sawatari  T., Mueller  R. K., “Fresnel-zone-plate spectrometer with central stop,” J. Opt. Soc. Am.. 62, (8 ), 945  (1972), CrossRef. 0030-3941 
Corbin  A., The Third Element: A Brief History of Electronics. ,  AuthorHouse ,  Bloomington, Indiana  (2006).
Kitaura  N., Ogata  S., Mori  Y., “Spectrometer employing a micro-Fresnel lens,” Opt. Eng.. 34, (2 ), 584 –588 (1995), CrossRef. 0091-3286 
Wenzel  R. G., “Effect of the aperture-lens separation on the focal shift in large-F-number systems,” J. Opt. Soc. Am. Opt. Image Sci. Vis.. 4, (2 ), 340 –345 (1987), CrossRef. 0740-3232 
Takeya  N. et al., “Holographic collimator lens with small F-number,” Optik. 88, (2 ), 80 –82 (1991). 0030-4026 
Guha  S., “Validity of the paraxial approximation in the focal region of a small-f-number lens,” Opt. Lett.. 26, (20 ), 1598 –1600 (2001), CrossRef. 0146-9592 
Schurig  D., “An aberration-free lens with zero F-number,” New J. Phys.. 10,  (2008), CrossRef. 1367-2630 
Park  Y. et al., “Miniaturization of a Fresnel spectrometer,” J. Opt. Pure Appl. Opt.. 10, (9 ) (2008), CrossRef. 1464-4258 
Goodman  J., Introduction to Fourier Optics. , pp. 158 –160,  Roberts and Company Publishers ,  Englewood Colorado  (2004).
Zhang  N. et al., “Spectral-domain optical coherence tomography with a Fresnel spectrometer,” Opt. Lett.. 37, (8 ), 1307 –1309 (2012), CrossRef. 0146-9592 
Yang  C. A. et al., “Proposal and demonstration of a spectrometer using a diffractive optical element with dual dispersion and focusing functionality,” Opt. Lett.. 36, (11 ), 2023 –2025 (2011), CrossRef. 0146-9592 
Jeon  S. C. et al., “A simplified fabrication process of fresnel zone plates with controlling proximity effect correction,” J. Nanosci. Nanotechnol.. 11, (1 ), 503 –506 (2011), CrossRef. 1533-4880 
Hagstrum  H. D. D., Pretzer  D., Takeishi  Y., “Focused slow ion beam for study of electron ejection from solids,” Rev. Sci. Instrum.. 36, (8 ), 1183  (1965), CrossRef. 0034-6748 
Nagamachi  S. et al., “Focused ion-beam direct deposition of metal thin film,” Rev. Sci. Instrum.. 67, (6 ), 2351 –2359 (1996), CrossRef. 0034-6748 
Fu  Y. Q. et al., “Experimental study of three-dimensional microfabrication by focused ion beam technology,” Rev. Sci. Instrum.. 71, (2 ), 1006 –1008 (2000), CrossRef. 0034-6748 
Petit  D. et al., “Nanometer scale patterning using focused ion beam milling,” Rev. Sci. Instrum.. 76, (2 ) (2005), CrossRef. 0034-6748 
Kunstmann  T. et al., “Focused ion beam milling monitored by an additional electrode,” Rev. Sci. Instrum.. 77, (8 ) (2006), CrossRef. 0034-6748 
Sawaragi  H. et al., “Performance of a combined focused ion and electron-beam system,” J. Vac. Sci. Tech. B. 8, (6 ), 1848 –1852 (1990), CrossRef. 0734-211X 
Muhle  R., “High-field ion sources and applications,” Rev. Sci. Instrum.. 63, (5 ), 3040 –3049 (1992), CrossRef. 0034-6748 
Seki  S. et al., “Development of an instrument for simultaneous detection of positive and negative scanning ion images,” Appl. Surf. Sci.. 231, , 976 –980 (2004), CrossRef. 0169-4332 
Fokkema Hagen  C. W. E., Kruit  P., “Brightness measurements of a gallium liquid metal ion source,” J. Vac. Sci. Tech. B. 26, (6 ), 2091 –2096 (2008), CrossRef. 0734-211X 
Santschi  C. et al., “Interdigitated 50 nm Ti electrode arrays fabricated using Xef2 enhanced focused ion beam etching,” Nanotechnology. 17, (11 ), 2722 –2729 (2006), CrossRef. 0957-4484 
Taniguchi  J. et al., “Focused-ion-beam-assisted etching of diamond in XeF2,” J. Vac. Sci. Tech. B. 16, (4 ), 2506 –2510 (1998), CrossRef. 0734-211X 
Harriott  L. R., “Focused ion-beam Xef2 etching of materials for phase-shift masks,” J. Vac. Sci. Tech. B. 11, (6 ), 2200 –2203 (1993), CrossRef. 0734-211X 
Nakamura  H., Komano  H., Ogasawara  M., “Focused ion-beam assisted etching of quartz in Xef2 without transmittance reduction for phase-shifting mask repair,” Japanese J. Appl. Phys.. 31, (Part 1, No. 12B ), 4465 –4467 (1992), CrossRef. 0021-4922 
Kettle  J., Hoyle  R. T., Dimov  S., “Fabrication of Step-and-Flash Imprint Lithography (S-FIL) templates using XeF2 enhanced focused ion-beam etching,” Appl. Phys. Mater. Sci. Process.. 96, (4 ), 819 –825 (2009), CrossRef. 0947-8396 
Brown  C. W., Lin  J., “Interfacing a fiberoptic probe to a diode-array uv-visible spectrophotometer for drug dissolution tests,” Appl. Spectros.. 47, (5 ), 615 –618 (1993), CrossRef. 0003-7028 
Moreau  F. et al., “Fiber-optic remote multisensor system based on an acousto-optic tunable filter (AOTF),” Appl. Spectros.. 50, (10 ), 1295 –1300 (1996), CrossRef. 0003-7028 
Chen  G. et al., “Design of a hybrid integrated microfiber spectrometer,” J. Microlithography Microfabrication and Microsystems. 2, (3 ), 191 –194 (2003), CrossRef. 1537-1646 
Lienert  B., Porter  J., Sharma  S. K., “Simultaneous measurement of spectra at multiple ranges using a single spectrometer,” Appl. Opt.. 48, (24 ), 4762 –4766 (2009), CrossRef. 0003-6935 
Zhou  H. Y. et al., “Z(eff) profile measurement system with an optimized Czerny-Turner visible spectrometer in large helical device,” Rev. Sci. Instrum.. 79, (10 ) (2008), CrossRef. 0034-6748 
Belz  M. et al., “Smart-sensor approach for a fibre-optic-based residual chlorine monitor,” Sensor. Actuator. B Chem.. 39, (1–3 ), 380 –385 (1997), CrossRef. 0925-4005 
Buric Falk  M. P. J., Woodruff  S. D., “Conversion of a TEM10 beam into two nearly Gaussian beams,” Appl. Opt.. 49, (4 ), 739 –744 (2010), CrossRef. 0003-6935 
Minassian  A. et al., “High-power scaling (>100  W) of a diode-pumped TEM00 Nd : GdVo(4) laser system,” IEEE J. Sel. Top. Quant. Electron.. 11, (3 ), 621 –625 (2005), CrossRef. 1077-260X 
Ait-Ameur  K., Sanchez  F., Brunel  M., “The transfer of TEM00 and TEM01 beams through a hard-aperture,” J. Mod. Opt.. 47, (7 ), 1203 –1211 (2000). 0950-0340 
Willke  B. et al., “Spatial and temporal filtering of a 10-W Nd : YAG laser with a Fabry-Perot ring-cavity premode cleaner,” Opt. Lett.. 23, (21 ), 1704 –1706 (1998), CrossRef. 0146-9592 
RGB-LaserSystem. Qstick, the world’s smallest USB spectrometer, http://www.rgb-laser.com/content_products/product_qstick.html (23  May 2013).
Chemisana  D., Ibanez  M., “Linear Fresnel concentrators for building integrated applications,” Energ. Convers. Manag.. 51, (7 ), 1476 –1480 (2010), CrossRef. 0196-8904 
Pfeiffer  F. et al., “Nanometer focusing properties of Fresnel zone plates described by dynamical diffraction theory,” Phys. Rev. B. 73, (24 ) (2006), CrossRef. 0163-1829 
Yun  W. et al., “Development of zone plates with a blazed profile for hard x-ray applications,” Rev. Sci. Instrum.. 70, (9 ), 3537 –3541 (1999), CrossRef. 0034-6748 
Kang  H. C. et al., “Nanometer linear focusing of hard x rays by a multilayer Laue lens,” Phys. Rev. Lett.. 96, (12 ) (2006), CrossRef. 0031-9007 
David  C., Nohammer  B., Ziegler  E., “Wet etching of linear Fresnel zone plates for hard X-rays,” Microelectron. Eng.. 61, , 62– 987 –992 (2002), CrossRef. 0167-9317 
Stein  A. et al., “Diffractive x-ray optics using production fabrication methods,” J. Vac. Sci. Tech. B. 21, (1 ), 214 –219 (2003), CrossRef. 0734-211X 
Onda  H., “Development of a unique, high precision linear motor integrated air slide table, and its application to laser beam writers,” Opt. Rev.. 6, (1 ), 88 –92 (1999), CrossRef. 1340-6000 
Serre  D., Deba  P., Koechlin  L., “Fresnel Interferometric Imager: ground-based prototype,” Appl. Opt.. 48, (15 ), 2811 –2820 (2009), CrossRef. 0003-6935 
Wang  Y. X. W., Yun  B., Jacobsen  C., “Achromatic Fresnel optics for wideband extreme-ultraviolet and X-ray imaging,” Nature. 424, (6944 ), 50 –53 (2003), CrossRef. 0028-0836 
Stern Liewer  R. A. K., Janesick  J. R., “Evaluation of a virtual phase charged coupled device as an imaging x-ray spectrometer,” Rev. Sci. Instrum.. 54, (2 ), 198 –205 (1983), CrossRef. 0034-6748 
Cammi  C. et al., “Custom single-photon avalanche diode with integrated front-end for parallel photon timing applications,” Rev. Sci. Instrum.. 83, (3 ) (2012), CrossRef. 0034-6748 
Griffiths  J. A. et al., “Characterization study of an intensified complementary metal-oxide-semiconductor active pixel sensor,” Rev. Sci. Instrum.. 82, (3 ), (2011), CrossRef. 0034-6748 
Vickers  J. S., Chakrabarti  S., “Silicon-anode detector with integrated electronics for microchannel-plate imaging detectors,” Rev. Sci. Instrum.. 70, (7 ), 2912 –2916 (1999), CrossRef. 0034-6748 
National Instruments, Data Acquisition (DAQ), http://www.ni.com/data-acquisition/ (23  May 2013).
Measurement Computing Co., M. C. Data Acquisition Products from MCC, http://www.mccdaq.com/ (23  May 2013).
Microsoft. Visual Studio, http://www.microsoft.com/visualstudio/en-us (23  May 2013).
Romoli  M. et al., “Stray-light suppression in a reflecting white-light coronagraph,” Appl. Opt.. 32, (19 ), 3559 –3569 (1993), CrossRef. 0003-6935 
Voigtman  E., Yuzefovsky  A. I., Michel  R. G., “Stray light effects in Zeeman atomic-absorption spectrometry,” Spectrochim. Acta B Atom. Spectros.. 49, (12 –14), 1629 –1641 (1994), CrossRef. 0584-8547 
Shuang  B. et al., “Stray light in low wavenumber Raman spectra and secondary maxima of grating diffraction,” J. Raman Spectros.. 42, (12 ), 2149 –2153 (2011), CrossRef. 0377-0486 
Shen  H. P. et al., “Stray light and bandpass correction in the spectral measurement for light emitting diodes,” Spectros. Spect. Anal.. 29, (6 ), 1493 –1497 (2009), CrossRef. 1000-0593 
Lin  M. et al., “Stray light characterization of an InGaAs anamorphic hyperspectral imager,” Opt. Express. 18, (16 ), 17510 –17520 (2010), CrossRef. 1094-4087 
Born  M., Wolf  E., Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light. , 7th ed.,  Cambridge University Press ,  New York  (1999).
Marathay  A. S., McCalmont  J. F., “Vector diffraction theory for electromagnetic waves,” J. Opt. Soc. Am. Opt. Image Sci. Vis.. 18, (10 ), 2585 –2593 (2001), CrossRef. 0740-3232 
Jackson  J. D., Classical Electrodynamics. , 3rd ed.,  Wiley ,  New York  (1998).
Gribbin  J. R., Gribbin  M., Richard Feynman: A Life in Science. ,  Plume ,  New York  (1998).
Yadugiri  V. T., Malhotra  R., “‘Plenty of room’–fifty years after the Feynman lecture,” Curr. Sci.. 99, (7 ), 900 –907 (2010). 0011-3891 
Crocombe  R. A., “Miniature optical spectrometers: the art of the possible, Part IV: new near-infrared technologies and spectrometers,” Spectroscopy. 23, (6 ), 26  (2008).
Crocombe  R. A., “Miniature optical spectrometers, Part III: conventional and laboratory near-infrared spectrometers,” Spectroscopy. 23, (5 ), 40  (2008).
Crocombe  R. A., “Miniature optical spectrometers: follow the money – Part II: the telecommunications boom,” Spectroscopy. 23, (2 ), 56  (2008).
Crocombe  R. A., “Miniature optical spectrometers: there’s plenty of room at the bottom Part I, background and mid-infrared spectrometers,” Spectroscopy. 23, (1 ), 1 –15 (2008).
Ashino  M., Ohtsu  M., “Fabrication and evaluation of a localized plasmon resonance probe for near-field optical microscopy/spectroscopy,” Appl. Phys. Lett.. 72, (11 ), 1299 –1301 (1998), CrossRef. 0003-6951 
APS-Group, Introduction iSPEX, http://ispex.nl/project/introductie-ispex/ (23  May 2013).
Sparkfun, Color Light Sensor – Avago ADJD-S371-Q999, http://www.sparkfun.com/products/8618 (23  May 2013).
Simpson  M. L. et al., “A photospectrometer realized in a standard integrated circuit process,” Rev. Sci. Instrum.. 69, (2 ), 377 –383 (1998), CrossRef. 0034-6748 
Hamamatsu, Mini-spectrometer, http://jp.hamamatsu.com/products/sensor-ssd/pd186/index_en.html (29  June 2012).
GLoptic, Mini-Spectrometer, Precise Light Measuring Technology in a handy size, http://www.gloptic.com/mini-spectrometer.php (29  June 2012).
Optics, O. STS Microspectrometer, http://www.oceanoptics.com/products/sts.asp (23  May 2013).
Hirai  Y. et al., “Novel mold fabrication for nano-imprint lithography to fabricate single-electron tunneling devices,” Japanese J. Appl. Phys.. 38, (Part 1, No. 12B ), 7272 –7275 (1999), CrossRef. 0021-4922 
Moore  S. K., “Imprint lithography for nano-components,” IEEE Spectrum. 39, (5 ), 25 –26 (2002). 0018-9235 
Hirai  Y. et al., “Nano-imprint lithography using replicated mold by Ni electroforming,” Japanese J. Appl. Phys.. 41, (Part 1, No. 6B ), 4186 –4189 (2002), CrossRef. 0021-4922 
Schift  H., Park  S., Gobrecht  J., “Nano-imprint molding resists for lithography,” J. Photopolymer Sci. Tech.. 16, (3 ), 435 –438 (2003), CrossRef. 0914-9244 
Lee  H., “Effect of imprinting pressure on residual layer thickness in UV nano-imprint lithography,” J. Vac. Sci. Tech. B. 23, (5 ), 2176 –2176 (2005), CrossRef. 0734-211X 
Lee  H. et al., “Fabrication of nano-sized resist patterns on flexible plastic film using thermal curing nano-imprint lithography,” Microelectron. Eng.. 83, (2 ), 323 –327 (2006), CrossRef. 0167-9317 
Hong  H. C. W., Hsu  H., Shy  J. T., “In-situ monitoring of pattern filling in nano-imprint lithography using surface plasmon resonance,” J. Nanosci. Nanotech.. 11, (6 ), 5279 –5284 (2011), CrossRef. 1533-4880 
Scheerlinck  S. et al., “Metal grating patterning on fiber facets by UV-based nano imprint and transfer lithography using optical alignment,” J. Lightwave Tech.. 27, (10 ), 1417 –1422 (2009), CrossRef. 0733-8724 

Grahic Jump LocationImage not available.

Yeonjoon Park is a senior staff researcher at the National Institute of Aerospace at Hampton, Virginia. He has supported NASA Langley Research Center after he received his master’s degree in physics at Seoul National University in South Korea and PhD in engineering at University of California at Berkeley. He received R&D 100 award and Solar Industry Award for new rhombohedral hybrid crystal epitaxy. He is a leading scientist in the Solar-Powered-Airship program of DoT and NASA. He has 18 US patents, 60 invention disclosures, 12 NASA Innovation awards, and numerous publications. He is a member of SPIE and AIAA.

Grahic Jump LocationImage not available.

Sang H. Choi has been a senior scientist at NASA Langley Research Center since 1980. He has received numerous awards from NASA and two Nano50 Awards in 2006 and 2007, as well as a Nano50 Award in the “Innovator” category. He has won the 2009 R&D100 Award and the 2010 SOLAR Award. He has published 186 papers and reports. He has filed 91 inventions with 22 patents and has 19 patents pending. He has given 16 invited and keynote talks. He serves as associate editor for six journals and as an editor-in-chief for a journal. He is a fellow of SPIE and an associate fellow of AIAA.

© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Citation

Yeonjoon Park and Sang H. Choi
"Miniaturization of optical spectroscopes into Fresnel microspectrometers", J. Nanophoton. 7(1), 077599 (Jun 03, 2013). ; http://dx.doi.org/10.1117/1.JNP.7.077599


Figures

Graphic Jump LocationF1 :

(a) Joseph von Fraunhofer demonstrating the spectroscope, a painting by Richard Wimmer (public domain media from wikimedia.org), (b) mathematical configuration diagram when light passes a spot, S(η,ξ,0) on an aperture and illuminates a point P(x,y,z) on a screen.12

Graphic Jump LocationF6 :

Comparison of different wavelength/energy interpretations from similar sinusoidal photon distribution on sensor pixel array of Fraunhofer and Fresnel spectrometers.

Graphic Jump LocationF5 :

Cross-sectional views of photon distributions (a) above and below the imaging sensor surface with 533 nm light passing through a 100-line Fresnel grating of 400 μm height located at the left side, (b) above the imaging sensor surface with light of three wavelengths, 432 nm, 532 nm and 632 nm, passing through a 100-line Fresnel grating of 400 μm height, and (c) for the same three lights (432, 532, 632 nm) passing through a 156-line Fresnel grating of 500 μm height.

Graphic Jump LocationF4 :

(a) Various types of Fresnel gratings (zone-plates), (b) 3-D configuration of conventional Fraunhofer spectrometer and (c) linear Fresnel spectrometer.

Graphic Jump LocationF3 :

(a) Optical performance test with mixed yellow light from green and red lasers, (b) configuration diagram of Fresnel circular spectrometer, (c) separation of 633 nm photons (red) from incoming yellow light, (d) separation of 533 nm photons (green) from incoming yellow light, (e) detector current measured while scanning optical distance, (f) theoretical spectral resolution versus number of rings in Fresnel circular grating.

Graphic Jump LocationF2 :

SEM image of (a) a single Fresnel circular grating of 750 μm diameter and (b) an array of Fresnel circular gratings.

Graphic Jump LocationF8 :

(a) Side-light-loading scheme and the definition of variables, (b) front-light-loading scheme, (c) optical microscope image of the linear Fresnel grating, (d) SEM image of the 90-deg mirror with a gradient linear Fresnel grating.

Graphic Jump LocationF9 :

Screen shot of the microspectrometer control program.

Graphic Jump LocationF10 :

Software algorithms including the Fresnel spectrum data acquisition/conversion routines, (a) after each single scan, (b) after all single scans are finished, and (c) after all multiscan pixel data streams are acquired.

Graphic Jump LocationF7 :

The size of the linear imaging sensor array chip which was used to fabricate a linear Fresnel spectrometer.

Graphic Jump LocationF11 :

Measured data from the first prototype linear Fresnel spectrometer chip: (a) data from all pixels with one laser, (b) comparison of measured spectra from two He-Ne lasers (1.96 eV and 2.29 eV) and (c) photo of the first prototype front-light-loading linear Fresnel spectrometer with a 90-deg mirror inside.

Tables

References

Purvis  T. L., Revolutionary America. , 1763–1800,  New York  (1995).
Jackson  M. W., Spectrum of Belief: Joseph von Fraunhofer and the Craft of Precision Optics. ,  MIT Press ,  Boston  (2000).
Green  M. J., Barner  B. J., Corn  R. M., “Real-time sampling electronics for double modulation experiments with fourier-transform infrared spectrometers,” Rev. Sci. Instrum.. 62, (6 ), 1426 –1430 (1991), CrossRef. 0034-6748 
Hartland  G. V. et al., “Time-resolved fourier-transform spectroscopy with 0.25  cm−1 spectral and less-than-10(-7) s time resolution in the visible region,” Rev. Sci. Instrum.. 63, (6 ), 3261 –3267 (1992), CrossRef. 0034-6748 
Feng  C. et al., “Miniaturization of step mirrors in a static Fourier transform spectrometer: theory and simulation,” J. Opt. Soc. Am. B Opt. Phys.. 28, (1 ), 128 –133 (2011), CrossRef. 0740-3224 
Davis  M. A., Kersey  A. D., “Application of a fiber Fourier-transform spectrometer to the detection of wavelength-encoded signals from Bragg grating sensors,” J. Lightw. Technol.. 13, (7 ), 1289 –1295 (1995), CrossRef. 0733-8724 
Qu  H. et al., “All photonic bandgap fiber spectroscopic system for detection of refractive index changes in aqueous analytes,” Sens. Actuat. B Chem.. 161, (1 ), 235 –243 (2012), CrossRef. 0925-4005 
Chitteboyina  M. M., Butler  D. P., “Tunable infrared microspectrometer based on Bragg grating,” IEEE J. Quant. Electron.. 44, (1–2 ), 182 –184 (2008), CrossRef. 0018-9197 
Rodrigo  J. A. et al., “Fresnel diffraction effects in Fourier-transform arrayed waveguide grating spectrometer,” Opt. Express. 15, (25 ), 16431 –16441 (2007), CrossRef. 1094-4087 
Zhao  J., McCreery  R. L., “Multichannel FT-Raman spectroscopy: noise analysis and performance assessment,” Appl. Spectrosc.. 51, (11 ), 1687 –1697 (1997), CrossRef. 0003-7028 
O’Connor  J. J., Robertson  E. F., Augustin-Jean Fresnel. MacTutor History of Mathematics archive, University of St. Andrews, http://www-history.mcs.st-andrews.ac.uk/Biographies/Fresnel.html.
Wimmer  R., Essay in Astronomy. ,  D. Appleton & Company ,  New York  (1900).
Baker  B. B., The Mathematical Theory of Huygens’ Principle. ,  Society Chelsea Publishing  (2003).
Ghatak  A. K., Thyagarayan  K., Optical Electronics. ,  Cambridge University Press ,  New York  (1989).
Stroke  G. W., “Fourier-transform spectroscopy using holographic imaging without computing and with stationary interferometers,” Phys. Lett.. 16, , 272  (1965), CrossRef. 0031-9163 
Samson  J. A. R., Techniques of Vacuem Ultraviolet Spectroscopy. ,  Wiley ,  New York  (1967).
Hirsch  P. M., Jordan  J. A., Lesem  L. B., IBM Technical Disclosure Bulletin. , Vol. 12, p. 1806  (1970).
Jordan  J. A. et al., “Kinoform lenses,” Appl. Opt.. 9, (8 ), 1883  (1970), CrossRef. 0003-6935 
Keating  P. N., Sawatari  T., Mueller  R. K., “Fresnel-zone-plate spectrometer with central stop,” J. Opt. Soc. Am.. 62, (8 ), 945  (1972), CrossRef. 0030-3941 
Corbin  A., The Third Element: A Brief History of Electronics. ,  AuthorHouse ,  Bloomington, Indiana  (2006).
Kitaura  N., Ogata  S., Mori  Y., “Spectrometer employing a micro-Fresnel lens,” Opt. Eng.. 34, (2 ), 584 –588 (1995), CrossRef. 0091-3286 
Wenzel  R. G., “Effect of the aperture-lens separation on the focal shift in large-F-number systems,” J. Opt. Soc. Am. Opt. Image Sci. Vis.. 4, (2 ), 340 –345 (1987), CrossRef. 0740-3232 
Takeya  N. et al., “Holographic collimator lens with small F-number,” Optik. 88, (2 ), 80 –82 (1991). 0030-4026 
Guha  S., “Validity of the paraxial approximation in the focal region of a small-f-number lens,” Opt. Lett.. 26, (20 ), 1598 –1600 (2001), CrossRef. 0146-9592 
Schurig  D., “An aberration-free lens with zero F-number,” New J. Phys.. 10,  (2008), CrossRef. 1367-2630 
Park  Y. et al., “Miniaturization of a Fresnel spectrometer,” J. Opt. Pure Appl. Opt.. 10, (9 ) (2008), CrossRef. 1464-4258 
Goodman  J., Introduction to Fourier Optics. , pp. 158 –160,  Roberts and Company Publishers ,  Englewood Colorado  (2004).
Zhang  N. et al., “Spectral-domain optical coherence tomography with a Fresnel spectrometer,” Opt. Lett.. 37, (8 ), 1307 –1309 (2012), CrossRef. 0146-9592 
Yang  C. A. et al., “Proposal and demonstration of a spectrometer using a diffractive optical element with dual dispersion and focusing functionality,” Opt. Lett.. 36, (11 ), 2023 –2025 (2011), CrossRef. 0146-9592 
Jeon  S. C. et al., “A simplified fabrication process of fresnel zone plates with controlling proximity effect correction,” J. Nanosci. Nanotechnol.. 11, (1 ), 503 –506 (2011), CrossRef. 1533-4880 
Hagstrum  H. D. D., Pretzer  D., Takeishi  Y., “Focused slow ion beam for study of electron ejection from solids,” Rev. Sci. Instrum.. 36, (8 ), 1183  (1965), CrossRef. 0034-6748 
Nagamachi  S. et al., “Focused ion-beam direct deposition of metal thin film,” Rev. Sci. Instrum.. 67, (6 ), 2351 –2359 (1996), CrossRef. 0034-6748 
Fu  Y. Q. et al., “Experimental study of three-dimensional microfabrication by focused ion beam technology,” Rev. Sci. Instrum.. 71, (2 ), 1006 –1008 (2000), CrossRef. 0034-6748 
Petit  D. et al., “Nanometer scale patterning using focused ion beam milling,” Rev. Sci. Instrum.. 76, (2 ) (2005), CrossRef. 0034-6748 
Kunstmann  T. et al., “Focused ion beam milling monitored by an additional electrode,” Rev. Sci. Instrum.. 77, (8 ) (2006), CrossRef. 0034-6748 
Sawaragi  H. et al., “Performance of a combined focused ion and electron-beam system,” J. Vac. Sci. Tech. B. 8, (6 ), 1848 –1852 (1990), CrossRef. 0734-211X 
Muhle  R., “High-field ion sources and applications,” Rev. Sci. Instrum.. 63, (5 ), 3040 –3049 (1992), CrossRef. 0034-6748 
Seki  S. et al., “Development of an instrument for simultaneous detection of positive and negative scanning ion images,” Appl. Surf. Sci.. 231, , 976 –980 (2004), CrossRef. 0169-4332 
Fokkema Hagen  C. W. E., Kruit  P., “Brightness measurements of a gallium liquid metal ion source,” J. Vac. Sci. Tech. B. 26, (6 ), 2091 –2096 (2008), CrossRef. 0734-211X 
Santschi  C. et al., “Interdigitated 50 nm Ti electrode arrays fabricated using Xef2 enhanced focused ion beam etching,” Nanotechnology. 17, (11 ), 2722 –2729 (2006), CrossRef. 0957-4484 
Taniguchi  J. et al., “Focused-ion-beam-assisted etching of diamond in XeF2,” J. Vac. Sci. Tech. B. 16, (4 ), 2506 –2510 (1998), CrossRef. 0734-211X 
Harriott  L. R., “Focused ion-beam Xef2 etching of materials for phase-shift masks,” J. Vac. Sci. Tech. B. 11, (6 ), 2200 –2203 (1993), CrossRef. 0734-211X 
Nakamura  H., Komano  H., Ogasawara  M., “Focused ion-beam assisted etching of quartz in Xef2 without transmittance reduction for phase-shifting mask repair,” Japanese J. Appl. Phys.. 31, (Part 1, No. 12B ), 4465 –4467 (1992), CrossRef. 0021-4922 
Kettle  J., Hoyle  R. T., Dimov  S., “Fabrication of Step-and-Flash Imprint Lithography (S-FIL) templates using XeF2 enhanced focused ion-beam etching,” Appl. Phys. Mater. Sci. Process.. 96, (4 ), 819 –825 (2009), CrossRef. 0947-8396 
Brown  C. W., Lin  J., “Interfacing a fiberoptic probe to a diode-array uv-visible spectrophotometer for drug dissolution tests,” Appl. Spectros.. 47, (5 ), 615 –618 (1993), CrossRef. 0003-7028 
Moreau  F. et al., “Fiber-optic remote multisensor system based on an acousto-optic tunable filter (AOTF),” Appl. Spectros.. 50, (10 ), 1295 –1300 (1996), CrossRef. 0003-7028 
Chen  G. et al., “Design of a hybrid integrated microfiber spectrometer,” J. Microlithography Microfabrication and Microsystems. 2, (3 ), 191 –194 (2003), CrossRef. 1537-1646 
Lienert  B., Porter  J., Sharma  S. K., “Simultaneous measurement of spectra at multiple ranges using a single spectrometer,” Appl. Opt.. 48, (24 ), 4762 –4766 (2009), CrossRef. 0003-6935 
Zhou  H. Y. et al., “Z(eff) profile measurement system with an optimized Czerny-Turner visible spectrometer in large helical device,” Rev. Sci. Instrum.. 79, (10 ) (2008), CrossRef. 0034-6748 
Belz  M. et al., “Smart-sensor approach for a fibre-optic-based residual chlorine monitor,” Sensor. Actuator. B Chem.. 39, (1–3 ), 380 –385 (1997), CrossRef. 0925-4005 
Buric Falk  M. P. J., Woodruff  S. D., “Conversion of a TEM10 beam into two nearly Gaussian beams,” Appl. Opt.. 49, (4 ), 739 –744 (2010), CrossRef. 0003-6935 
Minassian  A. et al., “High-power scaling (>100  W) of a diode-pumped TEM00 Nd : GdVo(4) laser system,” IEEE J. Sel. Top. Quant. Electron.. 11, (3 ), 621 –625 (2005), CrossRef. 1077-260X 
Ait-Ameur  K., Sanchez  F., Brunel  M., “The transfer of TEM00 and TEM01 beams through a hard-aperture,” J. Mod. Opt.. 47, (7 ), 1203 –1211 (2000). 0950-0340 
Willke  B. et al., “Spatial and temporal filtering of a 10-W Nd : YAG laser with a Fabry-Perot ring-cavity premode cleaner,” Opt. Lett.. 23, (21 ), 1704 –1706 (1998), CrossRef. 0146-9592 
RGB-LaserSystem. Qstick, the world’s smallest USB spectrometer, http://www.rgb-laser.com/content_products/product_qstick.html (23  May 2013).
Chemisana  D., Ibanez  M., “Linear Fresnel concentrators for building integrated applications,” Energ. Convers. Manag.. 51, (7 ), 1476 –1480 (2010), CrossRef. 0196-8904 
Pfeiffer  F. et al., “Nanometer focusing properties of Fresnel zone plates described by dynamical diffraction theory,” Phys. Rev. B. 73, (24 ) (2006), CrossRef. 0163-1829 
Yun  W. et al., “Development of zone plates with a blazed profile for hard x-ray applications,” Rev. Sci. Instrum.. 70, (9 ), 3537 –3541 (1999), CrossRef. 0034-6748 
Kang  H. C. et al., “Nanometer linear focusing of hard x rays by a multilayer Laue lens,” Phys. Rev. Lett.. 96, (12 ) (2006), CrossRef. 0031-9007 
David  C., Nohammer  B., Ziegler  E., “Wet etching of linear Fresnel zone plates for hard X-rays,” Microelectron. Eng.. 61, , 62– 987 –992 (2002), CrossRef. 0167-9317 
Stein  A. et al., “Diffractive x-ray optics using production fabrication methods,” J. Vac. Sci. Tech. B. 21, (1 ), 214 –219 (2003), CrossRef. 0734-211X 
Onda  H., “Development of a unique, high precision linear motor integrated air slide table, and its application to laser beam writers,” Opt. Rev.. 6, (1 ), 88 –92 (1999), CrossRef. 1340-6000 
Serre  D., Deba  P., Koechlin  L., “Fresnel Interferometric Imager: ground-based prototype,” Appl. Opt.. 48, (15 ), 2811 –2820 (2009), CrossRef. 0003-6935 
Wang  Y. X. W., Yun  B., Jacobsen  C., “Achromatic Fresnel optics for wideband extreme-ultraviolet and X-ray imaging,” Nature. 424, (6944 ), 50 –53 (2003), CrossRef. 0028-0836 
Stern Liewer  R. A. K., Janesick  J. R., “Evaluation of a virtual phase charged coupled device as an imaging x-ray spectrometer,” Rev. Sci. Instrum.. 54, (2 ), 198 –205 (1983), CrossRef. 0034-6748 
Cammi  C. et al., “Custom single-photon avalanche diode with integrated front-end for parallel photon timing applications,” Rev. Sci. Instrum.. 83, (3 ) (2012), CrossRef. 0034-6748 
Griffiths  J. A. et al., “Characterization study of an intensified complementary metal-oxide-semiconductor active pixel sensor,” Rev. Sci. Instrum.. 82, (3 ), (2011), CrossRef. 0034-6748 
Vickers  J. S., Chakrabarti  S., “Silicon-anode detector with integrated electronics for microchannel-plate imaging detectors,” Rev. Sci. Instrum.. 70, (7 ), 2912 –2916 (1999), CrossRef. 0034-6748 
National Instruments, Data Acquisition (DAQ), http://www.ni.com/data-acquisition/ (23  May 2013).
Measurement Computing Co., M. C. Data Acquisition Products from MCC, http://www.mccdaq.com/ (23  May 2013).
Microsoft. Visual Studio, http://www.microsoft.com/visualstudio/en-us (23  May 2013).
Romoli  M. et al., “Stray-light suppression in a reflecting white-light coronagraph,” Appl. Opt.. 32, (19 ), 3559 –3569 (1993), CrossRef. 0003-6935 
Voigtman  E., Yuzefovsky  A. I., Michel  R. G., “Stray light effects in Zeeman atomic-absorption spectrometry,” Spectrochim. Acta B Atom. Spectros.. 49, (12 –14), 1629 –1641 (1994), CrossRef. 0584-8547 
Shuang  B. et al., “Stray light in low wavenumber Raman spectra and secondary maxima of grating diffraction,” J. Raman Spectros.. 42, (12 ), 2149 –2153 (2011), CrossRef. 0377-0486 
Shen  H. P. et al., “Stray light and bandpass correction in the spectral measurement for light emitting diodes,” Spectros. Spect. Anal.. 29, (6 ), 1493 –1497 (2009), CrossRef. 1000-0593 
Lin  M. et al., “Stray light characterization of an InGaAs anamorphic hyperspectral imager,” Opt. Express. 18, (16 ), 17510 –17520 (2010), CrossRef. 1094-4087 
Born  M., Wolf  E., Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light. , 7th ed.,  Cambridge University Press ,  New York  (1999).
Marathay  A. S., McCalmont  J. F., “Vector diffraction theory for electromagnetic waves,” J. Opt. Soc. Am. Opt. Image Sci. Vis.. 18, (10 ), 2585 –2593 (2001), CrossRef. 0740-3232 
Jackson  J. D., Classical Electrodynamics. , 3rd ed.,  Wiley ,  New York  (1998).
Gribbin  J. R., Gribbin  M., Richard Feynman: A Life in Science. ,  Plume ,  New York  (1998).
Yadugiri  V. T., Malhotra  R., “‘Plenty of room’–fifty years after the Feynman lecture,” Curr. Sci.. 99, (7 ), 900 –907 (2010). 0011-3891 
Crocombe  R. A., “Miniature optical spectrometers: the art of the possible, Part IV: new near-infrared technologies and spectrometers,” Spectroscopy. 23, (6 ), 26  (2008).
Crocombe  R. A., “Miniature optical spectrometers, Part III: conventional and laboratory near-infrared spectrometers,” Spectroscopy. 23, (5 ), 40  (2008).
Crocombe  R. A., “Miniature optical spectrometers: follow the money – Part II: the telecommunications boom,” Spectroscopy. 23, (2 ), 56  (2008).
Crocombe  R. A., “Miniature optical spectrometers: there’s plenty of room at the bottom Part I, background and mid-infrared spectrometers,” Spectroscopy. 23, (1 ), 1 –15 (2008).
Ashino  M., Ohtsu  M., “Fabrication and evaluation of a localized plasmon resonance probe for near-field optical microscopy/spectroscopy,” Appl. Phys. Lett.. 72, (11 ), 1299 –1301 (1998), CrossRef. 0003-6951 
APS-Group, Introduction iSPEX, http://ispex.nl/project/introductie-ispex/ (23  May 2013).
Sparkfun, Color Light Sensor – Avago ADJD-S371-Q999, http://www.sparkfun.com/products/8618 (23  May 2013).
Simpson  M. L. et al., “A photospectrometer realized in a standard integrated circuit process,” Rev. Sci. Instrum.. 69, (2 ), 377 –383 (1998), CrossRef. 0034-6748 
Hamamatsu, Mini-spectrometer, http://jp.hamamatsu.com/products/sensor-ssd/pd186/index_en.html (29  June 2012).
GLoptic, Mini-Spectrometer, Precise Light Measuring Technology in a handy size, http://www.gloptic.com/mini-spectrometer.php (29  June 2012).
Optics, O. STS Microspectrometer, http://www.oceanoptics.com/products/sts.asp (23  May 2013).
Hirai  Y. et al., “Novel mold fabrication for nano-imprint lithography to fabricate single-electron tunneling devices,” Japanese J. Appl. Phys.. 38, (Part 1, No. 12B ), 7272 –7275 (1999), CrossRef. 0021-4922 
Moore  S. K., “Imprint lithography for nano-components,” IEEE Spectrum. 39, (5 ), 25 –26 (2002). 0018-9235 
Hirai  Y. et al., “Nano-imprint lithography using replicated mold by Ni electroforming,” Japanese J. Appl. Phys.. 41, (Part 1, No. 6B ), 4186 –4189 (2002), CrossRef. 0021-4922 
Schift  H., Park  S., Gobrecht  J., “Nano-imprint molding resists for lithography,” J. Photopolymer Sci. Tech.. 16, (3 ), 435 –438 (2003), CrossRef. 0914-9244 
Lee  H., “Effect of imprinting pressure on residual layer thickness in UV nano-imprint lithography,” J. Vac. Sci. Tech. B. 23, (5 ), 2176 –2176 (2005), CrossRef. 0734-211X 
Lee  H. et al., “Fabrication of nano-sized resist patterns on flexible plastic film using thermal curing nano-imprint lithography,” Microelectron. Eng.. 83, (2 ), 323 –327 (2006), CrossRef. 0167-9317 
Hong  H. C. W., Hsu  H., Shy  J. T., “In-situ monitoring of pattern filling in nano-imprint lithography using surface plasmon resonance,” J. Nanosci. Nanotech.. 11, (6 ), 5279 –5284 (2011), CrossRef. 1533-4880 
Scheerlinck  S. et al., “Metal grating patterning on fiber facets by UV-based nano imprint and transfer lithography using optical alignment,” J. Lightwave Tech.. 27, (10 ), 1417 –1422 (2009), CrossRef. 0733-8724 

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