2.1

PGP Design

The fundamental optical concept and the geometrical dimensions of the PGP are depicted in Figure 1. In particular, the projection of the system into the plane of dispersion is shown. The input beam is collimated, has a circular shape with diameter of 58mm and it propagates along the optical axis. The overall configuration is highly symmetric, i.e. both prisms are equilateral with side lengths of 130mm and the inner angles are always 60°, respectively. The depth of the overall element (“into the display”) is 60mm. The input beam hits the entrance facet under an angle of incidence (AOI) of 60° and thus its projection yields an elliptical beam with extension of ~117mm x 58mm.

Figure 1.

Sketch of the optical beam path through the PGP disperser.

In between both equilateral prisms the diffraction grating is encapsulated on a 3mm thick fused silica substrate. The grating is operated in Littrow configuration, i.e. AOI and angle of diffraction of the central wavelength (760nm) are equal. The same is true for the entire PGP element, meaning that the diffracted beam at Littrow wavelength propagates along the optical axis after leaving the element. This configuration is highly preferred in order to arrange all optical components of the instrument’s channel (disperser + imager + detector) on a common and single optical axis.

2.2

Grating Design

The particular grating design is driven by two major specifications. First, the minimum angular resolution of the overall PGP disperser and, second, the required throughput and polarization sensitivity of the PGP device. The first requirement translates into an upper threshold of the gratings period and the latter can only be influenced by the geometry of the grating’s nanostructure itself. In the current project, the goal was to design and built a PGO disperser with angular dispersion of >0.15deg/nm and a minimum diffraction efficiency of 90%.

In consequence, the diffraction grating is realized as a surface relief structure which is etched/patterned into a fused silica carrier substrate. A sketch of the structure is provided by Figure 2. It is a binary structure of air voids/grooves which are entirely encapsulated between two fused silica bulk domains on “top and bottom”. In the chosen configuration, the calculated diffraction efficiency of the grating lies well above 95%. The polarization sensitivity is smaller than 2% being determined according to |I_TE – I_TM | / | I_TE + I_TM | with I_TE/TM being the diffraction efficiencies in TE/TM polarization. In order to reach such good performance in diffraction efficiency and polarization sensitivity the grating is characterized by small and deep air grooves with an aspect ratio larger than 1:8.

Figure 2.

Diffraction efficiency of the ideal binary grating profile. The red (green) curve are the TE (TM) polarization efficiencies. The inset shows the fundamental geometrical layout.

3.1

Grating Manufacturing

We use electron-beam lithography and reactive ion etching (RIE) to realize surface relief structures on bulky substrates^4,5,6. In the present scenario, the substrate material is fused silica, but it might be also other materials as long as the surface is coated with SiO₂. The main process flow is depicted in Figure 3. The substrate is coated by a metal mask (Cr) which is again coated by an electron sensitive resist. The resist is exposed to electron beam radiation which defines the binary grating pattern. Two subsequent dry RIE-process are used to i) transfer the resist into the metal mask and ii) to transfer the metal mask into the underlying SiO₂ material.

Figure 3.

Manufacturing process of binary surface relief gratings by e-beam lithography (left image) and reactive ion etching of the chromium layer (middle image) and SiO2 (right image).

The entire manufacturing process is monitored and controlled by various inspection methods, e.g. SEM and AFM. This allows verifying and eventually correcting for correct feature size and depth level. Figure 4 shows an SEM image in cross-section of the final surface relief structure. The side walls of the grating ridges are not perfectly perpendicular, but the groove width becomes smaller with increasing depth. Most importantly, the top of the ridges is perfectly flat which is guaranteed by the protective functionality of the metal mask during the entire etching process. In particular, the surface roughness is equal to those of the initial substrate such that the bonding process will not be negatively affected (see also Section 3.X).

Figure 4.

Cross section profile of the grating ridges achieved by focused ion beam milling and SEM inspection. The platinum (Pt) is only deposited for inspection purposes in order to mechanically stabilize the nanostructures during milling process. The air voids are a consequence of the Pt-deposition process and they do not appear in the remaining grating. The red symbols highlight the outer surface of the grating ridges and they are directly extracted from the SEM image.

3.2

Straylight Performance

The stray light performance of the grating was measured via ALBATROSS, an angle-resolved light scattering setup capable of scanning the ARS over the entire hemisphere (in polar and azimuthal direction). Figure 5 shows a typical measurement of the ARS around the signal order (-1^st diffraction order in transmission). The scattering background shows a nearly constant profile throughout the entire scanning region between -20° and +20° around the signal order. Most interestingly, the signal peak drops down very rapidly within an interval of only 2° to 3° before it approaches the constant background level of ~10^-2 sr^-1 for the detector scanning in the plane of dispersion. When leaving the plane-of dispersion then ARS drops down between 1 and 2 orders of magnitude as also presented in Figure 5 by the red/orange curves.

Figure 5.

ARS-measurement results of the grating. The curves in blue/cyan (red/orange) are measured in-dispersion plane (45°-out of dispersion plane). The blue/red (cyan/orange) curves correspond to two different measurement positions on the grating surface. The black curves show the ARS of an equivalent plane mirror with RMS roughness 1nm and 2nm, respectively according to Wein’s formula⁷.

4.1

Test Samples and Mechanical Stability.

The final PGP will include two directly bonded interfaces, one between two uncorrugated surfaces and a second where the surface relief structure of the diffraction grating is bonded to a bulky fused silica counterpart. Especially, the interface which involves the surface relief structure will most likely have less mechanical stability (due to reduced contacting area) – a measure which must thus be determined quantitatively.

In order to do so, we have first manufactured small bonding test samples (see Figure 6a) of size 55x55x43mm³. The individual building blocks are two fused silica cuboids of size 55x55x20mm³ and a small test grating substrate of size 55x55x3mm³. One can see the labeling “FS8” or “FS6” on the cuboids’ side faces in Figure 6a. The final bonding test samples were then cut into small bars of size 9.5x9.5x43mm³. A Zwick-Roell testing machine of type Z020 and a 3-point bending test set-up as illustrated in Figure 6b were used and ensembles of 8 specimens were tested. Since the direction of load impact relative to the grating structure was found appreciable, the most sensible orientation (with the grating dispersion direction normal to the load direction) was chosen. Load was applied symmetrically to the middle of both bonding planes and fraction occurred neatly in the grating plane in almost all cases. The bond strength of the uncorrugated interface was measured to be ~1/2 of that of the bulk reference structure (small bulky bars without any bonded interface). The bond strength of the grating interface amounts to only 1/3 of the uncorrugated interface and, in consequence, to ~1/6 in comparison with the bulk reference structure. The factor of 1/3 corresponds nearly to the intrinsical loss in contacting area which is attributed to the encapsulated air voids (grating grooves).

Figure 6.

a) Photograph of a bonding test sample used for process development. b) Three-point bending test setup for determining the mechanical strength of the joined component. c) Typical fracture result after bending.

5.1

AR coating and Environmental Testing.

Due to high Fresnel losses of ~1.0-0.84² = 30% in TE polarization at the entrance and exit facet of the PGP, an AR coating must be applied in order to achieve overall efficiencies larger than 90%. An interference layer stack between high and low-index dielectrics was used in order to apply an equal AR-coating on entrance and exit facet of the PGP. Two photographs in Figure 8 show the result.

Figure 8.

The same PGP demonstrator after AR coating applied to entrance and exit surface of the prisms.AR coating can be identified by the thin circumventing edge in the right photograph where no coating is applied due to mounting issues.

After AR coating, the PGP underwent 8 thermal-vacuum-cycles at temperatures between 0°C and 40°C at pressures down to 10^-6 mbar. Measurements of the diffraction efficiency as well as visual inspection before and after thermal cycling revealed no degradation of the element and the diffraction efficiency was proven to be unaffected.

Table 1 presents the diffraction efficiency of the prototype at three discrete wavelength within the NIR band. The measurement was performed by using a narrow-band and tunable laser source. The efficiency was measured before and after AR coating. At the short wavelength limit, the overall efficiency in TE polarization slightly falls below 90%. However, it is a question of further process optimizations (AR coating and grating manufacturing) in order to come closer to the theoretical limits as provided by Figure 2.

Table 1.

Measured diffraction efficiency of the PGP at three wavelength. The performance of the double-pass AR coating can be recognized by measurements before and after the coating process. The presented values are the spatial average throughout the clear aperture of the element.

Finally, a second PGP prototype was exposed to gamma radiation of a Co-60 source. After irradiation of 20krad (Si) no visible degradation of the PGP was observed. More importantly, this observation was again verified by a diffraction efficiency measurement (performed 18h after irradiation).

The data in Table 1 is the spatial average throughout the clear aperture of the PGP. The actual measurement was done with a spot size of ~2mm and an equidistant spatial mapping was performed. Figure 9 shows a representative outcome of such measurement at the center of the NIR band. One can see that the efficiency is nicely uniform throughout the clear aperture with a slight gradient to the edges. However, the performance of a typical imaging spectrometer is mainly driven by the integrated rather the locally resolved efficiency of the main disperser.

Figure 9.

Spatial mapping of the diffraction efficiency over the clear aperture at a wavelength of 760nm.