Precision radial velocity measurement requires spectral calibration sources that exhibit short to long term wavelength stability and high modal density. Here we report on our efforts to develop a novel 400-1800 nm whispering gallery mode resonator etalon for calibration of optical or infrared spectrographs. The small crystalline MgF2 ring etalon is stable to better than 10-11 levels on 1 s timescales but requires environmental isolation and an referencing to an external standard to maintain long term performance. We will summarize our efforts thus far, as well as the performance achieved over 10 day measurements.
Directly imaging Earth-sized exoplanets with a visible-light coronagraph instrument on a space telescope will require a system that can achieve approximately 10−10 raw contrast and maintain it for the duration of observations (on the order of hours or more). We are designing, manufacturing, and testing Dual Purpose Lyot coronagraph (DPLC) masks that allow for simultaneous wavefront sensing and control using out-of-band light to maintain high contrast in the science focal plane. Our initial design uses a tiered metallic focal plane occultor to suppress starlight in the transmitted coronagraph channel and a dichroic-coated substrate to reflect out-of-band light to a wavefront sensing camera. The occultor design introduces a phase shift such that the reflected channel is a Zernike wavefront sensor. The dichroic coating allows higher-order wavefront errors to be detected which is especially critical for compensating for residual drifts from an actively-controlled segmented primary mirror. A second-generation design concept includes a metasurface to create polarization-dependent phase shifts in the reflected beam, which has several advantages including an extended dynamic range. We will present the focal plane mask designs, characterization, and initial testing at NASA’s High Contrast Imaging Testbed (HCIT) facility
The PAlomar Radial Velocity Instrument (PARVI) is a diffraction-limited, high-resolution spectrograph connected by single-mode fiber to the 200 inch Hale telescope at Palomar Observatory. Here, we present on-sky results for HD 189733 obtained during PARVI’s commissioning phase. We first describe the implementation of our spectral extraction and radial velocity (RV) generation codes. Through RV monitoring, we detect the Rossiter–Mclaughlin signal of the transiting planet HD 189733 b. We further detect the presence of water and carbon monoxide in the atmosphere of HD 189733 b via transmission spectroscopy. This work demonstrates PARVI’s high-resolution spectral capabilities at H band and current intra-night Doppler stability of ∼4 to 10 m s − 1 on an early K dwarf. Finally, we discuss the limitations to this work and ongoing efforts to characterize and improve the Doppler performance of PARVI to the design goal of ∼1 m s − 1 for late-type stars.
State-of-the-art semiconductor lasers can deliver average power, linewidth, and beam quality suitable for supporting differential absorption (DIAL) instruments that are competitive with fiber and solid-state lasers. An all-semiconductor transmitter architecture can enable a drastic reduction in size, weight, and power consumption (SWaP) of the instrument, while allowing for beneficial wavelength agility. Crucially, this reduction in SWaP can enable the implementation of compact airborne and spaceborne profiling DIAL instruments with high power output, while the broad spectral coverage of semiconductor laser technology allows the adaption and tuning of the transmitter design across a variety of operating scenarios. In this work, we present the first demonstration of volumetric ranging based on an all-semiconductor intensity-modulated CW (IMCW) transmitter. For this proof-of-concept demonstration, we used Rayleigh backscattering in optical fiber to emulate the atmospheric backscattering return echo. The range-resolved profile is reconstructed using matched filtering of the return echo, a technique widely adopted in CW radar. Finally, we present a theoretical analysis grounded in CW radar theory, showing excellent agreement with the results measured across a wide range of transmitted waveforms and return target configurations.
Frequency combs based on mid-infrared cascade lasers have been studied both experimentally and theoretically in recent years. So far only FM combs with quasi-cw output have been reported for interband cascade lasers (ICLs). We discuss the parameters that need to be achieved to realize passive mode locking in ICLs. The results are obtained from a comprehensive numerical model based on the wavevector-resolved Bloch equations coupled to the one-dimensional wave equation. We find that the design of the saturable absorber, in particular the carrier extraction time and length, is very important, while passive mode locking should already be achievable for the experimentally demonstrated values of group velocity dispersion. The leakage into the high-index GaSb substrate should also be controlled via the waveguide design.
KEYWORDS: Near infrared, Stars, Single mode fibers, Motion measurement, Frequency combs, Telescopes, Spectroscopy, Spectrographs, Spectral resolution, Signal detection
The field of precision radial velocities (PRVs) aims to detect radial velocity (RV) signals on the order of 1 m/s. The motivation for the push into PRV is to detect the reflex motion of stars induced by Earth-sized orbiting planets. Measuring PRVs in the near-infrared (NIR) provides a number of advantages over optical, such as reduced noise from stellar jitter, and wealth of RV information encoded in the NIR absorption features of cool, low-mass stars. The Palomar Radial Velocity Instrument (PARVI) implements three key strategies to achieve 1 m/s RV precision in the NIR: single-mode fiber (SMF) feeds, thermo-mechanical stabilization of the spectrograph, and a line-referenced, electro-optical modulation frequency comb (LR-EOFC). PARVI is a J & H band (1145-1766 nm) echelle spectrometer with spectral resolution 87,000–121,000. It was installed at the Hale 200" telescope summer 2019, and since then has undergone multiple hardware upgrades to maximize stability. Using the laser frequency comb (LFC) as a light source, we measure a science channel to reference channel stability of 0.001 pixels over the timespan of a single observing night. This measurement includes the motion of 2790 LFC lines over 17 spectral orders in the H band, and corresponds to a radial velocity precision of approximately 1 m/s at the LFC pump line (lambda_p = 1560 nm).
Mid-infrared semiconductor lasers have emerged as indispensable compact coherent sources for military and commercial applications. While much of the historical emphasis has been on maximizing the output power and/or spectral purity, a recent new focus has been on engineering these lasers to operate as optical frequency combs (OFCs) for broadband real-time spectroscopy. In particular, the combination of low-drive-power and broad gain bandwidth has made interband cascade laser (ICL) OFCs an attractive complement to quantum cascade laser OFCs operating at longer wavelengths. Moreover, ICL combs can potentially be incorporated into fully-integrated dual-comb spectrometers that employ fast, room-temperature IC photodetectors processed on the same chip. However, the high refractive index of the ICL’s GaSb substrate poses some challenges to the optical waveguiding. Because the modal index is considerably lower than that of the substrate, the optical field can penetrate the bottom cladding layer and leak into the GaSb, inducing wavelength-dependent interference that modifies the gain and group velocity dispersion (GVD) profiles. Even when the effect on lasing threshold is small, the comb properties can be adversely affected. Using the sub-threshold Fourier transform technique, we studied ICL combs with various ridge widths, substrate thicknesses, and center wavelengths. This allowed us to evaluate the effects of modal leakage on the GVD. We find that the resonant nature of the substrate modes induces oscillations, which affect both the spectral bandwidth and the phase-locking properties above threshold. Strategies to mitigate the GVD’s undesired and unpredictable spectral variation will be presented.
We report on our efforts to develop a whispering gallery mode resonator etalon as a tool for precision radial velocity observations to detect exoplanets. The crystalline MgF2 etalon will be referenced to a compact fiber laser frequency comb, and will serve as the wavelength calibration source for a stabilized, high resolution, visible band spectrograph. The extreme stability required for the detection and characterization of exo-Earths orbiting solar-type stars will be achieved by employing a composite resonator structure with a compensating material to balance the resonator’s coefficients of thermal expansion and thermal refractivity. Progress in modeling the etalon to achieve single mode-like performance, and experiments to demonstrate broad-band (octave-spanning) ling to a white light source, are described.
By exploiting the bi-functional operation capability of interband cascade laser (ICL) frequency combs, we have utilized the laser medium not only for comb generation, but also as a room-temperature photodetector with near-GHz bandwidth for multi-heterodyne beating of the comb lines. Our self-contained platform consuming less than 2 W of electrical power enables free-running room-temperature broadband dual-comb spectroscopy of 1,1 difluoroethane with ~3% standard deviation in 2 ms over 600 GHz of optical bandwidth around 3.6 µm. We will discuss progress toward the optimization of ICL combs for realizing compact, low-power chemical sensors operating in the MWIR.
Interband cascade laser (ICL) optical frequency combs are promising midwave infrared sources for dual comb spectroscopy probing the strong fundamental absorption lines of numerous chemical and biological agents. In this work, a 4-mm-long ICL frequency comb emitting at 3.3 μm was operated by single-cavity optical self-injection. The experiments employing a free-space optical self-injection length of 1.1m with fine-delay control demonstrated a tuning range of 119MHz around the free-running intermode beat frequency of 9.58 GHz. For resonant fine-delay the line width of the intermode beat frequency was reduced to 390 kHz, what is an improvement by a factor of 40 in comparison to the solitary laser line width.
Optical frequency combs have revolutionized the field of high resolution real-time molecular spectroscopy. Here, we demonstrate an electrically-driven optical frequency comb whose sub-picosecond pulses span more than 1 THz of spectral bandwidth centered near 3.3 mm. This is achieved by passively mode locking an interband cascade laser in a multi-contact architecture with gain and saturable absorber sections monolithically integrated on the same chip.
Many of the high accuracy Earth science survey missions are planned to use laser-based remote sensing instruments. The 2-μm laser wavelength is of particular interest due to the presence of many CO2 and H20 absorption lines in its vicinity1. Transmitter architectures are typically composed of an optically pumped, frequency-stable, solid-state seed laser and a high-power optical amplifier.2, 3 Taking advantage of the reliability and relative simplicity of semiconductor lasers, this architecture can substantially improve by replacing the solid-state light source with semiconductor lasers of comparable performance. This approach will greatly improve the system reliability, and will simplify instrument integration and space qualification. There are currently very limited semiconductor lasers operating in the 2-μm range with performance satisfactory enough for use as an injection seed in a laser absorption spectrometer. Optimally, seed lasers producing greater than 50 mW of continuous-wave (CW), with frequency jitter of less than 1 MHz are desired to reliably resolve the CO2 absorption lines near 2-μm. In this paper, we report the demonstration of high-power, single-longitudinal-mode laterally coupled distributed feedback (LC-DFB) lasers at 2.05 μm wavelength. We measured more than 80 mW of CW power at -10 ºC for devices with a 4-μm-wide ridge and 2-mm-long cavity.
The Lunar Flashlight (LF) mission will send a CubeSat to lunar orbit via NASA’s Space Launch System (SLS) test flight. The LF spacecraft will carry a novel instrument to quantify and map water ice harbored in the permanently shadowed craters of the lunar South Pole. The LF instrument, an active multi-band reflectometer which employs four high power diode lasers in the 1-2 μm infrared band, will measure the reflectance of the lunar surface near water ice absorption peaks. We present the detailed instrument design and system engineering required to deploy this instrument within very demanding CubeSat resource allocations.
We report efficient wave plates with different retardations and orientations of fast axes realized using transmitarrays
composed of a periodic arrangement of amorphous silicon elliptical cylinders on glass. We show that novel polarization
devices which locally rotate the polarization by different angles while preserving the wavefront can be demonstrated
using such a high contrast transmitarray. We present design, fabrication and experimental characterization results for
near infrared transmissive wave retarders with efficiencies in excess of 90%, and discuss the potential applications of atwill
local polarization control enabled by this technology.
We present design, fabrication, and characterization results of high numerical aperture (NA) micro-lenses based on a
high contrast transmitarray platform. The high contrast transmitarray is created by periodic arrangement of amorphous
silicon posts with different diameters on a fused silica substrate. We report near infrared high NA micro-lenses with spot
sizes as small as 0.57λ and focusing efficiencies in excess of 80%. We demonstrate a trade-off relation between NA and
efficiency of high contrast array flat micro-lenses, and attribute it to the spatial discretization of their phase profiles.
Device characteristics of photonic crystal lasers formed in InGaAsP membranes bonded to a sapphire substrate are discussed. Also discussed are waveguide loss mechanisms in type-A and type-B photonic crystal waveguides and the transmission properties of photonic crystal waveguide bends.
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