arXiv subjects

Matthew C. H. Leung

Publications and source records attributed to Matthew C. H. Leung.

7 recordsLinked to original sources

arXiv2026

Coupling loss and transmission in a multimode fiber-fed virtually imaged phased array (VIPA)

The Virtually Imaged Phased Array (VIPA) is a spectral disperser that has seen increasing adoption across various applications, including optical telecommunications, high-resolution spectroscopy, and LiDAR. Although VIPAs are typically fed by single-mode optical fiber in these applications, there is growing interest in using a multimode optical fiber feed to enable higher throughput. However, a multimode fiber feed introduces several challenges, one of which is a fundamental limit on how much light can be coupled into a VIPA. In this work, we derive a closed-form expression for the minimum tilt angle of a multimode fiber-fed VIPA required for no etendue coupling loss, and we show that this angle depends solely on the input fiber's etendue, the focal ratio of the injection cylindrical lens, and the VIPA's intrinsic parameters. Moreover, we derive the 2D and 3D etendue that can be coupled into a VIPA, as well as the VIPA's etendue coupling efficiency and total transmission, given a particular configuration. We validate our mathematical models using ray-tracing simulations in Zemax OpticStudio non-sequential mode. Our results provide practical guidance for the design of high-throughput optical systems based on multimode fiber-fed VIPAs.

physics.optics↗

arXiv2026

Optical design of VIPER: a high-resolution multimode fiber-fed VIPA spectrograph for characterizing exoplanet atmospheric escape

We present the optical design of VIPER: a high-resolution, multimode fiber-fed, narrowband, cross-dispersed, seeing-limited spectrograph based on a Virtually Imaged Phased Array (VIPA), a spectral disperser that provides higher dispersion and a more compact form factor than conventional diffraction gratings. VIPER is specifically designed to probe exoplanet atmospheric escape through the helium 1083 nm triplet line, with a resolving power of 300,000 over a 10 nm wavelength range. VIPER is intended for operation at the 1.5 m Tillinghast Telescope at the Fred Lawrence Whipple Observatory (FLWO) on Mount Hopkins, Arizona, USA, and is matched to this telescope's 100 $μ$m circular-core multimode optical fiber feed at f/6. VIPER's optical design is specifically optimized for high throughput, which, in a VIPA spectrograph, is more challenging with a multimode fiber feed than a single-mode fiber feed because of the larger etendue. To address this challenge, our design implements a novel use of a cylindrical beam expander and pupil slicer. We validate our design using analytic calculations and simulations in Zemax OpticStudio non-sequential mode. Our results demonstrate the feasibility of multimode fiber-fed VIPA spectrographs as compact, high-throughput alternatives to conventional grating-based spectrographs for exoplanet science and other astronomical applications.

astro-ph.IM↗

arXiv2025

Crank-rocker optical fiber mode scrambler prototype for the GMT-Consortium Large Earth Finder (G-CLEF)

When coherent light propagates through a multimode optical fiber, the modes interfere at the fiber exit boundary, producing a high-contrast speckle interference pattern called modal noise. This non-uniform interference pattern introduces systematic errors in fiber-fed precision radial velocity (RV) spectrographs which are detrimental to exoplanet mass measurement. Modal noise can be mitigated by a device called a fiber mode scrambler or fiber agitator, which dynamically perturbs the fiber to change the interference pattern over time, smoothing it over long exposures. In this paper, we present a prototype optical fiber mode scrambler based on a four-bar linkage crank-rocker mechanism, developed for the GMT-Consortium Large Earth Finder (G-CLEF). G-CLEF is a fiber-fed, high-resolution, precision RV spectrograph for the Magellan Clay Telescope and Giant Magellan Telescope (GMT). To support this effort, we developed a fiber testing setup capable of imaging the near-field and far-field output of fibers and measuring focal ratio degradation. We designed, built, and tested the mode scrambler, using our setup, on step-index multimode optical fibers with various shapes, including octagonal, square, and rectangular core cross-sections. We developed custom software utilizing alpha shapes to identify the boundary of an arbitrarily shaped fiber and to compute a signal-to-noise ratio metric for quantifying modal noise. We investigated the effects of different mode scrambler parameters, such as agitation frequency, on mitigating modal noise. Our results offer valuable insights into optimizing fiber mode scrambling for precision RV spectrographs.

astro-ph.IM↗

arXiv2025

VIPER: A high-resolution multimode fiber-fed VIPA spectrograph concept for characterizing exoplanet atmospheric escape

An increasing number of applications in exoplanetary science require spectrographs with high resolution and high throughput without the need for a broad spectral range. Examples include the search for biosignatures through the detection of the oxygen A-band at 760 nm, and the study of atmospheric escape through the helium 1083 nm triplet. These applications align well with the capabilities of a spectrograph based on a Virtually Imaged Phased Array (VIPA), a high-throughput dispersive element that is essentially a modified Fabry-Perot etalon. We are developing VIPER, a high-resolution, narrowband, multimode fiber-fed VIPA spectrograph specifically designed to observe the helium 1083 nm triplet absorption line in the atmospheres of gaseous exoplanets. VIPER will achieve a resolving power of 300,000 over a wavelength range of 25 nm, and will be cross-dispersed by an echelle grating. VIPER is intended for operation on the 1.5 m Tillinghast Telescope and potentially on the 6.5 m MMT, both located at the Fred Lawrence Whipple Observatory (FLWO) on Mount Hopkins, Arizona, USA. In this paper, we present VIPER's instrument requirements, derived from the primary science goal of detecting anisotropic atmospheric escape from exoplanets. We discuss the design methodology for VIPA-based spectrographs aimed at maximizing throughput and diffraction efficiency, and we derive a wave-optics-based end-to-end model of the spectrograph to simulate the intensity distribution at the detector. We present an optical design for VIPER and highlight the potential of VIPA-based spectrographs for advancing exoplanetary science.

astro-ph.IM↗

arXiv2024

Off-axis Hartmann wavefront sensing for the GMT-Consortium Large Earth Finder (G-CLEF) red camera optics

The Hartmann test is a method used to measure the wavefront error in a focal optical system, wherein a mask with a pattern of small holes is placed at the system's aperture stop. By taking an image at a defocused plane, the differences between the ideal and real positions of the reimaged holes (called the transverse ray aberrations) can be measured, which can then be used to estimate the wavefront error. However, the Hartmann test is usually used with an on-axis field. In this paper, we present a wavefront sensing method which generalizes the classical Hartmann test for off-axis field angles and arbitrary reference wavefronts. Our method involves taking images at two defocused planes, and then using the real reimaged hole positions on both planes to estimate the trajectories of rays from the system's exit pupil, at which the reference wavefront is situated. We then propagate the rays forward from the reference wavefront to one of the two defocused planes, in order to find the ideal reimaged hole positions, from which we can compute transverse ray aberrations. We derive and solve a pair of nonlinear partial differential equations relating transverse ray aberrations to wavefront error, using Zernike decomposition and nonlinear least squares. Our method has been verified on simulated data from the 7-lens f/2.25 red camera system of the GMT-Consortium Large Earth Finder (G-CLEF), a high resolution optical echelle spectrograph which will be a first light instrument for the Giant Magellan Telescope (GMT).

astro-ph.IM↗

arXiv2022

Optical Design and Wavelength Calibration of a DMD-based Multi-Object Spectrograph

The multi-object spectrograph (MOS) has been the benchmark for the current generation of astronomical spectrographs, valued for its ability to acquire the spectra of hundreds of objects simultaneously. In the last two decades, the digital micromirror device (DMD) has shown potential in becoming the central component of the MOS, being used as a programmable slit array. We have designed a seeing-limited DMD-based MOS covering a spectral range of 0.4 to 0.7 $μ$m, with a field of view (FOV) of $10.5^\prime \times 13.98^\prime$ and a spectral resolution of $R\sim1000$. This DMD-MOS employs all-spherical refractive optics, and a volume phase holographic (VPH) grism as the dispersive element for high throughput. In this paper, we present the optical design and optimization process of this DMD-MOS, as well as a preliminary wavelength calibration procedure for hyperspectral data reduction. Using simulated data of the DMD-MOS, a procedure was developed to measure hyperspectral imaging distortion and to construct pixel-to-wavelength mappings on the detector. An investigation into the relationships between DMD micromirrors and detector pixels was conducted. This DMD-MOS will be placed on a 0.5 m diameter telescope as an exploratory study for future DMD-based MOS systems.

astro-ph.IM↗

arXiv2022

Accurately Measuring Hyperspectral Imaging Distortion in Grating Spectrographs Using a Clustering Algorithm

Grating-based spectrographs suffer from smile and keystone distortion, which are problematic for hyperspectral data applications. Due to this, spectral lines will appear curved and roughly parabola-shaped. Smile and keystone need to be measured and corrected for accurate spectral and spatial calibration. In this paper, we present a novel method to accurately identify and correct curved spectral lines in an image of a spectrum, using a clustering algorithm we developed specifically for grating spectrographs, inspired by K-means clustering. Our algorithm will be used for calibrating a multi-object spectrograph (MOS) based on a digital micromirror device (DMD). For each spectral line in a spectrum image, our algorithm automatically finds the equation of the parabola which models it. Firstly, the positions of spectral peaks are identified by fitting Gaussian functions to the spectrum image. The peaks are then grouped into a given number of parabola-shaped clusters: each peak is iteratively assigned to the nearest parabola-shaped cluster, such that the orthogonal distances from the parabola are minimized. Smile can then be measured from the parabolas, and keystone as well if a marked slit is used. Our method has been verified on real-world data from a long-slit grating spectrograph with sub-pixel error, and on simulated data from a DMD-based MOS. Compared to traditional approaches, our method can measure distortions automatically and accurately while making use of more spectral lines. With a precise model and measurement of distortion, a corrected hyperspectral data cube can be created, which can be applied for real-time data processing.

astro-ph.IM↗