For the Space Interferometry Mission (SIM) Spectrum Calibration Development Unit (SCDU) testbed, wideband white light is used to simulate starlight. The white light source mount requires extremely stable pointing accuracy ( lights were coupled to a photonic crystal fiber (PCF).
In many optical systems, simulating a point star with broadband spectrum with stability of microradians for white light interferometry is a challenge. In this case, the cameras use the white light interference to balance two optical paths, and to maintain close tracking. In order to coarse align the optical paths, a laser light is sent into the system to allow tracking of fringes because a narrow band laser has a great range of interference.
The design requirements forced the innovators to use a new type of optical fiber, and to take a large amount of care in aligning the input sources. The testbed required better than 1% throughput, or enough output power on the lowest spectrum to be detectable by the CCD camera (6 nW at camera). The system needed to be vacuum-compatible and to have the capability for combining a visible laser light at any time for calibration purposes.
The red laser is a commercially produced 635-nm laser 5-mW diode, and the white light source is a commercially produced tungsten halogen lamp that gives a broad spectrum of about 525 to 800 nm full width at half maximum (FWHM), with about 1.4 mW of power at 630 nm. A custom-made beam splitter window with special coating for broadband wavelengths is used with the white light input via a 50-mm multimode fiber. The large mode area PCF is an LMA-8 made by Crystal Fibre (core diameter of 8.5 mm, mode field diameter of 6 mm, and numerical aperture at 625 nm of 0.083). Any science interferometer that needs a tracking laser fringe to assist in alignment can use this system.
This work was done by Alireza Azizi, Daniel J. Ryan, Hong Tang, Richard T. Demers, Hiroshi Kadogawa, Xin An, and George Y. Sun of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-46165
This Brief includes a Technical Support Package (TSP).
Vacuum-Compatible Wideband White Light and Laser Combiner Source System
(reference NPO-46165) is currently available for download from the TSP library.
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Overview
The document outlines the development and functionality of the Vacuum-Compatible Wideband White Light and Laser Combiner Source System, specifically designed for the Spectrum Calibration Development Unit (SCDU) testbed at NASA's Jet Propulsion Laboratory (JPL). The SCDU testbed is a two-arm interferometer housed in a vacuum chamber, aimed at calibrating wavelength-dependent errors in scientific measurements using starlight of varying spectra.
The system integrates two light sources: a broadband white light source and a single-wavelength laser. The white light, generated by a Tungsten lamp, is filtered to simulate different stellar spectra and is coupled to a multi-mode fiber before entering the chamber. The laser light, used for calibration purposes, is transmitted via a single-mode fiber. Both light sources are combined using a photonic crystal fiber (PCF) to produce a single-mode, Gaussian beam output with a required optical bandwidth of 450-950 nm and a minimum output power of 10 nW.
The document details the design requirements for the combiner system, emphasizing the need for high pointing stability (better than 4 micro-radians) and sufficient optical power levels for effective fringe tracking. The performance of the system is assessed through various optical measurements, including power output and spectral analysis, demonstrating improvements in bandwidth and output characteristics compared to traditional single-mode fibers.
The SCDU testbed's alignment and configuration are critical, as the setup is sensitive to perturbations. The document also discusses the testing of different photonic crystal fibers (PCFs) to evaluate their contributions to bandwidth limitations, with the LMA-8 PCF showing a wider bandwidth than alternatives.
In conclusion, the SCDU testbed and its associated combiner system are pivotal for achieving the precision astrometry goals of the SIM-PlanetQuest mission, which aims for 1 micro-arcsecond accuracy in astrometric measurements. The advancements in fiber optics and light source integration presented in this document highlight significant progress in the field of astronomical instrumentation, with potential applications extending beyond space exploration.
