We propose and have demonstrated a prototype high-reliability pump module for pumping a Non-Planar Ring Oscillator
(NPRO) laser suitable for space missions. The pump module consists of multiple fiber-coupled single-mode laser diodes
and a fiber array micro-lens array based fiber combiner. The reported Single-Mode laser diode combiner laser pump
module (LPM) provides a higher normalized brightness at the combined beam than multimode laser diode based LPMs.
A higher brightness from the pump source is essential for efficient NPRO laser pumping and leads to higher reliability
because higher efficiency requires a lower operating power for the laser diodes, which in turn increases the reliability
and lifetime of the laser diodes. Single-mode laser diodes with Fiber Bragg Grating (FBG) stabilized wavelength permit
the pump module to be operated without a thermal electric cooler (TEC) and this further improves the overall reliability
of the pump module. The single-mode laser diode LPM is scalable in terms of the number of pump diodes and is capable
of combining hundreds of fiber-coupled laser diodes. In the proof-of-concept demonstration, an e-beam written
diffractive micro lens array, a custom fiber array, commercial 808nm single mode laser diodes, and a custom NPRO
laser head are used. The reliability of the proposed LPM is discussed.
We describe the advantages of a nulling coronagraph instrument behind a single aperture space telescope for detection and spectroscopy of Earth-like extrasolar planets in visible light. Our concept synthesizes a nulling interferometer by shearing the telescope pupil into multiple beams. They are recombined with a pseudo-achromatic pi-phase shift in one arm to produce a deep null on-axis, attenuating the starlight, while simultaneously transmitting the off-axis planet light. Our nulling configuration includes methods to mitigate stellar leakage, such as spatial filtering by a coherent array of single mode fibers, balancing amplitude and phase with a segmented deformable mirror, and post-starlight suppression wavefront sensing and control. With diffraction limited telescope optics and similar quality components in the optical train (λ/20), suppression of the starlight to 10-10 is readily achievable. We describe key features of the architecture and analysis, present the status of key experiments to demonstrate wide bandwidth null depth, and present the status of component technology development.
The direct detection of Earthlike planets in the visible is a very challenging goal This paper describes a new concept for visible direct detection of Earths using a nulling interferometer instrument behind a 4m telescope in space. The basic concept is described along with the key advantages of the nulling interferometer over more traditional approaches, an apodized aperture telescope or coronagraph. In the baseline design, a 4 beam nuller produces a very deep theta^4 null. With perfect optics, the stellar leakage is less than 1e-11 of the starlight at the location of the planet. With diffraction limited (lambda/20) telescope optics suppression of the starlight to ~1e-10 would be possible.
This paper describes the latest progress for visible direct detection of Earth like extrasolar planets using a nulling coronagraph instrument behind a 4m class telescope. Such a system is capable of satisfying the scientific objectives of the Terrestrial Planet Finder mission In our design, a 4 beam nulling interferometer is synthesized from the telescope pupil, producing a very deep null proportional to θ4 which is then filtered by a coherent array of single mode fibers to suppress the residual scattered light. With diffraction limited telescope optics and similar quality components in the optical train (λ/20), suppression of the starlight to 10-10 is achievable. Such a telescope with this nulling interferometer as back-end instrument can image and detect planets, or provide the input to a low resolution spectrometer. Shown are key features of this system in a space mission, latest results of laboratory measurements demonstrating achievable null depth, and progress toward fabrication of coherent single mode fiber arrays.
We present the development of a single-mode spatial filter array for the nulling coronagraph application. The development consists of two generations of fiber array designs and a Zygo-interferometer based lens array to fiber array alignment methodology. We discuss the use of large mode field diameter (MFD) fibers to relax fiber placement
tolerance of the fiber array. The pros and cons of using the Photonic Crystal Fiber (PCF) for building the array are discussed. The future plan for implementing a 1000-channel class, single-mode spatial filter array is described.
Optical Planet Discoverer (OPD) is a 1.5m class space telescope concept working as a visible nulling-interferometer imager. It is designed to detect Jupiter-like planets orbiting main sequence stars 10pc away in a few minutes of integration and carry out a low resolution (~20) spectroscopy of their atmosphere. OPD would fit in the budget envelope of a discovery class mission. It would serve as an efficient precursor to a Visible Terrestrial Planet Finder (VTPF), a scaled-up 4m class version based on the same optical scheme and allowing direct detection of 10pc Earthlike planets in a few hours. We detail here OPD's optical principle layout, which is primarily driven by an integrated stellar light attenuation of 1e-6 in the final focal plane. The optical concept is based on a double-shearing nulling interferometer followed by an array of single-mode waveguides. The waveguides array ensures high residual starlight suppression - as already demonstrated at the 1e-6 level by preliminary JPL visible LASER nulling experiments - together with diffraction limited imaging of the circumstellar environment over a 2 arcsec field. During the observations, the telescope is spun around the line of sight to allow for proper detection of fixed planetary signatures against residual off-axis speckle patterns at the 1e-9 level. Use of the single-mode waveguide array to filter out scattered starlight eliminates the requirements for pristine λ/4000 rms wavefronts anywhere in the optical train. With OPD, stringent phase requirements apply only to scales larger than 5 cm - the equivalent size of the pupil regions to be recombined and nulled in a given fiber, so that phase specifications can be met using low order active optics.
The success of interferometry in space depends on the development of lasers that can survive launch conditions and the challenging space environment during missions that could last five years or more. This paper describes the fabrication of a rugged, laser-welded package for a 200mW, monolithic diode-pumped solid-state Nd:YAG laser operating at 1319nm. Environmental testing shows that the laser withstands non-operational thermal cycles over a temperature range from -20°C to 55°C, and 22.3 g-rms of random vibration, with little or no degradation of laser output power or performance. The novel packaging method employs a specially designed housing to which multi-mode or single-mode polarization-maintaining fiber pigtails can be aligned and laser-welded into place. To further enhance reliability, a redundant pumping system called the Multi-Fiber Pump Ferrule (MFPF) was developed and implemented. The MFPF allows multiple laser diode pump modules to be aligned to the laser crystal simultaneously, in order to accommodate either parallel or standby pump redundancy. This compact, lightweight design is well suited for space flight applications and the laser-welded technique can easily be adapted to a number of other fiber optic and electro-optic devices in which critical optical alignments must be maintained in a harsh environment.
We describe the development, functional performance, and space-qualification status of a Metrology Source suitable for implementation of space-based metrology systems with picometer-level relative displacement measurement and micron-level absolute displacement measurement resolution. The Metrology Source consists of the following components: lasers, frequency stabilization system, frequency shifters, and frequency modulators. All components are interconnected by polarization maintaining fibers to facilitate integration into a lightweight space-qualifiable module.
We present the latest developments on the concept of space adaptive optics coronography. We review the principle of deformable mirror used to suppress the scattered light in a 'dark hole' region around the optical axis. We describe the main limitations of this concept. A description of the experiment which has been built at JPL and its first results are given.
We present the characteristics of a prototype all-fiber sensor network that is useful in structure-health management and distributed sensor data acquisition. In this network, each remote sensor node is powered over a fiber by a laser in the base station. The sensor data are sent back to a base station through a different fiber. Issues concerning power consumption per node, data rate, fault-tolerance, packaging, cost, and network expandability will be discussed.
The concept of an all-fiber, IC-based sensor network is presented. Fundamental issues including topology tradeoffs, power budget, and power distribution subsystem are analyzed. Some potential applications of such sensor networks are discussed.
A new and simple mathematical model for describing radiation-induced absorption in optical fibers is presented. It treats the radiation-induced defect-generation and the decay process as a series of superposable infinitesimal growth and decay events. Unlike the existing power law growth equation, the new equation is non-empirical, dose-rate dependent, and describes the growth and decay of the induced defect at the same time. It is capable of predicting long-exposure, low-dose-rate induced fiber loss normally taking place in a space mission, using short-exposure, high-dose-rate results produced in a ground-based laboratory. Numerically, the derived equation is also capable of simulating those effects caused by irregular radiation events such as solar-flare radiation burst. In the case of constant dose rate, the general equation reduces to a simple analytical form which agrees reasonably well with the experiment.
An optical link can provide an interface channel for a focal plane array that is immune to electro-magnetic interference (EMI) and can lower the heat load on the dewar. Our approach involves the use of fiber optics and an on-focal-plane optical modulator to provide an interface to the focal plane array (FPA). The FPA drives the modulator with an electrical signal. We evaluated specially fabricated AlGaAs/GaAs multiple-quantum-well (MQW) optical modulators, operating near 840 nm, for analog modulation, and we have used the results to calculate the performance of an optical interface link using experimentally determined device parameters. Link noise and dynamic range for an analog link were estimated from a separate experiment using pigtailed fiber components. The performance of the MQW modulator system is compared to alternative strategies. Significant improvement in performance in comparison to conventional electronic interfaces appears to be possible.
An optical link can provide an interface channel for the focal-plane array that is immune to electromagnetic interference (EMI) and can lower the heat load on the dewar. Our approach involves the use of fiber-optics and an on-focal-plane optical modulator to provide an interface to the focal-plane array (FPA). The FPA drives the modulator with an electrical signal. We evaluated specially fabricated AlGaAs/GaAs multiple quantum well (MQW) optical modulators, operating near 840 nm, for analog modulation, and we have used the results to calculate the performance of an optical interface link using experimentally determined device parameters. Link noise and dynamic range for an analog link were estimated from a separate experiment using pigtailed fiber components. The performance of the MQW modulator system is compared to alternative strategies. Significant improvement in performance in comparison to conventional electronic interfaces appears to be possible.
A potential application of the photorefractive time-integrating correlator is the real-time radar jamming interference rejection system, using the adaptive filter method; a fast photorefractive crystal is needed for adapting a rapidly changing jamming signal. An effort is presently made to demonstrate and characterize a GaAs-based photorefractive time-integrating correlator, since GaAs crystals are 2-3 orders of magnitude faster than most other alternatives.
We demonstrate a new optical correlator in which the correlation peak intensity is increased when the matched input object is moving. The basic configuration of the correlator is the same as a VanderLugt optical correlator consisting of a photorefractive crystal. The principle of this new correlator is based on the dynamic grating erasure property of photorefractive materials. The detail of this principle is described.
The unique features of photorefractive compound semiconductors are presented. The advantages of this class of nonlinear optical materials for optical processing are illustrated with examples using GaAs and InP. The difference between GaAs and InP in the laser power density requirement is discussed.
The resolution of a target-tracking optical novelty filter is discussed in terms of the response time of the nonlinear medium, the speed of the target, and the resolution of the input device. Optical novelty filters using a faster nonlinear medium may have a higher output resolution. This is particularly true in the case of tracking high-speed targets. The potential of implementing high-resolution optical novelty filters using photorefractive GaAs is investigated experimentally.
A real-time optical correlator based on GaAs and liquid-crystal TV (LCTV) is demonstrated. The demonstrated system has a video-frame rate limited by the speed of the LCTVs; if faster spatial-light modulators are used, the potential frame rate of a GaAs-based correlator can be as fast as 1000 frames/sec under experimental conditions. Comparisons are made between VanderLugt and joint transform and between degenerate and nondegenerate four-wave mixing. The edge-enhancement effect and the Bragg diffraction effect are discussed.
Photorefractive semiconductors have the attractive features of fast response times and operation at near-infrared wavelengths. This has opened some new research opportunities in the field of photorefractive nonlinear optics which is significant for applications in real-time image processing and optical computing. This paper presents recent experimental demonstrations of several basic optical information processing techniques using photorefractive GaAs crystals. The results of these demonstrations illustrate that photorefractive compound semiconductors has a great potential as a new medium for light beam interaction based on the dynamic holographic principle.