The Laser Interferometer Space Antenna (LISA) mission is a space-based gravitational wave detector consisting of three spacecraft with two transceiver telescopes per spacecraft. In addition to tight wavefront error control as expected for an interferometric system, there are tight pupil imaging and optical path length specifications. We use concepts gleaned from pupil aberration theory to understand these latter two constraints and show how these concepts led to a successful design for the LISA transceiver.
We have comprehensively tested uncooled, free space coupled, InGaAs Quad Photoreceivers having 0.5 mm, 1 mm, and 2 mm diameter integrated with a low noise transimpedance amplifier (TIA) using 30 MeV Protons, 100 MeV Protons, 662 keV Gamma Rays, 1 GeV/n Helium, and 1 GeV/n Iron at room temperature of ~20°C. These devices find multiple applications in space for differential wavefront sensing as part of a Gravitational Wave Observatory, as well as instrumentation and control for next generation space telescopes. The bandwidth of all receivers was 20 MHz which was TIA limited.
All 0.5 mm and 1 mm devices were found to be fully functional at normal operating conditions and at room temperature for Protons, Gamma Rays, 1 GeV/n Helium, and 1 GeV/n Iron. Only one quadrant of a 2 mm InGaAs Quad had hard failure due to 1 GeV/n Helium Ions; otherwise it too survived all other radiation tests. Detailed test results follow in the manuscript including recommendations for future space flights. These radiation test results, combined with the earlier successful mechanical shock and vibration testing mean these devices have passed preliminary testing for space qualification.
The study of the Universe through gravitational waves will yield a revolutionary new perspective on the Universe, which
has been intensely studied using electromagnetic signals in many wavelength bands. A space-based gravitational wave
observatory will enable access to a rich array of astrophysical sources in the measurement band from 0.1 to 100 mHz,
and nicely complement observations from ground-based detectors as well as pulsar timing arrays by sampling a different
range of compact object masses and astrophysical processes. The observatory measures gravitational radiation by
precisely monitoring the tiny change in the proper distance between pairs of freely falling proof masses. These masses
are separated by millions of kilometers and, using a laser heterodyne interferometric technique, the change in their
proper separation is detected to ~ 10 pm over timescales of 1000 seconds, a fractional precision of better than one part in
1019. Optical telescopes are essential for the implementation of this precision displacement measurement. In this paper
we describe some of the key system level design considerations for the telescope subsystem in a mission context. The
reference mission for this purpose is taken to be the enhanced Laser Interferometry Space Antenna mission (eLISA), a
strong candidate for the European Space Agency’s Cosmic Visions L3 launch opportunity in 2034. We will review the
flow-down of observatory level requirements to the telescope subsystem, particularly pertaining to the effects of
telescope dimensional stability and scattered light suppression, two performance specifications which are somewhat
different from the usual requirements for an image forming telescope.
We describe our efforts to fabricate, test and characterize a prototype telescope for the eLISA mission. Much of our
work has centered on the modeling and measurement of scattered light performance. This work also builds on a
previous demonstration of a high dimensional stability metering structure using particular choices of materials and
interfaces. We will discuss ongoing plans to merge these two separate demonstrations into a single telescope design
demonstrating both stray light and dimensional stability requirements simultaneously.
Space-based gravitational-wave observatories will systematically study the source-rich band of gravitational waves from 0.0001 Hz to 1 Hz. All current designs require propagation of a laser beam from one spacecraft to another over immense distances. An optical telescope is needed for efficient power delivery and its design is driven by the interferometric displacement sensitivity requirements. Here we describe the design for a catoptric telescope that meets those requirements, emphasizing differences from the usual specifications for high quality image formation, and discuss design trade-offs as well as early results from research into scattered light suppression and modeling that may enable alternative designs.
Space-based observation of gravitational waves promises to enable the study of a rich variety of high energy astrophysical sources in the 0.0001 to 1 Hz band using signals complementary to traditional electromagnetic waves. Gravitational waves represent the first new tool for studying the sky since gamma ray telescopes debuted in the 1970s, and we expect compelling science to be the result. The fundamental measurement is to monitor the path length difference between pairs of freely falling test masses with laser interferometry to a precision of picometers over gigameter baselines. The test masses are arranged in an equilateral triangle to allow simultaneous measurement of both gravitational wave polarizations. The heliocentric orbital space environment enables the test masses to be shielded from large ground motions at low frequencies, and allows the construction of long measurement baselines that are well matched to the signal wavelengths. Optical telescopes play an important role in the measurement because they deliver laser light efficiently from one spacecraft to another. The telescopes are directly in the measurement path, so there are additional performance requirements to support precision metrology beyond the usual requirements for good image formation.
Quad photoreceivers, namely a 2 x 2 array of p-i-n photodiodes followed by a transimpedance amplifier (TIA) per diode,
are required as the front-end photonic sensors in several applications relying on free-space propagation with position and
direction sensing capability, such as long baseline interferometry, free-space optical communication, and biomedical
imaging. It is desirable to increase the active area of quad photoreceivers (and photodiodes) to enhance the link gain,
and therefore sensitivity, of the system. However, the resulting increase in the photodiode capacitance reduces the
photoreceiver's bandwidth and adds to the excess system noise. As a result, the noise performance of the front-end quad
photoreceiver has a direct impact on the sensitivity of the overall system. One such particularly challenging application
is the space-based detection of gravitational waves by measuring distance at 1064 nm wavelength with ~ 10 pm/√Hz
accuracy over a baseline of millions of kilometers.
We present a 1 mm diameter quad photoreceiver having an equivalent input current noise density of < 1.7 pA/√Hz per
quadrant in 2 MHz to 20 MHz frequency range. This performance is primarily enabled by a rad-hard-by-design dualdepletion
region InGaAs quad photodiode having 2.5 pF capacitance per quadrant. Moreover, the quad photoreceiver
demonstrates a crosstalk of < -45 dB between the neighboring quadrants, which ensures an uncorrected direction sensing
resolution of < 50 nrad. The sources of this primarily capacitive crosstalk are presented.
Observations of the Earth are extremely challenging; its large angular extent floods scientific instruments with high flux
within and adjacent to the desired field of view. This bright light diffracts from instrument structures, rattles around and
invariably contaminates measurements. Astrophysical observations also are impacted by stray light that obscures very
dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low
reflectance structural surface treatments and by using baffles and stops to limit this background noise. In 2007 GSFC
researchers discovered that Multiwalled Carbon Nanotubes (MWCNTs) are exceptionally good absorbers, with potential
to provide order-of-magnitude improvement over current surface treatments and a resulting factor of 10,000 reduction in
stray light when applied to an entire optical train. Development of this technology will provide numerous benefits
including: a.) simplification of instrument stray light controls to achieve equivalent performance, b.) increasing
observational efficiencies by recovering currently unusable scenes in high contrast regions, and c.) enabling low-noise
observations that are beyond current capabilities. Our objective was to develop and apply MWCNTs to instrument
components to realize these benefits. We have addressed the technical challenges to advance the technology by tuning
the MWCNT geometry using a variety of methods to provide a factor of 10 improvement over current surface treatments
used in space flight hardware. Techniques are being developed to apply the optimized geometry to typical instrument
components such as spiders, baffles and tubes. Application of the nanostructures to alternate materials (or by contact
transfer) is also being investigated. In addition, candidate geometries have been tested and optimized for robustness to
survive integration, testing, launch and operations associated with space flight hardware. The benefits of this technology
extend to space science where observations of extremely dim objects require suppression of stray light.
The Laser Interferometer Space Antenna mission is a planned gravitational wave detector consisting of three spacecraft
in heliocentric orbit. Laser interferometry is used to measure distance fluctuations between test masses aboard each
spacecraft to the picometer level over a 5 million kilometer separation. Laser frequency fluctuations must be suppressed
in order to meet the measurement requirements. Arm-locking, a technique that uses the constellation of spacecraft as a
frequency reference, is a proposed method for stabilizing the laser frequency. We consider the problem of arm-locking
using classical optimal control theory and find that our designs satisfy the LISA requirements.
This overview will discuss core network technology and cost trade-offs inherent in choosing between "analog" architectures with high optical transparency, and ones heavily dependent on frequent "digital" signal regeneration. The exact balance will be related to the specific technology choices in each area outlined above, as well as the network needs such as node geographic spread, physical connectivity patterns, and demand loading.
Over the course of a decade, optical networks have evolved from simple single-channel SONET regenerator-based links to multi-span multi-channel optically amplified ultra-long haul systems, fueled by high demand for bandwidth at reduced cost. In general, the cost of a well-designed high capacity system is dominated by the number of optical to electrical (OE) and electrical to optical (EO) conversions required. As the reach and channel capacity of the transport systems continued to increase, it became necessary to improve the granularity of the demand connections by introducing (optical add/drop multiplexers) OADMs. Thus, if a node requires only small demand connectivity, most of the optical channels are expressed through without regeneration (OEO). The network costs are correspondingly reduced, partially balanced by the increased cost of the OADM nodes. Lately, the industry has been aggressively pursuing a natural extension of this philosophy towards all-optical "analog" core networks, with each demand touching electrical digital circuitry only at the in/egress nodes. This is expected to produce a substantial elimination of OEO costs, increase in network capacity, and a notionally simpler operation and service turn-up.
At the same time, such optical "analog" network requires a large amount of complicated hardware and software for monitoring and manipulating high bit rate optical signals. New and more complex modulation formats that provide resiliency to both optical noise and nonlinear propagation effects are important for extended unregenerated reach. More sophisticated optical amplifiers provide lower noise for increased reach and increased spectral bandwidth for higher wavelength count lower wavelength blocking probability. Optical analog networks also require methods for mitigating optical power transients, for controlling optical spectral flatness, and for dynamically managing changes (e.g. in chromatic dispersion and polarization mode dispersion.) Since signals stay in the optical domain, optical performance monitoring techniques are required for fault isolation and correction. Efficient routing of optical signals also requires sophisticated switching nodes with an ability to selectively steer optical signals into several directions with single-channel spectral granularity. Most of these technologies are not modular and require interruption of service if not deployed at the initial system installation, thereby increasing first install costs substantially, even if initial capacity loading is small.
Further, validation of systems and software targeting a specific network design is complex. Only a small fraction of the total network may be reasonably reproduced in the lab, and many field configurations are not predictable or even dynamic. Thus, extra system margin has to be allocated to handle the behavior uncertainty.
To constrain the complexity of both hardware technology and software algorithms, regions of network transparency can be established with OEO forced at perimeters. Thus, "analog" regions are surrounded by "digital" interfaces.
Following are some example tradeoffs that will be discussed. What is a good modulation format choice, and does increased reach and impairment resiliency justify hardware and controls that are more complicated? What are reasonable amplifier choices to make under specific network assumptions? Can high transport system capacity be leveraged to simplify optical switch node design by reducing spectral efficiency?
Over the last decade, deployed core telecom networks have migrated from being based on single-channel SONET regeneration links to multi-span, multi-channel optically amplified systems. More recently, the industry has been aggressively pursuing a natural extension of this philosophy towards all-optical “analog” core networks, with each traffic demand touching electrical digital circuitry only at the in/egress nodes. This trend produced a substantial elimination of regeneration costs, increase in network capacity, and notionally simpler operation and service turn-up. At the same time, the optical “analog” network requires a large amount of sophisticated hardware and software for monitoring and manipulating high bit rate optical signals. The primary goal for current equipment suppliers is to provide cost effective system designs that are simple to deploy and operate. This paper will examine the trade-offs inherent in the technology and architecture choices needed to reach this goal through the “analog” transmission/all-optical ideal and concludes that it is difficult to improve on the present approach which uses a mix of transparent and opaque network elements.
Wavelength Division Multiplexing (WDM) is currently being investigated for ground-based fiber optic networks as a means to achieve high aggregate data rates and/or to allow the use of lower power lasers at a given rate by adding additional channels. A drawback to using WDM techniques for space applications has been that most of the WDM devices used to combine wavelengths are lossy, particularly when implemented with integrated optics or fiber gratings. The additional loss is not important for ground-based applications because it can be overcome by adding gain (with an erbium-doped fiber amplifier, for example). For space applications, where size, weight, and power consumption are critical, the excess loss is a serious drawback. We describe here a very low-loss technique for wavelength combining using standard fiber components. The technique is scalable to moderate numbers of wavelengths while maintaining low loss. Low loss means that these multiplexers can be used for applications such as power combining that are not feasible with traditional techniques. Experimental results confirming low loss power combining will be presented.
High data rate communications systems will soon be needed for space applications. Technology and applications which support high data rates are already in place for ground- based telecommunications, or will be in the future. The advantages of an optical system over a traditional RF link for free space communications are particularly compelling for high data rates. We have been developing the necessary technology to demonstrate the feasibility of high rate free-space optical communications technology at 1.5 micrometers . The existence of a large, mature technology base at 1.5 micrometers developed for the telecommunications industry has allowed us to focus our development effort on two key technologies needed for space applications that have not been developed for the ground: a 1 Watt class optical power amplifier and a near quantum limited receiver. This paper will describe the overall system design for a high data rate optical communications system and present experimental results demonstrating < 50 photons/bit sensitivity at 10 Gbps with 1 Watt of optical power. The existence of a feasibility demonstration at this data rate enables downward scalability to data rates of 1 Gbps or less with small, inexpensive terminals.
Applications such as high resolution image transmission and the aggregation of multiple bit streams have increased the interest in very high speed (Gbit/s) data communications for space applications. The ability to perform error correction on a data link can yield significant improvements in channel efficiency and can mitigate the effects of various bit error rate (BER) impairments afflicting many communications systems. A codec integrated circuit (IC) has been constructed which is capable of supporting up to 2 Gbit/s of data throughput. The device has been demonstrated in two optical communications systems, a 1 Gbit/s binary optically preamplified OOK system at 1.5 µm and an 800 Mbit/s FSK MOPA system at 0.98 µm. At a BER of 10-9, this coding provided a 3.7 dB sensitivity improvement for the OOK system, and a 6.2 dB improvement with the FSK system. The measured sensitivity of the coded OOK system was 37 photons/bit, which is better than could be realized with an ideal uncoded system. The viability of applying error correcting coding to higher data rate systems is discussed and a method of utilizing these ICs to provide four Gbit/s operation is shown.
Semiconductor laser devices with tapered gain regions have recently generated much interest because they promise high output power with near-diffraction-limited spatial beam quality and good electrical to optical conversion efficiency. We report recent progress on two specific applications: a ring laser and a high- power erbium-doped fiber amplifier (EDFA). The ring laser operates unidirectionally in a single longitudinal mode with an output power of 170 mW and without a Faraday isolator. The high- power EDFA has an output power of 520 mW at 1.55 micrometers , the highest power reported to dates for an erbium-doped fiber amplifier using all semiconductor pump lasers. The common theme for both of these applications is the development of optical systems that produce high power in near-diffraction-limited collimated beams and efficient coupling into single mode optical fiber. We present an experimental procedure for quantitatively predicting the optical fiber power coupling efficiency. We have measured 64% power coupling efficiency measure fiber fact to power in the single-mode fiber, or 51% laser facet to power in the fiber, in good agreement with the predictions.
For very high data rates, optical communications holds a potential performance edge over other technologies, especially for space applications where size, weight, and power are of prime importance. We report demonstrations of several Gigabit-per-second (Gbps) class all- semiconductor optical communications systems which have been developed for free-space satellite crosslink applications. These systems are based on the master-oscillator-power- amplifier (MOPA) transmitter architecture which resolves the conflicting requirements of high speed and high power on a single-laser coherent transmitter. A 1 Gbps, 1 Watt system operating at 973 nm with a frequency-shift-keyed (FSK) modulation format is the highest power coherent optical communications system using all semiconductor lasers reported to date. A 3 Gbps differential-phase-shift-keyed (DPSK) system uses a 2-stage injection-locked diode array as a power amplifier at 830 nm. At a wavelength of 1.5 micrometers , an optically- preamplified direct-detection on-off-keyed (OOK) receiver was demonstrated at both 3 and 10 Gbps. A 3 Gbps optically-preamplified direct-detection DPSK receiver was also demonstrated and represents, to our knowledge, the highest sensitivity DPSK receiver reported to date for data rates above 2 Gbps.