NASA conducts science investigations using spaceborne instruments on custom-built satellite platforms. The instruments enable global measurements of the Earth, moon, and planets, as well as investigations of astrophysical science. The satellites require communication and navigation systems. Accordingly, continuing advances in laser technology facilitate new satellite infrastructure. Our engineering goal is to make the highest-quality science product for the lowest cost (i.e., to minimize size, weight, and power). For laser-based systems this means using the highest electrical input to optical output efficiency lasers with the most sensitive near-IR detectors. We emphasize the near-IR regime because a combination of economics and materials science has enabled the most science-applicable, efficient, and reliable lasers at these wavelengths. However, devices still require improvements for high speed and sensitivity to qualify for space operation.
Both laser-based time-of-flight instruments (e.g., laser range systems and altimeters) and high-bandwidth laser-communication terminals have increased capability by employing short pulses (<1ns) and optimizing the laser receiver signal-to-noise ratio (SNR). Ideally, a laser receiver detector can detect a single photon, thus attaining near-ultimate receiver sensitivity. Some of NASA's laser-ranging and altimetry instruments have improved the system performance (including SNR) by frequency doubling the laser to allow use of single-photon-sensitive detectors—e.g., silicon avalanche photodiodes (APDs)1 or photomultiplier tubes—at visible (green) wavelengths. This is suboptimal, because it provides only half of the total number of photons per Watt. Moreover, losses are associated with the practical implementation of efficient frequency doubling at high power. Since 1990, all of NASA's space-based laser-altimetry missions have used versions of the near-IR-enhanced silicon APD detector manufactured by PerkinElmer Opto-Electronics Canada for use at 1064nm wavelength. The silicon band gap reduces absorption in the near-IR range, necessitating a thick absorption region. Unfortunately, this also increases the dark-current noise (i.e., electrical signals not caused by light), thus limiting the sensitivity. As a result, new materials and approaches are required to achieve single-photon sensitivity in the near-IR spectral range.
NASA's first such effort (the Lunar Laser Communication Demonstration, being designed and built at the Massachusetts Institute of Technology's Lincoln Laboratory for launch in March 2013) will use a 1.5μm-wavelength single-mode fiber pre-amplifier receiver with an indium gallium arsenide (InGaAs) p-i-n diode detector on the lunar orbiting spaceborne terminal. Performance is expected to be tens of photons per bit sensitivity.
For all of NASA's spaceborne near-IR laser receiver applications, our goal is to achieve single-photon sensitivity. We are investigating InGaAs, indium aluminum arsenide (InAlAs), InGaAs phosphide (InGaAsP), mercury cadmium telluride (HgCdTe), and resonant-cavity-enhanced silicon APDs, as well as InGaAs or InGaAsP photocathode photomultiplier tubes (see Figure 1).
Figure 1. Near-IR detectors. (a) Impact-ionization-engineered indium aluminum arsenide avalanche photodiode (APD). (b) Mercury cadmium telluride APD on dewar. (c) Hybrid photomultiplier tube.
None of the linear-mode APDs we have tested from numerous companies have been able to match the performance of the PerkinElmer silicon APD. We have achieved improved performance (and 1GHz electrical bandwidth) by developing an impact-ionization-engineered InAlAs APD. InGaAs Geiger-mode APDs can achieve single-photon detection but have recently been shown to be susceptible to space-radiation damage, which limits their applicability to multiyear NASA missions. We continue to develop HgCdTe APDs in collaboration with US industry. Recent results include >100MHz electrical bandwidth, tens-of-photons sensitivity, and low excess-noise factor (1.1). This challenges our gold-standard silicon APD. Recent international results2 indicate that numerous improvements are still achievable.
We have measured good performance from custom-selected dynode-chain (a set of metal plates inside the photomultiplier tube that provide gain at each plate) InGaAs photocathode photomultiplier tubes. These include >10% single-photon detection efficiency at 1550nm, near-GHz bandwidth, large area (1mm), low excess-noise factor (1.2), and reasonable dark-count rates (electrical noise in pulses per second) (<1Mcps). We achieved the best performance from InGaAsP photocathode hybrid photomultiplier tubes.3 We measured 25% single-photon detection efficiency at a wavelength of 1064nm with a dark-count rate of 60,000/s at −22°C. The single-photon response-output pulse width is 0.9ns with a timing jitter of 500ps. The maximum count rate exceeds 100Mcps. In summary, a new generation of high-sensitivity near-IR detectors for spaceborne laser instruments will enable new science, including high-precision global measurements of the Earth's and planetary topography, and atmospheric trace gases (e.g., carbon dioxide and methane). In addition, high-sensitivity near-IR detectors should make possible new spacecraft infrastructure with laser communications and navigation. NASA continues to invest in improved APD and photomultiplier technology for these efforts.
Michael Krainak, Xiaoli Sun, Guangning Yang
NASA Goddard Space Flight Center
Michael Krainak received his PhD in electrical engineering from Johns Hopkins University. He has worked at AT&T Western Electric, the Defense Department, and Quantum Photonics. For 19 years he has worked on laser instruments at NASA Goddard Space Flight Center, where he heads the Laser and Electro-Optics Branch.