A common problem in optical detection is determining the arrival time of a weak optical pulse that may comprise only one to a few photons. Currently, this problem is solved by using a photodetector to convert the optical signal to an electronic signal. The timing of the electrical signal is used to infer the timing of the optical pulse, but error is introduced by random delay between the absorption of the optical pulse and the creation of the electrical one. To eliminate this error, a time-to-space converter separates a sequence of optical pulses and sends them to different photodetectors, depending on their arrival time.
The random delay, called jitter, is at least 20 picoseconds for the best detectors capable of detecting the weakest optical pulses, a single photon, and can be as great as 500 picoseconds. This limits the resolution with which the timing of the optical pulse can be measured.
The time-to-space converter overcomes this limitation. Generally, the time-to-space converter imparts a timedependent momentum shift to the incoming optical pulses, followed by an optical system that separates photons of different momenta. As an example, an electro-optic phase modulator can be used to apply longitudinal momentum changes (frequency changes) that vary in time, followed by an optical spectrometer (such as a diffraction grating), which separates photons with different momenta into different paths and directs them to impinge upon an array of photodetectors. The pulse arrival time is then inferred by measuring which photodetector receives the pulse.
The use of a time-to-space converter mitigates detector jitter and improves the resolution with which the timing of an optical pulse is determined. Also, the application of the converter enables the demodulation of a pulse position modulated signal (PPM) at higher bandwidths than using previous photodetector technology. This allows the creation of a receiver for a communication system with high bandwidth and high bits/photon efficiency.
This work was done by Kevin Birnbaum and William Farr of Caltech for NASA’s Jet Propulsion Laboratory. NPO-45799
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Electro-Optic Time-to-Space Converter for Optical Detector Jitter Mitigation
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Overview
The document discusses the "Electro-Optic Time-to-Space Converter for Optical Detector Jitter Mitigation," developed by NASA's Jet Propulsion Laboratory. This technology addresses the challenges posed by optical detector jitter, which can lead to inaccuracies in timing measurements in various applications, particularly in optical communication and ranging systems like LIDAR.
The time-to-space converter operates by directing photons to different photodetectors based on their arrival time, effectively mitigating the effects of jitter. It utilizes a modulator that imparts a time-dependent momentum change to the photons, allowing for precise timing measurements. This is particularly beneficial in systems that employ pulse position modulation (PPM), where information is encoded in the timing of optical pulses.
One significant application highlighted is in LIDAR systems, which measure distances by sending short optical pulses that reflect off targets. The time-of-flight of these pulses is critical for accurate distance measurement. The time-to-space converter enhances the resolution of these measurements by overcoming detector jitter, allowing for more accurate timing of pulse emissions and returns.
Another application is in quantum key distribution (QKD), where the technology can improve the signal-to-noise ratio by gating photodetectors to reject spurious signals. In QKD, the transmission of weak optical pulses is susceptible to background noise, which can compromise the security of the key distribution. By using the time-to-space converter, the system can focus on detecting only the relevant signals while discarding background noise, thus enhancing the reliability and security of the communication.
The document also discusses the technical aspects of the modulator, including the use of single-mode waveguides for greater phase modulation efficiency and easier coupling to fiber-based systems. It mentions different modulation techniques, such as sinusoidal and piece-wise parabolic modulation, and their implications for performance.
Overall, the document emphasizes the potential of the electro-optic time-to-space converter to significantly improve the performance of optical systems by enhancing timing accuracy and reducing the impact of noise, thereby broadening the scope of applications in both commercial and scientific fields.
