Recirculating Advanced Coupled-cavity Etalon Receiver (RACER)

The new NASA LaRC Pulsed 2-Micron Laser Transmitter for Coherent 3-D Doppler Wind Lidar Systems is an innovative concept and architecture based on a Tm:Fiber laser end-pumped Ho:YAG laser transmitter. This transmitter meets the requirements for space-based coherent Doppler wind lidar while reducing the mission failure risks. A key advantage of this YAG based transmitter technology includes the fact that the design is based on mature and low-risk space-qualified YAG host crystal. The transmitter operates at a 2096 nm wavelength using Ho:YAG, resulting in high atmospheric transmission (>99%), versus a transmitter operating at 2053 nm using co- doped Tm:Ho:LuLiF, which suffers limited transmission (90%) due to water vapor interference. In-band pumping through Tm:Fiber pump Ho:YAG architecture offers lower quantum defect from 1908 to 2096 nm (9.1%) compared to traditionally used co-doped Tm:Ho:LuLiF of 792 to 2051 nm (61%). The transmitter has an efficient pump compared to LuLF, since YAG has 27% higher pump absorption and 52% lower reabsorption of the emitted 2-micron, resulting in higher efficiency and lower heat load. Being isotropic, YAG is amenable for spatial-hole burning mitigation which supports linear cavity architecture without compromising injection seeding quality. This attribute is important in designing a compact, stable, high seeding efficiency laser. A folded linear cavity for long pulse (>200 ns), transform limited line-width (2.2 MHz) and high beam quality (M2 = 1.04) - the most critical parameters for coherent detection - are easier to achieve using YAG compared to LuLF. Lower heat load results in high repetition rate (>300 Hz) operation, which allows higher probability of wind measurements through broken clouds, off clouds, and below clouds, thus reducing errors and increasing science data product quantity and quality.

State of the art space-based LIDARs typically require a telescope with sufficient area to increase the return signal on the detector to levels above the noise floor of the detectors. Two major drivers of the signal-to-noise ratio (SNR) on the detectors are the laser output energy and the round trip distance traveled by the laser signal. The SNR on the detectors can be increased by increasing the telescope reflector area or by decreasing the system noise. If these techniques are not an option, this method can be used to separate stray light from polarized laser light in the LIDAR system and improve the SNR. The method includes generating a beam of azimuthally polarized or OAM light utilizing an optical transmitter comprising a laser light source. The method includes providing an optical receiver including optical sensors at a focal plane with a photon sieve that produces a ring pattern on the focal plane corresponding to a laser return signal. The ring pattern comprises azimuthally polarized or OAM light that is transmitted by the transmitter and reflected towards the receiver. The photon sieve is utilized to cause stray light that is not polarized to cluster centrally, and away from the ring pattern created by the LIDAR signal. This technology could also be used with space based and terrestrial LIDAR for encrypted line of sight communications. The unique revolution frequencies of the LIDAR make any attempt to intercept the communication pointless for those who don&#39t know the specific mode of the source. The lidar system also has use cases for short range navigation for Urban Air Mobility (UAM) vehicles providing input as to whether there is significant enough clear air turbulence on a given path as to be dangerous to an aerial vehicle.

For decades, frequency modulation has been used to generate chirps, the signals produced and interpreted by sonar and radar systems. Traditionally, a radio or microwave signal is transmitted toward the target and reflected back to a detector, which records the time elapsed and calculates the targets distance. Reflected signals can be heterodyned (combined) with output signals to determine the Doppler frequency shift and the target velocity. Accuracy of these systems can be enhanced by increasing the bandwidth of the chirp, but noise generated during heterodyning at high frequencies decreases the signal-to-noise ratio, increasing measurement error. Previous attempts at laser frequency modulation that relied on adjusting the laser cavity length have resulted in only sine wave or imperfect triangle waveforms. Heterodyning of imperfect, non-linear waveforms or sine waveforms will significantly degrade the effective signal-to-noise ratio, making such systems impractical. In contrast, the current technology produces a single, high-frequency laser that is passed to an electro-optical modulator, which generates a series of harmonics. This range of frequencies is then passed through a band-pass optical filter so the desired harmonic frequency can be isolated and directed toward the target. By modulating the electrical signal applied to the electro-optical modulator, a near perfect triangular waveform laser beam can be produced. Transmission and detection of this highly linear triangular waveform facilitates optical heterodyning for the calculation of precise frequency and phase shifts between the output and reflected signals with a high signal-to-noise ratio. By combining this information with the time elapsed, the location and velocity of the target can be determined to within 1 mm or 1 mm/s.

The NDL uses homodyne detection to obtain changes in signal frequency caused by a target of interest. Frequency associated with each segment of the modulated waveform collected by the instrument is positive or negative, depending on the relative range and direction of motion between the NDL and the target. Homodyne detection offers a direct measurement of signal frequency changes however only the absolute values of the frequencies are measured, therefore additional information is necessary to determine positive or negative sign of the detected frequencies. The three segmented waveform, as opposed to conventional two-segmented ones, allows for resolving the frequency sign ambiguity. In a practical system, there are times when one or more of the three frequencies are not available during a measurement. For these cases, knowledge of the relative positions of the frequency sideband components is used to predict direction of the Doppler shift and sign, and thus make correct range and velocity measurements. This algorithm provides estimates to the sign of the intermediate frequencies. The instrument operates continuously in real time, producing independent range and velocity measurements by each line of sight used to take the measurement. In case of loss of one of the three frequencies, past measurements of range and velocity are used by the algorithm to provide estimates of the expected new range and velocity measurement. These estimates are obtained by applying an estimation filter to past measurements. These estimates are used during signal loss to reduce uncertainty in the sign of the frequencies measured once signals are re-established, and never to replace value of a measurement.

The developed FPGA modules discern time-of-flight of laser pulses for LiDAR applications through the correlation of a Gaussian pulse with a discretely sampled waveform from the LiDAR receiver. For GRSSLi, up to eight cross-correlation engines were instantiated within a FPGA to process the discretely sampled transmit, receive pulses from the LiDAR receiver, and ultimately measure the time-of-flight of laser pulses at 20-picosecond resolution. Engine number is limited only by the resources within the FPGA fabric, and is configurable with a constant. Thus, potential time-of-flight measurement rates could go well beyond the 200-KHz mark required by GRSSLi. Additionally, the engines have been designed in an extremely efficient manner and utilize the least amount of FPGA resources possible.