Jason R. Dahl

Characterization of an actively linearized ultrabroadband chirped laser with a fiber-laser optical frequency comb

The optical frequency sweep of an actively linearized, ultrabroadband, chirped laser source is characterized through optical heterodyne detection against a fiber-laser frequency comb. Frequency sweeps were measured over approximately 5 THz bandwidths from 1530 nm to 1570 nm. The dominant deviation from linearity resulted from the nonzero dispersion of the fiber delay used as a reference for the sweep linearization. Removing the low-order dispersion effects, the residual sweep nonlinearity was less than 60 kHz rms, corresponding to a constant chirp with less than 15 ppb deviation across the 5 THz sweep.

Synthetic aperture ladar imaging demonstrations and information at very low return levels

We present synthetic aperture ladar (SAL) imaging demonstrations where the return-signal level from the target is near the single-photon level per resolved pixel. Scenes consisting of both specular-point targets and diffuse-reflection, fully speckled targets are studied. Artificial retro-reflector-based phase references and/or phase-gradient-autofocus (PGA) algorithms were utilized for compensation of phase errors during the aperture motion. It was found that SAL images could reliably be formed with both methods even when the final max pixel intensity was at the few photon level, which means the SNR before azimuth compression is below unity. Mutual information-based comparison of SAL images show that average mutual information is reduced when the PGA is utilized for image-based phase compensation. The photon information efficiency of SAL and coherent imaging is discussed.

Shot noise statistics and information theory of sensitivity limits in frequency-modulated continuous-wave ladar

A theoretical analysis and experimental verification of the sensitivity limits of frequency-modulated continuous-wave (FMCW) ladar in the limit of a strong local oscillator is presented. The single-photon sensitivity of coherent heterodyne detection in this shot-noise dominated limit is verified to extend to linearly chirped waveforms. An information theoretic analysis is presented to estimate the information efficiency of received photons for the task of locating the range to single and multiple targets. It is found that the optimum receive signal level is proportional to the logarithm of the number of resolvable range locations and the maximum theoretical photon information efficiency for FMCW ranging with coherent fields is log(e)≈1.44 bits per received photon.

Demonstrations of analog-to-digital conversion using a frequency domain stretched processor

Proof-of-concept analog-to-digital conversion demonstrations are presented for a photonics based frequency-domain, stretched processor. Here 800 MHz bandwidths and >26 dB dynamic range are shown, with models suggesting 10-bit performance over 20 GHz bandwidths.

Maximum-likelihood estimation for frequency-modulated continuous-wave laser ranging using photon-counting detectors

We analyze the minimum achievable mean-square error in frequency-modulated continuous-wave range estimation of a single stationary target when photon-counting detectors are employed. Starting from the probability density function for the photon-arrival times in photodetectors with subunity quantum efficiency, dark counts, and dead time, we derive the Cramér-Rao bound and highlight three important asymptotic regimes. We then derive the maximum-likelihood (ML) estimator for arbitrary frequency modulation. Simulation of the ML estimator shows that its performance approaches the standard quantum limit only when the mean received photons are between two thresholds. We provide analytic approximations to these thresholds for linear frequency modulation. We also compare the ML estimators performance to conventional Fourier transform (FT) frequency estimation, showing that they are equivalent if the reference arm is much stronger than the target return, but that when the reference field is weak the FT estimator is suboptimal by approximately a factor of √2 in root-mean-square error. Finally, we report on a proof-of-concept experiment in which the ML estimator achieves this theoretically predicted improvement over the FT estimator.

Ultra-broadband chirp linearity characterization with an optical frequency comb

The linearity of actively stabilized laser chirp from approximately 1530 to 1570 nm is measured by use of an optical frequency comb showing residuals of <; 60 kHz after removing low order dispersion related effects.

High Resolution FMCW Ladar for Imaging and Metrology

Active stabilization of tunable lasers enables coherent FMCW ladar with sub-mm range resolution, which opens up new application areas in ladar imaging and metrology. Demonstrations, applications, and benefits of the technology are reviewed.

Sensitivity in Synthetic Aperture Ladar Imaging

Synthetic Aperture Ladar (SAL) imaging experiments show that cross-range compression, including image based phase error correction, can be performed at very low return light levels. This includes images where the precompression shot-noise-limited signal-to-noise ratio is much less than unity.

Multi-target compressive laser ranging

Compressive laser ranging (CLR) is a method that exploits the sparsity available in the range domain using compressive sensing methods to directly obtain range domain information. Conventional ranging methods are marred by requirements of high bandwidth analog detection which includes severe SNR fall off with bandwidth in analog-to-digital conversion (ADC). Compressive laser ranging solves this problem by obtaining sub-centimeter resolution while using low bandwidth detection. High rate digital pulse pattern generators and off the shelf photonic devices are used to modulate the transmitted and received light from a superluminescent diode. CLR detection is demonstrated using low bandwidth, high dynamic range detectors along with photon counting techniques. The use of an incoherent source eliminates speckle issues and enables simplified CLR methods to get multi-target range profiles with 1-3cm resolution. Using compressive sensing methods CLR allows direct range measurements in the sub-Nyquist regime while reducing system resources, in particular the need for high bandwidth ADC.

Novel MEMS Deformable Mirror for Focus Control and Aberration Correction

Electrostatically actuated MEMS mirrors for focus control have been fabricated. Deflection over 7.3 µm has been achieved using closed-loop control. Low-order aberrations were corrected and the devices were utilized in an imaging system.