Pulsed Coherent Laser Research Articles

Fluorescence-detected Fourier transform electronic spectroscopy by phase-tagged photon counting.

Fluorescence-detected Fourier transform (FT) spectroscopy is a technique in which the relative paths of an optical interferometer are controlled to excite a material sample, and the ensuing fluorescence is detected as a function of the interferometer path delay and relative phase. A common approach to enhance the signal-to-noise ratio in these experiments is to apply a continuous phase sweep to the relative optical path, and to detect the resulting modulated fluorescence using a phase-sensitive lock-in amplifier. In many important situations, the fluorescence signal is too weak to be measured using a lock-in amplifier, so that photon counting techniques are preferred. Here we introduce an approach to low-signal fluorescence-detected FT spectroscopy, in which individual photon counts are assigned to a modulated interferometer phase ('phase-tagged photon counting,' or PTPC), and the resulting data are processed to construct optical spectra. We studied the fluorescence signals of a molecular sample excited resonantly by a pulsed coherent laser over a range of photon flux and visibility levels. We compare the performance of PTPC to standard lock-in detection methods and establish the range of signal parameters over which meaningful measurements can be carried out. We find that PTPC generally outperforms the lock-in detection method, with the dominant source of measurement uncertainty being associated with the statistics of the finite number of samples of the photon detection rate.

Optical frequency deviation compensation using pulsed serrodyne technique on coherent laser transmitter for wind sensing lidar

We report a new pulsed coherent laser transmitter for wind sensing that combines a semiconductor optical amplifier (SOA) with a phase modulator operated at pulsed serrrodyne modulation. Using pulsed serrodyne modulation, we demonstrate the correction of the instantaneous frequency deviation caused by the direct modulation of the SOA within a single pulse duration. The residual frequency errors are reduced to 0.03 MHz, which corresponds to a wind velocity error of 0.02 m s−1.

Semianalytic pulsed coherent laser radar equation for coaxial and apertured systems using nearest Gaussian approximation

We present a semianalytic pulsed coherent laser radar (CLR) equation for coaxial and apertured systems. It combines the conventional CLR equation, numerical Fresnel integration (NFI), and nearest Gaussian approximation, using correction factors that correspond to beam truncation. The range dependence of the signal-to-noise ratio obtained by this semianalytic equation was found to agree well with the precise NFI solution for not only the focal range, but also the near-field range. Furthermore, the optimum beam truncation condition depending on the atmospheric refractive index structure constant is shown. The derived equation is useful for precisely predicting the CLR performance simply by its semianalytic expression.

Advanced signal processing methods for pulsed laser vibrometry

Although pulsed coherent laser radar vibrometry has been introduced as an improvement over its continuous wave (CW) counterpart, it remains very sensitive to decorrelation noises, such as speckle, and other disturbances of its measurement. Taking advantage of more polyvalent polypulse waveforms, we address the issue with advanced signal processing. We have conducted what we believe is the first extensive comparison of processing techniques considering CW, pulse-pair, and polypulse emissions. In this framework, we introduce a computationally efficient maximum likelihood estimator and test signal tracking on pseudo-time-frequency representations (TFRs), which, respectively, help deal with speckle noise and fading of the signal in harsh noise conditions. Our comparison on simulated signals is validated on a 1.55 microm all-fiber vibrometer experiment with an apparatus simulating vibration and strong speckle noise. Results show the advantage of the estimators that take into account actual noise statistics, and call for a wider use of TFRs to track the vibration-modulated signal.

Solid-state coherent laser radar wind field measurement systems

Diode-pumped solid-state pulsed coherent laser radar systems have recently been developed at Coherent Technologies, Inc., for the remote measurement of atmospheric wind fields. Flashlamp-pumped systems have been utilized since 1990 for obtaining wind field measurements. These flashlamp-pumped lidar systems have been applied to wind profiling, aircraft-wake vortex measurements, airport wind shear and gust front monitoring, military cargo air drops and many other applications. The diode-pumped coherent lidar systems currently available are capable of near turnkey operation. The Tm:YAG laser transceivers operate at with output pulse energies of 1-10 mJ with PRFs of 1000 to 100 Hz, respectively. Range resolutions of 30-75 m are typical. A real-time lidar signal processor has also been developed for collecting and analysing laser radar (lidar) data. The signal processor is based on a commercial PC architecture and offers a real-time data acquisition, analysis, display, recording and playback environment. Wind measurements and overall system performance results will be presented. Wind measurement performance, for a variety of applications, is presented using the flashlamp- and diode-pumped coherent lidars including measured wind profiles from ground and on aircraft, wake vortex tracking results, and example flows over mountain terrain.

Characterization of pulsed coherent Doppler LIDAR with the speckle effect

The relationships among heterodyne efficiency γ, number of speckle cells M, and the ratio of receiver area to coherence area S(R)/S(C) for a pulsed coherent laser radar (CLR) are written through the use of mutual coherence functions. It is shown that numerical values for S(R)/S(C) that follow Goodman's definition [J. W. Goodman, in Laser Speckles and Related Phenomena, J. C. Dainty, ed. (Springer-Verlag, Berlin, 1975), pp. 9-75] or that are obtained through the use of a transverse-field coherence length agree. In the frame of the Gaussian model proposed by Frehlich and Kavaya [Appl. Opt. 30, 5325 (1991)] a new equation is derived: M = (1 + S(R)/S(C)). This equation agrees with our experimental results. Our theoretical analysis shows that the number of speckle cells for an optimal monostatic CLR system is M ~ 4. An experiment has been conducted with a ground-based pulsed CO(2) LIDAR and remote hard targets to study the probability density function of LIDAR returns as a function of M and to study the dependence of M on S(R)/S(C). An assessment of CLR performance through the use of M or the collecting aperture S(R) is discussed.

Eye-safe coherent laser radar system at 21 μm using Tm,Ho:YAG lasers

An eye-safe pulsed coherent laser radar has been developed by using single-frequency Tm,Ho:YAG lasers and heterodyne detection. Returns from a mountainside located 145 km from the laser radar system and the measurement of wind velocity to ranges exceeding 20 km have been demonstrated with transmitted pulse energies of 22 mJ.