Kevin S. Repasky

Optical detection of honeybees by use of wing-beat modulation of scattered laser light for locating explosives and land mines

An instrument is demonstrated that can be used for optical detection of honeybees in a cluttered environment. The instrument uses a continuous-wave diode laser with a center wavelength of 808 nm and an output power of 28 mW as the laser transmitter source. Light scattered from moving honeybee wings will produce an intensity-modulated signal at a characteristic wing-beat frequency (170-270 Hz) that can be used to detect the honeybees against a cluttered background. The optical detection of honeybees has application in the biological detection of land mines and explosives, as was recently demonstrated.

Eye-Safe Diode-Laser-Based Micropulse Differential Absorption Lidar (DIAL) for Water Vapor Profiling in the Lower Troposphere

Abstract A second-generation diode-laser-based master oscillator power amplifier (MOPA) configured micropulse differential absorption lidar (DIAL) instrument for profiling of lower-tropospheric water vapor is presented. The DIAL transmitter is based on a continuous wave (cw) external cavity diode laser (ECDL) master oscillator that is used to injection seed two cascaded tapered semiconductor optical power amplifiers, which deliver up to 2-μJ pulse energies over a 1-μs pulse duration at 830 nm with an average power of ∼40 mW at a pulse repetition frequency of 20 kHz. The DIAL receiver utilizes a commercial 28-cm-diameter Schmidt–Cassegrain telescope, a 250-pm narrowband optical filter, and a fiber-coupled single-photon-counting Avalanche photodiode (APD) detector, yielding a far-field full-angle field of view of 170 μrad. A detailed description of the second-generation Montana State University (MSU) DIAL instrument is presented. Water vapor number density profiles and time–height cross sections collected w...

Testing carbon sequestration site monitor instruments using a controlled carbon dioxide release facility

Two laser-based instruments for carbon sequestration site monitoring have been developed and tested at a controlled carbon dioxide (CO(2)) release facility. The first instrument uses a temperature tunable distributed feedback (DFB) diode laser capable of accessing the 2.0027-2.0042 microm spectral region that contains three CO(2) absorption lines and is used for aboveground atmospheric CO(2) concentration measurements. The second instrument also uses a temperature tunable DFB diode laser capable of accessing the 2.0032-2.0055 mum spectral region that contains five CO(2) absorption lines for underground CO(2) soil gas concentration measurements. The performance of these instruments for carbon sequestration site monitoring was studied using a newly developed controlled CO(2) release facility. A 0.3 ton CO(2)/day injection experiment was performed from 3-10 August 2007. The aboveground differential absorption instrument measured an average atmospheric CO(2) concentration of 618 parts per million (ppm) over the CO(2) injection site compared with an average background atmospheric CO(2) concentration of 448 ppm demonstrating this instruments capability for carbon sequestration site monitoring. The underground differential absorption instrument measured a CO(2) soil gas concentration of 100,000 ppm during the CO(2) injection, a factor of 25 greater than the measured background CO(2) soil gas concentration of 4000 ppm demonstrating this instruments capability for carbon sequestration site monitoring.

High-conversion-efficiency, diode-pumped continuous-wave Raman laser

We demonstrate a diode-pumped cw Raman laser in H(2) with photon-conversion efficiency of (66+/-8)%. Pumped by an injection-locked diode laser at 792 nm, the Stokes laser produces a peak output power of ~16mW at 1180 nm. Accompanying the high Stokes power are deviations from the existing theory, which are believed to be caused by the thermal-lensing effect of the Raman gas.

High-finesse interferometers

A detailed plan for the construction and use of a nonconfocal cavity used as a high-finesse interferometer is presented. The interferometer has a free spectral range of l5 GHz, with a finesse of over 30,000.

Dual-polarization lidar using a liquid crystal variable retarder

We describe the design and characterization of a compact dual-polarization lidar that uses a liquid crystal variable retarder (LCVR) to discriminate between backscattered polarization states on alternate laser pulses (at 30 Hz). Measurements of the polarization discrimination of the system, including the liquid crystal and a Schmidt-Cassegrain receiver telescope, show that depolarization ratios can be determined with an additive error of less than 0.4%. The source is a Nd:YAG laser with a wavelength of 532 nm, pulse energy of 118 mJ, and pulse-repetition frequency of 30 Hz. The normal operating range is 15 km, with a 1.5-m range resolution. The full-angle receiver field of view is variable up to 8.8 mrad. Sample data from atmospheric clouds demonstrate the use of lidar depolarization measurements for distinguishing between ice and liquid water in thin clouds with low multiple scattering (with cloud phase verified using radiosonde profiles of atmospheric temperature and humidity). Also shown is a lidar observation of a depolarizing layer over Bozeman, Montana, identified as subvisual cirrus, aerosols transported from in or near China, or a combination thereof.

Extending the continuous tuning range of an external-cavity diode laser

The continuous tuning range of an external-cavity diode laser can be extended by making small corrections to the external-cavity length through an electronic feedback loop so that the cavity resonance condition is maintained as the laser wavelength is tuned. By maintaining the cavity resonance condition as the laser is tuned, the mode hops that typically limit the continuous tuning range of the external-cavity diode laser are eliminated. We present the design of a simple external-cavity diode laser based on the Littman-Metcalf external-cavity configuration that has a measured continuous tuning range of 1 GHz without an electronic feedback loop. To include the electronic feedback loop, a small sinusoidal signal is added to the drive current of the laser diode creating a small oscillation of the laser power. By comparing the phase of the modulated optical power with the phase of the sinusoidal drive signal using a lock-in amplifier, an error signal is created and used in an electronic feedback loop to control the external-cavity length. With electronic feedback, we find that the continuous tuning range can be extended to over 65 GHz. This occurs because the electronic feedback maintains the cavity resonance condition as the laser is tuned. An experimental demonstration of this extended tuning range is presented in which the external-cavity diode laser is tuned through an absorption feature of diatomic oxygen near 760 nm.

Accumulated programming of a complex spectral grating

A complex spectral grating is accumulated by repeated application of a pair of low-power optical programming pulses to a short-term persistent inhomogeneously broadened transition in Tm:YAG at 4.5 K and then probed to investigate the buildup dynamics. The necessary frequency stability is obtained by locking a cw Ti:sapphire laser to a regenerating transient spectral hole in the same transition. Grating accumulation is demonstrated for both a periodic spectral grating, representing a true-time delay, and a complex spectral grating, permitting correlation-based pattern recognition. This work is a step toward demonstrating an optical coherent transient continuously programmed continuous processor.

Tunable external cavity diode laser based on integrated waveguide structures

A tunable external cavity laser based on integrated optics produced in a KTP substrate is demonstrated. The laser operates single mode with side mode suppression greater than 35dB and an electrooptic tuning response of R=19.5MHz/V.

Micropulse water vapor differential absorption lidar: transmitter design and performance

An all diode-laser-based micropulse differential absorption lidar (DIAL) laser transmitter for tropospheric water vapor and aerosol profiling is presented. The micropulse DIAL (MPD) transmitter utilizes two continuous wave (cw) external cavity diode lasers (ECDL) to seed an actively pulsed, overdriven tapered semiconductor optical amplifier (TSOA). The MPD laser produces up to 7 watts of peak power over a 1 µs pulse duration (7 µJ) and a 10 kHz pulse repetition frequency. Spectral switching between the online and offline seed lasers is achieved on a 1Hz basis using a fiber optic switch to allow for more accurate sampling of the atmospheric volume between the online and offline laser shots. The high laser spectral purity of greater than 0.9996 coupled with the broad tunability of the laser transmitter will allow for accurate measurements of tropospheric water vapor in a wide range of geographic locations under varying atmospheric conditions. This paper describes the design and performance characteristics of a third generation MPD laser transmitter with enhanced laser performance over the previous generation DIAL system.