Douglas J. Bamford

Absolute calibration of a fluorescence collection system by Raman scattering in H2

A new method for absolute calibration of the detection efficiency of fluorescence collection systems is described for wavelengths covering the 180–2000-nm range. The radiation source for the technique is spontaneous Raman scattering in H2. An analytic expression is derived for the scattering cross section averaged over the solid angle of the fluorescence collection system. From this result, an optical system is calibrated at 845 nm, demonstrating that the technique is easy to use, is quick to implement, and gives accurate results.

Spectroscopic detection of methane by use of guided-wave diode-pumped difference-frequency generation

We report spectroscopic gas detection by the use of mid-infrared difference-frequency mixing of two diode lasers in a channel waveguide. The waveguide was fabricated by annealed proton exchange in periodically poled lithium niobate. We generated 3.43-3.73-microm tunable radiation in a single waveguide at room temperature by mixing diode lasers near 780 and 1010 nm. High-resolution spectra of methane were obtained in 2 s with electronically controlled frequency scans of 45 GHz. The use of highly efficient waveguide frequency converters pumped by fiber-coupled diode lasers will permit construction of compact, solid-state, room-temperature mid-infrared sources for use in trace-gas detection.

Multi-watt 589nm fiber laser source

We have demonstrated 3.5W of 589nm light from a fiber laser using periodically poled stoichio-metric Lithium Tantalate (PPSLT) as the frequency conversion crystal. The system employs 938nm and 1583nm fiber lasers, which were sum-frequency mixed in PPSLT to generate 589nm light. The 938nm fiber laser consists of a single frequency diode laser master oscillator (200mW), which was amplified in two stages to >15W using cladding pumped Nd3+ fiber amplifiers. The fiber amplifiers operate at 938nm and minimize amplified spontaneous emission at 1088nm by employing a specialty fiber design, which maximizes the core size relative to the cladding diameter. This design allows the 3-level laser system to operate at high inversion, thus making it competitive with the 1088nm 4-level laser transition. At 15W, the 938nm laser has an M2 of 1.1 and good polarization (correctable with a quarter and half wave plate to >15:1). The 1583nm fiber laser consists of a Koheras 1583nm fiber DFB laser that is pre-amplified to 100mW, phase modulated and then amplified to 14W in a commercial IPG fiber amplifier. As a part of our research efforts we are also investigating pulsed laser formats and power scaling of the 589nm system. We will discuss the fiber laser design and operation as well as our results in power scaling at 589nm.

Widely tunable rapid-scanning mid-infrared laser spectrometer for industrial gas process stream analysis

A mid-infrared spectrometer with a tuning range of >400 cm−1 in the C-H stretching region of the spectrum has been designed and constructed. The spectrometer is based on the difference-frequency generation of two tunable diode lasers in periodically poled lithium niobate waveguides. Tuning is achieved by varying a single parameter, the wavelength of one of the near-infrared input lasers. The instrument can be tuned over the entire tuning range in less than 1 s. By taking advantage of the wide tuning range, the instrument has been used to analyze a mixture of methane, ethylene, and propylene. Each of these major components was measured with an accuracy of better than 2% (where the error is defined as a percentage of the measured value) in a single 30 s long scan. When optimized, the spectrometer has the potential to meet both the performance requirements and the practical requirements for real-time process control in petrochemical manufacturing. The general principles for the design of mid-infrared spectrometers with wide tuning ranges are explained, including the use of variable waveguide fabrication recipes to create broad phase-matching resonances (which lead to broad tuning) in the desired location.

High-power solid-state sodium guidestar laser for the Gemini North Observatory

We report on the first successful installation of a commercial solid-state sodium guidestar laser system (GLS). The GLS developed at LMCT was delivered to Gemini North Observatory in February of 2005. The laser is a single beacon system that implements a novel laser architecture and represents a critical step towards addressing the need of the astronomy and military adaptive optics (AO) communities for a robust turn-key commercial GLS. The laser was installed on the center section of the 8 m Gemini North telescope, with the output beam relayed to a laser launch telescope located behind the 1 m diameter secondary mirror. The laser went through a three week performance evaluation between November and December 2005 wherein it consistently generated 12 W average power with measured M2 < 1.1 while locked to the D2 line at +/- 100 MHz. The system was required to perform during a 12-hour test period during three runs of 4-6 consecutive nights each. The laser architecture is based on continuous wave (CW) mode-locked solid-state lasers. The mode-locked format enables more efficient SFG conversion, and dispenses with complex resonant intensity enhancement systems and injection-locking electronics. The linearly-polarized, near-diffraction-limited, modelocked 1319 nm and 1064 nm pulses are generated in separate dual-head diode-pumped resonators. The two IR pulses are input into a single-stage, 30 mm PPSLT sum-frequency generation (SFG) crystal provided by Physical Science, Inc. Visible (589 nm) power of >16 W have been generated, representing a conversion efficiency of 40%.

Generation of high-average-power visible light in periodically poled nearly stoichiometric lithium tantalate

Sum-frequency generation (SFG) of >16 W of 589 nm light has been achieved by a single pass through a 20 mm long, undoped, periodically poled, nearly stoichiometric lithium tantalate (PPSLT) crystal. This, to our knowledge, represents the highest reported average power in the visible produced by a single-pass SFG crystal and the highest visible average power produced by PPSLT. The stoichiometric lithium tantalate crystal was grown directly from the melt without magnesium doping.

CO2 laser-based dispersion interferometer utilizing orientation-patterned gallium arsenide for plasma density measurements

A dispersion interferometer based on the second-harmonic generation of a carbon dioxide laser in orientation-patterned gallium arsenide has been developed for measuring electron density in plasmas. The interferometer includes two nonlinear optical crystals placed on opposite sides of the plasma. This instrument has been used to measure electron line densities in a pulsed radio-frequency generated argon plasma. A simple phase-extraction technique based on combining measurements from two successive pulses of the plasma has been used. The noise-equivalent line density was measured to be 1.7 × 10(17) m(-2) in a detection bandwidth of 950 kHz. One of the orientation-patterned crystals produced 13 mW of peak power at the second-harmonic wavelength from a carbon dioxide laser with 13 W of peak power. Two crystals arranged sequentially produced 58 mW of peak power at the second-harmonic wavelength from a carbon dioxide laser with 37 W of peak power.

High-average-power visible generation in periodically-poled nearly-stoichiometric lithium tantalate

Sum-frequency generation has been used to generate more than 12 W of average power at 589 nm in a single pass through a crystal of periodically-poled, undoped, nearly-stoichiometric lithium tantalate.

Periodic poling of stoichiometric lithium tantalate

The periodic poling of stoichiometric lithium tantalate, a nonlinear optical material with great promise for the frequency conversion of high-average-power solid state lasers, has been investigated. Two problems with commercially available stoichiometric lithium tantalate substrates have been identified: non-reproducibility of the coercive field from one wafer to the next, and susceptibility to the formation of micro-domain defects. Strategies for dealing with these problems have been developed. Wafer-scale poling has been carried out to produce quasi-phasematching gratings with periods as short as 7.3 microns on half-millimeter thick substrates and 25.4 microns on millimeter-thick substrates. The phase-matching properties of periodically poled stoichiometric lithium tantalate have been measured using nonlinear optical frequency conversion. For processes which generate visible radiation, good agreement with predictions based on the published Sellmeier equation for stoichiometric lithium tantalite has been obtained.

Efficient channel waveguide device for difference-frequency mixing of diode lasers

Summary form only given. We designed and fabricated a three-stage integrated channel waveguide array in periodically poled lithium niobate (PPLN) using the process of annealed proton exchange (APE). Our choice of fabrication conditions, as well as theoretical predictions of waveguide performance, were based on a prior systematic study of refractive index profiles and dispersion of planar APE waveguides. The channel waveguide array was used to produce continuous-wave, single-frequency tunable 2.8-/spl mu/m radiation for spectroscopic applications, by difference-frequency mixing of diode lasers at 790 nm and 1096 nm, each operated at 150 mW output power.