S Waltman

Absolute frequency of the molecular iodine transition R(56)32-0 near 532 nm

The absolute frequency of the hyperfine component a/sub 10/ in the transition R(56)32-0 of iodine has been measured using the D/sub 2/ line in Rb at 780 mm and an iodine-stabilized 633-nm He-Ne laser as references. This measurement provides a secondary frequency standard within the tuning range of a doubled Nd:YAG laser. The measured frequency of the a/sub 10/ component is 563 260 223.480 MHz /spl plusmn/70 kHz. >

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS2. IR frequency tuning between 3076 and 3183 cm−1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)−1/2.

External-cavity difference-frequency source near 3.2 μm, based on combining a tunable diode laser with a diode-pumped Nd:YAG laser in AgGaS 2

AgGaS2 has been used to generate more than 2 μW of cw mid-infrared radiation near 3.2 μm by difference-frequency mixing of the outputs of an extended-cavity diode laser near 795 nm (pump wave) an a compact diode-pumped Nd:YAG laser at 1064 nm (signal wave). An external ring enhancement cavity was used to build up the signal power inside the nonlinear crystal by as much as 14.5 times. The novel mid-infrared source incorporating a single diode laser could be angle-tuned from 3.155 to 3.423 μm (from 3170 to 2921 cm−1). This system was used to detect the Doppler-broadened fundamental ν3-asymmetric stretch vibration of methane (CH4) by both direct and wavelength-modulation absorption spectroscopy.

Stability and absolute frequency of molecular iodine transitions near 532 nm

A frequency-doubled Nd:YAG laser has been stabilized to hyperfine transitions in molecular iodine near 532 nm via modulation transfer spectroscopy. This technique, together with the low noise of the source, yields excellent SNR (500 in a d kHz bandwidth); thus, an impressive frequency stability is achieved. The nearly systematic-free resonance signals obtained by modulation transfer spectroscopy give a correspondingly encouraging reproducibility, estimated to be about +/- 300 Hz. With two such stabilized lasers were found a pressure shift of only -1.3 kHz/Pa over the range 0.4-4.0 Pa and a power-dependent frequency shift of 2.1 kHz/mW. We have also measured the absolute frequency of the component a10 in the transition R(56)32-0 using the D2 line in Rb at 780 nm and an iodine-stabilized 633 nm He-Ne laser as references. The measured frequency is 563 260 223.471 MHz +/- 40 kHz. In turn, the absolute frequency of the D2 line was measured via the frequency difference between the D2 line and the two-photon transition 5S1/2 - 5D5/2 at 778 nm in Rb. Thus we now have realized a pure frequency measurement of this interval and of the 532 nm frequency.

Demonstration of a phase-lockable microwave to submillimeter wave sweeper

The development of low-temperature-grown GaAs photomixers enables the construction of a microwave to submillimeter- wave source capable of large frequency sweeps. By utilizing semiconductor diode lasers to drive the photomixer, this source is all solid-state and compact, and has small power consumption. Frequency stabilization of the semiconductor diode lasers allows this source to be phase-locked to an external microwave reference. Two 805 nm extended-cavity- diode lasers are mixed in a low-temperature-grown GaAs photoconductive photomixer. The difference-frequency mixing product is radiated by a planar spiral antenna and collimated by a Si lens. This output is phase-locked to a microwave reference by downconverting it in a whisker- contacted Schottky-barrier diode harmonic mixer and using the output to offset-phase-lock one laser to the other. The photomixer output power is 300 nW at 200 GHz and 10 nW at 1.6 THz, as measured by a 4 K InSb bolometer calibrated with a methanol laser and a power meter at 526 and 812 GHz.

Precise optical frequency references and difference frequency measurements with diode lasers

Heterodyne methods have been used in conjunction with molecular calculations to accurately determine the wavelengths of more than 35,000 infrared transitions. We have used high speed whisker contract Schottky diodes to extend this technology to the 0.8 micrometers spectral region. Using microwave harmonic mixing we demonstrate that it is possible to detect beat notes between diode lasers to frequencies as high as 400 GHz.

Measurement of the absolute frequency of molecular iodine transitions near 532 nm

The optical frequencies of several transitions in iodine have been measured using the D2 line in Rb 780 nm and an iodine-stabilized 633 nm He-Ne laser. This measurement provides a useful, stable and highly reproducible optical frequency reference in the green.<<ETX>>

Diode lasers for frequency standards and precision spectroscopy

As they apply to frequency standards and precision spectroscopy the characteristics and technology of tunable diode lasers are briefly reviewed. It is now possible to use nonlinear optical techniques and high quality diode lasers to extend the useful wavelength coverage of semiconductor lasers into the UV, the IR and even millimeter-wave spectral regions. Progress in developing an all diode-laser system for cooling, trapping and precision spectroscopy of calcium is discussed. New measurements indicated that two-stage optical cooling of calcium may be feasible; this should improve the accuracy of the 657 nm optical wavelength/frequency reference.

DIODE LASERS AND METROLOGY

At NIST in Boulder we have been pursuing an active research program developing diode-laser technology for scientific applications. Commercial diode lasers are readily available in a few wavelength bands in the red and near IR region of the spectrum. Our work has focused on the AlGaAs, InGaAlP, and InGaAsP lasers that operate at room temperature in the red and near IR region of the spectrum between 650 nm and 1.5 microns. These lasers have a number of recognized advantages, including: high efficiency, low cost, tunability, and moderate power levels (~1 to 100 mW). Increasing interest in applying diode lasers to science in general and spectroscopy in particular has stimulated a number of recent reviews on the subject.1–4

Compact tunable midinfrared laser sources: technology and applications

Recent progress in design of diode-laser-pumped CW tunable narrowband mid-infrared sources based on difference-frequency generation (DFG) is addressed. Application of two such tunable sources to high-resolution spectroscopy and sensitive atmospheric trace detection of methane near 3.2 micrometers is reported. Methane detection limit in air of 12 ppb- m/(root)Hz is reported based upon the signal-to-noise ratio observed in the direct absorption spectra. Performance characteristics of these sources are examined including tuning range, phase matching bandwidth, output power, and amplitude stability. Applications of diode-laser-pumped CW tunable DFG may include atmospheric trace detection of several major hydrocarbons such as ethane, ethylene, and benzene, and toxic air pollutants such as carbon monoxide, nitric and nitrous oxide, sulfur dioxide, and methyl chloride.