Lithium isotope separation with tunable diode lasers

A laser-isotope-separation study of lithium has been performed with two-step excitation involving UV laser radiation and a visible tunable-diode laser. The method yields a high degree of selectivity by tuning the narrow-linewidth diode laser to the D1 or D2 levels of the lithium atom. Selective laser excitation is simplified by the use of the tunable diode laser and the overall approach benefits from the application of a compact mass selector that includes a precision magnetic sector and an ion beam that is designed specifically for light atoms such as lithium.     

Frequency-resolved absorption tomography with tunable diode lasers

A tunable diode laser was used for absorption tomography in an axisymmetric atmospheric pressure flat-flame burner. A rapid tomographic inversion algorithm was used to facilitate the many reconstructions at a relatively sparse set of projections typical of laser absorption tomography. Profiles of temperature and CO2 mole fraction were measured simultaneously in methane-air flames. Absorption measurements were made near the R-branch bandhead at 4.17 microm to minimize interferences with other species, while providing good temperature and concentration sensitivity at flame conditions. The procedure showed the advantage of reconstructing detailed spectra at each radial node.     

Frequency locking of a pulsed single-longitudinal-mode laser in a coupled-cavity resonator

The performance of a novel resonator that couples a grazing-incidence and a linear cavity is reported. The coupling secures single-longitudinal mode, TEM(00), higher-efficiency and lower-threshold operation. By use of Ti:sapphire as the gain medium, a slope efficiency of 23% and a 100-nm tuning range are reported. A model is explained that fully predicts the mode behavior of the resonator and that can be used to optimize the cavity for single-mode operation. We have developed computer control of the cavity, which is simple in design and is used to lock the <200-MHz bandwidth mode to +/-40 MHz. A 4.8-GHz scan has also been demonstrated.     

Fiber-amplifier-enhanced photoacoustic spectroscopy with near-infrared tunable diode lasers

A new approach to wavelength-modulation photoacoustic spectroscopy is reported, which incorporates diode lasers in the near infrared and optical fiber amplifiers to enhance sensitivity. We demonstrate the technique with ammonia detection, yielding a sensitivity limit less than 6 parts in 10(9), by interrogating a transition near 1532 nm with 500 mW of output power from the fiber amplifier, an optical pathlength of 18.4 cm, and an integration time constant of 10 s. This sensitivity is 15 times better than in prior published results for detecting ammonia with near-infrared diode lasers. The normalized minimum detectable fractional optical density, alphaminl, is 1.8 x 10(-8); the minimum detectable absorption coefficient, alphamin, is 9.5 x 10(-10) cm(-1); and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5 x 10(-9) W cm(-1)/square root Hz. These measurements represent what we believe to be the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.     

Stable injection locking of diode lasers through a phase-modulated double phase-conjugate mirror

The stable injection locking of 0.8-mum diode lasers with a double phase-conjugate mirror (DPCM) was achieved. Phase modulation by piezoelectric transducers allowed us to keep two input beams of the DPCM mutually incoherent during locking. We preserved the high performance of the DPCM and retained stable locking for more than an hour.     

High-sensitivity detection of tritiated water vapour using tunable diode lasers

A tunable lead-salt diode laser and a multipass optical cell are used to detect traces of tritiated water vapour in air. The present system has a sensitivity of ∼ 700 μCi/m³ if the air sample directly enters the cell, and an enhanced sensitivity of 35 μCi/m³ if the water vapour is precondensed from the air sample. The instrument is free from interference from HT or other radioactive species which may be present in the sample.     

Manifestation of active medium astigmatism at transverse mode locking in a diode end-pumped stable resonator laser

Transverse mode locking in a diode end-pumped Nd:YAG laser with up to approximately 140 cm resonator length was investigated. It was found that for each resonator degeneracy, there are two degenerate lengths where the fundamental mode is very different from the Gaussian mode. Fundamental mode intensity patterns for these lengths expand in directions perpendicular to each other. Experimental results are in a good agreement with numerical calculations, taking into account active medium (AM) astigmatism and inhomogeneous gain. Optical powers of astigmatic AM can be found directly from measurements of degenerate lengths without using numerical modeling.     

Detection and measurement of middle-distillate fuel vapors by use of tunable diode lasers

A sensor for the rapid (10-ms response time) measurement of vapors from the hydrocarbon-based fuels JP-8, DF-2, and gasoline is described. The sensor is based on a previously reported laser-mixing technique that uses two tunable diode lasers emitting in the near-infrared spectral region [Appl. Opt. 39, 5006 (2000)] to measure concentrations of gases that have unstructured absorption spectra. The fiber-mixed laser beam consists of two wavelengths: one that is absorbed by the fuel vapor and one that is not absorbed. Sinusoidally modulating the power of the two lasers at the same frequency but 180 degrees out of phase allows a sinusoidal signal to be generated at the detector (when the target gas is present in the line of sight). The signal amplitude, measured by use of standard phase-sensitive detection techniques, is proportional to the fuel-vapor concentration. Limits of detection at room temperature are reported for the vapors of the three fuels studied. Improvements to be incorporated into the next generation of the sensor are discussed.     

Mechanically tunable optical filters with a microring resonator

A new type of optical filter based on mechanical tuning and a microring resonator is proposed. The proposed filter is expected to consume much less standing power compared to the conventional thermo-optic and carrier-injection tunable filters. In this work, two methods are used to prove the concept of the proposed device: (1) the analytical method and (2) the finite-difference time-domain method. The dependence of the filter characteristics on some of the device parameters is studied as well.     

Application of tunable excimer lasers to combustion diagnostics: a review

Tunable excimer lasers are being used to produce species-, space-, and time-resolved images of complex gaseous media. These media may be analyzed for composition, density, temperature, or flow velocities. The techniques are, in general, highly selective, sensitive, and nonintrusive and are being made possible by recent technological developments in these UV lasers and in intensified cameras, imaging spectrographs, and fast digital image processing. We describe the needs for laser diagnostics in combustion, the physical mechanisms, the relevant spectroscopy, typical experimental setups, and equipment considerations. Precision and accuracy are discussed on the basis of some simple, but realistic, calculations intended to guide the experimentalist in design considerations and to reveal potential sources of errors in the often difficult conversion of raw data to values for such quantitative parameters as densities or temperatures. Finally we present an overview of previous results, select some examples that show the power of tunable excimer laser diagnostics in combustion, and present some suggestions for future directions.     

MASERATI: a RocketBorne tunable diode laser absorption spectrometer

The MASERATI (middle-atmosphere spectrometric experiment on rockets for analysis of trace-gas influences) instrument is, to our knowledge, the first rocket-borne tunable diode laser absorption spectrometer that was developed for in situ measurements of trace gases in the middle atmosphere. Infrared absorption spectroscopy with lead salt diode lasers is applied to measure water vapor and carbon dioxide in the altitude range from 50 to 90 km and 120 km, respectively. The laser beams are directed into an open multiple-pass absorption setup (total path length 31.7 m) that is mounted on top of a sounding rocket and that is directly exposed to ambient air. The two species are sampled alternately with a sampling time of 7.37 ms, each corresponding to an altitude resolution of approximately 15 m. Frequency-modulation and lock-in techniques are used to achieve high sensitivity. Tests in the laboratory have shown that the instrument is capable of detecting a very small relative absorbance of 10(-4)-10(-5) when integrating spectra for 1 s. The instrument is designed and qualified to resist the mechanical stress occurring during the start of a sounding rocket and to be operational during the cruising phase of the flight when accelerations are very small. Two almost identical versions of the MASERATI instrument were built and were launched on sounding rockets from the And?ya Rocket Range (69 degrees N) in northern Norway on 12 October 1997 and on 31 January 1998. The good technical performance of the instruments during these flights has demonstrated that MASERATI is indeed a new suitable tool to perform rocket-borne in situ measurements in the upper atmosphere.     

Frequency shifts in a crystal resonator due to submersion in a fluid

Shifts of resonant frequencies of a three-dimensional crystal body of arbitrary shape due to submersion in a fluid is analyzed by a perturbation procedure. A first-order perturbation integral is obtained for the frequency shifts. The perturbation integral is shown to be effective in an example.     

Fast tuning of external-cavity diode lasers

We report on a method for characterizing the tuning capabilities of small-bandwidth external-cavity diode lasers. The step response of a piezo-driven Littman-Metcalf external-cavity diode laser and the compensation of its optoelectromechanical frequency response is investigated. When an approximately 50-V compensated Gaussian pulse is applied to the piezo element, a detuning of 13.6 GHz is observed. This modification of the laser is useful for several spectroscopical applications and as a tunable seed laser for lidar applications.     

Potential for improving the Rubidium Frequency Standard with anovel optical pumping scheme using diode lasers

Frequency stability of a Rubidium Frequency Standard (RFS) is directly related to the signal-to-noise ratio (S/N) of the microwave-induced (6834 MHz) optical signal which is proportional to the fractional atomic population difference between the two M_F=0 ground state sublevels of ⁸⁷Rb. In the present RFSs this fractional population difference is small (<1%). S/N can be substantially improved by concentrating all of the atoms in one of the two M_F=0 sublevels. Potentially, this could lead to a significant improvement in the short-term performance of RFSs. We have developed a novel scheme for concentrating a large fraction of the Rb atoms in one of the two M_F=0 ground state sublevels. We optically pump the Rb vapor with circularly polarized light from a AlGaAs diode laser tuned to the D₁ transition (794.7 nm). Nearly all of the atoms are concentrated in one of the two high angular momentum states (M_F=2 or -2 sublevels depending on the handedness of the circular polarization). The pumping laser is switched off and two radiofrequency (RF) π-pulses are applied sequentially. The first π-pulse transfers the atoms from the 2,2(F,M_F) sublevel to the 2,1 sublevel and the second π-pulse transfers the atoms from the 2,1 sublevel to the 2,0 sublevel. The resulting population distribution is diagnosed using a second AlGaAs diode laser (weak probe) in conjunction with a microwave field tuned to the 0-0 transition (6834 MHz). We obtain a fractional population difference of 0.7-0.9 between the two M_F=0 sublevels. This should result in an improvement in the S/N by a factor of 70-90 over the lamp pumped RFSs. This could potentially be of considerable importance towards the development of future RFSs. Various relaxations and field inhomogeneities limit the transfer efficiency from being 100%. The details of the experimental technique and possible applications are discussed     

Analysis of broadly tunable coupled-cavity semiconductor lasers

We report on the spectral properties for above-threshold operation of broadly tunable, asymmetric multiple quantum well (AMQW), coupled-cavity InGaAsP/InP semiconductor diode lasers. We developed a traveling wave model to understand the mode selection that the lasers exhibit. We find that a weak, short external cavity (SXC) can be used to obtain single frequency operation on each longitudinal mode over the ～100 nm tuning range of the uncoated AMQW coupled-cavity lasers. We measured the spectral properties of AMQW coupled-cavity lasers with and without an SXC. In a synthesized optical coherent optical tomography application, the use of an SXC with an AMQW coupled-cavity laser reduces the coherence length and hence enhances the performance of the AMQW coupled-cavity laser for optical coherence tomography applications.     

Single-frequency Sb-based distributed-feedback lasers emitting at 2.3 microm above room temperature for application in tunable diode laser absorption spectroscopy

GaInAsSb/GaAlAsSb/GaSb distributed-feedback (DFB) laser diodes based on a type I active region were fabricated by molecular beam epitaxy at the Centre d'Electronique et de Micro-Optoélectronique de Montpellier (CEM2). The DFB processing was done by Nanoplus Nanosystems and Technologies GmbH. The devices work in the continuous-wave regime above room temperature around an emission wavelength of 2.3 microm with a side-mode suppression ratio greater than 25 dB and as great as 10 mW of output power. The laser devices are fully characterized in terms of optical and electrical properties. Their tuning properties made them adaptable to tunable diode laser absorption spectroscopy because they exhibit more than 220 GHz of continuous tuning by temperature or current. The direct absorption of CH4 is demonstrated to be possible with high spectral selectivity.     

Frequency stability degradation of an oscillator slaved to a periodically interrogated atomic resonator

Atomic frequency standards using trapped ions or cold atoms work intrinsically in a pulsed mode. Theoretically and experimentally, this mode of operation has been shown to lead to a degradation of the frequency stability due to the frequency noise of the interrogation oscillator. In this paper a physical analysis of this effect has been made by evaluating the response of a two-level atom to the interrogation oscillator phase noise in Ramsey and multi-Rabi interrogation schemes using a standard quantum mechanical approach. This response is then used to calculate the degradation of the frequency stability of a pulsed atomic frequency standard such as an atomic fountain or an ion trap standard. Comparison is made to an experimental evaluation of this effect in the LPTF Cs fountain frequency standard, showing excellent agreement.