Search

We verify the accuracy of femtosecond laser optical frequency synthesizers that employ microstructure fibers with those that directly generate a broadband output. No limitation of either system is found at fractional frequency levels approaching 1x10^{-19}.

Frequency stabilized cw lasers achieve exceptional frequency stability using high finesse Fabry-Perot cavities. That stability can be transferred to other optical frequencies with mode-locked lasers and provides optical and electronic pulses with ultra-low timing jitter.

We describe the current status of the Ca optical frequency standards with laser-cooled neutral atoms realized in two different laboratories for the purpose of developing a possible future optical atomic clock. Frequency measurements performed at the Physikalisch-Technische Bundesanstalt (PTB) and the National Institute of Standards and Technology (NIST) make the frequency of the clock transition of 40Ca one of the best known optical frequencies (relative uncertainty 1.2e-14) and the measurements of this frequency in both laboratories agree to well within their respective uncertainties. Prospects for improvement by orders of magnitude in the relative uncertainty of the standard look feasible., 13 pages, 11 figures, to appear in Comptes Rendus Physique

Optical frequencies of the hyperfine components of the ${D}_{2}$ line in $^{133}\mathrm{Cs}$ are determined using high-resolution spectroscopy and a femtosecond laser frequency comb. A narrow-linewidth probe laser excites the $6s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}(F=3,4)\ensuremath{\rightarrow}6p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}(F=2,3,4,5)$ transition in a highly collimated atomic beam. Fluorescence spectra are taken by scanning the laser frequency over the excited-state hyperfine structure. The laser optical frequency is referenced to a Cs fountain clock via a reference laser and a femtosecond laser frequency comb. A retroreflected laser beam is used to estimate and minimize the Doppler shift due to misalignment between the probe laser and the atomic beam. We achieve an angular resolution on the order of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\mathrm{rad}$. The final uncertainties $(\ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}5\phantom{\rule{0.3em}{0ex}}\mathrm{kHz})$ in the frequencies of the optical transitions are a factor of 20 better than previous results [T. Udem et al., Phys. Rev. A 62, 031801 (2000).]. We find the centroid of the $6s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}\ensuremath{\rightarrow}6p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}$ transition to be ${f}_{D2}=351\phantom{\rule{0.2em}{0ex}}725\phantom{\rule{0.2em}{0ex}}718.4744(51)\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$.

This paper discusses an optical atomic clock that uses a narrow atomic resonance to control the frequency of a spectrally narrow laser source. The atomically controlled frequency of the laser gives the clock oscillation frequency as an optical output and provides the stable reference for frequency and timing.

We report on the fabrication of millimeter-sized vapor cells and their performance in atomic clocks based on coherent population trapping (CPT). We discuss two fabrication techniques. The first one is based on hollow-core pyrex fibers, fused with a CO/sub 2/ laser or micro-torch, and the second one involves anodic bonding of micro-machined silicon wafers to pyrex. Key aspects of the discussion are the performance of the cell in frequency references, the potential for further miniaturization of the cells and the ability to manufacture them on a large scale with reproducible performance.

The repeated comparison of atomic frequency standards based upon different transitions enables the search for time variation of the fundamental constants that determine the transition frequencies. Over the course of two years we compared the frequency of the /sup 199/Hg/sup +/ 5d/sup 10/6s/sup 2/S/sub 1/2 /(F=0)/spl harr/5d/sup 9/6s/sup 2/ /sup 2/D/sub 5/2/(F=2) electric-quadrupole transition at 282 nm with the frequency of the ground-state hyperfine splitting in neutral /sup 133/Cs that defines the SI second. These measurements constrain any fractional time variation of the ratio/spl nu//sub Cs///spl nu//sub Hg/ between the two frequencies to be less than /spl plusmn/7/spl times/10/sup -15/yr/sup -1/ (1/spl sigma/ uncertainty). According to recent atomic structure calculations, this sets an upper limit to a possible fractional time variation of the product gCs(m/sub e//m/sub p/)/spl alpha//sup 6.0/ at the same level.

We demonstrate that femtosecond laser frequency combs (FLFCs) can have a subhertz linewidth across their entire emission spectra when they are phase locked to a reference laser with a similarly narrow line width. Correspondingly, the coherence time of the comb components relative to the reference laser can be of the order of a few seconds. Thus we are able to detect high-contrast spectral interferograms at up to 10-8 integration time between two FLFCs locked to a common optical reference.

We demonstrate the reduction of residual Doppler shifts for a laser-cooled neutral atom optical frequency standard to near negligible levels through use of second-stage laser cooling and atom interferometry. For the NIST Ca optical clock, this opens up the possibility for uncertainties of less than 1 part in 1016

Two types of comparisons using transportable femtosecond laser combs have been performed in France and the USA, respectively. The agreement among the combs, when referenced to a hydrogen maser, is on the sub-hertz level at 563 THz. When the combs are referenced to an optical standard, the agreement among four combs was much improved and found to be < 1 mHz at 456 THz

Optical frequencies of the hyperfine components of the D2 line in 133Cs are determined using high-resolution spectroscopy and a femtosecond laser frequency comb. The achieved frequency uncertainties are

We analyze the line shape and width of atomic coherent-population-trapping (CPT) resonances excited by laser beams with different transverse intensity profiles. A dramatic difference in the resonance line shape is found when comparing a beam with a 'steplike' profile to a beam with a Gaussian profile. In particular, for nonuniform profiles, a non-Lorentzian functional form is given that is more appropriate for describing the nonlinear resonance line shape than is a conventional Lorentzian. Our analysis is supported by measurements of CPT line shapes in a thermal vapor of {sup 85}Rb.

We present measurements of dark-line resonances excited in cesium atoms confined in submillimeter cells with a buffer gas. The width and contrast of the resonances were measured for cell lengths as low as 100 mm. The measured atomic Q factors are reduced in small cells because of frequent collisions of atoms with the cell walls. However, the contrast of coherent population trapping resonances measured in the small cells is similar in magnitude to that obtained in centimeter-sized cells, but substantially more laser intensity is needed to excite the resonance fully when increased buffer-gas pressure is used. The effect of the higher intensity on the linewidth is reduced because the intensity broadening rate decreases with buffer-gas pressure. OCIS codes: 020.1670, 300.6260. Compact frequency references have become increasingly important for military 1 and civilian 2 applications in recent years. For many applications in the f ield of navigation (e.g., global positioning systems) and communication systems, inexpensive miniature references with frequency stabilities in the 10 211 range at integration times approaching one day would be of great advantage. It has been predicted 3 that atomic clocks based on coherent population trapping (CPT) spectroscopy in millimeter-sized vapor cells might fulfill these requirements. It is well known that two phase-stable laser fields in resonance with a resonance line of the alkali atoms, and with a difference frequency equal to the hyperfine splitting, induce coherence between the two hyperfine ground

The performance and prospects for neutral-atom optical frequency standards are discussed based on our recent progress with a calcium optical frequency star~dard. Second stage narrow-line cooling to microkelvin (and even 300 nK) temperatures, combined with launched atoms, should reduce Doppler frequency errors to about 1x10 1 6 . Advanced femtosecond optical frequency combs allow direct comparisons between the Ca optical standard, the Hg* optical standard and the Cs primary standard. These comparisons provide independent "reality checks" on both the stability and accuracy. Relative frequency measurements also constrain the possible time variation of atomic energy levels and fundamental constants.

Recently, atomic clocks based on optical frequency standards have been demonstrated [1,2]. A key element in these clocks is a femtosecond laser that downconverts the petahertz oscillation rate into countable ticks at 1 GHz. When compared to current microwave standards, these new optical clocks are expected to yield an improvement in stability and accuracy by roughly a factor of 1000. Furthermore, it is possible that the lowest noise microwave sources will soon be based on atomically-stabilized optical oscillators that have their frequency converted to the microwave domain via a femtosecond laser. Here, we present tests of the ability of femtosecond lasers to transfer stability from an optical oscillator to their repetition rates as well as to the associated broadband frequency comb. In a first experiment, we phase-lock two lasers to a stabilized laser diode and find that the relative timing jitter in their pulse trains can be on the order of 1 femtosecond in a 100 kHz bandwidth. It is important to distinguish this technique from previous work where a femtosecond laser has been stabilized to a microwave standard [3,4] or another femtosecond laser [5]. Furthermore, we extract highly stable microwave signals with a fractional frequency instability of 2×10−14 in 1 s by photodetection of the laser pulse trains. In a second experiment, we similarly phase-lock the femtosecond laser to an optical oscillator with linewidth less than 1 Hz [6]. The precision with which we can make the femtosecond frequency comb track this reference oscillator is then tested by a heterodyne measurement between a second stable optical oscillator and a mode of the frequency comb that is displaced 76 THz from the 1 Hz-wide reference. From this heterodyne signal we place an upper limit of 150 Hz on the linewidth of the elements of the frequency comb, limited by the noise in the measurement itself.

Using microfabrication processes, we have been able to construct physics packages for vapour cell atomic frequency references 100× smaller than previously existing versions, with a corresponding reduction in power consumption. In addition, the devices offer the potential for wafer-level fabrication and assembly, which would substantially reduce manufacturing costs. It is anticipated that a complete frequency reference could be constructed based on these physics packages with a total volume below 1 cm3, a power dissipation near 30 mW and a fractional frequency instability below 10−11 over time periods from hours to days. Such a device would enable the use of atomically precise timing in applications that require battery operation and portability, such as hand-held global positioning system receivers and wireless communication systems.

Extremely broadband femtosecond pulse sources with well defined carrier-envelope phase evolution have been a field of intense research over the past years [1–3]. The time-domain motivation for the push towards increased bandwidth is the generation of optical pulses that approach the single-cycle regime and can be used for phase-sensitive nonlinear experiments [4]. The frequency-domain motivation is the interest in extending the bandwidth over which optical frequencies can be precisely synthesized and subsequently used for absolute optical frequency measurements or comparisons [1, 5, 6]. Nonlinear frequency conversion is one route to overcome the bandwidth limitations of current systems based on one femtosecond laser. A second attractive route is linking two femtosecond lasers with overlapping emission spectra. Phase-locking the repetition rates of two femtosecond lasers with different gain media has recently been demonstrated. Here, we use a Ti:sapphire laser and a Cr:forsterite laser to extend this approach and additionally equalize the rate at which the carrier-waves move underneath their pulse envelopes. In other words, we link the absolute position of the two frequency combs, thereby establishing a true phase-coherence between all modes of their combined spectrum. The laser repetition rates are phase-locked by using a nonlinear cross-correlation technique.

An optical clock based upon a single mercury ion in a radio-frequency trap has the potential for both high stability and high accuracy. The systematic uncertainty of this frequency standard is limited by contributions due to two types of external fields. The clock has previously been operated with a magnetic bias field that destabilizes dark states that limit the rates of laser cooling and state detection. This bias field has previously resulted in a substantial second-order Zeeman shift. We now destabilize the dark states with a polarization-scrambling technique that allows us to operate with only a minimal magnetic field. The second uncertainty that we address here is the magnitude of the quadrupole shift due to the interaction of stray electric-field gradients with the quadrupole moment of the ion. We describe the steps that we are taking to evaluate this shift in the near future.

We measure optical frequencies of the D2 and D1 lines in 133Cs using high-resolution spectroscopy and a femtosecond laser frequency comb. Narrow diode lasers probe the 6s 2S1/2-6p 2P1/2 and 6s 2S1/2-6p 2P3/2 transitions in a highly collimated atomic beam. Uncertainties are reduced by more than a factor of 10. Full-text article is not available.

In the past four years have shown a dramatic improvement in the performance of optical frequency references and in the methods by which they are calibrated and utilized. These revolutionary changes result from better stabilized lasers that probe narrow-linewidth transitions in laser-cooled atoms and ions, and from the development of a convenient method for synthesizing, measuring and distributing optical frequencies based on mode-locked lasers.

We consider theoretically the expected performance of ultra-small vapor-cell frequency references with cell volumes below 1 mm/sup 3/. The short-term stability is found to degrade as the size is reduced. The amount of degradation depends on whether a buffer gas or wall coating is used for the atomic confinement. Preliminary experimental results with all-optical excitation indicate that the atomic Q-factor behaves as expected.

We present results on optical atomic clocks based on lasers stabilized to optical transitions (in Ca and Hg/sup +/) and whose frequencies are phase-coherently divided down to the microwave domain with a spectrally-broadened mode-locked fs-laser.

The fidelity of a coherent link between optical and microwave frequencies is largely determined by noise processes in a mode-locked femtosecond laser. This work presents an experimental study of the noise properties of a Ti:sapphire femtosecond laser. It includes measurements of pulse repetition rate fluctuations and shot noise exhibited by the Ti:sapphire femtosecond laser. Based on the results of noise measurements, the fractional frequency stability of a microwave signal produced by the femtosecond laser has been evaluated.

We independently phase lock the repetition rates of two femtosecond lasers at their approximately 456, 000th harmonic to a common optical oscillator. The timing jitter of each individual laser relative to the optical reference is only 0.45 fs in a 100-Hz bandwidth. Our method takes advantage of the tremendous leverage that is possible when stability is transferred from the optical to the microwave domain. The low timing jitter is commensurate with the independently measured fractional frequency instability in the repetition rates ofor = 2.3 x 10(-15) in 1-s averaging time, limited by the measurement system. The microwave signals at 1 GHz that are extracted by photodetection of the pulse trains have a 10-times-greater instability, confirming the presence of excess noise in the photodetection.

We are developing laser frequency measurement technologies that should allow us to construct an optical frequency synthesis system capable of measuring optical frequencies with a precision limited by the atomic frequency standards. The system will be used to interconnect and compare new advanced optical-frequency references (such as Ca, Hg/sup +/ and others) and eventually to connect these references to the Cs primary frequency standard. The approach we are taking is to subdivide optical frequency intervals into smaller and smaller pieces until we are able to use standard electronic-frequency-measurement technology to measure the smallest interval.

In the present work, we establish the importance of amplifier phase noise and system environmental fluctuations in determining the low-frequency (f

We describe a high performance optical frequency standard at 657 nm based on laser-trapped Ca. Using two semiconductor laser systems (423 nm for trapping and 657 nm for spectroscopy), we have been able to obtain sub-kilohertz optical linewidths and a relative Allan variance of 1.3/spl times/10/sup -14/ at one second, with prospects for considerable improvement. This Ca standard can be used to make a phase-coherent frequency measurement of the 282 nm /sup 199/Hg/sup +/ optical standard.

We develop an analytical theory of dark resonances that accounts for the full atomic-level structure, as well as all field-induced effects such as coherence preparation, optical pumping, ac Stark shifts, and power broadening. The analysis uses a model based on relaxation constants that assumes the total collisional depolarization of the excited state. A good qualitative agreement with experiments for Cs in Ne is obtained., 16 pages; 7 figures; revtex4. Accepted for publication in PRA

Summary form only given. Measurements of the frequency of the /sup 199/Hg/sup +/ ion optical clock versus the cesium primary standard over a two year period can be interpreted as a stringent test of the stability of the fine structure constant.

We demonstrate a chromium-doped forsterite femtosecond ring laser that generates 30-fs pulses at a 420-MHz repetition rate with nearly 500 mW of average power. The compact solid-state design and broad spectral output make this laser attractive for telecommunications applications in the 1.3-1.5-micrometre region. Additional spectral broadening of the laser output in highly nonlinear optical fiber leads to octave-spanning spectra ranging from 1.06 to 2.17 micrometre. The octave is reached at a level of 18 dB below the peak. The underlying optical frequency comb can be linked to existing optical frequency standards.

Publisher Summary This chapter discusses the issues involved in diode laser locking. The chapter describes in detail the various steps needed to lock the laser to a cavity resonance: (1) Derivetion of the error (locking) signal, (2) design of the electronic feedback circuitry, (3) initial locking of the laser, (4) adjustment of the feedback design, and (5) evaluation of the lock performance. The chapter illustrates this discussion by frequency locking an extended-cavity diode laser, reducing the linewidth to a few hertz relative to the cavity. The chapter concludes with an example in which the locking apparatus is modified for a cavity ring-down demonstration. Included are results showing the laser repetitively locking and unlocking to the cavity.

This paper describes recent advances in the design and control of femtosecond laser combs for their use in optical clocks and in the synthesis of low-noise microwave and optical signals. The authors present a compact and technically simple femtosecond laser that directly emits a broad continuum and shows that it can operate continuously on the timescale of days as the phase-coherent "clockwork" of an optical clock. They further demonstrate phase locking of an octave-spanning frequency comb to an optical frequency standard at the millihertz level. As verified through heterodyne measurements with an independent optical frequency standard, this provides a network of narrow optical modes with linewidths at the level of /spl les/150 Hz, presently limited by measurement noise. Finally, they summarize their progress in using the femtosecond laser comb to transfer the stability and low phase-noise optical oscillators to the microwave domain.

Summary form only given. The development of future atomic clocks at the National Institute of Standards and Technology (NIST) is discussed. The atomic fountain clock called NIST-F1, the primary frequency standard at NIST, is designed to measure the energy splitting of the hyperfine ground states of the /sup 133/Cs atom with a very high degree of accuracy. The next generation of fountain clock is currently being designed at NIST and is in the early stages of testing. One modification to the apparatus will be to cool the drift region to cryogenic temperature, thereby reducing the correction for the black-body radiation frequency shift to a negligible level. An additional modification will be to reduce the atom density during the measurement while increasing the atom throughput in order to reduce the cold-collision frequency shift. Another up-and-coming frequency standard in progress is the Primary Atomic Reference Clock in Space (PARCS) project, which is an atomic-clock mission currently scheduled to fly aboard the International Space Station (ISS) in 2007. The mission involves a laser-cooled cesium atomic clock and a time-transfer system using the Global Positioning System satellites. The microgravity environment aboard the ISS will allow for observation times much longer than feasible on earth. The objectives of the mission are to test gravitational theory and to realize the second with an uncertainty of 1×10/sup -16/ - a factor of 10 better than that achieved on earth.

Summary form only given. Results of the measurements which compare the frequency ν/sub Hg/ of the /sup 199/Hg/sup +2/ S/sub 1/2/ (F=0) - /sup 2/D/sub 1/2/ (F=2, m/sub F/=0) optical transition to the SI second as realized at NIST are reported. Since the NIST time scale is calibrated with a cesium fountain primary frequency standard, the ratio of ν/sub Hg/ ( ≈ 10/sup 15/ Hz) to the cesium ground-state hyperfine splitting ν/sub Cs/ (≈ 9.2 GHz) is measured. These measurements show better reproducibility than 10 Hz at ν/sub Hg/, and constrain any possible variation of the ratio ν/sub Hg/ /ν/sub Cs/ to ± 7·10/sup -15/ yr/sup -1/.

We present a Cr:forsterite ring laser producing 30 fs pulses at a 420 MHz repetition rate. The output is broadened in highly nonlinear fiber yielding spectra covering 1050-2200 nm.

Advances in laser cooling of alkaline earth atoms have reduced trapped atom temperatures from the millikelvin to the microkelvin regime. Using such atoms, our optical atomic clock should have a fractional frequency uncertainty approaching 1 /spl times/ 10/sup -15/. Sub-recoil cooling and two-pulse atom interferometry are also discussed.

Experimental results are presented on a hybrid optical/electronic oscillator which uses an optical delay line to generate high spectral purity microwave signals. At Fourier frequencies above 10 kHz, the single sideband (SSB) phase noise spectrum decreases as roughly 1/f/sup 2/ attaining a value of -138 dB below the carrier in a 1 Hz bandwidth (dBc/Hz) at 20 kHz offset. The origin of this noise is in part the fundamental shot noise on the light itself, although other optical noise sources such as double Rayleigh scattering and stimulated Brillouin scattering also appear to be important under certain operating conditions. The frequency stability over several hours is dominated by changes in the fiber ambient temperature with a coefficient of about 10/sup -5//K originating mostly from changes in the fiber refractive index with temperature.

We have developed an electronic circuit for phase-locking extended-cavity diode lasers to a stable reference laser. The basic phase-lock operates with input frequencies between 5 MHz and 1.5 GHz and allows synthesizer tuning of the difference frequency between the lasers. An external mixing stage can be used for an arbitrary frequency offset. >

A low-cost, phase-lock circuit for slaving an extended-cavity diode laser to a stabilized reference laser has been developed. Grounded coplanar waveguide and surface mount technology have been used. An internal mixing stage allows continuous tuning of the laser difference frequency between 5 MHz and 1.5 GHz. Absolute phase locks exceeding 8 hours have been demonstrated. >

Diode lasers at 657 nm are used with an atomic beam and a high-flux calcium beam-cell to provide narrow saturated-absorption resonances. Stability of /spl ap/3/spl times/10/sup -14//spl tau//sup 1/2 / is projected based on signal-to-noise ratios. Laser cooling is possible with a frequency doubled diode laser system that produces a usable 35 mW at 423 nm. >

Summary form only given. We report here measurements of electromagnetically induced transparency (EIT) in a sample of cold trapped Rb/sup 87/ atoms. By using cold atoms, we avoid complications associated with Doppler broadening, which often exist in similar experiments performed with room-temperature vapors. The measurements were made on an 2mm-diameter sample of -10/sup 8/ laser-cooled Rb/sup 87/ atoms collected in a standard magnetooptic trap.

We present a theoretical comparison between a TE/sub 01n/ cavity and a traditional Ramsey cavity when used with a laser-cooled atom source in a microgravity clock.

The recent achievement of reliable quasi-phasematching (QPM) in periodically poled crystals, and in particular periodically poled lithium niobate (PPLN), has empowered even more the use of nonlinear optical mixings in frequency connections. As is well-known, QPM gives access to the largest nonlinear coefficients of the material (which are almost never usable for birefringent phasematching - BPM), suppresses walk-off, and gives great flexibility in the choice of the wavelengths involved in the nonlinear mixing. Simultaneous QPM of different interactions in the same crystal is now easy to obtain by poling different regions of the crystal with the different corresponding periods. But even a single-period (50% duty cycle) crystal offers many QPM cascading possibilities, which are impossible most of the time with BPM because of constraints on the optical polarizations. We experimentally realized third-harmonic generation (THG) by cascading second-harmonic generation (SHG: /spl omega//spl rarr/2/spl omega/) and sum frequency generation (SFG: /spl omega/+2/spl omega//spl rarr/3/spl omega/), which of course realizes a 3:1 frequency connection.

The Time and Frequency Division of the National Institute of Standards and Technology (NIST) begun operation of a laser-cooled cesium atomic fountain for evaluation as a possible primary frequency standard. This device is conceptually similar to the Zacharias caesium fountain and is similar in layout to the BNM-LPTF (Observatoire de Paris) caesium fountain. This paper gives a brief description of the NIST Time and Frequency Division's caesium fountain and presents some preliminary data.

The 657 nm calcium intercombination line is an attractive optical frequency/length reference due to its high line Q (10/sup 12/) and insensitivity to external perturbations. In this paper we describe progress towards an all-diode-laser-based reference using laser-trapped Ca. To generate 35 mW of 423 nm trap light, we double the frequency of a 150 mW, 846 nm diode laser with a nonlinear crystal (KNbO/sub 3/) inside a resonant build-up cavity. We use this light to trap 2/spl times/10/sup 7/ atoms in a compact, magneto-optic trap. Spectroscopy of the narrow 657 nm transition is performed with a frequency-stabilized diode laser system. With the trapped atoms we have measured optical Ramsey fringes with a linewidth of 6 kHz. We discuss future improvements as well as potential performance which could achieve a fractional frequency instability of

We will present the latest experimental results obtained with this 1.3 /spl mu/m oscillator that uses a diode laser optoelectronic oscillator; in addition to low frequency measurements of the phase noise of the 850 nm system. A configuration using two fiber loops used to further suppress the side modes will also be described.

An important application of diode lasers in the standards community is the development of the next generation of frequency/wavelength references, where semiconductor laser technology will enable higher performance in portable, inexpensive, and reliable systems. At NIST we are working on an all-diode-laser, optical frequency reference based on the /sup 1/S/sub 0/-/sup 3/P/sub 1/ intercombination line in Ca at 657 nm. This transition is one of those recommended for the realization of the meter, and its absolute frequency has been measured with an uncertainty of 450 Hz, the most precise of any visible reference.

Methane has a significant effect on the radiative balance of the troposphere and stratosphere because it has a strong absorption at 7.66 /spl mu/m where carbon dioxide and water absorb only weakly. Accurate measurements of the abundance of methane are thus of considerable interest to climatologists. Previously, we demonstrated a spectrometer capable of producing fast, accurate measurements of the methane mixing ratios in natural air samples. This system demonstrated a relative accuracy of less than 10/sup -9/ mol/mol (1 ppb by volume), which is comparable to what can be achieved using non-spectroscopic methods. This system employed bulk periodically poled lithium niobate pumped by a solitary diode laser at 808 nm and a diode pumped monolithic ring Nd:YAG laser at 1064 nm, and a multi-pass absorption cell with 18 m pathlength. We generated approximately 1 /spl mu/W of infrared radiation at 3019 cm/sup -1/ using 21 mW from a grating-tuned extended-cavity diode laser at 806 nm and 382 mW from a diode-laser-pumped Nd:YAG laser at 1064 nm to pump a periodically poled lithium niobate crystal. Because this system is compact, uses all solid-state lasers, and could be made transportable, it has potential applications for field measurements where rapid, nondestructive concentration and isotope ratio measurements are needed. Even though it is not competitive with state-of-the-art nonoptical instruments, this system can measure /sup 13/CH/sub 4///sup 12/CH/sub 4/ ratios at CH/sub 4/ concentrations of only 1.6/spl times/10/sup -6/ mol/mol without gas processing.

This work was done to develop a diode-laser-based technique for sensitive atmospheric trace detection gases such as CH/sub 4/, CO, N/sub 2/O, and NO. Spectroscopic detection of methane in the fundamental, and overtone stretch vibration bands using tunable infrared lasers has been reported. The /spl nu//sub 3/ band of methane near 3.2 /spl mu/m includes its strongest known molecular transition and therefore is better suited for sensitive detection. The band is accessible by either conventional spectroscopy or with Ar/sup +/-dye laser difference-frequency generation, the carbon monoxide overtone laser, the helium-neon laser near 3.39 /spl mu/m, lead-salt diode lasers, and color-center lasers. However, each one of these mid-infrared laser sources suffers from its own specific practical drawbacks such as large physical size, lack of portability, high cost, high power consumption, poor tunability, or the need for cryogenic cooling. In the work, detection of the methane in natural air (1.8 ppmv) was performed using diode-laser-pumped cavity-enhanced CW tunable difference-frequency generation (DFG) near 3.2 /spl mu/m.

A wide variety of new laser applications would be feasible if we had tunable, single-frequency laser sources that were compact, lightweight and operated with low power consumption. Some of these applications include: atmospheric and process monitoring, atomic and molecular spectroscopy, ultrasensitive detection and analytic applications, lidar and ranging, and new optical frequency/length references. Semiconductor diode lasers can now meet these challenges in many cases, particularly for laboratory experiments. Extension of this technology to higher powers, extended wavelength coverage, and transportable instrumentation is on the near term horizon. For some time now it has been possible to construct single-frequency tunable diode laser systems that operate at many wavelengths in the red and near IR spectral region. Even commercial tunable diode laser systems are now readily available at most of the wavelengths accessible with commercial semiconductor lasers chips.