Cavity ringdown spectroscopy with a continuous-wave laser: calculation of coupling efficiency and a new spectrometer design

For the efficient operation of a cavity ringdown spectroscopy (CRDS) system utilized with a continuous-wave (cw) laser, we numerically analyze the coupling efficiency of a cw laser to a ringdown cavity in terms of changes in the scanning rate, the laser linewidth, and the mirror reflectivity. We also demonstrate a new simple design for a CRDS system that can produce a CRDS signal with only a piezoelectric transducer (PZT), without the acousto-optic modulator that is usually adopted to switch off the cw laser beam that enters the cavity. Furthermore, we investigate the feasibility of the cw CRDS technique with a fast-scanning PZT by recording a CRDS spectrum of acetylene overtones. The detection sensitivity that corresponds to the noise-equivalent absorption is found to be ~3 x 10(-9)/cm.     

Laser with a nonlinear mirror: application for improving the output power stability of a cw laser

We put forward a new method for improving the output power stability of a cw laser. The basic idea is to replace the output mirror of a laser with a nonlinear mirror that is formed by a dispersive nonlinear Fabry-Perot étalon. Based on the semiclassical dynamic model for this system, first we derive the static conditions for this laser operating with stabilized output power, then we study the dynamic responses of output power to gain fluctuations. Both static and dynamic results show that, compared with normal lasers, the output power stability of our laser can be improved significantly when the output power fluctuations are caused by the fluctuations of the gain. The improvement of the output power can be explained by the fact that there exists an optical servoloop in this laser that is produced by the nonlinear mirror.     

Accurate measurement method of Fabry-Perot cavity parameters via optical transfer function

It is shown how the transfer function from frequency noise to a Pound-Drever-Hall signal for a Fabry-Perot cavity can be used to accurately measure cavity length, cavity linewidth, mirror curvature, misalignments, laser beam shape mismatching with resonant beam shape, and cavity impedance mismatching with respect to vacuum.     

Multipass grating interferometer applied to line narrowing in excimer lasers

The line narrowing of excimer lasers is discussed. The theory for an optical two-effect intracavity line narrowing device, the multipass grating interferometer (MGI), is presented. An MGI contains a grating aligned in its second-order Littrow configuration and a mirror aligned parallel to the grating surface reflecting back the beam normal to the grating corresponding to the first-order diffraction. The Littrow grating is doing the coarse line narrowing, and the mirror aligned parallel to the grating has similar line narrowing properties as tilted intracavity Fabry-Perot etalons. An MGI is applied to a KrF laser cavity to achieve a linewidth of 0.03 cm(-1).     

Active laser resonator performance: formation of a specified intensity output

We discuss the formation of a specified super-Gaussian intensity distribution of a fundamental mode by means of an intracavity controlled mirror, which is a water-cooled bimorph flexible mirror equipped with four controlling electrodes. Analysis has confirmed the possibility to form fourth-, sixth-, and eighth-order super-Gaussian intensity distributions at the output of the stable resonators of industrial cw CO(2) and YAG:Nd(3+) lasers. We present the results of the experimental formation of fourth-order and sixth-order super-Gaussian fundamental modes at the output of a cw CO(2) laser by means of an intracavity flexible mirror. We observed an increase in power up to 12% and an enlargement of the peak value of the far-field intensity by as much as 1.6 times that with a Gaussian TEM(00) mode of the cw CO(2) laser.     

Gaussian-optics-based optical modeling and characterization of a Fabry-Perot microcavity for sensing applications

A generalized study has been carried out on the modeling of a Fabry-Perot microcavity for sensing applications. Different analytical models on transmission characteristics of a Fabry-Perot microcavity are established by using plane-wave-based techniques, such as the Macleod characteristic matrix technique, the transfer matrix technique, and Smith's technique. A novel Gaussian-optics-based model for a Fabry-Perot microcavity illuminated by a laser beam is then developed and validated. The influence of laser beam waist on microcavity optical response is investigated, and the required minimal beam waist size is explored to ensure a useful optical response for sensing applications that can be accurately predicted by plane-wave optics. Also, the perturbations of microcavity performance induced by different types of microcavity mirror imperfections are discussed, based on the novel optical model. The prototype of the proposed Fabry-Perot microcavity for sensing applications has been successfully fabricated and characterized.     

Near shot-noise-level relative frequency stabilization of a laser-diode-pumped Nd:YVO(4) microchip laser

A laser-diode-pumped Nd:YVO(4) microchip laser was built and actively frequency stabilized relative to a Fabry-Perot cavity with the frequency-modulated sideband technique. The error signal reaches the shot-noise level of 7.4 mHz/√Hz around 1 kHz. Excess intensity noise sets a lower limit of16.5 mHz/√Hz for the relative frequency noise, corresponding to a spectral linewidth of 860 μHz. We discuss the method for reconstructing the actual frequency deviation from the observed error signal.     

Single actuator alignment control for improved frequency stability of a cavity-based optical frequency reference

We demonstrate a method of controlling the alignment of a laser beam to a Fabry-Perot resonator through synchronous detection of the misalignment arising from modulating the orientation of a single beam-steering mirror. The horizontal and vertical tilt of the mirror are modulated in quadrature to drive a circular motion of the beam orientation. A corresponding modulation of the intensity of the optical field circulating in the cavity is measured at either the reflected or transmitted port and demodulated synchronously to derive two error signals to indicate the vertical and horizontal misalignment. These signals are fed back to the beam-steering mirror to suppress fluctuations below 30 Hz. This method avoids the complexity of monitoring off-axis cavity modes and is particularly effective in the case where unwanted pointing fluctuations are introduced by one or two elements in the optical setup. We have applied the technique to two Fabry-Perot resonators in use as precision frequency references, delivering a result of 10 dB suppression of alignment fluctuations at 1 Hz and an improvement in frequency stability by up to a factor of 4.     

Single-frequency, high-power, continuous-wave fiber-laser-pumped Ti:sapphire laser

We demonstrate a high-power, continuous-wave (cw), single-frequency green source based on single-pass second-harmonic generation of a Yb-fiber laser in MgO:sPPLT as a viable pump source for a cw single-frequency Ti:sapphire ring laser. By careful design and optimization, the Ti:sapphire laser can provide as much as 2.3 W of cw single-frequency output across a 47 nm tuning range, limited by the reflectivity of the cavity mirrors. By implementing active stabilization of the laser frequency to an external reference, an ultrastable Fabry-Perot interferometer, we obtain a frequency stability better than 12 MHz over 10 min and continuous tunability greater than 180 MHz. Stable output power with peak-to-peak fluctuation of 5.4% over 75 min, in high spatial beam quality with M(2)<1.34, is achieved.     

Spectral characteristics of a regenerative semiconductor optical amplifier mutually injection locked with a Fabry-Perot laser diode

The anomalous injection locking of a traveling-wave semiconductor optical amplifier (SOA) and a fiber-pigtailed Fabry-Perot laser diode (FPLD) linked with single-mode and improved side-mode suppression ratio output is demonstrated. We achieve this injection locking by driving the FPLD slightly below threshold and by feedback injecting the FPLD with fractional output of a closed-loop SOA. The SOA-FPLD link lases in a single FPLD longitudinal mode with a reduced linewidth of 0.013 nm and a maximum side-mode-suppressing ratio of 39.7 dB. A precise 3-dB linewidth of 45-50 MHz is also observed from the self-homodyne mode-beating spectrum. The optimized feedback-injecting power for the FPLD is approximately 2% of the SOA-FPLD linked output power of >400 microW. The variations in output power and in peak wavelength are not more than 0.54% and 0.06%, respectively. The injection-locked SOA-FPLD link is insensitive to the temperature fluctuation within +/- 0.25 degrees C.     

Double-Pass-Pumped Tm:YAG Laser with a Simple Cavity Configuration

For a double-pass-pumped cw Tm:YAG laser, we developed a theoretical model, taking into account reabsorption loss and mode matching between the pumping light and the cavity mode. We also demonstrated that efficient operation can be obtained with a simple cavity configuration by using a dichroic output mirror, highly reflective at the pumping wavelength and partially reflective at the lasing wavelength. Experimental comparisons of this pumping method with single-pass pumping showed that the longitudinally double-pass-pumped Tm:YAG laser performs as well at room temperature as the single-pass-pumped laser performs at -10 degrees C.     

1053-nm-wavelength selection in a diode-laser-pumped Nd:YLF laser

We report on a Nd:YLF laser that operates at 1053 nm without optical intracavity elements for the suppression of the stronger 1047-nm transition. The Nd:YLF crystal is end pumped by a fiber optically coupled 10-W diode-laser bar. The different thermal-lensing focal lengths of the two main lasing wavelengths in a plane-parallel resonator were used to achieve the selection by tilting the end mirror slightly from its optimum position for maximum output power. With 9.8-W cw diode-laser-pumping power the 1053-nm Nd:YLF laser produces a maximum output power of 1.9 W in cw operation and nearly 1 W of average power at a Q-switch repetition rate of 15 kHz. The highest slope efficiency of 47% achieved in cw operation.     

Injection-seeded pulsed alexandrite laser for differential absorption lidar application

We describe a Q-switched alexandrite laser injection seeded with a cw single-mode titanium-sapphire laser. The reported experimental results show that this system meets the frequency stabilization required for differential absorption lidar measurement of humidity, pressure, and temperature. The emission of the cw titanium-sapphire master oscillator is locked to an atmospheric absorption line by means of a servoloop with derivative spectroscopy. The spectral position is stabilized within ±3.5 × 10(-4) cm(-1) (10 MHz) of the peak of the line over 1 hr. The alexandrite laser emits pulses of 30 mJ in 500 ns, with a spectral linewidth of ≈ 3.3 × 10(-3) cm(-1) (100 MHz). The position of the centroid of the emitted spectrum has a standard deviation of 6 × 10(-4) cm(-1) (18 MHz) and is held within ±1.3 × 10(-3) cm(-1) (40 MHz) of the peak of the absorption line over 1 h.     

Directly Single-Diode-Pumped Continuous-Wave Yb(3+):YAG Laser Tunable in the 1047-1051-nm Wavelength Range

We have demonstrated tunability in a wavelength range of 1047-1051 nm, using the lowest pumping power known to us (1.4 W at 972 nm), for a cw single-diode-pumped Yb:YAG laser. The output power was 166 mW in the multimode regime and 73 mW in the fundamental transverse-mode regime. The laser efficiency was 12.8% and 21% with respect to the incident and the absorbed pump powers, respectively. The linewidth of the laser was Dlambda = 0.05-0.07 cm(-1) (1.5-2.1 GHz), which corresponds to two to three longitudinal cavity modes. A theoretical model of the laser has been developed. Experimental results are in good agreement with the theoretical model.     

Ultimate linewidth reduction of a semiconductor laser frequency-stabilized to a Fabry-Pérot interferometer

We report a theoretical dynamical analysis on effect of semiconductor laser phase noise on the achievable linewidth when locked to a Fabry-Perot cavity fringe using a modulation-demodulation frequency stabilization technique such as the commonly used Pound-Drever-Hall frequency locking scheme. We show that, in the optical domain, the modulation-demodulation operation produces, in the presence of semiconductor laser phase noise, two kinds of excess noise, which could be much above the shot noise limit, namely, conversion noise (PM-to-AM) and intermodulation noise. We show that, in typical stabilization conditions, the ultimate semiconductor laser linewidth reduction can be severely limited by the intermodulation excess noise. The modulation-demodulation operation produces the undesirable nonlinear intermodulation effect through which the phase noise spectral components of the semiconductor laser, in the vicinity of even multiples of the modulation frequency, are downconverted into the bandpass of the frequency control loop. This adds a spurious signal, at the modulation frequency, to the error signal and limits the performance of the locked semiconductor laser. This effect, reported initially in the microwave domain using the quasistatic approximation, can be considerably reduced by a convenient choice of the modulation frequency.     

Measurements of the phase shift on reflection for low-order infrared Fabry-Perot interferometer dielectric stack mirrors

A simple technique based on a Fizeau interferometer to measure the absolute phase shift on reflection for a Fabry-Perot interferometer dielectric stack mirror is described. Excellent agreement between the measured and predicted phase shift on reflection was found. Also described are the salient features of low-order Fabry-Perot interferometers and the demonstration of a near ideal low-order (1-10) Fabry-Perot interferometer through minimizing the phase dispersion on reflection of the dielectric stack. This near ideal performance of a low-order Fabry-Perot interferometer should enable several applications such as compact spectral imagers for solid and gas detection. The large free spectral range of such systems combined with an active control system will also allow simple interactive tuning of wavelength agile laser sources such as CO(2) lasers, external cavity diode lasers, and optical parametric oscillators.     

Development of a Tunable, Narrow-Linewidth, CW 2.066-mum Ho:YLF Laser for Remote Sensing of Atmospheric CO(2) and H(2)O

A smoothly tunable, narrow-linewidth, cw, 32-mW, 2.066-mum Ho:YLF laser was constructed and used for the first time in preliminary spectroscopic measurements of atmospheric CO(2) and H(2)O. The laser was constructed with a 4.5-mm-long, TE-cooled, codoped 5% Tm and 0.5% Ho yttrium lithium fluoride crystal (cut at Brewster's angle) pumped by an Ar(+)-pumped 500-mW Ti:sapphire laser operating at 792 nm. Intracavity etalons were used to reduce the laser linewidth to approximately 0.025 cm(-1) (0.75 GHz), and the laser wavelength was continuously and smoothly tunable over approximately 6 cm(-1) (180 GHz). The Ho:YLF laser was used to perform spectroscopic measurements on molecular CO(2) in a laboratory absorption cell and to measure the concentration of CO(2) and water vapor in the atmosphere with an initial accuracy of approximately 5-10%. The measurement uncertainty was found to be due to several noise sources, including the effect of asymmetric intensity of the laser modes within the laser linewidth, fluctuations caused by atmospheric turbulence and laser beam/target movement, and background spectral shifts.     

Progress in the realization of a frequency standard at 192.1 THz(1560.5 nm) using 87Rb D2-line and second harmonicgeneration

Second harmonic generation of a 192.1 THz semiconductor distributed feedback (DFB) laser is achieved using a KNbO₃ crystal in a resonant ring cavity. Optical feedback from this cavity is used to stabilize the laser frequency and reduce its linewidth. A second harmonic power of 5.5 μW is generated with 38 mW incident on the cavity. We use the second harmonic signal to observe saturated absorption lines and orientation signals in rubidium vapor. Injection-locking of a 780 nm Fabry-Perot laser using the second harmonic signal is also demonstrated. With this scheme, we observe saturated absorption lines in rubidium     

Simple configuration to generate short bursts of picosecond optical pulses

We have developed an efficient method of generating short bursts of picosecond optical pulses. The multiplying stage consists of simple optical elements, including 50/50 beam splitters and a 100% reflecting mirror. Time delay between the pulses can be adjusted by changing the incident angle of the beam or the separation of the splitters. The multiplying scheme has been successfully applied to the output of a cw mode-locked Nd:YAG laser at 82 MHz. Multiplication of beam pulses up to a factor of 8 was demonstrated. This technique promises to increase the repetition rate of laser pulses up to hundreds of gigahertz.     

Evaluation of quantum-cascade lasers as local oscillators for infrared heterodyne spectroscopy

We report experiments evaluating the feasibility of quantum-cascade lasers (QCLs) at mid-infrared wavelengths for use as local oscillators (LOs) in a heterodyne receiver. Performance tests with continuous-wave (cw) lasers around 9.6 and 9.2 microm were carried out investigating optical output power, laser linewidth, and tunability. A direct comparison with a CO2 gas laser LO is presented as well. The achieved system sensitivity in a heterodyne spectrometer of only a factor of 2 above the quantum limit together with the measured linewidth of less than 1.5 MHz shows that QCLs are suitable laser sources for heterodyne spectroscopy with sufficient output power to replace gas lasers as LOs even in high-sensitivity astronomical heterodyne receivers. In addition, our experiments show that the tunability of the lasers can be greatly enhanced by use of an external cavity.