Antireflection-coated blue GaN laser diodes in an external cavity and Doppler-free indium absorption spectroscopy

Commercially available GaN-based laser diodes were antireflection coated in our laboratory and operated in an external cavity in a Littrow configuration. A total tuning range of typically 4 nm and an optical output power of up to 30 mW were observed after optimization of the external cavity. The linewidth was measured with a beterodyne technique, and 0.8 MHz at a sweep time of 50 ms was obtained. The mode-hop-free tuning range was more than 50 GHz. We demonstrated the performance of the laser by detecting the saturated absorption spectrum of atomic indium at 410 nm, allowing observation of well-resolved Lamb dips.     

Improved frequency stability of an external cavity diode laser by eliminating temperature and pressure effects

The construction of a passively stabilized external cavity diode laser operating at 780 nm is reported. The sensitivity of laser frequency to changes in air pressure was studied and subsequently eliminated. The relative frequency stability obtained was 4 × 10(-9) for an integration time of 4000 s.     

Analysis of dual-wavelength oscillation in a broad-area diode laser operated with an external cavity

Apodizing holographic gratings used in an external cavity have shown to be effective to control the modal content of multimode broad-area diode lasers, providing single longitudinal-mode and single lateral-mode emission. They can also be designed to provide Littrow reflection at two wavelengths. We observed stable oscillation at two wavelengths in a diode laser with an external cavity ended with such a grating. This is not a common behavior for homogeneously broadened gain media. We present simulations of the behavior of this laser based on a rate equation analysis. The effects of spatial hole burning and spontaneous emission are examined.     

Piezo-locking a diode laser with saturated absorption spectroscopy

We demonstrate modulation-based frequency locking of an external cavity diode laser, utilizing a piezo-electrically actuated mirror, external to the laser cavity, to create an error signal from saturated absorption spectroscopy. With this method, a laser stabilized to a rubidium hyperfine transition has a FWHM of 130 kHz over seconds, making the locked laser suitable for experiments in atomic physics, such as creating and manipulating Bose-Einstein condensates. This technique combines the advantages of low-amplitude modulation, simplicity, performance, and price, factors that are usually considered to be mutually exclusive.     

Absolute interferometry with a 670-nm external cavity diode laser

In the past few years there has been much interest in use of tunable diode lasers for absolute interferometry. Here we report on use of an external cavity diode laser operating in the visible (lambda ~ 670 nm) for absolute distance measurements. Under laboratory conditions we achieve better than 1-mum standard uncertainty in distance measurements over a range of 5 m, but significantly larger uncertainties will probably be more typical of shop-floor measurements where conditions are far from ideal. We analyze the primary sources of uncertainty limiting the performance of wavelength-sweeping methods for absolute interferometry, and we discuss how errors can be minimized. Many errors are greatly magnified when the wavelength sweeping technique is used; sources of error that are normally relevant only at the nanometer level when standard interferometric techniques are used may be significant here for measurements at the micrometer level.     

Frequency stabilization of an external cavity diode laser to molecular iodine at 657.483 nm

The saturation spectrum of the P(84) 5-5 transition of 127I2 at 657.483 nm is obtained with the third-harmonic demodulation method using an external cavity diode laser. The laser frequency is modulated by modulating the diode current instead of modulating the cavity length with a piezoelectric transducer (PZT). Current modulation allows a modulation frequency that is higher than PZT modulation. The signal-to-noise ratio of 1000 is better than previous results presented in the literature. The laser is frequency stabilized to the hyperfine component o of the P(84) 5-5 transition with a frequency stability of better than 10 kHz (2.2 x 10(-11) relative stability).     

Wavelength switching in an actively mode-locked Fabry-Perot laser diode with a highly dispersive external cavity

We demonstrate a simple and reliable scheme for wavelength switching in an actively harmonic mode-locked Fabry-Perot laser diode (FPLD) that is coupled to a highly dispersive external fiber cavity. Wavelength switching between FPLD modes is achieved by detuning of the modulation frequency applied to the FPLD. A side-mode suppression ratio of as much as 40 dB is maintained, and the supermode noise suppression ratio is more than 50 dB for the whole wavelength switching operation range of 4.7 nm when the pulse repetition rate is ~2 GHz. The wavelength-switching mechanism of our laser is based on the wavelength-dependent cavity resonance frequency shift that is due to chromatic dispersion, and we verify that our experimental results closely coincide with analytical results.     

Tunable diode laser absorption spectroscopy in coiled hollow optical waveguides

We demonstrate tunable diode laser absorption spectroscopy of CO2 and NH3 near 1.5 microm using a distributed feedback diode laser in conjunction with hollow optical waveguides as long-path sample cells. The waveguides are coiled to reduce the physical extent of the system. The small volume of the waveguide provides rapid instrument response to changes in gas concentration. To reduce the pressure drop associated with long lengths and high flow rates, we perforate the waveguides in a novel geometry providing parallel pneumatic paths while maintaining optical path length. A minimum detectable absorbance of 3.5 x 10(-5) in a 3-m section of waveguide is demonstrated.     

Gas-phase characterization in diamond hot-filament CVD by infrared tunable diode laser absorption spectroscopy

In hot-filament CVD the gas-phase composition is a vital parameter for diamond coating results. Concentrations of carbon-containing species have significant influence on growth rate, quality and morphology of deposited diamond. To learn more about the correlations between process parameters and gas species concentrations we applied the highly sensitive infrared tunable diode laser absorption spectroscopy (IR-TDLAS) technique. With a sophisticated compact IR-TDLAS unit, relative and absolute concentrations of CH₄, C₂H₂ and CO were simultaneously measured. Also, the absolute concentration of the methyl radical was determined in dependence on process parameters. Concentrations of CO₂, C₂H₆ and HCN were investigated but found to be lower than the detection limit. The influence of the typical diamond CVD-process parameters on various species concentrations is discussed. The applicability of IR-TDLAS for hot-filament process monitoring is evaluated. In context with diamond growth results, information for CVD process refinement was deducted from the IR-TDLAS measurements.     

Doppler-free nonlinear absorption in ethylene by use of continuous-wave cavity ringdown spectroscopy

We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. We tune the sideband frequencies to observe a saturated, Doppler-free Lamb dip in the nu7, 11(1,10) <-- 11(2,10) rovibrational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption.     

High-resolution absorption measurements by use of two-tone frequency-modulation spectroscopy with diode lasers

The capability of two-tone frequency-modulation spectroscopy (TTFMS) in deriving spectral line-shape information was investigated. Two oxygen A-band transitions at 760 nm were selected, and the Voigt profile and two different collisionally narrowed line profiles were employed in their analysis. By means of a least-squares fitting procedure, we obtained accurate information regarding transition strengths and pressure-induced broadening, shift, and narrowing coefficients. Both TTFMS and direct absorption line shapes were modeled with deviations as small as 0.3% over a wide pressure range by use of the collisionally narrowed line profiles. Line parameters measured with TTFMS showed excellent agreement with the parameters measured with direct absorption. The experimental technique used constant-current fast-wavelength scanning, which improved measurement accuracy.     

Performance analysis of a wavelength-locked Fabry-Perot laser diode by light injection of an external spectrally sliced Fabry-Perot laser diode

We have analyzed and experimentally demonstrated the performance of a wavelength-locked Fabry-Perot laser diode (FP-LD) with a spectrally sliced FP-LD as an external-injection optical source. A high side-mode suppression ratio exceeding 30 dB was obtained, and the noise characteristic of the wavelength-locked FP-LD was analyzed in terms of both mode partition noise and interferometric noise. The measured relative intensity noise had a minimum value of -140 dB/Hz. We also investigated the transmission characteristics such as the Q factor and extinction ratio for a 622 Mbits/s and 2.5 Gbits/s baseband digital signal.     

Effect of external cavity length on self-mixing signals in a multilongitudinal-mode Fabry-Perot laser diode

The effect of external-cavity length on self-mixing signals in a multilongitudinal-mode Fabry-Perot laser diode (FP-LD) was investigated experimentally. It has been shown that the output waveforms of self-mixing signals vary periodically when the length of the external cavity changes. This result agrees well with our theoretical calculations for the self-mixing effect of two adjacent longitudinal modes in a FP-LD. Moreover, the time-averaged output intensities of self-mixing signals has also been measured and compared with theoretical analysis.     

Low-frequency wavelength modulation spectroscopy with D2 transition of atomic cesium by use of an external-cavity diode laser

Low-frequency wavelength modulation spectroscopy is acquired with an external-cavity diode laser. The wavelength modulation is achieved with voltage tuning by means of scanning with the piezoelectric stepper motor, which rotates the end mirror in the laser cavity. With optimum 1-kHz frequency modulation and harmonic detection, direct absorption experiments for the 6S(1/2)(F = 4) --> 6P(3/2) transition of the cesium D(2) line were carried out. We found that 6f-harmonic detection is best here with a signal-to-noise voltage ratio of 460.     

Ammonia detection by use of quartz-enhanced photoacoustic spectroscopy with a near-IR telecommunication diode laser

A gas sensor based on quartz-enhanced photoacoustic detection and a fiber-coupled telecommunication distributed-feedback diode laser was designed and characterized for trace NH3 monitoring at a 1.53-microm wavelength (overtone absorption region). Signal and noise dependence on gas pressure were studied to optimize sensor performance. The ammonia concentration resulting in a noise-equivalent signal was found to be 0.65 parts per million by volume with 38-mW optical excitation power and a lock-in amplifier time constant of 1 s. This corresponds to a normalized absorption sensitivity of 7.2 x 10(-9) cm(-1) W/Hz1/2, comparable with detection sensitivity achieved in conventional photoacoustic spectroscopy. The sensor architecture can be the basis for a portable gas analyzer.     

Measurement of exhaled nitric oxide in beef cattle using tunable diode laser absorption spectroscopy

Measurement of nitric oxide (NO) in the expired breath of crossbred calves received at a research facility was performed using tunable diode laser absorption spectroscopy. Exhaled NO (eNO) concentrations were measured using NO absorption lines at 1912.07 cm(-1) and employing background subtraction. The lower detection limit and measurement precision were determined to be approximately 330 parts in 10(12) per unit volume. A custom breath collection system was designed to collect lower airway breath of spontaneously breathing calves while in a restraint chute. Breath was collected and analyzed from calves upon arrival and periodically during a 42 day receiving period. There was a statistically significant relationship between eNO, severity of bovine respiratory disease (BRD) in terms of number of times treated, and average daily weight gain over the first 15 days postarrival. In addition, breathing patterns and exhaled CO2 showed a statistically significant relationship with BRD morbidity.     

Wavelength-shift speckle interferometry for absolute profilometry using a mode-hop free external cavity diode laser

Abstract

In this paper we present an absolute surface topography measurement with a tuneable diode laser with an external cavity that has a tuning range of as much as 25 nm without mode hops and a very high tuning speed of less than 1 s for the whole range. With this laser, an absolute wavelength-shift speckle pattern interferometer was realized, capable of measuring optically smooth and rough surfaces. We briefly explain the principles of operation, and the importance of wavelength tuning without mode hops. We discuss the capabilities, possible measuring ranges and some results as well as calibration methods of wavelength-shift speckle profilometry.     

Stable isotopic analysis of atmospheric methane by infrared spectroscopy by use of diode laser difference-frequency generation

An infrared absorption spectrometer has been constructed to measure the stable isotopic composition of atmospheric methane samples. The spectrometer employs periodically poled lithium niobate to generate 15 microW of tunable difference-frequency radiation from two near-infrared diode lasers that probe the nu3 rotational-vibrational band of methane at 3.4 microm. To enhance the signal, methane is extracted from 25 l of air by use of a cryogenic chromatographic column and is expanded into the multipass cell for analysis. A measurement precision of 12 per thousand is demonstrated for both delta13C and deltaD.     

Direct monitoring of absorption in solution by cavity ring-down spectroscopy

Cavity ring-down spectroscopy is applied to the liquid phase by placing the target solution directly into the optical cavity. We demonstrate that solutions in the cavity can be stirred and more importantly monitored in a flow. We report a minimum detectable absorption of 10(-6) cm(-1) for a range of organic solvents. This detection limit corresponds to picomolar concentrations for strong absorbers.