Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

High frequency wavelength modulation spectroscopy with diode lasers

High frequency wavelength modulation spectroscopy with diode lasers is accomplished by dithering the drive current at RF frequencies as high as 250 MHz. This technique is useful for fast and sensitive detection of absorption lines in the near-and mid-infrared spectral regions. Detection of 300 MHz wide spectral features corresponding to 1% changes in transmission is accomplished in time intervals as short as 500 ns. A potential application is for fast reading of information contained in frequency domain optical memories based upon photochemical hole burning.     

Frequency-stabilized high-power violet laser diode with an ytterbium hollow-cathode lamp

We have demonstrated in an ytterbium laser cooling and trapping experiment a high-power violet extendedcavity diode laser (ECDL) stabilized to the Yb resonant transition at 398.9 nm in an Yb hollow-cathode lamp. A frequency-dispersion signal, which we obtained by applying a modulation-free dichroic-atomic-vapor laser lock technique, allowed us to stabilize the violet ECDL at a frequency stability below 1 MHz at 1-s average time and a useful output power of 15 mW.     

Watt-level continuous-wave and passively Q-switched red lasers pumped by a single blue laser diode

Praseodymium-ion-doped gain materials have the superiority of lasing at various visible wavelengths directly.Simple and compact visible lasers are booming with the development of blue laser diodes in recent years. In this Letter, we demonstrate the watt-level red laser with a single blue laser diode and Pr:YLiF_4 crystal. On this basis,the passively Q-switched pulse lasers are obtained with monolayer graphene and Co:ZnO thin film as the Q-switchers in the visible range.     

Infrared diode laser spectroscopy

Three types of lasers (double-heterostructure 66 K InAsSb/InAsSbP laser diode, room temperature, multi quantum wells with distributed feedback (MQW with DFB) (GaInAsSb/AlGaAsSb based) diode laser and vertical cavity surface emitting lasers (VCSELs) (GaSb based) have been characterized using Fourier transform emission spectroscopy and compared. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) for the strongest absorption line of the v₃ + v₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of v₁, v₅). Laser sensitive detection (laser absorption together with high resolution Fourier transform infrared technique including direct laser linewidth measurement, infrared photoacoustic detection of neutral molecules (methane, form-aldehyde) is discussed.     

Sub-ppb NO2 detection by optical feedback cavity-enhanced absorption spectroscopy with a blue diode laser

We report preliminary results on the first application of optical feedback cavity-enhanced absorption spectroscopy with a blue (411 nm) extended cavity diode laser (ECDL) for NO₂ detection. While this technique was originally developed to operate with distributed feedback diode lasers in the near infrared, it is here extended to ECDLs and applied to the blue spectral region. With a simple and compact optical setup, we demonstrate from the baseline noise a minimum detectable NO₂ concentration of 6×10⁹ molecules/cm³ for a single laser scan (70 ms), which extrapolated under atmospheric conditions corresponds to 200 pptv. Signal averaging should allow further lowering of this limit. Observed absorption spectra display more structure than previous spectra obtained at lower resolution by Fourier-transform spectroscopy at the same wavelength. PACS 07.88.+y; 42.55.Px; 42.62.Fi     

InGaN MQW violet laser diode performance with quaternary AlInGaN blocking layer

Violet laser diode performance with AlInGaN blocking layer has numerically been investigated by using ISE TCAD software simulation program. The effects of AlInGaN blocking layer have been studied from different perspectives, the threshold current, output power, optical intensity, and temperature characteristics. In this study, simulation results indicated that the use of AlGaInN instead of the conventional AlGaN blocking layer leads to decreasing the threshold current while this blocking layer increases the optical intensity and output power when the mole fractions of Al and Ga are carefully chosen. The laser diode survives above 370 K.     

Stable diode lasers for hydrogen precision spectroscopy

We report on an external cavity diode laser at 972 nmstabilized to a mid-plane mounted Fabry-Perot (FP) resonator with afinesse of 400000. The 0.5 Hz optical beat note line width betweentwo similar lasers (Allan deviation 2 × 10^-15) is limitedby thermal noise properties of two independent FP resonators. Thelong term drift of the FP resonator and mirror substrates made fromUltra-Low-Expansion glass (ULE) is small and can be well predictedon time intervals up to many hours if the resonator is stabilized atthe zero thermal expansion temperature T_c. Using a Peltierelement in a vacuum chamber for temperature stabilization allowsstabilization of the FP cavity to T_c which is usually below theroom temperature. Beat note measurements with a femtosecond opticalfrequency comb referenced to a H-maser during 15 hours have shown awell defined linear drift of the FP resonance frequency of about 60 mHz/s with residual frequency excursions of less than ±20 Hz.     

High-sensitivity transient spectroscopy using tunable diode lasers

Experimental techniques have been developed to monitor transient infrared absorptions using lead-salt tunable diode lasers. The techniques are easily implemented, yield sensitivities which are limited by detector noise at 10^–5 level of absorbance, and have a response time on the order of one microsecond. The transient absorption detection techniques are high frequency versions of the sweep integration technique pioneered by Jennings [Appl. Opt.19, 2695 (1980)]. TDL modulation rates of 100 kHz and 500 kHz allow for absorption sampling rates of 200 kHz and 1 MHz, respectively. In order to reproducibly achieve near-detector-noise-limited sensitivities for 100 kHz TDL modulation rates, an automated analog subtraction circuit has been developed which removes the effects of minor TDL power variations. At the 500 kHz modulation rate, digital filtering techniques are used to remove the effects of this power variation.     

High-power broad-area diode lasers for laser cooling

Received: 1 April 1996/Revised version: 24 September 1996     

灵敏的基于分布反馈式半导体激光波长调制光声光谱

利用室温下单模运行的近红外半导体二极管激光,报导了波长调制共振光声光谱结合二次谐波探测技术.实验系统应用到乙炔探测,在1个标准大气压和3毫瓦平均光功率以及3毫秒锁相积分时间条件下其探测灵敏度可达10ppm(体积比),归一化到激光功率和系统带宽最小可探测吸收为4.0×10-8Wcm-1/Hz,并且实验中发现系统最佳压力响应值在2.66×104Pa附近.本实验装置可有效的消除光声光谱系统中常见的窗片和光声腔壁吸收入射光而引起的背景噪声.此外,相对于其他方法我们描述的基于半导体激光共振光声光谱具有很大的优点,为进一步发展便利、实用、便携式环境监测仪器奠定了坚实的基础.     

Tunable diode laser spectroscopy with optimum wavelength scanning

In this paper, an improved signal processing method is given, aiming at a sensitivity improvement of the tunable diode laser spectroscopy based gas concentration measurement. The gas absorption spectrum is probed with an optimum wavelength scanning function which enables a more efficient curve fit than for traditional linear wavelength scanning. The wavelength scanning function is determined to be optimum, in the sense that the variance of the estimated concentration noise is minimized. This optimum scanning function depends on the signal model used in curve fitting. Several models including interfering gas spectra and etalon fringes are examined. Compared to the gas absorption spectrum recorded by ramping the wavelength linearly, the optimum spectrum waveforms have a cascade structure, which means that the optimum scanning preferably samples important points of the spectrum. The new method theoretically enables a factor of ～2 improvement on detection sensitivity of the estimated concentration. Furthermore, direct spectroscopy is superior to second harmonic detection, because the concentration noise can be an additional factor of ～2 lower.     

Application of pressure-tuned diode lasers to high-resolution spectroscopy

The application of the high-pressure chamber with a pressure transmitting medium (liquid at room temperature) to diode-laser tuning was found to be a useful method for high-resolution spectroscopy. Ammonia absorbtion spectra in the 10 μm region were recorded with a resolution approaching the Doppler limit.     

Wavelength modulation spectroscopy based on quasi-continuous-wave diode lasers

A modified wavelength modulation spectroscopy（WMS）based on the self-heating effect of the tunable diode laser when driven in quasi-continuous-wave（QCW）mode is investigated.A near-infrared distributed feedback（DFB）diode laser working at the QCW mode is employed as the QCW light source,and CO2 is selected as the target gas.The characteristic of the QCW second harmonic（2f）line profile is analyzed through a comparison with that of the traditional CW WMS with the same system.A noise-equivalent absorbance of 3.2×10-5 Hz-1/2 for CO2 at 1.58μm is obtained with 18-m optical path.The QCW WMS lowers the dependence on lasers and expands selectivity,thus verifying the feasibility of the method.     

Mid-infrared diode laser spectroscopy of SO

The absorption spectrum of the fundamental band of SO⁺ (X²Π) has been recorded using a mid-infrared tunable diode laser spectrometer with the velocity modulation technique in an AC glow discharge of He/SO₂. Forty-two lines of SO⁺ were identified in the spectral range of 1230-1330 cm⁻¹. The observed rovibrational transitions together with the microwave data published previously were fitted to a standard effective Hamiltonian for ²Π states. Molecular constants for the ground and υ = 1 vibrational states were derived. The band origin was determined to be 1291.5299(27) cm⁻¹.     

480-nm blue laser based on sum-frequency mixing of diode pumped Nd:ASL-Yb:YAG lasers

We report for the first time a continuous-wave (CW) coherent radiation at 480 nm by intracavity sum-frequency generation of 900 nm Neodymium Doped Strontium and Lanthanum Aluminate (Nd:ASL) laser and 1030 nm Yb:Y₃Al₅O₁₂ (Yb:YAG) laser. Blue laser is obtained by using a doubly cavity, type-I critical phase matching LiB₃O₅ (LBO) crystal sum-frequency mixing. With total pump power of 28.6 W, the blue laser at 480 nm of 170 mW is obtained. At the output power level of 170 mW, the blue power stability is better than 4.7% and laser beam quality M ² factor is 1.43.     

Xenon saturation spectroscopy by diode laser

Summary Commercial diode lasers have emission wavelengths which overlap some absorption xenon lines. We have performed saturation spectroscopy of four lines, leaving from the first excited atomic configuration 5p ⁵6s, at 823.16 nm, at 828.01 nm, at 834.68 nm and at 820.63 nm, in a weak glow discharge of natural xenon. Natural xenon is a mixture of several isotopes, and the two even isotopes, Xe(129) (26% of relative abundance) and Xe(131) (21%), have a nuclear spin (1/2 and 3/2, respectively) that produces a hyperfine structure. The complex resulting spectra have been resolved and the results are compared with the available literature data. The authors of this paper have agreed to not receive the proofs for correction.     

Doppler cooling a single Ca+ ion with a violet extended-cavity diode laser

We present a scheme for employing a violet extended-cavity diode laser in experiments with single, trapped ions. For this the grating-stabilised laser is spatially and spectrally filtered and referenced to a Fabry–Pérot cavity. We measure an upper limit to the line width by observing a 305-kHz FWHM beat note with the second harmonic of a titanium sapphire laser. The laser is subsequently used to optically cool a single ⁴⁰Ca⁺ ion close to the Doppler limit. PACS 03.67.Lx; 32.80.Pj; 42.55.Px     

Single-tone frequency-modulation spectroscopy with frequency-doubled current-modulated diode laser light

We demonstrate and characterize the generation of single-tone frequency-modulated and frequency-doubled radiation at 400 MHz and 430 nm. We obtained the radiation at 430 nm by frequency doubling light from a current-modulated 860-nm diode laser, using noncritical type I phase matching in a KNbO(3) crystal. The optical spectrum of the doubled light was found to be in keeping with our expectations based on the measured frequency- and amplitude-modulation indices of the fundamental radiation. The experimentally measured diode laser and crystal parameters were used to simulate the in-phase and quadrature signals that would be observed in a single-tone frequency-modulation spectroscopy experiment.