High-power 457-nm light source by frequency doubling of an amplified diode laser

We demonstrate the generation of 150-mW blue coherent single-mode radiation at 457 nm in a compact and inexpensive setup. The light is generated by frequency doubling the radiation of a master oscillator power amplifier (MOPA) system in an enhancement cavity with a potassium niobate (KNbO3) crystal. The MOPA consists of a 914-nm single-mode diode laser and a broad-area diode laser (BAL) as the amplifier. The BAL is a multimode laser with a specified wavelength of 938 nm. Sufficient gain at 914 nm is obtained by antireflection coating the BAL front surface and by cooling it to -10 degrees C.     

Backscatter-modulated laser diode for low-frequency small-amplitude vibration measurement

Feedback phenomenon in laser diodes is applied to low-frequency vibration measurement in the micrometer range on poorly reflecting targets. The laser diode is frequency modulated. Its beam is focused on the vibrating target, and backscattered light is fed back into the laser. The optical laser diode power variation is processed to measure the frequency and amplitude of low-frequency vibrations of amplitude, ~100 nm to 10 mum. Experimental results for sinusoidal and triangular vibrations are compared with theory.     

Frequency-stabilized 1520-nm diode laser with rubidium 5S(1/2) --> 7S(1/2) two-photon absorption

A 760-nm light source of >10 mW is obtained from a frequency-doubled external-cavity diode laser by use of using an erbium-doped fiber amplifier and a periodically poled lithium niobate waveguide. The 5S(1/2) --> 7S(1/2) two-photon transitions of rubidium are observed with such a light source. This laser frequency is locked to the Rb two-photon transitions with an instability of 10 kHz (1 s). Our experimental scheme provides a compact, high-performance frequency, standard in the S band (1480-1530 nm) for fiber-optic communication and sensing applications.     

Amplitude-stabilized frequency-modulated laser diode and its interferometric sensing applications

A direct frequency-modulated (FM) laser diode light source without light power variation is developed. The amplitude variation of the FM laser diode is compensated by means of a feedback system with use of a superluminescent diode as an external light power controller. Output power greater than 1 mW is obtained at the modulation frequency to 5 kHz with a >10 stabilization factor. By use of the amplitude-stabilized FM laser diode, we measured subfringes with high accuracy in FM continuous wave interferometry, increased the dynamic range of the displacement measurement, and improved the stabilization factor in the laser diode feedback interferometer.     

Iodine-stabilized extended cavity diode laser at λ=633 nm

An extended cavity diode laser at 633 nm has been frequency stabilized to I₂-Doppler-free absorption signals of the P(33)6-3 transition using a third-harmonic detection technique. The frequency was measured by the beat-frequency technique with an iodine-stabilized He-Ne laser as reference. A minimum value for the two-probe relative standard uncertainty of 1× 10^-11 (5 kHz) is reached after 100 s. We also report measurements of the hyperfine splittings of the P(33)6-3 transition and laser frequency dependence on modulation amplitude and iodine pressure     

Optoacoustic trace-gas monitoring with near-infrared diode lasers

An inexpensive resonant optoacoustic monitoring system using near-infrared laser diodes was developed. It was demonstrated that wavelength modulation at the resonance frequency of the cell provides a superior signal-to-noise ratio compared with amplitude modulation and eliminates background drifts and fluctuations. The system was tested out on ammonia. Its sensitivity is 8 parts in 10(9) (S/N = 1) at atmospheric pressure, which corresponds to a minimum detectable absorption coefficient of approximately 3.5 × 10(-11) cm(-1) W(-1). The pressure dependence of the optoacoustic resonance was also investigated. The monitor can be used as a continuous flow-through system up to a flow rate of approximately 3.5 L/min.     

Extended-cavity diode lasers with tracked resonances

We present a painless, almost-free upgrade to present extended-cavity diode lasers (ECDLs) that improves the long-term mode-hop-free performance by stabilizing the resonance of the internal cavity to the external cavity. This stabilization is based on the observation that the frequency or amplitude noise of the ECDL is lowest at the optimum laser diode temperature or injection current. Thus, keeping the diode current at the level where the noise is lowest ensures mode-hop-free operation within one of the stable regions of the mode chart, even if these should drift due to external influences. This method can be applied directly to existing laser systems without modifying the optical setup. We demonstrate the method in two ECDLs stabilized to vapor cells at 852 and 895 nm wavelengths. We achieve long-term mode-hop-free operation and low noise at low power consumption, even with an inexpensive non-antireflection-coated diode.     

Tunable diffraction-limited light at 488 nm by single-pass frequency doubling of a broad area diode laser

A laser system that is based on second-harmonic generation of a broad area laser diode and provides 23.2 mW of diffraction-limited light with narrow bandwidth is described. It is tunable from 487.4 to 489 nm. The broad area laser diode is frequency stabilized in an external cavity that yields 800 mW of diffraction-limited light. This infrared light is converted into the visible by use of a 1 cm periodically poled MgO:LiNbO(3) bulk crystal with a measured single-pass conversion efficiency of up to 3.6%/W x cm.     

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).     

Algorithm for automated analysis of surface vibrations using time-averaged digital speckle pattern interferometry

A fully automated algorithm was developed for the recording and analysis of vibrating objects with the help of digital speckle pattern interferometry utilizing continuous-wave laser light. A series of measurements were performed with increasing force inducing vibration to allow the spatial distribution of vibration amplitude to be reconstructed on the object's surface. The developed algorithm uses Hilbert transformation for an independent, quantitative evaluation of the Bessel function at every point of the investigated surface. The procedure does not require phase modulation, and thus can be implemented within any, even the simplest, DSPI apparatus. The proposed deformation analysis is fast and computationally inexpensive.     

Applications of an extremely short strong-feedback configuration of an external-cavity laser diode system fabricated with GaAs-based integration technology

A composite-cavity laser diode is used to monitor the reflectivity or the displacement of the external-cavity mirror for micromechanical photonics devices. Optical disk bits are read out in the near field from the difference in medium reflectivity with an antireflection-coated laser diode and a photodiode. Microbeam vibration is also detected in the near field from the phase difference with an uncoated laser diode and a photodiode. In both cases the carrier-to-noise ratio is very high (more than 45 dB) because of the lack of mode-hopping noise resulting from the extremely short (less than 3 μm) external-cavity length and strong light feedback. These composite-cavity laser diode microdevices are fabricated on a gallium arsenide substrate to eliminate the need for optical alignment.     

1 550 nm全光纤激光多普勒测振系统研制

针对目前国内激光测振系统价格昂贵、使用不便的问题,采用1 550 nm波段成熟的窄线宽光源和光纤元器件研制了一套低成本的全光纤激光测振系统原理样机。此原理样机光路部分采用马赫?泽德干涉仪结构,搭建了外差式激光干涉光路,参考光被40 MHz的声光调制器调制,与测量光在光电探测器表面发生干涉,产生原始的激光多普勒信号;信号解调部分采用相位解调法对原始激光多普勒信号进行解调,得到振动目标的运动特性,包括位移、速度和加速度信息。采用本单位的振动标准装置对其性能进行了测试,实验结果表明：在10 ~ 2 000 Hz的中低频振动范围内,1 550 nm全光纤激光多普勒测振系统峰值位移、速度和加速度的测量误差在-0.6% ~ 0.7%内。该系统在中低频段具有较高的测量准确度,且成本相对较低、操作便捷,具有技术借鉴价值。     

Sensitive trace gas detection with near-infrared laser diodes and an integrating sphere

We describe a simple spectrometer for sensitive trace gas detection in the atmosphere. A communication laser diode is used as a light source, and a commercial integrating sphere is used as a multipass absorption cell. We developed a theoretical formulation of the relative absorption of the optical power by trace gases in the sphere and applied it to two kinds of experimental result: one that is concerned with a structureless broad absorption band of butane with the use of a 1.2-μm multimode laser diode, and one that is related to the study of an isolated and sharp rovibrational line of water vapor in air at atmospheric pressure with the use of an 830-nm single-mode laser diode. With equivalent path lengths of several meters obtained with a 10-cm-i.d. integrating sphere we can demonstrate the usefulness of such a device as a broadband multipass cell for the measurement of small absorptions.     

Kalman filtering of tunable diode laser spectrometer absorbance measurements

A recursive Kalman time-series filter was applied to absorbance measurements obtained with a tunable diode laser spectrometer. The spectrometer uses frequency modulation spectroscopy and a nearinfrared diode laser operating at 1.604 μm to monitor the CO(2)-vapor concentration in a 30-cm absorption cell. The Kalman filter enhanced the signal-to-noise ratio of the spectrometer by an order of magnitude when an absorbance of 6 × 10(-5) was monitored.     

Ultrasensitive, visible tunable diode laser detection of NO(2)

Recent advances in room-temperature visible diode lasers and ultrasensitive detection techniques have been exploited to create a highly sensitive tunable diode laser absorption technique for in situ monitoring of NO(2) in the lower troposphere. High sensitivity to NO(2) is achieved by probing the visible absorption band of NO(2) with an AlGalnP diode laser at 640 or 670 nm combined with a balanced ratiometric electronic detection technique. We have demonstrated a sensitivity of 3.5 × 10(10) cm(-3) for neat NO(2) in a 1-m path at 640 nm and have estimated a sensitivity for ambient operation of 5 ppbv m (l0 ppbv m at 670 nm), where ppbvm is parts in 10(9) by volume per meter of absorption path length, from measured pressure-broadening coefficients.     

High-powered green light generation by intracavity frequency doubling using grating feedback optics

Grating feedback optics is shown to contribute to narrowing the spectral bandwidth of a multilongitudinal-mode laser diode to less than 0.2 nm and tuning the lasing wavelength to the peak absorption wavelength of Nd:YVO(4). A continuous green light of 31 mW was efficiently generated by intracavity frequency doubling of the Nd:YVO(4) laser with a KTiOPO(4) crystal. A relative intensity noise of less than -140 dB/Hz was obtained in the frequency region greater than 2 MHz. The noise characteristics of generated green light are discussed as compared with the case of using a single-longitudinal-mode laser diode as the pumping source.     

Direct observation of transient fluorine atoms with 25-µm wavelength-stabilized diode laser absorption

Through the use of continuous diode laser absorption, detection of transient fluorine atoms with an initial number density in the range of 10(14) cm(-3) has been demonstrated. A crucial part of the continuous-detection technique was laser frequency stabilization with a reference cell of atomic fluorine with Zeeman modulation of the absorption lines to generate a feedback signal. Long-term wavelength stability was demonstrated with second-harmonic phase-sensitive detection of the second-derivative signal for periods up to several hours. For determination of the short-term wavelength stability in the range of microseconds to seconds, a transient signal was generated by photolysis of F(2) with an excimer laser at 308 nm. The initial diode laser absorption was compared to a calculated value obtained from the measured excimer laser fluence, the known dissociation cross section of F(2), and the atomic fluorine absorption cross section, which included a statistical population distribution, the finite bandwidth of the laser dode, and the effects of pressure broadening. The observed absorption was approximately 33% less than the calculated value, possibly because of the diode laser's wavelength instability on the time scale of a few seconds, which is consistent with an observed amplitude instability from pulse to pulse when pulsed at 1-10 Hz.     

Antireflection coating with fluoropolymer for a novel organic nonlinear optical crystal: 8-(4'-acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (ADPA)

Prior to the frequency doubling of a laser diode through a ring resonator, reflection loss on the surface of a novel organic second-order nonlinear optical crystal, 8-(4'-acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (APDA), was decreased. An optically flat crystal surface with a flatness of λ/10 (λ = 633 nm) and surface roughness of 1.5 nm rms was obtained when an ultraprecision diamond turning lathe was used to machine the crystal surface. After this surface was coated antireflectively with an amorphous fluorinated polymer, Teflon AF-2400, the transmittance at 810 nm was increased to 98%. A laser-diode-pumped ring resonator was built with this crystal, and a blue second-harmonic emission at 405.5 nm wassuccessfully generated.     

Stabilization improvements of laser-diode closed-loop heterodyne phase-shifting interferometer for surface profile measurement

To stabilize the phase-shifting Fizeau-type interferometer against environmental disturbances (namely, vibration and temperature variations), the feedback scheme that uses the current-induced frequency modulation of a laser diode (lambda = 633 nm) and the two-frequency optical heterodyne method has been investigated, with particular attention to improvement of the achievable stabilization. It is demonstrated that introduction of the proportional-integral control into the feedback system improves stabilization against the proportional control case; e.g., stabilization is improved 5 times for 100-nm(p-p) vibration at the frequency range at 30 Hz. The surface profile measurement for a sample mirror was conducted with a reproducibility of 6.8 nm in the root mean square under the subwavelength-amplitude vibration at 100 Hz.     

Cold strontium atoms for an optical frequency standard

We report the capture of cold strontium atoms in a magnetooptical trap at a rate of 3/spl times/10/sup 10/ atoms/s. This is achieved with high power laser light at 461 nm generated by sum frequency mixing in a potassium titanyl phosphate (KTP) crystal using diode lasers at 813 nm and a Nd:YAG laser at 1064 nm. Up to 115 mW of blue light is obtained. Tuned to the /sup 1/S/sub 0//spl rarr//sup 1/P/sub 1/ of /sup 88/Sr, this light is used both to slow down an atomic beam in a Zeeman slower and to capture the atoms in a magnetooptical trap. Trapping of /sup 87/Sr and /sup 86/Sr is also reported.