Heterodyne measurements of submillimeter laser spectrometer frequencies

The frequencies of 46 CW laser lines commonly used for submillimeter spectroscopy, with wavelengths between 0.1 and 0.7 mm, have been measured by heterodyne methods. All the fines are optically pumped by a CO2laser, with threshold pump powers of 3 W or less. The precision of measurement, limited by the laser linewidth, is typically ± 1 part per million.     

用飞秒锁模激光测量光的频率

介绍了2005年诺贝尔物理学奖的获奖工作-用锁模飞秒激光光梳去测量光的频率.飞秒激光光梳技术大大简化了光频的测量.锁模飞秒激光通过光子晶体光纤时,由于自相位调制,在可见光和近红外区能够产生上百万等间隔的梳状频率,其频率间隔等于锁模脉冲的重复率.利用光频梳和倍频技术,把对光频的测量变为对射频的测量,这样就能够很容易地测出光的频率.使得光频测量精度和原子钟精度达到前所未有的高度,从而对物理学和计量学的发展有重要意义.     

Theory of optically pumped submillimeter lasers

Theoretical models of the operating characteristics of both pulsed and CW optically pumped submillimeter (SMM) lasers are reviewed. The SMM laser gain is discussed for both pressure and Doppler broadened transitions. A review is given of expressions for SMM laser gain based on quantum mechanical calculations in a three-level molecular system.     

Assignment of laser lines in optically pumped submillimeter lasers: HDCO, D2CO

Respectively, three and eight laser lines have been precisely measured and assigned in asymmetric-top molecules HDCO and D2CO. Frequency calculations of rotational transitions use parameters obtained from microwave studies. For frequency calculations of rovibrational transitions, vibrational energies derived from infrared band analysis have been also taken into account. The assignment is confirmed by infrared radio frequency double resonance experiments inside the submillimeter laser cavity.     

Application of submillimeter wave lasers to high voltage cable inspection

An experimental device designed for real-time, nondestructive inspection of cross-linked polyethylene insulation during cable manufacture has been built, analyzed, and tested under laboratory conditions. The device detects scattered far-infrared (FIR) laser energy at 118.8 μm from imperfections, such as voids and contaminants, known to have a serious deleterious influence on cable in-service lifetime. As presently configured, the device can readily detect voids as small as 100 μm in diameter in a cable 2.8 cm in diameter moving past the sensor at 5 cm/s. Better performance of the liquid helium cooled Ga-doped Ge detectors could lead to a significant improvement in void detection capability.     

Application of the optoacoustic effect in optically pumped submillimeter lasers

The dependence of the optoacoustic signal generated in an optically pumped submillimetre laser system on gas pressure and modulation frequency has been studied experimentally and the results accounted for theoretically. With care an extracavity optoacoustic cell containing the laser gas diluted with air can be used to stablize the pump laser frequency to maximum absorption but this does not always optimise laser output.     

Wavelength-tunable optical mixing experiments between 208 nm and 259 nm

Sum-frequency generation experiments in which the second harmonics of rhodamine and coumarin dye lasers are upconverted to the 208-234 nm by mixing with 1064 nm are described. The dye lasers were pumped by the second and third harmonics of a repetitively pulsed Nd:YAG laser. Both temperature-tuned and angle-tuned phase matching techniques were used with mixer crystals of ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP). Tunable light from 246 nm to 259 nm also was generated by mixing the output of a coumarin laser with the 532-nm harmonic of the Nd:YAG laser. Peak powers between about 500 W and 10 kW were obtained, and both theoretical and experimental tuning curves have been determined for these processes. Limitations imposed by mode beating effects in the Nd:YAG laser and by the relatively broad linewidth of the dye lasers are discussed.     

Investigations of optically pumped submillimeter wave laser modes

A complete theory for waveguide laser modes for oversized metallic and dielectric waveguides with circular cross section has been developed for the submm wavelength region. The experimental investigations have been done by a submm heterodyne technique for the first stage using a Schottky barrier diode in an open structure mixer.     

Electric field effects on the efficiency of optically pumped submillimeter lasers

The FIR power dependence on an applied electric field is carefully investigated for optically pumped lasers. The enhancement effect observed for many laser lines, both "orthogonal" and "parallel," is analyzed as a function of different parameters such as gas pressure, laser threshold, cavity tuning, and transitions selection rules. Measurements of the IR absorption of the gases are also carried out by a Stark optoacoustic technique. Absorption increases are observed at low electric field values. The experimental results disagree with the previously proposed linear Hanle effect explanation. They are explained with the increase in the pump saturation intensity, also detected with the transferred Lamb-dip technique. The combined effects of the absorption increase and of the small-signal gain curve decrease are shown to qualitatively explain the observed FIR enhancement features.     

Frequency pulling in an optically pumped submillimeter laser by Doppler induced dispersion

The dependence of the frequency of a CW optically pumped submillimeter laser on the frequency of its pump laser has been directly measured and shown to result largely from the dispersion produced by the two peaked gain spectrum resulting from pumping in the wings of a Doppler broadened absorption line.     

Modulation of room-temperature GaAs lasers at X-band frequencies

GaAs lasers operating at room temperature are directly modulated at X-band frequencies. A fast silicon avalanche photodiode is used to demodulate the laser light.     

Visible fiber lasers excited by GaN laser diodes

This paper describes and discusses visible fiber lasers that are excited by GaN laser diodes. One of the attractive points of visible light is that the human eye is sensitive to it between 400 and 700 nm, and therefore we can see applications in display technology. Of course, many other applications exist. First, we briefly review previously developed visible lasers in the gas, liquid, and solid-state phases and describe the history of primary solid-state visible laser research by focusing on rare-earth doped fluoride media, including glasses and crystals, to clarify the differences and the merits of primary solid-state visible lasers. We also demonstrate over 1 W operation of a Pr:WPFG fiber laser due to high-power GaN laser diodes and low-loss optical fibers (0.1 dB/m) made by waterproof fluoride glasses. This new optical fiber glass is based on an AlF₃ system fluoride glass, and its waterproof property is much better than the well known fluoride glass of ZBLAN.     

铜蒸气激光链中激光脉冲的抖动分析

运用概率论方法导出了铜蒸气激光链中激光脉冲抖动量的表达式,结果表明:当铜蒸气激光器并联时,两台激光器输出光脉冲之间的抖动量的平方等于每台激光器输出光脉冲相对于电触发脉冲抖动量的平方和;当多台铜蒸气激光器串联放大时,第m台激光器输出光脉冲相对于振荡器输出光脉冲的抖动量随m的增加而增加,但总是小于每台激光器放电脉冲抖动量的2~(1/2)百倍。实验结果与理论计算基本相符。     

Electronic control of the difference frequency between two semiconductor lasers

A method for controlling the difference frequency between two semiconductor lasers has been developed. The beat frequency is directly measured and current feedback applied to one laser to control this frequency. Stability to plus or minus 10 MHz was achieved. Applications include coherent optical communication systems.     

Secondary locking regions in laser diode subject to optical injection from two lasers

A new secondary locking region in a semiconductor laser when under optical injection from two master lasers is reported. Theory and experiment suggest this can be attributed to a frequency pulling effect. Also, a stability map containing instability threshold caused by the second signal, is generated.     

Transverse electron-beam wave excitation by photoelectric mixing of laser beams

It is shown theoretically that, by photoelectric mixing of laser beams which are incident obliquely on to a photosurface, transverse electron-beam waves may be excited. It should thus be possible to construct transverse-field microwave phototubes for detecting amplitude-modulated laser signals.     

Dynamic line broadening of semiconductor lasers modulated at high frequencies

Directly modulated semiconductor lasers show a pronounced linewidth broadening. A general and simple expression for this broadening is derived and the consequences for optical communication systems are discussed.     

Theory of two coupled lasers

The phase-locking of two coupled lasers is studied using the model of Spencer and Lamb. Tuning curves for the odd and even normal modes of two coupled empty resonators are presented to show the origin of the strong frequency-pulling of the coupled lasers. It is shown that the even mode is preferable for combining the two laser beams in the far field. Scaling relations, obtained for the locking range, reduce the number of effective parameters of the coupled system. In the limits of very large gain and very weak coupling the locking range is found to be narrower than predicted by the Spencer-Lamb coupled laser theory.     

Coherent combining of two pulsed fibre lasers in phase modulated mutually coupled fibre laser array

A scalable architecture for coherent combining of pulsed fibre lasers is reported. Stable synchronous and phase-locked pulsed fibre laser beams are generated in a phase modulated mutually coupled fibre laser array. The mechanism for generating a pulsed laser is different from previous Q-switching, modelocking or pump modulation methods. Two pulsed fibre lasers are coherently combined with 0.48 J pulse energy at the repetition rate of 115 kHz. The repetition rate can be controlled by adjusting the pump current. The combining efficiency of the two pulsed fibre lasers reaches 90 and the fringe contrast is larger than 40 .