Comparison of FM dye lasers in standing-wave and ring configuration

We have compared the operation of a frequency-modulated dye laser (FML) in a standing-wave and ring configuration. As expected, for a given frequency detuning (difference between the drive frequency for the phase modulator and the cavity mode spacing) the enhanced modulation index is twice as large for the standing-wave configuration as for the ring configuration. In addition, we have demonstrated that application of small amounts of phase modulation in the ring configuration leads to improved mode stability. The fine structure of the rf beats for both FM lasers, have been studied and the results suggest that spatial hole burning complicates the detailed structure.     

Generation of 10-GHz ultra-short pulses with low time jitter in an actively mode-locked fiber laser

We have experimentally achieved the 8.3-ps ultra-short pulse at 10 GHz repetition rate with the time jitter as low as 590 fs in an actively mode-locked fiber ring laser. The ring-cavity laser is mode-locked by a semiconductor optical amplifier based on cross-gain modulation. The external CW source is modulated with radio frequency signal by an amplitude modulator as the external optical pulses and, then, injected into the fiber ring cavity to achieve active mode locking. Further investigating the laser output characteristics, it indicates that the linewidth of employed CW source affects properties of the generated ultra-short pulse, such as phase noise and time jitter. Ultra-short pulse at high repetition rate with low time jitter can be generated by the optimization of CW laser source.     

光纤激光经过模清洁器后的强度噪声分析

对1560 nm单频光纤激光光源通过一个光学模清洁器后的强度噪声进行了分析研究.实验观察到模清洁器虽然对于激光的高频强度噪声有明显的抑制作用,但强度噪声特性随频率呈一定周期变化,且在低频处模清洁器对强度噪声有放大作用.本文认为这是由于模清洁器具有类似于光纤延迟线的时延效应,将激光源的部分相位噪声转化为强度噪声.通过理论分析,由相位噪声转化的相对强度噪声幅度与激光相干时间、模清洁器的平均延时参数以及分析频率相关.分析结果与实验测量结果符合良好.此外,通过在光路中加入声光调制器进行反馈调制,激光的线宽从26 kHz压窄至16 kHz,模清洁器的锁定质量明显提高,经过模清洁器后的激光强度噪声有所减小,与理论相符.该结果进一步证实了由相位噪声转化而来的强度噪声与模清洁器的锁定质量无直接关系.通过该研究,完善了光学模清洁器对激光的噪声抑制模型.     

Beat-note jitter suppression in a dual-frequency laser using optical feedback

An efficient locking technique based on optical feedback is demonstrated to suppress jitter on the rf beat note between the two modes of a dual-frequency Yb:Er glass laser. The method consists of a self-injection process in which one selected mode serves as a master oscillator to lock and stabilize the second mode via a frequency-shifted optical feedback. The beat note adjusted near 170 MHz was stabilized with an accuracy of 250 mHz using an optical feedback loop with a double pass through an acousto-optic modulator. The beating note can be tuned over 300 kHz by controlling the reference oscillator. The extensions and limitations of the technique are discussed.     

Generation of high order multi-bound solitons and propagation in optical fibers

We demonstrate experimental generation of multi-bound solitons of up to sextuple in an active FM mode-locked fiber ring laser operating under power saturation in the locking state. The ring laser consists of two booster optical amplifiers operating in saturation regime, an electro-optic phase modulator driven by a sinusoidal electrical wave and a length of dispersive fiber. The periodic phase modulation generates phase chirp of the generated lightwaves in the ring laser. The chirped phase state plays an important role in the phase matching condition for mode-locking as well as the stabilization and the determination of the bound states of multi-solitons. The formation of such high order multi-bound solitons is explained based on the chirping of the phase and the behavior of the optical pulse sequence in the near field region of the dispersive fiber. The propagation of these multi-bound solitons through single mode optical fibers is observed. Experimental and simulation results of bound solitons have been shown to follow similar trends.The propagation of these multi-bound solitons through single mode optical fibers is described. Their mutual interaction through such quadratic phase media shows the influence of the quadratic phase property on the differential phase of individual solitons of the bound group. Simulated results confirm the evolution of the bound solitons over dispersive single mode optical fibers.     

A parametric study of negative feedback Nd:YAG laser

The output pulse parameters of a mode-locked Nd:YAG laser with a passive negative feedback element were studied experimentally. The pulse evolution within the train was experimentally recorded using a modified second harmonic generation autocorrelator. By comparing the laser operation with and without an acousto-optic modulator, we found that with the later there is a significant increase in the mode locking probability and the pulse acquires a temporally Gaussian symmetric pulse shape. Further with the active modulator there is a relaxation in alignment tolerances and increase in the range of permissible dye concentrations for stable mode locking. It was also observed that the pulse width of the negative feedback laser depends on the saturation intensity of the mode locking dye and reduces for a dye with higher saturation intensity. The pulse width was also found to reduce linearly as the initial transmission of the dye is reduced.     

基于NICE-OHMS技术进行大气压气样直接检测的理论分析

噪声免疫腔增强光外差分子光谱技术(NICE-OHMS)是目前世界上最灵敏的激光吸收光谱技术,其在低压环境中具有极高的探测灵敏度。然而当测量样品处于大气压时,NICE-OHMS系统的探测灵敏度会大幅下降。主要原因之一是大气压下获取最大NICE-OHMS信号幅度的条件与低气压下不同。通过对大气压NICE-OHMS理论进行分析,分析了影响信号幅度的参数,并通过数值模拟来寻找最佳的实验条件。本文着重讨论影响信号的主要参数包括光学腔腔长L,调制系数β,探测相位θ。其中,由于在NICE-OHMS中使用DeVoe-Brewer技术将调制频率ν_m锁定到Fabry-Parot(FP)腔的自由光谱区(FSR)。因此FP腔的腔长决定了ν_m,同时还作用于信号幅度S■。模拟结果显示,当腔长增大时,由于ν_m随之减小,载波和边带的光谱成分相互重叠部分增大,因此线型函数的幅度逐渐减小。而吸收信号幅度随着腔长的增加而逐渐增加,色散信号幅度先增大后减小,并且在腔长等于8 cm时达到最大值。调制系数β会影响频率调制后激光载波和边带的幅度大小,并且影响信号线型。随着腔长的增加,最大信号幅度对应的β值也随之增加。在相同腔长下,色散信号的最佳β值小于吸收信号,更容易使用电光调制器实现。最后分析了参数的可实现性,分析了不同种类激光器的频率调谐能力,压电陶瓷的扫描宽度等。以乙炔气体为例,大气压下NICE-OHMS的谱线半宽达到~3 GHz,而光谱覆盖范围大于10 GHz。分布反馈式半导体激光器(DFB)与外腔二极管激光器(ECDL)的频率调谐范围可以达到30 GHz以上,但是由于激光线宽宽,得到的PDH锁定性能欠佳。回音壁模式激光器(WGM)和掺饵光纤激光器(EDFL)线宽为百Hz量级,是目前高灵敏NICE-OHMS系统中常用的光源。但是WGM目前可以实现了5 GHz的激光频率调谐范围,而EDFL的外部电压可控制的调谐范围仅为3 GHz。使用精细度为55000的腔进行模拟,调制系数β=1,腔长大于8 cm时,可使用WGM激光器实现,腔长大于25 cm时,可以使用EDFL激光器实现。而对于在设计光学腔中常用的伸缩长度为25μm的PZT,随着腔长的增加,对应的腔模频移范围逐渐减小,在腔长为典型的40 cm时,扫描范围大于12 GHz。     

Frequency offset locking of a synchronously pumped mode-locked dye laser

We have developed a scheme of frequency offset locking for transferring the frequency stability of a commercial dye laser to a synchronously pumped mode-locked dye laser. By observing the beat frequency between the two dye lasers, mode stability of less than 2 MHz has been observed for the mode-locked dye laser mode. The use of the mode-locked system as a spectroscopic tool has been demonstrated by observing Doppler-free multiple pulse two photon spectra of the sodium 3S-4D transition.     

Erbium-doped fiber laser simultaneously mode locked on more than 24 wavelengths at room temperature

We demonstrate simultaneous mode locking of more than 24 wavelengths at 3 GHz in an actively mode-locked erbium-doped fiber laser operating at room temperature. The multiwavelength operation is achieved when a frequency shifter and an all-fiber 50-GHz periodic filter are inserted into a ring cavity. Active mode locking is performed with an amplitude modulator, and pulses with a FWHM of 30 ps are obtained.     

激光二极管泵浦的全固体锁模激光器

报道了用国产激光二极管泵浦Ｎｄ：ＹＶＯ４激光器连续锁模的实验结果。在泵浦功率为１２２．４ｍＷ时获得稳定的锁模序列，脉冲宽度４３ｐｓ（ＦＷＨＭ），输出功率７ｍＷ，实验中在较低的泵浦强度下观察到了弛豫振荡的振幅调制现象，并导出了含有自相位调制时锁模宽度的自恰解。     

Dual-frequency pulsed laser with an accurate gigahertz-beat note

We report a novel nanosecond pulsed laser with a highly accurate gigahertz beat that is realized by injection locking to gigahertz sidebands generated from a single-frequency cw laser radiation using a Mach-Zehnder type intensity modulator. It is shown from both the spectrum of the pulsed sidebands and their coherent beat in the time domain that the frequency accuracy of an rf oscillator driving the intensity modulator is reflected in the injection-locked nanosecond pulse with an intense peak power of the megawatt class.     

Low-Frequency Fluctuation and Frequency-Locking in Semiconductor Lasers with Long External Cavity Feedback

The properties of low-frequency fluctuations in semiconductor lasers with optical feedback from a long external cavity are experimentally studied. Frequency-locking of the laser light output to the injection current modulation is observed when the modulation frequency approaches the external cavity mode. The modulation frequency for the successful frequency-locking is always less than the external cavity mode frequency and the locking domains as a function of the modulation amplitude is asymmetric with respect to the frequency detuning.     

Synchroscan streak camera study of potential saturable absorbers in the blue spectral region

The viscocity-dependent fluorescence lifetime of some cyanine dyes with peak absorption cross-sections in the 350–500 nm range have been examined using a synchronously operating strek camera in conjunction with the picosecond excitation pulses from a frequency doubled synchronously pumped c.w. dye laser. Through increases in the recovery time of a saturable absorber by employing viscous solutions, the necessary saturation flux is conveniently reduced and passive mode locking of laser systems can occur more easily. This is illustrated by the application of one of the dyes to the mode locking of the coumarin 102 dye laser.     

关于氦氖激光器声光调制锁模的实验研究

本实验中,将声光调制锁模器放置在氦氖激光器中的谐振腔里,控制谐振腔中的增益,使声波的频率调制与激光器固有频率间隔一致,从而达既可以到锁模的效果,又可以增加增益,减少能量损耗。通过功率计与示波器检测出激光脉宽、脉冲周、纵模间隔等物理量,与理论值进行比较,判断激光锁模实验的效果。本实验使用的氦氖激光器腔长大于1.55m,是双面全反射腔,可以利用激光锁模技术得到纵模进行比较,从而获得单色性好的超短脉冲激光。     

Fourier domain mode locking pulse fiber laser

A novel pulse fiber laser is proposed based on the modulation of the lasing wavelength. Different from conventional amplitude or phase modulated mode-locking fiber lasers, the proposed fiber laser is with a so-called Fourier domain mode locking (FDML) operation (i.e., the cavity round-trip time of the optical pulse should match the period that the open time window of the tunable filter appears for the lasing wavelength or its harmonics). An FDML pulse fiber laser with a cavity round-trip frequency of 19.46 kHz is demonstrated and optical pulses with repetition rate of 19.46 kHz are achieved when the wavelength modulation frequency is 9.73 kHz.     

光温双控太赫兹波调制特性研究

基于多频带金属开口谐振环结构,利用GaAs材料的光敏特性和VO₂薄膜的热致相变特性,设计了一种既能实现光控又能实现温控的太赫兹(Terahertz, THz)波调制器,研究了光强和薄膜温度对 THz波调制特性的影响。结果表明,随着光强的增加,谐振频率均出现蓝移且谐振强度减小,当光强达到0.2 μJ·cm-2时,第二个谐振点(0.52 THz)蓝移了0.14 THz,透射幅度增加达50%;随着VO₂温度增加至相变温度以上,THz波透射幅度急剧减小,在0.63 THz处透射幅度减小达45.5%;当光强和温度同时控制时,随着光强和温度的增加,谐振点频率蓝移且谐振点处的THz波透射幅度增加,但在温度超过相变温度后,则温度控制起主导作用。设计的THz波调制器能通过光控和温控实现对THz波的明显调制效果,可为实现多功能的THz波功能器件的设计及应用提供参考。     

Laser phase and frequency stabilization using an optical resonator

We describe a new and highly effective optical frequency discriminator and laser stabilization system based on signals reflected from a stable Fabry-Perot reference interferometer. High sensitivity for detection of resonance information is achieved by optical heterodyne detection with sidebands produced by rf phase modulation. Physical, optical, and electronic aspects of this discriminator/laser frequency stabilization system are considered in detail. We show that a high-speed domain exists in which the system responds to the phase (rather than frequency) change of the laser; thus with suitable design the servo loop bandwidth is not limited by the cavity response time. We report diagnostic experiments in which a dye laser and gas laser were independently locked to one stable cavity. Because of the precautions employed, the observed sub-100 Hz beat line width shows that the lasers were this stable. Applications of this system of laser stabilization include precision laser spectroscopy and interferometric gravity-wave detectors.     

Otpically illuminated dielectric interfaces as high-speed far-infrared modulators

A new device for the gigahertz modulation of far-infrared radiation is analytically and numerically analyzed. It consists of a thin layer of a high-mobility, direct-bandgap semiconductor, such as GaAs, in which a high-density electron-hole plasma is rapidly created and destroyed, thereby rapidly changing the free-carrier reflectivity of the active layer. Illumination by a high-power, near-infrared laser diode array generates the plasma through intrinsic photoconduction. It is shown that this device acis primarily as an amplitude modulator, and that its efficiency increases sharply with increasing far-IR frequency, in contrast to a Schottky diode, which acts primarily as a phase modulator, and whose efficiency falls off sharply with far-IR frequency. The breakeven frequency lies at about 1.5 THz, depending slightly on the assumed device parameters. The relative advantage of the new device increases rapidly with increasing far-infrared frequency. At an operating frequency of 2.5 THz (119 m), for example, a 1 GHz modulation bandwidth may be achieved with a single-sideband conversion loss of only-21 db, versus a Schottky's loss of-39 db, assuming a laser diode power of 1 W, which is readily available from recently developed laser diode arrays.     

Fringe Locking in a Laser Diode Interferometer by Optical Feedback During Modulation of Injection Current

A fringe locking phenomenon in a two beam interferometer using a semiconductor laser subject to optical feedback was, whose injection current is modulated. When a path difference of the interferometer is sufficient, fringes taken by a charge coupled device camera are seen to be stationary and the rms fluctuations of fringe phase is reduced to as low as 0.2ϖ radians from more than 8ϖ radians without the optical feedback. The rms phase fluctuation is independent of frequency and amplitude of the current modulation. The fringe locking has also been observed in the presence of both injection current modulation and piezoelectric transducer mirror vibration.     

Vertical-cavity saturable-absorber intensity modulator

We propose and demonstrate a reflection-type optical modulator, with surface-normal architecture, that exploits the optical saturation of absorption in semiconductor quantum wells. The modulation section of the modulator, which is composed of quantum wells placed within a Fabry-Perot cavity, is optically controlled by an intensity-modulated beam generated by an in-plane laser integrated monolithically on the same wafer and grown in a single epitaxial step. The modulation section and the in-plane laser share the same medium; therefore, efficient coupling between the control beam and the signal beam is achieved. The device was successfully used for active mode locking of an erbium-doped fiber laser.