基于纵模拍频控制的激光稳频技术

介绍了一种新的双纵模激光稳频技术:基于激光频率与纵模频率间隔的对应关系,通过精密锁相控制技术将两相邻纵模的拍频频率锁定在射频频率标准上,以控制激光谐振腔腔长,实现锁定激光频率的目的.理论分析表明,激光频率稳定度与两相邻纵模拍频频率的稳定度相同;实验上以射频频率标准为参考,精密锁定了He-Ne激光两相邻纵模的拍频频率及激光频率,且对采用该技术稳频的两套He-Ne激光系统进行了比对.实验结果表明,激光频率的稳定度为5×10-10(1 s积分时间),5×10-11(100 s积分时间).     

Coherence of pulsed microwave signals carried by two-frequency solid-state lasers

This paper studies the coherence of the radio-frequency (RF) beat note between the two frequencies emitted by a two-eigenstate solid-state laser operating in pulsed regime. Depending on how the pulsed operation regime is obtained-passive Q switching using a saturable absorber or resonant amplitude modulation using pump-power modulation-we show theoretically and experimentally that the RF beat note loses or maintains its coherence from pulse to pulse. Namely, RF coherence is shown to be lost when the laser intensity vanishes between pulses, while it can be maintained if a slight amount of intensity remains inside the cavity between pulses. We give experimental demonstrations of these results for both Nd:YAG and Er-Yb:glass two-frequency pulsed lasers, in connection with applications to lidar-radar systems.     

双频固体激光器产生相干脉冲输出的研究进展

光载波雷达（LidarRadar）是一种新型体制的激光雷达,它采用激光拍频产生的射频信号作为探测媒介,在保证激光探测的高空间分辨力优点的同时相对于普通激光雷达具有更强的抗大气干扰能力,其中作为探测光源的二极管抽运的双频固体激光器是其关键技术之一。针对光载波雷达的具体应用要求对光学脉冲相干的双频固体激光器进行了分析,并报道了最新研究成果。     

Frequency stabilization of the FM laser

A technique for the frequency stabilization of an FM laser is discussed. This technique employs a stabilization discriminant derived from a residual variation of the phase and amplitude of the small FM laser beat note at the modulation frequency, as the frequency of the FM carrier moves across the Doppler gain profile. A first-order analysis describes the dependence of that stabilization discriminant on δ, the mode coupling coefficient produced by the internal KDP phase modulator, and Γ, the resulting FM modulation index. Experimentally, a 50-mW He-Ne FM laser was frequency stabilized to better than one part in 10⁸on a long-term basis. In addition, the beat note at the modulation frequency was suppressed by 40 dB from its free-running value.     

THz-dual-frequency Yb/sup 3+/:KGd(WO/sub 4/)/sub 2/ laser for continuous wave THz generation through photomixing

A diode-pumped KGd(WO/sub 4/)/sub 2/ Yb/sup 3+/ doped laser has been developed for CW-THz generation. The frequency difference between the two modes is step tunable from DC to 3.1 THz. A maximum total optical output power of 120 mW CW has been obtained with a beat note linewidth narrower than 30 kHz.     

两台独立激光器拍频线型对线宽测量的影响

根据两台独立激光器拍频的理论模型,计算得到了洛伦兹线型和高斯线型的拍频线宽随衰减值的变化关系。为了提高激光线宽测量精度,在未知拍频线型时应选取较小的衰减值计算激光线宽;在已知拍频线型的情况下可选取较大的衰减值计算激光线宽。实验上采用窄线宽半导体激光器拍频测量其激光线宽,根据几个特殊衰减值处的线宽比值关系来判别拍频线型。选用较大衰减值处的拍频线宽计算激光线宽,测量结果与仪器出厂指标相比,误差仅为0.23%。该方法对其他线型仍然适用。     

Experimental study of additive-pulse mode locking in an Nd:glasslaser

Self-starting additive-pulse mode locking (APM) has been investigated experimentally in a continuous-wave Nd:glass laser. Stress has been put on the study of the self-starting process and relaxation oscillation instabilities. An intensity threshold for self-starting APM is observed and related to the linewidth of the first beat note of the power spectrum of the free-running laser output. Under steady-state conditions, two distinct operating regimes are obtained: mode locking with repetitive self-Q switching and pure mode locking. Increasing the intracavity power gives rise to a strong chirp developing on the mode-locked pulses and to a simultaneous disappearance of self- Q switching     

Optical clockworks and the measurement of laser frequencies with amode-locked frequency comb

Femtosecond laser-frequency comb techniques are vastly simplifying the measurement and synthesis of optical frequencies. A single mode-locked femtosecond laser, with its spectrum broadened by self-phase modulation in a microstructured or tapered nonlinear fiber, can produce millions of sharp laser lines in a precise evenly spaced grid spanning much of the visible and near-infrared spectrum. The absolute frequency of each line is determined by two observable radio-frequency signals. The pulse repetition rate gives the spacing of the comb lines and the rate at which the phase of the lightwave slips, relative to the intensity envelope from pulse to pulse determines the offset frequency by which each line is displaced from a precise integral multiple of the repetition frequency. This offset frequency can be measured most easily if the comb spans more than an optical octave so that one can observe a radio frequency beat note between the second harmonic of the infrared comb lines with the corresponding comb lines at the blue end. Such an optical-frequency synthesizer makes optical oscillations readily countable and provides the long-awaited compact optical clockwork for an all-optical clock     

Optical Kerr effect in a direction-switched fiber laser gyroscope

The operation of a direction-switched mode-locked fiber laser gyroscope is described. We observed a large bias in the beat frequency output, and found that it was caused by the optical Kerr effect when the two counterpropagating pulses in the laser cavity have different optical powers. The beat frequency bias was 65 Hz per 1 mW of power difference when the scale factor of the gyroscope was 0.43 kHz/(°/s). We found that a small amount of backscattering in the laser cavity could result in random fluctuations in the beat frequency. It is shown that the random frequency noise can be suppressed efficiently by applying push-pull phase modulation in the laser cavity     

Measurement of length shifts down to 10-5Å with a three-mode laser

The frequency of one axial mode of a three-mode He/Ne laser can be tuned independently from the other modes if a third mirror is added to the usual two laser mirrors. Provided the adjustment is correct, the frequency shift is proportional to a change of the optical path length between the third mirror and the neighboring laser mirror. By observing the beat frequency of the laser, measurements of optical path variations down to 10^-5Å are possible.     

两台同类型号独立激光器频率稳定度的测量

采用拍频法对两台同类型号独立激光器的稳定度进行了测量。从拍频理论出发,得到了拍频信号稳定度与待测激光器和参考激光器稳定度三者所满足的平方和关系,对于稳定度一致的两台同类型号激光器,由拍频信号稳定度可以得到待测激光器的稳定度。实验中将New Focus公司生产的两台同类型号激光器(TLB-6017)进行拍频,根据拍频信号稳定度测得激光器稳定度为1.36×10-8,频率漂移量为5.1MHz(1s积分时间)。实验结果与激光器出厂指标[稳定度1×10-8、频率漂移量5MHz(1s积分时间)]相比,稳定度在同一量级,频率漂移量的相对误差为2%。此方法避免了通常测量激光器稳定度时所需的高稳定度参考光源的限制,为激光器稳定度的测量提供了一定的参考。     

Hooting modes in a CO2 waveguide laser

CO₂ waveguide lasers `hoot' when, instead of the solitary resonator mode expected from a homogeneously broadened gas laser, two or more resonator transverse modes laser simultaneously on a common laser line. The unwanted mode or modes, even if very faint and present only occasionally, can impede heterodyne measurements where a clean frequency spectrum is required. The authors report measurements of transverse-mode beat frequencies from a small RF-excited CO₂ laser with a hybrid Al/Al₂O₃ square-bore waveguide and plane mirrors; these frequencies alter with mirror tilt by up to 70 MHz mrad^-1. A minimum beat frequency is seen when the laser is well assigned. The authors suggest that a model of the misaligned resonator may help to identify the faint modes responsible for hooting     

Measurement of the statistics of DFB laser frequency fluctuations

The statistics of the frequency of the bandpass filtered beat signal between a DFB-laser and an external cavity laser were measured. The distribution of frequency fluctuations from the centre frequency is shown to be very close to Gaussian, with the variance given by the product of beat signal linewidth and IF-filter bandwidth     

Linewidth, tunability, and VHF-millimeter wave frequency synthesis of vertical-cavity GaAs quantum-well surface-emitting laser diode arrays

The authors report the measurement of the laser linewidth, wavelength tunability, and generation of microwave frequencies between individually addressable elements of a vertical-cavity GaAs quantum-well surface-emitting laser diode array (lasing in the wavelength range 850-865 nm). Using heterodyne techniques, the authors obtain a deconvolved 65 MHz laser linewidth from the 109 MHz beat signal. The laser linewidth corresponds to a semiconductor laser linewidth enhancement factor alpha =5.7, which is in excellent agreement with that obtained independently from optical gain measurements and corresponding calculated refractive index changes. The authors measured heterodyne beat frequencies of 2-20 GHz. The bandwidth was limited by the microwave amplifiers. A simple calculation shows that a tuning range of 65 MHz to 3 THz can be achieved.<>     

DBR光纤激光器拍频传感及其组网技术研究

研究了基于单纵模分布Bragg反射(DBR)光纤激光器的拍频解调传感技术,并对其组网技术进行了探讨。由于光纤本身的固有双折射,DBR光纤激光器能够产生两种正交偏振模式的激光,而这两种偏振模式的频率差拍会在射频域形成对温度变化极为敏感的拍频(beatfrequency)。实验结果表明:在20～100℃的温度范围内,激光器拍频随温度的增加呈线性减小,其线性拟合度高达0.99981,拍频灵敏度为0.51063MHz/℃。这种偏振式光纤激光器具有高输出功率、高信噪比、低解调成本和窄输出线宽等特点,有助于构建大规模的光纤有源传感网络。参考近年关于光纤传感网络的报道,提出一种有源光纤环形传感网络的构想。     

Performances of a frequency offset locked He-Xe laser system at 3.51 µm

A highly stabilized frequency offset locked He-Xe laser system was constructed for high resolution laser spectroscopy of H2CO [5_{1,5}(upsilon = 0) rightarrow 6_{0,6}(upsilon_{5}= 1)] at 3.51μm. It is composed of three He-Xe lasers. The first laser is H2CO-stabilized and is used as a frequency reference in the system. The second laser is frequency offset locked to the first laser by using the beat frequency between these lasers, and is used as a local oscillator. The third laser is frequency offset locked to the second laser, and is used to observe the H2CO spectrum by slowly varying the beat frequency between these lasers. The frequency stability of the first laser, measured against a similarly stabilized and synchronously modulated laser, was1.0times10^{-14}attau = 100s, where τ represents the integration time. The frequency traceability of the second laser to the first laser was expressed as8.0times10^{-13} cdot tau^{-1}for 10 msleq tau leq 100s. It was found that this value of the traceability was independent of the frequency modulation of the first and second lasers. The frequency traceability of the third laser to the second laser was nearly equal to that of the second laser described previously. The variable range of the frequency of the third laser was 19 MHz. In this range, the frequency traceability of the third laser to the second laser was independent of the beat frequency between these two lasers. From these results, it was concluded that this system can be used for the observation of the H2CO spectrum.     

WDMA/subcarrier-FDMA lightwave networks: limitations due to opticalbeat interference

The authors present a general model of optical beat interference, and its contribution of channel outage in a WD-SFDMA (wavelength division-subcarrier frequency division multiple access) network. The probability of channel outage due to optical beat interference is determined from analysis and computer simulation for externally and directly modulated single-mode lasers. These results are used to calculate the ultimate limitation on the throughput and capacity of the network due to beat note interference, for a simple retransmission protocol. It is shown that, as more subcarrier channels are added, network capacity increases sublinearly and eventually saturates     

Fiber-coupling-operated orthogonal-linear-polarization Nd:YAGmicrochip laser: photothermal beat-frequency stabilization andinterferometric displacement measurement application

The Nd³⁺:YAG microchip laser is operated at orthogonal-linear-polarized two frequencies (λ=1.06 μm). A low crosstalk of 10^-4 is accomplished by means of the fiber-coupled laser diode pumping. The effect of another fiber-coupled laser diode irradiation on the beat frequency is investigated in terms of photothermally induced stress. The beat frequency of typically 100 MHz is stabilized within ±1 MHz by means of the photothermal feedback control of the cavity stress. The utility of the laser in heterodyne interferometric displacement measurement with a good linearity and high resolution is demonstrated     

A system for measuring absolute frequencies of up to 4.25 THz using a Josephson point contact

A system for measuring the absolute frequency of a far-infrared (FIR) laser is described. Josephson point contacts have been utilized in the system as a frequency harmonic mixer connecting microwaves and optically pumped CH₃OH laser lines. The Josephson point contacts are capable of generating beat signals of 90 GHz microwaves and FIR waves of up to 4.25 THz. To measure the frequency of the beat signals from the Josephson junction with a frequency counter, tracking oscillators have been developed, which tracks the beat signals by phase locking and regenerate clean signals for frequency counting. It is shown that the absolute frequency can be measured to an accuracy of about 100 Hz by using the tracking oscillators.     

Analyses of numerical and experimental results for optical signal generation using heterodyne scheme

In this paper, we have proposed a heterodyne technique to generate an optical millimeter-wave signal for ultra-wideband communication. First, we have investigated the characteristics of semiconductor lasers locked to another semiconductor under the RF modulation having many sidebands. The RF-modulated master laser is represented by a series of Bessel functions. This model is then inserted into Lang's rate equations. By numerically solving the resulting rate equations, we have determined the locked laser output characteristics as well as the RF spectrum of the beat signals. The result is that the unselected sidebands can produce undesired beat signals whose power may be comparable to that of the desired beat signal. Furthermore, their strength is affected by the injected ML light power. With reduced ML light, undesired beat signals and the injection-locking bandwidth can be suppressed. Second, we have experimented a new technique for generating millimeter-wave signals from a semiconductor laser. A 32 GHz signal is generated using a multisection semiconductor laser operated under a continuous wave by injecting optical pulses at a repetition rate equal to the fourth subharmonic (8 GHz). The generated millimeter-wave signal exhibits a large subharmonic suppression ratio, a large frequency detuning range, low levels of phase-noise and a large locking range. These simulation results are confirmed by experimental results. The high-frequency signal can be used in the field of ultra-wideband communication employing local multipoint distribution system (LMDS), wireless local loop (WLL) and mobile broadband system (MBS).