千赫兹量级窄线宽半导体激光器研究进展

窄线宽半导体激光器由于其高单色性、低频率噪声、高可调谐性等优点,广泛应用于高速相干光通信、分布式传感、激光雷达等领域.随着高品质因子(Q)光学谐振腔、硅光异构集成芯片等技术的发展,窄线宽半导体激光器近十年经历了革命式发展,线宽压缩至千赫兹(kHz)量级,甚至到亚千赫兹量级.文章阐述了千赫兹量级窄线宽半导体激光器的最新进展,针对不同压缩线宽机制的窄线宽激光器进行了分类介绍,深入讨论了优化耦合系数、减少外腔损耗等对窄线宽激光器性能的影响,并针对未来应用需求展望了千赫兹量级窄线宽激光器在进一步压缩线宽、提升输出光功率方面的发展方向.     

Frequency noise reduction of visible InGaAlP laser diodes bydifferent optical feedback methods

The emission spectra of several commercial InGaAlP laser diodes operating in the visible range were investigated. Strong relaxation sidebands at frequency spacings up to 2 GHz were observed in the field spectra. By using optical feedback from an external high-finesse resonator, the laser line-width was reduced from a few hundred megahertz to less than one megahertz. However, only a small part of the laser power was found within the narrow bandwidth, whereas most of the power finds itself in relaxation sidebands a few gigahertz apart from the carrier. The frequency noise reduction was investigated by using an external cavity configuration. With a short cavity length the relaxation oscillation sidebands were completely suppressed, and the laser linewidth was reduced to a few megahertz. Additional strong reduction in the laser linewidth to roughly 50 kHz was obtained using a combination of the external cavity laser with optical feedback from an external high-finesse resonator     

Stable single-mode compound-ring erbium-doped fiber laser

In this paper we report the fabrication of a robust, tunable single-longitudinal-mode compound-ring Er³⁺-doped fiber laser. The laser is fundamentally structured on an all-fiber compound-ring resonator in which a dual-coupler fiber ring is inserted into the main cavity. When combined in tandem with a mode-restricting intracavity tunable bandpass filter, the compound-ring resonator ensures single-longitudinal-mode laser oscillation. The laser can provide up to 20 mW output power over much of its 1525 to 1570 nm wavelength tuning range. The short-term linewidth of the laser output measured is less than 5 kHz. The laser frequency is actively stabilized for a long time by exploiting the inherent filter nature of the compound-ring resonator. The reduction of an associated frequency modulation is also discussed in detail     

Propriétés spectrales d’un laser semiconducteur soumis à une rétroaction optique externe

The rate equations of a semiconductor laser with external optical feedback are solved numerically. The method is applied to the case of simple optical feedback and to that of a high finesse external resonator. The spectral linewidth reduction, the damping of the relaxation oscillations and the influence of the resonant frequencies of the external resonator are discussed and compared for these two configurations.     

有源光纤环形腔的频率滤波特性

本文首先从理论上通过简单的分析得以了有源光纤环形腔的输出谱密度函数公式，该式清楚地表明光纤环形腔的频率滤波特性。从该公式出发，详细分析了环形腔输入光纤宽和环形腔参数对输出光谱谱型和线宽的影响。最后，通过设计不同的环形腔的自由谱宽和不同线宽的激光器作为输出光源，对上述理论分析做了实验验证。理论和实验表明，由于环形腔的强度传递函数是多峰的响应函数，环形腔自由谱宽和输入激光线宽的大小直接影响到环形腔输出光频谱形状，即是单峰输出还是多峰输出，对于不同形状的光谱输出，输出激光的线宽受环形腔的参数影响也各不相同。     

高稳定度窄线宽激光器的研究

介绍了3种不同类型的高稳定度窄线宽激光器的研究进展.基于Littman结构和饱和吸收光谱稳频技术,研制了稳频外腔半导体激光器系统,输出波长为780.2 nm,频率稳定度1 MHz,不失锁时间大于12 h.利用边带稳频技术将分布反馈(DFB)激光器的输出波长稳定在Cs原子的吸收谱线的边带处,引入数字信号处理器(DSP)全数字稳频控制技术,实现了自动找频和稳频,获得波长为852.3 nm的稳频激光输出,24 h内频率漂移为±2 MHz.利用国产磷酸盐玻璃光纤作为增益介质,实现了一台高功率单纵模光纤激光器,制作的厘米级激光器实现了最大输出功率100 mW,利用外部光反馈实现单偏振运转,测得输出线宽为2 kHz,偏振消光比优于35 dB.     

有源光纤环形腔进行频率调谐窄线宽激光的方法

本文提出用有源光纤环形腔来获得精确调谐窄线宽激光的新方法，理论分析结果表明：幅值（或相位）调制产生的频宽扩展对腔带宽的影响很小，滤波输出激光的线宽约等于入射光随机初相位决定的线宽，有源腔对滤波输出激光还有光放大作用。     

Phase variations due to dispersion in inhomogeneously broadened lasers

One-dimensional analytical solutions for irradiance and phase are obtained for a two-level ring resonator model of a long inhomogeneously broadened chemical laser with strong velocity changing collisions (momentum cross relaxation). The solutions give the longitudinal mode spectrum of the laser line in terms of the ratio of elastic to inelastic collision rate, the Doppler linewidth, the product of small-signal gain times the length of the gain medium, and the resonator loss. The intensity is given in terms of the width of the mode spectrum, the saturation intensity, and the gain-length product. The phase in a single pass is given in terms of the mode spectrum, the loss, and the ratio of homogeneous to inhomogeneous width. For typical conditions the phase difference relative to line center for a detuning frequency near the edge of a laser line is less than about one twentieth of a wavelength for a single pass through the resonator. Application to unstable resonator geometries suggests that the total phase difference at the edge of such a laser may reach a few tenths of a wave, although most of the power in the line has a much smaller phase variation.     

激光器线宽对光纤环形谐振腔谐振特性的影响

采用光波场叠加的方法计算了谐振腔光纤陀螺的核心敏感部件———光纤环形谐振腔 (FRR)的传递系数 ,详细分析了激光器光谱线宽对光纤环形谐振腔谐振特性的影响 ,并进一步分析了在一定激光器光谱线宽条件下 ,由光探测器散弹噪声所限制的谐振腔光纤陀螺极限灵敏度和光纤环形谐振腔光路参数之间的关系 ,从而为谐振腔光纤陀螺的优化设计提供了理论基础     

Stabilization of millimeter-wave frequencies from passivelymode-locked semiconductor lasers using an optoelectronic phase-lockedloop

The use of an optoelectronic phase-locked loop to stabilize the pulsation frequency of a three-section, passively mode-locked quantum well laser diode at 41 GHz is discussed. It is shown that the free-running mode-locked signal, with a radio frequency (RF) linewidth of 770 kHz, can be stabilized to the linewidth of a reference RF oscillator (<1 kHz). The stabilized mode-locked signal has a phase noise of -70 dBc/Hz at 370 kHz offset. Tracking of the mode-locked signal to the external reference RF oscillator is maintained over 11 MHz. This is, in effect, an actively mode-locked laser source at 41 GHz     

Spectral properties of a semiconductor laser coupled to a single mode fiber resonator

The effect of optical feedback from a single mode fiber resonator on the instantaneous frequency fluctuation spectrum and on the field fluctuation spectrum of an AlGaAs semiconductor laser is studied theoretically and experimentally. Single mode fiber resonators of different lengths and finesses are used for the experiments. The minimum linewidth conditions are given and it is shown that the linewidth reduction ratio increases with the fiber resonator selectivity. The limit for multistability operation depends on the resonator finesse. Theory and experiments are found to be in good agreement. Finally, the effect of resonator loaded semiconductor laser phase noise on BER performance in optical DPSK heterodyne systems is theoretically analyzed.     

一种可降低Littman-Metcalf光栅外腔半导体激光器衍射损耗的结构设计

基于原有Littman-Metcalf型光栅外腔半导体激光器的工作原理,设计了一种可以降低衍射损耗的外腔结构。在Littman-Metcalf结构的基础上增加一个反射镜,将闪耀光栅二次衍射产生的零级衍射光反馈回半导体激光器本征腔。推导了新结构模型外腔损耗的表达式,通过等效腔的概念对两种结构激光器的外腔损耗、阈值电流、输出线宽以及输出功率进行了仿真分析。结果表明：将二次衍射产生的零级光反馈回有源区可有效降低Littman-Metcalf结构激光器的外腔损耗,提高了系统的耦合效率,从而降低阈值电流,提高了激光器的输出功率。同时,由于提高了外腔反射效率,该外腔结构进一步压窄激光器的输出线宽。对影响低损耗Littman-Metcalf外腔激光器输出线宽以及输出功率的因素(端面反射率、内外腔长、闪耀光栅衍射效率以及反射镜反射率等)也进行了仿真分析,为后期激光器制作提高了理论指导。     

Laser frequency control using an optical resonator locked to anelectronic oscillator

A new method for controlling a laser's center frequency is reported. This method extends the Pound-Drever-Hall technique, which delivers exceptional laser linewidth. The new technique, applicable over a broad range of wavelengths, produces a laser beam with both narrow linewidth and stable center frequency. A diode-pumped ring laser is locked to an ultrahigh-finesse, tunable optical resonator which, in turn, is locked to an electronic (5 MHz) oscillator using a difference frequency developed by locking a second laser to the resonator. A 1064-nm beam is reported with linewidth less than 0.004 Hz and center-frequency stability given by the electronic reference. For the case of an oven-controlled quartz crystal, the laser center-frequency stability is less than 1 part in 10¹¹ per day with root Allan variance of 10^-12 for a 1000-s time delay. The entire system is compact, and all elements are solid-state, making it useful both in the laboratory and in a number of space-based applications     

Theory of active mode locking of a semiconductor laser in an external cavity

An analytical treatment is given for the active mode locking of a semiconductor laser in an external resonator. The width of the mode-locked pulses is obtained as a function of the laser and cavity parameters and the amount of frequency detuning. The effects of self-modulation and saturation are included in the treatment. The pulse output is compared with that obtained by a strong modulation of the laser diode with no external cavity.     

Multi-channel laser interferometer based on automatic frequency stabilization system for improving coordinate measurement accuracy

A multi-channel laser interferometer (MCLI) is proposed to improve the coordinate measurement accuracy. A 780 nm external cavity laser is locked on the D2 line of 87Rb atom by polarization spectroscopy, and a high frequency stabilized laser source is obtained with a linewidth of 385.8 kHz at root mean square (RMS). The interferometers share the stabilized source and individually install on 4 axes of a coordinate measuring system. As a result, the measurement uncertainty is reduced from 1.2 µm to 0.2 µm within the dynamic measurement range of 1.0 m. The MCLI is adept at integrate and flexible installation, which caters to various applications on precision measurement.     

Novel measuring method for spectral linewidth of laser diodes using fibre-optic ring resonator

A novel measuring technique for the spectral linewidth of a laser diode was developed by using a fibre-optic ring resonator. Although the method is simple, a measuring accuracy within ±10% was obtained in the wide spectral linewidth range from 1 to 100 MHz.     

The effect of side modes with linewidth and intensity fluctuationsin semiconductor lasers

The influence of side modes on the linewidth and intensity fluctuations of the main mode is studied using Monte Carlo techniques. Explicit analytic relations are developed for the side-mode induced linewidth, using impulse responses of the laser obtained from rate equations. It is found that both the absolute level of the main-mode too side-mode power ratio and the passive loss in the side mode influence the main-mode linewidth. A typical 3-mW laser has negligible linewidth enhancement from the side mode if the modal power ratio is about 300/1 (or greater), corresponding to a side-mode loss of 0.2-0.5 dB (or more) for one round trip within the laser resonator. Lower side-mode losses yield an abrupt catastrophic increase in main-mode linewidth     

Semiclassical theory of noise in multielement semiconductor lasers

We present a derivation of the noise spectra of multielement semiconductor lasers. We model the noise by a set of Langevin sources which drive a system of small-signal field equations. The Langevin sources are normalized to transition rates within the laser and general formulas for relative intensity, frequency fluctuation, and field spectra are produced. We evaluate the formulas for several specific cases of interest, including those of a passive-active resonator and active-active coupled cavity resonator. In each case, the linewidth is governed by effective α-parameter(s) which generally differ from the material parameter. In the active-active cavity, the linewidth consists of two parts, one which is similar to the Schawlow-Townes linewidth, and a second which is proportional to the FM modulation index.     

浅析元件参数对外腔半导体激光器输出谱宽影响

半导体抽运碱金属蒸气激光器（DPAL）需大功率窄线宽泵浦源,但市售半导体激光器输出线宽远远大于碱金属原子吸收谱宽,难以实现有效泵浦,因此需采用Littrow 外腔法压窄半导体激光器输出谱宽。Littrow 外腔系统中元件参数的选择直接影响大功率半导体激光器输出谱宽。为此文中沿入射光线方向构建外腔压窄模型,利用球面镜替代柱面镜,分析了微柱透镜阵列、光学系统和光栅元件对外腔输出谱宽的影响,模拟结果为微柱透镜阵列焦距越小、光栅刻线越密、球面镜焦距越大,外腔输出谱宽越窄,实验结果符合理论模型。     

Electrooptic narrow linewidth wavelength tuning and intensity modulation of an erbium fiber ring laser

We report an electrically tunable fiber laser featuring a 50-channel wavelength switching capability with a 0.2 nm linewidth and -20 dB crosstalk. The laser consists of an Er-doped fiber resonator and an electro-optic TE-TM convertor used as a wavelength tunable filter. It allows wavelength tuning over a spectral range of 10 nm with a tuning rate of 0.05 nm/V and a linewidth of 0.06 nm.