Investigation on the spectral characteristics of DFB lasers withdifferent grating configurations made by electron-beam lithography

The single-mode stability for distributed-feedback (DFB) lasers with various electron-beam-written grating configurations has been investigated theoretically and experimentally, for both as-cleaved and AR-coated lasers. Other laser properties interesting for coherent and multichannel communications systems, such as linewidth and tunability, have also briefly been investigated. Lasers with more sophisticated grating structures, such as an optimized multiple phase-shifted or a corrugation-pitch-modulated grating, did not exhibit performance significantly superior to that of λ/4-shifted DFB lasers with an appropriate coupling coefficient. Antireflection (AR)-coating of the end facets proved indispensable for obtaining a high yield for lasers with single-mode operation at high output power and for reducing the large chip-to-chip variation seen for the as-cleaved lasers. A theoretical investigation of the effect of end reflections on the stopband and of the problem of determining the coupling coefficient was also made     

Theoretical analysis of external optical feedback on DFB semiconductor lasers

The effect of external optical feedback on resonant frequency, threshold gain, and spectral linewidth of distributed feedback (DFB) semiconductor lasers is theoretically analyzed. The analysis applies to any type of laser cavity formed by a corrugated waveguide limited by partially reflecting facets. It is shown that the sensitivity to optical feedback on a facet is closely related to the power emitted through this facet. Numerical results on wavelength selectivity and on sensitivity to optical feedback are given for conventional DFB lasers having an AR-coated facet and for quarter-wave-shifted (QWS) DFB lasers with AR-coatings on both facets. Both laser types are found to be more sensitive to optical feedback on their AR-coated facet than Fabry-Perot lasers for lowkL. On the other hand, QWS-DFB lasers are found to be relatively insensitive to optical feedback for largekL.     

Coherent Array of Nonlinear Regenerative Fiber Amplifiers

A coherent array of high power fiber amplifiers (lasers) with a nonlinear refractive index is described and discussed. By adding reflectivity to the inputs of the fibers, each fiber laser is transformed into a regenerative amplifier, with an internal circulating power that varies depending on the round trip phase shift of the field within the fiber. The variation of the internal circulating power results in a preferential shift in the output phase of each amplifier due to the high nonlinearity within the fiber. The resulting phase shift is predicted to substantially compensate for the inherent randomness in the output phase distribution usually associated with the long lengths of fiber amplifiers. The individual outputs of the coherent array are shown to exhibit a ldquofuzzyrdquo phase distribution that can be partially quantified based on a statistical analysis and is dependent on the parameters of the array. A simple example is given of a coherent array of fiber lasers in a Self-Fourier cavity and the degree of coherence is predicted under simplifying assumptions.     

Characteristics of a semiconductor laser with external feedback

The theoretical analysis of the compound cavity of a semiconductor laser with external optical feedback is conducted. For large optical feedback, the output power from the laser and its oscillation frequency differ from those for small optical feedback. From the rate equations of the compound cavity, the conditions of the laser oscillation are derived in the presence of large optical feedback The dependencies of the output power and the laser oscillation frequency on the external-cavity length are investigated. Some new results involving laser oscillation depending on the external-cavity length are presented. The experimental results are compared with theoretical predictions. The dependence of the laser oscillation on the external-cavity length is qualitatively explained in the present model     

可用于差拍法珀干涉仪的633nm可调谐外腔半导体激光器

为更好地改善差拍法 珀干涉仪用于对纳米干涉仪进行非线性比对和校准场合的性能 ,提出用 6 33nm外腔半导体激光器代替He Ne激光器 ,并针对差拍法 珀干涉仪对工作激光器的要求 ,设计和实现了 6 33nm可调谐外腔半导体激光器。用前端镀增透膜的 6 37nm量子阱结构半导体激光器件及工作于Littrow自准直状态的光栅作为反馈元件 ,构成外腔激光器。实验结果表明 ,该激光器具有比较窄的光谱线宽 ;通过控温和旋转光栅 ,获得了覆盖 6 33nm的 4nm的调谐范围 ;通过控制频率调谐器 ,得到了 1 8GHz的连续调谐范围。     

Effects of feedback asymmetry in external-cavity semiconductorlaser systems

Presents the first systematic study of the effects of feedback asymmetry, as determined by misalignment of an external mirror, on the characteristics of external-cavity semiconductor lasers. Particular attention has been paid to output power, intensity noise and optical spectrum. The authors have observed and explained undulations in power as a function of external mirror tilt as well as regimes in which complex noise spectra occur     

Coherent optical frequency domain reflectometry usingphase-decorrelated reflected and reference lightwaves

We report a new scheme for coherent optical frequency domain reflectometry (C-OFDR) where the coherence length of the lightwaves does not limit the measureable fiber length. In this scheme, we use the beat spectrum which results when we mix reflected and reference lightwaves whose phases are not correlated. We demonstrated this scheme using a narrow-linewidth-lightwave source and an external electro-optical phase modulator. We measured Rayleigh backscattering and Fresnel reflections from a 30-Ion optical fiber, and achieved a spatial resolution of 5 m for two neighboring Fresnel reflectors located at the far end of the fiber. We estimated the expected spatial resolution and single-way dynamic range for our new scheme and show that it is capable of measuring long optical fibers with high-spatial resolution     

Nanosecond pulse generation in a passively Q-switched Yb-doped fiber laser by Cr/sup 4+/:YAG saturable absorber

We explored efficient nanosecond pulse generation in a passively Q-switched cladding-pumped Yb-doped fiber laser by use of an external-cavity configuration containing a Cr/sup 4+/:YAG saturable absorber crystal. By exploiting passive Q-switched regime and stimulated Brillouin scattering, pulses of durations as short as 2.7 ns, corresponding to a peak power of /spl sim/9 kW, have been achieved. The laser was tunable over 70 nm and green output power generation was obtained by external frequency doubling in a KTP crystal.     

Excess phase noise in self-heterodyne detection

The interpretation of self-heterodyne spectra is difficult if, apart from spontaneous emission, additional noise sources are presented. Measurements on an external-cavity semiconductor laser show how, for a relatively long delay, the high-frequency (Lorentzian) wings of the self-heterodyne spectrum are a sensitive measure for the quantum-limited (Schawlow-Townes) laser linewidth. The quantum-limited laser linewidth is shown to be inversely proportional to the output power. Values below 5 kHz are routinely measured. The full width at half maximum (FWHM) laser linewidth is larger than this due to excess low-frequency fluctuations, which are shown to result from the presence of side modes     

Frequency stabilization in FBG external cavity semiconductor laser based on acetylene absorption method

A frequency-stabilized 1.53 μm FBG external-cavity semiconductor laser by using acetylene absorption is presented and its basic principles are introduced. Graded refractive index fiber and pigtailed fiber are used in the absorption air chamber to enhance the coupling stability. The impact of the background power is eliminated by using the third-harmonic mode-locking technique. A lock-in amplifier is utilized to ensure that the output laser wavelength is locked at the C2H2 absorption line of 1530.37 nm. The frequency stability reaches 10^-8 within 24 h.     

High light output-power single-longitudinal-mode semiconductor laser diodes

Issues related to realizing high light output-power single-longitudinal-mode InGaAsP/InP DFB LD's have been studied experimentally and theoretically. Asymmetry in the reflections as the front and rear facets of DFB cavity has been found to result in enhanced single-longitudinal-mode selectivity and higher light output power from the front facet. Single-longitudinal-mode CW light output power as high as 103 mW has been obtained in a 1.3-μm DFB-DC-PBH LD with facet reflectivities of 2 and 80-90 percent. Low internal quantum efficiency is pointed out to be an important problem for 1.5-μm LD's.     

Instabilities and nonlinear L-I characteristics in complex-coupledDFB lasers with antiphase gain and index gratings

Complex-coupled DFB lasers can be designed to provide large bandwidths and low chirp by adjusting the strength and phase of their index and gain gratings. In this paper we develop a self-consistent method of calculating the coupling coefficient of complex-coupled DFB lasers with a corrugated active region. Basic geometrical and structural parameters are used as inputs to the model. We show that antiphase coupling implies lasing on the short-wavelength side of the Bragg wavelength which in turn leads to instabilities and/or nonlinearities in the light-current characteristic. The critical output power for onset of instabilities is typically in the order of a few milliwatts. Time-domain simulations are used to assess the potential effects of these instabilities on optical communication systems. We find that transitions from a state of equal output powers from the facets to a state of dramatically different output powers occur within a few nanoseconds of turn-on. The origin of these instabilities is explained using a simple physical model and possible ways of increasing the critical power for instabilities are discussed. For example, we clearly show that the critical power for instability increases when the carrier lifetime is decreased     

InP-based optically pumped VECSEL operating CW at 1550 nm

We present a vertical external-cavity surface-emitting laser operating at 1550 nm. The laser comprises an InGaAsP-based gain element, with a resonant periodic gain structure on top of a distributed Bragg reflector, and a high reflectivity spherical mirror as the external reflector. Optical pumping is achieved using a 1250-nm fiber Raman laser. A maximum continuous output power of 70 mW was obtained under multitransverse mode operation at 233 K. Under single-mode operation, we obtained a maximum power of 60 mW with a beam quality factor M/sup 2/ less than 1.1.     

平坦功率输出的宽带波长可调光纤环型激光器

为了获得一种平坦功率输出的宽带波长可调的掺铒光纤环型激光器,提出采用一高双折射光纤环镜获得其平坦功率输出,在高双折射光纤环镜中,采用了大量的高双折射光纤段和偏振控制器,它们的反射光谱可补偿掺铒光纤环型激光器输出功率光谱的不平坦,通过压缩或者延伸激光腔里的分布式光纤布喇格光栅即可实现波长调谐。实验验证可以获得一种宽达38nm的宽带波长调谐(1527nm～1565nm),输出功率的不平坦被控制在±0．8dB范围之内,总输出功率大约4dBm、3dB线宽为0．01nm、旁瓣抑制比为48dB的光纤环型激光器。     

利用全光纤自外差系统研究光纤光栅半导体激光器的线宽

以带光纤尾纤引出式微体声光移频器（ＡＯＦＳ）为基础,用全光纤结构的延迟自外差激光器线宽测量系统（分辨率为５０ｋＨｚ）,其中声光移频器插入损耗仅为２．９ｄＢ,大大提高了信噪比。采用该系统对窄线宽１．３μｍ光纤光栅半导体激光器（ＦＢＧ－ＬＤ）进行了测量,测得激光器线宽的典型值为１．５ＭＨｚ,同时分析了测量系统近端反射光对线宽测量精度的影响以及ＦＢＧ－ＬＧ的线宽压窄特性,最后验证了ＦＢＧ－ＬＤ 输出线宽     

掺镱双包层光纤激光器多波长输出性能分析

从理论上分析了由F-P滤波器实现的多波长双包层光纤激光器(DCFL)的边界条件和输出功率,在考虑了F-P滤波器中双色镜反射率的波长依赖关系的前提下,对影响多波长输出功率和消光比的参数如双色镜和光纤出光端面的反射率、抽运功率等进行了研究。结果表明,光纤出光端面的反射率存在一个最佳值,再配合适当的双色镜反射率,能使多波长输出的功率和消光比达到很好的均衡。适当增大抽运功率也能优化多波长输出功率和消光比。通过控制F-P滤波器的腔长,能够调节多波长输出的数目。     

External cavity controlled operation of a semiconductor diode gain element in series with an optical fiber

The series combination of a semiconductor diode gain element (a diode laser whose end facets have been antireflection coated) and an optical fiber has been placed inside an external cavity and the combined system has lased in a single spectral line whose width was less than the1.7 times 10^{-5}-nm (7.5-MHz) resolution of the scanning Fabry-Perot interferometer used in the measurement. This result has been achieved with either a single-mode or a multimode optical fiber in series with the diode gain element and for experiments in which a polarizer, oriented so its polarization is parallel to the diode gain element polarization, was placed at the other end of either fiber. The output of the external cavity is temporally stable as measured both by a 225-MHz-bandwidth detector system and a spectrum analyzer. Over a one minute interval the maximum fluctuation in the output frequency of the external cavity was found to be 2 MHz. Analysis of the threshold behavior of the external cavity for the cases where the multimode or single-mode fiber are in the cavity indicates that the coupling coefficients for either fiber are nearly the same. Without an external cavity, the coupling coefficient from the diode gain element (or from a similar, not anti-reflection-coated, laser) to the multimode fiber is over five times larger than that to the single-mode fiber. These coupling results are explained by postulating that for the multimode fiber, only one or a selected number of the many modes of the fiber can participate in the laser action of the external cavity. It is believed that only these mode(s) are reflected back into the fiber by the spherical mirror in appropriate phase and angle to participate in the laser emission.     

High light output-power single-longitudinal-mode semiconductor laser diodes

Issues related to realizing high light output-power single-longitudinal-mode InGaAsP/InP DFB LD's have been studied experimentally and theoretically. Asymmetry in the reflections as the front and rear facets of DFB cavity has been found to result in enhanced single-longitudinal-mode selectivity and higher light output power from the front facet. Single-longitudinal-mode CW light output power as high as 103 mW has been obtained in a 1.3-µm DFB-DC-PBH LD with facet reflectivities of 2 and 80-90 percent. Low internal quantum efficiency is pointed out to bean important problem for 1.5-µLD's.     

光纤激光阵列自组织相干合成的性能研究

研究了基于倏逝波耦合的自组织相干合成激光阵列。应用静态本征模/超模理论推导阵列模式;通过数值方法考察阵列激光系统的动态特性进而分析输出功率的稳定性、阵列的扩展能力和阵列模式;并对两种方法得到的阵列模式进行了比较分析。结果表明:输出功率的稳定性以及输出功率的大小受阵列的锁相性能和阵元间的相位差影响;随着激光单元数目的增加,阵列总输出功率会非线性增大。通过跟踪各个阵元的相位变化并考察阵列模式的近场和远场特性,初步验证了37单元阵列的相干合成,随着阵元数目的增加,中心阵元与其邻近层阵元的振幅差会逐渐减小。     

Line narrowing in a single-mode injection laser due to external optical feedback

The effect of external optical feedback on the linewidth of a single-mode injection laser is considered theoretically. A set of three rate equations with Langevin noise sources is used to obtain the power spectrum. If the high-frequency structure in the power spectrum is ignored, the line shape is Lorentzian and exhibits broadening or narrowing depending on the external-cavity phase shift. Particular attention is paid to the line narrowing after including the effect of carrrier-induced index changes.