大幅度增加弛豫振荡频率来实现毫米级外腔半导体激光器的外腔机制转换

混沌外腔半导体激光器输出明显存在弛豫振荡特征,弛豫振荡频率小于外腔振荡频率时,外腔半导体激光器输出态是短腔机制;反之,外腔半导体激光器输出态是长腔机制.首先对比分析了弛豫振荡频率为5.6 GHz,腔长对频谱有效带宽的影响.然后同时调节注入电流和载流子寿命来大幅度地增加弛豫振荡频率.最后在弛豫振荡频率为40 GHz、腔长为毫米级(4—20 mm)时,实现由短腔机制到长腔机制的转换,进而分析了外腔反馈率和外腔长对外腔半导体激光器频谱带宽的影响.分析结果表明:短腔机制下,输出混沌态不稳定,0.1 mm的偏差就会导致混沌态与非混沌态之间的转化;长腔机制下,输出混沌态稳定,输出混沌区域较大,证明长腔机制下更有益于获得宽带连续的混沌区域.在弛豫振荡频率为40 GHz、外腔长度为毫米级时,实现了外腔半导体激光器的长腔机制,从而增大了高带宽混沌的参数空间.     

Stability of an Er-Yb-doped fiber distributed-feedback laser with external reflections

The maximum tolerable amplitude backreflection coefficient r(ext,c) into a fiber distributed-feedback laser before the onset of instability has been investigated. r(ext,c) was found to decrease with increasing external cavity lengths up to ~320 m and to be proportional to the relative linewidth of the relaxation oscillation resonance. The tolerable length of Rayleigh backscattering standard telecom fiber was found to be 135-200 m. An observed degradation in the laser stability and slope efficiency at low pump powers is believed to be due to UV-induced saturable absorbers.     

Relaxation Oscillation Enhancement and Coherence Collapse in Semiconductor Lasers with Optical Feedback

The dynamic characteristics of a semiconductor laser with optical feedback are strongly dependent on the injection current and the reflectivity and position of the external feedback reflector. We investigated the relaxation oscillation enhancement and coherence collapse state of the laser oscillation based on the laser rate equations. It is well known that laser output power jumps with increase of the injection current due to external mode transition. But here for the first we time demonstrate the existence of a chaotic scenario within successive laser power jumps. The results calculated by numerical simulations based on the rate equations are compared with those of the experiments and good coincidence between them is found.     

The Study on the Performance of the Wavelength Conversion Based on a Tunable External Cavity Semiconductor Laser

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The Study on the Performance of the Wavelength Conversion Based on a Tunable External Cavity Semiconductor Laser

In this paper, the performance of the wavelength conversion based on a tunable external cavity semiconductor laser (ECL) was studied. The static wavelength conversion was achieved in the tunable wavelength range of the ECL. The dynamic wavelength conversions of 155 Mb/s and 622 Mb/s nonreturn to zero (NRZ) signals were realized. The simulation of the ECL output characteristics using modified compound cavity rate equation shows that the higher bit rate wavelength conversion can be realized by increasing the relaxation oscillation frequency of the ECL. When the modulated bit rate is much lower than the relaxation oscillation frequency, under a fixed input signal power, there is an optimal bias current for the wavelength converter based on ECL.     

Optical feedback in a Zeeman-birefringence HeNe laser and its application in ranging

The optical feedback characteristics in a Zeeman-birefringence HeNe laser have been studied systematically when the external cavity varies in a large range. The intensity variations of the two orthogonally polarized lights are observed during the laser cavity tuning in the dual frequency laser. When the external cavity length is an integral multiple of the laser cavity length, either polarized light's intensity modulation depth varies not obviously when the laser works in dual modes conditions or single mode condition. But when the external cavity length is far away from an integral multiple of the laser cavity length and the laser works in different dual modes conditions, polarized light's intensity modulation depth varies obviously. This characteristic can guide us to tune the laser to work in single mode condition to improve the sensitivity of the optical system when the system is used for a large displacement measurement. It can also be used for measuring the position of the target mirror. The theoretical analysis is in good agreement with experimental results.     

Short- and long-term frequency stabilization of a He-Ne laser using a Fabry-Perot cavity locked to the lamb dip

We have developed a frequency-stabilization system for the He-Ne laser using an external Fabry-Perot cavity which is locked to the Lamb dip of the laser. The cavity serves as a frequency discriminator for high-frequency noise suppression, while the length of the Fabry-Perot cavity is controlled, reffered to the Lamb dip, in order to eliminate low-frequency fluctuations. The frequency noise of a laser stabilized with the cavity suspended in a vacuum tank can be suppressed to about 1 $$0.1Hz/\sqrt {Hz} $$ at around 100 Hz by an FM-sideband locking method. The oscillation frequency is fixed to the Lamb dip at low frequency by controlling the cavity length thermally. The frequency fluctuation around the Lamb dip was estimated to be ～ 2.3 MHz.     

LD双端泵浦U型腔Nd∶YVO4激光器稳定性研究

 基于传输矩阵理论,针对端面抽运的U型腔Nd∶YVO4固体激光器谐振腔的稳定性进行了理论分析和数值模拟。通过改变“U”型腔各个臂长得到不同腔臂长条件下谐振腔的稳区范围和晶体中心处的振荡光斑,比较稳区范围以及振荡光斑,找出一组腔参数值使谐振腔能够在较高泵浦功率下保持稳定且振荡光斑足够理想。按照确定的腔参数值进行了具体实验,实验结果表明:按选定参数设计的谐振腔在调Q重复频率为100kHz,泵浦功率为60W时,激光器能够稳定工作并输出21.6W的高重频激光,谐振腔稳区范围的变化规律与理论计算基本吻合。     

Wavelength tuning characteristics of a fabry-perot semiconductor laser with an external short cavity

Wavelength tuning characteristics of a Fabry-Perot semiconductor laser with an external short cavity are analyzed, in which the oscillation wavelength can be changed by slightly altering the external cavity length. Analysis is based on rate equations for an optical power and carrier density. It is shown that the wavelength tuning range is dominated by a change of carrier density through the effect of carrier-induced refractive index change in the active layer of a laser diode. This depends on effective coupling coefficients of the optical field iteratively reflected back to the laser diode by the external mirror, and on reflection coefficients of an anti-reflection coated laser facet and the external mirror which compose the external cavity. The effective coupling coefficient is also derived using the waveguide theory and Kirchhoff ’s theory. Finally, an unstable condition which may limit a stable wavelength tuning range is shown by results of a linear stability analysis of rate equations.     

Multiwavelength Erbium doped fiber ring lasers

We demonstrate a multiple wavelength laser, employing wavelength dependent cavity losses, with up to four lasing wavelengths on the ITU-grid in the C-band and report on its stability characteristics. The linewidth of each wavelength was estimated to be less than 4 pm and we observed antiphase oscillations around 3 kHz and relaxation oscillations around 25 kHz. The antiphase oscillations in the multiwavelength laser lie on a limit cycle in a relative phase diagram. We present an analytic lumped model to determine the output power and relaxation oscillation frequency of the multiple lasing wavelengths in the Erbium doped fiber. The model is validated against our implementation of a multiwavelength ring laser.     

Self-sustained oscillations of turbulent pipe flow terminated by an axisymmetric cavity

Turbulent flow through a long pipe terminated by an axisymmetric cavity can give rise to self-sustained oscillations exhibiting a very strong coherence, as evidenced by the narrow-band character of corresponding amplitude spectra. These oscillations, associated with the turbulent axisymmetric jet passing through the cavity, are strongly influenced by the acoustic modes of the pipe. The frequencies of oscillation lie within or near the range of most “unstable” frequencies of the turbulent jet previously predicted by using concepts of inviscid hydrodynamic stability theory; consequently, these experiments show truly self-excited and strongly coherent “instability” of a fully turbulent, low Mach number (～10^?2), axisymmetric flow undergoing separation, corroborating previous experiments involving the external forcing of free turbulent jets. As flow velocity or cavity length is varied, both upward and downward jumps in oscillation frequency are observed; the sign (up or down) of these jumps tends to systematically alternate with increase of velocity or length. The role of these frequency jumps is, in effect, to allow the oscillation of the flow to remain “locked-on” to a pipe mode over a wide range of impingement length or flow velocity. Moreover, these jumps exhibit two types of behavior: for the first kind, the predominant frequency makes a relatively continuous transition between stages and the frequency of the neighboring stage appears as a secondary component; for the second kind, there is a dead zone (where no oscillation occurs) between stages. The consequence of externally exciting the system is strongly dependent on whether the self-sustaining oscillation is relatively near, or well away from, a frequency jump. During excitation, the amplitudes of pressure fluctuations in the cavity substantially exceed the corresponding no-flow values only in regions away from the frequency jumps; at locations of jumps, there can be significant attenuation of the no-flow excitation amplitude. For the type of frequency jump involving a “dead zone”, enhancement of a given mode of oscillation can be achieved by externally exciting not only the given mode, but also neighboring modes. For the other type of jump, involving a relatively continuous transition from one stage to the next, the predominant mode of oscillation following the jump is that mode giving maximum amplitude response to excitation before the jump.     

交叉相位调制提高半导体激光器混沌载波发射机带宽方法

提出半导体激光器混沌交叉相位调制(XPM)光反馈提高混沌载波发射机带宽方法,建立了有外腔光纤传输反馈XPM控制下的激光动力学物理模型.理论导出XPM作用下激光双反馈频率失谐公式,指出XPM产生的非线性相移影响了激光器增益和线宽增强因子,其光纤二阶非线性效应使激光振幅和相位变化更加丰富,而非线性相移的出现进一步增加了新的频率分量并使频谱展宽.数值结果表明,XPM使激光器混沌带宽增加到4倍以上,使激光混沌张弛振荡频率增加到2.85倍,其光纤长度、入纤光功率、面镜反射系数、光纤二阶非线性系数等都能影响激光混沌带宽.     

Observation of antiphase dynamics and harmonic resonance in a modulated dual-wavelength laser

We observe a nonlinear response of a dual-wavelength Nd:YAG laser when subjected to low-frequency periodic modulations of cavity losses. The modulation frequency is far from the relaxation oscillation frequency. The harmonic resonances of the two laser wavelengths associated with antiphase intensity oscillations are demonstrated and resonances up to the fourth order were observed. For relatively weak modulation, the intensity oscillation frequency of the laser is equal to the modulation frequency. Harmonic resonances occur under a stronger modulation. We find that more harmonic components appear when the modulation frequency is increased. Furthermore, with enhancing the modulation, the dominant frequency of the intensity oscillations of both wavelengths is shifted toward the higher-order harmonic frequency.     

Bifurcation and locking in an multi-quantum-well laser subjected to external injection

Bifurcation and dynamic stability as well as locking in an external light injection multi-quantum-well (MQW) laser are studied. Different dynamic regimes in locking diagram are analyzed with the injection level and frequency detuning. Bifurcate behavior is numerically simulated via the external injection light intensity, frequency detuning, current, linewidth enhance factor, photon loss rate and carrier loss rate, respectively. And the route to chaos from bifurcation, period-doubling and quasi-period are described by numerical analysis. A perturbation equation of four-dimension model and the bifurcation condition are demonstrated. Dynamic stability of the laser is theoretically and numerically analyzed. The bifurcate expression is theoretically given while the maximum locking frequency domain is given. The variational characteristic of the oscillation frequency in the self-pulse regimes versus the injection and detuning are numerically analyzed to find that the relaxation frequency is reduced with adding the detuning in the smaller detuning or with adding the injection in the smaller injection while the relaxation frequency will be increased with adding the detuning in the larger detuning or with adding the injection in the larger injection. We find also that the chaotic spectra are broadened with adding the detuning or narrowed with adding the injection or the current.     

Dual-wavelength external cavity laser with a sampled grating formed in a silica PLC waveguide for terahertz beat signal generation

We propose and demonstrate a dual-wavelength external cavity laser (ECL). In our design, a Fabry–Pérot laser diode (FP-LD) is hybrid-integrated with a sampled Bragg grating written in a silica planar lightwave circuit (PLC). The grating selects two laser wavelengths that share the same laser cavity. The dual-wavelength oscillation with a side mode suppression ratio >32 dB has been demonstrated experimentally. Experimental results indicate that the hybrid-integrated dual-wavelength ECL exhibits strong dual-wavelength emission corresponding to beat frequency of 1 THz. Simulations of this system also indicate good mutual coherence of the two modes and stability of the 1 THz beat signal. PACS 42.79.Dj; 42.60.By; 42.55.Px     

Dynamic characteristics in an external-cavity multi-quantum-well laser

This paper outlines our studies of bifurcation, quasi-periodic road to chaos and other dynamic characteristics in an external-cavity multi-quantum-well laser with delay optical feedback. The bistable state of the laser is predicted by finding theoretically that the gain shifts abruptly between two values due to the feedback. We make a linear stability analysis of the dynamic behavior of the laser. We predict the stability scenario by using the characteristic equation while we make an approximate analysis of the stability of the equilibrium point and discuss the quantitative criteria of bifurcation. We deduce a formula for the relaxation oscillation frequency and prove theoretically that this formula function relates to the loss of carriers transferring between well regime and barrier regime, the feedback level, the delayed time and the other intrinsic parameters. We demonstrate the dynamic distribution and double relaxation oscillation frequency abruptly changing in periodic states and find the multi-frequency characteristic in a chaotic state. We illustrate a road to chaos from a stable state to quasi-periodic states by increasing the feedback level. The effects of the transfers of carriers and the escaping of carriers on dynamic behavior are analyzed, showing that they are contrary to each other via the bifurcation diagram. Also,we show another road to chaos after bifurcation through changing the linewidth enhancement factor, the photon loss rate and the transfer rate of carriers.     

Wavelength Tuning Characteristics of a Vertival Cavity Surface Emitting Laser Diode with an External Short Cavity

The characteristics of a wavelength tunable vertical cavity surface emitting laser diode (VCSEL) with an external short cavity are analyzed, in which the oscillation wavelength can be changed over several tens of nanometers with a nearly constant optical power by slightly altering the external cavity length. Analysis is based on rate equations for the optical power and carrier density, taking the effect of carrier-induced refractive index change into consideration, together with the study of behaviors of a complex resonator. The reflection coefficient r₂ of a laser facet facing the external mirror is shown to affect notably the characteristics of wavelength tuning, optical power and carrier density for a change of the external cavity length. It is also noticed that the wavelength change for this length becomes slower with relatively larger r₂ due to an increasing contribution of the effect of carrier-induced refractive index change, within the optical gain spectrum of the laser diode.     

各向异性外腔微片Nd:YAG激光器调频回馈研究

研究了在各向异性外腔下微片Nd:YAG激光器调频回馈特性.作为对比,首先给出了在各向同性外腔下调频回馈结果,激光器输出功率的半波条纹数正比于外、内腔长度之比.在回馈外腔中内置一个四分之一波片,激光偏振态发生规律性跳变,每个纵模带宽内的偏振跳变次数正比于外、内腔长之比,相同外腔长下,回馈条纹再次倍频.进行了基于法布里-珀罗腔等效模型的理论分析,结果和实验相符合.此现象在精密测量领域有潜在应用. 关键词： 光回馈 调频回馈 各向异性外腔 半波条纹数     

Mode selectivity of multisegment composite-cavity semiconductor lasers

The single longitudinal mode (SLM) characteristics of narrow linewidth long-external-cavity semiconductor lasers has been improved significantly by using a multisegment composite-cavity configuration. In this paper, the mode selectivities of two-segment, three-segment and four-segment cavity semiconductor lasers have been analysed and compared. In experiments, a four-segment composite cavity laser exhibits SLM oscillation with side-mode suppression ratio of 30 dB even for total external cavity length of 75 cm. The oscillation linewidth is less than 200 kHz.     

Theoretical investigation into spectral characteristics of a semiconductor laser with dual-FBG external cavity

A new theoretical model of the dual-FBG external cavity based on the couple-mode theory has been proposed for controlling the output spectral characteristics of the semiconductor laser with dual-FBG external cavity. The output spectral characteristics have been studied through the analyses of the external cavity length, the FBG length and the fiber length between the two FBGs. The results reveal that the side mode suppression ratio (SMSR), the full wave at half maximum (FWHM), the longitudinal mode interval (LMI) and the stability of the central wavelength are mainly determined by the effective external transmissivity, the fiber length between the two FBGs and the external cavity length, respectively. Furthermore, the rising/falling tendencies of the SMSR and the FWHM are exactly opposite. There exist the maximum of the SMSR and the minimum of the FWHM in a semiconductor laser with dual-FBG external cavity. The increase of the fiber length between the two FBGs results in the exponential decrease of the LMI. The increase of the external cavity length leads to the better stability of the central wavelength.