Gain saturation in fiber distributed feedback lasers

Systematic numerical studies of the nonlinear operation of a fiber distributed feedback (DFB) Nd:YAG laser are presented. An approximate formula relating small-signal gain to output power and the parameters of the structure for this kind of laser is derived. Characteristics revealing saturation effects for weak and strong coupling regions are presented. The fiber DFB structure provides the discrimination between the different kinds of transverse modes. An interesting behavior of the laser characteristics is revealed. For large coupling strength the longitudinal mode spectrum is inversed to the one observed at the threshold or to the one above the threshold for weak coupling. This mode inversion strongly depends on the output power level and the normalized loss coefficient. Similar phenomena are observed for the transverse mode     

Simulation of the frequency behavior of external-cavity semiconductor lasers

A theoretical model of semiconductor lasers mounted in external cavities is developed. The modes' frequencies and gains are calculated. We study the frequency of the oscillating mode versus the laser injection current. We explain its hysteresis behavior by introducing in the model the coupling between the semiconductor gain and its index of refraction induced by fluctuations in carrier density. We also determine the influence of the laser diode output facet reflection coefficient on mode hops and frequency tuning. Theoretical simulations coincide well with experimental observations.     

A rigorous analysis of tunable semiconductor lasers with built-ingrating-assisted vertical coupler filter

A general theoretical model is established and analyzed for a tunable semiconductor laser with a built-in co-directional grating-assisted vertical coupler filter. Possible internal reflection at the gain-coupler junction is included. A rigorous coupled-mode formulation based on the composite modes in the absence of the grating perturbation and a mode-matching method at the waveguide junctions is combined with multimode rate equation analysis. The dispersion of the semiconductor material is also included in the model. The accuracy and the scope of validity of the model are verified for both weakly and strongly coupled filter structures. Simple expressions for equivalent mirror reflectance, lasing wavelength, threshold gain and side-mode suppression ratio (SMSR) are derived and typical examples examined. The effects of the filter and the internal reflection on the longitudinal mode selectivity are discussed     

Rotation of polarization axis of injection lasers with viewing direction

It is shown theoretically that in lasers with box modes and polarization determined by facet reflectivity, the axis of polarization rotates with viewing direction. For thin double-heterojunction (DHJ) cavities, the angle ρ between the junction plane and theEfield of the radiation depends on the angles θ and φ in the junction plane and transverse to it,rho approx 10^{-2}thetaphiin degrees. It is suggested that this behavior is a useful test for control of polarization by facet reflection. Measurements show the relation is followed in wide sawed lasers with good box modes prepared from a thin DHJ wafer and in a narrow stripe laser from the same wafer. A second narrow stripe laser from the wafer and a single-heterojunction (SHJ) cavity have no such dependence. Qualitative discussion of the measurements reveals information about polarization and mode properties not previously available.     

外腔锁相激光二极管阵列的一般分析

在不针对特定系统参量并考虑激光二极管阵列(LDA)前端面的剩余反射的情况下。对外腔锁相激光二极管阵列进行了一般性分析。在只考虑相邻发光单元间互相耦合及单元自耦合的近似下，把耦合参量表示为模和幅角的形式以便于分析各阶超模的阈值条件。对于和前端面的剩余反射系数有关的外腔锁相激光二极管阵列各阶超模的阈值，解析证明了阈值最低的模式或为最低阶超模，或为最高阶超模。说明了存在一些分立的外腔长度范嗣可以使激光二极管阵列运行于最低阶超模或者最高阶超模，这些结果和模式耦合理论的结果一致。     

Oxide defined TJS lasers in InGaAsP/InP DH structures

Transverse junction stripe lasers in the InGaAsP-InP systems have been fabricated by Zn diffusion through oxide windows into DH structures of InGaAsP/InP with all epitaxial layers n-type. The quaternary layer composition is such that room temperature laser emission is at 1.18 μm. TJS laser mode behavior with accompanying single longitudinal mode operation and kink-free light-output characteristics are seen at 77 K. As the temperature is increased, this behavior persists up to about 130 K, when parallel electron injection through the p-n junction in the InP layers becomes so large that, in parallel with the TJS laser filament, a normally operating DH laser filament starts operating. At higher temperatures only the latter is seen to operate.     

Closed form theory of multicavity reflectors and the output power of external cavity diode lasers

A closed form solution for the transmission and reflection coefficients of a double cavity Fabry-Perot resonator is given. The explicit expressions for these coefficients are used in a formula to give the output power of diode lasers coupled to multiple external cavities. Analysis of a cleaved or etched coupled-cavity laser shows that stable operation depends on proper choice of the phase length of both the gap and the control section. A study of a diode laser coupled to an external waveguide containing a Bragg reflector shows that for correct choice of grating reflectivity only modestly effective antireflection coatings (5 percent on laser facet and 2 percent on waveguide face) are required to allow the grating to dominate the operating wavelength of the laser diode. A single external cavity with loss coupled to a laser diode is also considered. In this case the theory indicates the necessity of proper control of loss or coupling fraction between diode and cavity for there to be effective control of wavelength.     

Dual mode coupling by square corner cut in resonators and filters

A method for realization of dual-mode coupling in rectangular waveguide cavities is described and analyzed. The method completely replaces the coupling screw, and therefore can be used to eliminate the need for tuning in dual-mode waveguide cavity filters. It also offers a wide range of coupling values and can achieve higher power-handling capability than coupling screws. Mode matching is used to calculate the mode chart of the infinitely long square corner cut rectangular waveguide (SCCRW), the field distributions of each mode, and the resonant frequencies of the cavity. An evanescent mode rectangular waveguide is used to provide dual-mode couplings between adjacent cavities. The junction discontinuity between the SCCRW and the rectangular waveguide is modeled by a double mode-matching method, yielding modal scattering parameters of the junction. A four-pole dual-mode elliptic-function rectangular waveguide cavity filter using the coupling method was constructed. The experimental filter results showed excellent agreement with theory     

Threshold and modal characteristics of composite-resonator vertical-cavity lasers

We report the operation of a vertical-cavity surface-emitting laser (VCSEL) with two active, coupled optical cavities. In contrast to a conventional single-cavity VCSEL, the coupling between the two electrically independent active regions produces emission into two longitudinal modes. The light output versus current characteristics are presented under various biasing conditions in each cavity. Laser operation is achieved with current injection into only one cavity, and additional current injected into the other cavity changes the effective threshold current measured. The longitudinal and transverse modes are characterized, and it is found that the modal evolution of the laser can be modified by current injection into each cavity.     

Characteristics of a grating-external-cavity semiconductor lasercontaining intracavity prism beam expanders

A grating tuned external cavity semiconductor laser incorporating a gradient-index rod lens and a pair of intracavity silicon prism beam expanders was constructed using a configuration aimed at simultaneously achieving the following objectives: the ability to operate at any external cavity longitudinal mode without tuning gaps caused by residual resonances of the laser diode cavity, stable feedback coupling between the diode and external cavities, narrow optical linewidth, and a high degree of external cavity longitudinal sidemode suppression     

Detecting mode hopping in semiconductor lasers by monitoringintensity noise

Mode-hopping semiconductor lasers exhibit intensity fluctuations which are correlated to the level of mode-hopping activity. It is shown that these fluctuations occur at a level that is easily measured and that mode hopping can be detected with a p-i-n photodiode and an AC voltmeter having microvolt sensitivity. A plot of intensity fluctuations versus laser case temperature and injection current displays periodicities in the conditions under which mode hopping occurs. These regularities are explained in terms of the peak gain wavelength passing the longitudinal mode wavelength as temperature changes. The occurrence of mode hopping is determined by junction temperature (which determines the relationship of the gain peak with longitudinal mode structure) and injection current (which determines mode-hopping frequency). The junction temperature is itself a function of case temperature and injection current. The stability map shows that precise control of the operating parameters is not sufficient to avoid mode hopping. It is necessary to have these parameters set properly     

Longitudinal mode-switching dynamics in a dual external-cavitylaser diode [optical neural network]

We study the potential speed of an optical neural network that uses the longitudinal cavity modes of an external-cavity laser diode as neurons. For this purpose, we used a laser diode coupled to two external cavities, each corresponding to one longitudinal cavity mode. The process of longitudinal mode switching is investigated for the case of intracavity optical modulation. In this experiment, the feedback for the mode in one cavity is modulated, and the length of the other cavity can be controlled. Three limitations are imposed on the switching speed. A number of external-cavity round trips are needed to switch from one mode to the other. It is observed that, depending on the amount of optical feedback in both cavities, between 7 and 21 round trips are needed. When the experimental results for varying cavity length are extrapolated to zero cavity length, a residual delay of a few nanoseconds remains. It is believed that this delay is due to a change in carrier density, needed to switch from one mode to another. Modified rate equations are used to model our experiments. The results of numerical simulations are in good agreement with the experimental results and predict the residual delay. The model also predicts a turn-on delay that is related to relaxation oscillations and imposes a third limitation on the operation speed of our optical neural network. Implications of our findings on the potential operation speed of the optical neural network are discussed and suggestions are made for optimization     

High power operation of phase-shifted DFB lasers with amplitude modulated coupling coefficient

The performance characteristics of amplitude modulated coupling (AMC) phase-shifted MQW DFB lasers emitting near 1.55 mu m are reported for the first time. The AMC is obtained by the holographic double-exposure technique, it provides single longitudinal mode oscillation up to 78 mW for I=500 mA. This result confirms that AMC flattens the power distribution along the laser cavity. Linewidths as low as 600 kHz have been obtained for cavities as short as 600 mu m at P=15 mW.<>     

Theoretical analysis of longitudinal mode coupling in external cavity semiconductor lasers

The single longitudinal mode behavior in long external cavity semiconductor lasers is discussed. Experimentally, the laser exhibits a single frequency oscillation even for an external cavity length of 100 cm. The mode selectivity of a composite cavity is shown to be insufficient to explain the experiments. Longitudinal mode coupling in semiconductor lasers is found to arise from an interference effect between the modes on the interband transition probability of electrons. Mode coupling equations are derived, which indicates that the single mode oscillation in long external cavity semiconductor lasers is brought about when the coupling strength goes beyond a critical value. It is shown that the effect of the hole burning in the external cavity semiconductor lasers is similar to that in the solitary laser.     

光纤耦合模块中光纤焊接技术的研究

大功率半导体激光器列阵的光纤耦合模块对光纤焊接的要求很高,在焊接避免使用有机粘接剂和有助焊剂的金属焊膏,因为其在激光器工作时易挥发出有机物质。这些有机物会污染激光器腔面,致使激光器工作时腔面温度过高。附着在腔面的有机物就会被碳化,影响激光器的出光率,甚至还会导致激光器烧毁。文章介绍用电场辅助焊接的方法,使光纤在硅片的V型槽中固定,在焊接中不使用助焊剂和有机粘接剂,取得了良好效果,减少了激光器腔面的污染,从而提高了半导体激光器光纤耦合模块的寿命。测试结果表明,剪切强度最大可达35MPa。     

Intermodal tuning characteristics of an InGaAsP laser with opticalfeedback from an external-grating reflector

A report is presented on the spectral characteristics of one-side antireflection coated 1.3-μm InGaAsP lasers exposed to strong feedback from an external grating, when the center wavelength of reflection of the grating is tuned continuously from one solitary laser longitudinal mode to the adjacent lower frequency mode. The effect of different residual reflectivities of the coated facet on the tuning characteristics of the laser was investigated experimentally. Bistability in the tuning characteristic is explained using a graphical representation of operating conditions. The stability of single-mode oscillation is found to depend on the offset between the resonance frequency of the solitary laser and the frequency of maximum reflection from the grating. An offset between the oscillation frequency of the laser and the frequency of maximum reflection from the grating is shown to occur     

可调谐单纵模多波长光纤激光器的研究

为了实现对台阶高度和绝对距离等物理量的高精度干涉测量,采用在一个光纤激光器中构建多个激光谐振腔的方法,构建了能同时发出多波长激光的光纤激光器。每个激光谐振腔都利用掺铒光纤作为增益介质,利用光纤光栅作为波长选择元件,改变光纤光栅的布喇格波长,即可改变对应谐振腔的激光波长。各个激光谐振腔独立但部分重叠,在重叠区域利用光纤耦合器构成复合子腔,使每个激光谐振腔都是复合激光谐振腔,从而使每个激光谐振腔都发出单纵模激光。结果表明,该光纤激光器能同时发出功率和频率都稳定的多波长激光,且每个波长都是单纵模激光;在4h内,每个波长的波长稳定性优于0.01nm。该设计对可调谐单纵模多波长光纤激光器的研究是有帮助的。     

单纵模多环形腔掺铒光纤激光器及其稳定性

介绍了一种多环形腔结构(MRC)的单纵模(SLM)掺铒光纤激光器(EDFL).这种激光器通过在主环形腔中插入充当模式滤波器的三个长度不等的无源次级环形腔,并结合腔内光纤布拉格光栅(FBG)形成多环形腔掺铒光纤激光器结构,多环形谐振腔可保证激光器的单纵模输出。讨论了使激光器运行在单纵模状态的谐振腔理论。同时,为了提高系统输出的频率稳定性,采用外光注入方法有效地抑制了模式跳变和拍噪声,改善了输出谱特性。实验得到在1550.225nm处输出功率约3.6dBm,信噪比(SNR)>35dB的单纵模输出光,且测得线宽小于500Hz.     

散射相位函数的近似计算

用ＭｏｎｔｅＣａｒｌｏ方法模拟光在介质中的传输时，常常会面临散射相位函数的选取问题。本文对几种散射相位函数进行了分析和比较。不难发现，Ｃｏｒｎｅｔｔｅ等提出的近似公式是一个较佳的散射相位函数，但由这个公式不能直接得到散射角的表达式，我们在ＭｏｎｔｅＣａｒｌｏ模拟中，了提出了三种散射相位函数的近似方法，尽管三咱方法都取得了较好的效果，但比较而言，在保证计算精度和计算效率的条件下，方法三是散射相位函数     

DBR单纵模光纤激光器波长温度调谐

报道了采用DBR方式,利用8 mm的高浓度掺Yb3+单模光纤,实现了波长为1 064 nm的单纵模调谐激光稳定输出的实验结果。该DBR谐振腔有效腔长为16 mm,输出最大功率为7.4 mW,通过半导体制冷器温控改变谐振腔的温度,实现了0.824 nm的单纵模无跳模调谐。采用光纤外差法,并利用低损耗环形器和光纤反射镜倍增延迟线长度提升测量精度的方式,测量得到激光最大线宽为4.4 kHz。单纵模激光的弛豫震荡峰位于900 kHz处,其相对强度噪声为-110 dB/Hz,当频率大于1.5 MHz时相对强度噪声为-145 dB/Hz。