Power fluctuations in the side modes of injection lasers

Small side modes which contain on the average only a few percent of the total output of injection lasers can cause high bit-error rates through power fluctuations. By measuring the RF spectrum and the bandwidth dependence of the statistical power distributions, we have clarified the nature of these fluctuations. We find that although a laser is driven above threshold, an individual small side mode behaves like a chaotic light source, i.e., a laser biased below threshold.     

Computer simulation of laser photon fluctuations: Single-cavity laser results

In this second paper of a series I present statistical results obtained from computer simulated experiments in single-cavity semiconductor lasers and show that the probability distribution for realistic lasers are much broader than the Poisson distribution. When the gain saturation parameter is properly chosen, the computer simulated probability distributions agree very well with published experimental results. The following is a summary of the results. 1) Only the longitudinal laser mode at the center of the gain peak shows a probability distribution whose peak coincides with the average photon number. The modes to either side of the central mode have noise-like character with probability maxima at zero photon number. 2) The photon probability distribution narrows with increasing power output, increasing cavity mirror reflectivity and decreasing cavity length. 3) For the photon probability distribution to approach the Poisson limit, the cavity mirrors must have reflectivity near unity and the cavity losses must be very low. 4) Two modes symmetrically located at either side of the gain peak have a probability distribution that is constant from zero photon number to a cutoff value, because the modes compete for power and fluctuate widely. 5) Pulsed single-cavity lasers do not turn on consistently in the same longitudinal mode even if they are biased above threshold. 6) The performance of coupled-cavity (C³) and distributed feedback lasers (to be discussed in a subsequent paper) is much better than that of single-cavity lasers.     

Partition noise in semiconductor lasers under CW and pulsed operation

A numerical model which simulates the mode partition noise in semiconductor lasers under CW as well as pulsed operation is presented. The noise levels in the individual modes under CW operation are in agreement with those predicted by an analytic small-signal model. Under simulated pulse modulation with pulse repetition frequencies of 633 MHz and 2.2 GHz, the probability density functions for the relative photon numbers in the individual modes are found. The data allow for an evaluation of the mode partition parameterk. For the central modes,kfactors are in the range of 0.31-0.42, depending on bias level and modulation frequency. Weaker side modes carrying less than 5 percent of the total output havekfactors smaller than 0.16.     

Amplitude fluctuations and photon statistics of InGaAsP injection lasers

We experimentally show that the statistical distribution of the light output from an injection laser can be adequately described by the superposition of a coherent electric field and narrow-band Gaussian noise. A technique for measuring these fluctuations in the time domain with a resolution of 120 ps is described. The results for several InGaAsP lasers show that intensity fluctuations in an injection laser are rather large, so that the probability of finding the output below 50 percent of the average power in a 120 ps time interval can be as high as 10^-5.     

Mode partition in semiconductor lasers with optical feedback

The authors point out that random noise processes induce mode partition fluctuations in semiconductor lasers. Mode partition depends on laser parameters and modulation current. However, external optical feedback can also increase mode partition noise. Here, a numerical solution of multimode noise-driven rate equations with time-delayed terms is utilized to investigate mode partition in semiconductor lasers with reflecting feedback. Photon statistics of the main and side modes in semiconductor lasers under both CW operation and dynamic operation are considered. Probability-density curves for the main and side modes are shown. The feedback-induced change of photon statistics of the main and side modes is clearly seen. Numerical results indicate that, if the laser used is exposed to reflections, a more stringent mode discrimination requirement for suppressing the buildup of laser-cavity longitudinal side modes may result. If mode discrimination is insufficient for avoiding the excitation of side modes, the feedback-induced power penalty depends on the fiber dispersion     

Computer simulation of laser photon fluctuations: DFB lasers

This fourth paper of the Series is devoted to photon fluctuations in the light output of distributed feedback (DFB) lasers. Statistics on laser photon fluctuations are collected from numerical solutions of noise driven rate equations. The approximate formula for the variance of the photon fluctuations, presented earlier [1], is also applicable to the DFB laser. Compared to the single-cavity injection laser, the DFB laser has the following features. 1) The order of magnitude of the photon fluctuations of the single lasing mode of a DFB laser is comparable to the fluctuations of the total power output of a single-cavity laser. 2) Contrary to single-cavity lasers, which cannot be made to Operate in a single longitudinal mode even under idealized conditions if they are driven by short current pulses, DFB lasers will readily deliver a single, pulsed mode. Thus, if thermal effects and backreflection of scattered light can be controlled, there will be no mode jumping during pulsed operation.     

Computer simulation of laser photon fluctuations: Coupled-cavity lasers

This third paper of the series deals with photon fluctuations in the light output of coupled-cavity lasers. As before, statistics on laser photon fluctuations are collected from numerical solutions of noise driven rate equations. Compared to the single-cavity injection laser, the coupled-cavity laser is found to have the following advantages. 1) The coupled-cavity laser can be made to operate in a single mode by driving one of its sections above and the other below threshold. For a given amount of output power from the high-current end, fewer photon fluctuations occur if it is the longer laser section that is driven above threshold. 2) The order of magnitude of the photon fluctuations of the single lasing mode of a coupled-cavity laser is comparable to the fluctuations of the total power output of a single-cavity laser. 3) Contrary to single-cavity lasers, which cannot be made to operate in a single mode if they are driven by short current pulses, coupled-cavity lasers will readily deliver a single, pulsed mode. Thus, if thermal effects and backreflection of scattered light (for example, from an attached fiber) can be ignored, mode jumping need not occur.     

Noise properties of semiconductor lasers due to optical feedback

It has been recognized for some time that reflections back into the cavity of an injection laser from a mechanically unstable external mirror cause noise in the output of the laser (reflection noise). In this paper, noise measurements are presented for index-guided lasers as functions of the output power, the degree of optical isolation and external cavity length (5-15 cm), while current modulation (using 2-4 mA at 50-200 MHz) has been used to reduce the noise levels by as much as 20 dB. It is demonstrated that the mechanism of noise reduction by modulation is the frequency modulation of the laser's spectrum and its interaction with the modes of the external cavity. It is shown that using an optical isolator can give a relative intensity noise of -110 dB while using modulation in addition can reduce this to -125 dB; the bandwidth performance of optical storage systems is discussed in terms of these two schemes.     

混合式CO_2激光器

众所周知,TEACO_2激光器可以在高气压下产生大体积均匀辉光放电,从而获得很大的脉冲输出能量和很高的峰值功率.但是,正是由于TEACO_2激光器工作在高气压下,因此不可避免地使激光介质的增益线型被气压大大地展宽,其增益线宽可达4千兆赫/大气压.如果这样的增益介质放在一个2米长的光学谐振腔(其纵模间隔为75兆赫)中,计算表明有20多个不同频率的纵模的增益系数之差小于5%,这样就可以有很多不同频率的纵模同时振荡,它们还会产生拍频,使TEACO_2激光器的脉冲输出呈现很强的调制.这在诸如脉冲激光雷达、光泵远红外激光器的光泵源、激光同位素分离、非线性光学等很多重要的领域都是不希望的.     

高功率横流CO2激光器的新型管板式电极

提出一种更适用于焊接等场合的低阶模输出的新型管板式电极。阳极块长边沿阴极管方向,提高了放电均匀性和稳定性,增加了有效放电长度;阳极沿气流方向的长度和阴阳两极之间的距离根据输出模式确定,可充分利用激活体积,提高选模激光光电转换效率;其放电均匀稳定,可减小限流电阻值,降低外电路电功损耗。与现有管板电极相比,可提高注入到激活体积内的电功率密度,明显提高选模激光输出功率和总体效率。多模输出时直接光电转换效率可达20%;低阶模TEM11输出时光电转换效率可达15%,激光器总效率达8%。该电极结构适用于国内生产的横流CO2激光器,在不同模式输出时均能获得较高的光电转换效率,从而使其更好地应用于焊接等低阶模输出场合。     

Mode-hopping noise in index-guided semiconductor lasers and its reduction by saturable absorbers

Mode-hopping noise in index-guided semiconductor lasers is investigated. It is found that random switching between lasing modes and output power differences in those modes cause mode-hopping noise. An effective method to suppress such mode-hopping noise is proposed. High Te doping to an n-type GaAlAs cladding layer completely suppresses the noise. Te in GaAlAs forms a DX center that acts as a saturable absorber. This property stabilizes the laser mode and prevents mode competition. The minimum loss difference between lasing and nonlasing modes to suppress mode-hopping noise is also discussed.     

Partition fluctuations in nearly single-longitudinal-mode lasers

A simple model of mode partition noise is developed, which is valid for semiconductor lasers operating CW, having a single active region and oscillating primarily in one dominant mode. It is shown that the intensity fluctuations in the nonlasing modes occur with characteristic times of a few nanoseconds and have exponential probability distributions. We show that the lasing mode is able to follow adiabatically fluctuations in the nonlasing modes in such a way that the total intensity remains constant. Our model quantitatively accounts for the observations of Linke et al. that the frequency of "dropouts" in the lasing-mode intensity decreases exponentially with the ratio of average intensities of the lasing and nonlasing modes, and that in digital communication applications a ratio of about 50 is required to reduce the resulting bit-error probability tolsim 10^{-9}. We also account for the probability distribution of the lasing-mode intensity, which was observed by Liu et al to be exponential at low intensity and Gaussian at high intensity. The exponential fluctuations constitute partition noise induced in the lasing mode by large Gaussian fluctuations in the field amplitude of the nonlasing mode. Additional noise sources associated with the carrier density and the lasing-mode field produce Gaussian behavior at high intensities plus a negligible broadening of the low-intensity exponential tail.     

On fluctuations and transients in injection lasers

A new computer model of the semiconductor injection laser is described. The electron-photon interactions are introduced using a Monte Carlo technique in combination with multimode rate equations to compute laser output versus time and to compute output power probability density. Laser turn-on initially in a side mode, followed by evolution to the normal equilibrium modal power distribution, is shown to be intrinsic and a natural consequence of fluctuations in the modal powers at the moment the laser passes through threshold. Calculations of fluctuations in the equilibrium power levels show that occasional dropouts of the main-mode power are also a natural consequence of electron-photon interactions. Reduction in equilibrium fluctuations and in turn-on fluctuations can be achieved by increasing the laser power-output level, by increasing the mirror reflectivities, and by decreasing the laser length. This work indicates that conventional injection lasers must be operated at or above the 1 mW level in order to assure an error rate of 10^-9in digital systems. The well-known transition from thermal-noise statistics below threshold to Poisson statistics well above threshold is still evolving in the 0.5-2 mW region in the injection laser.     

InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructuresquare ring lasers

The processing and optical characteristics of an InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructure square ring laser (4 μm width and 50-μm/side length) with two output waveguides and two narrow physical gap couplers are described. The ridges and the total internal reflection (TIR) mirrors of the square ring lasers are fabricated by two photolithographic steps with a single SiO₂ mask and planarized with polyimide such that the TIR mirrors are etched through the active region while the ridge waveguides are not. Single longitudinal mode operation is observed with a side mode suppression ratio of 20 dB. Asymmetric characteristics of emission spectra from the two output waveguides demonstrates that the square ring lasers operate in traveling wave modes     

Difference mode generation in injection lasers

The nonlinear generation of a difference mode in an injection laser is considered. A new design based on the InGaP/GaAs/InGaAs heterostructure is suggested in order to generate two laser modes with a wavelength of about 1 μm and a difference mode at a wavelength of about 10 μm. In lasers with a 100-μm-wide waveguide, the power output of the difference mode can be as high as ∼10 mW at ∼10 W in the short-wavelength modes. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 10, 2001, pp. 1256–1260. Original Russian Text Copyright ? 2001 by Aleshkin, Afonenko, Zvonkov.     

Influence of instantaneous mode competition on the dynamics ofsemiconductor lasers

Comprehensive theoretical investigations of the influence of instantaneous mode-competition phenomena on the dynamics of semiconductor lasers are introduced. The analyzes are based on numerical simulations of the multimode rate equations superposed by Langevin noise sources that account for the intrinsic fluctuations associated with the spontaneous emission. Numerical generation of the Langevin noise sources is performed in such a way as to keep the correlation of the modal photon number with the injected electron number. The gain saturation effects, which cause competition phenomena among lasing modes, are introduced based on a self-consistent model. The effect of the noise sources on the mode-competition phenomena is illustrated. The mode-competition phenomena induce instantaneous coupling among fluctuations in the intensity of modes, which induce instabilities in the mode dynamics and affect the state of operation. The dynamics of modes and the characteristics of the output spectrum are investigated over wide ranges of the injection current and the linewidth enhancement factor in both AlGaAs-GaAs and InGaAsP-InP laser systems. Operation is classified into stable single mode, stable multimode, hopping multimode, and jittering single mode. Based on the present results, the experimental observations of multimode oscillation in InGaAsP-InP lasers are explained as results of the large value of the linewidth enhancement factor     

High-power (AlGa)As strip-buried heterostructure lasers

The characteristics of index-guided strip-buried heterostructure lasers are described. The lasers are grown by a two-step process, consisting of the initial 5-layer planar growth by MOCVD, and after etching to define the strip buried wave guide, recovery by Al0.65Ga0.35As using LPE. The devices are highly uniform, having 20 mA thresholds, quantum efficiencies of 0.65, and T0of 163°C. The TE polarized emission was in the lowest order transverse mode and consisted of ∼ 6 longitudinal modes. CW output powers of 50 mW per facet were obtained from lasers with reduced output reflectivity, and pulsed powers of 95 mW per facet from uncoated lasers have been obtained. The far-field angular widths weresim 23degandsim 50degin, and perpendicular to the junction plane, respectively. The total optical conversion efficiency of the coated laser emitting 50 mW CW was 41.5 percent.     

太赫兹二级分布反馈量子级联激光器中的模式竞争与功率特性

结合实验和理论计算研究了太赫兹二级分布反馈量子级联激光器中的模式竞争和功率特性.研究表明,激光器在整个动力学范围内均稳定地工作在横向及纵向的基模.横向基模的产生原因是脊条两侧的吸收边界有效提高了高阶横模的损耗.纵向基模的产生原因主要是谐振腔内基模与高阶纵模的电磁场分布交叠较大,并且频率接近,从而有效避免了增益在空间和频域的烧孔效应.此外,激光器的辐射效率随分布反馈光栅长度的增加而减小,导致只有在特定的光栅长度才能获得最大的输出功率.该工作有助于高性能单模太赫兹激光器及锁相激光器阵列的研制.     

Modeling rare turn-on events of injection lasers

An efficient numerical method based on photon statistics and rate equations is developed to simulate rare turn-on events of injection lasers. Results indicate that: (1) during the transient stage, with a probability of 10^-9, the side-mode suppression ratio can deteriorate by three orders of magnitude from its steady-state value; (2) there is a turn-on jitter for the main mode, even when the side mode is missing; and (3) the slower the risetime of the laser output the less the transient mode partitioning. The effects of gain compression and driving condition on the transient turn on are discussed     

Single-mode characteristics of short coupled-cavity semiconductor lasers

The characteristics of single-longitudinal-mode selection in short coupled-cavity (SCC) semiconductor lasers with a plane reflector are analyzed theoretically. It is found that the power ratio of the main mode to other side modes can be increased significantly by reducing the reflectivity of a laser diode facet adjacent to the external mirror. Based on the theoretical results, a new type of SCC laser is developed; single-longitudinal-mode output with 35-40 dB side mode suppression ratio is obtained under CW and 150 MHz modulation conditions.