Miniature Nd:YAG lasers: noise and modulation characteristics

The frequency and intensity noise spectra, as well as the frequency modulation (FM) response, of 1320-nm laser-diode-pumped miniature Nd:YAG ring lasers have been measured. The frequency noise spectrum has a resonance peak at the relaxation oscillation frequency of the laser (between 123 and 150 kHz) and is flat beyond 200 kHz with a spectral density of 613 rad²-Hz, much smaller than that of semiconductor lasers; the corresponding laser linewidth is less than 49 Hz. The relative intensity noise is -140 dB/Hz at the valley and has a resonance peak at the relaxation oscillation frequency of the laser. The FM response is flat from DC to 110 kHz and is in the 0.65-3 MHz/V range; the modulation frequency is limited by the relaxation oscillation frequency of the laser     

Observation of multiple resonance frequencies in stripe geometryInGaAs/InGaAsP/InP quantum-well lasers

Intensity fluctuation noise in strained InGaAsP/InP multi-quantum-well lasers is analyzed for both ridge-guided and broad-area gain-guided structures. A single resonance peak is observed in the noise spectrum for the ridge-guided laser, as expected. However, the noise spectrum for the broad-area lasers shows multiple (~2-5) resonance peaks, distinctly spaced, from ~2 to ~5 GHz. Combined with near-field measurements, the experiments show that these peaks originate from lasing filaments having significantly nonuniform optical power. The authors also determined the resonance frequency of the single-mode laser from both small-signal modulation and turn-on relaxation oscillation measurements and found the results to be consistent with the measured peak noise frequency     

High-speed dynamics of GaAsSb vertical-cavity lasers

The intensity noise spectrum is measured for antimonide based 1.3 μm vertical-cavity surface emitting lasers. Analysis of the noise spectra indicates a modulation bandwidth of 5.7 GHz with an intrinsic maximum frequency response that exceeds 28 GHz. A discontinuity in the relaxation oscillation resonance frequency is observed, which suggests that the lasing behavior at threshold is assisted by nonlinearities in the gain medium     

Relative intensity noise of multitransverse-mode vertical-cavitysurface-emitting lasers

Relative intensity noise (RIN) spectra of weakly index guided vertical-cavity surface-emitting lasers (VCSELs) in a multitransverse-mode regime are analyzed by using a model that takes into account all the transverse modes supported by the waveguide. Several resonance peaks are obtained in the noise spectra that correspond to the relaxation oscillation frequencies of the transverse modes. It is shown that for low spatially overlapping transverse modes, low RIN operation can be maintained. However, the excitation of transverse modes with a significant spatial overlap leads to a clear enhancement of the RIN at low frequencies     

Theoretical calculation of relative intensity noise of multimode vertical-cavity surface-emitting lasers

An analytical calculation of relative intensity noise spectra of weakly index-guided vertical-cavity surface-emitting lasers (VCSELs) in a multimode regime is performed. We obtain analytical expressions that incorporate the spatial dependence of electrical field and carrier density profiles. Expressions are derived for single-mode and two-transverse-mode operation. Our analysis shows that single-mode noise spectrum depends on the spatial mode profile through a modal effective volume. Two-mode noise spectra also incorporate the dependence on the degree of spatial overlapping between the modes. Two resonance peaks appear in the noise spectra of the individual modes and total power of the two-mode VCSEL. We analytically show that those peaks appear at frequencies that correspond to the relaxation oscillation frequencies of the multimode laser. Simple analytical expressions are also derived for noise spectra at zero frequency.     

光电反馈抑制掺铒光纤激光器的低频强度噪声

通过光电反馈电路对掺铒光纤激光器的中低频噪声进行了抑制。根据速率方程理论,分析了影响掺铒光纤激光器强度噪声的因素,通过电路仿真分析优化反馈电路参数,重点讨论反馈信号的相位对噪声抑制的影响。实验表明：激光器的低频(小于20KHz)强度噪声平均降低了10dB,中频弛豫振荡峰处（30KHz附近）抑制达35dB,并且克服了光电反馈抑制强度噪声使弛豫振荡峰向高频移动,导致高频噪声增大的问题。优异的噪声特性使光纤激光器在光传感领域具有很高的实用价值。     

Influence of lateral field on the relaxation oscillation frequencyof semiconductor lasers

We demonstrate theoretically that the lateral field distribution can be utilized to enhance the relaxation oscillation frequency of semiconductor lasers. It is found, for some laser parameters, that gain-guided semiconductor lasers with narrow stripe can exhibit higher relaxation oscillation frequency than index-guided devices     

Dynamic behavior of fundamental-mode stabilized VCSELs using a shallow surface relief

An extensive theoretical study was performed on the dynamic behavior of 850-nm-wavelength oxide-confined fundamental-mode stabilized vertical-cavity surface-emitting lasers (VCSELs), using a shallow surface relief. The surface relief is used to provide lower mirror loss for the fundamental mode, thus acting as a mode discriminator. In this way, single-mode operation at high power levels can be obtained. We utilized a comprehensive model that includes the detailed epitaxial layer structure and device geometry when calculating the optical fields and that accurately accounts for the dynamic effects of carrier density and temperature on the modal distributions. Modulation response, eye diagrams, bit error rate (BER), and relative intensity noise (RIN) were simulated and compared to the performance of VCSELs without a mode discriminator, i.e., conventional multimode VCSELs. The fundamental-mode stabilized VCSELs are associated with a higher out-coupling, which lowers the relaxation oscillation frequency and damping, and strong spatial hole burning, which induces a low-frequency roll-off in the modulation response and contributes to the damping of the relaxation oscillation at low bias. However, their dynamics is fully competitive with conventional multimode VCSELs at both 2.5 and 10 Gb/s although they exhibit a slightly higher eye closure. We only found a 0.5-dB power penalty in the BER. The RIN is enhanced, with a peak that is about 10-15 dB higher, caused by the lower damping of the relaxation oscillation. It should be noted that in the comparison we assume that all modes are equally captured from the multimode VCSEL. A mode-selective loss can severely degrade its performance.     

Equivalent circuit model of quantum-well lasers

A study of equivalent circuit model of quantum well lasers derived from quantum well rate equation has been carried out. Pulse response simulation results obtained from a SPICE simulator indicates that various modulation properties, e.g., turn-on delay, relaxation oscillation frequency, and on/off aspect ratio, have a substantially different behavior than those of DH lasers. This model offers a fundamental building block for the simulation of the over all signal transmission when using quantum-well lasers as a light source     

Cavity length dependence of K-limited bandwidth in 1.6 mu m compressive-strained quantum well lasers

The intrinsic small signal frequency response of 1.6 mu m compressive-strained quantum well lasers is measured at various cavity lengths. The highest 3 dB bandwidth reached is approximately 18 GHz. Cavity length dependence on the K factor is investigated. It was found that the optimal condition for obtaining the highest possible K-limited is the same as that for obtaining the highest relaxation oscillation frequency for a given optical power.<>     

半导体抽运Nd:YAG激光器强度噪声抑制的研究

为了降低半导体抽运固体激光器的弛豫振荡噪声,提高其输出功率的稳定性,采用光电负反馈的方法来抑制半导体抽运的固体激光器的强度噪声,并对激光器强度噪声的理论特性进行了分析。根据理论分析结果设计了比例-积分-微分反馈控制电路,通过运用该反馈电路对激光器进行强度噪声抑制实验,得到了比较理想的实验数据,即当抽运功率为700mW、弛豫振荡峰频率为300kHz时,弛豫振荡峰值处和低频区域强度噪声分别降低了45dB和15dB;当抽运功率为550mW、弛豫振荡峰值为250kHz时,弛豫振荡峰值处和低频区域强度噪声分别降低了40dB和10dB。结果表明,该反馈控制电路能够有效地降低半导体抽运固体激光器的强度噪声,提高激光器输出功率的稳定性。     

High bandwidth small signal modulation response of two laterally mode-locked diode lasers

In this letter, we present the study of the small signal modulation response of a monolithic twin ridge laterally coupled diode lasers. Such structures have been extensively theoretically studied for their promising characteristics as high-speed optical sources for optical communications, and in this work we demonstrate the increase of the small signal bandwidth beyond the relaxation oscillation frequency with the appearance of a second peak in the small signal response due to the laterally mode locking of these two emitters.     

Intrinsic modulation bandwidth in ultra-high-speed 1.3 and 1.55 mu m GaInAsP DFB lasers

By using an ultra-low capacitance structure, the wavelength dependence, 1.3 or 1.55 mu m, of the intrinsic modulation bandwidth relating to the relaxation oscillation frequency and the nonlinear damping in GaInAsP DFB lasers is investigated.<>     

Dependence of high-speed properties on the number of quantum wellsin 1.55 μm InGaAs-InGaAsP MQW λ/4-shifted DFB lasers

The dependence of intrinsic dynamic properties, such as relaxation oscillation frequency, damping K-factor, and spectral chirping under 10-Gb/s direct modulation, on the number of quantum wells is systematically investigated in 1.55-μm multiquantum-well λ/4-shifted distributed-feedback lasers. The theoretical and experimental results indicate that the dependence of the above three factors on the number of quantum wells is clearly explained by the linear gain saturation of the quantum-well lasers     

Suppression of the relaxation oscillation in the modulated output of semiconductor lasers

Practical rates of direct pulse modulation of semiconductor lasers have so far been limited to below several hundred megahertz, owing to serious distortion in the output signal caused by the relaxation oscillation of the light intensity. Based on theoretical analysis of the dynamic properties of lasing under constant injection of external radiation into the cavity modes, light injection is proposed as a method for suppressing the relaxation oscillation. The effectiveness of this method has been confirmed in a series of preliminary experiments which employed GaAs injection lasers both as the modulated laser and as the external source of the injected radiation.     

Characteristics of chaotic masking in synchronized semiconductor lasers

Modulations imposed on a chaotic optical signal generated by a semiconductor laser can be suppressed by injecting the signal into another similar laser under conditions for chaos synchronization. This filter effect could be used to recover messages hidden in chaotic carriers for robust and secure communications. We use a numerical model to examine the filter properties and show that the filter can be described in terms of differences in characteristic transmission functions for imposed signal and chaotic carrier in the output of the synchronized laser. The filter effect is shown to be larger for lower frequencies and decreases as frequencies approach the relaxation oscillation frequency of the laser in the gigahertz regime, similar to the response of steady-state injection-locked lasers to small-signal modulation. The filter properties are confirmed in experiments using both single and multimode lasers.     

Low distortion 1.3 μm strained-layer MQW-DFB laser for AM-SCMtransmission systems with large channel capacity

1.3-μm strained-layer multiquantum-well distributed-feedback (SL-MQW-DFB) lasers with extremely low distortion and low noise characteristics up to high output-power as well as over wide frequency range were successfully fabricated. This was achieved by both the high relaxation oscillation frequency (f_r) characteristics of the laser with the optimized SL-MQW active structure and the suppression of nonradiative current with relatively long laser cavity length. By using the SL-MQW laser with a 400-μm long cavity as a light source for CATV transmission system, high quality AM-SCM transmissions with the largest channel capacity (151 channel) was demonstrated due to low distortion characteristics at high output power. Also, for the laser with 300 μm long cavity, extremely low bias-current operation (17 mA above threshold) is realized in high quality 100 channel AMSCM transmissions, due to high f_r at low bias-current     

半非平面单块激光器强度噪声及其抑制的研究

实验研究了半非平面单块固体环形Nd∶YAG激光器的强度噪声特性和抑制技术。结果表明,弛豫振荡仍是该种激光器强度噪声最主要的来源。噪声峰值频率随抽运和输出功率的增加向高频方向移动,同时噪声峰宽度增加幅度降低,且在峰值频率附近发生180°相位跃变。采用光电反馈方法设计和制作了包含低噪声宽带接收、比例积分微分(PID)调整等单元组成的噪声抑制电路,测量了整个系统及各部分的传递函数,实现了强度噪声的抑制。当弛豫振荡峰值频率为600kHz时,噪声峰值处和低频区域幅度降低分别接近40dB和15dB。     

Frequency dependence of the chirp factor in 1.55 μm distributedfeedback semiconductor lasers

The authors report on measurements of the chirp factor versus modulation frequency and bias for 1.55 μm distributed feedback lasers. A scanning Michelson interferometer is used to measure the phase response of the optical field under small-signal modulation of the injection current. The magnitude of the phase response is then quantified in terms of an effective chirp factor corresponding to the well known α-parameter. The authors find this factor to increase monotonically and nearly double as the modulation frequency increases from below to above the resonance frequency of the relaxation oscillation. The results contradict the commonly held view of α as a constant parameter for a given laser structure     

Cavity resonance shift and bandwidth enhancement in semiconductor lasers with strong light injection

We theoretically investigated the physical mechanism of significant bandwidth enhancement in injection-locked semiconductor lasers with strong light injection. Strong light injection can increase the semiconductor laser bandwidth to several times the free-running relaxation oscillation bandwidth. We focused on the fact that an injection-locked semiconductor laser can operate at an optical frequency different from its cavity resonance condition. Resonance is shifted from solitary resonance through the carrier-induced refractive-index change due to strong optical injection. We then framed a theory in which the enhanced resonance can be generated by transient interference between the injection-locked field and the field corresponding to the shifted cavity resonance. To examine the theory, we numerically investigated the rate equations and found that the numerical and theoretical results agree well over a wide range for strong injection. A stability analysis for the rate equations revealed that there is a mode transition from relaxation oscillation to interference-induced oscillation with increased injection, and the relaxation oscillation can become suppressed for higher injection levels. We believed that negligibly small fluctuations of the carrier suppressed the relaxation oscillation and examined it numerically. We also predict that, at strong injection levels, semiconductor lasers can be lower-dimensional systems because of the possibility of adiabatic elimination of the carrier. We further conclude that such low-dimensional systems can generate a high-frequency interference beat in the laser cavity and the resultant enhanced resonance.