Tuning characteristics of monolithic passively mode-lockeddistributed Bragg reflector semiconductor lasers

Tuning characteristics of multicontact passively mode-locked distributed Bragg reflector (DBR) semiconductor lasers are investigated in terms of center wavelengths and pulse repetition frequencies. It is shown that the center wavelength of the pulses can be tuned over a wide range by changing the refractive index of the Bragg reflector section either by means of carrier injection or by thermal effects while maintaining the pulses to be nearly transform-limited. Tuning of the pulse repetition rates is realized by using four different approaches, i.e., current injection to a passive phase-control section, varying of the reverse bias to the absorber, varying of the injection current to the gain region, and thermal effects. Injecting current into the phase-control region results in a tuning range of more than 400 MHz while maintaining the pulses at transform-limited condition. Varying the reverse bias voltage to the absorber is shown to be an alternative to achieve large repetition rate tuning, by which a tuning range of 600 MHz is obtained. By combining these tuning schemes, a total tuning range of more than 1 GHz is realized     

Coherent pulsed injection seeding of two gain-switchedsemiconductor lasers

High-power, narrow-spectrum, short pulses at a wavelength near 830 nm are needed for optical logic applications. The authors report the generation of two coherent trains of pulses by pulsed injection seeding of two gain-switched semiconductor lasers with one mode-locked master oscillator. The available power was doubled and both lasers emit in nearly identical spectral lines. Short pulses (30-50 ps) are generated at a repetition rate of 1.9 GHz. The spectrum is reduced to a single mode cluster and shows a 1.7 Å wide chirp suitable for pulse compression. The peak power is 0.1 W for each pulse train. The capability of this technique for coherent power combining is demonstrated     

Generation of transform-limited optical pulses using a combinationof a gain-switched diode laser and a semiconductor opticalamplifier

We introduce a novel method for generating nearly transform-limited picosecond optical pulses at multi-gigahertz repetition rates. Frequency-chirped optical pulses are first produced using a gain-switched semiconductor diode laser, and they are subsequently amplified by a semiconductor optical amplifier. The red frequency chirp in the gain-switched pulses is compensated for almost completely as a result of gain-saturation-induced self-phase modulation occurring in the amplification process. Experimentally, we have successfully generated nearly transform-limited 28-ps optical pulses at a 5-GHz repetition rate using a distributed feedback diode laser and a traveling wave semiconductor optical amplifier     

Chirp and stability of mode-locked semiconductor lasers

A numerical study of mode-locked semiconductor lasers is presented with special attention to the chirp characteristics and to dispersion-related criteria for stable pulse-train emission. The dependence of the pulse chirp upon the refractive-index change, both with carrier density and carrier temperature changes, is discussed. The experimental observation of blue-chirped pulses for passive mode-locking in contrast to red-chirped pulses for active mode-locking is found to be due to the different contributions of gain and absorber media to the refractive-index change. In addition, it is revealed that the boundary of the stable operation regime is critically influenced by the spectral characteristics of laser and external cavity. Design considerations toward the achievement of high pulse energy, narrow spectral bandwidth, and linear chirp are given     

Characterization of 1.55-μm pulses from a self-seededgain-switched Fabry-Perot laser diode using frequency-resolved opticalgating

The intensity and frequency chirp of picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode have been directly measured using the technique of frequency-resolved optical gating. Measurements over an output sidemode suppression ratio (SMSR) range of 15-35 dB show that higher SMSR's are associated with an increasingly linear frequency chirp across the output pulses. This complete pulse characterization allows the conditions for optimum pulse compression to be determined accurately, and indicates that transform-limited, pedestal free pulses can be obtained at an SMSR of 35 dB     

Ultrashort pulse generation by intracavity spectral shaping andphase compensation of external-cavity modelocked semiconductor lasers

Intracavity spectral shaping and external chirp compensation techniques were employed to generate nearly transform-limited optical pulses with a temporal duration of 250 fs from an external-cavity modelocked semiconductor laser. It was also demonstrated that intracavity spectral shaping techniques can be used for artificially tailoring the chirp of the output pulses     

Analysis of large-signal dynamic characteristics of 10-Gb/s tunable distributed Bragg reflector lasers integrated with electroabsorption modulator and semiconductor optical amplifier based on the time-dependent transfer matrix method

We investigate large-signal dynamic characteristics of tunable distributed Bragg reflector (DBR) lasers with different structures. Because of high chirp and complex structures of tunable DBR lasers, it is difficult to accurately analyze large signal chirp and output pulse shapes with analytical models. Therefore, we apply a numerical model based on the time-dependent transfer matrix method to tunable DBR lasers integrated with electroabsorption modulator (EAM) and semiconductor optical amplifier (SOA). Using this model, we find a suitable /spl alpha/-parameter range in the Bragg grating section (/spl alpha//sub Bragg/-parameter) that produces the tolerable wavelength chirp while maintaining a proper tuning range. To employ the tunable lasers in wavelength division multiplexed networks and improve flexibility of the networks, the tunable lasers should have high output power and low wavelength chirp. According to our simulation results, the EAM section had better be located in between SOA and DBR laser sections to obtain high output power and low wavelength chirp.     

Locking bandwidth of actively mode-locked semiconductor lasersusing fiber-grating external cavities

We demonstrate numerically that actively mode-locked semiconductor lasers employing a linearly-chirped fiber grating in an external cavity can exhibit multiple drive frequency ranges of stable pulse generation, and thus give a large locking bandwidth. The locking bandwidth defines the range of RF drive frequencies over which the laser will generate pulses with low timing jitter. The stable optical pulses are generated with leading or trailing subpulses due to optical energy circulating within the fiber grating. The multiple stable ranges merge if loss is introduced into the fiber-grating region. We also show that the locking bandwidth can be improved if chirp is introduced into the grating. Suprisingly, however, chirp of either sign improves the locking bandwidth     

Tunable picosecond frequency-shifted feedback fiber laser at 1550 nm

A novel frequency-shifted feedback (FSF) fiber laser cavity is proposed and experimentally verified. The laser generates stable mode-locked pulses of <2 ps, mainly limited due to chirp induced by the frequency-shifting element (i.e., the acoustooptic modulator) on the pulses. Fourier transform-limited subpicosecond pulses were obtained by compensating this excess chirp using a piece of single-mode fiber SMF-28 outside the laser cavity. The unique cavity design also allow us to tune the pulse wavelength throughout the Er-gain band and the ability of the laser to self-start gives us the opportunity to characterize transient behavior of mode-locked pulses in an FSF fiber laser cavity.     

Dependence of Linewidth Enhancement Factor on Duty Cycle in InGaAs–GaAs Quantum-Dot Lasers

The linewidth enhancement factor (alpha-factor) of InGaAs-GaAs quantum-dot (QD) lasers is studied as a function of the current pulse duty cycle. In this letter, we demonstrate the impact that current pulse duty cycle has on measured -factor in QD lasers due to increased thermal effects. Below threshold, while comparable quantum-well lasers blue-shift under all conditions, the QD lasers demonstrate an unusual sensitivity of chirp to duty cycle of the current, with a blue-shift at low duty cycles, and a red-shift at high duty cycles and dc current, which may lead to negative -factor. The assumption of Gaussian gain distribution and linear gain shift with temperature is used to analyze the -factor at different duty cycle currents. The result agrees well with the positive, zero, and negative alpha-factors reported for QD lasers. The effect of duty cycle is quite significant and can shift the alpha-factor substantially going from pulse to dc. This is due to the low thermal conductivity of QD regions and increased thermal effects in those devices.     

Frequency control and chirp compensation from an electro-opticalDBR laser

The variability of optical frequency modulation with electric field in an electro-optical phase section is analysed. When configured for maximum sensitivity, the electro-optical phase section was used to compensate for the chirp of the gain section: 48 ps and 8.5 GHz wide transform-limited pulses were obtained at 5 GHz     

短脉冲在色散平坦光纤中传输前后波形、相位和啁啾测量的实验研究

二次谐波频率分辨光学门(SHG-FROG)是能够准确测量短脉冲多项特性参量的新技术。利用二次谐波频率分辨光学门脉冲分析仪对在色散平坦光纤中传输前后的短脉冲进行了测量,得到了待测光脉冲的频率分辨光学门(FROG)图、自相关曲线、自相关频谱曲线、波形和相位曲线以及脉宽、谱宽、啁啾等反映短脉冲特性的信息,对实验结果进行了分析,并与高斯脉冲在单模光纤中的线性传输理论进行了比较。结果表明,激光器输出的短脉冲是具有负线性啁啾的近变换极限高斯脉冲,经过12.7 km色散平坦光纤传输后仍然为具有负线性啁啾的高斯脉冲,其谱宽在传输过程中基本保持不变,脉宽展宽了3.1倍,啁啾增大了4倍。实验测量结果和理论预期一致。     

由输出光脉冲选择半导体激光器泵浦电流

从实践的需要出发,用速率方程组探讨了如何选择半导体激光器的泵浦电流以产生预先设定的光脉冲,并以高斯、整形等形状的光脉冲为例,推导了所需泵浦电流的解析表达式。     

Compresion and dispersionless transmission of chirped 1.3 mu m DFB ps laser pulses in dispersion-shifted single-mode fibres

Optical pulse compression of high-speed gain-switched ps 1.3 mu m DFB semiconductor laser pulses in dispersion-shifted single-mode fibres is measured with a streak camera. The laser-red-shift chirp is compensated for by the negative chromatic fibre dispersion, resulting in pulse compression and 'dispersionless' transmission of 45 ps pulses over 12 km of fibre.<>     

Spectral broadening of pulsating AlxGa1-xAs double heterostructure lasers

The longitudinal mode spectrum of stripe geometry double heterostructure (DH) lasers exhibits a spectral broadening of ∼ 0.8 Å when the laser emits pulsations. The broadening results from a chirp to longer wavelengths which occurs during the ∼ 0.4 ns long pulse. The chirp is due to an increase of the refractive index resulting from the reduction in the carrier concentration during the pulse. Both plasma and band to band interactions affect the index change with the latter mechanism giving the dominant contribution. A comparison of the wavelength shift obtained during the pulse with the current dependent shift of the longitudinal modes at currents well below threshold confirms that the change in carrier concentration is responsible for the chirp.     

CW injection seeding of a modelocked semiconductor laser

The authors extended the injection locking technique to control the output of a modelocked semiconductor laser with an external continuous-wave (CW) signal. With this injection seeding technique, over 8 mW of average power in 30 ps pulses with side cluster suppression of over 20 dB was obtained from an actively modelocked AlGaAs semiconductor laser. This average output power compares favorably with the 12 mW W output power of the extended resonator. The frequency spectrum of the laser is determined by the background noise level as set by the spontaneous emission. Injection seeding overrides the noise and concentrates over 99% of the available energy in a single nearly transform-limited pulse     

Operation of an infrared dye laser synchronously pumped by a mode-locked CW Nd:YAG laser

Infrared dyes emitting around 1.3 μm exhibit fluorescence lifetimes τflof only ∼ 10 ps which are shorter than the duration of the pump pulses (t_{p} sim70ps). This fact results in performance characteristics different from those of visible dye lasers where the fluorescence lifetime is longer than tp. The pulse properties of IR dye lasers are investigated for various bandwidth-limiting elements. The generation of nearly transform-limited dye laser pulses as short as 6 ps-assuming a Gaussian temporal shape-is demonstrated.     

Dispersion of the group velocity refractive index in GaAs double heterostructure lasers

The dispersion of the refractive index corresponding to the group velocityn*_{1}has been measured as a function of wavelength. It is obtained from the longitudinal mode spacing of GaAs buried heterostructure lasers at threshold. The dependence ofn*_{1}on wavelength contains an approximately constant term due to the refractive index n1and a strongly dispersive component due to-lambda (partialn_{1}/ partiallambda). For a given spectral bandwidth, the dispersion ofn*_{1}causes a temporal broadening of a pulse as it passes through the medium. This dispersive effect is shown to contribute to the width of 0.65 ps long pulses obtained recently from mode locked semiconductor lasers. By reducing the length of the laser, the dispersive effect is reduced and it is suggested that pulses as short as 10^-13s should be obtainable from such mode locked semiconductor lasers.     

利用快速相位反馈控制输出变换极限飞秒激光脉冲

报道了利用简化的频率分辨光学开关法快速测量飞秒激光脉冲,并形成反馈,控制液晶空间光调制器,补偿非线性光纤飞秒激光放大器输出脉冲的啁啾,获得了脉冲宽度74 fs的变换极限脉冲。