Demonstration of down-chirped and chirp-free pulses fromhigh-repetition-rate passively mode-locked lasers

Knowledge and control of the chirp parameters of semiconductor lasers is a prerequisite to obtaining transform-limited pulses and/or to compensate for group velocity dispersion in fiber. Here, we report measurements of the sign and magnitude of chirp in high-repetition-rate mode-locked semiconductor lasers. The chirp of these monolithic lasers is measured in the frequency domain, using filtering and cross-correlation techniques. For different injection currents, a range of different chirp values is measured, including strongly down-chirped pulses at higher injection currents and transform-limited pulses to slightly up-chirped pulses at lower injection currents. The pulse chirp and the resulting broadening are due to the algebraic addition of opposite-signed chirps due to saturation of the absorption section and the gain section. These may cancel each other under some conditions, leading to a soliton-like transform-limited pulse     

Tunneling injection lasers: a new class of lasers with reduced hotcarrier effects

In conventional quantum-well lasers, carriers are injected into the quantum wells with quite high energies. We have investigated quantum-well lasers in which electrons are injected into the quantum-well ground state through tunneling. The tunneling injection lasers are shown to have negligible gain compression, superior high-temperature performance, lower Auger recombination and wavelength chirp, and better modulation characteristics when compared to conventional lasers. The underlying physical principles behind the superior performance are also explored, and calculations and measurements of relaxation times in quantum wells have been made. Experimental results are presented for lasers made with a variety of material systems, InGaAs-GaAs-AlGaAs, InGaAs-GaAs-InGaAsP-InGaP, and InGaAs-InGaAsP-InP, for different applications. Both single quantum-well and multiple quantum-well tunneling injection lasers are demonstrated     

Frequency dynamics of gain-switched injection-locked semiconductorlasers

The frequency dynamics of gain-switched single-mode semiconductor lasers subject to optical injection is investigated. The requirements for low time jitter and reduced frequency chirp operation are studied as a function of the frequency mismatch between the master and slave lasers. Suppression of the power overshoot, typical during gain-switched operation, can be achieved for selected frequency detunings     

DC and Dynamic Characteristics of P-Doped and Tunnel Injection 1.65-μm InAs Quantum-Dash Lasers Grown on InP (001)

We have studied the characteristics of 1.65-mum InAs self-organized quantum-dash lasers grown on InP (001) substrates, wherein special techniques of p-doping of quantum dashes and tunnel injection are incorporated for the first time. We measured a very large T₀ (196 K) in p-doped quantum-dash lasers, accompanied by an increase in threshold current density (J_th~1600 A/cm² ), compared to the undoped quantum-dash lasers (T₀=76 K and J_th~950 A/cm²). The p-doped lasers exhibit a maximum 3-dB bandwidth of 8 GHz, chirp ~1.0 Aring, and alpha-parameter ~1.0 (measured at subthreshold bias conditions) at a temperature of 278 K. Similar undoped quantum-dash lasers exhibit a 3-dB bandwidth of 6 GHz. A self-consistent model, that includes Auger recombination in quantum dashes, is developed to calculate the threshold current at various temperatures. A comparison of the calculated threshold current and T₀ with measured values reveals that Auger recombination in quantum dashes plays a major role in determining the values of threshold current and T₀ in both undoped and p-doped quantum-dash lasers. While p-doping increases the gain and differential gain, the presence of wetting layer states, the relatively large inhomogeneous broadening of quantum dashes, and the substantially increased Auger recombination upon p-doping severely limit the potential benefits. Superior characteristics, including large modulation bandwidth (f_{-3 dB}~12 GHz), near-zero alpha-parameter, and very low chirp (~0.3 Aring), are achieved when the technique of tunnel injection is also utilized     

Variation of frequency chirp with wavelength in an InGaAsP/InP multiple-quantum-well (MQW) waveguide electroabsorption modulator

Through measurements of the modulation index and the optical power spectra, the chirp parameter for an InGaAsP quantum-well electroabsorption waveguide modulator has been determined. The result shows that the frequency chirp parameter achieves a minimum value of 0.6 in the wavelength range 1509-1545 nm. This is significantly less than what is obtained from direct intensity modulation of injection lasers, making this a useful device in high-bit-rate, long-haul systems.<>     

Chirp reduction of directly modulated semiconductor lasers at 10Gb/s by strong CW light injection

The influence of strong light injection on the reduction of the dynamical linewidth broadening of directly current-modulated semiconductor lasers at high bit rates is theoretically investigated and experimentally verified for 10 Gb/s NRZ pseudorandom modulation with a large current swing of 40 mA pp. Significant chirp reduction and single-mode operation are observed for bulk DFB, quantum well DFB lasers at 10 Gb/s and a weakly coupled bulk DFB laser at 8 Gb/s, so that an improvement of the transmission performance using standard monomode fibers in the 1.55 μm low-loss wavelength region can be achieved for all these laser types, where dispersion otherwise causes severe penalties for long-haul transmission. The properties of injection-locked bulk DFB and quantum well DFB lasers with respect to high bit rate modulation have been systematically studied by the use of the rate equation formalism. A dynamically stable locking range of more than 30 GHz under modulation has been found for both laser types with injection ratios higher than 0.5     

Chirp characteristics of 10-Gb/s electroabsorption modulatorintegrated DFB lasers

We present a complete large-signal dynamic model of electroabsorption modulator integrated (EAMI) distributed feedback (DFB) lasers using the time-dependent transfer matrix method. With this model, it is possible to analyze dynamic characteristics depending on optical feedback and spatial hole burning. Also, we can separately calculate the laser and modulator chirp including the voltage-dependent modulator chirp parameter, the grating phase at the end of the laser section, the length of the waveguide region, and electrical coupling. Therefore, our model can provide better predictions regarding the laser and modulator chirp. The calculated large-signal chirp using our model has similar characteristics to the measured large-signal chirp for 10-Gb/s EAMI-DFB lasers     

Fabrication and performance characteristics of InGaAsP ridge-guide distributed-feedback multiquantum-well lasers

The fabrication and performance characteristics of InGaAsP ridge-guide distributed-feedback lasers with multiquantum-well active layers are reported. The lasers are 320 ?m long and have four active wells (?300 ? thick). The lasers have threshold current ?100 mA at 30°C, external differential quantum efficiency ?0.1 mW/mA/facet at 30°C and To? 60 K and can be operated in the same DFB mode up to 70°C. The measured frequency chirp is about a factor of 3 smaller than that for conventional double-heterostructure lasers. The smaller chirp should allow larger repeater spacing for high-bit-rate long-distance fibre transmission systems applications.     

Characterization of wavelength chirping in modelocked monolithicCPM lasers

A new method to measure the chirp of pulses emitted from repetitively driven lasers is described. The technique relies on spectrally filtering the output of the laser, detecting it, and then measuring its phase with respect to a reference signal via a mixing technique. We demonstrate the method by measuring the chirp of pulses emitted from monolithic CPM lasers. The measured chirp agrees with that predicted by a newly developed model of the CPM lasers     

Dynamic linewidth of amplitude-modulated single-longitudinal-mode semiconductor lasers operating at 1.5 ?m wavelength

We have measured the frequency chirp characteristics of single-longitudinal-mode semiconductor lasers operating at 1.5 ?m wavelength and subject to large signal amplitude modulation in the 50 to 800 MHz range. Measurements on cleaved-coupled-cavity (C3) and GRIN-rod external cavity lasers show that the wavelength chirp is large enough to cause a penalty in ultra-high-bit-rate long-haul optical-fibre systems.     

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     

从等效电路模型分析激光光源的啁啾特性

在已有的分布反馈激光器的电路模型中引入位相回路,从而使这种电路模型能够携带出射光的位相信息,进而反映激光器的啁啾特性.对修正后的模型进行仿真,结果与已报道的结果十分吻合.进一步将这种带位相回路的激光器模型与电吸收调制器的电路模型相结合,构成分布反馈激光器与电吸收调制器集成光源的等效电路模型.该模型同时考虑了激光器与调制器之间的电耦合.这种由于隔离电阻不够大而导致的电耦合将导致激光器激射波长发生漂移,即产生啁啾.通过对电路模型进行仿真,给出不同隔离电阻时的啁啾特性曲线,并对啁啾大小与隔离电阻的关系进行讨论.结果显示了集成光源相比于直接调制激光器在啁啾特性方面的显著改善.     

LD泵浦固体激光器的光谱匹配问题

实验研究了LD泵浦Nd:YAG固体激光器的光谱匹配问题,如匹配方法、匹配效率、匹配chirp现象等:考察了LD光谱随温度、电流、时间的变化,讨论了这些变化对光谱匹配效率的影响。     

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.     

Dynamic and CW linewidth measurements of 1.55-μm InGaAs-InGaAsPmultiquantum well distributed feedback lasers

Measurement of the CW linewidth and frequency chirp as functions of modulation data rate and bias level for 1.55-μm InGaAsP multiquantum-well distributed feedback lasers grown by low-pressure MOCVD are presented. The results show that the CW linewidth of asymmetric facet-coated multiquantum-well DFB lasers can be as low as 2.0 MHz at 13.5 mW output power. The frequency chirp increases with modulation data rate and is significantly larger if the laser off-state is below threshold than if it is above threshold. The 20 dB down chirp widths are in the range of 1.9-5 Å for 40 mA peak-to-peak modulation current at 10 Gb/s under above-threshold bias     

Design and optimization of strained-layer-multiquantum-well lasersfor high-speed analog communications

We report on how the contributions from spatial hole burning, gain suppression, and relaxation oscillations to the chirp and harmonic distortion of SL-MQW DFB lasers can be calculated and minimized. It is shown how, by taking into account the specific properties of strained-layer-multiquantum-well (SL-MQW) lasers, simple solutions of the rate equations point the way to a chirp reduction and an increase of the useful bandwidth for analog communications. In such lasers, the absorption is only weakly dependent on the carrier density and therefore the harmonic distortion at lower modulation frequencies is mainly caused by spatial hole burning. Our numerical simulations indicate that in many cases this distortion is seduced by the same measures that reduce the chirp and increase the bandwidth     

Low chirp observed in directly modulated quantum dot lasers

We have examined the dynamic properties of high-aspect-ratio InAs-quantum-dot (QD) lasers at room temperature. A novel characteristic of low chirp in the lasing wavelength under 1-GHz current modulation was found in the quantum dot lasers. This is more than one order of magnitude less than the typical chirp (0.2-nm) found in a conventional quantum well laser that we used as a reference. Low chirp was obtained not only in the ground state lasing but in the second level lasing of quantum dots as well.     

Effects of filter concatenation for directly modulated transmissionlasers at 2.5 and 10 Gb/s

We report computer simulation results of the effects on optical signal quality of passage through a cascade of wavelength division multiplexing (WDM) filters-multiplexers and demultiplexers-for directly modulated lasers with different chirp characteristics. In particular, lasers with transient or adiabatic chirp characteristics at 2.5 Gb/s and 10 Gb/s are investigated, and we find clear differences between the laser types with respect to filter concatenation effects. Filters with an optical bandwidth suitable for a 200-GHz channel-spacing system are considered, and we evaluate the system behavior as a function of laser frequency offset for a fixed number of filters. The reference network architecture used for the simulations is an optically transparent metropolitan scale network in which the WDM signals may be demultiplexed and then multiplexed again at multiple optical network elements. The signal quality is evaluated in terms of a distortion-induced eye-closure penalty as well as the excess attenuation or loss suffered. We find that transient chirp-dominated lasers show a generally symmetric distortion penalty response to laser frequency offset, whereas the response for adiabatic chirp dominated lasers is highly asymmetric. Furthermore, the extinction ratio for the latter class of lasers can be improved, in some cases, by the appropriate offset between laser and filter center frequencies     

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     

Optimum range for DFB laser chirp for fiber-optic AM videotransmission

Simple expressions for the impact of the distributed feedback (DFB) laser chirp on the system noise and distortion specifications of AM CATV fiber links are derived. These· expressions are found to agree well with measured results. There is a narrow range of chirp for which the degradation of both noise and distortion is low. 1310-nm lasers are found to have chirp within the acceptable range. 1550-nm lasers are found to have chirp that is unacceptably large for most CATV applications