Synchronous mode-locking in passively mode-locked semiconductor laser diodes using optical short pulses repeated at subharmonics of the cavity round-trip frequency

Synchronous mode-locking was achieved in passively mode-locked semiconductor lasers using optical pulses with repetition rates at subharmonics of the cavity round-trip frequency. Stable and continuous mode-locked pulses at 8.5 GHz were generated when the repetition rate of the control optical pulses was 4.25 GHz (2nd subharmonic). The timing jitter of the mode-locked pulses was reduced to 1 ps. The relationship between repetition rates of the control pulses and the realization of stable and continuous mode-locking was examined.     

Passively mode-locked laser diodes with bandgap-wavelength detunedsaturable absorbers

We fabricated monolithic passively mode-locked laser diodes with bandgap-wavelength detuned saturable absorbers using selective growth. The pulsewidth shortened with increasing detuning of the absorber toward shorter wavelengths. For the detuned absorber, the pulsewidth was reduced to 1.2 ps, compared to 2.6 ps for the nondetuned absorber. This originated in broadened mode-locked spectra and reduced absorption recovery time under larger reverse bias voltages for detuned saturable absorbers     

Generation of wavelength-tunable transform-limited pulses from amonolithic passively mode-locked distributed Bragg reflectorsemiconductor laser

Generation of wavelength tunable transform-limited picosecond pulses from a monolithic passively mode-locked distributed Bragg reflector (DBR) laser is demonstrated for the first time. A wide tuning range of 12 nm is realized by changing the refractive index of the passive DBR region through a thin-film heater embedded on the DBR section without causing significant changes in pulse characteristics     

Generation of synchronized subterahertz optical pulse trains byrepetition-frequency multiplication of a subharmonic synchronousmode-locked semiconductor laser diode using fiber dispersion

We describe the generation of a synchronized, low-jitter, subterahertz optical pulse train by repetition-frequency multiplication of a subharmonic synchronous mode-locked semiconductor laser diode using the group-velocity dispersion (GVD) of optical fiber. A 49-196 GHz optical pulse train was generated with low intensity fluctuation and with low timing jitter, by adjusting fiber length in accordance with the mode-locking frequency and the fiber's GVD     

Novel design scheme for high-speed MQW lasers with enhanceddifferential gain and reduced carrier transport effect

We present a theoretical analysis exploring the optimum design of high-speed multiple-quantum-well (MQW) lasers for 1.55-μm operation. Various combinations of well and barrier materials are examined for lattice-matched, strained-layered (SL), and strain-compensated (SC) MQW lasers with InGaAsP and InGaAlAs barriers. The gain characteristics are investigated for these MQW lasers with various barrier bandgap wavelengths and are used to evaluate the modulation characteristics based on the carrier dynamics model which includes a set of Poisson, continuity, and rate equations. The importance of band engineering aimed at simultaneously reducing the carrier transport effect and enhancing the differential gain is described. It is shown that SC-MQW lasers with InGaAlAs barriers have an advantage in reducing the density of states in the valence band by reducing the overlap integral between the heavy- and light-hole wave functions, which effect has previously been discarded as a minor correction in designing conventional InGaAsP-based MQW lasers. Furthermore, the hole transport rate across the barriers can be drastically reduced in SC-MQW lasers due to the reduced effective barrier height for the holes. Based on this novel design scheme, a 3-dB bandwidth approaching 70 GHz is expected for 20-well SC-MQW lasers with InGaAlAs barriers as a result of both the large differential gain and reduced transport effect     

30-GHz bandwidth 1.55-/spl mu/m strain-compensated InGaAlAs-InGaAsP MQW laser

High-speed 1.55 /spl mu/m laser diodes with a 3-dB modulation bandwidths of 30 GHz were fabricated by using short-cavity mushroom structures with undoped, strain-compensated InGaAlAs-InGaAsP twenty-quantum-well active regions. The bandwidths were achieved at low bias current of 100 mA. The laser exhibited a high differential gain of 1.54/spl times/10/sup -15/ cm/sup 2/ and a small K factor of 0.135 ns. These results were achieved by using an In/sub 0.386/Ga/sub 0.465/AlAs barrier with 0.83% tensile strain to reduce the thermal emission time of holes from wells and hence the hole transport time.     

Polarization-, Wavelength-, and Filter-Free All-Optical Clock Recovery in a Passively Mode-Locked Laser Diode With Orthogonally Pumped Polarization-Diversity Configuration

We proposed and demonstrated all-optical clock recovery system using a monolithic mode-locked laser diode (MLLD) that operated with less sensitivity to the polarization and the wavelength of the input data signals also with no bandpass filter to eliminate the input signal-components. The keys to this new technique are the MLLD integrated with a tensile-strained quantum-well saturable absorber and a new polarization-diversity setup by signal input orthogonally polarized to the lasing polarization of the MLLD. This approach was experimentally validated in the 40 Gbps clock recovery experiments. The results exhibited excellent performance of the clock recovery with low timing jitters (<0.3 ps) remaining small dependence on the wavelength and the polarization of the input data signals and input-signal suppression less than -30 dB with no use of the bandpass filter. We also succeeded in the stable clock recovery for the input of polarization-scrambled data signals.     

Extreme timing jitter reduction of a passively mode-locked laserdiode by optical pulse injection

Reduction of the timing jitter to less than that of the master laser pulses was achieved for a passively mode-locked laser diode stabilized by subharmonic-frequency optical pulse injection. Detailed investigation revealed that this phenomenon originates from the short-term stability of the mode locking frequency under passively mode-locking operation with suitable bias conditions of the saturable absorber and the gain sections     

Repetition-frequency tuning of monolithic passively mode-lockedsemiconductor lasers with integrated extended cavities

Repetition-frequency tuning characteristics of passively mode-locked laser diodes (LDs) were investigated in detail. Anomalous tuning characteristics were observed when the bias conditions of the LDs were changed. Theoretical analysis using conventional self-consistent mode-locking theory showed that the anomaly originated from the change of the detuning from the cavity's round-trip frequency due to gain/absorption saturation effects in the passively mode-locked lasers. A wide tuning range of 280 MHz (1.6% of the repetition frequency) was obtained by changing the bias conditions of the extended cavity