Gain-switched Semiconductor Laser Research Articles

Efficient ultraviolet-light (345-nm) generation in a bulk LiIO_3 crystal by frequency doubling of a self-seeded gain-switched AlGaInP Fabry–Perot semiconductor laser

A self-seeded gain-switched semiconductor laser is applied as a fundamental optical source to second-harmonic generation, and improvement of frequency-conversion efficiency is demonstrated. 345-nm ultraviolet light of 0.12-muW average power is generated in a bulk LiIO(3) crystal for 690-nm fundamental light of 10.5-mW average power, which is composed of 500-MHz pulse trains emitted from an AlGaInP diode laser with a 68-ps temporal width and a 0.11-nm spectral bandwidth. It is clearly shown that the narrowing of the spectral bandwidth by the self-seeding technique gives rise to a normalized conversion efficiency of 0.218%/Wcm , which is fourtimes larger than that for pulses without seeding.

Frequency dynamics of gain-switched injection-locked semiconductor lasers

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.

Generation and Propagation of Short Pulses in the 1.3 µm Optical Communication Window

A experimental study on short pulse generation from praseodymium-doped fluoride mode-locked fibre lasers and gain-switched DFB semiconductor laser is presented[l]. The fiber lasers were modelocked either by using a phase modulator (PM) or cross phase modulation (XPM). In the first case, pulses of 33 ps and 75 mW (peak power) at 419 MHz were obtained. In the second case, we used a 1.55 µn gain-switched diode laser to modulate the phase of the 1.3 um laser field via intracavity XPM. This produced 50 ps, 13 mW pulses at 2.8 GHz. In this second laser we also observed stable dark pulses with 140 to 220 ps duration, making this laser attractive as a dark soliton generator. In the case of the gain-switched (GS) diode laser, we obtained (after spectral filtering) 5 ps, 20 mW at 1 GHz. Using this GS laser we propagated solitons in 50 km of standard fibre, thus demonstrating its potentials as a simple and inexpensive soliton quality laser source.

Efficient generation of wavelength-tunable single-mode pulses at 1.3 and 1.55 μm by a simplified approach of self-injection seeding

A simple configuration to generate tunable single-mode pulses by self-injection seeding of a gain-switched semiconductor laser is demonstrated. The key part of the setup consists of a highly dispersive fiber and a fiber mirror. The configuration shows improved stability and promise to operate at both 1.3 and 1.55 /spl mu/m. The lasers at the respective wavelengths have been successively tuned over 13.43 and 12.81 nm with a side-mode suppression ratio >15 dB. Continuous access of wavelength with an enhanced tuning range is also achieved by combining both electrical and thermal control on the laser.

Low error rate return-to-zero direct modulation of gain-switched lasers

A simple scheme for data transmission by direct modulation of a gain-switched semiconductor laser is demonstrated. Experimental results are presented that show low error rate and low pattern dependence. The sensitivity of the error rate to changes in drive conditions is also measured. Comparison of our direct modulation scheme with external modulation of the same gain-switched laser shows nearly penalty-free operation.

Analysis of emission and gain saturation in gain-switched semiconductor lasers

We calculate the switch-on dynamics of a single-mode bulk GaAs laser diode including carrier kinetics with Boltzmann collision integrals for carrier–carrier and LO–phonon-carrier scattering. The dependencies of the dynamics on the electrical pump amplitude and on the laser frequency are determined. The shortest light pulse with 12.8 ps (FWHM) is obtained for a laser mode in the gain maximum and a pump rate of 15.5 times the threshold value. The resulting differential gain, transparency density, and gain saturation coefficient are calculated as functions of the pump rate and the mode detuning. The gain saturation during the laser switch-on is enhanced at the trailing edge of the light pulse. This is well described by an extended dynamical gain saturation model that includes correction in terms of the light intensity and its first time derivative. The coefficients of this dynamical gain saturation model are determined by a comparison of the detailed carrier kinetics in a laser with the corresponding rate equations.

Nonlinear optical compression of Er/sup 3+/-fiber amplified 1.5-μm laser diode pulses

Femtosecond optical pulses of 110-200-fs width have been produced using a gain-switched distributed-feedback semiconductor laser followed successively by a linear compression stage and a nonlinear compression stage. Analysis is focused on this last stage where pulses with peak powers corresponding to 10and 12-order solitons are fed at the fiber input end. Experimental results are well described using both the modified nonlinear Schrodinger equation and an accurate intensity and phase model of the gain-switched laser diode. Experiments are shown to be correctly described only if both intrapulse stimulated Raman scattering and third-order dispersion are taken into account. Guidelines are then given to optimize the nonlinear fiber compression using laser diodes and fiber amplifiers. The influence of the third-order dispersion in the fiber compressor is first evaluated. Second, the nonlinear self-phase modulation induced in the fiber amplifier is studied. It is shown to be the main factor limiting any further pulse shortening with this technique.

Generation and Propagation of Short Pulses in the 1.3 µm Optical Communication Window DOI: 10.14209/jcis.1996.7

A experimental study on short pulse generation from praseodymium-doped fluoride mode-locked fibre lasers and gain-switched DFB semiconductor laser is presented[l]. The fiber lasers were modelocked either by using a phase modulator (PM) or cross phase modulation (XPM). In the first case, pulses of 33 ps and 75 mW (peak power) at 419 MHz were obtained. In the second case, we used a 1.55 µn gain-switched diode laser to modulate the phase of the 1.3 um laser field via intracavity XPM. This produced 50 ps, 13 mW pulses at 2.8 GHz. In this second laser we also observed stable dark pulses with 140 to 220 ps duration, making this laser attractive as a dark soliton generator. In the case of the gain-switched (GS) diode laser, we obtained (after spectral filtering) 5 ps, 20 mW at 1 GHz. Using this GS laser we propagated solitons in 50 km of standard fibre, thus demonstrating its potentials as a simple and inexpensive soliton quality laser source.

Gain saturation and pulse statistics in single-mode semiconductor lasers.

The statistics of pulse height ${\mathit{I}}_{\mathrm{max}}$, pulse energy A, and pulse width of gain switched semiconductor lasers are obtained for different positive and negative values of the gain saturation factor by using the linear dependence of ${\mathit{I}}_{\mathrm{max}}$ and A on the switch-on time.

Jitter and dynamics of self-seeded Fabry-Perot laser diodes

We report an investigation of the time jitter and the switch-on dynamics of self-seeded gain-switched Fabry-Perot semiconductor lasers. Self-seeding leads to single-mode emission and is shown do reduce the turn-on delay time jitter to 210 fs, which is one order of magnitude lower than observed for gain-switched DFB-lasers. The number of round-trips necessary to reach the stable state after switch-on is determined using time resolved spectral measurements to be in the range of 5-10. The number of round-trips is 1-2 orders of magnitude shorter than for mode-locked lasers, thus opening the possibility of fast wavelength switching (i.e. over 5-10 bit periods). A theoretical model is in good agreement with all our results. >

Coherence and noise properties of gain-switched Fabry-Perot semiconductor lasers

We present a detailed study of the optical power spectrum and coherence properties of Fabry-Perot semiconductor lasers under gain-switched operation. We demonstrate that the distribution of longitudinal modes under gain-switched operation is described to high accuracy by a Gaussian envelope, in contrast to continuous wave (CW) lasers where the distribution is Lorentzian. We show that the minimum values of coherence under gain-switched operation are two orders of magnitude lower than under CW operation. We also demonstrate that intensity noise generated through interferometric conversion of mode partition noise is markedly different for gain-switched and CW lasers. The results are important for a host of potential applications that use low-coherence interferometry.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cavity optimization for minimum pulsewidth of gain-switched semiconductor lasers

The optimum photon lifetime for the minimum pulsewidth of a gain-switched laser is investigated both analytically and numerically. For a fixed pump power, the shortest pulse can be produced when the laser cavity is optimized to give an optical pulsewidth that is /spl sim/2.5 times the photon lifetime. This is in good agreement with experimental results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Improved expression for the time-bandwidth product of picosecond optical pulses from gain-switched semiconductor lasers

An improved expression for the time-bandwidth product (DeltatDeltaf) of picosecond optical pulses from gain-switched semiconductor lasers was obtained by a series of numerical simulations based on rate equations without making any assumption for a particular pulse shape and chirp condition. The relation can be expressed by [Equation] as a function of the linewidth-enhancement factor alpha. The influence of the spontaneous emission factor beta on the relation is also examined.

Effects of extra low-frequency noise injection on microwave signals generated by a gain-switched semiconductor laser

Low-frequency upconverted noise (1/f laser intensity noise and the low-frequency noise of the driver source) has been identified as the dominant noise generation mechanism in microwave signals generated by a Fabry–Perot gain-switched semiconductor laser. An experimental investigation is carried out using extra low-frequency noise injection added to the drive signal. Results show the dependence of the broadband intensity noise level, formed by the overlapping of the upconverted noise sidebands present at each harmonic, on gain-switching input parameters.

Special Issue on Laser Parameter Control. Estimation of Chirp in Optical Pulses from Gain-Switched Semiconductor Lasers.

Chirp occurring within optical pulses from gain-switched semiconductor lasers causes significantproblems in their applications. However, a common expression giving the chirp estimationmay not be fully applicable in actual experiments. Simple and precise methods for carryingout the chirp estimation are required in such a field.In this paper, problems and their solutions about chirp estimations from gain-switchingoptical pulses are reviewed laying stress on our results obtained from related studies.

Propagation of single-mode 1.5-μm gain-switched semiconductor laser pulses in normally dispersive fibers

The behavior of gain-switched semiconductor laser pulses propagating in normally dispersive fibers is analyzed both theoretically and experimentally. A simple gain switch model is analytically derived from rate equations including gain compression effects in order to predict the instantaneous optical intensity and frequency during the pulse. A great deal of attention is given to the phase equation. It is shown that carrier dependence of the phase-amplitude coupling factor /spl alpha/ must be taken into account to accurately describe experiments. By means of an Er/sup 3+/ doped amplifier, nonlinear propagation of such pulses in normally dispersive fibers is experimentally studied for various peak powers up to 3.2 W. Large Kerr-induced spectrum narrowing is demonstrated together with the production of pulses of adjustable width from 3 to 12 ps. Corresponding time-bandwidth products measured between 0.4 and 0.8 are close to the Fourier transform limit. These results are successfully compared to theory by means of computer simulation involving both the gain-switch model and the nonlinear propagation in the fiber. >

Generation of 2.5-ps light pulses with 15-nm wavelength tunability at 1.3 mu m by a self-seeded gain-switched semiconductor laser

The generation of 2.5-ps pulses by a Fabry-Perot semiconductor laser with 15-nm wavelength tunability is demonstrated. A commercial laser is switched to single longitudinal mode pulse emission by self-seeding, using an external grating. The sidemode suppression ratio is better than 15 dB, with a peak value of 21 dB. The linearly red chirped pulses are compressed from 21-30 ps to 2.5-3.5 ps in a dispersion-shifted fiber. The simple experimental setup consists of a fiber coupler, a grating, and the dispersion-shifted fiber.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Subpicosecond soliton compression of gain switched diode laser pulses using an erbium-doped fiber amplifier

Combining a gain-switched semiconductor laser, an erbium-doped fiber amplifier, and two cascaded fibers for compression, optical pulse trains of 0.63 ps FWHM in autocorrelation trace, with certain suppression of wing structures, were successfully generated. Evaluated energy in each pulse was >or=17 pJ. A computer simulation of the soliton-effect compression of gain-switched pulses containing substantial chirps was performed. Optimization of the pulse train by simply controlling the DC bias current on the master distributed-feedback (DFB) laser was clarified.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ultralow timing jitter in electrically gain-switched vertical cavity surface emitting lasers

We report that contrary to common belief, it is possible to achieve very low timing jitters of less than (≊100 fs) in gain-switched semiconductor lasers, which is comparable to actively mode-locked semiconductor lasers. A vertical cavity surface emitting laser was used to achieve these results, with a gain-switched pulse width of 12 ps.

Detection of a weak external signal via the switch-on-time statistics of a semiconductor laser.

We calculate the passage-time statistics of a gain-switched single-mode semiconductor laser with a weak, detuned injected signal. Results are given in terms of a relevant scaling parameter. We find that a weak injected signal can be detected by measurements of the mean passage time or its variance whenever its frequency is close to the one of the semiconductor lasers in the on state. In this case, the external signal is resonant with the laser field at the time when amplification becomes possible, and hence it triggers the switch-on more efficiently. The detection bandwidth is of the order of 50 GHz for the case considered.