Large-signal effective alpha factor of complex-coupled DFB semiconductor lasers

DFB lasers with complex (gain and index) coupling have been proposed as low-chirp devices because of a reduction in an effective alpha factor (linewidth enhancement factor) calculated using a small-signal analysis. The article represents comprehensive numerical simulations showing that there is a corresponding reduction in the large-signal effective alpha factor, calculated from the time-bandwidth product of transient optical pulses, and a significant effect on the transient response.<>     

Novel measurement method of linewidth enhancement factor in semiconductor lasers by optical self-locking

The authors propose a simple, accurate and novel method to measure the value of the linewidth enhancement factor in semiconductor lasers. It utilises the asymmetry of the frequency locking range of a semiconductor laser with the optical feedback from the external CFP cavity. The measured value for a 0.8 mu m CSP-type AlGaAs laser was 3.07+or-0.26, which showed good agreement with those measured by other conventional methods.<>     

Single-polarization operation of fiber distributed feedback (DFB)lasers by injection locking

This paper investigated injection locking characteristics of a fiber distributed feedback (DFB) laser operating in dual polarizations, and found that it could operate in a single polarization by injection locking. The locking bandwidth was less than 10 MHz, much narrower than DFB laser diodes as a consequence of longer cavity length and resulting longer photon lifetime. As a more practical method to realize single-polarization operation of the fiber DFB laser, we proposed self-injection locking with a polarization-selective optical feedback composed of a mirror and a polarizer, and demonstrated a stable single-polarization operation     

Dual-mode injection locking of two independent modelocked semiconductor lasers

A novel method of optical injection locking to obtain efficient synchronisation between independent sets of optical frequency combs from two modelocked semiconductor lasers is presented. Locking dynamics are experimentally observed and the performance of oscillator synchronisation is demonstrated by an optical heterodyne detection experiment.     

Chaos in coupled semiconductor lasers with an electronic delayedfeedback caused by injection locking

We proposed and demonstrated a novel hybrid chaotic system using injection locking in coupled laser diodes. This system consists of double laser diodes, a Fabry-Perot etalon, and an electronic delay feedback. The bifurcation diagram route to chaos and variation of the temporal waveform of the output light intensity were observed relative to the feedback gain. Some peculiar bifurcation properties, which were caused by a strong nonlinear effect due to injection locking, were observed. We also analyzed the experimental results using Van der Pole equations coupled with rate equations. The results of theoretical analysis showed good agreement with the experimental results     

Influence of inhomogeneous injection on sidemode suppression in strongly coupled DFB semiconductor lasers

A longitudinally nonuniform injection counteracting the spatial hole burning in quarter-wavelength shifted DFB semiconductor lasers with strong coupling has been obtained from a quasi-three-dimensional model. This effect enlarges with falling series resistances. It substantially lowers the degradation of sidemode suppression compared with former models that assume homogeneous injection.<>     

Mode instabilities in complex-coupled DFB semiconductor lasers

The authors investigate the mode stability of complex-coupled lasers with gain gratings and predict unstable mode solutions for moderate output powers when antiphase coupling is used. These instabilities are likely to have a negative impact on the performance of optical communications systems     

Simulation of DFB semiconductor lasers incorporating thermaleffects

Static and dynamic thermal processes are incorporated into the modeling and simulation of DFB lasers. Analytical expressions for AM/FM responses considering both carrier and thermal effects are obtained. The self-consistency of this model is verified by comparing with experiment. Thermal effects on the laser performance such as L-I characteristics, lasing wavelength chirp, small signal AM/FM and large signal modulation responses are examined. It is noted that there is a trade-off between static and dynamic thermal wavelength chirps     

Dynamic properties of push-pull DFB semiconductor lasers

Using the spatially dependent multimode rate equations, we present a systematic study of small-signal dynamics of push-pull DFB lasers. The various spatial effects such as the longitudinal spatial hole burning, nonlinear gain compression, side-mode contribution, and push-pull modulation are all analyzed in a self-consistent manner. With the closed form expressions for the AM and FM responses, we show explicitly that the resonance frequency and the first cut-off frequency of push-pull DFB lasers are determined by the frequency spacing and the threshold gain difference between the lasing mode and its closest antisymmetric side mode, respectively. Numerical results reveal that a high modulation speed with a very low frequency chirp can be achieved with the push-pull DFB lasers     

Design of asymmetric quarter-wave-shifted DFB semiconductor lasers

Quarter-wave-shifted distributed-feedback (QWS-DFB) semiconductor lasers with asymmetric structure are theoretically analysed in terms of wavelength selectivity, differential efficiency, linewidth and sensitivity to external optical feedback. The analysis is used to design narrow-linewidth lasers with low sensitivity to optical feedback for coherent transmission.     

Injection locking of broad-area angled grating DFB lasers

The injection locking properties of an angled-grating distributed feedback laser have been measured experimentally. Fringe visibility measurements were used to ascertain the quality of the phase lock between the master and slave laser as a function of master coupling level and frequency detuning. Experimental results show an asymmetric lockband with the stable lock region increasing in width as the coupling is increased     

Transmission line description of optical feedback and injection locking for Fabry-Perot and DFB lasers

A general theory of semiconductor lasers is presented in which the static and dynamic properties are expressed in terms of the effective left and right reflectivities at a given reference plane in the laser. The theory applies to Fabry-Perot and DFB solitary lasers and to laser configurations with strong optical feedback or strong light injection.     

Intermodal injection locking and gain profile measurement of GaAlAs lasers

Injection locking of a GaAlAs laser by coupling light into a nonlasing Fabry-Perot mode separated by up to 22 mode spacings (60 Å) from the free-running lasing mode is demonstrated. The gain spectra for operation above threshold are determined from measurements of the wavelength dependence of the injected power required for locking. These results are compared to gain curves deduced from spontaneous emission spectra measured slightly below threshold. For the greatest wavelength separation, the gain difference was found to be about 4 cm^-1, with injected power levels of 1 percent of the total slave laser power required to obtain 75 percent of the total energy at the master laser frequency.     

Mode locking of Brewster-angled semiconductor lasers

Brewster-angled semiconductor laser diodes have been actively mode locked in an external cavity to produce mode-locked pulses of 11 ps duration and 0.5 W peak power with no evidence of temporal or spectral substructure.     

Optical modulation technique for carrier lifetime measurement in semiconductor lasers

We propose a new accurate method for differential carrier lifetime measurement, in which the laser under test biased below threshold is optically modulated. Experimental results are very reproducible and show very high signal-to-noise ratio, allowing also direct measurements of carrier density and transparency carrier density N/sub 0/. No additional technological process for the laser under test are required.     

External-Q factor measurement of an oscillator by the injection locking ellipse

The usual relation between the external Q factor and the locking bandwidth of an oscillator is not valid when the device reactance is nonlinear. The correct value of the Q factor is then obtained by using a certain frequency interval that can be easily read on the locking ellipse in this relation, rather than the locking bandwidth.     

Frequency offset locking of AlGaAs semiconductor lasers

Frequency offset locking was proposed as a reliable electrical negative feedback technique for tracking and sweeping of a semiconductor laser frequency. A frequency stabilized laser was used as a master laser, whose residual frequency fluctuations were 140 (kHz) at the integration time (τ) of 100 msleq tau leq 100s. A digital phase comparator of a large dynamic range of2pi times 2^{11}(rad) was employed in the feedback loop to reduce the phase fluctuations of the beat signal between the master and slave lasers. Performances of frequency tracking and sweeping of the slave laser were quantitatively evaluated, and the results are: residual frequency fluctuations of the beat signal were reduced as low as 11 (Hz) attau = 100s, which meant that the residual frequency fluctuations of the slave laser were almost equal to those of the master laser, i.e., the slave laser frequency tracked accurately to the master laser frequency. Both the capture range and lock range of the beat frequency were 1.22 GHz. Frequency tunable range of the slave laser was 36.6 GHz under the condition of frequency offset locking, in which the slave laser frequency fluctuations were maintained as low as the one given above.     

Fabry-Perot semiconductor laser injection locking

Injection locking of a semiconductor laser is studied using a Fabry-Perot (FP) model. For low injection powers the FP model gives the same results as the rate equation model. At higher injection powers, the FP laser has an unlimited injection locking bandwidth; however, regions of the bandwidth are dynamically unstable. The influence of the linewidth enhancement factor on injection locking and its stability are also studied using the FP model. Finite values of the linewidth enhancement factor lead to increased locking bandwidth, asymmetry in the locking range and a saddle-node bifurcation in the injection locked solutions, of which only lower carrier density arm has linearly stable solutions.     

Analysis of DFB semiconductor lasers with external optical feedback

A theoretical analysis of the sensitivity of DFB semiconductor lasers to external optical feedback is presented. Numerical simulations allow the authors to determine the optimum facet reflectivity, for a given κL value, to minimise the sensitivity to external optical feedback