Longitudinal Spatial Hole Burning Research Articles

Reduction of the frequency chirp of two section distributed feedback laser by nonuniform current injection

The effect of nonuniform current injection on the frequency chirp of a two section distributed feedback (DFB) laser, which is under large signal modulation, is measured while the optical intensity and optical modulation depth are fixed. By adjusting the injection current distribution in the laser, the frequency chirp can be reduced since the effects of carrier density fluctuations in the two sections on the chirp partially compensate each other. The range of the injection current ratio of the two sections is found in which the chirp is at its minimum. The reduction of the frequency chirp is predicted by a computer simulation for two section DFB lasers which accounts for the longitudinal spatial hole burning.

Analysis of the second- and third-order intermodulation distortion in DFB lasers including dynamic spatial hole burning effect

Analysis of second and third order intermodulation distortion characteristics of semiconductor DFB lasers is, for the first time, performed, including dynamic longitudinal spatial hole burning (LSHB) effect. We have shown theoretically that the third order intermodulation distortion can be lower than the calculated curve without LSHB effect, which is confirmed with experiments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Second- and third-order harmonic distortion in DFB lasers

The second- and third-order harmonic distortion in distributed feedback (DFB) lasers is simulated using a time domain large signal dynamic model. The effects of the longitudinal spatial hole burning, nonlinear gain, spontaneous emission, and current leakage are included in the model. Composite second-order (CSO) and composite triple beat (CTB) are calculated for an 80 channel 60-540 MHz system with 3% modulation depth in each channel. The simulation results are compared with the experimental harmonic distortion measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamics of spatial hole burning effects in DFB lasers

A lumped small-signal model for intensity and frequency modulation response of semiconductor lasers, including the effects of longitudinal spatial hole burning (SHB), is presented. It is shown that the laser dynamics including SHB-effects can be accurately described by three small-signal rate equations. The simplicity of the model gives new insight into SHB-effects on modulation response and cavity state stability. It is shown that SHB-effects have a cut-off frequency that depends on the carrier lifetime (including stimulated recombination) and the feedback of perturbations in the longitudinal intensity distribution during modulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

External feedback sensitivity of partly gain-coupled DFB semiconductor lasers

External optical feedback sensitivity of partly gain-coupled DFB semiconductor lasers has been analyzed in above threshold operation regime. Both the longitudinal spatial hole burning and the nonlinear gain compression have been taken into account. A comparison has been made among /spl lambda//4-shifted, pure index-coupled and partly gain-coupled DFB laser diodes. Even though pure index-coupled and partly gain coupled DFB lasers exhibit similar sensitivity to external optical feedback at the threshold, however, gain grating can reduce the feedback sensitivity when the lasers operate well above the threshold specially when the /spl kappa/L parameter is high. >

Detailed large-signal dynamic modelling of DFB laser structures and comparison with experiment

This paper describes the first general large-signal dynamic multiple-mode laser model that incorporates all the main mechanisms known to influence the dynamic behaviour of DFB laser structures with the exception of thermal effects: longitudinal mode spatial hole burning, carrier transport effects, nonlinear gain, and laser and submount parasitics. The time evolution of the output power and wavelength of all modes is predicted, and full spectra can be plotted as a function of time. The model has been extended to include an approximation to the effects of propagation down dispersive fibre, thereby allowing the simulation of filtered received eye diagrams. Detailed comparison of the model with the experimental performance of 2×λ/8 DFB lasers has shown good agreement, allowing the performance to be optimized, particularly with respect to longitudinal hole burning and carrier transport. The model is also applied to gain-switched operation of 2×λ/8 DFB structures, fast pulsing of three-section λ/4 DFB lasers, and the dynamic behaviour of complex coupling coefficient DFB laser structures.

A tractable large-signal dynamic model-application to strongly coupled distributed feedback lasers

The modulation characteristics of DFB semiconductor lasers have been studied using a transfer matrix method combined with an appropriate rate equation analysis. The model takes into account longitudinal mode spatial hole burning, as well as the nonuniform current injection resulting from the axially varying Fermi voltage, and can be used for the efficient simulation of static, small-signal, and large-signal dynamic properties. The program is applied to the interpretation of experimental data from a strongly coupled InGaAsP/InP DFB laser. The experimental high-frequency properties of this device are well described by the simulations. >

Corrugation-Pitch-Modulated Distributed Feedback Lasers with Ultranarrow Spectral Linewidth

We demonstrate that the spectral linewidth of distributed feedback (DFB) semiconductor lasers can be reduced by suppressing the longitudinal spatial hole-burning (SHB) effect. We confirm that the minimum spectral linewidth of lattice-matched multiple-quantum-well λ/4-shifted DFB (lattice-matched MQW-λ/4-DFB) lasers is limited by the SHB effect and we show that a corrugation-pitch-modulated (CPM) grating structure reduces the SHB effect while maintaining stable single-mode oscillation: at an output power of 25 mW, a lattice-matched MQW-CPM-DFB laser gives a spectral linewidth of 56 kHz. We further introduced the strained MQW structure into the CPM-DFB laser to obtain a narrower spectral linewidth. Introducing a 1.0% compressively strained MQW active layer into a CPM-DFB laser gives a spectral linewidth of 3.6 kHz at 55-mW output power and a linewidth floor (residual linewidth for extrapolated infinite output power) of 2 kHz and results in a linewidth-power product of 140 kHz·mW.

A transfer matrix method based large-signal dynamic model for multielectrode DFB lasers

A large-signal dynamic model capable of modeling the transient behavior of the output power and wavelength of multielectrode DFB lasers is described here. The key feature of the model is the use of a modified form of the transfer matrix method resulting in a time-dependent implementation of this technique. Other features are the inclusion of longitudinal spatial hole burning and nonlinear gain in the model. The versatility of the model is demonstrated in an analysis of the response of a two-electrode DFB laser under large-signal direct current modulation which illustrates the important role played by longitudinal spatial hole burning. The limited use of wavelength tunability in controlling chirp is also demonstrated. However, a scheme to improve the damping mechanism through nonuniform excitation called backbiasing is proposed. Finally, wavelength switching is demonstrated using the model. >

Static and dynamic properties of InGaAsP-InP distributed feedback lasers-a detailed comparison between experiment and theory

We have investigated the static and dynamic characteristics of phase shifted InGaAsP-InP DFB lasers mainly focusing on a comprehensive comparison between experimental results and numerical simulations. Experimental data of InGaAsP-InP mushroom type DFB lasers have been recorded, such as optical spectra, variations of the mode wavelengths with continuous and pulsed injection current, side mode suppression ratio, relative intensity noise, small signal amplitude modulation, and the transient response to 10 and 15 Gb/s large signal modulation. The theoretical model calculations in this paper are based on the transfer matrix method in combination with a rate equation analysis and take into account longitudinal mode spatial hole burning which is modified by the inhomogeneous current injection resulting from the axially varying Fermi voltage in both the static and the dynamic case. A good agreement between the experimental data and the theoretical simulations has been obtained extracting a set of parameters which consistently describes the measurements of our devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Self-suppression effect of longitudinal spatial hole burning in absorptive-grating gain-coupled DFB lasers

Longitudinal spatial hole burning (LSHB) induces degradation of longitudinal-mode stability in distributed-feedback (DFB) lasers. Measurement of frequency modulation characteristics has revealed that, in absorptive-grating gain-coupled DFB lasers, the LSHB diminishes as power increases. This anomalous behavior has been qualitatively explained by a theoretical analysis that took into account the saturable nature of the absorption of the gain-coupled grating. This LSHB suppression effect is advantageous for high-power single-longitudinal-mode operation of DFB lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of external optical feedback on distributed-feedback semiconductor lasers above threshold

An analysis of external optical feedback based on distributed-feedback (DFB) semiconductor lasers above threshold is presented. It is based on a numerical model taking into account the longitudinal spatial hole burning (SHB) effect, which has been recently known to be an important phenomenon for DFB lasers above threshold. Numerical results for a typical index-coupled quarter-wave-shifted (QWS) DFB laser with a moderate coupling coefficient ( kappa L=3) are given. It was found that the SHB effect can affect the sensitivity to feedback for DFB lasers above threshold.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Calculation of combined lateral and longitudinal spatial hole burning in lambda /4 shifted DFB lasers

A quasi-3-D semiconductor laser model that takes into account both longitudinal and lateral spatial hole burning is presented and used to perform calculations of the longitudinal side-mode-suppression ratio, the lateral side-mode kink power, and the linewidth of devices with coupling up to kappa L=6. Detailed results on the effects of inhomogeneous injection, wavelength detuning, phase shift position, and optical losses on these quantities are discussed and used to draw conclusions on the regions of stable single-mode operation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamics of SHB-Induced mode instabilities in uniform DFB semiconductor lasers

Time-domain simulations show strong 1 GHz pulsations and mode hopping of a uniform-DFB semiconductor laser operated at high output powers. These pulsations are due to asymmetric longitudinal spatial hole burning (SHB) of the carrier density by a high-frequency mode, leading to a pulse in a low-frequency mode. The pulse relaxes the carrier density to a symmetrical profile, which prefers the original mode.

Analysis of the stability of multiple-phase-shift distributed-feedback semiconductor lasers

The effects of the longitudinal spatial hole burning on the static lasing characteristics of a specific configuration of distributed-feedback semiconductor laser with three phase-shift regions are investigated using a numerical approach. A serious degradation of the stability of the optimum design, having the flattest axial intensity distribution at low output power, is predicted for drive levels beyond a critical value. The lasing wavelength exhibits a sudden shift (wavelength chirping), along with a significant degradation of the single-mode character of the longitudinal-mode spectrum. Thus, the potentialities of this multiple-phase-shift structure to provide a stable narrow-linewidth emission at high output power appear to be less than expected from results calculated for the near-threshold regime. Nevertheless, it is found that a multiple-phase-shift configuration that departs slightly from the optimum case suffices to recover most of the promises expected from this distributed-feedback laser design.

Influence of the axially varying quasi-Fermi-level separation of the active region on spatial hole burning in distributed-feedback semiconductor lasers

The longitudinal-mode characteristics of distributed-feedback semiconductor lasers subjected to longitudinal spatial hole burning have been investigated using an improved numerical modeling scheme. The main new feature of the model is that it allows for the natural axial variations of the separation between the quasi-Fermi levels (Fermi voltage) in the laser’s active region. This gives rise to a current density injected into the active region that varies along the laser axis, even for uniformly biased lasers. It is found that compared to the results obtained by assuming an uniform current density, the detrimental influence of the longitudinal spatial hole burning on important static characteristics of quarter-wave-shifted distributed-feedback lasers, such as the gain margin and the lasing wavelength stability, is weakened. Therefore, the usual assumption of an uniform injected current density gives rise to an overestimated influence of the spatial hole burning, this overestimation being more important for long-cavity lasers having a grating structure with a large coupling coefficient.

A new large-signal dynamic model for multielectrode DFB lasers based on the transfer matrix method

A large-signal dynamic model capable of modeling the transient behavior of the output power and wavelength of multielectrode DFB lasers is described. The key feature of the model is the use of a modified form of the transfer matrix method. Other features are the inclusion of spontaneous emission, longitudinal spatial hole burning, and nonlinear gain in the model. Results from the model demonstrate the important role played by longitudinal spatial hole burning in the chirping of DFB lasers and the limited use of tunability in controlling chirp. >

The static and dynamic characteristics of single and multiple phase-shifted DFB laser structures

The influence of longitudinal mode spatial hole burning (LMSHB) on the performance of distributed feedback (DFB) laser structures is examined in detail. A comprehensive model has been used to interpret the experimental results and to construct a theoretical framework that was utilized to develop more advanced device designs. An increasing side mode intensity with output power, movement of the lasing mode relative to the stopband, and curvature of the light-current characteristic at low power can all be manifestations of the influence of LMSHB on the static device performance. The dynamic behavior can also be affected, with extended wavelength chirp and amplitude patterning effects on the timescale of the effective carrier recombination time being particularly important.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Eye closure due to spatial hole-burning dynamics in λ/4-shifted DFB lasers

The dynamics of longitudinal spatial hole-burning (SHB) in a DFB laser are shown to cause significant pattern-dependent eye closure during transmitted 'ones' at 2-5Gbit/s. This closure is caused by recovery dynamics of the SHB during several 'zeros' and the subsequent time to reburn the hole. This eye closure can be reduced by biasing above threshold during 'zeros' to prevent recovery of the SHB, however higher biases cause significant turn-on peaks during ‘zeros’.

Numerical analysis of static wavelength shift for DFB lasers with longitudinal mode spatial hole burning

The static wavelength shift induced by longitudinal mode spatial hole burning is analyzed numerically for lambda /4-shifted DFB lasers. The effective Bragg wavelength at each bias level is introduced to clarify the contribution of nonuniformity in carrier density distribution to the lasing wavelength shift. It is shown that the wavelength shift is caused by two separate factors: by the position-dependent deviation and by the average value in the exact N/sub eq/ distribution. The former factor induces both red- and blue-shifted tuning due to the nonuniformity itself in carried density distribution, while the latter results in blue-shifted tuning due to the increase in modal gain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>