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     

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.<>     

Stability of phase-shifted DFB lasers against hole burning

To study the above-threshold operation of DFB lasers, we have developed a multisection model which we present in detail. We have assessed the sensitivity to hole burning of various strongly coupled DFB lasers and explained their behavior both qualitatively and quantitatively, Good agreement is found with experimental results. It turns out that DFB lasers with a narrow stripe region are more stable than λ/4-shifted DFB lasers     

Analysis of the second- and third-order intermodulation distortionin 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     

Transient side-mode suppression in gain-coupled DFB lasers

Based on the spatially dependent multimode rate equations, we investigate the transient side-mode suppression in the gain-coupled distributed feedback (DFB) lasers. A simplified but accurate multimode dynamic analysis of gain-coupled DFB lasers is developed. To first order of perturbation approximation, the study includes various spatial effects, such as the distributed complex coupling, the nonuniform carrier distribution, and the nonlinear gain compression. It is found that gain coupling introduces high decay rates and low dynamic differential gains for the side modes, which effectively suppress their transient fluctuation and shorten the rise time of the transient side-mode suppression ratio (SMSR)     

Self-consistent Simulation of self-pulsating two-section gain-coupled DFB lasers

The role of cavity conditions in the dynamics of two-section gain-coupled distributed feedback (DFB) lasers is investigated using a self-consistent model. Self-sustained pulsation (SSP) exists only for devices with strongly coupled DFB gratings. As the coupling strength increases, multiple SSP regimes are developed. The SSP frequency tuning range increases as cavity length decreases. The frequency and modulation index predicted by the model agree well with experimental results. The facet condition of each section is found to affect SSP differently because of the asymmetrical behavior of the modes responsible for SSP.     

Improvement of single-mode gain margin in gain-coupled DFB lasers

Using the Bloch-wave analysis, this paper investigates the effect of the gain grating on the single-mode condition in DFB lasers. Various factors affecting the threshold gain of gain-coupled DFB lasers are analyzed in some detail. It is shown for the first time that unequal section lengths in the gain grating can have a significant effect on the single-mode gain margin of gain-coupled DFB lasers, especially when the linewidth enhancement factor α_M is large, because the long and shortwavelength Bloch waves are in phase and in antiphase with the index grating of DFB lasers, respectively     

Stabilization of the longitudinal mode against spatial hole burningin λ/4-shifted DFB lasers by quantum size effect

A novel method for stabilizing the spectral properties of λ/4-shifted distributed-feedback (DFB) lasers against the longitudinal spatial hole burning by using the quantum size effect in MQW structures is proposed and demonstrated experimentally and theoretically. The effect of the longitudinal spatial hole burning on the spectral behavior is shown to be determined by the α parameter, not by the differential index or by the optical confinement in the active layer. The reduction of the α parameter is found to be very effective in suppressing the spectral instabilities induced by the longitudinal spatial hole burning     

Sensitivity to external feedback for gain-coupled DFB semiconductor lasers

Performances of DFB semiconductor lasers with both index and gain spatial modulation are theoretically analysed in terms of threshold gain, emission wavelength, model discrimination and sensitivity to external optical feedback. It is found that gain coupling provides low threshold gain, high spectral selection in favour of the mode oscillating at the Bragg wavelength with a relative immunity from facet reflection, and the sensitivity to external feedback is not significantly reduced in comparison with the case of pure index coupling.<>     

Basic analysis of AR-coated, partly gain-coupled DFB lasers: thestanding wave effect

A theoretical analysis of distributed feedback (DFB) lasers with mixed gain and index coupling (partly gain-coupled DFB) is given for perfect antireflection (AR) coatings. Analytical expressions for the threshold gain, facet loss, and the relative depth of the standing wave pattern are derived. At the same time the importance of the standing wave effect and its consideration by coupled mode equations is shown. For purely gain-coupled DFB lasers, simple expressions for the effective linewidth enhancement factor and the longitudinal spontaneous emission factor are derived. In addition, various approximations describing the performance of purely gain-coupled DFB lasers are given     

Gain-coupled DFB lasers versus index-coupled and phase shifted DFBlasers: a comparison based on spatial hole burning corrected yield

A statistical yield analysis is presented for gain- and index-coupled distributed feedback (DFB) laser structures, allowing a comparison of their single longitudinal mode (SLM) yield capabilities. For the yield calculations, the threshold gain difference and the longitudinal spatial hole burning (SHB) are taken into account. By comparing the experimental and theoretical yield of index-coupled DFB lasers, the significance of SHB for correct yield predictions is illustrated. For the purpose of comparison, yield calculations for various λ/4-shifted DFB lasers (with low facet reflectivities) are presented. The most emphasis, however, is on partly gain-coupled DFB lasers. Estimations of practical gain coupling coefficient values for gain and for loss gratings are discussed     

Dynamic properties of partly gain-coupled 1.55-μm DFB lasers

The lasing characteristics and dynamic properties of partly gain-coupled 1.55-μm DFB lasers with a gain corrugation in the strained-layer MQW active region are presented. Narrow spectral linewidth, which is associated with the low linewidth enhancement factor, was experimentally measured. By analyzing data from RIN measurements, the damping rate, the damping factor, the intrinsic bandwidth and the effective differential gain were obtained. From the small-signal frequency response, a measured 3 dB bandwidth of 22 GHz at 10 mW output power was achieved. The high bandwidth is believed to be related to the high differential gain, resulting from the combination of longitudinal gain and index-coupling mechanisms and the reduction of the carrier transport time, which is due to an efficient lateral carrier injection along the longitudinal interface. Experimental results show that under 10 Gbit/s pseudorandom NRZ modulation, the devices have small wavelength chirp and clear eye openings making them suitable for long haul and high bit-rate applications     

Analytical formulation of distortion and chirp in CATV DFB lasersincluding spatial hole burning

Analytical formulas for distortions permitting the calculation of composite second order (CSO) and composite triple beat (CTB) in distributed feedback (DFB) laser diodes are given, including gain compression and longitudinal spatial hole burning (LSHB). An improved chirp expression including both effects is also proposed. Gain compression is compared to LSHB over the CATV band: LSHB effects on distortion and chirp are found to be dominant in actual CATV lasers. Nonmonotonous behavior (dips) function of frequency and bias current is verified in second- and third-order distortions in agreement with recent results     

Effects of spatial hole burning on gain switching invertical-cavity surface-emitting lasers

We present a numerical study of the effects of carrier diffusion and spatial hole-burning in vertical-cavity surface-emitting lasers under gain switching. Our model includes spatial and temporal dependences of both the optical field and the carrier density. Results show that spatial hole burning places a limit on the minimum achievable pulse width. We demonstrate that spatial hole-burning tan be avoided and shorter pulses can be obtained by using an appropriate pumping geometry. We also consider the case in which the laser operates simultaneously in two transverse modes and show that transverse-mode competition induced by spatial hole burning leads to period doubling and other interesting nonlinear behavior     

Er-fibre lasers: suppression of spatial hole burning by internal modulation

Spatial hole burning is suppressed in linear Er/sup 3+/ fibre lasers by two alternative modulation schemes. They move the standing wave pattern, formed by the two counterpropagating waves, along the gain medium in an oscillatory or continuous way. Output bandwidths <5 kHz result.<>     

Fabrication of AlGaAs-GaAs quantum-wire gain-coupled DFB lasers bya single MOCVD growth step

AlGaAs-GaAs quantum-wire (QWR) gain-coupled distributed-feedback (GC-DFB) lasers have been fabricated by a single metal-organic chemical vapor deposition growth step on 0.36-μm pitch V-groove arrays of GaAs. A record low-threshold current of 13 mA is achieved via DFB lasing from QWR gain at room temperature. The consistency of the photon energies of the lasing and the photoluminescence peaks from QWR, and about 4-nm-wide stopband with a large threshold gain difference observed in the near-threshold spectrum are presented as possible evidence for GC-DFB effects in these devices     

Side-mode suppression mechanism of gain-coupled DFB lasers with periodically etched quantum wells

The effect of the gain and index coupling on the side-mode suppression ratio (SMSR) is studied for gain-coupled DFB lasers with periodically etched quantum wells. An accurate expression for the SMSR based on the amplified spontaneous emission model is used with the local-normal-mode transfer-matrix method. The mechanism for the strong single-mode stability of the gain-coupled DFB lasers is explained by the difference between the effective gain and loss of the Bloch waves in the grating structures. This new view clearly shows the advantage of the gain-coupled DFB lasers in terms of single-mode stability.     

Dynamics of all-optical clock recovery using two-section index- and gain-coupled DFB lasers

The dynamics of coherent clock recovery (CR) using self-pulsing two-section distributed feedback (TS-DFB) lasers have been investigated. Both simulation and experimental results indicate fast lockup and walk-off of the clock-recovery process on the order of nanoseconds. Phase stability of the recovered clock from a pseudorandom bit sequence (PRBS) signal can be achieved by limiting the detuning between the frequency of free-running self-pulsation and the input bit rate. The simulation results show that all-optical clock recovery using TS-DFB lasers can maintain a better than 5% clock phase stability for large variations in power, bit rate, and optical carrier frequency of the input data and therefore is suitable for applications in optical packet switching.     

Cascaded strongly gain-coupled (SGC) DFB lasers with 15-nmcontinuous-wavelength tuning

More than 15 nm of continuous wavelength tuning range has been obtained by means of temperature tuning and current setting in three-section grating-cascaded strongly gain-coupled distributed-feedback lasers. Within the entire wavelength tuning range, a sidemode-suppression ratio beyond 50 dB, a linewidth of less than 10 MHz, and a fiber-coupled output power of more than 5 dBm were achieved     

Asymmetric gain induced by longitudinal spatial carrier burning in semiconductor lasers

Nonlinear gain caused by dielectric corrugation resulting from the cavity standing wave of a lasing mode in semiconductor lasers is investigated using the perturbation approach. The results show that the nonlinear gain spectrum is asymmetric when the linewidth enhancement factor alpha not=0, and the possibility of single mode operation is greater at alpha =0.<>