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     

Mode selectivity in DFB lasers with cleaved facets

The mode selectivity of distributed feedback lasers with cleaved facets is examined for various positions of the grating relative to the facets. The expected yield of single-mode lasers is calculated for several values of the coupling coefficient.     

Mode instability and linewidth rebroadening in DFB lasers

Results of a detailed linewidth theory for DFB lasers are presented. The authors show that the fluctuations in the longitudinal distributions of photon and carrier densities can have significant effect on the linewidth. A new type of linewidth rebroadening is identified, which is attributed to model instability. This may explain the rebroadening observed in cases with high sidemode suppression.<>     

Three-dimensional analysis of multiquantum well DFB lasers

A three-dimensional analysis is presented by using the transfer-matrix method for 1.55 mu m InGaAs BH MQW DFB lasers. It is shown that the coupling coefficient becomes maximum and the threshold current density becomes minimum at a certain number of wells, depending on the stripe width.<>     

A transfer matrix-based analysis of multielectrode DFB lasers

A numerical model for characterizing the steady-state operation of multielectrode distributed feedback lasers (DFB) is presented. The model is based upon the transfer matrix method which allows a wide range of structures to be analyzed. Using the model to compare a two-electrode device and a three-electrode device reveals that the latter has superior operating characteristics for tunability or FSK.<>     

Dynamic analysis of multiple wavelength DFB fiber lasers

The behavior of multi- and single-wavelength distributed feedback fiber lasers is simulated in a comprehensive dynamic model. The evolution of the spatial distribution of gain, saturation-induced gain gratings, and spontaneous emission is taken into account. Stability and relative intensity noise (RIN) of the different laser types and laser modes are compared, and the effect of varying degree of pump RIN and quenching of spontaneous emission lifetimes is analyzed     

A unified Green's function analysis of complicated DFB lasers

An efficient full-wave analysis technique for one-dimensional optical domains, known as the recursive Green's function method (RGFM), is presented for evaluation of distributed feedback (DFB) laser cavities with arbitrary material profiles. The method first constructs the Green's function of an inhomogeneous domain and subsequently uses Green's theorem to determine the laser optical field, lasing wavelength, and threshold gain. The technique is applied to investigate the performance of three DFB laser structures: a chirped-grating configuration, a modulated stripe width design, and a reduced duty cycle complex-coupled device. These structures are evaluated in terms of their single-mode lasing behavior and the uniformity of the optical field within the cavity     

Tunability of multisection DFB lasers

Using a comprehensive multisection model taking into account spatial hole burning, we study the stability and tunability of various multielectrode DFB lasers. For each structure, we determine the emission wavelength, the output power and the spectrum as a function of the injected currents and find a good agreement with experimental results. For the first time, different structures are examined with the same model, leading to fruitful comparisons     

Realization of dot DFB lasers

Dot lasers with first-, second- and third-order gain coupled distributed feedback (DFB) gratings have been realized by low damage dry etching in combination with wet chemical etching and epitaxial over-growth. This technique allows above room temperature (RT) operation of dot DFB lasers with dot diameters down to 85 nm. The laser spectra show the expected emission of gain coupled DFB lasers. Threshold current densities between 1.1 kA/cm/sup 2/ and 2.6 kA/cm/sup 2/ could be obtained depending on size of the active region. An improvement in T/sub 0/ could be demonstrated comparing 0-D/1-D/2-D lasers on the same wafer. Based on the dot grating geometry improvement of the side mode suppression ratio (SMSR) was observed for broad-area dot DFB lasers.     

Theoretical analysis of external optical feedback on DFB semiconductor lasers

The effect of external optical feedback on resonant frequency, threshold gain, and spectral linewidth of distributed feedback (DFB) semiconductor lasers is theoretically analyzed. The analysis applies to any type of laser cavity formed by a corrugated waveguide limited by partially reflecting facets. It is shown that the sensitivity to optical feedback on a facet is closely related to the power emitted through this facet. Numerical results on wavelength selectivity and on sensitivity to optical feedback are given for conventional DFB lasers having an AR-coated facet and for quarter-wave-shifted (QWS) DFB lasers with AR-coatings on both facets. Both laser types are found to be more sensitive to optical feedback on their AR-coated facet than Fabry-Perot lasers for lowkL. On the other hand, QWS-DFB lasers are found to be relatively insensitive to optical feedback for largekL.     

A rigorous analysis of DFB lasers with large and aperiodiccorrugations

A rigorous electromagnetic analysis of the distributed-feedback (DFB) laser in which the corrugations, seen as a cascade of interacting discontinuities, need not be periodic or small is presented. The analysis takes into account the continuous spectrum of radiation modes excited at each discontinuity and their interaction between successive discontinuities without assuming periodicity. The analysis of the individual discontinuity is based on the concept of the transition function. The analysis of the cascade of discontinuities takes place by generalized transfer-matrix methods (including the continuum), bypassing the need for a periodic unit cell. This enables the total diffraction generated by the grating to be modeled exactly. Particular attention is devoted to the threshold analysis of a ridge-waveguide DFB laser, for the case of a rectangular and a trapezoidal grating     

Dispersive self-Q-switching in self-pulsating DFB lasers

Self-pulsations reproducibly achieved in newly developed lasers with two distributed feedback sections and with an additional phase tuning section are investigated. The existence of the dispersive self-Q-switching mechanism for generating the high-frequency self-pulsations is verified experimentally for the first time. This effect is clearly distinguished from other possible self-pulsation mechanisms by detecting the single-mode type of the self-pulsation and the operation of one section near the transparency current density using it as a reflector with dispersive feedback. The operating conditions for generating this self-pulsation type are analyzed. It is revealed that the required critical detuning of the Bragg wavelengths of the two DFB sections is achieved by a combination of electronic wavelength tuning and current-induced heating. The previous reproducibility problems of self-pulsations in two-section DFB lasers operated at, in principle, suited current conditions are discussed, and the essential role of an electrical phase-control section for achieving reproducible device properties is pointed out. Furthermore, it is demonstrated that phase tuning can be used for extending the self-pulsation regime and for optimizing the frequency stability of the self-pulsation. Improved performance of the devices applied as optical clocks thus can be expected     

Broad-band continuously tunable all-fiber DFB lasers

Up to 27-nm continuous tuning is demonstrated from Er/Yb all-fiber distributed-feedback (DFB) lasers using a simple tuning technique for axial extension and compression. The demonstrated devices operate with powers up to 10 dBm and remain operating in single mode over the full tuning range. Our results represents the broadest tuning-range previously reported in any DFB laser configuration and demonstrate that uniform compression tuning of long Bragg gratings is possible with high reliability     

Superfast 1.55 mu m DFB lasers

Singlemode 1.55 mu m distributed feedback lasers with a modulation bandwidth of 22.5 GHz are reported. The devices use p-doped, strained multiquantum-well active regions to reduce the bandwidth limitations caused by hole transport, as well as increasing the intrinsic response of the device. The present 22.5 GHz bandwidth is limited by device parasitics.<>     

Fiber DFB lasers with ultimate efficiency

A novel method for designing distributed feedback (DFB) lasers with ultimate efficiency is presented. The method is based on the calculation of the optimum signal distribution for a given pumping scheme and derivation of the grating profile that sustains this distribution so that at every point in the cavity pump-to-signal conversion efficiency is maximum. Theory applied experimentally in Er-Yb co-doped fiber resulting in a 57% increase in the efficiency with 10% shorter device length compared with standard optimized co-pumped designs.     

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     

Performance limitations of high-power DFB fiber lasers

In this letter, we experimentally study several different configurations of high-power single-frequency sources based on distributed-feedback fiber laser (DFB FL). In particular, we have compared two schemes in terms of pumping efficiency, relative intensity noise (RIN) and optical signal-to-noise ratio (OSNR): directly pumped DFB FLs pumped by a high-power fiber-based pump source (stand-alone DFB FL) and a combination of a DFB FL and a power amplifier (amplified DFB FL). At the output powers below 40 mW, a stand-alone DFB FL has the highest OSNR (> 63 dB) and the lowest RIN (< -165 dB/Hz). The net efficiency of a stand-alone DFB FL can be doubled by using an amplified DFB FL at the expense of degraded OSNR and RIN. It is also shown that RIN below -160 dB/Hz and OSNR > 60 dB can be achieved by an amplified DFB with output power greater than 1 W.     

Theory of selfpulsations in two-section DFB lasers

A dynamic theory of asymmetrically pumped two-section distributed-feedback (DFB) lasers is presented, which gives a first explanation for the nature of the recently observed gigahertz-selfpulsations in terms of the relative shift between the feedback spectra of the two sections     

Measurement of mode partition in DFB lasers

Some DFB (distributed feedback) laser diodes have a satellite mode beside a main DFB mode even if FP modes are suppressed. In this paper, the mode partition noise is presented for several DFB lasers operating in multilongitudinal modes. The results show that under modulation at 140 Mbit/s, the mode partition coefficient k²of multimode DFB lasers is very small and at most 0.02 while that of FP lasers biased at the threshold level is 0.03 to 0.12. The numerical evaluation of the mode partition effect in two-mode DFB lasers suggests that a 20- dB suppression of the satellite mode power is enough to achieve a repeater spacing of over 100 km in the 280 Mbit/s fiber-optic transmission system with less than 0.1-dB power penalty.