Sampled grating conventional DFB lasers with fabricability and high immunity to the spatial-hole burning

Sampled grating has been used to enlarge the allowed range of coupling coefficient for high spatial hole-burning corrected single-frequency yield in conventional index-coupled DFB lasers with 1%-80% mirror combination. Theoretical calculation using an effective-index transfer matrix method shows that the allowed range of coupling coefficient increases approximately by 20% in the proposed sampled grating distributed-feedback lasers (DFB) lasers compared with conventional ones. Thus, the tolerance to the coupling coefficient variation improves to the level typically achieved in grating fabrication. The improvement is attributed to the flattened axial photon distribution of gap modes, and therefore stability in other lasing characteristics can be expected.     

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     

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     

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     

Self-suppression effect of longitudinal spatial hole burning inabsorptive-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     

Static and dynamic characteristics of DFB lasers with longitudinalnonuniformity

Static and dynamic characteristics of 1.55-μm distributed-feedback (DFB) lasers with longitudinal nonuniformity in optical intensity and carrier density are discussed. The nonuniformity is manifested in soft light-current threshold and varying differential quantum efficiency, blue-shifted static tuning, and free-carrier induced FM response     

Static and dynamic simulation for ridge-waveguide MQW DFB lasers

An efficient and versatile computer-aided simulator for the design and analysis of ridge-waveguide (RWG) multiple-quantum-well (MQW) distributed-feedback (DFB) lasers has been developed and is presented. This simulator combines spectral index method and Green's function-based transfer-matrix method (TMM) to deal with the transverse RWG MQW structure and longitudinal DFB structure, respectively. It is capable of simulating both static and dynamic behaviors for a variety of RWG MQW DFB lasers. The major difference from most of the existing models and analyses is that this simulator is capable of linking important device characteristics with practical material and geometrical parameters directly and self-consistently. For instance, the effects of lateral ridge width, vertical MQW layers and longitudinal nonuniformity are all explicitly included in the simulator. important laser characteristics, such as L-I curve, effective linewidth enchancement factor, static lasing wavelength shift, spectral linewidth, facet-power spectrum, AM and FM modulation responses, dynamic-wavelength chirping, as well as longitudinal photon and carrier distribution, can be predicted based on material and waveguide parameters. Therefore, this simulator may be used as an efficient and versatile tool for the systematic exploitation and optimization of a wide range of practical RWG MQW DFB lasers. Analysis of a λ/4 shifted SCH RWG MQW DFB laser is performed to illustrate the capability of this simulator     

Large-scale longitudinal spatial-hole burning contribution to lasergain compression

The impact of large-scale longitudinal optical power nonuniformities on the dynamics of a Fabry-Perot semiconductor laser is analyzed. New rate equations which incorporate this effect are derived. The effects of optical power nonuniformity are shown to be small compared to those of phenomenological gain compression at low frequencies, and insignificant at the relaxation oscillation frequency, except in the limit of low facet reflectivity     

A transfer matrix method based large-signal dynamic model formultielectrode 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     

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.<>     

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.     

Frequency control of a single-frequency fiber laser bycooperatively induced spatial-hole burning

A simple frequency control method of a single-frequency fiber laser is proposed and demonstrated. The mechanism of the frequency control arises from cooperatively induced spatial-hole burning (SHB) in a saturable absorber in the laser cavity. The SHB is formed by an external frequency-stabilized light source and the lasing light. Consequently, the lasing frequency can be tuned to the external light frequency. Moreover, narrow-linewidth operation is possible even when an external-source with a broad-linewidth is employed     

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     

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.     

Large-signal dynamic model for gain-coupled DFB lasers based on the transmission-line laser model

New scattering matrices are developed for the transmission-line laser model (TLLM) to enable the dynamics of gain-coupled DFB lasers to be studied. The resulting model is able to simulate large-signal transient responses and spectral characteristics of gain-coupled DFB semiconductor lasers, and gives threshold gains in excellent agreement with analytical formulations.<>     

Mode analysis of DFB SE lasers

This paper presents theoretical results on mode characteristics of surface-emitting (SE) lasers utilizing an active second-order grating section. Based on a coupled-mode approach, longitudinal modes and the associated space-harmonic transverse modes are calculated via a numerical technique. From these, the lasing-mode spectrum, near- and far-field patterns of the radiation mode, and the surface-emission power efficiency are obtained. Effects of the substrate reflector and the grating parameters are also investigated. Finally, comparisons are made with conventional, edge-emitting DFB lasers. The results indicate that with a suitable choice of structural parameter values, DFB SE lasers can be made to possess both the spectral discrimination of the conventional DFB lasers and the advantages of SE lasers at the same time and also that the second lowest longitudinal mode may be preferred over the fundamental longitudinal mode for many applications due to its symmetric field distribution     

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     

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)