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.     

Gain saturation in DFB lasers with end reflectors

Results of an analysis of gain saturation in distributed feedback (DFB) lasers with end reflectors are presented. The dependence of the gain and frequency-shift eigenvalues of DFB lasers on the properties of end reflectors is reduced by gain-saturation effects. The degree of gain saturation is sensitive to the reflector properties if the distributed feedback and the reflections from the ends are of approximately equal strength.     

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.     

High-frequency pulsations in DFB lasers with amplified feedback

We describe the basic ideas behind the concept of distributed feedback (DFB) lasers with short optical feedback for the generation of high-frequency self-pulsations and show the theoretical background describing realized devices. It is predicted by theory that the self-pulsation frequency increases with increasing feedback strength. To provide evidence for this, we propose a novel device design which employs an amplifier section in the integrated feedback cavity of a DFB laser. We present results from numerical simulations and experiments. It has been shown experimentally that a continuous tuning of the self-pulsation frequency from 12 to 45 GHz can be adjusted via the control of the feedback strength. The numerical simulations, which are in good accordance with experimental investigations, give an explanation for a self-stabilizing effect of the self-pulsations due to the additional carrier dynamic in the integrated feedback cavity.     

DFB lasers with monolithically integrated passive waveguide

Novel MQW integrated passive waveguide (IPW) DFB lasers have been fabricated using a selective area MOVPE growth technique. Detailed measurements of the absorption loss of the passive waveguide in the structure as well as its influences on the laser linewidth are reported for the first time. A linewidth reduction factor of more than 3 is observed as compared to a simple DFB laser despite the high absorption loss     

Tunable DFB lasers with ultra-narrow spectral linewidth

A spectral linewidth of less than 98 kHz was achieved in three-section corrugation-pitch-modulated multiquantum-well distributed feedback (CPM-MQW-DFB) lasers over a tuning range of 1.3 nm, while maintaining a constant output power of 10 mW.<>     

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     

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.     

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     

Single-mode stability of DFB lasers with longitudinal Braggdetuning

Threshold and above-threshold behavior of laser cavities with various pitch-modulated feedback gratings is analyzed. The optimum design for high power operation with large side mode suppression is shown to deviate considerably from the distributed λ/4-shift design     

Gain-coupled DFB lasers with truncated quantum well second-ordergratings

Static and dynamic properties of in-phase gain-coupled multiple quantum well distributed feedback (DFB) lasers with second-order gratings have been measured at 1.55 μm. Devices with both facets antireflection coated and with a κL product between 4 and 5 have achieved a sidemode suppression ratio of over 63 dB. Relative intensity noise measurements have confirmed that the dynamic properties of second-order truncated well gain-coupled (GC)-DFB lasers are comparable to those with first-order truncated-well gratings     

Optimization of DFB lasers integrated with Franz-Keldysh absorptionmodulators

The influence of the residual facet reflectance on electroabsorption modulators monolithically integrated with DFB lasers is analyzed, for both index- and partly loss-coupled lasers. The optimum laser design is discussed from the standpoint of system requirements in long-distance 10 Gb/s transmission systems, requiring a minimum of chirp. The integrated device's performance will depend as much on the laser design as on the modulator. The DFB laser should be long (L>500 μm) with a coupling strength of about κ·L=2-3 and should have a high relaxation oscillation frequency. The residual reflectance should be less than 1·10^-4     

Wavelength accuracy in distributed phase-shifted DFB lasers

In the wavelength-division-multiplexed transmission framework, we investigate the ability of the distributed phase-shifted distributed-feedback (DFB) structure by enlarging the ridge to fulfil the wavelength accuracy required for array fabrication. Measurements on three different phase-shift geometries indicate that the poor localization of the optical mode along the cavity is detrimental to the wavelength accuracy. This study points out the best solution for DFB structure implementable in arrays, using already validated low-cost and high-throughput technologies in a production environment     

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     

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     

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.     

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)     

Wavelength tuning mechanism in three-electrode DFB lasers

The wavelength tuning mechanism is investigated for three-electrode distributed feedback (DFB) InGaAs-InGaAsP lasers. It is shown that the side and center sections play different roles in wavelength tuning: the former determines the effective Bragg wavelength and the latter contributes through the round-trip phase condition. The lasing wavelength is expressed in a simple form that renders the wavelength shift behavior exceedingly understandable. The indispensability of the thermal contribution to continuous broad range tuning is also clarified both theoretically and experimentally     

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     

Gain-coupled DFB lasers with a titanium surface Bragg grating

The improved performance of gain-coupled DFB lasers incorporating a titanium surface Bragg grating and a strained layer multiquantum well active region for operation at 1.55 μm is presented. Besides the essentially simplified fabrication process, the incorporation of the metallic, absorptive grating yields stable singlemode operation with a high sidemode suppression