Azimuthal mode discrimination in radially chirpedconcentric-circle-grating distributed feedback lasers

We report a theoretical investigation analyzing the threshold modes of a concentric-circle-grating (CCG) distributed feedback laser with a radially chirped first-order Bragg grating. A numerical coupled-mode analysis of a chirped CCG laser shows improved azimuthal mode discrimination for chirped gratings with linear, quadratic, and square-root radial dependence. Negatively chirped gratings, in which the grating period decreases with radius, result in improved threshold discrimination between the circularly symmetric fundamental mode and higher order modes; positively chirped gratings, in which the grating period increases with radius, result in decreased threshold for higher order modes in general. Also, the intensity for the fundamental mode at the grating center can be reduced by an order of magnitude using linearly chirped gratings     

Radiation losses in distributed feedback lasers and longitudinal mode selection

We show that seemingly symmetric longitudinal modes symmetrical distributed feedback (DFB) lasers utilizing higher orc Bragg reflection actually have differing thresholds and resonant wavelength shifts from the Bragg condition as a result of power radiated by the grating.     

Multiple-waveguide distributed feedback lasers

Coupled wave theory is used to examine the performance of lasers formed from an array of coupled stripe waveguides with a grating overlay. It is found that such lasers exhibit inherent longitudinal mode discrimination, and, if the number of coupled guides is small, inherent transverse mode discrimination as well. The conditions for such selectivity are examined in detail, and it is shown that the correct grating distribution to use is one in which the grating fills the overlap region between the coupled guides.     

Performance characteristics of buried facet quarter-wave shifted distributed feedback lasers

The performance characteristics of quarter-wave shifted GaInAsP distributed feedback lasers emitting near 1.3 mu m are described. The quarter-wave shifted grating is fabricated on a substrate using the double-exposure holographic technique. The low reflectivity required for this quarter-wave shifted DFB laser is obtained using buried facets at both ends of the laser. The lasers have threshold current of 30 mA, quantum efficiency of 0.18 mW/mA/facet, bandwidth of 11.5 GHz at 10 mW and 10 dB chirp width of 2.5 AA under 40 mA modulation current at 5 Gbit/s.<>     

Tunable two-segment distributed feedback lasers

The authors describe a two-segment distributed feedback laser at 1.3 mu m with a tuning range of 12.8 AA (209 GHz) and an FM response of 7 GHz/mA. The observed tuning behaviour is in qualitative agreement with an earlier theoretical model. Such two-segment DFB lasers are useful in frequency-multiplexed, frequency-modulated optical networks.<>     

Principles of distributed feedback and distributed Bragg-reflector lasers

Wave propagation in periodic waveguides is analyzed by decomposing the eigen Bloch waves into traveling-wave components. It is shown that the principal components consist of a primary forward wave, a primary backward wave, and their Bragg-scattered secondary waves. One important parameter is the coupling constantsdue to Bragg scattering, which relates the secondary wave to the respective primary wave. Laser threshold condition is then obtained by applying the continuity of tangentialEandHat the two boundaries. The results thus obtained are general and applicable to thin-film lasers with various waveguide structures. The laser threshold condition of thin-film Bragg lasers is expressed in terms of two effective reflection coefficients for easy comparison with conventional lasers. For appreciable reflection, a significant change either in the propagation constant or in the coupling constant is required. Two basic types of thin-film Bragg lasers are distributed-feedback (DFB) lasers in which Bragg scattering is confined to the active medium and distributed-Bragg-reflector (DBR) lasers in which Bragg scattering is limited to regions beyond the active medium. The threshold gain, frequency control, and mode selectivity for both types are analyzed and the analyses are applied to GaAs and Nd lasers. It is shown that DBR lasers should have a lower threshold gain and a better mode selectivity than DFB lasers. For distributed-feedback effect to play a significant role in thin-film Bragg lasers, the product kLintmust be greater than unity whereKis the distributed-feedback coefficient and Lintis the interaction length. Advantages for having periodic structures outside the active medium so as to relax constraints onkand Lintare also discussed.     

Second-order distributed feedback lasers with mode selection provided by first-order radiation losses

We present a theoretical study of the effect of radiation losses on the mode selectivity of DFB lasers with second-order gratings. For a second-order grating, interference of the radiation due to first-order diffraction of oppositely propagating guided waves cancels the radiation loss at one of the edges of the spectrum gap. This provides threshold gain discrimination of order 10 cm^-1against one of the two dominant modes occurring near the edges of the gap. This should allow fabrication of DFB lasers with properties that are nearly independent of the positions of the facets relative to the grating corrugations, which are uncontrolled. By applying antireflection coatings to the two ends, differential quantum efficiencies close to those of conventional Fabry-Perot lasers should be achievable.     

Antisymmetric taper of distributed feedback lasers

In a distributed feedback (DFB) laser with spatial index modulation, an antisymmetric taper of the feedback parameter,k, removes the threshold degeneracy, which is characteristic of uniform structures, and leads to one mode of particularly low threshold. Exact solutions are presented for the special case of an antisymmetric step ofk. An approximate perturbation method is developed which gives simple expressions for the threshold gains, and externalQ's of some tapered distributed feedback structures. The method is tested against two exact solutions, the uniform, and the stepped-kDFB laser. It is shown that the threshold of the stepped-kDFB laser is the lowest of any structure with an antisymmetric taper and a prescribed maximum value of|k|.     

Analysis of ridge-waveguide distributed feedback lasers

An analysis concerning the coupling efficiency and the threshold current density is presented for a ridge-waveguide distributed feedback laser. Particular attention is paid to the grating efficiency that is different under and beyond the ridge region because of different lateral environments and generally different corrugation depths. The threshold current density and its optimization are discussed for 1.55 μm InGaAsP lasers after considering the design parameters, such as the ridge width and the layer thicknesses. Both the TE and TM modes are considered. A narrow ridge helps to reduce the device threshold if the grating under the ridge is absent or is partially washed out during the ridge-overgrowth procedure.     

Groove-cell analysis of distributed feedback lasers

The propagation of waves in a groove cell is analyzed. This propagation is a function of transfer matrix, which contains the gain, together with the phase constant, the length of a groove cell, and the groove reflection coefficient. When the transfer matrix is cascaded the transmission of an optical field and the reflection coefficient of a distributed feedback structure can be determined even with unequal grooves and any groove number. With the help of an eigenvalue equation the threshold gain and phase constant of the resonator modes are determined for a symmetrical laser. The increase of threshold gain is demonstrated for an inhomogeneous laser with changed mean value of effective refractive index along the laser length. Bloch waves exist in a long grating (or a closed loop) of periodic grooves     

InAs-based distributed feedback quantum cascade lasers

InAs/AlSb distributed feedback quantum cascade lasers are presented. The lasers can operate in the single frequency regime at 3.34-3.38 mum in the 0-100degC temperature range in pulse mode. The wavelength tuning rate of the lasers is 0.27 nm/K and continuous tuning range up to 10 nm can be achieved.     

Electromagnetically induced distributed feedback intersubband lasers

We propose a method to coherently generate uniform and phase-shifted complex-coupled (CC) semiconductor distributed feedback (DFB) lasers based on intersubband transitions in n-doped quantum-well structures. This is done by utilizing infrared-induced coherent optical processes in these structures including resonant enhancement of refractive index of the conduction intersubband transitions and generation of laser-induced transparency and gain without inversion. We show that these coherent phenomena can generate electromagnetically induced gratings where the index and gain/loss perturbations and their relative phases can be manipulated using an infrared laser beam. This allows us to coherently control optical feedback in a waveguide structure, switching from a case where there is no feedback in the absence of the infrared laser to the case where different types of CC optical feedbacks are generated as this field is properly adjusted. These include generation of gain and index perturbations (partly gain-coupled DFB laser), pure index corrugation (index-coupled DFB laser), and loss and index perturbations (loss-coupled DFB laser). We study these feedback mechanisms in the cases where the optically induced gratings are uniform along the cavity or have a /spl pi//2 phase shift. We discuss mode characteristics of such electromagnetically induced DFB intersubband lasers and find out how here the gain- and index-coupled DFB lasers are associated, respectively, with gain without inversion and laser-induced transparency in the conduction intersubband transitions of quantum-well structures.     

Coherently tunable mid-infrared distributed feedback lasers

We propose utilization of quantum interference effects in quantum well structures to tune lasing wavelengths of mid-infrared distributed feedback lasers. The interference effects are generated via interaction of an intense laser field with an n-doped quantum well, causing coherent suppression or enhancement of refractive indexes of the conduction intersubband transitions. We show that these processes allow us to shift lasing wavelength to shorter or longer wavelengths by adjusting the intensity and frequency of the intense laser. This study is done for two types of lasers: 1) an electromagnetically induced distributed feedback intersubband laser formed by embedding a longitudinal corrugation of several periods of the quantum well structure within a waveguide structure and 2) a phase-shifted distributed feedback laser where the quantum well is inserted in the middle of an index grating, forming an active phase shift region. In the former the intense laser field is responsible for generation of optical feedback while shifting the coherently induced stop-band. In the latter, however, this field changes the optical length of the phase shift region, tuning the lasing mode within the stop-band. We show that the amount of the wavelength shift, which can reach 17 nm, is controlled by the intensity of the intense laser. The sign of the tuning process (red or blue shift), however, is decided by the frequency of this field, after proper choice of the corrugation periods. We investigate the optical feedback mechanisms in such coherently tunable lasers and discuss how they are related to an electromagnetically induced transparency process that happens in the conduction intersubband transitions.     

Self-pulsation in multielectrode distributed feedback lasers

Measurements on multielectrode distributed feedback (DFB) lasers without a saturable absorber reveal the existence of a self-pulsation (SP) regime. In this regime, the laser remains in the same single-longitudinal mode with simultaneous intensity and frequency modulation. The laser spectrum is similar to that of a current-modulated single-mode laser. At the up-limit of the SP regime, the behavior between the output power and the injection current becomes bistable. In one branch of the bistable loop, the SP laser presents a very large spectrum without distinguishable peaks, a kind of chaotic state with coherence collapse. A qualitative explanation based on the effective differential gain is given for the origin of SP and associated phenomena in these lasers     

Two-dimensional theory of distributed feedback semiconductor lasers

A theory which takes account of the two dimensional waveguide structure of the distributed-feedback (DFB) laser is presented. Laser threshold conditions in the case of no external reflections are calculated for a three-layer model in which one cladding layer has a periodically changing dielectric constant. In contrast with the coupled-wave theory the threshold conditions are found to be asymmetric with respect to the Bragg frequency. The longitudinal mode which lies below and nearest the Bragg frequency has the lowest threshold gain. The difference between the threshold gains of the two adjacent longitudinal modes straddling the Bragg frequency has a maximum as a function of the coupled strength of the grating     

Nonlinear analysis of surface-emitting distributed feedback lasers

An analysis of surface-emitting distributed feedback lasers under conditions above threshold is presented. The coupled-wave equations are integrated numerically using a self-consistent technique that includes the effects of gain saturation, carrier diffusion, antiguiding, and free carrier loss. The laser threshold current and slope efficiency are determined as a function of the strip length and p-layer thickness where it is found that the device efficiency can be very high. Antiguiding, which varies when the grating is mistuned with respect to the gain peak, influences the slope, threshold, and shape of the emission far field. Small variations in the longitudinal index of refraction are shown to affect the far-field intensity profile significantly     

Modulation bandwidth of waveguide distributed feedback lasers

The effect of the gain saturation of the active medium and system parameters such as loss coefficient, coupling coefficient, and waveguide geometry on the 3-dB modulation bandwidth in planar and fiber waveguide distributed feedback lasers is discussed for various transverse laser modes. Additionally, the influence of the amplitude and phase of the end reflectivity on the modulation bandwidth is analyzed for both structures. Moreover, the spatial hole burning effect on the modulation bandwidth f_{3 dB} for the various system parameters is investigated. It is shown, that in general, the spatial hole burning effect increases the 3-dB modulation bandwidth in both structures     

Gain nonlinearities in semiconductor lasers: Theory and application to distributed feedback lasers

The gain spectrum in semiconductor lasers is affected by the intensity-dependent nonlinear effects taking place due to a finite intraband relaxation time of charge carriers. We obtain an analytic expression for the nonlinear gain in multimode semiconductor lasers using the density-matrix formalism. In general, the nonlinear gain is found to consist of the symmetric and asymmetric components. The asymmetry does not have its origin in the carrier-induced index change, but is related to details of the gain spectrum. The general expression for the nonlinear gain is used to discuss the range of single-longitudinal-mode operation of distributed feedback lasers. It is also used to obtain an analytic expression for the self-saturation coefficient and to compare the predicted value to the experimental value for both GaAs and InGaAsP lasers. The agreement between the theoretical and the experimental values supports the hypothesis that spectral hole burning is the dominant mechanism for the gain nonlinearities in semiconductor lasers.     

90 mW 660 nm distributed feedback lasers

High efficiency single spatial and spectral mode, 662 nm wavelength distributed feedback lasers have been fabricated from a GaInP/AlInP laser structure. Output powers of up to 90 mW at room temperature were achieved without any spectral mode jumps. These devices should be useful for short and/or visible wavelength applications that require spectral mode stability, such as spectroscopy, interferometry, and high-density optical storage     

Double-tapered-waveguide distributed feedback lasers for high-powersingle-mode operation

Theoretical analysis of double-tapered-waveguide distributed feedback semiconductor laser for stable single-mode and high-power operation is presented. It is found that the single-mode behavior of a distributed feedback laser with large coupling-length product can be improved significantly by the double-tapered-waveguide structure