Phase-shifted Distributed Feedback Laser Research Articles

Quasi-three-dimensional simulation of carrier dynamics in quantum well DFB lasers

A quasi-3D simulation for quantum well (QW) distributed feedback (DFB) laser diodes is presented. It takes into account both longitudinal effects such as the nonuniformity of the optical intensity, local gain, and spontaneous emission, and lateral effects such as current spreading, quantum carrier capture, and quantum carrier heating. The simulation is performed by solving several 2D cross-sections using Minilase II and a 1D longitudinal solver in a self-consistent fashion. As an example, an analysis of spatial hole burning in a λ/4 phase shifted DFB laser is given. It is shown that spatial hole burning (SHB) results in a local gain saturation, and is mainly restricted to electrons and holes within the QW. Investigations of electron heating at high power show that the temperature distribution of the bound electrons and the LO phonons in the QW is insignificant, and only slightly influenced by SHB.

Single phase‐shifted DFB laser diodes

The effects of phase shift value and position on the laser characteristics of single phase-shifted distributed feedback laser diodes (1PS DFB LDs) are investigated using the transfer matrix method. The results show that the quarterly wavelength-shifted (QWS) DFB laser oscillates at the Bragg wavelength, presenting the minimum threshold gain, a higher gain margin, and a flatter field intensity when the coupling coefficient is around KcL=1.25. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 17: 398–402, 1998.

Broad-band tunable electroabsorption modulated laser for WDM application

Design and fabrication of the first 1.55-/spl mu/m wavelength tunable electroabsorption modulated laser integrated with a bent waveguide distributed-feedback (DFB) laser is reported. A low-threshold high-efficiency and stable single-longitudinal-mode operation is obtained when the electrodes of the bent DFB waveguide were uniformly pumped. In a normal configuration where the modulator part is used as an intensity modulator (i.e., AR coating on the modulator side and HR coating on the DFB side), stable single longitudinal mode output power close to 40 mW in free space is obtained with a 0-V bias to the modulator and an extinction ratio of up to 15 dB at 2.5 V. This single mode stability is due to the continuously distributed phase shift implemented in the structure thus reducing the photon pile-up inside the laser cavity as compared to an abruptly quarter-wave phase shifted DFB laser. With a nonuniform injection in the multiple electrodes, it was possible to select one particular single longitudinal mode out of the three neighboring modes inside the broad reflection band of the reflector. A wavelength tuning range of about 3.5 nm was obtained while maintaining an optical output power of more than 2 dBm from each mode. The device has a very low chirp (0.01-nm peak-to-peak) when modulated with a 2.5 Gb/s pseudorandom binary sequence (PRES) data stream and an error-free transmission over 200 km of non-DSF fiber has been demonstrated for all four wavelength channels separated by 100-GHz spacing. Also, when the modulator part of the structure is used as a phase tuning element (i.e., antireflective (AR) coating on the DFB side and HR coating on the modulator side), we have successfully demonstrated high-speed optical packet switching with a fast and wide tunable wavelength range. The optical packets can be modulated at 2.5 Gb/s and may be switched among four wavelength channels in less than one bit period.

Characterization of semiconductor lasers by spontaneous emission measurements

The measurement of spontaneous emission is a powerful characterization technique of semiconductor lasers, which has been developed with the fabrication of the first lasers. It allows the determination of both material and structural parameters such as gain, refractive index, longitudinal effects, etc. Significant advance has been made recently on the understanding of material properties and lasing characteristics by using spontaneous emission measurements. In particular, the effect of spatial hole burning in phase-shifted distributed feedback lasers has been investigated quantitatively. The main origins of the gain suppression in both bulk and quantum-well structures have been identified by observing the evolution of spontaneous emission spectrum with output power. The paper presents the underlying principles of such a technique and gives a summary of these recent achievements as well as the main results.

Effects of structural parameters on the external optical feedback sensitivity in DFB semiconductor lasers

External optical feedback sensitivity in distributed feedback (DFB) semiconductor lasers is analyzed with special attention to phase-shifted and complex-coupled lasers. The effects of various structural parameters such as coupling strength, facet reflectivity, and corrugation phase angle on external optical feedback sensitivity are studied. The /spl lambda//4 phase-shifted index-coupled DFB laser exhibits low external optical feedback sensitivity for large index-coupling coefficient and high facet reflectivity. Pure gain-coupled DFB lasers perform better than the phase-shiftless uniform index-coupled DFB lasers but worse than /spl lambda//4 phase-shifted index-coupled lasers with high coupling strengths. External optical feedback sensitivity of complex-coupled lasers depends significantly on the index-to-gain coupling ratio, the phase between the index and gain gratings, and the total coupling.

Theoretical and experimental study of inhomogeneously pumped distributed feedback lasers

We have investigated inhomogeneously pumped index-coupled distributed feedback (DFB) laser structures both theoretically and experimentally. Using a transfer matrix method, an optimized device structure was designed which drastically suppresses the longitudinal sidemodes by concentrating the injection current in the region close to the phase-shift position. In this way, a high yield of monomode lasers together with an easy fabrication technique was achieved without the necessity of end facet antireflection coating. Experimental evidence of the improvement of the single-mode stability is given first by three-section DFB lasers operated by two different injection currents. Second, the proposed three-segment laser pumped with a single drive current was implemented simply by splitting up the contact of a mushroom type 2×λ/8 phase-shifted DFB laser. We observed the theoretically predicted enhanced stability of the Bragg mode with a sidemode suppression ratio of 52 dB and a higher yield of monomode lasers compared with uniformly pumped conventional DFB laser devices.

Analysis of polarization bistability of phase-shifted DFB laser due to TM light injection

As a method of high-bit-rate optical processing, the TM light injection-induced optical polarization bistability using Fabry-Perot type laser diodes has been experimentally demonstrated and theoretically analyzed. In these studies, although switching times less than 1 ns both for switch-up and switch-down have been reported, both of the TE and TM outputs exhibit longitudinal multimode spectra. In practical processing systems, a dynamically stable operation of the polarization switching is considered to be essential. It is the purpose of this paper to present an analytical study of the TM light injection-induced polarization bistability using a lambda /4-shifted distributed feedback (DFB) laser model and to describe the detailed picture of the mechanism. >

Distributed feedback lasers with distributed phase-shift structure

Distributed feedback (DFB) lasers with distributed phase-shift (DPS) structures are suggested for reducing the spatial hole burning in conventional phase-shifted DFB lasers. Solving the coupled-mode equations subject to suitable boundary conditions, we have determined the threshold gain condition of a symmetric distributed phase-shift distributed feedback (DPSDFB) laser as well as the internal photon intensity distribution. The results show that by distributing the phase shift to a greater extent, the spatial hole burning can be reduced significantly and a large threshold gain difference between lowest gain oscillation modes can be maintained over a large phase-shift range. Moreover, a properly designed DPSDFB laser does not pose extra requirements on its fabrication.

The static and dynamic characteristics of single and multiple phase-shifted DFB laser structures

The influence of longitudinal mode spatial hole burning (LMSHB) on the performance of distributed feedback (DFB) laser structures is examined in detail. A comprehensive model has been used to interpret the experimental results and to construct a theoretical framework that was utilized to develop more advanced device designs. An increasing side mode intensity with output power, movement of the lasing mode relative to the stopband, and curvature of the light-current characteristic at low power can all be manifestations of the influence of LMSHB on the static device performance. The dynamic behavior can also be affected, with extended wavelength chirp and amplitude patterning effects on the timescale of the effective carrier recombination time being particularly important.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Highly efficient single longitudinal-mode oscillation capability of gain-coupled distributed feedback semiconductor lasers-advantage of asymmetric facet coating

The gain-coupled (GC) distributed feedback (DFB) laser is found to possess an uncommon and useful feature; unlike the quarter-wave phase-shifted (index-coupled) DFB laser, it is possible to enhance single-facet power extraction efficiency by applying an asymmetric antireflection-high-reflection (AR-HR) coating without demolishing its intrinsic single-mode property. A 1-90% coating gives more than 92% of output power from one of the facets while preserving Delta gL, the difference between the lowest and second lowest normalized threshold gains, larger than 0.2 in 90% of the devices. >

Large-signal dynamic model of the DFB laser

A computer model is proposed to analyze the characteristics of distributed feedback (DFB) lasers. The model is based on time-dependent coupled wave equations, with spontaneous emission taken into account. In order to avoid uncertain phase factors in spontaneous emission, a method of converting field equations to power equations in a matrix format before computation is introduced. The steady-state LI curve and transient response to the pulse excitation are calculated in the lambda /4 phase-shifted DFB lasers. The longitudinal variations of the carrier and photon densities as well as of the refractive index are considered in the model. >

Gain-coupled DFB lasers versus index-coupled and phase shifted DFB lasers: 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 lambda /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. >

A tractable above-threshold model, for the design of DFB and phase-shifted DFB lasers

A model, based on the transfer matrix method, and extended to above-threshold conditions in order to be used in the analysis and design of distributed feedback (DFB) structures, is presented. It provides the oscillation parameters as lasing frequency and net material gain, the linewidth estimation and output power, the photon density, carrier density, and effective refractive index profiles. This model can be used with or without facet reflectivities and with a number of symmetric or asymmetric phase shifts. It takes into account nonuniform envelope material gain and nonuniform index saturation. It is easily adaptable to any structure modifications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Phase-shifted distributed feedback laser with linearly chirped grating for narrow linewidth and high-power operation

Phase-shifted distributed feedback laser with linearly chirped grating (PS-CG-DFB) was analyzed. The field distribution, threshold gain difference between the lasing mode and the next lowest threshold gain mode, linewidth of the lasing mode, and output power of PS-CG-DFB were self-consistently calculated under different injection levels. It is found that the lasing mode pattern and the carrier concentration distribution are ‘‘locked’’ at high injection level due to the strong interaction between carriers and photons. The chirped grating compensates the spatial hole burning effect to the mode stability and makes the mode stable in the whole injection region.

Transfer-matrix formulation of spontaneous emission noise of DFB semiconductor lasers

A unified formulation of the spontaneous emission noise in semiconductor DFB (distributed feedback) lasers is presented by using a transfer-matrix approach. Analytical expressions for the noise power per unit frequency bandwidth below threshold and the spontaneous emission rate into the lasing mode are obtained based on the Green's function method. Three DFB laser structures are analyzed: (1) a standard DFB structure with facet reflectivities, (2) a multisection DFB structure composed of n sections which models a phase-shifted DFB laser and a multielectrode (tunable) DFB laser, and (3) a periodic layered DFB structure which models a surface-emitting DFB laser. It is shown that the spontaneous emission noise of a complicated DFB laser structure can be calculated easily by the transfer matrix of each section of the structure and its derivative to frequency. >

Effect of phase shift position on spectral linewidth of the pi /2 distributed feedback laser diode

The dependence of the spectral linewidth on the phase shift position (PSP) of a phase-shifted distributed-feedback (DFB) laser diode is investigated theoretically, using the coupled wave model. In particular, the analysis is performed to find an optimum PSP so as to minimize the spectral linewidth of the laser diode. This optimum PSP is compared to that obtained by maximizing the stability of the laser oscillation. Since both narrow spectral linewidth and good stability are essential for the light source in coherent optical communications, the analysis revealed the suitability of using phase-shifted DFB laser diodes in coherent transmissions. >

Corrugation-pitch-modulated phase-shifted DFB laser

A corrugation-pitch-modulated phase-shifted distributed feedback (DFB) laser is proposed to reduce the spatial-hole-burning effect along the laser cavity. The structure of the proposed corrugation-pitch-modulated phase-shifted DFB laser is illustrated. A reduction in this effect is confirmed from its spectral behavior as a function of driving current. Consequently, stable single-longitudinal-mode operation up to high-output powers is obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The design assessment of lambda /4 phase-shifted DFB laser structures

A detailed nonlinear model of the performance of single-frequency laser structures operating both below and above threshold is discussed. Arbitrary series combinations of uniform pitch grating, linearly-chirped pitch grating and plane guide sections can be analyzed-taking longitudinal-mode spatial hole burning into account. The material properties of each section of a device structure can be specified. The output power from each end of the laser and the emission wavelength, and the longitudinal intensity, carrier density, and relative permittivity profiles are predicted as a function of drive current above threshold for each lasing mode. The linewidth is also estimated while allowing for the nonuniform longitudinal distribution of spontaneous emission into the mode in a physical manner. The model was used to provide the data with which to trade off the numerous interacting performance parameters of a lambda /4 phase-shifted DFB (distributed-feedback) laser. The design options are then summarized. >

Emission properties of surface-emitting distributed-feedback and distributed-Bragg-reflector semiconductor lasers

We use a single-mode transfer equation model to study the performance of vertical-cavity surface-emitting distributed- feedback (DFB) and distributed-Bragg-reflector (DBR) semiconductor lasers above threshold. We find that DBR lasers exhibit less envelope spatial hole burning than both single and double phase-shifted DFB lasers and therefore may achieve more stable single-longitudinal-mode operation.

Coupled phase-shift distributed-feedback semiconductor lasers for narrow linewidth operation

Mode properties of a coupled phase-shift distributed-feedback (DFB) structure are analyzed and the feasibility of narrow-linewidth emission is shown theoretically. The structure consists of a multiple number of phase-shift DFB laser units which are arranged in tandem and coupled with each other through additional phase shift of corrugation. Optimum structure parameters were determined for two coupled laser units so that the frequency detuning is zero and the mode pattern is the flattest. The mode analysis shows that the intensity distribution is flatter and the normalized coupling constant can be larger than those for the conventional DFB laser with a single phase shift. This indicates that the cavity length can be extended with less influence from the longitudinal spatial hole burning effect. >