Distributed Feedback Laser Structure Research Articles

Optically written solid-state lasers with broadly tunable mode emission based on improved poly (2,5-dialkoxy-phenylene-vinylene)

We report an optical written distributed feedback (DFB) structure with broadly and continuously tunable mode emission (Δλ=30 nm) based on polycondensation-type poly[2methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH–PPV). In a Lloyd-mirror configuration we realize first-order feedback by dynamic photoinduced in-plane gratings, which can also be imprinted permanently. This technique is a versatile fabrication tool for polymeric DFB laser structures while also providing an efficient method to probe the lasing performance of organic solid-state laser materials. Furthermore, features of the photoinduced absorption spectra of improved MEH–PPV indicate favorable material properties towards further homopolymer solid-state laser applications.

Analysis of TM-polarized DFB laser structures with metal surface gratings

An analysis of distributed feedback (DFB) laser structures with metallized surface grating structures in TM polarization is presented. The modal properties of these structures are described using coupled-mode theory where the coupling coefficients are derived from rigorously computed on-resonant Floquet-Bloch solutions of the waveguide grating problem. Based on this theory, first- and second-order DFB quantum cascade laser structures operating at a wavelength of 10 /spl mu/m are investigated numerically. We show that, utilizing a metal stripe grating structure, second-order laser structures are feasible showing efficient surface emission, whereas radiation into the substrate is strongly suppressed. The fraction of stimulated emission power being emitted via the surface can be as high as 17.5% whereas a low threshold gain of 20 cm/sup -1/ is maintained.

Complex-coupled DFB lasers: advantages and drawbacks of gain and radiation loss grating

Two types of InGaAs-GaAs-AlGaAs complex-coupled distributed feedback laser structures grown by molecular beam epitaxy are compared. The first structure uses interrupted quantum wells (QW's) grown over a grating. Its properties strongly depend on the interface cleaning resulting in nonreproducible threshold current densities. The second structure includes three uniform QW's, the gain coupling being induced by the vertical radiation of the second order grating. This last structure has reproducible, low continuous wave threshold current densities (jth=300 A/cm/sup 2/). The coupling strength and its origin are described for the two structures. Both structures have high mono-mode yield (>75%) and side-mode suppression ratios larger than 40 dB.

A probability-amplitude transfer matrix model for distributed-feedback laser structures

Two different treatments of spontaneous emission in distributed-feedback (DFB) lasers were found in the literature, but adequate explanations for the different treatments were not found. Using an approach that allows comparison of the two different treatments of spontaneous emission, we show that the different treatments can lead to different spectral predictions. The difference in spectral predictions is negligible in Fabry-Perot lasers and index-coupled DFB lasers. However, in truncated-well gain-coupled DFB lasers, the difference between the two treatments is noticeable, and one treatment is markedly better at fitting to data. The treatment that best fits the data is also the treatment that makes sense quantum-mechanically.

Matrix-grating strongly gain-coupled (MC-SGC) DFB lasers with 34-nm continuous wavelength tuning range

A matrix-grating strongly gain-coupled (MG-SGC) distributed-feedback laser structure has been demonstrated to exhibit a continuous wavelength tuning range of about 34 nm. A simple tuning mechanism was employed, and a sidemode-suppression-ratio of beyond 50 dB within the entire tuning range was achieved. This device can be potentially used in dense wavelength-division-multiplexed systems for cost effective applications.

Theory for optimum design and analysis of distributed-feedback lasers

Theory for optimum design of a distributed-feedback (DFB) laser is presented, based on a new set of coupled-power equations. A novel /spl lambda//4 phase shift index-coupled AR-coated DFB laser structure, which theoretically exhibits a longitudinally uniform power density, is proposed. It is shown that the theory is very simple and provides an easy way to analytically understand the physical mechanism crucial for the optimum design. Our theory can also be easily applied to the optimization of the design of gain-coupled, complex-coupled DFB laser structures as well as high power DFB lasers.

Analysis of radiation mode effects on oscillating properties of DFB lasers

The effects of radiation mode on the oscillating properties of distributed feedback (DFB) lasers with second-order corrugations are analyzed for designing a new type of DFB laser. A formulation based on the transfer matrix technique is applied to calculating Streifer's /spl zeta/-terms added to the coupled-wave equations. These terms represent the effects of radiation and evanescent modes. This formulation greatly simplifies the analysis of distributed resonance along multilayered waveguide structures with arbitrary-shaped second-order corrugations. The effects of vertical resonance are also incorporated into the formulation. Various types of DFB lasers with phase-shifted second-order corrugations are analyzed using this method. It is found that the phase shift and the blaze of the corrugations greatly affect the longitudinal mode selectivity. A new phase-shift DFB laser structure with two complementary blazing regions connected at the shift is proposed. It is demonstrated that this structure has small radiation loss resulting in low-threshold performance despite employing second-order corrugations.

Microwave circuit design for chirp or intensity fluctuation suppression in multielectrode DFB lasers

Realizable microstrip circuits have been designed to provide the required modulation current characteristics to suppress the chirp or intensity fluctuations occurring in a multielectrode distributed feedback laser structure under intensity or frequency modulation schemes, respectively. The designs have been performed using the combination of a commercial microwave circuit simulator and a time domain laser model. It is shown that good chirp or intensity fluctuation suppression can be achieved without significantly affecting the desired intensity or frequency modulation performance.

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.

Temperature dependence of the threshold current in gain-coupled distributed feedback lasers with periodically etched quantum wells: Mechanism for an appearance of infinite T

A phenomenological model describing the temperature dependence of the threshold current of 1.3 μm gain-coupled distributed feedback (DFB) lasers with periodically etched strained-layer quantum wells is presented and compared to experimental results. The model can predict the minimum threshold current as a function of temperature (infinite T0), which has been observed experimentally. The mechanism for the appearance of this minimum is explained by the decrease of the detuning of the DFB-mode wavelength from the material gain-peak wavelength as the temperature increases. The condition for single-mode operation is also discussed with use of the effective gain of DFB laser structures.

First-order gain-coupled (Ga,In)As/(Al,Ga)As distributed feedback lasers by focused ion beam implantation and in situ overgrowth

First-order gain-coupled distributed feedback lasers have been fabricated in the (Ga,In)As/(Al,Ga)As material system by maskless patterning with focused ion beam implantation. The laser devices are operating at 77 K and room temperature at emission wavelengths between 1.0 and 0.7 μm. The structures obtained are thermally stable up to annealing temperatures of 800 °C. A full in situ processing of gain-coupled distributed feedback laser structures was for the first time successfully demonstrated. This process includes molecular beam epitaxy, grating patterning by focused ion beam, and epitaxial overgrowth. All processed lasers show single mode emission at all operating conditions as expected for gain-coupled lasers.

Molecular beam epitaxy growth of lattice-matched [formula omitted] multiple quantum well distributed feedback laser structures with gratings defined by implantation enhanced intermixing

AlGaInAs GaInAs multiple quantum wells (MQWs) are particularly suited for planar device integration using local refractive index modification by masked implantation enhanced intermixing (MIEI). Suitable molecular beam epitaxy (MBE) growth conditions are elaborated to achieve 1.3 μm AlGaInAs GaInAs MQW distributed feedback (DFB) lasers taking advantage of the capability of the MIEI process to define the gratings. High-quality MBE overgrowth on AlGaInAs necessary for device completion after H ∗ radical treatment is demonstrated.

Deep Etched ZnSe‐Based Nanostructures for Future Optoelectronic Applications

AbstractBy electron beam lithography and a subsequent wet/dry etch process, optically active nanostructures are developed based on ZnSe with lateral extensions down to 35 nm. Dry etching using Ar+ ions is found to generate very smooth etch surfaces, while the photoluminescence efficiency of narrow wires is much higher in wet chemically etched structures. As a first application of deep etched nanostructures, index coupled distributed feedback laser structures of second order are realized with periods down to 185 nm on the base of (Cd, Zn)Se/ZnSe/(Mg, Zn) (Se, S) vertical waveguide heterostructures. A clear correlation between the resonator period and the emission wavelength is observed, indicating a high coupling coefficient of the structures. For optical pumping using a pulsed N2‐laser, the laser threshold at room temperature is about 100 kW/cm2.

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>

Transfer matrix analysis of the spectral properties of complex distributed feedback laser structures

A transfer matrix model for the spectral behavior of complicated distributed feedback lasers is presented. The rigorous numerical approach permits the accurate solution of the one-dimensional wave equation without any approximation. Hole burning effects are taken into account by splitting up the whole structure into subsections with uniform parameters and finding the self-consistent solution for the longitudinal mode intensity, the carrier density, and the carrier induced refractive index change by an iteration procedure. The resulting distributions are used to calculate the spectrum by a summation over the multiple reflected spontaneous emission originating from each subsection. The method can be applied to arbitrary complex device structures both below and above threshold and is easy to implement with high accuracy and good computational performance. >

Surface emitting InGaAsP/InP distributed feedback laser diode at 1.53 mu m with monolithic integrated microlens

Monolithic integration of a distributed feedback (DFB) surface-emitting laser diode with a microlens is demonstrated. The transverse and longitudinal cross-sectional views of the laser diode are illustrated. The microlens and a DFB laser structure are located on opposite sides of an n-InP substrate. 11 mA minimum continuous wave (CW) threshold current and 5 mW CW emission perpendicular to the InP substrate are achieved at room temperature using a chemically etched 45 degrees mirror. Single mode emission at 1.53 mu m is obtained. The integrated microlens, etched by ion beam and coated with aluminum oxide, provides optical beam collimation and an ultralow laser mode reflectivity of <10/sup -4/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of TE and TM modes in DFB lasers by two-dimensional theory

The two-dimensional theory of a distributed feedback (DFB) laser (which was previously presented and applied to the analysis of the laser threshold conditions for the transverse-electric (TE) mode in a simple three-layer waveguide structure) is developed to treat both TE and transverse-magnetic (TM) modes in a four-layer waveguide structure with a thin grating layer, which more closely reflects actual DFB laser structure. The differences between TE and TM modes for the dispersion relations and the laser threshold conditions are clarified. The effects of the waveguide structure (including grating layer thickness, refractive indexes of layers, coupling constant, and corrugation period) on the threshold gains and the gain differences between the two longitudinal modes on both sides of the Bragg frequencies are studied in detail for both TE and TM modes. >

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. >

Analysis of surface-emitting lambda /4-shifted DFB lasers with a distributed Bragg reflector

Threshold current density and differential quantum efficiency are analyzed for a surface emitting (SE) lambda /4-shifted distributed feedback (DFB) laser consisting of alternating active and passive layers with a distributed Bragg reflector (DBR). It is shown that the threshold current density can be reduced by using the DFB action induced by alternating active and passive layers in the active region of the SE DFB laser structure, as compared to SE DBR lasers with a homogeneous active region. It is also shown that the differential quantum efficiency of the DFB laser with a DBR can be higher than that of conventional DBR lasers without increasing the threshold current density. >

Frequency-tunable semiconductor lasers

Recent advances in frequency-tunable semiconductor lasers are reviewed. They are classified into hybrid and monolithic tunable lasers. Monolithic tunable lasers based on distributed Bragg reflector or distributed feedback laser structures are most attractive for practical applications. The device structures and the tuning characteristics are described, with emphasis on the tuning range, spectral linewidth and frequency-switching time. Recent system experiments using monolithic tunable lasers in an optical communication area are also described.