Laterally coupled-cavity semiconductor lasers

We analyze the threshold behavior of a pair of laterally coupled semiconductor lasers of different lengths. The predictions include longitudinal mode selectivity leading to single longitudinal mode operation with a periodicity determined by the length mismatch, and ripples in the equipower curves in the current plane due to carrier-induced index shifts. We present experimental measurements that confirm these predictions.     

Analysis of two-section coupled-cavity semiconductor lasers

Analysis and simulation studies of the longitudinal mode behavior of two-section coupled-cavity lasers in the steady-state are presented. The laser is analyzed with a resonance amplifier model and the scattering matrix formulation. The mode selection property of this laser is explained in terms of the wavelength-dependent effective reflectance. Expressions for the effective reflectance and the mode power are derived for two different cases: the case of weak reflection from the junction and strong coupling between individual cavities, and the case of strong reflection at the junction and relatively weak coupling between cavities. It is shown to be possible to simulate the longitudinal mode behavior with respect to temperature changes and wavelength tuning by separately pumping each section of the laser.     

Wavelength self stabilisation of coupled-cavity semiconductor lasers

A simple wavelength control circuit is demonstrated that maintains single-longitudinal-mode operation of coupled-cavity semiconductor lasers over wide ranges of temperature. Photodetection properties of the lasing system itself are used for generating the discriminating signal for the feedback loop. No external optical elements like photodetectors or spectrometers are required.     

Single-mode characteristics of short coupled-cavity semiconductor lasers

The characteristics of single-longitudinal-mode selection in short coupled-cavity (SCC) semiconductor lasers with a plane reflector are analyzed theoretically. It is found that the power ratio of the main mode to other side modes can be increased significantly by reducing the reflectivity of a laser diode facet adjacent to the external mirror. Based on the theoretical results, a new type of SCC laser is developed; single-longitudinal-mode output with 35-40 dB side mode suppression ratio is obtained under CW and 150 MHz modulation conditions.     

Grating-coupled GaAs-GaAlAs lasers with distributed Bragg reflectors

Grating-coupled GaAs-GaAIAs lasers with 4th-order distributed Bragg reflectors have been successfully demonstrated with high efficiencies. The threshold current density, lasing spectrum and grating-coupled output power have been measured at room temperature. The output power from the grating at 1.5 Jth and its differential quantum efficiency have been measured to be 13.5 mW and 1.1%, respectively.     

Mode-locking of in-line coupled-cavity fiber lasers usingintra-core Bragg gratings

The report the observation of self-mode-locked laser pulses from in-line coupled-cavity (ILCC) fiber lasers, consisting of an optically pumped Er-doped fiber main cavity and an auxiliary Si fiber cavity formed by using three intra-core Bragg gratings. Mode-locked pulses at a repetition rate of 100 MHz have been obtained with an input pump threshold as low as 10 mW     

An analysis of pulsation in coupled-cavity structure semiconductor lasers

Pulsation in a coupled-cavity structure (CCS) semiconductor laser, in which laser cavities are placed parallel with optical coupling between them, has been observed. The mechanism is simulated by solving rate equations for a model with two stripes of different emission energies. The pulsation in the simulation is based on the resonant oscillation, which originates in the nonlinear interaction between the electromagnetic field in the laser cavity and the population inversion, in cooperation with the perturbations caused by repetitively occurring optical pumping and quenching. Important parameters to define the properties of the pulsation are emission-energy difference, coupling coefficient, and excitation levels.     

Optically pumped GaAs lasers with two-dimensional Bragg reflectors

We report experiments on oscillations of a new two-dimensional distributed-feedback (DFB) laser. A pair of distributed Bragg reflectors (DBR) and another perpendicular corrugation were formed on a GaAs crystal and the sample was optically pumped at 77 K. We observed oscillation of a two-dimensional mode in addition to that of a one-dimensional mode.     

Strip buried heterostructure lasers with passive distributed Bragg reflectors

A GaAs-AlxGa1-xAs strip buried heterostructure laser with distributed feedback Bragg reflectors is described. In this structure, the active strip is present in the form of an isolated segment completely buried in wide band gap AlxGa1-xAs. The diode is shown to operate stably in single transverse (fundamental) and longitudinal modes when the Fabry-Perot modes are suppressed by saw cutting the mirrors.     

Design studies for distributed Bragg reflectors for short-cavityedge-emitting lasers

In conventional edge-emitting lasers, mirror reflectivity is ~0.3-0.4 so that very short cavities have an unacceptable loss. To decrease the mirror losses, one can conceive of an external distributed Bragg reflector (DBR) structure produced by etching. We examine the design considerations for such mirrors. Using a finite-difference time-domain (FDTD) scheme, we numerically simulate the propagation of a Gaussian wave packet in a laser cavity with an external DBR structure on one end. We find that despite the divergence of light at the semiconductor-air interface, high reflectivities (>90%) can be obtained using only a few mirrors, provided a low refractive index material (such as air) is one of the components of the DBR period. In our modeling, we include fluctuations based on factors such as lithographical error, etch error, and off-design wavelength lasing. Our results indicate that the external DBR is highly tolerant of such factors. In addition, simulations done using material with higher refractive index than air in the DBR structure indicate that it is possible for insulators to be deposited in the DBR structure, yet allow high reflectivity     

Multibranch frequency-selective reflectors and application totunable single-mode semiconductor lasers

Multibranch reflectors (MBRs) are proposed as a flexible wavelength-selective element for optical integrated circuits generally and for tunable single-mode lasers specifically. The undesired recurring peaks of reflectivity obtained with two-branch MBRs are spread out in the spectrum by using three or more branches. Design relations are given for the lengths of an arbitrary number of branches, for the selectivity near the peak response, for the wavelength interval to the nearest undesired peak, and for tuning changes in the various branches. It is shown that an MBR is not sensitive to amplitude differences of the order of 20% between the branches, but requires phase control in order to achieve tuning to a specific wavelength. The effects of semiconductor gain variation with wavelength are obtained from a rate-equation solution including MBR. By changing the semiconductor temperature (gain peak) at the same time as the MBR is tuned, a broad range of tuning without jumps is achievable     

Simulation of DFB semiconductor lasers incorporating thermaleffects

Static and dynamic thermal processes are incorporated into the modeling and simulation of DFB lasers. Analytical expressions for AM/FM responses considering both carrier and thermal effects are obtained. The self-consistency of this model is verified by comparing with experiment. Thermal effects on the laser performance such as L-I characteristics, lasing wavelength chirp, small signal AM/FM and large signal modulation responses are examined. It is noted that there is a trade-off between static and dynamic thermal wavelength chirps     

Performance of gain-guided surface emitting lasers withsemiconductor distributed Bragg reflectors

The performance limitations of gain-guided vertical cavity surface emitting lasers (VCSELs) which use epitaxially grown semiconductor distributed Bragg reflectors (DBRs) are discussed. The light-current ( L-I) characteristics and emission wavelength of such lasers are examined as a function of temperature and time under continuous wave (CW) and pulsed operation. The authors observed a sharp roll-over in the CW L-I characteristics which limits the maximum output power. The threshold current under CW operation is found to be lower than that obtained under pulsed conditions. Several microseconds long delay in lasing turn-on is also observed. It is shown quantitatively that these anomalies are a consequence of severe heating effects. It is shown that reduction of the series resistance and threshold current density can lead to significant improvements in the power performance of VCSELs     

Optically pumped epitaxial GaAs waveguide lasers with distributed Bragg reflectors

Fabrication and characteristics of epitaxial waveguide lasers with distributed Bragg reflectors are described. GaAs-(GaAl)As double-layered structures were grown by conventional liquid-phase-epitaxy (LPE) technique. Periodic corrugations were made by chemical etching through a photoresist grating mask. Laser action with clearly defined logitudinal modes was observed under optical pumping at liquid-nitrogen temperature.     

Threshold current reduction in InGaN MQW laser diode withλ/4 air/semiconductor Bragg reflectors

A 13% reduction in the threshold current density of InGaN laser diodes is demonstrated upon the introduction of two 5λ/4 air/nitride Bragg reflectors. These are defined at one end of the laser cavity by means of focused ion beam etching     

Continuous wavelength tuning of Brewster angled semiconductor lasers

A Brewster angled InGaAsP/InP semiconductor laser coupled to an external two grating cavity has been tuned continuously over 55 nm spectral range. This was made possible by making high spectral selectivity of the external feedback.<>     

Voltage drop in n- and p-type Bragg reflectors for vertical-cavitysurface-emitting lasers

An analysis of carrier transport in n- and p-type distributed Bragg reflectors (DBR) of vertical-cavity surface-emitting lasers that consist of stacks of quarter-wave GaAs-AlAs layers is presented. The analysis is based on the diffusion-drift approximation with the thermionic boundary conditions at heterojunction interfaces. The spatial distribution of carrier effective masses and mobilities has been taken into account. While the voltage drop in n-type DBR is determined mostly by thermionic emission at the interfaces, the drift-diffusion component of the voltage drop is comparable with the thermionic emission in p-type DBR. We present the calculated resistance as a function of graded-region thicknesses and doping levels, which can be useful for low-resistive DBR design     

Coupling coefficients for coupled-cavity lasers

We derive simple, analytic formulas for the field coupling coefficients in a two-section coupled-cavity laser using a local field rate equation treatment. We show that there is a correction to the heuristic formulas based on power flow calculated by Marcuse; the correction is in agreement with numerical calculations from a coupled-mode approach.     

Instability of semiconductor lasers due to optical feedback from distant reflectors

We explain an istability occurring in continuously operating lasers due to moderate feedback from distant reflectors. This instability occurs despite the fact that the laser is stable with respect to small deviations from steady-state operation. It is the result of finite phase and carrier number changes caused by fluctuations in spontaneous emission. We predict several properties that agree with recent experimental observations: 1) the instability only occurs when the laser reaches a steady state that maximizes coherent feedback and laser light intensity; 2) the instability vanishes at strong feedback levels; and 3) at moderate feedback levels, the laser will be nearly stable at threshold, but unstable when operated well above threshold. The latter behavior results in a nonlinear "kinked" shape in the light versus current relation.     

Narrow-stripe distributed reflector lasers with first-order vertical grating and distributed Bragg reflectors

Narrow-stripe 1.55-/spl mu/m wavelength distributed reflector lasers consisting of both distributed Bragg reflectors and vertical grating of the first Bragg order were fabricated. A low threshold current I/sub th/ of 2.8 mA, a differential quantum efficiency /spl eta//sub d/ of 28% from the front facet, and a submode suppression ratio of 44 dB were obtained for structures with a stripe width of 1.3 /spl mu/m and a cavity length of 150 /spl mu/m.