Analysis of surface-emitting λ/4-shifted DFB lasers with adistributed Bragg reflector

Threshold current density and differential quantum efficiency are analyzed for a surface emitting (SE) λ/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     

λ/4-shifted InGaAsP/InP DFB lasers

lambda/4-shifted InGaAsP/InP DFB lasers were studied theoretically and experimentally. The effect of reflectivities at the end of a DFB region and that of alambda/4shift position were analyzed in terms of stability of single-longitudinal-mode operation and asymmetric power distribution. The shift of thelambda/4-shift position from the center to a certain place in the DFB region, with the end reflectivities less than several tenths of a percent, seemed most effective for efficient power extraction and reproducible DSM operation. The devices emitting at 1.5 μm range were fabricated by using negative and positive photoresists and employing one-step holographic exposure. They exhibited single-longitudinal-mode operations just at or closely around the center of the stopband, i.e., the Bragg wavelength. The slight wavelength deviations from the center were found to be attributed to the accidental phase-shift variations from the optimal value. Concerning such deviations in the fabricated devices, a simple and useful criterion, for example,P_{0}/P_{1} geq 2-3atI/I_{th} = 0.9, for stable DSM operation was presented. Statistically, single-longitudinal-mode operations were observed in 95 devices out of 100, and the theoretical prediction was verified. The side-mode-suppression ratios under high-speed direct modulation were 35 dB or more.     

?/4-shifted AlGaAs/GaAs DFB lasers with double window structures

Single-mode oscillation was obtained by introducing a ?/4-shifted, double window structure into DFB lasers fabricated by two-step MOCVD. The resulting ?/4-shifted laser produced Bragg oscillation and was very stable up to 10 mW under CW conditions.     

Enhanced relaxation oscillation frequency and reduced nonlinear K-factor in InGaAs/InGaAsP MQW lambda /4-shifted DFB lasers

The relaxation oscillation frequency, f/sub r/, of 1.55 mu m InGaAs/InGaAsP MQW lambda /4-shifted DFB lasers was doubled by increasing the carrier injection efficiency into each quantum well, which results from an optimised bandgap energy and optimised thickness of the barrier layers. The nonlinear K-factor which determines the maximum modulation bandwidth through the damping phenomenon can be reduced by adopting a p-type modulation doped MQW structure in the active layer.<>     

Longitudinal-mode behaviour of ?/4-shifted InGaAsP/InP DFB lasers

Longitudinal-mode behaviour of ?/4-shifted InGaAsP/InP DFB lasers fabricated by the nega-posi method have been studied. The deviation of the main mode from the centre of the stopband, which was attributed to an accidental additional phase shift introduced around the ?/4-shifted position, was related to the intensity ratio of the main to side mode below the threshold, and it was much larger compared to conventional DFB lasers for the devices with small deviation. The intensity ratio above the threshold was about 35 dB or more in 100% direct modulation for most of the lasers.     

Asymmetric λ/4-shifted InGaAsP/InP DFB lasers

1.5 μm asymmetriclambda/4-shifted InGaAsP/InP DFB lasers, in which thelambda/4-shift position was moved from the center of the DFB region toward the front side, were made in order to obtain higher output power with high single-longitudinal-mode (SLM) yield. Statistical measurements revealed that it was effective for the increase of the differential quantum efficiency from the front facet without a remarkable decrease of the SLM yield to move thelambda/4-shift position to the front facet by 10-15 percent of the total DFB length. The output efficiencies of the diodes with AR coatings on the window structure were almost coincident to those expected from theoretical calculations. The main to submode ratioP_{0}/P_{1}in the rear spectra of the asymmetric devices noticeably decreased with increasing asymmetry. From an experimental point of view, a criterion for stable SLM operation, for example,P_{0}/P_{1} gsim 1.5in the rear spectrum atI = 0.9 Ithfor thelambda/4-shift positionl_{f} : l_{r} = 35 : 65, is presented.     

Frequency stabilisation of lambda /4-shifted DFB laser to stabilised optical resonator

The frequency of a 1.55 mu m-range GaInAsP lambda /4-shifted DFB laser is locked to the resonant frequencies of a confocal etalon of which the mirror spacing is stabilised with a frequency-stabilised HeNe laser. An Allan variance of less than 10/sup -11/ was achieved for averaging times longer than 1 s without frequency modulation of the laser diode.<>     

Amplified-spontaneous-emission spectrum of the radiation field in surface-emitting DFB lasers

In this paper, the authors calculate the amplified-spontaneous-emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers. The response of the laser cavity to the Langevin noise source in the frequency domain is obtained using the newly developed Green's functions for the slowly varying amplitudes of the guided waves. The authors show that the power spectra from the surface and the edge are different, and this discrepancy is due to excitation of the radiation field by the interference between the counter-propagating waves inside the cavity. This feature can be properly exploited in the design of surface-emitting DFB lasers for optical communications.     

Highly nondegenerate four-wave mixing in uniform andλ/4-shifted DFB lasers

A theoretical investigation of highly nondegenerate four-wave mixing (HNDFWM) in uniform and quarter-waveshifted distributed feedback (DFB) lasers in the weak injection regime has been performed using a numerical analysis of the coupled-wave equations. The effects of carrier spatial and spectral hole burning, carrier heating, and longitudinally variable pump field above the laser threshold are included. Pump-probe detunings up to 1.5 THz are investigated. It is found that, for small and moderate DFB coupling coefficients, the conjugate wave can be generated in a bandwidth of ~1 THz with an efficiency of -20 dB or more, compared to the injected probe power     

Analysis of wavelength conversion using highly nondegeneratefour-wave mixing in λ/4-shifted DFB lasers

We present a theoretical analysis of terahertz-range wavelength conversion using highly nondegenerate four-wave mixing in λ=1.55 μm, medium-coupling λ/4-shifted distributed feedback (DFB) lasers. Based on the coupled-wave approach, our model predicts up to ~2 THz probe-pump detuning with a -20 dB or higher conversion efficiency for quantum-well λ/4-shifted DFB. The influences of pump power and DFB coupling strength (κL) on wave mixing as well as the Bragg reflection effects are discussed. The predictions of the model are found to be in good agreement with recent experimental observations     

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.<>     

Spectral behaviour of aged 1.55 ?m ?/4-shifted DFB lasers

Side-mode suppression ratio and main-mode wavelength were measured in the 3000 h life test of 1.55 ?m ?/4-shifted InGaAsP/InP DFB lasers. Degradation of the single-wavelength property and the wavelength shift of the main mode were observed in lasers which showed relatively large degradation rates of about 10?4/h at 50°C.     

1.5-µm λ/4-shifted InGaAsP/InP DFB lasers

A single wavelength light source in 1.5-μm range was developed using InGaAsP/InPlambda/4-shifted distributed feedback (DFB) semiconductor heterostructures. Superiority of thelambda/4-shifted DFB structure in terms of stability of the main mode at the Bragg wavelength was shown theoretically, in which the threshold, the output, and the polarization characteristics were taken into account. Alambda/4-shifted corrugated grating was made by a newly developed negative and positive photoresists technique. Buried heterostructure (BH) diode lasers with nonreflective window ends were fabricated and highly stable single-mode operation with a low threshold was obtained reproducibly. Direct modulation properties and life-tests results indicated that thelambda/4-shifted DFB lasers could be a reliable single-mode light source in a long span lightwave transmission system in 1.5-μm range.     

New high-resolution positive and negative photoresist method for ?/4-shifted DFB lasers

High-resolution positive and negative photoresists were used to fabricate ?/4-phase-shifted corrugations by holographic exposure. To avoid mixing these photoresists, cyclised polyisoprene was used as a midlayer. The shape and depth of the corrugations on the positive and negative photoresist regions were almost the same. Single-mode operation at the Bragg wavelength was confirmed in lasers with the corrugation.     

Ultralow chirp and high-speed 1.55 μm multiquantum wellλ/4-shifted DFB lasers

An ultra-low-chirp, high-speed, 1.55-μm, multi-quantum-well (MQW), λ/4-shifted, InGaAsP distributed-feedback (DFB) laser is demonstrated. A record chirp width of 0.4-0.5 nm (20-dB down full width) is achieved at 10-Gb/s direct modulation. This low-chirp single-longitudinal-mode operation is attained by an increase in the quantum size effect in the MQW active layer     

Optically pumped surface-emitting DFB GaInAsP/InP lasers with circular grating

The lasing action in surface-emitting DFB lasers with circular grating (SE-CG DFB) is reported for the first time. 1.283 mu m laser oscillation is demonstrated by optical pumping. The threshold pumping power density is estimated to be 16.2 kW/cm/sup 2/. The pulsed output power is more than 25 mW.<>     

DFB and DBR lasers emitting at 980 nm

The performance of DFB and DBR ridge waveguide lasers operating at 980 nm is reported. The DBR devices used a single growth step and gave singlemode output powers as high as 30 mW. The DFB devices used a two-step growth process in which the lower cladding and active regions were grown fast, the gratings were etched and then the upper cladding and p-contact layers were regrown on top of the grating. The DFB devices gave output powers as high as 75 mW.<>     

?/4-shifted InGaAsP/InP DFB lasers by simultaneous holographic exposure of positive and negative photoresists

?/4-shifted InGaAsP/InP DFB lasers were fabricated by a novel method, that is, simultaneous holographic exposure of positive and negative photoresists. The ?/4-phase-shift in the first-order corrugations formed on an InP substrate was confirmed through SEM views and diffracted beam patterns. A strong resonance peak below the threshold and single-wavelength operation above it at the Bragg wavelength were observed.     

Complex-coupled λ/4-shifted DFB lasers with a flat FMresponse from 10 kHz to 17 GHz

Semiconductor lasers with a flat FM response are required for coherent FSK transmission systems. A complex-coupled λ/4-shifted DFB laser is described that gives a flat FM response from 10 kHz to I7 GHz at 15 mW, achieved by using the self-suppressing mechanism of the spatial hole-burning effect     

Correlation of relative intensity noise with linewidth floors in narrow linewidth DFB lasers

The relative intensity noise of narrow linewidth DFB lasers in the vicinity of linewidth floors is measured. A strong correlation between linewidth rebroadening and a corresponding increase in the relative intensity noise for frequencies below 3 GHz is observed for devices with mode suppression ratios in excess of 35 dB.<>