1.5 µm range InGaAsP/InP distributed feedback lasers

Lasing characteristics of InGaAsP/InP distributed feedback (DFB) lasers in the 1.5 μm range were studied theoretically and experimentally. Wave propagation in five-layer DFB waveguides were analyzed to estimate the effect of the structural parameters on threshold conditions. A brief consideration on designing a low threshold laser and its lasing wavelength was made. DFB buried heterostructure lasers with fundamental grating emitting at 1.53 μm were prepared by liquid phase epitaxial techniques. CW operation was confirmed in the temperature rangeof -20° to 58°C, and a CW threshold current was as low as 50 mA at room temperature. A stable single longitudinal mode operation was observed both in dc condition and in modulated condition by a pseudorandom pulse current at 500 Mbits/s. No significant increase in the threshold current was observed after 1400 h continuous CW operation at 20°C.     

MOVPE-grown 1.5 µm distributed feedback lasers on corrugated InP substrates

A single-step low-pressure metalorganic vapor phase epitaxy (MOVPE) was applied to the fabrication of 1.5 μm InGaAsP/InP distributed feedback laser diodes on corrugated InP substrates, accompanied by LPE for buried heterostructure formation. High probability of single longitudinal mode operation was obtained due to the uniformity of the active layer thickness. A typical threshold current was 35 mA with both facets cleaved. A maximum output power of up to 27 mW was also obtained under single longitudinal mode operation with anti-reflective/cleaved facet configuration. The laser diode had high spectral stability under high-frequency direct modulation of 1.4 GHz. Results of initial aging tests (APC of 5 mW at 25°C for longer than 3800 h) have shown no degradation in driving currents. It is found that low-pressure MOVPE is favorable for epitaxial growth on corrugated substrates.     

1.5-1.6 µm GaInAsP/InP dynamic-single-mode (DSM) lasers with distributed Bragg reflector

The theoretical bases and the experimental results of the dynamic behavior of1.5-1.6 mum GaInAsP/InP dynamic-single-mode (DSM) lasers with distributed Bragg reflector (DBR) are given. A condition for the single-mode operation of a rapidly modulated DBR laser, called a "dynamic-single-mode laser," and the dynamic spectral width were theoretically presented. Experimentally, buried-heterostructure distributed-Bragg-reflector integrated-twin-guide (BH-DBR-ITG) and buried-heterostructure butt-jointed-built-in distributed-Bragg-reflector (BH-BJB-DBR) lasers emitting at the wavelength of1.5-1.6 mum were directly modulated up to 3 GHz, and the stable single-mode operations were obtained in both types of lasers. The dynamic spectral width at the worst modulation condition was measured to be 0.27 nm, which was aboutfrac{1}{35}times smaller than that of conventional lasers.     

Distributed feedback InGaAsP/InP lasers with window region emitting at 1.5 µm range

Distributed feedback (DFB) InGaAsP/InP lasers with a window region formed at an end of the corrugated DFB region were made in order to overcome the problems inherent in the previous structures of DFB lasers with cleaved, sawed, etched, or AR-coated facets, or with an unexcited corrugation region. The window structure DFB lasers showed linear current versus output (I-L) curves, in contrast to those with a hysteresis or a kink appearing in a DFB laser with an unexcited region. Suppression of Fabry-Perot (F-P) resonances due to the two facets were sufficient enough to keep a single longitudinal mode property by DFB up to high excitation level. CW operation up to 65°C was achieved at the 1.5 μm wavelength range. Axial modes concerning the corrugated resonator were measured at about the threshold current. A stop band of a DFB laser was clearly observed with two dominant modes and much smaller submodes, which almost agreed with the axial modes predicted from a basic DFB theory.     

Spectral linewidth estimation of a 1.5 µm range InGaAsP/InP distributed feedback laser

Spectral linewidth of a 1.5 μm range distributed feedback buried heterostructure (DFB-BH) laser in CW operation is estimated theoretically and experimentally. Considering the equivalent mirror facet loss coefficient and the confinement factor in the active layer, etc., we modified the conventional formula for the spectral linewidth of single-mode semiconductor lasers and presented a formula for the linewidth of DFB lasers. Furthermore, power-dependent linewidth measurements of a 1.5 μm range InGaAsP/InP DFB-BH laser with a window region were carried out using Fabry-Perot interferometers. The linewidth was observed to increase linearly with inverse output power. The measured result was explained by the calculated result through the modified formula. The full width at half maximum was estimated to be 50 MHz at an output power of 1 mW.     

Lasing characteristics of 1.5 - 1.6 µm GaInAsP/InP integrated twin-guide lasers with first-order distributed Bragg reflectors

Lasing characteristics of1.5-1.6 mum GaInAsP/InP integrated twin-guide lasers with first-order distributed Bragg reflectors (DBR-ITG lasers) are given theoretically and experimentally. At this wavelength region the fiber loss is ultimately low, but the effect of the material dispersion is serious. It is theoretically found that single longitudinal mode operation over twice the threshold current is obtainable with high quantum efficiency by optimizing the coupling properties and configurational parameters of the DBR-ITG laser. The experimental results are also demonstrated. Single longitudinal mode operation of1.5-1.6mum DBR-ITG lasers under high-speed direct modulation was achieved with a pulsewidth of 1.5 ns.     

Static characteristics of 1.5 - 1.6 µm GaInAsP/InP buried heterostructure butt-jointed built-in integrated lasers

Lasing characteristics of GaInAsP/InP buried heterostructure butt-jointed built-in distributed Bragg reflector integrated lasers (BH-BJB-DBR lasers) intended for dynamic single mode operation in the wavelength range of1.5-1.6 mum are given. In the first section, the coupling property between the active region and the butt-jointed external waveguide region is calculated to show the possibility of large fabrication tolerance. A coupling coefficient of more than 95 percent is estimated. Secondly, the GaInAsP/InP integrated lasers were fabricated and tested in the view of the static characteristics and the axial mode selection property. Lateral mode control was achieved by the use of a buried heterostructure, so that the axial and lateral modes were maintained to a fixed single mode. The lasers thus fabricated were operated in CW conditions at room temperature with a threshold current of about 100 mA. Single longitudinal mode operation was observed with a temperature dependence of about 0.13 nm/deg with the temperature range of more than50-65degC at around 0°C. Differential quantum efficiency as high as 13 percent/ facet was obtained for the laser with power output of more than 5 mW/facet. The output spectrum below threshold indicated the strong wavelength selectivity of the DBR region, and the net gain difference between the main DBR mode and the adjacent submode was measured to be about 6 cm^-1. No appreciable degradation and change of characteristics have been observed even after CW operation of more than 9770 h at 20°C.     

1.6 µm wavelength GaInAsP/InP lasers prepared by two-phase solution technique

Reduction of the threshold current of GaInAsP lasers with an antimeltback layer was studied in the wavelength range1.50-1.65 mun. The two-phase solution growth technique was applied using a relatively low temperature and a slow cooling rate of 0.17°C/min to reduce the active layer thickness. The antimeltback layer with a bandgap wavelength of 1.35 μm resulted in a flat active layer and eliminated the melt-back problem completely. From experiments and calculations we found that both the carrier and the optical confinement of this structure, having an antimeltback layer, were almost the same as those for the conventional InP cladding structure. A threshold eurrent density as low as 1.2 kA/cm²and an active layer thickness of 0.20μm were obtained at these wavelengths. The lattice-match condition of low-temperature growth was studied. In the low-temperature growth, the longer wavelength lasers were grown with the same amount of InAs.J_{th}/dwas independent of the growth condition (T_{S}, T_{G}) and had a value of 5-6 kA/cm². μm.     

1.5 μm GaInAsP/InP distributed reflector (DR) lasers with SCHstructure

The large differential quantum efficiency η_df with one-directional output operation obtained in 1.5-μm distributed reflector (DR) lasers using a thin active layer of 50 nm and the separate-confinement heterostructure (SCH) structure is discussed. η _df of the DR laser was experimentally determined to be twice that of distributed-feedback (DFB) lasers cleaved from the same wafer, which indicates high efficiency and high power characteristics of DR lasers     

Effect of mirror facets on lasing characteristics of distributed feedback InGaAsP/InP laser diodes at 1.5 µm range

The effect of mirror facets on lasing properties of distributed feedback (DFB) InGaAsP/InP laser diodes was studied theoretically and experimentally. A DFB laser with a window region was found to be suitable to examine the effect of mirror facets. The effective reflectivity of a window structure was calculated to be very small, typically as small as 0.03 percent for a few tens of micrometers of the window region. These small effective reflectivities were experimentally confirmed. Three kinds of DFB lasers, i.e., a double-window (DW), a single-window (SW), and a Fabry-Perot (FP), were discussed. Two modes with a separation corresponding to a so-called stopband appeared in a DW-DFB laser, in which almost no reflection at both ends was estimated. On the other hand, the threshold and the resonant wavelength of an SW- and an FP-DFB laser were found to be sensitive to the phases of corrugation at the facets. It turned out, however, that the mirror facet contributed to the single-mode operation due to an asymmetric resonant spectrum and to the reduction in the threshold. Although a low-threshold-current FP-DFB laser was experimentally obtained, the coincidence between the gain peak and the Bragg wavelengths was essential in this type. The SW-DFB laser seemed the most promising among the three types in terms of the stability of the single-mode operation.     

Regimes of feedback effects in 1.5-µm distributed feedback lasers

We have measured the effects of feedback on the spectra of 1.5-μm DFB lasers from feedback power ratios as low as -80 dB up to -8 dB. Five distinct regimes of effects are observed with well defined transitions between them. The dependence of these effects on the distance to the reflection is also investigated.     

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.     

1.5 ?m GaInAsP/InP distributed Bragg reflector lasers with built-in optical waveguide

Room-temperature operation of distributed Bragg reflector (DBR) lasers emitting at 1.5 ?m is reported. The diode was fabricated by conventional techniques and has a simple configuration with a built-in waveguide grown by single-step liquid-phase epitaxy. Single-longitudinal-mode oscillation at 1.504?1.505 ?m was obtained at temperatures between ?10°C and 5°C with 310?360 mA threshold current.     

Narrow spectral linewidth 1.5 μm GaInAsP/InP distributed Braggreflector (DBR) lasers

The spectral linewidth of distributed Bragg reflector (DBR) lasers is discussed. The narrowest linewidth of 3.2 MHz was obtained at P =1.5 mW. It was found that narrow spectral linewidths can be obtained constantly for the DBR lasers with long and low optical loss corrugation regions, and with a small coupling coefficient. These results are explained by the external cavity model, which is used in the theoretical analysis. To increase the output power of the narrow-linewidth DBR lasers, AR (antireflection) coating was applied to the output facet. Improved quantum efficiency and submode suppression were achieved. Degradation of the linewidths by the AR coating was insignificant     

Growth and characterization of 1.3 µm CW GaInAsP/InP lasers by liquid-phase epitaxy

The variation of the threshold current density and its temperature dependence with acceptor concentration in GaInAsP/InP lasers emitting at 1.3 μm is described. Mechanisms responsible for the dependence are identified. A model is developed to predict the effect of the above dependence on the CW operation range of these devices. The validity of the model is verified experimentally.     

1.5 µm GaInAsP traveling-wave semiconductor laser amplifier

This paper presents a theoretical and experimental study in terms of small-signal gain, signal gain saturation, and noise characteristics of a 1.5 μm GaInAsP traveling-wave amplifier (TWA), realized through the application of SiOxfilm antireflection coatings. This TWA, having a residual facet reflectivity of 0.04 percent, exhibits a wide, flat signal gain spectrum and a saturation output power of +7 dBm at a 20 dB signal gain. The TWA also has a noise figure of 5.2 dB, which is the smallest value reported for semiconductor laser amplifiers. The experimental results are confirmed to be in good agreement with the theoretical predictions based on the multimode traveling-wave rate equations in conjunction with the photon statistic master equation analysis, which takes into account the amplifier material and device structural parameters. Signal gain undulation, saturation output power, and noise figure are also theoretically evaluated as functions of the facet reflectivity. The superior performance of the TWA demonstrates that the device is favorable for use in linear optical repeaters in fiber transmission systems.     

A GaInAsP/InP tapered-waveguide semiconductor laser amplifierintegrated with a 1.5 μm distributed feedback laser

A GaInAsP/InP index-guided type tapered-waveguide traveling-wave semiconductor laser amplifier (TTW-SLA) with exponential taper structure was realized for the first time and its integration with a 1.5 μm wavelength phase adjusted DFB laser was demonstrated. A reasonable amount of the single-pass gain, which was limited by the residual reflectivity at the output end of the amplifier region, was obtained with no serious problem with the lasing spectrum. A narrow beam divergence of FWHM=5.5° and stable longitudinal mode operation were obtained with the output waveguide width of 20 μm by adopting both a window structure and 7° angled facets. This value agrees well with the theoretically calculated one and proves that single transverse-mode propagation can be attained by an exponentially tapered TTW-SLA with a very wide output end     

Wavelength variation of 1.6 µm wavelength buried heterostructure GaInAsP/InP lasers due to direct modulation

Wavelength shift during the period of direct modulation (dynamic wavelength shift) for injection lasers having a BH structure has been investigated both experimentally and theoretically. A GaInAsP/InP BH laser emitting a nominal wavelength of 1.61 μm was modulated by a sinusoidal current at frequencies in the range of 0.2-2 GHz. The full width of the dynamic wavelength shift was 0.35 nm at a modulation frequency of 1.8 GHz, and a modulation depth of 63 percent at a bias current 1.14 times the threshold current. It was found that the width of the dynamic wavelength shift increases with proportion to the modulation depth, and with inverse proportion to the bias current at a frequency below 1 GHZ. The differential coefficientdn/dNof refractive indexnfor carrier densityNin the active region was measured for the purpose of the analysis. The value obtained is-1.2 times 10^{-20}cm³. The dynamic shift of the lasing wavelength was found to be characteristic of the change of the refractive index induced by the oscillation of carrier density in the active region during intensity modulation. The theoretical shift shows maximum value at a resonance-like modulation frequency. The peak height of the resonance wavelength shift is strongly affected by carrier diffusion in the transverse direction, and has a minimum value when stripe width is nearly equal to carrier diffusion length.     

1.5?1.6 ?m GaInAsP/InP Integrated twin-guide lasers with first-order distributed Bragg reflectors

The room-temperature operation of 1.5?1.6 ?m GaInAsP/InP integrated twin-guide lasers with first-order distributed Bragg reflectors is reported. Single-longitudinal-mode oscillation was observed up to 1.4 times the threshold current. The temperature dependence of lasing wavelength was only 0.8 ?/degree.