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.     

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.     

Dry-etched wire distributed feedback laser

We report on the realization and the properties of dry-etched wire and wire-DFB lasers and compare them with two-dimensional (2-D) reference lasers. The starting vertical structure, which was optimized for low threshold, consists of four GaInAs quantum wells embedded in a GaInAsP waveguide. Even for very small longitudinal confinement factors of the order 0.23, the dry-etched wire lasers show laser operation up to 60°C and a threshold current density at room temperature of 5 kA/cm ² for simple oxide stripe lasers. For the wire lasers, an increase of the differential material gain of more than one order of magnitude (about a factor of 20) was measured, compared to conventional 2-D lasers. The wires were also arranged periodically with different grating constants to allow gain-coupled DFB laser operation. Clear single-mode emission has been found, as expected, from the gain-coupling mechanism     

GaInAsP distributed feedback laser array

Reports the fabrication and performance characteristics of a gain-guided coupled distributed feedback laser array. The five-emitter lasers have threshold currents in the range of 300 to 500 mA and emit in a single wavelength with a single-lobed far field. Single-wavelength output powers of ~120 mW have been observed. The authors believe the frequency selective feedback provided by the diffraction grating helps in coherent coupling between the array elements, which results in a single-lobed far field     

Far-infrared distributed feedback gas laser

Theory and first operation of a distributed feedback gas laser is expanded. This goal has been achieved by an optically pumped 496μm CH3F laser equipped with a hollow metal waveguide with deep periodic corrugations. The period of corrugations is tuned by temperature variation. Single-mode operation is achieved. The working pressure of CH3F is up to 10 torr. Propagation of the laser beam is near the diffraction limit. The spectral linewidth of less than 22 MHz corresponds to the transform limit set by the 50 ns pulse duration. Temperature tuning curves of the laser output are in good agreement with theory.     

40 Gbit/s electroabsorption modulated distributed feedback laser

A 40 Gbit/s multi-quantum well (MQW) electroabsorption modulator (EAM) with a lumped electrode monolithically integrated with a distributed feedback (DFB) laser is demonstrated. Superior characteristics are exhibited for the device, such as low threshold current of 18 mA, over 40 dB side-mode suppression ratio at 1 550 nm and more than 30 dB extinction ratio when coupled into a single-mode fiber. By adopting deep ridge waveguide and planar electrode structures combined with buried benzocyclobutene (BCB), the capacitance of the EAM is reduced down to 0.18 pF and over 33 GHz modulation bandwidth at a small signal has been demonstrated. Negative chirp operation is realized when the bias voltage is beyond 1.6 V.     

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.     

Analysis of multiple-quantum-well distributed feedback laser

The doping and detuning dependences of the threshold, dynamic and spectral properties of multiple-quantum-well distributed feedback lasers were analysed theoretically. It was shown that optimised detuning improves the dynamic and spectral properties. It was also shown that donor doping significantly reduces the threshold current and spectral line-width.     

The heteroepitaxial ridge-overgrown distributed feedback laser

A new distributed feedback laser, the heteroepitaxial ridge-overgrown distributed feedback (HRO-DFB) laser, is proposed and demonstrated. The growth steps permit post-active-layer growth determination of the grating period, and the fabrication of this laser is simple with an automatic alignment of the current confinement to the ridge-overgrowths, which form the strip-loaded waveguide of the laser. The lateral overgrowth extending over the oxide films on both sides of the window stripe enhances the effect of grating feedback. Both LPE and MO-CVD have been employed successfully for the ridge overgrowth. The HRO-DFB laser was shown to operate in stable single-longitudinal mode with no observable mode partition events under 2 Gbit/s pseudorandom pulse modulation. The dynamic spectral width under 2 Gbit/s modulation was typically 0.5-2 Å. Other characteristics of these 1.5 μm HRO-DFB lasers include ∼8 mW/facet of single-longitudinal mode output power, and narrow beam divergence of 10° operating in the fundamental transverse mode in the junction plane. The CW thresholds were usually 50-100 mA. Transmission experiments with dispersive fibers (17 ps/km-nm) atsim1.55 mum at 2 Gbits/s over 82 km and at 420 Mbits/s over 203 km confirmed that the HRO-DFB laser is an excellent a single-frequency optical source for use in optical communication applications.     

Theory of a distributed feedback laser

This paper treats distributed feedback laser operation in which a classical electromagnetic field obeying Maxwell's equations interacts with an active medium according to the laws of quantum mechanics. The theory describes arbitrarily intense two-mode operation of fundamental and higher modes in both index and gain grating configurations. Spacial hole burning and finite atomic linewidth are included in the analysis and are shown to give marked changes in some laser intensity profiles.     

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     

Effect of phase shift position on spectral linewidth of the π/2distributed 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     

Longitudinal mode behaviors of 1.5 µm range GaInAsP/InP distributed feedback lasers

Longitudinal mode behaviors of asymmetric structure distributed feedback buried heterostructure (DFB-BH) lasers are examined theoretically and experimentally. A 1.5 μm range GaInAsP/InP DFB-BH laser was fabricated by a three-step LPE growth process. We measured the stopband in the spectrum of the DFB laser. It was found that no resonance mode emission occurred in the gain spectrum and its spectrum was asymmetric with respect to the Bragg wavelength. Most of the lasing power concentrated on the DFB mode adjacent to the stop-band which was determined by the Bragg condition. The measured spectrum was explained by the calculated results of the coupled wave theory with external reflectors. The asymmetric spectrum was caused by the relative position of the cleaved facet on the corrugation grating. It was shown that the asymmetric structure DFB laser, which consisted of two end facets with different reflection coefficients, gives a stable single longitudinal mode. There was no mode jump up to 2.3 times threshold. At a modulation depth of 100 percent, the ratio of the highest nonlasing mode intensity to the lasing DFB mode was estimated to be -16.0 dB.     

New 1.5 ?m wavelength GaInAsp/InP distributed feedback laser

A new 1.5 ?m-wavelength GalnAsP/InP distributed feedback buried-heterostructure laser was fabricated by a three-step LPE growth process. The second-order corrugation grating was formed on the waveguide layer grown on the active layer. High differential quantum efficiency of 13%/facet was obtained. Single-longitudinal-mode operation in the temperature range from ?20°C to 55°C was obtained.     

High-power operation of a ridge-waveguide AlGaAs/GaAs distributed feedback laser

We report high-power continuous-wave operation of a ridge-waveguide AlGaAs/GaAs distributed feedback laser as high as 40 mW. It was realised by an appropriate design of a ridge structure and by facet coatings.     

AlGaAs/GaAs visible distributed feedback laser operating at 770 nm

A visible-light AlGaAs/GaAs DFB laser with practical characteristics oscillating at 770 nm has been fabricated by using two-step liquid phase epitaxy for the first time. Threshold current of 90 mA measured at room temperature is satisfactorily low for its simple oxide stripe structure. The same DFB mode oscillation without mode hopping has been maintained in a considerably wide range of temperature. The results show a possibility of realising a high-performance DFB laser in the visible-wavaelength region.     

Two-dimensional surface emitting distributed feedback laser arrays

The demonstration of two-dimensional arrays of surface-emitting distributed feedback lasers is reported. A 30-element array is shown to produce up to 6.8 W of pulsed output power. A 27-element array bonded on a silicon microchannel water-cooled heat sink is shown to produce as much as 3 W of continuous-wave output power. The high continuous wave power is possible due to the low thermal resistance in the p-side down design of the surface-emitting distributed feedback lasers and effective heat extraction by a silicon microchannel heat sink     

Optimum design for multiple-phase-shift distributed feedback laser

An optimum design for the multiple-phase-shift distributed feedback (MPS-DFB) laser with three phase shifts in the corrugation was established theoretically. It was shown that the optimised MPS-DFB laser gives sufficient threshold gain difference and uniform field distribution even for strong coupling. Therefore, the long cavity MPS-DFB laser is expected to give stable single-longitudinal-mode operation at high injection levels and give a minimum linewidth of less than 500 kHz     

Wideband tuning of tunable distributed amplification distributed feedback laser array

A wide continuous tuning range of 6.7 nm was achieved with a tunable distributed amplification distributed feedback (TDA-DFB) laser by employing an asymmetric periodic structure. A 36-channel operation with a 100 GHz grid was realised for an array of six TDA-DFB lasers integrated monolithically with a coupler and an amplifier.