High-performance 1.55-μm wavelength GaInAsP-InPdistributed-feedback lasers with wirelike active regions

High-performance 1.55-μm wavelength GaInAsP-InP strongly index-coupled and gain-matched distributed-feedback (DFB) lasers with periodic wirelike active regions mere fabricated by electron beam lithography, CH₄/H₂-reactive ion etching, and organometallic vapor-phase epitaxial regrowth, whose index-coupling coefficient was more than 300 cm^-1. In order to design lasers for low threshold current operation, threshold current dependences on the number of quantum wells and the wire width mere investigated both theoretically and experimentally. A record low threshold current density of 94 A/cm² among 1.55-μm DFB lasers was successfully obtained for a stripe width of 19.5 μm and a cavity length of 600 μm. Moreover, a record low threshold current of 0.7 mA was also realized at room temperature under CW condition for a 2.3-μm-wide buried heterostructure with a 200-μm-long cavity. Finally, we confirmed stable single-mode operation due to a gain-matching effect between the standing-wave profile and the wirelike active region     

High-power highly reliable Al-free 940-nm diode lasers

Al-free diode lasers emitting at 930 nm having a broadened step-index waveguide structure and a single active InGaAs quantum well have been realized by MOVPE. The impact of waveguide thickness on device performance has been studied. The highest wall plug efficiency of about 60% has been obtained with diode lasers having a 1-μm-thick waveguide. Increasing the waveguide thickness to 1.5 μm resulted in record low degradation rates below 10^-5 h^-1 for 3-W output power (100 μm stripe width). The same diode lasers showed a good long-term reliability even at an output power of 4 W. The best beam quality had diode lasers with a 2-μm-thick waveguide, at the expense of a reduced temperature stability     

High-power highly-reliable operation of 0.98-μm InGaAs-InGaPstrain-compensated single-quantum-well lasers with tensile-strainedInGaAsP barriers

We compared 0.98-μm lasers with a strain-compensated active layer consisting of a compressive InGaAs well and tensile-strained InGaAsP barriers with identical lasers that have a conventional active layer with GaAs barriers. It was shown that the lasers with InGaAsP barriers have better temperature characteristics due to the larger energy gap difference between a well and barriers. Because of the high characteristic temperature, 200-mW operation was obtained with the InGaAsP-barrier laser even at 90°C without any significant deterioration. We also showed that the operation of the lasers with a strain-compensated active layer was highly reliable. The degradation rate of these lasers was four times smaller than that of the lasers with GaAs barriers due to the better crystal quality in their active laser. The estimated lifetime at 25°C for the lasers with a strain-compensated active layer was more than 170000 hours     

Recent Advances on InAs/InP Quantum Dash Based Semiconductor Lasers and Optical Amplifiers Operating at 1.55 μm

This paper summarizes recent advances on InAs/InP quantum dash (QD) materials for lasers and amplifiers, and QD device performance with particular interest in optical communication. We investigate both InAs/InP dashes in a barrier and dashes in a well (DWELL) heterostructures operating at 1.5 mum. These two types of QDs can provide high gain and low losses. Continuous-wave (CW) room-temperature lasing operation on ground state of cavity length as short as 200 mum has been achieved, demonstrating the high modal gain of the active core. A threshold current density as low as 110 A/cm² per QD layer has been obtained for infinite-length DWELL laser. An optimized DWELL structure allows achieving of a T₀ larger than 100 K for broad-area (BA) lasers, and of 80 K for single-transverse-mode lasers in the temperature range between 25degC and 85degC. Buried ridge stripe (BRS)-type single-mode distributed feedback (DFB) lasers are also demonstrated for the first time, exhibiting a side-mode suppression ratio (SMSR) as high as 45 dB. Such DFB lasers allow the first floor-free 10-Gb/s direct modulation for back-to-back and transmission over 16-km standard optical fiber. In addition, novel results are given on gain, noise, and four-wave mixing of QD-based semiconductor optical amplifiers. Furthermore, we demonstrate that QD Fabry-Perot (FP) lasers, owing to the small confinement factor and the three-dimensional (3-D) quantification of electronic energy levels, exhibit a beating linewidth as narrow as 15 kHz. Such an extremely narrow linewidth, compared to their QW or bulk counterparts, leads to the excellent phase noise and time-jitter characteristics when QD lasers are actively mode-locked. These advances constitute a new step toward the application of QD lasers and amplifiers to the field of optical fiber communications     

High-temperature operation of 1.3-μm tapered-active-stripe laserfor direct coupling to single-mode fiber

High-temperature operation of 1.3-μm wavelength multiquantum-well (MQW) lasers with an active stripe horizontally tapered over whole cavity, for direct coupling to single-mode fibers (SMFs), are reported. The lasers have reduced the output-beam divergence in a simple structure which does not contain an additional spot-size transformer. To improve high-temperature characteristics, we have investigated the influence of the thickness of separate-confinement-heterostructure layers and the number of quantum wells (QWs) on the threshold current and the output-beam divergence at high temperature. As a result, the fabricated lasers show low-threshold current (~18 mA) and high-slope efficiency (~0.4 mW/mA) with narrow output-beam divergence (~12°) at 85°C. Moreover, we have obtained maximum coupling efficiency of -4.7 dB in a direct coupling to a SMF, and the reliability of longer than 10⁵ h (MTTF) by a lifetime test of over 2000 h at 85°C     

High-Power All-Fiber-Based Narrow-Linewidth Single-Mode Fiber Laser Pulses in the C-Band and Frequency Conversion to THz Generation

Based on highly Er/Yb codoped phosphate fibers, we have implemented all-fiber-based narrow-linewidth single-mode (SM) pulsed fiber lasers in master oscillator and power amplifier configuration. Two approaches were used to achieve the narrow-linewidth pulsed fiber laser seeds: 1) an all-fiber-based active Q-switched fiber laser using an actuator and 2) a directly modulated single-frequency continuous-wave fiber laser. Both the fiber laser seed pulses from the two approaches have the transform-limited spectral linewidth. Based on a newly developed large-core SM highly Er/Yb codoped phosphate fiber, the peak power of SM pulses can be scaled to more than 50 kW with transform-limited linewidth and diffraction-limited beam quality. These high-power narrow-linewidth SM fiber laser pulses have been successfully used to generate coherent terahertz (THz) waves based on parametric processes in a nonlinear optical crystal. The peak power of this fiber-based THz source can reach 26.4 mW.     

Spectral Linewidth Reduction in Widely Wavelength Tunable DFB Laser Array

We have developed an L-band tunable distributed feedback laser array (TLA) with a new design to reduce the spectral linewidth. A wide wavelength tuning range of $sim$40 nm is obtained with a high fiber output power of 20 mW and a high side-mode suppression ratio of ≫50 dB in the TLA module. A narrow linewidth of less than 580 kHz is achieved over the entire tuning range. Furthermore, we investigated the causes of linewidth variation. We found that a TLA with a longer cavity is more tolerant to external feedback, which reduces the variation in linewidth.      

Monolithic integration of spot-size converters with 1.3-μmlasers and 1.55-μm polarization insensitive semiconductor opticalamplifiers

To reduce packaging costs, it is necessary to use passive alignment between the laser diodes and optical fiber. Such an alignment requires low-coupling loss and large positional alignment tolerances. This is achievable with integrated spot-size converters, which permit to match the near field of a laser to that of a flat-end single-mode fiber (SMF). In this paper, we first review briefly the different technological approaches to realize spot-size converters. Then, we focus on the double-core structure developed both for 1.3-μm Fabry-Perot lasers and 1.55-μm semiconductor optical amplifiers (SOAs). The spot-size expansion is simulated using a two-dimensional (2-D) beam propagation method analysis. Short spot-size converters (100 μm) integrated with 1.3-μm lasers and 1.55-μm SOAs exhibit beam divergences as low as 12°×12° and 12°×15°, respectively. The performances of devices with integrated spot-size converters are reported and discussed. A 2-in wafer process is used thanks to the versatility of the double-core structure and its compatibility with buried ridge stripe technology     

Stable single-mode operation of distributed feedback lasers with wirelike active regions

A stable single-mode operation mechanism in distributed feedback (DFB) lasers with wirelike active regions was theoretically investigated by taking into account "gain matching" between standing wave profiles of each resonant mode and periodic active regions. As a result, it was clarified that the resonant modes at the longer wavelength side of the stopband have higher modal gain than those at the shorter wavelength side and that the oscillation takes place at the longer wavelength side resonant mode nearest to the stopband. The influence of the cleaved facet with respect to the grating phase was also analyzed. The measured spectral properties of buried heterostructure GaInAsP/InP DFB lasers consisting of wirelike active regions, such as a subthreshold gain spectrum and the lasing wavelength with respect to the stop band, agreed well with theoretical results. Finally, it was confirmed that a stable single-mode operation was preserved even after a room temperature continuous wave aging of 7300 h at bias current of around 10/spl times/ the threshold.     

Cr4+ lasers: present performance and prospects for newhost lattices

Cr-doped lasers, based on forsterite and YAG, provide broadly tunable power in the 1.25-μm and 1.45-μm regions. Performance data on tuning range, pumping, output power, and thermal management for these lasers is reviewed. Potential new crystals for Cr⁴⁺ should have heavy atoms to reduce lattice phonon frequencies, a distorted tetrahedral cage for the Cr⁴⁺ ion, and possibly an octahedral site for Cr³⁺. Possible materials include monticellite and diopside     

The metal-organic chemical vapor deposition growth and propertiesof InAsSb mid-infrared (3-6-μm) lasers and LEDs

We describe the metal-organic chemical vapor deposition (MOCVD) growth of AlAs_1-xSb_x cladding layers and InAsSb-InAs multiple-quantum well (MQW) and InAsSb-InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. The AlAs_1-xSb_x cladding layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. By changing the layer thickness and composition of SLSs and MQWs, we have prepared structures with low temperature (<20 K) photoluminescence wavelengths ranging from 3.2 to 6.0 μm. We have made gain-guided injection lasers using undoped p-type AlAs_0.16Sb_0.84 for optical confinement and both strained InAsSb-InAs MQW and InAsSb-InAsP SLS active regions. The lasers and light emitting diodes (LEDs) utilize the semi-metal properties of a GaAsSb(p)-InAs(n) heterojunction as a source for electrons injected into active regions. A multiple-stage LED utilizing this semi-metal injection scheme is reported. Gain-guided, injected lasers with a strained InAsSb-InAs MQW active region operated up to 210 K in pulsed mode with an emission wavelength of 3.8-3.9 μm and a characteristic temperature of 29-40 K. We also present results for both optically pumped and injection lasers with InAsSb-InAsP SLS active regions. The maximum operating temperature of an optically pumped 3.7-μm strained-layer superlattice (SLS) laser was 240 K. An SLS LED emitted at 4.0 μm with 80 μW of power at 300 K     

1.55-μm InP-lattice-matched VCSELs with AlGaAsSb-AlAsSb DBRs

We review the design, fabrication, and characterization of 1.55-μm lattice-matched vertical-cavity surface-emitting lasers, operating continuous wave up to 88°C. For one embodiment, the threshold current is 800 μA, the differential quantum efficiency is 23%, and the maximum output power is more than 1 mW at 20°C and 110 μW at 80°C. The basic structure consists of AlAsSb-AlGaAsSb mirrors, which provide both high reflectivity and an InP-lattice-matched structure. The quaternary mirrors have poor electrical and thermal conductivities, which can raise the device temperature. However, a double-intracavity-contacted structure along with thick n-type InP cladding layers circumvents these drawbacks and finally leads to an excellent performance. The measured voltage and thermal impedances are much lower for the intracavity-contacted device than an air-post structure in which current is injected through the Sb-based quaternary mirror. The structure utilizes an undercut aperture for current and optical confinement. The aperture reduces scattering loss at the etched mirror and contributes to high differential efficiency and low threshold current density     

0.98-μm multiple-quantum-well tunneling injection laser with98-GHz intrinsic modulation bandwidth

We demonstrate GaAs-based 0.98-μm multiple-quantum-well (MQW) tunneling injection lasers with ultrahigh-modulation bandwidths. Electrons are injected into the active region via tunneling, leading to a “cold” carrier distribution in the quantum wells (QWs). The tunneling time (2 pS) measured by time resolved differential transmission spectroscopy agrees with the capture time extracted form the electrical impedance measurement. The tunneling barrier prevents electrons from going over the active region into the opposite cladding layer. The carrier escape time in tunneling injection lasers is larger than that in conventional QW lasers. Enhanced differential gain, minimized gain compression and improved high frequency performance have been achieved. The -3-dB modulation bandwidth is 48 GHz and the maximum intrinsic modulation bandwidth is as high as 98 GHz     

High-power VCSELs: single devices and densely packed 2-D-arrays

We report on vertical-cavity surface-emitting lasers (VCSELs) and laser arrays providing high output powers in the 980-nm wavelength regime. Extensive investigations on size scaling behavior of single top- and bottom-emitting devices concerning fundamental electrooptical and thermal properties show limits of attainable output characteristics. Maximum experimentally achieved continuous-wave (CW) optical output powers at room temperature are 180 and 350 mW for top- and bottom-emitting VCSELs, respectively. Detailed analysis on the thermal interaction between closely spaced elements have been carried out to describe the thermally induced power limitations of two-dimensional arrays. Fabricated heat sunk bottom-emitting arrays of 23 elements and 40-μm aperture size of individual elements show output powers of 0.56 W CW at room temperature and 0.8 W actively cooled, resulting in 0.33 kW/cm² and 0.47 kW/cm² maximum spatially averaged optical power density, respectively     

Ultralow threshold 1.3-μm InGaAsP-InP compressive-strainedmultiquantum-well monolithic laser array for parallel high-densityoptical interconnects

An ultralow-threshold 1.3-μm InGaAsP-InP 10-element monolithic laser array is achieved through careful optimization of a strained multiquantum-well active layer, especially the amount of strain, the well thickness, the barrier thickness, the number of wells, and the active laser width. This array has a record-low threshold current, highly uniform threshold current characteristics (1.3±0.09 mA and slope efficiency of 0.37±0.01 W/A), extremely low operating current of 14 mA under 5-mW output power, and long-term reliability. This array is suitable as light sources for a parallel high-density optical interconnection system. In addition, a record low CW threshold current of 0.58 mA at 20°C and 1.62 mA at 90°C, as a long-wavelength laser, is obtained by employing a short cavity (100 μm) uith high-reflection coatings     

Improved output performance of high-power VCSELs

The intention of this paper is to report on state-of-the-art high-power vertical-cavity surface-emitting laser diodes (VCSELs), single devices as well as two-dimensional (2-D) arrays. Both approaches are studied in terms of electrooptical characteristics, beam performance, and scaling behavior. The maximum continuous wave (CW) output power at room temperature of large-area bottom-emitting devices with active diameters up to 320 μm is as high as 0.89 W, which is to our knowledge the highest value reported for a single device. Measurements under pulsed conditions show more than 10-W optical peak output power. Also, the CW performance of 2-D arrays has been increased from 0.56 W for 23 elements to 1.55 W for 19 elements due to significantly improved heat sinking. The extracted power densities spatially averaged over the area close to the honeycomb-like array arrangement raised from 0.33 kW/cm² to 1.25 kW/cm². Lifetime measurements have proven acceptable reliability for over 10000 h at a degradation rate of less than 1% per 1000 h. The emission wavelength of bottom-emitting devices is restricted to about 900 nm or higher due to fundamental absorption in the GaAs substrate. Windowing of the substrate has been studied to allow for shorter wavelength emission     

High-power low-divergence semiconductor lasers for GaAs-based980-nm and InP-based 1550-nm applications

High-power diode lasers with low-vertical divergence and high-fiber coupling efficiency were developed for GaAs-based 980-nm pump lasers and InP-based 1550-nm Fabry-Perot and distributed-feedback (DFB) lasers. Narrow divergence at 980 nm was made possible by a large optical-mode waveguide design, with full-width at half-maximum (FWHM) far-field angles of 11.7°×17.8° and coupling efficiency of 80% into a cleaved single-mode fiber (SMF). A vertical taper processing technique was developed for InP-based laser structures. Fabry-Perot lasers produced over 90-mW output power, 17°×16° FWHM beam divergence angles, and 63% coupling efficiency into a lensed SMF. The vertical taper was successfully integrated in 1550-nm DFB lasers, and over 80 mW single-mode output power with beam divergence angles of 12°×14° was obtained     

Dual-Grating Spectral Beam Combination of High-Power Fiber Lasers

We describe a dual-grating spectral beam combination (SBC) system to combine multiple high-power fiber laser outputs while maintaining near-diffraction-limited beam quality. The two gratings are parallel in a grating rhomb configuration, with input and output beams that are parallel but shifted with wavelength, rather than the typical angular dispersion of a single grating. The resulting advantage of the dual-grating SBC over other beam combination systems is the relaxation of the linewidth requirement. We combined two fiber lasers with output powers of 115 W each and linewidths of about 0.15 nm ( ~40 GHz) to produce a combined beam of 190 W power with near-diffraction-limited beam quality (M ² ~ 1.18).     

Passively Mode-Locked 4.6 and 10.5 GHz Quantum Dot Laser Diodes Around 1.55 μm With Large Operating Regime

Passive mode-locking in two-section InAs/InP quantum dot laser diodes operating at wavelengths around 1.55 $mu$m is reported. For a 4.6-GHz laser, a large operating regime of stable mode-locking, with RF-peak heights of over 40 dB, is found for injection currents of 750 mA up to 1.0 A and for values of the absorber bias voltage of 0 V down to −3 V. Optical output spectra are broad, with a bandwidth of 6–7 nm. However, power exchange between different spectral components of the laser output leads to a relatively large phase jitter, resulting in a total timing jitter of around 35 ps. In a 4-mm-long, 10.5-GHz laser, it is shown that the operating regime of stable mode-locking is limited by the appearance of quantum dot excited state lasing, since higher injection current densities are necessary for these shorter lasers. The output pulses are stretched in time and heavily up-chirped with a value of 16–20 ps/nm. This mode of operation can be compared to Fourier domain mode-locking. The lasers have been realized using a fabrication technology that is compatible with further photonic integration. This makes such lasers promising candidates for, e.g., a coherent multiwavelength source in a complex photonic chip.      

Integrated GaInAsP laser diodes with monitoring photodiodes throughsemiconductor/air Bragg reflector (SABAR)

A 1.3-μm GaInAsP laser diode (LD) is integrated with a monitoring photodiode (M-PD) through a semiconductor/air Bragg reflector (SABAR). Instead of conventional cleavage, the SABAR can provide not only Fabry-Perot resonance with high reflectivity, but also possibility of integration of laser with other functional devices. The design, fabrication, and some characteristics including threshold current, monitoring photocurrent, SABAR reflectivity as a function of the number of semiconductor/air pairs N are reported. The threshold current of ridge waveguide laser with SABAR (cavity length L=160 μm, ridge width W=7 μm, SABAR pairs N=3) is 20 mA. The threshold current is reduced by improving butt-coupled interface between active and passive waveguides employed in this laser and is expected 2 mA/μm. The monitoring photocurrent responds linearly with output power from the laser and 0.024 mA at laser output power of 5 mW. From the threshold characteristics, SABAR reflectivity is determined to >80%. The increase of photocurrent can be achieved by optimizing the number of SABAR pairs to N=1. We have obtained threshold current of 22 mA in the followed laser structure (L=270 μm, W=7 μm, N=1), and detector photocurrent of 1.13 mA (@5 mW). The experimental SABAR reflectivity is ~50%, which is estimated by threshold characteristics and efficiency of light output power. The laser has a mode field converter section, resulting in narrow beam divergence 11° along vertical axis. This integrated laser is very promising candidate for coming optical module in low-power consumption and low-cost access network systems