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     

Lasing mechanism of InGaN-GaN-AlGaN MQW laser diode grown on SiC bylow-pressure metal-organic vapor phase epitaxy

We studied the lasing mechanism of an InGaN-GaN-AlGaN multiquantum-well (MQW) laser diode by making various optical characterizations on the diode. Excitation power dependence of photoluminescence (PL) intensity was obtained to investigate the carrier recombination process of the laser. Surface emission and edge emission were compared by optical pumping to clarify where the lasing lines were located in relation to the absorption continuum. From the results, we demonstrate that lasing phenomena in our laser are dominated by free carriers. PL mapping was also taken on the same laser chip to examine the in-cavity bandgap inhomogeneity. We found a very large bandgap scattering of 100 meV. We also found that the wavelength distribution has a periodic modulation. We clarified that the various stimulated emission lines observed in our lasers are caused by the in-cavity spatial bandgap inhomogeneity of the InGaN MQW     

Novel laser structures based on MQW interdiffusion using rapidthermal annealing technique

In this paper, experimental results for GaAlAs-GaAs multiple-quantum-well (MQW) interdiffusion using rapid thermal annealing (RTA) technique under different processing conditions are presented and discussed. Two kinds of novel laser structures based on such technique are also proposed and fabricated. First, a laser diode with window region for high-power operation is designed and fabricated. The maximum output power of such a device shows an increase by 18% over laser diodes without interdiffused window region. Then a transverse mode controlled laser structure taking advantages of the refractive index change induced by MQW interdiffusion is realized using RTA technique. Single-mode operation up to four times the threshold current has been demonstrated for this RTA treated laser diode     

Efficient 1.94-μm Tm:YALO laser

Laser emission from Tm:YALO is observed over the range 1.93-2.00 μm. A model including reabsorption loss and polarization effects, predicting the output wavelength as a function of laser parameters, is used to design a Tm:YALO laser with output restricted to 1.94 μm, without employing a tuning element. This laser is potentially useful for medical applications, offing to the strong absorption coefficient at 1.94 μm in liquid water (twice that of the 2.02-μm Tm:YAG laser and four times that of the 2.09-μm Ho:YAG laser)     

Area selectivity of InGaAsP-InP multiquantum-well intermixing byimpurity-free vacancy diffusion

Area selectivity of bandgap tuning in the InGaAsP-InP multiquantum-well structure has been investigated using low temperature photoluminescence (PL). The bandgap blue-shift in the intermixed region was as much as 170 meV for a rapid thermal anneal of 30 s at 850°C, and was controllable using annealing temperature and time. From samples with SiO₂ stripe patterns, clearly separated PL peaks were observed centered at 0.95 and 1.08 eV, each representing signals originating from the dielectric capped and exposed areas, respectively. In samples with stripes intervals less than 6 μm, PL signals did not separate, but formed one broad spectrum due to lateral diffusion. The lateral diffusion was found less than 3.0 μm     

CW and mode-locked integrated extended cavity lasers fabricatedusing impurity free vacancy disordering

A phosphorus-doped silica (P:SiO₂) cap containing 5 wt% P has been demonstrated to inhibit the bandgap shifts of p-i-n and n-i-p GaAs-AlGaAs quantum-well laser structures during rapid thermal processing. Bandgap shift differences as large as 100 meV have been observed between samples capped with SiO₂ and with P:SiO₂. The technique has been used to fabricate GaAs-AlGaAs ridge lasers with integrated transparent waveguides. With a selective differential blue-shift of 30 nm in the absorption edge, devices with 400 μm/2.73-mm-long active/passive sections exhibited an average threshold current of 9 mA in continuous-wave (CW) operation, only 2.2 mA higher than that of discrete lasers of the same active length and from the same chip. Extended cavity mode-locked lasers were also investigated and compared to all active devices. For the extended cavity device, the threshold current is a factor of 3-5 lower, the pulsewidth is reduced from 10.3 to 3.5 ps and there is a decrease in the free-running jitter level from 15 ps (measurement bandwidth 10 kHz-10 MHz) to 6 ps. In addition, the extended cavity lasers do not exhibit any self-pulsing modulation of the mode-locked pulse train, unlike the all-active lasers, and the optical spectra indicate that the pulses are more linearly chirped     

Strain-overcompensated GaInP-AlGaInP quantum-well laser structuresfor improved reliability at high-output powers

Strain-overcompensated multiple-quantum-well (MQW) laser structures have been analyzed theoretically as well as experimentally for the first time. Strain overcompensation reduces the bandgap shrinkage that normally takes place at the facets of compressively strained layers because of strain relaxation. This results in a lower absorption of the laser spot leading to a remarkable improvement of the reliability of high-power laser diodes     

Enhanced polymer ablation rates using high-repetition-rateultraviolet lasers

Etch rates (μm/pulse) for glycol-modified polyethylene terephthalate (PETG) under pulsed UV (255 nm) laser processing are measured as a function of pulse repetition frequency in the range 0.7-15 kHz. Materials removal rates (μm/s) scale approximately linearly with pulse repetition frequency at a fluence of 0.59 J/cm², and there appears to be no attenuation of the ablating laser beam by the ejected material plume for pulse rates up to 15 kHz. The instantaneous etch rate for pulses in a sequence increases markedly (~40%) for long pulse sequences (>100 pulses) at high PRF (15 kHz), an effect which can be used to increase machining rates while operating at a moderate laser fluence     

1.3-μm spot-size-converter integrated laser diodes fabricated bynarrow-stripe selective MOVPE

High-performance 1.3-μm spot-size-converter integrated laser diodes (SSC-LDs) have been developed by using narrow-stripe (<2.0 μm) selective MOVPE. In order to decrease leak current at high temperature, a p-n-p-n current blocking structure was added using a self-alignment process. These LD's no longer require a semiconductor etching process. Superior lasing characteristics, such as a low driving current of 56 mA for output power of 10 mW, and high-slope efficiency at 85°C, were achieved by using a high-quality multiple-quantum well (MQW) active layer of narrow-stripe selective MOVPE and a p-n-p-n current blocking structure. A narrow radiation angle of 12° was obtained by optimizing the tapered-waveguide profile. A high-coupling efficiency of -2.8 dB was achieved between a LD chip and a single-mode fiber (SMF). This SSC-LD is very appropriate as a light source for access network systems, which require a low-cost LD module. It has excellent coupling efficiency, using a SMF, and a simple fabrication process, using selective MOVPE     

Narrow linewidth, tunable Tm3+-doped fluoride fiberlaser for optical-based hydrocarbon gas sensing

Operation of an efficient continuous-wave (CW) thulium-doped fiber laser emitting at wavelength, λ=2.31 μm is reported. The fiber laser parameters are optimized with a view to ultimately producing a compact and efficient laser source for optical absorption based gas sensing. A number of fiber laser configurations are investigated to assess their suitability for narrow linewidth, tunable fiber laser operation emitting around λ=2.3 μm, which is a wavelength region of significant importance for hydrocarbon gas monitoring. Tuning ranges of 140 nm and linewidths of less than 210 MHz have been demonstrated with lasers with bulk external tuning grating. Preliminary hydrocarbon gas sensing investigation confirm the potential of this source for detection of ppb gas concentrations     

Wavelength demultiplexer using GaInAs-InP MQW-based variable refractive-index arrayed waveguides fabricated by selective MOVPE

A GaInAs-InP multiple quantum well (MQW)-based wavelength demultiplexer composed of an arrayed waveguide in which the refractive index varies across the array was fabricated. Since optical path length differences between waveguides in the array are achieved through refractive-index differences that are controlled by SiO/sub 2/ mask design in selective metal-organic vapor phase epitaxy (MOVPE), straight waveguide gratings having reduced optical propagation losses can be achieved. Furthermore, by employing MQW waveguides, variations in the refractive index may be induced through an applied electric field, allowing the device to manipulate wavelengths dynamically. A straight arrayed waveguide device having a 1.4% difference in refractive index was fabricated using an asymmetric side mask via a single selective MOVPE growth. The achievement of a diffraction angle difference of 4.40/spl deg/ between wavelengths of 1520 and 1580 nm was confirmed experimentally. In addition, a preliminary wavelength demultiplexer with a wavelength separation of approximately 25 nm and a free spectral range (FSR) of approximately 100 nm was also fabricated.     

High-performance small vertical-cavity lasers: a comparison ofmeasured improvements in optical and current confinement in devicesusing tapered apertures

We analyze the scaling characteristics of the optical and current confinement for three different vertical-cavity surface-emitting laser (VCSEL) structures with tapered apertures. The improvements in scaling have allowed devices with apertures <3 μm to have wall-plug efficiencies over 20% at output powers as low as 150 μW. The combination of low threshold (<200 μA), single modedness, and good wall-plug efficiency even at low output powers makes these devices excellent candidates for short distance (<1 m) interconnects within computers     

Photothermal-induced temperature changes in a model inner ear: acomparison of visible, infrared, and ultraviolet lasers

The temperature change in a model cylindrical vestibule (90 mm³) was measured following irradiation by argon (488-514 nm), CO₂ (10.6 μm), KTP (Nd:YAG) (532 nm), Er:YAG (2.9 μm), and XeCl (308 nm) lasers. Otic capsule bone was used to simulate the otosclerotic stapes footplate, and the thickness of each specimen was machined to variable thicknesses (0.20-0.90 mm). Thermocouples were used to measure the temperature below the air-bone surface at depths of 1.0, 3.0, and 5.0 mm. The time-dependent temperature change, thermocouple position, and bone thickness were measured following single pulse application from argon, CO₂, and KTP (Nd-YAG) lasers. The effect of infrared and ultraviolet lasers on vestibule fluid temperature changes were studied with several fluence and pulse sequences. The temperature change in the vestibule following pulsed laser irradiation decreased with increasing bone thickness and thermocouple depth. Laser irradiation from CO₂, argon, KTP-532, XeCl, and Er:YAG lasers produced minimal (less than 5°) vestibule temperature changes. Measured temperatures were in good agreement with an analytic model, based on a solution to the bio-heat equation in semi-infinite media. The results are discussed with relevance to ear surgery     

Picosecond SESAM-based ytterbium mode-locked fiber lasers

Using semiconductor saturable absorber mirrors and a grating-pair dispersion compensator, we obtain reliable self-starting mode locking of a ytterbium (Yb) fiber laser tunable over 125 nm. The 980-1105-nm tuning range is achieved by optimization of nonlinear reflection and bandgap characteristics of the multiple-quantum-well saturable absorber and by proper engineering of the laser cavity. A short-length Yb-doped double-clad amplifier seeded with mode-locked Yb-fiber laser produces picosecond pulses with energy of 30 nJ (700 mW of average power). A compact version of the fiber laser was built using a Gires-Tournois compensator and short length (1-cm long) of highly doped Yb fiber. Using a novel semiconductor saturable abserver mirror based on GaInNAs structure, self-started 1.5-ps pulse mode-locked operation was obtained at 1023 nm with a repetition rate of 95 MHz. A mode-locked Yb-doped fiber laser was also developed without using any dispersion compensation technique. Overall group-velocity dispersion was minimized by using highly doped Yb fiber in a compact amplifying loop cavity. Self-started mode-locked operation was obtained in 980-1030-nm wavelength range with a fundamental repetition rate of 140 MHz. Without using dispersion compensation, the lasers produced pulses in a range from 15 to 26 ps.     

Highly strained GaInAs-GaAs quantum-well vertical-cavitysurface-emitting laser on GaAs (311)B substrate for stable polarizationoperation

We have realized high-quality GaInAs-GaAs quantum wells (QWs) with high strain of over 2% on GaAs (311)B substrate for a polarization controlled vertical-cavity surface-emitting laser (VCSEL). By increasing the In composition in GaInAs, the optical anisotropy in photoluminescence (PL) intensity was increased. The anisotropy of 50% was obtained at 1.15 μm emission wavelength. We have demonstrated edge-emitting lasers and VCSELs emitting at over 1.1 μm on GaAs (311)B substrate for the first time. The 1.15-μm edge-emitting laser showed a characteristic temperature of 210 K and the threshold current density of 410 A/cm². The threshold current and lasing wavelength of VCSELs are 0.9 mA and 1.12 μm, respectively. The orthogonal polarization suppression ratio was 25 dB and CW operation up to 170°C without a heat sink was achieved     

WDM operation of a hybrid emitter integrating a wide-bandwidthon-chip mirror

A wide-bandwidth on-chip mirror based on a deeply etched Bragg grating localized over 10 μm is presented; it exhibits reflectivity over more than 25 nm. Integration within a laser-Mach-Zehnder modulator is shown and the wavelength-division multiplexing capability of the device demonstrated in a hybrid Bragg reflector configuration, by showing operation of the same InP chip with fiber gratings from 1534 to 1558 nm     

1.3-μm InGaAsP-InP n-type modulation-doped strainedmultiquantum-well lasers

The use of n-type modulation doping to reduce the threshold current, the carrier lifetime, and the internal loss in 1.3-μm InGaAsP-InP strained multiquantum-well (MQW) lasers is experimentally demonstrated. The threshold current density, the carrier lifetime, and the internal loss were reduced by about 33%, 36%, and 28%, respectively, as compared with an undoped MQW laser. Moreover, the turn-on delay time in the n-type modulation-doped MQW lasers with a low-leakage buried heterostructure was reduced by about 35%. These results confirm the suitability of this type of laser for use in the basic structure of a monolithic laser array used as a light source for high-density parallel optical interconnection     

Narrow-beam divergence 1.3-μm multiple-quantum-well laser diodeswith monolithically integrated tapered thickness waveguide

This paper describes the optimum design, fabrication, and performance of a 1.3-μm multiple-quantum-well (MQW) laser diode monolithically integrated with a tapered thickness spot-size transformer. The dependence of the lasing characteristics on the thickness distribution of the core layer and on the current injection profile of the device were analyzed. This integrated laser with its optimized structure performed at a low threshold current of 22.2 mA, even at 85°C. The integrated spot-size transformer effectively reduced the lateral and vertical far-field FWHM's to 8° and 9°, respectively. A very long lifetime of over 1×10⁵ h was estimated at 85°C and 8 mW under CW operation     

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     

Time-development of transient-carrier temperature, density, andgain spectrum in ultrashort optical pulse excited InGaAsmultiquantum-well laser structure

We present a novel transient gain-spectra measurement method based on the traditional variable pump-stripe technique. Using the pump-stripe technique with ultra-short optical pulse excitation, time-resolved amplified spontaneous emission spectroscopy of an InGaAs-InGaAsP multiquantum-well (MQW) laser structure was measured, and time-development of the transient optical net gain spectra was obtained accordingly. By fitting the measured gain spectra with a model for band-to-band transitions including momentum conservation and an energy- and density-dependent lifetime broadening, dynamics of band filling, carrier density, carrier temperature and bandgap renormalization have been obtained. This opens the possibility to study simultaneously the influence of transient-carrier density and, in particular, transient-carrier temperature on the transient optical gain. Strong gain compression in the whole gain-spectra region due to transient high carrier temperature after ultra-short pulse injection is clearly demonstrated for the first time