High-power broad-area tapered amplifier with a monolithicallyintegrated output focusing lens at 0.98-μm wavelength

We have demonstrated a 0.98-μm wavelength tapered broad-area amplifier with a monolithically integrated aspherical waveguide lens. CW output exceeding 1 W from the amplifier-lens chip was measured with 10 mW input from a 0.98-μm diode laser. The integrated semiconductor waveguide lens focused the amplifier output to a 8 μm×3 μm spot, which was measured at output power up to about 0.5 W, corresponding to 2.5 times the diffraction limit The beam propagation method was used to model the integrated amplifier-lens chip, and the calculated focal distances agree with the experiment to within 5%. The integrated lens may be used for output coupling to a single mode fiber with the requirement that the focal point should be positioned on the output facet. Based on BPM simulation, however, the focal point position becomes uncritical if a single mode output waveguide is integrated. Our results indicate that the waveguiding lens is a useful component for the design of high-power photonic integrated circuits     

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     

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     

Infrared microscopy studies on high-power InGaAs-GaAs-InGaP laserswith Ga2O3 facet coatings

InGaAs-GaAs separate confinement, heterostructure single quantum-well (SCH-SQW) lasers (λ=0.98 μm) with lattice-matched InGaP cladding layers, using a new Ga₂O₃ low reflectivity (LR) front-facet coating, are reported. The CW peak power density (17 MW/cm²) of 6 μm×750 μm ridge-waveguide lasers is limited by thermal rollover, and repeated cycling beyond thermal rollover produced no change in operating characteristics. The high-power temperature distribution along the active stripe has been measured by high-resolution infrared (3-5 μm) imaging microscopy. The temperature profile acquired for a very high optical power density P_D=11 MW/cm³ was found to be uniform along the inner active laser stripe, and revealed a local temperature increase at the LR front facet ΔT_f of only 9 K above the average stripe temperature ΔT_s=24 K. An excellent front-facet interface recombination velocity <10⁵ cm/s has been inferred from the measured low local temperature rise in the front facet     

High-power 1.55-μm mass-transport-grating DFB lasers forexternally modulated systems

High-output-power operation of 1.55-μm-wavelength distributed-feedback (DFB) lasers with a novel mass-transport grating (MTG) structure which is composed of InAsP buried with InP are reported. To improve high output power characteristics, we have investigated the influence of the width of the active layer on the light output power and the spectral linewidth at high injection current. It is confirmed that the increase of the active layer width is effective to realize high output power and to reduce the linewidth power product. The fabricated lasers show high single-longitudinal-mode output power of 180 mW, which is the highest value reported for 1.55-μm DFB lasers. They also exhibit narrow spectral linewidths less than 0.3 MHz and low noise characteristics of -159 dB/Hz. Moreover, we have obtained the mean time to failure of longer than 10⁵ h with a lifetime test over 200 h at 50°C     

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     

Influence of strain on lasing performances of Al-freestrained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers emitting at0.78<λ<1.1 μm

The influence of strain on lasing performances of Al-free strained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers is investigated for the first time over a large emission range of 0.78<λ<1.1 μm. GaAsP and InGaAs are used for tensile and compressive-strained quantum-well layers, respectively, while GaAs and GaInAsP lattice-matched to GaAs are applied for unstrained quantum wells. The laser structures were prepared by using gas-source molecular beam epitaxy, and broad-area and ridge waveguide Fabry-Perot laser diodes were fabricated. This study shows that applying both tensile and compressive strains in the quantum well reduces threshold current density for the Al-free strained-layer quantum-well lasers. However, it was found that the lattice relaxation set a limitation of maximum compressive strain (i.e., maximum lasing wavelength) for the compressive strained InGaAs lasers while the carrier confinement determined the acceptable maximum tensile strain (i.e., minimum lasing wavelength) and lasing performances for the tensile strained GaAsP lasers. Threshold current density as low as 164 A/cm² has been obtained for 1.4% compressive-strained InGaAs-GaInAsP-GaInP lasers having a 12-nm thick quantum well. However, excellent characteristics, such as low threshold current, high efficiency low internal loss, and high output power, have been achieved for the Al-free strained-layer quantum-well lasers     

Diode-pumped tunable Yb:YAG miniature lasers at room temperature:modeling and experiment

We present a simple design rule for diode-laser pumped quasi-three-level lasers by using the M² factor. The validity of this model was demonstrated by diode-pumped Yb:YAG laser experiments. The maximum output power of 1.33 W and optical slope efficiency of 63% were obtained in a 400-μm Yb:YAG chip miniature laser. Using a 200-μm Yb:YAG chip, a 70% optical slope efficiency was reached. In a coupled-cavity configuration, with a quartz birefringent tuning filter, 8.2 THz (29 nm) of tuning was obtained at room temperature. By changing to a calcite birefringent filter, single-axial-mode oscillation with an output power of 500 mW was observed     

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     

Advanced Ti:Er:LiNbO3 waveguide lasers

This paper reviews the latest developments of diode-pumped Ti,Er:LiNbO₃ waveguide lasers emitting at wavelengths around 1.5 μm. In particular, harmonically mode-locked lasers, Q-switched lasers, distributed Bragg reflector (DBR)-lasers, and self-frequency doubling lasers are discussed in detail. Supermode stabilized mode-locked lasers have been realized using a coupled cavity concept; a side mode suppression ratio of 55 dB has been achieved at 10-GHz pulse repetition rate with almost transform limited pulses. Q-switched lasers with a high extinction ratio (>25 dB) intracavity electrooptic switch emitted pulses with a peak power level up to 2.5 kW and a pulsewidth down to 2.1 ns at 1-kHz repetition frequency. Numerical simulations for both lasers are in a good, almost quantitative agreement with experimental results. A DBR-laser of narrow linewidth (≈3 GHz) with a permanent (fixed) photorefractive grating and 5 mW output power has been realized. Self frequency doubling lasers have been fabricated with a periodic microdomain structure inside an Er-doped laser cavity; simultaneous emission at the fundamental wavelength, 1531 nm, and at the second harmonic wavelength, 765 nm, has been obtained     

1.3-μm InAsP n-type modulation-doped MQW lasers grown bygas-source molecular beam epitaxy

The effect of n-type modulation doping as well as growth temperature on the threshold current density of 1.3-μm InAsP strained multiple-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy (GSMBE) was investigated for the first time. We have obtained threshold current density as low as 250 A/cm² for 1200-μm long devices. The threshold current density per well for infinite cavity length J_th/N_w∞ of 57 A/cm² was obtained for the optimum n-doping density (N_D=1×10¹⁸ cm^-3) and the optimum growth temperature (515°C for InP and 455°C for the SCH-MQW region), which is about 30% reduction as compared with that of undoped MQW lasers. A very low continuous-wave threshold current of 0.9 mA have been obtained at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest results grown by all kinds of MBE in the long-wavelength region. The differential gain was estimated by the measurement of relative intensity noise. No significant reduction of differential gain was observed for n-type MD-MQW lasers as compared with undoped MQW lasers. The carrier lifetime was also reduced by about 33% by using n-type MD-MQW lasers. Both reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay is very attractive for laser array application in high-density parallel optical interconnection systems     

Interferometric near-field imaging technique for phase andrefractive index profiling in large-area planar-waveguide optoelectronicdevices

A versatile, interferometric optical technique is described for nondestructively imaging the near-field output phase uniformity and refractive index profile in broad-area optoelectronic waveguide devices or heterostructure materials. In active traveling-wave optical power amplifier devices, measurements are presented for thermal lensing, solder bond inhomogeneities, heatsink impedance, and carrier-lensing effects due to nonuniform gain saturation by the amplifier input beam, transverse amplified spontaneous emission, or intensity filaments. The thermal performance of diamond and copper heatsinks for high-power optical amplifiers is compared. In passive devices, the technique is used to observe heteroepitaxial material compositional uniformity, defects, photoelastic stress, and intentional structural waveguide index modifications. The technique has a phase and spatial resolution as low as λ/100 and 1 μm. The corresponding refractive index and temperature resolutions (dependent on device length) are as low as Δn=10^-5 and ΔT=0.025°C for 1000-μm-long devices     

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     

Highly efficient diode-pumped 3-μm Er3+:BaY2F8 laser

A systematic investigation on a series of monoclinic Er³⁺ :BaY₂F₈ crystals with different dopant concentrations (C_Er=5%-30%) and crystal orientations was conducted to optimize the laser performance in this new 3-μm laser medium by laser diode pumping. The highest slope efficiency of 32% near the quantum defect (35%) was obtained with a 10% doped Er³⁺:BaY₂F₈ crystal with the orientation (010) and a length of 3.5 mm. A maximum output power of 160 mW was achieved at an absorbed pump power of 550 mW at a wavelength of 970 nm     

Transmission properties of hollow glass waveguides for the deliveryof CO2 surgical laser power

Hollow glass waveguides are an attractive fiber delivery system for a broad range of infrared wavelengths, including the 3-μm Er:YAG and 10.6 μm CO₂ lasers. The losses for these waveguides are as low as 0.1 dB/m at the 10.6-μm wavelength for waveguides with a 700-μm bore. The guides are suitable for delivering laser powers well in excess of 100 W. Continuous power delivery for over 250 h is possible for powers less than 35 W. When stored under normal laboratory conditions, the loss is seen to change only slightly over a period up to two years     

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     

Monolithic integration via a universal damage enhanced quantum-wellintermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO₂, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3°) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 μm are demonstrated with low loss (α=4.1 cm^-1) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration     

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A·cm ^-2 at 77 K and of 62 A·cm^-2 at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of ~80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to ~50 meV, ultrahigh material gain of ~10⁵ cm^-1, differential gain of ~10^-13 cm² and strong nonlinear gain effects with a gain compression coefficient of ~10^-16 cm³. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is ~0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 μm and 1.37 μm at 300 K     

High-power and high-efficiency operation of a CW-diode-side-pumpedNd:YAG rod laser

We have demonstrated high-power and high efficiency performance of a continuous-wave (CW) Nd:YAG laser with a simple and scalable side-pumping configuration. The maximum output power of 147 W was obtained in low brightness operation of M²=45. The corresponding electric efficiency is 14.8%. To our knowledge, this is the highest value reported for diode side-pumped Nd:YAG lasers. By using a rate and photon transport calculation, we have estimated the pumping efficiency of 72%. High brightness operation was also carried out by applying bifocusing compensation of the Nd:YAG rod. The brightness of 272 MW/cm² sr with beam quality of M²=5.9 and output power of 107 W was obtained at the electric efficiency of 11.6%. The brightness and the electric efficiency are comparable with those of industrial high-power CO₂ lasers that have been the first option for industrial applications     

Room-temperature operation of GaInAsN-GaAs laser diodes in the1.5-μm range

GaInAsN-GaAs double quantum-well (DQW) laser structures emitting in the 1.5-μm range were grown by solid source molecular beam epitaxy using a radio frequency plasma source for nitrogen activation. Lasing operation in the 1.5-μm wavelength region has been realized for fabricated ridge waveguide laser diodes (LDs) under pulsed condition up to record high temperatures of 80°C resulting in an emission wavelength of 1540 nm. This is the highest emission wavelength for laser diode operation based on GaAs. In addition, to investigate the optical properties of the active region, photoluminescence studies of underlying GaInAsN-GaAs QW structures emitting at wavelengths up to 1.55 μm are presented