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     

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     

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     

Continuous-wave low-threshold performance of 1.3-μm InGaAs-GaAsquantum-dot lasers

The understanding of material quality and luminescence characteristics of InGaAs-GaAs quantum dots (QD's) is advancing rapidly. Intense work in this area has been stimulated by the recent demonstration of lasing from a QD active region at the technologically important 1.3-μm wavelength from a GaAs-based heterostructure laser. Already, several groups have achieved low-threshold currents and current densities at room temperature from In(Ga)As QD active regions that emit at or close to 1.3 μm. In this paper, we discuss crystal growth, QD emission efficiency, and low-threshold lasing characteristics for 1.3-μm InGaAs-GaAs QD active regions grown using submonolayer depositions of In, Ga, and As. Oxide-confinement is effective in obtaining a low-threshold current of 1.2 mA and threshold-current density of 19 A/cm² under continuous-wave (CW) room temperature (RT) operation. At 4 K, a remarkably low threshold-current density of 6 A/cm² is obtained     

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)     

The temperature dependence of 1.3- and 1.5-μm compressivelystrained InGaAs(P) MQW semiconductor lasers

We have studied experimentally and theoretically the spontaneous emission from 1.3- and 1.5-μm compressively strained InGaAsP multiple-quantum-well lasers in the temperature range 90-400 K to determine the variation of carrier density n with current I up to threshold. We find that the current contributing to spontaneous emission at threshold I_Rad is always well behaved and has a characteristic temperature T₀ (I_Rad)≈T, as predicted by simple theory. This implies that the carrier density at threshold is also proportional to temperature. Below a breakpoint temperature T_B, we find I α n^Z, where Z=2. And the total current at threshold I_th also has a characteristic temperature T₀ (I_th)≈T, showing that the current is dominated by radiative transitions right up to threshold. Above T_B, Z increases steadily to Z≈3 and T₀ (I_th) decreases to a value less than T/3. This behavior is explained in terms of the onset of Auger recombination above T_B; a conclusion supported by measurements of the pressure dependence of I_th. From our results, we estimate that, at 300 K, Auger recombination accounts for 50% of I_th in the 1.3-μm laser and 80% of I_th in the 1.5-μm laser. Measurements of the spontaneous emission and differential efficiency indicate that a combination of increased optical losses and carrier overflow into the barrier and separate confinement heterostructure regions may further degrade T₀ (I_th) above room temperature     

Novel integrated photodetector on Si LSI circuits. Opticallycontrolled MOSFET

We have demonstrated a novel integrated photo-detector on Si large-scale integration circuits. This device integrated the light absorption region on the gate of metal-oxide-semiconductor field-effect transistor (MOSFET), and the long wavelength light controls the current of MOSFET. The GaInAs-InP multiple-quantum-well absorption region and SiO₂ of the MOSFET were directly bonded. From the experimental results, we confirmed that the light-controlled current was increased by shortening the gate length of the MOSFET, and that an 1850 A/W responsivity was obtained in a 3.5-μm gate length device using an irradiation of 1.5-μm wavelength light     

Realization and characterization of 8×8 resonant cavity LEDarrays mounted onto CMOS drivers for POF-based interchipinterconnections

An 8×8 array of resonant-cavity light emitting diodes (RCLED's) emitting at 980 nm and flip-chip mounted onto complimentary metal-oxide-semiconductor (CMOS) integrated drivers, is presented. The RCLED's are optimized for maximal extraction efficiency into the numerical aperture of polymer optical fibers (NA=0.5) and minimal optical crosstalk. Design of the optimal cavity structure is presented, and 8×8 arrays are realized and mounted directly onto standard CMOS chips using a solder reflow technique. The CMOS integrated drivers are designed for high-speed operation and low-power consumption, and are realized in 0.8 and 0.6-μm CMOS technology. The electrooptical modules have been realized and characterized, and over 50-μW optical power coupled to POF at 3-mA drive current is reported. Open eye diagrams at operation speed up to 250 Mb/s are presented. These characteristics are compatible with CMOS integrated low-power receivers     

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     

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     

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     

0.78- and 0.98-μm ridge-waveguide lasers buried with AlGaAsconfinement layer selectively grown by chloride-assisted MOCVD

The 0.78- and 0.98-μm buried-ridge AlGaAs laser diodes (LD's) with a high Al-content AlGaAs confinement layer selectively grown by using a Cl-assisted MOCVD are demonstrated. By employing the AlGaAs confinement layer, the threshold current and the slope efficiency of the 0.78-μm LD are improved by ~40%, compared to those of the conventional loss-guided LD with the GaAs confinement layer. In addition, the stable fundamental mode up to 150 mW and the small astigmatic distance less than 1 μm are obtained. The 0.78-μm LD also shows the excellent high-power and high-temperature characteristic such as 100 mW CW operation at 100°C and the reliable 2,000-hour operation under the condition of 60°C and 55 mW. In the 0.98-μm LD, the narrow beam with the low aspect ratio of 1.86 and the stable fundamental transverse mode over 200 mW are exhibited. As a result, the 0.98-μm LD realizes the high fiber-coupled-power of 148 mW. Moreover, the high-power and high-temperature operation of 150 mW at 90°C is obtained. In the preliminary aging test, the LD's have been stably operating for over 900 hours under the condition of 50°C and 100 mW     

High-power continuous-wave 3- and 2-μm cascadeHo3+:ZBLAN fiber laser and its medical applications

A new medical fiber laser oscillating at two useful wavelengths (3 and 2 μm) is reported. We have demonstrated highly efficient and high-power continuous-wave cascade oscillation at room temperature with a holmium ion-doped fluoride glass fiber laser pumped with a 1.15-μm fiber Raman laser. The simultaneous oscillation wavelengths were 3 and 2 μm, and their combined output power was 3.0 W with a slope efficiency of 65%. To our best knowledge, this is the first achievement of watt-level-output power in the mid-infrared region with ZrF₄-BaF₂-LaF₃-AlF₃-NaF (ZBLAN) glass fiber. In experiments to evaluate potential for medical applications, we tested the two wavelength beam as a laser surgical knife on soft rabbit tissues and demonstrated that it had strong cutting capability, and that the coagulation layer thickness could be controlled by varying the power ratio of the two-wavelength laser     

1.55-μm vertical-cavity laser arrays for wavelength-divisionmultiplexing

We report on the fabrication and operation of the first electrically pumped 1.55-μm vertical-cavity laser array for wavelength-division-multiplexing applications. The array consisted of four channels operating between 1509 and 1524 nm. Wafer bonding was used to integrate GaAs-AlGaAs distributed Bragg reflectors with an InP-InGaAsP active region     

Analysis of cavitation dynamics during pulsed laser tissue ablationby optical on-line monitoring

Flashlamp pumped mid-IR laser systems emitting in the 2-3-μm wavelength range are widely used for various medical applications, especially for tissue ablation. Explosive evaporation is inevitably associated with this process due to the short pulse durations of these laser systems and the high absorption of tissue and water in this spectral regime. Tissue displacement and dissection occur in liquid environment as a consequence of the induced cavitation. Depending on the application these processes might enhance the tissue ablation but can also cause adverse tissue effects. The ablation dynamics were investigated by evaluating the change in reflected probe-light intensity re-emitted from the application fiber tip. The ablated cavity and the signal was correlated to fast-flash photographs of the event. Based on this reflection signal a water/tissue discrimination system is introduced which can widely support medical laser applications. In laser sclerostomy ab externo, for example, this approach can be used as a feedback system to automatically control the ablation process. With such a system, adverse effects to adjacent tissue in the anterior chamber of the eye can be minimized     

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 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     

Orientation dependence of optical properties in long wavelengthstrained quantum-well lasers

The dependence of optical properties on crystal orientation is analyzed for long wavelength strained quantum-well (QW) GaAsP-InGaAsP lasers. The calculation is based on the multiband effective mass theory which enables us to consider the anisotropy and the nonparabolicity of the valence-band dispersions. It is found that the optical gain increases as the crystal orientation is inclined from (001) toward (110). This is due to the reduced valence-band density of states. The differential gain is about 1.6 times larger for the (110)-oriented 1.55-μm strained QW's than for equivalent (001)-oriented QW's. It is also shown that the threshold current density in 1.3-μm strained QW lasers decreases to two-thirds of that in the (001)-oriented laser as the orientation is inclined away from (001) by 40°-90     

Growth of highly strained GaInAs-GaAs quantum wells on patterned substrate and its application for multiple-wavelength vertical-cavity surface-emitting laser array

We carried out the growth of highly strained GaInAs-GaAs quantum wells (QWs) on a patterned substrate for extending emission wavelength on a GaAs substrate. We examined the shift of photoluminescence wavelength of the QWs and showed a large wavelength shift due to the spatial modulation in well thickness and indium composition. We demonstrated a single-mode multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) array on a patterned GaAs substrate covering a new wavelength window of 1.1-1.2 /spl mu/m. By optimizing pattern shape, we achieved multiple-wavelength operation with widely and precisely controlled lasing wavelengths. The maximum lasing span is as large as 77 nm. We carried out a data transmission experiment through 5-km of single-mode fiber with a 2.5 Gb/s/channel. The total throughput reaches 10 Gb/s. The VCSEL-based wavelength-division-multiplexing (WDM) source would be a good candidate for WDM-LAN beyond 10 Gb/s.     

Spectroscopic imaging and the characterization of the autofluorescence properties of human bronchus tissues using UV laser diodes

Ultraviolet laser diodes (UV-LD) were used for the excitation source of autofluorescence (AF) measurements and spectroscopic imaging of the AF originating from the human bronchus was obtained. The AF spectra from normal bronchus tissues were measured and a clear AF spectrum was obtained by using a short wavelength (400 nm) laser diode; the overlap of the AF signal and excitation source could be substantially eliminated. In order to study the origin of AF intensity deterioration from bronchus tissue due to the formation of tumor tissues, the fluorescence spectrum was measured for various AF substances under various conditions. The blue AF signal of elastin and NADH solutions, which could not be easily studied by conventional excitation light sources, as well as the green AF became weak by adding lactic acid. The AF spectrum was measured for 512/spl times/512 pixels and the intensity mapping as a function of emission wavelength was obtained. Two-dimensional information of the AF signal intensity distribution for a certain wavelength component was measured. The feature originating from a region as small as about 100 /spl mu/m could be recognized. Numerical calculations of the data were performed and precise features of the AF were revealed.