GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 μm and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T₀=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 μm     

Lasing characteristics of InGaAs-GaAs polarization controlledvertical-cavity surface-emitting laser grown on GaAs (311) B substrate

We have demonstrated an oxide confinement polarization controlled vertical-cavity surface-emitting laser (VCSEL) grown on a GaAs (311)B substrate. The polarization state was well controlled along the [2¯33] crystal direction due to an anisotropic gain in the (311)B plane. We fabricated a small oxide aperture VCSEL with a threshold of 260 μA and realized single-transverse mode and single-polarization operation for the first time. The sidemode suppression ratio (SMSR) was 35 dB and the orthogonal polarization suppression ratio (OPSR) was 25 dB. In addition, we have measured polarization and transverse mode characteristics of multi- and single-transverse mode devices under high-speed modulation. In the multimode device of 12 μm×12 pm oxide aperture, we have achieved stable polarization operation of over 25-dB OPSR up to 10 Gb/s and have observed no power penalty due to polarization instability under 2.5-Gb/s pseudorandom modulation. The single-mode device showed stable single-transverse mode and polarization under the modulation conduction up to 5 GHz of sinusoidal modulation. SMSR and OPSR were over 30 and 10 dB, respectively     

Polarization-controlled 850-nm-wavelength vertical-cavitysurface-emitting lasers grown on (311)B substrates by metal-organicchemical vapor deposition

We demonstrate the polarization stability of 850-nm-wavelength vertical cavity surface-emitting lasers (VCSELs) grown on (311)B substrates under continuous-wave (CW) and dynamic operation. To clearly verify the polarization stability of VCSELs on (311)B substrates due to the anisotropic optical gain, the characteristics of both VCSELs on (311)B and (100) substrates were compared experimentally. Under CW operation, very small difference in both orthogonal polarization suppression ratio and the distribution of polarization direction was observed between VCSELs on (311)B and (100) polyimide-buried structures. On the other hand, significantly larger orthogonal polarization suppression ratio was obtained for VCSELs on (311)B substrates than those on (100) substrates under zero-bias modulation. Time-dependent orthogonal polarization suppression ratio measurements also showed that the orthogonal polarization suppression ratios of the VCSEL on (311)B substrates were more stable than those on (100) substrates. The data transmission characteristics also indicate large differences in the dependence of the bit error rate on bias current and the power penalty between polarization resolved and unresolved systems between VCSELs on (311)B and (100) substrates. The beneficial effect of the polarization stability of VCSELs on (311)B substrates due to their anisotropic optical gain is clearly demonstrated     

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     

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     

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)     

Long-wavelength quantum-dot lasers on GaAs substrates: from media to device concepts

Recent progress in semiconductor quantum-dot (QD) lasers approaches qualitatively new levels, when dramatic progress in the development of the active medium already motivates search for new concepts in device and system designs. QDs, which represent coherent inclusions of narrower bandgap semiconductor in a wider gap semiconductor matrix, offer a possibility to extend the wavelength range of heterostructure lasers on GaAs substrates to 1.3 /spl mu/m and beyond and create devices with dramatically improved performance, as compared to commercial lasers on InP substrates. Low-threshold current density (100 A/cm/sup 2/), very high characteristic temperature (170 K up to 65/spl deg/C), and high differential efficiency (85%) are realized in the same device. The possibility to stack QDs (e.g., tenfold) without an increase in the threshold current density and any degradation of the other device parameters allow realization of high modal gain devices suitable for applications in 1.3-/spl mu/m short-cavity transmitters and vertical-cavity surface-emitting lasers (VCSELs). The 1.3-/spl mu/m QD GaAs VCSELs operating at 1.2-mW continuous-wave output power at 25/spl deg/C are realized, and long operation lifetime is manifested. Evolution of GaAs-based 1.3-/spl mu/m lasers offers a unique opportunity for telecom devices and systems. Single-epitaxy VCSEL vertical integration with intracavity electrooptic modulators for lasing wavelength adjustment and/or ultrahigh-frequency wavelength modulation is possible. Arrays of wavelength-tunable VCSELs and wavelength-tunable resonant-cavity photodetectors may result in a new generation of "intelligent" cost-efficient systems for ultrafast data links in telecom.     

Device performance and wavelength tuning behavior of ultra-short quantum-cascade microlasers with deeply etched Bragg-mirrors

The fabrication and characteristics of edge-emitting quantum-cascade (QC) lasers and microlasers with monolithically integrated deeply etched semiconductor-air Bragg-mirrors based on GaAs is reported. We observe a reduction of the threshold current density by 25% and an increase of the operation temperature by 23 K to a maximum of 315 K for 800 /spl mu/m long devices by employing Bragg-mirrors. Devices with ultra-short cavities of about 100 /spl mu/m (/spl sim/40 times the wavelength) operate up to 260 K. At 80 K, these devices show threshold currents as low as 0.63 A and output levels up to 56 mW. In these devices, longitudinal single mode operation with output levels exceeding 7.7, 5.6, and 2.8 mW was measured at 180, 200, and 240 K, respectively. This can be attributed to the limited gain bandwidth of QC lasers and the large mode spacing in these devices. By temperature control the emission wavelength can be tuned without mode jumps over 80 nm. The feasibility to pre-select the emission wavelength by a direct control of the Fabry-Perot mode was demonstrated by microlasers with 1 /spl mu/m cavity length difference.     

Densely arrayed eight-wavelength semiconductor lasers fabricated bymicroarray selective epitaxy

This paper describes a novel epitaxial growth technique, called microarray selective epitaxy (MASE), for fabricating extremely small integrated photonic devices. The MASE technique makes it possible to form densely arrayed (pitch <10 μm) multiple-quantum-well (MQW) waveguides without semiconductor etching as well as to control the bandgap energy of each waveguide. The technique is demonstrated for fabricating an eight-channel 10-μm-spacing microarray MQW structure, and the bandgap wavelength of each channel is successfully controlled by changing the SiO₂ mask pattern over a range of 90 nm. The technique is also applied to the fabrication of densely arrayed, eight-wavelength, Fabry-Perot laser diodes. The laser section is only 70 pm wide and 400 μm long. Eight different lasing wavelengths (each over 80 nm), a uniform threshold current of less than 9 mA, and an output power of over 10 mW are obtained     

Investigation of 1.3-/spl mu/m GaInNAs vertical-cavity surface-emitting lasers (VCSELs) using temperature, high-pressure, and modeling techniques

We have investigated the temperature and pressure dependence of the threshold current (I/sub th/) of 1.3 /spl mu/m emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) and the equivalent edge-emitting laser (EEL) devices employing the same active region. Our measurements show that the VCSEL devices have the peak of the gain spectrum on the high-energy side of the cavity mode energy and hence operate over a wide temperature range. They show particularly promising I/sub th/ temperature insensitivity in the 250-350 K range. We have then used a theoretical model based on a 10-band k.P Hamiltonian and experimentally determined recombination coefficients from EELs to calculate the pressure and temperature dependency of I/sub th/. The results show good agreement between the model and the experimental data, supporting both the validity of the model and the recombination rate parameters. We also show that for both device types, the super-exponential temperature dependency of I/sub th/ at 350 K and above is due largely to Auger recombination.     

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     

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     

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     

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     

Room-temperature lasing operation of GaInNAs-GaAssingle-quantum-well laser diodes

We have succeeded in demonstrating continuous-wave (CW) operation of GaInNAs-GaAs single-quantum-well (SQW) laser diodes at room temperature (RT). The threshold current density was about 1.4 kA/cm², and the operating wavelength was approximately 1.18 μm for a broad-stripe geometry. Evenly spaced multiple longitudinal modes were clearly observed in the lasing spectrum. The full-angle-half-power far-field beam divergence measured parallel and perpendicular to the junction plane was 4.5° and 45°, respectively. A high characteristic temperature (T₀) of 126 K under CW operation and a small wavelength shift per ambient temperature change of 0.48 nm/°C under pulsed operation were obtained. These experimental results indicate the applicability of GaInNAs to long-wavelength laser diodes with excellent high-temperature performance     

Single-mode distributed feedback and microlasers based on quantum-dot gain material

Quantum-dot gain material fabricated by self-organized epitaxial growth on GaAs substrates is used for the realization of 980-nm and 1.3-/spl mu/m single-mode distributed feedback (DFB) lasers and edge-emitting microlasers. Quantum-dot specific properties such as low-threshold current, broad gain spectrum, and low-temperature sensitivity could be demonstrated on ridge waveguide and DFB lasers in comparison to quantum-well-based devices. 980-nm DFB lasers exhibit stable single-mode behavior from 20/spl deg/C up to 214/spl deg/C with threshold currents < 15 mA (1-mm cavity length). Utilizing the low-bandgap absorption of quantum-dot material miniaturized monolithically integrable edge-emitting lasers could be realized by deeply etched Bragg mirrors with cavity lengths down to 12 /spl mu/m. A minimum threshold current of 1.2 mA and a continuous-wave (CW) output power of >1 mW was obtained for 30-/spl mu/m cavity length. Low-threshold currents of 4.4 mA could be obtained for 1.3-/spl mu/m emitting 400-/spl mu/m-long high-reflection coated ridge waveguide lasers. DFB lasers made from this material by laterally complex coupled feedback gratings show stable CW single-mode emission up to 80/spl deg/C with sidemode suppression ratios exceeding 40 dB.     

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     

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.     

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     

Coupling effects between lumped elements [MMICs]

The coupling effects between various lumped elements have shown that the effect on inductors is less than 1% for inductors having reactance of about 50 Ω and separated by 20 μm on a 75 μm thick GaAs substrate. Thus, a distance of 20 μm between lumped elements is safe for monolithic circuits at RF frequencies