Distributed-Feedback Quantum-Cascade Lasers at 9 $mu$m Operating in Continuous Wave Up to 423 K

Single-mode lasers operating at $lambdaapprox 9 muhbox{m}$ in continuous wave up to 423 K (150 $^{circ}hbox{C}$) were achieved by the combination of strong distributed-feedback coupling, a narrow gain active region design, low intersubband, and free-carrier losses as well as a good thermal management. Tuning of 10 $hbox{cm}^{-1}$ or 0.9% of the center frequency was achieved by heating the device. The threshold current density varies from 1.1 $hbox{kA/cm}^{2}$ at 303 K to 2.4 $hbox{kA/cm}^{2}$ at 423 K. Other devices with low electrical power consumption of 1.6 and 3.8 W for an optical output power of 16 and 100 mW have been demonstrated at 263 K.      

Continuous-wave operation of GaInAs-AlGaAsSb quantum cascade lasers

We report on the demonstration of continuous-wave (CW) operation of GaInAs-AlGaAsSb quantum cascade (QC) lasers. By placing a 2.5-/spl mu/m-thick gold layer on both sides of the laser ridge to extract heat from the active region in the lateral direction, together with mounting the device epilayer down, we have achieved CW operation of GaInAs-AlGaAsSb QC lasers composed of 25 stages of active/injection regions. The maximum CW operating temperature of the lasers is 94 K, and the emission wavelength is around /spl lambda//spl sim/4.65 /spl mu/m. For a device with the size of 10/spl times/2000 /spl mu/m/sup 2/, the CW optical output power per facet is 13 mW at 42 K and 4 mW at 94 K. The CW threshold current density is 1.99 kA/cm/sup 2/ at 42 K, and 2.08 kA/cm/sup 2/ at 94 K, respectively.     

High performance injectorless quantum-cascade lasers

Injectorless quantum-cascade (QC) lasers, for the first time comparable to conventional QC lasers, are presented. The samples are based on InP and the active region is designed as a four-level staircase, realised in the strain-compensated material system Ga/sub 0.4/In/sub 0.6/As/Al/sub 0.56/In/sub 0.44/As. Low threshold current densities and a maximum operating temperature of 350 K have been achieved, due to an optimised design. At 77 K the wavelength is about 10 /spl mu/m, while threshold current densities of 0.9 kA/cm/sup 2/ have been observed, which compare well with those of conventional QC lasers in this wavelength region.     

A Compact Four-Wavelength Quantum-Cascade Laser Source

A four-wavelength quantum-cascade (QC) laser source that operates using a single current channel is presented. The source includes two different heterogeneous cascade QC lasers, one with emission wavelengths of 7.0 $mu{hbox {m}}$ and 11.2 $mu{hbox {m}}$, and the other with 8.7 $mu{hbox {m}}$ and 12.0 $mu{hbox {m}}$ . For 3.0-mm and 3.5-mm cavity lengths, QC lasers with emission wavelengths of 8.7, 11.2, and 12.0 $mu{hbox {m}}$ have threshold current densities within less than a factor of 2, which allows them to be conveniently driven in series by a single current source.      

Short-Pulse High-Power Operation of GaSb-Based Diode Lasers

We report on the short-pulse high-power operation of GaSb-based diode lasers emitting in the 1.95- to 2.2-$muhbox{m}$ wavelength range. Both epi-side-down mounted broad-area lasers and epi-side-up mounted ridge waveguide lasers, sharing the same active region and epitaxial layer structure, have been studied. They show drastically different output power characteristics and thermal limitations in continuous wave (CW) or quasi-CW operation ($hbox{long ~current ~pulses}> 1~mu hbox{s}$) due to the vastly different laser geometries and mounting configurations. However, in short-pulse operation ($hbox{pulse ~length} le 50~hbox{ns}$), the maximum output power density was found to be almost independent of the device geometry and the mounting technique, even though still limited by thermal rollover due to self-heating of the device active region. With the heatsink temperature set to $-$30 $^{circ}hbox{C}$, a maximum single-emitter output power of 22.5 W was recorded at an injection current density of 110 $hbox{kA}/hbox{cm}^{2}$ (50-ns pulse length), and a maximum power density at the output facet of 26.7 $hbox{MW}/hbox{cm}^{2}$ (30-ns pulse length) was achieved, without encountering catastrophic optical mirror damage.      

InAs/AlSb quantum cascade lasers operating at 6.7 /spl mu/m

Quantum cascade lasers based on the InAs/AlSb material system have been realised. The optical confinement is obtained using a plasmon waveguide with n/sup +/-InAs cladding layers. In pulse mode the lasers emit close to 6.7 /spl mu/m with a threshold current density of 5 kA/cm/sup 2/ at 90 K. The maximum operating temperature is 220 K.     

Room-temperature InAs=AlSb quantum-cascade laser operating at 8.9 /spl mu/m

A room-temperature InAs/AlSb quantum-cascade laser operating at 8.9 mum is reported. The laser structure is grown on an n-InAs (100) substrate by solid-source molecular-beam epitaxy. The active region utilises a diagonal intersubband transition in an InAs/AlSb three-quantum-well structure. Observed threshold current density in pulse mode is 2.6 kA/cm² at 80 K and 12.0 kA/cm² at 300 K. The maximum operation temperature is 305 K     

2.7-$mu$ m GaSb-Based Diode Lasers With Quinary Waveguide

Type-I double-quantum-well (QW) GaSb-based diode lasers operating at 2.7 $mu hbox{m}$ with room-temperature continuous-wave (CW) output power of 600 mW and peak power-conversion efficiency of 10% were designed and fabricated. The devices employed 470-nm-wide AlGaInAsSb waveguide optimized for improved device differential gain. CW threshold current density about 100 $hbox{A}/hbox{cm}^{2}$ per QW and slope efficiency of 150 mW/A were demonstrated at 16 $^{circ }hbox{C}$.      

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.     

Mid-infrared (8.5 /spl mu/m) semiconductor lasers operating at room temperature

The room-temperature pulsed operation of a semiconductor laser emitting at 8.5 /spl mu/m is reported. This device is an optimized vertical transition quantum cascade (QC) laser. At 300 K the peak output power from a single facet is 15 mW, and the current density at threshold is /spl sim/8 kA/cm/sup 2/. The temperature dependence of the threshold current density is described by a high T/sub 0/ (107 K) in the 200-320 K temperature range.     

Room-temperature long-wavelength (λ=13.3 μm) unipolarquantum dot intersubband laser

Long-wavelength (λ=13.3 μm) unipolar lasing at 283 K from self-organised In_0.4Ga_0.6As/GaAs quantum dots, due to intersubband transitions in the conduction band, is demonstrated for the first time. The threshold current density under continuous wave operation is 1.1 kA/cm² for a 60 μm×1.2 mm broad-area plasmon-enhanced waveguide device and the maximum power output is ≈ μW. The long intersubband relaxation time in quantum dots, together with the short lifetime in the ground state, due to interband stimulated emission, help to achieve the necessary population inversion and gain     

GaInAs/AlGaInAs DH and MQW lasers with 1.5-1.7 mu m lasing wavelengths grown by atmospheric pressure MOVPE

The first successful growth and fabrication of long wavelength (1.5-1.7 mu m) DH and MQW lasers by atmospheric pressure MOVPE in a 'phosphorus-free' material system is reported. The GaInAs/AlGaInAs DH and MQW lasers were grown on InP substrates. DH lasers emitting at around 1690 nm exhibit threshold current densities down to 2.8 kA/cm/sup 2/ at 25 degrees C; the characteristic temperature is 50 K in the 15-55 degrees C range. First MQW lasers with 1565 nm emission wavelength have threshold current densities around 3.2 kA/cm/sup 2/.<>     

GaAs-based 1.53 μm GaInNAsSb vertical cavity surface emitting lasers

Electrically-pumped GaAs-based 1.53 mum vertical cavity surface emitting lasers operating in pulsed mode at room temperature and continuous wave (CW) up to 20degC are reported for the first time. The lasers employ a GaInNAsSb/GaNAs multiple quantum well active region, a selectively oxidised AlAs aperture and p- and n-doped Al(Ga)As/GaAs distributed Bragg reflectors. Typical devices have room-temperature pulsed threshold current densities of 8.3 kA/cm and threshold voltages of 5.5 V. CW threshold currents as low as 2.87 mA for a 7 mum aperture device were observed at 15degC.     

Optimized second-harmonic generation in quantum cascade lasers

Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 2 /spl mu/W of SHG light at 3.75 /spl mu/m wavelength at a fundamental peak power and wavelength of 1 W and 7.5 /spl mu/m, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 /spl mu/W/W/sup 2/. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 /spl mu/m, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 /spl mu/W/W/sup 2/. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7/spl times/10/sup -5/ esu (2/spl times/10/sup 4/pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.     

Bound-to-continuum and two-phonon resonance, quantum-cascade lasersfor high duty cycle, high-temperature operation

Recent advances in quantum-cascade (QC) laser active-region design are reviewed. Based on a rate equation model of the active region, we show why new gain regions. based on a two-phonon resonance or a bound-to-continuum transition exhibit significantly better performance than the traditional design based on a three-quantum-well active region. Threshold current densities as low as 3 kA/cm² at T=300 K, operation with a peak power of 90 mW at 425 K, single-mode high-power operation up to temperatures above 330 K at λ≈16 μm and continuous wave operation up to T=311 K are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300-MHz free-space optical link between two buildings separated by 350 m     

Low-Threshold Strained Quantum-Well GaSb-Based Lasers Emitting in the 2.5- to 2.7-$mu$ m Wavelength Range

GaInAsSb–GaSb strained quantum-well (QW) ridge waveguide diode lasers emitting in the wavelength range from 2.51 to 2.72 $ mu{hbox {m}}$ have been grown by molecular beam epitaxy. The devices show ultralow threshold current densities of 44 $hbox{A}/{hbox {cm}}^{2}$ (${L}rightarrow infty $) for a single QW device at 2.51 $ mu{hbox {m}}$, which is the lowest reported value in continuous-wave operation near room temperature (15 $^{circ}hbox{C}$) at this wavelength. The devices have an internal loss of 3 ${hbox {cm}}^{-1}$ and a characteristic temperature of 42 K. By using broader QWs, wavelengths up to 2.72 $mu{hbox {m}}$ could be achieved.      

Absorption, Gain, and Laser Action in Bismuth-Doped Aluminosilicate Optical Fibers

In this study, bismuth-doped fiber lasers operating at the wavelength of 1179 nm with an optical efficiency of up to 28% are realized. The fiber gain upon 1-$mu{hbox{m}}$ pumping declines, while the unsaturable absorption increases with increasing the small-signal absorption. We conclude that up-conversion and excited-state absorption are responsible for limiting the efficiency of such lasers.      

Temperature Characteristics of Gain Profiles in 1.3-$ muhbox{m} p$-Doped and Undoped InAs/GaAs Quantum-Dot Lasers

The modal gain and differential gain of 1.3-$muhbox{m}break p$ -doped and undoped InAs/GaAs quantum-dot (QD) lasers have been investigated as a function of injection current under different operation temperatures. The results show that $p$ -doping improves the modal and differential gains in QD lasers at high temperatures. Exponential decrease in the differential gain profiles were observed in both types of lasers from 20 $^{circ}hbox{C}$ to 80 $^{circ}hbox{C}$. Theoretical calculations based on the rate equation model for the undoped QD laser gain at different temperatures are presented.      

Threshold dependence on active-layer thickness in InGaAsP/InP d.h. lasers

Room-temperature threshold current density Jth against active-layer thickness d is reported for pulsed InGaAsP/InP double-heterostructure lasers operating at wavelengths near 1.23 ?m. For d ? 1 ?m, Jth/d is 5.0 kA cm?2 ?m?1, whereas the lowest threshold is 1.6 kA cm?2 for d ? 0.2 ?m. The threshold dependence is fitted by a numerical calculation based on a two-parameter gain model.     

InGaN multiple quantum well laser diodes grown by molecular beam epitaxy

The first InGaN multiple quantum well laser diodes produced by molecular beam epitaxy are reported. Ridge waveguide lasers have been demonstrated at room temperature under pulsed current injection conditions. The lasers emit at a wavelength of approximately 400 nm with a spectral line-width of less than 0.2 nm, and a threshold current density of /spl sim/30 kA cm/sup -2/.