Proceedings Volume 6909

Novel In-Plane Semiconductor Lasers VII

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Proceedings Volume 6909

Novel In-Plane Semiconductor Lasers VII

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Volume Details

Date Published: 7 January 2008
Contents: 14 Sessions, 38 Papers, 0 Presentations
Conference: Integrated Optoelectronic Devices 2008 2008
Volume Number: 6909

Table of Contents

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Table of Contents

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  • Front Matter: Volume 6909
  • Quantum Dots and InGaAsN
  • Quantum Dots and Modelocking
  • Integration
  • Nitrides
  • THz Lasers
  • Silicon Photonics I: Joint Session with Conference 6898
  • Silicon Photonics II: Joint Session with Conference 6898
  • High Brightness
  • Mid-IR Lasers
  • Quantum Cascade Lasers I
  • Quantum Cascade Lasers II
  • High Power
  • Applications Driven
Front Matter: Volume 6909
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Front Matter: Volume 6909
This PDF file contains the front matter associated with SPIE Proceedings Volume 6909, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
Quantum Dots and InGaAsN
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High-performance 1300-nm InAs/GaAs quantum-dot lasers
H. Y. Liu, M. Hopkinson, K. Groom, et al.
Quantum dot (QD) lasers incorporating the dot-in-a-well (DWELL) structures offer the prospect of lowcost and high-performance sources for telecom applications at 1300 nm. A number of significant advantages have been demonstrated to arise from the 0-D density of states, such as low threshold, low noise, low chirp and relative temperature insensitivity. However QD lasers suffer from a low modal gain per dot layer, which is a major factor of limiting high-speed performance. To address this, both a high inplane dot density and the use of multilayer structure are necessary and this presents a major challenge for molecular beam epitaxy (MBE) growth. In this work, to increase the gain of 1300-nm quantum-dot (QD) lasers, we first optimize the MBE growth of InAs/InGaAs QD structure for single-layer epitaxy structure with In composition within InGaAs well. Then we proposed a growth technique, high-growthtemperature spacer layer to suppress the dislocation formation for the multilayer QD structure. These lead to the realization of high-performance multilayer 1300-nm QD lasers with extremely low threshold current density (Jth) of 17 A/cm2 at room temperature (RT) under continuous-wave (cw) operation and high output power of over 100 mW. By combining the high-growth-temperature spacer layer technique with the p-type modulation doping structure, a negative characteristic temperature above RT has been demonstrated for a 5-layer QD laser structure. Further modification of the high-growth-temperature spacer layer technique, we realized a very low RT threshold current density of 33 A/cm2 for a 7-layer ptype- modulated QD laser. The temperature coefficient of ~0.11 nm/K over the temperature range from 20 to 130 °C has also been realized by modifying the strain profile of InGaAs capping layer. These techniques could find application in lasers designed for optical fiber systems.
High performance quantum dot distributed feedback laser diodes around 1.15 µm
J. Koeth, M. Fischer, M. Legge, et al.
Monomode laser diodes in the wavelength range around 1.15μm are of particular interest for various kinds of applications, including frequency doubling where a large part of the spectral range from yellow to green currently remains inaccessible. For efficient frequency conversion, single-frequency laser light, as e.g. obtained from Distributed Feedback Laser (DFB) laser diodes, is an essential prerequisite. One particular challenge at this wavelength range around 1.15μm is to find a gain medium with high internal efficiency. For broad area (BA) lasers, good results have recently been achieved using quantum dots (QDs) or highly strained InGaAs quantum wells (QWs) [1]. In the following, first results for high performance monomode QD DFB laser diodes in the wavelength range of interest are discussed. The spectral gain properties of the underlying QD active region allow to realize DFB lasers with emission spanning an extremely broad wavelength range of 65nm ranging from around 1095nm to 1160nm based on the identical laser structure.
High performance 1.3 µm GaInNAs quantum well lasers on GaAs
S. M. Wang, G. Adolfsson, H. Zhao, et al.
We present state-of-the-art performance of 1.3 μm GaInNAs lasers on GaAs grown by molecular beam epitaxy. The lowest achieved threshold current density is 297, 150 and 133 A/cm2 per quantum well (QW) for single, double and triple QW broad area lasers with a cavity length of 1 mm. The characteristic temperature is 93-133 K in the ambient temperature range of 10-80 °C for broad area lasers depending on the cavity length, and increases to 163-208 K for ridge waveguide lasers as a result of temperature insensitive lateral carrier diffusion in QWs. The maximum 3 dB bandwidth of 17 GHz is achieved in a double QW laser. Uncooled 10 Gb/s operation up to 110 °C has been demonstrated.
Quantum Dots and Modelocking
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Reconfigurable monolithic quantum dot passively mode-locked lasers
The dynamical response of a quantum dot photonic integrated circuit formed with a combination of passive and active gain cells is investigated. When these cells are appropriately biased and positioned within the multi-section laser cavity, fundamental frequency and harmonic mode-locking at repetition rates from 7.2 GHz to 115 GHz are found. Carefully engineered multi-section configurations that include a passive waveguide section significantly lower the pulse width up to 34% as well as increase the peak pulsed power by 49% in comparison to conventional two-section configurations that are formed on the identical device under the same average power. In addition an ultra broad operation range with pulse widths below ten picoseconds is obtained with a 3rd-harmonic mode-locking configuration. The fundamental design principles for using QDs in mode-locked lasers are presented to explain the observed results and to describe why QDs are particularly well-suited for reconfigurable laser devices.
Monolithic mode-locked quantum dot lasers
R. V. Penty, M. G. Thompson, I. H. White
Monolithic mode-locked laser diodes based on QD active regions are regarded as potentially suitable for a large range of photonic applications due to their compactness, mechanical stability and robustness, high potential repetition rates and low potential jitter. Their inherent properties, such as high differential gain, low chirp and fast saturable absorption have led to demonstration of improved performance over their QW equivalents. Low background loss and the relatively long lengths of quantum dot laser devices also have encouraged studies of mode-locking at repetition rates previously not explored in monolithic devices. Applications include biomedicine, high-speed data transmission, clock signal generation and electro-optic sampling. This paper reviews some of the work at Cambridge on the realization of such devices.
Low-noise monolithic mode-locked semiconductor lasers through low-dimensional structures
Kresten Yvind, David Larsson, Jesper Mørk, et al.
The design of a quantum well (QW) based high-saturation energy and low-loss gain region allows a high power density which ensures efficient saturation of the absorber, increases the efficiency, and lowers the noise of monolithic modelocked lasers. This is illustrated though 10 GHz all-active lasers with different number of quantum wells. By comparing a 40 GHz quantum dot and a 40 GHz quantum well laser we discuss the physical difference in the dynamics of the devices. The slow dynamics of quantum dots (QD), results in low self-phase modulation for picosecond pulses and a strong damping of intensity fluctuations, which gives rise to clean optical spectra and very low noise for passive mode-locking.
Integration
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High performance laser arrays for tunable and parallel link applications
Bardia Pezeshki, Gideon Yoffe
The integration of multiple lasers with different characteristics on the same semiconductor chip allows for very high performance tunable lasers as well as new low cost architectures for parallel data links. For applications in tunable lasers, we describe our geometry of laser array with MEMS switch than can cover a 53nm tuning range with high spectral purity. In parallel data links, the laser arrays are operated simultaneously and deliver multiple 10Gb/s channels.
Reduction of RIE induced damage of GaInAsP/InP DQW lasers fabricated by 2-step growth
D. Plumwongrot, M. Kurokawa, T. Okumura, et al.
In order to realize low damage fine structuring processes for the low-dimensional quantum structures, we investigated a process for reducing the degradations of optical properties, which was induced during a reactive-ion-etching (RIE) process with CH4/H2 gas mixture in the quantum-well (QW) structures. Quantitative studies of optical degradation were carried out by photoluminescence (PL) and electroluminescence (EL) measurements. We introduced a thicker upper optical confinement layer (OCL) to protect the QWs from the RIE-plasma. In practical, for the PL measurement, twotypes of strain-compensated single-quantum-well (SC-SQW) structures were prepared for 40-nm-thick- and 80-nmthick- upper OCL wafers and covered by 20-nm-thick SiO2. After the samples were exposed to CH4/H2-RIE for 5- minutes, a relatively stronger suppression of integral PL intensity as well as a spectral broadening was observed in the sample with 40-nm-thick OCL, while those did not change in the sample with 80-nm-thick OCL. For the EL measurements, using two types of SC-DQW structures, samples were exposed to CH4/H2-RIE plasma for 5-minute and then re-grown for other layers to form high-mesa stripe laser structures (Ws=1.5μm). As a result, the spontaneous emission efficiency of the lasers with 80-nm-thick OCL was almost 2 times higher than that of the lasers with 40-nmthick OCL. In addition, a lower threshold current as well as a higher differential quantum efficiency was obtained for the lasers with 80-nm-thick OCL , while that in lasers with 40-nm-thick OCL indicated poor efficiency and a slightly higher threshold.
Three-guide coupled rectangular ring lasers with total internal reflection mirrors
The lasing characteristics of three-guide coupled ring lasers using the self-aligned total internal reflection (TIR) mirrors were investigated numerically and experimentally. The rectangular laser cavity consists of four low loss TIR mirrors and an output coupler made out of passive three coupled waveguides. Two different lasers having active section lengths of 250 and 350 μm and total cavity lengths of 580 and 780 μm are fabricated. For both devices lasing thresholds of 38 mA is obtained at room temperature and under CW operation. Lasing is predominantly single mode with the side mode suppression ratio better than 20 dB.
Nitrides
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A GaN-based surface-emitting laser with 45degree-inclined mirror in horizontal cavity
Masao Kawaguchi, Satoshi Tamura, Masaaki Yuri, et al.
A novel GaN-based surface-emitting laser was realized by utilizing total internal reflection (TIR) by an inclined mirror formed at one end of the horizontal cavity of an edge-emitting laser. The inclined mirror was fabricated by focused ion beam (FIB) etching. The mirror was inclined by 45° with respect to the surface normal. The guided light propagating along the horizontal-cavity is reflected at the mirror to the surface normal. We analyzed optical losses in the laser. To increase the external quantum efficiency, removal of an FIB-damaged layer and precise control of the mirror angle are important. Argon-milling was applied to the FIB-etched surface to remove the FIB-damaged layer which causes an optical loss. The fabricated device with the stripe width of 8 µm and the cavity length of 600 μm lased at 390 nm with a threshold current of 260 mA. Surface-emission was obtained with beam divergence angles of 24.0° and 6.2°, corresponding to perpendicular and parallel to the junction plane, respectively. The presented surface-emitting laser is suitable to form high-power GaN-based 2D laser arrays.
Characterization of AlInGaN-based 405nm distributed feedback laser diodes
S. Masui, K. Tsukayama, T. Yanamoto, et al.
The first-order AlInGaN 405 nm distributed feed-back (DFB) laser diodes were grown on the low dislocation freestanding GaN substrates by a metal organic chemical vapor deposition method. The first-order diffractive grating whose period was 80 nm was formed into an n-type cladding layer. The fine tooth shape grating was obtained by the EB lithography and the dry etching. No additional threading dislocation could be found at the regrowth interface. As a result, we succeeded in demonstrating the first-order AlInGaN based 405 nm DFB laser diodes under cw operation. The threshold current and the slope efficiency were 22 mA and 1.44 W/A under continuous wave operation at 25 °C, respectively. The single longitudinal mode emission was maintained up to an output power of 60 mW. The fundamental transverse mode operation with a single longitudinal mode was observed in the temperature range from 15 °C to 85 °C at an output power of 30 mW. The lifetime was estimated to be 4000 h by the lifetime test which was carried out under the condition of a constant output power of 30mW at 25 °C for 1000 h. The single longitudinal mode emission was maintained for the life tested DFB laser diodes.
THz Lasers
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High-temperature and high-power operation of terahertz quantum-cascade lasers
Sushil Kumar, Alan W. M. Lee, Qi Qin, et al.
We summarize recent development of terahertz quantum-cascade lasers (QCLs) based on a resonant-phonon active region design and metal-metal waveguides for mode confinement. Maximum pulsed operating temperature of 169 K is demonstrated for a 2.7 THz design. Lasers processed with the semi-insulating surface-plasmon (SISP) waveguides and the metal-metal (MM) waveguides are experimentally compared. Whereas the SISP waveguides have higher out-coupling efficiencies, the MM waveguides demonstrate improved temperature performance owing to their lower-loss and near unity mode confinement; however, this comes at the cost of poor radiation patterns and low output power. The beam quality and the out-coupling efficiency of the MM waveguides is shown to be significantly improved by abutting a silicon hyperhemispherical lens to the cleaved facets of ridge lasers. Whereas peak pulsed power of 26 mW at 5 K was detected from a 4.1 THz laser without the lens (device Tmax = 165 K), the detected power increased to 145 mW with the lens with only a 5 K degradation in the maximum operating temperature (device Tmax = 160 K).
Silicon Photonics I: Joint Session with Conference 6898
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Monolithically integrated III-Sb based laser diodes grown on miscut Si substrates
J. Tatebayashi, A. Jallipalli, M. N. Kutty, et al.
We report the formation and growth characteristics of an interfacial misfit (IMF) array between AlSb and Si and their application to III-Sb based quantum well (QW) light-emitting devices including edge-emitting laser diodes and verticalcavity surface emitting lasers (VCSELs) monolithically grown on a Si (001) substrate. A III-Sb epi-structure is grown monolithically on the Si substrate via a thin (≅50 nm) AlSb nucleation layer. A 13% lattice mismatch between AlSb and Si is accommodated by using the IMF array. We demonstrate monolithic VCSELs grown on Si(001) substrates operating under room-temperature with optically-pumped conditions. A 3-mm pump spot size results in peak threshold excitation density of Ith= 0.1 mJ/cm2 and a multimode lasing spectrum peak at 1.62 μm. Moreover, broad-area edgeemitters consisting of GaSb/AlGaSb QWs are demonstrated under pulsed conditions at 77K with a threshold current density of ≅2 kA/cm2 and a maximum peak output power of ≅20 mW for a 1mm-long device. A use of 5° miscut Si substrates enables both IMF formation and suppression of an anti-phase domain, resulting in a drastic suppression of dislocation density over the III-Sb epi-layer and realization of electrically-injected laser diodes operating at 77 K. The current-voltage (I-V) characteristics indicate a diode turn-on of 0.7 V, which is consistent with a theoretical built-in potential of the laser diode. This device is characterized by a 9.1 Ω forward resistance and a leakage current density of 0.7 A/cm2 at -5 V and 46.9 A/cm2 at -15 V. This IMF technique will enable the realization of III-Sb based electrically injected VCSELs operating at the fiber-optic communication wavelength monolithically grown on a Si platform.
On-chip integration of InGaAs/GaAs quantum dot lasers with waveguides and modulators on silicon
Jun Yang, Pallab Bhattacharya, Guoxuan Qin, et al.
Compound-semiconductor-based photonic devices, including lasers and modulators, directly grown and on-chip integrated on Si substrates provide a promising approach for the realization of optical interconnects with CMOS compatibility. Utilizing quantum dots as efficient dislocation filters near the GaAs-Si interface, for the first time, we demonstrated high-performance InGaAs/GaAs quantum dot (QD) lasers on silicon with a relatively low threshold current (Jth = 900 A/cm2), large small-signal modulation bandwidth of 5.5 GHz, and a high characteristic temperature (T0 = 278 K). The integrated InGaAs QD lasers with quantum well (QW) electroabsorption modulators, achieved through molecular beam epitaxy (MBE) growth and regrowth, exhibit a coupling coefficient greater than 20% and a modulation depth ~100% at 5 V reverse bias. We achieved the monolithic integration of amorphous and crystalline silicon waveguides with quantum dot lasers by using plasma-enhanced-chemical-vapor-deposition (PECVD) and membrane transfer, respectively. Finally, preliminary results on the integration of QD lasers with Si CMOS transistors are presented.
Silicon Photonics II: Joint Session with Conference 6898
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Recess integration of micro-cleaved laser diode platelets with dielectric waveguides on silicon
Clifton G. Fonstad Jr., Joseph J. Rumpler, Edward R. Barkley, et al.
Ongoing research directed at integrating 1.55 μm III-V ridge waveguide gain elements (i.e. diode lasers and semiconductor optical amplifiers) co-axially aligned with, and coupled to, silicon oxy-nitride waveguides on silicon substrates is presented. The integration techniques used are highly modular and consistent with fabricating waveguides on Si-CMOS wafers and doing the integration of the III-V gain elements after all standard front- and back-end Si processing has been completed. A novel micro-cleaving technique is used to produce active ridge waveguide platelets on the order of 6 µm thick and 100 μm wide, with precisely controlled lengths, in the current work 300 ± 1 μm, and cleaved end facets. Typical ridge guide micro-cleaved platelet lasers have thresholds under 30 mA. Micro-cleaved platelets are bonded within dielectric recesses etched through the oxy-nitride (SiOxNy) waveguides on a wafer so the ridge and SiOxNy waveguides are co-axially aligned. Transmission measurements indicate coupling losses are as low as 5 db with air filling the gaps between the waveguide ends, and measurements made through filled gaps indicate that the coupling losses can be reduced to below 1.5 dB with a high index (n = 2.2) dielectric fill. Simulations indicate that with further optimization of the mode profile in the III-V waveguide the loss can be reduced to below 1 dB. The paper concludes with a discussion of device design and optimization for co-axial recess integration, and with a comparison of co-axial coupling with the hybrid evanescent vertical coupling III-V/Si integration approach recently introduced by researchers at UCSB and Intel.
High Brightness
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High-brightness diode lasers with very narrow vertical divergence
Götz Erbert, Frank Bugge, Bernd Eppich, et al.
A narrow vertical divergence of about 30° including 95% of power is highly desired in many applications. Principal designs for narrow divergence diode lasers like simple broad waveguide and more sophisticated resonant waveguide structures are discussed. Devices with narrow divergence could be realized in the wavelength range 800nm to 1060nm using very broad waveguide structures. More than 1W in fundamental mode and about 5W nearly diffraction limited output could be achieved from ridge waveguide laser and from diode lasers with tapered resonator structure, respectively.
Highly reliable, high-brightness 915nm laser diodes for fiber laser applications
Zuntu Xu, Wei Gao, Lisen Cheng, et al.
High brightness, high power, and highly reliable 915nm InAlGaAs laser diodes with optimized design are reported in this paper. The laser diodes exhibit excellent performance, such as, high slope efficiency, low threshold current, low voltage, etc., which make them suitable for high brightness operation. The aging test data shows no failures during aging test and more than 220,000 hours estimated lifetime for 90um emitter laser diodes at 8W CW operation. The aging test with the same emitter size at higher stress conditions showed sudden failure that corresponds to catastrophic optical damage (COD) on the facet. A novel large optical cavity (LOC) epi-structure with flat-top near field intensity distribution was developed. The maximum output power is up to 23W under CW testing condition at 25 °C, which is highest level achieved so far. The output power is limited by thermal roll over and there is no COD occurring. This data shows Axcel's technologies can further increase the brightness to over 110mW per micron for 915nm laser diodes. This type of laser diodes is essential for pumping fiber lasers to replace CO2 lasers for industry applications.
Asymmetric Al-free active region laser structure for high-brightness tapered lasers at 975 nm
N. Michel, M. Calligaro, M. Lecomte, et al.
We have realized an asymmetric laser structure at 975 nm, based on an Al-free active region. The quantum wells are located near the p-cladding, so that most of the waveguide is n-type, which allows for reduced optical losses and series resistance. On uncoated broad-area lasers, we have obtained very low optical losses of 0.4 cm-1, together with a high internal quantum efficiency of close to 100%. Based on this structure, we have realized index-guided tapered lasers delivering 1.3W CW, with a narrow far-field angle of 2.5° (FWHM) and 5.8° (at 1/e2) in the slow axis, and a good beam propagation ratio M2 = 1.6. The lasers reach a high maximum wall-plug efficiency of 56%. These tapered lasers deliver a maximum power of 1.5W CW with M2 < 3. The results on the asymmetric structure are compared with those of a symmetric laser structure.
High reliability, high power arrays of 808 nm single mode diode lasers employing various quantum well structures
B. C. Qiu, O. Kowalski, S. D. McDougall, et al.
Single mode laser diode arrays operating at 808 nm have been designed and fabricated using several different waveguide and quantum well combinations. In order to operate these devices at 200 mW per element a quantum well intermixing process has been used to render their facets non-absorbing and thus they do not suffer from mirror damage related failure. In this paper we demonstrate extremely high levels of reliability for GaAs and AlGaAs quantum well devices with arrays of 64 elements completing over 6000 hours continuous operation without any single laser element failure and a correspondingly low power degradation rate of <1% k/hr. In contrast we show extremely high power degradation rates for arrays using InGaAs and InAlGaAs 808 nm quantum wells laser arrays.
Grazing-incidence-slab semiconductor laser (GRISSL)
Anish K. Goyal, Robin K. Huang, Leo Missaggia
We report on a novel geometry for electrically driven semiconductor lasers called the GRISSL. The laser cavity is formed between two mirrors that are external to the semiconductor chip and the laser beam intercepts the quantum well (QW) gain region at a grazing angle-of-incidence. In this first demonstration of the GRISSL, the laser structure was grown on an n-type GaAs substrate and the gain region comprises three InGaAs QWs. The external cavity consists of a pair of lenses, a flat high-reflectivity mirror, and a flat R = 70% output coupler. Lasers emit a power of 30 mW CW in a single-mode, 35-μm-diameter beam at λ ~ 0.98 μm. Under pulsed conditions (1 μsec, 1 kHz), a peak output power of 0.37 W was measured. The beam is single-mode near threshold but becomes multi-mode at the maximum drive currents. The slope efficiency of these devices is about 10 times lower than the design value of 1 W/A. This discrepancy can be accounted for by higher than anticipated losses in the substrate and a poor overlap of the laser beam with the pumped region. Methods for overcoming both of these factors to regain a high wall-plug efficiency are discussed.
Mid-IR Lasers
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InAs-based quantum-cascade lasers
J. Devenson, R. Teissier, O. Cathabard, et al.
Different issues of the development of short wavelength quantum cascade lasers (QCLs) in the InAs/AlSb system, concerning both optical and electronic design, are considered. A plasmon enhanced waveguide is shown to be suitable for use in InAs-based QCLs operating at wavelengths near 3 μm. High performance lasers emitting near 3.3 μm are demonstrated, as well as short wavelength QCLs emitting down to 2.75 μm. It is shown that their performances are not limited neither by interband absorption across the small band gap of InAs, nor by the electron scattering into the L-minimum of the well material.
Short-wavelength quantum cascade lasers
Dmitry G. Revin, Shiyong Zhang, Matthew J. Steer, et al.
We report the first realization of short wavelength (λ ~ 3.05 - 3.6 μm) lattice matched In0.53Ga0.47As/AlAs0.56Sb0.44/InP quantum cascade lasers (QCLs). The highest-performance device (λ ~ 3.6μm) displays pulsed laser action for temperatures up to 300 K. The shortest wavelength QCL (λ ≈ 3.05 μm) operates in pulsed mode at temperatures only up to 110 K. The first feasibility study of the strain compensated InGaAs/AlAsSb/InP QCLs (λ ~ 4.1 μm) proves that the lasers with increased indium fractions in the InGaAs quantum wells of 60 and 70% display no degradation compared with the lattice matched devices having identical design. This strain compensated system, being of particular interest for QCLs at λ <~ 3.5μm, provides increased energy separation between the Γ and X conduction band minima in the quantum wells, thus decreasing possible carrier leakage from the upper laser levels by intervalley scattering. We also demonstrate that the performance of strain compensated InGaAs/AlAsSb QCLs can be improved if AlAsSb barriers in the QCL active region are replaced by AlAs layers. The introduction of AlAs is intended to help suppress compositional fluctuations due to inter diffusion at the quantum well/barrier interfaces.
Interband cascade laser progress
W. W. Bewley, C. L. Canedy, M. Kim, et al.
Recent advances in the development of mid-IR antimonide type-II "W" interband cascade lasers have led to a considerably improved high-temperature operation of the devices. We report an experimental investigation of four interband cascade lasers with wavelengths spanning the mid-infrared spectral range, i.e., 2.9-5.2 μm near room temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm at 78 K (λ = 3.6 μm) and 590 A/cm2 at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength device (λ = 2.6-2.9 μm) was even lower, 450 A/cm2. A cavity-length study of the lasers emitting at 3.6-4.1 μm yielded an internal loss varying from 7.8 cm-1 at 78 K to 24 cm-1 at 300 K, accompanied by a decrease of the internal efficiency from 77% to 45%. Preliminary cw testing led to a narrow-ridge device from one of the wafers with emission at λ = 4.1 μm operating to 288 K, a new record for interband devices in this wavelength range.
Quantum Cascade Lasers I
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Bloch gain in quantum cascade lasers
R. Terazzi, T. Gresch, M. Giovannini, et al.
Laser action by a weak to vanishing population inversion is observed in mid-infrared (7 μm) quantum cascade lasers. The origin of the optical gain is the same than for the Bloch oscillator predicted in superlattices. This special gain does not only relax the threshold condition for lasing, but also underlines the major role played by coherent processes in semiconductor heterostructures, as it is based on scattering assisted optical transitions.
Intracavity amplitude modulation of quantum-cascade lasers using intersubband absorption in the active region under reverse bias
We have analyzed light absorption in a quantum cascade laser structure under forward and reverse bias. Strong absorption modulation at the laser frequency is predicted and observed experimentally for the voltage variation within the high differential resistance voltage range. We propose to use this mechanism for monolithically integrated intracavity modulation of quantum cascade lasers, promising suppressed thermal chirp and fast modulation capability. In addition, the described method allows for extraction of the intersubband absorption from the total waveguide losses.
Quantum Cascade Lasers II
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On the coherence/incoherence of electron transport in semiconductor heterostructure optoelectronic devices
P. Harrison, D. Indjin, I. Savić, et al.
This paper compares and contrasts different theoretical approaches based on incoherent electron scattering transport with experimental measurements of optoelectronic devices formed from semiconductor heterostructures. The Monte Carlo method which makes no a priori assumptions about the carrier distribution in momentum or phase space is compared with less computationally demanding energy-balance rate equation models which assume thermalised carrier distributions. It is shown that the two approaches produce qualitatively similar results for hole transport in p-type Si1-xGex/Si superlattices designed for terahertz emission. The good agreement of the predictions of rate equation calculations with experimental measurements of mid- and far-infrared quantum cascade lasers, quantum well infrared photodetectors and quantum dot infrared photodetectors substantiate the assumption of incoherent scattering dominating the transport in these quantum well based devices. However, the paper goes on to consider the possibility of coherent transport through the density matrix method and suggests an experiment that could allow coherent and incoherent transport to be distinguished from each other.
Multimode regimes and instabilities in quantum-cascade lasers
Franz X. Kärtner, Ariel Gordon
A theoretical study of multimode operation regimes in quantum cascade lasers (QCLs) is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes: One is spatial hole burning (SHB), and the other one is related to the Risken-Nummedal-Graham-Haken (RNGH) instability predicted in the sixties. A model that can account for coherent phenomena, a saturable absorber and SHB is developed and studied in detail both analytically and numerically.
Coherent coupling of mid-infrared quantum cascade lasers
L. K. Hoffmann, C. A. Hurni, S. Schartner, et al.
A monolithic coupling scheme in which two active waveguides merge into a single waveguide to form a Y-shaped resonator is demonstrated for mid-infrared quantum cascade lasers. Lasers with emission wavelengths of 10.5 μm and 4.2 μm were processed from lattice-matched GaAs/AlGaAs and strain-compensated InP/InGaAs/AlAs/AlInAs structures. Phase-locking is observed in the laser cavities, resulting in coherent interference of the emitted radiation. Far fields were recorded on both sides of the devices and analyzed in respect to their radiative origin. By matching the recorded far field intensity profiles to corresponding near field distributions, the lateral mode distribution within the resonator is derived. Depending on the length of the coupling section, even or odd cavity modes evolve. Moreover, a comparison between the fabricated devices shows the emission wavelength's impact on the coupling performance of the Y-junction. The results demonstrate the feasibility of coherent laser resonators with prospective applications in interferometric sensing and high power laser arrays.
High Power
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High brightness quantum well and quantum dot tapered lasers
Several types of high-brightness near-infrared tapered quantum well and quantum dots tapered lasers are reviewed and compared. Recent developments include record-high brightness quantum well tapered lasers at 810 nm and 975 nm (up to 8.3 W CW, diffraction limited), high wall-plug efficiency gain-guided and index-guided tapered lasers (up to 57%), narrow slow axis far-field (2.5° FWHM) index-guided tapered lasers at 975 nm, wavelength-stable (down to 0.09 nm/K), high-brightness quantum dots tapered lasers at 920 and 975 nm, and quantum dots tapered laser bars (up to 14 W CW) at 920 nm, with narrow far-field in the slow axis (3° FWHM).
Master oscillator power amplifier 3D assemblies based on grating coupled laser diodes
We report on novel architectures of the hybrid master oscillator power amplifier (MOPA) assemblies incorporating vertically stacked surface-emitting laser diodes. Optical coupling between the MO and the PA is provided by nonresonant grating couplers integrated on both of the devices. The MOPA consists of a MO chip with dual grating reflector for single wavelength operation and a flared PA chip with two grating outcouplers. Optical peak power over 100W and spectral bandwidth of 0.2nm were achieved from the single MOPA while the MO operated in the gain-switching regime and the PA operated as a traveling wave amplifier. New designs of coherent MOPA arrays are proposed based on a phase locked MO bar and a single transverse mode PA bar. This MOPA assembly requires an optical cross-coupling between the bars provided by tilted gratings which have been developed and experimentally evaluated.
External cavity phase-locked semiconductor tapered lasers
I. Hassiaoui, N. Michel, M. Lecomte, et al.
This work relates to combining a phase corrected array of tapered laser diodes, emitting at λ = 975 nm, coherently using the Talbot effect. Diffractive coupling of semiconductor lasers by use of the Talbot effect provides a means for coherent beam addition of multiple elements in laser diode arrays and makes possible a very compact external cavity. We have used, in this work, fully index guided tapered laser diodes. They contain a ridge waveguide, which acts as a modal filter, and a tapered section of increasing width, which provides high power. We have realized arrays of several emitters (N=10), which are not optically coupled to each other. First, to improve the beam quality of the array, a phase correcting micro system, achieving collimation in the fast axis, correction of the wave front tilts in both directions and also a slow axis collimation, was added. The FWHM divergences of the array were reduced from 34° to 0.17° in the fast-axis and from 3.5° to 0.7° in the slow-axis at 6A, 3.7 W. Then, to be close to diffraction limit, we have combined this corrected array coherently using the Talbot effect. We have obtained quasi-monolobe slow axis far field profile for the in phase mode with a central peak divergence of only 0.27° at 1.5 A, 315 mW under CW operation and of only 0.20° at 2.5 A, 787 mW under pulsed operation.
High-power laser arrays with 100% fill factor emission facet
Dan A. Yanson, Stewart D McDougall, Bocang Qiu, et al.
Novel types of laser diode array with a 100% filling factor at the emission facet are reported. The arrays utilize both parallel and tapered cavity emitters that are connected via a common Laterally Unconfined Non-Absorbing Mirror (LUNAM) defined with quantum-well intermixing technology at 808 nm wavelength. Two LUNAM array types are considered: incoherent (weakly coupled) and coherent (diffraction coupled). Incoherent LUNAM arrays benefit from a reduced power density at the facet, improving reliability, and a near-uniform intensity distribution across the array aperture. Stacked laser diode arrays built with LUNAM bars deliver 950 W power under QCW operation with <5% degradation at 1.9×108 shots. Novel coherent arrays are realized using a monolithically integrated LUNAM Talbot cavity. The devices produce a single-lobed horizontal far-field pattern over a limited current range with <10% slope efficiency penalty compared to an uncoupled case. The LUNAM arrays are promising candidates for high-power, high-brightness and high-reliability operation.
Applications Driven
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700-730 nm InGaAsP quantum well ridge-waveguide lasers
E. Nomoto, T. Taniguchi, T. Ohtoshi, et al.
Single-mode InGaAsP QW lasers with wavelengths of 700 to 730 nm are demonstrated. These lasers are attractive as light sources for medical and biological applications because biological tissues are relatively transparent in the near-infrared range. The laser has a 2-micrometer-wide ridge-waveguide structure and achieves single-mode 100-mW CW operation at 80°C with an extremely low operating current of 130 mA and a high characteristic temperature of 182 K. These InGaAsP lasers enable innovative solutions in demanding applications by means of a light source with a compact size and low cost.
Chirped comb generation in frequency shifted feedback DFB lasers
Jon Paul, Yanhua Hong, Paul S. Spencer, et al.
The optical output of frequency shifted optical feedback lasers can exhibit a chirped structure covering a large continuous range of frequencies. These features can be used in various applications such as optical pumping, high precision optical frequencies measurement and optical frequency domain ranging (OFDR). For the first time, a broadband chirped frequency comb has been generated using a conventional DFB laser diode and an intracavity acoustooptic modulator. The experimental factors that affect the comb characteristics of this compact and robust system have been studied and quantified. The experimental results have also been compared with theoretical predictions and, in general, there is good agreement. The FSF DFB has also been evaluated in a simple OFDR system.
High power DFB lasers for D1 and D2 caesium absorption spectroscopy and atomic clocks
A. Klehr, H. Wenzel, O. Brox, et al.
Distributed feedback ridge-waveguide lasers, emitting up to 250 mW in a single lateral and longitudinal mode at 852 nm and 894 nm are presented. A threshold current of 40 mA and a differential quantum efficiency of 1 W/A is achieved. The distributed feedback is provided by a second order grating, formed into an InGaP/GaAs/InGaP multilayer structure. Owing to the stable lasing frequency, the large side mode suppression ratio (>40 dB) and the small spectral line width (<500 kHz) the lasers are well suited for atomic clocks and Caesium D1 and D2 spectroscopy. This was verified by the measurement of the hyperfine structure of the Caesium lines.
Narrow linewidth and demonstration of saturation spectra of the Cesium at 852 nm with high power Al-free DFB laser diodes
V. Ligeret, S. Bansropun, M. Lecomte, et al.
There is a growing demand for precise gyroscopes and atomic clocks for positioning, flight navigation systems and aerospatial applications. One of the prerequisites for atomic optical pumps is a laser diode with high power (≈100mW), narrow linewidth (<2MHz), and spatial qualities (M2<1.5). Another important factor for aerospatial applications is a very high reliability performance of the laser devices. With an aim to address these issues, we have laid down the technological foundation and further developed ridge waveguide distributed feedback (DFB) lasers with an emission wavelength of 852nm corresponding to the D2 cesium transition in atomic clocks. The epitaxy is based on an Al free active region with a GaInP large optical cavity and a single compressive strained GaInAsP quantum well. Fabricated DFB uncoated lasers have shown wavelength emission at 852.12nm with an output optical power of 40mW, a SMSR >30dB at the D2 line, at 37°C. Low self-heterodyne linewidths of 0.8MHz and 1.2MHz were measured respectively at 20mW 12°C and 40mW 37°C. With this uncoated diode, we have realized saturation spectra of cesium atoms to determine the resolution and the stability of the laser diode working on Cs. The saturation spectrum of the D2 line of 133Cs was recorded with a resolution close to the natural line width.
Integrated 1060nm MOPA pump source for high-power green light emitters in display technology
We present a compact green light emitter for laser displays and focus on the pump source for a SHG waveguide in single-pass configuration. The developed pump source has a RW-structure consisting of three sections: a DFB, a spacer and an amplifier section. The optical output power is 305mW for currents of 120mA and 400mA in DFB and amplifier section. The control of the current in the amplifier section allows a modulation of the output power from 5mW to 305mW. Spectral characteristics as well as measured beam divergence are well suited for pumping SHG waveguide crystals. Results on the hybrid 530nm emitter are summarized.