Ridge-waveguide Devices Research Articles

Substrate-emitting quantum cascade lasers with a distributed Bragg reflector and a spectrally detuned second-order outcoupler

Demonstration of substrate-emitting quantum cascade lasers (QCLs) with a distributed Bragg reflector (DBR) used for wavelength selection and a second-order outcoupler spectrally detuned from the reflector is reported. A 40-stage QCL structure with the gain spectrum centered at 4.1 μm was processed into 20 μm-wide ridge-waveguide devices with a 4 mm-long gain section, a 0.5 mm-long first-order DBR section with 644 nm period, and a 1.5 mm-long second-order outcoupler with 1100 nm period. Etch depth and duty cycle were 900 nm and 45%, respectively, for both gratings. The three sections shared the same electrical contact and were, therefore, pumped at the same current density. A device with a high-reflection coated back facet and anti-reflection coated front facet and substrate side delivered a peak power of 0.6 W from the substrate into a single-lobed beam with ∼1° × 18° angular full width at half maximum (FWHM). Due to the spectral detuning, the beam was extracted from the device at an angle of ∼−34° relative to the growth direction. The measured spectrum had a FWHM of 3 cm−1, and its peak was centered at 4.07 μm, close to the design value of 4.10 μm. The detuning between the two gratings allows for independent optimization of grating profiles to significantly improve the overall laser performance, which will be the focus of future work.

Importance of Overcoming MOVPE Surface Evolution Instabilities for >1.3 μm Metamorphic Lasers on GaAs

We investigated and demonstrated a 1.3 $\mu$m-band laser grown by metalorganic vapour phase epitaxy (MOVPE) on a specially engineered metamorphic parabolic graded In$_x$Ga$_{1-x}$As buffer and epitaxial structure on a GaAs substrate. Bottom and upper cladding layers were built as a combination of AlInGaAs and InGaP alloys in a superlattice sequence. This was implemented to overcome (previously unreported) detrimental surface epitaxial dynamics and instabilities: when single alloys are utilised to achieve thick layers on metamorphic structures, surface instabilities induce defect generation. This has represented a historically limiting factor for metamorphic lasers by MOVPE. We describe a number of alternative strategies to achieve smooth surface morphology to obtain efficient compressively strained In$_{0.4}$Ga$_{0.6}$As quantum wells in the active layer. The resulting lasers exhibited low lasing threshold with total slope efficiency of 0.34 W/A for a 500 $\mu$m long ridge waveguide device. The emission wavelength is extended as far as 1360 nm.

Impact of Film Stress and Film Thickness Process Control on GaAs-TiAu Metal Adhesion

The fabrication of GaAs-based optoelectronic ridge-waveguide devices requires deposition of a topside-contact metallization for proper device operation. Fabrication delays occurring during the processing of TiAu-contact pads have been linked to poor adhesion and metal blister formation, factors that negatively affect the final device yield. In this study, we examined sputter-deposited Ti and Au films to determine the impact of film-thickness process control and film stress as measured by wafer bow. We theorized that competing stress relaxation forces between the Ti and Au films would produce a post-deposition change in wafer bow, which affects the Au film, setting the stage for blister creation. We now report the development of a reduced-stress sputter-deposited TiAu-contact metallization and demonstrate the utility of the modified process with fabrication of blister-free ridge-waveguide devices with high device yield.

High-Performance Growth of Terahertz Quantum Cascade Laser Structures by Solid Source MBE

High performance growth of terahertz quantum cascade lasers (THz QCLs) based on hybrid bound-to-continuum transition and resonant phonon extraction is presented by solid source molecular beam epitaxy. The corresponding accurate control of layer thickness and alloy composition during growth, with precise calibration of the growth rate of Ga and Al by reflection high energy electron diffractometry and high resolution XRD techniques are studied in detail. By utilizing surface plasmon ridge waveguide device structures, the THz QCLs lasing at 3.3 THz with maximum powers of 426 mW at 10 K and 213 mW at 80 K are realized in pulsed mode and the maximum lasing temperature achieves 110 K.

InP-based quantum cascade lasers monolithically integrated onto silicon.

Lasing is reported for ridge-waveguide devices processed from a 40-stage InP-based quantum cascade laser structure grown on a 6-inch silicon substrate with a metamorphic buffer. The structure used in the proof-of-concept experiment had a typical design, including an Al0.78In0.22As/In0.73Ga0.27As strain-balanced composition, with high strain both in quantum wells and barriers relative to InP, and an all-InP waveguide with a total thickness of 8 µm. Devices of size 3 mm x 40 µm, with a high-reflection back facet coating, emitted at 4.35 µm and had a threshold current of approximately 2.2 A at 78 K. Lasing was observed up to 170 K. Compared to earlier demonstrated InP-based quantum cascade lasers monolithically integrated onto GaAs, the same laser structure integrated on silicon had a lower yield and reliability. Surface morphology analysis suggests that both can be significantly improved by reducing strain for the active region layers relative to InP bulk waveguide layers surrounding the laser core.

Mid-infrared quantum cascade laser arrays with electrical switching of emission frequencies

We present a design of quantum cascade laser arrays made of ridge-waveguide devices in which the emission frequency can be electrically switched between several specified values. Our approach relies on fabricating multiple independently-biased distributed feedback grating sections along the laser ridge waveguides. Switchable single-mode lasing from the laser facet is achieved by balancing the injection pumping of the different grating sections. Our method provides a robust solution that can increase the tuning bandwidth of the quantum cascade laser arrays without increasing the size of the array emission aperture.

Cascade Pumping of 1.9–3.3 μm Type-I Quantum Well GaSb-Based Diode Lasers

Cascade pumping of type-I quantum well gain sections was utilized to increase output power and efficiency of GaSb-based diode lasers operating in spectral region from 1.9 to 3.3 μm. Coated devices with ∼100-μm-wide aperture and 3-mm-long cavity demonstrated continuous wave (CW) output power of 1.96 W near 2 μm, 980 mW near 3 μm, 500 mW near 3.18 μm, and 360 mW near 3.25 μm at room temperature. The corresponding narrow ridge lasers with nearly diffraction limited beams operate in CW regime with tens of mW of output power up to 60 °C. Two step shallow/deep narrow/wide ridge waveguide devices showed lower threshold currents and higher slope efficiencies compared to single step narrow ridge lasers. Laterally coupled DFB lasers mounted epi-up generated above 10 mW of tunable single frequency CW power at 20 °C near 3.22 μm.

All-laser-micromachining of ridge waveguides in LiNbO3 crystal for mid-infrared band applications

The femtosecond laser micromachining of transparent optical materials offers a powerful and feasible solution to fabricate versatile photonic components towards diverse applications. In this work, we report on a new design and fabrication of ridge waveguides in LiNbO3 crystal operating at the mid-infrared (MIR) band by all-femtosecond-laser microfabrication. The ridges consist of laser-ablated sidewalls and laser-written bottom low-index cladding tracks, which are constructed for horizontal and longitudinal light confinement, respectively. The ridge waveguides are found to support good guidance at wavelength of 4 μm. By applying this configuration, Y-branch waveguiding structures (1 × 2 beam splitters) have been produced, which reach splitting ratios of ∼1:1 at 4 μm. This work paves a simple and feasible way to construct novel ridge waveguide devices in dielectrics through all-femtosecond-laser micro-processing.

Highly efficient phase-sensitive parametric gain in periodically poled LiNbO3 ridge waveguide.

We have developed a periodically poled LiNbO3 (PPLN) ridge waveguide device and experimentally demonstrated a phase-sensitive optical parametric amplification. The highly efficient net phase-sensitive parametric gain of +18.6 dB is obtained in a cascaded second harmonic generation and difference frequency generation configuration. The phase-sensitive gain is greater by +5.8 dB in comparison with the phase-insensitive gain. We successfully confirmed the phase-sensitive amplification with a high gain in the PPLN ridge waveguide.

InAs-based distributed feedback interband cascade lasers

Single mode emission of InAs-based interband cascade lasers using lateral metal gratings that provide distributed feedback is presented. A double ridge configuration was developed, where the grating is placed above the active region while a second, slightly wider ridge ensures current confinement in the active region. Side mode suppression ratios above 30 dB have been obtained, and single mode emission was observed at wavelengths around 6 μm in continuous-wave operation up to a temperature of −10 °C. Current- and temperature-tuning rates of 0.011 nm/mA and 0.50 nm/ °C, respectively, have been found, and a total tuning range of 6.5 nm has been covered by one device. As a reference, ridge waveguide devices without a grating made from the same material were able to emit in pulsed mode up to 49 °C and in continuous-wave operation at temperatures up to 0 °C around 6 μm.

Strain-compensated GaAs1−yPy/GaAs1−zBiz/GaAs1−yPy quantum wells for laser applications

GaAs1−zBiz/GaAs1−yPy strained-compensated quantum well (QW) structures for laser applications were grown by metalorganic vapor phase epitaxy. The band offsets for the GaAs1−zBiz/GaAs1−yPy heterojunction were calculated by the density functional theory, and the design of strain-compensated structures was undertaken by the zero stress analysis. The post-growth thermal annealing of the structures dramatically increases the photoluminescence intensity compared to that from as-grown GaAs1−zBiz QW samples. Transmission electron microscopy studies verified layer thicknesses as well as the presence of abrupt interfaces in the annealed GaAs1−zBiz/GaAs1−yPy QW structure. Electroluminescence measurements from ridge-waveguide devices show broad spectral emission characteristics and lasing was not observed up to a current injection of 4 kA cm−2.

High power continuous-wave GaSb-based superluminescent diodes as gain chips for widely tunable laser spectroscopy in the 1.95–2.45 μm wavelength range

We present high-power single-spatial mode electrically pumped GaSb-based superluminescent diodes (SLDs) operating in the 1.95 to 2.45 μm wavelength range in continuous-wave (CW). MBE grown GaSb-based heterostructures were fabricated into single-angled facet ridge-waveguide devices that demonstrate more than 40 mW CW output power at 2.05 μm, to >5 mW at 2.40 μm at room-temperature. We integrated these SLDs into an external cavity (Littrow configuration) as gain chips and achieved single-mode CW lasing with maximum output powers exceeding 18 mW. An extremely wide tuning range of 120 nm per chip with side-mode-suppression-ratios >25 dB was demonstrated while maintaining optical output power level above 3 mW across the entire tuning range.

High-Power and High-Efficiency 1.3- <named-content content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="TeX">$\mu\hbox{m} $</tex-math></inline-formula></named-content> Superluminescent Diode With Flat-Top and Ultrawide Emission Bandwidth

We report on a flat-top and ultrawide emission bandwidth of 125 nm from In- GaAsP/InP multiple quantum-well (MQW) superluminescent diode with antireflection coated and tilted ridge-waveguide device configuration. A total output power in excess of 70 mW with an average power spectral density of 0.56 mW/nm and spectral ripple � 1:2 � 0:5 dB is measured from the device. Wall-plug efficiency and output power as high as 14% and 80 mW, respectively, is demonstrated from this batch of devices. We attribute the broad emission to the inherent inhomogeneity of the electron-heavy-hole (e-hh) and electron-light-hole (e-lh) recombination of the ground state and the first ex- cited state of the MQWs and their simultaneous emission.

High-Performance 1.55- <inline-formula> <tex-math notation="TeX">$\mu\hbox{m}$</tex-math> </inline-formula> Superluminescent Diode Based on Broad Gain InAs/InGaAlAs/InP Quantum Dash Active Region

We report on the high-performance characteristics from superluminescent di- odes (SLDs) based on four-stack InAs/InGaAlAs chirped-barrier thickness quantum dash (Qdash) in a well structure. The active region exhibits a measured broad gain spectrum of � 140 nm, with a peak modal gain of � 41 cm � 1 . The noncoated two-section gain- absorber broad-area and ridge-waveguide device configuration exhibits an output power of 9 20 mW and 9 12 mW, respectively. The corresponding � 3-dB bandwidths span � 82 nm and � 72 nm, with a small spectral ripple of G 0.2 dB, related largely to the con- tribution from dispersive height dash ensembles of the highly inhomogeneous active region. These C-L communication band devices will find applications in various cross- disciplinary fields of optical metrology, optical coherent tomography, etc.

Nearly degenerate wavelength-multiplexed polarization entanglement by cascaded optical nonlinearities in a PPLN ridge waveguide device

In this paper we report the generation of wavelength-multiplexed polarization-entangled photon pairs in the 1.5-μm communication wavelength band by using cascaded optical second nonlinearities (sum-frequency generation and subsequent spontaneous parametric down-conversion, c-SFG/SPDC) in a periodically poled LiNbO(3) ridge waveguide device. The c-SFG/SPDC method makes it possible to fully use the broad spectral bandwidth of SPDC in nearly frequency-degenerate conditions, and can provide more than 50 pairs of wavelength channels for the entangled photon pairs in the 1.5-μm wavelength band, using only standard optical resources in the telecom field. Visibilities higher than 98% were clearly observed in two-photon interference fringes for all the wavelength channels under investigation (eight pairs). We further performed a detailed experimental investigation of the cross-talk characteristics and the impact of detuning the pump wavelengths.

Room‐temperature operation of InAs‐based interband‐cascade‐lasers beyond 6 µm

A report is presented on long wavelength room-temperature operation of InAs-based interband cascade lasers (ICLs) at 6.06 µm. The broad area and ridge waveguide devices operate in pulsed mode up to 25 and 40 °C, respectively, and therewith extend the available room-temperature operation range of ICLs to longer wavelengths.

Experimental investigation in transmission performance of polarization-entangled photon-pairs generated by cascaded χ^(2) processes over standard single-mode optical fibers

In this paper we report experimental investigation in transmission performance over standard single-mode optical fibers (SMFs) of polarization-entangled photon-pairs in a 1.5-μm band generated by cascaded second-harmonic generation and spontaneous parametric down conversion (c-SHG/SPDC) from a periodically poled LiNbO(3) (PPLN) ridge-waveguide device. Clear two-photon interference fringes were observed even after the transmission over 140 km of the SMF spools, remaining small degradation in the visibilities of less than 3%. The performance was also investigated by using optical attenuators, instead of the SMF spools, to study the maximum reach of the distribution of the entanglement in terms of loss penalty. The results show that the quantum entanglement could be distributed even with 50 dB of the transmission loss with violation of Bell inequality by using the c-SHG/SPDC-based photon-pair source.

15-μm band polarization entangled photon-pair source with variable Bell states

In this paper we report a polarization-entangled photon-pair source in a 1.5-μm band which can generate arbitrary entangled states including four maximum entangled states (Bell states) by using cascaded optical second nonlinearities (second-harmonic generation and the following spontaneous parametric down conversion) in a periodically poled LiNbO(3) (PPLN) ridge-waveguide device. Exchange among the Bell states was achieved by using an optical phase bias compensator (OPBC) in a Sagnac loop interferometer and a half-wave plate outside the loop for polarization conversion. Quantitative evaluation was made on the performance of the photon-pair source through the experiments of two-photon interferences, quantum state tomography, and test of violation of Bell inequality. We observed high visibilities of 96%, fidelities of 97%, and 2.71 of the S parameter in inequality of Clauser, Horne, Shimony, and Holt (CHSH). The experimental values, including peak coincidence counts in the two-photon interference (approximately 170 counts per second), remained almost unchanged in despite of the exchange among the Bell states. They were also in good agreement with the theoretical assumption from the mean number of the photon-pairs under the test (0.04 per pulse). More detailed experimental studies on the dependence of the mean number of the photon-pairs revealed that the quantum states were well understood as the Werner state.

Experimental studies in generation of high-purity photon-pairs using cascaded χ^(2) processes in a periodically poled LiNbO_3 ridge-waveguide device

We report detailed experimental studies in a single wavelength-band system of correlated photon-pair generation in a 1.5 μm telecommunication wavelength-band using cascaded χ(2):χ(2) processes, second-harmonic generation, and the following spontaneous parametric down conversion (c-SHG/SPDC), in a periodically poled LiNbO3 (PPLN) ridge-waveguide device. By using a PPLN module with 600%/W of the SHG efficiency, we achieved a coincidence-to-accidental ratio (CAR) of 2380 at 1.8×10−4 of the mean number of the signal photon per pulse. Detailed investigation on the origin of uncorrelated noise photons was also discussed in this paper. We revealed that the noise photons mainly originated from spontaneous Raman scattering induced in pigtail optical fibers and also that the PPLN device itself had poor contribution to the noise photons. This feature of the c-SHG/SPDC process is promising for the realization of a noise-photon-free, high-purity quantum entangled photon-pair source.

Room-temperature 3.73 µm GaSb-based type-I quantum-well lasers with quinternary barriers

Room-temperature operation of GaSb-based type-I lasers with emission wavelength of 3.73 µm is reported for the first time. Ridge-waveguide devices with five quantum wells and quinternary barriers for improved hole confinement lase in pulsed operation with a low transparency threshold current density of 676 A cm−2 for infinite stripe width. The presence of additional loss mechanism is observed above 20 °C, changing the characteristic temperature from 38 K to 16 K and limiting the maximum operating temperature to 32 °C.