This PDF file contains the front matter associated with SPIE Proceedings Volume 11301, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee list

A novel approach to realize DFB gratings on GaN based laser diodes is presented and single longitudinal mode operation is achieved. For lasers with plasma-etched surface gratings, single mode operation was maintained until 900 mA and the spectral width FWHM was less than 5 pm with a SMSR of more than 29 dB. Moreover, several issues limiting the performance of semipolar III-Nitride DFB laser diodes with the etched grating are also addressed in this work. Besides these first order gratings that were formed by electron beam lithography and shallow plasma etching, an improved grating design based on dielectric teeth imbedded into ITO is described, along with the design’s impact on power and spectral performance. Particularly, by utilizing the HSQ resist, we focus on reducing the high operating voltage by imbedding the grating inside the transparent conductive oxide layer without dry etching. This new design with a non-etched imbedded grating successfully reduces the threshold voltage and achieves an output power of more than 200 mW under pulsed operation from an anti-reflection coated facet.

The self-consistent 6-band k∙p calculations of AlGaInN barriers surrounding the InGaN quantum well (QW) emitting at ~495 nm show ~ 30% increase in material gain and ~ 40% reduction in threshold current density, compared to the conventional InGaN / GaN QW structure. Following the guidance of our computational study, the InGaN / AlGaInN multiple QW structures with different AlGaInN alloy compositions lattice-matched to GaN are grown via MOVPE. The use of InGaN / AlInN QW structure resulted in improved luminescence, and the results of InGaN / AlGaInN with larger compositional range will also be presented.

The development of laser diodes at visible wavelengths have revolutionized the technological era. Nowadays, specialized applications such as atomic cooling, sensing, and optical communication require not just a conventional laser but a narrow-line or single-wavelength laser source. Herein, we report the demonstration of narrow-line emission directly from commercial InGaN-based laser diodes by monolithic integration of distributed-feedback (DFB) surface grating. Our results obtained at continuous wave (CW) current injection and room temperature reveal spectral lasing linewidth in the picometer range on wavelengths between 460 nm and 520 nm.

Laser light sources have being used in sensing and medical applications, and it is required for higher power, high efficiency, good beam quality and high reliability. We demonstrated single-mode CW 200mW red laser diodes with wavelength of 660nm, 675nm, and 690nm. These were fabricated from AlGaInP-based material with narrow stripe. These showed good temperature characteristics that the highest power reached 250mW upto 70°C and 200mW at 90°C for 660nm laser, exceeded 250mW upto 90°C for 675nm and 690nm lasers. Each characteristic temperature T0 was 112K, 169K, and 201K, respectively. The wall plug efficiency exceeded 32% at 25°C. The far field pattern showed Gaussian-like single transverse mode. Stable operation at high temperature was demonstrated in 1,500hours at 60°C, CW 200mW for 660nm laser and in 1,000hours at 100°C, CW 200mW for 675nm and 690nm lasers. CW 200mW operation is the world’s first in single-mode 660nm to 690nm laser diodes to the best of our knowledge. In the case of AlGaInPbased material, although longer wavelength is concerned degrading crystal quality in the active layer due to increasing lattice mismatch, no degradation observed the characteristics and the reliability. These red laser diodes will contribute to progress for sensing and medical applications.

Visible diode lasers with wide wavelength tunability and narrow spectral linewidth are of high importance in bio-photonics and metrology. Hybrid integrated diode lasers, using waveguide circuits for spectrally selective feedback, provide wide tunability and sub-100-Hz intrinsic linewidths in a robust chip format. So far, these lasers have only been realized at infrared wavelengths. Here we present the first operation of a hybrid integrated diode laser in the visible. The laser, formed by a diode amplifier which is hybrid integrated with a Si3N4 ring-resonator based feedback circuit, is tunable over 11 nm around 685 nm and delivers 5 mW output power.

In this study, we fabricated QD laser diodes using a digital embedding method (DEM) in which InAs QDs were embedded in an InGaAs/InAlAs superlattice whose miniband acts as an effective barrier for the QDs. We stacked 15 QD layers by using DEM with four monolayers in each InGaAs/InAlAs superlattice. The stripe laser structures were fabricated using conventional laser diode processes. The laser with a 600-µm cavity showed lasing at 1600 nm with a threshold current of 474 mA. The internal loss of this laser was 16.2 cm-1, which was similar that of the laser that uses a conventional quaternary InGaAlAs barrier material.

The small and large signal responses of InP-based 1.55 μm high-speed quantum dot (QD) lasers with and without tunnel-injection (TI) quantum well (QW) and/or p-type doping in the active region (incorporating nominally identical QDs) were designed, manufactured and compared. The structures were grown by a molecular beam epitaxy system equipped with group-V valved cracker cells. In all cases, the active region consisted of six QD or TI-QD structures, which were embedded in InAlGaAs barriers lattice matched to InP. The InGaAs TI-QWs were separated by a thin InAlGaAs tunnel barrier from the InAs QDs. The laser structures were processed into ridge waveguide lasers and analyzed. The results show, that the bandwidth and maximum data rates were reduced by incorporation of TI-QWs. P-doping resulted in slightly worse performance of the simple QD laser, but in an improvement of the TI QD laser. Furthermore, the large signal response of the tunneling injection QD laser is one of the first reports of digital modulation of such a laser. An optimization of the doping profile is promising to further improve the laser performance over the undoped counterparts.

Monolithic integration of a laser diode with a modulator usually requires complicated and potentially defect-generating processing steps such as epitaxial regrowth or bonding. Alternatively, the same medium grown in a single epitaxial run can be used in both gain and electro-absorption section. We demonstrate a universal active medium with tunnel coupled InGaAs short period superlattice (SPSL) quantum well grown on InAs quantum dots (QW-on-QDs) in an AlGaAs waveguide. In this configuration, the QW serves as a tunnel injector of electrons into the QDs in the forward-biased gain section, while in the reverse-biased modulator section the InGaAs QW acts as a wavelength matched quantum Stark effect electro-absorber. The optimum QW and QD ground state separation is close to 30meV for efficient tunnel injection into QDs to increase the gain. The 3x(QW-on-QDs) double heterostructure with separate 0.3μm waveguide was grown by molecular beam epitaxy on n-type GaAs substrate. The device was processed into two-section dies with an intermediate distributed Bragg reflector (DBR) between the sections. Focused ion beam air-gap DBR fabrication with low electrical leakage and required (~30%) optical mode reflection was employed. The laser-modulator performance with slope efficiency over 50%, 6dB power modulation, linear dynamic range>24dB at 2GHz was shown with a 200μm long modulator section and low optical feedback from the modulator to laser.

The temperature behavior of operating characteristics in semiconductor lasers with a quantum-confined active region is studied with a proper account for (i) non-instantaneous capture of charge carriers from the waveguide region into the active region and (ii) internal optical loss that depends on the carrier densities. Because of (i), the carrier densities are not pinned in the lasing mode, i.e., they are functions of the injection current. In view of (ii) and as a result of pump-currentdependence of the carrier densities, so becomes the internal loss coefficient. This in turn leads to the roll-over of the light-current characteristic at high currents (i.e., decreasing optical power with increasing injection current) and, under certain conditions, appearance of the second branch in it. The laser characteristics are shown to transform qualitatively with varying temperature: they are conventional, i.e., consist of one branch, at low temperatures but they have two branches, i.e., are of a binary nature, at high temperatures. The two branches merge together at the maximum operating current beyond which the lasing quenches. In contrast to the first (conventional) lasing threshold, the threshold for emerging the second branch decreases with increasing temperature. The pump-current-dependence of the carrier densities and internal loss coefficient is also fascinating: these quantities decrease with increasing current in their second branches.

Experiments on QD lasers grown on GaAs and on Si have revealed the quenching of the GS optical power as the current overcomes the ES threshold. A common technique to mitigate this quenching is the modulation p-doping, but an excessive p-doping level results in a deterioration of the GS optical power and threshold current. Theoretical models based on rate equations have ascribed the GS power quenching to the de-synchronization between the electron and hole dynamics. However, these approaches resort to phenomenological transport times. In this contribution, we study a 1.3 um QD laser grown on silicon by employing a drift-diffusion model for the transport of carriers across the SCH region. We show that the unbalance of electron and hole mobilities in the GaAs barriers is responsible for the GS quenching. The simulations also emphasize the existence of an optimum modulation p-doping level minimizing the GS threshold current, which we ascribe to electrostatic effects induced by this doping.

Passively mode-locked InAs/InGaAs quantum dot on silicon lasers emitting at 1310nm are promising sources for high-speed high-capacity communication applications. Optical self-injection stabilization of a monolithic passively mode-locked quantum dot on Silicon laser with an absorber section length to total length ratio of 18% is investigated experimentally. A repetition rate tuning range of 24MHz around the free-running repetition rate of 9.4 GHz and a pulse-to-pulse timing jitter reduction by a factor of 2.5 from 150 fs to 59 fs are achieved for an external optical cavity length of 5.8m with fine-delay control. Obtained experimental results are in good quantitative agreement with simulation results obtained by a stochastic time-domain model.

We developed a Time Domain Traveling Wave model to properly study the dynamics of a hybrid lasers realized by coupling a III-V Reflective Semiconductor Optical Amplifier with a Silicon Photonics mirror providing a narrow effective reflectivity (<10GHz). In free running operation mode, we show that for realistic values of the Henry factor stable single mode emission only occurs around the maximum of the reflectivity slope. Very interestingly for applications, in presence of optical-feedback, we access a regime of ultra-stability with respect to unwanted reflections or to self-oscillations triggered by a photon-photon resonance phenomenon.

III-V based devices have provided unsurpassed performance enabling the rapid advancement of data-communications over the past decades. Yet the integration of these components is still primitive leading to high costs due to the packaging challenges. Heterogeneous integration using controlled release of the essential device layers from individual source wafers and engineered parallel transfer to a common platform is a very promising approach as it takes the best materials and devices, produced in a conventional foundry environment, to produce powerful photonic circuits on target waveguiding platforms such as silicon-on-insulator. The devices can be pre- or post-processed and optically integrated to the silicon waveguides using butt, evanescent or potentially grating coupling. Laser devices are the most critical since they cannot easily be realized in Si. We demonstrate the transfer of lasers based on InP quantum wells where the devices are bonded by van der Waals forces. We have also demonstrated the release and transfer of silicon microcircuits, GaN materials and dielectric layers. We study the interface property between the transferred materials (e.g. InP) and the target wafer (Si). There is an improvement in device performance after the transfer due to the high thermal conductivity of Si. This approach will allow more sophisticated circuits due to the ease of including multi-wavelength lasers as well as modulating and detecting functions along with specialty materials such as potentially lithium niobate or magnetic materials. The technique enables close integration of photonics with electronics platforms and thus a route to widespread consumer applications for III-V devices.

The monolithic integration of III-V semiconductors on on-axis silicon is currently under active consideration. In this work we propose a novel epitaxial procedure to grow high quality, anti-phase boundary free GaSb layers on on-axis Si. Broad-area laser diodes based on AlGaAsSb/GaInAsSb QWs exhibit threshold current densities lower than 1 kA.cm-2 whereas narrow-ridge lasers operate cw above room temperature. Our results open the way to the epitaxial integration of a variety of IR lasers on on-axis Si.

Hexagonal SiGe has been theoretically shown to feature a tunable direct bandgap in the range 0.4-0.8eV. We study arrays of site-selectively grown Si_(1-x)-Ge_x nanowires (NWs) grown using the crystal transfer method in which wurtzite GaP core NWs are used as template for SiGe growth. Our approach opens up routes towards photonic band-edge lasers using group-IV NWs. Low-temperature µPL studies of arrays of SiGe NW-arrays reveal strong emission at 0.395eV and linear power dependence for weak excitation levels (P_ex~0.01-1kW/cm^2). For P_ex>4kW/cm^2, a new peak emerges at 0.37eV with an intensity that increases according to ~(P_ex)^5, indicative of stimulated emission close to the photonic band-edge.

We investigate the degradation processes that limit the long-term lifetime of 1.3 μm quantum dot lasers grown on silicon substrate. The analysis is based on combined optical and electrical characterization, carried out before and during accelerated ageing tests. Specifically, we demonstrate that: (i) when submitted to constant current stress, the analyzed devices show a monotonic increase in threshold current; (ii) degradation kinetics are strongly dependent on stress current; a power-law dependence of TTF on stress current was extrapolated (TTF proportional to J^{^-3.9}). (iii) during stress time, a decrease in slope efficiency was detected, well correlated to the threshold current increase. This effect was ascribed to a decrease in injection efficiency of the devices. (iv) A detailed analysis of the degradation kinetics showed that the threshold current increase has a square-root dependence on stress time, indicating the presence of a defect-diffusion process, that degrades the properties of the active region. Finally (v), the analysis of the spectral characteristics plots indicates that stress is impacting quantum dots with high energy emission preferentially.

The results collected within this paper are explained by considering that stress promotes the diffusion of defects towards the active region of the devices. This mechanism results in a decrease in the SRH recombination lifetime, and in the subsequent increase in threshold current and drop in sub-threshold emission. An increase in the SRH rate next to the quantum dots can also reduce the injection efficiency into the QDs, thus inducing a drop in the slope efficiency of the lasers.

The results of designing, manufacturing and investigating characteristics of AlGaAs/InGaAs/GaAs lasers with ultranarrow waveguides are presented. Low threshold current density near 40 A/cm² has been observed for the lasers with quantum wells. We have demonstrated the possibility of obtaining up to 5 W of output power in continuous mode and up to 40 W in pulsed mode, with a beam convergence (FWHM) of 17.8° It is demonstrated that such lasers can exhibit main characteristics similar to conventional laser heterostructures and allow a potential for further improvement and optimization.

We present a detailed experimental investigation of 48 emitter distributed Bragg reflector laser bars, developed for LIDAR applications, that emit nanosecond pulses under pulsed high current excitation round 905 nm. The in-house developed electrical interface provides nanosecond pulses with peak currents up to 900 A. The influence of chip length, active region design, and operation parameters are investigated. Experimental results including the optical pulse power, the spectral emission characteristics, and temporal characteristics will be presented. Peak optical pulse powers exceeding 440 W at >600 A and 650 W at >900 A for 2 ns and 10 ns long pulses, respectively, at 10 kHz and 25 °C can be achieved.

We report a novel ultra-short light pulse emitters utilizing transient charge carrier behaviour in a multiple wide-quantumwell (WQW) heterostructure. The optical waveguide is implemented as a tandem-cavity laser diode with electro absorber section in the middle, surrounded by two end-firing gain sections. The ultrashort pulse production is achieved by employing the gain region with three wide GaAsP tensile strained quantum wells separated by GaInP barriers in an unintentionally doped active region of the p-i-n laser diode structure. At large negative absorber bias, lasing emission spiking starts with an unusually long delay of 7 μs. By applying the current pulses of duration smaller than 7 μs it is possible to quench entirely the lasing emission. With selection of the parameters of the electrical pump pulse and the absorber voltage it is possible to obtain ultra-short light pulse regime. This optical pulse appears at the end of the electrical pump pulse, as a single optical pulse on top of wide pedestal, due to amplified spontaneous emission. The duration of the pulse is 1.2 ps and pulse energy is 80 pJ. We attribute this behaviour to quantum confined Stark effect. Removal of the external bias field, enabling stronger overlap of carriers yields a sudden increase in the radiative recombination rate and optical gain enabling SR emission. We provide a detailed report on the pulse width and optical spectral behaviour as well as on possible nonclassical correlation in the emitted light state seen from comparison to CW lasing regime.

Electrical injection locking dynamics of a monolithic edge-emitting semiconductor quantum dash frequency comb laser are investigated experimentally by beat note spectroscopy. Spectrally resolved phase and amplitude characteristics across the 10 nm broad optical comb spectra and an inter-mode beat frequency locking dynamics are reported. A locking range of 2 MHz around the fundamental repetition rate of 20 GHz and an inter-mode beat line width reduction to the line width of the electrical radio-frequency signal source are attained.

We have demonstrated an operation and 65-GHz bandwidth simultaneously for three different designs of high-speed directly modulated lasers (DMLs), including distributed reflector (DR) laser, modified-grating DBR laser, and DFB laser with a reflection from an integrated passive waveguide. The DMLs maintained a RIN lower than -155 dBc/Hz under an optical reflection of as high as < -8 dB. The device physics behind the operation relates to the cavity design which has a strongly wavelength-dependent mirror loss. Modified-grating DBR laser was used to demonstrate an error-free 100 GBd PAM2 transmission, and also PAM4 transmission. This is the record fast data rate realized by DML as of today.

In this study, the 1.3 μm wavelength range super structure grating (SSG-) distributed Bragg reflector (DBR) laser was experimentally demonstrated for the first time. The use of 1.3 μm wavelength range tunable DBR lasers has been barely reported because of their small refractive index change compared with that of the 1.55-μm wavelength range, although 1.3 μm is a major wavelength range for optical communication. One reason is that increasing the carrier concentration in the InGaAsP core layer in the DBR sections is difficult. This is because the bandgap energy of the core layer for 1.3-μm wavelength range lasers should be relatively large compared with that of 1.55-μm wavelength range lasers, and it reduces the band offset between core and InP claddings. To efficiently trap the carrier inside the core layer, we introduced InAlAs carrier confinement layers (CCLs) to both boundaries between the core layer and cladding layers. As a result, the 1.3 μm wavelength range SSG-DBR laser was successfully demonstrated with a 4.9-nm quasi-continuous wavelength tuning range by using the single SSG-modes. Furthermore, the total wavelength tuning range of 31 nm and stable single mode operation for the entire tuning range were achieved. The introduced CCLs significantly enhanced the refractive index change due to the carrier-plasma effect and thus we can successfully demonstrate wide range tuning of the 1.3-μm wavelength range SSG-DBR laser.

Inclusion of a mirror based on Fano interference in photonic crystal lasers has shown rich dynamics, including controllable self-pulsing, extraordinary feedback stability and pinned single-mode lasing. The narrowband Fano mirror also results in a strong response to tuning of the nanocavity resonance, so that the Fano laser enables realisation of a variety of on-chip optical pulse generation and manipulation schemes. In this work initial numerical investigations of switching with Fano lasers is presented, demonstrating deterministic generation of optical pulses with pulsewidths in the few ps range at GHz repetition rates, as well as equalisation and transistor-like operation.

Timing stabilization of a photonic integrated circuit extended cavity passively mode-locked semiconductor ring laser with four gain sections and two saturable absorbers in a symmetric ring geometry by optical self-injection is presented. The laser has been fabricated using an InP generic integration technology platform. Repetition rate tuning up to 5.5MHz and a timing jitter reduction by optical self-injection from 99 fs (solitary laser operation) to 20 fs is demonstrated. The experimental results are in excellent agreement with results obtained by a stochastic time domain model which yet had been solely applied to edge-emitting straight waveguide semiconductor lasers.

By applying a recently proposed coupled-Bloch-mode approach, we have derived the resonance condition for the longitudinal modes of passive photonic crystal (PhC) line-defect cavities. We have derived simple expressions for the electric field depending on the size of the cavity and the order of the resonant mode. We have shown that, as the cavity becomes longer, the fundamental mode turns from FP-like to DFB-like and the fraction of its wavevector components within the light cone is gradually suppressed. Importantly, we have clarified the physical origin for this behaviour.

Photonic crystal surface emitting lasers (PCSELs) are a new class of laser diode, offering control over emission (wavelength, polarisation, beam shape) through photonic crystal design, as well as power scalability and low beam divergence. We present developments in 2D arrays of large scale (~150x150um) PCSELs, coherently coupled by 1mm long, 100 μm wide waveguides that can be electrically driven into loss or gain. By studying the spectral and current-power characteristics, we show coherent power scaling between multiple devices. We discuss injection locking between devices achieved through controllable 2D in-plane feedback and its effect on the near and far field emissions.

We outline advances in the design, manufacture, and characterisation of a range of all-semiconductor photonic crystal surface emitting lasers (PCSELs). We initially discuss AlAs/GaAs based PC devices that enjoy improved index contrast compared to our previous GaInP/GaAs devices. We will also discuss all-semiconductor InGaAsP/InP based PC PCSELs operating at 1550 nm. For these high aspect ratio structures, the infill of the PC is a significant epitaxial challenge, and optimisation of MOVPE growth conditions such as growth temperature, rate of temperature increase, and V:III ratio, is demonstrated through TEM structural analysis. We will also discuss the optoelectronic properties of our devices.

We present evaluation results of the 940nm 400mW transverse single-mode laser diodes (LDs) with real reflective index self-aligned (RISA) structure based on graded index separate confinement hetero structures (GRIN-SCH) for a three-dimensional (3D) depth sensor. The AlGaAs/InGaAs laser diodes that are adopted with RISA structure have many advantages over conventional complex refractive index guided lasers, what include low operating current, high temperature operation and stable fundamental transverse-mode operation up to high power levels.

Simultaneously, the RISA process is easy to control the waveguide channel width and does not require stable oxide mask for the regrowth of aluminum alloys, so it is possible to manufacture high output power and high reliability laser diodes.

At the optical power 400mW under the continuous-wave (CW) operation, Gaussian narrow far-field patterns (FFP) are measured with the full-width at half-maximum vertical divergence angle of 23°. A threshold current (I_th) of 33mA, slope efficiency (SE) of 0.81mW/mA and operating current (I_op) of 503mA are obtained at room temperature. Also, we could achieve catastrophic optical damage (COD) of 657mW and long-term reliability of 60°C with TO-56 package.

Ultra-narrow linewidth, ultra-stable lasers are the heart of precision high-end scientific systems. In this talk we describe a new class of photonic integrated stimulated Brillouin scattering laser that enables moving these systems to the chip-scale and their application to data center communications, atomic clocks, optical gyros and microwave synthesis.

Recent years have witnessed an inflow of ideas from quantum field theory and condensed matter physics into the field of optics. In this talk, we describe how notions from topological physics and supersymmetry can be used in designing novel laser arrays with properties that are of interest in some applications. Likewise, such lasing arrangements can be used to emulate a variety of topological and supersymmetric phenomena, beyond what is possible in their respective original platforms.

This Conference Presentation, Towards the experimental demonstration of topological Haldane lattice in microring laser arrays was recorded at Photonics West 2020 held in San Francisco, California, United States.

This Conference Presentation, Towards electrically pumped topological insulator lasers was recorded at Photonics West 2020 held in San Francisco, California, United States.

Changing the length of the cavity is perhaps the simplest way to tune the wavelength of a laser, but is almost never used for continuous tuning over a large fractional range. This is because, to avoid multi-mode lasing and mode hopping, the cavity must be kept optically short to ensure a large free-spectral-range compared to the gain bandwidth of the amplifying material. The metasurface VECSEL architecture is shown to be an effective approach for widely tunable lasers based upon cavities that operate on low-order longitudinal modes. Since the gain resides in the amplifying reflectarray metasurface, and not a bulk medium, there is no gain/loss penalty to making the cavity length wavelength scale. Fractional tuning of a THz quantum-cascade laser up to 25% is observed in a multi-mode regime, and up to 19% in a single-mode regime with high quality beam pattern. We discuss the fundamental limits to broadband single-mode tuning using this approach.

Thanks to their ultrafast gain dynamics, a unique feature among semiconductor lasers, quantum cascade lasers exhibit strong coherent oscillations of the population inversion with concomitant formation of mode skipping frequency combs (harmonic state) and time dependent spatial hole burning leading to new microwave transmitters (laser-radios). Finally we show that the frequency comb formation in QCLs obeys a simple variational principle, that relies on the maximization of the laser output power.

Most comb research is focused on the generation of pulses. However, frequency combs can also exhibit a very different behavior that is characterized by a continuous output intensity – the frequency modulated (FM) comb regime. Here, we present our theory including a new master equation to describe the involved physical mechanisms and explain which conditions need to be fulfilled to generate self-starting FM combs. Using our new insights we will discuss experimental observations of FM combs in quantum cascade lasers, as well as lasers with slower dynamics, such as interband cascade, quantum well and quantum dot laser.

WE WILL REVIEW RECENT ADVANCES IN THE REALIZATION OF WIDE BANDWIDTH, HIGH PERFORMANCE. FREQUENCY COMB SOURCES BASED ON QUANTUM CASCADE LASERS OPERATING BOTH IN THE THz AND MID-IR REGIONS OF THE E. M. Spectrum. In the Mid-IR, a grating compressor is employed to obtain pulses from a quantum cascade comb operating in CW. IN THE THz, COPPER-BASED DOUBLE METAL WAVE GUIDES ALLOW COMB OPERATION ABOVE LIQUID NITROGEN TEMPERATURE WITH RELATIVE Comb BANDWIDTHS OF 25%. Frequency COMB CONTROL BY MEANS of RF INJECTION AND COUPLED CAVITY SCHEMES WILL BE PRESENTED TOGETHER WITH NEW COMB characterization TECHNIQUES.

The road towards the realization of quantum cascade laser (QCL) frequency combs (QCL-combs) has undoubtedly attracted ubiquitous attention from the scientific community, as these devices promise to deliver all-in-one (i.e. a single, miniature, active devices) frequency comb (FC) synthesizers in a range as wide as QCL spectral coverage itself (from about 4 microns to the THz range), with the unique possibility to tailor their spectral emission by band structure engineering. For these reasons, vigorous efforts have been spent to characterize the emission of four-wave-mixing multifrequency devices, aiming to seize their functioning mechanisms. However, up to now, all the reported studies focused on free-running QCL-combs, eluding the fundamental ingredient that turns a FC into a useful metrological tool. For the first time we have combined mode-locked multi-frequency QCL emitters with full phase stabilization and independent control of the two FC degrees of freedom. At the same time, we have introduced the Fourier transform analysis of comb emission (FACE) technique, used for measuring and simultaneously monitoring the Fourier phases of the QCL-comb modes. The demonstration of tailored-emission, miniaturized, electrically-driven, mid-infrared/THz coverage, fully stabilized and fully-controlled QCL-combs finally enables this technology for metrological-grade applications triggering a new scientific leap affecting several fields ranging from everyday life to frontier-research.

Interband cascade laser (ICL) optical frequency combs are promising midwave infrared sources for dual comb spectroscopy probing the strong fundamental absorption lines of numerous chemical and biological agents. In this work, a 4-mm-long ICL frequency comb emitting at 3.3 μm was operated by single-cavity optical self-injection. The experiments employing a free-space optical self-injection length of 1.1m with fine-delay control demonstrated a tuning range of 119MHz around the free-running intermode beat frequency of 9.58 GHz. For resonant fine-delay the line width of the intermode beat frequency was reduced to 390 kHz, what is an improvement by a factor of 40 in comparison to the solitary laser line width.

GaSb based types of diode lasers may cover the spectral regime from below 1.8 μm up to 5 μm. For the wavelength regime of 1.8 μm to 2.5 μm InGaAsSb/GaSb MQW material is used. For 2.5 μm to 3.4 μm InAlGaAsSb/GaSb MQW material is used. For above 3μm, an ICL type of design is required. We realized a growth campaign of 10 GaSb based wavers for covering the wavelength regime from 1.9μm to 3μm. We report on the test, performance and applications results in molecular gas sensing of both, gain chips within an external cavity laser as well as on digital DFB lasers.

An overview is presented on the developments in semiconductor design and technology needed to reliably and reproducibly realize 1-cm GaAs-based diode laser bars with optical output in the kilowatt class, summarizing studies performed at the Ferdinand-Braun-Institut and elsewhere. In the more than ten years since the first demonstration in 2007, the operating conversion efficiency of kilowatt-class 1-cm laser bars has improved from 35% to over 65% and lateral far field has reduced from < 20° to below 9° (at 95% power) in quasi-continuous-wave testing. Greater than a kilowatt output is also demonstrated in continuous wave testing with ever-increasing efficiencies and brightness. In addition, when tailored for cryogenic operation, peak power increases to at least 2000 W and operating efficiency to above 70%. The highest performance is seen in the 940 nm to 980 nm wavelength range, with kilowatt operation also confirmed in the 780 nm to 900 nm range. We review the diode laser design and technological steps that were needed to enable steady performance progress, including both epitaxial design development (for low optical losses and reduced power saturation) and advances in 1-cm bar and packaging technology (for low stress, low electrical and thermal resistance). We also review status on efforts to increase power in other wavelength ranges, and discuss the path to further performance improvements.

Semiconductor laser diodes with a tapered gain region provide a beam quality near to the diffraction limit combined with high output power. They can be configured as lasers with a high-reflectivity coating on the rear facet as well as amplifiers with an antireflection coating on both facets. In amplifier configuration, they can be used in external cavity or Master-Oscillator-Power-Amplifier configuration with the advantage of a narrow linewidth. Today amplifiers are commercially established with an optical output-power of 1-3W in a wide range of applications in quantum optics, metrology or spectroscopy. By extension of the resonator length up to 5mm combined with optimised processing and coating a new class of high-power tapered amplifiers at different wavelengths between 750nm and 1060nm for master-oscillator- power amplifier configurations of 4-5W output power will be presented. In addition, the tapered concept has been successfully transferred to InP and GaSb based material systems to address the eye-safe spectral range between 1500 and 2000nm. Nearly diffraction limited tapered amplifiers and lasers will be demonstrated in the 1W power range for 1530nm, 1550nm and 1930nm.

A micro-integrated diode laser based dual-wavelength master oscillator power amplifier (MOPA) at 785 nm is presented. The device is realized on a 5 x 25 mm² micro-optical bench and consists of a Y-branch distributed Bragg reflector ridge waveguide (RW) diode laser as MO with a front facet reflectivity of 30%, micro-cylindrical lenses for beam shaping and a tilted RW amplifier as PA. This approach allows power scaling of 785 nm dual-wavelength diode lasers that have already been applied for Raman spectroscopy and terahertz frequency generation. The optical concept is designed to reduce unwanted optical feedback to the MO and avoids integrating an optical isolator, which was used in a previous tabletop configuration.

At T = 25°C and 20 mW pump power, diffraction limited laser emission with 0.5 W optical output power and beam propagation parameters of 1.3 (M²_4σ) are obtained. At both emission wavelengths of 784.6 nm and 785.2 nm, spectral bandwidths below 0.02 nm at full width at half maximum and side mode suppression ratios of 30 dB are measured. A negligible wavelength shift of < 0.02 nm/A between threshold and maximum power corresponds to a temperature rise during operation of only 0.3 K. This indicates a low thermal influence from the PA to the MO and allows a free choice of excitation power for applications. Compared to previously reported free-running Y-branch diode lasers, the MOPA does not show lateral spatial tilts between the two far field intensity distributions at both wavelengths.

This compact MOPA allows addressing applications such as shifted excitation Raman difference spectroscopy under in-situ conditions and confocal Raman microscopy without the need of a spectral recalibration during the measurements. In addition, simultaneous dual-wavelength operation also enables terahertz frequency generation.

The effect of the number and position of AlInAs energy barrier layers on the output characteristics of high-power multimode AlGaInAs / InP lasers, spectral range 1400–1600 nm, has been studied. It was shown that the use of energy barriers allows increasing the laser maximum output power 1.5-2 times. It was found that the barrier layer should be installed at the waveguide-cladding heterojunction from the p-side in order to localize electrons in the waveguide layer. The study was supported by the Russian Science Foundation, project No. 19-79-30072

Among the work done to improve laser performance by optimizing the efficiency of vertical laser structures and thermal management, the spectral quality of the emitted light is a key criterion for the usability of the device. Especially the exact adjustment of the emission wavelength, a small linewidth and a preferably small dependence of the wavelength on temperature changes are important for many applications. These requirements are met by implementing surface gratings into the devices. In the first part of the paper the theoretical tools which are used to simulate and optimize grating reflectivity are presented and discussed. 2D and 3D simulation models confirm possible high reflectivities (more than 90 %) of surface Bragg gratings, if the duty cycle is high (< 0.9). Wafer stepper i-line lithography is particular useful for high volume fabrication with a simple resist process. In order to ensure a sufficiently large duty cycle, high Bragg orders (> 9 @ λ = 630 nm) are required. Electron beam lithography allows better design flexibility and larger etch depth tolerances, but requires a more complex etch mask preparation. With both techniques gratings up to a depth of 1.7 μm can be etched, which is sufficient to reach a high reflectivity for typical AlGaAs based laser structures. A single grating can be thermally tuned up to 7.5 nm. As an example for high power applications a 6 mm long distributed feedback broad laser with a stripe width of 30 μm as part of a spectral beam combining setup is presented. This device achieved a maximum output power of 6 W and a spectral width of 0.3 nm.

Surface–trapped electromagnetic waves can be localized at a boundary between a semiconductor distributed Bragg reflector (DBR) and a homogeneous dielectric medium or air. These waves enable a novel class of both in–plane–emitting and vertically–emitting optical devices including edge–emitting lasers, disk microlasers, near–field fiber–coupled lasers as well as vertical cavity surface emitting lasers (VCSELs). We show that the surface–trapped modes can be controlled by tuning the thickness of a single DBR layer. Diagrams in variables “wavelength – thickness of the control layer” are constructed for both TM and TE optical modes revealing the parameter domains, in which surface–trapped modes exist. The domains contain cusps, in the vicinity of which surface–trapped modes are allowed only in a narrow spectral region, enabling wavelength–stabilized operation of a laser. For an edge–emitting structure designed for lasing at ~1 μm, the lasing wavelength shifts upon temperature at a rate ~0.07 nm/K. The fraction of the optical power of the surface–trapped mode accumulated in the air can reach ~60%. In oxide–confined VCSEL structures the surface–trapped mode can be used for engineering of the interaction with the VCSEL lasing modes. Deposition of an effective (3λ/4)–thick additional layer on top of the top DBR of the VCSEL allows surface–trapped modes to reach the wavelength of the VCSEL lasing modes. Interaction of these two types of generally non–orthogonal optical modes results in the lateral leakage of the VCSEL emission. Mapping of the VCSEL wafers in areas with the variable aperture diameters D shows non–monotonous behavior of side mode suppression ratio (SMRS) versus D oscillating in the range from 7 dB to ~30 dB with three clearly revealed maxima in the SMSR at particular aperture diameters varied in the range from ~3 μm to ~5 μm. Similar oscillatory behavior was previously predicted for a different type of leaky VCSELs. VCSELs with SMSR above 20 dB are tested for data transmission over a multimode fiber (MMF). 40Gb/s open eye data transmission over 1.4 km OM5 MMF without pre–emphasis or equalization is demonstrated is such device.

We report on high performance Terahertz Quantum Cascade Lasers with InGaAs and GaAs active regions. Modified doping profiles derived from symmetric structures allowed achieving record output powers of double metal InGaAs/InAlAs THz Quantum Cascade Lasers. The increase of the Al concentration of the barriers in GaAs/AlGaAs devices helped to increase the operating temperature to above 191 K while keeping the threshold current low. This has enabled laser operation by thermoelectric cooling which is very important for application. We demonstrate laser wavelength switching by magnetic field and wavelength selection in Random THz Quantum Cascade Lasers by spatially controlled near-infrared excitation

We report on development of the mid-infrared antimonide based laser technology targeting dual wavelength operation for intra-cavity difference frequency generation. The devices utilize Y-branch architecture with high order DBR reflectors controlling the laser emission spectrum. The device active region contain asymmetric tunnel coupled quantum well with built in resonant second order nonlinearity. The epitaxially regrown photonic crystal surface emitting GaSb-based lasers will be demonstrated.

This Conference Presentation, High-brightness GaSb-based quantum-well lasers with an unstable resonator was recorded at Photonics West 2020 held in San Francisco, California, United States.

Interband Cascade Lasers (ICLs) are gaining field in molecular sensing thanks to their compact sizes and low-power consumption. They emit in the 3-6 μm range, and are valid alternative to QCLs especially for on-field measurement. Their suitability for high-resolution spectroscopy and metrology critically depends on their frequency stability and linewidth.
Here we investigate this issues, reporting experimental studies of the frequency noise and intrinsic linewidth of an ICL at 4.6 μm. The main differences with respect to other semiconductor lasers are discussed, as well as quantum-limited operation achievable using low-noise current drivers and frequency locking to high-performance optical resonators.

Grating-coupled, surface-emitting (GCSE) quantum-cascade lasers (QCLs) offer a pathway towards realizing watt-range, surface-emitted output powers in the mid-infrared spectral region with high beam quality. Previously we have reported wide-ridge GCSE QCLs which employed metal/semiconductor, 2nd-order distributed feedback (DFB) gratings with distributed Bragg reflector (DBR) terminations. We report here on the lasing characteristics of narrow-ridge (~7 μm-wide) GCSE devices, which employ the STA-RE-type active-region design, for obtaining single-spatial-mode both laterally and longitudinally. The QCL structure was grown using Metalorganic Chemical Vapor Deposition (MOCVD) and the grating was defined using a combination of e-beam lithography patterning and wet-chemical etching, and the ridge (~7 μm) was dry-etched. The total length of the DFB + DBR regions is 5.1 mm, and was electrically isolated in the DBR regions by employing AlOx. Due to resonant coupling of the guided light to the antisymmetric surface-plasmon modes of the 2nd-order grating, the antisymmetric (A) modes are strongly absorbed; thus, allowing for the symmetric (S) mode to be favored to lase. Initial devices have demonstrated maximum pulse output power from the surface of ~150 mW at 4.88 μm, with only ~10% power emitted from the edge facets. An anti-reflective (AR) coating of a quarter-wavelength Y₂O₃ layer was applied on the emission window, drastically improving the far-field beam pattern, that resulting in a central, near-diffraction-limited single-lobe beam pattern. COMSOL simulations were performed to further optimize the SE-base design for high CW performance. Parameter sweeps of cladding-layer thickness, grating height, and grating duty cycle were performed, which identified design tradeoffs for the various structural parameters.

We report on a comparison between the piezoelectric and interferometric readouts of vibrations in quartz tuning forks (QTFs) when employed as sound wave transducers in quartz-enhanced photoacoustic trace gas sensors. We demonstrate the possibility to properly design the QTF geometry to enhance interferometric readout signal with respect to the piezoelectric one and vice versa. When resonator tubes are acoustically coupled with the QTFs, signal-to-noise ratio enhancements are observed for both readout approaches. These results open the way to the implementation of optical readout of QTF vibrations in applications where external electromagnetic field could distort the piezoelectric signal.

In this work, a comparison of self-mode-locking of a 100 GHz repetition-rate monolithic diode as a stand-alone laser source and whilst employed in an external cavity arrangement at 1550 nm is reported. We operated our chip at a forward current slightly above the monolithic chip's lasing threshold and compensated the chirp by a single mode fiber. Ultrashort pulses with 1 ps pulse-width were generated. Changes in the dispersion compensation parameters due to the changed cavity dispersion were analyzed.

Studies of multimode and single-mode semiconductor lasers with a surface distributed Bragg reflector (S-DBR) were carried out. S-DBR with a period of 2 μm was formed in the upper cladding layer by contact photolithography. The spectrum width for all laser designs did not exceed 0.3 nm both at continuous wave (CW) and pulse of 100 ns pump. Temperature stability of emission wavelength increase from a value of 0.35 nm/°С for a Fabry-Perot laser to a value of 0.075 nm/°С for a S-DBR laser was demonstrated. The relatively low output optical power of high-order S-DBR lasers is associated with the presence of diffraction modes emitting from the surface of the DBR.

Intermode beat frequency and line-width stabilization of a 1 mm long self mode-locked frequency comb quantum dash laser emitting at 1535 nm by external all-fiber based single-cavity optical self-injection time-delay control is presented. Self-injection optical delay tuning is conducted by piezo-crystal single-mode fiber stretching. An intermode beat frequency control by 2 MHz and an intermode beat frequency line width reduction from 67 kHz to approximately 900 Hz is achieved. The experimental results are confirmed by an independent free-space stabilization experiment and stochastic time-domain modeling.