This PDF file contains the front matter associated with SPIE Proceedings Volume 10939, including the Title Page, Copyright information, Table of Contents, Introduction, Author and Conference Committee lists

Dilute nitride technology has significantly progressed towards fulfilling their exceptional promises for the development of optoelectronic devices. Current technology level renders possible practical applications. At the same time it is increasingly becoming apparent that dilute nitrides are poised to make a stronger impact on other applications areas than the telecom field, which motivated many of the initial developments related to this material system. For example, features such as uncooled operation of laser diodes and semiconductor amplifiers could have a strong impact on the development of integrated circuits where the high level of integration and the thermal conditions would make uncooled operation of the III-V components extremely important. The benefits of dilute nitride technology have also been leveraged to high power lasers, in particular emitting within the 1150-1240 nm window, which is important for reaching yellow-orange domain via frequency doubling. The presentation reviews state-of-the-art developments of GaInNAs optolectronic technology with a focus on laser diodes and components with in-plane architecture. Highlights include laser diodes emitting more than 500 mW in single-mode operation near 1.2 µm transparent band of Si. A very important feature addressed is wavelength and power stability, which are kept almost constant in a temperature range extending to 80 °C. As a more advanced approach towards monolithic integration of GaAs light emitters on Si, we review the progress in developing 1.3 µm GaInNAs laser diodes on Ge/SiGe platform.

A comparison between QD lasers with and without tunnel-injection QW designs was performed. In both cases, six layers of a QD or TI-QD design were grown by molecular beam epitaxy equipped with group-V valved cracker cells. The InAs QDs are embedded in InAlGaAs barriers lattice matched to InP. The TI-QW consists of InGaAs separated by a thin InAlGaAs tunnel barrier. The lasers were processed into broad area and ridge waveguide lasers. Both laser designs exhibited high modal gain values in the range of 10-15 cm⁻¹ per dot layer. The static and dynamic device properties of the different QD laser designs were measured and compared against each other.

European Space Agency (ESA) considers Mode-Locked Semi-Conductor Lasers as a promising technology for precision metrology systems in space such as High Accuracy Absolute Long Distance Measurement. We report our progress towards challenging ESA requirements: picosecond pulse duration, pulse energy 200 pJ, Pulse Repetition Frequency (PRF) 1-3 GHz, PRF stability < 5·10^-9 at 1 second and PRF tunability 20 MHz. The laser should have small power consumption, be compact and robust against launch vibrations. We have reported in the past two such mode-locked (ML) laser diodes, each reaching only 90 pJ pulse energies: (i) very long (13.5mm) monolithic tapered laser and (ii) inverse bow-tie external cavity (EC) laser. The subject of the present communication is a novel passively mode-locked monolithic tapered laser achieving 201 pJ pulses. Large optical cavity with 2QWs heterostructure provides a low internal loss (~1 cm^-1) together with high quantum efficiency (< 90 %) and low series resistance. To reach high energy output pulses, the tapered gain section gets a low (< 0.1 %) reflectivity dielectric coating. For passive mode-locking at fundamental cavity frequency, the saturable electroabsorber section is located at the back side of the gain chip with a high reflectivity coating (< 95 %). The monolithic cavity is made 13.5mm long by introducing an intermediate section for PRF tuning around 3 GHz. We reached passive ML at 2.9 GHz PRF with pulse energy of 201 pJ, compressed pulse width of 2.6 ps and electric power consumption of 8.2 W. PRF can be continuously tuned by 9.8 MHz. Active current modulation for hybrid ML resulted in PRF relative stability at 9.16·10^-10 level on 1s intervals, while with a phase lock loop (PLL) acting on the DC gain section current we reached PRF stability of 1.15·10^-10 on 1 s measurement interval.

Semiconductor DFB or DBR lasers with narrow linewidths are of outmost importance for a variety of applications, the most important ones being communication and LIDAR. Conventional single mode lasers based on quantum wells have linewidths of the order of one to a few MHz; reducing the linewidth requires the addition of an external feedback, and a stabilization scheme by means of some control circuitry which enables to reach linewidths of about 100 kHz. A much better solution is a diode laser chip that can provide very narrow linewidths without the need for complex external additions. Recent works suggest that such lasers are possible provided that the gain medium comprises quantum dots (QDs). This paper describes the spectral properties of state of the art 1550 nm InAs/InP QD DFB lasers grown by solid source MBE and comprising five highly uniform dot layers. The linewidth of these lasers was tested using delayed self-heterodyne as well as by beating against a highly stabilized optical frequency comb. The lasers exhibit linewidths of the order of 20 kHz at room temperature and below 70 kHz at 80 degrees C.

Extensive research has established organic-inorganic hybrid perovskites as a promising material for optoelectronic device applications. Especially for lasers, many optically driven amplified spontaneous emission and lasing reports have been demonstrated across the near infrared to the green in various configurations (distributed feedback (DFB), distributed Bragg reflectors, photonic crystals, etc) and pumping regimes (from pulsed to continuous-wave excitation) at low threshold, allowed by the tunable bandgap energy by controlling halide stoichiometry or material dimensionality. However, most reports lack red-emitting lasing action at room temperature due to undesirable halide phase separation that produces iodide-rich and bromide-rich domains in mixed-halide perovskites under intense illumination, resulting in a ‘red gap’ problem where bandgap becomes pinned to the lower iodide-rich phase. Here we demonstrate a red-emitting 2nd-order DFB laser operating at room temperature from solution-processed organic-inorganic hybrid perovskite thin films for the first time. Ultra-flat mixed-halide perovskite layers were prepared on a quartz grating to achieve single-mode surface-emitting laser emission for at least a few tens of minutes (~ 10^6 pulses) with a threshold of 85 µJ/cm^6 and full-width-half-maximum of less than 1.5 nm under picosecond-pulsed optical excitation. We effectively suppress phase separation by properly choosing self-assembled long-chain organic ammonium halide additives to enable stable lasing action over a wide range of wavelengths near the red. Our results provide a significant step towards full-color visible laser device applications from cost-effective halide perovskite material systems.

Photonic-crystal surface emitting lasers (PCSELs) exhibit several unique features compared to conventional vertical cavity surface-emitting lasers (VCSEL) including scalability, high power, and high beam quality. They are promising candidates in power-demanding applications such as free-space sensing. Motivated by the experimental advances, there have been significant efforts in developing the simulation tools for PCSELs. In particular, a coupled mode theory (CWT) model for PCSEL was developed, which provides important insights into the operating mechanisms. However, CWT makes several uncontrolled approximations such as a small number of waveguide modes as the basis, which is difficult to justify since the index contrast in PCSELs can be quite large. Here we show that PCSELs, especially its lasing threshold, can be simulated from first-principles by using rigorous coupled-wave analysis (RCWA). Traditionally, RCWA has been widely used to simulate the transmission or reflection of such structures, where the frequency is real. Here, we use RCWA to calculate the scattering matrix (S-matrix) of PCSELs on the complex frequency plane. This approach builds upon the concepts from steady-state ab initio laser theory (SALT). On the complex frequency plane, the poles of the S-matrix correspond to various resonances of the structure, including Fabry-Pérot resonances and the guided resonances. We gradually increase the material gain in the PCSEL, and the threshold is retrieved as the gain value for which the first pole crosses the real axis. Other important characteristics such as the quality factor of each mode and the power efficiency of the laser can be computed as well.

Replacing a conventional mirror in a photonic crystal laser by one based on Fano interference leads to rich laser dynamics, including realization of stable self-pulsing and potential for ultra-fast modulation. In particular, the narrowband Fano mirror guarantees single-mode operation and significantly alters the modulation response compared to Fabry-Perot lasers. In this work the small-signal response is analyzed using a dynamical model based on coupled-mode theory and rate equations, which shows how the 3-dB bandwidth of the frequency modulation response may exceed tens of THz, orders of magnitude larger than for conventional semiconductor lasers.

Small semiconductor lasers have attracted a wide interest in academia owing to their potential as highly integrated components in photonic circuits. Particularly, microdisk lasers exploiting whispering gallery modes have been regarded as a good candidate because of ultrasmall modal volume and low threshold property. To exploit large index difference between gain and surrounding medium, microdisk resonators with an underlying post using undercut etching have been proposed and widely investigated in many previous studies. However, it has been challenging in microdisk laser to operate single mode due to the large number of exsiting whispering gallery modes. Here, we propose and demonstrate a novel subwavelength scale microdisk laser. InGaAsP multi-quantum wells microdisk is self-suspended in air by connecting bridges. The behavior of TE-like whispering gallery modes, which belong to most dominant class of mode in thin disk configuration, is both numerically and experimentally investigated. We highlight that bridge does not only provide mechanical stability, but the number of bridges can be an important factor to improve or suppress wave confinement of whispering gallery modes by protecting or breaking spatial symmetry of mode. Moreover, a suitable choice of bridges increases quality factor by up to 79% comparing to the microdisk resonator without bridges. Using this scheme, we numerically and experimentally investigate mode selectivity and further demonstrate single mode microdisk lasers operating at near-infrared telecommunication wavelength.

More and more applications are using GaN laser diodes. Visible blue laser devices are well established light sources for converter based business projection of several thousand Lumens. Additional laser-based concepts like near-to-eye projection push device requirements above heretofore limits. In 2017, threshold currents of 10 and 20mA were reported for single mode blue and green laser, respectively. We will present a drastic reduction of laser threshold of green R&D laser samples by more than a factor of 2 down to 10mA. We also will discuss turn-on delay as a limiting factor for modulation speed and spatial resolution of flying spot projection.

On the other side, new applications may occur in the near future. We will present research data on blue laser bars as a possible component for industrial applications like for materials processing. LIV characteristics are measured up to power levels of 107W. We observe power conversion efficiencies of 44% at 60W output power for our best samples.

High-speed visible light communications (VLC) has been identified at an essential part of communication technology for 5G network. VLC offers the unique advantages of unregulated and secure channels, free of EM interference. Compared with the LED-based VLC transmitter, laser-based photonic systems are promising for compact, droop-free, and high-speed white lighting and VLC applications, ideal for ultra-fast 5G network and beyond. Besides the potential for achieving high data rate free-space communication links, i.e. the Li-Fi network, laser-based VLC technology can also enable underwater wireless optical communications (UWOC) for many important applications. In this paper, the recent research progress and highlights in the fields of laser-based VLC and UWOC have been reviewed with a focused discussion on the performance of various light sources, including the modulation characteristics of GaNbased edge emitting laser diodes (EELDs), superluminescent diodes (SLDs) and vertical-cavity surface-emitting lasers (VCSELs). Apart from the utilization of discrete components for building transceiver in VLC systems, the development of III-nitride laser-based photonic integration has been featured. Such on-chip integration offers many advantages, including having a small-footprint, high-speed, and low power consumption. Finally, we discuss the considerations of wavelength selection for various VLC and UWOC applications. Comparison of infrared (IR) and visible lasers for channels with high turbulence and the study of ultraviolet (UV) and visible lasers for non-line-of-sight (NLOS) communications are presented.

Deep-ultraviolet (DUV) emitters have attracted enormous attentions for water/air purification, and sterilization. However, it is difficult to realize high-efficiency DUV emitters due to several material limitations. In addition, the band mixing effect is another crucial fundamental physics factor limiting device efficiency for 240-260 nm based on AlGaN/AlN quantum wells (QWs). Specifically, heavy hole (HH) and crystal-field split-off (CH) subbands are mixed together in this regime, which results in the insufficient conduction band to valence subband transitions and consequently the low transverse-electric (TE)- and transverse-magnetic (TM)-polarized optical gain. To resolve this issue, we have proposed and investigated the use of AlGaN-delta-GaN and AlN-delta-GaN QWs as active region. Alternatively, high-quality AlGaN substrates are being developed for potential UV applications. However, very limited analysis has been reported for AlGaN QWs on AlGaN substrates. Thus, here, we theoretically study the optical properties of AlGaN QWs on ternary substrates for 240-260 nm. The results show up to 12.16, 7.79 and 6.95 times optical gain enhancements by using Al_xGa_1-xN/Al_yGa_1-yN QWs on Al_yGa_1-yN substrate, as compared to Al_xGa_1-xN/AlN QWs on AlN substrate at 240 nm, 250 nm and 260 nm, respectively. It can be explained by the fact that the reduced strain in the active region shift HH/CH subbands crossover Al-content to lower Al-content, which ensures the topmost CH subband with large energy separation to HH subband and sufficient C-CH transition. As a result, large TM-polarized optical gain can be achieved, which indicates the great potential of using AlGaN substrate for high-efficiency 240-260 nm lasers.

Several hurdles to further enhance the performance of semipolar III-Nitride laser diodes are addressed in this work. Particularly, we focused on improving their high operating voltage by thinning the p-GaN cladding layer and utilizing a transparent conductive oxide p-contact. On-wafer optical absorption measurements showed that a further reduction in voltage with thinner p-GaN was limited by increased optical loss due to increased mode overlap with the ITO/metal anode. In separate attempts to minimize bulk-related optical losses, we implemented a new design that consisted of an AlGaN electron blocking layer (EBL) placed remotely from the quantum wells (QWs) and a low p-waveguide Mg doping profile. A very low optical loss of about 2 cm^-1 was extracted but the net improvement in differential efficiency was limited by lower internal injection efficiency due to carrier accumulation in the p-waveguide region. With an optimized design, that consisted of a lightly doped EBL close to the QWs and a UID p-waveguide, an improved light output power of 1.4 W at 1.5 A and a low threshold current density of 1.2 kA/cm² were obtained.

Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow linewidth blue laser source can be used to target the atomic cooling transition. We report on the continuous wave, room temperature operation of a distributed feedback laser diode (DFB-LD) with high-order notched gratings. The design, fabrication and characterization of DFB devices based on the (Al,In) GaN material system is described. A single peak emission at 408.6 nm with an optical power of 20 mW at 225 mA and a side mode suppression ratio (SMSR) of 35 dB was achieved. Additionally, we demonstrate the use of a GaN DFB-LD as a transmitter in visible optical communications system. We also present results from a DFB-LD optimized for laser cooling of Sr+.

Except for their primary emission, diode lasers frequently show emissions at lower photon energies. We present a study in which we record and analyze emission images of (In,Ga,Al)N-based 450 nm emitting diode lasers. Imaging is realized in the spectral ranges of two broad secondary emission bands, which are peaking in the yellow region at 580 nm (VIS) and in the infrared at 875 nm (IR). Both bands have their principal origin in the active region of the device. The VIS emission spectrum looks like the well-known yellow GaN-emission, but comes exclusively from the active region. It is very likely an electroluminescence that involves trapping of non-equilibrium carriers into defects located in the active region, followed by radiative recombination under emission of VIS photons. The IR emission involves also emission from the active region, but significant contributions are also observed in the substrate. The latter contribution could be generated by absorption of spontaneous primary emissions there. Moreover, we modelled emission images by raytracing. This allows the determination of absorption coefficients and refractive indexes of the active region, the unpumped epitaxial layer, and the substrate. The VIS signal from the active region proved to be proportional to the nonequilibrium carrier concentration. This makes it potentially interesting for analytical purposes, e.g., the imaging of carrier concentration profiles.

We consider the impact of strain and substrate orientation on the conduction band (CB) and valence band (VB) energies in (Al,Ga,In)P materials. We show that applying tensile strained GaP–rich insertions as barriers in lattice–matched heterostructures grown on high–index GaAs substrates, like (211), (322), and (111) ones, allows to overcome the main problem of (Al,Ga,In)P materials, namely the cross–over of the Γ and X minima of the CB in (In,Ga,Al)P alloy at ~55% AlInP content which hinders the possibility to extend the spectral range of the devices towards shorter wavelengths. Adding GaP–enriched insertion for structures grown on the (111) substrate allows to shift the X minimum upwards forming a potential barrier for electrons escaping the active region. Both the initial energy of the CB X minimum of GaP, the highest among III–V binaries, and the tensile strain help. For other orientations the effect is reduced or is even of opposite sign due to the splitting of the X band minimum upon applied strain. For high strain an impact on the Γ and L minima in the barrier insertion are to be considered, as both minima are strongly shifted downwards once the strained insertion is parallel to the (111) plane. Admixture of AlP helps to push the Γ and L minima to higher energies (additional 1 eV for L minimum for AlP compared to GaP) even at the expense of a smaller increase of the X band–induced barrier. Experimental results confirm the predictions, revealing lasing at 599 nm at room temperature. We show that with proper substrate orientation and the composition of the strained barriers one can strongly modify and tune the CB minima extending the spectral rage of InGaAlP lasers to wavelengths shorter than ~570–590 nm at room temperature.

In this paper, a thermally induced dielectric strain on quantum well intermixing (QWI) technique is employed on tensilestrained InGaP/InAlGaP laser structure, to promote inter-diffusion, in conjunction with cycle annealing at elevated temperature. A bandgap blueshift as large as large as ~250meV was observed for samples capped with a single and bilayer of the dielectric film (1μm-SiO₂ and 0.1μm-Si₃N₄) and annealed at a high temperature (700-1000^oC) for cycles of annealing steps. Samples subjected to this novel QWI technique for short duration and multiple cycle annealing steps shown a high degree of intermixing while maintaining strong photoluminescence (PL) intensity, narrow full wave at half maximum (FWHM) and good surface morphology. Laser devices fabricated using this technique, lased at a wavelength of 608nm with two facet power of ~46mW, indicating the high quality of the material. Our results show that thermal stress can be controlled by the engineering dielectric strain opening new perspectives for QWI of photonics devices.

In this report, we investigated the optical gain properties and lasing characteristics of a laser structure consisting of one single-layer of self-assembled InP quantum dots in Al_0.10GaInP barriers. The optical gain and absorption spectra are obtained by analyzing the amplified spontaneous emission. An internal optical loss value of 5±2 cm⁻¹ , and a maximum peak modal gain of 39.3 cm⁻¹ for a single-sheet of QD were obtained at room temperature. The influence of temperature on the gain properties was studied. A 2.24-mm-long laser with uncoated facets emitting at 660 nm was demonstrated. A low threshold current density of 281 A/cm² with an external differential quantum efficiency of 34.2% was also achieved.

The integration of optical functions on a microelectronic chip brings many innovative perspectives, along with the possibility to enhance the performances of photonic integrated circuits (PIC). Owing to the delta-like density of states, quantum dot lasers (QD) directly grown on silicon are very promising for achieving low-cost transmitters with high thermal stability and large insensitivity to optical reflections. This paper investigates the dynamical and nonlinear properties of silicon based QD lasers through the prism of the linewidth broadening factor (i.e. the so-called α-factor) and the optical feedback dynamics. Results demonstrate that InAs/GaAs p-doped QD lasers epitaxially grown on silicon exhibit very low α-factors, which directly transform into an ultra-large resistance against optical feedback. As opposed to what is observed in heterogeneously integrated quantum well (QW) lasers, no chaotic state occurs owing to the high level of QD size uniformity resulting in a near zero α-factor. Considering these results, this study suggests that QD lasers made with direct epitaxial growth is a powerful solution for integration into silicon CMOS technology, which requires both high thermal stability and feedback resistant lasers.

This paper reports on a preliminary investigation of the gradual degradation processes that may affect the lifetime of InAs quantum dot (QD) lasers epitaxially grown on silicon substrates. To this aim, a series of identical Fabry-Pérot lasers emitting at 1.31 μm have been subjected to current step-stress and constant-current aging experiments at an ambient temperature of 35°C. With the adopted stress conditions, the optical characteristics of the devices exhibited an increase in the threshold-current and a decrease in the slope efficiency. This latter process was found to be well correlated with the variation in the threshold current, suggesting that this specific degradation mode may be ascribed to a stress-induced reduction in the injection efficiency. Moreover, the linear dependence of the threshold-current variation on the square root of time observed for longer stress time highlighted the possible role of a charge/defects diffusion process in the optical degradation of the devices. Consistent with this hypothesis, the electrical characteristics of the devices exhibited an increase of the forward leakage current in the bias regime dominated by defect-assisted current conduction mechanisms. The degradation process was found to be heavily accelerated for bias values allowing excited-state operation: this peculiar behavior was ascribed to the higher rate of carriers escaping from the quantum dots that undergo Recombination Enhanced Defect Reactions (REDR) in proximity of the active region of the device.

We investigate the temperature and pressure dependence of a series of intrinsic and modulation p-doped InAs-based dot-in-well (DWELL) laser diodes grown on silicon substrates. Temperature dependence of the threshold current density (Jth) and pure spontaneous emission spectra provide an insight into inhomogeneity and non-radiative recombination mechanisms within the devices. Initial investigations showed that the intrinsic devices exhibited low temperature sensitivity in the range 170-200K. Above this, Jth increased more rapidly consistent with Auger recombination. P-doping increased the temperature at which Jth(T) started to increase up to 300K with a temperature insensitive region close to room temperature. P-doping delays the onset of carrier thermalization, leading to a high T0 but with an associated higher Jth. Temperature dependence of gain spectrum broadening was investigated by measuring the spontaneous emission spectral width parameter (1/e2) just below Jth (T). A strong direct correlation is found between the temperature dependence of peak width with the temperature dependence the radiative component of threshold, Jrad(T). At low temperature the correlation is consistent with strong inhomogeneous broadening of the carrier distribution. As temperature increases Jth reduces associated with carriers thermalizing to lower energy states. At higher temperatures homogeneous thermal broadening coupled with non-radiative recombination causes Jth to increase. Inhomogeneous broadening is more pronounced in the p-doped devices due to coulombic attraction between acceptor holes and injected electrons. A detailed analysis of recombination processes using high hydrostatic pressure and spontaneous emission in these lasers as a function of doping density will be presented and discussed at the conference.

Silicon-based optoelectronic devices have long been desired owing to the possibility of monolithic integration of photonics with high-speed Si electronics and the aspiration of broadening the reach of Si technology by expanding its functionalities well beyond electronics. To overcome the intrinsic problem of bandgap indirectness in the group-IV semiconductors such as Si and Ge, a new group-IV based material, GeSn alloy, has attracted increasing interests. The group-IV GeSn alloy has been demonstrated to become direct bandgap material with more than 8% Sn incorporation, which opens a new opportunity towards a Si-based light source with fully complementary metal-oxide-semiconductor (CMOS) compatibility. The GeSn laser contributes strongly to the progress of optoelectronic integration towards next-generation photonic integrated circuit on the Si platform, as it fills the deficiency of the efficient group-IV band-to-band lasers. Moreover, due to the tunable bandgap of GeSn, the lasing operation wavelength covers broad near- and mid-infrared range. Recently, the GeSn optically pumped lasers based on direct bandgap GeSn alloys have been demonstrated. In this work, the following aspects have been investigated: i) the novel growth strategy to obtain high Sn compositions based on spontaneous-relaxation-enhanced (SRE) Sn incorporation and the GeSn virtual substrate (VS) approaches. The maximum Sn composition of 22.3% was achieved; ii) the demonstration of GeSn optically pumped heterostructure lasers. The operation wavelength covers from 2 to 3 µm and the maximum lasing temperature is 265 K; iii) the demonstration of GeSn quantum well laser. The significantly reduced lasing threshold compared to heterostructure laser was achieved.

A common way of extracting the chirp parameter (i.e., the α-factor) of semiconductor lasers is usually performed by extracting the net modal gain and the wavelength from the amplified spontaneous emission (ASE) spectrum. Although this method is straightforward, it remains sensitive to the thermal effects hence leading to a clear underestimation of the α-factor. In this work, we investigate the chirp parameter of InAs/GaAs quantum dot (QD) lasers epitaxially grown on silicon with a measurement technique evaluating the gain and wavelength changes of the suppressed side modes by optical injection locking. Given that the method is thermally insensitive, the presented results confirm our initial measurements conducted with the ASE i.e. the α-factor of the QD lasers directly grown on silicon is as low as 0.15 hence resulting from the low threading dislocation density and high material gain of the active region. These conclusions make such lasers very promising for future integrated photonics where narrow linewidth, feedback resistant and low-chirp on-chip transmitters are required.

Si photonics technology is promising for reducing the size and cost of optical transmitters because we can use mature Si-CMOS technologies to fabricate compact Si photonics devices on a large-scale Si wafer. For the optical transmitters, integration of lasers and silicon photonic devices is essential. Recently, heterogeneously integrated devices consisting of InP-based lasers and silicon Mach-Zehnder modulators (MZMs) have been developed, where the thickness of the Si waveguide in the laser gain section needs to be ~500 nm for index matching. On the other hand, silicon waveguide thickness between 200 and 300 nm is typically used in Si photonic devices; therefore, a Si thickness transition is necessary between the laser gain section and silicon MZMs. For changing the Si thickness, additional etching, deposition, or growth of Si layers is needed. However, these are not suitable solutions because device performance would be degraded by increasing the surface roughness and thickness variations of the Si waveguide. In this work, we proposed a novel technique for integrating lasers and Si photonic devices without a Si thickness transition. We use a lateral current-injection membrane buried heterostructure (BH) as a laser gain section. This structure enables us to reduce the total thickness of the III-V region, resulting in the reduction of its effective refractive index. Therefore, the effective refractive index of the membrane BH laser can be matched to that of a 200-nm-thick Si waveguide, and the laser is suitable for integration with Si photonic devices.

We present a systematic study of Zn thermal diffusion and Si ion implantation with subsequent rapid thermal annealing as the methods to fabricate lateral p-i-n junctions in InP membranes on silicon for use in electrically pumped in-plane nanolasers. We describe in detail optimized fabrication steps, which include MOVPE growth of InGaAs/InP epilayers, 2” InP to 4” SiO₂/Si direct bonding, and several cycles of DUV lithography. Values for sheet resistance of p-InGaAs/InP and n-InP membranes are obtained, which correspond to carrier concentration levels higher than 10¹⁸ cm^-3 for both Zndiffused p-InP and Si-implanted n-InP.

Optical interconnect is essential for massive data communication in rapidly developed data center and high-performance computing infrastructures. Large bandwidth, high energy efficiency and low latency are intrinsic advantages in optics. But they are also present R&D challenges under new requirements such as low total solution cost and reliable operation in harsh computing environment. Recently we have developed hybrid microring lasers on silicon to enable high integration density, compact chip size, and potentially volume and cost-effective production in a CMOS foundry. Novel structures such as thermal shunts and hybrid metal-oxide-semiconductor (MOS) capacitors were integrated into the laser cavity to allow over 100 oC cw operation and "zero-power" laser wavelength and power control. Special CMOS driver with equalization functionality for direct microring laser modulation with good signal integrality was also designed and fabricated in a 65 nm foundry process. For the first time, we integrated all these designs and chips together to demonstrate a 5-channel hybrid transmitter with 0.5 nm channel spacing and overall 70 Gb/s direct modulation rate. A novel direct photon lifetime modulation with much larger bandwidth than conventional injection current modulation by modulating bias on the MOS capacitor is demonstrated for the first time as well. Finally we review our on-going progress on migrating the similar design from a standard quantum well laser active region to a superior quantum-dot one for further improved temperature and dynamic performance.

We have experimentally investigated the effects of sidewall corrugations on the beam quality and brightness of narrow-ridge interband cascade lasers (ICLs) emitting at λ ≈ 3.3 μm. We find that at this wavelength a corrugation period of 10 µm provides greater suppression of higher-order lateral modes than a shorter period of 2-4 µm. While the power and efficiency decrease modestly for the longer corrugation period, there is a net increase of the brightness defined as the output power divided by the beam quality factor M2 . However, the brightness degrades when the corrugation amplitude is increased from 2 µm to 3.5 µm, since lower output power offsets a relatively small improvement of the beam quality

Interband cascade lasers (ICLs) are becoming a leading semiconductor laser technology for the mid-infrared because of their high efficiency and low power consumption, especially as compared with conventional diode lasers and intersubband quantum cascade lasers (QCLs) in the wavelength range from 3-5 μm. Although a greater effort has been directed towards GaSb-based ICLs in the ~3-5μm range, recent work has highlighted the exciting potential for InAs-based ICLs for reaching longer emission wavelengths. In this work we report the development of low-threshold InAs-based ICLs with a room-temperature emission wavelength of 6.3μm. The devices were grown on n+-InAs (100) substrates by solid-source molecular beam epitaxy in a custom V90 system using valved crackers for Sb2 and As2. The ICL structures employ an improved waveguide design using intermediate AlAs/AlSb/InAs strain-balanced superlattice cladding layers surrounded by heavily-doped n+-InAs plasmonic claddings. The active region includes 15-stages with AlSb/InAs/In(0.35)Ga(0.65)Sb/InAs/AlSb type-II “W” quantum wells and optimized electron injector doping. In pulsed mode, broad-area devices lased at 300 K at a lasing wavelength of 6.26 μm and a threshold current density of 395 A/cm2 which is the lowest ever reported among semiconductor lasers at similar wavelengths. The broad-area devices lased up to 335K in pulsed mode at a wavelength of 6.45 μm. These results provide strong evidence of the potential for InAs-based ICLs as efficient sources in the mid-IR.

An InGaAs based distributed Bragg reflector (DBR) laser operating at 2 μm was fabricated and evaluated as a light source for real-time gas sensing for which mode-hop free operation throughout its lifetime is essential. We therefore performed an aging test to confirm mode-hop free operation. Current was injected into the active/SOA region and/or the DBR/phase control (PC) region at 85 °C. The changes in threshold current and wavelength were not significant after 5000 h of aging. As the wavelength shifts for DBR and PC current after aging are comparable, we confirmed mode-hop free operation of the laser throughout its lifetime as a gas-sensing light source.

We fabricated and characterized a grating-coupled external cavity laser with gain chips including self-assembled InAs quantum dots (QDs) for swept-source optical coherence tomography applications. By controlling the emission wavelength of the self-assembled InAs QDs, tunable lasing at a wavelength band of 1–1.1 μm was obtained, which represents an optimal balance between absorption and scattering in biological tissues. Straight and J-shaped edgeemitting ridge waveguides (RWGs) were fabricated on a GaAs-based waveguide layer containing four InAs QDs layers. A diffraction grating with the quasi-Littrow configuration was employed as an external cavity for the fiber-coupled diodes. Electroluminescence spectra from the QD-based diodes revealed that broadband amplified spontaneous emissions appeared in a J-shaped RWG, whereas Fabry–Perot lasing occurred in the straight RWG. The external cavity was then introduced for the diode with a J-shaped RWG, and a tuning range of 65 nm centered at approximately 1100 nm was obtained from the QD gain chip with the J-shaped RWG.

Step-wise tuning of a monolithically integrated widely tunable continuous wave semiconductor ring laser is investigated, for application in Fourier domain optical coherence tomography (OCT). The device operates around 1530 nm and was realized on an InP generic photonic integration technology platform. The laser is tuned using voltage-controlled electrooptic phase modulators with <100 μW thermal dissipation, which reduces time dependent thermal effects in the filter. Here we present a calibration method with progressively finer wavelength control steps and discuss the limits of wavelength accuracy and repeatability with respect to OCT requirements. It is shown that thermal effects due to light absorption in the phase modulators have a negligible effect on the tuning of the laser for six out of seven phase modulators. To bring the thermal dissipation of the seventh phase modulator in line with the others a design change is proposed. Wavelength switching dynamics are investigated with a numerical model of the laser. A simulation based on this model shows that it takes around 50 ns from the wavelength switching instant to establish a single mode operation with side mode suppression ratio of 30 dB.

Some spectroscopic applications require excitation light sources with dual-wavelength laser emission. For example, in absorption spectroscopy one on-resonance and one off-resonance wavelength are needed for concentration measurements. For shifted excitation Raman difference spectroscopy (SERDS), two excitation wavelengths are used to distinguish between disturbing light and Raman signals. In both cases an adjustable wavelength spacing allows optimizing the measurements according to the spectral width of the target. In addition, a tuning of the spectral distance also allows generating a tunable sum or difference frequency signal, enabling further applications.

In this paper, diode lasers with customized designs according to the spectral requirements of the applications will be presented. As basis for all devices, Y-branch diode lasers with an integrated grating for wavelength stabilization are realized. The emission of the two branches is combined in an implemented Y-shaped coupler. The bent waveguides are sine shaped S-bends. The spectral tuning is performed via implemented heater elements next to the Distributed Bragg Reflector (DBR) gratings or via the injection current when using a Distributed Feedback (DFB) grating. Powervoltage current characteristics, spectral and tuning properties will be shown.

The devices emitting at 671 nm and 785 nm are used for SERDS, whereas devices at 965 nm were tested as seed sources for pulsed master oscillator power amplifiers (MOPA) suitable for the detection of water vapor. Devices at 785 nm are also suitable for the generation of THz radiation using difference frequency generation. A widely tunable Y-branch diode laser near 972 nm is used for the sum frequency generation in an up-conversion system.

It has long been established that the combs emitted by quantum cascade lasers (QCLs) cannot be described as a train of short pulses, separated by the cavity roundtrip time. Instead, simulations made for typical device parameters suggest that, in steady state, these four-wave mixing driven combs have a constant temporal envelope, and undergo periodic rapid and complicated swings in frequency. Recent work in characterising the modal phases has revealed a state which, somewhat unexpectedly, has a simple parabolic phase profile, corresponding to a linearly chirped output field. Moreover, this phase relationship was shown to be stable over time, and to be recoverable after the laser’s power had been cycled; from the perspective of a fixed external pulse compression scheme, these last two properties are critical. In this work, we use a pair of gratings and lenses in a 4-f Martinez-type scheme to modify the phase of a high-power (~1 W) QCL comb emitted at 8.2 um with more than 100 cm-1 spectral bandwidth. By changing the position of the second grating, a parabolic phase can be added to or subtracted from the field. Employing this scheme, we demonstrate a compression of the QCL output from a 133 ps continuous wave waveform, to a train of pulses of width < 20 ps, and a peak power more than 10x that of the original. With this proof-of-principle work, we highlight the potential of the QCL system to deliver short, powerful pulses, with applications in nonlinear spectroscopy, for example.

Recently a new harmonic regime in quantum cascade lasers (QCLs) was discovered, in which a mid-infrared QCL generates an equidistant coherent comb of isolated modes separated by several hundred GHz. Here we use Maxwell equations in space-time domain coupled to a realistic active region model to analyze the physics and possible applications of the harmonic frequency combs. Our calculations show that the harmonic state can be self-supported due to four-wave mixing in a specific region of the parameter space. The properties of the harmonic state are discussed in detail. We also examine the beat oscillations of the electron populations in the active region, which can lead to current oscillations on the QCL chip and generation of (sub-)THz radiation.

Cascade pumping of type-I quantum well gain sections led to increase of the output power and efficiency of GaSb-based diode lasers operating in spectral region from 1.9 to 3.3 µm. The wide stripe multimode lasers based on cascade lasers heterostructures generate watt class output power levels up to 3 µm. The corresponding narrow ridge single spatial mode and single frequency mode distributed feedback devices generate tens of mW. The external cavity lasers utilizing gain chips based on cascade diode laser heterostructures demonstrate extra wide tuning range. The short pulse passively mode-locked lasers generate optical frequency combs.

Optical frequency combs have revolutionized the field of high resolution real-time molecular spectroscopy. Here, we demonstrate an electrically-driven optical frequency comb whose sub-picosecond pulses span more than 1 THz of spectral bandwidth centered near 3.3 mm. This is achieved by passively mode locking an interband cascade laser in a multi-contact architecture with gain and saturable absorber sections monolithically integrated on the same chip.

Optical frequency combs are coherent sources that emit a series of evenly spaced lines. In the mid-infrared, comb based spectroscopy is of particular interest and, without the need of any movable parts, will potentially lead to a breakthrough in miniaturization. Interband cascade lasers, with their low power consumption and inherent detection functionality, are an ideal candidate for practical implementations. Here, we present the generation of low-dissipation optical frequency comb utilizing interband cascade lasers. Other than one might have expected, the long lifetime of the interband transition does not automatically lead to slow gain dynamics that would favor in-phase mode-locking. We discuss why ICLs should be considered as fast gain media and why passive mode-locking is difficult or even impossible to be achieved. We applying shifted-wave interference Fourier transform spectroscopy to show that ICL frequency combs naturally favor repulsive intermode beat synchronization with the same chirped FM character recently found in QCL combs. Furthermore, we show first evidence of multiple normal modes of the intermodal beats in frequency combs and picosecond pulse generation from ICLs.

Thanks to their compactness and low-power consumption, Interband Cascade Lasers (ICLs) are emerging sources for mid-infrared (MIR) molecular sensing below 6 µm. Understanding their noise features is of fundamental importance for applications like high-sensitivity and high-resolution spectroscopy. It could unveil details of their intrinsic physical behavior and, similarly to what happened for Quantum Cascade Lasers (QCLs), lead to the development of frequency and phase stabilization techniques for linewidth reduction. In this manuscript, we discuss the importance of full frequency noise characterization for ICLs, pointing out the main similarities and differences with respect to QCLs, and we show preliminary noise measurements. The frequency noise spectrum is analyzed and discussed, and the laser linewidth over different timescales calculated.

Absolute frequency and phase control of a coherent terahertz (THz) source is desirable for high-resolution spectroscopy of atoms and molecules, coherent communications and advanced imaging techniques. Here we report on the phase control of a 2.0 THz quantum cascade laser (QCL). The QCL is optical injection locked (OIL) to an infrared frequency comb, which is generated by the successive modulation of a ‘C-band’ laser in a recirculating fibre loop. A stable microwave source defines the spacing between the comb lines resulting in the QCL being locked to an integer harmonic of the microwave frequency; the reference frequency. Within the locking range the frequency of the injection locked QCL is locked to the reference frequency, whereas the phase of the QCL undergoes a ‘π’ phase shift across the locking range. In this work, we control the phase of the QCL by introducing a phase lock loop to the OIL system to provide feedback to the QCL current forming an optical injection phase locked loop. This has several advantages over the bare OIL system: (1) the underlying frequency of the QCL is stabilised so that the QCL remains within the locking range for long periods of time. (2) The QCL frequency and phase track the microwave frequency so that the QCL may be tuned with extremely high precision. (3) By changing the locking point for the PLL the phase of QCL relative to the reference frequency could be controlled within a range 0.4π, limited by the PLL, with a constant amplitude.

Recently, modelocked THz QCLs have been shown to generate 4ps pulses using monolithically integrated Gires-Tournois Interferometer (GTI) dispersion compensation schemes. However, Fourier limit pulse trains were not achieved to date that is vital to realize shorter pulses. Here we show a Fourier-limited pulse train of 3.4ps obtained from an active-modelocked QCL by exactly matching the spectral bandwidth to that of an appropriate GTI. This is despite a spectral bandwidth that is much lower than previous demonstrations. A QCL based on a multiple stack hybrid active region was used and processed into a metal/metal waveguide. The emission frequency is centred at 3THz with a free-running bandwidth of ~0.1THz. The GTI is fabricated by etching a sub-wavelength air gap through the active region at one end of the QCL ridge. Electromagnetic simulations were performed to optimize the GTI size resulting in a 66.2µm long GTI compensating for the dispersion over a range of 0.3THz around 3.15THz. The pulse measurements are based on coherent sampling of the electric-field using electro-optic detection. A stable 3.4ps pulse train was obtained by actively modelock the THz QCL with a microwave modulation. Each pulse shows a spectrum with 0.13THz FWHM, exactly at the Fourier-transform limit. In the frequency domain, lasing action occurs only at the off-resonance condition of the GTI as this appoints the dispersion compensated region as the most favourable range for modelocked emission. In this work we attained the Fourier transform limit permitting the shortest demonstrated stable pulse train from a modelocked THz QCL.

Quantum cascade laser-based frequency combs have attracted much attention as of late for applications in sensing and metrology, especially as sources for chip-scale spectroscopy at mid-infrared fingerprint wavelengths. A frequency comb is a light source whose lines are evenly-spaced, and only two frequencies are needed to describe the system—the offset and the repetition rate. Because chip-scale combs have large repetition rates, for many spectroscopic applications is important to be able to change both parameters independently, without substantially changing the comb spectrum or spectral structure. Although it is possible to modulate both the offset and the repetition rate of a comb by tuning the laser current and temperature, both properties affect the laser by changing its index of refraction, and both frequencies will be affected. Here, we show that by integrating a mirror onto a MEMS comb drive, the dispersion and group delay associated with a quantum cascade comb’s cavity can be modulated at kilohertz speeds. Because the MEMS mirror primarily affects the group delay of the cavity, it is able to adjust the comb’s repetition rate while leaving the offset frequency mostly unaffected. Since this adjustment is linearly independent from current adjustments and can be adjusted quickly, this provides an avenue for mutual stabilization of both parameters. In addition, we show that dynamic modulation of the comb drive is able to allow the laser to recover from comb-destroying feedback, making the resulting comb considerably more robust under realistic conditions.

Distributed Bragg reflector tapered diode lasers (DBR-TPL) emitting at 1064 nm are highly efficient and narrowband light sources which e.g. can be used for the treatment of eye diseases including the two main causes of blindness worldwide (i.e diabetic retinopathy and age-related macular degeneration) by laser coagulation via second harmonic generation (SHG) to the green spectral region.

In this work, we present DBR-TPL with an additional internal DBR-grating at the output side of the tapered amplifiersection in comparison with DBR-TPL with standard facet coating of different reflectivities.

Our DBR-TPLs are based on an epitaxial structure with a large and asymmetric waveguide to realize a small vertical far field for optimal coupling into SHG-crystals. All DBR-TPL consist of a 2 mm long ridge waveguide section, which contains a 1 mm long passive DBR-grating section at the end for longitudinal mode filtering. The amplifier-section is 4 mm long and forms a taper with a full angle of 6°.

The influence of the front mirror reflectivity on laser performance including reliability is investigated. The DBR-TPL emit up to 9 W of optical output power in continuous wave operation at 25°C in a longitudinal single mode at 1064 nm and yet the spectral width remains below 20 pm. In favor of a high radiance the power content in the central lobe at a total optical output power of 8 W is greater than 75% for almost all devices. Aging tests revealed significant differences in the lifetime of the devices depending on their front facet reflectivity or length of the front DBR-grating, respectively.

Reliability step tests of two batches of 1030 nm DBR tapered diode lasers are presented. All 6 mm long devices are based on a previously published structure with a triple quantum well embedded in an asymmetric super large optical cavity. Layout variations include devices with straight waveguides and gratings, tapered waveguides and straight gratings, and straight waveguides and tapered gratings. The latter two enable enhanced DBR diffraction efficiencies, improving the spatial mode filtering within the device. Step tests are carried out with 1 W power increases every 1,000 h. In the first batch step test, devices with the all-straight design were operated at output powers from 5 W to 10 W, using a previously applied current density in the straight ridge waveguide of 75 A/mm ² . Here a demonstrated lifetime up to 5,700 h was measured, resulting in an estimated mean time to failure (MTTF) of 20,000 h at 5 W and 3,000 h at 8 W, respectively. The second batch step test contained devices with tapered waveguides or gratings. In these experiments, the ridge waveguide (RW) injection current densities were selected for highest spatial quality. The step tests were performed from 7 W to 9 W. Devices with tapered RWs had to be operated at 150 A/mm² , three times higher compared to devices with straight waveguides in this test, and failed after 2,000-2,500 h. This is a shorter lifetime compared to the demonstrated 2,600 h for devices with tapered gratings. The overall estimated MTTF of 2,800 h at 8 W was comparable to the first batch, indicating the reliability of the vertical layer structure. Intermediate measurements before failure show more or less comparable stable electro-optical, spectral and spatial performances. After failure, corresponding to a 20% current increase required for the selected optical output power, near field measurements as well as electroluminescence and front facet images indicated that internal damages in the vicinity of the front facet might have occurred. Based on changes in the spectral behavior, damages and subsequent device failures are potentially linked to damaged intersections between waveguides and amplifiers. Nevertheless, these results show the robustness of the layer structure and processed devices and indicate that straight waveguide designs, requiring low RW injection currents to achieve excellent beam quality, are to be preferred.

Laser based displays have gathered much attention because only the displays can express full color gamut of Ultra- HDTV, ITU-R BT.2020. There are many types of laser based displays. A projector with single spatial light modulator (SLM) is a main stream in the displays so far. This projector uses the laser light sources under pulse with duty of 20- 50 %. We improved a triple-emitter 638-nm high-power broad area (BA) laser diode (LD) for this application. Countermeasures were adopted to improve the output power characteristics and strengthen the facet to the fatal degradation. The improved triple emitter showed the output of 5.0 W at the injection current of 5 A under pulse with duty of 30%, case temperature of 25°C. The output was increased by 4.1% compared to the former. The peak wall plug efficiency reached to 41.5% under 25°C. The long term aging test was also performed. Mean time to failure of the improved LD was estimated 57,000 hours under the output of 3.5 W, pulse condition with duty of 30%. The value was approximately 2.6 times longer than that of the former.

Laser light source projector becomes popular due to the features of high brightness, wide color gamut and long lifetime of laser light source. One of the key performances that the market continues to request is the higher output power from the single laser diode. To meet the market demand and accelerate the laser projector installation, we developed AlGaInP based 638nm 3.5W pulse / 2.4W CW red laser diode for projector light source. We designed new chip with dual emitters in one chip with each emitter width of 75μm. The chip was then assembled into diameter 9mm TO-CAN package. The highest power at high temperature achieved 45°C 3.9W, 55°C 3.1W under pulsed operation with frequency 120Hz duty 30%, in case of CW operation 45°C 3.0W, 55°C 2.1W. The wall plug efficiency (WPE) at 25°C was reached each 43% under pulsed operation and 42% under CW operation. As a result of life test at 20°C 3.5W 5,500hours under CW operation and 45°C 3.5W 1,500hours under pulsed operation with frequency 120Hz duty 35%, the estimated life was exceeded MTTF 20,000 hours under 45°C 3.5W pulsed operation. The 3.5W pulsed operation and WPE of 43% are the world’s highest in 638nm LD to the best of our knowledge. This newly developed LD is suitable for red light source for projector.

Unwanted optical feedback is a common problem in many optical setups of laser systems. To quantify the effect, the influence of a controlled external feedback on the emission properties of a low power distributed feedback ridge waveguide and a high power distributed Bragg reflector tapered diode laser are analyzed. The measured influence of the phase dependent feedback over several orders of magnitude in feedback power attenuation on emission wavelength is discussed and compared to theory.

Lasing is reported for ridge-waveguide devices processed from a 40-stage InP-based quantum cascade laser structure grown on a 6-inch Ge-coated 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. A preliminary surface morphology analysis suggests that laser performance for QCLs-on-Si devices can be significantly improved by reducing strain for the active region layers relative to InP bulk waveguide layers surrounding the laser core. Additional experimental data on material quality, including threading dislocation density, will be presented in the talk and compared for the same design grown on three different substrates to demonstrate how material quality impacts laser performance. The three substrates to be studied are the following: a native InP substrate, a GaAs substrate (~ 4 % lattice mismatch with InP), and a Si substrate (~ 8 % lattice-mismatch with InP).

Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and ultrafast, miniband-like carrier extraction. In turn, that has led to very high internal-efficiency n_i values: ~ 77 % and 80-86 % from ~ 5.0 μm- and 8-9 μm-emitting QCLs, respectively. Based on extracted parameters that characterize the interfaceroughness (IFR) scattering, a study has been performed of the effects of elastic scattering, both IFR and alloy-disorder (AD) scattering, on 5.0 μm-emitting STA-QCLs. We find that the laser-transition efficiency n_tr is enhanced by ~15 % (i.e., from 83 % to ~ 95 %) due to the much stronger effect of elastic scattering on the lower-laser-level lifetime than on the effective upper-laser-level lifetime. In turn, the injection efficiency: n_inj = n_i /n_tr , reaches ~ 81 %; that is, the highest injection-efficiency value obtained to date from QCLs. Furthermore, we find that the projected upper limit for the pulsed wall-plug efficiency can reach values as high as 44.4 % for 4.6 μm-emitting devices; thus, raising the possibility of CW operation of 4.5-5.0 μm-emitting QCLs with wallplug-efficiency values as high as 40 %.

Quantum cascade lasers are often operated in pulsed regime for low-power applications due to the large thermal dissipation required for continuous wave operation. The typical pulse length is of the order of 100 ns with a duty cycle below 1%. Fourier transform infrared spectrometers, commonly used in the mid-infrared, typically have a spectral resolution of the order of 3 GHz and rely on the acquisition of a path-difference interferogram. As a consequence, when measuring devices operated in pulsed regime such spectrometers can only measure the spectrum averaged over several pulses. We propose a method to determine the absolute instantaneous frequency of a pulsed laser with a precision of 10 MHz. First, the light from the laser is sent through a 30 cm long Fabry-Perot resonator under vacuum. The temporal waveform of the transmitted signal, which is measured using an HgCdTe detector, contains fringes corresponding to constructive and destructive interference occurring as function of time. This experiment allows to determine the chirp rate. The Fabry-Perot cavity is then filled with a known gas exhibiting an absorption line lying within the laser emission range, which can be used as an absolute frequency reference. By combining this measurement with the chirp rate, we obtain the instantaneous frequency of the laser as a function of time. Complex spectral behavior of pulsed DFB lasers, such as mode-hopping and dual-wavelength lasing, can also be properly identified using this technique.

The paper focuses on the design, fabrication and characterization of monolithic, coupled cavity two-section quantum cascade lasers. The devices were fabricated by reactive ion etching from InP-based heterostructure designed for emission in 9.x micrometer range. To make the device attractive for sensing applications, the idea of the coupled-cavity device was employed, giving the possibility of single longitudinal mode operation. We have previously presented devices fabricated by means of focused ion beam post-processing. However, FIB etching is challenging and time-consuming. In order to overcome the relatively low throughput of the FIB process, in this work, gaps separating sections were defined by dry etching during the fabrication process. Careful optimization of the dry etching process resulted in very good control of gap geometry. Quality of mirrors formed by RIE did not introduce high scattering loss into the cavity, as the threshold current density was not increased significantly. Devices routinely exhibited side mode suppression ratio of more than 20 dB. Approach to fabricate two-section devices by dry etching resulted in improved yield as well as high repeatability of the performance of individual devices. Monolithic, electrically isolated, two-section devices were also fabricated and characterized. We will present a comparison of the performance of different designs and discuss their characteristics, fabrication challenges and stability against operating conditions.

The terahertz quantum-cascade (QC) VECSEL is a recently demonstrated approach to designing single-mode terahertz lasers based on the coupling of an amplifying reflect-array metasurface with an external optical cavity. The QC-VECSEL has demonstrated single-mode terahertz lasing with high output power and near-diffraction limited beam quality. The QC-VECSEL is also a natural candidate for demonstrating broadband, continuous, single-mode frequency tuning as the VECSEL’s lasing frequency is determined by the length of the its external cavity, which can be mechanically tuned. In this work, we use a piezoelectric translational stage to actively adjust the length of the QC-VECSEL’s external cavity and demonstrate >500 GHz of single-mode tuning around a center frequency of 3.5 THz (>20% fractional tuning). High-quality, circular beam patterns are observed with a divergence angle of ~15° throughout the tuning range, and tens of milliwatts of peak terahertz output power are observed. In order to maintain single mode behavior, the external cavity is made to be extremely short, increasing the spacing between the external cavity’s neighboring longitudinal resonances. Cavity lengths as short as 250 µm have been studied, but the free-spectral range of the external cavity could not be made larger than the gain bandwidth of the metasurface, providing testament to the bandwidth of both the metasurface and the QC-gain material.

We report on a quartz-enhanced photoacoustic (QEPAS) sensor employing a monolithic distributed-feedback quantum cascade laser array operating in a pulsed mode as a light source. The array consists of 32 quantum cascade lasers emitting in a spectral range from 1190 cm^-1 to 1340 cm^-1, which covers two absorption branches of nitrous oxide (N₂O) and several absorption features of (CH₄). The versatility of the QEPAS technique combined with the rapid wavelength tuning provided by the ultra-compact, low-power consuming laser source allowed the detection of N₂O and CH₄ with detection sensitivities below a part-per-million at atmospheric pressure.

Presently quantum-cascade (QC) lasers enable emission at the wavelengths ranging from infrared to terahertz making them ideal light source for the distant detection of harmful gases and free-space optical communication. In those applications, requirements for the lasers include: narrow, single-fundamental-mode operation, low-divergent emitted beam, low threshold current and high speed modulation. Those properties are inherently owned by vertical-cavity surface-emitting lasers (VCSELs). However, when a QC is embedded into conventional vertical cavity, stimulated emission is impossible, because of the absence of the vertical electromagnetic wave component, which makes fundamentally impossible fabrication of QC VCSELs in their conventional design. We propose a design of QC VCSEL in which top DBR mirror is replaced with a monolithic high-refractive-index contrast grating (MHCG). QCs are embedded within the MHCG stripes where the vertical component of the electromagnetic field is induced, enabling stimulated emission from the QCs. Using a three-dimensional, fully vectorial optical model combined with an electrical model and gain model we discuss the distribution of the optical field, threshold current, emitted optical power and wall-plug efficiency of a 9 micro m AlInAs/InGaAs/InP QC VCSEL. Our anticipation shows that threshold current of QC VCSELs can be as low as 0.09 mA and the wall-plug efficiency at the level of 4%. We consider methods of current injection to active regions as well as methods of current and optical confinement. The fabrication possibility of QC VCSELs opens new perspectives for merging the advantages of VCSELs with those of QCLs.

We experimentally study the emission dynamics of a monolithic multi-section semiconductor laser based on InAs/InGaAs quantum dot (QD) material in the regime of passive mode-locked (ML) operation obtained via saturable losses in the absorber (reversed biased). When the active section is biased above the lasing threshold we observe emission of a regular train of optical pulses at 1250nm with characteristic repetition rate of 6 GHz. By sweeping back the pump current below lasing threshold, we verify that the ML solution coexists with the zero intensity ("off") solution, even in absence of any external optical injection.¹ These evidences are very promising for the observation of temporal localized structures in compact, monolithic semiconductor photonics devices. Experimental results are validated by numerical simulations performed using a multi-section delayed differential equation (DDE) model to compute the evolution of the electrical field, coupled with the rate-equations that describe the carrier dynamics in the QD active and absorber media.