We report direct modulation results in a simple and compact widely tunable V-cavity laser. Chirp managed laser technology has been successfully applied to the V-cavity laser with an optical spectrum reshaping filter. The tunable V-cavity-laser employs a half-wave coupler to obtain single-mode operation with high side-mode suppression ratio and the Vernier effect to extend its tuning range. It does not require any grating structure and regrowth steps. In this experiment, we achieved single-electrode controlled wavelength tuning of about 18 channels at 100GHz spacing with a fixed temperature, and 32 channels using 2 temperature settings. Well-open eye diagrams with extinction ratio above 4.3dB in all channels are observed when the laser is directly modulated at 2.5Gb/s. Although the measured small-signal frequency response is only about 5.7GHz, more than 6.7dB extinction ratio under 10Gb/s modulation rate is achieved by using the chirp managed laser technology with an optical spectrum reshaping filter placed after the output of the laser to convert the frequency chirp accompanying the direct modulation to amplitude modulation. The advantages of compactness, fabrication simplicity, easy wavelength control algorithm, and simple direct modulation offer great potential for the chirped managed V-cavity laser to be used in low-cost WDM links.

The combination of low chirp Quantum dash directly modulated lasers with an optical filter have shown promising data transmission performances with 10Gb/s transmission distances in the range of 0-100 km, in full compliance with the current standardization for NGPON2. This paper describes in details the operation of such transmitters, including the enhancement in modulation bandwidth of laser induced by the passive filter.

Nitride based GaN and InGaN quantum dots are excellent single-photon emitters at high temperature owing to their wide bandgap and large exciton binding energy [1-5]. In this work, two different molecular beam epitaxy (MBE) grown nanostructures have been investigated for single-photon emission: InGaN/GaN disk-in-nanowire and InGaN/GaN self-organized quantum dot. Single-photon emission under both optical and electrical excitation has been observed from a single InGaN quantum contained in a GaN nanowire p-n junction. We demonstrate electrically driven single-photon emission, with a g ⁽²⁾(0) = 0.35, from a single InGaN quantum dot emitting in the green spectral range (λ=520 nm) up to 125 K. Additionally, a self-organized InGaN/GaN single quantum dot diode was grown and fabricated. Emission from a single quantum dot (λ=620 nm) shows single-photon emission with g⁽²⁾(0) = 0.29 at room temperature. On-demand single-photon emission by electrical pumping of the quantum dot at an excitation repetition rate of 200 MHz was demonstrated.

We study the effects of dual feedback on a passively mode-locked semiconductor laser using a delay differential equation model. Timing jitter reduction is achieved provided that one of the feedback cavities is resonant with the laser cavity. Maximal jitter reduction occurs when both feedback cavities are resonant. As with single cavity feedback there is a locking range in the feedback delay length about the main resonances. When both feedback delay lengths are tuned appropriately within this locking range simultaneous timing jitter reduction and repetition rate tuning can be achieved, making it interesting for device applications.

Optical emission spectra, optical puslewidths and optical gain spectra are compared as a function of temperature using different sections of gain length for a passively mode-locked InAs quantum dot device. By increasing the length of the gain section, we decrease the threshold condition for mode-locking which allows access to a greater gain bandwidth. At 300 K, when the dots states and wetting layer are in thermal equilibrium, a decrease in the threshold condition results in little change in the width of the optical emission spectra and the corresponding optical pulsewidths. At 80 K, where the dot states are randomly populated, the same decrease in the threshold condition, results in a near doubling of optical emission spectra and a reduction in the optical pulsewidth from 910 fs to 600 fs. These changes in pulsewidth were obtained with only a modest 20% reduction of threshold condition from 14 cm^-1 to 11 cm^-1 which corresponds to an increase of the gain section length from 2 mm to 2.6 mm. The experimental results are qualitatively explained using a simple set of rate equations, which take explicit account of the photon density in the cavity. A reduction in the cavity loss results in an increase in the width of the gain spectrum at and above threshold as dots of different sizes in the inhomogeneous distribution are no longer coupled via carrier escape to the wetting layer.

We present intra-cavity pulse shaping of external cavity mode-locked semiconductor lasers. In our approach, a pulse shaper utilizing a dual LC-panel spatial light modulator is used in the cavity of a mode-locked multi-quantum-well semiconductor laser to introduce spectrally resolved phase manipulation and losses to the pulse propagating in the cavity. Utilizing this, we generate pulses with broader spectra than obtained in conventional external cavity geometries without pulse shaping. The pulses can be compressed near to the transform limit using a grating compressor.

the magnitude of the change in threshold current with temperature in InP quantum dot lasers caused by the distribution of carriers among dot states is quantified and demonstrated. Samples with differing distributions of allowed states, as assessed using absorption spectra and achieved by varying the composition of the quantum well above each layer of quantum dots, are affected differently by this thermal broadening although the underlying mechanism is the same. This difference is shown to be a result of different optical loss and the different gain magnitude achieved at a similar inversion level in the different samples. Uncoated, cleaved facet Fabry-Perot lasers with 2 mm long cavities are demonstrated with a threshold current density of 138 Acm^-2 at 300 K that increases to 235 Acm^-2 at 350 K (77ºC).

Dynamic characteristics of double tunneling-injection (DTI) quantum dot (QD) lasers are studied. To reveal the potential of such lasers for high-speed direct modulation of their optical output by pump current, fast carrier injection into QDs and no carrier leakage from QDs are assumed. The small-signal analysis of rate equations is applied. The modulation bandwidth is calculated as a function of the dc component of the injection current density and parameters of the laser structure.

We employ a device which exploits the properties of InP quantum dots (QD), (emitting from 650-730 nm), to produce simultaneous dual-λ lasing from a single ridge-waveguide comprising two sections. Due to the effects of state-filling in an inhomogeneously broadened QD ensemble, the wavelength is strongly dependent on magnitude of the gain (or cavity loss). Therefore, by altering the loss of each section of the device we are able to demonstrate a large range of difference-wavelengths, up to 63 nm. Here, we test the performance of the device and measure effects of temperature and difference-wavelength on the stability of the two lasing modes.

Time-frequency applications are in need of high accuracy and high stability clocks. Compact industrial Cesium atomic clocks optically pumped is a promising area that could satisfy these demands. However, the stability of these clocks relies, among others, on the performances of laser diodes that are used for atomic pumping. This issue has led the III-V Lab to commit to the European Euripides-LAMA project that aims to provide competitive compact optical Cesium clocks for earth applications. This work will provide key experience for further space technology qualification. We are in charge of the design, fabrication and reliability of Distributed-Feedback diodes (DFB) at 894nm (D1 line of Cesium) and 852nm (D2 line). The use of D1 line for pumping will provide simplified clock architecture compared to D2 line pumping thanks to simpler atomic transitions and larger spectral separation between lines in the 894nm case. Also, D1 line pumping overcomes the issue of unpumped “dark states” that occur with D2 line. The modules should provide narrow linewidth (<1MHz), very good reliability in time and, crucially, be insensitive to optical feedback. The development of the 894nm wavelength is grounded on our previous results for 852nm DFB. Thus, we show our first results from Al-free active region with InGaAsP quantum well broad-area lasers (100μm width, with lengths ranging from 2mm to 4mm), for further DFB operation at 894nm. We obtained low internal losses below 2cm^-1, the external differential efficiency is 0.49W/A with uncoated facets and a low threshold current density of 190A/cm², for 2mm lasers at 20°C.

Real 3D display applications require RGB light sources which provide a spectral bandwidth of less than 5 MHz for recording (and playing-back) a holographic film. Furthermore, these emitters must be small in size, efficient, robust and reliable. Therefore, diode lasers with internal wavelength filters are the devices of choice.

In our paper, we will present a further development of our red-emitting distributed Bragg reflector ridge waveguide laser (DBR-RWL). DBR surface gratings are implemented into the 2 mm long RWL by BCl₃-Ar reactive ion etching. This fabrication process uses a single-epitaxy and is hence industry-compatible.

At a heat sink temperature of 15°C the DBR-RWL provides 120 mW of optical output power in longitudinal single mode emission. Heterodyne linewidth measurements of two identical DBR-RWL show a FWHM linewidth of less than 2 MHz. The excellent beam quality (M² = 1.1) enables the use of these chips for play-back of holographic films or as master oscillators in a future micro-integrated master oscillator power amplifier configuration for recording.

Hybrid Si/III-V is a promising platform for semiconductor narrow-linewidth lasers, since light can be efficiently stored in low loss silicon and amplified in III-V materials. The introduction of a high-Q cavity in silicon as an integral part of the laser's resonator leads to major reduction of the laser linewidth. However, the large intra-cavity field intensity resulting from the high-Q operation gives rise to non-linear effects. We present a theoretical model based on non-linear rate equations to study the effect of two-photon absorption and induced free-carrier absorption in silicon on the laser's performance. The predictions from this model are compared to the experimental results obtained from narrow-linewidth lasers fabricated by us. It is shown to have an effect on the linearity of the L-I curve, and to reduce the achievable Schawlow- Townes linewidth.

A pair of external cavity diode lasers are fabricated using an integrated planar Bragg grating. The planar waveguide and Bragg reflector is UV-written within a glass-on-silicon chip. Intensity isolated, continuous wavelength tuning at > 1kHz modulation rate is acheived using micro-heating elements fabricated directly over the Bragg grating. Low RIN (<140dB) and low linewidth (δν ~ 200 kHz) operation is found using a heterodyne measurement. We demonstrate the lasers operating in phase-locked loop configuration where one laser is frequency-offset locked to the other.

Recently, high-power broad-area laser diodes based on GaN with output powers beyond 1 W have become available. However, their broad spectral emission limits their applicability. Due to a lack of internal grating technology for GaN devices, narrowband emission with several hundreds of milliwatts in the blue-green spectral range has not been achieved with laser diodes thus far. In this work, a high-power external cavity diode laser (ECDL) system at 445 nm is presented. The system is based on a commercially available broad-area GaN laser diode and a surface diffraction grating in Littrow configuration for optical feedback. Using this configuration an output power of 400 mW with a reduced spectral emission bandwidth of 20 pm (FWHM) with a side-mode suppression ratio larger than 40 dB is obtained. With the above presented optical output power and narrowband laser emission at 445 nm, the ECDL is well suited as a pump light source for nonlinear frequency conversion into the deep ultraviolet spectral range.

We review recent theoretical and experimental work on InP membrane microdisk lasers heterogeneously integrated on SOI and coupled to a Si bus waveguide. After a general introduction on the fabrication and the operation principles, we will describe various improvements in the fabrication technology. This includes improvements in the yield of the bonding of the InP die on the SOI die and in the controllability of the bonding layer thickness, as well as an optimization of the alignment of the microdisk with respect to the silicon waveguide and some proposals for better heat sinking and loss reduction. Improvement in the alignment and the bonding has led to interesting results on the uniformity in device characteristics. In a second part, unidirectional behaviour and reflection sensitivity will be briefly discussed. Theoretical, numerical and experimental results will be shown about the unidirectional behavior and it will be explained how unidirectional microdisk lasers can be a lot less sensitive to external reflections than other lasers. We will also show how such lasers can be used as optical signal regenerators that can work with low optical input powers and that have small power consumption. We will end with a description of demonstrations of optical interconnects based on heterogeneously integrated microdisk lasers and heterogeneously integrated photodetectors. Optical interconnects on chip have been demonstrated at 10 Gb/s. An epitaxial layer stack that contains both the laser and the detector structure has been used for this purpose.

A silicon-based laser remains an important goal in science and technology. Unfortunately silicon is ill-suited as a light-emitter, prompting the need for alternative high quality light sources integrated with silicon. One such alternative, presented here, is a monolithic III-N edge-emitting laser comprised of a planarized nanowire array. Nanowire heterostructures with InGaN/GaN disk-in-nanowire active regions were grown on (001)silicon and planarized with parylene, forming a composite slab heterostructure supporting a guided mode propagating transverse to the growth direction. From this composite slab, ridge-geometry lasers were fabricated. Lasers with emission at 533 nm (green) and 610 nm (red) are presented here. The lasers are characterized by J_th = 1.76 kA/cm² (green) and 2.94kA/cm² (red) under continuous wave current injection. The green lasers have device lifetime of ~7000 hrs. Small-signal modulation measurements have also been performed. The -3dB modulation bandwidth of the green laser is 5.7 GHz.

Type-II light sources on InP substrate are an innovative concept for wavelengths ranging from 2 μm to the mid-IR. The concept is using the type-II band alignment between GaInAs and GaAsSb to exceed the limitation of type-I devices. Since the first demonstration of InP type-II heterostructure lasers above 2.3 μm in 2012, we have extended the emission wavelength to 2.7 μm. Furthermore, a drastic reduction in threshold current density down to 104 A/cm² per QW at infinite length was achieved (at 2.5 μm), which represents an improvement by more than a factor of two. Additionally CW operation up to 30°C and up to 80°C pulsed is presented. Furthermore, LEDs for 3.5μm peak emission wavelength and up to 86 μW output power are shown.

In recent years, the use of laser sources in gas sensing applications has been increasing continuously. Tunable Laser Absorption Spectroscopy (TLAS) has proven to be a versatile tool in a variety of sectors including industry, health and security and modern environmental analysis. Especially the mid-infrared wavelength range is of great interest for high accuracy gas sensing applications, since many technologically and industrially relevant gas species have their strongest absorption features in the spectral region between 3 and 6 μm. These include, e. g., important hydrocarbons like methane or propane, as well as nitric oxide and formaldehyde. Interband cascade lasers (ICL) provide mono mode continuous wave (CW) operation above room temperature in this wavelength range. Application-grade complex coupled distributed feedback (DFB) laser devices based on the ICL concept are presented, using lateral metal gratings as wavelength selective elements. The fabricated devices operate at specific, technologically relevant, emission wavelengths in the spectral region from 3 to 6 μm. CW operation up to 80 °C and mono mode wavelength tuning ranges above 20 nm were achieved with low energy consumption. Application examples in industry and research are presented that demonstrate the high potential of DFB ICLs for the use in TLAS. E. g., formaldehyde gas sensor systems based on DFB ICL devices operating around 3.6 μm can provide realtime in-situ measurements with resolution limits in the low ppb range, even in dense background atmospheres. The low power consumption of ICL based devices makes them especially favorable for battery-powered or portable sensor applications.

Cascade pumping schemes that utilize single-QW gain stages enhanced both the power conversion efficiency and the output power level of GaSb-based diode lasers that emit near and above 3 μm at room temperature. The cascade lasers discussed in this work had densely stacked type-I QWs gain stages characterized by high differential gain. The 3 μm emitting devices demonstrated CW threshold current densities near 100 A/cm², a twofold improvement over the previous world record, that resulted in peak power conversion efficiencies increasing to 16% at 17°C. Comparable narrow ridge two-stage devices generated more than 100 mW of CW power with ~10% power conversion efficiencies. Three-stage multimode cascade lasers emitted 960 mW of CW output power near 3 μm and 120 mW CW near 3.3 μm.

The intensity noise properties of injection locked mid-infrared quantum cascade lasers are experimentally investigated. The injection locking is realized below and near the threshold of the free running slave laser, resulting in an efficient technique to achieve low noise operation. It is found that below threshold the locking characteristics (locking range shape and bandwidth) are different in comparison to those above threshold. Furthermore, an alternative injection locking realization is also investigated: injection locking into longitudinal side modes of the slave laser. Here, similar characteristics were observed, however, with the potential to achieve even higher relative intensity noise reduction suppression with respect to the quantum noise limit. The measurements are confirmed by numerical simulations with a travelling-wave model which takes into account the multi-mode spectrum of the slave laser and the spectral profile of the material gain. The experiments give the perspective for the achievement of the relative intensity noise reduction of the slave laser of up to 10 dB (above threshold) and up to 20 dB (below threshold) in comparison to the free running slave laser noise level.

On-chip resonant leaky-wave coupling of quantum cascade lasers (QCLs) emitting at 8.36 μm has been realized by selective regrowth of interelement layers in curved trenches, defined by dry and wet etching. The fabricated structure provides large index steps (Δn = 0.10) between antiguided-array element and interelement regions. In-phase-mode operation to 5.5 W front-facet emitted power in a near-diffraction-limited far-field beam pattern, with 4.5 W in the main lobe, is demonstrated. A refined fabrication process has been developed to produce phased-locked antiguided arrays of QCLs with planar geometry. The main fabrication steps in this process include non-selective regrowth of Fe:InP in interelement trenches, defined by inductive-coupled plasma (ICP) etching, a chemical polishing (CP) step to planarize the surface, non-selective regrowth of interelement layers, ICP selective etching of interelement layers, and non-selective regrowth of InP cladding layer followed by another CP step to form the element regions. This new process results in planar InGaAs/InP interelement regions, which allows for significantly improved control over the array geometry and the dimensions of element and interelement regions. Such a planar process is highly desirable to realize shorter emitting wavelength (4.6 μm) arrays, where fabrication tolerance for single-mode operation are tighter compared to 8 μm-emitting devices.

Intersubband transitions in n-doped semiconductor heterostructures provide the possibility to quantum engineer one of the largest known nonlinear optical responses in condensed matter systems, limited however to electric field polarized normal to the semiconductor layers. Here we show that by coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered intersubband transitions in semiconductor heterostructures one can create ultra-thin highlynonlinear metasurfaces for normal light incidence. Structures discussed here represent a novel kind of hybrid metalsemiconductor metamaterials in which exotic optical properties are produced by coupling electromagneticallyengineered modes in dielectric and plasmonic nanostructures with quantum-engineered intersubband transitions in semiconductor heterostructures. Record values of effective optical nonlinearities of over 400 nm/V are experimentally measured for metasurfaces optimized for efficient second harmonic generation at 9.7 μm pump wavelength under normal incidence.

Terahertz quantum cascade laser (THz-QCL) is expected as a compact terahertz laser light source which realizes high output power, quite narrow emission linewidth, and cw operation. We are studying on THz-QCLs using GaAs/AlGaAs and GaN/AlGaN semiconductor superlattices. We demonstrated 1.9-3.8 THz GaAs/AlGaAs QCLs with double metal waveguide (DMW) structures. We developed a low-frequency high-temperature operation QCL (T<160K for 1.9 THz- QCL) by introducing indirect injection scheme design (4-level design) into GaAs/AlGaAs THz-QCLs. Nitride semiconductor is a material having potentials for realizing wide frequency range of QCL, i.e., 3～20 THz and 1～8 μm, including an unexplored terahertz frequency range from 5 to 12 THz, as well as realizing room temperature operation of THz-QCL. The merit of using an AlGaN-based semiconductor is that it has much higher longitudinal optical phonon energies (E_LO> 90meV) than those of conventional semiconductors (~ 36 meV). We fabricated high-quality AlGaN/GaN QC stacking layers by introducing a novel growth technique in molecular beam epitaxy (MBE). We fabricated a GaN/AlGaN QCLs with “pure three-level” design and obtained the first lasing action of nitride-based QCL from 5.4-7 THz.

This paper reviews recent advances in the research and development toward the graphene-based terahertz (THz) lasers. Mass-less Dirac Fermions of electrons and holes in gapless and linear symmetric band structures in graphene enable a gain in a wide THz frequency range under optical or electrical pumping. The excitation of the surface plasmon polaritons in the population-inverted graphene dramatically enhances the THz gain. Photon-emission-assisted resonant tunneling in a double-graphene-layered nano-capacitor structure also strongly enhances the THz gain. Novel graphene-based heterostructures using these physical mechanisms for the current-injection driven THz lasing are discussed. Their superior gain-spectral properties are analyzed and the laser cavity structures for the graphene THz laser implementation are discussed.

980 nm GaAs-based photonic crystal surface emitting lasers containing all semiconductor GaAs/InGaP and GaAs/air photonic crystals (PC) inside their cavity are theoretically investigated. We use a combination of an average index approach and optical coupled mode theory to optimize the PC interaction with optical modes of the laser waveguide and draw guidelines for design of PCSELs based on a range of material systems and operating wavelengths. Results show that the all-semiconductor PC provides a higher coupling with the optical mode in most cases.

Raman lines are often superimposed by daylight, artificial light sources or fluorescence signals from the samples under study. Shifted excitation Raman difference spectroscopy (SERDS), i.e. exciting the sample alternatingly with two slightly shifted wavelengths, allows to distinguish between the Raman lines and sources of interference. In this work, monolithic dual wavelength Y-branch DBR ridge waveguide diode lasers and their application in master oscillator power amplifier (MOPA) systems at 785 nm suitable for Raman spectroscopy and SERDS will be presented. The definition of the wavelengths is made by implementing deeply-etched 10^th order 500 μm long surface gratings with different periods using i-line wafer stepper lithography. Y-branch DBR lasers with a total length of 3 mm and a stripe width of 2.2 μm were manufactured and characterized.

The monolithic devices reach output powers up to 215 mW with emission widths of about 20 pm. At 200 mW the conversion efficiency is 20%, i.e. the electrical power consumption is only 1 W. The spectral distance between the two laser cavities is about 0.6 nm, i.e. 10 cm^-1 as targeted. The side mode suppression ratio is better than 50 dB. Amplifying these devices using a ridge waveguide amplifier an output power of about 750 mW could be achieved maintaining the spectral properties of the master oscillator.

Optically-pumped microsquare cavity laser on InP-based multiple-quantum-wells (MQW) material platform is demonstrated. Continuous wave operation is achieved for microsquare cavity with footprint as small as 4×4μm². Numerical mode analysis and experimental characterization of the microsquare laser are conducted, and the results are compared with the commonly used microdisk cavity laser fabricated on the same platform. The microsquare laser shows a lower lasing threshold and infers a higher differential efficiency than the microdisk counterpart. The microsquare cavity laser has sufficiently high quality factor, and higher pumping injection efficiency due to the more evenly distributed field profile as compared to that of the microdisk. Experimental result also shows that the microsquare laser has better temperature stability than the microdisk. These results promise a potential alternative laser structure for onchip light source applications.

The infrared emission from 980-nm single-mode high power diode lasers is analyzed in the wavelength range from 0.8 to 7.0 μm. A pronounced short-wavelength infrared (SWIR) emission band with a maximum at 1.3 μm is found to originate from defect states located within the waveguide of the devices. The SWIR intensity is verified to represent a measure of the non-equilibrium carrier concentration in the waveguide, allowing for non-destructive waveguide mapping in spatially resolved detection schemes. The potential of this approach is demonstrated by measuring spatially resolved profiles of SWIR emission and correlating them with mid-wavelength infrared thermal emission along the cavity of devices undergoing repeated catastrophic optical damage. The enhancement of SWIR emission in the damaged parts of the cavity is due to a locally enhanced carrier density in the waveguide and allows for in situ analysis of the damage patterns. Moreover, spatial resolved SWIR measurements are a promising tool for device inspecting even in low-power operation regimes.

The development of broad area laser diodes towards higher output power, efficiency and brightness is essential to gain progress in almost all laser applications because those devices provide the basis for high power laser sources. To systematically improve the characteristics of high power broad area laser diodes through a design process, it is necessary to have an accurate and efficient computation model self-consistently taking into account optical, electrical and thermal properties. In this publication we present numerical techniques to compute the optical properties of the multimode beam generated by high power AlGaAs broad area laser diodes with an operating wavelength of 970 nm. This simulation considers fluctuations of the carrier and power density as well as the temperature distribution. The numerical results show an excellent agreement to measured data of conventional and microstructured high power broad area lasers. The high computation speed of the model allows optimizing microstructures inside the laser resonator with the use of a genetic optimization algorithm. We show that this design approach potentially leads to a substantial performance gain of the device. In particular degradation of the beam quality due to thermal effects at high injection currents can be controlled.

Semiconductor based sources which emit high-power spectrally stable nearly diffraction-limited optical pulses in the nanosecond range are ideally suited for a lot of applications, such as free-space communications, metrology, material processing, seed lasers for fiber or solid state lasers, spectroscopy, LIDAR and frequency doubling.

Detailed experimental investigations of 975 nm and 800 nm diode lasers based on master oscillator power amplifier (MOPA) light sources are presented. The MOPA systems consist of distributed Bragg reflector lasers (DBR) as master oscillators driven by a constant current and ridge waveguide power amplifiers which can be driven DC and by current pulses.

In pulse regime the amplifiers modulated with rectangular current pulses of about 5 ns width and a repetition frequency of 200 kHz act as optical gates, converting the continuous wave (CW) input beam emitted by the DBR lasers into a train of short optical pulses which are amplified. With these experimental MOPA arrangements no relaxation oscillations in the pulse power occur. With a seed power of about 5 mW at a wavelength of 973 nm output powers behind the amplifier of about 1 W under DC injection and 4 W under pulsed operation, corresponding to amplification factors of 200 (amplifier gain 23 dB) and 800 (gain 29 dB) respectively, are reached. At 800 nm a CW power of 1 W is obtained for a seed power of 40 mW. The optical spectra of the emission of the amplifiers exhibit a single peak at a constant wavelength with a line width < 10 pm in the whole investigated current ranges. The ratios between laser and ASE levels were > 50 dB. The output beams are nearly diffraction limited with beam propagation ratios M²_lat ~ 1.1 and M²_ver ~ 1.2 up to 4 W pulse power.

A new approach to generation of high optical peak power by epitaxially and functionally integrated high-speed highpower current switch and laser heterostructure (so-called laser-thyristor) has been developed. This approach makes it possible to reduce the loss in external electrical connections, which is particularly important for the short-pulse highamplitude current pumping. In addition, it considerably simplifies the fabrication technology of pulsed laser sources as a commercial product and allows stacking of multiple-element systems. The epitaxially integrated AlGaAs/GaAs heterostructure of low-voltage laser-thyristor has been studied and optimized for generation of high-power pulses at a 900-nm wavelength. It is shown that the incomplete switch-on of the laserthyristor in the initial stage and the nonlinear dynamics of the emitted laser power are due to the insufficient efficiency of the vertical optical feedback in the epitaxially integrated heterostructure. Optimization of the composition and the interband absorption spectra of transistor base layers makes it possible to substantially raise the efficiency of control signals due to the rise in the photogeneration speed. Experimental laser-thyristor samples with a 200-μm aperture have been fabricated and studied. The maximum static blocking voltage does not exceed 20 V. It is shown that the generated laser pulses have a perfect bell-like shape without any indications of a nonlinear dynamics. This confirms that the changes introduced into the heterostructure design provide a sufficient efficiency of photogeneration of the control signal. As a result, the maximum optical peak power reaches 40 and 8 W at FWHM pulse durations of 95 and 13 ns, respectively. An analysis of the potential dynamics has shown that the heterostructure provides pumping of the active layer with up to 90-A pulses.

Micro-DIAL (differential absorption LIDAR) systems require light sources with peak powers in the range of several 10 W together with a spectral line width smaller than the width of absorption lines under study. For water vapor at atmospheric pressure this width should be smaller than 10 pm at 975 nm. In this paper, an all semiconductor master oscillator power amplifier system at an emission wavelength of 975 nm will be presented. This spectral range was selected with respect to a targeted absorption path length of 5000 m and H2O line strengths. A distributed feedback (DFB) ridge waveguide diode laser operated in continuous wave is used as master oscillator whereas a tapered amplifier consisting of a RW section and a flared section is implemented as power amplifier. The RW section acts as optical gate. The current pulses injected into the RW part have a length of 8 ns and the tapered part is driven with 15 ns long pulses. The delay between the pulses is adjusted for optimal pulse shape. The repetition rate is in both cases 25 kHz. A maximal pulse output power of about 16 W limited by the available current supply is achieved. The spectral line width of the system determined by the properties of the DFB laser is smaller than 10 pm. The tuning range amounts 0.9 nm and a SMSR of 40 dB is observed. From the dependence of the peak power on the power injected into the tapered amplifier, the saturation power is determined to 5.3 mW.

Three-dimensional above-threshold analyses of high-index-contrast (HC) photonic-crystal (PC) quantum-cascade-laser arrays (QCLA) structures, for operation at watt-range CW powers in a single spatial mode, have been performed. Threeelement HC-PC structures are formed by alternating active- antiguided and passive-guided regions along with respective metal-electrode spatial profiling. The 3-D numerical code takes into account absorption and edge-radiation losses. Rigrod’s approximation is used for the gain. The specific feature of QCLA is that only the transverse component of the magnetic field sees the gain. Results of above-threshold laser modeling in various approximate versions of laser-cavity description are compared with the results of linear, full-vectorial modeling by using the COMSOL package. Additionally, modal gains for several higher-order optical modes, on a ‘frozen gain background’ produced by the fundamental-mode, are computed by the Arnoldi algorithm. The gain spatial-hole burning effect results in growth of the competing modes’ gain with drive current. Approaching the lasing threshold for a competing higher-order mode sets a limit on the single-mode operation range. The modal structure and stability are studied over a wide range in the variation of the inter-element widths. Numerical analyses predict that the proper choice of construction parameters ensures stable single-mode operation at high drive levels above threshold. The output power from a single- mode operated QCLA at a wavelength of 4.7 μm is predicted to be available at multi-watt levels, although this power may be restricted by thermal effects.

We present the experimental characterization results of a 15-to-1 wavelength multiplexer for a Distributed Feedback Quantum Cascade Laser (DFB QCL) array operating in the 7-8.5 μm (mid-long) infrared (IR) range. This design is customized for its use to combine the output from a DFB QCL array with a 0.1 μm wavelength channel spacing for spectroscopy applications, and it is proposed in order to achieve a continuous tuning range overcoming the limited tunability of a single QCLs, required for multi-gas or complex molecule detection. This multiplexer is based on an Echelle diffraction mirror grating scheme, in which multiple output waveguides are deliberately implemented in the design to de-risk for wavelength deviations in the fabrication process. We optimized the location of the input and output guides in order to allow for monolithic integration of the DFB QCL arrays, which would provide for a number of advantages such as a higher stability, less complexity and lower cost over other technologies such as external cavities. We discuss the effects over the device performance of the design, such as the diffraction effects, input channel width overlapping/crosstalk and input channel profile, which are very important to address in order to avoid unaccounted transmission losses. Other parameters such as the profile of the input and output waveguides and fabrication limitations are also discussed as their effect on the device is observed. A series of characterization tests are presented in order to compare the simulation results to the experimental data, which suggests that these multiplexers are a suitable option compared to other IR multiplexer schemes in terms of size and power transmission.

Remarkable progress made in quantum cascade lasers (QCLs) has led them to find an increasing number of applications in remote sensing, chemical sensing, and free space communications, in addition to potential space applications. However, little has been reported on reliability and failure modes of QCLs although it is crucial to understand failure modes and underlying degradation mechanisms in developing QCLs that meet lifetime requirements for space missions. Focused ion beam (FIB) techniques have been employed to investigate failure modes in various types of laser diodes. Our group has also used FIB to study failure modes in single-mode and multi-mode InGaAs-AlGaAs strained QW lasers, but few groups have used this technique to investigate failure modes in QCLs. In our study, we report on destructive physical analysis (DPA) of degraded InGaAs-InAlAs QCLs using FIB and high-resolution TEM techniques. The active region of QCLs that we studied consisted of two-23 stage layers of InGaAs-InAlAs separated by a 0.5 μm thick InP spacer layer for 8.4μm QCLs and 30-stage layers of lattice-matched InGaAs-InAlAs heterostructure for 4.7μm QCLs. The MOVPE-grown laser structures were fabricated into deep-etched ridge waveguide QCLs. L-I-V-spectral characteristics were measured at RT under pulsed operation. Our 8.4μm QCLs with as-cleaved and HR-coated facets showed a laser threshold of 1.7 A and a threshold voltage of 13 V at RT, whereas our 4.7μm QCLs without facet coating showed threshold currents of 320 - 400 mA and threshold voltages of 13 - 13.5V. Failures were generated via short-term tests of QCLs. FIB systems were used to study the damage area on the front facet and also to prepare TEM cross sections at different locations along the waveguide for defect and chemical analyses using a HR-TEM. In contrast to the COMD damaged area showing as a blister on the front facet of QW lasers, the damaged area of QCLs was significantly extended into the InP substrate due to a much less absorption of lasing photons in QCLs. Our detailed destructive physical analysis results are reported including defect, structural, and chemical analysis results from degraded QCLs.

We present experimental comparison of Type-I diode lasers emitting <3 μm wavelength in room temperature with increased strain in quantum wells (QWs). Due to diminishing hole confinement in the barrier, the performance of mid- IR Type-I diode laser is generally poor. Here we improve the hole confinement using quinary alloy in the barrier in conjunction with highly strained QWs. By using molecular beam epitaxial growth method, we achieve up to 2.3% strain in the QWs. At near room temperature, highly strained laser structure shows approximately 4 times improved laser performance than regular strained laser under the same testing condition. The study demonstrates significant improvement in laser efficiency using highly strained QWs in the GaSb-based type-I mid-infrared laser diodes.

The analysis of I-V and I-L curves in mid-IR quantum cascade lasers operating at room temperature is performed. When the ohmic component of the device resistance in the I-V curve is subtracted, the current I can be approximated by the exponential function, I = I_s exp(eV_j/ε) where I_s is the saturation current, Vj is the n-n junction voltage, and ε is an energy parameter related to the tunneling mechanism which enables filling of upper states and emptying of lower states of the laser transition. Values of εare found to be in 0.68-1.45 eV range, and when divided by the number of stages in the cascade, the tunneling parameter of each stage is determined. The threshold related “kink” of differential I-V curves is shown. The effect of voltage saturation above the laser threshold is observed. Thus, the possibility of determination of the threshold using electrical measurements in quantum cascade lasers has been demonstrated.

The model of a new type of high-power laser light generators, based on epitaxially and functionally integrated fast highpower current switch and laser heterostructure, the so-called laser-thyristor, has been developed. In this model, the functional characteristics of the laser-thyristor were analyzed by considering the epitaxially integrated structure as an optoelectronic pair constituted by a heterophototransistor and a laser diode. It was demonstrated that the turn-on of lasing fundamentally affects the injection efficiency of the laser-thyristor. The dynamic characteristics of the laser-thyristor were examined by using analytical relations for the optical feedback. It is shown that the impact ionization can substantially raise the build-up rate of the through current across the laser-thyristor structure and, as a result, make shorter the leading edge of a laser pulse. It is demonstrated that the developed dynamic model is in good agreement with experimental results at the maximum blocking voltages.

A ZnSe/BeTe p-grading superlattice (p-GSL) with a low voltage drop is reported for BeZnCdSe quantum well laser diode (LD) in green-yellow visible range. A p-GSL is inserted between a p⁺-BeTe for ohmic contact layer and a ZnSe/BeMgZnSe p-cladding layer in a LD, for an efficient hole injection in spite of a large potential barrier height of ~0.8 eV between these layers. A GSL design has hence a great impact on a threshold voltage of lasing and thus reliability in LDs. Simple p-n junction devices with various GSL designs are fabricated, where a p-n junction is formed between p-ZnSe and a n-GaAs. In a p-GSL where a pair of ZnSe/BeTe is repeated, BeTe thickness increases with fixed monolayer (ML) step, while ZnSe thickness decreases with the same step when next pair of ZnSe/BeTe is grown. While a grading of 1 ML step is used in the previous LDs, the new GSL design with smaller grading step of 0.5 ML gives a 2 V lower voltage at 200 A/cm² current injection. Then, LDs characteristics are compared with the GSL of new and old designs, while other layers in LDs are kept nearly identical, which is confirmed by a similar threshold current of ~80 mA and an emission wavelength at ~540 nm in these LDs. The LD with the new GSL design showed a lower threshold voltage for a lasing as well as a higher output power due to a lower device heating.

In this work we demonstrate tunable red- or blue-shift emissions of GaAs_0.9P_0.1 quantum well (QW) heterostructures. The wavelength shift is achieved by ampoule sealed post-growth annealing of QW with different combinations of dielectric encapsulants and in various ambient conditions. For capless bare samples sealed in ampoules with little arsenic overpressure, furnace anneals at 800°C result in red photoluminescence (PL) shifts asymptotically as much as 75 meV with anneal time up to 40 hours. We attribute this redshift to the inter-diffusion of phosphorous and arsenic in QW and the neighboring confinement layers. For samples capped with a bilayer of SrF₂ and SiO_x, similar temporal red shifts appear suggesting the combined dielectrics either prohibit or slow down the diffusion of column III vacancies during anneals. For samples capped with either SiO_x or SiN_x alone their PL spectra first shift toward longer wavelength then toward shorter wavelength. The large turn-around blue-shift (up to 165meV for 40hr annealing under 800°C) is attributed to the intermixing of Ga in QW and Al in the confinement layers. Additional complexity arises when As overpressure is replaced with Ga overpressure. For samples similarly capped with either SiO_x or SiN_x films, the turn-around blue-shift proceeds much faster (up to 281meV for only 5-hr annealing at 800°C). We attribute the slower blue-shift to the generation and diffusion of column III vacancies while the faster blue-shift to the kick-out of Zn-dopants in the heavily doped contact layer.

Recent work has shown the Quantum Dot (QD) material system to be well-suited to support dual-mode lasing. In particular, optical injection from a master laser (ML) into the residual Fabry-Perot (FP) modes of a 1310 nm Quantum Dot Distributed Feedback (QD-DFB) laser has been recently demonstrated to offer a highly reliable platform for stable dual-mode lasing operation. External controls on the ML, such as operating temperature and bias current, can be used to precisely adjust the spacing between the two lasing modes. This tunability of modeseparation is very promising for a range of applications requiring the generation of microwave, millimeter wave and terahertz signals. Considering the versatility and utility of such a scheme, it is imperative to acquire a deeper understanding of the factors that influence the dual-mode lasing process, in order to optimize performance. Toward this end, this paper seeks to further our understanding of the optically-injected dual-mode lasing mechanism. For fixed values of optical power injected into each FP residual mode and wavelength detuning, the dual-mode lasing characteristics are analyzed with regard to important system parameters such as the position and the intensity of the injected residual mode (relative to the Bragg and the other residual FP modes of the device) for two similarly-fabricated QD-DFBs. Results indicate that for dual mode lasing spaced less than 5 nm apart, the relative intensity of the injected FP mode and intracavity noise levels are critical factors in determining dual mode lasing behavior. Insight into the dual-mode lasing characteristics could provide an important design guideline for the master and QD-DFB slave laser cavities.