This PDF file contains the front matter associated with SPIE Proceedings Volume 7953, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

In this effort, we present the most recent progress towards fabrication and testing of quantum dot composite materials. These materials are important steps towards easy integration of a light source on silicon. Quantum dot composites are formed by incorporating quantum dots in a host material that acts to protect and maintain the characteristics of the quantum dots, as well as to act as a patternable matrix for lithography and etching. In this effort, commercially available IR emitting PbS quantum dots (Evident Technologies) were incorporated with PMMA and poly (hexyl methacrylate) and their photoluminescence spectra was examined over time. A large shift in the photoluminescence emission peak over the course of several days was observed in all composites. This could be due to oxidation despite being in a host.

In this paper we report on the multi-section gain and absorption analysis of strain engineered molecular beam epitaxy (MBE) grown GaAs and InGaAs capped bilayers. The InGaAs capped bilayer quantum dot (QD) lasers extends the room temperature lasing wavelength to 1.45 μm. The spectral measurement of gain demonstrates that net modal gain is achieved beyond 1.5 μm at room temperature. Analysis of the temperature and current density dependence gain characteristics of a GaAs capped bilayer sample indicate that the temperature sensitivity of threshold current around room temperature is due to phonon assisted thermal escape of carriers from the QDs.

We employ an alternate approach to Stranski-Krastanow (SK) QD formation involving the use of nanopatterning with diblock copolymers combined with selective MOCVD growth, enabling QD formation over large surface areas intended for device application. This approach allows for increased control over the QD size and distribution and elimination of the problematic wetting layer associated with SK QDs. Cross-sectional TEM studies of the nanopatterned QD active regions confirm the absence of a wetting layer, and AFM/SEM measurements indicate high QD densities are achieved (>6x10¹⁰ cm^-2). Furthermore, the process is applicable to large surface coverage, showing promise for implementation into long wavelength (λ = 1.3-1.5μm) sources employing either lattice-matched or strained QDs. Preliminary device results demonstrate LT (up to 170K) InP-based laser operation from devices employing patterned lattice-matched InxGa1- xAs QD (~ 20 nm dia.) active regions. The formation of high density compressively strained InAs QDs on InP substrates is also demonstrated using the nanopatterning and selective growth process.

Using the segmented contact technique we determine the lateral out-diffusion current in shallow etched quantum dot ridges of different width and by fitting this out-diffusion data we obtain the ambipolar diffusion length as a function of the intrinsic carrier injection level. The measured data reveals two regimes of reduced diffusion that can be explained by a mechanism involving the thermal redistribution of carriers via the wetting layer. In a high performance 5 layer quantum dot structure operated at 350K the diffusion length varies between 0.75 and 1.5 μm as the peak modal gain increases from 5 to 11cm-1. In dot-in-well quantum dot samples fabricated into deep etched ridge lasers we demonstrate the improved performance and reduced ridge-width dependence of threshold current density as compared to quantum well material, which is identical except for the presence of the dots and wetting layer. For example in 6μm wide, 1500 μm long quantum dot ridges at 300K the threshold current density is 41% of that of the corresponding quantum well ridge. Furthermore, at 230K the threshold current density of the 6 μm ridge increases by a factor of 1.1 as compared to similar 20μm wide ridges whereas in quantum well ridge lasers the threshold current density increases by a factor of 3.2 for the same pair of ridge widths. This corresponds to a threefold decrease in the ridge width dependence of QD ridge lasers compared to QW based lasers. This different dependence on ridge width corresponds to the different lateral diffusion characteristics of the quantum well and self assembled quantum dot material.

By optimising an InP/AlGaInP quantum dot size distribution a broad and relatively flat topped gain spectra can be achieved. Using the segmented contact method we measure the optical gain spectra and use this to explain the range of lasing wavelengths that can obtained by varying the grating structure of deep etched DBR lasers. We describe the optimisation of a simple single stage ICP etch process suitable for producing anisotropic microstructures in this material system and the resulting deep-etched DBR lasers. Measurements of emission wavelength made between 220 and 320 K on a ridge laser, fabricated with cleaved facets, reveals a temperature dependence on of 0.14 nm/K. DBR structures have been used to improve this behaviour, with a dependence of peak wavelength with temperature of 0.07 nm/K, over the same temperature range. Measurements on a 4 μm wide DBR ridge laser show they can be operated up to 17 nm from the peak emission of a ridge laser operating at the same current density.

Quantum dot (QD) lasers have many attractive features including low-threshold current density, high gain, low chirp and superior temperature stability. In this paper, design, fabrication and characteristics of wafer-level index coupled holographically fabricated 1.3μm QD distributed feedback (DFB) lasers are reported. Previously, 1.3 μm QD-DFB lasers were fabricated with metal surface gratings, which are lossy and (being typically written by e-beam lithography) are difficult to fabricate. In this paper, devices are fabricated using molecular beam epitaxy (MBE) for QD growth, metalorganic chemical vapor deposition (MOCVD) for grating overgrowth, and wafer level interference lithography for grating fabrication. Design and fabrication methods for these devices are reported. Analysis of broad area devices gives a material transparency current density of ~150A/cm². Single mode ridge waveguide devices with cavity length of 500 μm were tested. Device characteristics were fairly uniform, with typical DC characteristics of the devices of threshold currents of ~35mA and slope efficiencies of ~0.11W/A. Measured bandwidths at room temperature were around 1.5 GHz, with very flat responses. Further analysis and design revision of the laser is ongoing.

Development of techniques such as atom optical pumping, for atomic clocks and matter-wave interferometers (gyrometer, accelerometer, gravimeter and gradiometer), requires laser diodes with high power and excellent spectral (narrow linewidth) and spatial qualities together with high reliability. We have developed high power Fabry-Perot and distributed Feedback lasers diodes (DFB) emitting at 780nm corresponding to the D₂ line of Rubidium. We have realized a Fabry-Perot laser diode, without aluminium in the active region, with a cavity length and a waveguide width of respectively 1mm and 4μm and an AR/HR coating on the facets. We obtain a low threshold current around 40mA and a high slope efficiency of 1W/A at 20°C. We obtain a good beam quality M² of 2 at 200mW. These lasers diodes are very interesting to be inserted in an external cavity. For DFB lasers, we used a second order diffraction grating in a GaInP/GaAsP/GaInP waveguide. We calculated the coupling coefficient KL of 1.5 for a length of 2mm, for a width of 4μm. These lasers show a low threshold current (around 65mA) with a slope efficiency around 0.37W/A. We have obtained at 25°C, 145mA an optical power of 25mW at the D₂ line of Rubidium with a side mode suppression ratio around 44dB. By the selfheterodyne method, we measured a low linewidth of our DFB laser at 780nm around 1.25MHz (lorentzian fit).

The conventional distributed feedback (DFB) edge-emitting lasers with buried gratings require two or more epitaxial growth steps. To avoid the problematic overgrowth we have used laterally-corrugated ridge-waveguide surface gratings, which also enable easy integration of the resulting laterally-coupled DFB (LC-DFB) lasers with other devices and are applicable to different materials, including Al-containing ones. The paper presents the modeling and design particularities of LC-DFB lasers, the fabrication process, involving a highly productive and cost-effective UVnanoimprint lithography technique, and the characteristics obtained for the LC-DFB lasers fabricated from GaAs-, GaSband InP-based epiwafers. The first batches of GaAs-based LC-DFB lasers, emitting at 894 nm, GaSb-based LC-DFB lasers emitting at 1.946 μm and InP-based LC-DFB lasers, emitting at 1.55 μm had relatively low threshold currents, a high side-mode-suppression-ratio and exhibited linewidths in the range of 1 MHz and below, showing that the LC-DFB lasers are an effective low-cost alternative for the conventional buried-grating DFB lasers.

The conventional distributed feedback and distributed Bragg reflector edge-emitting lasers employ buried gratings, which require two or more epitaxial growth steps. By using lateral corrugations of the ridge-waveguide as surface gratings the epitaxial overgrowth is avoided, reducing the fabrication complexity, increasing the yield and reducing the fabrication cost. The surface gratings are applicable to different materials, including Al-containing ones and can be easily integrated in complex device structures and photonic circuits. Single-contact and multiple contact edge-emitting lasers with laterally-corrugated ridge waveguide gratings have been developed both on GaAs and InP substrates with the aim to exploit the photon-photon resonance in order to extend their direct modulation bandwidth. The paper reports on the characteristics of such surface-grating-based lasers emitting both at 1.3 and 1.55 μm and presents the photon-photon resonance extended small-signal modulation bandwidth (> 20 GHz) achieved with a 1.6 mm long single-contact device under direct modulation. Similarly structured devices, with shorter lengths are expected to exceed 40 GHz small-signal modulation bandwidth under direct modulation.

To combine low-cost fabrication and high-speed data communication like 100 GBit/s, multi-section DBR lasers are developed with nanoimprint compatible surface defined gratings. This laser design has the potential to enhance the modulation bandwidth by exciting a higher order optical mode, the so-called photon-photon resonance (PPR). ICP-RIE etching was used to transfer the e-beam exposed surface pattern in one step into the semiconductor. High aspect ratios of > 1:15, vertical trenches with a width of about 140 nm and an etch depth of > 2 μm were obtained for the lateral gratings. Three-section DBR lasers are fabricated on an MOVPE grown 1.5 μm InP laser material exhibiting CW threshold currents of 94 mA for a 0.9 mm long device. A side mode suppression ratio of > 50 dB could be achieved demonstrating a high enough coupling strength of the lateral gratings. The influence of different operation conditions (currents, temperature) and dependence on the grating period on threshold current and emission wavelength are studied and will be discussed in this paper. First high frequency measurements in operation conditions without PPR enhancement show a - 3dB bandwidth of about 15 GHz.

Low cost and compact transmitters are key components for short reach and datacom applications in fiber communication systems. Bit rates of 40 Gbit/s are beyond the speed limitation of conventional directly modulated lasers. Lasers with an integrated electro-absorption modulator (EML) represent one possible solution. An attractive alternative are Passive Feedback Lasers (PFL). Here, the modulation bandwidth is significantly increased by an integrated feedback section. Underlying physics and functionality of the PFL are presented as well as the successful realization at wavelengths in the 1300 nm and 1550 nm regions. The performance of these PFLs is demonstrated in system experiments.

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and the wall plug efficiency of blue and green lasers. We report on improvements of the performance of true blue riedge waveguide InGaN lasers at 452nm with cw-output power up to 800mW in overstress and mono mode operation up to 500mW in a temperatures range of 20°C to 80°C. We succeeded in high and almost temperature independent wall plug efficiencies >20% at stable output power levels from 200 to 500mW in cw-operation. Due to several improvements of our blue laser diodes we now estimate life times is in the order of 40khrs for 80mW output power in cw-operation at 40°C. Additional overstress degradation tests at power levels up to 200mW show a strong dependency of lifetime with output power. Furthermore, we present pioneering results on true green InGaN laser diodes on c-plane GaN-substrates. The technological challenge is to achieve In-rich InGaN-quantum wells with sufficiently high material quality for lasing. We investigated the competing recombination processes below laser threshold like nonradiative defect recombination by electro-optical measurements, such confirming that low defect densities are essential for stimulated emission. A model for alloy fluctuations in In-rich InGaN-MQWs based on spectral and time resolved photoluminescence measurements yields potential fluctuations in the order of E₀=57meV for our blue laser diodes. To get a closer insight into the physics of direct green InGaN-Laser we investigated the inhomogeneous broadening of experimentally measured gain curves via Hakki-Paoli-measurements in comparison to calculated gain spectra based on microscopic theory showing the importance of strong LO-phonon coupling in this material system. Investigations of current dependent gain measurements and calculations yield a factor of 2 higher inhomogeneous broadening for our green lasers than for our blue laser diodes on c-plane GaN. Based on the improvements of the material quality and design we demonstrate true green InGaN-Laser in cw-operation at 522nm with more than 80mW output power on c-plane GaN. The combination of low laser threshold ~60-80mA, high slope efficiency ~0.65W/A and low operating voltage 6.9-6.4V of our green monomode RWG-Laser results in a high wall plug efficiency of 5-6% in a temperature range of 20-60°C.

The optical gain characteristics of high Al-content AlGaN quantum wells (QWs) are analyzed for deep UV lasers. The effect of crystal-field split-off hole (CH) and heavy-hole (HH) bands crossover on the gain characteristics of AlGaN QW with AlN barriers is analyzed. Attributing to the strong transition between conduction - CH bands, the TM spontaneous emission recombination rate is enhanced significantly for high Al-content AlGaN QWs. Large TM-polarized material gain is shown as achievable for high Al-content AlGaN QWs, which indicates the feasibility of TM lasing for lasers emitting at ~220-230 nm.

In the present work we demonstrate a concept of a "weak plasmonic cladding" for the improved transversal optical confinement in the structures of nitride lasers diodes emitting in the violet and blue spectral region. We show that by using highly doped GaN:O or GaN:Si layers we can induced the reduction of the refractive index by around 1-2% comparing to a lightly doped material. Such a material can be effectively used as optical cladding replacing thick, highly strained AlGaN layers. Plasmonic claddings can be grown by two methods: first of them is High Nitrogen Pressure Solution growth (an introduced donor is an oxygen) and Molecular Beam Epitaxy with silicon as a donor. In the both cases we can reach a free carrier concentration of up to around 10²⁰cm^-3. MOVPE method so far did not show capabilities for achieving so high doping level. We demonstrate the use of such layers for the construction of the violet and blue laser diodes and laser diodes mini-arrays showing a total suppression of the substrate mode leakage.

We present optical pulse generation with 100 W peak power, 3 ps temporal duration, and 1 GHz repetition from a GaInN master oscillator power amplifier (MOPA). An external cavity GaInN laser diode is passively mode-locked to generate 4 W peak power optical pulses as a master oscillator, and a semiconductor optical amplifier (SOA) effectively amplified these to a high peak power of more than 100 W. Two major factors that contributed to effective amplification of optical pulses were the generation of clean optical pulses without sub-pulse components by the GaInN mode-locked laser diode and suppression of amplified spontaneous emission in the SOA. This novel high-peak-power pulse source is used to induce multi-photon absorption to create sub-micrometer recording marks in a bulk plastic recording media. The realization of an all-semiconductor pulse source is a significant breakthrough towards a practical 3D optical data storage system.

We present a systematic study of the dynamics of nitride laser diodes with different emission wavelengths ranging from violet (409 nm) to green (512 nm). The current-dependent relaxation behavior of the laser diodes above threshold is analyzed with high temporal and spectral resolution and electroluminescence decay measurements below threshold are employed to measure the charge carrier lifetimes at low excitation. We compare the experimental results to rate equation simulations to investigate on the origin of the differences in the dynamical behavior as function of the laser wavelength. From fitting these simulations to the experiment we extract device parameters such as the carrier lifetime at threshold and the differential gain.

The equations for the threshold-current density J_th, differential quantum efficiency η_d and maximum wallplug efficiency η_wp,max for quantum-cascade lasers (QCLs) have been modified for electron leakage and backfilling. We used a thermalexcitation model of "hot" injected electrons from the upper laser state to upper active-region energy states to calculate leakage currents. Then the calculated characteristic temperature T₀ for J_th was found to agree well with experiment for both conventional and deep-well QCLs. The characteristic temperature T₁ for η_d was deduced to be due to both electron leakage and an increase in the waveguide-loss coefficient. For conventional mid-infrared QCLs η_wp,max is found to be strongly temperature dependent which explains experimental data. By using a new concept: tapered active-region (TA), deep-well QCLs have been optimized for virtual suppression of the electron-leakage currents. In turn, at room temperature, for continuous-wave (CW)-operating, 4.5-5.0 μm-emitting TA QCLs we estimate the threshold current to decrease by ~ 25 %, the active-region temperature rise at the η_wp,max point to decrease by ~ 30 %, and the single-ended, η_wp,max value to become at least 22 %. Preliminary results from TA QCLs include T1 values as high as 454 K, over the 20-60 oC heatsink-temperature range.

We demonstrate a terahertz quantum-cascade laser (QCL) operating significantly above the temperature of hv/kB, which had so-far been been an empirical limitation for the maximum operating temperature of these devices. With a design that employs a new scattering-assisted injection scheme, a 1.8 THz QCL operating up to a temperature of 1.9hv/kB (163 K) is realized with more than 3 mW of peak optical power output at 150 K. We also demonstrate continuous tunability over a frequency range of 137 GHz of a single-mode QCL operating at 3.8 THz in metal-metal waveguides. A unique concept of altering the lateral mode profile of the "wire laser" waveguide geometry was implemented to achieve tuning despite the strong mode confinement of metal-metal waveguides at terahertz frequencies.

We present a development of a broad gain heterogeneous quantum cascade laser for terahertz frequencies. By placing appropriate different active-regions based on a four-quantum-well design into a double-metal waveguide we obtained laser emitting gapless over a bandwidth of 1 THz, between 3.2 to 2.2 THz. This means that this single-device source covers an emission range of nearly 40 % around the center frequency. In pulsed mode operation, our devices show threshold current density as low as 285 A/cm² and they operate up to 125 K. We also report on continuous wave measurements.

We describe a monolithically integrated THz transceiver consisting of a Schottky diode embedded into a THz quantum cascade laser (QCL) waveguide. Besides functioning as a heterodyne receiver for externally incident radiation, the device is a useful tool for characterizing the performance and dynamics of the QCL. Here we present an overview of the device, demonstrate receiver operation, and present laser dynamics measurements especially related to feedback of the QCL's emission due to retroreflections.

We propose a novel room-temperature semiconductor source of narrowband coherent THz radiation that radiates via the Smith-Purcell effect and uses the Gunn effect to increase the radiated power (Figure 1). The radiation frequency of the device depends only on the grating period and the velocity of the Gunn domain, reaching low THz range with submicron grating. The Smith-Purcell effect is well-known in vacuum electronics and accelerator physics: a DC electron beam focused parallel to a metallic grating generates weak, incoherent electromagnetic radiation called spontaneous Smith-Purcell radiation (SPR). A bunched electron beam instead generates coherent stimulated SPR, with radiated power many orders of magnitude stronger than spontaneous SPR. Spontaneous SPR has been observed in semiconductors, but only at low temperature. The Gunn effect naturally provides space-charge bunching in semiconductors, and Gunn domains generate stimulated SPR at room temperature in the proposed devices. We develop a simple analytic model to describe the radiation which agrees well with device simulations. The small size of the radiative portion of the device suggests the use of an integrated antenna which significantly boosts the power of simulated devices. Our results indicate that the effect should be observable from a InP device, with power density of 30nW/μm of device width at 0.27 THz and 6nW/μm at 0.44THz. Results based on InN parameters look very promising for this approach with calculated power densities of 3.5μW/μm at 0.25THz, 20nW/μm at 1.0THz, and 3nW/μm at 2.0THz. We conclude that experimental investigation is warranted.

There is currently considerable interest in terahertz (THz) difference frequency generation (DFG) utilizing near-resonant intersubband nonlinearity in quantum cascade lasers and other quantum well heterostructures. Such devices were shown to operate at room temperature, but their power demonstrated so far has been rather low. In all previous works the intracavity configuration was studied, in which the nonlinear mixing region was placed inside a pump laser cavity. Here we obtain the upper limits on the THz DFG power that can be achieved in intersubband quantum well systems under external optical pumping. We consider strong optical fields and include all resonant absorption or pump depletion, and nonlinear saturation effects by self-consistently solving three coupled wave equations, the Poisson equation, and density matrix equations. Although we use a GaInAs/AlInAs double quantum well heterostructure as an example, our analysis is applicable to any material system possessing resonant second-order optical nonlinearity. The maximal THz DFG power is reached for intermediate pump intensities of the order of the saturation intensity. Further increase of the pump intensity results in the decrease of the maximum THz DFG power. We analyze the dependence of THz power on the length and doping of the nonlinear mixing region, and present the design of the optimal structures.

Highly efficient 670 nm-tapered lasers with a vertical divergence of 31° (FWHM) will be presented. The devices are based on a GaInP single quantum well embedded in AlGaInP waveguide layers. Compared to previously reported material, the structure has an improved material quality with a transparency current density j_tr = 165 A/cm², an internal efficiency ηi = 0.75, small internal losses αi = 1.2 cm^-1, and a good temperature stability with T₀ = 120 K. 2 mm long tapered lasers were fabricated in a standard process, using reactive ion etching for the index-guided structures and ion implantation for the definition of the contact window in the tapered section. The properties of devices with 500 μm or 750 μm long ridge waveguide (RW) section and a flared section with 3° or 4° taper angle will be compared. In CW-operation an output power up to P = 1 W with a conversion efficiency of 30% and a beam propagation ratio M² (2nd moments) smaller than 2.3 were obtained. In pulsed mode up to 3.3 W output power was measured.

We report on development of novel curved waveguide (CWG) laser devices, where the emission wavelength centered at ~976 nm is stabilized to a 20 dB bandwidth of less than 100 picometer by using fiber Bragg gratings (FBG). Radiation from the curved waveguide laser is coupled using an anamorphic fiber lens into a single mode polarization maintaining fiber containing the FBG, the latter acting as a front reflector. The high power gain chip is based on Oclaro's InGaAs/AlGaAs quantum well laser. Use of the curved waveguide geometry allows to eliminate residual reflections in the optical path of the cavity, which is formed by the rear chip facet and the FBG. It is well known that additional reflections lead to significant deterioration of the spectral and power stability. The devices, assembled in telecom type housings, provide up to 1 W of low-noise and kink-free CW power. In addition pulse operation in nanosecond range is also investigated. The spectral stabilization time to the wavelength of the FBG is limited by the external cavity roundtrip of ~2 ns. A side mode suppression ratio of about 40 dB and higher is achieved for pulsed and CW operation. Results are also presented for a device at 1064 nm. Numerous applications can be envisioned for these devices. For instance devices with high power and ultranarrow spectral bandwidth allow greater flexibility in the choice of parameters for frequency conversion applications. In pulsed mode the device can be used in the special sensing applications where spectral stability is crucial.

We demonstrate a compact high-power master oscillator (MO) power amplifier (PA) laser module emitting near 1064 nm. The module is micro-integrated on a footprint of 50 x 10mm². The oscillator is a distributed Bragg reflector laser optimized for narrow linewidth operation. The amplifier consists of a ridge waveguide entry and a tapered section. The module features stable single-mode narrow linewidth emission at an output power of 1W and can be tuned mode-hop free by 450GHz.

The optical pumping injection cavity (OPIC) laser concept was developed to enhance the efficiency of optical pump beam absorption, and this work focuses on epitaxial configurations designed for broadband absorption around 1850 nm, an optimal pump wavelength for transmission through GaSb substrates that allows for epi-down mounting for improved heat management, while minimizing the photon decrement. The OPIC devices presented in this work have InAs/GaSb/InAs/AlSb type-II W active regions with a thicker GaSb/AlAsSb distributed Bragg reflector on top in order to enhance reflection back into the active region for epi-down mounting. Results are presented for optical pumping at 1850 nm as well as for resonant optical pumping, as the cavity resonance varies with temperature due to shifts in lattice constant, refractive index, and gain. Pumping at 1850 nm resulted in lasing from 78 K up through 310 K. At 78 K, the actual pump cavity resonance is ~1840 nm, and with increasing temperature the resonance shifts to longer wavelengths beyond 1850 nm. Emission wavelengths range from 3.59 μm at 78 K to 4.01 μm at 310 K for 1850 nm optical pumping. The broadened OPIC configuration presents a distinct advantage over earlier reported OPIC devices as the broader resonance allows for efficient emission across a wide temperature range for a single pump wavelength (e.g., 1850 nm), providing over 400 nm of wavelength tuning. Results will be compared with a second broadened OPIC with emission wavelengths beyond 4 μm that temperature tunes across the carbon dioxide spectral line at 4.2 μm.

We discuss the design and performance of quantum cascade laser sources based on intra-cavity second harmonic generation operating in at wavelengths shorter than 3.7μm. A passive heterostructure tailored for giant optical nonlinearity is integrated on top of an active region and patterned for quasi-phasematching. We demonstrate operation of λ≈3.6μm, λ≈3.0μm, and λ≈2.6m devices based on lattice-matched and strain-compensated InGaAs/AlInAs/InP materials. Threshold current densities of typical devices with nonlinear sections are only 10-20% higher than that of the reference lasers without the nonlinear section. Our best devices have threshold current density of 2.2kA/cm² and provide approximately 35μW of second-harmonic output at 2.95μm at room temperature. The second-harmonic conversion efficiency is approximately 100μW/W². Up to two orders of magnitude higher conversion efficiencies are expected in fully-optimized devices.

QC laser active regions with multiple transitions from strongly coupled upper laser states to lower laser states were designed to achieve broad gain spectra as well as high performance. Two broad gain QC laser designs in the 7-9 μm wavelength region were demonstrated based on the continuum-to-bound design. The first embodiment of one design at ~8 μm enables external cavity tuning over 190 cm^-1 (7.5 - 8.8 μm) in pulsed mode operation at 0°C. We also demonstrated a QC laser structure based on a continuum-to-continuum active region in the 4-5 μm wavelength region with a gain spectrum of ~ 430 cm^-1. External cavity tuning over 340 cm^-1 (4.4 - 5.2 μm) was achieved with this design in pulsed mode operation at 15°C. In spite of the broad gain spectrum, a low threshold current density (1.6 kA/cm2), large slope efficiency (4.5 W/A), good temperature performance (T0=160 K), high peak power (up to 5 W) and high wall plug efficiency (WPE, up to 20%) were achieved for ridge lasers with as cleaved facets, in pulsed mode operation at 295 K, demonstrating that it is possible to make a wide gain spectrum compatible with high power and efficiency performance.

We demonstrate room temperature lasing of quantum cascade lasers (QCLs) operating at 3.45 and 3.55 μm under pulsed and continuous wave (CW) operations, respectively. To the best of our knowledge those are the shortest wavelengths ever achieved at room temperature from QCLs with strain balanced GaInAs/AlInAs material on InP substrate. With the back facet high reflection coated, a maximum output power of 60 mW was obtained at 10 °C under CW operation. A tuning range of 124 cm^-1 was obtained in a pulsed mode in external cavity configuration.

High power diode lasers in 7xx-nm region, have been needed for various applications. Compared to 9xx nm lasers that have been developed extensively in the last 20 years, high power lasers at 7xx-nm region presents much more challenges for operation power, efficiency, temperature performance and reliability. This paper will present recent progresses on 7xx nm laser diodes for the above attributes. Two laser designs will be reviewed and high power diode laser performance and reliability will be presented. Single emitter devices, with 200μm wide emitting width, show up to 10W reliable operation power, with peak efficiency more than 65%. Accelerated life testing at 12A, 50°C heatsink temperature has been running for thousands of hours. High temperature performance and high COMD threshold (> 20W) will also be shown. Life-test failure modes will also be discussed. In summary, with advanced epitaxial structure design and MOCVD process, critical facet passivation and advanced heatsink and bonding technology, 7xx-8xx nm devices have been demonstrated with high performance and reliability similar to those of 9xx nm devices.

High-power (more than 500 mW) and high-speed (more than 1 Gbps) tapered lasers at 1060 nm are required in freespace optical communications and (at lower frequencies of around 100 MHz) display applications for frequency doubling to the green. On a 3 mm long tapered laser, we have obtained an open eye diagram at 1 Gbps, together with a high extinction ratio of 11 dB, an optical modulation amplitude of 530 mW, and a high modulation efficiency of 13 W/A. On a 4 mm-long tapered laser, we have obtained an open eye diagram at 700 Mbps, together with a high extinction ratio of 19 dB, a high optical modulation amplitude of 1.6 W, and a very high modulation efficiency of 19 W/A. On a 6 mm-long tapered laser, we have obtained a very high power of 5W CW and a very high static modulation efficiency of 59.8 W/A.

A new compact picosecond light source is presented. It consists of a master oscillator, an ultra fast pulse picker element and integrated high-frequency electronics arranged on a micro bench with a footprint of 5 cm x 4 cm. The master oscillator is a 10 mm long laser consisting of 200 μm long saturable absorber, 1500 μm long gain, 8000 μm long cavity, 200 μm long DBR and 100 μm long monitor sections. Pulses with a width of 7 - 10 ps (depending on output power) and a peak power of 1 W are generated by hybrid mode locking. Despite the 10 mm long cavity, the repetition rate is still 4.3 GHz, which is too high for many applications. In order to reduce the repetition rate to values between 1 kHz and 100 MHz, one has to pick single pulses out of the continuous pulse sequence generated by the mode-locked laser. For this purpose a semiconductor pulse picking element based on a ridge waveguide, which acts as an optical gate, and a tapered section for pulse amplification is developed. Selective pulse picking with a free choice of the repetition rate is achieved by driving the RW section by a high frequency GaN high-electron mobility transistor with low capacitances and high current density. If a current pulse with a width of about 200 ps (smaller than the spacing of the pulses) is injected, the RW section becomes transparent and an injected optical pulse can pass the RW section. This pulse is subsequently amplified by injecting short current pulses with a width of 2 ns and a peak current up to 20 A into the tapered section. Pulse energies in the nJ range at a frequency of 16 MHz are obtained.

In this paper we present the 830nm single mode lasers for consumer electronics and computer to plate applications. One of the key criteria is to develop a robust as well as cost effective design that enables high yield and high reliability operation. We will present results obtained on single transverse mode, single emitters or arrays of Fabry Perot lasers and single longitudinal mode lasers emitting in the range of 830nm exhibiting superior reliability performance.

Recent efforts to improve the reliability of high-power broad-area diode lasers operating in the 9xx nm wavelength band have yielded single emitter devices with excellent reliability to >15 W. However, in applications requiring fiber coupling, the fiber coupling power of a single emitter device is rather limited by its linear power density, and hence brightness of the device. Unfortunately, due to a rapid increase in the slow-axis divergence, a typical broad-area diode laser offers a much lower brightness increase and an earlier rollover than its output power. In this work, we show that thermal lensing (rather than carrier- or gain-induced guiding) in the slow axis is the predominant cause of beam quality degradation at increased driving current. Some of the techniques which are presented include the use of cavity length scaling and thermal path engineering. It is expected these approaches are critical to enabling continued scaling of highbrightness fiber coupled diode lasers.

Diode lasers that deliver high continuous wave optical output powers (> 5W) within a narrow, temperature-stable spectral window are required for many applications. One technical solution is to bury Bragg-gratings within the semiconductor itself, using epitaxial overgrowth techniques to form distributed-feedback broad-area (DFB-BA) lasers. However, such stabilization is only of interest when reliability, operating power and power conversion efficiency are not compromised. Results will be presented from the ongoing optimization of such DFB-BA lasers at the Ferdinand-Braun- Institut (FBH). Our development work focused on 976nm devices with 90μm stripe width, as required for pumping Nd:YAG, as well as for direct applications. Such devices operate with a narrow spectral width of < 1nm (95% power content) to over 10W continuous wave (CW) optical output. Further optimization of epitaxial growth and device design has now largely eliminated the excess optical loss and electrical resistance typically associated with the overgrown grating layer. These developments have enabled, for the first time, DFB-BA lasers with peak CW power conversion efficiency of > 60% with < 1nm spectral width (95% power content). Reliable operation has also been demonstrated, with 90μm stripe devices operating for over 4000 hours to date without failure at 7W (CW). We detail the technological developments required to achieve these results and discuss the options for further improvements.

A novel concept of a Finite Aperture Tapered Unstable Resonator Laser (FATURL), based on two distributed Bragg reflectors (DBRs) at the rear and front facets with different lateral widths, is presented. The passive resonator analysis using Fresnel-Kirchhoff diffraction theory shows that with the grating widths the losses of the different lateral resonator modes change differently. This leads to resonator geometries, characterized by nearly integral Fresnel-numbers with higher modal discrimination of the fundamental lateral mode. In spatial-temporal simulations of the active device the influence of carrier-photon interaction on that effect is studied showing that the inhomogeneous index change by spatial hole-burning in the carrier density may become dominant over the geometric modal discrimination. Both, the influence of the grating widths on the suppression of higher order modes as well as the influence of spatial hole-burning is found in the experimental characterization of a first generation of fabricated devices.

One of the most common methods to increase the output power of semiconductor waveguide lasers is broadening the stripe width of the active region. However, this results in higher order transverse modes to be amplified which impairs the beam quality and increases the beam divergence. By integrating optical elements into the cavity, it is possible to control the amplitude shape and the number of modes which are amplified in the laser. This paper reports on aspects to integrate phase and amplitude modifying microstructures into a semiconductor waveguide resonator by adding an additional lithographic step to the fabrication process of broad area laser diodes. The latest experimental results of such structured InGaAIP broad area lasers revealed a significant improvement of the beam quality even at a high operation current. Hence, the expansion of the stripe width of the amplifying region without degrading the beam quality is possible. The demonstrated power-current characteristics of structured laser diodes exhibit a low threshold current and a high efficiency.

Broad tunability in the mid-IR (~3-12 μm) is desirable for a number of applications. We have built a number of External Cavity quantum cascade Lasers (ECqcL^TM) that maximize tuning range from the quantum cascade chip. As much as 525 cm^-1 of pulsed and 121 cm^-1 of cw, mode hop-free tuning has been obtained. Various mechanisms for the broadening of the gain of the QC chip are considered.

A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission at 4.0 μm using nonresonant extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. An air-cooled laser system incorporating a 10 mm by 11.5 μm laser with antireflection coated front facet and high reflection coated back facet delivered over 2 W of single-ended optical power in a collimated beam. Maximum continuous wave room temperature wallplug efficiency of 5.0% was demonstrated for a high reflection coated 3.65 mm by 8.7 μm laser mounted on an aluminum nitride submount. Lasers processed from a 3.5μm structure with a similar design delivered 50 mW in CW mode and 300 mW of average power in high duty cycle mode at 265K. The low performance of the 3.5 μm structure is attributed to the fact that the bottom of indirect profile is located below the upper laser level for this design.

We report the fabrication and characterizations of integrated distributed Bragg reflector (DBR) thermally tunable buriedheterstructure (BH) quantum-cascade-lasers (QCLs) emitting at λ ~ 4.7 μm. We implemented a special device bonding technique utilizing two submount heatsinks that allow us to separately control the temperatures on the gain and DBR grating sections, respectively. With a thermoelectric (TE) cooler dedicated for the grating section, a tuning range of 65 nm, equivalently 29.831 cm^-1, was achieved while the gain section was maintained at 293K under pulsed mode operation. This corresponds to a tuning efficiency of 0.0325 nm/mA. We also established a 2D thermal model to simulate the temperature profiles and the heat flows within two sections as a function of TE cooler and gain-section temperatures. The thermal response time as the TE cooler temperature changed was also investigated.

Broad area type-I GaSb based diode lasers have recently exceeded 100 mW continuous wave room temperature powers in 3.1-3.2 μm spectral region. Certain applications such as single frequency sources for spectroscopy and efficient coupling to single mode fiber require single lateral mode laser operation. We characterize and compare two types of lasers with similar structures and various ridge widths emitting at 3.1 and 3.2 μm. We obtain 35 and 25 mW of continuous wave single lateral mode power from 8 and 13 μm wide ridge lasers emitting at 3.1 and 3.2 μm respectively. This constitutes a threefold improvement compared to the previous result. Both devices had ridges etched to the depth leaving approximately 300 nm of the top p-cladding in the areas outside the ridges. For 3.2 μm emitting lasers the dielectric thickness was 220 nm while it was 510 nm for 3.1 μm emitting lasers. Gain spectra were measured by Hakki- Paoli technique for various ridge widths. From gain spectra we extract differential gain and internal loss. We find that the internal loss in thin dielectric, 3.2 μm emitting laser is about 14 cm^-1 while it is 7 cm^-1 in thick dielectric, 3.1 μm emitting laser for the ridge widths of 13 and 8 μm exhibiting single lateral mode operation respectively. Internal losses measured on broad area, 100 μm wide lasers processed from the same materials are similar and around 6-7 cm^-1. We discuss reasons for the internal loss increase with the aid of simulation of optical mode field and loss in our waveguide structures.

AlGaInAsSb-based broad area lasers (BALs) with a monolithically integrated Fourier-optical 4f set-up in a folded-resonator geometry are realized. The two resonator branches - each one d = 0.825 mm long - are connected through a dry-etched cylindrical total-internal-reflection (TIR) mirror acting as a Fourier-transform element. Transverse mode selection (TMS) is achieved by monolithically integrated spatial-frequency filters positioned in the back focal plane of the mirror (i.e. in the Fourier-transform plane). The whole resonator is gain section (active medium) as well as part of the TMS 4f set-up at the same time. The integration of TMS within the active BAL chip is shown to be successful. All employed BAL/TMS type-II heterostructure lasers are MBE-grown on GaSb substrates, designed for an emission wavelength in the mid-infrared around 2 μm. Different laser samples without any filter elements (no-TMS) and with filters for the selection of the fundamental transverse mode (#0; TMS0) are prepared and characterized. Just for a proof of principle also samples for the selection of higher order transverse modes, here exemplarily mode #6 (TMS6) and #8 (TMS8), have been processed and investigated. The free spectral range between the longitudinal modes is found to be around 0.33 nm corresponding to the BAL's total-resonator length 2d = 1.65 mm (with an effective refractive index n_eff ≈ 3.8). This result strongly emphasizes that both resonator branches act together as one entity.

We report in this work the near-Infrared (IR) induced negative photoconductance (NPC) on mid-IR Quantum Cascade Lasers (QCLs) and its relationship with the quenching of QCL. The NPC behaviour depends on the coupled near-IR intensity, wavelength and the QCL bias voltage amplitude. If the QCL is under pulse operations, it depends on the pulse width as well. From our interpretation, the NPC is related to electron-hole pair generation and hole-trapping, which produces an internal field counteracting to the external field. The created band-bending effect will reduce the current, increase the resistance, and reduce the mid-IR light output, which has been reported earlier as the "optical quenching effect." We observed that only a near-IR laser with good quenching ability can modulate the mid-IR laser. Since the modulation speed exceeds more than 100 MHz, this excludes the possibility of a thermal origin of these results. The near-IR induced optical quenching effect has the potential to provide all optical modulation of mid-IR QCLs.

Signal modulations and coherent signal detections are important for both communications and remote sensing applications. Photonic integration reduces size, weight, cost, and improves the performance of these systems as demonstrated by integrated DFB-laser modulators, receivers and transceivers at near-IR wavelength regions. However, due to the difficulties of waveguide coupling, photonic integration in the mid-IR wavelength range is under-developed. We developed a technique that can monitor mid-IR coupling based on the negative photoconductivity in quantum cascaded (QC) materials that can simplify and optimize laser waveguide coupling in the mid-IR wavelength range.

This paper reports on the direct comparison of Hakki-Paoli and segmented contact method gain spectra measurement techniques for InAs/GaAs 1.3μm quantum dot laser material. This was made possible by realizing a single-mode segmented contact device structure. The differences in required apparatus, ease of measurements, signal to noise ratio, and relative advantages and disadvantages of these two complementary techniques are discussed under continuous wave operation. As expected, at current densities (prior to possible self heating effects) the techniques are shown to give essentially identical results. However, the segmented contact method is demonstrated to be more accurate for internal loss measurements. A method to remove self heating effects in the Hakki-Paoli measurements is described and allows the gain and spontaneous emission measurements to be performed at carrier densities as high as 5.5kA/cm² with a constant cavity temperature.

In this effort, we describe our recent progress towards demonstrating optical gain in erbium-doped lithium niobate and its potential for use in Erbium-Doped Waveguide Amplifiers (EDWAs) and lasers. By introducing erbium into lithium niobate, we have the potential for both gain at telecom wavelengths and electro-optic modulation in a single material. This has many possible applications, including high speed, narrow band tunable lasers for RF generation and other applications. We focus on lithium niobate grown in the presence of erbium available with a uniform doping profile from Roditi International. In this work we discuss photoluminescence, amplified spontaneous emission, gain, and lasing in titanium-indiffused waveguides.

A broadly tunable MQW laser utilizing a combined impurity-free vacancy disordering and beam steering techniques is proposed and investigated experimentally. The device consists of a beam-steering section and an optical amplifier section fabricated on a GaAs/AlGaAs MQW p-i-n hetrostructure substrate. The beam steering section forms a reconfigurable single mode waveguide that can be positioned laterally by applying electrical currents to two parallel contact stripes. The active core of the gain section contains a GaAs/AlGaAs MQW that is progressively disordered such that an optical beam steered through the selected region experience a peak in the gain spectrum that is determined by the degree of disordering of the MQWs. Furthermore the MQW in the beam-steering section is disordered to the largest extent to minimize optical beam attenuation. The MQW structure was intermixed using an impurity-free vacancy induced disordering technique. The MQW sample is encapsulated with a SiO₂ film grown by plasma enhanced chemical vapor deposition (PECVD). The beam steering region is coated with a 400nm thick SiO₂ film whereas in the gain section, the SiO₂ film is selectively etched such that the thickness grades linearly ranging from 0 to 325nm. The disordering of the entire slab region is then induced by a single rapid thermal annealing step at 975°C for a 20s. Experimental results showed a controllable 10 to 60 nm wavelength blue shift of the peak of the photoluminescence spectrum corresponding to the change in SiO₂ caps thickness and a lateral beam steering range up to 20 μm over the slab region.

Low threshold quantum cascade lasers (QCLs) are important for low-power chemical sensor applications. We report a low-threshold high-period-number 4.8μm strain-balanced InGaAs/InAlAs quantum cascade laser. By growing more periods (up to 60 periods) of QCL stages, shortening the QCL cavity length, and applying high-reflectivity coatings on both facets of the laser cavity, low threshold QCLs can be obtained. At current stage a QCL threshold of <30mA at 20K, <50mA at 80K, and <120mA near room temperature were achieved. Low current threshold QCLs will have significant impacts to future chemical sensing systems.

The temperature effect on the characteristics of strain-balanced mid-infrared quantum cascade lasers is theoretically investigated and compared to experimental results. Our theoretical model simulates the temperature dependence of modal gain, current threshold density, and optical output power of a QCL at pulsed, continuous wave (CW), and quasi-CW operation conditions. Simulation results show that QCLs operated at quasi-CW driving condition expresses much lower laser core temperature than CW mode while generates comparable or even higher average power than CW mode. The model can be used to predict the optimum driving condition in order to generate maximum average output power.