This PDF file contains the front matter associated with SPIE Proceedings Volume 12021, including the Title Page, Copyright information, Table of Contents and Conference Committee list.

To realize a high-speed high-reliability wavelength switching at future WDM networks, we proposed the injection current/temperature cooperative control method of the tunable-distributed-feedback (DFB) laser array (TLA) for avoiding a large injection current, which is the primary cause of laser degradation. We successfully demonstrated a full-C-band wavelength switching within 124 ms by rapid increase of the injection current and following gradual decrease of the injection current to avoid laser aging. Compared to the conventional control method, the meantime to failure of the TLA in the proposed method is estimated to be extended by 44 times.

A wavelength adjustable distributed Bragg reflector diode laser suitable for background-free Raman spectroscopy at 785 nm is presented. It is based on a GaAsP single quantum well embedded in a 1 μm thick AlGaAs waveguide. The 3 mm long device consists of a 2.2 μm wide ridge waveguide and a 10^th order DBR surface grating as wavelength selective rear side mirror. On-chip resistors implemented as heater elements next to the grating allow applying a current that enables a flexible wavelength adjustment by Joule heating. At a heatsink temperature of 25°C, the laser provides 100 mW of optical output power and narrowband laser emission with spectral widths of 20 pm (0.3 cm^-1) along the whole power range. A current up to 0.6 A applied to the on-chip resistors shifts the excitation wavelength by 2.18 nm (35 cm^-1) with narrowband emission at all settings and an optical output power remaining within a span of 14 mW. Along that available wavelength range, alternating dual-wavelength operations for five wavelengths separated from another by about 8 cm^-1 are presented (f = 1 Hz, 50% duty cycle). At 50 ms after switching, the spectral distance between selected target wavelengths and measured peak wavelengths is ≤ 1.1 cm^-1 (≤ 0.07 nm). This enables flexible selections of excitation wavelengths with low latencies for background-free Raman spectroscopy.

This work investigates a monolithic slotted Y-branch diode laser as a beating source to drive a continuous wave Terahertz spectrometer. Both arms of the Y-branch laser exhibit spectral selective feedback, which causes simultaneous emission at two frequencies. At first, a thorough optical characterisation with 5400 individual setpoints is performed to find the best point of operation. Two operational regimes with difference frequencies of 1 THz ± 10.5 GHz and 0.85 THz ± 6.5 GHz were identified. While validating the laser as a beating source to drive a cw-THz spectrometer, it was demonstrated that the device supports current-induced tuning of the emitted difference frequency. This technique allows frequency sweeps in the terahertz regime that can be used to measure the transmitted field without a mechanical delay stage. Finally, this technique is demonstrated to independently determine the thickness and refractive index of high resistive float zone silicon wafers of 2, 3.5, 4 and 8 mm thickness without a priori knowledge.

Dilute bismides are a promising class of semiconductors that find applications in mid-infrared optoelectronic devices, especially lasers, photodetectors, and heterojunction solar cells owing to the tenability of the bandgap, the low temperature dependence of their physical properties, and the low Auger recombination coefficient. In this paper we investigate on the defects and deep levels in n-type GaAs1-xBix (x = 1.2 %) Schottky barrier diodes grown by low-temperature molecular beam epitaxy (MBE). Original results obtained by means of capacitance deep level transient spectroscopy (C-DLTS) indicate that: (a) only four majority and two minority carrier traps with concentration below 10¹⁴ cm^-3 can be detected in the probed regions and (b) the concentration of the defects is weakly position-dependent, thus indicating that MBE is an effective growth technique to control defect formation. The analysis of the carrier capture kinetics of the dominant electron and hole traps show that (c) they are associated to a dislocation and a point defect with capture barrier of 0.48 eV, respectively. Finally, by comparing the DLTS signatures of the detected defects, we proved (d) that only a deep level is possibly associated to the presence of the bismuth, while the others were already found in pure GaAs.

Multi-wavelength laser sources have gained significant interest for future high-bandwidth density DWDM optical links, enabling improved energy efficiency and bandwidth scaling. In this work, we present an integrated III-V/Si hybrid four-wavelength DFB laser with 200 GHz wavelength spacing and <10 dBm output power per wavelength. The wavelength spacing and total output power variations are <±25 GHz and <1 dB, respectively, for an ambient temperature change of 30°C. We also measured the relative intensity noise (RIN) and Lorentzian linewidth of the laser to be <-135 dB/Hz and <300 kHz, respectively.

Direct epitaxial integration of III-V lasers on Si offers substantial manufacturing cost and scalability advantages. We present our work towards fully monolithic active/passive Si photonics integration by metalorganic chemical vapor deposition (MOCVD) heteroepitaxy. 1.55 µm InP-based Fabry-Perot (FP) lasers on Si by blanket heteroepitaxy are firstly demonstrated, achieving electrically pumped continuous-wave (CW) lasing exceeding 65°C. Aging test shows a stable operation after 200 hours. As a compelling candidate for lasers on Si by virtue of their defect-forgiving nature, InAs/GaAs quantum dot (QD) lasers are then presented. Threshold current as low as 8 mA and high single-facet output power of 200 mW are obtained for devices on GaAs. The QD photoluminescence on Si exhibits the same intensity and full-width half-maximum as on GaAs. At last, we discuss the perspectives on fully integrated III-V/Si photonics by selective area heteroepitaxy (SAH) and present high-quality GaAs-based materials selectively grown in 7 µm ~ 30 µm wide recessed SiO2 on Si. A low defect density of 8.5×106 cm-2 is achieved for the GaAs buffer and the subsequently grown GaAs/InGaAs multi-quantum-well (MQW) microdisk lasers (MDLs) are demonstrating room-temperature lasing under optical pulsed pumping.

This paper investigates the temperature dependence of the optical degradation of InAs quantum-dot (QD) lasers grown on silicon, and its relation with impurity diffusion processes. This goal was achieved by submitting a group of identical 1.3 μm QD LDs on Si to a series of constant-current stress experiments at baseplate temperatures ranging from 15 °C to 75 °C. The analysis of the threshold current (I_th) kinetics revealed that the optical degradation process i) is not activated by temperature for junction temperatures (T_j) lower than 60 °C, ii) becomes temperature activated with E_a ≈ 0.6 eV up to 80 °C, iii) is further accelerated for higher operating temperatures, and iv) resembles a diffusion process, due to the squareroot dependence of the I_th variation on stress time. This peculiar temperature activation was explained in terms of a recombination-enhanced diffusion process, driven be the escape of carriers from the InAs QDs toward nearby semiconductor layers. This process, which is strongly inhibited at low/room temperature, becomes relevant only above a specific temperature threshold. In this condition escaped carriers can be captured by extended defects, where they recombine and release their excess energy non-radiatively. This energy release contributes to the generation of additional defects, and/or to the diffusion of impurities, whose physical origin could be preliminarily attributed to the p-dopant Be, or to the native defects limiting its diffusivity (V_Ga or Ga_I).

We study the stability of a hybrid laser source consisting of a III-V reflective semiconductor optical amplifier (RSOA) edge-coupled to a silicon photonic mirror, based on two coupled high-Q microring resonators, providing a narrow band effective reflectivity. We simulate the laser dynamics through a model of time-delayed algebraic equations accounting for the frequency-selective mirror reflectivity, demonstrating single-mode emission, self pulsing, and turbulent regimes. Further, we identify the regions of higher CW operation in terms of bias current and laser detuning with respect to the reflectivity peak. Finally, we test the CW laser stability with respect to optical feedback, mimicking the effect of spurious back-reflections from the passive parts of the circuit, and demonstrate ultra-stable CW operation for a sizeable range of detuning.

An InP integrated widely tunable laser is investigated for the use as a swept source in optical coherence tomography (OCT) applications. The laser is realized on a generic integration technology platform. It consists of a gain medium and a bandpass filter with 3 cascaded asymmetric Mach-Zehnder interferometers. The additional presence of a balanced Mach-Zehnder modulator as variable out-coupler is instrumental to increase the laser tuning range to 90 nm between 1480 and 1570 nm but can add to additional filtering effects in the laser cavity. In this work, we propose an optimized control strategy for the wavelength calibration of this widely tunable laser source, for a stepwise wavelength scan that is suitable for OCT. The aim is to obtain a wavelength scan with at least 1000 of 10 GHz equally spaced optical frequencies, having uniform power around 100 µW and 1 GHz accuracy. The control strategy is based on the a-priori knowledge of the coarse and the medium filter tuning and on an optimization of the fine filter tuning and the longitudinal cavity mode tuning that can be frequently updated. In this way, the calibration of the laser system can be kept sufficiently accurate and stability of the scan quality can be ensured. With this strategy, 10 GHz spaced optical lasing frequencies are obtained over 30 nm making the calibrated laser suitable as an OCT source

Photonic crystal surface emitting lasers emitting up to 2.6 µm have been designed and fabricated. A high-index-contrast photonic crystal layer was incorporated into the laser heterostructure by air-hole-retaining epitaxial regrowth. Transmission electron microscopy studies demonstrated uniform and continuous regrowth of the nano-patterned GaSb surface with AlGaAsSb alloy until air-pockets start being formed. The photonic crystal surface emitting lasers based on diode laser and cascade diode laser heterostructures generated narrow spectrum low divergence beams with mW-level output power. The angle-resolved electroluminescence analysis demonstrated well resolved photonic subbands corresponding to Γ2 point of square lattice and photonic gaps of several meV.

Surface emission lasing in pulse operation was realized with a quantum cascade laser using a two-dimensional photonic crystal at a wavelength of about 4 µm. The quantum well of the light-emitting layer was a strain-compensated system, and crystal growth was achieved while maintaining the strain balance. PC-QCLs lasing were realized on the single-mode vertical-emission at 77K under pulse operation. The threshold current density is 760 A/cm2 . The far-field radiation pattern showed a very small divergence angle of less than 1°. In addition, we observed the polarization dependence of the far-field profile, and analyzed for operating characteristics

Quantum cascade lasers (QCLs) are relevant optical sources for free-space communication because they can emit in the long-wave infrared (LWIR) domain, i.e. in the 8-12 µm region. The advantage of this optical domain is that it combines a high atmosphere transmission¹ with a reduced distortion for propagating beams,² thus the superiority of LWIR lasers in comparison with existing near-infrared systems is very dependent on link availability.³ Furthermore, QCLs are characterized by the absence of relaxation oscillation resonance.⁴ This peculiarity could imply a very large modulation bandwidth, even if QCL structures still need to be optimized to avoid parasitic effects.⁵ Recent experimental efforts have highlighted the potential of QCL-based free-space communication systems^6–8 and the current 4 Gbits/s record rate is expected to be outperformed in the near future with bandwidth-enhanced structures.⁹ This work describes a free-space live video broadcasting with a room-temperature QCL emitting at 8.1 µm. The video file is encoded in uncompressed high-definition format (1280 pixels x 720 pixels) and this corresponds to a data rate of 1.485 Gbits/s with on-off keying scheme. This high-speed electrical signal is directly injected in the QCL via the AC port of a bias tee. The modulated optical signal from the QCL is retrieved with a Mercury-Cadmium-Telluride detector and the resulting electrical signal is sent to a TV monitor where the video can be watched in live. The current findings demonstrate the versatility of a communication system with QCLs and this paves the way for real-field applications

Here we report on the multi-gas detection of carbon monoxide (CO), nitrous oxide (N₂O), carbon dioxide (CO₂), and water vapor (H₂O) by using a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor exploiting a Vernier effect-based quantum cascade laser as excitation source. The device emission wavelengths ranged from 2100 cm^-1 to 2250 cm^-1. The achieved minimum detection limits were 6 ppb, 7 ppb, and 71 ppm fr CO, N₂O, and CO₂, respectively, at 100 ms of integration time. Finally, QEPAS sensor performances were tested retrieving the concentrations of the target gases within laboratory air.

Asymmetric photon density (recombination-rate) along the high-power diode laser cavity leads to longitudinal-spatialhole- burning (LSHB), which limits maximum output power. Here, we summarize recent investigations on the impact of LSHB on current (longitudinal) and carrier (lateral and longitudinal) density distribution and hence total-recombination for continuous-wave (CW) operations. Custom diode lasers with 90 μm stripe and 3000-6000 μm resonator have been fabricated with segmented p-side contact to measure local current density and backside metallization window to measure relative carrier density (via spontaneous intensity) and infer temperature (via wavelength). Also, 98% back facet reflectivity and 0.8% and 20% front facet reflectivities have been used to vary the photon density profile and hence severity of hole-burning. We present data showing that current crowds at the front facet due to the high recombinationrate, which becomes more severe as the bias and resonator length increase. The current crowding effect is reduced using higher front facet reflectivity. Longitudinal one-dimensional simulation is broadly consistent with experiments at low bias; however, the current crowding effect is substantially stronger in the experiment than simulation at high bias. Further, spatially-resolved-spontaneous-emission measurements of intensity and wavelength demonstrate that the longitudinal carrier density is also non-uniform with a higher carrier density at the back facet for 0.8% front facet reflectivity, even at low bias, while it is flat for devices with 20%. At high bias, temperature increases at the front facet, leading to lateral carrier accumulation at the stripe edges, higher current and carrier density, which is not included in the simulation.

Tapered diode laser and amplifier structures feature an intrinsic occurrence of different beam waist positions in lateral and vertical direction. The effect, described as astigmatism, shows a changing magnitude in dependence of the device working point. Different working points may therefore require different optical setups to achieve the desired beam shaping and will also affect the overall amplifier setup performance. This work investigates the influences of thermal and charge carrier induced changes to the optical device properties for tapered diode amplifier structures, based on gallium arsenide, at an emission wavelength of λ = 980 nm and a tapered section length of lT P = 4 mm. An advanced beam propagation algorithm was utilized to simulate the optical behavior of the device. To address the dominant influences of localized temperature change and charge carrier distribution the optical model is coupled to a thermal and electrical solver algorithm. General applicable astigmatism mechanisms are described which are based on the insights to the microscopic device functionalities. This includes the influence of different injection current densities, different thermal heat sink conductivities as well as heat spreading on top of the device. The theoretically established outcomes give insight to fundamental tapered amplifier mechanisms, necessary for better understanding of experimental results. Furthermore, the approach opens up the ability to optimize the optical setup which is used to shape the emitted radiation. At the same time limits of the tapered device design will be discussed.

In this contribution, the properties of 4 mm long DBR-tapered lasers with different DBR grating lengths and different ridge waveguide lengths will be investigated. For the different device designs, the influence of the ridge waveguide current on the spectral and spatial parameters will be presented. The vertical layer structure of the devices is based on a GaAsP single quantum well in a large optical cavity. Three different grating lengths, i.e., 500 μm, 750 μm, and 1 mm, were manufactured using e-beam lithography. For the first two gratings, the tapered section has a length of 2.5 mm, for the latter 2.0 mm. All devices have a full tapered angle of 6°. The devices having the longer tapered sections reach output powers up to 7 W with a narrow spectral width. Increasing the ridge waveguide current increases the optical output power up to a saturation level. At saturation level, although the output remains approximately stable, differences in the spectral behavior and the beam quality occur. The dependence of spectral properties and beam parameters on the ridge waveguide current will be discussed. A correlation between spectral properties and lateral beam profile measured at beam waist position can be supposed. The measured emission width amounts to 19 pm, which is limited by the resolution of the used spectrometer. In best cases, at an output power of 5 W the lateral beam propagation ratio is below 3.5 (1/e²) and 8.4 (2^nd moments). These lasers are well-suited as pump lasers for Tm:YAG lasers and as excitation light sources in Raman spectroscopic experiments with large excitation areas.