LiDAR (light detection and ranging) technology has gained significant importance in various fields, including autonomous vehicles, environmental monitoring, and remote sensing. 905nm pulsed laser diodes are an essential component of a LiDAR system, which performance relies heavily on the laser power level and the characteristics of its emitted light beam. While the fabrication of high-power pulsed laser diodes is already mature, reliably combining a high-power level and a concentration of the laser beam (⪆90%) within a limited emitting width is an ongoing challenge. Another major concern is the development of facet coatings capable of withstanding the high-power density generated by these lasers. Facet degradation due to the excessive optical power leads to a reduced laser efficiency and a risk of increased failure rates. Here, we have devised novel optical waveguide designs for triple junction 905nm lasers with optimized mode profiles and an effective confinement structure. This enables lasers to have a power density that is four (4) times higher than the traditional ones and, to confine most of the beam energy within an emitting width of less than 60 µm. Furthermore, our research explores innovative approaches to facet coatings that enhance facet durability and minimize power-induced degradation. This results in exceptionally reliable lasers, as demonstrated by a thousand hours of life test data. Through this work, we have achieved significant advancements in design and fabrication of high-power laser diodes for LiDAR applications. Experimental results demonstrate improved power efficiency, reliable facet coatings, and effective energy confinement within the desired emitting width.
The micro-fabrication process for advanced GaAs-based Pulsed Laser Diodes (PLDs) necessitates the precise etching of trenches for patterning waveguides. Traditionally, we employed wet etching in our approach, which, unfortunately, does not allow for precise engineering of waveguide trench geometry. The isotropic nature of wet etching results in a sidewall angle of approximately 45°. To enhance device performance, achieving a steeper angle without compromising other process steps dry etching is preferable. Ideally, trenches with sidewall angles ranging from 60° to 70° would strike an optimal balance. In pursuit of this goal, we initiated a Design of Experiment (DoE) to optimize the etching by Inductively Coupled Plasma - Reactive Ion Etching (ICP-RIE). Through this experimentation, we identified an ICP-RIE recipe capable of producing trenches with sidewall angles within the desired range (60° to 70°), exhibiting low roughness and attaining a depth of 17 μm. After this optimization, we applied the new ICP-RIE process to fabricate PLDs and conducted a comparative analysis against devices produced using our conventional wet etching method. The PLDs etched with ICP-RIE showcased slightly superior performance compared to those etched with wet etching. The implementation of ICP-RIE not only enhances device performance but also allows for a reduction in footprint per device. Consequently, this optimization contributes to an increased yield of devices per wafer, thus demonstrating the potential for scalability and improved efficiency in our micro-fabrication process.
InGaAs PIN detectors are extensively used for detection of photons in the wavelength range between 1000 nm and 1600 nm. Epitaxial InGaAs layers are commonly grown by MOCVD on InP substrates and layers of only a few micrometers are needed to fully absorb all IR radiation. In many applications, large area single- or multi-element InGaAs detectors are required with diameters ranging from 1 mm up to 5 mm. While useful for tracking large spots of IR light, their thin active layers have the disadvantage of a relatively large capacitance, which causes higher noise and reduced bandwidth. A PIN structure was designed with the purpose to reduce said capacitance by half and thus effectively double the value of the bandwidth when compared to standard values of catalog devices. The growth structure will be detailed, electro-optical measurement results will be presented and the next steps for specific markets such as laser spot tracking, semi-active laser guided precision-guided munitions or laser beam alignment over long distances will be presented. The new diodes have half the capacitance of regular PIN photodetectors leading to twice as much bandwidth at a low operating voltage. The combination of controlled epitaxial growth parameters with low defect density and low intrinsic doping in material have yielded new devices with proven reliability at high temperatures. Finally, it will be demonstrated that the change to the structure did not impact other parameters of the photodiode like dark current, breakdown voltage, responsivity or series resistance.
Triple-junction AlGaInP/InGaAs/Ge solar cells with embedded InAs quantum dots are presented, where typical
samples obtain efficiencies of > 40% under AM1.5D illumination, over a range of concentrations of 2- to 800-suns
(2 kW/m2 to 800 kW/m2). Quantum efficiency measurements show that the embedded quantum dots improve the
absorption of the middle subcell in the wavelength range of 900-940 nm, which in turn increases the overall
operating current of the solar cell. These results are obtained with 1 cm2 solar cells, and they demonstrate the
solar cells' low series resistance, which and makes them ideal for the current generation in commercial
concentrator systems. The thermal management and reliability of the solar cell and carrier is demonstrated by
testing the experimental samples under flash (up to 1000-suns) solar simulator and continuous (up to 800-suns)
solar simulator. Under continuous solar illumination, the solar cell temperature varies between ~Δ3°C at 260-suns
linearly to ~Δ33°C at 784-suns when the solar cell is mounted with thermal paste, and ~Δ27°C at 264-suns linearly
to ~Δ91°C at 785-suns when no thermal paste is used. The solar cells experience the expected shift in open circuit
voltage and efficiency due to temperature, but otherwise operate normally for extended periods of time.
Photovoltaic solar cells are a route towards local, environmentally benign, sustainable and affordable energy solutions.
Antireflection coatings are necessary to input a high percentage of available light for photovoltaic conversion, and
therefore have been widely exploited for silicon solar cells. Multi-junction III-V semiconductor solar cells have achieved
the highest efficiencies of any photovoltaic technology, yielding up to 40% in the laboratory and 37% in commercial
devices under varying levels of concentrated light. These devices benefit from a wide absorption spectrum (300-
1800 nm), but this also introduces significant challenges for antireflection coating design. Each sub-cell junction is
electrically connected in series, limiting the overall device photocurrent by the lowest current-producing junction.
Therefore, antireflection coating optimization must maximize the current from the limiting sub-cells at the expense of
the others. Solar concentration, necessary for economical terrestrial deployment of multi-junction solar cells, introduces
an angular-dependent irradiance spectrum. Antireflection coatings are optimized for both direct normal incidence in air
and angular incidence in an Opel Mk-I concentrator, resulting in as little as 1-2% loss in photocurrent as compared to an
ideal zero-reflectance solar cell, showing a similar performance to antireflection coatings on silicon solar cells. A transparent conductive oxide layer has also been considered to replace the metallic-grid front electrode and for inclusion as part of a multi-layer antireflection coating. Optimization of the solar cell, antireflection coating, and concentrator system should be considered simultaneously to enable overall optimal device performance.
Novel algorithms based upon Mueller matrix terms are presented herein, which allow the recovery of the spectra associated with incident transverse electrical (TE) and transverse magnetic (TM) orthogonal states of polarization. These, in turn, give more insight into polarization-dependent frequency shifts and other performance-related parameters of interest. This elegant patent-pending solution entails little computational efforts and can significantly improved the testing capacity and capabilities of component designers and contract manufacturers.
Characterization of DWDM photonic integrated circuits presents many challenges and trade-offs. High channel numbers significantly increase the required time to screen and qualify devices at different stages within their production. The SurePath Monitor product requires accurate calibration of the absolute responsivity of each of its 43 100GHz-spaced channels, across the C-band (1525 to 1565nm), to meet its power monitoring accuracy specification of ±0.5dB. Further specifications such as input optical power range (0 to -40dBm) require photocurrent measurement capabilities ranging from mA down to sub-nA levels, whilst the high efficiency of the integrated demultiplexer filter requires large electrical dynamic range (>65dB). Characterization stations developed to ensure high throughput and low cost without sacrificing measurement accuracy and repeatability were based upon matrix-based measurement methodologies to ensure this balance is achieved.
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