A GaN surface emitting laser (SEL) based on angular-symmetry-breaking concentric-ring surface grating (ASB-CRSG) is proposed in this paper. The second-order CRSG located in the p-contact and p-cladding of an EPI wafer of GaN FP laser is adopted to select the radial mode and couple the optical power vertically out of the laser cavity. As the zero-order azimuthal CRSG with a two-lobe far field has the lowest mode loss in the angular-symmetric CRSG, the first-order ASB is adopted by the removal of two circular sections of GaN epitaxial layers to break the angular symmetry of the lasing modes. The simulation results show that degenerate modes in angular-symmetric CRSG have different mode losses with the help of the first-order ASB and the bigger breaking angles of CRSG results to higher loss difference between the first-order and other azimuthal modes. The loss and divergence angle decrease with the increasing area of CRSG, and the deeper CRSG results to the higher out-plane coupling. The first-order azimuthal mode has the lowest mode loss whose value is ~ 84% of that of the second-lowest-loss mode. A single-lobe far-field with a divergence angle of 1.33° in the wavelength of 450nm will be realized by an ASB-CRSG with the diameter of 10.6μm, the breaking angles of 12° and the depth of 325nm. Therefore, the single-mode operation of the first-order azimuthal mode which has a single-lobe far field is expected with the combination of the second-order CRSG and the first-order ASB.
The distributed feedback (DFB) laser is a key component for fiber communication due to its single-mode performance, but it usually requires complex and expensive regrowth after grating definition. The laterally-coupled distributed feedback (LC-DFB) laser has the advantage of a simple fabrication process without epitaxial regrowth, but the LC-DFB laser usually has a low coupling coefficient as the optical feedback is provided by the evanescent field and Fabry-Parot (FP) longitudinal modes arise from the pair of parallel cleaved facets. In this work, a triangular prism etched facet is proposed to suppress the FP longitudinal modes from cleaved facets of a 1.3 μm LC-DFB laser. The length-width ratio of a triangular prism facet is optimized on the compromise between the reflection and length by finite difference time domain (FDTD) method. The vertical etched facet with depth of 4 μm and tip curvature of 100 nm and the lateral gratings with depth of 1.8 μm and gap of 200 nm are fabricated by inductively coupled plasma (ICP) etching with the gas mixtures of Cl2/CH4/Ar and CH4/H2/Ar, respectively. The FP longitudinal modes of the etched-facet laterally-coupled distributed feedback (EF-LC-DFB) laser are effectively suppressed compared to the counterpart of cleaved facets, and the stable single-mode operation of EF-LC-DFB is demonstrated with the side mode suppression ratio (SMSR) of 54.35 dB.
Optical phased arrays (OPAs) are widely used in many applications to realize high-speed optical beam scanning. At present OPAs often suffer from limited scanning range. Here we propose a circular optical phased array (COPA) based on silicon photonics platform. According to our simulations, by positioning the OPA units in a circle and adopting a specific phase distribution, the COPA can realize 360° constant amplitude scanning. In addition, the design of the disk grating coupler, which is the key device of the COPA, is presented. The COPA is believed to have great potential for applications where a wide scanning range is mandatory.
The high-gain photomultiplier tube (PMT) is the most popular method to detect weak ultra-violet signals which attenuate quickly in atmosphere, although the vacuum tube makes it fragile and difficult to integrate. To overcome the disadvantage of PMT, an AlN/GaN periodically–stacked-structure (PSS) avalanche photodiode (APD) has been proposed, finally achieving good quality of high gain and low excessive noise. As there is a deep г valley only in the conduction band of both GaN and AlN, the electron transfers suffering less scattering and thus becomes easier to obtain the threshold of ionization impact. Because of unipolar ionization in the PSS APD, it works in linear mode. Four prototype devices of 5-period, 10-period, 15-period, and 20-period were fabricated to verify that the gain of APD increases exponentially with period number. And in 20-period device, a recorded high and stable gain of 104 was achieved under constant bias. In addition, it is proved both experimentally and theoretically, that temperature stability on gain is significantly improved in PSS APD. And it is found that the resonant enhancement in interfacial ionization may bring significant enhancement of electron ionization performance. To make further progress in PSS APD, the device structure is investigated by simulation. Both the gain and temperature stability are optimized alternatively by a proper design of periodical thickness and AlN layer occupancy.
The next generation infrared (IR) detection technology demands for very-large-format focal plane arrays (FPAs) with
high performance. Semiconductor up-converters can convert IR photons to near-infrared (NIR) photons, and can be
potential candidates for large-format IR imaging since the mechanical bonding with the read-out circuits can be avoided.
However, previously reported up-converters and corresponding up-conversion systems suffer from low detectivity
because of the trade-off between responsivity and dark current. To solve this issue, a cascade infrared up-converter
(CIUP) is demonstrated in this work. Based on a quantum cascade transport mechanism, high IR responsivity is achieved
while the dark current is maintained fairly low. A 4-μm InGaAs/AlGaAs CIUP has been fabricated, and both the CIUP
and up-conversion system are under background-limited infrared performance (BLIP) regime below 120 K. The upconversion
efficiency is 2.1 mW/W at 3.3 V and 78 K. Taking shot noise as the main noise in the up-conversion system,
the BLIP detectivity of the system is 2.4×109 Jones at 3.3 V and 78 K, higher than the semiconductor up-converters at
similar wavelengths reported so far. To further improve the CIUP performance, an AlInP hole-blocking layer is
introduced taking place of the AlAs layer. AlInP/GaAs has larger valence band discontinuity than AlAs/GaAs, showing
the advantage of tightly confining injected holes into the emission quantum well. By adopting the AlInP hole-blocking
layer, the quantum efficiency and detectivity of the up-conversion system can be enhanced.
We propose a novel method for designing reflectors with large-size spherical surface sources. The center portion of the reflector is designed using the edge-ray principle, while the rim portion is designed based on the variable-separation mapping method. Step discontinuities are introduced during rim surface construction to control the deviation caused by error in normal vectors, and a feedback modification is adopted to compensate for the illuminance deviation produced by the large size of sources. As an example, a streetlamp with a desired rectangular illuminance distribution on the road is designed using a spherical surface source (10 cm diam) and a compact reflector. It has an illuminance uniformity of 60.2% and utilance of 66.9%, considerably improved from the traditional values of 35 and 40%, respectively.
In this work, a systematic study on the plasma-induced damage on n-type GaN by inductively coupled plasma (ICP) etching is presented. After n-contact metal formation and annealing, electrical property is evaluated by the I-V characteristics. Room temperature photoluminescence (PL) measurement of etched GaN surfaces is performed to investigate the etching damage on the optical properties of n-type GaN. Investigation of the effect of additive gas RF chuck power on these characteristics has also been carried out. The better etching conditions have been obtained based on these results.
In this article, we report the successful fabrication of high-brightness blue LEDs with InGaN/GaN multiple quantum well structures grown by low pressure metalorganic vapor phase epitaxy on sapphire substrates. The active region is composed of five pairs of InGaN well and GaN:Si barrier. The epitaxial wafer is processed into mesa diodes by inductively coupled plasma etching technique, with SiO2 deposited by plasma-enhanced chemical vapor deposition as the etching mask. The diode chips are then encapsulated into transparent epoxy to form packaged LEDs. The typcial emitting spectrum of the blue LEDs shows a peak wavelength at 460 nm and a FWHM of 30 nm. The working voltage and output power of blue lEDs shows a peak wavelength at 460 nm and a FWHM of 30 nm. The working voltage and output power of blue LEDs at a forward current of 20 mA are 3.6V and 1.5mW, respectively. The reverse leakage current at 5V was about 5μA , and the wavelength uniformity is 0.25 nm.
The transmission window of the acousto-optic modulator was discussed theoretically. By injecting pulsed higher acoustic
power, the FW}{M of the transmission window of the modulator was reduced, so the mode-locked pulse width was reduced
to 55ps. In addition, through discharging the λ/4 voltage of the Pockels Cell, a single pulse was dumped out from the intracavity
of the laser: the energy was 2mj, the energy variation was ±4%. Besides, the repetition rate of the laser was improved
to 30Hz due to the reduction of the laser threshold. It is evinced that this kind of laser has potential for a wide variety of
application.
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