The advancements in integrated thin-film lithium niobate on insulator (LNOI) platform have significantly enhanced the performance of various integrated electro-optic devices, including modulators, external cavity diode lasers, and optical frequency comb generators. Additionally, the development of LNOI has facilitated applications at shorter wavelengths, due to its wide transparency window. The coupling efficiency between laser diodes (LDs) and LN chips is commonly enhanced by designing spot size converters (SSCs). However, achieving high-efficiency SSCs is more challenging at shorter wavelength due to the smaller mode area and the increased confinement of the optical field in LN waveguides. In this study, we present a spot size converter based on hybrid SiN-LN structure, for low-loss light coupling between a III– V gain chip and a LNOI waveguide at 780nm. The parameters of SiN waveguide, LN taper and SiO2 spacing layer has been optimized in order to enhance the matching of effective refractive indices. The entire SSC structure can be fabricated with two steps of photolithography and etching, demonstrating high fabrication tolerance. Simulations indicate that the coupling losses between the output mode of the LD and the fundamental mode of the LN waveguide are 0.41dB/facet for TE mode and 0.55dB/facet for TM mode at a wavelength of 780nm. Our design is intended to offer efficient light coupling from LD to LNOI chips at short wavelength range, characterized by its simple process and high fabrication tolerance.
We demonstrate waveguide Modified Uni-Traveling-Carrier (MUTC) Photodetectors (PDs) to complete optical-tomicrowave conversion. Ultra-high 3-dB bandwidths of 137GHz and 163GHz, high responsivities of 0.32A/W and 0.24A/W, and low dark currents of 3nA and 5nA have been achieved for 5×12μm2 and 5×8μm2 devices, respectively. These PDs demonstrate promising performance for photodetection of high-repetition-rate optical pulses, laying the foundation for the photonic chip-based ultra-stable and low-phase noise microwave generation.
KEYWORDS: Quantum chromodynamics, Transition metals, Quantum efficiency, Absorption, Sensors, Electric field sensors, Doping, Modulation, Quantum wells, Electron transport
This work proposes a novel way to regulate the electron quantum states of quantum cascade detectors (QCDs) by utilizing localized built-in electric field introduced by modulation doping. The mechanism that how the localized built-in electric field influences extraction efficiency is studied by analyzing the quantum transitions in a simplified three-quantum-well model. The calculation results show that, by introducing the localized built-in electric field, a transition energy close to the LO phonon energy can be more easily realized with almost unchanged transition matrix element. The transition matrix element can be enlarged by the localized built-in electric field with almost unchanged transition energy. The calculated extraction efficiency is below 65% for the standard QCD structures without localized built-in electric field, whereas for the structures with localized built-in electric field, the extraction efficiency can reach above 80%. From experimental results, a higher extraction efficiency of photo-generated electrons of 89% is obtained for the proposed QCD structure, comparing with 63% for the standard QCD structure. The peak response wavelengths of two structures are both around 4.5 μm. At temperatures ranging from 40K to 210K, the photocurrents of the structure with localized built-in electric field are over 55% larger than those of the standard structure. To sum up, the localized built-in electric field can be utilized to regulate the electron states besides the layer thickness and material composition of QCDs.
In this paper, we investigate the fabrication of high aspect ratio photonic crystal air holes in AlGaAs materials using general inductively coupled plasma (ICP) dry etching system. We propose dividing the long etching process into multiple short-time etching segments during the ICP etching process, so that there is enough time to exhaust the etch products out from the bottom of the holes before the next etching segment, which is beneficial for deep air hole etching. Simultaneously, a novel method to suppress lateral penetration of holes by in-situ sidewall passivation is proposed, which can be realized by inserting one oxygen plasma treatment between two etching segments. This method also allows the optimization of etch rate to be achieved independent of sidewall passivation. Our experiment results show that the sidewall passivation has a crucial influence on the etched morphology of air holes. Without sidewall passivation, the air holes are lateral penetrated in the middle. While with appropriate oxidation for sidewall passivation, deep air holes with high verticality are obtained. Finally, high-aspect-ratio air holes with a diameter of 130 nm and a depth of over 1.5 μm are successfully manufactured.
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.
A hybrid integration method of back-illuminated modified uni-traveling carrier photodiode (MUTC-PD) on silicon-oninsulator (SOI) is demonstrated. Compared with the die-to-die bonding of unprocessed III-V die, this hybrid bonding method, implemented by a flip-chip bonding machine, is more convenient and flexible, thus providing a more direct path to utilizing high-speed PDs in integrated microwave photonics on SOI. As a result, the integrated photodetector exhibits a 3-dB bandwidth of 30 GHz, showing no degradation compared with the bandwidth before bonding.
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.
High-speed AlGaInAs multiple-quantum-well (MQW) electroabsorption modulated lasers (EMLs) based on identical epitaxial layer (IEL) integration scheme are developed for 40 Gb/s optical fiber communication systems. The electroabsorption modulator (EAM) section adopts a narrow high-mesa waveguide formed by inductively coupled plasma (ICP) dry etching technique, and a self-aligned planarization technique is employed to further reduce the device capacitance. Resonances are observed in the small signal modulation response of the packaged EML module, which are attributed to parallel-plate modes of the coplanar waveguide (CPW) transmission line used for modulation signal feeding and the residual reflection at the modulator facet, respectively. The influence of such resonances on the large signal eyediagram performance of the device is studied, and methods for their suppression are presented. Clear eye opening under 40 Gb/s non-return-to-zero (NRZ) modulation has been demonstrated for the optimized EML module.
In this talk, identical epitaxial layer high speed electroabsorption (EA) modulator integrated light sources are studied systematically. Firstly, a 1.55 μm 10 Gb/s distributed feedback (DFB) laser integrated EA modulated laser is presented, which adopts a partially gain-coupled DFB laser to improve single-mode yield. The typical measured small signal response indicates a 3 dB electrical bandwidth over 10 GHz. Secondly, a 1.55 μm laterally coupled DFB laser is fabricated to eliminate the necessity of additional regrowth, in order to reduce the process complexity and fabrication cost. A stable single-mode operation has been demonstrated with a side mode suppression ratio over 45 dB at an injection level of 60 mA. Finally, an SOA integrated EA modulator with a planar ridge waveguide is fabricated for 40 Gb/s optical fiber communication systems. The device is chip-level packaged and tested on a coplanar waveguide based submount. The small signal frequency modulation responses of the integrated EA modulator are measured and simulated based on an equivalent circuit. The measured 3 dBe modulation bandwidth exceeds 40 GHz, and it is estimated to be over 45 GHz.
A novel microwave packaging technique for 10Gb/s electro-absorption modulator integrated with distributed feedback laser (EML) is presented. The packaging parasitics and intrinsic parasitics are both well considered, and the packaging circuit was synthetically designed to compensate for the intrinsic parasitic of the chip. A butterfly-packaged EML module has been successfully developed to prove that. The small-signal modulation bandwidth of the butterfly-packaged module is about 10 GHz. Optical fiber transmission experiments have shown that the module can be used for 10Gb/s optical transmission system. After transmission through 40km, the power penalty is less than 1 dBm at a bit-error-rate of 10-12.
Heat sinks with impedance matching circuit have been designed and fabricated for the packaging of high-speed electroabsorption (EA) modulators. Ti/Cu/Ni/Au metallization system is adopted for the coplanar waveguide (CPW) electrodes and a 50-ohm Ta2N thin-film resistor in parallel with the EA modulator is used for impedance matching. By a matching resistance optimization, a reflection coefficient S11 better than -21 dB has been demonstrated up to 40 GHz. The heat sinks are applied successfully in the 40 GHz Modulator packaging.
Low threshold semiconductor lasers with etched facets have been fabricated by inductively coupled plasma (ICP) dry etching technology. To ensure vertical and smooth etched-facets, a novel C12/CH4/Ar mixture has been adopted for the
ICP etching process. The typical threshold current of etched-facet lasers is about 1 8 mA, which is as low as that of lasers with cleaved-facets and similar cavity length.
Inductively coupled plasma (ICP) dry etching technique has been adopted to form narrow high-mesa ridge waveguide structure in a high-speed integrated EA modulator. A 3dBe bandwidth of over 12 GHz has been achieved without the use of polyimide in the DFB laser integrated EA modulator. Meanwhile, integrated device with a threshold current as low as 12 mA has been demonstrated by optimization ofthe wavelength detuning.
The optimization of wavelength detuning for better transmission capability in an identical expitaxial layer (IEL) integrated light source is presented. The absorption spectra of the multiple-quantum-well (MQW) material under varying electric fields are worked out by using the fractional dimensional method. The extinction ratio and the chirping parameter of a 200um-long modulator section are calculated accordingly for different working wavelengths. A voltage-related chirping parameter has been used in a 10Gb/s-transmission simulation to make a more realistic description of the performances of the EA modulator in actual fiber communication systems. A detuning value of about 30~50nm is found to be the optimum in the sense of low power penalty and high sensitivity according to the simulation results.
Electroabsorption modulator integrated distributed feedback lasers have been fabricated using identical epitaxial layer approach. The influence of wavelength detuning on device characteristics is investigated to achieve optimum device performance. Gain coupling is introduced into the device to improve single mode yield and wavelength stability. Integrated light source modules have also been fabricated for 2.5 Gb/s trunk line applications.
In this talk, we review recent achievements on compound semiconductor based optoelectronic devices in State Key Lab on Integrated Optoelectronics, Tsinghua University. The presentation will cover research work on electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers for wavelength-division multiplexing (WDM) systems and the growth of InP based materials by all-solid-source molecular beam epitaxy (SSMBE) system.
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