Input Optical Power Research Articles

Compact seven-level optical power quantizer using silicon-based nonlinear ring resonators

In this paper, an all-optical quantization scheme using the nonlinear Kerr effect in silicon-based ring resonators is proposed. In this structure, silicon nanocrystals embedded in silica (Si-nc/SiO2) are used as the Kerr materials because of their high nonlinear coefficient at the wavelength of 1550 nm and complementary metal–oxide–semiconductor (CMOS) fabrication compatibility. Simulation results obtained by the three-dimensional finite difference time domain (FDTD) method show seven quantization levels corresponding to the range of 0 to 75 W/mm2 of the optical input power. This is the maximum number of quantization levels in similar recent structures to the best of our knowledge, while the total footprint is reduced to 300 μm2.

Linearity optimization of multi-octave analog photonic links based on power weighting, polarization multiplexing and bias control.

We present and demonstrate an approach to linearizing analog photonic links (APLs) with substantially enhanced multi-octave spurious-free dynamic range (SFDR). Combining with power weighting, polarization multiplexing and bias control techniques, the proposed approach enables the second-order harmonic distortion (HD2) and third-order intermodulation distortion (IMD3) to be suppressed simultaneously. To maximize the RF output power, an optimization model is established. The simulation results indicate that the maximum RF power can be attained when the power weighting factor and polarization incident angle are equal to 0.5 and 0.34 radians, respectively. The link is validated with a proof-of-principle experiment. The third-order SFDR is 112.3 dB·Hz2/3, corresponding to the improvement of 15.5 dB as compared with a quadrature-biased link. The second-order SFDR reaches as high as 94.6 dB·Hz1/2. Furthermore, the adjacent channel power ratio (ACPR) is measured to be up to 54.6 dBc, which is 5.4 dB greater than that of a quadrature-biased link. Finally, the system tolerances for the RF and optical input power are also investigated in terms of error vector magnitudes (EVMs). Therefore, by introducing optimization model, our scheme provides further insight into the APL linearization technique and a better performance is also achieved.

Photonic crystal enabled manipulation of optical and electric field in germanium avalanche photodetectors

We demonstrate the use of a photonic crystal (PhC) structure to improve the performance of a germanium avalanche photodetector (APD) by simultaneously manipulating the distribution of the optical and electric fields. The PhC is fabricated at the top center of the vertical germanium APD. For a 14 μm diameter device, the 1550 nm responsivity increases from 0.2 to 0.6 A W−1 at unity gain, owing to the resonance-enhanced absorption. Moreover, the structure separates the absorption and multiplication regions of the device, resulting in an increase of the avalanche gain and the gain-bandwidth product. Under −10 dBm input optical power, a 3 dB bandwidth of 34 GHz before avalanche and a clear 40 Gbps eye diagram under avalanche demonstrates good high-speed performance of the device.

Photoresponse characteristics from vacuum deposited thin film of copper phthalocyanine: dependence on grain size

An organic photodetector (PD) is fabricated with single layer thin film of copper phthalocyanine (CuPc) on glass substrate. The thermal deposition of CuPc film on the substrate was realized under high vacuum and at different substrate temperature (room temperature, 40 ◦C, 70 ◦C, 100 ◦C). The comparison of photoresponse characteristics of CuPc thin film deposited under different conditions showed good improvement with increase in the substrate temperature, and the best performances being observed for the film deposited at substrate temperature of 100 ◦C. The thin film of CuPc characterized through X-ray diffraction (XRD) spectroscopy and atomic force microscopy (AFM) clearly indicated that with increase in substrate temperature, the grain size of CuPc film increased, which improved the crystallinity and hence the photoconductivity of the device. The CuPc thin film based PD displayed stable and reproducible photoswitching characteristics under white light irradiation, with photocurrent modulation by varying in input optical power. The highest photo to dark current ratio and responsivity of the devices was calculated to be, ca. 6.5 and 7.1 mA-W−1 at low optical power, ca. 14.5 mW-cm−2.

Stochastic Approximation Aided Adaptive Thresholding for Optical Detection in PAM4 Based FSO Transmission

This study proposes an adaptive thresholding optical detection technique using a stochastic approximation (SA) to enhance receiver performance in multi-level pulse amplitude modulation (PAM) based free-space optical (FSO) transmission. When the receiver detects the symbol by fixed threshold decision (FTD) scheme, the detection performance of high order modulation, including PAM4, becomes degraded due to a random fluctuation of the received signal. Owing to the slow varying characteristic of atmospheric turbulence, we used an adaptive linear prediction (ALP) filter to predict the decision threshold of the upcoming frame. The experiment was conducted on various degrees of turbulence channels. A Mach-Zehnder modulator (MZM) based turbulence channel emulator was used to experimentally investigate the turbulence-induced scintillation effect. The experiment results confirmed that the proposed technique yielded improved receiver performance compared to FTD. In the case of considering the amplified spontaneous emission (ASE) noise of erbium-doped fiber amplifier (EDFA), the adaptive threshold decision (ATD) scheme obtained about 3dB gain compared with an even-valued threshold (EVT) decision scheme when the average EDFA input optical power was below -16dBm.

Graphene- WS2 van der Waals Hybrid Heterostructure for Photodetector and Memory Device Applications

The intriguing electronic properties of graphene and the strong light-matter interaction in layered transition-metal dichalcogenide (TMDC) make them a natural partner for hybrid devices, not only for optoelectronic device applications, but also to understand the conversion of light to electricity in this atomic scale prototype of a donor-acceptor complex. Here, we describe graphene-on-${\mathrm{WS}}_{2}$ binary heterostructure FET device, displaying gate-tunable persistent photoconductivity. Our time-dependent photovoltage relaxation experiments suggest that the charge-transfer time scale in this heterostructure is dependent on the input optical power, contrary to what is observed for the bare TMDC and is orders of magnitude slower than that observed for various other vdW hybrids. The optoelectronic responsivity of this device at low optical power is found to be as high as ${10}^{10}\phantom{\rule{0.1em}{0ex}}\mathrm{V/W}$, and thus shows the potential to be one of the most sensitive visible range photodetectors, while the gate tunability of the persistent photoconductivity can be utilized in the memory device applications. We identify that the photoresponse is the outcome of a photogating mechanism, due to the exciton dissociation under optical excitation, followed by the trapping of holes in ${\mathrm{WS}}_{2}$ and subsequent electron transfer to graphene.

Support vector machine based machine learning method for GS 8QAM constellation classification in seamless integrated fiber and visible light communication system

Visible light communication (VLC) network over optical fiber has become a potential candidate in ultra-high speed indoor wireless communication. To mitigate signal distortion accumulated in optical fiber and VLC channel, we present to utilize support vector machine (SVM) for constellation classification in two kinds of geometrically-shaped 8QAM (quadrature amplitude modulation) seamless integrated fiber and VLC system. We introduce 4 sub-bands to simulate multi-user. Experimental results show that system performance can be significantly improved, and transmission at −2.5 dBm input optical power under 7% forward error correction (FEC) threshold can be realized employing Circular (7, 1) geometrically-shaped 8QAM and SVM. At overall capacity of 960 Mbps, Q-factor increases by up to 11.5 dB.

On-chip high-sensitivity photonic temperature sensor based on a GaAs microresonator.

We demonstrate an on-chip high-sensitivity photonic temperature sensor based on a GaAs microdisk resonator. Based on the large thermo-optic coefficient of GaAs, a temperature sensitivity of 0.142 nm/K with a measurement resolution of 10 mK and low input optical power of only 0.5 µW was achieved. It exhibits great potential for chip-scale biological research and integrated photonic signal processing.

Investigation and optimization of light trapping through hexagonal-shaped nanopillar (NP) array of indium gallium arsenide material based photodetector

Nanopillar arrays over the photodetector’s surface attribute an enhanced coupling efficiency of incident light due to its various light trapping mechanisms that contribute towards high-resolution optical detection. This paper reports the analytical investigation of the performance of Indium Gallium Arsenide material based hexagonal-shaped nanopillar (NP) array deployed over the 5 um × 5 um detector’s front surface for high photodetection infrared applications. The proposed structure exhibits an optimum 98% overall absorption of the incoming light. High EQE of about 75% and responsivity of 0.93 A/W has been obtained at 1.55 um operating wavelength with a minimum detector’s depletion width of 0.7 um at 1 mw optical input power.

Thermo-plasmonic lab-on-fiber optrodes

The thermoplasmonic effect causing the local overheating in nanostructured metallic substrates has been recently recognized as an intriguing technological tool for the development of light-controlled active micro and nano systems. Here, we present a study of thermoplasmonic effect in Lab-on-Fiber devices, consisting in nanostructured gold layers directly integrated onto the cleaved facet of an optical fiber tip. We analyze the effect of the metallic nanostructure parameters on the temperature distribution under different (in-fiber) illumination conditions. Heating and cooling temporal dynamics are also investigated. At the steady state, a linear relationship between input optical power and temperature with a linear coefficient in the order of 10 °C/mW is found. All the experimental results are in good agreement with numerical simulations based on a finite element method. Our findings lay the groundwork for the development of light triggered active Lab-on-Fiber probes, merging the unique characteristics of the optical fiber platform and the enormous potentiality offered by thermoplasmonics.

Self-biasing of carrier depletion based silicon microring modulators.

We report on the self-biasing effect of carrier depletion based silicon microring modulators (MRM) by demonstrating that a silicon MRM can generate open eye diagrams for non-return-to-zero (NRZ) on-off keying (OOK) modulation without an external reverse bias supplied to it. Two modulator configurations are investigated namely single-ended drive in a ground-signal-ground and differential drive in a ground-signal-signal-ground pad configurations. The single-ended modulator is designed with an on photonic integrated circuit (PIC) 50 Ω termination. Open eye diagrams are obtained at 25 Gbit/s and 36 Gbit/s NRZ OOK modulations. We carry-out thorough experimental characterization of the self-biasing of single-ended MRM under various operating conditions of input optical power, carrier wavelength, ring quality factor and extinction ratio as well as modulation speeds, driving voltage swing and pattern length. We demonstrate that the self-biasing is robust and works well in almost all tested conditions. The differential drive MRM is designed with a high impedance without an on-PIC 50 Ω termination. Open eye diagrams are obtained at 30 Gbit/s and 60 Gbit/s NRZ OOK modulations for modulating voltage swing of ∼2.5 Vpp. As demonstrated, the self-biasing works well in both single-ended and differential drive configurations as well as for on-PIC 50 Ω terminated and non-terminated MRMs. The electrical passive parts are all co-designed and fabricated on the same silicon chip as the PIC. The reported self-biasing eliminates the need of having bipolar DC biases supplied to the anode and cathode of the differential drive modulator and allows for simpler driver / modulator interfaces without inductive bias tees.

Thermo-optical dynamics of a nonlinear GaInP photonic crystal nanocavity depend on the optical mode profile

We measure the dynamics of the thermo-optical nonlinearity of both a mode-gap nanocavity and a delocalized mode in a Ga0.51In0.49P photonic crystal membrane. We model these results in terms of heat transport and thermo-optical response in the material. By step-modulating the optical input power we push the nonlinear resonance to jump between stable branches of its response curve, causing bistable switching. An overshoot of the intensity followed by a relaxation tail is observed upon bistable switching. In this way, the thermal relaxation of both the localized resonance and the delocalized resonance is measured. Significant difference in decay time is observed and related to the optical mode profile of the resonance. We reproduce the observed transient behavior with our thermo-optical model, implementing a non-instantaneous nonlinearity, and taking into account the optical mode profile of the resonance, as experimentally measured.

Multi-electrode quantum-dot semiconductor optical amplifier as an intensity modulator of signals in optical communication systems

In this paper, we are going to exhibit the system performance using the single, double and three-electrode quantum-dot semiconductor optical amplifier as an intensity modulator (3E-QD-SOA-IM) of the adaptively modulated optical orthogonal frequency division multiplexing (AMOOFDM) signals in the intensity-modulation direct-detection passive optical network (IMDD-PON) systems. Moreover, to compare the bit-rate versus transmission distance of the proposed fiber link model when using both configurations multi-electrode semiconductor optical amplifier intensity-modulator (ME-SOA-IM) and multi-electrode quantum-dot semiconductor optical amplifier intensity-modulator (ME-QD-SOA-IM), to study the improvement of the transmission capacity, distance reach and power efficiency when applying the proposed configurations for optical access networks for distances ranging from 20 km up to 120 km. The three-electrode configuration solved the nonlinearity of the two-electrode configuration and offered a considerably wider range of optimum operating conditions to achieve up to 35 Gbps. Furthermore, the 3E-QD-SOA-IM showed a better performance over the 3E-SOA-IM over all the 120 km, and a 5 Gbps enhancement in the bitrate with a 20 dB less optical input power at 20 km.

Ionization effect and displacement effect induced photoresponsivity degradation on α-In2Se3 based transistors for photodetectors

Two-dimensional (2D) van der Waals (vdW) materials-based photodetectors have attracted great attention because of quick response time, excellent photoresponsivity and high light/dark current ratio. α-In2Se3 is one of the interesting 2D vdW materials for photodetectors because of the interrelated in-plane (IP) and out-of-plane (OOP) polarizations. It has been demonstrated that the polarizations can be used to regulate photoresponsivity of the α-In2Se3 nanoflakes based photodetectors. In addition, domain evolution may be related to the photoresponsivity degradation when the α-In2Se3 nanoflakes based photodetectors are in radiation environment. However, it is still unclear the effect of ionization and displacement damage on photoresponsivity induced by radiation particles. In this article, the α-In2Se3 based transistors for photodetectors are exposed in the 60Co γ-rays environment to induce the photoresponsivity degradation, all the channel width is about 95–100 μm for observing significant radiation effects. In order to investigate the intrinsic photoresponsivity degradation of the α-In2Se3 based transistors, a 405 nm laser is used as the light source to measure the photoresponsivity. The photoresponsivity of the devices is significantly degraded with increasing irradiation dose. Besides these, the dark current and Se atoms ratio decrease as the increasing of total dose. It may show complicate coupling radiation effect that the domain evolution induced by both the ionization effect and displacement effect promotes the photoresponsivity degradation. After a total dose of 1 Mrad (Si) 60Co γ-rays, the photodetector can still be used to detect the light with a high photoresponsivity of about 107 A/W when the input optical power density is about 454 μW/cm2. Our results may promote the investigation of radiation effect in 2D vdW materials-based photodetectors.

A Novel Proposal for All-Optical OR/AND Gate Using Nonlinear Photonic Crystal Ring Resonators

AbstractIn this paper, we aim to design an all-optical structure that can be employed as optical OR/AND logic gates. To do so, a nonlinear photonic crystal-based ring resonator will be designed whose resonant wavelength depends on the variation of optical intensity. Then, by adding some optical waveguides, the optical logic circuit structure will be obtained that can function as optical OR/AND logic gates. The maximum time delay for the proposed structure is about 1.5 ps. Total footprint of the proposed structure is about 372 mm2. The proposed structure has lower time delay, lower footprint and lower optical input power compared with previously proposed structures.

OE-CAM: A Hybrid Opto-Electronic Content Addressable Memory

A content addressable memory (CAM) is a type of memory that implements a parallel search engine at its core. A CAM takes as an input a value and outputs the address where this value is stored in case of a match. CAMs are used in a wide range of applications including networking, cashing, neuromorphic associative memories, multimedia, and data analytics. Here, we introduce a novel opto-electronic CAM (OE-CAM) utilizing the integrated silicon photonic platform. In our approach, we explore the performance of an experimental OE-CAM and verify the efficiency of the device at 25 Gbit/s while maintaining the bit integrity under noise conditions. We show that OE-CAM enables a) two orders of magnitude faster search functionality resulting in b) a five orders of magnitude lower power-delay-product compared to CAMs implementations based on other emerging technologies. This remarkable performance potential is achieved by utilizing i) a high parallelism of wavelength-division-multiplexing in the optical domain, combined with ii) 10's of GHz-fast opto-electronic components, packaged in iii) integrated photonics for 10–100's ps-short communication delays. We further verify the upper optical input power limit of this OE-CAM to be given by parasitic nonlinearities inside the silicon waveguides, and the minimal detectable optical power at the back-end photoreceiver's responsivity given channel noise. Such energy-efficient and short-delay OE-CAMs could become a key component of functional photonic-augmented ASICS, co-processors, or smart sensors.

Configurations of Pump Injection and Reinjection for Improved Amplification Efficiency of Turbo Cladding Pumped MC-EDFA

Configurations of cladding pumped MC-EDFA with pump recycling have been researched to improve optical amplification efficiency. The difference between the four researched configurations is the pump injection and re-injection method. The optical amplification performance was compared experimentally by evaluating the optical gain, noise figure, and the maximum output signal power. The results show the spatial injection and the fiber re-injection with spatial splitting configuration improved the pump recycling efficiency the most, and the configuration offered an additional 3.5 dB of optical gain and boosted the maximum output signal power by 4.0 dB compared to the standard cladding pumping without pump recycling. The improvement in the optical gain due to the pump recycling was constant at 1.0 dB in the fixed experimental set-up even if the input optical signal power was changed. The effect of the spatial intensity distribution widening due to the fiber pump combiner is considered to be effective in improving the pump recycling efficiency. An additional 0.2 dB improvement (1.2 dB in total) in the optical gain was achieved by modifying the free-space lens optics design of the spatial pump splitter.

Material-to-device performance correlation for AlGaN-based solar-blind p–i–n photodiodes

We report on crystalline quality-to-device performance correlation for self-powered Al0.40Ga0.60N, p–i–n ultraviolet (UV) photodetectors on c-plane sapphire. The active p–i–n detector stack was grown over an AlN buffer. Careful optimization of the nucleation density on growth surface helped achieve a two-orders and one-order of magnitude reduction in the screw and edge dislocation density in the buffer layer, respectively. This resulted in a nine-orders of magnitude reduction in the reverse leakage current from 4.3 mA to 4.2 pA (at 10 V). Correspondingly, a thirteen-fold enhancement in the zero-bias external quantum efficiency (EQE) from 3.4% to 45.5%, when measured under 289 nm front-illumination was also observed. The detector epi-stack grown over the optimal AlN buffer layers led to the realization of high-performance p–i–n detectors with a dark current density below 4 nA cm−2 at 10 V and a zero-bias EQE of 74.7% under back-illumination. This is one of the highest zero-bias EQE reported for solar-blind detectors realized on template-free and mask-free III-nitrides grown using metal organic chemical vapor deposition on any substrate. The deep-UV-to-visible rejection ratio exceeded 106 while the deep-UV-to-near UV rejection exceeded 103. The thermal-noise limited detectivity was estimated to be 4 × 1014 cm Hz1/2 W−1. Hopping conduction along screw dislocation-mediated localized trap states was found to be the dominant carrier transport mechanism in the samples exhibiting high reverse leakage. For these samples, the responsivity (photocurrent) exhibited an exponential variation with reverse bias and a nonlinear variation with input optical power. This is explained using a hole-trapping associated gain mechanism and its impact on the transient characteristics of the detectors is investigated. A 6 × 1 linear array of the highest EQE detectors was realized and detector performance parameters were found to be comparable before and after wire bonding. This study is expected to enhance the understanding of III-nitrides-based vertical, self-powered detectors and benefit the development of high-performance, focal plane arrays using less complicated growth techniques.

Photothermal Effect in Plasmonic Nanotip for LSPR Sensing.

The influence of heat generation on the conventional process of LSPR based sensing has not been explored thus far. Therefore, a need exists to draw attention toward the heat generation issue during LSPR sensing as it may affect the refractive index of the analyte, leading to incorrect sensory conclusions. This manuscript addresses the connection between the photo-thermal effect and LSPR. We numerically analyzed the heat performance of a gold cladded nanotip. The numerical results predict a change in the micro-scale temperature in the microenvironment near the nanotip. These numerical results predict a temperature increase of more than 20 K near the apex of the nanotip, which depends on numerous factors including the input optical power and the diameter of the fiber. We analytically show that this change in the temperature influences a change in the refractive index of the microenvironment in the vicinity of the nanotip. In accordance with our numerical and analytical findings, we experimentally show an LSPR shift induced by a change in the input power of the source. We believe that our work will bring the importance of temperature dependence in nanotip based LSPR sensing to the fore.

Enhanced thermo-optic nonlinearities in a MoS2-on-silicon microring resonator

We experimentally investigated the thermal nonlinearity in a MoS2-on-silicon microring resonator. In the hybrid MoS2-on-silicon microring resonator, there was a threefold increase in the resonance shift rate as input optical power increased. The effective photothermal coefficient of the MoS2-on-silicon resonator was enhanced to 4.7 × 10−15 m2 W−1. The enhancement can be attributed to the presence of defect states of the MoS2 film, leading to additional absorption of light and enhanced heating, which changed the effective refractive index of the resonator. Efficient thermal bistability was observed in the resonator.