This paper reports the demonstration of optoelectronic mixing of micro/mm-wave frequencies (upto 45GHz) in Ge-on-Silicon waveguide photodetectors fabricated using silicon photonics platform. Experimental results are found to be promising for futuristic silicon photonic micro/mm-wave processors.

Thermo-optically tunable Mach–Zehnder interferometer (MZI) switches have been popular for building robust designs of programmable photonic integrated circuits (PPICs) for various applications. In general, microheaters integrated on both arms of the MZIs are preferred to minimize power consumption by accounting phase errors between MZI arms arising out of fabrication-induced process variabilities. However, only one of the two microheaters integrated in MZI arms is chosen to drive, for achieving desired phase imbalance with lower power consumption. Again, integrating microheaters on both arms doubles the electronic control channels and as many numbers of bond pads, posing challenges for scalability and packaging. To address this, we propose a diode-assisted microheater (DAM) architecture that enables selective driving of one of the two microheaters in an MZI without increasing the pad count or control complexity. The DAM is implemented on a standard silicon photonics technology platform and is fully compatible with foundry processes. Comprehensive electrothermal simulations and subsequent experimental results confirm the CMOS foundry fabrication feasibility of such Diode assisted microheater designs. The integrated diode design exhibits low series resistance and high breakdown voltage, ensuring efficient forward operation and negligible reverse leakage. Both the diode and microheater resistances were modelled as functions of the forward current, showing excellent agreement with experimental data. DC and AC characterizations reveal that the DAM achieves efficient thermal tuning with a total power dissipation of ~50 mW at ±3 V and a rise/fall time of ~20 μs. The DAM’s performance was validated in a 1X4 power splitter comprising cascaded 1X2 MZIs, fabricated through a commercial silicon photonics foundry. Using DAM-based tuning, we achieved precise phase correction, yielding uniform power splitting with deviations of only ±0.015 dB at 1550 nm and 0.025 dB across a 40 nm operating wavelength bandwidth. Furthermore, by employing bipolar driving and selectively activating one of the MZI arms, we experimentally demonstrated a reduction in power consumption exceeding 50% when compared with the experimental results for the same PPIC by assuming only one microheater was integrated in MZIs. These results demonstrate that the proposed DAM architecture offers a scalable, power-efficient, and foundry-compatible solution for next-generation PPICs.

The dimensional limit of a single-mode waveguide cross-section is the fundamental roadblock of designing compact and large-scale planar photonic integrated circuits (PICs). The other limitation is the choice of waveguide core materials for designing various passive and active components for the application specific PICs. Fortunately, besides crystalline Si, plenty of other options of waveguide core materials such as epitaxially grown Ge, PECVD/LPCVD grown SiN, AlN, etc., are available for the utilization of advanced CMOS fabrication process compatible silicon photonics technology in silicon-on-insulator (SOI) platform. However, because of the absence of a suitably designed optical via, hybrid structure waveguide components and multi-layered three-dimensional PIC designs couldn't be effectively implemented till date. Here, we have proposed a low-loss compact optical tunnel via (OTV) design as a solution for 3D photonic integration. Using a standard Maxwell's equation solver, we have shown that the insertion loss of such a OTV design can be $< $ 0.1dB for a broad wavelength range of operations around 1550nm. We have also shown low-loss performance of such OTVs can be arrayed in designing complex PICs with hybrid waveguide components. The materials and design dimensions have been chosen carefully such that the proposed 3D-PICs can be easily manufactured in conventional CMOS foundries. We have also presented wavelength dependent loss (WDL) and polarization dependent loss (PDL) of various OTV designs for hybrid photonic interconnects using 3D-FDTD simulations.

A compact high-Q silicon photonic micro-disc resonator design is proposed and demonstrated for energy-efficient tunable microwave photonic filters. The disc radius of 5- μ m is carefully chosen such that its fundamental whispering gallery mode is phase-matched with the guided fundamental mode of its bus waveguide of width 540 nm, which was fabricated in a silicon-on-insulator (SOI) substrate of device layer thickness 220 nm. A spiral Tungsten microheater (cross-section 600 nm × 400 nm) of length 103- μ m was integrated on the top cladding layer for an efficient thermo-optic tuning. The Q-value of the fabricated resonators are as high as ∼ 2× 10^5 for a resonance wavelength λ _r ∼ 1550 nm corresponding to the fundamental whispering gallery mode. The fabricated microdisc resonator devices were successfully used for the demonstration of a widely and continuously tunable microwave filter (tuning range of up to 50 GHz and filter bandwidth of ∼ 1 GHz with a thermo-optic tuning efficiency as high as ∼ 38 GHz/mW).

A compact and energy-efficient silicon photonic microdisc resonator (MDR) has been designed and demonstrated for widely tunable microwave photonic filters. The MDR was integrated with a meander type microheater for efficient thermo-optic tuning. The experimentally observed loaded Q value of the MDR (radius 5 μm) in all-pass configuration is 10^5 and its resonance wavelength (λr ∼ 1550 nm) could be thermo-optically tuned with a slope of 351 pm/mW (∼ 44 GHz/mW). In our experiment, we have demonstrated microwave bandpass filter of 3-dB bandwidth ∼1 GHz and tunable up to 50 GHz which is limited mainly due to the bandwidth of modulator and photodetector. However, the theoretical tuning range of the device is as high as 780 GHz, which is again limited by the higher order resonance mode coupling to the MDR in all-pass configuration. The device design parameters were optimized such that it could be fabricated in a conventional CMOS fabrication process-compatible silicon photonics technology platform.

We present a Verilog-A compact model of lateral-gate transistors on a 220 nm silicon-on-insulator platform, validated with TCAD simulations and silicon photonics foundry-specific process parameters, used to design logic gates and a multiplexer. Such transistor design enables co-integration for the large-scale programmable photonic integrated circuits.

A programmable silicon photonic multiband microwave filter with tunable bandwidths has been demonstrated. The filter has been successfully used to set up a radio-over-fiber link operating in the X-band as a proof of concept. For this purpose, we have investigated the programmable photonic integrated circuit comprising 14 tunable balanced Mach-Zehnder interferometers designed into a square mesh architecture and fabricated using CMOS-compatible silicon photonic technology. The photonic chip was electrically packaged for its reconfigurable operations using an external 16-channel programmable power supply by tuning the thermo-optic phase shifters integrated into both the arms of Mach-Zehnder interferometers. The operating temperature of the packaged chip is stabilized within +/- 0.002 degrees C using a PID controller to avoid any interference of ambient temperature fluctuations. The mesh architecture is programmed into three different microring resonators (MRRs) operating independently in all-pass configurations for three free spectral ranges (23.25 GHz, 11.75 GHz, and 8.75 GHz, respectively). The multiband microwave photonic filter characteristics have been experimentally obtained using a fiber-coupled offchip laser source (operating at )similar to 1550 nm), a modulator (bandwidth similar to 40 GHz), and a photodetector (bandwidth similar to 50 GHz). Thus, we have demonstrated microwave filters with two, three, and four bands within the modulator bandwidth used in the experiments. Among these, the two-band filter realized for the X- and Ka-bands was thoroughly investigated in the experiments to understand the MRR coupling condition (Q-factor) effect on the realized filter bandwidth and its tuning range, link gain, and rejection. The bandwidth of both the filter bands was tuned over the extensive range of 1.6-8.1 GHz by controlling the MRR Q-factor and the resonance spacing from the carrier. At the same time, the bandwidth tuning range of the three-band filter realized for the C-, Ku-, and Ka-band (2-6 GHz) and the four-band filter realized for the C-, Ku-, K-, and Ka-band (3-5 GHz) in our experiments was limited by the finesse of the ring resonator. Finally, the microwave filter response at the X-band is used for evaluating a radio-over-fiber receiver link as a proof of concept for its potential 5G/6G applications. With an optimized set of operating parameters of the programmable microwave filter, we could achieve a bit error rate as low as similar to 10-20 for a fiber-optic link length of 400 m (at 100 Mbps). (c) 2025 Chinese Laser Press

A compact microring resonator in all-pass configuration using a low-loss bimodal SiN waveguide design has been demonstrated and subsequently it was successfully used for the demonstration of high-performance tunable microwave photonic oscillator in dual-loop configuration. The microring resonator design was optimized through a detailed theoretical analysis (for multimode coupling interactions) such that its Q-value can be maximized for any given fabrication process parameters (CMOS fabrication process compatible). We have recorded an experimental Q-value as high as similar to 2x10(6) from such a device design by exciting fundamental mode into the bimodal bus and ring waveguides. Our experimental results also show that an equivalent device design with a singlemode waveguide and fabricated with same fabrication process technology exhibit nearly 7 times lower Q-value (similar to 2.5x10(5)) with an FSR of similar to 120 GHz. The cavity loss of the fundamental guided mode in a bimodal waveguide is reduced to similar to 0.1 dB/cm due to reduced scattering from fabrication-induced sidewall roughness and low bend-induced radiation losses. These results are matching well with our theoretical predictions. Such a device has been used further to demonstrate a widely tunable narrow-bandpass microwave photonic filter. The experimental results exhibit a narrowband filter with a 3-dB bandwidth as low as 290MHz with broad tunability from 5-40 GHz and the side-band rejection of similar to 30 dB has been recorded. Furthermore, the device was used to demonstrate a widely tunable microwave oscillator (up to 20 GHz) with a signal-to-sidemode suppression ratio of similar to 70 dB and phase noise of -125 dBc/Hz at 10 kHz offset. The typical 3-dB bandwidth of the generated microwave signal (at 10GHz) is similar to 4 kHz. Though both filter and oscillator can be tuned up to similar to 60 GHz, we are limited in characterizing it due to the phase modulator bandwidth (40 GHz) and the RF amplifier bandwidth (20 G...

We present an experimental demonstration of optoelectronic mixing of micro/mm-wave frequencies in Ge-on-Si waveguide photodetector fabricated using conventional silicon photonics technology platform. It has been shown that bias modulating input RF signal can be efficiently up/down-converted at the photodetector output in response to an incident local oscillator (LO) modulated laser light. In our experimental demonstration, the frequency conversion efficiency (at IF = 1.5 GHz) is observed from -19 dB to -41 dB while tuning the RF signal from 5 GHz to 45 GHz and -50 dB to -16 dB for varying IF from 2 to 45 GHz. This result is very promising for the futuristic demonstration of a fully integrated robust micro/mm-wave signal processing chip, by integrating widely tunable microwave photonic LO, high-speed modulator and further optimized Ge-on-Si waveguide photodetector using silicon photonics technology compatible with CMOS process.

An experimental investigation assisted with coupled mode theory for distributed Bragg reflector (DBR) in a single-mode SiN rib waveguide has been carried out in order to obtain narrow-linewidth and high-extinction filters operating in optical C-band (lambda similar to 1550 nm), for silicon photonics applications. A uniform sub-wavelength grating structure with a given periodicity is considered in the cladding region of the rib waveguide structure for the DBR response studies. The upper limit of periodic perturbation strength has been evaluated for designing narrow linewidth and high extinction DBR filters using coupled mode theory approximation. This study enables photonic integrated circuit design engineers to design a relatively simple and shorter length of DBR device (similar to 1 mm) exhibiting a 3-dB bandwidth as narrow as similar to 1-nm and rejection as high as >40-dB, which can be easily integrated for large-scale complex photonic integrated circuit functionalities.

A verilog-A compact model for Ge-on-Si waveguide photodetectors designed to operate in optical C-band has been developed. The model is validated with experimental results for both continuous wave laser power-dependent linear as well as saturation response characteristics and the laser modulation frequency-dependent performance characteristics.

A high-Q SiN waveguide resonator is investigated for its microwave photonic bandpass filter characteristics as a function of signal strength. It has been shown that the link gain of the filtered microwave signal reduces drastically at higher power levels, most likely because of thermo-optic bistability.

We present the concept of an 'optical-via' enabling 3D photonic integration with multi-material stacks. Low-loss, and compact optical-via designs are optimized for optical routing from conventional single-mode SOI waveguide to front-end defined single-mode SiN waveguide, and from SiN waveguide to back-end defined single-mode AlN waveguide.

An algorithm is developed to extract component level phase errors of photonic integrated circuits from its passive transmission characteristics. This algorithm successfully reduces the optical crosstalk from -10dB to -40dB in 4 x 4 MZI feedforward mesh architecture.

Four-wave mixing (FWM) in silicon photonics offers promising applications in optical signal processing, wavelength conversion, and quantum photonics, necessitating a comprehensive understanding of its underlying physics and limitations. Stimulated four-wave mixing in silicon photonic wire waveguides has been investigated theoretically by considering two-photon absorption, free carrier absorption, and free carrier dispersion, etc., operating near lambda similar to 1550 nm. It has been shown that these effects severely degrade the performance when operating at higher pump power levels. We present exact numerical calculations of idler wavelength conversion efficiency and bandwidth taking into account the abovementioned nonlinearities, to show the effects of various parameters within the waveguide structure. We extended to investigate silicon photonic microring resonators in terms of the four-wave mixing performance of high-Q-value microring resonators, which are very promising as photon sources (spontaneous four-wave mixing) for large-scale quantum photonic circuits. Our analysis illustrates the limitations of analytical models in describing FWM in ring resonator behavior at a moderate input power level, necessitating a rigorous numerical approach. Experimentally, we demonstrate stimulated FWM (1525 nm <= lambda(p) <= 1575 nm) in a silicon photonic wire with a cross-section of 500 nmx220 nm and with a small waveguide length of 2 mm. The idler-to-signal conversion efficiency of about -35 dB has been measured for an approximate launched pump power of 30 mW. The conversion efficiency is improved further in the microring resonator as predicted by the simulation results. (c) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI

We review the design and demonstration of distributed Bragg reflector (DBR)-based resonance filters developed at CoE-CPPICS, IIT Madras, with the in-house complementary metal oxide semiconductor (CMOS)-compatible silicon photonics technology platform. The proposed devices include two types of band-pass filter design approaches, i.e., the higher-order DBR coupled cavity filter and apodized DBR cavity filter with ultra-broad stopband for guided Fabry-P & eacute;rot resonance in a silicon-on-insulator rib waveguide structure. The device design parameters are optimized through semi-analytical simulation methods for a low insertion loss singly resonant transmission peak at a desired wavelength. Fourth- and fifth-order passive resonant filters are designed and demonstrated with nearly lossless, flat-top response (ripple <1 dB) with large out-of-band rejection (>40 dB), and a maximum shape factor of 0.9 without any active tuning. With optimized apodization parameters for the DBR cavity, a device of length as low as similar to 35 mu m exhibits a large rejection band of similar to 60 nm and an extinction of similar to 40 dB at the resonant wavelength peak at lambda r similar to 1550 nm (FWHM similar to 80 pm, IL similar to 2 dB). The demonstrated devices are potential candidates for many integrated photonic applications such as microwave filters, modulators, add-drop multiplexers, sensors, and broadband noise suppression.

A high-Q microring resonator in all-pass configuration using a low-loss bimodal SiN waveguide design has been demonstrated and explored for microwave photonic applications. We have recorded a Q- as high as ~2X10^6 from such a device design by exciting fundamental mode into the bimodal bus and ring waveguides which was fabricated using in-house conventional silicon photonics process technology. Our experimental results show that an equivalent device design with a single-mode waveguide and fabricated with same fabrication process technology exhibit nearly 7 times lower Q-value (~ 2.5X10^5). The cavity loss is reduced because of the fact that the fundamental guided mode in a bimodal waveguide experiences lower scattering and bend-induced radiation losses (~0.1 dB/cm); scattering losses occurs mainly due to the waveguide sidewall roughness which in fact fabrication process dependent. Such a device has been used further to demonstrate a widely tunable narrow-bandpass microwave photonic filter. The experimental results exhibit a narrowband filter with a 3-dB bandwidth as low as 290 MHz with broad tunability from 5-45 GHz and the side-band rejection of ~30 dB has been recorded. Furthermore, the device was used to demonstrate widely tunable microwave oscillator with a signal-to-sidemode suppression ratio of up to 57-dB. Though both filter and oscillator can be tuned up to 60 GHz, we are limited to characterize it due to the phase modulator bandwidth (40 GHz) and the RF amplifier bandwidth (20 GHz).

We report experimental demonstration of high performance DBR filters (operating in C-band) in SiN waveguides by cladding integrated grating structures. The gap between waveguide core and grating structure is shown to be an additional key parameter for the fine control of the DBR filter characteristics.

A programmable square mesh architecture of photonic integrated circuit comprised of 14 tunable balanced Mach-Zehnder interferometers has been investigated using silicon photonics technology platform for the demonstration of reconfigurable multiband microwave photonic filters. The photonic chip was electrically packaged for its reconfigurable operations using external16-channel programmable power supply for tuning the thermo-optic phase-shifters integrated in both the arms of Mach-Zehnder interferometers. The operating temperature of the packaged chip is stabilized within ±0.002º𝐶 using a PID controller, to avoid any interference of ambiance temperature fluctuations. The mesh architecture is programmable into three different microring resonators in all-pass configurations operating independently for three different free-spectral ranges (23.25 GHz, 11.75 GHz, 8.75 GHz), respectively. The multiband microwave photonic filter characteristics have been experimentally obtained using fiber-coupled off-chip laser source (operating at 𝜆 ∼ 1550 nm), modulator (bandwidth ∼ 40 GHz) and photodetector (bandwidth ∼50 GHz). Thus we have demonstrated microwave filters with two, three and four bands within the bandwidth of the modulator used in the experiments. Among these, the two-band filter realized for X and Ka-band was thoroughly investigated in the experiments to understand the effect of the MRR coupling condition (Q-factor) on the realized filter bandwidth and its tuning range, link gain and rejection. The bandwidth of both the filter bands was tuned over the extensive range of 1.6 GHz to 12.7 GHz by controlling the MRR Q-factor and the resonance spacing from the28 carrier. At the same time, the bandwidth tuning range of the three-band filter realized for C, Ku, and Ka-band (2 GHz to 6 GHz) and four-band filter realized for C, Ku, K and Ka-band (3 GHz to 5 GHz) in our experiments were limited by the MRR’s resonance bandwidth and free spectral range.

We have explored compact design of microring resonator in SiN platform for a demonstration of widely tunable narrow-bandpass microwave photonic filter and optoelectronic oscillator. Higher Q value (> 10 6 ) and wider FSR (> 100 GHz) of a microring resonator are the essential figure of merits for such applications.