Silicon photonics is a matured technology today for optical communication transceiver industries. Besides transceiver, other upcoming application areas for silicon photonics are depicted in the cartoon figure below. Present research focus of the CPPICS team is mainly cantering around design, demonstration and packaging of photonic chips for advanced microwave and quantum photonic applications.

• Novel Device Design
• Compact Modelling
• Microwave Photonic Processor
• Quantum Photonic Processor

One of the prime focuses of our group is the design of novel photonic devices for various applications such as computations and sensing to perform power splitting/combining, filtering, modulation, photon pairs generation/manipulation and detection. Apart from silicon platform, silicon nitride is also explored to fabricate devices spanning a multitude of functionalities. Devices can be categorized broadly into active and passive, depending on whether doping (n or p type) is present or not.

Our current focus is on the following research topics:

• Passive Devices: Passive devices constitute waveguides, splitters/combiners, MUX/DMUX, crossings, bends which are routing essentials, as well as stand-alone devices like microring/microdisc (MRR/MDR) resonator, Sub-wavelength gratings (SWG), Mach Zehnder interferometer (MZI), Distributed Bragg reflector (DBR), DBR cavity and much more.
• Active Devices:
  • High-speed optical modulators
  • Waveguide integrated photodetector

In the process of scaling photonic circuits, an in-depth exploration of fundamental photonic devices is of paramount significance to realize large-scale circuits. The development of compact models for individual devices is a pivotal step, enabling photonic designers to create process design kits (PDKs). These PDKs derived from the foundries, empower end-users to efficiently design the specific photonic circuits. In addition, accurate compact models of individual components will eventually enable photonic circuit designers to realize electro-optic circuits for applications like LIDAR, neuro-morphic computing, and quantum computing.

Our current focus is on the following research topics:

• Development of compact models of different active and passive integrated photonic components for electro-optic co-simulation in a single platform
• Wafer Scale Analysis for yield prediction, variability analysis, and design optimization from analysis of wafer scale characterization data

Owing to the success of high-speed silicon photonic transceivers in the data centres, demand for similar products in several other fields has emerged. Microwave photonics is one such field where the THz bandwidth of photonics can overcome the limitations in conventional microwave components. The futuristic communication systems target to operate in the high frequency (10s of GHz) to meet the ever-increasing demand of data consumption. The design of microwave components such as filters, oscillators, phase shifters to operate at these frequencies with a small footprint (cm x cm) is challenging.

Our current focus is on the following research topics:

• Programmable Photonic RF Filter: An RF filter is the basic component of any RF receiver architecture to separate the unwanted signal from the signal of interest. For this purpose, passive electrical filters are most commonly used because of their robustness and cost-effectiveness. However, these filters are designed to operate at a single frequency band with little to no scope for tuning and usually are power hungry especially at mm Wave frequencies. The programmable RF photonic filters show promise in terms of size, weight and power (SWaP), wide-band tunability and scalability, which are very crucial in 5G/6G and satellite communications and avionic applications.
  At CoE-CPPICS, we are working towards the development of a working prototype of a widely tunable (beyond 40 GHz) RF photonic filter. We have proposed a novel operational scheme of the programmable optical filter to realize a tunable RF photonic filter with improved shape factor.

Optoelectronic Oscillator: RF signal generation with low linewidth and phase noise is required in multiple applications such as radar systems, satellite communication and RF signal processing. In the conventional approach of optoelectronic oscillator (OEO) longer length of fiber delay lines are used with narrow bandpass electrical filter to generate spectrally pure RF signal. In this case, electrical filter with these critical specifications of narrowband and tunability is limiting factor. Here, in CoE-CPPICS we are working on the demonstration of integrated optoelectronic oscillator using microring resonators (MRRs).
An MRR with high-Q (~ 1.5 million) is used to demonstrate the functionality of bandpass filter response at 10 GHz with the 3-dB bandwidth of 340 MHz and rejection of 25 db. The filter is further used to demonstrate the tunable OEO frequency generation from 8-12 GHz with sidemode suppression ratio (SMSR) of 42 dB and FSR of 4.3 MHz for the longitudinal modes of OEO cavity.

Photonic Beamformer: In 5G/6G architecture, beamforming/steering is a powerful technique to improve the transmission range of small cells where the mm-waves operate (>3 GHz). A traditional electronic beamforming/steering network suffers from the problem of beam squinting (frequency dependent steering angle), thus limiting the operating bandwidth of the network. Microwave Photonics becomes advantageous here. Scalable circuitry and large bandwidth makes it suitable for realizing on-chip broadband optical beamforming networks (OBFNs). Integrated tunable optical delay lines are the key elements for OBFNs. Thus far, several schemes have been implemented with ring resonators or MZIs. The Figure below shows the schematic diagram of a typical optically controlled phased array antenna system using a m✕n OBFN chip in SiN.

Our current focus is on the following research topics:

• Quantum Photonic Sources: Photon pair generation through spontaneous four-wave mixing (FWM) in silicon waveguides or micro-ring resonators stands as a pivotal requirement in large-scale integrated quantum photonics.

Quantum Random Number Generator: Quantum Random Number Generator (QRNG) is a type of TRNG where the source of randomness is generated from quantum-mechanics driven physics, which is by nature having very high unpredictability. We have exploited the well developed CMOS technology in our lab to generate real-time truly random bit streams on silicon (SOI) platform.

Quantum Key Distribution: Classical cryptography depends on the time complexity of a mathematical problem such as the factorization of the product of two large prime numbers. But with the help of Shor's algorithm, quantum computers can solve the factorization problem in less time which is a huge threat to security.
However, quantum key distribution uses quantum mechanics to ensure unconditional security. Practical implementation of QKD protocols based on bulk optics has been shown. Bulk optics-based QKD implementations occupy large space and also face stability issues. Implementation of QKD on chip offers compact size, high stability, and robustness.

Boson Sampling: Boson Sampling (BS) is considered to be one the important milestones to demonstrate supremacy of quantum technologies over the classical computation paradigm in the NISQ era. BS is also relevant to domains like - quantum chemistry, cryptography, cryptocurrency and problems of graph optimization. Silicon photonics allows for the implementation of BS experiments in a very small footprint which ensures the phase stability of the system and makes the system more resilient to ambience fluctuations.
We are working towards an efficient demonstration of the BS experiment on the Silicon Photonic platform. Low-loss waveguides, high-quality indistinguishable photon sources, phase-stable and full-range interferometric circuits, and high-efficiency single-photodetectors are key ingredients to successfully realize this experiment.