RA3: Design and Development of custom tool for fiber-array coupling (6-Axis) & attach
Designed a system for attaching optical fiber array modules to photonic chips using multiple coupling schemes. The system comprises an optical table as a base. A sliding mount with an angular orientation mechanism, a laser source to check alignment through an input port, a photodetector to receive the laser beam from the fiber array confirming alignment, a tilt stage, a 3-axis translation stage, a fiber array mounting unit, UV source comprising a UV lamp for attaching a fiber array module to the photonic chip, a first microscope and a second microscope oriented to provide a top and side view of the first vacuum chuck for adjusting position of the chip during attachment. The system is highly cost effective and can hence be easily adopted for prototyping and small-scale product production. The custom tool was used to demonstrate fiber-array coupling and attachment to Si-PICs using Vertical coupling (VGA & QPC) as well as Edge coupling.
Research Areas
RA4: Design of Photonic System & Package for Quantum Random Number Generator (QRNG) application using Directional Coupler on a Si-PIC:
The focus of this research was on building a True Random Number Generator (TRNG) for quantum cryptographic applications using Silicon Photonics by exploiting the phenomenon of quantum vacuum fluctuations. A simple 3-dB balanced directional coupler with one input arm connected to a highly coherent laser source (1550nm) and other arm kept open will act as a mixer to modulate the input Local Oscillator (LO) signal of the coherent laser with quantum vacuum fluctuations in the open arm, see Figure below for system schematic. The output signals from the two arms of the 3-dB coupler are sent to a balanced photo detector which converts the light to electrical signals followed by differential amplification to remove the LO signal. This differential signal is truly random due to the random nature of vacuum fluctuations and can be digitized using an ADC and sent to an FPGA for post processing (to remove electronically induced signal correlations). The final 8-bit random signal, at a target generation rate of 100 Gbps, can then be used for Quantum Key Distribution (QKD). Some of the key technological aspects of this project are: (i) to design & fabricate a balanced 3dB Si-photonic directional coupler on a 100mm SOI wafer thus making a Photonic Integrated Circuit (PIC), (ii) grating design for coupling light in and out the PIC using optic fibers, (iii) fiber array attachment process for the light coupling, and (iv) full package design including the housing to hold the fiber arrays, TEC, & PIC as well as the DC PCBs for the PIC’s micro-heater control signals and RF PCBs for transmitting the high frequency random signals.
Published Patent:
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RA5: Thermoelastic modeling of GaN Laser Lift-Off (LLO) Process:
The focus of this research was on trying to model the stresses in GaN thin films grown on c-plane sapphire substrates during the Laser-Liftoff (LLO) process. The LLO process is used to release the thin films of GaN (~5 microns thick) from the sapphire to a target substrate. It is especially used to mass transfer GaN microLEDs to build microLED displays on different substrates & backplanes. During the LLO process, light from a high power, pulsed Excimer or Q-switched solid-state laser (200-350nm wavelength) is passed through the backside of the polished sapphire wafer. Due to the large band gap of sapphire, the light passes through without much absorption. However, when the light hits the GaN/Sapphire interface, the smaller band gap of GaN causes the light pulse to be absorbed. The high power density of the absorbed light causes localized heating and the GaN dissociates into metallic Ga and Nitrogen plasma (ablation). This process happens within a very short timeframe of < 10 nanoseconds and hence there is a thermo-mechanical shockwave created at the GaN/Sapphire interface. The high thermal gradient at the interface as well as the pressure of the expanding nitrogen gas/plasma contribute a dual effect and create a thermo-elastic shockwave that propagates up through the GaN film at the speed of sound. This shockwave can be so intense that it can cause micro-scale cracking of the GaN films especially when the stresses go beyond the yield strength of GaN. Thus, what is experimentally observed is that when the GaN microLEDs undergo LLO process, nearly > 50% of them are cracked thus providing poor process yields. By modeling how this shockwave propagates quantitatively, it is possible to design stress buffer layers on top of the GaN films (Ni layer) to absorb the incoming stress wave so that the peak stresses within the GaN film can be kept below the yield strength at all times. By using this Ni stress buffer layer, the LLO yield was improved to > 99%. This was the main achievement of this research.
