Scaling Silicon Photonics-based Laser Doppler Vibrometry with Multi-Beam Frequency Shifters

Introduction

Laser Doppler vibrometry (LDV) is a powerful technique for remotely detecting vibrations in diverse applications such as non-destructive testing and photoacoustic sensing. Silicon photonics-based LDVs offer a compact and scalable solution compared to traditional free-space optics approaches. However, current silicon photonics LDV implementations are limited to only a handful of detection beams due to challenges in scaling the number of electrical connections required.

This tutorial introduces an innovative architecture that overcomes these limitations by incorporating a multi-beam frequency shifter. We will explain the functionality of this frequency shifter, present simulated performance results, and illustrate how this novel design effectively reduces constraints on electrical connections, enabling large arrays of LDV beams.

Laser Doppler Vibrometer Overview

A homodyne LDV splits incoming light into reference and measurement arms, as shown in Figure 1a. The measurement light is directed towards the target by a grating coupler and subsequently recombined with the reference light via a 90-degree optical hybrid. Analysis of photocurrents from four photodiodes facilitates precise demodulation of the target's movement.

Fig. 1: a) Schematic of an on-chip homodyne LDV. b) Multibeam implementation of homodyne LDV

For applications requiring simultaneous vibration detection at multiple locations, the conventional approach is to implement multiple homodyne LDVs in parallel (Figure 1b). However, this architecture faces scaling challenges due to the linear relationship between the number of beams and required electrical connections or pads on the photonic chip.

The Multi-Beam Frequency Shifter

A novel on-chip multi-beam frequency shifter design, introduced in [5], enables the generation of multiple heterodyne frequency shifts from a single source (Figure 2a). Single-frequency light in a waveguide is split into an array of waveguides, each equipped with a modulator element. Modulation of each element generates harmonics, and a wave-like modulation pattern across the array, with a delay between consecutive modulators, separates these distinct frequency shifts. The phase relation of the harmonics at the input of the star coupler allows the collection of different frequency shifts in various output waveguides.

Fig. 2: a) Diagram of a multi-beam frequency shifter. b) Simulation results for s-parameters and harmonic content with perfect phase modulation (modulation depth 1.84, phase delay 1.25 radians), showing outputs 0 to 2; other outputs symmetric for negative harmonics. c) 2D FDTD simulation for s-parameter calculation of the star coupler's output 0 (upper right).

The performance of the multi-beam frequency shifter is calculated by determining the harmonic content resulting from periodic modulation and accounting for the phase modulation induced on each harmonic between adjacent star coupler inputs. With knowledge of the amplitudes and phases for each harmonic across all input ports, the output can be calculated using the star coupler's s-parameters obtained from 2D FDTD simulations (Figure 2c). Figure 2b illustrates the different outputs, demonstrating that distinct harmonics are collected, effectively serving as a multi-beam frequency shifter.

Scaling to Multi-Beam Architecture

The multi-beam frequency shifter introduces an alternative architecture for multi-beam LDVs on a chip, similar to rainbow heterodyne free-space architectures [6]. As shown in Figure 3a, the measurement light is processed by the multi-beam frequency shifter, while the reference beam is frequency-shifted by a single-beam frequency shifter. The reflected measurement beams, combined with the frequency-shifted reference, are detected by photodetectors.

Fig. 3: a) Multi-beam and single beam frequency shifters integrated in one architecture. b) Optical spectrum showing shifted reference (green) and measurement signals (blue) combined on a photodetector to separate signals electrically. c) Analysis of pad requirements vs. beam numbers, suggesting efficient pad sharing with multi-beam frequency shifters across different wavelengths.

Figure 3b illustrates the process in the optical and electrical frequency domains. The multi-beam frequency shifter generates different harmonics (in red), and interference with the frequency-shifted reference (in green) results in signals downshifted and separated in the electrical frequency domain.

While around 16 and 4 connections are necessary for driving the multi-beam frequency shifter and the reference frequency shifter, respectively, only two electrical contacts are required to read out 5 beams. Considering 5 beams, the number of electrical connections necessary is similar for both architectures. However, as the number of beams increases, and additional on-chip frequency shifters can be connected in parallel, sharing the electrical contacts from the chip, the multi-beam frequency shifter architecture offers a more efficient solution in terms of electrical connections/pads.

Scaling analysis in Figure 3c reveals that for hundreds of beams, the former would need approximately 400 pads, while the latter would require only around 60. Additionally, the wavelength stability of the multi-beam frequency shifter (as seen in Figure 2b) opens possibilities for sharing a single multi-beam frequency shifter among various wavelengths.

Conclusion

By introducing a multi-beam frequency shifter, we have presented a scalable architecture for multi-beam LDVs on the silicon photonics platform. Scaling analysis has shown that this approach substantially increases the number of beams, overcoming the limitations posed by the amount of electrical connections compared to homodyne LDV layouts, enabling the deployment of LDVs with high beam counts for advanced applications.

Reference

[1] E. Dieussaert, R. Baets, and Y. Li, "Scaling Silicon Photonics-based Laser Doppler Vibrometry with Multi-Beam Frequency Shifters," in Proc. of the IEEE Conference on Lasers and Electro-Optics, 2024, pp. 1-3, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.

[2] E. Dieussaert, R. Baets, and Y. Li, “On-chip multi-beam frequency shifter through sideband separation,” Optics Express, vol. 31, p. 29213, 8 2023.