On-chip multi-beam frequency shifter through sideband separation.

In this paper, we introduce a novel method to realize a multi-beam optical frequency shifting component for photonic integrated circuits, utilizing an array of parallel optical modulators and a free-propagation region (FPR), such as a slab waveguide-based star coupler. This component generates multiple optical beams with different frequency shifts, making it suitable for various systems, such as multi-beam laser Doppler vibrometry (LDV). We thoroughly elaborate on the working principle of the component through theoretical analysis and demonstrate that by applying periodic wave-like modulation in the modulator array, the discrete harmonic content of the light can be selectively directed to different outputs based on the delay between consecutive modulators. A design comprising a 16-element modulator array and 5 outputs will be presented. Simulations show that this design can generate and collect 5 different harmonics (-2, -1, 0, +1, +2) in the different outputs with a side band suppression ratio of 20 dB to 30 dB for each output. Our proposed design is just one possibility and the component can be modified and optimized for specific applications.

A Compensation Method for Nonlinearity Errors in Optical Interferometry.

Optical coherent detection is widely used for highly sensitive sensing applications, but nonlinearity issues pose challenges in accurately interpreting the system outputs. Most existing compensation methods require access to raw measurement data, making them not useful when only demodulated data are available. In this study, we propose a compensation method designed for direct application to demodulated data, effectively addressing the 1st and 2nd-order nonlinearities in both homodyne and heterodyne systems. The approach involves segmenting the distorted signal, fitting and removing baselines in each section, and averaging the resulting distortions to obtain precise distortion shapes. These shapes are then used to retrieve compensation parameters. Simulation shows that the proposed method can effectively reduce the deviation caused by the nonlinearities without using the raw data. Experimental results from a silicon-photonics-based homodyne laser Doppler vibrometry prove that this method has a similar performance as the conventional Heydemann correction method.

Miniaturization of Laser Doppler Vibrometers-A Review.

Laser Doppler vibrometry (LDV) is a non-contact vibration measurement technique based on the Doppler effect of the reflected laser beam. Thanks to its feature of high resolution and flexibility, LDV has been used in many different fields today. The miniaturization of the LDV systems is one important development direction for the current LDV systems that can enable many new applications. In this paper, we will review the state-of-the-art method on LDV miniaturization. Systems based on three miniaturization techniques will be discussed: photonic integrated circuit (PIC), self-mixing, and micro-electrochemical systems (MEMS). We will explain the basics of these techniques and summarize the reported miniaturized LDV systems. The advantages and disadvantages of these techniques will also be compared and discussed.