Recirculating frequency-shifting loop for flexible optical chirp generation and FMCW LiDAR

Abstract

A recirculating frequency-shifting loop (FSL) provides a highly flexible platform for generating coherent optically chirped light with tunable bandwidth, duration, chirp rate and repetition rate. The properties of the chirped light are controlled using low-frequency sinusoidal electronic drive signals, enabling deterministic waveform synthesis without complex high-speed electronics. We achieve chirp bandwidths of 10 GHz with a duration in the nanosecond regime, representing one of the fastest tunable laser sources to date. Using such chirped laser pulses, we demonstrate coherent FMCW LiDAR measurements over distances up to 3 m, highlighting the potential of FSL-based sources for compact, scalable and high-performance ranging systems.

Figures (6)

• Figure 1: Schematic of the frequency-shifting loop (FSL). Light at frequency $\nu_{\mathrm{p}}$ from a continuous-wave seed laser is injected into a fiber loop with round-trip time $T$, where the circulating field undergoes a controlled frequency shift of $f_{\mathrm{mod}}$ on each round trip while an EDFA compensates for loop losses. A bandpass filter (BPF) and an optical isolator (OI) ensure stable operation. After multiple circulations, the loop generates a discrete optical spectrum corresponding to linearly chirped pulses, which can be detected using a photodiode (PD) and oscilloscope. An additional output provides direct access to the frequency-chirped optical signal without the pump light.
• Figure 2: Simulated chirp rate versus modulation frequency for a round-trip time $T = 200~\mathrm{ns}$ and orders $m = 1, 2, 3,$ and $7$. The dashed lines indicate the mode-locking frequencies $f_{\mathrm{mod}} = m/T$. The chirp rate diverges near the mode-locking condition and changes sign across it. This behavior illustrates the strong dependence of the achievable chirp rate on both the modulation frequency $f_{\mathrm{mod}}$ and the order $m$.
• Figure 3: a Experimental schematic of the frequency-shifting loop (FSL) using two acousto-optic modulators (AOMs) driven by independent RF sources. The effective modulation frequency $f_{\mathrm{mod}}$ is monitored with an electrical mixer, a low-pass filter, a frequency counter, and an electrical spectrum analyzer (ESA). b Electrical spectra for first- and second-order operation, showing peaks at $f_{\mathrm{mod}}=4.8230~\mathrm{MHz}$ and $f_{\mathrm{mod}}=9.6410~\mathrm{MHz}$. c Optical spectra measured at two extraction points of the loop (Point I. and Point II.), exhibiting spectral broadening due to the accumulated frequency shifts.
• Figure 4: a Normalized beat signals and corresponding spectrograms for first- and second-order operation at modulation frequencies of 4.823 MHz and 9.641 MHz, respectively. b Chirp rates as a function of modulation frequency for first- and second-order components, exhibiting a sharp change in magnitude and sign near the mode-locking condition. The chirp rates corresponding to 4.823 MHz and 9.641 MHz are indicated by green stars.
• Figure 5: a Schematic of the FSL implementation using an electro-optic I/Q modulator operated in single-sideband (SSB) mode. b Spectrograms of the loop output for first-, second-, third-, and seventh-order operation at modulation frequencies of 4.21 MHz, 8.42 MHz, 12.61 MHz, and 29.42 MHz, respectively, showing linear frequency sweeps. c Chirp rates as a function of modulation frequency for the corresponding orders, with green stars indicating the spectrograms.