Noise characterization of mode-locked lasers by comparing the power spectra of the fundamental and second-harmonic pulses

Low-noise co-arm differential sensor for an optical frequency comb sampling an E-field test system

Optical frequency comb (OFC) technology can realize the rapid measurement of electric fields (E-fields) with large bandwidth. However, this technology suffers from the problem of high intensity noise, resulting in low sensitivity and a blind frequency region. In order to solve the above problems, a dual-path optical E-field sensor with a common reference arm based on a lithium niobate optical waveguide is proposed. The introduction of the reference arm improves the balance of optical paths and the degree of integration. A segmented electrode is also designed to ensure the generation of reverse electrical signals on two Mach-Zehnder interferometers (MZIs). After exiting from the differential photodetector (PD), the intensity noise can be removed, and the sensitivity of the sensor can be improved. After testing, the maximum intensity noise reduction is about 37 dB, the average noise reduction is about 22.3 dB, and the blind frequency region can be eliminated with the co-arm differential optical E-field (CDOE) sensor in the process of measuring the signal. This sensor can be used in the 1 MHz-12 GHz bandwidth with a sensitivity better than 10 mV/m·√Hz.

Wideband phase-noise measurement of mode-locked laser pulses by a demodulation technique

A new method is proposed and demonstrated for measuring the phase noise (pulse timing fluctuations) of mode-locked lasers. The instantaneous phases of the pulse intensity are extracted in a time domain, and the phase-noise power spectral density is calculated from the demodulated signals. Compared with the conventional method based on the single-sideband phase-noise measurement, the proposed method has a larger dynamic range and a wider frequency span. The phase noise of a mode-locked Cr:LiSAF laser is measured for 50-mHz-1-MHz Fourier frequencies.