Very long range quasi-Fourier spectroscopy for narrowband lasers

Abstract

The measurement of the spectral broadening, or temporal coherence property of very narrow linewidth lasers is not an easy task, while such a measurement is essential in any interferometric applications of the lasers. The beat note between two assumingly identical lasers only provides the convolutional spectral profile of the two lasers, but not characterizes the single laser. The delayed self-heterodyne interferometer (DSHI) would not be effective for kHz linewidth range because the finite delay cannot realize complete de-correlation. Here, we demonstrate, for the first time to our knowledge, the complete characterization of the modulus of the degree of coherence (DOC) of kHz linewidth lasers, with a self-referenced fashion where any other reference beam is not used, accordingly, characterize the spectral profile. The method is based on speckle statistical analysis of the Rayleigh scattering in the coherent fiber reflectometry, and would be a novel strong tool to characterize very narrow linewidth lasers.

Verification of Laser Phase Error Measurement Using Speckle Analysis in Coherent Fiber Reflectometry

“…The theory of the measurement principle has already been presented in [8]: a brief sketch is provided in the following. The measurement system setup ( Fig.…”

“…Let a q (t)e j 2πν q t and a s (t)e j 2πν s t be the complex amplitudes of the q-th and s-th samples of a laser emission with temporal duration T . If the fluctuation of the beam is ergodic and the observation time T is sufficiently large such that it includes every possible fluctuation, and the sample represents the entire ensemble, then the correlation between the two speckle amplitudes, e q (τ ) and e s (τ ), observed at the delay τ are given by [8] where…”

“…In [7] and [8], a novel method is demonstrated for characterizing the magnitude of the coherence function of a narrow linewidth laser based on Rayleigh speckle analysis using coherent fiber reflectometry, where no reference laser is needed. In this Letter, we examine a new method for measuring the phase errors of lasers, and compare the results with those obtained from the beat signal analysis of two lasers.…”

A Novel Speckle Analysis Scheme for Coherence Measurement Using a Standard Phase Modulator

“…However, this is not feasible if the delay is finite. Our group recently demonstrated that the degree of coherence (DOC) of a narrow linewidth laser can be accurately characterized with a beam based on Rayleigh speckle analysis using coherent optical frequency domain reflectometry (C-OFDR) with a single sideband (SSB) modulator [6]- [8]. However, a Rayleigh backscattering measurement needs the Manuscript optical amplifier to recover the power loss due to the modulation and connection losses in the SSB modulator.…”

“…The novel speckle correlation analysis method is proposed for evaluating the degree of coherence (DOC), by modifying the previously proposed theory [8] and adapting it to the standard phase modulator configuration.…”