Study on Frequency Coherence Properties of Light Beams | IEEE Journals & Magazine | IEEE Xplore

Study on Frequency Coherence Properties of Light Beams


Abstract:

This paper presents a new concept of frequency coherence in the frequency-time domain to describe the field correlations between two lightwaves with different frequencies...Show More

Abstract:

This paper presents a new concept of frequency coherence in the frequency-time domain to describe the field correlations between two lightwaves with different frequencies. The coherence properties of the modulated beams from lightwave sources with different spectral widths and the modes of Fabry-Perot (FP) laser are investigated. It is shown that the lightwave and its corresponding sidebands produced by the optical intensity modulation are perfectly coherent. The measured linewidth of the beat signal is narrow and almost identical no matter how wide the spectral width of the beam is. The frequency spacing of the adjacent FP modes is beyond the operation frequency range of the measurement instruments. In our experiment, optical heterodyne technique is used to investigate the frequency coherence of the modes of FP laser by means of the frequency shift induced by the optical intensity modulation. Experiments show that the FP modes are partially coherent and the mode spacing is relatively fixed even when the wavelength changes with ambient temperature, bias current and other factors. Therefore, it is possible to generate stable and narrow-linewidth signals at frequencies corresponding to several mode intervals of the laser.
Published in: IEEE Journal of Quantum Electronics ( Volume: 45, Issue: 5, May 2009)
Page(s): 514 - 522
Date of Publication: 17 April 2009

ISSN Information:


I. Introduction

Coherence in optics is an important parameter that quantifies the quality of interference [1], [2]. The most commonly used concepts are temporal and spatial coherence, which have been extensively studied in the past [3]– [6]. The concept of field correlations in the space-time domain has been expressed clearly in [1]. Spatial coherence describes the correlation between signals at different points in space. Temporal coherence describes the correlation between signals observed at different moments. In a typical interferometer, a lightwave is split into two beams, and the two beams are recombined together with different delay times. The two beams are perfectly coherent when the lengths of the two paths are identical. For a certain delay difference, the degree of coherence depends on the linewidth and wavelength stability of the light beam. There are other concepts on coherence in accordance with different physical parameters, such as polarization coherence, quantum coherence, and spectral coherence. Spectral correlation, which is not so widely used as temporal and spatial coherence, describes the correlation that exists between the spectral components at a given frequency in the light oscillations at two points in a stationary optical field [1].

References

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