A photonic frequency discriminator based laser linewidth estimation technique

Abstract

The conventional methods on the measurement of linewidth of an individual laser or free-running beat spectrum are unable to realize both direct and accurate measurement/estimation all at once. An optical measurement or estimation technique is necessary for very high frequency optical system. The direct measurement of the optical spectrum from an RF-SA suffers from the frequency jitter, hence provides an inaccurate measurement. On the other hand methods to estimate the RF beat linewidth without directly measuring the RF spectrum are limited in terms of frequency range. We propose to use an optical discriminator to estimate the beat spectrum linewidth of a free running heterodyning system. Using a dense wavelength multiplexing (DWDM) filter as an optical discriminator, the transformation of phase modulation (PM) to intensity modulation (IM) is achieved. So, transformed laser phase noise into RIN (relative intensity noise). The proposed concept for beat spectrum linewidth estimate is shown and compared to other direct/indirect beat spectrum measurement techniques. Our proposed technique estimates the beat linewidth more accurately and by direct measurements of the beat spectrum. Moreover, the proposed technique operates in optical domain, thus not limited in frequency range. This technique is not affected by frequency jitter unlike the other methods for beat spectrum linewidth measurement.

Highly Stable Photonic Local Carriers for Phased Array Receiver System

“…In all these applications, a precisely defined optical frequency is needed and long-term stability must be ensured for correct system operation, with a required degree of stability and accuracy that depends on the application. Practically speaking, a beat signal is produced between the laser to be frequencystabilized (slave) and a second laser with known frequency (master) [7]. To stabilize the optically generated carrier, the beat signal is often mixed down to a lower frequency which is far better to be taken care of electronically [8,9].…”