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In the late 70’s and early 80’s, Phase Shift Interferometry (PSI) was one of (if not the) hot topics in the Wyant lab. As my dissertation revolved around measuring the statistical properties of speckle patterns (using the newly invented CCD array), my work was somewhat out of the mainstream, and I developed a mild case of PSI envy.
Two years after graduating, I finally had a chance to use PSI, although for a very different purpose. In the mid 1980s, there was a great deal of interest in being able to build heterodyne receivers for optical communications. An optical heterodyne receiver is exactly analogous to FM radio, where the input frequency modulated signal is beat against a cw semiconductor laser (the local oscillator) with a slightly different frequency. Heterodyne receivers can achieve shot noise limited performance by turning up the power of the local oscillator to the point where the shot noise becomes larger than the receiver thermal noise. Moreover, changing the frequency of the local oscillator enables decoding one of many channels in the signal).
Among the many challenges facing the development of such systems was the problem that that semiconductor laser linewidths at the time were one or two orders of magnitude larger than the linewidth predicted by Schawlow-Townes for gas lasers, leading to unacceptable amounts of receiver phase noise. Moreover, the linewidth was not Lorentzian, but had structure - also contributing to excess phase noise with a resonance at the relaxation oscillation frequency of the laser.
Two groups (Chuck Henry[1] at Bell Labs, and Vahala and Yariv at Caltech) were working to extend the Schawlow-Townes theory to account for the change in a semiconductor’s index of refraction due to spontaneous emission. At that time, measurements of linewidth were typically made in the frequency domain using a Fabry-Perot Interferometer. However, it is extremely difficult to deconvolve the effect of changes in the cavity index from the linewidth caused by simply adding spontaneous photons to the field in the frequency domain. In contrast, by measuring the coherence function (delayed time domain), these effects are multiplicative, and can be separated by simple curve fitting. Using Phase-Shifting Interferometry (PSI) it was possible to measure the coherence function of semiconductor lasers, enabling quantitative validation of the Henry/Vahala theory. Moreover, it was also possible separate out total impact of stochastic phase fluctuations, from the frequency modulation caused by direct current modulation of the signal laser, to show that the coherence properties of the signal laser do not change under modulation.
I had the great fortune to work for Jim Wyant at the most pivotable point in my career. It led to the work described above, which in turn led to work on very high speed (>50 GHz) frequency modulation, receiver characterization, optical switching, and optical amplifiers. Perhaps more importantly (and something that took me many years following my graduate career to recognize), working for Jim provided a model for how to guide research groups, and how to mentor students. In this talk, I will discuss both the technical aspects of using PSI to measure the coherence properties of semiconductor lasers, as well as some of lessons learned (and a few of the humorous things that happened) during my apprenticeship in the Wyant lab.
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