Two distributed-feedback diode lasers feed adjacent channels of a dense…

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Coherence and statistical optics · Coherence and statistical optics

What an Instrument Can See, Ch Temporal coherence: correlation, spectrum and the interferometer

Problem

Two distributed-feedback diode lasers feed adjacent channels of a dense wavelength-division-multiplexed link near

\lambda_0 = 1550\ \mathrm{nm}

. Each emits a Lorentzian line of full width at half maximum

\Delta\nu = 5.0\ \mathrm{MHz}

, the two channels are

\delta\nu = 50\ \mathrm{GHz}

apart, and the lasers deliver equal power into the same fibre. Treat the combined light as one stationary source with a two-line spectrum, and take

c = 3.0\times10^8\ \mathrm{m\,s^{-1}}

. ((a)) Compute the coherence time

\tau_c

and coherence length

\ell_c

of one laser alone. ((b)) Write

|\gamma_{11}(\tau)|^2

for the combined light and, from the integral definition, find its coherence time and coherence length. Say why the beat term can be dropped from the integral.

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