An investigation is made into the signal-to-noise requirements of a photonic local oscillator (LO). Simple theoretical expressions are givenforthe RF power, relative-intensity-noise (RIN), shot noise and zero-point noise, and measurements are made that verify the simple theory. The theory is then used to determine what photomixer current and noise levels will be required for ALMA receivers, and to predict how much wideband noise will be added to the receiver by the photonic LO. For ALMA, implementation of the photonic LO includes the transmission of two wavelengths of light by optical fiber over distances as great as 25 km. There is an optical fiber nonlinearity called stimulated Brillouin scattering (SBS), which limits the amount of light that can be transmitted by fiber over long distances. As a result, in some cases, an optical amplifier may be required at the antenna end of the fiber.
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#319.
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Last modified: August 9, 2000
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