TABLE OF CONTENTS The laser was invented in 1960 [1], and since that time, different modulation techniques have been used to place information on the optical signal. Early optical transport layers used direct detection [2] ? [3] in which the optical receiver simply determined whether a pulse was present. As technology has developed, modulation techniques have developed that allow much more of the optical fiber's 20 THz capacity to be used. Wavelength Division Multiplexing (WDM) [4] is one such technique that multiplexes multiple optical signals onto a single fiber.
Other techniques are possible such as heterodyne or homodyne detection of BPSK modulated signals [5]. In most of these advanced optical modulations, the phase noise of the laser's signal is critical and can limit capacity [6]. Many of the high-capacity systems now use optical phase-locked loops to control the line widths of the modulated (or unmodulated) laser lights.
In general, the optical phase-locked loop allows
More optical signals on a single fiber (dense wavelength division multiplexing).
Longer distances between repeaters in a long-haul cable.
Higher data rates.
A different application of optical phase-locked loops is in optical beam forming networks for microwave antennas [20]. Precision amplitude and phase-controlled signals are applied to each individual element of a phase-array antenna to synthesize the desired antenna pattern. As the number of elements increases, it becomes physically impossible to provide coaxial cables to every element in the antenna. Coherent optical signals can be transmitted...
