Method and apparatus for bias and alignment control in an optical signal transmitter

An apparatus for transmitting an optical signal, said apparatus comprising:

• a modulator for modulating data on an optical signal with a modulation format having a periodic intensity modulation and a differential phase shift keying (DPSK) data modulation, said modulator comprising a Mach-Zehnder RZ modulator for imparting said periodic intensity modulation in response to a RZ bias signal, and a Mach-Zehnder NRZ modulator for imparting said DPSK data modulation in response to a NRZ bias signal;
• a phase shifter for selectively aligning said periodic intensity modulation and said data modulation in response to a phase bias signal;
• a low speed photo-detector, said low speed photo-detector having a bandwidth of at least one order of magnitude less than a data rate of said data and being configured to detect light scattered at an output of said NRZ modulator and provide an output representative of power in said light scattered at said output of said NRZ modulator;
• and
• a controller for providing said RZ bias signal, said NRZ bias signal, and said phase bias signal in response to said output of said low speed photodetector, said RZ bias signal comprising a low frequency RZ bias dither, said NRZ bias signal comprising a low frequency NRZ bias dither, and said phase bias signal comprising a low frequency phase bias dither, each of said RZ bias dither, said NRZ bias dither, and said phase bias dither having an associated frequency at least one order of magnitude less than said data rate,
• said controller being configured to provide said RZ bias signal in response to a RZ bias error value obtained by mixing a signal representative of said output of said photodetector with a signal representative of said RZ bias dither,
• said controller being configured to provide said NRZ bias signal in response to a NRZ bias error value obtained by mixing said signal representative of said output of said photodetector with a signal representative of said NRZ bias dither, and
• said controller being configured to provide said phase bias signal in response to a phase bias error value obtained by mixing said signal representative of said output of said photodetector with a signal representative of said phase bias dither.