Electronically-controlled optical feedback methods for laser linewidth

A coherent light source comprising:

• a coherent light emitter operable to emit a beam of coherent light;
• an optical feedback system comprising an integrated photonic circuit operative to redirect a fraction of the light of the beam of coherent light back into the coherent light emitter as an optical feedback signal;
• and,
• an electronic control operative to set an amplitude and/or a phase of the redirected fraction of the light;
• an optical filter connected to pass optical power in one or more sidebands of a spectrum of the coherent light to a photosensor wherein the electronic control is connected to receive an output signal from the photosensor that provides a measure of the optical power in the one or more sidebands of the spectrum of the coherent light.

A coherent light source comprising:

• a coherent light emitter operable to emit a beam of coherent light;
• an optical feedback system comprising an integrated photonic circuit operative to redirect a fraction of the light of the beam of coherent light back into the coherent light emitter as an optical feedback signal;
• and,
• an electronic control operative to set an amplitude and/or a phase of the redirected fraction of the light;
• a frequency/phase noise discriminator operable to measure optical phase fluctuations in the coherent light beam wherein the electronic control is configured to control the amplitude and/or the phase of the optical feedback signal to minimize the optical phase fluctuations.

A coherent light source comprising:

• a coherent light emitter operable to emit a beam of coherent light;
• an optical feedback system comprising an integrated photonic circuit operative to redirect a fraction of the light of the beam of coherent light back into the coherent light emitter as an optical feedback signal;
• and,
• an electronic control operative to set an amplitude and/or a phase of the redirected fraction of the light;
• wherein the coherent light emitter comprising a laser and the laser comprises a multi-wavelength laser operative to emit light of a plurality of different wavelengths, the optical feedback system is one of a plurality of optical feedback systems, each of the plurality of optical feedback systems configured to provide a corresponding optical feedback signal at a corresponding one of the plurality of wavelengths and the electronic control itself or in combination with one or more additional electronic controls is operative to set an amplitude and/or a phase of the optical feedback signals of each of the plurality of wavelengths.

A reflection endless phase shifter comprising:

• at least one input/output port,
• a first optical coupler connecting the at least one input/output port to first and second ports of a four port switch,
• a line phase shifter section having first and second ports connected by a second optical coupler to third and fourth ports of the switch,
• the line phase shifter section having third and fourth ports connected by a third optical coupler to first and second ports of a reflector
• wherein:
• the switch comprises a first arms extending from the first port of the switch to the third port of the switch and a second arm extending from the second port of the switch to the fourth port of the switch,
• the first and second arms respectively comprise first and second switch phase shifters;
• the switch has a first state in which the first and second phase shifters are controlled to provide a first relative phase of the first and second arms and a second state in which the first and second phase shifters are controlled to make provide a second relative phase of the first and second arms;
• the line phase shifter section comprises at least one line phase shifter, the line phase shifter controllable to set a phase shift at least in the range of 0 and 2π radians.