Coupling devices and methods, wavelength locking systems and methods, and phase unwrapping systems and methods
Configurations for an optical device used for light splitting and wavelength locking are disclosed. The optical device may be a two by three coupler with a first waveguide coupled to a second waveguide, and a third waveguide coupled to the second waveguide. The first and third waveguides may receive input light and optically couple light to the second waveguide. The output signals of the first, second, and third waveguides may have a constant phase difference from one another over a broadband wavelength range, which may allow for phase unwrapping. By phase unwrapping the output signals over an FSR and performing further phase unwrapping over the broadband wavelength range, a continuous signal may be produced and used to sequentially lock each wavelength of light emitted by light sources over the broadband wavelength range.
1. An optical device, comprising:
a light source configured to generate light;
a splitter for receiving and splitting the light received from the light source into a first light along a first light path and a second light along a second path;
a phase shifter positioned to receive the first split light along the first light path and to phase shift the first split light relative to the second split light;
a two by three coupler comprising:
a first waveguide configured to:
receive the first split light along the first light path; and
output a first output signal with a first wavelength response and a first phase shift;
a second waveguide optically coupled to the first waveguide and configured to output a second output signal with a second wavelength response and a second phase shift; and
a third waveguide optically coupled to the second waveguide and configured to:
receive the second split light along the second light path; and
output a third output signal with a third wavelength response and a third phase shift; and
a set of photodetectors positioned to receive the first output signal, the second output signal, and the third output signal, wherein:
the first phase shift and the second phase shift are offset by a first phase difference;
the second phase shift and the third phase shift are offset by a second phase difference;
the first phase shift and the third phase shift are offset by a third phase difference;
the first phase difference, the second phase difference, and the third phase difference are constant
the first waveguide, the second waveguide, and the third waveguide have constant widths in a coupling region; and
the second waveguide is wider than the first waveguide and the third waveguide in the coupling region.
2. The optical device of claim 1 , further comprising a phase shifter operable to phase shift the second light; wherein:
the phase shift of the second light controls wavelength locking efficiency of the first, the second, and the third output signals; and
the first phase shift, the second phase shift, and the third phase shift are constant over a wavelength range of approximately one micron.
3. The optical device of claim 1 , wherein the first waveguide and third waveguide are symmetric about the second waveguide.
4. An optical system for monitoring a wavelength of a light source, comprising:
the light source configured to generate light;
a splitter for receiving and splitting the light received from the light source into a first split light along a first light path and a second split light along a second path;
a phase shifter positioned to receive the first split light along the first light path and to phase shift the first split light relative to the second split light;
a two by three coupler comprising a first waveguide, a second waveguide, and a third waveguide, and configured to:
receive the first split light from the phase shifter at the first waveguide along the first light path;
receive the second split light from the splitter at the third waveguide along the second light path; and
output a first output signal from the first waveguide, a second output signal from the second waveguide, and a third output signal from the third waveguide, each of which has a respective intensity based on a respective interference between the first split light and the second split light;
a set of photodetectors positioned to receive the first output signal, the second output signal, and the third output signal; and
a controller configured to monitor a wavelength of the light received by the splitter using intensities of the first output signal, the second output signal, and the third output signal, wherein:
the first waveguide, the second waveguide, and the third waveguide have constant widths in a coupling region; and
the second waveguide is wider than the first waveguide and the third waveguide in the coupling region.
5. The optical system of claim 4 , wherein:
the controller comprises a set of photodetectors;
the set of photodetectors converts the first output signal, the second output signal, and the third output signal to a first digital output signal, a second digital output signal, and a third digital output signal;
the controller compares the first digital output signal, the second digital output signal, and the third digital output signal to a first target digital value, a second target digital values, and a third target digital value; and
the controller transmits a feedback signal to the light source to control the first output signal, the second output signal, and the third output signal at the first target digital value, the second target digital value, and the third target digital value.
6. The optical system of claim 4 , further comprising:
a phase extract block that receives the first output signal, the second output signal, and the third output signal and extracts an unwrapped phase signal;
a differentiator that receives the unwrapped phase signal and is configured to:
detect zero points in the unwrapped phase signal; and
generate a differentiated signal indicative of the detected zero points;
a set of comparators configured to adjust the zero points; and
an integrator configured to generate integrated signals for use in producing a continuous signal for wavelength locking;
a set of photodetectors operable to:
receive the first output signal, the second output signal, and the third output signal; and
transmit a corresponding sinusoidal signal for each of the first output signal, the second output signal, and the third output signal to the phase extract block;
a first summer for summing the first output signal, the second output signal, and the third output signal from the set of comparators; and
a second summer for summing an integrated signal with the differentiated signal, thereby generating a summed signal used to determine an amount of phase shift per wavelength used to lock a measured wavelength of light to a corresponding target wavelength of light.
7. The optical system of claim 6 , wherein the integrated signal is a correction value used to generate a one to one relationship between a particular wavelength of the first and second split light and the summed signal.
8. The optical system of claim 6 , wherein:
the set of comparators comprises:
a negative jump comparator; and
a positive jump comparator;
the negative jump comparator adds two π to the differentiated signal;
the positive jump comparator subtracts two π to the differentiated signal; and
the optical system further comprises a second summer configured to sum the integrated signals with the differentiated signal, thereby generating a summed signal containing information used to sequentially lock each measured wavelength of light across a wavelength range of approximately one micron.
9. The optical system of claim 4 , wherein:
the first output signal has a first sinusoidal wavelength response with a first phase shift;
the second output signal has a second sinusoidal wavelength response with a second phase shift;
the third output signal has a third sinusoidal wavelength response with a third phase shift; and
the first phase shift, the second phase shift, and the third phase shift are constant over a wavelength range of approximately one micron.
10. The optical system of claim 4 , wherein the first output signal, the second output signal, and the third output signal are offset from one another by a same phase difference.
11. The optical system of claim 4 , wherein a phase shift of the first output signal, the second output signal, and the third output signal is constant over a wavelength range of approximately one micron.