Wavelength locker
An apparatus and method for calculating the frequency of the light.
1. A method comprising:
measuring, by a wavelength locker, a phase in a period of a wavelength filter of a portion of light to produce a measured phase, wherein measuring the phase of the portion of light using the wavelength filter includes:
inputting the portion of the light into a length imbalanced Mach-Zehnder interferometer with a 90 degree optical hybrid coupled to a first photodetector and a second photodetector;
transmitting, from the 90 degree optical hybrid, an in-phase (I) signal to the first photodetector and a quadrature (Q) signal to the second photodetector;
receiving, from the first photodetector the I signal and receiving from the second photodetector the Q signal in response to the light; and
calculating a wavelength of the light based on the I and Q signal to produce a calculated wavelength;
comparing the calculated wavelength of light to a desired wavelength; and
changing the wavelength of the light to match the desired wavelength based on the calculated wavelength not matching the desired wavelength.
2. The method of claim 1 , wherein the 90 degree optical hybrid is connected to a third and fourth photodetector, wherein the third photodetector is complementary to the first photodetector to form a first differential pair, wherein the fourth photodetector is complementary to the second photodetector to form a second differential pair, and wherein signals from either a photodetector of the first, second, third, and fourth photodetectors or a differential pair of the first and second differential pair are treated as the I or Q signal.
3. The method of claim 1 , further comprising calculating a frequency of the light using the measured phase.
4. The method of claim 1 , splitting a portion of light produced by a laser to a periodic wavelength filter.
5. The method of claim 1 , wherein changing the wavelength comprises changing a cavity length of a laser.
6. The method of claim 1 , wherein changing the wavelength comprises changing a temperature of a heater near a laser.
7. A system comprising:
a length-imbalanced Mach-Zehnder interferometer;
a 90-degree optical hybrid coupled to the Mach-Zehnder interferometer;
a first photodetector and a second photodetector, wherein each of the first and second photodetectors is coupled to at least one output of the 90-degree optical hybrid, wherein the first photodetector outputs an in-phase (I) signal and the second photodetector outputs a quadrature (Q) signal;
a processor coupled to the first and second photodetectors, the processor is to:
determine a wavelength of light based on the I and Q signal to produce a determined wavelength of light;
compare the determined wavelength of light to a desired wavelength of light; and
change the wavelength of the light to match the desired wavelength of light based on the determined wavelength not matching the desired wavelength.
8. The system of claim 7 , wherein the 90-degree optical hybrid is connected to a third and fourth photodetector, wherein the third photodetector is complementary to the first photodetector to form a first differential pair, wherein the fourth photodetector is complementary to the second photodetector to form a second differential pair, and wherein signals from either a photodetector of the first, second, third, and fourth photodetectors or a differential pair of the first and second differential pair are treated as the I or Q signal.
9. The system of claim 7 , further comprising a laser, wherein changing the wavelength comprises changing a cavity length of the laser.
10. The system of claim 7 , further comprising a laser and a heater near the laser, wherein changing the wavelength comprises changing a temperature of the heater.
11. The system of claim 7 , further comprising a laser, wherein changing the wavelength of light tunes the laser.
12. The system of claim 7 , wherein determining the wavelength of light is based on a determined phase of the light.
13. The system of claim 12 , wherein the determined phase of light is calculated by
f
=
c
0
n
(
f
)
Δ
L
(
m
+
1
2
π
tan
-
1
Q
I
)
,
where c 0 is speed of light in a vacuum, n(f) is refractive index in the Mach-Zehnder interferometer as a function of frequency f, ΔL is a path-length difference in the Mach-Zehnder interferometer, and m is an integer.
14. The system of claim 13 , wherein n(f) and ΔL are calibrated and held at a set temperature by a thermoelectric cooler that holds a laser temperature constant.
15. A system comprising:
a laser;
a length-imbalanced Mach-Zehnder interferometer;
a 90-degree optical hybrid coupled to the Mach-Zehnder interferometer;
a first photodetector and a second photodetector, wherein each of the first and second photodetectors is coupled to two outputs of the 90-degree optical hybrid, wherein a first signal is sent to the first photodetector from one of the two outputs, and wherein a second signal is sent to the second photodetector from one of the two outputs;
an analog to digital converter coupled to each of the first and second photodetectors; and
a processor coupled to the analog to digital converters, wherein the analog to digital converters provide an input to the process, the processor is to:
determine a wavelength of light to produce a determined wavelength of light;
compare the determined wavelength of light to a desired wavelength of light; and
change the determined wavelength of the light to match the desired wavelength of light based on the wavelength not matching the desired wavelength.
16. The system of claim 15 , wherein the 90-degree optical hybrid is connected to a third and fourth photodetector, wherein the third photodetector is complementary to the first photodetector to form a first differential pair, wherein the fourth photodetector is complementary to the second photodetector to form a second differential pair, and wherein signals from either a photodetector of the first, second, third, and fourth photodetectors or a differential pair of the first and second differential pair are treated as an in-phase (I) or a quadrature (Q) signal.
17. The system of claim 15 , wherein changing the wavelength comprises changing a cavity length of the laser.
18. The system of claim 15 , further comprising a heater near the laser, wherein changing the wavelength comprises changing a temperature of the heater.
19. The system of claim 15 , wherein determining the wavelength of light is based on a determined phase of the light.
20. The system of claim 16 , wherein determining the wavelength of light is based on a determined phase of the light, wherein the determined phase of light is calculated by
f
=
c
0
n
(
f
)
Δ
L
(
m
+
1
2
π
tan
-
1
Q
I
)
,
where c 0 is speed of light in a vacuum, n(f) is refractive index in the Mach-Zehnder interferometer as a function of frequency f, ΔL is a path-length difference in the Mach-Zehnder interferometer, and m is an integer.