Optical filter for use in a laser transmitter
View Patent ↗In an embodiment, an optical communication system includes an optical transmitter and an optical discriminator. The optical transmitter is configured to emit a frequency modulated signal having a bit rate frequency and a frequency excursion between 20% and 80% of the bit rate frequency. The optical discriminator is configured to convert the frequency modulated signal to a substantially amplitude modulated signal and includes a delay line interferometer (DLI). The DLI includes an input, an output, a first optical path coupling optical signals from the input to the output and a second optical path coupling optical signals from the input to the output. The first and second optical paths have different lengths.
1. An optical communication system comprising:
an optical transmitter configured to emit a frequency modulated signal having a bit rate frequency and a frequency excursion between 20% and 80% of the bit rate frequency; and
an optical discriminator configured to convert the frequency modulated signal to a substantially amplitude modulated signal, wherein the optical discriminator includes:
a delay line interferometer (DLI) including an input, an output, a first optical path coupling optical signals from the input to the output, and a second optical path coupling optical signals from the input to the output, the first and second optical paths having different lengths; and
a filter positioned in the second optical path, wherein:
a combined transmission function of the DLI and the filter is effective to cause at least one of a rise time or fall time of the substantially amplitude modulated signal to be less than a corresponding rise time or fall time of the frequency modulated signal;
the combined transmission function has alternating high and low peaks; and the low peaks are between 1.5 and 4 dB lower than the high peaks.
2. The optical communication system of claim 1 , wherein the filter comprises an etalon positioned in the second optical path and oriented to transmit a portion of light from the input to the output.
3. The optical communication system of claim 2 , wherein the etalon includes first and second reflective surfaces each having a reflectivity of less than 40%.
4. The optical communication system of claim 2 , wherein the etalon includes first and second reflective surfaces each having a reflectivity of less than or equal to about 30%.
5. An optical communication system comprising:
an optical transmitter configured to emit a frequency modulated signal having a bit rate frequency and a frequency excursion between 20% and 80% of the bit rate frequency; and
an optical discriminator configured to convert the frequency modulated signal to a substantially amplitude modulated signal, wherein the optical discriminator includes:
a delay line interferometer (DLI) including an input, an output, a first optical path coupling optical signals from the input to the output, and a second optical path coupling optical signals from the input to the output, the first and second optical paths having different lengths; and
a filter positioned in the second optical path to reflect a portion of light from the input to the output, wherein:
a combined transmission function of the DLI and the filter is effective to cause at least one of a rise time or fall time of the substantially amplitude modulated signal to be less than a corresponding rise time or fall time of the frequency modulated signal; and
the filter comprises an etalon having first and second reflective surfaces, the first reflective surface having a reflectivity of less than 20% and the second reflective surface having a reflectivity of less than 60%.
6. The optical communication system of claim 1 , wherein the frequency excursion is between 25% and 75% of the bit rate frequency.
7. An optical communication system comprising:
an optical transmitter configured to emit a frequency modulated signal having a bit rate frequency and a frequency excursion between 20% and 80% of the bit rate frequency; and
an optical discriminator configured to convert the frequency modulated signal to a substantially amplitude modulated signal, wherein the optical discriminator includes:
a delay line interferometer (DLI) including an input, an output, a first optical path coupling optical signals from the input to the output, and a second optical path coupling optical signals from the input to the output, the first and second optical paths having different lengths; and
a filter positioned in the second optical path to reflect a portion of light from the input to the output, wherein:
a combined transmission function of the DLI and the filter is effective to cause at least one of a rise time or fall time of the substantially amplitude modulated signal to be less than a corresponding rise time or fall time of the frequency modulated signal; and
the filter comprises an etalon having first and second reflective surfaces, the first reflective surface having a reflectivity of less than 50% and the second reflective surface having a reflectivity of greater than 90%.
8. The optical communication system of claim 7 , wherein the combined transmission function has alternating high bandwidth and low bandwidth peaks.
9. The optical communication system of claim 8 , wherein the high bandwidth peaks have a 3 dB bandwidth greater than one half of a free spectral range of the DLI.
10. The optical communication system of claim 8 , wherein the high bandwidth peaks have a 3 dB bandwidth greater than 60 percent of a free spectral range of the DLI.
11. The optical communication system of claim 7 , wherein the first reflective surface has a reflectivity of less than or equal to about 40% and the second reflective surface has a reflectivity of at least 99%.
12. The optical communication system of claim 5 , wherein the etalon comprises a first etalon, the optical discriminator further comprising a second etalon positioned to receive optical signals from the output of the DLI.
13. The optical communication system of claim 5 , wherein the first reflective surface has a reflectivity less than or equal to about 10% and the second reflective surface has a reflectivity of at least 50%.
14. An optical communication system comprising:
an optical transmitter configured to emit a frequency modulated signal having a bit rate frequency and a frequency excursion between 20% and 80% of the bit rate frequency; and
an optical discriminator configured to convert the frequency modulated signal to a substantially amplitude modulated signal, wherein the optical discriminator includes:
a delay line interferometer (DLI) including an input, an output, a first optical path coupling optical signals from the input to the output, and a second optical path coupling optical signals from the input to the output, the first and second optical paths having different lengths; and
an etalon positioned to receive optical signals from the output of the DLI, wherein:
a combined transmission function of the DLI and the etalon is effective to cause a rise time or fall time of the substantially amplitude modulated signal to be less than a corresponding rise time or fall time of the frequency modulated signal;
the combined transmission function has alternating high and low peaks; and
the high frequency portions of the frequency modulated signal transmitted through the optical discriminator are located on a positive slope portion of a high peak.
15. A method comprising:
modulating an optical signal source according to a data signal to produce a frequency modulated signal, the frequency modulated signal including high frequency portions encoding a first bit value and low frequency portions encoding a second bit value, the high and low frequency portions separated by a frequency excursion between 20% and 80% of a bit rate frequency of the frequency modulated signal; and
transmitting the frequency modulated signal through an optical discriminator to convert the frequency modulated signal to a substantially amplitude modulated signal, wherein the optical discriminator comprises:
a delay line interferometer (DLI) including an input, an output, a first optical path coupling optical signals from the input to the output, and a second optical path coupling optical signals from the input to the output, the first and second optical paths having different lengths, and
a filter positioned in the second optical path, wherein:
a combined transmission function of the DLI and the filter is effective to cause at least one of a rise time or fall time of the substantially amplitude modulated signal to be less than a corresponding rise time or fall time of the frequency modulated signal;
the combined transmission function has alternating high and low peaks; and the low peaks are between 1.5 and 4 dB lower than the high peaks.
16. The method of claim 15 , wherein the frequency excursion is less than one half of a free spectral range of the DLI.
17. The method of claim 16 , wherein the frequency excursion is less than one fourth of the free spectral range of the DLI.
18. The method of claim 15 , wherein the high frequency portions are located on a positive slope portion of a high peak and wherein the low frequency portions are located at a local minimum between the high peak and an adjacent low peak.
19. The method of claim 15 , wherein the filter comprises an etalon positioned in the second optical path and oriented to transmit a portion of light from the input to the output.
20. The method of claim 19 , wherein the etalon includes first and second reflective surfaces each having a reflectivity of less than 40%.
21. The method of claim 19 , wherein the etalon includes first and second reflective surfaces each having a reflectivity of less than or equal to about 30%.
22. A method comprising:
modulating an optical signal source according to a data signal to produce a frequency modulated signal, the frequency modulated signal including high frequency portions encoding a first bit value and low frequency portions encoding a second bit value, the high and low frequency portions separated by a frequency excursion between 20% and 80% of a bit rate frequency of the frequency modulated signal; and
transmitting the frequency modulated signal through an optical discriminator to convert the frequency modulated signal to a substantially amplitude modulated signal, wherein the optical discriminator comprises:
a delay line interferometer (DLI) including an input, an output, a first optical path coupling optical signals from the input to the output, and a second optical path coupling optical signals from the input to the output, the first and second optical paths having different lengths; and
a filter positioned in the second optical path, wherein:
a combined transmission function of the DLI and the filter is effective to cause at least one of a rise time or fall time of the substantially amplitude modulated signal to be less than a corresponding rise time or fall time of the frequency modulated signal; and
the filter comprises an etalon having first and second reflective surfaces, the first reflective surface having a reflectivity of less than 50% and the second reflective surface having a reflectivity of greater than 90%.
23. The method of claim 22 , wherein the combined transmission function has alternating high bandwidth and low bandwidth peaks.
24. The method of claim 23 , wherein the high bandwidth peaks have a 3 dB bandwidth equal to between about 0.5 and 2 times a bit rate of the data signal.
25. The method of claim 23 , wherein the high bandwidth peaks have a 3 dB bandwidth equal to between about 0.7 and 1.5 times a bit rate of the data signal.
26. The method of claim 23 , wherein the high frequency portions are located on a positive slope portion of a high bandwidth peak.
27. The method of claim 22 , wherein the first reflective surface has a reflectivity of less than or equal to about 40% and the second reflective surface has a reflectivity of at least 99%.
28. A method comprising:
modulating an optical signal source according to a data signal to produce a frequency modulated signal, the frequency modulated signal including high frequency portions encoding a first bit value and low frequency portions encoding a second bit value, the high and low frequency portions separated by a frequency excursion between 20% and 80% of a bit rate frequency of the frequency modulated signal; and
transmitting the frequency modulated signal through an optical discriminator to convert the frequency modulated signal to a substantially amplitude modulated signal, wherein the optical discriminator comprises:
a delay line interferometer (DLI) including an input, an output, a first optical path coupling optical signals from the input to the output, and a second optical path coupling optical signals from the input to the output, the first and second optical paths having different lengths; and
a filter positioned in the second optical path, wherein:
a combined transmission function of the DLI and the filter is effective to cause at least one of a rise time or fall time of the substantially amplitude modulated signal to be less than a corresponding rise time or fall time of the frequency modulated signal; and
the filter comprises an etalon having first and second reflective surfaces, the first reflective surface having a reflectivity of less than 20% and the second reflective surface having a reflectivity of less than 60%.
29. The method of claim 28 , wherein the first reflective surface has a reflectivity less than or equal to about 10% and the second reflective surface has a reflectivity of at least 50%.