High-speed optical modulation using a quantum-well modulator
View Patent ↗An optical transmitter having a quantum-well (QW) modulator and a method of operating the same. The QW modulator is configurable to perform both amplitude and phase modulation. Using the disclosed methods, the length of the QW modulator, one or more drive voltages, and/or an operating wavelength can be selected to enable the optical transmitter to generate a modulated optical signal having a relatively high bit rate, e.g., an optical duobinary signal having a bit rate greater than about 80 Gb/s.
1. A method of operating an optical transmitter, comprising:
(A) generating different first and second drive voltages for a first quantum-well (QW) modulator to produce an optical signal modulated with data, wherein:
the optical transmitter comprises the first QW modulator;
the first drive voltage corresponds to a first binary level; and
the second drive voltage corresponds to a second binary level; and
(B) generating an input optical signal having an operating wavelength for the optical transmitter such that, for the first QW modulator, a ratio δ of a peak-to-peak change in electro-refraction to a peak-to-peak change in electro-absorption corresponding to the first and second drive voltages is greater than zero but smaller than about two.
2. The method of claim 1 , further comprising receiving instructions specifying a new operating wavelength, wherein:
step (B) comprises generating the optical input signal having the new operating wavelength; and
step (A) comprises:
(A1) limiting each of the first and second drive voltages to a specified voltage range; and
(A2) selecting the first and second drive voltages from the specified voltage range based on a bandgap of the first QW modulator.
3. The method of claim 1 , wherein:
the data has a bit rate; and
for an optical wave having the operating wavelength, transit time through the QW modulator is smaller than about 20% of the inverse of the bit rate.
4. The method of claim 3 , wherein the bit rate is greater than about 50 Gb/s.
5. The method of claim 1 , wherein, for the first QW modulator, a peak-to-peak phase shift corresponding to the first and second drive voltages is smaller than 180 degrees.
6. The method of claim 5 , wherein the peak-to-peak phase shift is smaller than about 90 degrees.
7. The method of claim 1 , further comprising adjusting at least one of (i) the first and second drive voltages and (ii) the operating wavelength to change the ratio δ.
8. The method of claim 1 , wherein the optical transmitter comprises:
a Mach-Zehnder (MZ) interferometer having first and second interferometer arms, wherein:
the first QW modulator is located in the first interferometer arm;
a second QW modulator is located in the second interferometer arm; and
at least one of the interferometer arms comprises one or more phase shifters; and
a laser optically coupled to the MZ interferometer to apply thereto the optical input signal having the operating wavelength.
9. The method of claim 8 , further comprising:
applying a first drive signal having the first and second drive voltages to the first QW modulator, wherein the first drive signal is based on a first data signal; and
applying a second drive signal having the first and second drive voltages to the second QW modulator, wherein the second drive signal is based on a second data signal.
10. The method of claim 9 , further comprising inverting the first data signal to generate the second data signal.
11. The method of claim 9 , further comprising subjecting at least one of the first and second drive signals to low-pass filtering.
12. The method of claim 9 , further comprising subjecting an optical output signal produced by the MZ interferometer to optical bandpass filtering.
13. The method of claim 9 , wherein the optical transmitter generates an optical duobinary signal stream having the operating wavelength.
14. The method of claim 8 , further comprising configuring the one or more phase shifters to produce a phase difference from a range between about 165 and about 195 degrees between the first and second interferometer arms.
15. An apparatus, comprising:
a Mach-Zehnder (MZ) interferometer having first and second interferometer arms, wherein:
the first interferometer arm comprises a first quantum-well (QW) modulator; and
the second interferometer arm comprises a second QW modulator;
a first driver circuit electrically coupled to drive the first QW modulator;
a second driver circuit electrically coupled to drive the second QW modulator, wherein the MZ interferometer is configured to be optically coupled to a laser to receive therefrom an optical input signal having an operating wavelength; and
a controller configured to cause:
the first driver circuit to generate first and second drive voltages for the first QW modulator; and
the second driver circuit to generate the first and second drive voltages for the second QW modulator, wherein:
the first drive voltage corresponds to a first binary level; and
the second drive voltage corresponds to a second binary level; and
wherein the first and second QW modulators are configured to cause a ratio δ of a peak-to-peak change in electro-refraction to a peak-to-peak change in electro-absorption corresponding to the first and second drive voltages to be greater than zero but smaller than about two.
16. The apparatus of claim 15 , wherein the controller is configured to:
receive instructions specifying the operating wavelength; and
select the first and second drive voltages corresponding to the operating wavelength to cause the ratio δ to be greater than zero but smaller than about two.
17. The apparatus of claim 15 , wherein at least one of the first and second QW modulators comprises:
a waveguide having interleaved N QW layers and N+1 barrier layers, where N is a positive integer; and
an electrode in electrical contact with the waveguide and configured to receive a drive signal having the first and second drive voltages.
18. The apparatus of claim 15 , wherein:
the driver circuits are configured to drive the QW modulators based on a data stream; and
for an optical wave having the operating wavelength, transit time through at least one of the first and second QW modulators is smaller than about 20% of the inverse of a bit rate of said data stream.
19. The apparatus of claim 15 , wherein at least one of the first and second QW modulators has a total length smaller than about 500 μm.
20. The apparatus of claim 15 , wherein:
the first and second interferometer arms comprise one or more phase shifters; and
the controller is configured to cause the one or more phase shifters to produce a phase difference from a range between about 165 and about 195 degrees between the first and second interferometer arms.
21. The apparatus of claim 15 , comprising an optical transmitter, wherein the MZ interferometer, the first driver circuit, and the second driver circuit are parts of said optical transmitter.
22. The apparatus of claim 15 , further comprising the laser, wherein the controller is configured to cause the laser to generate the operating wavelength.
23. The apparatus of claim 15 , further comprising a set of one or more electrical filters configured to subject at least one of the first and second drive signals to low-pass filtering.
24. The apparatus of claim 15 , further comprising an optical filter configured to subject an optical output of the MZ interferometer to optical bandpass filtering to generate an optical duobinary signal having the operating wavelength.