Comb laser
An optoelectronic device includes a reflective semiconductor optical amplifier (RSOA), which includes a gain medium to amplify laser radiation within a given gain band, a first reflector at a first end of the gain medium, and a waveguide coupled to convey the laser radiation into and out of a second end of the gain medium. An external laser cavity, disposed on an optical substrate, is optically coupled to the waveguide. The external laser cavity includes a second reflector, a comb filter, disposed between the second reflector and the RSOA and configured to pass a set of distinct wavelength sub-bands within the gain band, the set of distinct wavelength sub-bands defining a comb, and a bandpass filter between the second reflector and the RSOA in series with the comb filter, having a passband encompassing a subset of the wavelength sub-bands in the comb.
1 . An optoelectronic device, comprising:
a reflective semiconductor optical amplifier (RSOA), comprising:
a gain medium configured to amplify laser radiation within a given gain band;
a first reflector disposed at a first end of the gain medium; and
a waveguide coupled to convey the laser radiation into and out of a second end of the gain medium, opposite the first end;
an optical substrate; and
an external laser cavity, which is disposed on the optical substrate and is optically coupled to the waveguide, the external laser cavity comprising:
a second reflector;
a comb filter, disposed between the second reflector and the RSOA and configured to pass a set of distinct wavelength sub-bands within the gain band, the set of distinct wavelength sub-bands defining a comb; and
a bandpass filter, disposed between the second reflector and the RSOA in series with the comb filter, the bandpass filter having a passband encompassing a subset of the wavelength sub-bands in the comb.
2 . The device according to claim 1 , wherein the RSOA comprises a III-V semiconductor material, and wherein the external laser cavity comprises a silicon photonic integrated circuit (PIC).
3 . The device according to claim 1 , wherein the external laser cavity comprises a phase tuner disposed between the second reflector and the RSOA in series with the comb filter and the bandpass filter.
4 . The device according to claim 1 , wherein the comb filter comprises an optical ring resonator.
5 . The device according to claim 1 , wherein the bandpass filter comprises one or more optical ring resonators, each optical ring resonator among the one or more optical ring resonators configured to filter out a selected group of the wavelength sub-bands in the comb that are outside the passband.
6 . The device according to claim 5 , wherein the one or more optical ring resonators comprise multiple optical ring resonators having different, respective resonances, which are selected so that each of the optical ring resonators filters out a different, respective subset of the wavelength sub-bands that are outside the passband.
7 . The device according to claim 1 , wherein the passband of the bandpass filter is narrower than the gain band of the gain medium.
8 . A method for generating radiation, the method comprising:
providing a reflective semiconductor optical amplifier (RSOA), comprising a gain medium configured to amplify laser radiation within a given gain band and a first reflector disposed at a first end of the gain medium;
optically coupling the RSOA to an external laser cavity, which is disposed on an optical substrate and comprises a second reflector, a comb filter disposed between the second reflector and the RSOA and configured to pass a set of distinct wavelength sub-bands within the gain band, the set of distinct wavelength sub-bands defining a comb, and a bandpass filter, disposed between the second reflector and the RSOA in series with the comb filter, the bandpass filter having a passband encompassing a subset of the wavelength sub-bands in the comb; and
applying an electrical drive to the RSOA, whereby the laser radiation in subset of the wavelength sub-bands in the comb is emitted through the second reflector.
9 . The method according to claim 8 , wherein providing the RSOA comprises forming the RSOA on a III-V semiconductor substrate, and wherein optically coupling the RSOA comprises coupling the RSOA via a waveguide to a silicon photonic integrated circuit (PIC).
10 . The method according to claim 8 , and comprising coupling a phase tuner between the second reflector and the RSOA in series with the comb filter and the bandpass filter.
11 . The method according to claim 8 , wherein optically coupling the RSOA comprises coupling an optical ring resonator to serve as the comb filter.
12 . The method according to claim 8 , wherein optically coupling the RSOA comprises coupling one or more optical ring resonators to constitute the bandpass filter, each optical ring resonator among the one or more optical ring resonators configured to filter out a selected group of the wavelength sub-bands in the comb that are outside the passband.
13 . The method according to claim 12 , wherein coupling the one or more optical ring resonators comprises coupling together multiple optical ring resonators having different, respective resonances, which are selected so that each of the optical ring resonators filters out a different, respective subset of the wavelength sub-bands that are outside the passband.
14 . The method according to claim 8 , wherein optically coupling the RSOA comprises setting the passband of the bandpass filter to be narrower than the gain band of the gain medium.
15 . An integrated optical device, comprising:
an optical substrate; and
multiple optical ring resonators, which are disposed on the optical substrate and interconnected in series by waveguides, and which comprise:
a first optical ring resonator, configured to pass a set of distinct wavelength sub-bands, the set of distinct wavelength sub-bands defining a comb; and
one or more second optical ring resonators, each second optical ring resonator among the one or more second optical ring resonators configured to filter out a selected group of the wavelength sub-bands in the comb.
16 . The device according to claim 15 , wherein the one or more second optical ring resonators comprise a plurality of the second optical ring resonators having different, respective resonant wavelengths, which are selected so that each of the second optical ring resonators filters out a different, respective subset of the wavelength sub-bands in the comb by destructive interference at the respective resonant wavelengths.
17 . The device according to claim 16 , wherein the respective resonant wavelengths of the second optical ring resonators are selected so as to define a bandpass filter that passes a specified set of the wavelength sub-bands in the comb while attenuating the wavelengths that are outside the specified set by destructive interference in the second optical ring resonators.
18 . The device according to claim 16 , wherein the first optical ring resonator comprises a first resonant ring having a first length, which determines a spectral spacing of the wavelength sub-bands in the comb, and wherein each of the second optical ring resonators comprises a respective second resonant ring having a second length, which is an integer quotient of the first length.