Wavelength-stabilized semiconductor laser source

1. A semiconductor laser source comprising:

(a) a semiconductor substrate;

(b) an optical waveguide formed on the substrate and arranged so as to provide position-dependent optical gain or loss, for an optical signal that propagates along the waveguide within an operating wavelength range of the laser source, that varies according to a position-dependent level of electrical current density flowing into or out of the optical waveguide;

(c) an optical reflector arranged on the substrate or waveguide so as to reflect, to propagate along the optical waveguide in a forward direction, at least a portion of an optical signal propagating along the waveguide in a rearward direction within the operating wavelength range;

(d) an optical grating arranged on the substrate or waveguide so as to diffract, to propagate in the rearward direction along the waveguide toward the reflector, at least a portion of an optical signal propagating along the waveguide in the forward direction within the operating wavelength range, wherein the waveguide, reflector, and grating define a laser resonator arranged so that laser output from the laser resonator propagates (i) in the forward direction from the grating along the waveguide, (ii) in the rearward direction from the optical reflector, or (iii) in both directions, the laser output being characterized by a laser wavelength;

(e) a first laser electrode positioned over a first segment, of length L 1 , of the waveguide between the reflector and the grating, the first laser electrode being arranged so as to enable a substantially constant first laser current I 1 to flow through the first laser electrode into the first segment of the waveguide and produce optical gain therein;

(f) a second laser electrode positioned over a second segment, of length L 2 , of the waveguide that includes at least a portion of the grating, the second laser electrode being arranged so as to enable a substantially constant second laser current I 2 to flow through the second laser electrode into the second segment of the waveguide and produce optical gain therein;

(g) an optical detector and a wavelength discriminator, with the wavelength discriminator being arranged so as to direct a wavelength-dependent fraction of the laser output onto the optical detector so as to generate therefrom a laser-wavelength electrical signal; and

(h) a laser-control electrical circuit coupling the optical detector and one or both of the first or second laser electrodes in a feedback arrangement operative to control and stabilize the laser wavelength relative to a wavelength-dependent property of the wavelength discriminator, wherein the laser-control electrical circuit is arranged so as to (i) derive from the laser-wavelength electrical signal a wavelength-control current I WC , (ii) direct the first laser current I 1 to flow through the first laser electrode, (iii) direct the second laser current I 2 to flow through the second laser electrode, and (iv) direct the wavelength-control current I WC to flow through one or both of the first or second laser electrodes.

2. The laser source of claim 1 wherein no portion of the optical grating lies below the first laser electrode.

3. The laser source of claim 2 wherein the optical grating lies entirely below the second laser electrode.

4. The laser source of claim 1 wherein the laser-control electrical circuit is arranged so as to direct the wavelength-control current I WC to flow through one or both of the first or second laser electrodes in accordance with only one of: (i) a total current I 1 +I WC flows into the first segment of the waveguide and a total current I 2 flows into the second segment of the waveguide; (ii) a total current I 1 +I WC flows into the first segment of the waveguide and a total current I 2 −I WC flows into the second segment of the waveguide; (iii) a total current I 1 flows into the first segment of the waveguide and a total current I 2 +I WC flows into the second segment of the waveguide; and (iv) a total current I 1 −I WC flows into the first segment of the waveguide and a total current I 2 +I WC flows into the second segment of the waveguide.

5. The laser source of claim 1 wherein either: (i) a current density I 1 /L 1 is less than a current density I 2 /L 2 , so that the current density I 1 /L 1 results in the optical gain of the first segment of the waveguide being about zero or varying substantially linearly with respect to the current density I 1 /L 1 and so that the current density I 2 /L 2 results in the optical gain of the second segment of the waveguide being at least partly saturated with respect to the current density I 2 /L 2 ; or (ii) a current density I 1 /L 1 is greater than a current density I 2 /L 2 , so that the current density I 1 /L 1 results in the optical gain of the first segment of the waveguide being at least partly saturated with respect to the current density I 1 /L 1 and so that the current density I 2 /L 2 results in the optical gain of the second segment of the waveguide being about zero or varying substantially linearly with respect to the current density I 2 /L 2 .

6. The laser source of claim 1 wherein the laser resonator, the laser-control electrical circuit, and the first and second laser electrodes are arranged and connected so that operation of the feedback arrangement results in reduced phase noise of the laser output, relative to operation of the laser source without operation of the feedback arrangement, at offset frequencies exceeding about 1.0×10 6 Hz.

7. The laser source of claim 1 wherein the laser-control circuit includes an electrical low-pass filter, and the electrical low-pass filter is arranged so as to attenuate generation of the wavelength-control current I WC at wavelength-modulation frequencies above about 1.0×10 7 Hz.

8. The laser source of claim 1 wherein one or both of the wavelength discriminator or the optical detector are integrated onto the semiconductor substrate.

9. The laser source of claim 1 wherein the wavelength discriminator exhibits a wavelength-dependent reflection or transmission spectrum, and includes one or more of an optical low-pass filter, an optical high-pass filter, and optical bandpass filter, an optical notch-filter, a Fabry-Perot resonator, an optical interferometer, an optical grating, a fiber grating, an atomic or molecular absorption or emission line, or a spectroscopic feature.

10. The laser source of claim 1 wherein one or both of the wavelength discriminator or the laser-control circuit are arranged so as to sweep the laser wavelength.

11. The laser source of claim 10 wherein one or both of the wavelength discriminator or the laser-control circuit are arranged so as to sweep the laser wavelength over a wavelength range equivalent to an optical frequency range greater than about 1.0 GHz.

12. The laser source of claim 10 wherein one or both of the wavelength discriminator or the laser-control circuit are arranged so as to sweep the laser wavelength at a wavelength sweep rate equivalent to a laser optical frequency sweep rate greater than about than about 0.10 GHz/μs.

13. The laser source of claim 10 wherein the sweep of the laser wavelength results in sweep of the laser optical frequency that is substantially linear with respect to time.

14. The laser source of claim 10 wherein wavelength-dependent properties of the wavelength discriminator are substantially fixed.

15. The laser source of claim 14 wherein the wavelength discriminator exhibits a substantially fixed wavelength-dependent reflection or transmission spectrum, and includes one or more of an optical low-pass filter, an optical high-pass filter, and optical bandpass filter, an optical notch-filter, a Fabry-Perot resonator, an optical interferometer, an optical grating, a fiber grating, an atomic or molecular absorption or emission line, or a spectroscopic feature.

16. The laser source of claim 10 wherein the wavelength discriminator is arranged so as to exhibit one or more variable wavelength-dependent properties, and the wavelength discriminator and the laser-control circuit are arranged so as to sweep the laser wavelength according to sweeping of one or more of the one or more variable wavelength-dependent properties of the wavelength discriminator.

17. The laser source of claim 16 wherein the wavelength discriminator exhibits a variable wavelength-dependent reflection or transmission spectrum, and includes one or more of an optical low-pass filter, an optical high-pass filter, an optical bandpass filter, an optical notch-filter, a Fabry-Perot resonator, an optical interferometer, an optical grating, a fiber grating, an atomic or molecular absorption or emission line, or a spectroscopic feature.

18. The laser source of claim 16 wherein the wavelength discriminator includes a tunable Fabry-Perot resonator, and the laser-control circuit is arranged so as to sweep the laser wavelength according to sweeping a reflection or transmission spectrum of the tunable Fabry-Perot resonator.

19. The laser source of claim 18 wherein the tunable Fabry-Perot resonator includes a Fabry-Perot waveguide segment incorporating one or more electro-optic materials in, on, or near the Fabry-Perot waveguide segment and arranged so as to alter resonant optical frequencies of the Fabry-Perot resonator in response to a wavelength-sweep control signal applied to the Fabry-Perot waveguide segment that alters a refractive index of the electro-optic material.

20. The laser source of claim 18 wherein the tunable Fabry-Perot resonator includes a Fabry-Perot waveguide segment incorporating one or more semiconductor materials in, on, or near the Fabry-Perot waveguide segment and arranged so as to alter resonant optical frequencies of the Fabry-Perot resonator in response to a wavelength-sweep control signal applied to the Fabry-Perot waveguide segment that alters an electric current density flowing through the Fabry-Perot waveguide segment.

21. The laser source of claim 1 further comprising:

(i) an optical modulator arranged to modulate transmitted power of the laser output;

(j) a second optical detector arranged so as to receive a fraction of the transmitted power so as to generate therefrom a laser-power electrical signal; and

(k) a power-control electrical circuit coupling the second optical detector and the optical modulator so as to (i) generate from the laser-power electrical signal a power-control electrical signal, (ii) apply the power-control electrical signal to the optical modulator and thereby alter the transmitted power, and (iii) operate as a laser-amplitude-noise-reduction feedback mechanism.

22. The laser source of claim 21 wherein:

(i) the optical modulator comprises a modulator optical waveguide formed on a modulator semiconductor substrate and a modulator electrode positioned over at least a portion of the modulator waveguide;

(ii) the modulator waveguide is arranged so as to receive at least a portion of the laser output to propagate along the modulator waveguide; and

(iii) the power-control electrical circuit applies the laser-power electrical signal to the optical modulator by deriving from the laser-power electrical signal a power-modulation current, and directing the power-modulation current through the modulator electrode into or out of the modulator waveguide so as to alter optical gain or loss of the laser output propagating along the modulator waveguide.

23. The laser source of claim 22 wherein the laser and modulator substrates form a single, common substrate, and the laser and modulator waveguides form a single, common waveguide.

24. The laser source of claim 21 wherein the laser-control electrical circuit includes the power-control electrical circuit.

25. A method for operating the laser source of claim 1 , the method comprising:

(A) applying the first laser current I 1 to the first segment of the waveguide through the first laser electrode and applying the second laser current I 2 to the second segment of the waveguide through the second laser electrode, thereby producing the laser output to propagate along the waveguide; and

(B) operating the laser-control electrical circuit so as to direct the wavelength-control current I WC to flow through one or both of the first or second laser electrodes and thereby control and stabilize the laser wavelength.

26. A method for operating the laser source of claim 10 , the method comprising:

(A) applying the first laser current I 1 to the first segment of the waveguide through the first laser electrode and applying the second laser current I 2 to the second segment of the waveguide through the second laser electrode, thereby producing the laser output to propagate along the waveguide; and

(B) operating the wavelength discriminator or the laser-control electrical circuit so as to direct the wavelength-control current I WC to flow through one or both of the first or second laser electrodes and thereby control, stabilize, and sweep the laser wavelength.

27. A method for operating the laser source of claim 16 , the method comprising:

(A) applying the first laser current I 1 to the first segment of the waveguide through the first laser electrode and applying the second laser current I 2 to the second segment of the waveguide through the second laser electrode, thereby producing the laser output to propagate along the waveguide; and

(B) sweeping one or more of the one or more variable wavelength-dependent properties of the wavelength discriminator and operating the laser-control electrical circuit so as to direct the wavelength-control current I WC to flow through one or both of the first or second laser electrodes and thereby control, stabilize, and sweep the laser wavelength.

28. A method for operating the laser source of claim 21 , the method comprising:

(A) applying the first laser current I 1 to the first segment of the waveguide through the first laser electrode and applying the second laser current I 2 to the second segment of the waveguide through the second laser electrode, thereby producing the laser output to propagate along the waveguide;

(B) operating the laser-control electrical circuit so as to direct the wavelength-control current I WC to flow through one or both of the first or second laser electrodes and thereby control and stabilize the laser wavelength; and

(C) operating the power-control electrical circuit so as to reduce amplitude noise of the laser output.

29. A semiconductor laser source comprising:

(a) a semiconductor substrate;

(b) an optical waveguide formed on the substrate and arranged so as to provide position-dependent optical gain or loss, for an optical signal that propagates along the waveguide within an operating wavelength range of the laser source, that varies according to a position-dependent level of electrical current density flowing into or out of the optical waveguide;

(c) one or more optical reflectors or one or more optical gratings arranged on the substrate or waveguide so as to define along the waveguide a laser resonator arranged so that laser output from the laser resonator propagates along the waveguide away from the laser resonator in a forward direction, in a rearward direction, or in both directions, the laser output being characterized by a laser wavelength;

(d) one or more laser electrodes positioned over one or more corresponding segments of the laser resonator, each laser electrode being arranged so as to enable a corresponding laser current or wavelength control current to flow therethrough and into or out of the corresponding segment of the laser resonator;

(e) an optical detector and a wavelength discriminator, with the wavelength discriminator being arranged so as to direct a wavelength-dependent fraction of the laser output onto the optical detector so as to generate therefrom a laser-wavelength electrical signal; and

(f) a laser-control electrical circuit coupling the optical detector and one or more of the one or more laser electrodes in a feedback arrangement operative to control and stabilize the laser wavelength relative to a wavelength-dependent property of the wavelength discriminator,

wherein:

(g) the laser-control electrical circuit is arranged so as to (i) derive from the laser-wavelength electrical signal a wavelength-control current I WC , (ii) direct a corresponding substantially constant laser current to flow through each laser electrode, and (iii) direct the wavelength-control current I WC to flow through one or more of the one or more laser electrodes;

(h) the wavelength discriminator and the laser-control circuit are arranged so as to sweep the laser wavelength; and

(i) the wavelength discriminator is arranged so as to exhibit one or more variable wavelength-dependent properties; and

(j) the wavelength discriminator and the laser-control circuit are arranged so as to sweep the laser wavelength according to sweeping of one or more of the one or more variable wavelength-dependent properties of the wavelength discriminator.

30. The laser source of claim 29 wherein the wavelength discriminator exhibits a variable wavelength-dependent reflection or transmission spectrum, and includes one or more of an optical low-pass filter, an optical high-pass filter, an optical bandpass filter, an optical notch-filter, a Fabry-Perot resonator, an optical interferometer, an optical grating, a fiber grating, an atomic or molecular absorption or emission line, or a spectroscopic feature.

31. The laser source of claim 29 wherein the wavelength discriminator includes a tunable Fabry-Perot resonator; and the laser-control circuit is arranged so as to sweep the laser wavelength according to sweeping a reflection or transmission spectrum of the tunable Fabry-Perot resonator.

32. The laser source of claim 31 wherein either:

(i) the tunable Fabry-Perot resonator includes a Fabry-Perot waveguide segment incorporating one or more electro-optic materials in, on, or near the Fabry-Perot waveguide segment and arranged so as to alter resonant optical frequencies of the Fabry-Perot resonator in response to a wavelength-sweep control signal applied to the Fabry-Perot waveguide segment that alters a refractive index of the electro-optic material; or

(ii) the tunable Fabry-Perot resonator includes a Fabry-Perot waveguide segment incorporating one or more semiconductor materials in, on, or near the Fabry-Perot waveguide segment and arranged so as to alter resonant optical frequencies of the Fabry-Perot resonator in response to a wavelength-sweep control signal applied to the Fabry-Perot waveguide segment that alters an electric current density flowing through the Fabry-Perot waveguide segment.

33. The laser source of claim 29 wherein the sweep of the laser wavelength results in sweep of the laser optical frequency that is substantially linear with respect to time.

34. A method for operating the laser source of claim 29 , the method comprising:

(A) applying the one or more laser currents to the corresponding one or more segments of the laser resonator through the one or more corresponding laser electrodes, thereby producing the laser output to propagate along the waveguide; and

(B) sweeping one or more of the one or more variable wavelength-dependent properties of the wavelength discriminator and operating the laser-control electrical circuit so as to direct the wavelength-control current I WC to flow through one or both of the first or second laser electrodes and thereby control, stabilize, and sweep the laser wavelength.