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1. WO2002025782 - OPTICAL TRANSMITTER COMPRISING A STEPWISE TUNABLE LASER

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[ EN ]

What is claimed is:

1. An external cavity laser comprising:
a first mirror;
a homogeneously broadened gain region comprising multiple quantum wells, having a gain region optical axis, and having a length equal to a selected integer multiple of a selected design laser wavelength, where the first mirror faces and is adjacent to the gain region;
a second mirror, spaced apart from and facing the first mirror, with the gain region therebetween; and at least one of:
an intra-cavity frequency-selective optical element; and
a spacer configured to provide a selected mirror spacing between the first and second mirrors and to define an external cavity having a cavity length that provides at least first and second laser frequencies corresponding to a selected optical channel spacing.

2. The laser of claim 1, wherein said gain region is fabricated monolithically with said first mirror.

3. The laser of claim 1, wherein said first mirror has a reflectance of at least 90 percent.

4. The laser of any of claims 1-3, wherein said first mirror comprises a multilayer Bragg reflector.

5. The laser of any of claims 1-4, configured to receive optical energy from a laser pump, from a direction oblique to an output beam axis of said laser.

6. The laser of any of claims 1-5, further comprising a heat sink attached to said first mirror.

7. The laser of claim 6, further comprising a selected metal layer secured to said heat sink, wherein said first mirror has a reflectance of at least 99 percent and a combination of the metal layer and said first mirror have a reflectance of at least 99.5 percent.

8. The laser of any of claims 1-7, wherein said multiple quantum wells include at least one pair of quantum wells located at a peak amplitude location for a standing wave present in said gain region at said design wavelength.

9. The laser of any of claims 1-8, further comprising an anti-reflecting coating having a low reflectance at said design wavelength at a surface of said gain region facing said second mirror.

10. The laser of any of claims 1-9, wherein said second mirror is located by said spacer at a distance of between 0.5 mm and 10 mm from said gain region.

11. The laser of any of claims 1-10, further comprising an optical pump positioned to provide optical radiation for said gain region..

12. The laser of claim 11, wherein said optical pump is a laser diode pump and said optical pump radiation is received at an anti-reflective coating on an external surface of said gain region at an equivalent Brewster's angle, relative to said gain region axis.

13. The laser of any of claims 11 and 12, wherein said optical pump is integral with said spacer or is aligned with and secured in a side wall of said spacer.

14. The laser of claim 6, wherein at least one of said optical pump and said laser provides an optical beam to which said heat sink is substantially transparent.

15. The laser of any of claims 1-14, further comprising a thermoelectric cooler/heater, in thermal communication with said heat sink and configured to control an operating temperature of said gain region to facilitate single mode-to-single mode tuning of said laser.

16. The laser of any of claims 1-15, further comprising a piezoelectric element mounted on said heat sink, wherein said spacer and the piezoelectric element are connected so that said length of said external cavity can be change to thereby tune a wavelength of an optical beam emitted by said laser.

17. The laser of any of claims 1-16, wherein at least one of said first mirror, said gain region and said gain region anti-reflection coating is formed by at least one of molecular bean epitaxy and metal oxide chemical vapor deposition.

18. The laser of any of claims 1-17, wherein said laser operates in at least one of a single transverse mode and a longitudinal mode that corresponds to a selected frequency used in optical communication.

19. The laser of claim 18, wherein said gain region is pumped to transparency and said external cavity is a stable resonator having a finesse at least equal to 10 and configured to suppress power in any side mode to a level no greater than 10"4 of power in a selected central mode.

20. The ;laser of any of claims 1-19, wherein said gain region comprises at least one of AlJJij.y Ga^As^ and ImG.yAsj.yP^ , where x and y are selected fractions satisfying 0 < x < 1 and 0 < y < 1.

21. The laser of any of claims 1-20, wherein said gain region has a net gain that exceeds losses in said cavity over a frequency band whose bandwidth is less than a selected mode-to-mode spacing.

22. The laser of claim 21, wherein said laser is tunable from a first selected mode to a second selected mode and remains stable in the second mode.

23. The laser of any of claims 1-22, further comprising pump radiation absorption means, formed in or adjacent to said spacer, at an equivalent Brewster's angle for absorbing any of said pump radiation reflected from an external surface that receives said pump radiation.

24. The laser of any of claims 1-23, further comprising at least one of a thermoelectric cooler/heater and an intra-cavity etalon.

25. The laser of claim 24, wherein at least one of said thermoelectric cooler/heater and said intra-cavity etalon is controllable using a digital controller.

26. The laser of any of claims 1-25, further comprising a first reflective surface facing said gain region along said first optical axis, and a curved second reflective surface facing the first reflective surface along a second optical axis.

27. The laser of claim 26, wherein said first reflective surface comprises a mode-selective intra-cavity etalon.

28. The laser of any of claims 1-27, wherein said intra-cavity frequency-selective element comprises an electronically actuated, frequency-selective optical element within said cavity that induces optical loss for all but a selected potential mode of oscillation, that suppresses laser action on each loss-induced mode, and that switches from a first potential mode of oscillation to a second potential mode of oscillation.

29. The laser of any of claims 1-27, wherein said intra-cavity frequency-selective element comprises an electronically actuated, frequency-selective optical element within said cavity that reduces round trip optical gain for all but a selected potential mode of oscillation, that suppresses laser action on each mode for which round trip gain is reduced, and that switches from a first potential mode of oscillation to a second potential mode of oscillation in a time interval of length less than one msec.

30. The laser of any of claims l-29,wherein said first mirror and said gain region forma an epitaxially-grown, monolithic semiconductor structure, said gain region further comprises a plurality of active layers, spaced apart by spacer layers, arranged to provide optical gain in said cavity, and positioned adjacent to said first mirror, wherein at least two of the active layers have different indices of refraction, selected to provide optical radiation reflectivity at least equal to 95 percent within a selected optical bandwidth.

31. The laser of any of claims 1-30, further comprising a source of radiation that is directed onto at least one of said first mirror and said gain region.

32. The laser of any of claims 1-31, wherein said frequency-selective element comprises at least one of (i) a planar etalon including an etalon spacer comprising an electro-optically active material and (ii) a monolithic, planar, air-spaced etalon having at least one free-standing dielectric film that serves as a reflecting surface, wherein a length of the etalon spacer or a length of said cavity is chosen so that only one transmission maximum frequency lies within a selected gain band of the etalon.

33. The laser of claim 32, wherein said transmission maximum frequency is tunable over a selected frequency band by at least one of (i) application of a variable voltage to said spacer material and (ii) changing an angle of orientation of said etalon with respect to said gain region optical axis.

34. The laser of any of claims 33, wherein said etalon spacer comprises at least one of a nematic crystal and a smectic crystal.

35. The laser of any of claims 28 and 29, wherein said frequency-selective element comprises:
a polarization-selective element that receives and passes a light beam; and a birefmgent electro-optical medium having ordinary and extraordinary axes oriented at 45° with respect to a selected transmission axis of the polarization-selective element, the optical medium having anti-reflection-coated surfaces oriented substantially perpendicular to a path of light beam propagation through the medium and having a light propagation length such that light having a selected initial polarization that passes through the medium along the selected transmission axis will be in a high transmission polarization state for at most one selected mode; and
voltage means for applying a selected voltage to the electro-optical medium over a range of voltages.

36. The laser of claim 35, further comprising temperature control means, connected to said electro-optical medium, for controlling temperature of said medium.

37. The laser of any of claims 1-35, further comprising a beam focusing module for providing a secondary beam waist for a selected fundamental mode of said cavity.

38. The laser of claim 37, wherein said beam focusing module comprises at least one of (i) a lens or concave mirror with at least one multi-layer dielectric coating and (ii) an off-axis parabolic lens.

39. The laser of 35, wherein said electro-optical medium comprises at least first and second optical elements, with at least one birefringent element positioned between the first and second elements and oriented to increase angular acceptance of said frequency-selective element.

40. The laser of claim 39, wherein said birefringent element is a zero order, half wave plate, positioned between said first and second optical elements and oriented to rotate first and second directions of optical beam polarization with respect to a selected axis.

41. The laser of claim 35, wherein said electro-optical medium comprises at least one of lithium tantalate and lithium niobate.

42. The laser of claim 35, wherein said extraordinary axis is perpendicular to at least one of said anti-reflection coated surfaces and lies in a plane oriented at approximately 45° with respect to a plane defined by said high transmission polarization state axis of said polarization-selective element and a direction of light propagation through said frequency-selective element.

43. The laser of claim 35, further comprising:
an electro-optically active medium that provides a variable retardation of speed of propagation of light having a first polarization relative to speed of propagation of light having a second polarization, the retardation being controllable by an applied electrical voltage,
whereby a combination of said birefringent medium and the electro-optically active medium allows selection of a wavelength of light that returns to said polarization-selective component in said high transmission axis by application of a voltage to the electro-optically active medium.

44. The laser of claim 43, wherein said electro-optically active medium comprises at least one of a nematic crystal and a smectic crystal.

45. The laser of claim 35, wherein said electro-optical medium comprises an electro-optical crystal.

46. The laser of any of claims 1-45, wherein light reflected into a fundamental lasing mode from said first mirror and produced by any medium other than said gain region is reduced by a factor of at least 104 in power relative to a power level of light produced in said gain region.

47. The laser of any of claims 1-46, wherein said second mirror is planar and additionally comprises a focusing element, positioned within said cavity and configured so that said cavity forms a stable resonator centered within a stability range of said cavity.

48. The laser of any of claims 1-47, wherein said second mirror is concave.

49. The laser of claim 48, wherein said second mirror has a radius of curvature selected to maintain a single transverse mode of oscillation over a range of optical pumping power and is coated with at least two layers of dielectric materials having different indices of refraction to provide reflectance at least equal to 95 percent over a selected gain band of said second mirror.

50. The laser of any of claims 1-49, wherein at least one of said first mirror, said gain region and said spacer is attached to a heat sink.

51. The laser of any of claims 1-50, further comprising a housing that localizes at least one of said first mirror, said gain region, said second mirror and said frequency-selective element.

52. The laser of claim 35, wherein said electro-optical medium comprises at least one surface having an anti-reflection coating.

53. The laser of any of claims 1-52, further comprising a diode pump laser, aligned at an equivalent Brewster's angle relative to said optical axis and being integral with said spacer, wherein said spacer has a light absorption element.

54. The laser of any of claims 1-53, further comprising:
wavelength tuning means for tuning said laser from a first mode to a second mode;
an optical modulator that receives an optical beam from said laser and impresses an information-carrying modulated signal upon the received beam; and an optical coupler for receiving and coupling the modulated signal into an optical fiber.

55. A method for making a semiconductor structure for use as part of a laser, the method comprising:
forming a semiconductor structure, using at least one of molecular beam epitaxy and metal oxide chemical vapor deposition, the structure comprising:
at least two semiconductor crystalline layers on a semiconductor
substrate;
a multi-layer mirror positioned facing the crystalline layers;

a multi-layer active gain region having at least two quantum wells and having a thickness an odd integer times a selected design wavelength for the laser; and an anti-reflection coating having a low reflectance at the design wavelength,
removing the substrate to provide a modified semiconductor structure; and providing a heat sink for at least one of the mirror and the gain region.

56. A method of calibrating mode spacing of a laser, the method comprising the steps of:
providing a laser external cavity and positioning a first mirror at an end of the cavity, where the cavity has a selected length that defines a plurality of optical modes, with each mode having a corresponding wavelength;
providing a homogeneously broadened gain region, located adjacent to or within the external cavity, that is active over a wavelength band having a bandwidth less than a mode-to-mode spacing, where the gain region is tunable from a first mode to a second mode and is stable at the second mode;
providing optical tuning means, including wavelength-selective optical sensing means for tuning the laser from the first mode to the second mode and for detecting at least one pulse indicating a transition from the first mode to the second mode;
providing a mode controller for maintaining the laser in a selected mode;
varying a control parameter associated with the controller to sweep the tuning means between a selected longest wavelength mode and a selected shortest wavelength mode that can be generated by the laser;
recording in a memory associated with the controller a transition control parameter associated with the optical sensing means, when the optical sensing means detects a mode-to-mode transition; and
determining and storing at least one single mode value as an approximate half-increment magnitude between magnitudes of transition control parameter values for at least two adjacent modes.