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A tunable laser and laser tuning method, based on the interaction of a spectrally dependent beam distortion and a spatial filter within a laser cavity. One embodiment of this laser is an external cavity semiconductor laser in which broad tunability is obtained by the insertion of an acousto-optic tu

A tunable laser and laser tuning method, based on the interaction of a spectrally dependent beam distortion and a spatial filter within a laser cavity. One embodiment of this laser is an external cavity semiconductor laser in which broad tunability is obtained by the insertion of an acousto-optic tunable filter (AOTF) into the laser cavity such that the intra-cavity laser beam passes through the AOTF in zeroth order.

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1. A laser formed by an optical resonator comprising:a) an electrically pumped semiconductor gain medium comprising a single-mode optical waveguide having first and second endfaces, where said first endface is an output coupler of said optical resonator, from which a beam with total power Pa is emit

1. A laser formed by an optical resonator comprising:a) an electrically pumped semiconductor gain medium comprising a single-mode optical waveguide having first and second endfaces, where said first endface is an output coupler of said optical resonator, from which a beam with total power Pa is emitted from said second endface;b) a lens which receives said emitted beam and transmits it to;c) an acousto-optic device which receives said transmitted beam, wherein said received beam is distorted and attenuated, but not frequency shifted, in the course of transmission through the acousto-optic device, and wherein the extent of the distortion and attenuation is dependent on the received beam wavelength and the RF frequency applied to the acousto-optic device; andd) a return mirror which reflects said non frequency-shifted beam back through said acousto-optic device and said lens;whereby said reflected beam impinges on said second endface with a total power Pb, with a lesser optical power P0 being launched into the gain medium waveguide, such that P0/Pa has a maximum value at a wavelength λ0 where the total loss due to mode mismatching and attenuation in the external cavity is minimized, where λ0 is selected by said acousto-optic device in response to the RF frequency applied to said acousto-optic device,and wherein λ0 is the wavelength of laser emission from said first endface.2. The laser of claim 1 wherein said optical resonator also contains a grid fixing etalon.3. The laser of claim 1 where discrete tunability is achieved by adjusting the round trip optical path length of said optical resonator.4. A laser formed by an optical resonator comprising:a) a pumped gain medium comprising a single-mode optical waveguide having first and second endfaces, where said first endface is an output coupler of the optical resonator, from which a beam with total power Pa is emitted from said second endface;b) coupling optics which receive the beam emitted from said second endface and transmit it to;c) a spectrally dependent spatial filtering (SDSF) tuning element which receives said transmitted beam, and which allows said received beam to exit from the tuning element as a beam that is attenuated and distorted without a frequency shift, wherein the extent of the attenuation and distortion depends on said received beam wavelength, and wherein said SDSF tuning element includes control means to alter the wavelength dependence of the beam distortion and attenuation;d) a return mirror which reflects said distorted and attenuated beam back through said tuning element and said coupling optics;whereby said reflected beam impinges on said second endface with a total power Pb, with a lesser power P0 being launched into the gain medium waveguide,such that P0/Pa has a maximum value at a wavelength λ0 where the total loss due to mode mismatching and attenuation in the external cavity is minimized, where λ0 is selected by said tuning element,and wherein λ0 is the laser emission wavelength from said first endface.5. The laser of claim 4 wherein three wave mixing occurs in said SDSF tuning element, and said SDSF tuning element is aligned such that no frequency shift is incurred in transmission through said element.6. The laser of claim 4 wherein said optical resonator also contains a grid fixing etalon.7. The laser of claim 4 where discrete tunability is achieved by adjusting the roundtrip optical path length of said optical resonator.8. A laser formed by an optical resonator comprising:a) a pumped gain medium;b) a spectrally dependent spatial filtering (SDSF) tuning element, which allows said incident beam to exit from the tuning element as a beam that is attenuated and distorted without a frequency shift, wherein the extent of the attenuation and distortion depends on said incident beam wavelength, and wherein said SDSF tuning element includes control means to alter the wavelength dependence of the beam distortion and attenuation; andc) a spatial filter;such that the total round trip loss attains a minimum value at a wavelength λ0 selected by said tuning element,whereby λ0 is the laser emission wavelength.9. The laser of claim 8, wherein said SDSF tuning element is a volume hologram, aligned such that the propagation direction of the laser beam within said optical resonator is nominally unchanged in transmission through said hologram at λ0.10. The laser of claim 8 wherein three wave mixing occurs in said SDSF tuning element, and said SDSF tuning element is aligned such that no frequency shift is incurred in transmission through said element.11. The laser of claim 10 where the three wave mixing interaction is an acousto-optic interaction and wherein said SDSF tuning element is an acousto-optic tunable filter.12. The laser of claim 8 wherein said optical resonator also contains a grid fixing etalon.13. The laser of claim 8 where discrete tunability is achieved by setting the round trip optical path length of said optical resonator.14. A method for generating a tunable laser beam comprising:a) pumping a gain medium positioned within an optical resonator to thereby generate an optical beam;b) passing the beam through a spectrally dependent spatial filtering (SDSF) tuning element within said optical resonator;c) passing the beam through a spatial filter within said optical resonator;d) distorting and attenuating the beam passing through said SDSF tuning element in accordance with a beam distortion and attenuation control signal, such that the total round trip loss within said optical resonator is minimal at a wavelength λ0;e) emitting laser radiation having a wavelength λ0 from an output coupler within said optical resonator; andf) tuning the wavelength of said laser beam by selecting λ0 by said SDSF tuning element in response to the beam distortion and attenuation control signal.15. The method of claim 14 wherein said SDSF tuning element is a volume hologram, aligned such that the propagation direction of the laser beam within said optical resonator is nominally unchanged in transmission through said hologram at λ0.16. The method of claim 14 wherein three wave mixing occurs in said SDSF tuning element, and said SDSF tuning element is aligned such that no frequency shift is incurred in transmission through said element.17. The method of claim 16 where the three wave mixing interaction is an acousto-optic interaction and wherein said SDSF tuning element is an acousto-optic tunable filter.18. The method of claim 14 further comprising:g) inserting a secondary output coupler into said optical resonator such that a fraction of the circulating laser beam within said optical resonator is emitted from said secondary output coupler;h) dividing said emitted beam into a first sub-beam in a first optical path, and a second sub-beam in a second optical path;i) filtering said second sub-beam with a linear transmission filter placed in said second optical path to produce a filtered second sub-beam; andj) determining relative beam intensities of said first sub-beam in said first optical path and said second filtered sub-beam in said second optical path in order to measure the laser emission wavelength.19. The method of claim 14 further comprising:g) inserting a secondary output coupler into said optical resonator such that a fraction of the circulating laser beam within said optical resonator is emitted as a first beam from said secondary output coupler, and such that a fraction of the diffracted beam generated within said SDSF tuning element is emitted as a second beam from said secondary output coupler; andh) determining relative beam intensities of said first beam and said second beam in order to measure the difference between the laser emission wavelength and the center wavelength of the zeroth order spectral notch of said SDSF tuning element.20. The method of claim 14 further comprising:g) inserting a grid fixing etalon into said optical resonator in order to provide discrete tunability;h) inserting a secondary output coupler into said optical resonator such that a fraction of the circulating laser beam within said optical resonator is emitted as a first beam from said secondary output coupler, and such that a fraction of the beam generated within said optical resonator by reflection of the circulating laser beam from said grid fixing etalon is emitted as a second beam from said secondary output coupler; andi) determining relative beam intensities of said first beam and said second beam in order to measure the difference between the laser emission wavelength and a wavelength of the transmission peak of said grid fixing etalon.21. A method for generating a tunable laser beam comprising:a) pumping a gain medium positioned within an optical resonator, said gain medium comprising a single-mode optical waveguide;b) passing the beam through a spectrally dependent spatial filtering (SDSF) tuning element within said optical resonator;c) distorting and attenuating the beam passing through said SDSF tuning element in accordance with a beam distortion and attenuation control signal, such that the total round trip loss within said optical resonator is minimized at a wavelength λ0;d) emitting laser radiation having a wavelength λ0 from an output coupler within said optical resonator; ande) tuning the wavelength of said laser beam by selecting λ0 by said SDSF tuning element in response to the beam distortion and attenuation control signal.22. The method of claim 21 wherein three wave mixing occurs in said SDSF tuning element, and said SDSF tuning element is aligned such that no frequency shift is incurred in transmission through said element.23. The method of claim 21 where the three wave mixing interaction is an acousto-optic interaction and wherein said SDSF tuning element is an acousto-optic tunable filter.24. The method of claim 21 further comprising:f) inserting a secondary output coupler into said optical resonator such that a fraction of the circulating laser beam within said optical resonator is emitted from said secondary output coupler,g) dividing said emitted beam into a first sub-beam in a first optical path, and a second sub-beam in a second optical path,h) filtering said second sub-beam with a linear transmission filter placed in said second optical path to produce a filtered second sub-beam,i) determining relative beam intensities of said first sub-beam in said first optical path and said second filtered sub-beam in said second optical path in order to measure the laser emission wavelength.25. The method of claim 21 further comprising:f) inserting a secondary output coupler into said optical resonator such that a fraction of the circulating laser beam within said optical resonator is emitted as a first beam from said secondary output coupler, and such that a fraction of the diffracted beam generated within said SDSF tuning element is emitted as a second beam from said secondary output coupler; andg) determining relative beam intensities of said first beam and said second beam in order to measure the difference between the laser emission wavelength and the center wavelength of the zeroth order spectral notch of said SDSF tuning element.26. The method of claim 21 further comprising:f) inserting a grid fixing etalon into said optical resonator in order to provide discrete tunability;g) inserting a secondary output coupler into said optical resonator such that a fraction of the circulating laser beam within said optical resonator is emitted as a first beam from said secondary output coupler, and such that a fraction of the beam generated within said optical resonator by reflection of the circulating laser beam from said grid fixing etalon is emitted as a second beam from said secondary output coupler; andh) determining relative beam intensities of said first beam and said second beam in order to measure the difference between the laser emission wavelength and a wavelength of the transmission peak of said grid fixing etalon.