LOW SPECKLE NOISE MONOLITHIC MICROCHIP RGB LASERS

• US 20070291810A1
• Filed: 06/15/2006
• Published: 12/20/2007
• Est. Priority Date: 06/15/2006
• Status: Active Grant

• Current Assignee: Pavilion Integration Corporation
• Original Assignee: Pavilion Integration Corporation
• Inventors: Luo, Ningyi, Zhu, Sheng-Bai
• Granted Patent: US 7,457,330 B2
• Time in Patent Office: Days
• US Class Current: 372/50.121
• CPC Class Codes: H01S 2301/20 Lasers with a special output beam profile or cross-section, e.g. non-Gaussian
• CPC Class Codes: H01S 3/0606 with polygonal cross-section, e.g. slab, prism H01S3/0604 takes precedence
• CPC Class Codes: H01S 3/0621 Coatings on the end-faces, e.g. input/output surfaces of the laser light
• CPC Class Codes: H01S 3/0627 the resonator being monolithic, e.g. microlaser
• CPC Class Codes: H01S 3/08054 Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation quarter-wave plates in a Q-switch laser H01S3/1124, H01S3/115
• CPC Class Codes: H01S 3/0809 Two-wavelenghth emission
• CPC Class Codes: H01S 3/09415 the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
• CPC Class Codes: H01S 3/1083 using parametric generation
• CPC Class Codes: H01S 3/109 Frequency multiplication, e.g. harmonic generation
• CPC Class Codes: H01S 3/1673 YVO4 [YVO]
• CPC Class Codes: H01S 3/2383 Parallel arrangements
• CPC Class Codes: H01S 3/2391 emitting at different wavelengths

Abstract

A method for reducing speckle noise of a monolithic microchip laser with intracavity beam combining and sum frequency mixing is based on time averaging of uncorrelated speckle patterns generated from a large number of independent longitudinal modes and comprises schemes including selection of gain media and nonlinear optical materials to support broadband sum frequency mixing; adoption of gain-conjugated and/or chirped mirrors for flat-top spectra and/or mode phase diversification; multimode laser operation introduced by RF modulation; and multiplication of source modes in frequency mixing process featured with degeneration free and narrowed/uneven intervals. A device and an apparatus for generating low speckle noise red, green, blue lasers adaptable for color display systems are developed based on the inventive method.

Claims

1. A low speckle noise monolithic microchip laser comprising:

two independent laser sources to produce two fundamental beams linearly polarized with mutually orthogonal polarizations;

a birefringent crystal for tilting propagation direction of one fundamental laser beam with polarization along the principal plane of said birefringent crystal, referred to as e-ray, and for combining said e-ray with another fundamental laser beam with polarization normal to the principal plane of said birefringent crystal, referred to as o-ray, at the output surface; and

a nonlinear optical crystal to generate a new wavelength based on sum frequency mixing of the two fundamental laser beams;

wherein;

said fundamental laser beams are generated in separate cavities to eliminate the green problem;

said fundamental laser beams are preferably multimode and are narrowly spaced;

said nonlinear optical crystal possesses a wide phase-matching bandwidth, preferably, the phase matching condition can be satisfied for all or many of the input modes simultaneously.

2. A low speckle noise monolithic microchip laser as of claim 1, wherein:

at least one said laser source further comprising;

one active region for emission of laser at the desired wavelength and with wide fluorescence bandwidth;

one pumping source for activating said active region; and

a resonator, preferably formed with specially coated mirrors for gain flattening and/or chirped mirrors for phase shift diversification.

3. A low speckle noise monolithic microchip laser as of claim 1, wherein:

the two fundamental laser beams are generated from two active regions in separate cavities;

said two active regions can be in two individual gain media or in one gain medium;

said gain media (or medium) can be birefringent in nature, producing stimulated emission that is linearly polarized along certain direction, or simultaneously producing two stimulated emissions of different wavelengths, respectively from ordinary and extraordinary transitions;

alternatively, said gain media (or medium) can be isotropic in respect of emission polarization.

4. A low speckle noise monolithic microchip laser as of claim 2, wherein:

said pumping source further comprising;

a laser diode for emitting light with wavelength matching the absorption spectrum of the gain medium to be activated;

said laser diode is controlled by a drive current;

said drive current is preferably RF modulated for stabilized operation;

one or more optical coupling elements or mechanisms for beam shaping and for delivering the pump light to the gain medium to be activated.

5. A low speckle noise monolithic microchip laser as of claim 4, wherein:

one preferred embodiment using one laser diode for simultaneously pumping two active regions further comprises;

a laser diode for emitting light with wavelength matching the absorption spectra of the gain media to be activated;

a polarization controller for rotating the incident light polarization to a predefined direction for optimizing absorption and wavelength conversion efficiencies; and

a beam splitter for splitting an incident light into two components with mutually orthogonal polarizations;

wherein;

said polarization controller can be an element such as a wave plate, or a mechanism such as rotation of a free-space laser diode or randomization of polarization through an optic fiber;

said beam splitter can be a combination of polarization dependent elements such as mirrors coated with polarization sensitive materials or polarization beam splitter or prisms coated with polarization sensitive materials;

alternatively, said beam splitter can be a birefringent crystal;

wherein;

each pump beam is linearly polarized along the maximum absorption direction of the gain medium to be activated;

6. A low speckle noise monolithic microchip laser as of claim 1, wherein:

one laser source can be a laser diode, controlled by a drive current with radio frequency modulation for stabilization;

said laser diode provides for second fundamental beam with multimode;

said second fundamental beam is linearly polarized, acting as an o-ray or e-ray, depending on the polarization of the first fundamental beam, in the birefringent crystal for intracavity beam combination.

7. A low speckle noise monolithic microchip laser as of claim 1, wherein:

two or more crystal layers are stacked one above one, each layer represents an individual laser;

each individual laser emits a monochromatic light based on sum frequency mixing of two fundamental beams;

in each laser, at least one fundamental beam is generated by exciting an appropriate gain medium and circulating optical power from the stimulated emission in an independent cavity resonated at the fundamental wavelength and preferably coated with gain-conjugated and/or chirped mirrors;

in each laser, one fundamental beam can be generated by a laser diode controlled by an RF modulated drive current or by other devices such as OPO.

8. A low speckle monolithic microchip laser as of claim 7, wherein:

said individual lasers respectively emit red, green, and blue lights;

said low speckle monolithic microchip laser is adaptable for use with a color display system.

9. A method for producing low speckle noise light from a monolithic microchip laser, as described in claim 1, based on time averaging of uncorrelated speckle patterns from multiple longitudinal modes, comprising schemes of:

1) selection of gain media so that the emission spectra are centered at the desired fundamental wavelengths, preferably, with broadband fluorescence curves;

2) selection of nonlinear optical materials with phase-matching bandwidths wider than the corresponding gain profiles to support broadband sum frequency mixing;

3) adoption of chirped mirrors with positive dispersive properties for diversifying mode phase shifts;

4) adoption of specially coated mirrors with gain compensation for obtaining flat-top spectra;

5) repeated on-off operation of laser diode controlled by RF modulated drive current for stabilized multimode output;

6) mode multiplication from frequency mixing of two sources with unequally spaced multiple longitudinal modes.

10. A method for producing low speckle noise light from a monolithic microchip laser as of claim 9 further comprising steps of:

excitation of said first gain medium for creating population inversion corresponding to the radiative transition of said first fundamental wavelength;

excitation of said second gain medium for creating population inversion corresponding to the radiative transition of said second fundamental wavelength;

suppressing lasing at competitive wavelengths;

suppressing lasing at unwanted polarization;

stabilization of multimode operation of laser diode as pump source and/or one of the fundamental laser beams;

bandwidth broadening resulted from gain flattening mechanism;

mode phase diversification resulted from wavelength-dependent reflection;

generation of an extraordinary (e) laser beam from said first gain medium;

generation of an ordinary (o) laser beam from said second gain medium;

walk-off of e-beam in the birefringent crystal;

combination of o-beam and e-beam at the input surface of the nonlinear optical crystal;

wavelength conversion and mode multiplication via sum frequency mixing;

second-pass sum frequency mixing for efficiency improvement; and

extraction of the mixed laser beam at the outer surface of the nonlinear crystal through output coupler(s).

11. A method for producing low speckle noise light from a monolithic microchip laser as of claim 9;

wherein said repeated on-off operation of laser diode is realized by periodic change of drive current below or above the threshold;

wherein said drive current having an RF waveform such as a sine, a rectified sine, a distorted sine, or other waves, preferably with a high duty ratio;

wherein said repeated on-off operation results in stabilized multimode laser output;

wherein said stabilized multimode laser output is a pump source or one of the two fundamental laser sources;

wherein said stabilized multimode laser operation is not only for reducing optical noise but also for reducing speckle.

12. A method for producing low speckle noise light from a monolithic microchip laser as of claim 9, wherein said mode multiplication further comprising schemes for:

generating at least one fundamental beam(s) operated at multimode, furthermore, the mode structures including mode intervals of the two fundamental beams are different with each other, whether their spectral profiles including the central wavelengths are identical or not;

generating plurality of longitudinal modes in laser output from sum frequency mixing;

these longitudinal modes have narrowed spacing and are unevenly distributed;

said mode multiplication scheme eliminates mode degeneration;

said mode multiplication scheme eliminates risks associated with self mode locking and/or Q-switching;

said mode multiplication scheme results in mode structure covering a wavelength range narrower than the input spectra, whereby a less stringent requirement for phase matching bandwidth.

13. An apparatus for producing monochromatic light with low speckle noise comprising:

a solid-state monolithic device with two independent laser oscillation cavities for generating two fundamental wavelengths;

a birefringent crystal for spatially combining two orthogonally polarized fundamental laser beams via walk-off effect;

a nonlinear optical crystal for sum frequency generation;

an optional thermoelectric cooler for automatic temperature control of pump diodes and/or laser device;

wherein;

said solid-state monolithic device with independent laser oscillation cavities further comprises;

a first diode pumped solid-state laser to produce a laser beam with first fundamental wavelength and a polarization along the principal plane of said birefringent crystal, in conjunction with a resonant cavity to support oscillation of laser at said first fundamental wavelength and to suppress oscillation of laser at competitive wavelengths and/or unwanted polarization;

a second laser source to produce a laser beam with second fundamental wavelength and a polarization normal to the principal plane of said birefringent crystal, in conjunction with a resonant cavity to support oscillation of laser at said second fundamental wavelength and to suppress oscillation of laser at competitive wavelengths and/or unwanted polarization; and

an output coupler for extraction of laser output at the mixed wavelength;

wherein;

said second laser source can be a diode pumped solid-state laser or a laser diode or other light sources such as OPO;

optical pump sources can be two individual laser diodes or a single laser diode with polarization-sensitive beam splitting schemes;

said output coupler can be a coating on the exterior surface of the nonlinear optical crystal or an external concave lens for collimation improvement and/or high power output.

14. An apparatus as of claim 13, in particular, generating a green light, wherein:

said first diode pumped solid-state laser comprises;

an a-cut Nd;

YVO₄ laser gain medium with C-axis parallel to the principal plane of said birefringent crystal;

a pump source emitting light at 808 nm with π

polarization for end pumping; and

a resonant cavity to support laser oscillation at 1064 nm;

said second laser source is a diode pumped solid-state laser comprising;

an a-cut Nd;

YVO₄ laser gain medium with C-axis normal to the principal plane of said birefringent crystal;

a pump source emitting light at 808 nm with π

polarization for end pumping; and

a resonant cavity to support laser oscillation at 1064 nm;

said birefringent crystal is TiO₂ or un-doped YVO₄ or the like;

said nonlinear crystal is KTP or the like;

the exterior surface of the nonlinear crystal is coated anti-reflective to 532 nm;

wherein the first and second pump sources can be two separated laser diodes or a dual-emitter, or a single laser diode with polarization-sensitive beam splitter;

the pump light is delivered to the gain medium through free-space or fiber coupling;

beam shaping optics can be a set of separate optical elements or micro-lenses.

15. An apparatus as of claim 13, in particular, generating a blue light, wherein:

said first diode pumped solid-state laser comprises;

an a-cut Nd;

YVO₄ laser gain medium with C-axis parallel to the principal plane of said birefringent crystal;

a pump source emitting light at 808 nm with π

polarization for end pumping; and

a resonant cavity to support laser oscillation at 914 nm and to suppress oscillation at 1064 nm and 1342 nm

said second laser source is a diode pumped solid-state laser comprising;

an a-cut Nd;

YVO₄ laser gain medium with C-axis normal to the principal plane of said birefringent crystal;

a pump source emitting light at 808 nm with π

polarization for end pumping; and

a resonant cavity to support laser oscillation at 914 nm and to suppress oscillation at 1064 nm and 1342 nm;

said birefringent crystal is TiO₂ or un-doped YVO₄;

said nonlinear crystal is BBO or the like and its exterior surface is coated highly transmissive to 457 nm; and

a temperature controller for supporting 914 nm emission;

wherein the first and second pump sources can be two separated laser diodes or a dual-emitter, or a single laser diode with polarization-sensitive beam splitter;

the pump light is delivered to the gain medium through free-space or fiber coupling;

beam shaping optics can be a set of separate optical elements or micro-lenses.

16. An apparatus as of claim 13, in particular, generating a red light at 659 nm, wherein:

said first diode pumped solid-state laser comprises;

an active region of a Nd;

YAG laser gain medium;

a pump source emitting light at 808 nm; and

a resonant cavity to support laser oscillation at 1319 nm with polarization parallel to the principal plane of the birefringent crystal;

said resonant cavity suppresses oscillation at 1064 nm;

said resonant cavity suppresses oscillation of polarization normal to the principal plane of the birefringent crystal;

said second laser source is a diode pumped solid-state laser comprises;

another active region of the Nd;

YAG laser gain medium same as for the first laser source;

a pump source emitting light at 808 nm; and

a resonant cavity to support laser oscillation at 1319 nm with polarization normal to the principal plane of the birefringent crystal;

said resonant cavity suppresses oscillation at 1064 nm;

said resonant cavity suppresses oscillation of polarization parallel to the principal plane of the birefringent crystal;

said birefringent crystal is TiO₂ or un-doped YVO₄ or the like;

said nonlinear crystal is KTP or the like;

the exterior surface of the nonlinear crystal is coated highly transmissive to 659 nm;

wherein the first and second pump sources can be two separated laser diodes or a dual-emitter, or a single laser diode with polarization-sensitive beam splitter;

the pump light is delivered to the gain medium through free-space or fiber coupling;

beam shaping optics can be a set of separate optical elements or micro-lenses.

17. An apparatus as of claim 13, in particular, generating a red light at 528 nm, wherein:

said first diode pumped solid-state laser comprises;

an a-cut Nd;

YVO₄ laser gain medium with C-axis parallel to the principal plane of said birefringent crystal;

a laser diode emitting light at 808 nm with π

polarization for end pumping; and

a resonant cavity to support laser oscillation at 1064 nm;

said second laser source is a diode laser controlled by an RF modulator to emit light around 1530 nm;

said birefringent crystal is TiO₂ or un-doped YVO₄;

said nonlinear crystal is KTP or the like;

the exterior surface of the nonlinear crystal is coated anti-reflective to 628 nm.

18. An apparatus as of claim 13, wherein:

the laser gain media can be selected from the group including, but not limited to, Nd;

YAG, Nd;

YLF, Nd;

YVO₄, Nd;

GdVO₄, Nd;

KG(WO₄)₂, Nd;

KYW, Nd;

Cr;

GSGG, Nd;

Cr;

YSGG, Yb;

YVO₄, Yb;

KGW, Yb;

KYW, and Yb;

YAG.

19. An apparatus as of claims 14, 15, 16, 17, and 18 comprising three individual lasers, stacked one above one, wherein:

the first laser is constructed according to claim 16 or claim 17 to produce a red light;

the second laser is constructed according to claim 14 to produce a green light;

the third laser is constructed according to claim 15 to produce a blue light;

one or more of these layers can be made of the materials listed in claim 18 or a combination thereof,

optical pump sources can be independent for each individual gain medium or shared with several gain media through beam splitting elements;

said apparatus is adaptable for use of RGB color display.

20. An apparatus as of claim 13, can be modified by one or more elements for additional features including, but not limited to, tunability and Q-switching.