Abstract
A diode pumped solid state laser is provided which includes a ruby crystal optical gain medium and a high bandgap semiconductor laser diode (LD) or light emitting diode (LED) pump source to directly optically pump the gain medium. The high-bandgap semiconductor LD or LED is a semiconductor device whose chemical composition is chosen to provide output radiation at an approximate wavelength of 405 nm. The ruby crystal produces laser output at the relatively short wavelength of 694 nm.
Claims
1. An apparatus, comprising: a high bandgap semiconductor source of electromagnetic radiation; a ruby crystal gain element contained within an optical cavity resonant at a wavelength of 694 nm; and means for directly pumping said ruby crystal gain element with said electromagnetic radiation to produce an output beam having said wavelength of 694.3 nm.
2. The apparatus of claim 1, wherein said source is selected from the group consisting of at least one semiconductor laser diode and at least one light emitting diode.
3. The apparatus of claim 1, wherein said electromagnetic radiation comprises continuous wave radiation.
4. The apparatus of claim 1, wherein said electromagnetic radiation comprises QCW pulsed radiation.
5. The apparatus of claim 1, wherein said electromagnetic radiation comprises a wavelength of 405 nm.
6. The apparatus of claim 1, wherein said electromagnetic radiation comprises a wavelength of 530 nm.
7. The apparatus of claim 2, wherein said at least one semiconductor laser diode comprises an array of semiconductor laser diodes.
8. The apparatus of claim 2, wherein said at least one light emitting diode comprises an array of light emitting diodes.
9. The apparatus of claim 4, further comprising a Q-switch operatively located within said optical cavity.
10. The apparatus of claim 4, further comprising a saturable absorber mode-locker operatively located within said optical cavity.
11. The apparatus of claim 1, further comprising a non-linear optic (NLO) crystal located outside of said cavity, wherein said NLO crystal is phase matched at said wavelength of 694 nm and at a wavelength of 347 nm; and means for coupling said output beam to said first NLO crystal.
12. The apparatus of claim 1, further comprising a non-linear optic (NLO) crystal located inside said cavity, wherein said NLO crystal is phase matched at said wavelength of 694 nm and at a wavelength of 347 nm.
13. The apparatus of claim 4, wherein each QCW pulse of said pulsed radiation has a pulse duration in the range from 0.1 millisecond to 5 milliseconds.
14. The apparatus of claim 10, wherein said output beam is a sequence of mode-lock pulses, wherein each pulse of said mode-locked pulses has a pulse duration within a range from 1 picosecond to 100 picoseconds.
15. A method, comprising: producing electromagnetic radiation from a high bandgap semiconductor source of electromagnetic radiation; and directly pumping a ruby crystal gain element with said electromagnetic radiation to produce an output beam having a wavelength of 694 nm, wherein said ruby crystal gain element is contained within an optical cavity resonant at said wavelength of 694 nm.
16. The method of claim 15, wherein said source is selected from the group consisting of at least one semiconductor laser diode and at least one light emitting diode.
17. The method of claim 15, wherein said electromagnetic radiation comprises continuous wave radiation.
18. The method of claim 15, wherein said electromagnetic radiation comprises QCW pulsed radiation.
19. The method of claim 15, wherein said electromagnetic radiation comprises a wavelength of 405 nm.
20. The method of claim 15, wherein said electromagnetic radiation comprises a wavelength of 530 nm.
21. The method of claim 16, wherein said at least one semiconductor laser diode comprises an array of semiconductor laser diodes.
22. The method of claim 16, wherein said at least one light emitting diode comprises an array of light emitting diodes.
23. The method of claim 18, further comprising operatively locating a Q-switch within said optical cavity.
24. The method of claim 18, further comprising operatively locating a saturable absorber mode-locker within said optical cavity.
25. The method of claim 15, further comprising providing a non-linear optic (NLO) crystal outside of said cavity, wherein said NLO crystal is phase matched at said wavelength of 694 nm and at a wavelength of 347 nm; and coupling said output beam to said first NLO crystal.
26. The method of claim 15, further comprising providing a non-linear optic (NLO) crystal located inside said cavity, wherein said NLO crystal is phase matched at said wavelength of 694 nm and at a wavelength of 347 nm.
27. The method of claim 18, wherein each pulse of said QCW pulsed radiation has a pulse duration in the range from 0.1 millisecond to 5 milliseconds.
28. The method of claim 24, wherein said output beam is a sequence of mode-lock pulses, wherein each pulse of said mode-locked train of pulses has a pulse duration within a range of 1 picosecond to 100 picoseconds.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
[0015] FIG. 1 is a drawing of the first ruby laser by Maiman.
[0016] FIG. 2 shows the spectral absorption coefficient and cross section of ruby crystal, and the spectral absorption peaks of the 405 nm and 560 nm pump bands.
[0017] FIG. 3 is a block diagram of a prior art continuous wave ruby laser, pumped with a frequency doubled, near infrared laser diode pump neodymium solid state laser.
[0018] FIG. 4 is a block diagrams of a ruby laser directly pumped by a high bandgap GaN semiconductor laser diode.
[0019] FIG. 5 shows an optical configuration of a CW or QCW ruby laser directly pumped by a high bandgap GaN semiconductor laser diode emitting at a wavelength of 405 nm.
[0020] FIG. 6 shows an optical configuration of a Q-switched ruby laser directly pumped by a high bandgap GaN semiconductor laser diode emitting at a wavelength of 405 nm.
[0021] FIG. 7 is a block diagram of a source of pulsed DUV radiation at a wavelength of 347 nm produced by second harmonic nonlinear conversion of a ruby laser that is directly pumped by a high bandgap GaN semiconductor laser diode emitting at a wavelength of 405 nm.
[0022] FIG. 8 shows the measured continuous wave, CW, output power versus continuous wave pump power, of a ruby laser directly pumped by a high bandgap GaN laser diode continuously emitting pump radiation at a wavelength of 405 nm.
DETAILED DESCRIPTION OF THE INVENTION
[0023] A ruby laser that is directly pumped with a high bandgap semiconductor laser diode or LED is disclosed. The descriptor bandgap refers to the difference (or gap) in energy between the top of the valance band and the bottom of the conduction band of the semiconductor material from which a laser diode or LED is fashioned; the specific descriptor high bandgap used herein, refers to the class of semiconductors whose lattice anions are nitrogen ions, and are generally referred to as nitride or GaN semiconductors. Nitride based laser diodes and LEDs have bandgap energies that fall in the range of 3.5 eV to 2.3 eV (corresponding to devices that emit near UV and visible wavelengths in the range of 365 nm to 530 nm) depending on the relative amount of Al, Ga, and In incorporated into the nitride semiconductor device layers. This high bandgap class of semiconductors is distinct from the (legacy) class of semiconductors whose lattice anions are arsenic ions, and are generally referred to as arsenide or GaAs semiconductors. Arsenide based laser diodes and LEDs have lower bandgap energies that fall in the range of 2.0 eV to 0.75 eV (corresponding to emitted device infrared wavelengths of 750 nm to 1250 nm), depending on the relative amount of Al, Ga, and in incorporated into the arsenide semiconductor device layers.
[0024] FIG. 4 shows a functional block diagram of an embodiment of the present invention, where a high bandgap GaN semiconductor diode 35 produces pump beam 36 that pumps ruby laser 37 that, in turn, generates laser output beam 38 at a wavelength 694 nm. The disclosed laser of FIG. 4 may be operated in any of the known temporal modalities, continuous wave, quasi-CW, repetitively pulsed, Q-switched and mode-locked, applying techniques well known in the art. The system described in FIG. 4 is presented as a non-limiting example.
[0025] FIG. 5 shows the optical configuration of an embodiment of the present invention, where the high bandgap semiconductor laser diode 39 is specifically fashioned to emit a pure continuous wave at a wavelength of 405 nm. In this configuration, high bandgap semiconductor laser diode 39 emitting in a continuous wave at a wavelength of 405 nm generates continuous wave pump beam 40 that passes through pump beam coupling optics 41 and enters the optical cavity formed by end mirrors 42 and 43. Those skilled in the art, based on this disclosure, will recognize that other optical configurations can be utilized to direct beam 40 through mirror 42 to directly optically pump Ruby crystal 44. Such other optical configurations axe within the scope of the present invention. Ruby gain crystal 44 is contained in this optical cavity. Optical cavity end mirror 42 is a coated mirror that substantially transmits the 405 nm wavelength pump beam and highly reflects radiation at a wavelength of 694 nm. Optical cavity end mirror 43 is a partially transmitting mirror at a wavelength of 694 nm. The continuously pumped ruby gain crystal 44 emits stimulated emission beam 45 at a wavelength of 694 nm within the optical cavity, giving rise to continuous wave output laser beam 46 at a wavelength of 694 nm. Those skilled in the art, based on this disclosure, will recognize that, in addition to coupling optics 41, other optical configurations can be utilized to direct beam 40 through mirror 42 to directly optically pump Ruby crystal 44. For purposes of this disclosure, coupling optics 41 and such other optical configurations referred to above, including directly optically pumping the crystal without intervening optics between the laser 39 and mirror 42, are within the scope of the present invention and are considered to be means for directly pumping the ruby crystal gain element 44. Further, the system described in FIG. 5 is presented as a non-limiting example.
[0026] FIG. 6 shows the optical configuration of another embodiment of the present invention in which the output of the ruby laser is a Q-switched pulse at a wavelength of 694.3 nm. This embodiment employs a high bandgap semiconductor laser diode specifically fashioned to emit at a wavelength of 405 nm and operated in a so-called quasi-continuous-wave or QCW pulse mode. As used here, the QCW pump pulse produces a nominally constant power for a time duration the order of the energy storage lifetime of the gain medium being pumped, for ruby in the multi-millisecond range. In this embodiment, high bandgap semiconductor laser diode 47 emitting in the QCW pulse mode at a wavelength of 405 nm generates a pump beam 48 of multi-millisecond pulse duration that passes through pump beam coupling optics 49 and enters the optical cavity formed by end mirrors 50 and 51. Ruby gain crystal 52 is contained in this optical cavity. Optical cavity end mirror 50 is a coated mirror that substantially transmits the 405 nm QCW pump beam 48 and highly reflects radiation at a wavelength of 694 nm. Optical cavity end mirror 51 is a partially transmitting mirror at a wavelength of 694 nm. At the beginning of the QCW pump pulse, the Q-switch 53 is set to contribute a high optical loss at a wavelength of 694 nm to the optical cavity. During the duration of the QCW pulsed pump pulse, the ruby gain crystal 52 integrates and stores pump energy supplied by the pomp pulse to establish a population inversion at a wavelength of 694 nm. At the end of the QCW pump pulse duration, the Q-switch 53 is quickly switched to a low optical loss condition, initiating the buildup of stimulated emission beam 54 at a wavelength of 694 nm within the optical cavity, giving rise to Q-switched output laser beam 55 at a wavelength of 694 nm. Those skilled in the art, based on this disclosure, will recognize that, in addition to coupling optics 48, other optical configurations can be utilized to direct beam 48 through mirror 50 to directly optically pump Ruby crystal 52. For purposes of this disclosure, coupling optics 49 and such other optical configurations referred to above, including directly optically pumping the crystal without intervening optics between the laser 47 and mirror 49, are within the scope of the present invention and are considered to be means for directly pumping the ruby crystal gain element 52. Further, the system described in FIG. 5 is presented as a non-limiting example.
[0027] A variant of this embodiment renders the waveform of the output of the ruby laser as a sequence of mode-lock pulses with durations of a few picoseconds, obtained when the Q-switch 53 of FIG. 6 is replaced by a suitable saturable absorber, as known in the art.
[0028] Two additional embodiments of the present invention replace the high bandgap semiconductor laser diode emitting at a wavelength of 405 nm in the two embodiments of FIGS. 5 and 6, with high bandgap semiconductor laser diodes emitting in the green spectral region at a wavelength lying with the spectral breath of the broad green absorption band of ruby (see FIG. 2). When pumped at such a green wavelength, the quantum energy defect between pump and ruby laser photon energies is significantly reduced compared to that when a 405 nm pump wavelength is utilized. However, at present the wall plug efficiency of 530 nm green emitting high bandgap semiconductor laser is about 6%, and even much lower at the peak ruby absorption wavelength of 560 nm. In future, should the efficiency performance of green high bandgap semiconductor laser diodes be increased sufficiently, these additional embodiments may become more desirable than at present.
[0029] FIG. 7 shows a block diagram of the incorporation of the ruby laser embodiments of FIGS. 5 and 6 into a DUV laser system emitting radiation at a wavelength of 347 nm, generated by second harmonic conversion of the output radiation at a wavelength of 694 nm of a ruby laser of the embodiments of FIGS. 5 and 6 described above. In FIG. 7, high bandgap pump diode 56, pump diode beam 57, ruby laser 58 and ruby output beam 59 at a wavelength of 694 nm collectively represents the embodiments of FIGS. 5 and 6. The ruby laser output beam 59 is passed, in one embodiment, through a single NLO crystal 60 that, is phase matched to efficiently produce DUV output radiation beam 61 at a wavelength of 347 nm. In an alternate embodiment, crystal 60 comprises at least two non-linear optic (NLO) crystals that are cascade phase matched at a wavelength of 231 nm. In another embodiment, crystal 60 comprises at least three non-linear optic (NLO) crystals that are cascade phase matched at a wavelength of 173 nm.
[0030] A ruby laser of the embodiment of FIG. 5 has been constructed and operated. Referring to FIG. 5, the high bandgap semiconductor laser diode 39 is a Nichia NDV4B16 laser diode emitting 300 mW at a wavelength of 405 nm in a single spatial mode. Ruby laser crystal 44 is a rectangular parallel piped with a crystal thickness of 5 mm, doped with 0.05% Cr.sub.2O.sub.3. Optical cavity mirror 42 is a flat mirror with high transmission at a wavelength of 405 nm and high reflectivity at a wavelength of 694 nm. Optical cavity mirror 43 is a concave mirror with a 50 mm radius of curvature. The cavity mirrors are positioned to form a hemispherical optical cavity. FIG. 8 shows the achieved output power at a wavelength of 694 nm as a function of the pump laser diode pump power at a wavelength of 405 nm. The reflectivity of the output coupler mirror 43 used to obtain this performance was 95%. The output quantum slope efficiency is 27%, the power slope efficiency is 15%, and the optical-optical power conversion efficiency is greater than 10%. This test data confirms the efficacy of the disclosed ruby laser and validates that the disclose ruby laser may also be beneficially used construct various DUV sources described above.
[0031] As a further discussion of the manner of usage and operation of the present invention, the same should be apparent from the above description, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and describer in the specification are intended to be encompassed by the present invention.
[0032] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.
