A tunable, solid state laser device with both visible and infrared laser emission is developed with a trivalent ytterbium-doped yttrium calcium oxyborate crystal as the host crystal. The Yb:YCOB crystal generates an infrared fundamental light over a wide bandwidth, from approximately 980 nanometers (nm) to approximately 1100 nm. The bandwidth generated by the Yb:YCOB crystal is approximately 100 nm wide and supports the generation of pulsed infrared light or when self-frequency doubled provides a compact, efficient, source of tunable, visible, blue or green laser light in wavelengths of approximately 490 nm to approximately 550 nm.

BACKGROUND AND PRIOR ART
 The laser is a device for the generation of coherent, nearly
 single-wavelength and single-frequency, highly directional electromagnetic
 radiation emitted somewhere in the range from submillimeter through
 ultraviolet and x-ray wavelengths. The word laser is an acronym for the
 most significant feature of laser action: light amplification by
 stimulated emission of radiation.
 There are many different kinds of lasers, but they all share a crucial
 element: each contains material capable of amplifying radiation. This
 material is called the gain medium, because radiation gains energy passing
 through it. The physical principle responsible for this amplification is
 called stimulated emission. It was widely recognized that the laser would
 represent a scientific and technological step of the greatest magnitude,
 even before T. H. Maiman constructed the first one in 1960. Laser
 construction generally requires three components for its operation: (1) an
 active gain medium with energy levels that can be selectively populated;
 (2) a pumping process to produce population inversion between some of
 these energy levels; and usually (3) a resonant electromagnetic cavity
 structure containing the active gain medium, which serves to store the
 emitted radiation and provide feedback to maintain the coherence of the
 electromagnetic field.
 Many lasers have the capability to emit light over a tunable wavelength
 range. For a laser to be tunable in wavelength it must possess a laser
 gain medium whose spectral gain bandwidth is tunable, with temperature or
 by some other means. Alternatively, the spectral gain of the gain medium
 can be broad, and an additional wavelength-dependent loss element is added
 to the resonator to tune the laser emission to different wavelengths
 within the spectral gain curve.
 The lasers of the present invention use a new crystal material, trivalent
 ytterbium-doped yttrium calcium oxyborate crystals and are referred to
 herein as Yb.sup.3+ :YCa.sub.4 O(BO.sub.3).sub.3 or Yb:YCOB for easier
 reference.
 A Patent Corporation Treaty (PCT) application numbered WO 96/26464 reports
 the growth of calcium gadolinium oxyborate, GdCOB, as the first element of
 a new family of borate crystals, which includes YCOB. However, WO 96/26464
 does not disclose or suggest a tunable laser device comprising Yb:YCOB.
 In the prior art, there are no disclosures of Yb:YCOB being used as the
 active gain medium. Further, there are no teachings supporting the use of
 Yb:YCOB to generate tunable, self-frequency doubled, coherent, visible
 laser light or ultrashort infrared radiation pulses.
 Trivalent ytterbium-doped crystalline laser systems producing optical
 radiation are reported. U.S. Pat. No. 3,462,707 discloses Yb and Nd doped
 borate glass host for a non-radiative transfer of energy between Nd ions
 and Yb ions; there is no mention of frequency doubling. U.S. Pat. No.
 5,123,026 disclosed that a Yb-doped host crystal from the garnet family
 worked as a laser with a separate frequency doubling crystal located
 within the resonant cavity. Other Yb-doped host material are described in
 U.S. Pat. Nos. 5,280,492 and 5,381,428; the crystals from the classes of
 oxides, fluorides, fluoroapatite or glass. Frequency doubling is
 accomplished by a separate crystal placed in the laser cavity. Tuning is
 accomplished in U.S. Pat No. 5,381,428 with a birefringent tuning plate, a
 grating, or a prism also placed within the laser cavity. U.S. Pat. No.
 5,677,921 discloses a new class of laser crystals formed from Yb-doped
 borate fluoride host crystals; these crystals were found to be
 self-frequency doubling.
 More recently, the approach to generating high power, visible laser light
 has been to use nonlinear optical crystals to convert near-infrared
 radiation to the visible portion of the spectrum via second harmonic
 generation (SHG) (sometimes termed frequency doubling and used
 interchangeably, herein). This process generates a harmonic wavelength
 which is one-half of the fundamental wavelength. Since the SHG conversion
 efficiency is a function of the fundamental laser beam intensity, the
 nonlinear crystal is often placed inside the cavity of a low power
 continuous wave laser to benefit from the high intracavity fundamental
 beam intensity.
 Thus, in the search for smaller, less expensive, more powerful,
 multifunctional lasers, the discovery of a new class of laser hosts, the
 oxyborates, makes possible the combination of linear and nonlinear optical
 properties in a single active medium. More particularly, the
 ytterbium-doped oxyborate crystal (Yb:YCOB) of the present invention
 generates an infrared fundamental light over a relatively wide bandwidth,
 from approximately 980 nanometers (nm) to approximately 1100 nm. This
 approximately 100 nm range is a large bandwidth which could support the
 generation of pulsed infrared light or when self-frequency doubled
 provides a compact, efficient, source of tunable, visible blue or green
 laser light with wavelengths of approximately 490 nm to approximately 550
 nm.
 SUMMARY OF THE INVENTION
 The first objective of the present invention is to provide a tunable,
 infrared and visible light laser that combines the active gain medium and
 frequency doubler in one single element.
 The second objective of this invention is to provide a tunable
 self-frequency doubled (SFD) laser using the oxyborate family of crystals
 as the host crystal.
 The third objective of this invention is to provide a tunable, compact
 efficient source of visible laser light.
 The fourth objective of this invention would be to provide a source of
 ultrashort laser pulses by using Yb:YCOB active gain material in a
 mode-locked laser system.
 A preferred embodiment of the invention provides a ytterbium-doped
 oxyborate crystal (Yb:YCOB) pumped with continuous wave (cw) coherent
 titanium:Sapphire laser radiation or diode laser light at approximately
 905 nm or approximately 977 nm to efficiently generate approximately 530
 nm of green laser light.
 The optical pumping means which provides energy to the crystal can be
 selected from one of a coherent or incoherent light pumping source. The
 incoherent pumping source may be xenon or krypton lamps in the shape of a
 straight-line or spiral or annular or LED diodes, which can be of pulsed
 or continuous wave output. The coherent pumping source may be laser light
 source, such as a single laser diode or a matrix laser diode series, which
 can also be of pulsed or continuous wave output. Ytterbium systems are
 particularly well suited for diode pumping.
 Further objects and advantages of this invention will be apparent from the
 following detailed description of a presently preferred embodiment which
 is illustrated in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Before explaining the disclosed embodiment of the present invention in
 detail it is to be understood that the invention is not limited in its
 application to the details of the particular arrangement shown since the
 invention is capable of other embodiments. Also, the terminology used
 herein is for the purpose of description and not of limitation.
 The Czochralski method, as reported by Qing Ye and Bruce H. T. Chai in
 Journal of Crystal Growth, 197 (1999) 228-235; "Crystal growth of
 YCa.sub.4 O(BO.sub.3).sub.3 and its orientation" is used to grow the
 oxyborate crystal for the present invention. When rare-earth elements,
 such as, ytterbium are added during the crystal growing process, the
 crystal is said to be "doped" with the rare-earth ions. Doping changes the
 function of the crystal into an active gain medium. Undoped YCOB crystal
 is a nonlinear optical medium as disclosed in PCT application WO 96/26464.
 However, doping the YCOB crystal with a rare-earth, such as erbium or
 ytterbium, converts the crystal into a nonlinear optical laser crystal. To
 create a solid-laser device that is capable of tuning and self-frequency
 doubling, rare-earth elements for doping are selected from the group
 consisting of erbium, ytterbium, and mixtures thereof.
 When the newly formed crystal is grown from the melt, it is generally in
 cylindrical shape called a crystal "boule." The boule can be cut into a
 cylindrical rod or other geometric shapes. The flat ends are polished and
 given an appropriate reflective coating or anti-reflective coating. One
 end is more reflective than the other; laser light is emitted through the
 end mirror that is less reflective, i.e., the output coupler.
 Polarized absorption and emission spectra of Yb:YCOB are shown in FIG. 1;
 confirming that Yb:YCOB has a broad emission band between approximately
 980 nm and approximately 1100 nm, allowing for wide wavelength tunability.
 Undoped YCOB has been shown to have a nonlinear coefficient, deff of 1.1
 pm/V, which is between that of other nonlinear crystals KDP (0.37) and BBO
 (1.94 pm/V). See J. Appl. Phys. 36, 276 (1997) and W. Koechner, Solid
 State Laser Engineering, 4th ed. (Springer-Verlag New York, 1996), p. 579.
 Nonlinear crystals are needed for the frequency doubling laser action.
 In experiments, with ytterbium-doped YCOB crystal, several advantages of
 the Yb:YCOB active gain medium were explored. In addition to the
 approximately 100 nanometer (nm) wavelength tunability and the capability
 of self-frequency doubling the fundamental output, it was revealed that
 ytterbium possesses a wide range of 4f--4f vibrational transitions,
 resulting in broadband spontaneous emission. In the past, the usefulness
 of ytterbium was limited by the fact that there are no excitation
 manifolds accessible beyond the 4f manifold at 10,000 cm.sup.-1, therefore
 making flashlamp pumping inefficient. However, the broad absorption band
 near 900 nm is ideal for diode pumping with near infrared laser diodes,
 because it eliminates the need for precise control of diode temperature.
 Furthermore, the lack of higher energy levels is an advantage for diode
 pumping because it eliminates the possibility of energy loss due to
 excited-state absorption and up conversion. Another advantage of ytterbium
 is in the doping process; when ytterbium replaces yttrium, as in
 Yb:Y(yttrium)COB, there is virtually no lattice mismatch, and
 consequently, no concentration quenching.
 To reiterate, the broad spectral emission characteristics of ytterbium
 allow for wavelength tunability and an approximate 100 nm tuning range has
 been demonstrated using Yb:YCOB. This wide bandwidth has allowed the
 generation of ultrashort mode-locked pulses for applications in the
 infrared, and extension to visible wavelengths by virture of
 self-frequency doubling.
 A common approach to converting the laser wavelength to half its value, for
 example, from 1100 nm to 550 nm, often used to convert infrared lasers to
 laser emitting in the visible part of the spectrum, is to use intra-cavity
 frequency up conversion (IC). The most common IC approach is to
 incorporate a second crystal, a nonlinear optical crystal, correctly
 oriented for phase matching, inside the laser resonator, and to adjust the
 reflectivity of the cavity mirrors to maximize the wavelength converted
 laser light emission.
 It will be shown that the large bandwidth tunability of ytterbium and the
 nonlinear characteristics of YCOB are combined in Yb:YCOB to generate a
 tunable source of green visible light as well as an infrared light source.
 In FIG. 1 the polarized absorption and emission spectra of Yb:YCOB are
 shown. The two spectra capture the broad emission band of Yb:YCOB,
 allowing for a wide tunability range. The upperstate lifetime of Yb:YCOB
 is 2 to 3 ms, depending on dopant concentration; therefore, fewer diodes
 are required to store the same amount of energy.
 The laser apparatus into which the new laser materials may be incorporated
 is illustrated schematically in FIG. 2. This is just one of a number of
 optical cavity embodiments. Other embodiments of cavity configurations can
 include a simple linear cavity, hemispherical cavity, planar--planar
 cavity, or a ring cavity. In this particular embodiment, an X-Cavity
 configuration (20), contains a Yb:YCOB crystal (21) which is 10 mm long
 and was grown with a ytterbium dopant concentration of 10% in the melt and
 used as the active gain medium. Crystals were grown with Ytterbium dopant
 concentrations in a range from approximately 10 weight % to approximately
 44 weight % in the melt.
 The crystal cut is shown in detail in FIGS. 5(a) and 5(b). The crystal was
 cut such that the laser propagates parallel to the y-axis, and the crystal
 faces were cut with a 60.degree. Brewster angle to the x-axis,
 corresponding to a refractive index of 1.7, and with the E-field of the
 pump parallel to the z-axis of the crystal. As a surrogate for high power
 InGaAs laser diodes, the pump source was a Ti:Sapphire laser (22) tuned to
 900 nm with a maximum power of about 1.4 W. The pump beam (23) was focused
 into the cavity by a 12.5 cm focal length lens (24) into the cavity. The
 10 cm radius of curvature mirrors (25, 26) had a broadband reflectivity
 from 980 nm to 1220 nm and were positioned at the optimum astigmatic
 compensation angle of 24.degree. with respect to the pump beam. The high
 reflector (27) and the 2% output coupler (28) had about 100 nm bandwidth
 centered at 1064 nm. Tuning was accomplished by inserting a single plate
 birefringent filter (29) into the cavity between the focusing lens (26)
 and the output coupler (28). The crystal (21) was cooled on one side by a
 thermoelectric cooler set at 15.degree. C. The cavity was optimized for a
 minimum threshold of 184 mW absorbed pump power, with 0.33 W Ti:Sapphire
 incident upon the focusing lens.
 FIG. 3 shows the observed continuous wave (cw) output as a function of
 absorbed pump power, showing a slope efficiency of 24% and maximum output
 power to about 150 mW, which was reduced to about 120 mW by the
 introduction of the birefringent filter into the cavity.
 Another pumping embodiment utilizes single emitter diode pumping. In FIG.
 4, the diode-pumped output power versus the absorbed pump power for a 20%
 Yb:YCOB laser, with a 2% OC, is shown for 1050 nm and 1041 nm wavelengths
 and for 905 nm and 977 nm pumping, respectively. Slope efficiencies of 27%
 and 40% were obtained showing improved operation due to a smaller Stokes
 shift for 977 nm diode-pumping. Operation using both laser-diode pump
 wavelengths has shown tunable operation from 1030 nm to 1095 nm.
 Experiments demonstrating diode-pumped operation were performed using
 either a 905 nm, 940 nm, or 977 nm diode laser. The hemispherical laser
 resonator consisted of a flat, highly reflective rear mirror and a 10-cm
 radius of curvature output coupler (OC). The 20% Yb:YCOB laser rod was cut
 with the x-axis collinear with the laser axis. The temperature of the
 crystal was maintained at room temperature (.about.23.degree. C.) with a
 thermoelectric cooler. The pump laser polarization was parallel to the
 Z-axis and was focused into the crystal through the rear mirror. The rear
 mirror was highly reflecting from 1040 to 1150 nm and over 95% transparent
 at 977 nm.
 In addition to single emitter diode pumping, other pumping means may be
 used, such as, a diode laser bar; a diode laser array, including
 Ti:Sapphire diode; and a fiber-optically coupled diode laser source. The
 diode source wavelength is adjusted to a range between approximately 870
 nm and approximately 985 nm.
 FIG. 6 is an illustration of the tuning range of the Yb:YCOB laser. The
 high wavelength limit is set by the available gain at longer wavelengths
 and the reflectivity of the cavity mirrors. The lower wavelength limit can
 be affected by self absorption of unpumped laser material in the optical
 resonator. Optimal design of the pumping; crystal (i.e., length and dopant
 concentration); and resonator components should allow tuning over the
 entire gain curve from approximately 980 nm to approximately 1100 nm.
 As stated earlier, the resonant laser cavity configuration can be varied to
 include, but not be limited to, linear configurations, as well as
 hemispherical systems pumped by a tunable cw Ti:sapphire laser.
 The energy stored in a pumped solid-state laser medium can be delivered as
 a giant pulse in a short time by the use of Q-switching (quality factor
 switching). This technique can be applied in the present invention. It
 makes use of the idea that if the resonant cavity structure is maintained
 at a very low-Q level while the active medium is pumped, a high level of
 population inversion can be reached. If the cavity is then suddenly
 switched to a high-Q state, stimulate emission occurs rapidly, and
 radiation is emitted in a short pulse. Other means for generating short
 pulses that are suitable for the present invention, include, but are not
 limited to, electro-optic elements, or acousto-optic elements as the
 switch in the laser cavity. Mode-locking may also be applied to generate
 femtosecond pulses.
 It is shown that the new material, Yb:YCOB, is a promising laser crystal
 with wide emission bandwidth, broad absorption at approximately 900 nm,
 and non-linear properties that allow for the possibility of an
 inexpensive, rugged, and compact diode pumped tunable and/or mode locked
 laser system capable of generating ultrashort pulses, including sub-100
 femtosecond pulses in the infrared and coherent green visible light via
 self-frequency doubling.
 While the invention has been described, disclosed, illustrated and shown in
 various terms of certain embodiments or modifications which it is presumed
 in practice, the scope of the invention is not intended to be, nor should
 it be deemed to be, limited thereby and such other modifications or
 embodiments as may be suggested by the teachings herein are particularly
 reserved especially as they fall within the breadth and scope of the
 claims here appended.