YB-DOPED:YAB LASER CRYSTAL AND SELF-FREQUENCY DOUBLING YB:YAB LASER SYSTEM

Abstract: A nonlinear Yb:YAB laser material is disclosed. The material is capable of generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, the material being oriented for type 1 phase matching of the first wavelength laser light. Also disclosed is a laser system. The system comprises a pumping light source (102) emitting a pumping beam of light, a laser cavity (114) having an input coupler (108) operatively disposed with respect to the light source (102) so as to couple the pumping beam of light into the cavity (114), and a nonlinear Yb:YAB laser material (105) capable of lasing in response to a pumping beam of light thereby generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, the material being oriented for type 1 phase matching of the first wavelength laser light, the first wavelength laser light being in the range of 1020-1100 nm and the second wavelength laser light being in the range of 510-550 nm. The input coupler (108) comprises a reflector to at least partially reflect the first wavelength laser light and the second wavelength laser light into the cavity (114), and the laser cavity (114) further includes an output coupler (111) for coupling and outputting at least the second wavelength laser light from the cavity (114) as an output laser beam. Also disclosed are methods of providing an output laser beam and methods of using the beam.

Description: Yb-doped:YAB laser crystal and self-frequency doubling Yb:YAB laser system This invention relates to a nonlinear Yb:YAB laser material, a laser system, a method for providing an output laser beam from the laser system and methods of using 5 such an output laser beam. BACKGROUND ART Nd 3+ doped self-frequency-doubling (SFD) crystalline solid state lasers based on Nd :,' :LiNbO 3 (Nd: LN) and Nd 3+ :YAl 3 (BO 3 ) 4 (Nd: YAB), have been extensively studied since the first reports of their operation [1,2]. These lasers offer attractive simplicity for I D visible laser generation, but also suffer from a number of problems, largely associated with the active Nd 3+ ions, such as low quantum efficiency, high quantum defect, reabsorption loss in green and particularly difficulties of growth of the nonlinear laser material. As a result, SFD solid state lasers have not met with significant practical success. 1 5 More recently, Yb 3+ doped nonlinear crystalline materials have received attention as alternative SFD laser media. Yb 3+ has no concentration quenching, no excited state absorption, and no visible reabsorption loss [3], as well as offering high quantum efficiency, low quantum defect and potentially broad gain bandwidth. SFD green output of 60mW at 532nm has very recently been reported by Montoya et. al [4], for 2 0 Yb 3τ :LiNbO 3 :MgO crystals pumped by a Ti:sapphire laser; SFD green output at low power (<lmW) has also been observed for the nonlinear laser crystals Yb J+ :YCa 4 B 3 Oιo (Yb:YCOB) [5], and Yb 3+ :GdCa4B 3 O 10 (Yb:GdCOB) [6]. A recent report detailed studies of growth and spectral properties [7] and highly efficient diode-pumped infrared laser operation [8, 9] of the new nonlinear laser crystal 25 Yb 3 ":YAl 3 (BO 3 ) 4 (referred to herein as "Yb:YAB"). Yb:YAB has the advantages of comparatively easy growth (i.e. compared with Nd:YAB), large range of doping concentration (at least up to 20 at. %) at good crystal optical quality, large nonlinear optical coefficient (d ef r >1.4 pm/N), long radiative lifetime (~680μs) and good absorption and fluorescence spectral properties. Maximum output power of 654m W at 1040nm has 3o been obtained at an absorbed pump power-output power slope efficiency of 71 %, for pumping by a fibre-coupled 976nm diode [8, 9]. OBJECTS OF INVENTION Objects of the invention are to provide a nonlinear Yb:YAB laser material, a laser system, a method for providing an output laser beam from the laser system and methods 3 of using such an output laser beam. DISCLOSURE OF INVENTION According to one embodiment of the invention there is provided a nonlinear Yb:YAB laser material capable of generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light said material ^ being oriented for type 1 phase matching of the first wavelength laser light. According to another embodiment of this invention there is provided a laser system, said system comprising: a) a pumping light source emitting a pumping beam of light; b) a laser cavity having: I D (i) an input coupler operatively disposed with respect to the light source so as to couple the pumping beam of light into the cavity; and (ii) a nonlinear Yb:YAB laser material capable of lasing in response to a pumping beam of light thereby generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, said material 15 being oriented for type 1 phase matching of the first wavelength laser light, the first wavelength laser light being in the range of 1020 - 1 l OOnm and said second wavelength laser light being in the range of 510-550nm; (iii) said input coupler comprising a reflector to at least partially reflect the first wavelength laser light and second wavelength laser light into the cavity; and 2o (iv) the laser cavity further including an output coupler for coupling and outputting at least the second wavelength laser light from the laser cavity as an output laser beam. method of providing an output laser beam from a laser system, said method comprising: a " ) pumping a nonlinear Yb:YAB laser material capable of lasing in response to a 5 pumping beam of light thereby generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, said material being oriented for type 1 phase matching of the first wavelength laser light, the first wavelength laser light being in the range of 1020 - 1 1 OOnm and said second wavelength laser light being in the range of 510-550nm, with said pumping beam of light whereby the 3o Yb:YAB laser material lases in response to the pumping beam thereby generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, the first wavelength laser light being in the range of 1020 - 1 1 OOnm and said second wavelength laser light being at or about one half the wavelength of the first wavelength laser light, the second wavelength laser light being in the range of 510-550nm: and b ' ) coupling and outputting at least the second wavelength laser light from the laser cavity as an output laser beam. Thus one form of the laser system comprises: a) a pumping light source emitting a pumping beam of light, typically o- ^ polarised: b) a laser cavity having: (i) an input coupler operatively disposed with respect to the light source so as to couple the pumping beam of light into the cavity; and (ii) a nonlinear Yb:YAB laser material, generally a Yb:YAB laser crystal, i n oriented for type 1 phase matching, the Yb:YAB laser material being material which lases in response to the pumping beam thereby generating lundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, said second wavelength laser light being at about one half the wavelength of the first wavelength laser light, the second wavelength laser light being in the range of 510- i - 550nm: (iii) said input coupler comprising a reflector to at least partially reflect the first wavelength laser light and second wavelength laser light into the cavity; (iv) the laser cavity further including an output coupler for coupling and outputting at least the second wavelength laser light from the laser cavity as an output o laser beam. One form of the method of providing an output laser beam from a laser stem comprises: a " ) pumping a nonlinear Yb:YAB laser material, generally a Yb:YAB laser crystal, oriented for type 1 phase matching, with a pumping beam of light, typically o- ^ polarized, whereby the Yb:YAB laser material lases and generates fundamental o- polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, said second wavelength laser light being at about one half the wavelength of the first wavelength laser light, the second wavelength laser light being in the range of 510-550nm; and 30 b " ) coupling and outputting at least the second wavelength from the laser cavity as an output laser beam. The invention also includes a method of using laser light for displaying laser light on a selected area comprising illuminating the selected area with the output laser beam of the invention. The pumping beam of light may be unpolarised or polarised. In one form of the invention the pumping beam of light and the first wavelength laser light are o-polarised and the second wavelength laser light is e-polarised. In another form of the invention the pumping beam of light is unpolarised. the first wavelength laser light is o-polarised and 5 the second wavelength laser light is e-polarised. The efficiency of power conversion of the pumping beam oi " light to the second w avelength laser light is typically in the range of 0.1 - 13%+. The laser system of the invention may be in the form of discrete components or integral components or a combination of both. i n One of the advantages of the laser system of the invention is that it is scalable to high power of pumping light (e.g. from 0.001 Watt up to 60 Watt of pumping light. 10 Watt up to 60 Watt). An example of a suitable pumping light source is a fibre-coupled InGaAs diode laser, power in the range 1 W - 20W. more typicalh 10 - 1 5 W ( pumping pow er) will, in part, determine output power of the laser material), fibre diameter 400μm, i - numerical aperture 0.16, operating at a frequency in the range 975nm - 980nm or at 975nm. 976nm or 977nm with a bandwidth in the range of l -5nm. The pumping light source may be in the form of a diode array. Associated with the array are means to operate the array in a continuous or pulsed manner or other variable manner depending on the required use of the resultant output laser beam (i.e. whether a continuous or pulsed or 2o otherw ise variable output laser beam is required) from the laser cavity. The cavity may further include means to select and/or tune the wavelength of the output beam. Typically the means to tune is a quartz birefringent filter which is inserted into the cavity to tune the laser system. The means to select the wavelength of the output beam may be linked or coupled to the Yb:YAB laser material or may be separate from the 25 Yb:YAB laser material. The output coupler may be a highly reflecting output coupler. Typicalh an RoC coupler is used (coated HR at 1020-1 1 OOnm). Typically an output RoC coupler having a radius of curvature in the range of 1 - 12 cm. more typically 1 , 2.5. 5, 7.5 or 10 cm is used. 3o The Yb:YAB material may be cooled. Thus the laser system ma) include means for cooling the Yb:YAB material. The laser system may be gas cooled (e.g. air cooled). One means for cooling is a Peltier temperature controller. The Yb:YAB material may be cooled while it is being pumped with the pumping light. Typically the means for cooling is capable of cooling the Yb:YAB material to and maintaining the material at a 3^ temperature (during pumping of the laser material) in the range of -10°C to 40°C, or - 1 ()°C to 25°C. 0°C to 25°C, typically 0°C to 20°C, more typically 5 to 15°C and even more typically 0°C to 5°C. More typically the Yb:YAB material is cooled to and maintained at 0, 2, 4, 5. 8. 10. 12, 14, 15, 18, 20, 22, or 25°C. The invention includes a method of using laser light for displaying laser light on a selected area comprising illuminating the selected area with the output laser beam of the invention. The invention includes a method of using laser light for displaying laser light on a selected area comprising illuminating the selected area with the output laser beam of the ention. I he cavity of the laser system may be configured to operate within a narrow bandwidth or in a single axial mode. The invention includes a nonlinear Yb:YAB laser material cut and oriented for type 1 phase matching of the first wavelength laser light. Typically the Yb:YAB laser material is a crystal that is cut and oriented for type 1 phase matching of the first wavelength laser light. Typically the Yb:YAB laser material is a crystal that is cut and oriented for type 1 phase matching of the first wavelength laser light at normal incidence (θ ~ 31 °±5°, φ = 0°). The Yb:YAB crystals may be grown by the high temperature flux method to yield comparatively large crystals with high optical quality (see reference [7] ). The first wavelength laser light is in the range of 1020 - 1 l OOnm. More typicalh the first wavelength laser light is in the range of 1040 - 1068nm and the second w-avelength laser light is in the range of 510-550nm, typically 513-545.8nm and more typically 520 - 534nm. The second wavelength laser light may be tuned to specific w avelengths within these ranges if required e.g. 514nm or 532nm. The second wavelength laser light may be tuned to a bandwidth of 0.2nm if required. The type 1 phase matching angle is in the range θ ~ 31 °+5°, φ = 0°, or θ « 31 °±5°, φ = 0°. or θ * 3 1 °±3°. φ = 0°, typically θ ~ 31 °±2°. φ = 0°, more typically θ ~ 31 0 +1 °, φ = 0° and even more typically θ ~ 31 °, φ = 0°. Typically the type 1 phase matching angle is chosen for optimum operation of the laser system whereby the power output of the laser output beam is substantially optimum (however, the invention also includes a laser system and a method of providing an output laser beam from a laser system where the phase matching angle of the nonlinear Yb:YAB laser material oriented for type 1 phase matching, is non optimal). This will be dependent on the temperature range in which one operates the laser system. A type 1 phase matching angle of θ « 31°±2°, φ = 0°, more typically θ ~ 31 °±1 °. φ = 0° is typically chosen for an operating temperature range of 25°C±25°C. more typically 25°C±20°C. The amount of Yb doping in the Yb:YAB crystal is typically in the range l -30atom%. more typically l -20atom%. more usualh ' 10±7 atom%. usually 10±5 atom%. even more usually 10±2 atom%. Typically, the amount of Yb doping in the Yb:YΛB crystal is about 1 , 2. 3. 3.5, 4. 4.5, 5. 5.5, 6. 6.5. 7. 7.5. 8. 8.5, 9, 1 0. 1 1 . 12. 13. 14. 1 5 or 20 atom%. Typically the crystal is of the order of l -6mm long, more typically 2- - 4mm and usually about 3-4mm long. Typically the Yb ' AB cr\ stal is antirefiection coated for pump and laser wavelengths. The laser system may be a laboratory (e.g. scientific or medical laboratory ) or industrial scale. Alternatively, the laser system may be portable. The invention includes a method of using laser light for monitoring blood ιo comprising illuminating the blood with the output laser beam oi ' the invention and monitoring any changes in the laser beam after it has interacted w ith the blood. The invention includes a method of using laser light for treating, delecting or diagnosing a selected area requiring such diagnosis or treatment on or in a subject comprising illuminating the selected area with the output laser beam of the invention. Typically the i - method further comprises detecting the output laser beam after it has interacted w ith the subject. Typically the selected area is illuminated with the output laser beam having the second wavelength for a time and at a power level which is appropriate and effective for the diagnosis or therapeutically effective for the treatment. The output laser beam having the second wavelength may. depending on the application, be continuous, pulsed or o otherw ise variable. In the event that a pulsed output laser beam having the second wavelength is required for medical applications the pulses are typically in a range selected from the group consisting of 1 to 650, 1 to 600, 1 to 550. 1 to 500. 1 to 450. 1 to 400. 1 to 350. 1 to 250, 1 to 150. 1 to 100, 1 to 50, 1 to 25, 1 to 10. 1 to 5. 2 to 20, 2 to 10 and 5 to 10 milliseconds. For example, the pulsed output laser beam may be at a pulse rate 2^ selected from the group consisting of 1 , 2, 3, 4, 5, 6, 7, 8. 9. 10, 1 1 , 12, 13. 14, 15. 16. 17, 1 8, 19. 20. 25, 30, 35. 40. 45. and 50 milliseconds. The pulse rate is one selected as being suitable for the desired application. Examples of medical applications include dermatological applications, scalp applications and ophthalmic applications. For medical applications a typical pulse rate is 1 , 5, 10, 15 or 20 milliseconds. The output laser beam o having the second wavelength is particularly useful in medical applications (such as certain dermatological applications) requiring coagulation of blood because light of the second wavelength is absorbed by blood. The subject may be a mammal or vertebrate or other animal or insect, or fish or tissue from such an animal. Typically the subject is a mammal or vertebrate which is a 3 bovine, human, ovine, equine, caprine. Leporine, feline or canine vertebrate. Advantageously the vertebrate is a bovine, human, ovine, equine, caprine. Leporine, domestic fowl, feline or canine vertebrate. The cavity may include at least one Q-switch such as an active Q-switch or a passive 0 switch. An acousto-optical or electro-optical Q-switch can be used. 5 Alternatively a cavity dumping configuration or other suitable means can be adopted ( see "The Laser Guidebook '" by Jeff Hecht, 2 nd edition. McGraw-Hill 1992. the w hole content of which is incorporated by cross reference). The cavity ma\ include one or more etalons (e.g. (a) one or more li ce standing elalons: (b) an air etalon as shown in Figs. 1 and 5: and/or (c) an integral etalon which is I D added on to the nonlinear Yb:YAB laser material oriented for type 1 phase matching, via deposition or other suitable means (e.g. a composite microchip w ith an etalon grow n on i t ) ) . The cavity ma\ include al least one polariser (generally tw o polarisers ). In one form the cavity is configured b\ including means to mode lock the laser light i - such that the output laser beam is mode-locked Typically an active or passive mode locker is disposed in the cavity. It is particularly of advantage to mode lock the output laser beam to provide pulses in the range short pulses ( = 10 " ' seconds) to medium short (~ 10 ~6 -10 ~ ' seconds, typically -10 " ' seconds) to very short (- 10 " -1 0 " ' seconds, typically ~ 10 "1 ' seconds) to ultrashort (~10 ~ l 2 - 10 " ' ' seconds, typically - 1 0 " ' 1 seconds). 2o BRIEF DESCRIPTION OF DRAWINGS Figure 1 : The setup diagram of self-frequency-doubling Yb:YAB laser experiment; Figure 2: (a) Infrared laser emission spectrum, with the etalon effect; and (b) green laser emission spectrum of Yb:YAB SFD lasers: Figure 3 : Infrared and green output power as a function of incident pump pow er. 25 fhe crystal mount temperature is 20°C; Figure 4: Green output power as a function of wavelength tuned by a 1.32mm-thick quartz birefringent filter (B. F.); Figure 5 : Schematic diagram of a laser system of the invention; and Figure 6: Schematic diagram of an alternative laser system of the invention. o BEST MODE AND OTHER MODES FOR CARRYING OUT INVENTION Figure 5 depicts a laser system 100. System 100 comprises optical fibre 101 which is coupled to laser diode 102 (typically λ ~ 976nm±5nm). In use, a pumping beam of light (typically λ ~ 976nm) emerging from end 103 of optical fibre 101 (as an alternative to an optical fibre one could use a suitable combination of lenses or no lenses at all) is 3 5 imaged onto 10 atom % Yb doped Yb:YAB crystal 105 to give an appropriate pump mode diameter of the pumping beam of light on crystal 105 via collimating lens 104. dichroic mirror 106. focus lens 107 and flat input mirror 1 08. Crystal 105 is cut with a type 1 phase matching angle (θ ~ 31 °, φ = 0°) for l μm obtained by calculation from the Sellmeier equations of Yb:YAB refractive indices and is typically polished to give optimum type- 1 phase matching for normal incidence. Dichroic mirror 106 is typically highly reflecting ("HR") in the range 510-550nm. and highly transmitting for the frequency of the pumping beam of light (typicalh' λ = 976nm). A suitable combination of local lengths for collimating lens 104 and focus lens 107 is l ' L =3. 1 mm for lens 104 and l ' ι=4.5 1 mm for lens 107. although it will be understood that other suitable combinations of in l " c and I may be used as required. Flat input mirror 108 is highly transmitting for pump l ight (typically λ - 976nm), reflecting for light in the range 51 0-550nm and highly reflecting for fundamental first wavelength laser light (typically - l μm. more typically 1 20nm- l l OOnm) generated when crystal 105 lases in response to pumping with a pumping beam of light. Crystal 105 is held in holder and temperature controller 109 15 (typically a copper holder and a Peltier temperature controller) to control and maintain the temperature of crystal 105 in use at a desired temperature or within a desired temperature range in use. Crystal 105 is located within the laser cavity 1 14 (which is defined by mirror 1 8 and output coupler 1 1 1 as depicted) in close proximity to mirror 108 so as to form thin air-space etalon 1 13. As depicted in Fig. 5 coupler 1 1 1 is a 10cm radius of curvature 0 output coupler which is highly transmitting in the range 510-550nm and highly reflecting for fundamental first wavelength laser light generated when crystal 105 lases in response to pumping with a pumping beam of light. Cavity 1 14 includes birefringent filter 1 10 which may be used to tune cavity 1 14. Cavity 1 14 may also include an active or passive Q switch and/or an active or passive mode locker. Alternatively a Q sw itch may be 25 located outside cavity 1 14 either between coupler 1 1 1 and filter 1 12 or after filter 1 12. Filter 1 12 is typically a band pass filter which transmits light in the range 510-550nm and does not substantially transmit the fundamental first wavelength laser light. In use, a pumping beam of light from diode 102 which is coupled to optical fibre 101 is imaged onto 10 atom % Yb doped Yb:YAB crystal 105 to give an appropriate 3o pump mode diameter of the pumping beam of light on crystal 105 via collimating lens 104. dichroic mirror 106, focus lens 107 and flat input mirror 108. As a result the nonlinear Yb:YAB laser material oriented for type 1 phase matching, lases and generates fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light in cavity 1 14 the second wavelength laser light being at 35 about one half the wavelength of the first wavelength laser light, the second wavelength laser light being in the range of 510-550nm. At least the second wavelength laser light is coupled and outputted from cavity 1 14 as an output laser beam and is filtered by filter 1 1 2. Figure 6 depicts an alternative laser system 200. System 200 comprises optical fibre 201 which is coupled to laser diode 202 (typically λ = 976nm±5nm). In use. a pumping beam of light (typically λ ~ 976nm) emerging from end 203 of optical fibre 201 (as an alternative to an optical fibre one could use a suitable combination of lenses or no lenses at all) is imaged (typically the pumping beam of light is o-polarised) onto a 10*5 atom % Yb doped Yb:YAB crystal 205 to give an appropriate pump mode diameter of the pumping beam of light on crystal 205 via collimating lens 204. dichroic mirror 206. focus lens 207 and flat input coating 208 (HR^l μm. I l l » 976nm. HRX 510-550nm ). Crystal 205 is cut with a type 1 phase matching angle (θ ~ 1 °. φ = 0°) for l μm obtained by calculation from the Sellmeier equations of Yb:YAB refract e indices and is typicalh polished to give optimum type-1 phase matching for normal incidence. Dichroic mirror 206 is typically highly reflecting ("HR") in the range 510-550nm. and highly transmitting for the frequency of the pumping beam of light (typically λ ~ 976nm). A suitable combination of focal lengths for collimating lens 204 and focus lens 207 is f L =3.1 mm for lens 204. and, f f =4.51 mm for lens 207, although it will be understood that other suitable combinations of f c and fr may be used as required. Flat input coating 208 is highh transmitting for pump light (typically λ ~ 976nm). reflecting for light in the range 510- 550nm and highly reflecting for fundamental first wavelength laser light generated when crystal 205 lases in response to pumping with a pumping beam of light. Crystal 205 may be held in a holder and temperature controller (not shown but typically a copper holder and a Peltier temperature controller) to control and maintain the temperature of crystal 205 in use at a desired temperature or within a desired temperature range in use. Crystal 205 is located within the laser cavity 214 (which is defined between coating 208 and output coupler 21 1 as depicted). As depicted in Fig. 6 coupler 21 1 is a coating on a layer of passive dielectric material 212. Coupler 21 1 is highly transmitting in the range 510- 550nm and highly reflecting for fundamental first wavelength laser light generated when crystal 105 lases in response to pumping with a pumping beam of light (typically - l μm. Cavity 214 includes partially reflecting coating 213 to tune cavity 214. An optional passive Q switch 215 is located outside cavity 214 on coating 21 1 (Cr 4+ :YAG is a possible passive Q switch material). An optional mode locking material may also be included in the structure if required. Filter 216 is typically a band pass filter which transmits light in the range 510-550nm and does not substantially transmit the fundamental first wavelength laser light. The ratio of the length of crystal 205 to the length of material 212 should be chosen so as not to be an integer ratio. In use. a pumping beam of light from laser diode 202 (eg frequency of pumping beam of light of 975 or 976nm) which is coupled to optical fibre 201 is imaged onto 10±5 atom % Yb doped Yb:YAB crystal 205 to e an appropriate pump mode diameter of the pumping beam of light on crystal 205 via collimating lens 204. dichroic mirror 206. focus lens 207 and flat input coating 208. As a result the nonlinear Yb: YAB laser material oriented for t} pe 1 phase matching, lases and generates fundamental o-polarized first wavelength laser light (~1020nm - 1 1 OOnm ) and frequency doubled e-polarized second l o wavelength laser light in cavit\ 214 the second \va\ elength laser light being at or about one half the w avelength of the first wavelength laser light, the second wavelength laser l ight being in the range of 510-550nm. At least the second w avelength laser light is coupled and outputted from ca\ it} 214 (e.g. an appropriate radius of curvature (RoC) output coupler) as an output laser beam and is filtered filter 216. ι EXAMPLE In this example, we report for the first time efficient CW self-frequency-doubling green laser operation of a ty pe-I phase-matched 3mm-thick Yb:YAB crystal pumped by a 976nm fibre-coupled diode. Tunable green output from 513-545.8nm has also been demonstrated. o A 10 at.% Yb doped Yb:YAB crystal w as roughh cut with a type-I phase matching angle (θ~3 1 °. φ=0°) for l μm obtained by calculation from the Sellmeier equations of Yb:YAB refractive indices. The crystal was then carefully reoriented to give the strongest 532nm green output power with the input of a pulsed 1064nm Nd:YAG laser, and polished to gi\ e optimum type-I phase matching for normal incidence. The crystal of 25 dimension 3mmx3mmx3mm was uncoated for a later laser experiment. The polarized absorption coefficients at 976nm were 15cm " and 12cm " ' for o-ray and e-ray. respectively, with an absorption bandwidth 22nm (FWHM). The pump and laser cavity configuration used in the present experiments is shown in Figure 1 . The 976nm pump light from a 50μm core-fiber-coupled 1 .6W laser diode was o imaged through a flat end-mirror onto the crystal using a 1=3. l mm collimating lens and a f=4.51mm focusing lens to give.a pump mode diameter of approximately 73μm. The Yb:YAB crystal was held in a temperature controlled copper mount. The characteristics of the pump end-mirror coating are critical because a sharp edge between transmission at the pump wavelength and reflection at the laser wavelength (l Ol Onm- 3 1 l OOnm) is required. The coating used for the present experiment had transmission 93% at 976mn and reflection > 99.8% from 1010- 1 l OOnm, and also 80% transmission in the green ( Lambda Research Optics). A 10cm radius-oi ' -curvature output coupler (transmission~94% in the green and reflection >99.8% al 1 1 0-1 1 OOnm) was used to complete the Yb:YAB laser cavity, which w as of overall length approximately 10cm. A 1 .32mm-thick single-plate quartz birefringent filter w as inserted into for experiments in tunability. The SFD green output power was measured at both ends of the . at one end directly from the output coupler, and at other end. via a 45° dichroic mirror ( HR at green and HT at 976nm) placed between the collimating and focusing lenses. The green output powers quoted herein refer to the sum of SFD green obtained f rom both ends of the laser cavity (typicalh . output power from the coupler w as 80% of the total power, although this was quite dependent on adjustment). Because Yb ,+ :YAB is a quasi-four level sy stem, it is expected that laser emission at the fundamental (1R) will be shifted to longer w a\ elength for low loss ca\ ities due to the reduced reabsorption losses at longer wavelength. For example, the absorption coefficient at 1 061 nm is less than 0.07cm " : while the absorption coefficient at 1040nm is approximately 0.28 cm " 1 , for the Yb:YAB crystal used in the present experiment. For SFD operation in the present experiment where the output coupler used had a broad-band high reflective coating from 1010- 1 l OOnm. the fundamental output wa\ elength w as shifted to 1061 nm and operated with a bandw idth of up to 12nm (note that for a 4 % output coupler, the free-running wavelength of the fundamental was 1040nm [9]). To narrow the bandwidth of fundamental laser emission, the distance and parallelism between the flat-input mirror and uncoated input face of the Yb:YAB crystal ere aclμisted to form a thin air-space etalon. as shown in Figure 1 . The resulting infrared and green laser emission spectra are shown in Figure 2 (a) and (b) respectively, measured by an optical spectrum analyzer (Anritsu Co. MS9030A). Three infrared emission peaks were observed at 1061.3nm, 1060.2nm and 1059. Onm with a bandwidth of 0.4nm. and wavelength separation corresponding to a free spectral range of the etalon with spacing 510μm. The main emission peak in the green w as at 530.6nm with a bandwidth of 0.2nm at total output green power of 143mW. It is anticipated that single-frequency operation can be achieved readily with appropriately designed intraca\ it} * etalons. Figure 3 shows measured SFD green and residual infrared output powers as a function of incident pump power. The crystal mount temperature was set at 20°C using a Peltier temperature controller. The maximum incident pump power (unpolarized) onto the crystal was 1400mW and more than 90% of the pump power was absorbed by the crystal. The pump power at threshold for both infrared and green w as 1 50mW. A maximum of 80mW residual o-polarized infrared output was obtained after the output coupler. The maximum e-polarized SFD green output power was 143mW, corresponding to an incident pump power-green output power conversion efficiency of 10.2%. The green output power increases quadratically with the incident pump power, indicating that the pump-green conversion efficiency can be increased further with increasing pump po er. Table 1 shows results of an investigation of the effects of the Yb:YAB crystal mount temperature on threshold pump power, maximum green output power and pump-green conversion efficiency. Table 1 . Temperature effect of the crystal on threshold pump power. maximum green output power and pump-green conversion efficienc\ at incident pump power 1400mW The maximum green output power of 160mW was obtained for a crystal mount temperature of 8°C, giving an incident pump power-green output pow er conv ersion efficiency of 1 1.3% and electrical input to green power conversion efficienc} of 3.9%. Note the threshold pump power increased quite rapidly and green output pow er decreased for crystal mount temperature below 8°C; the reason for this is not clear at present. TEMQ O mode for the green output at full power was obtained for laser cavity alignment adjusted to minimize the effects of beam walk-off. To the limit of the presently available pump power, we saw no evidence of the effects of thermally-induced distortion, including thermal lensing or optical damage of the Yb:YAB crystal. For investigation of wavelength tunable operation, a 1 .32mm-thick quartz single- plate bircfringent filter was inserted into the cavity as indicated in Figure 1 . Green output power as a function of laser wavelength is shown in Figure 4. The total tunable range w as about 33nm. from 5 13.0nm to 545.8nm with a bandw idth typicalh ' 0.4nm and the maximum output power was 17.3mW at 529. l nm. The crystal w as not adjusted for optimum phase matching angle during the tuning process, demonstrating that Yb:YAB has a broad spectral acceptance bandwidth. The CW green output powers achieved in the present experiments are the highest reported for any Yb ,_ SFD materials by a considerable margin (factor of 3) and indeed compare favorably w ith the highest power reported for a diode-pumped Yb:YAG laser incorporating TP as the intracavity frequency -doubling medium [ 1 1 ]. The visible tuning range of 33nm achieved for Yb:YAB also exceeds that reported for the KTP/Yb:YAG configuration [12]. In summary, efficient CW self-frequency-doubled green laser output of 160mW has been obtained from Yb:YAl 3 (BO 3 ) crystals, pumped 1 .4W incident pow er from a liber-coupled 976nm laser diode. The incident pump power-green output pow er conversion efficiency is over 1 1.3% and electrical input-green conversion efficienc\ is 3.9%. Tunable green output from 513.0nm-545.8nm is also demonstrated, using a quanz bircfringent filter. In conclusion, we have demonstrated for the first time efficient CW self-frequenc} - doubling green laser output and wavelength tunabilty over 33nm in the visible from diode-pumped Yb:YAB lasers. Relative ease of growth and favorable optical and thermal properties suggest that Yb:YAB has considerable potential as a practical laser material. References: 1 . V. G. Dmitriev. E. V. Raevskii. N. M. Ruina, L. N. Rashkovich. O. O. Silicheyand A. A. Fomichev, Sow Tech. Phys. Lett., 5, 590 (1979). 2. L. M. Dorozhkin, I. I. Kuratev, N. I. Leonyuk, T. I. Timchenko and A. Y. Shestakov, Sov. Tech. Phys. Lett. 7. 555 (1981 ). 3. P. Lacovara, H. K. Choi, C. A. Wang, R. L. Aggarwal. and T. Y. Fan, Opt. Lett. 16. 1089 ( 1991 ). 4. E. Montoya, J. Capmany, L. E. Bausa, T. Kellner, A. Diening. and G. Huber. Appl. Phys. Lett. 74, 31 13 (1999). 5. D. Λ. Hammons. J. M. Eichenholz, Q. Ye. B. H. T. Chai. L. Shah. R. E. Peals, M. Richardson, and H. Qiu, Opt. Comm.156.327 (1998). 6 F. Mougel, K. Dardenne. G. Aka. A. Kahn-Harari. and D. Vivien. J. Opt. Soc. Am. B 16.164(1999). 7. P. Wang. J. M. Dawes. P. Dekker. D. S. Knowles. J. A. Piper, and B. S. Lu. J. Opt. Soc. Am. B 16.63 (1999). 8. P. Wang. J.M. Dawes. P. Dekker. and J. A. Piper. OSΛ TOPS. Vol.26.614(1999) 9. P. Wang. J. M. Dawes. P. Dekker. and J. A. Piper. Opt. Comm.174.467 (2000) 1 . Λ. A. Lagatsky. N. V. Kuleshov. and V. P. Mikhailov. Opt. Comm.165.71 (1999) 11. T. Taira. J. Saikawa. E. Yamaguchi T. Kobayashi. and R. L. >er. OSA TOPS 19. 1 9(1999) 12. J. Saikawa. S. Kurimura. I. Shoji. and T. Taira. OSΛ 1 echnical digest of ΛSSL'2000. MB1

Claims: CLAIMS 1 A nonlinear Yb:YAB laser material capable of generating fundamental o-polarized i rst w avelength laser light and frequency doubled e-polarized second wavelength laser light, said material being oriented for type 1 phase matching of the iirst wavelength laser l ight. 2. The laser material of claim 1 wherein the first wavelength laser light is in the range l ' 1 020 - 1 l OOnm and the second wavelength laser light is in the range of 5 10 - 550 nm. The laser material of claim 1 or 2 wherein the Yb:YAB laser material is oriented at a type 1 phase matching angle in the range selected from the group consisting of θ ~ 3 1 °=5° and cp = 0°. 0 * 31 °±4° and φ = 0°. θ * 31 °±3° and φ = 0°. 0 * 3 1 °±2° and φ = 0°. and 0 ^ 3 1 °±l o and φ = 0°. 4 The laser material of claim 1 wherein the Yb:YAB laser material is oriented at a ι\ pe 1 phase matching angle of θ ~ 3 1 ° and to = 0°. 5. The laser material of claim 1 wherein Yb is present in the Yb:YAB laser material in an amount in the range of 1 to 30atom%. 6 Λ laser system, said system comprising: a) a pumping light source emitting a pumping beam of light; b) a laser cavity having: (i) an input coupler operatively disposed with respect to the light source so as to couple the pumping beam of light into the cavity: and (ii) a nonlinear Yb:YAB laser material capable of lasing in response to a pumping beam of light thereby generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, said material being oriented for type 1 phase matching of the first wavelength laser light, the first avelength laser light being in the range of 1020 - 1 1 OOnm and said second wavelength laser light being in the range of 510-550nm: (iii) said input coupler comprising a reflector to at least partially reflect the first wavelength laser light and second wavelength laser light into the cavity; and (iv) the laser cavity further including an output coupler for coupling and outputting at least the second wavelength laser light from the laser cavity as an output laser beam. 7. The laser system of claim 6 wherein the Yb:YAB laser material is oriented at a type 1 phase matching angle in the range selected from the group consisting of θ « 3 1 °±5° and φ = 0°, θ « 31 °±4° and φ = 0°. θ « 31 °±3° and cp = 0°, 0 ~ 3 1 °±2° and φ = 0°. and θ ~ 31 °+l o and φ = 0°. 8. The laser system of claim 6 wherein the Yb:YAB laser material is oriented at a type 1 phase matching angle of θ ~ 31 ° and φ = 0°. 9. The laser system of claim 6 further including means to tune the wavelength of the output beam. 1 . The laser system of claim 6 further including means to select the w avelength of the output beam. I 1 . The laser system of claim 6 wherein the laser cavity has at least one etalon. 12. The laser system of claim 6 wherein Yb is present in the Yb:YAB laser material in an amount in the range of 1 to 30atom%. 1 3. A method of providing an output laser beam from a laser system, said method comprising: a " ) pumping a nonlinear Yb:YAB laser material capable of lasing in response to a pumping beam of light thereby generating fundamental o-polarizcd first w avelength laser light and frequency doubled e-polarized second wavelength laser light, said material being oriented for type 1 phase matching of the first wavelength laser light, the first wavelength laser light being in the range of 1020 - 1 l OOnm and said second wavelength laser light being in the range of 510-550nm, with said pumping beam of light whereby the Yb:YAB laser material lases in response to the pumping beam thereby generating fundamental o-polarized first wavelength laser light and frequency doubled e-polarized second wavelength laser light, the first wavelength laser light being in the range of 1020 - I I OOnm and said second wavelength laser light being at or about one half the wavelength of the first wavelength laser light, the second wavelength laser light being in the range of 5 1 0-550nm; and b " ) coupling and outputting at least the second wavelength laser light from the laser cavity as an output laser beam.

Inventors: BLOWS JUSTIN LAURENCE, DAWES JUDITH MARGARET, DEKKER PETER, PIPER JAMES AUSTIN, WANG PU, BLOWS, JUSTIN, LAURENCE, DAWES, JUDITH, MARGARET, DEKKER, PETER, PIPER, JAMES, AUSTIN, WANG, PU
Applicants: BLOWS JUSTIN LAURENCE, DAWES JUDITH MARGARET, DEKKER PETER, MACQUARIE RES LTD, PIPER JAMES AUSTIN, WANG PU, BLOWS, JUSTIN, LAURENCE, DAWES, JUDITH, MARGARET, DEKKER, PETER, MACQUARIE RESEARCH LTD, PIPER, JAMES, AUSTIN, WANG, PU
Application year: 2001
Main classification: H01S3
IPC codes: H01S3