Apparatus for providing optical radiation

Abstract

In one embodiment, a photo-darkening resistant optical fibre includes a waveguide having a numerical aperture less than 0.15. The waveguide includes a core having a refractive index n 1 and a pedestal having a refractive index n 2, and wherein the fibre includes a first cladding having a refractive index n 3 surrounding the pedestal, wherein n 1 is greater than n 2, n 2 is greater than n 3. The core includes silica, a concentration of alumina of between approximately 0.3 to 0.8 mole percent, a concentration of phosphate of substantially 15 mole percent, a concentration of ytterbium substantially in the range 20000 to 45000 ppm. The pedestal can include silica, phosphate and germania. The core can have substantially zero thulium dopant.

1 . An apparatus for providing optical radiation, said apparatus comprising a seed laser for emitting seeding radiation, at least one optical amplifier, a reflector and a controller, wherein:
 the seed laser comprises a semiconductor laser;   the seeding radiation comprises a plurality of seed laser pulses;   the seed laser is connected to the optical amplifier via the reflector;   the seed laser pulses are amplified by the optical amplifier to produce optical radiation;   the optical radiation comprises output pulses;   the output pulses are characterized by a peak power, a pulse energy, and a pulse repetition frequency;   the reflector is arranged to reflect a proportion of the seeding radiation emitted by the seed laser back into the seed laser; and   the controller controls the seed laser to vary the shape of the seed laser pulses to maintain the peak power above 3 kW, the pulse energy in excess of 0.04 mJ, and the pulse repetition frequency at 1 Hz-500 kHz.   
     
     
         2 . The apparatus according to  claim 1 , wherein the controller varies the shape of the seed laser pulses to maintain the peak power above 5 kW, the pulse energy in excess of 0.1 mJ, and the pulse repetition frequency at 1 Hz-200 kHz. 
     
     
         3 . The apparatus according to  claim 1 , wherein the controller varies the shape of the seed laser pulses to maintain the peak power at 18-26 kW, the pulse energy at 0.8 to 1 mJ, and the pulse repetition frequency at 1 Hz-25 kHz. 
     
     
         4 . The apparatus according to  claim 1 , wherein the apparatus comprises two optical amplifiers. 
     
     
         5 . The apparatus according to  claim 1 , wherein the reflector is arranged to reflect less than 20% of the seeding radiation emitted by the seed laser back into the seed laser. 
     
     
         6 . The apparatus according to  claim 5 , wherein the reflector is arranged to reflect between 1% and 10% of the seeding radiation emitted by the seed laser back into the seed laser. 
     
     
         7 . The apparatus according to  claim 1 , wherein the reflector is characterized by a bandwidth which is greater than 1 nm. 
     
     
         8 . The apparatus according to  claim 1 , wherein the reflector is located at a distance less than 2 m from the seed laser. 
     
     
         9 . The apparatus according to  claim 8 , wherein the distance is between 0.5 m and 1.5 m. 
     
     
         10 . The apparatus according to  claim 8 , wherein the distance is between 5 mm and 50 cm. 
     
     
         11 . The apparatus according to  claim 1 , wherein the optical amplifier comprises a cladding pumped optical amplifier which further comprises an optical fiber. 
     
     
         12 . The apparatus according to  claim 11 , wherein the optical fiber is a multimode waveguide at a signal wavelength. 
     
     
         13 . The apparatus according to  claim 11 , wherein the optical fiber is a single mode waveguide. 
     
     
         14 . The apparatus according to  claim 11 , the optical fiber comprises a waveguide having a numerical aperture less than 0.15. 
     
     
         15 . The apparatus according to  claim 11 , wherein the optical fiber is characterized by an increase in attenuation, which is no greater than 5% in 2000 hours at a wavelength between 1000 nm and 1100 nm when a 0.1 to 1 m length of the optical fiber is core pumped with approximately 400 mW of light at a wavelength of 976 nm. 
     
     
         16 . The apparatus according to  claim 1 , wherein the reflector is a fiber Bragg grating. 
     
     
         17 . The apparatus according to  claim 16 , wherein the fiber Bragg grating is chirped. 
     
     
         18 . The apparatus according to  claim 1 , further comprising a laser delivery fiber, and a processing head. 
     
     
         19 . The apparatus according to  claim 18 , wherein the controller controls the seed laser and the optical amplifier such that the output pulses have sufficient average power and peak power to process a material over the range of pulse repetition frequencies. 
     
     
         20 . The apparatus according to  claim 1 , wherein the output pulses are characterized by a pulse width, and the controller varies the pulse width as the pulse repetition frequency is varied. 
     
     
         21 . The apparatus according to  claim 11 , wherein the optical fiber comprises a core having a refractive index, n 1 , and a pedestal having a refractive index, n 2 , and wherein the optical fiber comprises a first cladding made of glass having a refractive index, n 3 , surrounding the pedestal, wherein
 n 1  is greater than n 2 ;   n 2  is greater than n 3 ; and   
       the pedestal guides optical radiation that escapes from the core, 
       and the amplifier is cladding pumped by coupling pump radiation from a pump into the first cladding. 
     
     
         22 . The apparatus according to  claim 1 , further comprising a pump for pumping the optical amplifier, and wherein the controller reduces power emitted by the pump. 
     
     
         23 . The apparatus according to  claim 22 , wherein the power emitted by the pump is reduced by modulation. 
     
     
         24 . The apparatus according to  claim 23 , wherein the modulation applied to the pump is synchronous with the seed laser pulses. 
     
     
         25 . The apparatus according to  claim 1 , wherein the apparatus is defined by a peak power obtained with a rectangular seed laser pulse, and the controller reduces the peak power by controlling the shape of the seed laser pulses. 
     
     
         26 . The apparatus according to  claim 1 , wherein a portion of the optical radiation is wavelength converted, and the controller controls the seed laser to vary the shape of the seed laser pulses to maintain the portion of the optical radiation that is wavelength converted to less than 50%. 
     
     
         27 . The apparatus according to  claim 26 , wherein the portion of the pulse that is wavelength converted is less than 10%. 
     
     
         28 . The apparatus according to  claim 1 , wherein the pulse repetition frequency is 20 kHz. 
     
     
         29 . An apparatus for providing optical radiation, the apparatus comprising a seed laser for emitting seeding radiation, at least one optical amplifier, and a controller, wherein:
 the seeding radiation comprises a plurality of seed laser pulses;   the seed laser is connected to the optical amplifier;   the seed laser pulses are amplified by the optical amplifier to produce optical radiation;   the optical radiation comprises output pulses;   the output pulses are characterized by a peak power, a pulse energy, and a pulse repetition frequency; and   the controller controls the seed laser to vary the shape of the seed laser pulses to maintain the peak power above 3 kW, the pulse energy in excess of 0.04 mJ, and the pulse repetition frequency at 1 Hz-500 kHz.   
     
     
         30 . The apparatus according to  claim 29 , wherein a portion of the optical radiation is wavelength converted, and the controller controls the seed laser to vary the shape of the seed laser pulses to maintain the portion of the optical radiation that is wavelength converted to less than 50%. 
     
     
         31 . The apparatus according to  claim 30 , wherein the portion of the optical radiation that is wavelength converted is less than 10%. 
     
     
         32 . The apparatus according to  claim 29 , further comprising a pump for pumping the optical amplifier, wherein a portion of the optical radiation is wavelength converted, and the controller reduces the power emitted by the pump to maintain the portion of the optical radiation that is wavelength converted to less than 50%. 
     
     
         33 . The apparatus according to  claim 32 , wherein the portion of the optical radiation that is wavelength converted is less than 10%. 
     
     
         34 . The apparatus according to  claim 32 , wherein the power emitted by the pump is reduced by modulation, and wherein the modulation applied to the pump is synchronous with the seed laser pulses. 
     
     
         35 . The apparatus according to  claim 29 , wherein the pulse repetition frequency is 20 kHz. 
     
     
         36 . An apparatus for providing optical radiation, said apparatus comprising a seed laser and at least one amplifier, wherein:
 the seed laser is configured to provide seeding radiation;   the seed laser is connected to the amplifier;   the amplifier is configured to amplify the seeding radiation;   wherein the amplifier comprises an optical fiber, said optical fiber comprises a core having a refractive index, n 1 , a pedestal having a refractive index, n 2 , and a first cladding having a refractive index n 3 , wherein:   n 1  is greater than n 2 ;   n 2  is greater than n 3 ;   and wherein the first cladding is made of glass and surrounds the pedestal;   the amplifier is cladding pumped by coupling pump radiation into the first cladding;   the pedestal is adapted to guide optical radiation that escapes from the core;   and wherein, when in use, the amplifier emits a pulse having a peak power greater than 1 kW when seeded by the seed laser.   
     
     
         37 . The apparatus of  claim 36 , wherein: the core comprises silica, a concentration of alumina in the range of 0.1 to 4 mole percent, and a concentration of phosphate in the range of 2 to 20 mole percent; and wherein, the pedestal comprises silica, phosphate and germania. 
     
     
         38 . The apparatus of  claim 37 , wherein the optical fiber is doped with at least one rare earth dopant disposed in at least the core or the pedestal. 
     
     
         39 . The apparatus of  claim 38 , wherein the rare earth dopant is ytterbium having a concentration in the range of about 2000 to about 60000 ppm. 
     
     
         40 . The apparatus of  claim 39 , wherein the concentration of ytterbium is between approximately 15000 to approximately 50000 ppm. 
     
     
         41 . The apparatus of  claim 40 , wherein the concentration of ytterbium is between approximately 20000 to approximately 45000 ppm. 
     
     
         42 . The apparatus of  claim 37 , wherein the concentration of phosphate in the core is between approximately 12 to approximately 17 mole percent. 
     
     
         43 . The apparatus of  claim 42 , wherein the concentration of phosphate in the core is approximately 15 mole percent. 
     
     
         44 . The apparatus of  claim 37 , wherein the concentration of alumina is between approximately 0.20 to approximately 1 mole percent. 
     
     
         45 . The apparatus of  claim 44 , wherein the concentration of alumina is between approximately 0.3 and approximately 0.8 mole percent. 
     
     
         46 . The apparatus of  claim 36 , wherein the optical fiber is a multimode waveguide at a signal wavelength. 
     
     
         47 . The apparatus of  claim 46 , wherein the optical fiber is configured to propagate single mode light without significant distortion over a substantial length. 
     
     
         48 . The apparatus of  claim 36 , wherein the optical fiber is a single mode waveguide. 
     
     
         49 . The apparatus of  claim 36 , further comprising at least one stress producing region for inducing birefringence in the core. 
     
     
         50 . The apparatus of  claim 36 , wherein the optical fiber comprises a waveguide having a numerical aperture less than 0.15. 
     
     
         51 . The apparatus of  claim 36 , wherein:
 the optical fiber is a photo-darkening resistant optical fiber comprising a waveguide having a numerical aperture less than 0.15;   the core comprises silica, a concentration of alumina of between approximately 0.3 and approximately 0.8 mole percent, a concentration of phosphate of substantially 15 mole percent, a concentration of ytterbium substantially in the range of 20000 to 45000 ppm; and   the pedestal comprises silica, phosphate and germania.   
     
     
         52 . The apparatus of  claim 36 , further comprising a laser delivery fiber and a processing head. 
     
     
         53 . The apparatus of  claim 52 , further comprising a controller configured to control the seed laser and the amplifier such that the optical radiation has sufficient average power and peak power to process a material over a range of pulse repetition frequencies. 
     
     
         54 . The apparatus of  claim 36 , further comprising a controller, wherein the optical radiation is characterized by pulses having a pulse width, and wherein the controller varies the pulse width as the pulse repetition frequency is varied. 
     
     
         55 . The apparatus of  claim 36 , wherein the optical fiber is a photodarkening resistant optical fiber characterized by an increase in attenuation which is no greater than 5% in 2,000 hours at a wavelength between 1000 nm and 1100 nm when a 0.1 to 1 m length of the optical fiber is core pumped with approximately 400 mW of pump light at 976 nm 
     
     
         56 . The apparatus of  claim 55 , wherein the increase in attenuation is less than 1% in 2,000 hours. 
     
     
         57 . The apparatus of  claim 36 , further comprising a controller, wherein the seeding radiation is characterized by a pulse shape, the optical radiation is characterized by a pulse energy, a pulse repetition frequency, and a pulse width, and wherein the controller controls the seed laser to vary the pulse shape to maintain the peak power above 3 kW, the pulse energy in excess of 0.04 mJ, and the pulse repetition frequency at 1 Hz-500 kHz. 
     
     
         58 . The apparatus of  claim 57 , further comprising a laser delivery fiber and a processing head. 
     
     
         59 . The apparatus of  claim 36 , wherein the amplifier is side pumped. 
     
     
         60 . The apparatus of  claim 36 , wherein the apparatus is configured to emit a peak power greater than 5 kW with a pulse energy greater than 0.04 mJ at pulse repetition frequencies from 1 Hz to 500 kHz, and the apparatus further comprises at least two optical fiber amplifiers which are connected to amplify the seeding radiation. 
     
     
         61 . The apparatus of  claim 36 , wherein the apparatus is configured to emit a peak power greater than 18 kW with a pulse energy greater than 0.8 mJ at pulse repetition frequencies from 1 Hz to 25 kHz, and the apparatus further comprises at least two optical fiber amplifiers which are connected to amplify the seeding radiation. 
     
     
         62 . The apparatus of  claim 36 , wherein the seed laser is a superluminescent light emitting diode. 
     
     
         63 . The apparatus of  claim 36 , wherein the seed laser is a semiconductor laser. 
     
     
         64 . The apparatus of  claim 36 , wherein the seed laser is defined by a wavelength less than 1350 nm. 
     
     
         65 . The apparatus of  claim 36 , wherein the seed laser is defined by a wavelength less than 1100 nm. 
     
     
         66 . A method for providing optical radiation comprising the steps of:
 providing a seed laser;   providing at least one amplifier which comprises an optical fiber, wherein the optical fiber comprises a core having a refractive index, n 1 , a pedestal having a refractive index, n 2 , and a first cladding having a refractive index, n 3 , wherein:
 n 1  is greater than n 2 ; 
 n 2  is greater than n 3 ; 
   the first cladding is made of glass and surrounds the pedestal; and   the pedestal is adapted to guide optical radiation that escapes from the core;   cladding pumping the amplifier by coupling pump radiation into the first cladding;   controlling the seed laser to generate seeding radiation; and   amplifying the seeding radiation with the amplifier to generate a pulse having peak power of greater than 1 kW.   
     
     
         67 . The method of  claim 66 , further comprising the step of using the pulse to mark a material.