Fiber Lasers: Technology, Applications and Associated Laser Safety

Transcription

1 Fiber Lasers: Technology, Applications and Associated Laser Safety Presentation to 2012 LSO Workshop Dr. Jay W. Dawson Fiber Laser Group Leader National Ignition Facility and Photon Sciences Directorate September 11, 2012 This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

2 Outline Optical fiber fabrication How fiber lasers work Applications 2

3 Conventional fibers are made by modified chemical vapor deposition (MCVD) The National Ignition Facility NIF ppt Author-Review 3

4 Light manipulated by refraction Light manipulated by diffraction Conventional fiber based on material variations ϕ= 125 µm PCF fiber based on geometrical variations ϕ= 7 µm In addition to total internal refraction, there are also optical fibers that operate based upon photonic crystal technology

5 LLNL has an operational facility for fabricating photonic crystal fibers (PCF) Stack and draw PCF preforms Geometry 8.2 m optical fiber draw tower Fiber Solution Mesh Modeling and design capability

6 After modeling has determined the pitch and hole size, we develop a preform design 6

7 The starting material is commercially available bulk silica glass 7

8 A rod or tube is inserted in the furnace and a drop starts the process 8

9 Once the pull is started, a tractor controls the draw diameter and an operator cuts the rods and tubes 9

10 The rods and tubes are then stacked into the array using a jig 10

11 The corners are pulled off to make a more circular stack 11

12 The circularized stack is then stuffed into an inner tube 12

13 The sleeved stack of rods and tubes is then put in a glass working lathe to tighten the stack 13

14 The resulting preform is then put in the furnace and the end is dropped off 14

15 The preform is flipped over and pressure fittings are attached to provide vacuum and pressure 15

16 The glass is threaded through a pressure coater and pulled and controlled by the capstan 16

17 The fiber is then threaded through a series of pulleys and wound up on a spool 17

18 Below are some examples of fibers made to date, the draw tower was commissioned in January 2012 Small Core, High NA Passive Ribbon Fiber High NA Multimode Fiber The goal for this year is a rare earth doped ribbon fiber surrounded by a high NA multimode fiber cladding. 18

19 How do fiber lasers work? Rare earth doped core absorbs pump light from cladding Light propagating in core stimulates emissions leading to brightness enhancement Optical fiber core defines output beam quality High surface area to volume ratio of core minimizes thermal effects High intensity over long lengths lead to highly efficiency process I>>Isat Yb 3+ fibers can achieve 85% optical to optical conversion efficiencies

20 Single-mode fiber lasers have reached powers of 10 kw and multi-mode fibers can carry kw

21 However, due to their small apertures, fibers typically carry relatively limited pulse energy Self-focusing and optical damage limit the pulse energy capacity of optical fibers Excessive power or pulse energy can cause failures, spraying light in random directions M.Y. Cheng, et al, Optics Letters, vol. 30, pp (2005) Diffraction limited fibers have produced nano-second pulses of 4 mj at the theoretical limit for self-focusing. Chirped pulse amplification has enabled few-mj pulses of < 500 fs. However, these pulses can be generated at very high repetition rates and average powers. 21

22 Fibers can carry light with a broad range of wavelengths Super-continuum sources can span 1500 nm. High power fiber lasers can operate at 1µm (Nd or Yb), 1.5µm (Er) or 2µm (Tm) 22

23 High power laser light may be carried by single mode or multi-mode fibers Multi-mode fibers emit non-diffraction limited light which is best characterized by the fiber core diameter and numerical aperture Single mode fibers emit diffraction limited light whose divergence is a function of the fiber mode field diameter and the laser wavelength Multi-mode fiber can carry significantly more laser power and pulse energy than single mode fibers. Multi-mode fibers are also commonly used to carry high power diode laser light and to transport the light from other lasers for some applications, particularly medical. 23

24 Commercial fiber lasers are a simple-to-use, lowmaintenance, compact sources of high-brightness, high-power laser light with wall plug efficiencies in excess of 30%

25 Commercial fiber lasers are a simple-to-use, lowmaintenance, compact sources of high-brightness, high-power laser light with wall plug efficiencies in excess of 30%

26 Exposed optical fibers carrying very high powers need to be kept clean and be properly heat sunk Spontaneous emission from high power lasers leads to significant, intense light at the surface of the fiber, any flammable contaminant will become a point of failure. 26

27 The NIF laser is seeded by a fully rack-mounted state-of-the-art LLNL-engineered 1053-nm fiber laser system that operates 24/7 under full computer control.

28 The NIF injection seed laser is a master oscillator power amplifier architecture FS AMP Failsafe Failsafe Grating Grating SBS SBS Failsafe Failsafe Pulser 10 mw 0.5nJ/100 ns 100nJ 10nJ 2.5nJ 43nJ 1.3nJ/30ns 22nJ Oscillator AO Modulator AMP-A Phase Modulaor FM-to-AM Optical Gate Comp AMP-B Optical Gate AMP-C 40nJ 675nJ 135nJ 400 mw Peak Power AMP-D 370nJ 58nJ Disp Comp 1x4 12nJ 200nJ AMP-E AMP-E AMP-E 1x4 1x4 1x4 AMP-F 1x4 1x5 1x5 1x5 1x5 1x5 1x5 1x5 1x5 1x5 1x5 1x5 48 Outputs to Main Laser System High gain fiber amplifiers can generate very well timed and stable pulses this way. However, care needs to be taken in designing the interlock systems for these lasers to ensure the laser is not damaged when the interlock is tripped. 28

29 The Advanced Radiographic Capability project will create a parallel short pulse front end for NIF

30 T-REX Seed Laser System World s first fiber-based photocathode driver Frequency and phase-locked to 3 ppm Foundation for future accelerators and FELs The MEGa-Ray project is expanding on this work We are also constructing a photocathode drive laser for LBL 1.8 mm-mrad rms state-ofthe-art emittance measured at 800 pc

31 The LLNL 589nm fiber laser at high power Blue light can also be made this way

32 Conclusions Optical fibers and fiber lasers can carry kilowatts of laser power and moderate pulse energies at high average power Fiber lasers are proliferating in commercial applications and in research The light out of a fiber diverges quickly, but is easily re-collimated or focused Optical fibers are relatively easily broken at which point light will scatter randomly High power fiber lasers can also ignite flammable materials they come into contact with due to the spontaneous emission and scattered light emitted through the sides of the fiber 32

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