Survey Report: Laser R&D Peter Moulton VP/CTO, Q-Peak, Inc. DLA-2011 ICFA Mini-Workshop on Dielectric Laser Accelerators September 15, 2011 SLAC, Menlo Park, CA
Outline DLA laser requirements (one version) Quick review of suitable laser technology Tm:fiber lasers for DLAs Current technology Prospects
One possible accelerator design needs efficient, high-power lasers The low-power laser components (optical clock, phase-locked oscillators) in the system can be engineered based on existing solid state laser technology The power amplifiers remain a challenge. The pulsewidth and wavelength range requires a solid state laser. The solid state solution is based on fiber-laser technology.
Highly stable optical clocks are old news
Hansch and Hall win Nobel Prize for Optical Combs Stockholm December 10, 2005
Variety of formats for high-beam-quality, high-power solid state lasers Slab (zig-zag) Laser
Step index fiber - single mode design θ max ( ) NA = sin θ max n c n c NA step = n f 2 n 2 c V = 2π a λ o NA a is core radius, λ is wavelength V < 2.405 for single-mode fiber In general, base fiber material is SiO 2 (fused silica)
Cladding-pumped fiber laser allows multimode pumping of single-mode cores Traditional single-mode fiber lasers need single-mode pumps but... Elias Snitzer first described cladding pumped lasers in 1988 Maurer, U.S Patent 3,808,549 (April 30, 1974) J. Kafka, U.S. Patent 4,829,529 (May 9, 1989)
High-power double-clad fiber lasers facilitated by advances in diode-lasers Signal output @~1.1 µm HT @972 nm HR @~1.1 µm Diode stack @972 nm, 1 kw HT @975 nm HR @~1.1 µm Double-clad Yb-doped fibre II HT @975 nm HR @~1.1 µm Ytterbium-doped large-core fiber laser with 1 kw continuous-wave output power Y. Jeong, J.K. Sahu, D. N. Payne, and J. Nilsson, ASSP 2004
SORC results: 1.4 kw single-fiber laser Signal power [kw] 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Measured Linear fit Slope efficiency: 83% M 2 = 1.4 Core: 40 um, NA <0.05 Cladding: (D-shaped) 650/600 um NA 0.48 Fiber length: 12 M 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Launched pump powwer [kw]
Thanks to Mike O Connor at IPG for these slides For Government use only Recent Progress in Scaling High Power Fiber Lasers at IPG Photonics 22 nd Annual Solid State and Diode Laser Technology Review Dr. V. Gapontsev, V. Fomin, A. Yusim Newton, Massachusetts, July`09 IPG Photonics Proprietary and Competition-sensitive Information
YLS-10000-SM Optical Schematic Master Oscillator High Power Amplifier Length 15 m Core diameter 30 µm Eff. mode area 700 µm 2 Delivery Fiber Core diameter 40 µm 1 kw 10 kw 2.5 m FBG HR Pump LDs FBG OC Pump LDs 57/1 Pump Combiner IPG connector YLS-5000 5000- YLS-10000 10000- SM SM Pump lasers quantity 24 47 Effective mode area 500 µm 2 700 µm 2 Delivery fiber length 5 m 2.5 m 270 W, 1018 nm SM Fiber Lasers IPG Photonics Proprietary and Competition-sensitive Information
YLS-10000-SM Output Power Pmax=10,150 W Output Power vs. Current IPG Photonics Proprietary and Competition-sensitive Information
Materials at long wavelengths may have higher damage thresholds (Si especially)
Rare-earth laser transitions used in fiber lasers Energy (wavenumber/10000) 1060 nm 930 nm 1550 nm 1950-2050 nm 1080 nm
Tm:silica has a broad gain cross section 5E-21 4E-21 Walsh/McCumber absorption Walsh emission data Q-Peak absorption data Cross section (cm2) 3E-21 2E-21 1E-21 0 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 Wavelength (nm)
Tm-ion cross relaxation allows excitation of two upper laser levels for one pump photon
fs-duration pulses generated by Tm:silica fiber Assume half-gain points at 1925 and 2100 nm 13 THz linewidth, 33 fs pulses
High-peak power amplifier- chirped pulse
Advances in Tm-doped fiber-laser efficiencies show levels approaching Yb fibers
Early Q-Peak results scaling to 300 W, single-mode power meter 2050 nm output Single-ended pump Active fiber coil focusing head clamp Dichroic mirror 793-nm pump HR at 2050 nm 400-um, 0.2 NA HT at 790 nm fiber delivery clamp focusing head Gain fiber: 5-m long, 3-m undoped ends (2) Core: 25 µm in diameter, NA: 0.08. Pump cladding: 400-µm in diameter Heat sink Meniscus 2.5-cm R concave surface HR at 2050 nm HT at 790 nm Pump Laser A 325 300 301 W Pump Laser B 275 250 61.8% slope Output power (W) 225 200 175 150 125 100 75 50 25 0 64.5% slope LMA HI2 fiber data conduction cooled, new clamps Linear fit LMA HI2 fiber data conduction cooled Linear fit LMA HI2 fiber data water cooled Linear fit 0 50 100 150 200 250 300 350 400 450 500 550 600 Launched pump power (W) 59.1% slope
Components for all-glass laser single stage 150-W fiber-coupled pump modules at 79X nm (6+1) to 1 Co-propagating Combiner Tm-doped 20/400 fiber 10-m length FBG oscillator 50 W at 2041 nm Angled end-cap on fiber
> 1 kw of power output at 2045 nm MOPA output power (W) 1200 1000 800 600 400 One stage Fiber #1 One stage Fiber #2 Two stage 61.6% slope efficiency 53.2% slope efficiency 200 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Pump power (W)
Picture of all-glass system
We are now on the same upwards path in power pioneered by Yb-doped fibers 10000 Money (to buy pump lasers) is now the major limit to scaling RELI? We are going here Power Output (W) 1000 100 BAA1, 2 BAA2, 1 all-glass BAA1, 1 DSTO data IPG NGAS 10 First double-clad 1 1990 1995 2000 2005 2010 2015 Date
V = 2π a λ o Nonlinear effects: wavelength scaling issues for fiber lasers NA a is core radius, λ is wavelength V < 2.405 for single-mode fiber Core area for constant V: scales as λ 2 Optical damage fluence: Raman gain, nonlinear phase: Brillouin gain (theory): scales as λ scales as 1/ λ constant in λ (smaller linewidth (1/ λ 2 ) cancels smaller gain) Brillouin gain (actual): reduces with λ (more sensitive to inhomogeneous effects)
Nonlinear effects: Tm-doped fibers compared to Yb-doped fibers For the same V parameter, compared to Yb-doped fibers, Tmdoped fibers can have: 8X higher fiber core damage threshold 8X higher stimulated Raman scattering threshold 8X lower nonlinear phase distortion At least 4X higher stimulated Brillouin threshold The challenge for fiber makers is to scale up the core diameter for Tm-doped fibers and keep single-mode operation IPG 10-kW single-mode laser reportedly has about a 30-µm core diameter and is near Raman limit With a 60-µm core, a cw Tm:fiber can operate at 80 kw (!?)
DLA fiber-laser driver uses chirped-pulse amplification to avoid phase distortion 8 uj, 10-ps pulses 1-m fiber length 25 um MFD 76 um MFD
Large core (50 um MFD) fibers can allow very high cw powers 40000 35000 30000 Power output (W) 25000 20000 15000 10000 5000 0 0 2 4 6 8 10 12 14 16 18 20 Distance along fiber (m)
Summary Optical accelerators employ dielectrics to create high fields Longer-wavelength laser sources may be able to drive dielectrics to higher fields before optical damage occurs to the dielectric The 2-µm-wavelength Tm:fiber laser may be suitable as an efficient high-power source for optical accelerators Efficiency is enhanced by cross-relaxation pumping process Long wavelength has added advantages in raising the limit to power set by fibers themselves Large gain-bandwidth supports generation/amplification of short pulses, carrier-phase control is possible