Compact Fiber Laser for 589 nm Laser Guide Star Generation

1. Compact Fiber Laser for 589 nm Laser Guide Star Generation Jay W. Dawson, Deanna M. Pennington, A. Brown Lawrence Livermore National Laboratory 2007 CFAO Spring Retreat March 26, 2007 * Work done under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. * This work has been supported by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST-9876783. * This work has been supported by the National Science Foundation Adaptive Optics Development Program, managed by the Association for Research in Astronomy.

2. 938 nm master oscillator Phase and amplitude modulator NDFA pre-amplifier NDFA Pump diodes SFG 589nm Pump diodes EDFA 1583 nm master oscillator Phase and amplitude modulator EDFA pre-amplifier We are developing CW and pulsed fiber laser technologies for next generation LGSs NIF fiber amplifier chassis • Fiber lasers provide an elegant solution : • Compact, rack mounted, fiber delivery • Efficient operation (limited electrical power and cooling) • Turnkey operation • Reliable (high MTBF) • Robust • Safe (all solid-state, no chemicals)

3. ELT designs include several laser guide stars • Baseline TMT architecture includes three 50 W CW lasers designed to produce 9 LGS • Preferred upgrade architecture includes six 50 W pulsed lasers with dynamic refocusing • Power requirements for upgrades can be reduced by: • Implementing AO on the LGS uplink to produce a smaller focus • This will be tested on the Nickel Telescope at Lick Observatory in 2007 • Mitigating spot elongation effects • AODP funded development of pulsed fiber laser and custom CCD capable of tracking laser pulse through Na layer • Pyramid wavefront sensing

4. t Na layer h z q laser subaperture s ground Spot elongation can be mitigated by tracking laser pulses in the Na layer • Key laser times • Time to Na layer: 300µs = (h/90km)/cosz • Round trip: 600µs = (h/90km)/cosz • Time through Na: 33µs = (t/10km)/cosz • Pulse separation for single pulse in Na • layer: 66µs = (t/10km)/cosz • Max pulse frequency: • 15Khz = (10km/t)cosz • Pulse duration + integration time: • < 8.7µs = (blur/0.5 arcsec)/(s/15m)/cos3z

5. Arbitrary pulse format can be achieved by adding modulators to the seed lasers Ppeak = Pavgtrep/to , Duty cycle = ton/trep , Repetition rate = 1/trep • Rep rate < 2 kHz is not optimal for CW pumping NDFA • - Nd3+ upper state lifetime ~ 470 ms • For efficiency, repetition rate should be > 2 kHz with >1% duty cycle • Consistent with proposed ELT pulse format (6 ms, 16.7 KHz)

6. Gain competition from the 1088 nm 4-level line make the 938 nm Nd3+ laser challenging • 938 nm operation requires an Al-free glass composition • Al or P pull the emission wavelength shorter to 915 nm • Significant limitation on the Nd ion concentration (<10 dB/m @ 808 nm) because of concentration quenching, forcing a long laser amplifier • 938 nm operation is hampered by ground-state absorption at 938 nm and parasitic emission at 1088 nm

7. (7.5 mm) (20 mm) (30 mm) Increasing the core/clad ratio increases the overlap between the pump and the core leading to a shorter amplifier and a higher operating inversion The difference in gain at 1088 nm and 938 nm is a minimum at full inversion Reducing core/clad ratio enables room temperature 938nm operation with manageable 1088nm gain 1088 nm peak has 40 dB lower gain than 938 nm peak

8. 200 mW 938 nm LD Isolator and filters 35 W 808 nm pump lasers 25 m, 20 mm core Nd3+ doped fiber Isolator and filters 3.5 W@938 nm 40 m, 30 mm core Nd3+ doped fiber 90 W 808 nm pump laser Isolator and filters CW Output: 15 W @ 938 nm Pulsed Output: 10 W @ 938 nm (20% duty cycle) We generated >15 W, CW at 938 nm with a 100 W, 808 nm pump, with narrow linewidth 938 nm output power at various spots in the system M2 < 1.01 Polarization 10:1

9. Initial 938 nm pulsed experiments yielded >10 W avg. power with ~500 MHz bandwidth • Pulsed at 100 kHz with 20% duty cycle • >95% of optical power was in 938 nm signal line • No sign of SBS with 500 MHz signal line width • Square pulse distortion will be implemented to scale to 10 W in the 10 kHz repetition rate regime

10. Isolator Pump Laser IPG Amplifier WDM Coupler Temperature Controlled Oscillator Amplifier Output 1583 nm fiber laser is constructed from commercially available components PM Lithium Niobate phase modulator Polarization sensitive, 20 dB extinction ratio Koheras SM 1 mW/1583 nm Single mode fiber pre-amplifier Lithium niobate phase modulator EAR-15k-1583-LP-SF IPG Fiber amplifier 14 W 14 W CW 10 W pulsed Isolator

11. 1583 nm system produces 14 W in CW mode with >98% of the power in the signal Power vs. Pump Current Output spectrum at full power

12. 1583 nm laser produced > 15 W at 20% duty cycle at 100 kHz However, the lithium niobate amplitude modulator is leaking significant CW light. So the peak power is only ~1/3 of expected value The cause of the CW leakage was poor polarization control from the oscillator

13. The front end of the 1583nm laser has now been improved to be all PM (no leakage issues) • Square pulse distortion increases the peak power, driving SBS • - Add more bandwidth to suppress (> 400 MHz) • By programming the modulator drive signal, we can pre-compensate for square pulse distortion • IPG 15W amplifier unit failed twice. It has been repaired and the root cause of the prior failures is believed to be a flakey key switch

14. 5 PPKTP SFM Data 4 3 589nm PPKTP 589nm power (W) Theoretical Fit 2 1 0 0 5 10 15 20 Total Combined 1583nm and 938nm Power (W) Preliminary SFG in PPKTP yielded 2.7 W of 589 nm light for CW format • 2.7 W @ 589 nm with 6 W @ 1583 nm and 11 W @ 938 nm • Power scaling in PPKTP limited by damage and available 1583 nm • Switch to PPSLT which is less susceptible to damage effects • Pulse laser to achieve higher conversion efficiency Na cell, 589nm

15. We generated 3.8 W at 589 nm in 3 cm of PPSLT at 100 kHz and 10% duty cycle • 1583 nm laser had significant CW leakage, so a large percentage of its power was not contributing to frequency conversion • PPSLT showed no signs of damage at these power levels

16. Where are we? Where are we going next? • 3.8W is less than the 10W original target • However, it does demonstrate the basic feasibility of the concept • It was hoped we could generate the full 10W in the breadboard phase, but enormous efforts were being undertaken to do this with little practical payoff in the long run • To this end, breadboard level experiments have been discontinued • Our recent internal experience with our short pulse laser systems indicate that packaging leads to much better performance and simplified ease-of-use • Our focus is now on engineering the system for packaging and turn-key operation and installation on the Nickel telescope at Lick in late 2007 or early 2008 • The 1583nm laser sub-system is essentially in this state now • The only thing the 1583nm subsystem needs to be ready to go to Lick is some software control and a better driver for the phase modulator • The 938nm laser system a bigger, but solvable challenge (see next slides) • It appears PPSLT will work for this laser • We now have AR coated PPSLT crystals and need only to design the sum-frequency mixing and diagnostics breadboard which should be straightforward • We will be working with the CFAO and Lick teams over the summer to ensure the final packaging will integrate well at the telescope

17. 938nm laser engineering and packaging • A new fiber coupled 938nm master oscillator has been ordered and received, along with a fiber pigtailed AOM and LiNbO3 phase modulator • We have also designed and ordered a PM 938nm fiber, an all fiber pump signal combiner and 7 LIMO pump diodes appropriate for driving the 938nm laser to full operating power of 15W • This task was tricky and involved simultaneous negotiation with several vendors in order to ensure the custom parts will be all created in a way that they operate together • We have also had some purchasing and institutional bureaucracy issues that have created some schedule delays • The project is currently dormant in order to conserve funds while we wait for these components to arrive • We have identified two people internally with appropriate skills in software control, optics and mechanical engineering who can work on this problem starting in late April when we expect the above components to arrive and we anticipate generating 589nm light with the new system by the early fall

18. 938nm system block diagram

19. Internal schematic of 938nm amplifier (up to 4 25W LIMO diodes can be employed)

20. Our laser will be installed on the Nickel Telescope at Lick Observatory in late 2007 or early 2008 for a visible light AO demonstration • Need ~10x more laser fluence per spot to do visible light AO • A factor of 100x – 200x gain is available from • Uplink AO correction • Pyramid sensing • 4x more beacons needed in the tomography constellation • Other gains? • CW vs micropulse format • Tracking beam in Na layer

21. Summary • We are developing CW and pulsed 589 nm laser systems for ELTs • We have achieved > 15 W at 938 nm in a Nd3+ based amplifier system • We demonstrated 11 W in a pulsed format with 20% duty cycle at 938 nm • 10% is achievable with an extra amplifier stage for additional gain • We constructed a 14 W, 1583 nm laser system from commercial components • Pre-compensation for square pulse distortion is being implemented to achieve 10% duty cycle • We have achieved 3.8 W at 589 nm with a 10% duty cycle via sum frequency mixing in PPSLT with no signs of optical damage in the crystal • Final pulse format of 3 ms at 15 kHz will enable tracking of pulses through the Na layer to mitigate spot elongation • System scheduled for installation on the Nickel telescope at Lick Observatory at end of 2007 for a visible AO demonstration with AO corrected laser uplink