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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.

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We are developing CW and pulsed fiber laser technologies for next generation LGSs

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)

938 nm master oscillator

Phase and amplitude modulator

NDFApre-amplifier

ND

FA

Pump diodes

1583 nm master oscillator

Phase and amplitude modulator

EDFApre-amplifier

ED

FA

Pump diodes

SFG 589nm

NIF fiber amplifier chassis

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Lasers

Centering Array

Pointing Array

Fold Flat

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

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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

h

t

s

z

ground

Na layer

subaperture

laser

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Arbitrary pulse format can be achieved by adding modulators to the seed lasers

Ppeak = Pavg rep/o , Duty cycle = on/rep , Repetition rate = 1/ rep

• Rep rate < 2 kHz is not optimal for CW pumping NDFA

- Nd3+ upper state lifetime ~ 470 s

• For efficiency, repetition rate should be > 2 kHz with >1% duty cycle

Consistent with proposed ELT pulse format (6 s, 16.7 KHz)

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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

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Reducing core/clad ratio enables room temperature 938nm operation with manageable 1088nm gain

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

(7.5 m) (30 m) (20 m)

1088 nm peak has 40 dB lower gain than 938 nm peak

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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

200 mW 938 nm LD

Isolator and filters

25 m, 20 m core Nd3+ doped fiber

35 W 808 nm pump lasers

Isolator and filters

3.5 W@938 nm

40 m, 30 m core Nd3+ doped fiber

90 W 808 nm pump laser

Isolator and filters

CW Output: 15 W @ 938 nmPulsed Output: 10 W @ 938 nm(20% duty cycle)

M2 < 1.01

Polarization 10:1

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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

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1583 nm fiber laser is constructed from commercially available components

Koheras SM1 mW/1583 nm

PM Lithium Niobate phase modulatorPolarization sensitive, 20 dB extinction ratio

Lithium niobate phase modulator

EAR-15k-1583-LP-SF IPG Fiber amplifier 14 WIsolator

Single mode fiber pre-amplifier

Pump Laser

WDM Coupler

Temperature Controlled Oscillator

Isolator

IPG Amplifier

Amplifier Output

14 W CW10 W pulsed

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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

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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

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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

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Preliminary SFG in PPKTP yielded 2.7 W of 589 nm light for CW format

• Switch to PPSLT which is less susceptible to damage effects

• Pulse laser to achieve higher conversion efficiency

• 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

Na cell, 589nm

PPKTP SFM Data

0

1

2

3

4

5

0 5 10 15 20

Total Combined 1583nm and 938nm Power (W)

589

nm p

ower

(W

)

589nm PPKTP

Theoretical Fit

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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

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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

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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

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938nm system block diagram

938nm fiber coupled oscillator

LLNL/OFRAOM

EOSPACEPhase modulator

Fiber Amp 1

Fiber Amp 2

Fiber Amp 3

SRS arbitrary waveform

Need to identify RF driver

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

Heat sink

LIMO 25W fiber coupled

Cooled fiber coil

OFR

Iso

lato

r an

d B

PF

Input

Output

Pump/signal combiner

Splices

LIMO 25W fiber coupled

Heat sink

LIMO 25W fiber coupledLIMO 25W fiber coupledLIMO 25W fiber coupled

Cooled fiber coil

OFR

Iso

lato

r an

d B

PF

Input

Output

Pump/signal combiner

Splices

LIMO 25W fiber coupledLIMO 25W fiber coupled

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• 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

Our laser will be installed on the Nickel Telescope at Lick Observatory in late 2007 or early 2008 for a visible light AO demonstration

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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 s 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