Linac Laser Notcher Status David Johnson, Todd Johnson, Kevin Duel, David Slimmer, Matt Gardner Sreenivas Patil, Jason Tafoya March 2, 2016 PIP weekly meeting Beams doc 5097

Photonics West System Layout Current Status Fiber splicing complete Transport enclosure installed for lab testing Re-starting alignment process At this point our main focus is safely working toward full power test Amplifier layout Diode stability Power amp stability Dips in power Power output spikes Spikes on reverse power Investigation Keep-alive frequency Keep-alive pulse width Replace single keep alive pulse with burst of 200 MHz at 450 kHz Plans Pre-amp design

Photonics West Largest Photonics conference and Exposition in US with over 1200 vendor exhibits Feb 14-18, 2016 San Franscisco Sunday 14 th course on “Improving Laser Reliability, an Introduction” – slides to follow course on “High-power fiber sources” purchased class notes Monday – met with Russell Wilcox (LBNL) and discussed our laser notcher laser system particularly our instability and what we did to resolve the issue. Wednesday- Southampton ORC reunion where I met with several experts in fiber design, amplifier design, and laser design, thanks to KV at PriTel. Vendors: Diamond: Met with Swiss Engineers responsible for the E-2000 power solutions connector- Optical Engines: met with Don and Jason to discuss our amplifier design and the issues we have been having. Northrup Grumman: Met with Jay and Ryan about our DPSS module operation Many fiber manufactures Many seed and pump diode manufactures EOM, laser diode controllers, spectrum analyzers, instrumentation

Photonics West – Improving Laser Reliability, and Introduction Many/most of the attendees were from industry (Coherent, IPG, Nufern, and Spectra Physics), small businesses, and university (MIT Lincoln Labs) Seemed to be focused on manufacturing for reliability Organized the discussion along the following stages Idea/Research phase Design Phase Prototype Phase Manufacturing/Production Phase Support to obsolescence What are reliability considerations for each phase of the laser product life cycle? How to identify the risks to reliability ? How do you estimate laser lifetime? How to utilize best practices in design and manufacturing? How to design qualification tests? How to troubleshoot problems? Discusses many case studies  tips and best practices

Design for Reliability Early teamwork makes the difference System AND component specifications at an early stage are key From prospective of hardware design- OPTICAL, MECHANICAL, CONTAMINATION, ELECTRONICS, SOFTWARE, AND CHILLERS – need to address each early in design phase Stray beams, optical weak points, performance margin (system AND component)

Notcher Laser System Layout

External Splice Box & Optical Engines Amplifier Sreenivas Patil (PriTel) – splice PtiTel power amplifier to Optical Engines Amplifier. Remove old delivery fiber from unit Splice new delivery fiber inside Pritel unit Re-install PriTel unit in rack Re-splice old delivery fiber to connect to power meter to verify splice Remove old delivery fiber Create splice in external splice box Use Spiricon camera to look at light out of Optical Engines Amplifier (un-pumped) Optical Engines Delivery fiber PriTel Delivery fiber Splice box

First Light Optical Engines  New PCF delivery fiber out of Optical Engines Amplifier – FUD matched to DC-200/40 DZ Yb gain fiber to inherently strip out cladding light ( aperture no required after fiber port) Previous delivery fiber (shown after fiber port)New delivery fiber (shown after fiber port & lens)

Optics Box and Mock-up of Transport Enclosure Install transport enclosure on temporary support rails Piezo steering mirrors Optical BPM’s Prototype gated integrating sphere / fast phot diode used to measure average power & pulse energy after each amplifier stage Fiber Port – delivery fiber from OE

Diode Stability Requirements Central wavelength nm (air) nm (vac)- Nd:YAG gain peak Spectral bandwidth: looking for about 0.2 nm FWHM Average Power: limited by modulator to 100 mW average power Stable output power Stable spectral bandwidth The instability has greatly improved as We are running the dipole at 200 mW and using an air gap attenuator to get the 100 mW needed for modulator We are running the TEC at 8 degrees Always on the lookout for a more stable seed that can be pulsed to increase peak power in the signal pulses

Pump: 600 mA 6 um SM CorActive YB-401-PM ~ 5m Gain fiber FP: Pre-Amp Input Pump: 7A (4.5W) Fused TC 99/1 PLMA 10/130 Yb gain fiber L=3.5 m Fiber port to free space FP: SBS Monitor FP: Power-Amp Input Monitor PM Power Monitor & ASD Control 976nm pass nm block Pump filter 3W 1064 nm isolator 6 um core 4nm band pass filter Center 1064 (ASE block) 3W 1064 nm isolator 6 um core 3W 1064 nm isolator 6 um core 1064 nm isolator 6 um core 976nm pass nm block Pump filter Back Panel spliced Pump dump 450 uW Seed External amp 40 mW 2W 2.1 GHz PD to scope Power Meter Connect to fiber coupled PD into scope PriTel Amplifier Configuration

Power Amp Stability We need to have a stable input into Optical Engines amplifier. Connect output of PriTel power amplifier to triplet collimator bring into free space Run keep-alive pulses + notch pulses to determine what is required for stable output Direct output into Thorlabs 3W integrating sphere power meter (measure average power) Beam sampler & PD to monitor pulse structure and peak power of output Photodiode Power meter Fiber port

Dips in output power from PriTel Power Amp OSA: modulator output Gray- spectral bandwidth Blue – power in peak Red – total power

Large forward power spikes test OSA: modulator output Gray- spectral bandwidth Blue – power in peak Red – total power

Large forward and reverse power spikes Set up scope to trigger on output PD (set trigger higher than signal) Yellow – RF waveform (keep-alive pulse] Magenta – amplifier output Green – reverse power monitor What is causing these spikes in forward & reverse power?  SBS & Why are we getting SBS? Let’s look into signals in a little more detail

Yellow- power amp input monitor Magenta – power amp output monitor Green – power amp reverse power Data from 2/5/16. power amp pump 5 A, standard keep-alive pulses.. We see same behavior an pump powers > 4.5A  Sometimes spikes in both forward and reverse and sometimes spikes only in reverse  Always associated with keep-alive pulse

With current keep-alive structure (46 ns 450 kHz) determine maximum pump current for stable operation Maximum pump current 4.5A to give 0.8W out of the power amplifier However, we want to run at 7.5 Amps What is required ? Frequency  higher is better for SBS but not for notch pulses 450 kHz Pulse width  smaller better but too small can’t pump gain fiber Average power  higher is better but we don’t want DC level Peak power  good Investigated each of these options including increasing the increasing input power to pre-amp/power amp with external amplifier

Keep-Alive Frequency Adjusted keep-alive frequency from 450 kHz to 1.7 MHz Spike in forward power at 450, 500, and 900 kHz KA 1.7 MHz (no spike) but non uniform notch peak pwr

What does ASE and SBS look like SBS spike Broad Spectrum ASE Generated in LMA amplifier

Create a burst of 200 MHz pulses at 450 kHz with AWG to play continuously… disable the current keep-alive Tried 100 MHz pulses at 450 kHz – but still got an “event” Go back to 200 MHz matches notch pattern  NO EVENTS

New Keep-Alive We found that by using a burst of 200 MHz pulses at 450 kHz we can eliminate the large SBS spikes (reverse direction) and large forward spikes. Todd has designed a new hardware keep-alive unit Runs continuously (crystal osc.) Marker pulses from AWG are used to suspend the KA when notch pulses are triggered KA pulses resume when the marker pulses are absent of 2.2 us [Hold off]

Plans GOAL To safely perform tests of all amplifiers leading to full power test ASAP Plans Complete alignment Install & commission instrumentation OBPM’s Integrating spheres Fast photodiodes Photodiode monitoring circuit IR video monitors Re-commission OEA Re-commission DPSS modules Prepare for full power test Dry fit transport & dump enclosure to the installed cavity and support structure Prepare for inert atmosphere in optics box Continue software development

External Amplifier Used external amplifier between modulator & pre-amp Increase seed into pre-amp and hence power amp Amplifiers run further in saturation Don’t get much more output from power amp. But Reverse power (ASE) reduced dramatically

F WDM IL= ~0.6 dB IL= 0.3 dB signal x x x x x x x x x x x xx x IL= 0.1 dB IL= 0.5 dB 5W isolator Mid-stage isolator IL= ~0.1 dB IL= 0.5 dB IL= 0.3 dB 5W isolator IL=~.6 dB pump IL= ~0.6 dB pump 70% 30% Pump 1.1 A 725 mW Booster Pre-amp LM A IL= ~2.8 dB x x x x x x 400 uW 8-10 mW mW (Do not need both) New section (contained in existing pre-amp fiber tray) IL= 0.5 dB 5W isolator ASE Filter IL= ~0.7 dB IL= ~1.0 dB IL= 2-3 dB Potential Technique for increasing signal power – pump 2 gain sections with single pump