A proposal for an improved laser system for the CEBAF photo-injector.

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A proposal for an improved laser system for the
CEBAF photo-injector.

• John Hansknecht
• Electron Gun Group

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A timeline of laser systems at Jlab

• Feb 1995. A 5mW He-Ne laser source produced the
  first photo emitted beam at Jlab. The beam was
  DC and the chopper chopped the beam for the
  three halls. There was no tune mode and viewer
  limited mode was achieved by inserting a neutral
  density filter on a pneumatic cylinder to reduce
  laser power.
• Pros
• We made polarized beam!
• Cons
• Most of the beam produced was thrown away on the
  chopper.
• Controls were not suited for production beam.

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1996

• April 1996. JLab source group, under the
  guidance of Dr. Charles Sinclair, was the first
  in the world to demonstrate high frequency
  polarized synchronous photoinjection from GaAs.
• The laser driving the gun was state of the art.
  A diode laser was rf gain-switched at 1497 MHz
  and subsequently amplified by a tapered-stripe
  laser diode amplifier 1.
• The laser provided tune and viewer limited pulse
  structures. These Macro-pulses were
  relatively easy to create electronically and met
  the requirements necessary for all beam
  diagnostics.
• This laser system was subsequently copied by
  other labs for use on their electron guns.
• 1 M. Poelker, Appl. Phys. Lett ., 67, 2762
  (1995).

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The 1996 CEBAF Laser Table
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1996 Laser Pulse Structure

• Pros
• Better photocathode lifetime vs. beam from a
  DC laser.
• Gain switching was simple.
• Cons All for One One for All
• The chopper is still required to intercept beam
  for amplitude control.
• The current drawn from the photocathode needed
  to be 3 times the current requested by the
  highest current hall.
• Wavelength not tunable. Diodes and amplifiers
  were only available at two important wavelength
  ranges.

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1997 Diode Laser system improvements

• The diode laser system was modified to provide 3
  separate lasers, each pulsed at 499MHz and phased
  120 apart.
• Space constraints forced the source group to
  design a new compact seeded amplifier.
• This design change was also needed to provide the
  ability to quickly swap lasers among the two
  wavelength selections.

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1997 Laser Table schematic
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Key points for 3 laser operationsBeam combining
methods
http//www.jlab.org/accel/inj_group/laserparts/Bea
m_combining_tutorial.pdf
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1997 laser table (open in lab)
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1997 Laser Pulse Structure

• Beam amplitude is customized for the specific
  halls at the laser rather than at the chopper.
• Individual hall laser can be shut-off if hall
  does not want beam.
• Individual Tune and viewer limited modes.
• Most efficient use of the precious resource of
  electrons. - Longest lifetime of photocathode.

Beam is now being routinely delivered for physics
from Bulk GaAs T-Gun Broken.
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1997 and 1998

• New problems introduced
• ASE (Amplified Spontaneous Emission) is not our
  friend
• 1. leakage to unintended hall
• 2. Polarization dilution
• 3. Tune mode cross-talk
• Laser powers of individual lasers are
  subsequently dropped to limit ASE.
• We are laser power limited further than before.
• Beam coincidence
• Vendor delivery problems
• Dripping sweat kills lasers
• Changes are needed soon

100uA 35 polarization delivered to HAPPEX from
Bulk GaAs
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1999 brings major changes

• Vertical Gun replaced with 2 horizontal guns
• (no more kneeling on the floor for laser
  work)
• An air-conditioned laser hut (inexpensive plastic
  curtain) is constructed to contain the laser
  table.
• (no more sweat dripping on the lasers)
• Safety No more vertical laser beam.
• Ti-Sapphire lasers are in testing phase. New
  Strained layer photo-cathodes require 10X more
  power than bulk GaAs for same current. We are
  severely power limited.
• No real changes to lasers other than physical
  layout.

50uA 70 polarization delivered to HAPPEX from
Strained GaAs
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1999 - Ready for some serious physics now
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1999 laser table schematic
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Y2K Introducing the Actively-modelocked
Ti-Sapphire laser (another first)
(C. Hovater and M. Poelker Nuclear Instruments
and Methods in Physics Research A 418 (1998)
280-284) 
Jlab Patented Technology C. Hovater, M. Poelker
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Y2K- Actively modelocked Ti-Sapphire laser
schematic
Jlab Patented Technology C. Hovater, M. Poelker
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Y2K laser table schematic
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Y2K Laser System

• Pros
• Ti-Sapphire laser is wavelength tunable to reach
  peak polarization of photo-cathode material
• Ti-Sapphire laser provides high power as compared
  to diode laser systems - (can deliver more
  Coulombs between cathode activations)
• Ti-Sapphire laser has no ASE

Nov. 2000. Delivered high current and high
polarization to two halls simultaneously. (GEn
GEp) This would not have been possible without
the new Ti-Sapphire laser.
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Y2K Laser System

• Cons
• Ti-Sapphire lasers are extremely sensitive to
  alignment and cleanliness. 1um changes of cavity
  will affect lock. Small changes in room
  temperature will change alignment of the cavity.
• The cavity length sets the fundamental repetition
  rate of the cavity. Injection modelocking relies
  on many parameters being perfect to achieve a
  good pulse structure on the output.
• If laser phase lock is lost, the beam can be sent
  to the wrong hall(s).
• Phase noise makes e- beam difficult to transport
• Difficult to produce a Tune structure. Diode
  used colinearity, phase differences and
  amplitude differences caused problems.
• Laser on-call is a full time job

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2 Halls from 1 Ti-Sapphire Laser (Nov 2000)
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2001 Laser System g0 preps
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2001-2002

• New technology available- (SESAM technology)
  Commercial Ti-Sapphire laser that provides
  superior reliability and performance over our
  injection-seeded and AOM mode-locked Ti-Sapphire
  lasers.
• g0 31MHz experiment would not have been
  successful were it not for this laser.
• Laser was so successful that we purchased several
  for 499MHz as well.
• Laser table controls upgraded to provide fine
  control for the correction of current and
  position asymmetries.

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2002 Present Laser Table
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Enhancements
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2004- A Safer and Cleaner environment
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Current Standings

• Pros
• The year is 2005. The injector is generally not
  the source of problems for the accelerator.
• The injector is providing the highest
  polarization, highest current synchronous
  photo-injected beam ever delivered in the world.
• We are now capable of delivering parity quality
  beam to 3 experiments simultaneously. (sort of)
• The injector area is Safe, Clean, and Cool
• A new load-locked gun is coming soon that will
  allow rapid exchange of previously prepared
  photo-cathodes.
• Cons
• The source group has lost several key personnel
  and the present budget does not support
  replacement. Innovation improvements now take
  a back seat to maintenance of the existing
  system. Everything takes longer.
• Although the Time-Bandwidth Products, Inc lasers
  are vastly superior to our previous lasers, they
  are still temperamental and require an expert to
  maintain.

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Laser Specific issues that need to be addressed

• Wavelength tunability and time required for
  changes
• Phase noise phase lock
• Vendor spares
• Beam Colinearity
• Polarization dilution
• Tune mode quality
• Parity system quality
• 1.497 GHz stable laser for Accelerator tuning has
  been requested
• Laser Power limit
• Amplified Spontaneous Emission (ASE)
• Laser sensitivity to its environment and laser
  safety.
• Time consumed to replace lasers
• Only 2 Laser experts in the group.
• Need to align multiple items on laser table.
• Confusion to operators when lasers are changed
  and have different performance characteristics.

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Lets build a new laser system from scratch.

• Our first point of action is to select a
  wavelength. Our recent success with the
  super-lattice cathodes proves that 780nM is
  ideal for high polarization and excellent QE.
• We are sticking with this decision and will place
  this cathode material in both guns, so our laser
  system will deliver 780nM light. (scratch 1
  from the list)
• We liked our gain-switched diode technique
  because of its phase noise and phase lock
  attributes. Lets start with three laser seeds
  and gain switch them. (scratch 2 from the
  list)
• We will be using lasers and fiber laser
  amplifiers designed for the cable TV and
  communications industry. (scratch 3 from the
  list)
• We are going to use new fiber laser technologies
  that allow us to combine all lasers in a single
  fiber with identical polarization and collinear
  travel. (scratch 4 and 5 from the list)
• We will use fiber-based electro-optic modulators
  with GHz bandwidth, so we should be able to
  produce any desired tune mode or other modulation
  on the beam with high quality. (scratch 6 and 7
  from the list)

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New laser system design. Lets see what we have
so far.
Pre-Amp
Pre-Amp
Pre-Amp
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More design thoughts

• Since combining these lasers appear to be so
  easy, lets throw in a fourth laser at 1.497
  GHz. We will use a fiber based MEMS optical
  switch to efficiently swap from the 3 laser
  system (500MHz) to the single 1.497 GHz laser
  with the press of a button.
• (and scratch 8 from the list)

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Laser system design continued
Pre-Amp
Pre-Amp
Pre-Amp
Pre-Amp
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Now we need some Power

• Thus far our system has four lasers that provide
  clean gain-switched light. We need to amplify it
  further to get sufficient power for operations.
• Erbium-Ytterbium fiber laser amplifiers are now
  commercially available. Their power level
  capabilities have been growing exponentially over
  the past few years. They will meet our immediate
  demand (for a price), but will become more
  powerful and cheaper as the technology and market
  demand grows.
• Previous worries about delivering quality
  single-mode TEM00 beam from a fiber are gone.
  New Panda fiber designs transmit pure single
  mode beam without worry.
• We are specifying an amplifier that should
  triple our present deliverable laser power.
  (scratch 9 from the list)

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The final pieces of the system
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The final system operation

• Light was produced with proper pulse structure,
  intensity control, modulation control.
• Light from all lasers was amplified and some ASE
  is present in amplified 1560nM light.
• Non-linear Second Harmonic Generation (SHG)
  crystal is used to frequency double the light
  from 1560nM down to 780nM. Non-linear gain of
  SHG crystal will cut off and not pass the low
  levels of ASE. (scratch 10 from the list)
• Light for all halls through the SHG crystal is
  linearly polarized and perfectly round. Now we
  can place a LP optic immediately before the
  helicity control Pockels Cell to obtain the
  highest purity polarization possible.
• All components are designed with quick disconnect
  polarization maintaining fiber connectors. When
  connected there are no laser safety issues except
  for the area of the fiber to air launch to the
  SHG crystal and subsequent beam delivery optics.
  An expert can be anyone with training on the
  system. (scratch the entire list)

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Final laser system operation continued

• The commercial fiber lasers, amplifiers and
  modulators often come with monitoring ports
  installed. There will be multiple points within
  the system to verify system operation and laser
  beam quality.
• We will be producing much more light than is
  needed, so we will now be able to afford placing
  fast photo-diodes and power taps at the output
  for phase feedback monitoring and control
• The system will consist of 19 rack mounted
  drawers that can easily be interlocked to power
  off when the lid is opened and thus eliminate any
  laser hazard.
• The main laser system could be remotely located
  (upstairs in the service building) and the main
  delivery fiber can be fed to the tunnel through a
  conduit.
• Operators will be able to select any laser for
  any hall. There will no longer be any confusion
  over the capabilities of a given laser.

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System is compact and easy to swap components
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Possible Pitfalls

• This is pure RD. To the best of our knowledge
  it has never been done before and may not work
  exactly as planned.
• Communications laser companies are making big
  from communications users. They have no interest
  in pursuing our little project, but have been
  helpful in offering to sell us components.
• There is a possibility that our mode of operation
  and PSS/FSD protection could change. Example
  If ASE passes amplifier when a given Hall is in
  Beam Sync, we would need to secure all halls by
  securing the main laser amplifier until the
  chopper slit could be fully inserted. This would
  be very similar to how we used to run the
  thermionic beam.
• We may find temperature induced phase or mode
  variations in the fiber system that we have never
  experienced before in a free space system.
• The halls will lose their ability to
  independently move a PZT mirror for their hall.
  It is envisioned that one PZT mirror would serve
  all halls and the 30Hz PZT functions.

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Laser system wide view
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Fiber laser launch on table
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Whats next?

• Matt Poelker and I will be performing laser
  studies and procuring components.
• Our group is short staffed and we need another
  PhD. One might consider finding one with
  experience on fiber lasers.
• We need input as early as possible from anyone
  who has any special needs for the beam. (i.e.
  special modulation schemes) So these can be
  designed into the system.
• If we want a quality product in the shortest
  amount of time we will need to form a team that
  consists of
• 1. Electrical engineering support
• 2. Software support (drivers and screens)
• 3. EECAD and FAB support
• 4. Rf Engineering support
• 5. PSS/MPS system support
  and.

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One Million Dollars!

• Actually much less, but we do need the labs
  commitment for funding of the project.

Which we shall call
The Alan Parsons Project
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Actual cost rough analysis (optical components)
15K to 30K
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Rough cost analysis
6K
57K