Transcript A proposal for an improved laser system for the CEBAF photo-injector. John Hansknecht Electron Gun Group Thomas Jefferson National Accelerator Facility Operated by the Southeastern.

A proposal for an improved laser system for
the CEBAF photo-injector.
John Hansknecht
Electron Gun Group
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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:
1. Most of the beam produced was
thrown away on the chopper.
2. Controls were not suited for
production beam.
Thomas Jefferson National Accelerator Facility
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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).
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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The 1996 CEBAF Laser Table
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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1996 Laser Pulse Structure
668 ps
A
A
C
B
B
C
38 dBm
-15 dBm
50 ps FWHM
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.
Thomas Jefferson National Accelerator Facility
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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.
Thomas Jefferson National Accelerator Facility
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1997 Laser Table schematic
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Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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Key points for 3 laser operations
Beam combining methods
http://www.jlab.org/accel/inj_group/laserparts/Beam_combining_tutorial.pdf
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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1997 laser table (open in lab)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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1997 Laser Pulse Structure
668 ps
A
B
C
A
B
C
38 dBm
-15 dBm
50 ps FWHM
• 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.
Thomas Jefferson National Accelerator Facility
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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
Thomas Jefferson National Accelerator Facility
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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
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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1999 - Ready for some serious physics now
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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1999 laser table schematic
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Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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Y2K – Introducing the Actively-modelocked
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
Thomas Jefferson National Accelerator Facility
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Ti-
Y2K- Actively modelocked Ti-Sapphire laser
schematic
Jlab Patented Technology – C. Hovater, M. Poelker
Thomas Jefferson National Accelerator Facility
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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.
Thomas Jefferson National Accelerator Facility
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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
Thomas Jefferson National Accelerator Facility
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2 Halls from 1 Ti-Sapphire Laser (Nov 2000)
plate
Glan
Thomson
Polarizer
Beam from Diode Laser
for Hall B. Phased 120 degrees from
A and C
Either
100%
reflector
Atten B
plate
plate
500Mhz pulse train
Linear polarized from
Ti-Sapphire laser
Glan
Thomson
Polarizer
Polarized
beam splitter
Polarized
beam splitter
2%
reflector
Atten C
plate
Glan
Thomson
Polarizer
Small A-C loss
Large B loss
100% reflector
100%
reflector
Atten A
Path Length of assembly is adjustable so the returning beam
will be delayed by 120 degrees of Rf phase
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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To
Pockels
Cell
and
Cathode
2001 Laser System g0 preps
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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2001-2002
•
•
•
•
New technology available- (SESAM technology) Commercial TiSapphire 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.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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2002 – Present Laser Table
Thomas Jefferson National Accelerator Facility
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Enhancements
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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2004- A Safer and Cleaner environment
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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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.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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Laser Specific issues that need to be addressed
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
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.
Thomas Jefferson National Accelerator Facility
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Let’s 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. Let’s 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)
Thomas Jefferson National Accelerator Facility
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New laser system design. Lets see what we have so far.
Fast Electronic Attenuator
500 MHz
Phase 0 degrees
(A)
1560 nM
Pre-Ampifier
Pre-Amp
Fast Electronic Attenuator
500 MHz
Phase 120 degrees
(B)
1560 nM
Pre-Ampifier
Pre-Amp
Fast Electronic Attenuator
500 MHz
Phase 240 degrees
(C)
1560 nM
Pre-Amp
Pre-Ampifier
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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More design thoughts
Since combining these lasers appear to be so easy,
let’s 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)
Thomas Jefferson National Accelerator Facility
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Laser system design continued:
Fast Electronic Attenuator
500 MHz
Phase 0 degrees
(A)
1560 nM
Pre-Ampifier
Pre-Amp
MEMS optical switch
Custom 1.5 GHz
Fast Electronic Attenuator
500 MHz
Phase 120 degrees
(B)
1560 nM
To Eb-Yb
Power Amplifier
Pre-Ampifier
Pre-Amp
Uniform 1.5 GHz
Fast Electronic Attenuator
500 MHz
Phase 240 degrees
(C)
1560 nM
Pre-Ampifier
Pre-Amp
Fast Electronic Attenuator
1.5 GHz
1560 nM
Pre-Ampifier
Pre-Amp
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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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)
Thomas Jefferson National Accelerator Facility
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The final pieces of the system
Erbium/Ytterbium
Fiber Amplifier
SHG Crystal
From Beam
Combining
Wavelength
Filter.
780nM Pass
1560nM
Block
780nM
Pulsed Beam
Delivery
Launch to free space
915 nM Diode
Pump Laser(s)
Thomas Jefferson National Accelerator Facility
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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)
Thomas Jefferson National Accelerator Facility
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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.
Thomas Jefferson National Accelerator Facility
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System is compact and easy to swap components
Thomas Jefferson National Accelerator Facility
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Possible Pitfalls
•
This is pure R&D. 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.
Thomas Jefferson National Accelerator Facility
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Laser system wide view
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Fiber laser launch on table
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What’s 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…………….
Thomas Jefferson National Accelerator Facility
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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…
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Actual cost rough analysis (optical components)
$15K to 30K
Fast Electronic Attenuator
500 MHz
Phase 0 degrees
(A)
1560 nM
Pre-Ampifier
MEMS optical switch
Custom 1.5 GHz
Fast Electronic Attenuator
500 MHz
Phase 120 degrees
(B)
1560 nM
To Eb-Yb
Power Amplifier
Pre-Ampifier
Uniform 1.5 GHz
Fast Electronic Attenuator
500 MHz
Phase 240 degrees
(C)
1560 nM
Pre-Ampifier
Fast Electronic Attenuator
1.5 GHz
1560 nM
Pre-Ampifier
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
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Rough cost analysis
Erbium/Ytterbium
Fiber Amplifier
SHG Crystal
From Beam
Combining
Wavelength
Filter.
780nM Pass
1560nM
Block
780nM
Pulsed Beam
Delivery
Launch to free space
915 nM Diode
Pump Laser(s)
$6K
$57K
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy
Page 43