Synchronization System for LUX

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Synchronization System for LUX John Staples,
LBNL 26 July 2004
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Fundamental Approach to Timing

• Distribute accurate clock to all accelerator
  elements and end-station pump lasers
• Measure residual jitter of accelerator
  components, sum them up, and transmit to
  end-station lasers
• All elements ride on top of common-mode clock
  jitter
• Differential jitter between X-ray pulse and
  end-station lasers is reduced to 50 femtosecond
  regime
• Long term mechanical drifts also very important
• 10 fsec is equivalent to a 3 micron motion
• 1 meter of aluminum or SS grows 3 microns for 0.1
  degree C temperature change, invar about 0.1
  microns
• transport systems geometric changes must also be
  monitored

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Why Should This Work?

• Spectral density of timing jitter dominated by
  low-frequency phenomena
• flicker and random walk of frequency and phase
• The coherence time of the significant spectral
  components is long (audio to sub-audio)
• The jitter components above this frequency range
  are usually below the noise floor of the monitors
• The integral over frequency space, the time
  jitter, is dominated by the low-frequency part of
  the spectrum.

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Available Technology

• Stabilized fiber laser links show promise for
  transporting timing signals with femtosecond
  jitter over short distances
• ML lasers have been synchronized to 1 fsec
  relative jitter using electrical techniques, at
  10-14 GHz
• Commodity fiber components are widely available
• but the fiber must be actively stabilized
• Don't need a super-stable clock
• Crystal oscillators useable, a Poseidon not
  necessary
• Common-mode jitter of a picosecond acceptable

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NLC Approach Frisch et al. Transmit 357 MHz
timing signal over 15 km About 1 degree X-band
over moderate time scales (240 fs)
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Hardware Approach

• Start with a good (lt1 psec jitter) clock
• Distribute clock very accurately (lt25 fsec) to
  all elements in accelerator and endstations
• Use 1550 nm fiber optics components
• Stabilize each fiber distribution link
• Will try to stabilize based on modulation, not
  optical carrier
• Fiber has much better bandwidth than coax cable
• Local loops stabilize elements within
  gain/bandwidth limitations, provide residual
  error signal
• Weighted sum of low-frequency error signal
  distributed over low-bandwidth digital link to
  TBCs
• End-station lasers follow low-bandwidth correctors

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ExampleCrab Cavity

• 3.9 GHz deflector
• include clock, microphonics
• I-Q LLRF system
• simulate residual noise after control loop is
  closed
• 350 watts klystron output power
• 12 fsec residual

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Timing Distribution

• Require timing distribution system that provides
  differential-mode jitter of a few femtoseconds to
  all clients over the entire facility.
• Demonstrate a stabilized fiber network that can
  satisfy this requirement
• fiber has wide bandwidth capability
• use RF techniques to achive jitter stabilization
• if inadequate, revert to interferometric
  techniques

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Proof-of-Principle Experiment

• The key is low-jitter distribution of a 1 GHz
  clock
• Demonstration is constrained by budget
• 1 GHz crystal clock (Wenzel 100 MHz
  multipliers)
• 1.5 mW 1550 nm DFB single-mode laser
• EDFA increases power up to about 40 mW
• Mach-Zehnder modulator, no chirp
• 100 m single-mode fiber, APC/PC connectors
• Low-noise (75K, 1 db NF) RF amplifiers
• Low-noise op-amps in LLRF circuits
• Piezo stabilization of fiber

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Contributions to Jitter in POP

• LLRF system operational bandwidth rolloff at 1
  kHz
• 1 dB noise figure of ZRL-1150LN amplifiers
  following photodiodes operating at -8 dBm adds
  about 9 fsec noise at 1 GHz over a 10 kHz
  bandwidth
• LMH6624 Low-noise op amps following phase
  detectors will add noise in the fsec range.
• Still characterizing ZFM-4 mixers (phase
  detectors), but contribution may be significant
• Wenzel clock is measured to have a 1.0 psec jitter

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Example Noise contribution of op-amp following
mixer
Integrate noise current and voltage, including
Johnson noise over working bandwidth.
With a 10 kHz 2-pole LPF, the noise integrates to
0.2 uV, for a sub-femtosecond phase jitter.
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Characterizing the Piezo Fiber Optic Phase
Modulator
Strong 18 kHz mechanical resonance, Q139,
measured interferometrically Shape and stabilize
feedback loop around modulator
Open-loop Bode Plot
Nyquist Stability Plot
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Present Status

• Have all the optical components
• have measured and verified their specifications
• LLRF system designed, not yet constructed
• all items are at hand, including PC boards
• Signal generators, spectrum analyzers, etc. are
  all acquired

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Future Developments

• Will establish capabilities of a fiber
  stabilization system at 1 GHz.
• Move on to 10 GHz region
• Improve laser itself, if budget permits
• Mode-locked
• A gas-stabilized reference would be nice
• Look at interferometric techniques if necessary