# Presentation: 149956

## SIST 2004 Final Presentation

- Phase Detector (Comparator) For the A0 Laser

- Dzifa Y. Duose

- Duose

- August 10, 2004

## Objective

- The main aim of the project is to confirm or refute the phase error signal from the commercial phase detector currently in use.

- Duose

- August 10, 2004

## Outline

- Fermilab
- A0 Photo-injector Background
- The A0 Laser System
-  The Mode locked oscillator
- Operation of Phase Locked Box
- The Project:Phase Detector
- Conclusion

- Duose

- August 10, 2004

##  Fermilab

- Fermi National Accelerator Laboratory (FNAL) is a renowned  research facility in high-energy physics.
- The A0 Photo-injector Project (A0PI) is to prototype the low energy stages of an electron-positron linear collider known as the TeV Electron Super-conducting Linear Accelerator (TESLA).
- FNAL collaborated with UCLA, INFN Milan, University of Rochester and DESY  to work on the A0PI project.

- Duose

- August 10, 2004

## The A0 Photo-Injector

- A0PI produces high quality electron bunches used in  advanced accelerator Research.

- Fig 1: A0PI Beam line

-     It Consists of: Laser System, RF Gun and Super-conducting Cavity

- Duose

- August 10, 2004

##  The A0 Laser System

- Oscillator: generates Laser pulse train
- 2.2Km Fiber- expands pulse and triples spectral BW
- Pockell Cells: Pulse Selection
- Nd:glass: amplified to 5.8 x 106 gain saturation.
- Pair of  parallel plate gratings compresses BW of laser pulse
- Crystals: Quadruple frequency from IR to UV

-  Laser System

- Duose

- August 10, 2004

## Mode locked Oscillator

- Free oscillation of laser modes occur in the oscillator cavity because it is a linear resonator.

- Each mode oscillates at a random phase compared to the others resulting in large intensity fluctuations at the laser output.

- Active mode locking is used to synchronize the laser output with a reference signal to ensure minimum intensity fluctuations (stable phase relationship among modes).

- It is important to have every laser pulse in phase with the RF because once the relative phase is set; every laser pulse will arrive at the photo-cathode at the correct launch phase for acceleration.
- TESLA stipulates a phase jitter of less than 1-degree of the RF.

- A commercial phase locked box is utilized to keep the laser phase locked to the RF signal

- Duose

- August 10, 2004

## Operation of Phase Locked Box

- The reference signal (81.25 MHz) is divided by 2 and applied as the drive signal to an acousto-optic mode-locker.
- The actual phase of the 81.25 MHz on the laser output is picked up from a fast photodiode (This is seen as a pulse signal).
- A phase detector then compares the actual 81.25 MHz with the reference.
- The phase error signal from the output of the phase detector is applied to a phase shifter, which modifies the drive signal.

- Fig 3: Phase Lock Box

- Duose

- August 10, 2004

## The Project:Phase Detector

- The Project is to design, build and test an external phase detector (comparator) for the laser system to confirm or refute the error signal obtained by the commercial phase locked box currently in use.
- What is a phase detector?
- A phase detector is a device that compares two input frequencies and generates an output that is a measure of their phase difference.
- There are three types of phase detectors: Analog or multiplier (mixer), Exclusive-OR  and Digital phase-frequency detector.
- Which Phase Detector is best to use?
- Because of the high noise associated with the photodiode signal, an analog mixer is the best choice because it is capable of recovering a signal from low signal-to-noise input.
- A double balanced frequency mixer can be used as a phase detector given that the two signals being compared have identical frequencies and unvarying amplitude.

- Duose

- August 10, 2004

## The Project:Phase Detector

-  What is a double balanced mixer?

- How does it work?

-  Mixer Operation

- Duose

- August 10, 2004

## The Project:Phase Detector

- What if the input Signal is not a sinusoidal?
- The 81.25 MHz PD signal is not Sinusoidal but the  81.25 MHz RF signal is.
- What Next?
- Convert the signal into a sinusoid.
- How?
- A 90 MHz Low Pass Filter is used to reshape the signal by eliminating higher order modes.

-  Conversion of PD Signal

- Duose

- August 10, 2004

## The Project:Phase Detector

- What if the signal power is too low for the mixer?
- The PD signal is a current signal and therefore must be converted to a voltage signal before it can  be amplified. A trans-impedance amplifier is used to do this.

- Trans-impedance Amplifier Design

- Duose

- August 10, 2004

## The Project:Phase Detector

- The design so far....

- Block Diagram of Intermediate Design

- Duose

- August 10, 2004

## The Project:Phase Detector

- How does a change in the amplitude of the input signal affect the phase error signal?
- The relative phase was set to 0° and measured over time. The results are shown below.

-   An increase in amplitude of the input signal causes an increase in the output signal (and vice-versa) thereby causing ambiguity in the phase error signal.

- Duose

- August 10, 2004

## The Project:Phase Detector

- How to solve this problem?
- Design a system whose phase error signal is independent of change in input amplitude.
- How?
- By the use of AD8302 RF/IF Gain Phase Detector
- Why?
- It consists of a limiter at its inputs, an amplifier and a LPF at the output.
- Its DC level ranges from 30mV to 1.77V implying:
- 180-degrees =  30mV
- 0-degrees = 1.77mV
- 90-degrees = 930 mV
- It was calibrated to have  9.7-millivolt of phase-detector output per degree.

- AD8302

- Duose

- August 10, 2004

## The Project:Phase Detector

- The Block Diagram of Final Design

- Final Design Block Diagram

- Duose

- August 10, 2004

## The Project:Phase Detector

- How does this compare with  the commercial phase detector currently in use?

- Duose

- August 10, 2004

## Conclusion

- It can be concluded that the phase error signal of the two systems agree within 0.1-degree.
- Although more investigation needs to be made, it would be advisable to replace the phase detection circuit of the commercial phase locked box with a more accurate circuit such as the one described in this talk.

- Duose

- August 10, 2004

## Acknowledgement

- I would like to thank:
-  My supervisor, James Santucci for his leadership and guidance.
-  Rene Padilla and Nikolai Barov for their invaluable assistance and practical advices on this project.
-  Dianne Engram, Elliott McCrory, David Peterson, James Davenport and the entire SIST committee for giving me this great opportunity.
- The entire SIST/GEM student interns for making my stay here enjoyable.

- Duose

- August 10, 2004