Characterization of an Electro-Optical Modulator for Next Linear Collider Photocathode Research
The SLAC Next Linear Collider design parameters call for a polarized electron source capable of outputting bunches of electrons 1.4 ns apart (714 MHz). Stanford Linear Accelerator Center (SLAC) currently uses a pulsed Ti:Sapphire laser incident on a GaAs photocathode to produce polarized electrons. The pulses produced by this laser are approximately 300 ns in duration and occur at a frequency of 60 or 120 Hz. The purpose of this research is to find a way to modulate the 300 ns pulse into a high-frequency pulse train. High-frequency modulation of the 300 ns laser pulses was achieved with an electro-optical (EO) modulator, which uses the Pockel's Effect to change the polarization of incoming light. The highest frequency possible was 200 MHz (limited by the RF amplifier). For this reason, the SLAC 119MHz timing signal was selected for input into the amplifier. The output of the amplifier was passed through the EO modulator to induce an alternating half-wave phase shift of the incident laser light. The cube polarizer at the exit of the modulator rejects any laser light not parallel to its polarization axis. These two effects coupled together produce amplitude modulation of the laser pulse. The modulation can be changed by several variables. The linearly polarized light incident into the modulator can be rotated by a half-wave plate to any polarization angle. The DC Bias could be changed to give the RF signal an offset. The input signal can also be changed to vary the voltage across the crystals. Finally, the RF setup (attenuators, splitters, cables, terminators) can be changed. This paper examines how all of these variables change the modulation characteristics of the EO modulator used. The rotation of the half-wave plate was found to change the percent modulation from 10% to 90%. Adding a DC Bias to the half-wave plate rotation caused the modulated signal to change shape as modulation depth still ranged from 10-90%. The modulation depth was shown to increase with increased input voltage. RF setup was only varied slightly but small changes still gave drastic changes in percent modulation. The EO modulator examined showed that high-frequency modulation of a Ti:Sapphire laser pulse is possible with a high percent modulation. This research should be extended up to 714 MHz and the RF setup should be optimized for maximum modulation in future studies.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (US)
- DOE Contract Number:
- AC03-76SF00515
- OSTI ID:
- 833116
- Report Number(s):
- SLAC-TN-04-062; TRN: US0406571
- Resource Relation:
- Other Information: PBD: 3 Sep 2004
- Country of Publication:
- United States
- Language:
- English
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