Measurement of Bunch-Length by a Mode-Locked Laser with a Time Resolution of 10 Picoseconds

In accelerators working at accelerating frequencies of several 100 MHz the bunch length is in the order of several hundred picoseconds. For obtaining good measurements the time resolution of the measuring apparatus should be 10 ps. With convenient electronic components such a high time resolution cannot be obtained. In this paper a method is described for measuring the bunch length optically by measuring the time structure of the light emitted by relativistic particles (synchrotron or Cerenkov radiation). The heart of the system is a wideband Nd3+-Glass Laser emitting infrared light at 1.06 ¿m. With a special technique, the so-called mode-locking, the laser is forced to emit high-intensity optical pulses of several GW peak power and a pulse-length of about 5 ps. These pulses enter a cell filled with a Kerr medium, e.g. CS2, and induce birefringence. Since the relaxation time of CS2 is small, the bire-fringent zone travels with c/n through the Kerr medium. Combinated with two polarizers the system works like a camera with a slit-shutter traveling at the velocity of light. By photographing a total time resolution of 10 ps is obtained.