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Pulse Pickers, Cavity Dumpers

Pulse Pickers, Cavity Dumpers

Pulse pickers and cavity dumpers are high speed acousto-optic modulators that can separate and divert a single laser pulse within a pulse train to a new optical path. Both require a very fast rise time with a low duty cycle. With rise/fall times as short as 4 ns and a variety of materials at our disposal, we can create a complete solution to select individual laser pulses with speed and synchronization.

A cavity dumper refers to an acousto-optic modulator used inside the cavity of an argon, dye, or Ti:Sapphire mode locked laser. A pulse picker describes the same function when used externally. Both use high-speed acousto-optic crystal and design techniques to keep rise time to a minimum. Similar to a fast modulator, tight focusing of the beam within the device is required to achieve the desired speed (typically Rise time is the primary selection criteria when selecting a single pulse from a pulsed laser. We offer material options in fused silica, tellurium dioxide (TeO2), and gallium arsenide (GaAs) to achieve rise times of < 7 ns for wavelengths from the near-UV (350 nm) to the near infrared (1550 nm).

Other requirements which may require trade-off decisions include:

  • Optical power
  • Desired throughput
  • Polarization of the input light (linear, circular, random)
  • Repetition rate of the laser
  • Pick rate required

Despite the fast rise time, the modulation rate for a pulse picker or cavity dumper is actually low (generally < 10 MHz). The acousto-optic device is “on” for a short time, followed by a long “off” time. It is designed specifically to maximize diffraction efficiency and minimize rise/fall time when operated at a low duty cycle, but can handle the high RF power only for the limited amount of time required.

Optical power handling is a very important selection criteria for a pulse picker or cavity dumper. The rise time is limited by the time needed for the acoustic wave to travel through the optical beam. Tight focusing into the device is essential. This results in very high input power density. Our high manufacturing standards and rigorous adherence to quality of AO materials reduces scatter and results in industry-leading optical power handling. Where applicable, antireflection coatings minimize insertion loss.
By using a high RF carrier frequency and an optimized lens design, we can provide large angular separation between the undeflected and deflected beams (up to 75 mrad). This makes it easy to separate the two beams in space and maintain a static contrast ratio of at least 500:1.
The choice and application of the RF driver is particularly important in pulse picking and cavity dumping to synchronize accurately with the laser pulse train. Controls on the driver optimize the RF drive pulse shape and delay. The modulation signal is synchronized and optimized using a control loop to select the best time at which to pick off the pulse.

Synchronization of a pulse picker or cavity dumper can be quite complex, as can achieving the stringent focusing required to achieve high speeds. Our team can provide guidance in selecting the optimal combination of device and RF drivers.

Applications of Pulse Pickers, Cavity Dumpers

Generation of high energy pulses, injection and extraction of pulses from regenerative amplifiers, femtosecond/picosecond lasers, reduction of pulse repetition rates

ProductTypeWavelengthRise / Fall TimeActive ApertureOperating FrequencyOptical Material