The Cavity Dumper Kit is an acousto-optical switch that can be integrated into the cavity of a mode-locked laser system for the reduction of the system’s pulse repetition rate. Unlike pulse pickers the intracavity operation increases the pulse energy to a multiple of the energy that is reached by the normal mode-locked pulses of the system. This makes it particularly relevant for non-linear optical processes like SHG and THG.

The Cavity Dumper Kit consists of an acousto-optical Bragg cell to be integrated into the laser cavity and the pulseSwitch control electronics. As an option APE offers a mounting kit for holding the Bragg cell, which provides all the necessary tools for adjusting the modulator.

The control electronics generate the RF carrier signal for the Bragg cell from a seed input signal with the repetition rate of the mode-locked system. An internal frequency divider allows for division rates between 1 : 2 and 1 : 260,000 of the seed frequency. External triggering and therefore single pulse operation is possible as well. The phase of the RF signal is fine-tuned by a phase shifter as a requirement for using the cavity dumper in double pass operation.

• Reduction of the pulse repetition rate while increasing the pulse energy
• Single pulse operation
• Generation of powerful laser pulses
• Fast rise times
• Tunable phase shift
• Suitable for a variety of different Ti:Sapphire, ion- and dye lasers as well as other laser systems

The cavity dumper pulseSwitch is an acousto-optical switch, which – unlike pulse pickers – is integrated into the laser resonator. It is designed specifically for use with the Coherent Mira 900 femtosecond Ti:Sapphire laser. Since 2017, pulseSwitch is no longer available but replaced by the Cavitiy Dumper Kit (see above).

A cavity dumper extends the usability of a mode-locked laser system by offering higher peak power at lower repetition rates.

Mode-locked lasers which are capable of generating very short laser pulses (e.g. picosecond or femtosecond pulses) have pulse repetition rates corresponding to the round trip time of the light in the laser cavity. This frequency is around 80 MHz for many tunable solid state-, ion-, and dye lasers used in scientific applications. This frequency corresponds to a pulse separation time of 12.5 ns. For a number of applications like the measurement of longer fluorescence delay times or some pump and probe experiments it is desirable, however, to have a lower repetition rate.

This can be achieved with different methods acting inside or outside the laser cavity. The APE Cavity Dumper Kit, for example, is well suited for pulse selection of mode-locked lasers with repetition rates between 50 MHz and 85 MHz inside the laser cavity. The pulse energy increases compared to simple mode-locked operation at the same time. The function is based on the acousto-optical effect. In a suitable crystal (e.g. fused silica) a modulation of density and thus a refractive index is introduced by applying a high frequency acoustic signal. This acts as a three dimensional optical grating on a laser beam passing the crystal and leads to a diffraction of the beam. The acoustic wave inside the crystal is generated by applying an electrical RF signal to a piezoelectric transducer cemented on the crystal. By using short RF pulses single laser pulses can be selected out of the pulse train and detected to the first diffraction order. That way they are separated from the other pulses and coupled out of the laser cavity and can be used in the experiment.

A typical Cavity Dumper Kit may consist of the fused silica Bragg cell and mounting kit and the driver and control electronics module. The Bragg cell as the most important part selects single pulses from the laser beam based on the acousto-optical effect. The intra-cavity beam must be focused into the Bragg cell by suitable spherical mirrors to achieve the necessary fast switching time. The driver electronics provides the modulated RF signal for the Bragg cell with a carrier frequency equal to the fivefold of the laser repetition rate and an output power of up to 16 W (peak). The division ratio of the mode-locked laser repetition rate is for example between 1:2 and 1:260,000. For synchronization, the user must provide a seed signal with the laser repetition rate (for instance from a fast photodiode) and with an amplitude of 30 mV … 300 mV at 50 Ohm. Instead of using the internal frequency divider, pulses can be selected by external triggering with a TTL signal at 50 Ohm external trigger input. In this case the “SEED” input signal is required as well. Depending on the applied modulator type diffraction efficiencies of > 50 % (fused silica, single pass at 800 nm) can be achieved. Usually the cavity dumper is operated in double pass mode to increase the efficiency. For this the Cavity Dumper Kit offers a phase adjustment of the RF carrier frequency.