OG-B/F. Pulse Pickers with Fixed Gate Open Time
|Electrooptic crystal||DKDP||BBO||RTP or KTP|
|Operational wavelength range (standard models)||510-540 nm / 700-1000 nm / 1000-1100 nm / 1250 nm||510-540 nm / 700-1000 nm / 1000-1100 nm / 1250 nm||1000-1100 nm / 1500-1600 nm / 1550+780 nm|
|Possible custom wavelengths||from 340 to 1280 nm||from 200 to 2200 nm||from 1000 to 2700 nm|
|Minimum repetition rate of picked pulses||single-shot|
|Maximum repetition rate of picked pulses (standard models)||1 kHz||3 kHz / 10 kHz / 50 kHz / 100 kHz||3 kHz / 10 kHz / 50 kHz / 100 kHz / 200 kHz / 1 MHz|
|Clear aperture||6 mm||6 mm (up to 20 mm upon request)||2.5 mm (up to 5 mm upon request)||3.5 mm (up to 10 mm upon request)|
|HV pulse (gate) shape||bell-shaped|
|HV pulse (gate) length
(at level 10%)
|3 ns||8 ns|
(at level 10-90%)
|700 ps||3.5 ns|
(at level 90-10%)
|2 ns||3.5 ns|
|Pockels cell voltage||up to 10 kV||up to 4 kV||up to 3 kV|
(quarter-wave upon request)
|half-wave/quarter-wave at λ<600 nm; at λ>600 nm - quarter-wave or half-wave with double crystal cell||half-wave
(quarter-wave upon request)
|Complete system transmission at central wavelength||>85% (PC + two Glan-Taylor prisms, default package)
>90% (PC + a Glan-Taylor and a Rochon prism, upon request)
>98% (Pockels cell only)
|Input optical train repetition rate||2 MHz …150 MHz with internal or external trigger with optical or electric sync to input train; 1 Hz…2 MHz only with external trigger signal(2)|
|Sync source for internal trigger mode||optical/electric|
|Additional control modes||by edge / by level / built-in burst generator with internal or external trigger|
|Channel delay time||0...10 us (up to 1 ms upon request)(3)||0...1 us (up to 5 us upon request)(3)|
|Cooling||passive at up to 3 kHz output rate, water-cooled(4) at 10 kHz and higher|
|(1) - energy ratio of a picked and any non-picked pulse. The given contrast values are valid for adjacent pulses;
(2) - when operating with 1 Hz…2 MHz input pulse train, an external trigger signal must be used. Such a signal must lead the optical pulse by 0.25-3 us, must be rigidly in sync with the pulse to be picked and must have jitter of less than 200 ps;
(3) - maximum channel delay time is defined by maximum repetition rate of picked pulses and cannot exceed the temporal distance between adjacent pulses at this frequency;
(4) - the HV generator unit requires cooling to support operating temperature below 35°C. Water cooling is required when operating at output frequency of 10 kHz and more (water flow 1 L/min, water temp. 20-22°C, tap or building-scale supply may be also be used if permitted). Heating power to be dissipated does not exceed 80 W even at the highest frequencies (a water chiller is not included and may be supplied separately);
An electro-optic modulator (EOM) is a pulse selection device based on the Pockels electro-optic effect. Electric field applied to an optical medium induces birefringence and allows fast selection of passing laser pulses. Thus, such a device might be used as a fast pulse picker solution.
Electro-optic pulse pickers with fixed gate duration are widely used in the following applications:
- picking of single laser pulses from a train of pulses;
- reduction of repetition rate of laser pulses;
- building regenerative laser amplifiers: injection of seed pulses into amplifier cavity and ejection of amplified pulses out of amplifier cavity;
- pulse slicer, i.e. output amplified pulse contrast improvement via cleaning of "parasitic" pulses.
The OG pulse picker consists of a certain optical setup and an electronic control and power supply unit.
Optical setup of a pulse picker comprises two crossed polarizers with a Pockels cell placed in between. Pulse selection is performed with the use of the Pockels effect — a short high-voltage electric pulse induces birefringence in the Pockels cell's optical crystal allowing precise rotation of the polarization plane of a certain laser pulse. Then this pulse may be separated from a train of consecutive pulses by using a polarizer. Since the response time of the Pockels effect is very short, such a unit is suitable for working with dense pulse trains and ultrashort laser pulses.
Electronic control unit of a pulse selector is built using programmable logic components and contains such built-in modules as optical and electronic synchronization, trigger pulse generation, frequency division, multi-channel delay signal generation, high-voltage drivers. The unit allows for multiple usage scenarios given below for integration in various laser setups:
- external or internal trigger;
- synchronization to optical (built-in photodetector) or electric signal;
- two independent delay channel groups, each with its trigger and synchronization signals; spare channels may be used as general purpose delay channels for triggering other experimental equipment (e.g. oscilloscopes, pump lasers, streak-cameras etc.);
- additional control modes with triggering by edge/level of an external signal or via a built-in burst generator allow formation of various output pulse patterns;
- up to 4 synchronized Pockels cells may be driven by a single electronic control unit.
Optional solutions for demanding applications:
- multi-channel Pockels cell, i.e. a cell in which up to 4 high-voltage generators may be connected to a single optical crystal with adjustable delay between high-voltage pulses of each generator; such a cell may be used to select several closely placed pulses in a train of pulses or used as a single Pockels cell both for injection and ejection purposes in a regenerative amplifier setup:
- combination of fixed HV pulse channel and adjustable/wide fixed HV pulse channel on a single Pockels cell;
- pulse pickers with two consecutive Pockels cells for suppression of depolarization at high average optical power ratings and increase of overall contrast ratio of the picker unit up to 50 dB;
- provision to install power supplies for additional Pockels cells (total up to 4 single-channel cells per one control unit) at time of purchase, but the additional cells themselves may be purchased later;
- extended choice of polarizers, namely Rochon prisms (increased transmission) or thin-film dielectric polarizers (increased damage threshold);
- optical setup may be combined onto a single optical breadboard with adjustable feet.
What is a quarter-wave scheme?
A quarter-wave scheme implements double pass of the laser beam through the Pockels cell, effectively allowing usage of quarter-wave voltage instead of half-wave voltage. It may be required to reach longer wavelengths for certain crystals or increase the output repetition rate of the pulse picker system while keeping the electric part of the system at reasonable cost level.
However, such scheme has a significant drawback: the idle beam of non-picked pulses in such a scheme propagates in backward direction towards the laser source. This might be mitigated by tilting the return mirror or by using a Faraday isolator instead of a simple polarizer.
Such a scheme also fits perfectly in such optical layouts where double pass opportunities exist by design (e.g. in linear cavities).
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