Del Mar Photonics - Wedge 50 titanium : sapphire (Ti:sapphire) amplifier system manual - Request a quote

This manual describes installation and operation of Wedge 50 titanium : sapphire (Ti:sapphire) amplifier system. The system is based on a femtosecond confocal multipass amplifier configuration and consists of the following parts:

1. Pulse stretcher
2. Multipass Ti : sapphire amplifier
3. Pulse compressor
4. Pulse picker and Pockels cell driver
5. Synchronization electronics

The seed pulses used in the system originate from a mode-locked Ti: sapphire seed oscillator, model Trestles 50 (or Trestles Finesse, Trestles Opus, Trestles LH or Trestles EL) are recommended. Before amplification, femtosecond pulses are stretched in time to avoid effects of peak power damage in high energy ultrafast amplifiers. Femtosecond pulses with pulse duration 100 fs are stretched to several tens of picoseconds before pulse selection and amplification. To decrease pulse repetition rate, a Pockels cell is placed between crossed polarizers. This pulse picker system permits transmission of a single pulse during a <6-ns window that is synchronized with a laser through the countdown and synchronization unit. The countdown electronics receives 80 90 MHz signal from the output pulses of the Ti : sapphire oscillator and divides this rate to 1000 Hz according to the pulse repetition rate of Nd:YLF laser pumping the multipass amplifier. After the Pockels cell, the pulse is injected into a two-mirror confocal multipass amplifier (MPA) that is effective device for amplification of femtosecond pulses in a Ti: sapphire crystal. After six, eight, or ten passes the seed pulse is amplified by a factor up to 106 and leaves the amplifier through the aperture in the output mirror. The pump radiation is focused by the lens through the aperture in the input mirror.

After amplification, the picosecond pulses are temporally compressed to 50 - 100 fs pulses (depending on the input pulse duration) by one-grating pulse compressor. At the pumping with a frequency doubled Nd: YLF pumping laser (1000 Hz, 20 mJ/pulse), the compressor gives 1.0 mJ pulses at 800 20 nm. Request a quote

Femtosecond Pulse Stretcher
Temporal pulse stretching is required in avoiding the effects of peak power damage in high energy ultrafast amplifiers. This peak power damage is due to the tendency of bright beams to self focusing (a result of non-linearity in the index of refraction), which makes it necessary to limit the intensity present in amplifiers. The technique of chirped pulse amplification (CPA) gives a possibility to avoid this obstacle. The idea of CPA is to stretch femtosecond pulse duration reducing peak power before amplification and to compress pulse duration back to femtoseconds after the amplification. A principal scheme of femtosecond pulse stretcher is shown in the Figure 2. This is purely optical device containing diffraction grating, spherical mirror and plane mirrors. Femtosecond pulse going into pulse stretcher has a broad bandwidth. For a 100 fs Gaussian pulse the corresponding bandwidth is about 9 nm. A diffraction grating sends different frequencies in different directions at different angles of diffraction shown in the Figure 2 for long wavelength (shown as red) and short wavelength (shown as blue) spectral components of femtosecond pulse. After double pass, bluer and redder components exit the stretcher as shown in the Figure 2. One can see from the figure that bluer frequency components have to travel further through the stretcher than the redder frequency components. The result is that the redder frequency components exit the stretcher first, the pulse has been stretched. In the Wedge 50 pulse stretcher the input pulse is dispersed in the horizontal plane. The stretched pulse is directed back to the stretcher with help of vertical retroreflector, and four passes through the stretcher are achieved. Four-pass configuration is necessary to ensure that the stretched beam is spatially reconstructed. Femtosecond pulses with pulse duration 100 fs are stretched to more than ten picoseconds pulses before amplification. High reflective gold coated holographic grating gives stretcher efficiency higher than 50% for specific wavelength regimes.

Femtosecond Pulse Compressor
The pulse compressor was designed for compression of picosecond pulses amplified by Wedge 50 multipass Ti: sapphire amplifier to pulses as short as 50-100 fs. The principle of pulse compressor operation is shown in the Figure 3. One can see that in contrast to the pulse stretcher, redder frequency components have to travel further through the compressor than the bluer frequency components. The result is that the pulse has been compressed. Varying distance between the gratings, the compression can compensate the stretching precisely giving almost the same pulse duration as obtained from the seed laser pulse. The Figure 3 shows a simplified pulse compressor. In the Wedge 50 pulse compressor some other optics are involved, i.e. horizontal and vertical retroreflectors give a possibility to use one grating and to achieve four-pass configuration. High reflective gold coated holographic grating gives stretcher efficiency higher than 50% for specific wavelength regimes. Femtosecond Confocal Multipass Ti:sapphire Amplifier The confocal multipass amplifier has been designed as an effective device for amplification of femtosecond pulses in different active media. Our unique design features two confocally placed concave mirrors of different radii of curvature (ROC) with central holes (Figure 4). This telescopic configuration provides six, eight, ten passes of the light beam through the common focus where Ti : sapphire crystal is placed. Due to different focal lengths of the mirrors, beam cross section is decreased after each pass and the beam waist diameter is increased accordingly. On the sixth pass the beam waist diameter is about four times more than on the first one. This is an important condition for getting the maximum gain, and one cannot find this feature in other multipass or regenerative amplifier systems. The pump radiation is focused by the lens through an aperture in the input mirror.

Pulse Picker
A pulse picker installed in the Wedge 50 femtosecond amplifier system is used for one pulse selection from a train of stretched pulses. As a result, seed pulses are formed for the amplification. The pulse picker utilizes well known electrooptical Pockels effect. Pulse train having horizontal polarization goes through the Pockels cell. Without applied voltage pulses do not change polarization and exit pulse picker with help of polarizers as shown in the Figure 5. When half wave voltage is applied to the Pockels cell, an input pulse changes its polarization from horizontal to vertical, goes through polarizer and is used as a seed pulse for the amplifier. Applied voltage is synchronized with femtosecond pulse train and Nd:YLF pump pulses, and seed pulses have pulse repetition rate equal to the repetition rate of Nd:YLF pump pulses. Input polarizer is used to increase polarization ratio for input pulses. Synchronization Electronics The unit is designed to trig high voltage window applied to the Pockels cell and to synchronize this window with pump pulse and femtosecond pulse train. The aim of synchronization is to select one femtosecond pulse from train and to amplify it at the maximum pump efficiency. The schematic diagram is shown in the Figure 5 and the unit description is in the Section 5.


Pump laser for Wedge 50 Multipass Amplifier
Darwin-527-30-M DPSS Laser
Wavelength: 527nm
Energy: 20mJ at 1kHz
Beam Quality (M^2): <16
includes Water/Air Heat Exchanger

Del Mar Photonics, Inc.
4119 Twilight Ridge
San Diego, CA 92130
tel: (858) 876-3133
fax: (858) 630-2376
Skype: delmarphotonics

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