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.
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PRINCIPLES OF OPERATION
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
sales@dmphotonics.com
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Mavericks femtosecond
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OAFP optical attenuator
Pearls femtosecond fiber laser
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Pismo pulse picker
Reef-M femtosecond scanning
autocorrelator for microscopy
Reef-RTD scanning
autocorrelator
Reef-SS single shot
autocorrelator
Femtosecond Second Harmonic Generator
Spectrometer ASP-100M
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femtosecond solid state laser system
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Trestles Finesse
femtosecond lasers system integrated with DPSS pump laser