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Femtochemistry Lab

 

Our Home-Built Ultrafast Laser: The Lab's Pride

Description    System Output    Some Results


 



 








 

 

 

 

General

(a) Ultrafast

 

(b) Our systems

 

1. Oscillator

2. CPA

(2a) Stretcher

 

(2b) Pulse Selector

 

(2c) Amplifier

 

(2d) Compressor

 

3. Par. Amplifiers

(3a) General

 

(3b) NOPA

 

(3c) TOPAS

 

4. Shaper

(4a) General

(4b) Prism Pair

(4c) DM

 

 

 

 

General

(a) Ultrafast Systems

The heart of ultrafast pulse generation is the phenomena of passive/active mode locking. A finite number of longitudinal modes, governed by the gain profile of the active medium, oscillate in the cavity. In CW mode, the relative phases of those modes fluctuate randomly, leading to random interference effects. "Zeroing" the relative phases of the modes in a specific region in the cavity results in the generation of transform-limited pulses, i.e. the shortest pulses available, limited only by the bandwidth based on Heisenberg's uncertainty principle.

The weak pulses coming out of a mode-locked laser are amplified, at a much lower repetition rate, using Q-switched laser in either a multipass or regenerative configuration.

Typical processes being studied are in the order of a few femtosecond to hundreds of femtosecond; e.g.: typical vibration (period of 15-100 fsec), photodissociation, bond fission and curve crossing (50-300 fsec), relaxation of excited states (100 fsec - picos) etc.

Therefore, the ability to monitor a chemical reaction in real-time fashion (time-resolved spectroscopy, as opposed to frequency resolved spectroscopy), requires an experimental system that meets the following requirements:

  •  The experimental time resolution must be one order of magnitude less or of the same order of magnitude of the process being studied.

  •  Wavelength tunability, both for the pump and the probe, is an evident advantage in trying to map the properties of both the reactants and the products.

  •  Power (flux) tunabilty is needed in order to be able to escort and map different processes at different molecules (photo-excitation, photo-dissociation etc.).

  •  Finally, averaging significantly improves signal to noise, making stability and high repetition rate advantageous.

 

(b) Our Systems

Currently, we are using two similar home-built ultrafast Ti:Sapphire lasers.

The systems is built of:

  •  Ti:Saphhire based Oscillators

  •  Chirped Pulse Amplification (CPA)  Apparatus:
    Stretcher, Single Pulse Selector, Multipass (9-passes) Amplifier (Ti:Sapphire), Compressor

  •  Wavelength Conversion by Optical Parametric Amplifiers (NOPA and TOPAS).

  •  Shaper (Prisms and Deformable-Mirror) - in the NOPA system only.

A Schematic Diagram of the System is shown below (for the NOPA system):

 

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1. Oscillator

The Oscillator is the source for generating ultrashort pulses. It is a fast pulse generator.

The Oscillator is a passive mode-locked laser, where the mode locked mechanism is based on the self-focusing effect (Kerr lensing) within the laser material (Ti:Sapphire). The mechanism is known as "Kerr-Lens Mode-locking" (KLM). The effect induces modification in the resonator, that translates to a power dependent loss (or gain), responsible for the mode-locking.

The laser is pumped with a commercial Nd:YVO CW laser (~530 nm), and is capable of generating ~20-fsec pulses centered at 790 nm at a repetition rate of 85 MHz and spectral bandwidth of 45 nm. The pulses are relatively low in intensity (5 nJ/pulse). The output is dominated by the mode TEM00 (gaussian shaped).

Our oscillator is based on the design originally suggested by Asaki et al. (Opt. Lett. 18(12), 977 (1993)).
 

A Schematic Diagram of the oscillator and a real-time (in work) photo are shown below:

 

 

 

 

 

 

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2. CPA (Chirped Pulse Amplification)

The amplification of nanojoule-level femtosecond pulses to the millijoule level and above is complicated by the extremely high peak powers involved, which are higher than the damage threshold of most optical materials.
The problem is overcome by stretching the pulse in time using dispersion, followed by amplification and subsequent recompression to approximately the original pulse duration.

A diagram showing the principle of CPA is shown below
 

(diagram taken from: Reid G.D. and Wynne K., Ultrafast Laser Technology and Spectroscopy in Myers R.A. (Ed.),
Encyclopedia of Analytical Chemistry
, John Wiley & Sons Ltd., Chichester 2000, pg. 1364
4).

 

(2a) Stretcher

The pulse stretcher uses all reflective optics to minimize uncontrolled chirp and undesirable abberations.
It is built from a single diffraction grating (1,200 mm-1), parabolic silver mirror (f=61 cm) and two gold flat mirrors to form an achromatic one to one telescope. The stretcher is adding Positive Chirp (PC) to the pulses, and stretches them to a time scale of 100-200 psec.

Our system is bases on the design originally described by 

 

(2b) Single Pulse Selector

Pulse selection is being done typically at 400Hz-1 kHz repetition rate from the 85 MHz Ti:Sapphire output. The selection is achieved using a Pockell's Cell, between two crossed glan-laser polarizers. The ratio between the leakages through the Pockells's Cell to the transmitted pulse was measured to be 1/3,000.

 

(2c) Multipass Amplifier

The amplifier assembled in a multipass configuration, following the design by Backus et. al. (Opt. Lett. 20(19), 2000 (1995)). The pulse passes through the gain medium several times without the use of a cavity.
The amplifier is pumped with adequate repetition rate, 10 mJ pulses from a frequency-doubled Q-switch YLF laser. The basic components of the amplifier are two dichroic spherical mirrors, which transmit the pumping radiation and reflect the near-IR, and one planar gold reflector in a ring configuration.

The amplification medium is a Ti:Sapphire crytal positioned at the focus of the spherical mirror. We utilize nine consecutive passes through the crystal, experimentally shown to be the best compromise between the desired pulse amplification and the ASE (Amplified Spontaneous Emission), after which the amplified pulse is ejected from the cavity. An additional spherical silver mirror positioned after the end dichroic mirror, is used to refocus the remainder of the pump back to the Ti:Sapphire crystal, effectively doubling the amplification power.

The amplified pulses are passed through a mask of 2.2 mm holes, in order to reduce the effects of both ASE and self-focusing.

The overall output of the amplifier is ~1-5 mJ/pulse (7 orders of magnitude as the amplification factor).

Aside from increasing the pulse energy, the amplification process shapes and shifts the pulse spectrum (red-shift) due to the finite gain bandwidth of the amplifier crystal, as demonstrated below in the graph:

A diagram showing the multipass amplifier is shown below:

And a photo of our multipass amplifier:

After the amplifier, the pulses are spatially filtered to improve the beam profile.

 

(2d) Compressor

The final stage in the ultrashort pulse generation in the frame of CPA is to recollect the frequency components, trying to get as close as possible to a transform-limited pulse.

The compensation is mostly for positive chirp (PC) created in the stretcher, but also for "normal" dispersion created by the path of the broadband laser beam through optical elements (crystals, lenses, beam splitters etc.).

In principal, the compressor setup could be similar to the stretcher, where the grating is placed behind the focal plane of the parabolic mirror instead of in-front it.

We chose a simpler and more flexible alternative where we use double reflections from a grating pair (1,200 mm-1). This affords us with an additional degree of freedom where we can tune the grating angle thus varying both GVD and TOD in an effort to obtain the shortest possible pulse (GVD is mainly affected by the distance between gratings, while TOD by the angle of deviation). The grating pair must be parallel to each other in order to avoid spatial chirp (different frequency components in different zones of the beam).

Typical output pulses are 25-30 fsec FWHM, 0.5 mJ/pulse in energy (at 1 kHz repetition rate), centered at 800 nm (40 nm FWHM). Peak to peak stabilities are generally on the order of ~1%.

A diagram of the compressor setup is shown below:

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3. Wavelength Conversion (Optical Parametric Amplifiers)

 

(3a) General

In order to be able to investigate versatile systems, we need a tool for wavelength conversion of the ~800 nm (oscillator originated) pulse.

We are using parametric amplifiers, based on nonlinear optics in crystals. The processes are based on Difference Frequency Mixing. Generally, those processes are described by a "summation" of two photons to obtain a third new one, while conserving the energy and the photons momentum (k-vector).

Naturally, those non-linear processes require high flux, which is acquired by the output pulses of the pulse generator.

Both our parametric converters are commercial in origin, but were updated to our needs.

 

(3b) NOPA (Non-Collinear Optical Parametric Amplification)

The NOPA is an optical parametric amplifier designed to generate ultrashort tunable visible pulses.

The NOPA consists of three main functional blocks (see figure below):

  •  Continuum generation in a Sapphire disk (pumped by NIR pump light).

  •  Parametric amplification of the seed light in a BBO crystal, pumped by the frequency doubled pump light.

  •  Compression of the broadband output pulses to sub-30-fs length.

Photons contained in a short wavelength (blue) pump beam are split into one signal (visible) and one idler photon (NIR) each. The active medium is a nonlinear optical crystal, such as BBO.
In linear OPAs,  the three different group velocities are hard to match, therefore very short and broadband pulses are not feasible
The NOPAs offer a solution to the problem of pulse lengthening in collinearly phase matched parametric intercation. Hence, the NOPAs allow a better matching of the phase and group velocity of the signal and idler pulses - therefore allowing for creation of shorter and more broadband pulses.

A typical NOPA spectrum (centered at 600 nm) has a bandwitdh of ~100 nm (FWHM), which is equivalent to ~6 fsec in time (TL). A typical energy is 5 mJ/pulse.

 

(3c) TOPAS (Traveling-Wave Optical Parametric Amplifier of Super-Fluorescence)

TOPAS operation is based on second order non-linear optics, namely 3-photon interaction, in a non-centrosymmetric crystal (the second harmonic generation process is a degenerate example of the same process). A strong pump at a frequency, w1, can simultaneously amplify two different frequencies, w2 and w3, provided  both energy and momentum are conserved (w1=w2+w3 and  k1=k2+k3). Extension of the frequency range is achieved via either doubling the IR output, or by mixing with the fundamental 800 nm. Finally, an additional second harmonic crystal can be inserted so as to generate the desired UV pulses for the experiment.

Naturally, those non-linear processes require high flux, which is acquired by the output pulses of the pulse

 

 

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MORE COMING SOON !!!

 

4. Shaper

 

(4a) General (Pulse Shaping)

 

 

(4b) Prisms Pair

 

 

(4c) Deformable Mirror

 

 

 

 

 

 

 

 

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Description    System Output    Some Results

 

 


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Last Updated: 11/06/11
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