Ultrafast laser pulses are
among the fastest man-made events. But there is more - ultrafast also
means ultrapowerful. And, as we all know, with great power comes
great tunability.
The high power within ultrafast
laser pulses allows us to make practical use of nonlinear optical effects
that are usually very weak. It is through such effects that the colour of
the laser pulses can be tuned. The ultrafast lasers described below are
based on the use of a Ti-doped sapphire crystal. Ti:Sapphire has very
broad absorption and emission spectra.
Just as Cr3+
ions do in ruby, Ti3+ ions replace the Al3+ ions of
sapphire leading to strong (26% larger) distortion of the local electric
fields. The energy levels of Ti are broadened by coupling to vibrational
levels in sapphire. Also, Ti3+:Al2O3 has
excellent thermal conductivity.
To many people, an ultrafast
laser is a black box. Often, this is because it is from Coheren, as
those from Newport - Spectra Physics usually come in a white box. In
order to help me visualize what goes on inside the boxes, I made these
animations and you might find them useful.
The Tsunami laser from Newport
Spectra-Physics
The RegA regenerative amplifier from Coherent
Fig. 1.
Because Ti:Sapphire has such broad absorption and emission
bands, lasing starts in many modes, which all compete for
amplification. Because of cavity loss, self-focusing and acousto-optical
modulation, the pulse with highest amplitude is eventually
identified and amplified. Ti:Sapphire lasers typically
emit light at 800 nm with a spectral range from 650 to 1000 nm.
Pulses are 100 to 200 fs long and repetition rate is about 80
MHz.
Although the laser is tunable, its spectral range
does not extend to the visible frequencies, nor does it go very
far in the infrared. Moreover, the power per pulse is not very
high. Happily, ultrafast laser amplification brings solutions to
all of these problems.
Fig. 2. In this
regenerative amplifier, the light pulses extract power from the Ti:Sapphire
crystal over a total of 22 amplification passes. Too much power in the
crystal can be a bad thing as it might damage the Ti:Sapphire. However, the
pulses stretch in time due to dispersion in all the optical component along
the cavity. This stretching brings their amplitude below
the damage threshold. Once a pulse has taken all the available energy from the
crystal, its amplification is maximal and it is then compressed to ultrashort again. The RegA produces 200 fs light pulses at 800 nm, with
repetition rate 250 kHz. The output power is about 1 W. Still higher power
per pulse can be achieved by lowering the repetition rate.
The Spitfire regenerative amplifier from
Newport - Spectra Physics
The visible Optical Parametric Amplifier from Coherent
Fig. 3. Rather
than relying on dispersion in all the optical components along
the cavity, this design from Newport - Spectra Physics begins by
purposefully stretching the laser pulse. The amplitude of the
pulse is then increased by consecutive passes through the
Ti:Sapphire crystal. Finally, the pulse is recompressed to
achieve very high peak powers and it exits the laser. The
Spitfire can also produce 1 W of average output power but at the
repetition rate of only 1 kHz, which means much more power per
pulse. The problem with this amplification is that it is
confined to a single frequency. Nevertheless, with so much power it
is possible to make use of nonlinear optical effects and achieve
wavelength tunable ultrafast laser pulses.
Fig. 4. In an optical
parametric amplifier (OPA), the parametric processes involve frequency (ω)
mixing, such as ω1+ω2=
ω3, or
ω1–ω2=
ω3. These are known as sum
frequency generation and difference frequency generation, respectively. The
parametric processes allow the color change in the laser beam. In this OPA,
the fundamental beam is split in two. One part is frequency doubled to 400
nm by passing into a BBO crystal. The other part is directed into a sapphire
crystal where it produces a white light supercontinuum. Subsequently, the
400 nm and the white light are directed into another BBO crystal where the
frequency mixing occurs. The resulting signal consists of ultrafast (about
200 fs) laser pulses that are tunable within the visible part of the
spectrum: 480 to 700 nm range. On the same principles it is possible to
build an OPA for tunable infrared light.
The infrared Optical Parametric Amplifier
from Newport - Spectra Physics
Fig. 5. In this schematic diagram, a
Spitfire regenerative amplifier provides light pulses at the entrance of the OPA. Again the amplified pulses are split in two parts. One to create the
white light supercontinuum and the other goes straight into a BBO crystal
for frequency mixing with the white light. Here there is no frequency
doubling to 400 nm, so the values for ω1+ω2=
ω3, or
ω1–ω2=
ω3 are different.
Consequently, the resulting signal consists of ultrafast laser pulses that
are tunable within the infrared part of the spectrum, i.e. between 1100 and
1600 nm.
In a
typical experiment, the pulses are guided through a variety of optical
components to a particular sample where unusual light-matter interaction
takes place. Subsequently, the light is directed to detectors, (ex.
photodiodes or photo multiplier tubes). The detectors convert the light
signal into an electric signal that can be measured with electronic
instruments and recorded onto a computer.