Transcript poster two color

Intra-cavity Pulse Shaping of Mode-locked Oscillators
Shai Yefet, Naaman Amer and Avi Pe’er
Department of physics and BINA Center of nano-technology, Bar-Ilan University, Ramat-Gan 52900, Israel
Ultrashort pulse shaping - the manipulation of a pulse in time by controlling its spectral amplitude and phase - is an indispensable tool in ultrafast and nonlinear
optics. As technology and applications progress, a need is clear for sources of femtosecond pulses with precisely controlled spectra. For example, a two lobed
spectrum is necessary for two-color two-photon microscopy, Raman spectroscopy and direct frequency comb spectroscopy. Standard extra-cavity shaping can be
impractically lossy for these applications, whereas intra-cavity techniques cannot achieve the desired spectrum. We demonstrate a simple and robust technique to
fully control the spectrum of mode-locked oscillators within the laser cavity in a lossless manner. A novel design of the cavity introduces a pulse shaper inside the
optical cavity, nulls mode competition between different frequencies in the oscillator and allows precise shaping of the spectral gain curve, thus shaping the intracavity spectrum. The technique is demonstrated by the generation of two lobed spectra in a titanium-sapphire (TiS) oscillator.
Novel design of a titanium-sapphire (TiS) oscillator
OC
M1
M2
M3
M4
HR
Spatial
shaper
1st gain
medium
2nd gain
medium
Beam
splitter
Pump source @ 532 nm
Cavity concept:
- The core principle of our method for shaping of pulses is tailoring the gain profile inside the optical cavity instead of the loss. By placing an additional gain
medium at a position in the cavity where the spectrum is spatially dispersed, each frequency passes this medium at a different location, and by proper spatial
shaping of the pump beam the spectral gain profile can be shaped. Pumping crystal #1 (where all frequencies are packed) introduces homogeneous gain
over the entire spectrum, whereas pumping crystal #2 introduces spectrally selective gain according to the spatial shape of the pump there.
CW and Mode-locked operations
Titanium-Sapphire
crystal
Power (a.u.)
100
80
Selective
gain
60
40
20
0
680
765
840
930
680
840
Wavelength (nm)
Wavelength (nm)
The laser was mode-locked with only the 1st crystal
pumped, resulting in a broad homogeneous spectrum (red
curve). Pump power is then gradually transferred from the
1st medium to the 2nd medium, where only two specified
spots were pumped. During the transfer, gain is increasing
only for certain frequency components while decreasing
for all other frequencies (blue curve). The final shape of
the spectrum has two clear and significant lobes (black
curve).
745
The novel designed oscillator enables flexible control
over the spectral power, width and center wavelength of
each lobe. Taking a two lobed spectrum as a reference
(black curve), the spectral width of each lobe is
controlled by spatially widening the pump spots in the
2nd medium (blue curve). The spectral power of each
lobe can also be controlled by adjusting the splitting
ratio of the pump power between the spots in the 2nd
medium (red curve). Shifting the center position of lobes
by shifting the pump spot laterally is also demonstrated.
Dispersion management
680
930
The cancellation of mode competition in the 2nd
medium is demonstrated by the continuous-wave
(CW) operation of the novel cavity. Pumping only the
2nd medium with an elliptically shaped pump spot
results in a unique ”multiple fingers” CW operation,
indicating that different frequency components
coexist. These ”fingers” span a bandwidth that
corresponds to the spatial width of the elliptically
shaped pump spot, and can be centered anywhere
within the TiS emission spectrum.
Kerr lens mode-locking
The optical Kerr effect takes
place in the 1st gain medium and
can be used as a passive modelocking technique.
Using chirped mirrors and prism pair compressor we
achieve dispersion compensation over the TiS emission
spectrum. Dispersion was the limiting factor to the ability
to shape the spectrum. We observed that each of the
lobes could be centered at any position, where dispersion
is reasonably compensated within the TiS emission
spectrum. Trying to pump frequencies which were not
well compensated resulted in the formation of CW
modes. Better compensation of the overall dispersion in
the cavity will therefore improve the bandwidth available
for gain shaping (ideally up to the entire emission
spectrum of the TiS crystal).
Wavelength (nm)
930
Titanium-sapphire crystal is a widely used
gain medium for tunable lasers and
femtosecond solid-state lasers. It has an
excellent thermal conductivity, and a very
large gain bandwidth, allowing the
generation of very short pulses. The
crystal is pumped in the green or blue
spectral region.
Optical pulses
characteristics
n  n0  n2 I
In spatial domain the optical
Kerr effect causes self-focusing
which discriminate mode-locked
operation against CW operation.
In time domain the optical
Kerr effect causes self phase
modulation.
In frequency domain the
optical Kerr effect causes four
wave mixing, leading to
spectral broadening.
In time domain:
- the laser output is characterized by a coherent
train of ultrashort pulses with app. 50 fs pulse
duration and a repetition rate of app. 100MHz.
In frequency domain:
- the laser output is characterized by an equally
spaced longitudinal cavity modes constructing
a frequency comb.