In order to obtain the spectral interferometry signal necessary for employing the FTSI algorithm described in chapter 4, the f-to-2f method is used. [61] The experimental layout for obtaining the fringes for the KLS laser pulses is shown in Figure 5.4.
Figure 5.4 f-to-2f for measuring CE phase drift: NDF: neutral density filter, SP: sapphire plate, BBO: SHG-crystal, P: polarizer, τ : delay
In the experimental setup, a small portion of the amplified laser pulse train (<1 µJ) was sent into the interferometer. The beam size was focused into the 2.3 mm thick sapphire plate by an f=75 mm lens. The NDF was used to adjust the beam power in order obtain a stable single- filament inside the sapphire. The strong self-focusing in the sapphire plate broadened the spectrum of the input pulses by over an octave. Since the sapphire was birefringent, the plate
was mounted in a rotational mount in order to adjust the spectrum, similar to that done for the PCF in the case of the oscillator.
The f and 2f components exhibited a delay, given by τ in the figure, due to creation at different times during the self-focusing process and propagation through the sapphire plate. The two components were then focused by an f=100 mm concave mirror into a 1 mm thick BBO crystal cut for Type-I phase matching at 1064 nm. Note that in this setup, unlike in the oscillator setup, a slightly broad second harmonic phase-matched spectrum was desirable in order to
measure the CE phase drift. This is because a broader spectrum gave a better sampling of the CE phase drift across the spectrum.
Once the pulses had exited the BBO crystal, they were focused by an f=70 mm lens into an imaging spectrometer after passing through a polarizer used to select a common polarization. A second NDF attenuated the beam in order not to saturate the CCD camera of the spectrometer. The image in the figure shows a typical SI signal obtained from the experimental setup.
A measurement of the white-light obtained from the sapphire-plate for different energies is shown in Figure 5.5.
Figure 5.5 Spectrum from the sapphire-plate at different energies.
The spectra were obtained by scanning across the CCD of the spectrometer down to around 500 nm. As is shown, the spectra cover more than an octave. In fact, the spectrum can actually go below 500 nm to around 460 nm, depending on the axial orientation of the sapphire
plate. Typically, the f-to-2f measurement is carried out around a 30 nm width centered on 500 nm. Also, the pulse energy required for the generation of an octave depended on the input pulse duration. This depended on the optimization of the compressor in the KLS amplifier. If the pulse duration was long, then more energy was required, usually around 700 nJ. For an optimized compressor, the energy needed was lower, usually 300 to 500 nJ. Theoretically, the required intensity on the sapphire plate for generating the white-light was ~ 12
10
2× W/cm2. The orientation of the sapphire plate’s optical axis, or equivalently the input laser polarization, would also be a factor in how much energy was required.
The experimental procedure, though, would involve an optimization of all of the aforementioned parameters. The feedback for the optimization would be the quality of the fringes obtained. The phase-matching angle would be rotated, the input power adjusted, the rotation of the sapphire plate, the rotation of the polarizer, and the position of the lens used to the focus the light into the imaging spectrometer would all be adjusted until a fringe pattern with high contrast was obtained. A typical problem, though, with the optimization was that focusing too much energy onto the sapphire plate caused the formation of multiple-filaments. The muli- filamentation produced fringe patterns unusable for measurement of the CE phase drift.
Once a stable fringe pattern was obtained, though, a comparison of the CE phase drift for different integration times of the spectrometer was obtained. Since each laser pulse came very 1 ms (1 kHz repetition rate), fringe patterns for 1, 50, and 100 ms integration times were obtained when the oscillator CE phase stabilization was on and when it was disengaged. The 1 ms integration time is shown in Figure 5.6.
Figure 5.6 1 ms integration time interference fringes.
The lineouts of the fringe patterns are shown on the right. Since, for 1 laser shot, the CE phase drift is not measureable. This is why the locked and unlocked versions are not different. The 50 ms and 100 ms cases are shown in Figures 5.7 and 5.8 respectively.
Figure 5.8 100 ms integration time interference fringes.
In figure 5.7, 50 laser shots were integrated. As is shown, when the laser oscillator CE phase lock was engaged, the fringe contrast remained high during the integration. However, the unlocked CE phase situation showed that the fringes began to blur, which was representative of CE phase drift. For the 100 ms integration time, the difference between the locked and unlocked cases was even more dramatic. Since the fringe contrast remained high when the oscillator was locked, the CE phase drift of the amplified laser pulses could be measured. The shift of the fringes could be used to correct the CE phase drift of the amplified laser pulses.