How do you know that you have made ultrashort pulses? The pulse are too fast for electronics to measure them so optical techniques using other ultrashort pulses were developed. The first is called autocorrelation. An autocorrelator utilizes second harmonic generation (SHG) in nonlinear crystals to measure a pulse. SHG occurs when two lasers pulses of the same wavelength are present in a nonlinear medium at the same time and in the same space. Those two pulses then produce a third pulse of light with the combined energy and at half the wavelength. The setup for an intensity autocorrelator is shown in Figure 1. The unknown incoming pulse is spilt into equal portions called the pump and the gate. The gate beam passes through a delay stage before meeting the pump beam in the nonlinear crystal. If the distance along the two paths is the same the pulses will overlap temporally and spatially in the crystal to create the signal at half the wavelength. Obtaining proper spatial alignment of the beams at the correct angle for SHG is the most difficult aspect of autocorrelation. The SHG signal pulse is emitted between the gate and pump pulse and is detected by a photodiode. Because light travels at a constant velocity a 0.3mm change in the path length results in a 1 ps change in the time the light pulse arrives at the crystal. By measuring the intensity versus the delay stage position it is possible to extract the temporal pulse shape.
Figure 1. Typical Autocorrelator setup.
Autocorrelation is an excellent diagnostic technique for any ultrashort laser system. However it only provides information about the pulses temporal behavior. There is not information about the spectral characteristics of the pulse. To learn if the pulses are chirped, another technique was developed called Frequency Resolved Optical Gating or FROG. FROG is an autocorrelator with a spectrograph and CCD detector instead of a photodiode. Figure 2 diagrams a sample FROG setup. FROG spectra require more time to collect as tens to hundreds of spectra must be collected at each delay setting. So they are not practical as a sensor while performing alignment on ultrafast lasers but can determine the pulse properties at intermediate steps. Calculated FROG spectra in Figure 3 illustrates how positive and negative chirpped pulses signals would appear.
Figure 2. Sample FROG Setup. The only change from the autocorrelator is a new detection method.
Figure 3. Sample FROG spectrum. In the negative chirpped pulse the shorter wavelengths come before the longer wavelengths and vise versa for the positive chirpped pulse.
The third common ultrashort pulse diagnostic technique is called GRENOUILLE or GRating-Eliminated No-nonsense Observation of Ultrafast Incident Laser Light E-fields. The acronym was chosen because grenouille is the French word for frog. A GRENOUILLE system is able to provide spectral and temporal information about a pulse without the sensitive alignment issues of FROG and autocorrelatior techniques. Figure 4 diagrams a GRENOUILLE system. First the beam is focused by a horizontal cylindrical lens to a stripe on the nonlinear crystal. Then a Fresnel biprism is inserted into the path. These prisms will create two virtual sources for the light beam and then spatially overlaps the virtual sources onto the crystal. Figure 5 shows a lens diagram for a Fresnal biprism to help visualize this effect. This optic replaces the beam splitter and the delay stage in an autocorrelator and places the time delay along the horizontal axis. Then the crystal produces the doubled light and also acts as the spectrograph because nonlinear processes are dependent on the incident angle for different wavelenghs. After the crystal a set of cylindrical lenses disperses the beam so that the wavelength is on the vertical axis and time on the horizontal axis. Using the Fresnel biprism eliminates the alignment dependence found in autocorrelator setups but does require a 2-D array for detection.
Figure 4. Sample setup for a GRENOUILLE experiment.
Figure 5. Fresnel biprism lens diagram. The single incoming beam is split and then spatially overlapped with the same optic.