Pulse measurement instrument Wizzler principle and usage skills
Figure 1: Wizzler internal optical path design
1. Wizzler basic principle
1. Cross-polarization filtering
To understand the basic principles of Wizzler, you have to first explain the cross-polarization filtering (XPW) technology, because this technology is an important knowledge point in the optical part of Wizzler.
The generation of cross-polarized waves is based on the degenerate third-order nonlinear principle of four-wave mixing. As shown in Figure 2, in the barium fluoride crystal with asymmetric third-order polarization characteristics, the polarization of the incident laser is rotated, so there will be a certain amount of output in the output direction of the orthogonal analyzer. Since XPW is a third-order nonlinear process, the conversion efficiency depends heavily on the intensity of the incident laser. It is precisely this strong intensity-dependent effect that the noise in the pulse structure will be greatly suppressed by the analyzer. Therefore, XPW technology was first used to improve the contrast of ultra-intense lasers, and is now a necessary technology for commercial TW/PW lasers.
In addition, XPW also has the functions of broadening the spectrum, spatial filtering and improving the spectral phase.
Figure 2: XPW technology based on nonlinear rotation principle (picture from Wikipedia)
2. Wizzler internal optical path design
Figure 3: Wizzler internal optical path design
As shown in Figure 3, the incident laser pulse first passes through the positioning aperture, which serves as the benchmark for Alignment. Wave plates and P1-P4 polarizers play the role of polarization rotation, energy attenuation and polarization purification. The birefringent plate is used to generate a pulse to be measured with a fixed delay and perpendicular polarization to the incident pulse. The pulse is then focused onto the XPW crystal BaF2 via M1. At this time, the XPW effect is triggered, producing a small pulse (labeled XPW pulse) with vertical polarization but no delay from the main pulse. The polarization direction of polarizer P5 is orthogonal to P1-P4. After the three pulses pass through P5, the main pulse is blocked from entering the spectrometer, leaving only the pulse to be measured and the XPW pulse with a fixed delay. Spectral interference occurs between the two.
3. Pulse measurement algorithm
Figure 4: Extract frequency domain electric field and spectral phase from reference spectral interference
After obtaining the spectral interference information, it can be processed to obtain complete spectral intensity and phase information.
1. As shown in Figure 4, first perform inverse Fourier transform on the spectral interference pattern to obtain the central main pulse and two side lobe pulses. Filter the side lobe pulses and delay to the center zero point before performing Fourier transform. Then, the phase difference of the two pulses participating in the interference can be obtained: ?1 (?) - ?2 (?) and the product of the electric field: E1 (?) * E1 (?).
2. Perform Fourier transform on the central main pulse to obtain the sum of squares of the electric field E12(?)+E22(?).
At this point, we can obtain the complete electric field amplitude information of the pulse to be measured, and only the phase information of the pulse to be measured needs to be calculated. Next, we need to consider another effect of XPW, which is the phase flattening effect on the left side of Figure
5. That is to say, the spectral phase ?2 (?) of the XPW pulse is close to zero, so the value of ?1 (?) is also known.
3. But is the hypothesis that the spectral phase of the XPW pulse is zero true? It is necessary to introduce an iterative algorithm for correction, as shown on the right side of Figure
5. After multiple cycles, the spectral phase can be corrected, and finally converged accurate phase information is obtained.
Figure 5: Spectral phase acquisition and accuracy correction of reference pulses
2. Wizzler usage skills
Due to its completely collinear design, Wizzler’s internal optical path adjustment is very simple. Especially after mastering the above principles, it will not take a long time to disassemble and assemble all the components. The following is a brief description of Wizzler adjustment methods and usage techniques.
1. Light path adjustment
1) Remove the XPW crystal and use the incident aperture and the auxiliary aperture provided by the manufacturer to align the light path. The way to verify alignment is to see if the intensity on the spectrometer is the strongest. Note: The small hole provided by the manufacturer has a quarter-wave plate attached to the back, so the wave plate needs to be rotated to make the initial intensity appropriate.
2) Remove the small hole. At this time, due to the orthogonality of the polarizing elements, there should be no signal on the spectrometer, or it may be very weak. If there is a strong signal, it means that the direction of the polarizer has been rotated and needs to be corrected. If there is a weak signal and spectral interference is found, it means that the angle of the birefringent plate is incorrect and needs to be rotated to remove the interference fringes.
3) Place the XPW crystal, and a relatively smooth spectrum should appear on the spectrometer. Of course, this is based on the premise of pulse comparison optimization. If the signal is weak, the pulse compressor in the laser system needs to be adjusted until the strongest spectral intensity signal and widest spectral width are obtained.
4) The last step is to use the birefringent plate to generate the signal to be measured, and rotate the angle of the birefringent plate appropriately to generate an interference pattern with a moderate modulation depth (the author's experience is about 30%).
5) At this point, a detailed pulse width value and phase information should be given on the Wizzler interface.
Figure 6: Wizzler typical measurement interface
2. Feedback with Dazzler
Figure 7: Wizzler-Dazzler active feedback system
An important advantage of Wizzler is that it can form a feedback system with Dazzler, ultimately obtaining strict Fourier transform limit pulses.
The key to operating this function is to install the Dazzler and Wizzler software on the same computer, or install them on different computers, and use remote control to establish communication.
Boliang Technology is the exclusive agent of the French Fastlite company in China. We provide free trial services for the Wizzler800 version for a long time. We also provide consulting services for existing Wizzler users. Welcome to contact us.
References
1. A. Jullien et al., Opt. Express, vol. 14, pp. 2760–2769 (2006)
2. “Self-referenced spectral interferometry，” Oksenhendler, T., Coudreau, S., Forget, N. et al. Appl. Phys. B (2010) 99: 7.
