[Instrument usage tips] DAZZLER, used by ultrafast laser systems around the world!

summary

Ultrafast lasers, especially femtosecond lasers, have been increasingly used in research fields such as biology, chemistry, and physics due to their excellent characteristics in the time domain and frequency domain in recent years. The extremely short pulse width and rich spectral components make them favored by scientific researchers. However, "a sword may be sharp, but it is difficult to be good at dancing." A femtosecond laser often only has a fixed pulse width and spectral shape, which is difficult to fully match all the conditions required in the experiment, making it difficult for expensive femtosecond lasers to perform. If there is such an instrument that can flexibly change the pulse width and spectral shape of femtosecond pulses, or even turn a single femtosecond pulse into a customized pulse train, then the practical significance can be said to be "extraordinary".

Let’s take a look at this powerful tool that can make the best use of femtosecond laser – DAZZLER.

1. The origin and market status of DAZZLER

The original meaning of DAZZLER is "dazzle, flash". It is actually the brand name of the commercial product of Acousto-opto Programmable Dispersive Filter (AOPDF, Acousto-opto Programmable Dispersive Filter). At present, everyone is more accustomed to calling it DAZZLER. The concept of AOPDF was proposed by Pierre Tournois in

1997. Then he and several partners founded the FASTLITE company and produced the first DAZZLER commercial product in 2000.

The name Pierre Tournois may not be as unfamiliar as you think. As early as 1964, Dr. Tournois and F. Gires invented the famous GTI mirror (Gires-Tournois Interferometer). The GTI mirror has now become the most common device used to compensate for dispersion in various ultrafast lasers. Otherwise, where do you think the "T" in "GTI" comes from? !

Coming back to the DAZZLER instrument, it may not be as famous as the GTI mirror (after all, it is 30 years younger...), but its functions are definitely superior. This product is used in almost all mainstream ultrafast laser systems in Europe. It has also become a standard product in the products of well-known laser companies such as Amplitude Technologies, Thales, FemtoLasers, etc. Its compact size and powerful functions have won widespread praise from users.

Figure 1 Actual picture of DAZZLER

2. Functions of DAZZLER

2.1 Improvement of the peak power of ultrafast amplifiers (For details on CPA technology, please refer to the previous article on the public account "How much do you know about ultrafast amplification technology?")

Taking titanium sapphire femtosecond amplifier as an example, in order to obtain as narrow a femtosecond pulse and high peak power output as possible, two issues need to be paid attention to during the amplification process. One is that the dispersion compensation must be accurate. If the femtosecond seed pulse is broadened and amplified, if the dispersion compensation is not good when compressed, it will cause the pulse to become wider.

The other is that there will be a gain narrowing effect during the CPA amplification process. That is, if corresponding measures are not taken during the amplification process, the spectral components far away from the central wavelength will not be effectively amplified, resulting in a narrowing of the pulse spectrum. Poor compression effect and narrowing of the spectrum will lead to unsatisfactory output pulse width, resulting in insufficient laser output peak power.

DAZZLER was born to solve these two thorny problems. First of all, DAZZLER can quantitatively compensate for each order of dispersion (up to the fourth order). Please note that this means that it can customize the size of a certain order of dispersion without changing other orders of dispersion. Currently, there is no structure or material that can achieve this function. By flexibly controlling each order of dispersion, it can compensate the spectral phase of the pulse very smoothly, that is, you can compress the femtosecond pulse to a pulse width close to the Fourier transform limit.

It can be said that "the shortest is only this short."

DAZZLER is also handy for the second problem. In order to solve the problem of spectrum narrowing during the amplification process, it can preprocess the spectral shape of the pulse, such as modulating the Gaussian-like spectrum into a saddle shape (low in the middle and high on both sides) to offset the narrowing of the spectrum caused by the marginal effect of the gain curve. In addition, DAZZLER also has an optimized version for OPCPA amplifiers. Its main function is to improve the compensation accuracy of high-order phases and accurately control the pulse width during the amplification process. At the same time, due to the precise requirements of OPCPA amplifiers for delay, DAZZLER also has an ultra-low time jitter design. In order to meet the needs of some scientific research users for the CEP locking function, DAZZLER can also be equipped with a peripheral locking circuit to perform precise closed-loop control of the CEP signal.

2.2 Multidimensional spectroscopy applications

Of course, the application of DAZZLER is not limited to the field of ultrafast laser amplification. Another function of DAZZLER comes into play in some experiments that use ultrafast lasers as pump detection tools. DAZZLER can separate a single incident pulse into pulse pairs with adjustable delay.

The two pulses contained in a pulse pair are phase locked, and you can tune the delay between the pulse pairs in rapid succession. This function is applicable to a very wide spectral range and can work from UV to MIR.

3. Principle of DAZZLER

Behind the powerful functions of DAZZLER is its ingenious principle design. Let's take a brief look at what the ultra-fast pulse goes through inside DAZZLER. First of all, we can summarize the working process of DAZZLER as the interaction of sound waves and light waves.

The interaction mechanism between sound waves and light waves [2] can be expressed by the following formulas (1) (2) (3). In the time domain, the electric field of the output light wave is the convolution of the input light field and the acoustic wavelength. In the frequency domain, the output light field is the product of the input light field and the acoustic wave field, and the incident light field, outgoing light field and acoustic wavelength satisfy phase matching (energy conservation, momentum conservation). For the theoretical background of sound-light interaction, please refer to Pierre Tournois's article in Optics Communications [3].

Then we can understand the phase and amplitude modulation process of the incident laser by AOPDF from the perspective of ultrafast (wide spectrum) laser. Only the sound waves that match the phase of each frequency component of the incident laser can interact with maximum efficiency. According to formula (3), it can be seen that the spectral phase and amplitude of the outgoing light field can be modulated by modulating the phase and amplitude of the acoustic wave field. This represents the two most basic functions of DAZZLER: spectral phase optimization (dispersion compensation) and spectral shape modulation (suppression of gain narrowing).

Figure 2 Schematic diagram of AOPDF compressed pulse width

As shown in Figure 2, a broadband pulse with positive chirp passes through a preset acoustic wave field. The long-wave (low-frequency) component is first diffracted and experiences a relatively long optical path. The short-wave (high-frequency) component is subsequently diffracted and experiences a relatively short optical path. Therefore, the effect of the acoustic wave field is equivalent to generating a negative chirp, thereby compressing the pulse. From the perspective of an actual ultrafast laser system, it is easy to imagine that if the residual dispersion (spectral phase distortion) of the entire laser system is obtained through a measurement device, and then the negative value of this dispersion is fed back to the DAZZLER, the dispersion of the system can be modulated to zero, and a pulse at the Fourier transform limit can be obtained.

In the same way, the spectrum narrowing information of the system can also be measured and fed back to the AOPDF to obtain the widest spectrum. In addition, by applying two or more acoustic wave fields with fixed phase and fixed time delay, multiple pulses with phase lock and fixed delay can be obtained, which provides an ideal tool for spectroscopy research.

4. DAZZLER installation and debugging skills

(1) DAZZLER initial installation

Carefully read the nominal light aperture in the instruction manual and adjust the position of DAZZLR. Generally, the light spot should pass through the light aperture near the bottom 1/3, otherwise the diffraction light spot may be cut (see Figure 3).

Figure 3 Diagram of DAZZLER clear aperture

For incident light of P polarization, the DAZZLER should be placed vertically. DAZZLER will change the polarization state of the incident light by 90°, so the outgoing light becomes S-polarized. On the contrary, for S-polarized incident light, the DAZZLER should be placed horizontally. DAZZLER will change the polarization state of the incident light by 90°, so the outgoing light becomes S-polarized.

Connect the DAZZLER controller trigger signal according to the requirements of the laser system. Take the 1 kHz regenerative amplifier as an example; the 1 kHz signal output by the signal source has certain trigger level requirements, and the TTL signal requires a voltage of above 3V. If you are using an older version of DAZZLER, a 50-ohm impedance matching connector is required for the software to correctly identify the repetition frequency (not required for the new version). Under normal circumstances, the L1 light should flash green.

Figure 4 Schematic diagram of DAZZLER power controller

(2) Synchronization signal setting

The purpose of the synchronization setting is to ensure the effective interaction between the incident laser and the acoustic wave light field, which means that the acoustic wave is already in a waiting state when the light pulse arrives. Open the DAZZLER software, first click setup, and perform Trigger and mode settings. Here, the delay between the external trigger signal and the regenerative amplified light export time is required to be within 1us. A certain amount of advance is required from software calculation to actual control, about 21us, which can be fine-tuned in the settings.

Figure 4 DAZZLER synchronization signal settings

(3) Wavelength correction

Due to the influence of phase matching and the existence of birefringence properties, different acousto-optic crystal angles correspond to different center wavelengths of the output spectrum (can be understood by referring to the nonlinear optical parametric amplification process). The manufacturer has already calibrated the corresponding sound and light field and output spectrum before leaving the factory. This calibration needs to be restored during installation.

First, set the hole position in DAZZLER software, such as 800nm. Then adjust the angle of the incident light entering DAZZLER, and you will find that the spectrum will shift significantly. Carefully adjust the center of the spectrum depression to the set value of 800nm ​​to fix it.

Figure 5 DAZZLER software operation interface

5. Precautions

The output pulse polarization is perpendicular to the input polarization, so when setting up the optical path, pay attention to selecting appropriate polarization components.

Determine the input laser energy and spot size strictly in accordance with the manufacturer's requirements.

Read the manufacturer's instructions carefully to prevent improper operation of DAZZLER from damaging the laser system, especially to prevent spectral cutting.

[1] F. Gires and P. Tournois, “Interferometreutilisable pour la compression d'impulsions lumineuses modulees en frequence”,C. R. Acad. Sci. Paris 258, 6112 (1964)

[2] Pierre Tournois, “Acousto-optic programmable dispersive filter for adaptive compensation of group delay time dispersion inlaser systems”, Optics Communications,Volume 140, Issues 4–6, 1997,Pages [3] F. Verluise, V. Laude, Z. Cheng, Ch.Spielmann, and P. Tournois, "Amplitude and phase control of ultrashortpulses by use of an acousto-optic programmable dispersive filter: pulsecompression and shaping," Opt. Lett. 25, 575-577 (2000)