[Laser Technology Sharing] High Energy Dual Grating Compressor System Design

With the development of laser technology, especially the emergence of titanium sapphire self-locking film technology (KLM) and chirped pulse amplification technology (CPA) in the 1990s, lasers can generate ultrashort pulses with pulse widths of femtoseconds and peak powers of more than terawatts. It provides an important means for the study of the microscopic world and ultrafast processes in physics, chemistry, biology, spectroscopy and other disciplines.

At present, increasing the peak power of laser pulses is still a goal pursued by scientific research institutions around the world. For example, in 2017, the Shanghai Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences took the lead in realizing laser pulse output with peak powers of 5PW and 10PW based on CPA technology. However, such a high peak power will inevitably cause damage to the optical components in the laser. Therefore, international PW-level laser systems are generally divided into two types of amplification methods, namely based on chirped pulse amplification technology (CPA) and based on chirped pulse optical parametric amplification technology (OPCPA). As shown in Figure 1, it is the CPA schematic diagram.

Figure 1. Schematic diagram of chirped pulse amplification technology

In order to reduce the damage of laser pulses to optical elements during the amplification process, these two technologies first use a stretcher to stretch the laser pulse from a femtosecond pulse into a picosecond or nanosecond pulse, then amplify the broadened seed pulse in the gain medium, and finally use a A corresponding compressor compresses the amplified pulse, thereby obtaining a laser pulse with extremely high energy (on the order of Joules), extremely short pulse width (on the order of femtoseconds), and extremely high peak power (on the order of one hundred terawatts or petawatts), providing a tool for exploring the microscopic world and ultrafast processes. Therefore, in high-energy laser amplification systems, the design of the compressor is crucial.

Here, we will design a reasonable and effective compressor for a high-energy petawatt-level system, taking the dual grating compression system as an example.

Notice:

1. This example is not based on any existing system. Any similarity is purely coincidental.

2. If you need to refer to this example for compressor design, please pay attention to inserting the correct laser parameters (wavelength, bandwidth, pulse width, energy, beam diameter, etc.), and carefully confirm the calculation results (if you have any questions, please contact our company for assistance). Do not directly apply the compressor dimensions for design or purchase.

1. Design goals

The specific parameters of the optical pulse emitted by the amplifier are shown in Table 1:

Table 1. Amplifier output light parameters

We adopt a dual-grating compression structure here, as shown in Figure

2. The broadband laser pulse output by the amplifier is incident on grating 1 for spectrum expansion, and then collimated by grating 2, lowered by a 90° folding roof mirror, collected by grating 2 and collimated by grating 1, and output to the compressor after passing through the reflector. Achieve pulse width compression of light pulses in time. The optical components used are: grating 1, grating 2, and roof mirror.

Figure 2. Dual grating compressor. G1, grating 1; G2, grating 2, RM, roof mirror;

2. Design steps

The specific implementation steps are as follows:

1. Determine the spot size

Since the incident broadband laser pulse is 28J, according to the empirical value of the damage threshold of the grating (for example, provided by JY Company, 0.4 J/cm2, femtosecond short pulse 0.12 J/cm2), in order to ensure that the grating is not damaged, through the following calculation, it can be obtained: R≥66.7mm, that is, Φ=133mm.

In order to increase the safety factor, the beam diameter is Φ=190mm.

2. Set the grating incident angle

Generally speaking, for diffraction gratings, the diffraction efficiency of the grating is maximum when the incident angle is the Littrow angle. According to the grating equation: sinγ+sin(γ-θ)=λ⁄d, its Littow angle (θ=0) is 26.7°. However, for a reflective compressor, when the Littrow angle is incident on the grating, the diffracted light and the incident light cannot be spatially separated. Therefore, in order to separate the diffracted light from the incident light, the incident angle is tentatively set to 40°.

Note: In the CPA system, when selecting the grating parameters and incident angle of the compressor, it is also necessary to consider the stretcher parameters and the dispersion of the amplifier itself to ensure that the overall dispersion of the CPA system (including second-order dispersion, third-order dispersion and higher-order dispersion) can be compensated.

3. Determine the grating pair spacing

According to Treacy’s formula [1], it can be calculated that when the compression amount of the compressor with the structure shown in Figure 2 is 2ns, the slant distance of the grating pair is:

Here, we will not go into details about the derivation of the dispersion formula for grating pairs. See reference [1] for details.

4. Determine the size of grating 1

The light spot is incident with an aperture of 190mm and an angle of 40°. The light spot on the grating is elliptical, with its long and short axes being 248mm and 190mm respectively. Consider separating the incoming pulse and the outgoing pulse up and down, and leaving enough space for the refracting mirror. Then determine the size of grating 1 as 350mm*450mm (width*height);

5. Determine the size of grating 2

After the incident light is expanded on grating 1, it propagates 1300mm in space, and the lateral size is ≥586mm. In order to ensure the adjustment margin, the size of grating 2 is determined to be 680mm*450mm (width*height);

6. Determine the size of the folding mirror

The transverse size of the 586mm beam after being collimated by grating 2 is 450mm. In order to ensure the adjustment margin, the transverse size of the folding mirror is determined to be 550mm and the longitudinal spot size is 269mm. Therefore, the size of the two folding mirrors is 550*305mm.

7. Grating efficiency

The efficiency of the compressor system designed here is 60%, so the diffraction efficiency of each grating must be above 88% within the entire bandwidth range (1300nm~1700nm) and near the corresponding incident angle (here 40°).

Figure 3. Schematic diagram of the optical path of the designed compressor.

As can be seen from the above figure, for PW-level compressors, due to the large spot size of the incident light, it is necessary to customize ultra-large-diameter gratings. In order to reduce the size of a single grating, a four-grating compressor structure can often be used. As shown in Figure 4, under the condition of four-grating structure, the compressor parameters remain unchanged.

Figure 4. Schematic diagram of the optical path of the four-grating compressor

8. Inspection

The above parameters can be input into ZEMAX software or other similar optical software for optical tracing. If there is no light leakage or light cut, the system parameters can be determined. It is summarized in Table 3 below:

Table 3. Parameters of each component in the compressor

In this way, you can go to various companies to order gratings and folding mirrors.

Note: Here the author only takes the most common four-pass structure dual grating structure compressor as an example to describe how to determine each parameter in the compressor. For other types of compressors and stretchers, such as dual-grating compressors with an eight-pass structure, compressors based on four gratings at high energy, Offner stretchers and Martinez stretchers, etc., we can all draw inferences from one example and apply them better in scientific research life.

The grating size parameters here are the most reasonable parameters. The specific grating size purchased should be determined based on the actual situation of the company from which the grating is purchased.

Cited documents:

[1] Treacy E B. Optical pulse compression with diffraction gratings [J]. IEEE J. Quantum Electron., 1969, QE-5(9): 454-458.