[Commonly used optical components] Principles and applications of chirped mirrors

1. Principles of chirped mirrors

To understand the principles of chirped mirrors, we first need to know where the chirped mirrors come from. Simply put, for a pulse containing many spectral components, as long as it propagates through the medium, chirp will inevitably be introduced. This is because different spectral components propagate at different speeds in the medium, and a group velocity delay will inevitably occur per unit time. At the same time, the longer the distance propagated in the medium, the greater the delay between wavelengths, and the greater the amount of chirp introduced, causing the pulse to become wider in the time domain. This is one of the reasons why we should try to avoid using transmissive elements when using ultrafast laser light sources (other reasons include nonlinear effects, etc.). After all, as the pulse width becomes wider, the ultrafast laser light source loses its role. The chirped mirror was first proposed by R. Szipöcs in an article in

1994. Its most basic principle is to coat multiple layers of film on the lens base. By designing the thickness and material (i.e. refractive index) of each layer of film, the penetration depth of different frequency components of the laser in the multi-layer film is controlled (this process is equivalent to artificially introducing a designed group delay) to achieve the purpose of controlling dispersion.

Figure 1. Multi-layer film structure of chirped mirror

2. Development and improvement of chirped mirror

Although the principle is simple, it was found during testing that this design actually has a serious problem. The group delay caused by the reflection of multi-layer films will have extremely large oscillations in the spectrum range, which will obviously cause the introduced chirp amount to behave as the same violent oscillation. This kind of oscillation is something we don't want to see. After the laser pulse passes through such a chirped mirror, its spectral phase will be distorted, which cannot achieve the expected effect. Although this oscillation can be attenuated to a certain extent by optimizing the structure of the multilayer film, this optimization is limited by the more stringent initial conditions, that is, the bandwidth and reflectivity that must meet the design requirements, so it is difficult to achieve effective improvements. With further research on the coating principle, two root causes of the group delay oscillation problem have been discovered. First, the surface coating, that is, the outermost coating in contact with the air, forms an F-P cavity-like structure due to Fresnel reflection. This structure introduces dispersion that oscillates regularly according to wavelength changes, as shown in Figure 2. (The famous GTI mirror uses this to artificially introduce such dispersion)

Figure 2. Dispersion that oscillates regularly according to wavelength changes

The second reason is that the light reflected by each layer in the multi-layer film structure will undergo a weak mismatch when it re-enters the air after coupling. This phenomenon will also cause dispersion oscillation. Having found the "cause", it is time to "prescribe the right medicine". The oscillation of group delay is a regular oscillation according to the change of wavelength, so a pair of chirped mirrors that can complement each other in terms of dispersion can be obtained by designing multi-layer films. The peak of your wave versus the trough of my wave ensures the gentleness of the dispersion curve, as shown in Figure

3. This design can also effectively reduce the number of coating layers and reduce design difficulty. In addition, the surface of the chirped mirror will be coated with an AR (Anti-Reflection) film to eliminate the effect of Fresnel reflection.

Figure 3. The designed chirped mirror complements dispersion

3. Application of chirped mirror

Chirped mirror has unique advantages compared to other structures of dispersion compensation devices. It is compact in size and has high reflectivity. However, it has a fly in the ointment. The amount of dispersion provided is relatively limited. After all, it only relies on a few thin layers of film to achieve its function. Therefore, the applicable range of chirped mirrors is often where the amount of dispersion is not large, but accuracy and space are required. For example, chirped mirror pairs are often used in ultrafast oscillators to accurately compensate for small intra-cavity material dispersions (which can be combined with prism pairs or wedge pairs). Chirped mirrors are also commonly used in the supercontinuum generation process based on ultrafast lasers. After ultrashort laser pulses pass through the supercontinuum material, the spectrum broadens, and chirped mirrors need to be used to compress the pulses to a shorter pulse width.