What is chirped mirror?
Chirp, the chirp of a light pulse is the characteristic that its instantaneous frequency changes with time. After the laser pulse propagates in a transparent medium, chirp will be generated due to the influence of dispersion and nonlinear effects. GDD (group delay dispersion) is usually used to measure the size of the chirp, and the common unit is fs^2. When the laser pulse is not chirped, it has the smallest pulse width. On the contrary, if the pulse is chirped, the pulse width will be broadened. This is a problem that must be solved in the ultrafast field of pursuing ultra-short pulses. The chirped mirror is to compensate for the chirp generated by the laser pulse.
What is a chirped mirror?
Chirped mirrors are also known as dispersion compensation mirrors. Chirped mirrors are mainly used in femtosecond lasers. A single reflection introduces a fixed amount of dispersion, which can correct the pulse broadening that occurs when ultra-short pulses pass through the optical system. They are very suitable for use as dispersion management tools (such as pre-compensation). The most basic thing about the design of chirped mirrors is that the Bragg wavelength is not constant, but changes spatially in the structure, mainly due to the function of the multi-layer film. Light of different wavelengths enters the mirror structure at different depths, so it has different group delays.
Classification of chirped mirrors
Dispersion compensation chirped mirror pair - The dispersion compensation pair is specially designed to solve the problem of dispersion floating. It consists of two different mirrors. Their GDD curve fluctuations have the same trend, but they are relatively shifted by half a period. This can ultimately minimize the fluctuation of the average GDD. For compensation mirror pairs with less than 1 octave, such a design is easy to implement. Dual-angle chirped mirror - two identical mirrors used in combination at different angles of incidence. Compared with the compensation mirror pair, it has the following advantages:
1. Higher stability and lower manufacturing deviation;
2. No need for two different coatings, low cost.
Single-angle/Brewster's angle chirped mirror - only allows the beam to be incident at one angle. If the angle is designed to be Brewster's angle of incidence, the Fresnel reflection ratio is low and a floating-free GDD can be achieved. The Brewster angle is generally ~56°, so it is easier to apply outside the cavity. On the contrary, it is difficult to obtain such a large reflection angle inside the cavity, so it is not suitable. Under subdivision, there are high dispersion compensation mirrors and positive dispersion compensation mirrors.
The design of the chirped mirror needs to consider the characteristics of bandwidth, dispersion compensation, dispersion oscillation and reflectivity. From an application perspective, wider bandwidth and higher dispersion compensation have wider uses. However, from a design perspective, bandwidth and dispersion compensation are inversely proportional, and dispersion compensation and dispersion oscillation are also directly proportional. Therefore, in actual use, we can only seek a balance. Ti:sapphire lasers usually introduce tens of fs2 of dispersion, and Yb lasers are usually high dispersion compensation mirrors.
Application scenarios of chirped mirrors
1. Periodic-level pulse compressors To obtain periodic-level pulse widths, hollow-core fibers or slice groups are usually selected to broaden the spectrum, and then chirped mirrors are used to compress the pulse width to periodic magnitude.
2. Compensate for material dispersion introduced by transmission. In ultrafast laser application systems, complex optical paths will inevitably introduce positive dispersion, causing the initial pulse width to be broadened. In order to regain the pulse width near the Fourier limit, a chirped mirror can be used to compensate for negative dispersion.
3. Resonator mirror When designing the laser resonant cavity, the resonator mirror can be designed as a reflector or output mirror with a certain negative dispersion, which can directly compensate for the negative dispersion introduced by the optical components inside the optical cavity.