定义：用来引入色散的线性光谐振腔。
 
Gires-Tournois干涉仪是类似于法布里-珀罗干涉仪的一种驻波谐振腔，但是利用的是反射效应，用来产生色散。前面的镜子是部分反射的，而后面的镜子则具有很高的反射率。如果谐振腔中不存在损耗，那么在所有波长情况下功率反射率都是一致的，但是反射光的相位由于存在共振效应而与频率有关，于是产生色散。反射光的相位变化和色散（包括群时延色散和高阶色散）随频率周期性的变化，这是在忽略材料色散的前提下。此时并没有二阶色散是共振或者反共振的，以及这些点之间无正常或者反常色散。 

图1：Gires-Tournois干涉仪示意图。 

图2：由5μm 厚的石英层制备在高反射镜上的GTI随波长变化的群时延色散曲线。 
可调谐的色散可以通过调节两反射镜之间的空气间隔而得到，但是需要仔细固定以避免不需要的移动。采用单片设计可以得到更稳定但是不能调节的GDD，采用包层电介质材料，例如TiO2和SiO2，尤其适用于飞秒激光器。 
GTI最主要的缺陷是其有限的带宽（与给定GDD幅值的平方根成正比），并且对高阶色散的控制程度很小。具有更大带宽的色散反射镜可以作为啁啾反射镜。

Acronym: GTI

Definition: linear optical resonators used for introducing chromatic dispersion

More general terms: interferometers, optical resonators

A Gires–Tournois interferometer is an optical standing-wave resonator similar to a Fabry–Pérot interferometer, which however is operated in reflection and with the purpose of generating chromatic dispersion. The front mirror is partially reflective, whereas the back mirror has a high reflectivity. If no losses occur in the resonator, the power reflectance is unity at all wavelengths, but the phase of the reflected light is frequency-dependent due to the resonance effect, causing chromatic dispersion. The phase change of reflected light and the dispersion (including group delay dispersion and higher-order dispersion) change periodically with the optical frequency, if material dispersion is negligible. There is no second-order dispersion exactly on-resonance or anti-resonance, and positive or negative dispersion between these points.

Figure 1: Schematic setup of a Gires–Tournois interferometer.

Ideally, a GTI is operated near a maximum or minimum of the GDD, and the usable bandwidth is some fraction (e.g. one-tenth) of the free spectral range, which is inversely proportional to the resonator length. In the time domain, an optical bandwidth well below the free spectral range implies that the pulse duration needs to be well above the round-trip time of the GTI. The maximum magnitude of GDD scales with the square of the resonator length.

Tunable dispersion can be obtained with a variable air gap between the mirrors, which however must be carefully stabilized to avoid unwanted drifts. More stable but in general not tunable GDD can be generated with monolithic designs, based e.g. on thin films of dielectric media such as TiO₂ and SiO₂, particularly for use in femtosecond lasers.

Figure 2: Wavelength-dependent group delay dispersion of a GTI made of a 5-μm thick silica layer on a high reflector. The diagram has been made with the software RP Coating.

The main drawbacks of the GTI are the fundamentally limited bandwidth (proportional to the square root of the given magnitude of GDD) and the limited amount of control of higher-order dispersion. Dispersive mirrors with significantly broader optical bandwidth can be designed as chirped mirrors.

Bibliography