Virtually imaged phased array

A virtually imaged phased array (VIPA)[1] is an angular dispersive device that, like a prism or a diffraction grating, splits light into its spectral components.

In contrast to prisms or regular diffraction gratings, the VIPA has a much higher angular dispersion but has a smaller free spectral range.

This is the fundamental difference from an Echelle grating, where a similar phased array is formed in the real space.

The virtual images of a light source in the VIPA are automatically aligned exactly at a constant interval, which is critical for optical interference.

[14] A drawback of the VIPA is its limited free spectral range due to the high diffraction order.

To expand the functional wavelength range, Shirasaki combined a VIPA with a regular diffraction grating in 1997 to provide a broadband two-dimensional spectral disperser.

After the mid 2000s, the two-dimensional VIPA disperser has been used in various fields and devices, such as high-resolution WDM (Weiner, 2004),[16] a laser frequency comb (Diddams, 2007),[17] a spectrometer (Nugent-Glandorf, 2012),[18] astrophysical instruments (Le Coarer, 2017, Bourdarot, 2018, Delboulbé, 2022, and Stacey, 2024),[19][20][21][22] Brillouin spectroscopy in biomechanics (Scarcelli, 2008, Rosa, 2018, and Margueritat, 2020),[23][24][25] other Brillouin spectroscopy (Loubeyre, 2022 and Wu, 2023),[26][27] beam scanning (Ford, 2008),[28] microscopy (Jalali, 2009),[29] tomography imaging (Ellerbee, 2014),[30] metrology (Bhattacharya, 2015),[31] fiber laser (Xu, 2020),[32] LiDAR (Fu, 2021),[33] and surface measurement (Zhu, 2022).

[34] The main component of a VIPA is a glass plate whose normal is slightly tilted with respect to the input light.

Similarly to the resolving power of a diffraction grating, which is determined by the number of the illuminated grating elements and the order of diffraction, the resolving power of a VIPA is determined by the reflectivity of the back surface of the VIPA and the thickness of the glass plate.

For large angular dispersion with high resolving power, the dimensions of the VIPA should be accurately controlled.

[35] A constant reflectivity of the partially transmissive mirror in the VIPA produces a Lorentzian power distribution when the output light is imaged onto a screen, which has a negative effect on the wavelength selectivity.