Holographic medium for the mid-IR narrow-band filters

Holographic medium for the mid-IR narrow-band filters

A.S. Shcheulin, A.E. Angervaks, A.I. Ryskin

St. Petersburg, Russia angervax@mail.ru

Abstract—Holographic medium for fabrication of narrow- band holographic filters for mid-IR spectral range is proposed. It can be used for stabilization of diode lasers radiation.

Keywords—volume hologram; mid-IR; calcium fluoride; filter Diode lasers of mid-IR spectral range need narrow-band filters for stabilization of their radiation. However existing holographic media can’t be used at wavelengths exceeding ~ 2.5 m. We propose a medium that, in principle, allows formation of such filters in the wavelength range up to 9–10

m. This medium is based on CaF₂ crystals with color centers.

There are various types of centers in this crystals and each of them has specific absorption bands. Under the impact of radiation resonant to one or another centers at an elevated temperature the centers transformation take place which is accompanied with modification of the absorption spectrum of the crystal. Photochromism of the crystal allows recording amplitude, phase and amplitude-phase holograms within its volume, which can be read out in visible and IR ranges of the spectrum up to fundamental absorption of the crystal lattice (~

10 m). Thickness of recorded holograms reaches 10 mm and more. The specific diffusion-drift mechanism of the hologram recording results in both center transformation and their redistribution over the crystal bulk. This mechanism enables also to carry out the post exposure photo-thermal treatment of the crystal with a hologram using an actinic incoherent radiation at temperatures of 85–200 °C. It results in modification of the absorption spectrum of the crystal without decaying the hologram.

This enables to read out the recorded hologram as a predominantly phase hologram at practically any wavelength within IR spectral range up to ~ 9 m. This opportunity makes possible to create narrow-band reflective filters for this range.

The example of post-exposure hologram transformation is shown in Figs. 1 and 2. Fig. 1 shows the absorption spectrum of CaF₂ crystal with color centers before (solid line) and after the hologram recording with 532 nm radiation (Hologram 1).

Absorption spectra of the crystal with several holograms formed as a result of series of successive photo-thermal treatments of the sample with Hologram 1 are presented in Fig.

2.

Fig. 1. Absorption spectrum of CaF2 crystal with color centers before (solid line) and after (dotted line) the hologram recording.

Fig. 2. Absorption spectra of the crystal with Holograms 2–5 (a), and the spectrum of the crystal with Hologram 2 in the extended wavelength range (b).

This work was partially financially supported by Government of Russian Federation, Grant 074-U01.