Abstract

Presently, one of the most promising techniques for generating coherent THz radiation relies on mixing two continuous-wave laser beams of suitable close-wavelength and same polarization in a photomixing antenna (photomixer) which subsequently radiates their difference-frequency in the THz range into free-space. The so generated THz radiation serves to acquire images in the THz band. Recent works show that imaging with two different THz frequencies enables improving the identification of materials [1,2]. In this work we report on the triple-wavelength operation a broadband Cr³⁺:LiCAF solid-state laser for the simultaneous generation of three different laser wavelengths. When combined with a suitable photomixer or DFG process it can be employed to generate two different THz frequencies simultaneously, or to switch the THz emission rapidly between two different frequencies. Figure 1 shows the experimental arrangement of the laser. It consists of a V-folded main cavity with three distinct externally-coupled cavities based on diffraction gratings that simultaneously lock the laser emission to three targeted wavelengths. Every external cavity has an anamorphic prism pair for narrowing the laser lines emitted thus enhancing the coherence of the THz waves. Due to the relatively low gain in Cr:LiCAF it is necessary to employ low loss optics and rather low values of the output coupling of cavity mirrors, so a trade-off between output coupling and feedback efficiency arises (to be discussed on site). Since the light inside the cavity experiences a double loss per rountrip because of the double reflection from the folding mirror (CM2, R=99%), a higher reflectivity is required for this mirror in comparison to the output mirror (CM3, R=98.5%) and, therefore, a higher amount of feedback from the external reflector is also necessary to obtain self-injection-locking. For this reason, a Littrow configuration is used in the external cavities 1 and 2. In the third external cavity a Littman configuration suffices for providing self-injection-locking of the third wavelength and it also provides a useful spatially fixed output beam that is not steered when the feedback mirror is angle tuned.

© 2011 Optical Society of America