2.1.2. AOTF vs. LCTF

An AOTF (Figure 3) is an electro-optical device that functions as an electronically tunable excitation filter to simultaneously modulate the intensity and wavelength of the laser beam. The AOTF relies on a specialized birefringent crystal whose optical properties vary upon interaction with an acoustic wave. Changes in the acoustic frequency alter the diffraction properties of the crystal, enabling very rapid (in microsecond level) wavelength tuning, limited only by the acoustic transit time across the crystal. The selected wavelength is determined by Equation (1):

$$
\lambda = \frac{\Delta nV}{f} \left[ \sin^2 2\theta\_i + \sin^4 \theta\_i \right]^{1/2} \tag{1}
$$

where the selected wavelength (λ) is a function of the difference of the refractive indexes due to birefringent Δn, the frequency of the applied RF signal *f*, the incident angle θ<sup>i</sup> and the variable speed of acoustic waves in the crystal material *V*.

Previously, a feasibility study for detecting REP values was also carried out with a Liquid Crystal Tunable Filter (LCTF)-based HSL (LCTF-HSL) [31]. The major parameter specifications of LCTF and AOTF are shown in Table 1. The AOTF has a wider spectrum, covering from 430 nm to 1450 nm, while the LCTF just covers 400 nm to 720 nm. The AOTF is capable of continuously selecting or tuning the spectral resolution from 2 nm to 10 nm, and the LCTF just has three independent selections of 7, 10 and 20 nm. Moreover, the response time of AOTF is 10 μs, which is at least three orders of magnitudes quicker than the LCTF (the typical response time is 50 ms). Obviously, AOTF is much more preferable while selecting a filter in HSL. Figure 3 presents the AOTF and LCTF devices employed in this research.

**Table 1.** The AOTF vs LCTF Major Parameters Specifications.

