**3. Laboratory measurement system**

To characterize the tuning methods, an optical measurement system was set up, whose implementation scheme is shown in **Figure 3**. It is based on the measurement of the spectral component reflected by the network when illuminated by an optical

#### **Figure 3.**

*Experimental setup used to determine the deviations in the central wavelength of the FBG, caused by the tuning system.*

**Figure 4.** *Optical spectrum of the ASE measured at the amplifier output for an optical pumping power value of 88 mW.*

source with high spectral bandwidth. The input optical signal is amplified and introduced into the Bragg grating through an optical circulator. The signal reflected by the FBG is then conveyed by the circulator to an optical spectrum analyzer (ANRITSU model MS9601A). The Bragg grating is tuned by a motorized system developed for this purpose, which is described in ref. [25].

A commercial optical amplifier (Photonetics model BT 13) with a pumping laser at 980 nm and an optical saturation power of 13 dBm was used.

**Figure 4** shows the ASE spectrum, measured with a resolution of 0.5 nm, where the dependence of the spectral amplitude with the wavelength is clearly visible. Therefore, all spectral measurements described in the following chapter are represented by subtracting the optical power of the source from the reflection spectrum, at each wavelength.

Bragg gratings are recorded on photosensitive single-mode fiber (Fibercore model PS1250/1500) with numerical aperture 0.13 and diameter 125 μm. The etching method exposes the fiber to ultraviolet radiation through a constant period phase mask. The recording optical source is an Argon laser operating in continuous mode at 244 nm and with an average power of 150 mW. A more detailed description of the recording system can be found in [26, 27]. The apodization format of the network is approximately Gaussian because the laser used has an optical beam with a Gaussian profile.
