Measurement setup and results

The LPFG structure based on a fiber taper with a side-contacted metal grating and the measurement setup are shown in Fig. 3.5. A uniform taper waist of 20-μm diameter is achieved by tapering down a standard single mode fiber (Corning SMF-28). The length of the uniform waist is around 1 cm, and the length of the taper transition region is approximately 1.75 cm. A one-dimensional thin-stannum-layer grating of total length about 1 cm is side-contacted with the taper fiber waist.

Fig.3.5 Diagram of measurement setup and the long period fiber grating composed of a fiber taper and a side-contact metal grating.

The fiber taper is mounted by UV-adhesives on a glass substrate, and the metal grating mounted on the translation stage is moved to contact the underside of the fiber taper. A broadband white light source containing superluminescent diodes (SLD) spanning from 1250 to 1650 nm was launched into one end of the fiber taper for performing measurement. The other end of fiber taper is connected to an optical spectrum analyzer (OSA) for measuring transmission spectra.

In order to understand the behavior of mode coupling, we have measured the transmission spectra of LPFGs when the surrounding medium is air or water. In the latter case we immerse both the taper waist and the grating in water and confirm that they are totally covered by the water. Fig.3.6 (a)-(d) shows the measured transmission spectra with different grating periods.

In Fig.3.6 (a), the resonant wavelengths of the LPFG with diameter of taper waist being 20μm, and the grating period being 421μm are 1392nm and 1532nm. The surrounding medium is air.

Comparing with the case when the surrounding medium is water (1289nm, 1451nm, and 1599nm), the loss peaks shift to longer wavelengths by 59nm and 67nm respectively.

Fig.3.6(a) Transmission spectra of the LPFG with diameter of taper waist being 20 μm, grating period being 421μm, and the surrounding medium being air (black line) and water (red line).

In Fig.3.6 (b), the resonant wavelengths of the LPFG with diameter of taper waist being 20μm, and the grating period is 430μm, are 1327nm and 1472nm. The surrounding medium is air. Comparing with the resonant wavelengths with the surrounding medium being water (1384nm and 1558nm), the loss peaks shift to longer wavelengths by 57nm and 86nm respectively.

Fig.3.6(b) Transmission spectra of the LPFG with diameter of taper waist being 20 μm, grating period being 430μm. The surrounding medium is air (black line) or water (red line).

In Fig.3.6 (c), the resonant wavelengths of the LPFG with diameter of taper waist being 20μm, and the grating period being 500μm are 1255nm, 1401nm and 1540nm. The surrounding medium is air. Comparing with the resonant wavelengths when the surrounding medium is water (1309nm, 1467nm and 1638nm), the loss peak shifts to longer wavelengths by 54nm, 66nm, and 98nm respectively.

Fig.3.6(c) Transmission spectra of the LPFG with diameter of taper waist being 20 μm, grating period being 500μm. The surrounding medium is air (black line) or water (red line).

From the above measurement results, we can conclude that the resonant wavelengths shift to longer wavelengths as the surrounding medium changes from air (n=1) to water (n=1.33).

The tendency is different from the conventional LPFGs where the resonant wavelengths shift toward shorter wavelength as the refractive index of the surrounding medium increases. In the case of conventional LPFGs, the effective index of the core mode is almost constant and the effective indices of cladding modes increase as surrounding index increases, and therefore the index difference between core and cladding modes decreases for larger surrounding indices.

In the similar way, the index difference of two cladding modes in a fiber taper is smaller in water than in air-surrounding at the same wavelength. In order to match the difference of propagation constants through the same grating, the phase-matching wavelengths become shorter in a standard single-mode fiber and larger in a fiber taper for higher surrounding indices.

The amount of the shifted wavelength by changing the surrounding index is larger in the longer resonant wavelengths than in the shorter resonant wavelengths. This is because that the mode field extended to the surrounding medium is wider with longer wavelengths than with shorter wavelengths. Therefore, the longer resonant wavelength is more sensitive to the surrounding medium as compared with the shorter resonant wavelengths.

In Fig.3.6 (d), we compared the transmission spectra of LPFGs with different grating periods in air, which are 421μm and 430μm respectively. For coupling with same modes, the

loss peaks shift toward shorter wavelengths by 65μm and 60μm as the grating period changes from 421μm to 430μm. The tendency is different from the conventional LPFGs. The phase-matching wavelengths shift toward longer wavelengths as the grating period increases.

This is caused by the different dispersion characteristics of the dispersion relations of a standard single mode fiber and a fiber taper, as we have already discussed in Chapter.2.

From the measured transmission spectra, we can see that the -3dB bandwidth is around 100nm, which is larger compared to the bandwidth several nm in conventional LPFGs. The reason is that the changes in effective index difference with wavelength is not as obvious in a fiber taper when compared with the case in a standard single-mode fiber, as has been shown in Fig. 2.13.

Fig.3.6(d) Transmission spectra of the LPFGs. The diameter of taper waist is 20 μm, grating period is 430μm(black line), 421μm (red line), and the surrounding medium is air.