Abstract

In this paper, we report a novel implementation for WDM add/drop multiplexing application based on acoustically modulated fiber Bragg grating. We use acoustic waves to excite lateral vibration and hence micro-bending of the fiber grating. By appropriately controlling the period of the micro-bending, the coupling between the core modes and certain cladding modes in the fiber is produced. Through such mechanisms, we can select the reflection window by switching on/off the acoustic wave. Before the input end of the grating section, the fiber was laterally glued onto the tip of a metal horn, whose base was attached to a piezoelectric transducer (PZT), driven by a voltage source, for receiving mechanical vibration. Such vibration translates to the fiber and generates micro-bending. To enhance the micro-bending effect and to control the cladding mode distribution, the fiber section of grating was etched with HF solution to reduce the cladding diameter down to 40 μm. The fiber grating was written with a slant angle of 2 degrees. The grating length was 1.7 cm. The etching length was 3.5 cm with its center coincident with the grating center. Figure 1 shows the reflection spectrum with zero applied voltage. The small side-lope at λ_s = 1539.7 nm beside the Bragg reflection peak at λ_b = 1541.5 nm represents the residual power of the backward-propagating cladding mode $\left({\text{β}}_{1\text{m}}^{-}\left({\text{λ}}_{\text{s}}\right)\right)$, which is coupled from the incident core mode $\left({\text{β}}_{01}^{+}\left({\text{λ}}_{\text{s}}\right)\right)$. When a voltage is applied, the generated micro-bending can phase-match the backward-propagating cladding mode $\left({\text{β}}_{1\text{m}}^{-}\left({\text{λ}}_{\text{s}}\right)\right)$ and the backward-propagating core mode $\left({\text{β}}_{01}^{-}\left({\text{λ}}_{\text{s}}\right)\right)$. Therefore, the out-coupled power can be coupled back to the backward­propagating core mode at λ_s. Meanwhile, because the micro-bending can also phase-match the coupling between the forward-propagating core mode at ${\text{λ}}_{\text{b}}\left({\text{β}}_{01}^{+}\left({\text{λ}}_{\text{b}}\right)\right)$ and the forward­propagating cladding mode $\left({\text{β}}_{1\text{m}}^{+}\left({\text{λ}}_{\text{b}}\right)\right)$, the reflected power at λ_b is reduced. Figures 2 shows the reflection spectrum when the applied voltage for the PZT is 8.4 V and the voltage frequency is 1.08 MHz. One can see the increasing and decreasing trends of reflection levels at λ_s and λ_b, respectively.

© 2000 IEEE