Acoustic vibrations switch reflection wavelength

John Wallace

A filter developed by scientists at the Chung Cheng Institute of Technology (Taoyuan, Taiwan), the Industrial Technology Research Institute (Hsinchu, Taiwan), and National Taiwan University (Taipei, Taiwan) uses acoustically generated vibration in combination with a fiber Bragg grating (FBG) to switch wavelengths. Such a device is potentially useful as a WDM add/drop filter.

A slant FBG was formed with a length of 17 mm and a slant angle of 2°. The grating had a Bragg wavelength of 1541.5 nm and a reflectivity of 63%. A 35-mm section of fiber including the FBG was etched down in diameter from 125 to 40 µm. A solid aluminum horn was attached to one side of the etched section; attached to a piezotransducer (PZT), the horn induced lateral vibration and microbending in the FBG. A 3-dB fiber coupler was spliced to the fiber section for measurements.

When the grating was driven by the PZT, phase matching induced by microbending together with the grating phase matching resulted in a decreased reflectivity at the Bragg wavelength and an increase at the cladding-mode coupling wavelengths. With no applied PZT voltage, the fiber grating exhibited a reflection peak at its Bragg wavelength; as the voltage was increased, the Bragg reflectivity peak dropped and a second microbending-related peak appeared at 1539.5 nm and grew. At 16 V, the Bragg reflection peak dropped to one-sixth the height of the second peak.

Couplings between various core and cladding modes cause the switching effect. When the acoustic wave is applied, a core mode at the Bragg wavelength is coupled with the opposite-propagating cladding mode through microbending phase-matching. Most of the coupled light radiates away through cladding modes. At the second wavelength, microbending phase-matches the core and cladding modes that are traveling in the same direction. Thus, the outcoupled power at this wavelength can be coupled back into the core mode and the reflectivity preserved. The modes at the two wavelengths can be phase-matched by the acoustically induced microbending because its phase-matching bandwidth is larger than a Bragg window.

The etching of the fiber section was done to improve microbending effects and control the cladding-mode characteristics. The cladding-mode coupling wavelength also can be controlled by changing the acoustic frequency, say the researchers. For more information, contact Chih-Chung Yang at ccy@cc.ee.ntu.edu.tw.

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