1.Introduction Vibration exists in a variety of engineering structures and mechanical systems,the measurement and analysis of vibration has an important significance and value[1].The traditional sensors can not content the immunity to electromagnetic interferences and high sensitivity.Optical fiber sensor with high sensitivity, immunity to electromagnetic interferences, resistance to chemical corrosion, compact size and fast response characteristics, so it has high research value and widespread application prospects in the sensor field.

Tilted fiber Bragg grating(TFBG) is one of members of the short-period fiber grating.The difference between general fiber Bragg grating is its grating wave vector is blazed by a certain angle in the relation to the direction of optical axis.TFBG not only can lead to counter-propagating guided modes coupling, but also can couple the fundamental mode to cladding mode and radiate out. Controlling the angle between the grating plane and the fiber axis can make different wavelengths of light propagated in optical fiber coupling to the fiber cladding at different angles.It's special structure and properties make it have the advantages of fiber Bragg grating (FBG) and long-period fiber grating (LPG) [2,3],and the distinctive advantages of temperature qualities and high sensitivity to external cladding mode.Recently, TFBG has been proposed for many sensing applications,such as refractive index, bending and others[4-7].

In this paper,a novel sensor for vibration measurement based on a TFBG interacting with a MMF is proposed.The sensor is composed by inserting a small section of MMF between a TFBG and a SMF. The MMF is used to couple back the counter-propagating cladding modes of the TFBG into the core modes of the SMF.Because the cladding mode is sensitive to external vibrations,so by measuring the output power of the cladding mode can obtain the vibration information.As using of instensity demodulation, the complexity of the sensor reduces greatly.

2.Production and operating principle of the sensor

The sensor head of the vibration sensor is shown in Fig.1.A section of 2mm MMF is spliced between the SMF and the TFBG,the MMF has a core diameter of 105 m and cladding diameter of 125 m.TFBG is the using of phase mask technique,the phase mask in the production will be tilted at an angle,make a Ge-loaded SMF exposed on a UV laser obtained. The cycle of the phase mask is 1066nm, tilt angle is 50,the length of the TFBG is 2cm.The measured transmission spectrum of the TFBG is shown in Fig.2.The right dip,on the long-wavelength side,corresponds to the Bragg resonance wavelength.The other dips are due to coupling