Abstract:
A grating-type tunable filter comprises a signal input terminal, a signal output terminal, a focusing component and gratings. An input collimator, an output collimator, a first grating, a first reflector, a light beam expanding component, a second grating, a polarization rotating component and a second reflector are arranged along an optical path. Wavelength selection is realized by changing the incident angles of the first grating and the second grating by virtue of the rotatable first reflector, and the first grating is inserted among the input collimator, the output collimator and the first reflector driven by MEMS to carry out pre-dispersion.

Description:
TECHNICAL FIELD  
       [0001]    The present patent application relates to a tunable filter, and particularly to a miniaturized grating-type tunable filter. 
       BACKGROUND 
       [0002]    The tunable filter is an instrument used for wavelength selection. It can pick out the light with desired wavelengths from many wavelengths, and deny the light in other wavelengths. As a wavelength-selective device, optical filter is playing an increasingly important role in the optical fiber communication system. 
         [0003]      FIG. 1  is an optical path diagram of a present 100 GHZ ITU wavelength tunable optical filter  10 . As shown in  FIG. 1 , the tunable optical filter  10  includes a signal input terminal  113 , a signal output terminal  111 , a focusing element  13 , a grating  15  and a reflector  17 . The optical signal inputted from the input terminal  113  converts to parallel light signal through the focusing element  13 . The parallel light signal is diffracted at the surface of the grating  15  and directed to the reflector  17 . After being reflected by the reflector  17 , the light comes back to the original way and finally inputs to the signal output terminal  111 . By rotating the grating  15  or reflector  17 , the optical signal enters into the grating  15  twice from entering the input terminal  113  to the output terminal  111 . 
         [0004]    If 50 GHZ ITU wavelength tunable optical filter uses the same optical structure with the above 100 GHZ ITU wavelength tunable optical filter, it will lead to greater diameter of the used lens and larger light spot size of the reflector. The diameter of the corresponding reflector must be increased. The increase of light spot size leads to an increase of the corresponding grating size. The increase of the diameter of the lens, reflector and grating must increase the overall height of the device. It will not only result in decreased versatility of the device but also increase the overall cost of the device comparing with 100 GHZ ITU wavelength tunable optical filter. 
       SUMMARY 
       [0005]    In view of this, there is a need for a grating-type tunable filter with small size and convenient adjustment. 
         [0006]    A grating-type tunable filter includes a signal input terminal, a signal output terminal and a focusing element. An input collimator, an output collimator, a first grating, a first reflector, a light beam expanding component, a second grating, a polarization rotating component and a second reflector are arranged along an optical path. The first reflector is a rotatable reflector. A wavelength selection is realized by changing incident angles of the first grating and the second grating by rotating the rotatable reflector. 
         [0007]    In one embodiment, the rotatable reflector is rotatable MEMS reflector or reflector having same functions as the rotatable MEMS reflector. Incident light beam from the input collimator which passes through the first grating, the MEMS reflector, the light beam expanding component, the second grating and the second reflector is received by the output collimator. 
         [0008]    In one embodiment, the light beam expanding component is a prism assembly or a lens assembly, a combination of the prism assembly and the lens assembly. 
         [0009]    In one embodiment, the input and output collimator is a dual fiber collimator. 
         [0010]    In one embodiment, the input and output collimator is a single fiber collimator. 
         [0011]    In one embodiment, the polarization rotating element is a quarter-wave plate. 
         [0012]    In one embodiment, the polarization rotating element is a Faraday rotation plate. 
         [0013]    Since the present patent application uses a small-sized MEMS reflector. In order to increase the number of grating dispersion by the light beam expanding component, the first grating is inserted among the input collimator, the output collimator and the first reflector to carry out pre-dispersion. The large dimension of the second grating due to the rotation of the MEMS reflector is not required. The structure of the second grating is much miniaturized. The change of the waveform of the tunable filter with the wavelength is small. The polarization rotating component is added between the second grating and the second reflector. The polarization state of a light beam passing through the first grating and the second grating at the second time is mutually vertical to the polarization state of the light beam passing through the first grating and the second grating at the first time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The embodiments of the present patent application are further described with reference to the drawings. 
           [0015]      FIG. 1  is an optical path diagram of a tunable optical filter of prior art. 
           [0016]      FIG. 2  is an optical path diagram of a grating-type tunable filter according to the first embodiment of the present patent application. 
           [0017]      FIG. 3  is an optical path diagram of a grating-type tunable filter according to the second embodiment of the present patent application. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present patent application will be further described below with reference to the drawings. 
         [0019]      FIG. 2  is an optical path diagram of a grating-type tunable filter according to the first embodiment of the present patent application. The tunable filter  20  includes a dual fiber collimator  21 . The dual fiber collimator  21  includes an input fiber  211  and an output fiber  212 . A first grating  22 , a first reflector  23 , a triangular prism  24 , a second grating  25 , a polarization rotating element  27  and a second reflector  26  are provided along an optical path. The first grating  22  receives input light beam from the input fiber  211  and divides the light beam into different small distance light beam with different wavelength. The different small distance light beam with different wavelength are reflected by the first reflector  23  to the second grating  25 . The second grating  25  further divides different small distance light beam based on the wavelength. The polarization state of the further divided light beam is changed by the polarization rotation element  27 . Then, the light beam is reflected by the second reflector  26 . After passing the polarization rotating element  27 , the second grating  25 , the triangular prism  24 , the first reflector  23  and the first grating  22 , the light beam are received by the output fiber  212  of the dual fiber collimator  21 . The first reflector  23  is used select the desired wavelength of the optical signal by adjusting to an appropriate angle. The micro rotator  231  is used to control the size of the angle of rotation. The control unit  232  is used to input voltage magnitude signal to the drive unit  233 . The driving unit  233  is used to control the rotation angle of the micro rotator  231  according to the received voltage magnitude signal. 
         [0020]    Optical principles of the present embodiment is as follows: during the operation, the optical signal input from the input optical fiber  211  is converted to parallel optical signal by the lens  213  in the collimator. The parallel optical signal hits the surface of the first grating  22  and diffracts at the surface. The diffracted optical signal sequentially passes the first reflector  23 , the triangular prism  24 , the second grating  25 , the polarization rotation element  27  and then diffracted again. After being reflected by the second reflector  26 , the optical signal returns along the original path, and eventually enters to signal output fiber  212 . The optical signal which is inputted from the signal input terminal  211  enters the signal output terminal  212 . 
         [0021]    In the above-described embodiment, the input fiber  211  and the output fiber  211  can be the dual fiber pigtail type. The spacing between the dual-fiber is adjustable. In this embodiment, the center distance of the dual-fiber is 125 μm. 
         [0022]    In the present embodiment, the triangular prism increases the dispersion angle of the received light beam. 
         [0023]    In the present embodiment, the polarization rotating element  27  includes a quarter wave plate or Faraday rotator plate for rotating the polarization state of the light beam which pass through the second grating  25 . 
         [0024]      FIG. 3  is an optical path diagram of a grating-type tunable filter according to the second embodiment of the present patent application. The tunable filter  30  includes a dual fiber collimator  31 . The dual fiber collimator  31  includes an input fiber  311  and an output fiber  312 . A first grating  32 , a first reflector  33 , a lens assembly  34 , a second grating  35 , a polarization rotating element  37  and a second reflector  36  are provided along an optical path. The first grating  32  receives input light beam from the input fiber  311  and divides the light beam into different small distance light beam with different wavelength. The different small distance light beam with different wavelength are reflected by the first reflector  33  to the second grating  35 . The second grating  25  further divides different small distance light beam according to the wavelength. The polarization state of the further divided light beam is changed by the polarization rotation element  37 . Then, the light beam is reflected by the second reflector  36 . After passing the polarization rotating element  37 , the second grating  35 , the lens assembly  34 , the first reflector  33  and the first grating  32 , the light beam are received by the output fiber  312  of the dual fiber collimator  31 . The first reflector  33  is used select the desired wavelength of the optical signal by adjusting to an appropriate angle. The micro rotator  331  is used to control the size of the angle of rotation. The control unit  332  is used to input voltage magnitude signal to the drive unit  333 . The driving unit  333  is used to control the rotation angle of the micro rotator  331  according to the received voltage magnitude signal. 
         [0025]    Optical principles of the present embodiment is as follows: during the operation, the optical signal input from the input optical fiber  311  is converted to parallel optical signal by the lens  313  in the collimator. The parallel optical signal hits the surface of the first grating  32  and diffracts at the surface. The diffracted optical signal sequentially passes the first reflector  33 , the lens assembly  34 , the second grating  35 , the polarization rotation element  37  and then diffracted again. After being reflected by the second reflector  36 , the optical signal returns along the original path, and eventually enters to signal output fiber  312 . The optical signal which is inputted from the signal input terminal  311  enters the signal output terminal  312 . 
         [0026]    In the above-described embodiment, the input fiber  311  and the output fiber  311  can be the dual fiber pigtail type. The spacing between the dual-fiber is adjustable. In this embodiment, the center distance of the dual-fiber is 125 μm. 
         [0027]    In the present embodiment, the polarization rotating element  37  includes a quarter wave plate or Faraday rotator plate for rotating the polarization state of the light beam which pass through the second grating  35 . 
         [0028]    Since the present patent application uses a small-sized MEMS reflector. In order to increase the number of grating dispersion by the prism  24  or lens assembly  34 , the first grating is inserted among the input collimator, the output collimator and the first reflector to carry out pre-dispersion. The large dimension of the second grating due to the rotation of the MEMS reflector is not required. The structure of the second grating is much miniaturized. The change of the waveform of the tunable filter with the wavelength is small. The polarization rotating component is added between the second grating and the second reflector. The polarization state of a light beam passing through the first grating and the second grating at the second time is mutually vertical to the polarization state of the light beam passing through the first grating and the second grating at the first time. 
         [0029]    The present patent application is described in connection with preferred embodiments. One of ordinary skill in the art should understand that the scope of the present inventions is defined by reference to the claims. Within the spirit and scope of the appended claims and without departing from the patent application as defined, forms and details may be changed.