Abstract:
The invention relates to an all-fiber add/drop filter. A wide-band light is input one port of a photosensitive fiber and affected by a Bragg grating, and then deviated a Bragg wavelength and a transmission light satisfying the Bragg condition. The Bragg wavelength and transmission light couple from one fiber to the other, wherein the Bragg wavelength is dropped at one port of another optical fiber and the transmission light is added to another port of another optical fiber.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an all-fiber add/drop filter. In particular, the invention relates to an add/drop filter employing an optical fiber with Bragg gratings.  
           [0003]    2. Description of the Related Art  
           [0004]    Because of the popularity of the Internet and demands on broad band communication networks, the fiber-optic communication system with higher speed and wider bandwidth becomes more and more critical. To meet these demands, the optical wavelength division multiplexing (WDM) system and dense wavelength division multiplexing (DWDM) system were proposed and implemented.  
           [0005]    As shown in FIG. 1A, a four-channel transmission architecture with three low-loss fiber Bragg gratings/optical circulators including one programmable add/drop multiplexer has been constructed and tested. However, the price of each circulator costs about US$1000 and it is too expansive.  
           [0006]    In FIG. 1B, a hybrid DWDM device combines fiber Bragg grating and dielectric-coated band-pass filters and can meet required specification in various cost-effective structures. However, it is hard to align the fiber gratings on the two arms of the fiber coupler.  
           [0007]    [0007]FIG. 2 shows an optical circuit of a 16×32 arrayed-waveguide grating [see “16×32 AWG with Cyclic-Frequency Response”, by K. Maru et. al., Third Optoelectronics and Communications Conference (OECC&#39;98) Technical Digest, pp. 54-55, July 1998, Makuhari Messe]. However, it also has an alignment problem.  
           [0008]    As shown in FIG. 3A, a device consists of a mismatched coupler with a Bragg grating written in one core over the coupling region. However, the related art can&#39;t form a long effective coupling length and has the problem of excess loss. FIG. 3B schematically shows an add-drop-multiplexer and has an effective coupling length of 2.5 mm [see “Compact All-Fiber Add-Drop-Multiplexer Using Fiber Bragg Gratings”, by Ingolf Baumann et. al., IEEE PHOTONICS TECHNOLOGY LETTERS, pp. 1331-1333, VOL. 8, NO. 10, October 1996]. However, the related art utilizes the glass as a substrate and also has the problem of excess loss. FIG. 3C schematically shows an Add-drop multiplexer, wherein the waveguides were fabricated on a Si substrate [see “An optical add-drop multiplexer with a grating-loaded directional coupler in silica waveguides” by Naoki OFUSA et. al., Third Optoelectronics and Communications Conference (OECC&#39;98) Technical Digest, pp. 52-53, July 1998, Makuhari Messe]. However, it is hard to align the fiber with the silica waveguides.  
         SUMMARY OF THE INVENTION  
         [0009]    An object of the invention is to solve the above-mentioned problems of the related art by providing an all-fiber add/drop filter. In addition, the invention has advantages of low losses and narrow drop bandwidth.  
           [0010]    A feature of the invention is to employ an optical fiber with a Bragg grating. Owing to multiple reflection, the invention can obtain the advantage of low losses.  
           [0011]    Another feature of the invention is to form a V-groove on a wafer by utilizing an E-beam mask and standard microelectronic techniques. The invention can adjust the depth and radius curvature of the V-groove by the etching step to locate a side-polished fiber therein.  
           [0012]    Another feature of the invention is to simultaneously form a plurality of V-grooves on a wafer by utilizing an E-beam mask and standard microelectronic techniques. The invention can accomplish a plurality of all-fiber add/drop filters by these V-grooves. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    This and other objects and features of the invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the drawings, in which:  
         [0014]    [0014]FIG. 1A schematically shows a four-channel transmission architecture with three low-loss fiber Bragg gratings/optical circulators including one programmable add/drop multiplexer;  
         [0015]    [0015]FIG. 1B schematically shows a hybrid DWDM device combines fiber Bragg grating and dielectric-coated band-pass filters;  
         [0016]    [0016]FIG. 2 schematically shows an optical circuit of a 16×32 arrayed-waveguide grating;  
         [0017]    [0017]FIG. 3A schematically shows a device consists of a mismatched coupler with a Bragg grating written in one core over the coupling region;  
         [0018]    [0018]FIG. 3B schematically shows an add-drop-multiplexer;  
         [0019]    [0019]FIG. 3C schematically shows an Add-drop multiplexer, wherein the waveguides were fabricated on a Si substrate;  
         [0020]    [0020]FIG. 4 schematically shows a pattern of a mask for forming a V-groove;  
         [0021]    [0021]FIG. 5A is a longitudinally sectional view of the Si substrate of the invention;  
         [0022]    [0022]FIG. 5B is a cross-sectional view of the Si substrate of the invention;  
         [0023]    [0023]FIG. 6 is a longitudinally perspective view showing the optical fiber locating in the V-groove;  
         [0024]    [0024]FIG. 7 is a cross-sectional view showing the optical fiber located in the V-groove;  
         [0025]    [0025]FIG. 8A schematically shows an all-fiber add/drop filter having two photosensitive fibers;  
         [0026]    [0026]FIG. 8B schematically shows an all-fiber add/drop filter having one photosensitive fiber;  
         [0027]    [0027]FIG. 9A is a diagram showing the operation of an all-fiber add/drop filter having one photosensitive fiber;  
         [0028]    [0028]FIG. 9B is an operation diagram showing the operation of an all-fiber add/drop filter having two photosensitive fibers;  
         [0029]    [0029]FIG. 10A is a diagram showing the spectrum of an all-fiber add/drop filter having one photosensitive fiber;  
         [0030]    [0030]FIG. 10B is a diagram showing the coupling efficiency of the output ports of an all-fiber add/drop filter having one photosensitive fiber;  
         [0031]    [0031]FIG. 10C is a diagram showing the spectrum of an all-fiber add/drop filter having two photosensitive fibers;  
         [0032]    [0032]FIG. 11 schematically shows an all-fiber add/drop filter employing piezoelectric substrate;  
         [0033]    [0033]FIG. 12A schematically shows an uncovered bound fibers;  
         [0034]    [0034]FIG. 12B schematically shows an all-fiber add/drop filter having exceptional temperature stability. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]    The manufacturing method of the embodiment of the invention uses crystal orientation material, such as (100)-oriented silicon wafer of the semiconductor substrate, as the side-polishing substrate. Besides, the Si can be replaced by the quartz, glass or piezoelectric material.  
         [0036]    Referring to FIG. 4, the mask pattern  40  is narrow in the intermediate zone and wide at both sides. The mask pattern  40  is transferred to a Si wafer by photolithography, so that the Si wafer also forms a narrow pattern in the intermediate zone and wide in both sides. In the embodiment, it is preferred to transfer the mask pattern on the plane (100) of Si substrate.  
         [0037]    Next, referring to FIG. 5A, a V-groove  51  with long radius curvature R, such as R=1000 cm, is precisely formed on Si substrate  50  by anisotropic etching. Further, the all-fiber add/drop filter can have a long interaction region. Referring to FIG. 5B, a V-groove  51  has an included angle θ=70.53°. Moreover, a plurality of V-grooves, which have the same specification or not, are formed simultaneously by photolithography.  
         [0038]    Next, glue  60  is positioned at both sides of the V-grove  51 . The V-groove  51  absorbs the glue  60  from both sides by capillarity, so that the glue  60  can uniformly fill the V-groove  51 . The glue  60  is an adhesive liquid and has approximate refraction index of cladding of a fiber. Next, referring to FIG. 6, an optical fiber  100  with a Bragg grating  130  is fixed in the V-groove  51 . It is preferred to locate the Bragg grating  130  in the shallow region of the V-groove  51 . The optical fiber  100  with the Bragg grating  130  is a photosensitive fiber.  
         [0039]    Next, polishing the cladding  110  of the fiber  100 , which is higher than the Si substrate  50 , forms a side-polished surface  115  contiguous to the core  120 . After polishing the cladding  110 , the side-polished surface  115  of the fiber  100  and the surface of Si substrate  50  have the same level. Referring to FIG. 7, the core  120  of the fiber  100  is quite near the side-polished surface  115 . The smallest distance between the side-polished surface and the fiber core is about one half of the specific operation wavelength or less.  
         [0040]    It can follow the above-mentioned steps to accomplish a fiber fixing in another V-groove. Next, referring to FIGS. 8A and 8B, the side-polished surfaces of two fibers are aligned and bound together. Further, an index matching liquid  70  is inserted between the interface of the side-polished surfaces. Referring to FIG. 8A, the side-polished surfaces of two photosensitive fibers  300  are aligned and bound together, wherein the Bragg gratings  130  in each fiber  300  exist under each side-polished surface. Referring to FIG. 8B, the side-polished surface of the photosensitive fiber  300  is aligned and bound with the side-polished surface of the ordinary (telecommunication-grade) fiber  200 .  
         [0041]    As a broadband light is injected in a photosensitive fiber, the wavelength of the broadband light satisfies the Bragg relationship 
         2Λ= mλ   
         [0042]    where Λ is the grating period and m is a positive integer, such as m=1, 2, 3, . . . .As the wavelength of the broadband light including the Bragg wavelength λ propagates in the photosensitive fiber, the Bragg grating reflects the Bragg wavelength.  
         [0043]    As shown in FIGS. 9A and 9B, if the port  1  is an input port and a broadband light is injected into the port  1  of the optical fiber  1 , the light is coupled into the optical fiber  2 . The Bragg wavelength λ g  included in the broadband light satisfies the Bragg relationship and is in phase to make constructive interference. Next, the Bragg wavelength λ g  is dropped at port  2  of the optical fiber  2 . The broadband light without Bragg wavelength λ g  propagating through the Bragg gratings is called the transmission light. Moreover, the transmission light is also coupled into the optical fiber  2 .  
         [0044]    As shown in FIG. 10A, the Bragg wavelength λ g  measured at port  2  is 1548.6 nm. A valley shown in the transmission light at port  4  is called the stop band, wherein the wavelength of the stop band measured at port  4  is 1548.6 nm. The full width at the half maximum (FWHM) of the Bragg wavelength is about 1.14 nm, and the FWHM of the stop band is about 0.6 nm. The output spectrums of FIG. 10A are normalized to the input light power and plot in FIG. 10B. Referring to FIG. 10B, the coupling efficiency of the add channel (port  4 ) is about 70%, and the coupling efficiency of the drop channel (port  2 ) is about 30%. However, the input light power remaining un-coupled and transmitted to the port  3  is about 10%. As shown in FIG. 10C, the all-fiber add/drop filter having two photosensitive fibers improves the coupling efficiency. At port  4 , the coupling efficiency is about 93%, and the FWHM is about 0.52 nm.  
         [0045]    In the embodiment of the invention, the index matching liquid can be adjusted by temperature. Further, the temperature adjusts the coupling efficiency. The index matching liquid can be replaced by a material having approximate refraction index of cladding of a fiber, such as glue, air. As shown in FIG. 11, the piezoelectric material  80  is formed on the Si substrate, the injected broadband light will be modulated by applying a modulation signal on the piezoelectric material  80 . Moreover, the photosensitive fiber further includes several Bragg gratings with different grating period, and then drops out several Bragg frequencies from another optical fiber.  
         [0046]    Furthermore, as shown in FIG. 12A, an uncovered bound fibers  400  is obtained by utilizing the solvent, which removes the Si substrate  50  from the photosensitive fiber  300  and the Si substrate  50  from the ordinary fiber  200 . Finally, as shown in FIG. 12B, an all-fiber add/drop filter having exceptional temperature stability is formed by packaging the uncovered bound-fibers  400  by utilizing a thermally compensated material  500 .  
         [0047]    Alignment of each polished fiber can be made on silicon wafer using the standard photolithography method.  
         [0048]    While the preferred embodiment of the present invention has been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.