Abstract:
An optical delay line of a configuration in which a number of components such as a circulator and an optical coupler is reduced is to be provided. To this end, the optical delay line has a configuration in which, after an optical fiber diffraction grating having a core subjected to refractive index modulation and a cladding partly cleared of its outer circumference and an optical fiber having a cladding partly cleared of its outer circumference are brought close to each other so that the cores have mutually parallel directions of an optical axis to fabricate a directional coupler, both ends of the optical fiber are connected into an optical fiber loop. In this way, an optical delay line to generate a specific time delay for controlling optical signals can be provided in small dimensions and with a small number of components.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical delay line, which is an optical control element for use in optical fiber communication, and a manufacturing method therefor.  
           [0003]    2. Description of the Prior Art  
           [0004]    Nowadays, a higher speed is sought after in optical fiber communication, not only on trunk lines but also on optical subscriber lines, and studies are made on capacity expansion by use of ultra-short pulses and photonic networks performing no photoelectric conversion at repeaters. Especially at each repeater (node) of such an optical communication system, an optical delay line to generate a specific time delay for controlling optical signals is required, and an optical control device for that purpose constitutes one of the key devices. Whereas a specific way of generating an optical delay is usually to have the light pass an optical fiber loop or the like having a specific distance, the method of matching with a specific wavelength is to distinguish the wavelength with a filter and to connect it to the optical fiber loop with a coupler or the like.  
           [0005]    On the other hand, an optical fiber diffraction grating (fiber grating) in the core of the optical fiber has a narrow band filter characteristic, and excels in stability and in the efficiency of the use of light. However, since the optical fiber diffraction grating uses a reflected spectrum, it is difficult to be made a transmission type element, and therefore has to be used in combination with an optical circulator. This makes it difficult to realize the element at a low cost.  
           [0006]    Waveguide couplers having a configuration of using no optical circulator includes one disclosed in the Published Japanese Translation of Unexamined PCT Application No. Hei 9-505673. Its elemental configuration is shown in FIG. 1, in which reference numerals  10  and  20  denote optical fibers;  30 , an input end;  40  and  50 , glass blocks to which the optical fibers  10  and  20  are respectively fixed;  25 , a diffraction grating arranged in the core of an optical fiber; and  45 , a coupling region having a length Lc of the two fibers  10  and  20  exposed from the surfaces of the glass blocks  40  and  50 .  
           [0007]    The optical fiber  10  and the optical fiber  20  constitute a directional coupler in which the cores of the two fibers are arranged close to each other. The coupling length Lc is so set that lights of a plurality of wavelengths coming incident from the input end  30  all transfer from the optical fiber  10  to the optical fiber  20 . However, the light of a Bragg wavelength (λ1=λB) from the diffraction grating  25  cannot transfer to the optical fiber  20 , and is outputted as T 1 . Therefore, lights of other wavelengths (λ2, λ3. . . ) than the Bragg wavelength are outputted as T 2 .  
           [0008]    The waveguide coupler of the configuration described above, though able to manifest a wavelength filtering function, cannot function as an optical delay line for a specific wavelength. It involves a further problem that, though it can constitute a delay line for a specific wavelength when combined with an optical fiber loop, an optical circulator and an optical coupler would be required separately. Still another problem is that, in order to form a directional coupler after the formation of an optical fiber diffraction grating, precise and uniform machining of a large area of cladding giving rise to no thermal process is needed, but there is scarcely a practical means to meet this need.  
           [0009]    An object of the present invention, intended to solve the problems of the prior art noted above, is to provide an optical delay line for generating a specific time delay for controlling optical signals and a manufacturing method therefor.  
         SUMMARY OF THE INVENTION  
         [0010]    An essence of an optical delay line according to the present invention is that it is provided with a directional coupler wherein a first waveguide consisting of an optical fiber diffraction grating having a core subjected to refractive index modulation and a cladding partly cleared of its outer circumference and a second waveguide consisting of an optical fiber having a cladding partly cleared of its outer circumference are brought close to each other to cause the cores to have mutually parallel directions of an optical axis, and with means for optically connecting both ends of the second waveguide.  
           [0011]    The invention can provide, in small dimensions and with a small number of components, an optical delay line to generate a specific time delay for controlling optical signals.  
           [0012]    Another essence of an optical delay line according to the invention is that, in the aforementioned optical delay line, the first waveguide has a plurality of diffraction gratings differing in refractive index modulation pitch. This configuration makes possible simultaneous delaying of lights having a plurality of different wavelengths.  
           [0013]    Another essence of an optical delay line according to the invention is that, in the aforementioned optical delay line, the first waveguide have two diffraction gratings of the same refractive index modulation pitch. As this configuration makes it possible to accurately control the position in which the diffraction gratings are formed in the optical fibers, an optical delay line capable of generating any desired extent of delay can be provided.  
           [0014]    Another essence of an optical delay line according to the invention is that, in the aforementioned optical delay line, the first waveguide has a plurality of pairs of diffraction gratings of the same refractive index modulation pitch. This configuration makes it possible to provide an optical delay line capable of generating any desired extent of delay for each of a plurality of lights of different wavelengths.  
           [0015]    An essence of a method for manufacturing an optical delay line according to the invention is that it comprises a step of embedding an optical fiber diffraction grating and an optical fiber respectively into grooves formed in surfaces of two glass substrates; a step of removing part of claddings of the optical fiber diffraction grating and of the optical fiber by lapping the surfaces of the glass substrates; a step of forming a directional coupler by sticking together the surfaces of the glass substrates in a state in which the optical fiber diffraction grating and the optical fiber are brought into contact with each other; and a step of connecting both ends of the optical fiber. This method for manufacturing an optical delay line can provide the advantages of giving rise to no thermal process and making it possible to machine a large area of cladding accurately and uniformly.  
           [0016]    Therefore, an object of the present invention is to provide an optical delay line for generating a specific time delay for controlling optical signals and a manufacturing method therefor.  
           [0017]    The object and the advantages of the invention will become more apparent from the embodiments thereof to be described below with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 schematically shows a section of a waveguide coupler in an embodiment of the prior art.  
         [0019]    FIGS.  2 :  
         [0020]    (a) A schematic perspective view of the configuration of an optical delay line in Embodiment 1 of the invention;  
         [0021]    (b) A schematic plan of the optical delay line in Embodiment 1 of the invention; and  
         [0022]    (c) A schematic sectional profile of the optical delay line in Embodiment 1 of the invention.  
         [0023]    [0023]FIG. 3 schematically shows a section of an optical delay line having two diffraction gratings differing in refractive index modulation pitch in Embodiment 2 of the invention.  
         [0024]    FIGS.  4 :  
         [0025]    (a) A spectral diagram of the input light in Embodiment 2 of the invention; and  
         [0026]    (b) A spectral diagram of the diffracted light in Embodiment 2 of the invention.  
         [0027]    [0027]FIG. 5 schematically shows a section of an optical delay line having four diffraction gratings differing in refractive index modulation pitch in Embodiment 3 of the invention.  
         [0028]    FIGS.  6 :  
         [0029]    (a) A spectral diagram of the input light in Embodiment 3 of the invention; and  
         [0030]    (b) A spectral diagram of the diffracted light in Embodiment 3 of the invention.  
         [0031]    [0031]FIG. 7 schematically shows a section of an optical delay line having three pairs of two diffraction gratings of the same refractive index modulation pitch in Embodiment 4 of the invention.  
         [0032]    FIGS.  8 :  
         [0033]    (a) A schematic process diagram of fabricating a directional coupler in Embodiment 5 of the invention.  
         [0034]    (b) A section in the lengthwise direction of another example of directional coupler fabricating process. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    (Embodiment 1)  
         [0036]    Preferred embodiments of the present invention will be described below with reference to accompanying drawings. FIGS.  2  schematically shows the configuration of an optical delay line in Embodiment 1 of the invention, in which FIG. 2( a ) is an overall perspective view of the optical delay line; FIG. 2( b ), a plan of its directional coupler portion; and FIG. 2( c ), a profile of its directional coupler portion. Reference numerals  1  and  2  denote glass substrates;  11  denotes an input light;  12  denotes an diffracted light ;  13  denotes an optical fiber diffraction grating;  14  denotes an optical fiber;  15  denotes a diffraction grating formed in the optical fiber diffraction grating  13 ;  16  denotes an optical fiber loop for giving a delay; and  17  denotes an output light.  
         [0037]    The operation of the optical delay line configured as described above will now be described. The input light  11  having a plurality of wavelength components (λ1, λ2, . . . λn) is brought to incidence on the optical fiber diffraction grating  13  from an input port Pin. The optical fiber diffraction grating  13  and the optical fiber  14  are cleared of the cladding on their respective fiber flanks, and their cores are brought close to each other to form a directional coupler. The propagation constants of the optical fiber diffraction grating  13  and the optical fiber  14  are represented by β1 and β2, and β1 and β2 are supposed to be unequal. When the diffraction grating  15  is subjected to refractive index modulation at a pitch Λ and a magnitude Δn and the distance between the cores is set to a few μm to tens of μm, out of the input light  11 , a wavelength component satisfying the phase matching condition of:  
         β1(λ)−α2(λ)=2π/Λ  (1) 
         [0038]    is supplied as the diffracted light  12 . Whereas the intensity of the diffracted light is determined by such factors as the propagation constant, inter-core distance, coupling length and refractive index modulation magnitude Δn, it can be brought close to 100%.  
         [0039]    Therefore, by forming the diffraction grating  15  of a pitch matching λ1 as the Bragg wavelength, a light having a wavelength λ1 can be emitted from a port P 2  as the diffracted light  12 . The diffracted light  12  passes the optical fiber loop  16  and is brought to incidence again on the optical fiber  14  from a port P 3 ; this time, in a reverse sequence to the previous, it is coupled to the optical fiber diffraction grating  13  to be emitted from a port P 1  as an output light  17 . Thus only the component of the wavelength λ1 can be caused to delay behind other wavelength components by the optical path length of the optical fiber loop  16 .  
         [0040]    In this way, this Embodiment 1 of the invention can realize in small dimensions and with a small number of components an optical delay line to generate a specific time delay for controlling optical signals.  
         [0041]    (Embodiment 2)  
         [0042]    Next will be described Embodiment 2 of the present invention with reference to FIG. 3 and FIGS. 4. FIG. 3 schematically shows an overall configuration of an optical delay line having two diffraction gratings differing in refractive index modulation pitch, wherein reference numerals  1  and  2  denote glass substrates;  21  denotes an input light;  22  denotes an diffracted light;  23  denotes an optical fiber diffraction grating;  24  denotes an optical fiber;  25  and  26  denote a first diffraction grating and a second diffraction grating, respectively, formed in the optical fiber diffraction grating  23 ;  27  denotes an optical fiber loop for generating a delay; and  28  denotes an output light. FIG. 4( a ) shows the spectrum of the input light  21  in a case where an actually fabricated element is used, and FIG. 4( b ), the spectrum of the diffracted light  22 . A super-luminescent diode is used as the source of the input light  21 , having a wideband spectral characteristic of 100 nm or more. The two kinds of diffraction gratings which were fabricated had pitches Λ of 536.4 nm and 538.5 nm, respectively matching wavelengths of 1552 nm and 1558 nm.  
         [0043]    The input light  21  is brought to incidence on the optical fiber diffraction grating  23  from the input port Pin. The optical fiber diffraction grating  23  and the optical fiber  24  are cleared of the cladding on their respective fiber flanks, and their cores are brought close to each other to form a directional coupler. Therefore, by forming the diffraction gratings  25  and  26  of pitches having a Bragg wavelength matching the wavelength of the input light  21 , lights having a plurality of matching wavelengths can be emitted from the port P 2  as the diffracted light  22 . The diffracted light  22  passes the optical fiber loop  27  and is brought to incidence again on the optical fiber  24  from the port P 3 ; this time, in a reverse sequence to the previous, it is coupled to the optical fiber diffraction grating  23  in the parts of the diffraction gratings  26  and  25  of the respectively matching pitches to be emitted from the port P 1  as an output light  28 . Thus the lights having a plurality of wavelengths passing the optical fiber loop  27  generate a delay by the optical path length of the optical fiber loop  27 .  
         [0044]    In this way, this Embodiment 2 of the invention can realize in small dimensions and with a small number of components an optical delay line matching lights having a plurality of wavelengths to generate a specific time delay for controlling optical signals.  
         [0045]    (Embodiment 3)  
         [0046]    Next will be described Embodiment 3 of the present invention with reference to FIG. 5 and FIGS. 6. FIG. 5 schematically shows an overall configuration of an optical delay line having four diffraction gratings differing in refractive index modulation pitch, wherein reference numerals  1  and  2  denote glass substrates;  41  denotes an input light;  42  denotes an diffracted light;  43  denotes an optical fiber diffraction grating;  44  denotes an optical fiber;  45 ,  46 ,  47  and  48  denote a first diffraction grating, a second diffraction grating, a third diffraction grating and a fourth diffraction grating, respectively, formed in the optical fiber diffraction grating  43 ;  49  denotes an optical fiber loop for generating a delay; and  50  denotes an output light. FIG. 6( a ) shows the spectrum of the input light  41  in a case where an actually fabricated element is used, and FIG. 6( b ), the spectrum of the diffracted light  42 . A super-luminescent diode is used as the source of the input light  41 . The four kinds of diffraction gratings which were fabricated had pitches Λ of 533.4 nm, 536.4 nm, 537.7 nm and 538.5 nm, respectively matching wavelengths of 1544 nm, 1552 nm, 1558 nm and 1560 nm.  
         [0047]    The input light  41  is brought to incidence on the optical fiber diffraction grating  43  from the input port Pin. The optical fiber diffraction grating  43  and the optical fiber  44  are cleared of the cladding on their respective fiber flanks, and their cores are brought close to each other to form a directional coupler. Therefore, by forming the diffraction gratings  45 ,  46 ,  47  and  48  of pitches having a Bragg wavelength matching the wavelength of the input light  41 , lights having a plurality of matching wavelengths can be emitted from the port P 2  as the diffracted light  42 . The diffracted light  42  passes the optical fiber loop  49  and is brought to incidence again on the optical fiber  44  from the port P 3 ; this time, in a reverse sequence to the previous, it is coupled to the optical fiber diffraction grating  43  in the parts of the diffraction gratings  48 ,  47 ,  46  and  45  of the respectively matching pitches to be emitted from the port P 1  as an output light  50 . Thus the lights having a plurality of wavelengths passing the optical fiber loop  49  generate a delay by the optical path length of the optical fiber loop  49 .  
         [0048]    In this way, this Embodiment 3 of the invention can realize in small dimensions and with a small number of components an optical delay line matching lights having a plurality of wavelengths to generate a specific time delay for controlling optical signals.  
         [0049]    (Embodiment 4)  
         [0050]    Next will be described Embodiment 4 of the present invention with reference to FIG. 7. FIG. 7 schematically shows an overall configuration of an optical delay line having three pairs of two diffraction gratings of the same refractive index modulation pitch, in which the extent of delay for each wavelength can be adjusted as desired according the positions of those diffraction gratings. In FIG. 7, reference numerals  1  and  2  denote glass substrates;  601  denotes an input light;  602  denotes an diffracted light;  603  denotes an optical fiber diffraction grating;  604  denotes an optical fiber;  605 ,  606 ,  607 ,  608 ,  609  and  610  denote respectively a first diffraction grating, a second diffraction grating, a third diffraction grating, a fourth diffraction grating, a fifth diffraction grating and a sixth diffraction grating formed in the optical fiber diffraction grating  603 ;  611  denotes, an optical fiber loop for generating a delay; and  612  denotes an output light. The first diffraction grating  605  and the sixth diffraction grating  610  have the same pitch matching a Bragg wavelength λ1; similarly, the second diffraction grating  606  and the fifth diffraction grating  609  have the same pitch matching a Bragg wavelength λ2, and the third diffraction grating  607  and the fourth diffraction grating  608  have the same pitch matching a Bragg wavelength λ3.  
         [0051]    The input light  601  having a plurality of wavelength components (λ1, λ2, . . . λn) is brought to incidence on the optical fiber diffraction grating  603  from the input port Pin. The optical fiber diffraction grating  603  and the optical fiber  604  are cleared of the cladding on their respective fiber flanks, and their cores are brought close to each other to form a directional coupler. Therefore, in the first diffraction grating  605  part of the pitch matching λ1 as the Bragg wavelength a light of the wavelength λ1 is emitted from the port P 2  of the optical fiber  604  as the diffracted light  602 . The diffracted light  602  passes the optical fiber loop  611 , and is again brought to incidence on the optical fiber  604  from the port P 3 . As the sixth diffraction grating  610  is formed in the vicinity of the port P 3 , the light of the wavelength λ1 is coupled to the optical fiber diffraction grating  603  in this part and emitted from the port P 1  as the output light  612 . Similarly, in the second diffraction grating  606  part of the pitch matching λ2 as the Bragg wavelength, a light of the wavelength λ2 is emitted from the port P 2  as the diffracted light  602 , passes the optical fiber loop  611 , is brought to incidence again on the optical fiber  604  from the port P 3 , and in the fifth diffraction grating  609  part a light of the wavelength λ2 is coupled to the optical fiber diffraction grating  603  to be emitted from the Port P 1  as the output light.  612 . Similarly, in the third diffraction grating  607  part of the pitch matching λ3 as the Bragg wavelength a light of the wavelength λ3 is emitted from the port P 2  as the diffracted light  602 , passes the optical fiber loop  611 , is brought to incidence again on the optical fiber  604  from the port P 3 , and in the fourth diffraction grating  608  part, a light of the wavelength λ3 is coupled to the optical fiber diffraction grating  603  to be emitted from the port P 1  as the output light  612 .  
         [0052]    In this way, to consider the optical path lengths of λ1, λ2 and λ3, the light of each wavelength travels between the optical fiber diffraction grating  603  and the optical fiber  604  at two different points, and though all of the three lights pass the same optical fiber loop  611 , there arise twice as great a difference in optical path length as the interval between the diffraction grating positions. Thus, the positional interval between consecutive diffraction gratings being represented by L as illustrated, the optical path length difference between λ1 and λ2 is 2L, and similarly the difference between λ2 and λ3 also is 2L. Accordingly, even if only one optical fiber loop  611  is used, a slight delay difference can be generated for each wavelength by positional adjustment of diffraction gratings.  
         [0053]    In this way, this Embodiment 4 of the invention can realize in small dimensions and with a small number of components an optical delay line to generate any desired extent of delay for lights having a plurality of different wavelengths.  
         [0054]    To add, it is evident that diverse optical delay lines in Embodiments 1, 2, 3 and 4 described above can be realized by appropriately setting the number of diffraction gratings and delay differences (diffraction grating intervals L), the length of the optical fiber loop and those of the optical fibers constituting the directional coupler, and the method of connecting the output ports and the optical fiber loop among other factors.  
         [0055]    (Embodiment 5)  
         [0056]    Next will be described Embodiment 5 of the present invention with reference to FIGS. 8. FIG. 8( a ) is a schematic diagram of a process of fabricating a directional coupler by bringing the cores of optical fibers close to each other, wherein reference numerals  71  and  72  denote glass substrates having the same surface areas and optical characteristics;  71   a  and  72   a , grooves of the same size formed in the same positions in the glass substrate  71  and  72 ;  73 , an optical fiber diffraction grating; and  74 , an optical fiber.  
         [0057]    First the grooves  71   a  and  72   a  of substantially the same depth as the diameter of each fiber for embedding the optical fiber diffraction grating  73  and the optical fiber  74  are formed in the glass substrates  71  and  72 , respectively; the optical fiber diffraction grating  73  and the optical fiber  74  are embedded into the grooves  71   a  and  72   a , respectively, and fixed with an adhesive or the like. Then, the surfaces of the glass substrates  71  and  72  are lapped to remove part of the cladding on the flanks of the optical fiber diffraction grating  73  and the optical fiber  74 . The apparatus used for the lapping may be an apparatus usually applied to semiconductor substrate lapping, with which a large area can be lapped accurately and uniformly in a relatively short period of time. Finally, the directional coupler is formed by sticking together the lapped surfaces of the glass substrates  71  and  72  with the cladding-cleared portions of the optical fiber diffraction grating  73  and  74  kept in contact with each other. Further by fusing together both ends of the optical fiber  74  with separately prepared other optical fibers, the optical fiber loop is formed.  
         [0058]    Whereas as much of the cladding of the optical fiber grating  73  and the optical fiber  74  as the width of the glass substrates  71  and  72  is removed by the method above, if it is desired to partly remove the cladding of the optical fiber grating  73  and the optical fiber  74 , the cladding only in the central part can be removed by, for instance as shown in FIG. 8( b ), forming the grooves  71   a  and  72   a  in the glass substrates  71  and  72 , respectively, in increasing gradually depths from the central part towards each end to cause the parts of the optical fiber grating  73  and the optical fiber  74  to be cleared of the cladding, because in this way only the central parts of the optical fibers  73  and  74  are lapped when the surfaces of the glass substrates  71  and  72  are lapped.  
         [0059]    Thus this Embodiment 5 can provide a method for manufacturing an optical delay line having the advantages of giving rise to no thermal process and making it possible to machine a large area accurately and uniformly.  
         [0060]    To add, regarding the glass substrate size and the characteristics of the directional coupler including the region to be cleared of cladding in this Embodiment 5, the fabricating conditions can be appropriately set and put into practice to realize a diversity of optical delay lines.  
         [0061]    Although the present invention has been described with reference to preferred embodiments thereof illustrated in the accompanying drawings, it is evident that persons skilled in the art can readily vary or modify these embodiments without deviating from the true spirit and scope of the invention. The invention includes such variations and modifications.