Abstract:
Disclosed herein is an optical multiplexing/demultiplexing device having a structure wherein a curved waveguide for input, linear waveguides for output, and a planar waveguide are provided within a substrate, and the curved waveguide is made discontinuous by the planar waveguide and the curved waveguide and planar waveguide are separated from each other with an equal interval interposed therebetween. A light signal inputted to the curved waveguide is reflected by discontinuous surfaces of the curved waveguide. Afterwards, the respective reflected light signals are distributed to their corresponding linear waveguides through the planar waveguide every wavelengths and focused thereon. Further, the light signals of the respective wavelengths, which are inputted to the linear waveguides, are multiplexed by and focused on the discontinuous portions of the curved waveguide.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical waveguide element or device for separating or demultiplexing a wavelength-multiplexed input light signal every wavelengths and outputting the same therefrom. The present invention relates particularly to an optical multiplexing/demultiplexing device having a waveguide formed within a planar waveguide.  
           [0003]    2. Description of the Related Art  
           [0004]    In the field of optical communications, a wavelength division multiplexing (WDM) system has been developed which brings a plurality of signals into signal form set as different lights and transmitting them via an optical fiber. The present system needs to multiplex or demultiplex the lights different in wavelength for their input/output. Various types of elements or devices such as an array waveguide grating device, a device using a grating, etc. have heretofore been known as such types of optical branching or demultiplexing devices. FIGS. 1 and 2 respectively show examples of optical demultiplexing devices each using a grating.  
           [0005]    The device shown in FIG. 1 has a structure in which a linear optical waveguide  2  is provided on a substrate  1  and a linear chirp grating  3  is formed thereon. A planar waveguide  4  is provided side by side with the linear optical waveguide  2 . The planar waveguide  4  takes a configuration in which optical waveguides  5  for output are connected thereto at its boundary surface. Light incident from one end of the linear optical waveguide  2  is reflected by the grating  3  and inputted to the planar waveguide  4  through the linear waveguide  2 .  
           [0006]    The cycle of the grating becomes small as it proceeds to its end. Thus, the light propagates so as to converge on the boundary portion of the planar waveguide  4  as shown in FIG. 1. Light-gathering points differ from one another every wavelengths depending on the state of interference of light dependent on the wavelengths at this boundary. The provision of the output waveguides  5  at the boundary makes it possible to take out the lights every wavelengths. A method of avoiding the use of such a special grating as seen in the structure of FIG. 1 has also been proposed as shown in FIG. 2.  
           [0007]    In a structure of FIG. 2, a curved waveguide  13  laid out in arc form, other than the linear waveguide is provided with equidistant gratings. Further, a planar waveguide  14  having a shape extending along the curved waveguide  13 , is provided, and output waveguides  15  are placed in a central position of a circular arc of the curved waveguide  13 . The gratings are set diagonally to the center of the waveguide in such a manner that lights reflected by the gratings converge on the center of the circular arc. If the structure of FIG. 2 is adopted, then the lights can be focused on one point even if the equidistant gratings are provided.  
           [0008]    However, the conventional structure has a drawback in that the optimum focal position is substantially one point, i.e., an output value decreases in the case of each wavelength deviate from the focal point. A problem arises in that the acquisition of a certain degree of output by wavelengths other than at the optimum focal position needs to reduce a change in focal position with respect to the distance extending from each grating to the focal point, thus leading to an increase in the overall length of the device.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention aims to replace a conventionally used grating by a reflecting surface producible according to a waveguide producing process and has a structure wherein a curved waveguide for input, output waveguides and a planar waveguide are provided within a substrate, the curved waveguide for input is rendered discontinuous by the planar waveguide, and the curved waveguide and planar waveguide are spaced away from each other with an equal interval interposed therebetween.  
           [0010]    Further, a light signal is reflected by each individual discontinuous surfaces and wavelength-demultiplexed through the substrate and planar waveguide. The demultiplexed lights are respectively focused on the output waveguides every wavelengths. As a result, a structure can be formed which includes an optimum waveguide shape and reflecting surfaces, and hence a device good in controllability can be formed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a plan view for describing an optical demultiplexing device having a linear grating structure, according to a prior art;  
         [0013]    [0013]FIG. 2 is a plan view for describing an optical demultiplexing device having a circular grating structure, according to a prior art;  
         [0014]    [0014]FIG. 3 is a plan view of an optical multiplexing/demultiplexing device for describing a first embodiment of the present invention;  
         [0015]    [0015]FIG. 4 is a coordinate system for describing the first embodiment of the present invention;  
         [0016]    [0016]FIG. 5 is an enlarged view of the multiplexing/demultiplexing device for describing the first embodiment of the present invention;  
         [0017]    [0017]FIG. 6 is a simulation result of output strengths of the optical demultiplexing device showing the first embodiment of the present invention;  
         [0018]    [0018]FIG. 7 is a simulation result of output strengths of the optical demultiplexing device illustrating the first embodiment of the present invention; and  
         [0019]    [0019]FIG. 8 is a plan view of an optical multiplexing/demultiplexing device for describing a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings.  
         [0021]    [0021]FIG. 3 is a plan view for describing a first embodiment. In FIG. 3, reference numeral  21  indicates a substrate, reference numeral  22  indicates a planar optical waveguide, reference numeral  23  indicates an input waveguide, and reference numeral  24  indicates a curved waveguide, which has such a structure as to trap or block up light with each portion high in equivalent refractive index in the substrate  21  and allows it to propagate. Discontinuous portions  25  are equivalent to discontinuous structures defined in the curved waveguide  24  and are those for reflecting light propagated in the curved waveguide  24 . Reference numerals  28  indicate output waveguides. The planar optical waveguide  22  is formed so as to be spaced away from the input waveguide  23  with an equal interval defined therebetween. On the other hand, a planar waveguide is provided over the whole surface of the substrate and an input waveguide may be formed therein.  
         [0022]    A waveguide structure having an opening defined in a light-focused position  27  guides light to each output waveguide  28 . Each of the discontinuous portions  25  has the function of reflecting light by both boundary surfaces  26   a, b.  The number of the reflecting surfaces can also be set to one according to their structures. The reflecting surface  27  has the function of reflecting the reflected light in the direction of the output waveguide. The reflecting surface is formed by an etching technique or the like.  
         [0023]    In the present invention, lights reflected by a number of reflecting surfaces are set so as to produce the maximum outputs at two most-suitable focal centers  27 . If the distance between the boundary surfaces  26   a, b  is cut short to some extent, it is then also possible to increase an output of light having a wavelength brought into focus at a position therebetween.  
         [0024]    [0024]FIG. 4 shows a curve structure of a block-in waveguide for each curved waveguide  24 . The curved waveguide  24  is separated into small intervals or sections each having a length of dL 1 . Discontinuous portions  25 =A, A′ corresponding to reflecting surfaces are provided at both ends of dL 1 . Lights from the reflecting surfaces are focused on an optimum focal center  27 . A triangle aAA′ has a side AA′ of a curved waveguide  24  inclined to a side bc of an isosceles triangle abc. The side AA′ is inclined only dΦ toward the side bc.  
         [0025]    A small section of another curved waveguide  24  is connected subsequent to the small section of one curved waveguide  24 . A triangle formed by the small section and the optimum focal center  27  similarly has a structure in which the side of the curved waveguide  24  is inclined to an isosceles triangle. This inclination dΦ′ may vary for each small section.  
         [0026]    The small sections are connected to one another in this way to thereby form the whole curve of the curved waveguides  24 . When the side AA′ of the curved waveguide  24  is made parallel (identical) to the side bc, i.e., dΦ=0, the curve designated at numeral  24  results in an circular arc. Incidentally, the apex angles of the triangles aAA′ and abc will be defined as dθ. Let&#39;s assume that the angle of each of these triangles with respect to the horizontal axis is θ and the length of the side ac line thereof is L 2 . Coordinates are represented as Z as viewed in a vertical direction and D as viewed in a horizontal direction.  
         [0027]    When the difference in phase between lights from the discontinuous portions  25  at the respective reflecting surfaces is constant at the optimum focal center  27 , lights emitted from an array waveguide to a planar waveguide by a conventional array waveguide diffraction grating element or device are gathered in the vicinity of the optimum focal center  27  every wavelengths under the same action as when the lights are caused to converge on many ends of the planar waveguide.  
         [0028]    Light introduced from the input waveguide  23  is successively reflected by the reflecting surfaces A and A′ and focused on the optimum focal center  27 . A phase difference corresponding to the sum of a phase difference caused by a propagation distance dL 1  and a phase difference caused by a difference dL 2  in propagation distance developed by the inclination of the side AA′ is developed between the light reflected at A and the light reflected at A′. Since the reflecting surfaces A and A′ are equally distant from the optimum focal center  27  when the curved waveguide  24  is represented in the form of a circular arc, no difference dL 2  in propagation distance occurs.  
         [0029]    When the curved waveguide  24  is not given as the circular arc, it is necessary to contrive the way of providing the curve of the curved waveguide  24  and the discontinuous portions  25  corresponding to the reflecting surfaces. When the output waveguide is provided in plural form, the phases must satisfy specific conditions with respect to a plurality of different optimum focal centers  27  in a single curved waveguide  24 . Since dL 1  is fixed, no problem occurs, whereas when the optimum focal center  27  is shifted, dL 2  varies.  
         [0030]    It is necessary to keep constant changes in phase due to the shifting of the optimum focal centers  27  in the respective small areas of the curved waveguide  24 . Since chirping occurs in the difference in phase between the discontinuous portions  25  corresponding to the respective reflecting surfaces when this condition is not met, an optical field distribution will diminished at each light-gathering point. This will bring about an increase in loss and an increase in crosstalk in terms of performance.  
         [0031]    Next, a phase error is analyzed with reference to FIG. 5 to produce or derive a curved structure of a curved waveguide  24 . The shifting of an optimum focal center  27  is associated with an angle dΦ is changed by an angle δ in FIG. 5. The following relation is derived from FIG. 5 between dL 1 , L 2 , dθ and dΦ.  
         sin( dθ/ 2)= dL   1 cos( d Φ)/(2 L   2   +dL   2 )  (1)  
         [0032]    Using an equation obtained from a cosine theorem in place of the expression also yields the same result.  
         [0033]    The following relation is established between dL 2  and dL 1 .  
           dL   2   =dL   1 sin( dΦ )/cos( dθ/ 2)  (2)  
         [0034]    Next, dL=n w dL 1 +n s dL 2  needs to be identical over all the small sections so that no phase chirping is developed at each optimum focal position. Now, n w  and n s  respectively indicate equivalent refractive indexes of the curved waveguide  24  and planar waveguide  22 .  
         [0035]    From the above, the following relations are obtained:  
           dL   1   =dL/[n   w   +n   s sin( dΦ )/cos( dθ/ 2)]  (3a)  
           dL   2   =dL/[n   s   +n   w cos( dθ/ 2)/sin( d Φ)]  (3b)  
         [0036]    An approximate calculation to the expression (1) is carried out. When dL 1  is very short and dL 1 /L 2 &lt;&lt;1, dθ is obtained as dθ&lt;&lt;1.  
         [0037]    Further, sin(dθ/2) is rewritten in the following manner using the expression (3):  
         sin( dθ/ 2)=cos( d Φ)/[2 L   2   /dL ]( n   w   +n   s sin( d Φ))+sin( dΦ)]   (4)  
         [0038]    dΦ can be set at random. Operating characteristics of the device can be set according to a change in small section number.  
         [0039]    dL 1  remains unchanged even in the case of the shifting of the optimum focal center  27 , and the difference dL 2  between the distances extending from A and A to  27  contributes to a change in phase difference. A change δdL 2  in dL 2  when the focal position is changed by S, is derived from the expression (2) as follows:  
         δ dL   2   =dL   1 [sin( dΦ+S/L   2 )−sin( d Φ)]/cos( dθ/ 2)= dL   2 [cos( S/L   2 )−1]+sin( S/L   2 )(2 L   2   +dL   2 )tan( dθ/ 2)  (5)  
         [0040]    δdL 2  is 0 at the optimum focal position of S=0 and no phase chirping is developed. Thus, the device produces an output at the maximum power. When S is not equal to 0, a phase error given by the expression (5) is normally developed and output power is reduced. However, δdL 2  can be set to a constant δdL 2 C with respect to a certain value of S without depending on a small section.  
         [0041]    When dL 1  and tan(dθ/2)≅sin(dθ/2) are represented as an expression of Φ from the expressions (3) and (4) under the above conditions, and a change in Φ at each small section number is determined, a device structure can be obtained in which δdL 2  is constant at S (focal position S C ) with respect to its change. Eventually, no phase error occurs at the two positions of S=0 and SC, and a device can be obtained which produces the maximum output.  
         [0042]    An expression for obtaining dΦ is represented as follows:  
         {[δ dL   2C   −dL cos( S   C   /L   2 )+ dL ]+sin 2 ( S   C   /L   2 ) dL   2 }sin 2 ( d Φ)+2δ dL   2C   [δdL   2C   −dL cos( S   C   /L   2 )+ dL ]sin( d Φ)+δ dL   2C   2 −sin 2 ( S   C   /L   2 ) dL   2 =0  (6)  
         [0043]    Waveguide patterns are designed as follows. D and z coordinates are given to a start position of a waveguide having reflecting surfaces. Thereafter, an initial value θ=atan(Z/D) of an angle θ and L 2  are determined from the coordinates. Further, dΦ is provided to determine a start angle of the waveguide. DΦ is set to θ (dΦ=θ) to start the waveguide perpendicularly to a chip end surface.  
         [0044]    Afterwards, θ of the next section is determined from dθ and θ+dθ determined from the expression (4). Coordinates D and Z at an end of a small section are determined from D−dL 1 sin(θ−dΦ+dθ/2) and Z+dL 1 cos(θ−dΦ+dθ/2). As a result, the new L 2  of the next small section is obtained. Thereafter, a new dθ is obtained from the expressing (4) using dΦ determined from the expression (6), whereby the end coordinates D and Z of the new small section are obtained.  
         [0045]    A design method for providing a third optimum focal point is difficult because the degree of freedom of a structure is insufficient (dΦ and L 2  are determined uniquely as is understood from the expression (6), and L 2  need to be different from each other every small sections except when dΦ=0, thus causing a contradiction). Only the provision of conditions under which second and third output characteristics are identical to each other, is allowed and hence the maximum output cannot be obtained.  
         [0046]    The analysis of design conditions will be explained below. S ca,b  indicate two optimum focal points other than 0.  
         [0047]    If L 2C  is an initial value of L 2 , then  
         δ dL   2ca,b   =dL   1 [cos ( S   ca,b   /L   2C )1]+sin( S   ca,b   /L   2C )(2 L   2C   +dL   2C )tan( dθ/ 2)  (7a)  
           dL   2   =[−δdL   2cb sin( S   ca   /L   2 )+δ dL   2ca sin( S   cb   /L   2 )]/{sin( S   cb   /L   2   −S   ca   /L   2 )−[sin( S   cb   /L   2 )−sin( S   ca   /L   2 )]}  (7b)  
         [0048]    As the waveguide becomes an arc structure, an output value increases. However, the arc structure is poor in wavelength resolution as shown below.  
         [0049]    A method of series-connecting the structures of the intput waveguides  23  respectively having a plurality of different optimum focal points as viewed in the direction in which light propagates through the input waveguide  23 , and dispersing the optimum focal points can be used for the placement of the output waveguides  28  in their corresponding optimum focal points. The output value is reduced in principle. However, when a shift in S is substantially equal to or smaller than S C , a prominent reduction in output value does not appear in terms of practical use.  
         [0050]    Alternatively, structures having optimum focal points different from one another may overlap. In other words, they are structures wherein a pair of input waveguides  23  with respect to respective wavelength are connected in parallel in plural form. The connection thereof in plural form allows an improvement in output strength.  
         [0051]    A dispersion indicative of the speed at which the focal point moves according to the wavelength, is given approximately from λΔS/Δλ=[n w /n s +sin(dΦ)]L 2a /cos(dΦ). In the expression, L 2a  indicates the mean value of L 2 . As DΦ becomes close to the right angle (close to a straight line), the dispersion of the focal points increases. It is understood from the result of simulation that the dispersion thereof becomes great as S decreases.  
         [0052]    [0052]FIGS. 6 and 7 respectively show examples of operating characteristics at a 1.55 μm band, according to the first embodiment. In the examples, five times the wavelength, 2000 and {fraction (π/2.1)} were respectively used as dL, the number of reflecting surfaces of curved waveguides  24 , and an initial value of θ. An example of a serial arrangement wherein two waveguides in which an initial value of Z is 50000 μm, Sc is −6 mm and Sc is shifted −6 mm against first waveguide, are combined into one, was used in FIG. 6. An example of a serial arrangement wherein two waveguides in which an initial value of Z is 40000 μm, Sc is −3 mm and Sc is shifted −2 mm against first waveguide, was used in FIG. 7.  
         [0053]    Shifting a structure designed under similar parameters and having only one optimum focal position from the optimum wavelength by 5 nm results in a half reduction in output value. It is however understood that a high output is produced within a range of 30 nm under the structure employed in the first embodiment. In the case of a 1.55 μm-band wavelength employed in optical communications, a device with small characteristic change between output waveguides over the range of 30 nm can be implemented with an overall length of about 6 cm.  
         [0054]    [0054]FIG. 8 shows a second embodiment of the present invention. In the present embodiment, a planar waveguide  32  and a connecting surface  36  of an output waveguide are designed so as to be inclined toward each output waveguide  37 . Since L 2a  is small at a small portion of S large in dispersion in the present structure, dispersions set every waveguides can be made uniform. Further, since output values can be outputted according to narrower changes in S position with the same output-waveguide pitches, they can easily be uniformized.  
         [0055]    In FIG. 8, a light-detecting element is used as the output waveguide  37 . Mirrors  39  each used as an optical-path changing means are provided to allow lights  38   a, b  reflected from discontinuous portions  35  corresponding to reflecting surfaces to well converge on their corresponding output waveguide  37 . Side faces of openings defined by etching are used as the mirrors  39  respectively.  
         [0056]    As described above, an optical multiplexing/demultiplexing device can be implemented which is simple in manufacturing process, and has a structure having optimum waveguide configurations and reflecting surfaces and is good in controllability.  
         [0057]    While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.