JP6572175B2 - Waveguide type optical coupler - Google Patents

Description

  The present invention relates to a waveguide type optical coupler. More specifically, the present invention relates to a waveguide type optical coupler that has a small variation in optical coupling factor characteristics with respect to a processing deviation and operates over a wide wavelength range.

  Planar lightwave circuits (PLCs) using optical waveguides are widely used as platforms for realizing integrated circuits such as optical multiplexers / demultiplexers, optical switches, and optical splitters used in transmission devices and the like. In these integrated circuits, an optical coupler, a phase shifter, a delay line, and the like are integrated as a basic circuit. Among them, the optical coupler is one of important optical signal processing basic circuits for branching / coupling light. A directional coupler type optical coupler composed of two adjacent waveguides has an excellent feature of low loss, but generally its coupling rate is wavelength dependent.

  As a configuration of an optical coupler with reduced wavelength dependence, as disclosed in Non-Patent Document 1, two arm waveguides having a slight waveguide length difference are cascaded between two directional couplers. 2. Description of the Related Art A wavelength-independent coupler (WINC) including a kind of connected Mach-Zehnder Interferometer (MZI) is known.

FIG. 1 is a diagram showing a configuration of a conventional winc. 1 has a configuration in which a first directional coupler 101, a phase difference providing unit 102, and a second directional coupler 103 are cascaded in this order. The phase difference providing unit 102 includes two arm waveguides 104 and 105 having a predetermined optical path length difference. Binding ratio kappa _1'of the first directional coupler 101, coupling ratio kappa _2'of the second directional coupler 103, a waveguide between two arm waveguides 104 and 105 in the phase difference providing section 102 By appropriately designing the length difference ΔL _{MZI ′} , broadband operation is realized. In practice, a quartz-based waveguide is used to realize an optical coupler capable of operating in a wide band with almost no change in optical coupling rate over a wavelength range of several hundred nm.

  In addition, as disclosed in Non-Patent Document 2, four directional couplers are provided as a configuration for mitigating fluctuations in the optical coupling ratio of the directional coupler due to manufacturing errors and fluctuations in the optical coupling ratio due to the wavelength dependency. There is also known a stabilized optical coupler including a lattice interferometer in which three sets of two arm waveguides are alternately connected.

K. Jinguji, et al., "Mach-Zehnder interferometer type optical waveguide coupler with wavelength-flattened coupling ratio," IEE electronics letter, vol.26, no.17, pp.1326-1327, 1990 M. Oguma, et al., "Compact and low-loss interleave filter configured lattice-form structure and silica-based waveguide," Journal of Lightwave Technology, vol.22, no.3, pp.895-902, 2004

The characteristics of the directional coupler are as follows: the refractive index difference Δn between the core and clad of the waveguide, the core width W, the core height H, the gap G between the cores of two adjacent waveguides, and the two adjacent ones. The length (coupling length) L _{DC of the} waveguide is determined. However, the actually produced directional coupler has a problem in that its characteristics vary due to production errors such as a deviation of the core width W and a gap G due to a processing deviation of the waveguide pattern.

  FIG. 2 is a diagram showing a typical wavelength dependence of the optical coupling rate of a 3 dB directional coupler manufactured using a silica-based waveguide. Three cases are shown with the wavelength (μm) on the horizontal axis and the optical coupling ratio (%) on the vertical axis. A curve 201 indicates a case where the waveguide core width is a design value, a curve 202 indicates a case where the waveguide core width is a design value + 0.2 μm, and a curve 203 indicates a case where the waveguide core width is a design value−0.2 μm. Yes. As can be seen from the curve 201, in this directional coupler, the core width is designed so that the optical coupling factor is 50% at the wavelength of 1550 nm. In the following description of the characteristics on the graph, unless otherwise specified, the term including a substantially linear one is referred to as a curve for simplicity. As indicated by the curves 201 to 203, the directional coupler generally has a characteristic that the coupling becomes stronger as the wavelength becomes longer. This characteristic varies depending on the processing error of the waveguide. For example, when the core width w in the isolated waveguide is 0.2 μm thicker than the design value, the optical coupling factor is 3% as shown by the curve 202 in FIG. It gets bigger. Although the core width variation value in the isolated waveguide and the core width variation value in the directional coupler in which the two waveguides are close to each other may not be exactly the same, it may be considered that they are almost the same. . In the case of the example of the directional coupler of FIG. 2, the core width W of the directional coupler is increased by about 0.2 μm, and the opposite sides of the two waveguides are increased by 0.1 μm, resulting in two waveguides. It can be considered that the gap G is narrowed by about 0.2 μm. The characteristic deviation of the directional coupler caused by such processing deviation becomes a problem that causes the deviation of the optical coupling ratio even in the WINC 100 using the directional coupler as an element circuit.

FIG. 3 is a diagram showing a typical wavelength dependence of the optical coupling rate of a WINC manufactured using a silica-based waveguide. Three cases are shown with the wavelength (μm) on the horizontal axis and the optical coupling ratio (%) on the vertical axis. Corresponding to the WINC 100 of FIG. 1, the waveguide length between the two arm waveguides 104 and 105 of the phase difference imparting unit 102 so as to have a coupling rate of 50% in the wavelength band of 1.5 to 1.6 μm. The difference ΔLMZI ′ is about 0.725 μm, the coupling length L _DC of the directional coupler 101 is about 125 μm, the coupling rate κ _{1 ′} is 19.6% at a wavelength of 1550 nm, and the coupling length L _DC of the directional coupler 103 is about 483 μm. in coupling ratio kappa _2'is designed to 74.2% at a wavelength of 1550 nm. With this configuration, an optical coupling factor of 50% is obtained over a wide wavelength range of 100 nm or more. In FIG. 3, the curve 301 is obtained when the core width of the arm waveguide is the design value, and the core width is the design value + 0. Curve 302 in the case of 2 μm, the core width is the design value− 0. A curve 303 in the case of 2 μm is shown. When the core width in the isolated waveguide is 0.2 μm thicker than the design value, the wavelength coupling is maintained to some extent, but the optical coupling factor is increased by about 2% as shown by the curve 302 in FIG.

  In the stabilized optical coupler comprising the lattice interferometer of the other prior art configuration example described above, the variation in the optical coupling ratio with respect to the processing deviation is small, but the number of interferometer stages is three and the element is enlarged. There was a problem to do. Furthermore, this stabilized optical coupler has a wavelength dependency of ± 5% in the 100 nm band with respect to the 50% coupling design, and has a problem that the wavelength dependency is larger than the WINC having one interferometer. There was also.

  The present invention has been made in view of such a problem, and the purpose thereof is to obtain a desired coupling ratio over a wide wavelength range, and the variation in the optical coupling ratio characteristics with respect to the fabrication processing deviation is small. An object of the present invention is to provide a waveguide type optical coupler having a large manufacturing tolerance with a small circuit scale.

According to the present invention, in order to achieve such an object, according to the first aspect of the present invention, in the waveguide type optical coupler for branching or coupling one or more lights, the one or more lights are input. A phase difference providing unit comprising a first directional coupler and two arm waveguides and cascade-connected to the first directional coupler, wherein one arm of the two arm waveguides A phase difference providing unit including a waveguide portion in which at least a part of the waveguide has a width different from that of the other arm waveguide; and a second directional coupler further connected in cascade to the phase difference providing unit Where R is the optical coupling rate of the entire optical coupler, and the optical coupling rate κ ₁ of the first directional coupler is an angle of κ ₁ = (sin (θ ₁ )) ^{2 according} to the coupling phase angle θ ₁ . expression, and combining the optical coupling ratio kappa ₂ of the second directional coupler phase angle theta ₂ by kappa ₂ = (sin when an angle expressed by θ _{²⁾⁾ 2,} for the waveguide width variation .delta.w, wherein the ratio of the variation δφ phase difference given by the phase difference providing section phi d.phi / dw is in a predetermined operating wavelength

A waveguide characterized in that a length of the waveguide portion, a waveguide width of the waveguide portion, and a waveguide length difference between the two arm waveguides are set so as to satisfy the relationship Type optical coupler.

Wherein said waveguide portion, the wide waveguide having a width L _B than the waveguide width of the two arm waveguides, or may be a narrow waveguide having a narrower width L _N. Further, the waveguide portion can be continuously connected to the two arm waveguides via a tapered waveguide.

The invention according to claim 2 is the waveguide type optical coupler for branching or coupling one or more lights, and the same waveguide width as the first directional coupler to which the one or more lights are input. A phase difference providing unit comprising two arm waveguides having W and cascade-connected to the first directional coupler, wherein at least one of the two arm waveguides has the same width W and Comprises a phase difference providing unit including waveguide portions having different waveguide widths, and a second directional coupler further cascade-connected to the phase difference providing unit, and R is an optical coupling as an entire optical coupler. The optical coupling rate κ ₁ of the first directional coupler is expressed as an angle by κ ₁ = (sin (θ ₁ )) ² by the coupling phase angle θ ₁ , and the light of the second directional coupler is expressed. When the coupling factor κ ₂ is expressed in terms of the coupling phase angle θ ₂ by κ ₂ = (sin (θ ₂ )) ² , the waveguide width The ratio dφ / dw, which is the ratio of the fluctuation δφ of the phase difference φ given by the phase difference providing unit to the fluctuation δw, at a predetermined operating wavelength.

A waveguide characterized in that a length of the waveguide portion, a waveguide width of the waveguide portion, and a waveguide length difference between the two arm waveguides are set so as to satisfy the relationship Type optical coupler.

  Invention of Claim 3 is the waveguide type optical coupler of Claim 1 or 2, Comprising: In the some wavelength of a predetermined | prescribed operating wavelength range,

The length of the waveguide portion, the waveguide width of the waveguide portion, and the waveguide length difference between the two arm waveguides are set so that the sum of the squares of And

  A fourth aspect of the present invention is the waveguide optical coupler according to any one of the first to third aspects, wherein the optical coupling factor R is differentiated with respect to the wavelength λ at one or more wavelengths in the predetermined operating wavelength range.

The length of the coupling of the directional coupler, the length of the waveguide portion, the waveguide width of the waveguide portion, and the distance between the two arm waveguides so that the sum of the squares of The difference in waveguide length is set.

The invention according to claim 5 is the waveguide type optical coupler according to claim 2 , wherein the waveguide portion is a wide waveguide having a width LB wider than the same waveguide width W, or the same It is a narrow waveguide having a width LN smaller than the waveguide width W.

The invention according to claim 6 is the waveguide type optical coupler according to claim 2 , wherein the waveguide portion is connected to the portion having the same waveguide width W of the two arm waveguides via a tapered waveguide. It is characterized by being connected continuously.

  A seventh aspect of the present invention is the waveguide type optical coupler according to any one of the first to sixth aspects, wherein the waveguide portion is formed of a tapered waveguide.

The invention of claim 8 is the waveguide type optical coupler according to claim 6 or 7, wherein the tapered waveguide is provided at an end of the first directional coupler or the second directional coupler. A part of the curved waveguide portion of the development portion where the distance of the waveguide increases is a tapered waveguide.
As another aspect of the waveguide optical coupler of the present invention, in the waveguide optical coupler that branches or combines one or more lights, a first directional coupler to which the one or more lights are input; A phase difference providing circuit comprising two arm waveguides including an arm waveguide having a waveguide width W1 and an arm waveguide having a waveguide width W2 different from W1 and cascade-connected to the first directional coupler. A phase difference providing unit, wherein at least one of the two arm waveguides includes a waveguide portion having a waveguide width different from the waveguide width of the one arm waveguide, and the phase difference And a second directional coupler connected in cascade to the adding unit, wherein R is an optical coupling factor of the entire optical coupler, and an optical coupling factor κ ₁ of the first directional coupler is a coupling phase angle θ. ₁ by _{κ 1 = (sin (θ 1} )) and the angle represented by ^two, the When the angle expressed in _{κ 2 = (sin (θ 2} )) 2 2 of the directional coupler of the optical coupling ratio kappa ₂ by coupling phase angle theta _2, for the waveguide width variation .delta.w, given by the phase difference generating portion Dφ / dw, which is the ratio of the fluctuation δφ of the phase difference φ obtained, at a predetermined operating wavelength

As a waveguide type optical coupler in which the length of the waveguide portion, the waveguide width of the waveguide portion, and the waveguide length difference between the two arm waveguides are set so as to satisfy the relationship Can be implemented.

  According to the present invention, it is possible to realize a waveguide type optical coupler that operates with a desired optical coupling factor characteristic over a wide wavelength range with a small circuit scale, with a small variation in optical coupling factor characteristic with respect to processing deviation. it can.

FIG. 1 is a diagram showing a configuration of a wavelength-independent coupler according to the prior art. FIG. 2 is a diagram showing a typical wavelength dependence of the optical coupling rate of a 3 dB directional coupler manufactured using a silica-based waveguide. FIG. 3 is a diagram showing a typical wavelength dependence of the optical coupling rate of a WINC manufactured using a silica-based waveguide. FIG. 4 is a diagram showing the configuration of the broadband waveguide type optical coupler of the present invention. FIG. 5 is a diagram showing how the optical coupling rate R varies in the entire MZI when the coupling phase angle of the directional coupler varies. FIG. 6 is a diagram showing a variation of the optical coupling rate R in the entire MZI when the phase difference φ of the phase difference imparting unit varies. FIG. 7 is a diagram showing how the coupling phase angle θi varies when the waveguide core width w varies in a typical design directional coupler in a silica-based waveguide. FIG. 8 is a diagram showing a variation mode of the optical coupling ratio R in the entire MZI when the waveguide core width w varies. FIG. 9 is a diagram illustrating a variation mode of the phase difference φ of the phase difference imparting unit when the waveguide core width w varies. FIG. 10 is a diagram illustrating a variation mode of the optical coupling ratio R in the entire MZI when the waveguide core width w varies in the case where the phase difference providing unit has ideal characteristics. FIG. 11 is a diagram showing an example of the dependency of the effective refractive index n on the waveguide core width w in the silica-based waveguide. FIG. 12 is a diagram illustrating a variation mode of the phase difference φ of the phase difference providing unit when the waveguide core width w varies in calculation examples having different configurations of the phase difference providing unit. FIG. 13 is a diagram showing an aspect of the phase difference φ of the phase difference providing unit designed with other target characteristics having different phase differences φ. FIG. 14 is a diagram showing an aspect of the phase difference φ of the phase difference providing unit designed with other target characteristics having different dφ / dw. FIG. 15 is a diagram showing an aspect of the change of the optical coupling ratio R with respect to the change of the wavelength λ in the optical coupler having the configuration of the parameter 1 of the present invention and the WINC of the prior art. FIG. 16 is a diagram showing, for each wavelength, how the optical coupling rate R fluctuates with respect to the wavelength λ change in the optical coupler by the wavelength-independent optimized parameter. FIG. 17 is a diagram showing a variation mode of the optical coupling ratio R with respect to the waveguide core width change Δw in the optical coupler by the wavelength-independent optimized parameter. FIG. 18 is a diagram showing the wavelength dependence of the optical coupling rate in the optical coupler with a 50% optical coupling rate designed in Example 1 of the present invention. FIG. 19 is a diagram showing the wavelength dependence of the optical coupling rate in the optical coupler of the optical coupling rate 20% design according to the second embodiment of the present invention. FIG. 20 is a diagram showing the wavelength dependence of the optical coupling rate in the optical coupler with a design of 70% optical coupling rate according to the second embodiment of the present invention. FIG. 21 is a diagram showing the wavelength dependence of the optical coupling rate of the optical coupler designed to have the wavelength dependence characteristic of the third embodiment of the present invention. FIG. 22 is a diagram showing the wavelength dependence of the optical coupling rate of the optical coupler having the configuration of Example 1 of the present invention that was actually manufactured. FIG. 23 is a diagram showing the wavelength dependence of the optical coupling rate of the actually manufactured conventional winc. FIG. 24 is a diagram showing an example of the shape of a waveguide portion constituted only by a tapered waveguide in the phase difference providing portion of the optical coupler of the present invention. FIG. 25 is a top view showing a configuration example drawn in a state close to an actual waveguide pattern of the optical circuit of the present invention. FIG. 26 is a top view showing still another configuration example drawn in a state close to the actual waveguide pattern of the optical circuit of the present invention.

  Hereinafter, the configuration of the waveguide type optical coupler of the present invention will be described in detail. In the directional coupler of an optical coupler, the design values of the core width W and the adjacent inter-waveguide gap G are the constraints required from pattern processing accuracy, the request for suppression of propagation excess loss, and the request for shortening the coupling length L. Often determined by factors such as Therefore, it is desirable to provide a waveguide type optical coupler under the condition that the core width W and the gap G as design values are given. In the following description of the waveguide type optical coupler according to the present invention, a novel configuration that has a wider band operability and satisfies a high tolerance characteristic against a processing error when given constant core width W and gap G conditions. And design techniques are presented.

  The waveguide type optical coupler according to the present invention includes a waveguide in which the width of at least a part of one of the two arm waveguides is different from the width of the other arm waveguide. A part is provided. Further, in the waveguide type optical coupler according to another aspect of the present invention, the width of at least a part of one arm waveguide of the two arm waveguides in the phase difference providing unit is equal to the width of the other arm waveguide. Different waveguide portions are provided. The waveguide portions having different widths may be wider or narrower than the common waveguide width common to the two arm waveguides. The widths of the waveguide portions having different widths described above are obtained in accordance with a procedure peculiar to the present invention, and the optical path length difference between the two arm waveguides. A waveguide type optical coupler that satisfies a wide range of operability and high tolerance characteristics against processing errors can be provided with only slight modification of the same components as those of the prior art optical coupler.

  FIG. 4 is a diagram showing the configuration of the waveguide type optical coupler of the present invention. The waveguide type optical coupler 400 with high processing tolerance has a configuration in which a first directional coupler 401, a phase difference providing unit 402, and a second directional coupler 403 are cascaded in this order. Here, although description of the operation of the optical coupler itself is omitted, one or more optical signals are input to the input port of the first directional coupler 401 and one is output from the output port of the second directional coupler 403. The above optical signals are output. The optical signal is split or combined depending on the input method of the input signal and the output method of the output signal. In terms of the overall configuration of the optical coupler 400, there is no difference from the conventional optical coupler 100 shown in FIG. The phase difference providing unit 402 of the optical coupler of the present invention is provided with a waveguide portion in which the width of at least a part of one of the two arm waveguides is different from the width of the other arm waveguide. ing. From another point of view, it is composed of two arm waveguides 404 and 405, and a waveguide width of a part of at least one of the arm waveguides is a normal waveguide of two arm waveguides. It is also different from the width. The ratio of the fluctuation δφ of the phase difference φ of the phase difference imparting section 402 to the waveguide width fluctuation δw, that is, the waveguide width and length of the waveguide portion so that the differential dφ / dw satisfies the following expression at the operating wavelength, and A waveguide length difference between the two arm waveguides 404 and 405 is set.

In Expression (1), R represents the optical coupling rate of the optical coupler 400 as a whole, the coupling phase angle θ ₁ is an amount expressing the optical coupling rate κ ₁ of the first directional coupler 401 as an angle, and the coupling phase angle θ. ₂ is an amount that the optical coupling ratio kappa ₂ and angle representation of the second directional coupler 403. There is a relationship of κ ₁ = (sin (θ ₁ )) ² and κ ₂ = (sin (θ ₂ )) ² between the coupling phase angle θ and the optical coupling rate κ.

In the prior art WINC 100 shown in FIG. 1, the phase difference providing unit 102 is composed of two arm waveguides 104 and 105 having a predetermined waveguide length difference ΔL _{MZI ′} . In contrast, in the optical coupler 400 of the present invention, in the two arm waveguides 404 and 405 of the phase difference imparting unit 402, the width of at least a part of one of the two arm waveguides. Is provided with a “waveguide portion” that is different from the width of the other arm waveguide. In other words, the waveguide width of the “waveguide portion” of at least one of the arm waveguides is different from the normal waveguide width of the two arm waveguides. The optical coupler of the present invention is configured in such a manner that the ratio of the variation δφ of the phase difference φ of the phase difference providing unit 402 to the waveguide width variation δw, that is, the differential dφ / dw satisfies the formula (1). This is different from WINC. Of the two arm waveguides 404 and 405, the width of at least a part of one arm waveguide has a waveguide portion that is different from the width of the other arm waveguide, or two arm waveguides At least one of the waveguides 404 and 405 has a waveguide portion having a different waveguide width, and the two arm waveguides 404 and 405 and the waveguide width and length of the waveguide portion. The waveguide length difference ΔL _MZI between 405 is set to satisfy the formula (1). In the configuration example shown in FIG. 4, as a waveguide portion, an example in which a thick width waveguide 406 of different waveguide width W _B to the upper arm waveguide 404 only for normal waveguide width W ing. Further, in order to avoid an increase in optical loss, the wide waveguide 406 and a normal waveguide are connected via a tapered waveguide portion in which the waveguide width gradually changes.

  The optical coupler 400 of the present invention operates with a desired optical coupling characteristic over a wider wavelength range than the prior art, and at the same time, the variation in the optical coupling characteristic with respect to manufacturing processing deviation is small, and the manufacturing tolerance is small. large. These excellent features can be explained as follows.

  Consider the 2-input 2-output MZI transfer matrix T, which is the basic configuration of the optical coupler 400 of the present invention of FIG. 4 and the prior art WINC 100 of FIG. The transfer matrix Cp of the directional coupler and the transfer matrix Ps of the phase difference adding unit are expressed by the following equations.

At this time, the transfer matrix T of MZI is described as follows:

When the entire MZI is regarded as an optical coupler, the optical coupling ratio R is the power transmittance | T ₁₂ | ² or | T ₂₁ | ² in the cross path of the 2-input 2-output circuit. For example, in FIG. 4, the power transmittance of the cross path from the input optical signal 407 to the output optical signal 408 is obtained. Therefore, the optical coupling rate R of the 2-input 2-output MZI is expressed by the following equation.

Here, how the optical coupling rate R varies with respect to the variation of the waveguide core width w will be examined. In the manufacturing process of the optical coupler, to variations in the waveguide core width w, if Kere near the rate of change dR / dw optical coupling ratio R obtained by Equation (4) is zero, it is greater tolerance for manufacturing process variation Means. As shown in Expression (4), the optical coupling ratio R is a function of θ ₁ , θ ₂ , and φ, and therefore dR / dw is expressed by the following expression.

The first term on the right side of equation (5) is the rate of change of the optical coupling ratio R due to the characteristic variation of the first directional coupler 401 with respect to the variation of the waveguide core width w, and the second term is the waveguide core width w. The change rate of the optical coupling rate R due to the characteristic variation of the second directional coupler 403 with respect to the variation of the second directional coupler 403, the third term is the light due to the characteristic variation of the phase difference providing unit 402 with respect to the variation of the waveguide core width w The change rate of the coupling rate R is shown respectively.

First, using equation (4) for the optical coupling rate R, how the optical coupling rate R varies when θ ₁ , θ ₂ and φ vary, that is, ∂R / ∂θ ₁ , ∂R / ∂θ. _2. Consider the behavior of ∂R / ∂φ. For simplicity, assuming the wavelength of interest only to 1550 nm, the optical coupling ratio κ _1'≒ 19.6% of the design parameters used in WINC100 prior _{art, κ 2'≒ 74.2%, ΔL} MZI' ≒ 0 Use .725 μm. These values correspond to θ ₁ ≈0.15π, θ ₂ ≈0.33π, and φ≈1.36π, respectively, when converted into the phase difference given by the coupling phase angle and the phase difference providing unit.

FIG. 5 is a diagram showing how the optical coupling rate R varies in the entire MZI when the coupling phase angle of the directional coupler varies. The horizontal axis represents the coupling phase angle fluctuation amounts Δθ ₁ and Δθ ₂ (0.01π radians), and the vertical axis represents the optical coupling rate R (%) of the entire MZI. A curve 501 shows the fluctuation of the optical coupling rate R when the coupling phase angle θ ₁ of the first directional coupler 401 fluctuates, and a curve 502 shows the fluctuation of the coupling phase angle θ ₂ of the second directional coupler 403. The fluctuation of the optical coupling rate R in each case is shown. The slopes of the lines in the curves 501 and 502 in FIG. 5 correspond to ∂R / ∂θ ₁ and ∂R / ∂θ ₂ in the equation (5), respectively.

  FIG. 6 is a diagram showing a variation of the optical coupling rate R in the entire MZI when the phase difference φ of the phase difference imparting unit varies. The horizontal axis indicates the phase difference variation Δφ (0.01π radians), and the vertical axis indicates the optical coupling rate R (%) of the entire MZI. The slope of the curve in FIG. 6 corresponds to ∂R / ∂φ in equation (5).

Next, consider the behavior of dθ ₁ / dw and dθ ₂ / dw in equation (5). dφ / dw will be described later. In a typical design example of the directional coupler in the silica-based waveguide as shown in FIG. 1, when the core width in the isolated waveguide is larger than the design value, the optical coupling rate of the directional coupler is the design value. Bigger than.

FIG. 7 shows a variation of the coupling phase angle θ _i at a wavelength of 1550 nm when the waveguide core width w is varied in a typical design directional coupler in a silica-based waveguide. It is a figure. The horizontal axis shows the fluctuation amount Δw (μm) of the core width, and the vertical axis shows the fluctuation amount (0.01π radians) of the coupling phase angle θ _i . A curve 701 shows a variation amount of the coupling phase angle θ _{i in} the case of a directional coupler having a design coupling rate of 19.6%, and a curve 702 shows a coupling phase angle θ in the case of a directional coupler having a design coupling rate of 74.2%. _For reference, the curve 703 shows the fluctuation amount of the coupling phase angle θ _{i in} the case of a directional coupler having a design coupling ratio of 50%. The slopes of the lines in the curves 701 and 702 in FIG. 7 correspond to dθ ₁ / dw and dθ ₂ / dw in Expression (5), respectively. A directional coupler having a high coupling rate is designed such that the adjacent portions of the two waveguides are long so that the working length of the directional coupler is long. For this reason, it is strongly influenced by the fluctuation of the waveguide core width w, and dθ _i / dw becomes larger than that of a directional coupler designed to have a low coupling rate.

The sum of the first term and the second term on the right side of Equation (5) is the characteristic variation of the first directional coupler 401 and the characteristic variation of the second directional coupler 403 when the waveguide core width w varies. Represents the change rate of the optical coupling rate R due to both. Each element of the sum of the first and second terms is ∂R / ∂θ ₁ and ∂R / ∂θ ₂ based on the slope of each curve in FIG. 5 and 、 based on the slope of each curve in FIG. It can be obtained from θ ₁ / ∂w and ∂θ ₂ / ∂w.

  FIG. 8 is a diagram showing a variation of the optical coupling rate R in the entire MZI at a wavelength of 1550 nm when the waveguide core width w varies. The horizontal axis represents the fluctuation amount Δw (μm) of the core width, and the vertical axis represents the optical coupling ratio R (%) in the entire MZI. Each curve in FIG. 8 is calculated from Equation (4), each curve in FIG. 5, and each curve in FIG. A curve 801 represents a change in the optical coupling rate R caused only by a characteristic variation of the first directional coupler 401 (design coupling rate 19.6%), and its slope represents the first term on the right side of the equation (5). . A curve 802 represents a change in the optical coupling ratio R caused only by a characteristic variation of the second directional coupler 403 (design coupling ratio 74.2%), and its slope represents the second term on the right side of the equation (5). . A curve 803 represents a change in the optical coupling ratio R when the characteristic fluctuations of both directional couplers occur at the same time, and the inclination thereof is approximately the sum of the inclination of the curve 801 and the inclination of the curve 802. The slope of this curve 803 represents the sum of the first and second terms on the right side of Equation (5). Therefore, in a typical design of a directional coupler in a silica-based waveguide, when the core width w is increased, the characteristic variation of the two directional couplers 401 and 403 increases the optical coupling ratio R of the optical coupler. Acts on direction. It should be noted that the above discussion of the variation in optical coupling rate R with respect to Equations (4) and (5) above relates to components common to the prior art optical coupler 100 and the optical coupler 400 of the present invention. I want to be.

FIG. 9 is a diagram showing a variation of the phase difference φ at the wavelength of 1550 nm of the phase difference imparting unit when the waveguide core width w varies in the conventional WINC. The horizontal axis shows the fluctuation amount Δw (μm) of the core width, and the vertical axis shows the fluctuation of the phase difference (0.01π radians). In the conventional WINC 100, since the waveguide length difference ΔL _{MZI ′} of the two arm waveguides 104 and 105 of the phase difference applying unit 102 is very small as 0.725 μm, the core width of the arm waveguides 104 and 105 is designed. Even if it deviates from the value, the phase difference φ of the phase difference providing unit 102 hardly fluctuates. A curve 901 in FIG. 9 shows the fluctuation of the phase difference φ when the waveguide length difference ΔL _{MZI ′} of the phase difference providing unit 102 is 0.725 μm. It can be seen that the slope of the curve 901 in FIG. 9 is dφ / dw, and the slope of the curve 901 is almost 0 and dφ / dw≈0. Therefore, in the prior art WINC 100, the third term on the right side of the equation (5) indicating the rate of change of the optical coupling rate R is zero. As a result, the fluctuation of the optical coupling ratio R shown by the curve 803 in FIG. 8 caused by the two directional couplers is the change in the optical coupling ratio R in the entire MZI with respect to the fluctuation of the core width as shown in FIG. It was.

  Here, the change rate dφ / dw of the phase difference φ of the phase difference imparting unit with respect to the core width w is not zero, and a characteristic that cancels the sum of the first term and the second term on the right side of the equation (5), that is, the equation ( If it has the characteristics shown in 1), the change in the optical coupling ratio R due to the fluctuation of the core width of the directional coupler is canceled out, and the entire dR / dw in the equation (5) becomes zero, and the waveguide The optical coupling ratio R does not vary with respect to the processing deviation of the core width w, and an optical coupler having a large manufacturing tolerance can be realized. Here, it is assumed that the phase difference providing unit has an ideal characteristic that cancels out the change in the optical coupling ratio R caused by the fluctuation of the core width of the directional coupler as shown by the curve 902 in FIG. To do. The ideal characteristic 902 is a characteristic that generally satisfies the relationship of the expression (1) with respect to the characteristic curve 803 caused by the two directional couplers shown in FIG.

  FIG. 10 shows the fluctuation of the optical coupling ratio R of the entire MZI when the waveguide core width w fluctuates when the phase difference providing unit has the ideal characteristic of the phase difference φ with respect to the fluctuation of the core width shown in FIG. It is the figure which showed the aspect of. The horizontal axis indicates the amount of fluctuation (μm) in the core width, and the vertical axis indicates the optical coupling rate R (%) in the entire MZI. A curve 1001 shows a change in the optical coupling rate R caused only by the characteristic variation of the phase difference providing unit. The slope of the curve 1001 represents the third term on the right side of Equation (5). A curve 1002 shows a change in the optical coupling ratio R caused only by the characteristic variation of the two directional couplers, and is the same as the curve 803 in FIG. The slope of the curve 1002 is the sum of the first and second terms on the right side of Equation (5) as described above. A curve 1003 shows a change in the optical coupling rate R when the characteristic variation of the phase difference providing unit and the directional coupler occurs simultaneously. The slope of the curve 1003 represents dR / dw expressed by the equation (5), and the slope is zero as is apparent from the curve 1003. Thus, if the phase difference φ with respect to the core width variation of the phase difference imparting unit has ideal characteristics such as the curve 902, this optical coupler can realize a state of dR / dw≈0. That is, it is possible to obtain an optical coupler having a large manufacturing tolerance in which the optical coupling ratio R does not vary with respect to the processing deviation of the waveguide core width w.

  Next, how the phase difference imparting unit can actually have the ideal characteristic of the phase difference variation with respect to the core width change as shown by the curve 902 will be described. In the optical coupler of the present invention, the phase difference imparting unit 401 in FIG. 4 has the ideal characteristic in which the variation in the phase difference with respect to the change in the core width has a curve 902, thereby providing high-bandwidth operability and high tolerance for processing deviation. It has gained. Compared with the optical coupler of the prior art, the feature of the present invention is realized by the structural difference in the phase difference providing unit 401 shown in FIG.

  FIG. 11 is a diagram showing an example of the dependency of the effective refractive index n on the waveguide core width w in the silica-based waveguide. The horizontal axis represents the core width w (μm), the vertical axis represents the effective refractive index n, and three cases of wavelengths 1450, 1550, and 1650 nm are illustrated. Note that the value of the core width w on the horizontal axis actually indicates the pattern width on the photomask. As is apparent from the three curves shown in FIG. 11, the slope of each curve, that is, dn / dw, is dependent on the core width w. For example, the value of dn / dw when the core width is 6 μm is about 4.3 times larger than the value of dn / dw when the core width is 12 μm. This means that the degree of influence on the effective refractive index due to the core width variation (variation) differs between the 6 μm wide waveguide and the 12 μm wide waveguide. By utilizing the difference in the size of dn / dw depending on the core width, the phase difference providing unit 401 can have an arbitrary dφ / dw.

For example, like the phase difference providing unit 402 of the optical coupler 400 of the present invention shown in FIG. 4, a part of the upper arm waveguide 404 is thicker than the normal waveguide width W over the length L _B. the waveguide width W _B to "waveguide portion". Hereinafter, in this specification, a portion having a waveguide width different from the normal waveguide width W is referred to as a “waveguide portion”. For simplicity, the waveguide portion 406 wider than the normal waveguide width will be referred to as a “thick waveguide”. In addition, “normal waveguide width W” means a common waveguide width when each arm waveguide of the phase difference providing portion composed of two or more arm waveguides has the same waveguide width. Say W. The arm waveguides of the phase difference imparting portion are normally set to the same waveguide width, that is, “normal waveguide width W”. In general, a waveguide for propagating an optical signal on a PLC and routing between element circuit blocks has a normal waveguide width W determined according to a core material, a core height, a refractive index, a mode of light to be propagated, and the like. The Exceptionally, the input / output waveguide connected to the optical fiber with low loss and two adjacent waveguides in the directional coupler use a waveguide width different from the normal waveguide width. In principle, however, a common "normal waveguide width" is used in the PLC. However, it should be noted that two-arm waveguides having different widths can be used in the phase difference providing unit 402 of the optical coupler of the present invention. In the following description, for the sake of simplicity, it is assumed that the phase difference providing unit 402 has the same waveguide width of the two arm waveguides and has a normal waveguide width W. In the following discussion, it is assumed that the thick waveguide 406 and the normal waveguide are directly connected without using a tapered waveguide, and no excess loss is caused by a sudden change in the waveguide width.

In FIG. 4, the difference between the waveguide length L ₁ of the lower arm waveguide 405 and the waveguide length L ₂ of the upper arm waveguide 404 including the thick waveguide 406 is expressed as ΔL _MZI (= L ₁ −L ₂ ), The light propagating through the upper arm waveguide 404 propagates through the waveguide having the effective refractive index n (W _B ) by the distance L _{B and} travels through the waveguide having the effective refractive index n (W) by the distance (L ₁ −L). _B ) Propagating. Therefore, the phase changes by the following while propagating through the upper arm waveguide 404.

Since the light propagating through the lower arm waveguide 405 is propagating through the waveguide having the effective refractive index n (W) through L ₂ , the phase is the following while the phase propagates through the lower arm waveguide 405. It will change by the amount of.

Accordingly, the propagation light phase difference φ of the upper and lower arm waveguides 404 and 405 in the phase difference imparting unit 402 is expressed by the following equation.

  Further, the change rate dφ / dw of the phase difference φ in the equation (6) with respect to the fluctuation of the waveguide core width w is expressed by the following equation where n ′ = dn / dw.

It can be said that the ideal characteristic 902 of the phase difference φ of the phase difference providing unit shown in FIG. 9 is substantially a straight line within the assumed range of fluctuation of the core width. In order to realize the ideal characteristic 902, φ and dφ / dw in the design core width w need only coincide with the ideal characteristic 902. Therefore, the normal waveguide width W and the width W _B of the wide waveguide (waveguide portion) 406, which are variables of the equations (6) and (7), are set so that φ and dφ / dw are ideal characteristics. The length L _B of the wide waveguide 406 and the waveguide length difference ΔL _MZI between the two arm waveguides may be determined. Since there are two simultaneous equations with four variables, there are multiple sets of solutions. As an example, here, the conventional waveguide width W is fixed to 7μm as a given value, the number of try to seek solutions for the width W _B of the wide waveguide, is a set of different solutions as follows can get.
Example 1: W _B = 12 μm, L _B = 180 μm, ΔL _MZI = 0.522 μm
Example 2: W _B = 10 μm, L _B = 225 μm, ΔL _MZI = 0.542 μm
Example 3: W _B = 8 μm, L _B = 462 μm, ΔL _MZI = 0.566 μm

FIG. 12 is a diagram illustrating a variation of the phase difference φ of the phase difference imparting unit 402 when the waveguide core width w is changed in different calculation examples of the optical coupler of the present invention. The horizontal axis indicates the amount of fluctuation Δw (μm) of the core width of the wide waveguide, and the vertical axis indicates the phase φ (0.01π radians) of the phase difference providing unit. FIG. 12 depicts three curves corresponding to Δw and ideal characteristic 902 as well as solutions of 3 kinds of L _B and [Delta] L _MZI calculation example of Examples 1 to 3 above set between φ in FIG. 9 ing. Since the four curves are close enough is difficult to distinguish substantially duplicate, even if the set of solutions of the three types in any of L _B and [Delta] L _MZI, substantially the same as the ideal characteristic 902, the core width change Δw- position It can be seen that fluctuation characteristics of the phase difference φ are obtained. The combination of L _B and [Delta] L _MZI case of W _B = 10 [mu] m of Example 2, since further utilized when considering the wavelength dependence of the optical coupler of the present invention after the combination of Example 2 L _B and [Delta] L _MZI Parameter 1 is set.

Three solutions of the width W _B of the above-described different thickness width waveguides are determined by the respective calculated as follows. Basically, the simultaneous equations of Equation (6) and Equation (7) may be solved. The ideal characteristic 902 (FIG. 9) of the core width variation Δw−phase φ to be realized by the phase difference providing unit 402 of the optical coupler of the present invention can be regarded as a straight line in the range of the core width variation Δw of ± 0.3 μm. Therefore, φ is a value at Δw = 0 of the ideal characteristic 902, and dφ / dw is a slope at Δw = 0 of the ideal characteristic 902. Since the normal waveguide width W and the width W _B of the wide waveguide are given values, the refractive index changes with changes in the effective refractive indexes n (W), n (W _B ), and the core width w. The ratios n ′ (W) and n ′ (W _B ) are the effective refractive index value and the inclination at the corresponding core width w in each curve shown in FIG. Here, the wavelength λ is 1550 nm. Using the above values, the formula (6) and (7) is two-way simultaneous linear equations for the L _B and [Delta] L _MZI as variables (unknown), be determined algebraically L _B and [Delta] L _MZI it can. Strictly speaking, the ideal characteristic 902 between Δw and φ in FIG. 9 is not a straight line, and the core width dependence of the effective refractive index shown in FIG. 11 is not a straight line. Therefore, in the actual calculation, the equation (6) regarding the phase difference φ at each point of the assumed core width variation Δw (in the calculation examples of Examples 1 to 3, Δw = −0.2, 0, +0.2 μm). ), That is, when the difference between the left side and the right side of Equation (6) at each Δw is E (Δw), the sum of squares expressed by Equation (7-2) below is minimized. In addition, L _B and ΔL _MZI are obtained by numerical calculation.

In the following calculation example, a solution is obtained in the same manner.

In the configuration (configuration example 1) of the phase difference providing unit 402 of the optical coupler 400 of the present invention described above, the wide waveguide 406 is provided in a part of the upper arm waveguide 404. However, the arm waveguide in the phase difference providing unit is provided. The difference in core width is relative. Therefore, the upper arm waveguide 404 is constituted only by a waveguide having a normal waveguide width W, and a waveguide having a length smaller than the normal waveguide width W over a length L _N in a part of the lower arm waveguide 405. A structure may be provided in which a waveguide portion having a width W _N is provided. For simplicity, a waveguide portion having a waveguide width W _N narrower than the normal waveguide width W is referred to as a “narrow waveguide”. For example, consider a case where the normal waveguide width W is 7 μm and the waveguide width W _N of the narrow waveguide is 6 μm. At this time, when the combination of the unknowns L _N and ΔL _MZI is obtained by solving the simultaneous equations, L _N = 274 μm and ΔL _MZI = 0.593 μm (Configuration Example 2). By designing the phase difference providing unit by this combination, the variation characteristic of the phase difference φ with respect to the core width change Δw can be obtained, which is almost the same as the ideal characteristic 902 between Δw and φ shown in FIG.

The configuration of the phase difference providing unit 402 of the optical coupler 400 of the present invention is not limited to the configuration including the waveguide portion only on one side of the arm waveguide as described above, but a part of the upper arm waveguide 404. It is also possible to provide a wide waveguide, a narrow waveguide in part of the lower arm waveguide 405, and a waveguide portion on both arm waveguides. In that case, the light propagating through the upper arm waveguide 404, the waveguide effective refractive index n (W _B) the distance L and _B propagates, the distance through the waveguide effective refractive index _{n (W) (L 1 -L} B ) Because it is propagating, its phase changes by:

The light propagating through the lower arm waveguide 405 propagates through a waveguide having an effective refractive index n (W _N ) through a distance L _{N and} propagates through a waveguide having an effective refractive index n (W) through a distance (L ₂ −L _N ). Therefore, the phase changes by the following amount.

Therefore, the general formulas of the phase difference φ and the change rate dφ / dw of the phase difference applying unit 402 are respectively expressed by the following two formulas.

In the equations (8) and (9), there are five variables, and the degree of freedom is increased by one compared to the cases of the equations (6) and (7). It is sufficient to obtain a solution by limiting the increased degrees of freedom assuming that the lengths are the same (L _BN = L _B = L _N ). When the normal waveguide width W is 7 μm, the waveguide width of the wide waveguide is W _B = 8 μm, and the waveguide width of the narrow waveguide is W _N = 6 μm, L _BN and ΔL _MZI are obtained. L _BN = 172 μm and ΔL _MZI = 0.587 μm are obtained (Configuration Example 3). Even in this design, characteristics substantially similar to the core width variation Δw−phase φ ideal characteristics 902 to be realized shown in FIG. 9 can be obtained. In the configuration example 2 and the configuration example 3 described above, as in the configuration example 1, the waveguide length difference ΔL _MZI between the two arm waveguides is the upper arm relative to the waveguide length of the lower arm waveguide 405. It is assumed that the difference between the waveguide lengths of the waveguide 404 is positive. That is, it means that the waveguide length of the upper arm waveguide 404 is longer than that of the lower arm waveguide 405. Conversely, when the waveguide length difference ΔL _MZI is negative, it means that the waveguide length of the upper arm waveguide 404 is shorter than that of the lower arm waveguide 405, and the same applies to the following examples. Further, the above-described method of limiting the degree of freedom in the equations (8) and (9) is not limited to L _B = L _N , but also by arbitrarily setting the ratio of L _B and L _N. Needless to say, you can.

  In the above-described configuration examples 1 to 3, in accordance with the curve 803 of the optical coupling rate R change due to the core width variation Δw caused by the characteristic variation of the two directional couplers illustrated in FIG. The target characteristic between Δw and φ to be given to is determined as the ideal characteristic 902 shown in FIG. The change of the optical coupling rate R due to the characteristic variation of the directional coupler varies depending on the specific design values of the optical coupling rates R of the first directional coupler 401 and the second directional coupler 403. Further, the change in the optical coupling ratio R due to the characteristic variation of the directional coupler is caused by the core width W, the core height H of the directional coupler, and the inter-core gap G in the adjacent portion of the two arm waveguides. It depends on the detailed configuration. Therefore, the target characteristic to be given to the phase difference adding unit in the optical coupler of the present invention also varies depending on the specific configuration of the two directional couplers and the characteristic variation of the optical coupling rate R with respect to the core width Δw variation. . The procedure of exemplarily explaining the configuration of the phase difference providing unit conforming to the ideal characteristic 902 between Δw and φ described in the above-described configuration example 1 to configuration example 3 by determining the unknowns of the simultaneous equations is the target characteristic. However, it can be applied to any other case different from the ideal characteristic 902.

FIG. 13 is a diagram illustrating an aspect of the phase difference φ change with respect to the core width variation of the phase difference providing unit 402 that realizes different target characteristics between Δw and φ. Compared with the example of the target characteristic shown in FIG. 9 (phase difference φ = 1.36π), the absolute value of the phase difference φ (phase difference φ = 0.36π) realized by the phase difference providing unit is greatly different. The horizontal axis represents the core width variation Δw (μm), and the vertical axis represents the phase difference φ (0.01π radians) of the phase difference providing unit. FIG. 13 shows target characteristics and actual characteristics of the design configuration obtained according to the present invention described below. Specifically, the phase difference providing unit 402, a configuration in which a large-width waveguide only in the upper arm waveguide 404, a normal waveguide width W = 7 [mu] m, the waveguide width of Futoshihabashirube waveguide W _B = 10 μm, the length of the wide waveguide is L _B = 224 μm, and the waveguide length difference between the two arm waveguides is ΔL _MZI = 0.010 μm. The target characteristics and the characteristics of the design configuration obtained in accordance with the present invention are almost the same.

FIG. 14 is a diagram showing an aspect of the phase difference φ change with respect to the core width variation of the phase difference providing unit 402 that realizes another different target characteristic between Δw and φ. Compared to the target characteristic example shown in FIG. 9 (inclination is positive), the inclination realized by the phase difference providing unit, that is, dφ / dw is different (inclination is negative). The horizontal axis represents the core width variation Δw (μm), and the vertical axis represents the phase difference φ (0.01π radians) of the phase difference providing unit. FIG. 14 shows target characteristics and actual characteristics of the design configuration obtained according to the present invention described below. Specifically, a thick waveguide is provided only in the lower arm waveguide 405, the normal waveguide width is W = 7 μm, the waveguide width of the wide waveguide is W _B = 10 μm, and the wide waveguide. The length of L _{B is} 223 μm, and the waveguide length difference between the two arm waveguides is ΔL _MZI = 0.906 μm. The target characteristics and the characteristics of the design configuration obtained in accordance with the present invention are almost the same. In both examples of the target characteristics shown in FIGS. 13 and 14, according to the present invention, the phase difference applying unit is set using the expressions (6) and (7) or the expressions (8) and (9). By designing, it can be seen that the target characteristics of the phase difference providing unit that can cancel out the change in the optical coupling rate R due to the core width variation Δw of the two directional couplers can be substantially realized.

As described above, in the optical coupler of the present invention, in the two arm waveguides 404 and 405 of the phase difference imparting unit 402, the width of at least a part of one arm waveguide of the two arm waveguides is the other. A “waveguide portion” (thick width) having a “waveguide portion” that is different from the width of the arm waveguide, or having a waveguide width different from the normal waveguide width on at least one arm waveguide A waveguide or a narrow waveguide). The ratio of the variation δφ of the phase difference φ of the phase difference imparting section 402 to the waveguide width variation δw, ie, the differential dφ / dw satisfies the formula (1), and the waveguide width and length of the “waveguide portion” region. And a waveguide length difference ΔL _MZI between the two arm waveguides 404 and 405 is set. As a result, an optical coupler can be realized in which dR / dw≈0, that is, the optical coupling ratio R hardly changes with respect to the waveguide core width fluctuation Δw. That is, it is possible to obtain an optical coupler having a large manufacturing tolerance by suppressing the fluctuation of the optical coupling rate R with respect to the processing deviation in the manufacturing process of the optical coupler.

Therefore, the optical coupler of the present invention is the same waveguide as the first directional coupler 401 to which the one or more lights are input in the waveguide type optical coupler that branches or couples one or more lights. A phase difference providing unit comprising two arm waveguides 404 and 405 having a width W and cascade-connected to the first directional coupler, the same as at least one of the two arm waveguides Including a waveguide portion 406 having a waveguide width different from the width W of the first and second directional couplers 403 connected in cascade to the phase difference providing portion, and The optical coupling rate of the entire coupler is expressed, and the optical coupling rate κ ₁ of the first directional coupler is expressed as an angle by κ ₁ = (sin (θ ₁ )) ² by the coupling phase angle θ ₁ , and the second by the optical coupling ratio kappa ₂ of the directional coupler coupling phase angle θ _{2 κ 2} = (si (Θ _{²⁾⁾ 2} when the angle expressed by, for waveguide width variation .delta.w, d.phi / dw is the ratio of change δφ phase difference φ given by the phase difference generating portion in a predetermined operating wavelength

The length of the waveguide portion, the waveguide width of the waveguide portion, and the waveguide length difference between the two arm waveguides are set so as to satisfy the relationship.

The design method of the optical coupler having a large tolerance with respect to the waveguide core width variation Δw described above can be applied to realize an optical coupler that has less wavelength dependency and can operate in a wide band. When an optical coupler having a large production tolerance is to be realized in a predetermined operating wavelength range, for example, 1.5 to 1.6 μm, the above-described unknown parameter (“ The length of the “waveguide portion” region and the waveguide length difference ΔL _MZI between the two arm waveguides) are selected. Even if the expression (1) is not strictly satisfied in all the operating wavelength ranges, the sum of the squares of dR / dw at the predetermined one or more wavelengths λ in the operating wavelength range (the following expression (9-2)) is By designing to be minimized, it is possible to realize an optical coupler that has a substantially sufficient manufacturing tolerance.

Further, in order to realize an operation in which the optical coupling rate R is substantially constant in a predetermined operating wavelength range, the change rate dR / dλ of the optical coupling rate R with respect to the variation of each wavelength λ is substantially zero in the operating wavelength range. If it is good. As shown in the equation (4) for the optical coupling rate R, since the optical coupling rate R is a function of θ ₁ , θ ₂ and φ, dR / dλ is expressed by the following equation.

  The first term on the right side of Equation (10) is the rate of change of the optical coupling factor R due to the first directional coupler with respect to the wavelength change, and the second term is the light due to the second directional coupler with respect to the wavelength change. The third term represents the rate of change of the coupling rate R, and the third term represents the rate of change of the optical coupling rate R caused by the phase difference imparting section with respect to the wavelength change. In the above formulas (5) to (7), the mode of variation of the optical coupling ratio R with respect to the wavelength change is obtained by the same procedure as that for obtaining the change of the optical coupling ratio R when the core width w varies. be able to.

  FIG. 15 is a diagram showing an aspect of the change of the optical coupling ratio R with respect to the change of the wavelength λ in the optical coupler having the configuration of the parameter 1 of the present invention and the WINC of the prior art. The horizontal axis represents the wavelength λ, and the vertical axis represents the optical coupling rate R of the entire optical coupler. Here, the waveguide core width w does not vary, that is, Δw = 0. A curve 1501 shows a change in the optical coupling rate R caused only by the wavelength dependency of the phase difference providing unit 402 in the configuration of the parameter 1 of the present invention. The slope of the curve 1501 represents the third term on the right side of Equation (10). Curve 1502 shows the change in optical coupling rate R due to the wavelength dependence of the two directional couplers only. The slope of the curve 1502 represents the sum of the first and second terms on the right side of Equation (10). A curve 1503 shows a change in the optical coupling rate R when the wavelength dependency of both the phase difference providing unit 402 and the two directional couplers in the configuration of the parameter 1 of the present invention is simultaneously affected. The slope of the curve 1503 represents the equation (10), that is, dR / dλ, and when this slope becomes zero, wavelength-independent optical coupling rate characteristics can be obtained. A curve 1504 shows a change in the optical coupling ratio R caused only by the wavelength dependency of the phase difference providing unit 102 in the conventional winc 100 shown in FIG. A curve 1505 shows a change in the optical coupling ratio R when the wavelength dependency of both the phase difference providing unit 102 and the two directional couplers in the WINC 100 is simultaneously affected.

  As can be seen from the comparison between the curve 1503 and the curve 1505, the wavelength-independent characteristic generally realized by the WINC 100 is rather deteriorated in the optical coupler 400 having the configuration of the parameter 1 of the present invention, and the optical coupling factor R is slightly wavelength. There is a dependency. As can be seen from the comparison between the curve 1501 and the curve 1504, the cause of this deterioration is that the wavelength characteristic of the phase difference providing unit 402 has changed from the wavelength characteristic of the phase difference providing unit 102 of the original WINC 100. As described above, the design parameters of the phase difference imparting unit 402 are designed in conformity with specific characteristics of individual directional couplers. The wavelength dependence is also changed by changing the parameters of the directional coupler. Therefore, in order to obtain the desired wavelength dependency of the optical coupling ratio R with the optical coupler of the present invention, it is necessary to optimize not only the phase difference providing unit but also the design parameters of the directional coupler. .

Specifically, each parameter was optimized as follows. The length (coupling length) L _DC1 of the waveguide adjacent to the directional coupler 401 is about 165 μm, and the coupling rate κ ₁ is 25.0% at a wavelength of 1550 nm. The coupling length L _DC2 of the directional coupler 403 was about 528 μm, and the coupling rate κ ₂ was 80.3% at a wavelength of 1550 nm. The configuration of the phase difference imparting unit 402 is a configuration in which a thick waveguide is provided only in the upper arm waveguide 404, the normal waveguide width is W = 7 μm, the waveguide width of the thick waveguide portion is W _B = 10 μm, The waveguide length of the wide waveguide portion was set to L _B = 189 μm, and the waveguide length difference between the two arm waveguides was set to ΔL _MZI = 0.569 μm.

Optimization of the wavelength dependence of this embodiment, a normal waveguide width W, the waveguide width W _B of Futoshihabashirube waveguide portion is set to a given _{value, L DC1, L DC2, L} B, parameters for determining the [Delta] L _MZI As an optimization. The square of the difference between the value R and the target optical coupling ratio R _t of the optical coupling ratio obtained from equation (4), integrated at each wavelength lambda (1.49～1.65Myuemu), further amounts of fluctuation of the core width In order to minimize the sum (formula (10-2) below) taken at Δw (three points of Δw = −0.2, 0, +0.2 μm in the following embodiment), L _DC1 , L _DC2 , L _B , and ΔL _MZI are obtained as solutions.

In the calculation examples in the following embodiments, the solution is obtained by the same method.

  FIG. 16 is a diagram showing a variation of the optical coupling ratio R with respect to a change in wavelength λ in the optical coupler 400 by the wavelength-independent optimized parameter. The horizontal axis indicates the wavelength λ, and the vertical axis indicates the optical coupling ratio R (%) of the entire optical coupler. A curve 1601 is a change in the optical coupling rate R caused only by the wavelength dependency of the phase difference providing unit 402 by the wavelength-independent optimized parameter. Curve 1602 shows the change in optical coupling rate R due solely to the wavelength dependence of the directional coupler due to the wavelength independent optimized parameters. A curve 1603 shows a change in the optical coupling rate R when the wavelength dependency of both the phase difference providing unit 402 and the two directional couplers due to the wavelength-independent optimized parameter is simultaneously affected. As can be seen from FIG. 16, the design parameter of the directional coupler is also optimized to change the optical coupling ratio R caused by the directional coupler, that is, the first term on the right side of the equation (10). And the sum of the second term and the change in the optical coupling ratio R due to the phase difference imparting unit, that is, the third term on the right side of the equation (10) cancel each other, so that dR / dλ≈0 and the wavelength-independent characteristic is obtained. can get.

  FIG. 17 is a diagram illustrating a variation of the optical coupling ratio R in the entire MZI when the waveguide core width w in the optical coupler 400 varies according to the wavelength-independent optimized parameter. In each of FIGS. 17A and 17B, the horizontal axis represents the core width variation Δw (μm), the vertical axis represents the optical coupling ratio R (%) of the entire optical coupler, FIG. 17A shows the wavelength of 1500 nm, and FIG. In the case of (1), (c) shows the case of 1600 nm, respectively. At any wavelength, changes 1701a, 1701b, 1701c of the optical coupling rate R due to the characteristic variation of the phase difference imparting unit 402, and changes 1702a of the optical coupling rate R due to the characteristic variation of the directional couplers 401, 403, 1702b and 1702c cancel each other. As a result, the slope dR / dw of the optical coupling factor R when the wavelength dependency of both the phase difference providing unit and the two directional couplers is simultaneously affected is dR / dw≈0, and the waveguide core width It can be confirmed that an optical coupler having a large manufacturing tolerance is realized by suppressing the fluctuation of the optical coupling rate R with respect to the processing deviation of w.

  It should be noted that the fact that the effective refractive index n is dependent on the waveguide core width w and the dn / dw is dependent on the core width as shown in FIG. 11 is not limited to the quartz waveguide. Even in waveguides made of other materials such as silicon (Si) waveguide, indium phosphide (InP) waveguide, and polymer waveguide, the technique of the present invention is used to provide high-bandwidth operability and high tolerance characteristics against processing errors. A satisfactory broadband waveguide optical coupler is provided. Hereinafter, more specific examples will be described.

[50% optical coupler]
As a first embodiment of the optical coupler of the present invention, a high processing tolerance broadband waveguide optical coupler having a coupling rate of 50% will be described in detail. In the design of an actual high processing tolerance broadband waveguide type optical coupler, a tapered waveguide is used when waveguide portions having different waveguide widths are formed on an arm waveguide. As shown in FIG. 4, the waveguide width is gradually converted using a tapered waveguide in order to reduce the loss generated at the connection portion between the arm waveguides 404 and 405 having a normal waveguide width and the waveguide portion. To do. In addition, in order to shorten the length of the arm waveguides 404 and 405, the tapered waveguide is not disposed only in the straight portion of the arm waveguide, but is a curved waveguide portion of the developed portion of the directional couplers 401 and 403. Place it so that it will also be applied. In general, at both ends of a directional coupler, a curved waveguide is used for coupling with separate optical fibers and optical circuits, and the waveguides are gradually separated so that a physical port interval can be obtained. . In the developed part of the directional coupler, the waveguide path is drawn by using a curved waveguide to create an S-shaped waveguide pattern, and the distance between the two waveguides is changed. A part of this curved waveguide part is a tapered waveguide. Thus, the waveguide is a curved waveguide and the waveguide width is gradually changed at the same time. Therefore, in the optical coupler of the present invention, the tapered waveguide is a curved waveguide in a developed portion where the distance of the waveguide increases at the end of the first directional coupler or the second directional coupler. A portion of the portion can be configured as a tapered waveguide.

  Further, in the optical coupler of the present embodiment, since the width of the change of the waveguide width is not so large as described later, the waveguide width is a wide waveguide having a constant waveguide width and a predetermined length as a “waveguide portion”. Instead of the narrow waveguide, only the tapered waveguide is used to obtain the effect of changing the width by the wide waveguide or the narrow waveguide.

FIG. 24 is a diagram showing an example of the shape of a waveguide portion constituted only by a tapered waveguide in the phase difference providing portion of the optical coupler of the present invention. FIG. 24A shows an example in which a waveguide portion corresponding to a wide waveguide is formed only by a tapered waveguide. As shown in (a) of FIG. 24, after began to thicker from a portion 2401 of normal waveguide width gradually W _TB a waveguide width at the first tapered waveguide 2402, a portion of constant width Instead, the taper width is gradually narrowed by the second taper waveguide 2403 immediately and returned to the original normal waveguide width. Hereinafter, the configuration shown in FIG. 24A will be referred to as a thick taper waveguide. FIG. 24B shows an example in which a waveguide portion corresponding to a narrow waveguide is formed only by a tapered waveguide. Similarly to (a) of FIG. 24, after the waveguide width is gradually reduced to W _TN from the normal waveguide width portion 2404 to the first tapered waveguide 2405, the constant width portion is not interposed. In addition, the taper width is gradually increased gradually by the second tapered waveguide 2406 to return to the original normal waveguide width. Hereinafter, the configuration of FIG. 24B is referred to as a narrow taper waveguide.

In this example, the design was performed in consideration of wavelength dependency so that the coupling rate was 50% in the wavelength range of 1.5 to 1.6 μm. Specifically, the coupling length L _DC1 of the directional coupler 401 is designed to be about 161 μm, the coupling rate κ _{1 ′} is 24.5% at a wavelength of 1550 nm, and the coupling length L _DC2 of the directional coupler 403 is designed. was approximately 524μm, were designed to bind factor kappa _2'is 79.9% at a wavelength of 1550 nm. The phase difference imparting unit 402 is configured to provide a thick taper waveguide only in the upper arm waveguide 404. The normal waveguide width is W = 7 μm, the waveguide width W _TB of the widest portion of the wide tapered waveguide is 9.35 μm, and the wide tapered waveguide length L _TB is The difference ΔL _MZI of the waveguide length of the upper arm waveguide 404 with respect to the waveguide length of the lower arm waveguide 405 of 440 μm was set to 0.579 μm.

  FIG. 18 is a diagram showing the wavelength dependence of the optical coupling rate in the optical coupler with a 50% optical coupling rate designed in Example 1 of the present invention. The horizontal axis represents wavelength (μm), and the vertical axis represents optical coupling rate R (%). Here, the wavelength dependence and the waveguide core width fluctuation dependence of the optical coupling rate of the directional coupler as the element circuit are the same as the characteristics of the directional coupler shown in FIG. 2 and FIG. did. Further, it is assumed that the wavelength dependency and the waveguide core width dependency of the effective refractive index of the waveguide are the same as the characteristics of the waveguide shown in FIG. FIG. 18 shows a curve 1801 when the waveguide width in the optical coupler is a design value, a curve 1802 when the design value is +0.2 μm, and a curve 1803 when the design value is −0.2 μm. ing. Even if the core width deviates from the designed value by ± 0.2 μm as shown by curves 1802 and 1803, there is almost no fluctuation in the optical coupling ratio R over a wide wavelength range of 100 nm or more, and tolerance to the change Δw in the waveguide width. An optical coupling rate R of 50% is obtained.

[20% optical coupler, 70% optical coupler]
Next, as a second embodiment of the optical coupler of the present invention, a high processing tolerance broadband waveguide type optical coupler having a coupling rate of 20% will be described in detail. The basic configuration is the same as that of the optical coupler of the first embodiment. As shown in FIG. 4, the phase difference providing unit 402 has a configuration in which a thick taper waveguide is provided only in the upper arm waveguide 404. . In this embodiment, the coupling length L _DC1 of the directional coupler 401 is set to about 131 μm, the coupling rate κ _{1 ′} is designed to be 20.4% at a wavelength of 1550 nm, and the coupling length L _DC2 of the directional coupler 403 is to about 333μm, were designed to bind factor kappa _2'is 51.0% of the design at the wavelength 1550 nm. The phase difference imparting section 402 has a normal waveguide width of W = 7 μm, a waveguide width W _TB of the thickest waveguide width portion at the center of the thick taper waveguide is 8.55 μm, and a wide taper guide. The waveguide length L _TB is 440 μm, and the difference ΔL _MZI of the waveguide length of the upper arm waveguide 404 with respect to the waveguide length of the lower arm waveguide 405 is 0.536 μm.

  FIG. 19 is a diagram showing the wavelength dependence of the optical coupling rate of the high processing tolerance broadband waveguide type optical coupler of Example 2 designed for 20% optical coupling rate. The horizontal axis represents wavelength (μm), and the vertical axis represents optical coupling rate R (%). FIG. 19 shows a curve 1901 when the waveguide width in the optical coupler is a design value, a curve 1902 when the design value is +0.2 μm, and a curve 1903 when the design value is −0.2 μm. ing. Even if the core width deviates from the design value by ± 0.2 μm as shown by curves 1902 and 1903, there is almost no variation in the coupling ratio over a wide wavelength range of 100 nm or more, and the tolerance for the change Δw in the waveguide width is large. A stable optical coupling rate R of 20% is obtained.

Similarly, as another example of the second embodiment, a high processing tolerance broadband waveguide type optical coupler having an optical coupling ratio of 70% is as follows. In another embodiment 2, the directional coupler 401 is designed so that the coupling length L _DC1 is about 137 μm and the coupling rate κ _{1 ′} is 21.2% at a wavelength of 1550 nm. the coupling length L _DC2 and about 652Myuemu, binding rate kappa _2'is designed to be 93.5% at a wavelength of 1550 nm. The phase difference imparting unit 402 has a normal waveguide width of W = 7 μm, a waveguide width W _TB of the thickest waveguide width portion at the center of the thick taper waveguide is 9.30 μm, and a wide taper guide. The waveguide length L _TB is 440 μm, and the difference ΔL _MZI of the waveguide length of the upper arm waveguide 404 with respect to the waveguide length of the lower arm waveguide 405 is 0.595 μm.

  FIG. 20 is a graph showing the wavelength dependence of the optical coupling rate of the high processing tolerance broadband waveguide type optical coupler of Example 2 designed for 70% optical coupling rate. The horizontal axis represents wavelength (μm), and the vertical axis represents optical coupling rate R (%). Similarly to the case of FIG. 19, a curve 2001 when the waveguide width in the optical coupler is a design value, a curve 2002 when the design value is +0.2 μm, and a curve 2003 when the design value is −0.2 μm. Each is shown. Even if the core width deviates by ± 0.2 μm from the design value as shown by curves 2002 and 2003, there is almost no variation in the coupling ratio over a wide wavelength range of 100 nm or more, and the tolerance for the change Δw in the waveguide width is large. A stable optical coupling rate R of 70% is obtained.

  As in the above-described embodiments, the optical coupler of the present invention can suppress the variation of the optical coupling rate R with respect to the processing deviation of the waveguide core width w even in the design of various optical coupling rates R. Can be obtained.

[Wavelength-dependent 50% optical coupler]
In the above-described first and second embodiments, the broadband optical coupler having a constant optical coupling ratio in a predetermined wavelength range and suppressing the wavelength dependency has been described. However, the scope of application of the present invention is not limited to optical couplers having a constant optical coupling ratio, and the present invention is also applicable to optical couplers having optical coupling characteristics that vary depending on the wavelength and have wavelength dependency. Can do. For example, a normal directional coupler has a characteristic that the optical coupling rate R increases as the wavelength becomes longer as shown in FIG. 2, but it is opposite to FIG. 2 as described below. It is also possible to construct an optical coupler having a wavelength dependence characteristic that falls to the right.

  The optical coupler of Example 3 of the present invention is an optical coupler having a wavelength dependency in which the optical coupling rate is 53% at a wavelength of 1500 nm, 48% at a wavelength of 1600 nm, and the optical coupling rate is decreased by 1% every 20 nm. The basic configuration is the same as that of the first embodiment, and the phase difference providing unit 402 has a configuration in which a thick taper waveguide is provided only on the upper arm waveguide 404.

In this embodiment, the coupling length L _DC1 of the directional coupler 401 is set to about 206 μm, the coupling rate κ _{1 ′} is designed to be 31.0% at a wavelength of 1550 nm, and the coupling length L _DC2 of the directional coupler 403 is designed. was approximately 549μm, were designed to bind factor kappa _2'is 83.0% at a wavelength of 1550 nm. The phase difference imparting section 402 has a normal waveguide width of W = 7 μm, a waveguide width W _TB of the thickest waveguide width portion at the center of the thick taper waveguide is 7.88 μm, and a wide taper guide. The waveguide length L _TB is 440 μm, and the difference ΔL _MZI of the waveguide length of the upper arm waveguide 404 with respect to the waveguide length of the lower arm waveguide 405 is 0.669 μm.

  FIG. 21 is a diagram showing the wavelength dependence of the optical coupling rate of the optical coupler of wavelength dependent design according to the third embodiment of the present invention. The horizontal axis represents wavelength (μm), and the vertical axis represents optical coupling rate R (%). FIG. 21 shows a curve 2101 when the waveguide width in the optical coupler is a design value, a curve 2102 when the design value is +0.2 μm, and a curve 2103 when the design value is −0.2 μm. ing. Like curves 2101 to 2103, the optical coupling rate is linear and has a negative slope over a wavelength range of 100 nm or more, and an optical coupling rate having a specific wavelength dependency can be realized.

  Finally, a high processing tolerance broadband waveguide type optical coupler having a 50% optical coupling ratio design of the configuration of the first embodiment of the present invention and WINC, which is a broadband waveguide type optical coupler having a configuration of the prior art for comparison, are quartz-based. The result produced using planar lightwave circuit (quartz PLC) technology is shown.

  The optical coupler of the present invention was designed with a minimum bending radius of 2 mm of the waveguide. The circuit length of the optical coupler is about 3.58 mm for the optical coupler of the present invention, and about 3.53 mm for the conventional WINC. The circuit length is hardly increased by the high tolerance configuration of the present invention, and the circuit is compact. Was realized.

  The optical coupler of the present invention was fabricated using a combination of a glass film deposition technique such as a flame deposition (FHD) method and a microfabrication technique such as reactive ion etching (RIE). Specifically, a glass film serving as a lower cladding layer was deposited / transparent on a silicon substrate, and subsequently a core layer having a refractive index slightly higher than that of the cladding layer was deposited. Next, a core pattern to be an optical waveguide circuit was patterned by a microfabrication technique, and a glass film to be an upper clad layer was deposited / transparent to produce a buried optical waveguide.

  In order to confirm the high processing tolerance characteristics of the present invention, a circuit was fabricated with three levels of core widths by changing only the waveguide core width of the same optical coupler. The fine processing of the core pattern is a photo process in which the circuit pattern drawn on the photomask is transferred to the photoresist applied on the upper surface of the core film, and the unnecessary core film is removed using the transferred photoresist pattern as a mask. The etching process is performed in two steps. In this photo process, the waveguide core width is changed by changing the exposure amount when transferring the circuit pattern in three stages, and three levels (# 1, # 2, # 3) with different core widths on the same substrate. A circuit was fabricated. Further, two circuits having the same level of core width are formed on the right side (R) and the left side (L) on the substrate. The high processing tolerance broadband waveguide type optical coupler of the present invention and the prior art WINC were both fabricated on the same substrate.

  FIG. 22 is a diagram showing the wavelength dependence of the optical coupling rate of the optical coupler having the configuration of Example 1 of the present invention that was actually manufactured. FIG. 23 is a diagram showing the wavelength dependence of the optical coupling rate of the actually manufactured conventional winc 100. 22 and 23, the horizontal axis represents wavelength (μm), and the vertical axis represents optical coupling ratio R (%). # 1, # 2, and # 3 show the characteristics of the three-level core width circuit, and the core width in the isolated waveguide is +0.22 μm, +0.04 μm, and −0.13 μm, respectively, compared with the design value. ing. R and L indicate the characteristics of the circuits produced on the right and left sides of the substrate, respectively.

  As can be seen from FIG. 23, in the wavelength range of 1500 to 1600 nm, the optical coupling factor characteristics of the conventional WINC depend on the change in the core width (# 1, # 2, # 3) and the circuit board position (R, L). Shows a variation of about ± 4%. On the other hand, as can be seen from FIG. 22, in the optical coupler of the present invention having the configuration of Example 1, the variation in the optical coupling rate characteristic can be reduced to about ± 1.5%, and the design value of the optical coupling rate of 50% can be obtained. On the other hand, stable optical coupling characteristic is obtained in a wide wavelength band.

  In the above-described design procedure of the optical coupler of the present invention and in the embodiments, the waveguide widths of the two arm waveguides of the phase difference providing unit have been described as having the normal waveguide W. However, the waveguide width W1 of one arm waveguide may be different from the waveguide width W2 of the other arm waveguide (W1 ≠ W2). Further, the entire one of the arm waveguides of the phase difference providing unit may be a “waveguide portion”. Such a configuration example will be described in detail in FIG. 25 and FIG.

  When the widths of the two arm waveguides (W1 ≠ W2) are different, the optical circuit of the present invention is a waveguide-type optical coupler that branches or combines one or more lights, and the one or more lights are The first directional coupler is composed of two arm waveguides including an input first directional coupler, an arm waveguide having a waveguide width W1, and an arm waveguide having a waveguide width W2 different from W1. A phase difference providing unit connected in cascade with a directional coupler, wherein at least one of the two arm waveguides has a waveguide width different from the waveguide width of the one arm waveguide. The present invention can be applied to an optical coupler that includes a phase difference providing unit including a second directional coupler further cascade-connected to the phase difference providing unit. At this time, the waveguide width and the length of the waveguide portion so that the ratio of the variation δφ of the phase difference φ of the phase difference providing unit to the waveguide width variation δw, that is, the differential dφ / dw satisfies the formula (1) at the operating wavelength. In addition, the waveguide length difference between the two arm waveguides may be set.

  The position of the “waveguide portion” on the arm waveguide is basically unrelated to the position on the arm waveguide. Whether it is arranged close to the first directional coupler side, arranged close to the second directional coupler, or whether the approach is reversed in the upper and lower arm waveguides, the fabrication of the present invention There is no change in the effect of high tolerance of optical coupling rate characteristics and wide band operation with respect to processing deviation. Further, the “waveguide portion” may be divided into two or more on one arm waveguide. The “waveguide portion” may include a tapered waveguide as a part in addition to a portion having a constant waveguide width. Furthermore, as shown in FIG. 24, a configuration in which a portion where the waveguide width gradually increases and a portion where the waveguide width gradually decreases are continuously connected without having a constant waveguide width portion may be employed. Furthermore, in the above description, only the case where there are two arm waveguides in the phase difference imparting portion is mentioned, but the same procedure can be applied to three or more configurations.

  Finally, various variations of the “waveguide portion” in the optical circuit of the present invention will be described. In the optical circuit of the present invention, the width of the “waveguide portion” may be different from the waveguide width of the arm waveguide opposite to the arm waveguide where the “waveguide portion” is installed. In addition to the optical circuit of the present invention shown in FIG. 4, generally, a gentle curve is used for bending the waveguide in order to suppress unnecessary loss and reflection unless there is a special purpose. In principle, the waveguide width is changed continuously and smoothly to form a waveguide pattern of an optical circuit for realizing a desired function. Therefore, for example, the boundary between the directional coupler 401 and the phase difference providing unit 402 in FIG. 4 is not clearly expressed on the drawing.

  In the directional coupler, there are adjacent portions of the upper and lower waveguides, and the upper and lower waveguides have a symmetrical structure up to a certain position while developing from the adjacent portions. Accordingly, the limit range in which the width of the upper and lower waveguides and the manner, length, and shape of the width change are vertically symmetrical may be considered as the boundary between the directional coupler and the phase difference providing portion. Within this range, there is no additional phase difference between the vertically symmetric waveguides, so that this range can be regarded as a directional coupler. Therefore, in all the embodiments described above, it can be considered that a region that passes this boundary (a limit range that is vertically symmetric) is included in the phase difference providing portion.

  FIG. 25 is a top view showing a configuration example drawn in a state close to an actual waveguide pattern of the optical circuit of the present invention. In the optical circuit 2500 in FIG. 25A, a directional coupler 2501, a phase difference providing unit 2502, and a directional coupler 2503 are cascaded in this order. Here, it should be noted that the coupling portion of the two directional couplers is configured to be narrower than the normal waveguide width W. The boundary between the three elements is indicated by a dotted line. It should be noted that the upper and lower waveguides have a symmetrical structure in the dotted line regions of the directional couplers 2501 and 2503.

In the phase difference providing unit 2502 of the example of (a) 25, with the exception of the tapered waveguide portion at both ends, the substantially entire width W _B of the upper arm waveguides of the phase difference providing section 2502 the wide waveguide ( "Waveguide part"). On the other hand, the lower arm waveguide has a normal waveguide width W. Therefore, the waveguide width of at least a part of one arm waveguide (substantially all upper arm waveguides) is different from the waveguide width of the other arm waveguide (lower arm waveguide portion). become.

  In the optical circuit 2510 in FIG. 25B, a directional coupler 2511, a phase difference providing unit 2512, and a directional coupler 2513 are cascaded in this order. Again, the coupling portions of the two directional couplers 2511 and 2513 are configured to be narrower than the normal waveguide width W. The boundary between the three elements is indicated by a dotted line. It should be noted that the upper and lower waveguides have a symmetric structure within the dotted region of the directional couplers 2511 and 2513.

Example In the phase difference providing unit 2512 of (b) in FIG. 25, except for the tapered waveguide portion at both ends, the substantially entire width W _B of the upper arm waveguides of the phase difference providing section 2512 the wide waveguide ( "Waveguide part"). On the other hand, almost the entire lower arm waveguide is a narrow waveguide having a width W _N (“waveguide portion”). Therefore, even in the case of FIG. 25B, the waveguide width of at least a part of one arm waveguide (substantially all the upper arm waveguide portions) is equal to that of the other arm waveguide (lower arm waveguide). This is different from the waveguide width.

FIG. 26 is a top view showing still another configuration example drawn in a state close to an actual waveguide pattern of the optical circuit of the present invention. In the configuration example of FIG. 26, the waveguide in the directional coupler is configured with a normal waveguide width W. FIG. 26A shows an optical circuit 2600 in which a directional coupler 2601, a phase difference providing unit 2602, and a directional coupler 2603 are cascaded in this order. The boundary between the three elements is indicated by a dotted line. It should be noted that the upper and lower waveguides also have a symmetrical structure in the dotted line region of the directional couplers 2601 and 2603. In the phase difference providing unit 2602 of the example of (a) 26, the entire upper arm waveguides is in the wide waveguide width W _B ( "waveguide portion"). On the other hand, the entire lower arm waveguide is a narrow waveguide having a width W _N (“waveguide portion”). In FIG. 26 (a), there is no tapered waveguide so that the difference in waveguide width can be easily seen. Even in this case, the waveguide width of at least a part of one arm waveguide (all upper arm waveguide portions) is different from the waveguide width of the other arm waveguide (lower arm waveguide). It will be.

In FIG. 26B, the optical circuit 2600 includes a directional coupler 2611, a phase difference providing unit 2612, and a directional coupler 2613 that are cascaded in this order. (A) Similarly, the boundary of three elements is shown by the dotted line. It should be noted that the upper and lower waveguides have a symmetric structure in the dotted line region of the directional couplers 2611 and 2613. Example In the phase difference providing unit 2612 of FIG. 26 (b), has become a part of the upper arm waveguides the thick waveguide width W _B ( "waveguide portion"), the remaining normal guide The waveguide width is W. On the other hand, a part of the lower arm waveguide is a narrow waveguide having a width W _N (“waveguide portion”), and the rest is a normal waveguide width W. FIG. 26B also shows a state without a tapered waveguide so that the difference in waveguide width can be easily seen. Even in this case, the waveguide width of at least a part of one arm waveguide (a part of the upper arm waveguide part) is different from the waveguide width of the other arm waveguide (lower arm waveguide). Will be.

  As described in various variations of the actual “waveguide portion” in FIGS. 25 and 26 described above, in the optical circuit of the present invention, one arm of the two arm waveguides of the phase difference providing portion is provided. It includes a waveguide portion in which the width of at least a part of the waveguide is different from the width of the other arm waveguide. Here, the “at least part” of one arm waveguide may include the entire one arm waveguide.

  As described above, by the optical coupler of the present invention, the variation in the optical coupling ratio can be sufficiently suppressed with respect to the processing deviation of the waveguide core width, and an optical coupler having a large manufacturing tolerance can be realized with a compact circuit size. .

  The present invention is generally applicable to communication systems. In particular, it can be used in an optical circuit of an optical communication system.

100, 400, 2500, 2510, 2600, 2610 Optical couplers 101, 103, 401, 403, 2501, 2503, 2511, 2513, 2601, 2603, 2611, 2613 Directional couplers 102, 402, 2502, 2512, 2602, 2612 Phase difference imparting section 104, 105, 404, 405, 2401, 2404 Arm waveguide 406 Waveguide portion (thick waveguide, narrow waveguide)
2402, 2403, 2405, 2406 Tapered waveguide

Claims (8)

In a waveguide type optical coupler for branching or coupling one or more lights,
A first directional coupler into which the one or more lights are input;
A phase difference providing unit comprising two arm waveguides and cascade-connected to the first directional coupler, wherein at least a part of one arm waveguide of the two arm waveguides has a width A phase difference providing unit including a waveguide portion different from the width of the other arm waveguide;
A second directional coupler connected in cascade to the phase difference providing unit,
R is an optical coupling rate of the entire optical coupler, and the optical coupling rate κ ₁ of the first directional coupler is expressed as an angle by κ ₁ = (sin (θ ₁ )) ² by a coupling phase angle θ ₁ , When the optical coupling factor κ ₂ of the second directional coupler is expressed as an angle κ ₂ = (sin (θ ₂ )) ² by the coupling phase angle θ ₂ , the phase difference providing unit for the waveguide width variation δw The ratio dφ / dw, which is the ratio of the fluctuation δφ of the phase difference φ, is given at a predetermined operating wavelength.

A waveguide characterized in that a length of the waveguide portion, a waveguide width of the waveguide portion, and a waveguide length difference between the two arm waveguides are set so as to satisfy the relationship Type optical coupler. In a waveguide type optical coupler for branching or coupling one or more lights,
A first directional coupler into which the one or more lights are input;
A phase difference providing unit comprising two arm waveguides having the same waveguide width W and cascade-connected to the first directional coupler, wherein at least one of the two arm waveguides A phase difference providing unit including a waveguide portion having a waveguide width different from the same width W;
A second directional coupler connected in cascade to the phase difference providing unit,
R is an optical coupling rate of the entire optical coupler, and the optical coupling rate κ ₁ of the first directional coupler is expressed as an angle by κ ₁ = (sin (θ ₁ )) ² by a coupling phase angle θ ₁ , When the optical coupling factor κ ₂ of the second directional coupler is expressed as an angle κ ₂ = (sin (θ ₂ )) ² by the coupling phase angle θ ₂ , the phase difference providing unit for the waveguide width variation δw The ratio dφ / dw, which is the ratio of the fluctuation δφ of the phase difference φ, is given at a predetermined operating wavelength.

A waveguide characterized in that a length of the waveguide portion, a waveguide width of the waveguide portion, and a waveguide length difference between the two arm waveguides are set so as to satisfy the relationship Type optical coupler. At multiple wavelengths in a given operating wavelength range,

The length of the waveguide portion, the waveguide width of the waveguide portion, and the waveguide length difference between the two arm waveguides are set so that the sum of the squares of The waveguide type optical coupler according to claim 1 or 2. Differentiation of optical coupling rate R with respect to wavelength λ at one or more wavelengths in the predetermined operating wavelength range

The length of the coupling of the directional coupler, the length of the waveguide portion, the waveguide width of the waveguide portion, and the distance between the two arm waveguides so that the sum of the squares of The waveguide type optical coupler according to any one of claims 1 to 3, wherein a difference in waveguide length is set. Said waveguide portion, the wide waveguide having a width L _B than the same waveguide width W, or a narrow waveguide having a width L _N than the same waveguide width W The waveguide type optical coupler according to claim 2 . 3. The waveguide according to claim 2, wherein the waveguide portion is continuously connected to the portion having the same waveguide width W of the two arm waveguides via a tapered waveguide. Type optical coupler.   7. The waveguide type optical coupler according to claim 1, wherein the waveguide portion is a tapered waveguide.   In the tapered waveguide, at the end of the first directional coupler or the second directional coupler, a part of the curved waveguide portion of the development portion where the distance of the waveguide increases is a tapered waveguide. 8. The waveguide type optical coupler according to claim 6, wherein the waveguide type optical coupler is configured.

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