The present invention relates to an optical phase shifter using an optical waveguide and operating in a wide wavelength range, and an optical switch and a 90 degrees optical hybrid using the optical phase shifter.
The optical switch is one of important optical components for switching a communication path in an optical communication system in which flexible operation is required in accordance with a communication demand. There are various means for realizing the optical switch. A waveguide optical type optical switch using a quartz-based optical waveguide formed on a substrate is characterized in that it is smaller in size, superior in integration, and high in long-term stability and reliability as compared with a free-space optical type optical switch for assembling bulk type optical components such as lenses and mirrors. The waveguide optical type optical switch is widely used in a practical system such as an optical communication device (see, for example, NPL 1). For example, as a large-scale optical switch using the quartz-based optical waveguide, an N×N matrix switch in which a large number of Mach-Zehnder interference type switch elements with variable phase shifters are integrated has been put into practical use (see, for example, NPL 2).
Therefore, the double gate type switch element 20 propagates the light to the cross path (input line waveguide 23a->input line waveguide 23b and output line waveguide 24a->output line waveguide 24b) in the OFF state, in the ON state, the light is propagated to the bar path (input line waveguide 23a->output line waveguide 24b). Here, a path propagating from the output line waveguide 24a to the input line waveguide 23b cannot be made. In the 4×4 matrix switch 10 shown in
In this way, the double gate type switch element 20 cuts off the propagation light from the input line waveguide 23a to the output line waveguide 24b by two switch elements of the first switch element 21 and the second switch element 22 at the time of OFF. Thus, the characteristics of extremely small crosstalk and high extinction ratio, i.e., the ratio of the transmittance at the time of on to the transmittance at the time of OFF, can be obtained. This double gate type switch element 20 is applied to the switch element 13 of the 4×4 matrix switch 10 shown in
The waveguide length LA of the interference arm 35 is longer than the waveguide length LB of the interference arm 34 by ΔL (=LA-LB). A so-called optical path length difference n·ΔL obtained by multiplying ΔL by the equivalent refractive index π of the waveguide is set to a half wavelength of an operation wavelength λ0.
Transmittance Tcross of cross paths (input port 31a->output port 32b and input port 32a->output port 31b) of this switch element, and transmittance Tbar of bar paths (input port 31a->output port 31b and input port 32a->output port 32b) is expressed by the following expression by a known interference principle.
Here, κ is the optical coupling rate of the optical couplers 33a and 33b, and Δφ is the inter-arm phase difference of the Mach-Zehnder interferometer, that is, the difference between the phase of the light propagating through the interference arm 34 and the phase of the light propagating through the interference arm 35. Since the optical path length of the interference arm 34 is n·LB and the optical path length of the interference arm 35 is n·LA, when the waveguide length difference is ΔL, the inter-arm phase difference Δφ is expressed by the following expression.
Therefore, at the operation wavelength λ0, Δφ=π is satisfied, so that Tcross=0 is always obtained regardless of the value of K. The value of K may be deviated due to the deviation of the manufacturing process, but in such a case, the switch element 30 can have a high extinction ratio in the cross path. Note that, in the ON state, the equivalent refractive index π is increased by the thermooptic phase shifter 36 to n′ in the following expression.
Thus, since the optical path length of the interference arm 34 is n′·LB, that is, from Expression 5, n·LA is established, the difference from the optical path length n·LA of the interference arm 35 becomes zero, and the inter-arm phase difference Δφ=0 is satisfied. Therefore, when κ=0.5 is satisfied, Tcross=1 is established, and the path is switched. The switch element 30 is used for the first switch element 21 and the second switch element 22 of the double gate type switch element 20 shown in
As an attempt to widen a wavelength range in which the high extinction ratio can be obtained, the configurations disclosed in PTL 1 to 3 are known. In the configuration described in PTL 1, in the double gate type switch element 20, the waveguide wavelength λ0 of the first switch element 21 is set to λ0A, and the operation wavelength λ0 of the second switch element 22 is set to λ0B. By shifting the operation wavelength (λ0A≠λ0B), the wavelength region in which the high extinction ratio can be obtained is shared by the first switch element 21 and the second switch element 22, thereby widening the band.
Specifically, when the normal waveguide width is W0, in the interference arm 45, a part of the waveguide is a wide waveguide 47 having a waveguide width Wbold (>W0) over the length Lbold. Although a tapered waveguide is formed at the connecting portion between the normal waveguide portion and the wide waveguide 47 so as not to cause an increase in loss, the tapered waveguide connection is ignored in the following characteristic calculation for simplifying the description.
In such a configuration, the inter-arm phase difference Δφ is expressed by the following expression.
Here, π (W) represents an equivalent refractive index of the waveguide in the waveguide width W. In addition, Δπ is a difference in equivalent refractive index due to a difference in waveguide width W.
As can be seen from
In the configuration described in PTL 2, a phase generating optical coupler (PGC) having a special configuration is used as an optical coupler used in a Mach-Zehnder interferometer. In a normal optical coupler, when light inputted from one input port is outputted to two output ports, the phase difference is fixed to π/2. On the other hand, in the phase generating optical coupler, wavelength dependence can be given to the phase difference by an appropriate design. Thus, the inter-arm phase difference Δφ in the OFF state is set to π in a wide wavelength range.
However, the means for widening the wavelength band in which a high extinction ratio can be obtained, described in the above-mentioned PTL, has the following problems. The means described in PTL 1 shares a wavelength range in which a high extinction ratio can be obtained in each switch element. Since the method is not a method for widening the wavelength region of the single switch element, the wavelength region where a high extinction ratio is obtained is kept at about 140 nm in the double gate type switch element 20 using two switch elements. In order to further widen the band, there is a problem that it is necessary to increase the number of gates.
The means described in PTL 2 has a problem that the inter-arm phase difference Δφ at the time of OFF is not strictly π but slightly wavelength dependence remains. Further, as a more serious problem, there is a problem that the manufacturing tolerance is small and the extinction ratio characteristic is greatly deteriorated due to a manufacturing error. In the former case, the wavelength dependence Δn(λ) of the difference in the equivalent refractive index at the different waveguide widths shown in
Here, a and b are coefficients determined by widths W1 and W2 of the two waveguides to be compared, in the example of
The deterioration of the extinction ratio characteristic due to the manufacturing error of the latter is caused by a difference in the amount of change in the equivalent refractive index π in the case where the waveguide is deviated when the waveguide width W is different from each other. As can be seen from
That is, the differential value dΔπ/dW≠0 is established at the difference Δπ between the equivalent refractive indices described in Expression 7. As shown in
In the means described in PTL 3, a phase generating optical coupler having a special configuration is used as the optical coupler, so that the configuration is complicated. Further, similar to the means described in PTL 2, there is a problem that the phase difference of the output light in the phase generating optical coupler varies depending on the manufacturing error, that is, the extinction ratio characteristic may deteriorate.
An object of the present invention is to provide an optical phase shifter in which a phase shift amount is kept constant over a wide wavelength range. Especially, the optical phase shifter is provided in which the amount of phase shift does not deviate and operates over a wide-band even if a waveguide width deviates due to the manufacturing error, that is, having a large manufacturing tolerance. Further, the optical switch is provided which uses the optical phase shifter, operates over the wide-band, and has a high extinction characteristic in a wide wavelength range.
In order to achieve such an object, an embodiment of the present invention is an optical phase shifter constituting of two waveguides of a basic width and having a phase difference between two lights propagating through each waveguide and is characterized in that the optical phase shifter is arranged in at least one of the two waveguides, includes a different type waveguide having a waveguide width different from the basic width and the configuration of the different type waveguide, and the parameters of the two waveguides and the different type waveguide are optimized.
Hereinafter, embodiments and examples of the present invention will be described in detail with reference to the accompanying drawings. In the present embodiment and example, an example using a quartz-based optical waveguide is shown, but the present invention does not specify the material of the waveguide. The present invention can be applied not only to a quartz optical waveguide but also to other material waveguides such as a silicon (Si) waveguide, an indium phosphorus (InP) waveguide and a polymer waveguide. Further, as a specific design example of a waveguide, a waveguide having a specific refractive index difference Δ of 2%, a core height of 3.4 μm, and a reference waveguide width of 5 μm will be described. The present invention is not limited to the basic parameters of these waveguides, and the same concept can be applied to other parameters.
A waveguide length LA of the interference arm 55 differs from a waveguide length LB of the interference arm 54 by a difference ΔL (=LA−LB). In addition, a waveguide width of the interference arm 54 and a waveguide width of the interference arm 55 are partially different. Specifically, when a reference waveguide width of a normal waveguide is W0 (basic width), a part of the waveguide is a wide waveguide 57 having a waveguide width Wbold (>W0) over a length Lbold in the interference arm 55. The waveguide of the interference arm 54 is constituted with a constant width W0. Although a tapered waveguide is formed at a connecting portion between the normal waveguide portion and the wide waveguide 57 so as not to cause an increase in loss, the tapered waveguide connection is ignored in the following characteristic calculation for simplifying the description.
Note that the difference between the waveguide width of the interference arm 54 and the waveguide width of the interference arm 55 is a relative difference, and the waveguide width of the interference arm 55 may be a normal waveguide width, and a narrow waveguide having a waveguide width narrower than the basic width W0 may be provided in a part of the interference arm 54. In addition, a narrow waveguide may be provided in a part of the interference arm 54, and a wide waveguide may be provided in the interference arm 55.
In this way, the portions having the different waveguide width is called a different type waveguide. The wide-band switch element 50 is different from the switch element 30 in that the waveguide width of the interference arm 54 and the waveguide width of the interference arm 55 are different from each other, and different from the wide-band switch element 40 in that the waveguide length LA of the interference arm 55 and the waveguide length LB of the interference arm 54 are different from each other.
In the configuration of the embodiment 1, the phase difference Δφ between the two interference arms is expressed by the following expression. When the position of the light propagation direction of the wide waveguide 57 is defined as 1, the following expression is established.
Here, n(W) represents an equivalent refractive index of the waveguide in the waveguide width W. In addition, the difference Δn between the equivalent refractive indices of the normal waveguide and the wide waveguide is expressed by Expression 7.
n(W0) can be expressed by a linear function of the wavelength λ as shown by the following expression when the first-order approximation is performed.
By substituting Expressions 8 and 10 into Expression 9, the following expression is obtained.
Therefore, by selecting ΔL and Lbold satisfying the simultaneous expressions (Expressions 12 and 13), Δφ=π can be satisfied in the first-order approximation.
This means that Expression 14 is minimized in a required wavelength range.
As can be seen from the line of linear approximation in
Thus, in the wide-band switch element 50, the wavelength dependence of the inter-arm phase difference Δφ at the OFF time can be completely eliminated in the first-order approximation manner, and an extremely high extinction ratio can be obtained in a wide wavelength range over 500 nm. In the conventional wide-band switch element 40, only one parameter which can be set is one of the lengths Lbold of the wide waveguide 47. In the wide-band switch element 50 of the embodiment 1, two parameters, i.e., the difference ΔL between the waveguide lengths of the interference arm 55 and the interference arm 54, can be set in addition to the length Lbold of the wide waveguide 57, and the degree of freedom in design is increased.
As can be seen from
A waveguide length LA of the interference arm 65 differs from a waveguide length LB of the interference arm 64 by a difference ΔL (=LA−LB). In addition, a waveguide width of the interference arm 64 and a waveguide width of the interference arm 65 are partially different. Specifically, when two different type waveguides are included and the reference waveguide width of a normal waveguide is W0 (basic width), a part of the waveguide becomes a wide waveguide 67 having a waveguide width Wbold (>W0) over a length Lbold in the interference arm 65. In addition, another part of the waveguide is a narrow waveguide 68 having a waveguide width Wnarrow(>W0) over a length Lnarrow. The waveguide of the interference arm 64 is constituted with a constant width W0. Although a tapered waveguide is formed at a connecting portion between the normal waveguide portion and the wide waveguide 67 and the narrow waveguide 68 so as not to cause an increase in loss, the tapered waveguide connection is ignored in the following characteristic calculation for simplifying the description.
Note that the difference between the widths W0, Wnarrow, and Wbold is a relative difference, and the expression that the basic width W0 is a normal waveguide width is convenient. When the waveguide width W0 of the interference arm 64 is used as a reference, it is sufficient that the waveguide width Wbold of the wide waveguide 67 and the waveguide width Wnarrow of the narrow waveguide 68 which are the different type waveguides satisfies Wnarrow<W0<Wbold. The wide-band switch element 60 is a similar configuration to the switch element 50 of the embodiment 1, but is different in that not only the wide waveguide 67 but also the narrow waveguide 68 are provided in the interference arm 65.
In the configuration of the embodiment 2, the phase difference Δφ between the two interference arms is expressed by the following expression. When the positions of the wide waveguide 67 and the narrow waveguide 68 in the propagation direction of light are defined as 1, the following expression is established.
Here, n(W) represents an equivalent refractive index of the waveguide in the waveguide width W. A difference Δnbold between the equivalent refractive indices of the normal waveguide and the wide waveguide, and a difference Δnnarrow between the equivalent refractive indices of the normal waveguide and the narrow waveguide are used.
Δnbold is the same as Δn shown in Expression 7, and when W0=5 μm and Wbold=6 μm are satisfied, the wavelength dependence of Δnbold is the same as that of
When the wide waveguide 67 and the narrow waveguide 68 are combined, the average equivalent refractive index navg is reflected by the respective lengths as weighting, and is expressed by the following expression.
Therefore, a difference Δnavg between the average equivalent refractive index navg between the wide waveguide 67 and the narrow waveguide 68 with respect to an equivalent refractive index n(W0) of the waveguide having the reference waveguide width W0 is as follows.
A differential value dΔnavg/dW of the waveguide width W of Δnavg is as follows.
Therefore, by selecting an appropriate ratio of Lbold and Lnarrow, dΔnavg/dW=0 can be obtained. For example, when W0=5 μm, Wbold=9 μm, and Wnarrow=3.5 μm are satisfied, the ratio of length in which dΔnavg/dW=0 is established Lbold: Lnarrow is 42:58. Strictly speaking, since Expression 20 has the wavelength dependence, the ratio is calculated by the least square method in the wavelength range of 1.2 to 1.7 μm.
In other words, the ratio that minimizes expression 21 is calculated.
That is, in the expression of Expression 8, b≠0 is clearly established. Therefore, in order to suppress the wavelength dependence of the inter-arm phase difference Δφ at the OFF time to be flat, it is very important to provide the difference ΔL between the waveguide lengths of the interference arm 65 and the interference arm 64. That is, although the wavelength independence of Δφ can be enhanced by providing ΔL in the wide-band switch element 50 of the embodiment 1, the provision of ΔL is essential for wavelength independence of Δφ in the wide-band switch element 60 of the embodiment 2.
Expression 15 can be expressed as Expression 22 by using Expression 19.
Further, Expression 22 can be modified into Expression 23 by using Expressions 8 and 10.
With the same concept as in the embodiment 1, by selecting ΔL, Lbold, and Lnarrow satisfying the simultaneous equations (Expressions 24 and 25), Δφ=π is established in the first-order approximation.
Of course, Lbold and Lnarrow are the ratio of dΔnavg/dW=0 in Expression 20.
In Expression 22, strictly speaking, the value of not only Δnavg but also n(W0) varies when the width deviation Werr occurs. That is, dn/dW≠0 is satisfied, but as describe below normally, ΔL multiplied to π (W0) is extremely small as compared with Lbold+Lnarrow multiplied to Δnavg, so that the variation caused by dn/dW≠0 can be ignored. Of course, the design may be performed in consideration of this. In this case, for the inter-arm phase difference Δφ shown by Equation 15 in the required wavelength range, ΔL, Lbold, and Lnarrow are calculated so that Expressions 26 and 27 are minimum.
As described above, in the wide band switch element 60, even when the finished width of the waveguide is deviated due to the manufacturing error, the wavelength dependence of the inter-arm phase difference Δφ at the time of OFF can be almost eliminated. Therefore, an extremely high extinction ratio can be stably obtained in a wide wavelength range over 500 nm. This is because, in the wide-band switch 60, the interference arm 65 is provided with the wide waveguide 67 and the narrow waveguide 68 having different waveguide widths, whereby the dependence of the inter-arm phase difference Δφ on the finished waveguide width can be greatly suppressed, and the manufacturing tolerance becomes large. In addition, as in the embodiment 1, in addition to the lengths Lbold and Lnarrow of the wide waveguide 67 and the narrow waveguide 68, the difference ΔL between the waveguide lengths of the interference arm 65 and the interference arm 64 can be set, and there is a sufficient degree of freedom in design. Therefore, the wavelength dependence of the inter-arm phase difference Δφ can be almost eliminated.
Note that in the embodiment 2, the interference arm 65 has two types of waveguides, i.e., the wide waveguide 67 and the narrow waveguide 68, as waveguides different from the normal waveguide width, but this is only necessary to have two types of waveguides as a minimum necessary degree of freedom, and three or more types of waveguides may be provided.
A waveguide length LA of the interference arm 75 differs from a waveguide length LB of the interference arm 74 by a difference ΔL (=LA−LB). In addition, a waveguide width of the interference arm 74 and a waveguide width of the interference arm 75 are partially different. Specifically, when the reference waveguide width of a normal waveguide is W0 (basic width), the interference arm 75 includes a different type waveguide, and a part of the waveguide is a first wide waveguide 77 having a waveguide width W1 (≠W0) over the length L1. The interference arm 74 also includes a different type waveguide, and a part of the waveguide is a second wide waveguide 78 having a waveguide width W2 (≠W0 and <W1) over the length L2. Although a tapered waveguide is formed at a connecting portion between the normal waveguide portion and the first wide waveguide 77 and the second waveguide 78 so as not to cause an increase in loss, the tapered waveguide connection is ignored in the following characteristic calculation for simplifying the description. The wide-band switch element 70 is a similar configuration to the switch element 50 of the embodiment 1, but is different in that not only the first wide waveguide 77 is provided in the interference arm 75 but also the second wide waveguide 78 is provided in the interference arm 74.
In the configuration of the embodiment 3, the phase difference Δφ between the two interference arms is as the following expression. When the positions of the first wide waveguide 77 and the second wide waveguide 78 in the light propagation direction are defined as 1, the following expression is established.
Here, n(W) represents an equivalent refractive index of the waveguide in the waveguide width W. A difference Δn1 between equivalent refractive indices of the normal waveguide and the first wide waveguide 77, and a difference Δn2 between equivalent refractive indices of the normal waveguide and the second wide waveguide 78 are used.
using Expression 31,
Expression 28 is written as Expression 32. Further, a differential value dΔndiff/dW of Δndiff with respect to the waveguide width W is as following expression.
As can be seen from the description in the embodiments 1 and 2, when W1≠W2, W1>W0, and W2>W0 are established, in the difference in equivalent refractive index Δn1 and Δn2, the differential values dΔn1/dW and dΔn2/dW with respect to the waveguide width W become negative, respectively. That is, dΔn1/dW<0, and d Δn2/dW<0 are satisfied. By selecting an appropriate ratio of L1 and L2, dΔndiff/dW=0 can be obtained, and Δφ described in Expression 32 can be prevented from varying with respect to width deviation Werr of the finished width of the waveguide. Also, as can be seen from the similarity between Expressions 22 and 32, by selecting appropriate ΔL, L1, and L2, Δφ=π is established in a first-order approximation. Of course, L1 and L22 are the ratio of dΔndiff/dW=0 in Expression 33.
As described above, even when the finished width of the waveguide is deviated due to a manufacturing error, the wavelength dependency of the inter-arm phase difference Δφ at the time of OFF can be almost eliminated in the wide-band switch element 70, similarly to the wide-band switch element 60 of the embodiment 2. Therefore, an extremely high extinction ratio can be stably obtained in a wide wavelength range over 500 nm. This is because, in the wide-band switch element 70 the interference arm 75 is provided with the second wide waveguide 78 in the interference arm 74 in addition to the first wide waveguide 77 having the different waveguide width.
This is because the dependence of the inter-arm phase difference Δφ on the finished waveguide width can be greatly suppressed, and the manufacturing tolerance becomes large. Also, similarly to the embodiment 1, in addition to the lengths L1 and L2 of the first wide waveguide 77 and the second narrow waveguide 78, the difference ΔL between the waveguide lengths of the interference arm 75 and the interference arm 74 can be set, and there is a sufficient degree of freedom in design. Therefore, the wavelength dependence of the inter-arm phase difference Δφ can be almost eliminated.
In this way, even the wide-band switch element 70 can obtain the same performance as that of the wide-band switch element 60 of the embodiment 2, but when the design parameters are viewed, the longer length of L1 and L2 is close to 3 mm. On the other hand, in the wide-band switch element 60, the sum of the lengths of the wide waveguide 67 and the narrow waveguide 68 (Lbold+Lnarrow) is 1 mm or less. Therefore, the wide-band switch element 70 tends to have a larger circuit size than the wide-band switch element 60.
Although the above embodiments have been described as an optical switch, this embodiment is an optical phase shifter for obtaining a constant phase difference over a wide wavelength range in an interference arm in a Mach-Zehnder interferometer constituting the optical switch. In particular, the present embodiment relates to a configuration of the optical phase shifter which is resistant to manufacturing errors such as variations in waveguide width, and is not limited to the optical switch, and can be applied to all optical circuits using the optical phase shifter. That is, the embodiment is the optical phase shifter constituting of two waveguides having the basic width W0, and constituted so that the two lights propagated through the respective waveguides have a phase difference ξ.
In the above-described optical phase shifter, since the configuration has been described in consideration of application to the optical switch, the parameters are optimized so that the phase difference Δφ between the interference arms becomes π. However, depending on the optical circuit, there is a case where the phase difference between the two waveguides is other than π. In this case as well, the present embodiment can be applied. In accordance with the required phase difference Δφ, the configuration of the different type waveguide arranged in at least one of the two waveguides and the parameters of the two waveguides and the different type waveguide should be optimized. Specifically, when the required phase difference is ξ, Expression 12 is replaced with following Expression 34, Expression 24 is replaced with following Expression 35, Expression 27 is replaced with following Expression 36, and the optimization of each parameter should be performed.
Also, in the above-described embodiment, the case where a simple directional coupler is used for the optical coupler is described, but the present invention is not limited thereto. For example, a multimode interferometer (MMI) type optical coupler which is an optical coupler using a multimode waveguide, or a wavelength independent optical coupler (Winc) consisting a plurality of directional couplers described in PTL 4 may be used.
Since the embodiment 2 is suitable as the configuration applied to the wide-band switch element of the three embodiments described above, the following examples will be described based on the configuration of the embodiment 2. Some examples of the optical switch to which the embodiment 2 is applied in an optical circuit using the quartz-based optical waveguide technique are shown. These optical circuits are manufactured on a silicon substrate by using a known combination of a glass film deposition technique such as flame hydrolysis deposition (FHD) method and a fine processing technique such as reactive ion etching (RIE). A waveguide having a specific refractive index difference Δ of 2% is used, and design is performed with a minimum bending radius of 1 mm of the bent waveguide.
In the example 1, N×N matrix switch in which the above-mentioned wide-band switch element is a base and a large number of wide band switch elements are integrated will be described.
The second point is that a thermooptic phase shifter is provided not only on the interference arm 84 but also on the interference arm 85. A thin film heater 86a for the thermooptic phase shifter is loaded on the surface of the clad 92 just above the waveguide 81 constituting the interference arm 84. In addition, a thin film heater 86b for the thermooptic phase shifter is loaded on the surface of the clad 92 just above the waveguide 82 constituting the interference arm 85. Stress is often applied to the waveguide from the thin film heaters 86a and 86b. In order to cancel the influence of this stress on the refractive index of the waveguide, the thin film heater is provided on both the interference arm 84 and the interference arm 85. When the switch is turned ON, the thin film heater 86a is basically driven, but the thin film heater 86b may be driven.
The third point is that the clad on both sides of the waveguide is removed along the optical axis direction of the waveguide on which the thin film heaters 86a and 86b are loaded, and the heat insulating groove 89 is formed. The heat generated by the thin film heater 86a is not diffused to the periphery by the heat insulating groove 89, and the temperature of the waveguide 81 can be effectively raised. In addition, it is possible to suppress the temperature rise of the peripheral waveguide, for example, the waveguide 82. Power consumption of the thermooptic phase shifter can be suppressed, and thermal crosstalk can be suppressed. The same applies to the thin film heater 86b.
A waveguide length LA of the interference arm 85 differs from a waveguide length LB of the interference arm 84 by a difference ΔL (=LA−LB), which is the same as that of the wide-band switch element 60. The waveguide of the interference arm 84 is constituted of a width W0. The interference arm 85 includes two different type waveguides, which are the same as the wide-band switch element 60. A part of the waveguide of the interference arm 85 becomes a wide waveguide 87 having a waveguide width Wbold (>W0) over a length Lbold, and another part of the waveguide becomes a narrow waveguide 88 having a waveguide width Wnarrow(>W0) over a length Lnarrow. In addition, at a connecting portion of the normal waveguide portion and the wide waveguide 87 and the narrow waveguide 88, respectively, in order not to cause an increase in loss, a tapered waveguide having a length TprLbold and a length TprLnarrow is formed. As specific design values, W0=5 μm, Wbold=9 μm, Wnarrow=3.5 μm are set, and Lbold=227 μm, Lnarrow=347 μm, and ΔL=0.2854 μm are optimum values. In addition, TprLbold=336 μm and TprLnarrow=148 μm. Therefore, the length of the wide waveguide 87 including the tapered waveguide is Lbold+2-TprLbold=899 μm, and the length of the narrow waveguide 88 is Lnarrow+2-TprLnarrow=643 μm.
The reason why the wavelength independent optical coupler (Winc) is used as the optical coupler will be described in detail. A transmittance Tcross of a cross path (input port 81a->output port 82b, and input port 82a->output port 81b) of the wide band switch element 80 and a transmittance Tbar of bar paths (input port 81a->output port 81b and input port 82a->output port 82b) of the wide band switch element 80 are expressed, similarly to the conventional switch element 30, by Expressions 2 and 3, when the optical coupling rate of the optical coupler 83a and the optical coupler 83b is defined as κ, and the phase difference between the interference arm 84 and the interference arm 85 is defined as Δφ. Therefore, if Δφ=π can be obtained when the switch is turned OFF, Tcross=0 is always obtained regardless of the value of κ, and a high extinction ratio can be provided in the cross path. On the other hand, when the switch is turned ON, the thermooptic phase shifter is driven to set Δφ=0, and the transmittance Tcross of the cross path at this time becomes as following Expression 37 using Expression 2.
If the coupling rate κ of the optical couplers 83a and 83b is deviated from 0.5, that is, from 50% coupling, the transmittance Tcross is correspondingly reduced and the loss is increased. Since the coupling rate κ of the optical couplers 83a and 83b greatly affects the loss, it is desirable to have a value as close as 0.5 as possible in the wavelength region to be used.
In a normal directional coupler (simple DC), the optical coupling rate has a relatively large wavelength dependence, and in general, the optical coupling rate becomes stronger when the wavelength becomes longer. An increase in loss due to this phenomenon is also conspicuous when the wavelength region exceeds about 120 nm. Therefore, it is desirable to use an optical coupler having a small wavelength dependence in order to obtain not only a high extinction ratio but also a low loss in the wide wavelength range. As described in PTL 4, Winc is constituted by a kind of Mach-Zehnder interferometer in which a first directional coupler and a second directional coupler are connected in cascade. By appropriately selecting a coupling length DcL1 of the first directional coupler, a coupling length DcL2 of the second directional coupler, and an optical path length difference ΔLwinc between interference arms of the Mach-Zehnder interferometer, a characteristic having a flat optical coupling rate in the wide wavelength range can be obtained.
In addition, the characteristics of an optical coupler called an asymmetric directional coupler (asymmetric Dc) are also shown. In the simple Dc, waveguides having the same waveguide width are used in an optical coupling portion where two waveguides are close to each other, but in the asymmetric DC, waveguides having different waveguide widths are used in the optical coupling portion. That is, the waveguide widths of the two waveguides in the optical coupling portion are not the same but different. Of course, the front and rear portions of the waveguide are connected by tapered waveguides so as not to cause an increase in loss when changing the waveguide width. The difference ΔW between the widths of the waveguides at the optical coupling portion of the asymmetric Dc having characteristic shown in
As can be seen from
When this Winc is used as the optical coupler of the wide-band switch element 80, the Winc is arranged in a direction called point symmetry arrangement. Specifically, as shown in
When the structure is vertically asymmetric like Winc, unlike a simple DC whose structure is vertically symmetric, when inputted from one input, the phase difference of light outputted from two ports is deviated from π/2 and outputted. This deviation can be canceled by performing point symmetry arrangement.
On the other hand, the transmittance of the cross path at the time of ON greatly varies in wavelength characteristics depending on the type of the optical coupler used. The wavelength range in which the transmittance is −0.3 dB or more is kept at about 160 nm when the simple Dc is used, whereas the wavelength range is 430 nm or more when the Winc is used, and 300 nm or more is obtained even when the asymmetric Dc is used. Thus, by using the optical coupler having small wavelength dependence of the optical coupling rate, not only the high extinction ratio but also the low loss can be obtained in the wide wavelength range.
The size of the optical coupler increases in the order of simple DC, asymmetric DC, and Winc. Therefore, what kind of optical coupler is used for the optical coupler used for the wide-band switch element may be determined by the size of the wavelength range requiring the low loss characteristic. If it is necessary to fully use a wavelength region 1.26 to 1.675 μm which can be used for a communication optical fiber, Winc is used. If only a C band (1.53 to 1.565 μm) or an L band (1.565 to 1.625 μm) used for a long distance communication is sufficient, the simple DC is used. If S band (1.46 to 1.53 μm) are to be added to C band and L band, the asymmetric Dc may be used.
Note that in a large-scale switch in which a large number of these switch elements are integrated, not only the switch elements of the cross path when the switch is turned ON but also the switch elements of the bar path when the switch is turned OFF pass through between the input and the output. However, the switch element which can obtain the high extinction ratio in the cross path is Tcross≈0 when the switch is turned OFF. As can be seen from Expression 3, Tbar, that is the transmittance of the bar path at the time of switch OFF, ≈1 is satisfied, and as shown in
Therefore, the double gate type switch element 110 propagates the light to bar paths (input line waveguide 113a->input line waveguide 113b and output line waveguide 114a->output line waveguide 114b) in the OFF state, and in the ON state, the light is propagated to the cross path (input line waveguide 113a->output line waveguide 114b). In this way, similarly to the wide-band switch element 20, since the double gate type switch element 110 cuts off propagation light from the input line waveguide 113a to the output line waveguide 114b by two switch elements of the first switch element 111 and the second switch element 112 at the time of OFF, a characteristic having extremely small crosstalk, that is, a characteristic having a high extinction ratio which is a ratio of transmittance at the time of ON to transmittance at the time of OFF, is obtained.
In the double gate type switch element 110, an intra-element intersection 26 in the double gate type switch element 20 is exposed to the outside of the switch element. In addition, the second switch element 112 is arranged in an inverted direction with respect to the vertical direction of the first switch element 111. This is because the longer side of the interference arm of the switch element is arranged in the opposite direction by two switch elements, as described in PTL 5, so that the influence of the refractive index distribution in the wafer plane can be reduced.
In this embodiment, a wide-band switch element 80 is used as the first switch element 111 and the second switch element 112 of the double gate type switch element 110. As described above, the wide-band switch element 80 is arranged in the first switch element 111 in the direction as it is, and the wide-band switch element 80 is arranged in the second switch element 112 in the direction inverted upward and downward.
Both of them show only the difference in the arrangement direction of the switch elements described later, and are basically the same in configuration. Specifically, in the switch element 123 indicated by the solid surrounding line, the double gate type switch element 110 shown in
In the manufactured 4×4 matrix switch 120, a layout folded back at the center of the four switch stages is adopted, and the input/output ports are arranged on the same side of the chip. The chip size of the 4×4 matrix switch 120 was about 44×4 mm. The 4×4 matrix switch 120 were manufactured three times using the same photomask. One circuit was manufactured so as to be finished to a waveguide width generally as designed. The remaining two circuits were manufactured by changing the exposure conditions and finishing the waveguide widths by shifting the waveguide widths from the designed values to +0.2 μm and −0.2 μm.
Even if the finish of the waveguide width is deviated by +0.2 μm from the design value, the transmittance in the OFF state is suppressed to −62 dB or less over the wavelength region of 1.25 to 1.68 μm, and the transmittance in the ON state is secured to −1.8 dB or more. That is, the high extinction ratio and the low insertion loss can be obtained in the entire wavelength range where the optical fiber for communication can be used. As described above, according to the example 1, it is possible to obtain a matrix switch having the large manufacturing tolerance and the high extinction ratio characteristic and the low insertion loss characteristic in the wide wavelength range.
As a large scale switch having a configuration similar to that of a matrix switch, a multicast switch (MCS) described in NPL 3 is known. The double gate type switch element 110 described in the present example can be applied to the MCS as it is. As in the matrix switch of the present example, it is apparent that the high extinction ratio and the low insertion loss can be obtained over the wide wavelength range with the large manufacturing tolerance even in the MCS.
The 1×4 switch 130 was also manufactured by using the same photomask to manufacture three circuits, and the finished width of the waveguide was shifted in the same manner as in the embodiment 1. In the manufactured 1×4 switch 130, the thin film heater was driven to evaluate the transmittance in the OFF state and the transmittance in the ON state of each path. A typical example of the transmittance characteristic was almost the same as that shown in
In addition to the construction of a large-scale integrated optical switch, there are a tree type 1×N switch in which a plurality of switch elements are connected in a binary tree shape. Also in such a switch, for example, the double gate type switch element 110 is used as a switch element, so that the high extinction ratio and the low insertion loss can be obtained over the wide wavelength range with the large manufacturing tolerance.
The optical phase shifter is constituted of a pair of waveguides of the interference path 145a and the interference path 145b, and the optical phase shifter is constituted of a pair of waveguides of the interference path 145c and the interference path 145d. A waveguide length difference between a waveguide length LA of the interference path 145a and a waveguide length LB of the interference path 145b is set to ΔLAB (=LB−LA), and a waveguide length difference between a waveguide length LD of the interference path 145c and a waveguide length LD of the interference path 145d is set to ΔLCD (=LD−LC). At this time, the difference between the waveguide length differences is defined as ΔL (=ΔLCD−ΔLAB). Typically, LA=LB=LC is established, that is, ΔLA≠0, ΔL=ΔLCD are satisfied in many cases, but LA≠LB≠LD may be used. The waveguides of the interference paths 145a to 145d are basically constituted of a width W0. In the interference path 145d, two different type waveguides are included, and a part of the waveguide is a wide waveguide 146 having a waveguide width Wbold (>W0) over a length Lbold. In addition, another part of the waveguide is a narrow waveguide 147 having a waveguide width Wnarrow (>W0) over a length Lnarrow. In addition, at a connecting portion of the normal waveguide portion and the wide waveguide 146 and the narrow waveguide 147, a tapered waveguide having a length TprLbold and a length TprLnarrow is formed.
The Winc described in the example 1 is used for the optical couplers 143a, 143b, 144a, and 144b to obtain characteristics of the coupling rate of 50% over the wide wavelength range. Note that, in the arrangement of the Winc, the optical couplers 143a and 143b of the front stage are arranged in the same direction in the vertical and horizontal directions each other, and the optical couplers 144a and 144b of the rear stage are also arranged in the same direction in the vertical and horizontal directions each other. On the other hand, the optical couplers 143a and 143b of the front stage and the optical couplers 144a and 144b of the rear stage do not necessarily have to be in the same direction, but they may be arranged in the same direction. In
When the phase of the signal light with respect to the reference light is ψ, for the phase ψA of the signal light outputted to the output port 148a, the phase ψB of the reference light outputted to the output port 148b, the phase ψC of the signal light outputted to the output port 149c, the phase ψD of the reference light outputted to the output port 149d, each of them is calculated as follows.
Here, θber1 is a phase change of propagation light in the bar path of the optical couplers 143a and 143b of the front stage, and θcross1 is a phase change of propagation light in the cross path of the optical couplers 143a and 143b of the front stage. θber2 is a phase change of propagation light in the bar path of the optical couplers 144a and 144b in the rear stage, and θcross2 is a phase change of propagation light in the cross path of the optical couplers 144a and 144b in the rear stage. The difference in the phase change between the bar path and the cross path is defined as Δθ1 (=θcross1−θber1) and Δθ2 (=θcross2−θber2), respectively. In addition, the phase change in the intersection 150 is completely canceled at the dummy intersections 151a and 151b. Further, in this expression, for the sake of simplicity, the tapered waveguide connection is ignored and described.
A phase difference ΔψIch (=ψB−ψA) of the signal light with respect to the reference light outputted to the output port 148b is called an Ich phase difference. When a phase difference ΔψQch (=ψD−ψC) of the signal light with respect to the reference light outputted to the output port 149b is called a Qch phase difference, the Ich phase difference ΔψIch and the Qch phase difference ΔψQch are calculated as follows.
Further, when a difference of the Qch phase difference ΔψQch with respect to the Ich phase difference ΔψIch is defined as an IQ phase difference ΔψIQ(=ΔψQch−ΔψIch), the IQ phase difference is calculated as follows.
Here, Expressions 16 and 17 described in the embodiment 2 are used for Δnbold and Δnnarrow. This expression 44 has the same type as that of expression 15 described in the embodiment 2 except for only the reference numerals. Therefore, by obtaining the parameters of ΔL, Lbold and Lnarrow, as in the procedure described in the embodiment 2, ΔψIQ can be set to an arbitrary phase difference in the wide wavelength range with a large manufacturing tolerance.
In the 90 degrees optical hybrid, ΔψIQ=−π/2 is set. That is, the Qch phase difference ΔψQch is delayed by 90° from the Ich phase difference ΔψIch. This corresponds to advancing the phase of the Qch reference light by 90° from the phase of the Ich reference light, and the input signal light can be detected as a complex signal as described below.
From the above, in the 90 degrees optical hybrid 140, ΔL, Lbold, and Lnarrow, which satisfies an expression in which dΔnavg/dW=0 is set in Expression 20, an expression in which ξ=π/2 is set in Expression 35, and the three expressions of Expression 25, may be calculated. Or, ΔL, Lbold, Lnarrow in which Expression 25 is minimized and an expression in which ξ=π/2 is set in Expression 36 is minimized may be calculated. As specific design values, W0=5 μm, Wbold=8 μm, and Wnarrow=3.5 μm are set, Lbold=120 μm, Lnarrow=151 μm, and ΔL=0.1437 μm were obtained as optimum values. Further, TprLbold=259 μm and TprLnarrow=148 μm are satisfied. In addition, ΔLAB=0 and ΔL=ΔLCD were set.
Note that even if ΔψIQ is +π/2, the Ich output and the Qch output can be made to correspond to each other on the complex plane by reversing positive and negative of the Ich output or the Qch output. Therefore, the interference path provided with the wide waveguide 146 and the narrow waveguide 147 is not limited to the interference path 145d, but any of the other interference paths 145a to 145c can be used.
Note that even if ΔψIQ is +π/2, the Ich output and the Qch output can be made to correspond to each other on the complex plane by reversing positive and negative of the Ich output or the Qch output. Therefore, the interference path provided with the wide waveguide and the narrow waveguide is not limited to the interference paths 175a and 175d, but may be the interference paths 175b and 175c.
When the length of the first wide waveguide 196 is defined as L1, the width is defined as W1, the length of the second wide waveguide 197 is defined as L2, and the width is defined as W2, the above-mentioned Expressions 40 and 41 are rewritten as follows.
The Qch phase difference ΔψQch of Expression 43 is rewritten as follow.
Accordingly, the IQ phase difference ΔψIQ of Expression 44 is defined as follows.
Here, for Δn1 and Δn2, Expressions 29 and 30 described in the embodiment 3 are used. This Expression 48 has the same type as that of Expression 22 described in the embodiment 3 except for only the reference numerals. Therefore, by obtaining the parameters of ΔL, L1 and L2, as in the procedure described in the embodiment 3, ΔψIQ can be set to an arbitrary phase difference in the wide wavelength range with a large manufacturing tolerance.
In the 90 degrees optical hybrid, a configuration in which the configuration of the 90 degrees optical hybrid 170 is applied to the configuration of the 90 degrees optical hybrid 190 may be considered. Further, a configuration in which the configuration of the 90 degrees optical hybrid 170 is applied by combining the configuration of the 90 degrees optical hybrid 140 and the configuration of the 90 degrees optical hybrid 190 may be considered.
As described above, it is possible to realize the optical phase shifter in which the phase shift amount is kept constant over the wide wavelength range. Further, it is possible to realize the wide-band operation optical phase shifter in which the phase shift amount does not vary even if the waveguide width varies due to the manufacturing error, that is, the optical phase shifter having the large manufacturing tolerance. By using these optical phase shifters, an optical circuit operating over the wide-band, especially the optical switch having high extinction ratio characteristics in the wide wavelength region can be provided. Furthermore, the optical switch having the low loss characteristic in the wide wavelength region can be provided. Furthermore, the 90 degrees optical hybrid operating in the wide wavelength range can be provided.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/JP2021/017082 | 4/28/2021 | WO |