JP5910186B2 - Wavelength multiplexing / demultiplexing element and optical apparatus using the same - Google Patents

Description

  The present invention relates to a wavelength multiplexing / demultiplexing element used for optical communication, an optical interconnect, and the like, and an optical apparatus using the same.

  In recent years, silicon (Si) photonics has attracted attention as a promising technology for large-capacity interconnects. The main advantage of Si photonics technology is that it has a cross-sectional area of several hundreds of nanometers, which enables high-density integration and wavelength division multiplexing (WDM) within the Si chip. The transmission capacity per unit can be expected to be improved. In order to transmit and receive WDM light within the Si chip, components such as a light source, a light modulation element, and a wavelength multiplexing / demultiplexing element are required.

  The wavelength multiplexing / demultiplexing element is an optical device that multiplexes and demultiplexes WDM light as necessary. The characteristics required of the wavelength multiplexing / demultiplexing element when transmitting / receiving WDM light within the Si chip include flatness of the multiplexing / demultiplexing spectrum, low channel deviation, and low channel crosstalk. As a candidate technique that satisfies these conditions, a wavelength multiplexing / demultiplexing element in which a delayed Mach-Zehnder interferometer (DMZI) is cascade-connected in multiple stages has been actively researched and developed.

  As an example of a conventional wavelength multiplexing / demultiplexing element, a DMZI type 1 × 8 Ch wavelength multiplexing / demultiplexing element as shown in FIG. 1 is known (for example, see Non-Patent Document 1). As shown in the schematic diagram of FIG. 1A, this wavelength multiplexing / demultiplexing element is composed of a DMZI of three stages, and a part of DMZI has a phase of ± π / 2 or ± π / 4 inside. A control area is provided. The delay waveguide length of each DMZI is adjusted so as to satisfy the channel spacing of 400 GHz. In such a wavelength multiplexing / demultiplexing element, as shown in the multiplexing / demultiplexing spectrum of FIG. 1 (B), good multiplexing / demultiplexing characteristics can be obtained, but transmission corresponding to each channel Ch-1 to Ch-8. Since the spectral shape of the band is determined in a Gaussian function, the peak portion where the transmittance is highest is rounded and not flattened, and the crosstalk is theoretically limited to about −13 dB. That is a problem.

  As one of the conventional techniques capable of coping with the above problem, a DMZI type 1 × 8 Ch wavelength multiplexing / demultiplexing element having a different interferometer architecture as shown in FIG. 2 has been reported (for example, see Non-Patent Document 2). . As shown in the schematic diagram of FIG. 2A, this wavelength multiplexing / demultiplexing element is a cascade connection of a plurality of DMZIs having different optical path lengths for the first and second stages of the three-stage DMZIs. And the optical coupling rate and the phase shift amount in each DMZI are optimized. As a result, as shown in the multiplexing / demultiplexing spectrum of FIG. 2 (B), a flat spectral shape is obtained in the high transmittance portion as the transmission band corresponding to each channel, and the spectral flatness of the multiplexing / demultiplexing characteristics is obtained. The crosstalk can theoretically be improved to about -18 dB.

Further, as a technique related to the conventional wavelength multiplexing / demultiplexing element as described above, a ring resonator is positioned in one arm waveguide in a one-stage delay interferometer as shown in the schematic diagram of FIG. There are optical circuit elements (see, for example, Patent Document 1). In this optical circuit element, when WDM light including a plurality of wavelengths λ _{1 to} λ ₆ is applied to one input port of the delay interferometer, light of each wavelength λ ₂ , λ ₄ , λ ₆ is output from one output port P1. A signal is output, and optical signals of wavelengths λ ₁ , λ ₃ , and λ ₅ are output from the other output port P2. FIG. 3B shows a transmission spectrum corresponding to each of the output ports P1 and P2, and the optical circuit element functions as an interleaver that distributes and outputs the input WDM light to the two output ports every other channel. To do.

JP 2000-298222 A

Dae Woong Kim et al., "Silicon-on-insulator eight-channel optical multiplexer based on a cascade of asymmetric Mach-Zehnder interferometers", OSA Optics Letters vol.33 no.5, pp.530-532, March 2008 1 day Folkert Horst, "Silicon Integrated Waveguide Devices for Filtering and Wavelength Demultiplexing", Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC / NFOEC), OWJ3, March 21-25, 2010

  However, in the wavelength multiplexing / demultiplexing device according to the prior art as shown in FIG. 2, the number of DMZIs in the entire device inevitably increases as compared with the case shown in FIG. There is a problem that it ends up. Each DMZI has an optical coupler before and after the pair of arm waveguides, and the optical coupler needs to adjust the optical coupling ratio (optical branching ratio). For this reason, there is also a problem that an increase in the number of DMZIs in the entire device results in an extremely large number of adjustment points of the optical coupling rate in the optical coupler.

  Further, the optical circuit element (interleaver) shown in FIG. 3 functions as a wavelength multiplexing / demultiplexing element, that is, generates a WDM light by multiplexing a plurality of optical signals having different wavelengths only by the configuration. At the same time, the function of demultiplexing the WDM light and generating the optical signal of each wavelength cannot be realized. Further, even if the above-described optical circuit element is formed as one unit configuration and simply connected in cascade, it is difficult to obtain a desired multiplexed / demultiplexed spectrum. In other words, when applying a delay interferometer equipped with a ring resonator to achieve a wavelength multiplexing / demultiplexing device with excellent spectral flatness and low crosstalk, the wavelength of optical signals to be multiplexed / demultiplexed, channel spacing, etc. Therefore, how to optimize the relative design conditions of the delay interferometers at each stage is an important issue.

  The present invention has been made by paying attention to the problems of the prior art as described above, and has wavelength multiplexing / demultiplexing characteristics that have excellent spectral flatness and low crosstalk, and are suitable for miniaturization. An object of the present invention is to provide an optical device using the wave element.

  In order to achieve the above object, one embodiment of the present invention includes a plurality of delay interferometers cascaded in multiple stages, and combines a plurality of optical signals having different wavelengths by the plurality of delay interferometers. And a wavelength multiplexing / demultiplexing element that generates a plurality of optical signals having different wavelengths by demultiplexing the WDM light by the plurality of delay interferometers. In this wavelength multiplexing / demultiplexing element, each of the plurality of delay interferometers includes a pair of optical couplers through which the plurality of optical signals are input and output, a first arm waveguide connecting the optical couplers, and Including a second arm waveguide and a loop waveguide disposed in the vicinity of the first arm waveguide, wherein the first arm waveguide constitutes a ring resonator in combination with the loop waveguide The second arm waveguide is a delay waveguide relative to the first arm waveguide, and the optical path length difference between the first and second arm waveguides in each delay interferometer, and The loop length of the ring resonator in each delay interferometer is set to a different value for each stage of the multistage cascade connection according to the wavelength and channel spacing of the plurality of optical signals.

  According to the wavelength multiplexing / demultiplexing element, delay interferometers having ring resonators on one arm waveguide are cascade-connected, and the optical path length difference in each delay interferometer and the circulation length of the ring resonator are determined for each stage. By using different values, it is possible to achieve wavelength multiplexing / demultiplexing characteristics with good spectral flatness and low crosstalk, while minimizing the total number of delay interferometers and miniaturizing the entire device. Is possible.

It is a figure which shows an example of the conventional wavelength multiplexing / demultiplexing element. It is a figure which shows the other example of the conventional wavelength multiplexing / demultiplexing element. It is a figure which shows the optical circuit element relevant to the said conventional wavelength multiplexing / demultiplexing element. It is a top view which shows the structure of one Embodiment of the wavelength multiplexing / demultiplexing element by this invention. It is a figure which shows the specific example of the delay interferometer of the 1st stage in the said embodiment. It is a figure for demonstrating an example of the manufacturing method of the waveguide in the said embodiment. It is a figure for demonstrating the other manufacturing method relevant to FIG. It is a figure which shows the multiplexing / demultiplexing spectrum characteristic with respect to the optical coupling factor of the ring resonator in the said embodiment. It is a figure for demonstrating the coupling characteristic of a directional coupler. It is a figure which shows schematic structure of an MMI coupler. It is a figure which shows an example of the multiplexing / demultiplexing characteristic when all conditions are satisfied in the said embodiment. It is a figure for demonstrating deterioration of the multiplexing / demultiplexing characteristic when the non-resonance condition of a ring resonator is not satisfy | filled in the said embodiment. It is a figure for demonstrating quantitatively the spectrum flatness and low crosstalk of the wavelength multiplexing / demultiplexing characteristic in the said embodiment. It is a top view which shows schematic structure of the optical transmitter which applied the structure of the said embodiment. It is a top view which shows schematic structure of the optical receiver which applied the structure of the said embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 4 is a plan view showing a configuration of an embodiment of the wavelength multiplexing / demultiplexing element according to the present invention.
In FIG. 4, the wavelength multiplexing / demultiplexing device 1 of this embodiment includes a plurality of delay interferometers 20 cascaded on a substrate 10 in multiple stages. Here, for example, seven delay interferometers 20 are cascade-connected in three stages. Specifically, the delay interferometer 20 ₁ of the first stage on the left side in the figure, two delay interferometers 20 _2A, 20 _2B of the second stage is located in the center is connected. Further, the delay interferometer _{20 2A} of the second stage, the third stage of the four delay interferometer ₂₀ 3A _{20 3B} 20 3C, ₂₀ among _3D of the delay interferometer ₂₀ 3A located on the right side in the figure, 20 _3B is connected, the delay interferometer _{20 2B} of the second stage, the remainder of the delay interferometer ₂₀ 3C of the third stage, _{20 3D} is connected.

Figure 5 is a diagram showing a specific example of the delay interferometer 20 ₁ of the first stage, a plan view showing an enlarged schematic configuration of the delay interferometer 20 ₁ (A), (B) transmission Spectral characteristics. In FIG. 5 (A), the delay interferometer 20 ₁ of the first stage, a pair with the optical coupler 21, first connecting the respective optical coupler 21 of the arm waveguides 23 and a second The arm waveguide 24 and the loop waveguide 25 disposed in the vicinity of the first arm waveguide 23 are provided. Each of the optical couplers 21 and 22 has an optical coupling rate of 50%. The first arm waveguide 23 forms an all-pass ring resonator in combination with the loop waveguide 25. The second arm waveguide 24 is a delay waveguide with respect to the first arm waveguide 23. That is, the delay interferometer 20 ₁ has a structure of the delay Mach-Zehnder interferometer equipped with a ring resonator in one arm waveguide (DMZI).

In the ring resonator, the circumference (the length of one round of the loop waveguide 25) L _M1 is set according to the optical path length difference L _D1 in DMZI (the difference between the optical path lengths of the arm waveguides 23 and 24). . Configuring circumferential length L _M1 of the ring resonator for transmission spectrum of DMZI, transmission spectrum of the ring resonator is made so as to satisfy the non-resonant condition. That is, the non-resonant wavelength (dashed line) in the transmission spectrum characteristic (dashed line) of the ring resonator shown in FIG. 5B is positioned at the center wavelength of each transmission band in the transmission spectrum characteristic (solid line) of DMZI. The circumference length L _M1 of the ring resonator is set according to the optical path length difference L _D1 in DMZI. The ring resonator is set so that the optical coupling ratio between the arm waveguide 23 and the loop waveguide 25 is 80% or more. The setting of the ring resonator circumference L _M1 and the optical coupling rate will be described in detail later.

For even the delay interferometers ₂₀ 3A _{to 20 3D} of the second stage the delay interferometers ₂₀ 2A of, _{20 2B} and the third stage, in the same manner as the delay interferometer 20 ₁ of the first stage shown in FIG. 5, Each arm waveguide has a DMZI structure with a ring resonator. In the following description, in the second the delay interferometers ₂₀ 2A of the stage, _{20 2B,} the circumferential length of the optical path length difference and the ring resonator with a _{L D2} and _{L M2,} the delay interferometers _{20 3A} of the third stage The optical path length difference and the ring resonator circumference in ˜20 _3D are denoted as L _D3 and L _M3 (see FIG. 4). The optical path length differences L _{D1 to} L _D3 in the delay interferometer of each stage are set in correspondence with the channel interval of the optical signal that is multiplexed / demultiplexed by the wavelength multiplexing / demultiplexing element 1.

Further, as shown in FIG. 4 above, the second stage delay interferometer _202B and the third stage delay interferometer _203B are arranged on one arm waveguide provided with a ring resonator by −π / A phase shifter 26 having a phase shift amount of 2 is provided. Similarly, the delay interferometer 20 _3C of the third stage, the phase shifter 26 is provided with a phase shift of + [pi / 4, the delay interferometer 20 _3D third stage, - [pi] / 4 phase A phase shifter 26 having a shift amount is provided.

Here, an example of a method of manufacturing a waveguide in the wavelength multiplexing / demultiplexing element 1 as described above will be described with reference to FIG. In the example of FIG. 6, first, an SOI wafer having a SiO ₂ (BOX) layer 102 and a Si core layer 103 (for example, a film thickness H of 0.25 μm) on the Si substrate 101 is used, and an optical multiplexer / demultiplexer is formed by an optical exposure process. The waveguide stripe structure in the corresponding region of the element 1 is patterned. The optical semiconductor waveguide pattern is defined by a photomask of an optical exposure apparatus. In this case, electron beam exposure may be used instead of light exposure.

Then, the drawn pattern is dry-etched by a method such as reactive ion etching to form a rib waveguide structure (for example, the waveguide width W is 0.48 μm) having a slab height of about 0.05 μm. Thereafter, when the upper portion of the waveguide stripe pattern is covered with the SiO ₂ film 104 using a vapor deposition apparatus or the like, the wavelength multiplexing / demultiplexing device 1 of the present embodiment is completed.

  Although FIG. 6 shows an example of the rib waveguide structure, the waveguide structure in the present invention is not limited to this, and may be a channel structure as shown in FIG. In this case, the channel waveguide structure can be formed by performing etching without leaving the slab thickness in the manufacturing process of the rib waveguide structure.

In FIG 4 wavelength mux 1 configured as shown in, the WDM light including a plurality of optical signals wavelength arranged at regular channel spacing is applied to the delay interferometer 20 ₁ of the first stage, The WDM light propagates through the first to third stage delay interferometers and is demultiplexed according to the wavelength, and the optical signals of the respective wavelengths are transmitted from the corresponding delay interferometers 20 _{3A to} 20 _{3D of} the third stage. Each is output. Here, the optical signals output from the delay interferometer _{20 3A} of the third stage as a channel Ch-1, Ch-2, the optical signals output from the delay interferometer _{20 3B} channels Ch-3, Ch - 4 and then, the optical signals output from the delay interferometer _{20 3C} as a channel Ch-5, Ch-6, the respective optical signals output from the delay interferometer _{20 3D} channel Ch-7, Ch-8. Moreover, contrary to the demultiplexing of the WDM light, an optical signal corresponding to each channel Ch-1 to CH-8 is supplied to the corresponding delay interferometer ₂₀ 3A _{to 20 3D} of the third stage, each of said light signals are multiplexed into one by propagating the corresponding delay interferometer of the third to the first stage, WDM light is output from the delay interferometer 20 ₁ of the first stage.

Here, specific settings of the first to third stage delay interferometers will be described in detail.
In the wavelength multiplexing / demultiplexing device 1 of the present embodiment, the transmission spectrum characteristics of the ring resonator in each delay interferometer are not compared to the transmission spectrum characteristics of each delay interferometer having a DMZI structure as described above. It is required to satisfy the resonance condition. Given the non-resonant condition for the delay interferometer 20 ₁ of the first stage, the optical path length difference L _D1 in the delay interferometer 20 _1, circumferential length L _M1 of the ring resonator becomes 2 × L _D1 or near .

As a specific example, assuming that a wavelength channel spacing in 1.55μm vicinity to demultiplexing in WDM device 1 the optical signals of 400 GHz, the optical path length in the delay interferometer 20 ₁ of the first stage The difference L _D1 is about 90 μm. The value of the optical path length difference L _D1 is a result obtained in consideration of the dispersion characteristics of the Si fine wire waveguides shown in FIGS. Circumferential length L _M1 of the ring resonator in the first stage of the delay interferometer 20 ₁ in this case is about 180 [mu] m.

For the second and subsequent stages, the number of delay interferometers increases at a rate of 2 ^S-1 (where S is the number of stages) each time the stages are stacked. That is, the number of delay interferometers in the second stage is 2 ^2-1 = 2, and the number of delay interferometers in the third stage is 2 ^3-1 = 4. The total number of delay interferometers constituting the wavelength multiplexing / demultiplexing element is 2 ^S −1. In this embodiment, since S = 3, the total number of delay interferometers is 7.

Optical path length difference in the second and other subsequent stages of the delay interferometers cascaded to the delay interferometer 20 ₁ of the first stage may be set 2 ^(1-S) to vary according to the relational expression × L _D1. That is, the optical path length difference L _D2 in each of the delay interferometers 20 _2A and _{202 B} of the second stage is set to L _D2 = 2 ^(1-2) × L _D1 = 0.5 × L _D1 , The optical path length difference L _D3 in each of the delay interferometers 20 _{3A to} 20 _3D can be set to L _D3 = 2 ^(1-3) × L _D1 = 0.25 × L _D1 . Under the assumption of the above-described specific example (multiplexing / demultiplexing of each optical signal having a wavelength of about 1.55 μm and a channel interval of 400 GHz), the optical path length difference L _D2 for the second stage is about 45 μm, and the optical path length for the third stage. The difference L _D3 is about 22.5 μm.

Moreover, what is necessary is just to set so that the circumference of the ring resonator in the delay interferometer after a 2nd stage may change according to the relational expression of 2 ^(2-S) * _LD1 . That is, the ring resonator circumference L _M2 for the second stage is set to 2 ^(2-2) × L _D1 = L _D1 , and the ring resonator circumference L _M3 for the third stage is 2 ⁽ 2 ^2-3) xL _D1 = 0.5 × L _D1 can be set. Assuming the specific example described above, the ring resonator circumferential length L _M2 for the second stage is approximately 90 μm, and the ring resonator circumferential length L _M3 for the third stage is approximately 45 μm.

Next, in order for the ring resonator in each delay interferometer after the second stage to satisfy the non-resonance condition for each delay interferometer, the ring resonator in the first stage delay interferometer Are set to λ ₀ + 0.5 × Δν and λ ₀ −0.5 × Δν, respectively, for the center wavelength (λ _1st = λ ₀ ) of the ring resonator in the second stage delay interferometer do it. However, Δν is the channel interval of the optical signal to be multiplexed / demultiplexed. The center wavelength of the ring resonator in the delay interferometer after the third stage is centered on the center wavelength of the delay interferometer of the previous stage, and the center wavelength of each adjacent delay interferometer each time the stage is overlapped. May be changed according to the relational expression 2 ^(S-2) × Δν.

The following formulas (1) to (7) are obtained specifically from the center wavelength of the ring resonator in each of the first to third stage delay interferometers.
First stage: λ _1st = λ ₀ (1)
Second stage: λ _2nd-A = λ ₀ + 0.5 × Δν (2)
λ _2nd-B = λ ₀ −0.5 × Δν (3)
Third stage: λ _3rd-A = λ ₀ + 1.5 × Δν (4)
λ _3rd−B = λ ₀ −0.5 × Δν (5)
λ _3rd-C = λ ₀ −1.5 × Δν (6)
λ _3rd-D = λ ₀ + 0.5 × Δν (7)
Here, λ _2nd-A and λ _2nd-B are the center wavelengths of the ring resonators in the delay interferometers 20 _2A and _{202B of} the second stage. Λ _3rd-A , λ _3rd-B , λ _3rd-C, and λ _3rd-D are the centers of the ring resonators in the third stage delay interferometers 20 _3A , 20 _3B , 20 _3C, and 20 _3D , respectively. Is the wavelength.

As a specific example, assuming that the channel spacing Δν is 400 GHz, the center wavelength of each ring resonator for the second stage is λ _2nd−A = λ ₀ according to the above equations (2) and (3). It can be set to +200 GHz and λ _2nd−B = λ ₀ −200 GHz. The center wavelength of each ring resonator for the third stage, according to the above (4) to _{(7), λ 3rd-A = λ 0} + 600GHz, λ 3rd-B = λ 0 -200GHz, λ 3rd- _C = lambda can be set to ₀ -600GHz and _{λ 3rd-D = λ 0 +} 200GHz. As described above, the difference in the center wavelength between the ring resonators in the delay interferometers adjacent to each other in the third stage is 2 × Δν (800 GHz), and λ _2nd-A and λ of the second stage Even with respect to _2nd-B , the arrangement is uniform (equal intervals of 400 GHz). That is, the center wavelength of the ring resonator in each delay interferometer is shifted by approximately half of the ring resonance frequency interval with respect to the center wavelength of the delay interferometer. Therefore, the ring resonators in all the delay interferometers of the three stage stage satisfy the non-resonant condition.

The center wavelength of the ring resonator used in each delay interferometer can be adjusted by the following method. Normally, the center wavelength λ ₀ of the ring resonator is determined as the following equation (8).
λ ₀ = (N _WG × L _MRR ) / m (8)
Here, _NWG is the effective refractive index of the loop waveguide, _LMRR is the circumference of the ring region, and m is the diffraction order of the ring resonator. That is, in the ring _region, by adjusting the _{N WG} or _{L MRR,} it is possible to control the center wavelength.

  Further, as described above, the ring resonators in the first to third stage delay interferometers need to be adjusted so that each optical coupling rate is 80% or more. More specifically, in order to obtain excellent multiplexing / demultiplexing characteristics with excellent spectral flatness and low crosstalk, a short arm guide having a curved long arm waveguide 24 and a ring resonator inside the delay interferometer is provided. For the waveguide 23, it is important to make the slope of the phase change in the transmission band equal. However, unlike the case of the long arm waveguide 24, in the case of the short arm waveguide 23 provided with the ring resonator, the slope of the phase change is not constant, and the slope of the phase change periodically changes due to the ring resonance phenomenon. Tend to. Normally, when the optical coupling factor of the ring resonator is set to 80% or more, the slope of the phase change of the short arm waveguide 23 tends to match the slope of the phase change of the long arm waveguide 24 within the transmission band. Therefore, good multiplexing / demultiplexing characteristics can be expected. On the other hand, as the optical coupling ratio of the ring resonator decreases below 80%, the phase change slopes do not match, and as a result, crosstalk increases and the multiplexing / demultiplexing characteristics deteriorate significantly. Resulting in.

  FIG. 8 is a graph plotting the combined / demultiplexed spectrum characteristics with respect to the optical coupling rate of the ring resonator in the delay interferometer for one channel. FIG. 8A shows the case where the optical coupling rate is 85%, and FIG. The case where the optical coupling rate is 35% is shown. By comparing the characteristics shown in FIGS. 8A and 8B, when the optical coupling factor of the ring resonator is reduced, crosstalk is remarkably increased, and the combined / demultiplexed spectrum characteristics are significantly deteriorated. I understand.

  Here, a configuration example in which the optical coupling rate of the ring resonator can be increased to 80% or more will be described. The optical coupling rate of the ring resonator is determined by the coupling characteristics of the optical coupler formed by the adjacent portions of the arm waveguide 23 and the loop waveguide 25 in the configuration of the delay interferometer shown in FIG. Examples of the configuration of this optical coupler include a directional coupler and a multimode interference (MMI) type coupler.

FIG. 9 shows an example of coupling characteristics of the directional coupler having the Si thin-line channel waveguide structure shown in FIG. As shown in FIG. 9, as the interval GAP _DC waveguides in the coupling region in which the pair of waveguides are close is reduced, it can be seen that high optical coupling rate with a short coupling length is obtained. In the example of FIG. 9, when the gap GAP _DC of the waveguide is 0.1 μm (solid line in the lower graph), an optical coupling ratio of 85% or more can be obtained with a coupling length of about 13 μm.

On the other hand, in the case of an MMI coupler, the operating principle is different from that of the directional coupler, and it is difficult to freely adjust the optical coupling rate between ports. However, by setting the shape of the MMI coupler to a predetermined one, it is possible to obtain discrete optical coupling rates such as 72% and 85%, for example. FIG. 10 is a schematic diagram of an MMI coupler that provides an optical coupling ratio of 85%. 10, for example, if you set the width _{W IO} of the input and output waveguides to 0.5 [mu] m, the width _{W MMI} of the MMI region becomes 2.0 .mu.m. Further, the length L _MMI of the MMI region follows the relationship expressed by the following equation (9), where N _{MMI is} the refractive index of the MMI region and λ is the optical wavelength.
L _MMI = (N _MMI × W _MMI ² ) / λ (9)

Next, the condition of the relative phase difference in the delay interferometer at each stage will be described in detail.
In the wavelength multiplexing / demultiplexing device 1 of the present embodiment, the functionality that WDM light is distributed for each wavelength is based on the optical interference effect of the delay interferometer. That is, in each delay interferometer cascaded in multiple stages, the relationship between the optical path length differences L _{D1 to} L _D3 of each stage described above and the center wavelength of the ring resonator shown in the expressions (1) to (7) In addition to the relationships λ _{1st to} λ _3rd-D , the relative phase difference between the delay interferometers is required to satisfy a predetermined relationship.

Specifically, the delay region in each of the first to third stage delay interferometers needs to have a phase change amount (unit: radians) as shown in the following equations (10) to (16).
First stage: φ _1st = 0 (10)
Second stage: φ _2nd-A = 0 (11)
φ _2nd−B = + 0.5 × π (12)
Third stage: φ _3rd-A = 0 (13)
φ _3rd−B = + 0.5 × π (14)
φ _3rd-C = −0.25 × π (15)
φ _3rd−D = + 0.25 × π (16)
Here, phi _1st is the proper phase variation of the delay area in the delay interferometer 20 ₁ of the first stage. φ _2nd-A and φ _2nd-B are appropriate phase change amounts of the delay regions in the delay interferometers 20 _2A and _202B of the second stage. φ _3rd-A , φ _3rd-B , φ _3rd-C, and φ _3rd-D are the appropriate phase change amounts of the delay regions in the delay interferometers 20 _3A , 20 _3B , 20 _3C, and 20 _3D of the third stage. is there.

  FIG. 11 is a diagram illustrating an example of multiplexing / demultiplexing characteristics in the wavelength multiplexing / demultiplexing element 1 that satisfies all the above-described conditions. Here, the channel interval of the optical signal to be multiplexed / demultiplexed is set to 400 GHz. As shown in FIG. 11, compared with the case of the conventional wavelength multiplexing / demultiplexing device shown in FIG. 2B, wavelength multiplexing / demultiplexing characteristics having good spectral flatness and low crosstalk can be obtained. ing. The wavelength multiplexing / demultiplexing device 1 of this embodiment can reduce the number of delay interferometers compared to the conventional configuration shown in FIG. 2A, and is suitable for downsizing the entire device.

  In the wavelength multiplexing / demultiplexing device 1 of the present embodiment, when any one of the above-described conditions is not satisfied, the multiplexing / demultiplexing characteristics are deteriorated. For example, when considering the non-resonance condition of the ring resonator in each delay interferometer, the multiplexing / demultiplexing characteristics obtained when the non-resonance condition is satisfied in all the delay interferometers are shown in FIG. The spectrum is as shown. However, in FIG. 12, only the multiplexing / demultiplexing spectrum corresponding to 4 out of 8 channels is plotted in order to make the change of the multiplexing / demultiplexing characteristics easy to understand. On the other hand, the multiplexing / demultiplexing characteristics obtained when the non-resonance condition is satisfied only by the first-stage delay interferometer and the non-resonance condition is not satisfied by the other delay interferometers are shown in FIG. Such a spectrum results in degradation of the multiplexing / demultiplexing characteristics. However, among the above-described conditions, for example, even if the optical coupling factor of the ring resonator does not satisfy the condition of 80% or more, the wavelength multiplexing / demultiplexing characteristics are improved as compared with the conventional case shown in FIG. It may be possible. In this case, it is possible to improve the wavelength multiplexing / demultiplexing characteristics while avoiding the increase in the element size and the increase in the number of adjustment points as in the prior art shown in FIG.

  Here, a quantitative explanation will be given regarding the spectral flatness and low crosstalk of the wavelength multiplexing / demultiplexing characteristics described above. Here, regarding the spectral flatness, as a figure of merit, the ratio of the bandwidth when the transmittance is -1 dB to the bandwidth when the transmittance in the transmission band of a certain channel is -10 dB (hereinafter referred to as "shape"). (Referred to as “factor”) and the shape factor is used to evaluate the degree of flatness.

  FIG. 13 exemplifies the spectral characteristics of the transmission band corresponding to one channel (here, Ch-2) in the wavelength multiplexing / demultiplexing element, and FIG. 13A shows the case of the wavelength multiplexing / demultiplexing element 1 of the present embodiment. (B) corresponds to the conventional wavelength multiplexing / demultiplexing device shown in FIG. 2, and (C) corresponds to the conventional wavelength multiplexing / demultiplexing device shown in FIG. In the example of FIG. 13, the shape factors can be estimated as (A) 0.73, (b) 0.64, and (c) 0.33. Further, the level XT of the crosstalk component can be estimated as (a) -32 dB, (b) -18 dB, and (c) -13 dB at maximum. Thus, it can be seen that the wavelength multiplexing / demultiplexing device 1 of the present embodiment is most excellent in terms of both spectral flatness and low crosstalk.

  In the above-described embodiment, an example of a wavelength multiplexing / demultiplexing element corresponding to an optical signal of 8 channels by cascading seven delay interferometers in three stages has been described. The number of stages of cascade connection is not limited to three, and the number of stages of cascade connection can be appropriately set according to the maximum number of channels of optical signals to be multiplexed / demultiplexed. In addition, although the case of 400 GHz has been described as a specific example of the channel spacing to which the wavelength multiplexing / demultiplexing element corresponds, any channel spacing other than 400 GHz can be obtained in the same manner as in the above-described embodiment. This can be dealt with by optimizing each parameter.

Next, various optical devices to which the configuration of the above-described embodiment is applied will be described.
FIG. 14 is a plan view showing a schematic configuration of an optical transmission apparatus to which the configuration of the wavelength multiplexing / demultiplexing element 1 is applied. The optical transmitting apparatus shown in FIG. 14 300, for example, a light source unit 310 for generating light of different wavelengths lambda ₁ to [lambda] ₄ together, the light modulation for modulating the light of each wavelength lambda ₁ to [lambda] ₄ from the light source unit 310 Unit 320 and a wavelength multiplexing unit 330 that multiplexes the optical signals modulated by the optical modulation unit 320.

The light source unit 310 includes, for example, four lasers 311 _{1 to} 311 ₄ and four Si waveguide type filters 312 _{1 to} 312 ₄ . Each of the lasers 311 _{1 to} 311 ₄ uses a semiconductor optical amplifier (SOA) as a gain medium, and supplies light amplified by the SOA to the corresponding Si waveguide type filters 312 _{1 to} 312 _4. Select the oscillation wavelength. Each of the Si waveguide filters 312 _{1 to} 312 ₄ has a ring resonator and a Bragg reflector formed on the common substrate 301. The common substrate 301 has the same Si wire waveguide structure (see FIG. 7 or FIG. 8) as the substrate 10 in the wavelength multiplexing / demultiplexing device 1 shown in FIG. In each of the Si waveguide filters 312 _{1 to} 312 ₄ , the oscillation wavelength of the laser light is selected by controlling each parameter of the ring resonator and the Bragg reflector. Here, different oscillation wavelengths λ _{1 to} λ ₄ are selected by the Si waveguide type filters 312 _{1 to} 312 ₄ , and an optical coupler in which the oscillation light of the wavelengths λ _{1 to} λ ₄ is located inside the laser resonance structure is provided. Via the optical modulation unit 320.

Light modulation unit 320 is, for example, has four all-pass ring resonator modulator ₃₂₁ 1-321 ₄ formed on the common substrate 301. In the optical modulation unit 320, the laser beams having the wavelengths λ _{1 to} λ ₄ output from the light source unit 310 are respectively modulated by the corresponding modulators 321 _{1 to} 321 ₄ , and the modulated light is converted into the wavelength multiplexing unit 330. Is output.

In the wavelength multiplexing unit 330, three delay interferometers 331 ₁ , 331 _2A , and 331 _2B having a DMZI structure including a ring resonator in one arm waveguide are cascade-connected on a common substrate 301. The delay interferometers 331 ₁ , 331 _2A , and 331 _2B respectively correspond to the delay interferometers 20 ₁ , 20 _2A , and _202B in the wavelength multiplexing / demultiplexing device 1 shown in FIG. The optical signals of wavelengths λ ₁ and λ ₂ output from the optical modulator 320 are given to the delay interferometer 331 _2B, and the optical signals of wavelengths λ ₃ and λ ₄ are given to the delay interferometer 331 _2A . In the wavelength multiplexing unit 330, optical signals of wavelengths λ _{1 to} λ ₄ given to the delay interferometers 331 _2A and 331 _2B are combined into one to generate WDM light, and the WDM light is delayed and interfered. A total of 331 ₁ is transmitted to the outside.

  In the optical transmission apparatus 300 as described above, the multiplexing characteristics of the wavelength multiplexing unit 330 are similar to the multiplexing characteristics of the wavelength multiplexing / demultiplexing element 1 shown in FIG. Since the talk is excellent, even if the wavelength of each optical signal applied from the light source unit 310 to the wavelength multiplexing unit 330 via the light modulation unit 320 slightly varies, good multiplexing characteristics can be maintained. Therefore, the optical transmission device 300 can function as a good multi-channel transmitter.

FIG. 15 is a plan view showing a schematic configuration of an optical receiving apparatus to which the configuration of the wavelength multiplexing / demultiplexing element 1 is applied. Optical receiving apparatus 400 shown in FIG. 15, for example, a wavelength demultiplexer 410 which WDM light is given including the optical signals of different wavelengths lambda ₁ to [lambda] ₄ together, each wavelength demultiplexed by the wavelength demultiplexing unit 410 and a light receiving unit 420 that receives optical signals of λ _{1 to} λ ₄ .

In the wavelength demultiplexing unit 410, three delay interferometers 411 ₁ , 411 _2A , and 411 _2B having a DMZI structure including a ring resonator in one arm waveguide are cascade-connected on a common substrate 401. The common substrate 401 and each delay interferometer 411 ₁ , 411 _2A , 411 _2B correspond to the substrate 10 and each delay interferometer 20 ₁ , 20 _2A , _202B in the wavelength multiplexing / demultiplexing device 1 shown in FIG. doing. In the wavelength demultiplexing unit 410, the WDM light is given to the delay interferometer 411 ₁ , the WDM light is demultiplexed, and optical signals of wavelengths λ ₁ and λ ₂ are output from the delay interferometer 411 _{2 A} to the light receiving unit 420. In addition, the optical signals of wavelengths λ ₃ and λ ₄ are output from the delay interferometer 411 _2B to the light receiving unit 420.

The light receiving unit 420 includes four light receiving units 421 _{1 to} 421 ₄ , and the light receiving units 421 _{1 to} 421 ₄ corresponding to the optical signals of the wavelengths λ _{1 to} λ ₄ output from the wavelength demultiplexing unit 410 are provided. To detect each. Each of the light receivers 421 _{1 to} 421 ₄ can be formed, for example, by crystal growth of Ge on a common substrate (Si substrate) 401. Usually, in the case of a Ge type photo detector on a Si substrate, it is possible to detect an optical signal in a wavelength band of 1.5 to 1.6 μm.

  In the optical receiving apparatus 400 as described above, the spectral demultiplexing characteristic of the wavelength demultiplexing unit 410 is the same as that of the wavelength demultiplexing element 1 shown in FIG. Since the talk is excellent, good demultiplexing characteristics can be maintained even if the wavelength of each optical signal included in the received WDM light varies somewhat. Therefore, the optical receiver 400 can function as a highly efficient multi-channel receiver.

  Regarding the above embodiments, the following additional notes are further disclosed.

(Supplementary Note 1) A plurality of delay interferometers cascaded in multiple stages are provided, and a plurality of optical signals having different wavelengths are combined by the plurality of delay interferometers to generate WDM light, and WDM light is A wavelength multiplexing / demultiplexing element that generates a plurality of optical signals having different wavelengths by demultiplexing with a delay interferometer,
Each of the plurality of delay interferometers is
A pair of optical couplers through which the plurality of optical signals are input and output;
A first arm waveguide and a second arm waveguide connecting between the optical couplers;
A loop waveguide disposed in the vicinity of the first arm waveguide,
The first arm waveguide constitutes a ring resonator in combination with the loop waveguide;
The second arm waveguide is a delay waveguide relative to the first arm waveguide;
The optical path length difference between the first and second arm waveguides in each delay interferometer, and the round length of the ring resonator in each delay interferometer are the wavelengths and channels of the plurality of optical signals. A wavelength multiplexing / demultiplexing element, wherein a different value is set for each stage of the multistage cascade connection according to the interval.

(Additional remark 2) It is a wavelength multiplexing / demultiplexing element of Additional remark 1, Comprising:
The loop length of the ring resonator in each delay interferometer is set so that the transmission spectrum characteristic of the ring resonator satisfies the non-resonance condition with respect to the transmission spectrum characteristic of the delay interferometer. A wavelength multiplexing / demultiplexing device characterized.

(Additional remark 3) It is a wavelength multiplexing / demultiplexing element of Additional remark 2, Comprising:
In the multistage cascade connection, each of the two delay interferometers located in the second stage is cascade-connected to one delay interferometer located in the first stage, and each time the second stage and subsequent stages are overlapped, The number of the delay interferometers is a connection configuration that increases at a rate of 2 ^S-1 where S is the number of stages.
When the optical path length difference between the first and second arm waveguides in the delay interferometer located in the first stage of the multistage cascade connection is L _D1 , the plurality of delay interferometers The optical path length difference between the first and second arm waveguides in the delay interferometer located in the S-stage is set to 2 ^(1-S) × _LD1, and the delay interferometer in the S-stage is included in the delay interferometer. The circumference of the ring resonator is set to 2 ^(2-S) × L _D1 , and the center wavelength of the ring resonator deviates from the center wavelength of the delay interferometer by approximately half of the ring resonance frequency interval. A wavelength multiplexing / demultiplexing element characterized by comprising:

(Supplementary note 4) The wavelength multiplexing / demultiplexing device according to supplementary note 3, wherein
The plurality of delay interferometers are arranged in each delay interferometer located in the second stage when the center wavelength of the ring resonator in the delay interferometer located in the first stage of the multistage cascade connection is λ ₀ . The center wavelength of the ring resonator is set to λ ₀ + 0.5 × Δν and λ ₀ −0.5 × Δν, where Δν is the channel interval of the plurality of optical signals, and is positioned after the third stage. With respect to the center wavelength of the ring resonator in each delay interferometer, the center wavelength difference in each adjacent delay interferometer is centered on the center wavelength in the delay interferometer of the previous stage each time the stage is overlapped. Is set to 2 ^(S-2) × Δν.

(Additional remark 5) It is a wavelength multiplexing / demultiplexing element as described in any one of Additional remark 1-4, Comprising:
The wavelength multiplexing / demultiplexing element according to claim 1, wherein the ring resonator in each delay interferometer has an optical coupling ratio between the first arm waveguide and the loop waveguide of 80% or more.

(Appendix 6) The wavelength multiplexing / demultiplexing device according to appendix 5,
The ring resonator in each delay interferometer is characterized in that optical coupling between the first arm waveguide and the loop waveguide is performed via a directional coupler or a multimode interference coupler. Wavelength multiplexing / demultiplexing device.

(Appendix 7) The wavelength multiplexing / demultiplexing device according to any one of appendices 1 to 6,
In the plurality of delay interferometers, the pair of optical couplers each have an optical coupling rate of 50%.

(Supplementary Note 8) A light source unit that generates a plurality of lights having different wavelengths, a light modulation unit that modulates light of each wavelength from the light source unit, and each optical signal modulated by the light modulation unit And a wavelength multiplexing unit that transmits WDM light,
The optical device, wherein the wavelength multiplexing unit includes the wavelength multiplexing / demultiplexing device according to any one of appendices 1 to 7.

(Supplementary note 9) The optical device according to supplementary note 8,
The optical device, wherein the light source unit, the light modulation unit, and the wavelength multiplexing unit are formed on a common silicon substrate.

(Supplementary Note 10) A wavelength demultiplexing unit that receives and demultiplexes WDM light including a plurality of optical signals having different wavelengths, and a light receiving unit that receives an optical signal of each wavelength demultiplexed by the wavelength demultiplexing unit And an optical device comprising:
An optical apparatus, wherein the wavelength demultiplexing unit includes the wavelength multiplexing / demultiplexing element according to any one of appendices 1 to 7.

(Supplementary note 11) The optical device according to supplementary note 10,
The optical device, wherein the wavelength demultiplexing unit and the light receiving unit are formed on a common silicon substrate.

1 ... wavelength mux 10 ... substrate _{20 1, 20 2A, 20 2B} , 20 3A ~20 3D ... delay interferometer 21, 22 ... optical coupler 23 ... arm waveguides 25 ... loop waveguide 26 ... phase shifter 101 ... Si substrate 102 _... SiO 2 (BOX) layer 103 ... Si core layer 104 ... SiO ₂ film 300 ... optical transmitter 301, 401 ... common substrate 310 ... light source 311 _1-311 4 _... laser 312 _{1-312 4} ... Si waveguide type filter 320... Optical modulation unit 321 _{1 to} 321 ₄ ... All-pass ring resonator type modulator 330 .. wavelength combining unit 331 ₁ , 331 _2A , 331 _2B , 411 ₁ , 411 _2A , 411 _2B . 400 ... optical receiver 410 ... wavelength demultiplexing unit 420 ... receiving portion 421 _1-421 4 _... photoreceiver L _D1 ~L _D3 ... Path length difference L _M1 ~L _{M3 ...} circumferential length

Claims (4)

A plurality of delay interferometers cascaded in multiple stages are provided, and a plurality of optical signals having different wavelengths are combined by the plurality of delay interferometers to generate WDM light, while WDM light is generated by the plurality of delay interferometers. A wavelength multiplexing / demultiplexing element that demultiplexes and generates a plurality of optical signals having different wavelengths,
Each of the plurality of delay interferometers is
A pair of optical couplers through which the plurality of optical signals are input and output;
A first arm waveguide and a second arm waveguide connecting between the optical couplers;
A loop waveguide disposed in the vicinity of the first arm waveguide,
The first arm waveguide constitutes a ring resonator in combination with the loop waveguide;
The second arm waveguide is a delay waveguide relative to the first arm waveguide;
The optical path length difference between the first and second arm waveguides in each delay interferometer, and the round length of the ring resonator in each delay interferometer are the wavelengths and channels of the plurality of optical signals. Depending on the interval, it is set to a different value for each stage of the multistage cascade connection ,
The round length of the ring resonator in each delay interferometer is set so that the transmission spectrum characteristic of the ring resonator satisfies the non-resonance condition with respect to the transmission spectrum characteristic of the delay interferometer,
In the multistage cascade connection, each of the two delay interferometers located in the second stage is cascade-connected to one delay interferometer located in the first stage, and each time the second stage and subsequent stages are overlapped, The number of the delay interferometers is a connection configuration that increases at a rate of 2 ^S-1 where S is the number of stages .
When the optical path length difference between the first and second arm waveguides in the delay interferometer located in the first stage of the multistage cascade connection is L _D1 , the plurality of delay interferometers The optical path length difference between the first and second arm waveguides in the delay interferometer located in the S-stage is set to 2 ^(1-S) × _LD1, and the delay interferometer in the S-stage is included in the delay interferometer. The circumference of the ring resonator is set to 2 ^(2-S) × L _D1 ,
The plurality of delay interferometers are arranged in each delay interferometer located in the second stage when the center wavelength of the ring resonator in the delay interferometer located in the first stage of the multistage cascade connection is λ ₀ . The center wavelength of the ring resonator is set to λ ₀ + 0.5 × Δν and λ ₀ −0.5 × Δν, where Δν is the channel interval of the plurality of optical signals, and is positioned after the third stage. With respect to the center wavelength of the ring resonator in each delay interferometer, the center wavelength difference in each adjacent delay interferometer is centered on the center wavelength in the delay interferometer of the previous stage each time the stage is overlapped. There ^{2 (S-2)} is set to × .DELTA..nu WDM device characterized Rukoto. The wavelength multiplexing / demultiplexing device according to claim 1 ,
The wavelength multiplexing / demultiplexing element according to claim 1, wherein the ring resonator in each delay interferometer has an optical coupling ratio between the first arm waveguide and the loop waveguide of 80% or more. A light source that generates a plurality of lights having different wavelengths, an optical modulator that modulates light of each wavelength from the light source, and an optical signal modulated by the optical modulator to multiplex WDM light An optical device comprising a wavelength multiplexing unit for transmitting
The wavelength multiplexing unit, an optical device which comprises a wavelength multiplexing and demultiplexing device according to claim 1 or 2. A wavelength demultiplexing unit that receives and demultiplexes WDM light including a plurality of optical signals having different wavelengths, and a light receiving unit that receives an optical signal of each wavelength demultiplexed by the wavelength demultiplexing unit. Optical device,
It said wavelength demultiplexing unit is an optical device which comprises a wavelength multiplexing and demultiplexing device according to claim 1 or 2.