JP4129911B2 - Wavelength filter and tunable filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength filter and a wavelength tunable filter, which are important optical components that support wavelength-multiplexed large-capacity optical communication.
[0002]
[Prior art]
In recent years, due to the explosive increase in traffic on the Internet, transmission capacity has been increased using wavelength multiplexing for transmission between nodes. A wavelength filter or a wavelength tunable filter is an important optical component indispensable in such wavelength division multiplexing transmission.
[0003]
Under such circumstances, as shown in FIG. 13, a tunable filter using an arrayed waveguide grating filter (AWGF) has been proposed. In FIG. 13, an AWG (arrayed waveguide grating) is formed by connecting a slab waveguide 101 forming a free space area on the input side and a slab waveguide 102 forming a free space area on the output side by a waveguide array 103. In the waveguide array 103, the optical path length difference between adjacent waveguides 104 is different by a certain length. Therefore, when the input light 105 enters the slab waveguide 101, it spreads in the slab waveguide 101 and enters each waveguide 104 of the waveguide array 103, and then enters each output side slab waveguide 102. The slab waveguide 102 interferes with the phase difference caused by the optical path length difference, and only the light having a specific wavelength corresponding to the optical path length difference becomes the output light 106. The selected wavelength can be made variable by forming the electrode 107 on the waveguide array 103 and performing current injection to change the refractive index of each waveguide 104 under the electrode.
[0004]
[Non-Patent Document 1]
ELECTRONICS LERTTERS Vol.36, No.7, pp.658-660 (2000), Fig.1
[0005]
[Problems to be solved by the invention]
In the arrayed waveguide grating filter, since the optical path length difference between adjacent waveguides 104 in the waveguide array 103 is different by a certain length, the arrayed waveguide grating filter is operated as a wavelength filter. In addition, it is necessary to change by a certain length Δs between the adjacent waveguides 104. For this reason, the length of the selective wavelength variable electrode 107 is the longest (N−1) × Δs, where N is the number of waveguides of the waveguide array 103, and the electrode length of the entire filter is N × (N− 1) It becomes large with xΔs / 2.
[0006]
For this reason, there is a problem that the power consumption increases and the problem of heat generation occurs, the problem that the electrostatic capacity increases and the high-speed response cannot be performed, and the yield of the filter decreases.
[0007]
The present invention has been made under the above background, and an object of the present invention is to provide a wavelength tunable filter that can reduce the electrode length of the entire filter without impairing the filter characteristics, and has a large wavelength tunable range. Is to provide. Another object of the present invention is to provide a low-loss and low-cost wavelength filter with a high extinction ratio that is the basis of such a wavelength tunable filter.
[0008]
[Means for Solving the Problems]
A first invention is a wavelength filter, an input waveguide, an output waveguide, a plurality of input waveguide side optical couplers arranged at regular intervals in the input waveguide, and the regular intervals in the output waveguide. A plurality of optical couplers on the output waveguide side, which are arranged at regular intervals different from each other, and between the input waveguide and the output waveguide, the optical couplers on the input waveguide side and the output waveguide side A plurality of connection waveguides connected via the optical coupler, and a plurality of optical path lengths from the incident end of the input waveguide to the output end of the output waveguide through the connection waveguide The length of the connection waveguide is increased or decreased in order from the path passing through the connection waveguide near the end, and the length of the connection waveguide is the same difference in order from the connection waveguide near the incident end. Increase or decrease in Thus, each of the plurality of optical couplers on the output waveguide side causes the light incident from the connection waveguide to interfere with the light incident from the output waveguide, so that the optical coupler on the input waveguide side Propagate only light of the same wavelength determined by the interval, the interval between the optical couplers on the output waveguide side, and the difference between the lengths of the connection waveguides. It is characterized by. As a result, it is possible to realize a wavelength filter with a high extinction ratio, low loss, and low cost.
[0009]
The wavelength filter of the second invention is the wavelength filter according to the first invention, wherein the coupling coefficient of the optical coupler on the input waveguide side is smaller as it is closer to the incident end, and the coupling coefficient of the optical coupler on the output waveguide side is smaller. The closer to the exit end, the smaller. This improves the filter characteristics such as the extinction ratio of the output spectrum.
[0010]
A third invention is a wavelength tunable filter. In the first or second invention, at least one of the refractive index heat, the current injection, and the voltage sign of the input waveguide and the output waveguide. In addition Refractive index changing means for changing it further is provided. As a result, it is possible to realize a large wavelength variable range with a shorter electrode length than before. In this case, in addition to the input waveguide or the output waveguide, the refractive index of the waveguide constituting the optical coupler on the input waveguide side or the optical coupler on the output waveguide side or both optical couplers is also changed by the refractive index changing means. By making it change at, the wavelength variable range becomes larger.
[0011]
A wavelength tunable filter according to a fourth aspect of the present invention is a wavelength tunable filter in which a first wavelength tunable filter having a large output wavelength interval and a second wavelength tunable filter having a small output wavelength interval are combined, and at least the first wavelength tunable filter. Is the wavelength tunable filter of the third invention. Thereby, the wavelength variable range is large and the transmission bandwidth of the selected wavelength is narrowed.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0013]
In the present invention, the input light, which is performed by a normal arrayed waveguide grating filter (AGFF), is not expanded by the incident-side slab waveguide and is incident on the waveguide array. Is branched from a common input waveguide into a plurality of connection waveguides one by one using a plurality of optical couplers, and each is joined to a common output waveguide using a plurality of optical couplers. Configure. Although details will be described later, by changing the refractive index of both or one of the input waveguide and the output waveguide, the optical path lengths of a plurality of paths from the incident end to the output end are sequentially changed, and the wavelength variable filter is obtained. Operate.
[0014]
[First embodiment]
FIG. 1 shows an embodiment of a wavelength filter related to the invention according to claim 1.
[0015]
The wavelength filter shown in FIG. 1 is formed, for example, by forming a waveguide constituting a wavelength filter on an InP-based compound semiconductor, and includes an input waveguide 1 and a plurality of n optical couplers 3 on the input waveguide 1 side. And an output waveguide 5, a plurality of n optical couplers 7 on the output waveguide 5 side, and a connection waveguide array 9. Similarly, the connection waveguide array 9 includes a plurality of n connection waveguides 11 arranged in an array.
[0016]
In FIG. 1, the input waveguide 1 and the output waveguide 5 are slightly non-parallel, and are slightly open from the parallel, for example, 5 degrees in the right direction. Of these, the left end 13 is an incident end, and in the output waveguide 5, the right end 19 of the left and right ends 17 and 19 is an exit end. Therefore, light enters the input end 13 in the input waveguide 1 and propagates from the left to the right as indicated by the arrow 21, and similarly propagates from the left to the right in the output waveguide 5 as indicated by the arrow 23. And exits from the exit end 19.
[0017]
The n optical couplers 3 are arranged on the input waveguide 1 at a constant equal interval ΔL1, and the n optical couplers 7 are arranged on the output waveguide 5 at a constant equal interval ΔL2. In this example, the coupling coefficients of the optical couplers 3 on the input waveguide 1 side are all set to the same value k1, and the coupling coefficients of the optical couplers 7 on the output waveguide 5 side are all set to the same value k2. The coupling coefficient k1 and the coupling coefficient k2 may be different from each other or the same. Further, the interval ΔL1 and the interval ΔL2 may be different from each other or the same.
[0018]
Each connection waveguide 11 of the connection waveguide array 9 connects the input waveguide 1 and the output waveguide 5 via one optical coupler 3 and one optical coupler 7.
[0019]
Specifically, the number i (i: 1 to n) is sequentially assigned to the n optical couplers 3 with the one closest to the incident end 13 being first, and the n optical couplers 7 are The number i is assigned in order with the one farthest from the output end 19 as the first, and the i-th connection is given to the n connection waveguides 11 with the number i closest to the incident end 13 in order. The waveguide 11 connects the input waveguide 1 and the output waveguide 5 via the i-th optical coupler 3 and the i-th optical coupler 7.
[0020]
Further, when the length of the i-th connection waveguide 11 is Li, in this example, the length Li is longer than the length Li-1 of the i-1th connection waveguide 11 by ΔL.
[0021]
Therefore, if the distance between the incident end 13 and the first optical coupler 3 is Lin and the distance between the emission end 19 and the first optical coupler 7 is Lout, the optical path length from the incident end 13 to the emission end 19 is Among them, since the optical path length Si of the path passing through the i-th connection waveguide 11 is Lin + (i−1) ΔL1 + Li + Lout− (i−1) ΔL2, the optical path of the path passing through the i−1th connection waveguide 11 It is longer by ΔS (= ΔL1 + Li−Li−1−ΔL2 = ΔL1−ΔL2 + ΔL) than the length Si−1 (= Lin + (i−2) ΔL1 + Li−1 + Lout− (i−2) ΔL2). In other words, in this example, a plurality of optical path lengths from the incident end 13 of the input waveguide 1 through the connection waveguides 11 to the output end 19 of the output waveguide 5 pass through the connection waveguide 11 close to the incident end 13. In order from the same by the difference of the length ΔS. Note that ΔL1 and ΔL2 have arbitrary lengths, but if ΔL1 = ΔL2, ΔS = ΔL.
[0022]
Next, the operation principle of the wavelength filter will be described with reference to FIG.
[0023]
First, the propagation of light from the input end 13 to the output end 19 will be described. Input light incident on the input end 13 is connected to the plurality of connection waveguides 11 one by one from the common input waveguide 1 by the plurality of optical couplers 3. Each of these branches into a common output waveguide 5 by a plurality of optical couplers 7 and exits from the exit end 19.
[0024]
In FIG. 1, the placement location of the first optical coupler 3 from the left on the input waveguide 1 is point A, and the placement location of the second optical coupler 7 from the left on the output waveguide 5 is the point B. Then, there are two paths in which light travels from point A to point B: a path passing through the first connection waveguide 11 from the incident end 13 and a path passing through the second connection waveguide 11 from the incident end 13. . The distance difference ΔS at this time is given by the following equation (1).
ΔS = ΔL 1 + L 2 −L 1 −ΔL 2 = ΔL 1 −ΔL 2 + ΔL (1)
[0025]
In each optical coupler 7 on the output waveguide 5, the light incident from the connection waveguide 11 interferes with the light incident from the output waveguide 5 due to the optical path length difference, and only the light having the same wavelength is output waveguide. 5 propagates to the next optical coupler 7 and light of other wavelengths is subjected to radiation loss. Therefore, when the length is increased by ΔL in order from the connection waveguide 11 closer to the incident end 13 between the adjacent connection waveguides 11, ΔS is always the same since ΔS = ΔL1−ΔL2 + ΔL. Since light of a wavelength propagates through the output waveguide 5 and light of other wavelengths receives a radiation loss, it operates as a good wavelength filter. At this time, the wavelength λ of light that can be output light ₀ Satisfies the following equation (2).
λ ₀ = N _eff ΔS / m (2)
Here, m is an arbitrary integer satisfying the formula (2), n _eff Is the effective refractive index. Effective refractive index n _eff Is determined by the waveguide material. ΔS and m are the desired wavelengths λ ₀ On the other hand, any value can be used as long as the ratio ΔS / m is constant.
[0026]
FIG. 2 shows the transmission characteristics of the wavelength filter manufactured with the configuration of FIG. In this case, the number of connection waveguides 11 is 50, and the integer m is 30. In order to set the center wavelength to 1.55 μm, ΔL was set to 13.7 μm, and ΔL1 and ΔL2 were set to the same value. After all, ΔS is 13.7 μm. At this time, a transmittance of 2.95 dB at the center wavelength and an extinction ratio of 13.2 dB of the output spectrum were obtained, and a low-loss and high extinction ratio wavelength filter was realized at low cost. If ΔL1 = ΔL2, these values may be arbitrary, but as an example, it is set to 100 μm. When ΔL1 ≠ ΔL2, ΔS satisfies Expression (1).
[0027]
In this example, the length of the adjacent connection waveguides 11 is set to increase by ΔL as the distance from the incident end 13 increases, but the same effect can be obtained by setting the length to decrease by ΔL. be able to. In this case, a plurality of optical path lengths from the incident end 13 to the output end 19 through the connection waveguides 11 decreases in order from the path passing through the connection waveguide 11 close to the incident end 13 by the same difference ΔS.
[0028]
Further, in FIG. 1, even if the input waveguide 1 is replaced with the output waveguide, the output waveguide 5 is replaced with the input waveguide, the right end 19 is replaced with the input end, and the left end 13 is replaced with the output end, the wavelength having the same effect is obtained. Acts as a filter. In this case, since the length of the adjacent connection waveguides 11 decreases or increases by ΔL as the distance from the incident end 19 increases, a plurality of connections from the incident end 19 through each connection waveguide 11 to the emission end 13 are performed. Is decreased or increased in order from the path passing through the connection waveguide 11 close to the incident end 19 with the same difference ΔS.
[0029]
[Second Embodiment]
In FIG. 1, the light propagation directions in the input waveguide 1 and the output waveguide 3 are the same as indicated by arrows 21 and 23, but the input light incident on the incident end is common even in the reverse direction. The plurality of optical couplers 3 and the plurality of optical couplings are branched from the input waveguide 1 to the plurality of connection waveguides 11 one by one, and merge into the common output waveguide 5 and exit from the exit end. As long as the device 7 is configured, the same effect can be obtained. For example, by making the incident end and the output end the same as the left end 13 of the input waveguide 1 and the output end 5 being the left end 18 of the output waveguide 5, the input waveguide 1 and the output are made the same. The light propagation direction in the waveguide 3 is reversed. An example of this aspect is shown in FIG.
[0030]
In the wavelength filter shown in FIG. 3, the input waveguide 1 and the output waveguide 5 are non-parallel, the input waveguide 1 is located on the lower right side above the output waveguide 5, and the output waveguide 5 is on the left. The left ends 13 and 17 which are positioned at the lower side and are spaced apart from each other between the input waveguide 1 and the output waveguide 5 are an entrance end and an exit end, respectively. Accordingly, the light enters the incident end 13 in the input waveguide 1 and propagates in the direction 21 descending from left to right, and the output waveguide 5 propagates in the direction 23 descending from right to left and exits from the exit end 17. .
[0031]
In FIG. 3, n optical couplers 3 are numbered i (i: 1 to n) in order, with the one farthest from the incident end 13 as the first, and the n optical couplers 4 The number i is assigned in order with the one farthest from the exit end 17 as the first, and the number i of the n connection waveguides 11 is given when the number i is given in order with the one farthest from the incident end 13 as the first. The connection waveguide 11 connects the input waveguide 1 and the output waveguide 5 via the i-th optical coupler 3 and the i-th optical coupler 7.
[0032]
In FIG. 3, when the length of the i-th connection waveguide 11 is Li, the length Li is longer than the length Li-1 of the i-1th connection waveguide 11 by ΔL. .
[0033]
However, in the case of FIG. 3, the expression satisfied as the wavelength filter is the following expression (3) instead of the expression (1). Wavelength λ of output light ₀ Is given by equation (2).
ΔS = ΔL1 + ΔL2 + L1-L2 = ΔL1 + ΔL2-ΔL (3)
[0034]
In the wavelength filter of FIG. 3, the length increases by ΔL in order from the connection waveguide 11 that is farther from the incident end 13 between the adjacent connection waveguides 11, and therefore passes through each connection waveguide 11 from the incident end 13. Speaking of a plurality of optical path lengths to the output end 18, when ΔS = ΔL1 + ΔL2−ΔL> 0, the lengths increase in the same length ΔS sequentially from the path passing through the connection waveguide 11 close to the input end 13, and ΔL1 + ΔL2−. In the case of ΔL <0, the distance decreases in the order of the same length difference ΔS from the path passing through the connection waveguide 11 close to the incident end 13. In either case, always λ ₀ = N _eff Wavelength λ satisfying | ΔS | / m ₀ The light of the other wavelength propagates through the output waveguide 5, and the light of other wavelengths suffers radiation loss, so that it operates as a good wavelength filter.
[0035]
In FIG. 3, even if the input waveguide 1 is replaced with the output waveguide, the output waveguide 5 is replaced with the input waveguide, the left end 17 is replaced with the input end, and the left end 13 is replaced with the output end, the wavelength having the same effect is obtained. Acts as a filter.
[0036]
In FIG. 3, the length of the adjacent connection waveguide 5 is set to increase by ΔL as the distance approaches the incident end 13. However, the same effect can be obtained by setting to decrease by ΔL. Obtainable.
[0037]
Furthermore, the aspect in which the length of the adjacent connection waveguides 5 decreases by ΔL as the distance approaches the incident end 13 is, for example, that the right end 15 of the input waveguide 1 is the incident end in FIG. The right end 19 is an exit end, and light incident on the entrance end 15 is branched from the common input waveguide 1 to the plurality of connection waveguides 11 one by one, and each merges with the common output waveguide 5 and exits. This can be realized by configuring the plurality of optical couplers 3 and the plurality of optical couplers 7 so as to emit from the end 19. However, in this case, the expression satisfied as a wavelength filter is the following expression (4) instead of the expression (3). Wavelength λ of output light ₀ Is given by equation (2). If the lengths of the adjacent connection waveguides 11 are increased by ΔL as the distance from the incident end 15 increases, a plurality of optical path lengths from the incident end 15 through the connection waveguides 11 to the output end 19 are incident. In order from the path passing through the connection waveguide 11 close to the end 15, the distance increases by the difference of the same length ΔS given by the following equation (4).
ΔS = ΔL1 + ΔL2 + ΔL (4)
[0038]
[Third embodiment]
Next, a wavelength filter related to the invention according to claim 2 is shown in FIG.
[0039]
Compared with the wavelength filter shown in FIG. 1, the wavelength filter shown in FIG. 4 has a smaller coupling coefficient of the optical coupler 3 disposed in the input waveguide 1 as it is closer to the left end (incident end) 13 and has a smaller output. The difference is that the coupling coefficient of the optical coupler 4 disposed in the waveguide 5 is smaller as it is closer to the right end (outgoing end) 19 and that directional couplers are used as these optical couplers 3 and 4. Are the same. In FIG. 4, the same functional parts as those in FIG.
[0040]
In the directional coupler, the coupling coefficient increases as the distance between the two waveguides is narrowed and as the distance between the portions where the light travels close to each other is longer.
[0041]
In FIG. 4, both the optical coupler 3 and the optical coupler 7 use a directional coupler configured using a strip-mounted waveguide 25 so as to enlarge and show one waveguide in the portion C. ing. This is to obtain a large coupling coefficient at a short distance.
[0042]
The strip-mounted waveguide 25 includes a core layer 27, an upper clad layer 29 that is etched to a position directly above the core layer 27 and has an uneven shape along the light propagation direction, and a lower clad layer 31.
[0043]
In the wavelength filter of FIG. 4, the end of the connection waveguide 11 is close to the input waveguide 1 at each position corresponding to the optical coupler 3, and the end of the connection waveguide 11 and the input are input in this proximity region. The waveguides 1 are both strip-mounted waveguides 25 and the recesses of the upper cladding layer 29 are arranged side by side. Therefore, light leaks from the concave portion of the upper clad layer 29 of the input waveguide 1 to the concave portion of the upper clad layer 29 of the connection waveguide 11, and a large coupling is obtained at a short distance.
[0044]
Similarly, for each position corresponding to the optical coupler 7, the end of the connection waveguide 11 is close to the output waveguide 5. In the close region, the end of the connection waveguide 11 and the output waveguide 5 are Both are strip-mounted waveguides 25 and the recesses of the upper cladding layer 29 are arranged side by side. Accordingly, light leaks from the concave portion of the upper clad layer 29 of the connection waveguide 11 to the concave portion of the upper clad layer 29 of the output waveguide 5, and a large coupling is obtained at a short distance.
[0045]
In the optical coupler 3 on the input waveguide 1 side, the waveguide interval is narrowed and the distance between the adjacent portions is increased so that the coupling coefficient increases as the distance from the incident end 13 increases. Also, the optical coupler 7 on the output waveguide 5 side is also narrowed in the waveguide interval and long in the adjacent portion so that the coupling coefficient increases as the distance from the output end 19 increases. . In other words, the closer to the incident end 13, the smaller the coupling coefficient of the optical coupler 3, and the closer to the exit end 19, the smaller the coupling coefficient of the optical coupler 4.
[0046]
Specifically, the coupling coefficients of the optical couplers 3 and 7 are changed as follows.
(1) If it is assumed that all the optical couplers 3 and 7 are manufactured by using directional couplers having the same coupling coefficient, that is, adjacent waveguide lengths, the adjacent optical couplers 3 are ΔL1. It is assumed that the connecting waveguides 11 constituting the directional coupler are arranged so that the adjacent optical couplers 7 are spaced at a constant interval of ΔL2. Each tip position P of the connection waveguide 11 at this time _o Is the reference position.
(2) As the coupling coefficient is smaller, the tip position of the connection waveguide 11 of the optical coupler 3 is set to the reference position P. _o The distance from the input end 13 is further reduced, and the length of the portion adjacent to the input waveguide 1 is shortened. Similarly, for the optical coupler 7, the smaller the coupling coefficient is, the more the position of the tip of the connection waveguide 11 is set to the reference position P. _o The distance from the output end 19 is further reduced, and the length of the portion adjacent to the output waveguide 5 is shortened.
(3) On the other hand, as the coupling coefficient is larger, the tip position of the connection waveguide 11 of the optical coupler 3 is set to the reference position P. _o Further, the length of the portion close to the input waveguide 1 is increased closer to the input end 13. Similarly, for the optical coupler 7, the larger the coupling coefficient, the more the position of the tip of the connection waveguide 11 is set to the reference position P. _o Further, the length of the portion close to the output waveguide 5 is increased closer to the output end 19.
[0047]
Thus, the closer to the incident end 13, the smaller the coupling coefficient of the optical coupler 3, and the closer to the exit end 19, the smaller the coupling coefficient of the optical coupler 7, so that the extinction ratio of the output spectrum is reduced. Thus, the characteristics of the wavelength filter are improved.
[0048]
The reason is as follows.
(1) As described above, in the optical coupler 7 on the output waveguide 5 side, the light propagating through the output waveguide 5 and the optical waveguide 3 branched from the input waveguide 1 and propagated through the connection waveguide 11. In this case, only light having a wavelength in phase propagates through the output waveguide 5 and propagates toward the next optical coupler 7, and this is repeated to repeat the wavelength having a strong resonance peak. Acts as a filter.
(2) However, the intensity ratio of the input light from the two waveguides (in FIG. 4, the output waveguide 5 and the connection waveguide) when this resonance occurs suddenly changes for each optical coupler 7. Then, side lobes are generated as shown in FIG.
(3) Therefore, by reducing the coupling coefficient of the optical coupler 3 closer to the incident end 13 and decreasing the coupling coefficient closer to the optical coupler 7 closer to the outgoing end 19, two guides for causing resonance are obtained. The intensity ratio of input light from the waveguide does not change or gradually changes in each optical coupler, and the side lobe is reduced. That is, the extinction ratio is improved.
[0049]
FIG. 5 shows the transmission characteristic 33 of the wavelength filter manufactured with the configuration of FIG. 4 by a solid line. The number of connection waveguides 11 is 50, and the integer m is 30. Further, ΔL was set to 13.7 μm and ΔL1 = ΔL2 so that the center wavelength was 1.55 μm. For reference, the transmission characteristic 35 when the coupling coefficient is the same for all optical couplers is indicated by a broken line, but the wavelength filter of the wavelength filter set by changing the coupling coefficients of the optical couplers 3 and 7 as described above. It can be seen that the extinction ratio is improved by 20 dB or more.
[0050]
In order to improve the extinction ratio, as shown in this example, the coupling coefficient closer to the optical coupler 3 closer to the incident end 13 and the coupling coefficient closer to the optical coupler 7 closer to the emission end 19 may be decreased. Although it is preferable to reduce the coupling coefficient closer to the input end 13 only for the optical coupler 3, or to reduce the coupling coefficient closer to the output end 19 only for the optical coupler 7, An effect of improving the extinction ratio can be obtained.
[0051]
[Fourth embodiment]
Next, another wavelength filter related to the invention of claim 2 is shown in FIG.
[0052]
The wavelength filter shown in FIG. 6 is a Mach-Zehnder interferometer using multimode couplers 37 and 39 in place of the directional coupler, as compared with the wavelength filter shown in FIG. The optical coupler 7 and the optical coupler 7 on the output waveguide 5 side are different, and the coupling coefficient of the optical coupler 3 is smaller as it is closer to the incident end 13, and the coupling coefficient of the optical coupler 7 is smaller than the outgoing end 19. Other points are the same, such as smaller points closer to. In FIG. 6, the same functional parts as those in FIG.
[0053]
Normally, in the Mach-Zehnder interferometer, when the optical path lengths of the two arms 41 and 43 are equal, all light is output to the cross port 45 and no output is output to the bar port 47. This corresponds to the case where the coupling coefficient of the optical couplers 3 and 7 is zero.
[0054]
In each Mach-Zehnder interferometer shown in FIG. 6, a thin waveguide 49 having a width different from that of the other arm 41 is provided only on one arm 43 of the two arms 41 and 43 each having a tapered waveguide. Connected. However, the lengths of the two arms 41 and 43 are physically the same.
[0055]
Specifically, both the arms 41 and 43 are provided with a tapered waveguide that is continuous with a thick waveguide and has a width that is gradually narrowed, and a tapered waveguide that is gradually thicker and that is continuous with a thick waveguide. In the arm 41, the two tapered waveguides are directly connected to each other through the thin portions, in other words, the waveguide having a length of zero. In the arm 43, the thin portions of the two tapered waveguides are connected by the thin waveguide 49 on condition that the physical lengths of the two arms 41 and 43 are the same. The width of the waveguide 49 is the same as the width at the connection point of the two tapered waveguides in the arm 41.
[0056]
As described above, the Mach-Zehnder interferometer in which only one of the two arms 41 and 43 is connected to the thin waveguide 49 having a different width operates as an optical coupler. That is, since the effective refractive index is different between the two arms 41 and 43 due to the difference in the waveguide width, the optical path length is physically different even in the arms 41 and 43 having the same length, and the arm is also in the bar port 47. Light is output according to the optical path length difference between 41 and 43.
[0057]
Further, in the Mach-Zehnder interferometer constituting each optical coupler 3 on the input waveguide 1 side, the length D1 of the waveguide 49 is increased as the optical coupler 3 is moved away from the incident end 13, thereby The closer the optical coupler 3, the smaller the coupling coefficient. Similarly, also in the Mach-Zehnder interferometer constituting each optical coupler 7 on the output waveguide 5, the length D <b> 2 of the waveguide 49 is increased as the optical coupler 7 moves away from the emission end 19, thereby the emission end 19. The coupling coefficient of the optical coupler 7 closer to is made smaller. The physical length of the entire arm 43 to which the waveguide 49 is connected is not changed.
[0058]
FIG. 7 shows the relationship between the lengths D1 and D2 of the narrow waveguide 49 and the amplitude of light output to the bar port 47 (corresponding to the coupling coefficient of the optical coupler). As is apparent from FIG. 7, it can be seen that the coupling coefficient increases almost in proportion to the lengths D1 and D2.
[0059]
In FIG. 6, such a Mach-Zehnder interferometer is arranged in the input waveguide 1 and the output waveguide 5 to form an optical coupler 3 and an optical coupler 7, and a bar port of the Mach-Zehnder interferometer on the input waveguide 1 side. 47 and the bar port 47 of the Mach-Zehnder interferometer on the output waveguide 5 side are connected by the connection waveguide 11.
[0060]
In other words, in the input waveguide 1, the length D 1 of the waveguide 49 is increased as the Mach-Zehnder interferometer moves away from the incident end 13, so that the coupling coefficient of the optical coupler 3 becomes smaller as the distance is closer to the incident end 13. ing. Similarly, in the output waveguide 5, the length D <b> 2 of the waveguide 49 is increased as the Mach-Zehnder interferometer moves away from the emission end 19, so that the coupling coefficient of the optical coupler 7 is smaller as the distance from the emission end 19 is closer. It has become.
[0061]
Therefore, the wavelength filter shown in FIG. 6 also has a transmission characteristic with a high extinction ratio similar to that in FIG.
[0062]
In this example, the two waveguides of the arm 41 are directly connected with each other between the thin portions, but may be connected with a thin waveguide having a fixed length instead.
[0063]
[Fifth embodiment]
Next, a wavelength tunable filter related to the invention according to claim 3 is shown in FIG.
[0064]
The wavelength tunable filter shown in FIG. 8 has the same waveguide structure as the wavelength filter shown in FIG. 1 or FIG. 4, but the refractive index of the input waveguide 1 is changed to the refractive index changing means 51 and the output waveguide. 5 is different in that a refractive index changing means 53 for changing the refractive index is provided. Reference numeral 52 denotes a power supply path to the refractive index changing means 51, and 54 denotes a power supply path to the refractive index changing means 53. In FIG. 8, the same functional parts as those in FIG. 1 or FIG.
[0065]
Even if only the refractive index of the input waveguide 1 changes, only the refractive index of the output waveguide 5 changes, or both refractive indexes change, the selected wavelength changes as described below. Therefore, it operates as a wavelength tunable filter. Further, not only the input waveguide 1 and the output waveguide 5 but also the optical couplers 3 and 7 thereof, that is, the optical couplers 3 and 7 on the side of the optical couplers 3 and 7 that are optically coupled to the input waveguide 1 and the output waveguide 5 are provided. By providing the refractive index changing means 51 and the refractive index changing means 53 in the waveguide, the selection wavelength similarly changes.
[0066]
In this example, the refractive index changing means 51 is provided with respect to the input waveguide 1 over the entire length from the left end optical coupler 3 to the right end optical coupler 3. Similarly, the refractive index changing means 53 is also provided over the entire length from the left end optical coupler 7 to the right end optical coupler 7 with respect to the output waveguide 5.
[0067]
Therefore, in the wavelength filter of FIG. 8 having the same waveguide structure as FIG. 1 or FIG. 4, the refractive index of the input waveguide 1 is n. _eff To Δn _eff Assuming that only a change has occurred, the wavelength change Δλ is given by the following equation (5) from the above equations (1) and (2).
Δλ = Δn _eff ΔL1 / m (5)
[0068]
Further, in the wavelength filter of FIG. 8 having the same waveguide structure as FIG. 1 or FIG. 4, the refractive index of the output waveguide 5 is n. _eff To Δn _eff Assuming that only a change has occurred, the wavelength change Δλ is given by the following equation (6) from the above equations (1) and (2).
Δλ = −Δn _eff ΔL2 / m Expression (6)
[0069]
In a wavelength filter having the same waveguide structure as that in FIG. 3, the refractive index of the input waveguide 1 is n. _eff To Δn _eff In the case where only the change is made, the wavelength change Δλ is given by the above-described equation (5), but the refractive index of the output waveguide 5 is n _eff To Δn _eff In the case where only a change occurs, the wavelength change Δλ is given by the following equation (7) instead of the above equation (6).
Δλ = Δn _eff ΔL2 / m (7)
[0070]
As apparent from the equations (5), (6), and (7), it can be seen that the wavelength variable amount is proportional to ΔL1 and ΔL2 in the wavelength tunable filter including the refractive index changing means 51 and 53. In this case, since ΔL1 and ΔL2 can take arbitrary values, they can have a large wavelength variable range as compared with a normal grating or the like.
[0071]
When the refractive index changing means 51 including the optical coupler 3 is used to change the refractive index of the input waveguide 1, the length of the optical coupler 3 is included in ΔL1 in the equation (5). Accordingly, the wavelength change Δλ increases accordingly. When the refractive index changing means 53 including the optical coupler 7 changes the refractive index of the output waveguide 5, the length of the optical coupler 7 is included in ΔL2 in the equations (6) and (7). It is done. Accordingly, the wavelength change Δλ increases accordingly.
[0072]
Note that the refractive index changing means 51 can be intermittently provided with respect to the input waveguide 1 by a certain length ΔL3 (ΔL3 <ΔL1) between the optical couplers 3. In this case, the wavelength change Δλ is given by replacing ΔL1 in equation (5) with ΔL3. Similarly, the refractive index changing means 53 can be provided intermittently with respect to the output waveguide 5 by a fixed length ΔL4 (ΔL4 <ΔL2) for each interval between the optical couplers 7. In this case, the wavelength change Δλ is given by replacing ΔL2 in the equations (6) and (7) with ΔL4.
[0073]
In this example, a waveguide constituting a wavelength filter is formed by the epitaxial semiconductor substrate 55 having the structure shown in FIG. 9, and an electrode is used as the input waveguide 1 in order to change the refractive index of the input waveguide 1 by current injection. On the other hand, the refractive index changing means 51 is configured by providing a diode over the entire length from the left end optical coupler 3 to the right end optical coupler 3, and the refractive index of the output waveguide 5 by current injection. In order to change the refractive index changing means 53, an electrode is provided over the entire length from the left optical coupler 7 to the right optical coupler 7 with respect to the output waveguide 5 to form a diode. Yes. Since the refractive index of the input waveguide 1 and / or the output waveguide 5 is changed by current injection, the selected wavelength is changed and the filter operates as a wavelength tunable filter. Note that if the semiconductor substrate on which the wavelength filter is formed has a PIN structure, a diode is formed only by providing an electrode.
[0074]
Specifically, the epitaxial semiconductor substrate 55 is formed on an n-type InP substrate 57, an n-doped InP layer 59, a non-doped InGaAsP layer 61, P ^- Doped InP layer 63, P ⁺ Doped InP layer 65 and P ⁺ A doped InGaAs layer 67 is laminated.
[0075]
The n-doped InP layer 59 is a cladding layer and has a doping concentration of 1 × 10 5. ¹⁸ cm ^-3 It is said. The non-doped InGaAsP layer 61 is a core layer and has a thickness of 0.5 μm. P ^- The doped InP layer 63 is a cladding layer and has a film thickness of 1.2 μm and a doping concentration of 5 × 10. ¹⁷ cm ^-3 It is said. P ⁺ The doped InP layer 65 is a cladding layer, and has a film thickness of 0.3 μm and a doping concentration of 1 × 10. ¹⁸ cm ^-3 It is said. P ⁺ The doped InGaAs layer 67 is a contact layer having a thickness of 30 μm and a doping concentration of 8 × 10 ¹⁸ cm ^-3 It is said.
[0076]
By this epitaxial semiconductor substrate 55, the input waveguide 1, the plurality of optical couplers 3 having the same interval of ΔL 1 on the input waveguide 1 side, the output waveguide 5, and the plurality of optical couplers 7 having the same interval of ΔL 2 on the output waveguide 2 side. A plurality of connection waveguides 11 are formed to connect the input waveguide 1 and the output waveguide 3 via the optical coupler 3 and the optical coupler 7.
[0077]
Then, P of the epitaxial semiconductor substrate 55 ⁺ An AuZnNi electrode layer is formed on the surface of the doped InGaAs layer 67 on the portion corresponding to the input waveguide 1 and the portion coupled to the input waveguide 1 of each optical coupler 3, and n n opposite to this. The refractive index changing means 51 is formed by forming an AuGeNi electrode layer on the back surface of the type InP substrate 57.
[0078]
Similarly, P ⁺ An AuZnNi electrode layer is formed on a portion of the surface of the doped InGaAs layer 67 corresponding to the output waveguide 5 and a portion coupled to the output waveguide 5 of each optical coupler 7, and is opposed to this n n layer. The refractive index changing means 53 is formed by forming an AuGeNi electrode layer having a length on the back surface of the type InP substrate 57.
[0079]
FIG. 10 shows tuning characteristics of the wavelength tunable filter provided with the refractive index changing means 51 and 53 by the current injection described above. In FIG. 10, a characteristic 69 indicates a transmission characteristic when no current is injected, and a characteristic 71 indicates a transmission characteristic when the refractive index changing means 51 changes the refractive index of the input waveguide 1 by -0.1% by current injection. The characteristic 73 shows the transmission characteristic when the refractive index changing means 53 changes the refractive index of the output waveguide 5 by -0.1% by current injection.
[0080]
Hereinafter, the wavelength variable amount Δλ in the wavelength tunable filter of FIG. 8 will be described in the case where the refractive index changing means 51 and 53 by current injection are provided.
[0081]
As described above, in the wavelength tunable filter having the waveguide configuration in FIG. 8, the refractive index is Δn by current injection. _eff When the wavelength is changed, the wavelength variable Δλ is given by the above-described equation (5), and the refractive index becomes Δn by current injection. _eff If it changes, the wavelength variable amount Δλ is given by the above-described equation (6).
Δλ = Δn _eff ΔL1 / m (5)
Δλ = −Δn _eff ΔL2 / m Expression (6)
[0082]
As is apparent from these equations (5) and (6), it can be seen that the wavelength variable amount shown in FIG. 8 is proportional to ΔL1 and ΔL2. In this case, since ΔL1 and ΔL2 can take arbitrary values, they can have a large wavelength variable range as compared with a normal grating or the like.
[0083]
When the refractive index is changed by current injection, the refractive index decreases according to the amount of current. _eff Is negative, so that when the current is injected from the electrode on the input waveguide 1 side, the center wavelength is shifted to the short wavelength side as shown in the transmission characteristic 71 of FIG. 10, and the current is injected from the electrode on the output waveguide 5 side. In this case, the center wavelength is shifted to the long wavelength side as shown by the transmission characteristic 73 in FIG. In this case, the refractive index change rate (Δn _eff / N _eff ) Of 0.1%, a large wavelength variable range of 30 nm or more was obtained. The current injection amount may be about 50 mA, and the power consumption can be greatly reduced as compared with the conventional example.
[0084]
That is, the wavelength variable amount Δλ in the conventional grating or ring resonator is Δλ / λ. _o = Δn _eff / N, and Δn _eff Since / n is about 0.3% for current injection, the center wavelength λ _o Is 1.55 μm, which is as small as about 5 nm, whereas 0.1% Δn _eff / N _eff A large wavelength tunable amount of 30 nm was obtained.
[0085]
Further, the total length of the electrodes constituting the refractive index changing means 51 is ΔL1 × (N−1), where N is the number of connection waveguides 11 (the number of optical couplers 3), compared to the total electrode length of the conventional AWGF. The total length of the electrodes constituting the refractive index changing means 53 is ΔL2 × (N−1), where N is the number of connection waveguides 11 (the number of optical couplers 7). .
[0086]
The difference between such effects is as follows. That is, in the grating and the ring resonator, the part that determines the resonance wavelength directly changes the refractive index, whereas in this example, the resonance wavelength (center wavelength) is mainly determined adjacent to the equation (1). The difference in the length of the matching connecting waveguide 11 is ΔL, and the wavelength is changed by the refractive index of the input waveguide 1 or the output waveguide 5.
[0087]
As described above, it is possible to realize an arrayed waveguide grating type semiconductor wavelength tunable filter that could not be realized conventionally.
[0088]
In the above description, means for changing the refractive index by current injection is used as the refractive index changing means 51 and 53. However, the invention is not limited to this, and the refractive index is changed by means of changing the refractive index by heat or by applying a voltage (electric field). It is possible to use a means for making it.
[0089]
In the case of changing the refractive index by heat, the five layers 59 to 67 of the epitaxial substrate 25 of FIG. 9 are all non-doped “non-doped layers” (non-doped InP layer, non-doped InGaAsP on the InP substrate). 8 is formed using a layer, a non-doped InP layer, a non-doped InP layer, and a non-doped InGaAs layer. Then, for example, a chromium film is deposited on the input waveguide 1 and / or the output waveguide 5 (the waveguide portions of the optical couplers 3 and 7 to be coupled to these) as heaters instead of the electrodes. Refractive index changing means 51 and 53 are used. In this case, when the temperature of the waveguide rises by applying heat with a heater, the refractive index increases unlike the above-described change in the refractive index of the semiconductor, so that the center wavelength shifts in the opposite direction. That is, when the input waveguide 1 is heated, the center wavelength is shifted to the longer wavelength side, and when the output waveguide 5 is heated, the center wavelength is shifted to the shorter wavelength side.
[0090]
When the refractive index is changed by voltage application, the wavelength filter shown in FIG. 8 is formed using an epitaxial substrate having the same configuration as that shown in FIG. 9 except for the core layer (non-doped InGaAsP layer) 61. For the core layer, for example, a non-doped multiple quantum well structure in which 30 layers of InGaAlAs (8.6 nm) / InAlAs (5 nm) are repeatedly stacked is used. That is, on the InP substrate 57, the n-doped InP layer 59, the non-doped multiple quantum well structure layer, the P ^- Doped InP layer 63, P ⁺ Doped InP layer 65 and P ⁺ The wavelength filter shown in FIG. 8 is formed from an epitaxial substrate on which the doped InGaAs layer 67 is laminated. As in the case of current injection, electrodes are formed on the front and back of the input waveguide 1 and / or the output waveguide 5 (if necessary, the waveguide portions of the optical couplers 3 and 7 to be coupled to them). Refractive index changing means 51 and 53 are used. A reverse bias voltage is applied between the front and back electrodes. In this case, the exciton peak wavelength is 1.44 μm. Since the refractive index increases as the reverse bias voltage increases, the center wavelength shifts in the reverse direction. That is, when a voltage is applied to the input waveguide 1, the center wavelength is shifted to the long wavelength side, and when a voltage is applied to the output waveguide 5, the center wavelength is shifted to the short wavelength side.
[0091]
[Sixth embodiment]
Next, a wavelength tunable filter related to the invention according to claim 4 is shown in FIG.
[0092]
The wavelength tunable filter shown in FIG. 11 is a combination of a first wavelength tunable filter 75a having a large wavelength interval (FSR) and a second wavelength tunable filter 75b having a small wavelength interval (FSR). It has a large wavelength variable range and can narrow the transmission band of the selected wavelength. For this reason, it becomes possible to narrow the frequency interval of the signal light in the case of wavelength multiplexing transmission, and it is possible to transmit a large number of wavelengths at a time.
[0093]
Although details will be described later, in this example, a wavelength tunable filter related to the invention according to claim 3 in which m = 30 (FSR 50 nm) is used as the first wavelength tunable filter 75a, and the second wavelength tunable filter 75b includes A wavelength tunable filter related to the invention according to claim 3 in which m = 300 (FSR 5 nm) is used, and the first wavelength tunable filter 75a is connected to the front stage, and the second wavelength tunable filter 75b is connected to the back stage.
[0094]
The waveguide structure of the first wavelength tunable filter 75a is the same as that of the wavelength filter shown in FIG. 8 except that the input waveguide 1 and the output waveguide 5 are non-parallel and narrow on the left side. A waveguide side refractive index changing means (for example, a refractive index changing means by current injection) 51 and an output waveguide side refractive index changing means (for example, a refractive index changing means by current injection) 53 are provided.
[0095]
Therefore, in the first wavelength tunable filter 75a shown in FIG. 11, the input light incident on the lower left incident end 13 is branched from the common input waveguide 1 to the plurality of connection waveguides 11 one by one by the plurality of optical couplers 3. Then, each of the optical waveguides 7 is joined to a common output waveguide 5 by a plurality of optical couplers 7 and operates as a wavelength filter by exiting from the exit end 19 at the upper right, and further, the refractive index changing means 51 The selected wavelength is variable by changing the refractive index or changing the refractive index of the output waveguide 5 by the refractive index changing means 53.
[0096]
In FIG. 12, the transmission characteristic 77a of the first wavelength tunable filter 75a when m = 30, FSR = 50 nm, and the center wavelength when the refractive index is not changed is 1.55 μm is indicated by a broken line.
[0097]
The waveguide structure of the second wavelength tunable filter 75b is the same as the wavelength filter shown in FIG. 8 except that the input waveguide 1 and the output waveguide 5 are non-parallel and narrow on the left side. A waveguide side refractive index changing means (for example, a refractive index changing means by current injection) 51 and an output waveguide side refractive index changing means (for example, a refractive index changing means by current injection) 53 are provided.
[0098]
Accordingly, as shown in FIG. 11, the second wavelength tunable filter 75b also receives input light incident on the lower left incident end 13 from the common input waveguide 1 to the plurality of connection waveguides one by one by the plurality of optical couplers 3. 11, each of which is joined to a common output waveguide 5 by a plurality of optical couplers 7, and operates as a wavelength filter by being output from the output end 19. Further, the refractive index changing means 51 is used to input the input waveguide 1. The selected wavelength is variable by changing the refractive index of the output waveguide 5 by the refractive index changing means 53.
[0099]
In FIG. 12, the transmission characteristic 77b of the second wavelength tunable filter 75b when m = 300, FSR = 5 nm, and the center wavelength when the refractive index is not changed is 1.55 μm is indicated by a broken line.
[0100]
By connecting the upper left entrance end 13 of the second wavelength tunable filter 75b to the upper right exit end 19 of the first wavelength tunable filter 75a, the entire wavelength tunable range and the transmission band of the selected wavelength are obtained. Constitutes a narrow wavelength tunable filter. The upper right exit end 19 of the first wavelength tunable filter 75a and the upper left entrance end 13 of the second wavelength tunable filter 75b may be connected by an appropriate waveguide or may be formed to coincide with each other.
[0101]
FIG. 12 shows the transmission characteristics 79 of the entire tunable filter shown in FIG.
[0102]
As shown in FIG. 12, it can be seen that only one wavelength (1.55 μm) can be selected in the measured wavelength range. In other words, only one wavelength can be selected from the FSR 50 nm of the first wavelength tunable filter 75a with m = 30.
[0103]
In FIG. 11, the second wavelength tunable filter 75b has a larger angle between the input and output waveguides than the first wavelength tunable filter 75a. _o In order to increase m, it is necessary to increase ΔS, and as a result, the difference in length ΔL between adjacent connection waveguides 11 increases.
[0104]
In the wavelength tunable filter shown in FIG. 11, the first wavelength tunable filter 75a having a large FSR is connected to the front stage and the second wavelength tunable filter 75b having a small FSR is connected to the subsequent stage, but conversely, the FSR The second wavelength tunable filter 75b having a smaller FSR may be used as a front stage and the first wavelength tunable filter 75a having a larger FSR may be used as a subsequent stage, and the same effect can be obtained.
[0105]
In the wavelength tunable filter shown in FIG. 11, the second wavelength tunable filter 75b having a small FSR can be realized not only by the wavelength tunable filter according to the third aspect of the invention, but also by, for example, a grating or a ring resonator.
[0106]
However, only the wavelength tunable filter of the invention according to claim 3 can realize the first wavelength tunable filter 75a having a large FSR.
[0107]
In the description of each of the above embodiments, the waveguide formed by the InP-based compound semiconductor is used for the configuration of the wavelength filter and the wavelength tunable filter. However, the waveguide formed by the GaAs-based compound semiconductor, or Si and SiO ₂ Similarly, a wavelength filter and a wavelength tunable filter can also be realized by using a silicon thin wire waveguide composed of, for example, polyimide. Further, by integrating the semiconductor optical amplifier in the input waveguide 1 and the output waveguide 5, it is possible to eliminate the loss of the wavelength filter and the wavelength tunable filter. In addition, it is possible to provide a light receiving element in an integrated manner at the output end of the wavelength filter and the wavelength tunable filter. In this case, an optical signal transmitted through the wavelength filter or the wavelength tunable filter can be converted into an electrical signal and taken out. it can.
[0108]
In the description of the above embodiments, the input waveguide 1 and the output waveguide 5 are non-parallel. However, in principle, the input waveguide 1 and the output waveguide 5 may be parallel. When the input waveguide 1 and the output waveguide 5 are non-parallel, the adjacent connection waveguides 11 are sequentially changed by the same difference ΔL without changing the shape of each connection waveguide 11 except for the length. There is an advantage that it can be increased or decreased, and the connection waveguide array 5 can be easily formed. In other words, if the connection waveguides 11 having substantially the same shape except the length are arranged in the order of the length, the input waveguide 1 and the output waveguide 5 become non-parallel at an angle corresponding to ΔL, ΔL1, and ΔL2. When the input waveguide 1 and the output waveguide 5 are parallel, the shape of each connection waveguide 11 is made different so that the adjacent connection waveguides 11 are sequentially increased or decreased with the same difference ΔL. it can.
[0109]
【The invention's effect】
According to the present invention, as can be seen from the above description, the wavelength filter and the wavelength tunable filter can be realized with a high extinction ratio, low loss, and low cost. The wavelength tunable filter of the present invention can realize a large wavelength tunable range.
[Brief description of the drawings]
FIG. 1 is a diagram showing a wavelength filter according to a first embodiment of the present invention.
2 is a graph showing the transmission characteristics of the wavelength filter of FIG.
FIG. 3 is a diagram showing a wavelength filter according to a second embodiment of the present invention.
FIG. 4 is a diagram showing a wavelength filter according to a third embodiment of the present invention.
5 is a graph showing the transmission characteristics of the wavelength filter of FIG.
FIG. 6 is a view showing a wavelength filter according to a fourth embodiment of the present invention.
7 is a graph showing the characteristics of the optical coupler used in FIG. 6. FIG.
FIG. 8 is a diagram showing a wavelength tunable filter according to a fifth embodiment of the present invention.
9 is a diagram showing a configuration of an epitaxial substrate used in the wavelength tunable filter of FIG.
10 is a diagram showing wavelength tuning characteristics of the wavelength tunable filter in FIG. 8. FIG.
FIG. 11 is a diagram showing a wavelength tunable filter according to a sixth embodiment of the present invention.
12 is a graph showing the transmission characteristics of the wavelength tunable filter shown in FIG.
FIG. 13 is a diagram showing a configuration of a conventional wavelength tunable filter.
[Explanation of symbols]
1 Input waveguide
3 Optical coupler on the input waveguide side
5 Output waveguide
7 Optical coupler on the output waveguide side
9 Connection waveguide array
11 Individual connection waveguides in a connection waveguide array
13 Left end of input waveguide
15 Right end of input waveguide
17 Left end of output waveguide
19 Right end of output waveguide
21 Arrow indicating light propagation direction on input waveguide
23 Arrow indicating light propagation direction on output waveguide
25 Strip-mounted waveguide
27 Core layer
29 Upper cladding layer
31 Lower cladding layer
33 Transmission characteristics of wavelength filter fabricated in the configuration of FIG.
35 Transmission characteristics when the coupling coefficient is the same for all optical couplers
37, 39 Multimode coupler
41, 43 Two arms of Mach-Zehnder interferometer
45 Crossport
47 Barport
49 Thin waveguide connected to one arm
51 Refractive index changing means on the input waveguide side
52 Feeding path
53 Refractive Index Changing Means on Output Waveguide Side
54 Feeding path
55 Epitaxial substrate
57 n-type InP substrate
59 n-doped InP layer
61 Non-doped InGaAsP layer
63 P ^- Doped InP layer
65 P ⁺ Doped InP layer
67 P ⁺ Doped InGaAs layer
69 Transmission characteristics of wavelength tunable filter fabricated with the configuration of FIG.
71 Transmission characteristics when current is injected into the input waveguide
73 Transmission characteristics when current is injected into the output waveguide
75a First wavelength tunable filter (wavelength tunable filter having a large wavelength interval)
75b Second wavelength tunable filter (wavelength tunable filter with a small wavelength interval)
77a Transmission characteristics of the first variable wavelength filter
77b Transmission characteristics of the second variable wavelength filter
79 Transmission characteristics of wavelength tunable filter manufactured with the configuration of FIG.

Claims (4)

An input waveguide;
An output waveguide;
A plurality of optical couplers (hereinafter referred to as optical couplers on the input waveguide side) disposed at regular intervals in the input waveguide;
A plurality of optical couplers (hereinafter referred to as optical couplers on the output waveguide side) disposed in the output waveguide at regular intervals different from or equal to the regular intervals;
A plurality of connection waveguides connecting the input waveguide and the output waveguide via the optical coupler on the input waveguide side and the optical coupler on the output waveguide side;
A plurality of optical path lengths from the input end of the input waveguide through the connection waveguide to the output end of the output waveguide increase in order of the same length from the path passing through the connection waveguide close to the input end. Alternatively, the plurality of lights on the output waveguide side are decreased and the length of the connection waveguide is increased or decreased in order from the connection waveguide close to the incident end by the same length difference. Each of the couplers causes the light incident from the connection waveguide to interfere with the light incident from the output waveguide, so that the distance between the optical couplers on the input waveguide side and the optical coupler on the output waveguide side A wavelength filter characterized by propagating only light having the same wavelength determined by the distance between the two and the length of the connection waveguide through the output waveguide . In claim 1,
The coupling coefficient of the optical coupler on the input waveguide side is smaller as it is closer to the incident end, and the coupling coefficient of the optical coupler on the output waveguide side is smaller as it is closer to the emission end. Wavelength filter. In claim 1 or 2,
Among said output waveguide and said input waveguide, a wavelength tunable filter, characterized in that it comprises at least one of the refractive index of heat, current injection, the refractive index change means to further change the voltage application pressure. A wavelength tunable filter in which a first tunable filter having a large output wavelength interval and a second tunable filter having a small output wavelength interval are combined,
The tunable filter according to claim 3, wherein at least the first tunable filter is the tunable filter according to claim 3.

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