Optical Module

Abstract

The optical module includes a ring resonator (13) having a first waveguide portion (13a) whose group refractive index is temperature independent, a second waveguide portion (13b) whose group refractive index is negative in temperature dependence, and a third waveguide portion (13c) whose group refractive index is positive in temperature dependence, and a heater (20) that heats either the second waveguide portion (13b) or the third waveguide portion (13c).

Description

[Technical Field]

[0001] The present disclosure relates to an optical module having a ring resonator filter. [Background technology]

[0002] As the capacity of optical communication systems increases, wavelength division multiplexing communication systems are being used. Non-Patent Document 1 shows a ring resonator that has achieved athermalization (temperature independence), which is one of the wavelength filters required for wavelength division multiplexing communication systems. The ring resonator shown in Non-Patent Document 1 is a ring resonator to which the structure of a Si slot waveguide is applied. The Si slot waveguide described in Non-Patent Document 1 is a slot waveguide in which benzocyclobutene (BCB) is filled into the gap between a pair of silicon (Si) slot optical waveguides having a gap on an SOI (Silicon On Insulator) substrate. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Yuki Atsumi, et. al., “Athermal wavelength property of Si-slot ring resonator embedded with benzocyclobutene”2009 6th IEEE International Conference on Group IV Photonics, ThB3 Summary of the Invention [Problem to be solved by the invention]

[0004] In the ring resonator shown in Non-Patent Document 1, the silicon that makes up the slot waveguide has a refractive index that is positively temperature dependent when compared with the silicon oxide (SiO2) clad layer. Therefore, by filling the gap with BCB, which has a refractive index that is negatively temperature dependent, and adjusting the gap, an athermal effect is achieved, which offsets the temperature dependence of the light propagating through the slot waveguide. However, in the wavelength filter using a ring resonator shown in Non-Patent Document 1, when creating a ring resonator, it is not possible to accurately create the phase state of light that determines the peak wavelength at which the filter resonates, and since the ring resonator is a temperature-independent waveguide, there are problems in that it is not possible to apply the heat-based method that is widely used to adjust the phase of light.

[0005] The present disclosure is intended to solve the above-mentioned problems, and aims to provide an optical module that realizes athermalization and can easily adjust the resonant peak wavelength of light propagating through a waveguide in a ring resonator. [Means for solving the problem]

[0006] The optical module according to the present disclosure includes a ring resonator having a first waveguide portion whose group refractive index is temperature independent, a second waveguide portion whose group refractive index is negatively dependent on temperature, and a third waveguide portion whose group refractive index is positively dependent on temperature, and a heater that heats either the second waveguide portion or the third waveguide portion. [Effects of the Invention]

[0007] According to the present disclosure, the ring resonator has waveguide sections that are temperature independent of the group refractive index, have a negative temperature dependency of the group refractive index, and have a positive temperature dependency of the group refractive index, and a heater is provided to heat at least one of the waveguide sections with a negative or positive temperature dependency of the group refractive index. This allows for athermalization and easy, highly accurate fine adjustment of the resonating peak wavelength in the ring resonator. [Brief explanation of the drawings]

[0008] [Figure 1]1 is a top view schematically showing a ring resonator filter in the optical module according to the first embodiment. FIG. [Figure 2] FIG. 2 is a cross-sectional view schematically illustrating a slot waveguide in the optical module according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Embodiment 1 The optical module according to the first embodiment will be described with reference to FIGS. The optical module according to the first embodiment is an optical module having a ring resonator filter, which is a wavelength filter. Wavelength filters are used in optical communication systems that use wavelength division multiplexing. In optical communications, wavelength filters have the function of extracting only the light of the required wavelength from wavelength-multiplexed light during transmission and carrying the signal, or extracting the light of the required wavelength from wavelength-multiplexed light during reception and inputting it into the optical receiver.

[0010] Therefore, wavelength filters are required to have steep filter characteristics and accurate center wavelength settings in order to obtain the necessary light from wavelength-multiplexed light that bundles light of many wavelengths. The ring resonator in the ring resonator type filter that constitutes the wavelength filter outputs light that has periodic wavelength dependency with the wavelength that forms a standing wave in the ring resonator as the resonance peak value. The ring resonator filter in the optical module according to the first embodiment can be made athermal (temperature independent), and fine adjustment of the resonating peak wavelength can be easily achieved with high precision.

[0011] The ring resonator filter can be used as a wavelength filter that extracts light of a required wavelength from wavelength-multiplexed light, as a filter that monitors the wavelength of a semiconductor laser that oscillates in single mode, or as an external resonator mirror with a filtering function that can be used in combination with a semiconductor optical amplifier to generate laser oscillation.

[0012] As shown in FIG. 1, the optical module according to the first embodiment basically includes a ring resonator filter 10 and a heater 20. The ring resonator filter 10 includes a first bus waveguide 11 , a second bus waveguide 12 , and a ring resonator 13 .

[0013] In the following description, first bus waveguide 11 will be described as an input optical waveguide that receives wavelength-multiplexed light and propagates the incident light, and second bus waveguide 12 will be described as an output optical waveguide that receives light having periodic wavelength dependency, with the wavelength that forms a standing wave in ring resonator 13 as its resonance peak value, and that guides and outputs the light.

[0014] First, the slot waveguides constituting first bus waveguide 11, second bus waveguide 12, and ring resonator 13 will be described with reference to FIG. The slot waveguide 1 is composed of a pair of narrow optical waveguides 1a and 1b arranged parallel to each other with a gap between them on the surface of an SOI substrate. The optical waveguides 1a and 1b are formed by a commonly known technique.

[0015] FIG. 2 shows a silicon oxide (SiO2) layer 2, which is the surface layer of the SOI substrate, and the silicon oxide layer 2 functions as a lower cladding for the optical waveguides 1a and 1b. The slot waveguide 1 is embedded on the surface of the SOI substrate, and an insulating layer 3 made of benzocyclobutene (hereinafter referred to as BCB) is formed to fill the gap between the pair of optical waveguides 1a and 1b. The insulating layer 3 is formed by a commonly known technique. The insulating layer 3 functions as an upper cladding for the optical waveguides 1a and 1b.

[0016] The refractive index of silicon forming the optical waveguides 1a and 1b is higher than the refractive index of silicon oxide forming the silicon oxide layer 2, and the refractive index of this material increases as the temperature rises. The refractive index of BCB forming the insulating layer 3 decreases as the temperature increases.

[0017] That is, the temperature dependence of light propagating through the optical waveguides 1a and 1b is offset by filling the gap between the optical waveguides 1a and 1b with BCB, which has a negative temperature dependence, in the optical waveguides 1a and 1b made of silicon, which has a positive temperature dependence (temperature coefficient). Therefore, by changing the width of the gap Gap between the optical waveguides 1a and 1b, the light confinement coefficient in the BCB 3 changes, and by utilizing this, the temperature dependence of the group refractive index in the slot waveguide 1 portion including the BCB can be made to be temperature independent, negative, or positive.

[0018] The material for forming the optical waveguides 1a and 1b may be any material whose refractive index increases with increasing temperature, and other than silicon, silicon nitride (SiN) or indium phosphide (InP) may also be used. Furthermore, the lower cladding (silicon oxide layer) 2 is not limited to silicon oxide, and may be made of any material having a lower refractive index than the material forming the optical waveguides 1a and 1b. The material for forming the upper cladding (insulating layer) 3 may be any material that has a refractive index that decreases as the temperature increases, and other than BCB, titanium oxide (TiO2) or the like may also be used.

[0019] The input optical waveguide 11 has a pair of optical waveguides 11a and 11b arranged in parallel and linear fashion on the surface of a silicon oxide layer 2 with a gap of the same width over the entire length, and an insulating layer 3 filling the gap between the optical waveguides 11a and 11b, and has a cross-sectional structure as shown in Figure 2. The gap between the optical waveguides 11a and 11b of the input optical waveguide 11 is set so that the temperature dependency of the group refractive index is temperature independent.

[0020] That is, the group refractive index in the input optical waveguide 11 is n g1 In this case, the gap Gap between the optical waveguides 11a and 11b is set so that the temperature dependency of the group refractive index in the input optical waveguide 11 becomes 0 as shown in the following equation (1). dn g1 / dT=0 (1) In the above equation (1), T is the temperature.

[0021] The input optical waveguide 11 has a first port 11c at one end and a second port 11d at the other end. In the following description, the first port 11c is described as an optical input port into which wavelength-multiplexed light is input, and the second port 11d is described as a through port.

[0022] An optical monitor 30 is connected to the through port 11d of the input optical waveguide 11 via a light receiving element (not shown). The optical monitor 30 also has a function of observing (monitoring) the light from the through port 11d of the input optical waveguide 11 and adjusting the amount of heat generated by the heater 20 according to the output level of the light receiving element that detects the light from the through port 11d.

[0023] The output optical waveguide 12 is arranged parallel to the input optical waveguide 11 with the ring resonator 13 interposed therebetween. The output optical waveguide 12 has a pair of optical waveguides 12a and 12b arranged in parallel and linear fashion on the surface of a silicon oxide layer 2 with a gap of the same width over the entire length, and an insulating layer 3 filling the gap between the optical waveguides 12a and 12b, and has a cross-sectional structure as shown in Figure 2. The gap between the optical waveguides 12a and 12b of the output optical waveguide 12 is set so that the temperature dependency of the group refractive index becomes temperature independent.

[0024] That is, the group refractive index in the output optical waveguide 12 is n g1 In this case, the gap Gap between the optical waveguides 12a and 12b is set so that the temperature dependency of the group refractive index in the output optical waveguide 12 becomes 0 as shown in the following equation (2). dn g1 / dT=0 (2)

[0025] The output optical waveguide 12 has a first port 12c at one end and a second port 12d at the other end. In the following description, the first port 12c is described as an optical output port from which light in a wavelength band resonated by the ring resonator 13 is output.

[0026] Ring resonator 13 has first waveguide portion 13a, second waveguide portion 13b, third waveguide portion 13c, fourth waveguide portion 13d, and connecting waveguide portions 13e to 13h. The first waveguide portion 13a and the second waveguide portion 13b are connected to the connecting waveguide portion 13e. The first waveguide portion 13a and the third waveguide portion 13c are connected to the connecting waveguide portion 13f. The fourth waveguide portion 13d and the second waveguide portion 13b are connected to the connecting waveguide portion 13g. The fourth waveguide portion 13d and the third waveguide portion 13c are connected to the connecting waveguide portion 13h.

[0027] As a result, the first to fourth waveguide portions 13a to 13d are connected by the connecting waveguide portions 13e to 13h to form a seamless ring-shaped optical waveguide. It should be noted that there is no physical boundary between the waveguide section and the connecting waveguide section, but only a virtual boundary surface. In addition, the seamless ring-shaped optical waveguide that constitutes the ring resonator 13 will be referred to as a ring-shaped optical waveguide in the following description.

[0028] The first waveguide portion 13 a is disposed opposite the output optical waveguide 12 . The first waveguide portion 13a has a slot waveguide formed on the surface of the silicon oxide layer 2, which is composed of a pair of optical waveguides 13a1 and 13a2 arranged in parallel with a gap of the same width over the entire length, and an insulating layer filled in the gap between the optical waveguides 13a1 and 13a2, and has a cross-sectional shape as shown in Figure 2.

[0029] In its planar shape, the first waveguide portion 13a has a first arc portion at one end, a second arc portion at the other end, and a straight portion between the first arc portion and the second arc portion, and the straight portion is arranged parallel to the output optical waveguide 12. In the first waveguide portion 13a, the gap Gap between the optical waveguides 13a1 and 13a2 is set so that the temperature dependency of the group refractive index becomes temperature independent.

[0030] That is, the group refractive index in the first waveguide portion 13a is ng0 In this case, the gap Gap between the optical waveguides 13a1 and 13a2 is set so that the temperature dependency of the group refractive index in the first waveguide portion 13a becomes 0 as shown in the following equation (3). dn g0 / dT=0 (3) In short, the temperature dependency of the group refractive index of the first waveguide portion 13a is set to be temperature independent.

[0031] The fourth waveguide portion 13d is disposed at a position in the ring-shaped optical waveguide opposite the first waveguide portion 13a, and is disposed opposite the input optical waveguide 11. The fourth waveguide portion 13d has a slot waveguide formed on the surface of the silicon oxide layer 2, which is composed of a pair of optical waveguides 13d1 and 13d2 arranged in parallel with a gap of the same width over the entire length, and an insulating layer filled in the gap between the optical waveguides 13d1 and 13d2, and has a cross-sectional shape as shown in Figure 2.

[0032] In its planar shape, the fourth waveguide portion 13d has a first arc portion at one end, a second arc portion at the other end, and a straight portion between the first arc portion and the second arc portion, and the straight portion is arranged parallel to the input optical waveguide 11. In the fourth waveguide portion 13d, the gap Gap between the optical waveguides 13d1 and 13d2 is set so that the temperature dependency of the group refractive index becomes temperature independent.

[0033] That is, the group refractive index in the fourth waveguide portion 13d is n g0 In this case, the gap Gap between the optical waveguides 13d1 and 13d2 is set so that the temperature dependency of the group refractive index in the fourth waveguide portion 13d becomes 0 as shown in the following equation (4). dn g0 / dT=0 (4) In short, the temperature dependency of the group refractive index of the fourth waveguide portion 13d is set to be temperature independent.

[0034] The second waveguide portion 13b is disposed between one end of the first waveguide portion 13a and one end of the fourth waveguide portion 13d. The second waveguide portion 13b has a slot waveguide formed on the surface of the silicon oxide layer 2, which is composed of a pair of optical waveguides 13b1 and 13b2 arranged in parallel and linear fashion with a gap Gap of the same width over the entire length, and an insulating layer filled in the gap Gap between the optical waveguides 13b1 and 13b2, and has a cross-sectional shape as shown in Figure 2.

[0035] The width of the gap between the optical waveguides 13b1 and 13b2 is set to be wider than the width of the gap between the optical waveguides 13a1 and 13a2 in the first waveguide portion 13a and the width of the gap between the optical waveguides 13d1 and 13d2 in the fourth waveguide portion 13d. As a result, the temperature dependence of the group refractive index of the second waveguide portion 13b is negative.

[0036] That is, the group refractive index in the second waveguide portion 13b is n g- In this case, the gap Gap between the optical waveguides 13b1 and 13b2 is set so that the temperature dependency of the group refractive index in the second waveguide portion 13b is smaller than 0 as shown in the following equation (5). dn g- / dT<0 (5)

[0037] The connecting waveguide portion 13e connecting the first waveguide portion 13a and the second waveguide portion 13b has a tapered structure in which the width increases adiabatically toward the second waveguide portion 13b. The connecting waveguide portion 13e has a slot waveguide made up of a pair of optical waveguides and an insulating layer filled in the gap between the pair of optical waveguides, and has a cross-sectional shape having the structure shown in FIG.

[0038] The width of the gap Gap between the pair of optical waveguides in the connecting waveguide portion 13e becomes wider from the width of the gap Gap between the optical waveguides 13a1 and 13a2 in the first waveguide portion 13a toward the width of the gap Gap between the optical waveguides 13b1 and 13b2 in the second waveguide portion 13b, and the pair of optical waveguides in the connecting waveguide portion 13e are arranged in a tapered shape that widens toward the second waveguide portion 13b. Since the first waveguide portion 13a and the second waveguide portion 13b are connected by a tapered structure in which the width is adiabatically widened by the connecting waveguide portion 13e, there are no reflection points in the connecting waveguide portion 13e, and light is not reflected at the connecting waveguide portion 13e when it propagates through the ring-shaped optical waveguide.

[0039] A connecting waveguide portion 13g that connects the fourth waveguide portion 13d and the second waveguide portion 13b has a tapered structure in which the width adiabatically increases toward the second waveguide portion 13b. The connecting waveguide portion 13g has a slot waveguide made up of a pair of optical waveguides and an insulating layer filled in the gap between the pair of optical waveguides, and has a cross-sectional shape having the structure shown in FIG.

[0040] The width of the gap Gap between the pair of optical waveguides in the connecting waveguide portion 13g becomes wider from the width of the gap Gap between the optical waveguides 13d1 and 13d2 in the fourth waveguide portion 13d toward the width of the gap Gap between the optical waveguides 13b1 and 13b2 in the second waveguide portion 13b, and the pair of optical waveguides in the connecting waveguide portion 13g are arranged in a tapered shape that widens toward the second waveguide portion 13b. Since the fourth waveguide portion 13d and the second waveguide portion 13b are connected by a tapered structure in which the width is adiabatically widened by the connecting waveguide portion 13g, there are no reflection points in the connecting waveguide portion 13g, and light is not reflected at the connecting waveguide portion 13g when it propagates through the ring-shaped optical waveguide.

[0041] The third waveguide portion 13c is disposed between the other end of the first waveguide portion 13a and the other end of the fourth waveguide portion 13d. The third waveguide portion 13c has a slot waveguide formed on the surface of the silicon oxide layer 2, which is composed of a pair of optical waveguides 13c1 and 13c2 arranged in parallel and linear fashion with a gap Gap of the same width over the entire length, and an insulating layer filled in the gap Gap between the optical waveguides 13c1 and 13c2, and has a cross-sectional shape as shown in Figure 2.

[0042] The width of the gap between the optical waveguides 13c1 and 13c2 is set to be narrower than the width of the gap between the optical waveguides 13a1 and 13a2 in the first waveguide portion 13a and the width of the gap between the optical waveguides 13d1 and 13d2 in the fourth waveguide portion 13d. As a result, the temperature dependence of the group refractive index of the third waveguide portion 13c is positive.

[0043] That is, the group refractive index in the third waveguide portion 13c is n g+ In this case, the gap Gap between the optical waveguides 13c1 and 13c2 is set so that the temperature dependency of the group refractive index in the third waveguide portion 13c is greater than 0 as shown in the following equation (6). dn g+ / dT>0 (6)

[0044] The connecting waveguide portion 13f connecting the first waveguide portion 13a and the third waveguide portion 13c has a tapered structure in which the width is adiabatically narrowed toward the third waveguide portion 13c. The connecting waveguide portion 13f has a slot waveguide made up of a pair of optical waveguides and an insulating layer filled in the gap between the pair of optical waveguides, and has a cross-sectional shape having the structure shown in FIG.

[0045] The width of the gap between the pair of optical waveguides in the connecting waveguide portion 13f narrows from the width of the gap between the optical waveguides 13a1 and 13a2 in the first waveguide portion 13a toward the width of the gap between the optical waveguides 13c1 and 13c2 in the third waveguide portion 13c, and the pair of optical waveguides in the connecting waveguide portion 13f are arranged in a tapered shape narrowing toward the third waveguide portion 13c. Since the first waveguide portion 13a and the third waveguide portion 13c are connected by a tapered structure in which the width is adiabatically widened by the connecting waveguide portion 13f, there are no reflection points in the connecting waveguide portion 13f, and light is not reflected at the connecting waveguide portion 13f when it propagates through the ring-shaped optical waveguide.

[0046] A connecting waveguide portion 13h that connects the fourth waveguide portion 13d and the third waveguide portion 13c has a tapered structure in which the width adiabatically narrows toward the third waveguide portion 13c. The connecting waveguide portion 13h has a slot waveguide made up of a pair of optical waveguides and an insulating layer filled in the gap between the pair of optical waveguides, and has a cross-sectional shape having the structure shown in FIG.

[0047] The width of the gap Gap between the pair of optical waveguides in the connecting waveguide portion 13h narrows from the width of the gap Gap between the optical waveguides 13d1 and 13d2 in the fourth waveguide portion 13d toward the width of the gap Gap between the optical waveguides 13c1 and 13c2 in the third waveguide portion 13c, and the pair of optical waveguides in the connecting waveguide portion 13h are arranged in a tapered shape narrowing toward the third waveguide portion 13c. Since the fourth waveguide portion 13d and the third waveguide portion 13c are connected by a tapered structure in which the width is adiabatically widened by the connecting waveguide portion 13h, there are no reflection points in the connecting waveguide portion 13h, and light is not reflected at the connecting waveguide portion 13h when it propagates through the ring-shaped optical waveguide.

[0048] In the ring resonator 13, the temperature dependence of the group refractive index of the ring-shaped optical waveguide as a whole (around the ring) is made temperature independent. In other words, the length L of the second waveguide portion 13b is set so that the temperature dependence does not appear around the ring. - and the length L of the third waveguide portion 13c + is set as follows: The sum of the length of the first waveguide portion 13a and the length of the fourth waveguide portion 13d is defined as L.

[0049] The resonant wavelength m·λ in the ring resonator 13 can be expressed by the following equation (7). n g0 L+n g+ L + +n g- L - =m λ (7) Differentiating the above equation (7) with respect to temperature gives the following equation (8).

[0050] TIFF0007745818000001.tif18166

[0051] By setting the right side of the above equation (8) to 0, the temperature dependence of the group refractive index of the entire ring-shaped optical waveguide becomes temperature independent. At this time, the physical lengths L and L + , L - is assumed to remain unchanged. If the right side of the above equation (8) is set to 0, the following equation (9) is obtained.

[0052] TIFF0007745818000002.tif16166

[0053] Since the temperature dependence of the group refractive index of the first waveguide portion 13a and the fourth waveguide portion 13d is set to be temperature independent, the above formula (9) can be expressed by the following formula (10).

[0054] TIFF0007745818000003.tif18166

[0055] The length L of the second waveguide portion 13b is set to satisfy the above formula (10). - and the length L of the third waveguide portion 13c + By setting the above, the temperature dependency of the group refractive index of the ring resonator 13 becomes temperature independent. That is, the length L of the second waveguide portion 13b - and the length L of the third waveguide portion 13c + teeth, The temperature dependence of the group refractive index dn in the second waveguide portion 13b g- / dT and the product of length L - and the temperature dependence value dn of the group refractive index in the third waveguide portion 13c. g+ / dT and the product of length L + The length is set to a value such that the sum of

[0056] The heater 20 is disposed inside the ring-shaped optical waveguide and adjacent to the third waveguide portion 13c in order to heat the third waveguide portion 13c. The heater 20 is embedded in the insulating layer 3 . By heating the third waveguide portion 13c, the heater 20 has the effect of extending the optical path length of one ring of the ring-shaped optical waveguide, and the peak wavelength, i.e., the resonance wavelength, of the ring resonator 13 can be changed, that is, fine-tuned. The heater 20 may be disposed on the outside or above the ring-shaped optical waveguide.

[0057] Furthermore, the heater 20 may be disposed close to the second waveguide portion 13b inside, outside or above the ring-shaped optical waveguide in order to heat the second waveguide portion 13b. By heating the second waveguide portion 13b, the heater 20 has the effect of shortening the optical path length of one ring of the ring-shaped optical waveguide, and the peak wavelength, i.e., the resonance wavelength, of the ring resonator 13 can be changed, that is, fine-tuned.

[0058] Furthermore, the heater 20 may be disposed adjacent to each of the third waveguide portion 13c and the second waveguide portion 13b in order to heat each of the third waveguide portion 13c and the second waveguide portion 13b. By selecting heaters that are arranged adjacent to the third waveguide portion 13c and the second waveguide portion 13b and that heat the third waveguide portion 13c and the second waveguide portion 13b, respectively, it is possible to lengthen or shorten the optical path length of one circuit of the ring-shaped optical waveguide, and to change the peak wavelength in the ring resonator 13, i.e., the resonance wavelength, to either a long or short wavelength, that is, to finely adjust it to either a long or short wavelength.

[0059] Next, an operation for finely adjusting the peak wavelength, that is, the resonance wavelength, in ring resonator 13 in which the temperature dependency of the group refractive index is set to be temperature independent in the optical module according to the first embodiment will be described. Now, let us assume that the ring resonator filter 10 is one in which light is input from the optical input port 11 c of the input optical waveguide 11 and output from the optical output port 12 c of the output optical waveguide 12 . When light is input into the optical input port 11c of the input optical waveguide 11 by changing the wavelength of the light, or when wavelength-multiplexed light is input, light having periodic wavelength dependence with a wavelength that becomes a standing wave in the ring resonator 13 as a resonance peak is output from the optical output port 12c of the output optical waveguide 12.

[0060] First, it will be described how the resonant wavelength m·λ in the ring resonator 13 can be changed by heating the third waveguide portion 13c with the heater 20. When the third waveguide portion 13c is not heated by the heater 20, the resonant wavelength m·λ in the ring resonator 13 when the environmental temperature changes is assumed to be given by the following equation (11). That is, the resonant wavelength m·λ assumed to be affected by the temperature change ΔT is given by the following equation (11): In the following equation (11), the second term in the three parentheses on the left side indicates the effect of the temperature change ΔT.

[0061] TIFF0007745818000004.tif35166

[0062] The above equation (11) can be transformed into the following equation (12). TIFF0007745818000005.tif28166

[0063] In the first embodiment, the temperature dependence of the group refractive index of ring resonator 13 is set to be temperature independent, and therefore the value in parentheses of the second term on the left side of equation (12) above is 0. Therefore, equation (12) above can be expressed by the following equation (13):

[0064] TIFF0007745818000006.tif14166

[0065] On the other hand, in a state where the environmental temperature has changed by ΔT, the third waveguide portion 13c is heated by the heater 20, and the environmental temperature (ambient temperature) of the third waveguide portion 13c is increased by T'. The resonant wavelength m'·λ in the ring resonator 13 is assumed to be given by the following equation (14):

[0066] TIFF0007745818000007.tif37166

[0067] The above equation (14) corresponds to the above equation (11) assumed when the environmental temperature increases by ΔT, and shows the resonant wavelength m′·λ in the ring resonator 13 when only the environmental temperature of the third waveguide portion 13c increases by a further T′.

[0068] By converting the above equation (14) in the same way as converting the above equation (11) to the above equation (12), and furthermore, since the temperature dependency of the group refractive index of ring resonator 13 is set to be temperature independent in embodiment 1, the value in parentheses of the second term on the left side of the above equation (12) is set to 0 in the same way, and then the above equation (14) can be expressed by the following equation (15). The following equation (15) corresponds to the above equation (13).

[0069] TIFF0007745818000008.tif24166

[0070] The above equation (15) can be expressed by the following equation (16). TIFF0007745818000009.tif14166

[0071] The value of m' in the above equation (16) is larger than the value of m in the above equation (13). This means that by heating the third waveguide portion 13c with the heater 20, the resonant wavelength m'·λ of the ring resonator 13 can be changed from the resonant wavelength m'·λ of the ring resonator 13 when the third waveguide portion 13c is not heated.

[0072] That is, by heating the third waveguide portion 13c but not the second waveguide portion 13b, a temperature gradient is generated between the third waveguide portion 13c and the second waveguide portion 13b, and the phase of the light in the ring resonator 13 can be changed, thereby changing the resonant wavelength in the ring resonator 13.

[0073] In short, in the ring resonator 13, even if the environmental temperature changes and the temperature of the entire ring resonator 13, particularly the temperatures of the third waveguide portion 13c and the second waveguide portion 13b, changes by the same temperature, the resonance wavelength mλ does not change. In other words, a ring resonator 13 in which the temperature dependence of the group refractive index is temperature independent can be realized. Then, by using the heater 20 to generate a temperature gradient between the third waveguide portion 13c and the second waveguide portion 13b, it is possible to fine-tune the resonance wavelength in the ring resonator 13.

[0074] Based on the above, the operation of finely adjusting the resonant wavelength in ring resonator 13 in which the temperature dependency of the group refractive index is set to be temperature independent will be described. When light is incident from the optical input port 11 c of the input optical waveguide 11 , light of a wavelength based on the resonance frequency in the ring resonator 13 is output from the optical output port 12 c of the output optical waveguide 12 .

[0075] On the other hand, the optical monitor 30, which observes (monitors) the light from the through port 11d of the input optical waveguide 11, determines that the resonant frequency of the ring resonator 13 deviates from the set resonant frequency when the intensity of the light from the through port 11d, for example, the output level of the photodetector that detects the light from the through port 11d of the input optical waveguide 11, is large, and controls the amount of heating by the heater 20 via a heater control circuit (not shown).

[0076] Heater 20, the amount of heating of which is controlled, heats third waveguide portion 13c, and the resonance wavelength in ring resonator 13 is changed, that is, finely adjusted, as shown in equation (16) above. When the resonance wavelength in ring resonator 13 is finely adjusted to the set resonance frequency and the output level of the light-receiving element that detects light from through port 11d of input optical waveguide 11 decreases, it is determined that light of the set wavelength is being output from optical output port 12c of output optical waveguide 12, and heating of third waveguide portion 13c by heater 20 is stopped.

[0077] In the first embodiment, the optical monitor 30 observes the light from the through port 11d of the input optical waveguide 11, but it may also observe the light output from the optical output port 12c of the output optical waveguide 12 and control the amount of heating by the heater 20 via the heater control circuit. In this case, if the intensity of the light of the resonance wavelength output from the optical output port 12c of the output optical waveguide 12 is low, the amount of heating by the heater 20 is controlled.

[0078] Therefore, the ring resonator filter 10 in the optical module according to the first embodiment is independent of changes in the environmental temperature, and can finely adjust the resonant wavelength in the ring resonator 13 using a control signal based on the light monitored by the optical monitor 30.

[0079] The optical module according to the first embodiment includes a ring resonator 13 having a first waveguide portion 13a whose group refractive index is independent of temperature, a second waveguide portion 13b whose group refractive index has a negative temperature dependency, and a third waveguide portion 13c whose group refractive index has a positive temperature dependency, and a heater that heats either the second waveguide portion 13b or the third waveguide portion 13c. Therefore, it is possible to realize a ring resonator 13 whose group refractive index is independent of temperature dependency without being dependent on changes in the ambient temperature, and further, it is possible to easily and precisely fine-tune the resonant wavelength of the ring resonator 13.

[0080] In the first embodiment, first bus waveguide 11 is used as input optical waveguide 11, in first bus waveguide 11, first port 11c is used as an optical input port, second port 11d is used as a through port, second bus waveguide 12 is used as output optical waveguide 12, and in second bus waveguide 12, first port 12c is used as an optical output port, but the following may also be adopted.

[0081] The second bus waveguide 12 may be the input optical waveguide 12, in which the first port 12c is an optical input port and the second port 12d is a through port, the first bus waveguide 11 may be the output optical waveguide 11, and in which the second port 11d is an optical output port.

[0082] The first port 11c of the first bus waveguide 11 and the first port 12c of the second bus waveguide 12 may each be an optical input port, and the second port 11d of the first bus waveguide 11 and the second port 12d of the second bus waveguide 12 may each be an optical output port.

[0083] In the first embodiment, the ring resonator 13 is configured as a planar waveguide system on a silicon platform, for example. Ring resonator filter 10, which includes first bus waveguide 11, second bus waveguide 12, and ring resonator 13, is optically connected by a directional coupler or a multi-mode interferometer (MMI). The filter characteristics of the ring resonator can be changed by changing the coupling coefficient here and the ring circumference.

[0084] In the first embodiment, ring resonator 13 is a ring-shaped optical waveguide that uses a slot waveguide composed of a pair of optical waveguides. However, by changing the width of the waveguide structure, a ring-shaped optical waveguide may be used that has first waveguide portion 13a whose group refractive index is temperature-independent, second waveguide portion 13b whose group refractive index is negative in temperature dependence, third waveguide portion 13c whose group refractive index is positive in temperature dependence, and fourth waveguide portion 13d whose group refractive index is temperature-independent in temperature dependence.

[0085] It should be noted that the embodiments may be freely combined, or any of the components in each embodiment may be modified, or any of the components in each embodiment may be omitted. [Industrial Applicability]

[0086] The optical module according to the present disclosure is suitable for an optical module having a wavelength filter used in an optical communication system using a wavelength division multiplexing communication method. [Explanation of symbols]

[0087] 10 ring resonator filter, 11 first bus waveguide (input optical waveguide), 12 second bus waveguide (output optical waveguide), 13 ring resonator, 13a first waveguide portion, 13a1, 13a2 optical waveguides, 13b second waveguide portion, 13b1, 13b2 optical waveguides, 13c third waveguide portion, 13c1, 13c2 optical waveguides, 13d fourth waveguide portion, 13d1, 13d2 optical waveguides, 13e to 13h connecting waveguide portion, 20 heater.

Claims

1. a ring resonator having a first waveguide portion whose temperature dependence of the group refractive index is temperature independent, a second waveguide portion whose temperature dependence of the group refractive index is negative, and a third waveguide portion whose temperature dependence of the group refractive index is positive; a heater that heats either the second waveguide portion or the third waveguide portion; An optical module comprising:

2. The first waveguide portion, the second waveguide portion, and the third waveguide portion each have a slot waveguide composed of a pair of optical waveguides arranged in parallel with a gap on the surface of an SOI substrate and made of a material whose refractive index increases as the temperature increases, such as silicon, silicon nitride, or indium phosphide, and an insulating layer formed of a material whose refractive index decreases as the temperature increases, such as benzocyclobutene or titanium oxide, and embedded in the gap between the pair of optical waveguides in the slot waveguide.

2. The optical module according to claim 1.

3. the length of the second waveguide portion and the length of the third waveguide portion are set so that the sum of the product of the temperature dependence value of the group refractive index of the second waveguide portion and the length of the third waveguide portion satisfies 0; 3. The optical module according to claim 1.

4. the first waveguide portion and the second waveguide portion are connected by a connecting waveguide portion having a tapered structure whose width adiabatically increases toward the second waveguide portion, the first waveguide portion and the third waveguide portion are connected by a connecting waveguide portion having a tapered structure in which the width thereof is adiabatically narrowed toward the third waveguide portion; 4. The optical module according to claim 3.

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