Optical module

Abstract

This optical module is provided with: a ring resonator (13) including a first waveguide section (13a) which is temperature-independent of a group refractive index, a second waveguide section (13b) which has a negative temperature dependence with the group refractive index, and a third waveguide section (13c) which has a positive temperature dependence with the group refractive index; and a heater (20) that heats either the second waveguide section (13b) or the third waveguide section (13c).

Description

Optical Module

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

[0002] Wavelength division multiplexing (WDM) communication systems are being used as the capacity of optical communication systems increases. A ring resonator that achieves athermalization (temperature independence), which is one of the wavelength filters required for WDM communication systems, is disclosed in Non-Patent Document 1. The ring resonator disclosed in Non-Patent Document 1 is a ring resonator that applies the structure of a Si slot waveguide. The Si slot waveguide disclosed in Non-Patent Document 1 is a slot waveguide in which benzocyclobutene (BCB) is filled in the gap between a pair of silicon (Si) slot optical waveguides that have a gap on an SOI (silicon-on-insulator) substrate.

[0003] 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

[0004] In the ring resonator shown in Non-Patent Document 1, the silicon constituting the slot waveguide is silicon oxide (SiO ２ Since the refractive index of the cladding layer of the slot waveguide is positively temperature-dependent, BCB, whose refractive index is negatively temperature-dependent, is embedded in the gap, and by adjusting the gap, an athermal filter is realized, which offsets the temperature dependence of 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 waveguide is temperature-independent, it is not possible to apply the heat-based method, which is widely used for phase adjustment 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.

[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.

[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.

[0008] 1 is a top view schematically showing a ring resonator filter in the optical module according to the first embodiment, and FIG. 2 is a cross-sectional view schematically showing a slot waveguide in the optical module according to the first embodiment.

[0009] First Embodiment An optical module according to the first embodiment will be described with reference to Figures 1 and 2. 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 communication, a wavelength filter has the function of extracting only light of a required wavelength from wavelength-multiplexed light during transmission and carrying the signal, or extracting light of a required wavelength from wavelength-multiplexed light during reception and inputting it into a photoreceiver.

[0010] Therefore, wavelength filters are required to have steep filter characteristics and accurate center wavelength setting in order to obtain the necessary light from wavelength-multiplexed light that bundles light of many wavelengths.The ring resonator in the ring resonator filter that constitutes the wavelength filter outputs light that has periodic wavelength dependence, with the wavelength that becomes 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 achieved with high precision and ease.

[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] 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 the first bus waveguide 11, the second bus waveguide 12, and the ring resonator 13 will be described with reference to Fig. 2. The slot waveguide 1 is composed of a pair of narrow optical waveguides 1a and 1b arranged parallel to each other and adjacent 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] In FIG. 2, a silicon oxide layer (SiO ２) 2, and the silicon oxide layer 2 functions as a lower cladding for the optical waveguides 1a and 1b. On the surface of the SOI substrate, an insulating layer 3 made of benzocyclobutene (hereinafter referred to as BCB) is formed, which embeds the slot waveguide 1 and fills the gap between the pair of optical waveguides 1a and 1b. The insulating layer 3 is formed by a generally 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 1 a and 1 b is higher than the refractive index of silicon oxide forming the silicon oxide layer 2, and is a material whose refractive index increases with increasing temperature. The refractive index of BCB forming the insulating layer 3 is a material whose refractive index decreases with increasing temperature.

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

[0018] The material forming the optical waveguides 1a and 1b may be any material whose refractive index increases with increasing temperature, and in addition to silicon, silicon nitride (SiN) or indium phosphide (InP) may be used. The lower cladding (silicon oxide layer) 2 is not limited to silicon oxide, and may be any material whose refractive index is lower than that of the material forming the optical waveguides 1a and 1b. The material forming the upper cladding (insulating layer) 3 may be any material whose refractive index decreases with increasing temperature, and in addition to BCB, titanium oxide (TiO ２ ) etc. 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 with a gap of the same width over the entire length on the surface of a silicon oxide layer 2, and an insulating layer 3 filled in the gap between the optical waveguides 11a and 11b, and has a cross-sectional structure as shown in Fig. 2. The gap between the optical waveguides 11a and 11b of the input optical waveguide 11 is set so that the temperature dependence 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): ｇ１ / dT=0 (1) In the above formula (1), T is 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 will be referred to as an optical input port into which wavelength-multiplexed light is input, and the second port 11d will be referred to as a through port.

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

[0023] The output optical waveguide 12 is arranged parallel to the input optical waveguide 11 with a ring resonator 13 sandwiched therebetween. The output optical waveguide 12 has a pair of optical waveguides 12a and 12b arranged parallel and linearly 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 filled in the gap between the optical waveguides 12a and 12b, and has a cross-sectional structure shown in Fig. 2. The gap between the optical waveguides 12a and 12b of the output optical waveguide 12 is set so that the temperature dependence of the group refractive index is temperature independent.

[0024] That is, the group refractive index in the output optical waveguide 12 is n ｇ１In 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): 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 will be described as an optical output port from which light in a wavelength band resonated by the ring resonator 13 is output.

[0026] The ring resonator 13 has a first waveguide portion 13a, a second waveguide portion 13b, a third waveguide portion 13c, a 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 waveguides 13a to 13d are connected by connecting waveguides 13e to 13h to form a seamless ring-shaped optical waveguide. Note that there are no physical boundaries between the waveguides and the connecting waveguides; only virtual boundary surfaces exist. In the following description, the seamless ring-shaped optical waveguide that constitutes the ring resonator 13 will be referred to as a ring-shaped optical waveguide.

[0028] The first waveguide portion 13a is disposed opposite the output optical waveguide 12. The first waveguide portion 13a is formed on the surface of the silicon oxide layer 2 by a pair of optical waveguides 13a arranged in parallel with a gap of the same width over the entire length. １ , 13a ２ The slot waveguide and the optical waveguide 13a １ , 13a ２ The insulating layer is filled in the gap Gap, and the cross section has the structure shown in FIG.

[0029] The first waveguide portion 13a has, in a planar shape, 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. The first waveguide portion 13a is configured such that the temperature dependence of the group refractive index is temperature independent. １ , 13a ２ A gap Gap is set.

[0030] That is, the group refractive index in the first waveguide portion 13a is n ｇ０ In this case, the temperature dependency of the group refractive index in the first waveguide portion 13a is set to 0 as shown in the following equation (3). １ , 13a ２ The gap Gap is set. 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 facing the first waveguide portion 13a in the ring-shaped optical waveguide, and is disposed opposite the input optical waveguide 11. The fourth waveguide portion 13d is formed on the surface of the silicon oxide layer 2 and is a pair of optical waveguides 13d arranged in parallel with each other with a gap of the same width over the entire length. １ , 13d ２ The slot waveguide and the optical waveguide 13d are configured as follows: １ , 13d ２ The insulating layer is filled in the gap Gap, and the cross section has the structure shown in FIG.

[0032] The fourth waveguide portion 13d has, in a planar shape, 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. The fourth waveguide portion 13d is configured such that the temperature dependence of the group refractive index is temperature independent. １ , 13d ２ A gap Gap is set.

[0033] That is, the group refractive index in the fourth waveguide portion 13d is n ｇ０In this case, the temperature dependency of the group refractive index in the fourth waveguide portion 13d is set to 0 as shown in the following equation (4). １ , 13d ２ The gap Gap is set. 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 is a pair of optical waveguides 13b arranged in parallel and linearly on the surface of the silicon oxide layer 2 with a gap of the same width therebetween over the entire length. １ , 13b ２ The slot waveguide and the optical waveguide 13b are configured as follows: １ , 13b ２ The insulating layer is filled in the gap Gap, and the cross section has the structure shown in FIG.

[0035] Optical waveguide 13b １ , 13b ２ The width of the gap Gap is the width of the optical waveguide 13a in the first waveguide portion 13a. １ , 13a ２ The width of the gap Gap and the optical waveguide 13d in the fourth waveguide portion 13d １ , 13d ２ 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 temperature dependency of the group refractive index in the second waveguide portion 13b is set to be smaller than 0 as shown in the following equation (5). １ , 13b ２ The gap Gap is set. ｇ－ / dT<0...(5)

[0037] The connecting waveguide 13e connecting the first waveguide 13a and the second waveguide 13b has a tapered structure in which its width increases adiabatically toward the second waveguide 13b. The connecting waveguide 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 as shown in FIG.

[0038] The width of the gap Gap between the pair of optical waveguides in the connecting waveguide portion 13e is １ , 13a ２ The width of the gap Gap is １ , 13b ２ The pair of optical waveguides in the connecting waveguide portion 13e are arranged in a tapered shape, widening toward the second waveguide portion 13b. The first waveguide portion 13a and the second waveguide portion 13b are connected by the connecting waveguide portion 13e in a tapered structure in which the width is adiabatically widened, so there are no reflection points in the connecting waveguide portion 13e, and no light is reflected in the connecting waveguide portion 13e when light propagates through the ring-shaped optical waveguide.

[0039] The connecting waveguide 13g connecting the fourth waveguide 13d and the second waveguide 13b has a tapered structure in which its width increases adiabatically toward the second waveguide 13b. The connecting waveguide 13g has a slot waveguide formed 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 as shown in FIG.

[0040] The width of the gap Gap between the pair of optical waveguides in the connecting waveguide portion 13g is １ , 13d ２ The width of the gap Gap is １ , 13b ２The pair of optical waveguides in the connecting waveguide portion 13g are arranged in a tapered shape, widening toward the second waveguide portion 13b. The fourth waveguide portion 13d and the second waveguide portion 13b are connected by the connecting waveguide portion 13g in a tapered structure in which the width is adiabatically widened, so there are no reflection points in the connecting waveguide portion 13g, and no reflection of light occurs in the connecting waveguide portion 13g when light 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 is a pair of optical waveguides 13c arranged in parallel and linearly on the surface of the silicon oxide layer 2 with a gap of the same width therebetween over the entire length. １ , 13c ２ The slot waveguide and the optical waveguide 13c are configured as follows: １ , 13c ２ The insulating layer is filled in the gap Gap, and the cross section has the structure shown in FIG.

[0042] Optical waveguide 13c １ , 13c ２ The width of the gap Gap is the width of the optical waveguide 13a in the first waveguide portion 13a. １ , 13a ２ The width of the gap Gap and the optical waveguide 13d in the fourth waveguide portion 13d １ , 13d ２ As a result, the temperature dependency 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 temperature dependency of the group refractive index in the third waveguide portion 13c is set to be greater than 0 as shown in the following equation (6). １ , 13c ２ The gap Gap is set. ｇ＋ / dT>0...(6)

[0044] The connecting waveguide 13f connecting the first waveguide 13a and the third waveguide 13c has a tapered structure in which its width is adiabatically narrowed toward the third waveguide 13c. The connecting waveguide 13f has a slot waveguide formed 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 as shown in FIG.

[0045] The width of the gap Gap between the pair of optical waveguides in the connecting waveguide portion 13f is １ , 13a ２ The width of the gap Gap is １ , 13c ２ The pair of optical waveguides in the connecting waveguide portion 13f are arranged in a tapered shape, narrowing toward the third waveguide portion 13c. The first waveguide portion 13a and the third waveguide portion 13c are connected by the connecting waveguide portion 13f in a tapered structure in which the width increases adiabatically, so there are no reflection points in the connecting waveguide portion 13f, and no reflection of light occurs in the connecting waveguide portion 13f when light propagates through the ring-shaped optical waveguide.

[0046] The connecting waveguide 13h connecting the fourth waveguide 13d and the third waveguide 13c has a tapered structure in which its width is adiabatically narrowed toward the third waveguide 13c. The connecting waveguide 13h has a slot waveguide formed 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 as shown in FIG.

[0047] The width of the gap Gap between the pair of optical waveguides in the connecting waveguide portion 13h is １ , 13d ２ The width of the gap Gap is １ , 13c ２The width of the gap Gap between the fourth and third waveguides 13d and 13c is narrowed toward the third waveguide 13c, and the pair of optical waveguides in the connecting waveguide 13h are arranged in a tapered shape that narrows toward the third waveguide 13c. Since the fourth waveguide 13d and the third waveguide 13c are connected by the connecting waveguide 13h in a tapered structure that adiabatically widens, there are no reflection points in the connecting waveguide 13h, and light is not reflected at the connecting waveguide 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 (one ring circumference) 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 in one ring circumference. － 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): ｇ０ L+n ｇ＋ L ＋ +n ｇ－ L － = m·λ (7) When the above equation (7) is differentiated with respect to temperature, the following equation (8) is obtained.

[0050]

[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. ＋ , L － The case where the right side of the above equation (8) is set to 0 is shown in the following equation (9).

[0052]

[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]

[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 length L of the second waveguide portion 13b, the temperature dependency of the group refractive index of the ring resonator 13 becomes temperature independent. － and the length L of the third waveguide portion 13c ＋ is the temperature dependence value dn of the group refractive index 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 close to the third waveguide portion 13c 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 circumference of the ring-shaped optical waveguide, thereby changing, that is, fine-tuning, the peak wavelength, i.e., the resonance wavelength, of the ring resonator 13. The heater 20 may also be disposed outside or above the ring-shaped optical waveguide.

[0057] Furthermore, heater 20 may be disposed close to second waveguide portion 13 b on the inside, outside, or upper portion of the ring-shaped optical waveguide to heat second waveguide portion 13 b. Heating second waveguide portion 13 b by heater 20 has the effect of shortening the optical path length of one circuit of the ring-shaped optical waveguide, thereby enabling the peak wavelength, i.e., the resonant wavelength, of ring resonator 13 to be changed, i.e., finely adjusted.

[0058] Furthermore, heaters 20 may be arranged adjacent to third waveguide portion 13 c and second waveguide portion 13 b to heat third waveguide portion 13 c and second waveguide portion 13 b, respectively. By arranging heaters adjacent to third waveguide portion 13 c and second waveguide portion 13 b and selecting heaters that heat third waveguide portion 13 c and second waveguide portion 13 b, 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 of 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 fine-tuning the peak wavelength, i.e., the resonance wavelength, in the ring resonator 13, in which the temperature dependence of the group refractive index is set to be temperature independent, in the optical module according to the first embodiment will be described. Assume that the ring resonator filter 10 receives light from the optical input port 11c of the input optical waveguide 11 and outputs light from the optical output port 12c of the output optical waveguide 12. When light having a different wavelength is input to the optical input port 11c of the input optical waveguide 11, or when wavelength-multiplexed light is input, light having periodic wavelength dependence with a resonance peak at a wavelength that forms a standing wave in the ring resonator 13 is output from the optical output port 12c of the output optical waveguide 12.

[0060] First, we will explain how the resonant wavelength m λ of 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 with the heater 20, the resonant wavelength m λ of the ring resonator 13 when the environmental temperature changes is assumed to be in accordance with the following equation (11). That is, the resonant wavelength m λ assumed to be affected by a temperature change ΔT is shown in the following equation (11). In the following equation (11), the second term in the three parentheses on the left side represents the effect of the temperature change ΔT.

[0061]

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

[0063] In the first embodiment, the temperature dependence of the group refractive index of ring resonator 13 is set to be temperature independent. Therefore, the value in the 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]

[0065] On the other hand, when 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 estimated by the following equation (14):

[0066]

[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 an additional 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, because the temperature dependence of the group refractive index of ring resonator 13 is set to be temperature independent in embodiment 1, the value in the parentheses of the second term on the left side of the above equation (12) is set to 0 in the same way, the above equation (14) can be expressed by the following equation (15), which corresponds to the above equation (13).

[0069]

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

[0071] The value of m' in the above formula (16) is larger than the value of m in the above formula (13). This means that by heating the third waveguide portion 13 c with the heater 20, the resonant wavelength m'·λ in the ring resonator 13 can be changed from the resonant wavelength m'·λ in the ring resonator 13 when the third waveguide portion 13 c 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, and the resonant wavelength in the ring resonator 13 can be changed.

[0073] In short, in ring resonator 13, even if the environmental temperature changes and the temperature of the entire ring resonator 13, particularly the temperatures of third waveguide portion 13 c and second waveguide portion 13 b, changes at 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 heater 20 to generate a temperature gradient between third waveguide portion 13 c and second waveguide portion 13 b, it is possible to fine-tune the resonance wavelength in ring resonator 13.

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

[0075] On the other hand, the optical monitor 30, which is observing (monitoring) the light from the through port 11 d 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 11 d, for example, the output level of the light receiving element that detects the light from the through port 11 d 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 resonant wavelength in ring resonator 13 is changed, that is, finely adjusted, as shown in equation (16) above. When the resonant wavelength in ring resonator 13 is finely adjusted to the set resonant 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 monitors the light from the through port 11d of the input optical waveguide 11, but it may also monitor 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 13 a whose group refractive index is independent of temperature, a second waveguide portion 13 b whose group refractive index has a negative temperature dependency, and a third waveguide portion 13 c whose group refractive index has a positive temperature dependency, and a heater that heats either the second waveguide portion 13 b or the third waveguide portion 13 c. 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 in 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. However, the following configuration may also be adopted.

[0081] The second bus waveguide 12 may be the input optical waveguide 12, the first port 12c of the second bus waveguide 12 may be an optical input port, the second port 12d may be a through port, the first bus waveguide 11 may be the output optical waveguide 11, and the second port 11d of the first bus waveguide 11 may be 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, ring resonator 13 is configured as, for example, a planar waveguide system on a silicon platform. 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 an MMI (Multi-Mode Interferometer). 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 13 a whose group refractive index is temperature-independent, second waveguide portion 13 b whose group refractive index is negative in temperature dependence, third waveguide portion 13 c whose group refractive index is positive in temperature dependence, and fourth waveguide portion 13 d 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.

[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.

[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 section, 13a １ , 13a ２ Optical waveguide, 13b Second waveguide portion, 13b １ , 13b ２ Optical waveguide, 13c Third waveguide section, 13c １ , 13c ２ Optical waveguide, 13d Fourth waveguide section, 13d １ , 13d ２ Optical waveguide, 13e to 13h connecting waveguide portion, 20 heater.

Claims

1. An optical module comprising: a ring resonator having a first waveguide section whose group refractive index has no temperature dependency, a second waveguide section whose group refractive index has a negative temperature dependency, and a third waveguide section whose group refractive index has a positive temperature dependency; and a heater that heats either the second waveguide section or the third waveguide section.

2. An optical module according to claim 1, wherein the first waveguide section, the second waveguide section, and the third waveguide section each have a slot waveguide consisting 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, embedded in the gap between the pair of optical waveguides in the slot waveguide.

3. An optical module according to claim 1 or claim 2, wherein the length of the second waveguide section and the length of the third waveguide section are set so that the sum of the product of the temperature-dependent value of the group refractive index of the second waveguide section and the length of the third waveguide section satisfies 0.

4. An optical module according to claim 3, wherein the first waveguide section and the second waveguide section are connected by a connecting waveguide section having a tapered structure whose width adiabatically increases toward the second waveguide section, and the first waveguide section and the third waveguide section are connected by a connecting waveguide section having a tapered structure whose width adiabatically decreases toward the third waveguide section.

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