Optical modulator and optical modulation method

Abstract

Provided are an optical modulator having excellent high-frequency characteristics and operating at low power, and an optical modulation method.　In this optical modulator, a light absorption region is provided to a portion of an optical waveguide. A pair of optically-coupled mirrors is respectively provided on each side sandwiching the light absorption region, and a ripple in space symbol transmittance with respect to a wavelength absorbed in the light absorbing region is wavelength-shifted into a ripple in mark symbol transmittance. In this optical modulation method, the pair of optically-coupled mirrors is respectively provided to each of the sides sandwiching the light absorption region, and a ripple in space symbol transmittance with respect to a wavelength absorbed in the light absorbing region is wavelength-shifted into a ripple in mark symbol transmittance.

Description

Optical Modulator and Optical Modulation Method

[0001] The present invention relates to an optical modulator and an optical modulation method, and particularly to an EA-type optical modulator and an optical modulation method utilizing the electroabsorption effect.

[0002] In optical communication for transmitting and receiving large-capacity data in data centers and the like, an optical transceiver equipped with an optical modulator that converts an electrical signal into an optical signal is used. In recent years, such optical transceivers are required to have excellent high-frequency characteristics for the purpose of increasing the bit rate of electrical signals to be converted. Furthermore, since the number of optical transceivers used in data centers is extremely large, low power consumption and low cost are also required (for example, Patent Document 1).

[0003] Incidentally, extinction of optical power according to an applied voltage is used for conversion from an electrical signal to an optical signal in an optical modulator. At this time, it is important to secure an optical modulation amplitude, which is a sufficient optical power difference between symbols "1" and "0", by reducing optical loss and increasing the extinction ratio. That is, in order to achieve low power consumption, it is necessary to obtain a large optical modulation amplitude with a small applied voltage; in other words, it is necessary to suppress optical loss and increase the efficiency of optical power extinction.

[0004] For example, Non-Patent Document 1 proposes a method of providing mirrors at the input and output of an MZ interferometer using an MZ (Mach-Zehnder) type optical modulator in order to obtain a large extinction ratio. Here, the principle is the same as that of a general MZ type in that a reverse bias applied to a pn structure formed in an arm of the MZ interferometer serves as a phase shifter, and symbols "1" and "0" are generated by MZ interference. On the other hand, by driving only one of the two arms of the MZ interferometer, utilizing the fact that the wavelength condition of resonance occurring between the mirrors changes between each symbol, the wavelength of resonance ripple generated in the transmittance of each symbol is changed. According to this method, in addition to the extinction ratio of a general MZ type optical modulator, the effect of resonance between the mirrors further acts to increase the extinction ratio.

[0005] Japanese Unexamined Patent Application Publication No. 2024-021920

[0006] T. Murao et al., “Mach-Zehnder interferometric engineering in silicon optical modulators: towards extrinsic OMA enhancement,” Appl. Opt. 62, 7387 (2023).

[0007] In the method described above, where mirrors are provided at the input / output section of the MZ interferometer of the MZ-type optical modulator, in addition to forming the symbol "1" and the symbol "0" by MZ interference, similar to conventional MZ-type optical modulators without mirrors, the effect of resonance between the mirrors will increase the extinction ratio.

[0008] Here, we considered that more efficient operation could be achieved by changing the wavelength conditions of the resonance between the mirrors for each symbol. However, even if both arms of the two MZ interferometers are driven in a push-pull configuration, it is not possible to change the wavelength conditions of the resonance between the mirrors for each symbol. Therefore, only one arm is driven, resulting in higher power consumption.

[0009] The present invention has been made in view of the above circumstances, and its object is to provide an optical modulator and an optical modulation method that have excellent high-frequency characteristics and operate with low power consumption.

[0010] The optical modulator according to the present invention is an optical modulator in which an optical waveguide is provided with an optical absorption region in part, and is characterized by having a pair of optically coupled mirrors on both sides of the optical absorption region, and by causing a wavelength shift of the ripple with respect to the wavelength of the space symbol transmittance absorbed in the optical absorption region with respect to the wavelength of the ripple with respect to the wavelength of the mark symbol transmittance.

[0011] These characteristics allow for the creation of an optical modulator with a large modulation amplitude in a short optical absorption region by combining it with a mirror. In other words, by shortening the optical absorption region, an increase in capacitance can be prevented, and by suppressing the resonator length between the mirrors, the Q factor of the resonance can be kept low, resulting in excellent high-frequency characteristics. Furthermore, by obtaining a large modulation amplitude, it is possible to create an optical modulator that operates with low power consumption.

[0012] In the invention described above, the mirror may be characterized by being a loop mirror or a Bragg grating. With such characteristics, the mirror can be formed with a simple configuration, and an optical modulator can be made that has excellent high-frequency characteristics and operates with low power consumption.

[0013] The invention described above may be characterized by providing a heater between the pair of mirrors. With this feature, the wavelength shift can be adjusted by adjusting the phase of the transmitted light by the thermo-optic effect, resulting in an optical modulator that has excellent high-frequency characteristics and operates with low power consumption.

[0014] The above-described invention may be characterized by including a VOA or SOA on the optical input side. With this feature, the temperature of the optical absorption region can be controlled by adjusting the incident optical power, thereby adjusting the phase of the transmitted light due to the thermo-optic effect and adjusting the wavelength shift, resulting in an optical modulator that has excellent high-frequency characteristics and operates with low power consumption.

[0015] In the above-described invention, the optical waveguide may be characterized by having a core containing Si. Furthermore, the optical absorption region may be characterized by containing a semiconductor of group IV elements or a compound semiconductor of group III-V elements. Alternatively, the optical absorption region may be characterized by having either a p-type region or an n-type region by impurity doping of Si. According to these features, an optical modulator can be easily obtained using silicon photonics technology, and an optical modulator with excellent high-frequency characteristics and low power consumption can be obtained.

[0016] Furthermore, the optical modulation method according to the present invention is an optical modulation method using an optical modulator having an optical absorption region in a part of an optical waveguide, characterized in that a pair of optically coupled mirrors are provided on both sides of the optical absorption region, and the ripple with respect to the wavelength of the space symbol transmittance absorbed in the optical absorption region is wavelength-shifted with respect to the ripple with respect to the wavelength of the mark symbol transmittance.

[0017] Based on these characteristics, by combining a mirror with an EA (electroabsorption) type optical modulator that utilizes the electrical absorption effect, a large optical modulation amplitude can be obtained in a short optical absorption region, and the optical modulator can be operated to have excellent high-frequency characteristics and low power consumption.

[0018] This is a block diagram of an optical modulator according to one embodiment of the present invention. This is a graph showing the relationship between the transmittance and wavelength of mark symbols and space symbols to explain the principle of the optical modulator according to one embodiment of the present invention. This is a diagram showing the optical modulation amplitude of a conventional EA-type optical modulator. This is a diagram showing the optical modulation amplitude of an optical modulator according to one embodiment of the present invention. This is a diagram showing the optical modulation amplitude when the optical absorption region has a pin structure and the injection current is 8 mA in a conventional EA-type optical modulator. This is a diagram showing the optical modulation amplitude when the optical absorption region has a pin structure and the injection current is 2 mA in a conventional EA-type optical modulator. This is a diagram showing the optical modulation amplitude when the optical absorption region has a pin structure and the injection current is 2 mA in an optical modulator according to one embodiment of the present invention. This is a block diagram of an optical modulator according to another embodiment of the present invention. This is a block diagram of an optical modulator according to yet another embodiment of the present invention.

[0019] Hereinafter, an optical modulator and optical modulation method, which are one embodiment of the present invention, will be described in detail with reference to Figures 1 to 7.

[0020] <Example 1> As shown in Figure 1, the optical modulator 10 has an optical waveguide 1 with an optical absorption region 2 provided in a part of it, and a pair of mirrors 3 optically coupled to both sides of the optical absorption region 2. The optical modulator 10 is formed on a silicon wafer, for example, by silicon photonics. In this case, the optical waveguide 1 has a core containing Si. Note that the core and the cladding covering the core are omitted in the figure. The optical waveguide 1 can also be formed as a compound semiconductor of a group III-V element such as InP. In this case, the optical modulator 10 may be part of an EML diode (electro-absorption type modulator laser diode) monolithically integrated with a laser element using a compound semiconductor of a group III-V element such as InP.

[0021] The light-absorbing region 2 can be formed as a semiconductor that can obtain optical modulation by changing the optical absorption coefficient using the Franz-Keldisch effect, for example. Such semiconductors can be, for example, those in which IV elements such as Ge, GeSi, and GeSn are deposited on Si, or those in which compound semiconductors of group III-V elements such as InP are hybrid-bonded. In addition, the light-absorbing region 2 can also be a semiconductor having a quantum well structure that can obtain optical modulation by changing the optical absorption coefficient using the quantum confinement Stark effect. In particular, if the optical waveguide 1 is a compound semiconductor of group III-V elements, the light-absorbing region 2 can also be a compound semiconductor of group III-V elements such as InP that has a quantum well structure.

[0022] As mirror 3, for example, a loop mirror can be used, which is formed by bringing the cores of optical waveguides close together to create a loopback. Alternatively, mirror 3 may be a Bragg grating.

[0023] According to the optical modulator 10, in response to incident light from the optical input side, it utilizes the change in transmittance corresponding to the drive voltage applied to the optical absorption region to generate signal light using a highly transmittance mark symbol transmittance (symbol "1") and a space symbol transmittance (symbol "0") whose transmittance is reduced by the optical absorption region 2 while being reflected between the pair of mirrors.

[0024] Referring to Figure 2, in the optical modulator 10, for both the mark symbol transmittance and the space symbol transmittance, a ripple occurs in the transmittance with respect to wavelength due to the presence of wavelengths at which resonance occurs between the pair of mirrors 3. In other words, an amplitude of transmittance with respect to wavelength is generated. At this time, in the optical absorption region 2, a change in the refractive index, as explained by the KK (Kramers-Kronig) relationship, occurs simultaneously with the change in the optical absorption coefficient between each symbol, and the wavelength condition of resonance between the pair of mirrors 3 changes between each symbol. Therefore, the wavelength at which the ripple peaks in the space symbol transmittance is shifted relative to the wavelength at which the ripple peaks in the mark symbol transmittance.

[0025] Therefore, the optical modulator 10 sets the wavelength at which the ripple of the mark symbol transmittance peaks as the operating wavelength. At this operating wavelength, the space symbol transmittance deviates from the peak of the transmittance ripple and approaches the trough. As a result, at the operating wavelength, the difference between the mark symbol transmittance and the space symbol transmittance becomes larger than the difference between the peaks of the ripple.

[0026] In addition, according to the optical modulator 10, the space symbol transmittance is reduced due to the interaction between resonance and light absorption between the mirrors 3. This can be explained by the principle of a Fabry-Perot resonator in the case of losses. In other words, the light passes through the light absorption region 2 multiple times by traveling back and forth between the mirrors 3, increasing light absorption and allowing for a space symbol transmittance with further reduced transmittance.

[0027] As described above, with the optical modulator 10, by combining the mirror 3 with an EA-type optical modulator that utilizes the electrical absorption effect, a large optical modulation amplitude can be obtained in a short optical absorption region. Shortening the optical absorption region prevents an increase in capacitance and also suppresses the resonator length between the mirrors. As a result, the Q factor of the resonance can be kept low, resulting in excellent high-frequency characteristics, and the large optical modulation amplitude allows the optical modulator to be operated with low power consumption.

[0028] Figure 3 shows the calculated wavelength dependence of the transmittance of signal light obtained in a conventional EA-type Si optical modulator without mirrors. Here, the calculation was performed assuming that the optical absorption region 2 is made of Ge, the length of the optical absorption region is 40 μm, the optical confinement coefficient is 1, and the electric field in the optical waveguide cross-section applied to the optical absorption region 2 in space symbol transmittance is 10 kV / cm, with an operating wavelength of 1558.1 nm. The black line shows the calculated results for mark symbol transmittance (symbol "1"), and the gray line shows the calculated results for space symbol transmittance (symbol "0"). The length of the arrow in the figure represents the optical modulation amplitude.

[0029] In contrast, Figure 4 shows the calculated wavelength dependence of the transmittance of the signal light obtained in the optical modulator 10. Here, the same assumptions as those for the conventional EA-type Si optical modulator described above were used, and the calculation was performed with a distance of 100 μm between the mirrors 3. The Q value of the resonance is 869. The black line represents the calculated transmittance of the mark symbol (symbol "1"), and the gray line represents the transmittance of the space symbol (symbol "0"). The dashed line shows the result for the conventional EA-type optical modulator for comparison, and the solid line shows the result for the optical modulator 10. As shown in the figure, the optical modulator 10 exhibits a ripple in the transmittance due to resonance between the mirrors 3. Furthermore, if the peak of the ripple in the mark symbol transmittance is taken as the operating wavelength (see arrow), then for the space symbol transmittance, the ripple is slightly outside the peak and approaching the trough. Also, the space symbol transmittance is smaller than that of the conventional EA-type optical modulator. From the above, it can be concluded that the extinction ratio in the optical modulator 10 is 1.1 dB higher than that of the conventional EA-type optical modulator. This corresponds to an increase of 0.8 dB in terms of the optical modulation amplitude.

[0030] Thus, the optical modulator 10 and the optical modulation method using the optical modulator 10 provide a larger optical modulation amplitude while maintaining good high-frequency characteristics compared to conventional EA-type optical modulators. Furthermore, with the optical modulator 10, there is no need to change the driving method compared to conventional EA-type optical modulators, and low power consumption can be maintained. In addition, because the resonator length between mirrors can be suppressed by the short optical absorption region, the Q factor of the resonance can be kept low, thereby preventing the narrowing of the wavelength-direction spacing of the ripple in the obtained transmittance. Therefore, the effect of averaging the transmittance of the ripple within the optical spectral band superimposed with the high-frequency signal can be suppressed, and a large extinction ratio can be obtained even for high-frequency signals.

[0031] Next, we will explain the results of calculations comparing a conventional EA-type optical modulator with optical modulator 10' (see Figure 1) when the optical absorption region has a pin structure. When the optical absorption region has a pin structure, it can be formed by doping the Si core with impurities. For example, the p-type region as a p-type semiconductor can be obtained by ion implantation of B, and the n-type region as an n-type semiconductor can be obtained by ion implantation of P. Note that a pn structure can also be used instead of a pin structure.

[0032] Figures 5 and 6 show the calculated wavelength dependence of the transmittance of signal light obtained in a conventional EA-type Si optical modulator without mirrors. Here, the optical absorption region was assumed to have a PIN structure and a length of 50 μm. Figure 5 shows the result for space symbol transmittance assuming a current of 8 mA injected into optical absorption region 2, and Figure 6 shows the result assuming the same current of 2 mA. The black line shows the calculated transmittance of mark symbol (symbol "1"), and the gray line shows the calculated transmittance of space symbol (symbol "0"). As shown, reducing the current injected into optical absorption region 2 from 8 mA to 2 mA also reduces the optical modulation amplitude.

[0033] In contrast, Figure 7 shows the calculated wavelength dependence of the transmittance obtained in the optical modulator 10'. Here, the same assumptions as those for the conventional EA-type Si optical modulator described above were used, with the distance between the mirrors 3 set to 100 μm, the reflectance of the mirrors 3 set to 12%, and the current injected into the optical absorption region 2 set to 2 mA. The Q value of the resonance is 751. The black line represents the calculated transmittance of the marked symbol (symbol "1"), and the gray line represents the calculated transmittance of the space symbol (symbol "0"). The dashed line shows the result from the conventional EA-type optical modulator for comparison, and the solid line shows the result from the optical modulator 10'. As shown in the figure, the optical modulator 10' exhibits a ripple in the transmittance due to resonance between the mirrors 3. Furthermore, if the peak of the ripple in the marked symbol transmittance is taken as the operating wavelength (see arrow), then the same wavelength approaches the trough of the ripple in the space symbol transmittance. From the above, the space symbol transmittance in the optical modulator 10' was 2.0 dB higher in extinction ratio compared to the conventional EA-type optical modulator. This corresponds to a 3.9 dB increase in optical modulation amplitude. In other words, it was found that with an injection current of 2 mA, the optical modulation amplitude equivalent to that obtained when an 8 mA current was injected into the conventional EA-type Si optical modulator can be obtained with the optical modulator 10'.

[0034] Furthermore, when a pn or pin structure is used in the optical absorption region 2, the carrier plasma effect during carrier injection is utilized, resulting in a larger change in refractive index associated with absorption compared to the above optical modulator 10 which uses the Franz-Keldisch effect or the quantum confinement Stark effect. Therefore, with the optical modulator 10' using a pn or pin structure in the optical absorption region 2, the operating wavelength (operating frequency) can be brought closer to the trough of the ripple in the space symbol transmittance, thereby obtaining a larger optical modulation amplitude. In addition, in conventional EA-type Si optical modulators, when a pn or pin structure is used in the optical absorption region, if the length of the optical absorption region is shortened to increase speed by reducing capacitance due to the junction area, a large injection current is required to obtain a sufficient optical modulation amplitude. As a result, this can lead to an increase in depletion layer capacitance and diffusion capacitance, which can conversely worsen high-frequency characteristics. In contrast, with the optical modulator 10', even if the length of the optical absorption region is shortened, a large optical modulation amplitude can be obtained with a small injection current, preventing deterioration of high-frequency characteristics.

[0035] <Example 2> Another embodiment of the present invention, an optical modulator and an optical modulation method, will be described in detail with reference to Figure 8.

[0036] As shown in Figure 8, the optical modulator 11 includes a heater 4 between the mirrors 3 of the optical modulator 10 described above, for adjusting the phase of the signal light obtained by the thermo-optic effect. The heater 4 may be located at any position between the pair of mirrors 3. As the heater 4, for example, a configuration in which a voltage is applied to a TiN or impurity-doped Si core can be used. With such an optical modulator 11, the conditions for the resonant wavelength can be controlled by the applied voltage. This makes it possible to adjust the resonant wavelength, which is the peak of the ripple in the mark symbol transmittance, to an arbitrary operating wavelength. Alternatively, instead of the heater 4, a pn structure or a pin structure may be provided in the optical waveguide 1, and the resonant wavelength may be adjusted by applying a change in refractive index using the carrier plasma effect.

[0037] <Example 3> Another embodiment of the present invention, an optical modulator and an optical modulation method, will be described in detail with reference to Figure 9.

[0038] As shown in Figure 9, the optical modulator 12 is equipped with an optical power regulator 5 at the optical input end of the optical modulator 10 described above for adjusting the optical power. For example, a VOA (Variable Optical Attenuator) can be used as the optical power regulator 5. When the optical power regulator 5 is an optical absorption type VOA, it can be obtained, for example, by doping a Si core with impurities to form a pn structure or a pin structure and obtaining a carrier plasma effect during carrier injection. Alternatively, it can be obtained using the Franz-Keldisch effect when a voltage is applied to a semiconductor of an IV element such as Ge, GeSi, or GeSn formed as a film on Si. It can also be obtained using the Franz-Keldisch effect when a voltage is applied to a compound semiconductor of a III-V element such as InP hybrid-bonded on Si. Furthermore, it can be obtained using the quantum confinement Stark effect in a compound semiconductor of a III-V element such as InP having a quantum well structure. As an optical power regulator 5, an SOA (Semiconductor Optical Amplifier) ​​can also be used. SOA can be obtained, for example, by hybrid bonding a compound semiconductor composed of Group III-V elements such as InP onto Si.

[0039] The optical modulator 12 allows the resonance wavelength, which is the peak of the ripple in the mark symbol transmittance, to be adjusted to any desired wavelength. In other words, the operating wavelength, when selecting the peak of the transmittance ripple as the operating wavelength, can be adjusted to any desired wavelength. This is explained by the fact that Joule heat is generated in the optical absorption region due to the photocurrent that is generated simultaneously with the absorption of light. The greater the power of the light incident on the optical absorption region, and the greater the amount of light absorbed in the optical absorption region, the more Joule heat is generated. On the other hand, when a driving voltage is applied to the optical absorption region and it is modulated at high speed, the amount of light absorbed and the resulting Joule heat change rapidly. However, this temperature change due to Joule heat cannot keep up with the rapid voltage change, and as a result, a constant temperature rise occurs in response to the voltage change. In other words, when the optical modulator 12 is modulated at high speed, the temperature of the optical absorption region 2 can be controlled by adjusting the incident light power with the optical power adjuster 5. Therefore, the resonance wavelength conditions between the mirrors 3 can be controlled by the thermo-optic effect.

[0040] Although embodiments and modifications based thereon have been described above, the present invention is not necessarily limited thereto, and those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the invention or the scope of the attached claims.

[0041] 1. Optical waveguide 2. Optical absorption region 3. Mirror 10. Optical modulator

Claims

1. An optical modulator having an optical absorption region in a part of an optical waveguide, wherein each of the optical modulator has a pair of optically coupled mirrors on both sides of the optical absorption region, and is characterized by wavelength shifting the ripple of the space symbol transmittance absorbed in the optical absorption region with respect to the wavelength of the ripple of the mark symbol transmittance with respect to the wavelength.

2. The optical modulator according to claim 1, characterized in that the mirror consists of a loop mirror or a Bragg grating.

3. The optical modulator according to claim 1, characterized in that a heater is provided between the pair of mirrors.

4. The optical modulator according to claim 1, characterized in that it includes a VOA or SOA on the optical input side.

5. The optical modulator according to claim 1, characterized in that the optical waveguide has a core containing Si.

6. The optical modulator according to claim 5, characterized in that the light-absorbing region includes a semiconductor of a group IV element or a compound semiconductor of group III-V elements.

7. The optical modulator according to claim 5, characterized in that the light absorption region has either a p-type region or an n-type region due to impurity doping of Si.

8. An optical modulation method using an optical modulator having an optical absorption region in a part of an optical waveguide, characterized in that a pair of optically coupled mirrors are provided on both sides of the optical absorption region, and the ripple of the space symbol transmittance absorbed in the optical absorption region with respect to the wavelength is wavelength-shifted with respect to the ripple of the mark symbol transmittance with respect to the wavelength.

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