JP2008268276A - Light modulator, light source device, and driving method of light modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light modulator which is capable of achieving stable modulation characteristics by reducing fluctuations of an input wavelength or the variance of extinction characteristics due to the variance of an environmental temperature, a light source device, and a driving method of the light modulator. <P>SOLUTION: The light modulator modulates an input optical wave to output an optical wave signal and includes cascaded first and second optical resonators 35 and 36 mutually different by resonance wavelength and a modulator 15 which modulates optical losses or refractive indexes of the first and second optical resonators synchronously in opposite directions. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical modulator, a light source device, and a method for driving the optical modulator that convert an electrical signal used for optical communication, optical information processing, optical interconnection, and the like into an optical wave signal.

Optical modulators that convert electrical signals into lightwave signals are key components that have played a vital role in the rapid increase in traffic in the recent optical communications market. As the optical modulator, an electroabsorption modulator using a compound semiconductor material and a Mach-Zehnder modulator using a LiNbO ₃ material have been put into practical use. As an example of an electroabsorption modulator using a compound semiconductor material, a modulator integrated light source using an InP material has been published in Non-Patent Document 1, and has been put to practical use as a light source for long-distance optical communication. Further, as an example of a Mach-Zehnder modulator made of a LiNbO ₃ material, Non-Patent Document 2 reports a particularly miniaturized one.

  However, the former has a difficulty in cost reduction in terms of wafer material and manufacturing cost, and the latter has a device size of 1 cm or more although it is miniaturized, and there is a limit to cost reduction. Optical modulators are important optical components in optical information processing and optical interconnection, but there is a demand for significant reduction in component costs compared to the optical communication field, and the application is extremely difficult at present. .

  On the other hand, Non-Patent Document 3 recently discloses an optical modulator using a Si semiconductor that can be manufactured at low cost. In addition to being able to use mature process technology, the advantages of Si semiconductors include the possibility of OEIC (Optoelectronic Integrated Circuit) using Si wafers due to its affinity with CMOS LSI (Complementary Metal-Oxide Semiconductor Large Scale Integration). This is a suggestion. The announced optical modulator was epoch-making in that not only the passive waveguide portion of light but also the active waveguide portion was formed on the Si wafer. However, as the element structure, a change in refractive index due to application of an electric field in the vicinity of the PN junction is merely converted into light intensity by a simple Mach-Zehnder interference system, and an element size of 1 cm or more is required. With element size of centimeter order, not only is there a limit to cost reduction in terms of yield obtained from each wafer, but also OEIC is difficult from the viewpoint of integration of CMOS LSI, etc. Have the problem of not being able to fully utilize the benefits of using.

On the other hand, Non-Patent Document 4 discloses an optical modulator that utilizes the wavelength resonance characteristics of an Si ring optical waveguide, and an element size of several tens of microns is realized. However, when using resonance characteristics such as a ring optical waveguide, the input light wavelength must be set near the resonance wavelength, and a highly accurate wavelength control mechanism is required. This causes a problem that the extinction characteristic (modulation dynamic characteristic) changes greatly due to a slight fluctuation of the resonance wavelength caused by the environmental temperature. In particular, in the optical interconnection, the athermal characteristic is an essential condition from the viewpoint of cost, and some measure for relaxing the temperature characteristic of the resonance wavelength is required. In addition, in wavelength division multiplexing (WDM (Wavelength Division Multiplex)) introduced to increase transmission capacity, preparing an optical modulator with suitable wavelength characteristics for each channel leads to an increase in system cost. An optical modulator whose characteristics do not depend on the channel wavelength is also required.
M.ISHIZAKA et al., IEEE PHOTONICS TECHNOLOGY LETTERS 1997, VOL.9, NO.10, p.1406 M.SUGIYAMA et al., 30th Eouropean Conference On Optical Communication, 2004, Post-Deadline Session3, Th4.3.5 Ansheng Liu et al., NATURE, VOL.427 12 FEBRUARY, 2004, p.615 Quianfan Xu et al., NATURE, VOL.435, 19 MAY 2005, p.325

  Optical components such as optical modulators, whose performance and cost have been advanced with the progress of optical communication technology, are considered to be applied to fields with higher usage quantities such as optical information processing and optical interconnection. Being started. In this field, a price reduction of about two orders of magnitude or more is required compared to the communication field, and it is necessary to further reduce costs including materials and element structures. In this sense, a modulator using a Si material is important. However, since the electro-optic effect is small, it is necessary to reduce the size by using a resonance structure such as a ring waveguide.

  However, when the resonance characteristics are used, there is a problem that the extinction characteristics (modulation dynamic characteristics) are greatly changed due to a slight change in the resonance wavelength caused by the environmental temperature change.

FIG. 10 is a schematic graph showing an example of the operation of an optical modulator using a conventional single optical resonator. In the conventional optical modulator, the input light wavelength λ ₀ is set in the vicinity of the resonance peak wavelength, and the transmitted light intensity is modulated by shifting the resonance peak by applying voltage, and the wavelength spectrum is inclined with respect to the wavelength fluctuation. Proportional fluctuations in extinction characteristics will occur.

  FIG. 11 is a graph for explaining a specific operation when a modulator according to an example of a conventional single ring resonator is used.

  FIG. 11A shows a power transmittance spectrum 60 of a single ring resonator for a voltage application of 0V. By applying voltage, the power transmittance spectrum 60 indicated by a dotted line is shifted to a power transmittance spectrum 61 indicated by a solid line as shown in FIG. 11B, and the resonance peak is also shifted. When the input light wavelength is set to the resonance peak in FIG. 6A, extinction characteristics such that the light intensity is increased by voltage application can be obtained.

  FIG. 12 is a graph showing a difference in extinction characteristics when a wavelength variation with a width of 0.06 nm occurs with respect to a setting center in a conventional single ring resonator. In the case of a single ring resonator, the fluctuation of the wavelength may cause the extinction characteristic to become a characteristic indicated by reference numeral 70 or a characteristic indicated by reference numeral 71, and a predetermined extinction characteristic can be obtained. Can not.

  Therefore, the present invention provides an optical modulator, a light source device, and a driving method for the optical modulator that can reduce fluctuations in the extinction characteristic with respect to fluctuations in input wavelength or environmental temperature fluctuations, and realize stable modulation characteristics. For the purpose.

  In order to achieve the above object, an optical modulator of the present invention is an optical modulator that modulates an input optical wave and outputs it as an optical wave signal, two optical resonators connected in cascade and having different resonance wavelengths, and 2 Modulation means for synchronizing and modulating optical loss or refractive index of two optical resonators in opposite directions.

  In the optical modulator of the present invention, the optical loss or refractive index of the two optical resonators is synchronized by the modulation means and modulated in opposite directions, thereby performing intensity modulation with a small wavelength dependence in a predetermined specific wavelength range. This can be realized and the tolerance of the input wavelength can be increased. That is, according to the optical modulator of the present invention, the fluctuation of the extinction characteristic with respect to the fluctuation of the input wavelength or the environmental temperature fluctuation can be mitigated, and a stable modulation characteristic can be realized.

  Further, in the optical modulator of the present invention, the waveguide lengths of the two optical resonators may be different from each other in order to make the resonance wavelengths of the two optical resonators different.

  Alternatively, in the optical modulator of the present invention, in order to make the resonance wavelengths of the two optical resonators different, the refractive indexes of the two optical resonators in a state where the modulation by the modulation means is not performed are different from each other. Also good.

  Further, in the optical modulator of the present invention, a heater electrode may be disposed adjacent to the two optical resonators in order to make the resonance wavelengths of the two optical resonators different.

  In the optical modulator of the present invention, two optical resonators include a ring-type optical waveguide and at least one linear optical waveguide optically coupled to the ring-type optical waveguide, and these two optical resonators May be connected in cascade between an input optical waveguide to which a light wave is input and an output waveguide that outputs a light wave signal. In this case, when a semiconductor ring optical waveguide is used as the optical resonator, the element size can be reduced and the modulation speed can be improved.

  In the optical modulator of the present invention, the two optical resonators may be composed of linear optical waveguides whose both end surfaces are reflecting surfaces.

  The optical modulator of the present invention may be one in which two optical resonators have a PIN junction structure made of a semiconductor material, and the modulation means applies a reverse bias voltage to the PIN junction structure.

  Further, the optical modulator of the present invention may include a semiconductor multiple quantum well structure inside the PIN junction structure.

  In the optical modulator of the present invention, the two optical resonators have a PIN junction structure or a PN junction structure made of a semiconductor material, and the modulation means applies a forward bias voltage to the PIN junction structure or the PN junction structure. It may be applied.

  In the optical modulator of the present invention, the two optical resonators may include a ferroelectric material, and the modulation unit may apply a voltage to the two optical resonators including the ferroelectric material. .

  Further, the optical modulator of the present invention may be one in which the period of the resonant wavelength of the two optical resonators is matched with the channel interval of wavelength division multiplexing transmission. Thereby, uniform extinction characteristics can be realized in any channel of wavelength division multiplexing transmission.

  The light source device of the present invention includes the light modulator of the present invention and a light source, the wavelength of the light wave emitted from the light source is variable, and the period of the wavelength of the light wave and the interval between the resonance wavelengths of the optical resonators Are matched.

  The optical modulator driving method of the present invention is a method of driving the optical modulator of the present invention, wherein the optical loss or refractive index of two optical resonators connected in cascade and having different resonance wavelengths are modulated. To modulate in the opposite directions.

  In the optical modulator of the present invention, the optical loss or refractive index of the two optical resonators is synchronized by the modulation means and modulated in opposite directions, thereby performing intensity modulation with a small wavelength dependence in a predetermined specific wavelength range. This can be realized and the tolerance of the input wavelength can be increased. That is, according to the optical modulator of the present invention, the fluctuation of the extinction characteristic with respect to the fluctuation of the input wavelength or the environmental temperature fluctuation can be mitigated, and a stable modulation characteristic can be realized.

  The optical modulator of the present invention changes the transmission spectrum intensity in a specific range by shifting the resonance peak wavelengths of the two optical resonators connected in cascade with each other in synchronization with an external modulation voltage. An electric signal is converted into a light intensity signal.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 is a schematic diagram showing an outline of a configuration of a light source device including an optical modulator according to the present embodiment. FIG. 2 is a schematic cross-sectional view taken along line A1-A2 in FIG.

  A linear optical waveguide 22, a first ring-type optical waveguide 26, and a second ring are formed on the same plane above an SOI (Silicon on insulator) substrate formed of an N-type silicon layer 4 and a silicon oxide film 2 formed thereon. A mold optical waveguide 28 is formed. The linear optical waveguide 22 and the first ring optical waveguide 26 and the second ring optical waveguide 28 are disposed so as to be close to each other. The linear optical waveguide 22 and the first ring optical waveguide 26 arranged close to each other in this way constitute a first optical resonator 35, and the linear optical waveguide 22 and the second ring optical waveguide 26. Constitutes a second optical resonator 36. The two first optical resonators 35 and the second optical resonators 36 connected in cascade form a resonance unit 37.

  The first optical resonator 35 and the second optical resonator 36 have different total lengths of the waveguides so that the resonance wavelengths are different from each other. In addition, the first optical resonator 35 and the second optical resonator 36 are different in refractive index from each other in a state where no voltage is applied in order to have different resonance wavelengths. Also good. Note that the period of the resonant wavelength between the first optical resonator 35 and the second optical resonator 36 may be made to coincide with the channel interval of wavelength division multiplexing transmission. The first optical resonator 35 and the second optical resonator 36 are connected in cascade.

  Anode electrodes 9 for applying a voltage are formed inside the circumferences of the first ring-type optical waveguide 26 and the second ring-type optical waveguide 28, respectively. Each anode electrode 9 is disposed with anode pad electrodes 20 and 21 connected thereto. On the other hand, a cathode electrode (ground electrode) 11 is formed outside the circumference of the first ring type optical waveguide 26 and the second ring type optical waveguide 28. A heater 8 is disposed above the first ring-type optical waveguide 26 and the second ring-type optical waveguide 28. The heater 8 is for heating the first ring-type optical waveguide 26 and the second ring-type optical waveguide 28 so that the respective resonance wavelengths are different.

  Note that the first ring-type optical waveguide 26 and the second ring-type optical waveguide 28 of the present embodiment are minute ring-type optical waveguides having a radius of about 10 microns. For this reason, the area of the modulation part can be reduced, and the electric capacity of the modulation unit can be reduced. That is, according to the ring type optical waveguides 26 and 28, the modulation speed of the optical modulator is inversely proportional to the electric capacity, so that the modulation speed can be improved. In addition, since the element size is mainly determined by the size of the optical resonator constituting the optical modulator, the semiconductor optical ring is used as the first optical resonator 35 and the second optical resonator 36. The element size can be reduced to about several tens of microns. Furthermore, since the first ring-type optical waveguide 26 and the second ring-type optical waveguide 28 are made of an inexpensive Si semiconductor, a large number of optical modulators can be formed on the Si wafer. The manufacturing cost per element can be reduced.

  In addition, it is preferable that the resonance wavelength period of the first optical resonator 35 and the second optical resonator 36 coincide with the wavelength interval of wavelength division multiplexing (WDM) transmission. In this case, in wavelength division multiplexing (WDM) transmission, the channel wavelength dependency of the extinction characteristic at the time of modulation can be reduced, and the characteristic difference between channels can be eliminated, and even in any channel of wavelength division multiplexing (WDM) transmission, it is uniform. Quenching characteristics can be realized.

  The modulation device 15 is electrically connected to the anode pad electrodes 20 and 21. The modulation device 15 synchronizes the optical loss or refractive index of the first optical resonator 35 and the second optical resonator 36 in opposite directions and synchronizes the anode pad electrodes 20 and 21 with each other. The modulated voltage having the opposite phase is applied.

  The light source 100 emits a light wave to the input end 30 of the linear optical waveguide 22. The light source 100 has a function of changing the wavelength of the emitted light wave.

  Next, a cross-sectional structure of the optical modulator of this embodiment will be described.

  As described above, in the optical modulator of this embodiment, the silicon oxide film 2 is formed on the N-type silicon layer 4. A second ring type optical waveguide 28 is formed on the silicon oxide film 2 as a ridge type optical waveguide. P-type silicon layers 3 and 5 are disposed on both sides of the second ring type optical waveguide 28. The N-type silicon layer 4 and the P-type silicon layers 3 and 5 are formed by impurity doping. The N-type silicon layer 4, the P-type silicon layers 3 and 5, and the first and second ring-type optical waveguides 26, 28 constitutes a PIN junction. The PIN junction structure includes a multiple quantum well structure.

  A cathode electrode 11 and an anode electrode 9 are stacked on the P-type silicon layer 3 and the N-type silicon layer 4, respectively. A silicon oxide film 6 is formed on the P-type silicon layer 3 excluding the cathode electrode 11 and the anode electrode 9. The silicon oxide film 6 is formed for the purpose of a protective film and a waveguide clad layer. A heater 8 is provided on the silicon oxide film 6 to play a role of adjusting the resonance peak wavelengths of the first ring type optical waveguide 26 and the second ring type optical waveguide 28.

  Next, the operation mechanism of the optical modulator of this embodiment will be described in detail.

  The light wave emitted from the light source 100 is input from the input end 30 of the linear optical waveguide 22. The light source 100 has a function of changing the wavelength of the emitted light wave.

  The light wave input to the straight optical waveguide 22 is optically partially coupled to the first ring optical waveguide 26 and resonates. As a result, the light wave attenuates its power due to the ring waveguide loss. The light wave whose power is attenuated subsequently resonates also in the second ring-type optical waveguide 28, further attenuates the power, and is output from the output end 31 as a light wave signal.

  The intensity of resonance between the input light wave and the first and second ring optical waveguides 26 and 28 is changed by applying a voltage between the anode pad electrodes 20 and 21 and the cathode electrode 11 by the modulation device 15. To do.

  This is due to the following principle.

  When a voltage is applied between the anode pad electrodes 20 and 21 and the cathode electrode 11, the first and second ring types that are ridge type optical waveguides between the N-type silicon layer 4 and the P-type silicon layers 3 and 5 are used. A current flows through the optical waveguides 26 and 28. That is, a current flows through the PIN structure, and the refractive indexes of the first and second ring optical waveguides 26 and 28 change due to the carrier plasma effect. As a result, the resonance peak wavelengths of the first and second ring-type optical waveguides 26 and 28 shift, and the intensity of resonance with respect to the input light wave changes. The strength of this resonance is the strength of the power of the input light wave.

  FIG. 3A shows the power transmittance spectra when the modulation device 15 applies the modulation voltages having opposite phases to each other in synchronization with the first optical resonator 35 and the second optical resonator 36. The combined wavelength spectrum of each power transmittance spectrum is schematically shown in FIG.

As shown in FIG. 3A, the power transmittance spectrum 201 of the first optical resonator 35 shifts to the high wavelength side with the application of voltage, and the power transmittance spectrum 202 of the second optical resonator 36. Shift to the lower wavelength side. The combined wavelength spectrum 203 of these power transmittance spectra 201 and 202 is transmitted light intensity in a certain wavelength range near a specific wavelength λ ₀ (center of resonance peak wavelength of two resonators) as shown in FIG. Will change. That is, when a light wave having this specific wavelength is input, it is possible to guarantee a certain extinction characteristic against fluctuation in a certain wavelength range.

  Hereinafter, the transmission spectrum intensity in the optical modulator of this embodiment will be described more specifically.

  FIG. 4 is a graph showing changes in power transmission spectra of two ring resonators with voltage application.

  FIG. 4A shows the power transmittance spectrum 40 of the first optical resonator 35 and the second optical resonator when a constant voltage is applied between the anode pad electrodes 20 and 21 and the cathode electrode 9. A power transmittance spectrum 41 of 36 and a power transmittance 42 synthesized by these two resonators 35 and 36 are shown. FIG. 4B shows a change in power transmittance when the voltages applied to the anode pad electrodes 20 and 21 are changed in opposite directions from the state of FIG. FIG. 4C also shows the change in power transmittance when the voltages applied to the anode pad electrodes 20 and 21 are synchronized and changed in opposite directions, but from the state of FIG. Furthermore, the state which applied voltage was raised is shown.

  As the voltage is applied, the power transmittance spectrum of the first optical resonator 35 is sequentially shifted to the high wavelength side (reference numeral 40 → reference numeral 44 → reference numeral 47), and the power transmittance spectrum of the second optical resonator 36 is Are sequentially shifted to the lower wavelength side (reference numeral 41 → reference numeral 43 → reference numeral 46). These combined power transmittance spectra change from a concave shape to a convex shape without shifting to either the high wavelength side or the low wavelength side (reference numeral 42 → reference numeral 45 → reference numeral 48). .

  Therefore, when the wavelength of the input light is set at the center of the concave shape of the combined power transmittance spectrum 42, it is possible to ensure a predetermined extinction characteristic with respect to wavelength fluctuation in an appropriate range.

  FIG. 5 is a graph showing differences in extinction characteristics (dependence of power transmittance on applied voltage) when the input light wavelength varies with respect to the center of the set wavelength in the optical modulator of the present embodiment. The wavelength variation is 0.06 nm corresponding to about 10% of the channel wavelength interval of DWDM (Dense Wavelength Division Multiplexing). Although there is a wavelength difference of 0.06 nm between the extinction curve 50 and the extinction curve 51, it shows substantially the same extinction characteristics.

  As described above, the optical modulator of the present embodiment having the resonating unit 37 including two resonators performs control to synchronize the applied voltages and change them in opposite directions. For this reason, the optical modulator according to the present embodiment is more uniform (having less wavelength dependency) in a specific wavelength range than a modulator (FIGS. 10 and 11) using a conventional single ring resonator. Intensity modulation can be realized, and the wavelength tolerance of input light can be greatly increased.

  Moreover, the optical modulator of this embodiment can increase the tolerance of the environmental temperature with respect to the extinction characteristic. That is, in order to perform modulation to synchronize and shift the resonance peak wavelengths of the first and second optical resonators 35 and 36 of the resonance unit 37 in the opposite directions, they are uniform in a specific wavelength range. It is possible to realize intensity modulation (small wavelength dependence), and to ensure a certain allowable range with respect to fluctuations in the resonance peak wavelength due to changes in the environmental temperature.

  Further, the optical modulator of this embodiment performs intensity modulation by shifting the resonance peaks of the first and second optical resonators 35 and 36. For this reason, unlike the conventional Mach-Zehnder optical modulator, it is not necessary to shift the light wave phase by π radians, so that the modulation drive voltage can be reduced.

  Further, by making the resonance wavelength period of the first and second optical resonators 35 and 36 coincide with the wavelength interval of the WDM, uniform extinction characteristics can be realized in any channel of the WDM.

  In the present embodiment, an example in which the first optical resonator 35 and the second optical resonator 36 have a PIN junction structure made of a semiconductor material and a reverse bias voltage is applied thereto has been described. The invention is not limited to this. That is, the first optical resonator 35 and the second optical resonator 36 may have a PN junction structure made of a semiconductor material. The modulator 15 applies a forward bias voltage in the opposite direction to each of the first optical resonator 35 and the second optical resonator 36 having the PIN junction structure or the PN junction structure in synchronization with each other. There may be.

  Further, the first optical resonator 35 and the second optical resonator 36 may include a ferroelectric material. Then, the modulators 15 may apply voltages opposite to each other in synchronization with the first optical resonator 35 and the second optical resonator 36 containing a ferroelectric material.

  In the present embodiment, the optical resonator having only one resonance part 37 has been described as an example. However, the present invention is not limited to this, and may have a plurality of resonance parts 37. .

In addition, the element structure of the present invention is not limited to materials used, and can be applied to Si materials, compound semiconductor materials, and LiNbO ₃ materials.
(Second Embodiment)
As a second embodiment of the present invention, an example of an optical modulator that suppresses fluctuations in extinction characteristics with respect to environmental temperature changes and has stable modulation characteristics will be described below.

  The wavelength accuracy of external input light can be controlled in the picometer order of wavelength by the recent progress of microprocessors, but the temperature independence of the optical modulator is not necessarily an effective technology. Absent. In particular, in the optical interconnection used inside and outside the computer equipment, making the electric-light wave signal conversion unit temperature independent is an essential condition. In order to solve this problem, this embodiment provides a temperature-independent configuration of an optical modulator.

  FIG. 6 is a cross-sectional view schematically showing a configuration in the vicinity of the ridge type optical waveguide of the present embodiment.

In the optical modulator of the first embodiment, a silicon oxide film 6 is a protective film and a cladding layer on top of the first ring type optical waveguide 26 and the second ring type optical waveguide 28 which are ridge type optical waveguides. Was laminated as. On the other hand, the optical modulator of this embodiment is different from the first embodiment in that a clad layer 80 made of a PbMoO ₄ film is laminated instead of the silicon oxide film 6, but the basics other than that are the same. The construction is the same. Therefore, the description regarding the same structure is abbreviate | omitted.

  Hereinafter, a mechanism for making the extinction characteristic of the present embodiment temperature independent will be described in detail.

The temperature dependence of the resonance peak wavelength of the ring type optical resonator is determined by the temperature dependence of the refractive indexes of the silicon core layer 81 and the cladding layer 80 constituting the ridge type optical waveguide. Here, the thermo-optic coefficient indicating the temperature dependence of the refractive index of the silicon core layer 81 is approximately 1.8 × 10 ⁻⁴ [K ⁻¹ ]. On the other hand, the thermo-optic coefficient of PbMoO ₄ forming the cladding layer 80 is −6 × 10 ⁻⁵ [K ⁻¹ ], and the signs of the thermo-optic coefficients of the silicon core layer 81 and the cladding layer 80 are positive and negative. It is reversed.

  The effective refractive index affected by the light wave propagating through the waveguide is approximately the weighted average of the refractive indexes of the silicon core layer 81 and the cladding layer 80 in a field distribution. Therefore, the waveguide size can be appropriately adjusted so that the effective thermo-optic coefficient is zero, that is, the temperature dependence of the effective refractive index can be eliminated.

  FIG. 7 is a graph showing the calculation result of the change in the effective refractive index of the waveguide when the environmental temperature is changed from 0 ° C. to 100 ° C. using the waveguide width W as a parameter. From FIG. 7, it is understood that the temperature dependence of the effective refractive index can be eliminated by setting the waveguide width W to about 0.26 μm.

  By the way, when an exposure technique using an electron beam is used, it is possible to suppress the creation variation of the waveguide width W to 3 nm or less. When the refractive index change resulting from this variation is estimated from FIG. 7 and calculated as a change in resonance peak wavelength, it is about 0.05 nm. As shown in the first embodiment, in the configuration of the present invention, the extinction characteristic can be made invariant even with respect to a wavelength variation of 0.06 nm. Therefore, even if the creation variation of the waveguide width W is taken into consideration. It is possible to configure an optical modulator that eliminates the temperature dependence of the effective refractive index.

As described above, the optical modulator according to the present embodiment can perform electric-light wave signal conversion in which the extinction characteristic does not change even when the environmental temperature changes by 100 [K], and is particularly suitable for optical interconnection. ing.
(Third embodiment)
In the first and second embodiments described above, a resonance spectrum of power transmittance is created using propagation loss inside the ring optical waveguide. For this reason, a certain amount of optical loss occurs even in the light transmission state (light ON state), so it is necessary to adjust the loss inside the waveguide from the balance between the extinction characteristic and the optical loss. In the present embodiment, an optical modulator having a configuration in which optical loss can be easily reduced will be described below.

  The optical modulator of this embodiment has a configuration that does not use the loss inside the waveguide, and a schematic diagram thereof is shown in FIG.

  Three linear optical waveguides 107, 108, and 112 are arranged on the same plane above the SOI substrate 101 made of a silicon substrate and a silicon oxide film formed thereon.

  In addition, a first ring-type optical waveguide 113 and a second ring-type optical waveguide 114 are formed so as to be close to each of the straight optical waveguides 107, 108, and 112 at two locations. That is, a first ring optical waveguide 113 is formed between the linear optical waveguide 108 and the linear optical waveguide 112, and a second ring optical waveguide is formed between the linear optical waveguide 107 and the linear optical waveguide 108. A waveguide 114 is formed.

  The two linear optical waveguides 112 and 108 and the first ring optical waveguide 113 constitute a first optical resonator 150. Similarly, the two straight optical waveguides 108 and 107 and the second ring optical waveguide 114 constitute a second optical resonator 151. As described above, in the optical modulator of the present embodiment, the first optical resonator 150 and the second optical resonator 151 are connected in cascade to form the resonance unit 152.

  In addition, anode electrodes 125 and 129 for applying a voltage are formed inside the circumferences of the first and second ring type optical waveguides 113 and 114. Anode pad electrodes 109 and 104 are connected to the anode electrodes 125 and 129, respectively. A cathode electrode 110 is formed outside the circumference of the first and second ring type optical waveguides 113 and 114. Further, the heaters 102 and 103 are arranged above the first and second ring type optical waveguides 113 and 114.

  A modulation device 115 is electrically connected to the anode pad electrodes 104 and 109. The modulation device 115 synchronizes the optical loss or refractive index of the first optical resonator 150 and the second optical resonator 151 in a reverse direction to each other, so that the anode pad electrodes 104 and 109 are synchronized with each other. The modulated voltage having the opposite phase is applied.

  The operation mechanism in the optical modulator of the present embodiment configured as described above will be described in detail below.

  The light wave emitted from the light source 300 is input from the input end 111 of the linear optical waveguide 112. Part of the input light wave propagates through the straight optical waveguide 108 by being resonantly coupled with the waveguide mode of the first ring-type optical waveguide 113. Thereafter, the light wave propagated through the straight optical waveguide 108 is resonantly coupled with the waveguide mode of the second ring optical waveguide 114 and propagates through the straight optical waveguide 107. Then, the light wave propagated to the linear optical waveguide 107 is output from the output end 106 of the linear optical waveguide 107.

  A component that is not resonantly coupled to the ring optical waveguide by the light wave propagating through the linear optical waveguide 112 is output from the waveguide end 130, and a component that is not resonantly coupled to the ring optical waveguide 113 by the linear optical waveguide 108 is output from the waveguide end 131. Is output. Components output from the waveguide ends 130 and 131 become optical losses.

  In the case of this embodiment, even if the ring type optical waveguide is lossless, the components output from the waveguide ends 130 and 131 generate optical loss, so that the power spectrum of the light wave propagating through the straight optical waveguides 108 and 107 Becomes a resonance curve convex to the wavelength.

The power transmission spectrum of the output end 106 with respect to the input end 111 is represented by superposition of the two convex resonance spectra of the first and second ring optical waveguides 113 and 114. Therefore, by synchronizing the respective resonance peak wavelengths and shifting them in opposite directions, it is possible to obtain a characteristic equivalent to the extinction characteristic shown in the first embodiment. Note that the two resonance peaks can be synchronized and shifted in opposite directions by applying modulation voltages having opposite phases to the anode pad electrodes 104 and 109 easily.
(Fourth embodiment)
Each optical resonator in the first to third embodiments described above has been described using a ring optical resonator, but the present invention is not limited to this. In other words, the same effect can be obtained even when a waveguide type optical resonator having an appropriate reflectivity at both end faces of the linear optical waveguide is used as the optical resonator.

  FIG. 9 is a schematic diagram showing a schematic configuration of the optical modulator of the present embodiment in which two linear waveguide type optical resonators are connected in cascade.

  In the optical modulator of this embodiment, a linear waveguide type optical resonator 301 and a linear waveguide type optical resonator 302 are connected in cascade. The linear waveguide type optical resonator 301 constitutes an optical resonator by providing reflection end faces 301a and 301b on both end faces of the linear waveguide. Similarly, reflection end faces 302a and 302b are also provided on both end faces of the linear waveguide type optical resonator 302 to constitute an optical resonator.

  The linear waveguide optical resonator 301 and the linear waveguide optical resonator 302 are optically coupled. That is, a light wave input from the reflection end face 301a of the linear waveguide type optical resonator 301 is emitted from the reflection end face 301b, enters from the reflection end face 302a of the linear waveguide type optical resonator 302, and is output from the reflection end face 302b. Is done.

  As described above, the linear waveguide optical resonator 301 and the linear waveguide optical resonator 302 arranged in a column so that the reflection end surface 301b and the reflection end surface 302a face each other are optically coupled to form the resonance unit 320. is doing.

  The light wave passes through the optical isolator 310 disposed on the reflection end surface 301a side and then is input to the reflection end surface 301a. The other basic configuration of the optical modulator of this embodiment is the same as that of the optical modulators of the above-described embodiments, and thus detailed description thereof is omitted.

  The light wave that has passed through the optical isolator 310 sequentially propagates through the linear waveguide optical resonator 301 and the linear waveguide optical resonator 302. In this process, the refractive index or loss of the linear waveguide type optical resonator 301 and the linear waveguide type optical resonator 302 are synchronized and modulated in opposite directions to pass through these optical resonators 301 and 302. The light wave is modulated with the same effects and operations as in the first to third embodiments.

  In the configuration of FIG. 9, the linear waveguide type optical resonator 301 constitutes a resonator and functions as an input optical waveguide, and the linear waveguide type optical resonator 302 constitutes a resonator and serves as an output optical waveguide. Is also functioning. However, the optical modulator of the present embodiment is provided with an input optical waveguide to which a light wave is input in front of the linear waveguide optical resonator 301, and an optical wave signal is output behind the linear waveguide optical resonator 302. The output optical waveguide may be provided.

  As described above, in the case of the present embodiment, since loss inside the waveguide is not used for intensity modulation, loss adjustment of the ring waveguide is unnecessary. In the case of this embodiment, the optical loss in the light ON state can be reduced by reducing the propagation loss of the waveguide.

  The effects obtained by the optical modulator of the present invention are summarized below.

  The first effect of the optical modulator of the present invention is to increase the input wavelength tolerance with respect to the extinction characteristic. In other words, in order to perform modulation to synchronize and shift the resonance peak wavelengths of the two optical resonators in the opposite directions, uniform intensity modulation (less wavelength dependency) is performed in a specific wavelength range. This can be realized and the tolerance of the input wavelength can be increased.

  The second effect of the optical modulator of the present invention is to increase the tolerance of the degree to the extinction characteristic. In other words, in order to perform modulation so as to shift the resonance peak wavelengths of the two optical resonators in opposite directions, uniform intensity modulation (small wavelength dependence) is realized in a specific wavelength range. It is possible to ensure a certain allowable range with respect to fluctuations in the resonance peak wavelength due to changes in the environmental temperature.

  The third effect of the optical modulator of the present invention is to reduce the driving voltage. That is, in the configuration of the present invention, intensity modulation is performed by shifting the resonance peak of the optical resonator. For this reason, unlike the conventional Mach-Zehnder optical modulator, it is not necessary to shift the light wave phase by π radians, and the modulation drive voltage can be reduced.

  The fourth effect of the optical modulator of the present invention is to reduce the channel wavelength dependency of the extinction characteristic at the time of modulation in wavelength division multiplexing (WDM) transmission and eliminate the characteristic difference between channels. That is, in the configuration of the present invention, uniform extinction characteristics can be realized in any channel of the WDM by matching the resonance wavelength period of the optical resonator with the wavelength interval of the WDM.

  In particular, the first to third embodiments further have the following effects.

  The fifth effect of the optical modulator of the present invention is to reduce the element size. In other words, the element size is mainly determined by the size of the optical resonator that constitutes the optical modulator. Therefore, when a semiconductor micro ring is used as the optical resonator, the element size is about several tens of microns. Can be reduced in size.

  The sixth effect of the optical modulator of the present invention is to improve the modulation speed. In other words, in the configuration of the present invention, the area of the modulation part can be reduced and the electric capacity of the modulation unit can be reduced by using a semiconductor micro-ring type optical waveguide as the optical resonator. . Since the modulation speed of the optical modulator is inversely proportional to the electric capacity, the modulation speed can be improved.

  The seventh effect of the optical modulator of the present invention is to reduce the element cost. That is, in the configuration of the present invention, since an inexpensive Si semiconductor micro-ring type optical waveguide can be used as an optical resonator, a large number of optical modulators can be formed on a Si wafer. The manufacturing cost per hit can be reduced.

It is a schematic diagram which shows the outline of a structure of the light source device containing the optical modulator of the 1st Embodiment of this invention. It is typical sectional drawing in the A1-A2 line | wire in FIG. It is a typical graph which shows an example of an effect | action of the optical modulator of the 1st Embodiment of this invention. It is the graph which showed the change of the power transmission spectrum of two ring type resonators accompanying voltage application. It is the graph which showed the difference in the extinction characteristic when input light wavelength fluctuates with respect to the center of setting wavelength. It is sectional drawing which shows typically the structure of the ridge type | mold optical waveguide vicinity of the 2nd Embodiment of this invention. In the optical modulator of the present invention, it is a graph showing the result of calculating the change in the effective refractive index of the waveguide with respect to the environmental temperature using the waveguide width W as a parameter. It is a schematic diagram which shows the structure outline of the optical modulator of the 3rd Embodiment of this invention. It is a schematic diagram which shows the structure outline of the optical modulator of the 4th Embodiment of this invention. It is a typical graph which shows an example of an effect | action of the optical modulator using the conventional single optical resonator. It is a graph explaining the specific operation | movement at the time of using the modulator by an example of the conventional single ring resonator. In the conventional single ring resonator, it is a graph which shows the result of having calculated the change of the effective refractive index of the waveguide with respect to environmental temperature by making waveguide width W into a parameter.

Explanation of symbols

2, 6 Silicon oxide film 3, 5 P-type silicon layer 4 N-type silicon layer 8, 102, 103 Heater 9, 11, 110 Cathode electrode 15, 115 Modulator 20, 104, 109 Anode pad electrode 22, 107, 108, 112 Linear optical waveguide 26, 114 First ring optical waveguide 28, 113 Second ring optical waveguide 30, 111 Input end 31, 106 Output end 35, 150 First optical resonator 36, 151 Second light Resonator 37, 152, 320 Resonator 100, 300 Light source 101 SOI substrate

Claims (13)

In an optical modulator that modulates an input light wave and outputs it as a light wave signal,
Two optical resonators connected in cascade and having different resonant wavelengths;
An optical modulator comprising: modulation means for synchronizing the optical loss or refractive index of the two optical resonators and modulating them in opposite directions.   The optical modulator according to claim 1, wherein the waveguide lengths of the two optical resonators are different from each other.   The optical modulator according to claim 1, wherein refractive indexes of the two optical resonators are different from each other in a state where the modulation by the modulation unit is not performed.   The optical modulator according to claim 2, wherein a heater electrode is disposed adjacent to the two optical resonators.   The two optical resonators have a ring-type optical waveguide and at least one linear optical waveguide optically coupled to the ring-type optical waveguide, and the two optical resonators receive the light wave. 5. The optical modulator according to claim 1, wherein the optical modulator is connected in cascade between an input optical waveguide and an output waveguide that outputs the light wave signal. 6.   5. The optical modulator according to claim 1, wherein the two optical resonators are optically connected to each other and are formed of linear optical waveguides whose both end surfaces are reflecting surfaces. 6. The two optical resonators have a PIN junction structure made of a semiconductor material,
The optical modulator according to claim 1, wherein the modulation unit applies a reverse bias voltage to the PIN junction structure.   The optical modulator according to claim 7, wherein a semiconductor multiple quantum well structure is included inside the PIN junction structure. The two optical resonators have a PIN junction structure or a PN junction structure made of a semiconductor material,
The optical modulator according to claim 1, wherein the modulation unit applies a forward bias voltage to the PIN junction structure or the PN junction structure. The two optical resonators include a ferroelectric material,
The optical modulator according to claim 1, wherein the modulation unit applies a voltage to the two optical resonators including the ferroelectric material.   11. The optical modulator according to claim 1, wherein a period of a resonant wavelength of the two optical resonators is matched with a channel interval of wavelength division multiplexing transmission. The light modulator according to claim 11 and a light source,
A light source device in which a wavelength of a light wave emitted from the light source is variable, and a period of the wavelength of the light wave is matched with an interval of resonance wavelengths of the optical resonator. A method for driving an optical modulator according to any one of claims 1 to 11,
A method of driving an optical modulator in which optical losses or refractive indexes of two optical resonators connected in cascade and having different resonance wavelengths are modulated in the opposite directions by being synchronized by the modulation means.

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