JP2006285121A - Optical waveguide, optical modulator, and optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve modulation efficiency of an optical modulator by using an optical waveguide with a structure which lowers group velocity of light, and increasing an interaction effect with a modulation signal. <P>SOLUTION: The optical modulator has an optical waveguide 6 composed of a material with an electro-optic effect, and a two-dimensional photonic structure 5 comprising a plurality of pits 3 arranged on a surface of the optical waveguide 6. Thereby the group velocity of the light passing through the optical waveguide is lowered even when no photonic band gap is generated, and consequently very high modulation efficiency is achieved by applying the waveguide to the optical modulator. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical modulation element suitably used in an optical communication system, an optical signal processing system, and the like, an optical waveguide used in the optical modulation element, and a communication system including the optical modulation element.

  An optical modulation element is a basic element in high-speed optical communication and an optical signal processing system, and it is considered that the need for an optical modulation element that can operate at high speed and high speed will increase in the future.

  Conventionally, direct modulation by a semiconductor laser that has been used is difficult to cope with ultra-high speed optical modulation, and recently, an external modulation type element capable of high-speed operation has been urgently developed. In particular, a waveguide type electro-optic light modulator using a dielectric crystal having a large Pockels effect is capable of ultra-high-speed operation, and there is little disturbance in the phase of the optical signal due to light modulation. It is very suitable for information transmission and long-distance optical fiber communication.

  In general, an electro-optic light modulation element includes a transmission conductor line that functions as a modulation electrode provided on an electro-optic crystal, and an optical waveguide formed in the vicinity of the transmission conductor line. When a high frequency signal for modulation is applied to the modulation electrode, when the refractive index of the optical waveguide portion changes according to the electric field induced around the modulation electrode, the phase of the light wave propagating in the optical waveguide changes.

  The electro-optic coefficient, which is one of the basic parameters that determine the efficiency of light modulation, is very small, about 30 pm / V at the maximum even for lithium niobate crystals. Therefore, in order to achieve high modulation efficiency with an optical modulation element that uses the electro-optic effect, it is important to increase the interaction between the modulation electric field and the light wave in the optical waveguide.

  FIG. 6 is a perspective view showing a conventional light modulation element described in Non-Patent Document 1. As shown in FIG. This light modulation element includes an optical waveguide (6a to 6d) formed on the surface of a substrate 1 made of a crystal material having an electro-optic effect, and an electric signal for modulation (modulation) to light propagating through the optical waveguide (6a to 6d). And a modulation electrode 10 for applying a wave). The modulation electrode 10 has a coplanar conductor line structure constituted by two conductor lines 10a and 10b parallel to each other.

  The optical waveguides 6a to 6d include an entrance-side optical waveguide portion 2c into which light to be modulated (input light) is introduced, an exit-side optical waveguide portion 6d from which modulated light is output, and an entrance-side optical waveguide portion 6c and an exit. Two branching waveguide portions 14a and 14b for coupling the side optical waveguide portion 6d are provided.

  The optical waveguides 6a to 6d are branched into two branch waveguides 6a and 6b at two branch points 14a and 14b, and the input light input from the entrance-side optical waveguide 6c is branched at one branch point 14a. After passing through the two branching waveguides 6a and 6b, the other branching point 14b is configured to travel along the common exit-side optical waveguide 6d.

  The inner ends of the conductor lines 10a and 10b constituting the modulation electrode 10 are located immediately above the central portions of the branch waveguides 6a and 6b, and a high frequency wave for modulation is provided at one end of each of the conductor lines 10a and 10b. The signal source 11 is connected to the other end, and the terminating resistor 12 is connected to the other end.

  When a high-frequency signal (modulated wave) is supplied from the signal source 11 to the modulation electrode 10, the modulated wave propagates on the modulation electrode 10 in the same direction as the light propagation direction and forms an electric field in the gap 13. For this reason, the refractive index of the material which comprises the branched waveguides 6a and 6b changes according to an electric field strength by the electrooptic effect. Since electric fields in opposite directions are applied to the branching waveguide 6a and the branching waveguide 6b, when the substrate 1 is made of, for example, a z-cut lithium niobate crystal, two branching waveguides 6a and 6b are used. The light passing through is subjected to opposite phase changes.

  According to the light modulation element shown in FIG. 6, the interaction between the light wave and the modulation signal wave is increased by causing the modulation wave propagating through the modulation electrode 10 and the light wave propagating through the optical waveguide 6 to travel in the same direction. The aim is to achieve high-efficiency optical modulation.

  However, since the electro-optic constant of the electro-optic crystal represented by lithium niobate is very small as described above, sufficient modulation can be obtained even if the modulation electrode 3 is extended to a length of about several centimeters. In this case, a high voltage of about several volts must be applied to the electro-optic crystal. In order to reduce the size of the light modulation element and reduce the necessary modulation voltage, it is necessary to improve the interaction between the light wave and the modulation electric field.

On the other hand, normally used optical waveguides have almost no velocity dispersion, so the propagation velocity (here, the group velocity of light waves has a meaning) is almost determined by the refractive index of the waveguide. While the refractive index of the electro-optic crystal is about 2.1, the dielectric constant for microwaves is as high as about 20 to 40, so the speed of light is more than twice as high as the speed of microwaves, As a result, there is a problem that even if a traveling wave type electrode is used, perfect speed matching between light and signal waves cannot be achieved in the light modulation element. Therefore, even if the modulation electrode is set long, appropriate modulation cannot be realized, and the modulation efficiency is not improved.
JP 2001-281480 A JP 2002-196296 A IEEE Journal of Quantum Electronics, Vol. QE-13, no. 4, pp287-290, 1977)

  The present invention has been made to solve the above-described problems, and a main object thereof is to provide an optical modulation element having high modulation efficiency suitably used in an optical communication system and an optical waveguide used therefor.

  Another object of the present invention is to provide a communication system including a small light modulation element capable of efficiently performing light modulation.

  An optical waveguide having a ridge portion formed of a material having an electro-optic effect, and a high refractive index portion formed on the surface of the ridge portion, and a group velocity of light propagating through the optical waveguide An optical waveguide further comprising a periodic structure for lowering the periodicity, wherein the periodic structure is constituted by a plurality of pits provided on a surface of the ridge portion, and the plurality of pits have a two-dimensional photonic crystal structure. It is characterized by that.

  In a preferred embodiment, the depth of the pit is smaller than the wavelength of the light wave used in the waveguide.

  In a preferred embodiment, the optical waveguide has an optical modulation element configuration including a modulation electrode for applying an electric field for electro-optic light modulation to the optical waveguide.

  Further, a communication system of the present invention includes any one of the light modulation elements described above, an optical fiber that transmits the modulated light output from the light modulation element, and a unit that provides an electric signal for modulation to the light modulation element. ing.

  According to the configuration of the present invention, an optical waveguide having an electro-optic effect and a low loss and a low group velocity characteristic can be obtained. By applying the optical waveguide to a waveguide portion of an optical modulation element, a high-frequency signal such as a millimeter wave band can be obtained. However, a small and highly efficient light modulation characteristic can be realized. By using this optical modulation element in a communication system, for example, an optical fiber radio system that transmits and distributes a high-frequency radio signal at a millimeter wave level through an optical fiber, an ultrahigh-speed optical fiber transmission system, and the like become possible.

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
First, refer to FIG. The substrate 1 is made of an electro-optic material such as lithium tantalate (LiTaO ₃ ) single crystal or lithium niobate (LiNbO ₃ ) single crystal. The optical waveguide of this embodiment is formed on the substrate 1 by a ridge portion 2 formed by, for example, etching and a high refractive index portion 4 formed by, for example, thermal diffusion of titanium metal. In this ridge type optical waveguide, the confinement of the light wave in the depth direction of the optical waveguide portion 6 is due to the refractive index difference (usually about 0.2%) due to thermal diffusion of titanium metal, so the light wave confinement effect is not so strong. In the lateral direction, since there is a large refractive index difference between the substrate and air, the light wave is confined very strongly. Therefore, there is a feature that a very strong confinement can be realized in a two-dimensional direction in the surface of the substrate 1.

  Further, pits 3 periodically arranged as shown in the figure are formed on the surface of the ridge portion 2 of the ridge-shaped optical waveguide, thereby constituting a photonic crystal structure portion 5. Now, this photonic crystal structure will be described. Depending on the frequency of the light wave used, the refractive index of the substrate 1, and the size, period, and depth of the pits 3, the relationship between the wave number and the frequency of the light wave, the specific dispersion characteristics Can be given. Such a structure having dispersion characteristics depending on the periodicity is generally called a photonic crystal structure. For example, when the depth of the pit 3 is large or when the difference in internal refractive index between the substrate 1 and the pit 3 is large, the light wave is generated. There may be a wavelength region that cannot exist, and this is called a photonic band gap. In a normal optical waveguide, since the wave number and the frequency are almost proportional, a linear dispersion curve is obtained, and the group velocity does not change. However, in the case of a photonic crystal structure, in the very vicinity of the photonic band gap, the light wave passing through the photonic crystal structure part has a much lower group velocity than the light wave propagating through the part without the photonic crystal structure. Become. On the other hand, when the difference in refractive index between the substrate 1 and the inside of the pit 3 is small, or when the depth of the pit 3 is shallow, the photonic band gap does not occur, and the light wave of all wavelengths can propagate. Show.

  In the configuration of the present invention, the low group velocity characteristic peculiar to the photonic crystal structure is exhibited by using a specific wavelength region. Such a structure can be applied to an electro-optic crystal such as lithium niobate having a relatively low refractive index, and a highly efficient light modulation element can be realized. Further, in this configuration, effective group speed reduction can be realized by using a relatively shallow pit instead of a deep pit penetrating the substrate as the pit 3.

  Returning to the optical waveguide of this configuration in FIG. 1, since the light wave propagating through the optical waveguide portion 6 passes through the photonic crystal structure portion 5, the group velocity of the passing light wave is as follows when there is no pit 3. In comparison, the group velocity is reduced by the principle of the photonic crystal. Therefore, the waveguide having this structure operates as a waveguide having a small group velocity (having a slow wave structure).

  Further, in this configuration, by using a ridge waveguide or a rib-type waveguide as the optical waveguide, it is possible to simultaneously realize low loss and a small group velocity. In general, as shown in FIG. 1 (c), a structure in which the high refractive index portion 4 is only at the tip of the ridge portion 2 and the optical waveguide portion 6 is accommodated in the ridge portion 2 is shown in FIG. A structure in which the high refractive index layer spreads on the surface other than the protrusion as shown in 1 (d) is called a rib-type waveguide. Both structures have strong lateral confinement, but the ridge waveguide can achieve relatively strong confinement. Further, in the rib-type waveguide, confinement is somewhat weakened, but there is an advantage that the waveguide can be formed even when the refractive index difference between the substrate 1 and the high refractive index portion 4 is small.

  Actually, using a lithium niobate substrate, it has been confirmed by calculation using a two-dimensional plane wave expansion method that the group velocity is small. The group velocity can control the group velocity characteristics of the light wave propagating through the optical waveguide portion 6 by changing the diameter, period and depth of the pit 3.

  The wavelength of the light wave to be used is not limited in order to exert the effect of the present invention, but infrared light (free space wavelength is 1.55 μm, which is often used in the field of communication or the like, or It is very effective when a normal electro-optic crystal is used using a light wave of a wavelength band centered on 1.3 μm.

  When the substrate 1 is formed of a material having an electrooptic effect such as lithium niobate, the ridge portion 2 and the pit 3 can be formed by reactive dry etching using a fluorine-based gas. .

  In the above embodiment, the case where the pits are hollow has been described. However, if there is a difference between the refractive index of the substrate and the pits, the effect can be exhibited similarly. In this case, as the refractive index difference between the substrate 1 and the material in the pit 2 is larger, the same effect can be exhibited with a smaller number of pits.

(Embodiment 2)
Please refer to FIG. An optical modulator configured using the optical waveguide described in the first embodiment will be described. The photonic crystal structure portion 5 of the waveguide described in the first embodiment is composed of modulation electrodes 10a and 10b made of a metal film formed by, for example, vacuum deposition and etching. Here, a vertical electric field is applied to the optical waveguide portion 6 by applying a voltage between the electrodes. Since the substrate 1 has an electro-optic effect, the refractive index of the optical waveguide portion 6 changes according to the electric field strength, causing a phase change in the light wave passing through the waveguide portion 6. Thereby, this element operates as an optical phase modulator.

The electro-optic crystal represented by the lithium niobate crystal also has a large anisotropy in the electro-optic coefficient, and the largest electro-optic coefficient is γ ₃₃ , which is polarized in the c-axis direction of the crystal. Similarly, the refractive index received by the light wave is a coefficient that varies with the electric field in the c-axis direction. Therefore, in this configuration, the substrate 1 is a crystal substrate (z-cut lithium niobate crystal) cut so that the surface perpendicular to the c-axis is the surface, and light waves (TM) polarized in the vertical direction (TM The effectiveness becomes higher when the wave is used.

  Here, as described in the first embodiment, in this waveguide configuration, the group velocity of the guided light wave can be reduced, and therefore the time for receiving the refractive index change due to the electro-optic effect is also increased in inverse proportion thereto. As a result, the light modulation effect is increased. Further, in order to expand the modulation frequency band of the light modulation element of FIG. 2, the modulation efficiency can be increased by using the modulation electrode 10 as a transmission line and propagating the modulation wave in the same direction as the light wave for modulation. However, when such a traveling wave electrode is used for the modulation electrode 10 in FIG. 2, the group velocity of light is reduced by adjusting the diameter, period, depth, etc. of the pits 3 of the photonic crystal structure portion 5. It is also possible to match the speeds of light and modulated waves.

As a result, a broadband and highly efficient optical modulator can be realized. In this configuration, when a voltage is applied between the modulation electrodes 10a and 10b, a strong electric field is generated in the optical waveguide portion 6 in the vertical direction, so that the substrate 1 (for example, z) in which a large refractive index change occurs with respect to the vertical electric field. -cut lithium niobate crystal) is effective.

(Embodiment 3)
The light modulation element of Embodiment 3 is illustrated in FIG. The light modulation element of this embodiment also operates as an optical phase modulator. In this configuration, when a voltage is applied between the modulation electrodes 10a and 10b, a configuration in which a strong electric field is generated in the optical waveguide portion 6 in the lateral direction is used. In the case of using a normal electro-optic crystal, in order to cause a large refractive index change with respect to the electric field in the lateral direction, unlike the previous examples, c
This configuration is very useful when using a substrate 1 (for example, y-cut lithium niobate crystal) whose axis is transverse to the waveguide and using light waves (TE waves) polarized in the same transverse direction. It is valid.

(Embodiment 4)
Reference is now made to FIG. In this configuration, the optical modulator configuration described in the second or third embodiment is developed, and the Mach-Zehnder interferometer is configured by the optical waveguide portion 6. In this case, when a voltage is applied between the modulation electrodes 10a and 10b, electric fields in opposite directions are generated in the photonic crystal structure portions 5 of the two optical waveguide portions 6a and 6b, so that the optical waveguide portions 6a and 6b are connected to each other. The propagating light waves undergo phase changes opposite to each other, and when they are combined, they interfere with each other and are converted into light intensity changes corresponding to the amount of phase change.

  As a result, the optical modulator of this configuration can modulate the light intensity according to the voltage between the modulation electrodes. In this configuration, although there is a difference between the phase modulation and the intensity modulation with the optical modulator described in the second or third embodiment, high-efficiency optical modulation is possible in the same manner.

  In the second to fourth embodiments, only the mode in which the optical waveguide structure of the present invention described in the first embodiment is applied to an optical modulation element has been described. However, the optical waveguide structure of the present invention has a low loss and a group velocity. Therefore, it is possible to realize a highly efficient nonlinear optical effect element by increasing the interaction between the light wave and the substrate material by using a small group velocity. It is. Nonlinear optical effect elements can be expected to realize harmonic generation, wavelength conversion, and photoelectric conversion elements.

(Embodiment 5)
Next, an embodiment of a fiber radio system according to the present invention will be described with reference to FIG.

  A fiber radio system 50 according to the present embodiment includes an optical modulator / demodulator 51 incorporating the light modulation element according to the first and second embodiments. The antenna 53 can directly perform communication with a normal data communication network such as the Internet, communication with a portable terminal, reception of a signal from a CATV, or the like using, for example, a millimeter wave carrier wave. The optical modulator / demodulator 51 includes a light demodulating element (for example, a photodiode) in addition to the light modulating element.

  On the other hand, a radio signal having a high frequency such as millimeter wave is difficult to transmit over a long distance, and is susceptible to signal blocking by an object. Therefore, communication with the data communication network 61, the CATV 62, and the mobile phone system 63 can be performed using the wireless device 60 and the antenna 64 attached to the wireless device. In this case, an optical modulator / demodulator 55 connected via the fiber radio communication system 50 and the optical fiber 70 and an antenna 54 attached thereto are further provided. Signals can be exchanged with the wireless device 60 via the antennas 54 and 64 and the optical modulator / demodulator 55. The light modulator / demodulator 55 includes a light demodulating element (for example, a photodiode) in addition to the light modulating element. When long-distance transmission is desired or when transmission is performed indoors that is partitioned by a wall or the like, it is effective to transmit an optical signal modulated with a radio signal such as a millimeter wave through the optical fiber 70.

  The light modulation element of the present invention is suitably used for high-speed optical communication, an optical signal processing system, and the like. Particularly suitable for high-speed information transmission and long-distance optical fiber communication, which can dramatically improve modulation efficiency, in an electro-optic optical modulation element that can generate an optical modulation signal that is faithful to the modulation signal and has little phase disturbance. .

(A) is a perspective view of a first embodiment of an optical waveguide according to the present invention, (b) is a plan view, and (c) and (d) are cross-sectional views taken along the line A-A ′ thereof. (A) is a plan view of the first embodiment of the light modulation element according to the present invention, and (b) is a cross-sectional view taken along line A-A ′ thereof. (A) is a plan view of a second embodiment of the light modulation element according to the present invention, and (b) is a cross-sectional view taken along line A-A ′ thereof. (A) is a plan view of a third embodiment of the light modulation element according to the present invention, and (b) is a cross-sectional view taken along line A-A ′ thereof. The figure which shows embodiment of the communication system by this invention A perspective view showing a conventional example of a light modulation element

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Ridge part 3 Pit 4 High refractive index part 5 Photonic crystal structure part 6 Optical waveguide part 10 Modulation electrode 11 Signal source 12 Termination resistor 13 Gap part 14a Branch point 14b Branch point

Claims (5)

  An optical waveguide having a ridge portion formed of a material having an electro-optic effect, and a high refractive index portion formed on the surface of the ridge portion, and a group velocity of light propagating through the optical waveguide An optical waveguide further comprising a periodic structure for lowering the periodicity, wherein the periodic structure is constituted by a plurality of pits provided on a surface of the ridge portion, and the plurality of pits have a two-dimensional photonic crystal structure. An optical waveguide characterized by that.   2. The optical waveguide according to claim 1, wherein the plurality of pits are hollow, or the inside of the pit has a dielectric constant different from a dielectric constant of the substrate.   3. The optical waveguide according to claim 1, wherein the depth of the pit is smaller than the wavelength of the light wave used in the waveguide.   4. The light modulation element according to any one of 1 to 3, further comprising a modulation electrode for applying an electro-optic light modulation electric field to the optical waveguide. 5. A communication system comprising: the light modulation element according to claim 4; an optical fiber that transmits the modulated light output from the light modulation element; and means for providing an electric signal for modulation to the light modulation element.

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