JP6597052B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator that compensates for chromatic dispersion of an optical fiber.

  In the optical communication field and the optical measurement field, a light wave modulated by an optical modulator is transmitted through an optical fiber. In general, since optical fibers have different light propagation speeds depending on the wavelength, chromatic dispersion occurs, and the waveform of the transmitted optical signal is distorted. For this reason, in high-speed communication exceeding 40 Gbps, a wavelength-multiplexed high-speed transmission system, and the like, a technique for compensating the chromatic dispersion of the optical fiber transmission line is indispensable.

  As a chromatic dispersion compensation method, an optical fiber for chromatic dispersion compensation (dispersion compensation fiber) is disposed immediately before an optical signal receiver, a fiber Bragg grating (FBG) as in Patent Document 1, or an etalon. And a method using a digital signal processing circuit such as Patent Document 2 and Non-Patent Document 1. The digital signal processing circuit generates an impulse response that is compensated by the digital signal processor in response to changes in the real part and imaginary part related to chromatic dispersion.

  A dispersion compensation module using an inverse dispersion fiber is installed for every certain span. However, if the span length to be introduced is large, the compensation accuracy is limited and the compensation amount differs for each wavelength. In addition, for more accurate chromatic dispersion compensation such as wavelength division multiplexing (WDM) light, an optical device that becomes a chromatic dispersion compensator corresponding to a compensation amount necessary for each wavelength is separately required. An optical device such as an FBG or an etalon is generally used, but an optical device such as an FBG not only has a limited wavelength band to handle but also has a large optical loss. Furthermore, the digital signal processing circuit has a problem that high-speed processing exceeding 40 Gbps is technically difficult.

  In order to solve these problems, the present applicants have proposed optical modulators in Patent Documents 3 to 5 that skillfully utilize the polarization inversion structure light modulation technique as shown in FIG. Specifically, an optical modulation having a substrate 1 made of a material having an electro-optic effect, an optical waveguide 2 formed on the substrate, and a modulation electrode 3 for modulating an optical wave propagating through the optical waveguide. In this device, the outgoing light L2 emitted from the optical waveguide is guided by an optical fiber (not shown), and along the optical waveguide, the waveform distortion has a characteristic opposite to the wavelength dispersion characteristic of the optical fiber. The chromatic dispersion characteristics of the optical fiber are compensated by inverting the polarization of the substrate with a predetermined pattern (10) (P1 and P2 indicate polarization directions).

  As a result, the chromatic dispersion of the optical fiber transmission line can be compensated, and its effectiveness has been confirmed even in high-speed transmission exceeding several tens of Gbps. Further, in Patent Document 5, an adjustment member made of a dielectric material or a metal material is disposed in the vicinity of the modulation electrode so that the compensation amount of the chromatic dispersion characteristic is adjusted to a predetermined level. It proposes a technology that can be adjusted and expand the range.

  However, the dispersion compensation range is about 25 km in terms of the distance of the fiber transmission line due to the size of the optical modulator and the limitation of the magnitude relationship between the refractive indexes of light and microwave. Moreover, the chromatic dispersion compensation methods disclosed in Patent Documents 1 and 2 cannot be put to practical use in high-speed transmission exceeding 40 Gbps as described above, and the optical fiber length of the transmission path is common in, for example, metro networks. A dispersion compensation technique that can be applied over the entire optical communication wavelength band such as the C band and the L band with a relay span of 80 km or more has not been realized.

JP 2004-012714 A JP 2010-226254 A JP 2012-189630 A JP 2014-066940 A JP 2014-066941 A JP 2007-171452 A

Robert I. Kelley, et al., "Electronic Dispersion Compensation by Signal Predistortion Using Digital Processing and a Duai-Drive Mach-Zehnder Modulator", IEEE Photonics Technology letters, Vol.17, No.3, pp714-716, 2005 H. Murata, et al, “Quasi-velocity-matched LiTaO3 guided-wave optical phase modulator for integrated ultrashort optical pulse generators,” Electron. Lett., Vol. 36, no. 17, pp. 1459-1460, 2000.

  The problem to be solved by the present invention is to be able to compensate for the chromatic dispersion of a long optical fiber transmission line exceeding several tens of km, and to provide an optical modulator applicable to high-speed transmission exceeding several tens of Gbps It is to be.

In order to solve the above problems, the optical modulator of the present invention has the following technical features.
(1) In an optical modulator having a substrate made of a material having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide, The outgoing light emitted from the waveguide is transmitted through the optical fiber for a predetermined distance, and a waveform distortion having a characteristic opposite to the wavelength dispersion characteristic of the optical fiber is imparted to the light wave propagating through the optical waveguide. The substrate is inverted in a predetermined pattern, the optical waveguide has a Mach-Zehnder type waveguide having two branch waveguides, and the polarization inversion pattern formed in one branch waveguide is: The pattern corresponding to the real part response of the impulse response 1 / h (t) for compensating the impulse response h (t) of the optical fiber, and the polarization inversion pattern formed in the other branching waveguide is the impulse Meet The pattern corresponding to the imaginary part responsiveness of the answer 1 / h (t), configured to multiplex the light waves that have passed through the two branch waveguides with a predetermined phase difference, and formed by the modulation electrode Is configured so that the propagation direction of the light wave propagating through the optical waveguide and the propagation direction of the modulation signal propagating through the modulation electrode are opposite to each other in the active region acting on the optical waveguide. .

( 2 ) In the optical modulator described in ( 1 ) above, the impulse response h (t) of the optical fiber is given by the following equation. However, H (ω) is a transfer function of the optical fiber, and H (ω) = exp (jβ (ω) L). β (ω) is a phase constant of the light wave propagating through the optical fiber, and L is the length of the optical fiber.

( 3 ) In the optical modulator described in the above (1) or (2) , the carrier wave component of the light wave propagating through the optical waveguide is changed with respect to the outgoing light that has passed through the optical waveguide whose polarization has been inverted in the predetermined pattern. It is characterized by having a configuration for suppressing.

( 4 ) In the optical modulator according to any one of (1) to ( 3 ), the group velocity of light in the optical waveguide in the working region is slower than the group velocity of the modulation signal propagating through the modulation electrode. It is characterized by.

( 5 ) In the optical modulator according to any one of (1) to ( 3 ), the group velocity of the modulation signal propagating through the modulation electrode in the working region is greater than the group velocity of the light wave propagating through the optical waveguide. Characterized by being slow.

  In the optical modulator of the present invention, an optical modulation having a substrate made of a material having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating an optical wave propagating through the optical waveguide In order to transmit the outgoing light emitted from the optical waveguide for a predetermined distance by an optical fiber, and to impart a waveform distortion having a characteristic opposite to the wavelength dispersion characteristic of the optical fiber to the light wave propagating through the optical waveguide, The substrate is inverted in a predetermined pattern along the optical waveguide, and the propagation direction of the light wave propagating through the optical waveguide in an action region where the electric field formed by the modulation electrode acts on the optical waveguide; and Since the propagation directions of the modulation signals propagating through the modulation electrode are opposite to each other, the impulse response time width ΔT of the optical modulator (in other words, the length of the linear convolution operation or the time of dispersion) Rising) it is possible to lengthen the transmission distance of the optical fiber which enables wavelength dispersion compensation, it is possible to longer. Further, the impulse response of the optical modulator for compensating the wavelength dispersion characteristic can be realized by a polarization inversion pattern formed on the substrate. Unlike the method of generating an impulse response that is compensated by a digital signal processor, this dispersion compensation method can provide an optical modulator that is not theoretically limited in modulation band and can be applied to high-speed transmission exceeding several tens of Gbps. .

It is a figure which shows an example of the conventional optical modulator. It is a figure which shows an example of the optical modulator (modulator chip | tip) of this invention. It is a graph which shows an example of (a) real part responsiveness (Reh ^* (t)) and (b) imaginary part responsiveness (Imh ^* (t)) which compensate the impulse response of an optical fiber. It is a figure explaining the model of the optical phase modulator provided with the traveling wave type electrode. (A) Model configuration, (b) Step response, (c) Impulse response. It is a figure which shows the (a) model which shows the relationship between a light wave and a modulation signal, (b) step response of light modulation, and (c) impulse response of a traveling wave type electrode optical phase modulator using a polarization inversion structure. It is an effect comparison (eye pattern) of dispersion compensation according to the optical modulator of the present invention, and shows simulation results of (a) immediately after output of the optical modulator and (b) after 80 km of transmission. It is a comparison effect (I-Q constellation diagram) of dispersion compensation related to the optical modulator of the present invention, showing (a) immediately after the output of the optical modulator and (b) a simulation result of a state after 80 km of transmission. Is. It is a figure which shows an example of the structure for suppressing a carrier wave component. It is a figure which shows the other example of the structure for suppressing a carrier wave component.

Hereinafter, the present invention will be described in detail using preferred examples.
As shown in FIG. 2, the present invention includes a substrate 1 made of a material having an electro-optic effect, an optical waveguide 2 formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide. In the optical modulator having (3, 30), the output light L2 emitted from the optical waveguide is transmitted for a predetermined distance by an optical fiber (not shown), and the waveform has a characteristic opposite to the wavelength dispersion characteristic of the optical fiber. In order to impart distortion to the light wave propagating through the optical waveguide, the substrate is inverted in a predetermined pattern 10 along the optical waveguide, and the electric field formed by the modulation electrode acts on the optical waveguide. In the region R, the propagation direction of the light wave propagating through the optical waveguide and the propagation direction of the modulation signal S propagating through the modulation electrode are configured to be opposite to each other.

  As the substrate using the material having the electro-optic effect of the present invention, for example, lithium niobate (LN), lithium tantalate (LT), electro-optic polymer, PLZT (lead lanthanum zirconate titanate), GaAs (arsenic) Gallium) and substrates combining these materials are available. In particular, it is a material that has a high electro-optic effect and can easily form an arbitrary domain-inverted structure, and has a large primary electro-optic effect (Pockels effect) and a small secondary electro-optic effect (optical Kerr effect). It is preferable. Specifically, lithium niobate, lithium tantalate, electro-optic polymer, and GaAs can be used. In particular, when the domain-inverted pattern is rewritten or when there is a need for adjustment during the manufacturing process, the electro-optic polymer can be suitably used. As will be described later, when the refractive index of the optical waveguide is increased and a slow wave waveguide is formed, a photonic crystal, particularly a semiconductor photonic crystal, can be used.

  As a method of forming the optical waveguide 2 on a substrate such as lithium niobate or lithium tantalate, a high refractive index material is diffused or proton exchanged on the substrate surface by a method such as thermal diffusion of Ti or the like or proton exchange. Can be formed. It is also possible to use a ridge-shaped waveguide having a convex portion corresponding to the optical waveguide, such as etching a substrate other than the optical waveguide or forming grooves on both sides of the optical waveguide. As a method of forming the optical waveguide 2 on a GaAs substrate, a ridge-shaped waveguide having a convex portion corresponding to the optical waveguide can be used. The optical waveguide of the electro-optic polymer is processed into a shape in which the optical waveguide is embedded in a clad material having a lower refractive index than that of the electro-optic polymer by spin coating, photolithography, and etching processes, and the core portion of the embedded ridge shape or embedded rectangular shape It can be formed as an optical waveguide having

On the substrate 1, modulation electrodes such as the signal electrode 3 and the ground electrode are formed. Such an electrode can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Further, if necessary, a buffer layer such as a dielectric SiO ₂ may be provided on the substrate surface after the optical waveguide is formed, and a modulation electrode may be formed on the buffer layer. A symbol S in FIG. 2 is a modulation signal and also indicates an input portion of the modulation signal.

  An optical fiber (not shown) is optically coupled to the optical modulator of the present invention (see FIG. 2). A method of directly bonding an optical fiber to a substrate having an electro-optic effect using a capillary or the like, or a quartz substrate having an optical waveguide formed on a substrate having an electro-optic effect, and the optical fiber being attached to the quartz substrate or the like. It is also possible to join. Furthermore, it is also possible to configure the outgoing light L2 to be introduced into the optical fiber via a spatial optical system on a substrate having an electro-optic effect, a quartz substrate, or the like. Illustration of the phase difference adjusting means of both branch waveguides is omitted. A bias voltage may be superimposed on the modulation signal waveform S to give a phase difference between the branched waveguides, or an electrode for adjusting the bias and an optical delay line portion may be separately provided.

  In the optical modulator of the present invention, a substrate made of a material having an electro-optic effect as shown in FIG. 2 is used, and a part of the substrate is subjected to polarization inversion 10. Arrows P1 and P2 indicate the polarization direction of the substrate. When such a domain-inverted structure is applied to a traveling wave electrode type optical modulator, useful processing of high-speed electric signals in the microwave band and millimeter wave band can be performed. The primary electro-optic effect (Pockels effect) is a physical quantity represented by a third-order tensor. When spatial inversion is performed by polarization inversion, the sign of the corresponding tensor component is inverted. On the other hand, the refractive index and the dielectric constant are the second-order tensor amounts, and the sign does not change even if they are inverted. In other words, when a domain-inverted region is formed in a ferroelectric optical crystal, the electrical and optical characteristics are the same in the inverted and non-inverted regions, but the sign of the action due to the primary electro-optic effect is in the inverted and non-inverted regions. Different from each other. Therefore, it is possible to adjust the polarity at which electricity acts on light without changing the characteristics of the electric circuit and the optical circuit. For example, consider the effect of an electrical signal propagating in the same direction on an optical signal, such as a traveling wave optical modulator. To make a modulator that operates efficiently at high frequencies, the speed of the two signals is usually matched. However, when a polarization inversion structure is introduced, there is a speed difference between the two due to compensation by polarity. Even in this case, an efficient modulation action can be obtained (see Non-Patent Document 2). Furthermore, by using polarization inversion, useful characteristics such as zero chirp intensity modulation and optical SSB (Single Side Band) modulation can be obtained.

  The action of the electrical signal on the optical signal by the electro-optic effect can be handled as an input / output of the linear system. There is a speed difference between the two signals, and the optical modulation characteristic is the temporal linear convolution of the modulation signal, whether the electrical signal acts while overtaking the optical signal or the electrical signal acts while overtaking the optical signal. . Therefore, the modulation frequency characteristic of the traveling wave electrode type optical modulator having the polarization reversal structure is given by the Fourier transform of the impulse response that directly corresponds to the polarization reversal pattern.

  That is, as in the present invention, if the optical modulator is formed by using polarization inversion of the impulse response of the inverse transfer function of the transmission line according to the dispersion amount of the optical fiber transmission line, the dispersion amount of the optical fiber transmission line is reduced. An optical modulator having a pre-equalizing function for compensation can be realized. In addition, unlike an ordinary baseband modulator, the optical modulator of the present invention does not need to match the group speed of the modulated light and the group speed of the modulation signal, so that the cross-sectional area and surface area of the signal electrode are increased and the loss is extremely low. By using a simple traveling wave type electrode, an ultra-high speed response exceeding several tens of GHz is possible. Further, pre-equalization at a frequency that is difficult with an electrical equalizing technique using a high-speed A / D conversion technique as in a conventional digital signal processing circuit is also possible. The optical modulator of the present invention does not require a high-speed digital signal processing circuit, and can be driven with low power consumption.

  In the following, an explanation will be made focusing on an optical modulator that performs dispersion compensation of an optical fiber. The optical modulator of the present invention uses an electro-optic modulation technique using polarization reversal, so that when an electrical signal is converted into an optical signal, the inverse characteristics of waveform distortion due to wavelength dispersion of an optical fiber at a predetermined distance are obtained in advance. By providing it, the characteristic deterioration is compensated.

The optical modulator of the present invention is also applicable to high-speed transmission of several tens of Gbps or more, and more than 100 Gbps. In addition, waveform deterioration can be compensated regardless of the wavelength. Therefore, the present invention is also an epoch-making technique that surpasses the conventional dispersion compensation technique. The features of the dispersion compensation technology used by the present invention can be listed as follows.
(1) Capable of handling high-speed electrons exceeding 40 Gbps, which is difficult to cope with digital signal processing technology (2) No wavelength band limitation as in the FBG method (3) The optical modulator itself has a dispersion compensation function

  The feature combining the above (1) and (2) is highly useful as a dispersion compensation technique in a wavelength division multiplexing high-speed transmission system.

A dispersion compensation technique in the optical modulator of the present invention will be described in detail.
When the phase constant of the light wave propagating in the optical fiber is β (ω), the transfer function H (ω) of the optical fiber having the length L is expressed by the following equation.
H (ω) = exp (jβ (ω) L)

Further, in dispersion compensation, a second-order term when β (ω) is Taylor-expanded around the carrier angular frequency ω = ω ₀ may be considered, and can be modified as follows.
H (ω) = exp (jβ ₂ ω ² L / 2)
Here, β2 means a quadratic term of Taylor expansion and represents group velocity dispersion.

  Thereby, the impulse response h (t) of the optical fiber can be expressed by the following equation (2).

In order to compensate for the dispersion of the optical fiber, the transfer function for compensating the dispersion of the optical fiber is 1 / H (ω) = H ^* (ω). Therefore, in the optical modulator, h is an impulse response of the dispersion compensation. ^{* The} modulation corresponding to (t) (= 1 / h (t)) may be performed. Specifically, with the Mach-Zehnder type waveguide 2 shown in FIG. 2, when using the MZ interferometer type optical modulator, a real part responsiveness Re {h of h ^{* (t)} in one branch waveguide 21 ^* ( t)} is modulated, the other branch waveguide is modulated with the imaginary part response Im {h ^* (t)}, and both are synthesized with a predetermined phase difference. Most preferably, the phase difference is set to 90 °.

FIG. 3 shows an impulse response h ^* (t) for dispersion compensation of an optical fiber when 80 km transmission is performed. FIG. 3A shows real part response Re {h ^* (t)}, and FIG. It is a graph which shows imaginary part responsiveness Im {h ^* (t)}.

  In general, it is difficult to set the impulse response freely. However, if a polarization inversion structure is used for a material having a first-order electro-optic effect such as a ferroelectric material, this impulse response can be realized approximately. It is possible.

Specifically, as shown in FIG. 2, the optical waveguide 2 has a Mach-Zehnder type waveguide having two branch waveguides, and the pattern of the polarization inversion 10 formed in one of the branch waveguides is the light described above. A pattern corresponding to the real part response of the impulse response h ^* (t) (= 1 / h (t)) for compensating the impulse response h (t) of the fiber, and the polarization inversion formed in the other branching waveguide The pattern may be a pattern corresponding to the imaginary part responsiveness of the impulse response h ^* (t) of dispersion compensation.

  The light waves that have passed through the two branch waveguides are combined with a predetermined phase difference. As a method of generating this phase difference, a method of adjusting the length of each branch waveguide, or a signal electrode or a DC bias electrode arranged along the branch waveguide is used to adjust the refractive index of the branch waveguide. Methods etc. are available.

  The optical modulator as shown in FIG. 2 operates as a dispersion compensation modulator having a pre-equalizing function. Further, when a dual MZ modulator (an optical modulator having a nested optical waveguide in which a sub-MZ optical waveguide is incorporated in a branch waveguide of a main MZ optical waveguide) is used, pre-equalizing vector light modulation is possible. Therefore, dispersion compensation with higher accuracy is possible. Moreover, combined use with QPSK modulation and duobinary modulation is also possible.

  In FIG. 2, a coplanar electrode in which the signal electrode 3 straddling the two waveguides is sandwiched between the ground electrodes 30 is shown as the modulation electrode. However, the optical modulator of the present invention is not limited to that shown in FIG. As shown in Document 4, it is possible to arrange a signal electrode corresponding to each optical waveguide. Also, depending on the type of substrate used, not only the Z-cut type as shown in FIG. 2 but also the X-cut type as shown in FIG. 1 can be used. It is possible to adopt. In FIG. 1, illustration of the ground electrode is omitted.

Next, a method for performing dispersion compensation for a long-distance optical fiber, which is a feature of the optical modulator of the present invention, will be described. The modulated wave electric field traveling along the coplanar electrode is in a pseudo TEM mode, and usually the effective refractive index of the signal and the frequency dispersion of the impedance are sufficiently small. Furthermore, when the attenuation of the modulated wave electric field can be ignored, the impulse response of the optical modulator can be obtained from the step response.

  FIG. 4A is a model showing the relationship between the light wave of the optical modulator and the modulation signal. First, consider a case where the optical modulator is driven with a step voltage. If the dispersion and attenuation of the modulation signal are sufficiently small and the step voltage reaches the end without distortion, the step response of the optical modulation (see FIG. 4B) shows a linear response. An impulse response can be obtained by differentiating the step response with respect to time. Therefore, the impulse response (see FIG. 4C) of the traveling wave electrode optical phase modulator that is not speed matched is a square.

The relationship between the impulse response time Tt (ΔT) shown in FIG. 4C and the time (Tg, Tm) related to the step response shown in FIG. 4B is expressed by the following equation.
Tt = Tm−Tg = Lt / vm−Lt / vg = (nm−ng) Lt / c
Here, vm is the propagation speed of the modulation signal, and vg is the propagation speed (group speed) of the light wave. Further, nm represents the effective refractive index of the modulation signal, ng represents the group refractive index of the light wave, and Lt represents the electrode length shown in FIG. 4A (the same as the length of the active region, and the length of the optical waveguide in the active region). Is the same). c indicates the speed of light.

The step response and impulse response of the traveling wave type electrode optical phase modulator using the domain-inverted structure can be obtained similarly. Since the sign of the electro-optic coefficient is reversed in the polarization inversion region, the polarity of the change in the step response (see FIG. 4B) is reversed. For this reason, the impulse response is a series of squares corresponding to the domain-inverted shapes as shown in FIG. That is, the impulse response of the phase modulator can be freely set by changing the polarization inversion shape. The relationship between the impulse response time width ΔT (= Tt) and the action region length ΔL (= Lt) is given by the following equation.
ΔT = (nm−ng) ΔL / c

  FIG. 5 shows (a) a model showing a relationship between a light wave and a modulation signal, (b) a step response of light modulation, and (c) an impulse response of a traveling wave type electrode optical phase modulator using a polarization inversion structure. Each is shown.

  In order to obtain dispersion compensation characteristics for a longer optical fiber, it is necessary to increase the time width ΔT of the impulse response of the optical modulator. In the conventional optical modulator as shown in FIG. 1, in order to increase ΔT, the electrode length (Lt) corresponding to the length ΔL of the active region may be increased. There are limitations due to the size of the electro-optic material substrate and the footprint of the transmitter board.

In order to cope with this problem, the present inventors have found that the propagation direction of the light wave and the propagation direction of the modulated signal are opposite to each other as shown in FIG. Even when the electrical signal acts while traveling in the opposite direction to the optical signal, the optical modulation characteristic is a temporal linear convolution of the modulated signal. That is, if the modulation signal propagation velocity vm and the light wave propagation velocity vg shown in FIG. 4A are set in opposite directions, the impulse response time width ΔT can be expressed by the following equation.
ΔT = (nm + ng) ΔL / c
That is, the impulse response time width is proportional to the sum of the modulation signal speed and the light wave speed.

  In an optical modulator in which a coplanar electrode is formed by using an LN or LT crystal as a substrate, it is usually nm to 4 and ng to 2, so that the difference is replaced by the sum, the length of the optical fiber capable of dispersion compensation is about 3 Means double. In other words, the conventional dispersion compensation range, which is about 25 km in terms of the distance of the fiber transmission line, extends to about 80 km.

  Naturally, when the modulation electrode is a slow wave waveguide that further slows down the speed of the modulation signal that propagates, the effective refractive index nm of the modulation signal increases and the impulse response time width ΔT also increases. As the coplanar electrode has more shape parameters than the microstrip electrode, the effective refractive index and characteristic impedance can be adjusted over a wide range. The design as a slow wave waveguide can be easily realized by increasing the width of the signal electrode, decreasing the height of the modulation electrode, or widening the interval between the signal electrode and the ground electrode.

  Also, when the optical waveguide is a slow wave waveguide that makes the speed of the propagating light wave slower, the group refractive index ng of the light wave is increased and the impulse response time width ΔT is also increased. As the slow wave waveguide, a photonic crystal (ng ≧ 5), in particular, a semiconductor photonic crystal (ng = 10 to 50000) can be used. Furthermore, it is possible to adjust the group index of light by adopting a diffraction grating structure or the like as disclosed in Patent Document 6.

  As described above, when the propagation direction of the modulation signal and the propagation direction of the light wave are opposite to each other, the degree of modulation of the light wave becomes small. In order to solve this problem, the effective modulation efficiency can be improved by removing or suppressing the carrier wave component of the modulated light wave. The depth of the optical phase modulation is defined by the ratio of the amplitude of the phase change to the phase difference π. In the optical signal transmission application, the modulation degree in the phase modulation unit is 1 (100%) or less. When the modulation degree is in the range of 0 to 100%, the primary sideband component with respect to the optical carrier component monotonously increases as the modulation degree increases. That is, when only the optical carrier component is attenuated, the light intensity is attenuated, but the degree of light modulation is increased. The attenuation of the amount of light can be sufficiently compensated with an optical fiber amplifier such as an erbium-doped fiber amplifier (EDFA) or a fiber Raman amplifier.

  As a method of suppressing (removing) the optical carrier component, as shown in FIG. 8, an optical waveguide 200 for deriving a part of the carrier is provided for the optical modulation portion 100 that performs dispersion compensation. In the middle of the optical waveguide 200, an attenuator 201 for adjusting the intensity of the extracted carrier wave is disposed. As the attenuator, it is also possible to use an MZ type intensity modulator. The carrier wave whose intensity has been adjusted is combined with and interfered with the dispersion-compensated light wave emitted from the light modulation portion 100 and acts to suppress the light carrier component. The optical waveguide 200 may incorporate a configuration for adjusting the phase difference between the light modulation portion 100 and the optical waveguide 200 as necessary. The intensity of the optical carrier component can also be achieved by changing the ratio of branching or combining the optical carrier component into the optical modulation portion 100 and the optical waveguide 200. A configuration for adjusting the branching ratio or the multiplexing ratio may be included.

  As another method of suppressing the carrier light component, as shown in FIG. 9, the light wave subjected to dispersion compensation can be passed through an optical filter 300 that attenuates a specific wavelength, and the carrier light component can be suppressed. Since it is necessary to effectively attenuate only the optical carrier component without attenuating the sideband component, it is desirable that the attenuation wavelength width of the optical filter 300 is narrower than the modulation frequency. The filter 300 may be a dielectric multilayer filter, an FBG filter, a wavelength separation element such as an AWG (array waveguide) or an asymmetric Mach-Zehnder coupler, an etalon, or the like. The filter 300 is not limited to an optical filter having a fixed cut wavelength, and an optical filter having a function of adjusting a transmission wavelength or a cut wavelength, such as a variable etalon, can also be used. In addition to the attenuation of the optical carrier component, the optical carrier component on the low frequency side is appropriately attenuated to relatively increase the high frequency modulation component in the modulated optical spectrum, and the high frequency attributed to the frequency characteristics of the optical modulator and drive circuit The lack of responsiveness may be compensated.

  Specifically, as shown in FIG. 2, a simulation (transmission characteristics analysis) was performed using a configuration using a Mach-Zehnder type optical waveguide and a coplanar type electrode with lithium niobate as a substrate. In the working region, a pattern for performing dispersion compensation of 80 km as shown in FIG. 3 was formed with a domain-inverted structure. The characteristics when this was operated with a 40 Gbps binary ASK (On / Off keying) signal were evaluated. As shown in FIG. 6 (eye pattern) and FIG. 7 (I-Q constellation diagram), the time waveform of the light wave immediately after the modulator output appears to be distorted and distributed at first glance. When the dispersion characteristics of the optical fiber having a length of 80 km were given, the waveform distortion and the like were compensated, and a good signal waveform and IQ signal distribution were obtained.

  For the optical modulator described above, as disclosed in Patent Document 5, adjustment made of a dielectric material or a metal material is provided in the vicinity of the modulation electrode constituting the optical modulator (directly above the substrate, etc.). It is also possible to arrange the members (arrange them so that the position can be adjusted). This makes it possible to greatly change the effective refractive index of the microwave that is the modulation signal propagating through the modulation electrode. Thereby, the dispersion compensation amount can be further greatly adjusted.

  As described above, according to the optical modulator of the present invention, it is possible to provide an optical modulator that can compensate for the chromatic dispersion of a longer optical fiber and can be applied to high-speed transmission exceeding several tens of Gbps. Become.

DESCRIPTION OF SYMBOLS 1 Substrate using material having electro-optic effect 2 Optical waveguide 3 Signal electrode 30 Ground electrode 10 Polarization inversion pattern L1 Incident light L2 Output light L2 ′ Output light S with suppressed carrier light S modulated signal

Claims (5)

In an optical modulator having a substrate made of a material having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide,
In order to transmit outgoing light emitted from the optical waveguide for a predetermined distance through an optical fiber, and to impart a waveform distortion having a characteristic opposite to the wavelength dispersion characteristic of the optical fiber to the optical wave propagating through the optical waveguide, the optical waveguide The substrate is inverted in a predetermined pattern along
The optical waveguide has a Mach-Zehnder type waveguide having two branch waveguides, and the polarization inversion pattern formed in one of the branch waveguides is an impulse that compensates the impulse response h (t) of the optical fiber. The pattern corresponds to the real part response of the response 1 / h (t), and the pattern of polarization inversion formed in the other branch waveguide corresponds to the imaginary part response of the impulse response 1 / h (t). A light wave that has passed through the two branch waveguides, and is configured to multiplex with a predetermined phase difference,
In the working region where the electric field formed by the modulation electrode acts on the optical waveguide, the propagation direction of the light wave propagating through the optical waveguide and the propagation direction of the modulation signal propagating through the modulation electrode are configured to be opposite to each other. An optical modulator characterized by comprising: 2. The optical modulator according to claim 1 , wherein the impulse response h (t) of the optical fiber is given by the following equation.
However, H (ω) is a transfer function of the optical fiber, and H (ω) = exp (jβ (ω) L). β (ω) is a phase constant of the light wave propagating through the optical fiber, and L is the length of the optical fiber.

3. The optical modulator according to claim 1, wherein a configuration for suppressing a carrier wave component of a light wave propagating through the optical waveguide with respect to the outgoing light that has passed through the optical waveguide polarized in the predetermined pattern is provided. An optical modulator comprising the optical modulator. The optical modulator according to any one of claims 1 to 3, the group velocity of light in the optical waveguide in said working area, light modulation, wherein slower than the group velocity of the modulation signal propagating modulation electrode vessel The optical modulator according to any one of claims 1 to 3, the group velocity of the modulated signal propagating modulation electrode in said working area, characterized in that lower than the group velocity of the light wave propagating through the optical waveguide Light modulator

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