JP2017003813A - Light modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light modulator capable of suppressing intermodulation distortion without significantly reducing modulation efficiency and capable of being easily produced.SOLUTION: A light modulator 100 comprises: a branch part 3 for branching an input light 1 into two lights; modulation parts 4, 5 for modulating intensity of the branched lights; a phase adjustment part 6 for adjusting phase difference between the modulated lights; and a combining part 7 for combining the modulated lights and outputting the combined light as a single intensity modulated light. The modulation parts 4, 5 and the phase adjustment part 6 perform intensity modulation so that, the two modulated lights output from the modulation parts 4, 5 have different chirp parameters from each other, and intermodulation distortion included in an electric signal is suppressed when the output light 8 output from the combining part 7 is converted into the electric signal on a light detector. The light modulator 100 performs intensity modulation for suppressing intermodulation distortion, based on the fact that, the intermodulation distortion generated on the two modulated lights depends on a wavelength chirp in a different manner of a fundamental wave based on a modulation signal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical modulator that modulates the intensity of light with an electric signal when transmitting a large amount of information or a radio signal through an optical fiber.

  In recent years, for the purpose of eliminating dead zones for broadcasting radio waves and distributing wireless signals to condominiums and offices, light is directly modulated with a radio signal using an optical modulator, and the modulated light is transmitted far away with an optical fiber. A system called an optical fiber wireless system that returns a wireless signal and distributes the wireless signal has attracted attention. Optical modulators used in such optical fiber radio systems and large-capacity optical fiber communication systems are required to operate at high speed, so electro-optic optical modulators are mainly used, and some of them are electroabsorption modulations. A vessel is used. However, all of these optical modulators have a non-linear modulation operation, and a third-order intermodulation distortion occurs when light intensity modulation is performed, resulting in deterioration of transmission characteristics. Since the third-order intermodulation distortion increases in proportion to the third power of the modulation degree, in many cases, the influence of the nonlinear distortion is reduced by operating the optical modulator at a very low modulation degree. is the current situation. Such usage is at the expense of significant dynamic range.

  Compensation of nonlinear distortion is one of the problems of improving system performance, and various methods for actively compensating nonlinear distortion have been proposed. The predistortion method often used in ordinary amplifiers uses an electric circuit for distortion compensation, so it is difficult to apply in a high frequency region such as a millimeter wave band. For high-speed modulation, the optical third order is used. It is conceivable to cancel the intermodulation distortion. As an optical modulator having a structure suitable for high-speed operation, there is an optical modulator that modulates light branched into two by two modulators and then combines them with each other to form modulated light for transmission. In such an optical modulator, when the light combined with the modulated light from the two modulators is detected by the photodetector, the third-order intermodulation distortion is canceled to the two modulators. A device that compensates for third-order intermodulation distortion by adjusting the intensity, modulation degree, and bias voltage of each input unmodulated light is known (see, for example, Non-Patent Document 1 and Non-Patent Document 2). .

  In the optical modulators of Non-Patent Documents 1 and 2 described above, two modulation units are configured by the electro-optic optical modulator. In addition, a similar optical modulator that uses two modulation units using an electroabsorption optical modulator instead of the electro-optic optical modulator is known (for example, see Patent Document 1). These optical modulators divide input light into two light waves having different intensities, perform light intensity modulation with different modulation degrees, and invert the phases to multiplex them. At the time of the multiplexing, strong modulation is applied to the light wave of weak intensity and weak modulation is applied to the light wave of strong intensity so that the intensity of the third-order intermodulation distortion matches, and the third-order intermodulation distortion is applied. Cancel. The distortion compensation method based on this principle optically cancels the third-order intermodulation distortion by branching, modulation, and multiplexing of light, and does not greatly depend on the frequency of the modulation signal. This is advantageous for high-speed operation.

SK Korotky, RMRidder "Dual Parallel Modulation Schemes for Low-Distortion Analog Optical Transmission", I. Triple E Journal of Selected Areas in Communication, Vol. 8, no. 7, pp. 1377-1380, (1990) JLBrooks et al. “Implementation and Evaluation of a Dual Parallel Linearization System for AM Implementation and Evaluation of a Dual Parallel Linearization System for AM” -SCM Video Transmission ", Journal of Lightwave Technology, Vol. 11, no. 1, pp. 34-41 (1993) Sugawara et al., “Analysis of temporal change of wavelength chirp in electro-optic modulator and examination of evaluation method of minute chirp”, IEICE Technical Report, Vol. 114, no. 14, MWP2014-2, pp. 7-12, April 2014

JP 2003-172907 A

  However, the conventional optical modulators in Non-Patent Documents 1 and 2 and Patent Document 1 described above have a problem that the modulation efficiency is greatly deteriorated. The modulator that performs optical modulation with a large modulation degree requires a large power modulation signal, but all of the signals are subject to third-order intermodulation distortion generated by the other weak modulation modulator. Used only for offsetting. Furthermore, since a considerable amount of the original basic component is canceled at the same time, the final modulation degree is further smaller than the modulation degree of the light modulating unit that performs weak modulation, and the modulation efficiency is greatly deteriorated. End up.

  The second problem is that an unequal distribution optical branching structure is required. An optical modulator that is in practical use normally guides light using an optical waveguide and branches light using a branched optical waveguide. An optical waveguide having an equally-distributed light branching structure can be easily manufactured because it only needs to be manufactured in a symmetric structure, and is widely used. However, it is difficult to realize a non-uniformly distributed optical branching structure. This is because, in order to obtain a specific branching ratio (distribution ratio), it is necessary to design and build a branching waveguide structure with very high dimensional accuracy below the wavelength of light. However, since an optical waveguide such as an electro-optic light modulator is formed using a refractive index distribution formed by metal thermal diffusion or the like, the refractive index distribution is controlled with high accuracy and has a predetermined branching ratio. It is difficult to form a branched structure of the optical waveguide.

  As another method of branching light in an unequal distribution, a coupler (directional coupler) by an optical waveguide is formed on an electro-optic crystal without depending on the optical branching structure of the optical waveguide described above, and the coupler is formed by an electrode or the like. Some have an electric field to adjust the distribution ratio. However, such a coupler is large in size and not easy to design. Another problem is that the distribution ratio depends on the wavelength of light. Non-Patent Document 3 is a document relating to a wavelength chirp and a chirp parameter that are generated when a Mach-Zehnder light intensity modulator is operated.

  An object of the present invention is to provide an optical modulator that solves the above-described problems, is easy to manufacture, and can suppress intermodulation distortion without greatly reducing modulation efficiency.

  In order to achieve the above object, an optical modulator according to the present invention includes a branching unit that splits input light into two branches, two modulation units that modulate the intensity of each light branched by the branching unit, and two modulation units. An optical modulator comprising: a multiplexing unit that combines two modulated lights output via the optical signal and outputs the resultant as one intensity-modulated light, wherein the two modulating units are output from each of the modulators The intensity modulation is performed so that the two modulated lights have different chirp parameters, and the phase difference between the two modulated lights is adjusted to detect the output light output from the multiplexing unit. The intermodulation distortion contained in the electric signal converted into the electric signal by the detector is suppressed.

  In this optical modulator, a phase adjusting unit may be provided on at least one of the two optical paths from the branching unit to the multiplexing unit as means for adjusting the phase difference between the two modulated lights.

  In this optical modulator, the two modulators may perform intensity modulation so that the chirp parameters of the two modulated lights have the same magnitude and the signs are reversed.

  In this optical modulator, the modulation unit has a Mach-Zehnder type configuration having two phase modulation units, and performs modulation by performing intensity modulation with different phase modulation indexes in the two phase modulation units. The modulated light output from the unit may have a chirp parameter.

  In this optical modulator, the branching ratio of the branching section and the multiplexing ratio of the multiplexing section may both be 1: 1.

  The optical modulator may include a power distributor that supplies the phase modulation power with different intensities to the two phase modulation units.

  In this optical modulator, the branching unit, the two modulating units, and the multiplexing unit include an optical waveguide formed on the electro-optic crystal substrate, and an electrode made of a conductive thin film for applying an electric field to the optical waveguide. It may be configured.

  According to the optical modulator of the present invention, the two modulating units output two modulated lights having mutually different chirp parameters and adjusting the phase difference so that the intermodulation distortion is suppressed. By combining the two modulated lights into one intensity modulated light, the intermodulation distortion can be suppressed without greatly reducing the modulation efficiency.

1 is a schematic configuration diagram of an optical modulator according to a first embodiment of the present invention. The schematic diagram explaining generation | occurrence | production of the general 3rd order intermodulation distortion in optical modulation. (A) is a figure of the optical spectrum of the modulated light intensity-modulated by the optical modulator of FIG. 2, (b) is a general explanatory view of the order of the spectral component. The figure which shows the dependence with respect to the chirp parameter (alpha) of phase difference (DELTA) (theta) of each mixing pair shown to Fig.3 (a). FIG. 5 is a schematic configuration diagram of an optical modulator according to a second embodiment. The figure showing the dependence of the fundamental wave component included in the modulated light after multiplexing and the intensity of the third-order intermodulation distortion on the phase difference Δφ between the modulated lights before multiplexing, with the chirp parameter difference Δα being 1 . The figure which shows the comparative example which made the difference (DELTA) (alpha) of the chirp parameter in FIG. 6 zero. The figure which shows the dependence with respect to the modulation index of various intensity | strengths including the intensity | strength of a basic component, and the intensity | strength of a 3rd order intermodulation distortion. FIG. 6 is a schematic configuration diagram of an optical modulator according to a third embodiment. The typical block diagram of the optical modulator which concerns on 4th Embodiment.

(First embodiment)
Hereinafter, an optical modulator according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an optical modulator 100 according to the first embodiment. The optical modulator 100 outputs from the branching unit 3 that splits the input light 1 into two, the two modulation units 4 and 5 that modulate the intensity of each light branched by the branching unit 3, and the two modulation units 4 and 5. And a combining unit 7 that combines the two modulated lights and outputs them as one intensity-modulated light (referred to as output light 8). The optical modulator 100 includes an optical waveguide 2, and guides the input light 1, each light branched by the branching unit 3, and the combined output light 8 by the optical waveguide 2. Further, the optical modulator 100 adjusts the phase difference between the two modulated lights output from the modulation units 4 and 5 on the optical path after the one modulation unit 4 and before the multiplexing unit 7. An adjustment unit 6 is provided. The modulation units 4 and 5 are assumed to have nonlinear characteristics that generate intermodulation distortion and the like.

  The input light 1 enters the optical waveguide 2, is branched into two at the branching section 3, and is intensity-modulated by the same modulation signal 9 at each of the modulation sections 4 and 5. One of the two intensity-modulated lights (in this example, the light on the modulation section 4 side) passes through the phase adjustment section 6, and the phase difference between the two modulation lights during the passage. Is adjusted. The two modulated lights that have been bifurcated and subjected to intensity modulation and phase adjustment are combined and interfered by the combining unit 7 and output as one output light 8. That is, the optical modulator 100 is an optical modulator that outputs the input light 1 as output light 8 modulated by the modulation signal 9.

  In this optical modulator 100, the modulation units 4, 5 and the phase adjustment unit 6 have two modulated lights output from the modulation units 4, 5 having different chirp parameters, and from the multiplexing unit 7. Intensity modulation is performed by adjusting the phase difference between the two modulated lights so that the intermodulation distortion contained in the electrical signal is suppressed when the output light to be output is converted into an electrical signal by the photodetector. . The optical modulator 100 combines the two intensity-modulated lights modulated and phase-adjusted in this way into one output light 8, thereby modulating light that has suppressed intermodulation distortion without greatly reducing the modulation efficiency. Can be output. The phase adjustment unit 6 is a means for adjusting the phase difference between the two modulated lights, and may be provided in at least one of the two optical paths from the branching unit 3 to the multiplexing unit 7.

(Explanation of intermodulation distortion)
FIG. 2 schematically shows a state of an optical spectrum and an electric signal spectrum when light is intensity-modulated with a two-frequency signal (two-tone signal) composed of two waves of angular frequencies ω ₁ and ω ₂ . The optical modulator 34 corresponds to the modulation unit 4 or the modulation unit 5 described above, and has nonlinear characteristics. An unmodulated light wave (carrier wave 32, angular frequency ω _c ) emitted from the light source 31 is incident on the optical modulator 34 and is intensity-modulated by a modulation signal 33 (angular frequencies are ω ₁ and ω ₂ ) composed of two frequency signals. Is done.

When the modulated light 35 obtained by intensity modulation is detected by the photodetector 39 and converted into an electric signal 40, the electric signal spectrum has a fundamental wave component 42 (the angular frequency is modulated) that is the same frequency component as the modulation signal 33. The same ω ₁ and ω ₂ ) as the signal appears. Further, on both sides of the fundamental wave component 42, third-order intermodulation distortion (components) 41 having angular frequencies 2ω ₁ −ω ₂ and 2ω ₂ −ω ₁ are mixed as a result of mixing between the components of the optical spectrum. appear. In the present application, for simplification of description, the carrier wave 32, the modulation signal 33, and the like, and the component of the carrier wave 32 and the component of the modulation signal 33 that appear in the spectrum corresponding to these waves and signals are simply referred to as the carrier wave 32. And the like, and the like, as in the modulation signal 33, etc. are appropriately used (the same applies to other waves, signals, and distortion components).

FIG. 3A shows details of the optical spectrum of the modulated light 35. The optical spectrum of the modulated light 35 appears due to the carrier 32 (angular frequency is ω _c ), the fundamental wave component 36 (a component that appears based on linear modulation), and the nonlinear characteristics, because the optical modulator 34 has nonlinear characteristics. Distortion components (all components other than the carrier wave and the fundamental wave component). FIG. 3A shows the distortion components up to the third order in the frequency range up to the secondary sideband. The equation described above for each spectral component represents the angular frequency difference between each spectral component and the component of carrier 32, ie, the increment of the angular frequency of each spectral component relative to the angular frequency of carrier 32 (decrease if the difference is negative). ing.

Due to the presence of nonlinear characteristics in the optical modulator 34, an angular frequency (n ₁ ω ₁ +) obtained by adding an arbitrary integer (n ₁ , n ₂ ) to each of the _two angular frequencies (ω ₁ , ω ₂ ). A spectral component serving as a distortion component is generated at a position separated from the carrier wave 32 by n ₂ ω ₂ ). There are infinite combinations of integers and infinite distortion components. As shown in FIG. 3B, the distortion components are classified by the order (| n ₁ | + | n ₂ |). However, the component with the order of 1 is the fundamental wave component 36 generated based on the linear modulation operation, and the component with the order of 2 or more is a distortion component generated by the nonlinear operation. 3A and 3B show distortion components up to the third order, that is, the second-order distortion component 37 and the third-order distortion component 38, because the spectral intensity rapidly decreases as the order increases.

The spectrum of the electrical signal 40 obtained by detecting the modulated light 35 with the photodetector 39 is obtained by mixing the components of the electrical signal generated by mixing the optical spectrum components of the modulated light 35 shown in FIG. Including. The frequency of the electrical signal component is the frequency difference between the corresponding components of the optical spectrum. As a result, as shown in FIG. 2, third-order intermodulation distortion 41 occurs on both sides of the fundamental wave component 42 as a distortion component that appears in the vicinity of the fundamental wave component 42 that is a component having the same frequency as that of the modulation signal 33. . The angular frequency of the fundamental wave component 42 is ω ₁ and ω ₂ that are the same as those of the modulation signal 33, and is generated by mixing the carrier wave 32 and the fundamental wave component 36 in the optical spectrum of FIG.

In addition, the two distortion components of the intermodulation distortion 41 in the spectrum of the electric signal 40 have angular frequencies of 2ω ₁ −ω ₂ (n ₁ = 2 and n ₂ = −1) and 2ω ₂ −ω ₁ (n ₁ = −1, n ₂ = 2), which is a third-order distortion component having a low order (high intensity). Among these two distortion components, for example, a combination of mixing between optical spectral components that generate an intermodulation distortion 41 having an angular frequency of 2ω ₁ −ω ₂ is a mixing pair 43, 44, 44 shown in FIG. 45 and mixing pairs 43 ′, 44 ′, and 45 ′ that are symmetrical with respect to the carrier 32 as a center. The intermodulation distortion 41 having an angular frequency of 2ω ₁ −ω ₂ is a sum of high-frequency electric signals generated by mixing such as the mixing pair 43.

  By the way, when an optical modulator is put into practical use, the modulation signal is not usually a two-frequency signal but a high-frequency signal spread with a certain frequency bandwidth. Accordingly, there are an infinite number of combinations of two frequencies of the two-frequency signal that are the basis for generating the third-order intermodulation distortion described above within the frequency band of the modulation signal. In this case, the electrical signal 40 converted by the photodetector 39 is obtained by superimposing third-order intermodulation distortion in the same band as the frequency band of the fundamental wave component in the frequency space. For this reason, it is impossible to remove only the third-order intermodulation distortion after conversion into the electric signal 40 by the photodetector 39.

(Explanation of wavelength chirp)
Wavelength chirp is a phenomenon in which the frequency (wavelength) of a light wave varies with changes in light intensity. In normal optical fiber communication, if the wavelength of the light wave is different, the propagation speed in the optical fiber (strictly speaking, the group speed of the propagation wave) varies due to the chromatic dispersion characteristics of the optical fiber, and the arrival time varies. . Therefore, if the optical wave of the optical signal has a wavelength chirp, the arrival time of each part in the series of waveforms forming the optical signal will be different, and the waveform when receiving light will be disturbed, thus hindering signal transmission. Come. For this reason, it is said that a smaller wavelength chirp is better for a general optical modulator. There is a chirp parameter α as an index indicating the degree of the wavelength chirp. The chirp parameter α is defined by the ratio between the time change of the phase of the light wave and the time change of the amplitude, and is expressed by the following expression (1) using the phase φ and the amplitude E of the light subjected to the amplitude modulation, that is, the intensity modulation.

  α = (dφ / dt) / {(1 / E) (dE / dt)} (1)

  Since the time variation of the phase corresponds to the frequency, it can be seen that α represented by the above equation (1) is an index that quantitatively represents the frequency variation accompanying the amplitude modulation. The sign of the chirp parameter α is plus (+) when the direction of the time change of the amplitude and the time change of the frequency is the same, and the minus is minus (−). The amount (size and sign) of the chirp parameter α can be set to a desired amount by appropriately configuring the structures and operating conditions of the modulators 4 and 5. The modulated light has a chirp parameter α means that the modulated light has a wavelength chirp and the chirp parameter of the wavelength chirp is α, and the modulated light has a wavelength chirp with a chirp parameter of α. ,That's what it means.

(Principle of intermodulation distortion suppression)
In the present embodiment, a wavelength chirp, which is a wavelength variation of a light wave that occurs during light intensity modulation, is actively used for suppression of intermodulation distortion. The inventor of the present application changes the phase relationship of each optical spectrum component in the optical spectrum of the modulated light shown in FIGS. 3A and 3B depending on the chirp parameter α of the modulated light 35, and It was discovered that the behavior of the change is different between the fundamental wave component and the distortion component. It has also been found that the direction of the change is reversed by the sign of the chirp parameter α.

This will be described. 4 shows mixing pairs 43, 44, and 45 between optical spectrum components of modulated light that generate the intermodulation distortion (component) 41 of the angular frequency 2ω ₁ −ω ₂ shown in FIG. 2 (see FIG. 3A). Is a relationship between the phase difference Δθ between the optical spectrum components forming the mixing pair and the chirp parameter α. Here, the modulation index A of intensity modulation is 0.1π, and is drawn with reference to Δθ when α = 0. The phase difference Δθ is related to the phase of the electrical signal having the angular frequency 2ω ₁ −ω ₂ that is the third-order intermodulation distortion generated by mixing.

  The following can be understood from FIG. As described above, the third-order intermodulation distortion 41 is formed by adding high-frequency electrical signals generated by mixing such as the mixing pair 43. In other words, the third-order intermodulation distortion 41 is synthesized from a plurality of electrical signals having different phases at the time of detection by the photodetector 39. It can be seen from FIG. 4 that the phase relationship of these electrical signals synthesized with each other varies depending on the chirp parameter α, and that the phase relationship is reversed by the sign of the chirp parameter α.

  As a result of examining the relationship between the chirp parameters α and Δθ in more detail, light obtained by mixing two intensity-modulated lights having different chirp parameters α into one intensity-modulated light is converted into an electrical signal by a photodetector. In this case, it is found that if the phase difference between the two modulated lights is appropriately set and mixed (combined), only the third-order harmonic distortion component can be canceled by the phase relationship. It was. This effect is more prominent if the chirp parameters α of the two intensity-modulated lights are set to the same magnitude and the signs are reversed. Further, higher-order intermodulation distortion can be canceled by the same principle.

  The optical modulator 100 of FIG. 1 is based on this new discovery, and the modulation units 4 and 5 modulate the intensity so that the two modulated lights to be output have different chirp parameters α, and the modulation unit 4 , 5 and the phase adjustment unit 6 adjust the phase difference between the two modulated lights so that the intermodulation distortion is canceled and suppressed. As a result of adjusting the phase difference, the output light 8 combined by the combining unit 7 is detected by a photodetector and converted into an electric signal, which is included in the electric signal when the phase difference is not adjusted. Only the intermodulation distortion components cancel each other, and the fundamental component remains without being largely canceled.

  Here, if the difference in chirp parameter α between the intensity-modulated light beams output from the modulators 4 and 5 is Δα, even if Δα is small, the distortion compensation effect (mutually more than the fundamental wave component in the converted electric signal). There is an effect of more greatly attenuating the modulation distortion and an effect of suppressing the intermodulation distortion. For example, when Δα is set to 0.5 or more, a good distortion compensation effect is confirmed in which the ratio of the third-order intermodulation distortion to the fundamental wave component is improved by 20 dB or more under the modulation index A = 0.1π. Yes. In addition, it is preferable that the chirp parameters α of the two modulated lights have the same magnitude and opposite signs.

  In the optical modulator 100 shown in FIG. 1, the function of the phase adjustment unit 6 may be provided in one or both of the modulation units 4 and 5. When the modulation units 4 and 5 have a phase adjustment function for adjusting the phase difference between the two modulated lights so that the intermodulation distortion is canceled and suppressed, the modulation units 4 and 5 Need not include a separate phase adjustment unit 6. Further, such a phase adjustment function may be distributed between the modulation units 4 and 5 and the phase adjustment unit 6.

  In addition, the phase difference between the two modulated lights when being multiplexed by the multiplexing unit 7 is determined by the difference in optical length of the two optical paths from the branching unit 3 to the multiplexing unit 7. The adjusting unit 6 does not necessarily have to exist clearly, and the phase difference may be adjusted by appropriately setting the optical length difference between the branching unit 3 and the combining unit 7 by some method. For example, an appropriate difference is provided in the lengths of the two optical waveguides between the branching section 3 and the multiplexing section 7, or a temperature adjusting means for giving a temperature difference to the two waveguides is provided, and the substantial difference due to the temperature difference. The phase difference may be adjusted by causing a difference in the optical length. As a result, the intermodulation distortion can be suppressed by adjusting the phase difference of the modulated light with a simpler structure.

  The optical modulator 100 performs distortion compensation based on the presence of the wavelength chirp and the phase adjustment between the intermodulation distortions. Therefore, unlike a conventional optical modulator that compensates for distortion by providing a difference in light intensity based on different branching ratios, the branching ratio at the branching unit 3 and the multiplexing at the multiplexing unit 7 in the optical modulator 100. Any ratio need not be intentionally asymmetric, but may be 1: 1. In addition, the adjustment of the phase difference between the two modulated lights by the phase adjusting unit 6 can be performed if the third-order intermodulation distortion is suppressed when the output light 8 is detected by a photodetector such as a photodiode. Good. The converter used when the output light 8 is converted into an electric signal and used may be a general photoelectric converter.

  In the optical modulator 100, optical elements such as the branching unit 3, the modulation units 4 and 5, the multiplexing unit 7, and the phase adjustment unit 6 are formed on the waveguide substrate, and the optical elements 2 are connected by the optical waveguide 2. An optical integrated circuit structure can be obtained. The optical modulator 100 is not limited to such a structure, and each optical element is arranged as a bulk structure element (discrete element), and each optical element is optically connected by a light wave propagating in space or a transparent medium. It is good.

  According to the optical modulator 100 of the present embodiment, the two modulators 4 and 5 have two chirp parameters α different from each other, and the phase differences are adjusted so that intermodulation distortion is suppressed. Since the modulated light is output, the two modulated lights are combined into one intensity modulated light, so that the intermodulation distortion can be suppressed without greatly reducing the modulation efficiency. Further, since the optical modulator 100 can selectively suppress the intermodulation distortion, in particular, the fundamental wave component is largely attenuated by the third-order intermodulation distortion existing in the same band as the frequency band of the fundamental wave component. And can be effectively removed.

  In addition, since the optical modulator 100 can use the symmetric branching section 3 and the symmetric combining section 7, it can be easily manufactured when an optical modulator using an optical waveguide is used. Since a special and difficult optical circuit such as this is not used, the production yield can be improved and the cost can be reduced. Further, the optical modulator 100 can have a smaller structure than that using an optical coupler, and a plurality of optical modulators can be integrated.

  Further, by using the optical modulator 100 of the present embodiment, it is possible to perform optical modulation with low distortion and high efficiency with a simple configuration. For example, an optical communication system that performs analog optical modulation such as an optical fiber radio system In this case, there are significant effects in increasing the speed, increasing the capacity, and transmitting over a long distance. Also, the optical modulator 100 can suppress the third-order intermodulation distortion that increases the wavelength band by distorting the waveform of the modulated light to generate unnecessary spectral components, and is therefore suitable for an optical transmission system that performs digital modulation. Can be applied. The optical modulator 100 can be applied to digital modulation and analog modulation regardless of the type of modulation signal. The optical modulator 100 can be applied not only to suppression of third-order intermodulation distortion but also to an optical modulator that suppresses other intermodulation distortion and modulation distortion.

(Second Embodiment)
FIG. 5 shows an optical modulator 200 according to the second embodiment. In the optical modulator 200, each of the modulation units 4 and 5 in the optical modulator 100 described above is a modulation unit 14 or 15 having a Mach-Zehnder type configuration. The input light 10, the optical waveguide 12, the branching unit 13, the modulation units 14 and 15, the phase adjustment unit 26, the multiplexing unit 27, and the output light 29 of the optical modulator 200 are the input light 1 of the optical modulator 100 described above, It corresponds to the optical waveguide 2, the branching unit 3, the modulation units 4 and 5, the phase adjustment unit 6, the multiplexing unit 7, and the output light 8. The optical modulator 200 operates in the same manner as the above-described optical modulator 100 except for the operations inside the modulation units 14 and 15.

  The optical modulator 200 is formed by providing each optical element such as an optical waveguide 12 on the surface of an electro-optic crystal substrate 11 having an electro-optic effect such as a lithium niobate single crystal. The optical waveguide 12 is formed, for example, by thermally diffusing metal titanium on the surface of the electro-optic crystal substrate 11. In this case, each of the modulators 14 and 15 has a Mach-Zehnder type electro-optic light modulator structure.

  The modulation unit 14 is branched by a branching unit 16 that further splits one optical waveguide that has been split into two optical waveguides 12, a multiplexing unit 24 that combines two optical waveguides that are branched by the branching unit 16, and a branching unit 16. Two phase modulation units 18 and 19 arranged in each optical waveguide, and a phase bias adjustment unit 22 arranged in the subsequent stage of the phase modulation unit 18 are provided. A modulation signal 28 is input to the phase modulation units 18 and 19. The modulation unit 15 has the same configuration as that of the modulation unit 14, and includes a branching unit 17, a multiplexing unit 25, phase modulation units 20 and 21, and a phase bias adjustment unit 23. A modulation signal 28 is input to the phase modulation units 20 and 21.

  The operation of the modulation unit 14 will be described. The light wave input to the modulation unit 14 is branched into two by the branching unit 16 and phase-modulated by the modulation signal 28 in the phase modulation units 18 and 19, respectively. Further, the phase bias adjusting unit 22 adjusts the phase difference bias amount between the two phase-modulated light waves. These light waves (phase-modulated light) are multiplexed and interfered by the multiplexing unit 24 to become one intensity-modulated modulated light, which is output from the modulation unit 14. For example, the phase bias adjusting unit 22 sets the bias so that the phase difference bias amount of the two phase-modulated modulated light incident on the multiplexing unit 24 becomes π / 2 or −π / 2. With this setting, it is possible to realize light intensity modulation with good modulation efficiency and less distortion. The operation of the modulation unit 15 is the same as the operation of the modulation unit 14, and the light wave input to the modulation unit 15 undergoes phase modulation by the modulation signal 28 and finally becomes modulated light that is intensity-modulated from the modulation unit 15. Is output.

(Generation of wavelength chirp)
Similar to the modulation units 4 and 5 in the optical modulator 100 described above, the modulation units 14 and 15 perform intensity modulation so that the intensity-modulated modulated light has a chirp parameter α. The modulation unit 14 operates as a Mach-Zehnder type light intensity modulator, and can generate wavelength chirp in the intensity modulated light that is the output light, that is, generate and apply wavelength chirp. Hereinafter, generation of wavelength chirp in the modulation unit 14 will be described (for example, see Non-Patent Document 3 for wavelength chirp and chirp parameter).

The two phase modulation units 18 and 19 of the modulation unit 14 perform phase modulation using two high frequency modulation signals 28 having the same frequency (angular frequency ω) and opposite phases. The phase modulation index of this phase modulation is represented by the amplitude of the phase change of the light wave caused by the modulation signal. The phase change amounts Δφ ₁ and Δφ ₂ generated by the phase modulation in the phase modulation units 18 and 19 are expressed by the following equations (2) and (3) using the phase modulation indexes A ₁ and A ₂ of the phase modulation units 18 and 19 and the time t. ). The phase modulation indexes A ₁ and A ₂ are expressed by the following equation (4) using an intensity modulation modulation index A in the modulation unit 14 and an index α ₀ indicating the degree of phase modulation unbalance between the phase modulation units 18 and 19. It is represented by (5). Here, A ₁ + A ₂ = A. Further, when the phase bias amount is set to −π / 2 in the phase bias adjustment unit 22, the intensity I of the output light from the modulation unit 14 is expressed by the following equation (6).

Δφ ₁ = A ₁ cos ωt (2)
Δφ ₂ = −A ₂ cos ωt (3)
A ₁ = (1 + α ₀ ) A / 2 (4)
A ₂ = (1−α ₀ ) A / 2 (5)
I∝1 + sin (Acosωt) ≈1 + Acosωt (6)

  From the above equation (6), it can be seen that the modulation unit 14 performs light intensity modulation of the modulation index A. The rightmost side of Expression (6) is an approximate expression when A is sufficiently small. In order to realize such light intensity modulation, for example, a modulation electrode may be provided on the optical waveguide constituting each of the phase modulation units 18 and 19, and an individual modulation signal may be applied to each modulation electrode. The individual modulation signal is, for example, a signal obtained by dividing the modulation signal 28 using a power distributor and giving a phase difference of 180 ° between the divided signals.

Here, when the phase modulation in the phase modulation units 18 and 19 is ideally symmetric and balanced with each other, α ₀ = 0, Δφ ₁ = (A / 2) cos ωt = −Δφ ₂ and the phase The phase change amounts of the modulators 18 and 19 are equal in magnitude and have opposite signs. Therefore, in the multiplexing unit 24, the phase change of the modulated wave before the multiplexing is completely converted into an intensity change, so that phase change, that is, wavelength chirp does not occur in the output light from the modulation unit 14. That is, in the above formula (1), dφ / dt = 0 and α = 0.

Further, when α ₀ ≠ 0, that is, when the phase modulation by the phase modulators 18 and 19 becomes unbalanced (Δφ ₁ ≠ −Δφ ₂ ), the intensity I shown in the above equation (6) does not change. The phase modulation component remains in the output light itself from the modulation unit 14. That is, dφ / dt ≠ 0, and wavelength chirp is generated. Here, in particular, the modulation index A is small, the branching ratio of the branching portion 16 is 1: when close to 1, the chirp parameter alpha is known to be equal to the index alpha ₀ representing the imbalance of the phase modulation (Non-Patent Document 3). Therefore, when an unbalance of the phase modulation indexes A ₁ and A ₂ of the phase modulation units 18 and 19 is made (α ₀ ≠ 0), the wavelength having the chirp parameter α = α ₀ without affecting the intensity I Chirp can be generated in the intensity modulated light.

In order to create an unbalance of the phase modulation indexes A ₁ and A _{2 so} that α ₀ ≠ 0, for example, a so-called two-electrode Mach-Zehnder in which modulation electrodes are separately provided in two optical waveguides of a Mach-Zehnder interferometer The configuration of the electro-optic light modulator may be applied to the modulation unit 14 and a modulation signal 28 having different power may be input to each modulation electrode of the phase modulation units 18 and 19. In this case, there is an advantage that α ₀ can be strictly controlled by the power ratio of the modulation signal 28 input to each modulation electrode. In addition, α ₀ can be controlled by making the structures of the two modulation electrodes asymmetric with each other so that the electric field strength under the two electrodes is unbalanced. By such a method, wavelength chirp can be generated in the output light of the modulation unit 14.

The sign of the chirp parameter α is defined as, for example, + when the direction of amplitude change of the intensity-modulated light is the same as the direction of frequency change, and − is the opposite. Therefore, if the sign of α ₀ is reversed, for example, if | Δφ ₁ |> | Δφ ₂ | is changed to | Δφ ₁ | <| Δφ ₂ |, the sign of the generated chirp parameter α is also reversed. Further, based on the symmetry of the Mach-Zehnder interferometer, the signs of the phase difference bias amounts set by the phase bias adjusting unit 22 are reversed to each other even when α ₀ is the same sign, for example, from + π / 2 to −π / 2. Or by changing from −π / 2 to + π / 2, the sign of the chirp parameter α can be reversed.

Further, as a special case, when only the phase modulator 18 is operated, that is, when Δφ ₂ = 0, α ₀ = 1, and conversely when Δφ ₁ = 0, α ₀ = −1. The same applies to the modulation unit 15.

(Description of third-order intermodulation distortion generation)
Next, third-order intermodulation distortion will be described. As can be seen from the above equation (6), when the modulation index A is sufficiently small, the intensity I of the intensity-modulated light can be approximated as 1 + A cos ωt. Accordingly, the intensity I varies linearly with respect to the modulation signal 28 (its time dependence is represented by cos ωt). That is, the intensity I is a linear function of cos ωt, and the modulation for obtaining the intensity I is approximately linear modulation and no distortion occurs.

However, when the modulation index A is not small, this approximation does not hold, and the intensity I changes in the form of a sine function with the modulation signal 28 as a variable, that is, sin (Acosωt), so that a non-linear modulation operation is performed. This is the cause of nonlinear distortion. Among nonlinear distortions, third-order intermodulation distortion is a very big problem in an actual communication system. The third-order intermodulation distortion in the modulation unit 14 will be described. Consider the phase modulation operation by the two-frequency signal (two-tone signal) composed of two waves of the angular frequencies ω ₁ and ω _{2 in} the phase modulators 18 and 19. In this case, the phase change amounts Δφ ₁ and Δφ ₂ are expressed by the following equations (7) and (8), and the intensity I of the output light of the modulation unit 14 is expressed by the following equation (9).

Δφ ₁ = (1 + α ₀ ) (A / 2) (cos ω ₁ t + cos ω ₂ t) (7)
Δφ ₂ = − (1−α ₀ ) (A / 2) (cos ω ₁ t + cos ω ₂ t) (8)
I∝1 + sin {A (cosω ₁ t + cosω ₂ t)} ... (9)

  The optical spectrum of the intensity-modulated light that has undergone phase modulation with the two-frequency signal is schematically shown in FIG. Here, the optical modulator 34 in FIG. 2 corresponds to the modulator 14 in FIG.

Here, with reference to FIGS. 6, 7, and 8, calculation results performed on the modulator having the configuration of the optical modulator 200 will be described. The spectral intensity on the vertical axis of each figure is expressed as a relative value normalized under appropriate conditions. FIG. 6 shows that the chirp parameter difference Δα is 1, and the intensity 51 of the fundamental wave component included in the modulated light after the combination and the intensity of the third-order intermodulation distortion with respect to the phase difference Δφ between the modulated lights before the combination. An example of the dependency of 52 is shown. The modulation unbalance index α ₀ in each of the modulation units 14 and 15 is set to +0.5 and −0.5, and Δα = 1. The modulation index A was set to 0.1π. The phase difference Δφ is a phase difference given by the phase adjustment unit 26. The fundamental wave component and the third-order intermodulation distortion intensities 51 and 52 are component intensities included in an electrical signal obtained by detecting the intensity-modulated light output from the optical modulator 200 with the photodetector. It can be seen that only the intensity 52 of the third-order intermodulation distortion is greatly suppressed in the vicinity of the phase difference Δφ = 0.72π.

For comparison, FIG. 7 shows an example in which no wavelength chirp is given (α ₀ = 0). In this case, the phase of the fundamental wave component and the third-order intermodulation distortion in the optical spectrum do not change each other. Therefore, the intensity 51 of the fundamental wave component and the intensity 52 of the third-order intermodulation distortion are obtained when the phase difference Δφ is ± π. Both will be suppressed. From this, it can be seen that when wavelength chirp is not given to the two intensity-modulated lights before multiplexing, it is not possible to leave the fundamental wave and suppress only the intermodulation distortion.

  FIG. 8 is a plot of the intensity 61 of the fundamental wave component and the intensity 62 of the third-order intermodulation distortion included in the electrical signal detected by the photodetector, with the horizontal axis representing the modulation index A and the vertical axis representing the spectral intensity. It is. In this example, Δα = 1. The strength 63 is the strength of the difference between the strength 61 and the strength 62. For comparison, thin lines 64, 65, and 66 are intensities corresponding to the intensities 61, 62, and 63 when it is assumed that only the modulation unit 14 is taken out and operated, and the normal Mach-Zehnder type electro-optics. The characteristic of an optical modulator is shown.

  From FIG. 8, according to the optical modulator 200, the intensity 63 of the difference between the intensity 62 of the third-order intermodulation distortion with respect to the intensity 61 of the fundamental wave component is the intensity of the difference in the case of a normal Mach-Zehnder optical modulator. Compared to 66, it can be seen that the entire region where the modulation index A <0.5π is improved by 20 dB or more. In particular, an improvement of several tens of dB or more is seen around A = 0.1π.

  Even if the optical modulator 200 is a combination of chirp parameters α with Δα ≠ 0 other than Δα = 1 shown in FIG. Accordingly, the third-order intermodulation distortion intensity 62 can be suppressed more greatly than the fundamental wave component intensity 61, and a distortion compensation effect can be exhibited.

  In addition, even if one of the modulated lights from the two modulators 14 and 15 is set so that the chirp parameter α is 0 and the other has the chirp parameter, a difference in chirp parameter occurs, so that a distortion compensation effect is obtained. Is done.

Further, when intensity modulation is performed by setting conditions so that the chirp parameters α of the wavelength chirps generated in the two modulation units 14 and 15 have the same magnitude and opposite signs, Since the wavelength chirp is canceled, the finally output intensity-modulated light does not include the wavelength chirp. Therefore, high-quality optical modulation with low distortion and no wavelength chirp can be realized. In order to set such a condition, for example, in the two modulation units 14 and 15 designed to perform the same intensity α ₀ light intensity modulation, the phase difference bias set by the phase bias adjustment units 22 and 23 is used. By reversing the signs of the quantities (eg, from + π / 2 to -π / 2 or from -π / 2 to + π / 2), the resulting chirp parameter α is the same magnitude, It can be easily reversed.

  According to the optical modulator 200 of the present embodiment, since the two Mach-Zehnder type modulators 14 and 15 generate the wavelength chirp by changing the phase modulation index, the wavelength chirp having the desired chirp parameter α can be easily achieved. Can be generated.

(Third embodiment)
FIG. 9 shows an optical modulator 300 according to the third embodiment. The optical modulator 300 includes a power distributor 30 that supplies phase modulation power to the two phase modulation units 18 and 19 with different intensities in the optical modulator 200 shown in FIG. The modulation units 20 and 21 are also provided with the same power distributor 30. This non-uniformly distributed power distributor 30 performs, for example, intensity modulation with different phase modulation indexes in the two phase modulation units 18 and 19 so that the modulated light output from the modulation unit 14 has a chirp parameter. Used to have.

When the optical modulator 300 is an electro-optic light modulation system, two optical waveguides that pass through the phase modulation units 18 and 19 are each provided with a modulation electrode, and the phase that is non-equally distributed by the power distributor 30 to each modulation electrode. What is necessary is just to supply the electric power for modulation. By making power non-uniformly distributed, the phase modulation index of the phase modulators 18 and 19 is differentiated to create an imbalance between the phase modulation indices, and a wavelength chirp having a chirp parameter α = α ₀ can be generated. it can.

  The modulation electrode of each optical waveguide may be the same shape or different shape. The same applies to the modulation unit 15. Further, even if the optical modulator 300 has a configuration other than the electro-optical light modulation method, the power distributor 30 can be applied. According to the optical modulator 300 of the present embodiment, since the power distribution ratio can be adjusted with high accuracy by the power divider 30, an imbalance between the electric field strengths by the two modulation electrodes can be made with high accuracy and adjusted with high accuracy. The wavelength chirp having the chirp parameter α can be generated.

(Fourth embodiment)
FIG. 10 shows an optical modulator 400 according to the fourth embodiment. The optical modulator 400 corresponds to, for example, the optical modulator 200 of FIG. 5 that does not include the phase modulation units 19 and 21. In this case, in each of the Mach-Zehnder type modulation units 14 and 15, the modulation is performed by only one of the phase modulation units 18 and 20, thereby realizing two states of α ₀ = 1 and −1. , 15 to achieve a distortion compensation operation by realizing a chirp parameter difference Δα = 2. According to the optical modulator 400 of the present embodiment, the phase modulators 19 and 21 in the optical modulator 200 can be omitted, so that the structure is simpler than that of the optical modulator 200.

In the optical modulators of the above-described embodiments, for example, in the optical modulator 100, the branching section 3, the two modulating sections 4, 5 and the multiplexing section 7 are the same as the optical waveguide 2 formed on the electro-optic crystal substrate. And an electrode made of a conductive thin film for applying an electric field to the optical waveguide 2. The electro-optic crystal substrate is, for example, a lithium niobate (LiNbO ₃ ) single crystal and a substrate having an electro-optic effect. This LiNbO ₃ substrate can be used by combining both an X cut and a Z cut in the orientation of the crystal structure with an appropriate electrode structure. The optical waveguide 2 is formed on this LiNbO ₃ substrate by, for example, thermally diffusing titanium. For example, a gold thin film can be used as the conductive thin film forming the electrode. The optical modulators 200, 300, and 400 of other embodiments can be similarly configured.

  As described above, the optical modulator of the present invention is based on the fact that a difference in phase change amount occurs between the fundamental wave component and the distortion component included in the primary sideband of the modulated light spectrum due to the chirp parameter α. Thus, only the third-order intermodulation distortion contained in the electrical signal detected by the photodetector is canceled and suppressed, and the suppression amount of the fundamental component is reduced as compared with the conventional optical modulator. And enables high-efficiency optical modulation with low distortion. Further, since the structure for branching and multiplexing light may be an equally distributed structure, an unequally distributed light branching structure is unnecessary, and when an optical modulator is manufactured by an optical waveguide structure on an optical substrate, Waveguide fabrication is easy. In addition, the optical modulator in each of the above-described embodiments can be configured by forming an optical waveguide, a modulation electrode (modulation electrode), a phase adjustment electrode, etc. on a single substrate. Therefore, an optical modulator with high efficiency and low distortion can be configured.

  Further, the present invention is not limited to the above configuration and can be variously modified. The optical modulator of the present invention is not limited to the one configured on a single substrate. For example, the modulation units 4 and 5 operate with the same modulation signal, and the intensity output from the respective modulation units 4 and 5. Any structure that operates so that the modulated light has different chirp parameters α may be used. In addition to the electro-optic light modulation type configuration, an electroabsorption type optical modulator may be used as the modulation units 4 and 5.

  Moreover, it is good also as a structure which mutually combined the structure of each part of each embodiment mentioned above. For example, the modulation unit 14 in the optical modulator 200 is replaced with the modulation unit 14 combined with the power distributor 30 in the optical modulator 300, or is replaced with the modulation unit 14 in the optical modulator 400, and the modulation unit 15 is left as it is. It is good. In the optical modulator of the present invention, any type of electrode structure including a lumped constant type, traveling wave type, and resonator type may be used as a modulation electrode for giving a high frequency modulation signal. The phase modulation units 18, 19, 20, and 21 may have an arbitrary number at arbitrary positions on the optical waveguide. In addition, when applying a high frequency modulation signal to the modulation electrode, a DC or low frequency bias voltage may be applied in a superimposed manner to replace the phase bias adjustment units 22 and 23 and the phase adjustment unit 26. Further, the power distributor 30 in FIG. 9 may be configured by a non-uniformly distributed distributor and a circuit having a function of inverting the phase of one output signal from the distributor, for example, a rat race circuit. Alternatively, a circuit having a 180 ° hybrid function may be used.

  Furthermore, the same modulation signal may be input to the two power dividers 30 in FIG. Therefore, the modulation signal input to the two power distributors 30 can be generated from one input signal using a separate power distributor. In this case, the separate power distributor may be an equally distributed power distributor. That is, the optical modulator of the present invention can be operated by a single modulation signal. Further, these power distributors can be configured by a planar circuit and integrated on a substrate such as an electro-optic crystal substrate of an optical modulator. Thus, a highly practical low distortion optical modulator that has a one-chip configuration and operates with a single input signal can be realized.

  Since the optical modulator and the optical modulator distortion compensation method of the present invention can generate intensity-modulated light with small intermodulation distortion, it can be applied to the use of modulated light in a communication system such as an optical fiber radio system. System performance can be improved. In addition, the optical modulator of the present invention can be miniaturized with a simple configuration and enables high-efficiency optical modulation with low distortion. It can be suitably applied to the system. The present invention is suitable not only for analog light modulation but also for a long-distance optical transmission system that performs digital light modulation.

DESCRIPTION OF SYMBOLS 1,10 Input light 11 Electro-optic crystal substrate 18, 19, 20, 21 Phase modulation part 100,200,300,400 Optical modulator 2,12 Optical waveguide 22,23 Phase bias adjustment part 3,13 Branch part 30 Power distribution 4, 5, 14, 15 Modulator 51, 61 Intensity of fundamental wave component 52, 62 Intensity of third-order intermodulation distortion 6, 26 Phase adjuster 7, 27 Combiner 8, 29 Output light 9, 28 Modulation Signal A ₁ , A ₂ Phase modulation index α Chirp parameter Δφ Phase difference

Claims (7)

A branching unit that splits the input light into two parts, two modulation units that modulate the intensity of each light branched by the branching unit, and two modulated lights that are output via the two modulation units are combined. An optical modulator comprising: a multiplexing unit that outputs as one intensity-modulated light,
The two modulators perform the intensity modulation so that the two modulated lights output from the modulators have different chirp parameters, and adjust the phase difference between the two modulated lights. Thereby, the intermodulation distortion contained in the electric signal obtained by converting the output light output from the multiplexing unit into an electric signal by a photodetector is suppressed.   2. The optical modulator according to claim 1, further comprising a phase adjusting unit on at least one of two optical paths from the branching unit to the multiplexing unit as means for adjusting the phase difference.   3. The light according to claim 1, wherein the two modulation units perform the intensity modulation so that chirp parameters of the two modulated lights have the same magnitude and have opposite signs. Modulator.   The modulation unit has a Mach-Zehnder type configuration having two phase modulation units, and outputs the modulation unit by performing the intensity modulation with different phase modulation indexes in the two phase modulation units. 4. The optical modulator according to claim 1, wherein the modulated light to be transmitted has a chirp parameter.   5. The optical modulator according to claim 1, wherein a branching ratio of the branching unit and a multiplexing ratio of the multiplexing unit are both 1: 1.   6. The optical modulator according to claim 4, further comprising a power distributor that supplies phase modulation power to the two phase modulation units with different intensities.   The branching unit, the two modulation units, and the multiplexing unit are configured to include an optical waveguide formed on an electro-optic crystal substrate, and an electrode made of a conductive thin film for applying an electric field to the optical waveguide. The optical modulator according to claim 1, wherein the optical modulator is a light modulator.

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