JP4793550B2 - Optical carrier suppressed double sideband (DSB-SC) modulation system capable of high extinction ratio modulation - Google Patents

Description

  The present invention relates to an optical carrier suppression double sideband system capable of high extinction ratio modulation.

  In optical communication, it is necessary to modulate light in order to place a signal on the light. Optical modulation includes direct modulation for modulating the driving power of the semiconductor laser and external modulation for modulating light from the semiconductor laser by means other than the light source. A modulator used for external modulation is generally called an optical modulator. In an optical modulator, a physical change is caused in the modulator in accordance with a signal to modulate light intensity, phase, and the like. Technical issues of optical modulators include reduction of drive voltage, high extinction ratio for improving modulation efficiency, wide bandwidth, high speed, and high light utilization efficiency for loss reduction. That is, development of an optical modulator having a high extinction ratio is desired. The extinction ratio means the ratio of the light intensity when the light intensity is the highest to the light intensity when the light intensity is the weakest.

An optical carrier-suppressed double sideband (DSB-SC) modulator is known as an optical modulator. Specifically, for example, in FIG. 37 of Japanese Patent Laid-Open No. 2004-252386 (Patent Document 1 below), MZ, PM provided on both arms, and a fixed phase shifter provided on one arm are shown. A DSB-SC modulator is disclosed. Light DSB-SC modulator, ideally, two side bands (double sideband) outputs a signal, the carrier (carrier wave) signal component is suppressed. However, there is a problem that the extinction ratio cannot be increased because the output of the actual optical DSB-SC modulator contains carrier components and high-order component signals that cannot be suppressed in addition to sideband signals. Therefore, the conventional optical DSB-SC modulator was intended to output an optical signal in which the carrier component and the higher-order signal component are suppressed as much as possible.
JP 2004-252386 A

  An object of the present invention is to provide an optical DSB-SC modulation system capable of suppressing a carrier signal (or higher-order component signal) and obtaining a high extinction ratio modulation.

  The present invention basically includes an optical DSB-SC modulator and a phase / intensity modulator (specifically, a two-electrode MZ modulator) that modulates the output light of the optical DSB-SC modulator. Related to optical DSB-SC modulation system. That is, since the carrier component that cannot be suppressed remains in the output of the optical DSB-SC modulator, the extinction ratio cannot be increased. Therefore, phase modulation and / or intensity modulation is performed on the output light from the optical DSB-SC modulator. At this time, the phase and timing are adjusted so as to cancel the carrier component (or cancel the high-order component) for the site band (which has the same frequency as the carrier or higher-order component) derived from the site band. In this way, since the carrier component (or higher-order component) can be suppressed, an optical DSB-SC modulation system capable of obtaining a high extinction ratio modulation can be obtained. The present invention is basically based on such knowledge.

  That is, the first aspect of the present invention is an optical carrier-suppressed double-sideband modulator and either a phase modulator or an intensity modulator for modulating the output light of the optical carrier-suppressed double-sideband modulator, or And an optical carrier suppressed double sideband modulation comprising: a control unit for adjusting a modulation time of the modulated signal of the optical carrier suppressed double sideband modulator and the modulated signal of the phase modulator or intensity modulator. About the system. The control unit will be described in more detail. In the control unit, the phase modulator or the intensity modulator modulates either or both of the double sideband signals of the output light from the optical carrier suppression double sideband modulator. Any of the double-sideband signals generated by the optical carrier signal is controlled so as to coincide with the frequency of the optical carrier signal or higher-order optical signal in the output light of the optical carrier-suppressed double-sideband modulator, and the optical carrier signal or higher-order optical signal is canceled. To do. As a result, the optical carrier signal or the higher-order optical signal can be canceled effectively, so that the optical carrier suppressed double sideband modulation system of the present invention can obtain a high extinction ratio.

  According to the present invention, the carrier signal (or higher order signal) of the output signal of the optical DSB-SC modulator itself is not suppressed, but the sideband signal that is a noise component of the optical DSB-SC modulator is further reduced. By effectively using the signal, the carrier signal (or higher order signal) of the optical DSB-SC modulation system can be suppressed, so that an optical DSB-SC modulation system capable of obtaining a high extinction ratio modulation can be provided.

1. Optical carrier suppressed double sideband modulation system (optical DSB-SC modulation system)
FIG. 1 is a block diagram of an optical carrier suppressed double sideband modulation system of the present invention. As shown in FIG. 1, an optical carrier suppressed double sideband modulation system (1) according to a first embodiment of the present invention includes an optical carrier suppressed double sideband modulator (2) and the optical carrier suppressed double sideband. An optical modulator (3) including either or both of a phase modulator and an intensity modulator for modulating the output light of the modulator, a modulation signal of the optical carrier-suppressed double-sideband modulator, and the phase modulator Or a control unit (4) for adjusting the modulation time with the modulation signal of the intensity modulator. Each optical modulator may be connected by an optical waveguide such as an optical fiber, or may be capable of transmitting and receiving light in a vacuum or in the atmosphere. Each optical modulator has a signal source (5; 5a to 5c) for applying a modulation signal and the like so that the signal source is electrically connected to the control unit and can exchange information. It only has to be done.

  The control unit (4) is preferably configured to convert either or both of the double-sideband signals of the output light from the optical carrier-suppressed double-sideband modulator (2) to the phase modulator or the intensity modulator. Any one of the double-sideband signals generated by the modulation by the optical modulator (3) matches the frequency of the optical carrier signal or the higher-order optical signal in the output light of the optical carrier-suppressed double-sideband modulator, and the optical carrier Control to cancel signal or higher order optical signal.

  Note that the optical modulator (3) including both the phase modulator (PM) and the intensity modulator (IM) is a preferred embodiment of the present invention, and the control unit detects the optical signal (carrier signal) to be canceled. Or higher-order component signals) and site band signals to be used for canceling them, and grasping the phase after modulation by the intensity modulator of those signals, and the phase modulator is an optical modulator (3 ) Is controlled so that the phase after the modulation by () is opposite in phase. By doing so, the signal to be canceled after being modulated by the optical modulator (3) and the site band signal have the same frequency and the phases are opposite to each other, so that the signal to be canceled is canceled and weakened.

2. Basic Operation of Optical DSB-SC Modulation System Hereinafter, an operation example of the optical DSB-SC modulation system according to the first embodiment will be described with reference to the drawings. In this example, the optical modulator (3) includes a phase modulator (3a) and an intensity modulator (3b) in this order. Of course, the order of the phase modulator and the intensity modulator may be reversed, or only one of the intensity modulators may be used, for example. In particular, when the output signal of an optical DSB-SC modulated signal is modulated by an intensity modulator, the phase of the signal that you want to cancel is opposite to the phase of the optical signal that was used to cancel the signal. Does not require a phase modulator.

First, light is input to the optical DSB-SC modulator. This light is usually called an optical carrier wave (carrier signal), and is usually a single wavelength light. Let the frequency of this carrier signal be f ₀ [Hz]. The signal source (5a) applies a modulation signal such as a radio frequency signal to the optical DSB-SC modulator, so that the optical DSB-SC modulator modulates the carrier signal. The frequency (modulation frequency) of this modulation signal is assumed to be f _m [Hz]. FIG. 2 is a conceptual diagram showing an ideal optical DSB-SC modulated signal output from the optical DSB-SC modulator. As shown in FIG. 2, in the ideal optical DSB-SC modulation signal (11), the carrier signal is suppressed (that is, the signal of f ₀ [Hz] is eliminated), and the double sideband (site band) signal (12 ) Is output. The frequency of the site band signal (12) is f ₀ ± f _m [Hz]. Of the site band signals, the signal having the frequency of f ₀ + f _m [Hz] is the upper sideband (USB) signal (12a), and the signal having the frequency of f ₀ -f _m [Hz] is the lower side wave. This is the band (LSB) signal (12b).

FIG. 3 is a conceptual diagram showing an actual DSB-SC modulated signal output from the optical DSB-SC modulator. As shown in FIG. 3, the actual DSB-SC modulation signal (13) output from the optical DSB-SC modulator is not completely suppressed but remains in the remaining carrier signal component (14) and the site band signal (12). There is also a higher-order component signal (15) that is higher-order modulated. The center frequency of the higher-order component signal (15) is usually f ₀ ± nf _m [Hz] (n is an integer other than 1). When n is 2 or more, the higher the higher-order component signal (15) The strength of becomes smaller. Theoretically, the modulation signal from the optical DSB-SC modulator is composed of odd-order sidebands, and even-order sideband signals are suppressed, but the intensity of the third-order or higher-order optical signal is become weak. In the figure, 15a indicates the second order upper sideband signal, 15b indicates the second order lower sideband signal, 15c indicates the third order upper sideband signal, and 15d indicates the third order lower sideband signal. 15e indicates the 4th order upper sideband signal, 15f indicates the 4th order lower sideband signal, 15g indicates the 5th order upper sideband signal, 15h indicates the 5th order lower sideband signal Signals are shown.

  FIG. 4 is a conceptual diagram illustrating an example of a modulation signal output from the phase modulator. This example shows an example of canceling the carrier signal component (14) by using the site band of the primary sideband signal (site band). As shown in FIG. 4, the modulation signal (21) output from the phase modulator is either a site band signal (12), a carrier signal component (14), or a higher order component signal (15), or 2 One or more optical phases are modulated. Specifically, the phase modulator of the optical modulator (3) is an optical signal (carrier signal or higher-order component signal) to be canceled and a site band signal used for canceling it after modulation by the intensity modulator. The phase modulator grasps the phase, and controls so that the phase after the modulation by the optical modulator (3) of these signals becomes an opposite phase. In other words, in the example of FIG. 4, the phase of the carrier signal component (14) that cannot be fully suppressed and the site band signal (12) is originally shifted by π / 2. In response to the command, the voltage applied to the electrode is controlled so that the phase of the site band signal (12) is further shifted by π / 2, so the phase of the carrier signal component (14) and the site band signal (12) are shifted by π. (That is, the phase is reversed).

  FIG. 5 is a conceptual diagram for explaining a modulation signal output from the intensity modulator. FIG. 6 is a conceptual diagram showing a modulation signal output from the intensity modulator. As shown in FIG. 5, the modulation signal (31) output from the intensity modulator has the same frequency as the optical signal (carrier signal or higher-order component signal) to be canceled and the site band signal for canceling it. , The phase is reversed. Therefore, as shown in FIG. 6, the intensity of the signal to be canceled is reduced (ideally suppressed) in the modulation signal (31) output from the intensity modulator. In FIG. 5, 16a indicates the USB of the USB (12a), 16b indicates the LSB of the USB (12a), 17a indicates the USB of the LSB (12b), and 17b indicates the LSB of the LSB (12b). As shown in FIG. 5, in this example, the phase of the carrier signal component (14) and the site band signal (16b, 17a) of the site band signal (12) are shifted by π (that is, opposite in phase). Thus, the intensity of the carrier signal component (14) is reduced.

3.1. Optical DSB-SC Modulator FIG. 7 is a schematic diagram showing an example of an optical DSB-SC modulator. As shown in FIG. 7, the optical DSB-SC modulator is, for example, a first DC signal or a low frequency signal for controlling the phase of light propagating through two arms of a Mach-Zehnder waveguide (MZ) and MZ. And a first RF electrode (RF electrode) for inputting a radio frequency (RF) signal to the two arms constituting the MZ. The phase of light is controlled by a signal applied to the DC electrode, and frequency modulation is performed by a radio frequency signal applied to the RF electrode. In FIG. 7, the DC electrode and the RF electrode are conceptually drawn as separate ones, but may actually be configured as one electrode. When monochromatic light or the like is input to the MZ, the monochromatic light is separated into each arm by a branch path at the input point of the MZ, and is subjected to phase modulation or frequency modulation by the electrode on each arm, and the output point of the MZ is branched. They are combined on the road and output. At that time, optical signals having the same frequency (frequency) and the same phase strengthen each other, and signals having opposite phases cancel each other.

FIG. 8 is a schematic diagram showing another example of the optical DSB-SC modulator. As shown in FIG. 8, the optical DSB-SC modulator includes an MZ, a phase modulator (PM) provided in both arms of the MZ, and a fixed phase modulator provided in one of the MZ arms. An optical DSB-SC modulator having
The phase difference between the optical phase modulators can be controlled by the DC component of the electric field injected from the electrode. The phase difference of the RF signal input to the phase modulator (PM) can be controlled by adjusting the electric circuit configuration from the signal source. Usually, the two optical phase modulators are adjusted so as to be in opposite phases, and RF signals are input to both arms. The phase modulation parameter of the fixed phase modulator is usually set to π. The fixed phase modulator may be a phase modulator that applies a constant electric field to the optical waveguide, for example.

3.2. Phase modulator (PM)
An example of the phase modulator is one that can control the amount of phase modulation of an optical signal to be modulated by applying an electric field to a waveguide. Specifically, there are those provided with a waveguide and an electrode adapted to apply an electric field to the waveguide.

3.3. Intensity modulator (IM)
In the present invention, the intensity modulator, f ₀ the center frequency of essentially the input _signal, when the modulation frequency is f _m, the frequency of the main output _{signal, f 0, f 0 ± f} m (f 0 + f _m and _f 0 are those -f _m) and composed. Also it is intended intensity of the optical signal of frequency f ₀ is the strongest of these, f ₀ the intensity of 1 / 10-1 / 1 of one or both of the light intensity of the light intensity f ₀ of ± f _m Those having are preferred. Note that what outputs an output in which the f ₀ component is suppressed is called an optical DSB-SC modulator. That is, the optical DSB-SC modulator outputs a double-sideband optical signal and suppresses the frequency component f ₀ of the carrier signal.

  As the intensity modulator in the present invention, for example, the optical carrier suppression double sideband (DSB-SC) modulator described above may be used. The intensity modulator is a device for controlling the intensity (amplitude) of an optical signal propagating through a waveguide. A known variable optical attenuator (VOA) can be used as the intensity modulator. As the intensity modulator, a VOA element using LN may be used (for example, see Japanese Patent Laid-Open No. 10-142569). As a specific configuration of an optical DSB-SC modulator as an intensity modulator, for example, a metal thin film heater formed on a waveguide is used as a heat source to refract one arm waveguide of a Mach-Zehnder waveguide by a thermo-optic effect. There is one that causes a rate change and adjusts the output intensity of the interferometer (see, for example, JP 2000-352699 A). As an optical DSB-SC modulator as an intensity modulator, an electric signal provided with a signal source and a phase adjuster for adjusting the phase of a signal output from the signal source, and applied to both arms of the Mach-Zehnder waveguide For example, the phase is adjusted so that the phase of the phase is different by 180 degrees. Since the phases of the electrical signals applied to both arms are 180 degrees different, an optical DSB-SC signal can be output. In the optical communication system of the present invention, the modulation frequency fm of the optical modulator is applied to the optical DSB-SC modulator.

3.4. Control device (PC)
The control device is configured to detect either of the double sideband signals of the output light from the optical carrier suppression double sideband modulator or any of the double sideband signals generated by modulating the phase modulator or the intensity modulator. In the output light of the optical carrier-suppressed double-sideband modulator, the output light coincides with the frequency of the optical carrier signal or the higher-order optical signal and is controlled so as to cancel the optical carrier signal or the higher-order optical signal. In order to control in this way, the control unit of the present invention is connected to, for example, the signal source of each optical modulator, and controls the timing at which the modulation signal of each optical modulator is applied.

  A computer as a control unit includes, for example, a calculation unit such as a CPU, a memory, a main memory, an input / output unit (I / O), and a display. The CPU, memory, I / O, and display are connected to the system bus so that data can be transferred between them. When predetermined information is input to the computer from the input unit, the program in the main memory is read in accordance with the command, and the CPU or the like receives the command of the program in the main memory and issues a processing command as appropriate. The control signal may be output from the output unit via the system I / F.

  The input / output unit of the computer is electrically connected to each signal source, and each signal source outputs a signal having a predetermined voltage at a predetermined timing in accordance with a command from the computer.

As a specific control method, for example, a primary sideband signal is used to suppress a carrier signal, and the primary sideband signal and the carrier signal are shifted in phase by π / 2. Will be described. The signals input to the optical modulators are synchronized by the control unit, and a predetermined signal is modulated. The control unit outputs a command for shifting the phase of the primary sideband signal by π / 2 to the phase modulator. The phase modulator receives the output and adjusts the electric field applied to the waveguide so that the phase of the primary sideband signal is shifted by π / 2 (that is, to cancel the signal to be canceled). Is controlled so that the phase of the signal used for the phase is opposite (the phase is shifted by π). Such a signal is input to the intensity modulator. The optical modulator generates a site band signal in which the frequency of the primary sideband signal is shifted by f _m [Hz], and the frequency of the site band of the signal used for cancellation and the signal ( The frequency of the carrier signal) matches. Therefore, when these lights are combined, they function to cancel the intensity of the signal to be canceled.

  As shown in the figure, it is preferable to use signals branched from the same signal source as the modulation signals of the phase modulator and the intensity modulator. Furthermore, a modulation signal source for the optical carrier suppression double sideband modulator (2) and one signal source as the modulation signal source of the phase modulator and the intensity modulator are used, and the signal output from the signal source is branched. What is used is preferable. When the signal source for the optical carrier suppression double-sideband modulator (2) is separated from the signal source for the optical modulator (3) and these signal sources are controlled separately, the frequency of all signal sources is Although it is difficult to perfectly match, if the optical carrier-suppressed double sideband modulator (2), phase modulator, and intensity modulator are output from one signal source and controlled by a branched electrical signal, the signal Because the modulation frequency of each modulator brought about by the same is easily matched, it is possible to easily adjust the canceling action of the final output light by controlling the intensity and phase of the modulation signal supplied to each modulator. Become.

4). Manufacturing Method Each optical modulator basically includes a waveguide and an electrode, and includes, for example, a Mach-Zehnder waveguide (MZ) including a light input portion and a modulated light output portion. Things can be raised. The Mach-Zehnder waveguide includes, for example, a substantially hexagonal waveguide (which forms two arms) and includes two phase modulators arranged in parallel.

Usually, the Mach-Zehnder waveguide and the electrode are provided on the substrate. The substrate and each waveguide are not particularly limited as long as they can propagate light. For example, a Ti-diffusion lithium niobate waveguide may be formed on an LN substrate, or a silicon dioxide (SiO ₂ ) waveguide may be formed on a silicon (Si) substrate. Alternatively, an optical semiconductor waveguide in which an InGaAsP or GaAlAs waveguide is formed on an InP or GaAs substrate may be used. As the substrate, lithium niobate (LiNbO ₃ : LN) cut out so as to achieve X-cut Z-axis propagation is preferable. This is because a large electro-optic effect can be used, so that low power driving is possible and an excellent response speed can be obtained. An optical waveguide is formed on the surface of the X cut surface (YZ surface) of the substrate, and the guided light propagates along the Z axis (optical axis). A lithium niobate substrate other than the X-cut may be used. Further, as the substrate, a triaxial or hexagonal uniaxial crystal having an electro-optic effect, or a material whose crystal point group is C _3V , C ₃ , D ₃ , C _3h , D _3h can be used. . These materials have a function of adjusting the refractive index so that the change in refractive index is different depending on the mode of propagating light by applying an electric field. Specific examples include lithium tantalate (LiTO ₃ : LT), β-BaB ₂ O ₄ (abbreviation BBO), LiIO _{3 and the} like in addition to lithium niobate.

  The size of the substrate is not particularly limited as long as a predetermined waveguide can be formed. The width, length, and depth of each waveguide are not particularly limited as long as the module of the present invention can exert its function. The width of each waveguide is, for example, about 1 to 20 micrometers, preferably about 5 to 10 micrometers. Further, the depth (thickness) of the waveguide is 10 nm to 1 micrometer, and preferably 50 nm to 200 nm.

  The first bias adjustment electrode (DC electrode) is an electrode for controlling the phase of light propagating through the two arms of the MZ by controlling the bias voltage between the two arms constituting the MZ. The DC electrode is preferably a direct current or low frequency electrode. Here, “low frequency” in the low frequency electrode means, for example, a frequency of 0 Hz to 500 MHz. The output of this signal source is preferably provided with a phase modulator so that the phase of the output signal can be controlled.

  The modulation electrode (RF electrode) is an electrode for inputting a radio frequency (RF) signal as a modulation signal to the two arms constituting the MZ. As the RF electrode, a traveling wave type electrode or a resonance type electrode can be mentioned, and a resonance type electrode is preferable.

  As described above, the DC electrode and the RF electrode may be separate electrodes, or one electrode may perform their functions. In the latter case, a bias voltage and a radio frequency signal are applied to one electrode.

The RF electrode is preferably connected to a high frequency electrical signal source. The high-frequency electric signal source is a device for controlling a signal transmitted to the RF electrode, and a known high-frequency electric signal source can be adopted. RF electrodes, and as the frequency _{(f m)} of the high frequency signal input to the RF electrodes, for example 1GHz~100GHz the like. As an output of the high frequency electric signal source, a sine wave having a constant frequency can be mentioned. It is preferable that a phase modulator is connected to the output of the high frequency electric signal source so that the phase of the output signal can be controlled.

The RF electrode is made of, for example, gold or platinum. Examples of the width of the RF electrode include 1 μm to 10 μm, specifically 5 μm. Examples of the length of the RF electrode include 0.1 to 0.9 times the wavelength (f _m ) of the modulation signal, 0.18 to 0.22 times, or 0.67 to 0.70 times, and more preferably, the resonance of the modulation signal. 20-25% shorter than the point. This is because, by using such a length, the combined impedance with the stub electrode remains in an appropriate region. A more specific RF electrode length is 3250 μm. Hereinafter, the resonant electrode and the traveling wave electrode will be described.

  A resonance type photoelectrode (resonance type optical modulator) is an electrode that performs modulation using resonance of a modulation signal. Known electrodes can be used as the resonance type electrodes, for example, Japanese Patent Laid-Open No. 2002-268025, “Tetsuya Kawanishi, Satoshi Oikawa, Masayuki Izutsu”, “Plane Resonance Type Optical Modulator”, IEICE Technical Report, Technical Report of IEICE. , IQE2001-3 (2001-05) ”.

  A traveling wave type electrode (traveling wave type optical modulator) is an electrode (modulator) that modulates light while guiding and guiding light waves and electrical signals in the same direction (for example, Hiroshi Nishihara, Haruna) Masamitsu, Toshiaki Sugawara, “Optical Integrated Circuits” (Revised Supplement), Ohmsha, pp. 119-120). As the traveling wave type electrode, known ones can be adopted. For example, JP-A-11-295674, JP-A-11-295674, JP-A-2002-169133, JP-A-2002-40381, JP-A-2000- Those disclosed in Japanese Patent No. 267056, Japanese Patent Laid-Open No. 2000-471159, Japanese Patent Laid-Open No. 10-133159, and the like can be used.

  As the traveling wave type electrode, a so-called symmetrical ground electrode arrangement (in which at least a pair of ground electrodes are provided on both sides of the traveling wave type signal electrode) is preferably adopted. Thus, by arranging the ground electrodes symmetrically across the signal electrode, the high frequency output from the signal electrode is easily applied to the ground electrodes arranged on the left and right sides of the signal electrode. Can be suppressed.

  The RF electrode may serve as both an RF signal electrode and a DC signal electrode. That is, either or both of the RF electrode and the RF electrode are connected to a power supply circuit (bias circuit) that supplies a mixture of a DC signal and an RF signal.

  As an optical waveguide formation method, a known formation method such as an internal diffusion method such as a titanium diffusion method or a proton exchange method can be used. That is, the optical modulator of the present invention can be manufactured, for example, as follows. First, titanium is patterned on a lithium niobate wafer by photolithography, and titanium is diffused by thermal diffusion to form an optical waveguide. The conditions at this time may be that the thickness of titanium is 100 to 2000 angstroms, the diffusion temperature is 500 to 2000 ° C., and the diffusion time is 10 to 40 hours. An insulating buffer layer (thickness 0.5-2 μm) of silicon dioxide is formed on the main surface of the substrate. Next, an electrode made of metal plating having a thickness of 15 to 30 μm is formed thereon. The wafer is then cut. In this way, an optical modulator having a titanium diffusion waveguide is formed.

  The electrode can be manufactured in the same manner as described above. For example, in order to form electrodes, the gap between the electrodes is set to about 1 to 50 micrometers with respect to both sides of many waveguides formed with the same width by photolithography technology in the same manner as the formation of the optical waveguide. Can be formed.

When a silicon substrate is used, for example, the waveguide and the electrode can be manufactured as follows. A lower cladding layer mainly composed of silicon dioxide (SiO ₂ ) is deposited on a silicon (Si) substrate by flame deposition, and then silicon dioxide (SiO ₂ ) doped with germanium dioxide (GeO ₂ ) as a dopant is deposited. A core layer as a main component is deposited. After that, it is made into transparent glass in an electric furnace. Next, an optical waveguide portion is fabricated by etching, and an upper clad layer mainly composed of silicon dioxide (SiO ₂ ) is deposited again. Then, a thin film heater type thermo-optic intensity modulator and a thin film heater type thermo-optic phase modulator are formed on the upper cladding layer.

  The present invention can be suitably used in fields such as optical information communication.

FIG. 1 is a block diagram of an optical carrier suppressed double sideband modulation system of the present invention. FIG. 2 is a conceptual diagram showing an ideal optical DSB-SC modulated signal output from the optical DSB-SC modulator. FIG. 3 is a conceptual diagram showing an actual DSB-SC modulated signal output from the optical DSB-SC modulator. FIG. 4 is a conceptual diagram illustrating an example of a modulation signal output from the phase modulator. FIG. 5 is a conceptual diagram for explaining a modulation signal output from the intensity modulator. FIG. 6 is a conceptual diagram showing a modulation signal output from the intensity modulator. FIG. 7 is a schematic diagram illustrating an example of an optical DSB-SC modulator. FIG. 8 is a schematic diagram showing another example of the optical DSB-SC modulator.

Explanation of symbols

1 Optical carrier suppressed double sideband modulation system
2 Optical carrier suppressed double sideband modulator
3 Optical modulator; 3a phase modulator; 3b intensity modulator
4 Control unit
5 Signal source; 5a ~ 5c

Claims (1)

Optical carrier suppressed double sideband modulator (2) ,
A phase modulator (3a) and an intensity modulator (3b) for modulating the output light of the optical carrier-suppressed double sideband modulator (2) ;
A control unit (4) for controlling modulation signals applied to the optical carrier suppressed double sideband modulator (2), the phase modulator (3a) and the intensity modulator (3b) ,
The phase modulator (3a) includes a first lower sideband signal and a first upper sideband signal which are first double sideband signals included in the output light from the optical carrier suppressed double sideband modulator (2). Adjust the phase of either or both of the waveband signals,
The intensity modulator (3b) includes a second double side wave derived from one or both of the first lower sideband signal and the first upper sideband signal whose phase is adjusted by the phase modulator (3a). A band signal,
The control unit (4)
Controlling the application timing of modulation signals applied to the optical carrier suppressed double sideband modulator (2), the phase modulator (3a) and the intensity modulator (3b), respectively;
The frequency of one of the second lower sideband signal and the second upper sideband signal included in the second double sideband signal is set to an optical carrier wave of the output light of the optical carrier suppression double sideband modulator. Match the frequency of the signal or higher-order optical signal,
The phase of one of the second lower sideband signal and the second upper sideband signal included in the second double sideband signal is an optical carrier of the output light of the optical carrier suppression double sideband modulator. Adjust the signal or the higher-order optical signal so that it is opposite in phase,
Thus, either the second lower sideband signal or the second upper sideband signal included in the second double sideband signal is included in the output light of the optical carrier suppressed double sideband modulator. Optical carrier- suppressed double-sideband modulation system that controls to cancel optical carrier signals or higher-order optical signals .