JP6032699B2 - Optoelectric oscillator by chirped light modulation and optoelectric oscillation method - Google Patents

Description

  The present invention relates to an optoelectric oscillator and an optoelectric oscillation method. More specifically, the present invention relates to a feedback type photoelectric oscillator composed of a loop including a light source, an optical modulator, and a photodetector. In the present invention, an optical signal from a light source is modulated by an optical modulator, the optical signal is converted into an electrical signal by a photodetector, and then the electrical signal is fed back to the modulator as a modulation signal.

  In order to perform advanced optical signal processing in a transmitter or a receiver included in an optical communication system or an optical wireless communication system, a local oscillator is required. This is because a function for generating a carrier of an optical signal or an optical wireless signal is required.

  An optoelectric oscillator is a device that oscillates an optical signal or an electrical signal having a predetermined frequency when an optical signal is input, and an oscillation operation is obtained by feeding back (modulating) a modulation component of the optical signal. Since the feedback loop of the oscillator is composed of an optical circuit and an RF (high frequency) circuit, an optical signal having the same frequency and an RF signal can be simultaneously obtained as an oscillation signal during an oscillation operation. It is also well known that the oscillation frequency can be controlled by injection locking operation for both optical signals and RF signals.

  As an optoelectric oscillator having a feedback circuit, for example, a technique disclosed in Patent Document 1 is known. The photoelectric oscillator disclosed in Patent Document 1 includes an optical modulator that modulates an optical signal output from a light source, a photodetector that receives an optical signal from the optical modulator and converts the optical signal into an electrical signal, and optical detection. A frequency divider for dividing the electric signal from the optical device, a lead wire for inputting the frequency-divided electric signal to the modulation electrode of the optical modulator as a modulation signal, and a bias power source for controlling the bias point of the optical modulator doing. The invention of Patent Document 1 divides a feedback signal that does not include a modulation frequency component when generating an optical two-tone signal in an optoelectric oscillator under an arbitrary bias condition, particularly when the bias point is set to zero. By generating a modulation frequency component, oscillation under an arbitrary bias condition can be performed.

JP-A-2005-236539

  The invention described in Patent Document 1 is also a useful technique. However, if an optoelectric oscillator can be realized with a lower cost configuration, this technology will become more widespread. For example, in the invention of Patent Document 1, a frequency divider is required to obtain a modulation frequency that is fed back to the optical modulator. In this regard, there is a demand for realizing an optoelectric oscillator without using this frequency divider.

  Therefore, an object of the present invention is to provide an optoelectric oscillator and an optoelectric oscillation method that can be realized with a lower cost configuration.

The inventor of the present invention intensively studied the means for solving the above problems, and as a result, optical modulation was performed using an optical modulator capable of outputting an optical signal accompanied with chirp, and the output signal from the optical modulator was if input via a dispersion medium to the photodetector, will be able to detect the f _m components required for the optical-electrical oscillation from the chirp components, fed back to the optical modulator of the detected f _m component as a modulation signal The knowledge that it was possible was acquired. With such a configuration, it is not necessary to use a frequency divider in the photoelectric oscillator, so that the cost can be reduced. Then, the present inventor has come up with the idea that the above problem can be solved based on the above knowledge, and has completed the present invention.
More specifically, the present invention has the following configuration.

The first aspect of the present invention relates to an optoelectric oscillator.
The photoelectric oscillator of the present invention includes an optical modulator 20, a dispersion medium 40, a photodetector 50, and a conductive wire 80.
The optical modulator 20 includes a modulation electrode to which a modulation signal having a modulation frequency f _m [Hz] is applied, modulates the optical signal output from the light source 10 with the modulation signal, and converts the optical signal accompanied with the chirp. Output.
The dispersion medium 40 receives an optical signal modulated by the optical modulator 20.
Photodetector 50 receives light signals from the dispersion medium 40, converts the modulated component at the modulation frequency f _m into an electric signal.
Conductor 80, an electric signal converted by the photodetector 50 is input to the modulation electrode of the optical modulator 20 as a modulation signal of a modulation frequency f _m.

  In the photoelectric oscillator of the present invention, the bias point of the optical modulator 20 is preferably a bias point where the phase difference of the optical signal guided through the optical modulator 20 is π.

  In the optoelectric oscillator of the present invention, the optical modulator 20 is preferably a Mach-Zehnder optical modulator.

  In the optoelectric oscillator of the present invention, it is preferable that the dispersion medium 40 is an optical fiber and / or a fiber Bragg grating.

Optoelectronic oscillator of the present invention, the conductor 80 preferably has a band-pass filter 60 for selecting the electrical signals of a predetermined area including the modulation frequency f _m which is used as a modulation signal.

The second aspect of the present invention relates to a photoelectric transmission method.
The photoelectric transmission method of the present invention comprises:
Applying a modulation signal having a modulation frequency f _m [Hz] to the modulation electrode of the optical modulator 20, modulating the optical signal output from the light source 10, and outputting an optical signal with chirp;
Inputting the optical signal modulated by the optical modulator 20 into the dispersion medium 40;
The optical signal from the dispersion medium 40 and received by a photodetector 50, a step of converting the modulated component at the modulation frequency f _m into an electric signal,
The electric signal converted by the photodetector 50, and a step of inputting the modulation electrode of the optical modulator 20 via a line 80 as a modulation signal of a modulation frequency f _m, the.

In general, when the modulated optical signal by setting the bias point of the optical modulator chirp does not occur to zero, the output optical signal, 2f _m component is doubled modulation frequency f _m is included mainly . However, this output optical signal has almost no _fm component that needs to be applied to the optical modulator in the photoelectric oscillator. This, f _m component is the modulation frequency of the optical modulator is highly dependent on the bias condition, especially when the optical modulator is biased at around zero bias point, because the resulting almost disappeared. Therefore, using a Mach-Zehnder type optical modulator chirp does not occur, when obtaining a feedback signal required to light the electric oscillation (f _m component), as in the invention described in Patent Document 1, frequency divider for dividing (division 2 min) into an electric signal of a frequency f _m of the electric signal of frequency 2f _m is required.

In this regard, in the present invention, the light modulator has a generation configurable chirp component, in the chirp component in the output signal of the optical modulator, f _m components required to light the electric oscillation Will be included. Then, the f _m component in the chirp component (phase modulation component), through a dispersive medium disposed downstream of the optical modulator is converted to intensity modulation component, if so can be detected in the light detector, the feedback signal required to light the electric oscillation (f _m component) can be obtained appropriately. The _fm component contained in the optical signal is converted into an electrical signal by the photodetector and applied to the optical modulator. In this way, the present invention can realize photoelectric oscillation with an inexpensive configuration.

FIG. 1 is a schematic configuration diagram of an optoelectric oscillator according to an embodiment of the present invention. FIG. 2 is a schematic diagram for explaining the principle of the photoelectric oscillation method according to an embodiment of the present invention.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The present invention is not limited to the embodiments described below, but includes those appropriately modified by those skilled in the art from the following embodiments.

(1. Photoelectric oscillator)
FIG. 1 is a schematic configuration diagram of an optoelectric oscillator 100 according to an embodiment of the present invention. As shown in FIG. 1, the photoelectric oscillator 100 according to this embodiment includes a light source 10, an optical modulator 20, an optional optical amplifier 30, a dispersion medium 40, a photodetector 50, and an arbitrary band. A pass filter 60, an arbitrary RF amplifier 70, a conducting wire 80, and an optional optical output waveguide 90 are provided.

An optical signal from the light source 10 is input to a Mach-Zehnder optical modulator 20. The optical modulator 20 has a modulation electrode 27 modulated signal of the modulation frequency f _m is applied, the bias power source 29. The optical modulator 20, together with the bias point is controlled by the bias power source 29, driven by the modulation signal of the modulation frequency f _m applied to the modulating electrode 27 to modulate the optical signal output from the light source 10. Further, the optical modulator 20 has a configuration for outputting an optical signal accompanied with chirp (phase modulation component generated along with intensity modulation). In this chirp component output from the optical modulator 20 includes a frequency f _m component. The output signal accompanied with the chirp from the optical modulator 20 is amplified by an arbitrary optical amplifier 30 and enters the dispersion medium 40. The dispersion medium 40, an optical signal including a modulated component at the modulation frequency f _m by the optical modulator 20 is input. Dispersion medium 40 converts the modulated component at the frequency f _m in the phase modulation component included in the input optical signal into an intensity modulation component. Photodetector 50 receives the light signal passing through the dispersive medium 40, converts the modulated component at the modulation frequency f _m into an electric signal. The electric signal converted by the photodetector 50 is inputted by any of the band-pass filter 60, an electrical signal of a predetermined region including the modulation frequency f _m which is used as the modulation signal is selected. Electrical signal through the band-pass filter 60 is amplified by any of the RF amplifier 70, is applied to the modulation electrodes 27 of the optical modulator 20 via a line 80, it is fed back as a modulation signal having a frequency f _m. Further, the bias power source 29 of the optical modulator 20 controls the bias point of the optical modulator 20 so that the phase difference of the optical signal guided through the optical modulator 20 becomes π, so that the optical modulator 20 is second harmonic signal having a frequency twice the frequency f _m of the modulation signal (signal of a frequency 2f _m) are generated with a chirp. Therefore, the second harmonic signal can be obtained by demultiplexing the second harmonic signal with an optical coupler or the like and inputting it to the optical output waveguide 90. The second harmonic signal obtained by the optoelectric oscillator 100 can be used for a low-cost device made for a low frequency, for example.

In photoelectric oscillator according to the present embodiment, by employing the in the optical modulator 20 for example Mach-Zehnder modulator causing a predetermined chirp, from the light modulator 20, light including modulated component at the frequency f _m A signal is output. When the output signal from the optical modulator 20 enters the dispersion medium 40, the phase modulation component is converted into an intensity modulation component due to the wavelength dispersion effect of the dispersion medium 40. Modulation component of the frequency f _m, since those that occur as a chirp component (phase modulation component), the modulation component of the frequency f _m is converted into intensity modulation component by the dispersion medium 40. Then, the intensity modulation component of the output light intensity in the optical signal passed through the dispersion medium 40 by detecting by the photodetector 50, it is possible to obtain an electrical signal of frequency f _m. Electrical signal converted frequency f _m to the optical detector 50, via a line 80, is fed back to the modulation electrodes 21 of the optical modulator 20 is used as a modulation signal. In this way, photoelectric oscillation can be performed with a relatively inexpensive configuration. By adopting the modulator with chirp as an optical modulator 20, when converted into electrical signals, can be components of the frequency f _m is well-suppressed signal purity obtain a high optical signal.

  In the photoelectric oscillator 100 according to the present embodiment, the bias power source 21 is a constituent device that may be installed as appropriate. This is because the photoelectric oscillator according to the present embodiment can oscillate regardless of the bias voltage, as will be described later. In recent years, an optical modulator in which a bias point is designed in advance to eliminate the need for a bias voltage has been developed, and such an optical modulator in which the bias point is fixed in advance may be applied. Further, a temperature regulator for adjusting the temperature may be installed in the optical modulator, and the bias point may be controlled by the temperature of the optical modulator. As the temperature regulator, a known one can be used, and for example, it can be controlled by a temperature regulator such as a thin film heater.

(1.1. Light source)
A known light source can be used as the light source 10 of the photoelectric oscillator 100. A preferred light source 10 is a diode, a laser diode, or the like.

  An example of a light source is one that outputs a pseudo-random signal. As the pseudo-random signal, those described in JP-A-5-45250, JP-A-7-218353, JP-A-2003-50410, and the like can be used. If a pseudo-random signal is used, a signal having various characteristics can be generated. Another preferable aspect of the light source is one that outputs optical signals arranged with periodicity. A pulse signal is an example of an optical signal arranged with periodicity. A continuous light source may also be used.

  Examples of the wavelength of light output from the light source include 1200 nm to 1900 nm, preferably 1300 nm to 1800 nm, more preferably 1400 nm to 1700 nm, and 1500 nm to 1600 nm. The intensity of light output from the light source is 0.1 mW or more, preferably 1 mW or more, and more preferably 10 mW or more. Specific examples of the light source include HP 8166A and 81589A manufactured by Agilent.

(1.2. Optical modulator)
As the optical modulator 20 of the photoelectric oscillator 100, an optical modulator capable of controlling chirp relating to optical modulation is used. In the present embodiment, the optical modulator 20 is a device for modulating at least one of light intensity, light frequency, and light phase. As such an optical modulator, an intensity modulator, frequency modulation, and the like are used. And phase modulators. However, in the present invention, it is preferable to employ an intensity modulator that can efficiently generate a chirp component as the optical modulator.

  As such an optical modulator 20, it is preferable to employ a Mach-Zehnder type optical modulator. In the embodiment shown in FIG. 1, the Mach-Zehnder type optical modulator 20 is branched by a light input section 21, a branch section 22 where light input to the input section 21 branches, and a branch section 22. From the first waveguide 23 through which light propagates, the second waveguide 24 through which light different from the above branched by the branching section 22 propagates, the first waveguide 23 and the second waveguide 24 A multiplexing unit 25 that combines the output optical signals, an optical signal output unit 26 that outputs the optical signals combined by the combining unit 25, and a modulation signal that drives the first waveguide 23. Applied to the first modulation electrode 27, the second modulation electrode 28 to which the modulation signal for driving the second waveguide 24 is applied, the first waveguide 23 and the second waveguide 24. And a power supply 29 for obtaining a bias signal to be used. In the present embodiment, an electric signal supplied from the power source 29 can be applied to the second modulation electrode 28, and this electric signal can be used as a modulation signal. However, a power source for obtaining a bias signal and a power source for obtaining a modulation signal applied to the second modulation electrode 28 can be provided separately.

In the photoelectric oscillator 100 of the present invention, an optical modulator 20 that can generate an optical signal accompanied with a chirp is employed. Chirp is a phase modulation component generated with intensity modulation. The chirp parameter α representing the chirp amount represents how much the phase difference modulation component is superimposed when intensity modulation is performed in the optical modulator, and is represented by the following [Equation 1].
α = (dφ / dt) / (1 / 2S · dS / dt)... [Formula 1]
S represents the light intensity, φ represents the amount of phase change, and t represents time.

In the case of a Mach-Zehnder type optical modulator as shown in FIG. 1, the chirp amount is obtained by using the phase change amounts φ1 and φ2 of the branching waveguides 23 and 24 and the chirp parameter α as shown in [Equation 2]. Can be represented.
α = (φ1 + φ2) / (φ1-φ2)... [Formula 2]

Methods for controlling chirp in an optical modulator are known. For example, LiNbO ₃ (hereinafter referred to as “LN”) is used as a substrate of an optical modulator having an electro-optic effect, an optical waveguide as shown in FIG. 1 is formed using the LN substrate, and two branching waveguides 23 are formed. , 24 are arranged symmetrically with respect to the signal electrode and the ground electrode, the electric field applied to each branch waveguide 23, 24 has the same strength and the opposite direction, so the phase of each branch waveguide A relationship of φ1 = −φ2 is established for the change amount. In this case, the value of the chirp parameter α is zero and no chirp is generated. On the other hand, if you place the two branching waveguides 23 and 24 with respect to the signal and ground electrodes on the left and right non-symmetrical, because the strength of the electric field applied to the branching waveguides 23 and 24 are different, the branching waveguides 23 , 24 has a relationship of φ1 ≠ −φ2, and chirp is generated by the non-uniformity of the applied electric field. As described above, in order to generate an optical signal with a chirp in the optical modulator, the optical circuit and the modulation electrode need only be asymmetric.

  In order to generate an optical signal with chirp in the optical modulator, in the Mach-Zehnder type optical modulator, the first modulation electrode 27 provided in the first waveguide 23 or the second waveguide is provided. The modulation signal may be applied to only one of the second modulation electrodes 28 provided in the waveguide 24. In the present embodiment, a conductor 80 is connected to the first modulation electrode 27, and a modulation signal fed back by a feedback circuit is applied. For this reason, chirp can be generated by not applying a modulation signal to the second modulation electrode 28. In such a case, since the phase change amount φ2 in the second waveguide 24 expressed by the above equation 2 is 0, the value of the chirp parameter α is 1. Thus, it is preferable to adjust so that the chirp parameter α of the optical modulator becomes 1.

  In addition, in order to generate an optical signal with a chirp in the optical modulator, the thickness of the substrate in the region where the two branch waveguides of the Mach-Zehnder optical waveguide are formed is the same, By adjusting the dielectric constant and thickness of the arranged dielectric, the distribution of the electric field applied to the two branch waveguides can be made different to generate chirp. Further, the thickness of the substrate in the region where the two branch waveguides of the Mach-Zehnder type optical waveguide are formed is the same, and a member having a higher refractive index (dielectric constant) than the adhesive layer on the back side of the substrate or a conductive material is used. By arranging the members, it is possible to generate chirp by differentiating the distribution of the electric field applied to the two branch waveguides.

  When an intensity modulator is employed as the optical modulator 20, examples of the intensity modulator include an optical single sideband modulator (optical SSB modulator), an optical frequency shift keying modulator (optical FSK modulator), or an optical carrier wave. A modulator such as a suppressed double sideband modulator (optical DSB-SC modulator) can 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. 10-14269).

Intensity modulator is basically the center frequency of the input signal and _{f 0,} when the modulation frequency is _{f m,} the output _{signal, f 0 ± f m (f} 0 + f m and _f 0 _-f m) Frequency components. Among those that output an output signal having such a frequency, those that output particularly those in which the f ₀ component is suppressed are called optical DSB-SC modulators. 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 a specific configuration of the optical DSB-SC modulator, for example, a metal thin film heater formed on the waveguide is used as a heat source to cause a refractive index change in one arm waveguide of the Mach-Zehnder waveguide by a thermo-optic effect, Examples include adjusting the output intensity of the interferometer (see, for example, Japanese Patent Laid-Open No. 2000-352699). As an optical DSB-SC modulator, a signal source and a phase adjuster for adjusting the phase of a signal output from the signal source are provided, and a phase difference between electrical signals applied to both arms of the Mach-Zehnder waveguide is, for example, Examples are those adjusted to be π (180 degrees). The optical DSB-SC signal can be output when the phase difference between the electrical signals applied to both arms is π.

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 InGaP or GaAlAs waveguide is formed on an InP or GaAs substrate may be used. As the substrate, lithium niobate (LiNbO ₃ : LN) cut out 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. In addition, a uniaxial crystal such as a trigonal crystal system and a hexagonal crystal system 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 as the substrate. . 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 when an electric field is applied. Specific examples include lithium tantalate (LiTaO ₃ : LT), β-Ba 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 modulation electrodes 27 and 28 are made of, for example, gold or platinum. The width of these electrodes is 1 μm to 10 μm, specifically 5 μm. The lengths of these electrodes include 0.1 to 0.9 times the wavelength of the modulation signal, 0.18 to 0.22 times, or 0.67 to 0.70 times, More preferably, it is 20 to 25% shorter than the resonance point of the modulation signal. This is because, by using such a length, the combined impedance with the stub electrode remains in an appropriate region. A more specific length of these electrodes is 3250 μm. In order to generate a chirp component by the optical modulator, the modulation electrodes 27 and 28 may be provided with asymmetry. For example, the length and width of the electrodes may be varied.

In the present embodiment, as the first modulation electrode 27 to which the modulation signal fed back by the feedback circuit is applied, a traveling wave electrode or a resonance electrode can be used, but it is preferable to use a resonance electrode. . A resonance type photoelectrode (resonance type optical modulator) is an electrode that performs modulation using resonance of a modulation signal. As the resonant electrode, a known electrode can be used. For example, Japanese Patent Application Laid-Open No. 2002-268025, “Tetsuya Kawanishi, Satoshi Oikawa, Masayuki Izutsu”, “Plane Resonant Optical Modulator”, IEICE Technical Report, TECHNICAL REPORT OF IEICE. , IQE2001-3 (2001-05) ”. By employing the resonance type optical electrode as the first modulation electrode, it can be obtained efficiently in response to characteristics only the frequency f _m component.

  The power source 29 is a power source for applying a bias to the optical modulator 20, and may be an AC bias power source in addition to a DC bias power source. The power source 29 may be any power source that can control the optical signal guided through the waveguides 23 and 24 of the optical modulator 20 to an arbitrary phase difference. The bias (zero bias point) of the phase difference π (pi). It is preferable to use a power supply that provides

The Mach-Zehnder type optical modulator is an optical modulator in which an optical waveguide is formed with a Mach-Zehnder interferometer structure, and converts a phase modulation signal obtained by an electro-optic effect into an intensity modulation signal, on a LiNbO ₃ substrate. The formed optical modulator is called a Mach-Zehnder LN optical modulator. One of the features of the Mach-Zehnder LN optical modulator is that various optical signals can be obtained by appropriately controlling the bias point and the drive electric signal. That is, in the bias point of the phase difference of [pi (pi) of the first waveguide and the optical signal guided through the second waveguide branch (zero bias point), introducing a sinusoidal driving signal of a frequency f _m Then, it is possible to obtain an optical signal of frequency 2f _m.

However, as in the prior art, when the modulated optical signal by setting the bias point of the Mach-Zehnder optical modulator chirp does not occur to zero, the output optical signal is a doubling of the modulation frequency f _m although 2f _m component is included primarily required f _m component as the modulation frequency of the Mach-Zehnder optical modulator will almost disappear. This is f _m component is the modulation frequency of the optical modulator, greatly depends on the bias conditions, particularly when the optical modulator is biased at around zero bias point, since become almost disappears, mainly in the output signal 2f _m component is because the remains. Therefore, the use of Mach-Zehnder type optical modulator chirp does not occur, it was not possible to obtain a feedback signal required to light the electric oscillation (f _m component). In this regard, in the present invention, by Mach-Zehnder optical modulator to generate a chirp component, and may include the f _m components required to light the electric oscillation in the chirp components. Therefore, as described later, the f _m component in chirp component (phase modulation component), converted into an intensity-modulated component through the dispersion medium disposed downstream of the Mach-Zehnder optical modulator, the optical detector if so as to detect a feedback signal (f _m component) required for the photoelectric oscillation can be obtained.

  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 FSK modulator can be manufactured as follows, for example. 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.

(1.3. Dispersion medium)
An optical signal that is modulated by the optical modulator 20 and has a chirp (a phase modulation component generated in accordance with intensity modulation) enters the dispersion medium 40. The dispersion medium 40 converts the phase modulation component into an intensity modulation component by the wavelength dispersion effect.

  As the dispersion medium 40, a known medium having a property of dispersing predetermined light among the input light can be used. For example, an optical fiber, a fiber Bragg grating, a photonic crystal fiber, a liquid crystal spatial light phase modulator And an AWG type optical phase modulator.

  In the present invention, phase noise can be reduced by using an optical fiber whose dispersion amount and length can be adjusted as necessary as a dispersion medium. In other words, the phase noise can be reduced as the length of the oscillator is increased. Therefore, by using an optical fiber for the photoelectric oscillator, a long oscillator can be obtained with a small loss. In this way, by using an optical fiber as the dispersion medium, it is possible to give the dispersion medium two roles: the role of a peripheral and the gain of the oscillator length. As the optical fiber, a single mode fiber (SMF), a dispersion shifted fiber (DSF), or a dispersion compensating fiber (DCF) can be adopted depending on the required dispersion amount and length. As shown in FIG. 1, when an optical fiber is used as a dispersion medium, a loop can be formed by this optical fiber. The number of loops formed by the optical fiber as the dispersion medium may be one or more. Also, if the length of the optical fiber as the dispersion medium is too long, the number of modes that can oscillate in the loop increases so that the problem of lack of stability for a long time may occur. Adjustment may be made by forming a plurality of loops having a long loop length and a plurality of short loops.

  Further, a fiber Bragg grating (FBG) can be used as the dispersion medium. Examples of the fiber Bragg grating include those using a uniform fiber grating, a chirped grating or a multi-section grating, and a fiber grating which can be modulated. The fiber Bragg grating can be obtained, for example, by irradiating ultraviolet rays through a phase mask and changing the refractive index of the core at a predetermined pitch.

  In the present invention, the optical fiber and the fiber Bragg grating can be used in combination as a dispersion medium. In this case, an optical fiber having a plurality of loops in which the dispersion amount and length are adjusted may be used.

  Further, the dispersion medium 40 may include a mechanism for stabilizing the optical path length. As a mechanism for stabilizing the optical path length, for example, a combination of a temperature sensor such as a thermistor sensor and a heater or a cooler such as a Peltier element or a thin film heater can be used. This mechanism measures the temperature around the dispersion medium 40 with a temperature sensor, controls the heater and cooler, maintains the temperature around the dispersion medium 40 constant, and makes the refractive index of the dispersion medium 40 constant, etc. As a result, the optical path length can be stabilized.

(1.4. Photodetector)
The photodetector 50 is a means for detecting the output light from the dispersion medium 40 and converting it into an electrical signal. As the photodetector 50, a known one can be adopted as appropriate. As the photodetector 50, for example, a device including a photodiode can be employed. The photodetector 50 detects an optical signal and converts it into an electrical signal. The light detector 50 only needs to be capable of detecting at least the intensity (intensity modulation component) of the optical signal. Further, the photodetector 50 may be capable of detecting the frequency of the optical signal. As the photodetector 50, for example, those described in “Hiroo Yonezu“ Optical Communication Element Engineering ”—Light Emitting / Receiving Element—, Engineering Books Co., Ltd., 6th edition, published in 2000” can be appropriately employed.

In the present embodiment, the photodetector 50 is at least as long as they respond to the modulated component at the frequency f _m by the optical modulator 20. In particular, the optical detector 50 is preferably one that responds to f _m component is a feedback signal required to light the electric wave (frequency f ₀ ± f _m).

Light detector 50 need not be a detectable except f _m component is a feedback signal, for example if also responsive to 2f _m component (frequency f ₀ ± 2f _m), the input side of the photodetector 50 it may be removed 2f _m component contained in the electrical signal by providing a RF filter at the output side of either removing 2f _m component included in the optical signal, or an optical detector 50 is provided in the optical filter.

  In the embodiment shown in FIG. 1, a band-pass filter 60 is provided on the output-side conductor of the photodetector 50. The band-pass filter 60 is an electronic circuit that removes unnecessary frequency components from the electrical signal in which various frequency components output from the photodetector 50 are mixed and passes only the necessary frequency components. A known filter can be used as the band-pass filter 60, and a low-pass filter (LPF), a high-pass filter (HPF), or a combination of LPF and HPF can be used. The low-pass filter is a circuit that extracts a low frequency component, and is realized by an RC integration circuit that combines a resistor (R) and a capacitor (C), an LC integration circuit that combines a coil (L) and a capacitor (C), or the like. The high-pass filter is a circuit for extracting a high frequency component, and is realized by an RC differentiating circuit combining a resistor (R) and a capacitor (C) or an RL differentiating circuit combining a resistor (R) and a coil (L). The cut-off frequency for the RC differentiating circuit is f = 1 / (2πRC), and the cut-off frequency for the RL differentiating circuit is f = R / (2πL). A signal having a frequency lower than the frequency f can be attenuated and a signal having a high frequency can be passed.

(2. Photoelectric oscillation method)
Next, the principle of the photoelectric transmission method using the photoelectric oscillator 100 described above will be described with reference to FIG. In the present embodiment, the operation principle when the optical modulator 20 is biased so that the phase difference between optical signals guided through two waveguides is π (pi) will be described.

First, an optical signal of a frequency f ₀ output from the light source is input to the optical modulator. The optical modulator is given a bias (zero bias point) by which a phase difference of an optical signal guided through the optical modulator is π (pi) by a bias power source. Further, the optical modulator, the modulation signal of the frequency f _m is applied. That is, as will be described later, an electrical signal (frequency f _m ) that has passed through the dispersion medium and the photodetector is fed back to the optical modulator as a modulation signal. Thus, the optical modulator, an optical signal of frequency f ₀ ± 2f _m is output. In the present invention, the optical modulator has a configuration capable of generating chirp relating to optical modulation. Thus, the optical modulator, the phase modulation component is generated by superimposing the 2f _m component as intensity modulation component. The phase modulation component, includes at least f _m component. In this way, even when a bias (zero bias point) is applied so that the phase difference of the optical signal guided through the optical modulator is π (pi), the optical modulator generates chirp, it can contain f _m component in the output signal of the modulator.

The optical signal modulated by the optical modulator and accompanied by chirp (phase modulation component generated along with intensity modulation) enters the dispersion medium. The dispersion medium can convert a phase modulation component into an intensity modulation component by a wavelength dispersion effect. Therefore, through the dispersive medium, f _m component in the phase-modulated component included in the output signal of the optical modulator is converted into an intensity-modulated component. As shown in FIG. 2, the optical signal that has passed through the dispersion medium has a phase relationship that is the same as the optical signal output from the optical modulator, although the optical spectrum remains unchanged. Thus, even f _m component contained as a phase modulation component in the output signal of the optical modulator, by converting the intensity modulated component through the dispersion medium, it becomes possible to detected by a photodetector .

The optical signal that has passed through the dispersion medium is input to the photodetector and converted into an electrical signal. The photodetector may be one that is responsive to the modulated component at the frequency f _m by at least an optical modulator. That is, the photodetector need not be capable of detecting anything other than the _fm component that is the feedback signal. Therefore, when the light detector has a characteristic that is responsive to e.g. 2f _m component, on the input side of the photodetector provided in the optical filter for removing 2f _m component included in the optical signal or light provided RF filter at the output side of the detector may be removed 2f _m component contained in the electrical signal. In this manner, in the photodetector, f _m component of the optical signal passing through the dispersive medium is converted into electrical signals.

The electrical signal converted by the photodetector is fed back to the optical modulator via a conducting wire and used as a modulation signal. As described above, since the optical detector is to convert into electrical signals f _m component of the optical signal passing through the dispersive medium, the electric signal is a modulated signal of the modulation frequency f _m. In this way, an optical signal with a chirp is generated by the optical modulator, passed through the dispersion medium, and then converted into an electric signal by the optical detector. f _m) can be applied to. As a result, an appropriate feedback signal can be obtained even when a bias (zero bias point) having a phase difference π (pi) is applied, and the oscillation operation of the photoelectric oscillator can be performed. Further, the optical modulator is capable of generating a second harmonic signal having a frequency twice the frequency f _m of the modulation signal. The second harmonic signal obtained by the photoelectric oscillator of the present invention can be used for a low-cost device made for a low frequency, for example. Further, the second harmonic signal obtained by photoelectric oscillator of the present invention is f _m component is well suppressed, it becomes signal high purity.

  The present invention has been described with reference to preferred embodiments. However, the present invention is not limited to the above-described embodiments, and those skilled in the art can make appropriate modifications and alterations within the obvious range based on the above-described embodiments.

  The optoelectric oscillator of the present invention can be used in optical communication and telecommunication equipment using a reference millimeter wave / microwave signal source, a fiber radio signal source, and the like. The present invention also provides an optical fiber radio signal source, optical clock generation and signal processing in optical fiber transmission, a long-distance optical fiber transmission system, a large-capacity wavelength division multiplexing optical fiber network, an access optical fiber radio system, an optical measurement system, and It can be used in fields such as optical measurement signal sources.

DESCRIPTION OF SYMBOLS 10 Light source 20 Optical modulator 30 Optical amplifier 40 Dispersion medium 50 Photo detector 60 Band pass filter 70 RF amplifier 80 Conductor 90 Optical output waveguide 100 Photoelectric oscillator

Claims (6)

It has a modulation electrode to which a modulation signal of modulation frequency f _m [Hz] is applied, and the optical signal output from the light source (10) is intensity- modulated with the modulation signal and accompanied by a phase modulation component of frequency f _m an optical intensity modulator for outputting a light intensity modulation signal (20),
Said optical modulator (20) modulated optical intensity modulated signal is input, the dispersion medium that converts a phase modulation component of the frequency f _m contained in the light intensity modulated signal into an intensity-modulated component (40),
By receiving the optical signal from the dispersion medium (40), a photodetector for converting the intensity modulation component of the frequency f _m contained in the optical signal into an electric signal (50),
The electric signal converted by the light detector (50), the modulation frequency f _m optoelectronic oscillator having a, a wire (80) to be input to the modulation electrode of the optical modulator (20) as a modulation signal. The light intensity modulator (20) is branched by a light input section (21), a branch section (22) where light input to the input section (21) branches, and the branch section (22) The first waveguide (23) through which one light propagates, the second waveguide (24) through which the other light branched by the branch section (22) propagates, and the first waveguide (23 ) And an optical signal output from the second waveguide (24) are combined, and an optical signal is output from the optical signal combined by the combining unit (25). Output unit (26), a first modulation electrode (27) to which a modulation signal for driving the first waveguide (23) is applied, and a modulation signal for driving the second waveguide (24) A Mach-Zehnder light intensity modulator having a second modulation electrode (28) to which is applied,
The photoelectric oscillator according to claim 1, wherein a phase change amount of the first waveguide (23) is different from a phase change amount of the second waveguide (24) . When the phase change amount of the first waveguide (23) is φ1, the second waveguide (24) is φ2, and the chirp parameter α is expressed by the following [Equation 2], the value of the chirp parameter α optoelectronic oscillator according to claim 2 but which becomes 1.
[Formula 2] α = (φ1 + φ2) / (φ1-φ2) The optoelectric oscillator according to claim 1, wherein the dispersion medium (40) is one or both of an optical fiber and a fiber Bragg grating. It said wire (80), optoelectronic oscillator according to claim 1 having a band-pass filter (60) for selecting an electrical signal of a predetermined region including the modulation frequency f _m which is used as the modulation signal. Optical intensity modulator modulating signal the modulation frequency f _{m [Hz]} to the modulation electrode (20) is applied, the optical signal output from the light source (10) to intensity modulation with a phase modulation component of the frequency f _m Outputting a modulated light intensity signal;
The optical intensity modulated signal modulated by the optical intensity modulator (20) input to the dispersion medium (40), the step that converts the phase modulated component of the frequency f _m contained in the light intensity modulated signal into an intensity-modulated component When,
A step of an optical signal received by the photodetector (50), converts the intensity modulation component of the frequency f _m contained in the optical signal to an electrical signal from said dispersion medium (40),
Light including the steps of inputting to the modulation electrode of the optical detector the electric signal converted by (50), the optical modulator via a line (80) as a modulation signal of the modulation frequency f _m (20) Electric oscillation method.

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