JP4686785B2 - Photoelectric oscillator and optoelectric oscillation method - Google Patents

Description

  The present invention relates to a photoelectric oscillator and a photoelectric oscillation method.

  An optoelectric oscillator is a device that oscillates an optical signal or an electric signal having a predetermined frequency when an optical signal is input.

  FIG. 4 is a schematic configuration diagram of a conventional optoelectric oscillator. As shown in FIG. 4, the conventional photoelectric oscillator (30) has, for example, an optical modulator (32) that modulates an optical signal output from a light source, and a sideband component from the optical modulator (32). A photodetector (for example, a photodiode) (35) that receives an optical signal and converts it into an electrical signal of a predetermined frequency, an amplifier (37) that amplifies the electrical signal from the photodetector (35), and is amplified It comprises a conductor (36) that positively feeds back an electrical signal to the electrode of the optical modulator (32), and oscillates by obtaining a sufficient loop gain.

  The photoelectric oscillator (30) positively feeds back an electric signal of a predetermined frequency from the photodetector as a modulation signal to the electrode of the optical modulator, thereby generating a modulated sideband component in a free-running oscillation operation in the optical modulator. Therefore, there is an advantage that an external oscillator is not necessary.

  However, the conventional optoelectric oscillator has a problem that the accuracy decreases due to continuous use. For example, there is a problem that the frequency of the electric signal fluctuates as a result of fluctuations in the loop length due to heat generated by the device. This frequency variation makes the free-running oscillation operation of the optical modulator unstable.

  For this reason, in the conventional optoelectric oscillator, the electrical signal output from the photodetector is analyzed, the frequency variation of the electrical signal is corrected, and then fed back to the optical modulator. Unstable operation was solved. That is, the conventional optoelectric oscillator has a problem that the apparatus becomes complicated, for example, an additional apparatus for analyzing an electric signal is required to suppress the fluctuation of the frequency.

  The present invention has been made in view of the above situation, and provides an optoelectric oscillator capable of automatically and simply correcting fluctuations in oscillation frequency by using an optical filter such as a fiber Bragg grating (FBG). The purpose is to do.

  In the present invention, basically, when stabilizing the oscillation frequency of an opto-electric oscillator, a change occurs in the optical path length along which an optical signal is guided by disturbance such as heat, and this change is automatically and easily eliminated. This is based on the knowledge that the oscillation frequency can be effectively stabilized.

<Invention of Photoelectric Oscillator>
The photoelectric oscillator according to the present invention that solves the above problems is as follows.
A first optical modulator (1) for modulating an optical signal output from a light source;
A second optical modulator (2) for modulating the optical signal from the first optical modulator (1);
An optical path length adjuster (3, 4) for changing an optical path length of an optical signal from the second optical modulator (2);
A photodetector (5) for receiving an optical signal from the optical path length adjuster (3, 4) and converting it into an electrical signal of a predetermined frequency;
A conducting wire (6) for inputting an electric signal of a predetermined frequency from the photodetector (5) as a modulation signal to the electrode of the first optical modulator (1) and the electrode of the second optical modulator (2). And
The optical path length adjuster (3, 4) is configured to adjust the electrical signal from the first optical modulator (1) or the first signal so as to adjust the electrical signal from the photodetector (5) to the predetermined frequency. The photoelectric oscillator (10) changes the optical path length according to the frequency of the optical signal from the second optical modulator (2).

In addition, another photoelectric oscillator according to the present invention that solves the above problems is
An optical modulator (2) for modulating an optical signal output from the light source;
An optical path length adjuster (3, 4) for changing an optical path length of an optical signal from the optical modulator (2);
A photodetector (5) for receiving an optical signal from the optical path length adjuster (3, 4) and converting it into an electrical signal of a predetermined frequency;
A conductor (6) for inputting an electric signal of a predetermined frequency from the photodetector (5) to the electrode of the optical modulator (2) as a modulation signal;
According to the frequency of the optical signal from the optical modulator (2), the optical path length adjuster (3, 4) adjusts the electrical signal from the photodetector (5) to the predetermined frequency. An optoelectric oscillator that changes the optical path length.

  Here, the optical path length adjuster includes an optical filter (4) that reflects the optical signal from the second optical modulator (or the optical modulator) with a different delay time according to the frequency of the optical signal. It is preferable to comprise.

  The optical filter (4) is more preferably a chirped fiber Bragg grating (chirped FBG).

  Further, the chirped FBG changes the refractive index period in the longitudinal direction so that it is reflected at a position where the optical path length is shorter as the frequency of the optical signal from the second optical modulator (or the optical modulator) becomes higher. The chirped FBG is preferable.

  An optical circulator is used to guide an optical signal from the second optical modulator (or the optical modulator) to the chirped FBG and to guide an optical signal reflected by the chirped FBG to the photodetector. (3) may be used.

  In addition, a device for reducing the frequency of the electrical signal to an integer is provided on a lead wire that inputs the electrical signal of the predetermined frequency to the electrode of the second optical modulator (or the optical modulator) as a modulation signal. May be.

<Invention of Photoelectric Oscillation Method>
The photoelectric oscillation method according to the present invention for solving the above-described problems is as follows.
Modulating the optical signal output from the light source by the first optical modulator (1);
Modulating the optical signal modulated by the first optical modulator (1) by the second optical modulator (2);
An optical path length adjustment step of changing an optical path length of an optical signal from the second optical modulator (2);
Receiving the optical signal that has undergone the optical path length adjustment step by a photodetector (5) and converting it into an electrical signal of a predetermined frequency;
Inputting the electrical signal of the predetermined frequency to the electrode of the first optical modulator (1) and the electrode of the second optical modulator (2) as a modulation signal by a conducting wire (6),
In the optical path length adjustment step, the optical signal from the first optical modulator (1) or the optical signal from the second optical modulator (2) is adjusted so as to adjust the electrical signal to the predetermined frequency. It is a photoelectric oscillation method which is a step of changing the optical path length according to the frequency.

In addition, another photoelectric oscillation method according to the present invention that solves the above problems is as follows:
Modulating the optical signal output from the light source by the optical modulator (2);
An optical path length adjusting step of changing an optical path length of an optical signal from the optical modulator (2);
Receiving the optical signal that has undergone the optical path length adjustment step by a photodetector (5) and converting it into an electrical signal of a predetermined frequency;
Inputting the electrical signal of the predetermined frequency to the electrode of the optical modulator (2) as a modulation signal by a conducting wire (6),
The optical path length adjusting step changes the optical path length according to the frequency of the optical signal from the optical modulator (2) so as to adjust the electrical signal from the photodetector (5) to the predetermined frequency. This is a photoelectric oscillation method that is a step of causing the

  Here, in the optical path length adjusting step, the optical signal from the second optical modulator (or the optical modulator) is reflected with different delay times in the optical filter (4) according to the frequency of the optical signal. It is preferable that it is the process of making it.

  The optical filter (4) is more preferably a chirped fiber Bragg grating (chirped FBG).

  Further, the chirped FBG changes the refractive index period in the longitudinal direction so that it is reflected at a position where the optical path length is shorter as the frequency of the optical signal from the second optical modulator (or the optical modulator) becomes higher. The chirped FBG is preferable.

  An optical circulator is used to guide an optical signal from the second optical modulator (or the optical modulator) to the chirped FBG and to guide an optical signal reflected by the chirped FBG to the photodetector. (3) may be used.

  Including the step of reducing the frequency of the electrical signal to an integer when the electrical signal of the predetermined frequency is input to the electrode of the second optical modulator (or the optical modulator) as a modulation signal by a conducting wire. Good.

  The above-mentioned “reflect at different delay times” means, for example, “reflect at different positions”, and an optical filter such as chirped FBG has an effect of changing the optical path length by reflecting at different positions. On the other hand, an optical filter such as a multilayer film or a photonic crystal that reflects the physical position at the same position and changes the optical path length may be used even if the positions are not different.

  According to the optoelectric oscillator according to the present invention, by using an optical path length adjuster such as an optical filter, for example, an element such as a fiber Bragg grating (FBG), in an optical path through which an optical signal is guided, fluctuations in oscillation frequency are automatically detected. And it can correct simply. As a result, the oscillation frequency can be effectively stabilized by a compact device configuration.

  In addition, since the photoelectric oscillator according to the present invention can be used as, for example, a reference millimeter wave / microwave signal source or a signal source for fiber radio, stable reference millimeter wave / microwave oscillation and stability Fiber radio is possible.

<First Embodiment>
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

<1. Photoelectric Oscillator>
FIG. 1 is a schematic configuration diagram of an optoelectric oscillator according to the first embodiment. As shown in FIG. 1, the photoelectric oscillator (10) according to the present embodiment includes an optical single sideband modulator (optical SSB (Single SSB)) that is a first optical modulator that modulates an optical signal output from a light source. (Slide-Band) modulator) (1), optical modulator (2) which is a second optical modulator for modulating the optical signal from optical SSB optical modulator (1), and optical modulator (2) An optical path length adjuster (3, 4) for changing the optical path length of the optical signal, and a photodetector (5) that receives the optical signal from the optical path length adjuster (3, 4) and converts it into an electrical signal of a predetermined frequency. ) And a conductor (6) for inputting an electric signal of a predetermined frequency from the photodetector (5) to the electrode of the optical SSB modulator (1) and the electrode of the optical modulator (2) as a modulation signal. An electric oscillator (10), wherein the optical path length adjuster (3, 4) is configured to output the electric signal from the photodetector (5) according to the frequency of the optical signal. Changing the optical path length to adjust to a constant frequency.

  Note that, in the photoelectric oscillator according to the present embodiment, the amplifier (7) and the multiplier (8) may or may not be provided as appropriate.

  The optical path length adjuster includes an optical circulator (3) and a chirped FBG (4) which is an example of an optical filter. The optical circulator (3) converts an optical signal from the optical modulator (2) into a chirped FBG (4). ) And the optical signal reflected by the chirped FBG (4) is guided to the photodetector (5).

  The conducting wire (6) is connected to the conducting wire (6b) for inputting an electric signal of a predetermined frequency from the photodetector (5) to the electrode of the optical SSB modulator (1) as a modulation signal, and to the electrode of the optical modulator (2). It consists of the conducting wire (6a) to input.

  When an electric signal of a predetermined frequency output from the photodetector (5) is input as a modulation signal to the electrode of the optical SSB modulator (1) or the electrode of the optical modulator (2), the amplifier (7) Amplified and input. The electrical signal of a predetermined frequency output from the photodetector (5) is multiplied (n times) by the multiplier (8) when input to the electrode of the optical SSB modulator (1) as a modulation signal. Is input.

<1.1. Light source>
A known light source can be used as the light source of the photoelectric oscillator. A preferable light source of the photoelectric oscillator is a diode, a laser diode, or the like.

  An example of the 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. An example of an optical signal arranged with periodicity is a pulse signal. 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 HP8166A and 81689A manufactured by Agilent.

<1.2. First optical modulator>
As a first optical modulator, there is an optical single sideband modulator (optical SSB (Single Side-Band) modulator) that shifts and outputs the frequency of an optical signal. Optical SSB modulators and their operations are described in, for example, “Tetsuya Kawanishi, Masayuki Izutsu,“ Optical Frequency Shifter Using Optical SSB Modulator ”, IEICE Technical Report, Technical Report of IEICE, OCS2002-49, PS2002-33, OFT2002. -30 (2002-08) "," Hisumi et al., X-cut Lithium Niobium Optical SSB Modulator, Electron Letter, vol. 37, 515-516 (2001). " That is, according to the optical SSB modulator, it is possible to obtain an upper sideband signal (USB) and a lower sideband signal (LSB) whose frequency is shifted by a predetermined amount.

  The first optical modulator is not limited to the optical SSB modulator (1), and for example, an optical frequency shift keying modulator (optical FSK modulator) or an optical carrier suppressed double sideband modulator (optical DSB-SC). An element such as a modulator may be used. As the optical DSB-SC modulator, an optical DSB-SC modulator described later can be used.

The optical FSK modulator includes, for example, a first sub Mach-Zehnder waveguide (MZ _A ), a second sub-Mach-Zehnder waveguide (MZ _B ), the MZ _A and the MZ _B , A modulator comprising a main Mach-Zehnder waveguide (MZ _C ) comprising an input part and a modulated light output part. The modulator includes, for example, a first sub MZ electrode (electrode A) for inputting a radio frequency (RF) signal to two arms constituting the MZ _A, and two arms constituting the MZ _B. A second sub-MZ electrode (electrode B) for inputting a radio frequency (RF) signal and the MZ _C are controlled, and the voltage value or phase of the input RF signal is controlled to control the output from the output unit. And an electrode (electrode C) for controlling the frequency of the output light.

  Each Mach-Zehnder waveguide includes, for example, a substantially hexagonal waveguide (which forms two arms), and is configured to include two phase modulators 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-diffused lithium niobate waveguide may be formed on the LN substrate, or a silicon dioxide (SiO ₂ ) waveguide may be formed on the silicon (Si) substrate. Further, 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. Further, as the substrate, a triaxial or hexagonal uniaxial crystal having an electrooptic 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 (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 electrodes A, B, and C are made of, for example, gold or platinum. Examples of the width of these electrodes include 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 setting 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.

  Examples of the electrode A and the electrode B include a traveling wave type electrode or a resonance type electrode, preferably a resonance type 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) ”.

  Traveling wave type electrodes (traveling wave type optical modulators) are electrodes (modulators) that modulate light while 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, a known one 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. A traveling wave type modulator is preferable because it can modulate with the same characteristics to light input from either direction by inputting modulation signals from the electrodes at both ends.

  As the traveling wave 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 signal electrode) is preferably adopted. Thus, by arranging the ground electrodes symmetrically across the signal electrodes, the high frequency output from the signal electrodes is easily applied to the ground electrodes arranged on the left and right sides of the signal electrodes. Can be suppressed.

  The electrode C is preferably a traveling wave electrode. This is because the switching speed of the electrode C becomes the data speed of the optical FSK modulator, so that the switching can be performed at a high speed (switching between USB and LSB) by using the electrode C as a traveling wave type electrode.

  As a method for forming the optical waveguide, a known forming 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 a photolithography method, and the titanium is diffused by a thermal diffusion method 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 in which a titanium diffusion waveguide is formed is formed.

  The optical FSK modulator can be manufactured as follows, for example. First, a waveguide is formed on a substrate. The waveguide can be provided by subjecting the lithium niobate substrate surface to a proton exchange method or a titanium thermal diffusion method. For example, Ti metal stripes on the order of several micrometers are formed on the LN substrate in a state of being arranged in rows on the LN substrate by photolithography. Thereafter, the LN substrate is exposed to a high temperature around 1000 ° C. to diffuse Ti metal into the substrate. In this way, a waveguide can be formed on the LN substrate.

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

In addition, when using a silicon substrate, it can manufacture as follows, for example. A lower cladding layer mainly composed of silicon dioxide (SiO ₂ ) is deposited on a silicon (Si) substrate by a flame deposition method, 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. Then, it is made into transparent glass by an electric furnace. Next, an optical waveguide portion is produced 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.

<1.3. Second optical modulator>
An optical modulator is a device for modulating at least one of light frequency, light intensity, and light phase. Examples of such an optical modulator include a frequency modulator, an intensity modulator, and a phase modulator. In this embodiment, an intensity modulator (IM) is applied as the optical modulator (2) that is the second optical modulator, but the optical modulator is not limited to IM. The optical modulator exemplified as the first optical modulator can be used as the second optical modulator. Further, the optical modulator includes a resonance type modulator and a traveling wave type modulator, and either modulator may be used.

  An example of the intensity modulator is an optical carrier suppressed double sideband (DSB-SC) modulator. An 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. A VOA element using LN may be used as the intensity modulator (see, for example, Japanese Patent Laid-Open No. 10-142569). 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 thermooptic effect, Examples include adjusting the output intensity of the interferometer (see, for example, Japanese Patent Laid-Open No. 2000-352699). The optical DSB-SC modulator includes a signal source and a phase adjuster that adjusts the phase of the signal output from the signal source, and the phase of the electric signal applied to both arms of the Mach-Zehnder waveguide is, for example, 180. One that is adjusted differently. 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 present invention, basically intensity modulator, f ₀ the center frequency of the input signal modulation frequency of the modulated signal, when the modulation frequency is f _m, and its output signal is mainly f ₀ ± f _m ( f ₀ + f _m and f ₀ −f _m ). Among those that output an output signal having such a frequency, one that outputs an output signal whose 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.

  Such an optical DSB-SC modulator is a Mach-Zehnder waveguide, and more preferably a push-pull type Mach-Zehnder waveguide, which is manufactured in the same manner as the optical FSK modulator described above. it can. This is because the Mach-Zehnder waveguide can be provided on the same substrate as the optical FSK modulator. In addition, the Mach-Zehnder waveguide can avoid unnecessary optical phase change (frequency chirp) during intensity modulation. As such a Mach-Zehnder waveguide, a Mach-Zehnder waveguide used in a known optical SSB modulator or the like can be used.

<1.4. Optical circulator, optical filter>
In the present embodiment, an optical path length adjuster is configured by the optical circulator (3) and the chirped FBG (4). The optical circulator (3) guides the optical signal from the optical modulator (2) to the optical filter (4) and guides the optical signal reflected by the chirped FBG (4) to the photodetector (5). Is. Therefore, the optical circulator is not limited as long as it can perform such an optical signal waveguide operation.

  The chirped FBG (4), which is an example of an optical filter, reflects the optical signal from the optical modulator (2) incident from the optical circulator (3) and emits it again to the optical circulator (3). However, at this time, it is designed so that the position of reflection inside differs according to the frequency of the optical signal. For example, a chirped with a refractive index period changed in the longitudinal direction so that it is reflected at a position where the optical path length is shorter (position closer to the optical circulator (3)) as the frequency of the optical signal from the optical modulator (2) becomes higher. FBG and the like are preferable.

As a specific optical fiber grating, a step type described in Japanese Patent Application Laid-Open No. 9-005544, in which the difference in refractive index between the core and the clad is 0.015 to 0.03 and the core diameter is 4 μm to 6 μm. An optical fiber grating formed in a profile optical fiber, wherein the refractive index is periodically changed in the length direction by ultraviolet laser light in the core portion, and the period itself is changed in the length direction. "Optical fiber grating characterized in that", "Two-wavelength blocking optical fiber grating characterized in that a grating portion is formed on a polarization-maintaining fiber" described in Japanese Patent Laid-Open No. 10-288715, No. 119041 “Long-period refractive index modulation in a certain region including at least a core region in a predetermined range in the longitudinal direction of a silica-based optical fiber A formed optical fiber grating, the GeO ₂ and F element constant region is added together, an optical fiber grating, characterized in that ", described in JP-A-2000-249851" Optical fiber In the long-period optical fiber grating formed by forming a periodic change in the refractive index of the core in the length direction, a phase shift is introduced in the period of the periodic change in the refractive index of the core. A long-period optical fiber grating that performs a perturbation with a grating pitch of 20 to 80 μm along the length of the optical fiber core described in Japanese Patent Application Laid-Open No. 2002-243957. An optical fiber grating characterized by comprising "a refractive index periodically along the longitudinal direction of the optical fiber" described in JP-A-2003-050322 In a slant short-period optical fiber grating in which a fiber grating portion is formed such that a diffraction grating is formed and the grating vector of this diffraction grating has an inclination angle with respect to the longitudinal direction of the optical fiber. And a slant-type short-period optical fiber grating characterized in that generation of ripples in the transmission spectrum is suppressed by making the outermost diameter of each of the layers larger than 125 μm.

  The optical fiber grating may be provided on a PLC (planar lightwave circuit). Such an optical fiber grating is, for example, one having an optical waveguide and a multipoint phase shift structure optically coupled to the optical waveguide. More specifically, a planar multipoint phase shift grating having a planar optical waveguide and a plurality of uniform pitch Bragg diffraction gratings provided in the planar optical waveguide can be mentioned.

  In the present embodiment, the chirped FBG used as an example of the optical filter has an effect of changing the optical path length by reflecting the optical signal at different positions inside. On the other hand, an optical filter such as a multilayer film or a photonic crystal that reflects the physical position at the same position and changes the optical path length may be used even if the positions are not different. Therefore, the general function of the optical filter may be an optical filter that reflects with a different delay time depending on the frequency.

<1.5. Photodetector>
The photodetector is means for detecting the output light from the optical modulator (2) and converting it into an electrical signal. A well-known thing can be employ | adopted as a photodetector. For example, a device including a photodiode can be employed as the photodetector. Examples of the photodetector include one that detects an optical signal and converts it into an electrical signal. The optical detector can detect the intensity, frequency, and the like of the optical signal. As the photodetector, for example, those described in “Hiroo Yonezu“ Optical Communication Element Engineering ”—Light Emitting / Receiving Element—Kogaku Shobun Co., Ltd., 6th edition, published in 2000” can be appropriately employed.

<1.6. Divider>
In addition, it is also possible to provide a device for reducing the frequency to a fraction of an integer on the lead wire (6a) connected to the modulation electrode of the second optical modulator (2). An example of this device is a frequency divider.

  An example of a frequency divider is one using a toggle flip-flop (T-FF). Examples of the T-FF include a master slave (MS) T-FF and a dynamic T-FF. Specifically, a frequency divider (90 GHz InPHBT dynamic frequency divider manufactured by NTT) in which dynamic T-FF, buffer, MST-FF, buffer, MST-FF, dynamic T-FF, MST-FF, and driver are connected in this order. Is given. This frequency divider can generate a low-speed clock signal using a clock signal extracted from the received signal in a high-speed optical communication system exceeding 40 Gbit / s. Further, a frequency divider formed of a CMOS flip-flop (for example, FIG. 3 and FIG. 4 of Japanese Patent No. 3477844) may be used. This frequency divider is a cascade connection of three D flip-flops. Each D flip-flop is composed of a plurality of inverters. More specifically, a CMOS P-channel transistor and an N-channel transistor are push-pull connected by an inverter.

  As another frequency divider, as shown in FIG. 6 of Japanese Patent No. 3437551, a counter having a set value M (M = 2, etc.), an input signal of the counter, and an output signal of the counter are input. And a circuit including a circuit. The counter counts the input signal, and outputs one cycle every M cycles as a Low output. The AND circuit outputs a signal obtained by taking the logical sum of the input signal and the output signal of the counter, so that one cycle of pulses from the AND circuit is removed every M cycles. In this way, for example, when M = 2, one cycle of pulses is removed every two cycles, so that the signal cycle is doubled and the signal with the input signal frequency halved is output. Become.

  As another frequency divider, as shown in FIGS. 5 and 6 of Japanese Patent No. 3585114, one using a frequency divider integrated circuit can be cited. This frequency divider has a terminal SW1 and a terminal SW2 in the frequency divider integrated circuit 1 constituting the frequency divider, and divides the frequency based on the H level and L level of the control signal applied to the terminal SW1 and the terminal SW2. The ratio can be switched to 1/2, 1/4, 1/8. One example of the frequency divider integrated circuit 1 is MC12093 manufactured by Motorola. Further, this frequency divider includes a terminal SB (terminal for standby mode), a terminal IN (input terminal), a terminal IN bar, a terminal Vcc (power supply terminal), a terminal GND (ground terminal), and a terminal OUT (output terminal). It comprises. An input signal is input to the terminal IN via a coupling capacitor. The terminal IN bar is grounded through a coupling capacitor. When the control signal levels applied to the terminals SW1 and SW2 are both H level, the frequency division ratio is set to 1/2.

<2. Operating Principle of Photoelectric Oscillator>
Next, the operation principle of the photoelectric oscillator according to this embodiment will be described. Figure 2 is a conceptual diagram showing a state of the optical signal guided through the optical electric oscillator according to the first embodiment, an optical signal of frequency f ₀ + nf _m input signal light of a frequency f ₀ is the optical SSB modulator the modulated, further showing a state in which modulated into an optical signal having a sideband component of the frequency optical signal by the optical modulator _{f 0 + (n ± 1)} f m.

First, an optical signal of a frequency f ₀ output from the light source and is incident on the optical SSB modulator (1), is output after being modulated into an optical signal of frequency f ₀ + nf _m. This is because the optical SSB modulator (1) performs a modulation operation by a modulation signal (frequency nf _m ) from the multiplier (8) as will be described later.

Optical signal having a frequency f ₀ + nf _m, by being inputted to the optical modulator (2), the frequency f ₀ + (n ± 1) optical signal having a sideband component of f _m (frequency f ₀ + (n ± 1) the original optical signal sideband component of f _m and the frequency f ₀ + nf _m) are modulated. This is because the optical modulator (2) performs a modulation operation by a modulation signal (frequency f _m ) from the photodetector (5) as will be described later.

An optical signal having a frequency _{f 0 + (n ± 1)} f sideband component of _m, the optical circulator (3) is guided in a chirped FBG (4), the frequency of the optical signal inside the chirped FBG (4) Accordingly, the light is reflected at different reflection positions and then output to the photodetector (5) via the optical circulator (3).

In the photodetector (5), the input optical signal is converted into an electrical signal and output. At this time, an electric signal of a frequency f _m generated corresponding to sideband component of the frequency _{f 0 + (n ± 1)} f m is the modulation signal, the optical SSB modulator via an amplifier 7 and multiplier 8 It is input to the modulation electrode of (1) and the modulation electrode of the optical modulator (2).

More specifically, after the modulation signal of the frequency f _m is amplified by an amplifier (7), the modulation operation of the conductor (6a) by an optical modulator (2) is inputted optical modulator modulating electrode (2) Used. Moreover, after the modulated signal of a frequency f _m is amplified by an amplifier (7), (being n times) is multiplied by the further multiplier (8) is the modulation signal of the frequency nf _m, by a conductive wire (6b) The light is input to the modulation electrode of the optical SSB modulator (1) and used for the modulation operation of the optical SSB modulator (1).

  Based on the operation of such an opto-electric oscillator, when the conditions of the free-running oscillation operation such as the efficiency of the optical modulator is sufficiently high, the amplification degree of the amplifier is large, and the input optical power is large, noise, etc. Oscillation starts with a disturbance. It is also possible to supply a signal from somewhere outside the electric circuit portion and the optical portion of the opto-electric oscillator to oscillate in synchronization with the signal.

Thus, the form in which the electrical signal output from the photodetector (5) is input to the optical SSB modulator (1) via the multiplier (8) is a preferred embodiment of the present invention. This is because it is preferable frequency modulation is large in the optical modulator (2) (f _m than the frequency f ₀ of the input signal deviates region) sideband region of the optical SSB modulator than (1), multiplier (8 According to), because it is possible to frequency f _m inputs a modulation signal n times shifted frequency nf _m to the optical SSB modulator (1).

In the photoelectric oscillator according to this embodiment, the optical signal from the optical modulator (2) guided to the chirped FBG (4) is reflected at different reflection positions in the chirped FBG (4) according to the frequency of the optical signal. Reflected. As a result, even if the modulation frequency f _m (oscillation frequency from the photodetector 5) when the photoelectric oscillator is stably operated is shifted, the optical path length is adjusted in the chirped FBG (4), The modulation frequency can be corrected automatically and simply.

  For example, when the loop length of the photoelectric oscillator increases due to disturbance such as heat generation, the oscillation frequency from the photodetector (5) increases, and the modulation frequency input to the optical modulator (1, 2) As a result, the frequency of the optical signal output from the optical modulator (2) increases. However, in the present embodiment, the chirped FBG (4) is designed to be reflected at a position where the optical path length is shorter (position closer to the optical circulator (3)) as the frequency of the optical signal becomes higher. Can be corrected so as to lower the oscillation frequency.

  On the other hand, when the loop length of the photoelectric oscillator is decreased, the oscillation frequency from the photodetector (5) is lowered and the modulation input to the optical modulator (1, 2) is reversed, contrary to the above-described operation. As a result of the lower frequency, the frequency of the optical signal output from the optical modulator (2) is lowered. However, in this embodiment, the chirped FBG (4) is designed to be reflected at a position where the optical path length is relatively long (a position far from the optical circulator (3)), so that the oscillation length is increased by increasing the loop length. Can be corrected to be higher.

  In the photoelectric oscillator according to the present embodiment, the optical SSB modulator (1) generates USB (modulated optical signal shifted in the direction of increasing frequency) to increase the group delay in the chirped FBG (4). However, an operation in which the LSB (a modulated optical signal shifted in the direction of decreasing the frequency) is generated by the optical SSB modulator (1) and the group delay is reduced in the chirped FBG (4) may be used.

<Second Embodiment>
FIG. 3 is a schematic configuration diagram of the photoelectric oscillator according to the second embodiment. In FIG. 3, the same constituent elements as those used in the photoelectric oscillator according to the first embodiment are denoted by the same reference numerals, and redundant description is omitted.

  As shown in FIG. 3, the photoelectric oscillator (20) according to this embodiment includes an optical modulator (2) that modulates an optical signal output from a light source, and an optical path of the optical signal from the optical modulator (2). An optical path length adjuster (3, 4) for changing the length, a photodetector (5) for receiving an optical signal from the optical path length adjuster (3, 4) and converting it into an electric signal of a predetermined frequency, and optical detection A conductor (6) for inputting an electric signal of a predetermined frequency from the optical device (5) to the electrode of the optical modulator (2) as a modulation signal, and the optical path length adjuster (3, 4) is a frequency of the optical signal. Accordingly, the photoelectric oscillator (20) changes the optical path length so as to adjust the electrical signal from the photodetector (5) to a predetermined frequency.

In this embodiment, the input signal light of a frequency f _0, by being inputted to the optical modulator (2), the side of the optical signal (frequency f ₀ ± f _m having a sideband component of the frequency f ₀ ± f _m Band component and frequency f ₀ of the original optical signal). This optical signal is reflected through the optical circulator (3) in the chirped FBG (4) at different reflection positions depending on the frequency of the optical signal, and then output to the photodetector (5). In the optical detector (5), an electrical signal of frequency f _m generated corresponding to sideband component of the frequency f ₀ ± f _m is the modulation electrode as a modulation signal, through an amplifier 7 optical modulator (2) Is input.

  For example, the optoelectric oscillator according to the present invention can be used in the fields of optical communication and telecommunication using a reference millimeter wave / microwave signal source, a fiber radio signal source, and the like.

FIG. 1 is a schematic configuration diagram of an optoelectric oscillator according to the first embodiment. Figure 2 is a conceptual diagram showing a state of the optical signal guided through the optical electric oscillator according to the first embodiment, an optical signal of frequency f ₀ + nf _m input signal light of a frequency f ₀ is the optical SSB modulator the modulated, further showing a state in which modulated into an optical signal having a sideband component of the frequency optical signal by the optical modulator _{f 0 + (n ± 1)} f m. FIG. 3 is a schematic configuration diagram of the photoelectric oscillator according to the second embodiment. FIG. 4 is a schematic configuration diagram of a conventional optoelectric oscillator.

Explanation of symbols

1 optical SSB modulator 2 optical modulator 3 optical circulator 4 chirped FBG
5 Photodetector 6 Conductor 7 Amplifier 8 Multiplier 10 Photoelectric Oscillator 32 Optical Modulator 35 Photodetector 36 Conductor 37 Amplifier 30 Photoelectric Oscillator

Claims (8)

A first optical modulator (1) for modulating an optical signal output from a light source;
A second optical modulator (2) for modulating the optical signal from the first optical modulator (1);
Chirped fiber designed to reflect the optical signal from the second optical modulator (2) at different positions inside depending on the frequency of the optical signal from the second optical modulator (2) An optical path length adjuster (3, 4) that includes a Bragg grating (chirped FBG) (4) and changes the optical path length of the optical signal from the second optical modulator (2) by the chirped FBG (4); ,
A photodetector (5) for receiving an optical signal from the optical path length adjuster (3, 4) and converting it into an electrical signal of a predetermined frequency;
A conducting wire (6) for inputting an electric signal of a predetermined frequency from the photodetector (5) as a modulation signal to the electrode of the first optical modulator (1) and the electrode of the second optical modulator (2). And
The optical path length adjuster (3, 4) adjusts the frequency of the optical signal from the second optical modulator (2) so as to adjust the electrical signal from the photodetector (5) to the predetermined frequency. In response, the optical path length is changed ,
The first optical modulator (1) includes an optical single sideband modulator (optical SSB modulator), an optical frequency shift keying modulator (optical FSK modulator), or an optical carrier suppressed double sideband modulator (optical DSB). -SC modulator),
A multiplier (8) is provided on the conductor (6b) that inputs the electrical signal of the predetermined frequency to the electrode of the first optical modulator as a modulation signal, and the modulation signal is multiplied by the multiplier (8), An optoelectric oscillator (10) for inputting the multiplied modulation signal to the first optical modulator (1 ).
The photoelectric oscillator according to claim 1,
The chirped FBG is a chirped FBG whose refractive index period is changed in the longitudinal direction so that the optical path length is reflected at a shorter position as the frequency of the optical signal from the second optical modulator becomes higher. .
In the photoelectric oscillator according to claim 1 or 2,
An optoelectric oscillator comprising an optical circulator (3) for guiding an optical signal from the second optical modulator to the chirped FBG and guiding an optical signal reflected by the chirped FBG to the photodetector.
The photoelectric oscillator according to claim 1,
The second optical modulator (2) is an optoelectric oscillator which is an intensity modulator.
Modulating the optical signal output from the light source by the first optical modulator (1);
Modulating the optical signal modulated by the first optical modulator (1) by the second optical modulator (2);
Chirped fiber designed to reflect the optical signal from the second optical modulator (2) at different positions inside depending on the frequency of the optical signal from the second optical modulator (2) An optical path length adjusting step of changing an optical path length of an optical signal from the second optical modulator (2) by a Bragg grating (chirped FBG) (4);
Receiving the optical signal that has undergone the optical path length adjustment step by a photodetector (5) and converting it into an electrical signal of a predetermined frequency;
Inputting the electrical signal of the predetermined frequency to the electrode of the first optical modulator (1) and the electrode of the second optical modulator (2) as a modulation signal by a conducting wire (6),
The first optical modulator (1) includes an optical single sideband modulator (optical SSB modulator), an optical frequency shift keying modulator (optical FSK modulator), or an optical carrier suppressed double sideband modulator (optical DSB). -SC modulator),
A multiplier (8) is provided on the conducting wire (6b) that inputs the electric signal of the predetermined frequency to the electrode of the first optical modulator (1) as a modulation signal,
The step of inputting the electrical signal of the predetermined frequency to the electrode of the first optical modulator (1) and the electrode of the second optical modulator (2) as a modulation signal by the conducting wire (6) includes the multiplier ( multiplying the modulated signal by 8), viewed including the step of inputting the multiplied the modulated signal to the first optical modulator (1),
The optical path length adjusting step, the electrical signal to adjust said predetermined frequency, depending on the frequency of the optical signal from the second optical modulator (2) is a step of changing the optical path length of light Electric oscillation method.
In the photoelectric oscillation method according to claim 5 ,
The chirped FBG is a chirped FBG whose refractive index period is changed in the longitudinal direction so that the optical path length is reflected at a position where the optical path length is shorter as the frequency of the optical signal from the second optical modulator is higher. Method.
In the photoelectric oscillation method according to claim 5 or 6 ,
A photoelectric oscillation method in which an optical signal from the second optical modulator is guided to the chirped FBG and an optical signal reflected by the chirped FBG is guided to the photodetector by an optical circulator (3).
In the photoelectric oscillation method according to claim 5 ,
The photoelectric oscillation method, wherein the second optical modulator (2) is an intensity modulator.

Images