JP2000278218A - Frequency modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a phase noise included in an FM modulation signal while realizing frequency modulation with high frequencies at a broad band. SOLUTION: A 1st optical modulator 102 converts an electric signal Si into an optical frequency modulation signal L1, and gives the signal L1 to a 1st optical receiver 110 together with a non modulation light L2 outputted from a local oscillation light source 103. The 1st optical receiver 110 generates an FM modulation signal that is a beat signal between the two lights. A filter 111 receives this FM modulation signal, extracts only its carrier component, which is given to a frequency converter 112, where the component is frequency- converted as a signal Sx. A 2nd optical modulator 113 uses the signal Sx after frequency conversion to apply optical amplitude modulation or optical intensity modulation to the non modulation light L2 from the local oscillation light source 103. A 2nd optical receiver 114 receives a signal L2X after the optical modulation and the optical signal L1 from the 1st optical modulator 102 and provides an output of the MF modulation signal Sfm that is a beat signal between the two optical signals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a broadband frequency modulation signal (hereinafter referred to as "FM modulation signal") utilizing optical frequency modulation by a semiconductor laser and optical heterodyne detection.
And a modulation device for generating the following.

[0002]

2. Description of the Related Art FIG. 14 is a block diagram showing a configuration of a conventional frequency modulation device. This frequency modulation device is described, for example, in the literature (K. Kikushima, et al, "Optical Super Wide-Ba
nd FMModulation Scheme and Its Application to Mult
i-Channel AM Video Transmission Systems ", IOOC'95
Technical Digest, Vol.5 PD2-7, pp.33-34) describes its operation in detail. The frequency modulation device shown in FIG. 14 includes a signal source 700 and an optical modulator 702.
, A local light source 703, a first optical waveguide 706, and a second
708 and an optical receiver 714.

In the above-described frequency modulation device, the signal source 7
00 outputs an electric signal Si which is an original signal to be FM-modulated. The optical modulator 702 is formed of, for example, a semiconductor laser. In general, a semiconductor laser outputs light at a constant optical frequency f1 under the condition of a constant injection current, and when the injection current is amplitude-modulated, the optical frequency is also modulated, and the optical frequency f1
An optical frequency modulation signal centered at 1 is output. Due to this property, the optical modulator 702 converts the electric signal Si input from the signal source 700 into an optical frequency modulation signal L1, and outputs the signal. The local light source 703 outputs unmodulated light L2 having a constant optical frequency f2. Optical modulator 702 and local light source 70
The lights L1 and L2 output from 3 are input to the optical receiver 714 via the first optical waveguide 706 and the second optical waveguide 708, respectively. The optical receiver 714 is composed of a photodiode having a square-law detection characteristic or the like, and a beat signal of the two lights L1 and L2 at a frequency fs = | f1-f2 | corresponding to an optical frequency difference between the two lights. (This operation is called “optical heterodyne detection”). The beat signal thus obtained becomes a frequency modulation signal Sfm using the electric signal Si from the signal source 700 as an original signal.

As described above, the conventional frequency modulation device shown in FIG. 14 utilizes the high frequency modulation efficiency of a semiconductor laser (10 times or more the frequency modulation efficiency of a general electric circuit system). In addition, it is possible to easily generate an FM modulation signal having a very high frequency and a wide band (having a large amount of frequency shift), which is difficult to generate with a general electric circuit.

[0005]

However, since a light source such as a semiconductor laser generally has a larger phase noise than an electric oscillator, the above-mentioned conventional frequency modulation apparatus has a problem in demodulating an FM modulation signal generated thereby. In addition, there is a specific problem that the white noise component increases. That is,
If the optical frequency modulation signal L1 from the optical modulator 702 and the unmodulated light L2 from the local light source 703 have a frequency spectrum as shown in FIG.
The frequency spectrum of the FM-modulated signal Sfm output from 4 is as shown in FIG. FIG. 15 (A)
As shown in (B) and (B), the phase noise included in the FM modulation signal Sfm is the sum of the phase noises included in each of the optical frequency modulation signal L1 and the unmodulated light L2. When demodulating, the white noise component increases.

Therefore, an object of the present invention is to realize a high-frequency and wide-band frequency modulation while realizing a high-frequency and wide-band frequency modulation by a configuration combining optical frequency modulation by a semiconductor laser and optical heterodyne detection. An object of the present invention is to provide a frequency modulation device that suppresses noise and has excellent noise characteristics.

[0007]

Means for Solving the Problems and Effects of the Invention A first invention is an optical frequency modulation and optical system using first and second light sources respectively outputting first and second lights having different optical frequencies. What is claimed is: 1. A frequency modulation device that converts an input electric signal into a frequency modulation signal by heterodyne detection, and outputs the frequency-modulated signal as an FM modulation signal, wherein the first light frequency-modulated by the input electric signal is output as a first optical signal. A first optical modulator, a beat signal generation unit that generates an unmodulated beat signal corresponding to a carrier component of a beat signal obtained by optical detection based on the squared detection characteristic from the first and second lights, A frequency converter that generates a frequency conversion signal by converting the frequency of an unmodulated beat signal; and one of the first optical signal and the second light is the frequency conversion signal. A second optical modulator that generates a second optical signal by performing amplitude modulation or optical intensity modulation, and outputs the other of the first optical signal and the second light and the second optical signal. An optical receiver that receives the signal and generates the FM modulation signal by optical detection based on the squared detection characteristic.

According to the first aspect, two optical heterodyne systems are realized by two light sources (first and second light sources) and two square-law detectors (beat signal generator and optical receiver). A carrier component of a beat signal generated in a first optical heterodyne system including a first light modulator corresponding to the first light source, a second light source corresponding to a local light source, and a beat signal generation unit (Center frequency component) is a frequency conversion signal obtained by frequency conversion, and is used in a second optical heterodyne system including a first optical modulator, a second light source (local light source), and an optical receiver. Either one of the two lights is subjected to amplitude modulation or intensity modulation. As a result, the phase noise of the FM modulation signal, which is a beat signal generated in the second optical heterodyne system, is suppressed, and frequency modulation with excellent noise characteristics can be realized.

In a second aspect based on the first aspect, the first optical modulator generates the first optical signal by direct modulation, and the beat signal generating section generates the first optical signal. And a modulated electric signal having a center frequency corresponding to a difference between an optical frequency of the first optical signal and an optical frequency of the second light by optical detection based on the square-law detection characteristic upon receiving the second light. A light receiving element for generating a modulated beat signal, and extracting the carrier component from the modulated beat signal;
A filter that outputs the carrier component as the unmodulated beat signal.

According to the second aspect of the present invention, the first optical signal is generated by the direct modulation, and the non-modulated carrier wave component is obtained from the modulated beat signal generated from the first optical signal and the second light. A beat signal is extracted by a filter, and is a frequency-converted signal obtained by frequency-converting the unmodulated beat signal.
Either of the two lights is subjected to amplitude modulation or intensity modulation. As a result, phase noise of the FM modulation signal generated by the optical receiver can be suppressed, and frequency modulation with excellent noise characteristics can be realized.

[0013] In a third aspect based on the first aspect, the first optical modulator comprises: a first light source that outputs the first light that is unmodulated light; An external optical modulator that generates the first optical signal by modulating the output first light with the input electric signal, wherein the second light source converts the unmodulated light into the second optical signal. Output as light, and the beat signal generating section outputs the first unmodulated light.
And a light receiving element that receives the second light and generates the non-modulated beat signal by optical detection based on the squared detection characteristic.

According to the third aspect, the first light is converted into the first light signal which is the modulated light by the external modulation,
In the beat signal generation unit, an unmodulated beat signal is generated from the first and second lights that are unmodulated lights. Therefore, a filter for extracting the carrier component from the modulated beat signal becomes unnecessary.

In a fourth aspect based on the first aspect, the first optical modulator uniquely converts a change in the amplitude of the input electric signal into a change in the optical frequency of the first light. The second light source generates the first light signal having a predetermined center frequency f1, the second light source outputs light having a predetermined frequency f2 as the second light, and the beat signal generation unit generates the first light signal. An optical signal and the second light are received, and a frequency fs = | f1 corresponding to a difference between the optical frequencies of the first optical signal and the second light is obtained by optical detection based on the squared detection characteristic.
−f2 |, including a light receiving element that generates a modulated beat signal that is a modulated electric signal, and a filter that extracts a carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal. The frequency converter converts the unmodulated beat signal to a predetermined frequency fx
And the converted signal is output as the frequency-converted signal. The second optical modulator converts one of the first optical signal and the second light into a signal having a frequency fx. A second optical signal is generated by performing optical amplitude modulation or optical intensity modulation with a frequency conversion signal, and the optical receiver includes the first optical signal.
The other of the optical signal and the second light and the second
At the frequency fL = | fs-fx | by the optical detection based on the squared detection characteristic.
A modulation signal is generated.

According to the fourth aspect, the input electric signal is converted into the optical frequency modulation signal by the first optical modulator, and the first optical modulator and the second light source corresponding to the local light source are connected to the beat light source. In a first optical heterodyne system including a signal generation unit, a carrier component which is an unmodulated beat signal output from the beat signal generation unit is frequency-converted, and the first optical modulator and the second light source (station) In a second optical heterodyne system composed of a light-emitting source) and an optical receiver, one of the light output from the first optical modulator and the light from the second light source is converted into the frequency-converted carrier wave component. Is subjected to amplitude modulation or intensity modulation. As a result, the phase noise of the FM signal, which is the beat signal output from the optical receiver, is suppressed, and frequency modulation with excellent noise characteristics can be realized.

In a fifth aspect based on the first aspect, the duplexer branches the input electrical signal into first and second electrical signals having phases opposite to each other, and optical intensity modulation of the second electrical signal. A third optical modulator for converting into a third optical signal by
And the first optical modulator uniquely converts a change in the amplitude of the first electric signal into a change in the optical frequency of the first light by direct modulation, thereby obtaining a predetermined center frequency f1. The first light signal is generated, the second light source outputs light having a predetermined frequency f2 as the second light, and the beat signal generation unit outputs the first light signal and the second light signal. And modulated by the optical detection based on the squared detection characteristic at the frequency fs = | f1-f2 | corresponding to the difference between the optical frequencies of the first optical signal and the second light. A light-receiving element that generates a modulated beat signal that is a signal; and a filter that extracts a carrier component of the modulated beat signal and outputs the carrier component as the unmodulated beat signal. Place the modulated beat signal And outputs the converted signal as the frequency-converted signal. The second optical modulator converts one of the first optical signal and the second light into a frequency fx signal. generating a second optical signal by performing optical amplitude modulation or optical intensity modulation with the frequency-converted signal of fx, wherein the optical receiver includes the other of the first optical signal and the second optical signal and the second optical signal. The second optical signal is received, the FM modulation signal is generated at a frequency fL = | fs-fx | by optical detection based on the squared detection characteristic, and the third optical signal is received and the optical detection is performed. Thus, an electrical signal corresponding to a light intensity modulation component included in the third optical signal is generated.

According to the fifth aspect, the input electric signal, which is the original signal of the frequency modulation, has the first and second phases opposite to each other.
The first optical signal is converted into a first optical signal which is an optical frequency modulation signal by the first optical modulator, and the first optical signal is converted into the first optical signal and the second optical signal corresponding to the local light source. In the first optical heterodyne system including the light source and the beat signal generator, the carrier component, which is the unmodulated beat signal output from the beat signal generator, is frequency-converted, and the first optical modulator and the second optical modulator are used. In a second optical heterodyne system including a light source (local light source) and an optical receiver, one of the light output from the first optical modulator or the second light source is:
Amplitude modulation or intensity modulation is performed on the frequency-converted carrier wave component. Further, the second electric signal is converted by the third optical modulator into a third optical signal, which is an optical intensity modulation signal, and input to the optical receiver, where the intensity modulation-direct detection component (IM- DD component) is generated. With the IM-DD component, the IM-DD component corresponding to the light intensity modulation component (which occurs due to the direct modulation) included in the first optical signal is canceled in the optical receiver. Thereby, the beat signal F which is output from the optical receiver is obtained.
It is possible to suppress the phase noise of the M-modulated signal and reduce the unnecessary components to realize good quality frequency modulation.

In a sixth aspect based on the fifth aspect, the light included in the first optical signal in the intensity modulation-direct detection component generated by optical detection based on the square detection characteristic of the optical receiver. First IM-D corresponding to intensity modulation component
The D component and the second IM-DD component corresponding to the light intensity modulation component included in the third optical signal among the generated intensity modulation-direct detection components have opposite phases and the same amplitude. A phase / amplitude adjusting means is further provided.

According to the sixth aspect, the first IM-DD component corresponding to the light intensity modulation component included in the first optical signal and the light intensity modulation component corresponding to the third optical signal are included. The phase and amplitude of the second IM-DD component are adjusted so that they have the opposite phase and the same amplitude. For this reason, the IM-DD component output from the optical receiver is more reliably suppressed, and a higher quality high frequency / broadband FM modulated signal can be generated.

According to a seventh aspect based on the first aspect, there is provided a duplexer for splitting the input electric signal into first and second electric signals having phases opposite to each other, and a modulated light having a predetermined center frequency f2. A fourth optical modulator that outputs the second light as a fourth optical signal, wherein the first optical modulator detects a change in the amplitude of the first electrical signal by the direct modulation. 1
The first optical signal having a predetermined center frequency f1 is generated by uniquely converting the optical signal into a change in the optical frequency of the light, and the fourth optical modulator generates the first optical signal by direct modulation. By uniquely converting the change in amplitude into a change in the optical frequency of the second light, the fourth change of the predetermined center frequency f2 is obtained.
And the beat signal generating unit generates the first optical signal.
And the fourth optical signal, and by means of optical detection based on the square-law detection characteristic, an electric signal modulated at a frequency fs = | f1-f2 | corresponding to a difference between the optical frequencies of the first and fourth optical signals. A light-receiving element that generates a modulated beat signal that is a signal; and a filter that extracts a carrier component of the modulated beat signal and outputs the carrier component as the unmodulated beat signal. The modulated beat signal is converted into a signal having a predetermined frequency fx, and the converted signal is output as the frequency-converted signal. The second optical modulator includes the first optical signal and the fourth optical signal. One of which is subjected to optical amplitude modulation or optical intensity modulation with the frequency conversion signal having the frequency fx to generate a second optical signal, and the optical receiver includes the first optical signal and the fourth optical signal. Out of Receiving the second optical signal and the second optical signal, and generating the FM modulated signal at a frequency fL = | fs-fx | by optical detection based on the squared detection characteristic.

According to the seventh aspect, the input electric signal, which is the original signal of the frequency modulation, has the first and second phases opposite to each other.
The first electric signal is converted into a first optical signal which is an optical frequency modulation signal by direct modulation by the first optical modulator, and is directly converted by the fourth optical modulator. Second
Is converted into a fourth optical signal which is an optical frequency modulation signal. Then, in the first optical heterodyne system including the first optical modulator, the fourth optical modulator, and the beat signal generator, the carrier component that is the unmodulated beat signal output from the beat signal generator is used. The frequency is converted, and a second optical modulator including a first optical modulator, a fourth optical modulator, and an optical receiver is formed.
In the optical heterodyne system, one of the light output from the first optical modulator and the light output from the fourth optical modulator is amplitude-modulated or intensity-modulated by the frequency-converted carrier component. As a result, the phase noise of the FM signal, which is the beat signal output from the optical receiver, is suppressed, and frequency modulation with excellent noise characteristics can be realized. Further, as described above, since the first optical modulator and the fourth optical modulator perform a push-pull operation to generate a frequency modulation signal, unnecessary frequency modulation components caused by direct modulation are unnecessary. The IM-DD component, which is the component, is canceled. As a result, it is possible to realize high-quality frequency modulation in which unnecessary components are reduced.

In an eighth aspect based on the seventh aspect, the light included in the first optical signal in the intensity modulation-direct detection component generated by optical detection based on the square detection characteristic of the optical receiver. First IM-D corresponding to intensity modulation component
The D component and the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components have opposite phases and the same amplitude. A phase / amplitude adjusting means is further provided.

According to the eighth aspect, the first IM-DD component corresponding to the light intensity modulation component included in the first optical signal and the light intensity modulation component included in the fourth optical signal. The phase and amplitude of the third IM-DD component are adjusted so that the third IM-DD component and the third IM-DD component have opposite phases and the same amplitude. For this reason, the IM-DD component output from the optical receiver is more reliably suppressed, and a higher quality high frequency / broadband FM modulated signal can be generated.

According to a ninth aspect, in the first aspect, an optical signal is inserted between the first optical modulator and the beat signal generating section to extract an optical carrier component from the first optical signal. The apparatus further includes a filter, wherein the second light source outputs unmodulated light as the second light, and the beat signal generator includes:
A light receiving element that receives the optical carrier component extracted by the optical filter and the second light, and generates the unmodulated beat signal by optical detection based on a square detection characteristic.

According to the ninth aspect, the input electric signal is converted into the first optical signal, which is an optical frequency modulation signal, by the first optical modulator, and corresponds to the first optical modulator and the local light source. In a first optical heterodyne system composed of a second light source and a beat signal generator, the first optical signal is inserted by an optical filter inserted between the first optical modulator and the beat signal generator. The unmodulated beat signal is extracted from the optical carrier component and the unmodulated beat signal is generated in the beat signal generating unit from the optical carrier component and the second light without using a filter. By doing so, a frequency conversion signal is generated. Then, in the second optical heterodyne system including the first optical modulator, the second light source, and the optical receiver, the first optical modulator or the second optical heterodyne system is used.
Either light output from the light source is amplitude-modulated or intensity-modulated by the frequency-converted signal (frequency-converted carrier component). As a result, the phase noise of the FM signal, which is the beat signal output from the optical receiver, is suppressed, and frequency modulation with excellent noise characteristics can be realized.

According to a tenth aspect, in the first aspect, a splitter for splitting the input electric signal into first and second electric signals having phases opposite to each other, and modulated light having a predetermined center frequency f2. A fourth output of the second light as a fourth optical signal;
An optical modulator, a first optical filter inserted between the first optical modulator and the beat signal generator,
And a second optical filter inserted between the optical modulator and the beat signal generation unit, wherein the first optical modulator detects an amplitude change of the first electric signal by direct modulation. The fourth optical modulator generates the first optical signal having a predetermined center frequency f1 by performing a unique conversion to a change in the optical frequency of the first light, and the fourth optical modulator directs the second optical signal by a direct modulation.
By uniquely converting the change in the amplitude of the electric signal into a change in the optical frequency of the second light, the fourth optical signal having a predetermined center frequency f2 is generated, and the first optical filter includes:
The second optical filter extracts the optical carrier component from the first optical signal, the second optical filter extracts the optical carrier component from the fourth optical signal, and the beat signal generator includes the first and second optical signals. Receiving the two optical carrier components extracted by the second optical filter and performing optical detection based on the squared detection characteristic,
A frequency f corresponding to the difference between the optical frequencies of the two optical carrier components
s = | f1−f2 |, a light receiving element for generating the unmodulated beat signal, the frequency converter converts the unmodulated beat signal into a signal of a predetermined frequency fx, and converts the converted signal to the signal Outputting as a frequency-converted signal, wherein the second optical modulator modulates one of the first optical signal and the fourth optical signal with the frequency-converted signal having the frequency fx by light amplitude modulation or light intensity modulation. Generates a second optical signal, and the optical receiver generates the second optical signal by using the first optical signal
Receiving the other one of the optical signals and the second optical signal, and performing optical detection based on the squared detection characteristic, the frequency fL
= | Fs-fx |, wherein the FM modulation signal is generated.

According to the tenth aspect, the input electric signal, which is the original signal of the frequency modulation, is branched into the first and second electric signals having phases opposite to each other, and is directly modulated by the first optical modulator. The first electric signal is converted into a first optical signal that is an optical frequency modulation signal, and the second electric signal is converted into a fourth optical signal that is an optical frequency modulation signal by direct modulation in a fourth optical modulator. Is converted. Then, in the first optical heterodyne system including the first optical modulator, the fourth optical modulator, and the beat signal generating unit, the first optical filter converts the optical carrier component of the first optical signal from the first optical signal. Extracted, and the optical carrier component thereof is extracted from the fourth optical signal by the second optical filter,
The beat signal generator generates an unmodulated beat signal from these optical carrier components without using a filter. A frequency-converted signal is generated by frequency-converting the unmodulated beat signal, that is, the carrier wave component. In the second optical heterodyne system composed of the first optical modulator, the fourth optical modulator, and the optical receiver, , One of the light output from the first optical modulator or the fourth optical modulator is amplitude-modulated or intensity-modulated by the above-mentioned frequency conversion signal (frequency-converted carrier component). As a result, it is possible to suppress the phase noise of the FM modulation signal, which is the beat signal output from the optical receiver, and realize high-quality frequency modulation in which unnecessary components are reduced.

According to an eleventh aspect, in the tenth aspect,
A first IM corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver.
The DD component and the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components are set to have the same amplitude and the opposite phase to each other. And a phase / amplitude adjusting means for performing the adjustment.

According to the eleventh aspect, as in the eighth aspect, the first IM-DD component corresponding to the light intensity modulation component included in the first optical signal and the fourth optical signal The phase and amplitude of the third IM-DD component corresponding to the included light intensity modulation component are adjusted such that the third IM-DD component has the opposite phase and the same amplitude. For this reason, the IM-DD component output from the optical receiver is more reliably suppressed, and a higher quality high frequency / broadband FM modulated signal can be generated.

According to a twelfth aspect based on the first aspect, the light and electric propagation time of a path from the first light source to the optical receiver via the beat signal generation section,
First propagation time adjusting means for equalizing the light propagation times of the path directly from the light source to the optical receiver, and from the second light source to the optical receiver via the beat signal generator. The apparatus further includes a second propagation time adjusting unit that makes light and electric propagation times of the path equal to light propagation times of a path from the second light source directly to the optical receiver.

According to the twelfth aspect, each of the two branched optical signals output from the first light source passes through each component or performs processing such as photoelectric conversion or electro-optical conversion. Then, the propagation times until reaching the optical receiver are set to be equal to each other, and each of the lights output from the second light source and branched into two passes through each component, or After processing such as electrical conversion and electro-optical conversion,
The propagation times to reach the optical receiver are set to be equal to each other. This makes it possible to more appropriately suppress the phase noise of the FM modulation signal, which is the beat signal output from the optical receiver, and to realize frequency modulation with excellent noise characteristics.

[0031]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

(First Embodiment) FIGS. 1 and 2A
The frequency modulation device according to the first embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a block diagram showing the configuration of the frequency modulation device according to the first embodiment. The frequency modulation device includes a signal source 100, a first optical modulator 102, a local light source 103, a first optical branch circuit 104, a second optical branch circuit 105, and a first optical waveguide 106. , A second optical waveguide 107, a third optical waveguide 108, a fourth optical waveguide 109, a first optical receiver 110 and a filter 11
1 including the beat signal generation unit 200 and the frequency converter 11
2, a second optical modulator 113, and a second optical receiver 114
And This frequency modulation device includes two lights having different optical frequencies (a first light L1 and a second light L).
The electric signal Si output from the signal source 100 is converted into a frequency-modulated signal by optical frequency modulation and optical heterodyne detection using two light sources (first and second light sources) respectively outputting 2). Output as FM modulation signal Sfm.

Next, the operation of the frequency modulation device shown in FIG. 1 will be described in detail. The first optical modulator 102 is an optical modulator of a direct modulation system, and is usually formed of a semiconductor laser which is a first light source having an optical frequency modulation effect, and has a signal source 1
By amplitude-modulating the injection current to the semiconductor laser with the electric signal Si output from 00, the optical frequency modulation signal L1 having the central optical frequency f1 is output as the first light. The optical signal L1 output from the first optical modulator 102
Is split into two optical signals by the first optical splitter 104,
One of these optical signals is transmitted via the second optical waveguide 107 to the first optical receiver 1 in the beat signal generator 200.
10, the other optical signal is input to the second optical receiver 114 via the first optical waveguide 106, respectively. The local light source 103 corresponds to a second light source, and outputs unmodulated light L2 having an optical frequency f2 as second light. The light L2 output from the local light source 103 is split into two lights by a second optical splitting circuit 105, and one of these lights is transmitted through a fourth optical waveguide 109 to the second light splitter 105 in the beat signal generator 200. One optical receiver 110 and the other light are input to the second optical modulator 113 via the third optical waveguide 108, respectively.

The beat signal generation section 200 generates the first optical signal L1 branched by the first optical branching circuit 104 and the second optical signal L1.
And the second light L2 branched by the optical branching circuit 105 of the first and second lights L1 and L2 by the following operation.
Fs = | f1-f2 | corresponding to the difference between the optical frequencies of
Unmodulated electric signal (hereinafter referred to as “unmodulated beat signal”)
Sb). That is, the first optical receiver 1
Reference numeral 10 denotes a light receiving element such as a photodiode having a square detection characteristic, which receives the optical signal L1 from the first optical modulator 102 and the light L2 from the local light source 103, and based on the square detection characteristic. By optical heterodyne detection, a beat signal having the optical frequency difference fs (= | f1−f2 |) as a center frequency (hereinafter, referred to as “modulated beat signal”)
Generate Sbfm. This modulated beat signal Sbfm is
Optical frequency modulation signal L1 output from the optical modulator 102 of FIG.
Is a down-converted FM modulated signal. FIG.
(A) shows the case where the frequency spectrum of the modulated beat signal Sbfm is assumed to be a signal centered on the frequency fp from the electric signal Si output from the signal source 100. The modulated beat signal Sbfm is applied to the filter 11
1 is input. The filter 111 is a band-pass filter, and extracts only a carrier component (a component of the center frequency fs) of the modulated beat signal Sbfm, which is an FM modulated signal, and outputs it as a non-modulated beat signal Sb.

The frequency converter 112 has a frequency fL, for example.
The non-modulated beat signal Sb, which is a carrier component output from the filter 111, is frequency-converted (for example, down-converted) by the local oscillation signal SL, and the frequency fx (= |
fs−fL |) is output as a frequency conversion signal Sx. FIG. 2B shows the frequency spectrum of the unmodulated beat signal Sb as the input signal of the frequency converter 112 and the frequency spectrum of the frequency converted signal Sx as the output signal together with the frequency spectrum of the local oscillation signal SL.

The second optical modulator 113 is inserted and arranged on the way of the second optical waveguide 108, and converts the unmodulated light L 2 output from the local light source 103 into a frequency conversion output from the frequency converter 112. Optically modulated by the signal Sx.
That is, the second optical modulator 113 converts the unmodulated light L2 into an optically modulated signal L2x by optical amplitude modulation or optical intensity modulation, and outputs this.

The second optical receiver 114 is constituted by a light receiving element such as a photodiode having a square-law detection characteristic,
The optical signal L1 from the optical modulator 102 and the second optical modulator 1
13 and receives the optical signal L2x from the optical fiber 13 and performs the optical heterodyne detection based on the squared detection characteristic to obtain the frequency fL (=
| Fs−fx |) is generated as a beat signal having a center frequency of FM modulation signal Sfm. The FM modulation signal Sfm generated in this manner is a signal obtained by down-converting the optical frequency modulation signal L1 output from the first optical modulator 102, and has a phase noise as shown in FIG. Have been oppressed.

The effect of suppressing the phase noise of the FM modulation signal in the present embodiment will be described below using mathematical expressions. The optical frequency modulation signal L output from the first optical modulator 102
Field components _{1, E 1 (t) = A} 1 [1 + m 1 cos (2πf p t)] 1/2 cos [2πf 1 t + ΔFsin (2πf p t) + φ 1] ... (1) station The electric field component of the unmodulated light L2 output from the light emitting source 103 is expressed by E ₂ (t) = A ₂ cos (2πf ₂ t + φ ₂ ) (2). Here, m1 represents the degree of light (intensity) modulation in the first optical modulator 102, fp represents the frequency of the electric signal Si (assuming a sine wave) output from the signal source 100, and f1 and f2. Represent the optical frequencies of the light output from the first optical modulator 102 and the local light source 103, respectively, and φ1 and φ2 represent the first optical modulator 102
Represents phase noise of light output from the local light source 103, and ΔF is a frequency shift amount provided in the first optical modulator 102 corresponding to the electric signal Si output from the signal source 100. Represents

The first optical receiver 110 detects these two lights L
1 and L2, the first optical receiver 1
Based on the square-law detection characteristic of the light receiving element used in 10, a photocurrent expressed by the following equation flows through the light receiving element. I ₁ (t) = R ₁ | E ₁ (t) + E ₂ (t) | ² = R ₁ A ₁ ² [1 + m ₁ cos (2πf _p t)] cos ² [2πf ₁ t + ΔFsin (2πf _p t) + φ ₁ ] + R ₁ A ₂ ² cos ² (2πf ₂ t + φ ₂ ) + 2R ₁ A ₁ A ₂ [1 + m ₁ cos (2πf _p t)] ^1/2 cos [2πf ₁ t + ΔFsin (2πf _p t) + φ ₁ ] × cos (2πf ₂ + φ ₂ ) (3) where R1 represents the conversion efficiency of the light receiving element used in the first optical receiver 110. The signal components corresponding to the first term and the second term in the equation (3) are expressed by the first optical receiver 110
Is not detected due to the frequency response limitation of the light-receiving element used in step (1). The signal component corresponding to the first term includes an IM-DD (intensity modulation-direct detection) component generated by directly detecting the light intensity modulation component in the optical signal L1 from the first optical modulator 102. Included, but this IM-DD
The components are removed by a filter 111 described later and may be ignored.

Further expanding the third term in equation (3), I _c1 (t) = R ₂ A ₁ A ₂ [1 + m ₁ cos (2πf _p t)] ^1/2 × [cos [2π (f ₁ + f ₂ ) t + ΔFsin (2πf _p t) + φ ₁ + φ ₂ ] + cos [2π (f ₁ -f ₂ ) t + ΔFsin (2πf _p t) + φ ₁ -φ ₂ ]] ... (4) The signal component corresponding to the first term in the square bracket "[]" in the equation (4) is not detected due to the frequency response limit of the light receiving element used in the first optical receiver 110, and the square bracket "[]" Only the signal component corresponding to the second term is output. This second term uses the electric signal Si output from the signal source 100 as the original signal and sets the carrier frequency to fs = | f1-
f2 | represents an FM modulation signal with a frequency shift amount ΔF. The FM signal of the second term has a phase noise φ1 of the optical signal L1 output from the first optical modulator 102.
And the phase noise φ2 of the light L2 output from the local light source 103.
And has phase noise determined by

The filter 111 has an F value represented by the equation (4).
It is a signal component expressed as I _s (t) = cos [2π (f ₁ −f ₂ ) t + φ ₁ −φ ₂ ] (5) as a carrier component (a component of the center frequency fs) in the M-modulated signal. Only the unmodulated beat signal Sb is passed.

The frequency converter 112 converts the frequency of the unmodulated beat signal Sb expressed by the equation (5) using the local oscillation signal SL having the frequency fL. For example, by down-converting the unmodulated beat signal Sb, the frequency f
x = | f1-f2-fL | (Frequency conversion signal Sx).

The second optical modulator 113 modulates the unmodulated light L2 (see equation (2)) output from the local light source 103 by, for example, optical amplitude modulation using the frequency conversion signal Sx represented by equation (6). Then, an optical modulation signal L2x having an electric field component expressed by E _2x (t) = A ₂ cos (2πf _x t + φ ₁ -φ ₂ ) cos (2πf ₂ t + φ ₂ ) (7) is output.

The second optical receiver 114 receives the optical modulation signal L1 (see equation (1)) from the first optical modulator 102 and the second
And the optical modulation signal L2x (see Equation (7)) from the optical modulator 113, and based on its square detection characteristics, I ₂ (t) = R ₂ | E ₁ (t) + E _2x (t ) | ² = R ₂ A ₁ ² [1 + m ₁ cos (2πf _p t)] cos ² [2πf ₁ t + ΔFsin (2πf _p t) + φ ₁ ] + R ₂ A ₂ ² cos ² (2πf _x t + φ ₁ -φ ₂ ) cos ² (2πf ₂ t + φ ₂ ) + 2R ₂ A ₁ A ₂ [1 + m ₁ cos (2πf _p t)] ^1/2 cos [2πf ₁ t + ΔFsin (2πf _p t ) + φ ₁ ] × cos (2πf _× t + φ ₁ −φ ₂ ) cos (2πf ₂ + φ ₂ ) (8) A photocurrent is output. Here, R2 represents the conversion efficiency of the light receiving element used in the second optical receiver 114. The signal components corresponding to the first and second terms in the equation (8) are not detected due to the frequency response limitation of the light receiving element used in the second optical receiver 114. The signal component corresponding to the first term includes an IM-DD component generated by directly detecting a light intensity modulation component in the optical signal L1 from the first optical modulator 102.
When the frequency band of the M-DD component is far from the frequency band of the FM modulation signal Sfm, this component may be ignored. The frequency band of this IM-DD component is FM modulated signal S
When the frequency band overlaps with the frequency band of fm, this component can be suppressed by the configuration of another embodiment described later.

When the third term in the equation (8) is further expanded, I _c2 (t) = R ₂ A ₁ A ₂ [1 + m ₁ cos (2πf _p t)] ^1/2 × [cos (2πf _x t + φ ₁ -φ ₂ ) cos [2π (f ₁ + f ₂ ) t + ΔFsin (2πf _p t) + φ ₁ + φ ₂ ] + cos (2πf _x t + φ ₁ -φ _{2 ) cos [2π (f 1 -f} 2) t + ΔFsin (2πf p t) + φ 1 -φ 2]] = (R 2 A 1 A 2/2) [1 + m 1 cos (2πf p t)] ^1/2 × [cos [2π (f ₁ + f ₂ + f _x ) t + ΔFsin (2πf _p t) + 2φ ₁ ] + cos [2π (f ₁ + f ₂ -f _x ) t + ΔFsin (2πf _p t ) + 2φ ₂ ] + cos [2π (f ₁ -f ₂ + f _x ) t + ΔFsin (2πf _p t) +2 (φ ₁ -φ ₂ )] + cos [2π (f ₁ -f ₂ -f _x ) t _{+ ΔFsin (2πf p t)]} ] ... it is (9). The signal components corresponding to the first term and the second term in the square brackets “[]” in the equation (9) are output to the second optical receiver 11
The signal is not detected due to the frequency response limit of the light receiving element used in No. 4 and only the signal components corresponding to the third and fourth terms in square brackets “[]” are output as the FM modulation signal Sfm. In the fourth term, the electric signal Si output from the signal source 100 is used as an original signal, and the carrier frequency is set to fL = | f1.
−f2−fx |, which represents an FM modulation signal having a frequency shift amount ΔF. The FM modulated signal of the fourth term does not depend on the phase noises φ1 and φ2 of the lights L1 and L2 output from the first optical modulator 102 and the local light source 103, respectively, and is a signal having no phase noise.

Note that the second optical modulator 113 is given by the following equation (6).
The local light source 103 is generated by the frequency conversion signal Sx expressed by
When the unmodulated light L2 (see equation (2)) output from the optical modulator ₁₀₃ is _subjected to light intensity modulation, the electric field component of the optical modulation signal output from the second optical modulator 113 is E _2y (t) = A ₂ [1 + m ₂ cos (2πf _x t + φ ₁ -φ ₂ )] ^1/2 cos (2πf ₂ t + φ
₂ ). The above equation can be approximated as the following equation. _{E 2y (t) = A 2} [1+ (m 2/2) cos (2πf x t + φ 1 -φ 2)] cos (2πf 2 t + φ 2) ... (10) where, m2 is the 2 is a light (intensity) modulation degree in the second light modulator 113. Braces "@" in equation (10)
In the following, only the second term will be considered with respect to the inside of the curly bracket “｛｝”, since the first term in the above indicates an unmodulated light component.

The second optical receiver 114 receives an optical modulation signal L1 (see equation (1)) from the first optical modulator 102 and a second
Of the optical modulation signal L2x from the optical modulator 113 (see the second item in the curly bracket “｛｝” in the equation (10)), and based on its square detection characteristic, I ₂ (t) = R ₂ | E ₁ (t) + E _2y (t) | ² = R ₂ A ₁ ² [1 + m ₁ cos (2πf _p t)] cos ² [2πf ₁ t + ΔFsin (2πf _p t) + φ ₁ ] + ^{_{R 2 A 2 2 (m 2}} 2/4) cos 2 (2πf x t + φ 1 -φ 2) cos 2 (2πf 2 t + φ 2) + 2R 2 A 1 A 2 [1 + m 1 cos (2πf _p t)] ^1/2 cos [2πf ₁ t + ΔFsin (2πf _p t) + φ ₁ ] _{× (m 2/2) cos} (2πf x t + φ 1 -φ 2) cos (2πf 2 + φ 2) ... and outputs a photocurrent represented by (11). The first in equation (11)
The signal component corresponding to the term and the second term is not detected due to the frequency response limitation of the light receiving element used in the second optical receiver 114. The signal component corresponding to the first term includes:
IM-D generated by directly detecting the light intensity modulation component in the optical signal L1 from the first optical modulator 102
Although the D component is also included, if the frequency band of the IM-DD component is far from the frequency band of the FM modulation signal Sfm, this component may be ignored. When the frequency band of the IM-DD component overlaps the frequency band of the FM modulation signal Sfm, this component can be suppressed by the configuration of another embodiment described later.

[0049] With the third term the further development in equation _{(11), I c2 (t} ) = (R 2 A 1 A 2 m 2/2) [1 + m 1 cos (2πf p t)] 1/2 × [cos (2πf _x t + φ ₁ -φ ₂ ) cos [2π (f ₁ + f ₂ ) t + ΔFsin (2πf _p t) + φ ₁ + φ ₂ ] + cos (2πf _x t + φ ₁ -φ _{2 ) cos [2π (f 1 -f} 2) t + ΔFsin (2πf p t) + φ 1 -φ 2]] = (R 2 A 1 A 2 m 2/4) [1 + m 1 cos (2πf p t ^{)] 1/2 × [cos [2π} (f 1 + f 2 + f x) t + ΔFsin (2πf p t) + 2φ 1] + cos [2π (f 1 + f 2 -f x) t + ΔFsin (2πf _p t) + 2φ ₂ ] + cos [2π (f ₁ -f ₂ + f _x ) t + ΔFsin (2πf _p t) +2 (φ ₁ -φ ₂ )] + cos [2π (f ₁ -f ₂ -f _{x ) t + ΔFsin (2πf p t} )]] ... it is (12). The fourth term in square brackets “[]” in the equation (12) uses the electric signal Si output from the signal source 100 as the original signal and sets the carrier frequency to fL = | f1 as in the equation (9).
−f2−fx |, which represents an FM modulation signal having a frequency shift amount ΔF. The FM modulated signal of the fourth term does not depend on the phase noises φ1 and φ2 of the lights L1 and L2 output from the first optical modulator 102 and the local light source 103, respectively, and is a signal having no phase noise.

As apparent from the comparison between Expressions (9) and (12), the magnitude (amplitude) of the FM modulated signal L2x output from the second optical receiver 114 is equal to that of the second optical receiver 114. Applying the optical amplitude modulation in the modulator 113 is twice as large, and when transmitting this FM modulation signal, the CNR
(Carrier to noise power ratio). Therefore,
In the following, only the case where optical amplitude modulation is applied in the second optical modulator 113 will be described.

As described above, according to the first embodiment, in an optical heterodyne configuration using two light sources, a high-frequency / broadband FM modulation signal without phase noise can be obtained without depending on the phase noise of the light sources. Can be generated.

(Second Embodiment) FIGS. 3 and 4A
A frequency modulation device according to a second embodiment of the present invention will be described below with reference to FIGS.

FIG. 3 is a block diagram showing the configuration of the frequency modulation device according to the second embodiment. This frequency modulation device has a demultiplexer 301, a third optical modulator 315, and a fifth optical waveguide 3 in addition to the configuration of the first embodiment shown in FIG.
17 are provided. In FIG. 3, the same components as those in the first embodiment are denoted by the same reference numerals.

Next, the operation of the present embodiment shown in FIG. 3 will be described. However, since the configuration of the present embodiment conforms to the above-described first embodiment, only differences will be described below. In the frequency modulation device of the present embodiment, the demultiplexer 301 branches the electric signal Si output from the signal source 100 into the first and second electric signals Si1 and Si2 so that the electric signals Si have a phase opposite to each other. Output. The first optical modulator 102 converts the first electric signal Si1 output from the demultiplexer 301 into an optical frequency modulation signal L1, and outputs the same, as in the first embodiment. The third optical modulator 315 is usually composed of a semiconductor laser, converts the second electric signal Si2 output from the demultiplexer 301 into an optical intensity modulation signal L3, and outputs it. This optical signal L3 is input to the second optical receiver 114 via the fifth optical waveguide 317.

The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113, and performs optical heterodyne based on the square detection characteristic. From the detection, those optical signals L1 and L1
As a beat signal having a 2x optical frequency difference as the center frequency, F
An M modulation signal Sfm is generated, and the third optical modulator 315
From the light intensity modulation signal L3, and the light intensity modulation signal L3
, An IM-DD (intensity modulation-direct detection) component is generated. When the first optical modulator 102 employs the direct modulation method using the semiconductor laser as described in the first embodiment, the optical signal L1 from the first optical modulator 102 is
The light intensity modulation component is included together with the light frequency modulation component. Therefore, the second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113, as shown in FIG. The optical heterodyne detection generates an FM modulation signal which is a beat signal of the two optical signals, and the direct detection simultaneously generates an FM intensity signal corresponding to an optical intensity modulation component included in the optical signal from the first optical modulator 102. One IM-DD (intensity modulation-direct detection) component Simdd1 is generated. This first IM-D
The D component Simdd1 is an unnecessary component for the FM modulation signal Sfm that is originally required. In particular, as shown in FIG. 4A, the occupied frequency band of the first IM-DD component Simdd1 is the frequency of the FM modulation signal Sfm. If it overlaps the band, FM
The quality of the modulated signal Sfm is greatly degraded. The second optical receiver 114 also receives the light intensity modulated signal L3 from the third optical modulator 315, and by direct detection, corresponds to the light intensity modulated signal L3 as shown in FIG. IM-
DD component, that is, the first IM-DD component Simdd1
And a second IM-DD component Simdd2 having a phase opposite to the above. The second IM-DD component Simdd2 forms the first IM-DD component.
As the DD component Simdd1 is offset, FIG.
As shown in (1), the first IM-DD component Simdd1 is suppressed.

As described above, according to the second embodiment, the generation of the IM-DD component, which is an unnecessary component due to the light intensity modulation component included in the optical signal L1, is suppressed, and high quality without phase noise is achieved. A high frequency / broadband FM modulation signal Sfm can be generated.

(Third Embodiment) A frequency modulation device according to a third embodiment of the present invention will be described below with reference to FIG.

FIG. 5 is a block diagram showing the configuration of the frequency modulation device according to the third embodiment. This frequency modulation device includes a demultiplexer 301 and a fourth optical modulator 516 in place of the local light source 103 in the configuration of the first embodiment shown in FIG. Note that, in FIG. 5, the same components as those in the first embodiment are denoted by the same reference numerals.

Next, the operation of the present embodiment shown in FIG. 5 will be described. In the frequency modulation device of the present embodiment, the duplexer 301
Converts the electric signal Si output from the signal source 100 into the first and second electric signals S
The output is branched to i1 and Si2. First optical modulator 102
Is the first electric signal Si1 output from the duplexer 301.
Is converted to an optical frequency modulation signal L1 and output as in the first embodiment. The optical signal L1 output from the first optical modulator 102 is split into two optical signals by a first optical splitter 104, and one of these is beat-beated via a second optical waveguide 107. First in the signal generation unit 200
The other optical signal is transmitted to the first optical waveguide 1
06 to the second optical receiver 114 respectively.

The fourth optical modulator 516 is an optical modulator of a direct modulation system, and is usually composed of a semiconductor laser or the like.
The second electric signal Si2 output from the demultiplexer 301 is converted into an optical frequency modulation signal L2 and output. Since the fourth optical modulator 516 employs the direct modulation method, the output optical signal L2 is, like the first optical modulator 102,
The light intensity modulation component is included together with the light frequency modulation component. 4th
The optical signal L2 output from the optical modulator 516 is divided into two optical signals by the second optical branching circuit 105, and one of these optical signals is generated through the fourth optical waveguide 109 to generate a beat signal. The first optical receiver 110 of the unit 200 and the other optical signal are input to the second optical modulator 113 via the third optical waveguide 108, respectively.

The beat signal generation unit 200 is configured to output the optical signal L1 branched by the first optical branching circuit 104 and the second optical branching circuit 1
05, and receives the optical signal L2 branched at 05, and by the following operation, an unmodulated electric signal having a frequency fs = | f1-f2 | corresponding to the difference between the optical frequencies of the first and second optical signals L1 and L2. Generate Sb. That is, the first optical receiver 110 is constituted by a light receiving element such as a photodiode having a square-law detection characteristic, and the optical signal L1 from the first optical modulator 102 and the optical signal from the fourth optical modulator 516. L2 and the optical frequency difference fs (= | f1-f2) by optical heterodyne detection based on the squared detection characteristic.
A modulated beat signal Sbfm having a center frequency of |) is generated. This modulated beat signal Sbfm is transmitted to the first optical modulator 10
2 is an FM-modulated signal obtained by down-converting the optical frequency-modulated signal L <b> 1 output from the signal L <b> 2. The filter 111 extracts only the carrier component (the component of the center frequency fs) of the modulated beat signal Sbfm and outputs the same as the unmodulated beat signal Sb.

The frequency converter 112 converts the frequency of the unmodulated beat signal Sb (for example, down-converts).
Then, a frequency conversion signal Sx, which is a signal of the frequency fx (= | fs-fL |), is output.

The second optical modulator 113 is inserted and arranged on the way of the second optical waveguide 108, and the fourth optical modulator 516 is provided.
Is optically modulated by the frequency conversion signal Sx output from the frequency converter 112. That is, the second optical modulator 113 converts the optical signal L2 into an optical modulation signal L2x by optical amplitude modulation or optical intensity modulation, and outputs this.

The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113, and performs optical heterodyne based on its square detection characteristics. By detection, the frequency fL (= | fs-f
x |) is generated as the beat signal having the center frequency as the FM signal Sfm in which the phase noise is suppressed (FIG. 2C).
reference). In addition, the second optical receiver 114 performs the direct detection based on the squared detection characteristic to obtain the first IM-DD component corresponding to the light intensity modulation component included in the optical signal L1 from the first optical modulator 102. And also receives an optical intensity modulation component included in the optical signal L2 from the fourth optical modulator 516, and performs direct detection based on its squared detection characteristic to perform an inverse phase with the first IM-DD component. The third IM that is
Generate a DD component. The third IM-DD component cancels out the first IM-DD component, thereby
An FM modulation signal Sfm with a suppressed -DD component is obtained.

As described above, according to the third embodiment, the generation of the IM-DD component, which is an unnecessary component due to the light intensity modulation component included in the optical signal L1, is suppressed, and high quality without phase noise is achieved. A high frequency / broadband FM modulation signal Sfm can be generated.

(Fourth Embodiment) A frequency modulation device according to a fourth embodiment of the present invention will be described below with reference to FIG.

FIG. 6 is a block diagram showing the configuration of the frequency modulation device according to the fourth embodiment. This frequency modulation device includes an optical filter 617 instead of the electric filter 111 in the configuration of the first embodiment shown in FIG. In FIG. 6, the same components as those in the first embodiment are denoted by the same reference numerals.

Next, the operation of the present embodiment shown in FIG. 6 will be described. However, since the configuration of the present embodiment conforms to the above-described first embodiment, only differences will be described below. In the frequency modulation device according to the present embodiment, the optical filter 617 splits the optical signal L <b> 1 from the first optical modulator 102 by the first optical splitter 104 and goes to the first optical receiver 108. Only the optical carrier component L1c of L1 is passed. First
The optical receiver 110 receives the optical carrier component L1c that has passed through the optical filter 617 and the light L2 from the local light source 103, and performs optical heterodyne detection based on its square detection characteristics to obtain an optical frequency difference fs. An unmodulated beat signal Sb having a corresponding frequency is generated. The frequency converter 112 converts the frequency of the unmodulated beat signal Sb,
The frequency conversion signal Sx having the frequency fx is output. The second optical modulator 113 outputs the light L2 output from the local light source 103.
Is subjected to light modulation (light amplitude modulation or light intensity modulation) using the frequency conversion signal Sx output from the frequency converter 112 to output a light modulation signal L2x. The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102.
And the optical signal L2x from the second optical modulator 113, and by optical heterodyne detection based on its square detection characteristic, a phase noise is obtained as a beat signal having a frequency fL (= | fs-fx |) as a center frequency. Is generated.

As described above, according to the fourth embodiment, in an optical heterodyne configuration using two light sources, a high-frequency / broadband FM modulation signal without phase noise can be obtained without depending on the phase noise of the light sources. Can be generated.

In the configuration in which the fourth light modulator 516 is provided instead of the local light source 103 as in the third embodiment (see FIG. 5), as shown in FIG.
As in the case of the first embodiment, the first
And the fourth optical waveguide 1
09, a second optical filter 627 is provided.
The second optical filter 627 may pass only the optical carrier component of the optical signal L2 branched to the first optical receiver 110. According to such a configuration, as in the third embodiment, since the unmodulated beat signal Sb is output from the first optical receiver 110, no phase noise is generated without using the electric filter 111. A high frequency / broadband FM modulation signal can be generated.

(Fifth Embodiment) A frequency modulation device according to a fifth embodiment of the present invention will be described below with reference to FIG.

FIG. 8 is a block diagram showing the configuration of the frequency modulation device according to the fifth embodiment. This frequency modulation device outputs unmodulated light of the optical frequency f1 as the first light L1 instead of the first optical modulator 102 of the direct modulation method in the configuration of the first embodiment shown in FIG. Light source 10
1 and an optical modulator (hereinafter referred to as "external optical modulator") 102e that performs frequency modulation by an external modulation method, and the filter 111 is omitted. As shown in FIG.
01 is output from the first optical branching circuit 1
04 is input. Further, the external optical modulator 102e is inserted and arranged on the way of the first optical waveguide 106, and the signal source 1
From 00, an electric signal Si as an original signal is received. In FIG. 8, the same components as those in the first embodiment are denoted by the same reference numerals.

Next, the operation of the present embodiment shown in FIG. 8 will be described. In the frequency modulation device of the present embodiment, the unmodulated light L1 output from the light source 101 is split into two lights by the first optical splitter 104, and one of the two lights is changed to the external light modulator. 102e, and the other light is input to the first optical receiver 110.

The external light modulator 102e modulates the unmodulated light L1 from the light source 101 with the electric signal Si from the signal source 100 to generate an optical frequency modulation signal L1m having the center light frequency f1. This optical signal L1m is input to the second optical receiver 114. The first optical receiver 110 receives the unmodulated light L1 from the light source 101 and the unmodulated light L2 from the local light source 103, and performs an optical heterodyne detection based on the square detection characteristics thereof to obtain an optical frequency difference fs. =
An unmodulated beat signal Sb having a frequency corresponding to | f1-f2 | is generated. The frequency converter 112 converts the frequency of the unmodulated beat signal Sb and outputs a frequency converted signal Sx having a frequency fx. The second optical modulator 113 converts the light L2 from the local light source 103 into the frequency converter 1
12 is subjected to optical modulation (optical amplitude modulation or optical intensity modulation) using the frequency conversion signal Sx output from the
Output 2x. This optical modulation signal L2x is also transmitted to the second optical receiver 1
14 is input. The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the second optical modulator 113
From the optical signal L2x, and the frequency fL (= | f
An FM modulated signal Sfm in which phase noise is suppressed is generated as a beat signal having a center frequency of s-fx |).

As described above, according to the fifth embodiment,
As in the fourth embodiment, since the unmodulated beat signal Sb is output from the first optical receiver 110, a high-frequency / broadband FM modulated signal without phase noise is generated without using the electric filter 111. can do. Moreover, in the fifth embodiment, since the unmodulated light L1 from the light source 101 is input to the first optical receiver 110, unlike the fourth embodiment, no optical filter is required.

(First Modification) In each of the above embodiments, the second optical modulator 113 is inserted in the middle of the second optical waveguide 108 and the unmodulated light L 2 output from the local light source 103. Alternatively, the optical signal L2 output from the fourth optical modulator 516 is optically modulated by the frequency conversion signal Sx, but instead, the second optical modulator 113 is connected to the first optical waveguide 106. By inserting the optical signal L1 on the way, the optical signal L1 output from the first modulator 102 may be optically modulated by the frequency conversion signal Sx. FIG.
FIG. 1 is a block diagram illustrating an example of a configuration of such a frequency modulation device. This frequency modulation device is a modification of the first embodiment shown in FIG. 1 (hereinafter, referred to as a “first modification”).
In the first embodiment, the second optical waveguide 10
The second optical modulator 113 inserted on the way of No. 8 is provided on the way of the first optical waveguide 106.

In the first embodiment and the like, the second optical modulator 1
Reference numeral 13 denotes an optical frequency as shown in FIG. 10A by modulating the unmodulated light L2 of the optical frequency f2 with the optical amplitude modulation or the optical intensity with the frequency conversion signal Sx of the frequency fx (= | fs-fl |). An optical modulation signal L2x having a spectrum is generated (where f1> f2).

On the other hand, in the frequency modulation device according to the first modification, the second optical modulator 113 controls the frequency fx (= | f
An optical modulation signal L1x having an optical frequency spectrum as shown in FIG. 10B is generated by optically modulating or optically modulating the optical signal L1 having the center frequency f1 with the frequency conversion signal Sx of (s-fL |). (Similarly, f1> f2). The second optical receiver 114 in the first modification example
This optical modulation signal L1x and the unmodulated light L2 of frequency f2 output from the local light source 103 are received, and the frequency fL is obtained by optical heterodyne detection based on the square detection characteristic.
An FM modulation signal Sfm is generated as a beat signal having (= | fs−fx |) as a center frequency. That is, an FM modulated signal substantially the same as the FM modulated signal Sfm obtained in the first embodiment is generated. Therefore, according to the configuration of the first modified example, similarly to the case of the first embodiment and the like, it is possible to generate a high-frequency / broadband FM modulation signal without phase noise without depending on the phase noise of the light source.

(Second Modification) In each of the embodiments described above, it is more desirable to output from the first optical modulator 102 (more precisely, a light source such as a semiconductor laser constituting the optical modulator 102). The first light L1 is transmitted to the first optical waveguide 1
06, the propagation time until the light reaches the second optical receiver 114 directly, and the light L1 is transmitted to the second optical waveguide 107,
The propagation times until reaching the second optical receiver 114 via the first optical receiver 110 and the second optical modulator 113 are equal to each other, and the local light source 103 or the fourth optical modulation The second light L2 output from the optical modulator 516 (more precisely, the light source constituting the optical modulator 516) is transmitted to the second optical receiver 114 via the third optical waveguide 108 and the second optical modulator 113. And the time that the light L2 reaches the second optical receiver 114 via the fourth optical waveguide 109, the first optical receiver 110, the second optical modulator 113, and the like. Are set to be equal to each other.

For example, the first and second lights L1, L
Such a setting is realized by inserting one or a plurality of optical or electrical delay circuits as means for adjusting the propagation time into an optical path or an electric path such as an optical waveguide constituting the second propagation path. can do. As the means for adjusting the propagation time, an electric adjusting means is preferable to an optical adjusting means in terms of cost and the like. FIG.
FIG. 1 is a block diagram showing a configuration of a modification (hereinafter, referred to as “second modification”) of the first embodiment shown in FIG. Shows the configuration of a frequency modulation device inserted between the frequency converter 112 and the second optical modulator 113 as the adjusting means. Note that the insertion position of the delay adjuster 710 is not limited to the above-described position. For example, instead of the position between the frequency converter 112 and the second optical , Or between the first optical receiver 11 and the filter 111,
10 may be inserted.

The above-mentioned setting is realized by such a propagation time adjusting means, so that the second optical receiver 114
, It is possible to more reliably suppress the phase noise of the FM modulation signal Sfm output from the LM and generate a high-quality high-frequency / broadband FM modulation signal.

(Third and Fourth Modifications) In the above-described second and third embodiments, more preferably, the first I signal generated in the second optical receiver 114 is used.
The M-DD component and the second IM-DD component, or the first IM-DD component and the third IM-DD component are set so that they are out of phase with each other and have the same amplitude. More specifically, in order to adjust the phases of the IM-DD components, the first electric signal Si1 which is one of the signals branched by the demultiplexer 301 is converted into an optical signal L1 by the first optical modulator 102.
And the propagation time required to reach the second optical receiver 114 via the first optical waveguide 106;
Is converted into an optical signal L3 or L2 by the third optical modulator 315 or the fourth optical modulator 516, and the fifth optical waveguide 3
The propagation times until reaching the second optical receiver 114 via the third or third optical waveguide 108 are set to be equal to each other. Also, the first IM-DD component and the second I-DD in the optical signal received by the second optical
The amplitudes of the light intensity modulation components respectively corresponding to the M-DD components are equal to each other, or the first IM-DD component and the third IM-DD component have the same amplitude.
Are set so that the amplitudes of the light intensity modulation components respectively corresponding to the IM-DD components are equal to each other.

For example, one or a plurality of electric or optical delay circuits and a variable gain amplifier (or attenuator) may be added to the electric path or the optical path (optical waveguide) constituting the signal propagation path. Is inserted as a means for adjusting the phase and amplitude of the signal, such a setting can be realized. As such a phase and amplitude adjustment unit, an electric adjustment unit is more preferable than an optical adjustment unit in terms of cost and the like. FIG. 12 is a block diagram showing a configuration of a modification (hereinafter, referred to as a “third modification”) of the second embodiment shown in FIG. 3, and is configured by an electric delay circuit and a variable gain amplifier. This shows a configuration of a frequency modulation device in which a delay / amplitude adjuster 720 is inserted between the demultiplexer 301 and the third optical modulator 315 as the adjusting means. FIG. 13 is a block diagram showing a configuration of a modification (hereinafter, referred to as a “fourth modification”) of the third embodiment shown in FIG. 5. The configuration of the frequency modulation device in which the amplitude adjuster 730 is inserted between the demultiplexer 301 and the fourth optical modulator 516 is shown. The delay / amplitude adjuster 720,
The insertion position of 730 is not limited to the above position, and may be inserted, for example, between the duplexer 301 and the first optical modulator 102.

By realizing the above setting by such a phase and amplitude adjusting means, the IM-DD component output from the second optical receiver 114 is more reliably suppressed, and a higher-quality high-frequency A wideband FM modulated signal can be generated.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a frequency modulation device according to a first embodiment of the present invention.

FIG. 2A is a schematic diagram (A) showing a frequency spectrum of an output signal from a first optical receiver in the frequency modulation device according to the first embodiment, and a frequency converter in the frequency modulation device according to the first embodiment; FIG. 2B is a schematic diagram illustrating the frequency spectrum of the input / output signal of FIG. 1B, and FIG. 2C is a schematic diagram illustrating the frequency spectrum of the output signal from the second optical receiver in the frequency modulation device according to the first embodiment.

FIG. 3 is a block diagram showing a configuration of a frequency modulation device according to a second embodiment of the present invention.

FIG. 4 shows a frequency spectrum of a signal after detection corresponding to an optical signal output from the first and second optical modulators and input to the second optical receiver in the frequency modulation device according to the second embodiment. FIG. 9A is a schematic diagram showing a frequency modulation device according to a second embodiment.
(B) showing a frequency spectrum of a signal after detection corresponding to an optical signal input to the optical receiver of FIG.
FIG. 11C is a schematic diagram (C) illustrating a frequency spectrum of an output signal from the second optical receiver in the frequency modulation device according to the embodiment.

FIG. 5 is a block diagram showing a configuration of a frequency modulation device according to a third embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of a frequency modulation device according to a fourth embodiment of the present invention.

FIG. 7 is a block diagram showing another configuration of the frequency modulation device according to the fourth embodiment.

FIG. 8 is a block diagram showing a configuration of a frequency modulation device according to a fifth embodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of a frequency modulation device according to a first modification which is a modification of the first embodiment.

FIG. 10 is a schematic diagram (A) illustrating an optical frequency spectrum of an optical signal input to a second optical receiver in the frequency modulation device according to each of the first, second, fourth, and fifth embodiments; In the frequency modulation device according to the first modification, the second
FIG. 3B is a schematic diagram illustrating an optical frequency spectrum of an optical signal input to the optical receiver illustrated in FIG.

FIG. 11 is a block diagram showing a configuration of a frequency modulation device according to a second modification which is another modification of the first embodiment.

FIG. 12 is a block diagram showing a configuration of a frequency modulation device according to a third modification which is a modification of the second embodiment.

FIG. 13 is a block diagram showing a configuration of a frequency modulation device according to a fourth modification which is a modification of the third embodiment.

FIG. 14 is a block diagram showing a configuration of a conventional frequency modulation device.

FIG. 15A is a schematic diagram illustrating an optical frequency spectrum of an optical signal input to an optical receiver in a conventional frequency modulation device, and FIG. 15B is a diagram illustrating a frequency spectrum of an output signal from the optical receiver in the conventional frequency modulation device. Schematic diagram (B) shown.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 signal source 102 first optical modulator 102 e external optical modulator 103 local light source 104 first optical branch circuit 105 second optical branch circuit 106 first optical waveguide 107 second , A third optical waveguide 109, a fourth optical waveguide 110, a first optical receiver 111, a filter 112, a frequency converter 113, a second optical modulator 114, a second optical receiver 200 ... Beat signal generation unit 301... Demultiplexer 315... Third optical modulator 516... Fourth optical modulator 617, 627. Optical filter 710. ) Electric signal Sbfm: Modulated beat signal Sb: Unmodulated beat signal Sx: Frequency conversion signal Sfm: FM modulation signal (frequency modulation signal)

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H04B 10/02 10/18

Claims (12)

[Claims] 1. First and second optical frequencies different from each other.
A frequency modulation device that converts an input electric signal into a frequency modulation signal by optical frequency modulation and optical heterodyne detection using first and second light sources that respectively output light, and outputs the signal as an FM modulation signal, A first optical modulator that outputs the first light frequency-modulated by the input electric signal as a first optical signal; and a beat obtained from the first and second lights by optical detection based on a square detection characteristic. A beat signal generation unit that generates an unmodulated beat signal corresponding to a carrier component of a signal; a frequency converter that generates a frequency conversion signal by converting the frequency of the unmodulated beat signal; A second optical modulator that generates a second optical signal by performing optical amplitude modulation or optical intensity modulation on one of the second light with the frequency conversion signal; Receiving the other and the second optical signal among the signal and the second light, and an optical receiver for generating the FM-modulated signal by an optical detection based on the square-law detection characteristic,
A frequency modulation device comprising: 2. The first optical modulator generates the first optical signal by direct modulation, the beat signal generator receives the first optical signal and the second light, A light-receiving element that generates a modulated beat signal that is a modulated electric signal having a center frequency corresponding to a difference between optical frequencies of the first optical signal and the second light by optical detection based on a squared detection characteristic; The frequency modulation device according to claim 1, further comprising: a filter configured to extract a carrier component from the modulated beat signal and output the carrier component as the unmodulated beat signal. 3. The first light modulator includes: a first light source that outputs the first light that is unmodulated light; and the first light modulator that outputs the first light that is output from the first light source. An external optical modulator that generates the first optical signal by modulating with an input electric signal, wherein the second light source outputs unmodulated light as the second light, and the beat signal generating unit 2. The frequency modulation device according to claim 1, further comprising: a light receiving element that receives the first and second lights that are unmodulated lights and generates the unmodulated beat signal by optical detection based on a square detection characteristic. 3. 4. The first optical modulator according to claim 1, wherein the first optical modulator uniquely converts a change in the amplitude of the input electric signal into a change in the optical frequency of the first light, so that the first optical modulator has a predetermined center frequency f1.
The second light source outputs light having a predetermined frequency f2 as the second light, and the beat signal generation unit outputs the first light signal and the second light. And the frequency fs = corresponding to the difference between the optical frequencies of the first optical signal and the second light by optical detection based on the squared detection characteristic.
| F1−f2 |, a light-receiving element that generates a modulated beat signal that is a modulated electric signal, and a filter that extracts a carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal. The frequency converter converts the unmodulated beat signal into a signal of a predetermined frequency fx, outputs the converted signal as the frequency converted signal, and the second optical modulator includes the One of the first optical signal and the second light is subjected to optical amplitude modulation or optical intensity modulation with the frequency conversion signal having a frequency fx to generate a second optical signal, and the optical receiver includes: Receiving the other of the first optical signal and the second optical signal and the second optical signal, and performing optical detection based on the square-law detection characteristic, the frequency fL = | fs−
generating the FM modulated signal at fx |.
The frequency modulation device according to claim 1. 5. A splitter for splitting the input electric signal into first and second electric signals having phases opposite to each other, and a demultiplexer for converting the second electric signal into a third optical signal by light intensity modulation. A first optical modulator, wherein the first optical modulator uniquely converts a change in the amplitude of the first electrical signal into a change in the optical frequency of the first light by direct modulation. Generating the first optical signal having a predetermined center frequency f1, the second light source outputting light having a predetermined frequency f2 as the second light, and the beat signal generating section generating the first light signal. And the second light, and by optical detection based on the squared detection characteristic, the frequency fs = corresponding to the difference between the optical frequencies of the first optical signal and the second light.
| F1−f2 |, a light-receiving element that generates a modulated beat signal that is a modulated electric signal, and a filter that extracts a carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal. The frequency converter converts the unmodulated beat signal into a signal of a predetermined frequency fx, outputs the converted signal as the frequency converted signal, and the second optical modulator includes the One of the first optical signal and the second light is subjected to optical amplitude modulation or optical intensity modulation with the frequency conversion signal having a frequency fx to generate a second optical signal, and the optical receiver includes: Receiving the other of the first optical signal and the second optical signal and the second optical signal, and performing optical detection based on the square-law detection characteristic, the frequency fL = | fs−
fx | to generate the FM modulated signal;
The frequency modulation device according to claim 1, wherein the frequency modulation device receives the third optical signal and generates an electric signal corresponding to a light intensity modulation component included in the third optical signal by the optical detection. 6. A first IM corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver. The DD component and the second IM-DD component corresponding to the light intensity modulation component included in the third optical signal among the generated intensity modulation-direct detection components are set to have the same amplitude and the opposite phase to each other. The frequency modulation device according to claim 5, further comprising a phase / amplitude adjustment unit that performs the adjustment. 7. A splitter for splitting the input electric signal into first and second electric signals having phases opposite to each other, and converting the second light, which is a modulated light having a predetermined center frequency f2, into a fourth light. A fourth optical modulator that outputs the signal as a signal, wherein the first optical modulator uniquely converts a change in the amplitude of the first electric signal into a change in the optical frequency of the first light by direct modulation. To generate the first optical signal having a predetermined center frequency f1, and the fourth optical modulator converts the change in the amplitude of the second electric signal into the second optical signal by direct modulation. The beat signal generating unit generates the fourth optical signal having a predetermined center frequency f2 by uniquely converting the optical signal into a change in optical frequency. The beat signal generating unit receives the first and fourth optical signals and performs square-law detection. By the optical detection based on the characteristic, the light of the first and fourth optical signals is Frequency fs = corresponds to the difference of the wave | f1-f2 |
A light receiving element that generates a modulated beat signal that is a modulated electric signal, and a filter that extracts a carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal. The converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, outputs the converted signal as the frequency converted signal, and the second optical modulator includes the first optical signal and the first optical signal. The second optical signal is generated by subjecting one of the fourth optical signals to optical amplitude modulation or optical intensity modulation with the frequency conversion signal having the frequency fx, and the optical receiver includes the first optical signal and the second optical signal. The other of the fourth optical signal and the second optical signal are received, and the frequency fL = | fs is detected by optical detection based on the squared detection characteristic.
The frequency modulation device according to claim 1, wherein the FM modulation signal is generated at -fx |. 8. A first IM corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver. The DD component and the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components are set to have the same amplitude and the opposite phase to each other. The frequency modulation device according to claim 7, further comprising a phase / amplitude adjustment unit that performs the operation. 9. The second light source further comprising an optical filter inserted between the first optical modulator and the beat signal generator, for extracting an optical carrier component from the first optical signal. Outputs unmodulated light as the second light, the beat signal generation unit receives the optical carrier component extracted by the optical filter and the second light, and performs optical detection based on the squared detection characteristic. The frequency modulation device according to claim 1, further comprising a light receiving element that generates the non-modulated beat signal according to the following. 10. An input electric signal having a phase opposite to that of a first electric signal.
A branching filter that branches into a second electric signal and a second electric signal; a fourth optical modulator that outputs the second light that is modulated light having a predetermined center frequency f2 as a fourth optical signal; A first optical filter inserted between an optical modulator and the beat signal generator, a second optical filter inserted between the fourth optical modulator and the beat signal generator, The first optical modulator further comprises: a first modulator that directly converts a change in the amplitude of the first electrical signal into a change in the optical frequency of the first light by direct modulation, thereby obtaining a predetermined center frequency f1. The first optical signal is generated, and the fourth optical modulator uniquely converts an amplitude change of the second electric signal into a change of an optical frequency of the second light by direct modulation, Generating the fourth optical signal having a predetermined center frequency f2; Extracts the optical carrier component from the first optical signal, the second optical filter extracts the optical carrier component from the fourth optical signal, and the beat signal generation unit And receiving both optical carrier components extracted by the second optical filter,
The frequency fs = | f1-f2 corresponding to the difference between the optical frequencies of the two optical carrier components is obtained by optical detection based on the squared detection characteristic.
| Includes a light receiving element that generates the unmodulated beat signal, the frequency converter converts the unmodulated beat signal into a signal of a predetermined frequency fx, and outputs the converted signal as the frequency converted signal. The second optical modulator modulates one of the first optical signal and the fourth optical signal with the frequency converted signal having a frequency fx to modulate the second optical signal. The optical receiver receives the other of the first optical signal and the fourth optical signal and the second optical signal, and performs optical detection based on a squared detection characteristic, thereby obtaining a frequency fL = | Fs
The frequency modulation device according to claim 1, wherein the FM modulation signal is generated at -fx |. 11. A first IM corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver. The DD component and the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components are set to have the same amplitude and the opposite phase to each other. The frequency modulation device according to claim 10, further comprising a phase / amplitude adjustment unit that performs the phase / amplitude adjustment. 12. The light and electric propagation time of a path from the first light source to the optical receiver via the beat signal generating unit, and a light path and an electric propagation time of a path directly from the first light source to the optical receiver. First propagation time adjusting means for making light propagation times equal to each other; light and electric propagation times of a path from the second light source to the optical receiver via the beat signal generation unit; 2. The frequency modulation device according to claim 1, further comprising: a second propagation time adjusting unit configured to make light propagation times of a path from the light source directly to the optical receiver equal to each other. 3.