JPH09321700A - Method and device for generating electromagnetic wave - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a electromagnetic wave generating method and a device in which the frequency is stably varied. SOLUTION: An output beam of a light source 1 whose frequency is f1 is modulated by a modulation signal whose frequency is Δf by an optical modulator 2 and two side band waves are generated and amplified by an optical amplifier 3 and the side band waves whose frequency difference is NΔf are generated by a single mode optical fiber 4 based on the ternary nonlinear optical effect. Then two side band waves whose frequency is f1±NΔf are produced by a multistage optical frequency filter 5 and subject to optical heterodyne detection at a photo-detector 7, then an electromagnetic wave whose frequency is 2NΔf is generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for generating electromagnetic waves having a high frequency such as millimeter wave band.

[0002]

2. Description of the Related Art Conventionally, two methods have been tried for generating light and radio waves. In both cases, the principle is to generate two light waves having different frequencies, modulate one of the light waves with an information signal, and finally detect the beat frequencies of the two to generate a microwave corresponding to the frequency difference. . At this time, it is an important condition that the phases and frequencies of the two light waves are stable.

One of them is a method using two coherent light sources whose oscillation frequencies are stable and different from each other and whose phase relationship between the two lights is fixed. FIG. 4 is a block diagram showing the method, 41 is a coherent first light source,
42 is a coherent second light source, 43 is an optical modulator, 4
4 is a 3 dB coupler, and 45 is a photodetector.

The output beam of the light source 41 is modulated with data by an optical modulator 43, and a light source 42 is output by a 3 dB coupler 44.
Is combined with the output beam of (1) and is heterodyne detected by the photodetector 45. As a result, an electromagnetic wave signal having a frequency corresponding to the difference beat frequency is obtained.

The biggest problem with this method is that phase synchronization is difficult. Phase synchronization requires precise gain adjustment of the loop filter, and the configuration of the phase synchronization circuit is complicated. Also 2
Since two lasers are used, it is difficult to make them compact and economical. For the above reasons, this method is not suitable for practical use.

To solve the above-mentioned drawbacks, there is a method of generating light waves of two different frequencies by using one light source. In this case, the above-mentioned problem of phase synchronization is eliminated. FIG. 5 is a block diagram showing the method.
Is a coherent light source, 52 is a first optical modulator, 53 is a first optical frequency filter, 54 is a second optical frequency filter, 55 is a second optical modulator, and 56 is a photodetector.

The output beam of the light source 51 is modulated by the optical modulator 52 with Δf corresponding to ½ of the required carrier frequency to generate two sidebands. Optical frequency filter 5
3 removes only the frequency component of the light source 51, and the optical frequency filter 54 separates both sidebands. Next, only one of the sidebands is modulated with data using the optical modulator 55, and finally the photodetector 56 performs heterodyne detection. As a result, an electromagnetic wave signal having a frequency of 2Δf can be obtained.

[0007]

However, this method has the advantage that no phase synchronization is required, but since the carrier frequency is limited by the band of the optical modulator, the carrier frequency that can be generated is at most several. There was a problem that the upper limit was 10 GHz.

The present invention solves these drawbacks, and relates to a carrier generation method and apparatus in which phase synchronization is unnecessary and the frequency of a radio carrier that can be generated is not limited.

The present invention has been made in view of the above, and a continuous light (c) as a pump light is supplied to an optical fiber.
w light) and cw seed light whose phase is synchronized with the pump light and whose frequency can be continuously varied are simultaneously incident, and is based on the third-order nonlinear optical effect which is one of the nonlinear optical phenomena of the optical fiber. By the light wave mixing, the pump light and the cw
By generating a cw signal light having a frequency different by an integer multiple of the frequency difference from the seed light and being in phase with the cw seed light, and performing optical heterodyne detection of the cw seed light and the cw signal light, the cw signal is generated. Seed light and above c
Provided is an electromagnetic wave generation method for generating an electromagnetic wave whose frequency corresponding to the frequency difference of w signal light is variable.

Further, according to the present invention, the pump light is branched and one of them is frequency-modulated or phase-modulated by an optical modulator so that the phase is synchronized with the pump light and the frequency is applied to the optical modulator. There is provided an electromagnetic wave generation method for generating cw seed light that changes according to the frequency of the modulated signal.

Further, according to the present invention, an optical modulator for optically modulating the continuous light output from the coherent light source, an optical amplifier for amplifying the output light of the optical modulator, and a pump which is the output light of the optical amplifier. Continuous light (cw light) as light, and cw seed light whose phase is synchronized with the pump light and whose frequency can be continuously varied are simultaneously incident, and a third-order nonlinear optical phenomenon that is one of nonlinear optical phenomena A single-mode optical fiber that generates a cw signal light whose frequency is different by an integer multiple of the frequency difference between the pump light and the cw seed light and whose phase is synchronized with the cw seed light by the effect-based light wave mixing; An optical frequency filter for extracting the cw seed light and the cw signal light from the output light of the optical fiber, and an optical heterodyne detection of the cw seed light and the cw signal light which are the output light of the optical frequency filter. That provides an electromagnetic wave generating device comprising a light receiving element.

[0011]

1 is a block diagram of an electromagnetic wave generator according to an embodiment of the present invention, in which 1 is a coherent light source, 2 is a first optical modulator, 3 is an optical amplifier, 4 is an optical fiber, Reference numeral 5 is a multistage optical frequency filter, 6 is a second optical modulator, and 7 is a photodetector.

The operating principle of this embodiment will be described below. The output beam of the light source 1 has a required carrier frequency of 1 by applying a modulation signal of frequency Δf to the optical modulator 2.
After being modulated by Δf corresponding to / N, two sidebands are generated, the optical intensity is amplified by the optical amplifier 3, and then the optical signal is incident on the optical fiber 4.

Sidebands having a frequency difference of NΔf (N = 1, 2, ...) Are generated in the optical fiber 4 due to the third-order nonlinear optical effect, and the frequency is f1 ± from the multistage optical frequency filter 5. Two sidebands of NΔf are separated and extracted.

The two frequency components are amplified as needed, only one sideband is modulated with data using the optical modulator 6, and finally the photodetector 7 performs heterodyne detection. As a result, an electromagnetic wave signal having a frequency of 2NΔf can be obtained. By changing the frequency Δf of the modulation signal applied to the optical modulator 2, the frequency of the carrier wave is changed by N.
It is possible to change it twice.

Next, due to the third-order nonlinear optical effect of the optical fiber, the frequency is f1 ± NΔf (N = 1, 2, ...
The principle of generation of sidebands in () will be explained.

In FIG. 2, three light waves and an optical frequency f1
When ± Δf and f1 are incident, a light wave of a new frequency is generated by the induced three-wave mixing. The frequency of the light wave is determined by the law of conservation of energy. For example, if the frequency is f1-Δ
The frequency f of the light wave generated from the two light waves f and f1 must satisfy the following equation.

[0017]

[Equation 1]

[0018]

[Equation 2]

The solutions of equations 1 and 2 are f1 + Δ, respectively.
f and f1-2Δf. In this way, f1 ± NΔf
(N = 1, 2, ...) Sidebands are generated. on the other hand,
In guided three-wave mixing, the three light waves to be mixed must simultaneously satisfy the law of conservation of momentum. In the zero-dispersion frequency band where the dispersion of the optical fiber is small, the law of conservation of momentum is satisfied almost automatically.

The optical fiber 4 used in this embodiment is a single-mode fiber, and is a zero-dispersion fiber with zero dispersion in the wavelength range used, or a dispersion over a wide wavelength range centered on the wavelength range used. A so-called dispersion flat fiber with zero is used. The zero-dispersion fiber in the 1.55 μm band has been put into practical use, and the dispersion flat fiber has already been experimentally manufactured.

In this method, since the light waves incident on the optical fiber 4 are in phase with each other, it is important that all the light waves generated in the optical fiber 4 are also in phase.
Therefore, the carrier wave obtained as the beat frequency of the difference obtained by heterodyne detection of any two light waves from these has no fluctuation in frequency or intensity.

Next, the structure and design method of the multistage optical frequency filter 5 will be described. FIG. 2 shows a Mach-Zehnder interferometer (MZI) type optical frequency filter, 11 is a 3 dB coupler, 12 is a first arm of MZI, 13 is a second arm of MZI, 14 is a phase shifter, and 15 is 3 dB. It is a coupler.

For example, the outputs of the output terminals O1 and O2 with respect to the input of the input terminal I1 are sin2 (πf
τ), cos2 (πfτ). In order to block fc by the filter at the previous stage and pass fs and fa to the output terminal O1, the optical path length difference of MZI is v / (2Δf) (v
Is the speed of light in the waveguide). Here, the adjustment of the optical path length difference can be performed by a phase shifter, and is normally adjusted by using TO.

In the latter filter, the optical path length difference is v / (4Δ
If the same f) is used, fs is output to the output terminal O1 and fa is output to the output terminal O2, so that both waves can be separated. A method of separating and extracting only two frequency components from the spectrum arranged at equal intervals with the frequency interval Δf as shown in FIG. 3 using this (MZI) type multi-stage optical frequency filter will be described.

The adjacent spectrum is separated and removed by the first stage (m = 1) filter. The optical path difference is v / (2Δ
f). In the next stage (m = 2), the optical path difference is further reduced to v / (4Δ
The spectrum adjacent to f) is separated and removed.
The final m-th stage filter has an optical path difference of v / (2mΔf), and only two desired frequency components are obtained from both output arms.

The method has the advantage that it does not require phase synchronization and has the advantage that the frequency of the radio carrier is not limited by the band of the optical modulator and can be changed. The drawbacks of the two methods can be solved at once.

As described above, the present invention has been described based on the embodiments. However, the present invention is not limited to the above-described embodiments, and may be implemented in any way unless the configuration described in the claims is changed. it can.

As described above, in the present invention,
Since a single coherent light source and the non-linear optical effect of the optical fiber are used, a stable frequency variable electromagnetic wave signal can be generated with a simple configuration.

In addition, the electromagnetic wave generator has a great effect that it can be used as a so-called optical fiber feeder, for example, by performing communication between a radio base station provided with an antenna and a control station which integrates the radio base station with an optical fiber in radio communication. Play.

[Brief description of drawings]

FIG. 1 is a block diagram showing an electromagnetic wave generator according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a multistage optical frequency filter.

FIG. 3 is a conceptual diagram showing a method of designing a multistage optical frequency filter for separating and extracting only two frequency components from a spectrum arranged at equal intervals with a frequency interval Δf.

FIG. 4 is a block diagram showing a conventional electromagnetic wave generator.

FIG. 5 is a block diagram showing a conventional electromagnetic wave generator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 2, 6 optical modulator 3 optical amplifier 4 optical fiber 5 multi-stage frequency filter 7 photodetector 11, 15 3 dB coupler 12, 13 arm 14 phase shifter

Claims (3)

[Claims] 1. A continuous light (c) as a pump light for an optical fiber.
w light) and cw seed light whose phase is synchronized with the pump light and whose frequency can be continuously varied are simultaneously incident, and is based on the third-order nonlinear optical effect which is one of the nonlinear optical phenomena of the optical fiber. By the light wave mixing, the pump light and the cw
By generating a cw signal light having a frequency different by an integer multiple of the frequency difference from the seed light and being in phase with the cw seed light, and performing optical heterodyne detection of the cw seed light and the cw signal light, the cw signal is generated. Seed light and above c
A method for generating an electromagnetic wave, characterized in that an electromagnetic wave having a variable frequency corresponding to the frequency difference of w signal light is generated. 2. The electromagnetic wave generation method according to claim 1, wherein the pump light is branched and one of the pump light is frequency-modulated or phase-modulated by an optical modulator to synchronize the phase with the pump light. An electromagnetic wave generation method characterized in that cw seed light whose frequency changes according to the frequency of a modulation signal applied to the optical modulator is generated. 3. An optical modulator that optically modulates continuous light output from a coherent light source, an optical amplifier that amplifies output light of the optical modulator, and continuous light as pump light that is output light of the optical amplifier. (Cw light) and the pump light are in phase with each other,
In addition, cw seed light whose frequency can be continuously changed is simultaneously incident, and the frequency difference between the pump light and the cw seed light is changed by light wave mixing based on the third-order nonlinear optical effect, which is one of nonlinear optical phenomena. The frequency differs by an integer multiple,
A single mode optical fiber that generates a cw signal light whose phase is synchronized with the cw seed light, an optical frequency filter that extracts the cw seed light and the cw signal light from the output light of the optical fiber, and the optical frequency An electromagnetic wave generator comprising: a light receiving element that optically heterodyne-detects the cw seed light and the cw signal light, which are output lights of a filter.