JP2000013313A - Optical subcarrier transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical subcarrier transmission system that separates a transmission signal and secondary intermodulation distortion without signal deterioration. SOLUTION: A reference laser diode 10 generates a reference optical signal with a carrier frequency fopt0, a laser diode 11 generates a 1st optical signal with a subcarrier frequency fopt1, and a laser diode 12 generates a 2nd optical signal with a subcarrier frequency fopt2. An optical modulator 111 modulates the 1st optical signal according to another data signal. An optical multiplexer 80 multiplexes a reference optical signal with the 1st and 2nd optical signals and transmits the multiplexed signal via an optical fiber cable 60 and provides an output to a photo diode 90 of a photoelectric conversion element via an optical fiber cable 60. The frequencies fopt0, fopt1, fopt2 are selected so that a difference frequency fopt0-fopt1 and a difference frequency fopt2-fopt0 are not in duplicate and a relation of fopt1<fopt0<fopt2 holds.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is applicable to, for example, an optical fiber link system for transmitting a transmission signal between a radio control station and a radio base station of a mobile communication system. The present invention relates to an optical subcarrier transmission system for transmitting using a subcarrier.

[0002]

2. Description of the Related Art Recently, mobile communication systems and wireless LANs have been developed.
For example, expectations for the use of the millimeter wave band are increasing. In the millimeter wave band, long-distance transmission was difficult due to very high propagation loss. Therefore, an optical fiber-millimeter wave link system in which a millimeter wave signal is superimposed as a subcarrier on an optical signal using an optical fiber has been proposed, and research for high speed, wide band, and long distance transmission has been advanced.

There are many reports on the optical fiber-millimeter wave link system relating to an intensity modulation-direct detection (IM-DD) system that generates a millimeter wave serving as a subcarrier signal using an external modulator. As the external modulator, a Mach-Zehnder type using a LiNbO ₃ substrate or an optical electroabsorption type is used. In the IM-DD system using an external modulator, intermodulation distortion occurs due to nonlinearity of the modulator. In two-channel signal transmission, the second-order intermodulation distortion occurs far away from the signal frequency,
It can be easily removed with a filter. On the other hand, the third-order distortion is generated near the frequency of the signal, so that it is difficult to remove it by a filter. This is one of the important characteristics that determine the dynamic range of the system, and various studies have been reported on its reduction.

For example, in an optical fiber-millimeter wave link system using heterodyne detection, a millimeter wave signal equal to the difference frequency between the two laser lights is generated by heterodyne detection of two laser beams. FIGS. 1 and 2 show the relationship among the optical carrier, the millimeter-wave subcarrier, and the modulation signal in each system. FIG. 1 is a spectrum diagram showing the relationship between the carrier frequencies of the IM-DD system in the link system of the first conventional example, and FIG. 2 is the carrier frequency of the heterodyne system in the link system of the second conventional example. FIG. 4 is a spectrum diagram showing the relationship of FIG.

The link method of the second conventional example has the following advantages. (1) Resists chromatic dispersion of optical fibers. In the case of the IM-DD system, two millimeter-wave signals as sub-carriers are generated on both sides of the optical carrier. When this propagates through the optical fiber, it may cancel each other due to chromatic dispersion of the optical fiber.
In the case of the heterodyne method, the frequency of the millimeter-wave signal, which is a subcarrier, is determined by the relative relationship with the difference frequency between the two lasers. (2) Deep modulation is applied. In an external modulator using LiNbO ₃ , distortion increases as the degree of modulation increases, so that it was difficult to apply deep modulation. In the case of the heterodyne system, 100% modulation is possible in principle, and the light use efficiency is high. (3) An optical signal processing antenna can be used. This is an antenna that uses a plurality of photodiodes (PDs) and feeds the power of the electrical signal photoelectrically converted by each PD to each element of the array antenna by controlling the phase relationship. Thereby, the directivity of the millimeter wave antenna can be controlled.

FIG. 3 is a spectrum diagram showing a frequency relationship in the case of a single sub-carrier in the heterodyne system of the third conventional example, and FIG. 4 is a diagram showing a large number of sub-carriers in the heterodyne system of the fourth conventional example. FIG. 9 is a spectrum diagram showing a frequency relationship in the case of FIG. In order to transmit multi-channel information in an optical fiber-millimeter wave link system by heterodyne detection, a method of superimposing a large amount of information on one sub-carrier as shown in FIG. Using a carrier,
There is a method of superimposing a signal on each of them, or a fusion thereof. In the former, even if the number of channels is increased, it is not necessary to add a new laser. However, since a band occupied by information is widened, a wideband modulator is required. On the other hand, the latter requires the addition of a new laser, but does not require a wideband modulator because it is modulation in baseband. Since the frequency range of the subcarrier is wide, when applied to an optical signal processing antenna, a wide range of beam scanning is possible.

[0007]

For example, the above-described IM-
In the DD system, there is a problem that signal degradation is caused by chromatic dispersion. In the heterodyne link system,
When a two-channel millimeter wave transmission signal is generated when three optical signals are input, as described later in detail, in the conventional method, a second-order intermodulation distortion occurs near the transmission signal, and the transmission signal And the second-order intermodulation distortion cannot be separated. In such multi-channel signal transmission, it is important to consider intermodulation distortion, but there has been no report of such a disclosure or suggestion for the heterodyne system.

An object of the present invention is to solve the above problems and to provide an optical subcarrier transmission system capable of separating a transmission signal from secondary intermodulation distortion without deteriorating a signal. .

[0009]

An optical subcarrier transmission system according to the present invention comprises: a reference light generating means for generating a reference optical signal having a predetermined carrier frequency; and a first light having a first predetermined subcarrier frequency. At least three light generating means including first light generating means for generating a first optical signal, and second light generating means for generating a second optical signal having a predetermined second subcarrier frequency; First modulation means for modulating and outputting the first optical signal generated by the first light generation means in accordance with an input data signal, and second light generated by the second light generation means Second modulating means for modulating a signal in accordance with an input data signal and outputting the same; optical multiplexing means for multiplexing and outputting the reference optical signal, the first optical signal, and the second optical signal And the multiplexed output from the optical multiplexing means. An optical sub-carrier transmission system comprising: an optical fiber cable that transmits a signal; and a photoelectric conversion unit that photoelectrically converts a signal transmitted by the optical fiber cable into an electric signal and outputs the electric signal. The first and second subcarrier frequencies are set so that a difference frequency between the reference carrier frequency and the first subcarrier frequency does not overlap with a difference frequency between the reference carrier frequency and the second subcarrier frequency. The first sub-carrier frequency is set to be lower than the reference sub-carrier frequency, and the second sub-carrier frequency is set to be higher than the reference sub-carrier frequency.

[0010]

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

FIG. 5 is a block diagram showing a basic configuration of an optical fiber link system 100 according to a first embodiment of the present invention, and FIG. 6 is an overall view of the optical fiber link system according to the first embodiment. FIG. 3 is a block diagram illustrating a configuration. The optical fiber link system according to the present embodiment can be broadly divided, as shown in FIGS.
0 and the radio base station 300, and are connected therebetween by using a single mode (or multimode) optical fiber cable 60 capable of transmitting optical signals of a plurality of optical wavelengths. In this optical fiber link system, in the radio control station 200, the reference laser diode 10 generates a reference optical signal having an optical carrier frequency f _{opt0 as} an optical carrier, and the laser diode 11 generates an optical sub-carrier frequency f _{opt as} an optical sub-carrier. A first optical signal of _opt1 is generated, and the laser diode 12 generates a second optical signal of an optical subcarrier frequency f _opt2 to be an optical subcarrier.
It is called a PLL circuit. ) 21 is the laser diode 10,
11 While each controlling a bias current of a predetermined frequency f _{op t0,} f _opt1 a and the laser diode 10, 11 to a phase synchronized with each other, PLL circuit 22, a predetermined laser diode 10, 12 respectively frequency f _opt0 ,
The bias currents of the laser diodes 10 and 12 are controlled so as to be f _opt2 and to be in phase synchronization with each other. In this embodiment, each frequency is _defined as f _opt1 <f _opt0 , f _opt2 > f
_By setting to be _opt0 , the radio base station 30
The second embodiment is characterized in that the secondary intermodulation distortion in the electric signal output from the zero photodiode 90 (in this embodiment, used as a millimeter wave band transmission signal) is reduced.

First, the principle of generation of intermodulation distortion in an optical heterodyne millimeter-wave link will be described. FIG. 5 shows a basic configuration of a heterodyne system that generates two-channel millimeter waves by three light wave inputs. A part of these two optical signals, a reference optical signal generated by the reference laser diode 10 and a first optical signal generated by the laser diode 11 having a slightly different oscillation frequency, is multiplexed by the optical multiplexer 81. After that, the photodiode (P
D) Heterodyne detection at 31 and based on the detected signal,
The PLL circuit 21 controls the oscillation wavelength (i.e., oscillation frequency) of each of the laser diodes 10 and 11 to perform phase synchronization with each other. Similarly, a part of these two optical signals, a reference optical signal generated by the reference laser diode 10 and a second optical signal generated by the laser diode 12 having a slightly different oscillation frequency, is After that, the photodiode (PD) 32 performs heterodyne detection, and based on the detected signal, the PLL circuit 22 controls the oscillation wavelength (ie, oscillation frequency) of each of the laser diodes 10 and 12 to synchronize the phases with each other. . As a result, a millimeter-wave signal or a microwave signal equal to the frequency difference between the laser diodes 10 and 11 and between the laser diodes 10 and 12 is generated. In order to generate a millimeter wave signal of a desired frequency, the oscillation frequency of each of the laser diodes 10, 11, 12 is controlled by PLL circuits 21, 22. Each laser diode 10,
The optical signals generated by 11 and 12 are
After transmission from 0 to the distant radio base station 300 via the optical fiber cable 60, heterodyne detection is performed by the photodiode 90, whereby a millimeter wave band transmission signal can be obtained, for example.

In FIG. 6, 11f, 12f, 40, 5
0, 51, 52, 70fa, 70fb, 71f, 72
f, 81f, and 82f are optical fiber cables. The reference optical signal generated by the reference laser diode 10 enters the optical multiplexer 80 via the optical fiber cable 40, the optical splitter 70, and the optical fiber cable 50. here,
A part of the reference optical signal optically demultiplexed by the optical demultiplexer 70 enters the photodiode 31 via the optical fiber cable 70fa, the optical multiplexer 81, and the optical fiber cable 81f. Another part of the demultiplexed reference optical signal enters the photodiode 32 via the optical fiber cable 70fb, the optical multiplexer 82, and the optical fiber cable 82f.

The first optical signal generated by the laser diode 11 includes an optical fiber cable 11f, an optical demultiplexer 71, an optical modulator 111, and an optical fiber cable 51.
And enters the optical multiplexer 80. A part of the first optical signal optically demultiplexed by the optical demultiplexer 71 enters the photodiode 31 via the optical fiber cable 71f, the optical multiplexer 81, and the optical fiber cable 81f. Here, the optical modulator 111 performs, for example, intensity modulation on the passing first optical signal in accordance with, for example, the first data signal to be transmitted, which is generated by the modulation signal generator 121.
The first data signal is transmitted by the optical sub-carrier using the optical fiber cable 60 using the first optical sub-carrier frequency f _opt1 . The photodiode 31 photoelectrically converts the two incident optical signals and outputs the electric signal after the photoelectric conversion to the PLL circuit 21 as an error control signal. In response,
The PLL circuit 21 controls the bias currents of the laser diodes 10 and 11 so that the laser diodes 10 and 11 have predetermined frequencies f _opt0 and f _opt1 and are in phase synchronization with each other.

Further, the second optical signal generated by the laser diode 12 is connected to an optical fiber cable 12f,
Optical demultiplexer 72, optical modulator 112, optical fiber cable 5
The light is incident on the optical multiplexer 80 via 2. A part of the second optical signal optically demultiplexed by the optical demultiplexer 72 enters the photodiode 32 via the optical fiber cable 72f, the optical multiplexer 82, and the optical fiber cable 82f. Here, the optical modulator 112 performs, for example, intensity modulation on the passing second optical signal according to, for example, the second data signal to be transmitted generated by the modulation signal generator 122,
The second data signal is transmitted by the optical subcarrier using the optical fiber cable 60 using the second optical subcarrier frequency f _opt2 . The photodiode 32 photoelectrically converts the two incident optical signals and outputs the electric signal after the photoelectric conversion to the PLL circuit 22 as an error control signal. In response,
The PLL circuit 22 controls the bias current of the laser diodes 10, 12 so that the laser diodes 10, 12 have predetermined frequencies f _opt0 , f _opt2 , respectively, and are in phase synchronization with each other.

Then, the three optical signals incident on the optical multiplexer 80 are multiplexed, and then transmitted to the photodiode 90 of the wireless base station 300 via the optical fiber cable 60 and incident. The transmission here is an optical subcarrier transmission since the data signal is transmitted using the optical subcarrier signal, and the system is generally called an optical subcarrier system.
The photodiode 90 is a photoelectric conversion element having at least tertiary nonlinear photoelectric conversion characteristics. After photoelectrically converting an incident optical signal into a millimeter wave band transmission signal as an electric signal, the transmission power amplifier 91 and an unnecessary By outputting to an antenna 93 via a band-pass filter (BPF) 92 for removing an electric signal component, the signal is radiated from the antenna 93 toward a wireless terminal such as a mobile phone.

Next, the principle of generation of intermodulation distortion will be described below. The electric field amplitudes E0, E1, and E2 of the optical signals generated by the laser diodes 10, 11, and 12 are given by the following equations.

[0018]

[Number 1] _{E 0 = A 0 exp (-j2πf} opt0 t) E 1 = A 1 exp (-j2πf opt1 t) E 2 = A 2 exp (-j2πf opt2 t)

Here, A and fopt are the amplitude and frequency of the optical signal generated by each of the laser diodes 10, 11, and 12, and the suffixes 0, 1, and 2 correspond to the laser diodes 10, 11, and 12. Represents a number. After multiplexing these optical signals, the intensity (or optical power) I [W] of the multiplexed optical signal when entering the photodiode 90 is represented by the following equation.

[0020]

[Number 2] _{I = (E 0 + E 1} + E 2) (E 0 + E 1 + E 2) * = A 0 2 + A 1 2 + A 2 2 + 2A 1 A 2 cos (2πf rf0 t)
_{+ 2A 0 A 1 cos (2πf} rf1 t) + 2A 0 A 2 cos
(2πf _rf2 t)

Here, the frequency difference f _rf1 = f _opt1 −f
_{opt0, f rf2 = f opt2 -f} opt0 is the frequency of the desired millimeter-wave transmission signal. However, besides this, f _rf0 =
It can be seen that a millimeter wave signal or a microwave signal is generated at f _opt2 −f _opt1 . FIG. 7 shows the relationship between these optical frequencies and the millimeter wave signal frequency generated from them. Here, assuming that the input / output characteristics of the photodiode 90 have non-linearity and can be expressed by a polynomial with respect to the amount of incident light, the output current i of the photodiode 90 can be assumed as follows.

[0022]

(Equation 3)

Here, each of the laser diodes 10, 11,.
The intensity of 12 is assumed to equal to A, the output current i _sig signal frequency f _rf1, the output current i _dis distortion frequency 2 _FRF1 -f _rf2 is definitive as follows.

[0024]

I _sig = (2A ² k ₁ + 16A ⁴ k ₂ + 120A ⁶ k
₃ + ...) cos (2πf _rf1 t)

## _EQU5 ## i _dis = (4A ⁴ k ₂ + 48A ⁶ k ₃ +...) Cos
_{[2π (2f rf1 -f rf2)} t]

In a standard Mach-Zehnder type external modulator, only the third and higher order terms appear near the signal frequency.
The heterodyne system, in addition to the third-order distortion coefficient k ₃ according, second-order intermodulation distortion comprising coefficient k ₂ as in the first term of the 5 carbon of the even revealed that occur.

Next, the measurement results of a comparative example of intermodulation distortion will be shown. Three LD-pumped Nd: YAG lasers capable of changing the wavelength in the 1.3 μm band were used as the laser diodes 10, 11, and 12 as light sources. As the photodiode 90, p
−in-type photodiode (NEL KEPD151)
0 VPG), the output signal of the photodiode 90 was directly connected to a spectrum analyzer (HP 8565E) via a bias tee. The frequencies of the two generated millimeter wave signals are 38.5 GHz and 38.6 GHz, and the frequencies at which intermodulation distortion occurs are 38.4 GHz and 38.7 GHz. FIG. 8 shows a measurement result of a spectrum distribution including a signal and a distortion, and FIG. 9 shows a spectrum distribution near a 38.6 GHz signal. From FIG. 8, it can be confirmed that intermodulation distortion has occurred around the signal. Also,
It can be seen from FIG. 9 that a millimeter wave signal with a small line width is generated.

Next, FIG. 10 shows an example of measuring the millimeter wave signal and the intermodulation distortion output with respect to the power of the incident laser light signal when the powers of the three laser diodes 10, 11, and 12 are changed while maintaining the same value. Show. Here, the frequency of the millimeter-wave _{signal, f rf1 = 38.5GHz, f rf2} = 3
8.6 GHz. The straight lines in FIG. 10 are approximation lines of the measurement points. The slope of each straight line is 2.0 in the signal output, the distortion output is 6.3 in the high incident light range and 3.7 in the low incident light amount, and the error 10 % Or less, the characteristics of Formulas 4 and 5 were matched. From this experimental result, it can be seen that the third order term is dominant when the incident light amount is large, but the intermodulation distortion becomes dominant when the incident light quantity is low.

Assuming that the dynamic range of the system is the maximum value of the difference between the signal and the distortion output, the maximum occurs when the intermodulation distortion becomes equal to the noise level. From the measurement results of FIG. 10, the dynamic range is limited by the second-order intermodulation distortion, and is about 80 dB. The noise level at this time is the noise level of the spectrum analyzer.
Also, assuming that no second-order intermodulation distortion occurs,
From FIG. 10, the dynamic range is limited by the third-order distortion, and can be expected to be a value near 90 dB.

Next, a description will be given of the reduction of the secondary intermodulation distortion in the case of three light wave inputs according to the present embodiment. As described above, in a two-channel millimeter-wave link system using heterodyne of three light waves, secondary intermodulation distortion occurs near the signal frequency. Next, secondary distortion is removed from the vicinity of the signal frequency by changing the relationship between the oscillation frequencies of the lasers. The frequency of the laser diodes 11 and 12 with respect to the reference carrier frequency of the reference laser diode 10 is f _opt1 <
The laser diodes 11 and 12 are controlled so that f _opt0 , f _opt2 > f _opt0, and the frequency of each of the laser diodes 11 and 12 is divided into positive and negative with respect to the frequency of the reference laser diode 10. Here, f _opt1 ≪f _opt0 , f _opt2 ≫f _opt0
And the difference frequency f _rf1 = f _opt0 −f
_{opt1, f rf2 = f opt2 -f} opt0 leaves so as to be separated by the band pass filter together and at least second-order intermodulation distortion 2f _rf1 -f _rf2, 2 _fr2 bandpass filter each other so as not to overlap with the -f _rf1 F so that they can be separated by
_opt1よ_う f _opt0 , f _opt2 ≫f _opt0 . FIG. 11 shows the relationship between the oscillation frequency of each of the laser diodes 10, 11, and 12 and the generated millimeter wave frequency.

The signals and the output current of the photodiode 90 of the intermodulation distortion in the vicinity thereof, the desired millimeter-wave signal of the frequency (the difference _{frequency) f rf1 = f opt0 -f opt1} , f rf2 = f
_{opt2− fopt0} and _Equations 2 and 3 above _{give Equations} 6 and 7. The amplitude of each of the laser diodes 10, 11, and 12 was the same value A.

[0031]

I _sig ′ = (2A ² k ₁ + 16A ⁴ k ₂ + 120A ⁶
k ₃ + ...) cos (2πf _rf1 t)

I _dis ′ = (6A ⁶ k ₃ +...) Cos [2π (2
f _rf1 -f _rf2) t]

As described above, the output of the signal is the same as that of Equation 4, but it can be seen from _Equation 7 that no second-order intermodulation distortion appears in i _dis ′ near the signal frequency. Note that the second-order intermodulation distortion occurs near twice the signal frequency, but this can be easily reduced by a band-pass filter.

Assuming the coefficients k ₁ , k ₂ , and k ₃ of the above formulas 4 and 5 from the measurement results of FIG. 10, the signals for the amount of incident light and the intermodulation distortion output when these are substituted into the formulas 6 and 7 Was calculated. FIG. 12 shows the result. Here, the frequency of the millimeter-wave _{signal, f rf1 = 38.5GHz, f rf2} = 38.
6 GHz. The broken line in FIG. 12 is the intermodulation distortion measured in the comparative example, and the solid line is the calculation result of the distortion output using Expression 7. Since the secondary intermodulation distortion does not occur as shown in FIG. 12, the dynamic range is improved by about 10 dB, and 90 dB or more is expected.

In the above embodiment, the PLL circuit 21
Controls the laser diodes 10 and 11,
Only the laser diode 11 may be controlled. Similarly,
The PLL circuit 22 controls the laser diodes 10 and 12, but may control only the laser diode 12.

To summarize the frequency of the signal and the second-order intermodulation distortion, as shown in FIGS. 7 and 11, the frequency (wavelength) of the optical signal and the frequency of the millimeter-wave or microwave signal generated therefrom are arbitrary. A millimeter wave signal having a difference between the frequencies of the two sets of optical signals is generated. In the comparative example of FIG. 7, the laser diode 10 is
A millimeter wave signal having a difference frequency between the laser diodes 11 and 12 and a difference frequency between the laser diodes 10 and 12 are generated, and a signal having a difference frequency between the laser diodes 11 and 12 is also generated.
On the other hand, in the embodiment of FIG. 11, the same is true, but in this case, since the frequencies of the laser diodes 11 and 12 are far apart, the difference frequency component between the laser diodes 11 and 12 is generated at a high frequency far away. . Here, the frequency in the vicinity of the signal _{(2 frf1 -f rf2, 2f rf2} -f rf1), 2 -order intermodulation distortions disappear.

Further, in the comparative example of FIG. 7, the intermodulation distortion is caused by the second-order or higher-order distortion of the second, third,..., But in the embodiment of FIG. 11, the intermodulation distortion in FIG. This is due to third-order or higher distortion. Although second-order distortion is present in the other frequency band (such as _{f rf2 -f r f1, 2f rf1} , 2f rf2), to leave substantially from the signal frequency (f _rf1, f _rf2), bandpass filter Can be removed. In the conventional example, second-order intermodulation distortion was generated in the vicinity of the signal, so that it was difficult to remove the signal with a band-pass filter.

As described above, it has been theoretically shown that the second-order intermodulation distortion occurs near the signal frequency in the heterodyne optical fiber millimeter-wave link system. Three
Experiments on generation of two-channel millimeter waves using light wave input have demonstrated that third- and second-order intermodulation distortions occur near the signal frequency. Furthermore, in the case of a two-channel millimeter-wave link with three lightwave inputs, the wavelength (frequency) of each light
By appropriately selecting the arrangement of
It has been shown that the following distortions can be removed.

Therefore, in the present embodiment, the frequency of the laser diodes 11 and 12 is set to f _opt1 <f _opt0 , f _opt2 > f with respect to the reference carrier frequency of the reference laser diode 10.
_{Since the} laser diodes 11 and 12 were controlled to be _opt0, and the frequency of each laser diode 11 and 12 was divided into positive and negative with respect to the frequency of the reference laser diode 10,
Second-order intermodulation distortion can be removed from the vicinity of the signal frequency, and the transmission quality of signal transmission in the optical subcarrier transmission system can be greatly improved.

FIG. 13 is a block diagram showing a basic configuration of an optical fiber link system 101 according to a second embodiment of the present invention. FIG. 14 is a diagram showing an optical frequency and a millimeter wave transmission signal according to the second embodiment. FIG. 4 is a spectrum diagram showing a frequency relationship of FIG. The second embodiment is different from the first embodiment having three laser diodes 10, 11, and 12 in that
This is extended to a system including the laser diodes 10 to 16, and the concept of the system configuration is the same as that of the first embodiment.

Referring to FIG. 13, the optical fiber link system 101 of the present embodiment is roughly divided into a radio control station 201 and a radio base station 301, as shown in FIG. The connection is made using a single-mode (or multi-mode) optical fiber cable 60 capable of transmitting an optical signal. 21 to 26 are PLL circuits, 31 to 36 are photodiodes, 50 to 56 are optical fiber cables, 70a, 71 to 76 are optical demultiplexers, and 80 to 86 are optical multiplexers.

Here, the PLL circuit 21 converts the reference optical signal of the laser diode 10 and the first optical signal of the laser diode 11 into a reference optical signal based on a combined error signal photoelectrically converted by the photodiode 31. first optical signal each frequency f _opt0, f _{op t1} a and mutually phase-locked to such oscillation is allowed as a laser diode 10, 11
Control. The PLL circuit 22 also generates a reference optical signal and a second optical signal based on a combined error signal obtained by photoelectrically converting the reference optical signal of the laser diode 10 and the second optical signal of the laser diode 12 by the photodiode 32. The laser diodes 10 and 12 are controlled so that signals are oscillated at frequencies f _opt0 and f _opt2 and in phase synchronization with each other. Further, the PLL circuit 23 generates a reference optical signal and a third optical signal based on a combined error signal obtained by photoelectrically converting the reference optical signal of the laser diode 10 and the third optical signal of the laser diode 13 by the photodiode 33. The laser diodes 10 and 13 are controlled so that signals are oscillated at frequencies f _opt0 and f _opt3 and in phase synchronization with each other. The PLL circuit 24 also generates a reference optical signal and a fourth optical signal based on a combined error signal obtained by photoelectrically converting the reference optical signal of the laser diode 10 and the fourth optical signal of the laser diode 14 by the photodiode 34. The laser diodes 10 and 14 are controlled so that signals are oscillated at frequencies f _opt0 and f _opt4 and in phase synchronization with each other. Further, the PLL circuit 25 generates a reference optical signal and a fifth optical signal based on a combined error signal obtained by photoelectrically converting the reference optical signal of the laser diode 10 and the fifth optical signal of the laser diode 15 by the photodiode 35. The laser diodes 10 and 15 are controlled so that signals are oscillated at the frequencies f _opt0 and f _opt5 and in phase synchronization with each other. Further, based on the combined error signal obtained by photoelectrically converting the reference optical signal of the laser diode 10 and the sixth optical signal of the laser diode 16 by the photodiode 36, the PLL circuit 26 The laser diodes 10 and 16 are oscillated at frequencies f _opt0 and f _opt6 and in phase synchronization with each other.
Control.

In the second embodiment, as shown in FIG. 14, the oscillation frequency of each of the laser diodes 10 to 16 is f
_opt5 <f _opt3 <f _opt1 <f _opt0 <f _opt2 <f _opt4 <f
Each difference frequency that is set to be _opt6 and is the frequency of the millimeter wave transmission signal

[Equation 8] _{f rf1 = f opt0 -f opt1 f} rf2 = f opt2 -f opt0 f rf3 = f opt0 -f opt3 f rf4 = f opt4 -f opt0 f rf5 = f opt0 -f opt5 f rf6 = f opt6 - f _opt0 are not the same as each other and are set to be separable by a band-pass filter. That is, with respect to the reference carrier frequency of the reference laser diode 10, the other laser diodes 11 to 16 are set so as to be halved on the high frequency side and the low frequency side of the reference carrier frequency.

In the second embodiment configured as described above, the level of intermodulation distortion is low, as in the first embodiment. Further, the level of the intermodulation distortion can be reduced to about half as compared with the case where all the other laser diodes 11 to 16 are set to the high frequency side based on the laser diode 10.

In the above embodiment, the oscillation frequency (wavelength) is controlled by controlling the bias current of the laser diodes 10 to 16 using the laser diodes 10 to 16 which are semiconductor lasers. The invention is not limited to this, and a solid-state laser may be used instead of the laser diode. In this case, the oscillating optical frequency (wavelength) may be controlled by changing the temperature or the pressure to be applied.

[0045]

As described above in detail, according to the optical subcarrier transmission system of the present invention, the reference light generating means for generating the reference optical signal having the predetermined carrier frequency,
Generating a first optical signal having a subcarrier frequency of
And at least three light generating means including a second light generating means for generating a second optical signal having a predetermined second subcarrier frequency, and the first light generating means First modulation means for modulating the generated first optical signal in accordance with the input data signal and outputting the modulated signal, and converting the second optical signal generated by the second light generation means in accordance with the input data signal A second modulating means for modulating and outputting, an optical multiplexing means for multiplexing and outputting the reference optical signal, the first optical signal and the second optical signal, and an output from the optical multiplexing means An optical sub-carrier transmission system including an optical fiber cable for transmitting the combined signal, and a photoelectric conversion unit for photoelectrically converting the signal transmitted by the optical fiber cable into an electric signal and outputting the electric signal, Above reference carrier frequency The first and second sub-carrier frequency,
The difference frequency between the reference carrier frequency and the first sub-carrier frequency and the difference frequency between the reference carrier frequency and the second sub-carrier frequency do not overlap, and the first sub-carrier frequency is The second subcarrier frequency is set lower than the reference subcarrier frequency and higher than the reference subcarrier frequency.

Therefore, according to the present invention, the secondary intermodulation distortion can be removed from the vicinity of the signal frequency at the output of the photoelectric conversion means, and the transmission quality of the signal transmission in the optical subcarrier transmission system is greatly improved. Can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an IM-D in a first conventional link system.
FIG. 3 is a spectrum diagram showing a relationship between carrier frequencies in a D system.

FIG. 2 is a spectrum diagram showing a relationship between carrier frequencies of a heterodyne system in a link system of a second conventional example.

FIG. 3 is a spectrum diagram showing a frequency relationship in the case of a single subcarrier in a heterodyne system of a third conventional example.

FIG. 4 is a spectrum diagram showing a frequency relationship in the case of a large number of subcarriers in a fourth conventional heterodyne method.

FIG. 5 is a block diagram illustrating a basic configuration of an optical fiber link system according to the first embodiment of the present invention.

FIG. 6 is a block diagram showing the overall configuration of the optical fiber link system according to the first embodiment of the present invention.

FIG. 7 is a spectrum diagram showing a frequency relationship between an optical frequency and a millimeter wave transmission signal in a comparative example.

FIG. 8 is a spectrum diagram showing measurement results of a millimeter wave transmission signal and intermodulation distortion in a comparative example.

FIG. 9 is a spectrum diagram showing a measurement result of a millimeter wave transmission signal generated by a heterodyne method in a comparative example.

FIG. 10 is a graph showing measurement results of an input optical power level, a millimeter wave transmission signal, and intermodulation distortion in a comparative example.

FIG. 11 is a spectrum diagram showing a frequency relationship between an optical frequency and a millimeter wave transmission signal in the first embodiment.

FIG. 12 is a graph showing a millimeter wave transmission signal and an intermodulation distortion with respect to an input optical power level when an optical frequency is changed in the first embodiment.

FIG. 13 is a block diagram illustrating a basic configuration of an optical fiber link system according to a second embodiment of the present invention.

FIG. 14 is a spectrum diagram showing a frequency relationship between an optical frequency and a millimeter wave transmission signal in the second embodiment.

[Explanation of symbols]

11, 12, 13, 14, 15, 16 ... laser diode, 11f, 12f ... optical fiber cable, 21, 22, 23, 24, 25, 26 ... PLL circuit, 31, 32, 33, 34, 35, 36 ... Photodiodes, 40, 50, 51, 52, 53, 54, 55, 56 ... optical fiber cables, 70, 71, 72, 73, 74, 75, 76 ... optical demultiplexers, 70 fa, 70 fb, 71 f, 72 f ... Optical fiber cable, 80, 81, 82, 83, 84, 85, 86: optical multiplexer, 81f, 82f: optical fiber cable, 90: photodiode, 91: transmission power amplifier, 92: band-pass filter (BPF), 93: Antenna, 100, 101: Optical fiber link system, 200, 201: Radio control station, 300, 301: Radio base station.

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[Procedure amendment]

[Submission date] April 22, 1999 (1999.4.2
2)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction contents]

[0009]

An optical subcarrier transmission system according to the present invention comprises: a reference light generating means for generating a reference optical signal having a predetermined carrier frequency; and a first light having a first predetermined subcarrier frequency. At least three light generating means including first light generating means for generating a first optical signal, and second light generating means for generating a second optical signal having a predetermined second subcarrier frequency; First modulation means for modulating and outputting the first optical signal generated by the first light generation means in accordance with an input data signal, and second light generated by the second light generation means Second modulating means for modulating a signal in accordance with an input data signal and outputting the same; optical multiplexing means for multiplexing and outputting the reference optical signal, the first optical signal, and the second optical signal And the multiplexed output from the optical multiplexing means. An optical subcarrier transmission system comprising: an optical fiber cable for transmitting a signal; and a photoelectric conversion means for photoelectrically converting a signal transmitted by the optical fiber cable into an electric signal and outputting the electric signal. And the second sub-carrier frequency, so that the difference frequency between the reference carrier frequency and the first sub-carrier frequency, the difference frequency between the reference carrier frequency and the second sub-carrier frequency do not overlap, The first sub-carrier frequency is set to be lower than the reference sub-carrier frequency, and the second sub-carrier frequency is set to be higher than the reference sub-carrier frequency. It is characterized in that second-order intermodulation distortion is removed or reduced from the vicinity of the signal frequency.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0045

[Correction method] Change

[Correction contents]

[0045]

As described above in detail, according to the optical subcarrier transmission system of the present invention, the reference light generating means for generating the reference optical signal having the predetermined carrier frequency,
Generating a first optical signal having a subcarrier frequency of
And at least three light generating means including a second light generating means for generating a second optical signal having a predetermined second subcarrier frequency, and the first light generating means First modulation means for modulating the generated first optical signal in accordance with the input data signal and outputting the modulated signal, and converting the second optical signal generated by the second light generation means in accordance with the input data signal A second modulating means for modulating and outputting, an optical multiplexing means for multiplexing and outputting the reference optical signal, the first optical signal and the second optical signal, and an output from the optical multiplexing means An optical subcarrier transmission system comprising: an optical fiber cable that transmits the combined signal; and a photoelectric conversion unit that photoelectrically converts the signal transmitted by the optical fiber cable into an electric signal and outputs the electric signal. Carrier frequency and above The first and second sub-carrier frequency,
The difference frequency between the reference carrier frequency and the first sub-carrier frequency and the difference frequency between the reference carrier frequency and the second sub-carrier frequency do not overlap, and the first sub-carrier frequency is By setting the second sub-carrier frequency to be lower than the reference sub-carrier frequency and the second sub-carrier frequency to be higher than the reference sub-carrier frequency, the second mutual carrier frequency can be changed from near the signal frequency of the photoelectrically converted electric signal. Eliminate or reduce modulation distortion.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0046

[Correction method] Change

[Correction contents]

Therefore, according to the present invention, secondary intermodulation distortion can be removed or reduced from the vicinity of the signal frequency at the output of the photoelectric conversion means, and the transmission quality of signal transmission in the optical subcarrier transmission system can be reduced. Can be greatly improved.

Continuing on the front page (72) Inventor Keizo Inagaki 5 Shiraya, Inaya, Koza-cho, Seika-cho, Kyoto Prefecture Inside the ATR Eco-friendly Communication Research Laboratories (72) Inventor Yoshihiko Mizuguchi, Seika-cho, Kyoto Prefecture 5 Futam (Reference) 5K002 BA05 BA13 BA15 CA01 CA05 CA07 CA14 DA02 FA01

Claims (1)

[Claims] 1. A reference light generating means for generating a reference optical signal having a predetermined carrier frequency, a first light generating means for generating a first optical signal having a predetermined first subcarrier frequency, At least three light generating means including: a second light generating means for generating a second optical signal having a second subcarrier frequency; and a first light generated by the first light generating means. A first method of modulating a signal according to an input data signal and outputting the modulated signal
Modulating the second optical signal generated by the second light generating means in accordance with the input data signal, and outputting the modulated second optical signal.
Modulating means, an optical multiplexing means for multiplexing and outputting the reference optical signal, the first optical signal, and the second optical signal, and a multiplexed signal output from the optical multiplexing means An optical sub-carrier transmission system, comprising: an optical fiber cable that transmits the reference carrier frequency; and a photoelectric conversion unit that photoelectrically converts a signal transmitted by the optical fiber cable into an electric signal and outputs the electric signal. The first and second sub-carrier frequencies are a difference frequency between the reference carrier frequency and the first sub-carrier frequency, the reference carrier frequency and the second
So that the difference frequency with the sub-carrier frequency of
An optical subcarrier transmission system, wherein the first subcarrier frequency is set lower than the reference subcarrier frequency, and the second subcarrier frequency is set higher than the reference subcarrier frequency. .