JPH11112423A - Analog optical fiber transmitting method and transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the third intermodulation distortion of a semiconductor laser at a low cost by converting a frequency multiplex signal to a signal in an intermediate frequency band, electrooptically converting this signal and the signal of a local oscillator respectively by the semiconductor laser and summing its output to transmit. SOLUTION: A frequency-multiplexed transmission signal 3 is mixed with a local oscillation signal generated by the local oscillator 12 with a mixer 13 and passes through a band-pass filter 14 synchronized with the intermediate frequency of a difference between a transmission signal 3 and the local oscillation signal to be electrooptically converted by a semiconductor laser 2A. The local oscillation signal being the output of the oscillator 12 is electrooptically converted by a semiconductor laser 2B. The outputs of the lasers 2A and 2B are summed by a 2×1 optical coupler 15 and transmitted through an optical fiber 4. The output of the fiber 4 is electrooptically converted by the photodiode 5 of a photodetective part, rectified by a rectifier 16 and outputted as the original signal 3 through a band-pass filter 17 in a transmission signal band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to transmission of a frequency multiplex signal through an optical fiber.

[0002]

2. Description of the Related Art In an analog optical fiber transmission system, when a signal obtained by frequency-multiplexing signals of center frequencies f1, f2... Fn in a specific frequency band is transmitted, as shown in FIG. The optical frequency conversion is performed by an LD, the signal is transmitted by an optical fiber, and the light is converted into an electrical signal by a photodiode (PD) to obtain an original frequency multiplexed signal. However, when transmitting a frequency-multiplexed signal, the center frequency f
Third-order intermodulation distortion (hereinafter, referred to as third-order distortion) between signals having 1, f2... Fn degrades transmission quality. This is because the (2fi-fj) component (i, j is a value of 1 to n), which is the third-order distortion caused by nonlinearity in the transmission device, falls within the signal band to be transmitted.

[0003] FIG. 3 shows a general analog optical fiber transmission device, in which a semiconductor laser (LD) shown in FIG.
A more specific configuration in a section up to signal output is shown. In the figure, reference numeral 1 denotes an amplifier, reference numeral 2 denotes a semiconductor laser, reference numeral 3 denotes a frequency-multiplexed transmission signal, reference numeral 4 denotes an optical fiber, reference numeral 5 denotes a photodiode of a light receiving element, and reference numeral 6 denotes an amplifier. The frequency-multiplexed transmission signal 3 is amplified by the amplifier 1 of the light emitting unit, is subjected to electro-optical conversion by the semiconductor laser 2, and is transmitted by the optical fiber 4. In the light receiving section, the transmission signal 3 which is photoelectrically converted by the photodiode 5 and amplified by the amplifier 6 to obtain the original frequency-multiplexed transmission signal 3 is obtained. In this system, the third-order distortion is generated by the semiconductor laser 2 and the amplifiers 1 and 6, but the one generated by the semiconductor laser is the most serious problem. Therefore, conventionally, attention has been paid to (a) use of a semiconductor laser with less distortion, (b) correction of the distortion of the semiconductor laser, and (c) reduction of the input level of the semiconductor laser.

[0004] Of these, the cost of (a) directly affects the cost because the semiconductor laser is one of the most expensive parts in the analog optical fiber transmission device. In (b), because the phase difference between the original signal and the compensation signal cannot always be accurately adjusted for the signal in the transmission band of the semiconductor laser, the improvement cannot be taken so much, and the circuit configuration becomes complicated. It is not realistic at all. (c) is realized by reducing the amplification of the amplifier 1 and increasing the amplification of the amplifier 6 in FIG. However, the output portion of the photodiode 5 used for photoelectric conversion, which is located downstream of the semiconductor laser, which is the source of distortion, is the point where the signal level is the lowest in the present transmission system. There is contamination. Also,
Since the semiconductor laser itself generates noise in principle, it causes deterioration in transmission quality in terms of noise. As described above, there are problems, and it is not easy to improve the performance as a whole.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of reducing the third-order distortion in the prior art, such as high cost, complicated circuit configuration, and increase in noise. An object of the present invention is to provide an analog optical fiber transmission method and its device.

[0006]

To achieve the above object, the present invention provides an analog optical transmission method for analog transmission of a frequency-division multiplexed signal over an optical fiber. The multiplexed signal is converted into a signal of an intermediate frequency band by a local oscillator and a mixer, the signal of the intermediate frequency band is electro-optically converted by a first semiconductor laser, and the signal of the local oscillator is converted to an electric signal by a second semiconductor laser. -Optical conversion, and the sum of the first and second semiconductor laser outputs and transmits them with one optical fiber, and on the receiving side, the output of the optical fiber is subjected to optical-electrical conversion with one light receiving element, An analog optical fiber, wherein an output of the light receiving element is rectified by a rectifier, and a frequency multiplexed signal transmitted by a filter is extracted from the rectified output. It discloses a transmission method.

Further, the present invention is characterized in that the output wavelengths of the first and second semiconductor lasers are set so that the difference does not fall within the band of the frequency multiplexed signal extracted on the receiving side. An optical fiber transmission method is disclosed.

Further, the present invention relates to an analog optical transmission device for analog transmission of a frequency multiplexed signal over an optical fiber, wherein a local oscillator, an input frequency multiplexed signal and an output of the local oscillator are provided on the transmission side. A mixer for mixing, a band-pass filter for extracting a difference frequency component from the output of the mixer, a first semiconductor laser that converts an output of the band-pass filter into an optical signal by electro-optical conversion, and A second semiconductor laser that converts the output of the local oscillator into an optical signal by electro-optical conversion, and an optical coupler that sums the optical signals output from the first and second semiconductor lasers and sends the sum to an optical fiber; At the same time, on the receiving side, a light receiving element that converts an optical signal transmitted through the optical fiber into an electric signal by photoelectric conversion, an amplifier that amplifies the electric signal from the light receiving element, It discloses a rectifier for rectifying an output of the wide unit, an analog optical fiber transmission system, characterized in that a bandpass filter for extracting a frequency-multiplexed signal transmitted from the rectifier output.

[0009]

Embodiments of the present invention will be described below in detail. FIG. 1 shows an example of the configuration of an analog optical fiber transmission apparatus according to the present invention. Reference numeral 1 denotes an amplifier, reference numerals 2A and 2B denote semiconductor lasers, reference numeral 3 denotes a frequency-multiplexed transmission signal, reference numeral 4 denotes an optical fiber, and reference numeral 5 denotes light receiving. Element photodiodes, 6A and 6B are amplifiers, 11 is an attenuator, 12 is a local oscillator, 13 is a mixer, 14 is an intermediate frequency band-pass filter, 15 is a 2 × 1 optical coupler. Reference numeral 16 denotes a rectifier, reference numeral 17 denotes a bandpass filter of a transmission signal band, and reference numeral 18 denotes a wavelength control mechanism for controlling a wavelength difference of the semiconductor laser.

The frequency-multiplexed transmission signal 3 is applied to an attenuator 11
Through a band-pass filter 14 tuned to the intermediate frequency of the difference between the frequency-multiplexed transmission signal 3 and the local oscillation signal, which is mixed by the mixer 13 with the local oscillation signal generated by the local oscillator 12. Then, after being amplified by the amplifier 1, it is subjected to electro-optical conversion by the semiconductor laser 2A. On the other hand, a local oscillation signal, which is an output of the local oscillator 12, is converted into an electric signal by the semiconductor laser 2B. The outputs of both semiconductor lasers are added together by the 2 × 1 optical coupler 15 and transmitted by the optical fiber 4. The wavelength control mechanism 18 includes the semiconductor lasers 2A and 2A.
B operates so as to keep the difference between the oscillation wavelengths of B at a certain value or more. The output of the optical fiber 4 is photoelectrically converted by a photodiode 5 as a light receiving element of a light receiving unit, amplified by an amplifier 6A, rectified by a rectifier 16, passed through a bandpass filter of a transmission signal band, and then amplified. The signal is amplified at 6B and output as the original frequency-multiplexed transmission signal 3.

Next, the operation of each section will be described in detail. The frequency at the center of the band of the frequency-multiplexed transmission signal 3 is fs,
Let the local oscillation frequency of the output of the local oscillator 12 be fl, and
When fl is mixed by the mixer 5, the spectrum also has fs ± fl due to the nonlinearity of the mixer. Of these, the components passing through the band-pass filter 14 are components centered on the frequency of fs-fl. Although the third-order distortion occurs during mixing, the occurrence in the mixer depends on the attenuation of the attenuator 11 and the amplifier 1.
By adjusting the amplification degree, it is possible to keep the third-order distortion and noise in a state where the influence is small. The signal passing through the band-pass filter 14 is amplified by the amplifier 1 and then electro-optically converted by the semiconductor laser 2A. It is known that the third-order distortion generated by the semiconductor laser is proportional to the third power of the transmission frequency. I have. Conventionally, the frequency applied to the semiconductor laser is fs-frequency, but in the present invention, fs-f
l, it is 3 even if the same semiconductor laser as the conventional one is used.
The amount of generation of the secondary distortion is greatly reduced. The local oscillation signal is also electro-optically converted by the semiconductor laser 2B, but since this signal has a single frequency, the occurrence of distortion does not matter.

The outputs from the two semiconductor lasers 2A and 2B are added by the optical coupler 15 and transmitted. However, when outputs from the two semiconductor lasers 2A and 2B are transmitted through the optical cable 4 and applied to the photodiode 5 as a light receiving element, beat noise of both optical signals occurs. If the oscillation wavelength of the semiconductor laser 2A is λa and the oscillation wavelength of the semiconductor laser 2B is λb, this beat noise has a frequency f = c / λa
It has the center of the spectrum at -c / λb (c: speed of light) and may be mixed into the transmission band. For this reason, the wavelength difference control mechanism 18 is provided so that the wavelength difference between the semiconductor lasers 2A and 2B, λa-λb, is a certain value or more and the beat noise is outside the transmission band. As a method of wavelength difference control, a method is known in which beat noise is monitored as needed, and the temperature and bias current of both semiconductor lasers are controlled so that the frequency becomes higher than the transmission band. Alternatively, a semiconductor laser selected so as to increase the difference between the oscillation wavelengths of the semiconductor lasers 2A and 2B and λa−λb so that the beat noise is outside the transmission band may be used in advance.

The transmission signal 3 and the local oscillation signal, which is the output of the local oscillator 5, are converted to electric light by the semiconductor lasers 2A and 2B, transmitted by the optical fiber 4, and transmitted by the photodiode 5 as the light receiving element of the light receiving section. -Each of the conversions includes a DC component, as shown in FIG.
In FIG. 4, the diagram on the left is a diagram of the electro-optical conversion of the semiconductor laser (LD). The vertical axis represents the optical output of the semiconductor laser (LD), and the horizontal axis represents the input current of the semiconductor laser (LD). The input current of the semiconductor laser (LD) needs a bias current equal to or higher than Ith shown in the figure in order to secure the linearity of the conversion. In the figure, the bias current Ih is flowing. When the input current changes as shown in the figure, an optical output is obtained according to the magnitude of the input current, and the optical output becomes the optical input of the right photodiode (PD). The diagram on the right is a diagram of the photoelectric conversion by the photodiode. The vertical axis is the light input of the photodiode (PD), and the horizontal axis is the output current of the photodiode (PD). An output current is obtained according to the magnitude of the input light. When the bias current Ih on the left side changes, the average output current in the figure on the right side also changes, indicating that a DC component is included. Therefore, the output of the photodiode 5 as the light receiving element in FIG. 1 is input to the amplifier 6A. The amplifier 6A needs to have a function of cutting a DC component as well as an amplification function.

Amplified to a predetermined level by an amplifier 6A,
The signal from which the DC component has been cut is rectified by the rectifier 16, and this rectification operation generates a sum or difference component between the intermediate frequency component (fs-fl) and the local oscillation frequency fl. This is the same as the operation principle of a mixer such as a normal single balanced mixer or double balanced mixer.
The generated sum component (fs−fl) + fl, that is, only the restored original transmission signal 3 is the bandpass filter 1
7 and after being amplified to a predetermined level by the amplifier 6B,
Is output. Although the rectifier 16 also generates third-order distortion, similar to the mixer on the transmission side, by setting the signal level of this portion appropriately, the occurrence of distortion can be suppressed without increasing the noise level. Is possible.

As described above, the third-order intermodulation distortion of the semiconductor laser, which increases in proportion to the cube of the frequency, can be greatly reduced by using the intermediate frequency. On the other hand, the use of the intermediate frequency requires a mixer and a rectifier, and new distortion from the mixer and the rectifier occurs.However, it is possible to avoid the problem by properly performing the level distribution. As a whole, a transmission system with little distortion can be constructed.

[0016]

According to the present invention, the third-order intermodulation distortion of a semiconductor laser can be reduced without using an expensive semiconductor laser.
Can be significantly reduced. Further, since an inexpensive semiconductor laser can be used, the economic efficiency of the analog optical fiber transmission device is improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an analog optical fiber transmission device of the present invention.

FIG. 2 is an explanatory diagram of analog optical fiber transmission.

FIG. 3 is an explanatory diagram of a conventional analog optical fiber transmission device.

FIG. 4 is an explanatory diagram of electrical-optical conversion and optical-electrical conversion.

[Explanation of symbols]

 Reference Signs List 1 amplifier 2A, 2B semiconductor laser 3 frequency-multiplexed transmission signal 4 optical fiber 5 photodiode 6A, 6B amplifier 11 attenuator 12 local oscillator 13 mixer 14 intermediate band bandpass filter 15 optical coupler 16 rectifier 17 transmission signal band Bandpass filter 18 wavelength control mechanism

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/26 10/14 10/04 10/06

Claims (3)

[Claims] 1. An analog optical transmission method for analog transmission of a frequency multiplexed signal over an optical fiber, wherein a transmitting side converts an input frequency multiplexed signal into an intermediate frequency band signal by a local oscillator and a mixer. The signal of the intermediate frequency band is electro-optically converted by a first semiconductor laser, the signal of the local oscillator is electro-optically converted by a second semiconductor laser, and the outputs of the first and second semiconductor lasers are added. At the receiving end, the output of the optical fiber is subjected to opto-electric conversion by one light receiving element, and the output of the light receiving element is rectified by a rectifier. An analog optical fiber transmission method, wherein a frequency multiplex signal transmitted by a filter is extracted. 2. The apparatus according to claim 1, wherein output wavelengths of the first and second semiconductor lasers are set so that a difference between the output wavelengths does not fall within a band of a frequency multiplexed signal extracted on a receiving side. Analog optical fiber transmission method. 3. An analog optical transmission device for analog transmission of a frequency multiplexed signal through an optical fiber, comprising: a local oscillator on a transmitting side; and a mixer for mixing an input frequency multiplexed signal and an output of the local oscillator. A band-pass filter for extracting a difference frequency component from the output of the mixer; a first semiconductor laser that converts an output of the band-pass filter into an optical signal by electro-optical conversion; and an output of the local oscillator. A second semiconductor laser that converts an optical signal into an optical signal, and an optical coupler that sums the optical signals output from the first and second semiconductor lasers and sends the resultant signal to an optical fiber; A light-receiving element that converts an optical signal transmitted through the optical fiber into an electric signal by performing optical-electrical conversion, an amplifier that amplifies the electric signal from the light-receiving element, and an output of the amplifier. A rectifier to an analog optical fiber transmission system, characterized in that a bandpass filter for extracting a frequency-multiplexed signal transmitted from the rectifier output.