JP2002152144A - Optical transmitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitting device which reduces mixed modulation caused by third distortion and suppresses the deterioration of service life by using an inexpensive semiconductor laser device. SOLUTION: Average power P0 of an input signal is detected by an envelope detector 5, and a bias driving current controller 6 outputs a bias driving current I0 which increases as the power P0 increases. The semiconductor laser device comprising a light emitter 4 is driven by a driving current superimposing the bias driving current I0 and a current signal (i) proportional to the input signal, and the strength of output light is modulated and the light is outputted to an optical cable 7. Therefore, when the power of the input signal increases, the bias driving current is increased and the influence of the non-linear distortion of the semiconductor laser device is reduced. When the power decreases, the bias driving current is decreased and the deterioration of the semiconductor laser device is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device, and more particularly to an optical transmission device suitable for superimposing a plurality of high-frequency signals and transmitting the signals at once.

[0002]

2. Description of the Related Art In recent years, with the development of mobile communication such as mobile phones, a large number of wireless base stations for mobile phones have been installed to reduce blind spots where radio waves are difficult to reach, and have high traffic in downtown areas and the like. In order to cope with the problem, measures are taken to cover the entire service area by installing a large number of radio base stations in a small service area. In such a system configuration, it is necessary to reduce the size of the wireless base station and reduce restrictions on the installation location. However, in a wireless base station including a modem unit and a wireless unit, when the number of channels to be handled increases, the size of the modem unit increases, making it difficult to reduce the size of the device.

As a solution to this problem, the modulation and demodulation units of a plurality of radio base stations are collectively installed in one place, and each radio base station is provided with only a radio unit and an antenna. 2. Description of the Related Art A system in which radio frequency signals to and from a radio base station are connected via a wired transmission path is used. In the case of such a system, a coaxial cable can be used as a wired transmission path. However, if the radio frequency of the mobile phone system is several hundred MHz or more, a large distance from a centralized location of the modem unit to a radio base station far away is large. To transmit a radio frequency signal without attenuation requires a large-diameter coaxial cable, which is not preferable in terms of cost. On the other hand, if an optical cable is used as a transmission line of a radio frequency signal, a transmission line with a wide band and low loss can be easily realized at low cost. The present invention relates to an optical transmission device for transmitting a plurality of radio frequency signals having different frequencies through an optical cable.

As an optical transmission device using an optical cable, a semiconductor laser device is used as an optical modulator, and a drive current of the semiconductor laser device is varied in amplitude in proportion to a radio frequency signal to change the emission intensity of the semiconductor laser device. A device that transmits an optical signal whose optical intensity has been modulated in this way via an optical cable is widely used.

[0005]

As described above, a plurality of radio base stations are composed of only a radio section and an antenna, and the modulation and demodulation sections of these radio base stations are centrally installed in one place, and the radio base stations and each radio base station are arranged. In a system that connects an office with an optical cable,
Since each radio base station houses a plurality of radio frequency signals, that is, radio channels, the radio frequency signals received by the radio base station are usually plural, and radio frequency signals having different frequencies from each other are superimposed. . Therefore, if there is non-linear distortion, particularly third-order distortion, in the optical transmission line, intermodulation distortion occurs between the radio frequency signals, causing distortion in the radio frequency signals of each channel. The nonlinear distortion of the optical transmission line is mainly due to the non-linear characteristics of the semiconductor laser device. Therefore, it is necessary to use a semiconductor laser device having good characteristics.
For this purpose, a semiconductor laser device having good linearity may be used, but there is a problem that a semiconductor laser device having good linearity becomes more expensive. Also, semiconductor laser devices are generally
The higher the light output level is, the smaller the third-order distortion is. Therefore, when used at a high light output level, the linearity is improved. However, for this purpose, the drive current increases and the life of the semiconductor laser device is shortened. There is a problem of doing.

An object of the present invention is to provide an optical transmission device capable of sufficiently suppressing intermodulation distortion between radio frequency signals without using a semiconductor laser device having particularly good linearity.

[0007]

SUMMARY OF THE INVENTION The present invention relates to an optical transmission apparatus for modulating the light intensity of output light with an input signal in which a plurality of signals having different frequency bands are superimposed on each other and transmitting the modulated signal. Input power detection means for detecting the input signal, bias drive current control means for outputting a bias drive current that increases as the average power of the input signal detected by the means increases, and output of the drive current from the bias drive current control means A semiconductor laser device that is driven by a drive current in which a current proportional to the input signal is superimposed on the bias drive current that has been output, and that outputs an optical modulation signal whose output light intensity is modulated by the input signal. An optical transmission device characterized by the above is disclosed.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration example of an optical transmission device according to the present invention.
And a radio frequency signal received by the radio unit 2 is transmitted to the light emitting device 4 using the semiconductor laser device via the distributor 3 by the modulation signal S.
Is entered as A part of the received radio frequency signal is branched by the distributor 3 and input to the envelope detector 5, where the average power P0 of the input radio frequency signal is detected. The bias drive current controller 6 controls the drive current bias value of the semiconductor laser device in the light emitter 4 according to the value of the average power P0 detected by the detector 5. In the light emitting device 4, the sum of the bias drive current I0 from the controller 6 and the current i (hereinafter also referred to as a modulation signal) according to the modulation signal S is defined as drive current I = I0 + i. An optical signal having a high intensity is output as an optical modulation signal LS and transmitted to the optical cable 7.

FIG. 2 is a diagram showing the modulation characteristics of the semiconductor laser device. The emission intensity L changes along the characteristic curve C as the drive current I changes. That is, when the bias drive current is I01 and the modulation signal is i1, the output intensity changes like L1,
When the bias drive current is I02 and the modulation signal is i2, the output intensity changes like L2. However, the arrow in the figure indicates the direction of time. Here, if the characteristic curve C of the semiconductor laser device is a perfect straight line, the above-described cross-modulation distortion does not occur, but in fact, deviation from the straight line cannot be avoided.

FIG. 3 shows a third-order distortion component generated due to the non-linearity of the characteristic curve of the semiconductor laser device. The horizontal axis represents the light output (the average value of the light emission intensity L in FIG. 2, that is, the bias driving current I0). And the vertical axis represents the ratio of the power of the third-order distortion component to the power of the modulation signal i when the modulation factor is fixed. As shown in this figure, in a semiconductor laser device, generally, the higher the output, the smaller the distortion component with respect to the signal. FIG. 4 shows the noise characteristics with respect to the optical output of the semiconductor laser device. The horizontal axis represents the same optical output as in FIG. 3, and the vertical axis represents the ratio of the noise power to the power of the modulation signal i when the optical modulation factor is fixed. It is. Also in this case, generally, as the optical output increases, the noise component of the signal decreases. From these characteristics, it is possible to obtain an optical modulation signal having a large S / N ratio if the semiconductor laser device is used with a large optical output, that is, a large bias driving current. As the size increases, the deterioration rate also increases, and the life of the semiconductor laser device is shortened.

In the optical transmission apparatus of FIG. 1 according to the present invention, the average power P0 of the received radio frequency signal input by the envelope detector 5 is detected, and the bias drive current controller 6 has a large average power P0. The bias drive current I0 is controlled so as to increase as the bias drive current I0 increases. For example, as shown in FIG. 2, when the power of the modulation signal i2 is larger than that of the modulation signal i1, the control is performed so that the bias drive current I02 is larger than the bias drive current I01. Non-linear distortion generally decreases sharply as the input amplitude decreases, so that in the case of the modulation signal i1 of FIG. 2 where the amplitude of the input modulation signal is relatively small (and therefore P0 is also small), a large optical output is required. Even without it, third-order distortion does not matter. Therefore, in this case, the bias drive current is set to I01 which is sufficiently small. On the other hand, in the case of the modulation signal i2 of FIG. 2 in which the amplitude of the input modulation signal is relatively large (and therefore P0 is also large), the third-order distortion is suppressed as a large optical output. In this case, the bias drive current has a large value I02.

As described above, when the bias drive current I0 is controlled in accordance with the average power P0 of the input radio frequency signal, the optical modulation output is always obtained irrespective of the level of the radio frequency signal from the portable telephone received by the radio base station. Is suppressed, and the bias drive current has a small value when the average power P0 of the received radio frequency signal is small. The level of the radio frequency signal received from the mobile phone fluctuates greatly. While this level is low, the bias drive current of the semiconductor laser device can be kept small. An optical transmission device with high reliability can be realized.

In the above description, a case has been described as an example where a radio frequency signal is optically transmitted from a radio base station of a portable telephone system to a place where modulation and demodulation units are centrally installed, but the light intensity is modulated by a signal having a large level fluctuation. It is needless to say that the present invention is similarly effective as long as the system performs optical transmission.

[0014]

According to the present invention, when optically transmitting a signal on which a multi-frequency signal is superimposed, an optical transmission device having a small intermodulation distortion and a long life can be realized even if a low-cost semiconductor laser device is used. There is.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration example of an optical transmission device according to the present invention.

FIG. 2 is a diagram showing a modulation characteristic of a semiconductor laser device;

FIG. 3 is a diagram showing the relationship between the optical output of a semiconductor laser device and third-order distortion;

FIG. 4 is a diagram showing a relationship between an optical output of a semiconductor laser device and a relative noise level.

[Explanation of symbols]

 2 Radio unit 3 Distributor 4 Light emitter 5 Envelope detector 6 Bias drive current controller 7 Optical cable

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04Q 7/36

Claims (1)

[Claims] 1. An optical transmission apparatus for modulating the optical intensity of output light with an input signal on which a plurality of signals having different frequency bands are superimposed and transmitting the modulated signal, comprising: an input power detecting means for detecting an average power of the input signal; A bias drive current control unit that outputs a bias drive current that increases as the average power of the input signal detected by the unit increases; and that the drive current is input to the bias drive current output from the bias drive current control unit. An optical transmission device, comprising: a semiconductor laser device that is driven by a driving current in which a current proportional to a signal is superimposed, and outputs a light modulation signal in which the intensity of output light is modulated by the input signal.