JPS60130233A - Fdm signal optical transmitter - Google Patents

Abstract

PURPOSE:To keep the modulation rate of a light emitting element to an optimum value by extracting the level of distortion at the outside of a signal band includeding an output light from the light emitting element and controlling the level of an FDM signal to make the level constant. CONSTITUTION:An output of a generating source 8 of the FDM signal is subject to level change by an AGC amplifier 9, then amplified by a main amplifier 10, added with a bias current generated from a constant current source 11 and the result is fed to the light emitting element 13. The output of the light emitting element 13 is led to an optical fiber, through which the output is transmitted, and a part of it is detected and amplified by a photo diode 14, the distortion component is detected by a filter 15 extracting a signal at the outside of the transmission signal band, detected and amplified by a detection amplifier 16 and fed back to the AGC amplifier 9. In this case, since the amplifier 16 has a function comparing the input with a predetermined reference value and outputting a compared voltage in addition to the detection and amplification, the amplifier 16 controls the output distortion of the element 13 to a constant value at all times.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a device for transmitting an intensity-modulated signal from a light source as a frequency multiplexed signal (FDM signal).

Configuration of a conventional example and its problems A conventional example of an FDM optical transmission device is shown in a block diagram in FIG. Reference numeral 1 denotes a signal generation source to be transmitted, which is shown to have oscillators of frequencies f1 to f corresponding to the number of channels to be transmitted. These signals are added by an adder 2 to obtain an n-channel frequency 4 signal (FDM signal). This signal may be a VHF band TV signal. After this is amplified by the amplifier 3, it is added to the bias current of the laser diode (LD) 5 supplied from the variable current source 7 and the second adder 4, and then the bias current of the laser diode (LD) is added as a light emitting element. )5. The light emitted from the LD 5 is guided to an optical fiber and transmitted. A portion of the LD output light is received by a photodetector (PD) e, and the output current controls a variable constant current source 7 to keep the light output of the LD constant.

In such an FDM optical transmission device, a light emitting element (
In this example, the setting of the modulation degree of LD5) is important.

10 hours when received using an actual FDM optical signal transmission device
It is desirable for the C/N ratio to be large, and for this purpose it is convenient to make the modulation degree large, but if it is too large, the distortion of the light emitting element will increase, causing distortion noise. As the modulation degree decreases, the C/N deteriorates, and as the modulation degree increases, distortion components increase, resulting in deterioration of transmission characteristics. Especially when using an LD as a light emitting element,
Due to the threshold characteristics of the LD, when the degree of modulation approaches 1, distortion increases rapidly. In order to avoid this phenomenon, conventionally the LD
APC (automatic output control device) is applied to keep the light emission output constant, an AGC circuit is used to keep the FDM signal size constant, and an oscillator or modulator with stable output is used to control the modulation degree of each channel. I am trying to keep it constant. However, even with this method, it is difficult to always keep the modulation degree constant because each part changes over time. Also FD
Since the modulation degree of the light emitting element substantially changes depending on the modulation depth of the M signal (in the case of AM), it is difficult to always set the modulation degree to the optimum value. In addition, when this device is used for retransmitting TV double signals, the input signal is the signal received by the antenna, so its level may fluctuate considerably due to weather etc. will change.

Purpose of the Invention The purpose of the present invention is to constantly set the modulation degree of the light emitting element to the optimum level without being affected by level fluctuations of the FDM signal.
The object of the present invention is to provide an optical transmission device for FDM signals having high quality transmission characteristics.

Structure of the Invention The present invention detects a part of the output light from a light emitting element, extracts the level of distortion outside the signal band included in the output, and adjusts the magnitude of the FDM signal so that the level is constant. Alternatively, the modulation degree of the light emitting element is maintained at an optimum level by controlling the IA flow of the light emitting element.

DESCRIPTION OF EMBODIMENTS FIG. 2 is a block diagram of an FDM signal optical transmission apparatus according to an embodiment of the present invention. Signal source 8 is a source of FDM signals. After the output is subjected to a level change by the AGC amplifier 9, it is amplified by the main amplifier 10, added to the bias current generated from the constant current source 11 by the adder 12, and applied to the light emitting element 13.

The output of the light emitting element 13 is guided to an optical fiber and transmitted, but a portion of it is detected and amplified by an A-to-diode (PD) 14, and a filter 15 extracts signals outside the transmission signal band.
The distortion component is detected, amplified by the detection amplifier 16, and fed back to the AGC amplifier 9.

FIG. 3 shows the Norworth vector of the light output of the light emitting element. FD of multiple channels to which 11 to fn in the figure are to be transmitted
It is an M signal. Distortion components generated by such a signal are illustrated in fa-f9. The frequency component of distortion occurs in integral multiple harmonics of the frequency of each channel, the sum of each channel frequency, and the difference component (intermodulation). Next, distortion when the light emitting element is an LD will be explained based on FIG. 4. The seam characteristics of the LD are shown in Figure 4A. A bias current and a high frequency signal are applied to the LD in a superimposed manner. This waveform is shown at the bottom of FIG. In the figure, only one sine wave signal is shown for simplicity, but the same thing occurs with FDM signals. In the figure, the waveform when the modulation degree is close to 1 is slightly exaggerated. At this time, the optical output waveform is greatly distorted at a point where the optical output is close to "O" as shown in FIG. At this time, large distortion occurs. At this time, if the modulation degree is slightly reduced or the bias current is slightly increased, the distortion component will decrease rapidly.

If the distortion components a-fg shown in FIG. 3 are detected and controlled so that their values are constant, the modulation degree of the LD can be optimally controlled. In principle, the same thing is possible no matter which distortion component is detected.

Next, let's consider the optimal distortion component to detect. Usually, the frequency difference between each channel of an FDM signal is often constant. At this time, in the part of the frequency equal to the frequency difference,
This results in many distortion components. Therefore, this frequency component is filtered by a narrow band filter (B, P, F).
Since the distortion component is large if extracted with PD141 filter 15. The gain of the feedback system constituted by the detection amplification 16 may be small, which is advantageous in terms of design. In an actual system, the maximum value of the distortion component may be determined using a spectrum analyzer or the like, and the frequency component thereof may be extracted and controlled.

Since the distortion component extracted in this way is a high frequency signal, it is detected by the detection/amplifier 16 and its DC component is extracted, and the AGC amplifier 9 is controlled using it. Therefore, the detection amplifier 16 includes a comparator that detects and amplifies the distortion component and at the same time compares the value with a predetermined reference value and outputs a voltage. By doing this, the magnitude of the signal is controlled so that the distortion component of the light emission output of the LD is always constant. The AGC amplifier 9 may be an electrically controlled variable attenuator.

Furthermore, it is clear that the same effect can be obtained even if the output of the detection amplifier 16 is fed back to the constant current source 11 instead of the AGC amplifier.

The above method is sufficient when the amplitude of the carrier wave of each channel is considered to be constant (for example, when an FM signal or a signal with a constant average value is AM modulated). In the case of a signal whose amplitude is not constant even as an average value, the level after detection amplification will fluctuate, so
At this level, overmodulation may occur instantaneously, and good image transmission cannot be achieved.

This distortion component contains signal components of each channel, resulting in a very complex waveform. Therefore, if this distortion component is envelope-detected, the peak is detected by a peak hold circuit with a long time constant, and the AGC amplifier is controlled based on the detected value, extremely stable transmission becomes possible.

In addition to the effects of the invention, according to the present invention, since the distortion component generated in the light emitting element output is controlled to be constant, the modulation degree is always optimal with respect to input signal level fluctuations, aging changes of the entire system, and temperature fluctuations. Therefore, a very stable and good optical transmission device can be constructed.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a conventional FDM signal optical transmission device, Figure 2
3 is a block diagram of an FDM signal optical transmission device according to an embodiment of the present invention, FIG. 3 is a signal spectrum diagram in FIG. 2, and FIG. 4 is a current-optical output characteristic diagram of the LD in FIG. 2. 8--signal source, 9...AGC amplifier, 1o...
...Main amplifier, 11... Constant current source, 12.
... Adder, 13 ... Light emitting element, 14.
... Photodiode, 15 ... Filter, 16 ... Detection amplifier. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 7 Figure 2 Figure 41! lI

Claims (1)

[Scope of Claims] 0) A light emitting element whose analog intensity is modulated by FDM signals of a plurality of channels, a photoelectric conversion section that photoelectrically converts and amplifies a part of the output light of the light emitting element, and an output of the photoelectric conversion section. A detection section that detects and amplifies frequency components other than the band of the FDM signal from the signal, and F flowing to the light emitting element with the output of the detection section.
FDM signal optical transmission device having a control section for controlling the magnitude of the DM signal; (2) FD according to claim 1, characterized in that the bias current flowing through the light emitting element is controlled by the power of the detection section.
M signal optical transmission device (3) The detection unit detects and amplifies only the frequency component that is approximately equal to the frequency difference between each channel of the FDM signal. FDM signal optical transmission equipment. (4) Claims 1, 2, or 3 include a peak hold circuit that peak-holds the output of the detection unit, and the control unit is controlled by the output of the peak hold circuit. FDM signal optical transmission equipment.