JPH0342736B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to an APC (auto power control) device that keeps the oscillation output of a semiconductor laser (LD) constant in a device that optically transmits a PFM signal using a semiconductor laser. It is related to.

Conventional configuration and its problems The method of pre-modulating the LD with PFM and then modulating the brightness of the LD is superior in that it is less affected by the relay distance and non-linear distortion of the light source. Such relay transmission of optical pulse signals is performed using a repeater having only amplification or amplification and pulse regeneration functions.

In PFM transmission, increasing the maximum frequency deviation increases the degree of S/N improvement during demodulation. Therefore, when transmitting a video signal using PFM, the video signal is clamped so that the oscillation frequency at the sink potential is constant.

FIG. 1 shows the voltage-frequency conversion characteristics (V-F converter) of a PFM modulator, and a and b show the cases where the signal is not clamped and when the signal is clamped, respectively.

In the unclamped case b, the average value
The sink potential changes in state A where APL is large and state B where APL is small. Therefore, if the upper limit _nax (generally 80 MHz due to the band limit of optical fiber) and lower limit _nio ( _nio 3 ₀ for the maximum frequency of the transmission signal, ₀ ) of the oscillation frequency of the PFM pulse are determined, b is the maximum frequency. Frequency deviation ΔF can be increased.
C is the average voltage.

The oscillation frequency at the white peak of the video signal is
If the oscillation frequency at _{the w} ′ sink level is _s , then ΔF=( _s − _w )/ ₂₀ .

On the other hand, a semiconductor laser has a current-emission intensity characteristic as shown in FIG. 2, and the emission intensity increases linearly with increasing current from a point where the threshold current i _th is exceeded. i _th also changes depending on temperature, and i _th tends to increase due to long-term deterioration. Therefore, when transmitting light using an LD, automatic adjustment of the optical output (APC) is required to keep the emission intensity constant.
Therefore, a circuit that changes the bias current of the LD is generally added.

Now, consider the case where APC is applied so that the average output of light is constant. In the case of Figure 1a, the average voltage C of the signal is always constant, so the average oscillation frequency is constant, and the average optical power is also constant, so there is no problem, but in the case of Figure 1b, the average voltage of the image signal is constant. The average voltage changes depending on the value (APL),
The average oscillation frequency is the average value of the image signal (APL)
It changes depending on. Therefore, the average optical power is also
Varies depending on APL. In this way, in a PFM signal where the oscillation frequency of the sink potential is constant,
APC in which the average optical power is constant cannot be performed.

In such a case, a method of performing APC so that the peak power of the optical pulse is constant can be considered.
It requires a wideband amplifier, and circuits for clamping the pulse signal and holding the peak of the PFM pulse, making APC complicated and expensive.

As explained above, in order to increase the S/N improvement (wideband gain) in PFM demodulation, it is better to clamp the video signal at the sync level.
The disadvantage is that the circuit becomes complicated.

OBJECTS OF THE INVENTION The present invention was made to solve the above-mentioned drawbacks, and performs APC using a relatively simple circuit.

Structure of the Invention The present invention includes a separation circuit for a horizontal synchronization signal of a video signal, a gate circuit for extracting a part of the horizontal synchronization signal portion of a photoelectrically converted PFM pulse train using a pulse generated from this synchronization signal, and It consists of a detection circuit for the average power or peak power of a pulse train.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. FIG. 3 is a block diagram of the optical transmitter.

A synchronizing signal is extracted from the video input signal 6 by a synchronizing signal separation circuit 7, and the extracted synchronizing signal is applied to a gate pulse generating circuit 17, which generates a gate pulse narrower than a horizontal synchronizing pulse. This pulse is also used as a clamp pulse for the video signal and a gate pulse to extract a portion of the PFM pulse. On the other hand, the LD is driven with a PFM pulse modulated by a video signal, which is converted into an optical pulse by an electro-optic converter 11. A part of the light from the laser, for example, the backlight, is again converted into electricity by the optical-to-electrical converter 12, and this PFM
When the pulse passes through a video filter 13, it becomes a video signal. This video signal is the previously described gate pulse, and only the horizontal synchronizing signal is extracted, and the bias current of the LD is controlled by the magnitude of this average power. With this configuration, the band of the optical-to-electrical converter may be at most the video band (4 MHz).

Next, the operation of this device will be explained using a fourth timing chart. In FIG. 4A, the polarity of the signal input to the input terminal 6 of the present invention is inverted. Signal A is input to the V-F converter 9, where it is converted into a PFM pulse train D, which drives the LD. On the other hand, a gate pulse (c) in FIG. 4 is generated from the horizontal synchronization signal (b) separated by the synchronization separation circuit 7. A part of the LD light is photoelectrically converted, and when it passes through the video filter 13, it is demodulated into a video signal as shown in FIG. When the sink potential of this signal is extracted using the gate pulse (c) in Figure 4 and smoothed, this voltage is proportional to the amplitude A of the PFM pulse.

Although the video filter 13 is inserted in FIG. 3, the same operation is possible even without the video filter 13. Further, it is clear that the average power detection circuit 15 performs the same operation even when detecting the peak of the signal.

Effects of the Invention As explained above, according to the present invention, the response frequency of the light-receiving element, the optical-to-electrical conversion circuit, etc. may be about 4 MHz, making it cheaper than peak power detection. In addition, there is no need for a PFM pulse clamp circuit or peak detection circuit, and APC can be achieved with a simple circuit.
can be configured. Note that the same applies even if the signal is not only a video signal but also a multiplexed signal such as audio.
It is clear that APC operation is performed.

[Brief explanation of drawings]

Figures 1a and b are diagrams explaining the operation of the V-F converter with and without clamping the horizontal synchronizing signal, Figure 2 is a diagram showing the current-optical output characteristics of a semiconductor laser (LD), and Figure 3 is a diagram showing the current-optical output characteristics of a semiconductor laser (LD). FIG. 4 is a block diagram showing the configuration of the automatic light output adjustment device according to the embodiment of the present invention, and FIG. 4 is a timing chart for explaining the operation. 7...Synchronization separation circuit, 8...Clamp circuit, 9
... V-F converter, 10 ... laser drive circuit, 12 ... optical-electric conversion circuit, 14 ... gate circuit, 15 ... average power detection circuit, 17 ... gate pulse generation circuit.

Claims (1)

[Claims] 1. A clamp circuit that keeps the sink potential constant, a pulse frequency modulator that pulse frequency modulates the video signal whose sink potential has become constant in the clamp circuit, and a semiconductor laser that converts the pulse frequency modulated signal into an optical signal. , an opto-electrical converter having at most a video signal band that converts a part of the output of the semiconductor laser into an electric signal; An optical output automatic adjustment device characterized by changing the bias current of a laser.