JPH0659042B2 - Baseband IM optical image transmitter - Google Patents

Description

The present invention relates to a baseband optical image transmission device using a laser diode, and more particularly to an optical transmitter used in the optical image transmission device.

[Background of the Invention]

NTSC using a laser diode (hereinafter referred to as LD)
In an optical image transmission device that intensity-modulates (IM) a video signal in a base band, conventionally, in order to reduce modal noise, an optical transmitter switches a light output with a high frequency pulse. The part that has no light and the part that has a light output) is used. FIG. 1 shows the driving method of the LD, and the LD driving current is the sum of the bias current Ia and the signal current Ib (pulse). The ratio between the bias current Ia and the oscillation threshold of the LD affects the optical pulse waveform, pulse width, etc., and therefore affects the distortion, etc. In the case of this modulation method, optical output stabilization (APC) is performed by making the leading edge of the sync signal of the video signal constant.
APC is performed by operating the bias current Ia. In this case, when the LD IL characteristic changes due to changes in ambient temperature and aging, it is sufficient to change only the LD oscillation threshold, but when the LD differential efficiency changes, the ratio of the bias current to the oscillation threshold is changed. There is a drawback that (Ia / Ith) changes greatly and causes an increase in optical output distortion.

[Object of the Invention]

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to provide an optical transmitter that alleviates an increase in optical output distortion due to ambient temperature, aging, and the like.

[Outline of Invention]

In the present invention, the change in the differential efficiency of the LD is detected by monitoring the change in the pilot level of the LD monitor light (synchronization signal), and the clamp level of the LD signal current is manipulated by this signal to monitor the synchronization signal of the optical output. The peak value detected signal
By stabilizing the optical output by manipulating the bias current of the LD, the ratio of the bias current to the oscillation threshold current Ia /
It is characterized by reducing the fluctuation of Ith and reducing the increase in the distortion of the optical output due to temperature changes and aging changes.

Example of Invention

An embodiment of the present invention will be shown below in FIG. In FIG. 2, 1 is a signal current drive circuit, 2 is a bias current drive circuit, 3 is an LD,
4 is a photodiode (hereinafter referred to as PD) for monitoring the synchronizing signal of the optical output of the LD 3, 5 is a peak value detection circuit, 6 is a bandpass filter, 7 is a peak value detection circuit, and 8 and 9 are comparison amplification circuits. is there. The video input signal is peak-clamped by the signal current drive circuit 1 and becomes the signal current of the LD 3. This is added to the output of the bias current circuit 2 and LD3
To obtain the optical output. The optical output (synchronization signal of NTSC video signal) is monitored by the PD 4, the peak value of the optical output is detected by the peak value detection circuit 5, and the bias current Ia is manipulated by the comparison and amplification circuit 9 to respond to the change of the oscillation threshold. Apply stabilization (APC). On the other hand, the output of the PD4 is extracted by the bandpass filter 6 as the pilot signal of the NTSC video signal, the peak value is detected by the peak value detection circuit 7, and the clamp level of the signal current circuit is operated by the output of the comparison and amplification circuit 8. Stabilizes the light output against changes in ambient temperature and aging of the differential efficiency. By doing so, Ia / Ith is small even if the differential efficiency changes due to changes in ambient temperature, changes over time, and the like, and it is possible to reduce an increase in distortion of optical output.

〔The invention's effect〕

As described above, according to the present invention, it becomes possible to provide an optical transmitter in which distortion is less deteriorated with respect to ambient temperature and aging.

[Brief description of drawings]

FIG. 1 shows a baseband IM using a laser diode.
Explanatory drawing explaining the modulation method of the modulation laser diode,
FIG. 2 is a block diagram of an optical transmitter showing an embodiment of the present invention. 1 ... Signal current drive circuit 2 ... Bias current drive circuit 3 ... LD, 4 ... PD 5,7 ... Peak value detection circuit 6 ... Band pass filter 8,9 ... Comparison amplifier

Claims (1)

[Claims] 1. A baseband I for transmitting an NTSC video signal intensity-modulated by a baseband using a laser diode.
In the M optical image transmitter, the modulation of the laser diode is negatively modulated, and the monitor of the back light or forward light of the laser diode is for monitoring the synchronizing signal of the negatively modulated NTSC video signal. A negative feedback is applied to the bias current of the laser diode so that the level of the synchronizing signal becomes a constant value through a comparison amplifier, while the NTSC video signal pilot is fed from the monitored synchronizing signal by a bandpass filter. A signal is extracted, and the peak value of the pilot signal is controlled through a comparison amplifier to operate the clamp level of the signal drive current of the laser diode so that the level of the pilot signal becomes a constant value and stabilize the optical output. A baseband IM optical image transmission device characterized by the above.