JPS601937A - Optical converting circuit of fm signal made into pulse - Google Patents

Abstract

PURPOSE:To prevent a residual low frequency component by feeding back negatively an output of a laser diode output light monitor circuit as a threshold voltage comparator of a pulse-converted FM modulator via a low-pass filter LPF and an amplifier so as to control the pulse width. CONSTITUTION:A pulse-converted FM signal is amplified by an amplifier 2, applied to an optical monitor circuit 3, the light irradiated from a laser diode LD in the optical monitor circuit 3 is modulated and transmitted, and also, the light is received by a photodetector PDT in the optical monitor circuit 3 and converted into an electric signal output, which is fed back negatively as a threshold voltage of a voltage comparator 8 via an LPF12 and an amplifier 13. Thus, the residual low frequency component is prevented by applying the threshold voltage of the voltage comparator 8 in a direction suppressing the fluctuation of pulse width in response to the fluctuation in the pulse width due to the delay in the leading edge of the laser diode LD in this way.

Description

Detailed Description of the Invention (b) Technical Field of the Invention The present invention relates to an electrical-to-optical conversion circuit using a pulsed FM modulator, and more particularly, to an optical conversion circuit for a pulsed FM signal that prevents residual low frequency components. .

(b) Prior art and problems Figure 1 shows block 1 of a conventional pulsed FM electric-to-optical conversion circuit, and Figure 2 shows the rise delay of the laser diode (hereinafter referred to as LD) when the modulation frequency becomes high. A pulse waveform diagram is shown.

In the figure, 1 is a pulsed FM modulator, 2, 4, and 6.10 are amplifiers, 3 is an optical monitor circuit, and 5 and 9 are low-frequency P wave generators (hereinafter referred to as LP).
7.8 is a voltage comparator, 11 is a constant current circuit,
Tγ1 to Tγ3 are transistors, C is a capacitor, Vs
indicates the reference voltage.

To explain the pulsed FMi regulator 1, when an analog signal is input to the transistor Tr, the capacitor C is charged, and when the charged voltage becomes greater than the threshold voltage 8, the output of the voltage comparator 7 turns on the transistor Tr3. Discharge the charge in capacitor C. By performing such an operation, a triangular wave having a repetition frequency proportional to the level of the input signal is generated at both ends of the capacitor C. The triangular wave generated across the capacitor C is input to the voltage comparator 8, and its output is passed through the LPF 9 and amplified by the amplifier 10.
The voltage at the center level of the triangular wave is obtained and fed back negatively as the threshold of the voltage comparator 8. In this way, a pulsed FM signal with no residual low frequency components and a spectrum symmetrical with respect to the center frequency of the spectrum of the pulsed FM signal is obtained.

This pulsed FM signal is amplified by the amplifier 2 and sent to the optical monitor circuit 3, and the light emitted from the laser diode LD in the optical monitor circuit 3 is modulated and transmitted. After receiving this light and amplifying it with the electric signal amplifier 4, the LPF 5. The optical output is controlled by an APC (Automatic power Co.) via an amplifier 6.
ntrol) circuit. However, as the modulation frequency increases, the rise of the laser diode LD is delayed, so the pulsed FM waveform has a delay in the rise of the pulse as shown by the dotted line in Figure 2, and the interval between the high level and the low level as shown in Figures 2A and 4. The balance will be disrupted and unnecessary low frequency components will remain. Since the frequency of this low frequency component changes depending on the frequency of the pulsed FM signal, this is a disadvantage that requires countermeasures on the receiving side.

Conventional optical conversion circuits for pulsed FM signals have such drawbacks.

(c) Object of the Invention In view of the above drawbacks, an object of the present invention is to provide an optical conversion circuit for pulsed FM signals that prevents residual low frequency components.

(d) Structure of the Invention In order to achieve the above object, the present invention pulses the output of an LD output light monitor circuit through an LPF and an amplifier.
Negative feedback is applied as the threshold voltage of the voltage comparator of the M modulator to control the pulse width and prevent low frequency components from remaining.

(e) Examples of the invention Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a block diagram of an optical conversion circuit for a pulsed FM signal according to an embodiment of the present invention.

Components in the figure that have the same functions as those in FIG. 1 are indicated by the same symbols.

1' is a pulsed FM modulator, 12 is an LPF, and 13 is an amplifier.

The difference between the circuit shown in FIG. 3 and that shown in FIG. This is the point where negative feedback is applied as the threshold voltage of the voltage comparator 8 via the amplifier 13. By doing this, the threshold voltage of the voltage comparator 8 changes in a direction that suppresses the pulse width variation in accordance with the pulse width variation due to the delay in the rise of the laser diode LD, thereby preventing the residual of low frequency components.

FIG. 4 is a block diagram of an optical conversion circuit for a pulsed FM signal in which the circuit of the present invention has good linearity at the optical signal level.

Components in the figure that have the same functions as those in FIG. 3 are indicated by the same symbols.

Reference numeral 15 indicates a bandpass p-wave filter (hereinafter referred to as BPF) for passing the pulsed FM signal component, reference numeral 14.17 an amplifier, and reference numeral 16 an FM demodulator.

The difference between FIG. 4 and FIG. 3 is that an amplifier 14 is provided on the input side to monitor the light, and the band to be transmitted as an electrical signal is passed through the BPF 15 and demodulated by the FM demodulator 16. The difference is that this analog signal is used as an analog signal, and negative feedback is applied to the amplifier 14.

By doing so, the distortion generated in the internal circuit is improved, and an optical signal with good linearity can be obtained.

(f) Effects of the Invention As explained in detail above, the present invention has the effect of providing an optical conversion circuit for pulsed FM signals that prevents the generation of low frequency components.

[Brief explanation of drawings]

Figure 4 is a block diagram of a conventional optical conversion circuit for pulsed FM signals, Figure 2 is a Norms waveform diagram due to the rise delay of the laser diode when the modulation frequency becomes high, and Figure 3 is a diagram of the present invention. FIG. 4 is a block diagram of an optical conversion circuit for pulsed FM signals in which the circuit of the present invention has good linearity at the optical signal level. be. In the figure, 1.1' is a pulsed FM modulator, 2.4.6.10
゜13.14.17 is an amplifier, 3 is an optical monitor circuit, 5,
9. 12 is a low-pass P-wave device, 7 and 8 are voltage comparators, 11 is a constant current circuit, 15 is a band-pass f-wave device, 16 is an FM demodulator,
Tγ1~Tr are transistors, C is a capacitor, Vs
indicates the reference voltage.

Claims (1)

[Claims] In an electrical-to-optical conversion circuit using a pulsed FM modulator, the output of a laser diode output optical monitor circuit is
1. An optical conversion circuit for a pulsed FM signal, characterized in that negative feedback is applied as a threshold voltage of a voltage comparator of the pulsed FM modulator via a wave generator and an amplifier.