JPH05244119A - Frequency converter by optical external modulator - Google Patents

Abstract

PURPOSE:To provide the frequency converter for dispensing with a high frequency amplifier having a high band, and also, high linearity, while avoiding complication of a circuit configuration. CONSTITUTION:The receiving equipment is provided with a photoelectric converter 1, a first band pass filter 4, a first intermediate frequency amplifier 5, a second frequency mixer 6, a second band pass filter 7, a second intermediate frequency amplifier 8, and a detector 9, which are connected in this array order. To the frequency mixer 6, a local oscillating circuit 11 is connected. In the pre-stage of the photoelectric converter 1, an optical external modulator 20 is provided, and to this optical external modulator 20, a high frequency signal is added in accordance with a carrier-frequency of an optical signal, and a modulating signal generator 21 for executing intensity modulation to the optical signal is connected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency converter using an optical external modulator.

[0002]

2. Description of the Related Art Conventionally, in a receiver for optical communication, it is general that an intensity-modulated optical signal is first converted into an electric signal by a photoelectric converter and then processed such as amplification. FIG. 10 shows a double heterodyne configuration that is a typical system of this type of receiving device.
A photoelectric converter 1, a high frequency amplifier 2, a first frequency mixer 3, a first band pass filter 4, which are connected in this order of arrangement.
It has a first intermediate frequency amplifier 5, a second frequency mixer 6, a second bandpass filter 7, a second intermediate frequency amplifier 8 and a detector 9, and each frequency mixer 3, 6 has a local oscillator circuit 10 respectively. , 11 are connected.

FIG. 11 shows the state of the signal in the portion indicated by to in FIG. 10 in this receiving apparatus. The optical input signal containing the frequency signals f _{1 to} f _n is The signals are frequency-converted by the first frequency mixer 3 and the second frequency mixer 6, and finally the signal of the portion indicated by is selected by the detector 9, and this signal is sent to the outside as an output signal.

However, the device having such a configuration is
Although it is suitable for avoiding image interference when receiving a wideband signal, it has the technical problems described below.

[0005]

That is, in the device shown in FIG. 10, when the number of channels of the input signal is large, in order to make the output level per channel constant in the high frequency amplifier 2, A high level of output is required as compared with the case of a small number. Further, in the high frequency amplifier 2, it is necessary to suppress the occurrence of distortion as much as possible in order to prevent the occurrence of intermodulation distortion between channels of the input signal.

However, in order to meet such a request, the high frequency amplifier 2 is required to have a wide band and a high linearity, which is difficult to realize. As one of the solutions to such a problem, as shown in FIG. 10 between the photoelectric converter 1 and the high frequency amplifier 2.
It is conceivable to interpose a filter 12 and select a frequency band as indicated by a phantom line in FIG. 1, but this solution complicates the circuit configuration.

As a conventional receiving apparatus, there is a single heterodyne system configuration in which the second frequency mixer 6 to the second intermediate frequency amplifier 8 and the local oscillation circuit 11 in the configuration of FIG. 10 are removed. In this case, there was the same problem as in the double heterodyne system.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to:
An object of the present invention is to provide a frequency conversion device that does not require a high-frequency amplifier having a wide band and a high linearity while avoiding a complicated circuit configuration.

[0009]

To achieve the above object, the present invention relates to a frequency conversion device for frequency converting an intensity-modulated optical signal, wherein an external modulator receives the intensity-modulated optical signal as an input signal. A modulation signal generator that is connected to the external modulator, applies a high-frequency signal according to the carrier frequency of the optical signal, and performs intensity modulation on the optical signal; and connects the external modulator to the modulation signal. And a photoelectric converter for receiving the optical signal whose intensity is modulated by the generator and converting the optical signal into an electric signal.

[0010]

According to the frequency converter using the optical external modulator having the above structure, the external modulator and the modulation signal generator are provided in front of the photoelectric converter for converting an optical signal into an electric signal. The conversion takes place at the stage of the optical signal.

[0011]

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 1 to 3 show a first embodiment of a frequency conversion device using an optical external modulator according to the present invention. The frequency conversion apparatus shown in the same figure is one in which the present invention is applied to a receiving apparatus of a double heterodyne system, and the same or corresponding parts as those of the above-mentioned conventional example are designated by the same reference numerals.

The frequency conversion device of this embodiment, unlike the conventional device of this kind, receives an optical signal intensity-modulated by a frequency-multiplexed signal, and does not immediately convert it into an electric signal, but an optical signal. Frequency conversion is performed at the signal stage. For this reason, an optical external modulator 20 is provided in the preceding stage of the photoelectric converter 1, and a high-frequency signal is added to the optical external modulator 20 according to the carrier frequency of the optical signal to modulate the intensity of the optical signal. The generator 21 is connected.

FIG. 2 is a diagram showing a main part of FIG. 1. Based on this diagram, the basic principle of the frequency conversion device will be described. In the example shown in FIG. 2, the output signal of the external modulator 20 is the product of the input optical signal and the modulation signal s from the modulation signal generator 21. Now, considering an optical signal intensity-modulated by an FM-modulated signal as an input optical signal,
The intensity of the input optical signal is expressed by the following equation.

P _r = P (1 + acos ((Ω ₁ + mcos (ωt)) t + φ)) (1) where P: average light intensity, Ω ₁ : central angular frequency of FM wave, a : Modulation degree of intensity modulation, m: FM modulation index, ω: instantaneous angular frequency of signal, and φ: phase constant.

The modulation signal s from the modulation signal generator 21 is expressed by the following equation: s = cos (Ω ₂ t) (2) In the range where the loss and the specific linearity of the external modulator 20 can be ignored, The output P ₀ of the modulator 20 is represented by P ₀ = (α + β · s) · P _r = (α + β · cos (Ω ₂ t)) · P _r (3).

Here, α and β are constants set at the time of design, and α is a bias value and β is a modulation factor.
Substituting equations (1) and (2) into equation (3) and transforming Becomes

In the equation (4), the term represents the up-converted signal by s, and the term represents the down-converted signal. It can be seen that the frequency of the input optical signal can be converted by passing an appropriate filter after photoelectric conversion.

In the above description, the input optical signal is AM
In the case of an IM modulated signal, P _r = P (1 + m · cosωt · cosΩ ₁ t) (5) It can be seen that the frequency conversion can be performed by

Further, according to the equations (4) and (6), even if an optical signal intensity-modulated by a signal obtained by frequency-multiplexing the FM or AM-modulated signal is applied to the input, the frequency conversion is similarly performed. Furthermore, if the optical signal is intensity-modulated with a signal with a limited frequency band, the above-mentioned FM-IM, AM
-Frequency conversion can be performed as in the IM case.
The same applies to a signal obtained by frequency-multiplexing these.

FIG. 3 shows a frequency spectrum in each of the parts 1 to 3 of the apparatus shown in FIG. The optical signal input to the optical external modulator 20 includes a large number of intensity-modulated frequency signals fa, fb, fc ... Fn. The modulation signal generator 21 adds the modulation signal s having the frequency fl _{1 to} the optical signal. Therefore, the input and output signals of the photoelectric converter 1 are converted into a spectrum centered on fl ₁ as shown in FIG.

In this case, the frequency of the modulation signal s of the modulation signal generator 21 can be arbitrarily set. In this embodiment, the frequency of (fl ₁ + fb) is the first.
It is set to match the pass frequency of the bandpass filter 4. The signal that has passed through the die 1 bandpass filter 4 is amplified by the first intermediate frequency amplifier 5, and then is re-modulated by the local oscillation circuit 11 at the frequency fl ₂ by the second frequency mixer 6, and its spectrum is Figure 3
The state shown in FIG.

This signal then passes through the second band-pass filter 7 to be in the state of FIG. 3 (e), and the detector 9 detects only the signal of frequency fb and outputs it to the outside. It will be.

According to the frequency converter configured as described above, the external modulator 20 and the modulation signal generator 21 are provided in front of the photoelectric converter 1 for converting an optical signal into an electric signal. Frequency conversion is performed at the optical signal stage. This eliminates the need for a wide band, high output amplifier as in the conventional system. In addition, a complicated frequency selection filter is not required, and a complicated electric circuit is avoided.

Further, in the conventional method of performing frequency conversion by electricity, since the connecting cable has frequency characteristics, it is necessary to take a frequency characteristic correction measure when the frequency conversion device and the bandpass filter are installed separately. Moreover, even if frequency correction measures are taken, the loss in the metal cable is large, and therefore the installation distance of these cannot be made very large. However, according to the method of the present invention, the external modulator 20 and the photoelectric converter 1 are Since it can be connected with an optical fiber cable, it is possible to increase the installation distance without taking correction measures.

FIGS. 4 and 5 show a second embodiment of the present invention, and FIGS. 6 and 7 show the third embodiment. In the embodiments shown in these drawings, the frequency conversion device of the present invention is used as a heterodyne relay device.

In the example shown in FIG. 4, band pass filters BPF1 to BPFn having different pass frequency bands are provided in the subsequent stage of the frequency conversion device, and amplifiers AMP1 to AMPn and an electro-optical converter are provided in the subsequent stage of each band pass filter. E / O1
To E / On respectively, and NO. 1-NO. Signals are distributed to n stations at different frequencies.
Shows the state of the frequency spectrum.

Further, in the example shown in FIG. 6, the optical branching device 22 is installed in front of the frequency conversion device, and the photoelectric converter 1 and the band pass filters BPF1 to BPF1 connected in series with the frequency conversion device on the subsequent stage side. BPFn, amplifiers AMP1 to AMP
n, electro-optical converters E / O1 to E / On are provided, and NO. 1 to
NO. Signals are distributed to the n stations at the same frequency, and the state of the frequency spectrum is shown in FIG.

FIGS. 8 and 9 show an example in which the repeater shown in FIGS. 4 to 6 is further developed. In this example, a signal of any desired frequency can be transmitted to any desired terminal. Therefore, by providing frequency-multiplexed signals from different terminals as input side signals, it becomes possible to use as a switching device.

[0029]

As described above in detail in the embodiments, according to the frequency conversion device using the optical external modulator of the present invention,
Since the external modulator and the modulation signal generator are provided in front of the photoelectric converter for converting an optical signal into an electric signal, frequency conversion is performed at the optical signal stage. This eliminates the need for a wide band, high output amplifier as in the conventional system. In addition, a complicated frequency selection filter is not required, and a complicated electric circuit is avoided.

Further, in the conventional method of performing frequency conversion by electricity, since the connection cable has frequency characteristics, when the frequency conversion device and the bandpass filter are installed separately from each other, it is necessary to take a frequency characteristic correction measure. In addition, even if frequency correction measures are taken, the loss in the metal cable is large, so it is not possible to make these installation distances too large.However, according to the method of the present invention, the external modulator and the photoelectric converter are not connected to each other. Since it can be connected with a Faber cable, the installation distance can be increased without taking any correction measures.

Furthermore, if optical branching devices such as optical couplers are installed before and after the optical external modulator, it is possible to collectively frequency-convert signals supplied to receivers installed at a plurality of locations.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing an embodiment of a frequency conversion device according to the present invention.

FIG. 2 is an enlarged view of a main part of FIG.

3 is a frequency spectrum of the device of FIG.

FIG. 4 is a functional block diagram showing a second embodiment of the frequency conversion device according to the present invention.

5 is a frequency spectrum of the device shown in FIG.

FIG. 6 is a functional block diagram showing a third embodiment of the frequency conversion device according to the present invention.

7 is a frequency spectrum of the device shown in FIG.

FIG. 8 is a functional block diagram showing an application example of FIGS.

FIG. 9 is a functional block diagram showing another application example of FIGS.

FIG. 10 is a functional block diagram showing a conventional frequency conversion device.

11 is a frequency spectrum of the device shown in FIG.

[Explanation of symbols]

 1 Photoelectric Converter 20 Optical External Modulator 21 Modulation Signal Generator 22 Optical Divider

Claims (1)

[Claims] 1. A frequency conversion device for frequency-converting an intensity-modulated optical signal, wherein the external modulator receives the intensity-modulated optical signal as an input signal, and the optical modulator is connected to the external modulator and carries the optical signal. A high-frequency signal is added according to the frequency, and a modulation signal generator for intensity-modulating the optical signal, and an optical signal connected to the external modulator and intensity-modulated by the modulation signal generator are received. And a photoelectric converter for converting the electric signal into an electric signal. A frequency conversion device using an optical external modulator.