JPH0630469B2 - Optical receiver - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical receiver used for optical fiber communication, and more particularly to an optical receiver suitable as an optical receiver for long-distance high-speed optical fiber communication.

[Conventional technology]

Frequency modulation optical fiber communication is one of the optical fiber communication methods, and frequency modulation optical heterodyne communication (FSK optical heterodyne communication) can achieve high reception sensitivity and is therefore suitable for long-distance optical fiber communication. Such FSK
In heterodyne optical fiber communication, information to be transmitted is encoded by changing the optical frequency. That sends light with the frequency f ₁ to transmit a digital signal "1", and to transmit the "0" may be considered to transmit light of a frequency f ₁ and a different frequency f _2.

In this way, light having different optical frequencies depending on the information to be transmitted is transmitted to the receiving side through the optical fiber.

[Problems to be solved by the invention]

By the way, since an optical fiber which is a communication medium in this kind of optical fiber communication has a wavelength dispersion characteristic, there are the following difficulties in transmitting light of different frequencies.

That is, since the optical fiber has chromatic dispersion characteristics,
The lights of the different frequencies f ₁ and f ₂ described above propagate at different speeds in the optical fiber. Therefore, if the bit rate of the signal to be transmitted is high (the repetition rate of f ₁ and f ₂ per second is high) and the length of the optical fiber is long, the frequency f
₁ the wavelengths of the _first things lambda, the wavelength of one of the frequency _{f 2} lambda ₂
Then, the light of wavelength λ ₁ (or λ ₂ ) catches up with the light of wavelength λ ₂ (or λ ₁ ) one bit before during the propagation of the optical fiber, and overlaps. That is, high-speed long-distance F
In SK optical fiber communication, intersymbol interference occurs due to dispersion of optical fibers, so that the achievable speed and transmission distance are limited.

As described above, the FS which is essentially capable of high reception sensitivity
When the bit rate of K-heterodyne optical fiber communication is increased, intersymbol interference occurs due to the chromatic dispersion characteristic of the optical fiber, which hinders achievement of high reception sensitivity. Therefore, the bit rate of optical fiber communication cannot be increased so much. .

An object of the present invention is to provide an optical receiver capable of mitigating the influence caused by chromatic dispersion of an optical fiber and preventing deterioration of reception sensitivity.

[Means for solving problems]

The optical receiver of the present invention comprises a separating means for separating intermediate frequency signals of different frequencies obtained from the combined light obtained by multiplexing the local oscillation light and the input signal light, and the detection output of the signal component separated by the separating means. A differential combiner for differentially combining, and by giving a predetermined time difference to the transfer time until each of the intermediate frequency signals is transferred to the difference combiner, signal components with different optical frequencies in the input signal light are generated. It is characterized by comprising a correction means for correcting a relative propagation time difference received due to wavelength dispersion of the optical fiber during propagation of the optical fiber.

[Operation] The signal light transmitted through the optical fiber reaches the optical receiver under the influence of dispersion of the optical fiber, and there is a propagation time difference between the signal components having different optical frequencies. . The input signal light is combined with the locally oscillated light, the intermediate frequency signals of different frequencies are separated by the separating means, and the respective detection outputs are supplied to the difference synthesizing device for differential synthesizing. In the meantime, since a time difference is given to the propagation time by the correction means and the relative propagation time difference received by the chromatic dispersion of the optical fiber is corrected, the time lag occurring during the optical fiber propagation is canceled, The influence of dispersion of the optical fiber can be significantly reduced.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the optical receiver of the present invention.

As shown in FIG. 1, the optical receiver includes a fiber coupler 3, a balanced receiver 4, an amplifier 5, band pass filters (BPF) 6 and 7, envelope detectors 8 and 9.
The low pass filters (LPF) 10 and 11 and the difference amplifier 13 as a difference combiner are provided, and the low pass filter 11 and the difference amplifier 13 are provided as means for correcting the influence of intersymbol interference caused by the chromatic dispersion of the optical fiber. Delay line inserted in
Have 12

The signal light 1 and the local oscillation light 2 are incident on the fiber coupler 3, and the signal light 1 is frequency-modulated light transmitted from the transmission side through the optical fiber. The frequency-modulated signal light 1 contains mark components and space components of different optical frequencies as transmission information as described later. The local oscillation light 2 is incident from a local oscillator (light source) (not shown) and is combined with the signal light 1.

The balanced receiver 4 receives the output light of the fiber coupler 3, converts it into an intermediate frequency signal, and supplies it to the amplifier 5.

The output of the amplifier 5 is applied to band pass filters 6 and 7 having different filtering characteristics, thereby separating two kinds of intermediate frequency signals having different frequencies corresponding to the mark code and the signal code. After being separated into two systems by the two band pass filters 6 and 7, each is separately envelope-detected by the envelope detectors 8 and 9, and further passed through the low-pass filters 10 and 11, respectively, and the difference synthesis is performed. Code identification is performed by code identification. When the signal light 1 propagates into the optical fiber in the process up to the difference combining in this optical receiver, the mark component and the space are separated by the wavelength dispersion of the optical fiber. Time adjustments are made to offset the relative time difference experienced by the components.

This adjustment can be performed by means such as introducing a delay line between the band pass filters 6 and 7 and the device for difference synthesis, and in the present embodiment, the low pass communication filter 11 of one system is used. A delay line 12 is provided between the delay line 12 and the differential amplifier 13. Then, with respect to the delay time of the delay line 12, the transmission time difference between the two types of intermediate frequency signals separated by the band pass filters 6 and 7 as described above from the filters 6 and 7 to the difference amplifier 13 is calculated to obtain the optical fiber. The value is set so as to correct the above-mentioned relative time difference received during propagation.

Further, a specific description will be given with reference to FIGS. 2 and 3.

First, the signal modulation rate and the optical fiber transmission path length are described. Here, the signal modulation rate is 5 Gb / s (NRZ).
Also, the optical fiber transmission line length is 200 km. The optical transmitter is a semiconductor laser with an oscillation wavelength of 1.55 μm, and frequency modulation is performed by minutely changing the drive current of the semiconductor laser. The output spectrum has a bimodal spectrum having peaks at wavelengths corresponding to digital codes "1" (mark) and "0" (space), as shown in FIG. The frequency difference between the mark and the space is 15GH _Z.

The signal light 1 in FIG. 1 is the above frequency-modulated light propagated through the optical fiber. This optical fiber has a minimum optical loss of 0.2 dB / km at 1.5 μm, but the chromatic dispersion at that wavelength is 15 (ps / nm) /
It is km. Therefore, the signal light 1 is affected by the dispersion of the optical fiber before reaching the optical receiver. That is, the second showing the spectrum of the frequency-modulated signal light
In the figure, the spectrum component centered on the optical frequency f ₁ is composed of the optical component corresponding to the digital signal “1” (mark), and the spectrum component centered on the optical frequency f ₂ is the digital signal “0” ( Space) composed of light components. When light having such bimodal spectrum characteristics is propagated through an optical fiber having wavelength dispersion characteristics, interference occurs between different types of marks and signals. In other words, a typical optical fiber is 15 (ps
/ Nm) / for having a km of the wavelength dispersion, if a 15GH _Z optical frequency difference between the mark and space, the time required to each example 100km propagation, a difference occurs in approximately 150ps between the two Therefore, when the mark-signal repetition period is about 200 ps (5 Gb / s,
The NRZ signal) causes light of these different optical frequencies to completely overlap.

Now, the signal light 1 as described above is introduced into one input port of the fiber coupler 3. The local oscillation light 2 having a frequency lower than that of the signal light 1 by about 10 GHz enters from the other port of the fiber coupler 3. The output of the fiber coupler 3 is converted into an intermediate frequency signal by the balanced receiver 4 and amplified by the amplifier 5. This amplified intermediate frequency signal is shown in FIG. The mark component 31 and the space component 32 in FIG. 3 correspond to those in FIG. 2, but are downshifted by the frequency of the local oscillation light 2. Also the third
The figure also shows the characteristic F of the bandpass filter 6 and the characteristic F'of the bandpass filter 7. As is clear from FIG. 3, the bandpass filter 6 includes
Cut out only the space component 32 of a frequency about 25GH _Z, also band-pass filter 7 cuts out only mark components 31. The mark and space signals thus separated into different systems are respectively detected by envelope detectors 9 and 8 and shaped by low-pass filters 11 and 10.

As described above, the optical receiver first combines the signal light 1 propagating through the optical fiber and the local oscillation light from the light source that is CW-oscillated at an optical frequency slightly different from that, and then combines the combined light. Receive with a light receiver such as a photodiode. Here, in order to minimize the loss due to the multiplexing, as shown in FIG. 1, it is only necessary to use the optical fiber coupler 3, which should be called a two-terminal pair circuit, and receive the pair output from the balanced receiver 4. The output of the balanced receiver 4 is composed of the sum and difference frequency components of the signal light 1 and the local oscillation light. Of these, two band-pass filters 6 include a component downshifted from the frequency corresponding to the mark of the signal light 1 by the frequency of the local oscillation light 2 and a component downshifted from the frequency corresponding to the space by the frequency of the signal oscillation light 2. , 7 are separated into two systems, and the outputs of these two filters 6, 7 are separately envelope-detected to obtain a baseband signal corresponding to a mark and a baseband signal corresponding to a space. Since these are envelope detection outputs, they become in-phase signals, and therefore the transmitted baseband signal is obtained by inverting and adding one of them with the differential amplifier 13 or the like.

By the way, as described above, the signal light 1 has a chromatic dispersion of 15 (p
The optical fiber of s / nm) / km is propagated over 200 km. The frequency difference between the mark and space components is 15G.
In H _Z , the space component is located on the high frequency side. Therefore, when it reaches the receiver, the mark component precedes the space component by about 150 ps. The delay line 12 shown in FIG. 1 is used to cancel this relative time difference.

That is, when the time required for the fiber propagation of the mark and space signals differs depending on the dispersion of the optical fiber as described above, the envelope detection output corresponding to the mark reproduced in the receiver and the envelope corresponding to the space. The relative time relationship of the detection outputs is shifted by the propagation time difference due to dispersion.
In this way, the original baseband signal cannot be reproduced even if the waveforms whose time is shifted from each other are differentially combined, so that the reception sensitivity is deteriorated.

Therefore, in the optical receiver of the present embodiment, the delay line 12 is provided in one of the circuit systems separated into two systems by the bandpass filters 6 and 7.
Is inserted to correct the relative time shift, and after that, the difference synthesis is performed to reproduce the correct baseband signal.

Needless to say, the delay time due to the inserted delay line 12 is 150 ps in this embodiment. In this way, the local oscillation light 12 interferes with the input signal light 1, and the resulting intermediate frequency signals of two different frequencies corresponding to the mark code and the signal code are dispersed by the filters 6 and 7, respectively. When performing code detection after envelope detection of the outputs 6 and 7 and differential combination, the difference in transmission delay time of the above-mentioned two types of intermediate frequency signals from the filters 6 and 7 to the differential amplifier 13 as the differential combiner is determined. The delay time of the delay line 12 is set so as to correct the relative difference in propagation delay time that these signals receive due to the chromatic dispersion of the optical fiber during the propagation of the optical fiber. Without such a correction means, as described immediately above, intersymbol interference occurs due to the chromatic dispersion characteristic of the optical fiber, which interferes with the achievement of high reception sensitivity, but there is a limit to increasing the bit rate. ,
In the case of the configuration shown in FIG. 1, by correcting the influence of intersymbol interference caused by the chromatic dispersion of the optical fiber, it is possible to prevent the deterioration of the receiving sensitivity of the long-distance high-speed FSK heterodyne optical fiber communication, Higher speed and longer distance are possible. Actually, the mark and space signals time-adjusted as described above are subjected to anti-phase addition amplification by the differential amplifier 13 and led to the code identification system, and an error test is performed, and it is confirmed that good reception sensitivity is achieved. Was confirmed.

On the other hand, a test was also conducted with the delay line 12 removed, but due to the above-mentioned time lag, it was not possible to identify at all.

〔The invention's effect〕

As described above, according to the present invention, the influence of dispersion of optical fibers in optical fiber communication can be significantly mitigated, deterioration of reception sensitivity can be prevented, and optical fiber communication can be speeded up and lengthened.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical receiver according to an embodiment of the present invention, FIG. 2 is a diagram showing an example of a spectrum of frequency-modulated signal light, and FIG. 3 is a spectrum of an output of the amplifier of FIG. It is a figure which shows an example. 1 ... Signal light 2 ... Local oscillation light 3 ... Fiber coupler 4 ... Balanced receiver 5 ... Amplifier 6,7 ... Bandpass filter (BPF) 8,9 ... Envelope detector 10,11 ... … Low-pass filter (LPF) 13 …… Differential amplifier

Claims (2)

[Claims] 1. Separation means for separating intermediate frequency signals of different frequencies obtained from combined light obtained by combining local oscillation light and input signal light, and differential combination of detection outputs of signal components separated by this separation means. By providing a predetermined time difference to the transmission time until each of the intermediate frequency signals is transmitted to the difference synthesizer, the signal components with different optical frequencies in the input signal light propagate through the optical fiber. An optical receiver for frequency-modulation optical fiber communication, comprising: a correction unit that corrects a relative propagation time difference received due to wavelength dispersion of an optical fiber. 2. The optical receiver according to claim 1, wherein the separating means causes the local oscillation light and the input signal light to interfere with each other,
2 corresponding to the resulting mark and signal codes
It is a filter that separates intermediate frequency signals of different frequencies, and the difference synthesizer sends the output of each filter output to a code identification system that performs differential synthesis of the detection outputs obtained by envelope detection. An optical receiver for frequency-modulation optical fiber communication, which is a differential synthesizer, and wherein the correction means is a delay line inserted between the filter and the differential synthesizer.