JPH0285830A - Coherent light receiving system - Google Patents

Abstract

PURPOSE:To realize a coherent light communication system which can be applied to a high speed system and consists of a simple constitution by multiplying a first intermediate frequency signal obtained by multiplexing a signal light and a local oscillation light by an optical coupler by a sine wave signal and a cosine wave signal from an oscillator, respectively and multiplexing its multiplication signals. CONSTITUTION:A first intermediate frequency signal is obtained by multiplexing a signal light 1 and a local oscillation light 2 to which a sufficiently small frequency difference is given against a base band zone by an optical coupler 3 and thereafter, bringing them to opto-electric conversion by a photodetector 4. This first intermediate frequency signal is multiplied by a sine wave signal and a cosine wave signal from an oscillator 5, respectively, and by multiplexing these multiplication signals by a 90 deg. hybrid coupler 6, a second intermediate frequency signal is obtained. This second intermediate frequency signal is demodulated asynchronously. In such a way, a coherent light receiving system which is suitable for improving a transmission rate, whose request to spectral line width of a light source is not strict, and also, whose constitution is simplified can be offered.

Description

[Detailed explanation of the invention] Next overview ・ ・ ・ ・ ・ 2 Pages… ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ P. 3 ・ 頁 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・Means to solve problems ・
・・Page 11 Effect ・ ・ ・ ・ ・ ・
・ ・ ・ ・ 12 truth examples ・ ・
・ ・ ・ Effect of the invention on page 14
・・・・Page 21 Overview Regarding coherent optical reception systems, we aim to provide a coherent optical reception system that is suitable for improving transmission speed, does not place strict requirements on the spectral linewidth of the light source, and can be simplified in configuration. The purpose is to combine signal light and local light with a sufficiently small frequency difference with respect to the baseband band using an optical coupler, and then perform optical-to-electrical conversion using an optical receiver to obtain a first intermediate frequency signal. 1st
A second intermediate frequency signal is obtained by multiplying the intermediate frequency signal by a sine wave signal and a cosine wave signal from an oscillator, and combining these product signals with a 90° hybrid coupler. The configuration is such that the signal is demodulated asynchronously.

INDUSTRIAL APPLICATION FIELD The present invention relates to a receiving system in a coherent optical communication system.

In the field of optical communication or optical transmission, an intensity modulation m/direct detection method is common in which intensity modulated light transmitted through an optical transmission line is directly received by a light receiving element and converted into an electrical signal. In recent years, however, due to demands for improved optical frequency usage efficiency and longer transmission distances, laser light sources with high spectral purity are used as carrier light for transmission and local light sources for reception. Research on coherent optical communication systems that mixes local light and local light to perform heterodyne detection, homodyne detection, etc. is intensifying.

According to this method, it is expected that the receiving sensitivity will be improved compared to the intensity modulation/direct detection method, so it will be easy to increase the repeating interval, reduce the number of repeaters, or increase the number of branches in the optical transmission line, and It becomes possible to construct transmission lines economically.

However, it has become increasingly clear that it is difficult to increase the transmission speed in coherent optical communication systems. Currently, the highest bit rate in a coherent optical communication system is reported to be 4 Gb/s in the CPFSK system (K, Iwashi-ta and
N., Takachio, OFCM18. PD15
), On the other hand, with the intensity modulation/direct detection method, 10Gb/
s has been realized, albeit at the laboratory level (S, Fu
Jita et al.

, OFC''88. PD16).Therefore, there is a need for a coherent optical reception system suitable for improving transmission speed.

2. Description of the Related Art A heterogain detection method, a homodyne detection method, and a phase diversity method will be explained as coherent optical reception methods that have been proposed in the past with reference to FIG.

FIG. 5A is a block diagram of a hetero gain detection method. On the transmission side, 12 is a light source such as a semiconductor laser (LD), 14 is a modulator thereof, and 16 is a drive circuit that drives the modulator 14 based on an input signal, and the modulated light is transmitted as an optical transmission path. is guided into fiber 18. On the receiving side, 24 is a local light source, 22 is a polarization controller that controls the polarization plane of the local light from the local light source 24 to match the polarization plane of the received light, and 20 is a light beam transmitted through the optical fiber 18. An optical coupler 26 combines the incoming light and the local light; 26 is a light receiver that receives the light from the optical coupler 20 and converts it from light to electricity; 28;
52 is an amplifier, 52 is a bandpass filter, 40 is a demodulation circuit, 42 is a low-pass filter, 44 is an identification circuit, and 46 is a frequency stabilization circuit that controls the oscillation frequency of the local light source 24 so that the intermediate frequency is constant. . The signal component of the received light can be extracted as an intermediate frequency signal with a frequency that is the difference between the frequency of the light and the frequency of the local light (for example, several GHz) due to the square characteristic of the light receiver 26 such as a photodiode. A demodulation circuit 40 performs asynchronous demodulation. According to the hetero gain detection method, as in other coherent optical reception methods, the optical receiver 26
As an output, a signal with an amplitude proportional to the product of the amplitude of the received light and the amplitude of the local light can be obtained, so
High receiving sensitivity can be achieved by using local light of appropriate intensity.

However, if the hetero gain detection method is used, Fig. 7(a)
As shown in , the average frequency of the intermediate frequency (IF) signal whose electric waste vector is indicated by diagonal lines is more than 1.5 times the bit rate (Byo), and therefore the band is 0.5B, ~
2.5B is required. For example, if the bit rate is 4Gb/s, 2GHz to 10GHz
, and considering the fact that it is difficult to provide a receiver with a flat frequency response and good noise characteristics in this band, the heterogain method is not suitable for improving transmission speed. I can't say that. However, the heterodyne detection method has the advantage that it does not place strict requirements on the spectral linewidth of the light source because it can easily perform asynchronous demodulation using envelope detection or the like.

FIG. 6(b) is a diagram showing a PLL loop method in the homodyne detection method. In this method, the local light source 240
The phase is controlled to directly obtain the baseband signal. That is, the distributed lights whose phases are shifted by π in the optical coupler 20 are optical-to-electrically converted by separate optical receivers 26 and amplified by the amplifier 54, and the difference signal is obtained by the differential amplifier 56 to remove the DC component. The phase of the local light source 240 is controlled via the loop filter 58 so that no beat component occurs in the obtained signal.

FIG. 6(C) is a diagram showing the Costas loop method in the homodyne detection method. In this method, multiplexed lights whose phases are shifted by π/2 from each other, which are distributed by a 90° optical coupler 60, are converted into electricity and then multiplied by a mixer 62, thereby canceling out the modulation components and transmitting them to the local light source.
The phase drift of 4 is detected and the phase is controlled.

In these homodyne detection methods, the baseband signal is obtained directly at the photoreceiver 26, so the required band of the photoreceiver is as shown in FIG. 7(b).
It is sufficient if it is similar to the intensity modulation/direct detection method.

For this reason, the homodyne detection method can be said to be suitable for high-speed systems, but it requires a phase-locked loop, and due to limitations such as the band of that loop, it can be used as a light source with an extremely narrow linewidth (for example, 0.01 of the bit rate). %) is required. In addition, in the PLL loop method, the effects of DC drift and intensity noise of the local light source cannot be ignored, and it is difficult to realize a method of changing the phase of the local light source using the PLL loop. The Costas loop method requires a 90'' optical coupler, which has a more complex configuration than a general optical coupler, and requires two sets of light receivers.

Figure 6(d) shows the 1/
FIG. 2 is a diagram showing a Q (Inphase Quadrature) detection method. In this method, the frequency of the local light is set to be slightly different from the frequency of the carrier wave of the received light (for example, 10!Jf
lz) The received light and the local light are distributed by a 90° optical coupler 60, each subjected to optical-to-electrical conversion, asynchronously demodulated by the demodulation circuit 40, and then added by the adder 64, so that the demodulated signal is always I'm trying to get it.

FIG. 6(e) is a diagram showing a three-vote system in the phase diversity system. In this method, a 120° optical coupler 66 is used to perform asynchronous demodulation of one distributed light and a distributed light whose phase is shifted by 2π/3.4π/3 with respect to that distributed light, and then the adder 64 performs asynchronous demodulation. I'm trying to add it up. With this method as well, a demodulated signal can always be obtained, similar to the I/Q detection method.

According to these phase diversity methods, Fig. 7(C)
As shown in Figure 2, the receiver band can be made almost the same as that of the homodyne method, which not only makes it possible to apply it to high-speed systems, but also allows for asynchronous demodulation (unlike the homodyne method).
(detection), there is no need for a light source with a narrow linewidth spectrum. However, it is necessary to configure a multi-channel receiving system including a demodulation circuit, which complicates the configuration of the receiver.In addition, since the local light is separated into multiple parts, the influence of thermal noise in the amplifier and the local light The problem is that it is easily affected by intensity noise. Further, the 90° optical coupler and the 120° optical coupler generally have optically complicated structures.

Problems to be Solved by the Invention The main problems in the conventional technology are listed below.

(1) Although the heterodyne detection method does not have strict requirements for the spectral linewidth of the light source, it requires a broadband receiver, making it difficult to apply to high-speed systems.

(2) The homodyne detection method requires the same receiver bandwidth as the intensity modulation/direct detection method, so it can be applied to high-speed systems; however, it requires a light source with a narrow linewidth spectrum. is necessary.

(3) Although the phase diversity method can be applied to high-speed systems and does not place strict requirements on the spectral linewidth of the light source, it does require a complex configuration.

The present invention was created in view of these circumstances.
It is an object of the present invention to provide a coherent optical communication system that can be applied to high-speed systems, does not place strict requirements on the spectral line width of a light source, and can be simplified in configuration.

Means to solve problems FIG. 1 is a diagram showing the principle of the present invention.

In this coherent optical reception system, signal light 1 and local light 2, which have a sufficiently small frequency difference with respect to the baseband band, are multiplexed in an optical coupler 3, and then optical-to-electrically converted in a light receiver 4. 1 intermediate frequency signal is obtained, this first intermediate frequency signal is multiplied by a sine wave signal and a cosine wave signal from the oscillator 5, respectively, and these product signals are multiplexed by a 90° hybrid coupler 6. 2 intermediate frequency signals are obtained, and this second intermediate frequency signal is asynchronously demodulated.

Function In the configuration of the present invention, the reason why a sufficiently small frequency difference with respect to the baseband band is given to the signal light and the local light, as in the case of the phase diversity method, is that these combined lights are converted into optical-electrical signals by the optical receiver. When the first intermediate frequency signal having a frequency corresponding to the frequency difference is obtained by conversion, the band is made almost equal to the baseband band, and the necessary band of the optical receiver is minimized. be.

The reason why the first intermediate frequency signal is multiplied by the sine wave signal or cosine wave signal from the oscillator is to up-convert the first intermediate frequency signal so that the signal component can always be obtained. This is to eliminate the need for a 90° optical coupler, a 120° optical coupler, etc. in the phase variegation system.

The reason why the first intermediate frequency signal is multiplied by a sine wave signal and a cosine wave signal, and these product signals are combined using a 90° hybrid coupler is to avoid harmful images that may occur during up-conversion. This is to obtain a second intermediate frequency signal that can be asynchronously demodulated by removing the signal. In other words, since the frequency difference between the signal light and the local light is small compared to the baseband band, the electrical signal generated in the optical receiver is an electrical scum vector in which the signals folded back at DC overlap, as shown in Figure 2. The image signal resulting from this up-conversion is removed by the above-mentioned series of means.

As described above, the present invention can be applied to high-speed systems because it does not require a broadband receiver like the conventional phase diversity method, and because it performs asynchronous demodulation, it can be applied to high-speed systems that have strict requirements for the spectral line width of the light source. Furthermore, the configuration is simplified because it does not require the use of a 90° optical coupler, a 120° optical coupler, etc., and the configuration of a receiving system for multiple channels is not required.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 3 is a block diagram of a coherent optical communication system constructed by applying the present invention. Note that parts that are substantially the same as those in the conventional example are given the same reference numerals, and some explanations thereof will be omitted. As the transmission light source 12, a DFB-LD (distributed feedback semiconductor laser) can be used, and as the modulator 14, a phase modulator can be used. In this case, the modulation method is the PSK method. The modulator 14 may be an intensity modulator, in which case the AS
It will be K method. The CPFSK method may be used by directly modulating the transmitting light source 12 without using the modulator 14. Optical signals modulated by the above-mentioned modulation method or other methods are transmitted to the receiving side via an optical transmission line 18 made of, for example, a single mode optical fiber, and this signal light is transmitted to the polarization controller 2.
The optical coupler 20 combines the local light from the local light source 24 whose plane of polarization has been matched by the optical coupler 20 . The multiplexed optical signal is detected by a photoreceiver 26 configured using a photodiode or the like, that is, subjected to optical-to-electrical conversion. At this time, the optical receiver 26 has a square law detection characteristic, and the local light and the signal light have a sufficiently small frequency difference with respect to the baseband band, for example, 1 OMHz, so the optical receiver 26
A first intermediate frequency signal corresponding to the frequency difference is obtained.

FIG. 4 is a diagram for explaining a specific configuration example of the light receiver 26. As shown in FIG. Light-receiving elements 26a and 26b having the same characteristics are connected in series, and light from the optical coupler 20 whose phase is shifted by 180° is input to each of the light-receiving elements 26a and 26b with the same optical path length. According to this configuration, the signal components input to the light receiving elements 26a and 26b have opposite phases, and the intensity noise components of the local light are in phase, so the signal components are added, the intensity noise components are canceled, and the local light The intensity noise can be significantly reduced. Moreover, each light receiving element 26a, 2
The DC component generated at 6b can be removed.

In the present invention, since the receiving system has one channel, it is possible to easily configure a double-balanced optical receiver as described above.

Referring again to FIG. 3, the first intermediate frequency signal from the optical receiver 26 is amplified with low noise by the amplifier 28 and then branched into two branches. One of the branched signals is converted into a cosine wave signal (cos 2πf,
t).

The other branched signal is also multiplicatively modulated in the mixer 36 by the sine wave signal (sin2πf, t) from the oscillator 30 whose phase is shifted by the phase shifter 34. For example, in the case of the DPSK system, the oscillation frequency f of the oscillator 30 is f a + f offset = n B72 (n =
2. 3. 4. ) can be set so that Here, for
fset is the first intermediate frequency, and Bm is the band of the baseband signal, that is, the bit rate. The first intermediate frequency signals multiplicatively modulated by the sine wave signal and the cosine wave signal are transmitted to the manual parts 38a and 38 of the 90° hybrid coupler 38, respectively.
b.

The 90° hybrid coupler 38 can be formed from a passive waveguide as a component for microwave signals, and its specific operation will be explained with reference to FIG. As shown in FIG. 3(a), when a signal is input manually to one input section 38a,
A signal is outputted from the corresponding output section 38C without a phase delay, a signal is outputted from the other output section 38d with a phase difference of 90°, and no signal is outputted from the other input section 38b. Further, as shown in FIG. 3(b), the input section 38b
When a signal is input manually from the output section 38d, the signal is output without phase delay, and from the output t138c, the phase is 90.
A signal is output from the input section 38a, and no signal is output from the input section 38a.

Referring again to FIG. 3, one of the second intermediate frequency signals obtained by removing the image signal by the 90° hybrid coupler 38, for example, the signal output from the output section 38c, is
The demodulation circuit 40 asynchronously demodulates the signal, and the low-pass filter 42 and identification circuit 44 reproduce the original information. Since the other second intermediate frequency signal outputted from the output section 38d includes the frequency of the first intermediate frequency signal before upconversion as information, it is sent to the local light source 24 via the frequency stabilization circuit 46. AF with feedback
By performing C (automatic frequency control), the first intermediate frequency can be kept constant. Note that both of the second intermediate frequency signals may be used for demodulation, and the first intermediate frequency signal may be fed back to perform AFC.

The principle of image wave removal will be explained below. Now, the electric field E of the signal light and the electric field E of the local light on the light receiving surface of the light receiver are given by the following equation.

Es= (L'Ps) exp [j (2rr f
s t+ψs (t)+θ, (1))]...(1)E
L= ((PL) exp [j (2rr ftt
+θL(1))]...(2) Here, Ps, P is the intensity of the signal light, local light, f5
rfL is the frequency of signal light and local light, θ5(t).

θ, (t) is the phase noise in the signal light and local light, ψ, (
t) is the signal phase. In PSK, (1) s
(t) takes the value A (0, π) 0), and in CPFSK, ψs (t) takes any two values, and A
In SK, ψ5(t)-〇, and Jps changes.

At this time, the electrical signal generated in the light receiver 26 is 1=E
S+EL 2 =PS+PL+ 2 (FPsPL) CO9(2π
(rs-fL)t+ψs (t)+θs (t)θL
(t) )...Represented by (3). Here, the first intermediate frequency f. ffatt=f
s fL is set sufficiently small with respect to baseband band B. [3] First, the signal expressed by the formula is directly modulated (multiplied) by cos 2πf, t. However,
f, ≧1.5B,), I+ = 2 ((PS PL) cos (2rr
fotrs*tt +ψS+Δθ) cos(2πf, t
) = (J PSPL) cos (2rr f, t+
ψS+Δθ+2πf orrsst t ) + (r
Ps Pt) cos ('2πf, t-ψS-Δθ
−2πf offset j ) (4). Here ψS=ψ5(t), Δθ= θ.

(1)-θL(t)l. Also, for the convenience of the following explanation, cos(2πf, t+ψS+Δθ +2πf, tt,, t t ) = A ・(5)
cos (2rr f, −ψS−Δθ−2π f
, tr-at t ) = B (6). In this way, by up-converting, an image whose phase is reversed appears, so it cannot be demodulated as is.

On the other hand, when the signal expressed by equation (3) is modulated by sin 2πf, t, Iz= (rPsPL) sin (2πf, t+ψS+
Δθ+2πf arrsat t) <(P s
Pi,) Sln (2πf, −ψS−Δθ−2πf
offset j )...(7) Here, sin (2rr f, t+ψS+Δθ+2πf,
rt-ct) = C・”(8) sin(2πf,
Let t-ψS-Δθ-2πf,tt,,tt)=D...(9). From equations (4) to (9), C has a phase of 90 with respect to A.
It is leading by 90 degrees, and D has a phase difference of 90 degrees with respect to W. If we represent the conversion that delays the phase by 90 degrees, A=C"D=B".

A”=　(C”)”Knee C.

D"= (B") "=-B, so the outputs J1 and J2 of the 9o° hybrid coupler 38 are j+= N++
I2”)/2 = (JpspL) (A+B+C”+D”)
/2= (JP, PL) A - (10
)Jz=(I+12)=<Ipspt>D...(11)
Therefore, the images can be separated and each can be demodulated and detected.

Effects of the Invention As detailed above, according to the present invention, a phase-locked loop is not required and a light source with a wider spectral linewidth can be used compared to the homodyne detection method. . In addition, compared to the phase diversity method, etc., the receiving system only requires one channel, and there is no need to use a 90" optical coupler for the optical signal stage or signal, so the configuration is simpler.Furthermore, Since the receiver has a narrow band, it is easier to increase the bit rate compared to the heterodyne detection method, and it is also possible to narrow the frequency interval when used in conjunction with the FDM method (frequency division multiplexing method). In addition, since the frequency f of the oscillator in the electrical signal processing section can be selected relatively freely, the design becomes easy.

[Brief explanation of drawings]

Fig. 1 is a diagram of the principle of the present invention, Fig. 2 is an auxiliary diagram for explaining the principle of the present invention, Fig. 3 is a block diagram of a coherent optical communication system showing an embodiment of the present invention, and Fig. 4 is shown in Fig. 3. 5 is a diagram for explaining the operation of the 90° hybrid coupler shown in FIG. 3. FIGS. 6 and 7 are for explaining the prior art. This is a diagram for 3.20... Optical coupler, 5.30... Oscillator, 6.38... 90° hybrid 12... Transmission light source, 2 6... Light receiver, Rado coupler, 4... Local light source .

Claims (1)

[Claims] Signal light (1) and local light (2) given a sufficiently small frequency difference with respect to the baseband band are combined by an optical coupler (3) and then sent to a light receiver (4). A first intermediate frequency signal is obtained by optical-to-electrical conversion, and this first intermediate frequency signal is multiplied by a sine wave signal and a cosine wave signal from the oscillator (5), respectively, and these product signals are connected to a 90° hybrid coupler. A second intermediate frequency signal is obtained by combining in (6), and this second
A coherent optical reception system characterized by asynchronous demodulation of intermediate frequency signals.