JP2003309520A - Optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a 40 Gbit/s optical communication system for transmitting signals at 80 km relay interval in a single-mode fiber having 0.5 dB/km loss. <P>SOLUTION: Data are subjected to NRZ-I coding, are bandlimited, and then are modulated by a phase modulator. The data are converted to light intensity modulation signals at a reception end by a light interferometer. The transmission of fixed envelope signals is strong against a nonlinear phenomenon. Additionally, due to bandlimiting to be performed in a light transmitter, a waveform inputted into a light receiving element in light receiving equipment becomes a wideband pulsive shape, thus improving an S/N ratio after reception. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system capable of high speed transmission.

[0002]

2. Description of the Related Art With the expansion of the broadband communication market, higher speed networks are being advanced. In the current optical communication, the maximum bit rate per wavelength is 40 Gbit / s if it is limited to the technology at the practical stage. The 40 Gbit / s system is first introduced from a high-cost system capable of optimizing the transmission line such as a submarine system.
Both will be introduced to the land system.

In the land system, the already laid fiber is often used, and the optical fiber laid in the past is 1.3.
It is optimized for the wavelength in the μm band, and there is no guarantee of loss for the wavelength in the 1.55 μm band, which became the standard wavelength for long-distance transmission. Even if the fiber is optimized for the 1.3 μm band, in most cases, the loss is low even in the 1.55 μm band, but the loss is not as low as that of the fiber optimized for the 1.55 μm band. . In addition, since the length of terrestrial fiber that can be laid at one time is limited, splice points occur at intervals of about 1 km, resulting in splice loss.

With such a laid land-based fiber, 1.
When transmitting wavelengths in the 55 μm band, its average loss is 0.5 dB / k
It will be about m. The loss is more than doubled compared to about 0.2 dB / km when using a fiber optimized for 1.55 μm with almost no connection points.

[0005]

Generally, the relay interval of the land system depends on the installation interval of the relay station, and is usually 40 km or 80 km.
Is. The number of repeaters in the link is 80km for 40km intervals.
The cost of the link increases because the distance is doubled compared to the relay of the interval. Therefore, it is desirable that the relay interval be 80 km or more as much as possible.

However, since the bandwidth is wide at 40 Gbit / s, a higher repeater incoming optical power is required to obtain a signal-to-noise ratio (S / N) equivalent to a low bit rate.
In order to secure a high optical power received by the repeater, it is necessary to increase the optical power input to the optical fiber. However, the nonlinear phenomenon of the optical fiber becomes remarkable and a large waveform distortion occurs, which impairs the transmission quality.

As a method for ensuring the incoming optical power level of the repeater without increasing the input power to the optical fiber, there is known a method of injecting pump light into the optical fiber from the incoming repeater side to generate Raman amplification. There is. The Raman amplifier has a very low gain efficiency with respect to the pump light, requires a sub-watt class pump light, is expensive, and complicates the configuration of the repeater.

Up to now, it has been difficult to carry out transmission at an 80-km repeater interval without using Raman amplification with an optical fiber having a high loss of 0.5 dB / km.

The present invention has been made in view of such conventional problems, and it is possible to transmit 40 Gbit / s in a fiber having a loss of 0.5 dB / km at a relay interval of 80 km without Raman amplification. An object is to provide an optical communication system.

[0010]

In order to solve such a problem, in the present invention, a light source, an optical phase modulator connected to the light source, and an electric signal input to the optical phase modulator are generated. An optical communication system including an optical transmitter having an electric signal generating section, an interferometer, an optical receiver having a light receiving element for photoelectric conversion, the optical transmitter, and a transmission line connecting the optical receiver. The electric signal generation unit has an encoder that encodes an input data string in reverse NRZ, and a band limiting unit that band limits the encoder output, and the band limiting unit is an instantaneous output of the phase modulator output light. Phase change φ
The maximum absolute value of | φmax | is from πRΨ / 2 to 3πRΨ / 4 (ra
d / s) (where R is the bit rate (bit / s), Ψ is the degree of phase modulation: peak-to-peak phase modulation amount of phase modulator output light (rad)) In the interferometer, a delay time D corresponding to a propagation time difference between two paths for forming interference in the interferometer is set so that the non-modulated signal input is substantially totally transmitted or totally blocked. In addition, the delay time D is

[Equation 2] There is provided an optical communication system characterized by satisfying the following.

According to such a configuration, the output light from the optical transmitter is a substantially constant envelope signal data-modulated by phase modulation. Compared with the NRZ and RZ signals used in normal optical communication, the peak power in 1 bit is less than half even with the same optical power. As a result, the non-linear phenomenon is less likely to occur, the input power to the optical fiber transmission line can be increased, the incoming optical power level can be increased compared to the conventional one, and the signal-to-noise ratio is improved.

Further, in this configuration, the band limiting means limits the band of the electric signal input to the phase modulator in the optical transmitter. By doing so, the waveform after passing through the optical interferometer in the optical receiver becomes a pulse shape instead of a rectangular shape when band limitation is not applied. Therefore, the electric signal spectrum after being converted into an electric signal by the light receiving element is broadened, and the band of the noise elimination filter in the subsequent stage can be narrowed, so that the signal-to-noise characteristic is improved.

Furthermore, since the band is limited by the electric stage, the spectrum of the phase modulator output light becomes narrow. In addition to the effect of increasing the dispersion tolerance, phase modulation-amplitude modulation conversion (PM-A
The degree of (M conversion) becomes small, and it is expected that an adverse effect such as jitter due to a non-linear phenomenon is less likely to occur as compared with the case where band limitation is not performed in the electric stage.

Further, in the present invention, the Mach / interferometer is used as the interferometer.
A Zender interferometer is used. Among various interferometers, the Mach-Zehnder interferometer has an input port and an output port that are completely separated from each other, so that it is easy to handle signals. Furthermore, the degree of freedom in designing the distance between the two output ports and the output direction is high, and it is easy to design to match the configuration of the photoelectric converter in the subsequent stage, and the module mounting becomes easy.

Further, according to the present invention, the light receiving element is a balanced type receiver, an inductance is connected to an output terminal between two photodiodes forming the balanced type receiver, and a DC photocurrent flowing through the two photodiodes. The optical communication system according to claim 1, wherein the difference between the two is shunted.

According to this structure, the two outputs of the optical interferometer are photoelectrically converted by the balanced receiver. Although there are various configurations of the optical interferometer, the Mach-Zehnder interferometer and the Michelson interferometer have two outputs. The balanced receiver is a photodiode that operates differentially, and an optical interferometer having two outputs is used to input each output to the two photodiodes of the balanced receiver. In the two outputs of the optical interferometer, the signal components are output with their intensities reversed and are thus added by the differential operation, but the intensity noise components are canceled by the differential operation, so the signal-to-noise characteristics are Be improved.

In order to improve the common mode rejection ratio of a balanced receiver, the optical system of the input section is usually designed so that the optical powers input to the two photodiodes constituting the balanced receiver are equal. As a result, all of the direct current will pass through the balanced receiver,
The DC potential at the output end does not change.

However, in the configuration of the first invention of the present application, the DC optical powers of the two output lights of the optical interferometer cannot be equal. This is because one output waveform of the optical interferometer is pulsed, and the waveform obtained by subtracting that waveform from the input light is output from the other, so even if the mark ratio is 1/2, the interferometer This is because the other output power becomes larger. Furthermore, in this configuration, depending on the phase modulation degree Ψ of the optical transmitter and the delay time D of the optical interferometer, one output of the optical interferometer may have a waveform with a DC offset. In both cases, the amplitudes of the AC signals are the same and only the DC values are different. Therefore, even if an optical attenuator is inserted in one of them and the optical powers are made equal, the balance is not improved.

In the present invention of the present application, an inductance is connected to the output terminal of the balanced receiver to shunt a DC current corresponding to the difference in DC photocurrent. By doing so, even if an unequal photo-DC current flows, unnecessary DC current does not flow in the circuit of the next stage. If the inductance is not provided, a DC voltage is generated by the DC input impedance of the circuit in the next stage, and the potential between the two photodiodes of the balanced receiver varies depending on the generated DC voltage. It is desirable that the two photodiodes of the balanced receiver have the same characteristics over all frequencies, but if a DC voltage is generated at the output end, the bias voltages of the two photodiodes will become uneven and the high frequency characteristics will differ. turn into. With the configuration according to the present invention, no DC voltage is generated, and such a situation can be avoided.

Further, in the present invention, the transmission line is a space, the delay time D is smaller than 1 / R, and the phase modulation degree Ψ is π.
An optical communication system according to claim 3, wherein the optical communication system is larger.

In the structure of the present invention, the transmission line connecting the optical transmitter and the optical receiver is defined as a free space. In that case, the background light noise is mixed into the optical receiver, but since the background light noise has low coherence and is mainly intensity noise, most of the background light noise is canceled by the balanced receiver. As a result, the degree of improvement in the signal-to-noise characteristic becomes large.

Further, the bit rate required for the optical space transmission is much smaller than the bit rate required for the optical fiber trunk system and is several 100 Mbit / s at most. In the configuration of the present application, since information is added to the phase, it becomes noise when the coherence of the light source is low. How much coherence is maintained between two lights that have passed through two paths in the optical interferometer and are shifted by the time D is a factor that determines the amount of noise. The effects of coherence are mitigated. If the bit rate is high, D will naturally decrease, so 4
At 0 Gbit / s, the effect of coherence is small. (Of course, when applying to a system where the reception sensitivity is so severe that coherence becomes a problem even at 40 Gbit / s, D is set to be smaller than 1 bit time as in the third aspect of the present application, and the phase modulation degree Ψ is set as necessary. It should be larger than π.) Several hundred Mbi
At a low bit rate such as t / s, D is made smaller than 1 bit time in order to reduce the effect of coherence. Simply D
If the value is only small, the efficiency of conversion into an intensity-modulated signal by the optical interferometer decreases, so the phase modulation degree Ψ is increased to compensate for it. In the present application, since band limitation is performed in the optical transmitter, waveform distortion or the like does not occur even if Ψ is larger than π as long as the instantaneous phase change amount φ satisfies the expression (1).

By doing so, it is possible to perform excellent transmission with little influence of background light noise and coherence of the light source.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an embodiment of the first invention of the present application. The light source 1, the phase modulator 2, the NRZ-I encoder 3 and the band limiting means 4 constitute an optical transmitter 7. The optical signal transmitted from the optical modulator 7 is the optical fiber 6
And is sent to the optical receiver 8. The optical receiver 8 includes an optical interferometer, for example, a Mach-Zehnder interferometer 9 and a light receiving element 10.
It consists of and.

Light having a high coherence suitable for phase modulation is output from the light source 1. This is data-modulated by the phase modulator 2 and output from the optical transmitter 7 to the optical fiber 6 which is an optical transmission line. In the optical transmitter 7, the original signal data string input from the data input terminal 5 is encoded by the NRZ-I encoder 3 into a reverse NRZ signal (NRZ-I signal). The encoded NRZ-I signal is band-limited by the band limiting means 4 and then applied to the phase modulator 2.

The phase-modulated light is transmitted to the optical receiver 8 via the optical fiber 6. In the optical receiver 8, the Mach-Zehnder interferometer 9 converts the phase modulation signal of the NRZ-I code into the intensity modulation signal of the original signal, and the light receiving element 10 converts it into an electric signal.

According to such a configuration, since it is a phase modulation signal at the stage of passing through the optical fiber 6, it is an optical signal having a substantially constant envelope. Since there is no rise or fall of the signal strength that causes jitter due to the nonlinear phenomenon of the optical fiber, jitter is less likely to occur. Furthermore, compared to the RZ and NRZ waveforms used in normal optical communication, even if the average optical power is the same, the peak power in the bit is small, so it is less susceptible to nonlinear effects.

Therefore, a booster amplifier (not shown) installed at a portion where the optical transmitter 7 inputs the optical fiber 6
Alternatively, it is possible to increase the output power of a relay amplifier (not shown) inserted in the middle of the transmission path, and it is possible to lengthen the relay interval even with an optical fiber having a high loss of 0.5 dB / km.

The NRZ-I encoder 3 converts the original signal into NRZ-I
Figure 2 shows how the code is converted. 2A shows an original signal input from the data input terminal 5, which is converted into an NRZ-I signal as shown in FIG. 2B by the NRZ-I encoder 3. This NRZ-I signal is as shown in FIG. 2 (c) after being phase-modulated by the phase modulator 2, and as shown in FIG. 2 (d) after being transmitted through the Mach-Zehnder interferometer 9. It becomes a waveform.

The NRZ-I encoder 3 is constructed, for example, as shown in FIG. The original signal input from the input 12 is exclusive ORed by a 1-bit delay of its own output by an exclusive OR (EXOR) circuit 15. The result is output at output 13. This is a step corresponding to integration, and it is shown in Fig. 2 (b).
The NRZ-I signal is a signal in which 1,0 is inverted every time the original signal becomes 1.

The band limiting means 4 is a means for lengthening the rise / fall time of the NRZ-I signal waveform shown in FIG. 2 (b). In this embodiment of the invention, the bandwidth is limited as follows. That is, the bit rate of the original signal is R (bit /
s), the degree of phase modulation, that is, the peak of the phase modulator output light
When the to-peak phase modulation amount is Ψ (rad), the maximum absolute value of the instantaneous phase change φ of the phase modulator output light | φm
Band limitation is performed so that ax | becomes πRΨ / 2 to 3πRΨ / 4 (rad / s).

The meaning of this band limitation will be described with reference to FIG. FIG. 4 is a diagram showing a phase change of the output light of the phase modulator. The phase changes in response to changes in the signal 1,0.
Its amplitude is Ψ (rad) peak-to-peak. The instantaneous phase change amount φ is the derivative of the waveform in FIG. 4, and φmax is the maximum value of the derivative. The differential coefficient has an absolute value because the sign is inverted at the rising and falling edges of the waveform.

| Φmax | corresponds to the slope of the steepest part near the center of the rising or falling of the waveform of FIG. The output of the NRZ-I encoder 3 uses the band limiting means 4
The band is limited by, and the amplitude is appropriately adjusted (amplitude adjusting means is not shown) and applied to the phase modulator 2 to perform phase modulation on the light from the light source 1. At this time, if | φmax | is smaller than πRΨ / 2, the waveform rises (falls) during 1 bit time.
It is not desirable because it is likely not to cut. As | φmax | becomes larger, the rise / fall of the phase modulation waveform becomes steeper. As a result, the output waveform of the optical interferometer 9 in the optical receiver 8 changes from a pulse shape to a rectangular shape. In the present application, by setting the maximum value of | φmax | to 3πRΨ / 4, a pulse-like waveform is maintained and a wide-band electric spectrum after photoelectric conversion is secured. Since the electric spectrum has a wide band, it is possible to improve the S / N by narrowing the band of the noise removing electric filter (not shown).

The band limiting means 4 is actually a low-pass filter inserted before or after a phase modulator driver amplifier (not shown), or a driver amplifier intentionally manufactured in a narrow band.

Mach-Zehnder interferometer 9 in the optical receiver 8
The configuration is as shown in FIG. 5, for example. The light input from the input 16 is divided into two parts by the branching part 21, one of which passes through the route 117 and the other of which passes through the route 218 and is coupled by the coupling part 22. Route 1 and route
2 has a different optical path length, and the delay time corresponding to the optical path length difference is D
(sec). In the present application, this delay time D is relative to the instantaneous phase change amount φ of the phase-modulated light and the arbitrary time t.

[Equation 3] Designed to meet. Further, the stop center wavelength or the transmission center wavelength of the Mach-Zehnder interferometer 9 is set or controlled so as to substantially match the center wavelength of the light source 1. Specifically, since the characteristics of the outputs 1 19 and 2 20 are complementary, when the output light is at the transmission center wavelength on one side, it is at the stop center wavelength on the other side.

When this phase-modulated light passes through the Mach-Zehnder interferometer 9 configured as described above, the light that has passed through the path 2 interferes with the light that has been delayed by the time D that has passed through the path 1. How much interference occurs is determined by the amount of phase change during time D. For example, during time D
If there is a phase change of (180 °), the total optical power is output from either one of the outputs 1 and 2, and is not output from the other. The amount of phase change during time D depends on the phase modulation waveform. For example, if Ψ = π and D = 1 bit time, the light that was not transmitted at time t1 in the waveform shown in FIG. 4 is completely transmitted at time t2 and is not transmitted again at time t3. The Mach-Zehnder interferometer transmitted light thus obtained is the phase modulation converted to the intensity modulation. Since the operation of the Mach-Zehnder interferometer corresponds to the difference, the waveform integrated by the NRZ-I encoder is differentiated and the original signal is reproduced.

An example of the eye pattern after passing through the Mach-Zehnder interferometer is shown in FIG. As shown in FIG. 4, when there is no phase change before and after the phase change portion, the pulse becomes a little wider, and when there is continuous phase change, the pulse becomes a narrow pulse. Therefore, the eye pattern is a mixture of these. The difference in pulse width between wide and narrow pulses is the delay time
Depending on D, the shorter D becomes, the smaller the difference becomes, and the narrower the pulse waveform becomes, the narrower the pulse width becomes.

On the contrary, the upper limit of D is 1 bit time. NRZ-I
Since encoder 3 is an integrator with a 1-bit delay, D
This is because when exceeds 1 bit time, interference with adjacent bits is included.

The light receiving element 10 converts such a waveform into an electric signal. Since the signal is pulsed, the optical transmitter
The spectrum width is wider than that of the rectangular wave shape in the case where the band limiting means 4 is not inserted within 7. Before inputting the received signal to the discriminator, parts other than the effective signal band are filtered out. However, if the signal spectral bandwidth of the filter input is wide enough, a narrower band filter can be used. . As a result, more noise can be removed and the S / N of the signal input to the discriminator is improved. Therefore,
According to this embodiment of the present invention, the reception quality is improved by providing the band limiting means on the transmission side.

Further, in recent years, in ultrahigh-speed optical communication, high-density wavelength division multiplexing has been introduced in most systems. When wavelength-multiplexing signals, avoid leaks into adjacent wavelengths,
An optical filter (or wavelength multiplexer itself, etc.) removes other than the required wavelength band. When the phase-modulated optical signal is limited by the optical filter, the PM-A of the optical filter
An intensity modulation component is generated by the M conversion action. As described above, intensity fluctuation causes jitter after optical fiber transmission, and therefore it is desirable that the intensity modulation component is small. The degree of the intensity modulation component that is generated depends on the rate of being cut by the optical filter, and the greater the rate of removal, the greater the intensity modulation component that is generated.

In the configuration of this embodiment, since the modulated signal is band-limited in the electric stage, the optical spectrum after phase modulation is smaller than that in the case where the band limitation is not applied.
As a result, the rate of shaving when passing through the optical filter is small, the generated intensity modulation component is small, and as a result, it is possible to perform good transmission with less jitter.

If the center wavelength of the Mach-Zehnder interferometer does not substantially match the wavelength of the light source, good operation cannot be obtained, so it is necessary to control the center wavelength of the optical interferometer in the optical receiver 8. . This can be controlled by almost the same control as the AC control method applied to the Mach-Zehnder type optical intensity modulator, and can be controlled by the configuration schematically shown in FIG.
That is, the center wavelength of the Mach-Zehnder interferometer 9 is modulated by the low frequency signal output from the oscillator 23. Part of the output of the Mach-Zehnder interferometer 9 is branched and the receiver 2
The signal is converted into an electric signal at 5, and the low frequency signal from the oscillator 23 and the synchronous detector 24 are synchronously detected. The output is added to the low frequency signal by the adder 27 via the low pass filter 26,
Control in addition to the Mach-Zehnder interferometer 9.

Although the Mach-Zehnder interferometer is used as the optical interferometer in the above-mentioned example, the same operation can be performed by using the reflection type Michelson interferometer having almost the same function as the Mach-Zehnder interferometer. is there.

There is a Fabry-Perot interferometer as another optical interferometer, but it is also possible to use a Fabry-Perot interferometer having a low finesse. The case of using this type of interferometer will be described with reference to FIG. FIG. 8 shows a configuration example of the Fabry-Perot interferometer, in which light is incident on the etalon plate 28 like an incident beam 29. Reflective films are attached to both side surfaces a and b of the etalon plate, and a part of the incident beam is reflected by the surface b as shown by a dotted line and the rest is transmitted as a transmitted light 30. A part of the light reflected on the surface b is reflected again on the surface a, and a part of the light reflected on the surface b is transmitted as the once-reflected transmitted light 31. If the delay of one round trip of the etalon plate is designed to be D, the transmitted light 30 and the once-reflected transmitted light 31 deviate by D and interfere. A general Fabry-Perot interferometer has a sufficiently high reflectance of a and b, and is used in a region where the beam incident angle is as close to perpendicular to the etalon plate as possible to increase the number of multiple reflections and to increase the transmission bandwidth of the filter. And increase the ratio of the transmission period (finesse). However, in the present application, conversely, by tilting the etalon plate or lowering the finesse by lowering the reflectance,
It is configured to mainly extract the transmitted light that is transparent and the transmitted light that is reflected once, and realizes the same operation as a Mach-Zehnder interferometer. As can be seen from FIG. 8, the Fabry-Perot interferometer is often configured in free space instead of an optical waveguide, and is suitable for a light receiving system for spatial transmission.

In the structure of the present application, the interferometer does not necessarily have to be installed in the optical stage, and as shown in FIG.
A local light source 46 is installed at the input part of 45 to receive heterodyne, and the light receiving element 10 converts the signal into an electric signal, and then an interferometer,
That is, the same effect can be obtained even if the frequency discriminator 47 is installed. The configuration of the frequency discriminator 47 is, for example, FIG.
As shown in.

The input 16 is split into two by the splitter 48, and after passing through the path 1 and the path 2 to which the delay time difference D is given, they are combined by the multiplier 49 and output from the output 50. This is a component that operates exactly like a Mach-Zehnder interferometer in the optical stage. Even in such a configuration, the frequency discriminator output can be made into a pulse-like waveform by performing band limitation on the transmitting side.

The frequency discriminator output uses the AMI (Al
ternate Mark Inversion) Encoded.

Next, a characteristic part of the second embodiment of the present invention will be described with reference to FIG. Mach-Zehnder interferometers and Michelson interferometers usually have two interferometer output ports. These are in a complementary relationship such that the total output power of the two ports is equal to the input power (excluding excess loss). By receiving both of them by the balanced receiver as shown in FIG. 9, it is possible to cancel the intensity noise component generated in the optical fiber amplifier or the like in the transmission path and obtain a higher S / N.

The relationship between the phase modulation and the delay time D of the interferometer is (1)
When the relations satisfy the equal sign of the equation, the output of the optical interferometer is as shown in FIGS. 10 (a) (b). If band limitation is not performed by the band limiting means in the transmitter, the waveforms are rectangular in FIGS. 10 (a) and 10 (b), and the two output light powers of the interferometer are substantially equal. However, in the present application, since the band is limited, the waveform becomes a pulse and the two output light powers are not equal.

When the relationship between the phase modulation and the delay amount D of the interferometer is such that the inequality sign in the equation (1) is satisfied,
As in (c) and (d), the waveform has a DC offset on one side.

In either case, since the DC optical powers of the output 1 and the output 2 are different, equal optical power cannot be input to the two photodiodes of the balanced receiver.

However, these waveforms are different only in the DC value, and the AC waveforms are balanced in the outputs 1 and 2 in this state. Therefore, in the present application, only the DC difference is released. As shown in FIG. 9, the inductance 33 is connected to the output of the balanced receiver 32. Inductance 33
The end of is connected to the ground in the figure. This is because the ± V power supply voltage is supplied to both ends of the balanced receiver, so the connection destination of the inductance is set to the intermediate potential, and the bias voltages applied to the two photodiodes are made equal.

When direct current is output from the balanced receiver, if there is no inductance, the preamplifier in the subsequent stage
A voltage corresponding to the DC input impedance of 34 is generated.
When the inductance is connected, almost all the DC current is shunted to the ground through the inductance 33 because the DC impedance of the inductance is almost zero. As a result, the potential at the output of the balanced receiver is
It is always constant without depending on the difference in the optical power entering the two photodiodes, and it is possible to keep the bias voltage of the two photodiodes, and hence the frequency characteristics, equal.

Further, the amplifier often has a blocking capacitor at the input for stabilizing the internal bias voltage. If the preamplifier 34 has such a configuration,
When the DC optical powers incident on the two photodiodes are different, there is no escape area for the DC photocurrent from the photodiode receiving the large DC optical power. The escape-free photocurrent accumulates in the photodiode for a certain period of time and then recombines. When photocurrent is accumulated in the photodiode, the high frequency characteristics of the photodiode are significantly deteriorated. By connecting the inductance 33 as in the present application, it is possible to release the DC current even if the input portion of the preamplifier 34 is provided with a blocking capacitor. Therefore, the high frequency characteristics are not impaired, and
It is possible to operate the two photodiodes in a well-balanced state.

It was confirmed by simulation that the configuration of the present application is resistant to the nonlinear phenomenon and noise of the optical fiber and can realize a relay span of 80 km with a land-based fiber having a loss of 0.5 dB / km. The optical transmitter 7 was driven at 40 Gbit / s in the system as shown in FIG. 13, the relay span in parentheses was transmitted for 4 spans, then amplified by the optical preamplifier 44 and received by the optical receiver 8. The optical receiver 8 uses a Mach-Zehnder interferometer with a delay amount of 25 ps as an optical interferometer, and the light receiving element is a balanced receiver.
The wavelength of the light source is 1.55 μm, the output optical power of the optical amplifier 42 in the relay span is 10 dBm, the noise figure is 5 dB, and the optical fiber loss is 0.5 d.
B / km 1.31 μm zero dispersion single mode fiber 43 (SM
F) It was set to 80km. The dispersion compensator 41 compensated the dispersion of each span of the SMF. The total transmission is 320 km. As a comparison target, the Mach-Zehnder interferometer in the optical receiver 8 is converted into the RZ intensity modulation waveform in the optical transmitter in the optical transmitter, then passed through the same transmission line, and amplified by the optical preamplifier, We also simulated the system for receiving with the PIN photodiode. The results are shown in Fig. 14.

FIG. 14 (a) shows an R-shaped eye pattern after transmission.
FIG. 14B shows the data transmitted after being converted into the Z intensity modulation waveform, and FIG. 14B shows the data transmitted with the configuration shown in FIG. In Fig. 14 (a), the jitter is very large and the eye is completely collapsed, whereas in Fig. 14 (b), a good eye opening is obtained, which is 0.5 dB.
It turns out that / km is possible.

FIG. 11 is a diagram for explaining the third embodiment of the present application. This is an embodiment in which optical space transmission is performed. The configurations of the optical transmitter 35 and the optical receiver 36 are almost the same as those in FIG. 1, but the output of the phase modulator 2 is converted into parallel light via the lens 38 instead of the optical fiber, and the optical receiver 36 The difference is that the parallel light that has arrived is condensed by the lens 39 and is incident on the Mach-Zehnder interferometer. In such a configuration, the lens 39 would be the optical transmitter 35.
Not only the transmitted light from the sun but also background light noise such as sunlight and fluorescent lights will be collected. However, since these background lights have low coherence, most of them are intensity noise and can be removed by the balanced receiver. Therefore, by using the light receiving element as a balanced type receiver, higher S / N transmission can be performed as compared with a normal intensity modulation optical space transmission system.

Since it is difficult to collect the Mach-Zehnder interferometer 37 efficiently so as to match with the propagation mode of the waveguide, it is preferable that the Mach-Zehnder interferometer 37 is constructed of micro-optics rather than the waveguide type.

Since the bit rate is low in the optical space transmission,
It is easily affected by the coherence of the light source. Therefore, in the present application, as shown in FIG. 15, the delay time D of the optical interferometer is set to 1 bit time.
The effect of coherence is mitigated by making it smaller than 1 / R. At this time, in order to prevent the deterioration of the efficiency of conversion from phase modulation to intensity modulation, the phase modulation degree Ψ is set to be larger than π in the range satisfying the expression (1). In the present application, since band limitation is performed within the optical transmitter, the rise / fall of the phase modulation waveform is blunt as shown in FIG. Therefore, if the amount of phase change does not exceed π within the delay time D, even if Ψ exceeds π, adverse effects such as waveform distortion do not occur. By doing so, the influence of the coherence of the light source can be mitigated without lowering the conversion efficiency of the phase modulation-intensity modulation, and good sensitivity can be obtained.

FIG. 12 shows an example in which the optical transmitter 7 and the optical receiver 8 of the present application are applied to a multipoint-to-multipoint optical network. The optical receiver is a balanced receiver according to the second embodiment of the present application, an inductance is connected to the receiver output, and the potential of the output terminal is stabilized. Light from the plurality of optical transmitters 7-i (i = 1,2, ..., n) is transmitted through the optical network 40 to the plurality of optical receivers 8-i (i = 1,1,
2, ..., n) is reached. Light from different optical transmitters arrives at each optical receiver in time series. These have different optical powers and optical modulation degrees, but by adopting the balanced receiver configuration as in the present application, the potential of the central value of the output electric signal becomes almost constant even if the optical powers vary. The circuit configuration of the subsequent stages can be simplified.

In the above description, only the portion directly related to the configuration of the invention of the present application has been described. Means such as an amplifier that do not directly contribute to the function of the present invention are not shown, but they are used as necessary in constructing an actual system.

[0062]

As described above, according to the present invention, since transmission is performed by the phase-modulated signal of the constant envelope, it is resistant to the non-linear phenomenon. Also, due to the band limitation performed in the optical transmitter, the waveform input to the light receiving element in the optical receiver becomes pulsed,
The S / N after receiving can be improved. These results result in a loss of 0.5 dB / k
Even with a high-loss land fiber such as m, a relay interval of 80 km can be realized without performing Raman amplification, and it is possible to greatly simplify the system.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of an embodiment of the first invention of the present application.

FIG. 2 is a diagram for explaining the principle of the present invention.

FIG. 3 is a diagram showing a configuration example of an NRZ-I encoder.

FIG. 4 is a diagram for explaining a phase modulation waveform of the present application.

FIG. 5 is a diagram for explaining a Mach-Zehnder interferometer.

FIG. 6 is a diagram showing an example of an optical interferometer output eye pattern of the present application.

FIG. 7 is a diagram showing an example of a system for controlling the center frequency of a Mach-Zehnder interferometer.

FIG. 8 is a diagram showing a configuration example of a Fabry-Perot interferometer suitable for the invention of the present application.

FIG. 9 is a diagram showing a part of the configuration of the second embodiment of the present application.

FIG. 10 is a diagram showing waveforms of two output lights of the optical interferometer.

FIG. 11 is a diagram showing a configuration example of a third embodiment of the present invention.

FIG. 12 is a diagram showing a configuration of an application example of the present invention.

FIG. 13 is a diagram showing a system used in a simulation for verifying the characteristics of the present invention.

14 is a diagram showing an eye pattern as a result of simulation of the system shown in FIG.

FIG. 15 is a diagram for explaining a case where the degree of phase modulation exceeds π.

FIG. 16 is a diagram showing the configuration of another embodiment of the present invention.

17 is a diagram showing a configuration example of a frequency discriminator in the configuration of FIG.

[Explanation of Codes] 1 ... Light source, 2 ... Phase modulator, 3 ... NRZ-I encoder, 4 ... Band limiting means, 5 ... Data input terminal, 6 ... Optical fiber , 7 ... Optical transmitter, 8 ...
・ Optical receiver, 9 ・ ・ ・ Mach-Zehnder interferometer, 10
... Light receiving elements, 11 ... Output terminals, 12, 16 ...
・ Input, 13 ... Output, 14 ... 1-bit delay element, 15 ... EXOR circuit, 17 ... Path 1, 18 ...
・ Routes 2, 21 ... Branching part, 22 ... Coupling part, 23
... Oscillator, 24 ... Synchronous detector, 25 ... Photoreceiver, 26 ... Filter, 27 ... Adder, 28 ...
・ Etalon plate, 29 ... Incident beam, 30 ... Transmitted light, 31 ... Single reflected / transmitted light, 32 ... Balanced receiver, 33 ... Inductor, 34 ... Preamplifier, 35. ..Optical transmitters, 36, 45 ... Optical receivers,
37 ... Mach-Zehnder interferometer, 38, 39 ...
-Lens, 40 ... Optical network, 41 ... Dispersion compensator, 42 ... Optical amplifier, 43 ... Single mode fiber, 44 ... Optical preamplifier, 46 ... Local light source, 47 ... -Frequency discriminator, 48 ... Splitter, 49 ... Multiplier.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H079 AA12 BA03 CA04 FA02 HA11                       KA13 KA19 KA20                 2K002 AA02 AB18 AB19 BA01 DA08                       EA30 EB15 GA10                 5K102 AA01 AA15 AH27 AL23 PH00                       PH12 PH31 PH37 PH49 RD03                       RD05 RD15 RD27

Claims (4)

[Claims] 1. A light source, an optical phase modulator connected to the light source, an optical transmitter having an electric signal generating section for generating an electric signal to be inputted to the optical phase modulator, an interferometer, and a photoelectric converter. An optical communication system comprising an optical receiver having a light-receiving element, and a transmission line connecting the optical transmitter and the optical receiver, wherein the electric signal generation unit encodes an input data string in reverse NRZ. And an band limiting means for band limiting the encoder output, wherein the band limiting means is such that the maximum absolute value | φmax | of the instantaneous phase change amount φ of the phase modulator output light is πRΨ / 2. 3πR
Ψ / 4 (rad / s) (where R is the bit rate (bit / s), Ψ
Is the degree of phase modulation: the amount of phase modulation (rad) of the output light of the phase modulator is limited to a band, and the interferometer is approximately totally transmitted or totally transmitted with respect to the unmodulated signal input. A delay time D corresponding to a propagation time difference between two paths for forming interference in the interferometer is set so as to be blocked, and the delay time D is expressed by ] An optical communication system characterized in that 2. The optical communication system according to claim 1, wherein the interferometer is a Mach-Zehnder interferometer. 3. The light-receiving element is a balanced receiver, an inductance is connected to an output terminal between two photodiodes forming the balanced receiver, and a shunt is a difference between direct currents flowing through the two photodiodes. Claim 1 or claim 2 characterized in that
The optical communication system described. 4. The transmission path is a space, and the delay time D
Is less than 1 / R and the degree of phase modulation Ψ is greater than π.