JP4053473B2 - Optical transmitter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is used in a high capacity optical link system. Optical transmitter In particular, the transmission capacity can be doubled and the frequency utilization efficiency can be increased without increasing the optical frequency band of the conventional optical link system. Optical transmitter It is about.
[0002]
[Prior art]
An example of the prior art for increasing the transmission capacity is shown in FIG. 16 (see, for example, Patent Document 1). According to this method, light from the light source is first converted into a pulse train by the clock generation means. Thereafter, it is branched and encoded by the first data signal and the second data signal, one of which is delayed by one time slot so that the pulses do not overlap, and these are multiplexed by the optical multiplexer. . As a result, an OTDM (Optical Time Division Multiplexed) signal having a bit rate twice that of the data signal before multiplexing is generated.
[0003]
When multiplexed by this method, the bit rate per wavelength increases, and various transmission restrictions become severe. One reason is that waveform degradation due to wavelength dispersion of the optical fiber increases in proportion to the square of the bit rate. It also strongly depends on the increase of the transmission spectrum width. For example, the dispersion tolerance of a 40 Gbit / s NRZ optical signal is about 80 to 100 ps. In order to generate an OTDM signal by multiplexing these two, it is necessary to first convert it to an RZ signal, and further, the pulse width needs to be sufficiently within a time slot of 80 Gbit / s. Therefore, the dispersion tolerance of the 80 Gbit / s RZ OTDM signal becomes 1/10 or less of the dispersion tolerance of the 40 Gbit / s NRZ signal. For this reason, the accuracy required for equalizing the chromatic dispersion of the optical transmission line is increased, and the chromatic dispersion compensation device and its control cost are increased. In addition, since the operation margin of the transmission system itself is reduced, the stability of the system may be impaired and reliability may be reduced. In addition, it has been reported that the chromatic dispersion of the optical transmission line fluctuates with environmental temperature fluctuations [KSKim et al., Journal of Appl. Phys., Vol. 73, No. 5, p. 2069, 1993]. In such a small dispersion tolerance, the chromatic dispersion fluctuation due to the environmental temperature fluctuation exceeds this.
[0004]
Further, the influence of polarization mode dispersion becomes obvious, and transmission characteristics are degraded. Also, at the time of reception, a high-speed timing extraction circuit, a high-speed photoelectric converter, an electric amplifier, and a logic circuit are required, which causes a great increase in cost.
[0005]
In addition to the conventional techniques described above, there is a method for expanding the transmission capacity by using a multilevel code exceeding binary values, such as DQPSK. However, first, when increasing the multiplexing number from DPSK to DQPSK, it is necessary to increase the optical power by 3 dB in order to achieve the same minimum intersymbol distance. In addition to this, there is a penalty due to excess noise when using an autocorrelation type reception method using an MZ (Mach-Zehnder) interferometer [Y. Okunev, Phase and phase difference modulation in digital communications, Arctech House, London. , 1997]. This penalty can be ignored in binary DPSK, but cannot be ignored in quaternary DQPSK, and about 2 dB is theoretically estimated. This penalty has been verified experimentally and limits the transmission performance of DQPSK [C. Wree et al., OFC2003 ThE5, 2003].
[0006]
[Patent Document 1]
JP-A-10-79705
[0007]
[Problems to be solved by the invention]
The present invention has been made under such a background, and it is possible to increase the transmission capacity while alleviating the cost increase due to the increase in the speed of the electric circuit and the increase in the number of wavelengths, the reduction of the operation margin, and the deterioration of the transmission performance. Can be realized Optical transmitter The purpose is to provide.
[0008]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problems, and an optical transmitter according to the present invention includes a CW light source that generates continuous light and two light sources that branch the continuous light of the CW light source and perform phase modulation on each. A phase modulator and an asymmetric multiplexing unit that multiplexes the two phase-modulated optical signals with an asymmetric multiplexing ratio, and the optical intensity and optical phase of the communication light due to the light interference effect at the time of multiplexing Independent digital signals are simultaneously superimposed and transmitted.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
Also, Thereby, it is possible to configure an optical link system that doubles the transmission capacity without increasing the operation speed of the electric circuit. Also, the frequency utilization efficiency can be dramatically increased. For this reason, high efficiency and low cost of the optical link system can be achieved.
[0009]
The optical transmitter of the present invention includes a CW light source that generates continuous light, two phase modulators that branch the continuous light and phase-modulate each of the CW light source, Said An asymmetric multiplexing unit that multiplexes two phase-modulated optical signals with an asymmetric multiplexing ratio, and a multiplexing ratio adjuster that makes the multiplexing ratio arbitrarily variable, and an optical interference effect at the time of multiplexing Thus, independent digital signals are simultaneously superimposed on the optical intensity and optical phase of the communication light and transmitted.
Thereby, the signal-to-noise ratio (SNR) of the intensity modulation component and the SNR of the phase modulation component can be relatively changed.
[0010]
An optical transmitter according to the present invention includes a CW light source that generates continuous light, amplitude control means that receives two digital signals and controls the logical amplitude of one digital signal based on the logic of the other digital signal. A Mach-Zehnder type modulator and means for driving the Mach-Zehnder type modulator biased at or near the transmission zero point by the multiplexed digital signal generated by the amplitude control means; With independent digital signals superimposed on the optical intensity and optical phase of the communication light and transmitted. It is characterized by that.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
[0011]
The optical transmitter according to the present invention also includes a CW light source that generates continuous light and two digital signals. input First amplitude control means for controlling the logical amplitude of one digital signal based on the logic of the other digital signal, and an inverted signal of the two digital signals input The second amplitude control means for controlling the logical amplitude of one digital signal based on the logic of the other digital signal, a push-pull type Mach-Zehnder type modulator, and the multiplexed digital signal generated by the amplitude control means And means for driving the push-pull type Mach-Zehnder type modulator biased at or near its transmission zero point by its inverted signal, Independent digital signals are simultaneously superimposed and transmitted on the optical intensity and optical phase of communication light It is characterized by that.
As a result, the drive amplitude to be applied to the electrode of the Mach-Zehnder modulator is almost halved. Make It becomes possible.
[0012]
The optical transmitter of the present invention is The light intensity of the communication light is X and 1 Are two values, X satisfies 1 <X <3.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
[0013]
The optical transmitter of the present invention uses a pulsed light source that generates pulsed light synchronized with a bit rate in place of the CW light source, and generates and transmits an RZ signal by the pulsed light source. .
Thereby, the RZ light intensity / phase hybrid encoded optical signal can be generated.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
[0025]
[Basic concept of the present invention]
The present invention expands the transmission capacity without increasing the bit rate or increasing the number of wavelengths. For example, an optical transmitter is configured with a CW (Continuous Wave) light source and MZ (Mach-Zehnder) modulation. A light intensity / phase hybrid coded optical signal. Furthermore, the transmission capacity is expanded by separating and receiving the light intensity component and the optical phase component.
[0026]
Conventionally, in order to realize an increase in transmission capacity, the main method has been to increase the bit rate per channel or to increase the number of wavelengths in a wavelength multiplexing system. However, in order to increase the bit rate, it is necessary to increase the speed of various electric circuits and optical circuits related to the transceiver, which is generally very expensive. Furthermore, as described in the conventional example, the influence of various transmission limiting factors becomes severe in terms of acceleration, making it difficult to stably realize good transmission characteristics. On the other hand, when the number of wavelengths is increased, the cost is increased by the increase in the number of systems, and transmission limitations due to various nonlinear optical effects and the influence of excessive noise due to wavelength multiplexing / demultiplexing circuits also appear. Furthermore, if an attempt is made to expand the transmission capacity by using a multi-level code such as DQPSK, the transmission performance suffers from excessive deterioration under the influence of excessive noise found in autocorrelation reception.
[0027]
According to the present invention, since the transmission capacity per wavelength can be expanded without increasing the bit rate per channel, the transmission cost is increased due to the increase in device cost accompanying the increase in the speed of the electric circuit and the transmission speed per channel. Demerits such as a decrease in margin and deterioration in transmission characteristics can be alleviated. In addition, it is not affected by excessive penalties seen in autocorrelation reception of DQPSK codes. Furthermore, since the transmission capacity per wavelength channel can be greatly increased, when considered as a wavelength multiplexing system, the total transmission capacity can be dramatically increased, which is also effective in reducing costs in terms of frequency utilization efficiency. It is.
[0028]
[First Embodiment]
An optical transmitter according to the first embodiment of the present invention will be described. FIG. 1 is a diagram illustrating an example of an optical transmitter according to a first embodiment of the present invention, which is an example of an optical transmitter that generates a light intensity / phase hybrid encoded optical signal. The optical transmitter shown in FIG. 1 includes a CW light source 10 that generates continuous light, a precoding circuit 11 that converts an input electric signal to have correct logic at the output of the optical transmitter, and an LN substrate (X-cut) 12. The optical branching unit 13, the two MZ modulators 14 and the MZ modulator 15, and the asymmetrical multiplexing unit 16 that multiplexes at an asymmetrical light intensity ratio. The operation of the precode circuit 11 will be described in the section on operation description.
[0029]
The CW light source 10 generates continuous light such as a DFB-LD (Distributed-Feed-Back-Laser-Diode) or an external resonator type laser. Each optical functional block described above can be realized by forming an optical waveguide on the LN substrate 12, for example.
[0030]
The operation of the optical transmitter according to the first embodiment of the present invention will be described with reference to FIGS. In FIG. 1, continuous light generated from the CW light source 10 is guided to an X-Cut type MZ modulator 14 and an MZ modulator 15. On the other hand, the electrical signal 1 (CH 1) and the electrical signal 2 (CH 2) are subjected to code conversion by the precoding circuit 11 and then guided to the aforementioned MZ modulator 14 and MZ modulator 15. Here, with these electric signals, the continuous light is DPSK (Differential-Phase-Shift-Keying) encoded. These two encoded DPSK signals are combined in the asymmetric multiplexing unit 16 at the rear end in such a manner that the relative strength is asymmetric at a constant ratio. In the example of the optical transmitter shown in FIG. 1, since the X-Cut type MZ modulator 14 and the MZ modulator 15 are employed, it is possible to generate a DPSK signal having no optical frequency chirp with a single drive.
[0031]
Referring to FIG. 10, FIG. 10A shows the state of the optical signal intensity and optical phase in arm 1 before multiplexing, and FIG. 10B shows the state of the optical signal intensity and optical phase in arm 2. Indicated. As can be seen from the figure, the intensity of the optical signal in each arm is constant over time. At this time, the data signal is superimposed on the relative optical phase. Naturally, since the electric signal 1 (CH1) and the electric signal 2 (CH2) are independent signals, the one obtained by replacing them with the optical phase is also independent, and these two have no correlation. Therefore, as shown in FIG. 10C, when these two optical signals are combined, the light interference effect differs depending on the relative optical phase difference between them, and the intensity varies. At this time, information of two independent electric signals 1 (CH1) and electric signal 2 (CH2) is converted into the intensity and optical phase of one optical signal.
[0032]
This will be described with reference to the operation explanatory diagram of the precoder in FIG. First, the possible values of the electric signal 1 (CH1) and the electric signal 2 (CH2) are {0, 1}, respectively. Therefore, there are four combinations as shown in FIG. These four combinations are converted into the combinations described in the middle of FIG. 13 by the precoder.
[0033]
In this converted combination, the signal {0} corresponds to the phase {0}, and the signal {1} corresponds to the phase {π}. Then, as shown in FIG. 10, the combination having a relative phase difference of π is extinguished because the intensity is reduced, and in the combination having a relative phase difference of 0, the intensity is added and increased. As a result, the intensity of the generated optical signal has two values {1, X} (X> 1), the relative optical phase has two values {0, π}, and the combination thereof is Based on the correspondence table of FIG. 13, this corresponds to the combination of the codes of the original two electrical signals. That is, with the encoding of the present embodiment, information of two binary digital signals can be superimposed on one optical signal without causing an increase in bit rate.
[0034]
[Second Embodiment]
Next, an optical transmitter according to a second embodiment of the present invention will be described. FIG. 2 is a diagram illustrating an example of an optical transmitter according to the second embodiment of the present invention, which is an example of an optical transmitter that generates an optical intensity / phase hybrid encoded optical signal. The optical transmitter shown in FIG. 2 includes a CW light source 20 that generates continuous light, a precoding circuit 21 that converts an input electric signal to have correct logic at the output of the optical transmitter, and an LN (X-cut) substrate 22. 2, two MZ modulators 24 and 25, an asymmetrical multiplexing unit 26 that multiplexes with an asymmetrical light intensity ratio, and a multiplexing ratio that adjusts the combined light intensity ratio The controller 27 is configured. The difference from the optical transmitter of the first embodiment shown in FIG. 1 is that the multiplexing ratio adjuster 27 can adjust the generated optical signal so as to have an appropriate intensity modulation degree.
[0035]
The operation of the optical transmitter shown in FIG. 2 basically follows the same operating principle as that of the optical transmitter of the first embodiment shown in FIG. The optical transmitter shown in FIG. 2 is characterized in that the multiplexing ratio of the optical signal shown in FIG. 10 can be arbitrarily changed because the multiplexing ratio of the optical signal can be changed by the multiplexing ratio adjuster 27. can do. Thereby, the signal-to-noise ratio (SNR) of the intensity modulation component and the SNR of the phase modulation component can be relatively changed. The calculation of SNR will be described later.
[0036]
[Third Embodiment]
Next, an example of the optical transmitter according to the third embodiment of the present invention will be described. FIG. 3 is a diagram illustrating an example of an optical transmitter according to the third embodiment of the present invention, and is an example of an optical transmitter that generates an optical intensity / phase hybrid encoded optical signal. The optical transmitter shown in FIG. 3 has a CW light source 30 that generates continuous light, an MZ modulator 33 generated on an LN substrate (X-cut) 32, and a CH1 electrical signal whose amplitude is in accordance with the logic of CH2. It is comprised from the amplitude control means 31 to control.
[0037]
The operation of the optical transmitter shown in FIG. 3 will be described with reference to FIGS. Continuous light from the CW light source 30 is converted by the MZ modulator 33 into a light intensity / phase hybrid encoded optical signal based on the drive amplitude applied to the MZ modulator 33. The drive amplitude applied to the modulator 33 is generated as shown in FIG.
[0038]
The intensity of the signal of CH1 (electric signal before input of amplitude control means) shown in FIG. 11A is modulated by the amplitude control means 31 along the logic of CH2 shown in FIG. The drive amplitude shown in c) is obtained. Here, for example, if the logic of CH2 is 0, the drive amplitude is small, and if the logic of CH2 is 1, the amplitude is controlled to be large.
[0039]
The drive amplitude generated here is input to the X-cut type MZ modulator 33 shown in FIG. The operation in the MZ modulator 33 will be described with reference to FIG. The bias voltage is set so that the reference point of the input drive amplitude signal (FIG. 12B) becomes the zero point of the output light intensity of the response characteristic curve of the MZ modulator 33 (FIG. 12A). Then, as shown in FIG. 12C, it is possible to generate a light intensity / phase hybrid encoded optical signal in which the data of CH1 is superimposed on the optical phase and the data of CH2 is superimposed on the light intensity.
[0040]
[Fourth Embodiment]
An optical transmitter according to a fourth embodiment of the present invention will be described. FIG. 4 is a diagram illustrating an example of an optical transmitter according to a fourth embodiment of the present invention, and is an example of an optical transmitter that generates an optical intensity / phase hybrid encoded optical signal. The optical transmitter shown in FIG. 4 has the amplitude of the CW light source 40 that generates continuous light, the push-pull type MZ modulator 44 generated on the LN substrate (Z-cut) 43, and the CH1 electric signal. The amplitude control means 41 controls according to the logic of the CH2 electrical signal, and the amplitude control means 42 controls the amplitude of the inverted electrical signal of CH1 according to the logic of the inverted electrical signal of CH2.
[0041]
The operation of the optical transmitter shown in FIG. 4 basically follows the same operating principle as that of the optical transmitter according to the third embodiment of the present invention shown in FIG. The optical transmitter shown in FIG. 4 is characterized in that the drive amplitude to be applied to the electrode can be almost halved by driving the push-pull type MZ modulator 44 in pairs with the inverted signal. .
[0042]
[Fifth Embodiment]
An example of the optical transmitter according to the fifth embodiment of the present invention will be described. FIG. 5 is a diagram illustrating an example of an optical transmitter according to a fifth embodiment of the present invention, which is an example of an optical transmitter that generates an optical intensity / phase hybrid encoded optical signal. The optical transmitter shown in FIG. 5 includes a CW light source 50 that generates continuous light, two MZ modulators (DPSK) 52 and an MZ modulator (IM) 53 generated on an LN substrate (X-cut) 51. Is done.
[0043]
The operation of the optical transmitter shown in FIG. 5 will be described. In the first stage MZ modulator 52, the input continuous light is DPSK encoded by the CH1 signal by setting the bias voltage in the vicinity of the minimum transmittance. At this time, the drive amplitude is driven at about twice the half-wave voltage of the MZ modulator 52.
[0044]
In the subsequent MZ modulator 53, intensity modulation (IM) is performed with the CH2 signal by setting the bias voltage in the vicinity of the midpoint between the minimum and maximum transmittance. At this time, the drive amplitude is driven at about the half-wave voltage of the MZ modulator 53. As a result, the optical signal output from the two-stage MZ modulator is an optical intensity / phase hybrid encoded optical signal equivalent to that shown in FIG.
[0045]
[Sixth Embodiment]
An example of the optical transmitter according to the sixth embodiment of the present invention will be described. FIG. 6 is a diagram illustrating an example of an optical transmitter according to a sixth embodiment of the present invention, which is an example of an optical transmitter that generates an optical intensity / phase hybrid encoded optical signal. The optical transmitter shown in FIG. 6 includes a CW light source 60 that generates continuous light, two push-pull type MZ modulators (DPSK) 62 generated on an LN substrate (Z-cut) 61, and an MZ modulator (IM ) 63.
[0046]
The operation of the optical transmitter shown in FIG. 6 basically follows the same operation principle as that of the optical transmitter according to the fifth embodiment of the present invention shown in FIG. In the optical transmitter shown in FIG. 6, the first-stage push-pull MZ modulator 62 sets the bias voltage in the vicinity of the minimum transmittance, thereby DPSK-encoding the input continuous light by the CH1 signal. At this time, the drive amplitude per electrode is driven by the half-wave voltage of the MZ modulator 62.
[0047]
In the subsequent MZ modulator 63, intensity modulation (IM) is performed by the CH2 signal by setting the bias voltage in the vicinity of the midpoint of the minimum and maximum transmittance. At this time, the drive amplitude is driven at about ½ of the half-wave voltage of the MZ modulator 63. As a result, the optical signal output from the push-pull type MZ modulator having the two-stage configuration is an optical intensity / phase hybrid encoded optical signal equivalent to that shown in FIG. The optical transmitter according to the sixth embodiment shown in FIG. 6 is characterized in that the drive voltage per electrode can be almost halved as compared with the optical transmitter shown in FIG.
[0048]
[Seventh embodiment]
An example of the optical transmitter according to the seventh embodiment of the present invention will be described. FIG. 7 is a diagram illustrating an example of an optical transmitter according to a seventh embodiment of the present invention, which is an example of an optical transmitter that generates an optical intensity / phase hybrid encoded optical signal. The optical transmitter shown in FIG. 7 controls the amplitude of a pulsed light source 70 that generates pulsed light, the MZ modulator 72 generated on the LN substrate (X-cut) 71, and the CH1 electric signal according to the logic of CH2. It is comprised from the amplitude control means 73 to do.
[0049]
The operation of the optical transmitter shown in FIG. 7 basically follows the same operation principle as that of the optical transmitter according to the third embodiment of the present invention shown in FIG. The optical transmitter shown in FIG. 7 is characterized in that an RZ light intensity / phase hybrid encoded optical signal can be generated by using a pulsed light source as compared with the optical transmitter shown in FIG.
[0050]
[Eighth Embodiment]
Next, an example of an optical receiver according to the eighth embodiment of the present invention will be described. FIG. 8 is a diagram showing an example of an optical receiver according to an eighth embodiment of the present invention, which receives an optical intensity / phase hybrid encoded optical signal and separates multiplexed signals. It is an example.
[0051]
The optical receiver shown in FIG. 8 includes an optical amplifier 80 for amplifying an optical signal that has propagated through a transmission line and has a reduced intensity, an optical demultiplexer 81 that branches the amplified optical signal, and one optical signal that has been branched. A direct-detection photoelectric converter 82, the other optical signal as input, a 1-bit delayed MZ interferometer 83 that converts the intensity of the DPSK signal component, and two photoelectric conversions that receive the balance between the two converted optical intensity signals And an electric amplifier 86 that differentially amplifies signals from the photoelectric converter 84 and the photoelectric converter 85.
[0052]
The operation of the optical receiver shown in FIG. 8 will be described with reference to FIGS. First, CH2 which is an intensity modulation component is received by directly detecting as it is. On the other hand, CH1 which is a DPSK modulation component is input to a 1-bit delay type MZ interferometer, whereby the phase modulation component is converted into an intensity modulation component as shown in FIG. In this interferometer, the optical signal divided into two arms is combined with a delay difference of 1 bit, thereby realizing conversion. In this converted output, in addition to the phase modulation component, an intensity modulation component is also mixed, so that various intensity patterns are generated at the output of the 1-bit delay MZ interferometer. FIG. 15 shows an arrangement of patterns generated at that time.
[0053]
To 1-bit delay type MZ interferometer 83 input As shown in FIG. 15A, for example, when the intensity is 1 and X as shown in FIG. 15A, the output intensity is as shown in FIG. 15B when there is no phase inversion between adjacent bits in the input pattern. If there are three types {2X, 1 + X, 2} and there is a phase inversion as shown in FIG. 15C, there are two types {X-1, 0}. Here, since the CH1 signal component is superimposed on the presence / absence of this phase inversion, the CH1 signal can be demodulated if this presence / absence is determined. Therefore, the SNRs of the CH1 and CH2 signals superimposed on the optical phase and the optical intensity are respectively expressed by the following equations. The SNR mentioned here means the ratio of the optical phase to the total light intensity amplitude and the dynamic range of the light intensity modulation component. This dynamic range refers to the minimum value of the fluctuation range of the light intensity due to the logical amplitude of the digital optical signal.
SNR (phase) = (2- (X-1)) / 2X) Formula (1)
SNR (intensity) = (X−1) / X Equation (2)
However, X> 1 here.
[0054]
As can be seen from this equation, in order for this scheme to function at least, that is, to achieve multiplexing of two channels, SNR> 0, and X is
1 <X <3 Formula (3)
Must be in the range. By the way, the condition that the SNR of the light intensity component and the optical phase component are equal is that the equation (1) is equal to the equation (2) and the equation is solved.
X = 5/3 Formula (4)
It becomes. From this, in the invention described in the first embodiment, the multiplexing ratio of the asymmetrical multiplexer that satisfies Equation (4) can be calculated. The legal ratio is
α: 1-α Formula (5)
If the relative optical phase difference is 0, the intensity is
α + (1−α) = 1 Formula (6)
When the relative optical phase difference is π,
α- (1-α) = 2α-1 Formula (7)
It becomes. Here, from equation (4), when the intensity modulation component mark and space are {5/3, 1}, respectively, the SNR of the light intensity component and the optical phase component are equal. In the case of Expression (6), that is, when the relative optical phase difference is 0, the light intensity increases. In other words, since it is a mark bit, if the mark side is normalized to be 1, the case of {1, 3/5} is obtained. Therefore, if 3/5 and equation (7) are equal,
α = 4/5 Formula (8)
Is obtained.
[0055]
That is, in the first embodiment (see FIG. 1), if the signals are multiplexed at a ratio of 4: 1, an SNR having the same light intensity component and optical phase component is obtained. However, in this calculation, it is assumed that balanced reception is not used when optical phase components are received. Assuming balanced reception, equation (1) becomes
SNR (phase) = (2- (X−1)) / X Formula (1) ′
It becomes. In the case of balanced reception, the above calculation may be performed using Equation (1) ′.
[0056]
[Ninth Embodiment]
An example of an optical receiver according to the ninth embodiment of the present invention will be described. FIG. 9 is a diagram showing an example of an optical receiver according to a ninth embodiment of the present invention, which receives an optical intensity / phase hybrid encoded optical signal and separates multiplexed signals. It is an example. The optical receiver shown in FIG. 9 includes an optical amplifier 90 for amplifying an optical signal that has propagated through a transmission line and has a reduced intensity, an optical demultiplexer 91 that branches the amplified optical signal, and one optical signal that has been branched. A photoelectric converter 92 for directly detecting the signal, a 1-bit delay type MZ interferor 94 for inputting the other optical signal and converting the intensity of the DPSK signal component, and two photoelectric conversions for receiving the balance between the two converted optical intensity signals And a decoding circuit 93 for reproducing a correct code pattern.
[0057]
The operation of the optical receiver shown in FIG. 9 basically follows the same operating principle as that of the optical receiver of the eighth embodiment shown in FIG. The feature of the optical receiver shown in FIG. 9 is that the optical transmitter on the receiving side (see FIG. 8) is used when the precoding circuit provided in the optical transmitter of the first embodiment (see FIG. 1) is not used. If the optical signal is received as it is, the correct code pattern cannot be received. Therefore, the optical receiver has a decoding circuit 93 for converting it into a correct code string.
[0058]
Although the embodiments of the present invention have been described above, the optical communication method, the optical transmitter, and the optical receiver of the present invention are not limited to the illustrated examples described above, and do not depart from the gist of the present invention. Of course, various changes can be made within the range.
[0059]
【The invention's effect】
As described above, in the optical communication method of the present invention, independent digital signals are simultaneously superimposed and transmitted on the light intensity and optical phase of communication light, and are superimposed on these light intensity and optical phase on the receiving side. Separate and receive independent digital signals.
Thereby, the transmission capacity can be doubled without increasing the operating speed of the electric circuit. Also, the frequency utilization efficiency can be dramatically increased. For this reason, high efficiency and low cost of the optical link system can be achieved.
[0060]
In the optical communication method of the present invention, when the intensity of the light intensity component of communication light is X and 1, X satisfies 1 <X <3.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
[0061]
In the optical transmitter of the present invention, independent digital signals are simultaneously superimposed and transmitted on the light intensity and optical phase of communication light.
Thereby, it is possible to configure an optical link system that doubles the transmission capacity without increasing the operation speed of the electric circuit. Also, the frequency utilization efficiency can be dramatically increased. For this reason, high efficiency and low cost of the optical link system can be achieved.
[0062]
In the optical transmitter of the present invention, when the intensity of the light intensity component of the communication light is X and 1, X satisfies 1 <X <3.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
[0063]
Further, in the optical transmitter of the present invention, the continuous light of the CW light source that generates continuous light is branched, each of which is phase-modulated, and two phase-modulated optical signals are combined with an asymmetrical multiplexing ratio, Due to the interference effect of light at the time of multiplexing, independent digital signals are simultaneously superimposed and transmitted on the light intensity and optical phase of communication light.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
[0064]
Further, in the optical transmitter of the present invention, the continuous light of the CW light source that generates continuous light is branched, each of which is phase-modulated, and two phase-modulated optical signals are multiplexed with an asymmetrical multiplexing ratio. In addition, a multiplexing ratio adjuster that arbitrarily varies the multiplexing ratio is provided.
Thereby, the signal-to-noise ratio (SNR) of the intensity modulation component and the SNR of the phase modulation component can be relatively changed.
[0065]
In addition, the optical transmitter of the present invention includes a precoding circuit that converts the digital signal in advance so that the logic of the digital signal is correctly reflected in each of the generated light intensity and optical phase components.
Thereby, a code pattern in which the logic of the digital signal is correctly reflected can be generated. Therefore, no special decoding circuit is required on the receiver side.
[0066]
In the optical transmitter of the present invention, two digital signals are input, and a multiplexed digital signal in which the logic amplitude of one digital signal is controlled based on the logic of the other digital signal is generated. The signal drives a Mach-Zehnder modulator biased at or near the zero transmission point.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
[0067]
In the optical transmitter of the present invention, two digital signals are input A multiplexed digital signal in which the logic amplitude of one digital signal is controlled based on the logic of the other digital signal and its inverted signal are generated. A push-pull type Mach-Zehnder modulator biased in the vicinity is driven.
As a result, the drive amplitude to be applied to the electrode of the Mach-Zehnder modulator is almost halved. Make It becomes possible.
[0068]
The optical transmitter of the present invention includes a CW light source and two serial Mach-Zehnder modulators, phase-modulates the first-stage Mach-Zehnder modulator using the first digital signal, and uses the second digital signal to perform the next-stage Mach-Zehnder modulator. Intensity-modulates the Mach-Zehnder modulator.
Thereby, independent digital signals can be simultaneously superimposed on the light intensity and the optical phase of the communication light.
[0069]
The optical transmitter according to the present invention further includes a CW light source and two serial push-pull Mach-Zehnder modulators, phase-modulates the first-stage push-pull Mach-Zehnder modulator using a first digital signal, The next-stage push-pull Mach-Zehnder modulator is intensity-modulated by a digital signal.
As a result, the drive amplitude to be applied to the electrode of the Mach-Zehnder modulator is almost halved. Make It becomes possible.
[0070]
In the optical transmitter of the present invention, a pulse light source that generates pulsed light synchronized with the bit rate is provided instead of the CW light source, and an RZ signal is transmitted.
Thereby, the RZ light intensity / phase hybrid encoded optical signal can be generated.
[0071]
The optical receiver of the present invention separates and receives independent digital signals simultaneously superimposed on the light intensity and optical phase of communication light.
Thereby, it is possible to configure an optical link system that doubles the transmission capacity without increasing the operation speed of the electric circuit. Also, the frequency utilization efficiency can be dramatically increased. For this reason, high efficiency and low cost of the optical link system can be achieved.
[0072]
In the optical receiver of the present invention, one optical signal branched by the optical demultiplexer is directly detected by the photoelectric converter, and the remaining one optical signal is input to the 1-bit delay type Mach-Zehnder interferometer. The two optical signals output from the interferometer are directly and independently detected by the two photoelectric converters, and the output signals from the two photoelectric converters are differentially amplified.
Thereby, it is possible to separate and receive independent digital signals simultaneously superimposed on the light intensity and optical phase of communication light.
[0073]
The optical receiver of the present invention further includes a decoding circuit that performs code conversion so that the logic of the electrical signal extracted from each of the light intensity component and the optical phase component of the communication light matches the logic before transmission.
This realizes a system configuration that does not use a precoding circuit on the optical transmitter side.
[0074]
The optical receiver of the present invention further includes an optical amplifier that amplifies the optical signal intensity of the input communication light.
Thereby, it is possible to give the optical receiver good reception performance.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an example of an optical transmitter according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating an example of an optical transmitter according to a second embodiment of the present invention.
FIG. 3 is a diagram illustrating an example of an optical transmitter according to a third embodiment of the present invention.
FIG. 4 is a diagram illustrating an example of an optical transmitter according to a fourth embodiment of the present invention.
FIG. 5 is a diagram illustrating an example of an optical transmitter according to a fifth embodiment of the present invention.
FIG. 6 is a diagram illustrating an example of an optical transmitter according to a sixth embodiment of the present invention.
FIG. 7 is a diagram illustrating an example of an optical transmitter according to a seventh embodiment of the present invention.
FIG. 8 is a diagram illustrating an example of an optical receiver according to an eighth embodiment of the present invention.
FIG. 9 is a diagram illustrating an example of an optical receiver according to a ninth embodiment of the present invention.
FIG. 10 is an operation explanatory diagram of an asymmetric multiplexing unit of the optical transmitter of the present invention.
FIG. 11 is an operation explanatory diagram of amplitude control means of the optical transmitter of the present invention.
FIG. 12 is an operation explanatory diagram of an MZ modulator of the optical transmitter according to the present invention.
FIG. 13 is an operation explanatory diagram of a precoding circuit in the optical transmitter of the present invention.
FIG. 14 is an operation explanatory diagram of a 1-bit delay type MZ interferometer of the optical receiver of the present invention.
FIG. 15 is an explanatory diagram of a reception operation of the optical receiver of the present invention.
FIG. 16 is a diagram illustrating an example of a conventional technique.
[Explanation of symbols]
10 CW light source
11 Precoding circuit
12 LN substrate
13 Optical branch
14 MZ modulator
16 Asymmetric multiplexing section
20 CW light source
21 Precoding circuit
22 LN substrate
23 Optical branch
24, 25 MZ modulator
26 Asymmetric multiplexing section
27 Combiner ratio adjuster
30 CW light source
31 Amplitude control means
32 LN substrate
33 MZ modulator
40 CW light source
41, 42 Amplitude control means
43 LN substrate
44 MZ modulator
50 CW light source
51 LN substrate
52, 53 MZ modulator
60 CW light source
61 LN substrate
62, 63 MZ modulator
70 Pulse light source
71 LN substrate
72 MZ modulator
73 Amplitude control means
80 Optical amplifier
81 Optical demultiplexer
82 Photoelectric converter
83 1-bit delay type MZ interferometer
84, 85 Photoelectric converter
86 Electric amplifier
90 Optical amplifier
91 Optical demultiplexer
92 Photoelectric converter
93 Decode circuit
94 1-bit delay type MZ interferometer
95, 96 Photoelectric converter
97 Electric amplifier

Claims (6)

A CW light source that generates continuous light;
Two phase modulators for branching the continuous light of the CW light source and phase modulating each of the continuous light;
An asymmetric multiplexing unit that multiplexes the two phase-modulated optical signals at an asymmetric multiplexing ratio;
An optical transmitter characterized in that an independent digital signal is simultaneously superimposed and transmitted on the optical intensity and optical phase of communication light due to the interference effect of light at the time of multiplexing. A CW light source that generates continuous light;
Two phase modulators for branching the continuous light and phase-modulating each of them;
Asymmetric which combines the two optical signals the phase-modulated in an asymmetric multiplexing ratio,
And a multiplexing ratio adjuster that arbitrarily varies the multiplexing ratio,
An optical transmitter characterized in that an independent digital signal is simultaneously superimposed and transmitted on the optical intensity and optical phase of communication light due to the interference effect of light at the time of multiplexing. A CW light source that generates continuous light;
Amplitude control means for taking two digital signals as inputs and controlling the logical amplitude of one digital signal based on the logic of the other digital signal;
A Mach-Zehnder type modulator,
Means for driving the Mach-Zehnder type modulator biased at or near the transmission zero point by the multiplexed digital signal generated by the amplitude control means ,
An optical transmitter characterized in that an independent digital signal is simultaneously superimposed and transmitted on the light intensity and optical phase of communication light . A CW light source that generates continuous light;
First amplitude control means for taking two digital signals as inputs and controlling the logical amplitude of one digital signal based on the logic of the other digital signal;
Second amplitude control means for controlling the logical amplitude of one digital signal based on the logic of the other digital signal, with the inverted signals of the two digital signals as inputs ;
Push-pull type Mach-Zehnder type modulator,
Means for driving the push-pull type Mach-Zehnder modulator biased at or near the transmission zero point by the multiplexed digital signal generated by the amplitude control means and its inverted signal;
An optical transmitter characterized in that an independent digital signal is simultaneously superimposed and transmitted on the light intensity and optical phase of communication light . 5. The optical transmitter according to claim 1, wherein the light intensity of the communication light has two values of X and 1 , and X satisfies 1 <X <3. Instead of the CW light source, a pulsed light source that generates pulsed light synchronized with the bit rate is used,
6. The optical transmitter according to claim 1, wherein an RZ signal is generated and transmitted by the pulse light source.

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