JP4251355B2 - Optical transmitter - Google Patents

Description

  The present invention relates to an optical transmission apparatus of an optical transmission system that transmits an optical signal using an optical fiber, and more particularly to an optical transmission apparatus that realizes a large capacity, high speed, and long distance of information transmission.

Conventionally, as a technical obstacle in increasing the distance of information transmission by optical signals, waveform deterioration of optical signals due to a combined effect of chromatic dispersion and nonlinear effects in an optical fiber as an optical transmission line has been pointed out.
With chromatic dispersion, optical pulses of an optical signal are not strictly single wavelengths but have a certain width, and the propagation speed differs depending on the wavelength, so successive optical pulses interfere with each other and increase the communication speed. This phenomenon is a limiting factor.
Nonlinear effects include self-phase modulation in single-wavelength optical transmission, four-wave mixing and cross-phase modulation in optical wavelength division multiplex transmission as phenomena showing this. Any phenomenon causes interference between channels adjacent to the optical signal transmission channel, and becomes a limiting factor of DWDM (High Density Wavelength Division Multiplexing) for the purpose of increasing transmission capacity.

In self-phase modulation, optical phase modulation occurs in the optical signal itself by changing the refractive index of the optical fiber according to the intensity of the transmitted optical signal, the optical frequency of the rising portion of the optical signal is lower than the optical carrier frequency, This is a phenomenon in which the frequency of the falling portion of the optical signal increases.
Four-wave mixing and cross-phase modulation are phenomena that occur due to interaction between optical signals when a plurality of optical signals having different wavelengths propagate through an optical fiber. Due to this interaction, in four-wave mixing, a phenomenon in which an optical signal component corresponding to the wavelength difference of each optical signal is newly generated is observed. In cross-phase modulation, a phenomenon in which optical phase modulation occurs in each optical signal. Is observed.

  As conventional means for improving the waveform deterioration due to the combined effect of the nonlinear effect and chromatic dispersion of such an optical fiber, there are known techniques shown below. First, as a technique for suppressing chromatic dispersion, a method is known in which a dispersion value of an optical fiber is set to be large and a dispersion compensator having a dispersion value opposite to that of the optical fiber is arranged at a certain distance (for example, non-dispersion). Patent Document 1).

  Further, a CRZ (Chirped Return to Zero) modulation method has been proposed as means for suppressing the nonlinear effect of an optical fiber (for example, Patent Document 1). Further, a CS-RZ (Carrier Suppressed Return to Zero) modulation system has been proposed as an optical modulation system that narrows the optical spectrum of an optical signal to suppress chromatic dispersion or suppress interference between channels.

FIG. 7 is a block diagram showing a configuration of a conventional optical transmission device 10c in order to explain the CRZ method and the CS-RZ method, which are conventional optical modulation methods.
The random data generator 13 'generates a digital signal "0" or "1" based on the oscillation frequency f of the clock oscillator 14'.
The intensity modulator 12 ′ modulates the intensity of the CW light (continuous light) output from the light source 11 ′ in response to the generated “0” and “1” signals to generate an optical signal S composed of a sequence of optical pulses. 'Is formed.

  The phase modulator 19 ′ has an action of optical phase modulating the optical carrier constituting the optical pulse of the optical signal S ′. Further, since the phase modulator 19 ′ operates with a sine signal c ′ based on the oscillation frequency f of the clock oscillator 14 ′, the optical phase is given optical phase modulation having a uniform sinusoidal distribution. The phase shifter 17 ′ changes the phase of the sine signal c ′ that operates the phase modulator 19 ′. The phase shifter 17 ′ changes the optical phase modulation distribution applied to the optical pulse and transmits the optical signal S transmitted through the optical fiber. The improvement level of the waveform deterioration of ′ can be adjusted.

  Here, when the repetition frequency of the sine signal c ′ for operating the phase modulator 19 ′ is made to coincide with the oscillation frequency f of the clock oscillator 14 ′, the CRZ modulation method is adopted, and one half of the oscillation frequency f (f / 2). ), The CS-RZ modulation method is used.

Hideki Maeda and three others, “Signal Waveform Optimization in Long-distance Optical Amplified Repeater Transmission System”, IEICE Technical Report, IEICE, November 4, 1997, Vol.97 No.357 OCS97-44 JP-A-11-252013 (paragraphs 0015 to 0032, FIG. 1)

  However, the conventional means for improving the waveform deterioration of the optical signal described above has the following problems. In other words, in the method of disposing a dispersion compensator, the optical signal relay interval is determined by the inter-station distance, so that the chromatic dispersion value varies for each relay section, and further varies due to temperature changes. . For this reason, it has been necessary to optimize the dispersion compensation amount at a plurality of points following such variation with time.

In the CRZ modulation method, although the nonlinear effect of the optical fiber is suppressed, it is insufficient for suppressing the spread of the optical spectrum. Therefore, even if the CRZ modulation method is applied to long-distance transmission in DWDM (High Density Wavelength Multiplexing Transmission System), it does not provide a sufficient solution to the problem of crosstalk in which adjacent channels interfere.
Since the CS-RZ modulation method has an effect of narrowing the optical spectrum of the optical signal, it is effective in avoiding the above-described problem of crosstalk for long-distance transmission in DWDM. However, since the CS-RZ modulation method does not perform optical phase modulation synchronized with each optical pulse, the nonlinear phenomenon of the optical fiber cannot be suppressed. Therefore, the CS-RZ modulation method is not a sufficient solution for signal quality degradation associated with long-distance transmission of optical signals.

  The present invention has been made for the purpose of solving the above-mentioned problems, and suppresses the waveform deterioration of an optical signal in a long-distance transmission by a DWDM (Dense Wavelength Division Multiplexing) system, and achieves a stable, high speed and high quality. It is an object of the present invention to provide an optical transmission device capable of communication.

  The present invention was devised to achieve the above-described object. First, the optical transmission device according to claim 1 is provided at one end of an optical transmission path for transmitting an optical signal, and outputs the optical signal. An optical transmission apparatus for transmitting to the other end of the optical transmission line includes an oscillator (oscillation frequency f) and a phase modulator, and the phase modulator applies a frequency f / to a first sine signal of the frequency f. The optical carrier is optically phase-modulated periodically based on an addition signal obtained by adding two second sine signals.

  According to this configuration, the CW light (continuous light) is intensity-modulated by the oscillation frequency f of the oscillator, and an optical signal composed of a train of optical pulses is formed. The phase modulator periodically optically modulates an optical carrier in an optical pulse based on an addition signal formed by adding a first sine signal having a frequency f and a second sine signal having a frequency f / 2. As a result, the optical signal is optically phase-modulated so that the optical frequency of the first half of the optical pulse is higher than the optical carrier frequency and the optical frequency of the second half of the optical pulse is lower than the optical carrier frequency every other optical pulse. The Accordingly, the optical spectrum of the optical signal is narrowed, and chromatic dispersion and nonlinear effects that occur when the optical signal is transmitted through the optical fiber are also suppressed. That is, a new light modulation method that has the advantages of both the CRZ modulation method and the CS-RZ modulation method can be obtained.

Further optical transmission apparatus according to claim 1, characterized in that it has a first amplitude adjuster, a first phase shifter, a second amplitude adjuster, a second phase shifter, a.

  According to this configuration, the first amplitude adjuster and the second amplitude adjuster independently adjust the amplitudes of the first sine signal and the second sine signal, respectively. The first phase shifter and the second phase shifter independently adjust the phases of the first sine signal and the second sine signal, respectively. Thereby, the addition signal can arbitrarily set the amplitude ratio and the phase difference between the first sine signal and the second sine signal which are constituent components. Therefore, it becomes possible to adjust and optimize the optical transmission apparatus in accordance with the optical communication system to which the present invention is applied.

The optical transmission device according to claim 2 is the optical transmission device according to claim 1 , wherein an amplification factor of the second amplitude adjuster is set larger than an amplification factor of the first amplitude adjuster. And

  According to such a configuration, the degree of phase modulation optically modulated by the oscillation frequency f / 2 is greater than the degree of phase modulation optically modulated by the oscillation frequency f. Thereby, the carrier component at the center of the optical spectrum is suppressed, and an effect of promoting narrowing of the optical spectrum can be obtained.

The optical transmitter according to the present invention has the following excellent effects.
That is, when an optical signal is transmitted through an optical fiber, chromatic dispersion and non-linear effects that cause problems of waveform deterioration of the optical pulse are suppressed, and the optical spectrum is further narrowed. This enables high-speed and stable high-quality communication in long-distance transmission using the DWDM (Dense Wavelength Division Multiplexing) method.

Embodiments of the present invention will be described below with reference to the drawings.
(First embodiment)
A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram showing a basic configuration of an optical transmission system including an optical transmission apparatus in the first embodiment.

The optical transmission system 1A includes an optical transmission device 10a that transmits an optical signal S, an optical transmission path 20 that transmits the optical signal S, and an optical receiver 30a that receives the transmitted optical signal S.
The optical transmission device 10a includes a light source 11, an intensity modulator 12, a random data generator 13, a clock oscillator (oscillator) 14, a 1/2 frequency divider 15, amplitude adjusters 16a and 16b, phase shifters 17a and 17b, and an adder. 18 and a phase modulator 19.

The light source 11 outputs CW light (continuous light) composed of an optical carrier having a wavelength λ ₀ (here, λ ₀ = 1550 nm).

  The intensity modulator 12 modulates the intensity of the CW light output from the light source 11 in correspondence with the signals “0” and “1” generated by the random data generator 13 to generate an optical signal S composed of a sequence of optical pulses. Is generated. Here, as the modulation method of intensity modulation, an RZ (Return to Zero) method is employed in which the end of a pulse is temporarily returned to zero so that adjacent pulses do not continue.

  The random data generator 13 generates digital signals “1” and “0” based on the clock signal c 1 (frequency f) generated by the clock oscillator 14. Here, it is assumed that signals of “1” and “0” are randomly generated so that the bit rate becomes 10 Gbit / sec corresponding to the clock frequency f. Thus, the intensity modulation frequency of the optical signal S is determined in accordance with the oscillation frequency f of the clock oscillator 14.

  The 1/2 frequency divider 15 divides the clock frequency f of the clock signal c1 by 1/2 and outputs a clock signal c2 (frequency f / 2) that is phase-synchronized with the clock signal c1. .

The amplitude adjuster 16a and the phase shifter 17a generate a first sine signal c3 (Asin (2πft−φ)) having an amplitude A and a phase delay φ based on the frequency f of the clock signal c1.
The amplitude adjuster 16b and the phase shifter 17b generate a second sine signal c4 (Bsin (πft−θ)) having an amplitude B and a phase delay θ based on the frequency f / 2 of the clock signal c2 (t: time). ).
The adder 18 generates an addition signal c5 obtained by adding the first sine signal c3 and the second sine signal c4 and applies the addition signal c5 to the phase modulator 19.

  The phase modulator 19 periodically optically modulates the optical carrier of the optical signal S input from the intensity modulator 12 based on the addition signal c5. Note that the optical carrier constituting the optical pulse is optically phase-modulated corresponding to the displacement of the addition signal c5, and the frequency of the optical carrier changes corresponding to the degree of optical phase modulation.

The optical transmission line 20 includes an optical fiber 21 and an optical amplifier 22 as shown in FIG.
Here, it is assumed that the optical fiber 21 is provided with an interval of 200 km between the optical transmitter 10a and the optical receiver 30a, and is a single mode fiber (SMF) having a zero-dispersion wavelength of 1.3 μm.
The optical amplifier 22 optically amplifies the optical signal S attenuated due to the transmission loss of the optical fiber 21. The optical amplifier 22 is, for example, a distributed Raman amplifier that superimposes and amplifies excitation light having a wavelength shorter than the wavelength of the optical signal S by about 100 nm on the optical signal S, or a concentrated Raman amplifier.

  The optical receiver 30 a receives the optical signal S transmitted from the optical transmission device 10 a and transmitted through the optical transmission path 20.

  Next, the operation of the optical transmission device 10a will be described based on FIG. 1 and FIG. 2A and 2B are explanatory diagrams showing the relationship between the light intensity and the optical phase modulation of the optical signal S, where FIG. 2A is a CRZ modulation method, FIG. 2B is a CS-RZ modulation method, and FIG. It is a graph which shows this light modulation system.

  FIG. 2A is a graph showing the case where only the first sine signal c3 having a phase delay of φ = 0 in FIG. In this case, the relative phase difference between the optical intensity and the optical phase modulation in the optical signal S is such that the optical frequency of the first half of the optical pulse is higher than the carrier frequency, as shown in FIG. This is a CRZ modulation method in which the frequency is lower than the optical carrier frequency.

  FIG. 2B is a graph showing the case where only the second sine signal c4 having a phase delay of θ = 0 in FIG. 1 is applied to the phase modulator 19. In this case, the CS-RZ modulation method is employed in which the optical carrier phases between adjacent optical pulses of the optical signal S are reversed (reverse phase).

  In FIG. 2C, the amplification factors A and B are adjusted by the amplitude adjusters 16a and 16b in FIG. 1 so that B = 2A, and the phase delays of the phase shifters 17a and 17b are φ = 0 and θ =. 6 is a graph showing a case where the first sine signal c3 and the second sine signal c4, which are π / 2, are added and applied to the phase modulator 19. In this case, the relative phase difference between the optical intensity and the optical phase modulation in the optical signal S is such that the optical frequency of the first half of the optical pulse is higher than the optical carrier frequency every other optical pulse, and the optical frequency of the second half of the optical pulse is This is a novel optical modulation method in which optical phase modulation is performed so as to be lower than the optical carrier frequency.

  Here, the amplification factor A is determined so that the phase modulation degree m = π / 4. This is experimentally clear that, in the CRZ modulation system, when the phase modulation degree m = π / 4, the suppression function against the waveform deterioration of the optical pulse due to the nonlinear effect of the optical fiber is exhibited to the maximum. (See JP-A-11-252013, FIG. 2, etc.). Further, the amplification factor B of the amplitude adjuster 16b is set to be larger than the amplification factor A (here, twice). This is because the carrier component at the center of the optical spectrum is suppressed by making the phase modulation degree optical phase modulated by the clock frequency f / 2 larger than the phase modulation degree optical phase modulated by the clock frequency f. This is because the effect is obtained.

  As described above, this embodiment is an example for explaining the present invention, and the present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the gist of the invention. In particular, the amplification factors A and B and the phase delays θ and φ in the process of forming the addition signal c5 are not limited to the values described above, so that the transmission of the optical signal S is most stable and of high quality. The optimum value may be determined experimentally.

(Second embodiment)
Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a configuration diagram showing a basic configuration of an optical wavelength division multiplexing transmission system including an optical transmission device according to the second embodiment. 3 that are the same as or correspond to those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  The optical wavelength division multiplexing transmission system 1B includes an optical wavelength division multiplexing transmission terminal station (optical transmission device) 10b, an optical transmission line 20, and an optical wavelength division multiplexing reception terminal station 30b. The optical wavelength division multiplex transmission terminal 10b includes light sources 11a ... 11d, intensity modulators 12a ... 12d, phase modulators 19a, 19b, and multiplexers 41a, 41b. The random data generator 13, the clock oscillator 14, the 1/2 frequency divider 15, the amplitude adjusters 16a and 16b, the phase shifters 17a and 17b, and the adder 18 shown in FIG. 1 are provided for each phase modulator 19a and 19b. It is assumed that it is installed in, and the description is omitted.

Here, the light sources 11a... 11d output CW light having wavelengths λ ₁ , λ ₂ , λ ₃ , and λ ₄ . Each wavelength is set at regular intervals (wavelength interval 1 nm) around the wavelength λ ₀ (= 1550 nm). Here, λ ₁ and λ ₂ are shorter than λ ₀ , and λ ₃ and λ ₄ are longer than λ ₀ . The CW light of each wavelength is intensity-modulated using a 10 Gbit / s RZ random pulse signal by the intensity modulators 12a.

Then, the intensity-modulated signal lights having the wavelengths λ ₁ and λ ₂ are multiplexed by the multiplexer 41a and optical phase modulated by the phase modulator 19a. The intensity-modulated signal lights having the wavelengths λ ₃ and λ ₄ are multiplexed by the multiplexer 41b and optical phase modulated by the phase modulator 19b. The intensity-modulated signal light optically phase-modulated by the phase modulators 19a and 19b is multiplexed by the multiplexer 17c to become a multiplexed optical signal R.

Then, the multiplexed optical signal R output from the multiplexer 17c is transmitted to the optical wavelength multiplexing receiving terminal station 30b through the optical transmission line 20.
WDM receiving terminal 30b includes a demultiplexer 31 for demultiplexing each wavelength lambda every ₁ ... lambda ₄ multiplexed wave optical signal R, an optical receiver 33a for receiving the respective wavelengths λ ₁ ... λ ₄ of the signal light, respectively ... 33d.

  In this way, the multiplexed optical signal R is demultiplexed for each wavelength and is received by a separate optical receiver. And the optical spectrum of each received optical signal is narrowed, so the problem of crosstalk is alleviated, and the problem of chromatic dispersion and nonlinear effect due to transmission is also avoided, so the large capacity with excellent signal quality High-speed communication.

  Next, an embodiment in which the effect of the present invention has been confirmed will be described with reference to FIGS. 4 and 6. FIG. 4 is a graph showing an optical spectrum of the optical signal S received by the optical receiver 30a (FIG. 1). Here, FIG. 4A shows the case of the CRZ system which is a conventional modulation system. FIG. 4B shows a case where the CS-RZ method, which is a conventional modulation method, is used. FIG. 4C shows a case where a novel optical modulation system based on the addition signal c5 by the optical transmission apparatus of the present invention is used.

As shown in FIG. 4, the optical spectrum of the received optical signal S is represented as an aggregate of a large number of discrete line spectra in which the wavelength of the central optical carrier is λ ₀ . Here, when spectral widths (10 dB widths) w _a , w _b , and w _c in each method are examined, as shown in FIG. 4C, according to the new modulation method, the optical carrier at the center of the spectrum is obtained. Is suppressed, and w _c = about 0.04 nm. Therefore, the new optical modulation method is equivalent to the conventional CS-RZ method (FIG. 4B: w _b = about 0.04 nm) and the CRZ method (FIG. 4A: w _a = about 0.09 nm). It is clear that a more narrowed light spectrum is realized. Such narrowing of the optical spectrum provides an effect of suppressing mutual interference and chromatic dispersion of adjacent channels in multiwave transmission.
The 10 dB width is a scale indicating the sharpness of the optical spectrum, expressed by the spectral width at an intensity of −10 dB from the peak top value of the optical spectrum.

FIG. 6 is a graph obtained by calculating the eye opening deterioration of an optical signal transmitted by the novel optical modulation system and an optical signal transmitted by the conventional CS-RZ system by the optical transmission apparatus of the present invention by computer simulation.
As shown in FIG. 5A, the eye opening deterioration is the ratio of the eye opening a of the eye pattern of the transmission signal (random signal) to the eye opening b of the eye pattern of the reception signal of FIG. 5B (− 20 log (b / a)). The smaller the numerical value of the eye opening deterioration, the higher the signal quality of the optical signal S received by the optical receivers 30a, 33a (FIGS. 1 and 3).

  As shown in FIG. 6, in the novel optical modulation system using the optical transmission apparatus of the present invention, the eye opening deterioration is smaller than in the conventional CS-RZ. From this, it is shown that the new optical modulation method can transmit a high-quality optical signal with less waveform deterioration by suppressing nonlinear effects due to fiber transmission.

It is a block diagram which shows the basic composition of the optical transmission system containing the optical transmission apparatus in 1st embodiment of this invention. 2A and 2B are explanatory diagrams showing the relationship between the optical intensity and optical phase modulation of an optical signal, where FIG. 2A is a CRZ system, FIG. 2B is a CS-RZ system, and FIG. 2C is a new light according to the present invention. It is a graph which shows a modulation system. It is a block diagram which shows the basic composition of the optical wavelength multiplexing transmission system containing the optical transmission apparatus (optical wavelength multiplexing transmission terminal station) in 2nd embodiment of this invention. The graph which shows the optical spectrum of the received optical signal, (a) is a case by the CRZ system which is a conventional modulation system, (b) is a case by the CS-RZ system which is a conventional modulation system, c) is a graph showing a case in which a novel optical modulation system by the optical transmission apparatus of the present invention is used. It is a figure explaining eye opening deterioration, (a) shows the eye pattern of a transmission signal, (b) is a graph which shows the eye pattern of a received signal. It is the graph which calculated | required the eye opening degradation of the optical signal transmitted by the novel modulation system by the optical transmitter of this invention, and the optical signal transmitted by the conventional CS-RZ system by computer simulation. It is a block diagram which shows the conventional optical transmitter.

Explanation of symbols

10a Optical transmitter 10b Optical wavelength division multiplexing terminal (optical transmitter)
14 Clock oscillator
16a Amplitude adjuster (first amplitude adjuster)
16b Amplitude adjuster (second amplitude adjuster)
17a Phaser (first phaser)
17b Phaser (second phaser)
19 phase modulator 20 optical transmission line c3 first sine signal c4 second sine signal c5 addition signal S optical signal R multiwave optical signal (optical signal)

Claims (2)

An optical transmission device provided at one end of an optical transmission path for transmitting an optical signal, and outputting the optical signal to be transmitted to the other end of the optical transmission path,
An oscillator that oscillates the intensity modulation frequency f of the optical signal;
The phase modulator that periodically optically modulates the optical carrier of the optical signal based on an addition signal obtained by adding the second sine signal of the frequency f / 2 to the first sine signal of the frequency f ;
A first amplitude adjuster for adjusting the amplitude of the first sine signal;
A first phase shifter for adjusting the phase of the first sine signal;
A second amplitude adjuster for adjusting the amplitude of the second sine signal;
A second phase shifter for adjusting the phase of the second sine signal;
Optical transmitter you, comprising a. 2. The optical transmission device according to claim 1 , wherein an amplification factor of the second amplitude adjuster is set larger than an amplification factor of the first amplitude adjuster.

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