JP2010154223A - Optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission system capable of satisfactorily maintaining transmission quality of signal light. <P>SOLUTION: The optical transmission system 1 includes a light-transmitting section 10, an optical transmission line 20, and light-receiving section 30. The light-transmitting section 10 includes transmitters 11, 12 and a multiplexer 13. The transmitter 11 outputs first signal light S<SB>1</SB>of a wavelength λ<SB>1</SB>subjected to intensity modulation by a first transmission signal of a frequency ω<SB>1</SB>. The transmitter 12 outputs second signal light S<SB>2</SB>of a wavelength λ<SB>2</SB>subjected to intensity modulation by a second transmission signal of a frequency ω<SB>2</SB>. The multiplexer 13 multiplexes the signal light S<SB>1</SB>, S<SB>2</SB>outputted from the transmitters 11, 12, respectively, for outputting to the optical transmission line 30. In this case, ω<SB>1</SB><ω<SB>2</SB>, and λ<SB>1</SB>≠λ<SB>2</SB>should be satisfied. The optical transmission line 20 transmits the signal light S<SB>1</SB>, S<SB>2</SB>outputted from the light-transmitting section. The light-receiving section 30 receives the signal light S<SB>1</SB>, S<SB>2</SB>transmitted from the optical transmission line 20 for arrival by one light-receiving section to output first and second transmission signals. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical transmission system in which signal light transmitted from an optical transmitter is transmitted through an optical transmission path and received by an optical receiver.

2. Description of the Related Art There is known an optical transmission system that transmits a plurality of video signals by using a wavelength division multiplexing (WDM) technique. In this optical transmission system, the first signal light having the wavelength λ ₁ modulated with the first transmission signal having the frequency ω ₁ and the second signal light having the wavelength λ ₂ modulated with the second transmission signal having the frequency ω ₂ are used. Multiplexed by the optical transmitter, output to the optical transmission path, and transmitted to the optical receiver through the optical transmission path. Then, the first signal light and the second signal light transmitted through the optical transmission path are received by the optical receiving unit, and the first transmission signal and the second transmission signal are output from the optical receiving unit. However, “ω ₁ ≠ ω ₂ ” and “λ ₁ ≠ λ ₂ ”.

However, in the WDM optical transmission system as described above, crosstalk occurs between a plurality of signal lights during transmission through the optical transmission path. Due to this crosstalk, the first signal light having the wavelength λ ₁ that reaches the optical receiver not only contains the original signal component that is intensity-modulated by the first transmission signal, but also noise that is intensity-modulated by the second transmission signal. Ingredients will be included. For this reason, the transmission quality of signal light in this optical transmission system deteriorates.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical transmission system that can maintain good transmission quality of signal light.

The optical transmission system according to the present invention includes (1) a first signal light having a wavelength λ ₁ that is intensity-modulated by a first transmission signal having a frequency ω _{1 and} a second transmission having a frequency ω ₂ (where ω ₁ <ω ₂ ). An optical transmitter that multiplexes and outputs the second signal light having the wavelength λ ₂ (where λ ₁ ≠ λ ₂ ) that is intensity-modulated with a signal; and (2) the first signal light output from the optical transmitter and An optical transmission path for transmitting the second signal light; and (3) receiving the first signal light and the second signal light transmitted through the optical transmission path with one light receiving unit, and receiving the first transmission signal and And an optical receiver that outputs the second transmission signal. Furthermore, in the optical transmission system according to the present invention, the power of the first signal light output from the optical transmission unit is P ₁ (true number), and the power of the second signal light output from the optical transmission unit is P ₂ ( (True number), the following relationship (1) is satisfied.

In this optical transmission system, the first signal light having the wavelength λ ₁ that has been intensity-modulated by the first transmission signal having the frequency ω ₁ and the second signal light having the wavelength λ ₂ that has been intensity-modulated by the second transmission signal having the frequency ω ₂ are included. , Multiplexed from the optical transmitter and output to the optical transmission line. However, “ω ₁ <ω ₂ ” and “λ ₁ ≠ λ ₂ ”. The first signal light and the second signal light transmitted through the optical transmission path and reaching the optical receiving unit are received by one light receiving unit, and the first transmission signal and the second transmission signal are output. In such an optical transmission system, the transmission quality of the signal light can be kept good by satisfying the relationship of the expression (1).

In the optical transmission system according to the present invention, when the maximum transmission distance from the optical transmission unit to the optical reception unit is L [km] and the power of the first signal light output from the optical transmission unit is P ₁ [W] In addition, it is preferable that the relationship represented by the following formula (2) is satisfied.

The difference between the wavelength λ ₁ of the first signal light and the wavelength λ ₂ of the second signal light is preferably 10 nm or less. Power _{P 1} of the first signal light output from the optical transmission unit is suitable not more than 25.2mW than 10 mW. It is preferred that the power P ₂ of the second signal light output is less than 4mW or more 1mW from the light transmitting unit. In addition, it is preferable that the first transmission signal is a signal of any modulation scheme among AM-VSB, QAM, OFDM and QPSK.

  The optical transmission system according to the present invention can maintain good transmission quality of signal light.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a configuration diagram of an optical transmission system 1 according to the present embodiment. The optical transmission system 1 shown in this figure includes an optical transmitter 10, an optical transmission line 20, and an optical receiver 30. The optical transmission unit 10 includes transmitters 11 and 12 and a multiplexer 13.

The transmitter 11 outputs the first signal light S ₁ having the wavelength λ ₁ that is intensity-modulated by the first transmission signal having the frequency ω ₁ . The transmitter 12 outputs the second signal light S ₂ having the wavelength λ ₂ that is intensity-modulated by the second transmission signal having the frequency ω ₂ . The multiplexer 13 multiplexes the signal lights S ₁ and S ₂ output from the transmitters 11 and 12 and outputs them to the optical transmission line 30. However, “ω ₁ <ω ₂ ” and “λ ₁ ≠ λ ₂ ”.

The optical transmission line 20 transmits the signal lights S ₁ and S ₂ output from the optical transmission unit. Then, the optical receiver 30 receives the signal lights S ₁ and S ₂ transmitted through the optical transmission path 20 and received by one light receiver, and outputs a first transmission signal and a second transmission signal.

  By the way, any one of an external modulation type transmitter and a direct modulation type transmitter is used as the transmitters 11 and 12 for outputting signal light whose intensity is modulated with a transmission signal. The external modulation type transmitter can realize low distortion optical transmission by using an external modulation type intensity modulator having excellent linearity. The external modulation type transmitter also performs stimulated Brillouin scattering (SBS) that occurs in the optical transmission line by performing phase modulation on the signal light using an external modulation type phase modulator or dithering the signal light source. : Stimulated Brillouin Scattering) and the effect of optical interference due to multiple reflections can be improved.

  FIG. 2 is a diagram illustrating an arrangement of intensity modulation frequencies of signal light output from the external modulation type transmitter. The transmission signal to be originally transmitted has a frequency from DC to around 800 MHz. A phase modulation signal for SBS suppression and an RF signal in several GHz band are used for dithering. When an ideal phase modulator is used, the phase modulation signal used for modulation does not exist on the spectrum of intensity modulation, but normally the phase modulator has a residual intensity modulation component. A tone signal spectrum is displayed on the frequency axis. For example, when a BS signal or a CS signal is input to such a transmitter as a transmission signal, beat noise is generated between a tone signal near 2 GHz and the transmission signal, so that the distortion characteristics are deteriorated and desired. The transmission characteristics cannot be obtained.

  On the other hand, a direct modulation transmitter that performs intensity modulation when driving a laser light source is limited in that distortion characteristics are determined by the chirp characteristics of the laser light source. This distortion characteristic also varies depending on the number of channels of the input transmission signal, the modulation degree, and the modulation frequency.

Therefore, an external modulation transmitter having a low distortion characteristic is used as the transmitter 11 that outputs the first signal light S ₁ having the wavelength λ ₁ that is intensity-modulated by the first transmission signal having the low frequency ω ₁ , and the first signal light S ₁ having the high frequency ω ₂ is used. It is preferable to use a direct modulation type transmitter as the transmitter 12 that outputs the second signal light S ₂ having the wavelength λ ₂ that is intensity-modulated with two transmission signals. In this way, an optical transmission system can be constructed even if an optical transmission line or an optical amplifier having a relatively large distortion is used. Low-distortion external modulation transmitters can be used for propagation of low-frequency signal light, which is relatively weak against distortion, so transmission lines with poor distortion characteristics (transmission with large chromatic dispersion and polarization mode dispersion) Or an optical amplifier having a gain slope exceeding 0.1 dB / km.

FIG. 3 is a diagram illustrating an arrangement of intensity modulation frequencies in the optical transmission system 1 according to the present embodiment. FIG. 4A shows the arrangement of intensity modulation frequencies of the first signal light S _{1 having} the wavelength λ ₁ output from the transmitter 11. FIG. 4B shows the arrangement of intensity modulation frequencies of the second signal light S _{2 having} the wavelength λ ₂ output from the transmitter 12. FIG. 3C shows the arrangement of the intensity modulation frequencies of the first signal light S _{1 having} the wavelength λ ₁ input to the optical receiver 30. FIG. 4D shows the arrangement of intensity modulation frequencies of the second signal light S _{2 having} the wavelength λ ₂ input to the optical receiver 30. FIG. 5E shows the arrangement of intensity modulation frequencies of the signal lights S ₁ and S ₂ input to the optical receiver 30.

As shown in FIGS. 4A to 4D, the external modulation type transmitter 11 outputs the first signal light S ₁ having the wavelength λ ₁ that is intensity-modulated with the first transmission signal having the low frequency ω ₁ . To do. The direct modulation type transmitter 12 outputs the second signal light S ₂ having the wavelength λ ₂ that is intensity-modulated by the second transmission signal having the high frequency ω ₂ . However, in this case, it is necessary to pay attention to the following events that are not problematic in single-wave signal light transmission because the optical receiver 30 simultaneously receives the signal light of two wavelengths by the second power detection. That is, in the optical transmission line 20, when the signal light power is high, nonlinear optical phenomena such as power transition due to stimulated Raman scattering (SRS), cross-phase modulation (XPM), and the optical Kerr effect (OKE) occur. A phenomenon occurs in which the frequency component of intensity modulation of one signal light is transferred to the other signal light, and noise is superimposed ((c) and (d) in the figure).

In the optical receiver unit 30, and a first signal beam S ₁ and the second signal light S ₂ wavelengths lambda ₂ wavelength lambda ₁ after demultiplexing, when received by the light receiving portion of each of the signal light detection is, The first transmission signal and the second transmission signal can be obtained with a high S / N ratio. However, when the detection is received by the first signal beam S ₁ and the second signal light having a wavelength lambda ₂ S ₂ and a single light receiving portion of the wavelength lambda ₁ in the optical receiver 30 (FIG. (E)) is Since noise is superimposed on the intensity modulation frequency of each of the first transmission signal and the second transmission signal, the first transmission signal and the second transmission signal cannot be obtained with a high S / N ratio.

The optical transmission system 1 according to the present embodiment can solve the above-described problems after clarifying the relationship between power and crosstalk between two wavelengths of signal light. The first transmission signal and the second transmission signal can be obtained with a high / N ratio. That is, in this embodiment, the power of the first signal light output from the optical transmission unit 10 is P ₁ (true number), and the power of the second signal light output from the optical transmission unit 10 is P ₂ (true number). ) Satisfies the relationship of the following expression (3). Also, the maximum transmission distance from the light transmitting unit 10 and the light receiving section 30 and L [miles], the power of the first signal light output from the optical transmission unit 10 is taken as P _{1 [W],} the following ( 4) It is preferable that the relationship of the equation is satisfied.

The difference between the first signal light _{S 1} of wavelength lambda ₁ and wavelength lambda ₂ of the second signal light _{S 2} is 10nm or less of it is preferred. The power P _{1 of} the first signal light S ₁ output from the optical transmitter 10 is preferably 10 mW or more and 25.2 mW or less. It is preferred that the power _{P 2} of the second signal light _{S 2} output from the optical transmitting unit 10 is equal to or less than 4mW than 1 mW. In addition, it is preferable that the first transmission signal is a signal of any modulation scheme among AM-VSB, QAM, OFDM and QPSK.

Hereinafter, this embodiment will be described in more detail. First, the disturbance frequency component at square law, consider only the case where the frequency omega ₁ component of the intensity modulation of the signal light S ₁ of wavelength lambda ₁ is possess the signal light S ₂ wavelengths lambda _2, the following two frequencies Investigate the detected current when only the component is present.
Equation (5) below represents the electric field component e ₁ of the first signal light S ₁ having the wavelength λ ₁ that has been intensity-modulated by the first transmission signal having the frequency ω ₁ . The following equation (6) represents the electric field component e ₂ of the second signal light S ₂ having the wavelength λ ₂ to which the intensity modulation frequency ω ₁ component of the signal light S ₁ is transferred.

The parameters appearing in the equations (5) and (6) are as follows. P ₁ is the optical power of the first signal light S ₁ . P ₂ is the optical power of the second signal light S ₂ . m ₁ is a light modulation degree of intensity modulation of the first signal light S ₁ . m ₁ ′ is the light modulation degree of the intensity modulation component that has changed from the first signal light S ₁ to the second signal light S ₂ . ω ₁ is the central angular frequency of the first signal light S ₁ . φ ₁ is the phase of intensity modulation of the first signal light S ₁ . φ ₁ ′ is a phase of intensity modulation of the second signal light S ₂ . ω _c1 is the optical angular frequency of the first signal light S ₁ . omega _c2 is the second optical angular frequency of the signal light _{S 2.} φ _c1 is the phase of the first signal light S ₁ . phi _c2 is the second signal light _{S 2} phases.

The detection current I at the time of square detection in the optical receiver 30 is given by the following equation (7). From this equation (7), a component actually falls into the same frequency omega ₁ at square law becomes two and represented by the following equation (8) and (9).

Equation (8) represents the first transmission signal that is originally transmitted, and Equation (9) represents the noise component that has been transferred to the second signal light due to crosstalk. Since the phase of equation (9) is different from that of equation (8), it simply acts as an interference wave. When the expressions (8) and (9) are compared, the ratio of the detected current intensity is “P ₂ m ₁ ′ / P ₁ m ₁ ”. Considering the electric field components E ₁ and E ₂ (P ₁ = E ₁ ² , P ₂ = E ₂ ² ), it is expressed by the following equation (10).

  Therefore, DUR [dB], which is the ratio of the interference wave to the desired signal component to be obtained, is given by the following equation (11).

CTB or the like is well known as one of interference waves that fall into the same frequency component. If the interference component of this time also has a phase component that is randomly deviated from the signal component, or if the DUR has deteriorated, the screen becomes whitish in the same manner as CTB in terms of subjective video evaluation. Therefore, the DUR is desirably -60 dB or less. If RIN transfer is −40 dB, “P ₂ / P ₁ ” is preferably 0.1 or less.

FIG. 4 is a diagram illustrating a relationship between the power P _{2 of} the second signal light S ₂ input to the optical receiver 30 and the CNR in the intensity modulation component of the first signal light S ₁ . The power P ₁ of the first signal light S ₁ input to the optical receiver 30 is set to −3 dBm. As can be seen from this result, if the power P _{2 of} the second signal light S ₂ is about 10 dB smaller than the power P _{1 of} the first signal light S ₁ , it does not affect the CNR degradation of the original signal. I can confirm.

FIG. 5 is a diagram illustrating the relationship between the power P ₂ and the CNR of the second signal light S ₂ input to the optical receiver 30. Here, the second signal light _{S 2} and the BS signal. As can be seen from this result, it can be confirmed that the input signal light power at which the CNR is 16 dB or more is −14 dBm or more when at least the modulation degree m ₂ per channel is 4% or more.

The transmitter 12 for outputting a second signal light S ₂ of the high-frequency side, using a directly modulated transmitter without using a phase modulation. In this case, the line width of the signal light does not increase so much unless the laser light source is dithered, and the SBS threshold does not increase so much. The SBS threshold of a normal DFB laser is about 5 to 6 dBm for a 20 km long single mode optical fiber. Accordingly, the second signal light S ₂ of the high frequency side, the input light power to the optical transmission line 20 to as low as possible, it is desirable to increase the degree of modulation.

Next, the results of a theoretical study on RIN transfer will be described. FIG. 6 is a graph showing the intensity modulation frequency dependence of RIN transfer and overall RIN transfer due to each nonlinear optical phenomenon. Here, the transmission distance is 30 km, the wavelength λ ₁ of the first signal light on the low frequency side is 1555 nm, the wavelengths λ ₂ of the second signal light on the high frequency side are 1550 nm and 1560 nm, and the input light to the optical transmission line 20 The power was 14 dBm. From this result, it can be confirmed that in the wavelength arrangement as in this time, the influence of OKE is larger than that of SRS and XPM. It can also be confirmed that the amount of RINtransfer is larger on the low frequency side.

FIG. 7 is a graph showing the intensity modulation frequency dependence of RIN transfer. In FIG. (A), and the wavelength interval between the second signal beam S ₂ of the first signal beam S ₁ and the high-frequency side of the low frequency side and each value. Further, in FIG. 5B, the signal light input power is set to each value. As can be seen from this figure, when the wavelength interval is widened, the effect of SRS increases, so the amount of RIN transfer on the low frequency side increases. On the other hand, the input power P ₁ and RIN transfer of the first signal light on the low frequency side is proportional.

FIG. 8 is a graph showing the transmission path length dependency of RIN transfer. Here, the signal light input power to the optical transmission line 20 is 12 dBm. The observation frequency was 90 MHz as the lower limit of the frequency used in normal video transmission. As can be seen from this result, from the theoretical calculation result of this time, the worst value when the wavelength interval is changed is approximated by using the transmission distance L and the power P ₁ of the first signal light (12) It can be expressed by a formula.

If RIN transfer is −35 dB or less, it is possible to suppress DUR by adjusting the relationship of the input signal light power. Therefore, it is desirable that F [L, P ₁ ] is −35 dB or less. It will be.

1 is a configuration diagram of an optical transmission system 1 according to the present embodiment. It is a figure which shows arrangement | positioning of the intensity | strength modulation frequency of the signal beam | light output from an external modulation type transmitter. It is a figure which shows arrangement | positioning of the intensity | strength modulation frequency in the optical transmission system 1 which concerns on this embodiment. And power P ₂ of the second signal light S ₂ to be input to the optical receiver 30 is a diagram showing the relationship between the CNR in the intensity-modulated component of the first signal light S _1. FIG. 4 is a diagram illustrating a relationship between power P ₂ and CNR of second signal light S ₂ input to the optical receiver 30. It is a graph which shows the intensity modulation frequency dependence of each RIN transfer resulting from each nonlinear optical phenomenon, and the whole RINtransfer. It is a graph which shows intensity modulation frequency dependence of RIN transfer. It is a graph which shows the transmission path length dependence of RIN transfer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical transmission system, 10 ... Optical transmission part, 11, 12 ... Transmitter, 13 ... Multiplexer, 20 ... Optical transmission path, 30 ... Optical reception part.

Claims (6)

The first signal light having the wavelength λ ₁ modulated with the first transmission signal having the frequency ω ₁ and the wavelength λ ₂ modulated with the second transmission signal having the frequency ω ₂ (where ω ₁ <ω ₂ ) (where λ ₁ ≠ λ ₂ ) and a second signal light that is multiplexed and output,
An optical transmission path for transmitting the first signal light and the second signal light output from the optical transmitter;
An optical receiver that receives the first signal light and the second signal light that have been transmitted through the optical transmission path and received by one light receiving unit, and outputs the first transmission signal and the second transmission signal When,
With
When the power of the first signal light output from the optical transmitter is P ₁ (true number) and the power of the second signal light output from the optical transmitter is P ₂ (true number) ,

An optical transmission system characterized by satisfying the following relationship. When the maximum transmission distance from the optical transmitter to the optical receiver is L [km] and the power of the first signal light output from the optical transmitter is P ₁ [W],

The optical transmission system according to claim 1, wherein: The optical transmission system according to claim 1, wherein the difference between the wavelength lambda ₂ and wavelength lambda ₁ of the first signal light and the second signal light is equal to or is 10nm or less. 2. The optical transmission system according to claim 1, wherein the power P ₁ of the first signal light output from the optical transmission unit is 10 mW or more and 25.2 mW or less. _2. The optical transmission system according to claim 1, wherein a power P ₂ of the second signal light output from the optical transmission unit is 1 mW or more and 4 mW or less. 2. The optical transmission system according to claim 1, wherein the first transmission signal is a signal of a modulation scheme of AM-VSB, QAM, OFDM, or QPSK.

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