JP3302924B2 - Chromatic dispersion compensation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a chromatic dispersion compensating circuit for compensating chromatic dispersion of a transmission line in high-speed time division multiplexing (TDM) optical communication.

[0002]

2. Description of the Related Art Ultra-high speed TD for future multimedia
In the M optical communication system, compensation of chromatic dispersion of a transmission line is one of important issues.

FIG. 7 shows a schematic configuration of a conventional optical transmission system. In the figure, 71 is a transmission system, 72 is a transmission optical fiber, 73 is an optical repeater, 74 is a chromatic dispersion compensator, and 75 is a reception system. The optical signal (A in the figure) generated by the transmission system 71 is transmitted to the transmission optical fiber 72 and transmitted to the reception system 75 while being optically amplified by the optical repeater 73 inserted at a predetermined distance.

An optical signal immediately after transmission is transmitted through a transmission optical fiber 7.
2 and the wavelength of the optical repeater 73 degrade the waveform (B in the figure). When this optical signal is directly received, an error occurs in signal reading due to interference between adjacent optical pulses. Also,
As the speed of an optical signal increases, one time slot (the time width occupied by one bit) decreases, so that the influence of chromatic dispersion on transmission characteristics increases. Therefore, a chromatic dispersion compensator 74 for compensating the chromatic dispersion of the transmission path (the transmission optical fiber 72 and the optical repeater 73) is used.

The conventional chromatic dispersion compensating means 74 compensates for the chromatic dispersion of the transmission line using an optical fiber or an optical fiber grating having a dispersion value equal to the absolute value of the sign opposite to that of the transmission line (C in the figure). (Reference 1: Kunio Ogura,
"Recent development of dispersion compensating optical fiber", Applied Physics, 64, 1, p. 28, 1995, Reference 2: J. Williams, "F
iber dispertion compensation using a chirped in-fi
ber Bragg grating ", Electron. Lett., 30, 12, p.98
5, 1994).

[0006]

In the conventional chromatic dispersion compensating means 74, the amount of dispersion compensation can be adjusted by the length of the optical fiber or the grating interval of the optical fiber grating.
However, once fabricated, the amount of dispersion compensation is fixed. Therefore, the method is suitable for compensating the initial dispersion value of a specific transmission line, but cannot cope with the fluctuation of the dispersion value due to exchange of the transmission line or a temperature change.

On the other hand, as a conventional compensation method for dispersion value fluctuation, there is a method of controlling a wavelength of a light source of a transmission system so as to always obtain an optimum transmission characteristic in accordance with dispersion value fluctuation. However, this requires an expensive tunable light source. Further, in the optical repeater including the optical bandpass filter, there is a problem that the optical signal power after optical amplification relay changes due to the change of the optical signal wavelength.

The present invention provides a chromatic dispersion compensating circuit capable of compensating for a temporal variation in chromatic dispersion of a transmission line in an ultra-high-speed TDM optical communication system and expanding an allowable amount of dispersion compensation. The purpose is to:

[0009]

The chromatic dispersion compensation circuit of the present invention utilizes the fact that the chromatic dispersion value changes in accordance with the pumping light or pumping current (pumping level) injected into the optical amplification medium. On the other hand, the optical signal is converted into an electric signal to extract a clock frequency component, a ratio (Pc / Pa) between the clock frequency component power Pc and the total power Pa of the electric signal is obtained, and is provided for chromatic dispersion compensation control. That is, when dispersion compensation of an optical signal is optimal, Pc / Pa is maximized, so that the pumping light or excitation current injected into the optical amplification medium is controlled so that Pc / Pa is maximized. Thereby, the chromatic dispersion compensation amount is optimized.

If an optical amplifier for controlling the output power to be always constant is provided downstream of the optical amplifying medium, the above Pa is always constant, and the optical amplifier is controlled so that Pc is maximized. The excitation light or the excitation current injected into the amplification medium may be controlled. In this case, the control may be performed so that the Q value of the electric signal is maximized, or the bit error rate is minimized.

Here, control of the chromatic dispersion compensation amount by measuring the Q value will be described. When an optical signal from the transmission path is converted into an electric signal, and the electric signal is input to an electric signal processing circuit such as a sampling oscilloscope together with the extracted clock signal, an eye pattern is obtained. In this eye pattern, the signal amplitude (for example, voltage value) is μ, the standard deviation of “mark” or “1” level noise in binary transmission is σ ₁ , the standard deviation of “space” or “0” level noise the when the sigma _0, Q _{value, Q = μ / (σ 1} + σ 0) ... is defined as (1). Assuming a Gaussian noise amplitude distribution,
Transmission error rate P and Q values in a low error rate region is related to P = (1 / Q (2π ) 1/2) exp (-Q 2/2) ... (2), if measured Q values The bit error rate of the road can be estimated.

This Q value (bit error rate) is a value indicating the degree of transmission quality. A high Q value is obtained only when the signal-to-noise ratio of an optical signal is high and proper dispersion compensation is performed. can get. Therefore, the amount of chromatic dispersion compensation can be optimized by controlling the pumping light or the pumping current injected into the optical amplification medium so that the Q value is maximized (the bit error rate is minimized).

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment: Claim 1,
2, 3, 4) FIG. 1 shows a first example of the chromatic dispersion compensating circuit of the present invention.
An embodiment will be described.

In the figure, 10 is an optical amplifying medium, 20 is an exciting means for generating an exciting light or an exciting current to be injected into the optical amplifying medium, 31 is an optical splitter for branching a part of output light of the optical amplifying medium, 32 Is a light receiver for converting the split light into an electric signal, and 33 is an electric signal processing for extracting a clock frequency component of the optical signal from the electric signal output from the light receiver and controlling the excitation means 20 according to the clock frequency component. Department.

FIGS. 2 and 3 show examples of the configuration of the optical amplification medium 10 and the excitation means 20. FIG. FIG. 2 shows a pumping means 20 using a rare earth-doped optical fiber 11 doped with a rare earth such as erbium (Er) or neodymium (Nd) as an optical amplifying medium 10.
An excitation light source 21 for generating excitation light, an optical multiplexer 2 for multiplexing the excitation light and the signal light and guiding the multiplexed light to the rare-earth-doped optical fiber 11
2. This is an example using a current source 23 for adjusting an injection current to an excitation light source. 2 (a), 2 (b) and 2 (c) show forward excitation,
Shows backward excitation and bidirectional excitation. The excitation light power is adjusted by the injection current from the current source 23, and the chromatic dispersion compensation amount in the rare-earth-doped optical fiber 11 is controlled.

FIG. 3 shows a semiconductor optical amplifier 12 as an optical amplifying medium 10 and a semiconductor optical amplifier 1 as an excitation means 20.
This is an example in which an excitation current source 24 that generates an excitation current to be injected into a sample 2 is used. The semiconductor optical amplifier 12 includes an excitation current source 2
The pump current is adjusted by 4 to control the amount of chromatic dispersion compensation.

As described above, the waveform of the optical signal passing through the transmission path is deteriorated by the influence of the chromatic dispersion of the transmission path (FIG. 7B). The deteriorated optical signal is transmitted to the optical amplifying medium 10.
To enter. At this time, the excitation means 20 sends the light amplifying medium 1
The pumping light or the pumping current to be injected into 0 is adjusted, the chromatic dispersion value of the optical amplification medium 10 is changed, and the chromatic dispersion compensation amount is adjusted. The dependence of the wavelength dispersion value of the optical amplification medium 10 on the excitation level will be described below.

The relationship between the refractive index and the absorption coefficient of the optical amplifying medium 10 is given by Kramers-Kronig (Kramers-Kronig).

[0019]

(Equation 1)

## EQU1 ## Here, δn (ω) is the change in refractive index, δα (ω) is the change in absorption coefficient (gain coefficient), PV
Is the principal value of the integral. This equation means that the amount of change in the refractive index depends on (the wavelength dependence of) the change in the absorption coefficient. For example, in the case of an optical amplifying medium having a gain absorption characteristic as shown in FIG. 4A, the refractive index changes as shown in FIG. 4B. The solid line indicates the case where the pumping light power is large, and the dotted line indicates the case where the pumping light power is small. Since this refractive index change corresponds to the group delay time difference T, the chromatic dispersion D (= d
T / dλ) has characteristics as shown in FIG.

FIG. 5 shows an example in which gain and refractive index change are calculated from the absorption / emission cross section (actually measured values at room temperature) of an erbium-doped optical fiber (L = 1).
m) (Reference 3: E. Desuirvier, "Study of the comp
lex atomic susceptivity of EDFA ", JLT-8 (10), p.151
7, 1990, reference 4: SCFleming et al., "Measurm
ent and analysis of pump-dependent refractive inde
x and dispertion effects in Erbium-doped fiber amp
lifier ", JQE-32 (7), p.1113, 1996.
The case where the ratio of the pumping light power P to the pumping light power _Pth when the gain is 1 is 40 is shown. Similarly, the broken line shows the case of 20, and the dotted line shows the case of 10.

The dispersion of the refractive index change δn due to the excitation is
It becomes maximum near the gain peak wavelength (1530 nm), and its value is approximately δn / dλ = 0.1 × 10 ⁻⁶ [1 / nm].
The change in group velocity dispersion due to excitation is the gain change (δG)
Is 30 dB,

[0023]

(Equation 2)

## EQU1 ## Due to the Kramers-Kronig relationship, even if the length L of the erbium-doped optical fiber is changed, the change in the refractive index is the same if the gain change of the total length is the same. In an actual optical fiber, the Kramers-Kro
Since the material dispersion characteristics determined by the nig relationship also have the effect of thermal dispersion change due to structural dispersion and excitation, the dispersion amount changes more greatly. That is, by changing the excitation level to the optical amplification medium, the chromatic dispersion value of the optical amplification medium can be changed, and a variable chromatic dispersion compensation circuit can be realized.

The chromatic dispersion compensation amount in this chromatic dispersion compensation circuit is controlled to an optimum value as follows. As shown in FIG. 1, the light output from the optical amplifying medium 10 is split into two by an optical splitter 31. One of the lights is converted into an electric signal by the light receiver 32 and input to the electric signal processing unit 33. The electrical signal processing unit 33 extracts the clock frequency component of the optical signal from the electrical signal, and obtains the ratio (Pc / Pa) between the clock frequency component power Pc and the total power Pa of the electrical signal. Here, when the dispersion compensation of the optical signal is optimal, Pc / Pa becomes maximum. Therefore, the pumping means 20 is controlled so that Pc / Pa becomes maximum. In the pumping means 20, as described above, the optical amplifying medium 10
To control the amount of chromatic dispersion compensation by controlling the pumping light or the pumping current injected into the substrate.

In the case of the optical signal of the RZ code, since the electric signal itself after the photoelectric conversion includes a clock frequency component, this is extracted. On the other hand, in the case of the optical signal of the NRZ code, since the electrical signal after the photoelectric conversion has no clock frequency component, the clock frequency component is extracted using the nonlinear extraction circuit after the photoelectric conversion.

In the optical amplifying medium 10, since the amplified stimulated emission light (ASE) is output, the signal-to-noise ratio of the optical signal is degraded. In order to reduce the influence of this noise component, an optical bandpass filter 34 that transmits only the wavelength range of the optical signal may be disposed downstream of the optical amplifying medium 10 as shown in FIG. As a result, the measurement accuracy of the power ratio Pc / Pa can be improved.

(Second Embodiment: Claims 1, 2, 5,
6, 7) In the first embodiment, the amount of chromatic dispersion compensation is changed by changing the gain of the optical amplification medium 10. However, in this case, the output power of the chromatic dispersion compensating circuit is not constant, and changes according to the amount of chromatic dispersion compensation. Normally, there is no problem since an optical amplifier for performing constant optical output control is arranged at the subsequent stage of the chromatic dispersion compensating circuit. However, the second embodiment includes this optical amplifier in the chromatic dispersion compensating circuit. is there.

FIG. 6 shows a second embodiment of the chromatic dispersion compensating circuit according to the present invention.
An embodiment will be described. The configuration of the present embodiment is the same as the configuration of the first embodiment shown in FIG. 1 except that an optical amplifier 35 is arranged between the optical amplification medium 10 and the optical splitter 31. This is the same as the embodiment. This optical amplifier 35 is a general one that performs constant optical output control. With such a configuration, even if the power of the output optical signal changes due to the control of the excitation level of the optical amplification medium 10, an optical signal having a constant power can be always output.

The power of the optical signal incident on the photodetector 32 and the total power Pa of the photoelectrically converted electric signal are also constant. Therefore, the electric signal processing unit 33 does not need to obtain the power ratio Pc / Pa as in the first embodiment, but simply monitors only the clock frequency component power Pc, and sets the excitation unit 20 so that it becomes maximum. What is necessary is to control. Also,
Instead of the clock frequency component power Pc, the Q value or bit error rate of an optical signal or an electric signal is monitored, and control is performed so that the Q value is maximized or controlled so that the bit error rate is minimized. Good.

[0031]

As described above, according to the present invention, a tunable chromatic dispersion compensating circuit can be realized by utilizing the excitation level dependence of the chromatic dispersion value of the optical amplification medium. By using this chromatic dispersion compensating circuit, it is possible to compensate for the temporal variation of the chromatic dispersion of the transmission line in real time. Further, the allowable amount of dispersion compensation can be expanded. As a result, the deterioration of the transmission characteristics of the optical communication system can be reduced, so that the present invention can be applied to ultra-high-speed TDM optical communication.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of a chromatic dispersion compensation circuit of the present invention.

FIG. 2 is a diagram showing a configuration example of an optical amplification medium 10 and an excitation unit 20.

FIG. 3 is a diagram showing another configuration example of the optical amplification medium 10 and the excitation unit 20.

FIG. 4 is a diagram showing the excitation level dependence of the wavelength dispersion value of the optical amplification medium.

FIG. 5 is a diagram showing a specific example of the excitation level dependence of the wavelength dispersion value of the optical amplification medium.

FIG. 6 is a block diagram illustrating a chromatic dispersion compensating circuit according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a schematic configuration of a conventional optical transmission system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Optical amplification medium 11 Rare earth-doped optical fiber 12 Semiconductor optical amplifier 20 Excitation means 21 Excitation light source 22 Optical multiplexer 23 Current source 24 Excitation current source 31 Optical splitter 32 Optical receiver 33 Electric signal processing unit 34 Optical bandpass filter 35 Optical amplifier

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02B 6/00 H01S 3/10

Claims (7)

(57) [Claims] 1. An optical amplifying medium whose chromatic dispersion changes in accordance with an excitation level, and a control means for adjusting an excitation level of the optical amplifying medium and compensating for dispersion of an input optical signal. Wavelength dispersion compensation circuit. 2. An optical amplification medium whose chromatic dispersion changes according to an excitation level; an excitation unit for generating excitation light or an excitation current for exciting the optical amplification medium; and a part of output light of the optical amplification medium. Optical branching means for branching; photoelectric conversion means for converting the branched light into an electric signal; and extracting a clock frequency component from the electric signal, the clock frequency component power Pc and the total power Pa of the electric signal.
A chromatic dispersion compensating circuit comprising: an electric signal processing unit that controls the pumping means so that a ratio (Pc / Pa) of the chromatic dispersion is maximized. 3. The chromatic dispersion compensating circuit according to claim 1, wherein a rare-earth-doped optical fiber is used as an optical amplifying medium, and the control means or the pumping means controls the pumping light input to the rare-earth-doped optical fiber. A chromatic dispersion compensating circuit having a configuration for controlling a level. 4. The chromatic dispersion compensating circuit according to claim 1, wherein a semiconductor optical amplifier is used as an optical amplifying medium, and the control means or the pumping means controls an injection current to the semiconductor optical amplifier. A chromatic dispersion compensating circuit characterized by the following. 5. The chromatic dispersion compensating circuit according to claim 1, further comprising an optical amplifier that controls output power to be constant at a subsequent stage of the optical amplifying medium. 6. The chromatic dispersion compensating circuit according to claim 2, wherein an optical signal is provided between the optical amplifying medium and the optical branching means.
An optical amplifier (35) for controlling the output power to be constant, and the electric signal processing unit is configured to control the pumping means so that the clock frequency component power Pc of the electric signal is maximized. Compensation circuit. 7. The chromatic dispersion compensating circuit according to claim 2, wherein light is transmitted between the optical amplifying medium and the optical branching means.
An optical amplifier (35) for controlling the output power to be constant, the electric signal processing unit controls the pumping means so that the Q value of the optical signal or the electric signal is maximized or the bit error rate is minimized. A chromatic dispersion compensating circuit characterized in that it has a configuration that performs the following.