JP3971197B2 - Chromatic dispersion compensator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a chromatic dispersion compensator that compensates for chromatic dispersion in an optical transmission line.
[0002]
[Prior art]
With the increase in communication demand in recent years, the bit rate per channel has been increased with the increase in the number of channels in the optical communication trunk network. The effect of chromatic dispersion on an optical transmission line increases as the bit rate per channel increases, so chromatic dispersion compensation is an important issue.
[0003]
FIG. 9 shows a schematic configuration of a conventional optical transmission system. In the figure, 51 is a transmission system, 52 is a transmission optical fiber, 53 is an optical repeater, 54 is chromatic dispersion compensation means, and 55 is a reception system. The optical signal (A in the figure) generated by the transmission system 51 is transmitted to the transmission optical fiber 52 and transmitted to the reception system 55 while being optically amplified by the optical repeater 53 inserted at every predetermined distance.
[0004]
The waveform of the optical signal immediately after transmission deteriorates due to the wavelength dispersion of the transmission optical fiber 52 and the optical repeater 53 (B in the figure). When this optical signal is received directly, an error occurs in signal reading due to interference between adjacent optical pulses. Also, as the speed of the optical signal increases, one time slot (the time width occupied by one bit) decreases, so the influence on the transmission characteristics due to chromatic dispersion increases. Therefore, chromatic dispersion compensation means 54 for compensating the chromatic dispersion of this transmission line (transmission optical fiber 52 and optical repeater 53) is used.
[0005]
The conventional chromatic dispersion compensation means 54 compensates the chromatic dispersion of the transmission line by using an optical fiber or an optical fiber grating having the same sign as the transmission line and having the same absolute value as the transmission line (C in the figure) (reference) Reference 1: Kunio Ogura, “Recent Development Status of Dispersion Compensating Optical Fiber”, Applied Physics, 64, 1, p.28, 1995, Reference 2: J. Williams, “Fiber dispertion compensation using a chirped in-fiber Bragg grating ", Electron. Lett., 30, 12, p.985, 1994).
[0006]
[Problems to be solved by the invention]
In the conventional chromatic dispersion compensation means 54, the dispersion compensation amount can be adjusted by the optical fiber length or the grating interval of the optical fiber grating. However, since the dispersion value after fabrication is fixed, it is suitable for compensation of the initial dispersion value of a specific transmission line, but it cannot cope with fluctuations in the dispersion value due to exchange of the transmission line or temperature change.
[0007]
On the other hand, as a conventional compensation method for variation of the dispersion value, there is a method of controlling the wavelength of the light source of the transmission system so as to always obtain optimum transmission characteristics according to the variation of the dispersion value. However, this requires an expensive wavelength variable light source. In addition, the optical repeater including the optical bandpass filter has a problem that the optical signal power after the optical amplification relay is changed due to the change of the optical signal wavelength.
[0008]
It is an object of the present invention to provide a chromatic dispersion compensator that can compensate for temporal fluctuations of chromatic dispersion in a transmission line in an optical communication system and can increase the allowable amount of dispersion compensation.
[0009]
[Means for Solving the Problems]
The chromatic dispersion compensator of the present invention has a chromatic dispersion characteristic that periodically changes due to a resonance structure having reflection portions at both ends, and the magnitude of the dispersion of the chromatic dispersion characteristic changes according to the gain, so that the resonator length is increased. The resonant structure semiconductor optical amplifier in which the chromatic dispersion characteristic is shifted on the optical frequency axis in response to this, and the gain and resonator length of the resonant structure semiconductor optical amplifier are controlled, and the magnitude of dispersion of the chromatic dispersion characteristic and the position on the optical frequency axis And a control means for controlling the chromatic dispersion of the input signal light by the chromatic dispersion characteristic set in the resonant structure semiconductor optical amplifier.
[0010]
A configuration may be adopted in which the gain is adjusted by controlling the injection current to the resonant structure semiconductor optical amplifier to control the magnitude of dispersion of the wavelength dispersion characteristic. Further, the gain may be adjusted by controlling the optical power input to the resonant structure semiconductor optical amplifier, and the dispersion of the chromatic dispersion characteristic may be controlled. Further, the temperature of the resonant structure semiconductor optical amplifier may be controlled to adjust the resonator length to control the position of the wavelength dispersion characteristic on the optical frequency axis.
[0011]
The reflectivity of the reflection part of the resonant structure semiconductor optical amplifier is R, the resonator length is L, the gain when the light traverses the semiconductor optical amplifier once is G, the refractive index is n, the speed of light is c (m / s), When the bit rate of the input signal light is B (bit / s),
L <c / (4nB)
R ² G ² <1
It is the composition which satisfies.
[0012]
In addition, when performing wavelength dispersion compensation by inputting wavelength multiplexed signal light having an optical frequency interval Δf as input signal light,
Δf = c / (2 nL)
The resonator length (optical length) nL of the resonant structure semiconductor optical amplifier is set so as to satisfy the above relationship.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1 shows a first embodiment of a chromatic dispersion compensator of the present invention.
[0014]
In the figure, 1 is a resonant structure semiconductor optical amplifier, 3 is an optical coupler that branches a part of the output light of the resonant structure semiconductor optical amplifier 1, 4 is a light receiving circuit that converts the branched light into an electrical signal, and 5 is a light receiving circuit 4 2 is an optical bandpass filter arranged between the resonant structure semiconductor optical amplifier 1 and the optical coupler 3, and 6 is a resonant structure. This is optical power control means for controlling the incident power to the semiconductor optical amplifier 1. The optical bandpass filter 2 and the optical power control means 6 are arbitrary and are used according to the case described later.
[0015]
The waveform of the optical signal that has passed through the transmission line deteriorates due to the influence of chromatic dispersion of the transmission line (B in FIG. 9). This deteriorated optical signal is input to the resonant structure semiconductor optical amplifier 1. At this time, the control unit 5 adjusts the injection current to the resonant structure semiconductor optical amplifier 1 and the temperature of the resonant structure semiconductor optical amplifier 1 to change the chromatic dispersion characteristic of the resonant structure semiconductor optical amplifier 1, thereby changing the chromatic dispersion compensation amount. adjust.
[0016]
Here, the principle of adjusting the wavelength dispersion characteristic of the resonant structure semiconductor optical amplifier 1 will be described with reference to FIGS. In FIG. 2, the resonant structure semiconductor optical amplifier 1 constitutes an optical resonator (Fabry-Perot resonator) by reflecting portions 11 and 12 at both ends thereof. Examples of the reflective portion include end face coating, a grating structure such as DBR (distributed Bragg reflector), and a junction structure with other regions such as a spot size conversion coupling portion.
[0017]
Due to such a resonator structure, the complex amplitude At of the transmitted wave is expressed by the following equation (reference: A. Yariv, Optoelectronics Basics, Chapter 4, Maruzen Co., Ltd.).
At = [(1-R) G ^0.5 / (1-RGexp (iδ))] Ai (1)
δ = 4πnL / λ = 4πnLf / c
[0018]
Where Ai is the complex amplitude of the input light, T is the transmittance of the optical intensity of the semiconductor optical amplifier, R is the reflectance of the optical intensity at both ends of the semiconductor optical amplifier, and G is the gain of the optical intensity of the semiconductor optical amplifier (semiconductor optical amplifier) ) Is the phase difference generated by one round trip of the resonator, n is the refractive index of the semiconductor, L is the resonator length of the resonant structure semiconductor optical amplifier 1, and λ is the wavelength of the signal light. , F is the optical frequency of the signal light.
[0019]
The wavelength dispersion characteristic D of the resonant structure semiconductor optical amplifier 1 is as follows. When the phase term of the complex amplitude ratio At / Ai of the transmitted wave and the incident wave is φ (= arg [At / Ai])
D = ∂t / ∂λ = − (2πc / λ ² ) (∂ ² φ / ∂ω ² ) (2)
Is required. Here, t is a group velocity delay time, and ω is an angular frequency (= 2πc / λ). Therefore, from the equations (1) and (2), the wavelength dispersion characteristic of the resonant structure semiconductor optical amplifier 1 is as shown in FIG.
[0020]
As can be seen from FIG. 3, the chromatic dispersion characteristics change periodically, and this period is δλ = λ ² / ( ² nL) on the wavelength axis and Δf = c / ( ² nL) [Hz] in the optical frequency region. . By making the period Δf larger than, for example, twice the bit rate B of the optical signal (Δf> 2B), the chromatic dispersion compensation of the optical signal can be performed using the resonant structure semiconductor optical amplifier 1. That is, as a condition of the resonator length L, L <c / (4 nB) (3)
Set to satisfy.
[0021]
Here, when the temperature of the resonant structure semiconductor optical amplifier 1 is changed, the resonator length (optical length) nL is changed, and the period Δf of the wavelength dispersion characteristic is changed. Due to the change (Δf + α) of the period Δf of the wavelength dispersion characteristic, as shown in FIG. 4, the position of the peak wavelength of the dispersion slope (the transmission center wavelength of the Fabry-Perot resonator) (A and B in FIG. 4) is slightly shifted. Thus, in a predetermined optical frequency region (for example, 1.55 μm band), the chromatic dispersion characteristic is shifted on the optical frequency axis. In addition, it is good also as a structure which expands / contracts the space | interval (resonator length) of the reflection parts 11 and 12 shown in FIG.
[0022]
On the other hand, when the injection current to the resonant structure semiconductor optical amplifier 1 is increased, the gain increases, so that the dispersion of the wavelength dispersion characteristic changes. However, in order not to cause laser oscillation, the following condition R ² G ² <1 (4)
It is necessary to satisfy. The solid line in FIG. 3 is the case where the injected current is high and the gain G is large, and the dotted line is the case where the injected current is low and the gain G is small.
[0023]
As described above, the resonant structure semiconductor optical amplifier 1 can adjust the magnitude of dispersion of the wavelength dispersion characteristic and the position on the optical frequency axis (wavelength axis) by adjusting the injection current and temperature. It can be applied to chromatic dispersion compensation of optical signals.
[0024]
When actually applied, a resonant structure semiconductor optical amplifier having a resonator length L satisfying the expression (3) is first manufactured according to the bit rate B of the optical communication system to be applied. Then, the position on the optical frequency axis (wavelength axis) is adjusted by the temperature, and the degree of dispersion of the wavelength dispersion characteristic is adjusted by the injection current.
[0025]
Further, in the case of a WDM (wavelength multiplexing) system in which optical signals of a plurality of wavelengths are arranged at equal intervals on the optical frequency axis, as shown in FIG. By setting the period Δf of the chromatic dispersion characteristic, it is possible to perform chromatic dispersion compensation collectively.
[0026]
FIG. 6 is an experimental result using the configuration of the present embodiment, and shows eye patterns before and after dispersion compensation when a 5 Gbit / s NRZ optical signal is transmitted by a single mode fiber at 70 km. As is apparent from this eye pattern, the spread of the mark portion is suppressed, and it can be seen that dispersion compensation is effectively performed. Since the gain of the resonant structure semiconductor optical amplifier 1 varies depending on the incident power, if necessary, the optical power control for controlling the incident power to be constant at the front stage of the resonant structure semiconductor optical amplifier 1 as shown in FIG. Means 6 may be arranged. As the optical power control means 6, an optical fiber amplifier that performs constant output power control or the like can be used.
[0027]
The amount of chromatic dispersion compensation in this chromatic dispersion compensator is controlled to an optimum value as follows. As shown in FIG. 1, the light output from the resonant structure semiconductor optical amplifier 1 is branched into two by an optical coupler 3. One of the lights is converted into an electrical signal by the light receiving circuit 4 and input to the control unit 5. The control unit 5 extracts the clock frequency component of the optical signal from the electrical signal, and obtains the ratio (Pc / Pa) of the clock frequency component power Pc and the total power Pa of the electrical signal. Here, when dispersion compensation of the optical signal is optimum, Pc / Pa is maximized. Therefore, the injection current and temperature are controlled so that Pc / Pa is maximized, and the chromatic dispersion compensation amount is optimized.
[0028]
In the case of the optical signal of the RZ code, the electrical signal itself after photoelectric conversion includes a clock frequency component, so that it is extracted. On the other hand, in the case of an optical signal of NRZ code, since the electrical signal after photoelectric conversion has no clock frequency component, the clock frequency component is extracted using a nonlinear extraction circuit after photoelectric conversion.
[0029]
Further, as a feedback signal monitored by the control unit 5, a bit error rate or Q value is used in addition to the clock frequency component, and the control unit 5 performs control so that the bit error rate is minimized, or the Q value is maximized. You may control so that it may become. In that case, the light receiving circuit 4 uses a bit error rate detection circuit and a Q value measurement circuit, respectively.
[0030]
Further, since the resonant structure semiconductor optical amplifier 1 outputs amplified stimulated emission light (ASE), the signal-to-noise ratio of the optical signal is deteriorated. In order to reduce the influence of this noise component, an optical bandpass filter 2 that transmits only the wavelength region of the optical signal is disposed between the resonant structure semiconductor optical amplifier 1 and the optical coupler 3 as shown in FIG. Also good. Thereby, the measurement accuracy of the power ratio Pc / Pa can be increased.
[0031]
(Second Embodiment)
FIG. 7 shows a second embodiment of the chromatic dispersion compensator of the present invention.
[0032]
In the figure, 1 is a resonant structure semiconductor optical amplifier, 3 is an optical coupler that branches part of the output light of the resonant structure semiconductor optical amplifier 1, 4 is a light receiving circuit that converts the branched light into an electrical signal, and 7 is a light receiving circuit 4. A control unit for controlling the temperature of the resonant structure semiconductor optical amplifier 1 according to the output of the optical power; 8 an optical power control means for controlling the incident power to the resonant structure semiconductor optical amplifier 1 according to the output of the light receiving circuit 4; An optical band-pass filter disposed between the structural semiconductor optical amplifier 1 and the optical coupler 3. The optical bandpass filter 2 is optional and is used according to the case shown in the first embodiment.
[0033]
In the first embodiment, the control unit 7 adjusts the injection current of the resonant structure semiconductor optical amplifier 1 to control the magnitude of dispersion of the chromatic dispersion characteristic. However, in this embodiment, the optical power control means 8 The gain is adjusted by controlling the incident power of the resonant structure semiconductor optical amplifier 1, and the magnitude of dispersion of the wavelength dispersion characteristic is similarly controlled.
[0034]
As the optical power control means 8, a configuration for controlling the pumping light power of the erbium-doped optical fiber amplifier, a configuration for controlling the injection current of the semiconductor optical amplifier (normal type without a resonance structure), or the like can be used. Further, the incident power of the resonant structure semiconductor optical amplifier 1 may be controlled by combining these optical amplifiers and an optical attenuator with variable attenuation.
[0035]
(Third embodiment)
FIG. 8 shows a third embodiment of the chromatic dispersion compensator of the present invention. The present embodiment is a configuration example when applied to collective dispersion compensation of WDM signals.
[0036]
In the figure, a repeater 20 that performs chromatic dispersion compensation of a WDM signal whose waveform has deteriorated due to chromatic dispersion of a transmission line includes a fixed chromatic dispersion compensator 21, a chromatic dispersion compensator 22 of the present invention, and an optical amplifier as necessary. 23. The WDM signal is compensated for the dispersion slope by the fixed chromatic dispersion compensator 21 and is compensated for by the chromatic dispersion compensator 22 of the present invention. Since the chromatic dispersion compensator 22 of the present invention has a resonance structure, it is possible to amplify an optical signal simultaneously with dispersion compensation. However, if further amplification is required, an optical amplifier 23 may be used in the subsequent stage.
[0037]
【The invention's effect】
As described above, the present invention uses a resonant structure semiconductor optical amplifier having periodic chromatic dispersion characteristics, and controls its injection current or input optical power and temperature (resonator length), thereby changing the variable wavelength. A dispersion compensator can be realized. By using this chromatic dispersion compensator, it is possible to compensate in real time for temporal variations in chromatic dispersion in the transmission path. Further, it can be applied to collective dispersion compensation of WDM signals with a simple configuration.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a first embodiment of the present invention.
FIG. 2 is a diagram showing a configuration example of a resonant structure semiconductor optical amplifier.
FIG. 3 is a graph showing chromatic dispersion characteristics of a resonant structure semiconductor optical amplifier.
FIG. 4 is a diagram showing a state of shift of chromatic dispersion characteristics of a resonant structure semiconductor optical amplifier.
FIG. 5 is a diagram for explaining an application example to chromatic dispersion compensation of a WDM system.
FIG. 6 is a diagram showing experimental results.
FIG. 7 is a block diagram showing a second embodiment of the present invention.
FIG. 8 is a block diagram showing a third embodiment of the present invention.
FIG. 9 is a diagram showing a schematic configuration of a conventional optical transmission system.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Resonant structure semiconductor optical amplifier 2 Optical band pass filter 3 Optical coupler 4 Light receiving circuit 5, 7 Control part 6, 8 Optical power control means 11, 12 Reflecting part 20 Repeater 21 Fixed chromatic dispersion compensator 22 Chromatic dispersion compensation of this invention 23 Optical Amplifier

Claims (6)

It has a chromatic dispersion characteristic that changes periodically due to the resonant structure with reflecting parts at both ends, the dispersion of the chromatic dispersion characteristic changes according to the gain, and the chromatic dispersion characteristic changes according to the resonator length. A resonant structure semiconductor optical amplifier that shifts up;
Control means for controlling the gain of the resonant structure semiconductor optical amplifier and the resonator length, and controlling the magnitude of dispersion of the chromatic dispersion characteristic and the position on the optical frequency axis;
A chromatic dispersion compensator characterized in that chromatic dispersion compensation of input signal light is performed by chromatic dispersion characteristics set in the resonant structure semiconductor optical amplifier. The chromatic dispersion compensator according to claim 1, wherein
A chromatic dispersion compensator, characterized in that the gain is adjusted by controlling an injection current to the resonant-structure semiconductor optical amplifier to control the magnitude of dispersion of the chromatic dispersion characteristic. The chromatic dispersion compensator according to claim 1, wherein
A chromatic dispersion compensator characterized in that the gain is adjusted by controlling the optical power input to the resonant structure semiconductor optical amplifier to control the magnitude of dispersion of the chromatic dispersion characteristic. The chromatic dispersion compensator according to claim 1, wherein
A chromatic dispersion compensator, characterized in that the temperature of the resonant structure semiconductor optical amplifier is controlled to adjust the resonator length to control the position of the chromatic dispersion characteristic on the optical frequency axis. The chromatic dispersion compensator according to claim 1, wherein
The reflectivity of the reflection part of the resonant structure semiconductor optical amplifier is R, the resonator length is L, the gain when the light crosses the semiconductor optical amplifier once is G, the refractive index is n, and the speed of light is c (m / s). When the bit rate of the input signal light is B (bit / s),
L <c / (4nB)
R ² G ² <1
A chromatic dispersion compensator characterized by satisfying the above requirements. The chromatic dispersion compensator according to claim 5,
When performing wavelength dispersion compensation by inputting wavelength multiplexed signal light having an optical frequency interval Δf as the input signal light,
Δf = c / (2 nL)
The chromatic dispersion compensator is characterized in that the resonator length (optical length) nL of the resonant structure semiconductor optical amplifier is set so as to satisfy the above relationship.

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