JP2012048044A - Optical signal regenerator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To execute in-line compensation for and reduction of chirp, delay and noise including ASE in an optical signal that dynamically changes under an optical network environment, in an optical signal regenerator using a parametric process.SOLUTION: An optical signal regenerator comprises a phase retaining wavelength converter that uses a parametric process of optical waveguide which converts a frequency of carrier wave in a variable manner while retaining a phase of optical signal. The phase retaining wavelength converter is provided with an optical waveguide that has a dispersion slope S, a wavelength λ0 exhibiting a zero dispersion value and a third non-linear constant γ, for obtaining a relational expression so as to arrange an input wavelength near a peak wavelength λS in a short wavelength of parametric gain or a peak wavelength λL in a long wavelength of parametric gain and to arrange an output wavelength near the peak wavelength λL in the long wavelength of parametric gain or the peak wavelength λS in the short wavelength of parametric gain, respectively, and for using the relational expression to determine a wavelength λp of pump light and instantaneous intensity Pp of pump light of the phase retaining wavelength converter.

Description

  The present invention relates to an optical signal reproducing apparatus that suppresses or compensates for deterioration factors of an optical signal such as spontaneous emission optical noise, group velocity dispersion, and delay fluctuation received by an optical signal in an optical communication field and reproduces a signal waveform.

  Information and communication network traffic increases year by year and is expected to continue to increase in the future. In such a situation, a concern is an increase in power consumption of the electronic router. In the future, as services for distributing large-capacity content such as ultra-high-definition video become widespread, it is desired to realize an optical network using an optical switch whose power efficiency is several orders of magnitude higher than that of an electronic router.

  In order to construct a network with an optical switch and an optical fiber, it is necessary to guarantee the quality of optical transmission between any two users against line switching by the optical switch. However, when the line is switched by an optical switch, the optical transmission path changes, so that the group velocity dispersion between two points, the delay, and the spontaneous emission high noise (ASE noise) generated by the optical amplifier at the relay and the optical switch Also, the cumulative degree of factors that hinder transmission such as multipath interference noise (MPI noise) in the network changes. As a result, there is a problem in that optical transmission may or may not be possible depending on how a route is selected by the optical switch, and this adversely affects network operation.

  Therefore, there is a demand for a mechanism that compensates for group velocity dispersion / delay and removes / suppresses ASE noise and MPI noise in optical switch nodes and transmission lines. In particular, in order to use it in an optical network, we do not want to use as much as possible that which limits the transmission band. There is a need for a variable dispersion / delay compensation and optical signal regeneration device that operates in a wide band. Furthermore, since it is expected that optical signals of various modulation formats will be exchanged in an optical network, an apparatus that is as independent of the modulation format as possible is desirable. In that respect, in the parametric process, the phase information of the input optical signal is retained, and the operation independent of the modulation format can be easily realized. Therefore, it can be said that the optical signal processing apparatus using this has an unparalleled advantage.

An optical signal regenerator (P-OR) using a parametric process is already known (see Non-Patent Document 1). However, in order to properly operate the optical signal reproducing device, it is necessary to appropriately control the chirp of the input signal. In addition, in optical 3R signal regeneration including retiming, it is necessary to reliably synchronize the timing delay of the input signal and the local clock.
In particular, in an environment such as an optical network where the transmission path is dynamically switched and the chirp and delay of the optical signal change greatly each time, dynamic dispersion and flexible control of the delay are indispensable functions. Although the chirp of the input signal can be controlled by variable dispersion compensation, a variable dispersion compensation technique that has a sufficiently wide band and operates at a sufficiently high speed has been difficult in the past.

On the other hand, a parametric variable dispersion compensator (P-TDC) and a parametric delay dispersion tuner (PDDT) (see Patent Document 1, Non-Patent Documents 2 to 3) have been invented, and variable dispersion compensation that can operate satisfactorily in an optical network environment. And delay control became possible.
However, assuming that it operates properly in an optical network environment, a parametric optical signal regenerator (P-OR) is combined with P-TDC / PDDT to comprehensively suppress degradation factors of optical signals. The technology has not been studied so far.

JP 2009-65570 A

K. Inoue, “Optical level equalization based on gain saturation in fiber optical parametric amplifier,” Electron. Lett., 36, 1016-1017 (2000). S. Namiki, “Wide-Band and -Range Tunable Dispersion Compensation Through Parametric Wavelength Conversion and Dispersive Optical Fibers,” J. Lightwave Technol., 26, 28-35 (2008) S. Namiki and T. Kurosu, “17 ns Tunable Delay for Picosecond Pulses through Simultaneous and Independent Control of Delay and Dispersion Using Cascaded Parametric Processes,” ECOC2008, PDP Th.3.C.3.

  An object of the present invention is to compensate and suppress in-line noise such as chirp, delay, and ASE of an optical signal that dynamically changes in an optical network environment in an optical signal regeneration device using a parametric process.

The above problem is solved by the following optical signal reproducing device.
(1) In a phase-maintaining wavelength converter that uses a parametric process of an optical waveguide that variably changes the frequency of a carrier wave while maintaining the phase of the optical signal, the dispersion slope S and the wavelength λ ₀ , 3 at which the dispersion value becomes zero Prepare an optical waveguide with the following nonlinear constant γ, input wavelength near the short wavelength peak wavelength λ _S of the parametric gain or near the long wavelength peak wavelength λ _L of the parametric gain, and the output wavelength the length of the parametric gain. The wavelength λ _{p of the} pump light and the pump light of the phase-maintaining wavelength converter that can be arranged near the peak wavelength λ _L of the wavelength or near the peak wavelength λ _S of the short wavelength of the parametric gain, respectively. An optical signal regeneration device equipped with a phase-maintaining wavelength converter characterized by determining the instantaneous intensity P _p of the optical signal.
Here, π is the circular ratio, and c is the speed of light in vacuum.
(2) The optical signal regeneration device according to (1), wherein a variable optical attenuator or an optical amplifier is disposed before and after the phase maintaining wavelength converter.
(3) Two variable optical attenuators or optical amplifiers arranged before and after the phase maintaining wavelength converter are connected in series, and the output wavelength at the connection point is λ _L. (2) The optical signal reproducing device according to (2).
(4) The optical signal regeneration according to (3), wherein the pump light of each of the two phase-maintaining wavelength converters is configured to be subjected to phase modulation with exactly opposite phase when viewed from the optical signal. apparatus.
(5) A first dispersion medium for transmitting an optical signal, a phase maintaining wavelength converter, a second dispersion medium, and the phase maintaining wavelength converter are further connected in cascade. (1) The optical signal reproducing device according to any one of (4).
(6) The optical signal according to (5), wherein a third dispersion medium and a phase-maintaining wavelength converter are further connected in series before the first dispersion medium that transmits the optical signal. Playback device.
(7) The optical signal regenerating apparatus according to any one of (1) to (6), wherein an optical pulse train synchronized with an input optical signal is used as pump light used in the phase maintaining wavelength converter.

According to the present invention, optical signal degradation factors such as spontaneous emission optical noise, group velocity dispersion, and delay fluctuation received by an optical signal during optical transmission are dynamically and flexibly suppressed or compensated, and transmission is performed in response to path switching of the transmission path. The distance can be extended dramatically.
In addition, according to the present invention, it is possible to realize a dispersion variable amount that is not limited to a band and sufficient high speed necessary for an incoming optical network, and to reduce noise accumulated during transmission. It is expected to contribute greatly to the realization of networks.

Basic principle diagram of the present invention Basic configuration of fiber-type optical parametric wavelength converter Transfer function of input power and output power and principle of optical signal regeneration Output spectrum of spontaneous emission due to parametric gain when the pump wavelength is in the anomalous dispersion region of nonlinear fiber Relationship between pump wavelength and maximum gain wavelength for nonlinear fiber with zero dispersion wavelength at 1542nm. Results of experimental verification of [Formula 1] Experimentally measured transfer function Uniform transfer function curve obtained by adjusting the attenuation value by placing a variable optical attenuator at the input / output part of the wavelength tunable optical regenerator. Code error rate curve for input signal wavelength 1531nm Relationship between power penalty improvement effect (optical signal regeneration effect) and input signal wavelength according to the present invention Wavelength tunable optical signal regeneration device of embodiment 1 Wavelength variable optical signal regenerating apparatus of embodiment 2 Wavelength tunable optical signal regeneration device of embodiment 3 Wavelength tunable optical signal regeneration device of embodiment 4 Wavelength variable optical signal regenerator of embodiment 5 Wavelength tunable optical signal regenerator of embodiment 6 Wavelength tunable optical signal regenerator of Example 7 Wavelength tunable optical signal regenerating apparatus of embodiment 8 Wavelength variable optical signal regenerator of embodiment 9 Wavelength variable optical signal regenerating apparatus of embodiment 10 Wavelength tunable optical signal regeneration device of example 11

(Basic principle of the present invention)
The present invention is an optical signal regeneration that enables dynamic dispersion compensation, delay compensation, and removal and suppression of ASE and MPI noise at the same time for various optical signals in order to operate optical networks freely. Is realized.
A basic principle diagram of the present invention is shown in FIG.
Repairs signals that have deteriorated due to dispersion, delay, and ASE noise on the transmission line. The input pulse chirp is adjusted with the P-TDC or PDDT at the front stage, the delay is controlled, and the wavelength variable optical signal regenerator at the rear stage returns the signal wavelength to the input wavelength, and at the same time, the intensity noise is suppressed and the optical signal is output as it is To do.

The present invention employs a variable wavelength converter having an optical signal regeneration function in a wavelength conversion unit that plays a role of returning an output optical signal wavelength arranged at the end of an inline P-TDC or PDDT to an input wavelength. Take. In order to use P-TDC and PDDT in-line, it is necessary to output the same wavelength as that of the input optical signal. To this end, a parametric variable wavelength converter must be added after these devices. . In order to add an optical signal processing effect to this function, it is necessary to realize an optical signal regeneration function having a variability that converts an arbitrary wavelength into a desired wavelength (that is, an input wavelength to P-TDC or PDDT). . In the existing P-OR technology, how to achieve wavelength variability has not been studied. In particular, there is a problem if the optical signal reproduction effect changes depending on the input wavelength.
A method for obtaining a uniform optical signal reproduction effect with respect to an arbitrary input signal wavelength, which is the core of the present invention, will be described below.

P-OR uses a saturation phenomenon in a fiber-type optical parametric amplification (FOPA) process based on the third-order nonlinear effect of an optical fiber (see Non-Patent Document 1).
In P-OR using the FOPA saturation process, a copy of the signal light is generated at different wavelengths as idler light in the four-wave mixing process. When configured to handle idler light as an output, it functions as a wavelength converter having an optical signal reproduction effect. The saturation phenomenon is a phenomenon in which the energy of the FOPA pump light is depleted due to the interaction with the input optical signal, and saturation occurs as the input optical signal increases, so an optical signal with a high optical intensity is an optical signal with a low optical intensity. Compared to, it will receive a small gain. In FOPA, the wavelength of an ordinary optical signal is not changed, but here, the discussion will be focused on idler light generated in a parametric process, that is, wavelength-converted light.
The configuration of a basic fiber type optical parametric wavelength converter is shown in FIG.

FIG. 3 shows the relationship (transfer function: transfer function) between the input light instantaneous intensity and the output light instantaneous intensity (after wavelength conversion) in the saturation type parametric wavelength conversion process. When the input light intensity increases, the output light intensity saturates, and a region where the instantaneous intensity is stable with respect to input changes occurs. For example, even if the input signal intensity fluctuates due to noise, if the average power of the input light is adjusted so as to be optimal with respect to the transfer function, the noise of the output optical signal can be made smaller than that at the time of input. This phenomenon is utilized in optical signal regeneration.
In the present invention, in order to apply this phenomenon to the wavelength conversion process at the last stage of P-TDC / PDDT, a method for realizing a uniform optical signal reproduction effect for any input wavelength is disclosed. That is, a method for uniformly designing the curve shown in FIG. 3, that is, the transfer function, is provided for any input wavelength and conversion wavelength.

In order to obtain a uniform transfer function, in principle, a uniform or highly efficient parametric gain should be realized for any input wavelength. In the parametric gain, the nonlinear phase shift of the pump light cannot be ignored in the saturation region. Therefore, in order to obtain an efficient gain, it is effective to operate the wavelength of the pump light in an anomalous dispersion region of a nonlinear fiber that generates a parametric process. At that time, as shown in FIG. 4, gain peaks occur on both sides of the pump light on the optical spectrum.
In order to realize efficient optical signal regeneration, the signal wavelength and the conversion wavelength may be arranged so as to substantially match the gain peaks on both sides.
FIG. 4 shows an output spectrum of spontaneous emission light due to parametric gain when the pump wavelength is in the anomalous dispersion region of the nonlinear fiber.
The spectral shape of spontaneous emission light corresponds to the spectral shape of parametric gain, and peaks of parametric gain occur on both sides of the pump wavelength.
If a condition where this relationship holds for an arbitrary input optical signal wavelength is searched, variability of the input wavelength and highly efficient gain saturation, that is, an optical signal regeneration effect can be obtained. When the peak wavelength of the gain is calculated in order to find this condition, the peak wavelength λ _S of the short wavelength of the parametric gain and the peak wavelength λ _L of the long wavelength of the parametric gain are expressed by the following [Equation 1].
Here, p represents pump light, and λ ₀ represents a wavelength at which the dispersion value becomes zero. π is the circumference, c is the speed of light in vacuum, γ is the third-order nonlinear constant of the fiber, and Pp is the instantaneous light intensity of the pump light. S is the dispersion slope (ps / nm ² / km) of the optical fiber.

Prepare an optical fiber with a dispersion slope S, a wavelength λ ₀ where the dispersion value is zero, and a third-order nonlinear constant γ, and each input wavelength is the short wavelength peak wavelength λ _S , and the output wavelength is the parametric gain length What is necessary is just to determine the wavelength λ _p of the pump light and the instantaneous intensity P _p of the pump light that satisfy the formula [1] such that they can be respectively arranged at the peak wavelength λ _L of the wavelength.
What is important here is that the input wavelength and the output wavelength do not work unless they exactly match λ _S or λ _L. The input wavelength and the output wavelength may be in the vicinity of λ _S and λ _L , that is, the parametric gain may be within 3 db lower than the values at λ _S and λ _L.
Further, the input wavelength can be arranged in the vicinity of the long wavelength peak wavelength λ _L of the parametric gain, and the output wavelength can be arranged in the vicinity of the short wavelength peak wavelength λ _S of the parametric gain.

FIG. 5 shows the formula [Formula 1] that is actually calculated with specific numerical values.
FIG. 5 shows the relationship between the pump wavelength and the maximum gain wavelength for a nonlinear fiber having a zero dispersion wavelength of 1542 nm. The left figure shows a curve when γP _{p is changed} to 4.1 (solid line), 6.5 (dotted line) and 10.3 km ⁻¹ (broken line) when the dispersion slope S = 0.026 ps / nm ² / km. The right figure shows a curve when the dispersion slope S is changed to 0.004 (solid line), 0.013 (dotted line), and 0.026 ps / nm ² / km (broken line) when γP _p is fixed at 10.3 km ⁻¹ .
This figure shows that when the pump wavelength is shifted, the peak wavelength of the parametric gain is moved accordingly. While the peak gain wavelength on the short wavelength side changes relatively large, it can be confirmed that the peak gain wavelength on the long wavelength side hardly changes. That is, for example, if signal light is arranged on the short wavelength side peak gain wavelength line and converted light is arranged on the long wavelength side line, the present invention can always operate under the condition of maximum parametric gain.

FIG. 6 shows the result of experimental verification of the formula [1]. The left figure of FIG. 6 shows the spontaneous emission light spectrum generated by the parametric gain when the pump wavelength is changed. The maximum gain wavelength at each pump wavelength is plotted on the right side of FIG.
The zero-dispersion wavelength of the nonlinear fiber used in the experiment is 1542 nm, and the slope is 0.026 ps / nm ² / km. γP _p is 10.3 km ⁻¹ . The figure on the left is the nonlinear fiber output light spectrum at pump wavelengths 1542, 1544, 1546, 1548, 1550, 1552, and 1554 nm. The right figure shows the relationship between the pump wavelength and the maximum gain wavelength. The curve is a theoretical value according to the formula [1].
FIG. 6 shows that a high-efficiency and uniform wavelength-variable optical signal reproduction effect in which the output wavelength is fixed at 1561 nm over the continuous wavelength range from the signal wavelength 1521 nm to 1543 nm can be realized.

FIG. 7 shows the result of measuring the transfer function of light output under each condition using this fiber. The conversion wavelength was fixed at 1561 nm.
In FIG. 7, it can be seen that the curves of the respective transfer functions show saturation characteristics and a desired optical signal regeneration effect can be expected, but a uniform output curve cannot be obtained for the same input power. Therefore, in the present invention, a uniform transfer function can be obtained for the same input power by arranging variable optical attenuators at the input and output of the optical signal reproducing apparatus.
FIG. 8 shows that the variable optical attenuators are actually arranged in the input / output section and the transfer function curves at the respective pump wavelengths almost overlap.
In this way, it is possible to obtain a uniform optical signal reproduction effect with respect to an arbitrary input wavelength while fixing the output wavelength to 1561 nm.

FIG. 9 shows a measurement comparison of the code error rate of a 10.75 Gbit / s Return-to-Zero (RZ) optical signal that actually uses this curve. From FIG. 9, it was confirmed that the transmission penalty was also reduced by passing the optical signal having the transmission noise caused by the intensity noise through the optical signal regenerator.
Back-to-back (●), after wavelength conversion (▲), back-to-back of signal with intensity noise added (■), after wavelength conversion with optical signal regeneration effect (★). The inset shows each eye pattern and its histogram. It is confirmed that the intensity noise is suppressed by the optical signal reproduction effect.

Similarly, the results of the optical signal reproduction effect obtained for different input signal wavelengths are shown in FIG. It can be seen that the effect of improving the transmission characteristics was obtained over the range of the input signal wavelength of 20 nm.
As described above, optical signal reproduction with variable wavelength can be realized. In the present invention, as shown in FIG. 1, this part is combined with P-TDC and PDDT to realize an optical signal reproduction technique applicable to a dynamic optical network.
Although an optical fiber has been described as an example of the optical waveguide, it is needless to say that the present invention is not limited to the optical fiber but can be a general optical waveguide.

Examples of the present invention are illustrated below.
Example 1
FIG. 11 shows a variable wavelength optical signal regeneration having a function of adjusting the optical signal regeneration effect with respect to the wavelength by arranging a variable optical attenuator or an optical amplifier before and after the fiber optical parametric wavelength converter. Device. In the figure, BPF is a variable band-pass filter.

(Example 2)
As shown in FIG. 12, by connecting two wavelength-variable optical signal reproducing devices prepared so as to satisfy the [Equation 1] in series, λ _L in the [Equation 1] is used as an intermediate wavelength. Both wavelength and output wavelength can be made freely variable. In the figure, BPF is a variable band-pass filter.

(Example 3)
As shown in FIG. 13, the two pump lights of the second embodiment are phase-modulated so as to have opposite phases when viewed from the optical signal, thereby suppressing the generation of stimulated Brillouin scattering (SBS) in the highly nonlinear fiber, Signal transmission quality can be kept good. When SBS occurs, the quality of the optical signal is significantly degraded by SBS noise. However, the generation of SBS can be suppressed by performing phase modulation on the pump light. However, when phase modulation is applied to the pump light, phase noise is added to the optical signal light. However, by applying phase modulation to the pump light so that the phases are opposite to each other in the wavelength conversion process configured in a two-stage column. The phase modulation added to the signal light can be canceled. In the figure, BPF is a variable band-pass filter.

Example 4
FIG. 14 shows a bidirectional optical signal regeneration device that realizes the configuration of the third embodiment with one highly nonlinear fiber (HNLF). In the figure, TLS is a wavelength tunable light source, EDFA is an erbium-doped fiber amplifier, VOA is a variable optical attenuator, and PC is a polarization controller.

(Example 5)
FIG. 15 shows a configuration in which stimulated Brillouin scattering is suppressed by applying phase modulation to the output of the TLS that is the excitation light source of the fourth embodiment. The two phase modulators (PM) are driven by the same RF source and adjust the timing so as to cancel the influence of the phase modulation received by the converted light.

(Example 6)
FIG. 16 shows a common pump light in Examples 4 and 5. According to this, a signal always having the same wavelength as the incident light signal is output.

(Example 7)
FIG. 17 shows an optical 3R reproduction by adding a retiming effect to the optical signal reproduction portions of Examples 4 to 6. In the figure, PD is a light receiving element, CR is a clock extractor, and MLLD is a mode-locked laser.

(Example 8)
FIG. 18 shows a variable dispersion compensation / optical 2R signal reproducing apparatus. DCF controls the wavelength in conjunction with dispersion compensating fiber and BPF also works in conjunction with TLS. The output wavelength is not necessarily the same as the input wavelength.

Example 9
FIG. 19 shows a variable dispersion compensation / optical 3R signal reproducing apparatus. DCF controls the wavelength in conjunction with dispersion compensating fiber and BPF also works in conjunction with TLS. The output wavelength is not necessarily the same as the input wavelength. In the figure, ISO is an optical isolator, Pol is a polarizer, and Δτ is a delay device.

(Example 10)
FIG. 20 shows a variable delay dispersion compensation / optical 3R signal regeneration device. DCF controls the wavelength in conjunction with dispersion compensating fiber and BPF also works in conjunction with TLS. The output wavelength is not necessarily the same as the input wavelength.

(Example 11)
FIG. 21 shows a variable dispersion compensation / optical 3R signal reproducing apparatus. DCF controls the wavelength in conjunction with dispersion compensating fiber and BPF also works in conjunction with TLS. The output wavelength is not necessarily the same as the input wavelength. In the figure, WC is a wavelength converter.

As described in the background art, the high-speed and bandwidth-restricted optical signal regeneration apparatus of the present invention is not limited to point-to-point optical transmission, but rather to an optical network in which optical signals are dynamically switched and routed. Since it is indispensable, it can be applied to a technology in which the communication capacity is increased, which requires high-speed variable dispersion compensation without band limitation.

Claims (7)

In a phase-maintaining wavelength converter that uses a parametric process of an optical waveguide that variably changes the frequency of a carrier wave while maintaining the phase of an optical signal, a dispersion slope S, a wavelength λ _{0 at} which the dispersion value is zero, a third-order nonlinearity An optical waveguide having a constant γ is prepared, the input wavelength is near the short wavelength peak wavelength λ _S of the parametric gain or the long wavelength peak wavelength λ _L of the parametric gain, and the output wavelength is the peak of the long wavelength of the parametric gain. The pump light wavelength λ _p and the instantaneous intensity of the pump light of the phase-maintaining wavelength converter that can be placed near the wavelength λ _L or near the peak wavelength λ _S of the short wavelength of the parametric gain, respectively. An optical signal regeneration device including a phase-maintaining wavelength converter characterized by determining P _p .
Here, π is the circular ratio, and c is the speed of light in vacuum. 2. The optical signal reproducing apparatus according to claim 1, wherein a variable optical attenuator or an optical amplifier is disposed before and after the phase maintaining wavelength converter. Two variable optical attenuators or optical amplifiers arranged before and after the phase maintaining wavelength converter are connected in series, and the output wavelength at the connection point is λ _L. The optical signal reproducing device according to claim 2. 4. The optical signal regenerating apparatus according to claim 3, wherein the pump light of each of the two phase-maintaining wavelength converters is configured to be subjected to phase modulation of exactly opposite phase when viewed from the optical signal. 2. The first dispersion medium for transmitting an optical signal, a phase-maintaining wavelength converter, a second dispersion medium, and the phase-maintaining wavelength converter are further connected in cascade. 5. The optical signal regeneration device according to any one of items 1 to 4. 6. The optical signal reproducing apparatus according to claim 5, wherein a third dispersion medium and a phase-maintaining wavelength converter are further connected in series before the first dispersion medium for transmitting an optical signal. . 7. The optical signal regeneration apparatus according to claim 1, wherein an optical pulse train synchronized with an input optical signal is used as pump light used in the phase maintaining wavelength converter.