JPWO2018155017A1 - Wavelength conversion method and wavelength converter - Google Patents

Abstract

With the all-optical wavelength converter method reported so far, it is not possible to perform operation of converting a wavelength multiplexed signal to an arbitrary output wavelength without distortion, such as when the input signal light is limited to a single wavelength channel. Was. In an all-optical wavelength converter having a configuration in which the degenerate FWM is cascaded in two stages, the wavelength to be allocated to the intermediate signal light obtained after the first-stage conversion is clearly specified, so that the wavelength multiplexed signal having an arbitrary band in the C band can be obtained. On the other hand, a conversion operation for an arbitrary input / output wavelength is performed without distortion.

Description

  The present invention relates to an all-optical wavelength conversion method and all-optical wavelength converter that can be used for optical fiber communication.

For the purpose of improving the wavelength resource utilization efficiency in the optical network, the introduction of a wavelength converter that converts the wavelength of the optical signal at the node is being studied.
An optical-electrical / electrical-optical (OE / EO) conversion type wavelength conversion method using an optical transceiver can be realized based on the prior art, but the bit rate and modulation method of an inputable signal are fixed. There is no flexibility and multi-wavelength signals cannot be processed collectively.

Thus, research and development have been conducted on all-optical wavelength converters that convert the wavelength of an optical signal as it is.
Above all, low-loss and wide-band operation is possible, so all-optical wavelength converters using four-wave mixing (FWM), which is a nonlinear effect generated in optical fibers, are promising, and various methods have been proposed. I came.

  Two stages of degenerate FWM cascade, the input signal light is once converted to another wavelength by the first-stage wavelength conversion (the signal light obtained here is called the intermediate signal light), and the intermediate signal light is converted by the second-stage conversion. An all-optical wavelength converter configured to convert to an arbitrary output wavelength within the operating band is known (Non-Patent Document 1 to Non-Patent Document 3).

  This all-optical wavelength converter has a configuration that enables a conversion operation corresponding to an arbitrary input / output wavelength without a “guard band” meaning a wavelength band in which the wavelength conversion operation is impossible, and further, an input signal. This is the most promising because the phase of light is maintained and polarization independent operation is possible.

S. Petit et al., “Low penalty uniformly tunable wavelength conversion without spectral inversion over 30 nm using SBS-suppressed low-dispersion-slope highly nonlinear fibers,” IEEE Photon. Technol. Lett., Vol. 23, no. 9, pp. 546-548, May 2011. T. Inoue et al., “Guard-band-less and polarization-insensitive tunable wavelength converter for phase-modulated signals: demonstration and signal quality analyzes,” J. Lightw. Technol., Vol. 32, no. 10, pp. 1981-1990, May 2014. H. Nguyen Tan et al., “On the Cascadability of All-Optical Wavelength Converter for High-Order QAM Formats,” J. Lightw. Technol., Vol. 34, no. 13, pp. 3194-3205, July 2016.

  Considering practicality, a wavelength multiplexed signal having an arbitrary band and a center wavelength in the wavelength band called a C band having a wavelength range of 1530 to 1565 nm or a frequency range of 195.9 to 191.6 THz. On the other hand, it is important to be able to perform an operation of converting to an arbitrary output wavelength without distortion.

  However, the all-optical wavelength converter methods reported so far have not clarified a method for reliably performing the required operation, for example, the input signal light is limited to a single wavelength channel.

  In the present invention, in the all-optical wavelength converter configured to cascade the degenerate FWM in two stages, in particular, the wavelength allocated to the intermediate signal light obtained after the first stage conversion is clearly defined to have an arbitrary band within the C band. A method and a wavelength converter for performing a conversion operation on an arbitrary input / output wavelength without distortion on a wavelength multiplexed signal are proposed.

(1) An operation band is a predetermined frequency range, and input signal light having an arbitrary wavelength (carrier optical frequency) within the operation band is held in an optical region while holding information held by the input signal light. A wavelength conversion method for converting a wavelength of an input signal light into a wavelength of an arbitrary output signal light within the operating band,
Amplifying the input signal light by a first optical amplifier having a gain flattening design within the operating band;
Next, the input signal light is converted by the first-stage wavelength conversion by degenerate four-wave mixing generated in the first nonlinear medium by the amplified input signal light and the first-stage pump light whose wavelength is set according to the wavelength. Is converted into an intermediate signal light wavelength outside the operating band,
Next, the intermediate signal light is amplified by a second optical amplifier,
Next, the intermediate signal light by the second-stage wavelength conversion by degenerate four-wave mixing generated in the second nonlinear medium by the amplified intermediate signal light and the second-stage pump light whose wavelength is set according to the wavelength. In the case of converting the wavelength of the signal light into the wavelength of the arbitrary output signal light within the operating band,
The maximum frequency of the input signal light is f1, the minimum frequency is f2, the signal band is B = f1-f2, the maximum frequency of the intermediate signal light is f3, and the minimum frequency is f4 = f3-B. The minimum frequency is set to a value greater than 191.2 THz, and the maximum frequency f3 of the intermediate signal light is fixed to a value equal to or less than 191.2 THz,
Then, by applying a gain flattening design to the second optical amplifier in the frequency band below f3,
Controlling to output the wavelength of the output signal light without distortion,
A wavelength conversion method characterized by the above.

(2) The wavelength conversion method according to (1),
The input signal light is a wavelength multiplexed signal;
A wavelength conversion method characterized by the above.

(3) The wavelength conversion method according to any one of (1) and (2),
Immediately after the first-stage wavelength conversion is performed, using a low-pass filter having a cutoff frequency smaller than the minimum frequency of the first-stage pump light used in the first-stage wavelength conversion, Removing only the intermediate signal light by removing the input signal light and the first-stage pump light;
A wavelength conversion method characterized by the above.

(4) The wavelength conversion method according to any one of (1) and (3),
When the minimum frequency of the operating band is fmin,
satisfy fmin−f3>2B;
A wavelength conversion method characterized by the above.
(5) The method according to any one of (1) and (4),
The nonlinear medium is an optical fiber;
A wavelength conversion method characterized by the above.

(6) From the input signal light amplifying unit including the first optical amplifier as the input unit, the first-stage wavelength conversion unit and the intermediate signal light amplification unit including the second optical amplifier and the second-stage length conversion unit connected in order Become
The first-stage wavelength converter and the second-stage wavelength converter are connected to the first-stage pump light generator and the second-stage pump light generator connected to the pump signal frequency dithering RF signal generator, respectively.
The input signal light is held in the optical region while maintaining the information of the input signal light with respect to the input signal light having an arbitrary wavelength (carrier optical frequency) within the operation band as a predetermined frequency range. A wavelength converter that converts the wavelength of the output signal into a wavelength of any output signal light within the operating band,
Amplifying the input signal light by a first optical amplifier having a gain flattening design within the operating band in an input signal light amplifier;
Next, the first-stage wavelength by degenerate four-wave mixing generated in the first nonlinear medium by the amplified input signal light and the first-stage pump light whose wavelength is set according to the wavelength in the first-stage wavelength conversion unit By converting the wavelength of the input signal light into the wavelength of the intermediate signal light outside the operating band,
Next, the intermediate signal light amplification unit amplifies the intermediate signal light by a second optical amplifier,
Next, in the second stage length conversion unit, the amplified intermediate signal light and the second stage pump light whose wavelength is set in accordance with the wavelength of the amplified intermediate signal light are generated by degenerate four-wave mixing generated in the second nonlinear medium. In the case of converting the wavelength of the intermediate signal light into the wavelength of the arbitrary output signal light within the operating band by wavelength conversion of the stage,
The maximum frequency of the input signal light is f1, the minimum frequency is f2, the signal band is B = f1-f2, the maximum frequency of the intermediate signal light is f3, and the minimum frequency is f4 = f3-B. The minimum frequency is set to a value greater than 191.2 THz, and the maximum frequency f3 of the intermediate signal light is fixed to a value equal to or less than 191.2 THz,
Then, by applying a gain flattening design to the second optical amplifier in the frequency band below f3,
Controlling to output the wavelength of the output signal light without distortion,
A wavelength converter characterized by.

(7) The wavelength converter according to (1),
The wavelength converter, wherein the input signal light is a wavelength multiplexed signal.
(8) The wavelength converter according to any one of (6) and (7),
Using the low-pass filter having a cut-off frequency smaller than the minimum frequency of the first stage pump light in the intermediate signal light amplification unit, the input signal light and the first stage pump light are removed to remove the intermediate signal Allowing only light to pass through,
A wavelength converter characterized by.
(9) The wavelength converter according to any one of (6) and (8),
When the minimum frequency of the operating band is fmin,
satisfy fmin−f3>2B;
A wavelength converter characterized by.
(10) The wavelength converter according to any one of (6) and (9),
The nonlinear medium is an optical fiber;
A wavelength converter characterized by.

  An all-optical wavelength conversion operation corresponding to an arbitrary input / output wavelength can be performed without distortion on a wavelength multiplexed signal having an arbitrary band in the C-band.

2 is a diagram illustrating an internal configuration of a wavelength converter 16. FIG. 2 is a diagram illustrating a configuration of a polarization diversity module 12. FIG. It is the figure which showed on the frequency axis the spectrum of the signal light at the time of wavelength conversion, pump light, and intermediate | middle signal light. 4 (a), 4 (b), and 4 (c) representing the present invention correspond to the case where the frequency of the signal light is set to the general condition, the lowest frequency side and the highest frequency side, respectively. The spectrum at the time of wavelength conversion with respect to the signal light whose band is B when fmax = 195.9 THz, fmin = 191.6 THz, and f3 = 191.2 THz as operating bands is shown on the frequency axis. FIG. 4D shows, as a comparative example of the present invention, the spectrum at the time of wavelength conversion for the signal light whose band is B when the maximum frequency f3 of the intermediate signal light is set to 191.6 THz in the C band. This is shown on the axis. It is the schematic diagram which showed the mode of the wavelength conversion by a single pump light on the frequency axis by using the WDM signal of the zone | band B as input light. It is a figure which shows the various frequency settings used in experiment. It is a figure which shows the result of having measured the transfer function of (a) HPF and (b) LPF1 and LPF2. It is a figure which shows the experimental system 26 for performing the experiment which inputs the 8-channel WDM signal into a wavelength converter, and evaluates the quality of the output signal light in which the wavelength was converted. It is a figure which shows the result of having measured and overdrawing the optical spectrum of the 8-channel WDM signal as input signal light about Case 1,2,3 each. It is a figure which shows the measurement result of the optical spectrum output from the polarization diversity module of the 1st step | paragraph wavelength conversion part in a wavelength converter. When the frequency arrangement of the input signal light is Case 1 and the frequency arrangement of the output signal light is Case 1, 2, 3, the measurement result of the optical spectrum output from the polarization diversity module of the second-stage wavelength converter is shown. FIG. The measurement result of the optical spectrum output from the polarization diversity module of the second-stage wavelength converter when the frequency arrangement of the input signal light is Case 2 and the frequency arrangement of the output signal light is Case 1, 2, 3 FIG. The measurement result of the optical spectrum output from the polarization diversity module of the second-stage wavelength converter when the frequency arrangement of the input signal light is Case 3 and the frequency arrangement of the output signal light is Case 1, 2, 3 FIG. It is a figure which shows the result of having measured the Q value for 3 channels of 8 channel WDM signals when the frequency arrangement | positioning of input-output signal light is either Case 1, 2 or 3, respectively. It is a figure which shows the result of having measured Q value and bit error rate (BER) when receiving OSNR is changed about the channel whose frequency is 193.5 THz when the frequency arrangement | positioning of input-output signal light is Case2.

First, a basic configuration of a wavelength converter for carrying out the present invention will be described (Non-Patent Document 3).
FIG. 1 is a diagram illustrating the internal configuration of the wavelength converter.

  The input signal light has a center wavelength (carrier optical frequency) within a certain operating band, and when this is input to the wavelength converter, the input signal light amplifying unit 19, the first-stage wavelength converting unit 20, the intermediate signal Through the optical amplifying unit 21 and the second-stage wavelength converting unit 22, the center wavelength (carrier optical frequency) is converted into an arbitrary value within the operation band and output.

The input signal light amplifier and the intermediate signal light amplifier are composed of an optical amplifier and an optical filter.
Although not shown in FIG. 1, an optical amplifier and an optical filter may also be provided as an output signal light amplification unit after the second-stage wavelength conversion unit.
On the other hand, when the input signal light has a necessary and sufficient optical power, the wavelength converter does not have to include the input signal light amplification unit, and the input signal light is directly input to the first-stage wavelength conversion unit. There may be.

  The first-stage and second-stage wavelength conversion units include a polarization diversity module 12, a polarization-maintaining highly nonlinear fiber 11 (PM-HNLF) connected thereto, and an optical filter 4. Is a wavelength-variable and high-power continuous light generated by the first-stage and second-stage pump light generators, respectively, as pump light.

  In each stage of PM-HNLF, a degenerate FWM is generated by a single pump light, and the input signal light is converted as an intermediate signal light having a wavelength outside the operation band by the wavelength conversion of the first stage, and then the second stage. The intermediate signal light is converted into output signal light having a certain wavelength within the operating band by wavelength conversion.

By changing the wavelength of the pump light supplied to the first-stage and second-stage wavelength converters to an appropriate value, a variable wavelength conversion operation corresponding to any combination of input and output wavelengths can be performed within the operation band.
In FIG. 1, the use of an optical fiber having polarization maintaining characteristics and high nonlinearity as a nonlinear medium for generating FWM and based on quartz glass as a material is considered. A waveguide device made of silicon, an optical fiber using chalcogenide glass, a waveguide device, or a semiconductor optical amplifier (SOA) can also be used. Among these, at present, the PM-HNLF is the best nonlinear medium from the viewpoint of polarization maintaining characteristics, cumulative nonlinear effect, insertion loss, connectivity with other optical fibers, and the like.

  By using the polarization diversity module 12 having the configuration shown in FIG. 2, a polarization-independent wavelength conversion operation in which the operation of the FWM does not depend on the polarization plane (polarization plane) of the input signal light is possible.

  Furthermore, in the wavelength conversion process at each stage, the wavelength (frequency) of the pump light is phase-modulated to suppress stimulated Brillouin scattering (SBS) caused in the optical fiber by the high-power pump light (this is dithered). For this purpose, the optical phase modulator 7 is used in the pump light generation units 23 and 24 of each stage, and the frequency generated by the RF signal generation unit 25 for pump optical frequency dithering is an RF having a frequency of about 300 MHz. A signal is applied.

  At this time, the RF signal is bifurcated, the intensity difference and relative phase of each are adjusted, and the phase modulation for the first-stage and second-stage pump lights is synchronized, so that the phase modulation added to the output signal light through the FWM "Counter dither" that cancels the effect of can be enabled.

  It is another object of the present invention to convert a WDM signal having an arbitrary center wavelength within a C-band to meet any input / output wavelength condition without generating waveform distortion on the frequency (wavelength) axis. The operation is performed, and various frequency settings necessary for this purpose will be described below.

  FIG. 3 shows on the frequency axis the spectrum of signal light, pump light, and intermediate signal light during wavelength conversion.

  When the operation band is set to fmax> = f> = fmin, for example, fmax = 195.9 THz (153.334 nm) and fmin = 191.2 THz (1567.952 nm), the operation band covers the C band. .

  Among these operating bands, there is a WDM signal having a band B, and assuming that the maximum and minimum frequencies of the WDM signal are f1 and f2, respectively, B = f1-f2.

  On the other hand, if the maximum and minimum frequencies of the intermediate signal light are f3 and f4, respectively, B = f3-f4. At this time, the frequency of the pump light must be fp = (f2 + f3) / 2.

  Note that the wavelength conversion situation shown in this figure is seen in both the first-stage and second-stage wavelength conversion sections. When considering the first-stage wavelength conversion, the frequency is f1> = f> = f2. The signal light means input signal light, which is converted into intermediate signal light of f3> = f> = f4.

When the second stage is considered, the signal light means output signal light, and is converted from intermediate signal light to output signal light.
Since the wavelength conversion considered here is based on degenerate FWM, the signal light of frequency f1 and the intermediate signal light of f4 correspond before and after wavelength conversion, and the signal light of f2 and the intermediate signal light of f3 correspond. Thus, the corresponding signal lights before and after wavelength conversion are in a phase conjugate relationship.

By the way, in the conventionally known wavelength conversion method, there is no restriction on the frequencies f3 and f4 of the intermediate signal light, and the intermediate signal light may exist in the C band.
In this case, if the input signal light is a single channel, the frequency of the intermediate signal light is set flexibly so that the operation is not hindered.

  However, for the input of a WDM signal having a finite band B, the frequency band of the intermediate signal light is not limited to the C band and the frequency is f <191.6 THz, depending on the center frequency and bandwidth conditions of the input signal light. In some cases, the L band band is reached.

Since an apparatus for amplifying a WDM signal extending over the C and L bands with a flat gain is not readily available, a problem arises in the amplification operation after wavelength conversion.
Therefore, the present invention solves the problem by fixing the maximum frequency f3 of the intermediate signal light to, for example, f3 = 191.2 THz (1567.952 nm) in the L band and setting f4 = f3-B.

  4 (a) to 4 (c) show the wavelength conversion operation for the signal light whose band is B [THz] when fmax = 195.9 THz, fmin = 191.6 THz, and f3 = 191.2 THz. Is shown on the frequency axis.

FIG. 4A is a diagram corresponding to a general condition regarding the frequency setting of the signal light. FIGS. 4B and 4C show that the frequency of the signal light is set to the lowest frequency side and the high frequency side, respectively. Corresponds to the case.
In either case, the maximum frequency f3 of the intermediate signal light is fixed at 191.2 THz.
At this time, the frequency fp of the pump light is determined by the minimum frequency f2 of the signal light, and is given as fp = (f2 + f3) / 2 = (f2 + 191.2) / 2 [THz].

  In the case of FIG. 4B, that is, when the minimum frequency f2 of the signal light matches the minimum frequency fmin = 191.6 THz of the operating band, the pump light frequency is the minimum value fp = (191.6 + 191.2) / 2 = 191. .4 THz.

  On the other hand, in the case of FIG. 4C, for a single channel signal light with B = 0, that is, a frequency of 195.9 THz, the pump light frequency is the maximum value fp = (195.9 + 191.2) / 2 = 193. Takes .55 THz.

The important point here is that the input / output signal light is always present in the C band and the intermediate signal light is always present in the L band where f <191.2 THz. Therefore, as EDFA1 and EDFA2 in FIG. In addition, by using an optical amplifier with a gain flattening design for the L band, it is possible to amplify a WDM signal with a flat gain for each channel during wavelength conversion operation. On the other hand, wavelength conversion operation for any input / output wavelength without distortion on the frequency axis is possible for the first time.
On the other hand, when wavelength conversion operation that is not flat on the frequency axis is performed on the wavelength multiplexed signal and the waveform of the entire wavelength multiplexed signal is distorted on the frequency axis, the signal power of some channels of the wavelength multiplexed signal is reduced. As the signal becomes smaller and the signal-to-noise ratio of the channel deteriorates, the waveform on the time axis is buried in noise and has a large distortion.
Note that the value of the maximum frequency f3 of the intermediate signal light does not need to be 191.2 THz, and may be a value of 191.2 THz or less in the L band.

  Moreover, even if the gain flattening design is not necessarily made as the optical amplifier, distortion on the frequency axis is allowed by using an optical amplifier designed for C and L bands and having a flatness with a constant gain. It is possible to perform the wavelength conversion operation on the WDM signal within the range.

  Furthermore, the method of the present invention operates effectively not for a WDM signal but also for an ultra-high band signal having a band of several nanometers or more although it is a single channel.

  When outputting the polarization diversity module of the first-stage wavelength converter, it is necessary to extract only the intermediate signal light and to remove the input signal light and the pump light. In the case of FIGS. 4 (a) to 4 (c) Since the maximum frequency of the intermediate signal light is 191.2 THz and the minimum frequency of the pump light is 191.4 THz, the cut-off frequency is about 191.3 THz and the transfer function is fixed (LPF; described as LPF1 in FIG. 1). If the low-pass filter is used, the purpose of removing the input signal light and the pump light can be achieved with respect to an arbitrary condition regarding the frequency arrangement of the input signal light and the pump light frequency.

  Such an LPF can be manufactured relatively inexpensively using a dielectric multilayer film, is readily available, and is resistant to high-power input light. Is very useful.

  At the time of output of the polarization diversity module of the second-stage wavelength conversion unit, the pump light may exist in the C band (for example, in the case of FIG. 4C), so that it is transmitted with a cut-off frequency of about 191.3 THz, for example. It is not possible to extract only the output signal light after removing the intermediate signal light and the pump light by the high-pass filter 2 (HPF, high-pass filter) having a fixed function. It is necessary to use a band pass filter 4 (BPF).

  By the way, when performing wavelength conversion of a WDM signal using an FWM in an optical fiber, not only a degenerate FWM that is desired to be used as an operation principle but also a non-degenerate FWM is generated at the same time, and as a result, an undesired frequency component is output as output light Will occur.

FIG. 5 is a schematic diagram on the frequency axis showing a state of wavelength conversion by a single pump light using a WDM signal in band B as input light.
However, in FIG. 5, as a simple example, input light corresponding to a 4-channel WDM signal is considered as four line spectra indicated by a, b, c, and d.

As a result of the degenerate FWM, wavelength-converted signal light is obtained as d ^* , c ^* , b ^* , a ^* (where the asterisk “*” indicates that the phase conjugate light of the original signal light has been generated. At the same time, the non-degenerate FWM generates a non-degenerate FWM component in the band of fp ± B around the pump optical frequency fp.
For example, when the electric field complex amplitudes of the input signal lights a and d and the pump light p are set to E _a , E _d and E _p , respectively, (E _p E _a E _d ^* ) and (E _p E _d E _a ^* ) The represented component is due to non-degenerate FWM.

  If the frequency band where the signal light exists overlaps the band fp ± B where the non-degenerate FWM component is generated, the intermediate signal light or output signal light generated by wavelength conversion interferes with the non-degenerate FWM component and is distorted. And the signal quality is significantly degraded.

In order to avoid this, the signal light channel closest to the pump light frequency on the frequency axis may be separated from the pump light by a frequency difference larger than the band B of the WDM signal.
In the case of FIG. 3, values of various frequencies may be set so that f2−fp = fp−f3> B is always satisfied.
That is, it is only necessary that the maximum frequency f3 of the intermediate signal light and the minimum frequency fmin of the operation band satisfy the relationship of fmin−f3> 2B.

  By using the configuration described above, a wavelength conversion operation in which an input / output wavelength is arbitrarily set can be performed without distortion on the frequency axis for a WDM signal having an arbitrary band within the C band. become.

As a comparative example of the present invention, the case where the maximum frequency f3 of the intermediate signal light is set to 191.6 THz in the C band instead of the L band and a WDM signal having a finite band B is input is shown in FIG. ), The signal light, the pump light, and the intermediate signal light cannot be made close to each other on the frequency axis, so that the frequency of the signal light cannot be in the vicinity of 191.6 THz, and the input / output wavelength is within the C band. It turns out that it is impossible to set arbitrarily.
In particular, when various frequencies are set so as to satisfy the above-described condition, f2−fp = fp−f3> B, as shown in FIG. 4D, the minimum frequency f2 of the signal light is set to 191.6 + 2B [ Since it is necessary to set a value equal to or higher than [THz], the wavelength band that can be taken by the signal light within the C band is greatly limited.

Hereinafter, the effectiveness of the present invention will be described based on specific experimental results.
FIG. 6 shows various frequency settings used in the experiment.
The operating band is set to fmax = 196 THz (1529.553 nm) and fmin = 192.1 THz (156.606 nm), the band of the input WDM signal is B = 1 THz, and the maximum frequency of the intermediate signal light is f3 = 189.8 THz (1579). .518 nm).
At this time, the minimum frequency of the intermediate signal light is automatically determined as f4 = 188.8 THz.

  Three patterns are considered as the frequency arrangement of the input / output signal light. The frequency is 196, 195.8, 195.6, 195.45, 195.4, 195.35, 195.2, 195 THz and Case 2 as Case 1. 194.5, 194.3, 194.1, 193.95, 193.9, 193.85, 193.7, 193.5 THz, and Case 3 as 193.1, 192.9, 192.7, 192.55 , 192.5, 192.45, 192.3, and 192.1 THz, which are 8-channel WDM signals arranged at unequal intervals.

  Either Case 1, 2 or 3 is used as the frequency arrangement of the input signal light. In any case, the first wavelength conversion unit converts the signal light into intermediate signal light, and the second wavelength conversion unit converts the intermediate signal light into Case 1. , 2 and 3 are converted into output signal light and output.

Eventually, a total of nine wavelength conversion experiments are carried out regarding the frequency arrangement of the input / output signal light.
In both the first-stage and second-stage wavelength conversion units, when the frequency arrangement of the input or output signal light is Case 1, 2, 3, the pump light frequency is fp = 192.4, 191.65, 190.95 THz, respectively. is there.

  Also, when the input / output signal light has the same frequency arrangement, the first wavelength conversion unit converts the signal light into intermediate signal light, and the second stage wavelength conversion unit reconverts it to the original frequency arrangement. .

The configuration of the wavelength converter actually constructed is as shown in FIG.
The input signal optical amplifying unit is designed to flatten the gain in the C band, the optical amplifier EDFA1 having a maximum output of 17 dBm, the high frequency component of the C band with a cutoff frequency of 190.95 THz (1570.005 nm) ( It consists of HPF that allows only short wavelength components) to pass through.

FIG. 7A shows HPF transfer function measurement results.
By using this HPF, the noise component in the L band where the intermediate signal light is located can be removed, and good signal quality can be expected when the intermediate signal light is generated after the first wavelength conversion.

The first-stage wavelength conversion unit includes a polarization diversity module whose configuration is shown in FIG. 2, a PM-HNLF having a length of 100 m and a nonlinear constant of about 10 W ⁻¹ km ⁻¹ connected thereto, and a cutoff frequency of 190 It consists of a low-pass filter (LPF1) that passes only a low frequency component (long wavelength component) at .3 THz (1575.368 nm).

  FIG. 7 (b) shows the LPF1 transfer function measurement result. By using this, the input signal light and the residual pump light at the output of the first-stage wavelength conversion unit can be obtained under any of the frequency conditions of Case 1, 2, 3 It is possible to always remove and output only intermediate signal light.

Pump light generated and amplified by the first-stage pump light generator is input to the polarization diversity module in the first-stage wavelength converter.
In the first-stage pump light generation unit, a certain frequency (one of the pump light frequencies, fp = 192, 4, 191.65, 190.95 THz) generated by the narrow line width variable wavelength light source 5 (TLS). The light is phase-modulated by the optical phase modulator 7 (P Mod.).

  The optical phase modulator includes a phase shifter 8 (PS), an RF signal generated by the oscillator 1 (OSC) of the RF signal generator for pump optical frequency dithering, which is bifurcated. It is applied through an amplifier and an RF low pass filter (LPF) having a cutoff frequency of 400 MHz.

  There is an effect of suppressing stimulated Brillouin scattering (SBS) in PM-HNLF by phase modulation of pump light. The frequency of the RF output from the oscillator 1 does not need to be 300 MHz, and an arbitrary value of about 300 to 400 MHz may be used.

  The continuous light output from the optical phase modulator is amplified to an optical power of 2 W by an optical amplifier, and spontaneous emission light 14 (ASE) noise generated at that time is removed by the BPF, and then the polarizer 6 (Pol.) Is supplied. Then, it is input to the polarization diversity module as pump light.

  The intermediate signal light amplifying unit includes an optical amplifier EDFA2 having a gain flattening design in the L band and a maximum output of 17 dBm, and a low-pass filter LPF2 having a transfer function equivalent to LPF1.

The LPF 2 can remove noise components existing in the C band, and when the output signal light is generated after the second-stage wavelength conversion, good signal quality can be expected.
The second-stage wavelength conversion unit can be operated by changing the polarization diversity module and PM-HNLF as in the first stage, and the center wavelength, and the band-pass filter 4 (BPF) having a pass bandwidth of 1.7 THz. Composed.

  However, the pass bandwidth is not limited to this value. By using this BPF, it is possible to remove the intermediate signal light and the residual pump light at the output of the second-stage wavelength converter and extract only the desired output signal light. However, instead of the BPF, the cutoff frequency is used. May be a value slightly smaller than the minimum frequency f2 of the output signal light, and a high-pass filter 2 (HPF) capable of changing the cutoff frequency according to the output signal light frequency condition may be used.

  The polarization diversity module in the second-stage wavelength converter receives the pump light generated and amplified by the second-stage pump light generator having the same configuration as the first-stage pump light generator. The optical phase modulator of the pump light generator includes the variable attenuator 9 (the other of the two branches of the 300 MHz frequency RF signal generated by the OSC of the pump light frequency dithering RF signal generator 25. ATT), an RF amplifier, and an RF LPF having a cutoff frequency of 400 MHz.

  At this time, by appropriately adjusting the intensity and phase of the RF signal applied to each optical phase modulator of the first-stage and second-stage pump light generation units, the phase modulation of the pump light is the signal light after wavelength conversion. The effect added as phase noise to the first and second stages can be canceled.

  FIG. 8 shows an experimental system 26 for performing an experiment for inputting the 8-channel WDM signal as input signal light to the wavelength converter and evaluating the quality of the output signal light whose wavelength is converted.

  The continuous light output from the eight variable wavelength light sources 5 (TLS) whose oscillation frequency is set to any one of the cases 1, 2, and 3 is combined by the 8 × 1 polarization maintaining (PM) coupler 18 and polarization multiplexed. (DP) Lithium niobate (LN) IQ modulator (IQM) 27 inputs.

The 4-channel RF signal generated by the arbitrary waveform generator 13 (AWG) is applied to the DP-LN-IQM 27, and an 8-channel WDM signal is generated in the DP-QPSK format with a symbol rate of 32 Gbaud by modulation. The
FIG. 9 shows the result of measuring the optical spectrum of the 8-channel WDM signal as the input signal light and overlaying it for the cases 1, 2, and 3.

After the optical power of the input signal light is amplified by the optical amplifier, the polarization of the signal light is randomly rotated at about 400 kHz by the polarization scrambler 15 (Pol. Scr.), And the wavelength converter 16 shifts the polarization of the input signal light. Make sure it works independently of the wavefront.
The optical power of the output light whose wavelength has been converted by the wavelength converter is amplified by an optical amplifier, ASE noise outside the signal light band is removed by a band pass filter, and then received by the digital coherent receiver 28 (Rx).

  In order to confirm the quality of the received signal when the OSNR value is changed, optical noise of variable power is added to the signal light before the receiver. At that time, the ASE light source 14 is band-limited by the BPF. It is possible to change the OSNR value of the received signal by amplifying with an optical amplifier and adjusting the optical power with a variable optical attenuator 17 (VOA).

  FIG. 10 shows the measurement result of the optical spectrum output from the polarization diversity module of the first-stage wavelength conversion unit in the wavelength converter.

  In the case where the frequency arrangement of the input signal light is either Case 1, 2 or 3, the middle of the same frequency (wavelength) arrangement 189.8> f> 188.8 [THZ] (in terms of wavelength, 1579.518 to 15878.884 nm) It can be seen that it has been converted to signal light.

  In Cases 2 and 3 shown in FIGS. 10B and 10C, the power of the long-wavelength side channel of the intermediate signal light is attenuated and appears distorted on the frequency axis. This is due to the PM-HNLF. This is because the wavelength conversion efficiency by the degenerate FWM is deteriorated due to the dispersion effect.

  On the other hand, in the case of Case 1, it seems that a flat output spectrum is obtained, but this is because the gain flatness of the optical amplifier on the short wavelength side is poor, and as shown in FIG. 9, the power of the input signal light is on the short wave side. This is because a large amount of light is emitted from the channel, and the slope of the wavelength conversion efficiency cancels out, and each channel of the output light appears flat on the frequency axis.

By using PM-HNLF with a smaller dispersion slope, it is possible to cancel the frequency slope of the wavelength conversion efficiency and obtain a flatter and distortion-free intermediate signal light.
On the other hand, in each case, it can be seen that many components due to non-degenerate FWM are generated in the peripheral band of the pump light.

  In particular, when the frequency arrangement of the input signal light shown in FIG. 10C is Case 3, the non-degenerate FWM component appears to be quite close to the input signal light or the intermediate signal light.

  However, the difference between the pump light frequency fp = 190.95 THz and the minimum frequency f2 = 192.1 THz of the input signal light, and the pump light frequency fp and the maximum frequency f3 = 189.8 THz of the intermediate signal light are both set to 1.15 THz. Since this difference exceeds the signal band B = 1 THz, the non-degenerate FWM component and the signal light do not interfere with each other, and the signal quality is not affected.

  FIG. 11 shows the optical spectrum output from the polarization diversity module of the second-stage wavelength converter when the frequency arrangement of the input signal light is Case 1 and the frequency arrangement of the output signal light is Case 1, 2, and 3. The measurement results are shown.

  Next, in FIG. 12, when the frequency arrangement of the input signal light is Case 2 and the frequency arrangement of the output signal light is Case 1, 2, 3, it is output from the polarization diversity module of the second-stage wavelength converter. The measurement result of an optical spectrum is shown.

  FIG. 13 shows the light output from the polarization diversity module of the second-stage wavelength converter when the frequency arrangement of the input signal light is Case 3 and the frequency arrangement of the output signal light is Case 1, 2, and 3. The measurement result of a spectrum is shown.

  In either case, the 8-channel WDM signal is wavelength-converted as intended. The power of the short-wavelength side channel of the output signal light is attenuated due to the dispersion characteristics of PM-HNLF as in the first stage conversion, and by using PM-HNLF with a small dispersion slope, the frequency is reduced. It is possible to obtain a flatter waveform on the axis.

  As a result of evaluating the quality of the signal light output from the wavelength converter in FIG. 14, when the frequency arrangement of the input / output signal light is one of Cases 1, 2, and 3, 3 of the 8-channel WDM signals The result of having measured the Q value for a channel is shown.

  In all cases, the Q value exceeds 16.5 dB, and it can be said that the output signal light is of high quality. Further, when the bit pattern was verified with respect to the number of bits of about several million under each condition, no bit error was observed.

Since the theoretical value of the bit error rate when the Q value is 16.5 dB is 1.17 × 10 ⁻¹¹ , one bit error is observed when the number of bits to be observed is increased to 10 billion bits. It is predicted.

  Finally, FIG. 15 shows the results of measuring the Q value and bit error rate (BER) when the received OSNR is changed for a channel whose frequency is 193.5 THz when the frequency arrangement of the input / output signal light is both Case 2. Indicates.

  Comparing the values before and after the wavelength conversion with respect to the Q value measurement results shown in FIG. 15A, the Q value is deteriorated when the OSNR is a large value of 25 dB or more. This is performed before and after the wavelength conversion. As a result of optical amplification, the ASE noise is slightly added and the waveform is distorted, or the phase modulation component applied to the pump light for SBS suppression is transferred to the phase of the signal light through the FWM, and the waveform is distorted as a slight phase error. It is the result.

  On the other hand, when the received OSNR is 20 dB or less, waveform degradation due to ASE noise added for OSNR adjustment before the receiver is dominant, and there is almost no influence of slight waveform distortion caused by the wavelength conversion operation. As a result, there is almost no penalty in the Q value before and after wavelength conversion.

  FIG. 15B shows the BER measurement result when the received OSNR is 20 dB or less, but the penalty before and after wavelength conversion is very small for the same reason as described above.

  From the above results, the wavelength converter of the configuration proposed in the present invention is correct for the input WDM signal without depending on the polarization plane of the input signal light under any input / output wavelength conditions within the operating band. It was confirmed that a high-quality output signal light was obtained.

  In the design of an all-optical wavelength converter used in the field of optical fiber communications, a wavelength band called C-band having a wavelength range of 1530 to 1565 nm or a frequency range of 195.9 to 191.6 THz is obtained by using the proposed method. Thus, it is possible to perform a conversion operation for an arbitrary input / output wavelength without distortion on a wavelength multiplexed signal having an arbitrary band.

1 OSC: Oscillator 2 HPF: High pass filter, band pass filter 301 LPF 1: Low pass filter 302 LPF 2: Low pass filter 303 LPF: RF signal low pass filter 304 LPF: RF signal low pass filter 4 BPF: Band pass filter, band pass filter 5 TLS: (Narrow line width) wavelength tunable light source
6 Pol .: Polarizer
7 Phase Mod .: Optical phase modulator
8 PS: Phase shifter
9 ATT: Variable attenuator,
101 EDFA1: Erbium-doped optical fiber amplifier (first optical amplifier)
102 EDFA2: Erbium-doped optical fiber amplifier (second optical amplifier)
11 PM-HNLF: Polarization-maintaining highly nonlinear fiber 12 Pol. Div. Module: Polarization diversity module 13 AWG: Arbitrary waveform generator 14 ASE: ASE light source, spontaneous emission optical noise source 15 Pol. Scr .: Polarization scrambler 16 Wavelength Converter: Wavelength converter 17 VOA: Variable optical attenuator 18 8 × 1 polarization maintaining (PM) coupler 19 Input signal optical amplifier 20 First stage wavelength converter 21 Intermediate signal optical amplifier 22 Second stage wavelength converter 23 First stage Eye pump light generating unit 24 Second stage pump light generating unit 25 Pump optical frequency dithering RF signal generating unit 26 Experimental system 27 DP-LN-IQM: polarization multiplexed lithium niobate IQ modulator 28 Rx: digital coherent receiver

Claims (10)

The input signal light is held in the optical region while maintaining the information of the input signal light with respect to the input signal light having an arbitrary wavelength (carrier optical frequency) within the operation band as a predetermined frequency range. A wavelength conversion method for converting the wavelength of the output signal into a wavelength of any output signal light within the operating band,
Amplifying the input signal light by a first optical amplifier having a gain flattening design within the operating band;
Next, the input signal light is converted by the first-stage wavelength conversion by degenerate four-wave mixing generated in the first nonlinear medium by the amplified input signal light and the first-stage pump light whose wavelength is set according to the wavelength. Is converted into an intermediate signal light wavelength outside the operating band,
Next, the intermediate signal light is amplified by a second optical amplifier,
Next, the intermediate signal light by the second-stage wavelength conversion by degenerate four-wave mixing generated in the second nonlinear medium by the amplified intermediate signal light and the second-stage pump light whose wavelength is set according to the wavelength. In the case of converting the wavelength of the signal light into the wavelength of the arbitrary output signal light within the operating band,
The maximum frequency of the input signal light is f1, the minimum frequency is f2, the signal band is B = f1-f2, the maximum frequency of the intermediate signal light is f3, and the minimum frequency is f4 = f3-B. The minimum frequency is set to a value greater than 191.2 THz, and the maximum frequency f3 of the intermediate signal light is fixed to a value equal to or less than 191.2 THz,
Then, by applying a gain flattening design to the second optical amplifier in the frequency band below f3,
Controlling to output the wavelength of the output signal light without distortion,
A wavelength conversion method characterized by the above. The wavelength conversion method according to claim 1,
The input signal light is a wavelength multiplexed signal;
A wavelength conversion method characterized by the above. It is the wavelength conversion method of any one of Claim 1 or Claim 2, Comprising:
Immediately after the first-stage wavelength conversion is performed, using a low-pass filter having a cutoff frequency smaller than the minimum frequency of the first-stage pump light used in the first-stage wavelength conversion, Removing only the intermediate signal light by removing the input signal light and the first-stage pump light;
A wavelength conversion method characterized by the above. The wavelength conversion method according to any one of claims 1 and 3,
When the minimum frequency of the operating band is fmin,
satisfy fmin−f3> 2B,
A wavelength conversion method characterized by the above. The wavelength conversion method according to claim 1, wherein:
The nonlinear medium is an optical fiber;
A wavelength conversion method characterized by the above. The input signal light amplifying unit including the first optical amplifier is used as the input unit, and the first signal wavelength converting unit and the intermediate signal light amplifying unit including the second optical amplifier and the second stage length converting unit which are sequentially connected,
The first-stage wavelength converter and the second-stage wavelength converter are connected to the first-stage pump light generator and the second-stage pump light generator connected to the pump signal frequency dithering RF signal generator, respectively.
The input signal light is held in the optical region while maintaining the information of the input signal light with respect to the input signal light having an arbitrary wavelength (carrier optical frequency) within the operation band as a predetermined frequency range. A wavelength converter that converts the wavelength of the output signal into a wavelength of any output signal light within the operating band,
Amplifying the input signal light by a first optical amplifier having a gain flattening design within the operating band in an input signal light amplifier;
Next, the first-stage wavelength by degenerate four-wave mixing generated in the first nonlinear medium by the amplified input signal light and the first-stage pump light whose wavelength is set according to the wavelength in the first-stage wavelength conversion unit By converting the wavelength of the input signal light into the wavelength of the intermediate signal light outside the operating band,
Next, the intermediate signal light amplification unit amplifies the intermediate signal light by a second optical amplifier,
Next, in the second stage length conversion unit, the amplified intermediate signal light and the second stage pump light whose wavelength is set in accordance with the wavelength of the amplified intermediate signal light are generated by degenerate four-wave mixing generated in the second nonlinear medium. In the case of converting the wavelength of the intermediate signal light into the wavelength of the arbitrary output signal light within the operating band by wavelength conversion of the stage,
The maximum frequency of the input signal light is f1, the minimum frequency is f2, the signal band is B = f1-f2, the maximum frequency of the intermediate signal light is f3, and the minimum frequency is f4 = f3-B. The minimum frequency is set to a value greater than 191.2 THz, and the maximum frequency f3 of the intermediate signal light is fixed to a value equal to or less than 191.2 THz,
Then, by applying a gain flattening design to the second optical amplifier in the frequency band below f3,
Controlling to output the wavelength of the output signal light without distortion,
A wavelength converter characterized by. The wavelength converter according to claim 6, wherein
The wavelength converter, wherein the input signal light is a wavelength multiplexed signal. The wavelength converter according to any one of claims 6 and 7,
Using the low-pass filter having a cut-off frequency smaller than the minimum frequency of the first stage pump light in the intermediate signal light amplification unit, the input signal light and the first stage pump light are removed to remove the intermediate signal Allowing only light to pass through,
A wavelength converter characterized by. The wavelength converter according to any one of claims 6 and 8,
When the minimum frequency of the operating band is fmin,
satisfy fmin−f3>2B;
A wavelength converter characterized by. The wavelength converter according to any one of claims 6 and 9,
The nonlinear medium is an optical fiber;
A wavelength converter characterized by.

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