Dispersion determination device and dispersion determination method

Abstract

In this dispersion determination device and dispersion determination method, carrier light is generated by means of a laser, the intensity of the carrier light is modulated by means of an electric field absorption modulator to generate signal light, and the signal light is inputted to a transmission path. The signal light that has passed through the transmission path is received, the signal light is converted to an electric signal, and the wavelength dispersion of the transmission path is determined by means of a controller on the basis of the electric signal.

Description

Distribution decision device and distribution decision method

[0001] The present disclosure relates to a distribution determination device and a distribution determination method.

[0002] Non-Patent Document 1 discloses an electroabsorption modulator having an absorption layer with a multiple quantum well (MQW) structure. Intensity modulation of light using an electroabsorption modulator generates fluctuations in the optical frequency called chirp. Optical fiber, which serves as a transmission path, has chromatic dispersion, and the transmission speed varies depending on the frequency of the transmitted light. Therefore, when an intensity-modulated pulse is transmitted through an optical fiber, the pulse waveform after transmission changes. This causes interference between consecutive pulses, which is known to result in transmitted signal determination errors.

[0003] F. Dorgeuille and F. Devaux, “On the Transmission Performances and the Chirp Parameter of a Multiple-Quantum-Well Electroabsorption Modulator”, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 30, NO. 11, NOVEMBER 1994.

[0004] According to the electroabsorption modulator described in Non-Patent Document 1, it is possible to change the chirp by superimposing a reverse bias on the modulation signal. Therefore, if the chromatic dispersion of the transmission line is known, it is possible to improve the transmission characteristics and reduce judgment errors of the transmission signal by controlling the applied reverse bias. However, the chromatic dispersion of an installed optical fiber varies depending on differences in the optical fiber structure, stress on the optical fiber, environmental temperature, etc., and the chromatic dispersion is often unknown. Therefore, there is a problem in that it is difficult to control the reverse bias in accordance with the chromatic dispersion of the transmission line.

[0005] The present disclosure has been made in view of the above-mentioned problems, and has as its object to provide a dispersion determination device and a dispersion determination method that are capable of determining the chromatic dispersion of a transmission line.

[0006] To solve the above-mentioned problems, a dispersion determination apparatus and a dispersion determination method according to an aspect of the present disclosure generate a carrier light using a laser, intensity-modulate the carrier light using an electro-absorption modulator to generate a signal light, input the signal light into a transmission line, receive the signal light after passing through the transmission line, convert the signal light into an electrical signal, and determine the chromatic dispersion of the transmission line based on the electrical signal using a controller.

[0007] According to the present disclosure, it is possible to provide a dispersion determining device and a dispersion determining method that are capable of determining chromatic dispersion of a transmission line.

[0008] FIG. 1 is a diagram illustrating a configuration of a dispersion determination device according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram illustrating the relationship between reverse bias and transmitted light intensity in an electroabsorption modulator. FIG. 3 is a schematic diagram illustrating the relationship between reverse bias and chirp in an electroabsorption modulator. FIG. 4A is a diagram illustrating an example of an eye pattern based on a signal waveform before transmission. FIG. 4B is a diagram illustrating an example of an eye pattern based on a signal waveform after transmission through a negative dispersion transmission line. FIG. 4C is a diagram illustrating an example of an eye pattern based on a signal waveform after transmission through a positive dispersion transmission line. FIG. 5 is a diagram illustrating an example of a histogram generated based on a signal waveform. FIG. 6A is a diagram illustrating an example of a normalized histogram based on a signal waveform before transmission. FIG. 6B is a diagram illustrating an example of a normalized histogram based on a signal waveform after transmission through a negative dispersion transmission line. FIG. 6C is a diagram illustrating an example of a normalized histogram based on a signal waveform after transmission through a positive dispersion transmission line. FIG. 7 is a schematic diagram illustrating the relationship between chromatic dispersion and cross points of an eye pattern.

[0009] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and redundant description will be omitted.

[0010] 1 is a block diagram of a dispersion determination device according to an embodiment of the present disclosure. The dispersion determination device 1 includes a laser 11, an electroabsorption modulator 13, a converter 41, and a controller 43. In addition, the dispersion determination device 1 may include an amplifier 15, a combiner 21, a modulation signal generator 23, a bias voltage generator 25, and a database 45.

[0011] The laser 11 generates a carrier light. For example, the laser 11 is a distributed feedback laser.

[0012] The electroabsorption modulator 13 generates signal light by intensity-modulating the carrier light generated by the laser 11. The electroabsorption modulator 13 has a characteristic that the transmission loss changes depending on the applied reverse bias, and this characteristic can be utilized to perform intensity modulation of the carrier light.

[0013] The electroabsorption modulator 13 performs intensity modulation of the carrier light based on the modulation signal generated by the combiner 21. Here, the combiner 21 may generate the modulation signal by superimposing a reverse bias generated by a bias voltage generator 25 on the modulation signal generated by the modulation signal generator 23.

[0014] For example, the modulated signal generated by the combiner 21 is a 50 Gbaud NRZ (Non-return-to-zero) signal. Note that the dispersion determination method by the dispersion determination device 1, which will be described later, does not qualitatively depend on the signal rate. Therefore, other signal rates may be used as the modulated signal. The electroabsorption modulator 13 modulates the optical pulse with a binary peak intensity of optical ON / OFF based on the NRZ signal. In the following, the lower peak intensity is referred to as level L0, and the higher peak intensity is referred to as level L1.

[0015] FIG. 2 is a schematic diagram showing the relationship between the reverse bias and the transmitted light intensity in an electroabsorption modulator. Ｌ０ and reverse bias V Ｌ１ and NRZ signals can be generated by alternately applying these signals to the electroabsorption modulator 13.

[0016] The electroabsorption modulator 13 generally has an absorption layer having a multi-quantum well (MQW) structure, and modulates the light intensity by utilizing the shift in the absorption edge due to the quantum confined Stark effect.

[0017] The amplifier 15 amplifies the signal light generated by the electroabsorption modulator 13. The amplified signal light is introduced into the optical fiber FB (transmission path). Amplification by the amplifier 15 can improve the SNR (Signal-to-Noise Ratio) of the measured waveform after passing through the optical fiber FB. Note that the amplifier 15 may be omitted, in which case the signal light generated by the electroabsorption modulator 13 may be introduced into the optical fiber FB.

[0018] In the electroabsorption modulator 13, a refractive index change occurs that is expressed by the Kramers-Kronig relationship (K-K relationship), which modulates the phase of the light and causes frequency chirp. The refractive index change with respect to the reverse bias is parabolic, and in the low bias voltage region, the refractive index increases with increasing bias, and in the high bias voltage region, the refractive index decreases with increasing bias.

[0019] Figure 3 is a schematic diagram showing the relationship between reverse bias and chirp in an electroabsorption modulator. As shown in Figure 3, the low bias region shows positive chirp characteristics, and the high bias region shows negative chirp characteristics. The chirp parameter for the two reverse bias values ​​that make up the NRZ signal is α Ｌ０ and α Ｌ１ (α Ｌ０ <α Ｌ１ A positive chirp causes a blue shift in wavelength on the rising edge of the pulse and a red shift on the falling edge. Conversely, a negative chirp causes a red shift on the rising edge and a blue shift on the falling edge.

[0020] It is known that chirp affects the pulse waveform of a modulated signal transmitted through a dispersive medium such as an optical fiber FB. Because the group velocity of light in a dispersive medium varies depending on the wavelength, the waveform of a chirped pulse is distorted after transmission.

[0021] In the case of a positively chirped pulse transmitted through a negative dispersion optical fiber, the group velocity is low at the rising edge (level L0 → level L1) and high at the falling edge (level L1 → level L0). Because the pulse is transmitted late at the rising edge and fast at the falling edge, the pulse width narrows on the time axis as if it were compressed from both sides. This compresses the pulse and increases the peak intensity.

[0022] On the other hand, in the case of a negatively chirped pulse, the transmission speed is faster at the rising edge and slower at the falling edge, causing the pulse to broaden, resulting in a decrease in the peak intensity of the pulse.

[0023] Therefore, when the NRZ signal generated by the electroabsorption modulator 13 is transmitted, the waveform of the eye pattern changes as shown in Figures 4A to 4C. Figure 4A is a diagram showing an example of an eye pattern based on the signal waveform before transmission. Figure 4B is a diagram showing an example of an eye pattern based on the signal waveform after transmission through a negative dispersion transmission line. Figure 4C is a diagram showing an example of an eye pattern based on the signal waveform after transmission through a positive dispersion transmission line.

[0024] As shown in FIG. 4A, the cross point XP of the eye pattern before transmission (back-to-back) is usually adjusted to be around 50%.

[0025] As shown in Figure 4B, when an NRZ signal is transmitted through a negative dispersion transmission line, pulse compression occurs at level L1. As a result, the peak intensity of the pulse increases, causing the cross point XP of the eye pattern to drop below its level before transmission.

[0026] On the other hand, as shown in Figure 4C, when the signal is transmitted through a transmission line with positive dispersion, the opposite tendency is observed, and the pulse broadens due to the positive chirp characteristic at level L1. As a result, the peak intensity decreases and the cross point XP of the eye pattern rises from before transmission.

[0027] The converter 41 receives the signal light that has passed through the optical fiber FB and converts the signal light into an electrical signal. The converter 41 is a device that converts an optical signal into an electrical signal, and is, for example, a photodiode.

[0028] The controller 43 determines the chromatic dispersion of the optical fiber FB based on the electrical signal obtained by the converter 41. More specifically, the controller 43 calculates the amount of crosspoint change in the eye pattern indicated by the electrical signal, and determines the chromatic dispersion corresponding to the amount of crosspoint change.

[0029] Here, the controller 43 may refer to the correspondence relationship between the crosspoint change amount of the eye pattern indicated by the electrical signal and the chromatic dispersion, and determine the chromatic dispersion corresponding to the crosspoint change amount. The correspondence relationship may be stored in the database 45, for example.

[0030] Next, we will explain how to calculate the amount of change in the crosspoints of the eye pattern. The controller 43 discretizes the electrical signal received from the converter 41 and converts it into a decimal number. If the number of quantization bits used during discretization is 8 bits, this decimal number data string will take integer values ​​within the range of 0-255, so a histogram can be generated by counting the number of integer values ​​in the data string.

[0031] Fig. 5 shows an example of a histogram generated based on an electrical signal. As shown in Fig. 5, the generated histogram HS has two peaks corresponding to levels L0 and L1. For reference, Fig. 5 also shows the signal waveform SG of the electrical signal.

[0032] In a histogram generated based on a signal waveform, the distribution centered on a peak tends to be a normal distribution due to thermal noise and shot noise. Therefore, the distribution function of the entire histogram is the sum of two Gaussian functions. Therefore, the controller 43 detects a discretized intensity having two peaks by performing peak detection based on the histogram.

[0033] The controller 43 also detects the minimum point between two peaks where two Gaussian functions intersect. In the histogram, the minimum point between the peaks corresponds to the cross point of the eye pattern of the NRZ signal.

[0034] Next, the controller 43 normalizes the histogram using the intensities of the two peaks corresponding to levels L0 and L1 to generate a normalized histogram. At this time, normalization is performed so that the intensity of the peak corresponding to level L0 is −1 and the intensity of the peak corresponding to level L1 is +1. Therefore, the intermediate intensity between the intensity of the peak corresponding to level L0 and the intensity of the peak corresponding to level L1 is 0.

[0035] 6A, 6B, and 6C are diagrams showing examples of normalized histograms based on signal waveforms before, after, and after transmission through a negative-dispersion transmission path, respectively.

[0036] When the crossing point of the eye pattern of the NRZ signal is 50%, the intensity VP of the minimum point between two peaks in the normalized histogram is 0, as shown in Fig. 6A. When the chromatic dispersion of the optical fiber FB is 0, the crossing point of the eye pattern remains 50%, and the intensity VP of the minimum point remains 0.

[0037] When the optical fiber FB has negative dispersion, the effect of dispersion on the chirp described above causes the level L1, which has a larger chirp value than the level L0, to undergo more pulse compression, resulting in an increase in the peak intensity of the level L1. In this case, the peak intensity in the normalized histogram remains unchanged due to the normalization effect. The cross point of the eye pattern decreases by less than 50%, and the intensity VP of the minimum point between the two peaks changes to a negative value, as shown in FIG. 6B.

[0038] When the optical fiber FB has positive dispersion, the effect of dispersion on the chirp described above causes the pulse at level L1, which has a larger chirp value than level L0, to expand more, and the peak intensity at level L1 decreases. In this case, the peak intensity in the normalized histogram remains unchanged due to the normalization effect. The crossover point of the eye pattern increases beyond 50%, and the intensity Vp of the minimum point between the two peaks changes to a positive value, as shown in FIG. 6C.

[0039] 7 is a schematic diagram showing the relationship between chromatic dispersion and cross points of the eye pattern. The change in the cross points is represented by the amount of change in the intensity of the minimum point between two peaks from the state before transmission, and changes linearly with dispersion. From the relationship shown in FIG. 7, it can be seen that when the intensity of the minimum point between two peaks changes to a positive value, the optical fiber FB has positive chromatic dispersion. Also, when the intensity of the minimum point between two peaks changes to a negative value, it can be seen that the optical fiber FB has negative chromatic dispersion.

[0040] 6A to 6C, the relationship between chromatic dispersion and the normalized histogram is determined by the characteristics of the electroabsorption modulator 13. Therefore, if the relationship between chromatic dispersion and the normalized histogram is known, the chromatic dispersion of the optical fiber FB can be evaluated.

[0041] For example, the relationship between the known chromatic dispersion and the normalized histogram is acquired in advance based on the optical fiber FB whose chromatic dispersion is known, and the relationship between the known chromatic dispersion and the normalized histogram is stored in the database 45 or the like.

[0042] Then, the relationship between the unknown chromatic dispersion and the normalized histogram is obtained based on the optical fiber FB, whose chromatic dispersion is unknown. By comparing the relationship between the known chromatic dispersion and the normalized histogram with the relationship between the unknown chromatic dispersion and the normalized histogram, the unknown chromatic dispersion can be determined based on the amount of change in intensity at the minimum point between the two peaks.

[0043] For example, based on the relationship between chromatic dispersion and cross points of the eye pattern shown in FIG. 7, when the intensity of the minimum point between two peaks of the waveform that has passed through the optical fiber FB changes by −10%, the chromatic dispersion of the optical fiber FB can be estimated to be −50 ps / nm.

[0044] In the above-described method, an NRZ signal with a modulation rate of 50 Gbaud is used as the modulation signal for intensity modulation, but this is not limited to this. For example, modulation rates other than those listed here are also applicable. Furthermore, the intensity modulation is not limited to NRZ, and the present technology can also be applied to multi-level intensity-modulated optical signals such as PAM4 and PAM8. In such cases, for example, the present technology can be applied to the cross points between the inner eyes of a PAM4 signal.

[0045] As described above in detail, the dispersion determination device and dispersion determination method according to this embodiment generate a carrier light using a laser, intensity-modulate the carrier light using an electro-absorption modulator to generate a signal light, and input the signal light into a transmission line. The signal light after passing through the transmission line is received and converted into an electrical signal, and the controller determines the chromatic dispersion of the transmission line based on the electrical signal.

[0046] This makes it possible to determine the chromatic dispersion of the transmission line, especially in cases where the chromatic dispersion fluctuates due to differences in the structure of the optical fiber, the stress on the optical fiber, the environmental temperature, etc., as in the case of installed optical fiber.

[0047] Furthermore, in the dispersion determination device and dispersion determination method according to this embodiment, the controller may calculate the amount of crosspoint change in the eye pattern based on the electrical signal, and determine the chromatic dispersion corresponding to the amount of crosspoint change by referring to a database that stores the correspondence between the amount of crosspoint change in the eye pattern indicated by the electrical signal and chromatic dispersion.

[0048] As a result, the chromatic dispersion is determined based on the eye pattern that reflects the characteristics of the transmission line, and the chromatic dispersion of the transmission line can be determined.

[0049] Furthermore, in the dispersion determination device and dispersion determination method of this embodiment, the controller may generate a histogram based on the electrical signal, extract peaks of the histogram corresponding to levels contained in the modulated signal related to intensity modulation, normalize the histogram based on the intensity of the peaks to generate a normalized histogram, and calculate the intensity of the bottom of the normalized histogram sandwiched between the peaks based on the normalized histogram as the cross-point change amount.

[0050] This allows normalization based on the intensity of the peak corresponding to the level included in the intensity-modulated signal, making it possible to calculate the cross-point change amount more accurately, and as a result, to accurately determine the chromatic dispersion of the transmission line.

[0051] Furthermore, in the dispersion determination apparatus and dispersion determination method according to this embodiment, the controller may discretize the electrical signal and generate a histogram based on the discretized electrical signal. This allows the crosspoint change to be calculated more accurately. As a result, the chromatic dispersion of the transmission line can be determined with high accuracy.

[0052] Furthermore, in the dispersion determining apparatus and dispersion determining method according to the present embodiment, the laser may be a distributed feedback laser, which allows the chromatic dispersion of the transmission line to be determined with high accuracy.

[0053] Each of the functions described in the above embodiments may be implemented by one or more processing circuits, including programmed processors, electrical circuits, and even devices such as application specific integrated circuits (ASICs), or circuit components arranged to perform the described functions.

[0054] Although the contents of the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which will be apparent to those skilled in the art. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0055] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description.

[0056] REFERENCE SIGNS LIST 1 dispersion determination device 11 laser 13 electroabsorption modulator 15 amplifier 21 combiner 23 modulation signal generator 25 bias voltage generator 41 converter 43 controller 45 database FB optical fiber (transmission path) L0, L1 level

Claims

1. A dispersion determination device comprising: a laser that generates a carrier light; an electroabsorption modulator that generates a signal light by intensity modulating the carrier light; a converter that receives the signal light after passing through a transmission line and converts the signal light into an electrical signal; and a controller that determines the chromatic dispersion of the transmission line based on the electrical signal.

2. The dispersion determination device according to claim 1, further comprising a database in which a correspondence relationship between a crosspoint change amount of an eye pattern indicated by the electrical signal and chromatic dispersion is stored, and the controller calculates the crosspoint change amount of the eye pattern based on the electrical signal, and determines the chromatic dispersion corresponding to the crosspoint change amount by referring to the database.

3. The dispersion determination device of claim 2, wherein the controller: generates a histogram based on the electrical signal; extracts peaks of the histogram corresponding to levels contained in the modulation signal related to the intensity modulation; normalizes the histogram based on the intensity of the peak to generate a normalized histogram; and calculates, based on the normalized histogram, the intensity of a bottom of the normalized histogram sandwiched between the peaks as the cross-point change amount.

4. The apparatus of claim 3, wherein the controller discretizes the electrical signal and generates the histogram based on the discretized electrical signal.

5. An apparatus for determining dispersion according to any one of claims 1 to 4, wherein the laser is a distributed feedback laser.

6. A dispersion determination method comprising: generating carrier light by a laser; generating signal light by intensity modulating the carrier light by an electroabsorption modulator; inputting the signal light into a transmission line; receiving the signal light after passing through the transmission line; converting the signal light into an electrical signal; and determining chromatic dispersion of the transmission line based on the electrical signal by a controller.

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