JP2004045296A - Method and system for measuring modulation factor in spm - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure the modulation factor of self phase modulation in an optical fiber transmission path. <P>SOLUTION: An output laser beam having λp of wavelength from a wavelength variable light source 14 is intensity-modulated by an intensity modulator 18, and phase-modulated by a phase modulator 22. A dummy light source 24 outputs dummy light having λ1-λn of wavelength range. A photocoupler 26 multiplexes output light from the phase modulator 22 with the dummy light from the dummy light source 24, and a multiplexed light is input into an optical BPF 32 via a photocoupler 28, a gain eqaulizer 30 and the optical fiber transmission path 10. An optical BPF 32 extracts a measuring light having the λp of wavelength from the input light, and an output therein is input into a BER measuring instrument 38 via a photoreceiver 36. A controller 12 measures the modulation factor of phase modulation in a phase modulator 22 by a light spectrum analyzer 30, after adjusting a phase and an amplitude of a drive signal for the phase modulator 22 to bring a BER into the minimum. The measuring wavelength λp is swept to measure a wavelength change in the optimum phase modulation factor. The measured phase modulation factor represents the SPM modulation factor of the optical fiber transmission path 10. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and a system for quantitatively measuring the modulation degree of self-phase modulation (SPM) of an optical fiber transmission line.
[0002]
[Prior art]
In an optical fiber transmission line, a carrier phase shifts in an optical pulse due to an optical Kerr effect, and self-phase-modulation (SPM) occurs. The optical waveform deteriorates due to the synergistic effect of the self-phase modulation and the chromatic dispersion of the optical fiber. As the transmission distance increases, this waveform degradation cannot be ignored.
[0003]
[Problems to be solved by the invention]
In order to obtain good optical signal transmission, it is important to know the SPM modulation degree. Conventionally, however, the SPM modulation degree of an optical fiber transmission line, particularly the SPM modulation degree of an actually operated optical fiber transmission line, is quantitatively determined. No known method is known.
[0004]
An object of the present invention is to provide a method and a system for quantitatively measuring the SPM modulation degree of an actually operated optical fiber transmission line.
[0005]
[Means for Solving the Problems]
The SPM modulation degree measuring method according to the present invention is a method of data-modulating measurement light having a measurement wavelength with an intensity modulator, phase-modulating the data-modulated measurement light with a phase modulator, and adding a dummy light to the phase-modulated measurement light. The combined light consisting of the measurement light and the dummy light is input to the optical fiber transmission line to be measured, the measurement light propagated through the optical fiber transmission line is extracted, and the transmission error rate (BER) of the extracted measurement light is extracted. ) Is measured, and the amplitude and phase of the drive signal of the phase modulator are adjusted so that the transmission error rate is minimized, and the phase is adjusted under the adjusted amplitude and phase of the drive signal of the phase modulator. The method includes the steps of measuring a phase modulation degree of the modulator.
[0006]
An SPM modulation degree measurement system according to the present invention includes a measurement light source that generates measurement light of a measurement wavelength, an intensity modulator that modulates output light of the measurement light source with transmission data, and an optical phase of output light of the intensity modulator. A phase modulator that modulates the drive signal according to the drive signal, a drive signal generator that generates the drive signal, an optical multiplexer that multiplexes the dummy light with the output light of the phase modulator, and an output light of the optical multiplexer. A connection device for supplying the optical fiber transmission line, an optical filter for extracting the component of the measurement wavelength from the output light of the optical fiber transmission line, and a transmission error for measuring the transmission error rate of the transmission data from the output of the optical filter. Rate measuring apparatus, a phase modulation degree measuring apparatus for measuring the phase modulation degree of the measuring light phase-modulated by the phase modulator, and a transmission error rate measured by the transmission error rate measuring apparatus for a plurality of measurement wavelengths. A control device that adjusts the amplitude and phase of the drive signal so as to be minimized, and measures the phase modulation degree of the phase modulator under the adjusted amplitude and phase of the drive signal of the phase modulator. It is characterized by the following.
[0007]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0008]
(First embodiment)
FIG. 1 shows a schematic block diagram of an embodiment of the present invention. In the present embodiment, a WDM (wavelength division multiplexing) transmission state is formed, and under this state, the measurement wavelength λp is swept within the transmission wavelength band of the optical fiber transmission line 10 to measure the SPM modulation at each wavelength. .
[0009]
In FIG. 1, it is assumed that the optical fiber transmission line 10 to be measured is designed to transmit n wavelengths of the signal wavelengths λ1 to λn. FIG. 2 shows an arrangement example of the measurement wavelength λp and the signal wavelengths λ1 to λn. The optical fiber transmission line 10 may be a non-repeater optical transmission line composed of only optical fibers or an optical amplification relay transmission line in which optical repeater amplifiers are arranged at predetermined intervals. In FIG. 2, the solid line indicates the measurement light, and the broken line indicates the dummy light.
[0010]
The controller 12 controls the laser oscillation wavelength (measurement wavelength) λp of the tunable laser 14, and the tunable laser 14 outputs continuous laser light (CW laser light) having the wavelength λp. Although details will be described later, in this embodiment, the control device 12 covers the transmission wavelength band of the optical fiber transmission line 10 while setting a discrete wavelength not equal to the signal wavelengths λ1 to λn as the measurement wavelength λp. The measurement wavelength λp is swept. The measurement wavelength λp is generally set to an intermediate value between adjacent signal wavelengths as illustrated in FIG.
[0011]
The data generator 16 generates dummy data of the actual data rate for measuring the transmission error rate BER of the optical fiber transmission line 10, and the intensity modulator 18 generates the dummy data of the laser 14 based on the output data of the data generator 16. The output light is intensity-modulated. As a result, an optical pulse signal carrying dummy data is formed.
[0012]
The clock generator 20 generates a clock having a frequency X (Hz) corresponding to an actual signal transmission rate in synchronization with output data of the data generator 16. The control device 12 can control the amplitude of the output clock of the clock generation device 20 and the phase of the output data of the data generation device 16 with respect to the output data. The output clock of the clock generator 20 is applied to the phase modulator 22 as a drive signal or a modulation signal. The phase modulator 22 modulates the optical phase of the optical pulse signal from the intensity modulator 18 according to the output clock of the clock generator 20. As is well known, modulation of the optical phase causes fluctuations in the optical carrier wavelength. FIG. 3 shows an example of the relationship between the optical pulse, the drive signal of the phase modulator 22 (the output clock of the clock generator 20), and the frequency variation of the optical pulse. The horizontal axis indicates time, and the vertical axis indicates amplitude and frequency change Δf.
[0013]
The dummy light source 24 outputs dummy laser light having wavelengths λ1 to λn. The amplitude of each wavelength is set to approximately the same level as during signal transmission. The optical coupler 26 combines the measurement light output from the phase modulator 22 and the output light from the dummy light source 24. The dummy light source 24 includes, for example, n fixed-wavelength lasers that oscillate at individual wavelengths λ1 to λn, and an optical coupler that multiplexes output lights of these fixed-wavelength lasers.
[0014]
The optical coupler 28 supplies a part of the multiplexed light from the optical coupler 26 to the optical spectrum analyzer 30 and supplies the rest to the gain equalizer 32. The optical spectrum analyzer 30 checks the wavelength and intensity of the measurement light output from the phase modulator 22 and the dummy light output from the dummy light source 24, and measures the SPM modulation degree of the optical fiber transmission line 10 at the measurement wavelength λp. Set for purpose. The optical spectrum analyzer 30 outputs the measurement result to the control device 12.
[0015]
The gain equalizer 32 changes the gain profile of the light (the measurement light having the wavelength λp and the dummy light having the wavelengths λ1 to λn) input from the optical coupler 28 into a shape specified by the control device 12, and converts the gain profile into the optical fiber 10. Output. The relationship between the self-phase modulation of the optical fiber 10 and the gain profile can be measured by the gain equalizer 32, the details of which will be described later.
[0016]
The light that has propagated through the optical fiber 10 enters an optical bandpass filter (OBPF) 34. The transmission center wavelength of the optical bandpass filter 34 is selectable, and the control device 12 controls the transmission center wavelength of the optical bandpass filter 34 to the measurement wavelength λp. The optical bandpass filter 34 extracts the measurement light having the wavelength λp from the light input from the optical fiber 10 and applies the same to the light receiver 36. The photodetector (PD) 36 converts the input measurement light into an electric signal, and applies the electric signal to a BER (bit error rate) measuring device 38. The BER measuring device 38 measures the BER of the input signal and notifies the control device 12 of the result.
[0017]
In this manner, the WDM transmission state of the signal light having the wavelengths λ1 to λn is formed on the optical fiber 10, and the BER of the wavelength λp can be measured. In this state, the control device 12 adjusts the phase and the amplitude of the output clock of the clock generation device 20, and optimizes the phase modulation in the phase modulator 22 so as to minimize the BER.
[0018]
The spectrum analyzer 30 can be used to measure the optimized phase modulation. For example, the output of the data generator 16 is turned off with the phase modulation being optimized. Thereby, the intensity modulator 18 outputs the CW output of the laser 14 as it is, and the phase modulator 22 changes the optical phase of the CW light having the wavelength λp from the intensity modulator 18 in accordance with the clock from the clock generator 20. Modulate. In this state, the optical spectrum analyzer 30 measures the degree of phase modulation at the measurement wavelength λp. Specifically, the ratio of the carrier to the first harmonic is measured from the spectrum measured around the wavelength λp, and the modulation degree m of the phase modulation is measured. In general, in the case of sine wave phase modulation, the peak power Jn of the nth harmonic is expressed by the nth Bessel function with the modulation factor m as a variable. Thus, the modulation degree m of the phase modulation can be measured.
[0019]
In this embodiment, the phase modulator 22 modulates the optical phase of the CW light by turning off the output of the data generator 16, so that the optical spectrum analyzer 30 can easily measure the degree of phase modulation.
[0020]
The modulation degree m of the phase modulation optimized for the BER cancels out the self-phase modulation in the optical fiber 10 as it were. Therefore, the measured modulation degree m of the phase modulation can be a quantitative index of the SPM modulation degree of the optical fiber.
[0021]
FIG. 4 shows an operation flowchart of the control device 12. As an initial value, a value corresponding to (λ1 + λ2) / 2 is set to the measurement wavelength λp (S1), the laser oscillation wavelength of the laser 14 is controlled to the measurement wavelength λp (S2), and the transmission center of the optical bandpass filter 34 is set. The wavelength is set to the measurement wavelength λp (S3). As described above, the BER at the measurement wavelength λp is measured by the BER measuring device 38, and the phase and amplitude of the output clock of the clock generating device 20 are adjusted so that the BER is minimized (S4). After optimizing the phase and amplitude of the output clock of the clock generator 20, the data output of the data generator 16 is turned off (S5), and the optical spectrum analyzer 30 measures the degree of phase modulation at the measurement wavelength λp (S6). ).
[0022]
The measurement wavelength λp is changed (S7). In the second measurement, the measurement wavelength λp is set to (λ2 + λ3) / 2. If the new measurement wavelength λp is smaller than λn (S8), the steps after step S2 are executed again, and if it is larger than λn (S8), the measurement is terminated.
[0023]
As described above, the measurement wavelength λp is discretely changed within the range of the wavelengths λ1 to λn, and the optimum phase modulation at each measurement wavelength is measured, thereby obtaining the wavelength of the SPM modulation of the optical fiber transmission line 10. The distribution can be measured.
[0024]
It should be noted that the SPM modulation degree of the optical fiber transmission line 10 has a non-linear constant n₂(M²/ W), effective core area Aeff (μm²) And the chromatic dispersion at the measurement wavelength λp, and if an optical amplifier is provided, it also depends on the output power of the optical amplifier for the measurement light. The provision of the dummy light source 24 controls the output power of the optical fiber amplifier with respect to the measurement light to a desired constant value.
[0025]
(Second embodiment)
FIG. 5 shows a schematic configuration diagram of the second embodiment of the present invention. In this embodiment, a plurality of laser light sources that output laser light of wavelengths (for example, signal wavelengths λ1 to λn) that are appropriately sampled in the transmission band of the optical fiber transmission line to be measured are prepared, and the output light is used. Data modulation and phase modulation are collectively performed. On the receiving side, one desired wavelength is extracted and its BER is measured. Then, as in the first embodiment, the phase modulation is optimized so that the BER is minimized, and the degree of modulation of the optimized phase modulation is measured.
[0026]
The configuration and operation of the embodiment shown in FIG. 5 will be described. Also in FIG. 5, it is assumed that the optical fiber transmission line 110 to be measured is designed to transmit n wavelengths of the signal wavelengths λ1 to λn. Similarly to the optical fiber transmission line 10, the optical fiber transmission line 110 may be a non-repeater optical transmission line composed of only optical fibers or an optical amplification relay transmission line in which optical repeater amplifiers are arranged at predetermined intervals.
[0027]
The fixed wavelength lasers 114-1 to 114-n oscillate at wavelengths λ1 to λn, respectively, and output CW laser light. The output lights of the lasers 114-1 to 114-n are input to the optical coupler 118, with the polarizations controlled in the same direction by the polarization controllers 116-1 to 116-n. The optical coupler 118 multiplexes each output light of the polarization controllers 116-1 to 116-n, and applies the multiplexed light to the intensity modulator 122.
[0028]
The data generator 120 generates dummy data of an actual data rate for measuring the transmission error rate BER of the optical fiber transmission line 110, and the intensity modulator 122 generates an optical coupler 118 based on the output data of the data generator 120. The intensity of the multiplexed light from the light sources is collectively modulated. As a result, a wavelength division multiplexing (WDM) optical signal, which carries the same dummy data and includes optical pulse signals of different wavelengths λ1 to λn, is formed.
[0029]
The clock generator 124 generates a clock having a frequency X (Hz) corresponding to an actual signal transmission rate in synchronization with output data of the data generator 120. The control device 112 can control the amplitude of the output clock of the clock generation device 120 and the phase of the data generation device 120 with respect to the output data. The output clock of the clock generator 120 is applied to the phase modulator 126 as a drive signal or a modulation signal. The phase modulator 126 modulates the optical phase of each of the wavelength division multiplexed optical signals of the wavelengths λ1 to λn output from the intensity modulator 122 according to the output clock of the clock generator 124.
[0030]
The optical coupler 128 supplies a part of the output light of the phase modulator 126 to the optical spectrum analyzer 130, and supplies the rest to the gain equalizer 132. The optical spectrum analyzer 130 is set for the purpose of confirming the spectrum of light input to the optical fiber 110 and measuring the SPM modulation degree of the optical fiber transmission line 110 at the measurement wavelength λp. The optical spectrum analyzer 130 outputs the measurement result to the control device 112.
[0031]
The gain equalizer 132 sets the gain profile of the light (signal light of the wavelengths λ1 to λn, which carry the same data and whose optical phase is similarly modulated) input from the optical coupler 128, in a shape specified by the control device 112. And output to the optical fiber 110. The relationship between the self-phase modulation of the optical fiber 110 and the gain profile can be measured by the gain equalizer 132, the details of which will be described later.
[0032]
The light propagating through the optical fiber 110 is input to the optical bandpass filter 134. The transmission center wavelength of the optical bandpass filter 134 is selectable, and the control device 12 controls the transmission center wavelength of the optical bandpass filter 134 to one of the wavelengths λ1 to λn in order. The transmission center wavelength set in the optical bandpass filter 134 is the measurement wavelength λp. The optical bandpass filter 134 extracts the signal light of the wavelength λp set by the control device 112 from the light input from the optical fiber 110, and applies the signal light to the photodetector (PD) 136. The light receiver 136 converts the input light into an electric signal and applies the electric signal to the BER measuring device 138. The BER measurement device 138 measures the BER of the input signal, that is, the BER of the measurement wavelength λp, and notifies the control device 112 of the result.
[0033]
Thus, the WDM transmission state of the signal light having the wavelengths λ1 to λn is formed on the optical fiber 10, and the BER of each of the wavelengths λ1 to λn can be measured. In this state, similarly to the first embodiment, the control device 112 adjusts the phase and the amplitude of the output clock of the clock generation device 124 to optimize the phase modulation in the phase modulator 126 so that the BER is minimized. Become
[0034]
As in the first embodiment, the spectrum analyzer 130 can be used to measure the optimized degree of phase modulation. For example, the output of the data generator 120 is turned off with the phase modulation being optimized. As a result, the intensity modulator 122 outputs the CW light having the wavelengths λ1 to λn from the optical coupler 118 as it is, and the phase modulator 126 converts the optical phase of the CW light from the intensity modulator 122 into a clock generator. It modulates according to the clock from. In this state, the degree of phase modulation at the measurement wavelength λp is measured by the optical spectrum analyzer 130 in the same manner as in the first embodiment. Also in the present embodiment, the phase modulator 126 modulates the optical phase of the CW light by turning off the output of the data generator 120, so that the optical spectrum analyzer 130 can easily measure the degree of phase modulation.
[0035]
At each wavelength of λ1 to λn, the phase and the amplitude of the output clock of the optical clock generator 124 are optimized so as to minimize the BER, and the modulation degree of the phase modulation in that state is measured.
[0036]
In the second embodiment shown in FIG. 5, a wavelength tunable light source becomes unnecessary. Further, when using a wavelength tunable light source, it is necessary to secure time until the wavelength is stabilized. However, in the present embodiment, the measurement wavelength λp can be determined only by the optical bandpass filter 134, so that the first embodiment Measurement results can be obtained more quickly than in the examples. As described above, generally, the output wavelengths of the lasers 114-1 to 114-n need not be equal to the signal wavelengths λ1 to λn.
[0037]
(Third embodiment)
FIG. 6 is a schematic block diagram of a third embodiment in which the configuration of the first embodiment shown in FIG. 1 is combined with the configuration of the second embodiment shown in FIG. In the embodiment shown in FIG. 6, the same components as those in the embodiment shown in FIG. 5 are denoted by the same reference numerals.
[0038]
In the embodiment shown in FIG. 6, lasers 140-1 to 140-n that output CW laser lights of n wavelengths λ1 to λn in the transmission wavelength band of the optical fiber transmission line 110 are prepared, and those output lights are used. The light is multiplexed by the optical coupler 142. The configuration including the lasers 140-1 to 140-n and the optical coupler 142 corresponds to the dummy light source 24 in the embodiment shown in FIG.
[0039]
Further, n lasers (measurement light sources) that output CW laser beams having wavelengths slightly different from these wavelengths λ1 to λn, for example, wavelengths λ1 + Δ to λn + Δ different by half Δ of the wavelength interval between these wavelengths λ1 to λn. 144-1 to 144-n are prepared. FIG. 7 shows an example of the wavelength arrangement of the dummy light output from the lasers 140-1 to 140-n and the measurement light output from the lasers 144-1 to 144-n. In FIG. 7, the solid line indicates the measurement light, and the broken line indicates the dummy light.
[0040]
The output lights of the lasers 144-1 to 144-n are input to the optical coupler 148 with their polarizations controlled in the same direction by the polarization controllers 146-1 to 146-n. The optical coupler 148 multiplexes the output lights of the polarization controllers 146-1 to 146 -n and applies the multiplexed light to the intensity modulator 122. As in the embodiment shown in FIG. 5, the intensity modulator 122 modulates the intensity of the lightwave light by the optical coupler 148 according to the transmission data from the data generator 120, and the phase modulator 126 outputs the output of the intensity modulator 122. The optical phase of the light is modulated by the clock from the clock generator 124.
[0041]
The optical coupler 150 multiplexes the output light (dummy light) of the optical coupler 142 with the output light (measurement light) of the phase modulator 126 and outputs the multiplexed light to the optical coupler 128. The flow and processing of the measuring light after the optical coupler 128 are the same as in the embodiment shown in FIG. The control device 112 sequentially sets the laser wavelengths λ1 + Δ to λn + Δ of the lasers 144-1 to 144-n as the transmission center wavelength of the optical bandpass filter 134 as the measurement wavelength λp, and minimizes the BER at each wavelength λ1 + Δ to λn + Δ. The optimum degree of phase modulation is measured.
[0042]
(Fourth embodiment)
An ASE (Amplified Spontaneous Emission) light source may be used as the dummy light source. FIG. 8 is a schematic block diagram showing a configuration in which the embodiment shown in FIG. 1 is modified so that an ASE light source is used as the dummy light source 24. The same components as those in the embodiment shown in FIG. 1 are denoted by the same reference numerals. FIG. 9 shows an example of the wavelength arrangement of the dummy light and the measurement light having the wavelength λp output from the variable wavelength light source 14.
[0043]
The non-input optical amplifiers 40-1 and 40-2 generate broadband ASE light. The optical filters 42-1 and 42-2 extract the ASE light centered on the wavelength λa and the ASE light centered on the wavelength λb shown in FIG. 9 from the output lights of the optical amplifiers 40-1 and 40-2, respectively. . The optical coupler 44 multiplexes the output lights of the optical filters 42-1 and 42-2, and supplies the multiplexed light to the optical coupler 26. The optical power of the combined dummy ASE light output from the optical coupler 44 is set to such an extent that the nonlinear action in the optical fiber transmission line 10 in the WDM transmission state in the optical fiber transmission line 10 can be reproduced.
[0044]
In the embodiment shown in FIG. 8, the control device 12 sweeps the measurement wavelength λp between the dummy ASE light on the short wavelength side and the ASE dummy light on the long wavelength side and in a range not overlapping with these ASE dummy lights. . Fewer light sources are required, but the measurement range is narrower. Since the power distribution is different from the actual WSM transmission state, it is necessary to re-evaluate the measurement result.
[0045]
(Fifth embodiment)
The SPM modulation degree generally depends on not only the wavelength but also the quadratic coefficient of the refractive index of the optical fiber, the effective area of the core, the effective distance, and the effective optical power. Depends also on the amplification characteristics of the optical amplifier. In order to evaluate an optical fiber transmission line and to compare and compare a plurality of optical fiber transmission lines, a simple index including such a large number of parameters is required. By using the pre-emphasis by the gain equalizers 32 and 132, an index that can be used for evaluation and comparison can be obtained.
[0046]
Self-phase modulation of an optical fiber transmission line is caused by nonlinearity in components of the optical fiber transmission line. Accordingly, light having a high light peak power is susceptible to the non-linear effect, so that the degree of modulation of self-phase modulation increases. Conversely, light having a low optical peak power has a small non-linear effect, so that the degree of modulation of self-phase modulation is small.
[0047]
For example, as shown in FIG. 10, the control devices 12 and 112 control the gain equalizers 32 and 132 such that the peak power (Y) of the dummy light and the measurement light linearly changes with respect to the wavelength (X). Suppose the case. In FIG. 10, it is assumed that the slope of the change in peak power with respect to the wavelength is a. a is a so-called pre-emphasis amount. Under the gain profile set as described above, the self-phase modulation degree m of the optical fiber transmission line is measured at a plurality of wavelengths in the transmission wavelength band as described above. As a result, as shown in FIG. 11, the wavelength dependency of the SPM modulation degree m is typically obtained. In FIG. 11, the vertical axis indicates the SPM modulation degree m, and the horizontal axis indicates the wavelength (X). In general, the SPM modulation degree m is a function of the wavelength (X), but can be simply represented by a linear expression of m = sX + t as illustrated in FIG. s and t are coefficients for the optical fiber transmission lines 10 and 110 to be measured.
[0048]
By measuring the wavelength dependence of the modulation degree m of the self-phase modulation in several gain profiles, as shown in FIG. 12, the coefficient a of the gain profile, that is, the pre-emphasis amount a, and the wavelength dependence of the SPM modulation degree m The correlation with the sex s can be obtained. In FIG. 12, the vertical axis indicates the wavelength dependency s of the SPM modulation degree m, and the horizontal axis indicates the pre-emphasis amount a. The correlation as shown in FIG. 12 is representative of the nonlinear characteristic of the optical fiber transmission line to be measured, and the optical fiber transmission line having the same inclination of the correlation shown in FIG. We can evaluate that we have. When the optical fiber transmission lines 10 and 110 are optical amplification relay transmission lines, the correlation shown in FIG. 12 includes the optical amplification characteristics of an optical amplifier installed on the way.
[0049]
The case where the gain profiles of the gain equalizers 32 and 132 are linear has been described, but may be an n-th order or higher order curve. For example, in the case of a quadratic curve, the correlation between the quadratic coefficient, that is, the curvature, and the SPM modulation degree m is measured.
[0050]
(Measurement example 1)
FIG. 13 shows a measurement example according to the first embodiment. The channel power was set to -2.8 dBm, and the SPM modulation m was measured for WDM transmission of 17 wavelengths at 1549.39 nm near the entire dispersion wavelength of the transmission line where the accumulated chromatic dispersion can be ignored. The horizontal axis indicates the transmission distance, and the vertical axis indicates the measured modulation degree m of the self-phase modulation. The degree of SPM modulation, ie, non-linearity, increases with distance. For the transmission distance L (km), the SPM modulation degree m is
m = 0.334 × e^0.0002L
It has become.
[0051]
(Measurement example 2)
The correlation between the pre-emphasis amount and the SPM modulation degree m was measured by the method described as the fifth embodiment. Here, the gain profile is a quadratic curve in which the gain decreases at the center wavelength. FIG. 14 shows the wavelength dependence of the SPM modulation degree m when the gain reduction amount at the center wavelength is 0 dB, 4 dB, and 7 dB. FIG. 14 corresponds to FIG. 14, the vertical axis indicates the SPM modulation degree, and the horizontal axis indicates the wavelength. When the decrease amount of the center wavelength is 0 dB, the gain profile is flat with respect to the wavelength.
[0052]
FIG. 15 shows the correlation between the wavelength dependence of the SPM modulation degree and the pre-emphasis amount, obtained from the measurement results shown in FIG. The vertical axis in FIG. 15 shows an index of the wavelength dependence of the SPM modulation degree, and the horizontal axis shows the pre-emphasis amount. Here, the curvature of the wavelength dependence shown in FIG. 14, that is, the quadratic coefficient shown in FIG. 14 was employed as an index of the wavelength dependence of the SPM modulation degree, and the gain reduction amount at the center wavelength of the gain profile was employed as the pre-emphasis amount. .
[0053]
【The invention's effect】
As can be easily understood from the above description, according to the present invention, the nonlinearity of an actually operated optical fiber transmission line can be measured by a simple method. In addition, an index for quantitatively evaluating the nonlinearity of the optical fiber transmission line can be obtained, which makes it easy to compare and contrast a plurality of optical fiber transmission lines.
[Brief description of the drawings]
FIG. 1 is a schematic block diagram of a first embodiment of the present invention.
FIG. 2 is an arrangement example of a measurement wavelength λp and signal wavelengths λ1 to λn in the first embodiment.
FIG. 3 shows an example of a relationship between an optical pulse in a phase modulator 22, a drive signal, and a frequency variation of the optical pulse.
FIG. 4 is an operation flowchart of the control device 12.
FIG. 5 is a schematic configuration block diagram of a second embodiment of the present invention.
FIG. 6 is a schematic block diagram of a third embodiment.
FIG. 7 is an example of a wavelength arrangement of dummy light output from lasers 140-1 to 140-n and measurement light output from lasers 144-1 to 144-n.
FIG. 8 is a schematic block diagram of a fourth embodiment.
FIG. 9 is an example of wavelength arrangement in the fourth embodiment.
FIG. 10 is a schematic diagram of a gain profile by gain equalizers 32 and 132.
FIG. 11 is a schematic diagram of the wavelength dependence of the SPM modulation degree m.
FIG. 12 is a schematic diagram of a correlation between a pre-emphasis amount a and a wavelength dependency s of an SPM modulation degree m.
FIG. 13 is a measurement example of the distance dependence of the SPM modulation degree m.
FIG. 14 is a measurement example of a change in the wavelength dependence of the SPM modulation degree m with respect to a change in the pre-emphasis amount.
FIG. 15 is a measurement example of a correlation between the wavelength dependence of the SPM modulation factor and the amount of pre-emphasis.
[Explanation of symbols]
10: Optical fiber transmission line
12: Control device
14: Tunable laser
16: Data generator
18: Intensity modulation device
20: Clock generator
22: Phase modulator
24: dummy light source
26: Optical coupler
28: Optical coupler
30: Optical spectrum analyzer
32: Gain equalizer
34: Optical bandpass filter (OBPF)
36: Receiver
38: BER measurement device
40-1, 40-2: Optical amplifier
42-1 and 42-2: Optical Filter
44: Optical coupler
110: Optical fiber transmission line
112: Control device
114-1 to 114-n: laser (measurement light source)
116-1 to 116-n: Polarization control device
118: Optical coupler
120: Data generator
122: intensity modulator
124: Clock generator
126: phase modulator
128: Optical coupler
130: Optical spectrum analyzer
132: Gain equalizer
134: Optical bandpass filter
136: Receiver
138: BER measurement device
140-1 to 140-n: laser
142: Optical coupler
144-1 to 144-n: laser (measurement light source)
146-1 to 146-n: Polarization controller
148: Optical coupler
150: Optical coupler

Claims (2)

Data modulation of the measurement light of the measurement wavelength by the intensity modulator,
The data-modulated measurement light is phase-modulated by a phase modulator,
The dummy light is multiplexed with the phase-modulated measurement light,
The combined light consisting of the measurement light and the dummy light is input to the optical fiber transmission line to be measured,
Extract the measurement light that has propagated through the optical fiber transmission line,
Measure the transmission error rate (BER) of the extracted measurement light,
Adjust the amplitude and phase of the drive signal of the phase modulator so that the transmission error rate is minimized,
A method of measuring the degree of modulation of SPM, comprising: measuring the degree of phase modulation of the phase modulator under the adjusted amplitude and phase of the drive signal of the phase modulator. A measurement light source (14) for generating measurement light of a measurement wavelength;
An intensity modulator (18) for modulating output light of the measurement light source with transmission data;
A phase modulator (22) for modulating the optical phase of the output light of the intensity modulator according to a drive signal;
A drive signal generator (20) for generating the drive signal;
An optical multiplexer (26) for multiplexing the dummy light with the output light of the phase modulator;
A connection device (32) for supplying output light of the optical multiplexer to an optical fiber transmission line,
An optical filter (34) for extracting a component of the measurement wavelength from output light of the optical fiber transmission line;
A transmission error rate measuring device (38) for measuring a transmission error rate of the transmission data from an output of the optical filter;
A phase modulation degree measuring device (30) for measuring a phase modulation degree of the measurement light phase-modulated by the phase modulator;
For a plurality of measurement wavelengths, the amplitude and phase of the drive signal are adjusted so that the transmission error rate measured by the transmission error rate measurement device (38) is minimized, and the adjusted drive signal of the phase modulator is adjusted. A control device for measuring the degree of phase modulation of the phase modulator under the amplitude and the phase (12)
A SPM modulation degree measurement system, comprising:

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