JP2003172907A - Nonlinear distortion compensation method for optical modulator, low distortion optical communication system and low distortion optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means and a method by which inter-modulation distortion caused by the nonlinear characteristic of an optical modulator is reduced. <P>SOLUTION: In the characteristic of an electro-absorption type modulator (an EA modulator) that is one type of optical modulators, the phase of inter- modulation distortion is suddenly shifted for 180° with respect to a bias voltage. The characteristic of the EA modulator is used in the following scheme, i.e., two EA modulators are operated in parallel and distortion generated by the modulators is mutually canceled with each other to reduce the distortion. Having constituted the above, a distortion generating circuit and a distortion phase reversal circuit, which are essentially required in a conventional distortion compensation method such as a predistortion distortion compensation method, are eliminated and distortion compensation is realized with a simple configuration. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonlinear distortion compensation method for an optical modulator, which reduces nonlinear distortion noise generated in an optical modulator used in an optical fiber communication system, a low distortion optical communication system, and a low distortion optical modulator. It is about.

[0002]

2. Description of the Related Art In an optical fiber communication system, as a means for multiplex transmission of a plurality of signals, an SCM (Sub-carrier Mult) for frequency-multiplexing each signal in an electric stage and converting it into an optical signal
iplexed) method is applied. In this SCM multiplex transmission, since optical modulation is performed with a plurality of carrier signals, intermodulation (IM: IM:
Inter-modulation) has the problem of limiting the number of multiplex signals that can be transmitted and the transmission distance. Therefore, linearization of the characteristics of the optical modulator is required.

Since the nonlinear characteristics of an optical modulator are determined by its structure, material, etc., it is often difficult to linearize the optical modulator itself. Therefore, distortion compensation methods such as the predistortion method have been studied (Reference: [1] Mamoru Ogasawara, Kaji Watanabe, Takaaki Ichikawa "Study on Nonlinear Distortion Compensation in Subcarrier Optical Transmission", The Institute of Electronics, Information and Communication Engineers. Magazine B-11 Vo
lJ-75-B-II No.9 pp.658-660 September 1992, [2] T. Iwa
ki, K.Sato, K.Suto “Reduction of Dispersion-Induced
Distortion in SCM Transmission Systems by Using P
re-distortion-Linearized MQW-EA Modulators ”, J.of
Lightwave Technology, Vol.15.No.2, February 199
7.).

FIG. 6 shows an example of the basic configuration of a distortion compensation circuit based on the predistortion distortion compensation method. As shown in the figure,
The configuration is divided into two parts. The pre-stage is a pre-distortion circuit (hereinafter referred to as PD circuit) 1 for generating distortion for compensation in advance, and the post-stage is an optical modulation / demodulation system 10 requiring distortion reduction. The PD circuit 1 has a radio frequency band (RF)
Hybrid (HYB) that splits input 2 into two paths
3, a main signal path including the delay line 4, a distortion generation path including a distortion generator 5, a variable attenuator (ATT) 6 and a variable phase shifter (PS) 7, and a hybrid (resynthesizing the two path signals) HYB) 8.

The distortion generated by the distortion generator 5 is amplitude-adjusted by the variable attenuator (ATT) 6 so as to satisfy the condition for canceling the distortion generated by the optical modulation / demodulation system 10 in the subsequent stage, and the variable phase shifter (P
The phase is adjusted (shifted by 180 °) in S) 7 and then combined with the main signal component in hybrid (HYB) 8. The electric lengths of the main signal path and the distortion generation path are adjusted to be equal in the variable delay line 4.

The output signal of the PD circuit 1 is converted into an optical signal by the optical modulator 11 and is demodulated by the optical demodulator 13 via the optical fiber 12 to become an RF output signal. With this configuration, the distortion generated in the optical modulation / demodulation system 10 is canceled by the distortion added in the PD circuit 1, and the distortion generated in the optical modulator is reduced. The distortion noise of the optical modulation / demodulation system 10 is dominated by the non-linear characteristic of the optical modulator 11.

When a two-wave input having a spectrum 40 shown in the lower part of FIG. 6 is applied as the RF input 2, the signal spectrum of the output at the output terminal 9 of the PD circuit 1 and the signal spectrum of the optical modulation / demodulation system RF output 20 are 41 and 42, respectively. . As shown in this figure, the amplitudes of the distortions generated in both the PD circuit 1 and the optical modulation / demodulation system 10 are equal, and the phases are in an antiphase relationship (relative phase is different by 180 °). Therefore, the RF output 2
It can be seen that the distortion of the optical modulator 11 is canceled at 0.

[0008]

The configuration of the conventional predistortion distortion compensation method shown in FIG. 6 has the following problems. That is, the distortion generation system included in the pre-distorter circuit 1 generates only the distortion component, and the distortion phase is exactly the opposite phase (18
Since it is 0 °), there is a problem that a phase shift adjusting function excellent in wide band property is required. The predistorter circuit 1 has a problem that it is necessary to equalize the electrical lengths of the main signal path and the distortion generation path. There is a problem that it is difficult to secure the similarity between the distortion generated by the optical modulator 11 and the distortion characteristic of the distortion generator 5 in a wide dynamic range.

The present invention has been made in view of the above circumstances, and a non-linear distortion compensation method for an optical modulator that does not require a predistorter circuit having a complicated structure due to the occurrence of anti-phase distortion, and a low distortion compensation method An object is to provide a distorted optical communication system and a low distortion optical modulator.

[0010]

In order to achieve the above object, the invention according to claim 1 is a distortion compensating method for reducing intermodulation distortion caused by non-linear characteristics of an optical modulator. Is a configuration in which two optical modulators are operated in parallel using an optical modulator having a characteristic that the phase of distortion generated in the optical modulator is inverted by a bias voltage supplied, that is, shifted by 180 °. The bias voltage of both optical modulators is set so that the phase of the distortion generated in one optical modulator is in the range where the phase of the distortion generated in the other optical modulator is inverted. The bias voltages are set so that the amplitudes of the distortions generated by the optical modulators are equalized, and the outputs of the optical modulators are combined to cancel the distortions generated by the optical modulators.

Further, in the invention described in claim 2, in the low distortion optical communication system capable of reducing the intermodulation distortion generated by the non-linear characteristic of the optical modulator, the optical modulator generates in the optical modulator. A hybrid in which two optical modulators are operated in parallel by using an optical modulator having a characteristic in which the phase of the distortion to be performed is inverted by the bias voltage supplied, that is, shifted by 180 °, and the input signal is branched into two ( 3), the two optical modulators that receive the two outputs of the hybrid (3), two optical lines that respectively transmit the output signals of the two optical modulators, and the two optical beams. Two optical modulators, each comprising two optical demodulators for respectively demodulating an optical modulation signal from the optical path, and a hybrid (27) for combining and outputting two outputs of the two optical demodulators. Is one light The bias voltage of both optical modulators is set so that the phase of the distortion generated in the other optical modulator is in the range in which the phase of the distortion generated in the other optical modulator is inverted. The bias voltage is set so that the amplitudes of the distortions are equalized.

According to a third aspect of the present invention, there is provided a low distortion optical modulator capable of reducing intermodulation distortion caused by the non-linear characteristic of the optical modulator, wherein the optical modulator has the optical modulator. A hybrid in which two optical modulators are operated in parallel by using an optical modulator having a characteristic in which the phase of the distortion to be performed is inverted by the bias voltage supplied, that is, shifted by 180 °, and the input signal is branched into two ( 3)
And two optical modulators that respectively receive the two outputs of the hybrid (3), and an optical hybrid (28) that combines and outputs the outputs of the two optical modulators. In the optical modulator of, the bias voltage of both optical modulators is set so that the phase of the distortion generated in the other optical modulator is in the range where the phase of the distortion generated in the other optical modulator is inverted. Further, the bias voltage is set so that the amplitudes of distortions generated by both optical modulators are equalized.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, prior to the description of the embodiments of the present invention, an outline of the present invention will be described.

The present invention is characterized in that the distortion phase of the optical modulator / demodulator itself is reversed and the distortion of the optical modulator / demodulator system is reduced with a simple structure. The difference from the conventional predistortion distortion compensation method is that the predistorter function (anti-phase distortion generating function), which requires complicated circuits and functions, is not required.

As an optical modulator shown in FIG. 3, an electric field absorption (EA:
Third-order intermodulation distortion (IM) when an electro-absorption type optical modulator (hereinafter referred to as EA optical modulator) is used
3) A characteristic example of the bias voltage dependence of the level will be shown. Figure 3
The measured value shown in (b) is a two-frequency signal (f ₁ = 5.795).
GHz, f ₂ = 5.805 GHz) is input to the EA optical modulator, and the IM3 level at the output end of the optical demodulator is normalized (IM3 / C) with the main signal level C. The distortion generated in the optical modulation / demodulation system is dominated by the distortion generated in the EA optical modulator, and the distortion generated by the optical demodulator is a value lower by about 30 dB.

The third-order distortion level of the EA optical modulator shows two minimum points with respect to the bias voltage, and the relative phase is 180 ° as shown in FIG.
It has the characteristic of abrupt shift. That is, the strain and the region C generated when the bias voltage is set to the region B in the figure
The phase relationship of the distortion when set to 1 indicates that the phase is in the antiphase state. The same applies to the relationship between the areas B and A. Therefore, by utilizing the characteristic that the distortion generated by the bias voltage setting becomes the anti-phase distortion, it becomes possible to eliminate the anti-phase distortion generating circuit having the most complicated structure in the pre-distortion distortion compensation method.

FIG. 1 shows the configuration of the low distortion optical communication system according to the first embodiment of the present invention. In the figure, the low-distortion optical communication system according to the present embodiment has a configuration in which two optical modulators whose distortion is desired to be operated are operated in parallel. In FIG. 1, reference numeral 2 is an RF (radio frequency signal) input terminal, and reference numeral 3 is a hybrid (HYB) that branches an RF input input from the RF input terminal 2. Also,
Reference numerals 21 and 22 are EA optical modulators, and reference numerals 23 and 24 are used.
Are bias voltage regulators of the EA optical modulators 21 and 22, respectively. Reference numerals 25 and 26 are light sources of the EA optical modulators 21 and 22, respectively, and reference numerals 12 and 14 are optical fiber lines (optical lines) connected to the EA optical modulators 21 and 22, respectively. Reference numerals 13 and 15 are optical demodulators connected to the optical fiber lines 12 and 14, respectively.
Reference numeral 27 is a combination of outputs of the optical demodulators 13 and 15 and a reference numeral 2
It is a hybrid that outputs from the 0 RF output terminal.

Further, in FIG. 1, reference numeral 40 is an input signal frequency spectrum of the RF input terminal 2, and reference numeral 43 is E.
The RF demodulation spectrum of the path from the A optical modulator 21 to the optical demodulator 13, the reference numeral 44 is the RF demodulation spectrum of the path from the EA optical modulator 22 to the optical demodulator 15, and the reference numeral 45 is the spectrum of the RF output terminal 20. Is shown.

In the above structure, the RF signal applied to the input terminal 2 is branched into two paths by the hybrid 3 and E
The A optical modulators 21 and 22 convert the same RF signal into an optical signal that is intensity-modulated. Both EA optical modulators 21, 22
Are supplied with optical signals of wavelengths λ1 and λ2 from the light sources 25 and 26, respectively. The outputs of both EA optical modulators 21 and 22 are restored to RF signals by the optical demodulators 13 and 15 via the optical fiber lines 12 and 14, respectively. Then, the outputs of the optical demodulators 13 and 15 are combined by the hybrid 27, and the output is obtained from the RF output terminal 20.

When two waves (frequency f ₁ and f ₂ ) shown by reference numeral 40 are applied as an RF input signal, the EA optical modulators 21 and 22 have the third characteristic due to the non-linear characteristic of the EA optical modulators 21 and 22. Third-order intermodulation distortion IM as shown in the figure
3 occurs, and the phase thereof shows a characteristic of being shifted by 180 ° by the bias voltage.

Therefore, for example, the bias voltage V _b1 of the EA optical modulator 21 is set within the region B in FIG. 3 by the bias voltage adjuster 23, and the bias voltage V _b2 of the EA optical modulator 22 is set.
Is set in the region C of FIG. 3 by the bias voltage adjuster 24. As a result, the third-order intermodulation distortion (frequency 2f ₁ −f ₂ , 2f ₂ −f ₁ ) generated in the EA optical modulators 21 and 22 is generated.
Are represented by the spectra shown by reference numerals 43 and 44 in FIG. 1, respectively. Here, the phase of the distortion component indicated by the reference numeral 43 has an antiphase relationship with the distortion component indicated by the broken line of the reference numeral 44, and at the output of the hybrid 27 that combines the two, the two cancel each other out, and the RF indicated by the reference numeral 45. Distortion reduction is realized as shown in the output spectrum. It is necessary to match the amplitudes of the strains in order to completely cancel the strains of the both, but this can be realized by finely adjusting the bias voltages _Vb1 and _{Vb2 in} the regions B and C, respectively.

As described above, according to the present embodiment, the EA
By utilizing the feature that the phase of the mutual adjustment distortion generated in the optical modulators 21 and 22 is shifted by 180 ° by the bias voltage, it is possible to extremely easily achieve the function of inverting the distortion phase, which is essential in the distortion compensation method by cancellation. Occurs. In addition, as a secondary merit of this configuration, the EA optical modulator
Main signal output increases (voltage addition) to operate in parallel with each other
It is also possible to improve the C / N ratio. This eliminates the need for the distortion generation circuit, phase inversion circuit, and fine phase adjustment that were most difficult to realize in the conventional cancellation distortion compensation method such as the predistortion distortion compensation method. Simplification and improved stability are realized.

The light sources 25 and 26 have the configuration of this embodiment.
It is also possible to equalize the wavelengths of (λ1 = λ2),
In this case, one light source can be supplied in parallel. When using different wavelengths (λ1 ≠ λ
In 2), it is also possible to perform a multiplex transmission using a single optical fiber (optical line) by performing wavelength demultiplexing.

FIG. 4 shows an example of actually measured data in which the distortion of the EA optical modulator is canceled by the present embodiment. For measurement,
An EA optical modulator having a strained quantum well (MQW) structure made of InGaAsP in the light absorption region was used. Also, as the RF input signal, a dual frequency signal (f ₁ = 5.795) is used.
GHz, f ₂ = 5.805 GHz), and the light source has wavelengths λ1 = 1553.33 nm and λ2 = 1550.12 nm.
The optical fiber used was a 1.55 μm band single mode fiber.

In FIG. 4, the distortion noise characteristic at the output of the optical demodulator when the bias voltage V _b1 of the EA optical modulator 21 is set to the value in the region B (see FIG. 3) is shown in FIG. , (B) shows the distortion noise characteristic in the output of the optical demodulator when the bias voltage V _b2 of the EA optical modulator 22 is set to the value in the region C (see FIG. 3). In addition, FIG.
Indicates an output obtained by combining the two with the hybrid 27. As shown in this figure, fine adjustment of the bias voltage completely canceled the third-order intermodulation distortion of both, demonstrating the feasibility of this distortion compensation method. The first embodiment of the present invention has been described above.

Next, FIG. 2 shows the configuration of a low distortion optical communication system according to the second embodiment of the present invention. In this embodiment,
The configuration of the first embodiment is further simplified by combining and transmitting the outputs of the two EA optical modulators.
The difference between the low-distortion optical communication system according to the present embodiment and the low-distortion optical communication system according to the first embodiment is that an optical hybrid 28 is added and an optical fiber line and an optical demodulator are configured by only one series. This is the point that I was able to do. Note that FIG.
In FIG. 1, parts common to those in FIG.
The description is omitted.

In the above structure, the RF signal applied to the input terminal 2 is branched into two paths by the hybrid 3 and E
The A optical modulators 21 and 22 convert the same RF signal into an optical signal that is intensity-modulated. Both EA optical modulators 21, 22
The wavelengths from the light sources 25 and 26 are λ1 and λ2, respectively.
An optical signal of (λ1 ≠ λ2) is supplied. The outputs of both EA optical modulators 21 and 22 are combined by the optical hybrid 28,
The RF signal is restored by the optical demodulator 13 via the optical fiber 12.

Here, for example, the bias voltage V _b1 of the EA optical modulator 21 is set within the region B of FIG. 3 by the bias voltage adjuster 23, and the bias voltage V _b2 of the EA optical modulator 22 is set to the bias voltage adjuster. It is set in the area C of FIG. 3 by 24. As a result, the third-order intermodulation distortion (frequency 2f ₁ −f ₂ , 2f ₂ −f ₁ ) generated in the EA optical modulators 21 and 22 is
The spectrum is represented by the reference numerals 43 and 44 in FIG. 2, respectively. Here, the phase of the distortion component indicated by the reference numeral 43 has an antiphase relationship with the distortion component indicated by the broken line of the reference numeral 44, the two cancel each other at the output of the optical demodulator 13, and the spectrum 45 at the RF output 20. Distortion reduction is realized as shown in. It is necessary to match the amplitudes of the distortions in order to completely cancel the distortions of the both, but this can be realized by finely adjusting the bias voltages _Vb1 and _{Vb2 in} the regions B and C, respectively.

In addition to the merits shown in the first embodiment, the merits of this embodiment are two EA optical modulators 2.
By integrating 1, 22, the light sources 25, 26, the hybrid 3, and the optical hybrid 28 into an integrated circuit, the low distortion optical modulator 30 can be integrated.

FIG. 5 shows an example of actually measured data in which the distortion of the EA optical modulator is canceled by the present embodiment. The measurement conditions are the same as the example shown in FIG. The distortion noise characteristic at the output of the optical demodulator when the bias voltage of the EA optical modulator 21 is set to the value in the region B (see FIG. 3) is shown in FIG.
FIG. 5B shows the distortion noise characteristic in the output of the optical demodulator when the bias voltage V _b2 of the EA optical modulator 22 is set to the value in the region C (see FIG. 3). Further, FIG. 5C shows an output obtained by combining the both by the optical hybrid 28.
As shown in this figure, also in the present embodiment, fine adjustment of the bias voltage completely cancels the third-order intermodulation distortion of the two, demonstrating the feasibility of this distortion compensation method. The second embodiment of the present invention has been described above.

[0031]

As described above, according to the present invention, as a nonlinear distortion compensation method for an optical modulator, the phase of the third-order intermodulation distortion generated in an EA (electroabsorption) type optical modulator depends on the bias voltage value. By utilizing the characteristic of inverting, that is, shifting by 180 °, the anti-phase distortion generating function that is essential in the cancellation distortion compensation method such as the predistortion distortion compensation method becomes unnecessary, and the electrical length matching becomes unnecessary. This has the effect of greatly simplifying the configuration for distortion compensation.

Further, the simplification of the structure brings about an effect of enabling the distortion improving characteristic to have a wide band. As a result, by applying the present invention to various optical communication systems in which the non-green distortion generated in the optical modulator is an obstacle, the transmission quality is improved with a simple configuration, the transmission distance is increased, and the transmission capacity is increased. The advantage is that it can be expanded.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 3 is a diagram showing a characteristic example of bias voltage dependency of distortion generated in an electro-absorption (EA) type optical modulator.

FIG. 4 is a diagram showing experimental data demonstrating the operation according to the first embodiment of the present invention.

FIG. 5 is a diagram showing experimental data demonstrating the operation according to the second embodiment of the present invention.

FIG. 6 is a diagram showing a basic configuration of a conventional predistortion distortion compensation method.

[Explanation of symbols]

1 ... Pre-distorter circuit, 2 ... RF input, 3, 8,
27 ... Hybrid, 4 ... Delay line, 5 ... Distortion generator, 6 ...
Variable attenuator, 7 ... Variable phase shifter, 9 ... Predistorter circuit output terminal, 10 ... Optical modulation / demodulation system, 11 ... Optical modulator, 1
2, 14 ... Optical fiber line (optical line), 13, 15 ... Optical demodulator, 20 ... RF output, 40 ... RF input spectrum, 41 ... Predistorter circuit output spectrum,
42, 45 ... RF output spectrum, 21, 22 ... EA
(Electroabsorption) optical modulator, 23, 24 ... EA bias voltage regulator, 25, 26 ... Light source, 43 ... EA optical modulator 21 spectrum, 44 ... EA optical modulator 22 spectrum, 28 ... Optical hybrid, 30 ... Low distortion optical modulator

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04B 10/152 10/18 (72) Inventor Eiharu Okamoto 2-31-19 Shiba, Minato-ku, Tokyo Communications・ Inside the broadcasting organization (72) Inventor Ryoichi Miyamoto 2-31-19, Shiba, Minato-ku, Tokyo Communication and broadcasting agency F-term (reference) 2H079 AA02 AA13 BA01 CA04 DA16 EA05 EB04 FA02 GA03 HA11 5K002 CA01 CA16

Claims (3)

[Claims] 1. A distortion compensating method for reducing intermodulation distortion generated by a nonlinear characteristic of an optical modulator, wherein the phase of distortion generated in the optical modulator is inverted by a bias voltage supplied to the optical modulator. That is, by using an optical modulator having a characteristic of shifting by 180 °, two optical modulators are operated in parallel, and the phase of the distortion generated in one optical modulator in the other optical modulator is The bias voltage of both optical modulators is set so that the phase of the generated distortion is in the range that is inverted, and further, the bias voltage is set so that the amplitudes of the distortions generated by both optical modulators are equalized. A nonlinear distortion compensation method for an optical modulator, wherein the distortions generated by both optical modulators are canceled by combining the outputs of the modulators. 2. A low-distortion optical communication system capable of reducing intermodulation distortion caused by non-linear characteristics of an optical modulator, wherein the phase of distortion generated in the optical modulator is supplied to the optical modulator. A hybrid (3) configured to operate two optical modulators in parallel by using an optical modulator having a characteristic of inverting by voltage, that is, shifting by 180 °, and a hybrid (3), Of the two optical modulators, two optical lines for transmitting the output signals of the two optical modulators, and demodulated optical modulated signals from the two optical lines, respectively. Two optical demodulators, and a hybrid (27) that synthesizes and outputs two outputs of the two optical demodulators, and the two optical modulators are generated in one optical modulator. Distortion The bias voltage of both optical modulators is set so that the phase of the distortion generated in the other optical modulator is inverted with respect to the phase, and the amplitude of the distortion generated by both optical modulators is made equal. A low-distortion optical communication system characterized in that the bias voltage is set to. 3. A low-distortion optical modulator capable of reducing intermodulation distortion caused by non-linear characteristics of the optical modulator, wherein the phase of the distortion generated in the optical modulator is supplied to the optical modulator. A hybrid (3) configured to operate two optical modulators in parallel by using an optical modulator having a characteristic of inverting by voltage, that is, shifting by 180 °, and a hybrid (3), Two optical modulators respectively receiving the two outputs of the above, and an optical hybrid (28) for combining and outputting the outputs of the two optical modulators, and the two optical modulators are The bias voltage of both optical modulators is set so that the phase of the distortion generated in the other optical modulator is in the range where the phase of the distortion generated in the other optical modulator is inverted. Occur Low distortion light modulator the bias voltage is characterized by comprising been configured to equalize the amplitudes of.