KR100959398B1 - Optical transmitter for optimizing an input dc bias voltage of optical modulator with duo-binary modulation and method thereof - Google Patents

Abstract

Disclosed is an optical transmission device for optimizing a DC bias voltage input to an optical modulator to which a duo binary data modulation scheme is applied. The optical transmission device of the present invention comprises a duo binary type optical modulator; A sine wave generator configured to generate a sine wave added together with a DC bias voltage input to the optical modulator; And a bias voltage corrector configured to correct the value of the DC bias voltage by analyzing an electrical signal that is electrically converted and output after being modulated by the external optical modulator. As a result, the DC bias voltage can be optimized.

Description

Optical transmitter for optimizing an input dc bias voltage of optical modulator with duo-binary modulation and method

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical transmission technology, and more particularly, to a technique for stabilizing a DC bias voltage of an external modulator used in a duo binary data modulation scheme.

This study was derived from the research conducted as part of the IT growth engine technology development project of the Ministry of Information and Communication and the Ministry of Information and Communication Research and Development. [Task Management Number: 2006-S-059-02, Development of ASON-based Metro Optical Line Distribution Technology] ]

Dense Wavelength Division Multiplexing (DWDM) refers to a technology that greatly increases capacity and channel by narrowing the density between wavelengths as the wavelength division multiplexing (WDM) technology develops. Wavelength Division Multiplexing (WDM) is a light transmission system that bundles multiple channels of different wavelengths of light and transmits them through a single optical fiber. Channels are arranged by dividing the wavelength of light transmitted through the optical fiber at regular intervals, and the signals are loaded on each channel, and then optically multiplexed and transmitted through one optical fiber. On the receiving side, each channel is separated into wavelengths and each channel is used separately. Previously, only one wavelength was sent to one optical fiber. However, in WDM transmission, optical signals having multiple wavelengths are bundled and transmitted, thereby using the existing network as it is, and having a new cable network. It is a high density wavelength division multiplexing, which is an advanced step of such WDM technology. Such optical transmission system using DWDM is a system that is useful for high speed internet network which is increasing in recent years.

Meanwhile, modulators of an optical transmission device for high speed long haul transmission include a direct modulation method for controlling current applied to a light source and an external modulation method using an external modulator. At low transmission speeds, direct modulation is often used because of its ease of use, but moving to high speeds, chirps of direct modulation become obstacles to long-distance transmission.

Data modulation using non-return (Non Return-to-Zero) as a conventional external modulation method is well known. However, due to the rapid increase in data traffic and the need for high speed data transmission, there is a limit to the expansion of the transmission capacity due to the rapid inter-channel interference and distortion in data modulation using the existing non return-to-zero. In addition, the DC frequency component of the existing binary NRZ transmission signal and the high frequency component diffused during modulation cause nonlinearity and dispersion during propagation in the optical fiber medium, which has a limitation in transmission distance in high speed transmission.

The duo binary technology is attracting attention as an optical transmission technology that can overcome the transmission distance limitation due to chromatic dispersion. The main advantage of duo binary transmission is that the transmission spectrum is reduced compared to normal binary transmission. In a dispersion limiting system, the transmission distance is inversely proportional to the square of the transmission spectral bandwidth. This means that if the transmission spectrum is reduced by half, the transmission distance is quadrupled. Furthermore, the carrier frequency is suppressed in the Dewar binary transmission spectrum, thereby mitigating the limitation on the output optical power due to stimulated Brillouin Scattering in the optical fiber.

However, external modulators, especially Mach-Zender type optical modulators, used in the duo binary data modulation technique, cause the transfer curve to shift from side to side as the temperature changes (DC bias drift). . FIG. 1 is a diagram illustrating an eye pattern of an optical signal in an optimal DC bias voltage and a non-optimal DC bias voltage. As can be seen in Figure 1, it is confirmed that the eye pattern of the optical signal is different. This phenomenon eventually leads to degradation of the transmission system.

SUMMARY OF THE INVENTION The present invention is derived from this background, and an object of the present invention is to provide an optical transmission apparatus and method for optimally maintaining a bias voltage so that an optical modulator of a duo binary data modulation method can output a stable signal even with temperature changes.

The optical transmission device of the present invention for achieving the above technical problem, a duo binary type optical modulator; A sine wave generator configured to generate a sine wave added together with a DC bias voltage input to the optical modulator; And a bias voltage corrector configured to correct the value of the DC bias voltage by analyzing an electrical signal that is electrically converted and output after being modulated by the external optical modulator.

The bias voltage corrector may include: an amplifier configured to amplify the electrical signal; A filtering unit outputting only a frequency component of a signal generated by the sine wave generator from the amplified electrical signal; A detector including a phase detector configured to detect a phase between a sine wave output from the filtering unit and a sine wave signal generated by the sine wave generator; A correction controller for outputting a control signal for correcting the value of the DC bias voltage according to the detected information; And a DC bias control unit which receives a control signal output from the correction controller and adjusts the value of the DC bias voltage.

The detector further includes a voltage detector configured to detect the peak-peak voltage of the sine wave output from the filtering unit.

Meanwhile, a method for optimizing a DC bias voltage input to an optical modulator to which the duo binary data modulation scheme of the present invention is applied to achieve the above technical problem is added to the DC bias voltage input to the optical modulator to provide the optical modulator. Detecting phase information between a sine wave extracted from an electrical signal output from the sine wave and a sine wave input to the optical modulator in addition to the DC bias voltage; Determining whether the detected phase information coincides with reference phase information; And adjusting the DC bias voltage input to the optical modulator if the determination result does not match.

The method includes detecting a peak-peak voltage of a sine wave extracted from the electrical signal; Determining whether the detected peak-peak voltage coincides with a reference peak-peak voltage; And adjusting the DC bias voltage input to the optical modulator if the peak-peak voltage does not match as a result of the determination.

Since the present invention maintains the optimization by quickly correcting the DC bias voltage input to the optical modulator to which the duo binary data application method is applied, it is possible to achieve stabilization of output power and transmission signal form of the optical modulator.

Furthermore, a characteristic advantage of the present invention is to input an arbitrary sine wave to the optical modulator at the same time for the DC bias voltage optimization, and output the peak-peak voltage as well as the phase change information of the detected sine wave output from the optical modulator. The information was used at the same time. This compensates for the disadvantage that the DC bias voltage cannot be optimally maintained with phase information only at the point where the phase of the detected sine wave is changed at or near the point where the BER performance error occurs. We also confirmed that the point where the detected sine wave is the smallest in the range without the BER performance error is not in the middle of the range without the performance error. Therefore, the present invention can create the effect of tracking and controlling a more optimal point of the optimum range of the DC bias voltage.

In addition, the present invention has the effect that can be easily and simply control the DC bias voltage in a non-complicated design using the characteristics of the modulated signal.

The foregoing and further aspects of the present invention will become more apparent through the preferred embodiments described with reference to the accompanying drawings. Hereinafter, the present invention will be described in detail to enable those skilled in the art to easily understand and reproduce the present invention.

2 is a block diagram of an optical transmission device according to the present invention.

As shown, the optical transmission device includes an optical modulator 100, a sine wave generator 200, and a bias voltage corrector 300. The optical modulator 100 is an external modulator of a duo binary data modulation method, and is preferably a Mach-Zender type modulator. As shown, the optical modulator 100 includes a continuous wave (CW) light source in, a precoded 10Gb / s electrical data in, modulated 10Gb / s Duo binary optical signal output terminal, and an output terminal (Photo Diode (PD) output port) for converting the output part of the optical output branched inside the optical modulator 100 converted to an electrical signal and output.

The sine wave generating unit 200 generates a sine wave, and preferably generates a sine wave of several kHz. The sine wave generator 200 according to the present invention generates a sine wave and outputs the sine wave to be mixed with the DC bias voltage generated by the DC bias voltage generation circuit. Accordingly, the sine wave is added to the DC bias voltage and input to the DC bias input port of the optical modulator 100. The bias voltage corrector 300 analyzes the electrical signal output through the embedded PD output port of the optical modulator 100. The DC bias voltage input to the optical modulator 100 is adjusted to optimize the result of the analysis.

According to a characteristic aspect of the present invention, the bias voltage corrector 300 includes an amplifier 500, a filter 600, a detector 710, a correction controller 720, and a DC bias adjuster 400. . The amplifier 500 amplifies the electrical signal output through the embedded PD output port of the optical modulator 100. The filtering unit 600 is a band pass filter and outputs only frequency components of the signal generated by the sine wave generator 200. The detector 710 receives a sine wave output from the filtering unit 600 and detects necessary information. In one embodiment, the detector 710 includes both the voltage detector 715 and the phase detector 713, or has only one. The phase detector 713 receives a sine wave output from the filtering unit 600 and a part of the sine wave output from the sine wave generator 200, and provides phase information between the two sine wave signals. For example, the phase difference is detected. The voltage detector 715 receives a sine wave output from the filtering unit 600, and detects a peak-peak voltage of the input sine wave.

The correction controller 720 outputs a control signal for adjusting the DC bias voltage input to the optical modulator 100 to the DC bias controller 400 according to the value detected by the detector 710. In the present embodiment, the correction controller 720 has reference phase information and a reference peak-peak voltage value. It also has a DC bias adjustment that needs to be adjusted once. These reference values and adjustment values are preferably stored in the memory. In particular, the reference value should be a value for satisfying the optimization of the DC bias voltage input to the optical modulator 100, which will be described later.

The correction controller 720 compares the phase information detected by the phase detector 713 with the reference phase information and maintains the current DC bias voltage as it is. If it does not match, a control signal for raising or lowering the DC bias voltage by the DC bias adjustment value is output. Also, the correction controller 720 compares the peak-peak voltage detected by the voltage detector 715 with the reference peak-peak voltage, and maintains the DC bias voltage as it is. If it does not match, a control signal for raising or lowering the DC bias voltage by the DC bias adjustment value is output. Finally, the DC bias controller 400 increases or decreases the DC bias voltage by the adjustment value according to the control signal of the correction controller 720 so that the optimal DC bias voltage is stably input to the optical modulator 100.

Hereinafter, in order to satisfy the optimization of the DC bias voltage input to the optical modulator 100, how the reference phase information and the reference peak-peak voltage value are determined will be described with reference to FIGS. 3 to 8.

3 is a graph showing a change in the performance error rate (10 Gb / s duo-binary optical signal out) of the optical signal output from the optical modulator 100 according to the change of the DC bias voltage.

Reference numerals 301 and 302 denote changes in bit error rate (BER) of the output optical signal (10Gb / s duo-binary optical signal out) of the optical modulator 100 according to the voltage change of the DC bias voltage adjusting unit 400. The reference numerals 303 and 304 indicate the characteristics of the sine wave peak-peak voltage detected by the voltage detector 715 according to the voltage change of the DC bias voltage controller 400.

When the voltage of the DC bias voltage controller 400 is gradually adjusted from low to high, the BER of the output optical signal of the optical modulator 100 decreases rapidly as shown by reference numeral 301, and then rapidly increases as shown by reference numeral 302. . Points 'A' and 'E' are the points where performance errors occur. Points 'B' and 'D' are the points where there are no performance errors, but errors can occur instantaneously when the DC bias voltage changes, and point 'C' In the range where there is no performance error, even a slight change in the DC bias voltage does not cause a change in performance error at all. When the voltage of the DC bias voltage controller 400 is gradually adjusted from low to high, the peak-peak voltage gradually decreases as shown by reference numeral 303 and then gradually increases as shown by reference numeral 404.

4 to 8 illustrate the output sine wave of the filtering unit 600 compared to the output sine wave of the sine wave generator 200 of FIG. 1 at the points where the DC bias voltages of FIG. 3 are A, B, C, D, and E, respectively. It is a waveform diagram. The upper waveform diagram shows the output sine wave of the sine wave generator 200, and the lower waveform diagram shows the output sine wave of the filtering unit 600. Here, the output sine wave of the sine wave generator 200 of FIG. 1 is not affected by the DC bias voltage changed by the DC bias control unit 400, and thus is always constant as shown.

In the DC bias voltage in the range of B to D where the performance error of the 10Gb / s duo binary optical signal output from the optical modulator 100 does not appear, the peak-peak voltage gradually changes as shown in FIGS. 5 to 6 and 7. 8, it can be seen that the change of the peak-peak voltage gradually appears. However, when the DC bias voltage is changed from the point B to the point A where the optical signal does not show an error in performance, the peak-peak voltage rapidly changes as shown in FIG. 5 to FIG. 4. In FIG. 4, when the output sine wave of the sine wave generator 200 and the output sine wave of the filtering unit 600 are compared, it can be seen that the phase has changed drastically.

With the results obtained from FIGS. 3 to 8, the control unit 700 stores the phase information of the sine wave detected in the optimal BER in the memory as reference phase information, and the peak-peak of the sine wave detected in the optimal BER. The voltage is stored in memory as the reference peak-peak voltage value. In addition, the voltage control value of one step that is adjusted step by step by the DC bias control unit 400 is stored in the memory. In addition, the DC bias voltage value at the optimal BER is stored in the memory as the initial DC bias voltage value.

9 is a flowchart for optimizing a DC bias voltage input to an optical modulator to which a duo binary data modulation scheme is applied according to the present invention.

When the optical transmission device according to the present invention is started, the control unit 700 controls the DC bias control unit 400 to input the initial DC bias voltage value stored in the memory to the optical modulator 100. Thereafter, the control unit 700 receives the sine wave output from the sine wave generator 200 and the sine wave output from the light modulator 100 and extracted by the filtering unit 600. Then, phase information, for example, a phase difference between two input sine waves is detected (step S910). In addition, the control unit 700 detects the peak-peak voltage of the sine wave output from the filtering unit 600 (step S920). The controller 700 compares whether the phase information detected in step S910 and the reference phase information stored in the memory match (step S930). If not, the control unit 700 instructs the DC bias control unit 400 to add the current DC bias voltage by the adjustment value stored in the memory (step S940). The DC bias control unit 400 increases the current DC bias voltage by an adjustment value. Steps S930 and S940 are repeated until the detected phase information coincides with the reference phase information.

If the detected phase information coincides with the reference phase information, the controller 700 compares whether the peak-peak voltage detected in step S920 and the peak-peak voltage stored in the memory match (step S950). If the detected peak-peak voltage is smaller than the reference peak-peak voltage, the control unit 700 instructs the DC bias control unit 400 to add the current DC bias voltage by an adjustment value (step S960). On the contrary, if the detected peak-peak voltage is greater than the reference peak-peak voltage, the control unit 700 instructs the DC bias control unit 400 to reduce the current DC bias voltage by an adjustment value (step S970). The DC bias control unit 400 increases or decreases the current DC bias voltage by an adjustment value. Steps S950, S960, and S970 are repeated until the detected phase information coincides with the reference phase information.

Meanwhile, in the above-described example, the controller 700 detects both the phase and the peak-peak voltage of the sine wave. However, only one of them may be detected and the DC bias voltage may be adjusted according to the detection result.

So far I looked at the center of the preferred embodiment for the present invention. Those skilled in the art will appreciate that the present invention can be implemented in a modified form without departing from the essential features of the present invention. Therefore, the disclosed embodiments should be considered in descriptive sense only and not for purposes of limitation. The scope of the present invention is shown in the claims rather than the foregoing description, and all differences within the scope will be construed as being included in the present invention.

1 is an exemplary diagram illustrating a change characteristic of an eye of an optical signal according to a change of a DC bias voltage in a duo binary data modulation scheme.

2 is a block diagram of an optical transmission device according to the present invention.

3 is a graph showing a change in the performance error rate of the optical signal output from the optical modulator according to the change of the DC bias voltage.

4 is an output sine wave waveform diagram of an output sine wave versus a sine wave generator of FIG. 1 at a point where the DC bias voltage of FIG. 3 is A;

5 is an output sine wave waveform diagram of an output sine wave versus a sine wave generator of FIG. 1 at a point where the DC bias voltage of FIG. 3 is B; FIG.

6 is an output sine wave waveform diagram of an output sine wave versus a sine wave generator of FIG. 1 at a point where the DC bias voltage of FIG. 3 is C. FIG.

7 is an output sine wave waveform diagram of an output sine wave versus a sine wave generator of FIG. 1 at a point where the DC bias voltage of FIG. 3 is D; FIG.

8 is an output sine wave waveform diagram of an output sine wave versus a sine wave generator of FIG. 1 at a point where the DC bias voltage of FIG. 3 is E; FIG.

9 is a flowchart for optimizing a DC bias voltage input to an optical modulator to which a duo binary data modulation scheme is applied in accordance with the present invention.

<Explanation of symbols for the main parts of the drawings>

100: optical modulator 200: sine wave generator

300: DC bias control unit 400: amplification unit

500: filtering unit 600: control unit

610: detector 613: phase detector

615: voltage detector 620: correction controller

Claims (9)

delete delete Duo binary type optical modulator; A sine wave generator configured to generate a sine wave added to the DC bias voltage input to the optical modulator; And An amplifier for amplifying an electrical signal that is electrically converted and output after being modulated by the optical modulator, a filtering unit outputting only a frequency component of a signal generated by the sine wave generator from the amplified electrical signal, and outputting from the filtering unit A detector including a phase detector for detecting a phase between the generated sine wave and the sine wave signal generated by the sine wave generator, and a voltage detector for detecting the peak-peak voltage of the sine wave output from the filtering unit. The DC bias voltage is corrected and controlled if the phase information detected by the phase information and the predetermined reference phase information are not matched. If the phase information detected by the phase detector and the predetermined reference phase information match, the voltage detection unit detects the phase information. Peak-to-peak voltage and preset reference peak-peak voltage match A bias voltage corrector including a correction controller for correcting and controlling the DC bias voltage if not identical, and a DC bias adjuster for adjusting the DC bias voltage according to a control command of the correction controller; Optical transmission device comprising a. delete The method of claim 3, The optical modulator is a Mach-Zender type optical modulator. delete delete A method for optimizing a DC bias voltage input to an optical modulator to which a duo binary data modulation scheme is applied, Detecting phase information between a sine wave added to a DC bias voltage input to the optical modulator and extracted from an electrical signal output from the light modulator and a sine wave added to the DC bias voltage and input to the optical modulator; Detecting a peak-peak voltage of a sine wave extracted from the electrical signal; Comparing the detected phase information with reference phase information and adjusting a DC bias voltage input to the optical modulator if they do not match; And Comparing the detected peak-peak voltage with the reference peak-peak voltage when the detected phase information and the reference phase information coincide, and adjusting the DC bias voltage input to the optical modulator when the detected phase information does not match; DC bias voltage optimization method comprising a. delete

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