KR100469740B1 - Duobinary optical transmitter - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duobinary optical transmission apparatus capable of generating a duobinary optical signal using a general phase modulator without using an electric low pass filter. A duobinary optical signal generator configured to input a signal and output a modulated optical signal obtained by modulating the optical carrier according to the NRZ signal; And an RZ signal generator for converting the NRZ signal into a Return to Zero (RZ) signal, and outputting a duobinary RZ signal, wherein the duobinary optical signal generator comprises: a precoder for encoding the NRZ signal; ; An amplifier for amplifying the encoded two-level NRZ signal; A light source for outputting a carrier wave; A phase modulator configured to receive the amplified two-level NRZ signal and phase modulate the carrier; And a broadband pass filter configured to output a duobinary optical signal by limiting a bandwidth of the phase modulated optical signal.

Description

Duobinary optical transmission device {DUOBINARY OPTICAL TRANSMITTER}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device, and more particularly to a duobinary optical transmission device for transmitting a duobinary Return to Zero (RZ) signal.

Dense Wavelength Division Multiplexing (DWDM) optical transmission system improves transmission efficiency by transmitting optical signals composed of multiple channels having different wavelengths in one optical fiber, regardless of transmission speed. Since the signal can be transmitted, it is a system that is useful for the high-speed Internet network which is increasing in recent years. Currently, a system that transmits more than 100 channels using a single optical fiber using DWDM has been commercialized, and research on a system having a transmission speed of 10Tbps or more by simultaneously transmitting 200 or more 40Gb / s channels in one optical fiber It's going on.

However, due to the rapid increase in data traffic and the demand for high-speed data transmission of 40 Gbps or more, there is a limit in expansion of transmission capacity due to rapid inter-channel interference and distortion at 50 GHz channel interval or less when the light intensity is modulated using the existing NRZ. The DC frequency component of the 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 of 10 Gbps or more.

Recently, optical duobinary technology has attracted attention as an optical transmission technology that can overcome the transmission distance limitation due to chromatic dispersion. The main advantage of duobinary 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 to 1/2, the transmission distance is quadrupled. In addition, the duobinary RZ transmission technique, which has excellent nonlinear components and dispersion characteristics, has been actively studied in high speed, medium and long distance transmission of 10 Gbps or more than binary NRZ transmission signals.

1 shows an example of a configuration of a conventional duobinary RZ optical transmission device.

As shown in FIG. 1, the duobinary RZ optical transmission device generates a duobinary optical signal generator 10 that receives a carrier wave and an NRZ electrical signal and outputs a duobinary optical signal, and generates an RZ signal that outputs an RZ pulse signal. It consists of a part 20.

The duobinary optical signal generator 10 amplifies a pre-coder 11 for receiving and encoding an NRZ type electrical signal and a 2-level signal output from the precoder 11. A low pass filter 13 for converting the amplified 2-level signal into a 3-level signal and reducing the bandwidth of the signal, a laser light source 14 for outputting a carrier wave, And a Mach-Zehnder interferometer type optical intensity modulator 15 for receiving the amplified three-level signal and converting the carrier to a two-level optical signal.

The duobinary optical signal generator 10 may be classified into two types, a Z-cut structure and an X-cut structure, according to the electrode structure of the interferometer-type Mach-gender optical modulator. In the case of an X-cut Mach-gender optical modulator having a single electrode, as shown in FIG. 1, one arm includes a modulator drive amplifier and an electrical low pass filter. Allows the application of a 3-level signal to the electrode. Although not shown, the Z-cut Mach-Gender optical modulator with dual electrodes is provided with a driving amplifier and an electrical low pass filter on each arm, respectively. Allows the application of a level electrical signal.

The RZ signal generator 20 is a chirp-free Mach-gender optical modulator 21 and a clock signal generator 22 for generating a clock signal having a period of a transmission bit rate (T) of NRZ data. It is composed of

Operation of the duobinary RZ optical transmission device having the above configuration is as follows.

The electrical pulse signal of the NRZ method is encoded into a binary signal having information of 0 or 1 by the precoder 11, and the encoded two-level binary signal is amplified by the driving amplifier 12. The amplified two-level binary signal is input to the low pass filter 13, and the low pass filter 13 has a bandwidth corresponding to about one quarter of the clock frequency of the two-level binary signal. This excessive bandwidth limitation results in inter-code interference, and the inter-code interference converts the two-level binary signal into a three-level duo-binary signal. The converted three-level duobinary signal is applied to the modulation terminal of the Mach-gender optical modulator 15, modulates the carrier wave output from the laser light source 14, and outputs it as a two-level optical duobinary signal.

The duobinary optical signal output from the duobinary optical signal generator 10 is input to the chirp-free Mach-gender optical modulator 21 located in the RZ signal generator 20. In general, in order to convert an NRZ signal into an RZ signal, a clock having a transmission bit rate period is applied to a modulation terminal of an optical modulator, and the chirp pre-mach-gender optical modulator 21 is inputted in synchronization with the clock signal. Convert the signal to an RZ signal.

2A and 2B show the output and light spectral characteristics of a conventional duobinary optical transmission device. FIG. 2A is a graph of time (ps) versus optical power (mW), and FIG. 2B is of wavelength / frequency (GHz) versus power (dBm). will be.

However, the above-described duobinary optical transmission device generates a three-level electric signal by using an electric low pass filter when converting a two-level binary signal into a duo binary signal. Depending on the transmission quality and the length of the pseudo random bit sequence (PRBS), the difference in characteristics may cause a fatal problem in the system. This occurs equally in conventional structures with Z-cut or X-cut structures, and the dependence of this signal pattern makes the margin of the system margin in actual optical transmission.

In addition, the Bessel-Thomoson type low pass filter, which is mainly used as the low pass filter, has a high price, and thus, the price competitiveness of the entire optical transmission device is weakened.

Accordingly, an object of the present invention devised to solve the above problems of the prior art overcomes the dependence of the transmission quality and the bit pattern on the filter transmission characteristics of the duobinary optical transmitter using the low pass filter, The present invention provides a duobinary optical transmission device that greatly improves nonlinearity and dispersion characteristics in high-speed, long-range, WDM transmissions of 10Gbps or more.

In order to achieve the above object, the present invention generates a duobinary optical signal for inputting a light source for outputting a carrier, a non-return to zero (NRZ) signal, and outputting a modulated optical signal for modulating the optical carrier according to the NRZ signal part; And an RZ signal generator for converting the NRZ signal into a Return to Zero (RZ) signal, and outputting a duobinary RZ signal, wherein the duobinary optical signal generator comprises: a precoder for encoding the NRZ signal; ; An amplifier for amplifying the encoded two-level NRZ signal; A light source for outputting a carrier wave; A phase modulator configured to receive the amplified two-level NRZ signal and phase modulate the carrier; And a broadband pass filter configured to output a duobinary optical signal by limiting a bandwidth of the phase modulated optical signal.

Preferably, the RZ signal generation unit includes a clock signal generator for generating a clock signal of a predetermined period, and a Mach-Zender type for converting and outputting an NRZ signal input in synchronization with the clock signal to an RZ signal. And an optical modulator.

Preferably, the bandwidth of the wideband pass filter is characterized in that 0.7 / T of the transmission bit rate of the NRZ data.

More preferably, the modulation is performed by applying a clock signal at a quad point between a maximum point and a minimum point of a transfer curve of the Mach-Zender type optical modulator. .

1 is a view showing an example of a configuration of a conventional duobinary optical transmission device

2a and 2b is a view showing the output characteristics of a conventional duobinary optical transmission device,

3 is a diagram showing the configuration of a duobinary optical transmission device according to an embodiment of the present invention;

4A to 4E are views illustrating a process of converting a duobinary RZ output signal according to an embodiment of the present invention;

5A and 5B show output characteristics of a duobinary optical transmission device using the optical phase modulator of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the same components in the drawings are represented by the same reference numerals and symbols as much as possible even though they are shown in different drawings. In addition, in describing the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

3 is a diagram illustrating a configuration of a duobinary optical transmission device according to an embodiment of the present invention. The duobinary RZ optical transmitter of the present invention includes a light source 50 for generating and outputting a carrier wave, a duobinary signal generator 100, and an RZ signal generator 200. In the present embodiment, a structure in which the duo binary signal generator 100 is located at the front end and the RZ signal generator 200 is located at the rear end will be described. However, the positions of the signal generators may be interchanged.

The light source 50 generates and outputs a carrier wave capable of carrying information, and may use a laser diode.

The duo binary signal generator 100 converts the input NRZ electrical signal into a duo binary optical signal and outputs the differential binary encoder 110, the modulator driving amplifier 120, the phase modulator 130, and the broadband pass filter 140. It is configured to include).

The differential encoder 110 encodes an input NRZ electrical signal, and encodes a two-level binary signal to enable duo-binary transmission and reception without changing a receiver.

The modulator driving amplifier 120 amplifies and outputs the encoded binary signal, and the amplified signal is used as a driving signal of the phase modulator 130.

The phase modulator 130 phase modulates and outputs the carrier wave according to a two-level binary signal input through a modulation terminal RF, and adjusts a modulation index indicating a degree of modulation to adjust the degree of phase modulation. I can regulate it.

The wideband pass filter 140 has a bandwidth of 0.7 / T of the transmission bit rate of NRZ data and limits the bandwidth of the phase modulated duobinary optical signal by removing a signal that is out of the bandwidth.

The RZ pulse generator 200 converts an NRZ modulated signal to RZ, and transmits a chirp free Mach-Zender type optical modulator 210 and NRZ data. And a clock signal generator 220 for generating a clock signal having a period of bit rate (T).

4A to 4E illustrate a process of converting a duobinary RZ output signal according to an embodiment of the present invention, and through this, the operation of the duobinary optical transmission device of the present invention will be described.

3 and 4A, the NRZ type electrical signal is input to the differential encoder 110 of the duobinary optical signal generator 100 and encoded into a binary signal having 0 or 1 information. The encoded NRZ signal is amplified by the modulator drive amplifier 120 to an appropriate size necessary for driving the modulator, and then applied to the general phase modulator 130 to modulate the phase of the optical carrier. 4A is a diagram showing a result of phase modulation in which bits of 0 or 1 are optically output in the same size without intensity modulation, while bits of 0 or 1 have a phase difference of 0 or π in an electric field during modulation. It can be seen that it is converted to phase information. The phase modulated optical signal is input to the broadband pass filter 140 having a bandwidth of 0.7 / T of the transmission bit rate of NRZ data, and due to the excessive limitation of the bandwidth, the input binary signal is converted into a duo binary signal. do. The process at this time is a process of performing the same function as the step of passing through the electrical low pass filter of FIG. FIG. 4B shows a process of filtering the phase modulated optical signal using the wideband pass filter 104, and FIG. 4C shows the result after filtering. By leaving only the portion indicated by the dotted line in FIG. 4B and filtering out the rest, the duobinary signal as shown in the eye diagram of FIG. 4C may be obtained. In the present embodiment, the duobinary optical signal is generated using a wideband pass filter having a bandwidth of 0.7 / bit rate, but the characteristics of the output duobinary optical signal can be adjusted by adjusting the bandwidth of the wideband pass filter.

Referring back to FIG. 3, the optical signal converted into the duobinary signal by the duobinary optical signal generator 100 is a chirp-free Mach-gender optical modulator located in the RZ signal generator 200 for conversion to the RZ signal. 210 is entered. In general, in order to convert an NRZ signal into an RZ signal, a clock having a period of Vπ amplitude and a bit rate is applied to a modulation terminal of an optical modulator. Convert a binary optical signal to an RZ signal. FIG. 4D illustrates a conversion process to an RZ signal. As shown in FIG. 4D, a bit rate at a quad point (Q), which is a midpoint between a maximum point and a minimum point of a transmission curve of the optical modulator 210, is shown. A clock signal having a period of T) is applied to generate an RZ signal. 4E is an eye diagram after the RZ conversion, and the conversion to the duobinary RZ signal can be confirmed.

5A and 5B show output characteristics of a duobinary optical transmission device using the optical phase modulator of the present invention. FIG. 5A is a graph of time (ps) versus optical power (mW), and FIG. 5B is of wavelength / frequency (GHz) vs. power (dBm).

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

As described above, the duobinary optical transmission device of the present invention can generate a duobinary optical signal using a general phase modulator without using an electric low pass filter. Therefore, it is possible to overcome the limitations of the transmission quality by the electric low pass filter and to improve the nonlinearity and dispersion characteristics in the high-speed, long-range WDM transmission.

Claims (4)

A duobinary optical signal generator for inputting a light source for outputting a carrier, a Non Return to Zero (NRZ) signal, and outputting a modulated optical signal obtained by modulating the optical carrier according to the NRZ signal; And an RZ signal generator for converting the NRZ signal into a Return to Zero (RZ) signal, and outputting a duobinary RZ signal, wherein the duobinary optical signal generator A precoder for encoding the NRZ signal; An amplifier for amplifying the encoded two-level NRZ signal; A light source for outputting a carrier wave; A phase modulator configured to receive the amplified two-level NRZ signal and phase modulate the carrier; A broadband pass filter for outputting a duobinary optical signal by limiting the bandwidth of the phase modulated optical signal Duobinary optical transmission device, characterized in that configured to include. The method of claim 1, wherein the RZ signal generating unit A clock signal generator for generating a clock signal of a predetermined period; And a Mach-Zender type optical modulator for converting an NRZ signal input in synchronization with the clock signal to an RZ signal and outputting the same. The bandwidth of claim 1, wherein the bandwidth of the broadband pass filter is The duobinary optical transmission device, characterized in that 0.7 / T of the transmission bit rate of the NRZ data. The method of claim 1, wherein a modulation is performed by applying a clock signal at a quad point between a maximum point and a minimum point of a transfer curve of the Mach-Zender type optical modulator. Duobinary optical transmission device.