KR101931957B1 - Optical transmission method and system using polarization-time coding for polarization diversity multiplexed optical transmission - Google Patents

Optical transmission method and system using polarization-time coding for polarization diversity multiplexed optical transmission Download PDF

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KR101931957B1
KR101931957B1 KR1020160182949A KR20160182949A KR101931957B1 KR 101931957 B1 KR101931957 B1 KR 101931957B1 KR 1020160182949 A KR1020160182949 A KR 1020160182949A KR 20160182949 A KR20160182949 A KR 20160182949A KR 101931957 B1 KR101931957 B1 KR 101931957B1
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polarization
symbol
optical transmission
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aptc
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KR20180078014A (en
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이준구
아메드 갈립 레자
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한국과학기술원
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2569—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to polarisation mode dispersion [PMD]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/29—Repeaters
    • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
    • H04B10/293—Signal power control
    • H04B10/294—Signal power control in a multiwavelength system, e.g. gain equalisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/614—Coherent receivers comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/616—Details of the electronic signal processing in coherent optical receivers
    • H04B10/6162—Compensation of polarization related effects, e.g., PMD, PDL
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/10—Polarisation diversity; Directional diversity

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Abstract

The present invention relates to a method and system for optical transmission using specimen optical coding for polarization multiplexed optical transmission, and a method for optical transmission using specimen optical coding for polarization multiplexed optical transmission according to the present invention includes a pair of orthogonally coded symbols (S 1 , S 2 ); Simultaneously transmitting each of the symbols S 1 and S 2 with a horizontal polarization and a vertical polarization in a first symbol interval; And symbols (" H " and " H ") in the second symbol interval

Figure 112016129332832-pat00047
, S 1 ) at the same time.

Figure R1020160182949

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an optical transmission method and a system using a specimen optical coding for polarization multiplexed optical transmission,

The present invention relates to a receiver model for an optical transmission system, and more particularly to a receiver model for an optical transmission system using intensity modulation / direct detection using an Alamouti space-time code modified in the form of polarization- (PMD) impairments by using a polarization diversity multiplex (PDM) gain in an (IM / DD) optical transmission system. And an optical transmission method and system using the same.

As the demand for fiber channel capacity increases to over 40 Gbps, the performance of today's high-speed fiber-based optical communication systems is expected to overcome capacity limitations due to polarization mode dispersion (PMD). On the other hand, polarization mode dispersion (PMD) gives the diversity that can enhance the channel capacity by using polarization multiplexing (PDM). The simple polarization multiplexing (PDM) technique multiplexes two optical data streams on two orthogonal polarization modes and demultiplexes them to orthogonally cross at the receiving end. However, this technique does not meet the main requirements of the receiver to accurately compensate for fast polarization rotations, which can lead to polarized crosstalk in optical fiber transmission systems.

Therefore, most of the direct detection systems use a large-capacity, high-cost LiNbO3 (niobate) polarization converter and a polarization beam splitter (PBS) attached thereto in order to dynamically adjust the state of polarization (SOP) , polarization beam splitter (PC) and polarization controller. This technique is described in B. Koch et al., &Quot; Versatile endless optical polarization controller / tracker / demultiplexer " (Opt. Express 22 (7), 8259-8276 (2014) compensator for 112 Gb / s direct-detect PDM RZ-DQPSK systems "(J. Lightwave Technol. 28 (22), 3282-3293 (2010)).

This commercial solution attempt has not been realized since seamless tracking of the polarization state (SOP) in long polarized mode dispersion (PMD) damaged fibers is nearly impossible. Optical coherent detection techniques can simultaneously perform polarization multiplexing (PDM) demultiplexing and polarization mode dispersion (PMD) penalty compensation using digital electrical processing. These techniques are described in Jeffrey T. Rahn et al., "Real-time PMD tolerance measurements of a PIC-based 500 Gb / s coherent optical modem", J. Lightwave Technol. 30 (17), 2907-2912 ≪ / RTI > However, optical coherent communication systems are very complex, costly and require accurate channel estimation to handle high capacity channels at data rates above 10 Gbps.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide a method and apparatus for reducing the requirements of polarization controllers (PCs) and channel estimators using a modified Alamouti code And a method and system for optical transmission using specimen optical coding for polarization multiplexed optical transmission.

According to an aspect of the present invention, there is provided an optical transmission method using a sample optical coding for polarization multiplexed optical transmission according to an aspect of the present invention, including the steps of: inputting a pair of symbols S 1 , S 2 ); Simultaneously transmitting each of the symbols S 1 and S 2 with a horizontal polarization and a vertical polarization in a first symbol interval; And symbols (" H " and " H ") in the second symbol interval

Figure 112016129332832-pat00001
, S 1 ) at the same time.

The optical transmission method decodes a code according to Alamouti-type polarization-time coding (APTC) using horizontal and vertical polarization currents of each symbol interval without estimating channel state information (CSI) And estimates the received symbol.

The input signal is a non-return-to-zero ON-OFF keying (NRZ-OOK) signal.

The optical transmission method includes: demultiplexing polarization multiplexed symbols received through an optical transmission path into two orthogonal polarized signals; Photoelectrically converting the orthogonal polarized signals demultiplexed using photodetectors; Low-pass-filtering the photoelectrically converted signals; And estimating a symbol of each of the two symbol periods corresponding to a sum of horizontal and vertical polarization currents of the photodetectors in each of the two symbol periods from the low-pass filtered signals.

Alternatively, the optical transmission method may include: photoelectrically converting the polarization multiplexed symbols received through the optical transmission path using one photodetector; Low-pass-filtering the photoelectrically converted signal; And estimating a symbol of each of the two symbol periods corresponding to a summation value of horizontal and vertical polarization currents output by the one photodetector in each of the two symbol periods from the low-pass filtered signal.

The estimated symbol

Figure 112016129332832-pat00002
Is an equalized symbol.

In estimating the symbol, each symbol is estimated using a one-bit delay-and-add and a one-bit delay-and-subtract filter .

According to another aspect of the present invention, there is provided an optical transmission system using specimen optical coding for polarization multiplexed optical transmission, comprising: an encoder for generating a pair of symbols (S 1 , S 2 ) orthogonally coded from an input signal; A laser diode for generating a laser beam; A polarizing beam splitter for generating a beam separated into horizontal and vertical polarized lights from the laser beam; Two modulators for modulating each symbol using the beams of the horizontally polarized light and the vertically polarized light; (S 1 , S 2 ) modulated by the modulators in a first symbol interval and transmits the combined symbols S 1 , S 2 of the horizontally polarized light and the vertical polarized light modulated by the modulators in a first symbol interval, And vertically polarized symbols (

Figure 112016129332832-pat00003
, S 1 ).

The optical transmission system decodes a code according to Alamouti-type polarization-time coding (APTC) using horizontal and vertical polarization currents of each symbol interval without estimating channel state information (CSI) at the receiving side And estimates the received symbol.

Each of the modulators is a Mach-Zehnder modulator.

The input signal is a non-return-to-zero ON-OFF keying (NRZ-OOK) signal.

The optical transmission system includes: a polarization beam splitter for demultiplexing the polarization multiplexed symbols received through an optical transmission path into two orthogonal polarization signals; Two photodetectors each photoelectrically converting the demultiplexed orthogonal polarized signals; Two low-pass filters for respectively low-pass-filtering the photoelectric-converted signals; And a decoder for estimating a symbol of each of the two symbol periods corresponding to the sum of the horizontal and vertical polarization currents of the photodetectors in each of the two symbol periods from the low-pass filtered signals.

Alternatively, the optical transmission system may include: one optical detector for photoelectrically converting the polarization multiplexed symbols received through the optical transmission path; A low-pass filter for low-pass-filtering the photoelectrically converted signal; And a decoder for estimating a symbol of each of the two symbol periods corresponding to the sum of the horizontal and vertical polarization currents output by the one photodetector in each of the two symbol periods from the low-pass filtered signal have.

The estimated symbol

Figure 112016129332832-pat00004
Is an equalized symbol.

The decoder estimates each symbol using one-bit delay-and-add and one-bit delay-and-subtract filters.

According to the optical transmission method and system using the specimen optical coding for the polarization multiplexing optical transmission according to the present invention, it is possible to provide an IM / DD (Optical Phase Locked Loop) using Alamouti type specimen optical coding (APTC), which is difficult due to polarization crosstalk and polarization mode dispersion The polarization diversity gain can be used in the environment. Because of the unified nature of the polarization mode dispersion (PMD) channel, the present invention can mitigate polarized crosstalk without a dynamic polarization controller (PC) on the receiver side.

In addition, the decoding complexity in the present invention is significantly lower than other specimen optical techniques. This is because the optical channel estimator can be eliminated by using the optical channel model as a 2x2 MIMO model in the polarization mode dispersion (PMD) channel. The APTC-IM / DD method of the present invention can achieve a gain of 3-dB power reduction in a short / long range (<800 km) 40 Gbps optical fiber transmission system with a DGD of less than 6 ps, The PMD tolerance can be significantly improved.

Figure 1 is a series of various birefringent segments having random rotations of birefringence axes to illustrate the physical model of polarization multiplexing (PDM) for long optical fibers, where light propagates from left to right.
2A is a block diagram illustrating an Alamouti type specimen optical coding (APTC) method for polarization mode dispersion (PMD) compensation in an IM / DD optical transmission system of the present invention.
2B is a block diagram illustrating an IM / DD optical transmission system according to another embodiment of the present invention.
2C is a flowchart illustrating an operation of the APTC IM / DD optical transmission system of the present invention.
3A is a diagram for explaining a general IM / DD system (IM / DD).
3B is a view for explaining an IM / DD system (PC-IM / DD) having a polarization beam splitter (PBS) behind an optimally adjusted polarization controller (PC).
Figure 4 is a graph comparing BER results for OSNR in a 40 Gbps fiber optic transmission system with differential group delay (DGD) of 8.5 ps.
5 is a graph showing the results of first-order PMD tolerance on differential group delay (DGD) after transmission over various length PMD-damaged SMF paths in the range of 0 to 7200 km.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same components are denoted by the same reference symbols as possible. In addition, detailed descriptions of known functions and / or configurations are omitted. The following description will focus on the parts necessary for understanding the operation according to various embodiments, and a description of elements that may obscure the gist of the description will be omitted. Also, some of the elements of the drawings may be exaggerated, omitted, or schematically illustrated. The size of each component does not entirely reflect the actual size, and therefore the contents described herein are not limited by the relative sizes or spacings of the components drawn in the respective drawings.

First we will briefly describe Alamouti coding. Alamouti coding is described by S. M. Alamouti in the journal IEEE J. Select. Areas Commun. 16 (8), 1451-1458 (1998). &Quot; A simple transmitter diversity scheme for wireless communications " Alamouti coding does not need to know about the channel conditions at the transmitter and is designed for applications in the wireless domain to exploit spatial diversity capacity gains with the use of space-time coding.

The original Alamouti coding can be applied directly to the optical coherent system which exhibits the main results in mitigating polarization mode dispersion (PMD). Expression 16 (18), 14163-14172 (2008)) by E. Ivan B. Djordjevic et al., &Quot; Alamouti-type polarization-time coding in coded-modulation schemes with coherent detection & In this paper, we propose a new method for complex coherent detections (PDL), which is related to the polarization dependent loss (PDL) and the cost of accurate channel estimation. Express 21 (19), 22773-22790 (2013)).

In general Alamouti (Alamouti) coding principle, since the transmission to detect the negative (negatives) and a pair (conjugates) of the signal, intensity modulation (IM / DD) (intensity modulation ) / direct detection (direct detection) optical transmission system, It can not be implemented directly. Nevertheless, the IM / DD system only handles non-negative real-valued signals in the time domain. Modified Alamouti coding techniques are described by Simon and Vilnrotter in "Alamouti-type space-time coding for free-space optical communication with direct detection" (IEEE Trans. Wireless Commun. 4 (1), 35-39 ), Which can utilize channel capacity from space-time diversity in a free space IM / DD optical system. Free space channel fading is very similar to that observed in wireless channels. The same technique can be applied with polarization-time diversity Alamouti coding as presented here. However, implementing polarization multiplexed (PDM) transmissions in an IM / DD optical system without a polarization controller or optical channel estimator requires a differential group delay (DGD, differential group delay) is extremely difficult.

The Alamouti-type polarization-time coding (APTC) -IM / DD technique according to the present invention can be implemented in a polarization multiplexed (PDM) capable IM / DD transmission system in which Alamouti type coding can be embodied It is proposed based on the theoretical modeling to show the method. Here, by using the optical channel model as a 2x2 MIMO system, polarization multiplexing (PDM) and polarization crosstalk can be alleviated in common.

In optical fibers, polarization diversity is used to obtain APTC throughput enhancement. Hereinafter, the polarization diversity channel model will be described first and a method of applying the model to the APTC-IM / DD system will be described.

&Lt; Optical fiber model with polarization diversity >

Optical fiber channel models with polarization diversity include for coupling in fibers with random polarization state (SOP) rotation and weak birefringence. Here, the optical fiber channel represents the random direction of the polarization state (SOP) and represents the differential group delay (DGD) between the two orthogonal polarization modes. This property is referred to as a polarization mode dispersion (PMD) channel model. In short single mode fiber (SMF), differential group delay (DGD) is non-uniform and additive since there is no polarization state (SOP) random coupling. However, in typical communication systems, such as access networks, terrestrial networks, submarine cable systems, etc., the fiber lengths range from tens to hundreds of kilometers.

Figure 1 is a series of various birefringent segments having random rotations of birefringence axes to illustrate the physical model of polarization multiplexing (PDM) for long optical fibers, where light propagates from left to right.

Polarization mode dispersion (PMD) of long-haul fibers is often seen as a sequence of multiple birefringent segments. The birefringent axes and sizes of these segments change randomly, which results in random coupling between the fast axis and the slow axis in the fiber as in Fig. Thus, the differential group delay (DGD) of long-haul fibers does not increase linearly along the fiber length. Instead, the increase in differential group delay (DGD) in the fiber can be studied as a three-dimensional random walk and the average differential group delay (DGD) is approximated as the square root of the propagation distance . The polarization mode dispersion (PMD) of the long-distance fiber can be expressed by the following equation (1): &quot; (1) &quot; The channel response function H (?) Can be modeled using a Jones matrix.

[Equation 1]

Figure 112016129332832-pat00005

In Equation (1), N is the number of fiber segments (natural number) and? Is the optical frequency (rad / sec). τ n is the differential group delay (DGD) of the birefringent segment, θ n is the angle between the principle axes of the n-th and n-1 th segments and is randomly uniformly distributed between [0, π] This model characterizes the polarization mode dispersion (PMD) damage of all orders including the differential phase delay (DPD) and the differential group delay (DGD) of the 0th and first orders respectively.

The output signal S out (?) In the fiber can be expressed as in Equation (2) in the frequency domain. here,

Figure 112016129332832-pat00006
Is a unit vector,
Figure 112016129332832-pat00007
silver
Figure 112016129332832-pat00008
Is a polarization vector in the frequency domain of the scalar input signal S (?) In either the horizontal or vertical axis represented by?

&Quot; (2) &quot;

Figure 112016129332832-pat00009

<APTC-IM / DD System Model>

2A is a block diagram illustrating an Alamouti type specimen optical coding (APTC) method for polarization mode dispersion (PMD) compensation in an IM / DD optical transmission system 500 of the present invention. 2B is a block diagram illustrating an IM / DD optical transmission system according to another embodiment of the present invention. Figure 2a, in Figure 2b, the signal waveforms are each the transmit symbol in a time slot t 1, t 2. 2C is a flowchart illustrating an operation of the APTC IM / DD optical transmission system 500 of the present invention.

2A, an IM / DD optical transmission system 500 for performing square-law direct detection according to an embodiment of the present invention includes a transmitter 300, a polarization mode dispersion (PMD) A single mode fiber (SMF) -28 fiber transmission system 190, and a receiver 400, which are shown in FIG.

The transmitter 300 includes a non-return-to-zero ON-OFF keying transmitter 110, an APTC encoder 120, a laser diode (LD) 130, a polarization beam splitter 140, a Mach-Zehnder modulator 150, a polarization beam combiner 170, an erbium-doped fiber (EDFA) amplifier 180. [0029]

 The optical fiber transmission system 190 is a single mode fiber (SMF) in which the EDFA 192 under the control of a dispersion compensation module (DCM) 191 for every 8 km fiber is n 10), and it is assumed that there is an 80km fiber.

2A, the receiver 400 includes an optical bandpass filter (OBPF) 200, a polarization beam splitter (PBS) 210, a first and a second photo- (PD) photodetectors 220 and 230, first and second low pass filters 240 and 250, an APTC decoder 260, and a decision unit 270.

As shown in FIG. 2B, the polarization beam splitter 210, the second photodetector 230, and the second low-frequency filter 250 in the receiver structure of FIG. 2A may be omitted.

It is assumed that the chromatic dispersion damage of the optical fiber transmission system is compensated by dispersion compensators. In the APTC-IM / DD system 500 of the present invention, polarization mode dispersion (PMD) and polarized crosstalk are relieved only through coding and the polarization controller (PC) is not used.

<Technology of Transmitter 300>

The NRZ-OOK transmitter 110 generates an NRZ-OOK signal from the input data sequence and transmits the NRZ-OOK signal to the APTC encoder 120 at step S210. The APTC encoder 120 converts the two symbols s 1 , s 2 ( T 1 , t 2 ) as a code block for encoding, and each code in each time slot combines a pair of bits to be transmitted via two orthogonal polarizations (S220).

Let s = [s 1 , s 2 , ..., s N ] be the vector of N transmitted symbols and s i ∈ {0,1} for NRZ-OOK, where i = 1,2, ..., , And N. A pair of symbols (s x (t), s y (t)) orthogonally coded to the code block by the APTC encoder 120 in the APTC-IM / DD technique. For the first symbol of the code block slot t 1, the horizontal (x) - / vertical (y) - a code sent by using the polarization

Figure 112016129332832-pat00010
(S230). In the second symbol slot t 2 after the symbol period T, the transmitted code is
Figure 112016129332832-pat00011
, Where
Figure 112016129332832-pat00012
Denotes a complement of the symbol s 2 ∈ {0, 1} (S240). The superscript T is a matrix transpose operation. Where the channel data rate is 1 bit / time slot. These two orthogonal codes are modulated by an LD 130, a polarization beam splitter (PBS) 140 and a Mach-Zehnder modulator 150 (MZM) 150, respectively, and combined by a polarization beam combiner (PBC) And transmitted to the PMD-damaged fiber transmission system 190 through an erbium-doped fiber amplifier (EDFA) 180. That is, the symbol S 2 transmitted in the vertical polarization (y-polarization) during the previous symbol interval is complementarily (complementarily) transmitted (horizontally polarized) (x-polarized) to the next symbol interval
Figure 112016129332832-pat00013
), For the previous symbol interval, the horizontal polarized light (the symbols transmitted in x- polarized light) (S 1) is the next symbol interval, and transmitted as vertically polarized light (polarized y-) (S 1).

To this end, when the APTC encoder 120 generates a pair of symbols (S 1 , S 2 ) orthogonally coded from the input data stream, the laser beam generated by the LD 130 passes through the polarization beam splitter (PBS) 140 (X-polarized) and vertically polarized (y-polarized) by a polarizer (not shown).

A Mach-Zehnder modulator (MZM) 150, 160 modulates each of the symbols S 1 , S 2 using a beam of horizontally polarized light and vertically polarized light at a symbol slot t 1 and a polarized beam combiner (PBC) To transmit the respective symbols S 1 , S 2 to the optical fiber transmission system 190 at the same time. In addition, the Mach-Zehnder modulators (MZM) 150 and 160 generate respective symbols

Figure 112016129332832-pat00014
, S 1 ) in the symbol slot t 2 using beams of horizontal and vertical polarized light and combined by a polarization beam combiner (PBC) 170 to generate respective symbols
Figure 112016129332832-pat00015
, S 1 ) to the optical fiber transmission system 190 at the same time.

On the other hand, in the time domain, the received codes r (t) can be expressed by Equation (3). Here, * is a convolution operation, and s (t) is a transmitted signal vector of a code string including s (t 1 ) = s t1 , s (t 2 ) = s t2 . Also,

Figure 112016129332832-pat00016
Figure 112016129332832-pat00017
Represents the optical channel matrix in the time domain for the long optical fibers given in Equation (1).

&Quot; (3) &quot;

Figure 112016129332832-pat00018

Where h xx and h yy represent the complex channel gains of the signals received in the originally transmitted polarization and h xy and h yx represent the complex channel gains of the signals received in different polarization for the originally transmitted polarization. n ( t ) = [ n x ( t ) n y ( t )] T is an ASE (amplified spontaneous emission) noise vector in x- / y- polarized light. ASE is used for channel characterization.

<Technique of Receiver 400>

In the receiver 400, the optical bandpass filter (OBPF) 200 removes ASE noise outside the channel of the received signal. The noise-filtered polarization multiplexed (PDM) signals are demultiplexed into two orthogonal polarized signals by a polarization beam splitter (PBS) (S250). Unlike single-transmitter and single-receiver systems, this arrangement provides two copies of received signals (cpoy), each detected and processed. However, the optical carriers that are passed through the optical fiber and sent in orthogonal polarization states (SOP) do not maintain the input SOP due to the random direction of birefringence, and the respective outputs e x (t) of the polarization beam splitter (PBS) ), e y (t)) of the data streams. If the modulated symbol vector s is unchanged within one symbol period, it can be seen as a constant. Thus, at time t 1 , i.e.,

Figure 112016129332832-pat00019
The received signals at the two output ports of the polarization beam splitter (PBS) 210 can be expressed by Equation (4) and Equation (5).

&Quot; (4) &quot;

Figure 112016129332832-pat00020

&Quot; (5) &quot;

Figure 112016129332832-pat00021

Here, r x and r y are the received signal vectors in the x- and y-polarized light, respectively, and n x and n y represent the corresponding AES noise. Similarly, at time t 2 = t 1 + T,

Figure 112016129332832-pat00022
The received signals at the two output ports of the polarization beam splitter (PBS) 210 can be expressed as shown in Equation (6). Here, the two expressions are expressed by one, that is, x (y) indicates that x can be replaced with y, and the rest of parameters are the same.

 &Quot; (6) &quot;

Figure 112016129332832-pat00023

The demultiplexed received signals e x (t) and e y (t) are detected (S260) by two photodetectors (PD) 220 and 230 for photoelectric conversion, The converted signal is subjected to low-pass filtering in the low-pass filters (LPF) 240 and 250 (S270). If the responsivity of the photodetector (PD) 220 or 230 is represented by 1 / ?, the photocurrent of the photodetector PD 220 or 230

Figure 112016129332832-pat00024
Can be expressed as [Expression 7] and [Expression 8] at t 1 and t 2 , respectively.

 &Quot; (7) &quot;

Figure 112016129332832-pat00025

&Quot; (8) &quot;

Figure 112016129332832-pat00026

Here, the last three portions in Equations (7) and (8) represent detected electrical noise, and {.} Represents polarized crosstalk. Generally, in order to decode an Alamouti code, channel state information (CSI) including the phase and size information of a signal in a 2-input 1-output (2x1) channel receiver is required. However, the present invention shows that it is possible to decode the APTC-IM / DD code without using the CSI estimation even with the use of the 2-input 1-output (2x1) channel. That is, the receiver can estimate the received symbol by decoding the code according to APTC using the horizontal and vertical polarization currents of each symbol interval without estimating the channel state information. Further, in a general IM / DD transmission system, phase information is lost in the detection process due to the square-law nature of the photodetector. PDM transmission in an IM / DD transmission system is difficult due to the random coupling between two orthogonal polarizations due to the random direction of the polarization state (SOP), even though there is no differential group delay (DGD). Therefore, it is desirable to eliminate or reduce polarized crosstalk in order to reduce the demultiplex penalty.

At a negligible polarization dependent loss (PDL) level, a frequency dependent zone matrix (Jones matrix) is unified as shown in equation (1). therefore,

Figure 112016129332832-pat00027
You can write as follows,
Figure 112016129332832-pat00028
Is a Hermitian transpose. If the slope of the channel response function is small enough,
Figure 112016129332832-pat00029
, The time domain waveform can be approximated as shown in Equation (9).

&Quot; (9) &quot;

Figure 112016129332832-pat00030

Also,

Figure 112016129332832-pat00031
(10) can be obtained.

&Quot; (10) &quot;

Figure 112016129332832-pat00032

To alleviate both polarization crosstalk and PMD, the APTC method of the present invention uses two polarizations at the receiver 400 side.

Figure 112016129332832-pat00033
, Assuming a finite impulse response within the time delta,
Figure 112016129332832-pat00034
.

Now, the following simplified in accordance with the approximation of Equation 10] Chemistry [Equation 11] [Equation 12], and as shown in the [Equation 7], [Expression 8], a time t = t 1, t = t 2 , can be obtained by adding photocurrents at the x-, y- polarizations at time t = t 1 , t = t 2 .

&Quot; (11) &quot;

Figure 112016129332832-pat00035

&Quot; (12) &quot;

Figure 112016129332832-pat00036

Here, for simplification,? = 1,

Figure 112016129332832-pat00037
. α = β = 1, γ = 0. This eliminates the requirement that the power sum of the two orthogonal polarization components measure the channel matrix H.

The photocurrent-converted signals are subjected to low-pass filtering of low-pass filters (LPF) 240 and 250, and the photocurrents coupled in the APTC decoder 260 are decoded by the APTC decoder 260 (S280). The APTC decoder 260 performs a one-bit delay-and-add operation on the photocurrents summed and combined as shown in Equations (13) and (14) one-bit delay-and-subtract filter. At this time, when there is no polarization dependent loss (PDL)

Figure 112016129332832-pat00038
Equalized symbols using Equation (13) and Equation (14) can be expressed as Equation (14).

&Quot; (13) &quot;

Figure 112016129332832-pat00039

&Quot; (14) &quot;

Figure 112016129332832-pat00040

here,

Figure 112016129332832-pat00041
,
Figure 112016129332832-pat00042
Represents the symbols estimated by the APTC decoder 260 for the symbols s 1 and s 2 , respectively.

As shown in FIG. 2B, the polarization beam splitter 210, the second photodetector 230, and the second low-frequency filter 250 in the receiver structure of FIG. 2A may be omitted. At this time, if ASE noise is removed from the optical band pass filter (OBPF) 200 in the receiver 400, the received signal e (t) is detected by the photodetector (PD) 220 for photoelectric conversion, The photoelectrically converted signal i x (t) + i y (t) is subjected to low-pass filtering in a low-pass filter (LPF) Since the photoelectric converted signal i x (t) + i y (t) includes the photocurrents (the sum of the horizontal and vertical polarization currents) summed and combined as described above, the APTC decoder 260 From the low-pass filtered signal, the symbol of each of the two symbol periods corresponding to the sum of the horizontal and vertical polarization currents in each of the two symbol periods can be estimated. At this time, the APTC decoder 260 compares the 1-bit delay with the 1-bit delay for the photoelectrically converted signal i x (t) + i y (t) (sum of horizontal and vertical polarization currents) bit delay-and-add, and a one-bit delay-and-subtract filter scheme, the symbols of each symbol interval can be estimated.

The determination unit 270 determines a digital value by 1 bit for the symbols estimated by the APTC decoder 260 (S290). The noise samples n 1 , n 2 are uncorrelated and independent. The combined signals of (13) and (14) are similar to each other and are more simplified than in the conventional case due to the characteristics of the polarization mode dispersion (PMD) channel.

In order to verify the performance of the APTC-IM / DD method of the present invention in a PDM-capable IM / DD transmission situation with polarization mode dispersion (PMD) impairment, NRZ-OOK modulation with a wavelength of 1550 nm in the NRZ- A signal is generated. For example, the NRZ-OOK modulated signal may be word length 2 15 - 1 and may be generated at 10 Gbps, 40 Gbps, or the like using a pseudorandom binary sequence (PRBS) generator. In turn, the NRZ-OOK modulated signal is encoded in the APTC encoder 120 and transmitted to a Mach-Zehnder modulator (MZM) 150, 160, a polarized beam combiner (PBC) 170, an erbium-doped fiber amplifier (EDFA) (E.g., SMF-28 links) with polarization mode dispersion (PMD) damage, randomly coupled and of varying length, of the optical fiber transmission system 190 ranging from 0 to 7200 km. Although most applications will be implemented on short-haul optical paths, optical fiber lengths have been extended to explore the full range of differential-group delay (DGD) performance. At the same time, the 80-km single mode fiber (SMF) section can be modeled with random coupling of 10 8-km SMF sections. An EDFA 192 under the control of a dispersion compensating module (DCM) 191 is provided n times (for example, 10 times) to amplify and transmit the data every 8 km fiber. Here chromatic dispersion damage will be compensated and ignored. The resulting optical channel (eg, simulation) follows higher-order polarization mode dispersion (PMD).

(PC-IM) system with a polarization beam splitter (PBS) behind an optimally tuned polarization controller (PC), and the following three systems: a general IM / DD system / DD), and an Alamouti type specimen optical coding (APTC) based IM / DD system (APTC-IM / DD) of the present invention.

3A is a diagram for explaining a general IM / DD system (IM / DD).

3B is a view for explaining an IM / DD system (PC-IM / DD) having a polarization beam splitter (PBS) behind an optimally adjusted polarization controller (PC).

The general IM / DD system (IM / DD) of FIG. 3A and the PC-IM / DD system of FIG. 3B do not use two orthogonal polarized beams, and the APTC encoder 120, the polarization beam splitter PBS 140 and an APTC decoder 260 are not provided.

The PC-IM / DD system has a polarization beam splitter (PBS) behind a polarization controller (PC) optimally tuned between an optical bandpass filter (OBPF) on the receiver side and a photodetector (PD).

In the PC-IM / DD system of FIG. 3B, one polarized light (horizontal polarization) is detected to calculate a BER (Bit Error Rate). This assumes that the polarization controller PC is able to track the exact polarization state (SOP) of the received signal so that the signal components received at the other polarization axis (vertical axis) may increase to some extent, but to a negligible degree, the ASE noise power Because.

4 is a graph comparing BER results for optical signal-to-noise ratio (OSNR) in a 40 Gbps optical fiber transmission system with differential group delay (DGD) of 8.5 ps.

In FIG. 4, a signal is transmitted at a transmission rate of 40 Gbps on a PMD-damaged SMF path with a polarization mode dispersion (PMD) coefficient of 0.2 ps / sqrt (km), corresponding to an average differential group delay (DGD) of 8.5 ps. BER results for OSNR are shown. As shown in the drawing, for each of the general IM / DD system, the PC-IM / DD system, and the APTC-IM / DD system, in pre-FEC (pre-forward error correction) BER 10-3, the OSNR value is 14.6 dB, 13.72 dB, and 11.8 dB, respectively. It can be seen that the APTC-IM / DD system of the present invention has a remarkable performance improvement of 2.8 dB and 1.92 dB compared with the general IM / DD system, PC-IM / DD system and APTC-IM / DD system . Thus, it can be seen that the PC-free APTC method as in the present invention not only shows a strong recovery against PMD damage, but also reduces polarized crosstalk to a tolerable level.

5 is a graph showing the results of first-order PMD tolerance for differential group delay (DGD) after transmission over various length PMD-damaged SMF paths in the range of 0 to 7200 km. That is, the results of OSNR required to obtain 10 -3 pre-FEC BER for differential group delays (DGD) are shown. At 40 Gbps and 10 Gps, the results for a typical IM / DD system, a PC-IM / DD system, and an APTC-IM / DD system, respectively, are shown. Polarization mode dispersion (PMD) channels in simulation follow a higher-order PMD model. It is known that, in higher-order PMD channels, average differential group delay (DGD) and frequency dependence increase with increasing transmission distances in fixed PMD coefficients. In the simulation, the PMD coefficient of each SMF path is assumed to be 0.2 ps / sqrt (km).

At a transmission rate of 40 Gbps, the APTC-IM / DD method of the present invention gains about 3.0 dB over a general IM / DD system in the case of B2B (back-to-back) with DGD = This is because the two polarizations are used at the receiver 400 side in order to decode the code by the Alamouti type specimen optical coding (APTC) of the present invention.

In the case of the PC-IM / DD system, there is a gain of about 0.6 dB compared with the IM / DD system due to the noise canceling effect. In the 0 ps? DGD? 6 ps region where the DGD is low, the APTC-IM / DD system of the present invention has more tolerances than the general IM / DD system and the PC-IM / DD system. This is because the standard deviation of H x ( y ) x ( y ) (ω) is about 0.29 rad << π in a low DGD region with a transmission distance of 800 km. This shows the effectiveness of the approximation in (9). Simulation results show that the APTC-IM / DD method of the present invention can perform polarization demultiplexing without a PC. However, in the high DGD region (8 ps &lt; = DGD &lt; = 17 ps), the APTC-IM / DD system shows more dependence on DGD because this approximation begins to lose effectiveness in this region. The main cause is interchannel crosstalk that grows with increasing DGD. Generally, depending on the presence of DGD, the polarization state (SOP) of the polarization multiplexed signals in the middle of the symbol interval changes rapidly as opposed to unmultiplexed signals. For a zero demultiplexing penalty, the rising and falling edges of the multiplexed signal must be aligned in time. In the high DGD region, the polarization multiplexed signals are heavily dispersed over time, resulting in a high OSNR penalty.

Next, we see a case where the data transmission rate is reduced to 10 Gbps and the transmission is carried out from 0 to 7200 km. At this time, it can be grasped as a low DGD region in the case of the transmission path (for example, SMF-28 links). In FIG. 5, the graph for 10 Gbps is consistent with the tendency seen in the low DGD region for 40 Gbps, except that there is an improvement in the OSNR requirement of almost 6 dB at the same BER performance of 10 -3 . This is because the data transmission rate has been reduced to 1/4.

As described above, in the optical transmission system using the specimen optical coding for the polarization multiplexing optical transmission of the present invention, the modified Alamouti type specimen optical coding (APTC) of the specimen optical coding type is used to perform polarization multiplexing PDM) gain. This makes it possible to prevent polarization mode dispersion (PMD) damage. In the present invention, polarization multiplexing (PDM) transmission is achieved by combining two orthogonal coded symbols using a polarized beam combiner (PBC) 170 and simultaneously transmitting them in two orthogonal polarization modes in a given symbol interval . The symbols transmitted in the vertical polarization (y-polarized) during the previous symbol interval are compensated (complementarily) with the horizontal polarization (x-polarized) in the next symbol interval, ) Is transmitted in vertical polarization (y-polarization) in the next symbol interval.

On the receiver 400 side, the optical channel model is mapped to a 2 × 2 MIMO (Multiple Input Multiple Output) and is mapped to a 1-bit delay-and-add in the APTC decoder 260, It is possible to decode a signal simply transmitted by a one-bit delay-and-subtract filter. The technique of the present invention can achieve a gain of 3 dB in the low DGD range without a polarization controller (PC), a coherent receiver, a high-speed analog-to-digital converter (ADC), a digital signal processor (DSP) Can be obtained.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a semiconductor processor, or in a combination of the two. A software module may reside in a storage medium (i.e., memory, storage, etc.) such as a RAM memory, a flash memory, a ROM memory, an EPROM memory, an EEPROM memory, a register, a hard disk, a removable disk, a CD-ROM, An exemplary storage medium is coupled to the processor, which is capable of reading information from, and writing information to, the storage medium. Alternatively, the storage medium may be integral with the processor. The processor and the storage medium may reside within an application specific integrated circuit (ASIC). The ASIC may reside within the user terminal. Alternatively, the processor and the storage medium may reside as discrete components in a user terminal.

As described above, the present invention has been described with reference to particular embodiments, such as specific elements, and specific embodiments and drawings. However, it should be understood that the present invention is not limited to the above- Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the essential characteristics of the invention. Therefore, the spirit of the present invention should not be construed as being limited to the embodiments described, and all technical ideas which are equivalent to or equivalent to the claims of the present invention are included in the scope of the present invention .

The transmitter (300)
The NRZ-OOK transmitter 110,
APTC encoder 120,
The LD (130)
The polarization beam splitter (PBS)
A Mach-Zehnder modulator (MZM) 150, 160,
The polarization beam combiner (PBC)
The erbium-doped fiber amplifier (EDFA)
Fiber-optic transmission systems (190)
Polarization multiplexing (PDM)
Polarization mode dispersion (PMD)
Differential Group Delay (DGD)
Single Mode Fiber (SMF)
The polarization controller (PC)
Alamouti type specimen optical coding (APTC)
The polarization state (SOP)
Receiver 400,
An optical band pass filter (OBPF)
The polarization beam splitter (PBS)
The photodetector (PD) 220, 230,
A low-pass filter (LPF) 240, 250,
APTC decoder 260,
The determiner 270,

Claims (15)

Generating a pair of symbols (S 1 , S 2 ) orthogonally coded from the input signal;
Simultaneously transmitting each of the symbols S 1 and S 2 with a horizontal polarization and a vertical polarization in a first symbol interval; And
In the second symbol interval, the symbols horizontally polarized and vertically polarized (
Figure 112018104720210-pat00043
, S 1 ), respectively,
Estimating a symbol of each of the two symbol periods corresponding to a sum of horizontal and vertical polarization currents using at least one photodetector at the receiving end,
Figure 112018104720210-pat00056
, &Lt; / RTI &gt; is an equalized symbol using &lt; RTI ID =
The receiving side decodes a code according to Alamouti-type polarization-time coding (APTC) using horizontal and vertical polarization currents of each symbol interval without estimating channel state information (CSI) including signal phase and size information Estimates the received symbol,
Wherein the APTC decoding is performed for APTC decoding regardless of demultiplexing the polarization multiplexed symbols received at the receiving side into two orthogonal polarized signals. delete The method according to claim 1,
Wherein the input signal is a non-return-to-zero ON-OFF keying (NRZ-OOK) signal. The method according to claim 1,
Demultiplexing the polarization multiplexed symbols received through the optical transmission path into two orthogonal polarization signals;
Photoelectrically converting the orthogonal polarized signals demultiplexed using photodetectors;
Low-pass-filtering the photoelectrically converted signals; And
Estimating a symbol of each of the two symbol intervals corresponding to a summation value of horizontal and vertical polarization currents of the photodetectors in each of the two symbol intervals from the low pass filtered signals
Further comprising the steps of: (a) providing a polarization beam splitter for polarization multiplexing; The method according to claim 1,
Performing photoelectric conversion on the polarization multiplexed symbols received through the optical transmission path using one photodetector;
Low-pass-filtering the photoelectrically converted signal; And
Estimating a symbol of each of the two symbol intervals corresponding to a summation value of horizontal and vertical polarization currents output by the one photodetector in each of two symbol intervals from the low-pass filtered signal,
Further comprising the steps of: (a) providing a polarization beam splitter for polarization multiplexing; delete The method according to claim 1,
On the receiving side,
And for each of the sum of the horizontal and vertical polarization currents, one-bit delay-and-add and one-bit delay-and-subtract filters are used, Wherein the symbol estimating unit estimates a symbol for the polarization multiplexing optical transmission. An encoder for generating a pair of symbols (S 1 , S 2 ) orthogonally coded from an input signal;
A laser diode for generating a laser beam;
A polarizing beam splitter for generating a beam separated into horizontal and vertical polarized lights from the laser beam;
Two modulators for modulating each symbol using the beams of the horizontally polarized light and the vertically polarized light; And
(S 1 , S 2 ) modulated by the modulators in a first symbol interval and transmits the symbols S 1 , S 2 modulated by the modulators in the second symbol interval, Vertical polarization symbols (
Figure 112018104720210-pat00045
, S 1 ), wherein the polarization beam combiner
Estimating a symbol of each of the two symbol periods corresponding to a sum of horizontal and vertical polarization currents using at least one photodetector at the receiving end,
Figure 112018104720210-pat00057
, &Lt; / RTI &gt; is an equalized symbol using &lt; RTI ID =
The receiving side decodes a code according to Alamouti-type polarization-time coding (APTC) using horizontal and vertical polarization currents of each symbol interval without estimating channel state information (CSI) including signal phase and size information Estimates the received symbol,
Wherein the APTC decoding is performed for APTC decoding regardless of demultiplexing the polarization multiplexing symbols received at the receiving side into two orthogonal polarization signals. delete 9. The method of claim 8,
Wherein each of the modulators is a Mach-Zehnder modulator. 2. The optical transmission system of claim 1, wherein the modulator is a Mach-Zehnder modulator. 9. The method of claim 8,
Wherein the input signal is a non-return-to-zero ON-OFF keying (NRZ-OOK) signal. 9. The method of claim 8,
A polarization beam splitter for demultiplexing the polarization multiplexed symbols received through the optical transmission path into two orthogonal polarization signals;
Two photodetectors each photoelectrically converting the demultiplexed orthogonal polarized signals;
Two low-pass filters for respectively low-pass-filtering the photoelectric-converted signals; And
A decoder for estimating a symbol of each of the two symbol periods corresponding to the sum of the horizontal and vertical polarization currents of the photodetectors in each of the two symbol periods from the low-pass filtered signals,
Further comprising a polarization beam splitter for splitting the polarization beam splitter. 9. The method of claim 8,
One optical detector for photoelectrically converting the polarization multiplexed symbols received through the optical transmission path;
A low-pass filter for low-pass-filtering the photoelectrically converted signal; And
A decoder for estimating a symbol of each of the two symbol periods corresponding to a sum of horizontal and vertical polarization currents output from the one photodetector in each of two symbol periods from the low-pass filtered signal,
Further comprising a polarization beam splitter for splitting the polarization beam splitter. delete 9. The method of claim 8,
At the receiving end, one-bit delay-and-add and one-bit delay-and-subtract filters are applied to the sum of the horizontal and vertical polarization currents And estimates each of the symbols by using the polarization multiplexing optical transmission system.
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