JP2000209186A - Optical code division multiplex communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To substantially simplify a system device by using the optical pulse strings of different wavelengths for respective bits for payload information and encoding it. SOLUTION: The payload information is encoded by using the i (i=1, 2,..., N) pieces of the optical pulse strings of the different wavelengths for the respective bits and setting different codes in a specified code sequence for respective channels. For optical encoding, a multi-wavelength light source for repeatedly oscillating the optical pulses of the plural different wavelengths by a high frequency such as a mode lock laser is required. In this case, as shown in the figure, after an oscillation vertical mode for which a pulse width is about 3 ps and a pulse interval is about 17 ps is modulated by data signals to be transmitted by an optical modulator, it is made incident on FBG (chirped fiber Bragg). Then, when the cycle of a grating is matched with the Bragg diffraction condition of the optional three vertical modes of an oscillation spectrum, only the three vertical modes diffracted by the FBG is diffracted and reflected and a desired optical code is generated and made incident on an optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical code division multiplex communication system, and more particularly to a broad basic technology for constructing a future large-capacity, high-bit-rate, asynchronous lightwave network. Regarding use in the field.

[Prior art]

The multiplex communication technique is a technique for effectively utilizing a transmission band of a transmission path by sharing a same transmission path by a plurality of communication channels and expanding a communication capacity. FIG. 1 is a diagram showing an architecture configuration of a multiplex communication system, where 1-1 is a transmitter, 1-2 is a multiplexing apparatus, and 1-
3 indicates a transmission line, 1-4 indicates a demultiplexer, and 1-5 indicates a receiver. As shown in FIG. 2, in the case of time division multiplexing (hereinafter, referred to as TDM), time slots are assigned as communication channels.

On the other hand, in wavelength division multiplexing (hereinafter referred to as WDM), a plurality of signals are simultaneously multiplexed and transmitted using one transmission line by assigning wavelengths to channels, and a predetermined time slot is demultiplexed. Alternatively, it is a communication method of receiving by detecting only a signal of a wavelength.

[0004] In TDM, multiplexing / demultiplexing (MUX / DEMU) which is multiple times faster than the bit rate of information is performed.
X) The number of multiplexes is limited by the speed of the electronic circuit, since it requires a device. Therefore, the existing SDH (Synchron)
(Our Digital Hierarchy)
At present, research and development of WDM is being actively promoted. SD
When the H signal is further multiplexed by WDM and transmitted by one optical fiber, the communication capacity per optical fiber becomes (TD
(M multiplex number) × (WDM multiplex number).

An optical code division multiplex shown in FIG. 2 (c) has attracted attention as a third multiplex communication technique. Since optical code division multiplexing uses a time waveform as a communication channel, it is characterized in that a plurality of communication channels can be set on the same time slot and the same wavelength.

FIG. 3 is a diagram showing a configuration of a general optical code division multiplexing transmission system, where 3-1 is a transmitter, 3-2 is an optical encoder, 3-3 is a multiplexing coupler, and 3-4 is a multiplexing coupler. An optical fiber, 3-5 is a demultiplexing splitter, 3-6 is an optical decoder, and 3-7 receiver. The coded signals sent from each transmitter are multiplexed and transmitted over an optical fiber. At the receiving side, the data is demultiplexed and decoded at each receiver to reproduce the original data.

FIG. 4 is a diagram showing the configuration of a conventional optical code, where 4-1 is a demultiplexer, 4-2 is an optical delay line, 4-3 is a gate switch, and 4-4 is a multiplex. Are shown. The input optical pulse is separated by the demultiplexer, and as a result, is demultiplexed to each optical delay line, and the pulse is cut off or passed by each optical switch to generate a desired optical code sequence composed of 0 and 1. You. The configuration of the optical decoder is the same as that of the optical encoder.

[0008]

FIG. 5 is a diagram showing a decoded waveform when a prime code is used. FIG. 5A is obtained when the codes of the optical encoder and the optical decoder are the same. The autocorrelation waveform and FIG. 6B are cross-correlation waveforms obtained when the codes of the optical encoder and the optical decoder are different. The presence of a side lobe of the autocorrelation function and the peak of the cross-correlation function / peak value of the autocorrelation function =
Since it is as large as 2/5, there is a problem that it is difficult to determine a threshold value in decoding. Table 1 shows the number of codes for prime codes and the number of users who can communicate at the same time.
The code length N = p ² (p: prime number) and the number of users shall guarantee a bit error rate BER <10 ⁻⁹ .

[0009]

[Table 1]

On the other hand, the time spreading and wavelength hopping codes have the following characteristics, and can solve the problem of the prime codes. 1) Side lobe of autocorrelation function
Is zero. 2) The value of the cross-correlation function is at most one. 3) The number of codes is about one digit larger than the prime code (Table 1)
And Table 4), and 4) As a result, the number of users who can communicate at the same time is about one digit larger than that of the prime code (see Tables 1 and 4).

However, the biggest disadvantage is that the configuration of the apparatus for encoding / decoding becomes complicated.
FIGS. 6A and 6B are diagrams showing a configuration example of a conventional optical encoder / decoder for time spreading / wavelength hopping code. FIG. 6A shows an optical encoder, and FIG. 6B shows an optical decoder. 1 is a 2 × 2 optical switch, 6-2 is an optical delay line, and 6-3 is an optical filter. Taking the prime code as an example, it is necessary to use a mode-locked laser whose repetition frequency is p times the information bit rate as a light source and use a high-speed optical switch whose switching speed is p times the information bit rate. Further, there is a disadvantage that the number of switch stages increases in proportion to the code length.

The present invention seeks to greatly simplify the device configuration of a system when introducing an optical code division multiplexing communication technique aiming at increasing the transmission capacity in optical fiber communication and further functionalizing an optical network. That is the purpose.

[0013]

In order to solve the above-mentioned problems, the present invention encodes payload information using different optical codes, multiplexes a plurality of encoded information signals, and simultaneously transmits one signal. In optical coding using a so-called time spreading / wavelength hopping code in optical code division multiplexing communication in which a channel is transmitted and a channel is separated by decoding an optical code given to the channel on the receiving side, payload information is transmitted. I (i =
Using 1, 2,..., N) optical pulse trains (herein, each optical pulse is referred to as a chip pulse) and encoding by setting a different code in a specific code sequence for each channel; As a method for optically decoding a received signal, a matched filtering in the time domain [matched file] is performed.
After obtaining an autocorrelation function by performing a thresholding, each is determined according to the presence or absence of a peak of the autocorrelation function by threshold determination.
It is characterized by reproducing a bit of "0" or "1".

According to the present invention, there is provided an optical encoding method of the above time spreading / wavelength hopping code, wherein a so-called chirped fiber Bragg grating in which a plurality of diffraction gratings having different periods are formed in the longitudinal axis direction of the fiber. The period of the grating is の of the wavelength of the incident light to be diffracted (however, the wavelength in the fiber), and the position of the grating is set along the longitudinal axis of the chirped fiber Bragg grating at the input end. By setting the difference between the group delay times of round trip from the optical fiber to each diffraction grating to correspond to the time difference on the time axis of each chip pulse of the optical code, optical chip pulses of a plurality of wavelengths can be simultaneously transmitted to the captured fiber Bragg. When incident on the chip, chip pulses of different wavelengths diffracted by each diffraction grating generate an optical code with a predetermined wavelength arrangement at the incident end. To have.

Furthermore, as an optical decoding method of the above-mentioned time spreading / wavelength hopping code, a chirped fiber black having a lattice arrangement having a mirror image relation with the incident end and a chirping fiber black used for optical encoding is decoded. Injecting the received optical code, threshold judgment is performed for matched filtering in the time domain, and "1" of the original bit data is reproduced. When a chirped fiber Bragg having a grating arrangement that is not a mirror image of the incident end with the grating arrangement of the fiber Bragg is used for decoding, matched filtering [matched filter] is performed.
ring] to obtain a cross-correlation function as an output,
It is characterized in that "0" of the original bit data is reproduced by the threshold value judgment for the center value of the waveform.

The present invention has been known for its excellent properties of the code itself. However, the present invention provides a simple apparatus for time spreading and wavelength hopping, which has been difficult in actual optical coding and decoding. Provide configuration technology.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a method for generating time spreading and wavelength hopping will be described ^(note ). Taking a prime code as an example, the prime code of code number p has a code length of p ²
It is. Prime code sequence C _x = (C _x0 , C _{x1 ,.
··, C x (p2-1))} is _{i = S xj + jp (j} =
0, 1 · · · p-1 position) of the 1, and all other 0, or 1, is produced in a way that the fori _{= S xj} tens jp Cxj = 0, otherwise. Note that _Sxj is Sx =
(S _x0 , S _x1 ,..., S _{x (p−1)} , and are determined by the calculation of a modulo x mod of x × j. For example, when p = 3, The results are shown in Table 2. (Note) L. Tancevski and I. Ando
novic, “Wavelength hopping”
/ Time spreading c-mode divi
sion multiple access system
ems, "Electron. Lett., vol.
30, pp. 1388-1390 (1994).

[0018]

[Table 2]

A prime code C generated by using this,
Has a code length of 9 (= p ² ), 3 (= p) 1 and the other 6 (= p (p−1)) chips have a code of 0, and the following three are generated. C ₀ = (1,0,0,1,0,0,1,0,0,0) C ₁ = (1,0,0,0,1,0,0,0,1,0) C ₂ = (1,0,0,0,0,1,0,1,0,0)

[0020] Based on this, by operating the pattern H _y of the wavelength hopping prime symbol C _x, time spread / wavelength-hopping code can be generated. The number of wavelength-hopping pattern _{H y} is (p-1) pieces is obtained. In the above example where p = 3, six time spreading / wavelength hopping codes are generated as shown in Table 2.

The numbers ₁ , ₂ and ₃ in Table 2 correspond to different wavelengths λ ₁ , λ ₂ and λ ₃ respectively. Table 3 shows the number of codes for the prime number p and the bit error rate BER <
This is the number of users who can communicate at the same time as guaranteeing ^10-9- . Compared with the case of the prime code in Table 1, the number of codes is about one digit larger, and as a result, the number of users who can communicate simultaneously is about one digit larger.

[0022]

[Table 3]

[0023]

[Table 4]

Next, the system configuration will be described. The configuration of the optical code division multiplex transmission system is the same as that of FIG. FIG. 7 is an example of a device configuration of the transmitter. 7-1 is a light source,
7-2 denotes an optical modulator, 7-3 denotes an FBG (chirp fiber black, hereinafter abbreviated as FBG), 7-4 denotes an optical fiber, and 7-5 denotes an optical circulator.

Optical encoding requires a multi-wavelength light source that repeatedly oscillates a plurality of optical pulses of different wavelengths at a high frequency. A promising candidate is a mode-locked laser. FIG.
FIG. 3 is a diagram illustrating an example of an oscillation spectrum of a semiconductor mode-locked laser, in which a plurality of longitudinal modes oscillate at equal frequency intervals of 60 GHz.

FIG. 9 is a diagram showing a time waveform of the laser output, wherein the pulse width is about 3 ps and the pulse interval is about 17 ps.
(= 1/60 [GHz ⁻¹ ]). This oscillation longitudinal mode is modulated by a data signal (2.4 Gb / s in FIG. 7) to be transmitted by the optical modulator, and then is incident on the FBR.

For example, assume a time spreading and wavelength hopping code using the above p = 3 prime code. When the period of the grating is matched to Bragg diffraction conditions of any three longitudinal modes of the oscillation spectrum, only the three longitudinal modes diffracted by the FBG are diffracted and reflected, and a desired optical code is generated. And incident on the optical fiber. However, all the remaining longitudinal modes pass through the FBR and are removed, so that they do not enter the optical fiber.

FIG. 10 is a diagram showing a device configuration of the receiver. 10-1 is an FBG, 10-2 is an optical circulator,
10-3 photo detector and 10-4 are threshold elements. The demultiplexed light is incident on another FBG and matched-ed filter (match-ed filter).
The output is converted to an electric signal by a photodetector, and then subjected to threshold processing to reproduce one bit of the original data. In each receiver, a signal coded with a different optical code of another channel also exists at the same time, but due to the characteristics of time spreading and wavelength hopping as described above, a matched filter (matched filter) is used.
By thresholding the output of (tering), only those signals that must be received are output.

Here, it should be noted that:
That is, the band of the photodetector must be sufficiently narrower than the longitudinal mode interval of the mode-record laser of the light source. Thus, noise caused by interference between chips having the same code but different wavelengths can be removed. For example,
When a semiconductor mode-locked laser having a repetition frequency of 60 GHz is used, the 3 dB
Needs to be approximately 30 GHz. However, when the frequency falls below the band, the waveform of the chip pulse spreads in the output electric signal. As a result, in the worst case, the bit error rate BER may deteriorate due to the interference between bits.

Next, optical coding will be described. FIG.
FIG. 1 is a configuration diagram illustrating an operation principle of optical encoding. P above
As an optical encoding method using a prime code of = 3, diffraction gratings having different periods を_i (i = 1, 2, 3) are arranged along the z-axis in the longitudinal direction of the fiber (∧ ₃ , ∧ _1). ,
∧ ₂ ), in the above-described manner, each of the positions (L ₃ , L ₁ , L ₂ ) of the diffraction grating is determined by the difference Δn of the round-trip group delay time from the incident end of the FBG to each diffraction grating. _e
By setting (L ₁ −L ₃ ) / c, ne (L ₂ −L ₁ ) / c} to correspond to the time difference on the time axis of each chip pulse of the optical code, an optical chip having a plurality of wavelengths can be obtained. When the pulses are simultaneously incident on the FBG, chip pulses of different wavelengths (λ ₃ , λ ₁ , λ ₂ ) diffracted by the respective diffraction gratings are arranged in a predetermined order at the incident end to generate an optical code. The period of the diffraction grating is A _{i1 which} is one wavelength of the incident light to be diffracted from the Bragg diffraction condition.
/ 2 (however, the wavelength in the fiber), and is given by the following equation.

[0031]

(Equation 1)

FIG. 12 shows the principle of operation of optical decoding. The FBG for optical decoding is the FBG used for optical encoding.
A lattice arrangement of the BG lattice arrangement and a mirror image relation with respect to the incident end,
That is, (L ₂ , L ₁ , L _3. For this FBG,
By injecting the received optical code, a matched filter in the time domain is provided.
ring), and a threshold value determination (= 3) is performed on the median value 3 (however, the peak value of the case chip pulse is 1) of the waveform of the autocorrelation function obtained as the output, and the original bit data is determined. "1" is reproduced. However, when the lattice arrangement of the FBG used for optical decoding is different from (L ₂ , L ₁ , L ₃ ), matched filtering (matched filter) is performed.
As a result of the ering, a cross-correlation function is obtained as an output, and “0” of the original bit data is reproduced by threshold determination (<3) for the center value of the waveform.

FBGs have recently been commercialized as WDM wavelength filters, and have already begun to be introduced into commercial systems. Initial experiments have confirmed that the FBG used in the present optical encoder / decoder can be manufactured by the same UV laser exposure method as in the past.

When the configuration of the optical code division multiplexing communication system according to the present invention is compared with the configuration of the conventional time spreading / wavelength hopping code encoder / decoder shown in FIG. 6, the following advantages are obtained. 1) No switch is required in optical coding, and 2)
No optical filter is required for optical decoding. ,

[0035]

As described above, according to the present invention, when introducing the optical code division multiplexing communication technology aiming at increasing the transmission capacity in the optical fiber communication and further increasing the throughput of the optical network, the time spreading is required. In addition, the configuration of the apparatus using the method using the wavelength hopping code can be further simplified than the configuration of the conventional apparatus. Although time spreading and wavelength hopping codes are known for their excellent properties, they have difficulty in actual optical coding and decoding, and as a result, they have not been put to practical use. It is expected that the realization of an optical code / decoder for time spreading and wavelength hopping code will be extremely realistic by using the FBG.

[0036]

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an architecture of a multiplex communication system.

2A is a conceptual diagram of a TDM channel, FIG. 2B is a conceptual diagram of a WDM channel, and FIG. 2C is a conceptual diagram of a CDM channel.

FIG. 3 is a diagram showing a configuration of a general optical code division multiplex transmission system.

FIG. 4 is a diagram illustrating a configuration example of a conventional optical encoder.

5A and 5B are diagrams showing a decoded waveform when a prime code is used, and FIG. 5A is a diagram showing an autocorrelation waveform obtained when the codes of the optical encoder and the optical decoder are the same, and FIG. FIG. 4 is a diagram showing a cross-correlation waveform obtained when codes of an optical encoder and an optical decoder are different.

FIGS. 6A and 6B are diagrams showing a configuration example of a conventional optical encoder / decoder for time spreading / wavelength hopping code. FIG. 6A shows an optical encoder, and FIG. 6B shows an optical decoder.

FIG. 7 is a diagram illustrating an example of a device configuration of a transmitter.

FIG. 8 is a diagram illustrating an example of an oscillation spectrum of a semiconductor mode-locked laser, in which a plurality of longitudinal modes oscillate at equal frequency intervals of 60 GHz.

FIG. 9 is a diagram showing a time waveform of a semiconductor mode-locked laser output, in which a pulse width is about 3 ps and a pulse interval is about 17;
ps (= 1/60 [GHz- ¹ ]).

FIG. 10 is a diagram showing a configuration of a receiver device.

FIG. 11 is a diagram illustrating an operation principle of optical encoding.

FIG. 12 is a diagram illustrating an operation principle of optical decoding.

[Explanation of symbols]

 1-1 Transmitter 1-2 Multiplexer 1-3 Transmission line 1-4 Demultiplexer 1-5 Receiver 3-1 Transmitter 3-2 Optical encoder 3-3 Multiplexing coupler 3-4 Optical fiber 3- 5 Demultiplexer 3-6 Optical decoder 3-7 Receiver 4-1 Demultiplexer 4-2 Optical delay line 4-3 Gate switch 4-4 Combiner 6-1 2x2 optical switch 6-2 Optical delay line 6-3 Optical filter 7-1 Light source 7-2 Optical modulator 7-3 FBG 7-4 Optical fiber 7-5 Circulator 10-1 FBG 10-2 Optical circulator 10-3 Photodetector 10-4 Threshold element

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[Procedure amendment]

[Submission date] November 22, 1999 (1999.11.
22)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction contents]

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a method for generating time spreading and wavelength hopping will be described ^(note) . Taking prime example, prime symbol code number p is the code length is P ^2. Prime code sequence C _{x j} = (C _x0 , C _x1 ,...,
C _{x (p2-1)} ) is i = S _xj + jp (j = 0, 1,..., P
The position of -1) is set to 1 and the others are set to 0, that is, C _xj = 1, for i = S _xj + jp C _xj = 0, otherwise. Note that S _xj is S _x = (S
_x0, S _x1, ···, an element of the _{S x (p-1))} , is determined by calculation of modulo p of x × j (Modulo -p). For example, when p = 3, the following Table 2 is obtained. (Note) L. Tancevski and I. Andonovic, “Wavelength ho
pping / time spre a dingcode division multiple access
systems, ”Electron. Lett., vol. 30, pp. 1388-1390 (19
94).

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction contents]

[0020] Based on this, by operating the pattern H _y of the wavelength hopping prime symbol C _x, time spread / wavelength-hopping code can be generated. The number of wavelength-hopping pattern H _y is (p-1) pieces is obtained. In the above example of p = 3, as shown in Table 3 , six time spreads /
A wavelength hopping code is generated.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction contents]

The numbers 1, 2 and 3 in Table 3 correspond to different wavelengths λ ₁ , λ ₂ and λ ₃ respectively. Table 4 shows the number of codes for the prime number p, and the bit abuse rate BER <1.
0 ^-9 is a number of users can communicate at the same time guarantee. Compared with the case of the prime code in Table 1, the number of codes is about one digit larger, and as a result, the number of users who can communicate simultaneously is about one digit larger.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0023

[Correction method] Change

[Correction contents]

[0023]

[Table 4]

[Procedure amendment 5]

[Document name to be amended] Drawing

[Correction target item name] FIG.

[Correction method] Change

[Correction contents]

FIG. 11

Claims (3)

[Claims] 1. A method for encoding payload information using different optical codes, multiplexing a plurality of encoded information signals,
In optical coding using a so-called time spreading / wavelength hopping code in optical code division multiplexing communication in which transmission is performed using one transmission path at the same time and a channel is separated by decoding an optical code given to the channel on a receiving side. ,
The payload information is transmitted at a different wavelength for each bit i (i = 1, i = 1
2,..., N) optical pulse trains (herein, individual optical pulses are referred to as chip pulses) are encoded by setting different codes in a specific code sequence for each channel, and received. As an optical decoding method of a signal, matched filtering in the time domain [matchedfilter]
ing] to obtain an autocorrelation function, and then, by threshold value judgment, reproduce bits of “0” or “1” according to the presence or absence of a peak of the autocorrelation function. 2. A so-called chirped fiber Bragg grating in which a plurality of diffraction gratings having different periods are formed in a longitudinal axis direction of a fiber as an optical encoding method of a time spreading / wavelength hopping code according to claim 1. Of the wavelength of the incident light to be diffracted by 1 /
2 (however, the wavelength in the fiber), and the position of the diffraction grating is set along the longitudinal axis of the chirped fiber Bragg grating. By setting to correspond to the time difference on the time axis of each chip pulse of the optical code, when the optical chip pulses of a plurality of wavelengths are simultaneously incident on the chirped fiber Bragg, the chips of different wavelengths diffracted by each diffraction grating An optical code division multiplex communication system characterized in that a pulse generates an optical code having a predetermined wavelength arrangement at an incident end. 3. A chirped fiber having a lattice arrangement of a chirped fiber black used for optical encoding and a lattice arrangement of a mirror image with respect to an incident end as an optical decoding method of the time spreading / wavelength hopping code according to claim 1. Black is used for decoding, and the received optical code is incident, threshold matching is performed for time-domain matched filtering, and “1” of the original bit data is reproduced. When a chirped fiber Bragg having a grating arrangement that is not mirror-image related to the incident end and the grating arrangement of the used chirped fiber Bragg is used for decoding, matched filtering [matched?
An optical code division multiplexing communication method characterized in that a cross-correlation function is obtained as an output as a result of the tering, and "0" of the original bit data is reproduced by a threshold value judgment with respect to the center value of the waveform.