JP2011182052A - Optical pulse time spreading device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent an influence of fluctuation of wavelength of an optical pulse with single wavelength even when the wavelength fluctuates and to reduce crosstalk between optical signals of channels to which the adjacent wavelengths are assigned, respectively. <P>SOLUTION: This optical pulse time spreading device is constituted by being provided with first to U-th optical pulse time spreading units in which SSFBs to be constituted by arranging N unit FBGs in series along the length direction of an optical fiber are utilized. To each of the unit FBGs constituting the optical pulse time spreading units, refractive index modulation is given as a function obtained by multiplying the refractive index modulation as a diffraction grating by a sinc function, and phase difference between Bragg reflection light from a refractive index modulation area to be enveloped by a peak curve which takes the largest maximal values of the sinc function, which gives an envelope for connecting the maximal values of the function and Bragg reflection light from a refractive index modulation area to be enveloped by a peak curve which takes the largest maximal values of the sinc function in the adjacent unit FGB is set to odd times of half-wavelength of the Bragg reflection light. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical pulse time spreading device used for optical multiplex transmission, and in particular, a unit fiber black grating (FBG: Fiber Bragg Grating) which is a periodic refractive index distribution structure is provided in the waveguide direction of an optical fiber. The present invention relates to an optical pulse time spreading device comprising a plurality of superstructured fiber Bragg grating (SSFBG) type optical pulse time spreaders.

  In recent years, the demand for communication is rapidly increasing due to the spread of the Internet and the like, and the capacity of communication is increased correspondingly. As a technique for increasing the communication capacity, attention is focused on an optical multiplex transmission technique for transmitting optical pulse signals for a plurality of channels collectively on a single optical fiber transmission line. As optical multiplex transmission technology, active research is being conducted on optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), and optical code division multiplexing (OCDM). Has been.

  All of these optical multiplex transmission technologies are technologies that enable optical pulse signals for a plurality of channels to be transmitted together on a single optical fiber transmission line. Large capacity can be realized. Further, by combining these optical multiplex transmission technologies, communication capacity can be further increased. For example, a multiplex transmission system combining the OCDM system and the OTDM system (for example, see Non-Patent Document 1) or a multiplex transmission system combining the OCDM system and the WDM system (for example, see Non-Patent Document 2) is being studied. Yes.

  In a WDM optical multiplex transmission system, it is necessary to narrow the wavelength grid interval in response to an increase in the number of multiplexed channels. When the wavelength grid interval is narrowed, it is necessary to make the absolute value of the drift of the light source generating the optical carrier wave sufficiently small so as to be within the range of the wavelength grid interval. This is the first problem. The drift of the light source occurs with a change in ambient ambient temperature as time elapses. If the fluctuation range of the absolute value of the drift of the light source exceeds the range of the wavelength grid interval, the channel identification capability is lost in the WDM multiplex transmission system.

  By the way, in the ITU international standard established by the International Telecommunication Union (ITU), WDM grids are set to 100 GHz (approximately 0.8 nm interval), 50 GHz (approximately 0.4 nm interval), etc. Has been.

  In addition, a WDM multiplex transmission system requires a light source that generates a multi-wavelength optical carrier wave. Multi-wavelength light sources are expensive and the wavelength resources available for multiplex transmission systems are limited. In addition, it is necessary to use advanced technology and high manufacturing costs to ensure long-term stability of the wavelength of the multi-wavelength light source. This is the second problem.

  The inventor of this application divides the wavelength spectrum into a plurality of wavelength components as long as the wavelength spectrum has a finite width even if the wavelength spectrum is a light pulse having the only maximum. Note that it is possible to assign a channel to each of these. The inventor of the present application also described the difference in the wavelength components described above even when the wavelength of the single-wavelength optical carrier generation light source that generates the optical pulse having the only maximum in the wavelength spectrum varies with the variation in the ambient temperature. It was found that it is possible to realize an optical multiplexing system that does not affect the channel identification ability based on.

  This optical multiplexing system is a pseudo WDM system because the wavelength spectrum is divided into a plurality of channels and a channel is assigned to each of the divided wavelength components. Further, as will be described below, the method of dividing the wavelength spectrum into a plurality is common to the OCDM method. Therefore, in the following description, this wavelength multiplexing method is referred to as an OCDM / WDM hybrid multiplexing method.

  The OCDM / WDM hybrid multiplex method is a method that divides the wavelength spectrum of a single wavelength optical carrier generation light source into multiple wavelength components by SSFBG, and the divided wavelength components are determined by the structure of SSFBG, and single wavelength optical carrier The influence of the fluctuation of the wavelength of the generated light source does not appear. That is, since the wavelength component divided by the SSFBG is determined by the structure of the SSFBG, the wavelength component divided by the SSFBG does not change even if the wavelength fluctuation of the single wavelength optical carrier wave generation light source occurs. Therefore, in the OCDM / WDM hybrid multiplex system, channel identification is possible without being affected by the fluctuation of the wavelength of the single wavelength optical carrier generation light source, so that the first and second problems described above are solved.

Klaus Grobe, Jorg-Peter Elbers, "PON Evolution from TDMA to WDM-PON", OFC NThD6 (2008) Taro Hamanaka, Xu Wang, Naoya Wada, and Ken-ich Kitayama, "Demonstration of 16-user OCDMA over 3-wavelength WDM using 511-chip, 640 Gchip / s SSFBG en / decoder and single light source", OFC OMO1 (2007 )

  As a result of further research, the inventors of this application have found that, in the above-described OCDM / WDM hybrid multiplexing system, when the interval of Bragg reflection wavelengths set in each SSFBG is narrowed and the number of multiplexed channels is increased, It was confirmed that the base part of the wavelength spectrum of the optical signal of the channel to which the wavelength to be assigned overlapped and crosstalk occurred between the two channels, resulting in a decrease in communication accuracy. Therefore, a method for reducing this crosstalk is required. Here, the channel to which the adjacent wavelength is assigned refers to a channel having a relationship in which the wavelength difference between the assigned wavelengths is the smallest.

  The inventor of this application described above by apodizing the refractive index modulation of the periodic refractive index distribution structure with a sinc function for the periodic refractive index distribution structure forming each of the units FBG constituting the SSFBG. I came up with the idea that crosstalk can be reduced.

  Therefore, an object of the present invention is to use a single-wavelength optical pulse, and even if the wavelength of a single-wavelength optical pulse varies with changes in ambient temperature, the effect of this variation is In the OCDM / WDM hybrid multiplex system, which can realize a pseudo WDM transmission system that is not subject to interference, the cross-section generated by overlapping the base portions of the wavelength spectrum of the optical signal of the channel to which the adjacent wavelength is assigned An object of the present invention is to provide an optical pulse time spreading device capable of performing encoding and decoding capable of reducing talk.

  Here, the terms “single wavelength optical pulse” and “pseudo WDM” are defined as follows. That is, a single-wavelength optical pulse refers to an optical pulse whose wavelength spectrum has a single maximum. In addition, pseudo-WDM means that the wavelength spectrum of a single-wavelength optical pulse is divided and assigned to each channel, and is used in combination with the OCDM system, so that channel identification is performed using a difference in wavelength spectrum components and a code described later. It shall mean a method of identification based on the difference.

  A wavelength spectrum is an optical spectrum expressed with respect to the wavelength axis, whereas a frequency spectrum is an optical spectrum expressed with respect to the frequency axis. In the description of the pseudo WDM, it is not necessary to distinguish between the two, and therefore, the wavelength spectrum is described in the following description.

  In a normal WDM system, an optical pulse having a plurality of maximums in the wavelength spectrum is distributed to each channel in correspondence with the maximum wavelength, and channel identification is realized by identifying these wavelengths. In other words, the wavelength spectrum of the optical carrier of the wavelength multiplexed signal in the normal WDM system takes a plurality of maximums, whereas the wavelength spectrum of the optical carrier of the signal in the pseudo WDM system of the present invention takes a single maximum. There are differences.

  In the following description, the terms “code” and “decode” will be used in a broader sense than the conventional practice. That is, the rule for diffusing the optical pulse constituting the optical pulse signal on the time axis is not limited to the code in the normal sense (sometimes referred to as a code in a narrow sense), and is an arbitrary rule that is uniquely determined ( Even in a broad sense, the above-mentioned names of encoding and decoding are used. Accordingly, even in the case of codes in a broad sense, terms such as encoded optical pulse signals and chip pulses are used.

  In addition, the sequence of chip pulses output from the SSFBG constituting the optical pulse time spreading device of the present invention described below is a strict sequence like the sequence of chip pulses output from the SSFBG in which a narrowly-signed code is set. The optical pulse is not generated by time spreading based on the meaning sign. However, in the following description, for the sake of convenience, conversion of an optical pulse into a sequence of chip pulses may be referred to as encoding, and generation of a sequence of chip pulses as an autocorrelation wave or cross-correlation wave may be referred to as decoding.

  According to the gist of the present invention, an optical pulse time spreading device having the following configuration is provided.

  The optical pulse time spreading device of the present invention is a chip pulse train composed of N chip pulses from the first to the N-th chip pulse, in which the input optical pulses are sequentially time-diffused on the time axis (N is 2 or more). 1st to Uth optical pulse time spreader (U is an integer equal to or greater than 1 satisfying U ≦ N), and the first to Uth optical pulse time spreaders are connected in parallel or in series. An optical pulse time spreading device configured.

  Each of the first to U-th optical pulse time spreaders is configured to include an SSFBG including N unit FBGs, and is output from the first to U-th optical pulse time spreaders, respectively. In each of the p-th optical pulse time spreaders (p is an integer from 1 to U) so that the spectra of the U-chip pulse trains are different from each other, the interval between adjacent unit FBGs, and this p-th The Bragg reflection wavelength of the unit FBG in the optical pulse time spreader is set.

  Each unit FBG has a refractive index modulation as a function obtained by multiplying a refractive index modulation as a diffraction grating by a sinc function, and takes the maximum maximum value of the sinc function that gives an envelope connecting the maximums of this function. Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve and Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve taking the maximum maximum value of the sinc function in the adjacent unit FBG Is set to an odd multiple of the half wavelength of the Bragg reflected light.

  In the optical pulse time spreading device of the present invention, preferably, the spectra of the first to U-th chip pulse trains output from the first to U-th optical pulse time spreaders are different from each other, and the energy of the chip pulse train is equal. In each of the p-th optical pulse time spreaders (p is an integer from 1 to U), the interval between adjacent unit FBGs and the p-th optical pulse time spreader It is better to set the Bragg reflection wavelength of the unit FBG at.

  Further, in the optical pulse time spreading device of the present invention, preferably, each unit FBG included in the SSFBG is sequentially increased along the length direction of the optical fiber so that the maximum maximum value of the refractive index modulation of the unit FBG is large. It is set so that the maximum maximum value of the refractive index modulation of this unit FBG decreases sequentially along the length direction of the optical fiber when it is increased and becomes maximum at the center position of the SSFBG, and when the center position is exceeded. Is good.

  Preferably, in the p-th optical pulse time spreader of the optical pulse time spreading device of the present invention, the interval between adjacent unit FBGs and the Bragg reflection of the unit FBG in the p-th optical pulse time spreader The wavelengths should be configured as follows:

The Bragg reflection wavelength λ _Bp set in the unit FBG included in the p-th optical pulse time spreader is expressed by the following equations (1a) and (1b) with the peak wavelength of the wavelength spectrum of the input optical pulse as λ _s
λ _Bp = λ _s + k (Δλ _s / U) (1a)
(However, if U is an odd number, k is an integer that satisfies | k | <U / 2)
λ _Bp = λ _s + (2k + 1) (Δλ _s / 2U) (1b)
(However, when U is an even number, k is an integer satisfying | k | <U / 2)
Given in.

The difference λ _R = λ _Bmax −λ _Bmin between the maximum wavelength λ _Bmax and the minimum wavelength λ _Bmin of the Bragg reflection wavelength λ _Bp set in the unit FBG is _given by the following equation (2)
λ _R ≦ λ _s ² / (2 × L × n _eff ) (2)
And λ _s is given by the following equation (3)
λ _Bmin ≦ λ _s ≦ λ _Bmax (3)
Is set to the range given by.

The phase difference φ of the chip pulse reflected from the adjacent unit FBG of the p-th optical pulse time spreader is expressed by the following equation (4), where m is an integer of 0 or more:
φ = (2m + 1) (λ _Bp / 2) (4)
In the p-th optical pulse time spreader, an interval between adjacent unit FBGs and a Bragg reflection wavelength of the unit FBG in the p-th optical pulse time spreader are set.

  The optical pulse time spreading device according to the present invention outputs an input optical pulse as a chip pulse train composed of N chip pulses from the first to the N-th chip pulses, which are sequentially time-diffused on the time axis. Each of the p-th optical pulse time spreaders includes a U-th optical pulse time spreader, and the spectrums of the first to U-th chip pulse trains output from the first to U-th optical pulse time spreaders are different from each other. , The interval between adjacent unit FBGs and the Bragg reflection wavelength of the unit FBG in the p-th optical pulse time spreader are set.

  Therefore, the wavelength spectrum of the chip pulse output from the optical pulse time spreading device of the present invention is determined by the structure of the first to Uth optical pulse time spreaders and is not affected by the fluctuation of the wavelength spectrum of the input optical pulse. Therefore, if the optical pulse time spreading device of the present invention is used as an encoder and a decoder, even if the wavelength of a light source that generates an optical carrier fluctuates with a change in ambient temperature, etc., communication that is not affected by this fluctuation. A highly accurate optical multiplexing optical communication system is realized.

  Further, each of the first to U-th optical pulse time spreaders of the optical pulse time spreading device of the present invention is configured to include an SSFBG, and each of the unit FBGs configuring the SSFBG is a unit of the unit FBG. Refractive index modulation is given as a function of refractive index modulation as a diffraction grating multiplied by a sinc function, and the refractive index enveloped by a peak curve taking the maximum maximum of the sinc function giving an envelope connecting the maximum of this function The phase difference between the Bragg reflected light reflected from the modulation region and the Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve taking the maximum maximum value of the sinc function in the adjacent unit FBG is the Bragg reflected light. Is set to an odd multiple of a half wavelength.

  With this configuration, the shape of the wavelength spectrum of the Bragg reflected light reflected from each SSFBG becomes a rectangular shape in the wavelength space. Therefore, according to the optical pulse time spreading device of the present invention, the shape in the wavelength space of the wavelength spectrum of the mutual optical signal encoded by the SSFBG of the adjacent channel is a rectangle whose center wavelength is the wavelength assigned to each channel. It becomes.

  Therefore, the portion where the wavelength spectra of the optical signals encoded by the SSFBGs of adjacent channels are overlapped is not set to the shape of the envelope that connects the maximum of the refractive index modulation amount as described above. Compared to using. That is, if an optical multiplex communication system using the optical pulse time spreader of the present invention as an encoder and a decoder is constructed, a system with small crosstalk between adjacent channels and high communication accuracy is realized.

  Here, configuring the refractive index modulation as a function obtained by multiplying the refractive index modulation as a diffraction grating by the sinc function may be apodizing the refractive index modulation as the diffraction grating by the sinc function. In general, apodization means that the envelope of the peak of the refractive index change amplitude as a periodically changing diffraction grating is modulated by a Gaussian error function or the sinc function described above.

It is a figure where it uses for description about the structure of the optical pulse time spreading | diffusion apparatus of embodiment of this invention, and its operation | movement. (A) is a schematic configuration diagram of SSFBG that is a component of the optical pulse time spreader that constitutes the optical pulse time spreading device of the embodiment of the present invention, (B) is the first to U-th optical pulse time spreading It is a schematic block diagram of an optical pulse time spreading device of an embodiment of the present invention configured by connecting devices in parallel, (C) is a first to U-th optical pulse time spreading device connected in series 1 is a schematic block configuration diagram of an optical pulse time spreading device according to an embodiment of the present invention configured. FIG. It is a schematic diagram with which it uses for description about OCDM / WDM hybrid multiplexing system. It is a figure for explanation of refractive index modulation of SSFBG, (A) is a diagram schematically showing the state of refractive index modulation of SSFBG, (B) is taken out two of the unit FBG It is the figure which expanded and showed the mode of refractive index modulation. It is a figure where it uses for description about the distribution of the magnitude | size of the refractive index modulation | alteration of the unit FBG sequentially arrange | positioned in SSFBG. It is a diagram provided for explanation of refractive index modulation for each unit FBG, (A) is a diagram schematically showing the state of refractive index modulation of SSFBG, (B) is for two of the unit FBG It is a diagram that shows the state of refractive index modulation taken out and enlarged, and (C-1), (C-2), and (C-3) are diagrams for explaining apodization by a Gaussian error function. . It is a figure for explanation about the case where the refractive index modulation of the periodic refractive index distribution structure forming each of the unit FBGs is apodized by the sinc function, and (A) schematically shows the state of the refractive index modulation of the SSFBG. (B) is an enlarged view showing the state of refractive index modulation by taking out two of the unit FBGs, and (C-1), (C-2) and (C -3) is a diagram for explaining apodization by the sinc function. Refraction of the periodic refractive index distribution structure in which the maximum points of the refractive index modulation from the first unit FBG to the 32nd unit FBG are apodized with the Gaussian error function described above, and form each of the unit FBGs The rate modulation is also a diagram for explaining the reflection spectrum of the SSFBG that is apodized with the sinc function, and (A) to (C) are diagrams showing the reflection spectra of the SSFBG when Q = 1, 2, and 3, respectively. It is. It is a diagram for explaining the encoding and decoding operations by the optical pulse time spreading apparatus of the embodiment of the present invention, (A) is any one of the optical pulse time spreading apparatus of the embodiment of the present invention. It is a figure which shows a mode that an optical pulse is converted into a chip pulse train by an optical pulse time spreader and is encoded, and (B) is an optical pulse time spreading having the same configuration as the optical pulse time spreading device that has performed encoding. FIG. 4 is a diagram showing a state of decoding by any one of the optical pulse time spreaders included in the apparatus, and (C) shows that each of the N chip pulses constituting the chip pulse train input to the decoder is a decoder. FIG. 6 is a diagram for explaining a process in which an autocorrelation wave or a cross-correlation wave is generated by interference of these total (N × N) chip pulses. It is a figure with which it uses for description about the characteristic of the wavelength spectrum of the output light output from an optical pulse time spreader, and (A) is light which has SSFBG formed with unit FBG which is not apodized for refractive index modulation. It is a figure which shows the wavelength spectrum of the output light output from a pulse time spreader, and (B) is light which has SSFBG formed with unit FBG in which apodization based on a Gaussian error function is given to refractive index modulation. It is a diagram showing the wavelength spectrum of the output light output from the pulse time spreader, (C) includes SSFBG formed by unit FBGs that are subjected to refractive index modulation and apodized based on the sinc function as described above. It is a figure which shows the wavelength spectrum of the output light output from an optical pulse time spreader. It is a figure used for explanation about the size of crosstalk generated by overlapping the base part of a single wavelength band, and (A) includes SSFBG formed by unit FBGs that are not apodized for refractive index modulation. It is a figure which shows the mode of the crosstalk between the single wavelength bands in the optical pulse time spreader comprised by the optical pulse time spreader, and (B) is apodized based on the sinc function for refractive index modulation. It is a figure which shows the mode of the crosstalk of a single wavelength band in the optical pulse time spreading | diffusion apparatus comprised by the optical pulse time spreading | diffusion device provided with the SSFBG formed of unit FBG.

  Embodiments of the present invention will be described below with reference to the drawings. Each figure is a diagram for explaining one configuration example according to the embodiment of the present invention and data related thereto, and the present invention is not limited to the illustrated example.

<Optical pulse time spreading device>
With reference to FIG. 1 (A) to FIG. 1 (C), the structure and operation of an optical pulse time spreading device according to an embodiment of the present invention will be described.

  FIG. 1A is a schematic configuration diagram of an SSFBG that is a component of an optical pulse time spreader that constitutes an optical pulse time spreader according to an embodiment of the present invention. FIG. 1 (A) schematically shows an example of an SSFBG in which N unit FBGs are arranged in series along a length direction of an optical fiber with a constant interval L therebetween. The FBG is schematically shown as a vertically striped rectangle, and the detailed shape of the core or clad of the optical fiber is omitted. The length of each unit FBG is D.

  FIG. 1B shows an example in which the first to U-th optical pulse time spreaders are connected in parallel in the optical pulse time spreading device of the embodiment of the present invention.

  The optical pulse time spreading device according to the embodiment of the present invention shown in FIG. 1 (B) divides the input optical pulse 11 into U and outputs the first input optical pulse 13-1 to the Uth input optical pulse 13-U. An optical splitter 12 is provided. The division of the input optical pulse 11 by the optical splitter 12 is executed by a normal 1-input U-output optical splitter that divides the intensity of the input light.

  Each of the first input optical pulse 13-1 to the Uth input optical pulse 13-U is installed in the first stage of the first optical pulse time spreader 16-1 to the Uth optical pulse time spreader 16-U, respectively. The light is input to the first optical circulator 14-1 to the U-th optical circulator 14-U. The first input optical pulse 13-1 to the Uth input optical pulse 13-U are respectively transmitted through the first optical circulator 14-1 to the Uth optical circulator 14-U to the first optical pulse time spreader 16-1 to Input to the U-th optical pulse time spreader 16-U.

  The first optical pulse time spreader 16-1 to the Uth optical pulse time spreader 16-U receive the first input optical pulse 13-1 to the Uth input optical pulse 13-U, respectively, on the time axis. The first chip pulse train 17-1 to the U chip pulse train 17-U, which are composed of N chip pulses from the first to the Nth chip pulses, which are sequentially arranged in a time-spread manner, are converted into the first optical circulator 14-1 to the Uth light. Output via the circulator 14-U.

  Each of the first optical pulse time spreader 16-1 to the Uth optical pulse time spreader 16-U includes an SSFBG including N unit FBGs from the first unit FBG to the Nth unit FBG. ing. The spectra of the first chip pulse train 17-1 to the U-th chip pulse train 17-U output from the first optical pulse time spreader 16-1 to the U-th optical pulse time spreader 16-U are different from each other. In addition, in the p-th optical pulse time spreader 16-p, the interval between adjacent unit FBGs and the Bragg reflection wavelength of the unit FBG in the p-th optical pulse time spreader 16-p are set. .

  In the optical pulse time spreading device of the embodiment of the present invention, the first chip pulse train 17-1 to the U-th chip respectively output from the first optical pulse time spreader 16-1 to the U-th optical pulse time spreader 16-U The intervals between the unit FBGs arranged adjacent to each other and the p-th optical pulse time spreader so that the spectra of the pulse trains 17-U are different from each other and more preferably the energy of these chip pulse trains is equal. The Bragg reflection wavelength of the unit FBG at 16-p is set.

  Each of the unit FBGs constituting the first optical pulse time spreader 16-1 to the Uth optical pulse time spreader 16-U has a refractive index modulation multiplied by a refractive index modulation as a diffraction grating by a sinc function. Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve taking the maximum maximum value of the sinc function, which gives the envelope connecting the local maximum of this function, and the adjacent unit FGB The phase difference of the Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve taking the maximum maximum value of the sinc function is set to an odd multiple of the half wavelength of the Bragg reflected light.

  In FIG. 1 (C), in the optical pulse time spreader according to the embodiment of the present invention, the first optical pulse time spreader 26-1 to the Uth optical pulse time spreader 26-U are connected in series. An example is shown.

  In the optical pulse time spreading device of the embodiment of the present invention shown in FIG. 1 (C), the input optical pulse 21 is first installed in the first stage of the first optical pulse time spreader 26-1, first optical circulator 24-1. Is input to the first optical pulse time spreader 26-1 through the first optical pulse, is Bragg-reflected, and is a first consisting of N chip pulses from first to N-th chip pulses that are sequentially time-diffused on the time axis. The chip pulse train 27-1 is output to the outside again through the first optical circulator 24-1.

  The input optical pulse 21 input to the first optical pulse time spreader 26-1 is not only the component that is Bragg-reflected as described above and output as the first chip pulse train 27-1, but also the first optical pulse time spread There is also a first optical pulse time spreader transmission component 29-1 that is transmitted through the device 26-1. This first optical pulse time spreader transmission component 29-1 is input to the second optical pulse time spreader 26-2 via the second optical circulator 24-2, is Bragg reflected, and is time-diffused on the time axis. As a second chip pulse train 27-2 composed of N chip pulses from the first to Nth chip pulses sequentially arranged, it is output to the outside again through the second optical circulator 24-2.

  Similarly, the p-th chip pulse train 27-p is output from the p-th optical pulse time spreader 26-p to the outside via the p-th optical circulator 24-p, and finally from the U-th optical pulse time spreader 26-U. The U-th chip pulse train 27-U is output to the outside via the U-th optical circulator 24-U.

  The configurations of the first optical pulse time spreader 26-1 to the Uth optical pulse time spreader 26-U are the same as the first optical pulse time spreader 16-1 to the Uth optical pulse time spreader 16-U described above. It is the same. In the following description, the first optical pulse time spreader 16-1 to the Uth optical pulse time spreader 16-U and the first optical pulse time spreader 26-1 to the Uth optical pulse time spreader 26-U are distinguished. When there is no need to do this, it is described as a first to U-th optical pulse time spreader.

In the first to U-th chip pulse trains, in order to set the interval between unit FBGs and the Bragg reflection wavelength of unit FBGs so that the spectra are different from each other and the energy of these chip pulse trains is equal. The Bragg reflection wavelength λ _{Bp of the} unit FBG provided by the pth optical pulse time spreader and the adjacent unit FBG of the pth optical pulse time spreader so as to satisfy the expressions (1a) and (1b) to (4) What is necessary is just to set each value of phase difference (phi) of the chip pulse reflected from.

When actually creating the optical pulse time spreading device of the embodiment of the present invention, the first to U-th chip pulse trains respectively output from the first to U-th optical pulse time spreaders by using a numerical simulator or the like While confirming the characteristics of the wavelength spectrum, etc., the Bragg reflection wavelength λ _{Bp of the} unit FBG provided with the p-th optical pulse time spreader in the range satisfying the above formulas (1a), (1b) to (4), and Each value of the phase difference φ of the chip pulse reflected from the adjacent unit FBG of the p-th optical pulse time spreader may be set.

  With reference to FIG. 2, an OCDM / WDM hybrid multiplexing system in which a spectral component of an input optical pulse having a single maximum is divided into a plurality of channels and a channel is assigned to each of the divided spectral components will be described. In FIG. 2, the horizontal axis shows the wavelength of the input light pulse on an arbitrary scale, and the vertical axis is omitted, but the magnitude of the light intensity is shown on the vertical axis in an arbitrary scale.

In FIG. 2, a bell-shaped curve indicated by a broken line is a wavelength spectrum of the input light pulse, and its full width at half maximum is Δλ _s . When an optical pulse having such a wavelength spectrum is incident on the SSFBG, a reflection wavelength band determined by the Bragg wavelength λ _B of the unit FBG and the arrangement interval L of the unit FBG that constitute the SSFBG is determined. In FIG. 2, the Bragg reflection spectrum by the SSFBG constituting the i th optical pulse time spreader and the Bragg reflection spectrum by the SSFBG constituting the j th optical pulse time spreader are shown with different hatchings. Here, i and j are integers satisfying i ≠ j, 1 ≦ i ≦ U, and 1 ≦ j ≦ U.

It is possible to set the Bragg reflection wavelength λ _B of the unit FBG and the arrangement interval L of the unit FBG so that there are a plurality of reflection wavelength bands of the SSFBG in the wavelength spectrum range of the input optical pulse. In FIG. 2, there are two reflection wavelength bands of SSFBG constituting the i-th and j-th optical pulse time spreaders. The bandwidth of the SSFBG reflection wavelength band (bandwidth in each of the two reflection wavelength bands) is determined to be a value proportional to the reciprocal of the time waveform of the chip pulse reflected from the unit FBG, and is in two places. The reflection wavelength band intervals Δλ _Bi and Δλ _Bj are respectively determined by the arrangement interval L of the unit FBGs.

  The first to U-th chip pulse trains output from the first to U-th optical pulse time spreaders by setting the reflection wavelength band of the SSFBG to exist in a plurality of locations in the range of the wavelength spectrum of the input optical pulse. It is possible to set so that the energy of each becomes equal.

  The code length N of the code set in the optical pulse time spreader constituting the optical pulse time spreader of the embodiment of the present invention is used in the optical communication system in which the optical pulse time spreader of the embodiment of the present invention is used. It is set to a value larger than the possible number of channels U. This is because the number of channels U that can be identified by a code cannot exceed the code length N.

The total length of SSFBG, which is a constituent element of an optical pulse time spreader formed by arranging N unit FBGs in series, is given by code length N × arrangement interval L of adjacent unit FBGs. A diffusion time width in which one input optical pulse input to the optical pulse time spreader is spread on the time axis by the optical pulse time spreader is given by the following equation (5).
(2 × N × L × n _eff ) / c (5)
Here, c is the speed of light in vacuum, and n _eff is the value of the effective refractive index of the optical fiber constituting the SSFBG.

  Each of the optical pulses that make up the input optical pulse train is time-spread on the time axis as a sequence of chip pulses arranged on the time axis by the optical pulse time spreader, but adjacent optical pulses that make up the input optical pulse train are time-spread In order to prevent the chip pulse trains from overlapping each other, the diffusion time width needs to be narrower than the data bit interval which is the interval on the time axis between the adjacent optical pulses of the input optical pulse train.

Accordingly, a condition for preventing the chip pulse trains in which the adjacent optical pulses constituting the input optical pulse train are time-spread from overlapping each other is given by the following equation (6).
L ≦ c / (2 × DR × N × n _eff ) (6)
Here, DR is a value that provides a data bit rate that is given by the reciprocal of the interval on the time axis of adjacent optical pulses that constitute the input optical pulse train. The data bit rate has a dimension of reciprocal time (1 / second).

  The adjacent unit FBGs are arranged so that the phase of the Bragg reflected light from the adjacent unit FBG is shifted by ½ wavelength. That is, the adjacent unit FBGs are arranged at intervals obtained by adding a length corresponding to a half wavelength of the Bragg reflected light to an integral multiple of the wavelength of the Bragg reflected light. In the case of designing the unit FBG with the arrangement interval L being L and the length D of each unit FBG being equal to L, strictly speaking, the arrangement interval of the unit FBG is 1/2 wavelength longer than L, The length corresponding to 1/2 wavelength is 1 μm or less, and the arrangement interval L of typical unit FBGs is about 1 mm, which is negligible. Therefore, in the following description, the arrangement interval of the unit FBGs and the length of each unit FBG may be set equal.

Therefore, the relationship between the arrangement interval L of adjacent unit FBGs and the Bragg reflection wavelength λ _Bp is given by the following equation (7). Here, assuming that a p-th optical pulse time spreader is used as the optical pulse time spreader, the Bragg reflection wavelength is λ _Bp .
2 × L × n _eff = λ _Bp × (m + (1/2)) (7)
Here, m is an integer of 0 or more.

When the optical pulse time spreading apparatus according to the embodiment of the present invention is used as an encoder and a decoder in an optical communication system of the OCDM / WDM hybrid multiplexing system, the Bragg reflection wavelength λ _Bp becomes a channel identification mark. To set λ _Bp in the optical pulse time spreader, the period Λ _p of the periodic refractive index structure (grating period) of the unit FBG constituting this optical pulse time spreader is set to λ _Bp = 2 × n _eff Λ _What is necessary is just to set so that _p may be satisfy | filled.

The Bragg reflection wavelengths set in the first to U-th optical pulse time spreaders constituting the optical pulse time spreading device of the embodiment of the present invention are λ _{B1 to} λ _BU , respectively, and the maximum value of λ _{B1 to} λ _{BU Is set} to λ _Bmax and the minimum value is set to λ _Bmin , the peak wavelength λ _s of the wavelength spectrum of the input optical pulse is set so as to satisfy the above-described equation (3).

Each chip pulse train output from the optical pulse time spreader, which is generated by time-spreading each optical pulse constituting the input optical pulse train, is output on the time axis (2 × L × n _eff ). Arranged with periodicity given by / c. Therefore, the difference λ _R = λ _Bmax −λ _Bmin between the maximum wavelength λ _Bmax and the minimum wavelength λ _Bmin of the Bragg reflection wavelength λ _Bp set in the unit FBG of the p-th optical pulse time spreader is expressed by the equation (2 )
λ _R ≦ λ _s ² / (2 × L × n _eff ) (2)
Is set to the range given by.

That is, the λ _Rmax value is given by λ _Rmax = λ _s ² / (2 × L × n _eff ) as described below.
(c / λ _Bmin ) − (c / λ _Bmax ) = c / (2 × L × n _eff )
λ _Rmax / (λ _Bmin × λ _Bmax ) ≒ λ _Rmax / λ _s ² = 1 / (2 × L × n _eff )
That is, λ _Rmax = λ _s ² / (2 × L × n _eff ).

From the above discussion, for λ _{B1 to} λ _BU set in the first to U-th optical pulse time spreaders, the spacing Δλ _B between adjacent Bragg reflection wavelengths is expressed by the following equation (8):
Δλ _B = λ _Rmax / U = λ _s ² / (2 × L × n _eff × U) (8)
Given in.

When the optical pulse time spreading device of the embodiment of the present invention is used in an optical communication system, the Bragg reflection wavelength λ _Bp of the unit FBG provided in the p-th optical pulse time spreader is the input optical pulse train or the input around the central wavelength lambda _s of the wavelength spectrum of the chip pulse train, it takes integer times by different values of [Delta] [lambda] _B given by Δλ _B = λ _Rmax / U. Hereinafter, the Bragg reflection wavelength λ _Bp set in the unit FBG included in the p-th optical pulse time spreader may be simplified to be the wavelength λ _Bp assigned to the p-th optical pulse time spreader.

When the value of U, which is a channel identification parameter, is an odd number, the value of the Bragg reflection wavelength λ _B is λ _s −kΔλ _B ,..., Λ _s −2Δλ _B , λ _s −1Δλ _B , λ _s , Λ _s + 1Δλ _B , λ _s + 2Δλ _B ,..., Λ _s + kΔλ _B. That is, the wavelength λ _B1 assigned to the first optical pulse time spreader is λ _s −kΔλ _B , and thereafter, the above-mentioned wavelengths are sequentially assigned in order from the second optical pulse time spreader, and the U-th optical pulse time spread. The wavelength λ _BU assigned to the device is λ _s + kΔλ _B.

On the other hand, when the value U of the channel identification parameter is an even number, the value of the Bragg reflection wavelength λ _B is λ _s − (2k + 1) Δλ _B / 2,..., Λ _s −3Δλ _B / 2, λ _s −1Δλ _B / 2, λ _s + 1Δλ _B / 2, λ _s + 3Δλ _B / 2,..., Λ _s + (2k + 1) Δλ _B / 2. That is, the wavelength λ _B1 assigned to the first optical pulse time spreader is λ _s − (2k + 1) Δλ _B / 2, and thereafter, the above-described wavelengths are sequentially assigned from the second optical pulse time spreader. The wavelength λ _BU assigned to the U-th optical pulse time spreader is λ _s + (2k + 1) Δλ _B / 2.

As described above, the Bragg reflection wavelength λ _Bp set in the unit FBG included in the p-th optical pulse time spreader is expressed by the following equations (1a) and (1b), where λ _s is the peak wavelength of the wavelength spectrum of the input optical pulse.
λ _Bp = λ _s + k (Δλ _s / U) (1a)
(However, if U is an odd number, k is an integer that satisfies | k | <U / 2)
λ _Bp = λ _s + (2k + 1) (Δλ _s / 2U) (1b)
(However, when U is an even number, k is an integer satisfying | k | <U / 2)
Given in.

  As a specific example of the optical pulse time spreading device of the embodiment of the present invention, a case where the number of unit FBGs arranged in the optical pulse time spreading device is 32 and the number of codes (the number of channels U) is 16 is taken up. explain. The first to sixteenth optical pulse time spreaders are assigned with the first to sixteenth channels in a one-to-one correspondence. Accordingly, the wavelength assigned to the first to sixteenth optical pulse time spreaders is sometimes referred to as the wavelength assigned to the first to sixteenth channels.

  Referring to FIGS. 3A and 3B, the effective refractive index modulation of the SSFBG of the optical pulse time spreader constituting the optical pulse time spreading device of the embodiment of the present invention (hereinafter, simply referred to as refractive index modulation). Yes.) 3 (A) and 3 (B) are diagrams for explaining the refractive index modulation of SSFBG, and FIG. 3 (A) shows the position coordinates in the length direction of the optical fiber in which SSFBG is built. It is the figure which showed typically the mode of refractive index modulation for the direction. The unit FBG is schematically indicated by a rectangle, and the length of the side of the rectangle in the vertical axis direction is shown in proportion to the degree of refractive index modulation. Further, the length of the side of the rectangle in the horizontal axis direction indicates the length of the unit FBG. In FIG. 3 (A), the center line drawn in parallel with the horizontal axis indicates the average value of the refractive index of SSFBG.

  FIG. 3B is an enlarged view showing the state of refractive index modulation by taking out two of the unit FBGs. The amplitude of the corrugated curve indicates the magnitude of the refractive index modulation, and the center of the corrugated curve indicates the average value of the refractive index of SSFBG. The diffraction grating period Λ of the unit FBG is given by an interval between adjacent maxima of a wave-shaped curve that gives refractive index modulation.

Bragg reflection set in the unit FBG, _assuming that the peak wavelength λ _s of the wavelength spectrum of the input optical pulse is 1550 nm, the spacing L of the unit FBG is 1 mm, and the effective refractive index n _eff of the optical fiber constituting the SSFBG is 1.45 The difference λ _R = λ _Bmax −λ _Bmin between the maximum wavelength λ _Bmax and the minimum wavelength λ _Bmin of the wavelength λ _Bp is expressed by the above equation (2).
λ _R = 1550 ² / [2 × (1 × 10 ⁶ ) × 1.45 ≒ 0.8 (nm)
It becomes. That is, λ _Rmax = 0.8 (nm). When U = 16, the interval Δλ _B between adjacent Bragg reflection wavelengths is Δλ _B = λ _Rmax /U=0.8/16=0.05 nm based on the above equation (8).

Here, since the value of U, which is a channel identification parameter, is 16 and is an even number, the Bragg reflection wavelength assigned to the first to 16th channels is an integer satisfying | k | <16/2 = 8, k = 7, λ _s −15Δλ _B / 2, λ _s −13Δλ _B / 2, λ _s −11Δλ _B / 2, λ _s −9Δλ _B / 2, λ _s −7Δλ _B / 2, λ _s −5Δλ _B / 2 , Λ _s −3Δλ _B / 2, λ _s −1Δλ _B / 2, λ _s + 1Δλ _B / 2, λ _s + 3Δλ _B / 2, λ _s + 5Δλ _B / 2, λ _s + 7Δλ _B / 2, λ _s + 9Δλ _B / 2, λ _s + 11Δλ _B / 2, λ _s + 13Δλ _B / 2, and λ _s + 15Δλ _B / 2. That is, the Bragg reflection wavelength λ _s −15Δλ _B / 2 is given to the first channel, and then the Bragg reflection wavelength is given sequentially from the second channel to the 15th channel in the order described above, and the Bragg reflection wavelength λ is given to the 16th channel. _s + 15Δλ _B / 2 is given.

  In the SSFBG that is a component of the optical pulse time spreader that constitutes the optical pulse time spreader according to the embodiment of the present invention, the magnitude of the refractive index modulation of the unit FBG arranged in the central portion of the SSFBG is maximized. Apodized. With reference to FIG. 4, the distribution of the magnitude of the refractive index modulation of the unit FBGs sequentially arranged in the SSFBG will be described. In FIG. 4, the horizontal axis shows the unit FBG arranged at one end of the SSFBG as `` 1 '', the unit FBG arranged at the other end as `` 32 '', and the vertical axis shows the refractive index modulation of each unit FBG. The magnitude is shown as a percentage of the refractive index modulation intensity.

  The vertical axis in FIG. 4 normalizes the refractive index modulation of the 16th unit FBG and the 17th unit FBG arranged in the central part of the SSFBG to 1, and the refractive index modulation of the other unit FBGs. The magnitude is shown as a ratio of the refractive index modulation of the 16th unit FBG and the 17th unit FBG.

In Fig. 4, when the maximum local maximum of refractive index modulation from the first unit FBG to the 32nd unit FBG is connected with a smooth curve, the function that gives this curve is given by the following equation (9): It is done.
exp [-1 × ln2 × {2 × (M-32 / 2) / (B × 32)} ² ] = exp [-1 × ln2 × {2 × (M-16) /41.6} ² ] (9)
Here, exp is a symbol representing an exponential function, and ln2 represents a natural logarithm of 2. Further, M is a number in the arrangement order of the unit FBGs. For example, M = 1 in the first unit FBG and M = 32 in the 32nd unit FBG. Here, B = 1.3.

  Next, referring to FIG. 5 (A), FIG. 5 (B), FIG. 5 (C-1), FIG. 5 (C-2) and FIG. 5 (C-3), an optical pulse time spreader is configured. The refractive index modulation for each unit FBG, which is a component of the SSFBG, will be described. FIG. 5 (A) is a diagram schematically showing the state of refractive index modulation with the position coordinate in the length direction of the optical fiber in which the SSFBG is built in the horizontal axis direction. In FIG. 5 (A), an envelope that smoothly connects the maximum positions of the refractive index modulation is schematically shown. FIG. 5 (B) is an enlarged view showing the state of refractive index modulation by extracting two of the unit FBGs. The amplitude of the wavy curve shown by the solid line indicates the magnitude of the refractive index modulation. The diffraction grating period Λ of the unit FBG is given by an interval between adjacent maxima of a wave-shaped curve that gives refractive index modulation.

  5 (C-1) to 5 (C-3), for the sinusoidal refractive index modulation whose diffraction grating period is Λ shown in FIG. 5 (C-1), FIG. 5 (C-2) FIG. 5C shows that the apodized refractive index modulation shown in FIG. 5C is obtained by calculating the product of the Gaussian error function that gives the apodization shown in FIG. The refractive index modulation shown in FIG. 5C-3 is set as the refractive index modulation of the unit FBG.

  The Gaussian error function, which is a function that gives the apodization of the unit FBG, is given by the following equation (10).

exp [-1 × ln2 × {2 × (xD / 2) / (B × D)} ² ] (10)
Here, x is a value of a position coordinate set along the longitudinal direction (waveguide direction) of the optical fiber in which the SSFBG is formed, and D is the length of the unit FBG. Here, B = 0.5.

  As described above, as shown in FIG. 4, the refractive index modulation from the first unit FBG to the 32nd unit FBG is smoothly modulated, and each unit FBG is shown in FIG. By apodizing the refractive index modulation as described above, an effect of suppressing the sideband lobe of the wavelength spectrum of the output light output from the optical pulse time spreader can be obtained.

  In the SSFBG of the optical pulse time spreader that constitutes the optical pulse time spreader according to the embodiment of the present invention, in the OCDM / WDM hybrid multiplex system, the portion of the wavelength spectrum of the optical signal of the channel to which the adjacent wavelength is allocated In order to solve the problem of reduced communication accuracy due to crosstalk occurring between both channels due to overlapping, the periodic refractive index distribution structure forming each of the unit FBGs constituting the SSFBG The refractive index modulation of the periodic refractive index distribution structure is apodized with a sinc function.

  6 (A), 6 (B), 6 (C-1), 6 (C-2) and 6 (C-3), the periodic refraction forming each of the unit FBGs A case where the refractive index modulation of the periodic refractive index distribution structure is apodized with a sinc function will be described. FIG. 6 (A) is a diagram schematically showing the state of refractive index modulation with the position coordinate in the length direction of the optical fiber in which the SSFBG is built in the horizontal axis direction. In FIG. 6 (A), an envelope that smoothly connects the maximum positions of the refractive index modulation is schematically shown. In FIG. 6B, the amplitude of the wavy curve indicated by the solid line indicates the magnitude of the refractive index modulation. The diffraction grating period Λ of the unit FBG is given by an interval between adjacent maxima of a wave-shaped curve that gives refractive index modulation.

  6 (C-1), FIG. 6 (C-2) and FIG. 6 (C-3) are diagrams for explaining apodization by the sinc function of the unit FBG, and are shown in FIG. 6 (C-1). By calculating the product of the sinusoidal refractive index modulation whose grating period is Λ and the sinc function giving the apodization shown in FIG. 6 (C-2), the apodization shown in FIG. 6 (C-3) is obtained. The obtained refractive index modulation is obtained. The refractive index modulation shown in FIG. 6C-3 is set as the refractive index modulation of the unit FBG.

  The sinc function, which is a function that gives the apodization of the unit FBG, is given by the following equation (11).

sin [Q × {x- (D / 2)}] / [Q × {x- (D / 2)}] = sinc [Q × {x- (D / 2)}] (11)
However, when x = D / 2, sinc [Q × {x− (D / 2)}] = 1 is defined. Here, x is a position coordinate value set along the longitudinal direction (waveguide direction) of the optical fiber in which the SSFGB is formed, and D is the length of the unit FBG. Q is a positive integer. Here, the case of Q = 1, Q = 2, and Q = 3 was examined.

<Operating characteristics of optical pulse time spreading device>
With reference to FIG. 7 (A) to FIG. 7 (C), the maximum point of the refractive index modulation from the first unit FBG to the 32nd unit FBG is apodized by the above equation (10), The refractive index modulation of the periodic refractive index distribution structure forming each of the unit FBGs will also be described with respect to the reflection spectrum of the SSFBG that is apodized by the sinc function given by the above equation (11).

  The horizontal axis of FIGS. 7A to 7C shows the relative wavelength in units of nm with the center wavelength of the incident light spectrum wavelength being 0. Here, the center wavelength of the incident light spectrum wavelength is 1550 nm. That is, the coordinate indicated by 0 on the horizontal axis in FIGS. 7A to 7C means the wavelength of 1550 nm. Further, the arrangement interval L of the unit FBG and the length D of the unit FBG are both 1 mm.

  As is clear from FIGS. 7A to 7C, it can be seen that the bandwidth of the reflection spectrum of SSFBG increases as the value of Q increases. That is, when Q = 1, there are two significant peaks of the reflected spectrum intensity across the center wavelength of the incident light spectrum wavelength, whereas four when Q = 2, Q = 3 Is increased to six. From this, it is understood that by selecting the value of Q, it is possible to appropriately select the reflection spectrum band of the SSFBG that is a constituent element of the optical pulse time spreader.

  As shown in FIGS. 7A to 7C, the refractive index modulation of the periodic refractive index distribution structure forming each of the unit FBGs is reflected and output from the SSFBG that is apodized by the sinc function. The light intensity time waveform of each chip pulse has a shape given by the square of the sinc function used for the apodization described above.

Further, since the arrangement interval of the unit FBG is L, the interval t _c on the time axis of the adjacent chip pulses constituting the chip pulse train output by Bragg reflection from the SSFBG is t _c = 2 × L × It is given by n _eff / c. Since the repetition frequency of the chip pulse is given by 1 / t _c , the repetition frequency of the chip pulse is given by c / (2 × L × n _eff ). Here, since L = 1 mm and n _eff = 1.45, the repetition frequency of the chip pulse output by Bragg reflection from SSFBG is about 100 GHz.

<Encoding and Decoding by Optical Pulse Time Spreading Apparatus of Embodiment of the Present Invention>
With reference to FIGS. 8A to 8C, encoding and decoding operations by the optical pulse time spreading apparatus according to the embodiment of the present invention will be described.

  In FIG. 8C, the horizontal axis represents the time axis T. Further, in the vertical direction, N chip pulse trains obtained by converting each of the N chip pulses constituting the chip pulse train input to the decoder into N chip pulses by the decoder, These are shown in N stages, reflecting the relationship of the time delay output from the decoder. The chip pulse train shown as i = 0 is a chip pulse train generated by the decoder as the first chip pulse of the chip pulse constituting the chip pulse train output from the encoder. Similarly, for the chip pulse train indicated as i = 1 to (N-1), the second to Nth chip pulses of the chip pulse that are output from the encoder are generated as a chip pulse train by the decoder. It has been done. As shown in FIG. 8 (C), the chip pulse train indicated as i = 0 to (N-1) overlaps and interferes on the time axis. As a result, the optical pulse time spreader, which is a decoder, causes autocorrelation. Wave or cross-correlation wave is output.

8A to 8C, the SSFBG that constitutes the optical pulse time spreader is configured to include N unit FBGs, each unit FBG has a Bragg reflection wavelength λ _B , and the unit FBG arrangement interval. Is L. Here, when specifying the Bragg reflection wavelength generally set in the optical pulse time spreader without distinguishing the Bragg reflection wavelength set in each of the first to Uth optical pulse time spreaders, simply λ _B It shall represent.

  As shown in FIG. 8 (A), an optical pulse is input to optical circulator 40 from port 1 of optical circulator 40, output from port 2 of optical circulator 40, and an optical pulse time spreader (shown by a broken line in FIG. 8 (A)). Are enclosed in a square.). The optical pulse input to the optical pulse time spreader is generated as a chip pulse train, and this chip pulse train is input from the port 2 of the optical circulator 40 to the optical circulator 40 and output from the port 3.

The phase difference between adjacent chip pulses of the chip pulse train output from the optical pulse time spreader shown in FIG. 8 (A) is 2π × 2 × L × n _eff / λ _B = 2π × (m + (1/2)) The Bragg reflection wavelength λ _B of each unit FBG and the arrangement interval L of the unit FBG are set (where m is an integer greater than or equal to 0).

When the center wavelength of the wavelength spectrum of the input optical pulse is λ _s , the phase difference φ between chip pulses that are Bragg reflected from the adjacent unit FBG is given by the following equation (12).
φ (Δλ) = 2π × 2 × L × n _eff / λ _s = 2π × (m + (1/2)) × λ _B / λ _s
= 2π × (m + (1/2)) × (1+ (Δλ / λ _s ))
= Π + {2π × (m + (1/2)) × (Δλ / λ _s ) (12)
Here, Δλ = λ _B −λ _s . That is, the value of φ (Δλ) is determined by the magnitude of Δλ. When λ _B = λ _s , φ = π.

  When the input optical pulse is input to the optical pulse time spreader that is an encoder, a chip pulse train composed of a number of chip pulses equal to the number of unit FBGs is generated. As described above, in the diagrams shown in FIGS. 8A and 8B, the number of unit FBGs is N.

  The phase difference between adjacent chip pulses of the chip pulse constituting this chip pulse train propagates through the optical waveguide with the value given by the above equation (12), and is transmitted to the optical pulse time spreader as a decoder. Entered. With reference to FIG. 8B, a process in which the above-described chip pulse train is decoded in the decoder will be described. Here, it is assumed that the chip pulse train encoded and generated by the i-th optical pulse time spreader is decoded by the j-th optical pulse time spreader. Here, i and j are integers that satisfy 0 ≦ i ≦ N−1 and 0 ≦ j ≦ N−1, respectively.

The chip pulse encoded and generated by the i th optical pulse time spreader and given the phase Δφ _e is decoded by the j th optical pulse time spreader and given with the phase Δφ _d added further. Assuming that the chip pulse is based on the first chip pulse in the chip pulse train,
Δφ _ij = i × Δφ _e + j × Δφ _d (13)
Has a phase Δφ _ij given by

  As shown in FIG. 8C, each of the N chip pulses constituting the chip pulse train input to the decoder is converted into N chip pulses by the decoder. As a result, a total of (N × N) chip pulses are generated. Each chip pulse of the N chip pulses generated by encoding becomes a chip pulse train composed of N chip pulses again by the decoder, and the autocorrelation wave is caused by interference caused by overlapping these chip pulse trains on the time axis. Or a cross-correlation wave is generated.

The phase Δφ _ij (T) of the chip pulses overlapping on the time axis at time T = i + j is expressed by the following equation (14)
Δφ _ij (T) = T × Δφ _d + (Δφ _e −Δφ _d ) × i (14)
Given in.

When the encoder and the decoder are optical pulse time spreaders having the same structure, that is, when Δφ _e = Δφ _d = Δφ, Δφ _ij (T) = T × Δφ and does not depend on the value of i At time T, all chip pulses interfere with each other in the same phase. That is, a very large peak is formed at time T, and an autocorrelation wave is formed.

On the other hand, if the encoder and the decoder are optical pulse time spreaders having different structures, that is, if Δφ _e and Δφ _d are different, chip pulses overlapping on the time axis at time T = i + j As is clear from the above equation (14), the phase Δφ _ij (T) differs depending on the value of i, so that the values of Δφ _ij (T) differ from each other, and the peaks of the chip pulses cancel each other due to interference. Not formed, but a cross-correlation wave is formed.

  As described above, coding / decoding is realized by arranging two optical pulse time spreading apparatuses of the embodiment of the present invention having the same configuration and using them as an encoder and a decoder, respectively. The optical pulse time spreader having the same configuration means that the configurations of the first to U-th optical pulse time spreaders included in each optical pulse time spreader are composed of equal combinations. That is, it means that the configuration of the p-th optical pulse time spreader of one optical pulse time spreading device is equal to the configuration of the p-th optical pulse time spreader of the other optical pulse time spreading device. Here, p is any integer that satisfies 1 ≦ p ≦ U.

  The encoding step assigns the first to U-th channels to the first to U-th optical pulse time spreaders included in the optical pulse time spreading device of the embodiment of the present invention, and encodes the transmission signals of the respective channels. This is realized.

  The multiplexing step is realized by combining the first to U-th chip pulse trains output from the first to U-th optical pulse time spreaders with an optical multiplexer or the like.

  In the decoding step, for example, in the case of the configuration shown in FIG. 1 (B), the multi-chip pulse train obtained by multiplexing the first to U-th chip pulse trains described above is U-divided (intensity division), and this is a decoder. This is realized by supplying each of the first to Uth optical pulse time spreaders included in the optical pulse time spreader of the embodiment of the invention. An auto-correlation wave is generated in an optical pulse time spreader of the same structure as the optical pulse time spreader used for encoding, and a cross-correlation wave is generated in an optical pulse time spreader of a different structure. Thus, a code division multiplexing system is realized.

  Also, as described above, different spectrum bands are assigned to the first to U-th chip pulse trains, respectively, so in the communication method using this optical pulse time spreading device as an encoder and a decoder, WDM is also being executed at the same time. That is, according to the optical multiplex transmission system using the optical pulse time spreading device of the embodiment of the present invention, it is possible to realize optical communication by the OCDM / WDM hybrid multiplex system.

<Utilization of Optical Pulse Time Spreading Device of Embodiment of the Present Invention>
By narrowing the half width of the time waveform of the input optical pulse input to the optical pulse time spreading device of the embodiment of the present invention, the half width of the wavelength spectrum of the input optical pulse is widened, and the wavelength band of the wavelength spectrum of the input optical pulse It is possible to cover the wavelength band of the reflection spectrum of SSFBG which is a component of the optical pulse time spreader. In this case, if the center wavelength of the input optical pulse is near the center wavelength of the reflection spectrum band of the SSFBG, even if the wavelength of the light source that generates the optical carrier wave of the input optical pulse fluctuates slightly, the optical signal of multiple channels is output. Optical communication by the OCDM / WDM hybrid multiplex system for optical multiplex transmission becomes possible.

In optical communication based on the OCDM / WDM hybrid multiplex system realized by using the optical pulse time spreading device of the embodiment of the present invention as an encoder and decoder, an input optical pulse constituting an optical signal of this optical communication Even if the wavelength of the optical carrier varies slightly, the encoding and decoding operations are not affected. That is, even if the wavelength of the optical carrier wave of the input optical pulse fluctuates, it does not affect the encoding and decoding operations unless the Bragg reflection wavelength λ _B of the unit FBG of the SSFBG constituting the optical pulse time spreader fluctuates. Absent.

  Here, the wavelength of the optical carrier of the input optical pulse varies slightly. The optical carrier of the input optical pulse does not violate the condition that the wavelength band of the wavelength spectrum of the input optical pulse covers the wavelength band of the reflection spectrum of SSFBG. This means that the wavelength of fluctuates.

The wavelength of the output light of the semiconductor laser diode, which is a light source for generating the optical carrier wave of the input optical pulse, varies depending on the temperature of the semiconductor laser diode, and the variation is 0.8 nm per 1 ° C, whereas SSFBG The amount of fluctuation due to the temperature of the Bragg reflection wavelength λ _B of the unit FBG is 0.01 nm per 1 ° C and 1/80, which is very small. That is, in the optical communication by the OCDM / WDM hybrid multiplex system realized by using the optical pulse time spreading apparatus of the embodiment of the present invention as an encoder and a decoder, the reliability of the encoding and decoding operations is high. It can be seen that it is much higher than conventional optical communication using the same type of WDM.

  Further, each of the first to U-th optical pulse time spreaders of the optical pulse time spreading device of the embodiment of the present invention is configured to include an SSFBG, and each of the unit FBGs configuring the SSFBG is a diffraction grating. Is reflected as a product of the sinc function as a curve representing intensity and phase, and is reflected from the refractive index modulation region enveloped by a peak curve taking the maximum value of the absolute value of this sinc function. The phase difference between the Bragg reflected light and the Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve taking the maximum of the absolute value of the sinc function in the adjacent unit FGB is the half wavelength of the Bragg reflected light. It is set to an odd multiple of.

  Compared to the case where the apodization is not applied to the refractive index modulation of the unit FBG and the case where the apodization based on the Gaussian error function is applied to the refractive index modulation of the unit FBG, the optical pulse time spreader satisfying this condition is used. The characteristics of the wavelength spectrum of the output light to be output will be described with reference to FIGS. 9 (A) to 9 (C).

  In each of FIGS. 9A to 9C, the horizontal axis indicates the wavelength in units of nm, and the vertical axis indicates the reflectance in units of dB.

  In FIG. 9 (A) to FIG. 9 (C), the wavelength band divided by a broken line parallel to the vertical axis indicates a wavelength band treated as a single wavelength in normal WDM optical multiplex communication, that is, a wavelength grid.

  As shown in FIG. 9 (A), the wavelength spectrum of the output light output from the optical pulse time spreader including the SSFBG formed by the unit FBG that is not apodized for the refractive index modulation is the wavelength of the central portion. Peaks indicated by A1 and A2 exist in adjacent wavelength bands on the short wavelength side of the grid, and peaks indicated by A7 and A8 exist in adjacent wavelength bands on the long wavelength side. The wavelength components of these peaks A1, A2, A7, and A8 are noise components at the time of decoding, and the magnitude of this noise component is estimated to be -10 dB or more. The magnitude of this noise cannot be ignored in normal WDM optical multiplex communication.

  As shown in FIG. 9 (B), the wavelength spectrum of the output light output from the optical pulse time spreader including the SSFBG formed by the unit FBG in which the refractive index modulation is apodized based on the Gaussian error function is One peak indicated by B2 exists in the adjacent wavelength band on the short wavelength side of the wavelength grid in the central portion, and one peak indicated by B7 exists in the adjacent wavelength band on the long wavelength side. The magnitude of the noise component is estimated to be about -20 dB, and the magnitude of the noise is much smaller than that in the case where the above-described refractive index modulation is not apodized.

  On the other hand, the wavelength spectrum of the output light output from the optical pulse time spreader shown in FIG. 9 (B) is also present on the long wavelength side and the peak indicated by B3 existing on the short wavelength side in the central portion of the wavelength grid. The peak size indicated by B6 is smaller than the peak size indicated by B4 and B5 present in the central portion of the central wavelength grid. This means that the signal component is small, and even if the noise component is small, this is a characteristic that does not contribute to the improvement of the S / N ratio of the decoded signal.

  In the optical pulse time spreader constituting the optical pulse time spreading device of the embodiment of the present invention, the refractive index modulation of the unit FBG forming the SSFBG is apodized based on the sinc function. In this case, as shown in FIG. 9 (C), the wavelength spectrum of the output light output from the optical pulse time spreader is C1 and C2 in the adjacent wavelength band on the short wavelength side of the central wavelength grid. In the adjacent wavelength band on the long wavelength side, there are peaks indicated by C7 and C8, but both peaks are in the wavelength spectrum shown in FIGS. 9 (A) and 9 (B). Smaller than similar peaks (A1 and A2, B2, A7 and A8, B7). In addition, the peaks indicated by C3, C4, C5, and C6 existing in the central wavelength grid are large and all have the same size.

  As described above, according to the wavelength spectrum of the output light output from the optical pulse time spreader that constitutes the optical pulse time spreader according to the embodiment of the present invention, the adjacent short wavelength side of the wavelength grid in the central portion and Since the peak existing in the wavelength band on the long wavelength side is small, it means that the magnitude of the noise component is small. In addition, the peaks indicated by C3, C4, C5, and C6 existing in the central wavelength grid are large and all have the same size, which means that the signal component is sufficiently large and decoded. It shows that the S / N ratio of the signal can be secured sufficiently larger than the wavelength spectrum shown in FIGS. 9 (A) and 9 (B).

  Next, referring to FIGS. 10 (A) and 10 (B), six single wavelength bands, which are wavelength grids in conventional WDM, are included, and each single wavelength band includes 16 channels of OCDM / A description will be given of the magnitude of crosstalk that occurs when the base portions of adjacent single wavelength bands overlap when a WDM hybrid multiple channel is set. There are 16 OCDM / WDM hybrid multiplex channels in each adjacent single wavelength band. Therefore, the crosstalk generated by overlapping the base portions of the wavelength spectrum of the optical signal of the channel to which the adjacent wavelength component is assigned refers to the 16 channels included in the adjacent single wavelength band and the other adjacent single wavelength band. It means crosstalk that occurs between 16 channels included in one wavelength band.

  In FIGS. 10A and 10B, the horizontal axis indicates the wavelength in an arbitrary scale, and the vertical axis indicates the reflectance in dB. The six single wavelength bands are indicated as WDM1 to WDM6. The magnitude of crosstalk from other WDMs in the WDM3 region is shown as hatched areas in FIGS. 10 (A) and 10 (B).

  It is clear when comparing the size of the overlapping part of the skirts of adjacent single wavelength bands in the single wavelength band shown by WDM1 to WDM6 shown in FIG. 10 (A) and FIG. 10 (B), that is, the size of crosstalk. It can be seen that the crosstalk in the optical pulse time spreader including the SSFBG formed by the unit FBG in which the apodization based on the sinc function shown in FIG.

  This comprises an SSFBG formed of a unit FBG on which apodization based on the sinc function of the optical pulse time spreader constituting the optical pulse time spreader of the embodiment of the present invention is applied to the refractive index modulation. It is based on the feature. By having such a feature, the shape of the wavelength spectrum of the Bragg reflected light reflected from the SSFBG which is each component of the first to U-th optical pulse spreaders becomes a rectangular shape in the wavelength space. Therefore, as shown in FIG. 10 (A), each single wavelength band distinguished as shown as WDM1 to WDM6 does not take a bell-shaped shape in which the base of the wavelength spectrum becomes long in the wavelength space, and FIG. As shown in B), it is understood that the crosstalk is reduced by the rectangular shape in the wavelength space.

12: Optical splitter
14-1 to 14-U, 24-1 to 24-U: 1st to Uth optical circulator
16-1 to 16-U, 26-1 to 26-U: 1st to Uth optical pulse time spreader
40, 42: Optical circulator

Claims (4)

The first to second input light pulses are output as a chip pulse train (N is an integer of 2 or more) composed of N chip pulses from the first to the Nth chip pulse, which are sequentially arranged on the time axis by time diffusion. An optical pulse time spreader comprising a U optical pulse time spreader (U is an integer equal to or greater than 1 satisfying U ≦ N),
Each of the first to Uth optical pulse time spreaders includes a superstructured fiber black grating (SSFBG) including N unit fiber black gratings (FBGs). Has been
The p-th optical pulse time spreader (p is an integer from 1 to U) so that the spectra of the first to U-th chip pulse trains respectively output from the first to U-th optical pulse time spreaders are different from each other. In each of these, an interval between the unit FBGs arranged adjacent to each other, and a Bragg reflection wavelength of the unit FBG in the p-th optical pulse time spreader are set, and each of the unit FBGs is subjected to refractive index modulation. The refractive index modulation as a diffraction grating is given as a function obtained by multiplying the sinc function, and from the refractive index modulation region enveloped by the peak curve taking the maximum maximum value of the sinc function giving an envelope connecting the maximum of the function The phase difference between the reflected Bragg reflected light and the Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve taking the maximum maximum value of the sinc function in the adjacent unit FGB An optical pulse time spreading device, wherein the optical pulse time spreading device is set to an odd multiple of a half wavelength of the lag reflected light, and the first to U-th optical pulse time spreaders are connected in parallel or in series. The first to second input light pulses are output as a chip pulse train (N is an integer of 2 or more) composed of N chip pulses from the first to the Nth chip pulse, which are sequentially arranged on the time axis by time diffusion. An optical pulse time spreader comprising a U optical pulse time spreader (U is an integer equal to or greater than 1 satisfying U ≦ N),
Each of the first to Uth optical pulse time spreaders includes a superstructured fiber black grating (SSFBG) including N unit fiber black gratings (FBGs). Has been
The p-th optical pulse time so that the spectrums of the first to U-th chip pulse trains respectively output from the first to U-th optical pulse time spreaders are different from each other and the energy of the chip pulse train is equal. In each of the diffusers (p is an integer from 1 to U), the interval between the unit FBGs arranged adjacent to each other and the Bragg reflection wavelength of the unit FBG in the p-th optical pulse time spreader are set. Each of the unit FBGs is provided with a refractive index modulation as a function obtained by multiplying a refractive index modulation as a diffraction grating by a sinc function, and a maximum maximum value of the sinc function giving an envelope connecting the maximums of the function. Bragg reflected light reflected from the refractive index modulation region enveloped by the peak curve taking, and the bending enveloped by the peak curve taking the maximum maximum value of the sinc function in the adjacent unit FGB The phase difference of the Bragg reflected light reflected from the refractive index modulation region is set to an odd multiple of the half wavelength of the Bragg reflected light,
The optical pulse time spreading device, wherein the first to Uth optical pulse time spreaders are connected in parallel or in series.   Each unit FBG provided by the SSFBG has a maximum maximum value of the refractive index modulation of the unit FBG sequentially increased along the length direction of the optical fiber, and becomes maximum at the center position of the SSFBG, and 3. The maximum local maximum value of the refractive index modulation of the unit FBG is sequentially set to decrease along the length direction of the optical fiber when the center position is passed. An optical pulse time spreading device according to claim 1. The Bragg reflection wavelength λ _Bp set in the unit FBG included in the p-th optical pulse time spreader is expressed by the following equations (1a) and (1b), where λ _s is the peak wavelength of the wavelength spectrum of the input optical pulse.
λ _Bp = λ _s + k (Δλ _s / U) (1a)
(However, if U is an odd number, k is an integer that satisfies | k | <U / 2)
λ _Bp = λ _s + (2k + 1) (Δλ _s / 2U) (1b)
(However, when U is an even number, k is an integer satisfying | k | <U / 2)
Given in
A difference λ _R = λ _Bmax −λ _Bmin between the maximum wavelength λ _Bmax and the minimum wavelength λ _Bmin of the Bragg reflection wavelength λ _Bp set in the unit FBG is expressed by the following equation (2)
λ _R ≦ λ _s ² / (2 × L × n _eff ) (2)
And λ _s is given by the following equation (3)
λ _Bmin ≦ λ _s ≦ λ _Bmax (3)
Is set to the range given by
The phase difference φ of the chip pulse reflected from the adjacent unit FBG of the p-th optical pulse time spreader is expressed by the following formula (4), where m is an integer of 0 or more.
φ = (2m + 1) (λ _Bp / 2) (4)
As given in
In the p-th optical pulse time spreader, an interval between adjacent unit FBGs and a Bragg reflection wavelength of the unit FBG in the p-th optical pulse time spreader are set. 4. The optical pulse time spreading device according to any one of 1 to 3.

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