JP4479719B2 - Optical pulse time spreading device - Google Patents

Description

  The present invention relates to an optical pulse time spreading device such as an OCDM encoder or an OCDM decoder that converts an input optical pulse into an encoded pulse train indicating an optical phase code.

  Optical code division multiplexing (OCDM) is known as an optical multiplexing technique for transmitting optical pulses of a plurality of channels through a single optical fiber. OCDM is an optical multiplexing technique in which an optical pulse is encoded with a different code for each communication channel on the transmission side, and decoded on the reception side to return to the original optical pulse (see, for example, Non-Patent Document 1). As encoding in OCDM, a phase encoding method using a binary phase code is known (for example, see Non-Patent Document 2). Further, as the OCDM encoder or the OCDM decoder, a passive optical element that does not consume power, such as a superstructure fiber Bragg grating (SSSFBG), can be used. The fiber Bragg grating (FBG) is a filter in which a Bragg diffraction grating having a periodic refractive index modulation is formed on an optical fiber core, and is a filter that reflects light of a specific wavelength (see, for example, Non-Patent Document 3). . In the OCDM encoder or the OCDM decoder, a plurality of phase shift units are formed at a desired position in the refractive index modulation of the FBG, and the number of phase shift units in the FBG refractive index modulation is determined by the code length and the code pattern. The position changes. It is possible to use a transversal filter structure PLC (for example, see Non-Patent Document 3) or AWG (for example, see Non-Patent Document 4) as an optical pulse time spreading device, but the optical loss is lower than that of SSFBG. It is large and it is difficult to downsize the entire device.

  FIG. 1 shows encoding by which an encoder 201 using SSFBG generates a binary phase code encoded pulse train (sequence of chip pulses 111 to 114) from an input optical pulse 101, and encoding by a decoder 203 using SSFBG. It is a figure for demonstrating the principle of the decoding which reproduces | regenerates 121 optical pulses from a generalized pulse train. FIG. 1 shows a case where the FBG encoder 201 and the FBG decoder 203 handle a binary phase code having a code length of “4”. Each of the FBG encoder 201 and the FBG decoder 203 includes four unit FBGs ( Unit diffraction grating). Each unit FBG has the same relative phase of the unit FBG (phase amount 0) or different relative phase (phase amount π) according to the sign. As shown in FIG. 1, when the optical pulse 101 is input to the FBG encoder 201, for example, four chip pulses 111 to 114 are generated with a time difference. This is also referred to as the input light pulse 101 being diffused. Here, the four chip pulses 111 to 114 each have a relative optical phase of <0, 0, π, 0>. In FIG. 1, a chip pulse (phase 0) whose optical phase is not shifted is drawn by a solid line, and a chip pulse (phase π) whose optical phase is relatively shifted by π is drawn by a broken line. When the chip pulses 111 to 114 are input to the FBG decoder 203 via the optical cable 202, four chip pulses 111a to 111d, 112a to 112d, 113a to 113d, 114a to the chip pulses 111 to 114, respectively. 114d occurs with a time difference (that is, a total of 16 chip pulses 111a to 111d, 112a to 112d, 113a to 113d, and 114a to 114d are generated).

  The FBG decoder 203 shown in FIG. 1 is configured such that the optical phase of the second chip pulse from the left (second from the last in time) is shifted by π, and is reflected by the FBG decoder 203. The obtained chip pulses have an optical phase of <0, π, 0, 0> or <π, 0, π, π> relative to each other. Some of the 16 chip pulses 111a to 111d, 112a to 112d, 113a to 113d, and 114a to 114d output from the FBG decoder 203 overlap each other. When the overlapping chip pulses are in phase, they are strengthened. Negate each other. As shown in FIG. 1, when the codes of the FBG encoder 201 and the FBG decoder 203 are the same, four chip pulses 111d, 112c, 113b, 114a having the same phase are overlapped and strengthened at a specific time, and at other times. Since in-phase and out-of-phase light pulses overlap and cancel each other, an optical pulse 121 having a high intensity peak occurs at a specific time. Further, when the codes of the FBG encoder and the FBG decoder are different, four in-phase pulses do not overlap at a specific time, so that an optical pulse having a high intensity peak does not occur at a specific time.

FIGS. 2A and 2B are diagrams for explaining the periodic refractive index modulation structure and the phase shift unit 304 of the core 302 of the optical fiber 301 of the conventional FBG encoder (or FBG decoder) 303. FIG. 2 (c) is a partially enlarged view of FIG. 2 (b). The diffraction grating formed in the core 302 is composed of a periodic refractive index modulation Δn. The diffraction grating period Λ is the Bragg reflection formula λ = 2 × n _eff × Λ (1) at the desired reflection wavelength λ.
Follow. Here, n _eff is the effective refractive index of the optical fiber core.

The refractive index modulation Δn of the FBG encoder 303 shown in FIGS. 2A to 2C has a structure in which a plurality of phase shift units 304 are provided according to the code. The FBG encoder 303 has seven phase shift units 304. The FBG encoder 303 is a binary phase code <0, 0, 0, π, π, π, π, 0, π, 0, π, π, 0, 0, π>.
Are generated. As shown in FIG. 3, this binary phase code has seven phase shift portions with a phase shift amount of 0.5.

  In the FBG encoder having a high reflectivity, the further the stage after the FBG encoder, the greater the attenuation of the input signal due to the loss due to the reflection of the optical signal at the previous stage, and therefore the amount of signal reflection by the unit FBG at the subsequent stage is small. As a countermeasure, the reflectance is increased as the unit FBG in the subsequent stage is made, and the reflection amount of the optical signal from the entire FBG region is made uniform (see, for example, Non-Patent Document 5).

  Further, in the FBG encoder, when an optical pulse is reflected by the unit FBG, the optical pulse spreads, and the reflected light pulses of adjacent unit FBGs overlap each other, causing interference between the pulses. As a result, the encoded pulse train has a pulse intensity distribution having a large variation. When the phase code inversion of <..., 0, π, 0, π,. On the contrary, if the same phase code is repeated like <..., 0, 0, 0, 0, ...> or <..., π, π, π, π, ...> The pulse intensity of becomes very large. That is, the pulse intensity distribution and pulse intensity of the encoded pulse train depend on the code arrangement. Therefore, when the phase code of the adjacent unit FBG is the same as <0, 0> or <π, π>, the refractive index modulation of the adjacent unit FBG has a phase difference of 0.25 * Λ, and If the phase code of the matching unit FBG is different from <0, π> or <π, 0>, the refractive index modulation should have a phase difference of 0.75 * Λ (ie, −0.25 * Λ). Therefore, a structure for reducing interference between chip pulses has been proposed (see, for example, Patent Document 1). According to this proposal, although interference between chip pulses occurs, the intensity distribution of the encoded pulse train is averaged by the code arrangement, and the dependence of the code arrangement on the chip pulse intensity can be reduced.

Hideyuki Tonobayashi, “Optical Code Division Multiplexing Network”, Applied Physics, Vol. 71, No. 7, 2002, pp. 853-859 Akihiko Nishiki et al., “Development of phase encoder for OCDM using SSFBG”, IEICE Tech., Technical Report of IEICE. 0FT2002-66 (2002-11), pp. 13-18 Naoya Wada et al. , “A 10 Gb / s Optical Code Division Multiplexing Using 8-Chip Optical Bipolar Code and Coherent Detection”, Journal of Lightwave Technology. 17, no. 10, pp. 1758-1765, 0 ctober 1999. Jing Cao et al. "Special Encoding and Decoding of Monolithic InP OCDMA Encoder", Paper We 3.6.6, Vol. 3, ECOC 2005. Koji Matsushima et al. , "Experimental Demonstration of Performance Improvement of 127-Chip SSFBG En / Decoder using Apodization Technology", IEEE Photonoltechnol. 16, no. 9, pp. 2192-2194, September 2004. JP 2006-197067

  However, in the FBG encoder, when the optical pulse is reflected by the unit FBG, the optical pulse spreads, the reflected light pulses of adjacent unit FBGs overlap, causing interference between the optical pulses, and the encoding of FIG. An ideal encoded pulse train as shown in the decoding principle cannot be obtained. FIGS. 4A to 4D are diagrams for explaining interference between reflected light pulses by two adjacent unit FBGs. As shown in FIG. 4A, when an optical pulse 131 is input to an FBG 240 having two unit FBGs, the optical pulse 142 reflected by the unit FBG 242 adjacent to the optical pulse 141 reflected by the unit FBG 241 is different in time. However, as shown in FIG. 4B, a part of the two reflected light pulses 141 and 142 overlap each other. When the grating phase difference between adjacent units FBGs 241 and 242 is π, the reflected light pulses 141 and 142 have a light phase difference π. In this case, as shown in FIG. 4C, the overlapping of the reflected light pulses cancels each other, and the pulses are deformed to reduce the intensity. When there is no grating phase difference between adjacent unit FBGs, the reflected light pulses 141 and 142 have the same phase, and as shown in FIG. 4D, the reflected light pulses overlap each other and the light pulses are combined. . These two interferences also depend on the pulse width of the input light. The narrower the pulse width of the input light, the smaller the interference area. As a result, in the encoded pulse train by the FBG encoder having the refractive index modulation structure of the conventional example, interference occurs between the reflected light pulses of the unit FBG.

FIGS. 5A and 5B are diagrams showing an encoded pulse train by a FBG encoder having a code length of “31” having a conventional refractive index modulation structure. FIG. 5A shows the input optical pulse width. In the case of 25 ps, FIG. 5B shows the case where the input optical pulse width is 6.3 ps. This FBG encoder is a 31-chip binary phase code <0, π, π, π, 0, 0, 0, π, π, π, 0, π, π, 0, 0, 0, 0, 0, 0, π, π, π, π, 0, π, 0, π, π, 0,0,0>
It is a structure corresponding to. The encoded pulse train in FIG. 5A and the encoded pulse train in FIG. 5B are each composed of 31 encoded pulses, but adjacent optical pulses overlap each other. When the pulse width of the input optical pulse is 6.3 ps (FIG. 5B), the intensity distribution with a smaller variation in the peak intensity of the optical pulse is shown, and when the pulse width of the input optical pulse is 25 ps ( Compared to the ideal optical pulse train as compared with FIG. However, the intensity of the encoded pulse when the pulse width of the input optical pulse is 6.3 ps (FIG. 5B) is the same as the code when the pulse width of the input optical pulse is 25 ps (FIG. 5A). It is smaller than the intensity of the activation pulse. This indicates that the loss is large when the pulse width of the input light pulse is 6.3 ps (FIG. 5B).

  Therefore, the present invention has been made to solve the above-described problems of the prior art, and an object of the present invention is to provide an optical pulse time that can generate a chip pulse train with low loss and less interference between adjacent chip pulses. It is to provide a diffusion device.

  The optical pulse time spreading device of the present invention is formed by an optical waveguide to which an optical pulse is input and the same periodic refractive index modulation structure formed in the optical waveguide, each of which is continuous in the waveguide direction of the optical waveguide. A plurality of unit diffraction gratings, and a phase shift unit that is provided in the optical waveguide between the plurality of unit diffraction gratings and that shifts the phase of the chip pulse, and sequentially arranges the optical pulses on a time axis. An optical pulse time spreading device for converting into a sequence of chip pulses, wherein an interval is provided between the plurality of unit diffraction gratings, and the length of the interval is set in the waveguide direction of one unit diffraction grating. It is characterized by having a length equal to or longer than the unit diffraction grating length.

<< 1 >> Configuration of Embodiment FIGS. 6A and 6B show the refraction of the optical fiber core 2 of the FBG encoder (or FBG decoder) 3 which is an optical pulse time spreading device of the embodiment of the present invention. It is a figure for demonstrating a rate modulation structure and the phase shift part 4, FIG.6 (c) is a partially expanded view of the same figure (b). The optical pulse time spreading apparatus according to the embodiment is, for example, an SSFBG encoder or decoder used for OCDM.

  As shown in FIG. 6A, the FBG encoder 3 includes an optical fiber 1 to which an optical pulse is input, and a plurality of unit FBGs (unit diffraction gratings) formed on the core (optical waveguide) 2 of the optical fiber 1. ) 5 and a phase shift unit 4 formed in the core 2 between the plurality of unit FBGs. The plurality of units FBG5 are formed by a periodic refractive index modulation structure continuous in the waveguide direction of the core 2 (longitudinal direction of the optical fiber 1), reflect the input light pulse, and are sequentially arranged on the time axis. To an encoded pulse (a sequence of chip pulses). The phase shift unit 4 is provided between the plurality of units FBG 5 and shifts the phase of the chip pulse. The phase shift unit 4 exists as a part of the interval between the plurality of units FBG 5, that is, in a region where no grating is formed. Actually, when the interval 6 between the plurality of units FBG5 has a length that gives a phase difference, the phase shift unit 4 exists, and the interval between the plurality of units FBG5 does not give a phase difference. In this case, the phase shift unit 4 does not exist.

As shown in FIG. 6 (b), in the FBG encoder of the present embodiment, there is an interval (a portion where the refractive index is constant) between the plurality of unit FBGs (that is, the periodic refractive index modulation unit) 5. 6 is provided, the length _{L SP} of the spacing 6, the unit FBG length of the waveguide direction is a length of one unit FBG (unit diffraction grating _length) is set to more than _{L GR.}

Further, when a plurality of unit segments which are sections obtained by dividing the core 2 of the optical fiber 1 into equal lengths in the waveguide direction are determined, one unit FBG 5 and one unit FBG 5 are included in each of the plurality of unit segments. The unit FBG length L _GR is set to be ½ or less of the unit segment length L _SEG that is the length of one unit segment in the waveguide direction.

The FBG encoder 3 shown in FIGS. 6A and 6B has a binary phase code <0, 0, 0, π, π, π, π, 0, π, 0, π with a code length of “15”. , Π, 0, 0, π>
It is constituted by. The FBG encoder 3 shown in FIGS. 6A and 6B is formed by a diffraction grating of the core 2 of the optical fiber 1, and the refractive index modulation structure of the unit FBG 5 has a Bragg reflection equation λ = 2n _eff. × Λ
Follow. Here, Λ is the refractive index modulation period of the diffraction grating, and is about 540 nm in the present embodiment. In the present embodiment, the unit segment length L _SEG is about 2.4 mm, and the unit FBG length L _GR is about 1.2 mm.

  Further, the FBG encoder 3 of the present embodiment is configured to have the phase shift unit 4 between the unit FBGs 5 according to the code. The shift amount between the unit FBGs 5 is the refractive index of the adjacent unit FBG 5 when the phase code of the adjacent unit FBG 5 is the same as <0, 0> or <π, π> with respect to the refractive index modulation period Λ. The modulation has a phase difference of 0, and the phase shift unit 4 does not exist. When the phase code of the adjacent unit FBG is different from <0, π> or <π, 0>, the refractive index modulation has a phase difference of 0.5 * Λ, as in FIG. The phase shift unit 4 exists.

  FIG. 7A is a diagram showing the waveform of the optical pulse 11 input to the FBG encoder (or FBG decoder) according to the embodiment of the present invention and the refractive index modulation structure of the optical fiber core. ) Is a diagram illustrating waveforms of the reflected light pulses 31 and 33.

The reflection of the input light pulse 11 occurs continuously at each point in the unit FBGs 21 and 23. As shown in FIGS. 7A and 7B, the input light pulse 11 is continuously reflected by the gratings of the unit FBGs 21 and 23. The reflection of light differs in the reflection position on the input side 21a of the unit FBG 21 and the reflection position on the end side 21b. Therefore, the output light pulse 31 causes a time difference between the reflected light pulse 31a on the input side 21a and the reflected light pulse 31b on the termination side 21b. Here, the output light pulse width W _out (unit: time) can be obtained by the following equation (2) using the input light pulse width W _in (unit: time).
W _out = W _in + (2 × L _GR × n _f / C) (2)
Here, n _f represents the refractive index of the fiber core, and C represents the speed of light.

Therefore, the pulse width spread ΔW can be obtained by the following equation (3).
ΔW = W _out −W _in = 2 × L _GR × n _f / C (3)

In order to reduce the overlapping of the chip pulses adjacent the twice the length L _SP spacing 22 of adjacent unit FBG (i.e., the optical path length of the reciprocating), the length ΔW corresponding spread ΔW pulse width × must be at least C / _{n f.} That is, the following formula (4)
2 × L _SP ≧ ΔW × C / n _f (4)
It is necessary to satisfy.

By substituting equation (3) into equation (4), the right side of equation (4) becomes
2 × L _GR × n _f / C × C / n _f = 2 × L _GR
And the following equation (5)
L _SP ≧ L _GR (5)
Can be obtained. When the length L _{SP of the} interval between the plurality of unit FBGs is equal to or longer than the unit FBG length L _GR that is the length in the waveguide direction of one unit FBG, the expression (5) It means that the overlap of can be reduced.

Further, between the length L _SP of the interval between the unit FBGs, the unit FBG length L _GR, and the unit segment length L _SEG , the following formula (6)
L _SP = L _SEG -L _GR (6)
There is a relationship. By substituting equation (6) into equation (5), the following equation (7)
L _SEG -L _GR ≧ L _GR (7)
Is obtained. From equation (7), the following equation (8)
L _SEG / 2 ≧ L _GR (8)
Can be obtained. Formula (8) shows that if the unit FBG length L _{GR is set} to ½ or less of the unit segment length L _SEG that is the length of one unit segment in the waveguide direction, the overlap between adjacent chip pulses is reduced. Means that you can.

<< 2 >> Operation of Embodiment FIG. 8 is a configuration diagram of an observation system of an encoded pulse train used for evaluating the function of the FBG encoder (or FBG decoder) of the embodiment of the present invention.

  Pulses 42 spaced at 625 MHz are continuously generated from the optical pulse generator 41 of FIG. The pulse width of this optical pulse is 6.3 ps or 25 ps. This optical pulse is input to the FBG encoder 44 through the optical circulator 43. The reflected light of the FBG encoder 44 is output through the optical circulator 43 and the encoded pulse train is observed by the oscilloscope 45.

  FIGS. 9A and 9B are diagrams showing an encoded pulse train by an FBG encoder having a code length of “31” having a refractive index modulation structure according to the embodiment of the present invention. When the input light pulse width is 25 ps, FIG. 4B shows the case where the input light pulse width is 6.3 ps.

The FBG encoder is a 31-chip binary phase code <0, π, π, π, 0, 0, 0, π, π, π, 0, π, π, 0, 0, 0, 0, 0, 0. , Π, π, π, π, 0, π, 0, π, π, 0, 0, 0>
It is a structure corresponding to. The encoded pulse train in the case of an input optical pulse width of 25 ps shown in FIG. 9A is a binary phase code having a code length of “31”, but adjacent optical pulses overlap each other.

  Here, the input light pulse width of 25 ps corresponds to the time for light to pass through the round trip length of the unit segment. In the conventional example shown in FIG. 5 (a), the encoded pulse train has a large pulse intensity distribution due to interference between pulses, but the encoded pulse train by the FBG encoder of the present embodiment shown in FIG. 9 (a) is The intensity distribution shows a somewhat small variation in the peak value of the pulse.

  FIG. 9B shows a 6.3 ps encoded pulse train in which the input optical pulse width is narrowed by the FBG encoder of this embodiment. In the conventional example shown in FIG. 5B, the encoded pulse train shows an intensity distribution in which the variation in the peak value of the pulse is small due to interference between pulses, but in the present embodiment shown in FIG. The pulse train encoded by FBG encoding shows an intensity distribution with even smaller variations in pulse peak values.

<< 3 >> Effect of Embodiment Even if the half width on the time axis of the input optical pulse is narrowed, in the conventional FBG encoder, since the FBG continues continuously, adjacent chip pulse trains to be generated are adjacent to each other. Overlap between chip pulses occurs. Therefore, in the conventional FBG encoder, the occurrence of overlap between adjacent chip pulses of the generated chip pulse train cannot be prevented only by adjusting the half width on the time axis of the input optical pulse. Therefore, in the present invention, an interval (space) that satisfies the above equation (5) is provided between the unit FBGs of the FBG encoder. Also, if the half-value width on the time axis of the input optical pulse is narrowed, the half-value width on the frequency axis is widened, so that the half-value width of the filter on the frequency axis of the FBG encoder is narrower than that of the input optical pulse. A big loss occurs.

  FIG. 10A shows a filter spectrum (solid line) indicating the filter characteristics of a conventional FBG encoder, a spectrum of an input optical pulse having a half-value width on the time axis of 6.3 ps (dotted line), and a time axis. FIG. 10B shows a spectrum (solid line) of the input optical pulse having a half width of 25 ps, and FIG. 10B shows a filter spectrum (solid line) showing the filter characteristics of the FBG encoder according to the embodiment of the present invention. The spectrum (dotted line) of the input optical pulse whose upper half-value width is 6.3 ps and the spectrum (dotted line) of the input optical pulse whose half-value width on the time axis is 25 ps are shown.

  As shown in FIG. 10A, the filter spectrum of the conventional FBG encoder of FIG. 2 and the input light show a spectrum with a half-value width of 6.3 ps and 25 ps on the time axis. As shown in FIG. 10A, the half width of the spectrum (dotted line) of the input optical pulse having a half width of 25 ps on the time axis is substantially the same as the half width of the filter spectrum (solid line) of the conventional FBG encoder. is there. Since the output optical pulse of the FBG encoder with respect to the input optical pulse (dotted line) having a half width of 25 ps is obtained by multiplying the intensity of the input optical pulse having a half width of 25 ps (dotted line) and the filter spectrum (solid line) of the FBG encoder, The FBG encoder can efficiently convert an input optical pulse (dotted line) having a half width of 25 ps into an output optical pulse with low loss. As shown in FIG. 10A, the half-value width of the input optical pulse spectrum (one-dot chain line) having a half-value width of 6.3 ps on the time axis is half the filter spectrum (solid line) of the conventional FBG encoder. It is wider than the price range. The output optical pulse of the FBG encoder with respect to the input optical pulse (one-dot chain line) having a half-value width of 6.3 ps is the product of the intensity of the input optical pulse (half-dot chain line) having a half-value width of 6.3 ps and the filter spectrum (solid line) of the FBG encoder. Therefore, the filtering of the conventional FBG encoder causes a large loss in a range other than the center wavelength of the input optical pulse (one-dot chain line) having a half-value width of 6.3 ps.

  On the other hand, as shown in FIG. 10B, the filter spectrum and input light of the FBG encoder of this embodiment show a spectrum with a half-value width of 6.3 ps and 25 ps on the time axis. As shown in FIG. 10B, the half-value width of the spectrum (dotted line) of the input optical pulse having a half-value width of 25 ps on the time axis is greater than the half-value width of the filter spectrum (solid line) of the FBG encoder of the present embodiment. Is too narrow. The output light pulse of the FBG encoder with respect to the input light pulse (dotted line) having a half width of 25 ps is obtained by multiplying the intensity of the input light pulse (dotted line) having a half width of 25 ps with the filter spectrum (solid line) of the FBG encoder. The FBG encoder according to the embodiment can efficiently convert an input optical pulse (dotted line) having a half width of 25 ps into an output optical pulse with little loss. Also, as shown in FIG. 10B, the half-value width of the spectrum of the input optical pulse having a half-value width of 6.3 ps (dashed line) on the time axis is the filter spectrum (solid line) of the FBG encoder of the present embodiment. ) Half-width. The output optical pulse of the FBG encoder with respect to the input optical pulse (one-dot chain line) having a half-value width of 6.3 ps is the product of the intensity of the input optical pulse having a half-value width of 6.3 ps (one-dot chain line) and the filter spectrum (solid line) of the FBG encoder. Therefore, the FBG encoder of the present embodiment can efficiently convert an input optical pulse (dotted line) having a half-value width of 6.3 ps into an output optical pulse with little loss.

<< 4 >> Modifications In the above description, the case where a Bragg diffraction grating is formed in the optical fiber core is exemplified. However, any refractive index modulation structure other than the Bragg diffraction grating can be used as long as it is an optical waveguide capable of performing refractive index modulation. May be adopted.

An explanation is provided for explaining the principle of encoding for generating a binary phase code encoded pulse train from an input optical pulse by an encoder using SSFBG and decoding for reproducing an optical pulse from the encoded pulse train by a decoder using SSFBG. FIG. (A) And (b) is a figure for demonstrating the refractive index modulation structure and phase shift part of the optical fiber core of the conventional FBG encoder (or FBG decoder), (c) is the figure ( It is a partially expanded view of b). It is a figure which shows the binary phase code of code length '15', and the phase shift amount of a phase shift part. (A) thru | or (d) is a figure for demonstrating the interference of the reflected light pulses by two adjacent unit FBGs. (A) And (b) is a figure which shows the encoding pulse train by the FBG encoder of the code length '31' with the conventional refractive index modulation structure, (a) is a case where input optical pulse width is 25 ps, (B) is a case where the input light pulse width is 6.3 ps. (A) And (b) is a figure for demonstrating the refractive index modulation structure and phase shift part of the optical fiber core of the FBG encoder (or FBG decoder) of embodiment of this invention, (c) FIG. 4 is a partially enlarged view of FIG. (A) is a figure which shows the waveform of the optical pulse input into the FBG encoder (or FBG decoder) of embodiment of this invention, and the refractive index modulation structure of an optical fiber core, (b) is reflected light It is a figure which shows the waveform of a pulse. It is a block diagram of the observation system of the encoding pulse train used in order to evaluate the function of the FBG encoder (or FBG decoder) of embodiment of this invention. (A) And (b) is a figure which shows the encoding pulse train by the code length '31' FBG encoder which has the refractive index modulation structure of embodiment of this invention, (a) is an input optical pulse width. (B) is the case where the input optical pulse width is 6.3 ps. (A) shows the filter spectrum (solid line) showing the filter characteristics of the conventional FBG encoder, the spectrum of the input optical pulse whose half-value width on the time axis is 6.3 ps (dotted line), and the half-value width on the time axis. Shows the spectrum (dotted line) of the input optical pulse having a ps of 25 ps, and (b) shows the filter spectrum (solid line) indicating the filter characteristics of the FBG encoder according to the embodiment of the present invention and the half-value width on the time axis. Shows a spectrum (dotted line) of an input optical pulse having a 6.3 ps and a spectrum (dotted line) of an input optical pulse having a half-value width of 25 ps on the time axis.

Explanation of symbols

1 optical fiber, 2 optical fiber core, 3 FBG encoder or FBG decoder (optical pulse time spreading device), 4 phase shift unit, 5 unit FBG (refractive index modulation structure), 6 interval between unit FBGs (space) , 11 input light pulse, 21, 23 unit FBG (unit diffraction grating), 22 interval between unit FBGs, 31,33 reflected light pulse, L _SEG unit segment length, L _GR unit FBG length (unit diffraction grating length), L The length of the interval between _SP units FBG.

Claims (6)

An optical waveguide to which an optical pulse is input; and
Formed in the optical waveguide, each formed by a periodic refractive index modulation unit of the same structure that is continuous in the waveguide direction of the optical waveguide, and converts the optical pulse into a plurality of chip pulses arranged sequentially on the time axis A plurality of unit diffraction gratings,
An optical pulse time spreading device provided in the optical waveguide between the plurality of unit diffraction gratings, and having a phase shift unit that shifts the phase of any of the plurality of chip pulses,
An interval is provided between the plurality of unit diffraction gratings,
The length of the interval is set to be equal to or longer than the unit diffraction grating length which is the length in the waveguide direction of one unit diffraction grating.   The optical pulse time spreading device according to claim 1, wherein the optical waveguide is a core of an optical fiber. The unit diffraction grating, the optical pulse time spreading apparatus according to claim 1 or 2 characterized in that it is a full § i Bas Bragg grating.   4. The light according to claim 1, wherein the interval between the plurality of unit diffraction gratings is a portion where a periodic refractive index modulation portion of the optical waveguide is not formed. 5. Pulse time spreader. The phase shift unit is formed as a part of the interval between the plurality of unit diffraction gratings,
When the distance between the adjacent unit diffraction gratings is a length that gives a phase difference, the phase shift unit exists, and the distance between the adjacent unit diffraction gratings does not give a phase difference. If this is the case, the optical pulse time spreading device according to any one of claims 1 to 4, wherein the phase shift unit does not exist. A plurality of unit segments that are sections obtained by dividing the optical waveguide into equal lengths in the waveguide direction are determined, and one unit diffraction grating is disposed in each of the plurality of unit segments,
6. The unit diffraction grating according to claim 1, wherein the unit diffraction grating length is ½ or less of a unit segment length which is a length of one unit segment in a waveguide direction. Optical pulse time spreading device.

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