JP2005072868A5 - - Google Patents

Description

Optical decoder

  The present invention relates to an optical decoder in an optical communication system using optical code division multiplexing (OCDM), and in particular, receives a plurality of accommodating stations to which OCDM is applied and signals from the accommodating stations via optical fibers. The present invention relates to an optical decoder used in a PON (Passive Optical Network) which is a one-to-many connection optical network including a central office.

  The configuration of an optical encoder and an optical decoder used in an optical code multiplexing system for encoding in the wavelength region by arranging a plurality of reflectors that reflect light of a predetermined wavelength is shown in FIG. FIG. 3 is an operation explanatory diagram in which the state of conduction including the delay time is added to the optical transmission line for convenience. The first and second optical encoders 500A and 500B in FIGS. 2 and 3 input an FBG (Fiber Bragg Grating) 501 having portions that reflect light of different wavelengths, and input light to the FBG 501 to reflect from the FBG 501. It comprises a circulator 502 (or an optical splitter) that outputs light to an optical transmission line. The first and second optical decoders 600A and 600B also have an FBG 601 having portions that reflect light of different wavelengths, and a circulator 602 that inputs input light to the FBG 601 and outputs reflected light from the FBG 601 to the optical transmission line (or Optical branching device). Reference numeral 700 denotes an optical multiplexer / demultiplexer.

  First optical encoder 500A and first optical decoder 600A include FBGs 501 and 601 that reflect light of wavelengths λ0, λ3, λ4, and λ5 according to a code of “1001110” that is a third-order M-sequence code. . The interval between the reflection positions of light of each wavelength from λ1 to λ6 is an interval obtained by multiplying the subscripts 1 to 6 by a certain distance from λ0. FBGs 501 and 601 in FIG. 3 indicate the presence or absence of a reflector corresponding to light of each wavelength by the presence or absence of a vertical line.

  The light input to the first optical encoder 500A is reflected by the FBG 501 through the circulator 502 in the order of wavelengths λ0, λ3, λ4, and λ5. When the reflection position interval of each wavelength is 1 mm and the refractive index is 1.5, as shown in FIG. 3, the wavelength λ0 is output from the FBG 501 as a reference, λ3: 30 ps, λ4: 40 ps, λ5: The delay time is superimposed on 50 ps.

  The variation in delay time for each wavelength is reflected by the FBG 601 of the first optical decoder 600A in which the reflection position from the circulator 602 is arranged in the reverse order to that of the first optical encoder 500A. By making the sum of the delay time due to reflection at the optical encoder 500A and the delay time due to reflection at the first optical decoder 600A the same for all wavelengths, they are aligned.

The first optical decoder 600A in FIG. 3 has FBGs 601 having the same wavelengths and in reverse order as those of the first optical encoder 500A, and λ5, λ4, λ3, and λ0. The first optical decoder 600A has 10 ps, 20 ps, and 30 ps, respectively. By superimposing the delay time of 60 ps, the same delay time 60 ps is superimposed on all wavelengths, and the sum of the delay times becomes the same. Such an optical encoder and optical decoder are described in Non-Patent Document 1, for example.
Jen Hwa Hung et al., “Reducing Multiple Access Interference in Optical CDMA Networks with Fiber Grating”, IEEE Bulletin of Communications, Vol. 50, No. 10, pp. 1680-1687, October 2002 (Jen-Fa Huang et al "Reductions of Multiple-Access Interference in Fiber Grating-Based Optical CDMA Network" IEEE TRANSACTIONS ON COMMUNICATIONS Vol.50, No.10 pp.1680-1687, OCTOBER 2002)

However, when the light from the first optical encoder 500A in FIG. 3 is received by the second optical decoder 600B, as shown in FIG. 3, since the codes are different, light of half the wavelength constituting the code ( The light of [lambda] 3, [lambda] 4) is reflected by the FBG 601 and the light of half wavelength ([lambda] 0, [lambda] 5) is transmitted. Here, the second optical decoder 600B conforms to “0011101” obtained by shifting the code of the first optical decoder 600A. Therefore, the light of λ3 and λ4 reflected by the FBG 601 of the second optical decoder 600B has a delay time of 60 ps, but the light of λ0 and λ5 that transmits the FBG 601 of the second optical decoder 600B is 0 ps and 50 ps. It is a delay time, and when the delay time is not uniform, the transmission pulse is
Only approximately (number of wavelengths −1) × reflector spacing is expanded.

With this pulse spread,
((Number of wavelengths−1) × reflector interval) / ((number of wavelengths−1) × reflector interval + (bit time))
Thus, the power of the transmitted pulse will be out of the bit time to which the original pulse belongs. In the optical decoder of Non-Patent Document 1, differential detection is performed by subtracting the transmitted pulse intensity from the reflected pulse intensity of the optical decoder, and interference pulses from other optical encoders that are not to be decoded are removed.

However, the amount of transmitted pulse power that protrudes from the bit time due to pulse spread, that is,
0.5 × (interference pulse intensity) × ((number of wavelengths−1) × reflector interval) / ((number of wavelengths−1) × reflector interval + (bit time))
Since the bit protrudes from the bit where the interference pulse originally arrived to the front bit, in the bit where the interference pulse should arrive originally,
0.5 × (interference pulse intensity) × ((number of wavelengths−1) × reflector interval) / ((number of wavelengths−1) × reflector interval + (bit time))
Interference component remains.

  FIG. 4 shows the effect of residual interference components when an M-sequence with a code length of 63 is used to accommodate 32 users, the reflection position interval of light of each wavelength is assumed to be 1 mm, and the refractive index of the FBG is 1.5. The residual interference component for one pulse of one interference pulse and the code error rate (BER) in the case of 1 and 31 users simultaneously communicating are shown. A thick solid line A is a ratio in which the mark value is increased by the interference light, a light black solid line B is a ratio in which the space value is decreased by the interference light, and the broken broken line C is the code error rate when the simultaneous communication user is 1. The fine broken line D is the code error rate when the number of simultaneous communication users is 31.

As shown in FIG. 4, when a code error rate of 10 ⁻⁹ is requested, C when the number of simultaneous communication users is 1 has a transmission rate of 500 Mbit / s, and D in the case of 31 has a transmission rate of 10 Mbit / s. It becomes the upper limit. Therefore, there is a problem that the increase in transmission speed is limited.

  Further, as shown in Non-Patent Document 1, since the reflectance of the FBG of the optical decoder is not 100%, a part of the reflected wave is transmitted, the transmitted light intensity is increased, and the reflected wave and the transmitted wave are increased. However, there is a problem that a special configuration for removing interference due to imperfect reflection is required because sufficient interference removal cannot be performed by the differential detection.

  An object of the present invention is to provide an optical decoder that enables high-speed transmission and eliminates the need for a configuration that eliminates interference due to incomplete reflection of the reflection function.

The invention according to claim 1 is an optical decoder used in an optical communication system using optical code multiplexing, and receives input light arriving at different timings for each wavelength and outputs light of a plurality of wavelengths according to a predetermined code. A first reflection function unit that selects and reflects with a different delay time for each wavelength, and light having a wavelength that is received by the first reflection function unit and is not selectively reflected by the first reflection function unit differs for each wavelength. A second reflection function unit that reflects with a delay time; and a delay function unit that superimposes a time until light input to the second reflection function group is output on the reflected light of the first reflection function unit, The first reflection function unit and the second reflection function unit are reflected by the first reflection function unit and transmitted through the delay function unit and transmitted through the first reflection function unit and the second reflection function unit. The output timing of the light of the wavelength reflected by the reflection function unit is Characterized in that it is arranged to match the light slowest arriving wavelength of the input light arriving at different times.
According to a second aspect of the present invention, in the optical decoder according to the first aspect, the first optical waveguide is configured to input the input light to the first reflection function unit, and receive and output the reflection light of the first reflection function unit. A function unit; and a second optical waveguide function unit that inputs the transmitted light of the first reflection function unit to the second reflection function unit and receives and outputs the reflected light of the second reflection function unit. And
According to a third aspect of the present invention, in the optical decoder according to the first or second aspect, the first reflection function unit selects light of a plurality of wavelengths corresponding to a code opposite to the predetermined code for each wavelength. The second reflection function unit is replaced with a reflection function unit that selects light of a plurality of wavelengths according to the predetermined code and reflects with a different delay time for each wavelength. It is characterized by that.

  According to the present invention, even when the optical decoder receives a pulse having a different arrival time for each wavelength, the sum of the delay times is the same, so that high-speed transmission is possible. Further, since all the wavelengths are reflected once by the reflection function, there is an effect that a configuration for removing interference due to incomplete reflection of the reflection function becomes unnecessary.

  In the present invention, when the optical decoder receives a pulse having a different arrival time for each wavelength, in order to make the total delay time the same, the light of each wavelength of the transmitted light of the first reflection function unit of the optical decoder is used. A second reflection function unit for reflecting a predetermined delay time is provided. This will be described in detail below.

  The optical encoder and optical decoder according to the first embodiment (corresponding to claims 1 and 2) will be described with reference to FIG. Here, a state of conduction including a delay time for each light having a predetermined wavelength will be described by adding it to the optical transmission line for convenience. FIG. 1 shows first and second optical encoders 100A and 100B, first and second optical decoders 200A and 200B, and an optical multiplexer / demultiplexer 300. The first optical encoder 100A is the same as the optical encoder 500A shown in FIGS.

  The first and second optical decoders 200A and 200B each have a portion that reflects light of different wavelengths, and selects FBG (first delay) that reflects light with a predetermined delay time by selecting light of a wavelength included in a predetermined code. Functional unit) 201, a circulator 203 (or an optical branching unit) (first optical waveguide functional unit) that inputs input light to the FBG 201 and outputs reflected light from the FBG 1, and at least FBG 201 out of the transmitted light of the FBG 201 An FBG 210 (second delay function unit) that reflects light of a wavelength not selected when reflected as a code with a predetermined delay time, and a circulator 211 (or light) that inputs the transmitted light of the FBG 201 to the FBG 210 and outputs the reflected light from the FBG 210 (Branch unit) (second optical waveguide function unit) and an optical delay line (delay function unit) 212 that delays the reflected light of the FBG 201.The reflection position of each wavelength from λ5 to λ1 from the reflection position of λ6 of the FBGs 201 and 210 of the first and second optical decoders 200A and 200B is multiplied by a certain distance to the number obtained by subtracting the subscripts 5-1 from 6. It is an interval. The reflection position interval of each wavelength is 1 mm and the refractive index is 1.5.

  Similar to the first optical encoder 100A, the FBG 201 of the first optical decoder 200A reflects light of wavelengths λ0, λ3, λ4, and λ5 according to a code of “1001110” that is a third-order M-sequence code. The FBG 210 of the first optical decoder 200A reflects light of wavelengths λ1, λ2, and λ6 that are not selected when the FBG 201 reflects as a code.

  In order to decode a code different from the first optical encoder 100A, the second optical decoder 200B reflects the wavelengths λ2, λ3, λ4, and λ6 according to the code “0011101” shifted by one chip from the first optical decoder 200A. To do. The FBG 210 of the second optical decoder 200B reflects light of wavelengths λ0, λ1, and λ5 that are not selected when the FBG 201 reflects as a code.

  In the first optical decoder 200A, similarly to the conventional example, reflection is performed at λ5, λ4, λ3, and λ0 of the same wavelength and in reverse order as in the first optical encoder 100A, and the FBG 201 delays 10 ps, 20 ps, 30 ps, and 60 ps, respectively. The time is superimposed and the sum of the delay time superimposed by the first optical encoder 100A becomes the same delay time 60 ps for all wavelengths (λ0, λ3, λ4, λ5), and the optical delay line 212 further reduces the delay time to 30 ps. By superimposing, the total delay time becomes 90 ps.

  In the FBG 201 of the second optical decoder 200B, delay times of 0 ps, 20 ps, 30 ps, and 40 ps are superimposed on the wavelengths λ6, λ4, λ3, and λ2, respectively. For this reason, the sum of the delay time of the light of the wavelength reflected by the FBG 201 and the delay time superimposed by the first optical encoder 200A is 60 ps, and further, the delay time of 30 ps is superimposed by the optical delay line 212, so that λ3, λ4 The delay time of light having a wavelength of 90 ps is 90 ps. In the FBG 210, delay times of 10 ps, 50 ps, and 60 ps are superimposed on light of wavelengths λ5, λ1, and λ0, respectively. For this reason, the light of wavelengths λ0 and λ5 is superimposed with a delay time of 30 ps required for transmission through the FBG 201, and the sum of the delay time of the light of the wavelength reflected from the FBG 210 and the superimposed delay time of the first optical encoder 100A is 90 ps.

  In this way, the delay times of the light of all wavelengths transmitted from the first optical encoder 100A are 90 ps and are the same by the first and second decoders 200A and 200B. For this reason, in the first embodiment, there is an effect that the transmission rate can be improved by suppressing the code error rate deterioration that may occur when the delay times experienced by the transmitted light and the reflected light are different.

  In the first embodiment, the FBG 210 of the first and second optical decoders 200A and 200B reflects light having a wavelength that the FBG 201 does not select as a code, but selects it as a code in addition to the wavelength that is not selected by the code. An FBG that reflects light of a wavelength may be used. However, in this case, since the light selected as the code leaks to the light of the FBG 210 due to the incomplete reflection of the FBG 201, a special configuration for removing interference due to insufficient reflection as shown in the conventional example is required. There is a problem that cannot be solved. In the first embodiment, the optical delay line 212 is used to align the delay times of the wavelengths reflected by the FBG 201 and the FBG 210. However, the reflected light of the FBG 201 is received by the second detector, and the reflected light of the FBG 210 is received. In the case of a configuration in which each output is received by the first detector and the respective outputs are compared by the comparator, a delay line that applies the same delay as that provided by the optical delay line 212 is provided instead of the optical delay line 212. It may be provided between the detector and the comparator.

  In the first embodiment, the reflection position interval for each wavelength is constant. However, the sum of the delay time due to reflection at the FBG 101 and the delay time due to reflection at the FBG 201 and FBG 210 may be the same, and the opposite encoders. If it is compatible with the FBG 101, the generality is not lost even if the positions of the wavelengths are not changed even if the wavelength positions are kept constant.

  Furthermore, if the delay time when arriving at the optical decoder for each wavelength differs due to the chromatic dispersion of the optical transmission line connecting the optical encoder and the optical decoder, the optical decoder is used in advance at the opposite optical encoder or after transmission. Even if the difference between the delay time generated by the optical transmission line dispersion and the delay time obtained by reversing the positive / negative are added to the sum of the delay time due to reflection at the FBG 101 and the delay time due to reflection at the FBG 201 and FBG 210 The effect of the present invention for correcting the delay time difference for each wavelength generated by the optical encoder or the optical decoder is not impaired.

  As described above, in the first embodiment, when the optical decoder receives a pulse having a different arrival time for each wavelength, each wavelength of the transmitted light of the optical decoder is set so that the sum of the delay times is the same. The FBG 210 is reflected so as to be superimposed with the above delay time. In this way, by reflecting light having a wavelength not selected as a code so as to correct the delay time, it is possible to correct a difference in delay time for each wavelength of light from an optical encoder that is not a decoding target. Therefore, since the delay times related to the interference light as shown in the conventional example are the same, interference can be removed by differential detection, and high-speed transmission is possible. Furthermore, since all the wavelengths (λ0 to λ6) are once reflected by the FBG, there is an effect that it is not necessary to remove interference pulses due to incomplete reflection of the FBG.

  In the second embodiment (corresponding to claim 3), the wavelength reflected by the FBG 201 and the wavelength reflected by the FBG 210 in the first embodiment are interchanged. In this case, since the light having the wavelength of the code to be decoded passes through both the FBG 201 and the FBG 210, the signal is attenuated as compared with the third embodiment, but the other effects are the same as those of the first embodiment. .

FIG. 3 is an explanatory diagram of configurations and operations of an optical encoder and an optical decoder according to the first embodiment. It is a block diagram which shows the structure of the conventional optical encoder and optical decoder. It is operation | movement explanatory drawing of the conventional optical encoder and optical decoder. It is a characteristic figure of the code error rate (BER) with respect to a transmission rate, and a residual interference component in an optical decoder.

Explanation of symbols

100A: 1st optical encoder 100B: 2nd optical encoder 101: FBG (reflection function part)
102: Circulator (or optical splitter) (optical waveguide function unit)
200A: first optical decoder 200B: second optical decoder 201, 210: FBG (reflection function unit)
203, 211: Circulator (or optical splitter) (optical waveguide function unit)
212: Optical delay line (delay function unit)
300: Optical multiplexer / demultiplexer

Claims (3)

An optical decoder used in an optical communication system using optical code multiplexing,
A first reflection function unit that receives input light arriving at different timings for each wavelength, selects light of a plurality of wavelengths according to a predetermined code, and reflects the light at different delay times for each wavelength, and the first reflection function unit A second reflection function unit that reflects light having a wavelength that is not selectively reflected by the first reflection function unit with a different delay time for each wavelength, and until light input to the second reflection function group is output A delay function unit that superimposes the time of time on the reflected light of the first reflection function unit,
The first reflection function unit and the second reflection function unit are reflected by the first reflection function unit and transmitted through the delay function unit and transmitted through the first reflection function unit and the first reflection function unit. The output timing of the light having the wavelength reflected by the two-reflection function unit is arranged so as to coincide with the light having the latest arrival wavelength among the input lights that arrive at different timings for each wavelength. Optical decoder. The optical decoder according to claim 1.
A first optical waveguide function unit that inputs the input light to the first reflection function unit, receives and outputs the reflection light of the first reflection function unit, and transmits transmitted light of the first reflection function unit to the second reflection function. And a second optical waveguide function unit that receives the reflected light of the second reflection function unit and outputs the reflected light. The optical decoder according to claim 1 or 2,
The first reflection function unit is replaced with a reflection function unit that selects light of a plurality of wavelengths according to a code opposite to the predetermined code and reflects the light with a different delay time for each wavelength, and the second reflection function unit is replaced with the second reflection function unit. An optical decoder characterized in that light having a plurality of wavelengths corresponding to a predetermined code is selected and replaced with a reflection function unit that reflects light with different delay times for each wavelength.

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