JP2012227763A - Optical fiber transmission system and optical receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology of materializing long-distance transmission in the optical fiber transmission using an optical MIMO technology.SOLUTION: In the optical receiver R, group delay difference of optical fiber transmission is compensated preliminarily by using a pre-stage FBG (Fiber bragg grating) device 9, and group delay difference of optical fiber transmission is compensated with high accuracy by using an FIR(Finite inpulse response) filter device 6 in the subsequent stage. Consequently, the computation time is shortened by reducing the number of taps in the FIR filter device 6, and long-distance transmission is materialized in the optical fiber transmission using the optical MIMO technology.

Description

  The present invention relates to a technique for compensating for a group delay difference in optical fiber transmission.

  In optical fiber transmission systems, nonlinear effects and fiber fuses that occur in optical fibers become a problem, and the increase in transmission capacity is limited. In order to alleviate these restrictions, it is necessary to reduce the density of light guided to the optical fiber. As shown in Non-Patent Documents 1 and 2, a large core fiber has been studied.

  However, bending loss reduction, single mode operation area expansion, and effective area expansion are in a trade-off relationship with each other, and there is a problem that the amount of effective area expansion under certain conditions is limited. Therefore, an attempt has been made to apply a multi-input multi-output (MIMO) technique, which is a technique for increasing capacity in radio, to optical fiber transmission (see, for example, Non-Patent Documents 3 and 4).

  Optical MIMO technology uses a multimode optical fiber as a transmission medium and can increase the transmission capacity and eliminate the need for the single-mode operating condition that was the limiting factor of the large-core optical fiber described above. It is also a feature that is possible.

T.A. Matsui, K .; Nakajima and C.I. Fukai, “Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Candion Convergence Omm. Lightwave Technol. 27, 5410-5416, 2009. K. Mukasa, K .; Imamura, R.A. Sugizaki and T.A. Yagi, "Comparisons of merits on wide-band transmission systems between using extremely improved solid SMFs with Aeff of 160μm2 and loss of 0.175dB / km and using large-Aeff holey fibers enabling transmission over 600nm bandwidth", the Proceedings of OFC2008, OthR1 , Feb. 2008. Akhil R.D. Shah, Rick C.I. J. et al. Hsu, Alireza Tarihat, Ali H. et al. Sayed and Bahr Jalali, “Coherent Optical MIMO (COMIMO)”, J. Am. Lightwave Technol. 23, 2410-2419, 2005. B. C. Thomsen, “MIMO enabled 40Gb / s Transmission Usage Mode Division Multiplexing in Multimode Fiber,” in Optical Fiber Communication Conferencing, OSA Tec. M.M. Taylor, "Coherent Detection for Fiber Optic Communications Using Digital Signal Processing", in Optical Amplifiers and Their Applications / Coherent Optical Technologies and Applications, Technical Digest (CD) (Optical Society of America, 2006), paper CThB1.

  However, in optical fiber transmission using optical MIMO technology, intersymbol interference due to mode dispersion occurs as the transmission distance becomes longer, and signal recovery becomes difficult, and the transmission distance is limited to several kilometers. Is a problem.

  Therefore, in order to solve the above-described problems, an object of the present invention is to provide a technique for realizing long-distance transmission in optical fiber transmission using an optical MIMO technique.

  In order to achieve the above object, the optical receiver uses Fiber Bragg Grating (FBG) in the preceding stage to preliminarily compensate for the group delay difference of the optical fiber transmission, and the Finite Impulse Response (FIR) in the latter stage. A filter was used to compensate for the group delay difference of optical fiber transmission with high accuracy. This reduces the number of taps in the FIR filter and shortens the calculation time, and realizes long-distance transmission in optical fiber transmission using optical MIMO technology.

  Specifically, the present invention relates to N (N is an integer equal to or greater than 1) optical transmitters that transmit optical signals and M (M is an integer equal to or greater than N) optical receivers that receive the optical signals. And the optical signals transmitted from the N optical transmitters are combined, and the combined optical signals are P (P is an integer equal to or greater than N. However, when N is 1, P is equal to or greater than 2. An optical fiber having a propagation mode of an integer), a duplexer for demultiplexing the optical signal propagated by the optical fiber to the M optical receivers, and receiving by each optical receiver For the optical signal, the P propagation modes are incident on the entrance and exit ports, and the P propagation modes are input to the entrance and exit ports using a fiber Bragg grating that reflects each of the P propagation modes. Reflect and emit the P propagation modes from the entrance / exit to compensate for the group delay difference A FIR filter comprising a stage group delay difference compensation device and a delay element for adjusting a delay time and an amplitude phase adjuster for adjusting the amplitude and phase of the optical signal compensated for the group delay difference by the preceding stage group delay difference compensation device. An optical fiber transmission system comprising: a latter-stage group delay difference compensation device that uses and compensates for a group delay difference.

  The present invention also provides an optical transmitter for transmitting optical signals (N is an integer equal to or greater than 1), an optical transmitter for receiving optical signals (M is an integer equal to or greater than N), A multiplexer for multiplexing the optical signals transmitted by N optical transmitters and the optical signal combined by the multiplexer are propagated, and P (P is an integer greater than or equal to N, where N is 1 is an integer greater than or equal to 2), an optical fiber having a propagation mode, a duplexer for demultiplexing the optical signal propagated by the optical fiber to the M optical receivers, and each optical receiver For the optical signal received by the machine, the P number of propagation modes are incident from the entrance and exit, and the P number of the propagation signals are input to the entrance and exit using a fiber Bragg grating that reflects each of the P propagation modes. Reflects the propagation mode, emits the P propagation modes from the entrance and exit, and determines the group delay difference. A first stage group delay difference compensator for compensation, and a FIR comprising a delay element for adjusting a delay time and an amplitude phase adjuster for adjusting the amplitude and phase of the optical signal compensated for the group delay difference by the previous stage group delay difference compensator. An optical fiber transmission system comprising: a post-stage group delay difference compensation device that compensates for a group delay difference using a filter.

  Further, the present invention propagates the optical signals after transmission and multiplexing from N (N is an integer of 1 or more) optical transmitters that transmit optical signals, and P (P is an integer of N or more). However, when N is 1, P is an optical fiber having the propagation mode after propagation and demultiplexing via an optical fiber having a propagation mode of P (M is an integer of N or more). And the optical signal received by each optical receiver using a fiber Bragg grating that enters the P propagation modes from the entrance and exit and reflects each of the P propagation modes. A grating group delay difference compensation device that reflects the P propagation modes to the entrance and exit, emits the P propagation modes from the entrance and exit, and compensates for a group delay difference, and the preceding group delay difference compensation For the optical signal, the device compensates for the group delay difference, And an FIR filter group delay difference compensating device for compensating for a group delay difference by using an FIR filter comprising a delay element for adjusting the interval and an amplitude phase adjuster for adjusting amplitude and phase. Device.

  According to this configuration, in order to preliminarily compensate for the group delay difference of the optical fiber transmission in the FBG, the number of taps is reduced in the FIR filter, the calculation time is shortened, and in the optical fiber transmission using the optical MIMO technology, the long distance Transmission can be realized.

  In the present invention, the optical signal after transmission is propagated from one optical transmitter that transmits the optical signal through an optical fiber having P propagation modes (P is an integer of 2 or more). M optical receivers (M is an integer greater than or equal to 2) that receive the optical signals after and after demultiplexing, and the P propagation modes from the input / output port for the optical signals received by each optical receiver The P propagation modes are reflected to the entrance / exit port using a fiber Bragg grating that reflects each of the P propagation modes, and the P propagation modes are emitted from the entrance / exit ports. And a grating group delay difference compensation device for compensating for the group delay difference.

  According to this configuration, in order to fully compensate for the group delay difference of the optical fiber transmission in the FBG, it is possible to realize a long distance transmission in the optical fiber transmission using the optical MIMO technology without requiring an FIR filter. it can.

  Further, according to the present invention, in the optical fiber, effective refractive indexes for the P propagation modes are different from each other, and in the front-stage group delay difference compensator, the grating pitch of the refractive index in the fiber Bragg grating and the input / output are In the optical fiber transmission system, a distance from the mouth to the fiber Bragg grating is different among the P propagation modes.

  Further, according to the present invention, in the optical fiber, the effective refractive indexes for the P propagation modes are different from each other. In the grating group delay difference compensating device, the grating pitch of the refractive index in the fiber Bragg grating and the incident and output are In the optical receiver, the distance from the mouth to the fiber Bragg grating is different among the P propagation modes.

  According to this configuration, the group delay difference of the optical fiber transmission can be compensated with higher accuracy for each propagation mode in the FBG.

  Further, the present invention is the optical fiber transmission system, wherein the number of propagation modes at the used wavelength is 2 or more and 7 or less in the optical fiber.

  The present invention is the optical receiver characterized in that the number of propagation modes in the used wavelength is 2 or more and 7 or less in the optical fiber.

  According to this configuration, the number of FBGs can be further reduced.

  Further, according to the present invention, in the optical fiber, effective refractive indexes for at least two of the P propagation modes are equal to each other, and in the front group delay difference compensator, the refractive index in the fiber Bragg grating is obtained. The optical fiber transmission system is characterized in that the pitch of the grating and the distance from the incident / exit port to the fiber Bragg grating are equal between the at least two propagation modes.

  According to the present invention, in the optical fiber, effective refractive indexes for at least two of the P propagation modes are equal to each other. In the grating group delay difference compensating device, the refractive index in the fiber Bragg grating is used. The optical receiver is characterized in that the pitch of the grating and the distance from the incident / exit port to the fiber Bragg grating are equal between the at least two propagation modes.

  According to this configuration, the number of FBGs can be further reduced.

  The present invention can provide a technique for realizing long-distance transmission in optical fiber transmission using optical MIMO technology.

It is a figure which shows the structure of the optical fiber transmission system and optical receiver which compensate a group delay difference between several modes using a FIR filter apparatus. It is a figure which shows the structure of a FIR filter apparatus. It is a figure which shows the adaptive equalization algorithm for updating a tap coefficient. It is a figure which shows the training symbol for updating a tap coefficient. Using a FIR filter device, it was examined whether or not the group delay difference of 0.5 ns between the fundamental mode and the higher order mode can be compensated when there is one optical transmitter and one optical receiver. It is a figure which shows a simulation result. A simulation using a FIR filter device to examine whether a group delay difference of 5 ns between a fundamental mode and a higher-order mode can be compensated when there is one optical transmitter and one optical receiver. It is a figure which shows a result. It is a figure which shows the structure of an optical fiber transmission system and an optical receiver which compensates the group delay difference between several modes using an FBG apparatus and an FIR filter apparatus. It is a figure which shows the structure of a 1st FBG apparatus. It is a figure which shows the structure of a 2nd FBG apparatus. It is a figure which shows the relationship between an optical signal wavelength and an effective refractive index in the fundamental mode and higher order mode of a multimode optical fiber. It is a figure which shows the relationship between the normalized frequency and the number of propagation modes in a multimode optical fiber. Using the FIR filter device, when two optical transmitters and two optical receivers are used, it was examined whether or not the group delay difference of 0.04 ns between the fundamental mode and the higher-order mode can be compensated. It is a figure which shows a simulation result. Using the FIR filter device, it was examined whether or not the group delay difference of 0.1 ns between the fundamental mode and the higher-order mode can be compensated when there are two optical transmitters and two optical receivers. It is a figure which shows a simulation result.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Group delay difference compensation using FIR filter device)
FIG. 1 shows the configuration of an optical fiber transmission system and an optical receiver that compensate for group delay differences among a plurality of modes using an FIR filter device. The optical fiber transmission system includes optical transmitters 1-1, 1-2, ..., 1-N, optical receivers 2-1, 2-2, ..., 2-M, multiplexer 3, and light. The fiber 4, the duplexer 5, and the FIR filter device 6 are included. The optical receiver R includes optical receivers 2-1, 2-2,..., 2-M and an FIR filter device 6.

  , 1-N, multimode optical fiber 4 and M optical receivers 2-1, 2-2,..., 2-M , N input M output MIMO transmission can be performed, and N types of optical signals can be transmitted in parallel.

Optical transmitter 1-1 and 1-2, · · ·, 1-N, respectively the optical signals _{x 1 (n), x 2} (n), and transmits · · _{·, x} N a (n). Here, N is an integer of 1 or more, and the optical signals x ₁ (n), x ₂ (n),..., X _N (n) are optical signals transmitted as the nth symbol.

Optical receiver 2-1 and 2-2, · · ·, 2-M, respectively the optical signals _{y 1 (n), y 2} (n), ···, receives the _y M (n). Here, M is an integer greater than or equal to N, and the optical signals y ₁ (n), y ₂ (n),..., Y _M (n) are optical signals received as the nth symbol.

  The multiplexer 3 multiplexes the optical signals transmitted by the optical transmitters 1-1, 1-2, ..., 1-N. Here, even if the number of the optical transmitters 1 is one, the term multiplexing is used. The optical fiber 4 propagates the optical signal combined by the multiplexer 3 and has P propagation modes. Here, P is an integer greater than or equal to N. However, when N is 1, P is an integer of 2 or more. The demultiplexer 5 demultiplexes the optical signal propagated through the optical fiber 4 to the optical receivers 2-1, 2-2,. Here, even if the number of optical receivers 2 is one, the term demultiplexing is used.

The FIR filter device 6 includes optical signals y ₁ (n), y ₂ (n),..., Y _M (n) received by the optical receivers 2-1, 2-2,. ) Is compensated for using a FIR filter comprising a delay element for adjusting the delay time and an amplitude / phase adjuster for adjusting the amplitude and phase. Then, the FIR filter device 6 restores the optical signals u ₁ (n), u ₂ (n),..., U _N (n). Here, the optical signals u ₁ (n), u ₂ (n),..., U _N (n) are optical signals restored as the n-th symbol.

The received signals y ₁ (n), y ₂ (n),..., Y _M (n) may include group delay differences between multiple modes. The restored signals u ₁ (n), u ₂ (n),..., U _N (n) are preferably compensated for the group delay difference between the plurality of modes, and the transmitted signals x ₁ (n ), X ₂ (n),..., X _N (n).

The configuration of the FIR filter device is shown in FIG. The FIR filter device 6 includes FIR filters 61-1, 61-2, ..., 61-M and a multiplexer 62. Since the configurations for generating the restored signals u ₁ (n), u ₂ (n),..., U _N (n) are the same, only the configuration for generating the restored signals u ₁ (n) is shown in FIG. explain.

FIR filters 61-1 and 61-2, · · ·, 61-M, respectively received signals _{y 1 (n), y 2} (n), enter · · _{·, y} M a (n), respectively an optical signal Generate z ₁ (n), z ₂ (n),..., z _M (n). The multiplexer 62 multiplexes the optical signals z ₁ (n), z ₂ (n),..., Z _M (n) to generate a restored signal u ₁ (n).

  The FIR filters 61-1, 61-2, ..., 61-M are each composed of taps from the first to the Lth. The first to L-th taps are each composed of a delay element (indicated by τ in FIG. 2) and an amplitude phase adjuster (indicated by w in FIG. 2). The delay element adjusts the delay time, and the amplitude / phase adjuster adjusts the amplitude and phase. Although a direct FIR filter is used in FIG. 2, a transposed FIR filter may be used.

In the i-th tap for the received signal y _B (n), the delay time in the delay element is τ _i, and the tap coefficient in the amplitude phase adjuster is w _B (i). By adjusting the delay time τ _i and the tap coefficient w _B (i), signal degradation caused by interference, mode dispersion, chromatic dispersion, polarization mode dispersion, etc. from the non-target optical transmitter 1 generated in the optical fiber 4 Can be compensated. By using a local light source, a 90 ° hybrid, a balance receiver, and an analog-digital converter, the amplitude and phase of the electric field of the received signals y ₁ (n), y ₂ (n),..., Y _M (n) Information can be acquired (for example, refer nonpatent literature 5).

An adaptive equalization algorithm for updating tap coefficients is shown in FIG. A training symbol for updating the tap coefficient is shown in FIG. The transmission signal x has training symbols x (1),..., X (N _training ) in the previous stage, and data portions x (N _training +1),..., X (N _all ) in the _subsequent stage. Here, it is necessary that N _training > L. The optical receiver R makes the data part x (N _training +1),..., X (N _all ) unknown, but the training symbols x (1),..., X (N _training ) are known. Therefore, the tap coefficient can be updated as follows.

First, a case where a training symbol is being received and n ≦ N _training is described. The FIR filter device 6 converts the received signal y (n) into the restored signal u (n) by using the tap coefficient that has already been set. The subtracter 8 generates an error signal e (n) by generating a difference between the restored signal u (n) and the known training signal x (n). The adaptive equalization algorithm 7 updates the tap coefficient so as to reduce the error signal e (n). This process is repeated for all training symbols.

Next, a case where the data part is being received and n ≧ N _training +1 is described. The setting of the tap coefficient is completed during reception of the training symbol. The FIR filter device 6 converts the received signal y (n) into the restored signal u (n) by using the tap coefficient that has already been set. This process is repeated for all data portions.

  As the adaptive equalization algorithm, a Least Mean Square (LMS) algorithm or a Recursive Least Square (RLS) algorithm described in Non-Patent Document 2 can be used.

  Whether or not the group delay difference of 0.5 ns between the fundamental mode and the higher order mode can be compensated by using the FIR filter device 6 when there is one optical transmitter 1 and one optical receiver 2 FIG. 5 shows a simulation result obtained by examining the above. Using the FIR filter device 6, when there is one optical transmitter 1 and one optical receiver 2, it is examined whether the group delay difference of 5 ns between the fundamental mode and the higher order mode can be compensated. The simulation results are shown in FIG.

In FIG. 5 and FIG. 6, the transmission / reception signal is 4096 bits of a non-return to zero (NRZ) signal modulated by 10 Gbps binary phase shift keying (BPSK). The number of propagation modes of the optical fiber 4 is two, and the fundamental mode and the higher-order mode are excited at a ratio of 10: 4, respectively. N _training is set to 100 and L is set to 20. τ _i is 10 Gbps symbol length T = 0.1 ns, τ ₁ = 0, τ ₂ = 0.1 ns, τ ₃ = 0.1 × 2 ns,..., τ _L = 0.1 × (L -1) ns. RLS is used as the adaptive equalization algorithm. The step size and forgetting factor in RLS are 0.001 and 1, respectively.

  The case of FIG. 5 in which the group delay difference between the basic mode and the higher-order mode is 0.5 ns will be described. A constellation map of the transmission signal x (n) is shown in the upper left of FIG. In the upper right of FIG. 5, a constellation map of the received signal y (n) is shown. The received signal y (n) does not correctly reflect the transmitted signal x (n) due to mode dispersion. In the lower left of FIG. 5, a constellation map of the restoration signal u (n) is shown. The restored signal u (n) correctly restores the transmission signal x (n) by using the FIR filter device 6. The lower right of FIG. 5 shows the relationship between the tap number and the absolute value of the tap coefficient. The absolute value of the tap coefficient takes a large value periodically, and it can be seen that the tap coefficient of the tap number corresponding to an integral multiple of the group delay difference of 0.5 ns greatly contributes to the restoration.

  The case of FIG. 6 in which the group delay difference between the basic mode and the higher-order mode is 5 ns will be described. In the upper left of FIG. 6, a constellation map of the transmission signal x (n) is shown. In the upper right of FIG. 6, a constellation map of the received signal y (n) is shown. The received signal y (n) does not correctly reflect the transmitted signal x (n) due to mode dispersion. A constellation map of the restoration signal u (n) is shown in the lower left of FIG. The restoration signal u (n) does not correctly restore the transmission signal x (n) even if the FIR filter device 6 is used. The lower right of FIG. 6 shows the relationship between the tap number and the absolute value of the tap coefficient. The absolute value of the tap coefficient does not change periodically. That is, it can be seen that the number of taps L is insufficient compared to the group delay difference of 5 ns. Here, it is sufficient that the number of taps L is sufficient as compared with the group delay difference of 5 ns. However, since the calculation amount in the adaptive equalization algorithm 7 increases, it is not preferable.

(Group delay difference compensation using FBG device and FIR filter device)
FIG. 7 shows a configuration of an optical fiber transmission system and an optical receiver that compensate for a group delay difference between a plurality of modes using an FBG device and an FIR filter device. The optical fiber transmission system includes optical transmitters 1-1, 1-2, ..., 1-N, optical receivers 2-1, 2-2, ..., 2-M, multiplexer 3, and light. The fiber 4, the duplexer 5, the FIR filter device 6, and the FBG device 9 are included. The optical receiver R includes optical receivers 2-1, 2-2,..., 2-M, an FIR filter device 6, and an FBG device 9.

  .., 1-N, optical receivers 2-1, 2-2,..., 2-M, multiplexer 3, optical fiber 4, and duplexer 5 is the same as in FIGS. 1 and 7.

The FBG device 9 is an optical signal y ₁ (n), y ₂ (n),... Received by the optical receivers 2-1, 2-2,. .. For y _M (n), P propagation modes are incident from the entrance / exit and the P propagation modes are reflected to the entrance / exit using an FBG that reflects each of the P propagation modes. , P propagation modes are emitted from the entrance and exit, and the group delay difference is compensated. Then, the FBG device 9 generates optical signals y ₁ ′ (n), y ₂ ′ (n),..., Y _M ′ (n). Here, the optical signals y ₁ ′ (n), y ₂ ′ (n),..., Y _M ′ (n) are optical signals generated as the nth symbol.

The FIR filter device 6 is a delay that adjusts the delay time for the optical signals y ₁ ′ (n), y ₂ ′ (n),..., Y _M ′ (n) for which the FBG device 9 compensates for the group delay difference. The group delay difference is compensated by using an FIR filter including an element and an amplitude phase adjuster for adjusting the amplitude and phase. Then, the FIR filter device 6 restores the optical signals u ₁ (n), u ₂ (n),..., U _N (n). Here, the optical signals u ₁ (n), u ₂ (n),..., U _N (n) are optical signals restored as the n-th symbol.

  As described above, in the optical receiver R, the FBG is used in the first stage to preliminarily compensate for the group delay difference in the optical fiber transmission, and the FIR filter is used in the second stage to increase the group delay difference in the optical fiber transmission. Compensate for accuracy. Therefore, the number of taps in the FIR filter is reduced to shorten the calculation time, and long distance transmission is realized in optical fiber transmission using the optical MIMO technology.

  Here, the group delay difference compensated by the FBG device 9 may or may not coincide with the group delay difference generated in the optical fiber 4. In the former, the burden on the FIR filter device 6 is significantly reduced, but in the latter, the burden on the FIR filter device 6 is sufficiently reduced.

The configuration of the first FBG device is shown in FIG. The configuration of the second FBG device is shown in FIG. The first FBG device 9 includes FBGs 91-1, 91-2, 91-3, ..., 91-P and an optical circulator 92. The second FBG device 9 includes FBGs 91-1, 91-2, 91-3, ..., 91-P and an optical coupler 93. Since the configurations for generating the intermediate signals y ₁ ′ (n), y ₂ ′ (n),..., Y _M ′ (n) are the same, the representative intermediate signal y ′ in FIGS. Only the configuration for generating (n) will be described.

  The optical circulator 92 or the optical coupler 93 functions as an entrance / exit. The optical circulator 92 or the optical coupler 93 receives the reception signal y (n) from the optical receiver 2 and outputs it to the FBGs 91-1, 91-2, 91-3,..., 91-P. The FBGs 91-1, 91-2, 91-3,..., 91-P reflect each of the P propagation modes. The optical circulator 92 or the optical coupler 93 receives the intermediate signal y ′ (n) from the FBGs 91-1, 91-2, 91-3,..., 91 -P and outputs it to the FIR filter device 6.

Assume that the FBGs 91-1, 91-2, 91-3,..., 91-P are arranged in the order from the optical circulator 92 or the optical coupler 93. The distance from the FBG91-1 to FBG91-2 and _{L 1,} the distance from the FBG91-1 to FBG91-3 and _{L 2,} ···, the distance from the FBG91-1 to FBG91-P and _{L P-1} To do.

Between the propagation mode reflected by the FBG 91-1 and the propagation mode reflected by the FBG 91-2, the compensated delay time is 2L ₁ / (c / n _eff ) = 2n _eff L ₁ / c. Between the propagation mode reflected by the FBG 91-1 and the propagation mode reflected by the FBG 91-3, the compensated delay time is 2L ₂ / (c / n _eff ) = 2n _eff L ₂ / c. Between the propagation mode reflected by the FBG 91-1 and the propagation mode reflected by the FBG 91 -P, the compensated delay time is 2L _P-1 / (c / n _eff ) = 2n _eff L _P-1 / c Become. Here, c is the speed of light in vacuum, and n _eff is the effective refractive index of the propagation mode.

The period of the grating of the refractive index of the FBG 91-1 is Λ ₁ , the period of the grating of the refractive index of the FBG 91-2 is Λ ₂ , the period of the grating of the refractive index of the FBG 91-3 is Λ ₃ ,. the period of the grating of the refractive index of -P and lambda _P.

The effective refractive index n _eff of the propagation mode reflected by the FBG 91-1 is λ _p / (2Λ ₁ ). The effective refractive index n _eff of the propagation mode reflected by the FBG 91-2 is λ _p / (2Λ ₂ ). The effective refractive index n _eff of the propagation mode reflected by the FBG 91-3 is λ _p / (2Λ ₃ ). The effective refractive index n _eff of the propagation mode reflected by the FBG 91 -P is λ _p / (2Λ _P ). Here, λ _p is the wavelength of the optical signal in the optical MIMO technology.

  An example will be described in which two propagation modes, a fundamental mode and a higher-order mode, are present. FIG. 10 shows the relationship between the optical signal wavelength and the effective refractive index in the fundamental mode and the higher-order mode of the multimode optical fiber. In FIG. 10, the core radius a = 7 μm, the relative refractive index difference Δ of the core with respect to the cladding is 0.4%, and two propagation modes, LP01 mode and LP11 mode, at wavelengths of 1.53 to 1.56 μm. Exists.

When Λ is 0.5353 μm, the reflection wavelength of the LP01 mode is 1.550 μm, and the reflection wavelength of the LP11 mode is 1.548 μm. When Λ is 0.536 μm, the reflection wavelength of the LP01 mode is 1.552 μm, and the reflection wavelength of the LP11 mode is 1.550 μm. In other words, when the wavelength of the optical signal in the optical MIMO technology is 1.550 μm, if the Λ is 0.5353 μm and 0.536 μm in the order from the optical circulator 92 or the optical coupler 93, the effective refractive index of the nearest FBG 91 is The LP01 mode having a higher effective refractive index is reflected, and the LP11 mode having a lower effective refractive index is reflected by the next closest FBG 91. If the delay time to be compensated between the LP01 mode and the LP11 mode is D, the distance between the two FBGs 91 is (cD) / (2n _eff ).

つまり、図１０に挙げた例において、１個のＦＢＧ９１において１個の伝搬モードを反射させるときには、Δλが２ｎｍ以下となるように、Δｎ及びｌを適切に設定する。一般には、ＦＢＧ９１の反射帯域を数ｎｍの帯域とすることができるため、各伝搬モードを個別に反射する各ＦＢＧ９１は、現状の技術で実現可能であることが分かる。 Here, the reflection band Δλ of the FBG 91 is expressed as Equation 1 by the amplitude Δn and the region 1 with respect to the change in the refractive index grating of the FBG 91.

That is, in the example shown in FIG. 10, when one propagation mode is reflected by one FBG 91, Δn and l are appropriately set so that Δλ is 2 nm or less. Generally, since the reflection band of the FBG 91 can be set to a band of several nm, it can be seen that each FBG 91 that individually reflects each propagation mode can be realized by the current technology.

  In the above description, in the optical fiber 4, the effective refractive indexes for the P propagation modes are different from each other. In the FBG device 9, the pitch of the refractive index grating in the FBG 91 and the distance from the optical circulator 92 or the optical coupler 93 to the FBG 91 are different among the P propagation modes. Thereby, in each FBG 91, the group delay difference of the optical fiber transmission can be compensated with higher accuracy for each propagation mode.

  As another embodiment, in the optical fiber 4, the number of propagation modes at the used wavelength may be 2 or more and 7 or less. FIG. 11 shows the relationship between the normalized frequency and the number of propagation modes in a multimode optical fiber. When the normalized frequency is 6.38, the number of propagation modes is 7. When the normalized frequency is 6.38 or less, the increase in the number of propagation modes is moderate with respect to the increase in the normalized frequency, but when the normalized frequency is 6.38 or more, with respect to the increase in the normalized frequency. Therefore, the number of propagation modes increases rapidly. Therefore, in the optical fiber 4, the number of propagation modes at the used wavelength is set to be 2 or more and 7 or less. Thereby, the number of FBGs 91 can be further reduced.

  As another embodiment, in the optical fiber 4, the effective refractive indexes for at least two of the P propagation modes may be equal to each other. In the FBG device 9, the pitch of the refractive index grating in the FBG 91 and the distance from the optical circulator 92 or the optical coupler 93 to the FBG 91 may be equal between at least two propagation modes. An example of such an optical fiber 4 is a graded index multimode optical fiber in which the refractive index of the core decreases in a quadratic function with respect to the moving distance from the center to the periphery. Thereby, the number of FBGs 91 can be further reduced.

  Whether the group delay difference of 0.04 ns between the fundamental mode and the higher-order mode can be compensated by using the FIR filter device 6 when there are two optical transmitters 1 and two optical receivers 2 FIG. 12 shows a simulation result obtained by examining the above. Whether the group delay difference of 0.1 ns between the fundamental mode and the higher-order mode can be compensated by using the FIR filter device 6 when there are two optical transmitters 1 and two optical receivers 2 FIG. 13 shows a simulation result obtained by examining the above.

  In FIG. 12 and FIG. 13, the transmission / reception signal is 4096 bits of the NRZ signal subjected to 10 Gbps BPSK modulation. The number of propagation modes of the optical fiber 4 is two. For the transmission signal of the optical transmitter 1-1, the basic mode and the higher order mode are excited at a ratio of 10: 5, respectively, and for the transmission signal of the optical transmitter 1-2, the basic mode and the higher order mode are respectively 10: 3. Excited at a rate of The basic mode is demultiplexed by the optical receivers 2-1 and 2-2 at a ratio of 3: 7, and the higher-order mode is demultiplexed by the optical receivers 2-1 and 2-2 at a ratio of 6: 4, respectively. The

N _training is 400 and L is 20. τ _i is 10 Gbps symbol length T = 0.1 ns, τ ₁ = 0, τ ₂ = 0.1 ns, τ ₃ = 0.1 × 2 ns,..., τ _L = 0.1 × (L -1) ns. RLS is used as the adaptive equalization algorithm. The step size and forgetting factor in RLS are 0.001 and 1, respectively.

The case of FIG. 12 in which the group delay difference between the basic mode and the higher-order mode is 0.04 ns will be described. The upper part of FIG. 12 shows a constellation map of the transmission signals x ₁ (n) and x ₂ (n). The constellation map of the received signal y ₁ (n) is shown on the left side of the middle stage of FIG. The received signal y ₁ (n) does not correctly reflect the transmitted signal due to mode dispersion. A constellation map of the restored signal u ₁ (n) is shown on the right side of the middle stage of FIG. The restored signal u ₁ (n) uses the FIR filter device 6 to correctly restore the transmission signal x ₁ (n). A constellation map of the received signal y ₂ (n) is shown on the left side of the lower part of FIG. The received signal y ₂ (n) does not correctly reflect the transmitted signal due to mode dispersion. A constellation map of the restored signal u ₂ (n) is shown on the right side of the lower part of FIG. The restoration signal u ₂ (n) correctly restores the transmission signal x ₂ (n) by using the FIR filter device 6.

The case of FIG. 13 in which the group delay difference between the basic mode and the higher-order mode is 0.1 ns will be described. The upper part of FIG. 13 shows a constellation map of the transmission signals x ₁ (n) and x ₂ (n). A constellation map of the received signal y ₁ (n) is shown on the left side of the middle stage of FIG. The received signal y ₁ (n) does not correctly reflect the transmitted signal due to mode dispersion. A constellation map of the restored signal u ₁ (n) is shown on the right side of the middle stage of FIG. Even if the restoration signal u ₁ (n) uses the FIR filter device 6, the restoration signal x ₁ (n) is not correctly restored. The constellation map of the received signal y ₂ (n) is shown on the left side of the lower part of FIG. The received signal y ₂ (n) does not correctly reflect the transmitted signal due to mode dispersion. A constellation map of the restored signal u ₂ (n) is shown on the right side of the lower part of FIG. The restoration signal u ₂ (n) does not correctly restore the transmission signal x ₂ (n) even if the FIR filter device 6 is used.

Therefore, in the case of FIG. 13, not only the FIR filter device 6 but also the FBG device 9 is used. In the FBG device 9, there are two FBGs 91, and the structural parameters of the _first FBG (referred to as FBG 91-1) from the side on which y (n) is incident are Λ ₁ , Δn ₁ , l ₁ , The structural parameters of the _second FBG (referred to as FBG 91-2) from the side where y (n) is incident are Λ ₂ , Δn ₂ , and l ₂ . The core radius of the fiber is 7 μm, and the relative refractive index difference Δ with respect to the cladding of the core is 0.4%. The n _eff of the LP01 mode in the fiber is 1.44926, and the n _eff of the LP11 mode in the fiber is 1.44717. It is assumed that l ₁ = 1 mm, Λ ₁ = 0.5353 μm, Δn ₁ = 0.001, l ₂ = 1 mm, Λ ₂ = 0.536 μm, and Δn ₂ = 0.001.

Then, from Formula 1, in the FBG 91-1, the reflection wavelength of the LP01 mode is 1.55 μm, and the reflection wavelength of the LP11 mode is 1.548 μm. Further, from Formula 1, in the FBG 91-2, the reflection wavelength of the LP01 mode is 1.552 μm, and the reflection wavelength of the LP11 mode is 1.55 μm. Since the reflection band of each FBG 91 is 1.1 nm, only the LP01 mode is reflected by the FBG 91-1, and only the LP11 mode is reflected by the FBG 91-2. At this time, if the distance L ₁ between the FBG 91-1 and the FBG 91-2 is 6.2 mm, the compensated delay time is 2L ₁ / (c / n _eff ) = 2n _eff L ₁ /c=0.06 ns. . Therefore, the group delay difference of the received signal is changed from 0.1 ns before the application of the FBG device 9 to 0.04 ns after the application of the FBG device 9, and the group delay difference of the received signal is 0.04 ns as in FIG. It is possible to compensate using the FIR filter device 6.

In this way, by arranging the FBG device 9 and the FIR filter device 6 at the front stage and the rear stage of the optical receiving device R, respectively, without increasing the number of taps of the FIR filter device 6, the restored signals u ₁ (n), u ₂ (n) can correctly restore the transmission signals x ₁ (n) and x ₂ (n), respectively.

(Group delay difference compensation using FBG device)
In the above description, the group delay difference compensation using the FIR filter device 6 has been described first, and then the group delay difference compensation using the FBG device 9 and the FIR filter device 6 has been described. In the following description, group delay difference compensation using the FBG device 9 will be described.

  The number of optical transmitters 1 is one, the number of optical receivers 2 is M (M is an integer of 2 or more), and the number of propagation modes of the optical fiber 4 is P (P is an integer of 2 or more). Here, the group delay difference compensated by the FBG device 9 is equal to the group delay difference generated in the optical fiber 4. In other words, since the FBG device 9 fully compensates for the group delay difference of the optical fiber transmission, the long distance transmission can be realized in the optical fiber transmission using the optical MIMO technology without the need for the FIR filter device 6. it can. The configuration of the FBG device 9 is the same as that shown in FIGS.

  The optical fiber transmission system and the optical receiver according to the present invention are applied to optical fiber transmission that increases the transmission capacity and lengthens the transmission distance while reducing nonlinear effects and fiber fuses generated in the optical fiber. be able to.

R: optical receivers 1, 1-1, 1-2, 1-N: optical transmitters 2, 2-1, 2-2, 2-M: optical receiver 3: multiplexer 4: optical fiber 5: Demultiplexer 6: FIR filter device 7: Adaptive equalization algorithm 8: Subtractor 9: FBG devices 61, 61-1, 61-2, 61-M: FIR filter 62: Multiplexers 91, 91-1, 91 -2, 91-3, 91-P: FBG
92: Optical circulator 93: Optical coupler

Claims (10)

N optical transmitters (N is an integer of 1 or more) for transmitting optical signals;
M optical receivers (M is an integer equal to or greater than N) for receiving the optical signal;
The optical signals transmitted from the N optical transmitters are combined, and the combined optical signals are P (P is an integer equal to or greater than N. However, when N is 1, P is an integer equal to or greater than 2. And a multiplexer for sending to an optical fiber having a propagation mode of
A demultiplexer for demultiplexing the optical signal propagated by the optical fiber to the M optical receivers;
For the optical signal received by each optical receiver, the P propagation modes are incident from the entrance and exit, and the fiber Bragg grating that reflects each of the P propagation modes is used to enter the entrance and exit. A pre-stage group delay difference compensation device that reflects P propagation modes, emits the P propagation modes from the entrance and exit, and compensates for a group delay difference;
For the optical signal whose group delay difference is compensated by the preceding stage group delay difference compensation device, the group delay difference is calculated by using a delay element that adjusts the delay time and an FIR filter that adjusts the amplitude and phase. A rear-stage group delay difference compensator to compensate,
An optical fiber transmission system comprising: N optical transmitters (N is an integer of 1 or more) for transmitting optical signals;
M optical receivers (M is an integer equal to or greater than N) for receiving the optical signal;
A multiplexer for multiplexing the optical signals transmitted by the N optical transmitters;
An optical fiber that propagates the optical signal combined by the multiplexer and has P propagation modes (P is an integer greater than or equal to N, where P is an integer greater than or equal to 2 when N is 1);
A demultiplexer for demultiplexing the optical signal propagated by the optical fiber to the M optical receivers;
For the optical signal received by each optical receiver, the P propagation modes are incident from the entrance and exit, and the fiber Bragg grating that reflects each of the P propagation modes is used to enter the entrance and exit. A pre-stage group delay difference compensation device that reflects P propagation modes, emits the P propagation modes from the entrance and exit, and compensates for a group delay difference;
For the optical signal whose group delay difference is compensated by the preceding stage group delay difference compensation device, the group delay difference is calculated by using a delay element that adjusts the delay time and an FIR filter that adjusts the amplitude and phase. A rear-stage group delay difference compensator to compensate,
An optical fiber transmission system comprising: In the optical fiber, the effective refractive indexes for the P propagation modes are different from each other,
In the preceding-stage group delay difference compensator, the pitch of the refractive index grating in the fiber Bragg grating and the distance from the incident / exit port to the fiber Bragg grating are different among the P propagation modes. The optical fiber transmission system according to claim 2.   4. The optical fiber transmission system according to claim 2, wherein the optical fiber has two or more propagation modes and seven or less propagation modes at a used wavelength. 5. In the optical fiber, effective refractive indices for at least two of the P propagation modes are equal to each other,
In the preceding-stage group delay difference compensator, the pitch of the refractive index grating in the fiber Bragg grating and the distance from the input / output port to the fiber Bragg grating are equal between the at least two propagation modes. The optical fiber transmission system according to claim 2. From the N optical transmitters (N is an integer equal to or greater than 1) transmitting optical signals, the optical signals after transmission and multiplexing are propagated and P (P is an integer equal to or greater than N, provided that N is 1). M (where M is an integer equal to or greater than N) optical receivers that receive the optical signals after propagation and demultiplexing via an optical fiber having a propagation mode of P (an integer greater than or equal to 2),
For the optical signal received by each optical receiver, the P propagation modes are incident from the entrance and exit, and the fiber Bragg grating that reflects each of the P propagation modes is used to enter the entrance and exit. A grating group delay difference compensation device that reflects P propagation modes, emits the P propagation modes from the entrance and exit, and compensates for a group delay difference;
For the optical signal whose group delay difference is compensated by the preceding stage group delay difference compensation device, the group delay difference is calculated by using a delay element that adjusts the delay time and an FIR filter that adjusts the amplitude and phase. An FIR filter group delay difference compensator for compensating,
An optical receiving device comprising: From one optical transmitter that transmits an optical signal, the optical signal after transmission is propagated through an optical fiber having P propagation modes (P is an integer of 2 or more), and after propagation and demultiplexing. M optical receivers (M is an integer of 2 or more) for receiving the optical signal;
For the optical signal received by each optical receiver, the P propagation modes are incident from the entrance and exit, and the fiber Bragg grating that reflects each of the P propagation modes is used to enter the entrance and exit. A grating group delay difference compensation device that reflects P propagation modes, emits the P propagation modes from the entrance and exit, and compensates for a group delay difference;
An optical receiving device comprising: In the optical fiber, the effective refractive indexes for the P propagation modes are different from each other,
In the grating group delay difference compensating device, the pitch of the refractive index grating in the fiber Bragg grating and the distance from the input / output port to the fiber Bragg grating are different among the P propagation modes. The optical receiver according to claim 6 or 7.   9. The optical receiver according to claim 6, wherein the optical fiber has two or more propagation modes and no more than seven propagation modes at a used wavelength. In the optical fiber, effective refractive indices for at least two of the P propagation modes are equal to each other,
In the grating group delay difference compensator, the pitch of the refractive index grating in the fiber Bragg grating and the distance from the entrance / exit to the fiber Bragg grating are equal between the at least two propagation modes. The optical receiver according to claim 6 or 7.

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