JP2007150942A - Data transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a data transmission system which can realize high confidentiality by an optical physical level. <P>SOLUTION: An optical signal which conveys data D1 is generated by a laser diode 12 and an optical intensity modulator 14. A nonlinear group delay encoder 16 transforms the waveform of the optical signal. The nonlinear group delay decoder 32 of an optical receiver 30 reconstructs the waveform deterioration by the encoder 16 of the optical signal from an optical fiber transmission line 28. A photodiode 40 changes the output signal light of the decoder 32 into an electrical signal, and a data demodulation circuit 42 restores the data. The N pieces of the same ring resonators of a cavity length L and a ring resonator of a cavity length N×L are prepared, one or more of them is arranged to the encoder 16, and the residue is arranged to the decoder 32. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a data transmission system with improved secrecy at the optical physical level.

As a data transmission system that enhances secrecy at the photophysical level, a method using the photon number fluctuation (Patent Document 1) and a method using optical wavelength diffusion (Patent Document 2) are known.
JP 2004-302210 A JP 2003-209533 A

  However, the method described in Patent Document 1 requires a special light source, and the receiving apparatus is complicated and expensive. In addition, the method described in Patent Document 2 hops between a plurality of wavelengths at regular intervals to convert data into an optical signal, which complicates the configuration of the transmission apparatus. A wavelength filter corresponding to a plurality of wavelengths must also be prepared in the receiving apparatus, resulting in a complicated configuration.

  Therefore, an object of the present invention is to provide a data transmission system that can realize high secrecy at the optical physical level while having a simple configuration.

  A data transmission system according to the present invention includes a signal light generator that generates signal light that carries data, an optical transmitter that includes a nonlinear group delay encoder that applies a nonlinear group delay to the signal light, and the optical transmitter An optical receiving device that receives signal light output from a nonlinear group delay decoder that compensates for nonlinear group delay by the nonlinear group delay encoder, and photoelectric that converts an output optical signal of the nonlinear group delay decoder into an electrical signal A converter, and an optical receiver including a data demodulator that demodulates the data from an electrical signal output from the photoelectric converter.

  Preferably, the first resonator resonator has N first ring resonators (N is an integer of 2 or more), and a second resonator length equal to N times the first resonator length. One or more of the ring resonators are arranged in the nonlinear group delay encoder, and the rest are arranged in the nonlinear group delay decoder. The second ring resonator includes a nonlinear group delay that compensates for a combined nonlinear group delay of the N first ring resonators.

  According to the present invention, high concealment capability can be obtained by giving a nonlinear group delay to an optical signal. By using the ring resonator, the data can be concealed with low loss, and the authorized user can easily restore the data.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a schematic block diagram of an embodiment of the present invention. The optical transmission device 10 includes a laser diode 12 that is a light source. The light intensity modulator 14 modulates the intensity of the laser light from the laser 12 according to the data D1, and outputs pulse signal light carrying the data D1 to the nonlinear group delay encoder 16.

  The nonlinear group delay encoder 16 gives the optical signal output from the intensity modulator 14 a nonlinear group delay that periodically fluctuates within the band, and generates a distorted signal light in which the pulse waveform of the optical signal is degraded. . This distorted signal light is output to the optical fiber transmission line 28 as output signal light of the optical transmitter 10.

  The nonlinear group delay encoder 16 includes a connection waveguide 18 that connects the output of the intensity modulator 14 and the optical fiber transmission line 28, and three ring resonators having a resonator length L that are serially disposed along the connection waveguide 18. 20, 22, 24.

  Each ring resonator 20, 22, 24 includes ring waveguides 20 a, 22 a, 24 a, optical couplers 20 b, 22 b, 24 b that optically couple the ring waveguides 20 a, 22 a, 24 a to the connection waveguide 18, and ring guides. The phase shifters 20c, 22c, and 24c are arranged on the waveguides 20a, 22a, and 24a and shift the phase of light propagating through the ring waveguides 20a, 22a, and 24a by a predetermined amount. In this embodiment, the resonator lengths of the ring resonators 20, 22, and 24 are equally L, and the nonlinear group delay characteristics of the ring resonators 20, 22, and 24 are the same except that the resonance wavelengths are different from each other. is there. Details of the group delay characteristic of the nonlinear group delay encoder 16 will be described later.

  The signal light transmitted through the optical fiber transmission path 28 enters the nonlinear group delay decoder 32 of the optical receiver 30. The non-linear group delay decoder 32 is a device that eliminates the non-linear group delay caused by the non-linear group delay decoder 16 of the optical transmitter 10 and has a non-linear group delay characteristic that is substantially opposite to the non-linear group delay characteristic of the non-linear group delay decoder 16. .

  The nonlinear group delay decoder 32 includes a connection waveguide 34 that connects the optical fiber transmission line 28 and the photodiode 40, and two ring resonators 36 and 38 that are serially disposed along the connection waveguide 34. The ring resonator 36 includes a ring waveguide 36a having the same optical path length as the ring waveguides 20a, 22a, and 24a, an optical coupler 36b that optically couples the ring waveguide 36a to the connection waveguide 34, and the ring waveguide 36a. The phase shifter 36c is arranged and shifts the phase of light propagating through the ring waveguide 36a by a predetermined amount. The resonator length of the ring resonator 36 is L. The nonlinear group delay characteristic of the ring resonator 36 is theoretically the same as the nonlinear group delay of the ring resonators 20, 22, 24 except that the resonance wavelength is different from any of the ring resonators 20, 22, 24. is there.

  The ring resonator 38 includes a ring waveguide 38a having an optical path length approximately four times that of the ring waveguides 20a, 22a, and 24a, an optical coupler 38b that optically couples the ring waveguide 38a to the connection waveguide 34, and a ring. The phase shifter 38c is disposed on the waveguide 38a and shifts the phase of light propagating through the ring waveguide 38a by a predetermined amount. The resonator length of the ring resonator 38 is 4L.

  The group delay characteristic of the nonlinear group delay decoder 32 will be described later together with the nonlinear group delay encoder 16.

  The nonlinear group delay decoder 32 eliminates the distortion caused by the nonlinear group delay decoder 16 to the extent that data demodulation is possible. The nonlinear group delay decoder 32 may have a group delay characteristic that simultaneously eliminates waveform distortion due to group delay in the optical fiber transmission line 28.

  The photodiode 40 converts the output optical signal of the nonlinear group delay decoder 32 into an electrical signal. The data demodulator 42 demodulates the data D1 from the output electrical signal of the photodiode 40.

  The group delay characteristic that periodically varies within the band of the optical signal can be realized by a plurality of ring resonators connected in a single or serial manner.

  Further, in order to avoid data leakage due to optical signal leakage on the optical fiber transmission line 28, it is desirable that the waveform distortion is large on the optical fiber transmission line 28. In addition, it is desirable that there is no or little waveform distortion at the output stage of the nonlinear group delay decoder 32. In order to satisfy these requirements, the present embodiment basically uses a combination of two types of ring resonators having different ring resonator lengths.

  First, the structure and characteristics of the ring resonator will be described. FIG. 2 is a plan view of a single ring resonator, and FIG. 3 is a cross-sectional view taken along line AA of FIG.

  A Mach-Zehnder interferometer 52 is disposed on the substrate 50. The Mach-Zehnder interferometer 52 connects two input / output two optical multiplexer / demultiplexers 54 and 56, two output ports of the optical multiplexer / demultiplexer 54, and two input ports of the optical multiplexer / demultiplexer 56, respectively. Two arms 58 and 60 are provided. An input waveguide 62 is connected to one input port of the optical multiplexer / demultiplexer 54, and an output waveguide 64 is connected to one output port of the optical multiplexer / demultiplexer 56. The other output port of the optical multiplexer / demultiplexer 56 is connected to the other input port of the optical multiplexer / demultiplexer 54 via the waveguide 66.

  The portion composed of the input waveguide 62, the arm 60, and the output waveguide 64 corresponds to the connection waveguide 18 of the encoder 16 and the connection waveguide 34 of the decoder 32, and the portion composed of the waveguide 66 and the arm 58 is a ring waveguide. It corresponds to 20a, 22a, 24a, 36a, 38a. In this embodiment, the ring waveguides 20a, 22a, 24a, 36a, and 38a are optically coupled to the connection waveguides 18 and 34 at two locations of the optical multiplexers / demultiplexers 54 and 56.

  A thin film heater 68 as a phase shifter is disposed on the arm 60, and a voltage controller 70 applies a voltage to the thin film heater 68. The optical path length of the arm 60 is adjusted by the thin film heater 68, and as a result, the optical coupling ratio between the waveguides 62 and 64 and the waveguide 66, that is, the connection waveguides 18 and 34 and the ring waveguides 20 a and 22 a. The optical coupling ratio between 24a, 36a and 38a can be adjusted. The Mach-Zehnder interferometer 52 corresponds to the optical couplers 20b, 22b, 24b, 36b, and 38b.

  A thin film heater 72 corresponding to the phase shifters 20 c, 22 c, 24 c, 36 c, and 38 c is disposed on the waveguide 66, and the voltage controller 74 applies a voltage to the thin film heater 72. The thin film heater 72 can adjust the optical phase of light propagating through the waveguide 66, that is, the ring waveguides 20a, 22a, 24a, 36a, and 38a.

The group delay characteristics of the ring resonator as shown in FIGS. 2 and 3 are expressed by the following equation τ (ω) = T (1−K ² ) / {1 + K ² −2K cos (ωT−φ)}.
However,
K = (1-κ) ^1/2
κ = 0.5 + 0.5cosθ
Given in. ω is the angular frequency of the light, T is the time required for the signal light to make one round of the ring waveguide, and φ is the phase shift amount (relative value) by the thin film heater 72. T depends on the optical path length of the waveguide 66, that is, the optical path lengths of the ring waveguides 20a, 22a, 24a, 36a, and 38a. If the speed of light is c, the wavelength λ = 2πc / ω. The free spectral region FSR (Free Spectral Region) is represented by 1 / T. θ is a phase shift amount (relative value) by the thin film heater 68. κ represents the coupling rate of the Mach-Zehnder interferometer 52. By adjusting the phase shift amount θ by the thin film heater 68, the coupling rate κ can be selected between 0 and 100%.

  FIG. 4 is a group delay characteristic of a single ring resonator, and shows a variation example with respect to the coupling rate κ. FIG. 5 shows a group delay characteristic of a single ring resonator, and shows a variation example with respect to the phase shift amount φ. 4 and 5, the horizontal axis indicates the relative wavelength, and the vertical axis indicates the relative group delay. As is well known, the group delay characteristic of the ring resonator has a group delay that periodically varies with respect to the wavelength. The period is determined by the optical path lengths of the ring waveguides 20a, 22a, 24a, 36a, and 38a, the peak wavelength is determined by the phase shifters 20c, 22c, 24c, 36c, and 38c, the phase shift amount φ, and the fluctuation amplitude is It is determined by the binding rate κ.

  From FIG. 4, when the coupling rate κ is increased, the group delay characteristic approaches a sine waveform. Further, as the coupling rate κ decreases, the upward convex portion becomes steep, and the difference between the minimum value and the maximum value of the group delay increases. FIG. 5 shows that the resonance wavelength can be changed by the phase shift amount. The group delay characteristic of φ = 2π matches the group delay characteristic of φ = 0. 4 and 5 that the height difference of the group delay and the resonance wavelength can be arbitrarily controlled.

  When a plurality of ring resonators are connected in series, the overall group delay characteristic is the sum of the group delay characteristics of the individual ring resonators. The group delay characteristic obtained by serially arranging N ring resonators (N is an integer of 2 or more) with the same resonator length L and shifted resonance wavelengths by 2π / N is basically a resonator. It is possible to approximate the group delay characteristic of one ring resonator having a length of N × L. Therefore, the group delay can be substantially eliminated by shifting the latter group delay characteristic by π and adding the latter to the former combined group delay characteristic. However, since the group delay characteristics realized by the ring resonator have slightly different shapes between the upward convex portion and the downward convex portion, some ripple remains. In the embodiment shown in FIG. 1, N = 4.

  In the embodiment shown in FIG. 1, for example, the coupling rate κ of the optical couplers 20b, 22b, 24b, 36b of the ring resonators 20, 22, 24, 36 is 0.6, and the phase shifters 20c, 22c, 24c, 36c The phase shift amounts φ are set to 0, π / 2, π, and 3π / 2, respectively. FIG. 6A shows a combined group delay characteristic 80 of the ring resonators 20, 22, and 24 and a group delay characteristic 82 of the ring resonator 36. The horizontal axis represents the relative wavelength, and the vertical axis represents the relative delay delay. FIG. 6B shows a group delay characteristic obtained by synthesizing the group delay characteristics 80 and 82. Since the parameters of the ring resonators 20, 22, 24, and 36 are the same except for the phase shift amount φ and the phase shift amount φ is different by π / 2 (= 2π / 4), the combined group delay characteristic is The period is 1/4 compared to the case of the single ring resonator, and the change is almost a sinusoidal waveform. In other words, the coupling rate κ is adjusted so that the combined group delay characteristic approaches a sine waveform.

  On the other hand, the coupling factor κ of the ring resonator 38 is 0.975, and the phase shift amount φ is π. FIG. 6C shows the group delay characteristic 86 of the ring resonator 38. The horizontal axis represents the relative wavelength, and the vertical axis represents the relative delay delay. By increasing the coupling rate κ, the group delay characteristic 86 approaches a sine waveform. In addition, by setting the phase shift amount φ to π, a group delay characteristic close to a characteristic obtained by inverting the composite group delay characteristic 84 shown in FIG. 6B can be obtained. 6D synthesizes the combined group delay characteristic 84 by the ring resonators 20, 22, 24, and 36 shown in FIG. 6B and the group delay characteristic 86 by the ring resonator 38 shown in FIG. 6C. The resulting group delay characteristics 88 are shown. As described above, the group delay characteristic realized by the ring resonator has a slightly different shape between the upwardly convex part and the downwardly convex part. In this example, the peak delay is 80 ps. Ripple remains. Such a ripple will not interfere with data demodulation.

Briefly explain the theoretical relationship. The group delay characteristic of a single ring resonator can be expanded by a Fourier series and can be expressed by a synthesis of trigonometric functions. When the group delay characteristic of the ring resonator 20 is F ₁ (ω),
F ₁ (ω) = Σ _k E _k cos (kωT + kφ ₁ )
However, Σ _k indicates the cumulative addition of k = 1, 2, 3,.

Similarly, the group delay characteristics F ₂ (ω), F ₃ (ω), and F ₄ (ω) of the ring resonators 22, 24, and 36 are
F ₂ (ω) = Σ _k E _k cos (kωT + kφ ₂ )
F ₃ (ω) = Σ _k E _k cos (kωT + kφ ₃ )
F ₄ (ω) = Σ _k E _k cos (kωT + kφ ₄ )
It is. Since the amplitudes are equal, the coefficients E _k of the group delay characteristics F ₁ (ω), F ₂ (ω), F ₃ (ω), and F ₄ (ω) are equal.

In these equations, φ ₁ = 0, φ ₂ = π / 2, φ ₃ = π, and φ ₄ = 3π / 2 are substituted. These are integer multiples of 2π divided by the number N (here, 4) of ring resonators 20, 22, 24, 36 having the same resonator length.

In the sum of F ₁ (ω), F ₂ (ω), F ₃ (ω), and F4 (ω), cos ((4n + 1) ωT), cos ((4n + 2) ωT) and cos ((4n + 3) ωT) Ingredients cancel and disappear. However, n is an integer. Therefore,
F ₁ (ω) + F ₂ (ω) + F ₃ (ω) + F 4 (ω)
= Σ _k 4E _{4 k} cos ( ₄ kωT)
It becomes. The remaining ripple component mainly consists of 4E ₄ cos (4ωT). The period of this component is 1 / N times (here, 1/4 times) the period T of a single ring resonator.

On the other hand, by setting the resonator length of the ring resonator 38 to N × L (here, 4L), the FSR of the ring resonator 38 is one of that of the ring resonators 20, 22, 24, 36. / N. To counteract 4E ₄ cos (4ωT), sets the amount of phase shift by the phase shifter 38c to π (rad), modulates the binding ratio of the optical coupler 38b so that the amplitude matches the 4E ₄ kappa. FIG. 6C shows the group delay characteristic 86 of the ring resonator 38 thus obtained.

  In FIG. 6D, the combined group delay characteristic 84 (FIG. 6B) by the ring resonators 20, 22, 24, and 36 is canceled by the group delay characteristic 86 of the ring resonator 38 (FIG. 6C). As a result, the remaining group delay characteristic 88 is shown. The remaining ripple of the group delay characteristic 88 is about 80 ps peak-to-peak.

In the case of 10 Gbit / s NRZ signal light, the influence of the encoder 16 and the decoder 32 was examined. When the encoder 16 and the decoder 32 are not provided, the received light intensity at which the bit error rate (BER) is 10 ⁻⁹ is set as a reference value, and the magnification (dB) of the light intensity required to obtain the same BER is set as a power penalty. Define. FIG. 7 shows the calculation result of ripple vs. power penalty. The horizontal axis represents the peak-to-peak ripple (ps), and the vertical axis represents the power penalty. When the remaining ripple is 260 ps, that is, when the decoder 32 of the receiving device 30 includes only the ring resonator 36, the power penalty is 2.7 dB, and when the ripple is 80 ps, that is, the decoder 32 is In the case of the resonator 36 and the resonator 36, the power penalty was 0.2 dB. Thereby, it can be seen that the decoder 32 can receive a good signal.

  The ring resonator 38 can be replaced with four ring resonators connected in series with the resonator length L. However, in this case, the adjustment of the coupling ratio κ and the phase shift amount φ of each ring resonator becomes troublesome, and control is also possible. It becomes complicated.

  How many of the N ring resonators having the resonator length L are arranged on the transmission side can be determined as appropriate, and the ring resonator having the resonator length N × L is set on the transmission side or the reception side. It can also be determined as appropriate.

  An example of changing the arrangement of the ring resonator will be briefly described. FIG. 8 shows a first modified arrangement example. Two ring resonators 110 and 112 having a resonator length L are arranged in the encoder 16a on the transmitting side, and two ring resonators 114 and 116 having a resonator length L and a ring having a resonator length of 4L are arranged on the decoder 32a on the receiving side. A resonator 118 is arranged.

  FIG. 9 shows a second modified arrangement example. In FIG. 9, one ring resonator 120 having a resonator length L is arranged in the encoder 16b on the transmission side, and three ring resonators 122, 124, 126 having a resonator length L and a resonator are arranged in the decoder 32b on the reception side. A ring resonator 128 having a length of 4L is arranged.

  FIG. 10 shows a third modified arrangement example. In FIG. 10, three ring resonators 130, 132, 134 having a resonator length L and a ring resonator 136 having a resonator length 4L are arranged in the encoder 16c on the transmission side, and the resonator length L is arranged in the decoder 32c on the reception side. One ring resonator 138 is arranged.

  The change arrangement example shown in FIG. 10 can be changed like the change arrangement example shown in FIG. That is, two ring resonators 140 and 142 having a resonator length L, a ring resonator 144 having a resonator length of 4L, and a ring resonator 146 having a resonator length of L are serially arranged in the encoder 16d on the transmission side and received. One ring resonator 148 having a resonator length L is arranged in the decoder 32d on the side. Since the plurality of ring resonators 140 to 148 are serially connected and the respective nonlinear characteristics are linearly added, the ring resonator 144 having the resonator length 4L may be disposed anywhere (for example, at the head) of the encoder.

  Although an embodiment has been described in which four ring resonators having a resonator length L and one ring resonator having a resonator length 4L are used in combination, as described above, in general, N of the resonator length L By combining a single ring resonator and one ring resonator having a resonator length of N × L, the same effect can be obtained. N may be 2 or more in principle, but N is preferably 3 or more in terms of data confidentiality.

  Furthermore, ring resonators having different resonator lengths L may be combined. FIG. 12 shows an example of the changed arrangement. Two ring resonators 150 and 152 having a resonator length La and two ring resonators 154 and 156 having a resonator length Lb are serially arranged in the encoder 16e on the transmission side, and the resonator is provided in the decoder 32e on the reception side. One ring resonator 158 having a length La, a ring resonator 160 having a resonator length 3La, a ring resonator 162 having a resonator length Lb, and a ring resonator 164 having a resonator length 3Lb are serially arranged. The arrangement order of the ring resonators 150, 152, 154, and 156 in the encoder 16e is not limited. Similarly, the arrangement order of the ring resonators 158, 160, 162, and 164 in the decoder 32e is not limited.

  Ring resonator 160 compensates for group delay caused by ring resonators 150, 152, and 158, and ring resonator 164 compensates for group delay caused by ring resonators 154, 156, and 162.

  Although the invention has been described with reference to specific illustrative embodiments, various modifications and alterations may be made to the above-described embodiments without departing from the scope of the invention as defined in the claims. This is obvious to an engineer in the field to which the present invention belongs, and such changes and modifications are also included in the technical scope of the present invention.

It is a schematic block diagram of one Example of this invention. 1 shows a plan view of a single ring resonator. Sectional drawing seen from the AA line of FIG. 2 is shown. It is a group delay characteristic of a single ring resonator, and shows an example of a change with respect to the coupling rate κ. A group delay characteristic of a single ring resonator, which shows a variation example with respect to the phase shift amount φ. It is a figure which shows the group delay characteristic of each part of a present Example. It is a graph which shows the relationship between a ripple and a power penalty. It is an example of change arrangement of a ring resonator. It is an example of change arrangement of a ring resonator. It is an example of change arrangement of a ring resonator. It is an example of change arrangement of a ring resonator. It is an example of change arrangement of a ring resonator.

Explanation of symbols

10: Optical transmitter 12: Laser diode 14: Light intensity modulator 16, 16a, 16b, 16c, 16d, 16e: Non-linear group delay encoder 18: Connection waveguides 20, 22, 24: Ring resonators 20a, 22a, 24a : Ring waveguides 20b, 22b, 24b: optical couplers 20c, 22c, 24c: phase shifter 28: optical fiber transmission line 30: optical receivers 32, 32a, 32b, 32c, 32d, 32e: nonlinear group delay decoder 34: Connection waveguides 36, 38: ring resonators 36a, 38a: ring waveguides 36b, 38b: optical couplers 36c, 38c: phase shifter 40: photodiode 42: data demodulator 50: substrate 52: Mach-Zehnder interferometers 54, 56 : Optical multiplexer / demultiplexer 58, 60: arm 62: input waveguide 64: output waveguide 66: waveguide 68: thin film 70: Voltage controller 72: Thin film heater 74: Voltage controller 110, 112, 114, 116, 118: Ring resonators 120, 122, 124, 126, 128: Ring resonators 130, 132, 134, 136, 138 : Ring resonators 140, 142, 144, 146, 148: Ring resonators 150, 152, 154, 156, 158, 160, 162, 164: Ring resonators

Claims (3)

An optical transmitter (10) comprising a signal light generator (12, 14) for generating signal light for carrying data and a nonlinear group delay encoder (16, 16a-16e) for giving a nonlinear group delay to the signal light When,
An optical receiver that receives signal light output from the optical transmitter, and includes a nonlinear group delay decoder (32, 32a to 32e) that reduces nonlinear group delay by the nonlinear group delay encoder, and the nonlinear group delay decoder. Optical receiver (30) comprising an optical / electrical converter (40) for converting an output optical signal into an electric signal, and a data demodulator (42) for demodulating the data from the electric output signal of the photoelectric converter
A data transmission system comprising:   N first ring resonators (20, 22, 24, 36) of the first resonator length (L) (N is an integer of 2 or more), and N of the first resonator length (L) A second ring resonator (38) having a resonator length (N × L) equal to twice, comprising a nonlinear group delay that compensates for the combined nonlinear group delay of the N first ring resonators. 2. The data transmission system according to claim 1, wherein one or more of the second ring resonators are arranged in the nonlinear group delay encoder, and the rest are arranged in the nonlinear group delay decoder.   M (M is an integer of 2 or more) third ring resonators (154, 156, 162) having a second resonator length (Lb) different from the first resonator length (La), And one or more of the fourth ring resonators (164) having a resonator length (M × Lb) equal to M times the second resonator length (Lb) are used as the nonlinear group delay encoder. The data transmission system according to claim 2, wherein the data transmission system is arranged and the rest is arranged in the nonlinear group delay decoder.

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