Orthogonal frequency division multiplexing signal separation system

Abstract

To provide an orthogonal frequency division multiplexing signal demultiplexing system that can separate terahertz OFDM signals in real time directly in a terahertz band, improve processing speed without being subject to band restrictions in the RF band, and reduce power consumption.SOLUTION: A plurality of Mach-Zehnder interferometer (first Mach-Zehnder interferometer 2, second Mach-Zehnder interferometer 3, and third Mach-Zehnder interferometer 4) that can perform discrete Fourier transform on a terahertz-band orthogonal frequency division multiplexed signal (S) with multiple subcarrier signals (d0, d1, d2, d3) in the terahertz domain are used to separate the plurality of subcarrier signals (d0, d1, d2, d3).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an orthogonal frequency division multiplexing (OFDM) signal demultiplexing system, and more particularly to an orthogonal frequency division multiplexing (OFDM) signal demultiplexing system capable of demultiplexing an Orthogonal Frequency Division Multiplexing (OFDM) signal in the terahertz band. [Background technology]

[0002] Conventionally, an invention such as that described in Patent Document 1 is known as a circuit capable of separating an Orthogonal Frequency Division Multiplexing (OFDM) signal. This invention comprises a receiving antenna unit that receives an Orthogonal Frequency Division Multiplexing (OFDM) signal in the terahertz band, an RF conversion unit that converts the Orthogonal Frequency Division Multiplexing (OFDM) signal in the terahertz band received by the receiving antenna unit into an Orthogonal Frequency Division Multiplexing (OFDM) signal in the RF band, an optical conversion unit that converts the Orthogonal Frequency Division Multiplexing (OFDM) signal in the RF band converted by the RF conversion unit into an Optical Orthogonal Frequency Division Multiplexing (OFDM) signal, and a separation circuit that separates a plurality of subcarrier signals from the Optical Orthogonal Frequency Division Multiplexing (OFDM) signal converted by the optical conversion unit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-121107 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned invention has the characteristic of being able to demultiplex a terahertz band OFDM signal in real time. However, since the above-mentioned invention down-converts a signal to an RF band OFDM signal, outputs the RF band OFDM signal to an optical converter, and then demultiplexes the optical orthogonal frequency division multiplexed signal into multiple subcarrier signals using a demultiplexing circuit, it is subject to band limitations in the RF band, resulting in problems such as longer processing times and increased power consumption.

[0005] In view of the above problems, the present invention aims to provide an orthogonal frequency division multiplexing signal demultiplexing system that can demultiplex terahertz band OFDM signals directly in the terahertz band in real time, improve processing speed without being subject to band limitations in the RF band, and further reduce power consumption. [Means for solving the problem]

[0006] The above object of the present invention can be achieved by the following means: Note that the parentheses indicate reference symbols of embodiments to be described later, but the present invention is not limited to these.

[0007] The orthogonal frequency division multiplexing signal demultiplexing system according to claim 1 comprises a plurality of subcarrier signals (d0, d1, . . . , d N-1 The terahertz band orthogonal frequency division multiplexed signal (S) with the terahertz band is subjected to a discrete Fourier transform in the terahertz region by a Mach-Zehnder interferometer (first Mach-Zehnder interferometer 2, second Mach-Zehnder interferometer 3, third Mach-Zehnder interferometer 4). Duplicate Michelson interferometers capable of discrete Fourier transform in the terahertz region (first Michelson interferometer 2A, second Michelson interferometer 3A, third Michelson interferometer 4A) Duplicate The plurality of subcarrier signals (d0, d1, . . . , d N-1 ) and separate them. the law of nature, When separating the plurality of subcarrier signals, a delay time is provided in the Mach-Zehnder interferometer or the Michelson interferometer to separate the signals, when separating the plurality of subcarrier signals, the first-stage delay time of the Mach-Zehnder interferometer (first Mach-Zehnder interferometer 2) or the Michelson interferometer (first Michelson interferometer 2A) having a first-stage delay time is 1 / 2Δf, where Δf is a frequency interval between the plurality of subcarrier signals; The second-stage delay time of the Mach-Zehnder interferometer (first Mach-Zehnder interferometer 2) having the first-stage delay time, or the Mach-Zehnder interferometer (second Mach-Zehnder interferometer 3, third Mach-Zehnder interferometer 4) having the second-stage delay time set to the same delay time and located downstream of the Michelson interferometer (first Michelson interferometer 2A), or the Michelson interferometer (second Michelson interferometer 3A, third Michelson interferometer 4A) is 1 / 2 the first-stage delay time. It is characterized by the following.

[0008] Furthermore, an orthogonal frequency division multiplexing signal demultiplexing system according to claim 2 is the orthogonal frequency division multiplexing signal demultiplexing system (1) according to claim 1, wherein the Mach-Zehnder interferometer is The orthogonal frequency division multiplexed signal (S) in the terahertz band is received, the received orthogonal frequency division multiplexed signal (S) in the terahertz band is branched into two, and one of the branched orthogonal frequency division multiplexed signals (S) in the terahertz band is delayed by a first delay time (for example, 2Δt) relative to the other orthogonal frequency division multiplexed signal (S) in the terahertz band, and the one orthogonal frequency division multiplexed signal (S) in the terahertz band and the delayed other orthogonal frequency division multiplexed signal (S) are caused to interfere with each other, thereby generating the plurality of subcarrier signals (d0, d1, . . . , d N-1 ) is separated.

[0009] Furthermore, an orthogonal frequency division multiplexing signal demultiplexing system according to claim 3 is the orthogonal frequency division multiplexing signal demultiplexing system (1A) according to claim 1, wherein the Michelson interferometer is The orthogonal frequency division multiplexed signal (S) in the terahertz band is received, the received orthogonal frequency division multiplexed signal (S) in the terahertz band is branched into two, and one of the branched orthogonal frequency division multiplexed signals (S) in the terahertz band is delayed by a first delay time (for example, 2Δt) relative to the other orthogonal frequency division multiplexed signal (S) in the terahertz band, and the one orthogonal frequency division multiplexed signal (S) in the terahertz band and the delayed other orthogonal frequency division multiplexed signal (S) are caused to interfere with each other, thereby generating the plurality of subcarrier signals (d0, d1, . . . , d N-1 ) is separated. [Effects of the Invention]

[0010] Next, the effects of the present invention will be described with reference to the drawings. Note that the reference symbols in parentheses are those of the embodiments described below, but the present invention is not limited to these.

[0011] According to the invention of claim 1, it is possible to realize a Mach-Zehnder interferometer capable of performing a discrete Fourier transform in the terahertz region without the need to down-convert to an RF band OFDM signal, output the RF band OFDM signal to an optical converter, and separate a plurality of subcarrier signals from the optical orthogonal frequency division multiplexed signal converted by the optical converter using a separation circuit, as in the conventional case. Duplicate (First Mach-Zehnder interferometer 2, Second Mach-Zehnder interferometer 3, Third Mach-Zehnder interferometer 4), or Michelson interferometers capable of discrete Fourier transform in the terahertz region (First Michelson interferometer 2A, Second Michelson interferometer 3A, Third Michelson interferometer 4A) Duplicate By simply using the above, multiple subcarrier signals (d0, d1, . . . , d N-1 ) can be separated.

[0012] Therefore, according to the present invention, it is possible to separate terahertz band OFDM signals directly in the terahertz band in real time, improve processing speed without being subject to band limitations in the RF band, and further reduce power consumption.

[0013] Incidentally, the configuration described in claim 2 is preferable for such a Mach-Zehnder interferometer, and the configuration described in claim 3 is preferable for such a Michelson interferometer. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a block diagram showing an embodiment of an orthogonal frequency division multiplexing signal demultiplexing system according to the present invention; [Figure 2] FIG. 1(a) is a diagram showing the spectral shape of an orthogonal frequency division multiplexing (OFDM) signal in the terahertz band, and (b-1) to (b-4) are explanatory diagrams for performing serial-to-parallel conversion of a signal using delay. [Figure 3] FIG. 10 is a block diagram showing another embodiment of an orthogonal frequency division multiplexing signal demultiplexing system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] An orthogonal frequency division multiplexing signal demultiplexing system according to the present invention will be specifically described below with reference to the drawings. In the following description, when directions such as up, down, left, and right are indicated, they refer to up, down, left, and right when viewed from the front of the illustration.

[0016] <Explanation of Orthogonal Frequency Division Multiplexing Signal Separation System> The orthogonal frequency division multiplexing signal demultiplexing system is capable of demultiplexing an orthogonal frequency division multiplexing (OFDM) signal in the terahertz band, and specifically, as shown in Fig. 1, the orthogonal frequency division multiplexing signal demultiplexing system 1 is mainly composed of a first terahertz band Mach-Zehnder interferometer 2, a second terahertz band Mach-Zehnder interferometer 3, and a third terahertz band Mach-Zehnder interferometer 4. Note that the orthogonal frequency division multiplexing signal demultiplexing system 1 shown in this embodiment exemplifies a case where the number of demultiplexed channels is four.

[0017] As shown in Fig. 2(a), an orthogonal frequency division multiplexing (OFDM) signal in the terahertz band is composed of multiple subcarrier signals d0, d1, . . . , d N-1 The orthogonal relationship in Orthogonal Frequency Division Multiplexing (OFDM) means that each subcarrier signal d0, d1, . . . , d N-1 When the relationship Δf=1 / T holds between the symbol rate 1 / T (T is the symbol time of each subcarrier signal) of the modulated signal and the subcarrier signal frequency interval Δf, it becomes possible to separate and identify each subcarrier signal even if the transmission spectrum of each subcarrier signal overlaps.

[0018] It is known that such terahertz band orthogonal frequency division multiplexing (OFDM) signals cannot be separated without a Fourier transform. Furthermore, in order to directly separate terahertz band orthogonal frequency division multiplexing (OFDM) signals into individual subcarrier signals at high speed in the terahertz region, an element capable of performing a discrete Fourier transform (DFT) in the terahertz region or a fast Fourier transform (FFT) in the terahertz region is essential as a component.

[0019] Therefore, in this embodiment, in order to realize a discrete Fourier transform in the terahertz region, a first Mach-Zehnder interferometer 2, a second Mach-Zehnder interferometer 3, and a third Mach-Zehnder interferometer 4 are configured.

[0020] More specifically, the discrete Fourier transform is expressed by the following equation:

[0021]

number

[0022] I(t) is the input signal of the first Mach-Zehnder interferometer 2, O n (t) represents the output signals of the second Mach-Zehnder interferometer 3 and the third Mach-Zehnder interferometer 4. Furthermore, N is the number of separated channels (the number of subcarrier signals to be separated), and j is the imaginary unit.

[0023] In this case, the discrete Fourier transform includes I(kΔt), as shown in Equation 1 above. Therefore, in this embodiment, when a terahertz band orthogonal frequency division multiplexing (OFDM) signal is demultiplexed in real time without using an electronic circuit such as a memory, serial-to-parallel conversion of the signal is performed using delay, as shown in FIGS. 2(b-1) to 2(b-4). Specifically, to explain this using a specific example, if a terahertz band orthogonal frequency division multiplexing (OFDM) signal S consisting of four subcarrier signals d0, d1, d2, and d3 has a symbol time of 4Δt, the signal will be in the state shown in FIG. 2(b-4) when it is not delayed. When it is delayed by Δt, the signal will be in the state shown in FIG. 2(b-3). When it is further delayed by 2Δt, the signal will be in the state shown in FIG. 2(b-2). When it is further delayed by 3Δt, the signal will be in the state shown in FIG. 2(b-1).

[0024] Thus, by aligning a portion of the signal in the state shown in Figures 2(b-1) to 2(b-4) with the portion showing the effective time, it becomes possible to convert the terahertz band orthogonal frequency division multiplexing (OFDM) signal S into four time domains I(0), I(Δt), I(2Δt), and I(3Δt) in parallel.

[0025] In this embodiment, taking the above points into consideration, the first Mach-Zehnder interferometer 2, the second Mach-Zehnder interferometer 3, and the third Mach-Zehnder interferometer 4 are configured. Each configuration will be described in detail below.

[0026] <Explanation of the first Mach-Zehnder interferometer> The first Mach-Zehnder interferometer 2 is capable of performing a discrete Fourier transform in the terahertz band, and is mainly composed of beam splitters 20 and 25 and mirrors 21 to 24, as shown in Fig. 1. To explain this using a specific example, as shown in Fig. 1, when a terahertz band orthogonal frequency division multiplexing (OFDM) signal S consisting of four subcarrier signals d0, d1, d2, and d3 is input to the first Mach-Zehnder interferometer 2 via a lens 10, the beam splitter 20 branches the terahertz band orthogonal frequency division multiplexing (OFDM) signal S into two, a first path 2a and a second path 2b, as shown in Fig. 1. The terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched to the first path 2a is reflected by a mirror 21 to the third path 2c and enters the beam splitter 25.

[0027] By the way, the characteristics of a beam splitter are that when a signal is transmitted, the phase shift is 0, and when the signal is reflected from the side to the top or from the top to the side with the reflecting surface as the top, the phase shift is π, and when the signal is reflected from the bottom to the side or from the side to the bottom, the phase shift is also 0. Also, the phase shift when reflected by a mirror is always π.

[0028] On the other hand, the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched to the second path 2b side is reflected by mirror 22 to the fourth path 2d side and is incident on mirror 23. Further, the terahertz band orthogonal frequency division multiplexing (OFDM) signal S incident on mirror 23 is reflected to the fifth path 2e side and is incident on mirror 24. Furthermore, the terahertz band orthogonal frequency division multiplexing (OFDM) signal S incident on mirror 24 is reflected to the sixth path 2f side and is incident on beam splitter 25.

[0029] The arrangement distance is set so that there is a relative delay time difference of 2Δt between the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20 to the first path 2a side, passed through the third path 2c and entered the beam splitter 25, and the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20 to the second path 2b side, passed through the fourth path 2d to the sixth path 2f and entered the beam splitter 25. In other words, the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20 to the second path 2b side, passed through the fourth path 2d to the sixth path 2f and entered the beam splitter 25 is delayed by 2Δt compared to the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20 to the first path 2a side, passed through the third path 2c and entered the beam splitter 25.

[0030] Thus, in this manner, a filter with a free spectral spacing twice the subcarrier channel spacing can be configured. As a result, if the terahertz band orthogonal frequency division multiplexing (OFDM) signal S, which is branched to the first path 2a side and passes through the third path 2c to be incident on the beam splitter 25, and the terahertz band orthogonal frequency division multiplexing (OFDM) signal S delayed by 2Δt are both incident on the beam splitter 25 and interfere with each other, separation becomes possible. As a result, the subcarrier signals d0 and d2 separated from the terahertz band orthogonal frequency division multiplexing (OFDM) signal S are output from the first output path 11 side shown in FIG. 1, and the subcarrier signals d1 and d3 separated from the terahertz band orthogonal frequency division multiplexing (OFDM) signal S are output from the second output path 12 side shown in FIG. 1.

[0031] <Explanation of the second Mach-Zehnder interferometer> The second Mach-Zehnder interferometer 3 is capable of performing a discrete Fourier transform in the terahertz band, and is mainly composed of beam splitters 30 and 35 and mirrors 31 to 34, as shown in Fig. 1. Explaining this using a specific example, as shown in Fig. 1, when subcarrier signals d0 and d2 are input to the second Mach-Zehnder interferometer 3 via the first output path 11, the beam splitter 30 splits the subcarrier signals d0 and d2 into two, one to a first path 3a and one to a second path 3b. The subcarrier signals d0 and d2 split onto the first path 3a side are reflected by the mirror 31 onto the third path 3c side and enter the beam splitter 35.

[0032] On the other hand, the subcarrier signals d0 and d2 branched to the second path 3b side are reflected by the mirror 32 to the fourth path 3d side and are incident on the mirror 33. The subcarrier signals d0 and d2 incident on the mirror 33 are reflected to the fifth path 3e side and are incident on the mirror 34. The subcarrier signals d0 and d2 incident on the mirror 34 are further reflected to the sixth path 3f side and are incident on the beam splitter 35.

[0033] The subcarrier signals d0 and d2 branched by the beam splitter 30 to the first path 3a side, passed through the third path 3c and entered the beam splitter 35, and the subcarrier signals d0 and d2 branched by the beam splitter 30 to the second path 3b side, passed through the fourth path 3d to the sixth path 3f and entered the beam splitter 35 are arranged at an arrangement distance such that a difference of Δt occurs as a relative delay time difference between them. In other words, the subcarrier signals d0 and d2 branched by the beam splitter 30 to the second path 3b side, passed through the fourth path 3d to the sixth path 3f and entered the beam splitter 35 are delayed by Δt compared to the subcarrier signals d0 and d2 branched by the beam splitter 30 to the first path 3a side, passed through the third path 3c and entered the beam splitter 35.

[0034] Thus, in this manner, a filter with a free spectral spacing four times the subcarrier channel spacing can be configured, which satisfies the discrete Fourier transform equation shown in Equation 1. As a result, if subcarrier signals d0 and d2 branched to the first path 3a side and passed through the third path 3c to enter the beam splitter 35, and subcarrier signals d0 and d2 delayed by Δt are both made to enter the beam splitter 35 and interfere with each other, they can be separated. As a result, subcarrier signal d0 is output from the third output path 13 side shown in FIG. 1 via lens 13a, and subcarrier signal d2 is output from the fourth output path 14 side shown in FIG. 1 via lens 14a.

[0035] <Explanation of the third Mach-Zehnder interferometer> The third Mach-Zehnder interferometer 4 is capable of performing a discrete Fourier transform in the terahertz band, and as shown in FIG. 1, is mainly composed of beam splitters 40 and 45 and mirrors 41 to 44. To explain this using a specific example, as shown in FIG. 1, when subcarrier signals d1 and d3 are input to the third Mach-Zehnder interferometer 4 via the second output path 12, the beam splitter 40 splits the subcarrier signals d1 and d3 into two, one to a first path 4a and the other to a second path 4b. The subcarrier signals d1 and d3 split onto the first path 4a are reflected by the mirror 41 onto the third path 4c and then incident on the beam splitter 45. At this time, as shown in FIG. 1, an element 46 that shifts the phase by π is provided on the third path 4c.

[0036] On the other hand, the subcarrier signals d1 and d3 branched to the second path 4b side are reflected by the mirror 42 to the fourth path 4d side and are incident on the mirror 43. The subcarrier signals d1 and d3 incident on the mirror 43 are reflected to the fifth path 4e side and are incident on the mirror 44. The subcarrier signals d1 and d3 incident on the mirror 44 are further reflected to the sixth path 4f side and are incident on the beam splitter 45.

[0037] The subcarrier signals d1 and d3 branched by the beam splitter 40 to the first path 4a side, passed through the third path 4c, and entered the beam splitter 45 are disposed at a distance such that a difference of Δt occurs as a relative delay time between the subcarrier signals d1 and d3 branched by the beam splitter 40 to the second path 4b side, passed through the fourth path 4d to the sixth path 4f, and entered the beam splitter 45. In other words, the subcarrier signals d1 and d3 branched by the beam splitter 40 to the second path 4b side, passed through the fourth path 4d to the sixth path 4f, and entered the beam splitter 45 are delayed by Δt compared to the subcarrier signals d1 and d3 branched by the beam splitter 40 to the first path 4a side, passed through the third path 4c, and entered the beam splitter 45.

[0038] Thus, in this manner, a filter with a free spectral spacing four times the subcarrier channel spacing can be configured, which satisfies the discrete Fourier transform equation shown in Equation 1. As a result, if subcarrier signals d1 and d3 branched to the first path 4a side and passed through the third path 4c to enter the beam splitter 45, and subcarrier signals d1 and d3 delayed by Δt are both made to enter the beam splitter 45 and interfere with each other, they can be separated. As a result, subcarrier signal d1 is output from the fifth output path 15 side shown in FIG. 1 via lens 15a, and subcarrier signal d3 is output from the sixth output path 16 side shown in FIG. 1 via lens 16a.

[0039] Thus, by configuring the first Mach-Zehnder interferometer 2, the second Mach-Zehnder interferometer 3, and the third Mach-Zehnder interferometer 4 as described above, it is possible to separate a terahertz band orthogonal frequency division multiplexing (OFDM) signal S consisting of four subcarrier signals d0, d1, d2, and d3. If the number of channels (number of subcarrier signals) to be separated increases, the number of Mach-Zehnder interferometers capable of discrete Fourier transform can be increased accordingly. Furthermore, although this embodiment illustrates an example of a terahertz band orthogonal frequency division multiplexing (OFDM) signal S consisting of four subcarrier signals d0, d1, d2, and d3, separation of two subcarrier signals is possible using only one Mach-Zehnder interferometer.

[0040] According to the present embodiment described above, instead of down-converting an RF band OFDM signal, outputting the RF band OFDM signal to an optical conversion unit, and separating a plurality of subcarrier signals from the optical orthogonal frequency division multiplexed signal converted by the optical conversion unit using a separation circuit, as in the conventional art, it is possible to obtain a plurality of subcarrier signals d0, d1, . . . , d2 of an orthogonal frequency division multiplexed (OFDM) signal in the terahertz band simply by using one or more Mach-Zehnder interferometers (in the present embodiment, the first Mach-Zehnder interferometer 2, the second Mach-Zehnder interferometer 3, and the third Mach-Zehnder interferometer 4) capable of performing a discrete Fourier transform in the terahertz region. N-1 can be separated.

[0041] Therefore, according to this embodiment, it is possible to separate terahertz band OFDM signals directly in the terahertz band in real time, improve processing speed without being subject to band limitations in the RF band, and further reduce power consumption.

[0042] In this embodiment, the delay time difference in the first Mach-Zehnder interferometer 2 is set to 2Δt, and the delay time difference in the second Mach-Zehnder interferometer 3 and the third Mach-Zehnder interferometer 4 is set to Δt, so that the delay time difference in the first Mach-Zehnder interferometer 2 is longer than the delay time differences in the second Mach-Zehnder interferometer 3 and the third Mach-Zehnder interferometer 4. This is because the longer the delay time difference, the more the multiple subcarrier signals d0, d1, . N-1 This is because, among the above, a subcarrier signal that is close to the other can be separated, and a subcarrier signal that is farther away can be separated when the delay time difference is shorter. Therefore, by doing this, even if the number of channels (number of subcarrier signals) to be separated increases, when adding a Mach-Zehnder interferometer, it is only necessary to add a Mach-Zehnder interferometer with a shorter delay time difference than the previously configured Mach-Zehnder interferometer, making it possible to simply and easily construct an orthogonal frequency division multiplexing signal separation system.

[0043] <Description of Modifications> It should be noted that the contents illustrated in this embodiment are merely examples, and various modifications and alterations are possible within the scope of the gist of the present invention as defined in the claims. For example, in this embodiment, an example using a Mach-Zehnder interferometer has been described, but the present invention is not limited to this. A configuration combining a Mach-Zehnder interferometer and a Michelson interferometer may also be used, or a Michelson interferometer may be used instead of a Mach-Zehnder interferometer. A specific example of this is the configuration shown in FIG. 3. An orthogonal frequency division multiplexing signal demultiplexing system 1A shown in FIG. 3 will be described below. In describing the orthogonal frequency division multiplexing signal demultiplexing system 1A shown in FIG. 3, the same components as those in the orthogonal frequency division multiplexing signal demultiplexing system 1 shown in FIG. 1 will be assigned the same reference numerals, and description thereof will be omitted.

[0044] <Explanation of other orthogonal frequency division multiplexing signal separation systems> 3 is capable of separating orthogonal frequency division multiplexing (OFDM) signals in the terahertz band, and specifically, is mainly composed of a first terahertz band Michelson interferometer 2A, a second terahertz band Michelson interferometer 3A, and a third terahertz band Michelson interferometer 4A. Note that the orthogonal frequency division multiplexing signal separation system 1A shown in this embodiment also exemplifies a case in which the number of separation channels is four.

[0045] <Explanation of the first Michelson interferometer> The first Michelson interferometer 2A is capable of performing a discrete Fourier transform in the terahertz band, and is mainly composed of a beam splitter 20A and mirrors 21A to 22A, as shown in Fig. 3. Explaining this using a specific example, as shown in Fig. 3, an orthogonal frequency division multiplexing (OFDM) signal S in the terahertz band, which is composed of four subcarrier signals d0, d1, d2, and d3, is incident on the beam splitter 11A via a lens 10. In response, the beam splitter 11A transmits the orthogonal frequency division multiplexing (OFDM) signal S in the terahertz band to a first branch path 12Aa shown in Fig. 3. The orthogonal frequency division multiplexing (OFDM) signal S in the terahertz band transmitted through the first branch path 12Aa is then input to the first Michelson interferometer 2A. In response to this, the beam splitter 20A splits the terahertz band orthogonal frequency division multiplexing (OFDM) signal S into two, a first path 2Aa and a second path 2Ab, as shown in Fig. 3. Then, the terahertz band orthogonal frequency division multiplexing (OFDM) signal S split onto the first path 2Aa side is reflected onto the first path 2Aa by the mirror 21A and enters the beam splitter 20A.

[0046] On the other hand, the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched to the second path 2Ab side is reflected by the mirror 22A onto the second path 2Ab and enters the beam splitter 20A.

[0047] The beam splitter 20A is disposed at a distance such that a difference of Δt occurs as a relative delay time difference between the distance of the orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20A to the first path 2Aa side and incident on the mirror 21A and the distance of the orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20A to the second path 2Ab side and incident on the mirror 22A. Therefore, the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20A to the second path 2Ab side, passed through the second path 2Ab again and incident on the beam splitter 20A, is delayed by 2Δt compared to the terahertz band orthogonal frequency division multiplexing (OFDM) signal S branched by the beam splitter 20A to the first path 2Aa side, passed through the first path 2Aa again and incident on the beam splitter 20A.

[0048] Thus, in this manner, a filter with a free spectral spacing twice the subcarrier channel spacing can be configured. Thus, if the terahertz band orthogonal frequency division multiplexing (OFDM) signal S, which is branched to the first path 2Aa side and passes through the first path 2Aa again to be incident on the beam splitter 20A, and the terahertz band orthogonal frequency division multiplexing (OFDM) signal S delayed by 2Δt are both incident on the beam splitter 20A and interfered with, separation becomes possible. As a result, the subcarrier signals d0 and d2 separated from the terahertz band orthogonal frequency division multiplexing (OFDM) signal S are output from the first branch path 12Aa side shown in FIG. 3, and the subcarrier signals d1 and d3 separated from the terahertz band orthogonal frequency division multiplexing (OFDM) signal S are output from the second branch path 12Ab side shown in FIG. 3.

[0049] <Explanation of the second Michelson interferometer> The second Michelson interferometer 3A is capable of performing a discrete Fourier transform in the terahertz band and is mainly composed of a beam splitter 30A and mirrors 31A to 32A, as shown in FIG. 3. To explain this using a specific example, as shown in FIG. 3, subcarrier signals d0 and d2 pass through a first branch path 12Aa, are branched by a beam splitter 11A to a third branch path 12Ac, and are incident on a beam splitter 13A. In response, the beam splitter 13A transmits the subcarrier signals d0 and d2 to a fourth branch path 12Ad shown in FIG. 3. The subcarrier signals d0 and d2 transmitted through the fourth branch path 12Ad are then input to the second Michelson interferometer 3A. In response, the beam splitter 30A branches the subcarrier signals d0 and d2 into two paths, a first path 3Aa and a second path 3Ab, as shown in FIG. 3. Then, the subcarrier signals d0 and d2 branched to the first path 3Aa side are reflected by the mirror 31A onto the first path 3Aa and enter the beam splitter 30A.

[0050] On the other hand, the subcarrier signals d0 and d2 branched to the second path 3Ab side are reflected by the mirror 32A onto the second path 3Ab and enter the beam splitter 30A.

[0051] The beam splitter 30A is positioned at a distance such that the distance of the subcarrier signals d0 and d2 branched by the beam splitter 30A to the first path 3Aa side and incident on the mirror 31A, and the distance of the subcarrier signals d0 and d2 branched by the beam splitter 30A to the second path 3Ab side and incident on the mirror 32A, have a relative delay time difference of Δt / 2. Therefore, the subcarrier signals d0 and d2 branched by the beam splitter 30A to the second path 3Ab side, passed through the second path 3Ab again, and incident on the beam splitter 30A are delayed by Δt compared to the subcarrier signals d0 and d2 branched by the beam splitter 30A to the first path 3Aa side, passed through the first path 3Aa again, and incident on the beam splitter 30A.

[0052] Thus, this configuration allows for the construction of a filter with a free spectral spacing four times the subcarrier channel spacing, thereby satisfying the discrete Fourier transform equation shown in Equation 1 above. Thus, subcarrier signals d0 and d2 branched to the first path 3Aa, passed through the first path 3Aa again, and then input to the beam splitter 30A. Subcarrier signals d0 and d2 delayed by Δt can be separated by interfering with each other through the beam splitter 30A. As a result, subcarrier signal d0 is output to the fourth branch path 12Ad shown in FIG. 3, and subcarrier signal d2 is output to the fifth branch path 12Ae shown in FIG. 3. Then, subcarrier signal d0 is branched by the beam splitter 13A to the sixth branch path 2Af shown in FIG. 3 and output via lens 13a. Furthermore, subcarrier signal d2 is output via lens 14a.

[0053] <Explanation of the third Michelson interferometer> The third Michelson interferometer 4A is capable of performing a discrete Fourier transform in the terahertz band and is mainly composed of a beam splitter 40A and mirrors 41A to 42A, as shown in FIG. 3. To explain this using a specific example, as shown in FIG. 3, subcarrier signals d1 and d3 pass through a second branch path 12Ab and are incident on a beam splitter 14A. In response, the beam splitter 14A transmits the subcarrier signals d1 and d3 to a seventh branch path 12Ag shown in FIG. 3. The subcarrier signals d1 and d3 transmitted through the seventh branch path 12Ag are then input to the third Michelson interferometer 4A. In response, the beam splitter 40A splits the subcarrier signals d1 and d3 into two paths, a first path 4Aa and a second path 4Ab, as shown in FIG. 3. Then, the subcarrier signals d1 and d3 branched to the first path 4Aa side are reflected by the mirror 41A onto the first path 4Aa and enter the beam splitter 40A.

[0054] On the other hand, the subcarrier signals d1 and d3 branched to the second path 4Ab side are reflected by the mirror 42A onto the second path 4Ab and enter the beam splitter 40A.

[0055] The beam splitter 40A is positioned at a distance such that the distance of the subcarrier signals d1 and d3 branched by the beam splitter 40A to the first path 4Aa side and incident on the mirror 41A, and the distance of the subcarrier signals d1 and d3 branched by the beam splitter 40A to the second path 4Ab side and incident on the mirror 42A, have a relative delay time difference of Δt / 2. Therefore, the subcarrier signals d1 and d3 branched by the beam splitter 40A to the second path 4Ab side, passed through the second path 4Ab again, and incident on the beam splitter 40A are delayed by Δt compared to the subcarrier signals d1 and d3 branched by the beam splitter 40A to the first path 4Aa side, passed through the first path 4Aa again, and incident on the beam splitter 40A.

[0056] Thus, a filter with a free spectral spacing four times the subcarrier channel spacing can be constructed, satisfying the discrete Fourier transform equation shown in Equation 1 above. As a result, subcarrier signals d1 and d3 branched to the first path 4Aa, passed through the first path 4Aa again, and then input to the beam splitter 40A. Subcarrier signals d1 and d3 delayed by Δt can be separated by interfering with each other through the beam splitter 40A. As a result, subcarrier signal d1 is output to the seventh branch path 12Ag shown in FIG. 3, and subcarrier signal d3 is output to the eighth branch path 12Ah shown in FIG. 3. Then, subcarrier signal d1 is branched by the beam splitter 14A to the ninth branch path 12Ai shown in FIG. 3 and output via lens 15a. Furthermore, subcarrier signal d3 is output via lens 16a.

[0057] Thus, by configuring the first Michelson interferometer 2A, the second Michelson interferometer 3A, and the third Michelson interferometer 4A as described above, it is possible to separate an orthogonal frequency division multiplexed (OFDM) signal S in the terahertz band consisting of four subcarrier signals d0, d1, d2, and d3. Note that, if the number of channels (number of subcarrier signals) to be separated increases, the number of Michelson interferometers capable of performing discrete Fourier transform in the terahertz band can be increased accordingly. Furthermore, although this embodiment illustrates an example of an orthogonal frequency division multiplexed (OFDM) signal S in the terahertz band consisting of four subcarrier signals d0, d1, d2, and d3, separation of two subcarrier signals is possible using only one Michelson interferometer.

[0058] Thus, in this embodiment, instead of down-converting an RF band OFDM signal, outputting the RF band OFDM signal to an optical conversion unit, and separating a plurality of subcarrier signals from the optical orthogonal frequency division multiplexed signal converted by the optical conversion unit using a separation circuit, it is possible to obtain a plurality of subcarrier signals d0, d1, . . . , d2 of an orthogonal frequency division multiplexed (OFDM) signal in the terahertz band by simply using one or more Michelson interferometers capable of discrete Fourier transform in the terahertz region (in this embodiment, a first Michelson interferometer 2A, a second Michelson interferometer 3A, and a third Michelson interferometer 4A). N-1 can be separated.

[0059] Thus, in this embodiment, terahertz band OFDM signals can be separated directly in the terahertz band in real time, the processing speed can be improved without being subject to band limitations in the RF band, and power consumption can be reduced.

[0060] Also in this embodiment, the delay time difference in the first Michelson interferometer 2A is set to 2Δt, and the delay time difference in the second Michelson interferometer 3A and the third Michelson interferometer 4A is set to Δt, so that the delay time difference in the first Michelson interferometer 2A is longer than the delay time differences in the second Michelson interferometer 3A and the third Michelson interferometer 4A. This is because the longer the delay time difference, the more the multiple subcarrier signals d0, d1, . N-1 This is because, among the above, a subcarrier with a shorter delay time difference can separate nearby subcarrier signals, and a subcarrier with a shorter delay time difference can separate farther subcarrier signals. Therefore, by doing this, even if the number of channels (number of subcarrier signals) to be separated increases, when adding a Michelson interferometer, it is only necessary to add a Michelson interferometer with a shorter delay time difference than the previously configured Michelson interferometer, making it possible to simply and easily construct an orthogonal frequency division multiplexing signal separation system.

[0061] <Description of Modifications> Incidentally, the Mach-Zehnder interferometer in the orthogonal frequency division multiplexing signal demultiplexing system 1 shown in FIG. 1 and the Michelson interferometer in the orthogonal frequency division multiplexing signal demultiplexing system 1A shown in FIG. 3 are configured using beam splitters and mirrors, but the present invention is not limited to this. Waveguides or waveguides, delay elements, phase-adding elements, etc. may be used to integrate and configure a discrete Fourier transform circuit in the terahertz band so as to perform the same processing as described above.

[0062] Furthermore, in this embodiment, an example in which a lens is used is shown, but it is not necessary to use a lens unless there is a particular need. [Explanation of symbols]

[0063] 1,1A Orthogonal Frequency Division Multiplexing Signal Separation System 2. First Mach-Zehnder Interferometer (Mach-Zehnder Interferometer) 3 Second Mach-Zehnder Interferometer (Mach-Zehnder Interferometer) 4. Third Mach-Zehnder Interferometer (Mach-Zehnder Interferometer) 2A First Michelson Interferometer (Michelson Interferometer) 3A Second Michelson Interferometer (Michelson Interferometer) 4A Third Michelson Interferometer (Michelson Interferometer) d0~d N-1 Subcarrier Signal S Terahertz band orthogonal frequency division multiplexing signal

Claims

1. an orthogonal frequency division multiplexed signal in the terahertz band having a plurality of subcarrier signals is separated into the plurality of subcarrier signals by using a plurality of Mach-Zehnder interferometers capable of performing a discrete Fourier transform in the terahertz region or a plurality of Michelson interferometers capable of performing a discrete Fourier transform in the terahertz region; When separating the plurality of subcarrier signals, a delay time is provided in the Mach-Zehnder interferometer or the Michelson interferometer to separate the signals, when separating the plurality of subcarrier signals, the first-stage delay time of the Mach-Zehnder interferometer or the Michelson interferometer is set to ½Δf, where Δf is a frequency interval between the plurality of subcarrier signals; An orthogonal frequency division multiplexing signal separation system, comprising: a Mach-Zehnder interferometer having a first-stage delay time; a Mach-Zehnder interferometer having a second-stage delay time set to the same delay time as the Michelson interferometer and located downstream of the Mach-Zehnder interferometer; or a Mach-Zehnder interferometer having a second-stage delay time set to the same delay time as the Michelson interferometer, wherein the second-stage delay time of the Michelson interferometer is 1 / 2 the first-stage delay time.

2. The Mach-Zehnder interferometer comprises:

2. The orthogonal frequency division multiplexing signal separation system according to claim 1, wherein the system receives an orthogonal frequency division multiplexing signal in the terahertz band, branches the received orthogonal frequency division multiplexing signal in the terahertz band into two, delays one of the branched orthogonal frequency division multiplexing signals in the terahertz band by a first delay time relative to the other of the branched orthogonal frequency division multiplexing signals in the terahertz band, and causes interference between the one orthogonal frequency division multiplexing signal in the terahertz band and the delayed orthogonal frequency division multiplexing signal in the other terahertz band, thereby separating the plurality of subcarrier signals.

3. The Michelson interferometer comprises:

2. The orthogonal frequency division multiplexing signal separation system according to claim 1, wherein the system receives an orthogonal frequency division multiplexing signal in the terahertz band, branches the received orthogonal frequency division multiplexing signal in the terahertz band into two, delays one of the branched orthogonal frequency division multiplexing signals in the terahertz band by a first delay time relative to the other of the branched orthogonal frequency division multiplexing signals in the terahertz band, and causes interference between the one orthogonal frequency division multiplexing signal in the terahertz band and the delayed orthogonal frequency division multiplexing signal in the other terahertz band, thereby separating the plurality of subcarrier signals.

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