JPH11112473A - Circuit for optical cdma - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the unnecessary side lobe component of a decoded signal pulse light and to reduce the output level of a non-decoded signal pulse light by providing a threshold element outputting a decoded signal pulse higher than a prescribed level among the decoded signal pulses given inverse spread demodulation. SOLUTION: This circuit is provided with short pulse light sources 11-1 to 11-3, inverse modulators 12-1 to 12-3, optical star coupler 13 and inverse spreading demodulators 14-1 to 14-3. In addition the circuit is provided with threshold elements 20-1 to 20-3 outputting decoded signal optical pulse (main peak pulse) at a level respectively higher than a prescribed level. The threshold element 20 utilizes an optical time division multiplex separation circuit separating a pulse light synchronized with a controlled pulse light from a pulse light string given time division multiplexing. Thereby, a main peak pulse is transmitted to a waveguide on an outputting side and the side lobe component is reflected to a waveguide on an inputting side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spread modulator for performing spread modulation of a signal in an optical domain, and a spread modulated optical CDMA.
The present invention relates to an optical CDMA circuit including a despread demodulator for despreading and demodulating a signal.

[0002]

2. Description of the Related Art In an optical CDMA system, random access or self-routing operation can be realized by a code without using an element such as an optical switch.
Therefore, application to an optical LAN or an optical switching system is being studied.

FIG. 7 shows a configuration of a conventional optical CDMA circuit. In the figure, the conventional optical CDMA circuit comprises short light pulse light sources 35-1 to 35-3 and each short light pulse light source 35-1.
Lattice type optical circuit 36-1 connected to -1 to 35-3
36-3 and each of the lattice type optical circuits 36-1 to 36-3.
Star coupler 37 connected to the
7 is constituted by lattice type optical circuits 36-4 to 36-6. The lattice-type optical circuit 36 has a configuration in which asymmetric Mach-Zehnder interferometers having optical path length differences of ΔL, 2ΔL,..., JΔL (J is a natural number) are cascaded (FIG. 7: J = 2). The lattice optical circuits 36 on the left and right sides of the optical star coupler 37 correspond to a spread modulator and a despread demodulator for an optical pulse signal, respectively.

Here, the lattice type optical circuits 36-1, 36 are provided.
4 illustrates an operation example. Lattice type optical circuits 36-1, 36
The coupling ratio of the directional couplers 38-1 to 38-6 constituting the asymmetric Mach-Zehnder interferometer of No. -4 is set to 0.5.
The repetition frequency f (H
z) (<c / (2 ^J nΔL), where c is the speed of light, and n is the refractive index of the waveguide), a 2 ^J optical pulse train ( Period Δt (= nΔL
/ C)) is newly generated to form a code sequence. This code sequence includes phase information of the phase shifters 39-1 and 39-2 of the lattice type optical circuit 36-1. When this coded optical pulse train enters the opposing lattice type optical circuit 36-4 via the optical star coupler 37, the individual optical pulses are separated into 2 ^J optical pulses and then the coherent electric field components are added. Will be The setting of the phase shifters 39-3 and 39-4 of the lattice type optical circuit 36-4 is performed by the phase shifter 39.
When the decoding conditions are satisfied with respect to −1 and 39-2, the optical power is concentrated at the center of the optical pulse, and the decoding is performed. Spread in time, no decoding is performed.

FIG. 8A shows an incident light pulse train, and FIG.
= 2, generated code sequence after passing through the spread modulator, FIG.
8C shows an output result when the setting of the despread demodulator satisfies the decoding condition, and FIG. 8D shows an output result when the setting of the despread demodulator does not satisfy the decoding condition. This figure shows the effect of one pulse of the incident light pulse train for simplicity.

[0006]

In the demodulation process of the conventional optical CDMA circuit, a side lobe component occurs around the decoded signal light pulse even when the decoding condition is satisfied, and the S / N ratio on the receiving side is increased. However, there was a problem of deterioration. Also, in the case of non-decoding, an optical pulse train is generated, which degrades the S / N ratio of the decoded optical signal. This is because the auto-correlation characteristic and the cross-correlation characteristic of the CDMA spreading modulator and despreading demodulator deviate from ideal states.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical CDMA circuit capable of reducing unnecessary side lobe components of a decoded signal light pulse and reducing the output level of a non-decoded signal light pulse.

[0008]

An optical CDMA circuit according to the present invention comprises a spread modulator for performing spread modulation of a signal in an optical domain, and a despread demodulator for despread demodulating a spread modulated optical CDMA signal. And a threshold element for selecting and outputting a decoded signal light pulse (main peak pulse) having a predetermined level or more from the despread demodulated decoded signal light pulse (claim 1). As a result, unnecessary side lobe components included in the decoded signal light pulse can be reduced. Further, the output level of the non-decoded signal light pulse can be reduced.

The threshold element is an optical time-division multiplexing / demultiplexing circuit for separating an optical pulse synchronized with the control optical pulse from the time-division multiplexed optical pulse train, and a predetermined signal synchronized with the despread demodulated decoded signal optical pulse. Control light pulse generating means for generating a control light pulse having a level, and inputting the despread demodulated decoded signal light pulse and the control light pulse to an optical time division multiplexing / demultiplexing circuit, and outputting a decoded signal having a predetermined level or more. The light pulse (main peak pulse) is separated (claim 2). That is, the optical time division demultiplexing circuit separates the main peak pulse by the control light pulse synchronized with the main peak pulse.

As the optical time division multiplexing / demultiplexing circuit, TO
Using any one of AD, optical Kerr switch, 4-wave mixing switch, cross-phase modulation switch, and nonlinear loop mirror (NOLM), control light pulse synchronized with main peak pulse changes phase to main peak pulse, polarization rotation, optical frequency It is separated by giving a change or the like (claim 3). Also, TOAD
A semiconductor laser amplifier is used as a non-linear optical element arranged in the loop waveguide that constitutes (Claim 4).

The spreading modulator and the despreading modulator include a conventionally used lattice type optical circuit (claim 5), a plurality of waveguides having different optical path lengths from an optical splitter, a phase shifter, and an optical coupler. An optical circuit composed of a device (claim 6), an optical circuit composed of an optical demultiplexer and a plurality of waveguides, and an optical circuit composed of a phase shifter and an optical multiplexer (claim 7) can be used. Further, an arrayed waveguide diffraction grating is used as the optical demultiplexer and the optical multiplexer.

[0012]

FIG. 1 shows an embodiment of an optical CDMA circuit according to the present invention. In the figure, an optical CD of the present embodiment is shown.
The circuit for MA includes short light pulse light sources 11-1 to 11-3,
Spreading modulators 12-1 to 12-3 and optical star coupler 13
In addition to the same conventional optical CDMA circuit constituted by despread demodulators 14-1 to 14-3, decoded signal light pulses output from despread demodulators 14-1 to 14-3 Among them, there are provided threshold elements 20-1 to 20-3 which output decoded signal light pulses (hereinafter, referred to as “main peak pulses”) each having a predetermined level or more.

The threshold element 20 utilizes an optical time division multiplexing / demultiplexing circuit for separating an optical pulse synchronized with a control optical pulse from a time division multiplexed optical pulse train. A control light pulse generating means for generating a control light pulse having a predetermined level in synchronization with the decoded signal light pulse subjected to the despread demodulation; In this configuration, an optical pulse is input and a main peak pulse is separated.

FIG. 2 shows a configuration example of the threshold element 20. Here, TOAD (Terahe) is used as an optical time division demultiplexing circuit.
An example of the configuration using rtz Optical Asymmetric Demultiplexer) is shown.

The threshold element 20 is composed of a directional coupler 21-1 that branches the decoded signal light pulses 1a, 1b, and 1c output from the despread demodulator 14 into two, and a directional coupler 21-2.
The two output ports are connected in a loop, and the loop waveguide 24 in which the polarization beam splitters 22-1 and 22-2 and the nonlinear optical element 23 are arranged in the loop, and the optical amplifier 2
5 and a TE / TM mode converter 26. The nonlinear optical element 23 is arranged at a position shifted by Δx from the middle point 27 of the loop waveguide 24.

Here, one output port of the directional coupler 21-1 and one input port of the directional coupler 21-2 are connected via the input side waveguide 28, and
The output side waveguide 29 is connected to the other input port of -2.

The other output port of the directional coupler 21-1 and the input port of the polarization beam splitter 22-1 are connected via an optical amplifier 25, a TE / TM mode converter 26, and a waveguide 30. . That is, the directional coupler 21-1, the optical amplifier 25, the TE / TM mode converter 2
6 and the waveguide 30 are synchronized with the decoded signal light pulses 1a-1c and have control light pulses 2a-2c having predetermined levels.
Is a control light pulse generation means for generating the control light pulse. The length of the waveguide 30 is equal to the length of the decoded signal light pulse 1a to 1c.
Adjustable to an appropriate length for synchronization with Also,
A light source for the control light pulse may be separately prepared, and the output control light pulse may be synchronized with the decoded signal light pulse.

The polarizations of the decoded signal light pulses 1a to 1c and the control light pulses 2a to 2c are set by the TE-TM mode converter 26 so that one of them is orthogonal to the TE mode of the waveguide and the other is the TM mode of the waveguide. You. Thus, the control light pulses 2a to 2c input from the polarization beam splitter 22-1 to the loop waveguide 24 can be separated and extracted from the polarization beam splitter 22-2. In addition,
The wavelength of the control light pulses 2a to 2c may be changed in place of the wavelength converter instead of the TE-TM mode converter 26, and the WDM coupler may be used to multiplex and demultiplex with the decoded signal light pulse. Further, the polarization beam splitter 22-2 and the corresponding WDM coupler may be arranged at the subsequent stage of the output side waveguide 29.

The directional couplers 21-1 and 21-2 are realized by a configuration in which two waveguides are arranged close to each other, a configuration in which one or more symmetric Mach-Zehnder interferometers are connected in cascade, or the like. Is done. The optical amplifier 25 is realized by a rare earth element-doped optical fiber amplifier, a rare earth element-doped waveguide amplifier, a semiconductor laser amplifier, or the like. The nonlinear optical element 23 is realized by a semiconductor laser amplifier or the like (claim 4).

The operation of the TOAD will be described below with reference to FIGS. 2 (a) and 2 (b). In FIG. 2, the directional coupler 2
When the coupling ratio of 1-2 is set to 0.5, the loop waveguide 2
The decoded signal light pulses 1a to 1c input to 4 are equally divided into clockwise light and counterclockwise light. When the clockwise light and the counterclockwise light return to the directional coupler 21-2 again, when the phases are equal to each other, the input side waveguide 28
When the phases are different from each other, the light is transmitted to the output side waveguide 29.

The decoded signal light pulse 1b is a main peak pulse. The control light pulse 2b is set to a level sufficient to give a change in the refractive index to the nonlinear optical element 23 by the optical amplifier 25 in synchronization with the main peak pulse. At this time, the control light pulses 2a and 2c are set to a level that does not change the refractive index of the nonlinear optical element 23.

Here, the bit interval between the decoded signal light pulses 1a to 1c is T _b , the interval for time division multiplexing / demultiplexing is T _f , the rise time of the refractive index change of the nonlinear optical element 23 by the control light pulse 2b is τ _r , the fall time when the τ _f, τ _r relationship of <T _b «τ _f <T _f is deemed to have been established. At this time, when the control light pulse 2b incident on the loop waveguide 24 from the polarization beam splitter 22-1 propagates clockwise and reaches the nonlinear optical element 23, the nonlinear optical element 23 has a refractive index with a fast rise time. A rapid change occurs, which then relaxes over a long period of time. When the control light pulse 2b reaches the nonlinear optical element 23, the demodulated signal light pulses 1a, 1b, and 1c propagating clockwise are after passing through the nonlinear optical element 23, before passing, and before passing, respectively.
Demodulated signal light pulse 1a, 1b, 1 propagating counterclockwise
c is after passing through the nonlinear optical element 23, after passing through, and before passing through, respectively.

Accordingly, both the clockwise and counterclockwise decoded signal light pulses 1a are not subjected to a phase shift due to a change in the refractive index of the nonlinear optical element 23. The clockwise decoded signal light pulse 1b undergoes a phase shift, whereas the counterclockwise decoded signal light pulse 1b does not receive a phase shift. In addition, both the clockwise and counterclockwise decoded signal light pulses 1c undergo a phase shift. As a result, the decoded signal light pulse 1b as the main peak pulse is transmitted through the output side waveguide 29, and the decoded signal light pulses 1a and 1c as the side lobe components are reflected on the input side waveguide 28 to separate them. Can be. Assuming that the speed of the optical pulse propagating through the loop waveguide 24 is ν (= c / n), the switching gate time Δτ is Δτ = 2Δx / ν (JPSokoloff et al., “A Terahertz Optical A
symmetric Demultiplexer (TOAD) ", IEEE Photonics Te
chnology Letters, vol.5, no.7, pp.787-790, July, 1
993).

(Another Configuration Example of Threshold Element 20) Threshold Element 2
0 uses the optical time division multiplexing / demultiplexing circuit as described above,
This is to separate the main peak pulse from the despread demodulated decoded signal light pulse. The optical time-division multiplexing / demultiplexing circuit includes an optical Kerr switch, a four-wave mixing switch, a mutual phase modulation switch, a non-linear loop mirror (NOLM), etc. in addition to the TOAD. Hereinafter, each will be briefly described.

The optical Kerr switch changes the polarization direction of the signal light pulse that is temporally overlapped with the control light pulse by 90 degrees by the optical Kerr effect of the control light pulse, and only the component (light peak pulse) whose polarization direction has changed is changed. Is separated by a polarizing beam splitter (T. Morioka et al., "Ultra
fast all-optical switching utilizing the opticalKe
rr effect in polarization-maintaining single-mode
fibers ", IEEE J. Select. Areas. Commu., vol. 6, pp. 1
186-1198, 1988).

The four-wave mixing switch causes a frequency shift of the signal light pulse temporally overlapped with the control light pulse due to the four-wave mixing process between the control light pulse and the signal light pulse, and converts this component (light peak pulse). It is separated by an optical frequency filter (PAAndrekson et al., "16G
b / s all-optical demultiplexing using four-wave mix
ing ", Elect. Lett., vol. 27, pp. 922-924, 1991).

The cross-phase modulation switch is an optical frequency that extracts a desired signal light pulse (light peak pulse) frequency by subjecting the signal light pulse train to a time-varying frequency shift through cross-phase modulation with the control light pulse. (T. Morioka et al., "An ultrafas
t polarization-independent optical demultiplexerut
ilizing optical carrier frequency shift through cr
oss-phase modulation ", Elect. Lett., vol. 28, pp. 107
0-1072, 1992).

The nonlinear loop mirror (NOLM)
Like the AD, a loop mirror (Sagnac interferometer) configuration is used, but a nonlinear optical element is not provided in a part of the loop, but the entire loop is configured by a nonlinear optical element such as an optical fiber. The signal light pulse propagates while being split into two-way light by the directional coupler, is coupled again by the directional coupler, and returns to the input side. Here, when the control light pulse is input in one direction, only the signal light pulse that temporally overlaps in the same direction as the control light pulse undergoes a phase shift by the cross-phase modulation, and under appropriate conditions, the two-way light The phase difference can be π.
This signal light pulse (optical peak pulse) is separated as transmitted light (NJ Doran et al., "Nonlinear-optical loop m
irror ", Opt. Lett., vol.13, pp.56-58, 1988).

(Exemplary Configurations of Spreading Modulator and Despreading Modulator) Next, exemplary configurations of the spreading modulators 12-1 to 12-3 and the despreading demodulators 14-1 to 14-3 shown in FIG. 1 will be described. I do.

First, the lattice type optical circuits 36-1 to 36-6 shown in FIG. 7 can be used as the spread modulators 12-1 to 12-3 and the despread demodulators 14-1 to 14-3. Here, the phase shift values of the phase shifters 39-1 to 39-4 of the lattice type optical circuits 36-1 and 36-4 are set to φ ₁ to
and φ _4.

When a short optical pulse having a repetition frequency f (Hz) is incident on the lattice type optical circuit 36-1, an optical pulse train having a period Δt (= nΔL / c) is generated as shown in FIG.
Phase information of the phase shifters 39-1 and 39-2 (for example, φ ₁
= Φ ₂ = π). This coded optical pulse train enters the opposing lattice type optical circuit 36-4 via the optical star coupler 37 (13). Here, the phase shifters 39-3 and 39- of the lattice type optical circuit 36-4 are used.
4 is set so as to satisfy the decoding condition for the phase shifters 39-1 and 39-2 (for example, φ ₃ = φ ₄ =
0), as shown in FIG. 3B, the optical power is concentrated at the center of the optical pulse and decoding is performed, but a side lobe component also appears.
On the other hand, when the setting is different from the decoding condition (for example, φ ₃ =
In the case of (π, φ ₄ = 0), the incident coded pulse train is further temporally spread as shown in FIG. FIG. 3 shows the effect of one pulse of the incident light pulse train for simplicity.

Therefore, by setting the switching gate time Δτ as Δτ <Δt using the above-described threshold element 20, the output light pulse of the lattice type optical circuit 36-4 shown in FIG. As shown in d), there is only the main peak pulse from which the side lobe component has been removed.

On the other hand, when the decoding condition is not satisfied, the output light pulse of the lattice type optical circuit 36-4 becomes as shown in FIG. 3C, and then the refractive index of the nonlinear optical element 23 of the threshold element 20 is changed. No control light pulse to be changed is generated, the output light pulse of the lattice type optical circuit 36-4 is reflected by the threshold element 20, and the output light intensity of the threshold element 20 is shown in FIG.
It becomes 0 as shown in (e).

By using such a threshold element 20, unnecessary side lobe components of the decoded signal light pulse can be reduced, and the output level of the non-decoded signal light pulse can be reduced. The ratio can be improved.

FIGS. 4 and 5 show spread modulators 12-1 to 12-1.
12 shows another configuration example of 12-3 and despread demodulators 14-1 to 14-3. Since the spread modulator and the despread demodulator have the same configuration, only one of them is shown.

A spread modulator (despread demodulator) shown in FIG.
Is the input / output of the 1 × 4 optical splitter 41 and the 4 × 1 optical coupler 42, and the optical path length difference with respect to one waveguide is ΔL, 2Δ.
In this configuration, four waveguides L and 3ΔL are connected, and phase shifters 43-1 to 43-4 are inserted into each waveguide.
Here, the optical splitter 41 and the optical coupler 42 have a configuration in which an optical star coupler, an MMI coupler, and a 2 × 2 directional coupler are cascaded in multiple stages, a configuration in which Y-branch waveguides are cascaded in multiple stages, and the like. Can be used.

When an optical pulse train is incident on the spread modulator and the despread demodulator having such a configuration, the spread modulation of the optical pulse train is performed on the time axis due to the delay time difference of the waveguide, similarly to the lattice type optical circuit shown in FIG. And despread demodulation. Similarly, by combining the set values of the phase shifters, decoding and non-decoding of the spread CDMA optical CDMA signal can be performed.

A spread modulator (despread demodulator) shown in FIG.
Has a configuration in which two arrayed waveguide diffraction gratings 51-1 and 51-2 are symmetrically connected via phase shifters 52-1 to 52-4. Each arrayed waveguide grating has an input waveguide 5
3. An input slab waveguide 54, an array waveguide 55, an output slab waveguide 56, and an output waveguide 57,
Each functions as an optical demultiplexer and an optical multiplexer. An optical pulse input from the input waveguide 53 of the arrayed waveguide diffraction grating 51-1 is split into output waveguides 57 for each optical frequency component, and the corresponding phase shifters 52-1 to 52-2 are used.
-4, is input from the output waveguide 57 of the arrayed waveguide diffraction grating 51-2, is multiplexed with the input waveguide 53, and is output.

When an optical pulse train is incident on the spread modulator and the despread demodulator having such a configuration, it is possible to perform spread modulation and despread demodulation of the optical pulse train on the optical frequency axis. Similarly, by combining the set values of the phase shifter, decoding and non-decoding of the spread modulated optical CDMA signal can be performed.

When a quartz glass waveguide is used as each waveguide section of the optical CDMA circuit of the present invention described above, the hybrid integration of the quartz glass waveguide and the semiconductor laser amplifier (nonlinear optical element 23) PLC whose section is shown in FIG.
(Planar Lightwave Circuit) Performed using the platform. The formation process is as follows.

After the Si terrace region 62 is formed on the Si substrate 61 by anisotropic etching, the SiO ₂ lower cladding layer 6 is formed by a flame deposition method in which the Si terrace region 62 is completely buried.
7 is formed. Next, flattening polishing is performed to form a height reference plane when the semiconductor laser amplifier 65 is mounted, and Si
The surface of the terrace region 62 is exposed. Subsequently, the SiO ₂ height adjusting layer 68 and the GeO _{2 are} adjusted so that the height of the core of the optical waveguide coincides with the height of the active layer 66 of the semiconductor laser amplifier 65.
The SiO ₂ is deposited core layer 69 added as dopant. Subsequently, in order to obtain an optical circuit pattern as shown in FIGS. 1, 2, 4, 5, and 7, the core layer is etched to form a core portion. Further, an SiO ₂ upper cladding layer 70 is deposited to form a waveguide portion. Next, a portion serving as a mounting portion of the semiconductor laser amplifier 65 is etched to expose the surface of the Si terrace region 62. Finally, an insulating SiO ₂ film is formed on the surface of the Si terrace region 62, an Au electric wiring 63 and an AuSn solder pattern 64 are formed thereon, and the semiconductor laser amplifier 65 is mounted.

The phase shifter based on the thermo-optic effect is formed by depositing a thin film heater and electric wiring on a predetermined waveguide. The waveguide portion of the optical CDMA circuit of the present invention is not limited to a glass waveguide, but may be a dielectric waveguide,
It can be realized using a semiconductor waveguide, a polymer waveguide, or the like. It can also be realized by using a hybrid integrated configuration in which several types of waveguides are combined.

[0043]

As described above, the optical CDM of the present invention is
The circuit for A is a threshold element connected to the output side of the despread demodulator, and outputs a decoded signal light pulse (main peak pulse) having a predetermined level or higher among the despread and demodulated decoded signal light pulses. Thus, unnecessary side lobe components included in the decoded signal light pulse can be reduced. Further, the output level of the non-decoded signal light pulse can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a CDMA circuit of the present invention.

FIG. 2 is a diagram showing a configuration example of a threshold element 20.

FIG. 3 is a diagram showing an operation example of a CDMA circuit of the present invention.

FIG. 4 is a diagram showing another configuration example of a spread modulator and a despread demodulator.

FIG. 5 is a diagram showing another configuration example of the spread modulator and the despread demodulator.

FIG. 6 is a cross-sectional view of a CDMA circuit according to the present invention in which a silica-based glass waveguide is used as a waveguide portion and a semiconductor laser amplifier is hybrid-integrated as a nonlinear optical element.

FIG. 7 is a diagram showing a configuration of a conventional CDMA circuit.

FIG. 8 is a diagram showing an operation example of a conventional CDMA circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 Short light pulse light source 12 Diffusion modulator 13 Optical star coupler 14 Despread demodulator 20 Threshold element 21 Directional coupler 22 Polarized beam splitter 23 Nonlinear optical element 24 Loop waveguide 25 Optical amplifier 26 TE / TM mode converter 27 Midpoint of loop waveguide 28 Input waveguide 29 Output waveguide 35 Short optical pulse light source 36 Lattice type optical circuit 37 Optical star coupler 38 Directional coupler 39 Phase shifter 41 Optical splitter 42 Optical coupler 43 Phase shifter Reference Signs List 51 array waveguide diffraction grating (optical demultiplexer, optical multiplexer) 52 phase shifter 53 input waveguide 54 input slab waveguide 55 array waveguide 56 output slab waveguide 57 output waveguide 61 Si substrate 62 Si terrace area 63 Au electric wiring 64 AuSn solder pattern 65 semiconductor laser amplifier 66 active layer 67 S SiO ₂ lower cladding layer 68 SiO ₂ height adjusting layer 69 SiO ₂ core layer 70 SiO ₂ upper cladding layer

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI G02B 6/293 G02B 6/28 B G02F 1/35 H01S 3/18

Claims (8)

[Claims] 1. A spread modulator for performing spread modulation of a signal in an optical domain, and a spread modulated optical CDMA (Code Division Multiplexed Code).
In an optical CDMA circuit including a despread demodulator for despreading and demodulating an iple Access (code division multiple access) signal, a decoded signal light pulse having a predetermined level or more among the despread and demodulated decoded signal light pulses. A circuit for optical CDMA, comprising a threshold element for outputting a main peak pulse. 2. A threshold element, comprising: an optical time-division multiplexing / demultiplexing circuit for separating an optical pulse synchronized with a control optical pulse from a time-division-multiplexed optical pulse train; Control light pulse generation means for generating a control light pulse having a level of: the optical time division multiplexing / demultiplexing circuit, wherein the despread demodulated decoded signal light pulse and the control light pulse are input to a predetermined level The light C according to claim 1, wherein the light C is configured to separate the decoded signal light pulse (main peak pulse).
DMA circuit. 3. The optical time division multiplexing / demultiplexing circuit is one of a TOAD, an optical Kerr switch, a four-wave mixing switch, a mutual phase modulation switch, and a non-linear loop mirror (NOLM). Circuit for optical CDMA. 4. The semiconductor laser amplifier according to claim 3, wherein when the TOAD is used as the optical time division multiplexing / demultiplexing circuit, a semiconductor laser amplifier is used as the nonlinear optical element arranged in the loop waveguide constituting the TOAD. 3. The circuit for optical CDMA according to 1. 5. The spread modulator and the despread demodulator have a configuration in which one asymmetric Mach-Zehnder interferometer or a plurality of asymmetric Mach-Zehnder interferometers having different optical path length differences between two waveguides are cascaded, 2. The optical CDMA circuit according to claim 1, wherein at least one of the two waveguides of each asymmetric Mach-Zehnder interferometer includes a phase shifter for changing an optical phase. 6. A spread modulator and a despread demodulator, comprising: an optical splitter for splitting input light into a plurality of optical paths; and a plurality of waveguides having different optical path lengths through which the lights split by the optical splitter propagate. A waveguide, a plurality of phase shifters that change an optical phase of each light propagating through the plurality of waveguides, and an optical coupler that couples each light propagating through the plurality of waveguides and the plurality of phase shifters. 2. The optical CDMA circuit according to claim 1, wherein: 7. A spread modulator and a despread demodulator, comprising: an optical demultiplexer for demultiplexing input light into a plurality of optical frequency components; and light of each optical frequency component demultiplexed by the optical demultiplexer. A plurality of waveguides that propagate; a plurality of phase shifters that change the optical phase of each optical frequency component that propagates through the plurality of waveguides; and each optical frequency component that propagates through the plurality of waveguides and the plurality of phase shifters 2. The optical CDMA circuit according to claim 1, further comprising: an optical multiplexer that multiplexes the light. 8. A structure in which array waveguide gratings are symmetrically arranged as an optical demultiplexer and an optical multiplexer, and a phase shifter is connected between output waveguides of each array waveguide grating. The optical CDMA circuit according to claim 7, wherein