JP3333425B2 - Optical CDMA circuit and optical communication system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical CDMA (Code Division Multiple Access) for performing spread modulation and despread demodulation of an optical signal in an optical spread spectrum communication system.
iple Access) circuit and an optical communication system using an optical CDMA circuit.

[0002]

2. Description of the Related Art When a spread spectrum system used in the field of wireless communication is applied to optical communication, random access and self-routing operation can be realized systematically by codes without using devices such as optical switches. Therefore, it has a great advantage in an optical LAN (local area network) and optical switching.

As a conventional optical spread spectrum circuit,
One using an optical fiber branching and optical fiber delay line as shown in FIG. 7 is known. On the transmitting side of the circuit shown in FIG. 7, the 1 × N optical branching unit 24 is divided into N fiber delay lines 2 having different lengths ΔL (delay times Δτ).
5, and the delay lines 25 are collected again in the N × 1 optical multiplexing unit 26. When one optical pulse enters the 1 × N optical branching unit 24, it is divided into N pulse trains,
A code sequence is formed by delaying the length and number of the fiber delay line 25.

Similarly, on the receiving side in FIG. 7, a 1 × N optical fiber branching section 27, N fiber delay lines 28, and N
When a matched filter is formed using the × 1 optical fiber multiplexing unit 29 and a coded pulse train is incident on the 1 × N optical branching unit 27, each pulse is divided into N pulses, but non-coherent addition is performed. And the power is concentrated in the center of the pulse. As a result, a correlation waveform between the spread code constituted by the fiber delay line 28 and the incident signal is obtained.

[0005]

However, in the above-described conventional optical signal spread modulation / despread demodulation circuit using a fiber delay line, since the correlation is realized by the addition of power, a unipolar signal is used. It is correlated with the pseudo noise sequence. Therefore, since the power cannot be subtracted, there is a problem to be solved that the characteristics of the cross-correlation are poor and the SN ratio (signal-to-noise ratio) is poor.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has as its object to make it possible to perform substantial subtraction by enabling addition in an electric field instead of addition of power. An object of the present invention is to provide an optical CDMA circuit capable of performing spread modulation and despread demodulation of an optical signal having a high SN ratio and an optical communication system using the optical CDMA circuit.

[0007]

In order to achieve the above object, the invention of claim 1 comprises an input optical waveguide, an N channel array grating for demultiplexing, N 2 × 2 optical switches,
In an optical circuit in which a multiplexing N-channel array grating and an output optical waveguide are formed, the N-channel array grating for demultiplexing and the N-channel array grating for multiplexing have N equal length optical waveguides. The 2 × 2 optical switch and the phase shifter are arranged in a part of each of the optical waveguides.

Here, 2 of the N 2 × 2 optical switches are
Among the one input port and the two output ports, any one of the input ports and one output port can be a monitor port of a 2 × 2 optical switch.

According to a third aspect of the present invention, there is provided an optical CDMA communication system according to the first or second aspect, wherein
A star coupler is connected between an optical CDMA circuit for transmission that performs spread modulation of an optical signal and an optical CDMA circuit for reception that performs despread demodulation of an optical signal by using a plurality of circuits.

The present invention comprises an input optical waveguide, an N-channel array grating for demultiplexing, N 2 × 2 optical switches, an N-channel array grating for multiplexing, and an output optical waveguide. The demultiplexing array grating and the multiplexing array grating are connected by N equal-length optical waveguides, and a 2 × 2 optical switch and a phase shift are arranged in a part of each optical waveguide. In addition, it is possible to perform addition in an electric field instead of power, that is, substantial subtraction, and perform spread modulation and despread demodulation of an optical signal having a high SN ratio in an optical spread spectrum communication system.

According to the present invention, of the two input ports and two output ports of the N 2 × 2 optical switches, one of the input ports and one output port is a monitor of the 2 × 2 optical switch. Since the port is a port, the parameters of each optical switch can be easily set when forming a code sequence.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows the number of channels of the array grating N = 32
1 shows an embodiment of the optical CDMA circuit of the present invention in the case of (1).
The optical CDMA circuit includes an input optical waveguide 1, a first slab 2 of a demultiplexing array grating, an array waveguide 3 of a demultiplexing array grating, a second slab 4 of a demultiplexing array grating, and N connections. Waveguide 10, N 2 × 2 optical switches 5, phase shifts 6 and 7, N connecting waveguides 11, second array slab 12 for multiplexing, array waveguide 1 for multiplexing array grating
3. The first slab 14 of the multiplexing array grating and the output optical waveguide 15 are formed on a substrate 16.

Reference numeral 8 denotes N 2 × 2 optical switches 5 each.
Is an input port for monitoring, and 9 is an output port for monitoring, wherein one output port of each of the N 2 × 2 optical switches 5 is used as a monitoring output port.

The optical CDMA circuit of the present invention was manufactured using a quartz-based optical waveguide manufacturing technique as follows.

First, S is deposited on a Si substrate by a flame deposition method.
Deposit an iO ₂ lower cladding layer, and on top of that layer,
After depositing a core layer of SiO ₂ glass to which eO ₂ was added as a dopant, vitrification was performed in an electric furnace.

Next, the core layer is etched using the pattern shown in FIG.
3, 10, 11, 13, and 15 were produced. Finally, the SiO ₂ upper cladding layer was deposited again. The thin film heaters used for the 2 × 2 optical switch 5 and the phase shifters 6 and 7 were arranged on the upper clad layer.

FIG. 2 shows the 2 × 2 optical switch 5 shown in FIG.
FIG. 2A is a plan view of a Mach-Zehnder interferometer constituting the 2 × 2 optical switch 5, and FIGS. 2B and 2C are cutaways of FIG. 2A. It is an expanded sectional view in line AA 'and BB'. Silica-based glass optical waveguide 52 on substrate 51
And 53 are close to each other at a distance of several μm at two places,
Two directional couplers 54 and 55 are configured. The coupling ratio of the directional couplers 54 and 55 is set to 50% by adjusting the waveguide interval and length of the coupling portion. An optical waveguide 52, between two directional couplers 54, 55,
The lengths of 53 are set equal, and thin film heaters 56a and 56b are provided above the cladding layer 57 of this optical waveguide (see FIG. 2C).

This optical switch element constitutes a Mach-Zehnder interferometer as a whole, and the thin film heaters 56a, 56
In the off state in which no current flows through 6b, the signal light input from the left end of the optical waveguide 52 is output from the right end of the optical waveguide 53, and the signal light input from the left end of the optical waveguide 53 is output from the right end of the optical waveguide 52. In a so-called crossport state.

However, a current is applied to one of the thin-film heaters 56a and 56b to raise the temperature of the optical waveguide below the thin-film heaters 56a and 56b.
When a phase difference equivalent to π (radian) is generated between 4, 55, the signal light input from the left end of the optical waveguide 52 is output as it is from the right end of the optical waveguide 52, and the signal light input from the left end of the optical waveguide 53 Is switched from the right end of the optical waveguide 53 to a so-called through port state. When a phase difference between 0 and π (radian) is generated, the amount of light input from the left end of the optical waveguide 52 (or 53) and output from the right end of the optical waveguide 53 (or 52) is calculated. It can be set to any value between 0 and 1 (when the maximum value is normalized to 1).

Next, FIG. 3 shows the phase shifters 6 and 6 in FIG.
7 is an enlarged view and a sectional view of FIG. Above the cladding layer 64 of the optical waveguide 61 on the substrate 63, a thin film heater 62 is provided. When a current flows through the thin-film heater 62, the temperature of the optical waveguide 61 thereunder is increased, and the refractive index of the optical waveguide 61 is increased by the thermo-optic effect. Assuming that the change in the refractive index at this time is δn, the length of the heater 62 is 1 (letter of alphabet), and the wavelength of light is λ

[0022]

(Equation 1)

The following phase change φ can be obtained.

The optical CDM of the present invention thus produced
The operation of the spread-spectrum modulation / de-spread demodulation circuit for the optical signal of the optical spread-spectrum communication system performed using the A-circuit will be described below.

FIG. 4 is a diagram showing the configuration of a communication system using the optical CDMA circuit of the present invention for transmission and reception. here,
17 is an optical CDMA circuit for transmission, 18 is a star coupler, and 19 is an optical CDMA circuit for reception. Optical CD for transmission
Assuming that the repetitive pulse waveform of the signal pulse 20 (detailed in FIG. 5) of the repetition time T incident on the input optical waveguide 1 of the MA circuit 17 is f (t), f (t) is represented by the following equation (2).
It is represented as

[0026]

(Equation 2)

Here, sech (t) represents a pulse waveform, and A is a constant.

The frequency spectrum of the repetitive pulse waveform f (t) is given by the following equation (3) as a Fourier transform of f (t).

[0029]

(Equation 3)

Here, δ () is a delta function, and f ₀
= Ω ₀ / 2π. As is apparent from equation (3), F
(F) is a line spectrum every 1 / T (Hz).

Since the frequency channel spacing of the array gratings of FIGS. 1 and 4 is set to 1 / T (Hz), l =
Each of the spectrum components 1 to N is a demultiplexing array grating 2,
Demultiplexed by 3 and 4, the connection waveguide 10, N 2 × 2
The light is guided to the optical switch 5 and the phase shifters 6 and 7. here,
An encoding function C such as
(F) is multiplied.

[0032]

(Equation 4)

The amplitude value a _l of the above equation (4) is realized in the 2 × 2 optical switch 5, the phase shift amount phi _l phase shifter 6
Is realized.

The spectrum component G (f) = F (f) × C (f) multiplied by the encoding function C (f) is represented by the following equation (5).

[0035]

(Equation 5)

The encoded spectral component G (f) is N
Waveguides 11 for connection, array gratings 12 and 13 for multiplexing,
The light is guided to an output optical waveguide 15 through 14. The optical electric field g (t) at this time is given by the following equation (6) as the sum of the sinusoidal electric fields obtained by the encoded line spectra.

[0037]

(Equation 6)

Reference numeral 21 in FIG. 4 (shown in detail in FIG. 5) is an example of a coded pulse coded by a suitable combination of coded amplitude and phase shift (a _l , φ _l ). Such a coding pulse 21 is distributed to the CDMA network by the star coupler 18 of FIG.

Next, reception of a desired signal by the optical CDMA circuit of the present invention will be described. 4 is the receiving light C.
This is a DMA circuit, which is essentially the same as the transmission CDMA circuit. An encoding pulse 21a from another user is incident on the input optical waveguide of the receiving optical CDMA circuit 19. Now, it is assumed that the optical electric field of the encoding pulse 21a is represented by the above equation (6). Each line spectral component of the encoding pulse 21a is demultiplexed by the demultiplexing array gratings 2 ', 3', and 4 'of the receiving optical CDMA circuit 19, and is connected to the connecting waveguide 1.
0 ', N 2 × 2 optical switches 5', phase shifters 6 ',
It is led to 7 '. Here, the l-th spectral component is multiplied by a decoding function D (f) as shown in the following equation (7).

[0040]

(Equation 7)

The spectral component H (f) = G (f) × D (f) multiplied by the decoding function D (f) is represented by the following equation (8).

[0042]

(Equation 8)

The spectral component H (f) multiplied by the decoding function D (f) passes through the N connecting waveguides 11 'and the multiplexing array gratings 12', 13 ', and 14', and the output light guide. It is guided to the wave path 15 '. The optical electric field h (t) at this time
Is given as the following equation (9) as the sum of the sinusoidal electric fields of the respective line spectra.

[0044]

(Equation 9)

Here, in order to obtain another expression of the repetitive pulse waveform f (t) of the above equation (2), the inverse Fourier transform of the frequency spectrum F (f) of the above equation (3) is obtained. Equation (10) is obtained.

[0046]

(Equation 10)

F '(t) is another expression of the repetitive pulse waveform f (t) of the equation (2). By comparing equation (9) and equation (10),

[0048]

[Equation 11]

In the case of, except for the constant term,

[0050]

(Equation 12)

It can be seen that the signal pulse f (t) is detected. This is shown by the decoding pulse 22 in FIG.

The condition of the above equation (11) is that the receiving optical CDMA
The optical switch 5 'of the circuit 19 and the phase shifter 6',
This can be achieved by setting the same amplitude condition as that of the optical switch 5 of the CDMA circuit on the transmitting side to receive 7 'and the phase condition of complex conjugate with the phase shifters 6 and 7. At this time, since signals other than the desired light pulse do not satisfy the above equation (11), the noise becomes lower than the detection level as shown by the non-detection pulse 23 in FIG. 6, and is eliminated by electrical processing such as a limiter. Is done.

[0053]

As described above, the optical CDM of the present invention is
According to the circuit A, it is possible to provide a circuit that performs addition, that is, substantial subtraction, not in power but in an electric field, and that performs spread modulation and despread demodulation of an optical signal having a high SN ratio. It has a great advantage in the spread communication system.

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration of one embodiment of an optical CDMA circuit of the present invention.

2 is an enlarged plan view (a) and sectional views (b) and (c) of the 2 × 2 optical switch 5 in FIG.

FIG. 3 is an enlarged plan view (a) and a cross-sectional view (b) showing the phase shifters 6 and 7 in FIG.

FIG. 4 is a configuration diagram showing a configuration of a communication system using the optical CDMA circuit of the present invention for transmission and reception.

FIG. 5 is a waveform chart showing a detailed example of a signal pulse and a coding pulse in FIG. 4;

FIG. 6 is a waveform chart showing a detailed example of a decoding pulse and a non-detection pulse in FIG. 4;

FIG. 7 is a schematic diagram showing a configuration of a conventional optical spread spectrum circuit using an optical fiber coupling / branching and an optical fiber delay line.

[Explanation of symbols]

 Reference Signs List 1 input optical waveguide 2 first slab of demultiplexing array grating 3 array waveguide of demultiplexing array grating 4 second slab of demultiplexing array grating 5 2 × 2 optical switch 6,7 phase shifter 8 2 × 2 Input port for monitoring optical switch 9 Output port for monitoring 2 × 2 optical switch 10, 11 Connecting waveguide 12 Second slab of multiplexing array grating 13 Arraying waveguide of multiplexing array grating 14 Multiplexing array First slab of grating 15 Optical waveguide for output 16 Substrate 17 Optical CDMA circuit for transmission 18 Star coupler 19 Optical CDMA circuit for reception 20 Signal pulse 21 Encoding pulse 22 Decoding pulse 23 Non-detection pulse 24 Optical fiber branch 25 Optical fiber Delay line 26 Optical fiber multiplexing section 27 Optical fiber branching section 28 Optical fiber delay line 29 Optical fiber multiplexing section

Continuation of the front page (56) References JP-A-9-265060 (JP, A) JP-A-7-318987 (JP, A) JP-A-2-127833 (JP, A) JP-A-5-300124 (JP) , A) Koichi Takiguchi et al., Principle experiment on coherent optical CDMA using arrayed waveguide grating pairs, 1998 IEICE General Conference, Electronics 1, Japan, The Institute of Electronics, Information and Communication Engineers, March 6, 1998 , p. 281 Hiroyuki Tsuda et al., Application of Arrayed Waveguide Grating to Optical Code Division Multiplexing, The 1st Workshop on Next Generation Internet Technology Based on Photonic Network, Japan, The Institute of Electronics, Information and Communication Engineers, June 1999, p. 54-60 (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02B 6/122 G02F 1/313 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. Light having an input optical waveguide, a demultiplexing N-channel array grating, N 2 × 2 optical switches, a multiplexing N-channel array grating, and an output optical waveguide formed on a substrate. In the circuit, the N-channel array grating for demultiplexing and the N-channel array grating for multiplexing are connected by N optical waveguides having the same length, and a part of each of the optical waveguides includes the 2 × An optical CDMA circuit comprising a two-optical switch and a phase shifter. 2. One of two input ports and two output ports of the N 2 × 2 optical switches, one of which is a monitor port of a 2 × 2 optical switch. The optical CDMA circuit according to claim 1, wherein: 3. In an optical spread spectrum communication system,
A star coupler between a transmission optical CDMA circuit for performing spread modulation of an optical signal and a reception optical CDMA circuit for performing despread demodulation of an optical signal by using a plurality of the optical CDMA circuits according to claim 1 or 2. An optical communication system, comprising: