JP3154396B2 - Optical CDM circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical CDM (Code Division Multiplexed) circuit for performing spread modulation and despread demodulation of an optical signal in an optical spread spectrum communication system.

[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 are realized systematically by codes without using devices such as optical switches. Therefore, there is a great advantage in optical LAN and optical switching.

As a conventional optical spread spectrum circuit,
A device using an optical fiber coupling / branching and an optical fiber delay line as shown in FIG. 7 (optical CDM circuit) is known.

That is, in FIG. 7, in the optical CDM circuit on the transmitting side, N optical fiber delay lines (in other words, delay times Δτ) whose lengths are different in the 1 × N optical fiber branching section 13 by ΔL (in other words, delay time Δτ). Fiber Delay Lines) 14 are connected to each other, and each optical fiber delay line 14 is collected again in an N × 1 optical fiber multiplexing unit 15. Thus, when one optical pulse enters the 1 × N optical fiber branching unit 13, it is divided into N pulse trains, the length of the optical fiber delay line 14,
A code sequence (spread optical signal) is formed by adjusting the number.

Similarly, in the optical CDM circuit on the receiving side, 1
A matched filter (despread demodulation circuit) using an XN optical fiber branch section 16, N optical fiber delay lines 17 and an Nx1 optical fiber multiplexing section 18.
Is configured. Thus, when the optical pulse train (spread optical signal) encoded on the transmitting side is incident on the 1 × N optical fiber branching unit 16, each pulse is divided into N pulses, but the optical fiber delay line The non-coherent power is added by the 17 and the N × 1 optical fiber multiplexing unit 18, and the power is concentrated at the center of the optical pulse train. That is,
A correlation waveform between the spread code formed by the optical fiber delay line 17 and the incident signal is obtained.

[0006]

However, the conventional optical fiber delay lines 14 and 17 use spread modulation of optical signals and / or
Alternatively, in the despreading demodulation circuit, since correlation is realized by addition of power as described above, correlation is obtained with a unipolar pseudo-noise sequence. That is, since the power cannot be subtracted, the cross-correlation characteristics are poor and the SN ratio of the optical signal is poor.

Conventionally, 1 × N branching sections 13 and 16 and N × 1 multiplexing sections 15 and 18 are serially connected with a two-branch type optical element such as a Y-branch optical waveguide, a directional coupler, or a Mach-Zehnder interferometer. Connection or a combination of series / parallel connection. However, it is very difficult to fabricate the branching ratio of these two-branch optical elements as designed in an optical waveguide, and in practice, the branching ratio is adjusted by installing a phase adjuster using the thermo-optic effect. I have. Therefore, when such two-branch optical elements are connected in series, the deviation of the branch ratio of each of the two branch portions from the design value is integrated (accumulated), and the overall branch ratio is likely to greatly deviate from the design value. Had.

[0008]

In order to solve the above-mentioned problems, an optical CDM circuit according to the present invention comprises one input channel waveguide, a multimode interference type 1 × N branching element, and N optical paths each having a different optical path length ΔL. And a multimode interference type N × 1 multiplexing element is provided on the substrate so as to optically couple in this order, and a phase shifter is used for each of the delay waveguides (where N is A positive integer, ΔL is a positive number (substantially the same even with a negative number)), or an optical switch is provided in each of the delay waveguides.
Alternatively, the optical switch is a 2 × 2 optical switch,
The 2 × 2 optical switch is characterized in that the delay waveguide is one of an input section and an output section, and the remaining input section and output section are monitor ports.

As described above, the present invention provides a 1 × N branch section and N
Since the × 1 multiplexing unit is composed of both multimode interference optical elements and the phase shifter is arranged in the delay waveguide, addition in an electric field, that is, substantial subtraction, rather than addition of power, can be performed. This makes it possible to provide a circuit that performs spread modulation and / or despread demodulation of an optical signal having a high SN ratio in an optical spread spectrum communication system.

In the present invention, the amplitude of the optical signal in each of the delay waveguides is arbitrarily set (including zero) by an optical switch provided in each of the delay waveguides so that a good SN ratio can be obtained. ) Can be adjusted and weighted. In particular, when a 2 × 2 optical switch is used, N 2 × 2 optical switches are used.
Since any one of the two input ports and the two output ports of the two-optical switch is one of the input ports and one of the output ports is a monitor port of the 2 × 2 optical switch, a code sequence is formed. In this case, there is an advantage that parameter setting of each optical switch for the amplitude adjustment or the like can be easily and accurately performed.

Further, as shown in FIG. 2 or FIG. 3, the multimode interference type element used in the present invention performs all optical branching or optical multiplexing at a time, so that a 1.times. N branching sections 13 and 16 and N × 1 multiplexing section 1
In the case of 5, 18, the problem of "the deviation from the design value of the branch ratio is integrated" can be solved.

[0012]

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

FIG. 1 shows an embodiment of an optical CDM circuit according to the present invention when N = 16, showing one input channel waveguide 1 and a 1 × channel waveguide 1 composed of a multimode interference element. N
An optical branching unit 2, N 2 × 2 optical switches 3, N delay waveguides 4 whose optical path lengths are different by ΔL, and an N × 1 optical multiplexing unit 5 composed of a multi-mode interference element; A single output channel waveguide 6 is formed on a substrate 7 so as to be optically coupled in this order, and a phase shifter 12 is arranged in each delay waveguide 4. 2 optical switch 3
Is provided. In FIG. 1, reference numeral 8 denotes N 2 × 2
One input port of the optical switch is used as a monitor input port, and reference numeral 9 indicates that one output port of each of N 2 × 2 optical switches is used as a monitor output port. The 1 × N optical branching section is for branching an optical signal from one input port to N output ports, and the N × 1 optical multiplexing section is for splitting an optical signal from N input ports to one output port. The 2 × 2 optical switch is an optical switch having two input ports and two output ports.

FIG. 2 shows one embodiment of a multimode interference type device.
FIG. 3 is an enlarged view of the vicinity of the × N optical branching unit 2, wherein reference numeral 1 denotes the input channel waveguide, and reference numeral 10 denotes N = 16 channel waveguides. Assuming that the distance between the waveguide centers of the channel waveguide 10 is s, the width Wsi and the length L of the 1 × N optical branching unit 2
si is given by the following equation (1).

[0015]

[Number 1] Wsi = N × s, Lsi = (n e · Wsi 2) / (λN) ··· formula (1)

Here, _ne is the equivalent refractive index of the slab waveguide of the 1 × N optical branching unit 2, and λ is the wavelength of the optical signal.

Next, FIG. 3 is an enlarged view of the vicinity of the N × 1 optical multiplexing section 5 composed of a multi-mode interference type element. Reference numeral 6 denotes the output channel waveguide, and reference numeral 11 denotes N = 16. Channel waveguide. Also in this case, the channel waveguide 11
If the distance between the center of the waveguides is s, the N × 1 optical multiplexing unit 5
The width Wsi and the length Lsi are given by Equation (1) of Equation 1 above.

In these multi-mode interference type elements, when the above equation (1) is satisfied, one light spot is divided into N light spots by interference of many modes. 1 × N optical branching and N × 1 optical multiplexing are all achieved at once.

The optical CDM circuit according to the embodiment of the present invention is manufactured by using, for example, a quartz-based optical waveguide manufacturing technique as follows.

First, an SiO ₂ lower cladding layer is deposited on the Si substrate 7 by a flame deposition method, and then a SiO ₂ glass core layer doped with GeO ₂ as a dopant is deposited. did. Next, the core layer was etched using the patterns shown in FIGS. 1 to 3 to form an optical waveguide portion. Finally, the SiO ₂ upper cladding layer was deposited again. In this manner, one input channel waveguide 1, the multi-mode interference type 1 × N optical branching unit 2, and the optical path length are different by ΔL, and the phase shifter 12
Are arranged, and N delay waveguides 4 each having a 2 × 2 optical switch 3, a multimode interference type N × 1 optical multiplexing unit 5, and one output channel waveguide 6 are arranged in this order. The optical switch 3 is provided on the substrate 7 in an optically coupled manner.
An optical CDM circuit in which a monitor input port 8 and an output port 9 using the input / output ports of the above were also provided on the substrate 7 was manufactured.

The operation of the optical signal spread modulation / despread demodulation circuit of the optical spread spectrum communication system performed using the optical CDM circuit manufactured as described above will be described below.

FIG. 4 shows the optical CDM of N = 16 shown in FIG.
The figure shows how one optical pulse is modulated into a code sequence of N = 16 bits when the circuit is used as a spread modulation circuit. t indicates a time axis. In FIG. 4, each symbol of “π” and “0” shown in each optical output pulse is subjected to phase shift keying (PSK) modulation in which each optical pulse electric field is “π” and “0”. It represents that it is. The symbol S (abbreviation for Space) indicates an optical electric field shifted by “π”, and the symbol M (Ma
rk (abbreviation for mark) indicates an optical electric field having a phase shift of “0”. The PSK modulation of the light pulse is performed using each phase shifter 12 of the delay waveguide 4 in FIG.

That is, assuming that the phase of light passing through the n-th delay waveguide counted from the shorter one of the N delay waveguides is Φn, the phase Φn is given by the following equation (2).

[0024]

[Number 2] _{Φn = Φo + 2πn (n e} ΔL / λ) + (2π / λ) δn · d = Φo + 2πnm + (2π / λ) δn · d ··· formula (2)

Here, in the symbols in the equation (2), Φo is a constant phase shift amount, m is an integer, d is the length of the phase shifter 12, and δn is the refraction of the channel waveguide for causing the phase shift. The rate change amount, _ne is the equivalent refractive index of the slab waveguide of the 1 × N branch portion 2, and λ is the wavelength of the optical signal. Optical path length difference Δ
L is determined so as to satisfy the following equation (3).

[0026]

[Number 3] _{n e ΔL / λ = m (} m: integer) Equation (3)

As can be seen from the above equation (2), the refractive index change δn of the channel waveguide and the wavelength λ of the optical signal
And the length d of the phase shifter 12 as expressed by the following equation (4), the optical electric field φn is expressed by the following equation (5). Here, An is the amplitude of the optical electric field determined by the 2 × 2 optical switch 3 using, for example, a heater or the like of the n-th delay waveguide 4 in FIG.

[0028]

Δn = λ / (2d) Equation (4)

[0029]

Equation 5 φn = An · exp [−jΦn] = An · exp [−jΦo−jπ] = − An · exp [−jΦo] Equation (5)

When δn = 0, the optical electric field φ
It can be seen that n is represented by the following equation (6).

[0031]

Equation 6 φn = An · exp [−jΦn] = An · exp [−jΦo] Equation (6)

Therefore, as can be seen from Equations (5) and (6) of Equations (5) and (6), an optical electric field shifted by “π” is called “Space”, and an optical electric field having a phase shift of “0” is referred to as “Space”. To Mark
(Mark) ", since they have opposite polarities, when the optical electric field Space and the optical electric field Mark are added, unlike the addition of power, it is substantially a subtraction, and the optical electric field is canceled to zero. become.

The optical pulse train shown in FIG. 4 corresponds to the delay waveguide 4 shown in FIG.
1, 4, 5 counting from the shorter one in the phase shifter 12 of FIG.
, 7, 9, 10, 11, 12, and 16th phase shifters are driven to provide a phase shift by π.

As described above, the amplitude An of the optical electric field can be determined by the optical switch of the delay waveguide 4.
By using the optical switch 3, the amplitude of input / output light can be known using the monitoring ports 8 and 9 when forming a code sequence.
The parameter setting of the × 2 optical switch 3 can be performed easily and accurately.

Next, the operation characteristics when the optical CDM circuit shown in FIG. 1 is used as a despread demodulation circuit will be described with reference to FIG.
Will be described with reference to FIG.

FIG. 5 shows the phase shifter 12 of the delay waveguide 4 of FIG.
The despreading demodulation circuit in which the fourth, fifth, seventh, ninth, tenth, eleventh, twelfth and sixteenth phase shifters are driven so as to provide a phase shift by π is applied to the optical pulse train shown in FIG.
SMMSSMSSMSSSSMMM in order from the first bit
9 shows a light intensity waveform when S) is incident.

In the “π” phase shifter, the optical electric field of the incident pulse is converted from Mark (indicated as “M” in the figure) to Space, and Space (indicated as “S” in the figure) is converted into Mark. . On the other hand, in the “0” phase shifter, the optical electric field Mark and Space remain unchanged.

The tap arrangement of the phase shifter of the despreading demodulation circuit used here is, as shown as "tap coefficient (in order from the maximum delay arm, π00ππ0π0ππππ000π)" in FIG. 5, as shown in FIG. It corresponds to tap arrangement. Therefore, a sharp correlation peak is detected at the center of the output light pulse train as shown in FIG.

On the other hand, as shown in FIG. 6, a tap arrangement different from the tap arrangement of the phase shifter of the spread modulation circuit of FIG. 4 (in the example of FIG. Πππ00ππ000000π0π in order)
As can be seen from the figure, the 1, 2, 3, 6, 7, 14, and 16th phase shifters counted from the longest in the phase shifter 12 of the delay waveguide 4 are driven so as to give a phase shift by π. When the correlation is detected using the despread demodulation circuit, the same optical pulse train (SMMSSMSMSSSSSMM) is used.
It can be seen that the cross-correlation output is hardly detected even if (MS) is incident.

From the above, it can be seen that by using the optical CDM circuit of the present invention, only a desired signal sequence can be detected from a large number of coded and multiplexed signals.

In the example of FIG. 4, the amplitudes of all the optical signals are made equal. However, it is clear that the SN ratio can be made higher by changing the amplitude of each optical signal by setting the parameters of the 2 × 2 optical switch 3. is there.

[0042]

As described above, in the optical CDM circuit of the present invention, both the 1 × N branching section and the N × 1 multiplexing section are constituted by multimode interference type optical elements. By disposing the phase shifter in the delay waveguide, it is possible to perform addition in an optical electric field, that is, substantial subtraction, instead of non-coherent power addition. A circuit for performing spread demodulation can be provided, which has a great advantage in an optical spread spectrum communication system.

Also, since the multimode interference type element performs all branching or multiplexing at one time, there is a problem in the case of a 1 × N branching section and an N × 1 multiplexing section conventionally formed by combining two branching elements. This solves the problem that “the deviation from the design value of the branch ratio is integrated”.

Further, the amplitude of the optical signal can be set by the optical switch provided in each delay waveguide so that a good SN ratio can be obtained. In particular, 2 × as an optical switch
When the two-optical switch is used, the amplitude of the input / output signal can be known using the monitor port, so that the parameters of each optical switch can be easily and accurately set when forming the code sequence.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of an optical CDM circuit of the present invention.

FIG. 2 is an enlarged view of the vicinity of a 1 × N optical branching unit constituted by the multimode interference type element in FIG.

FIG. 3 is an enlarged view of the vicinity of an N × 1 optical multiplexing unit constituted by the multi-mode interference type element in FIG. 1;

FIG. 4 is a diagram illustrating a state in which one optical pulse is modulated into a 16-bit code sequence when the optical CDM circuit in FIG. 1 is used as a spread modulation circuit.

FIG. 5 shows 1, 4, 5, 7, 9, 10, 11, 12, 1 in the phase shifter of the delay waveguide of FIG.
FIG. 5 is a diagram illustrating a light intensity waveform when the optical pulse train of FIG. 4 is incident on a despread demodulation circuit in which a sixth phase shifter is driven by π.

FIG. 6 is a diagram illustrating a despread demodulation circuit in which the first, second, third, sixth, seventh, twelfth and sixteenth phase shifters counted from the longest in the delay waveguide phase shifter of FIG. 1 are driven by π; 4
FIG. 4 is a diagram showing a light intensity waveform when an optical pulse train of FIG.

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

[Description of Signs] 1 Input channel waveguide 2 1 × N optical branching unit 3 2 × 2 optical switch 4 Delay waveguide having optical path lengths different by ΔL 5 N × 1 optical multiplexing unit 6 Output channel waveguide 7 Substrate 8 One input port of a 2 × 2 optical switch used as a monitor input port 9 One output port of a 2 × 2 optical switch used as a monitor output port 10 N (= 16) channel waveguides 11 N (= 16) channel waveguides 12 phase shifters arranged in delay waveguides

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04J 13/00 (56) References JP-A-7-87027 (JP, A) JP-A-5-37542 (JP, A) Kaihei 5-323246 (JP, A) JP 7-61057 (JP, B2) U.S. Pat. No. 4,866,699 (US, A) Proceedings of the IEICE General Conference (March 1995) C-253 page Hiroaki Yamada Others (58) Surveyed fields (Int.Cl. ⁷ , DB name) G02F 1/01-1/313 H04B 10/00-10/28 H04J 13/00 INSPEC (DIALOG) JICST file (JOIS) WPI (DIALOG)

Claims (3)

(57) [Claims] When N is a positive integer and ΔL is a positive number,
One input channel waveguide, a multimode interference type 1 × N branch element, N delay waveguides having optical path lengths different by ΔL, and a multimode interference type N × 1 multiplexing element are optically coupled in this order. An optical CDM circuit, wherein the optical waveguide is provided on a substrate, and a phase shifter is used for each of the delay waveguides. 2. The optical CDM circuit according to claim 1, wherein an optical switch is provided in each of the delay waveguides. 3. The optical switch is a 2 × 2 optical switch, wherein the 2 × 2 optical switch uses the delay waveguide as one of an input section and an output section, and the remaining input section and output section are monitor ports. 3. The optical CDM according to claim 2, wherein:
circuit.