Description

VARIABLE OPTICAL DISTRIBUTORS USING MULTI-MODE

INTERFERENCE

Technical Field

[1] The present invention relates to a variable optical distributors using multi-mode interference. Background Art

[2] A variable optical distributor is one of very important elements of an optical circuit for an optical communication to control an optical signal transmission. The variable optical distributor performs an add/drop function to equalize an optical power of channels of a wavelength division multiplexing (WDM) system. Also, when a total power of an erbium doped fiber amplifier (EDFA) is varied, the variable optical distributor flats a gain in order to prevent an output variation according to a wavelength. The variable optical distributor is also used to distribute or control an arbitrary optical power to a plurality of transmitting ends.

[3] As the optical distributor, a fiber optical is widely used. However, the fiber optical distributor is large in size and is complicated in manufacturing process. Therefore, manufacturing yield is low, and a production cost is high. Also, a conventional optical attenuator based on a planar lightwave circuit (PLC) technique (e.g., disclosed in Korean Patent Publication No. 1999-0020073) uses a directional coupler. Thus, the optical attenuator is large in length and wide in width, and is narrow in manufacturing process margin. In addition, since a bent waveguide has to be used at various locations due to the directional coupler, a length of the optical attenuator becomes larger and a loss also becomes larger. Further, the optical attenuator can be expected to have an improved operation characteristic, production yield, and low cost compared with another conventional optical attenuator (disclosed in Korean Patent No. 0424606 and U.S. Patent No. 6,728,463 B2). Disclosure of Invention

Technical Problem

[4] Accordingly, the present invention is directed to a variable optical distributor that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

[5] An object of the present invention is to provide an optical distributor having a PLC waveguide structure with a multi-mode waveguide, a single-mode waveguide, and a thin film heater for temperature control. The optical distributor can distribute an optical power with a desired magnitude by varying a phase of an input optical signal by a

multi-mode interference and a thermo-optic effect of the temperature control heater. Technical Solution

[6] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided an optical distributor having a strong guiding multi-mode waveguide structure with a predetermined width, a predetermined length, and a predetermined thickness, the optical distributor including: a first multi-mode waveguide (3) receiving an optical signal at one side thereof, wherein the received optical signal is distributed in one or more self- images at periodic intervals in a proceeding direction, and a predetermined optical power is outputted to the other side of the first multi-mode waveguide (3); one or more transmission lines to which a temperature control heater (6) is attached; and a second multi-mode waveguide (7) receiving the optical signal at one side thereof through the one or more transmission lines, wherein the received optical signal is distributed in one or more self-images at periodic intervals in a proceeding direction, and a predetermined optical power is outputted through the other side of the second multi-mode waveguide (7).

[7] One or more optical signals are directly inputted to the first multi-mode waveguide

(3) or are inputted through a transmission line coupled to the first multi-mode waveguide 3.

[8] One or more optical signals are directly outputted from the second multi-mode waveguide (7) or are outputted through a transmission line coupled to the second multi-mode waveguide (7).

[9] The temperature control heater (6) attached to one or more transmission lines for connecting the first multi-mode waveguide (3) and the second multi-mode waveguide (7) varies phases of the respective optical signals proceeding in the one or more transmission lines to 0-180°by a thermo-optic effect.

[10] The length (L MMl ), width (W MMl ), and thickness (d) of the first multi-mode waveguide (3) and the input/output position of the optical signal are determined by wavelength (λ) of the applied optical signal, refractive index (n) of the waveguide, and general interference. [11] The length (L ), width (W ), and thickness (d) of the first multi-mode

MMl MMl waveguide (3) and the input/output position of the optical signal are determined by wavelength (λ) of the applied optical signal, refractive index (n) of the waveguide, and paired interference. [12] The length (L ), width (W ), and thickness (d) of the first multi-mode w

MMl MMl aveguide (3) and the input/output position of the optical signal are determined by wavelength (λ) of the applied optical signal, refractive index (n) of the waveguide, and

symmetric interference. [13] The length (L ), width(W ), and thickness (d) of the second multi-mode

MM2 MM2 waveguide (7) and the input/output position of the optical signal are determined by wavelength (λ) of the applied optical signal, refractive index (n) of the waveguide, and general interference. [14] The length (L ), width (W ), and thickness (d) of the second multi-mode

MM2 MM2 waveguide (7) and the input/output position of the optical signal are determined by wavelength (λ) of the applied optical signal, refractive index (n) of the waveguide, and paired interference. [15] By adding a predetermined number of the optical distributors in series and in parallel, the optical power distribution can be outputted through a plurality of output terminals.

Advantageous Effects

[16] A length of the optical distributor according to the present invention becomes short.

In addition, a phase variation range due to a thermo-optic effect of a temperature control heater can be decreased. Production yield can be improved by extending a manufacturing process margin.

Brief Description of the Drawings [17] FIG. 1 is a view illustrating an optical distributor/coupler according to a first embodiment of the present invention; [18] FIG. 2 is a view illustrating an optical distributor/coupler according to a second embodiment of the present invention; [19] FIG. 3 is a view illustrating an optical distributor/coupler according to a third embodiment of the present invention; [20] FIG. 4 is a view illustrating an optical distributor/coupler according a fourth embodiment of the present invention; and [21] FIG. 5 is a view illustrating an optical distributor/coupler according to a fifth embodiment of the present invention.

Best Mode for Carrying Out the Invention [22] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to accompanying drawings. [23] When an optical signal is input to a multi-mode waveguide and proceeds, the optical signal is distributed, and an image of a wave excited in an input optical signal is reproduced in a predetermined length by a constructive interference between modes.

This is an inherent characteristic of light called a self-image formation, and is a phenomenon that one or more images are periodically formed along a proceed direction of the waveguide. A basic principle of this self-image formation is described

in J. Lightwave Technol., vol. 13, p. 615, 1995. The optical wave distribution within the multi-mode waveguide can be expressed as Equation (1): [24] MathFigure 1 m- \

[25] where ψ denotes an excited mode, and v denotes order of mode.

[26] When the input position of the optical signal is located at a general location of the multi-mode waveguide, the length that a desired number of self-images are formed is given by Equation (2): [27] MathFigure 2

[28] where M is an integer representing period, N denotes the number of self-images, and L denotes a beat length and is obtained by Equation (3): [29] MathFigure 3

[30] where β and β denote a propagation constant of a standard mode and a propagation constant of a first mode, respectively, λ denotes a wavelength, n denotes

0 e an effective refractive index, and W denotes a substantial width of the standard mode. e

In case that I/O terminals are located at a W/3 position or a 2W/3 position of the multi- mode waveguide, when a paired interference is used, and a length that a desired number of the self-images are formed is obtained by Equation (4): [31] MathFigure 4

L = L.

[32] When the I/O terminals are located at a W/2 position of the multi-mode waveguide, a symmetric interference is used, a length that a desired number of self-images are formed is obtained by Equation (5):

[33] MathFigure 5

[34] As shown in FIG. 1, in an optical distributor having a strong guiding waveguide structure according to a first embodiment of the present invention, a first single-mode waveguide 1 and a second single-mode waveguide 2 are directly coupled to one side of a first multi-mode waveguide 3 and an optical signal is inputted thereto. The optical signal is distributed to one side of a third single-mode waveguide 4 to which a temperature control heater 6 directly coupled to the other side of the first multi-mode waveguide 3 is attached and a fourth single-mode waveguide 5 to which the temperature control heater 6 is attached. One side of the second multi-mode waveguide 7 is directly coupled to the other side of the third single-mode waveguide 4 and the fourth single-mode waveguide 5, so that the distributed optical signal is inputted to the second multi-mode waveguide 7. The optical signal is distributed to one side of a fifth single-mode waveguide 8 and a sixth single-mode waveguide 9 directly coupled to the other side of the second multi-mode waveguide 7. By the combination of the phases of the optical signals varied due to the phase variation at the input optical signal at the first multi-mode waveguide 3 and the second multi-mode waveguide 7 and the thermo- optic effect at the third single-mode waveguide 4 and the fourth single-mode waveguide 5, a predetermined optical power distribution is outputted to the fifth single-mode waveguide 8 and the sixth single-mode waveguide 9.

[35] The widths (W SM ) and thickness (d) of the first to sixth single-mode waveguides 1,

2, 4, 5, 8 and 9 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide. Also, the width (W ), thickness (d) and

MMl length (L ) of the first multi-mode waveguide 3 and the width (W ), thickness (d)

MMl MM2 and length (L ) of the second multi-mode waveguide 7 are determined by the

^& MM2 ^{& J} wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide and by Equations (2) and (3) in the case of the general interference. [36] In the case of the general interference, the optical signal is inputted to the first multi-mode waveguide 3 through the first single-mode waveguide 1 and the second single-mode waveguide 2 directly coupled to a width-direction edge of the first multi- mode waveguide 3. The optical signal gets to have a phase difference of 270°by a general interference while proceeding in the first multi-mode waveguide 3 and are distributed in a rate of 50:50 and then are transferred to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 directly coupled to the width- direction edge of the first multi-mode waveguide 3. The temperature control heater 6 is attached to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 so as to vary an effective refractive index by a thermo-optic effect. The optical signal proceeding in the third single-mode waveguide 4 and the fourth single-mode waveguide 5 has a predetermined phase variation of 0°to 180°due to the varied effective refractive index. The optical signal having the varied phase is directly

coupled to the width-direction edge of the second multi-mode waveguide 7, and the respective input optical signals have a phase difference of 270°by a general interference while proceeding in the second multi-mode waveguide 7 and are distributed in a rate of 50:50 and then are transferred to the fifth single-mode waveguide 8 and the sixth single-mode waveguide 9 directly coupled to the width-direction edge of the second multi-mode waveguide 7. The optical signals that are transferred to the output terminal make a desired optical power distribution in such a manner that the phase of 0°to 180°varied by the thermo-optic effect and a phase of 270° varied by the multi- mode interference are offset or added up.

[37] As shown in FIG. 2, in an optical distributor having a strong guiding waveguide structure according to a second embodiment of the present invention, a first single- mode waveguide 1 and a second single-mode waveguide 2 are directly coupled to one side of a first multi-mode waveguide 3 and an optical signal is inputted thereto. The optical signal is distributed to one side of a third single-mode waveguide 4 to which a temperature control heater 6 directly coupled to the other side of the first multi-mode waveguide 3 is attached and a fourth single-mode waveguide 5 to which the temperature control heater 6 is attached. One side of the second multi-mode waveguide 7 is directly coupled to the other side of the third single-mode waveguide 4 and the fourth single-mode waveguide 5, so that the distributed optical signal is inputted to the second multi-mode waveguide 7. The optical signal is distributed to one side of a fifth single-mode waveguide 8 and a sixth single-mode waveguide 9 directly coupled to the other side of the second multi-mode waveguide 7. By the combination of the phases of the optical signals varied due to the phase variation of the input optical signal at the first multi-mode waveguide 3 and the second multi-mode waveguide 7 and the thermo- optic effect at the third single-mode waveguide 4 and the fourth single-mode waveguide 5, a predetermined optical power distribution is outputted to the fifth single-mode waveguide 8 and the sixth single-mode waveguide 9.

[38] The widths (W ) and thickness (d) of the first to sixth single-mode waveguides 1,

SM

2, 4, 5, 8 and 9 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide. Also, the width (W ), thickness (d) and

MMl leng ^& th (L MMl ) of the first multi-mode waveg ^& uide 3 are determined by J the wavelengbth

(K) of the applied optical signal and the refractive index (n) of the waveguide and by Equations (2) and (3) in the case of the general interference. The width (W ),

MM2 thickness (d) and length (L MM2 ) of the second multi-mode waveguide 7 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide and by Equations (2) and (4) in the case of the paired interference. [39] In the case of the general interference, the optical signal is inputted to the first multi-mode waveguide 3 through the first single-mode waveguide 1 and the second

single-mode waveguide 2 directly coupled to a width-direction edge of the first multi- mode waveguide 3. The optical signal gets to have a phase difference of 270°by a general interference while proceeding in the first multi-mode waveguide 3 and are distributed in a rate of 50:50 and then are transferred to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 directly coupled to the width- direction edge of the first multi-mode waveguide 3. The temperature control heater 6 is attached to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 so as to vary an effective refractive index by a thermo-optic effect. The optical signal proceeding in the third single-mode waveguide 4 and the fourth single-mode waveguide 5 has a predetermined phase variation of 0°to 180°due to the varied effective refractive index. The optical signal having the varied phase is directly coupled to W /3 and 2W /3 of the width direction of the second multi-mode

MM2 MM2 waveguide 7, and the respective input optical signals have a phase difference of 270°by a paired interference while proceeding in the second multi-mode waveguide 7 and are distributed in a rate of 50:50 and then are transferred to the fifth single-mode waveguide 8 and the sixth single-mode waveguide 9 directly coupled to W /3 and 2W MM2 /3 of the width direction of the second multi-mode waveg °uide 7. The o ^r ptical signals that are transferred to the output terminal make a desired optical power distribution in such a manner that the phase of 0°to 180°varied by the thermo-optic effect and a phase of 270°varied by the multi-mode interference are offset or added up. [40] As shown in FIG. 3, in an optical distributor having a strong guiding waveguide structure according to a third embodiment of the present invention, a first single-mode waveguide 1 is directly coupled to one side of a first multi-mode waveguide 3 and an optical signal is inputted thereto. The optical signal is distributed to one side of a third single-mode waveguide 4 to which a temperature control heater 6 directly coupled to the other side of the first multi-mode waveguide 3 is attached and a fourth single-mode waveguide 5 to which the temperature control heater 6 is attached. One side of the second multi-mode waveguide 7 is directly coupled to the other side of the third single- mode waveguide 4 and the fourth single-mode waveguide 5, so that the distributed optical signal is inputted to the second multi-mode waveguide 7. The optical signal is distributed to one side of a fifth single-mode waveguide 8 and a sixth single-mode waveguide 9 directly coupled to the other side of the second multi-mode waveguide 7. By the combination of the phases of the optical signals varied due to the phase variation of the input optical signal at the first multi-mode waveguide 3 and the second multi-mode waveguide 7 and the thermo-optic effect at the third single-mode waveguide 4 and the fourth single-mode waveguide 5, a predetermined optical power distribution is outputted to the fifth single-mode waveguide 8 and the sixth single- mode waveguide 9.

[41] The widths (W ) and thickness (d) of the first, third, fourth, fifth and sixth single-

SM mode waveguides 1, 4, 5, 8 and 9 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide. Also, the width (W ),

MMl thickness (d) and length (L ) of the first multi-mode waveguide 3 are determined by

MMl the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide and by Equations (2) and (5) in the case of the symmetric interference. The width (W MM2 ), thickness (d) and leng σth (vL _MM2 ) ^/ of the second multi-mode waveg σuide 7 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide and by Equations (2) and (3) in the case of the general interference. [42] In the case of the symmetric interference, the optical signal is inputted to the first multi-mode waveguide 3 through the first single-mode waveguide 1 directly coupled to W MMl /2 of the width direction of the first multi-mode waveguide 3. The o xp-tical sigσnal gets to have a phase difference of O°by the symmetric interference while proceeding in the first multi-mode waveguide 3 and are distributed in a rate of 50:50 and then are transferred to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 directly coupled to W MMl /4 and 3W MMl /4 of the width direction of the first multi-mode waveguide 3. The temperature control heater 6 is attached to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 so as to vary an effective refractive index by a thermo-optic effect. The optical signal proceeding in the third single-mode waveguide 4 and the fourth single-mode waveguide 5 has a predetermined phase variation of 0°to 180°due to the varied effective refractive index. The optical signal having the varied phase is directly coupled to a width-direction edge of the second multi-mode waveguide 7, and the respective input optical signals have a phase difference of 270°by the general interference while proceeding in the second multi-mode waveguide 7 and are distributed in a rate of 50:50 and then are transferred to the fifth single-mode waveguide 8 and the sixth single-mode waveguide 9 directly coupled to the width-direction edge of the second multi-mode waveguide 7. The optical signals that are transferred to the output terminal make a desired optical power distribution in such a manner that the phase of 0°to 180°varied by the thermo-optic effect and a phase of 270°varied by the multi-mode interference are offset or added up. [43] As shown in FIG. 4, in an optical distributor having a strong guiding waveguide structure according to a fourth embodiment of the present invention, a first single- mode waveguide 1 is directly coupled to one side of a first multi-mode waveguide 3 and an optical signal is inputted thereto. The optical signal is distributed to one side of a third single-mode waveguide 4 to which a temperature control heater 6 directly coupled to the other side of the first multi-mode waveguide 3 is attached and a fourth single-mode waveguide 5 to which the temperature control heater 6 is attached. One

side of the second multi-mode waveguide 7 is directly coupled to the other side of the third single-mode waveguide 4 and the fourth single-mode waveguide 5, so that the distributed optical signal is inputted to the second multi-mode waveguide 7. The optical signal is distributed to one side of a fifth single-mode waveguide 8 and a sixth single-mode waveguide 9 directly coupled to the other side of the second multi-mode waveguide 7. By the combination of the phases of the optical signals varied due to the phase variation of the input optical signal at the first multi-mode waveguide 3 and the second multi-mode waveguide 7 and the thermo-optic effect at the third single-mode waveguide 4 and the fourth single-mode waveguide 5, a predetermined optical power distribution is outputted to the fifth single-mode waveguide 8 and the sixth single- mode waveguide 9. [44] The widths (W ) and thickness (d) of the first, third, fourth, fifth and sixth single-

SM mode waveguides 1, 4, 5, 8 and 9 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide. Also, the width (W MMl ), thickness (d) and length (L MMl ) of the first multi-mode waveguide 3 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide and by Equations (2) and (5) in the case of the symmetric interference. The width (W MM2 ), thickness (d) and leng °th (L MM2 ) of the second multi-mode waveg °uide 7 are determined by the wavelength (λ) of the applied optical signal and the refractive index (n) of the waveguide and by Equations (2) and (4) in the case of the paired i nterference. [45] In the case of the symmetric interference, the optical signal is inputted to the first multi-mode waveguide 3 through the first single-mode waveguide 1 directly coupled to W MMl /2 of the width direction of the first multi-mode waveguide 3. The o xp-tical sigσnal gets to have a phase difference of O°by the symmetric interference while proceeding in the first multi-mode waveguide 3 and are distributed in a rate of 50:50 and then are transferred to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 directly coupled to W /4 and 3W /4 of the width direction of the first

MMl MMl multi-mode waveguide 3. The temperature control heater 6 is attached to the third single-mode waveguide 4 and the fourth single-mode waveguide 5 so as to vary an effective refractive index by a thermo-optic effect. The optical signal proceeding in the third single-mode waveguide 4 and the fourth single-mode waveguide 5 has a predetermined phase variation of 0°to 180°due to the varied effective refractive index. The optical signal having the varied phase is directly coupled to W /3 and 2W /3 of the width direction of the second multi-mode waveguide 7, and the respective input optical signals have a phase difference of 270°by the paired interference while proceeding in the second multi-mode waveguide 7 and are distributed in a rate of 50:50 and then are transferred to the fifth single-mode waveguide 8 and the sixth single-

mode waveguide 9 directly coupled to W /3 and 2W /3 of the width direction of

MM2 MM2 the second multi-mode waveguide 7. The optical signals that are transferred to the output terminal make a desired optical power distribution in such a manner that the phase of 0°to 180°varied by the thermo-optic effect and a phase of 270°varied by the multi-mode interference are offset or added up.

[46] As shown in FIG. 5, in an optical distributor according to a fifth embodiment of the present invention, the optical distributor of the first embodiment is added in series and in parallel to the output terminal of the optical distributor of the first embodiment, so that the optical power can be distributed to a plurality of output terminals.

[47] While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention that come within the scope of the appended claims and their equivalents.