WO2001053888A2

WO2001053888A2 - Optical attenuator having geometrically biased branching waveguide structure

Info

Publication number: WO2001053888A2
Application number: PCT/US2000/041751
Authority: WO
Inventors: William K. Burns
Original assignee: Codeon Corporation
Priority date: 2000-01-24
Filing date: 2000-11-02
Publication date: 2001-07-26
Also published as: AU2001229201A1; WO2001053888A3

Abstract

An optical attenuator to be used in a fiber optic communication system, includes a waveguide system formed in a crystal substrate, including an input channel for receiving an optical input, a first output branch with a first width, and a second output branch with a second width; electrodes for selectively applying electric fields of opposite directions to the first and second output branches, respectively, wherein the first width and the second width are sufficiently different so that 100 % of the optic input goes through one of the first and second output branches when no electric fields are applied by the electrodes.

Description

OPTICAL ATTENUATOR HAVING GEOMETRICALLY

BIASED BRANCHING WAVEGUIDE STRUCTURE

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an optical attenuator, and more particularly, to an

electro-optic attenuator that has maximum transmission rate with zero voltage applied. The

present invention also relates to an optical attenuator that does not suffer from bias drift or bias offset problems. Discussion of the Related Art

In a general fiber optical communication system, optical signals are sent along an

optical fiber communication line to a desired location. One type of the fiber optical

communication system that can handle optical signals of multiple channels through

wavelength multiplexing is called a wavelength division multiplexed (WDM) system.

One example of a WDM system is shown in Fig. 1. A plurality of laser sources L„

L₂, ... L_n each generates an optical signal having a wavelength of λl, λl, . . . λn, respectively.

Each of the optical signals then goes through a modulator M,, M₂, . . . M., respectively,

before its amplitude gets adjusted to a desired level by its corresponding attenuator A,, A₂, . .

. A„. All signals then go into a multiplexer, and the multiplexed signals go into an existing

fiber optical trunk line to be sent to a desired location. Finally, the signals are demultiplexed

and received as separate signals having wavelengths of λl, λl, . . . λn, respectively.

As described above, in a WDM system, each signal from a laser source is generally

individually modulated with a separate modulator, and the different wavelength signals are

then combined, optically amplified, and transmitted over an optical fiber. To flatten the overall gain of such a system, it is convenient to provide an individual optical attenuator

integrated with each modulator as shown in the system of Fig. 1.

In the system shown in Fig. 1, modulators M_t, M₂, . . . M_n are used to modulate or

vary the intensity of the light (i.e., optical signal) transmitted through them. A modulator

used in this type of system is a very broad band device and it works at very high frequencies.

A modulator is used to encode information on the light which goes down the optical fiber.

The function of an attenuator is to adjust the level of an optical signal to a desired

level.

In the system shown in Fig. 1, all the signals will go through an optical amplifier at some

point (not shown in Fig. 1). An optical amplifier often has a gain characteristic which varies

as a function of wavelength. As a result, a signal having a wavelength λl is amplified by a

different amount compared to another signal having a wavelength λl To compensate for this difference and other differential losses in the system, and to achieve a constant intensity level

for different wavelength signals, attenuators are used to adjust the individual signal levels.

In this case, an attenuator is a low frequency device, and it is usually set at a particular

desired level. Also, it is not a broad band device, so it is not intended to encode information

like a modulator. It is just intended to set a particular intensity level for the light going

through the attenuator.

An optical attenuator is typically an electro-optic switch, controlled by an applied

voltage. Commonly desired characteristics of such an attenuator are low switching voltages, a short switching length, long term stability (over tens of years), and maximum transmission

rate (i.e., transmission of approximately 100% of input power) when no attenuation is

applied, e.g., when zero voltage is applied to an electro-optic attenuator.

There are several types of known attenuators, including Mach Zehnder interferometers, directional couplers, and active branching modulators (also called 3 -port

digital switches). Each of the above devices, when used as an attenuator, has its own disadvantages.

A Mach Zehnder interferometer typically suffers from long term bias drifts, thus

requiring periodic adjustment by an user. In addition, it can have geometrical biases arising

in the course of fabrication which can limit the zero voltage transmission capability.

A directional coupler is an optical analog of an earlier microwave device which can

also be used as an attenuator. However, a directional coupler must be fabricated to a precise

length to achieve both of the switched states, i.e., the full switched-off state and the full

switched-on state. Since the precise length of the directional coupler depends on complicated

fabrication conditions, it is difficult to fabricate directional couplers which can function as high quality attenuators.

An active branching modulator is described in detail in U.S. Patent No. 4,070,092,

and was invented by the inventor of the present invention. As shown in Fig. 2, an active

branching modulator is formed of a waveguide having a single input arm and two

symmetrical output branch arms separated by an angle θ. The active branching modulator

also includes an electrode structure which can apply an electric field across the two output

branch arms. As shown in Fig. 2, the electrode located in the middle portion between the two

output branch arms may have a voltage V applied thereon, and the electrodes on the side of

the two output branch arms may be grounded. Since the two output branching arms are

completely symmetrical, the optical power split between the two output branching arms is

approximately 50-50% when zero voltage is applied. When a voltage V is applied to the

middle electrode and the two side electrodes are grounded, the input power tends to exit in

one of the output branching arms. The switching state can be reversed by changing the polarity of the voltage applied.

The output branch arm transmission as a function of the applied voltage for the active

branching modulator is shown in Fig. 3. As shown in Fig. 3, when the applied voltage is

approximately zero, the power split between the two output branch arms is about 50-50%.

When the applied voltage V increases, more of the input power tends to exit in one of the

output branch arms until almost 100% of the input power goes through that particular arm.

When the polarity of the applied voltage is switched, more of the input power starts to exit in

the other output branch arm in a similar fashion.

Since the active branching modulator is not an interferometer, it does not have the

type of undesirable bias drift problem associated with an interferometer. However, it is still

undesirable to use an active branching modulator as an attenuator, because the active

branching modulator only allows approximately 50% of the input power to go through each

output branch arm when zero voltage is applied. As mentioned earlier, an ideal attenuator

should allow maximum transmission (i.e., approximately 100%) of input power when zero

voltage is applied, and allow gradual decreasing transmission of input power when a voltage

is applied.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an optical attenuator that

substantially obviates one or more of the problems due to limitations and disadvantages of the

related art.

An object of the present invention is to provide an electro-optic attenuator that allows

maximum transmission of input power with zero voltage applied.

Another object of the present invention is to provide an optical attenuator that does

not suffer from bias drift or bias offset problems. Additional features and advantages of the invention will be set forth in the description

which follows, and in part will be apparent from the description, or may be learned by

practice of the invention. The objectives and other advantages of the invention will be

realized and attained by the structure particularly pointed out in the written description and

claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purpose of the

invention, as embodied and broadly described, an optical attenuator of the present invention

includes an input waveguide channel for receiving an optical input; a first waveguide output

branch with a first width; a second waveguide output branch with a second width, the second

width being different from the first width; and electrodes for selectively applying electric

fields of opposite directions to the first and second waveguide output branches.

In another aspect, an optical attenuator to be used in a fiber optic communication

system, includes a waveguide system formed in a crystal substrate, including an input channel

for receiving an optical input, a first output branch with a first width, and a second output

branch with a second width; electrodes for selectively applying electric fields of opposite

directions to the first and second output branches, respectively, wherein the first width and

the second width are sufficiently different so that 100% of the optic input goes through one of

the first and second output branches when no electric fields are applied by the electrodes.

In a further aspect, a fiber optic communication system according to the present

invention includes a plurality of laser sources each for transmitting an optic signal; a plurality

of modulators each for receiving a respective optic signal from one of the plurality of the

laser sources; a plurality of optical attenuators each for receiving a modulated signal from one

of the plurality of modulators; a multiplexer for receiving attenuated signals from the

plurality of optical attenuators; a fiber optical line for transmitting a multiplexed signal received from the multiplexer, wherein each of the plurality of optical attenuators includes an

input waveguide channel for receiving the modulated signal from each of the plurality of

modulators, a first waveguide output branch with a first width, a second waveguide output

branch with a second width which is different from the first width, and electrodes for

selectively applying electric fields to the first and second waveguide output branches.

It is to be understood that both the foregoing general description and the following

detailed description are exemplary and explanatory and are intended to provide further

explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of

the invention and are incorporated in and constitute a part of this specification, illustrate

embodiments of the invention and together with the description serve to explain the principles

of the invention. In the drawings:

Fig. 1 illustrates a general WDM fiber optical communication system;

Fig. 2 illustrates the layout of a conventional active branching modulator;

Fig. 3 shows the output branch arm transmission as a function of an applied voltage

for the device of Fig. 2;

Fig. 4A is a top view of a waveguide branch formed in a crystal substrate;

Fig. 4B is a side view along line I-I of Fig. 4A;

Fig. 4C is a side view along line II-II of Fig. 4A;

Figs. 5 A and 5B show alternative configurations of the present invention;

Fig. 6 illustrates the layout of an optical attenuator with X- or Y-cut material

according to the present invention; Fig. 7 shows a waveguide supporting only a single transverse mode;

Fig. 8 illustrates the layout of an optical attenuator with Z-cut material according to the present invention; and

Fig. 9 shows the output branch arm transmission as a function of an applied voltage

for an optical attenuator according to the present invention.

PET ATI ED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of the present

invention, examples of which are illustrated in the accompanying drawings.

An optical attenuator according to the present invention can be designed for X-cut, Y-

cut, or Z-cut LiNbO₃ or LiTaO₃ waveguide systems. As shown in Figs. 4A-4C, for example,

an optical attenuator includes a waveguide formed in a crystal substrate of LiNbO₃ material. The waveguide is formed in the substrate by diffusing titanium (Ti) into the crystal substrate

according to a specific pattern. The diffused titanium raises an optical refractive index of the

crystal in the waveguide portion so that a laser projected onto one end of the crystal substrate

will travel along the specific pattern of the waveguide and be guided by the waveguide

structure because of its higher optical index. Alternatively, the optical refractive index of the

crystal substrate in the waveguide portion can also be raised by proton exchange.

As shown in Fig. 4A, the waveguide has one input branch arm for receiving input

optical power, and two output branch arms separated by a constant angle θ. Assuming the

width of the input branch arm is W, the two output branch arms have different widths, being

W+ΔW and W-ΔW, respectively. In the embodiment shown in Fig. 4A, the two output

branch arms are situated in such a way that they are symmetric with respect to the input

branch arm. In other words, each output branch arm forms a θ/2 angle with respect to the input branch arm. In the present invention, the waveguide can have other configurations. For

example, as shown in Fig. 5 A, one output branch arm may form a straight line with the input

branch arm, the other output branch arm can form an angle θ with respect to the input branch

arm. Or alternatively, one output branch arm may form a larger angle with respect to the

input branch arm, and the other output branch arm may form a smaller angle with respect to

the input branch arm. Furthermore, as shown in Fig. 5B, the two output branch arms are not

necessarily separated by a constant angle θ. In other words, the angle between the two output

branch arms may be a variable and the two output branch arms may have a curved shape

extending away from the input branch arm.

As mentioned above and as shown in Figs. 4B and 4C, the waveguide region (i.e., the

shaded portions in Figs. 4B and 4C) has an increased optical refractive index as compared to

the other portions of the crystal substrate. When light travels in the waveguide, an effective

optical index, which is an average optical index of different portions of the substrate, is

sensed by the light. In the waveguide branch the light tends to travel along the branch arm

where the effective optical index is the highest. This effective optical index also varies in

accordance with an electric field applied thereon since the crystal substrate is an electro-optic

material. As a result, an electric field applied to a branch arm of the waveguide changes the

effective index of that particular branch arm. The magnitude and the direction of the change

in the effective index of a branch arm depends on the magnitude and the direction of the

electric field applied, and the particular orientation of the crystal substrate (i.e., whether it is

X-cut, Y-cut, or Z-cut material).

In addition, the effective optical index of a branch arm depends on the width of that

particular branch arm. For a branch arm having a wider width, the effective optical index is

higher. For a branch arm having a narrower width, the effective optical index is smaller. Therefore, in the embodiment shown in Fig. 4A, the upper branch arm having the

width W+ΔW has a higher effective optical index as compared to the lower branch arm

having a width of W-ΔW. In the present invention, the difference between the widths of the

two branch arms, t.e., 2 ΔW, must be large enough so that the effective optical index of the

upper branch arm is sufficiently high as compared to that of the lower branch arm so that

almost 100% of the input power received through the input branch arm exits through the

upper branch arm when no voltage is applied. This allows the optical attenuator of the

present invention to have the maximum transmission capability when no voltage is applied.

As discussed above, the effective optical index also depends on electric fields applied

through the material. Fig. 6 illustrates an optical attenuator with X-cut or Y-cut material

according to the present invention. As shown in Fig. 6, various electrodes (shaded portions in

Fig. 6) can be set up along the two output branch arms. For the optical attenuator using X-cut

or Y-cut material, three electrodes are arranged as shown in Fig. 6 so that a horizontally

directed electric field can be applied to each output branch arm. As shown in Fig. 6, the

electrode located in the middle between the two output branch arms can be connected to a

voltage source having ±V voltages, and the two side electrodes can be grounded. This allows

an electric field having a horizontal direction (as represented by the dashed arrows in Fig. 6)

to be applied to the two output branch arms when a +V voltage is applied to the middle

electrode. When a -V voltage is applied the middle electrode, a horizontal directed electric

field having the opposite direction to that shown by the dashed arrows is applied to the two

output branch arms. The two side electrodes can be two separate electrodes or one single

wrapped-around electrode.

As a result of the applied electric field, the effective optical index of the two output

branch arms is either reduced or increased, depending on the particular direction and the magnitude of the electric field applied and the crystal orientation of the substrate.

Accordingly, the effective optical index of one output branch arm increases while the

effective optical index of the other output branch arm decreases since the direction of the

electric fields applied is opposite for the two output branch arms. Consequently, it is possible

to gradually reduce the power exited through the upper output branch arm having the wider

width (W+ΔW), and to gradually increase the power exited through the lower output branch

arm having the narrower width (W-ΔW). As the magnitude of the voltage V gradually

increases, the level of attenuation achieved by the optical attenuator of the present invention

gradually increases so that a desired level of attenuation can be achieved and a specific

amount of input power exits through the upper output branch arm having the wider width.

The portion of input power exiting through the lower output branch arm can be guided to

elsewhere or discarded.

For the present invention to work the way it is intended, the width W of the input arm

of the waveguide branch should be small enough so that it supports only a single transverse

mode. As shown in Fig. 7, the first order transverse mode, which is excited for a single mode

input, has a Gaussian distribution wave form of optical intensity, across the waveguide of

width W. For an input waveguide which can support two transverse modes(for example one

of width 2W), with exitation by the second order transverse mode, the invention can still be

made to perform its desired function. However, in that case the optical mode will tend to

travel through the output waveguide arm with the lower effective index rather then the output

waveguide arm with the higher effective index. In both cases the output arms of the branch

are always constructed with a width that supports only a single transverse mode.

When such a single transverse mode input comes from the input branch arm, since the

upper output branch arm is wider than the lower one, the mode will travel through the upper arm due to its higher effective index. When the upper output branch arm is sufficiently wide

as compared to the lower output branch arm, almost 100% of the input power will travel

along the upper branch arm. As mentioned earlier, an ideal optical attenuator allows a

maximum transmission of input power when zero voltage is applied. Therefore, the optical

attenuator of the present invention requires one of its output branch arms to be sufficiently

wide so that 100% of input power travels along that output branch arm when zero voltage is

applied. However, if only a certain percentage of input power is desired to go through one

branch arm when zero voltage is applied, the widths of the two branch arms can be varied

accordingly to achieve that particular goal. For example, the width of the two output branch

arms can be designed so that a 60-40% or 70-30% power split between the two output branch

arms are achieved when zero voltage is applied.

By applying a certain magnitude of electric field in a certain direction, it is possible to

lower the effective optical index in the wider output branch arm and raise the effective optical

index in the narrower output branch arm. This way, more and more of the input power can be

steered into the narrower output branch arm so that a desired level of attenuation can be

achieved. When 100% of the input power is steered into the narrower output branch arm, the

attenuator functions as an optical switch, and completely shuts off the transmission.

The embodiment shown in Fig. 6 is suitable for X-cut or Y-cut LiNbO₃ material. The

different cut of a crystal material affects the crystal orientation and the electro-optical

response of a crystal substrate. Depending on the specific crystal material and orientation

used, different electrode arrangements can be used in the present invention to apply the

required electric field so that the desired characteristics of the optical attenuator can be

achieved.

Fig. 8 shows another embodiment of the present invention where a Z-cut LiNbO₃ material is used. Similar to the embodiment shown in Fig. 6, one output branch arm has a

wider width as compared to the other one so that 100% (or whatever desired percentage) of

the optical input power exits through the wider output branch arm when zero voltage is

applied. The electrode arrangements are different from the embodiment shown in Fig. 6,

however. Here, a vertically directed electric field is required to change the effective optical

index of the Z-cut material so that a desired level of attenuation can be achieved. Although

Fig. 8 shows that a ±V voltage is applied to the electrode above the wider output branch arm

and a zero voltage is applied to the electrode above the narrower output branch arm, this

arrangement can be reversed. This specific arrangement of the electrodes and the voltages

applied depend on the specific orientation of the crystal used. However, as long as a

vertically directed electric field is applied to the two output branch arms in opposite

directions, the effective optical index of one output branch arm can be increased and that of

the other can be reduced.

In the present invention, the desired angle θ between the two output branch arms

depends on the specific length of the device, the width difference between the two output

branch arms, and the magnitude of the voltage applied to the optical attenuator. Typically,

the angle θ is in the range of approximately 0.06 to 0.24 degrees for output branch arms with

a length of approximately 0.36 - 1.43 cm, when proton exchanged LiTaO₃ material is used. In

other words, the angle θ depends on the voltage-induced asymmetry and the geometrical

asymmetry that can be achieved in the optical attenuator of the present invention.

In addition to LiNbO₃ and LiTaO₃ waveguide systems, other material can also be

used, for example, various polymer waveguide materials, Indium Phosphide (InP), Gallium

Arsenide (GaAs), Strontium Barium Niobate (SBN), various other ferroelectric waveguide

materials, and various other semiconductor waveguide materials. In addition, other than the use of the electro-optic effect in crystals described here, the thermo-optic effect in thermo-

optically active substrates, such as glass, may also be employed to practice the principles of

the present invention.

In the present invention, the typical width differences 2Δw between the two output

branch arms are about 1-2 microns, for a nominal branch arm with a width of 6-10 microns.

This also depends on the separation angle of the two branch arms and other factors in the

waveguide system. The numbers given are based on a Ti diffused or proton exchanged

LiNbO₃ or LiTaO₃ waveguide system operating at a wavelength of 1.3 or 1.55 microns.

Fig. 9 shows the output branch arm transmission as a function of the applied voltage

for the optical attenuator according to the present invention. As shown in Fig. 9, when the

applied voltage is zero, one output branch arm, i.e., the branch arm with the wider width, has

a maximum transmission while the other output branch arm has a transmission of

approximately zero. In other words, when no voltage is applied, maximum transmission of

input power is achieved by the optical attenuator of the present invention. When a certain

voltage is applied to the electrodes, the optical attenuator of the present invention achieves a

desired attenuation so that only a percentage of the input power exits through one of the

output branching arms. This is reflected by the curves in Fig. 9.

In operation with an optical modulator in a fiber optical communication system such

as that shown in Fig. 1, the optical attenuator of the present invention can be used either

before or after the modulator. In addition, the optical attenuator according to the present

invention can be fabricated simultaneously with a modulator on a single substrate. Since the

current trend is to integrate the attenuator with the modulator, this is the preferred choice.

Alternatively, the attenuator of the present invention can be fabricated separately with a

modulator on a different substrate. When used in a fiber optical communication system, both the optical attenuator and

the modulator will need to be optimized for the desired optical polarization for the particular

material orientation used. A waveguide will generally support a traverse-electric (TE) mode

and a traverse-magnetic (TM) mode. For X-cut or Y-cut material, assuming LiNbO₃ or

LiTaO₃ is used as the substrate, TE mode is the optimal mode. For Z-cut material, TM mode

is the optimal mode. In other words, for these specific crystal orientations, in order to

optimize the performance of the optical attenuator, the input light has to use the specific

polarization. Therefore, for X-cut or Y-cut material, when a TE polarized mode input is

combined with the electrode structure shown in Fig. 6 (i.e., horizontally directed electric

fields), the optimal performance is achieved. Similarly, for Z-cut material, when a TM

polarized mode input is combined with the particular electrode structure of Fig. 8 (i.e.,

vertically directed electric fields), the optimal performance is achieved.

The operation of the optical attenuator of the present invention can be described

briefly as follows. Optical energy having a specific polarization is received at the input

branch arm of a single transverse mode waveguide. When zero voltage is applied to the

electrodes, approximately 100% of input power goes through one output branch arm of the

attenuator because of the geometric asymmetry between the widths of the two output branch

arms. When a voltage is applied to the electrodes in a specific direction and with a specific

magnitude, the amount of the input power going through the initial preferred output branch

arm slowly decreases until a desired attenuation level is reached.

The present invention can be applied to any electro-optic material system in which

vertically or horizontally directed electric fields are employed. In addition, the present

invention can be used as an attenuator in other configurations. For example, it can be used as

an attenuator in a switching array, besides being used as an attenuator for a single modulator as described in detail in this application. The present invention can also be used as a single

polarity switch, i.e., only voltage of a single polarity is required, in an application where such

a feature is desired. As shown in Fig. 9, only the specific polarity represented by the arrow at

the bottom of the curve is required to operate this optical attenuator.

In summary, the present invention utilizes an asymmetrical active branching

modulator as an optical attenuator. Accordingly, the optical attenuator according to the

present invention has a geometrical bias between its two branching arms so that maximum

transmission of input power along one branching arm is automatically achieved in the zero

applied voltage state. Due to the nearly flat slope of the switching curve (as shown in Fig. 9)

in the fully switched state, the transmission of the optical attenuator according to the present

invention is very insensitive to small changes in geometry. In contrast, the bias of a Mach

Zehnder interferometer is very sensitive to small changes in geometry, which can drastically

change the transmission rate. In addition, since the optical attenuator of the present invention

is not an interferometric device, there is no analog to the long term bias drift which affects the

output transmission of Mach Zehnder interferometers. Consequently, the optical attenuator

of the present invention has very good long term stability without need for trimming or

adjustment of the applied voltage in order to maintain a fixed transmission value.

It will be apparent to those skilled in the art that various modifications and variations

can be made in the optical attenuator of the present invention without departing from the

spirit or scope of the invention. Thus, it is intended that the present invention cover the

modifications and variations of this invention provided they come within the scope of the

appended claims and their equivalents.

Claims

What is claimed is:

1. An optical attenuator comprising:

an input waveguide channel for receiving an optical input;

a first waveguide output branch with a first width;

a second waveguide output branch with a second width, the second width being

different from the first width; and

electrodes for selectively applying electric fields to the first and second waveguide

output branches.

2. The optical attenuator according to claim 1, wherein the first waveguide output

branch and the second waveguide output branch are separated by a constant angle.

3. The optical attenuator according to claim 2, wherein the constant angle is in

the range of approximately 0.06 to 0.24 degrees.

4. The optical attenuator according to claim 1, wherein the first waveguide output

branch and the second waveguide output branch are separated by a variable angle and at least

one of the first waveguide output branch and the second waveguide output branch has a

curved shape.

5. The optical attenuator according to claim 1, wherein the input waveguide

channel, the first waveguide output branch, and the second waveguide output branch are

formed in a crystal substrate.

6. The optical attenuator according to claim 5, wherein the crystal substrate

includes one of LiNbO₃ and LiTaO₃.

7. The optical attenuator according to claim 6, wherein the crystal substrate has a

X-cut or Y-cut crystal orientation.

8. The optical attenuator according to claim 7, wherein the electric fields applied

to the first and second waveguide output branches are horizontally directed electric fields.

9. The optical attenuator according to claim 6, wherein the crystal substrate has a

Z-cut crystal orientation and the electric fields applied to the first and second waveguide

output branches are vertically directed electric fields.

10. The optical attenuator according to claim 1, wherein the input waveguide

channel supports only a single transverse mode input.

11. The optical attenuator according to claim 1, wherein the first width is

sufficiently larger than the second width so that approximately 100% of the optic input goes

through the first waveguide output branch when no electric fields are applied by the

electrodes to the first and second waveguide output branches.

12. The optical attenuator according to claim 5, wherein the crystal substrate is

formed of one of InP, GaAs, SBN, other ferroelectrics, other polymers, and other

semiconductor materials.

13. An optical attenuator to be used in a fiber optic communication system,

comprising:

a waveguide system formed in a crystal substrate, including an input channel for

receiving an optical input, a first output branch with a first width, and a second output branch with a second width;

electrodes for selectively applying electric fields of opposite directions to the first and

second output branches, respectively,

wherein the first width and the second width are sufficiently different so that

approximately 100% of the optic input goes through one of the first and second output

branches when no electric fields are applied by the electrodes.

14. The optical attenuator according to claim 13, wherein the first waveguide

output branch and the second waveguide output branch are separated by a constant angle.

15. The optical attenuator according to claim 13, wherein the first waveguide

output branch and the second waveguide output branch are separated by a variable angle and

the first waveguide output branch and the second waveguide output branch have curved

shapes.

16. The optical attenuator according to claim 13, wherein the crystal substrate

includes one of LiNbO₃ and LiTaO₃.

17. The optical attenuator according to claim 16, wherein the crystal substrate has

a X-cut or Y-cut crystal orientation and the electric fields applied to the first and second

waveguide output branches are horizontally directed electric fields.

18. The optical attenuator according to claim 16, wherein the crystal substrate has

a Z-cut crystal orientation and the electric fields applied to the first and second waveguide

output branches are vertically directed electric fields.

19. A fiber optic communication system comprising:

a plurality of laser sources each for transmitting an optic signal;

a plurality of modulators each for receiving a respective optic signal from one of the

plurality of the laser sources;

a plurality of optical attenuators each for receiving a modulated signal from one of the

plurality of modulators;

a multiplexer for receiving attenuated signals from the plurality of optical attenuators;

a fiber optical line for transmitting a multiplexed signal received from the multiplexer,

wherein each of the plurality of optical attenuators includes an input waveguide

channel for receiving the modulated signal from each of the plurality of modulators, a first

waveguide output branch with a first width, a second waveguide output branch with a second

width which is different from the first width, and electrodes for selectively applying electric

fields to the first and second waveguide output branches.

20. An optical attenuator comprising:

an input waveguide channel for receiving an optical input; a first waveguide output branch with a first width;

a second waveguide output branch with a second width, the second width being different from the first width; and

thermal sources for selectively changing temperatures of the first and second waveguide output branches.

21. The optical attenuator according to claim 20, wherein the input waveguide

formed in a thermo-optically active substrate.

22. The optical attenuator according to claim 21, wherein the thermo-optically active substrate includes glass.