WO1996016350A1

WO1996016350A1 - Sagnac interferometer and reflective modulator comprising same

Info

Publication number: WO1996016350A1
Application number: PCT/EP1995/004515
Authority: WO
Inventors: Tsjerk Hans Hoekstra
Original assignee: Akzo Nobel N.V.
Priority date: 1994-11-21
Filing date: 1995-11-15
Publication date: 1996-05-30
Also published as: TW296430B

Abstract

Disclosed is a Sagnac Interferometer comprising a waveguide loop section and a modulation section wherein the modulation section is positioned outside the waveguide loop and both have a modulation function and a splitting function, the modulation function being based on electro-optical or thermo-optical modulation. This SI can be used as a versatile light modulator using thermo-optical and electro-optical effects, both in the transmission mode (performing the same functions as a Mach-Zehnder Interferometer) and in the reflection mode (performing the same functions as a Michelson Interferometer).

Description

SAGNAC INTERFEROMETER AND REFLECTIVE MODULATOR COMPRISING SAME

The invention pertains to a Sagnac Interferometer (SI) comprising a waveguide loop section and a modulation section.

Sagnac Interferometers are known. E.g. EP 297 338 discloses a Sagnac type optical fibre interferometer system comprising: an optical fibre constituting an interferometer loop, the optical fibre loop being provided with a modulation section; a light source and a photodetector which are colinear; optical coupling means for dividing a light wave emitted by the light source into two beams, each of which is applied to a respective end of the optical fibre, and for recombining the two beams emerging from the two ends of said optical fibre into a light wave which is colinear with the light wave emitted by the source but propagating in the opposite direction and directed towards the photodetector; The modulation section comprises a phase modulation means imparting periodic and symmetrical phase modulation to the two beams travelling along the optical fibre. The disclosed SI is used for gyrometry (gyroscopy).

Sis for use in gyroscopy have been described also in other publications. Thus, EP 551 537 discloses a method and device for measuring rotary motion employing an optical fibre Sagnac Interferometer. The device comprises a beam-splitting section comprising one input port and two output ports in which a single beam generated by a light source is divided into two beams, the output ports being interconnected via a fibre loop in which the two beams are counterpropagating. A phase modulation section is included at the sides of the output ports that are connected to the fibre loop. Thus, the modulation sections in the disclosed Sis are positioned in between the output ports of a beam splitter section and the fibre loop section. In effect, they form an integral part of the fibre loop, which extends from one output port of the beam splitter section to the other.

Sis in which the fibre loop is connected to a single output section rather than to the two output ports of a beam splitter, and in which a

10 modulation section is included literally within the fibre loop, are also known. Thus, in Applied Optics, Vol. 25, No.2 (1986), 209-214, Yao Li et al . suggest the use of an SI instead of a Mach-Zehnder Interferometer (MZI) in optical switching. See also US 5,274,488, in which an Si-based optical fibre communication system is disclosed wherein modulation takes places in the SI loop.

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A drawback to the known Sis of the above-disclosed type is that the modulation opportunities are restricted:

The known Si can only be used to modulate light using non-reciprocal

20 effects such as magneto-optic Farraday effects, fast time varying effects, and relativic rotation (Sagnac) effects such as described in EP 297 338. When non-reciprocal effects are involved, a phase difference arises from the fact that light traveling counterclockwise in a loop exhibits different properties than light traveling clockwise _?[- in a loop. Said phase difference is detected.

Reciprocal effects such as electro-optic and thermo-optic effects do not give rise to a phase difference because counterclockwise and clockwise the beams exhibit the same properties and therefore, these effects cannot be used in the known Sagnac Interferometers for light modulation.

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The invention now aims at an SI that allows the modulation of any signal, both using reciprocal and non-reciprocal effects, particularly modulation using reciprocal effects such as the thermo-optical and electro-optical effects. Further, the invention aims at providing a versatile modulator that can not only be used in the place of an MZI, i.e., in the form of a transmission modulator, but also, and in particular, as a reflective modulator (commonly based on a Michelson Interferometer).

In these reflective modulators, the output ports are terminated by a mirror. Thus the light is reflected back through the modulation section. Reflective modulators have the important property that, by virtue of the mirrors, sending back information in the form of a light wave does not require a light source (laser power) on the spot. This is particularly of advantage in the area of optical telecommunication, notably cable TV: a central light source can provide subscribers with the appropriate data in the form of light waves. Subscribers who desire to modify data (e.g., selecting a TV programme) do not need to have a light source in their home appliance (TV set). Thus, the all- important light sources can be placed in well-conditioned and protected locations, where they will be less prone to current interruption, fire hazards, or other potentially harmful local conditions.

Reflective modulators, however, are subject to a complicated preparation process requiring very precise making of mirrors at the ends of waveguide channels. This is a cumbersome process step, and the production of reflective modulators is prone to low yields.

Reflective modulators in which fibre loops are employed have been disclosed. See IEE Proceedings, Vol.137, Pt.J, No.2, April 1990, 108-114, and Journal of Lightwave Technology, Vol 6, No.7, July 1988, pages 1217-1224. The fibre loop reflectors shown comprise a directional coupler, the output ports of which have been interconnected by means of a fibre loop. The directional coupler cannot modulate the light before entering the loop and therefor does not comprise a modulation section: The modulation is done by EP95/04515

introducing birefringence in the fibre loop. This does not meet the invention's objective to provide a Sagnac Interferometer that can be used to modulate light on the basis of an electro-optical modulator or as a thermo-optical modulator.

In IEE Proceedings, Vol. 135, Pt. J. No. 4, August 1988 a laboratory set-up comprising a fibre-loop reflector is described wherein the coupler is heated for wavelength tuning. The set-up is not suitable for thermo-optic or electro-optic modulation.

In EP-A1-0376449 an optical fibre interferometer is described which is formed from an optical fibre Mach-Zehner interferometer whose output ports are coupled by an optical loop. The modulation takes place within the Mach-Zehner either by means of a piezzo-electric stretcher or by means of a movable prism. The use of a piezzo-electric stretcher for modulation has the disadvantage that loose optical fibres have to be used, rather than integrated planar waveguides in order to avoid tensile stress. Even if loose optical fibres are used, the joints of the device are permanently under mechanical stress by the stretching and shrinking of the device. The same can be said about the use of a movable prism for modulation. Moving parts are undesirable in optical devices.

In order to meet the above objectives, the invention consists in that in a Sagnac Interferometer of the type comprising a waveguide loop section and a modulation section, the modulation section is positioned outside the waveguide loop and has both a modulation function and a splitting function, wherein the modulation function is based on either thermo-optic or electro-optic modulation. In other words, the light is modulated by non-moving parts and divided in the splitter into two different beams and then guided into the waveguide loop in which the two beams are counterpropagating. Devices which both contain a modulating and a splitting function are switches. These switches may be in the form of an optical waveguiding device. All known optical waveguide device switches based on thermo-optic or electro-optic modulation such as a directional coupler switch, thermo-optical switches etc., may be used to modulate and split the light before entering the waveguide loop.

Crucial aspects of the Sis according to the invention include the configuration of the device, i.e., the lay-out of the confinements (channels) through which light is propagated, the build-up of the device (i.e., the way in which a waveguiding structure is realized, irrespective of the channel configuration), and the waveguiding materials chosen.

In principle, the Sagnac Interferometer of the invention can be prepared in any suitable waveguiding material in which propagating light waves can be modulated thermo-optically or electro-optically.

For thermo-optical modulation, the waveguiding material should display a thermo-optical effect, i.e. heating of the material results in a reversible change of refractive index. Examples of such materials include inorganic materials such as ion-exchanged glass and titanium- doped lithium niobate, but particularly all-polymeric waveguides are preferred. This, int. al., in view of the fact that a modest increase in temperature may result in a large index of refraction change. As all polymers exhibit a thermo-optical effect, basically any polymer having sufficient transparency for the wavelength used can be employed as the waveguiding material. Particularly suitable optical polymers include polyacrylates, polycarbonates, polyimides, polyureas.

For electro-optical modulation, the waveguiding material should display an electro-optical effect, i.e., applying a voltage to the material results in a reversible change of refractive index. Examples of such materials include inorganic materials that are intrinsically electro-optical, such as lithium niobate and gallium arsenide. Particularly preferred, however, are polymeric electro-optical materials, also known as non-linear optical polymers (NLO polymers). As is known, obtaining the desired NLO effect in polymeric materials macroscopically requires that first the groups present in such a material, mostly hyperpolarisable side groups, be aligned (poled). Such poling is usually effected by exposing the polymeric material to electric (dc) voltage, the so-called poling field, with such heating as will render the polymeric chains sufficiently mobile for orientation. Examples of particularly suitable NLO polymers are polyurethanes, polycarbonates, polyimides, polyesters, and polyacrylates having side groups comprising a delocalized rr-system to which are attached an electron donating group (at the side of the polymeric backbone) and an electron accepting group (at the terminus of the side group). Suitable NLO polymers are known in the art.

In the present invention it is highly preferred, both for thermo- optical and for electro-optical modulation, to employ polymeric waveguide materials. It should be noted that this is not only for their easy handling and processing in the making of waveguides, but also for specific advantages in the making of a Sagnac Interferometer according to the invention.

The waveguiding structure preferably is built up as a layered polymeric structure comprising a guiding layer (core layer) sandwiched between two deflection layers (cladding layers), e.g., as follows: Underneath the polymeric waveguide is a support, e.g., a glass or silicon substrate. On the substrate the following successive layers can be identified: a lower cladding layer, which may be of glass but preferably is polymeric, a polymeric core layer (guiding layer), and an upper cladding layer, which also is preferably polymeric but can be made up of other materials, e.g., glass. The polymeric core layer is the actual waveguiding layer, the two cladding layers having an index of refraction which is lower than that of the core layer. In a particularly preferred embodiment of an all-polymeric waveguide structure the lower cladding layer is made up of two sublayers, the lower of which (i.e., the one adjacent to the substrate) is a thin layer (e.g. about 3 μm) having a lower index of refraction than the other sublayer (i.e., the one adjacent to the core layer). This additional low index layer has the advantage of preventing the propagated light from radiating into the substrate. Thus, the actual waveguiding structure is "optically insulated" from the substrate.

The desired waveguide configuration generally is defined by way of making a pattern of laterally defined channels in the guiding layer. Methods of achieving this are known. For instance, such patterns may be provided by removing portions of the waveguide structure, e.g., by means of wet-chemical or dry etching techniques, and filling the formed voids with a material having a lower index of refraction (thus forming a channel of core layer material enclosed on all sides by deflection layer material). Alternatively, it is possible to use photosensitive material, which can be developed after irradiation; for instance, a negative photoresist, that is to say, material which is resistant to a particular solvent (developer) after being irradiated. The developer in that case may be used to remove non-irradiated material. However, it is preferred to employ a positive photoresist and have the developer remove the portion that has been irradiated.

The preferred technique, however, involves making use of a core material in which a waveguide pattern can be provided without any material being removed by etching. For instance, there is core material which is chemically converted into a material with a different index of refraction under the influence of heat, light or UV irradiation. If this concerns an increase in the index of refraction, the treated material will be used as core material. This may take the form of carrying out the treatment using a mask, with the holes in the mask being identical with the desired waveguide pattern. If, on the other hand, a reduction of the index of refraction is involved, the treated material will be suited for use as deflection material. The treatment in question in that case may be carried out using a mask of which the closed portions are identical with the desired waveguide pattern. At any rate the channels made in a guiding layer sandwiched between two deflection layers of lower refractive index will thus be surrounded by material having a lower index of refraction on all sides.

It is preferred to employ a waveguide structure of which the core layer comprises a polymer bleachable under the influence of irradiation. This is a particular type of light- or UV-sensitive core layer material. Probably because of a chemical rearrangement reaction, irradiation, preferably generally using blue light, lowers the index of refraction of such a material without affecting the remaining physical and mechanical properties. Preferably, the waveguide structure is provided with a mask covering the desired pattern of channels, so that the surrounding material can have its index of refraction lowered ("be bleached") by means of irradiation. Thus, as desired, waveguide channels are formed which are enclosed on all sides by material having a lower index of refraction (the bottom and top deflection layers and the surrounding bleached core layer material). As indicated above, it should be possible for such a polymer to be made electro-optically active. Preferred bleachable polymers in this respect have been described, int. al., in EP 358476.

The waveguide channels configuration actually determines the working of the SI according to the invention.

It should be noted that, according to the invention, the modulation section has at least one input port, through which light is passed to reach the actual modulation section, and at least two output ports. The intensity and/or phase of the light waves reaching either or both of the output ports is determined in the actual modulation section. In the case of a thermo-optical modulator, this modulation section comprises appropriately positioned heating elements. In the case of an electro-optical modulator, it comprises appropriately placed electrodes.

The versatility of layered, all-polymeric waveguides in making Sis according to the invention can be employed with advantage in either of the following two embodiments.

In one embodiment, the modulation section (but not the waveguide loop section) is made in a layered polymeric waveguide as described above. The output ports are provided with optical fibre ends (known in the art as "pigtails"). These fibre ends in turn are interconnected by means of an optical fibre loop. An advantage of this embodiment is that in a known manner there is provided a -waveguide component ("optoboard") that can be handled with ease and to which a fibre loop of any desired length can be coupled through conventional coupling means.

In the preferred embodiment, both the modulation section and the waveguide loop section are provided in the same layered polymeric waveguide, preferably integrated on one and the same substrate. While this will limit the length of the waveguide loop, this embodiment has the considerable advantage of being a compact, ready-to-use, optical component.

In order to explain various embodiments of the SI according to the invention, reference is made to the drawings. All the drawings show a top view (i.e., the topography) of a waveguide configuration made as channels in a waveguiding material. Figure 1 depicts a conventional Sagnac Interferometer comprising a waveguide single channel section (101) connected to a waveguide loop section (102) by a beam splitting section (104), the waveguide loop section (102) comprising a modulation section (103). As discussed earlier, this SI cannot be employed for the modulation of any signalby using thermo-optic or electro-optic effects.

Figure 2 depicts a conventional SI comprising a waveguide single channel section (201), a beam splitter section (202), and two output channels (203) each comprising one or two phase modulation sections (204), the output channels being interconnected by a waveguide loop section (205). As explained below, in such an SI the modulation sections (204) form an integral part of the waveguide loop section (205). Again, such an SI does not meet the requirement of the present invention. It does not allow the modulation of any signal by using thermo-optic or electro-optic effects.

Figure 3 depicts an SI according to the invention. It comprises a modulation section (301) and a waveguide loop section (302), the modulation section (301) being placed outside the waveguide loop section (302): the modulation section (301) is a Mach-Zehnder interferometer connected on one side to an input port (303) and on the other to a beam splitter section (304) connected to two output ports (305). The output ports in turn are interconnected by the waveguide loop section (302). The modulation section (301) in this example is a Mach-Zehnder interferometer based 2X2 switch consisting of a coupling section (306), two parallel waveguides (307) comprising modulating means (308), such as electrodes for electro-optical modulation, and an inverse coupler (309). As will be clear from figure 3, the modulation section (301)performs the beam splitting as (104) and thus does not form a part of the loop. Figure 4 also depicts an embodiment of an SI according to the invention. It comprises a modulation section (401) positioned adjacent to, and outside, a waveguide loop section (402). The modulation section (401) is a bidirectional switch comprising two input ports (403), a switching section (404) consisting of two adjacent waveguide channels (405) comprising modulating means (406) for electro-optical modulation modulation in the form of electrodes, the channels (405) being placed sufficiently close to each other to allow coupling forming a directional coupler, and two output ports (407). The output ports (407) are interconnected by means of the waveguide loop (402). The modulation section performs the beam splitting function as in (104) and thus does not form part of the waveguide loop.

Figure 5 also depicts an embodiment of an SI according to the invention. It comprises a modulation section (501) positioned adjacent to, and outside, a waveguide loop section (502). The modulation section (501) is a thermo-optical switch comprising one input port (503), a switching section (504) consisting of two adjacent waveguide channels (505) comprising modulating means (506) for thermo-optical modulation in the form of heating elements, the channels (505) being placed sufficiently close to each other to allow coupling forming a thermo-optical switch, and two output ports (507). The output ports (507) are interconnected by means of the waveguide loop (502). The modulation section performs the beam splitting function as in (104) and thus does not form part of the waveguide loop.

As indicated above, the SI according to the invention can be used in transmission, performing the same functions as a Mach-Zehnder Interferometer, and also in reflection, performing the same functions as a Michelson Interferometer. In the first case, it is essential that the modulation section have two input ports (which are also the output ports of the device). Thus, the light can be coupled in through one of the input ports, and, after having travelled through the modulation 5/04515

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section, the waveguide loop, and back through the modulation section, be coupled out via the other "input" port. In the second case, one port may serve both as an input port and as an output port for the device: light coupled in via this port will be coupled out through the same port after having travelled through the modulation section, the waveguide loop, and back through the modulation section. The SI according to the invention has several advanatges over a Mach-Zehnder interferometer: the SI can be used in the transmision mode and the reflective mode, the fiber connections are only on one side of the device, which also means that only one side of the device has to be provided with end-facets. The advantage over a Michelson Interferometer lies mostly in the fact that with the SI according to the invention the device does not have to be provided with mirrors.

The Sagnac interferometer according to the invention may be obtained by using existing designs and devices. For instance, by connecting a waveguide loop to the output ports of a 1 X 2 switch a reflective modulator is obtained. By connecting a waveguide loop to the outputports of a directional coupling switch a modulator is obtained which can be used as a reflective modulator and as a transmission modulator.

The invention is further illustrated with reference to the following unlimitative example.

A reflective modulator according to Figure 6 is prepared con-necting a fiber loop (602) to a 1 X 2 thermo-optical switch (601). The reflective modulator was coupled to a 95/5 (608) coupler which was part of the measuring equipment. The insertion loss was 6-7 dB, the drive voltage was 4 V, the extinction was 7 dB, the wavelength of the laser light used was 1310 nm. The modulation obtained by varying the voltage on the upper-electrode of the thermo-optical switch is given 0415

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in Figure 7. The modulation obtained by varying the voltage on the lower-electrode of the thermo-optical switch is given in Figure 8.

Claims

Claims:

1. A Sagnac Interferometer comprising a waveguide loop section and a beam splitting section, characterized in that a modulation section is positioned outside the waveguide loop and both has a modulation function and a splitting function, the modulation function being based either on thermo-optical or electro-optical modulation.

2. A Sagnac Interferometer according to claim 1, characterized in that the modulation section is an electro-optical switch having at least one input port and at least two output ports, the waveguide loop interconnecting the two output ports.

3. A Sagnac Interferometer according to claim 1, characterized in that the modulation section is a thermo-optical switch having one input port and two output ports, the waveguide loop interconnecting the two output ports.

4. A Sagnac Interferometer according to claim 2, characterized in that the electro-optical switch is a bidirectional switch.

5. A Sagnac Interferometer according to any one of the preceding claims, characterized in that the modulation section comprises waveguide channels made in a layered waveguide structure, the layered waveguide structure comprising a guiding layer sandwiched between two deflection layers.

6. A Sagnac Interferometer according to claim 5, characterized in that the layered waveguide structure is supported by a substrate. 15

7. A Sagnac Interferometer according to any one of claims 1-6, characterized in that the optical waveguide loop is an optical fibre.

8. A Sagnac Interferometer according to any one of claims 1-6, characterized in that the optical waveguide loop is in the form of a channel made in a layered waveguide structure, the layered waveguide structure comprising a guiding layer sandwiched between two deflection layers.

9. A Sagnac Interferometer according to claims 6 and 8, characterized in that the waveguide loop and the modulation section are integrated on a single substrate.

10. A light modulator comprising a modulation section having at least one input port and at least two output ports, characterized in that the output ports are interconnected by means of an optical waveguide loop.

11. A light modulator according to claim 10, characterized in that the modulation section comprises an electro-optical switch.

12. A light modulator according to claim 11, characterized in that the electro-optical switch has two input ports.

13. A light modulator according to claim 10, characterized in that the modulation section is a thermo-optical switch.

14. A light modulator according to any one of claims 9-13, characterized in that the modulation section comprises waveguide channels made in a layered waveguide structure, the layered waveguide structure comprising a guiding layer sandwiched between two deflection layers. CI7EP95/04515

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15. A light modulator according to claim 14, characterized in that the layered waveguide structure is supported by a substrate.

16. A light modulator according to any one of claims 10-15 characterized in that the optical waveguide loop is an optical fibre.

17. A light modulator according to any one of claims 10-15, characterized in that the optical waveguide loop is in the form of a channel made in a layered waveguide structure, the layered waveguide structure comprising a guiding layer sandwiched between two deflection layers.

18. A light modulator according to claims 15 and 17, characterized in that the waveguide loop and the modulation section are integrated on a single substrate.