Device for applying electromagnetic microwave radiation in a plasma cavity

A device for applying, in use, electromagnetic microwave radiation in a plasma cavity. The device includes a plasma cavity present within a housing, being substantially cylindrically symmetric about a first axis, and which plasma cavity includes a cylindrical wall provided with a circumferential slit. The plasma cavity is in communication, via the slit, with a first end of an elongated microwave guide having a longitudinally extending second axis. The device is in communication in use with a microwave generating device via the other end of the microwave guide, in which the electromagnetic microwave radiation being generated may include several modes, where the device can, in use, generate microwave radiation in the slit, which microwave radiation has only one electromagnetic field distribution at least in one direction perpendicularly to the propagation direction of the microwave radiation in the microwave guide.

DESCRIPTION

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for applying, in use, electromagnetic microwave radiation in a plasma cavity, which plasma cavity is present within a housing, being substantially cylindrically symmetric about a first axis, and which comprises a cylindrical wall provided with a circumferential slit, said plasma cavity being in communication, via said slit, with a first end of an elongated microwave guide having a longitudinally extending second axis, said device being in communication in use with microwave generating means via the other end of said microwave guide, in which the electromagnetic microwave radiation being generated may comprise several modes.

The invention also relates to an apparatus for depositing, by means of a Plasma Chemical Vapour Deposition process, one or more glass layers on the inner side of a hollow glass tube disposed in the plasma cavity of the device according to the invention.

The invention furthermore relates to a method for depositing, by means of a Plasma Chemical Vapour Deposition process, one or more glass layers on the inner side of a hollow glass tube disposed in the plasma cavity of the device according to the invention.

2. Discussion of the Background

Such a device, which is also referred to by the technical term microwave applicator, the slit being called a radial wave duct, can be used in particular with a sufra guide or a sufraton for generating a plasma in the plasma cavity. Another practical application of a microwave applicator according to the introduction is disclosed in International patent application PCT/EP98/07798, published under No. WO 99/35304 in the name of the present applicant.

WO 99/35304 shows a PCVD (Plasma Chemical Vapour Deposition) apparatus, in which a hollow cylindrical glass tube is accommodated in the plasma cavity of the microwave applicator, through which tube a gaseous mixture comprising O2, SiCl4and GeCl2for example, is passed. The microwave radiation applied in the slit by the microwave applicator locally generates a plasma in the hollow glass tube. By moving the microwave applicator back and forth with respect to the glass tube—possibly with simultaneously rotation of the tube—glass layers, whether or not of different composition, can be deposited on the inner side of the cylindrical glass tube by means of the plasma. In this way a so-called hollow preform having one or more glass layers deposited on the inner side thereof is obtained. The preform is subjected to a collapsing treatment so as to obtain a massive rod, from which an optical fibre is subsequently drawn, which fibre is intended for telecommunication purposes, for example.

In the PCVD application, the microwave radiation being used or being applied in the slit has a frequency of 890 MHz or 2.45 GHz and may comprise several so-called modes. The term modes as used herein is understood to mean the propagating modes in the radial propagation direction as well as the modes that have just been cut off. The propagating modes comprises a number of zero-axis crossings in the electrical field in both directions perpendicularly to the radial propagation direction.

If the width of the radial wave slit amounts less than half the wavelength of the microwave radiation being used, only one propagating mode will occur if the inner circumference of the radial wave slit has a length equal to or smaller than the wavelength of the microwave radiation being used, whilst three propagating modes will occur if the inner circumference of the radial wave slit has a length of 1-2 wavelengths of the microwave radiation being used.

It is very difficult with the current applications to determine which modes the microwave radiation being applied in the slit (radial wave duct) comprises. If the microwave radiation applied in the plasma cavity or the radial wave duct comprises several modes, the geometric properties of the plasma generated in the plasma cavity will not be known very exactly. If said modes exhibit the same electromagnetic field distribution, seen in the direction of rotation relative to the first axis, the geometric property of the applied microwave radiation and thus also of the generated plasma in the aforesaid direction of rotation will be clearly defined. Said plasma properties depend, among other things, on the excitation ratio of the modes, and a small change in the environment (temperature, pressure, etc) may lead to a different excitation ratio, causing the geometric properties of the plasma to change. All this may lead to an undesirable geometry of the plasma, amongst other things, or cause the plasma to become unstable or become extinguished altogether.

In particular in the case of PCVD applications such phenomena affect the composition and the structure of the glass layers being deposited on the inner side of the hollow glass tube, as well as the final quality of the optical fibre drawn from a preform thus obtained.

One solution for the problem of the modes of the microwave radiation applied in the plasma cavity and the unpredictable properties of the generated plasma can be found in the maximum allowable internal diameter of the radial wave duct, in which the inner circumference is smaller than or equal to the wavelength of the microwave radiation being used, this in order to prevent the occurrence of several modes. However, said maximum constructional dimension will also limit the maximum allowable external diameter of the preform to be manufactured in that case, as well as the length of the optical fibre drawn from said preform.

SUMMARY OF THE INVENTION

The object of the invention is to obviate the above drawbacks and to provide a microwave applicator by means of which a stable plasma having the aforesaid advantageous geometric properties can be generated and maintained on the one hand and which furthermore does not have any restrictions as regards its constructional dimensions.

In order to accomplish that objective, the microwave applicator is according to the invention characterized in that the device comprises means which, in use, generate microwave radiation in the slit, which microwave radiation has only one electromagnetic field distribution at least in one direction perpendicularly to the propagation direction. Since the field distribution properties of the electromagnetic microwave radiation no longer constitute an unknown and unpredictable factor, a more stable plasma having desired geometric properties in the aforesaid direction is thus generated in the plasma cavity.

In a specific embodiment, the microwave radiation in the slit comprises only one mode. Since only one mode exists in the slit, a stable plasma is thus generated. Not only does the microwave applicator thus have a much broader range of applications, but it furthermore makes it possible to control the deposition of glass layers on the inner side of a hollow glass tube much better, in particular in the case of PCVD applications, so that eventually a higher quality optical fibre can be drawn from the preform thus obtained.

In addition to that, the use of the microwave applicator is no longer restricted by its constructional dimensions, as a result of which in particular the dimensions of the plasma cavity can be increased, so that also larger-diameter preforms can be worked and obtained in the case of PCVD applications.

More in particular, the microwave radiation has only one specific electromagnetic field distribution, seen in the direction of rotation relative to the first axis or in a direction parallel to the first axis. In addition to that, the two axes can form a plane of symmetry for the device and orient the microwave radiation relative to the plane of symmetry of the device.

In a specific embodiment, said means form part of the microwave guide.

In a specific embodiment of the device according to the invention, the microwave guide is to that end of rectangular cross-section, with the short walls of the microwave guide extending perpendicularly to the plane of symmetry. From the viewpoint of symmetry, it has appeared that this construction of the microwave applicator directly excludes a number of modes and only passes microwave radiation of one mode.

In another embodiment, the microwave guide is of circular cross-section or of square cross-section.

All embodiments may comprise a metal partition disposed in the plane of symmetry, which partition divides the microwave guide into two chambers. The fact that microwave radiation comprising only one mode is passed whilst other modes are excluded has proved to be effective in these embodiments, too.

Another embodiment, in which microwave radiation comprising only one mode is generated in the plasma cavity, is according to the invention characterized in that the microwave guide is a coaxial guide comprising a rod portion extending along the second axis and a cylindrically symmetric guide tube extending round said rod portion.

In a specific embodiment, in order to obtain symmetric control of the plasma cavity, the microwave guide splits near its first end into an even number of supply guides present within the housing, which terminate in the slit.

In order to obtain simultaneous excitation of the plasma cavity, it is desirable that the difference in length between the paths of the microwave radiation propagating through the two supply guides equals n·½·λg, with n∈{0, 1, 2, . . . } and λgbeing the wavelength of the microwave radiation in the microwave guide. In a specific, simple and inexpensive construction, the supply guides are symmetrically arranged with respect to the plane of symmetry.

More in particular, each supply guide comprises a coaxial bore extending parallel to the first axis in the housing and terminating in the slit, with the cylindrical wall of the coaxial bore possibly coinciding with the external cylindrical wall of the slit.

In suitable, functional embodiments of the device according to the invention, the slit in the cylindrical wall extends along part of the circumference or along the entire circumference thereof.

The invention also relates to an apparatus for depositing, by means of Plasma Chemical Vapour Deposition, one or more layers of silica on the inner side of a hollow glass tube which is disposed in the plasma cavity of the device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.

The microwave applicator comprises a housing1in which a plasma cavity2is present, which plasma cavity2is circularly symmetric about a first axis of symmetry4. The plasma cavity2has a cylindrical wall3, which is provided with a slit5along its entire circumference in this embodiment, which slit functions as a radial wave duct.

The housing1is connected to a first side6aof an elongated wave guide7, in such a manner that the radial wave duct5is in communication with the wave guide7. The wave guide7has a longitudinally extending axis8.

A hollow cylindrical glass tube can be accommodated within the cylindrical, elongated plasma cavity2in the case of Plasma Chemical Vapour Deposition, through which tube a gaseous mixture comprising O2, SiCl4and GeCl2, for example, can be passed.

Since the elongated waveguide7is connected to microwave generating means (not shown) with its other end6b, electromagnetic microwave radiation can be applied in the plasma cavity2via the elongated waveguide7and the radial wave duct5, in which plasma cavity a plasma is generated in the hollow glass tube. By moving the microwave applicator back and forth with respect to the glass tube—with possible simultaneous rotation of the tube—glass layers, whether or not of different composition, can be deposited on the inner side of the cylindrical glass tube by means of said plasma. In this way a so-called hollow preform having one or more glass layers deposited on the inner side thereof is obtained, which preform is subjected to a collapsing treatment so as to obtain a massive rod from which an optical fibre is drawn, which fibre is intended for telecommunication purposes, for example.

Since electromagnetic microwave radiation may comprise various modes with the associated various electromagnetic field distributions, the geometric properties of the plasma generated in the plasma cavity2are not known very exactly. Desirably, in particular in the case of the PCVD application, the plasma generated in the plasma cavity2is not only rotationally symmetric to the desired extent but stable as well, so as to create conditions favourable for the deposition of glass layers on the inner side of the hollow cylindrical glass tube.

The plasma that has been generated in the plasma cavity2may become unstable or even become extinguished altogether, in particular due to minor changes in the environment (temperature, pressure, etc).

According to the invention, in order to generate a stable plasma having the desired geometry in the plasma cavity2, microwave radiation is generated in the slit, which microwave radiation has only one specific electromagnetic field distribution in one direction perpendicularly to the propagation direction.

In the embodiment as shown inFIGS. 1A and 1B, the elongated microwave guide7is to that end rectangular in shape with short walls10aand long walls10b. In order to obtain microwave radiation comprising the specific mode that is known in advance in the plasma cavity2, the short side10aextends perpendicularly to the plane of symmetry9of the device in this embodiment, which plane of symmetry9is formed by the longitudinal axis8of the waveguide7and a cylindrical axis4of the plasma cavity2. The plane of symmetry9defined by the two axes4and8functions as the plane of symmetry for the entire device.

The rectangular construction of the elongated waveguide7and the orientation thereof with respect to the plane of symmetry9of the overall device orients the microwave radiation propagating through the elongated waveguide7, in such a manner that the microwave radiation in the plasma cavity2comprises only one electromagnetic field distribution at least in one direction perpendicularly to the propagation direction.

Another embodiment of such a waveguide7is shown inFIG. 1C, in which the waveguide is shown to be of square cross-section, with sides10aof identical length. In order to orient the microwave radiation propagating through the waveguide7in such a manner that microwave radiation having only one electromagnetic field distribution is generated in the plasma cavity2in this embodiment as well, the elongated waveguide7is divided into two sections by means of a metal partition7a, which metal partition7ais positioned exactly in the plane of symmetry9of the device. The metal partition7afunctions as an electrical barrier for the microwave radiation propagating through the waveguide7, and consequently it orients said microwave radiation in such a manner that the radiation in the plasma cavity2has only one electromagnetic field distribution at least in one direction perpendicularly to the propagation direction.

FIGS. 2A and 2Bshow another embodiment of the microwave applicator according to the invention, in which a co-axial waveguide is used rather than a rectangular or square waveguide7. To that end, a cylindrically symmetric guide tube extending about the axis of symmetry8is used as the microwave guide, in which case a rod portion11is arranged in and along the axis of symmetry8.

Near the first end6a(the transition between the waveguide7and the housing1or the plasma cavity2), the coaxial waveguide is split into two supply guides14aand14barranged within the housing1, which are preferably disposed symmetrically with respect to the plane of symmetry9in the housing1. Each supply guide14a,14bcomprises a respective co-axial bore17a,17bcomprising a respective guide tube15a,15bcontaining a respective elongated rod portion16a,16b. The two bores17aand17bextend parallel to the axis4as well as to each other, and they terminate in the radial wave duct5, as is shown inFIG. 2B. In order to achieve a symmetric control of the radial wave duct5, so as to generate a homogeneous and symmetric plasma in the plasma cavity2, the cylindrical wall15cof each respective coaxial bore17a,17bcoincides with the external cylindrical wall5a(seeFIG. 2B) of the radial wave duct5.

In this embodiment, two chokes13aand13bmay furthermore be arranged in the housing1, which chokes are positioned on either side of the radial wave duct5. The use of chokes on either side of the radial wave duct5has a number of advantages, among which a more efficient enclosure of the generated plasma in a direction parallel to the cylindrical axis4and a reduction of the leakage of microwave radiation from the plasma cavity2to the outside, thus diminishing any health hazards for the operators.

Another embodiment is shown inFIG. 3, in which the waveguide7is of cylindrical section. In order to generate microwave radiation having only one specific electromagnetic field distribution in this embodiment as well, the cylindrical waveguide is split, similarly to the embodiment ofFIG. 1C, by means of a metal partition18which divides the cylindrical waveguide7into two sections. The metal partition18, which functions as an electrical barrier for the electromagnetic microwave radiation propagating through the waveguide7in this embodiment, lies in the plane of symmetry9as defined by the axis8and the axis4in the plasma cavity2.

Although only the splitting into two supply guides14aand14bof the microwave guide according to the embodiment as shown inFIGS. 2A and 2Bis described herein, this technical feature can be used without any problem with any type of microwave guide, such as rectangular, square and even cylindrical microwave guides as shown inFIGS. 1A-1Cand3. Although the microwave guide ofFIGS. 2A and 2Bis furthermore split into two supply guides, the microwave applicator can be extended by splitting up the microwave guide into any even number of supply guides, which all terminate in the slit (radial wave duct).

It should furthermore be noted that also elliptical microwave guides can be used without any problem in one or more of the embodiments that are described herein.

FIGS. 4A and 4Bshow another embodiment of a microwave applicator according to the invention. This embodiment corresponds in large measure to the embodiments ofFIGS. 1A-1C, so that like parts are indicated by the same reference numeral for a better understanding of the Figure.

This microwave applicator, too, comprises a housing1with a plasma cavity2present therein, which plasma cavity2is circularly symmetric about a first axis of symmetry4. The plasma cavity2comprises a cylindrical wall3which, in this embodiment, is provided with a slit5which does not extend along the entire circumference thereof, which slit functions as a radial wave duct.

The circular slit5is interrupted in at least one place, on the other hand; in this embodiment it is interrupted in two places, which are indicated at20aand20b. Said places20aand20b, in which the wall3of the circularly symmetric plasma cavity2normally continues, lie in the plane of symmetry9, so as to enable the device according to the invention to function correctly.