LU502413B1 - Fiber-optical sensor device and system for monitoring of a thin probe-film - Google Patents
Fiber-optical sensor device and system for monitoring of a thin probe-film Download PDFInfo
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
- G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
- G01B11/0616—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating
- G01B11/0675—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating using interferometry
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
- G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
- G01B11/0616—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating
- G01B11/0683—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating measurement during deposition or removal of the layer
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B2290/00—Aspects of interferometers not specifically covered by any group under G01B9/02
- G01B2290/25—Fabry-Perot in interferometer, e.g. etalon, cavity
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- General Physics & Mathematics (AREA)
- Measuring Temperature Or Quantity Of Heat (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
Abstract
The invention relates to a fiber-optical sensor device (10) for monitoring of a thin probe-film (109) receivable at a sensing side of a tip of the fiber-optical sensor device by measuring of a thin probe-film’s (109) optical parameter, the fiber-opti- cal sensor device (10) comprising: a lead-in optical fiber (101) and an optical sensor provided to the lead-in optical fiber (101). According to the invention the optical sensor is a fiber-optical sensor (100) formed by an optical fiber of the tip, said tip-optical-fiber (111) extending from the lead-in optical fiber (101), and the fiber-optical sensor (100) comprises: - a reference section of the tip-optical-fiber (111) of the fiber-optical sensor (100) with a first fiber section (107) with a first semi-reflective mirror (104), wherein the first semi-reflective mirror (104) is provided as an in-fiber mirror of the tip-optical-fiber (111), and/or a reflective reference section (112) with a reflective fiber optical structure - a second fiber section (108) of the tip-optical-fiber (111), with a second semi-reflective mirror (105), wherein the second semi-reflective mirror (105) is provided as an in-fiber mirror of the tip-optical-fiber (111), and a fiber-optical sensor’s tip end-surface (106) at a sensing side of the fiber-optical sensor (100) adapted to receive the thin probe-film (109).
Description
Berlin, 29.06.2022
University of Maribor, Dr. Stephan Nüsse/anb
LU502413
Fiber-optical sensor device and system for monitoring of a thin probe-film
The present invention relates to a fiber-optical sensor device for monitoring of a thin probe-film receivable at a sensing side of a tip of the fiber-optical sensor device by measuring of a thin probe-film’s optical parameter. The invention also relates to a fiber-optical sensor system for monitoring of thin probe-film parame- ters comprising the fiber-optical sensor device and an optical signal interrogation system.
Such a fiber-optical sensor device comprises a lead-in optical fiber and an optical sensor provided to the lead-in optical fiber.
Prior Art
The rising complexity of industrial processes, bio-medical systems, and environ- mental monitoring methods often require sensing of more than one physical or chemical parameter. Furthermore, a harsh environment compatibility is often re- quired in sensing systems that address gas measurements. This compatibility often includes chemical compatibility, broad temperature operational range, elec- tromagnetic interference immunity, dielectric design, and explosion safety. Re- cent advances in micro-fluidic and micro-reactor systems also require miniaturi- zation of available fluidic sensors’ solutions. Miniaturization brings additional challenges in cases of small sensing devices designs, as those devices are more sensitive to oxidation and other degradation processes. Fiberoptic sensors can address many of the aforementioned challenges. Fiber-optic Fabry-Perot inter- ferometers (FPIs) are one of the fiber sensing technologies that provide possibil- ities to build and design dual parameter sensors.
Caranto, N.R.Y., et al., An Optical-Fiber Thin-Film Thickness Monitor. Measure- ment Science and Technology, 1993, 4(8): p. 865-869 discloses a fiber optic probe consisting of a perpendicularly cleaved optical fiber end for film thickness measurements. Authors show fiber-optical sensor’s high potential for calibration a quartz crystal sensors, which are standardly used for monitoring depositions rates in vacuum-assisted systems. The sensor has a simple design and is simple to operate, having straightforward signal processing that needs only high preci- sion optical power detection. However, this method does not overcome the low resolution, mainly at the starting point of layer thickness growth. The oscillations in the intensity of the light source are directly reflected in measured reflection as it is the intensity-based principle.
US 5,804,453 A discloses a fiber-optic interferometer assay device designed to detect analyte binding to an optical fiber end-surface; this relates to the thickness measurement of analyte molecules that are bound to the end-surface 106of an optical fiber. The device is simple to operate and provides a rapid assay method for analyte detection, which is however limited only to one forming Fabry-Perot interferometer which can be efficiently used in many biochemical applications where the temperature remains constant during the device operation. Since the
Fabry-Perot cavity is short, determined by the thickness of the sensed layer, ap- plicable signal interrogation methods are limited and thus only a limited measure- ment performance is available.
US 7,394,547 B2 discloses a fiber-optic interferometer assay device designed to detect analyte binding to an optical fiber end-surface; this relates to improving the measurement sensitivity of the device by adjusting the thickness and refractive index of the optical element to form a short cavity Fabry-Perot Interferometer, which length including binding analyte typically does not overcome a few hundred nanometers. That allows low-cost interrogation with low-resolution optical spec- trum analyzers. The assay device need to use the functionality of flexible gripping arms, that are designed to slide over the end of the fiber and grip the fiber by engagement of an annular rim, which enables the replacement of optical assem- bly whith a fresh element for a new assay. Single Fabry-Perot cavity. However, this exhibits a large temperature dependence, which limits use of this approach to environment with known or stabilized temperatures, as for example biochemi- cal applications. Structurally the same approaches of similar kind like in US -2-
2010/0093106 A1, US 2011/0236911 A1 and US 2014/0322819 A1 experience the same limitations.
US 9,719,936 B2 discloses an assembly used for detecting an analyte based on thin probe-film spectral interference; this relies on improved coupling without air gap (e.g. using index matching gel) between waveguide (e.g. lead-in optical fiber 101) and replaceable monolithic substrate, which serve for a thin probe-film layer directly bonded to the sensing side of the monolithic surface.
Compared to these basic approaches other improved dual parameter sensors are based on FPls (FPI, “Fabry-Perot-Interferometer”) and include, for example, sensors for simultaneous measuring of refractive index and temperature, sensors for simultaneous measurement of pressure and temperature, sensors for simul- taneous measurement of strain and temperature, sensors for simultaneous measurement of humidity and temperature, and sensors for simultaneous meas- urements of pressure and refractive index.
Specifically even in these improved approaches a problem arises in that only some of those sensors feature partly an all silica design, which address compat- ibility with challenging environments and small size requirements successfully.
Further, there are only a few known solutions that provide the opportunity to sense more than two parameters and most of these solutions are either large in size and/or do not use an all-silica design or not allow simultaneous measurement of all parameters.
As an example of such an even improved approach in the article of S. Pevec and
D. Donlagic in Optics Letters Vol. 40, No. 24 (2015) (Doc. ID 249644) a miniature fiber-optic sensor is created at the tip of an optical fiber suitable for simultaneous measurement of relative humidity and temperature. The proposed sensor is based on two cascaded Fabry-Perot interferometers, the first configured as a relative humidity sensing element made from silica micro-wire coated with thin porous SiO2 layer, and the second as a temperature sensing element made from a segment of a standard single-mode fiber. The sensor has linear characteristics -3-
for both measurement parameters and a sensitivity of 0.48 degree/%RH and 3.7 degree/°C.
Further in the article of S. Pevec and D. Donlagic in IEEE Photonics Journal, Vol 9, No.1 (2017) (DOI: 10.1109/JPHOT.2017.2651978) a miniature, all-silica four/multiparameter sensor for simultaneous measurements of thermal conduc- tivity, pressure, refractive index, and temperature of gases is described. The sen- sor is composed of multiple Fabry-Perot interferometers (FPIs) that were created at the tip of a standard optical fiber by a micromachining process based on se- lective etching and standard fiber manipulation steps. The experimental sensor length was below 3.4 mm, while the diameter did not exceed 125um. Interrogation of the sensor utilized acquisition and appropriate signal processing of the back- reflected optical spectrum, which allowed for crosstalk free extraction of individual resonators’ lengths. The sensor utilizes three different FPIs stacked on the tip of a lead-in optical fiber for measurements of pressure, refractive index, tempera- ture and thermal conductivity. Each FPI has a different length. This allows for spectral interrogation of the individual FPIs’ lengths by application of the Inverse
Fourier Transform (IFT) over the acquired optical spectrum. The first FPI is made of a thin-wall glass capillary (OD/ID 125/85um) with a length of 390um and a flexible and thin silica diaphragm located on the top of the sensor structure. The diaphragm deflects proportionally to the absolute surrounding pressure and, thus, modulates the first FPI length. The second FPI is a 165um long open path U- shaped all-silica microcell, which allows for free access of the surrounding gas in-between the FPIs’ semi-reflective surfaces and, thus, for gas RI measure- ments.
A short section of Vanadium-Doped Fiber placed in between an in-fiber mirror and semi-reflective surface of the second FPI defines the third FPI. The length of the Vanadium-Doped Fiber corresponds to 2290 um, while the distance between mirrors is slightly larger, i.e., 2320 um, as the semi-reflective surface is separated by a short segment of core-less fiber from the Vanadium-Doped Fiber. This third
FPI performs two functions: Temperature sensing and thermal conductivity meas- urement. Temperature is measured by observing the optical path length change -4-
of the third FPI, which is mainly temperature dependent due to the silica’s refrac- tive index dependence on the temperature (i.e. dn/dT fo silica corresponds to about 107° K). On the other hand, the thermal conductivity is measured by ap- plication of active heating to this temperature sensitive segment.
Also these promising designs can be further improved; in particular the all-silica integration aspect and the measurement approach for several parameters can be even more improved.
This is where the invention comes in, wherein it is an objective of the present invention to provide a fiber-optical sensor device and a system with improvement based on an all-silica integration aspect and the measurement approach for sev- eral parameters. In particular, a measurement’s reliability should be improved whilst resistance to harsh environments is to be taken care for.
In particular it is an object to provide a fiber-optical sensor device and a system for a thin probe-film characterization with a FP-Interferomenter (FPI) on optical fiber integrated and/or cascaded with a reference section. The fiber-optical sen- sor device and a system should be suited for an application in CVD (Chemical
Vapor Deposition).
This objective is achieved in a first aspect by the invention by a fiber-optical sensor device according to claim 1.
As defined in the claim the invention starts from a fiber-optical sensor device for monitoring of a thin probe-film receivable at a sensing side of a tip of the fiber- optical sensor device by measuring of a thin probe-film’s optical parameter, the fiber-optical sensor device comprising: - a lead-in optical fiber, - an optical sensor provided to the lead-in optical fiber.
According to the invention it is provided that -5-
- the optical sensor is a fiber-optical sensor formed by an optical fiber of the tip, said tip-optical-fiber extending from the lead-in optical fiber, and the fiber-optical sensor the fiber-optical sensor is adapted to provide an optical signal indicative of an optical path length at the tip-optical-fiber and comprises: - a reference section of the tip-optical-fiber of the fiber-optical sensor with a first fiber section with a first semi-reflective mirror, whereinthe first semi- reflective mirror is provided as an in-fiber mirror of the tip-optical-fiber, and/or a reflective reference section with a reflective fiber optical structure - a second fiber section of the tip-optical-fiber, with a second semi-reflective mirror, wherein the second semi-reflective mirror is provided as an in-fiber mirror of the tip-optical-fiber, and a fiber-optical sensor’s tip end-surface at a sensing side of the fiber- optical sensor adapted to receive the thin probe-film.
To achieve the object, the invention also in a second aspect leads to a fiber-op- tical sensor system of claim 20.
The invention in this second aspect provides a fiber-optical sensor system for monitoring of thin probe-film parameters comprising the aforementioned fiber-op- tical sensor device of the invention and an optical signal interrogation system.
Therein, according to the invention the optical signal interrogation system is con- nected to the fiber-optical sensor formed by said tip-optical-fiber extending from the lead-in optical fiber via the lead-in optical fiber, and wherein optical signal interrogation system performs readout of an optical signal of the fiber-optical sen- sor of the fiber-optical sensor device, the optical signal being indicative of an op- tical path length at the tip-optical-fiber.
The subject matter of the invention is based on an all-silica integration aspect and the measurement approach for several parameters. An all-silica fiber has -6-
shown up to be particular resistent to heat and adaptable in length for specific use.
The fiber-optical sensor device and a system for a thin probe-film characterization is provided with a FP-Interferomenter (FPI) on optical fiber which is integrated and/or cascaded with a reference section; thus in close neighborhood a sensed optical path length or change of optical path length of the reference section can be considered as significant due to change of length of the reference section. The environment temperature can be considered as relevant for the change of length; thus this environmental situation can also be considered as being relevant for the second fiber section of the tip-optical-fiber in close neighborhood of the reference section.
Thus, the fiber-optical sensor formed by an optical fiber of the tip, said tip-optical- fiber extending from the lead-in optical fiber is particular useful based on an all- silica to resist the harsh environments is to be taken care for. The fiber-optical sensor device and a system should be suited for an application in CVD (Chemical
Vapor Deposition).
Further, the fiber-optical sensor is adapted to provide an optical signal indicative of an optical path length (OPL) at the tip-optical-fiber. The tip-optical-fiber can be readily be adapted to the application case in length and thereby addresses the spectral range of interest for the thin probe-film and/or the demands and prefer- ences of the light source used.
It should be noted that “semi-reflective” is meant in the meaning of “partially re- flective”; thus, “semi-reflective” in principle means “reflection in part” and “trans- mission in part” as compared in contradistinction to total reflection of light (except for unavoidable losses). The reason is, that the light coupled-in the sections of the tip-optical-fiber is to be coupled-out again for measurement purposes.
Still nevertheless a “semi-reflective mirror” also embraces developments wherein part of the mirror (respectively preferably a surface or structure thereof) is meant -7-
to be total reflective and part of the same mirror is meant to be semi-reflective in the above meaning of partially reflective.
Thus, generally a “reflective mirror” is meant to embrace a “semi-reflective” mirror but also a total-reflecting mirror. Therein, where applicable in the technical sense, it should be understood that a balance of reflectivity and transmission should be sufficient to allow for an interferometric effect for measurement purposes.
Thus, the present invention discloses fiber-optic sensor device and a system adapted for detection and characterization of a thin probe-film, which can be op- erated over broad range of operating conditions, including high and variable tem- peratures.
This device and system are adapted to be used in wide variety of applications including but not limit to monitoring of thin probe-film deposition in industrial, re- search and similar applications, bio-chemical and biomedical sensing, and envi- ronmental monitoring. Presented devices form a universal sensing platform for accurate monitoring of thin probe-film formation, growth, change or evaluation.
Developments of the invention are found in the dependent claims and indicate in detail advantageous possibilities to realize the concept described above within the scope of the task as well as with regard to further advantages.
In particular the optical signal interrogation system is adapted to interpret the op- tical signal of the fiber-optical sensor of the fiber-optical sensor device in terms of a thin probe-film’s parameters and a sensor temperature.
In particular, in this case the in-fibre "close neighborhood" or even integration of the second fiber section, preferably forming a Fabry-Perot interferometer, is adapted for measuring a thin-film-thickness. Further the "close neighborhood" or even integration of the reference section with the first fiber section and/or the -8-
semi-reflective reference section, is meant to measure its (the reference sec- tion's) optical path length change. Thus thereby additionally the reference section is adapted to address a temperature change of the reference section and the second fiber section; respectively. This provides the improved results as apparent from the reasons outlined below, in particular as explained with the fomulas (5)- (7) below.
In a most preferred development advantageously - the second semi-reflective mirror and the end-surface form a Fabry-Perot inter- ferometer adapted to sense the thin probe-film’s optical parameter at the tip end- surface, and - for sensor temperature sensing of the tip-optical-fiber the first semi-reflective mirror and the semi-reflective mirror, and/or the reflective reference section with the reflective fiber optical struc- ture define an optically reflecting element of the reference section adapted to sense an optical path length of the reference section.
Even further advantageous developments of the invention are found in the de- pendent claims and indicate in detail advantageous possibilities to realize the concept described above within the scope of the task as well as with regard to further advantages. In the following, preferred developments of the converter building block will be described.
In a first variant of development advantageously - the first semi-reflective mirror is placed at a coupling side of the fiber-optical sensor where said tip-optical-fiber extends from the lead-in optical fiber. This de- velopment, with preferred embodiments shown in FIG.1A, FIG.1B, is particular useful in view of providing a reference section of the tip-optical-fiber and a second fiber section in cascade extending therefrom at the distal end thereof.
In a second variant of development advantageously -9-
- the second semi-reflective mirror is placed at a coupling side of the fiber-optical sensor where said tip-optical-fiber extends from the lead-in optical fiber. This de- velopment, with preferred embodiments shown in FIG.1C, FIG.1C, is particular useful in view of providing a second fiber section and integrated therewith a ref- erence section of the tip-optical-fiber. Therein preferably at the distal end of the tip-optical fiber basically the second fiber section is provided; possibly also the integrated reference section.
Generally and as broadly claimed with the inventive concept the tip-optical-fiber extends from the lead-in optical fiber, and the fiber-optical sensor is adapted to provide an optical signal indicative of an optical path length (OPL) at the tip-opti- cal-fiber. This concept embraces that preferably the tip-optical-fiber with its sec- tions as such, but preferably also its extension from the lead-in fiber can be exe- cuted by any kind.
In a preferred development the term of “extension” embraces that the tip-optical- fiber with its sections as such, but optionally also the tip-optical-fiber extending from the lead-in optical fiber, means a monolithic fiber. A monolithic fiber body of the tip-optical-fiber, optionally also the tip-optical-fiber extending from the lead-in optical fiber may provide sections wherein the semi-reflective mirrors and/or re- flective fiber optical structures, in particular also end-surface are applied by in- scribing. In of such or other kind of fiber optical structures can be applied prefer- ably to a fiber --in particular a monolithic fiber body of the tip-optical-fiber, option- ally also the tip-optical-fiber extending from the lead-in optical fiber-- by applying high peak power of a short-pulse laser source, preferably Femtosecond-Laser source. Also possibly a fiber optical structure can be drilled or applied into said monolithic fiber body thereby.
Generally the fiber of use is adapted for broad use adapted to receive light from a coherent (like laser) or non-coherent (like diode) light source and/or pulsed or
CW- light source. - 10 -
In a preferred development advantageously the lead-in optical fiber is joint to the second semi-reflective mirror, wherein the second semi-reflective mirror is joint to the second fiber section of the tip-optical-fiber. Joining in particular embraces any kind of gluing, attaching or the like but also in particular any sort of welding.
In particular fusion splicing of two fibers or fiber sections is preferred, which is considered as any act of joining two optical fibers end-to-end. Thereby preferably it is possible to fuse two fibers together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and so that the splice and the region surrounding it are almost as strong as the intact fiber. The source of heat used to melt and fuse the two glass fibers being spliced is usually an electric arc, but can also be a laser, a gas flame, or a tungsten filament through which current is passed.
Advantageously in a preferred development the distal end of the second fiber section of the tip-optical-fiber defines a fiber-optical sensor’s end-surface adapted to accept a monitored thin probe-film, wherein the monitored thin probe-film is allowed to deposit to the fiber-optical sensor's end-surface which is adapted to accept the monitored thin probe-film.
Advantageously in a first kind of preferred development, additionally or alterna- tively to the second kind of development, the first semi-reflective mirror and the second semi-reflective mirror define a Fabry-Perot interferometer adapted for temperature sensing. A Fabry-Perot interferometer has been shown to be partic- ular advantageous for detecting film-thickness but also any change of optical path length. Thus, a Fabry-Perot interferometer may be provided not only in the sec- ond fiber section but also in the reference section of the tip-optical-fiber.
Advantageously in a second kind of preferred development, additionally or alter- natively to the first kind of development, the reflective reference section is adapted for temperature sensing, wherein the reflective fiber optical structure of the reflective reference section forms a Bragg-reflector. Generally any kind of op- tical reflector can possibly to be provided in the reflective reference section. A - 11 -
Bragg-reflector has shown up to be of particular use in fiber optics. A Bragg grat- ing to the Bragg-reflector can be adapted in length. The Bragg-reflector thus, even when integrated with the second fiber section of the tip-optical-fiber, may preferably be of length greater then the second fiber section.
Preferably, as will be apparent from the further description, a Bragg-reflector (adapted for measurement of temperature; thus change of optical path length) may have a length largely exceeding the length of the second fiber section (adapted for thickness measurement). A Bragg-reflector typically would be very much preferred but not restricted to a length of between 1mm to 1cm.
A mirror provided at the tip-optical-fiber can be an in-fiber mirror; e.g. a mirror can be drawn with the fiber of a mirror can be inscribed as outlined above. A mirror also can be applied by any known technique upon joining a two fibers or fiber sections. A mirror may extend about the whole cross section of the fiber (like a mirror in an embodiment shown in FIG.1) or only part of the cross section of the fiber, preferably only on the core of the fiber (like a mirror in an embodiment shown in FIG.8).
A mirror according to a development can be provided in preferred places at the fiber.
Advantageously in a further kind preferred development, the reference section provides for the second semi-reflective mirror. A preferred embodiment and the advantageous of this development is described with FIG.1A.
Advantageously in a further kind preferred development, the reference section provides for a second semi-reflecting mirror part and/or a first semi-reflecting mir- ror part.
Advantageously in a further kind preferred development, additionally or alterna- tively, -12 -
- the second semi-reflective mirror is provided as a part of a mirror arrangement, wherein the mirror arrangement has a first and second semi-reflecting mirror part with a part of the tip-optical-fiber extending therebetween the first and second semi-reflecting mirror part. À preferred embodiment and the advantageous of this development is described with FIG.1B.
Preferably, in a still further advantageous variation of this development, one of the first and/or second semi-reflecting mirror part and the end-surface form a
Fabry-Perot interferometer. The other one of the first and/or second semi-reflect- ing mirror part in this regard then forms a semi-reflective mirror of the reflective reference section. À section between surfaces formed by the semi-reflective first and second semi-reflecting mirror part and can be a piece of a fiber; in particular any kind of fiber. Preferably, the fiber is basically of identical or at least similar structure as the fiber of the Fabry-Perot interferometer and the reflective refer- ence section; i.e. in particular along a fibers axis the provision of core and clad- ding the fiber is basically of identical or at least similar structure as the fiber of the
Fabry-Perot interferometer and the reflective reference section. Thus, advanta- geously the fiber can be drawn as such in one unified process.
It should be noted, that the fiber-optical sensor is not necessarily to have one common semi-reflective surface in form the second semi-reflective mirror of the second fiber section. As should be clear therefrom both sensing sections of the tip-optical-fiber —i.e. the reference section and the second fiber section-- can be separated by a fiber portion of any kind; still not too far away respectively not too long to keep the neighborhood for measurement purposes, of course all as part of said tip-optical-fiber. In the instant development, as an example, the separating fiber portion of said tip-optical-fiber is realized between said first and/or second semi-reflecting mirror part. A preferred development thereof is shown in FIG.1B.
Preferably in a development - between a second semi-reflecting mirror part and a first semi-reflecting mirror part extends the tip-optical-fiber. Preferably in a development -13-
- the reflective reference section is placed into the second fiber section of the tip- optical-fiber. À preferred embodiment and the advantageous of this development is described with FIG. 1C and FIG. 1D.
In particular the reflective reference section is placed into the second fiber section of the tip-optical-fiber. Preferably in a variant of a development - the reflective reference section extends in between the end-surface and at least partially the semi-reflecting mirror. Thus the reflective reference section can be of same length basically or longer than the second fiber section. This is particular preferred when the reflective reference section is to be functionally applied for measuring a temperature.
Preferably in a variant of a development - the reflective reference section is placed into the second fiber section of the tip- optical-fiber and is distant from the end-surface and semi-reflecting mirror of the tip-optical-fiber. Thus, the reflective reference section can be of same length ba- sically or shorter than the second fiber section. This is particular preferred when the second fiber section is of rather long extension for some purposes where scanning of the FPI can be accepted in favor of spectral broadness and thus still the reflective reference section is able to be long enough to be functionally ap- plied for measuring a temperature.
According to a preferred development advantageously the length of the first and the second fiber section of the tip-optical-fiber is between 10 um and 5 cm, in particular between 100 um and 1 cm, in particular between 200 um and 1 mm.
According to a preferred development advantageously the ratio of lengths of the first and the second fiber section of the tip-optical-fiber are between 1:100 and 100:1 or between 1:10 and 10:1 or between 0.3:1 and 1:5. - 14 -
In a preferred development advantageously for monitoring of thin probe-film pa- rameters the thin probe-film parameters are one or more parameters or their com- binations from the group of thin probe-film thickness, thin probe-film refractive index, thin probe-film density, thin probe-film optical absorption.
In a preferred development advantageously for monitoring of thin probe-film pa- rameters an active thin probe-film is pre-formed at the surface adapted to accept monitored thin probe-film. In a particular preferred development advantageously the fiber-optical sensor device is adapted for monitoring of the thin probe-film parameters, in particular when a physical application like in a CVD application.
In a preferred development advantageously the fiber-optical sensor device is adapted for monitoring of thin probe-film parameters, the active thin probe-film has ability to selectively capture target monitored species. In a particular pre- ferred development advantageously the fiber-optical sensor device is adapted for monitoring of the thin probe-film parameters, in particular when a target species is chemical, bio-chemical or biological species.
Thus, preferably there are two possible applications and/or configurations.
In a first kind of a application and/or configuration a thin probe-film does not exist at first, i.e. at the beginning. The thin probe-film is formed during the deposition process that is monitored. Especially, this configuration is related with an appli- cation that is targeted primarily. A use in inorganic deposition --like in physical deposition processes in semiconductor or similar-- is preferred.
In a second kind of an application and/or configuration a thin probe-film does already exist at first, i.e. at the beginning. But the thin probe-film is changed du- ring a monitored process. In an examplifying example; this situation is typical for chemical and biochemical applications. - 15 -
As to the fiber-optical sensor system for monitoring of the thin probe-film param- eters the system comprises the fiber-optical sensor device as mentioned with the invention and an optical signal interrogation system.
According to the concept of the invention the optical signal interrogation system is connected to the fiber-optical sensor formed by said tip-optical-fiber extending from the lead-in optical fiber via the lead-in optical fiber and wherein optical signal interrogation system performs readout of thin probe-film’s parameters and a sen- sor temperature.
According to a preferred development advantageously the optical interrogation system is a spectrally resolved system that records the optical signal of the fiber- optical sensor as a recorded spectrum. The recorded spectrum preferably is formed from the optical signal as back-reflected optical power versus optical fre- quency over a limited spectral range of the recorded spectrum.
Thus, advantageously the optical interrogation system is a spectrally resolved system that records back-reflected optical power versus optical frequency over limited spectral range.
According to a preferred development, advantageously characteristics of the rec- orded spectrum are further processed to track positions of spectral interference fringes positions or phases with frequencies. Preferably therein the spectral in- terference fringes and/or frequencies are assigned to belong to a second fiber section of the tip-optical-fiber, in particular thin probe- film monitoring, and/or a reference section of the tip-optical-fiber, in particular a temperature monitoring, preferably by a Fabry-Perot interferometer or a reflective ref- erence section forming a Bragg-reflector.
Thus, with advantage, the recorded spectra characteristics is further processed to track positions of spectral interface fringes positions or phases with frequen- - 16 -
cies that belong to thin probe-film monitoring and temperature monitoring; in par- ticular by a reflective reference section forming a Bragg-reflector and/or a Fabry-
Perot interferometer.
According to a preferred development advantageously a limited spectral range of the fiber-optical sensor is between 0.5 nm and 200 nm. More precisely the limited spectral range of the recorded spectrum of the optical signal of the fiber-optical sensor is between 0.5 nm and 200 nm. Advantageously the limited spectral range of the fiber-optical sensor is adapted with the adapting among others a tunable wavelength of a light source and/or the optical path length of the interferometric and/or reflective structure implemented therewith.
According to a preferred development advantageously the thin probe-film prop- erties are measured independently measured from the temperature. Additionally or alternatively - according to a preferred development advantageously the thin probe-film properties and temperature are measured simultaneously.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter with reference to the enclosed drawings. The embodiments of the invention are described in the fol- lowing on the basis of the drawings in comparison with the state of the art, which is also partly illustrated. The latter is not necessarily intended to represent the embodiments to scale. Drawings are, where useful for explanation, shown in schematized and/or slightly distorted form. With regard to additions to the les- sons immediately recognizable from the drawings, reference is made to the rel- evant state of the art. It should be borne in mind that numerous modifications and changes can be made to the form and detail of an embodiment without de- viating from the general idea of the invention. The features of the invention dis- closed in the description, in the drawings and in the claims may be essential for the further development of the invention, either individually or in any combina- tion. In addition, all combinations of at least two of the features disclosed in the description, drawings and/or claims fall within the scope of the invention. The general idea of the invention is not limited to the exact form or detail of the pre- ferred embodiment shown and described below or to an object which would be „17 -
limited in comparison to the object claimed in the claims. For specified design ranges, values within the specified limits are also disclosed as limit values and thus arbitrarily applicable and claimable. Further advantages, features and de- tails of the invention result from the following description of the preferred em- bodiments as well as from the drawings.
In the following, a summary of the figures is given. In the following it is shown in:
FIG.1A - FIG.1D: a schematic view of a preferred first, second, third and fourth em- bodiment of a fiber- optical sensor device with a fiber-optical sen- sor formed by an optical fiber of a tip, said tip-optical-fiber extend- ing from the lead-in optical fiber;
FIG.2 - FIG.4: a schematic view of a preferred first, second and third embodiment of a fiber-optical sensor system with a preferred fiber-optical sen- sor device as shown exemplary in FIG.1A — FIG.1D;
FIG.5: a nearly sinusoidal spectral characteristics in the domain of optical frequencies generated by a Fabry-Perot interferometer according to an embodiment of the invention;
FIG.6: an exemplary sensor spectral characteristics according to the first embodiment with a thin-film observing first Fabry-Perot interferom- eter and a temperature measuring second Fabry-Perot interferom- eter;
FIG.7: a Fourier transform of the exemplary spectral characteristic;
FIG.8: operational situation of a preferred fiber-optical sensor system for monitoring of thin probe-film deposition in an magnetron sputtering system with a preferred embodiment of a fiber-optical sensor de vice; -18 -
FIG.9A - FIG.9D: optical path length changes and length changes (FIG.9C,
FIG.9D) as demonstrated with data (FIG.9B) of the fiber- optical sensor device and system of the preferred embodi- ments, when light is applied as shown in FIG.9A.
With reference to FIG.1 in general --i.e. in detail FIG.1A, FIG.1B, FIG.1C, FIG.1D- - there are shown some varied examples, each of an fiber-optical sensor 100 of a fiber-optical sensor device 10 according to an embodiment of the present in- vention; the embodiments in detail shown in Fig.1A, FIG.1B, FIG.1C, FIG.1D are labeled with the same reference mark, although still they are varied as explained with their variations below. For the features in common reference is made to the fiber-optical sensor device 10 and the fiber-optical sensor 100, wherein this ap- plies to the various embodiments.
A fiber-optical sensor 100 of the present invention is meant for sensing one or more parameters related to the thin probe-film 109 located at the tip of the fiber- optical sensor 100. More specifically the optical sensor is a fiber-optical sensor 100 formed by an optical fiber of the tip, said tip-optical-fiber 111 extending from the lead-in optical fiber 101. Thus, the embodiments all start as basis from a fiber- optical sensor device 10 for monitoring of a thin probe-film 109 receivable at a sensing side of a tip of the fiber-optical sensor device by measuring of a thin probe-film’s 109 optical parameter. The fiber-optical sensor device 10 comprises a lead-in optical fiber 101 and an optical sensor provided to the lead-in optical fiber 101. Thus, according to the concept of the invention therein the optical sen- sor is a fiber-optical sensor 100 formed by an optical fiber of the tip, said tip- optical-fiper 111 extending from the lead-in optical fiber 101.
The parameter or parameters may be for example thin probe-film thickness, thin probe-film refractive index, optical absorption of the thin probe-film, or their com- binations. One or more of these parameters can be further related to other phys- ical, chemical, bio-chemical quantities or biological species in cases when the -19-
thin probe-film is designed to attract, adsorb, absorb, or respond in another meas- urable way to the presence of physical, chemical, biochemical or biological spe- cies or quantities.
As explained with the embodiments in detail below generally according to the concept of the invention the fiber-optical sensor 100 comprises: - a reference section of the tip-optical-fiber 111 of the fiber-optical sensor 100 with a first fiber section 107 with a first semi-reflective mirror 104, wherein the first semi-reflective mirror 104 is provided as an in-fiber mirror of the tip-optical-fiber 111 (embodiments of FIG.1A, FIG.1B), and/or a reflective reference section 112 with a reflective fiber optical structure 114 (embodiments of FIG.1C, FIG.1D); - a second fiber section 108 of the tip-optical-fiber 111, with a second semi-reflective mirror 105, wherein the second semi-reflective mirror 105 is provided as an in-fiber mirror of the tip-optical-fiber 111, and a fiber-optical sensor's tip end-surface 106 at a sensing side of the fiber- optical sensor 100 adapted to receive the thin probe-film 109.
In the following the embodiments of FIG.1A, FIG.1B and FIG.1C, FIG.1D respec- tively will be described in detail.
Fiber-optical sensor
A fiber-optical sensor 100 generally —like shown as an example in the first em- bodiment shown in FIG.1-- is formed at the tip of a lead-in optical fiber 101 having a core 102 and a cladding 103 that is adapted to guide light towards and away from the fiber-optical sensor 100. It will be clear to one skilled in the art that the designation of “lead” optical fiber refers to the use of this fiber as a conduit for the sensing light signal and is not meant as representative of any particular property -20 -
of the optical fiber itself. Further embodiments of the fiber-optical sensor 100 fol- low the same principle and will be described further in detail with some variations in structure.
The fiber-optical sensor 100 of the embodiment of FIG.1A and FIG.1B includes a first semi-reflective mirror 104, a second semi-reflective mirror 105, and a fiber- optical sensor's end-surface 106. More specifically a first fiber section 107 with a first semi-reflective mirror 104 is provided, wherein the first semi-reflective mirror 104 is provided as an in-fiber mirror of the tip-optical-fiber 111. In particular the first semi-reflective mirror 104 is placed at a coupling side of the fiber-optical sen- sor 100 where said tip-optical-fiber 111 extends from the lead-in optical fiber 101.
Generally therein the fiber-optical sensor device 10 provides for the second semi- reflective mirror 105 and the end-surface 106 form a Fabry-Perot interferometer adapted to sense the thin probe-film’s optical parameter at the tip end-surface 106, and - for sensor temperature sensing of the tip-optical-fiber 111 the first semi-reflective mirror 104 and the semi-reflective mirror 105 define an optically reflecting element of the reference section 112 adapted to sense an optical path length OPL of the reference section 112.
In the embodiment of FIG.1A a section of the sensor in-between the fiber-optical sensor's end-surface 106 and the first mirror 104 is composed of the first fiber section 107 of an optical fiber having a core 102 and a cladding 103, preferably but necessarily of similar diameter and composition as the lead-in fiber 101. Sim- ilarly, the fiber-optical sensor’s section in-between the second semi-reflecting mir- ror 105 and the end-surface 106 is also composed of the second fiber section 108 of an optical fiber, preferably but necessarily of similar diameter and compo- sition as the lead-in fiber 101.
The second fiber section 108 of the optical fiber can have a core and a cladding.
The chosen length of this section is short, typically 20 um or less than 100 um, or - 21 -
even less than 1 mm. In special cases longer length can be also applicable since they might allow applications of standard DDFB diodes (Distributed Feedback
Diode) for optical signal interrogation.
This fiber second fiber section 108 can have the cladding only, i.e. can be com- posed of the coreless fiber.
The fiber-optical sensor’s end-surface 106 is adapted to accept thin probe-film, which is measured, monitored or otherwise characterized. The fiber-optical sen- sor’s end-surface 106 can be covered by a thin probe-film 109 either during or before monitoring, measuring or other characterization process, as it will be dis- cussed further below.
The fiber-optical sensor’s end-surface 106 and the monitored thin probe-film 109 define a monitored optical semi-reflective surface 110. The relative phase and magnitude of the light-wave reflected from the optical semi-reflective surface 110 depends on the thin probe-film’s properties, such as thin probe-film’s thickness, thin probe-film’s refractive index, and thin probe-film’s optical absorption.
The fiber-optical sensor's end-surface 106 or the fiber-optical sensor's end-sur- face 106 covered by a monitored thin probe-film, i.e. the monitored optical semi- reflective surface 110, and second semi-reflective mirror 105 defines the thin-film observing Fabry-Perot interferometer 108. Changes in the thin probe-film’s thick- ness and/or refractive index, modulate optical path length OPL of the thin-film observing Fabry-Perot interferometer 108. These changes can be tracked and/or determined by observing and analyzing the properties of the light reflected back from the fiber-optical sensor 100 as discussed further below.
The thin probe-film properties are, however, not the only parameters that affect the optical path length OPL and phase of the thin-film observing Fabry-Perot in- terferometer 108. Any change in the fiber-optical sensor’s temperature will also significantly modulate the refractive index (and to lesser extension also the length) of the fiber that interconnects the monitored optical semi reflective surface -29-
110 and the second mirror 105. This will limit the ability to unambiguously distin- guish among changes in interferometer’s phase/path length caused by changes in the thin probe-film properties from the changes that are caused by temperature fluctuation.
Due to the typical configuration represented by FIG.1A, an operation involving two FPls sequentially will be in detail presented below.
Therefore, the sensor according to the present embodiment of FIG.1A contains a reference section of the tip-optical-fiber 111 of the fiber-optical sensor 100 with a first fiber section 107 as a temperature measurement/compensation section that provides information on the fiber-optical sensor’s temperature that is required for unambiguous and temperature in-depended determination of the character- ized thin probe-film properties.
According to present invention, close proximity of measurement and temperature sensing sections provides accurate temperature readout and compensation in challenging environment where large temperature gradients can occur or when temperatures are difficult to measure by separate sensors. Frequently encoun- tered environments where this problem is of particular concern are for example vacuum plasma assisted systems. Introduction of high energy release in reactors with very low pressures can lead to high temperature and poorly defined temper- ature profiles within reactors, which are further impacted by thermodynamic prop- erties of the sensors used for temperature sensing. According to the present in- vention, the temperature sensing section can be used for independent tempera- ture measurement, which is additional advantage of proposed sensor invention.
Especially in vacuum plasma-assisted systems, the information about the tem- perature, which is directly measured in plasma at the deposition point, has a high potential for optimization of target deposition layer characteristics. The target deposition layer might have temperature limitations, as maximum allowable tem- perature, or temperature elevation is needed for various structural properties such as are for example density, color, brightness, porosity, smoothness, refrac- tive index, etc. -23-
In vacuum plasma-assisted systems, the proposed sensor with two Fabry-Perot interferometers enables high-resolution measurement of the layer thickness (in the nanometer range) already in the initial phase of thin probe-film formation. This is hardly possible with principles based only on measurements of the reflectivity of the perpendicularly shaped/cleaved or angled shaped/cleaved fiber tip, or prin- ciples with one Fabry Perot resonator.
In the case of measuring the reflected optical power with perpendicularly shaped/cleaved or angled shaped/cleaved fiber tip, reflected optical power will go along the cosine curve from the extreme (max or min) onwards, according to the formation of a thin probe-film as explained in US 5,804,453 A which is incorpo- rated by reference herewith . When applying materials with a higher refractive index than fiber, the reflected optical power increases from the minimum towards the maximum, but when applying materials with a lower refractive index than the fiber, the reflected optical power will decrease from the maximum towards the minimum. However, in the case of a single Fabry-Perot resonator, high resolution cannot be achieved due to large temperature fluctuations in vacuum plasma-as- sisted systems.
The temperature measurement and compensation section 107 is defined by the fiber inserted in-between the first 104 and the second semi-reflective mirror 105, which together define the temperature measuring Fabry-Perot interferometer 107. Since the light is guided and confined by the fibers’ core in-between the first and the second mirror, the optical phase/path length in-between the two mirrors depends only on the fiber-optical sensor's temperature (assuming that fiber-opti- cal sensor 100 is not intentionally or un-intentionally exposed to external strains or stresses). This temperature related change in the fiber-optical sensor's phase/optical path length OPL is mainly consequence of the fiber’s refractive in- dex change. The temperature measuring Fabry-Perot interferometer thus allows for temperature measurement of the sensor and its immediate surrounding.
The fiber-optical sensor 100 according to a preferred embodiment modulates and partially reflects incident/incoming light that is guided to the sensor by the lead-in fiber 101. The reflected light is also guided through the led-in fiber in a backward - 24 -
direction, from the sensor to the optical signal interrogation system 200 as pre- sented on FIG.2.
The signal interrogation system performs light detection and extraction of meas- urement data from the back-reflected light signal. Depending on the applied sig- nal interrogation technique, data on the optical path-length change can be, for example, extracted independently from the temperature measurement interfer- ometer 107 and from the thin probe-film observing Fabry-Perot interferometer 108. Since the temperature measurement Fabry-Perot interferometers path- length depends only on the temperature, and while the thin-film observing Fabry-
Perot interferometer’s path-length depends on both the temperature and the thin probe-film’s properties, one can use the data on the path length change of the temperature measurement interferometer 107 to determine the path length change of thin-film observing Fabry-Perot interferometer 108 that is related only to the changes in thin-film properties.
In other words, the temperature measurement Fabry-Perot interferometer 107 path length change can be used to compensate, i.e. remove, temperature related influence from the path length change of the thin probe-film's observing Fabry-
Perot interferometer's 108. Such compensated path length change can be then directly related to the observed parameters of the characterized thin probe-film, regardless of the sensor temperature. Detailed example of temperature compen- sation process will be described further below. It shall be also stressed that tem- perature effects can be significant and can severely limit possibilities to monitor thin probe-film properties by similar fiber optic sensors without proper tempera- ture compensation. Therefore, temperature compensation process not only sig- nificantly improves, but also enables the effective measurements using fiber optic thin-film sensor. This is even more pronounced when Fabry-Perot interferometers with lengths well exceeding the optical wavelength are used for monitoring thin- film properties. Such longer interferometers can be interrogated by high-resolu- tion spectral interrogators 200 that enable very high-resolution tracking and de- termination in thin probe-film properties. „25 -
The temperature measurement and the thin probe-film characterization Fabry-
Perot interferometers can have a very broad range of lengths, depending on de- sired application and signal interrogation technique. In general, both resonators lengths can be typically between 10 um and 5 cm, more typically between 100 um and 1000 um and even more typically between 30 um and 500 um. Further- more, to allow for efficient and independent readout of temperature measurement and the thin probe-film characterization Fabry-Perot interferometer, both interfer- ometers can have different lengths. The typical length ratio between the thin probe-film characterization Fabry-Perot interferometer 108 and temperature measurement Fabry-Perot interferometer 107 can span over broad range of val- ues, for example between 1:100. However, more typical values would be in the range from 1:10 or between 1:5 and 1:1.5.
The small size and physical closeness of both interferometers also assure that temperature of both interferometers are substantially the same even when tem- perature gradients exists in the fiber-optical fiber-optical sensor’s environment.
This allows for highly-efficient temperature compensation and elimination of tem- perature influence from the measurement result, which further allows for stable and high resolution observation of thin probe-film properties, regardless of the environmental temperature.
The fiber-optical sensor 100 described above can be used in general applied in two principal applications: a) as a system that can monitor thin probe-film growth in various deposition or similar systems, for example, in physical deposition sys- tems as for example supporting or evaporation systems an in particular in all plasma assisted vacuum deposition systems; b) as an active thin probe-film mon- itoring systems in a pre-formed thin probe-film. In the latter case, an active thin probe-film is formed on the sensor tip that can selectively adsorb, absorb or oth- erwise interact with the species present in the sensor surrounding. The later use can be mainly found in chemical, biochemical, environmental and in bio-sensing systems. - 26 -
Signal interrogation and signal interpretation
There are several possible ways to realize temperature compensated thin probe- film characterization using the sensor described above. For example, both inter- ferometers can be chosen in a way to have different lengths. In this case, an especially convenient technique for the resolution of individual interferometer’s lengths is based on the acquisition and analysis of the sensors’ optical spectral response. In this instance, the back-reflectance of the sensor versus wavelength or optical frequency is acquired, for example by application of swept laser source and detector or combination of broadband source and optical spectrum analyzer as depicted in FIG.3 and FIG.4.
In the first instance a laser source 301 optical frequency is swept over predeter- mined frequency range, while the light reflected from the sensor is captured by the detector 302 to allow for recording of sensors spectral characteristic within limited spectral range. Similarly, when the broadband source 401 is used, optical spectrum analyzer 402 records the back reflected power as function of optical frequency (or wavelength), which also yields in sensors’ spectral characteristic with limited spectral range. Spectral ranges in which spectral characteristics can be recorded are limited by technical limitations of the spectral interrogation sys- tems (for example tunable source tuning range, or broadband source bandwidth).
Proper analysis of the recorded optical spectral characteristics allows unambigu- ous reconstruction of individual interferometers lengths.
In general, a single, low-finesse Fabry-Perot interferometer generates nearly si- nusoidal spectral characteristics in the domain of optical frequencies, as shown in FIG.5; it should be noted in this regard that the finesse of an optical resonator (cavity) is a measure for how narrow the resonances are in relation to their fre- quency distance is: unlike the situation depicted in Fig.5 a high finesse means sharp resonances.
The period of this optical spectral characteristic, also referred to as resonator’s free spectral range (FSR), and can be described by as: - 27 -
-
Av= 2nL (1)
Where Av is period of back-reflected optical spectral characteristics, n is the ef- fective index of the mode propagating in the resonator-forming fiber and L is the length of the resonator defined by two semi reflective surfaces that define the resonator.
Since in practice the expression for FSR is often expressed in wavelength à, thus the FSR can be also calculated as:
FSR = — (2)
Where A is central wavelength of used optical source. Also it should be noted, that L is to be distinguished from the optical path length (OPL), also known as optical length (or optical distance). The optical path length (OPL) is the product of the geometric length L of the optical path followed by light and the refractive index n of homogeneous medium through which a light ray propagates; for inho- mogeneous optical media, the product above is generalized as a path integral as part of the ray tracing procedure. A difference in OPL between two paths is often called the “optical path difference” (AOPL or OPD). OPL and OPD determine also the phase of the light and governs interference and diffraction of light as it prop- agates.
When multiple Fabry-Perot interferometers are cascaded, as it is the case in the presented invention, multiple frequencies will appear in the back-reflected optical spectrum. It should be noted that each combination of pairs of mirrors and/or semi reflective surfaces in the cascaded interferometric structure contributes to a for- mation of new frequency in the back-reflected optical spectrum. For example, in the present invention there are three semi reflective surfaces, two mirrors and the monitored optical semi-reflective surface. -08 -
This will yield in three frequency components present in the back-reflected optical spectrum: one frequency component will correspond to the thin-film observing
Fabry-Perot interferometer optical length, one frequency will correspond to the temperature measuring Fabry-Perot interferometer optical length, while the high- est frequency in the spectrum will be defined by the sum of optical path lengths
OPLs of thin-film observing Fabry-Perot interferometer and the temperature measuring Fabry-Perot interferometer.
To make this clearer, FIG.6 shows an example of a sensor spectral characteris- tics according to the present invention, where the thin-film observing Fabry-Perot interferometer and the temperature measuring Fabry-Perot interferometer are 20 um and 200 um long respectively. It is thus convenient to perform spectral domain analysis, i.e. applying Fourier transform of fast Fourier transform to the recorded spectral characteristics.
An example of a Fourier transform of the exemplary spectral characteristic is pre- sented in FIG.7. Three distinctive peaks in the characteristics are present, corre- sponding to three different optical path lengths OLs: the first peak 701 (peak ap- pearing at the lowest frequency) corresponds to the shortest (20 um) resonator (OPL1), the middle peak 702 corresponds to the longer (200 um) resonator (OPL2), while the third peak 703 corresponds to the sum OPL_sum of both res- onator’s path lengths (OPL1+OPL2=OPL_sum; 20pm +200um =220 um).
Both the temperature changes and/or the changes in any of a thin probe-film properties will cause changes in the optical paths of interferometers, which further leads to the changes in the spectral fringe frequency and position change. By applying Fourier analysis to the back-reflected spectrum, overall length and length changes of individual interferometers can be thus unambiguously deter- mined. - 29.
Thus, small changes in optical length/phase of the observed interferometer causes changes in the position (phase) of the sinusoidal pattern in the back-re- flected spectrum, which can be further determined by applying Fourier transfor- mation over the acquired spectral characteristics, while observing the phase com- ponent of the Fourier transform at the frequency that corresponds to the optical length of the observed interferometer. This phase change, could be calculated by
Fourier transform and can be directly converter in optical path length change (AOPL) by:
AOPL = 424 (3)
ATT
Where À is optical wavelength used for spectral interrogation, A® is phase change in the Fourier transformed spectrum at frequency that corresponds to the absolute peak position, further corresponding to the initial interferometer length.
AOPL is linearly proportional to the phase change in the observed interferometer, caused by temperature change or thin probe-film properties change like thin probe-film thickness, refractive index or density change.
The optical path length change of the temperature measuring Fabry-Perot inter- ferometer AOPL1 can be calculated as the product of thermos-optical coefficient, interferometer’s length and the change in temperature:
AOPL =dn/dT-L,-AT (4)
Where dn/dT represent refractive index change over temperature that corre- sponds in case of silica fiber to about 10° K*, AT is the temperature change, and
L1 is length of the temperature measuring Fabry-Perot interferometer.
The optical path length change in the thin-film observing Fabry-Perot interferom- eter AOPL:z is function of both temperature change and change of the thin probe- film’s length TFL: -30 -
AOPL, = 1 AT +n, TFL (9) ar
Where dn/dT represent refractive index change of the fiber due to temperature, which corresponds in the case of silica fiber to about 10° K”, AT is the tempera- ture change, Lz is length of the thin-film observing Fabry-Perot interferometer and ntre is the refractive index of the observing thin-film.
To obtain temperature independent sensing of thin probe-film’s optical properties other than absorption, optical path length changes AOPL1 , AOPL2 of both inter- ferometers shall be subtracted. However, this subtraction shall be weighted by interferometers’ lengths ratios as interferometers’ length directly influence the temperature sensitivities of spectral phase fringes.
The observed change in the thin probe-film’s length (TFL) can be then expressed:
L, 1 6
TFL =| AOPL, — 22. AOPL, |— (6)
L, Mrez,
Since the changes in optical lengths are usually very small in the present inven- tion, tracking of phases of the interference fringes in the optical spectrum can be one of methods to determine very small changes in thin probe-film properties and temperature. With respect to equation 3 and equation 6, the observed change in the thin probe-film’s length TFL can be also written with introduction of phase changes of both interferometers as:
TFL=| Ad, ag, | A (7)
L 47,
Where A®2 is phase change of thin-film observing Fabry-Perot interferometer and
A® 1 is phase change of the temperature measuring Fabry-Perot interferometer. - 31 -
Other signal interrogation methods, processing of analysis of back-reflected op- tical signals are also possible and the above described method is not the only possible approach in determining the thin probe-film’s length. Direct tracking of individual peaks in the back-reflected spectrum, measuring and analyzing reflec- tions at the pre-set and different optical wavelengths or similar approaches could be also applied.
Observing of the magnitude of the interface fringes could be for example used to assess the optical absorption in monitored thin probe-film, for example if the am- plitude of the interface fringe corresponding to the 2nd interferometer 108 decays over time and during thin probe-film deposition, this decay can be correlated to the appearance of the optical loss in the monitored thin probe-film It shall be ap- parent to the person skilled in the art that different signal interrogation and pro- cessing approaches shall be applicable to the sensor to extract desired thin probe-film parameter that is independent of the surrounding temperature.
It should be also stressed that according to the present invention, longer lengths of thin-film monitoring and temperature monitoring resonators can be chosen, while not composing temperature stability of the sensing system (longer resona- tors yield in higher temperature sensitivity, which can be efficiently compensated according to the present invention). Longer resonators provide a distinctive ad- vantage: according to Equ.2, longer resonators provide spectral responses with narrower free spectral ranges. This means that longer resonators require spectral integrators with narrower frequency acquisition ranges, which might significantly simplify and reduce the cost of an interrogation system. For example, at 1550 nm free spectral range of 1 mm long fiber based resonator will be about 0.8 nm.
To fully analyze position of the spectral fringe, the interrogator shall be in principle capable of acquiring spectral characteristics of the sensor over at least one full spectral fringe, i.e. 0.8 nm in above described example. Such a narrow spectral range can be acquired with very cost effective systems based, for example on, standard telecom DFB diodes, as for example described in US 9,948,061 B2 and
US 9,373,933 B2, which are incorporated by reference herewith. It should be thus -32-
apparent to the person skilled in the art that the freedom in choosing the resonator length within hundreds of micrometers or even millimeter range can significantly contribute to the simplification and cost reduction of the entire sensing system, which is distinctive advantage of the present invention.
Further fiber-optical sensors
Since the reference sensor can be any sensor with a waveguide structure that contains a core 102 and cladding 103 and allows temperature measurement, the first FPI sensor 107 can be replaced, for example, with Bragg Grating-based tem- perature sensor.
Two further possible configurations of the fiber-optical sensor device 10 are pre- sented in preferred embodiments of FIG.1B and FIG.1C.
Also therein the fiber-optical sensor 100 is formed at the tip of a lead-in optical fiber 101 from a tip-optical-fiber 111 extending from the lead-in optical fiber 101.
Therein the tip-optical-fiber 111 has a core 102 and a cladding 103 that is adapted to guide light towards and away from the tip-optical-fiber 111.
It will be clear to one skilled in the art that the designation of “lead” optical fiber refers to the use of this fiber as a conduit for the sensing light signal and is not meant as representative of any particular property of the optical fiber itself.
The tip-optical-fiber 111 includes a reflective reference section 112 consisting of the fiber structure that has characteristics of the temperature sensor and sensing second fiber section 108 which defines the thin-film observing Fabry-Perot inter- ferometer.
The reflective reference section 112 is preferably as closest as possible to the fiber-optical sensor's end-surface 106. In a preferred embodiment the reflective reference section 112 is placed somewhere in between end-surface 106 and semi-reflecting mirror 104. -33-
In a second embodiment as shown in FIG.1B the reflective reference section 112 is placed between a second semi-reflecting mirror part 105.2 and semi-reflecting mirror 104. As explained a first fiber section 107 with a first semi-reflective mirror 104 is provided, wherein the first semi-reflective mirror 104 is provided as an in- fiber mirror of the tip-optical-fiber 111.
Further a second fiber section 108 of the tip-optical-fiber 111, with a first semi- reflective mirror part 105.1 is provided, wherein the first semi-reflective mirror part 105.1 is provided as an in-fiber mirror of the tip-optical-fiber 111, and a fiber- optical sensor’s tip end-surface 106 at a sensing side of the fiber-optical sensor 100 adapted to receive the thin probe-film 109.
Therein a first semi-reflecting mirror part 105.1 and the fiber-optical sensor’s end- surface 106 is defining the second fiber section 108 of the tip-optical-fiber 111 of the fiber-optical sensor 100.
Between the first semi-reflecting Mirror part 105.1 and the second semi-reflecting mirror part 105.2 the tip-optical-fiber 111 is extending. A section between sur- faces formed by the semi-reflective first and second semi-reflecting mirror part 105.2 and 105.1 can be a piece of a fiber; in particular any kind of fiber. Preferably however the fiber is basically of identical structure as the fiber of the Fabry-Perot interferometer and the reflective reference section.
It should be noted, that the fiber-optical sensor is not necessarily to have one common semi-reflective surface in form the second semi-reflective mirror of the second fiber section. As should be clear therefrom both sensing sections of the tip-optical-fiber —i.e. the reference section and the second fiber section-- can be separated by a fiber portion of any kind; still not too far away respectively not too long to keep the neighborhood for measurement purposes, of course all as part of said tip-optical-fiber. In the instant development, as an example, the separating fiber portion of said tip-optical-fiber is realized between said first and/or second semi-reflecting mirror part. - 34 -
As shown in the embodiments of FIG.1C and FIG.1D the fiber-optical sensor de- vice 10 provides for the second semi-reflective mirror 105 and the end-surface 106 to form a Fabry-Perot interferometer adapted to sense the thin probe-film’s optical parameter at the tip end-surface 106.
For sensor temperature sensing of the tip-optical-fiber 111 the reflective refer- ence section 112 with the reflective fiber optical structure 114 is provided to define an optically reflecting element of the reference section 112 adapted to sense an optical path length OPL of the reference section 112. The reflective reference section 112 is provided with a reflective fiber optical structure 114 in form of a first and second semi-reflective mirror part 104.1, 104.2 as shown in FIG.1C in prin- ciple.
Further the second semi-reflective mirror 105 is placed at a coupling side of the fiber-optical sensor 100 where said tip-optical-fiber 111 extends from the lead-in optical fiber 101.
Also in a second embodiment the reflective reference section 112 is placed near the second fiber section 108 as presented in FIG.1C.
In particular, therein the reflective reference section 112 is placed in between end-surface 106 and the semi-reflecting mirror 105.
As shown in the embodiment of FIG.1D also the reflective reference section 112 can be provided with a reflective fiber optical structure 114 in form of a Bragg- reflector’s grating.
If the temperature sensor is Bragg Grating, the Bragg Grating can be also in- scribed within or over the sensing FPI as presented in FIG.1D. Thus, also in a third embodiment the reflective reference section 112 is placed near the second fiber section 108 as presented in FIG.1D. -35-
In particular therein the reflective reference section 112 is placed in between end- surface 106 and semi-reflecting mirror 105 wherein the Bragg Grating extends along the second fiber section 108.
The signal processing which is used for extracting the thin probe-film’s length can be similar as described above, with monitoring whole reflected spectrum, and can be for example of Bragg Grating as temperature sensor expressed in form:
TFL =(AOPL, - K - Ady, 1) + (8)
Argy,
Where AOPLa is the optical path length change in the thin-film observing Fabry-
Perot interferometer, AAsc is Bragg grating’s wavelength shift, La is length of the thin-film observing Fabry-Perot interferometer, n7r: is the refractive index of the observing thin-film, and K is constant obtained during the temperature calibra- tion of the sensor.
Sensor applications
Sensors according to present invention can be append in various and different applications.
The fiber-optical sensor device 10 with the fiber-optical sensor 100 formed by an optical fiber of the tip, said tip-optical-fiber 111 extending from the lead-in optical fiber 101, can for example be applied to measure optical thickness (OPL) of the thin probe-films formed at the tip of the sensor structure.
On-line and off-line monitoring is possible. For example, the sensor can be used to on-line monitor formation and growth of thin probe-films, for example in great varieties of vacuum deposition processes (sputtering, CVD, MOCVD, etc.). In this case, the sensor shall be exposed to a space where the thin probe-film is formed, for example, it might be mounted into deposition vacuum chamber, where thin probe-film deposition occurs. Furthermore, all-slica structure of the sensor in this group of applications offers and unique advantage: in case of many deposited - 36 -
film types, the sensor can be recovered or reset after the film deposition by clean- ing of the sensor, for example by chemical cleaning in a suitable chemical solution that removes the deposited thin probe-film, but does not affect the sensor/fiber.
This is possible since the silica fiber is inert and can tolerate cleaning with a broad variety of chemical agents, such as acids (except hydrofluoric acid), solvents, etc. even at elevated temperatures. Even cleaning for a limited time by hydrofluoric acid might be possible in some cases. Mechanical cleaning, such as polishing of the sensor tip, might be also applied.
The active thin probe-film can be also pre-deposited to the sensor according to the present invention to form an integral part of the sensor. Active thin probe-film can be designed to selectively absorb, adsorb or otherwise, preferably selec- tively, capture physical, chemical, biochemical and biological species. Such films can be deposited using any suitable thin-film formation or deposition process, including but not limited to physical deposition (like sputtering, evaporation, dip- coating, etc.) or chemical depositions (for example using chemical vapor deposi- tion n (CVD), sol-gel process, various surface reactions, solution-bonding, etc.).
Active thin probe-films can be designed to selectively capture/absorb/adsorb tar- get species, which are adapted to be sensed. These might include monitoring of active thin biofilms, including but not limited to label free detection systems. Ac- cording to the present invention, monitoring of active thin probe-films with high resolution shall be possible.
Sensor manufacturing
There are many ways how the sensor can be produced. For example, short sec- tions of optical fibers can be spliced to the lead-in fiber, while creating semi-re- flective mirrors in between spliced segments. Semi-reflective mirrors in between spliced segments can be created by different methods. For example, semi-reflec- tive mirror can be created by vacuum deposition of high refractive index material on the fiber tip followed by splicing of such fiber with the lead-in fiber as described in US 5,237,630 A or by etching of standard fiber and re-splicing it to the lead-in - 37 -
fiber as described in Cibula, E. and D. Donlagic, Low-loss semi-reflective in-fiber mirrors. Optics Express, 2010. 18(11): p. 12017-12026; which is incorporated by reference herewith. Other methods for production in fiber mirrors might be also used.
Typical application demonstration
FIG.8 demonstrates an operational situation of a fiber-optical sensor system 20 with a preferred embodiment of the fiber-optical sensor device 10 for monitoring of thin probe-film deposition in an magnetron sputtering system. The fiber-optical sensor 100 formed by an optical fiber of the tip, said tip-optical-fiber 111 extend- ing from the lead-in optical fiber 101, was placed in an deposition system (Semi- cor 450SC) while engaging deposition of silica films.
The temperature measurement interferometer and thin probe-film monitoring in- terferometers were 280 um and 103 um long respectively in this particular exam- ple. The interrogation of the sensor was accomplished by a fiber optic spectral interrogation system (Micron Optics si155).
FIG.9A shows power values applied to the lead-in fiber and thus to the fiber- optical sensor over time. FIG.9B shows correspondingly optical path length changes AOPL of the Fourier transform component of the acquired optical spec- trum at frequencies corresponding to temperature monitoring reference section of the tip-optical-fiber 111 of the fiber-optical sensor 100 and the thin-film moni- toring second fiber section’s 108 interferometer respectively.
FIG.9C shows a respective optical path length change of the thin probe-film mon- itoring interferometer with included temperature compensation; this being deter- mined according to the equations mentioned above and data from FIG.9B. Re- spectively FIG.9D shows a length change corresponding to the optical path length change of FIG.9C and as indicated with the formula in FIG.9D. - 38 -
List of reference signs (Part of the description) fiber-optical sensor device fiber-optical sensor system 100 fiber-optical sensor 101 lead-in optical fiber 104 first semi-reflective mirror 104.1, 104.2 first and second semi-reflecting mirror part 105 second semi-reflective mirror 105.1, 105.2 first and second semi-reflecting mirror part 106 fiber-optical sensor’s end-surface 107 first fiber section 108 second fiber section 109 monitored thin probe-film 111 tip-optical-fiber 112 reflective reference section 114 reflective fiber optical structure
OPL, AOPL optical path length, optical path length change -39-
Claims (25)
1. À fiber-optical sensor device (10) for monitoring of a thin probe-film (109) re- ceivable at a sensing side of a tip of the fiber-optical sensor device by measuring of a thin probe-film’s (109) optical parameter, the fiber-optical sensor device (10) comprising: - a lead-in optical fiber (101), - an optical sensor provided to the lead-in optical fiber (101), characterized in that - the optical sensor is a fiber-optical sensor (100) formed by an optical fiber of the tip, said tip-optical-fiber (111) extending from the lead-in optical fiber (101), and the fiber-optical sensor (100) is adapted to provide an optical signal indicative of an optical path length (OPL) at the tip-optical-fiber (111) and comprises: - a reference section of the tip-optical-fiber (111) of the fiber-optical sensor (100) with a first fiber section (107) with a first semi-reflective mirror (104), wherein the first semi-reflective mirror (104) is provided as an in-fiber mirror of the tip-optical-fiber (111), and/or a reflective reference section (112) with a reflective fiber optical structure (114); - a second fiber section (108) of the tip-optical-fiber (111), with a second semi-reflective mirror (105), wherein the second semi-reflective mirror (105) is provided as an in-fiber mirror of the tip-optical-fiber (111), and a fiber-optical sensor’s tip end-surface (106) at a sensing side of the fiber- optical sensor (100) adapted to receive the thin probe-film (109).
2. The fiber-optical sensor device (10) according to claim 1, wherein - the second semi-reflective mirror (105) and the end-surface (106) form a Fabry- Perot interferometer adapted to sense a thin probe-film’s optical parameter at the tip end-surface (106), and - 40 -
- for sensor temperature sensing of the tip-optical-fiber (111) the first semi-reflective mirror (104) and the semi-reflective mirror (105) and/or the reflective reference section (112) with the reflective fiber optical struc- ture (114) define an optically reflecting element of the reference section (112) adapted to sense an optical path length (OPL) of the reference section (112).
3. The fiber-optical sensor device (10) according to claim 1 or 2, wherein - the first semi-reflective mirror (104) is placed at a coupling side of the fiber- optical sensor (100) where said tip-optical-fiber (111) extends (FIG.1A, FIG.1B) from the lead-in optical fiber (101).
4. The fiber-optical sensor device (10) according to claim 1 or 2, wherein - the second semi-reflective mirror (105) is placed at a coupling side of the fiber- optical sensor (100) where said tip-optical-fiber (111) extends (FIG.1C, FIG.1D) from the lead-in optical fiber (101).
5. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein the lead-in optical fiber (101) is joint to the second semi-reflective mirror (105), wherein the second semi-reflective mirror (105) is joint to the second fiber section (108) of the tip-optical-fiber (111).
6. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein the distal end of the second fiber section (108) of the tip-optical- fiber (111) defines a fiber-optical sensor's end-surface (106) adapted to accept a monitored thin probe-film (109), wherein the monitored thin probe-film (109) is allowed to deposit to the fiber-optical sensor’s end-surface (106) which is adapted to accept the monitored thin probe-film (109).
7. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein - 41 -
- the first semi-reflective mirror (104) and the second semi-reflective mirror (105) define a Fabry-Perot interferometer adapted for temperature sensing, and/or - the reflective reference section (112) is adapted for temperature sensing, wherein the reflective fiber optical structure of the reflective reference section (112) forms a Bragg-reflector.
8. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein - the second semi-reflective mirror (105) is provided as a part of a mirror arrange- ment, wherein the mirror arrangement has a first and second semi-reflecting mir- ror part (105.1, 105.2) with a part of the tip-optical-fiber (111) extending there- between the first and second semi-reflecting mirror part (105.1, 105.2), and wherein - the first and/or second semi-reflecting mirror part (105.1, 105.2) and the end- surface (106) form a Fabry-Perot interferometer.
9. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein - the reference section (112) provides for the second semi-reflective mirror (105) or - a second semi-reflecting mirror part (105.2) and/or a first semi-reflecting mirror part (105.1).
10. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein - between a second semi-reflecting mirror part (105.2) and a first semi-reflecting mirror part (105.1) extends the tip-optical-fiber (111).
11. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein - the reflective reference section (112) is placed into the second fiber section (108) of the tip-optical-fiber (111). „42 -
12. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein - the reflective reference section (112) extends in between (FIG.1D) end-surface (106) and semi-reflecting mirror (105) and is placed into the second fiber section (108) of the tip-optical-fiber (111).
13. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein - the reflective reference section (112) is placed into the second fiber section (108) of the tip-optical-fiber (111) and (FIG.1D) is distant from the end-surface (106) and semi-reflecting mirror (105) of the tip-optical-fiber (111).
14. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein for monitoring of thin probe-film parameters the thin probe-film parameters are one or more parameters or their combinations from the group of thin probe-film thickness, thin probe-film refractive index, thin probe-film density, thin probe-film optical absorption.
15. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein for monitoring of thin probe-film parameters an active thin probe- film is preformed at the surface adapted to accept monitored thin probe-film.
16. The fiber-optical sensor device (10) according to any one of the preceding claims, adapted for monitoring of thin probe-film parameters, the active thin probe-film has ability to selectively capture target monitored species.
17. The fiber-optical sensor device (10) according to any one of the preceding claims, adapted for monitoring of the thin probe-film parameters, in particular when a target species is chemical, bio-chemical or biological species. - 43 -
18. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein the length of the first and the second fiber section (107, 108) of the tip-optical-fiber (111) is between 10 um and 5 cm, in particular between 100 um and 1 cm, in particular between 200 um and 1 mm.
19. The fiber-optical sensor device (10) according to any one of the preceding claims, wherein the ratio of lengths of the first and the second fiber section (107, 108) of the tip-optical-fiber (111) are between 1:100 and 100:1 or between 1:10 and 10:1 or between 0.3:1 and 1:5.
20. A fiber-optical sensor system (20) for monitoring of thin probe-film parameters comprising the fiber-optical sensor device of any of the claims 1 to 19 and - an optical signal interrogation system wherein the optical signal interrogation system is connected to the fiber-optical sensor (100) formed by said tip-optical-fiber (111) extending from the lead-in op- tical fiber (101) via the lead-in optical fiber (101), and wherein the optical signal interrogation system performs readout of an optical signal of the fiber-optical sensor (100) of the fiber-optical sensor device (10), the optical signal being indicative of an optical path length at the tip-optical-fiber (111).
21. The fiber-optical sensor system (20) according to claim 20, wherein the optical signal interrogation system is adapted to interpret the optical signal of the fiber- optical sensor (100) of the fiber-optical sensor device (10) in terms of a thin probe- film’s (109) parameters and a sensor temperature.
22. The fiber-optical sensor system (20) according to claim 20 or 21, wherein the optical interrogation system is a spectrally resolved system that records the opti- cal signal of the fiber-optical sensor (100) as a recorded spectrum, namely the - 44 -
optical signal as back-reflected optical power versus optical frequency over a lim- ited spectral range of the recorded spectrum.
23. The fiber-optical sensor system (20) according to any of the claims 20 to 22, wherein characteristics of the recorded spectrum are further processed to track positions of spectral interference fringes positions or phases with frequencies, wherein the spectral interference fringes and/or frequencies are assigned to be- long to a second fiber section (108) of the tip-optical-fiber (111), in particular thin probe-film monitoring, and/or a reference section of the tip-optical-fiber (111), in particular a temperature monitoring, preferably by a Fabry-Perot interferometer or a reflective ref- erence section (112) forming a Bragg-reflector.
24. The fiber-optical sensor system (20) according to claim 22 or 23, wherein the limited spectral range of the recorded spectrum of the optical signal of the fiber- optical sensor (100) is between 0.5 nm and 200 nm.
25. The fiber-optical sensor system (20) according to any one of the claims 20 to 24, wherein - the thin probe-film properties are measured independently from the tempera- ture, and/or - the thin probe-film properties and temperature are measured simultaneously. -45-
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