Planar ultra thin glass seals with optical fiber interface ...

Optics and Lasers in Engineering 98 (2017) 89–98

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Planar ultra thin glass seals with optical fiber interface for monitoring tamper attacks on security eminent components M. Thiel a, G. Flachenecker a,∗, W. Schade a,b, C. Gorecki c, A. Thoma c, R. Rathje c a

Department of Fiber Optical Sensor Systems, Fraunhofer Heinrich-Hertz-Institute, Am Stollen 19 H, 38640 Goslar, Germany Department of Applied Photonics, Institute of Energy Research and Physical Technologies, Clausthal University of Technology, Am Stollen 19 H, 38640 Goslar, Germany c OHB-System AG, Universitätsallee 27-29, 28359 Bremen, Germany b

a b s t r a c t Optical seals consisting of waveguide Bragg grating sensor structures in ultra thin glass transparencies have been developed to cover security relevant objects for detection of unauthorized access. For generation of optical signature in the seals, femtosecond laser pulses were used. The optical seals were connected with an optical fiber to enable external read out of the seal. Different attack scenarios for getting undetected access to the object, covered by the seal, were proven and evaluated. The results presented here, verify a very high level of security. An unauthorized detaching and subsequent replacement by original or copy of the seals for tampering would be accompanied with a very high technological effort, posing a substantial barrier towards an attacker. Additionally, environmental influences like temperature effects have a strong but reproducible influence on signature, which in context of a temperature reference database increases the level of security significantly. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction In general, seals are used for guaranteeing the integrity of the protected object. Seals must allow the detection of unauthorized access to protection areas or for identification of manipulation on objects with security relevance. For discovering such actions, the seals have to be examined regularly, and have to be compared with their original status. For this, an inspection strategy for securely proving the originality status of the seals, as well as secure techniques for mounting the seal at the object have to be developed. For prevention of replacement by a copy or reconstruction of the original status, seals often are configured with an individual patterns or signatures. Depending on the seal technology, the status of the seals can be evaluated either simply by naked eyes or with the help of a reading system. Johnston [1] divided the type of seals in active and passive categories. Typical passive seals are metal and plastic stripes with a special coinage or adhesive labels, which in theory cannot be opened without causing obvious damage of the seal. Active seals are for example electronic seals, which continuously monitor for some kind of change indicative of tampering. Fiber optic seals periodically or randomly send light pulses down a fiber optic bundle to check continuity. Radio frequency identification devices (RFID) can be used either as active or passive seals. The advantage of RFID systems is a contact-free communication, information storage and remote control [2]. Optical techniques like holograms, color shift inks or UV inks and prints widespread are used for security and product authentica-

∗

Corresponding author. E-mail address: guenter.fl[email protected] (G. Flachenecker).

http://dx.doi.org/10.1016/j.optlaseng.2017.06.007 Received 31 October 2016; Received in revised form 2 May 2017; Accepted 10 June 2017 0143-8166/© 2017 Elsevier Ltd. All rights reserved.

tion. However, since hologram-producing equipments are commercially available the effort for making counterfeits is getting less. Aggarwal et al. [3] have proposed a method for incorporating concealed coding features in security holograms in the form of moiré patterns, which need an encoded key hologram to decode them. Alvine et al. [4] have prepared thin films, containing optical resonant nanostructures, which cannot be produced by visible imaging technology. These films can be tailored to have resonant features anywhere within the visible, near infrared spectrum, and beyond or can be designed to be sensitive to the polarization of light collected from surface reflection. But simpler, a diffuse reflecting surface of a package or label itself can be used as random pattern and identified by the use of coherent light sources, acquiring with laser speckle scattering or white light scattering photography. Similar techniques for analysis of such speckles can be applied for sprayed UV ink speckles as a purely random pattern [5]. In summary, current optical seals in combination with optical reading systems have very high capabilities for identification of original state of seal, manipulations or replacement by counterfeits. However, for practical use of these optical seals restrictions in real world applications like the inadequate accessibility, weight and size of the reading systems or the technological effort very often prevents commercial applicability. In this publication, we report about an optical seal, based on ultra thin glass slides, which can be read out by fiber optics with an external reading system. For this, we have introduced in the glass slides an optical signature of sensor elements via femtosecond laser technology. A

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Table 1 The signature of the optical seals depends sensitively on the state of polarization of the light source, deformation of the glass plate as well as the surrounding temperature of the seal.

Fig. 1. Layout of an optical seal with sensory Bragg grating structures implemented in optical waveguides in the seal. Both, waveguides and Bragg gratings are directly generated with femtosecond laser pulses in a depth of 50 μm below the surface of the ultra thin glass substrate.

Fig. 2. The signature of the seal is characterized by the back reflection spectra of four Bragg gratings (BG1-4). Additionally a Bragg grating was introduced in a glass fiber (FBG), coupled to the seal for compensating temperature effects in connection with a thermal database.

demonstration system of optical seals was developed for covering sensitive electronic elements on electronic boards for a satellite communication system. A manipulation of such communication electronics is a high risk for security. For that reason, a security check for tamper attacks has to be performed directly before the takeoff of the rocket containing the satellite. Opening or disassembling the satellite for tamper detection would be a big effort and less practicable, as the communication electronics is deeply integrated inside the satellite. The fiber coupled optical seals, we present here enable a proof of state by connection of an optical fiber to an easily accessible interface outside or may be used for remote access as well as permanent monitoring.

pulses at 800 nm with a repetition rate of 5 kHz. The glass samples were mounted on top of three precision linear stages (XMS series from Newport). With a 𝜆/4 zero order waveplate the linear polarization of the laser was transformed into circular polarization. The laser was focused with a 20×, 0.4 NA microscope objective 50 μm below the surface of the glass sample, which was moved linearly horizontal with constant velocity of 1 mm/s. By moving the glass sample under the laser focus a single line (scan) with a positive refractive index contrast to the bulk material is generated. To enhance the transmission for 800 nm NIR light, the diameter of the waveguide has to be increased. To achieve this, we used a multi scan technique to form an effective waveguide with larger diameter consisting of a set of 49 parallel single waveguide scans in the volume, which we call waveguide bundle. The diameter of this waveguide bundle was around 6 μm. For the central scan, we introduced a periodically segmented scan for generation of a Bragg grating structure. By this way, parts of the central scan are modified by single laser pulses, while the glass keeps unaffected in between and a maximum refractive index contrast can be achieved. The effect of this Bragg grating is like a spectral mirror: If light with a broad spectrum is coupled into the waveguide Bragg grating a spectrally small part is reflected back, depending on the periodicity of Bragg grating. The reflection wavelength is calculated by the Bragg equation: m𝜆 = 2ne Λ where m is the grating order, 𝜆 the Bragg wavelength, ne the effective refractive index and Λ the distance between the grating points. The Bragg grating signals depend sensitively on external effects like temperature changes or mechanical stress in the glass substrate.

2. Experimental 2.1. Femtosecond laser processing of sensory waveguide structures in ultra thin glass Ultrafast Pico- and Femtosecond lasers are very well suited for cutting, drilling or structuring glass materials due to low thermal effects and nonlinear optical interactions during laser processing [6,7]. Furthermore, a structural change of transparent material can be introduced within the laser focus of a femtosecond lasers in bulk glass materials. Depending on the material, in most cases this results in a slight increase of refractive index. In the past this effect was successfully used for direct implementation of three–dimensional optical waveguide structures in transparent bulk glass for sensory applications or for miniaturized integrated optics [8–10]. We have applied this technique for implementation of waveguide Bragg grating elements in 100 μm thick AF32® Eco Thin Glass plates from Schott. For this we used a multi scan procedure, which we have published in Thiel et al. [11] and will be explained in short here: we used a Ti: Sapphire laser providing 100 fs laser 90

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nature of the seal, i.e. the measurable security pattern. Characteristic attributes are the wavelength, amplitudes as well as the line form of the Bragg signals. In addition, every line has an individual dependency on the polarization of the light, distortion of the seal and temperature.

2.3. Optical reading system The optical reading system consists of a fiber coupled broadband SLED with a peak wavelength at 840 nm (3 dB bandwidth of 50 nm), a 2 × 2-fiber coupler and a grating spectrometer (“Flame” from Ocean Optics, spectral range 277 nm, resolution 0.4 nm). The reading system is a separate unit, which is connected to the glass fiber of the optical seal for evaluation of the signals (Fig. 3). The broad band light from the SLED is transmitted to the optical seal and partially reflected at the waveguide Bragg grating sensors in the ultra thin glass slide. The response of the seal is a characteristic spectral pattern. This spectrum is reflected back and coupled into the spectrometer for evaluation.

Fig. 3. Concept of an optical seal, connected to read out system (SLED, Spectrometer and Coupler). A broadband SLED light source (FWHM ≈ 100 nm) is connected to the optical seal via a glass fiber at the front end of the ultra thin glass. The light of the SLED is partially spectrally reflected at the Bragg gratings. The signature of the seal is interrogated, by meaning the spectral analysis of the Bragg grating reflections. For this, the reflected light of the seal is guided via an optical coupler into a spectrometer for analysis.

2.4. Environmental conditions and evaluation of signature The finger print characteristics of the signature are the spectral position, amplitude and line shape of the Bragg signals which additionally depend individually from the polarization state of the light source, the deformations of the glass slight as well as from temperature (Table 1). This additional set of environmental parameters can be used to increase the individuality of the signature. On the other hand, these parameters have to be measured very precisely, as well. In our work, we kept the state of polarization always constant by fixing the fiber optics or using polarization maintaining optical components. For proofing the state of seal, a new measurement of the signature has to be repeated and compared to the original signature. The evaluation of the signature has to be acquired automatically and checked if the measured values are within a defined band of tolerance. While an automatic measurement of the peak’s position and amplitude is straight forward, the comparison to the line shape of the original spectrum and evaluation for identification is more sophisticated, and can be done by the following procedure: Each peak in the spectrum of the new measure is compared to the corresponding line shape of the original spectrum. For this, the peaks are normalized and subtracted from each other. For the evaluation of

2.2. Optical design of the seals and signature The geometrical size of the seals was 50 × 50 × 0.1 mm3 . The optical waveguides and the Bragg grating sensor structures were generated via femtosecond laser pulses inside the ultrathin glass seal, 50 μm below the surface. The following design was realized: We introduced a straight waveguide, starting from the border of the glass substrate, which is divided into two symmetrical waveguide branches. In each waveguide branch, two serial arranged Bragg grating structures are implemented (see Fig. 1). Each of the four Bragg gratings was created with 18 mm length and different reflection wavelengths. The light of a spectral broad light source is butt coupled via glass fiber from the front face of the seal to the single ended waveguide. The glass fiber is fixed with UV-curable adhesive. Additionally, for compensation of temperature induced signal changes a fiber Bragg grating was written into the glass fiber with femtosecond lasers. spectrum of such an optical seal is shown in Fig. 2. The individual spectrum of the Bragg reflections represents the sig-

Fig. 4. Comparison of line shapes. Similar spectra (A,A2 or B,B2) are represented by a much smaller line shape parameter F in comparison to non similar spectra (A,B or A2,B2).

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Fig. 5. A security relevant module on electronic board, covered with the optical seal. The electronic component is surrounded by a double-wall frame. Inside the fiber, connected to the seal, is winded several times, in order to prevent for a side access by drilling. The optical seal is fixed on top of the frame and on top of the electronic component by high temperature resistant adhesive.

the residual a “line shape parameter” F is calculated:

2.5. Integration of optical seal on electronic board For identification of a tamper attack on an electronic communication chip with a size of 30 × 30 mm2 , first the ultra thin glass was prepared with a signature. Subsequently, one side of the optical seal was contacted at the front face with an optical fiber by using UV-curable adhesive (OP-4-20,632-GEL, DYMAX Europe GmbH). A double-wall frame made of composite material (FR4) was glued on the electronic board, surrounding the electronic chip. The glass fiber, connected with the seal, was coiled inside the gap of the double-wall and fixed with adhesive. The end of this fiber was lead out for connecting the optical reading system. The optical seal was fixed in the center of the electronic chip as well as on top of the frame with special high temperature resistant epoxy adhesive (3 M, Scotch-Weld 2216) in a bended state. A defined deformation of the optical seal was introduced by loading it with 200 g during hardening of the adhesive (Fig. 5).

𝜆1

𝐹 =

|𝑆 (𝜆) − 𝑆 (𝜆)| 𝑑𝜆 𝑁 | ∫ | 𝑂

𝜆0

Here SO (𝜆) is the normalized original spectrum of the signal and SN (𝜆) is the normalized spectrum of the new measure. For illustration, we have compared the line shape parameter for an optical seal without deformation (original) with a spectrum with the same seal, but with a defined deformation. For generation of the deformations we have charged the seal area with a mass of 200 g. This measurement for the charged/uncharged seal was repeated twice (Fig. 4). As can already be seen by eyes the uncharged seal (original) spectrum A and the repetition measures A2 are very similar. Therefore, the line shape parameter F is only small. The same is true for the charged seal’s spectra B and B2. As can be seen clearly a comparison between charged and uncharged seal’s spectra results in an increased line shape parameters F, by a factor 4 to 10 and higher. Therefore, the line shape parameter delivers a clear indication that the seal is not in its original state anymore.

3. Results and discussion 3.1. Scenarios of attacks The following scenarios, listed in Table 2, are considered: In the simplest case, access to the electronic component is obtained by destruction 92

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Table 2 Tamper scenarios, related security actions and consequences. Tamper scenario

Security action

Consequence

Breaking the optical seal Making a copy of the seal

Thin glass with waveguide Bragg grating High technological effort and challenge for remaking exactly the individual signature of the Bragg grating Winding of optical fiber in frame Fixing the optical seal on top of the frame and on top of electronic component in a bended form

Signal loss or change Signature of copy is different.

Lateral intrusion (through frame) Detachment and replacement of original seal

Drilling through the seal

Mechanical stress elements in Ultra thin glass, due to the bending form. Waveguides across the glass transparency.

Loss of signal after intrusion Signal loss or change: Ultra thin glass breaks easily. In case of successfully detaching the seal, the changed bending form causes change of signature. Loss of signal by crack or changed signature by changed strain

Fig. 6. Drilling attack: A hole of 3 mm diameter was carefully cut into the ultra thin glass cover by ablation with femtosecond laser pulses. Due to the fixation of the seal in a bend form, stress elements in the glass substrate lead to a crack in the seal. This is monitored immediately by a significant change of signature: Due to the crack, the connection to the second Bragg grating is interrupted as well as parts of the first Bragg grating, leading to a strong loss of the signals.

of the thin glass cover. A break of the seal is immediately recognizable, because the optical signature is changing or even may result in a total loss of signal. As well, a careful separation of the seal from the frame and electronic component is extremely difficult, because of the fragility of the thin glass. The separation of the seal together with the frame is also hardly possible due to the additional adhesive contact with the electronic chip in the center. The space-certificated epoxy we used is strong adhesive and has a very high temperature resistance (specified to 177 °C). Therefore, for thermal detachment of the adhesive the seal ensemble has to be heated up. The adhesive contact of the fiber to the optical seal will be dissolved already at temperatures > 80 °C. If the attack shall be unnoticed, the fiber has to be reconnected and the bending form of the seal has to be reproduced exactly, otherwise the signature of the seal will be changed which is shown below in more detail. Other scenarios we have tested are drilling attacks, counterfeiting of the seals and the replacement of original seals by a copy or recycled seals.

3.2. Drilling attack A smarter attempt for an unnoticed access to the electronic chip may be prepared by a small borehole in the glass foil. We proofed the sensitivity of our optical seals for a bore attack experimentally by cutting carefully with our femtosecond laser a hole with a diameter of 5 mm in the optical seal, close to the pins of the electronic chip (Fig. 5) and more than 1 cm aside of the waveguide structures in the optical seal. The optical design of the seals was varying from our standard design; by meaning, that only one branch with two Bragg gratings in serial arrangement was realized here. Therefore, the original signature of this optical seal had a signature of only two Bragg reflections (Fig. 6). This signature dramatically changes after realization of the borehole. By fixing the ultra thin glass on the electronic chip and the surrounding frame, intentional stress is applied to the seal. In consequence of this the laser cut of this hole induced a crack and a slightly deformation of the glass foil. This can

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be seen clearly in the signature of the seal. One signal has completely lost, because a crack interrupted the back part of the waveguide. The amplitude and shape of the Bragg grating signal, still connected with the reading system, was reduced significantly. As demonstrated, the access via borehole in the optical seal is very challenging. This also applies to a drilling attack through the side frame seems, because in this case, the glass fiber, coiled in the double-wall of the frame has to be kept intact; otherwise, a total loss of signal will occur. 3.3. Replacement by a copy or recycling of seals We produced three optical seals with identical process parameters for investigation on the possibility of easy reproducibility. These seals were connected at the front face with an optical fiber. As a result, the Bragg signal spectra of the reproduced seals show significant different amplitudes and line forms (Fig. 7). The deviations of the signatures stem almost from imprecise connection of the glass fiber to the optical seals. The relative position of the glass fiber to the waveguide structure in the glass substrate determines very sensitive the amplitude and line form of the Bragg spectrum. For demonstration in Fig. 8 the change of line shape, amplitude and wavelength position of a Bragg reflection peak is shown in dependence of the horizontal displacement of the fiber position. Fixing the fiber in a defined position to the waveguide of the seal introduces variances for the position, which cannot be controlled precisely by hand, because during the hardening of the adhesive intentional stress is induced. This gluing process can be made more reproducible by automation engineering, but in the case of the seals, the opposite is wanted, in order to make production of a copy more difficult. In summary, the production of a copy supposes a very high technological effort, by meaning a femtosecond laser system, process equipment and special knowledge has to be available. Even in this case, the production of a copy of the optical seal with an identical spectrum is very challenging. Furthermore, this can be enhanced by fixing the optical seal with a defined shape on the electronic chip. To demonstrate this we have tested the influence of deformation of optical seals on the optical signature. For

Fig. 7. Generation of three optical seals A, B, C with similar process parameters.

Fig. 8. A slightly changed horizontal position of the connected fiber in relation to the entrance waveguide of the seal effects a significant change of Bragg signal in amplitude and line form. Here the horizontal position was varied by two micrometers.

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Fig. 9. Change of Bragg signals by introducing a bend form of the seal by charging with different weights.

this experiment, an optical seal was put on the frame and charged with different weights. As can be seen in Fig. 9, deflection under load, results in a change of Bragg Grating signals. These changes of signals from the four Bragg gratings are not uniform, because the positions of the gratings are different and the induced mechanical stress in the glass transparency is inhomogeneous. While Bragg grating 2, 3 and 4 show a significant shift of wavelength position, amplitude and change of line form of Bragg Grating 1 remains nearly unchanged. The signals could be reproduced very well by repeating the experiments (Fig. 10). The signature of each peak (wavelength position, amplitude, line form) depends sensitively and individually from the deformation of the seal. Replacement of the original seal is difficult, because the original bending form has to be reproduced exactly.

3.4. Thermal effects The glass-foil slightly expands or contracts, if temperature changes. Moreover, the refractive index of the glass depends on temperature, too. Therefore, the signature of the seal will be affected by temperature changes. Consequently, the temperature itself has to be monitored to avoid false alarms induced by temperature changes. This was realized by introducing a fiber Bragg grating structure (FBG) with a femtosecond laser into the fiber, connected to the seal. The FBG temperature sensory structure in the glass fiber, which was approximately 3 mm long, was located close to the optical seal and not fixed to other parts to avoid stress induced by temperature. The FBG temperature sensor was read out together with the signature of the seal. For test of thermal effects, the signature of the seal was measured at two different temperatures. The 95

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Fig. 10. Reproducibility of signal change in dependence of recovering the deformation of the seal.

sample was placed in a climatic chamber. Several thermal cycles were applied, each with the seal’s temperature being changed between 10 °C and 25 °C. The resulting signatures of the seal were measured, when the temperature was stable at the turning points of the temperature cycle. In Fig. 11 the signature of a seal, consisting of four Bragg Grating signals, of two temperature cycles are shown. The spectrum could be reproduced very well within the temperature cycle for all temperatures. A clear change of the signatures can be seen at different temperatures. The shift of the wavelength position for all peaks increases around 0.1 nm (+/−0.01 nm) with temperature. The first two peaks have a decrease of amplitudes about 10%, while the third and fourth peak’s amplitudes are not affected by change of temperature. The line shapes of the signals almost are maintained. Slight differences in the peak forms are caused by the limited resolution of the spectrometer. We used for the experimental investigation of thermal effects a spectrometer with a 1800 l/mm grating from StellarNet (spectral range 80 nm, resolution 50 pm). It was very essential for this experiment to keep the polarization state of the incoming light constant due to the fact, that all reflected Bragg signals depend significantly from polarization of the incoming light. If standard glass fibers are used, thermal change and mechanical stress induce a nonaxial symmetrical change of refractive index in the bend fiber, causing a change of polarization state of the incoming light, which overlays the thermal effects in the spectra. To avoid this, we used a complete setup with polarization maintaining (PM) fibers and fiber optical components. The FBG temperature sensor was written in the PM-fiber with femtosecond laser pulses at 815 nm (not shown in Fig. 11). For the FBG a red-shift of 30 pm was measured for heating from 15 °C to 25 °C. For quantitative analysis, the signature of seals has to be evaluated automatically, by meaning a comparison between the original signature and the current measure. A spectral database of the Bragg grating signals from the seal and FBG for a range of temperature has to be measured and saved as reference initially. The corresponding reference spectra can be selected automatically from the spectral database by comparing the FBG-sensor

signals. For comparison of the measured spectra with the original spectra, the procedure, explained in 2.4 can be used by calculation of the line shape parameters F for a pairwise comparison. It should be mentioned here, that the line shape parameter F considers both, change of line form and/or shift of peak position. For evaluation, additionally a change of the amplitudes relative to each other should be considered as well. 4. Conclusion We could show the sensitive waveguide Bragg grating structures of the optical seals to have the ability for elucidation of finest manipulations. For practical usage on the other side the original seal in original position must be recognized without doubt and unwanted false alarms should be avoided. Reading out the signature of seal is an experimental process of measurement, which means the data have to be compared to be within a tolerance band of uncertainty. If a measurement of a signature shows that data are outside this range of tolerance, then a manipulation attempt is detected. It has been shown that thermal effects cause significant change of signature. Therefore, the temperature correlation of the signature must be known very well. By adding a calibrated FBG sensor close to the seal for temperature measurement, a comparison of current signature with a temperature-based database is possible. By this, temperature dependence of signature can be used as an additional security element, which would need to be reproduced in case of a tampering attempt. A source of false alarms can be an unwanted change of polarization state of light. The polarization dependence of signature is due to the Bragg gratings, which have an anisotropic elliptic structure. Using standard fiber optics can cause slight change of polarization state by moving the fiber, but temperature changes can also introduce a change of polarization state in a glass fiber, both caused by stress induced anisotropic refractive index in the glass fiber. The effect of polarization change can be significant for the signature. Therefore, the polarization state has to 96

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Fig. 11. The signature of optical seals depends of temperature. Reproducibility of signals in dependence of temperature, tested in a thermal cycle between 10 °C and 25 °C.

be defined very well. Using polarization maintaining fiber optics eliminates these issues. Moreover, switching state of polarization in welldefined states can be used for increasing the dimension of signature parameters and therefore increasing the level of security. We used in our experiments two spectrometers with different resolution. In general, a higher resolution of spectrometer enables a more sensitive recognition of manipulations. The range of trust for signals hast o be adjusted in dependence of the used spectrometer and should on the one hand notice a tamper attack, but on the other hand must be not too sensitive in order to avoid unwanted false alarms. In summary we could show that the optical seals based on sensory waveguide Bragg grating structures in ultra thin glass transparencies are very well suited for detection of mechanical manipulations, like

breaking, removing or drilling holes, which leads to a lasting change of signature. The reproduction of individual signature of the seals is very challenging and needs high technical effort and knowledge, and even we were not able to reproduce a perfect copy with identical signature twice. Moreover, the seals were fixed in a bended state over the electronic chip, which gives a characteristic pattern of signature. In case of a successful removal of the seal, the originally applied glass deformation has to be reproduced as well. For future generation of optical seals the line shape of the signature can be formed more complex by introducing a second Brag grating structure, spatially overlapping with the first one, but differing slightly with Bragg grating wavelength. Such interference pattern, caused by spatially overlapping gratings have been demonstrated in glass fibers 97

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with superimposed Bragg gratings as basic elements for a tunable FabryPerot filter [12]. Defined amplitude and phase changes in a single Bragg grating structure is an additional possibility for increasing the complexity of the line shape of signature. Amplitude and phase changes can be realized with minor technical efforts with our femtosecond laser pointby-point writing technology, which was demonstrated by us [13] and other groups [14] in glass fibers. One of the main advantages beside of the high sensitivity and highlevel fulfillment of security requirements of these optical seals is the ability of proofing the state of the seal simply by connection of the reading system to a fiber optical interface. The electronic device, or in general, the security relevant object has not to be checked by opening the box or container, but can be performed remotely.

[4] Alvine KJ, Suter JD, Bernacki BE, Bennett WD. Optically resonant subwavelength films for tamper-indicating tags and seals. In: Carapezza EM, editor. Proc. of SPIE, 9456; 2015. 94560C. [5] Cozzella L, Simonetti C, Schirripa Spagnolo G. Drug packaging security by means of white-light speckle. Opt Lasers Eng 2012;50:1359–71. [6] Wlodarczyk KL, Brunton A, Rumsby P, Hand DP. Picosecond laser cutting and drilling of thin flex glass. Opt Lasers Eng 2016;78:64–74. [7] Beresna M, Gecevičius M, Kazansky PG. Ultrafast laser direct writing and nanostructuring in transparent materials. Adv Opt Photonics 2014;6:293. [8] Zhang H, Ho S, Eaton SM, Li J, Herman PR. Three-dimensional optical sensing network written in fused silica glass with femtosecond laser. Opt Express 2008;16:14015–23. [9] Maselli V, Grenier JR, Ho S, Herman PR. Femtosecond laser written optofluidic sensor: Bragg grating waveguide evanescent probing of microfluidic channel. Opt Express 2009;17:11719. [10] Femtosecond laser micromachining, 123. Berlin Heidelberg: Springer; 2012. [11] Thiel M, Flachenecker G, Schade W. Femtosecond laser writing of Bragg grating waveguide bundles in bulk glass. Opt Lett 2015;40:1266. [12] Liu S, Dong X, Sun J, Shum P. Free-spectral range tunable Fabry – Perot filter with superimposed fiber Bragg gratings. Opt Commun 2009;282:4729–32. [13] Burgmeier J, Waltermann C, Flachenecker G, Schade W. Point-by-point inscription of phase-shifted fiber Bragg gratings with electro-optic amplitude modulated femtosecond laser pulses. Opt Lett 2014;39:540–3. [14] Marshall GD, Williams RJ, Jovanovic N, Steel MJ, Withford MJ. Point-by-point written fiber-Bragg gratings and their application in complex grating designs. 18, 1864– 1866 (2010).

References [1] Johnston RG. Tamper-indicating seals for nuclear disarmament and hazardous waste management. Sci Glob Secur 2001;9:93–112. [2] Rizzo F, Barboni M, Faggion L, Azzalin G, Sironi M. Improved security for commercial container transports using an innovative active RFID system. J Netw Comput Appl 2011;34:846–52. [3] Aggarwal AK, Kaura SK, Chhachhia DP, Sharma AK. Concealed moiré pattern encoded security holograms readable by a key hologram. Opt Laser Technol 2006;38:117–21.

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