Influence of chemical polymer composition on integrated waveguide formation induced by excimer laser surface irradiation

Applied Surface Science 356 (2015) 532–538

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Influence of chemical polymer composition on integrated waveguide formation induced by excimer laser surface irradiation S. Hessler a,∗ , M. Rosenberger a , S. Belle a , B. Schmauss b , R. Hellmann a a b

Applied Laser and Photonics Group, University of Applied Sciences Aschaffenburg, Wuerzburger Strasse 45, 63743 Aschaffenburg, Germany Institute of Microwaves and Photonics, University of Erlangen-Nuremberg, Cauerstrasse 9, 91058 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 9 April 2015 Received in revised form 7 July 2015 Accepted 3 August 2015 Available online 6 August 2015 Keywords: Optical polymers PMMA UV surface modification Optical planar waveguides Integrated optics Residual monomer content Cut-back method

a b s t r a c t We show that the chemical composition and the amount of residual monomers in polymethylmethacrylate significantly affect the evolution of optical waveguide formation induced by UV surface irradiation. We employ an interferometric approach in Mach-Zehnder configuration to determine the refractive index depth profile in different planar polymethylmethacrylate materials. Our results reveal a distinctive different surface and buried waveguide formation for materials having different monomer content. In particular, we find that for smaller residual monomer content buried waveguide formation is less pronounced, which is in turn preferential for a selective light guidance in planar polymer structures. Attenuation measurements confirm a difference in attenuation coefficient of 0.5 dB/cm. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Polymer optical waveguides recently attract considerable interest in telecommunication and sensor systems. Beside polymer optical fibers (POF), planar polymer optical substrates offer the unique possibility to realize optical integrated circuits (OIC). The waveguides in which can be formed by either conventional lithographic techniques, embossing and imprint technologies or direct writing using UV radiation [1–4]. For the later process a detailed knowledge of the UV induced refractive index modification and its dependencies on irradiation parameters is indispensable as it determines the waveguide properties such as mode field distribution and attenuation. Especially polymethylmethacrylate (PMMA) is a suitable and well explored material for polymer optical devices. Particularly, the possibility to alter the refractive index (RI) by UV irradiation well below 300 nm facilitates the generation of passive optical components and OIC [5–9]. The underlying complex photochemical processes of this UV induced modification have been studied in [10–13] comprising competing processes such as UV induced direct photopolymerisation, main chain scission and side chain cleavage, respectively. Various studies have shown that these processes in

PMMA lead to a refractive index modification that yields a surface waveguide as well as a buried waveguide [15–17]. However, previous studies focused on one PMMA material only and a difference of the material composition and particularly of the content of residual monomers have, though being repeatedly reported relevant in the underlying processes, to the best of our knowledge, not been studied so far. In this contribution, we report on the UV excimer laser induced refractive index modification in different PMMA with varying composition. In particular, we study commercially available PMMA samples with different amount of copolymers and residual monomers. The refractive index profile has been measured by phase shifting interferometry in a Mach-Zehnder configuration as described by Shams El-Din et al. [17]. Compared to the research of Shams El-Din et al. who investigated refractive index depth profiles for one special polymethylmethacrylate material only, our results reveal that the generation of buried waveguides is significantly reduced for PMMA with a smaller amount of residual monomers.

2. Experimental 2.1. Materials

∗ Corresponding author. http://dx.doi.org/10.1016/j.apsusc.2015.08.011 0169-4332/© 2015 Elsevier B.V. All rights reserved.

To investigate the influence of the chemical composition of polymethylmethacrylate on the UV induced optical waveguide

S. Hessler et al. / Applied Surface Science 356 (2015) 532–538

formation, we studied two different commercially available PMMA grades, namely normal PMMA grade – subsequently referred to as standard PMMA – and impact-resistant PMMA grade (both by Goodfellow). According to the supplier, the latter is modified by an unspecified elastomer copolymer to enhance the impact resistance. The spectral dependence of the absorption is determined by carrying out transmission and reflection measurements using a spectro-radiometer equipped with an integration sphere (Instrument Systems, transmission (T) measurement accuracy T = ±0.1%). In addition, dispersion characteristics of both materials are measured by multi-wavelength refractometry using a white light Abbe refractometer (Atago DR-M2/1550, refractive index (n) measurement accuracy n = ±0.0001). The chemical composition of the PMMA, in particular the content of residual monomers, i.e. methylmethacrylate (MMA), and the content of possible copolymers are determined by employing 1 H NMR spectroscopy using a 400 MHz Bruker Avance DRX 400 NMR spectrometer. For these measurements, samples were prepared by dissolving 20 mg of each polymer type in chloroform. Standard PMMA needed a longer time for complete dissolution as compared to impact-resistant PMMA already indicating a different composition. The NMR spectrometer was calibrated by internal standard tetramethylsilane (TMS, chemical shift ı = 0 ppm). The residual monomer content was determined by evaluating the signals at 6.10 ppm and 5.55 ppm resulting from the two hydrogen atoms at the CH2 double bond of residual MMA and comparing these to the ester signal of the entire polymer body which appears around 3.60 ppm ( CH3 group, detected signal range from 3.39 to 3.85 ppm). Eq. (1) was applied to calculate the fractional residual MMA content by the use of integrated relative intensities of the relevant signals [14]. %MMA =

(A6.1 ppm + A5.55 ppm )/2 (A3.39–3.85 ppm )/3

(1)

2.2. Mach-Zehnder interferometer for refractive index profiles In order to determine UV induced refractive index alterations at the irradiated surface a Mach-Zehnder interference microscope is employed. Fig. 1 shows the setup of the Mach-Zehnder measurement system based on the pioneering work by Shams El-Din et al. [15–17]. The beam of a HeNe laser ( = 632.8 nm) is expanded by a 10× telescope allowing complete illumination of the relevant area on the sample. The widened beam is split into object and reference beam by beam splitter BS1 . The object beam travels through the

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sample waveguide area which alters the phase of the object beam relative to that of the reference beam. This results in a bend of the interference fringes according to the refractive index distribution. The sample is placed inside an optical glass cuvette filled with an immersion liquid matching the refractive index of the unilluminated PMMA substrates. This immersion oil with known refractive index serves as reference for the measured phase values and provides continuous interference fringes. Furthermore, by this approach only the shift of interference fringes due to the waveguide is visible. Two 10x microscope objectives (MO) with NA = 0.25 enlarge the beams such that MO1 magnifies the viewed waveguide area and MO2 equalizes the wavefront of object and reference beam. The resulting interferogram at the interferometer output is recorded by a monochromatic CCD camera (Allied Vision Marlin). In order to calculate refractive index values, the determination of phase difference values between object and reference arm out of the interferogram are necessary. For this purpose, Phase Shifting Interferometry (PSI) as a common approach for direct determination of phase values is applied. To facilitate this, mirror M2 in the reference arm of the interferometer is mounted on a piezoelectric transducer (PZT) enabling an accurate phase shift of the reference beam. The phase is shifted 5 times by steps of 90◦ whereas an interferogram for each step is recorded. From these 5 different intensity values at each pixel the wrapped phase differences are calculated by the software Fringe Processor from BIAS GmbH (Fig. 2a) After unwrapping (Fig. 2b) and normalization (Fig. 2c) the phase values are converted into refractive index values. For better visualization of the UV-modified PMMA area the obtained phase map can be displayed as a 3D plot (Fig. 2d). Integrated waveguides in PMMA substrate are usually written by irradiating the planar surface with a KrF excimer laser at 248 nm. However, for the measurement of the refractive index depth profile with the Mach-Zehnder interferometer the waveguides are generated in an optically polished edge of the PMMA substrate. This approach shortens the optical path length of the lateral measurement beam through the PMMA samples, which enables the use of interferometry and allows the application of the multi-layer waveguide model according to [15]. The waveguide is written using an amplitude mask (width 150 ␮m) in contact exposure mode. According to our previous work on polymer waveguides in PMMA, waveguides are written using the following excimer laser parameters: exposure 8 mJ/cm2 and pulse frequency 200 Hz [8]. The number of pulses has been varied between 1000 and 8000 shots. To increase the speed of the light induced chemical reactions and to stabilize the refractive index modification, the samples are tempered for 12 h at 60 ◦ C after UV illumination. Afterwards,

Fig. 1. Mach-Zehnder interference microscope setup for measurement of refractive index depth profiles of UV illuminated polymer samples. A microscope image shows the UV modified polymer sample area.

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Fig. 2. Typical results from the interferogram processing steps: (a) wrapped phase difference map, (b) unwrapped phase difference map, (c) normalized phase difference map, (d) 3D plot of normalized phase difference.

refractive index depth profiles for both materials were measured and compared on refractive index increase at the surface and deeper regions. In contrast to the measurements in a single polymer material by Shams El-Din et al., it is thereby possible to evaluate the influence of the chemical polymer composition on refractive index depth modification.

2.3. Waveguide characterization In a further step the fabricated waveguide areas in both materials are analyzed in terms of surface compaction and surface roughness. Since excimer laser radiation influences the molecular structure of the polymer chains especially by induced side chain cleavage, the irradiated polymer area densifies. Therefore, beside an increased refractive index also a surface compaction results which appears as a trench in the surface after the temper step. Due to ongoing excimer laser irradiation the polymer surface additionally gets locally corrugated which increases average surface roughness. Since using unpolarized UV light there is no spatial periodicity of the corrugations and surface roughness proves to be homogeneous over the irradiated area. An increasing surface roughness in turn leads to higher waveguide attenuation. Hence, both surface compaction and average surface roughness are precisely determined by laser scanning microscopy (Keyence VK-X210) after tempering the samples.

a

Finally, to evaluate and compare light guiding performances, the surface waveguides are coupled to a single-mode fiber in order to investigate optical attenuation. Subsequently, the output signal of the waveguide is collected by a multi-mode fiber. Using a combined source and detector device (Micron Optics) at  = 1550 nm together with an optical circulator the attenuation of the whole measurement system is determined. By employing the cut-back method, i.e. cutting back the waveguides and measuring the attenuation for different waveguide lengths, the attenuation coefficient for the waveguide under test is obtained by best fit straight line.

3. Results and discussion 3.1. Material characterization Fig. 3a shows the absorption spectra of the two differently composed PMMA materials under study (both samples of 0.5 mm thickness). Whereas impact-resistant PMMA shows a stronger UV absorption in the range from 250 to 370 nm, both materials have the same absorption characteristic (97.8%) at the employed KrF excimer laser wavelength of 248 nm. This in turn assures comparable energy impacts on the polymers upon excimer irradiation. Assuming an exponential intensity decrease inside the polymer material the absorption coefficient at 248 nm is calculated to

b

100 90

1.500

80 70

refractive index n

absorption in %

1.505

60 50 40

standard PMMA

30

impact-resistant PMMA

20

1.495

standard PMMA impact-resistant PMMA Sellmeier curve

1.490 1.485 1.480

10 1.475

0 200

250

300

350

wavelength λ in nm

400

450

400 500 600 700 800 900 1000 1100 1200 wavelength λ in nm

Fig. 3. Optical material characterization: (a) absorption spectra deduced from transmission and reflection measurements and (b) dispersion measured by multi-wavelength Abbe refractometry with comparison to Sellmeier approximation according to [18].

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Fig. 4.

1

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H NMR spectra of samples dissolved in chloroform: (a) standard PMMA and (b) impact-resistant PMMA.

Table 1 Evaluation of 1 H NMR signals.

COOCH3 – ester signal IRI at 3.75 ppm (MMA) IRI at 3.60 ppm (PMMA) IRI from 3.39 – 3.85 ppm Calculated residual monomer content in % MMA

Impactresistant PMMA

1.0000 1.0186

1.0000 1.2011

4.7837 517.7613 547.2478

5.5885 728.3002 837.4309

1,497 1,496

0.55%

0.39%

refractive index n

CH2 – signals IRI at 6.10 ppm IRI at 5.55 ppm

Standard PMMA

N = 7000

1,495

N = 6000

1,494

N = 5000

1,493

N = 4000

1,492

= 3000

1,491

N 2000

1,490

N = 1 00

1,489

7633 m−1 which corresponds to an initial penetration depth of 131 ␮m into the unirradiated materials. Fig. 3b compares the dispersion of the two materials as measured by multi-wavelength Abbe refractometry with the refractive indices given by the Sellmeier approximation according to [18]. While standard PMMA features a dispersion curve very similar to the Sellmeier approximation, impact-resistant PMMA clearly reveals an offset compared to standard PMMA indicating a different chemical composition and a different molecular weight. Fig. 4a and b show the results of 1 H NMR spectroscopy of standard and impact-resistant PMMA, respectively. The chemical shift ı is calibrated by internal TMS representing peaks at ı = 0 ppm. Peaks occurring at 7.28 ppm identify chloroform as applied polymer solvent for the NMR measurements. Signal peaks at 3.39–3.85 ppm represent typical chemical shifts of COOCH3 ester side chain bonds from PMMA. Both spectra show signal peaks for a chemical shift of 5.55 ppm and 6.10 ppm which indicate the presence of CH2 hydrogens of MMA and the thus residual monomer content. Comparison of the integrated relative intensities of these signals with the entire integrated ester signal from 3.39 to 3.85 ppm gives quantitative information on the MMA content of the polymer material. Table 1 lists all relevant integrated relative intensities (IRI) given by the peaks of interest as well as the calculated residual monomer content. While standard PMMA exhibits 0.55% MMA content, impact-resistant PMMA only contains 0.39% MMA. Moreover in the NMR spectrum of standard PMMA remarkable peaks appear at 4.08 ppm, 7.55 ppm and 7.73 ppm, respectively. These signals can be attributed to an aromatic copolymer or additive with an amount of about 1.5%. In contrast, the NMR spectrum of impact-resistant PMMA does not show these typical aromatic signals but reveals a broad signal around 4.05 ppm. The

0

10

20

30

40

50

60

70

80

90

100

110

120

dept z in μm

Fig. 5. Refractive index depth profiles in standard PMMA for varying number of excimer laser pulses N. Two constituting waveguide areas can be clearly observed.

latter results from side chain COOCH2 hydrogens belonging to an aliphatic copolymer with a content of about 4%. As a typical elastomer copolymer to increase the impact resistance of normally brittle PMMA, polybutylacrylate (PBA) is used [14,19]. The threefold appearance of side chain CH2 groups in PBA (see chemical formula of PBA side chain in Fig. 3b) vindicates the observed broad signal at 4.05 ppm. Consequently, increased impact resistance of impact-resistant PMMA can be attributed to PBA content.

3.2. Refractive index depth profiles after UV illumination Refractive index depth profiles of UV illuminated standard PMMA obtained from phase shifting Mach-Zehnder interferometry are shown in Fig. 5. Standard PMMA exhibits a strong and growing surface RI increase with increasing number of excimer laser pulses. This results in a strong surface waveguide with a depth ranging from 10 ␮m at 1000 pulses to 17 ␮m at 7000 pulses. However, also a much broader but weaker RI increase in a quite deeper region with maximum at a depth of about 35 ␮m can be observed. This results in an undesirable buried second waveguide which increases attenuation of the primary surface waveguide due to light coupling effects. The two generated waveguides are separated by a local minimum in the RI depth profile. The maximum depth of a detectable RI modification is about 100 ␮m which corresponds to the calculated penetration depth of 131 ␮m of the 248 nm UV laser irradiation.

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1,496

refractive index n

1,495

N = 8000

1,494

N = 7000

1,493

N = 6000

1,492

N = 5000 N = 4000

1,491

N = 3000

1,490

N = 2000

1,489

N = 1000

1,488 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

depth z in μm

The occurrence of two waveguide areas (surface and buried waveguide) due to UV illumination has also been reported by Shams El-Din et al. [15–17] and can be attributed to the concurring photochemical processes of direct photo-polymerization, side chain cleavage and depolymerization. Direct photopolymerization of residual monomers in the polymer matrix is the first mechanism to take place resulting in an exponential depth decaying RI increase due to absorption by Lambert-Beers law. Owing to an increasing absorption coefficient of illuminated areas, the polymer surface region absorbs most of the impinging radiation thereby shielding deeper regions. With further illumination PMMA side chain cleavage occurs at the surface region. The free separated side chains fill the gap volumina between the linear PMMA main chains thus increasing the RI by material densification. While the surface RI is growing strongly with ongoing irradiation, the second waveguide does not further increase significantly. The local RI minimum between the two waveguide areas is attributed to depolymerization enabled by the special amount of UV energy in this region. In contrast, Fig. 6 shows RI depth profiles for impactresistant PMMA. Again a strong surface waveguide but a much less pronounced secondary waveguide can be observed. This can be associated to the smaller residual monomer content in impact-resistant PMMA, which is responsible for direct photopolymerization as the first refractive index modification process. Furthermore having a bigger RI modification depth of up to 120 ␮m the influence of different chemical composition, i.e. less monomer and different copolymer content, is reconfirmed. The surface waveguide exhibits a wider extension as compared to standard PMMA with 20 ␮m at N = 1000 up to 40 ␮m at N = 8000. Nevertheless, the buried, second waveguide as well as the RI dip in between the waveguides are significantly smaller leading to advantageous light guiding properties in the primary waveguide. The associated surface refractive index modification of standard and impact-resistant PMMA as a function of the number of laser pulses impinging the specimen is illustrated in Fig. 7, respectively. In standard PMMA the increase of the surface RI ranges from 0.0019 at 1000 pulses to 0.0078 at 7000 pulses. Before saturating at 7000 laser pulses, the surface RI increases linearly from 1000 to 6000 pulses with a slope of ca. 0.001 per 1000 pulses. This behavior offers a precise linear refractive index tuning possibility of the waveguide surface in a range of n = 0.0058. A surface RI tuning is thereby always accompanied by a depth variation of the surface waveguide.

relat. refractive index increase Δnsurf

Fig. 6. Refractive index depth profiles in impact-resistant PMMA for varying number of excimer laser pulses N. A secondary buried waveguide is less pronounced than in standard PMMA.

0.009 0.008 0.007 y = 1E-06x R² = 0.9847

0.006 0.005 0.004 0.003 0.002

standard PMMA

0.001

impact-resistant PMMA

0.000 0

1000

2000

3000

4000

5000

6000

7000

8000

number of excimer laser pulses N Fig. 7. Surface refractive index increases linearly with the number of excimer laser pulses until saturation at 6000 pulses.

In impact-resistant PMMA the surface RI starts to increase linearly with increasing number of excimer pulses in a same manner as standard PMMA. Relative surface RI increase ranges from 0.0017 at 1000 pulses to 0.0065 at 5000 pulses. After linear RI increase of about 0.001 per 1000 pulses (same to standard PMMA), RI already begins to saturate at 5000 pulses reaching a maximum value of n = 0.0067 against substrate RI level at 8000 laser pulses. In summary both materials under investigation offer a similar surface refractive index range which can be linearly altered by the impinging number of excimer laser pulses which in turn also affects the depth of the primary surface waveguide. The different compositions of the PMMA materials with different residual monomer content on the one hand as well as aromatic or elastomer content on the other hand turn out to have great influence on UV induced waveguide formation. 3.3. Waveguide characterization In Fig. 8a surface compaction, i.e. the trench depth of the UV irradiated surface in dependence of the impinged laser pulses is plotted. These distinct compaction depths can only be observed after tempering the samples with a laser energy impact below the ablation threshold of the PMMA materials being ensured. Both materials under investigation exhibit very similar linear increases

S. Hessler et al. / Applied Surface Science 356 (2015) 532–538

a

b average roughness Ra in nm

1400

Compaction C in nm

1200

1000 800 600 400

standard PMMA

200

impact-resistant PMMA

0 0

2000

4000

6000

8000

537

90 80

standard PMMA

70

impact-resistant PMMA

60 50 40 30 20 10 0 0

number of excimer laser pulses N

2000

4000

6000

8000

number of excimer laser pulses N

Fig. 8. Characterization of irradiated surface after a 12 h temper step at 60 ◦ C: (a) surface compaction and (b) surface roughness.

in compaction. This fact supports the dose dependent side chain cleavage theory of the very similar primary waveguide RI increases irrespective of the slight chemical differences in standard and impact-resistant PMMA. Both materials experience quite the same amount of material densification due to chemical bond breaking in the surface waveguide region leading to surface compaction. Additionally, Fig. 8b shows the measured average surface roughness of the irradiated polymer areas. Up to 6000 laser pulses standard PMMA features lower roughness than impact-resistant PMMA, though beyond 6000 pulses impact-resistant PMMA seems to be superior. However, in the range of 1000–5000 pulses both materials show roughness values well below 30 nm up to where scattering losses can be tolerated for optical integrated circuits. In order to identify the optimum laser parameters for a wellguiding waveguide with low losses a compromise between high refractive index increase and low roughness has to be made. For the attenuation measurements using the cut-back method the number of pulses were chosen to be N = 3000, since both materials have almost the same relative surface RI increase of about 0.004 and a comparable surface roughness (with Ra = 18 nm in impact-resistant PMMA it is slightly higher than Ra = 11 nm in standard PMMA). Considering the fact that the strong surface waveguide and the buried waveguide areas are separated by a small gradient index profile only, we can assume both waveguides being in close contact. Subsequently there is the possibility of a guided mode coupling from the surface waveguide over to the broad buried waveguide. This adverse mechanism inevitably leads to a significant loss in guided light power in the surface waveguide. Fig. 9 summarizes the results of the cut-back measurements. The slopes of the best fit straight lines yield an attenuation 33 standard PMMA

32

impact-resistant PMMA

attenuation in dB

31

y = 0.174x + 23.634 R² = 0.9861

30 29

28 y = 0.1245x + 24.924 R² = 0.991

27 26 25 0

10

20

30

40

50

waveguide length in mm Fig. 9. Cut-back attenuation measurements of the coupled surface waveguides reveal lower loss of impact-resistant PMMA.

coefficient of ˛ = 1.2 dB/cm for impact-resistant PMMA and ˛ = 1.7 dB/cm for standard PMMA. Although impact-resistant PMMA exhibits a higher average surface roughness of the waveguide, it features a significantly lower attenuation coefficient by 0.5 dB/cm. This can be assigned to the significant mode coupling influence of the buried second waveguide area, in turn proving the impact of the chemical polymer composition on the wave guiding properties. 4. Conclusion We have demonstrated that the chemical composition, in particular the amount of residual monomer and copolymer content, plays a crucial role in the UV induced refractive index modification of PMMA. Especially, the spatial in depth evolution of surface and buried waveguide formation is influenced by the content of residual monomers with a strong buried waveguide being preferential with an increased amount of monomers. In contrast, smaller residual monomer content leads to a significant weaker buried waveguide region, in turn significantly reducing waveguide attenuation in the primary surface waveguide significantly. Thus, the implementation of process steps for detraction of residual monomer content inside the polymethylmethacrylatematrix is favorable to improve direct waveguide fabrication significantly. References [1] L. Eldada, L.W. Shacklette, Advances in polymer integrated optics, IEEE J. Sel. Top. Quantum Electron. 6 (1) (2000) 54–68. [2] H. Ma, A.K.Y. Jen, L.R. Dalton, Polymer-based optical waveguides: materials, processing, and devices, Adv. Mater. 14 (19) (2002) 1339–1365. [3] Y. Ichihashi, P. Henzi, M. Bruendel, J. Mohr, D.G. Rabus, Polymer waveguides from alicyclic methacrylate copolymer fabricated by deep-UV exposure, Opt. Lett. 32 (4) (2007) 379. [4] T. Han, S. Madden, M. Zhang, R. Charters, B. Luther-Davies, Low cost nanoimprinted polymer waveguides, in: Conference on Optoelectronic and Microelectronic Materials and Devices (COMMAD), Sydney, Australia, 2008, pp. 185–188. [5] W.J. Tomlinson, Photoinduced refractive index increase in poly(methylmethacrylate) and its applications, Appl. Phys. Lett. 16 (12) (1970) 486. [6] P.J. Scully, R. Bartlett, S. Caulder, P. Eldridge, R. Chandy, J. McTavish, UV laser photo-induced refractive index changes in poly-methyl-meth-acrylate and plastic optical fibres for application as sensors and devices, in: OFS2000: 14th International Conference on Optical Fibre Sensors, 2000, pp. 854–857. [7] M. Koerdt, Fabrication and characterisation of waveguide and grating structures induced by ultraviolet radiation in polymers with a shortened writing process, Proc. SPIE 7213 (2009). [8] M. Rosenberger, G. Koller, S. Belle, B. Schmauss, R. Hellmann, Planar Bragg grating in bulk polymethylmethacrylate, Opt. Express 20 (25) (2012) 27288. [9] M. Rosenberger, S. Hessler, S. Belle, B. Schmauss, R. Hellmann, Fabrication and characterization of planar Bragg gratings in TOPAS polymer substrates, Sens. Actuators A: Phys. 221 (2015) 148–153.

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