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Characterization of the optical-functional properties of a waveguide written by an UV-laser into a planar polymer chip
Optical Materials 27 (2005) 1138–1148 www.elsevier.com/locate/optmat
Characterization of the optical-functional properties of a waveguide written by an UV-laser into a planar polymer chip M.A. Shams-Eldin a, C. Wochnowski b,*, M. Koerdt b, S. Metev b, A.A. Hamza a, W. Ju¨ptner b b
a University of Mansoura, Mansoura, Egypt BIAS (Bremer Institut fu¨r Angewandte Strahltechnik), Klagenfurter Strasse 2, 28359 Bremen, Germany
Received 5 April 2004; accepted 14 September 2004 Available online 18 November 2004
Abstract The optical-functional properties of an integrated-optical strip-waveguide in a planar polymer chip are presented in this article. The waveguide was directly written into the surface of a planar polymer chip by UV-laser irradiation. The refractive index depth profile of the waveguide was examined by a two-beam-interferometric method. Also the mode field distribution and the loss rate of the waveguiding structure has been characterized. The study shows that the optical-functional properties strongly depend on the UV-irradiation parameters. Several mostly independently occurring photochemical processes competing with one another are proposed to explain the formation and shape of the refractive index distribution. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Polymethylmethacrylate; Excimer laser; Polymer integrated optics; Two-beam-interferometry (Mach–Zehnder); Refractive index depth profile
1. Introduction The optical properties of an UV-modifiable polymer can be photochemically changed in a controllable way by UV-laser irradiation [1,2]. Especially the UV-laserassisted local modification of the refractive index of polymethylmethacrylate (PMMA) permits the fabrication of integrated-optical waveguides in a planar polymer chip [3]. For the design and fabrication of an integrated-optical component the exact characterization of the optical-functional properties, especially the refractive index depth profile, of the integrated-optical polymer waveguides are urgently required.
*
Corresponding author. Tel.: +49 4212185078; fax: 4212185095. E-mail address: [email protected] (C. Wochnowski).
+49
0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.09.019
Refractive index depth profile measurements of integrated-optical gradient waveguiding samples prepared by a diffusion method are usually performed in transmission modus [4–9]. Other research groups report about refractive index depth profile measurements by an interferometric method done in reflection modus [10–12]. Simmens [13] described an interferometric technique to determine the refractive indices of complex structure fibres employed in transmission modus. Hamza [14] presented a method in order to measure the mean refractive indices and birefringence of fibre optics by the use of a two-beam-interferometry-technique. The mathematical expressions employed in this work are derived by Hamza et al. [15–17] and are generally valid for all kinds of two-beam-interferometry techniques. The same mathematical expressions are also used in our work in which we study the refractive index distribution of a waveguide prepared by UV-excimer laser irradiation in
M.A. Shams-Eldin et al. / Optical Materials 27 (2005) 1138–1148
a planar polymer chip by employing a two-beam interference technique in transmission modus. Two mostly independent photochemical reaction mechanisms are proposed in order to explain the experimental results.
2. Theoretical treatment The UV-laser-generated integrated-optical waveguide is supposed to be of gradient-index type. Its refractive index depth profile is measured by an optical free-space Mach–Zehnder interferometer (Fig. 1) which is described in detail in the next passage. The integrated-optical polymeric waveguide is a gradient waveguide which can be considered as being composed of many high index layers ni varying in dependence on the depth and being surrounded by low index materials (cladding substrate) (Fig. 2). The slab waveguide is infinitely extended in x-direction and finitely in y- and z-directions. The waveguide is inserted into an immersion liquid of refractive index nL near to that of substrate nS since ni > nS nL . We assume that the cross section of the waveguide is rectangular (Fig. 2). The top surface of the waveguide is
1 - Spatial Filter 2 - Diaphram 3 - Polarizer 4 - Collimated Lens 5 - Screen 6 - Cell contains liquid 7 - Cell contains liquid and the sample
LaserSource s
SpatialFilter
A2
M1
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7
6
M2
1
2 L1
3 A1
Fig. 1. Experimental set-up of the optical free-space Mach–Zehnder interferometer.
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at z = 0 and the bottom side of the waveguide is at z = d (depth). The refractive index of the waveguide changes in the z-direction starting from the surface at z = 0 to z = d (depth). The zone inside the substrate below the waveguiding structure having the refractive index nS starts from z = d in the positive direction of z. Measuring the values of the refractive index depth profile of the waveguide requires the value of substrate refractive index nS. The optical path difference DCS between the substrate and the immersed liquid is represented as DCS ¼ ðnS nL ÞaS
ð1Þ
with aS as the distance travelled in the substrate region. In case of two-beam interferometry, the optical path difference DCS is given by the following formula [13,14,17]. xS DCS ¼ k ð2Þ h where xS, h and k are the fringe shift inside the substrate, interfringe spacing and the wavelength of the test light (Fig. 3). The combination of the Eqs. (1) and (2) yields xS k ¼ ðnS nL ÞaS ð3Þ h By the same method the refractive index depth profile of the waveguide can be determined. The waveguide is divided into individual layers of constant thickness (Fig. 2). Monochromatic light emitted by a HeNe-laser is directed parallel to the y-axis of this waveguide. The optical path difference for each layer can be determined by the following formula: xi k ¼ ðni nS Þa; i ¼ 1; 2; . . . ; N ð4Þ h where i and a are the layer number and the distance travelled by the beam of light inside the waveguide, respectively. The resulting shift occurs in the x–z plane. By using Eq. (4) the refractive index depth profile of the waveguide is graphically reconstructed.
UV-excimerlaser beam top surface
y
n1 n2 n3
UV-laser modified area incident beam
HeNe-laser beam
150 µm
500 µm
matching-liquid optical cell (paraffins und α-bromine-naphthalene)
d n i-1 ni ns
PMMA-chip
z
a
Fig. 2. Optical cell with polymeric gradient waveguide sample consisting of many high index layers.
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the mirror M1 the first of the two partial laser beams is directed normally on the sample immersed in the liquid with a refractive index quite close to the refractive index of the substrate material nS as shown in Fig. 2. The second partial laser beam serves as the reference beam and is reflected by the mirror M2. The two beams are recombined at the beam splitter A2, which is identical to A1. The fringe pattern, which can be observed on the screen 5, resulting from the interference of these two beams is a family of equally spaced parallel fringes covering the shift through the entire channel waveguide structure. An example of such an interferogram can be seen in Fig. 3. 3.3. Experimental set-up for the mode field investigation and loss rate measurements Fig. 3. Sample interferogram recorded by the Mach–Zehnder interferometer.
3. Experimental 3.1. Fabrication of the waveguides As the basic polymer material PMMA provided by Goodfellow (product number: ME 303010) was employed. The UV-laser assisted generation of the integrated-optical waveguide in a planar PMMA chip is done by an UV-lithographic method which is detailly described elsewhere [1,2]. In the UV-illumination facility a KrF-excimer is employed emitting a wavelength of 248 nm. The laser beam is directed on the polymer sample situated in a sample holder. A quartz glass mask with an inherent aperture structure in the chrome layer is directly positioned on the polymer chip surface. Incident UV-photons illuminate the uncovered regions of the polymer chip surface where direct electronic bond breaking induces a photochemical modification mechanism in the polymer material. So the UV-irradiation results in a local and controllable increase of refractive index in the illuminated areas of the polymer surface generating the integrated-optical waveguiding structures in the planar polymer chip [1]. 3.2. Experimental set-up for refractive index depth profile measurements The experimental set-up for the refractive index depth profile measurements is based on an optical free-space Mach–Zender interferometer (Fig. 1). Polarized light is emitted by a He–Ne laser S (wavelength k = 632.8 nm). The spatial filter is situated in the focal plane of a collimating lens L1 in order to improve the beam quality. The parallel laser beam is divided by the beam splitter A1 into two partial laser beams. After reflection from
The mode propagation and the loss rate of the light guided by the integrated-optical waveguide is measured by a similar experimental set-up: A laser test beam was coupled into the waveguide by fibre-chip-coupling. A fibre with a diameter of 4 lm was employed as a coupling device. This fibre is monomode for red light. A laser diode (632.8 nm) served as a light source. In case of mode field characterization the light exiting from the back side of the polymer chip was directed to a freespace standing objective lense magnifying and projecting the image onto the chip of a CCD camera. The data were electronically recorded and evaluated by a computer. In case of loss rate measurements the light was coupled into a multimode fibre (diameter: 62.5 lm) on the back side of the polymer chip. The optical signals were transmitted to a photo diode by fibre optics, where they are converted into electrical data. The data were recorded and evaluated by a computer.
4. Results 4.1. Refractive index depth profile measurements The refractive index of the unirradiated polymeric substrate is measured for several unirradiated polymer samples by the Mach–Zehnder interferometry technique yielding an average value of nS = 1.49154 ± 0.0001 with the immersion liquid having a refractive index nL = 1.4912. A series of waveguide samples is prepared with the following irradiation parameters: wavelength k = 248 nm, fluence I = 17 mJ/cm2, repetition rate R = 5 Hz and the number of laser pulses N ranging from 1000 to 8000 increased by step of 1000 for each sample. The refractive index depth profiles of the waveguide samples is recorded in dependence on the irradiation dose (number of laser pulses) (Fig. 4).
M.A. Shams-Eldin et al. / Optical Materials 27 (2005) 1138–1148 1.500
λ = 248 nm, I = 17 mJ/cm2, N = 1000, R = 5 Hz
1.498 1.496 1.494 substrate
1.492
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refractive index
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λ = 248 nm, I = 17 mJ/cm 2, N = 4000, R = 5 Hz
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λ = 248 nm, I = 17 mJ/cm2, N = 5000, R = 5 Hz
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1.492
40
(d)
refractive index
refractive index
1.500
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upper waveguide
40
depth (µm)
(a)
λ = 148 nm, I = 17 mJ/cm2, N = 6000, R = 5 Hz
1.502 upper waveguide
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λ = 248 nm, I = 17 mJ/cm2, N = 8000, R = 5 Hz
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λ = 248 nm, I = 17 mJ/cm2, N=2000, R=5 Hz
1.498
1.490
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1.490 0
(g)
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40
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(h)
40
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Fig. 4. The measured refractive index depth profiles of the samples irradiated by the following parameters: k = 248 nm, I = 17 mJ/cm2, R = 5 Hz, and (a) N = 1000, (b) N = 2000, (c) N = 3000, (d) N = 4000, (e) N = 5000, (f) N = 6000, (g) N = 7000 and (h) N = 8000.
At 1000 laser pulses a moderate refractive index increase of about 0.0021 occurs from the polymer sample surface to a depth of 10 lm. Below 10 lm the refractive index difference Dn steadily decreases until it totally vanished at a depth of 50 lm (Fig. 4a). At 2000 laser pulses a local minimum of the refractive index arises at a depth of 13 lm (Fig. 4b). Below the local minimum the situation has not changed in comparison to the first sample (Fig. 4a), except for the fact that the region of modified refractive index now reaches to a depth of 90 –100 lm. But above this minimum in the area from the sample
surface to the depth of 13 lm the refractive index has strongly increased to a value of 1.4957 (Fig. 4b). By ongoing UV-irradiation the refractive index at the sample surface continues to increase, although in a non-uniform and inconsistent way: at a laser pulse number of 3000 the refractive index at the sample surface decreases to 1.495 (Fig. 4c), then at a pulse number of 4000 the refractive index increases to 1.4956 (Fig. 4d) and continue to rise to 1.4982 after 5000 pulses (Fig. 4e) until it reaches a large refractive index maximum of 1.5027 at 6000 laser pulses (Fig. 4f). In the meantime the local
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refractive index minimum shifted from a depth of 13 lm to a depth of 30 lm. The profile of the refractive index distribution below the local maximum does not change significantly in regard to shape, depth and value. At more than 6000 laser pulses the refractive index of the sample surface begins to decrease slightly passing the value of 1.4981 after 7000 pulses (Fig. 4g) until it reaches a value of 1.4974 at 8000 pulses (Fig. 4h). The upper part of the refractive index depth profile of the samples irradiated with more than 8000 pulses are difficult to characterize due to the roughening of the waveguide surface by the UV-laser irradiation. The refractive index at the surface of the waveguide samples in dependence on the number of laser pulses recorded by the Mach–Zehnder interferometer are shown in Fig. 5. One can observe two peaks: a small maximum at 2000 pulses and a big one at 6000 pulses. The refractive indices obtained by the Mach–Zehnder interferometer match the refractive index values of comparable samples examined by an Abbe´-white-light refractometer in [1]. 4.2. Mode field measurements The mode field distribution of a sample series of integrated-optical waveguides are seen in Fig. 6. Five samples are prepared under the same irradiation conditions (wavelength k = 248 nm, fluence f = 20 mJ/cm2, repetition rate R = 2 Hz, number of laser pulses N = 12000), but with different widths: 5 lm, 10 lm, 15 lm, 30 lm and 45 lm. Two intensity maxima of the light propagating inside the first waveguide sample (width: 5 lm) can be observed (Fig. 6a), also two maxima can be seen inside the second waveguide (width: 10 lm) (Fig. 6b). In case of the 15 lm wide waveguide three maxima arise in the near field (Fig. 6c), while at least 5 maxima can be observed inside the waveguide of a width of 30 lm (Fig. 6d) and at least seven maxima of the waveguide (width: 45 lm). In the last case the inten-
refractive index (n)
1.5040 1.5020 1.5000
sity maxima appear to be parallel like an interference pattern (Fig. 6e). 4.3. Loss rate measurements in dependence on the irradiation dose The loss rate of the waveguides were measured in dependence on the irradiation dose. For this purpose two sample series A and B were prepared with the following irradiation parameters: series A: k = 248 nm, f = 20 mJ/cm2, R = 2 Hz, number of laser pulses N ranging from 5000 to 15000; series B: k = 248 nm, f = 100 mJ/ cm2, R = 1 Hz, number of laser pulses N ranging from 800 to 1200. All samples have a sample length of 1 cm and a width of 25 lm. In Fig. 7a the loss rate of the ‘‘low-fluence’’ sample series A (f = 20 mJ/cm2) is drawn against the number of laser pulses N. One can observe that at relatively low pulse numbers the loss rate decreases with increasing pulse numbers until it reaches a minimum at 12000 pulses. After this minimum the loss rate increases moderately. A similar behavior can be seen at the ‘‘high fluence’’ sample series B (f = 100 mJ/ cm2) (Fig. 7b), yet with a loss rate minimum at 900 pulses. The loss rate of the sample series B is significantly higher than the loss rate of the sample series A. By light microscope observations the surface of the waveguide of series A appears to be more homogeneous and transparent than those ones of series B. The waveguide surface of the samples in series B are featured by many cracks and blisters and have grown grey and blurred during the UV-laser irradiation. 4.4. Loss rate measurements in dependence on the waveguide length (‘‘cut-back’’-method) The waveguide samples of a third series C are illuminated by the following irradiation parameters: k = 248 nm, f = 20 mJ/cm2, R = 4 Hz, N = 6000. All the samples have the same width of 45 lm, but different sample lengths. By the ‘‘cut-back’’-method one can separate the coupling loss and the waveguide loss from one another [18]. The slope of the linear regression curve yields a loss rate of 4 dB/cm for the waveguide loss, while from the ordinate intersection a coupling loss of 2.1 dB can be concluded (Fig. 7c).
1.4980 1.4960
4.5. Loss rate measurements in dependence on the waveguide width
1.4940
substrate
1.4920 1.4900 0
2000
4000
6000
8000
10000
pulses number Fig. 5. Refractive index of the polymeric substrate surface in dependence on the irradiation dose measured by the free-space Mach– Zehnder interferometer.
The waveguide samples of a forth series D are illuminated by the following irradiation parameters: k = 248 nm, f = 20 mJ/cm2, R = 4 Hz, N = 6000. All samples have the same length of 1 cm. The measurements indicate that the loss rate decreases with increasing waveguide width (Fig. 7d).
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Fig. 6. Mode field of integrated-optical polymeric waveguides (UV-irradiation parameters: k = 248 nm, f = 20 mJ/cm2, R = 2 Hz, N = 12000) with different widths recorded by a CCD-camera. (a) near field of a waveguide (width = 5 lm), (b) near field of a waveguide (width = 10 lm), (c) near field of a waveguide (width = 15 lm), (d) near field of a waveguide (width = 30 lm) and (e) near field of a waveguide (width = 45 lm).
4.6. Loss rate measurements in dependence on the coupling parameters The loss rate dependence on the z-position (height) of the incoupling fibre was investigated. Another sample series denoted as series E was generated whose irradiation parameters are presented in Fig. 7e. The smallest loss rate can be attained by incoupling shortly beneath the substrate surface. The length of the waveguide was 1 cm and the width of the
waveguide was 25 lm. The sample thickness is about 1 mm.
5. Discussion 5.1. Refractive index depth profile measurements Several photochemical processes can be proposed which could explain the shape of the refractive index
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wavelength = 248 nm fluence = 100 mJ/cm2 repetition rate = 1 Hz sample length = 1 cm sample width = 25 µm
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wavelength = 248 nm
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loss rate [dB]
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number of laser pulses N
f = 20 mJ/cm 2 R = 4 Hz N = 6000 sample length: 1 cm
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wavelength = 248 nm fluence = 20 mJ/cm2 repetition rate = 4 Hz l ser pulse number = 12000 a sample length = 1 cm sample width = 25 µm
6 4 2 0 -4
(e)
-2
0
2
4
z-position (height) [µm]
Fig. 7. Loss rate measurements of integrated-optical polymeric waveguides. (a) series A: loss rate in dependence on the irradiation dose (‘‘lowfluence’’ series), (b) series B: loss rate in dependence on the irradiation dose (‘‘high-fluence’’ series), (c) series C: incoupling and waveguide loss (‘‘cutback-method’’), (d) series D: loss rate in dependence on the waveguide width and (e) series E: loss rate in dependence on the z-position of the incoupling fibre.
profile. In every polymer sample a rest monomer content exists. In case of PMMA there is a non-negligible amount of Methylmethacrylate (MMA) monomer molecules [19–22]. According to [21–28] a very low UV-irradiation dose is sufficient to initiate a direct UV-photopolymerization (Fig. 8a). The incident UVphotons photolytically excite the double bonding between the two carbon atoms resulting in a direct UVphotopolymerization. The molecule polymer chain
growth could explain the modest increase of the refractive index after 1000 pulses (Figs. 4a and 5). But the incident UV-photons also activate the main polymer chain implying a main chain scission as well as a direct UVdepolymerization (unzipping) process (Fig. 8b) [19,20] [29,30] which result in a decrease of the refractive index. The unzipping process is boosted by the temperature rise induced by the ongoing laser irradiation [31,32]. The main chain scission and the unzipping process have a
M.A. Shams-Eldin et al. / Optical Materials 27 (2005) 1138–1148 CH3
CH3
hv (248 nm) CH2
.CH
C
C
2
.
C
C O
O
O
CH3
O
CH3
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.C
.
CH2
CH2
O
O
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C
[
C
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n-2
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C
.
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CH3
(b)
CH3
CH3
+
CH2
C
C O
CH3
n-m-1
O
CH3
hv (248 nm)
.
O
O
C
O
CH3
n-1
CH3
CH3
(2)
CH3 O
O
CH3
hv (248 nm)
CH2 ]
C
[
C
O
CH3
C O
CH2
C
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CH3
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(1)
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C C
O
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O
CH3
complete side chain scission a
[ CH2 C
.
CH3
CH3 ]
C
hv n
[ CH C 2
]
.
248 nm
+
O
O
CH3
C O
O
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high dose CH3
[...]
n
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+H
CH3 CH2
+
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CO + HOCH3 +
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(c)
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Fig. 8. (a) UV-induced direct photopolymerization of MMA, (b) (1) UV-induced direct main chain scission of PMMA, (2) UV-induced direct photodepolymerization of PMMA (unzipping) and (c) UV-laser induced photochemical modification process by complete side chain cleavage from the polymer main chain (Norrish I).
decreasing effect on the refractive index which could explain the decrease of the refractive index at the polymer surface between 1000 and 4000 pulses (Fig. 4b and c). It cannot be excluded that other UV-photon-induced mechanisms can attribute to the increase of the refractive index at low irradiation dose: after-cross-linking [29,30] (having an increasing effect on the refractive index) and a-photodissociation of the methylgroup (also affecting the refractive index) [29,30] could explain the alternating and inconsistent behaviour of the refractive index between 1000 and 4000 pulses: so the occurrence of the local refractive index minimum at 3000 pulses (Fig. 5) does not match exactly with the occurrence of
the local refractive index minimum between 1000 and 2000 pulses (Fig. 4b and c). But by ongoing UV-irradiation a photochemical modification of the polymer structure occurs by which the side chain is cleaved from the main chain (Fig. 8c). This cleavage is a photochemical reaction of Norrish Type I [33] implying a mechanical densification of the polymer material by dipole–dipole molecule interaction [34] after the degraded volatile polymeric molecule fragments are diffused out of the bulk material. The entire UV-laser-assisted modification mechanism of the PMMA-polymer is described in details elsewhere [1]. This photochemical process can explain the increase of
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the refractive index to its maximum (n = 1.5027) at the substrate surface at a pulse number of 6000 (Figs. 4c–f and 5). By continuing UV-irradiation more and more side chains are split from the main chain, so the modification effect becomes more and more dominant and finally strongly prevails over the remaining photoinduced effects like photo(de)polymerization which does not play a role any longer. By ongoing UV-irradiation many side chains are separated from the polymer main chain which makes the polymer main chain very unstable [1,35,36]. So main chain scission becomes more and more likely which explains the rapid decrease of the refractive index at the substrate surface at pulse numbers higher than 6000 (Figs. 4g–h and 5). During the UV-irradiation process the continuous change of the refractive index is also accompanied by the creation of double bonds in the PMMA polymer main chain (incubation) [37] which can be observed by the photo-yellowing of the UV-illuminated area. According to Ref. [37] the generation of the carbon double bondings lowers the threshold of ablation accelerating the degradation process of the polymer. Thus the continuing UV-irradiation results in the total defragmentation of the polymer structure. The depth of the region in which the direct UV-photopolymerization occurs is much deeper than the area of UV-modified PMMA, because it is required much less fluence to induce a direct UV-photopolymerization process of PMMA than to induce a modification process of PMMA featured by side chain cleavage. This can be explained as follows: After the side chain cleavage the defragmentation products of the broken side chain diffuse away yielding a mechanical densification of the polymeric material and thus an increase of the refractive index. A diffusion process is generally considered as an activated process. Although the cleavage of the side chain is a non-activated photochemical reaction, the entire UVlaser assisted modification process of the refractive index is partly thermally activated due to the thermally activated diffusion process. Without the diffusion of the defragmentation products out of the substrate no refractive index increase would occur. A relatively high fluence of UV-irradiation is required to trigger a thermally activated process thus the modification process only takes place in the upper layer of the polymer substrate. At more than 1000 laser pulses the shape of the bottom part of the refractive index profile does not change significantly. This can be explained by the following: before the photopolymerization in the lower part of the refractive index profile is finished, it is totally shielded by the upper part of the polymeric substrate as a result of an increasing absorption coefficient of the UV-irradiated area at the polymer substrate surface (incubation) [37,38]. Summarizing the lower part of the refractive index profile is mainly generated by direct UV-photo(de)poly-
merization, while the upper part is formed by direct UVmodification (side chain cleavage) (Fig. 4c–h). Simplified expressed the first small refractive index maximum at 2000 pulses (Fig. 5) is due to the direct UV-photo(de)polymerization and the second big one is caused by the direct UV-modification of the polymer structure (side chain cleavage with subsequent material densification of the polymeric material). 5.2. Mode field measurements According to [39] the number of modes of light propagating inside a waveguiding structure is determined by its wavelength, the incoupling angle of the lightwave, the refractive index profile and especially the geometrical feature of the waveguide e.g. the width of it. The measurements of the mode field distribution confirm that the maximal number of modes rises with increasing waveguide width (Fig. 6). The near field measurement of the 45 lm wide waveguide (Fig. 6e) shows an interference-like mode field distribution. Eventually the waveguide is sufficiently broad so that interference effects could occur between the existing modes. 5.3. Loss rate measurements in dependence on the irradiation dose The loss rate measurements indicate that the loss rate minimum is set at quite a high irradiation dose (N = 12000) (Fig. 7a). The refractive index has its maximum at 6000 pulses (Fig. 5). After 6000 pulses the refractive index starts to decrease, so the loss rate minimum (Fig. 7a) does not coincide with the refractive index maximum. This can be explained as follows: the refractive index curve (Fig. 5) only presents the refractive index values of the polymeric surface, not the refractive index inside the volume. But for the light transport the refractive index hub Dn inside the volume is relevant. At high irradiation dose (N 12000) the maximum refractive index hub Dn does not occur directly at the surface, but some microns beneath the surface of the waveguiding structure, because directly at the surface the polymer starts to be defragmented due to a too high UV-irradiation dose. No refractive index depth measurements were performed for waveguide samples generated with more than 8000 pulses, because the polymer surface in the UV-illuminated area became too rough after a too long UV-irradiation. So no information could be received about the refractive index profile shortly beneath the substrate surface of the waveguiding structure generated by more than 8000 pulses. The loss rate values of the sample series B (‘‘high-fluence’’) (Fig. 7b) are higher than the ones of the sample series A (‘‘low-fluence’’) (Fig. 7a). During the UV-laser assisted waveguide generation at high fluences photothermal effects cannot be inhibited due to the confined
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heat transport capacity of polymer. So thermal degradation of the waveguiding structure occurs being visible under the optical light microscope.
total light exiting from the fibre is inserted into the waveguide. In both cases the attenuation rises.
5.4. Loss rate measurements in dependence on the waveguide length (‘‘cut-back’’-method)
6. Conclusion
The total loss of a waveguide Atotal is composed of the coupling loss Acoupl and the intrinsic waveguide loss Aintr. = aintr*l with aintr as the intrinsic waveguide loss per length (intrinsic waveguide loss rate) and l as the waveguide length: Atotal = aintr*l + Acoupl. Consequently the slope of the regression curve (Fig. 7c) yields the intrinsic waveguide loss rate, while the point of intersection of the curve with the ordinate axis is identical with the coupling loss (Fig. 7c). Consequently the average coupling loss (incoupluing and outcoupling loss) of the sample series C is 2.1 dB and the average intrinsic waveguide loss rate is 4 dB/cm. 5.5. Loss rate measurements in dependence on the waveguide width Contrary to a monomode waveguide, in a multimode waveguide the defects of the waveguiding structure can be compensated by mode coupling without losing much light energy. The higher the number of modes the better the waveguide defects are compensated by mode coupling. According to [39] the maximal number of modes by which the light can propagate inside the waveguide rises with increasing waveguide diameter. Thus the loss rate of the waveguide decreases with increasing waveguide width (Fig. 7d). Besides the coupling loss of a broad waveguide is smaller than the coupling loss of a thin waveguide because in the case of a waveguide with a small diameter even a small misadjustment of the coupling fibre results in big coupling losses. 5.6. Loss rate measurements in dependence on the coupling parameters The refractive index depth profile is very asymmetric and of gradient nature (Fig. 4). The optimal light transport is performed by coupling light shortly beneath the waveguide surface (Fig. 7e) where the refractive index hub Dn is bigger than deep inside the bulk of the polymer substrate (Fig. 4). If the incoupling fibre is located in a deeper position, the incoupling light encounters a waveguiding structure whose refractive indes hub Dn and also the numerical aperture is inferior to those one occurring shortly beneath the waveguide surface. Thus the coupling losses into the waveguide is higher when the incoupling fibre is located in a deep incoupling position. In the other case if the incoupling fibre is positioned partly above the polymeric waveguide, not the
In this paper the functional-optical properties of UVlaser generated integrated-optical waveguides in the surface of a planar polymer chip have been characterized. Especially the refractive index profile strongly depends on the irradiation dose. At medium and high irradiation doses a double waveguide structure has been formed due to several mostly independent photochemical processes occurring in the UV-irradiated region of the polymer chip surface. All the photochemical processes competing with one another have different influences on the refractive index: some have an increasing effect, the others have a decreasing effect on the refractive index. So by the right choice of irradiation parameters the refractive index of the polymer can be increased controllably and locally, which enables the fabrication of integrated-optical structures by UV-laser light irradiation. Acknowledgments This work was supported by the Egypt Ministry of Science and Education and by the Deutsche Forschungsgemeinschaft (DFG) under contract No. ME 991/10-1 which the authors gratefully acknowledge.
References [1] C. Wochnowski, S. Metev, G. Sepold, Appl. Surf. Sci. 154–155 (2000) 706. [2] M. Schmidt, M. Dirscherl, S. Kaufmann, M. Stark, in: Proc. ICALEO Õ03, Jacksonville, USA, LIA Pub #: 595 (CD-version) Vol. 95 (2003) Paper-ID M107. [3] S. Metev, C. Wochnowski, F. Vollertsen, in: Proc. of ICALEO Õ03, Jacksonville, USA, LIA Pub #: 595 (CD-version) Vol. 95 (2003) Paper-ID M101. [4] E.M. Conwell, Appl. Phys. Lett. 23 (1973) 328. [5] D. Marcuse, IEEE J. Quant. Elect. QE-9 (1973) 1000. [6] G.B. Hocker, W.K. Burns, IEEE J. Quant. Elect. QE-11 (1975) 270. [7] J.M. White, P.F. Heidrich, Appl. Opt. 15 (1976) 151. [8] K.S. Chiang, Lightwave Technol. LT-3 (1985) 385. [9] P. Mathey, P. Jullien, Opt. Comm. 122 (1996) 127–134, p. 127. [10] W.E. Martin, Appl. Opt. 13 (1974) 2112. [11] B.E. Allman, A.G. Klein, K.A. Nugent, G.I. Opat, Appl. Opt. 33 (1994) 1806. [12] W.A. Ramadan, E. Fazio, M. Bertolotti, Appl. Opt. 35 (1994) 6173. [13] S.C. Simmens, Nature, London 181 (1958) 1260. [14] A. Hamza, Textile Res. J. 50 (1980) 731. [15] A. Hamza, T.Z.N. Sokkar, M.A. Kabeel, J. Phys. D: Appl. Phys. 18 (1985) 2321. [16] A. Hamza, M.M. El-Tonsy, S.K. El-Labany, Polym. Testing 8 (1989) 329. [17] A. Hamza, in: N. Barakat, A. Hamza (Eds.), Interferometry of Fibrous Materials, Adam Hilger, 1990, p. 37.
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[18] S. Geckeler, in: ‘‘Lichtwellenleiter fu¨r die optische Nachrichtenu¨bertragung’’, vol. 16 Springer Verlag Berlin (1987) 22. [19] R.B. Fox, L.G. Isaacks, S. Stokes, US Naval Research Lab. Rep. No. 5720 (1961). [20] J.R. MacCallum, C.K. Schoff, J. Chem. Soc. Faraday Trans. 68 (1972) 499. [21] M. Kopietz, M.D. Lechner, D.G. Steinmeier, Makromol. Chem. Rap. Comm. 4 (1983) 113. [22] M. Kopietz, M.D. Lechner, D.G. Steinmeier, Polym. Photochem. 5 (1984) 109. [23] H.W. Melville, Proc. Royal Soc. A 163 (1937) 511. [24] S. Jo´nsson, W. Schaeffer, P.-E. Sundell, M. Shimose, J. Owens, C.E. Hoyle, in: Proc. Conf. Rad Tech Int. North America, Orlando, 1 (1994) 194. [25] J.L Garnett, Radiat. Phys. Chem. 46 (1995) 925. [26] J.Y. Tsao, D.J. Ehrlich, Appl. Phys. Lett. 42 (12) (1983) 997. [27] H. Morita, T. Sadakiyo, J. Photochem. Photobiol. A: Chemistry 87 (1995) 163. [28] F.D. Lewis, M.J. Nepras, H.L. Hampsch, Tetrahedron 43 (7) (1987) 1635. [29] W.B. Hardy, in: N. S. Allen (Ed.), Developments in Polymer Photochemistry—3, Appl. Sci. Publ., London/New Jersey.
[30] B. Ranby, J.F. Rabek, in: J.F. Rabek (Ed.), Photodegradation, Photooxidation and Photostabilization of Polymers-Principles and Applications, John Wiley and Sons, 1996, p. 153. [31] T. Lippert, R.L. Webb, S.C. Langford, J.T. Dickinson, J. Appl. Phys. 85 (3) (1999) 1838, February. [32] J.L. Graham, J.M. Berman, B. Dellinger, J. Photochem. Photobiol. A 71 (1993) 65. [33] J.F. Rabek (Ed.), Photodegradation of Polymers, Springer, 1996, p. 77. [34] A. Schmoldt, Thesis work, Techn. Univ. Berlin, Germany (1994) 95. [35] M. Dole, in: M. Dole (Ed.), Reaction Chemistry of Macromolecules, vol. II, Academic Press NY, London, (1973) 97. [36] J.A. Moore, J.O. Choi, in: R.L. Clough, S.W. Shalaby (Eds.), Radiation Effects on Polymers, American Chemical Society, 1991, p. 158. [37] S. Ku¨per, M. Stuke, Appl. Phys. A 49 (1989) 211. [38] W.F.X. Frank, A. Scho¨sser, A. Stelmaszyk, J. Schulz, in: R.A. Lessard, W. F. X. Frank (Eds.), Polymers in Optics, Physics, Chemistry and Applications, SPIE-Vol. CR 63, USA (1996) 65. [39] W. Karthe, in: ‘‘Integrierte Optik‘‘ Akadem. Verlagsgesell, Geest & Portig (1991) 11.
Characterization of the optical-functional properties of a waveguide written by an UV-laser into a planar polymer chip M.A. Shams-Eldin a, C. Wochnowski b,*, M. Koerdt b, S. Metev b, A.A. Hamza a, W. Ju¨ptner b b
a University of Mansoura, Mansoura, Egypt BIAS (Bremer Institut fu¨r Angewandte Strahltechnik), Klagenfurter Strasse 2, 28359 Bremen, Germany
Received 5 April 2004; accepted 14 September 2004 Available online 18 November 2004
Abstract The optical-functional properties of an integrated-optical strip-waveguide in a planar polymer chip are presented in this article. The waveguide was directly written into the surface of a planar polymer chip by UV-laser irradiation. The refractive index depth profile of the waveguide was examined by a two-beam-interferometric method. Also the mode field distribution and the loss rate of the waveguiding structure has been characterized. The study shows that the optical-functional properties strongly depend on the UV-irradiation parameters. Several mostly independently occurring photochemical processes competing with one another are proposed to explain the formation and shape of the refractive index distribution. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Polymethylmethacrylate; Excimer laser; Polymer integrated optics; Two-beam-interferometry (Mach–Zehnder); Refractive index depth profile
1. Introduction The optical properties of an UV-modifiable polymer can be photochemically changed in a controllable way by UV-laser irradiation [1,2]. Especially the UV-laserassisted local modification of the refractive index of polymethylmethacrylate (PMMA) permits the fabrication of integrated-optical waveguides in a planar polymer chip [3]. For the design and fabrication of an integrated-optical component the exact characterization of the optical-functional properties, especially the refractive index depth profile, of the integrated-optical polymer waveguides are urgently required.
*
Corresponding author. Tel.: +49 4212185078; fax: 4212185095. E-mail address: [email protected] (C. Wochnowski).
+49
0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.09.019
Refractive index depth profile measurements of integrated-optical gradient waveguiding samples prepared by a diffusion method are usually performed in transmission modus [4–9]. Other research groups report about refractive index depth profile measurements by an interferometric method done in reflection modus [10–12]. Simmens [13] described an interferometric technique to determine the refractive indices of complex structure fibres employed in transmission modus. Hamza [14] presented a method in order to measure the mean refractive indices and birefringence of fibre optics by the use of a two-beam-interferometry-technique. The mathematical expressions employed in this work are derived by Hamza et al. [15–17] and are generally valid for all kinds of two-beam-interferometry techniques. The same mathematical expressions are also used in our work in which we study the refractive index distribution of a waveguide prepared by UV-excimer laser irradiation in
M.A. Shams-Eldin et al. / Optical Materials 27 (2005) 1138–1148
a planar polymer chip by employing a two-beam interference technique in transmission modus. Two mostly independent photochemical reaction mechanisms are proposed in order to explain the experimental results.
2. Theoretical treatment The UV-laser-generated integrated-optical waveguide is supposed to be of gradient-index type. Its refractive index depth profile is measured by an optical free-space Mach–Zehnder interferometer (Fig. 1) which is described in detail in the next passage. The integrated-optical polymeric waveguide is a gradient waveguide which can be considered as being composed of many high index layers ni varying in dependence on the depth and being surrounded by low index materials (cladding substrate) (Fig. 2). The slab waveguide is infinitely extended in x-direction and finitely in y- and z-directions. The waveguide is inserted into an immersion liquid of refractive index nL near to that of substrate nS since ni > nS nL . We assume that the cross section of the waveguide is rectangular (Fig. 2). The top surface of the waveguide is
1 - Spatial Filter 2 - Diaphram 3 - Polarizer 4 - Collimated Lens 5 - Screen 6 - Cell contains liquid 7 - Cell contains liquid and the sample
LaserSource s
SpatialFilter
A2
M1
4
5
7
6
M2
1
2 L1
3 A1
Fig. 1. Experimental set-up of the optical free-space Mach–Zehnder interferometer.
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at z = 0 and the bottom side of the waveguide is at z = d (depth). The refractive index of the waveguide changes in the z-direction starting from the surface at z = 0 to z = d (depth). The zone inside the substrate below the waveguiding structure having the refractive index nS starts from z = d in the positive direction of z. Measuring the values of the refractive index depth profile of the waveguide requires the value of substrate refractive index nS. The optical path difference DCS between the substrate and the immersed liquid is represented as DCS ¼ ðnS nL ÞaS
ð1Þ
with aS as the distance travelled in the substrate region. In case of two-beam interferometry, the optical path difference DCS is given by the following formula [13,14,17]. xS DCS ¼ k ð2Þ h where xS, h and k are the fringe shift inside the substrate, interfringe spacing and the wavelength of the test light (Fig. 3). The combination of the Eqs. (1) and (2) yields xS k ¼ ðnS nL ÞaS ð3Þ h By the same method the refractive index depth profile of the waveguide can be determined. The waveguide is divided into individual layers of constant thickness (Fig. 2). Monochromatic light emitted by a HeNe-laser is directed parallel to the y-axis of this waveguide. The optical path difference for each layer can be determined by the following formula: xi k ¼ ðni nS Þa; i ¼ 1; 2; . . . ; N ð4Þ h where i and a are the layer number and the distance travelled by the beam of light inside the waveguide, respectively. The resulting shift occurs in the x–z plane. By using Eq. (4) the refractive index depth profile of the waveguide is graphically reconstructed.
UV-excimerlaser beam top surface
y
n1 n2 n3
UV-laser modified area incident beam
HeNe-laser beam
150 µm
500 µm
matching-liquid optical cell (paraffins und α-bromine-naphthalene)
d n i-1 ni ns
PMMA-chip
z
a
Fig. 2. Optical cell with polymeric gradient waveguide sample consisting of many high index layers.
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the mirror M1 the first of the two partial laser beams is directed normally on the sample immersed in the liquid with a refractive index quite close to the refractive index of the substrate material nS as shown in Fig. 2. The second partial laser beam serves as the reference beam and is reflected by the mirror M2. The two beams are recombined at the beam splitter A2, which is identical to A1. The fringe pattern, which can be observed on the screen 5, resulting from the interference of these two beams is a family of equally spaced parallel fringes covering the shift through the entire channel waveguide structure. An example of such an interferogram can be seen in Fig. 3. 3.3. Experimental set-up for the mode field investigation and loss rate measurements Fig. 3. Sample interferogram recorded by the Mach–Zehnder interferometer.
3. Experimental 3.1. Fabrication of the waveguides As the basic polymer material PMMA provided by Goodfellow (product number: ME 303010) was employed. The UV-laser assisted generation of the integrated-optical waveguide in a planar PMMA chip is done by an UV-lithographic method which is detailly described elsewhere [1,2]. In the UV-illumination facility a KrF-excimer is employed emitting a wavelength of 248 nm. The laser beam is directed on the polymer sample situated in a sample holder. A quartz glass mask with an inherent aperture structure in the chrome layer is directly positioned on the polymer chip surface. Incident UV-photons illuminate the uncovered regions of the polymer chip surface where direct electronic bond breaking induces a photochemical modification mechanism in the polymer material. So the UV-irradiation results in a local and controllable increase of refractive index in the illuminated areas of the polymer surface generating the integrated-optical waveguiding structures in the planar polymer chip [1]. 3.2. Experimental set-up for refractive index depth profile measurements The experimental set-up for the refractive index depth profile measurements is based on an optical free-space Mach–Zender interferometer (Fig. 1). Polarized light is emitted by a He–Ne laser S (wavelength k = 632.8 nm). The spatial filter is situated in the focal plane of a collimating lens L1 in order to improve the beam quality. The parallel laser beam is divided by the beam splitter A1 into two partial laser beams. After reflection from
The mode propagation and the loss rate of the light guided by the integrated-optical waveguide is measured by a similar experimental set-up: A laser test beam was coupled into the waveguide by fibre-chip-coupling. A fibre with a diameter of 4 lm was employed as a coupling device. This fibre is monomode for red light. A laser diode (632.8 nm) served as a light source. In case of mode field characterization the light exiting from the back side of the polymer chip was directed to a freespace standing objective lense magnifying and projecting the image onto the chip of a CCD camera. The data were electronically recorded and evaluated by a computer. In case of loss rate measurements the light was coupled into a multimode fibre (diameter: 62.5 lm) on the back side of the polymer chip. The optical signals were transmitted to a photo diode by fibre optics, where they are converted into electrical data. The data were recorded and evaluated by a computer.
4. Results 4.1. Refractive index depth profile measurements The refractive index of the unirradiated polymeric substrate is measured for several unirradiated polymer samples by the Mach–Zehnder interferometry technique yielding an average value of nS = 1.49154 ± 0.0001 with the immersion liquid having a refractive index nL = 1.4912. A series of waveguide samples is prepared with the following irradiation parameters: wavelength k = 248 nm, fluence I = 17 mJ/cm2, repetition rate R = 5 Hz and the number of laser pulses N ranging from 1000 to 8000 increased by step of 1000 for each sample. The refractive index depth profiles of the waveguide samples is recorded in dependence on the irradiation dose (number of laser pulses) (Fig. 4).
M.A. Shams-Eldin et al. / Optical Materials 27 (2005) 1138–1148 1.500
λ = 248 nm, I = 17 mJ/cm2, N = 1000, R = 5 Hz
1.498 1.496 1.494 substrate
1.492
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λ = 248 nm, I = 17 mJ/cm2, N = 5000, R = 5 Hz
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1.492
40
(d)
refractive index
refractive index
1.500
refractive index
upper waveguide
40
depth (µm)
(a)
λ = 148 nm, I = 17 mJ/cm2, N = 6000, R = 5 Hz
1.502 upper waveguide
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40
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depth (µm)
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λ = 248 nm, I = 17 mJ/cm2, N = 8000, R = 5 Hz
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λ = 248 nm, I = 17 mJ/cm2, N = 7000, R = 5 Hz
1.492
40
(f)
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λ = 248 nm, I = 17 mJ/cm2, N=2000, R=5 Hz
1.498
1.490
1.490
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1.490 0
(g)
1141
40
0
80
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(h)
40
80
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Fig. 4. The measured refractive index depth profiles of the samples irradiated by the following parameters: k = 248 nm, I = 17 mJ/cm2, R = 5 Hz, and (a) N = 1000, (b) N = 2000, (c) N = 3000, (d) N = 4000, (e) N = 5000, (f) N = 6000, (g) N = 7000 and (h) N = 8000.
At 1000 laser pulses a moderate refractive index increase of about 0.0021 occurs from the polymer sample surface to a depth of 10 lm. Below 10 lm the refractive index difference Dn steadily decreases until it totally vanished at a depth of 50 lm (Fig. 4a). At 2000 laser pulses a local minimum of the refractive index arises at a depth of 13 lm (Fig. 4b). Below the local minimum the situation has not changed in comparison to the first sample (Fig. 4a), except for the fact that the region of modified refractive index now reaches to a depth of 90 –100 lm. But above this minimum in the area from the sample
surface to the depth of 13 lm the refractive index has strongly increased to a value of 1.4957 (Fig. 4b). By ongoing UV-irradiation the refractive index at the sample surface continues to increase, although in a non-uniform and inconsistent way: at a laser pulse number of 3000 the refractive index at the sample surface decreases to 1.495 (Fig. 4c), then at a pulse number of 4000 the refractive index increases to 1.4956 (Fig. 4d) and continue to rise to 1.4982 after 5000 pulses (Fig. 4e) until it reaches a large refractive index maximum of 1.5027 at 6000 laser pulses (Fig. 4f). In the meantime the local
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refractive index minimum shifted from a depth of 13 lm to a depth of 30 lm. The profile of the refractive index distribution below the local maximum does not change significantly in regard to shape, depth and value. At more than 6000 laser pulses the refractive index of the sample surface begins to decrease slightly passing the value of 1.4981 after 7000 pulses (Fig. 4g) until it reaches a value of 1.4974 at 8000 pulses (Fig. 4h). The upper part of the refractive index depth profile of the samples irradiated with more than 8000 pulses are difficult to characterize due to the roughening of the waveguide surface by the UV-laser irradiation. The refractive index at the surface of the waveguide samples in dependence on the number of laser pulses recorded by the Mach–Zehnder interferometer are shown in Fig. 5. One can observe two peaks: a small maximum at 2000 pulses and a big one at 6000 pulses. The refractive indices obtained by the Mach–Zehnder interferometer match the refractive index values of comparable samples examined by an Abbe´-white-light refractometer in [1]. 4.2. Mode field measurements The mode field distribution of a sample series of integrated-optical waveguides are seen in Fig. 6. Five samples are prepared under the same irradiation conditions (wavelength k = 248 nm, fluence f = 20 mJ/cm2, repetition rate R = 2 Hz, number of laser pulses N = 12000), but with different widths: 5 lm, 10 lm, 15 lm, 30 lm and 45 lm. Two intensity maxima of the light propagating inside the first waveguide sample (width: 5 lm) can be observed (Fig. 6a), also two maxima can be seen inside the second waveguide (width: 10 lm) (Fig. 6b). In case of the 15 lm wide waveguide three maxima arise in the near field (Fig. 6c), while at least 5 maxima can be observed inside the waveguide of a width of 30 lm (Fig. 6d) and at least seven maxima of the waveguide (width: 45 lm). In the last case the inten-
refractive index (n)
1.5040 1.5020 1.5000
sity maxima appear to be parallel like an interference pattern (Fig. 6e). 4.3. Loss rate measurements in dependence on the irradiation dose The loss rate of the waveguides were measured in dependence on the irradiation dose. For this purpose two sample series A and B were prepared with the following irradiation parameters: series A: k = 248 nm, f = 20 mJ/cm2, R = 2 Hz, number of laser pulses N ranging from 5000 to 15000; series B: k = 248 nm, f = 100 mJ/ cm2, R = 1 Hz, number of laser pulses N ranging from 800 to 1200. All samples have a sample length of 1 cm and a width of 25 lm. In Fig. 7a the loss rate of the ‘‘low-fluence’’ sample series A (f = 20 mJ/cm2) is drawn against the number of laser pulses N. One can observe that at relatively low pulse numbers the loss rate decreases with increasing pulse numbers until it reaches a minimum at 12000 pulses. After this minimum the loss rate increases moderately. A similar behavior can be seen at the ‘‘high fluence’’ sample series B (f = 100 mJ/ cm2) (Fig. 7b), yet with a loss rate minimum at 900 pulses. The loss rate of the sample series B is significantly higher than the loss rate of the sample series A. By light microscope observations the surface of the waveguide of series A appears to be more homogeneous and transparent than those ones of series B. The waveguide surface of the samples in series B are featured by many cracks and blisters and have grown grey and blurred during the UV-laser irradiation. 4.4. Loss rate measurements in dependence on the waveguide length (‘‘cut-back’’-method) The waveguide samples of a third series C are illuminated by the following irradiation parameters: k = 248 nm, f = 20 mJ/cm2, R = 4 Hz, N = 6000. All the samples have the same width of 45 lm, but different sample lengths. By the ‘‘cut-back’’-method one can separate the coupling loss and the waveguide loss from one another [18]. The slope of the linear regression curve yields a loss rate of 4 dB/cm for the waveguide loss, while from the ordinate intersection a coupling loss of 2.1 dB can be concluded (Fig. 7c).
1.4980 1.4960
4.5. Loss rate measurements in dependence on the waveguide width
1.4940
substrate
1.4920 1.4900 0
2000
4000
6000
8000
10000
pulses number Fig. 5. Refractive index of the polymeric substrate surface in dependence on the irradiation dose measured by the free-space Mach– Zehnder interferometer.
The waveguide samples of a forth series D are illuminated by the following irradiation parameters: k = 248 nm, f = 20 mJ/cm2, R = 4 Hz, N = 6000. All samples have the same length of 1 cm. The measurements indicate that the loss rate decreases with increasing waveguide width (Fig. 7d).
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Fig. 6. Mode field of integrated-optical polymeric waveguides (UV-irradiation parameters: k = 248 nm, f = 20 mJ/cm2, R = 2 Hz, N = 12000) with different widths recorded by a CCD-camera. (a) near field of a waveguide (width = 5 lm), (b) near field of a waveguide (width = 10 lm), (c) near field of a waveguide (width = 15 lm), (d) near field of a waveguide (width = 30 lm) and (e) near field of a waveguide (width = 45 lm).
4.6. Loss rate measurements in dependence on the coupling parameters The loss rate dependence on the z-position (height) of the incoupling fibre was investigated. Another sample series denoted as series E was generated whose irradiation parameters are presented in Fig. 7e. The smallest loss rate can be attained by incoupling shortly beneath the substrate surface. The length of the waveguide was 1 cm and the width of the
waveguide was 25 lm. The sample thickness is about 1 mm.
5. Discussion 5.1. Refractive index depth profile measurements Several photochemical processes can be proposed which could explain the shape of the refractive index
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wavelength = 248 nm fluence = 100 mJ/cm2 repetition rate = 1 Hz sample length = 1 cm sample width = 25 µm
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6 4 2 0 -4
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Fig. 7. Loss rate measurements of integrated-optical polymeric waveguides. (a) series A: loss rate in dependence on the irradiation dose (‘‘lowfluence’’ series), (b) series B: loss rate in dependence on the irradiation dose (‘‘high-fluence’’ series), (c) series C: incoupling and waveguide loss (‘‘cutback-method’’), (d) series D: loss rate in dependence on the waveguide width and (e) series E: loss rate in dependence on the z-position of the incoupling fibre.
profile. In every polymer sample a rest monomer content exists. In case of PMMA there is a non-negligible amount of Methylmethacrylate (MMA) monomer molecules [19–22]. According to [21–28] a very low UV-irradiation dose is sufficient to initiate a direct UV-photopolymerization (Fig. 8a). The incident UVphotons photolytically excite the double bonding between the two carbon atoms resulting in a direct UVphotopolymerization. The molecule polymer chain
growth could explain the modest increase of the refractive index after 1000 pulses (Figs. 4a and 5). But the incident UV-photons also activate the main polymer chain implying a main chain scission as well as a direct UVdepolymerization (unzipping) process (Fig. 8b) [19,20] [29,30] which result in a decrease of the refractive index. The unzipping process is boosted by the temperature rise induced by the ongoing laser irradiation [31,32]. The main chain scission and the unzipping process have a
M.A. Shams-Eldin et al. / Optical Materials 27 (2005) 1138–1148 CH3
CH3
hv (248 nm) CH2
.CH
C
C
2
.
C
C O
O
O
CH3
O
CH3
CH3
.C
.
CH2
CH2
O
O
CH3
CH3 CH2 ]
C
[
C
O
CH3
CH3
.C
+
C
(a)
CH3 O
[ CH2 C
O
]
[.... ]
n
m-1
H2C
C
.
[.... ]
n-1
. CH
+
2
C O
H2C
C
[....]
C
O
CH3
O
[.... ]
n-2
H2C
C
.
O
CH3
(b)
CH3
CH3
+
CH2
C
C O
CH3
n-m-1
O
CH3
hv (248 nm)
.
O
O
C
O
CH3
n-1
CH3
CH3
(2)
CH3 O
O
CH3
hv (248 nm)
CH2 ]
C
[
C
O
CH3
C O
CH2
C
C O
CH3
.C
n-2
CH3
(1)
1145
C C
O
CH3
O
O
CH3
complete side chain scission a
[ CH2 C
.
CH3
CH3 ]
C
hv n
[ CH C 2
]
.
248 nm
+
O
O
CH3
C O
O
CH3
high dose CH3
[...]
n
CH2 C
+H
CH3 CH2
+
.C
[...]
CO + HOCH3 +
CO2 + CH4
n
C O
(c)
O
CH3
main chain scission
Fig. 8. (a) UV-induced direct photopolymerization of MMA, (b) (1) UV-induced direct main chain scission of PMMA, (2) UV-induced direct photodepolymerization of PMMA (unzipping) and (c) UV-laser induced photochemical modification process by complete side chain cleavage from the polymer main chain (Norrish I).
decreasing effect on the refractive index which could explain the decrease of the refractive index at the polymer surface between 1000 and 4000 pulses (Fig. 4b and c). It cannot be excluded that other UV-photon-induced mechanisms can attribute to the increase of the refractive index at low irradiation dose: after-cross-linking [29,30] (having an increasing effect on the refractive index) and a-photodissociation of the methylgroup (also affecting the refractive index) [29,30] could explain the alternating and inconsistent behaviour of the refractive index between 1000 and 4000 pulses: so the occurrence of the local refractive index minimum at 3000 pulses (Fig. 5) does not match exactly with the occurrence of
the local refractive index minimum between 1000 and 2000 pulses (Fig. 4b and c). But by ongoing UV-irradiation a photochemical modification of the polymer structure occurs by which the side chain is cleaved from the main chain (Fig. 8c). This cleavage is a photochemical reaction of Norrish Type I [33] implying a mechanical densification of the polymer material by dipole–dipole molecule interaction [34] after the degraded volatile polymeric molecule fragments are diffused out of the bulk material. The entire UV-laser-assisted modification mechanism of the PMMA-polymer is described in details elsewhere [1]. This photochemical process can explain the increase of
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the refractive index to its maximum (n = 1.5027) at the substrate surface at a pulse number of 6000 (Figs. 4c–f and 5). By continuing UV-irradiation more and more side chains are split from the main chain, so the modification effect becomes more and more dominant and finally strongly prevails over the remaining photoinduced effects like photo(de)polymerization which does not play a role any longer. By ongoing UV-irradiation many side chains are separated from the polymer main chain which makes the polymer main chain very unstable [1,35,36]. So main chain scission becomes more and more likely which explains the rapid decrease of the refractive index at the substrate surface at pulse numbers higher than 6000 (Figs. 4g–h and 5). During the UV-irradiation process the continuous change of the refractive index is also accompanied by the creation of double bonds in the PMMA polymer main chain (incubation) [37] which can be observed by the photo-yellowing of the UV-illuminated area. According to Ref. [37] the generation of the carbon double bondings lowers the threshold of ablation accelerating the degradation process of the polymer. Thus the continuing UV-irradiation results in the total defragmentation of the polymer structure. The depth of the region in which the direct UV-photopolymerization occurs is much deeper than the area of UV-modified PMMA, because it is required much less fluence to induce a direct UV-photopolymerization process of PMMA than to induce a modification process of PMMA featured by side chain cleavage. This can be explained as follows: After the side chain cleavage the defragmentation products of the broken side chain diffuse away yielding a mechanical densification of the polymeric material and thus an increase of the refractive index. A diffusion process is generally considered as an activated process. Although the cleavage of the side chain is a non-activated photochemical reaction, the entire UVlaser assisted modification process of the refractive index is partly thermally activated due to the thermally activated diffusion process. Without the diffusion of the defragmentation products out of the substrate no refractive index increase would occur. A relatively high fluence of UV-irradiation is required to trigger a thermally activated process thus the modification process only takes place in the upper layer of the polymer substrate. At more than 1000 laser pulses the shape of the bottom part of the refractive index profile does not change significantly. This can be explained by the following: before the photopolymerization in the lower part of the refractive index profile is finished, it is totally shielded by the upper part of the polymeric substrate as a result of an increasing absorption coefficient of the UV-irradiated area at the polymer substrate surface (incubation) [37,38]. Summarizing the lower part of the refractive index profile is mainly generated by direct UV-photo(de)poly-
merization, while the upper part is formed by direct UVmodification (side chain cleavage) (Fig. 4c–h). Simplified expressed the first small refractive index maximum at 2000 pulses (Fig. 5) is due to the direct UV-photo(de)polymerization and the second big one is caused by the direct UV-modification of the polymer structure (side chain cleavage with subsequent material densification of the polymeric material). 5.2. Mode field measurements According to [39] the number of modes of light propagating inside a waveguiding structure is determined by its wavelength, the incoupling angle of the lightwave, the refractive index profile and especially the geometrical feature of the waveguide e.g. the width of it. The measurements of the mode field distribution confirm that the maximal number of modes rises with increasing waveguide width (Fig. 6). The near field measurement of the 45 lm wide waveguide (Fig. 6e) shows an interference-like mode field distribution. Eventually the waveguide is sufficiently broad so that interference effects could occur between the existing modes. 5.3. Loss rate measurements in dependence on the irradiation dose The loss rate measurements indicate that the loss rate minimum is set at quite a high irradiation dose (N = 12000) (Fig. 7a). The refractive index has its maximum at 6000 pulses (Fig. 5). After 6000 pulses the refractive index starts to decrease, so the loss rate minimum (Fig. 7a) does not coincide with the refractive index maximum. This can be explained as follows: the refractive index curve (Fig. 5) only presents the refractive index values of the polymeric surface, not the refractive index inside the volume. But for the light transport the refractive index hub Dn inside the volume is relevant. At high irradiation dose (N 12000) the maximum refractive index hub Dn does not occur directly at the surface, but some microns beneath the surface of the waveguiding structure, because directly at the surface the polymer starts to be defragmented due to a too high UV-irradiation dose. No refractive index depth measurements were performed for waveguide samples generated with more than 8000 pulses, because the polymer surface in the UV-illuminated area became too rough after a too long UV-irradiation. So no information could be received about the refractive index profile shortly beneath the substrate surface of the waveguiding structure generated by more than 8000 pulses. The loss rate values of the sample series B (‘‘high-fluence’’) (Fig. 7b) are higher than the ones of the sample series A (‘‘low-fluence’’) (Fig. 7a). During the UV-laser assisted waveguide generation at high fluences photothermal effects cannot be inhibited due to the confined
M.A. Shams-Eldin et al. / Optical Materials 27 (2005) 1138–1148
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heat transport capacity of polymer. So thermal degradation of the waveguiding structure occurs being visible under the optical light microscope.
total light exiting from the fibre is inserted into the waveguide. In both cases the attenuation rises.
5.4. Loss rate measurements in dependence on the waveguide length (‘‘cut-back’’-method)
6. Conclusion
The total loss of a waveguide Atotal is composed of the coupling loss Acoupl and the intrinsic waveguide loss Aintr. = aintr*l with aintr as the intrinsic waveguide loss per length (intrinsic waveguide loss rate) and l as the waveguide length: Atotal = aintr*l + Acoupl. Consequently the slope of the regression curve (Fig. 7c) yields the intrinsic waveguide loss rate, while the point of intersection of the curve with the ordinate axis is identical with the coupling loss (Fig. 7c). Consequently the average coupling loss (incoupluing and outcoupling loss) of the sample series C is 2.1 dB and the average intrinsic waveguide loss rate is 4 dB/cm. 5.5. Loss rate measurements in dependence on the waveguide width Contrary to a monomode waveguide, in a multimode waveguide the defects of the waveguiding structure can be compensated by mode coupling without losing much light energy. The higher the number of modes the better the waveguide defects are compensated by mode coupling. According to [39] the maximal number of modes by which the light can propagate inside the waveguide rises with increasing waveguide diameter. Thus the loss rate of the waveguide decreases with increasing waveguide width (Fig. 7d). Besides the coupling loss of a broad waveguide is smaller than the coupling loss of a thin waveguide because in the case of a waveguide with a small diameter even a small misadjustment of the coupling fibre results in big coupling losses. 5.6. Loss rate measurements in dependence on the coupling parameters The refractive index depth profile is very asymmetric and of gradient nature (Fig. 4). The optimal light transport is performed by coupling light shortly beneath the waveguide surface (Fig. 7e) where the refractive index hub Dn is bigger than deep inside the bulk of the polymer substrate (Fig. 4). If the incoupling fibre is located in a deeper position, the incoupling light encounters a waveguiding structure whose refractive indes hub Dn and also the numerical aperture is inferior to those one occurring shortly beneath the waveguide surface. Thus the coupling losses into the waveguide is higher when the incoupling fibre is located in a deep incoupling position. In the other case if the incoupling fibre is positioned partly above the polymeric waveguide, not the
In this paper the functional-optical properties of UVlaser generated integrated-optical waveguides in the surface of a planar polymer chip have been characterized. Especially the refractive index profile strongly depends on the irradiation dose. At medium and high irradiation doses a double waveguide structure has been formed due to several mostly independent photochemical processes occurring in the UV-irradiated region of the polymer chip surface. All the photochemical processes competing with one another have different influences on the refractive index: some have an increasing effect, the others have a decreasing effect on the refractive index. So by the right choice of irradiation parameters the refractive index of the polymer can be increased controllably and locally, which enables the fabrication of integrated-optical structures by UV-laser light irradiation. Acknowledgments This work was supported by the Egypt Ministry of Science and Education and by the Deutsche Forschungsgemeinschaft (DFG) under contract No. ME 991/10-1 which the authors gratefully acknowledge.
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