Investigation of refractive index modifications in CW CO2 laser written planar optical waveguides

Optics Communications 281 (2008) 3686–3690

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Investigation of refractive index modifications in CW CO2 laser written planar optical waveguides L.Ç. Özcan, Francis Guay *, Ludvik Martinu Department of Engineering Physics, École Polytechnique de Montréal, Montréal, QC, Canada H3C 3A7

a r t i c l e

i n f o

Article history: Received 11 October 2007 Received in revised form 27 January 2008 Accepted 24 March 2008

OCIS: 120.0120 130.0130 310.0310

a b s t r a c t This paper presents the measurement of the refractive index profile of buried channel waveguides fabricated by a CW CO2 direct writing technique. A reflectance method is used to assess the refractive index distribution n(x,y) in these structures as it is a key parameter determining the propagation properties of guided wave devices. Beam propagation method (BMP) is used with experimentally determined crosssectional data and refractive index profile to model the waveguides. The spatial resolution is 1.3 lm and, 5.104 for the refractive index. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Over the last few years, direct laser writing techniques have been proposed to realize planar optical waveguides without the need for complex photolithographic processes. One of these techniques is based on the focusing of UV irradiation [1,2] onto Germania doped planar films on silica to increase the refractive index in the exposed region to form a directly written waveguide. The refractive index modification forms the core of the waveguide and the refractive index change can be of the order of 103. The writing speeds are usually a few 100 l/s. The advantage of this technique is that it is direct and allows the production of a variety of devices such as couplers and splitters. On the other hand, the refractive index change has to be stabilised by annealing at an elevated temperature, after the UV exposure. Femto-second lasers [3– 5] can also cause a change in the structure of the silica, which in turn, leads to an increase in the refractive index in the laser exposed region (103). This change depends on experimental parameters such as exposure time, optical power and focused beam size. Writing speeds using this method is usually of the same order as for the UV written waveguides. In an alternative technique the refractive on either side of the core is reduced by laser ablating two trenches. This has been proposed and applied for the fabrication of ridge optical waveguides in different materials [6–8]. This has also been used to create optical waveguides in thin films on silicon using a femto-second laser for the ablation process [9]. This process is distinct from other laser writing techniques as the struc* Corresponding author. Tel.: +1 514 340 4711; fax: +1 514 340 3218. E-mail address: [email protected] (F. Guay). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.074

ture of the core region remains preserved since only the cladding areas are modified. On the other hand, this process is rather slow with writing speeds of less than 100 l/s and further post-processing is required to achieve smooth walls, as the roughness causes high optical propagation loss. Recently, we proposed a similar scheme in which a simple CW CO2 laser is used to ablate two trenches to create single-mode waveguides. This technique enables the production of low loss single-mode waveguides (0.1 dB/cm) at high writing speeds (50 mm/ s) [10,11]. As with other techniques, it has limitations as well, the main one being the laser beam diameter which is of the order of 20 lm and creates 20 lm trenches: therefore low loss couplers cannot be created simply as the spacing between adjacent waveguides is too large. Another question that arises is if there is indeed modification in the core area as it is well known that CO2 lasers melt silica which may lead to changes in the refractive index. It has been previously shown [12] that a 1% decrease in refractive index is possible on densified silica glass by CO laser irradiation. In SiON films, a refractive index lowering of 2% has been reported [13] with CO2 laser irradiation and used for laser induced trimming of waveguides. A reduction in the refractive index is accompanied by a reduction in the volume of the material as well. These observations imply that not only the core region can be affected but more interestingly, a cladding may be formed around the core region effectively burying the waveguide provided the refractive index change is large enough. This could explain the low losses that have been measured in our waveguides. The refractive index profile (RIP) of a waveguide cross-section is an important factor which can provide useful information on the waveguide fabrication process as well as on the transmission prop-

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2. Fabrication process in silica thin films on silicon A tightly focused CO2 laser beam ablates the material locally and therefore creates a low index (air) trench with a Gaussian profile adjacent to a high index region [10,11]. This thermal process affects the material in a region of a few microns adjacent to the created cavities, as there is a strong temperature gradient during the ablation process. A buried waveguide is formed extending a few microns into the buffer layer (Fig. 1), if two such trenches are fabricated adjacent to each other. These trenches permit light confinement horizontally while vertical confinement is ensured by the higher core index layer. This technique is similar to photolithography-based waveguide fabrication, but it offers several advantages. Here, the pattern mask is replaced by an XYZ translation stage that directly defines the desired pattern on the surface of the sample, adding great flexibility to the process since only the trajectory of the stage needs to be controlled in order to define a new pattern. The overall fabrication time of the waveguides is shorter because there is only one process step, with no further chemical or mechanical processing. In addition, the writing speed is 50 mm/s, currently only limited by the translation stages used for the process. Since the laser used for production of these waveguides is inexpensive and low maintenance, there maybe an advantage on the grounds of cost which may lead to potentially lower cost planar optical waveguide devices. Silica thin films on silicon were chosen for micro-machining as they are the most widely used structures in planar light-wave circuits. The CO2 laser radiation at 10.6 lm is strongly absorbed in silica making it a good choice for micro-machining. Additionally, the absorption coefficient of the silicon substrate is very low at this wavelength which in turn ensures that the substrate is not affected by the writing process. Fig. 1 shows such a waveguide fabricated by the CO2 laser method. This waveguide is back illuminated with white light and the exit facet is captured on a camera. As can be seen in Fig. 1, it appears that a buried waveguide is obtained by this method and which has a trapezoidal shape, significantly different from the standard rectangular profiles made by photolithographic techniques. The trapezoidal shape is analysed by intensity discriminating software to arrive at the physical dimensions of the guiding region. By adjusting the spacing between the two trenches, single-

45

Silicon Substrate

40 35

Depth (Microns)

erties of the waveguide. Therefore, for the accurate design of optical circuits, knowledge of the two-dimensional distribution of the refractive index of the waveguide is extremely useful. Several techniques have been proposed to measure the refractive index profile of optical fibers and waveguides, such as refracted near-field (RNF) method [14], interferometry, and reflectance measurement (RM) [15]. This paper reports the measurement of the two-dimensional refractive index profile of CO2 laser written waveguides with the RM technique, with respective spatial and index resolution of 1.3 lm and 5.104, and it is shown that the ablation process does indeed generate truly buried waveguides.

Buffer 30 25

Core

20 15 10

Cladding

5 0 1.20

1.40

1.60

1.80

2.00

2.20

2.40

2.60

Power (Watt) Fig. 2. The dependence of ablation depth as a function of vs. laser power. The figure also shows the positions of the different layers on the substrate.

mode or multi-mode waveguides may be fabricated. The knowledge of the refractive index profile in the fabricated structure is then required to compute the optical properties of the waveguide as well as to predict the number of modes supported by such a structure. We have performed a series of experiments to determine the depth of the ablated region as a function of power of the laser. These data are shown in Fig. 2. The various layers are indicated on the figure. To form waveguides it is necessary to achieve an ablation depth to reach just below the core layer, as it ensures lateral confinement. The power and speed are fixed for all the experiments so that comparable data may be produced. The translation speed was 50 mm/s, the power in the laser was 2.2 W and the depth of ablation of 28lm. The waveguides were then written by adjusting the distance between the trenches, to reduce the core size. This allows single or multi-mode waveguides to be written. As the purpose of our paper is to measure the refractive index change in the heat affected zone, it is not important to necessarily have single-mode waveguides. The physical dimensions and the measured refractive index changes are required to model the waveguides used for the measurements.

3. Reflectance measurement on fabricated waveguides Several papers have reported this simple and accurate reflection technique [15]. The reflected power from a focused light spot on a polished surface is directly related to the refractive index of the sample. In the case of normal incidence, the reflectivity R is given as  2 1  nðx; yÞ Rðx; yÞ ¼ ð1Þ 1 þ nðx; yÞ  2  nref  1 DR Dn ¼ ð2Þ 4 Rref

Fig. 1. Schematic of the waveguide with two trenches defining the core region in one of the guiding structure fabricated with the CW CO2 laser. Refractive index values are given at 1550 nm.

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Fig. 3. Reflectance measurement setup composed of: 635 nm Laser Diode, Polarization Beam Splitter, Quarter Wave Plate, 16X lens, 60X objective lens, CCD camera, XYZ translation stages and photo-detector.

Eq. (1) is used to convert the obtained reflectance R(x,y) into the refractive index profile n(x,y). Knowing the refractive index nref of the buffer layer at 635 nm, it is easy to obtain a two-dimensional refractive index profile from the reflectance measurement. The setup used for the reflectance measurement is shown in Fig. 3. A beam from a pigtailed 635 nm laser diode was expanded and collimated using a 16X microscope objective with a numerical aperture of 0.25. An isolator in the form of a quarter-wave plate and polariser was placed in front to prevent any light from returning to the laser. The beam was focused onto the sample surface by a 60X microscope

objective with a numerical aperture of 0.65. The sample was mounted on a XYZ translation stage with a minimal scanning step size of 0.1 lm. The light reflected from the end-facet of the sample was redirected with a non-polarizing cube beam-splitter onto a photo-detector. The reflectance profile along the transverse direction of the sample was then used to obtain the refractive index difference using Eq. (2). The main problem of this method is that the roughness of the polished end-surface disrupts the reflection and it is difficult to maintain the focus across the scan, as it requires a surface perfectly perpendicular to the beam. The setup was calibrated by taking a few measurements on single-mode fibres, from CorningÒ SMF28 (Fig. 4), and shown to be in good agreement with the manufacturer’s data. The 2-D refractive index mapping represented in Fig. 4b shows a slightly elliptical core. This artefact is caused by the fact that one axis in our measurement is not in focus throughout the scan. Some contamination is also visible in the same figure. The SMF28Ò fiber core diameter was measured to be 10.6 lm along the X axis and 9 lm along the Y axis instead of 8.2 lm, owing to the limitations in spatial resolution and the aforesaid focusing problem. As we previously mentioned, the dimensions of the fabricated waveguides depend on the spacing between the two trenches. We have made a simple setup to inspect the facets of the waveguides after polishing to assess surface quality which plays a key role in the accuracy of the refractive index measurements. The back facet of the waveguides is illuminated with a white light source as per Fig. 1 and a CCD camera equipped with a 50X microscope objective collects light transmitted through the front facet. Fig. 5 shows two fabricated waveguides: the first one was made with a 38 lm trench separation and the second one with a 44 lm separation. A greyish area is clearly visible in both pictures on either side of what remains of the core region which is white. These three areas originally had the same refractive index but it

b a 0.0052

Δn

0.0034

0.0016

-0.0002 -15

-10

-5

0

5

10

15

Position (micron) Fig. 4. (a) 1-D index profile of a single-mode SMF28 fiber from CorningÒ, obtained from a cross-section of the 2-D index profile. (b) 2-D refractive index mapping of the same fiber.

Fig. 5. Pictures of two fabricated waveguides with: (a) 38 lm and (b) 44 lm trench separation.

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0.007 0.006

Delta N

0.005 0.004 0.003 0.002 0.001 0.000 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Scan X (πm) Fig. 6. 2-D refractive index mapping of buried optical waveguide written by CW CO2 laser: (a) 38 lm separation and (b) 44 lm separation between trenches. Contour lines are also present for both measurements with a refractive index increment of 103 on the refractive index. All refractive index values are given at 635 nm.

is now clear that there appears to be a significant refractive index change near the ablated zone. In fact, this region is the zone thermally affected by the laser irradiation. To quantify the refractive index change, we performed reflectance measurement on these planar optical waveguides written by a CW CO2 laser. The trapezoidal core dimensions measured on these pictures are 10.75 and 17 lm at the bottom of waveguides 1 and 2, respectively, as measured by our intensity discriminating software. As these waveguides are used at 1550 nm and the dimensions of the waveguide are relatively large, the resolution achieved by white light illumination is more than adequate for our purposes. Fig. 6 shows the two-dimensional contour mapping of the refractive index of the fabricated waveguides. The dimension of the guiding structure was measured to be 14 lm and 20 lm at the bottom and 9 lm in height with a trapezoidal shape. The real value of the planar waveguide height is 7 lm. The difference between the dimension taken by the RM technique and the manufacturer data comes from the spatial resolution of our RI measurement setup which is 1.3 lm. The two refractive index profiles presented in Fig. 6 confirm the results from our inspection setup, that the refractive index has indeed been reduced significantly near the ablated zones and that the waveguide is buried. More importantly, within the resolution of our measurement of refractive index change (5  104), the waveguide core refractive index remains unchanged regardless of the spacing between the trenches in our samples. The behavior in the heat affected zone is somewhat different as its refractive index is lower than the core but slightly different from the cladding and buffer for a larger spacing (44 lm) and it becomes uniform for a 38 lm spacing. On both measurement in Fig. 6, color contour plot are shown starting at 1.4693 @ 635 nm (heat affected core region) to 1.4753 @ 635 nm (preserved core region) with a difference of 1.103 in RI. A gradient in the RI profile is clearly visible in both the X and Y directions. It comes in part from the fact that the fabrication process is a thermal one and in part it is an artifact due to the spatial resolution of the measurement technique (convolution of the RI distribution with the spot size of the measurement) The RI gradient is more important in the X direction since the waveguide is heated from the side and not from the top. In this configuration, the thermally induced RI change helps to reduce propagation loss in the fabricated waveguides in two ways. Firstly, it isolates the waveguide from the trench and potential roughness that could induce losses. Secondly, it reduces the asymmetry of the waveguide, again contributing in a reduction in propagation losses. Therefore, the buried nature of the waveguides explains the measured low losses. The RI data collected on these two waveguides can be used with BPM simulation to accurately model the behavior of the wave-

Fig. 7. Line scan in the X direction for waveguide 2. Dashed line represents the step index profile assumed for the simulation.

guides. Assuming that the trapezoidal step index profile between the cladding (nCladding = 1.4446 @ 1550 nm) and the core region (nCore = 1.4508 @ 1550 nm) is conserved. Fig. 7 shows the approximated step index profile. To assess propagation at 1550 nm in such a structure, we have performed simulations based on the BPM technique for our trapezoidal shaped profile, but with the step refractive index profile (instead of graded index). To arrive at the dimensions of the trapezoidal shape, the distance between the mid-points at the edges of the RI profile is used from the yellow contour plot of the refractive index of the wafer before processing. For comparison, these two waveguides were measured using butt coupling. A HI1060 fiber was used for the input and a standard SMF28 fiber used for the output. The length of the fabricated waveguide is 2.5 cm. Experimental measurements show fiber to fiber insertion losses of 1.62 dB and 0.9 dB @ 1550 nm, respectively for waveguide 1 and 2. These are in good agreement with simulations of 1.38 dB and 1.11 dB total loss, respectively @1550 nm. No account of the impact of surface roughness has been taken into account. The slight difference is believed to come from the spatial resolution of the RM setup which is used to find the dimensions of the trapezoidal shape of the waveguide and the assumption of a step index profile. 4. Conclusion We have investigated the optical properties of buried waveguides fabricated by a simple novel method using a CW CO2 laser. To get more information on the fabrication process, we used the reflectance measurement techniques to measure the refractive index profiles of the fabricated waveguides. The geometrical data of the buried waveguide have been taken and used as input for BPM calculations of waveguides devices assuming a trapezoidal step index profile. This thermal process produces a gradient index in the X direction. The spatial and refractive index resolution of the setup should be improved in order to increase the accuracy for simulating waveguide devices. The simulated and experimental total losses over a length of 2.5 cm waveguides are in reasonably good agreement. Other structures such as MMI or Y-Junctions are being investigated and will be reported on in a future publication. Acknowledgments This work was financially supported by a Grant from the Natural Sciences and Engineering Research Council of Canada, (I2I program), Univalor, PhotoNova Inc. and the Canada Research Chairs Program. The technology presented in this paper is protected by a PCT application No. CA2004/001798.

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