Arrayed waveguide collimators for integrating free-space optics on polymeric waveguide devices

Array-type optical devices are important for wavelength-division multiplexing optical communication system to achieve small footprint, mass production, and reliability. For fabricating transmitter module in an array configuration, it is difficult to achieve a passive alignment of isolator, collimating lens, and laser diode. To facilitate array isolator integration, a waveguide collimator is proposed in this work by using a low-contrast, large-core polymer waveguide. The diffraction of a guided mode propagating through a free-space region is suppressed by enlarging the guided mode. The fiber coupling loss due to the enlarged mode was overcome by incorporating an adiabatic taper structure. The excess loss of waveguide collimator including the loss through a 400-μm free-propagation region was less than 1.0 dB. ©2014 Optical Society of America OCIS codes: (130.5460) Polymer waveguides; (130.3120) Integrated optics devices; (080.1238) Array waveguide devices. References and links 1. R. Nagarajan, C. H. Joyner, R. P. Schneider, J. S. Bostak, T. Butrie, A. G. Dentai, V. G. Dominic, P. W. Evans, M. Kato, M. Kauffman, D. J. H. Lambert, S. K. Mathis, A. Mathur, R. H. Miles, M. L. Mitchell, M. J. Missey, S. Murthy, A. C. Nilsson, F. H. Peters, S. C. Pennypacker, J. L. Pleumeekers, R. A. Salvatore, R. K. Schlenker, R. B. Taylor, H.-S. Tsai, M. F. V. Leeuwen, J. Webjorn, M. Ziari, D. Perkins, J. Singh, S. G. Grubb, M. S. Reffle, D. G. Mehuys, F. A. Kish, and D. F. Welch, “Large-scale photonic integrated circuits,” IEEE J. Sel. Top. Quantum Electron. 11(1), 50–65 (2005). 2. A. Himeno, K. Kato, and T. Miya, “Silica-based planar lightwave circuits,” IEEE J. Sel. Top. Quantum Electron. 4(6), 913–924 (1998). 3. X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, and Y. Sun, “Sensitive optical biosensors for unlabeled targets: A review,” Anal. Chim. Acta 620(1-2), 8–26 (2008). 4. D. Shimura, R. Sekikawa, K. Kotani, M. Uekawa, Y. Maeno, K. Aoyama, H. Sasaki, T. Takamori, K. Masuko, and S. Nakaya, “Bidirectional optical subassembly with prealigned silicon microlens and laser diode,” IEEE Photon. Technol. Lett. 18(16), 1738–1740 (2006). 5. Y. Inoue, T. Oguchi, Y. Hibino, S. Suzuki, M. Yanagisawa, K. Moriwaki, and Y. Yamada, “Filter-embedded wavelength-division-multiplexer for hybrid-integrated transceiver based on silica-based PLC,” Electron. Lett. 32(9), 847–848 (1996). 6. H. Ishii, K. Kasaya, and H. Oohashi, “Spectral linewidth reduction in widely wavelength tunable DFB laser array,” IEEE J. Sel. Top. Quantum Electron. 15(3), 514–520 (2009). 7. D. Felipe, C. Zawadzki, Z. Zhang, W. Brinker, H. N. Klein, F. Soares, M. Moehrle, N. Keil, and N. Grote, “Polymer hybrid integrated devices for WDM-PON,” in Proceedings of the 17th Opto-Electronics and Communications Conference (Busan, Korea, 2012), pp. 232–233. 8. J.-Y. Park, H.-S. Lee, S.-S. Lee, and Y.-S. Son, “Passive aligned transmit optical subassembly module based on a WDM incorporating VCSELs,” IEEE Photon. Technol. Lett. 22(24), 1790–1792 (2010). 9. K. Watanabe, Y. Hashizume, Y. Nasu, M. Kohtoku, M. Itoh, and Y. Inoue, “Ultralow power consumption silicabased PLC-VOA/switchs,” J. Lightwave Technol. 26(14), 2235–2244 (2008). #222345 $15.00 USD Received 3 Sep 2014; revised 16 Sep 2014; accepted 16 Sep 2014; published 22 Sep 2014 (C) 2014 OSA 6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.023801 | OPTICS EXPRESS 23801 10. Y. Inoue, Y. Ohmori, M. Kawachi, S. Ando, T. Sawada, and H. Takahashi, “Polarization mode converter with polyimide half waveplate in silica-based planar lightwave circuits,” IEEE Photon. Technol. Lett. 6(5), 626–628 (1994). 11. M.-C. Oh, S.-H. Cho, and H.-J. Lee, “Fabrication of large-core single-mode polymer waveguide connecting to a thermally expanded core fiber for increased alignment tolerance,” Opt. Commun. 246(4–6), 337–343 (2005). 12. N.-S. Son, K.-J. Kim, J.-W. Kim, and M.-C. Oh, “Near-infrared tunable lasers with polymer waveguide Bragg gratings,” Opt. Express 20(2), 827–834 (2012). 13. J.-W. Kim, S.-H. Park, W.-S. Chu, and M.-C. Oh, “Integrated-optic polarization controllers incorporating polymer waveguide birefringence modulators,” Opt. Express 20(11), 12443–12448 (2012). 14. M.-C. Oh, W.-S. Chu, K.-J. Kim, and J.-W. Kim, “Polymer waveguide integrated-optic current transducers,” Opt. Express 19(10), 9392–9400 (2011). 15. G.-H. Huang, J. W. Kim, W. S. Chu, M. C. Oh, J. K. Seo, Y. O. Noh, and H. J. Lee, “Low-crosstalk high-density polymeric integrated optics incorporating self-assembled scattering monolayer,” Opt. Express 22(12), 14237– 14245 (2014). 16. T. Mizuno, T. Kitoh, M. Itoh, T. Saida, T. Shibata, and Y. Hibino, “Optical spotsize converter using narrow laterally tapered waveguide for planar lightwave circuits,” J. Lightwave Technol. 22(3), 833–839 (2004).


Introduction
Photonic IC technology enables manipulation of light signal on a small footprint substrate, and produces sophisticated optical devices by integrating various functional building blocks.The integrated optic device extends its application over optical communications, photonic sensors, and optical signal processing [1][2][3].However, for practical applications, many essential optical components, such as collimating lens, wavelength filter, isolator and wave plate are still difficult to be implemented as a waveguide component [4,5].If these free-space planar optics could be integrated with the waveguide device on a single substrate, there will be significant progress in the production method of optical components.
In the wavelength division multiplexing (WDM) optical communication system, array type devices of laser diodes (LDs), optical switches, and optical attenuators are preferred because of the reliability, cost-reduction, and small footprint [6][7][8][9].For producing array LDs, however, an isolator device has to be inserted as an array, which would have a thickness of few hundred microns, and requires a collimating lens to couple the light into the optical fiber.The alignment of these optical components is a large impediment in the production yield, and the alignment between an array LD and an array-collimating lens is practically not feasible.
Without using the collimating lens, we propose a method to integrate the free-space planar optics into the polymer waveguide devices that could be attached on the array LD.When a polyimide film was inserted in the silica waveguide device, the thickness of planar optics had to be thinner than 20 μm to keep the excess loss less than 0.5 dB [10].The isolator with a thickness of 500 μm cannot be inserted by the same way because it will introduce significant excess loss.If one can reduce the diffraction of light, the free-space planar optics could be inserted in the beam path of waveguide with a small excess loss.
When the guided light exits a waveguide and propagates through a free propagation region (FPR), diffraction is determined by the initial mode size.For reducing the diffraction, the larger waveguide is the better, which could be produced effectively in terms of low-loss polymers.The refractive index of such a polymer can be precisely adjusted to have a small contrast for producing a single mode waveguide with a large core dimension, over tens of microns [11].Moreover, the polymer device has excellent flexibility in structure and simple fabrication method, which has been demonstrated in prior experiments [12][13][14][15].The large thermo-optic effect and similarity in refractive index with optical fibers make the polymer waveguide device attractive.In the current work, with a view to integrating the free-space planar optics in the polymer waveguide, a large core single mode waveguide is incorporated to produce a collimated light with small diffraction.By using such a large-sized guided mode, diffraction loss through the FPR could be greatly reduced to be 0.8 dB through a 400 μm FPR.

Design and fabrication of the arrayed waveguide collimator
To reduce the diffraction of guided mode at the end of waveguide, it is necessary to increase the dimension of the waveguide, while maintaining the single mode condition.An index contrast, on the order of 10 −3 , between the core and the cladding could be easily achieved in a large core waveguide by simply blending two polymer materials with different refractive indices [11].However, the large core waveguide with low diffraction will have significant fiber coupling loss, and it should be prevented by incorporating a taper structure providing adiabatic mode conversion [16].
A 4-channel arrayed waveguide collimator with free-space planar optics inserted in the middle of the large core collimating waveguide is represented in Fig. 1.Three polymers with different refractive indices were used for implementing the device.The polymer with the highest refractive index formed the small waveguide core at the input and output.The dimension of the small waveguide was reduced along the propagation direction, so as to increase the size of the guided mode gradually.At the end of the taper, the small core waveguide disappeared, and then the guided mode was confined by the large core waveguide, made of the second highest refractive index polymer.The lowest refractive index polymer formed the cladding layer of the large core waveguide.
The polymers used in this experiment had refractive indices of 1.450, 1.445, and 1.444.To design a single mode waveguide with small and large cores, the effective index calculation was conducted.The small core waveguide buried within the large core had a contrast of 0.005 and a size of 6 × 6 μm 2 .It was designed to have similar mode size as the single mode optical fiber.The large core waveguide was designed as an oversized-rib waveguide with a contrast of 0.001.The small index contrast was carefully achieved by controlling the blending ratio of two polymers with different refractive indices.The oversized-rib waveguide structure had a core size of 25 × 25 μm 2 , and the remaining lateral core layer thickness was 10 μm.The large core waveguide will be effective to reduce the diffraction of the light through the FPR.
Efficient adiabatic mode conversion by the taper structure was important to realize a lowloss device.The taper structure was divided into 2 sections with length L 1 and L 2 as shown in Fig. 2. At the 1st taper, an initial 6 μm waveguide was tapered to a width of W a , followed by a further decrease in width at the 2nd taper, from W a to 0.2 μm, considering the limit of photolithography.The variation of mode conversion loss as a function of W a was then calculated by beam propagation method (BPM), as shown in Fig. 2(a).The two tapers had the same length of 800 μm, and the minimum loss occurred at W a = 2 μm.Then the loss was calculated for the two variables, L 1 and L 2 as shown in Fig. 2(b).It was found that the mode conversion loss became 0.23 dB for a total taper length of 1 mm (L 1 = L 2 = 500 μm).By including the FPR between the two tapers, which had a refractive index of 1.5 simulating the gap filled up with epoxy, the insertion loss was simulated.For L 1 = L 2 = 800 μm, and L f = 400 μm, the insertion loss was 0.58 dB, as shown in Fig. 2(c).Without the FPR, when the large core waveguide was connected, the insertion loss was as low as 0.09 dB.Then the excess loss by the FPR was calculated to be 0.49 dB.When the distance of the FPR was increased, the insertion loss was increasing as shown in Fig. 2(d).For L f = 500 μm, the insertion loss was still less than 1.0 dB, for L 1 = L 2 = 800 μm.If there's no mode expansion, the small core waveguide will introduce an insertion loss of 7.5 dB due to the strong diffraction at FPR.According to the BPM simulation results, it has been found the insertion loss changes only by 10% until the misalignment between the waveguides becomes 5 μm, which shows that the waveguide collimator is designed to have large fabrication tolerance.
The length from the end of the taper to the FPR is not important because the mode conversion is completed at the end of the taper.Slight amount of the length is inevitable considering the alignment tolerance of dicing process to form the FPR.The planar free-space optics may have high refractive index such as the YIG isolator material.However, the surface of planar optics has to be coated to reduce the reflection from the epoxy material before it is inserted in the FPR.The waveguide collimator device was fabricated by using three ZPU polymers (ZPU13-450, 445, 444) supplied by ChemOptics Co.The schematic fabrication procedure of the arrayed waveguide collimators has been outlined in Fig. 3.The lower cladding layer of the collimator was made with the polymer ZPU-444.A silicon substrate was first coated with a ZPU-444 layer of thickness 13 μm, followed by curing under UV exposure of 5 mW/cm 2 for 5 min in a nitrogen chamber.The sample was then baked at 100°C and 160°C for 5 min and 30 min, respectively.In order to form the large core layer, ZPU-445 polymer was coated on the lower cladding to have a thickness of 10 μm.ZPU-450 polymer for small core was coated over the large core layer to have a thickness of 6 μm.The small core waveguide and the taper structure were formed by photolithography and oxygen plasma etching.A ZPU-445 coating of thickness 10 μm was further applied on the small core waveguide with the taper structure so that the final thickness of the large core became 20 μm.The thickness of large core layer was decreased to 20 μm instead of 25 μm as the simulation because the thinner waveguide is easier to fabricate through the spin coating and the dry etching.Following this, the large core layer was etched by 10 μm to produce a rib waveguide with 10 μm remained core layer at the lateral cladding.ZPU-444 was then coated to cover the large core rib waveguide.Finally, for inserting free-space planar optics, the 400 μm wide FPR was formed by using a dicing machine with a blade width of 362 μm produced by DISCO Co. (DAD-320).Due to the vibration of the rotating blade, the FPR width became slightly thicker than the blade width.

Characteristics of the arrayed waveguide collimator
To measure the characteristics of the waveguide collimator, light from a 1550 nm DFB laser was launched into the small core via butt-coupling.A fiber-optic polarization controller was used for controlling the input polarization.The microstage used for the fiber alignment had good repeatability to provide the insertion loss accuracy of about 0.1 dB.The guided mode profile was observed by cleaving the sample at the end of the taper.For TE polarization, the mode profile was measured as shown in Fig. 4. The mode profile of the small contrast large core waveguide shown in Fig. 4(b) was much larger than that of the input and the output waveguide as Fig. 4(a) and Fig. 4(c).As the optical fiber was moved transversally, no higher order mode excitation was observed at the large core waveguide.
To find the mode conversion efficiency, insertion loss was measured and compared to that of small core straight waveguide before groove dicing.The straight waveguide with a length of 1 cm had an insertion loss of 1.5 dB, which could be roughly divided into 0.3 dB of propagation loss in 1-cm long waveguide and 2 times of 0.6 dB of fiber coupling loss, which was due to the rough end-facet just formed by dicing.The waveguide collimator device without the FPR exhibited a total loss of 2.0 ~2.2 dB as summarized in Table 1, which was not significantly dependent on the taper length variations.The excess loss by the two tapers was as small as 0.5 ~0.7 dB.
With the purpose of inserting free-space planar optics, 400 μm wide FPR was formed by using a dicing blade.Initially, at the FPR, the large core mode propagated through air, and experienced significant diffraction and Fresnel reflection, which resulted in a loss of 4.4 dB.When the FPR was filled up with an epoxy with a refractive index of 1.5, the insertion loss dropped to 2.8 dB.When compared to the insertion loss value before the groove formation, the loss was increased only by 0.8 dB.The polarization dependence of the device was measured by using a fiber-optic polarization controller.The polarization dependent loss was 0.4 dB.The insertion loss of the waveguide collimator with FPR of 400 μm could be reduced to less than 2.0 dB if the fabrication condition is optimized, including the waveguide polishing.

Conclusion
An array-type waveguide collimator was proposed and demonstrated by using a low-contrast, large-core polymer waveguide.The diffraction of the guided mode, propagating through a free-space region could be greatly reduced by increasing the size of the mode to about 25 μm.
In this regard, the extra loss by the free propagation region of 400 μm was reduced to 0.8 dB.An adiabatic taper structure was incorporated to reduce the fiber coupling loss, and the extra loss by the two tapers was as small as 0.5 dB.The waveguide collimator has the potential to play an important role in the integration of free-space planar optics, such as the isolator and to provide a low-cost solution to array-type transceiver devices.

Fig. 1 .
Fig. 1.Schematic diagram of the 4-channel arrayed waveguide collimator, consisting of a large core waveguide, free-propagation region, waveguide tapers, and input-output small core waveguides.The upper cladding of the large core waveguide is not shown in this figure.

Fig. 2 .
Fig. 2. BPM simulation results of the insertion losses of the arrayed waveguide collimators, calculated for half of the device consisting of small core waveguide, single taper, and large core waveguide as a function of (a) W a with L 1 = L 2 = 800 μm, (b) L 1 and L 2 with W a = 2 μm; Calculated results for the complete waveguide collimator structure as a function of (c) L 1 and L 2 , (d) distance of free-propagation region (FPR), L f .

Fig. 4 .
Fig. 4. Measurement setup and CCD images and contour plots of the mode profiles of the arrayed waveguide collimator at (a) input small core, (b) large core, and (c) output small core waveguides.