We demonstrate the feasibility of the process for fabricating a single-mode waveguide and a large-core multimode waveguide aligned vertically on the same substrate. Using this process, we propose and demonstrate a filter that drops optical signal propagating in a single-mode waveguide to a multimode waveguide in the specific wavelength interval by a long-period grating. We use perfluorocyclobutane and benzocyclobutane for the cladding and core of the single-mode waveguide, respectively. The large core of the multimode waveguide is made of Norland Optical Adhesive 61. For the grating period of 315.9 µm, the fabricated filter has the center wavelength of 1537.7 nm, at which the maximum attenuation is 17.8 dB.
©2003 Optical Society of America
The coupling phenomena between two single-mode waveguides (SMWs) have been important for implementing integrated optical switches, modulators, and filters [1,2]. However, the coupling between a single-mode waveguide and a multimode waveguide (MMW) is a phenomenon that is observed mainly in fiber-based devices. One example is a long-period fiber grating  by which the core mode of a single-mode fiber is coupled to a cladding mode. The other is a side-polished single-mode fiber covered with a planar multimode waveguide overlay . As shown in these examples, the coupling between a SMW and a MMW may be used to implement novel devices since various characteristics of many guided modes in the MMW can be employed.
However, such a coupling phenomenon has not been used much in integrated optical devices. By using this coupling, it will be possible to make a compact integrated optical gain-flattening filter (GFF) for an optical amplifier. Such a GFF is schematically shown in Fig. 1. In this filter, several modes in the MMW can be used like cladding modes in a fiber. Light propagating in the SMW is coupled out to different modes of the MMW by individual long-period gratings (LPGs) so that distinct resonance or attenuation bands in the transmission spectrum of the GFF are obtained. Even though LPGs are closely spaced, the overall transmission spectrum is almost equal to the product of the individual spectrums of such resonance bands. This feature can be obtained only from a structure based on the coupling between a SMW and a MMW .
Despite the attraction of this filter, there is a difficulty in its implementation. Because of the large cross-section area of the MMW, its fabrication process is different from the process for the SMW. Therefore, it is not simple to fabricate the SMW and the MMW on the same substrate. Consequently, we need a simple process to realize such a device. In this paper, we demonstrate the feasibility of this process by designing and fabricating a filter as shown in Fig. 2. This filter can be a part of the GFF shown in Fig. 1. It consists of a SMW and a MMW, which are aligned vertically on the same substrate. On the SMW core, a diffraction grating with a period Λ is formed. This vertically coupled structure has several advantages. For example, the separation between two waveguides can be controlled easily and precisely. In addition, this structure is more tolerable to misalignment due to the wide width of the MMW in comparison with that of the SMW. The SMW is made of low-loss thermo-curable polymers by using a conventional fabrication method. On the fabricated SMW, the MMW with about 50 µm by 50 µm cross-section area is made by the molding method with a polydimethlysiloxane (PDMS) mold [6–8].
2. Design and fabrication
We design a SMW with the oversized-rib structure  because there is a large difference of refractive indices between core and cladding materials. The waveguide parameters such as rib width, rib height, and slab height are determined by Eq. (9) in Reference . The thickness of the upper cladding of the SMW is determined to make the coupling between the SMW and the MMW effective. The period Λ of the LPG is obtained to satisfy the phase matching condition given by
where NSMW and NMMW are the effective indices of the SMW mode and a MMW mode, respectively. The grating order m is set to unity and λ 0 is the wavelength at which the coupling occurs. When we determine the period, we should also consider which MMW mode is coupled to the SMW mode. Since the SMW mode and a MMW mode are coupled evanescently, a MMW mode with a high order in vertical direction and with an even order in lateral direction is desirable for efficient mode overlap. Actually, exact determination of Λ is not easy since there are too many modes in the MMW and it is difficult to precisely analyze them. Hence, we determine roughly the period and make the proposed filter for various periods deviating from the determined value.
The procedure of implementing the proposed filter consists of two steps: SMW fabrication and MMW fabrication. To make the SMW, we used perfluorocyclobutane (PFCB) and benzocyclobutane (BCB) for the cladding and the core polymers, respectively. Their refractive indexes at the wavelength of 1.55 µm are 1.480 and 1.538, respectively. For the lower cladding and the core layer, PFCB and BCB were spin-coated and cured thermally in turn on a silicon substrate. After formation of two layers, the gold mask for the SMW pattern was made by vacuum evaporation of gold, a conventional photolithography, and dry etching of the gold. Then, reactive ion etching (RIE) in O2 and CF4 was carried out until the slab height reached a proper value. After the gold mask was removed, the grating was formed on top of the etched BCB layer by the O2 and CF4 RIE. For an etching mask, we used photoresist patterned by a standard photolithography with a chrome mask. As the last step for the SMW fabrication, the upper cladding PFCB was spin-coated and cured thermally. The fabricated SMW had a propagation loss of about 3 dB/cm. The loss is larger than the intrinsic material loss of BCB. The loss was increased by the roughness of the etched sidewalls. In addition, the corrugated grating induced an additional loss. To reduce loss, it is required to optimize a RIE condition and a grating depth.
On the fabricated SMW, we made the MMW using the molding method with a PDMS mold. This method consists of three steps: a master fabrication, a mold formation, and a transferring process to polymeric material . First, the master was made of thick photoresist AZ9260 on a new silicon substrate and the mold was made by pouring and curing the PDMS elastomer (Sylgard 184, Dow-Corning, A:B=1:10) on this master. The PDMS mold used in this process should be thick enough to avoid deformation due to its elastomeric nature and thin enough to avoid the image distortion of aligning marks. The PDMS mold used in this study was 3 mm thick. With this PDMS mold, the MMW pattern on it was transferred to the UV-curable polymer Norland Optical Adhesive 61 (NOA 61) on the fabricated SMW. The refractive index of NOA61 at the wavelength of 1.55 µm is 1.550. Finally, the PDMS mold was peeled off and the end facets were made by cleaving the silicon substrate for the light coupling.
While transferring to the MMW core polymer NOA 61 from the PDMS mold, we have to align the MMW to the SMW. This aligning process is described in Fig. 3. Using micro-translation stages, we aligned the aligning marks on the PDMS mold to the ones on the substrate where the SMW had been fabricated already. After finishing alignment, the PDMS mold was kept very close to the substrate and stuck to it by Van der Waals force. Then, a few drops of liquid prepolymer NOA61 was applied to the open ends of the channels formed by the dented regions on the mold and the substrate, and liquid filled them automatically by capillarity [7,8]. Viscosity of NOA61 is 300 cps at 25 °C which is too sticky to fill the channels, so we reduced it by increasing operating temperature to 70 °C. In this way, a 3.5 cm long channel with a 47 µm×50 µm cross-section area was filled completely after about 1 hour. Heating caused little change of optical properties.
Figure 4 shows the microscope photograph of the cleaved facet of the fabricated filter. The small SMW as well as the large rectangular MMW are shown in this figure. The small damage on top of the cleaved facet of the MMW was inflicted due to the large thickness of the MMW when the filter was cleaved. Since light is launched to the SMW and the MMW is used just as a wavelength-selective loss element in this filter, this damage does not affect the filter characteristics. The rib height, slab height, and width of the SMW are 6, 4, and 6 µm, respectively. The distance, which is denoted by t 0 in Fig. 2(b), from the top of the rib to the bottom of the MMW is 1 µm and the grating depth is 0.5 µm. Even though we want to align exactly, sometimes this aligning process may have a little alignment error, that is, the lateral distance between the MMW and the SMW. That is within 25 µm in our process. This alignment error seems to occur because of low precision of the used alignment setup and the slight deformation of the PDMS mold. This error could be reduced by using more precise commercial aligner.
3. Results and discussion
We measured the characteristics of the fabricated filter with amplified spontaneous emission (ASE) noise of an erbium-doped fiber amplifier (EDFA). Lens-coupled into the SMW through a half-wave plate and a polarizer, the ASE noise was used as a broadband light source. We monitored the output power of the SMW with an optical spectrum analyzer. Figure 5 shows the measured filter response of the device with a grating period of 315.9 µm for TE polarization. To observe the filter response according to a grating-interaction length, we changed it by cleaving a short piece from the same fabricated device and measured the filter response for each grating-interaction length.
Figure 5(a) is the transmission spectrum in case of the grating-interaction length of 11 mm. As shown in the figure, there are three distinct attenuation bands, which are related to couplings of the SMW mode to different MMW modes. If the grating-interaction length changes, so does the filter response. This fact is observed in Fig. 5(b), which is the transmission spectrum in case of the grating-interaction length of 16 mm. Among the three dips, especially, the middle one undergoes significant changes. The attenuation of the main lobe decreases slightly and those of the side lobes increase so that these side lobes affect and distort the first and the third dips. From this result, we can deduce that the grating-interaction length for full power transfer from the SMW mode to the MMW mode related to the second dip is between 11 and 16 mm.
In Introduction, we have mentioned that this filter can be a unit of the GFF shown in Fig. 1. For that purpose, it is desirable there is only one attenuation band for a given grating period in the wavelength range from 1500 nm to 1600 nm. However, the fabricated filter has three distinct bands. This is because there are too many modes in the MMW. From the effective index method, the number of MMW modes is estimated to be about 600. Therefore, it is required to reduce MMW dimensions appropriately, if we want to use this filter as a part of the GFF.
where L is the grating-interaction length. In this filter structure, the calculated derivatives of NMMW and NSMW with respect to λ at the wavelength of λ0 are -0.01169 µm-1 and -0.0061 µm-1, respectively. These values yield the bandwidth of 13 nm. The measured bandwidth is 15.7 nm, which is close to the calculated value.
Unfortunately, the clear filter characteristics for TM polarization could not be observed. This fact may be explained by the modal birefringence due to intrinsic birefringence of materials. The difference of the effective indices between TE and TM modes of the MMW is quite less than that of SMW. Therefore, the center wavelength for TM polarization is much different from the one for TE polarization as anticipated from Eq. (1). Consequently, we have to choose waveguide materials and design both waveguides carefully to reduce polarization dependency.
We have demonstrated the feasibility of the process for fabricating a SMW and a MMW aligned vertically on the same substrate. This process consists of conventional fabrication of a SMW and fabrication of a MMW by molding method with a PDMS mold. Using this process, we have proposed and demonstrated a filter that drops optical signal propagating in a SMW to a MMW in the specific wavelength interval by a LPG. On the one hand, this filter characteristics may be employed for a compact integrated optical GFF if MMW dimensions are adjusted appropriately. On the other hand, it may have potential applications for short distance communication that utilizes multimode fibers since the MMW constituting the filter has a large core size comparable to them.
This work was supported by Novera Optics, Inc..
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