1. Introduction

In response to the never-ending increase in data traffic over the internet, which has been caused by the extensive deploy of the smart phones and mobile computers, the capacity of optical communication system is forced to be increased by expanding the number of wavelength channels used in wavelength division multiplexing system. With an increase in the number of channels used, the configuration of optical device layers is getting complicated, thereby increasing the demand for cost-effective, array-type optical devices with smaller device footprint [1].

Variable optical attenuators (VOAs) have been used in WDM systems to control the optical signal levels of each wavelength channel independently. Various kinds of VOAs have been implemented in terms of silicon wire waveguides [2], silica waveguides [3], MEMS structures [4], and polymer waveguides [5]. Among these, polymer waveguides have a number of advantages such as low electric power consumption, low dependence on wavelengths, and convenient fabrication process [6, 7].

For use in a high capacity optical communication system that requires many VOA devices for controlling optical signal level of each channel individually, array-type VOAs are preferred because of the reduced complexity of module assembly and cost-effectiveness. On the contrary, the array device suffers from high crosstalk between the adjacent channels, which is an inherent obstacle of integrated optics. In particular, if the array devices are used in a cascaded communication network, the effect of wavelength crosstalk is significantly amplified [8, 9].

Modern WDM optical communication systems are required to have a crosstalk of less than −45 dB in order to maximize the transmission distance; however, this is hard to achieve [10, 11]. In polymeric VOAs, the crosstalk is mainly caused by the planar guided mode, which is hard to remove. In this work, the crosstalk is reduced by incorporating a self-assembled monolayer of a microsphere array in the middle of the planar waveguide. This microsphere monolayer scatters and diffracts the planar guided mode out of the device so as to reduce the source of the crosstalk into the adjacent channels. In array-type VOA devices that comprise the microsphere array, we could reduce the crosstalk to less than −50 dB.

2. Device structure and operating principle

By virtue of their high thermo-optic coefficient and slow heat dissipation, polymeric VOAs have the advantage of low operating power consumption, which is attractive for multi-channel VOA arrays. A schematic diagram of the proposed VOA structure is shown in Fig. 1(a) , in which single-mode waveguides are connected to the multi-mode waveguide section using the tapers. The heating electrode places at an angle over the multi-mode waveguide section. Initially, the fundamental mode launched at the single-mode waveguide propagates through the multi-mode waveguide sections with no inherent loss as long as the transition is adiabatic. As the temperature at the heating electrode increases, the refractive index under the electrode decreases, and this causes refractive-index perturbation in the multi-mode waveguide section. Subsequently, strong mode coupling occurs into the higher-order modes. These excited higher-order modes are filtered out as the light propagates through the output single-mode waveguide, thereby attenuating the optical signal.

 

Fig. 1 (a) Schematic diagram of the polymeric VOA device, wherein the sources of crosstalk are indicated as dotted arrows; (b) side view of the VOA exhibiting the channel guided and planar guided modes confined by the interfaces between air and the silicon surface; (c) diffraction of the planar guided mode in the outward direction by the microsphere array as a function of microsphere size.

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An arrayed VOA fabricated in this work consists of 10 or 40 individual VOAs integrated on a single chip. In the arrayed VOA, the radiated light at the attenuator can cross over to adjacent channels, thereby producing crosstalk, as shown in Fig. 1(a). The light radiated from a VOA channel waveguide does not escape from the planar waveguide structure consisting of thick polymer cladding layers, which are sandwiched between air and a silicon substrate as in Fig. 1(b). Hence, planar guided light is the major contributor to an increase in the crosstalk. Another source of crosstalk is the non-ideal input coupling due to the misalignment and mode mismatch.

The crosstalk could be significantly reduced if one can radiate the planar guided mode out of the device. For this purpose, a microsphere monolayer is incorporated in the arrayed VOA device to diffract the planar mode in the outward direction, as shown in Fig. 1(c). Such a diffraction grating structure was recently adopted in organic light emitting diodes in order to increase the out-coupling efficiency [12]. The diffraction efficiency of the microsphere monolayer depends on the period and size of the microsphere array and the position of the microsphere monolayer.

Finite difference time domain (FDTD) simulation was performed to find the effect of microsphere array. In this calculation, the propagation loss through the cladding layer without the core is calculated when a light source was excited from a single mode fiber. The polymer waveguide had an index of 1.43, and polystyrene (PS) of 1.59. Initially, the waveguide had loss due to the radiation into silicon substrate, and then the loss was increased by the diffraction of microsphere array. The amount of increased loss was defined as additional loss, which is calculated as shown in Fig. 2 for the two cases of microsphere locations. The additional loss becomes higher than 1 dB for the propagation length in FDTD simulation is limited to 500 μm, which corresponds to 20 dB/cm.

 

Fig. 2 FDTD simulation result of the diffraction efficiency as a function of the position of the microsphere monolayer.

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3. Experimental results

For the experimental demonstration, we fabricated 10-channel arrayed VOA devices with the microsphere layer embedded in the middle of the cladding. Because of the PS microsphere had better adhesion on ZPU polymer than silicon substrate, type 1 structure shown in Fig. 2 was chosen in this initial demonstration. The period determined by initial size of particle was 2.1 μm, and sphere size was adjusted to about 1.7 μm by plasma etching. The VOA array was fabricated by the conventional fabrication procedures such as spin coating, UV curing, photolithography, and oxygen plasma etching, as shown in Fig. 3 . For the core and cladding layers, we used UV-curable ZPU-series polymers (ChemOptics, Co.). PS microspheres were purchased from Duke Scientific, and used with no additional chemical treatment [13, 14].

 

Fig. 3 Schematic fabrication procedures of the array-type polymeric VOA device with a self-assembled monolayer of polystyrene microspheres.

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In order to prevent guided-mode scattering in the VOA device, we need to ensure that the microspheres are not placed near the waveguide core. For this purpose, AZ5214 photoresist (PR) was patterned to cover the region close to the waveguide core. PS microspheres dispersed in DI water with 10 wt% were coated on the ZPU polymer surface, which was treated by oxygen plasma to increase the surface adhesion property. After the coating, the surface was rinsed with DI water to remove the multi-layer stacked microspheres. The size of the microspheres was reduced to 1.7 μm by oxygen plasma etching. The PR was then lifted off along with the over-coated PS microspheres. Consequently, only the microspheres in the areas between the waveguides remained. Figure 4 shows the optical microscopy and SEM images of the microsphere patterns after the lift-off. One can observe that the PS microsphere is clearly lifted-off at the region close to the waveguide core of the VOA; the area corresponding to the microsphere looks dark because of the strong scattering. The microsphere is densely packed as a monolayer with reduced size by plasma etching.

 

Fig. 4 Optical microscopy and SEM photographs of the microsphere patterns incorporated in the device. The area of the microsphere array looks dark as a result of the strong scattering effect.

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The arrayed VOAs with and without microsphere patterns were characterized at 1550 nm. The mode profiles observed in a CCD are shown in Fig. 5 . For the reference sample, the light remained at lateral cladding was clearly observed. The light was almost completely vanished in the device with the microsphere layer. Additional loss in the channel-guided mode was negligible as compared by the intensity. This result implies that the microsphere array diffracts the planar guided mode successfully. The multiple spots in Fig. 5(a) appears at the center of the waveguide in vertical direction because the thin core layer of 1.2 μm supports a loosely confined fundamental mode with a long tail of evanescent field.

 

Fig. 5 CCD images of guided mode profiles observed from the samples (a) without microsphere layer, and (b) with microsphere monolayer, wherein the planar guided light is almost invisible. The multiple spots below the mode are appeared by the reflections at the objective lens.

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A two-channel V-groove fiber was connected to the VOA device to monitor the signal power and crosstalk from the adjacent channels simultaneously. Regardless of the presence or absence of the microspheres, the VOAs exhibit similar characteristics, with an attenuation of 30 dB for the applied power of 18 mW, as shown in Fig. 6 . However, the crosstalks of the two devices are significantly different. While the ordinary VOA had a crosstalk of about −30 dB, the microsphere-incorporated VOA exhibited a crosstalk of less than −50 dB.

 

Fig. 6 Comparison of the attenuation characteristics of the VOAs and the crosstalks measured from an adjacent channel in the presence and absence of the microsphere array. The crosstalk in the ordinary VOA device was about −30 dB, while that in the microsphere-incorporated VOA device was reduced to less than −50 dB.

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The device structure shown in Fig. 1 is a basic structure of the VOA, but in practice, a multimode waveguide was included to collect the attenuated power to a specific direction rather than the radiation into the substrate layer. Because of this pattern, the crosstalk was not increasing with the heating power increase. Hence the major source of crosstalk may come from the modal mismatch. However, the precise adjustment of mode profile to keep the crosstalk below −50 dB is quite difficult. To suppress the modal mismatch, the alignment tolerance of fiber pigtail becomes very small, which will affect the productivity in the manufacturing. Hence, the microsphere diffractor is quite attractive to suppress the crosstalk in terms of a relatively simple fabrication process without sacrificing the production yield.

4. Conclusion

Extremely low crosstalk was demonstrated in an array-type polymeric VOA device by incorporating a self-assembled microsphere scattering layer. In order to form the microsphere scattering layer inside the polymer waveguide cladding, a simple procedure such as spin coating and PR lift-off process was employed. Polystyrene microspheres providing a refractive-index contrast of 0.16 were introduced; a strong diffraction effect was obtained by adjusting the size of the microsphere by oxygen plasma etching.

The reference VOA exhibited an attenuation of 30 dB for a heating power of 18 mW. The crosstalk between the adjacent channels was dramatically reduced from −30 dB to −50 dB by adopting the microsphere monolayer. Moreover, there was no change in the insertion loss or the attenuation characteristics. The proposed microsphere diffractors can be incorporated in various integrated optic devices where the crosstalk between individual components is to be suppressed.