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Abstract
We propose a waveguide frontend with integrated polarization diversity optics for a wavelength selective switch (WSS) array with a liquid crystal on silicon switching engine to simplify the free space optics configuration and the alignment process in optical modules. The polarization diversity function is realized by the integration of a waveguide-type polarization beam splitter and a polarization rotating half-wave plate in a beam launcher using silica-based planar lightwave circuit technology. We confirmed experimentally the feasibility of using our proposed waveguide frontend in a two-in-one 1 × 20 WSS. The experimental results show that the fabricated waveguide frontend provides a polarization diversity function without any degradation in optical performance.
© 2017 Optical Society of America
1. Introduction
Internet traffic has been increasing greatly because of the spreading use of various services such as cloud computing and IP-based video streaming. To deal with this growth in traffic while maintaining or reducing networking costs, optical transport networks are evolving to achieve more flexibility as regards wavelength routing and wavelength assignment. In such networks, multi-degree reconfigurable optical add/drop multiplexing (ROADM) with colorless, directionless and contentionless functions has been attracting considerable attention with a view to realizing dynamic capacity allocation [1]. Wavelength selective switches (WSSs) are now widely used to provide the colorless function in these advanced ROADM nodes [2]. A WSS is generally composed of a micro-lens-array-based fiber frontend, a diffraction grating, Fourier optics components and a switching engine. Recently, several waveguide frontends have been reported with the aim of realizing a high port count WSS [3,4]. In [3], K. Suzuki et al. proposed a spatial and planar optical circuit platform and reported a 1 × 95 WSS as a proof of concept. In this platform, a silica-based waveguide frontend is used instead of a conventional fiber frontend. Since the waveguide frontend has an integrated beam-forming function and the design flexibility of port separation, an ultra-high port count WSS can be realized. Furthermore, the waveguide frontend can increase the integration level of optical modules while taking advantage of free space optics. As an example of the integration of functions, Y. Ikuma et al. reported an 8 × 24 WSS where the frontend was integrated with twenty-four 8 × 1 thermo-optic switches [5]. The applications of waveguides as a free-space optics frontend are summarized in [6].
The other approach to increasing the integration level is the integration of components, namely reducing the number of components in the WSS module. This approach will help to simplify the alignment process and reduce cost. At the European Conference on Optical Communication held in 2016, we proposed a waveguide frontend with integrated polarization diversity optics (PDO) as an example of component integration [7]. In this paper, we describe the design concept and operating principle of each optical circuit element of the waveguide frontend in more detail, for which we could not give a satisfactory explanation in [7] due to space limitations. The rest of this paper is organized as follows. In Section 2, we describe the need for PDO in the WSS module and the circuit design of our proposed waveguide frontend. Section 3 presents experimental results showing that the fabricated waveguide frontend causes no significant penalty for optical performance as a two-in-one 1 × 20 WSS. Section 4 concludes this paper.
2. Design of waveguide frontend
In this section, we explain the design concept and operating principle of a waveguide frontend including circuit elements such as a beam launcher and a polarization beam splitter (PBS).
2.1 Basic concept
First, we explain why PDO is an indispensable component of a WSS module. There are several technologies that can provide the switching engine in a WSS module, including micro electro-mechanical system (MEMS) mirrors, liquid crystal on silicon (LCOS) and a digital mirror device (DMD). Among them, LCOS-based switching engines have become the standard way to deal with flexible-grid wavelength channel assignment, because segmented tilting mirrors cause scattering losses and phase mismatches resulting in spectral ripples within the passband [8]. Since LCOS can operate only one linearly polarized beam, the PDO function is needed to enable us to launch polarization-multiplexed optical signals into the LCOS. Figure 1(a) shows the configuration of the PDO components. The input beam is separated into two orthogonal linearly polarized beams by a PBS, and then the polarization axis of one beam is rotated by a half-wave plate (HWP). In this configuration, any misalignment of the two beams degrades such aspects of optical performance as the polarization dependent loss (PDL). This paper proposes a waveguide frontend with a PDO function as shown in Fig. 1(b) to eliminate the need for PDO components and the alignment process.
2.2 Spatial beam transformer (SBT)
In our proposed waveguide frontend, we integrated a beam launcher circuit, which we call a spatial beam transformer (SBT) [9]. Figure 2(a) shows the SBT configuration. The SBT consists of input or output waveguides, a slab waveguide, and arrayed waveguides. The circuit layout is almost the same as that of an arrayed waveguide grating except that each arrayed waveguide has a uniform length. The SBT can radiate a collimated and anamorphic beam. The beam profile and port arrangement can be flexibly designed and precisely controlled with the waveguide fabrication technologies, for example photolithography. In addition, the SBT can emit or receive multiple beams at different angles. This enables us to achieve multiple WSSs in a single optical module. Here, we explain how to realize multiple WSSs in detail using Fig. 2(b). The green solid line from the bottom of the input port outputs at the upper angle to the free space optics via the SBT, and focuses at the top area of the LCOS. Then, the light is steered towards the selected output port by the LCOS and propagates through the SBT in reverse. The pink dashed line from the middle of the input port outputs at a different angle, and focuses at the middle area of the LCOS, and the light is connected independently to an output port of another WSS. The blue dot line from the top of the input port is output at a lower angle and focuses at the bottom area of the LCOS. The deflected light is similarly connected to an output port of another WSS. Therefore, multiple WSSs that work at different angles can be incorporated in simple optics because the lenses and grating are shared in this configuration.
Fig. 2 (a) SBT configuration and (b) three-in-one 1 x 4 WSS configuration as an example of multiple WSSs using an SBT array
2.3 Polarization beam splitter (PBS)
Figure 3 shows the configuration of a waveguide-type PBS. The PBS consists of a 1 × 2 Y-branch, two waveguide arms and a 2 × 2 directional coupler. To separate the input beam into two orthogonal linearly polarized beams, or TE and TM modes, the path length difference between the two arms for the TM mode differs by half a wavelength from that for the TE mode [10]. Thus, we have to adjust the waveguide birefringence of one arm to satisfy the following equation:
where n is the refractive index, L is the waveguide arm length, B is the waveguide birefringence, m is an integer and λ is the wavelength. The first and second subscripts of n mean the polarization axis and the waveguide arm number, respectively. The subscript of B corresponds to the arm number. It is generally known that the birefringence of silica waveguides is caused by the stress around the waveguide core that results from the difference between the thermal expansion coefficients of the silica glass layer and the substrate [11]. In our waveguide frontend, we adjusted the birefringence by introducing stress releasing grooves beside the waveguide core to satisfy Eq. (1) [12]. The insertion loss and the polarization extinction ratio are less than 2.5 dB and more than 25 dB, respectively, over the C-band [13].2.4 Frontend design
Figure 4 shows the configuration of the frontend circuit. Each input/output port consists of one PBS and two SBTs. For the input port, the input beam is launched into the PBS, and then the linearly polarized beams are launched into the upper-side and lower-side SBTs. The HWP is attached to the output-side facet of only the upper SBT to rotate the polarization axis at 90 degrees. Therefore, two beams with the same polarization state are radiated from the facet of the frontend chip into free space. The radiation angles are the same for both beams because the inputs into the slab waveguide in both SBTs are set at the same position. The HWP attachment procedure will be explained later. For the output port, the beams deflected by the LCOS propagate through this frontend in reverse. Figure 5 shows the operating principle of the frontend for a two-in-one WSS as an example of multiple WSSs. The thick and thin lines are the beams for WSS1 and WSS2, respectively. For the two-in-one WSS, a two-PBS array is needed for the respective WSSs. The input beams for WSS1 and WSS2 are launched into the lower-side and upper-side PBSs, respectively. Then, the linearly polarized beams of WSS1 and WSS2 are launched into the upper-side SBT. The other linearly polarized beams are launched into the lower-side SBT. The beams for WSS1 and WSS2 are input into different input ports in the slab waveguide of the SBT, and then they are output from the SBT at different output angles. In Fig. 5, the beams for WSS1 and WSS2 are directed upper and lower angle in free-space, respectively.
Figure 6 is a schematic illustration of a two-in-one WSS. An important feature is that an HWP with a striped design is attached to the SBT-side facet. As a result, all the beams between the frontend and the LCOS are aligned so that they have the same polarization axis. The beams that radiate from the waveguide frontend pass through a collimating lens, a diffraction grating, a port selecting lens, a focusing lens and an LCOS. Two triangles adjacent to the left side of the LCOS in the switching plane show the LCOS phase pattern to steer the beams as an example. All the lenses are cylindrical. The collimating lens and port selecting lens have optical power in the dispersion axis (y-axis) of the diffraction grating, and the port selecting lens refracts rays in the switching axis (x-axis). In the free-space optics, the beams for the same WSS, which were originally orthogonal to each other but with the same polarization in free space, have the same propagation direction, so that they are launched at the same position on the LCOS. The LCOS diffracts and reflects the incoming beams in a designated direction on the x-axis so that the reflected beams couple to the output ports in the frontend. The output ports combine the two beams into an orthogonal polarization in the way explained above.
3. Experimental results
We fabricated the waveguide frontend for the two-in-one 1 × 20 WSS using silica-based planar lightwave circuit technology. The refractive index difference was 1.5% and the waveguide core was 4.5 μm thick. The chip size was 18 mm × 47 mm. Figure 7 shows the HWP fabrication procedure. Since the polarization rotation has to be performed for every SBT, we need a comb-shaped HWP. We fabricated the comb-shaped HWP from polyimide. First, we cut off an HWP from its baseplate. Then, the HWP was held in a jig and incisions were made with a dicing blade. Next, we sandwiched the HWP between glass plates to facilitate handling. One side of the glass plate was coated with anti-reflection (AR) coating. Finally, the HWP block was attached to the waveguide facet.
The focusing length of the collimating, port selecting and focusing lenses were 12, 150 and 100 mm, respectively. The 1/e2 beam radius of the intensity distribution at the LCOS was 1042 μm in the switching direction. We installed the fabricated frontend into the optical setup shown in Fig. 6, which is the same setup used in [3]. Then, we evaluated the characteristics of the WSS, such as insertion loss, crosstalk, bandwidth and PDL.
Figures 8(a) and 8(b) show the measured transmittance spectra for WSS1 and WSS2, respectively. Each WSS was set to switch each port on the grid from 50 to 200 GHz. The minimum value of the variable grid width is theoretically 2.4 GHz which is limited by the sizes of the LCOS and its pixel. There were no differences between the optical characteristics of the two WSSs. The insertion loss ranged from 13.6 to 20.0 dB, which is higher than we expected. This loss can be broken down into the losses of 4.0 dB with the waveguide-frontend, 1.8 dB with the AR coating of the optics, 4.4 dB with the diffraction loss for the diffraction grating, 0.7 to 3.0 dB with the LCOS, 0.5 to 1.3 dB with the optics aberration loss and 5.5 dB with the alignment loss and vignetting on the lens. Because of our limited experimental equipment, the AR coating and diffraction grating were not optimized for this experiment. We believe that the insertion loss could be reduced by about 4 dB by optimizing the experimental setup. The wavelength dependent loss resulted from the difference in the deflection angle of the LCOS between the inner port and the outer port among other reasons such as miss alignments. For the outer port with the port number of one or twenty, the large diffraction loss was caused because of the large deflection angle compared to the inner port with the port number of ten or eleven.
The maximum crosstalk of −15 dB is the same level as that of previously reported WSS using bulk PDO components [3]. The crosstalk is assumed to be caused by imperfect LCOS phase pattern settings, for example the finite flybacks of the sawtooth waveform, and thus we think that there is room for further crosstalk reduction by optimizing the LCOS phase pattern. The crosstalk between two WSSs was below −30 dB.
Figure 9(a) shows the transmission spectrum for the 50-GHz grid and the PDL for the eleventh port of WSS1. The 0.5 and 3.0-dB bandwidths are 35.8 and 47.3 GHz, respectively. The PDL is less than 0.4 dB within the 0.5-dB bandwidth. The 0.5-dB bandwidth is improved from the previously reported value of 30.5 GHz [3]. The reasons of this improvement are as follows. In [3], the PDO components were implemented in the dispersion plane. Thus, the focusing lens with the small f-number was required, because the beams pass through near an outer edge of the lens. In contrast, as shown in Fig. 6, since the configuration of this work can implement the PDO components in the switching plane, we could use the focusing lens with the large f-number. As a result, the aberration causing the bandwidth degradation was eliminated. In addition, for the PDO components in the dispersion plane, two beams from orthogonal polarization have different optical path lengths and cannot be landed on the same position on the LCOS. This defocus degrades the bandwidth.
Fig. 9 (a) Transmission spectrum and PDL for the eleventh port of WSS1 (b) PDL for the 20 output ports of the two WSSs at 1550 nm (c) PDL for the eleventh port of WSS1 over the C-band
Figure 9(b) shows the PDL at a wavelength of 1550 nm for the 20 output ports of the two WSSs. Figure 9(c) shows the PDL for the eleventh port of WSS1 over the C-band. The measured PDL was less than 0.4 dB. These results prove that the fabricated waveguide frontend successfully provides the PDO function.
4. Conclusion
We proposed a waveguide frontend with an integrated polarization diversity function for a WSS array to simplify the free space optics configuration and the alignment process in a WSS module. We fabricated the silica-based waveguide frontend for a two-in-one 1 × 20 WSS, and the measured PDL was the same level as that of the previously reported WSS using bulk PDO components [14]. Our proposed frontend not only simplifies the optical configuration and the alignment process but also provides a wider transmission bandwidth because it eliminates the aberration in optics. This result indicates that the waveguide frontend can integrate a polarization diversity function without any degradation in the optical characteristics of the WSS array.
Acknowledgments
We thank Hiroshi Kudo, Mitsumasa Nakajima, Keita Yamaguchi and Kazunori Seno for their supports with the experiment.
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