As optical networks have evolved from point-to-point systems to ring or mesh networks, the optical devices that are needed to construct optical nodes have become more important and need to be more ...scalable. Hybridization of waveguide and free-space optics, or spatial and planar optical circuits (SPOCs), may provide the solutions for such needs. An SPOC platform is attractive because it can take advantage of both waveguide technology and free-space optics. Waveguide technology provides a high degree of integration of optical functionality for devices such as splitters and non-wavelength selective switches while free-space optics supplies a high degree of parallelism with two-dimensional spatial light modulators such as a liquid crystal on silicon (LCOS) devices. In this paper, we summarize the basics of SPOC technology and review its application to reconfigurable optical add-drop multiplexing (ROADM) devices. The key elements of a waveguide on an SPOC platform are an arrayed-waveguide grating and a spatial beam transformer. The latter functions as a microlens array and provides attractive features such as dense integration of switches. An LCOS device has numerous phase modulating pixels, enabling flexible manipulation of lightwaves. Using an SPOC platform, we constructed and demonstrated devices for ROADM applications including a wavelength filter, tunable optical dispersion compensators, and wavelength selective switches (WSSs). The WSSs range from an ultrahigh port count WSS to a single module wavelength cross connect.
As traffic volumes carried by optical networks continue to grow by tens of percent year over year, we are rapidly approaching the capacity limit of the conventional communication band within a ...single-mode fiber. New measures such as elastic optical networking, spectral extension to multi-bands, and spatial expansion to additional fiber overlays or new fiber types are all being considered as potential solutions, whether near term or far. In this tutorial paper, we survey the photonic switching hardware solutions in support of evolving optical networking solutions enabling capacity expansion based on the proposed approaches. We also suggest how reconfigurable add/drop multiplexing nodes will evolve under these scenarios and gauge their properties and relative cost scalings. We identify that the switching technologies continue to evolve and offer network operators the required flexibility in routing information channels in both the spectral and spatial domains. New wavelength-selective switch designs can now support greater resolution, increased functionality and packing density, as well as operation with multiple input and output ports. Various switching constraints can be applied, such as routing of complete spatial superchannels, in an effort to reduce the network cost and simplify the routing protocols and managed pathway count. However, such constraints also reduce the transport efficiency when the network is only partially loaded, and may incur fragmentation. System tradeoffs between switching granularity and implementation complexity and cost will have to be carefully considered for future high-capacity SDM–WDM optical networks. In this work, we present the first cost comparisons, to our knowledge, of the different approaches in an effort to quantify such tradeoffs.
We propose a compact transponder aggregator (TPA) consisting of a wavelength selective switch (WSS) array and a direction switch array. It has no intrinsic splitting loss and therefore the number of ...ports can exceed that of a conventional multicast switch. A waveguide-based frontend that incorporates spatial beam transformers enables us to increase the number of ports and integrate multiple WSSs in a simple optical system. The direction switch array is integrated in the frontend. We describe an 8 × 24 prototype, which is the largest port count yet achieved for a TPA with no intrinsic splitting loss.
Digital coherent transmission features a very large transmission bandwidth and has played a main role in core optical transmission networks. With the progress of semiconductor technologies, practical ...coherent transceivers with rates over 100 Gbaud are becoming feasible. With such advances, the transceiver components must have lower power consumption and lower costs, and it becomes important to know how each component contributes to the overall transmission performance. Here, to decompose the effects of noise factors in high-baud-rate DP-16QAM transmissions, we used the theoretical relationship between the bit error rate (BER) and noise-to-signal ratio (NSR) and performed linear analyses. The NSR could be decomposed into individual noise contributions according to dependences on the inverse signal and local photocurrents. The obtained parameters were shown to be useful for predicting required optical signal-to-noise ratio (ROSNR) characteristics.
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.
We report a multi-mode interference-based optical gate switch using a Ge(2)Sb(2)Te(5) thin film with a diameter of only 1 µm. The switching operation was demonstrated by laser pulse irradiation. This ...switch had a very wide operating wavelength range of 100 nm at around 1575 nm, with an average extinction ratio of 12.6 dB. Repetitive switching over 2,000 irradiation cycles was also successfully demonstrated. In addition, self-holding characteristics were confirmed by observing the dynamic responses, and the rise and fall times were 130 ns and 400 ns, respectively.
A wavelength selective switch (WSS) is proposed based on a Bragg reflector waveguide array, which has a footprint of only 2 × 6 mm 2 . We demonstrated arbitrary switching among 182 output ports by ...using a liquid crystal on silicon as a switching engine. The proposed WSS module length is only 10 cm, taking an advantage of the large angular dispersion in the Bragg reflector waveguide. Wavelength selective switching of 60 channels is carried out, exhibiting the device's excellent capability in providing ultra-large number of port count and wavelength channel operations at the same time. The crosstalk between adjacent ports are below -20 dB by a suitable alignment, which can be further improved by optimizing the module optics and alignments. The flexible lithography defined waveguide array design also reveals versatile potentials in integrating with other functional components.
A compact integrated 1×2 wavelength-selective switch (WSS) is demonstrated for which the structure is designed such that the WSS has no waveguide crossings. By employing a loopback configuration, ...only one arrayed-waveguide grating is needed in the WSS. The channel spacing is 100 GHz and the number of channels is 40. The measured transmission loss of the selected wavelength was <; 8.1dB, and the average crosstalk from two output ports were 23.0 and 40.7 dB.
We propose an arrayed-waveguide grating (AWG)-based tunable optical dispersion compensator (TODC) that uses a multiple lens structure with two lens materials. The lenses are realized by filling ...lens-shaped trenches in a slab waveguide with optical resins. The thermooptic effect provided by the lens materials realizes the desired dispersion tuning function. The multiple lens structure enables us to design the center dispersion and the dispersion tuning range independently. We fabricate a TODC based on a 10-ch, 10-GHz spacing AWG that uses resins with refractive indexes of 1.393 and 1.510. Its dispersion range is 0 to + 125 ps/nm. We also perform a transmission experiment using a 12.5 Gbps RZ-OOK signal. The power penalty at the bit error rate of 10 -9 is less than 3 dB; and error-free transmission is confirmed after dispersion compensation.
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