Virtually image phased array based on Bragg reflector waveguide for large-port optical switching

Abstract

ABSTRACT We review a new type of dispersion elements based on a Bragg reflector waveguide, which provides a large angular dispersion of 1~2°/nm. The device functions as sub-wavelength virtually imaged phased array grating. We obtain a number of resolution-poi nts (possible channel-count in demultiplexing) over 1,000. We demonstrate a large-scale wavelength switch based on a Bragg reflector waveguides array. The waveguides array has a small footprint of 2 4 mm 2 , but provides both ultra-large numbers (>100) of output -ports and wavelength-channels at the same time. Prospects for further increase in the wavelength channel count and output ports will be discussed. Keywords: beam steering , VCSEL, Bragg reflectors, phased array , wavelength selective switch 1. INTRODUCTION Mechanical beam steering devices such as polygonal mirror scanners have been widely used because of their wide deflection angle operations with high-resolutions [1-4]. However, they are bulky and their beam-steering speed is limited because of their moving parts. A non-mechanical beam steering approach is useful for various applications where the beam direction changes rapidly to random locations or when a system needs to be compact with good mechanical stabilities. Various approaches for non-mechan ical beam steering have been reported [5- 21], which include an optical phased array, a liquid crystal phase modulator, an electro-optic crystal deflector, an in-plane laser array, a photonic crystal laser, a phase -locked VCSEL and so on. The beam steering performance can be figured by a number of resolution -points N, which is defined as distinguishable spot counts in far -field patterns. The N for mechanical deflectors can be larger than 1,000 for practical application. It has been difficult to get a number of resolution points over 100 for non- mechanical beam steering techniques. An optical phased -array has been an interesting approach for realizing high -resolution beam steering [5, 18-21]. The recent Si-photonics approach enables compact 2D beam scanners [20]. However, it is difficult in increasing the inter-elements in the array for increasing N and there is a limitation in free spectrum range (FSR) of operating wavelengths. We proposed and demonstrated a high-resolution beam steering device based on a slow-light Bragg waveguide [22]. The structure is the same as that of vertical cavity surface emitting lasers (VCSELs) [23]. The scheme as a dispersive element is similar to a Virtually Imaged Phased Array (VIPA) [24] but in a different structure with a vertical microcavity. Wavelength swept b eam steering can be obtained in such a dispersive element with wavelength tuning. We demonstrated a high- resolution beam steering device with N of over 1,000 for a few mm long devices [25 -28]. A unique feature is its large angular dispersion of 1~2°/nm, which is an order larger than a conventional diffraction grating [29] and several times larger than that of VIPA [24]. Also, the FSR is over 100 nm thanks to its vertical microcavity. On the other hand, in recent long- haul and metro optical communication systems, the network architecture becomes more complex and desires for a flexible way in intelligent routing and dynamic traffic control [30], [31]. Reconfigurable optical switching is indispensable for realizing such functionalities. Rather than manually controlling the optical paths, pre -embedded software can be used for realtime monitoring and adjusting. At every node of the systems, add/drop -function is also needed for signal processing so such a node is named a reconfigurable optical add/drop multiplexer (ROADM). Optical switches and routing devices based on a planar lightwave circuit (PLC) have been studied but there still remain difficulties in handling a large number of spatial port counts [32-34]. A wavelength selective switch (WSS) is an alternative choice to realize such a functionality, which offers a much simpler way in routing the wavelengths.