Mode division multiplexed (MDM) waveguide link scheme with cascaded Y-junctions Alexander M. Bratkovsky a,n , Jacob B. Khurgin b , Ekaterina Ponizovskaya a , Wayne V. Sorin a , Michael R.T. Tan a Q1 a Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, CA 94304, USA Q2 b Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, MD 21218, USA article info Article history: Received 14 February 2013 Received in revised form 23 April 2013 Accepted 22 June 2013 Keywords: Mode division multiplexing/demultiplexing Y-junctions Mode conversion Waveguide Fiber Beam propagation method abstract A potentially high throughput waveguide mode multiplexer/demultiplexer (MUX/DEMUX) exemplified by two cascaded Y-junctions is shown to be capable of efficiently transforming modes from single mode waveguides into a two-dimensional set of the orthogonal ones in a multi-mode waveguide, and back. The design parameters of the cascade and the mode transformations are found by accurate beam- propagation modeling for possible realization with optical polymers. The proposed MUX/DEMUX can enable multifold increase of the data rates of multimode links used in datacom and optical computer communications. & 2013 Published by Elsevier B.V. 1. Introduction Optical interconnects are becoming indispensable in state-of-the art short reach networks, and as the complexity of the latter grows the amount of information passing through the interconnect increases exponentially creating a need for bandwidth expansion [1]. Since individual sources and detectors used in interconnects are limited at present by the speed of electronic components to 10– 40 Gbps bit rates, the only way to satisfy the bandwidth require- ments is to adapt a multi-channel architecture, i.e. to multiplex at the transmitter and de-multiplex at the receiver a number of individual channels. Most often multiplexing is performed in the spectral (wavelength) domain – as in wide spread WDM systems. But even coarse WDM requires rather sophisticated optical components (gratings and filters) and sources (lasers) with stable output that does not depend on temperature, thus raising the cost of the system. Alternatively, one can increase the cumulative data rates by using advanced modulation formats (QPSK, M-QAM, etc.) [2], in most cases at the cost of having to use coherent detection which necessitates even higher quality components, especially narrow bandwidth lasers. In addition to the aforementioned drawbacks, there is the limitation of the multimode fiber itself – the information carrying capacity of an inexpensive multimode fiber (MMF) or waveguide is restricted by the inter-modal dispersion and hence a single mode fiber (SMF) is often used. Here, we address this shortcoming of MMF and turn it into advantage by building upon a concept of spatial diversity used in wireless communications [3]. As explained above, it is precisely the inter-modal dispersion in standard MMF that limits its bandwidth-length product, in the same way the multi-path inter- ference causes distortion and fading of the signal in the wireless communications. It is also well known in wireless field that by using multiple antennae at both transmitter and receiving ends the information carrying capacity of the link can be increased manifold because one can think of different paths as separate channels. As a result, spectral efficiencies in excess of 100 s of bits per second per Hertz Q3 had been obtained [4]. It has been noticed as early as 10 years ago that the environment inside the MMF similarly provides multiple paths (or channels) for the light, and, in principle one can use N sources and detectors to increase information carrying capacity by a factor of N [5]. Fundamentally, the idea behind spatial multiplexing is straight- forward – since the link is a linear system, the relation between the N transmitted signals – a vector T and the N received signals – a vector R is represented by the complex N Â N matrix M. Obviously, once one detects the received signals the transmitted signals can be recovered by inverting the matrix M. However, while the recovery is easily accomplished in the RF domain, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications 0030-4018/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.optcom.2013.06.060 n Corresponding author. Tel.: +1 650 857 7355; fax: +1 650 813 3312. E-mail address: alex.bratkovski@hp.com (A.M. Bratkovsky). Please cite this article as: A.M. Bratkovsky, et al., Optics Communications (2013), http://dx.doi.org/10.1016/j.optcom.2013.06.060i Optics Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎