P2.14.pdf
ECOC Technical Digest © 2012 OSA
Spatial Mode-Division-Multiplexing of Few-Mode Fiber (1)
Nicolas Riesen (1) (2)
(2)
and John D. Love
RSPE, The Australian National University, ACT 0200, Australia,
[email protected] PEC, The Australian National University, ACT 0200, Australia,
[email protected]
Abstract The independent excitation and detection of spatial modes in few-mode fiber networks presents major challenges. This paper presents novel spatial mode–division–multiplexing techniques based on planar couplers and asymmetric Y-junctions. These developments may play a role in future high–capacity few-mode fiber telecommunications. Introduction The ever–increasing demands for high– bandwidth optical fiber communications are destined to challenge the limits of single–mode fiber. This has led to a renewed interest in the use of few–mode fiber to exploit the extra degrees of freedom provided by several propagating modes. Few–mode fiber avoids the severe modal crosstalk experienced in highly– multimode fibers, thus in principle allowing for each mode to be considered an independent data channel. The independent excitation and detection of modes however presents major challenges, especially for more than two spatial modes. Reported techniques involve, for instance, the 1 use of spatial filters , Liquid Crystal on Silicon 2 3 modulators and phase plates , amongst 4–6 others . The available techniques are however typically plagued by high cost, complexity, performance issues or are based on bulky freespace optics. This paper presents novel spatial modedivision-multiplexing (MDM) techniques that use simple waveguide structures. Simple waveguide structure candidates for spatial-MDM include mode-selective couplers (MSC)7,8, asymmetric Y-junctions9 and multimode interference devices (MMI)10,11. This paper only addresses the former two, as MMI devices are highly wavelengthdependent and are largely restricted to the multiplexing/demultiplexing of only two modes. Planar Mode-Selective Couplers The typical half-polished, fiber-based modeselective couplers are highly susceptible to temperature changes and other environmental variables and thus often have high loss and low coupling efficiency. This makes them largely impractical for real-world applications7,8. These issues can be addressed by instead using fused-type mode-selective couplers, which however introduce significant fabrication challenges7,8. In addition, mode-selective couplers are also inherently wavelength-
dependent, which can limit the application of wavelength-division multiplexing (WDM). They are also dependent on the orientations of the asymmetric modes, which are random in long distances of circularly-symmetric fiber12. The resulting demultiplexing issue of a randomly orientated asymmetric mode in a circularlysymmetric fiber, is addressed in this paper by using pairs of perpendicularly-positioned planar couplers (see Fig. 2). The use of planar couplers requires that the actual fiber be interfaced with square buried–channel waveguides at either end. The minimization of losses at the interfaces can be achieved by ensuring equivalency in the waveguide cross– sectional areas and core and cladding indices13. The proposed mux/demux (see Fig. 2) relies on the nature of the approximate adiabatic conversion of fiber modes into square waveguide modes, and vice versa. We therefore consider the case of a circular fiber to square waveguide interface, but by reciprocity the results apply in reverse. The fundamental linearly–polarized mode (LP01) transforms into the fundamental planar mode (T0). This planar mode could in principle then be decoupled using a single coupler. The behavior of the second linearly– polarized mode (LP11) is slightly more complex because of its asymmetry. At a fiber to square waveguide interface the LP11 mode transforms into two perpendicular, second planar modes (T1), with powers dictated by the LP11 azimuthal orientation, θLP11 (see Fig. 1). The azimuthal orientation of the LP11 mode (or any other asymmetric mode), is however unstable in a circularly–symmetric fiber because of the azimuthal degeneracy. This instability can be thought of as random coupling between an Even (horizontal)–LP11 and an Odd (vertical)–LP11 spatial mode. Moreover, the LP11 mode may lose all sense of orientation over a long length of fiber, resulting in an annular–like intensity pattern. However this can be thought of as merely a continuum of all LP11 orientations. In general a random distribution of the two perpendicular second planar modes (T1)
978-1-55752-950-3/12/$31.00 ©2012 Optical Society of America
P2.14.pdf
therefore results. This poses somewhat of a problem because, as mentioned, a mode– selective coupler is highly dependent on the orientation of the mode (see Fig. 1).
Fig. 1. The behaviour of mode–selective couplers (as used in Fig. 2c,e) is highly dependent on the azimuthal angle of the asymmetric modes. Shown is a Beam Propagation Method (BPM) simulation for the LP11 fiber mode and two perpendicularly–positioned planar couplers
Assuming that a planar coupler, located beyond the interface, is inline with the LP11
ECOC Technical Digest © 2012 OSA
perpendicular planar couplers. In principle this concept could be extended to more modes, although the spread in modal indices will 14 decrease with increasing number of modes . This complicates mode isolation and results in increased modal crosstalk within the fiber. Applying these concepts (which are shown in the Beam Propagation Method (BPM) simulations of Fig. 2a and Fig. 2b), the directdetection few–mode fiber network of Fig. 2 is proposed. The pairs of planar couplers at the multiplexer (Fig. 2c) independently excite the square waveguide modes that evolve into the second and third fiber modes. These modes are then demultiplexed using an identical planar coupler arrangement (Fig. 2e) at the end of the fiber. Since we are dealing only with three modes, the fundamental mode can be directly excited/detected at the square buried-channel waveguide (see Fig. 2f).
Fig. 2. The few–mode fiber mux/demux (structure is surrounded by cladding)
lobes at θLP11 = 0, the relative output power of the coupler (PX) is given to a very good 2 approximation by cos (θLP11) (Fig. 1). This explains why two perpendicularly–positioned couplers (X and Y) are required to decouple an arbitrarily–rotated LP11 fiber mode (Fig. 1, Fig. 2f). A perpendicular arrangement of couplers can therefore decouple all the power from an LP11 mode with a random orientation. As for the second symmetric linearly– polarized fiber mode (LP02), it transforms into two perpendicular third planar modes (T2). These two planar modes can then be decoupled, again, using two identical
Asymmetric Y-Junctions The second mux/demux simulated is based on the asymmetric Y-junction. Asymmetric Yjunctions are planar devices which can be used to adiabatically split or combine modes by means of phase matching15-17. In other words, the effective index of each mode in the stem of a Y-junction closely matches the fundamental mode in a distinct output arm. A sufficiently small branching angle ensures minimal radiation losses or modal crosstalk16,17. Multiple-arm Y-junctions can separate three or four planar modes with minimal dependence on wavelength, index contrast, or polarization in the
P2.14.pdf
case of weak-guidance9,16,17. Asymmetric Yjunctions are however, again, stronglydependent on the orientation of the asymmetric planar modes in the stem. This therefore prevents their use as demultiplexers when interfaced with circular few-mode fibers because of the inherent spatial-instability of the asymmetric fiber modes. Instead, we propose their use for the multiplexing/demultiplexing of the modes of few-mode elliptical-core fibers. The direct-detection elliptical-core fiber network multiplexed/demultiplexed using asymmetric Y-junctions17 is shown in Fig. 3. The Y-junctions can be fabricated using tried and tested lithography, etching, and deposition techniques. The configuration shown allows for the straightforward polarization-multiplexing of each of three spatial modes, without the need for active polarization controllers. The obvious drawback is the requirement of specialty fiber as the transmission medium, although virtually any type of polarization-maintaining fiber could be used. Pol. (x,y) Pol. (x,y) Pol. (x,y)
(d) T2
T1
LP01
LP02
LP11
LP01
(f) Demux
LP11
T0
LP02
(e) EDFA
(c) T0
T0
T0
(a) Mux
Pol. (x,y)
(b) Polarization
LP02 Pol. (x,y)
walk-off splitter
LP11 Pol. (x,y)
LP01 Pol. (x,y)
Pol. x
Pol. y
Fig. 3. Few-mode elliptical-core fiber mux/demux17
Assuming low surface/edge roughness the asymmetric Y-junction multiplexer (Fig. 3a) can 17 be fabricated short enough , to allow for the approximate preservation of input polarizations. The fundamental mode inputs to the arms of the Y-junction can be polarization-multiplexed (or demultiplexed) using polarization walk-off splitters (Fig. 3b) or simple polarization beamsplitters. The wavelength-insensitivity of the asymmetric Y-junction17 also allows for the use of wavelength-division multiplexing (WDM) over broad EDFA bands. The fundamental modes of the widest, second-widest, and narrowest Yjunction arms evolve into the fundamental (T0), second planar (T1), and third planar (T2) modes of the Y-junction stem, respectively, as shown in the Beam Propagation Method (BPM) simulation of Fig. 3c. The simulations predict radiation losses of a few percent for millimetre lengthscale devices, with almost negligible mode crosstalk (i.e.
