Reconfigurable 2 × 2 Orbital-Angular-Momentum-Based Optical Switching of 50-Gbaud QPSK Channels

We experimentally demonstrate a reconfigurable 2×2 switch for four OAM-multiplexed beams carrying 50-Gbaud QPSK data channels. The performance of the switch is measured for different switch configurations. An OSNR penalty of < 2.5 dB is observed for the switched channels.


Introduction
Orbital-angular-momentum (OAM) has emerged in the past few years as a potential approach to efficiently multiplex many spatially collocated optical data-carrying beams [1][2][3].Each optical OAM beam possesses a unique twisting of its phase front, such that multiple beams, each carrying a different amount of phase twist are orthogonal to each other [4,5].OAM light beams have azimuthal phase dependence given by exp(il ), in which 'l', the azimuthal mode order, corresponds to the number of 2 phase jumps across the wavefront of the OAM beam.High-bit-rate free-space and fiber point-to-point links have been demonstrated in which multiple OAM modes are spatially overlapping as they propagate [6,7].However, there might be a value in performing networking functions in the optical domain on OAM-based data channels.This is similar in concept to the advantages that wavelength-selective networking and switching provided historically on top of simple wavelength multiplexed high-capacity point-to-point systems [8,9].Recently, various OAM-based networking functions have been demonstrated, including reconfigurable add/drop multiplexing [10], selective switching of a single mode [11], and multicasting [12].A laudable goal would be to extend such functions to include a fully reconfigurable N×N OAM-selective optical switch.In this paper, we demonstrate reconfigurable 2×2 orbital-angular-momentum-based optical switching of 50-Gbaud QPSK channels.With the use of spatial light modulators (SLMs), we selectively redirect different combinations of the OAM beams at the input ports to same or different output ports.

Concept
In WDM networks, a 2×2 switch is capable of redirecting one of the input wavelengths channels to one of the output ports.It is also possible that all of the input channels simply pass through the switch without being redirected.Fig. 1 shows the concept of a 2×2 OAM based switch is analogous to the WDM switch.One of the two multiplexed OAM beams (l 1 , l 2 ) and (l 3 , l 4 ) arriving at the input ports A and B, can be independently redirected to appear at output ports B and A, respectively.If switching is not desired, then input OAM beams pass through the switch without being redirected.A functional block diagram of the 2×2 switch is depicted in Fig. 2. In each path, the multiplexed input beams go through the mode down conversion block [11], which "unwraps" the phase of one of the incoming OAM beams.In the far-field region, the phase unwrapped beam transforms into a non-OAM beam (l = 0) with planar phase front (represented as a spot).On the contrary, the other collinear OAM beam in the multiplexed pair, transforms into a higher order OAM beam (shown as a circle).Grating based switch spatially separates collinear beams by redirecting the inner non-OAM beam and outer OAM beam in different directions such that non-OAM beam from one path aligns with the higher order OAM beam from the other path.The mode orders of newly aligned beams are

Experimental Setup
The experimental setup is shown in Fig. 3.A 50 Gbaud NRZ-QPSK signal is split into four paths and is de-correlated by using a piece of fiber.Two SLMs (SLM-1 and SLM-2) are used to convert four QPSK data-carrying fundamental Gaussian beams (w o = 1.1 mm) into four datacarrying OAM beams.The effective area of each of the SLMs is divided so that it can simultaneously convert two Gaussian beams into OAM beams of desired order.The two OAM beams are then superposed by using a 3 dB non-polarizing beam-splitter.
An afocal setup with unity magnification (f = 200 mm), is used to keep the beams collimated over a longer distance.This constitutes the setup for one of the two input ports of the switch.Similar setup is duplicated to generate the second input for the switch.Inside the 2×2 switch, SLM-3 is used to perform the down conversion operation.An SLM with larger dimensions (600 x 792 pixels) was used to simultaneously down convert OAM beams in each path.After down conversion, the two collinear beams in each path transform into a non-OAM beam and a higher order OAM beam.SLM-4 is programmed with a phase mask that has two different blazed grating regions.In each path, a mirror is used to adjust the angle of beams incident on SLM-4.The angles of the mirrors and the blazed gratings were calculated such that one of the beams from each path, after reflecting off SLM-4, propagates collinearly with a beam from the other path.While one set of collinear beams forming output port A propagates towards SLM-5, the other set of collinear beams (shown as output port B with a dotted line), was aligned by using two mirrors.The phase mask on SLM-5 also has two regions to simultaneously up convert beams in each path.Although all data channels were transmitted simultaneously, one pair of collinear beams in each path was up converted at a time.At the receive end, SLM-6 is used to select only one of the incoming OAM beams by unwrapping its phase and transforming it into a focused spot.The phase unwrapped beam is then coupled into a single mode fiber and sent to coherent detection setup.We used OAM beams with l 1 = +4 and l 2 = -4 for input port A, and l 3 = +2 and l 4 = -6 for input port B. As shown in the Tab. 1, the switch can be configured in 5 different configurations.Four of the configurations correspond to redirecting one of the input OAM beams from each input port to arrive at the opposite output port.The 5 th configuration is for the "no-switching" case.We first aligned SLM-5 and SLM-6 to up convert and select OAM beams intended for the output port A, and measured bit error rate (BER) performance for all of the five switch configurations with both of the input ports turned 'ON'.We then re-aligned the SLM-5 and SLM-6 to form output port B. The BER for few selected OAM beams with 50 Gbaud QPSK data channels at output ports A and B are shown in Figs.4a-b.Please note that the number in parentheses signifies the OAM beam that was selected for BER measurement at the output port.For example, C1(+4) means configuration 1 and OAM +4 is selected at the output port.OSNR penalties of < 2.5 dB and < 1 dB at BER of 2e-3 were observed for the output ports A and    B, respectively.Fig. 4c shows the BER performance of both the output ports for the noswitching configuration C5.An OSNR penalty for no-switching configuration was < 3dB.Similar performance for the remaining channels was observed.We also calculated the crosstalk between different OAM beams after coupling the selected beam into a single mode fiber.In order to measure the crosstalk, we first measured the received power of the desired OAM beam while the sources of other OAM beams were blocked.We then blocked the source of the desired OAM beam and measured the power leaked from each of the other OAM beams, one at a time.

Conclusions
In conclusion, we have demonstrated a 2×2 OAM switch by combining down conversion, grating based switching and up conversion operations.The performance of the switch is measured in terms of BER for different switch configurations.It is also possible to build an OAM switch that has more than 2 ports by cascading 2×2 fundamental blocks.

Fig. 1 .
Fig. 1.Block diagram depicting the concept of 2x2 OAM switch.One of the two OAM beams at any input port can be redirected to other output port.

Fig. 4 (Fig. 5 (
Fig. 4(a)-(b).Measured BER for different switching configurations.The selected OAM beam for BER measurement in each configuration is shown inside the parenthesis, and configurations are describe in Tab.1; (c).Measured BER for noswitching configuration at output ports A and B.
Fig. 5a is calculated by taking the difference between the power in the desired beam and sum of the powers in all other beams.Figs.5(b) -(g) depict the constellation diagrams for different switch configurations.Figs.6(a) -(f) show different OAM beams after being redirected and up converted.The interferograms of different OAM beams after up conversion are shown in Figs.6(g) -(j).