We experimentally demonstrate a reconfigurable 2 × 2 switch for orbital angular momentum (OAM) multiplexed data-carrying optical beams. The switch can be configured to operate in either ‘cross’ or ‘bar’ state for each of the input OAM-multiplexed channels. The switching operation is demonstrated by operating the switch in five different configurations for the four OAM-multiplexed 50 Gbaud QPSK channels. An OSNR penalty < 2.5 dB is observed for the switched beams.
© 2014 Optical Society of America
Widely increasing capacity demands on optical networks have encouraged researchers to explore new dimensions in multiplexing optical data channels. While wavelength-division multiplexing (WDM) and polarization-division multiplexing (PDM) schemes are quite mature, multiplexing independent data channels using spatial degree of freedom, namely spatial-division multiplexing (SDM), has recently gained substantial interest . Two SDM techniques are under rigorous investigation: 1) multiplexing data channels using higher order modes ; and 2) the use of separate data streams transmitted over different cores in a multi-core fiber . Increased capacity is achieved by exploiting the inherent orthogonality among WDM, PDM, and SDM, which permits multidimensional multiplexing of data channels transmitted over the same link.
In recent years, multiplexing of data channels using orbital angular momentum (OAM) carrying light beams has emerged as a potential SDM approach to multiplex many spatially collocated optical data-carrying beams [4–8]. The OAM-carrying optical beams have azimuthal phase dependence, given by, and therefore possess helical wave front. The helicity of different OAM beams is governed by their azimuthal mode order,. Since OAM beams with different values are orthogonal to each other [9,10], it is possible to efficiently multiplex and demultiplex data channels using OAM beams.
Although high-bit-rate, free-space, and fiber links have been demonstrated in a point-to-point setup using multiple spatially overlapping OAM beams, many tools are required to implement reconfigurable optical networks in a multiuser environment. This is similar in concept to the advantages that wavelength-selective networking and switching historically provided on top of simple wavelength multiplexed high-capacity, point-to-point links [11,12]. Various OAM-based networking functions have recently been demonstrated, including reconfigurable add/drop multiplexing [13,14], selectively switching a single OAM beam , and multicasting . It might be useful to extend the employability of an OAM-based communications link in a multi-user environment, in which different OAM data channels could be selectively manipulated.
In this paper, we describe reconfigurable 2 × 2 OAM-based switching of 50-Gbaud QPSK channels . Our method uses multiple reflective spatial-light-modulators (SLMs) to spatially separate multiplexed OAM beams. After spatial separation, the beams are redirected, recombined, and sent toward the desired output port. The switch can be reconfigured to selectively redirect different combinations of the OAM beams to different output ports.
The remainder of the paper is organized as follows. Section 2 describes the principle of the switching scheme, and section 3 presents the experiment setup. In section 4, the results of operating the switch in various different configurations are presented. Concluding remarks are made in section 5.
2. Principle of operation
A 2 × 2 OAM-based switch is analogous to a 2 × 2 WDM switch. In WDM networks, a 2 × 2 switch either redirects one of the input wavelength channels to appear at the opposite output port (‘cross’ state) or allows a wavelength channel to simply pass through the switch without being redirected (‘bar’ state). As shown in Fig. 1,the concept of a 2 × 2 OAM-based switch is similar to a 2 × 2 WDM switch. Each input port of the switch receives two multiplexed OAM beams and depending on the switch state, OAM beams can be redirected to appear at a desired output port. The switch can be reconfigured to operate in either ‘cross’ or ‘bar’ state for each of the input OAM beams. As shown in Fig. 1, an exemplary switch configuration would be to switch with , while and simply pass through the switch. If switching is not desired, then the switch could be configured in ‘bar’ state for all of the input OAM beams, in which case all of the input OAM beams simply pass through the switch without being redirected.
Figure 2 depicts a functional block diagram of the 2 × 2 OAM-based switch. In each path, multiplexed OAM beams go through a mode down-conversion stage . Mode down-conversion refers to transforming one of the incoming OAM beams (a donut-like transverse intensity profile with helical wave front) into a Gaussian-like beam with = 0 (a spot-like transverse intensity profile with planar wave front). After passing through the down-conversion stage, the second collinear OAM beam in the multiplexed pair remains as an OAM beam (shown as a circle in Fig. 2), but with a different OAM mode order. Once spatially separated, the beams are passed through a programmable mode-dependent beam-steering element. This element spatially separates the two collinear beams by redirecting the inner Gaussian-like beam and outer OAM beam in different directions, such that the Gaussian-like beam from one path aligns with the OAM beam from the other path. The mode orders of the newly aligned beams are corrected at the up-conversion stage such that out going OAM beams have similar OAM values as their corresponding input beams. Up-conversion process is opposite of down-conversion process as it transforms an incoming Gaussian beam into an OAM beam with desired OAM value. After up-conversion stage, the beams are sent toward the corresponding output ports.
3. Experiment setup
The experiment setup is shown in Fig. 3.A 50 Gbaud NRZ-QPSK signal was split into four paths and de-correlated by using optical fibers of different lengths. Collimators were then used to generate four beams with Gaussian intensity profiles and a beam waist (w0) of 1.1 mm. Two SLMs, SLM-1 and SLM-2 (512 × 512 pixels, 15 micron pixel pitch), were used to generate the OAM beams at the two input ports for the switch. The phase mask on each SLM was designed to have two different fork holograms  in two different regions, so that beam shined on each region was transformed into the desired OAM beam. Half-wave plates (HWP) were used to align the incoming polarization to maximize the diffraction through SLMs. The two OAM beams were superposed with a 3 dB non-polarizing beam-splitter. An afocal setup with unity magnification (f = 200 mm) was used to keep the beams collimated over a longer distance. A pinhole was used in the Fourier plane of first lens in the afocal setup to suppress the zeroth order diffracted beam. This constitutes the setup for one of the two input ports of the switch. A similar setup generated the second input for the switch. SLM-3 was used inside the 2 × 2 switch to perform the down-conversion operation. For this purpose, we used an SLM with larger dimensions (600 x 792 pixels, 20 micron pixel pitch). The hologram on SLM-3 was also divided into two spatial regions, so that each region could down-convert one of the multiplexed OAM beams coming from each input port. After down-conversion, the beams were passed through another afocal system (f = 200 mm) and were shone onto SLM-4. SLM-4 was used as a mode-dependent, variable beam-steering element and was programmed with a phase mask having two different blazed grating regions. The incoming beams from the two input ports were made to have different incidence angles at SLM-4. In conjunction with the steering angles of the blazed gratings, these incidence angles allowed redirection of the beams , such that the down-converted beam from one input port aligned and propagated collinearly with the OAM beam from the other input port.
Mode up-conversion was performed by SLM-5. Like SLM-3 it was divided into two parts, each serving one output port. Each part was programmed with a phase mask having two regions to properly up-convert the incident 'bar' and 'crossed' beams. In the experiments reported below, SLM-5 was implemented with an available SLM whose pixel count was not enough to simultaneously handle both output ports but only one at a time. SLM-5 formed the last stage of the 2 × 2 switch.
Power levels at the input and output ports of the switch were 19 dBm and 8 dBm (for both the 'bar' and 'cross' states and for all four modes within ± 0.5 dB), respectively, representing an insertion loss of 11 dB, whose main source was the SLMs' diffraction efficiencies. As shown below, these power levels were sufficient to achieve error levels below the FEC limit. At the receive end, SLM-6 was used to select only one of the incoming OAM beams and transform it into a Gaussian-like beam with = 0. The selected beam was coupled into a single-mode fiber (SMF) by using a collimator. The received signal was then sent to the coherent detection setup for bit error rate (BER) measurements.
4. Results and discussion
Two multiplexed pairs of OAM beams with = + 4, −4, and + 2, −6 were used for the input ports A and B, respectively. The capability of operating the switch in ‘cross’ and/or ‘bar’ state for each of the four OAM beams suggests five configurations as shown in Table 1.In order to implement output ports, we first aligned SLM-5 to up-convert the beams directed toward output port A. In the meantime, all data channels were transmitted simultaneously. The measured BER performances for the four switch configurations (C1 – C4) for output port A are shown in Figs. 4(a) and 4(b). An optical signal to noise ratio (OSNR) penalty < 2.5 dB was observed at a BER of 2 × 10−3 for the switched channels appearing at output port A.
Figure 4(c) shows the measured BER performance for the channels appearing at output port A, while the switch was configured in the ‘bar’ state. SLM-5 and SLM-6 were then realigned to form output port B. Figures 5(a) and 5(b) show the BER for the four switching configurations for output port B. An OSNR penalty < 1 dB at a BER of 2 × 10−3 was observed for channels appearing at output port B. Measured BER performance for port B, while switch was operated in ‘bar’ state, is shown in Fig. 5(c).
To observe the transverse phase profiles of different OAM beams, OAM beams were interfered with a diverging Gaussian beam after up-conversion stage. The Interferograms shown in the Fig. 6 verified the correct up-conversion of OAM beams for different switch configurations.
The crosstalk among different OAM beams was also measured after demultiplexing the desired OAM beam and measuring the received power by coupling it into an SMF, while other undesired OAM beams were blocked. The difference between the received power in the desired OAM beam and the sum of the power leaked from each of the undesired OAM beams is shown as the crosstalk in Fig. 7.
We have demonstrated a 2 × 2 switch for OAM-multiplexed data channels. The switch consists of a mode down-conversion stage to separate incoming OAM beams. A programmable grating was used as a beam-steering element to perform the switching operation, followed by an up-conversion stage to up-convert the incoming OAM beams to the same OAM value as that of the input OAM beams. The re-configurability of the switch is demonstrated by its operation in five different configurations to switch OAM-multiplexed 50 Gbaud QPSK channels.
This research was supported by DARPA under the InPho (Information in a Photon) program.
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