Reconfigurable orbital angular momentum based switching amog multiple 100 Gbit data channel

Reconfigurable Orbital-Angular-Momentum-Based

Switching among Multiple 100-Gbit/s Data Channels Yang Yue, Nisar Ahmed, Hao Huang, Yan Yan, Yongxiong Ren, Dvora Rogawski, and Alan E. Willner Department of Electrical Engineering, Univ. of Southern California, Los Angeles, CA 90089, USA, Email: yyue@https://www.360docs.net/doc/b55263287.html,

Abstract:We demonstrate OAM-based reconfigurable optical switching functions among

multiple OAM modes. Three-pair OAM modes exchange shows 3.6-dB conversion efficiency and

selective data exchange among three 100-Gb/s QPSK OAM channels has a 2.1-dB OSNR penalty.

?2013 Optical Society of America

OCIS codes: (060.2605) Free-space optical communication; (200.4560) Optical data processing.

1. Introduction

Orbital angular momentum (OAM) has emerged as a potential multiplexing technique to increase system capacity as well as spectral efficiency [1-8]. In general, photons can carry orbital angular momentum (OAM), which is associated with an azimuthal phase dependence (i.e., helical phase front) of the complex electric field. Light beams carrying OAM can be described in the spatial phase form of exp(i l ) (l=0, ±1, ±2,…) [9]. Since OAM-based systems can be designed to have an infinite number of orthogonal eigenstates (l), it provides another degree of freedom to multiplex as well as manipulate the optical field. Both free-space and fiber OAM communication systems have been demonstrated for spectral and high-capacity communication links [5-8]. For example, multiplexing Laguerre-Gaussian (LG) modes carrying OAM has been demonstrated in free-space optical communication links at Tbit/s capacities and 90 bits/s/Hz efficiencies [6].

In general, research published in OAM-mode-based free-space communications systems involves relatively static point-to-point links [2, 5-7]. However, similar to the historical advancements made in wavelength-division-multiplexed (WDM) systems, reconfigurable networking functions for a multi-user environment might bring advantages for OAM systems. Potentially desirable functions include switching and manipulation of data channels.

Recently, data exchange between two different OAM beams has been demonstrated using a tunable spatial light modulator (SLM) [6]; this scheme represents a reconfigurable 2×2 OAM switch. A laudable goal would be to reconfigurably manipulate multiple OAM channels, thereby potentially enabling a dynamic and efficient communications network for multi-user applications.

In this paper, we experimentally demonstrate OAM-based reconfigurable optical switching functions when multiple OAM modes are used. We show multi-pair data channel exchange and selective manipulation of OAM modes. Multi-pair OAM modes can exchange their information with respect to the symmetric axis of their OAM charge number. Three-pair OAM mode data exchange shows an average conversion efficiency of -3.6 dB and the OSNR penalties for the channels before and after exchanges are 1.5 dB and 2.4 dB, respectively. Selective OAM manipulation can be realized by spatially separate the OAM modes and implement desired patterns at different regions of the SLM.We demonstrate selective data exchange among OAM-5, OAM+2 and OAM+8 channels, and the average OSNR penalties for the channels without and with exchanges are 1.3 dB and 2.1 dB, respectively.

2. Concept and Experimental Setup

Figure 1(a) is the schematic diagram of n×n OAM switch, which shows architecture to switch n input modes with arbitrary OAM charges to n OAM modes with desired charges. By using the OAM-based reconfigurable optical networking functions as shown in Fig. 1(b), this building block can be realized with 2n+2 SLMs. “charge shift” can shift all the OAM modes with the same charge step m-l, and it can be realized by using a SLM with topological

Fig. 1. (a) Schematic diagram of n×n OAM switch. (b) Concept of OAM-based reconfigurable optical networking functions, charge shift, charge exchange, and charge selective manipulation.

charge m-l and a mirror. “Charge exchange” can reverse the order of the OAM modes. The physical implementation of this function is simply using one SLM. A function can be realized is the exchange information of two OAM modes [6], and can be potentially extended to exchange information among symmetrically distributed multi-pair OAM modes on the OAM spectrum. “Charge shift” and “charge exchange” operate on all the OAM modes simultaneously. To realize a fully functional OAM-based reconfigurable optical network, more advanced functions to manipulate individual OAM modes are in great demand. “Charge selective manipulation” function can potentially be implemented to meet this goal. We can manipulate the charge of one OAM mode, while not affecting the others’, by simply using 2 SLMs. By effectively cascading the above function, we can relocate any input n OAM modes to any desired output state with a maximum of 2n+2 SLMs.

Figure 2 illustrates the experimental setup for reconfigurable switching among channels carried by OAM modes. To generate a 100-Gbit/s QPSK signal, a continuous-wave (CW) laser is modulated by an I/Q modulator at 2×50 Gbit/s. The signal is then split into three paths, decorrelated, and delivered over three OAM modes, which are converted from the collimated Gaussian beams by SLMs (SLM-1, -2 and -3) loaded with different phase patterns. For multi-pair exchange experiment, together with another three SLMs (SLM-1’, -2’ and -3’), 6 SLMs at the multiplexing stage are used. The polarization states of the Gaussian beams are optimized with half-wave plates (HWPs) before the SLMs. Three 100-Gbit/s QPSK-carrying OAM modes are combined using three non-polarizing beam splitters (BS). For multi-pair exchange, only SLM-4 is used. For the selective OAM mode manipulation, three SLMs (SLM-4, -5 and -6) are used to perform the functions of baseband conversion, selective manipulation and back conversion, respectively. Another SLM (SLM-7) loaded with a specified phase pattern is used to demultiplex one of the superposed OAM modes back to the Gaussian beam and coupled into the fiber for coherent detection. A flip mirror is used to reflect the OAM beams into the camera for monitoring the intensity and phase information.

Fig. 2. Experimental setup for 100-Gbit/s QPSK multi-pair data exchange and selective manipulation among OAM mode channels. QPSK: quadrature phase-shift keying; CW: continuous-wave; PC: polarization controller; EDFA: erbium-doped fiber amplifier; BPF: band-pass filter; OC: optical coupler; SMF: single-mode fiber; Col.: collimator; HWP: half-wave plate; SLM: spatial light modulator; BS: beam splitter; Att: attenuator; OSA: optical spectrum analyzer; DLI: delay-line interferometer; Rx: receiver; LO: local oscillator; ADC, analog-to-digital converter; DSP: digital signal processing.

3. Multi-pair OAM Exchange

The concept of exchanging multi-pair OAM modes is shown in Fig. 3(a). We are exchanging OAM modes between charges “-18, -15, -12” and “4, 7, 10”, only one SLM with topological charge 8 is used to realize this function. As an example, OAM mode with charge -15 adds 8 from the topological charge of the SLM and then flips the sign after the reflection. This gives the original OAM-15 a final charge of 7. As shown in Fig. 3(b), we experimentally demonstrate three-pair exchange among OAM modes. The average conversion efficiency is -3.6 dB. At a BER of 2×10?3 (enhanced FEC (EFEC) threshold), the average OSNR penalties for the channels before and after exchanges are 1.5 dB and 2.4 dB, respectively.

Fig. 3. (a) Concept, (b) power transfer, and (c) BER curves of multi-pair OAM exchange among 100-Gbit/s QPSK channels.

4. Selective OAM Manipulation

Figure 4 depicts the concept and principle of selective manipulation among OAM modes. As shown in Fig. 4(a), one OAM mode (within the dotted red region) is firstly converted down to the center as a Gaussian beam, while the other modes maintain the ring-shape intensity profile. The SLM is spatially separated into a center circular region 1 and outer region 2. By implementing desired patterns to different regions of the SLM, selectively manipulation of the OAM modes can be realized. A back conversion is used to convert the OAM modes, that we do not want to change, to their original states. Selective data exchange among signals carried by different OAM modes is an example of this function. The data carried on any two of the OAM channels can be exchanged while not affecting the other channel. The steps of operation for selective data exchange are as following. At the input, OAM -5, OAM +2 and OAM +8 are spatially overlapped. As an example, we are planning to exchange the information on OAM -5 and OAM +8, while not affecting OAM +2. The OAM +2 mode is firstly converted to a Gaussian beam by launching the multiple OAM modes into the 1st SLM with a topological charge of -2, while OAM -5 and OAM +8 are converted OAM +7 and OAM -6. The 2nd SLM is spatially separated into two regions. The center region has uniform phase, while the outer region has a topological charge of -1. After reflecting from 2nd SLM, original OAM -5 and OAM +8 are flipped to OAM -6 and OAM +7, while the original OAM +2 maintains the Gaussian profile. After the up conversion from the 3rd SLM with topological charge of -2, original OAM -5, OAM +2 and OAM +8 are converted to OAM +8, OAM +2 and OAM -5, and thus the information on OAM -5 and OAM +8 are selectively exchanged.

Figure 5(a) depicts the intensity profile of multiplexed OAM beams and the patterns on the SLMs in the selective mode manipulation stage. Forked diffraction grating is used to avoid the crosstalk induced from the zero-order diffraction of the other OAM modes due to the non-perfect diffraction efficiency. The pattern on SLM-5 has a buffer circular layer with uniform phase between the center and outer regions to reduce the inter-mode crosstalk. As shown in Fig. 5(b), interferograms between exchanged OAM beams (OAM -5 and OAM +8) and referenced Gaussian beam clearly indicate the exchange of their topical charges. The temporal waveforms as shown in Fig. 5(c), further verifies the successful exchange of 100-Gbit/s QPSK signals carried by OAM -5 and OAM +8. Reconfigurability of the proposed scheme is demonstrated by selectively exchange between OAM +2 and OAM +8, as well as between OAM -5 and OAM +2. Figure 5(d) shows the measured BER curves of selective data exchange among 100-Gbit/s QPSK channels carried by OAM -5 and OAM +8. The average OSNR penalties for the channels without and with exchanges are 1.3 dB and 2.1 dB, respectively.

We acknowledge the support of DARPA under InPho (Information in a Photon) program.

5. References

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[5] I. M. Fazal et. al., FIO’2011, FTuT1.

[6] J. Wang et. al., NP 6, 488, (2012). [7] T. H. Su et. al., OE 20, 9396 (2012). [8] N. Bozinovic et. al., ECOC’2012, Th.3.C.6. [9] A. M. Yao et. al., AOP 3, 161 (2011).

Fig. 4. Concept and principle of selective manipulation among OAM modes.

Fig. 5. OAM selective exchange between OAM -5 and OAM +8. (a) Intensity profile of the multiplexed OAM beams and the pattern on SLMs; (b) Interferograms between exchanged OAM beams (OAM -5 and OAM +8) and a Gaussian beam reference; (c) Demodulated temporal waveforms before and after selective exchange; (d) BER curves of 100-Gbit/s QPSK channels.