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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 9 — Apr. 23, 2012
  • pp: 9396–9402
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Demonstration of free space coherent optical communication using integrated silicon photonic orbital angular momentum devices

Tiehui Su, Ryan P. Scott, Stevan S. Djordjevic, Nicolas K. Fontaine, David J. Geisler, Xinran Cai, and S. J. B. Yoo  »View Author Affiliations


Optics Express, Vol. 20, Issue 9, pp. 9396-9402 (2012)
http://dx.doi.org/10.1364/OE.20.009396


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Abstract

We propose and demonstrate silicon photonic integrated circuits (PICs) for free-space spatial-division-multiplexing (SDM) optical transmission with multiplexed orbital angular momentum (OAM) states over a topological charge range of −2 to +2. The silicon PIC fabricated using a CMOS-compatible process exploits tunable-phase arrayed waveguides with vertical grating couplers to achieve space division multiplexing and demultiplexing. The experimental results utilizing two silicon PICs achieve SDM mux/demux bit-error-rate performance for 1‑b/s/Hz, 10-Gb/s binary phase shifted keying (BPSK) data and 2-b/s/Hz, 20-Gb/s quadrature phase shifted keying (QPSK) data for individual and two simultaneous OAM states.

© 2012 OSA

1. Introduction

Since the initial work of Allen et al. [1

1. L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,” Phys. Rev. A 45(11), 8185–8189 (1992). [CrossRef] [PubMed]

], the orbital angular momentum (OAM) of light has generated significant interest in multiple areas of research, including cold atom confinement [2

2. S. Franke-Arnold, L. Allen, and M. Padgett, “Advances in optical angular momentum,” Laser Photonics Rev. 2(4), 299–313 (2008). [CrossRef]

], nonlinear optics [3

3. A. M. Yao and M. J. Padgett, “Orbital angular momentum: origins, behavior and applications,” Adv. Opt. Photonics 3(2), 161–204 (2011). [CrossRef]

], and communications [4

4. G. Gibson, J. Courtial, M. Padgett, M. Vasnetsov, V. Pas’ko, S. Barnett, and S. Franke-Arnold, “Free-space information transfer using light beams carrying orbital angular momentum,” Opt. Express 12(22), 5448–5456 (2004). [CrossRef] [PubMed]

9

9. P. Martelli, A. Gatto, P. Boffi, and M. Martinelli, “Free-space optical transmission with orbital angular momentum division multiplexing,” Electron. Lett. 47(17), 972–973 (2011). [CrossRef]

]. Figure 1(a)
Fig. 1 (a) Visualization of the electric field of OAM beams. (b) Illustration showing how a beam encoded with an OAM state is sampled and demultiplexed by a circular arrangement of apertures, length-matched waveguides and a star coupler.
shows examples of a light beam with different OAM states which have an azimuthal (i.e., in the transverse plane) phase variation of φ(r,φ)=exp(iφ), where i2=1, φ is the azimuthal (angular) coordinate and ℓ can be any positive or negative integer value (known as the topological charge) [3

3. A. M. Yao and M. J. Padgett, “Orbital angular momentum: origins, behavior and applications,” Adv. Opt. Photonics 3(2), 161–204 (2011). [CrossRef]

]. This type of beam has helical phase fronts, where the handedness (direction of twist) depends on the sign of ℓ and the number of intertwined helices depends on the magnitude of ℓ (i.e., the number of 2π phase shifts that occur in one revolution of the azimuthal angle φ). In beams with OAM, the electromagnetic field that is transverse to the phase fronts will have axial components. Consequently, the Poynting vector, which is always perpendicular to these phase fronts, has an azimuthal component around the beam and hence an angular momentum along the beam axis equal to ℓħ [1

1. L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,” Phys. Rev. A 45(11), 8185–8189 (1992). [CrossRef] [PubMed]

]. The helical phase of a beam with nonzero OAM (i.e., ℓ ≠ 0) leads to a phase singularity on the beam axis, and therefore the intensity has to vanish and the beam will have a dark central spot [10

10. M. S. Soskin, V. N. Gorshkov, M. V. Vasnetsov, J. T. Malos, and N. R. Heckenberg, “Topological charge and angular momentum of light beams carrying optical vortices,” Phys. Rev. A 56(5), 4064–4075 (1997). [CrossRef]

].

2. Device design and fabrication

Figure 2(a)
Fig. 2 (a) Waveguide layout of silicon OAM device for multiplexing five OAM modes ( ℓ = +2, +1,0,−1,−2). (b) Fabricated silicon OAM device. Inset shows SEM photo of grating.
shows the device’s waveguide layout which is designed for a silicon-on-insulator (SOI) material platform, is optimized for TE polarization, and uses a 1-µm-wide silicon rib waveguides (effective index of 3.27). The upper inset of Fig. 2(a) shows a quarter of the circular grating which is formed by concentric etched circles that have a grating period of 0.47 μm, a 50% duty cycle, and an outer radius of 25 μm. The circular grating converts the vertically incident optical beam (azimuthal polarization) into a horizontally propagating beam. Sixteen tapered waveguides surround and capture the light from the grating and send it to length-matched waveguides (20-mm long) that terminate at the FPR [lower inset of Fig. 2(a)]. Depending on the OAM state of the input beam, the 16 guided modes will have a specific linear phase variation. Since the FPR is designed based on the Rowland circle principle, it focuses the 16 beams onto five waveguide outputs according to the linearly varying phase associated with the five different OAM states (labeled as ℓ = +2, +1, 0, −1, −2). The five waveguide outputs are tapered to a 3-µm width at the edge of the chip and have 250-µm spacing. The device also includes aluminum contact pads and traces that connect to sixteen Ti/Pt heaters. The heaters, located just above the waveguides, thermo-optically change the local index of refraction to compensate for optical phase errors (<π rad) in the waveguides.

3. Device characterization

Initial device characterization was performed on straight waveguides fabricated on the same silicon chip. The Fabry–Pérot resonance measurement method yielded the straight waveguide loss of 1.9 dB/cm. Figure 3(a)
Fig. 3 Intensity of near-field output from the OAM device (input port 0) from (a) a simulation and (b) measurement. Intensity of far-field output from OAM device for ℓ = 0 for (c) a simulation, (d) measured without phase-error correction (PEC), and (e) with PEC.
shows a simulation of the near-field intensity pattern of the circular grating output when light is coupled into the ℓ = 0 input. Figure 3(b) is the near-field intensity pattern imaged by an infrared vidicon camera for comparison. The 16 radial stripes indicate that the beam coupled from the waveguides to air as expected. Each radial stripe shows a periodic dark-bright pattern which is caused by interference from light that leaks across the center of the grating from the opposing waveguides. Figure 3(c) shows a simulation of the far-field intensity pattern for ℓ = 0. For comparison, Fig. 3(d) shows the measured far-field pattern with no phase-error correction (PEC) and Fig. 3(e) shows the more evenly distributed far-field pattern after applying appropriate PEC.

As an initial test of a single OAM device’s performance, we used an objective [20 × , 0.57 numerical aperture (NA)] to collect the light from the grating output, followed by a gold-coated mirror that retro-reflected the OAM mode back through the objective and into the OAM device. Thus, the same OAM device acts as both a transmitter and a receiver. In this retro-reflection configuration, when light is coupled into the +l port, the reflected beam is directed to −l port; similarly the input on the +2 port gives output on the −2 port. Using this configuration to measure the crosstalk performance, we coupled light into the +2 port and measured output ports −2, −1, and 0. The crosstalk without PEC from the −2 port to the −1 port was only −3 dB and −5 dB for the 0 port. After PEC was applied, the crosstalk improved significantly, the −2 port to the −1 port was −10.5 dB and −12 dB for the 0 port.

4. Transmission experiment

Although not investigated here, similar to most free-space OAM implementations [2

2. S. Franke-Arnold, L. Allen, and M. Padgett, “Advances in optical angular momentum,” Laser Photonics Rev. 2(4), 299–313 (2008). [CrossRef]

,4

4. G. Gibson, J. Courtial, M. Padgett, M. Vasnetsov, V. Pas’ko, S. Barnett, and S. Franke-Arnold, “Free-space information transfer using light beams carrying orbital angular momentum,” Opt. Express 12(22), 5448–5456 (2004). [CrossRef] [PubMed]

,6

6. J. A. Anguita, M. A. Neifeld, and B. V. Vasic, “Turbulence-induced channel crosstalk in an orbital angular momentum-multiplexed free-space optical link,” Appl. Opt. 47(13), 2414–2429 (2008). [CrossRef] [PubMed]

,7

7. I. B. Djordjevic and M. Arabaci, “LDPC-coded orbital angular momentum (OAM) modulation for free-space optical communication,” Opt. Express 18(24), 24722–24728 (2010). [CrossRef] [PubMed]

], misalignments between the demonstrated OAM device pair result in increased signal losses and increased crosstalk between demultiplexed states. A full analysis of the free-space beam and optics will provide insight into the specific sensitivities of these OAM devices in a realistic free-space communications system.

5. Conclusion

We fabricated silicon PICs and demonstrated free-space SDM coherent optical transmission using OAM state multiplexing and demultiplexing. The device design allows operation as either an OAM multiplexer or demultiplexer for a topological charge range of −2 to +2 and easily interfaces with fiber-pigtailed components. The single-channel BER statistics for both 10-Gb/s BPSK data and 20-Gb/s QPSK data indicated similar performance for all topological charges. In all cases, the BER was well below the forward error correction (FEC) limit of 2 × 10−3 (RS(255,239) coding [17

17. T. Mizuochi, “Forward error correction,” in High Spectral Density Optical Communication Technologies, M. Nakazawa, K. Kikuchi, and T. Miyazaki, eds. (Springer, 2010), 303–333.

]), even for the two simultaneous channels with crosstalk suffering a significant power penalty. Future work includes reducing coupling and on-chip losses, increasing the addressable number of OAM states, and improving the free-space optical system used to relay the OAM encoded beam between devices.

Acknowledgments

This work was supported in part by DARPA DSO under the contract HR0011-11-1-0005.

References and links

1.

L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,” Phys. Rev. A 45(11), 8185–8189 (1992). [CrossRef] [PubMed]

2.

S. Franke-Arnold, L. Allen, and M. Padgett, “Advances in optical angular momentum,” Laser Photonics Rev. 2(4), 299–313 (2008). [CrossRef]

3.

A. M. Yao and M. J. Padgett, “Orbital angular momentum: origins, behavior and applications,” Adv. Opt. Photonics 3(2), 161–204 (2011). [CrossRef]

4.

G. Gibson, J. Courtial, M. Padgett, M. Vasnetsov, V. Pas’ko, S. Barnett, and S. Franke-Arnold, “Free-space information transfer using light beams carrying orbital angular momentum,” Opt. Express 12(22), 5448–5456 (2004). [CrossRef] [PubMed]

5.

J. Lin, X. C. Yuan, S. H. Tao, and R. E. Burge, “Multiplexing free-space optical signals using superimposed collinear orbital angular momentum states,” Appl. Opt. 46(21), 4680–4685 (2007). [CrossRef] [PubMed]

6.

J. A. Anguita, M. A. Neifeld, and B. V. Vasic, “Turbulence-induced channel crosstalk in an orbital angular momentum-multiplexed free-space optical link,” Appl. Opt. 47(13), 2414–2429 (2008). [CrossRef] [PubMed]

7.

I. B. Djordjevic and M. Arabaci, “LDPC-coded orbital angular momentum (OAM) modulation for free-space optical communication,” Opt. Express 18(24), 24722–24728 (2010). [CrossRef] [PubMed]

8.

S. Slussarenko, E. Karimi, B. Piccirillo, L. Marrucci, and E. Santamato, “Efficient generation and control of different-order orbital angular momentum states for communication links,” J. Opt. Soc. Am. A 28(1), 61–65 (2011). [CrossRef] [PubMed]

9.

P. Martelli, A. Gatto, P. Boffi, and M. Martinelli, “Free-space optical transmission with orbital angular momentum division multiplexing,” Electron. Lett. 47(17), 972–973 (2011). [CrossRef]

10.

M. S. Soskin, V. N. Gorshkov, M. V. Vasnetsov, J. T. Malos, and N. R. Heckenberg, “Topological charge and angular momentum of light beams carrying optical vortices,” Phys. Rev. A 56(5), 4064–4075 (1997). [CrossRef]

11.

C. E. Shannon, “A mathematical theory of communication,” Bell Syst. Tech. J. 27, 379–423 (1948).

12.

R. Čelechovský and Z. Bouchal, “Optical implementation of the vortex information channel,” New J. Phys. 9(9), 328 (2007). [CrossRef]

13.

C. R. Doerr and L. L. Buhl, “Circular grating coupler for creating focused azimuthally and radially polarized beams,” Opt. Lett. 36(7), 1209–1211 (2011). [CrossRef] [PubMed]

14.

K. Okamoto, Fundamentals of Optical Waveguides (Academic, 2006).

15.

C. R. Doerr, N. K. Fontaine, M. Hirano, T. Sasaki, L. L. Buhl, and P. J. Winzer, “Silicon photonic integrated circuit for coupling to a ring-core multimode fiber for space-division multiplexing,” in European Conference on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.A.3.

16.

G. Li, “Recent advances in coherent optical communication,” Adv. Opt. Photonics 1(2), 279–307 (2009). [CrossRef]

17.

T. Mizuochi, “Forward error correction,” in High Spectral Density Optical Communication Technologies, M. Nakazawa, K. Kikuchi, and T. Miyazaki, eds. (Springer, 2010), 303–333.

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.4230) Fiber optics and optical communications : Multiplexing
(050.4865) Diffraction and gratings : Optical vortices

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: February 6, 2012
Revised Manuscript: March 26, 2012
Manuscript Accepted: March 26, 2012
Published: April 9, 2012

Citation
Tiehui Su, Ryan P. Scott, Stevan S. Djordjevic, Nicolas K. Fontaine, David J. Geisler, Xinran Cai, and S. J. B. Yoo, "Demonstration of free space coherent optical communication using integrated silicon photonic orbital angular momentum devices," Opt. Express 20, 9396-9402 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-9-9396


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References

  1. L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,” Phys. Rev. A45(11), 8185–8189 (1992). [CrossRef] [PubMed]
  2. S. Franke-Arnold, L. Allen, and M. Padgett, “Advances in optical angular momentum,” Laser Photonics Rev.2(4), 299–313 (2008). [CrossRef]
  3. A. M. Yao and M. J. Padgett, “Orbital angular momentum: origins, behavior and applications,” Adv. Opt. Photonics3(2), 161–204 (2011). [CrossRef]
  4. G. Gibson, J. Courtial, M. Padgett, M. Vasnetsov, V. Pas’ko, S. Barnett, and S. Franke-Arnold, “Free-space information transfer using light beams carrying orbital angular momentum,” Opt. Express12(22), 5448–5456 (2004). [CrossRef] [PubMed]
  5. J. Lin, X. C. Yuan, S. H. Tao, and R. E. Burge, “Multiplexing free-space optical signals using superimposed collinear orbital angular momentum states,” Appl. Opt.46(21), 4680–4685 (2007). [CrossRef] [PubMed]
  6. J. A. Anguita, M. A. Neifeld, and B. V. Vasic, “Turbulence-induced channel crosstalk in an orbital angular momentum-multiplexed free-space optical link,” Appl. Opt.47(13), 2414–2429 (2008). [CrossRef] [PubMed]
  7. I. B. Djordjevic and M. Arabaci, “LDPC-coded orbital angular momentum (OAM) modulation for free-space optical communication,” Opt. Express18(24), 24722–24728 (2010). [CrossRef] [PubMed]
  8. S. Slussarenko, E. Karimi, B. Piccirillo, L. Marrucci, and E. Santamato, “Efficient generation and control of different-order orbital angular momentum states for communication links,” J. Opt. Soc. Am. A28(1), 61–65 (2011). [CrossRef] [PubMed]
  9. P. Martelli, A. Gatto, P. Boffi, and M. Martinelli, “Free-space optical transmission with orbital angular momentum division multiplexing,” Electron. Lett.47(17), 972–973 (2011). [CrossRef]
  10. M. S. Soskin, V. N. Gorshkov, M. V. Vasnetsov, J. T. Malos, and N. R. Heckenberg, “Topological charge and angular momentum of light beams carrying optical vortices,” Phys. Rev. A56(5), 4064–4075 (1997). [CrossRef]
  11. C. E. Shannon, “A mathematical theory of communication,” Bell Syst. Tech. J.27, 379–423 (1948).
  12. R. Čelechovský and Z. Bouchal, “Optical implementation of the vortex information channel,” New J. Phys.9(9), 328 (2007). [CrossRef]
  13. C. R. Doerr and L. L. Buhl, “Circular grating coupler for creating focused azimuthally and radially polarized beams,” Opt. Lett.36(7), 1209–1211 (2011). [CrossRef] [PubMed]
  14. K. Okamoto, Fundamentals of Optical Waveguides (Academic, 2006).
  15. C. R. Doerr, N. K. Fontaine, M. Hirano, T. Sasaki, L. L. Buhl, and P. J. Winzer, “Silicon photonic integrated circuit for coupling to a ring-core multimode fiber for space-division multiplexing,” in European Conference on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Th.13.A.3.
  16. G. Li, “Recent advances in coherent optical communication,” Adv. Opt. Photonics1(2), 279–307 (2009). [CrossRef]
  17. T. Mizuochi, “Forward error correction,” in High Spectral Density Optical Communication Technologies, M. Nakazawa, K. Kikuchi, and T. Miyazaki, eds. (Springer, 2010), 303–333.

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