## Fourier relationship between angular position and optical orbital angular momentum

Optics Express, Vol. 14, Issue 20, pp. 9071-9076 (2006)

http://dx.doi.org/10.1364/OE.14.009071

Acrobat PDF (391 KB)

### Abstract

We demonstrate the Fourier relationship between angular position and angular momentum for a light mode. In particular we measure the distribution of orbital angular momentum states of light that has passed through an aperture and verify that the orbital angular momentum distribution is given by the complex Fourier-transform of the aperture function. We use spatial light modulators, configured as diffractive optical components, to define the initial orbital angular momentum state of the beam, set the defining aperture, and measure the angular momentum spread of the resulting beam. These measurements clearly confirm the Fourier relationship between angular momentum and angular position, even at light intensities corresponding to the single photon level.

© 2006 Optical Society of America

## 1. Introduction

## 2. Fourier conjugate pairs

_{Mask}(

*φ*) its phase and/or intensity profile is modified such that

*ℓ*, this simplifies to

*A*

_{n},

*A*

_{n}|

^{2}is the total intensity transmission of the mask. Upon transmission, each OAM component of the incident light acquires OAM sidebands shifted by δℓ=

*n*, where the amplitude of each component is given by the corresponding Fourier coefficient of the mask,

_{Mask}(

*φ*)=1 for-Θ/2<

*φ*≤Θ/2 and 0 elsewhere. A single-segment mask can be expressed in terms of its Fourier coefficients as

*m*-fold symmetry only has angular harmonics at multiples of

*m*. Extending the design of the masks to comprise

*m*identical equi-spaced apertures with Fourier components

*N*is an integer. Consequently, in apertures with two-fold symmetry, only every second OAM sideband is present, and in three-fold symmetric apertures only every third. In our experiments we use masks comprising

*m*hard-edge segments so that only every

*m*

^{th}sideband within the sinc envelope is present.

_{Mask}may also include phase information. Specifically we consider the situation where each of the

*m*hard-edged aperture segments has a definite and non-zero relative phase, Φ

_{q}. It is instructive to consider the

*m*-fold symmetric composite mask as a superposition of m single-segment apertures, each giving rise to its own set of OAM sidebands which may constructively or destructive interfere. This interference between the individual OAM sidebands constitutes a test of the Fourier relationship between angle and angular momentum. In our experiments we investigate one representative case, when the phase of the

*m*segments advances in discrete steps so that Φ

_{q}=α2

*πq*/

*m*. The Fourier components and hence OAM sidebands can then be calculated according to

*δℓ*=α, constructive interference will generate OAM sidebands at multiples of

*m*, and destructive interference will cancel any other sidebands. If

*α*is not integer, interference between light passing through the different segments will modulate the sidebands. Note that if

*m*tends to infinity only the central peak at

*δℓ*=α remains, turning the mask effectively into a spiral phase plate with optical step height αλ [11

11. S.S.R. Oemrawsingh, X. Ma, D. Voigt, A. Aiello, E.R. Eliel, G.W. ’t Hooft, and J.P. Woerdman “Experimental demonstration of fractional orbital
angular momentum entanglement of two photons,” Phys. Rev.
Lett. **95**, 240501 (2005) [CrossRef] [PubMed]

*mAδℓ*=1. Such masks generate OAM modes of α modulo

*m*, where

*m*is given by the rotational symmetry, and the integer α shifts between the different sets.

## 3. Experimental configuration

*ℓ*-state by transforming a collimated He-Ne laser (

*ℓ*=0) with a spatial light modulator (see figure 1). Spatial light modulators act as reconfigurable phase gratings, or holograms, giving control over the complex amplitude of the diffracted beams. As is standard practice, our modulator is programmed with a diffraction grating containing a fork dislocation to produce a beam with helical phase front in the first diffraction order [8]. A second spatial light modulator is used to analyse the

*ℓ*-state. If the index of the analysing hologram is opposite to that of the incoming beam, it makes the helical phase fronts of the incoming beam planar again. A significant fraction of the resulting beam can then be coupled into a single-mode optical fibre. If the beam and analysing hologram do not match, the diffracted beam has helical phase fronts and therefore no on-axis intensity, resulting in virtually no coupling into the fibre. To deduce the

*ℓ*-state, the analysing hologram is switched between various indices whilst monitoring the power transmitted through the fibre. It should be emphasised that the cycling of the hologram index makes the detection process inherently inefficient, where even with perfect optical components, the quantum detection efficiency cannot exceed the reciprocal of the number of different

*ℓ*-states to be examined [9

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

*π*addition of the two holograms. We measure the light coupled into the single mode fibre with an avalanche photodiode which enables photon counting. Inserting a neutral density filter immediately after the laser restricts the maximum count rate to less than 100kHz so that at any one time there is on average less than one photon within the apparatus.

*m*equi-spaced segments of defined width and phase. For each aperture function the transmitted photons are analysed for the orbital angular momentum states-18<

*ℓ*+

*δℓ*<18. One complication is that the manufacturing limitations of the spatial light modulators result in deviation from optical flatness by three or four lambda over the full aperture. This degrades the point-spread function of the diffracted beam and hence the efficiency of the mode coupling into the fibre, spoiling the discrimination between different

*ℓ*-states. Therefore, prior to their use within this experiment, we optimise each of the spatial light modulators by applying a hologram of the Zernike terms compansating for astigmatism to give the smallest point spread function of the diffracted

*HG*

_{00}mode.

## 4. Experimental results & discussion

*ℓ*of the incident mode, as given by (8). Figure 2 shows the measured OAM sidebands for a hard-edge aperture of width Θ=

*π*/4. We find almost perfect agreement between the observed distribution and that predicted from the Fourier-relation. However, as discussed, a more subtle test of the Fourier-relation is when the aperture function is multi-peaked and when these peaks are offset in phase. Introducing an aperture comprising two segments of the same width generates an OAM sideband distribution with the same envelope function, but if the Fourier relationship holds true the sidebands can interfere either constructively or destructively depending on the relative phase of the individual components. Figure 3 compares the angular momentum distribution as predicted from (11) with the one observed in the experiment for the case of two (i.e.

*m*=2) diametrically opposed hard-edge apertures, each of angular width 2

*π*/9. During the experimental sequence their relative phase δΦ=Φ

_{2}-Φ

_{1}is varied from 0 to 2

*π*(α is varied from 0 to 1). As discussed, the OAM sidebands of the two segments differ in their phase by (

*δℓ*+

*α*)×

*π*. When α=0, the OAM sidebands with odd

*δℓ*interfere destructively. As the relative phase increases, the light intensity in the odd modes rises at the expense of the even modes until all the even modes disappear when δΦ=

*π*. At intermediate positions when δΦ=

*π*/2 or 3

*π*/2 even and odd sidebands have equal weights. The width of the aperture Θ changes the width of the sinc distribution but not the underlying interference effects. Figure 4 shows the results for four (i.e.

*m*=4) equi-spaced apertures, at a phase difference of

*απ*/2. Increasing

*α*from 0 to 4 gives OAM sideband distributions in excellent agreement to that predicted by (11).

13. J. Leach, J. Courtial, K. Skeldon, S. M. Barnett, S. Franke-Arnold, and M. J. Padgett, “Interferometric methods to measure orbital and spin,
or the total angular momentum of a single photon,” Phys.
Rev. Lett. **92**, 013601 (2004). [CrossRef] [PubMed]

## 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 |

2. | A. T. O’Neil, I. MacVicar, L. Allen, and M. J. Padgett, “Intrinsic and extrinsic nature of the orbital
angular momentum of a light beam,” Phys. Rev.
Lett. |

3. | A. E. Mair, A. Vaziri, G. Weihs, and A. Zeilinger, “Entanglement of the orbital angular momentum states
of photons,” Nature |

4. | E. Merzbacher, |

5. | L. Allen, M. J. Padgett, and M. Babiker, “The orbital angular momentum of
light,” in |

6. | S. Franke-Arnold, S. M. Barnett, M. J. Padgett, and L. Allen, “Two-photon entanglement of orbital angular momentum
states,” Phys. Rev. A |

7. | L. Allen, S. M. Barnett, and M. J. Padgett, |

8. | V.Y. Bazhenov, M.V. Vasnetsov, and M. S. Soskin, “Laser beams with screw dislocations in their
wavefronts,” JETP Lett. |

9. | G. Gibson, J. Courtial, M. J. Padgett, M. Vasnetsov, V. Pas’ko, S. M. Barnett, and S. Franke-Arnold, “Free-space information transfer using light beams
carrying orbital angular momentum,” Opt. Exp. |

10. | S. Franke-Arnold, S. M. Barnett, E. Yao, J. Leach, J. Courtial, and M. J. Padgett, “Uncertainty principle for angular position and
angular momentum,” New J. Phys. |

11. | S.S.R. Oemrawsingh, X. Ma, D. Voigt, A. Aiello, E.R. Eliel, G.W. ’t Hooft, and J.P. Woerdman “Experimental demonstration of fractional orbital
angular momentum entanglement of two photons,” Phys. Rev.
Lett. |

12. | S. M. Barnett and D.T. Pegg, “Quantum theory of rotation angles,”
Phys. Rev. A |

13. | J. Leach, J. Courtial, K. Skeldon, S. M. Barnett, S. Franke-Arnold, and M. J. Padgett, “Interferometric methods to measure orbital and spin,
or the total angular momentum of a single photon,” Phys.
Rev. Lett. |

**OCIS Codes**

(070.2590) Fourier optics and signal processing : ABCD transforms

(230.6120) Optical devices : Spatial light modulators

**ToC Category:**

Fourier Optics and Optical Signal Processing

**History**

Original Manuscript: August 1, 2006

Manuscript Accepted: September 8, 2006

Published: October 2, 2006

**Citation**

Eric Yao, Sonja Franke-Arnold, Johannes Courtial, Stephen Barnett, and Miles Padgett, "Fourier relationship between angular position and optical orbital angular momentum," Opt. Express **14**, 9071-9076 (2006)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-20-9071

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### References

- 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 45,8185-8189 (1992). [CrossRef] [PubMed]
- A. T. O’Neil, I. MacVicar, L. Allen and M. J. Padgett, "Intrinsic and extrinsic nature of the orbital angular momentum of a light beam," Phys. Rev. Lett. 88, 053601 (2002) [CrossRef] [PubMed]
- A. E. Mair, A. Vaziri, G. Weihs, and A. Zeilinger, "Entanglement of the orbital angular momentum states of photons," Nature 412, 313-316 (2001). [CrossRef] [PubMed]
- E. Merzbacher, Quantum Mechanics (John Wiley & Sons, Brisbane, 1998).
- L. Allen, M. J. Padgett, and M. Babiker, "The orbital angular momentum of light," in Progress in Optics XXXIX, E. Wolf, ed. (Elsevier Science, New York,1999), pp. 291-372.
- S. Franke-Arnold, S. M. Barnett, M. J. Padgett and L. Allen, "Two-photon entanglement of orbital angular momentum states," Phys. Rev. A 65, 033823 (2002). [CrossRef]
- L. Allen, S. M. Barnett, and M. J. Padgett, Optical Angular Momentum (Institute of Physics, Bristol, 2003)
- V.Y. Bazhenov, M.V. Vasnetsov, and M. S. Soskin, "Laser beams with screw dislocations in their wavefronts," JETP Lett. 52, 429-431 (1990).
- G. Gibson, J. Courtial, M. J. Padgett, M. Vasnetsov, V. Pas’ko, S. M. Barnett and S. Franke-Arnold, "Free-space information transfer using light beams carrying orbital angular momentum," Opt. Exp. 12, 5448-5456 (2004). [CrossRef]
- S. Franke-Arnold, S. M. Barnett, E. Yao, J. Leach, J. Courtial and M. J. Padgett, "Uncertainty principle for angular position and angular momentum," New J. Phys. 6, 103 (2004). [CrossRef]
- S.S.R. Oemrawsingh, X. Ma, D. Voigt, A. Aiello, E.R. Eliel, G.W. ’t Hooft, and J.P. Woerdman "Experimental demonstration of fractional orbital angular momentum entanglement of two photons," Phys. Rev. Lett. 95, 240501 (2005) [CrossRef] [PubMed]
- S. M. Barnett and D.T. Pegg, "Quantum theory of rotation angles," Phys. Rev. A 41, 3427-3435 (1990). [CrossRef] [PubMed]
- J. Leach, J. Courtial, K. Skeldon, S. M. Barnett, S. Franke-Arnold and M. J. Padgett, "Interferometric methods to measure orbital and spin, or the total angular momentum of a single photon," Phys. Rev. Lett. 92, 013601(2004). [CrossRef] [PubMed]

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