Manipulation of Orbital-Angular-Momentum Spectrum Using Pinhole Plates

The orbital angular momentum (OAM) of light has attracted a great amount of interest in recent times, due to the wide variety of applications it has made possible. Generally, the OAM spectrum is produced by coaxial superposition of several vortex beams, which might increase the complexity of the system. Here, we propose and experimentally demonstrate a simple way to produce a state of light with a controllable OAM spectrum using a binary array of pinholes. More specifically, we show that a spiral structure can convert a plane wave into a beam with a wide OAM spectrum, which can be easily tuned to pure or multiple OAM modes by adapting the structure of the pinhole plate. Furthermore, we show that a simple pinhole plate can produce structured beams with particular OAM states, such as photonic gears (superposition of OAM modes with opposite topological charges ±ℓ) and OAM combs (an optical mode formed by a series of discrete and equally spaced OAM modes, akin to an optical frequency comb). It is worth noting that we demonstrate the OAM comb experimentally. This study provides an avenue for the flexible generation of OAM spectra and the simplicity of the setup could make this approach convenient for many applications, such as optical communications and quantum information.

Figure 1

Schematics of the binary slits. (a) Ring slit and (d) and (g) spiral slits of different radii; (b),(e),(h) the corresponding simulated intensity profiles, and (c),(f),(i) the OAM spectra of the diffracted beam at z = 1 m, when the plates are illuminated with a plane wave of wavelength λ = 532.8 nm.

Figure 2

Schematics of the discretized binary slits. (a) Single spiral, (d) five spirals, and (g) six spirals; (b),(e),(h) the corresponding simulated intensity profiles; and (c),(f),(i) the OAM spectra of the diffracted beam at z = 1 m, when the plates are illuminated with a plane wave of wavelength λ = 532.8 nm.

Figure 3

Schematics of the generation of petal-like beams with opposite OAM modes. (a) Simulated mask for $\ell =\pm 2$; (b),(c) the corresponding simulated intensity profiles and OAM spectrum; (d)–(f) same as (a)–(c), but $\ell =\pm 4$; and (g)–(i) same as (a)–(c), but $\ell =\pm 5$.

Figure 4

Schematics of the generation of OAM comb. Two and three pinhole plate (left column). The insets in (a),(d) represent a plane wave, whereas those in (g) represent an optical vortex of topological charge ℓ = 1. Corresponding simulated intensity profile (middle row). OAM spectrum of the generated output field (right column).

Figure 5

Schematic of the experimental setup. NA denotes neutral attenuators, BE denotes the beam expander, AS denotes the aperture slot, BS denotes the beam splitter, and CCD denotes the charge-coupled device. The sum of distances L₁ and L₂ is equal to the distance of z in Eq. (5).

Figure 6

Experimental results of generated structured beams with OAM modes. (a)–(c) The corresponding experimental results to those in Figs. 2, 2 and 2. (d)–(f) The corresponding experimental results to those in Figs. 3, 3 and 3.

Figure 7

(a) Experimental setup used to measure OAM spectra of photonic gears. f is the focal length of the lens, BS₁ and BS₂ are beam splitters, SLM₂ denotes the phase-only spatial light modulator, CCD₂ is the image sensor used to detect the mode decomposition by SLM₂. (b) Far-field intensity of multiplexed set of modes projected by topological input modes of ℓ  = ±5.

Figure 8

Experimentally measured OAM spectra. (a)–(c) The corresponding experimental results to those in Figs. 3, 3, and 3, respectively.