Efficient sorting of orbital angular momentum states of light

Efficient sorting of orbital angular momentum states of light

2010 | Berkhout, G.C.G.; Lavery, L.A.; Courtial, J.; Beijersbergen, M.W.; Padgett, M.J.
This paper presents a method for efficiently sorting orbital angular momentum (OAM) states of light using two static optical elements. The method involves transforming the light beam from Cartesian to log-polar coordinates, converting the helically phased light beam into one with a transverse phase gradient. A subsequent lens then focuses each input OAM state to a different lateral position. The method is demonstrated experimentally using two spatial light modulators to create the desired optical elements, applied to the separation of eleven OAM states. OAM states of light are characterized by a phase cross section of exp(iℓφ), where ℓ is an integer. These states have an unbounded state space, offering a large potential for information capacity. However, measuring OAM states requires a single photon to be in one of many different states. Generating helically phased beams with OAM is typically done using a diffractive optical element, such as a hologram. However, this method is limited in its ability to test for multiple states simultaneously. The proposed method uses a geometric transformation to convert the azimuthal position in the input beam into a transverse position in the output beam. This transformation is achieved using two custom optical elements: one to transform the image and a second to correct for phase distortion. The first element performs a mapping (x,y) → (u,v), where (x,y) and (u,v) are the Cartesian coordinate systems in the input and output planes, respectively. The second element corrects for phase distortion introduced by the first. The method is implemented using diffractive spatial light modulators (SLMs) to create the desired phase profiles. The system is tested with Laguerre-Gaussian (LG) beams as input states. The results show that the position of the elongated spots is proportional to the OAM state of the incident beam. The system is able to identify a superposition of OAM states, as shown in the final row of the results. The channel capacity of the system is calculated and found to be slightly lower than the theoretical maximum due to experimental imperfections. The method has potential applications in various fields, including quantum entanglement, astrophysics, and microscopy, all of which make use of the OAM state basis. The system is limited by the slight overlap of the spots for different states, which could be reduced by applying an additional diffraction grating. The method offers a novel system for efficiently measuring the OAM state of light using two bespoke optical elements.This paper presents a method for efficiently sorting orbital angular momentum (OAM) states of light using two static optical elements. The method involves transforming the light beam from Cartesian to log-polar coordinates, converting the helically phased light beam into one with a transverse phase gradient. A subsequent lens then focuses each input OAM state to a different lateral position. The method is demonstrated experimentally using two spatial light modulators to create the desired optical elements, applied to the separation of eleven OAM states. OAM states of light are characterized by a phase cross section of exp(iℓφ), where ℓ is an integer. These states have an unbounded state space, offering a large potential for information capacity. However, measuring OAM states requires a single photon to be in one of many different states. Generating helically phased beams with OAM is typically done using a diffractive optical element, such as a hologram. However, this method is limited in its ability to test for multiple states simultaneously. The proposed method uses a geometric transformation to convert the azimuthal position in the input beam into a transverse position in the output beam. This transformation is achieved using two custom optical elements: one to transform the image and a second to correct for phase distortion. The first element performs a mapping (x,y) → (u,v), where (x,y) and (u,v) are the Cartesian coordinate systems in the input and output planes, respectively. The second element corrects for phase distortion introduced by the first. The method is implemented using diffractive spatial light modulators (SLMs) to create the desired phase profiles. The system is tested with Laguerre-Gaussian (LG) beams as input states. The results show that the position of the elongated spots is proportional to the OAM state of the incident beam. The system is able to identify a superposition of OAM states, as shown in the final row of the results. The channel capacity of the system is calculated and found to be slightly lower than the theoretical maximum due to experimental imperfections. The method has potential applications in various fields, including quantum entanglement, astrophysics, and microscopy, all of which make use of the OAM state basis. The system is limited by the slight overlap of the spots for different states, which could be reduced by applying an additional diffraction grating. The method offers a novel system for efficiently measuring the OAM state of light using two bespoke optical elements.
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