An optical vortex beam has a phase singularity with orbital angular momentum presenting helical phase fronts [1,2]. The vortex phase term
Photonics Research, Volume. 12, Issue 1, 172(2024)
Mutual aid instead of mutual restraint: interactive probing for topological charge and phase of a vortex beam of large aberrations
An imperfect propagation environment or optical system would introduce wavefront aberrations to vortex beams. The phase aberrations and orbital angular momentum in a vortex beam are proved to be mutually restrictive in parameter measurement. Aberrations make traditional topological charge (TC) probing methods ineffective while the phase singularity makes phase retrieval difficult due to the aliasing between the wrapped phase jump and the vortex phase jump. An interactive probing method is proposed to make measurements of the aberrated phase and orbital angular momentum in a vortex beam assist rather than hinder each other. The phase unwrapping is liberated from the phase singularity by an annular shearing interference technique while the TC value is determined by a Moiré technique immune to aberrations. Simulation and experimental results proving the method effective are presented. It is of great significance to judge the characteristics of vortex beams passing through non-ideal environments and optical systems.
1. INTRODUCTION
An optical vortex beam has a phase singularity with orbital angular momentum presenting helical phase fronts [1,2]. The vortex phase term
Several methods have been developed to determine the TC of vortex beams, such as diffraction methods [4–9], interferometry [10–18], mode transformation [19,20], and deep learning [21]. As the most intuitive method, interference and diffraction methods become mainstream procedures, which include self-interference [14,18], conjugated beam interference [16], multiple-pinhole interference [10,17], double-slit interference [11], single-slit diffraction [5], triangular aperture diffraction [4,6,9], annular aperture diffraction [13], and so on. All these methods depend on the judgment of subsequent regular intensity patterns, such as bifurcations of interference fringes [14],
Figure 1.Mutual restraining of TC determination and phase recovery in a vortex beam of aberrations, in which (a)–(d) are the effect of aberrations on TC measurements while (e) is the effect of TC on the aberration phase measurement. (a) Bifurcations of self-interference fringes, (b) conjugated vortex beam interference patterns, (c) triangular aperture diffraction patterns, (d) hollow intensity image from which TC is determined by deep learning, (e) phase unwrapping dilemma due to the phase jump aliasing in the vortex phase.
Meanwhile, phase recovery in a vortex beam of large aberrations is another challenge. Different from the traditional smooth and continuous phase fronts, the phase jump boundary naturally exists in the helical phase fronts due to the singularity. Interferometry provides an elegant performance with the phase-shifting technique for pixel-level accurate phase demodulation [24,25]. However, the wrapped phase jump and the vortex phase jump are aliased together and unable to be distinguished, which would lead to the failure of phase unwrapping [26]. As is shown in Fig. 1(e), paths 1 and 2 in the wrapped phase provide distinct different solutions for the phase jump compensation from area A to B, respectively. That is, the phase unwrapping would be ambiguous due to the phase jump aliasing. Pre-designed unwrapping paths [26] would relieve the phase jump aliasing but be inoperative in the case of large aberrations because the complex phase jump boundaries make the pre-design of unwrapping paths impossible.
Sign up for Photonics Research TOC Get the latest issue of Advanced Photonics delivered right to you!Sign up now
Therefore, we conclude that the aberrated phase and TC restrict measurements of each other. Previous studies have focused on the respective measurement of the two characteristics, without concerning the mutual restraint between the two measurements in a beam of large aberrations. The simultaneous recovery of the aberration phase and TC number has not been reported previously. In this paper, we propose an interactive probing method in a dual-interferometer structure to determine the aberration phase and TC value, which separates the cross-impact between the two parameters. A singularity-immune annular radial shearing interferometer separates the phase unwrapping from the vortex phase jump, which makes the phase recovery as simple as the traditional smooth phase. With the recovered aberration phase, an aberration-immune virtual Moiré probe is generated to determine the TC. The relationship between the two characteristic measurements has changed from being mutually restrictive to complementary. Simulation and experimental results proving the method effective are presented.
2. PRINCIPLE
As a kind of LG beam, the electric field of the vortex beams of aberrations can be simplified as
Figure 2.Principle of the interactive probing method. The system consists of a Twyman–Green interferometer and an annular radial shearing interferometer. The two interferometers capture the purified interferogram and radial shearing interferogram, respectively. The RCP and LCP beams before the P-cameras in the two interferometers are designed to meet the synchronous phase shift condition. The aberration phase extracted from the shearing interferogram is used to generate a virtual interferogram. The Moiré probes then can be extracted from the product of the purified interferogram and virtual interferogram.
The Twyman–Green interferometer is employed to acquire the direct interferogram of the vortex beam and reference collimated beam. The polarized beam splitter (PBS 1) divides the incident vortex beam with circular polarization into two parts. The reflected
Sign up for Photonics Research TOC Get the latest issue of Advanced Photonics delivered right to you!Sign up now
A. Phase Recovery with Singularity Isolation
The annular radial shearing interferometer is employed to capture the interferogram with the phase singularity influence removed. The
B. TC Determination
With the recovered phase
It is obvious that the vortex phase (
The carrier phase
The sign of the TC is determined by a simple digital phase shifting. With the recovered
3. SIMULATION
Simulation examples (
Figure 3.Phase recovery and TC determination results with increasing incident aberrations. (a) Incident vortex phases with aberrations, (b) direct interferograms with the carrier, (c) shearing interferograms, (d) recovered phases and recovered errors, (e) Moiré probes, (f) far-field spots, (g) triangular aperture diffraction spots, (h) conjugated interference petals.
Figure 4 presents the performance of the method in the case of different TCs and aberration types. The four-row images refer to the results in the case of
Figure 4.Simulation results of TC determination and phase recovery in the case of
The TC sign determination is simulated in
Figure 5.Sign determination of TC. These pictures are the keyframes of
We then examined the resolution of our proposed method for fractional TCs with a camera of one megapixel. A complete probe refers to the same angular width as others while the fractional TC would induce several incomplete probes splitting from the complete one. Figure 6 illustrates the TC estimation simulation results corresponding to
Figure 6.Determination of the fractional TC with a camera of one megapixel. (a) Probes with TC between 6 and 7 spaced 0.1 apart, (b) GS curves which indicate the split process of corresponding TC probes. (c)
Figure 6(a) presents probe images with TC between 6 and 7 spaced 0.1 apart, which shows us the new probe splitting process. It suggests the inaccuracy of determining the TC only by the number of probes. The quantitative angular width of probes or adjacent probe angular spacing can act as the new index to describe the fractional TC. To quantify the angular width of these petals and their septa, the gray sum curve (GS curve) of the radial pixel along the polar angle is introduced. The angular coordinate of the GS curve is the polar angle from 0° to 360° and the radius coordinate is the sum of normalized radial pixel gray at the corresponding polar angle. Each peak of the GS curve refers to a radius of the maximum gray sum, which implies a petal. Figure 6(b) presents the GS curves of TC values 6–7 with 0.1 apart. Each petal splitting represents the growth of two new petals and a new petal spacing. The growing petal spacing is narrower than other complete ones. After a lot of simulation verification, we proposed an empirical formula to calculate the fractional part of the TC, as a correction to simple petal counting:
Determination Results of the Fractional Part of Topological Charge with a Camera of One Megapixel
Real | 6.1 | 6.2 | 6.3 | 6.4 | 6.5 | 6.6 | 6.7 | 6.8 | 6.9 | 7 |
---|---|---|---|---|---|---|---|---|---|---|
4.01 | 10.03 | 17.05 | 22.08 | 28.09 | 33.10 | 38.11 | 42.11 | 47.12 | 52.12 | |
6.078 | 6.195 | 6.332 | 6.429 | 6.546 | 6.644 | 6.741 | 6.819 | 6.916 | 6.999 | |
Error | −0.022 | −0.005 | 0.032 | 0.029 | 0.046 | 0.044 | 0.041 | 0.019 | 0.016 | −0.001 |
4. DISCUSSION
For the Moiré probe images, the measurable maximum TC limit depends on the pixel number of the camera. In theory, three pixels in a circle allow two petals to be distinguished. A camera of one megapixel has 2260 pixels counted in the outermost circle at the sensor, promising about 1130 petals counting and thus the largest measurable TC
Figure 7.TC resolution analysis with a one-megapixel camera. (a) TC resolution variations with the increasing TC number, (b) error performance of TC determination in the cases of
Another error consideration is the system structure. Just like diffraction methods for TC determination, the alignment of the beam singularity and the diffraction aperture is a must [4,17]. In our method, the beam wavefront singularity must be aligned with the shearing center of the annular shearing interferometer to ensure the vortex phase can be eliminated completely by the phase shearing. Figure 8 shows the simulation results of misalignment. The shearing center is generally the center of the phase matrix by default. A new phase matrix, truncated eccentrically from a phase matrix of a singularity at the center, is employed to simulate the misalignment. The incident phase with an exocentric singularity is shown in Fig. 8(a), which implies the misalignment of the singularity and shearing center. The purified interferogram and the shearing interferogram are presented in Figs. 8(b) and 8(c), respectively. The shearing interferogram shows obvious double singularity separation due to the misalignment, which means the phase singularity is not removed completely. Therefore, multiple phase jumps that should not exist appear in the recovered phase
Figure 8.Simulation of TC determination and phase recovery in the case of misalignment and phase recovery error. (a) Incident vortex phases with exocentric singularity, (b) purified interferogram, (c) shearing interferogram, (d) recovered phase referring to aberrations, (e) virtual interferograms, (f) Moiré fringes, (g) Moiré probes, (h) TC determination error due to 1% phase recovery error in the cases of different beam aberrations and TC.
Even with an aligned system, the influence of the inherent phase recovery error deserves discussion. The phase recovery in the radial shearing interferometer has been focused on by many researchers and the relative rms error can achieve about 1% [27,31]. With the increasing beam aberration, the absolute error of the recovered phase rises as well. The error of the
Although Moiré probes achieve aberration immunity, the technique requires the participation of an additional reference beam. Therefore, it cannot be applied where a reference beam is not available. If the aberration is accompanied by the spectrum dispersion [32,33] of the orbital angular momentum when the beam passes through a strong random turbulence, the proposed method may be ineffective due to multiple singularities to be measured.
5. EXPERIMENT
We set up an experimental system to validate the proposed method, as shown in Fig. 9(a). The system is built following the two interferometer structures as shown in Fig. 2. The difference is that the incident vortex beam is generated by a reflective spatial light modulator (SLM). The specific beam path is illustrated in Fig. 9(b). A He–Ne laser (
Figure 9.Verification experimental setup.
The comparative experiments were carried out in the previously mentioned methods and the proposed method. The SLM provided vortex beams of
Figure 10.Comparative experiment results with different aberrations in different methods. Each set of three images corresponds to the cases of three different aberrations. (a) Triangular aperture diffraction spots, (b) far-field spots, (c) conjugated interference petals, (d) Moiré probes.
Figure 11.Experiment results of TC determination and phase recovery in the case of
Figure 12.Determination results of the TC number from 3.1 to 4 with 0.1 space.
6. CONCLUSIONS
Measurements of the wavefront phase and topological charge of vortex beams carrying large aberrations are restricted by each other. We proposed an interactive probing solution with a dual-interferometer structure. The phase singularity-immune radial shearing interferometer is employed to recover the aberration phase. The phase recovery accuracy is the same as the traditional shearing interference phase recovery in case of alignment. With the recovered aberration phase, the aberration-immune Moiré probe is proposed to characterize the TC number. The measured TC number would be used to complement the vortex phase feature. The aberration and TC are proven not to restrict measurements of each other in this method. To achieve fine Moiré probe counting, the GS curve is employed to calculate the TC (especially fractional TC) number. With a tolerance of 12 pixels per two probes, 0.01 resolution and a maximum 200 measurable range of the TC number are achievable with a camera of one megapixel. The measurement accuracy of the integer TC is higher than that of the fractional TC. The beam wavefront singularity must be aligned with the shearing center of the shearing interferometer to ensure phase recovery accuracy. Even in the case of misalignment, the integer TC number can be determined accurately. Experiments achieved measurement of maximum
Get Citation
Copy Citation Text
Shengyang Wu, Benli Yu, Lei Zhang, "Mutual aid instead of mutual restraint: interactive probing for topological charge and phase of a vortex beam of large aberrations," Photonics Res. 12, 172 (2024)
Category: Instrumentation and Measurements
Received: Jun. 26, 2023
Accepted: Nov. 15, 2023
Published Online: Dec. 22, 2023
The Author Email: Lei Zhang (optzl@ahu.edu.cn)
CSTR:32188.14.PRJ.498502