Ghost Imaging with Different Speckle Sizes of Thermal Light
 Author: Jue Wang, Renlong Yu, Yu Xin, Yanming Shao, Yanru Chen, Qi Zhao
 Publish: Current Optics and Photonics Volume 20, Issue1, p8~12, 25 Feb 2016

ABSTRACT
In this paper, we theoretically and experimentally analyze the impact of speckle size of pseudothermal light source on ghost imaging. A larger size of speckle can bring improvements in SNR and visibility. At the same time, the edge blur of the retrieved image will become more serious. We also present a setup which can mitigate the edge blur of larger speckle while maintaining the advantages of higher SNR and visibility by changing the speckle size of the object beam with a concave lens.

KEYWORD
Ghost imaging , Coherence , Speckle size , Edge blur

I. INTRODUCTION
In the past decade, ghost imaging (GI) has attracted a lot of attention. In ghost imaging, the object image is retrieved by using two spatially correlated beams: the reference beam and the object beam. The reference beam never interacts with the object and is measured with a pixelated detector. The object beam interrogates the object and then illuminates a bucket detector, with no spatial resolution. By correlating the reference beam with the bucket signal, the ‘ghost’ image is retrieved. In 1995, the first GI was found in quantum optics [1], and it was once considered as a unique phenomenon of quantum optics. However, in 2002, Bennink achieved ghost imaging with scanning of a pair of collimated laser beams [2], which sparked a debate [35] of whether GI was a quantum effect or not. Then a variety of thermal light sources [38] were used to realize GI. Recently some teams focused on the application of GI, such as GI by measuring reflected photons [9], compressive GI [1012], GI through turbulent atmosphere [13], GI in turbid media [14]. However, only a few teams [7, 15, 16] focused on the impact of the speckle size on GI. In this paper, we theoretically and experimentally analyze the pros and cons of GI with different speckle sizes, and try to present a setup which can at the same time make the best use of the advantages and bypass the disadvantages caused by different sizes of speckles.
II. METHODS & RESULTS
In order to get speckles with different sizes, we used the setup in [17] (see Fig. 1(a)). The laser and rotating ground glass constituted a pseudothermal light source. The ground glass was in the focal plane of the Lens F while D was an aperture. With the formula
δx _{0} =πλf /ω _{0}, we can get desired speckle sizeδx _{0} by changingf of Lens. Here,λ is the wavelength of light andω _{0} represents the waist radius. The role of beam splitter is to form two spatially correlated beams. Beam 1 (the object beam) is collected with a bucket detector after illuminating the object. Beam 2 (the reference beam) never interacts with the object and measured with a pixelated detector. The distance between Aperture D and the two detectors arez _{1} andz _{2} respectively. Let us indicate withI _{1}(x _{1}) andI _{2}(x _{2}), the intensity distributions of the speckle fields on the object and reference planes, respectively. IfT (x ) indicates the transmission function, the bucket signalS _{1} isAccording to the result of [7], we have the following formula
Where <
⋯ > indicates the average over independent speckle configurations.µ _{Δz}(x _{1},x _{2}) is the crosscorrelation coefficient of the electric fields at the planesz _{1} andz _{2}.A_{coh} is the area of the speckle, which is related to the speckle size by . ⊗ denotes convolution,N_{obj} =A_{obj} /A_{coh} denotes the number of the speckles falling into the object. Eq. (2) shows that the visibility of GI [i.e., the maximum value ofv _{Δz}(x _{2})] is equal to 1/N_{obj} and, except for convolution effects (occurring whenγ _{Δz}(x _{2},x _{1}) and T(x_{2}, x_{1}) have similar widths), is independent of Δz . Another indicator to evaluate the GI is SNR [15, 16]While
N_{speckle} =A_{beam} /A_{coh} is the number of the speckles in the beam, ΔT _{min} is the minimum variation of the object transmission function to be detected. is the average quadratic transmission function of the object. R represents the independent sampling number. With Eqs. (2) and (4), we know that for the case of using the same average number, a larger speckle size will lead to higher visibility and SNR. However, in experiments we found that a larger speckle size will get a more blurred edge of the object as well. We call this phenomenon edge blur of GI. We will theoretically analyze the reason of this phenomenon and experimentally show the edge blur of GI in the following.Considering the case shown in Fig. 2, there is a point
x _{2} where the transmission function isT (x _{2}) = 0, and the distance from the edge isd .δx _{1} andδx _{2} are the sizes of two different speckles whereδx _{1} <δx _{2}. The shaded areas indicate that the region can be received by the bucket detector. Whenδx _{0} =D , the mutual coherence function [18] isWhere
I _{0} is a real function representing the mean transverse intensity distribution at the source plane, varying on the large scale D the source diameter.C _{0} is a real function with the propertyC _{0}(0) = 1, which dies out over the small scaleδx _{0} (the speckle transverse size). In the deepFresnel zone，the coherence factor characterizing the granular structure of the source has a Gaussian profile of the formHere, we only consider the case
z _{1} =z _{2}. Then, we obtain the complex coherence factor and its squared modulus reads as:Eq. (7) illustrates that in the case where
z _{1} =z _{2}, │μ │^{2} is a Gaussian function described by the sizeδx _{0}. The graph of │μ │^{2} and │x _{1} –x _{2}│ is shown in Fig. 3. We can see that for the same │x _{1} –x _{2}│, a larger speckle leads to a bigger │μ │^{2}.The correlation between
S _{1} andI _{2} (x _{2}) isAssuming that the bucket detector is pixelated, so we obtain discrete result
Where N denotes the number of the pixels in the shaded areas in Fig. 2. From Fig. 3 we can see that
Combining Eqs. (10) and (11), we have the result
On the other hand, from Fig. 2, the number of pixels has the relation that
N _{δx1} <N _{δx2}, so we obtain the final resultEq. (13) shows two key features of GI with a pseudothermal light source: Firstly, because of the Gaussian distribution of the correlation radius of the speckle,
I _{2}(x _{2}) and bucket signalS _{1} are still related and the retrievedT (x _{2}) > 0, although the idealT (x _{2}) = 0. Theoretically, whend >δx _{0} / 2, the transmission functionT (x _{2}) = 0, and we call this distanceδx _{0} / 2 as the width of edge blur. Secondly, while Eqs. (2) and (4) illustrate that the ghost imaging will have a higher visibility and SNR with lager speckle, Eq. (13) shows that it will bring a more serious edge blur as well.In order to experimentally demonstrate the edge blur, we implement the experiment shown in Fig. 1(a). The source is a HeNa laser (
λ = 632.8nm ,ω _{0} = 2mm ). By selecting different lenses (f = 50mm andf = 100mm , respectively), we obtain two kinds of speckle withδx _{1} = 49.7µm andδx _{2} = 99.4µm . The object was an aperture whose diameter is 900µm . The experimental result is shown in Fig. 4. Where Panel a1 shows the 2Dghost image retrieved by averaging R=12800 independent realizations of the speckle field (δx _{1} ≈ 50µm ). Panel b1 shows the horizontal profile of the aperture (blue continuous line), this profile compares with the expected one (dotted line) and its visibility is 0.0039. Panel a2 shows the 2Dghost image retrieved by averaging R = 12800 independent realizations of the speckle field (δx _{2} ≈ 100µm ). Panel b2 shows the horizontal profile of the aperture (blue continuous line), this profile compares with the expected one (dotted line) and its visibility is 0.0156.Comparing the results with different speckles, we have the following conclusions: Firstly, compared with Fig. 4(a1), the retrieved image with larger speckle (see Fig. 4(a2)) has a higher SNR. Secondly, compared with Fig. 4(b1), the retrieved image with lager speckle (see Fig. 4(b2)) has a higher visibility, which is about four times of the visibility retrieved with smaller speckle. Thirdly, compared with Fig. 4(b1), Fig. 4(b2) has a wider rising edge. This shows that the larger speckle used, the more serious edge blur exists.
Based on the analysis above, we try to present a setup (Fig. 1(b)) which can eliminate the adverse effects of speckle size. In Fig. 1(b), the reference beam remains intact, and an optical modulator M is added to the object beam. The M is used to modulate the distribution of the light, and it can be a spatial light modulator (SLM) or lens, simply. Then the bucket signal can be rewritten as
Where
I _{1}'(x _{1}) =mI _{1}(nx _{1}) represents the modulated light field,m andn are the modulation coefficients. By converting the variable, we have the following resultThe difference between Eq. (1) and Eq. (15) is the transmission function
T '(x ), which indicates that the modulation of the light is equivalent to the modulation of the object. Whenn > 1, the effect is to narrow the object. Whenn < 1, the effect is to amplify the object. So we have the following formulaThis setup has at least three advantages: Firstly, we can arbitrarily zoom the object by setting different
n . This feature allows us to image an object whose size is lager than the size of the detector. Secondly, according to Eq. (17), whenn >, 1 /N'_{obj} >N_{obj} which illustrates that the improved setup has a higher visibility than the traditional setup. Thirdly, because the reference beam remains intact, the width of the edge blur remainsδx _{0} / 2.Two independent experiments were implemented with the setups shown in Figs. 1(a) and (b), respectively. The two objects have the same size (slit, 1
mm × 3mm ). In Fig. 1(b), we used a concave lens as the modulator M andn = 2.12. The results are shown in Fig. 5. Panels a1 and b1 are results of the traditional ghost imaging while Panels a2 and b2 represent results of the improved method. The visibility of results in Panels a1 and b1 is about 0.0026, and the visibility of results in Panel a2 and b2 is about 0.012. We can see the improved ghost imaging has higher SNR and visibility (about 4.5 times) than the traditional ghost imaging by comparing the results. The improvement of visibility is due to the change of modulation coefficient n. Moreover, Panels b1 and b2 show that the two retrieved image have the same width of edge blur. However, there is a price to pay, which is the retrieved image is a narrowed object. The experimental results are in good agreement with the theoretical analysis results.III. CONCLUSION
In this paper, the impact of speckle size of pseudothermal light source on ghost imaging is theoretically and experimentally analyzed. For the same number of independent realizations, the larger speckle size of thermal source is, the higher visibility and SNR of the retrieved image and the more serious edge blur of the retrieved image will be. This means that the impact of the speckle size of the pseudothermal on GI is doublesided. In order to eliminate the adverse effects of the speckle size, we use a concave lens to modulate the object beam, which is equivalent to modulating the object. By the improved setup, we implement the experiment in which the experimental results are in good agreement with the theoretical analysis results.

[FIG. 1.] Experimental setups for ghost imaging. (a) the traditional setup for ghost imaging. (b) the improved setup for ghost imaging.

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[FIG. 2.] The schematic view of the point x2 where the transmission function is T(x2) = 0.

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[FIG. 3.] The graph of │μ│2 vs │x1 ？ x2│.

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[FIG. 4.] The ghost imaging reconstruction of an aperture whose diameter is 900 μm. (a1) and (b1) are the reconstruction images which obtain 12800 realizations, when the speckle size of the thermal source is 50 μm. (a2) and (b2) are the reconstruction image which obtain 12800 realizations, when the speckle size of the thermal source is 100 μm.

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[FIG. 5.] The ghost imaging reconstruction of a 1 mm × 3 mm slit. (a1) and (b1) are the reconstruction image of the traditional setup, obtained with 16000 realizations. (a2) and (b2) are the reconstruction image of the improved setup, obtained with 16000 realizations.