Polarimetric Measurement of Jones Matrix of a Twisted Nematic Liquid Crystal Spatial Light Modulator
 Author: KhosOchir Tsogvoo, Munkhbaatar Purevdorj, Yang Byung Kwan, Kim Hyun Woo, Kim Jin Seung, Kim MyungWhun
 Organization: KhosOchir Tsogvoo; Munkhbaatar Purevdorj; Yang Byung Kwan; Kim Hyun Woo; Kim Jin Seung; Kim MyungWhun
 Publish: Current Optics and Photonics Volume 16, Issue4, p443~448, 25 Dec 2012

ABSTRACT
A polarimetric experimental method was developed to determine the Jones matrix elements of transparent optical materials without sign ambiguity. A set of polarization dependent transmittance data of the samples was measured with polarizer ？ sample ？ analyzer system and another set of data was measured with polarizer ？ sample ？ quarterwave plate ？ analyzer. Two data sets were compared and mathematically analyzed to obtain the correct signs of the elements of the matrix. The Jones matrix elements of a quarterwave plate were determined to check the validity of the method. The experimentally obtained matrix elements of the quarterwave plate were consistent with the theoretical expectations. The same method was applied to obtain the Jones matrix elements of a twisted nematic liquid crystal panel.

KEYWORD
Jones matrix element , Liquid crystal , Spatial light modulator , Polarimetry

I. INTRODUCTION
Liquid crystal panels (LCPs) are optoelectronic devices that can modulate the state of light. They are used for many practical applications including flat panel display, holography and optical information processing because of their inherent ability to spatially modulate a light beam in a programmable manner [14]. Twisted nematic (TN) type LCP spatial light modulators (SLMs) are good for intensity modulation and parallel aligned LCP SLMs are suitable for phase modulation. Some application areas of SLMs such as ultrashort laser pulse control require phase modulation [5,6]. However parallel aligned LCP SLMs are expensive, while TNLCP SLMs are relatively lowcost and easily available. This is why much effort has been made to use TNLCP SLMs for many applications [7,8].
For programmable operation of a TNLCP spatial light modulator (SLM), the Jones matrix of the TNLCP needs to be determined accurately. However, manufacturers do not usually provide that information, and the user must determine the matrix. According to the literature, the TNLCP can be considered as a stack of thin slices acting as homogeneous uniaxial crystals [9]. The Jones matrix of a homogeneous uniaxial crystal slice can be written as a function of its ordinary and extraordinary indices of refraction as well as the orientation angle of the liquid crystal molecules [9]. The Jones matrix of the whole TNLCP can be described as the product of the individual Jones matrices of each homogeneous uniaxial crystal slice [10]. According to the approximation, the Jones matrix of the whole TNLCP can be written as
Here
c _{1} is a constant used to define the intensity loss,i isand
Δ？ is the phase change induced by the LCP at a given wavelength. The matrix describes the change in polarization state of the incoming beam. The goal of this study is to determine the signs as well as the absolute values of the matrix elements can be determined by polarimetric methods experimentally, but it has been thought that the signs of the elements can not be determined uniquely by polarimetric methods only. However it has been shown that the signs can be determined by polarimetric methods [11].In this paper, we demonstrated a polarimetric method for unambiguous determination of signs of the Jones matrix elements. By measuring the transmittance with an optical system of polarizerLCPanalyzer, we obtained the set of the unitary matrix elements. In addition, by measuring the transmittance after inserting a quarter wave plate in the system, we obtained another set of unitary matrix elements. By comparing two sets of unitary matrix elements, we could determine the unitary matrix elements accurately without the sign ambiguity.
Our method has some advantages with respect to other previous similar methods. First, our method improved the accuracy. In particular, polarizer and analyzer rotate in the same and opposite directions with 1degree angular increment from 0° to 180°, so Jones matrix elements were obtained from fitting a trigonometric function with many data points. Also, our method uses only two rotating elements (polarizer and analyzer) and except for the rotation of the polarizers the alignments of all the other optical elements were not disturbed during the measurement, which can reduce potential errors due to realignment.
II. THEORY
2.1. Transmittance Intensity of the First Part of the Optical System (Quarter Wave Plate + Polarizer)
The polarimetric method to determine the Jones matrix is based on the transmittance measurement of the optical system. Let us first explain how to calculate the transmittance of the optical system. Our whole optical system consists of a HeNe laser light source, a quarter wave plate (QWP1), polarizer (P), TNLCP, analyzer (A) and a silicon photodiode detector as shown in Fig. 1.
First we considered the transmittance intensity of the optical system consists of a light source, a quarterwave plate (QWP1) and a polarizer. The laser light source was linearly polarized, so the QWP1 was used to make the linearly polarized light into the circularly polarized light. The circularly polarized light was the input beam of the polarizer. The azimuth angle of the polarizer was defined as
ψ_{P} . The relation between the Jones vector of the input beam (a ,b ？e ^{iδ} ) and that of the output beam (x _{out1},y _{out1}) for the optical system can be written as Eq. (2)Here
a andb are the electric field components on the vertical (x ,z ) and horizontal (y ,z ) planes, respectively.δ is a phase difference between the electric field components.Q (π /4) is the Jones matrix of the QWP1 whose fast axis was aligned 45° with respect to the vertical (x ) axis.P (ψ_{P} ) is the Jones matrix of the polarizer whose azimuthal angle isψ_{P} . The output beam intensity was determined by following equation,The laser light was almost linearly polarized in the vertical (
x ,z ) plane, so the electric field components satisfied a condition,a >>b .2.2. Transmittance Intensity of the Whole Optical System (Quarter Wave Plate + Polarizer + TNLCP + Analyzer)
We calculated the transmittance intensity of the whole optical system consists of a light source, a quarterwave plate (QWP1), a polarizer, a TNLCP and an analyzer. The TNLCP was placed between a polarizer and an analyzer whose azimuth angles were
ψ_{P} andψ_{A} , respectively. The output beam of the polarizer described in Eq. (2) was used as the input beam of the later optical system. The Jones vector of the output beam for the whole optical system can be written as Eq. (4).2.3. Definition of the Transmittance of the Optical System Composed of TNLCP and Analyzer
We defined the transmittance of the optical system composed of TNLCP and analyzer,
T_{ψP} ,_{ψA} , as the relative ratio between the output (I) beam intensity of Eq. (5) and the input beam intensity (I_{1}) of Eq. (3).T_{ψP} ,_{ψA} can be expressed as Eq. (7),Here
c _{3} is a constant.III. EXPERIMENT
The optical system used in our study was shown in Fig. 1. The light source was a linearly polarized HeNe laser. The laser beam was expanded by using the beam expander (BE) and then was sent to the quarter wave plate (QWP1). The quarterwave plate made the linearly polarized light circularly polarized. The circularly polarized beam was directed to a rotating linear polarizer (P) and then was sent to the sample. The polarization state of the transmitted beam was investigated with an analyzer (A) and an additional quarter wave plate (QWP3). The intensity of the beam passed through the analyzer was measured with the photodiode (D). A quarterwave plate (QWP2) and a TNLCP SLM (Holoeye LC2002, SONY LCX016AL6) were used as samples.
The transmittance of the optical system (sample + analyzer) depends only on the sum (
ψ_{A} +ψ_{P} ) and the difference (ψ_{A} ψ_{P} ) of the angles of the polarizer and analyzer. After substitutingφ =ψ_{A} ψ_{P} andψ =ψ_{A} +ψ_{P} into Eq. (7), the transmittance of the optical system,T_{φ, ψ} takes a simpler form.Here C is a constant.
φ _{0},ψ _{0} are the initial phases of the trigonometric functions and A, B are the amplitudes of the functions. A set of transmittanceT_{φ, ψ} data was collected by changingφ andψ from 0° to 360°. The coefficients A and B, also the initial phase anglesφ _{0} andψ _{0} in Eq. (8) were determined by the NewtonRaphson fitting method.IV. RESULTS AND DISCUSSION
4.1. Determination of the Jones Matrix Elements of a Quarterwave Plate
To check the validity of our method, the Jones matrix elements of a quarter wave plate (QWP2) is experimentally determined and compared with the theoretical one. The fast axis of the QWP2 is set to make 45º with the
x axis. If the quarterwave plate is ideal, then the Jones matrix should befrom Eq. (1), where
c _{4} is the constant representing the intensity loss. The matrix should be unitary, so the elementsf ,h ,g andj should satisfy the condition,f ^{2} +h ^{2} +g ^{2} +j ^{2} = 1. To satisfy the constraintsf ,j ,h andg should beh =g = 0. Eqs. (6), (7), and (8) show thatf ,j ,h andg are connected to the coefficients A and B also the initial phase anglesφ _{0} andψ _{0} of the trigonometric functions representing the transmittance of the optical system, i.e.The transmittance
T_{φ, ψ} of the quarterwave plate (QWP2) and the analyzer was measured and fitted with Eq. (8) to determine A, B,φ _{0} andψ _{0}. To make Eq. (8) simpler, some conditions are chosen to make the cosines of the sum and the difference of the angles constant. First the analyzer and polarizer rotated in the opposite direction with same angle increment, so that the ‘sum angle (ψ_{A} +ψ_{P} )’ is set to be zero, i.e.ψ =ψ_{A} +ψ_{P} =0. Then the transmittance as a function of the difference angleφ =ψ_{A} ψ_{P} is measured as presented in Fig. 2(a). By fitting the data points with acosine function, the information about
f andh is obtained.Similarly, the analyzer and the polarizer are rotated in the same direction with the same angle increment so that the ‘difference angle (
ψ_{A} ψ_{P} )’ is set to be zero, i.e.φ =ψ_{A} ψ_{P} =0. Then the transmittance as a function of the sum angleψ =ψ_{A} +ψ_{P} is measured as presented in Fig. 2(b). By fitting the data points with a cosine function, the information aboutg andj is obtained.It should be noted however that, the relative signs of
g andj elements to those off orh were not uniquely defined because of the ambiguity of the phase of the trigonometric functionsφ _{0} andψ _{0} in Eq. (10).To eliminate the sign ambiguity, an additional quarterwave plate (QWP3) was inserted between the quarterwave plate (QWP2) and the analyzer. The fast axis of QWP3 was aligned 45° with respect to the x axis. Including the contribution of the QWP3, the transmittance became a function of
φ andψ as shown in Eq. (11).Here
c _{5} is a constant.m ,q ,p andl were connected tof ,h ,g andj by comparing Eq. (11) with Eq. (6). As a result,Also the sum of the squares of the new elements should
satisfy
m ^{2} +q ^{2} +p ^{2} +l ^{2} = 2 because of the unitarity.To determine
m ,p ,q andl , first the analyzer and the polarizer are rotated in the opposite directions with the same angle increment so that the sum angle remains zero, i.e.ψ =ψ_{A} +ψ_{P} =0. The transmittance as a function of the difference angleφ =ψ_{A} ψ_{P} is presented in Fig. 3(a). By data fitting, the information aboutm andq is obtained. Similarly, the analyzer and the polarizer are rotated together so that the difference angle remains zero, i.e.φ =ψ_{A} ψ_{P} =0. Then the transmittance as a function of the sum angleψ =ψ_{A} +ψ_{P} is presented in Fig. 3(b). By data fitting, the information aboutp andl is obtained.The sign of the
f ,g ,j , andh can be determined by comparing four element values of the first method (without QWP3) with four element values of the second method (with QWP3). The experimentally obtained Jones matrix elements of QWP2 aref = 0.706,j = 0.707,h = 0.017 andg = 0.026 consistent with the theory, which confirms the validity of our method.4.2. Determination of the Jones Matrix Elements of a TNLCP Spatial Light Modulator
The same method was used to determine the Jones matrix elements of a programmable TNLCP SLM. A programmable TNLCP SLM was installed between the polarizer and analyzer as shown in Fig. 1. The TNLCP can change the intensity and the phase of the input beam as the grey level is controlled by an external device. The corrected transmittances
T _{0} of the TNLCP and the analyzer for several different grey levels were measured. The meaning of the corrected transmittanceT _{0} will be discussed in subsection 4.3. Fig. 4 shows the difference angle (φ =ψ_{A} ψ_{P} ) and the sum angle (ψ =ψ_{A} +ψ_{P} ) dependence of theT _{0} forone of the grey levels (grey level 192 of our LCP device). As demonstrated for the quarterwave plate (QWP2) case, the absolute values of the
f ,g ,j , andh of the Jones matrix were determined by fitting the transmittance with Eq. (8).To determine the signs of the Jones matrix elements, additional set of data were measured. Fig. 5 shows the sum angle (
ψ =ψ_{A} +ψ_{P} ) and the difference angle (φ =ψ_{A} ψ_{P} ) dependence ofT _{0}. Note that additional quarterwave plate QWP3 was inserted between the TNLCP and the analyzer for obtaining the data set. The absolute values of them ,q ,p , andl of the Jones matrix were determined by fitting the transmittance with the Eq. (11).The sign of the
f ,g ,j , andh can be determined by comparing four element values of the first method (without QWP3) with four element values of the second method (with QWP3). The experimentally obtained Jones matrix elements of TNLCP (at grey level 192 of our LCP device) weref = 0.64,h = 0.63,g = 0.44 andj = 0.02. The matrix elements of other grey levels can be obtained similarly.4.3. Corrections for the Transmittance Measurement
There is discrepancy between the theoretical transmittances described by Eqs. (8) and (11) and the measured transmittance. Such discrepancy is negligible for the quarterwave (QWP2), but is not negligible for the TNLCP. Fig. 6 shows the measured transmittance (
T , solid asterisks) of TNLCP, the errorcorrected transmittance (T _{0}, solid circles), and the theoretical fitting function (solid line). They do not overlap perfectly and the discrepancy is especially large around theπ /4 of the difference angle.The intensity loss occurred at the TNLCP is the most
dominant reason of the discrepancy. The intensity loss can be caused by many factors including absorption, scattering or reflection. However, the intensity loss does not affect the fitting process significantly in our method.
Figure 7 shows the polarizer angle dependence of the transmittance of the TNLCP (
T _{1}). Note thatT _{1} was measured without the analyzer and the additional quarterwave plate QWP3. There are two polarizer rotation directions. Fig. 7(a) shows theT _{1} for the clockwise polarizer rotation (See the inset of Fig. 7(a)). This corresponds to the polarize rotation for the difference angle (φ =ψ_{A} ψ_{P} ) measurement case shown in Figs. 4(a) and 5(a). Fig. 7(b)T _{1} for the counterclockwise polarizer rotation (See the inset of Fig. 7(b)). This corresponds to the polarize rotation for the sum angle (ψ =ψ_{A} +ψ_{P} ) measurement case shown in Figs. 4(b) and 5(b). TheT _{1} data show the oscillating behavior deviated from the constant value about 2%.The final transmittance (
T ) of the whole optical system can be considered as the multiplication of theT _{1} and the transmittance (T _{0}) of the analyzer (or analyzer + the additional quarterwave plate QWP3, when QWP3 is inserted). To remove the polarizer dependence in the final transmittance of the whole optical system the transmittanceT was divided with transmittance (T _{1}) of TNLCP. This is the corrected transmittance (T _{0} =T /T _{1}) of the optical system also the transmittance (T _{0}) of the analyzer (or analyzer + the additional quarterwave plate QWP3). Errors were negligible for the quarterwave plate QWP2 data, so the Jones matrix elements of QWP2 are obtained without the correction.V. CONCLUSION
A polarimetric method was developed for determining the Jones matrix element without the sign ambiguity. The credibility of the method was checked by comparing the theoretical and experimental Jones matrix elements of a quarterwave plate. The method was used to determine the Jones matrix elements of a twisted nematic liquid crystal panel (TNLCP) successfully. The proposed method can be used to determine the Jones matrix elements of any other transparent optical materials.

[FIG. 1.] The optical system used to measure the transmittance of a TNLCP. Azimuth angles of the polarizer and analyzer are ψP and ψA, respectively. BE: beam expander, QWP1 and QWP3: quarter wave plates, P: polarizer, LCP: liquid crystal panel, A: analyzer, L: lens, I: Iris, D: photodiode.

[FIG. 2.] Transmittance (T) of the optical system composed of a quarterwave plate (QWP2), and an analyzer. Instead of the LCP shown in FIG. 1, a quarter wave plate (QWP2) was inserted. (a) The analyzer and the polarizer rotated in the opposite directions. (b) The analyzer and polarizer rotated together. The solid circles are the experimental result, and the line is the theoretical estimation.

[FIG. 3.] Transmittance (T) of the optical system composed of a quarterwave plate (QWP2), additional quarter wave plate (QWP3), and an analyzer. Instead of LCP shown in FIG. 1, a quarter wave plate (QWP2) was inserted. (a) The analyzer and the polarizer rotated in the opposite directions. (b) The analyzer and the polarizer rotated together. The solid circles are the experimental result, and the line is theoretical estimation.

[FIG. 4.] The corrected transmittance (T0) of the optical system composed of a TNLCP, and an analyzer as shown in FIG. 1. (a) The analyzer and polarizer rotated in the opposite directions. (b) The analyzer and the polarizer rotated together. The solid circles are the corrected experimental result, and the line is the theoretical estimation.

[FIG. 5.] The corrected transmittance (T0) of the optical system composed of a TNLCP, a quarterwave plate (QWP3), and an analyzer as shown in FIG. 1. (a) The analyzer and the polarizer rotated in the opposite directions. (b) The analyzer and the polarizer rotated together. The solid circles are the corrected experimental result, and the line is the theoretical estimation.

[FIG. 6.] Uncorrected and corrected transmittance of the optical system composed of a TNLCP, and an analyzer as shown in FIG. 1. The solid asterisks are the transmittance without correction (T). The analyzer and polarizer rotated in the opposite directions. The solid circles are the transmittance (T0) of the optical system with correction, and the line is the theoretical estimation.

[FIG. 7.] Transmittance (T1) of the TNLCP for (a) the clockwise polarizer rotation and (b) for the counterclockwise polarizer rotation as illustrated in the insets. ‘D’ represents the detector.