光学仪器  2018, Vol. 40 Issue (1): 49-54   PDF    
基于电压影响下圆偏振激光束的特性
郝蕊蕊1,2,3, 赵新未1,2,3, 潘倩1,2,3, 柏雪1,2,3, 焦新兵1,2,3     
1. 上海理工大学 光电信息与计算机工程学院, 上海 200093;
2. 上海理工大学 上海市现代光学系统重点实验室, 上海 200093;
3. 上海理工大学 教育部光学仪器与系统工程研究中心, 上海 200093
摘要: 采用狭缝光束分析仪和偏振仪测量了从铝薄膜表面反射的圆偏振光的光斑位置和偏振特性随外界直流电压变化情况,理论拟合激光从铝薄膜反射后的光斑位置与直流电流电压的关系。测量和多项式拟合结果表明,当外部电压值从0变化到2.5 V时,圆偏振激光在x轴和y轴上的位置分别是从-45 μm移动到-95 μm,从35 μm移动到75 μm,而激光束的光学偏振度基本上不变化。
关键词: 圆偏光     直流电流     位移     斯托克斯参数     光斑    
Characteristics of circularly polarized laser beam based on voltage
Ruirui HAO1,2,3, Xinwei ZHAO1,2,3, Qian PAN1,2,3, Xue BAI1,2,3, Xinbing JIAO1,2,3     
1. School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China;
2. Shanghai Key Laboratory of Modern Optical System, University of Shanghai for Science and Technology, Shanghai 200093, China;
3. Engineering Research Center of Optical Instruments and Systems(MOE), University of Shanghai for Science and Technology, Shanghai 200093, China
Abstract: Here, optical spot position and the polarization properties of a circularly polarized laser beam reflected from an aluminum film as a function of the external voltage are measured by a slit beam profiler and a polarimeter.The relation between the spot position of laser reflected from aluminum film and the direct current and voltage is theoretically fitted.The measured and polynomial fitting results show that the x-and y-optical position of circularly polarized laser moves from -45 μm to -95 μm and from 35 μm to 75 μm, repectively, when the external DC value changes from 0 to 2.5 V, while the degree optical polarization of the laser beam basically do not change.
Key words: circularly polarized laser beam     direct current     displacement     Stokes parameter     optical spot    
Introduction

Garnet is widely researched and used because of its excellent optical, electrical and magnetic properties[1-4].The polarization plane of a linearly polarized laser beam can be rotated through garnet or reflected from the garnet surface.The garnet surface is linearly proportional to the magnetic field in the laser propagation direction, known as the Faraday effect and the Kerr effect.In our previous work, the interfaces of garnet and graphite can be generated as a function of external direct current(DC) voltage, and the splitting effect is investigated[5-9].The changes in optical properties of a circularly polarized laser beam attribute to external conditions.In this paper, the optical properties of a circularly polarization laser beam reflects from a aluminum film, one weakly absorbing media, are researched.The optical position and optical polarization properties of the laser as a function of external DC voltage are researched by slit beam profiler and polarimeter.The optical shift of the circularly polarized laser beam and polarization properties imply a good approach for the design and analysis of optical devices.

1 Experimental principle

Figure 1 is a schematic diagram of the experimental setup.A continuously laser beam with a wavelength of 1 550 nm, with a intensity of 18 mW, goes through Thorlabs mounted zero order compound WPQ10-1550 wave plate, then turns into circularly polarized laser.The circularly polarized laser beam goes into a prism vertically, and reflects from an aluminum film on the oblique side of the prism.After that the circularly polarized laser beam arrives at the Thorlabs CCM1-BS015 non polarized beam splitter.The laser beam is splitted into two optical laser beams, one beam arrives at the Thorlabs BP209-IR slit beam profiler and another one arrives at the Thorlabs PAX5710 polarimeter.Garnet(Granopt Co., Ltd., GLB1550, 2 mm×2 mm×0.39 mm) and graphite(10 mm×10 mm×0.3 mm) are fixed under the aluminum film of the prism.A pair of copper electrodes are in contact with graphite at a distance of 1 mm.The polarization characteristics of the laser beam are measured by applying a DC voltage to the copper electrode.In order to character the optical spot conveniently, it is assumed that, the z axis is in the perpendicular direction of the garnet, the y axis is in the plane of the garnet and along the propagation direction of the laser beam and the y axis is perpendicular to the direction of the propagation direction of the laser beam.The optical spot energy distribution of the circularly polarized laser beam are measured by Thorlabs BP209-IR slit beam profiler.

Figure 1 Schematic of the experimental setup
2 The change of optical position

To evaluate the optical spot position of the laser beam, which reflects from the aluminum film on the prism, the optical spot position and optical spot energy distribution of the circularly polarized laser beams as a function of external DC must be measured.In this experiment, the position of the maximum energy of the circularly polarization laser beam is used to describe the position of the optical spot, as the initial state of 2-dimensional and 3-dimensional optical spot energy show Gaussian distribution shown in Fig. 2.

Figure 2 2-dimensional and 3-dimensional optical spot energy distribution

As the optical spot position can be described by the maximum energy of the circularly polarized laser beam.Figure 3 shows the measuring and polynomial fit of the optical spot position when the external DC value changes from 0 to 2.5 V.The x position of the circularly laser beam moves from -45 μm to -95 μm when the external DC voltage increases from 0 to 2.5 V as shown in Fig. 3(a).The polynomial fit fomular of the x position of the circularly laser beam can be described by equation (1).A1, B11, B12 are constant parameters and the parameter values are shown in table 1.

(1)
Figure 3 Measuring and polynomial fit of optical position of the circularly polarized laser beam as a function of external DC voltage

Table 1 The parameter values of the polynomial fit formula of x position

The y position of the circularly laser beam moves from 35 μm to 75 μm when the external DC voltage increases from 0 to 2.5 V as shown in Fig. 3(b).The polynomial fit of the y position of the circularly laser beam can also be described by equation (2).A2, B21, B22 are constant parameters and the parameter values are shown in table 2.

(2)
Table 2 The parameter values of the polynomial fit formula of y position

From table 1 and table 2, the value of B11 is more than zero, and the value of B21 is less than zero, so it can be seen that the x position of the circularly polarized laser beam moves toward to the x negative axis, while the y position of the circularly polarized laser beam moves toward to the y positive axis.So the optical spot position of the circularly polarized laser beam can be controlled by external DC voltage, as shown in Fig. 3.

Figure 4 shows the optical energy distribution of the circularly polarized laser beam at different external voltage.The optical energy intensity in the x position increase from 63.7% to 76.8%, when the voltage increases from 0 V to 2.5 V, while the optical energy intensity in the y position decrease from 50% to 46.6%.So, the optical energy intensity in the x position and y position have different change trend when the external voltage change.

Figure 4 Optical energy distribution of the circularly polarized laser beam at different external voltage
3 Polarization properties of the circularly laser beams

The optical spot positions and the optical energy in the x position and the y position of the circularly polarized laser beam show different trends when the external DC voltage value increases from 0 V to 2.5 V.The optical polarization properties of the circularly polarized laser beam also be researched as a function of the external DC voltage value.The degree of polarization may change on propagation precessure in free space[10-11] and dependent on the spatial location[12-14].Figure 5 illustrates the relationship between the Stokes parameter S1, S2, S3, DOP of the circularly polarized laser beam and the external DC voltage value.The Stokes parameters of S1, S2, and S3 can describe all the polarization states of electromagnetic waves[15-16].S1 is the intensity difference between horizontally polarized laser beam and vertically polarized laser beam, S2 is the intensity difference between +45°polarized laser beam and -45°polarized laser beam, and S3 is the intensity difference between right-circularly polarized laser beam and left-circularly polarized laser beam.The degree of linear polarization(DOLP), degree of circular polarization(DOCP) and degree of polarization can be described by Stokes parameters:

(3)
(4)
(5)
Figure 5 Polarization properties of the circularly laser beams reflect from aluminum film on the prism as a function of DC voltage

The Stokes parameters S1 of the circularly polarized laser beam is 0.6, and basically unchanged when the the external DC is less than 1.0 V, then, the S1 changes from 0 to 0.6 when the external DC voltage change from 1.0 V to 2.5 V.The Stokes parameters S2 of the circularly polarized laser beam changes from 0 to -0.9, when the external DC voltage change from 0 V to 1.5 V, while, the variation range of S2 is small.The Stokes parameters S3 of the circularly polarized laser beam decreases from 0.8 to 0, when the external DC voltage change from 0 V to 2.5 V.The DOLP, DOCP and DOP of the circularly polarized laser beam can be got from equations (3), (4), (5).The DOLP and DOCP can be changed, while the DOP basically unchange, when the external DC voltage changes from 0 V to 2.5 V.

4 Conclusion

In conclusion, optical spot position and the polarization properties of circularly polarized laser beam reflected from an aluminum film as a function of the external direct current voltage are measured by a slit beam profiler and a polarimeter.The measured and polynomial fit results show that the x and y optical position of circularly polarized laser moves from -45 μm to -95 μm and moves from 35 μm to 75 μm.The optical energy intensity in the x position and y position have different change trend when the external DC value changes from 0 V to 2.5 V.The Stokes parameter of S1, S2, and S3 can be changed, while the DOP of the laser beam basically do not change.The optical shift of the circularly polarized laser beam and polarization properties imply a good approach for the design and analysis of optical devices.

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