An LCVR consists of a nematic liquid-crystal (LC) material layer sandwiched between two fused-silica substrates (illustrated in Fig. 1). The outer surfaces of both substrates are coated with an antireflective film to reduce reflection. The inner surfaces of both substrates are coated with a transparent indium tin oxide (ITO) electrode and a polyimide layer in turn. Some spacers are used to control the thickness of the liquid crystal accurately. The polyimide layer forces an initial orientation of the LC molecules in a direction parallel to that of the silica plates, which define the optics axis of LCVR. The application of a voltage through the ITO films generates a homogeneous electric field inside the cavity that forces the LC molecules to tilt to a given angle with respect to the substrate surface. Thus the birefringence index is changed, and the retardations of LCVR will vary with the voltages (as shown in Fig. 2). In other words, LCVR is an electro-optic modulator, and it can modulate the linear polarization to circular or vice versa by changing the voltage.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. The construction of LCVR.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. The relations of retardation of LCVR with voltages.

Table 1 lists the materials, refractive index, and thicknesses of the LCVR we have used in FMG, which is manufactured by Institute of Fluid Physics, China Academy of Engineering Physics. The liquid crystal molecule is a kind of mixture materials 5CB(PP5CN), taking 5CB as the main. Its molecular structure is shown in Fig. 3.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. Molecular structure of liquid crystal used in FMG.

Table 1.

Table 1.

Table 1. Materials, refractive index, and thickness of LCVR in FMG. . Material Refractive index Thickness Fused silicon no = 1.46 5 mm ITO no = 2.0 50 nm Polyimide no = 1.5 300 nm Liquid crystal no = 1.5, ne = 1.76 5 μm

Table 1. Materials, refractive index, and thickness of LCVR in FMG. .

ASO-S will be launched to a Sun-synchronous orbit (SSO) at an attitude of 720 km. The harsh space environments on the orbit are the radiation belts protons, the radiation belts electrons, solar protons, UV radiation, atomic oxygen, and space debris. It is difficult for atomic oxygen and space debris to get inside of optic box of FMG, so what we need to consider is the influence of the radiation belts protons, the radiation belts electrons, solar protons and UV radiation on LCVRs. According to the structure characteristics and operating principle of LCVR and analysis of environmental effects of space radiation, the possible impacts and dose are as follows.

2.2.4. UV irradiation

Because FMG is a solar telescope, UV radiation must be taken into account. Moreover, LC material is also sensitive to UV spectra. The ultraviolet irradiation may change the chemical properties of LCVR, leading to performance degradation. In order to reduce the influence of UV radiation and stray light on FMG, a front widow with a pass band 532.4 nm±5 nm is designed and used to prevent the light from other wavelengths entering the optical system. The transmittance of front window is given Fig. 4. There is a peak near 370 nm and 200 nm respectively, where the peak near 370 nm is a real sub-maximum from coatings of the front widow, but another one near 200 nm is from measurement error and can be ignored. Finally, the UV radiation dose of LCVR is about 2.5 × 10⁻⁴ ESH at [200 nm 360 nm] and 13.3 ESH at [360 nm 400 nm] over 11-year.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. Transmissivity of the front widow in FMG.

Table 2 lists the main radiation effects and dose we need to test in LCVR of FMG according to the above analysis. Three kinds of radiation resources, λ radiation resource and high-energy proton and UV lamps, were used to done the test for total ionization dose effect, displacement damage effect, and UV irradiation respectively.

Table 2.

Table 2.

Table 2. The main radiation effects and dose of LCVR in FMG. . Radiation effects Dose Total ionization dose 20.7 krad[Si] Displacement damage 5.3 × 108 MeV/g or 21.2 × 1010 p/cm2 UV radiation 2.5 × 10−4 ESH @[200 nm 360 nm] 13.3 ESH @[360 nm 400 nm]

Table 2. The main radiation effects and dose of LCVR in FMG. .

A total of twelve identical LCVRs were used for irradiation testing: four LCVRs for λ irradiation (LCVR#1–LCVR#4), three for proton irradiation (LCVR#5–LCVR#7), and four for UV irradiation (LCVR#8–LCVR#11). The remaining one sample (LCVR#12) was designated as controls and was not exposed to any irradiation but was shipped between locations together with the irradiation samples to make their history as similar as possible. Besides, seven identical liquid crystal (LC) materials were exposed to λ irradiation to test the material’s chemical properties.

To verify the fulfilment of LCVRs requirements for FMG the following seven key parameters were measured in irradiation experiments: (i)

Retardation versus voltage;

The first four parameters represent the optical properties of LCVRs and the last three represent the chemical properties of LC material we used.

All the parameters were not measured in situ. In each irradiation, parameters (i)–(iii) had to be measured during all the intermediate doses, the time-lapse from the start of the optical parameters measurements to the start of the next irradiation exposure (another intermediate doses) is limited to one hour. By contrast, parameter (iv) was measured only before and after each irradiation test and parameters (iv)–(vii) measured only after γ irradiation test.

The experimental setups employed to measure all the optical parameters included an automatic retardation-voltage measurement system that can measure retardation–voltage curves and response time, a transmission measuring system with fiber-spectrometer and a Zygo interferometer. Response time was required to measure at 35°C where LCVRs work in FMG. The other parameters were measured in room temperature.

The LCs samples after γ irradiation were measured by Xi’an Modern Chemistry Research Institute to test the chemical change of LCVRs, including clearing point and birefringence and ion concentration.

It is noted that, a simple, fast but robust method was applied to measure retardation–voltage curves in the automatic retardation–voltage measurement system as shown in Fig. 5. The transmission directions of both polarizers are parallel, and the optic axis of LCVR is set at 45° relative to the polarizer. The retardation, corresponding to each driving voltage, is related to the detection intensity as I_V = I_ocos²(δ/2). Thus, retardation–voltage curves will be easy to calculate. In addition, we have carried out a lot of stability tests and comparison with the Soleil compensation method in the laboratory, and made sure the measurement accuracy within ±1°, meeting the requirements of this irradiation tests.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. An automatic retardation–voltage measurement system.

The γ irradiation was tested in Peking University (Fig. 6(a)). A ⁶⁰Co source was used at atmospheric pressure and room temperature. The samples under irradiation included 2 LCVRs with derived voltage during irradiation, 2 LCVRs without derived voltage, and 6 homologous LC materials. Besides, another LC material was used as a reference without irradiation. The samples were undergone to 6 intermediate doses in order to assess the tolerance of the LCVRs to ionizing radiation. Table 3 lists the intermediate doses and dose rates for all the samples received, where “Y” indicates the sample underwent the corresponding intermediate dose point, but “N” not.

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. Radiation test pictures: (a) γ irradiation; (b) proton irradiation; (c) UV irradiation.

Table 3.

Table 3.

Table 3. The intermediate doses and dose rates for all the samples received in γ irradiation. . Dose/krad Dose rate/(krad/h) Samples LCVR#1 without voltage LCVR#2 without voltage LCVR#3 with voltage LCVR#4 with voltage LC#2 LC#3 LC#4 LC#5 LC#6 LC#7 4 5 Y Y Y Y Y Y Y Y Y Y 10 5 Y Y Y Y N Y Y Y Y Y 20 5 Y Y Y Y N N Y Y Y Y 60 36 Y Y Y Y N N N Y Y Y 130 5 Y Y Y Y N N N N Y Y 500 180 Y N Y N N N N N N Y

Table 3. The intermediate doses and dose rates for all the samples received in γ irradiation. .

The proton irradiation was tested at 100-MeV compact cyclotron (CYCIAE-100) in China Institute of Atomic Energy (Fig. 6(b)). The samples under irradiation include two LCVRs without derived voltage and one with derived voltage. Table 4 lists the intermediate doses and dose rates for all the samples received as Table 3.

Table 4.

Table 4.

Table 4. The intermediate doses and dose rates for all the samples received in proton irradiation. . Dose/(1010 p/cm2) Dose rate/(107 p/cm2⋅s) Samples LCVR#5 without voltage LCVR#6 without voltage LCVR#7 with voltage 5 2 Y Y Y 10 2 Y Y Y 20 2 Y Y Y 50 20 Y Y Y 100 20 Y N Y

Table 4. The intermediate doses and dose rates for all the samples received in proton irradiation. .

The UV irradiation was tested in Harbin Industrial University (as shown in Fig. 6(c)). The three LCVRs are irradiated at a vacuum ultraviolet irradiation setup in the spectral range of [200 400] nm. However, it is known from Table 2 that the UV radiation of FMG may mainly come from the spectra range of [360 400] nm. Therefore, another LCVR is irradiated by a narrowband UV light with center wavelengths at 365 nm to verify its characteristics.

Table 5.

Table 5.

Table 5. The intermediate doses and dose rates for all the samples received in the vacuum ultra-violet irradiation setup at the spectral range of [200 400] nm. . Dose/ESH Dose rate/(ESH/h) Samples LCVR#8 without voltage LCVR#9 without voltage LCVR#10 without voltage 0.25 3 Y Y Y 0.50 3 Y Y Y 0.75 3 Y Y Y 1.00 3 Y Y Y 2.00 3 Y Y Y 5.00 3 Y Y Y

Table 5. The intermediate doses and dose rates for all the samples received in the vacuum ultra-violet irradiation setup at the spectral range of [200 400] nm. .

Table 6.

Table 6.

Table 6. The intermediate doses and dose rates of sample received in narrow band UV light with centre wavelengths at 365 nm. . Dose/ESH Dose rate/(ESH/h) Sample LCVR#11 without voltage 2.8 8.5 Y 5.6 8.5 Y 8.5 8.5 Y 11.3 8.5 Y 14.1 8.5 Y 16.9 8.5 Y

Table 6. The intermediate doses and dose rates of sample received in narrow band UV light with centre wavelengths at 365 nm. .

Tables 5 and 6 list the intermediate doses and dose rates for all the samples received in both UV irradiation tests.

Figure 7 gives the optical retardation variation, relative to those before tests, versus applied voltage measured for all the 11 LCVR samples irradiated. No significant changes were found after irradiations except for proton irradiation.

Fig. 7.

	Figure Option ViewDownloadNew Window
	Fig. 7. Retardation variation curves versus applied voltage for all the 11 samples irradiated. In the figure, the abbreviation of number 5E10 means 5 × 1010.

In order to analyze these changes in detail we got the retardation variation curves with irradiation dose for every sample by only extracting one voltage point from each sample. The voltage value chosen corresponds to the retardation about 360°, which is the most sensitive position to irradiation over the retardation range of 90°–360° and can completely represent the trend of the whole retardation–voltage curve we need. The results are shown in Fig. 8, where the first column indicates the retardation variation and the second column indicates the corresponding variation rates. It should be noted that the logarithmic coordinate (x axis) is selected in the first three rows for the sake of clear display.

Fig. 8.

	Figure Option ViewDownloadNew Window
	Fig. 8. Retardation variation with irradiated dose. In the figures, “Pre” means before the irradiation test and “Aft” means after the irradiation test.

The response time is defined as the time needed for the LCVR to reach a second polarization state defined by voltage applied to the LCVR V₁ from the first polarization state defined by voltage V₂.

In irradiation testing, the exposure time is 1 ms; V₁ and V₂ are set in 2 V and 5 V respectively, which corresponds to the retardation about 360° and 90°. The measured results are shown in Fig. 9.

Fig. 9.

	Figure Option ViewDownloadNew Window
	Fig. 9. Response time versus irradiated dose.

It is observed that: the rising time from V₁ to V₂ (< 32 ms) is significantly less than the falling time from V₂ to V₁ (< 50 ms). There are three typical response times found in the measurements, 16 ms, 32 ms, and 47 ms. They are almost multiple relationships, which are very interesting phenomena that deserve further study. However, there was no obvious change in response time before and after the irradiation test. The response time is less than 50 ms, meeting the scientific requirements of FMG payload.

Figure 10 represents the transmittance of each sample versus irradiated dose. The reason why the transmission oscillates with wavelength mainly comes from the mismatch of the refractive index of the TIO film and polyimide in LCVR. There is no obvious degradation during irradiation tests.

Fig. 10.

	Figure Option ViewDownloadNew Window
	Fig. 10. Transmittance versus irradiated dose.

The main results for wavefront error of all the samples can be seen in Table 7. There is no regular change, and the largest variation before and after irradiation tests is 0.089λ for PV and 0.013λ for RMS.

Tables 8 and 9 display the results of chemical properties measured by Xi’an Modern Chemistry Research Institute, including clearing point, birefringence, and ion density. The clearing point is defined as a temperature at which the liquid crystal material is transparent during the process of changing from liquid crystal state to isotropic liquids state, representing the chemical performance; the birefringence index Δn represents the polarization properties; and the ion density represents the possible variation of voltage added to LCVR. The main results from Tables 8 and 9 are the following ones by comparing with reference sample LC#1: i) There is no obvious change for clearing point and birefringence index Δn before and after λ irradiation; ii) The ION almost has no significant variation below 60 krad, but begin to increase dramatically above 60 krad.

Table 7.

Table 7.

Table 7. The wavefront error of all the samples before and after irradiation tests. . Samples Before After Variation PV(λ) RMS(λ) PV(λ) RMS(λ) PV(λ) RMS(λ) LCVR#1 0.220 0.020 0.114 0.016 −0.106 −0.004 LCVR#2 0.120 0.015 0.128 0.023 0.008 0.008 LCVR#3 0.258 0.033 0.169 0.021 −0.089 −0.012 LCVR#4 0.119 0.020 0.080 0.014 −0.039 −0.006 LCVR#5 0.177 0.033 0.150 0.025 −0.027 −0.008 LCVR#6 0.135 0.040 0.172 0.027 0.037 −0.013 LCVR#7 0.252 0.050 0.227 0.054 −0.025 0.004 LCVR#8 0.096 0.013 0.086 0.015 −0.010 0.002 LCVR#9 0.110 0.017 0.093 0.011 −0.017 −0.006 LCVR#10 0.149 0.029 0.151 0.033 0.002 0.004 LCVR#11 0.206 0.034 0.184 0.022 −0.022 −0.012

Table 7. The wavefront error of all the samples before and after irradiation tests. .

Table 8.

Table 8.

Table 8. The clearing point and birefringence Δn of LCs after γ irradiation. . Samples Dose/krad Clearing point (start/peak)/°C Δn @25°C, 589 nm LC#1-ref 0 82.90/83.62 0.2451 LC#2 4 81.89/84.37 0.2451 LC#3 10 82.81/83.69 0.2454 LC#4 20 82.59/83.46 0.2453 LC#5 60 82.67/83.52 0.2453 LC#6 130 82.58/83.62 0.2452 LC#7 500 82.37/83.52 0.2452

Table 8. The clearing point and birefringence Δn of LCs after γ irradiation. .

Table 9.

Table 9.

Table 9. The ion density of LCs measured after λ irradiation. . Samples Dose/krad ION LC#1-ref 0 27.66 LC#2 4 27.18 LC#3 10 28.02 LC#4 20 28.52 LC#5 60 27.15 LC#6 130 31.29 LC#7 500 37.57

Table 9. The ion density of LCs measured after λ irradiation. .

For optical properties of LCVR, error requirements for LCVR are listed in Table 10 in order to meet the scientific goals of FMG payload. It is easy to find out that the main error comes from retardation change. Although there was no drastic change in the retardation versus voltage during three irradiation processes, the amount of retardation change was still much higher than the instrument requirements. It has to be taken into account that an in-flight retardation versus voltage calibration method should be added in FMG payload, reducing or even avoiding the impact of retardation change.

Table 10.

Table 10.

Table 10. Error requirements for LCVR in FMG payload. . Parameter Requirement Retardation change ≤ 0.6% Response time ≤ 50 ms Wavefront error variation ≤ 0.025λ (RMS) Transmission change ≤ 10

Table 10. Error requirements for LCVR in FMG payload. .

For chemical properties of LCVR, the measured results in Tables 8 and 9 show that we do not need to care about LC’s chemical changes below 60-krad λ-irradiation doses.