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Proton irradiation of liquid crystal based adaptive optical devices
Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Proton irradiation of liquid crystal based adaptive optical devices E.J. Buis a,⇑, G.C.G. Berkhout a,b, G.D. Love c, A.K. Kirby c, J.M. Taylor c, S. Hannemann d, M.J. Collon d a
cosine Science & Computing BV, Niels Bohrweg 11, 2333 CA Leiden, The Netherlands Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands c Department of Physics, Durham University, South Road, Durham DH1 3LE, UK d cosine Research BV, Niels Bohrweg 11, 2333 CA Leiden, The Netherlands b
a r t i c l e
i n f o
Article history: Received 19 October 2010 Received in revised form 8 September 2011 Available online 1 October 2011 Keywords: Liquid crystal Proton irradiation Index of refraction
a b s t r a c t To assess its radiation hardness, a liquid crystal based adaptive optical element has been irradiated using a 60 MeV proton beam. The device with the functionality of an optical beam steerer was characterised before, during and after the irradiation. A systematic set of measurements on the transmission and beam deflection angles was carried out. The measurements showed that the transmission decreased only marginally and that its optical performance degraded only after a very high proton fluence ð1010 p=cm2 Þ. The device showed complete annealing in the functionality as a beam steerer, which leads to the conclusion that the liquid crystal technology for optical devices is not vulnerable to proton irradiation as expected in space. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystal (LC) devices can be used as phase and polarization modulators, as well as their more traditional use in displays as intensity modulators. LC phase modulators most commonly use the nematic phase with homogeneous or anti-parallel alignment, in which there is bulk alignment of the LC molecules producing an approximately uniaxial birefringent medium. The application of an electric field of appropriate frequency induces a dipole moment in the LC molecules and a resulting molecular torque. The equilibrium orientation of the molecules and thus of the optic axis of the bulk LC medium depends on the applied electric field. If the polarisation of the incident light is aligned with the extraordinary axis of the LC material, the rotation of the optic axis changes the effective refractive index of the LC material and therefore the effective optical thickness of an LC device can be varied by the application of an external field. This principle has been applied in a variety of applications involving controllable optics including lenses [1–5], adaptive optics [6,7], optical tweezers [8], variable axicons [9], tunable filters [10] and beam steerers [11,12]. Of the above mentioned applications in particular tunable filters and beam steerers could be of interest for space applications. Tunable filters may be applied in remote-sensing spectrometers or spectral imagers. The filters can then be used to dynamically select a small pass band (spectrometers) or a wide pass-band around the red, green and blue colours (spectral imagers). Optical beam steering may be applied ⇑ Corresponding author. Present address: TNO, P.O. Box 155, 2600 AD, Delft, The Netherlands. E-mail address: [email protected] (E.J. Buis). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.09.021
in laser altimetry or free space optical communications, where active steering of laser light is required. Typically, monochromatic lasers are used in the visible or the near-infrared wavelength ranges. Low operating voltages, a lack of moving mechanical parts, simplicity of operation, good physical robustness and relatively low cost of production make LC devices attractive for use in space research. However, any device operating in space is subjected to a relative harsh radiation environment. Space instrumentation will receive a mixture of high energy radiation, such as protons, gamma rays and X-rays. In order to test the suitability of LC based optics for operation in space, their radiation tolerance has to be assessed. In a previous study the radiation hardness w. r. t. gamma’s, X-rays and fast neutrons has been assessed in which only modest space radiation effects were found [13,14]: liquid crystals were irradiated using a.o. gamma’s from a 60 Co source up to a total dose of 2.2 Mrads with no measurable degradation. Proton irradiation effects are in general different from the ionizing effects from X-ray or gamma ray irradiation as protons can break up molecules and hence significantly alter material properties. Because protons are largely abundant in the ‘van Allen radiation belts’ or in solar proton events, space qualification of instrumentation should include the assessment of proton irradiation damage. In this paper we present the results of proton irradiation of LC beam steerer devices. Because of the similarity of the structure of the LC devices in general, it is expected that results on the radiation hardness of the tested devices as presented here, apply for other LC devices as well. Before describing the experimental setup and the results of the measurements in Section 4, we first discuss the structure of the LC
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E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
beam steerer devices in Section 2 and the proton irradiations and the proton beam in Section 3. We conclude in Section 5.
2. Liquid crystal based adaptive optics Fig. 1 shows the structure of the LC beam steerer, which in itself is a novel design. A tilt is induced between the upper and lower substrates by using different sized spacers on each side of the cell. This structure effectively produces a prism in which the refractive index and thus the effective tilt angle can be changed by the application of an external voltage. The design is similar to the modal prism [11] in which case the prism is induced by distributing a voltage ramp across the device. That has the advantage that the induced beam steering can be in 2 directions (positive and negative), but the applied voltage must be limited so that the device is operated in the quasi-linear part of the electro-optical response of the LC. The advantage of the design shown here is that the control is simpler as only a uniform voltage needs to be applied across the cell and the full electro-optical response of the LC can be utilised. This must be balanced against the drawback that the device can only deflect light on one direction. From the point of view of space-based applications the design described here has a much lower power consumption which is very important. Both glass substrates are coated with a transparent Indiumdoped tin-oxide layer (ITO) which acts as an electrode. On top of the ITO layer a rubbed polyimide layer is placed to align the LC. A voltage signal is applied between the two substrates which creates a refractive index change in the cell, causing the direction of the refracted beam to change. The beam steerer is driven with AC voltage with a frequency of f ¼ 1 kHz and an amplitude of V ¼ 0 20 V peak-to-peak. The steering angle a is determined by the wedge angle ¼ arctan ðDt=wÞ and the refractive index n of the liquid crystal material. Here Dt is the difference in spacer thickness and w is the width of the cell as shown in Fig. 1.
a ¼ ðn 1Þ
Dt ðn 1Þ w
ð1Þ
where the approximation holds for small angles. E7 was chosen as the liquid crystal material. The material, which is manufactured by Merck under the LiCrystal brand is a mixture of 5CB (40 -n-pentyl-n-cyanobiphenyl) and 7CB (40 -n-heptyl-n-cyanobiphenyl). For this specific design Dt ¼ 44 lm;w ¼ 12 mm and 1:521 6 n 6 1:746, resulting according to Eq. (1) in a steering angle range of 1:9 6 a 6 2:7 mrad. Note that the maximal steering angle is obtained when no bias voltage is applied and the minimal angle when the voltage is set 20 V.
LC glass ITO t spacer PI
w Fig. 1. Schematic drawing of a wedged beam steerer, indicating the parameters (t the thickness and w the width) that determine the beam steering properties of the device. The space between the substrates is filled with LC material. The vertical scale is strongly exaggerated (i.e. t w). As mentioned in the text the glass substrate are coated with ITO and polyimide.
3. Proton irradiation The proton irradiation was carried out at KVI [15] (Dutch Institute for proton and heavy ion accelerators for nuclear physics), where a proton beam with an energy of 190 MeV was extracted from the AGOR cyclotron [16]. A 96 mm aluminum degrader is placed between the end of the beam pipe and the irradiated device to tune the energy of the protons to 60 MeV. The choice of this particular proton energy is driven by both the satellite shielding and the expected spectrum of the proton flux. A typical satellite shield thickness of 2–3 g/cm2 is fully penetrated if the energy of the proton is above 40–50 MeV. On the other hand, the fluence of protons in Solar Energetic Protons (SEP) events drops rapidly at energies higher than 90–100 MeV. Hence, the energy of the proton should lie between 50 and 100 MeV. Note that protons with an energy in this range will not be stopped completely in the beam steerer device. Although damage due to protons is expected to be large when the protons are completely stopped, the probability of a Bragg peak in the LC cell is low as its thickness is small. A typical solar proton event at 1 AU has a total fluence between 107 and 108 protons/cm2 integrated over all energies above 10 MeV. However, large events, albeit rarely, have been recorded at 1 AU with proton fluences above 1010 protons/cm2 [17]. The irradiation was carried out in several steps. In this way a possible breakdown dose can be determined. In Table 1 we list the various irradiation steps and the relevant parameters. The measurements in between the irradiation steps took about 30 to 45 minutes, so that the complete irradiation cycle was carried out on the very same day. The systematic error on the proton fluence is estimated to be less than 3%. This estimation is based on the homogeneity of the beam, which is better than 2% and the absolute dose accuracy of 2% [16]. 4. Experimental setup and measurements A beam steerer device was constructed as described in Section 2 and a photograph of the device is shown in Fig. 2. For ease, a dedicated mechanical support was designed and manufactured to fit the delicate glass substrates in to the two inch. mounts in the optical setup. As mentioned above, the proton irradiation was divided into several steps. After each irradiation step, two measurements were carried out to characterise the device, a transmission measurement and a beam deflection angle measurement. 4.1. Transmission The transmission was measured using a broadband light source, an integrating sphere and a spectrometer. The experimental setup Table 1 Proton beam parameters during irradiation. Irradiation step
Energy [MeV]
Duration [s]
Fluence [p/ cm2]
Integrated [p/ cm2]
1
60
100
1 108
1 108
2
60
100
1 10
8
2 108
3
60
100
3 108
5 108
4
60
167
5 10
8
1 109
5
60
100
2 109
3 109
6
60
100
3 10
9
6 109
7
60
100
4 109
8
60
100
2 10
10
3 1010
9
60
150
3 1010
6 1010
10
60
200
10
1 1011
4 10
1 1010
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Fig. 2. Photograph of the LC beam steerer (a). The effective size of the device is 12 12 mm2 . In (b) a mechanical support for a 2 in. mount is shown.
white light source
compact spectrometer polarizer
LC−element
integrating sphere
fiber
Fig. 3. Setup to measure the transmission of the beam steerer.
(a)
(b)0.95
1
0.9
0.9
transmission
transmission
0.8 0.7 beamsteerer:
0.6
pre-irrad
0.5
glass:
1e10
0.4
pre-irrad
6e10
0.3
post-irrad
1e11
0.85 0.8 0.75 wavelength range
0.7
600-1000 nm @ 20V
0.2 0.65
0.1 0 300
600-1000 nm @ 0V
0.6 400
500
600
700
800
900
1000
wavelength [nm]
300-1000 nm @ 0V 300-1000 nm @ 20V
9
10
10
10
1011
proton fluence [cm-2]
Fig. 4. (a) Transmission of the liquid crystal cell as a function of the wavelength at various levels of irradiation. Indicated are the transmission of the complete device (black) and the transmission of two glass substrates (grey), which is calculated from the measured transmission of a single glass substrate. (b) The average transmission of a complete device as a function of the received dose. The transmission is averaged over two spectral ranges.
is schematically depicted in Fig. 3. The light source in this experiment is a combined Deuterium and Halogen light source (Avantes AvaLight DHS). An integrating sphere is used to collect all transmitted light, which is analysed using an Avantes AvaSpec spectrometer. The determination of the transmission consists of three individual measurements of the background (no light source and no sample), a reference signal (light source switched on, no sample) and a transmission measurement (light source switched on and sample in the light path). In Fig. 4 we plot the results of the transmission measurements. (a) shows the transmission as a function of the wavelength after various received fluences. One can see that the transmission of the sample is reduced by about 10% at the highest dose. This is becomes clear in Fig. 4(b), where the transmission averaged over two spectral ranges (300–1000 nm and 500– 1000 nm, resp.), is plotted as function of the received dose. In addition, Fig. 4(b) shows that when a bias voltage is applied, the index
of refraction of the liquid crystal is changed and consequently the transmission of the device. In addition to the transmission of a complete device, a single piece of glass (fused silica), coated with ITO was irradiated to verify if the a loss in transmission was due to the glass material. Note that the glass material was not selected for its radiation hardness and that more radiation resistant glasses are available. A single piece of glass was irradiated with the highest proton flux (i.e. 1 1011 p=cm2 ) and its transmission was determined before and after irradiation. In Fig. 4(a) the transmission of two stacked glass substrates (calculated from the measurements on a single substrate) is plotted next to the transmission of a complete device. Note that for wavelengths above 400 nm the transmission of a complete irradiated device is higher than the transmission for an irradiated empty cell, which is due to a better matching of the refractive indices in a filled cell.
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E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
(a)
(b) Z [wavelengths]
Z [wavelengths]
20 10 0
20 10 0
-10
-10
-20 Y 2 po sit 1 ion 0 [m m] -1
-20 Y 2 po sit 1 ion 0 [m m] -1
2 [mm]
0 -2
X
ion posit
2 [mm]
0 -2
X
ion posit
Fig. 5. (a) Wavefront images of the light path leaving the beam steerer which was kept under a bias voltage of 0 V (a) and 20 V (b).
After irradiation the device is left to anneal at room temperature for a month and was re-measured using the same setup. There was no recovery of the transmission.
2.8 fluence [p/cm2 ] 1e11
2.6
1e10
1.75
1.7
1e9
4.2. Beam deflection angle The beam deflection angle was determined using a wavefront sensor. A helium–neon laser ðk ¼ 633 nmÞ and a collimating lens are used to create a flat reference wavefront on a Thorlabs ShackHartmann wavefront sensor. The RMS wavefront error of the flat wavefront is less than k=10. The wavefront sensor was aligned perpendicular to the collimated beam and subsequently the beam steerer was placed in front of the sensor. In this way the wavefront was measured at various voltages applied to the beam steerer. Examples of wavefront images are depicted in Fig. 5. To characterise these wavefront images, Zernike polynomials were fitted to the data. The resulting Zernike polynomials Z 11 and Z 1 1 correspond to a tilt about the x- and y-axes, respectively, and are directly related to the steering angle of the device. In the measurements described here the beam steerer was aligned such that they tilt the beam about the y-axis and
a ¼ arctan
2k
q
Z 1 1
2k
q
Z 1 1
ð2Þ
where q ¼ 3:0 mm is the diameter of the pupil on which the Zernike polynomials are defined and the approximation again holds for small angles. The deflection angle a as it is derived from the Zernike coefficients using the above equation and is shown in Fig. 6 as a function of the bias voltage. The device seems to be unaffected by proton irradiation up to a fluence of 1 1010 p=cm2 , after which the power to steer the beam is much reduced. Note that at low fluences the beam steering angle is close the expected values as mentioned in Section 2. Using Eq. (1) one can determine the index of refraction of the device. The difference between the index of refraction ðDnÞ at a bias voltage of 0 and 20 V as determined using the measurement of the beam deflection angle is about Dn ¼ 0:21 when the device is not damaged by the proton irradiation. At large proton fluence the difference in index of refraction drops to Dn ¼ 0:11. The calculation of Dn can also be carried out using the results of the transmission measurement by applying the Fresnel equations for transmittance. In that case Dn ¼ 0:27 for unirradiated devices, while the value for a damaged device would be Dn ¼ 0. Since the latter value deviates from the results from the deflection angle measurements it is clear that the transmission is not only affected
α [mrad]
2.5
1e8 unirradiated/annealed
2.4
1.65
2.3 2.2
1.6
2.1
index of refraction
2.7
1.55
2
1.9 1.8
1.5 0
2
4
6
8
10 12 V [V]
14
16
18
20
Fig. 6. Measurements of the deflection angle of the beam steerer device. The curve for the annealed device is also shown.
by the change in the index of refraction, by the absorption of light in the device as well. Also shown in Fig. 6 is the beam deflection angle of the device, which was left to anneal during a month at room temperature. The device had completely recovered and shows beam deflection angles similar to the pre-irradiation values. 5. Conclusions In this paper we have shown the results of the assessment of the radiation hardness of liquid crystal adaptive optics. We have carried out proton irradiation on both a fully assembled beam steerer device and separate glass substrates. The measurements which were carried immediately after the irradiations, showed that the optical properties of the beam steerer only degraded significantly after a high proton fluence: from beam deflection measurements it was inferred that the index of refraction of the liquid crystal was affected after a fluence of about 1010 p=cm2 . After annealing the beam deflection angles were recovered to pre-irradiation values. Transmission measurements showed a drop in transmission was about 10% after the highest received dose, which could not be explained by the change in the index of refraction of the liquid crystal alone. It is therefore concluded that other effects, such as absorption in the glass substrate, contribute. No recovery in the transmission was observed after annealing.
E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
The overall conclusion is that, given the observation that the performance of the measured device only decreased after a proton fluence which is rarely observed in space, optical devices based on liquid crystals are well suited for beam steering applications in space with respect to the possible damage to protons. The drop is transmission might induce a risk, when liquid crystals are applied in optical devices in which transmission is a critical item, such as in tunable filters. That risk could be mitigated by using more radiation tolerant glass substrates. Acknowledgements This work was supported by the European Space Agency Contract AO/1-5476/07/NL/EM. We thank Alex Short and Ilias Manolis at ESA for their help and advice. We thank Emiel van der Graaf and Reint Ostendorf for their support at KVI during the irradiation campaign. References [1] S.T. Kowel, D.S. Cleverly, P.G. Kornreich, Focusing by electrical modulation of refraction in a liquid crystal cell, Appl. Opt. 23 (1984) 278–289. [2] A.F. Naumov, M.Y. Loktev, I.R. Guralnik, Liquid-crystal adaptive lenses with modal control, Opt. Lett. 23 (13) (1998) 992–994. [3] A.F. Naumov et al., Control optimisation of spherical modal liquid crystal lenses, Opt. Express 4 (9) (1999) 344–352.
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[4] Y.H. Huang, C.H. Wen, S.T. Wu, Polarization-independent and submillisecond response phase modulators using a 90 degrees twisted dual-frequency liquid crystal, Appl. Phys. Lett. 89 (2) (2006) 021103. [5] K. Asatryan et al., Optical lens with electrically variable focus using an optically hidden dielectric structure, Opt. Express 18 (13) (2010) 13981–13992. [6] M. Langlois et al., Multiconjugate adaptive optics laboratory experience, Opt. Express 12 (8) (2004) 1689–1699. [7] S.R. Restaino et al., On the use of dual frequency nematic material for adaptive optics systems first results of a closed-loop experiment, Opt. Express 6 (1) (2000) 2–6. [8] P.J.W. Hands et al., Modal liquid crystal devices in optical tweezing 3D control and oscillating potential wells, Opt. Express 14 (10) (2006) 4525–4537. [9] A.K. Kirby, P.J.W. Hands, G.D. Love, Liquid crystal multi-mode lenses and axicons based on electronic phase shift control, Opt. Express 15 (21) (2007) 13496–13501. [10] J.Y. Hardeberg, F. Schmitt, H. Brettel, Multispectral color image capture using a liquid crystal tunable filter, Opt. Eng. 41 (10) (2002) 2532–2548. [11] G.D. Love, J.V. Major, A. Purvis, Liquid crystal prisms for tip-tilt adaptive optics, Opt. Lett. 19 (15) (1994) 1170–1172. [12] N.A. Riza, Acoustooptic liquid crystal analog beam former for phase-array antennas, Appl. Opt. 33 (17) (1994) 3712–3724. [13] C. Uber et al., Radiation testing of liquid crystal optical phase shifters for space survivability, in: Proceedings of the IEEE, Military Communications Conference, 2008, pp. 1–7. [14] S. Lane et al., Radiation testing of liquid crystal optical devices for space laser communication, Opt. Eng. 48 (2009) 114002. [15] Available from:. [16] S. Brandenburg et al., The irradiation facility at the AGOR cyclotron, Nucl. Instr. Meth. B 261 (2007) 82–85. [17] J. Wilson et al., Shielding Strategies for Human Exploration, NASA Conference Publications 3360 (1997) 109–149.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Proton irradiation of liquid crystal based adaptive optical devices E.J. Buis a,⇑, G.C.G. Berkhout a,b, G.D. Love c, A.K. Kirby c, J.M. Taylor c, S. Hannemann d, M.J. Collon d a
cosine Science & Computing BV, Niels Bohrweg 11, 2333 CA Leiden, The Netherlands Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands c Department of Physics, Durham University, South Road, Durham DH1 3LE, UK d cosine Research BV, Niels Bohrweg 11, 2333 CA Leiden, The Netherlands b
a r t i c l e
i n f o
Article history: Received 19 October 2010 Received in revised form 8 September 2011 Available online 1 October 2011 Keywords: Liquid crystal Proton irradiation Index of refraction
a b s t r a c t To assess its radiation hardness, a liquid crystal based adaptive optical element has been irradiated using a 60 MeV proton beam. The device with the functionality of an optical beam steerer was characterised before, during and after the irradiation. A systematic set of measurements on the transmission and beam deflection angles was carried out. The measurements showed that the transmission decreased only marginally and that its optical performance degraded only after a very high proton fluence ð1010 p=cm2 Þ. The device showed complete annealing in the functionality as a beam steerer, which leads to the conclusion that the liquid crystal technology for optical devices is not vulnerable to proton irradiation as expected in space. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystal (LC) devices can be used as phase and polarization modulators, as well as their more traditional use in displays as intensity modulators. LC phase modulators most commonly use the nematic phase with homogeneous or anti-parallel alignment, in which there is bulk alignment of the LC molecules producing an approximately uniaxial birefringent medium. The application of an electric field of appropriate frequency induces a dipole moment in the LC molecules and a resulting molecular torque. The equilibrium orientation of the molecules and thus of the optic axis of the bulk LC medium depends on the applied electric field. If the polarisation of the incident light is aligned with the extraordinary axis of the LC material, the rotation of the optic axis changes the effective refractive index of the LC material and therefore the effective optical thickness of an LC device can be varied by the application of an external field. This principle has been applied in a variety of applications involving controllable optics including lenses [1–5], adaptive optics [6,7], optical tweezers [8], variable axicons [9], tunable filters [10] and beam steerers [11,12]. Of the above mentioned applications in particular tunable filters and beam steerers could be of interest for space applications. Tunable filters may be applied in remote-sensing spectrometers or spectral imagers. The filters can then be used to dynamically select a small pass band (spectrometers) or a wide pass-band around the red, green and blue colours (spectral imagers). Optical beam steering may be applied ⇑ Corresponding author. Present address: TNO, P.O. Box 155, 2600 AD, Delft, The Netherlands. E-mail address: [email protected] (E.J. Buis). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.09.021
in laser altimetry or free space optical communications, where active steering of laser light is required. Typically, monochromatic lasers are used in the visible or the near-infrared wavelength ranges. Low operating voltages, a lack of moving mechanical parts, simplicity of operation, good physical robustness and relatively low cost of production make LC devices attractive for use in space research. However, any device operating in space is subjected to a relative harsh radiation environment. Space instrumentation will receive a mixture of high energy radiation, such as protons, gamma rays and X-rays. In order to test the suitability of LC based optics for operation in space, their radiation tolerance has to be assessed. In a previous study the radiation hardness w. r. t. gamma’s, X-rays and fast neutrons has been assessed in which only modest space radiation effects were found [13,14]: liquid crystals were irradiated using a.o. gamma’s from a 60 Co source up to a total dose of 2.2 Mrads with no measurable degradation. Proton irradiation effects are in general different from the ionizing effects from X-ray or gamma ray irradiation as protons can break up molecules and hence significantly alter material properties. Because protons are largely abundant in the ‘van Allen radiation belts’ or in solar proton events, space qualification of instrumentation should include the assessment of proton irradiation damage. In this paper we present the results of proton irradiation of LC beam steerer devices. Because of the similarity of the structure of the LC devices in general, it is expected that results on the radiation hardness of the tested devices as presented here, apply for other LC devices as well. Before describing the experimental setup and the results of the measurements in Section 4, we first discuss the structure of the LC
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E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
beam steerer devices in Section 2 and the proton irradiations and the proton beam in Section 3. We conclude in Section 5.
2. Liquid crystal based adaptive optics Fig. 1 shows the structure of the LC beam steerer, which in itself is a novel design. A tilt is induced between the upper and lower substrates by using different sized spacers on each side of the cell. This structure effectively produces a prism in which the refractive index and thus the effective tilt angle can be changed by the application of an external voltage. The design is similar to the modal prism [11] in which case the prism is induced by distributing a voltage ramp across the device. That has the advantage that the induced beam steering can be in 2 directions (positive and negative), but the applied voltage must be limited so that the device is operated in the quasi-linear part of the electro-optical response of the LC. The advantage of the design shown here is that the control is simpler as only a uniform voltage needs to be applied across the cell and the full electro-optical response of the LC can be utilised. This must be balanced against the drawback that the device can only deflect light on one direction. From the point of view of space-based applications the design described here has a much lower power consumption which is very important. Both glass substrates are coated with a transparent Indiumdoped tin-oxide layer (ITO) which acts as an electrode. On top of the ITO layer a rubbed polyimide layer is placed to align the LC. A voltage signal is applied between the two substrates which creates a refractive index change in the cell, causing the direction of the refracted beam to change. The beam steerer is driven with AC voltage with a frequency of f ¼ 1 kHz and an amplitude of V ¼ 0 20 V peak-to-peak. The steering angle a is determined by the wedge angle ¼ arctan ðDt=wÞ and the refractive index n of the liquid crystal material. Here Dt is the difference in spacer thickness and w is the width of the cell as shown in Fig. 1.
a ¼ ðn 1Þ
Dt ðn 1Þ w
ð1Þ
where the approximation holds for small angles. E7 was chosen as the liquid crystal material. The material, which is manufactured by Merck under the LiCrystal brand is a mixture of 5CB (40 -n-pentyl-n-cyanobiphenyl) and 7CB (40 -n-heptyl-n-cyanobiphenyl). For this specific design Dt ¼ 44 lm;w ¼ 12 mm and 1:521 6 n 6 1:746, resulting according to Eq. (1) in a steering angle range of 1:9 6 a 6 2:7 mrad. Note that the maximal steering angle is obtained when no bias voltage is applied and the minimal angle when the voltage is set 20 V.
LC glass ITO t spacer PI
w Fig. 1. Schematic drawing of a wedged beam steerer, indicating the parameters (t the thickness and w the width) that determine the beam steering properties of the device. The space between the substrates is filled with LC material. The vertical scale is strongly exaggerated (i.e. t w). As mentioned in the text the glass substrate are coated with ITO and polyimide.
3. Proton irradiation The proton irradiation was carried out at KVI [15] (Dutch Institute for proton and heavy ion accelerators for nuclear physics), where a proton beam with an energy of 190 MeV was extracted from the AGOR cyclotron [16]. A 96 mm aluminum degrader is placed between the end of the beam pipe and the irradiated device to tune the energy of the protons to 60 MeV. The choice of this particular proton energy is driven by both the satellite shielding and the expected spectrum of the proton flux. A typical satellite shield thickness of 2–3 g/cm2 is fully penetrated if the energy of the proton is above 40–50 MeV. On the other hand, the fluence of protons in Solar Energetic Protons (SEP) events drops rapidly at energies higher than 90–100 MeV. Hence, the energy of the proton should lie between 50 and 100 MeV. Note that protons with an energy in this range will not be stopped completely in the beam steerer device. Although damage due to protons is expected to be large when the protons are completely stopped, the probability of a Bragg peak in the LC cell is low as its thickness is small. A typical solar proton event at 1 AU has a total fluence between 107 and 108 protons/cm2 integrated over all energies above 10 MeV. However, large events, albeit rarely, have been recorded at 1 AU with proton fluences above 1010 protons/cm2 [17]. The irradiation was carried out in several steps. In this way a possible breakdown dose can be determined. In Table 1 we list the various irradiation steps and the relevant parameters. The measurements in between the irradiation steps took about 30 to 45 minutes, so that the complete irradiation cycle was carried out on the very same day. The systematic error on the proton fluence is estimated to be less than 3%. This estimation is based on the homogeneity of the beam, which is better than 2% and the absolute dose accuracy of 2% [16]. 4. Experimental setup and measurements A beam steerer device was constructed as described in Section 2 and a photograph of the device is shown in Fig. 2. For ease, a dedicated mechanical support was designed and manufactured to fit the delicate glass substrates in to the two inch. mounts in the optical setup. As mentioned above, the proton irradiation was divided into several steps. After each irradiation step, two measurements were carried out to characterise the device, a transmission measurement and a beam deflection angle measurement. 4.1. Transmission The transmission was measured using a broadband light source, an integrating sphere and a spectrometer. The experimental setup Table 1 Proton beam parameters during irradiation. Irradiation step
Energy [MeV]
Duration [s]
Fluence [p/ cm2]
Integrated [p/ cm2]
1
60
100
1 108
1 108
2
60
100
1 10
8
2 108
3
60
100
3 108
5 108
4
60
167
5 10
8
1 109
5
60
100
2 109
3 109
6
60
100
3 10
9
6 109
7
60
100
4 109
8
60
100
2 10
10
3 1010
9
60
150
3 1010
6 1010
10
60
200
10
1 1011
4 10
1 1010
159
E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
Fig. 2. Photograph of the LC beam steerer (a). The effective size of the device is 12 12 mm2 . In (b) a mechanical support for a 2 in. mount is shown.
white light source
compact spectrometer polarizer
LC−element
integrating sphere
fiber
Fig. 3. Setup to measure the transmission of the beam steerer.
(a)
(b)0.95
1
0.9
0.9
transmission
transmission
0.8 0.7 beamsteerer:
0.6
pre-irrad
0.5
glass:
1e10
0.4
pre-irrad
6e10
0.3
post-irrad
1e11
0.85 0.8 0.75 wavelength range
0.7
600-1000 nm @ 20V
0.2 0.65
0.1 0 300
600-1000 nm @ 0V
0.6 400
500
600
700
800
900
1000
wavelength [nm]
300-1000 nm @ 0V 300-1000 nm @ 20V
9
10
10
10
1011
proton fluence [cm-2]
Fig. 4. (a) Transmission of the liquid crystal cell as a function of the wavelength at various levels of irradiation. Indicated are the transmission of the complete device (black) and the transmission of two glass substrates (grey), which is calculated from the measured transmission of a single glass substrate. (b) The average transmission of a complete device as a function of the received dose. The transmission is averaged over two spectral ranges.
is schematically depicted in Fig. 3. The light source in this experiment is a combined Deuterium and Halogen light source (Avantes AvaLight DHS). An integrating sphere is used to collect all transmitted light, which is analysed using an Avantes AvaSpec spectrometer. The determination of the transmission consists of three individual measurements of the background (no light source and no sample), a reference signal (light source switched on, no sample) and a transmission measurement (light source switched on and sample in the light path). In Fig. 4 we plot the results of the transmission measurements. (a) shows the transmission as a function of the wavelength after various received fluences. One can see that the transmission of the sample is reduced by about 10% at the highest dose. This is becomes clear in Fig. 4(b), where the transmission averaged over two spectral ranges (300–1000 nm and 500– 1000 nm, resp.), is plotted as function of the received dose. In addition, Fig. 4(b) shows that when a bias voltage is applied, the index
of refraction of the liquid crystal is changed and consequently the transmission of the device. In addition to the transmission of a complete device, a single piece of glass (fused silica), coated with ITO was irradiated to verify if the a loss in transmission was due to the glass material. Note that the glass material was not selected for its radiation hardness and that more radiation resistant glasses are available. A single piece of glass was irradiated with the highest proton flux (i.e. 1 1011 p=cm2 ) and its transmission was determined before and after irradiation. In Fig. 4(a) the transmission of two stacked glass substrates (calculated from the measurements on a single substrate) is plotted next to the transmission of a complete device. Note that for wavelengths above 400 nm the transmission of a complete irradiated device is higher than the transmission for an irradiated empty cell, which is due to a better matching of the refractive indices in a filled cell.
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E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
(a)
(b) Z [wavelengths]
Z [wavelengths]
20 10 0
20 10 0
-10
-10
-20 Y 2 po sit 1 ion 0 [m m] -1
-20 Y 2 po sit 1 ion 0 [m m] -1
2 [mm]
0 -2
X
ion posit
2 [mm]
0 -2
X
ion posit
Fig. 5. (a) Wavefront images of the light path leaving the beam steerer which was kept under a bias voltage of 0 V (a) and 20 V (b).
After irradiation the device is left to anneal at room temperature for a month and was re-measured using the same setup. There was no recovery of the transmission.
2.8 fluence [p/cm2 ] 1e11
2.6
1e10
1.75
1.7
1e9
4.2. Beam deflection angle The beam deflection angle was determined using a wavefront sensor. A helium–neon laser ðk ¼ 633 nmÞ and a collimating lens are used to create a flat reference wavefront on a Thorlabs ShackHartmann wavefront sensor. The RMS wavefront error of the flat wavefront is less than k=10. The wavefront sensor was aligned perpendicular to the collimated beam and subsequently the beam steerer was placed in front of the sensor. In this way the wavefront was measured at various voltages applied to the beam steerer. Examples of wavefront images are depicted in Fig. 5. To characterise these wavefront images, Zernike polynomials were fitted to the data. The resulting Zernike polynomials Z 11 and Z 1 1 correspond to a tilt about the x- and y-axes, respectively, and are directly related to the steering angle of the device. In the measurements described here the beam steerer was aligned such that they tilt the beam about the y-axis and
a ¼ arctan
2k
q
Z 1 1
2k
q
Z 1 1
ð2Þ
where q ¼ 3:0 mm is the diameter of the pupil on which the Zernike polynomials are defined and the approximation again holds for small angles. The deflection angle a as it is derived from the Zernike coefficients using the above equation and is shown in Fig. 6 as a function of the bias voltage. The device seems to be unaffected by proton irradiation up to a fluence of 1 1010 p=cm2 , after which the power to steer the beam is much reduced. Note that at low fluences the beam steering angle is close the expected values as mentioned in Section 2. Using Eq. (1) one can determine the index of refraction of the device. The difference between the index of refraction ðDnÞ at a bias voltage of 0 and 20 V as determined using the measurement of the beam deflection angle is about Dn ¼ 0:21 when the device is not damaged by the proton irradiation. At large proton fluence the difference in index of refraction drops to Dn ¼ 0:11. The calculation of Dn can also be carried out using the results of the transmission measurement by applying the Fresnel equations for transmittance. In that case Dn ¼ 0:27 for unirradiated devices, while the value for a damaged device would be Dn ¼ 0. Since the latter value deviates from the results from the deflection angle measurements it is clear that the transmission is not only affected
α [mrad]
2.5
1e8 unirradiated/annealed
2.4
1.65
2.3 2.2
1.6
2.1
index of refraction
2.7
1.55
2
1.9 1.8
1.5 0
2
4
6
8
10 12 V [V]
14
16
18
20
Fig. 6. Measurements of the deflection angle of the beam steerer device. The curve for the annealed device is also shown.
by the change in the index of refraction, by the absorption of light in the device as well. Also shown in Fig. 6 is the beam deflection angle of the device, which was left to anneal during a month at room temperature. The device had completely recovered and shows beam deflection angles similar to the pre-irradiation values. 5. Conclusions In this paper we have shown the results of the assessment of the radiation hardness of liquid crystal adaptive optics. We have carried out proton irradiation on both a fully assembled beam steerer device and separate glass substrates. The measurements which were carried immediately after the irradiations, showed that the optical properties of the beam steerer only degraded significantly after a high proton fluence: from beam deflection measurements it was inferred that the index of refraction of the liquid crystal was affected after a fluence of about 1010 p=cm2 . After annealing the beam deflection angles were recovered to pre-irradiation values. Transmission measurements showed a drop in transmission was about 10% after the highest received dose, which could not be explained by the change in the index of refraction of the liquid crystal alone. It is therefore concluded that other effects, such as absorption in the glass substrate, contribute. No recovery in the transmission was observed after annealing.
E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research B 270 (2012) 157–161
The overall conclusion is that, given the observation that the performance of the measured device only decreased after a proton fluence which is rarely observed in space, optical devices based on liquid crystals are well suited for beam steering applications in space with respect to the possible damage to protons. The drop is transmission might induce a risk, when liquid crystals are applied in optical devices in which transmission is a critical item, such as in tunable filters. That risk could be mitigated by using more radiation tolerant glass substrates. Acknowledgements This work was supported by the European Space Agency Contract AO/1-5476/07/NL/EM. We thank Alex Short and Ilias Manolis at ESA for their help and advice. We thank Emiel van der Graaf and Reint Ostendorf for their support at KVI during the irradiation campaign. References [1] S.T. Kowel, D.S. Cleverly, P.G. Kornreich, Focusing by electrical modulation of refraction in a liquid crystal cell, Appl. Opt. 23 (1984) 278–289. [2] A.F. Naumov, M.Y. Loktev, I.R. Guralnik, Liquid-crystal adaptive lenses with modal control, Opt. Lett. 23 (13) (1998) 992–994. [3] A.F. Naumov et al., Control optimisation of spherical modal liquid crystal lenses, Opt. Express 4 (9) (1999) 344–352.
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