Effects related to Dose Deposition Profiles in Integrated. Optics Structures. R H West and S Dowling. Cranfield University. Royal Military College Of Science.
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 3, JUNE 1996
Effects related to Dose Deposition Profiles in Integrated Optics Structures R H West and S Dowling Cranfield University Royal Military College Of Science Shrivenham Swindon SN6 8LA UK
ionizing radiation. This would be so particularly for the lower energy photons in th flash X-ray(FXR)exposures.Variation of the mass energy absoTtion cd%%nt with Photon e n e r g is Shown Results from exposures of lithium tantalate and lithium niobate materials in Figure 1. integrated optic structures to pulses of high energy X-rays and fast for the Accordingly, results for both tantalate and fiobate devices, electrons are related to dose and charge deposition profiles. Anomalous effects in the tantalate are ascribed to induced electric exposed to flash X-rays and brief Pulses Of&& e n e r g electrons, fields. are presented and discussed.
Abstract
I. IiTIXODUCTION At RADECS 93[l]the effects of flash X ray exposures on a simple LiTaO, optical waveguide structure were described.These measurements showed that the early losses(-dB/100Gy)could become a problem for systems employing such devices in senes,or using more complex integrated optics. An initial attempt was made to exp!ain some oi'these effects in terms of electric fields induced in the device duringthe exposure. Because of the relatively high effective atomic number of the material, and because the optical channel is at the surface of the material slice, these fields are associated with the dose build-up region. They can be strong enough to nod@ the refractive indices in the channel, and in the neighbouring material, so as to cause rotation of the plane of polarization of the light within the channel. Thus the transmission of that light will be modified, ir addition to any losses arising from the generation of absorbing or scattering centres. Similar induced electricfield effects have been observed in LiNbO, integrated optic couplei-s[2] [3] In the build-up region, the dose will differ from the expected bulk value, and will vary with depth into the device.A dose gradient across the channel could mod@ the propagating mode,either by selective absorption, or by altering the refiactive index profile. Additionally, there will be a charge deposition gradient which gives rise to an electric field normal to the surface on which the radiation falls. Either way,we mnight expect to see a distinction between effects related to the average dose level and those dependent on the gradients. Some attempt was made in our earlier work to acheve different dosefdose gradient contributions by filtering the flash X ray spectrum with Pb/Al absorbers, but it was felt that firther variation could be achieved by exposure to high energy electrons. Lithium niobate(LiNb0,)is the more usual material employed in integrated optic devices(I0D). It has similar electro-optic properties to those of the tantalate, but, with a lower atomic number and density, would be expected to respond differently to
Figure 1.Mass energy absorption coefficientsfor lithium tantalate, lithium niobate and, for comparison, bulk sillca. Bulk dosehnit fluence is proportional to this coeflicient for a given photon energy. Dose(Si) is commonly used in the radiation hardening context, but note the increased values in the IOD material at energies 4MeV.
11.EXPERIMENTAL The LiTaO, device is a single channel in the surface of an X cut slice of material 6 0 x 5 ~ 1mm. The LiNbO, sample, on the other hand, is an evanescent mode coupler with the guiding channels -55" long and an interaction length, between the channels, of -10". Both devices were irradiated in their entirety, without any externally applied electrical fields. Both were mounted in an aluminium housing, Imm thick. The flash X ray generator ( F X R ) gave a pulse, approximately 1OOns long, of photons with a range of energies downward from 4MeV accordingto whether the Pb/Al absorber was used (Figure 2). By varymg the distance of the device under test fiom the anode of the machine, uniform doses up to about lOOGy (LS) could be delivered.
0018-9499/96$05 .OO
0 1996 IEEE
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FXR SPECTRA 1oo
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10-2
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1oo
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ENERGY MeV
Figure 2. Energy spectrum (the FULL version) for the flash X-ray generator.Also shown is the effect of transmitting this spectrum through a lcm Pb/lcm AI absorber. The fast (1OMeV) electrons used were generated in a linear accelerator.Limitations in beam current and the relatively narrow spread of the beam meant that the maximum dose, that could be delivered uniformly over the device length, was about lOGy(Si). Using silicon dose diode arrays to measure dose variation over the device length, this limit could be lifted to an effective value of around lOOGy(Si) with the dose to the ends of the device being greater than 60% of the maximum at the centre. Beyond the ends the dose falls rapidly so there is a negligible contribution to the observed effects from the connecting fibre optics. In the FXR exposures, where a larger length of fibres would be irradiated, calculations suggest that there would still have been no problem, but the connections were shielded as a precaution.
.Pole
Dosimetq was provided by the source operators.Forthe X-ray exposures, LiF thermoluminescencedosimeters were used. Silicon pin diodes were used for the electron measurements.These were calibrated by simultaneously exposing them and LiF dosimeters to the electron beam.Observation of the pin output current arising from the exposure then yields the average dose rate and pulse length for the essentially rectangular radiation pulses
The results for the FXR exposures of the LiTaO, waveguide have been given elsewhere[l].They showed that the degree of transmission loss, and the rate of recovery, depended on the orientation of the device with respect to the radiation flux. It also varied with the energy spectrum of the photons for a given dose as measured by the dosimeters. That said, there was a consistent, slightly sub-linear variation of the loss with dose ,when observed at 10-4safter exposure(for doses
