1. STATUS ON Optical fiber: Preliminary tests on Optical Fiber and Fiber Optic Feedthrough Massimo Rossella, Marco Prata, Tommaso Cervi INFN, Sezione di Pavia - Dipartimento di Fisica dell’ Universtà di Pavia WA104 meeting January, 27th 1016

2. The Optical Fibers in Liquid Argon 2 A multimode 50um core optical fiber has been chosen on the entire PMT calibration system in order to reduce time dispersion No data were found on this kind of fiber working in liquid Ar or liquid N2, so tests must be done on it. Consider that this kind of components are qualified for TELECOM use (Infrared, while we have to work on PMT quantum efficiency peak i.e. in the blue) Considering costs, two different optical fiber has been purchased for a test (4 sample each kind of fiber): IRVIS 50/125um 0.22 Numerical Aperture, Step Index multimode fiber IRVIS 50/125um 0.20 Numerical Aperture, Graded Index multimode fiber

3. Both the fiber are produced by Oz Optics, both are 9meter long; Both are terminated on one side (the gas Argon side) with a flat polished FC connector; On the other side (Liquid Argon side, in front of the PMT) a flat polished 1.25mm diameter, ceramic ferrule has been chosen, instead of a standard FC connector in order to avoid mechanical stress and damage due to the presence of metal, ceramic, plastic and glue on a standard FC connector; Regarding the protection jacket, we chose a 0.9mm diameter loose white hytrel instead of the 3mm standard “orange” jacket: the 3mm one is very loose in comparison to the fiber and could create an air trap with problems during the vacuum phase. On the contrary the only drawback of the 0.9mm one is the possibility of (low) light losses along the fiber.

4. The Optical Fiber 4

5. The connector - the ceramic ferrule 5 The FC connector To be mounted near the fiber optic feedthrough in the gas argon zone The ceramic ferrule To be mounted in front of Photomultiplier tube in Liquid Argon

6. Test Set-up 6 We made tests in order to verify that those kind of fiber and termination (ferrule side) are not damaged by Liquid Nitrogen/Argon and that they can work properly in those conditions; First of all, we injected in the optical fiber a 405nm CW laser radiation and measured the output optical power by means of a Power Meter; In order to get more stable and repeatable results, the laser is first injected in a mode scramble that provides a modal distribution that is independent of the optical source; The intensity of the measured power after several thermal cycles and during the immersion in liquid Nitrogen has been investigated

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8. The optical fiber immersed in liquid Nitrogen, with 405nm laser Measurements set-up

9. 1 st measurement: Optical fiber thermal cycles results All the 8 optical fibers have been immersed in Liquid Nitrogen except the FC connector; The 9 meter of the fiber and the ferrule has been immersed for few minutes and then taken back to ambient temperature.; After this cryogenic shock, the fiber has been placed on the measurements system and the output power has been compared to the one before the immersion; Four thermal cycles have been done for each fiber No change in output power has been seen for all the fibers, after all the thermal cycles; Moreover a deep observation of the fiber and of the ferrule has shown no damage on them;

10. 2 nd : Attenuation length in Liquid Nitrogen The optical fibers have been immersed in Liquid Nitrogen keeping outside: the FC connector in order to let light injection by the laser the ferrule connected to the photometer under measurements The output power has been measured with different length of the fiber immersed: 0 m 1.75 m 3.50 m 5.25 m 7.00 m In order to evaluate the attenuation length of the fiber at ambient temperature and in a cryogenic environment

11. The Step Index optical fiber has less attenuation at ambient temperature, but show an higher attenuation length when immersed in liquid Nitrogen in comparison to the graded index one. Considering our needs, the gradex index seems better…. Step Index attenuation Length in LN2: 12.7 %/m Graded Index attenuation Length in LN2: 7.8 %/m

12. Fiber Optic Vacuum Feedthrough The light for PMT calibration come from a laser source that lies on the cryostat top, so several fiber optic high vacuum feedthroughs are needed; The device should contain a 50/125um optical fiber and have FC connectors on both side for compatibility.; Usually those components are optimized for TELECOM (infrared) wavelength, so test must be done for validate it in the blue region. Here the tests done on feedthroughs produced by VACOM (Vakuum Komponenten & Messtechnik GmbH) and purchased by the LNGS group; VACOM standard 50um product (manifacture code CF35- MM50-FCPC-1) mounts a graded index fiber and is validated only in the optical range 500nm-1300nm, so we decided to purchase also the step index one that, in VACOM opinion, should be better suited for the transmission at 400nm (manifacture code CF35-MM50SIR- FCPC-1).

13. VACOM Feedthrough

14. Measurements set-up Tests have been done on: 2 standard feedthroughs with Graded Index FO 3 custom feddthroughs with Step Index FO Light from the 405nm CW laser is injected in the mode scramble, then there will be the feedthrough under test (or alternatively a FC I) then a 2m long, 50um graded index fiber that bring the optical power to the Power meter A comparison of the measured optical power in out of the system when there is the feedthrough or the FC I, gives information about the attenuation of the device

16. Conclusions on Feedthrough Output Power with FC I Output Power with Feedthrough TRANSMISSION (ratio) Feedthrough STEP Index Fiber n.1 1230uW1090uW88.6% Feedthrough STEP Index Fiber n.2 1230uW1120uW91.0% Feedthrough STEP Index Fiber n.3 1220uW1090uW89.3% Feedthrough GRADED Index FO n.1 1240uW1175uW94.8% Feedthrough GRADED Index FO n.2 1240uW1175uW94.8% The Transmission (T%) is very high for both the feedthrough: STEP Index FeedthroughT = 90% GRADED Index FeedthroughT = 95% Both could be OK for our application.