DPF 2009

STUDY OF THE RADIATION-HARDNESS OF VCSEL/PIN ARRAYS

30 Jul 2009, 16:40

25m

Wayne State University

Detector Technology and R&D Detectors I

Speaker

Waruna Fernando (The Ohio State University)

Description

We investigate the feasibility of using VCSEL and PIN for the optical links at the silicon trackers of the ATLAS experiment at the planned upgrades of the LHC, CERN. We irradiated VCSEL and PIN with 24 GeV/c protons at CERN up to the equivalent SLHC fluence of 2.6 x 10^15 p/cm2, including a 50% safety factor. The GaAs VCSEL arrays were fabricated by Optowell and Advanced Optical Component (AOC). The latter vendor fabricates arrays with two different bandwidths, 5 and 10 Gb/s. The GaAs PIN arrays are fabricated by Optowell, AOC, ULM Photonics, and Hamamatsu. The silicon PINs were fabricated by Taiwan and Hamamatsu. The optical power of VCSEL arrays decreases significantly after the irradiation but can be partially annealed with high drive currents. The responsivities of the GaAs PIN arrays decrease by 40-90% after irradiation. As expected, for the silicon PIN diodes, the degradation in the responsivities is smaller, 15-45%. However, it should be noted that the bandwidth of the silicon PIN diodes is somewhat smaller. Overall, most of the degradations are acceptable and hence we have identified candidate VCSELs and PINs for the SLHC applications.

Summary

Optical links are now widely used in high energy physics experiments for data transmission. The links substantially reduce the volume of metallic signal cables freeing up valuable detector space. In addition, the fibers eliminate the cross talk between metallic cables and electrical ground loops between the front-end electronics and the data acquisition system. The high bandwidth of opto-electronics is well suited for multiplexing many input channels and allows for introduction of error checking and error recovery transmission protocols. These features are especially important in experiments where radiation can induce Single Event Effects (SEE) in the digital electronics. The silicon trackers of the ATLAS experiment at the LHC use VCSELs to generate the optical signals at 850 nm and PIN diodes to convert the signals back into electrical signals for further processing. The devices have been proven to be radiation-hard for operation at the LHC.

The LHC will start operation in 2009. However, an upgrade of the collider, Super LHC, is already being planned for 2016. The SLHC is designed to increase the luminosity of the LHC by a factor of ten to 10^35 cm-2s-1. Accordingly, both the required data bandwidth and radiation-hardness of the detector are expected to increase by a similar factor. We use the Non Ionizing Energy Loss (NIEL) scaling hypothesis to estimate the SLHC fluences at the present optical link location (PP0) of the ATLAS pixel detector. The estimate is based on the assumption that the main radiation effect is bulk damage in the VCSEL and PIN with the displacement of atoms. After five years of operation at SLHC (3,000 fb-1), we expect a silicon device to be exposed to a maximum total fluence of 1.5 x 10^15 1-MeV neq/cm2. The corresponding fluence for a GaAs component (VCSEL) is 8.2 x 10^15 1-MeV neq/cm2. We study the response of the optical link to a high dose of 24 GeV protons. The expected equivalent fluences at SLHC are 2.6 and 1.6 x 10^15 p/cm2, respectively.

We packaged the PIN and VCSEL at OSU for the irradiation. The VCSEL arrays were mounted on a shuttle to allow the VCSEL arrays to be moved out of the beam for periodic annealing. The VCSEL arrays irradiated include two from Optowell and one 5 and one 10 Gb/s array from AOC. The optical power decreased during the irradiation as expected. We annealed the arrays by moving the arrays out of the beam and passing the maximum allowable current (~ 10 mA per channel) through the arrays for several hours each day. The optical power increased during the annealing. Unfortunately, there was insufficient time for a complete annealing but the AOC arrays still have good optical power at the SLHC dosage. However, the situation is less clear with the Optowell device because of an intermittent connectivity problem. We believe the Optowell arrays will have good optical power when we characterize them in the lab after the radiation cool down.

For the PIN diodes, we irradiated two GaAs arrays from AOC, Optowell, ULM Photonics, and Hamamatsu. We irradiated the arrays from the first three vendors in 2007. The devices were powered but we did not monitor the responsivities online. The responsivities decreased up to 90%. This year we monitored the responsivities online by illuminating the devices with light from VCSELs and measuring the PIN current. The responsivities decrease by 40-90%, with Optowell and Hamamatsu having the highest responsivities after the irradiation.
In addition to the GaAs PIN diodes, we also irradiated silicon PINs, two Taiwan arrays and eleven single-channel silicon diodes from Hamamatsu (five S5973 and six S9055). The decrease in the responsivities is smaller, 15-45%, with S5973 having the highest responsivities after the irradiation. The degradation is consistent with the expectation that a silicon device is more radiation-hard than a GaAs device. However, it should be noted that the bandwidth of the silicon PIN diodes is somewhat smaller.

In summary, we identify the several VCSEL and PIN candidates for the SLHC applications. The results of the study will be presented at the conference.

Presentation materials

Slides

Radiation-Hardness_of_PIN_VCSEL_waruna.pdf