Dear Serge, All silicon urethral catheter (foley or nelaton ) are partially transparent. I think for your desired wavelength which ranges through red and infra red,  these catheters satisfy the partial ( and not complete) transparency.

Dear Manouchehr,

Thanks! Ideally, I'd like to have more than a partial transparency since I'm detecting small (optical) signals and already have a photon deficit. However, I'll check optical properties of silicone catheters.