Apparatus to treat an eye comprises an annular retention structure to couple to an anterior surface of the eye. The retention structure is coupled to a suction line to couple the retention structure to the eye with suction. A coupling sensor is coupled to the retention structure or the suction line to determine coupling of the retention structure to the eye. A fluid collecting container can be coupled to the retention structure to receive and collect liquid or viscous material from the retention structure. A fluid stop comprising a porous structure can be coupled to an outlet of the fluid collecting container to inhibit passage of the liquid or viscous material when the container has received an amount of the liquid or viscous material. The coupling sensor can be coupled upstream of the porous structure to provide a rapid measurement of the coupling of the retention structure to the eye.