Over the last decade, the U.S. navy has been converting its submarine periscopes from the traditional one using lenses and prisms to allow the user in the control room to see what is 10-15 meters above, at surface level, to all-digital sensors (vidcams, thermal imager and laser range finder) at the top of a telescoping mast that doesnt penetrate the pressure hull. These new "photonics masts" use a standard telescoping masts, and each American Virginia class SSN carries eight of them. Two of them carry sensors for the usual periscope functions (in case one breaks down). But the other six carry communications or electronic listening devices, or one is a spare for the top-of-the mast modules (some of which can be quickly swapped in or out). Thus an SSN comes to periscope depth (about 15 meters/45 feet beneath the surface) and deploys several masts to see what's out there via the vidcams and electronic eavesdropping. One of the masts can also connect with communications satellites, to send and receive email. This is a big morale booster for the crew.
The new masts make it easier to handle information. Everything picked up by the new system is instantly sent down into the control room using fiber optics. The images, and other information, can be viewed on flat screen displays in the control room, or anywhere else on the sub. This digital data can also be studied in more detail, and enhanced if needed. The Navy is upgrading existing periscopes on older submarines by putting electronics at the top of a traditional hull-piercing periscope, and replacing the optical components with fiber optic cable. Removing all these mechanically operated optics increases reliability.
The navy is also experimenting with small buoys that can be sent to the surface via a cable (containing a power and fiber optic link with the sub). These buoys can be expendable (used once) or retrievable. Another system in development uses a light sensor on the top of the sub that can, during daylight, capture images of what's on the surface while the sub is at a depth of 60 meters (183 feet).
Meanwhile, the types of sensors carried on the photonics mast continue to improve, both because of improvements in digital cameras and thermal (heat) imagers, and because new types of sensors are being developed.