Semiconductor Cylinder Fiber Light Amplifiers Break New Ground
Our research group has developed a Semiconductor Cylinder Fiber Light Amplifier (SCFLA), which consists of a glass fiber with a thin, about 5 nm thick, semiconductor layer at the clear-glass core cladding boundary. This is in contrast to fibers whose core is doped with semiconductor nanoparticles. Ours have a smooth semiconductor film at the core cladding boundary.
To date, we have used three semiconductor materials: CdTe, Cd3P2 and GaSb. CdTe was used primarily to develop the fabricating processes. Both Cd3P2 and GaSb cylinder fibers have gain in the wavelength region of interest to telecommunication. Most of our testing so far has been on Cd3P2 cylinder fibers. We have shown a gain of about 10 dB at 1550 nm and a bandwidth of several hundred nanometers in pieces of Cd3P2 fiber that are 4 mm in length. Theoretical calculations predict a gain of 30 dB with a bandwidth of 300 nm in an 8-mm-long piece of Cd3P2 cylinder fiber.
Energy gap increased
The long wavelength cut off of the gain occurs at the wavelength corresponding to the energy gap of the semiconductor in the fiber. The quantum size effect in the very thin semiconductor layer in these fibers increases the energy gap of the semiconductor. The thinner the film the
larger the energy gap. Bulk Cd3P2 has an energy gap of 0.61 eV corresponding to a wavelength of 2033 nm. We have typically measured energy gaps in our Cd3P2 cylinder fibers of 0.697 eV corresponding to a wavelength of 1780 nm. The thin, uniform semiconductor layer in our fiber exhibits a unique energy gap, while a continuously varying energy gap spectrum is observed in semiconductor doped glass fibers. Further tests of these devices are under way at several places.
To achieve these results, we have developed a fabricating process that produces fibers with smooth continuous semiconductor layers at the core cladding boundary. This differs substantially from the standard methods. Because these devices are only about 6 mm long, about 250,000 devices can be cut from a single draw of semiconductor cylinder fiber. One can draw about 1.5 km of fiber from one of our current fiber preforms.
The SCFLAs, as well as the fabricating process, have been patented by Syracuse University. These devices will most likely be fabricated by a joint venture of Dove Photonics Inc. in Rome, N.Y., and SDO Communication Corp. in Fremont, Calif. SCFLAs have several substantial advantages over Er doped FLAs. A comparison of these two devices is as follows:
| ||SCFLA||Er doped FLA|
|Gain||2.5 dB per mm||0.003 dB per mm (= 0.3 dB per m)|
|Shape of gain curve:||Flat||Highly curved|
|Bandwidth:||300 nm||30 nm|
|Pumping method:|| From side||Axially|
|Pump source:|| 3 Banks of LEDs (inexpensive)||Narrow band laser (expensive)|
|WDM couplers required:||None||Two|
Because this FLA uses semiconductors it can be tailored to work at many
different wavelengths, including the 1320-nm band. The wavelength range can be selected by the appropriate choice of semiconductor and semiconductor film thickness. Moreover, applications other than in optical communication are possible. For example, a plate consisting of about 1 million SCFLA can be used in the future as the first-ever true image light amplifier. These would be fabricated by combining our preform fabricating process with some of the processes that are used in fabricating the microchannel devices currently used in night vision goggles.
Current microchannel devices are not true light amplifiers. A very dim image impinges on one surface of the microchannel plate and is converted to electrons. The electrons are greatly multiplied by a photomultiplier process in about 1-million-micrometer size tubes. The electrons emerge from these tubes and impinge on a phosphorous screen. Thus, the image from a microchannel plate is viewed on a monochromatic phosphorous screen. The image, in color and in three dimensions, would be amplified by a SCFLA plate.
Philipp Kornreich, James Flattery, Douglas Keller and John Dove are with the Department of Electrical Engineering and Computer Science at Syracuse University.
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