Cryptography Reference
In-Depth Information
other research teams, we have selected Epitaxx EPM 239 AA APDs for our
detectors. Even with this special treatment, they suffer considerably from low
quantum efficiency (QE), relatively high dark noise, and serious after-pulsing
problems. Even so, they provide adequate performance for a 5-MHz pulse
rate quantum cryptography system. Since custom cooling and electronics
are required, we designed and built our own cooler package to maintain
the InGaAs APDs at the requisite operating temperatures. We had two key
goals in mind for the cooler package: it should be able to operate reliably
and repeatably over a wide range of temperatures down to -80
◦
C, to enable
exploration of detector behavior over a range of operating conditions; and
it should be suitable for prototype deployment and thus should not require
human intervention on a regular basis, e.g., to refill liquid nitrogen reservoirs.
Figure 4.7 shows a schematic of the core housing, which is a vacuum-
pumped chamber containing a pair of InGaAs detectors brought to operating
temperature by two Peltier thermoelectric coolers (TECs).
The housing itself is machined aluminum, consisting of a base container
plus a removable lid. The lid fits snugly with an O-ring so that a vacuum can
be maintained in the inner chamber. A number of holes pass through the base
container to allow hermetic feedthrough of fibers and electrical connections.
A large connection leads to the vacuum line. At the center of the chamber
rests a cooled block of copper with holes drilled out for two detectors. The
Side
Top
APD
Fiber hermetic
Spring washers
To vacuum line
To vacuum line
feedthrough
apply pressure
TEC
Base
Cover
to APD
to TECs stack
APD
mounting
O-Ring
block
seal
(Top TEC removed)
Wire hermetic
RF hermetic
Thermal
feedthrough to
Microstrip RF
O-Ring seal
feedthrough
interface
TEC and
clamp ring
connections
connection
material
thermocouple
to APDs
to APD
Figure 4.7
Side and top cutaway views of DARPA thermoelectric cooler package.
