Vacuum Encapsulations
Design and manufacture of specialised cryogenic vacuum encapsulations, or Dewars, is a core capability of SELEX Galileo's Infrared Detector Centre of Excellence. The highest performance CMT detectors are operated at temperatures below 120K to minimise thermally generated currents in the narrow-gap semiconductor. Our Product Design Group provides specialist skills in encapsulation design, cryogenics, and vacuum technology, and our manufacturing facility employs highly specialised encapsulation techniques including laser welding, cold welding, wire bonding, brazing, vacuum sealing, soldering, adhesives, and glass-to-metal seals.
Cooling to cryogenic temperatures is most often achieved by miniature Stirling engines or Joule-Thomson coolers where fast cool-down is required. SELEX Galileo has developed close collaborative relationships with a range of leading manufacturers of coolers and cooling engines so that we can offer detector solutions to meet the widest range of applications and budgets.
Cryogenic operation drives the need for hermetically sealed vacuum encapsulations that exclude the atmosphere and insulate the detector array for efficient cooling, in addition to providing a robust package that will survive harsh environments. SELEX Galileo has many decades of experience in the design and manufacture of vacuum Dewars for infrared detectors, many examples of which are still in the field after years of operation.
A Dewar design must satisfy the optical demands of the detector application; it must admit signal photons while excluding non-signal electromagnetic radiation which contributes to noise. The useful part of the electromagnetic spectrum is selected using an optical filter. Cryogenic bandpass filters are now usually favoured for this purpose since they minimise self-emission of non-signal radiation. Standard cold filters in the short-wave, medium-wave and long-wave band (1.5µm, 3-5µm and 8-10µm wavelength respectively) are routinely incorporated into our detector assemblies.
Out-of-field radiation is normally excluded using a cold shield with an aperture sized to achieve the required detector field-of-view. The cold shield is held at the same temperature as the sensitive array and its internal surfaces are often blackened with advanced infrared-black coatings and equipped with baffles to prevent stray, non-signal infrared radiation from impinging on the sensor.
The Dewar must provide a mechanically robust encapsulation and a very stable mechanical environment for the sensor. SELEX Galileo Dewars are designed for use in the most demanding environments specified for military applications as well as for space, science and industrial applications. Our Dewar structures are carefully designed to ensure that the sensor is mechanically stabilised which helps achieve image stability, boresight accuracy and to prevent microphonic phenomena.
In order to offer the best cooling performance and fastest possible cool-down, low input power and long cooler life, it is essential to minimise parasitic heat transport to the cold assembly. SELEX Galileo uses advanced materials coupled with careful thermal and mechanical design to manage the thermal conductance, emissivity and mass of each component and optimise the thermal properties of our Dewars.
In support of our overall detector design activities, SELEX Galileo's Product Design Group has a comprehensive suite of software modelling tools to aid Dewar design. These tools allow the mechanical, thermal and optical properties and performance of our Dewars to be modelled in detail before manufacture.
In addition to Dewars, SELEX Galileo also designs and manufactures other encapsulation types for our detectors, both vacuum and non-vacuum. These range from customised transistor-can packages for our uncooled single-element devices to very high integrity sealed packages for space use.
