Novel solid state modulator for the infrared : the germanium chopper
Uncooled thermal imaging cameras in the 8 to 14 µm band use pyroelectric array detectors as the preferred technology. The systems advantages offered by these uncooled detectors are those of reduced cost, complexity and power consumption. The price paid for this uncooled technology is that the image must be modulated in order to generate a change in signal intensity on the detector. Currently this is achieved using a rotating chopper blade, which introduces moving parts to the system, and has the drawbacks of size and image degradation as camera motion changes the blade's rotational speed. A need has existed for considerable time for solid state broad optical bandwidth modulation in the infrared. The application requires modulation of the 8 to 14 µm band with high 'on' state transmission, although not necessarily a very low 'off' state transmission. Crucially this must be over a relatively large (1 cm2) aperture. The modulator should be insensitive to polarisation since this would halve the image intensity from a randomly polarised scene, reducing the system signal to noise ratio. Low power consumption, compactness, and the incorporation into a low f-number optical system are the systems requirements for this application. The specification is eased only by the moderate chopping frequencies (<100 Hz) characteristic of pyroelectric detector operation. Adequate modulation using a novel method has been achieved in the 8 to 14 µm region by introducing moderate levels of excess carriers to suitably prepared germanium by excitation from a diode laser source. The process utilises inter valence band transitions from the light-hole to heavy-hole band, requiring excitation power densities in the order of Watts cm-2. This has been exploited by the construction of the first solid state modulator satisfying the requirements of thermal imaging cameras in the 8 to 14 µm band. A depth of modulation of from 95.4% (AR coated) to 4.7% transmission at 10µm using a total power density of 5.6Wcm-2 from 980nm diode lasers has been achieved in the centre of a 1cm2 aperture. The 'on' state loss at this wavelength is limited by the single-layer quarter-wavelength AR coating properties, and may be eliminated through use of a multilayer coating. There is presently no solid state modulator in the required thermal image band that even approaches this performance. The possibility of using electrical pumping to generate the required holes for the induced absorption is investigated. The design of an electrical-injection area-based modulator, modulating radiation perpendicular to the junction plane is affected by the conflicting requirements of uniform absorption across the aperture and high 'on' state transmission. A design for such a device is outlined. Modulation of a beam of radiation parallel to the junction plane is demonstrated by a reconfigured p-n junction diode to illustrate the principle.