Heterodyne Detectors

In optical domain, preamplifier is no more an utopia and is in actual use in fiber communication. However quantum physics prohibits the noiseless cloning of photons an amplifier must have a spectral density of noise greater than 1 photon/spatial mode (a "spatial mode" corresponds to a geometrical extent of A /4). Most likely, an optical heterodyne detector will be limited by the photon noise of the local oscillator and optical preamplifier will not increase the detectivity of the system. 

The main source of noise of such a heterodyne detector is the photon noise that takes place at the splitting of the local oscillator. Quantum physicists see this noise as originating from vacuum fluctuation on the input arm. This gives directly the spectral density of noise at input hv/2. 

Here Ck represents a proportionality constant for the fc-photon process. The leading dc terms are proportional to ip and 2l and may be associated with the absorption of k monochromatic photons, each of which arises from a given beam (1 and 2, respectively). The highest frequency current component is proportional to eil 5l cos[lc(< i — associated with both of the beams. It is evident from the above that multiple- and sum-frequency terms do not appear in the fc-photon absorption heterodyne detector output, in analogy with the result for the one-quantum case [7.12-14],... 

We recall from (7.4) that rjj is itself proportional to the irradiance of the LO, and we must have 4(72 <1. The result is therefore similar to that for the singlequantum heterodyne detector given in (7,1) in that case, however, is... 

The SNR at the fundamental-difference-frequency (nji-ojj) obtained for coherent beam mixing in the /c-photon absorption heterodyne detector. Following a series of steps similar to those given above, we find... 

Heterodyne detectors in the microwave and millimeter regions (hv< kT) include square-law mixers such as the crystal diode detector [7.93], the InSb photoconductive detector [7.94-96], the Golay cell [7.95], the pyroelectric detector [7.95], the metal-oxide-metal diode, and the bolometer [7.87]. The latter three types of detectors have also been used successfully in the middle infrared (at 10.6 pm) [7.97-100]. For this type of detector Johnson noise generally predominates, and the input SNR is given by [7.100]... 

The widespread application of 10.6 pm heterodyne detection for communications, radar and infrared active imaging is presently restricted by the need to cool the detectors below temperatures obtainable from thermal electric coolers ( 180 K). Presently the main reason for cooling 10.6 pm heterodyne detectors is to reduce the thermal generation-recombination rate in the photoconductor or the diffusion current in diodes to a level that can be overridden by available laser local oscillator power. In general the use of photoconductors (with band gaps tailored for 10.6 pm radiation) seems to be a realistic approach to operation at 180 K or above. Since the thermal generation-recombination rate is of the order of 10 /t hole-electron pair per cm at 180 K it follows that a 1 pm thick, 10 cm area photoconductor with a carrier lifetime (t) of 10" s would require 1 milliwatt of LO power to approach hvB detector sensitivity at bandwidths of... 

The two radiation fields f[ and are again mixed in a heterodyne detector with a strong, coherent, and polarized LO signal (at frequency yielding two electrical beat signals at and I/2 —/lI, along with a dc component which... 

C. Elliott, N. Gordon, T. Phillips, H. Steen, A. White, D. Wilson, C. Jones, C. Maxey, N. Metcalfe, Minimally cooled heterojunction laser heterodyne detectors in metalorganic vapor phase epitaxially grown Hgj.xCdxTe. J. Electron. Mat. 25(8), 1146-1150 (1996)... 

---