Characteristics of a laser-pumped 1.5-μm infrared quantum counter

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Abstract
The characteristics of room-temperature laser-pumped Er3+ infrared quantum counters (IRQC's) have been investigated. One such detector, employing BaY2F8 : Er3+ and a GaAs laser pump, exhibited an NEP of 3 × 10−9 W/Hz1/2 at 1.493 μm, the smallest NEP reported thus far for an IRQC. The limiting source of noise was found to be a generation of visible emission by the laser light alone, which provides an output equivalent to the dark current of a conventional detector. It is proposed that this output, which has a quadratic dependence on the laser power, arises largely from absorption of the 854-nm pump light by the anti-Stokes sideband of the 4I9/2 state of Er3+. The absorption coefficient for this process was determined to be approximately 3 × 10−6 cm−1. Minor modifications of the present system appear capable of improving the NEP to 2 × 10−10 W/Hz1/2, and further improvements may be possible by selecting an IRQC host crystal that has a smaller absorption of the pump radiation than does BaY2F8 : Er3+.