Task 1 IRLED

Visible light emitting diodes (LEDs) are very common, but they have only recently become available in the infrared. These IR LEDs are available in various infrared bands including a band centered at a wavelength of 3.3 microns. UC Berkeley has had success in quantifying hydrocarbon concentration by using a 3.39 micron HeNe laser as an optical probe. It may be possible to replace the laser with a solid state 3.3 micron LED. The advantages of LEDs over use of the HeNe laser are lower cost (currently LEDs are about $400 each vs. $1500 each for the HeNe Laser), increased ruggedness and reduced size.

Status: Experiments, both at vacuum and high pressures, have been performed using a 3.3

m m LED. These experiments incorporated a 24" pathlength cell at varied pressures to determine the effect of pressure on overall transmission of light from the LED, through various concentrations of methane in nitrogen. The absorption cell was vacuum-sealed with sapphire windows on each end through which the LED light was passed. The cell had a relatively long pathlength to assure significant ( ยป 50%) attenuation of the LED light occured. In order to measure LED radiation with such a long pathlength, a chopper wheel was used with a phase-lock amplifier. In this way, the very low levels of radiation from the LED are measured (the phaselock amplifier filters out all components of the detector signal that are not near the frequency of the chopper wheel, and amplifies components of the signal that are close in frequency to that of the chopper wheel; in this way, very small signals can be extracted in spite of large levels of noise).

At the same output power, the LED has a weaker signal at the detector due to the wide divergence of the LED light, compared to a laser. Thus, unlike the laser, most of the light emitted by the LED is not received at the detector.

Figure 1, below, is a schematic of the test setup used for characterizing the transmission of the LED vs. pressure for a given concentration of methane. Focusing mirrors were also used to reduce the effect of distance on measured LED radiation. The absorption cell shown in Figure 1 was a vacuum-tested cylinder with sapphire windows on either end. The sample gases were passed through the absorption cell at controlled pressures.

Figure 1, below, is a schematic of the test setup used for characterizing the transmission of the LED vs. pressure for a given concentration of methane. Focusing mirrors were also used to reduce the effect of distance on measured LED radiation. The absorption cell shown in Figure 1 was a vacuum-tested cylinder with sapphire windows on either end. The sample gases were passed through the absorption cell at controlled pressures.

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