Figure 1.

Circuit for driving 1060-nm LED and monitoring direct and reflected current waveforms.

51 Ω

43 Ω

33 Ω

24 Ω

25 ns

Figure 2a.

Examination of direct and reflected current waveforms as function of series resistance.

Diode is a

gure 2b. Optical pulse emitted when LED is driven by current waveform shown.
GAL 100, with a 43-2 series resistor and approximately 1-A peak current.

changes of the peak current are not serious. We matched the impedance of the system at the peak current of 1 A but frequently operate satisfactorily at 2 A.

To detect the radiation, we use an avalanche photodiode, usually followed by a transimpedance amplifier and sometimes by a radio-frequency amplifier as well. The circuit is shown in Fig. 3. As before, the waveform is detected by the sampling oscilloscope and transcribed by the xy recorder.

For many experiments, we wanted to measure the optical power at a given time (such as the peak of the pulse) as a function of some parameter such as wavelength or source or detector position. For this purpose, we fashioned a mechanical-to-electrical transducer and used the transducer output to drive the horizontal motion of the xy recorder. For the vertical motion, we set the sampling oscilloscope on "manual" and set the display at a fixed time; we use the output of the oscilloscope to drive the vertical amplifier of the recorder.

The sampling oscilloscope displays about 5 mV of noise, peak to peak; when we use the rf amplifier, it adds another 5 mV of noise. Many times, this results in a signal-to-noise ratio no better than 3 or 5. In part to reduce the effect of this noise, we record our waveforms with the xy recorder instead of the more-common oscilloscope photography. By scanning sufficiently slowly and integrating with a capacitor, we perform an analog average of the noise and improve the signal-to-noise ratio by an order of magnitude or more.

We will describe further details of the experiments as we need them. The rest of the report is organized parallel to Table 1.

Wavelength

The YAG laser emits radiation at the wavelength of 1064 nm; its linewidth is of the order of 0.5 nm. The LEDs, on the other hand, display a peak wavelength in the neighborhood of 1060 nm and a linewidth of about 50 nm. In addition, the peak wavelength depends on temperature, so there is the possibility that the spectrum will drift during the course of the pulse (this is known as "chirping").

To assess these problems, we first calibrated a 1/4 meter grating monochromator with a continuous-wave YAG laser. We excited the LEDS with 2-A, 20-ns pulses and observed the spectrum at various times during the pulses.