NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1031 Nat. Bur. Stand. (U.S.), U.S. GOVERNMENT PRINTING OFFICE For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 Price $2.50 (Add 25 percent for other than U.S. mailing) THE USE OF LEDS TO SIMULATE WEAK YAG-LASER BEAMS M. Young Electromagnetic Technology Division National Bureau of Standards Boulder, Colorado 80303 Abstract The purpose of this report is to determine whether and under what conditions a light-emitting diode may be used to simulate a weak YAG-laser beam that has been scattered by a distant reflecting object. By examining the differences between laser radiation and LED radiation, we conclude that there is no theoretical reason that a LED may not be used in place of the laser beam. Key words: Laser simulator; light-emitting diodes; YAG laser. THE USE OF LEDS TO SIMULATE WEAK YAG-LASER BEAMS The purpose of this report is to determine the conditions under which a light-emitting diode (LED) may be used to simulate a weak laser pulse at the wavelength of 1064 nm. The pulse is presumed to have originated from the diffuse scattering of a laser beam off a distant object. We seek to determine the feasibility of testing the receiver of such a pulse with the suitably collimated and filtered radiation of a 1060-nm LED. Much of the report is theoretical; the remainder is either experimental or supported by experimental evidence. Because of the organization of the document, theoretical and experimental considerations are not divided into separate sections; rather, we take the approach outlined below. We begin by asking the question, What are the potential differences between laser radiation and LED radiation? The laser radiation in question is presumed to be Nd:YAG radiation that has been scattered from a distant object. The LED radiation is presumed to have originated from one of several (nominally) 1060-nm LEDs that are on the market. To answer the question, we have compiled a list of parameters that will have to be examined. We have tried to make the list as exhaustive as possible, even though in some areas potential problems may be dismissed out of hand. The list is presented as Table 1. Other ways of organizing the list are possible; this organization best suits our needs. 2. Beam divergence. a. Degree of collimation of simulator, compared with laser return. Before going on to discuss the entries in Table 1 in detail, we must describe the experimental apparatus. We obtained six LEDs whose wavelength was nominally 1060 nm. We excited the LEDs with a signal generator that could provide electrical pulses up to 2 A and as short as 20 ns. The circuit is shown in Fig. 1. The diode provides protection for the LED in case of a strong, negative reflection. The series resistor provides an impedance match to the 50-2 coaxial cable; its value is chosen experimentally by time domain reflectometry. A microwave tee with a 270-2 resistor is used to sample the electrical pulse in the coaxial cable. We use enough cable between the LED and the generator to provide a 35-ns round-trip delay and operate the generator at the 20-ns setting. We use a sampling oscilloscope to observe the direct and reflected waveforms as they pass the tee. Following [1], we adjust the value of the series resistor until the waveform shows slight undershoot; when possible, we also ensure that none of the secondary oscillations becomes positive, because this will cause a small, secondary optical pulse. Figure 2 shows the effect of changing the series resistor on the electrical waveform of a particular LED.* The data were taken with the horizontal and vertical outputs of the sampling oscilloscope connected to an xy recorder; the vertical output was filtered with a capacitor to reduce noise (more on this later). In principle, the effective impedance of the LED and of the diode are functions of peak current and waveform. However, we have found that slight *The use of product names is intended for the convenience of the reader and does not imply endorsement by the National Bureau of Standards. Products of other manufacturers may work as well or better. |