Speed of Light with Nanosecond Pulsed 650 nm Diode Laser

The speed of light has been measured many different ways using many ingenious methods. The following note describes a method which is conceptually very easy to understand and fairly easy to implement. The technique is the simple time-of-flight optical pulse delay method using a fairly short (nanosecond) optical pulse and an oscilloscope with bandw
Speed of Light with Nanosecond Pulsed 650 nm Diode Laser - schematic

idth between 50 - 100 MHz. Common low power laser pointers, typically emit at a wavelength of 650 nm and operate from two to four 1. 5 V button cells. Many of these lasers can be easily extracted from the pointer assembly and pulse-modulated to several hundred megahertz. The laser used here was removed from a low power (< 5mW) laser pointer assembly from a popular retail outlet. The laser is prebiased below threshold, at 5 - 10 mA current (threshold current for the laser used here is 24 mA) using an inductor as a bias insertion element. A short (< 5 ns) electrical pulse modulates the laser. Since a very low duty cycle is used for pulsing the laser, fairly high current pulses are possible without degrading the laser. The actual forward current and voltage achieved during the drive pulse are dependent on the details of the I-V characteristic of the specific laser used, but are typically in the range of 50 - 100 mA and 6 - 10 V respectively. The short electrical pulse is generated using a simple avalanche transistor circuit. Due to the high frequency content of the short pulse, the actual shape of the current pulse driving the laser will depend on the circuit components (series resistors etc. ) and parasitic electrical effects (series inductance of connection wires etc. ) The circuit has been described by Jim Williams in a Linear Technology Measurement and Control Circuit Collection and has many other uses. A suitable choice...

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