Leno Pedrottionesey

(3/27/2000)

    Some instructors in the field prefer to cover the coherence length of a laser in Module 6 or Module 9 of Course 1, "Introduction to Lasers", but not in this module, Module 8. It is included here in Module 8 since the longitudinal coherence length is a measure of the temporal coherence of the laser, and the subject of Module 8 is "Temporal Characteristics of a Laser". It is, nevertheless, your choice of where you wish to teach/emphasize the concept of longitudinal coherence length.

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Leno Pedrottitwosey

(3/27/2000)

In the place of light choppers, you may wish to have your students cut out circular cards with certain sections removed to represent different duty cycles. For example, you can remove a quarter of the card for a 25% duty cycle, three-quarters of the card for a 75% duty cycle, and so on. You can mount the cards on cheap DC motors and measure the angle or speed of the motor (RPM) with an appropriate stroboscope.

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Leno Pedrottithreesey

3/27/2000

One instructor in the field, who had neither a Q-switched laser setup or a bare plasma tube without Brewster windows had his student follow a substitute precedure.

• Build a switch using a zener diode, a 555 chip, several appropriate resistor and a power supply. Hook up a neon light and/or an LED. Control pulse time with the resistor.

• Fabricate light choppers by cutting circles out of 3"x5" cards and sectioning the circles to represent different duty cycles. One can easily vary the number of slits (1, 2, 3 and 4) and also the width of the slits. A 12-V motor and power supply complete the setup. With Thor Labs photodetection and Tektronex oscilloscopes, the duty cycles show up nicely on the oscilloscope.