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MEASUREMENTOFLASER-L1-A

Optical power meters are used to measure the power from a CW laser. They are commercially available for use at all laser wavelengths and for power ranges from nanowatts to kilowatts. All power meters contain the four basic elements shown in Figure 1. Light striking the detector results in a current flow through the detector. The amplitude of the current is proportional to the total optical power striking the detector for power within the detector design range. This current is amplified and used to drive the meter readout device. This may be a microammeter or a digital display. The power range of the meter normally is selected with a range switch that controls the gain of the internal amplifier.

Pulse-Duration Measurement

To accurately display the time history of a laser pulse, the detector must respond quickly to changes in incident power. Response time of the detector usually is limited by the RC time constant of the electrical circuit. The capacitance is the capacitance of the detector itself and may be reduced by reducing the detector size. Thus, fast detectors for pulse-shape monitoring are generally small and cannot withstand much input energy. The two detector types used in most cases are the PIN photodiode and the pyroelectric detector. They’re commonly used by scientists and engineers

working with photodetectors. These quantities include:

• Responsivity

• Noise equivalent power

• Detectivity and D*

• Quantum efficiency

• Response time

 Responsivity

Responsivity gives a measure of the detector’s sensitivity to radiant energy. It’s specified as the ratio of the detector output to the light input. For a detector that generates a current output, the responsivity is the ratio of the root mean square (rms) signal current (measured in amperes) produced by the detector to the incident radiant power (measured in watts) at the entrance to the detector. If the signal output is represented as a voltage, it’s the ratio of rms signal voltage per watt of incident radiation. Equations 1 and 2 show these relationships . Thus, the responsivity is essentially ameasure of the effectiveness of the detector for converting electromagnetic radiation to electrical current or voltage. Responsivity will vary with changes in wavelength, bias voltage, and temperature. Responsivity changes with wavelength since the reflection and absorption characteristics of the detector’s sensitive material change with wavelength. Temperature changes affect both the optical constants of the detector material and its collection efficiency. For detectors that generate an electrical current. The most frequently encountered type of photodiode is silicon. Silicon photodiodes are widely used as the detector elements in optical disks and as the receiver elements in optical-fiber telecommunication systems operating at wavelengths around 800 nm. Silicon photodiodes respond over the approximate spectral range of 400-1100 nm, covering the visible and part of the near-infrared regions. The spectral responsivity of typical commercial silicon photodiodes is shown in Figure 2. The responsivity reaches a peak value around 0.7 amp/watt near 900 nm, decreasing at longer and shorter wavelengths. Optional models provide somewhat extended coverage in the infrared or ultraviolet regions. Silicon photodiodes are useful for detection of many of the most common laser wavelengths, including argon, He-Ne, AlGaAs, and Nd:YAG. As a practical matter, silicon photodiodes have become the detector of choice for many laser applications. They represent well-developed technology and are widely available. They represent the most widely used type of laser detectors for lasers operating in the visible and near-infrared portions of the spectrum.