RP-Sensoren – Für welchen Zweck sollte man sie einsetzen?

Von Dr. Ephraim Greenfield, CTO, Ophir Optronics

The RP sensors (RP for Rapid Pulse) are a specialty but unique type of energy sensor that can fill an important niche in laser power/energy measurement.  RP sensors are rather expensive and bulky and have other limitations and therefore are not the preferred solution when a pyroelectric sensor will do. However, there are situations where it is the only energy sensor that can be used.

The main situations in which RP sensors are useful are

1. For very long pulses >10ms and very large duty cycles  such as are typical in many pulsed diode laser applications.
2. For very high average powers greater than 50W with repetitive lasers.
3. When you want to measure the temporal pulse shape as well as the power and energy.
4. When you want to monitor for missing pulse

Principle of Operation
RP heads incorporate an innovation (patented) that allows measurement of the energy of high energy repetitively pulsed lasers which is of advantages over pyroelectric measurement in certain cases: higher average powers than a pyroelectric head can stand up to and high duty cycle lasers with up to 50% or more duty cycle which cannot be measured with a pyroelectric detector .The basic approach is to incorporate two sensors in one measurement head.
The RP measurement head has a standard thermopile type detector enabling it to measure average laser power to a high degree of accuracy, generally ±3%. In addition, it contains a fast photodiode detector, which measures the energy of the laser pulses in real time. The photodiode detector is mounted so that a fraction of the radiation falling on the thermopile absorber, 4% to 15% depending on head type, is scattered from the absorber. A fraction of that falls on the photodiode (see illustration below). (There is also a larger, fast photodiode connected directly to an oscilloscope via the BNC connector enabling it to measure the temporal pulse shape of the beam to a resolution of ns.)

The RP photodiode and its associated circuitry give a signal proportional to the energy of arriving pulses. The photodiode signal also shows the time between adjacent pulses. This information is fed into the Laserstar or USB Interface microprocessor and is averaged over two seconds enabling the Laserstar to calculate the average pulse rate, average energy and the energy per pulse, according to:

Average pulse rate = (number of pulses in time period) / (time period)
Average energy = (average power) / (pulse rate)
Energy of pulse (i) = (average energy) x (photodiode signal i)
average (2 sec) of photodiode detector signals
The RP can be adjusted to measure at the highest possible pulse rate for a given laser pulse width as shown in the table below

The RP sensors operate with either Laserstar displays or the USB Interface.

What models of RP are available and for what are they useful?

	30A-P-RP The 30A-P-RP is primarily for short pulse Q switched lasers up to 30W average power and up to 15KHz pulse rate. It is useful where you want to continuously monitor pulses for irregularities or missing pulses
	FL250A-LP1-RP The FL250A-LP1-RP is for monitoring continuously pulsing long pulse lasers such as industrial YAG lasers, fiber lasers and diode lasers with millisecond type pulses. This is the only pulsed laser energy meter to be able to handle average powers up to 250W and energy densities up to hundreds of J/cm² depending on pulse widths.
	L1500W-LP-RP This is the only energy meter available to continuously monitor pulses of industrial YAG and fiber lasers at up to 1500W average powers with the long pulses and high duty cycles typical of such lasers