Heads to measure Laser Peak Power and Pulse Shape
- Laser Peak Power Measurements and Laser Pulse Shape Measurements
- Various systems cover 100nW-1MW in 5ns - 20µs with beam diam. up to 60mm
- Absolute Calibration Accuracy of ±2.5% or better
- Response times down to 1ns (10-90%)
- Heads accept parallel laser beams or diverging light from an optical fibre
- Repetition rates from single shot to 10MHz
Types of Laser Measured
All our Laser Peak Power Meters are aimed at measuring the peak power of a pulsed laser. The Head might be receiving about 10W in 50ns for the most common semiconductor lasers or a far higher power (perhaps 1MW in 10ns from a Nd:YAG laser) or a far lower power returning from a very distant target (may be only 1µW).
Normally, the output of the receiving Head goes to a fast 'Scope With a 'Scope you can measure the peak power, pulse shape and pulse duration. Most of the heads have a response time of < 2.0ns. For single pulses a digitising 'Scope is required to capture the pulse, but at high repetition rates you can use a fast analogue 'Scope.
If you prefer a digital read-out of the peak power, then the optional Display Module (pictured below) gives a direct reading of the peak laser power.
Peak Power Measuring Head on optional Power Acquisition Unit
Peak Power Measurement Heads
All of these Heads are designed to measure the peak power of a pulsed laser. Most have a head with an integrating sphere to attenuate the light (see sketch below). This system gives a very wide acceptance angle so that it can measure accurately the diverging light from the end of an optical fibre (See separate page Principles of Integrating Spheres). For most of these Diffusing Heads the response time is close to 1ns.
For measurements at very low peak powers we use a lens to collect the light directly onto the photo diode. This allows the measurement of the very low power returning from a distant diffuse target (often less than a microwatt). Unfortunately, the acceptance angle is now only a few degrees, but this alignment is actually quite easy to achieve. The most sensitive Heads also have amplifiers but this then limits the rise time to 5-15ns depending on the gain required.
All the Heads are intended for use with either a fast 50 Ohm ’Scope or the optional Peak Power Module (see Specification at end of this web page).
Calibration and Accuracy.
All the Heads are designed to allow accurate calibration and traceability. Each design provides an accurate link between the peak power in a short high power pulse and the National Standards of CW power at about 1mW maintained at the UK National Physical Laboratory in Teddington. The final absolute accuracy in the measurement of the peak laser power is typically better than ±2.5%. A separate web page describes the principles and accuracy of the calibration ( Cal Accuracy ).
The standard Heads for the higher powers (LD, SD) can be operated with a simple battery pack giving 54V bias for the photo-diodes. The others have internal amplifiers and must be powered by either our Stand Alone Power Supply or by the Supply Rack (pictured above) which also accepts the optional Signal Analysis Modules.
Diffuse Receiving Head (SD)
Lens Receiving Head (SL)
Peak Power Module
The optional Peak Power Module (PPM/1) captures the peak value of the signal from the Head and displays the peak power of the laser directly in Watts. This it does by a successive approximation process and hence is only suitable for a laser with PRF above about 20Hz. Its limited dynamic range means that in many cases the measurement is better done by a modern digitising 'Scope. However, it can still be useful when the pulses from the laser are slightly inconsistent because it has modes to display either the average peak power or the maximum peak power or the minimum peak power. It is also easier to use for Quality Assurance checks at a fixed power level when the reduced dynamic range is not a problem.
It is completely automatic in action - two LEDs light up when the signals are being correctly processed.
The Display on the Module is Digital. There is also a steady output (for use with an external DVM or chart Recorder) and a fast 50Ω output to allow monitoring of the signal on a 'Scope.
Pulse Duration: 6ns to 2.5µs FWHM (less than 3% variation in readings)
Pulse Rep Rate: at PRFs up to 300 kHz every pulse is sampled. Above this frequency the pulses are sampled every 3µs. The minimum PRF for full accuracy is 20Hz.
Input Impedance: 50Ω (BNC on front of Module).
Sensitivity: An input of 200mV gives Full Scale Reading (2.000) on the DPM in the Module. The minimum input for correct operation is 10mV. The decimal point will be placed as appropriate to match the power range of the Head, reading in Watts. Other values of the sensitivity are possible.
Operational Modes: To allow for slight inconsistencies in the pulses from the laser, there is a choice of displaying the mean peak power or the maximum peak power, or the minimum peak power in the stream of pulses. The ‘max’ and ‘min’ modes find the 80 and 20 percentile points. Thus in ‘max’ setting the unit displays the value which has 80% of all pulses below it, while in ‘min’ setting it displays the value which has only 20% of pulses below it (80% above).
Meter Display: 3.5 Digit DVM
Fast Signal : 1.5 ns rise time pulse from 50Ω. Amplitude is 1/2 that of the input pulse.
Trigger Output: 0.3V for 2.5µs from reverse terminated 50Ω BNC. This is useful for 'scopes or External DVM
Display Time: Signal decays in about 30 sees if input pulses stop.
Signal Output: 2.0V from 1kΩ corresponds to FSD on the DVM.
Mean Power Module
Input: The input is via an internal link from the Peak Power Module.
Sensitivity: The sensitivity will be arranged to match the peak powers pulse widths and repetition rates of the pulsed input from the Head. Signal Output: 2.0V from 1 kΩ corresponds to FSD on the DVM.
Ambient Light: Because of the wide acceptance angle of the Head, ambient light may give an offset signal when measuring mean power. An offset knob allows this ambient light to be backed off.
Meter Display: 3.5 Digit DVM.