Peak Power Measurement of Pulsed Lasers


Heads to measure Laser Peak Power and Pulse Shape


 

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 1W).

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 Monitor for Semiconductor Lasers

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.

 

Signal Acquisition.

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 ).

 

Power Supplies

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.

Standard Heads
  1. Spectral Range is as for Silicon detectors (4001070nm).   Alternatively, we can supply GaInAs detectors giving a Spectral Range of 9001700nm.
  2. Speed is given as the FWHM in response to a very short optical pulse.   Overall pulse width observed with your optical pulse is obtained by adding the two FWHM in Quadrature.   E.g. SL Head has FWHM of 11ns which with a 50ns FWHM optical pulse gives a resultant FWHM of SQR{(11)2 + (50)2} = 51.2ns.
  3. Sensitivity is given for a 'Scope with 50Ωinput impedance.   When using our Peak Power Module, remember that it accepts signals from 10mV peak to 200mV peak.   Thus with the Standard Diffuse Head (SD) at 10mV/W it can be used with powers from 1W up to 20W.   A wider range of powers can be obtained with coaxial attenuators which we can supply as required.
  4. Power range.  The lower limit on the power is set by the requirement of 10:1 Signal/Noise.   The noise is the greater of 1mV 'scope noise or the output noise of the Head.   The upper limit on power is set by the maximum linear output of the Head.
  5. Acceptance Angle. For Diffuse Heads there is less than 2% variation in sensitivity over the whole input aperture or the whole acceptance angle.   For Lens Heads the signal is within 5% of the on-axis value for the range of angles given.
  6. Lens Heads (SL, HL) are normally calibrated for use in a uniform parallel field which overfills the aperture.
  7. High Sens Lens Head (HL) has option of a telescope to help in aligning on the distant source.
  8. To match your exact requirements, other sensitivities may be available on request.
  9. All the Heads have mounting threads of M6 (= '0'BA) centred under the input port.
Model Head Type Speed (FWHM) Sens. Power Range Accept. Angle Aperture Head Size Power Supply
(2) (3) (4) (5) (D L)
LD Low Sens Diffuse 1.2ns 1mV/W 10W-1000W 26 37mm 90120mm Batt or PSU
SD Standard Diffuse 1.5ns 10mV/W 1W-350W 26 37mm 90120mm Batt or PSU
SD60 Large Ap. Diffuse 1.5ns 10mV/W 1W-200W 26 60mm 116119mm Batt or PSU
MD Med. Sens Diffuse 4.0ns 100mV/W 0.1W-12W 26 37mm 90120mm PSU only
HD High Sens Diffuse 6.3ns 1.0V/W 10mW-1.2W 26 37mm 90120mm PSU only
78 High Speed Diffuse 0.7ns 1.6mV/W (1060nm) 10W-100W 15 12mm 7075mm PSU only
78A Small Diffuse 4.0ns 35mV/W (1060nm) 0.3W-20W 15 12mm 7075mm PSU only
SL Standard Lens (5)(6) 11ns l00mV/W/cm2 0.5-15W/cm2 3.0 20mm 7268mm PSU only
HL High Sens Lens (5)(6) 9-28ns 1V/W/cm2 0.2-2W/cm2 0.3 (7) 68mm 94240mm PSU only

Diffuse Receiving Head (SD)

Diffuse receiving head

Lens Receiving Head (SL)

 Receiving Head with collecting Lens

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 2.5s 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 2.5s 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.