Accuracy and Traceability in Pulsed Laser Measurements


 

Delta Developments has developed many Instruments for the measurement of laser output such as CW power, pulsed peak power, pulse shape and pulsed energy, beam width or beam position.   This web page describes just one of our areas of expertise:- the measurement of the Peak Power of a Pulsed Laser with traceability to UK National Standards of CW power.

There are many commercial applications for lasers with pulse durations of about 4-80ns, peak powers of 1-100W and wavelengths from 750-1550nm. These include military range finders, target markers, golf course rangers, weapon simulators, road speed cameras, wave height monitors and line of sight communications. There are no direct standards of laser peak power at the levels and wavelengths required by all these end users and so we derive all our peak power measurements from CW Standards.

In contrast to many other companies, who seem to add the calibration method almost as an afterthought, we build the calibration system into the design from the very beginning.   This is absolutely essential because the Instrument will have to be calibrated at just a few mW of CW power (being the only accurate standard available) but will then be used at peak laser powers in the kW - MW range.

To use a particular Measuring Head at about 100W of peak power when it can only be calibrated at about 1mW we use moderately fast photo-diodes such as Silicon or GaInAs.   When correctly made so that they are totally depleted ** these diodes can have a very large linear range.   Using our computer model of the carrier drift and electric fields in these PIN diodes we can calculate the maximum linear current for each particular bias voltage, laser wavelength and pulse duration (see web page How Photo-diodes Work ). This theoretical model has been confirmed experimentally for all those wavelengths which are available in short pulses.

A separate web page describes the use of Integrating Spheres with which we achieve three key points in the optical arrangement:-.

Integrating Sphere Receiving Head

Diffuse receiving head

Each photo diode drives a strip line of 66.7Ω impedance leading round the circumference of the sphere towards the output socket.   At the output socket resistive T-attenuators ensure that each strip line is driving 66.7Ω and yet the impedance seen looking back into the output socket is 50Ω.   This arrangement ensures that any reflections from the 'Scope are absorbed and don't rattle up and down the cable connecting the Head to the 'Scope.   Having measured the CW sensitivity for the Head, the attenuation of the T-network is then chosen to give a convenient figure for the pulsed sensitivity such as 10mV/W.

Peak Power Meter Circuit

Circuit of Peak Power Meter

To calibrate, we open the 4 low inductance switches (formed from screws to the ground plane) so that the impedance to ground at the photo diodes is quite high.   Using a stabilised beam passed through a narrow band filter at the laser wavelength we measure the absolute sensitivity of the photo-diodes at a few mW of power.   To measure this CW beam we use a Black Disc Thermopile ( Preston, 1971) as our local standard.   It is regularly calibrated at the UK National Physical Laboratory in Teddington.

We now know the absolute sensitivity of the Peak Power Head and can insert the correct resistors in the T-networks to give the desired pulsed sensitivity.   There is then a final calibration with the Head in its working configuration with the screw switches connected again.

With this system we can have an overall response time of close to 1.0ns for 10-90%.   By adjusting the parameters of the integrating sphere we can ensure that the maximum peak power expected will generate a photo-current well below the saturation current for that particular bias voltage and wavelength.

The long term stability of this system is quite good and is mostly determined by the long term stability of the paint on the integrating sphere.   A typical, sphere has a Bounce Round Factor (or "Gain") of x5.   Unfortunately, this means that a 1% change in paint reflectivity gives a 5% change in the sensitivity of the Head.   Despite this, we typically get only 1-3% sensitivity change in the first year after manufacture and then less than 1.5% change each year after that.

 

Traceabilty

With the design features above we can provide an accurate link between the peak power of the pulsed laser in a short high power pulse and the UK National Standards of CW power at about 1mW maintained at the National Physical Laboratory in Teddington, UK.   The final absolute accuracy in the laser peak power measurement is typically better than 2.5%.


Last Update: 21 Feb 2018