Delta Developments has now developed many Instruments for the measurement of output parameters of lasers such as beam width, beam position, CW power, pulsed peak power, pulse shape and pulsed energy. 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 existing 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, 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 the 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 then be used at a few kW of peak laser power.
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. These can have a large linear range when correctly made with a totally depleted structure**. 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. This theoretical model has been confirmed experimentally for all those wavelengths which are available in short pulses.
By using an Integrating Sphere we achieve three key points in the optical arrangement:-.
- The beam needs to be diffused so that the signal is independent of beam position.
- The photo-diode should be flooded in light ensuring the maximum possible linear current because all the active area is being used.
- The light should be attenuated to keep within the allowed range.
In the Diffusing Heads used in our laser peak power meters the two photo-diodes are set facing the centre of the integrating sphere. This arrangement ensures that each diode gives a signal independent of where the laser beam strikes the sphere surface.
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
To calibrate, we open the 4 low inductance switches (formed from screws to ground) so that the impedance to ground of 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. The correct resistors are then inserted in the T-networks to give the desired pulsed sensitivity. There is then a final calibration with the Head of the peak laser power meter in its working configuration.
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 for the integrating sphere. A typical, sphere has a Bounce Round Factor (or "Gain") of x5 which unfortunately 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.
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 at Teddington. The final absolute accuracy in the laser peak power measurement is typically better than ±2.5%.
Last Update: 3 Feb 2015