Delta Developments

Heads for Measurement of Peak Power

• Laser Peak Power and Laser Pulse Shape Measurements
• Wide choice of Heads cover 100nW-100W and beams up to 60mm diam.
• Absolute Calibration Accuracy of ±2.5%
• Response times down to 1ns (10-90%)
• Heads accept parallel laser beams or wide angle light from an optical fibre
• Optional Digital Readout of peak power

Types of Laser Measured

All our Peak Power Measurement Heads are aimed at measuring the peak power of pulsed lasers. For semiconductor lasers, these might typically have a peak power of about 10W in 50ns but there are options for higher or lower powers and different pulse durations.

For pulsed laser measurements at high repetition rates the peak power can be displayed on the optional Display Module. For single pulses a digitising 'Scope is required.

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Peak Power Measurement Heads

The Peak Power Measurement Heads are designed to measure the peak power of lasers with high repetition rates (diode lasers). Typically these emit about 10W in 50ns but there are different Heads to cover the range from 1µW to 1kW

The Heads for the higher powers use an integrating sphere to attenuate the light which also gives them a very wide acceptance angle so that they can measure accurately the diverging light from the end of an optical fiber. The response time is close to 1ns.

The Heads for the lower powers have a lens to collect the light onto the photo diode. This allows the measurement of the very low power returning from a distant diffuse target (usually less than a microwatt).  Although the lens reduces the acceptance angle to just a few degrees, in practice this is quite easy to achieve . The most sensitive Heads have amplifiers limiting the rise time to 6-20ns.

 

Signal Acquisition.

All the Heads are intended for use with either a fast 50 Ohm ’Scope or our Peak Power Module (see Specification at end of this web page).

 

Calibration.

All the Heads have built in Calibration facilities which we use to provide an accurate link between the power in a short high power pulse and the UK National Standards of CW power at about 1mW. The final absolute accuracy in the laser peak power measurement is typically better than ±2.5%.   A sheet discussing the principles and accuracy of the calibration is available on request.

 

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 (picured above) which also accepts the optional Signal Analysis Modules.

Standard Heads Available

The response time 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. Eg. 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. The Sensitivity is given when using a 50Ω CRO. When using our Peak Power Module, remember that it accepts signals from 10mV peak to 200mV peak. Thus with the Standard Diffuse Head at 10mV/W it can be used with powers from 1W up to 20W. A wider range of powers can be obtained with electrical attenuators which we can supply as required. The lower limit on the power is set by the requirement of 10:1 Signal/Noise. The noise being either 1mV CRO noise or the output noise of the Head, whichever is the greater. The upper limit is set by the maximum linear output of the Head. For Diffuse Heads there is less than 2% variation in sensitivity over the whole input aperture or the acceptance angle. For Lens Heads the signal is within 5% of the on axis value for the range of angles given. Lens Heads (SL, HL) are normally calibrated for use in a uniform parallel field which overfills the aperture. High Sens Lens Head (HL) has option of a telescope to allow alignment on the distant source. Spectral Range is as for Silicon detectors (300–1070nm). For some Heads GaInAs detectors are available giving a Spectral Range of 900–1700nm. Heads 78 and 78A are normally supplied with GaInAs detectors. Please enquire for the power ranges, response times and prices of other Heads with GainAs detectors. To match your exact requirements, other sensitivities may be available on request. All Heads have mounting threads of M6 (= 'O'BA) centred under the input port.
Model	Head Type	Speed (FWHM)	Sens.	Useable Range	Accept. Angle	Aperture	Head Size	Power Source
		(1)	(2)	(3)	(4)	(4)	(Diam. × length)
LD	Low Sens Diffuse	1.2ns	1mV/W	10W-1000W	±26°	37mm	90×120mm	Batt or PSU
SD	Standard Diffuse	1.5ns	10mV/W	1W-350W	±26°	37mm	90×120mm	Batt or PSU
SD60	Large Ap. Diffuse	1.5ns	10mV/W	1W-200W	±26°	60mm	116×119mm	Batt or PSU
MD	Med. Sens Diffuse	4.0ns	100mV/W	0.1W-12W	±26°	37mm	90×120mm	PSU only
HD	High Sens Diffuse	6.3ns	1.0V/W	10mW-1.2W	±26°	37mm	90×120mm	PSU only
78	High Speed Diffuse	0.7ns	1.6mV/W (1060nm)	10W-100W	±15°	12mm	70×75mm	PSU only
78A	Small Diffuse	4.0ns	35mV/W (1060nm)	0.3W-20W	±15°	12mm	70×75mm	PSU only
SL	Standard Lens (5)	11ns	l00mV/µW/cm2	0.5-15µW/cm2	±3.0°	20mm	72×68mm	PSU only
HL	High Sens Lens (6)	9-28ns	1V/µW/cm2	0.2-2µW/cm2	±0.3° (7)	68mm	94×240mm	PSU only

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Peak Power Module

The Peak Power Module (PPM/1) captures the peak value of the signal from one of the Heads. 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. Min 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 o/p: 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.

IEEE Interface

For both Signal Analysis Modules an IEEE-488 option is available for easy data collection by computer.