Measuring Pulsed Lasers with NanoScan 2s

Kevin Kirkham, NW Regional Sales Manager, Ophir-Spiricon, LLC

When operating in pulsed mode, the peaks of the individual pulses in the profile are connected to form a smooth profile. All parameter computations are performed on the resulting smooth profile. Measurement accuracy depends on the Pulse-to-Pulse Repeatability, Profile Averaging, and the number of pulses in the profile during a single scan, which in turn depends on the laser repetition rate and beam diameter.

The NanoScan-2s is designed for two types of laser pulse widths: Short Pulse <10nS and Long Pulse >10nS.

In Short Pulse operation, the amplitude (GAIN) must be set to < 4096 to avoid amplifier nonlinearity.

In Long Pulse, Full Scale Amplitude is limited to 32,768 counts to avoid saturation, non-linearity, and measurement error.

A minimum of 15 pulses per single scan is required to obtain the specified accuracy. This condition depends on the laser spot size and the scanhead scan rate. If there are not enough pulses present in a single scan (<15), the software computes the corresponding head scan rate and displays a message in the Message Window (see below) recommending a lower head speed.

The following table gives a list of calculated minimum beam diameters at a given pulse rate for a desired number of pulses per profile. The more pulses per profile, the more accurate the measurement is likely to be. Due to the 45° angle of the slits to the direction of rotation, the actual velocity of the slits is the drum speed divided by the square root of two.

The pulsed mode of operation is recommended for all pulsed beams with pulse frequencies greater than 10 kHz, unless they fall into the category of "quasi-CW," i.e. > 1 MHz. Results can also vary depending on signal-to-noise ratio and pulse-topulse repeatability. The use of Frame Averaging and Rolling Frame Averaging can improve the measurement of pulsed beam profiles.

The NanoScan 2s pulsed operation can operate at any scan rate, however, it is recommended that the scan rate be 1.25 or 2.5Hz unless the laser pulse repetition rate is above 50 kHz.

The peak connect algorithm finds the highest peak pulse, then, using the frequency value entered by the operator, it finds the other peaks and connects them to generate a smooth beam profile. It is important that the exact pulse rate be entered into pulse acquisition parameters. The software connects the pulse peaks based on the user input Laser Pulse Rate. If the actual and input frequencies are significantly different, the resulting profiles may be jagged (not smooth).

When selected in software, the actual Laser Pulse Rate is measured and displayed as a check for a possible incorrectly entered Laser Pulse Rate value. When jagged profiles appear, try using the measured rate value and the beam profiles may become smoother.

At high laser repetition rates, it may be better to operate the NanoScan 2s in CW mode and let the auto-filter smooth the beam. When this is preferable is dependent on the individual laser's pulse performance. If inconsistent results are seen with a high rep rate laser (e.g., >80kHz), it would be advisable to try the measurement both ways.