A GUIDE
Laser Quantum’s Noise
Measurement:
How Laser Quantum measures andreports
One of Laser Quantum’s goals has always been to create lasers with thelowest possible noise.
Intracavity frequency-doubled lasers have an inherent noise of about 3% RMS. Several
techniques have been developed over the years to reduce this noise, known as the“Green Problem”, and at Laser Quantum we have worked for many years to develop
cavity innovations and electronic solutions to progressively reduce the noise level tobelow 0.02% RMS (Figure 1). Our techniques are applied to all of our visible lasers andmany have been patented.
All these advances in noise reduction have made it much harderto actually measure the noise using existing commercial systems,
as it is a very small signal. We have, therefore, developed ourown system to measure noise accurately.
There are three parts to the noise detection method:
1.

The detection

2. The fast analysis		
3. The long-term computer analysis
Figure 1: Timeline showing noise
optimisation over a 17 year period.

RMSThe detection
Our detectors are specially made to measure 100% of the laser’s power, with no ND filters or pick
off wedges between the laser and our detectors. ND filters placed in the laser beam path can leadto unwanted interference effects which can result in spurious noise contributions.
To attenuate the laser power, we use a specifically designed ceramic plate of a thickness suitable
for the measured power that diffuses the laser beam. This reduces its power and removes the needfor an ND filter with parallel optical surfaces (Figure 2).

Figure 2: Laser Quantum’s finesse pure continuous
wave laser.

Figure 3: Magnitude of the measured andtheoretical complex AM NTF response. Reprintedwith permission from OSA1.

©2020 Novanta Corporation. All rights reserved.

LAser Quantum | Noise Measurement

One of the applications for our lasers is Ti:Sapphire pumping.
Ti:Sapphire crystals have a noise transfer function: laser noiseoccurring at frequencies beyond the frequency range of thetransfer function is not seen in the output of the Ti:Sapphirelaser beam. In the case of Ti:Sapphire, this is ~800 kHz (Figure3), so it is imperative to minimise the amplitude noise of thepump source below this value.
Behind the front ceramic plate is our photodetector (Figure4) that has a bandwidth of 6 MHz, more than sufficient for
applications like Ti:Sapphire pumping. The photodiode signal isthen amplified locally in the detector and a sample is sent to ourfast acquisition board every 0.02 seconds.
The fast analysis

Figure 4: Photodiode used in the laser power
detection.

We have designed our own data acquisition cards that contain afast data capture board that powers and reads the detector.
The board collates 1024 samples at a time, to calculate average power and RMS % noise.

Figure 5 highlights both the average power and RMS % using all 1024 data points. Peak to peakmeasurements are measured using two data points and then RMS % is determined over a longtest period, allowing any steps or trends in power amplitude to be analysed over many samples toprovide a more statistically valid data set.
Long-term computer analysis
The average power and RMS % noise are captured in real time and are also stored for analysis,alongside other key parameters for our lasers.
This burn-in test (Figure 6) is experienced by every production laser for a minimum of 300 hours
and will only be passed as Quality Assured when it has met its design specification which is thenadded to the Certificate of Conformity supplied with each laser (Figure 7).

Figure 5: (a) Equations used to generate thedata points, and (b) the graph of the 1024
samples taken.

Figure 6a and 6b: Computer acquisition andanalysis showing soak test data.

Figure 7: The certificate of conformity
provided with each laser, detailing the low
RMS noise.

©2020 Novanta Corporation. All rights reserved.