Gemma Simpson assesses the array of methods by which one can test or calibrate a laser once operational
Lasers are tricky things to test. Not only does a user need to make sure the right amount of energy and the correct sort of beam is produced, but the laser’s application – be that manufacturing, research or clinical – can also have an impact on how it is tested.
To make sure a laser ‘does what it says on the tin’, an operator has to ensure the light source is providing exactly what the manufacturer says it should. The testing should primarily check the power and wavelength stability, with emphasis on mode stability for some applications, such as holography. Lasers are usually tested and calibrated against known standards; which might be done via a power meter or piece of equipment that is regularly calibrated against a known standard. This equipment is often sold as a system comprising a sensor (the device that is exposed to the laser beam) and a meter (the instrument that analyses and displays the measurement).
But it’s not just a power meter that is required, as the beam also needs to be assessed using a profiler, which uses a camera to image the laser beam and then analyse the information from that image using specific software.
So, in broad terms, both a power meter and a beam profiler are needed when checking a laser. And while users seem to test lasers happily using power meters, not so many are clued up on beam profilers, according to Dr Carlos Roundy, founder emeritus of Spiricon. He says: ‘Every laser user knows that they need a power meter, but relatively few have profilers, so subsequently there are more power meters sold than profilers, but users are catching onto the need for profilers nowadays.’
And not using a profiler can be a dangerous move, as Roundy explains: ‘The power starts to go down as the laser deteriorates, but the beam profile often deteriorates sooner, and not a lot of people realise this. As the beam profile alters, some users tend to increase the power of the laser beam to compensate for this deterioration, but this often causes the beam to be even less accurate. That’s not to say users do not need a power meter – it is also a very important component of laser testing. The changes within a laser beam are not necessarily what we expect so we need both pieces of kit,’ Roundy adds.
Think before you buy
Before purchasing any laser testing equipment, a user has to factor in many different aspects to work out which bit of kit is most appropriate for a specific laser, as Sean Bergman, product line manager at Coherent, explains: ‘The most common things to consider include: deciding if a continuous-wave or average power measurement is most appropriate, or if the energy measurement of individual pulses is desired; confirming the beam size to ensure a large enough sensor is selected; checking the wavelength of the laser to ensure the right sensor coating is selected; checking the average power of the laser to ensure the sensor can dissipate the heat to which it will be exposed; and checking the actual power and/or energy levels that need to be measured to ensure the sensor has enough resolution, if the measurement is low, and also to ensure the sensor will not saturate, if the measurement is high.’
Bergman adds: ‘You need to determine what type of analysis is desired. This is where the choice of meter is performed. There are various types of meters available, ranging from simple meters designed for quick visual checks to meters that can log data, attach to a PC and display statistics and power stability onboard the meter. Manufacturers will typically offer several different levels of meters of increasing complexity, and hence cost, to address these various customer needs.’
Profilers have improved throughout the years, as Roundy explains: ‘The first profilers were scanning pinhole and these were used before CCD cameras became inexpensive. Although scanning pinhole profilers were good at the time, they only produce a single line through a beam, whereas a camera can get full, two-dimensional information.’
Camera-based profilers are also more suitable for pulsed lasers compared with their scanning counterparts, as Roundy adds: ‘You cannot catch a single laser pulse with a scanning profiler, but with a camera-based profiler you can get the full information and an instantaneous profile.’
Easy does it
Once an operator has all their laser testing kit, it’s not just a case of switching everything on to check the laser is working, as the set-up of the equipment can also be challenging. For example, one of the many conundrums is making sure the sensor is not damaged by the beam it is meant to be testing, as Bergman explains: ‘Many laser processing systems focus the beam at the work surface to perform a cutting, welding or marking operation. Most people are interested in knowing the power output at the work surface, but at this specific point the laser beam is focused and it can damage sensors. So one challenge is to ensure the laser does not damage the measurement device. This is achieved through a combination of using high damage threshold coatings and also measuring above or below the work plane.’
So coatings also play an important part within laser testing systems, but this is not the only thing to consider, as Bergman adds: ‘Some other challenges can include the repetition rate of pulsed lasers. Very high repetition rate lasers of >10kHz are best measured using a power sensor, because it is difficult to resolve the energy of specific pulses.’
The Pulsar, from Ophir Spiricon, can turn a PC into an energy meter.
Another problem facing operators is that many are unsure of the lasers and diagnostic equipment they are using and are subsequently losing out on cash, as Roundy explains: ‘Users are not often familiar with changing the cavity of a laser, and so on, and are hesitant to do so, and therefore spend money on calling out the manufacturer.’
And it’s not just user-awareness of the lasers which is changing, as more and increasingly diverse sectors of the electro optics industry are recognising the need for laser testing. As Roundy says: ‘Twenty years ago our customers were all scientists who had to know exactly what their lasers were doing and we were the laser manufacturers’ worst enemies as we told the customers, by using our laser testing equipment, what was wrong with their lasers. But now the laser manufacturers are coming to us to buy our meters and profilers to assure the quality of their products. Now, we are having a similar issue with industrial laser manufacturers, but more and more of them are realising that they need to have beam profilers,’ Roundy adds.
Of course, lasers are used within a plethora of applications, so a laser may be tested to ensure its design performs to the specifications of that application. Nowadays, the laser could also be subjected to lifetime and reliability testing, as Coherent’s Bergman says: ‘There is increasing demand for monitoring the health and status of lasers used in certain processes such as laser welding in the automotive industry.’
There are also numerous uses of laser testing and monitoring within laser processing applications, according to Bergman, who adds: ‘This can range from sensors located at numerous pick-off points within the beam path providing real-time feedback to monitor for potential degradation of optics or other equipment, to a sample taken before each laser process is performed to ensure the laser energy hitting the work is within acceptable parameters.’
Laser measurement is common with field service engineers too, according to Bergman: ‘The equipment is used to monitor the laser output before and after repair and is also used to certify the performance of the laser.’
And these applications are all becoming increasingly strict with their testing as users require a far more stable laser than has been accepted in past years, according to Grahame Rogers, managing director at Laser Support Services. He says: ‘The reason why people are looking for more stringent testing is that applications are becoming more stringent with more precise measurement and reliability issues. In addition, some lasers are used to optically pump other lasers, and these need a higher degree of stability and beam pointing.’
Rogers adds it is also challenging to ensure the environment the laser is tested in remains the same.
As with any sector in the electro optics industry, the laser testing market is evolving, with a few trends popping up. For example, there is a tendency towards more computer interfacing and data analysis, as Bergman explains: ‘More customers want to be able to retain test data in manufacturing or take data and analyse it on their PC later.’
Ophir-Spiricon’s Quasar is a wireless device that allows users to place the meter away from their PC or laptop.
And as users want to play with laser testing data, the vendors are also getting more savvy on how to get the data to the PCs in the first place, as Kenneth Ferree, VP of sales and marketing for Ophir Spiricon, says: ‘A popular device with our customers is Quasar, which is a Bluetooth device so you can have a wireless connection with your PC and wireless is one of the emerging trends within the industry.’
Quasar lets operators separate the placement of a remote laser energy meter and their PC or laptop. The wireless interface therefore reduces cable clutter and can accommodate hard-tomeasure areas such as glove boxes or high power lasers behind barriers.
Increasingly higher power lasers are also affecting the laser testing market, as Ferree adds: ‘Another trend is users’ need to test lasers in the THz region. This can be done only with a pyroelectric camera.’
And Coherent’s Bergman has also noticed the high power shift, as he adds: ‘We are seeing a trend towards higher power lasers, especially in the laser cutting field and from fibre lasers and large diode arrays, which poses unique challenges to dissipate the heat generated by the laser beam. These lasers are also highly divergent.’
So as lasers increase in power and size, while the range of applications and environments they are used in also grows, it looks like the laser testing manufacturers will have their work cut out to keep up with this increasingly demanding market.