Measuring Energy of High Rep Rate Pulsed Lasers - Ophir Photonics

There’s a common misconception when it comes to choosing an energy sensor to measure high frequency pulsed lasers.

Goto Beamtech to know more.

The confusion begins when we look at the energy sensor specs and compares them to our laser.

• Maximum power and damage threshold… high enough.
• Wavelength range… includes my wavelength.
• Etc.

Then we get to maximum repetition frequency, and notice it’s 10 or 20 kHz, 25 kHz in the best case.  But every sensor needs a power meter display or a PC interface, and they have a maximum limit too.  For instance, imagine you’re interested in the PE9-C and the Vega meter. What is the maximum repetition rate measurable: 25 kHz (sensor limit), or 4 kHz (meter limit)?

As usual, it is important to understand what each spec means. What happens if we go beyond the specs?  The answer is different for different specs, for example:

• Violating damage threshold can cause sensor damage
• Going beyond the maximum power or below the minimum power can reduce accuracy

How to measure a high frequency pulsed laser

In our case, going beyond the maximum frequency of the meter will cause no harm to the sensor, and won’t change the accuracy either, as such.  All it means is the sensor will not be able to measure every single pulse.

However, if the laser pulses faster than the sensor’s max rep rate, this will compromise the accuracy of your measurements.

Think of them is two different limits:

The limit of the sensor determines what laser can be measured.  If it pulses too fast, it won’t be measured at all.  (At least, not accurately.)

The limit of the meter or interface is just how fast the device can log the laser pulses.  So if your laser pulse four times faster than this limit, you will record only one in four pulses (approximately).

For example, imagine you are using a 2 kHz PC interface and 25 kHz sensor, but your laser is pulsing at 20 kHz.  The sensor will measure the laser pulses accurately, but the PC interface will only be able to log one in ten pulses.

Is this good enough?

Obviously, I can’t answer this question for you. It depends on what you need to measure, how stable your laser is, etc.

The company is the world’s best High Repetition Pulsed Laser supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

If you just want to see what a typical pulse looks like, this is certainly sufficient.

If you want to know how many pulses are abnormal or missing or weak, this might also be enough for you.  You won’t be able to determine the exact number of missed pulses, but you can extrapolate from the statistical data you get from even 10% of the pulses.  In many cases, this is also enough.

There are some cases, though, when you need to measure every single pulse.  In those cases, you’ll want to choose an energy sensor with a high maximum pulse energy, such as the PD10-C, PE9-C, or PE10-C.  (Just be careful: a high-speed absorber usually means compromising on damage threshold. Be sure your lasers don’t exceed the damage specs.)

Here’s a complete chart of Ophir’s energy sensor repetition rates:

The other concern is that you don’t lose this advantage with a slow interface.  For the very highest pulsed lasers (>10 kHz), you will need to use the Pulsar PC interface, which can log pulses (via USB) at up to 25 kHz.

PC Interface Quasar Juno USBI Pulsar (per channel) Max Rep Rate 500 Hz 10 kHz 2 kHz 25 kHz

What about StarLab software?  On-board power meter storage?

All the specs mentioned above are for USB (or RS232) data transfer to a PC log file.  There are other ways to store your measurements, and it bears pointing out that these other methods may have different maximum logging rates.

StarLab software has a logging feature, while the on-screen graph updates at only 10 Hz.  When you want every data point, you should look at the log, since that is where you’ll find all the measurements – up to the maximum logging rate of your PC interface.

When using power meters, you have two options: on-board data logging or transferring the measurements to a PC log file (with StarLab). The maximum rep rate for these is not always the same:

Power Meter Vega Nova II LaserStar Nova StarLite Max Rep Rate PC 2 kHz 2 kHz 1.5 kHz 10 Hz 20 Hz On-board 4 kHz 4 kHz 1.5 kHz 10 Hz N/A

Bottom Line

As always, before you know which sensor and meter or interface you need, you must determine what kind of measurement is necessary for your laser application.

Need help?  Tell us about your specific setup and measurement needs.

1. Get educated on the theory of lasers, IPL, safety and clinical presentations. If you understand wavelengths, principles of photothermolysis, pulse width, fluence, spot size, clinical end points, Fitzpatrick Skin Types, expected side effects and possible adverse outcomes then you will be able to be discerning and make a well-educated decision. Choose theory courses that are accredited, endorsed or recognised by radiation health departments or health professional bodies to ensure they are quality and evidence based.
2. Check out your state/territory regulations. Are you able to operate a Class 4 laser in your state? Are there licensing requirements? Did you know that IPL are only regulated in Tasmania and that regulation is under national scrutiny by ARPANSA currently?
3. Be a critical reader. Examine the claims and the clinical papers that are associated with them. Don’t take anything for face value and get independent information too.
4. Decide what services you’d like to deliver. Will it be hair removal, skin rejuvenation, skin resurfacing, tattoo removal or all of these? There is no one machine (yet) that does it all. How much can your population afford to pay? Where will you find your customers?
5. Machine settings should be customisable. You want the machine with parameters that are able to be adjusted to suit the different targets and skin types to get the results you really want.
6. Ongoing education and support from the manufacturer is essential. After they show you the basics of how to use the machine will they return to show you advanced applications? Do they have marketing in place that supports your practice? They should provide signage for the door for controlled access and laser safety eye wear as standard. Just because you have been orientated to the use of the machine and its accessories, doesn’t mean you have completed a Laser Safety Officer course, if you don’t have one, get one (see number 1).
7. Consider the costs both initial outlay and ongoing. The initial cost of your equipment isn’t the end of the story. Consider disposable handpieces, handpieces that need refurbishment or repair and other disposables such as gel, spatulas and linen. Ask for information on the numbers of treatments needed to start to get a return on your investment.
8. Practice. Provided consent is obtained, you work within your scope of practice/skill set under supervision and safety protocols are adhered to; colleagues, friends, family, volunteers, paid models or paying clients can all help you hone your skills. Some of the RTOs have short practical workshops that you can do as stand-alone modules. Choose practical workshops that are accredited, endorsed or recognised by radiation health departments or health professional bodies to ensure they are quality and evidence based.
9. Determine your budget. Shop around, there are hundreds of choices on offer. Some are TGA approved, some are not. Some are in the hundreds of thousands of dollars and some are not. Consider if you will buy outright, rent to buy, seek finance or how you will pay your machine off.
10. Cover your legal bases. Have adequate professional liabilities insurance. Work within your legal scope. Develop your practice based on exceptional education, evidence based practice and the discerning choice of equipment.

Download these tips now.

If you want to learn more, please visit our website High Repetition DPSS Laser.