Our blog post this week will cover the recurring concept of laser power stability, which is critical to better understand when you want to optimize your current or future setup involving detectors from our partner Gentec-EO .
Laser stability is an idea that tends to be associated more with continuous wave (CW) lasers. This article will therefore refer more to lasers of this type in particular, but most points discussed below also apply to pulsed lasers as well. Concepts specific to pulsed laser applications are gathered in the last section of this article.
Understanding laser power stability
Laser power stability is any metric that allows the operator of a laser system to evaluate how stable the output of the laser is. There’s no real formal definition based on units or a universal mathematical function for that matter. Generally, the operator tends to have his/her own idea of what makes a laser sufficiently stable for a specific application.
Laser power stability per se is an important thing to consider in most, if not all applications, because it directly affects the quality control of any process incorporating a laser source. For example, laser system integrators in industrial settings would not want the power of a laser source to be too far off the specified value because it would not only impact negatively the usage of the end user, but also take much time to support and help customers in the field using the system. Some of the applications requiring very fine power levels will also require a particularly stable laser power output to ensure quality and consistency.
The concepts discussed below will help you find the best way to ensure proper laser power stability. We’ll see how one can use a Gentec-EO detector to make it so.
The most basic step that does not even require a Gentec-EO detector
It’s important to distinguish firsthand that a Gentec-EO power detector can indeed give you an indicator of laser power stability, but most of the work to ensure stability is going to happen on the laser itself.
Laser radiation is achieved thanks to stimulated emission of photons after all, so there are no actual natural sources of laser radiation, if you will. The physical principle behind the emission of laser radiation of a certain wavelength tends to be different from the one of another wavelength. All in all, the medium that generates the laser radiation, may it be a gas or a crystal, needs to be controlled and monitored at all times, otherwise one should expect laser stability to be affected.
The most basic step is to ensure the laser has been properly warmed up before usage.
This is a well-known fact that is too often forgotten by laser operators. One can expect quite a difference in power measurement between the moment you turn on the laser and tens of minutes later. You can imagine this is not necessarily the first thing that will appear in the manual of your brand-new laser source, so it is understandably easy to skip this step when you first fire it up.
If you’ve already put aside this factor as a potential cause of laser instability, you can go straight to the more philosophical side of the subject.
An exercise in statistics and philosophy
You thought you wouldn’t need to go back to statistics and philosophy after college, right? I guess some things just won’t go away.
Laser sources and laser power detectors each have a set of properties (i.e. specifications) that will dictate what you can conclude about laser power stability for a laser source.
Laser manufacturers will often indicate power stability as part of their specifications. One would need to inquire the manufacturer to know the exact implication of the given values for this parameter, but for all practical purposes, we know this refers to the maximum relative power fluctuations one should observe over time.
Now, Gentec-EO power detectors also have a parameter that is closely related to power stability: repeatability.
Repeatability is a property that refers to how much a power measurement may vary when repeatedly measuring a theoretical “perfectly stable” laser beam (i.e. laser power value is perpetually the same over time) using the same Gentec-EO detector. As you can imagine, this can get a bit philosophical because there is no such perfect laser source (unfortunately), so repeatability can only be evaluated in highly controlled laser setups.
Combine those 2 together, and things can be difficult to interpret. This is the point where one can wrongly assume things about either the laser source, the Gentec-EO detector, or even both. Those 2 properties of laser and/or detectors cannot be added or multiplied in any way you might expect or hope for. They are random errors (i.e. random variations). Unlike systematic errors, there is no way to quantify by how much they are currently impacting your measurements. You just have to deal with the fact that “they’re just there”.
All in all, this implies that a certain level of observable laser power fluctuations, which are equivalent to saying this laser is unstable, cannot be totally attributed to the laser itself or the detector. These fluctuations can be direct manifestations of these properties that are inherent to the laser setup and cannot be systematically quantified.
Not your typical how-to, but still in your interest to understand
Some guidelines still need to be followed before one can distillate the problem down to this fundamental level of uncertainty about the laser source or detector.
First off, like we discussed above, laser source should be properly and thoroughly warmed up before usage. It is not uncommon to find that 20 to 30 minutes may be required to ensure so.
Secondly, and in the same line of thought as the previous point, the laser should always be used in the same environmental conditions. We include not only ambient temperature and cooling mechanisms here, but also the idea that the steps before using the laser are the same. This would include warm-up time, and also power levels, wavelength, etc.
Thirdly, and once again a follow-up on the previous point, one must take into consideration the uncharacterizable fluctuations that may be observed and that are inherent to the laser source and detectors, which are called random errors. Generally, a detector, especially one from Gentec-EO, tends to fluctuate less (assuming a perfect laser can allow us to characterize this) than a laser source would, assuming a perfect detector can allow us to characterize this. Numerically, this is coming from the fact that a laser power detector has better repeatability than the specified power stability of laser source when you check with your manufacturer. You might have also noticed that linearity with power was not mentioned in this article: this is coming from the fact that there is indeed an observable additional error on the power measurement given by a power detector when one checks at different power levels, but this is actually corrected automatically when it is used in conjunction with a Gentec-EO meter. Nothing to worry about on the linearity matter then.
Fourthly, one should always check power stability over an extended period of time. Power fluctuations are usually coming from environmental or design factors, so they are bound to manifest themselves better if you let it go for a while.
Fifthly, make good use of the statistics function of Gentec-EO meters. MAESTRO has a statistics interface that tracks useful information, such as root mean square (RMS) stability, peak-to-peak stability and standard deviation. The mathematical implications behind these parameters is outside the scope of this article, but they put a number on laser stability in their own way. It is definitely in the interest of most users to get accustomed to these parameters to fully optimize the usage of Gentec-EO meters and better tune their lasers to narrow down what might be causing laser instability.
Finally, if you are still unsure whether the laser source is actually unstable or the power detector is tricking you into thinking so, feel free to contact us to find answers specific to your situation.
Laser energy fluctuations, or how to philosophize with an energy detector
As promised, laser energy stability is a subject of its own that must be discussed.
The concept of interest here is peak-to-peak stability. Some laser applications may require each singular pulse of a pulsed laser to not be outside of a certain range of energy. A notable example of such an application is the LiDAR. This technology probes the environment to return information about it, and we find it in cases where it is critical to have the right measurements, such as self-driving cars. Our ultrafast joulemeter, Mach 6, is an amazing solution for such lasers because it incorporates fine electronics that can pick up pulse-to-pulse information of ultrafast lasers. In this particular example, the Mach 6 software gives peak-to-peak stability information in a visual way to help the operator quickly assess the stability of the laser source.
Most other concepts discussed above also apply to energy meters, to the point that one might consider using a power detector in certain pulsed applications. In the previous example, we have assumed that information about singular pulses was of use, but some pulsed laser applications only require the user to know the stability of the laser over extended periods of time. When pulse-to-pulse monitoring is not needed, one should simply use a power detector, since pulsed lasers do generate average power after all. This is also very nice if you wish a solution that is simpler to use. Integrators may also prefer this solution for this very reason because it requires less data processing and still returns sufficient information about laser stability.