For those interested in using our deformable mirrors with laser applications, there are a few common questions which are asked in regards to the AR-coated window and if it is/can be removable. Below is a summary of what's standard and what's possible:
Both the optical modulator and our DMs are protected by a 3mm thick window, which are standard BK-7 windows fromThorlabs. The options for windows are:
This can also be customized upon request. The windows are mounted on a 6° angle in order to prevent ghosting.
A lot of requests are in regards to removing the protective window. For our standard DMs, the window is not removable as it is attached with an epoxy. For our modulators, the windowIS removable. We highly recommend youDO NOT REMOVE the protective window. The only exception to not having an AR-coated window would be if the DM was operated in a clean room environment. In this case, we can deliver the modulator or DM without the window and include a protective removable lid instead. In addition, we recommend flowing Nitrogen at a very slow pace around the mirror to ensure the humidity remains low around the DM. The required humidity is <30% as the mirror is made of polysilicon which needs to be protected from corrosion.
In our second installment of this series designed to boil down the questions that need to be answered before selecting the right mirror, we will review some of the past categories with alterations specific to laser beam shaping and introduce a few new ones that pertain only to beam shaping. We plan to focus on pulse shaping applications in our third and final installment of this series.
So you have a beam (CW or pulsed) and you want to control it. Below are the fundamental questions that need to be asked in order to ensure that you’re on the path to obtaining great results in your research or manufacturing application. This list should be combined with Part 1 of this series to get the total picture of what’s needed. I have left out the “pitch” and “response” categories, assuming that you have read the previous installment. Click here, in case you haven’t.
1) Aperture: How big is your beam?
The size of the wavefront is the first and foremost issue to understand. Some applications have no control over this while others can change the size of their wavefront through the use of some simple focusing optics. Before doing research into your alternatives, you should figure out what your limitations are in relation to this.
2) Control: Phase control? Beam steering?
This will greatly affect the basic type of mirror you will need. For phase control, most modern phase-only mirrors will work, depending on your requirement of resolution (see “2. Resolution” from Part 1 of this series). However, if you get into beam steering, the amount you need to move your beam will greatly affect the type of mirror you need. For example, if you’re trying to move the beam multiple degrees, a fast-steering mirror is probably a good place to start. However, if you’re looking to only make very fine adjustments (milliradians), you can benefit from MEMS-based solutions which are usually referred to as tip-tilt-piston (TTP) devices or piston-tip-tilt, if you’re from one other particular company out there (you know who you are J). Many customers have come to us asking about using our entire mirror surface to steer a beam. For those asking for big angles, we unfortunately have to turn them away, but some want to steer it a very slight angle at high levels of precision and we can do that.
3) Speed: Do you want to make fine adjustments? Are you looking to phase-wrap?
If you’re shaping a beam that is pretty much static, then some low-cost solutions will work. However, if you’re looking to change the profile at high speeds with high precision, MEMS solutions are a great bet. The stroke is sufficient to accomplish phase-wrapping, using our SLM model (segmented surface). With sub-nanometer precision, very precisely-shaped beams are possible.
This is a biggie: If you have a high-powered laser, your options become limited very quickly as most of the very precise devices are a bit fragile as well. Lots of research is being conducted to steer big, powerful lasers and the bulk of the technologies out there fall short due the fact that they are made of thin-film surfaces and temperature-sensitive materials. My recommendation for this is to make sure you know the “big three” properties and contact individual manufacturers to see what their experience is. They are:
1) Peak power (in W/cm2)
2) Average power
3) Pulse width (if applicable)
Most manufacturers probably can’t guarantee much, but if your application has beam characteristics close to some of the data points they have, then it will make you much more comfortable that you won’t be frying mirrors when you fire things up. BMC has a database that is constantly being updated with new experience that we would be happy to discuss. Also, see this paper for the latest published results from our friends at the UCO/Lick Observatory.
As I mentioned before, this is not exhaustive, but if you have these questions answered, your first conversation with either us or one of our competitors will be a pleasant one which will make you more confident of your purchase.
Please chime in and let me know what you think of this series! Again, stay tuned for the final installment where I will talk about pulse-shaping and the different ways that deformable mirror technologies can be used to create the perfect pulse!
Michael Feinberg is the Director of Product Marketing at Boston Micromachines Corporation. He has over 10 years of marketing and engineering experience in various technology fields. He can be reached at firstname.lastname@example.org and welcomes any comments about the content presented herein.