Ultrafast lasers have been extensively used in ground breaking research including two Nobel Prizes. Applications within spectroscopy, photochemistry, laser processing and microscopy are widespread. However, to capitalize on such short laser pulses, a pulse compressor is required to compensate for the dispersion induced by optical elements. Liquid crystal based spatial light modulators are most commonly used in laser pulse compressors. Although a proven technology in display applications, liquid crystals have drawbacks including phase jitter and a limited fill factor. Researchers at the Cui Lab at HHMI’s Janelia Farm Research Campus looked to Boston Micromachines Corporation’s prototype Linear Array Deformable Mirror (DM) to address these challenges.
To evaluate the performance of the pulse compressor, the laser pulses were analyzed with frequency resolved optical gating (FROG) using a commercial instrument (Grenouille, Swamp Optics, Atlanta, GA). In Figure a and b, the temporal and spectral profile of the pulse is shown when a flat wavefront is displayed on the DM. Evidently, the pulse is distorted and the spectral phase is not flat at all (a flat spectral phase is required for a transform limited pulse). Next, the beam returning from the pulse compressor was focused with a concave mirror onto a GaAsP photodiode and the resulting nonlinear signal was used as a feedback for the correction algorithm. After optimization using a technique called Phase resolved interferometric spectral modulation (PRISM), the temporal profile (Figure c) shows a dramatically shorter, Gaussian shaped pulse. The spectral phase is perfectly flat (Figure d) with less than 0.01 radians phase error and is stable in time. These results suggest that the precision and stability of the Linear Array DM allows close to perfect restoration of transform limited laser pulses. For more information on the optimization technique, you can access a scientific publication here.
In our next blog post, we will discuss the results of the use of the Linear Array DM in an interesting two-photon microscopy experiment.
More details can be found in our Linear Array white paper which includes a more detailed description of this application.
This past summer, Boston Micromachines Corporation conducted a survey of nearly 300 members of the business and scientific community to find out what features were valued in a deformable mirror for adaptive optics and other wavefront correction applications. Respondents came from our three major vertical markets: microscopy, astronomy and laser science. In this survey, we asked some fundamental questions and had respondents choose between three DMs with properties varying in categories of actuator count, stroke, response time and price in various combinations. We were able to drill down to what each respondent valued. Here are some of our key findings:
1) Actuator count was the most valued property
Across all verticals, this was true. Overall, respondents preferred an average of 1000 actuators. While microscopists preferred 140 actuators by almost 2 to 1 over other models, those who identified as laser scientists were looking for an average of 1001 actuators and astronomers preferred, on average, 1800 actuators.
This was very interesting to us considering we are the only player in the market to provide deformable mirrors with these actuator counts as standard products or are developing DM systems which meet these specific needs (we have a 2000 element mirror in the works).
2) High speed is important
The most frequently chosen option for response time amongst laser scientists was 50μs and all other disciplines preferred average response better than 300μs. This is great news for the industry considering that most mirror architectures can respond adequately to meet the needs of the users. Our DM architectures are available with response times up to 22μs and we are able to drive these mirrors with our X-Driver (response time down to 4μs), satisfying high speed requirements as well.
3) Low price is desired
As we hear so often, most users were looking for low-priced devices. This was the second preferred property after actuator count. While those of us in the industry talk about lower prices with higher volumes, the volumes just haven’t been there yet to make this prophecy come true. The hope in the future is that the DMs based on scalable technologies, such as MEMS, will take off and lower-priced devices will be available.
We definitely learned a lot from this survey, above and beyond what is mentioned above. If you have any questions about our methods or are interested in discussing more specifics about the responses, I would be glad to chat further. Just contact me at firstname.lastname@example.org.
Up until recently nearly all adaptive optics (AO) systems used wavefront sensors for correction. But with recent advances, off-the-shelf wavefront sensorless AO is becoming a reality. Benefits of this type of AO include enhanced aberration correction due to the elimination of non-common path errors and wavefront sensor noise.
BMC has developed a Wavefront Sensorless AO Demonstrator (WS-AOD) which provides a platform for utilizing metric-based wavefront control with BMC MEMS deformable mirror (DM) technology. While conventional AO systems perform closed-loop DM control using direct measurements of the wavefront as feedback, the metric-based approach uses details in the aberrated light to improve clarity. Two versions are available; one is optimized for beam shaping applications and the other is designed for imaging applications.
We see laser beam shaping as a key area in this exciting technology and our demonstrator is built to address the unique challenges of this field. Our WS-AOD serves as an introduction to wavefront sensorless adaptive optics principles. It allows users to understand the details involved in properly implementing a metric-based adaptive optics solution on an optical system. The demonstrator can also be used as a stand-alone aberration compensator. By introducing aberrations in the sample stage, the system can be optimized for a multitude of use cases from laser research applications to scanning laser microscopy. Additionally, the user can easily integrate the hardware into an existing optical system and utilize the open source software code for metric-based correction.
Schematic of WS-AOD for beam shaping applications. Also available for imaging applications.
To compensate for phase aberrations the WS-AOD uses BMC’s deformable mirror (DM) technology. BMC’s DM is a continuous facesheet deformable mirror that is controlled by hysteresis-free electrostatic actuators located on a square grid. The full DM active aperture can be as little as 1.5 mm to as much as 25 mm across. Each actuator can provide up to 5.5 µm of mechanical stroke, which corresponds to about 11 µm of phase control. The electrostatic actuator array is driven using independent high voltage channels with 14-bit resolution. This corresponds to sub-nanometer displacement precision. The drive electronics can provide frame rates of from about 4.6 kHz up to 100 kHz.
The control software for WS-AOD allows the user to correct for aberrations introduced as well as generate a random aberration using the DM. The software is open source code based in Mathwork’s Matlab and runs on platforms using Windows operating systems. By utilizing the included algorithm to manipulate the mirror surface, the mirror compensates for aberrations and converges to an optimal profile. The user has access to the open source code to balance correction capability between maximum signal and minimal time.
To learn more please click here for a copy of our Wavefront Sensorless Adaptive Optics white paper.
The SPIE Mirror Technology SBIR/STTR Workshop was held in Rochester, NY this year at the end of July. This is always a good conference for BMC, and we go every year. The conference can best be summarized from their website:
Tech Days annually summarizes the USA Government's investment strategies and activities in developing technology for any application (such as telescopes, imaging systems, seeker/trackers, high-energy laser systems, solar energy, etc.) which requires optical components. Tech Days covers technology investment efforts in: optical materials; substrate design & manufacture; optical fabrication and metrology technology; optical coatings; wavefront sensing and control via adaptive optics; nano-technology imaging technologies; etc.
I highlighted the text for emphasis as to why we attend: You can see why this is a great place for BMC to be. We get to present the latest progress on our NASA SBIRs (of which we have 4 ongoing), see some of the other great research that is going on in the field, and learn from the NASA Program Scientist what the future needs are for mirror technology. Also this year, BMC was a sponsor/exhibitor. This gave us a chance to set up a table displaying some mirrors and information about our products and technology. It was in a great spot at the conference where lunch, coffee breaks and the Tuesday night reception were held. While the conference was not as big as some other SPIE events (e.g. Photonics West and Optics and Photonics), it was a great opportunity to meet with some key people.
A couple of takeaways from the meeting were
(1) NASA SBIR/STTR program is strong and growing.
They are using the research funding they have for strategic programs that will help with technology development, which was called out in the decadal survey as an, if not the, important push for the next ten years.
(2) There is a continuing need for BMC mirror technology.
There are a number of projects that will require the wavefront control that our DMs can provide.
Both of these items point to a rich future for BMC and the deformable mirror industry as a whole. We look forward to connecting with these folks again next year and for many years to come.
Our customers are constantly making exciting scientific discoveries and we’re proud of the part our deformable mirrors play in their research. Dr. Meng Cui, Lab Head at Howard Hughes Medical Institute, Janelia Farm Research Campus recently presented the Iterative Multiphoton Adaptive Compensation Technique (IMPACT) that his team has developed for deep tissue microscopy at a webinar on “Advances in Biomedical Photonics”. In Dr. Cui’s presentation he discussed IMPACT which utilizes iterative feedback and the nonlinearity of two-photon signals to measure and compensate wavefront distortion introduced in tissue. He gave details on the imaging results on a variety of biological tissue including brain tissue through mouse skull and labeled T cells inside lymph nodes and compared his team's technique with conventional adaptive optics methods. For more details on Dr. Cui’s research you can view the entire webinar which was presented by Photonics Media at http://www.photonics.com/Webinar.aspx?WebinarID=21. Details of the research can be downloaded from the following site: http://www.pnas.org/content/early/2012/05/09/1119590109.full.pdf
This year, the CLEO Conference had its normal interesting character: A variety of users from hardcore laser scientists to focused business interests to laser scanning imaging folks. Boston Micromachines took our position within the Thorlabs booth for the 5th year(thanks again, guys!) and demonstrated some great technologies that we think will make an impact in the industry. Our MEMS Optical Modulator generated a fair amount of interest and prompted some great questions about its capabilities and possibilities. We showed its flexibility by demonstrating how with a simple input signal and an amplifier, a reflective diffractive element can be used to couple light into a fiber at varying frequency and amplitude. We went into the show thinking this would be the big topic of conversation. While we did have some great conversations and we’re more than happy with the response, the real star of the show was our Wavefront Sensorless Adaptive Optics Demonstrator for Beam Shaping. Users from all walks of the laser industry approached me with potential uses from wavefront characterization techniques to photon counting. I learned that the simplistic nature of the kit (maximize a signal through a pinhole) allows researchers with very different backgrounds to think of interesting ways to take advantage of its versatility. We found that the simple, clear spot on the screen was enough to entice microscopists and laser scientists alike to brainstorm interesting ways in which to integrate the deformable mirror, detector and algorithm of the system into their latest work. I am looking forward to some great follow-up conversations!
I did get the chance to venture out of the booth for a few talks as well as touch base with some new and old friends. Major impressions:
1) AO is still not a major player in laser science
While there were some interesting topics and uses of deformable mirrors and spatial light modulators, the technique is by no means pervasive as in other industries such as astronomy or biological imaging. Other techniques such as MIIPS (congratulations again, Dr. Dantus) serve the industry and are well proven to be able to satisfactorily shape pulses. Another theory: Laser scientists prefer to go after the laser for improvements instead of supplementary hardware. This could be for a variety of reasons such as: a) extra hardware means lost light, b) this is where they are comfortable and love to tweak things or c) the cost is just too high right now.
2) Beam characterization is becoming more affordable
With a few companies introducing higher speed and lower cost wavefront sensors, the market is becoming more accessible to more researchers. This can only be good for everyone.
3) Booth traffic is down, but more focused
In past years, my conversations were usually an even split between educating the visitor about the basics and having in-depth discussions about the capabilities and possibility of integrating devices into optical systems. This year, the split was more like 75/25 in favor of detailed discussions. Many are well aware of the background and of the 75%, at least half approach me with well thought-out ideas. It is very refreshing and encouraging to have these discussions and I suspect the ratio will continue to grow as years go by.
Overall, it was a productive show. I look forward to returning to San Jose next year and introduce exciting products that we have in our product roadmap and get more people to shape their light!
Boston Micromachines has been awarded US$ 1.2M in contracts by NASA's Small Business Innovation Research Program (SBIR) to support space-based astronomy research. Now, one might wonder, considering this is astronomy, “why is anyone spending any money to develop deformable mirrors for use in space telescopes looking out at the stars? I thought that deformable mirrors were used to remove aberrations introduced by the atmosphere. There’s no atmosphere up there!”
Well, you’d be right: There is no atmosphere up there. But, there are aberrations: In the optical system. When systems are designed, none of them are completely perfectly aligned. There’s no such thing. But, when you’re looking for planets around stars in other galaxies, you need things to be well aligned. VERY well aligned. So, to compensate for any misalignments introduced either during transit to space or thermal variations in the instrument, you can use a deformable mirror as part of an adaptive optics system. So, for this, we are designing a 1021 segment (3063 actuator), tip-tilt-piston deformable mirror. It is the largest ever designed of this type and is expected to be used as part of a coronagraph telescope for exo-planet research.
Two other things to worry about are power and weight. In satellites, there is a very limited amount of power, and the weight greatly affects the cost to put the satellite into orbit. In terms of the mirror, MEMS has that taken care of: The power is extremely low and the weight is many orders of magnitude lower than traditional piezo-electric-based mirrors. So, the other half of the equation is the drive electronics. As part of this research, BMC is designing multiplexed drive electronics which will reduce the size and weight significantly and lower the power even further.
You put the two parts together, mirror hardware and drive electronics, and you have a low-cost, low-power, lightweight solution for the most cutting edge astronomy research going on right now.
If you have any questions, please contact me directly: I would be happy to talk about these products or refer you to the folks who are attacking this challenging problem.
Link to press release.
(Image credit: A giant Hubble mosaic of the Crab Nebula, a supernova remnant)
Tags: deformable mirror, adaptive optics, boston micromachines, turbulence, resolution, response time, CW, pulse, pulse width, peak power, average power, laser beam, SLM, spatial light modulator
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!
Although William Wallace shouted this in the movie, “Braveheart,” as he died in the hopes to inspire others to join his fight against tyranny, our interest here at Boston Micromachines is related to “degrees of freedom” rather than individual rights. I chose this as a post because numerous times we have come across customers who miscalculate the minimum number of control points (re: actuators) needed to satisfy their wavefront control needs because the forget about this simple concept. Here’s an example:
A customer recently came to us insisting that they needed one of our advanced tip-tilt-piston devices over a continuous surface deformable mirror. Their reasoning was that with the same number of actuators, to correct for their wavefront, tipping and tilting the individual segments would give them more control than using the simple piston mechanism of our continuous device. This is true if you compare the number of segments (with three actuators) to the number of single actuators of a continuous DM. However, if you choose to use a tip-tilt-piston device over a continuous surface mirror, you actually need more actuators (if you work out the numbers, 1.8 times more) to achieve the same level of correction capability. This has been explained and presented very well by Claire Max at UC Santa Cruz. See the presentation materials here (Slide 47 goes over the equations).
If we step back from the equations and look for the basic concept behind this, it boils down to the ability to change the profile of the surface in many more ways using a continuous membrane as opposed to stiff, separate segments. Even more simply put: If you want to approximate a curvy line using only a few points (five, let’s say), would you prefer to use a slightly less curvy line, or a series of straight lines?
The figure below illustrates this concept.
So, the next time you’re considering wavefront control, keep this in mind before hitching your wagon to a particular architecture. THINK FREEDOM!!!!!
We received notice yesterday that an Adaptive Optics Optical Coherence Tomography device for retinal imaging built by Lawrence Livermore National Labs won a 2010 R&D 100 Award!
Why do I mention this?
Because our Multi-DM is a central wavefront correcting component in the system. For those of you looking for nitty-gritty details, this is a single-deformable mirror system with a Badal optometer used for low-order corrections. The DM is 140 actuators with an aperture of 3.3mm x 3.3mm and stroke of 1.5 microns (wondering what these terms mean? Refer to this previous post for definitions and our website here for more on the mirror model). Check out the other award-winning inventions at the link below.
Congrats, guys and we’re looking forward to the awards dinner later this year!
Picture: Retina in 3D shown with 3-micron resolution, smaller than most eye cells.
Link for more information: https://publicaffairs.llnl.gov/news/news_releases/2010/NR-10-07-02.html
Update: We're on TV! Check out the video below from ABC 7 News in San Francisco:
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