This is more of a practical question about what optics are on the market. I have a 1550 nm femtosecond laser in fiber (100 fs pulses, 200 mW, 100 MHz, spectrum ~1500-1600 nm).
I'd like to focus this laser as tightly as possible (for coupling into hundred-nm-scale waveguides). Currently, we are using an aspheric lens from Thorlabs with NA=0.6.
https://www.thorlabs.com/thorproduct.cfm?partnumber=C660TME-C
I've gotten quotes for custom aspheric lenses with NA>0.6, but they are very expensive. Is there anyway to beat NA of 0.6 without spending many thousands of dollars?
Hello,
Take a look at this web and choose the lens that has a CA(Clear Aperture) that matches your beam diameter; this is the condition required for having the NA of the lens. You probably will need to adjust your beam diameter by using an identical lens. Nevertheless, reaching sub-micron diameters is far away as using the 0.68 NA lens you would obtain a focused spot size of 1.2 microns. Anyway, this is the closer you can go by means of a cheap solution.
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3812
Regards and good luck!
Thanks Afshan and Efrain.
Of course, the beam must fill the lens, but this is pretty each to achieve since there are so many different collimators available for 1550 nm light. The Thorlabs 0.68 NA aspheric lenses have a design wavelength of 780 nm, so I'd expect that these would have huge aberration at 1550 nm. The Newport lenses have a maximum NA of 0.5, so worse than what I already have.
Thanks for your input though!
Hi Daniel,
When working with broad spectrum the lens assembly will have to be designed to have a minimal focal shift with color, i.e. achromatic but also to focus each frequency to diffraction limit on the same spot, i.e. you end up needing something akin to a microscope objective. (Rule of thumb would be to have the focal shift across your spectrum be (a bit) smaller than the Rayleigh length of your beam.)
The good news is that there are cheap microscope objectives with high NA, bad news is that you get what you pay for. See [1] for a cheap NA=1.2 objective - note that I have never used this objective and have no idea what its performance is like, I just present it as an example of how cheap it can be and still somehow fulfil the spec.
As far as I know everyone in multiphoton microscopy uses brand name high NA objectives (Olympus/Nikon,etc), and those cost several thousands of dollars.
More things to look for in your case is temporal chromatic dispersion from the lens/objective (if your application is very sensitive to peak power - a 100fs pulse chirped by 4000fs2/high NA objective stretches to 150fs, i.e. peak power is reduced to 66%), focal shift across spectrum which will lead to weird space-time profiles which can make coupling hard (different pulse durations across beam profile, outside part of the beam arriving significantly after/before the center, and each frequency focusing at a distinct distance), and if you can send your pulse through the objective without damaging it (more of a problem with amplifiers, less with oscillators).
This all depends on how sensitive your setup is to laser/optical performance (e.g. if you can just crank up the power and get the similar results as if your focus was perfect, then all these problems are secondary)
Good Luck!
[1] http://www.edmundoptics.eu/microscopy/finite-conjugate-objectives/international-standard-microscope-objectives/33441/
Thanks Francisco!
We have used cheap Newport microscopy objectives [1] for a similar focusing 1000 nm light. However, it's difficult to find objectives that are designed to work with 1550 nm light. I think that most objectives, designed for the visible, would have pretty high loss and aberration at 1550 nm.
Hi Daniel,
You could possibly try these microscope objectives: http://www.microthek.de/index.html- email them and ask what they have available. The lenses are typically cheap (few hundred euro) and go to high NA. They contain relatively few lens elements, which actually for your application might be highly beneficial, giving higher transmission and more predictable dispersion. The problem with the high end objectives is that they are extensively engineered with lots of lens elements to create a flat field and chromatic correction for visible wavelengths. This can make the dispersion and transmission much worse for IR beams where you are often not concerned about the field curvature. Hope this helps!
Hi Ummer,
Do you mean lensed fibers, like these: http://shop.ozoptics.com/single-mode-taperedlensed-fibers
These work well, but I believe that they are not available with NA higher than about 0.4.
I recently received the following e-mail from Pietro Vahramian, which provides the best answer yet.
Dear Daniel,
Months afterwards I've finally found two solutions:
I've contacted many other industries, but none had the same compentece/knowledge/price than Thorlabs. I am ready to bet my left kidney that there is nothing better on the market, as regards CNC glass aspheric lenses.
The only drawback is the delivery time of 4 months...but for custom products it's quite normal.
I do not have a ReaserchGate account, but it would be cool to add these information to the question you raised years ago, such that my (small) knowledge acquired solving this stupid problem does not get lost.
Cheers,
Pietro
Dear Daniel,
I hope you considered using Edmund Optics aspheres. If so, I would greatly appreciate your feedback on standard aspheric lenses we have in the catalog and our manufacturing capabilities for build-to-print manufacturing services. I can be reached directly at oleonov(AT)edmundoptics.com
Thank you,
Oleg
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