Hello. Can anybody let me know what are the differences between a low quality laser and a high quality one, in terms of single molecule microscopy, with the same power (about 500 mW) and wavelength? I am talking about $20000 from Coherent and same $300 from Thorlabs, for epi and light sheet illumination, what major differences will they make?
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some laser quality metrics that may vary across different models include:
- Temporal stability of beam intensity (i.e. power fluctuations)
- Temporal stability of beam direction (i.e. beam wandering)
- Beam divergence / collimation
- Beam diameter
- Beam quality factor (M2) (i.e. roughly how gaussian the beam profile is)
(hereâs some decent info on laser parameters from coherent.)
The degree to which you need any particular property depends a lot on the application. For instance, if youâre doing something where interference is critical (perhaps like SIM, or lattice), then beam quality factor is important⌠If youâre trying to do very careful quantitative work, then temporal stability in beam power and (low) wandering is very important. If youâre just trying to âlight up some fluorescenceâ for qualitative purposes (or perhaps very low NA stuff?), and donât really care about some stray interference and/or intensity fluctuations, maybe you donât need to splurge on the 20K modelsâŚ
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Many thanks for your informative reply.
The thing that I cannot understand is that are these the problem of controllers and electronics, or laser source? And what is the difficulty of âbeam divergen / collimationâ? Even I used same lens system?
Good questions⌠and Iâm not able to answer them!
@Nicola, @nico.stuurman, @JonD @Hazen_Babcock ?
Hi,
a bit guessing here, but probably the beam divergence/quality is related to how the semiconductor laser cavity is manufactured (for solid state laser). It could even be that the companies selling expensive lasers only select the cavities which pass a quality threshold: if they are manufacturing their own laser cavities, they might trash a lot of under-spec cavities, or if they buy from other manufacturers they pay a premium price for the best items. Instead, I think the stability is more related to how the cavity is properly thermally stabilized and how the current running through the junction is stable: to get a stable emission, a laser needs to be both kept at a stable temperature and run at a fixed current. Only keeping one variable constant will result in âmode hoppingâ (wavelength or power fluctuating).
Is there anyone here with some âinsiderâ knowledge about the subject?
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Which lasers specifically are you looking at?
Part of the price difference is likely packaging. Iâm guessing that the $300 laser from Thorlabs is a (bare) diode laser. In order to get this work in your setup you will need a constant current source (~ $2k from Thorlabs), some sort of thermally heat-sinked mount ($500 - $1000), possibly an active temperature control unit (~ $2k from Thorlabs) and the collimation optics. The $20k laser from Coherent is probably complete, it comes with a power supply, etc⌠and you will get at least 500mW from it with reasonable beam quality. You turn it on and it just works. If the Thorlabs diode is also rated for 500mW you will probably get less than 500mW of âusableâ power from it due to losses in the collimation, etc. necessary to achieve a reasonable quality beam.
-Hazen
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Thanks for explanation. And could you please let me know whatâs the actual effect of low quality cavity and divergence on illumination and final images? (Can I simply say that I canât focus this laser to a perfect thiny gaussian spot and why?)
Great! Youâre right. But I think I can still find low price plug and play lasers from other companies, which of course comes from the low quality.
So, can anybody suggest me a threshold? With TIRF, epi and, light sheet illumination for single molecule quantitative imaging with high (ms) temporal resolution, should I go for best available laser quality, or a simple good one can perfectly work for me?
A semiconductor laser has an astigmatic output because its cavity has a rectangular cross section. Since its cavity is small (~mm) compared to gas lasers cavities (~cm or tens of cm), the beam also diverges much more quickly and the spatial coherence is smaller for semiconductor lasers vs. gas lasers. When focusing a semiconductor laser, you will get an astigmatic focus if no additional correction optics, specifically designed for that laser, are used. I guess that the design and alignment of such correction optics also plays a role in pushing the price upâŚ
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With TIRF, epi and, light sheet illumination for single molecule quantitative imaging with high (ms) temporal resolution, should I go for best available laser quality, or a simple good one can perfectly work for me?
i know the concept of buying something expensive âjust in caseâ is frustrating ⌠but itâs nearly impossible for someone else to answer this for you. It depends on so many factors, including your appetite for risk, sensitivity to downtime, ability to notice and fix problems that might be arising with your laser, precision demands and magnitude of change in your experiments (i.e. if youâre looking at massive intensity changes, then you may still have sufficient precision even with a fluctuating laser). Additionally, even if you could put a precise number on the laser metrics required for your experiment, the matching performance metrics may or may not be measured and reported for all of the lasers youâre considering.
âSingle molecule quantitative imagingâ demands (at least in theory) pretty good equipment, particularly the quantitative part⌠so youâre certainly in a situation where high quality stable equipment would be nice, but thatâs a budgetary decision only you can really make.
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Dear Talley, many thanks for your advice and helpful reply.
And happy birthday to you. 
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