According to which criteria should I choose a laser diode?

[Aymangh994]

Hello everyone,
I don't know which laserdiode and lens system should I choose for my Project. It is all about AFM Microscopy with a cantilever ( Length= 450 um, width = 50 um and thickness= 2um). So as you see it is not a small cantilever. As far as I Know, the larger the cantilever is, a larger spot will be recommended. The AFM will be also operated in contact mode. Any Help will be appreciated.

 

[Twigg]

My limited understanding of AFM is that the laser reflects off the cantilever and that pointing is used to measure deflections of the cantilever. Is that the laser you're shopping for?

As far as criteria, I can offer you two:

1) Make sure your laser's wavelength is within the bandwidth of your photodetector.

2) Ideally, you want a laser with a pure Gaussian mode. This is specified by the $M^2$ parameter. For a pure Gaussian (TEM00) beam, $M^2 = 1$. However, this isn't a hard requirement, as you can always couple your laser into a single-mode fiber to clean up the beam profile at the cost of losing some power (which you probably don't need much of).

As far as the lenses go, figure out the laser first, and then worry about the lenses. Do you know what beam waist is optimal for your given cantilever dimensions? We'll need that information to be able to help.

 

[DaveE]

I'll second the suggestion to consider fiber coupling. That could really simplify the mechanical design; or perhaps not, since we don't know about your size requirements etc.

 

[Aymangh994]

My limited understanding of AFM is that the laser reflects off the cantilever and that pointing is used to measure deflections of the cantilever. Is that the laser you're shopping for?

As far as criteria, I can offer you two:

1) Make sure your laser's wavelength is within the bandwidth of your photodetector.

2) Ideally, you want a laser with a pure Gaussian mode. This is specified by the $M^2$ parameter. For a pure Gaussian (TEM00) beam, $M^2 = 1$. However, this isn't a hard requirement, as you can always couple your laser into a single-mode fiber to clean up the beam profile at the cost of losing some power (which you probably don't need much of).

As far as the lenses go, figure out the laser first, and then worry about the lenses. Do you know what beam waist is optimal for your given cantilever dimensions? We'll need that information to be able to help.

Hello, Thank you for you answer. I really appreciate it. I have another question: Is this laser diode https://www.sukhamburg.com/products/details/51nano-N-660-1-M01-P-5-2-28-0-150
a single mode laser ?? I found an article, which is about laser diode in AFM. As recommendation from my professor was this laser diode, but the problem is, that the fiber type is polarization maintaining and not a single mode fiber.

 

[Twigg]

All polarization-maintaining fibers are single mode fibers. Not all single mode fibers are polarization maintaining.

Huh, I didn't know Schaefter Kirchoff was making lasers. Cool!

Polarization maintaining (PM) fiber is probably unnecessary but not harmful for your application (if I understand it correctly, of course).

I don't see anything obviously wrong with this choice of laser. The output power is pretty low. Make sure your detector can reasonably resolve this little optical power. This is especially true if you intend to use a quadrant photodetector.

From the noise floor of the detector and the detector's responsivity at 660nm (and transimpedance gain), you can estimate how sensitive your setup will be to displacements of the laser beam. Divide that number by the total distance between the cantilever and the detector, and you have your setup's sensitivity to deflections of the cantilever.

 

[Twigg]

Also, just a note about terminology:

Is this laser diode https://www.sukhamburg.com/products/details/51nano-N-660-1-M01-P-5-2-28-0-150
a single mode laser ??

A "laser that has been coupled to a single mode fiber" and a "single mode laser" are two very different things. A single mode laser (usually) means a laser that only emits light in one resonator mode at a time (when tuned properly). In contrast, a multimode laser would have a spectrum that consists of several resonator modes, each with different frequency. Just a heads up to save you confusion down the road.

 

[Aymangh994]

thanks a lot for you valuable information. I have chosen this quadrant detector. https://www.hamamatsu.com/resources/pdf/ssd/s1880_s2044_kpsd1015e.pdf
Do you see any problem with that?

 

[Twigg]

Did you mean the S1880 or the S2044? These are two different detectors.

This detector looks good! Here's how you can tell:

Look at the spectral responsivity curve in the datasheet, and see what the responsivity (in amperes of photocurrent per watt of incident optical power, so A/W) is at the laser wavelength of 660nm:

The red markup is me. You can see the responsivity (they call it photosensitivity, I've always seen it called responsivity) is about 0.45 A/W. So the full output power that S&K diode, which was 0.9mW, will produce 405$\mathrm{\mu A}$ of photocurrent.

Now look at the following table:

Notice that the saturation photocurrent is 0.5 mA. This means that with the full power of your laser (0.9mW, which creates 0.405mA of current), you will be using 80% of the dynamic range of the detector. That's a nice, beefy signal to start with.

It's not clear to me exactly how good a position resolution you will get with your setup, but based on their specification and the conditions they measured it in, I would be surprised if you saw much worse than a few $\mathrm{\mu m}$ resolution.

I don't know how fast an AFM moves, but one other thing to maybe check with your advisor or coworkers is whether 1.5 microseconds rise time is fast enough.

 

[Aymangh994]

Thank you very much for these Information. I really appreciate it
Actually this helped me a lot so a massive thank you once more