Is it possible to scan a sacher diode laser in certain wavelength range?

For example, when piezo offset is 0 volt, the wavelength of laser (read from a wave-meter) is x. When I set the piezo offset to -6 volt, I see laser wavelength shifting to x-x'. I would assume, when I change piezo offset from 0 to -10v, wavelength of diode laser is changing continuously. But I only see wavelength change starting at -6 volt.

So my question is, how to scan a diode laser wavelength in certain range?

  • $\begingroup$ If you're asking how to use a tunable laser that you bought, that's more of an engineering problem than physics. And also you need to tell us exactly what product you are using if you want us to have any hope of helping you. $\endgroup$ – The Photon Oct 7 '15 at 2:01
  • $\begingroup$ AFAICT, Sacher has at least 7 different tunable laser models available. $\endgroup$ – The Photon Oct 7 '15 at 2:03
  • $\begingroup$ It is "Manual Tunable Littman/Metcalf Diode Laser System - Lion". I would assume the ones using piezo to change wavelength is quite general in their products, or even other brands. Someone please help. Thanks! $\endgroup$ – Sean Oct 7 '15 at 2:20
  • $\begingroup$ The tunable laser in my lab has a 0 to 100 V tuning range. If voltages below 6 V didn't change the wavelength, the solution would just be to use the range from 6 to 100 V. $\endgroup$ – The Photon Oct 7 '15 at 2:23
  • $\begingroup$ The one I'm using has piezo offset from -13 to 13 V. However, from -7 to 0 V, the wavelength is just the same. From 0 to 13 V, the laser power is too low for me to do anything useful. And in the -13 to -7 V range, the wavelength only changes 3-4 times. Considering that I set the piezo step low as 0.01 V, I would assume the wavelength should change quite continuously. Do all these sound normal to you? What would be wrong? Thanks! $\endgroup$ – Sean Oct 7 '15 at 2:30

In my experience (from diode laser systems I built myself and some commercial systems), if changing the piezo voltage doesn't change the wavelength, it means that your feedback into the laser is not good and probably needs to be adjusted. I'm not sure what kind of access you have to the inside of the laser (if you're not comfortable opening the system, you should call customer support if its a commercial system), but if the feedback isn't good enough, you can try to "re-flash" the laser. Again, only do this if you are comfortable opening up the system, as you can cause a lot of problems if you don't know what you're doing. Also, try to find out what your warranty is. They may not want you to open the system at all, which in that case, don't do it and call for service. If you get to a point where you need to do it though, I'll describe the procedure below. Be sure to where a wrist-strap to ground yourself to the optical table. Diode lasers will die easily from static shock, and depending on the model of laser you use, they can be either dirt cheap or really expensive.

To flash the laser, you can turn down the current of the laser to just below the lasing threshold. Using an card with a small hole in it, you should be able to put the card in front of the laser output with the beam exiting through the hole in the card. If the alignment of the grating is off, you should see that the feedback beam hits the card away from the hole. Align the grating so that the feedback beam re-enters the laser. If you align it well, the laser will then go above the lasing threshold, and you should see the output suddenly get brighter, "flashing" the laser. You can then turn down the laser current again to just below threshold and repeat this process until you flash the laser at the lowest possible current. When you turn the laser current back up to the operating current, you should have good enough feedback to tune the laser over a wide range.

For very wide tuning ranges, make sure you also temperature control the laser. It also helps to have an electronics system to change the current of the laser in tandem with the voltage to prevent mode hopping. Some commercial systems have this already on there.

  • $\begingroup$ Thank you for your help. I'm able to flash the laser. After that, I've noticed that when I use piezo at different values, the laser power varies a lot. For instance, at -13 volt, it is 87.2 mW; at 0 volt, it is 67.4 mW; at 13 volt, it is 46.4 mW. Is this something normal? Btw, I've turned on current coupling, because I think this would give me a wider wavelength range. $\endgroup$ – Sean Oct 23 '15 at 22:03
  • $\begingroup$ This is what you would expect if you turned the current coupling on. It changes the current with the piezo voltage to keep the mode from hopping. Generally, you should be able to change the direction of the coupling, and amplitude of it to make sure that your tuning range is at a maximum. It does seem to be varying a lot, but I think your laser has a wider tuning range than what I am used to. $\endgroup$ – tmwilson26 Oct 23 '15 at 22:13
  • $\begingroup$ I always keep current coupling on because I assume I want to use this function so I can scan a range as wide as possible each time. This goes back to what I want to do: I want to use this Sacher diode laser to scan a range from 1100 to 1102 nm, and at highest possible resolution (with this laser). From what you are saying, if I want to do this without power fluctuation, then I should turn off current coupling (CC). However, I may have lower scan speed because CC is off. I will try CC off anyways. $\endgroup$ – Sean Oct 23 '15 at 22:38
  • $\begingroup$ 2 nm is a large range without the CC but it may work. I assume that you have a spectrum analyzer or wavemeter. You can monitor it to see if there are mode hops. If it does have a lot of mode hops, you can try changing the temperature set point of the laser to see if it makes it more stable. There should be some specs that tell you the safe temperature range for the diode you are using. $\endgroup$ – tmwilson26 Oct 23 '15 at 23:15
  • $\begingroup$ Yes, I have a wavemeter. I don't quite understand about the piezo part. For example there are two modes: one is piezo, the other is external. When I use piezo, I can see the wavelength is moving up and down. When I use external, there are offset to adjust. I would assume when I change this offset, then the wavelength is changing. I'm not sure if this is correct. Sacher doesn't have good manual, and I'm not familiar with this. $\endgroup$ – Sean Oct 27 '15 at 20:52

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