Published: 2019-07-27 | Categories: [»] Engineeringand[»] Optics.

It has been a long time since my last update on ThePulsar but I was unfortunately extremely busy. I have had a lot of unrelated stuff (most of them happy :-)) which kept me away from the keyboard. Nonetheless, I have some good news that I can finally share with you!

Mid of June I proceeded to an upgrade of the laser source for the [»] Raman spectrometer setup and managed to get a 12 cm-1 on the nitromethane peak near 1000 cm-1. This is not as good as the potential 6-7 cm-1 of the optical system but it is already competitive with commercial systems that range in the 8-16 cm-1 region. I don’t know the origin of the limitation at the moment but I will continue investigating to see if it is possible to squeeze some more juice out of the setup :)

An experimental spectrum of nitromethane along with SDBS data are given in Figure 1. Each of the peaks are clearly resolved with a resolution very close to the one of SDBS.

Figure 1 – Nitromethane spectrum compared to SDBS data

The problem faced with the previous laser system was that it was multimode with a bandwidth on the order of 0.6 nm limiting the resolution to about 20-25 cm-1. The only way to overcome this was to switch the laser by one with a bandwidth smaller than 0.15 nm. Such lasers are however more expensive with typical prices above 1500€ for a 0.1 nm 50 mW laser with 5% stability. Singlemode laser units being even more expensive with prices above 5000€ but have extremely narrow bandwidth and extreme stabilities. All these laser units (e.g. CNI, Cobolt, Coherent… DPSS lasers) are therefore not an option for a low-cost DIY Raman spectrometer such as ours. You may find cheaper alternatives ranging in the 100-500€ range but according to a sales engineer from Roithner Lasertechnik, they all have typical bandwidth on the order of 1 nm. The guy also told me that by tuning the laser currents and cooling them properly it might be possible to get something better than 1 nm but that he could not commit on any figures. It seems however not realistic to go below a 0.15 nm resolution with these units and they are therefore not an option either here.

Since the solution with complete commercial units seems to be doomed, I turned myself to different options. One of the options would have been to take a multimode laser and to filter its output with a monochromator setup. After some math, I quickly realized that this option would turn out to be as expensive as to buy an entry level pro unit… As a consequence, the only option left was to work directly from a single mode laser diode and to drive it manually. Ondax does produce VHG (Volume Holographic Grating) feedback laser diode that are single frequency but they are horribly expensive with prices on the order of 1000€. They do however produce these diodes at several frequencies which can be an advantage if you are not working at 532 nm.

Finally, after some discussions with Thorlabs engineers, it occurred that their DJ532-10 (10 mW) and DJ532-40 (40 mW) laser diodes have very small bandwidths! They were kind enough to perform a coherence length measurement for me which gave 23 meters for the DJ532-10 and 10 meters for the DJ532-40. Clearly, this was way beyond any expectation because the price for these diodes are about 140€ and 170€ respectively!

The only problem with these diodes is that you need both a LD driver and a TEC driver. To address problems one by one I decided to invest in their laser driver kit which includes a LD driver and a TEC driver plus a TEC mount and some handy accessories. Unfortunately, this derisking approach had a price which is about 3000€… uch! You may ask what is the point in buying such expensive gear while a 1500€ CNI laser could have done the job, but you should remember that, ultimately, the commercial driver kit will be replaced by a much cheaper custom version designed to perform just well enough for our Raman spectrometer. Put differently, I am the one taking the financial risk such that you can get the cheap version later :) Considering these repetitive investments I am doing for the website, I will probably open a Patreon account by the end of the year to help you support me if you’d like to (even single dollar bills will be welcome!).

Using the commercial driver kit I already performed a few experiments to check what would be the temperature and current sensitivity of the laser to draw some specifications for the custom drivers. Up to now, I limited the operating current to about 250 mA which should be about 5-10 mW. I prefer staying on the safe side at the moment since I don’t have any powermeter to check the laser output and that Thorlabs recommends not exceeding 40 mW with the DJ532-40. This also makes the results comparable to the previous results obtained with the 4.5 mW laser.

Concerning the temperature effect, Thorlabs recommend operating the diode between 20°C and 25°C. These should be considered as safe values but be warned that below some and above some thresholds the laser will stop emitting light. In my experiments, the absolute boundaries were about 17-28°C. I would therefore recommend using a set-point of 22.50°C to stay as much as possible in the recommended range.

I also measured the effect of temperature on signal strength. The results are shown in Figure 2 with a nitromethane spectrum. The temperature effect is relatively large with a 9.67%/°C. Achieving a 1% change would therefore require a temperature stability of 0.1°C.

Figure 2 – Temperature effect on signal strength

Finally, I also checked the effect of current change on signal strength. The results are shown in Figure 3 with an iso-propanol spectrum. The spectrum is a bit noisier than the previous one because I did not care measuring a blank spectrum first which tends to correct some hot/cold pixels effects. At 22.50°C, the system is not so sensitive to current change with only 1.65%/mA. Achieving a 1% stability would therefore require a 0.5 mA current stability.

Figure 3 – Current effect on signal strength

I decided to start with the custom current driver by adapting my [»] 1A LED driver to a 400 mA LD driver. I added several safety systems such as an overshoot protection, a transient suppression diode, polarity change protection and an interlock system (required by EU regulations). This driver had a 0.6 mArms at 400 mA and I could therefore have hoped to reach the 0.5 mA without any problem using the improved damping of the new circuit.

I first tested the new driver with a LED and it was working fine but, unfortunately, it killed my test LD in about 20 seconds… I still don’t know what happened :-/ I’ve checked my drawings with electrical engineers at the office but they were not able to find any flaw. Since I don’t want to spend loads of money on test LDs (25€ fried in 20 seconds is not what  I would call a good usage of money), I may go for a commercial LD driver which are about 150€. But this will be for a later post!

Anyway, I hope you enjoyed this post and that you are as excited as I am with the outcome of these Raman spectroscopy experiments :) Stay tuned for updates!

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