Beam combiner for EPR spectrometer

Scientists in the MEL (Materials-Envi Lab) group led by Professor Zbořil at the Center for Nanotechnology at VŠB-TUO are actively engaged in electron paramagnetic resonance (EPR) spectroscopy and its applications in modern low-dimensional materials. EPR examines the distribution of spin energies of unpaired electrons in a magnetic field and transitions between individual spin states induced by microwave radiation. This method can be further improved by exciting the sample with visible light (so-called photo-EPR), which allows, for example, the study of defects in semiconductors.

This is precisely the kind of improvement that scientists at VŠB wanted to make to their existing EPR spectrometer. The requirement was to obtain a laser source that would allow excitation using four selected wavelengths ranging from red (637 nm) to UV (375 nm). The lasers were to be fed into the spectrometer via optical fiber. The main criterion was to achieve homogeneous illumination with sufficient power over an area of at least 5x5 mm in the plane of the sample.

Similar to other recent implementations, we chose to manufacture a beam combiner. We used diode lasers with relatively high power (> 100 mW for 3 of the 4 wavelengths), which meant that the lasers needed to be actively cooled and protected from back reflections. In line with previous implementations, we equipped the beam combiner with a central key switch, an interlock connector, and buttons allowing individual switching on/off of the lasers. Laser management (e.g., setting the nominal power) can be performed remotely via a PC using the prepared USB connector.

In this case, the customer did not require fast modulation (pulsing) of the lasers or single-mode (SM) fiber output, so the only major technical challenge was to optimize the output profile. Given the nature of the application, where cuvettes with samples inside an EPR spectrometer are to be irradiated, the goal was to ensure the most homogeneous profile possible at the output in the required area. The main obstacle to achieving this goal is the formation of an interference pattern at the output of multimode fibers (known as speckle). We ultimately solved this problem by designing and manufacturing our own "despeckler," a device that ensures the "blurring" of the output pattern and thus its homogenization over time. Our despeckler operates at a time resolution of 0.1 ms (10 kHz), which is more than sufficient for the customer's needs, as EPR measurements usually take place on a time scale that is several orders of magnitude slower. In addition, we have combined the despeckling device with careful adjustment of the conditions for coupling lasers into the optical fiber, thanks to which the intensity profile at the output has a "top-hat" shape rather than the commonly used "Gaussian-like" shape. A comparison of the optimized and normal output in a multimode fiber can be seen in the figure below.

Device parameters

• Lasers: (wavelength, power)
  • 375 nm, 70 mW
  • 422 nm, 100 mW
  • 532 nm, 150 mW
  • 637 nm, 160 mW
• Common output to MM fiber with a length of 2 m
• Nominal power coupling efficiency > 70% for all lasers (power at the end of the fiber / power at the free space output of the laser)
• Homogenization of the output profile by a despecler with a time resolution of up to 0.1 ms
• Central switch (key)
• Interlock