There is an increasing demand for portable gas sensors, both in the fields of environmental and health sciences, as well as in the industrial sector. Planar micro-resonators, specifically resonant optical sensors, are highly sensitive and have a small footprint, making them ideal for such applications.
These guided-wave sensors operate based on changes in their spectral response when exposed to target molecules. To effectively probe these spectral shifts, a laser source is required that emits a single-mode and polarization-stable beam, while also being spectrally tunable over a few nanometers.
A team of researchers from the University of Toulouse in France set out to develop a compact optical microsystem for detecting ammonia gas. They decided to use a near-infrared single-mode laser diode source, known as a vertical cavity surface emitting laser (VCSEL).
VCSELs are semiconductor laser diodes that are compact and can be easily tuned spectrally by adjusting the operating current. The specific VCSEL chip utilized in their research included a grating relief etched on its surface, ensuring stable polarization of the emitted beam. However, the beam divergence of this VCSEL chip was too large for most practical applications, despite being smaller than that of an LED or a standard edge-emitting laser diode.
In their study, the researchers found that the spot size at the desired working distance (2 mm) exceeded 250µm. To achieve optimal coupling with the detection area, it needed to be reduced to less than 100µm. Unfortunately, commercially available VCSEL chips with reduced beam divergence and stable polarization in single-mode were not yet accessible. Consequently, the challenge was to devise an accurate method of directly integrating a collimation microlens onto the small-sized VCSEL chip (200x200x150 µm3), which was already mounted on a printed circuit board.
The researchers published their work in the Journal of Optical Microsystems, showcasing the application of 2-photon-polymerization 3D printing to fabricate such a microlens in a single step, with a writing time of only 5 minutes. They optimized the lens design and fabrication conditions to achieve satisfactory surface quality and an appropriate focal length.
Through this approach, the laser chip’s beam divergence could be reduced from 14.4° to 3°, resulting in a beam spot size of only 55µm at a distance of 2 mm. The team also conducted experimental and theoretical studies on the effects of lens integration on the device’s spectral properties. They proposed a new design to prevent a reduction in the tuning range.
The researchers’ work highlights the potential of 2-photon-polymerization 3D printing as a rapid and precise technique for VCSEL collimation during the post-mounting stage. It opens up possibilities for the development of optimized laser chips that can be directly integrated into portable optical sensing systems.