Miniaturized red-emitting laser module with polarization maintaining single-mode fiber output
Applications such as holography or interferometry are dependent on coherent laser beams at wavelengths visible to the human eye. So far, gas lasers like HeNe lasers are utilized for such purposes, but they suffer from drawbacks such as large package sizes, fragile glass tubes, and high driving voltages. Using a semiconductor laser instead would offer a smaller and more rugged design along with a simpler battery driven usage.
The FBH recently developed red-emitting lasers with internal gratings (DBR-RWL) delivering single-mode optical output powers of up to 100 mW . The linewidth of these lasers is below 10 MHz , thus suitable for holographic and interferometric applications. However, semiconductor lasers are sensitive to external feedback and therefore need to be protected by optical isolators. Also, a sealed package with a fiber pig tail is necessary in order to allow user-friendly out-of-the-lab usage.
To this end, a laser module in a small-sized package was developed by the FBH in the framework of the project FINDLING . This module comprises a laser diode, a newly developed miniaturized optical isolator (µ-isolator), and coupling optics for a polarization maintaining single-mode fiber (PM-fiber) output (see Fig. 1).
To avoid feedback to the laser chip, a purpose-built µ-isolator is employed based on a Faraday rotator using a CdMnTe crystal with a very high Verdet constant. Hence, the µ-isolator could be built with an outer diameter as small as 5 mm and a length of only 12 mm. It features an isolation of more than 20 dB and a damping in forward direction of less than 2 dB, i.e. a transmission of more than 65% (see Fig. 2).
The light is coupled into a PM-fiber after the µ-isolator using an aspheric lens, offering a coupling efficiency of about 45%. The final laser module is capable of emitting more than 1 mW ex PM-fiber for a current of 100 mA and an internal temperature of 20°C (see Fig. 3). At this point, the emission wavelength is at 633.1 nm and the side mode suppression ratio exceeds more than 35 dB (see inset in Fig. 3). The electrical power consumption is less than 250 mW at a driving voltage of little more than 2 V, making the laser ideal for a future out-of-the-lab usage.
FINDLING is funded by the German Federal Ministry of Education and Research (BMBF) under contract no. 13N13954 (01.04.2016 - 31.03.2019).
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