The SM VCSEL laser diode achieved a commercial breakthrough at a dimension hardly thought possible back in 2000. This breakthrough is ultimately due to tracking applications (i.e. the optical PC mouse).
The next generation of tracking systems requires an even higher resolution (i.e. an absence of polarization flips). How does one recognize a "polflip"?
Figure 1 should help: The optical power of a conventional SM VCSEL over time is depicted here. The laser beam, which is pulsed, passes through a polarization filter. It is notable that in some pulses the power suddenly fails. This is the result of a sudden change in the polarization, more precisely a polflip. This effect has been known for quite some time.
In the past polflips disturbed the spectroscopic measurement of oxygen in particular. Because only a certain percentage of these devices exhibits said effect, one used to manage by simply selecting a suitable VCSEL. However, in mass applications this method is not feasible. Here the battle against the polflip begins with the design.
Why is it that polflips exist in conventional SM VCSELs? This effect is essentially due to the high amount of symmetry in the SM VCSEL; the polarization state is unstable. As a result, the symmetry has to be broken. Ten possible methods were gathered in [1] for this purpose.
Grating Structure Inhibits Polflip
The additional application of a "subwavelength grating" on the output coupler mirror was successfully implemented (see Figure 2) by the scientists at ULM-Photonics [2]. ULM chose this method because it allows the established production platform for SM VCSELs to remain unchanged. One can imagine the physical mechanism as follows: The grating structures the reflection of the output coupler. Because the lateral expansion of the polarization directions of the optical field varies, the reflectivity of the output coupler is also different for both polarization directions. Thus the optical amplification is likewise different. As a result only one polarization direction can resonate. This means that the polarization is stabilized by the design, but unfortunately not defined. Ironically, precisely this method did not appear to be very promising for mass production in [1].
Such polarization stabilized SM VCSELs are available at 850 nm. In the article number, these lasers can be identified by the abbreviation "PL" for "polarization lock."
References:
[1] F. Monti di Sopra, Optical properties of VCSELs and phase-coupled VCSEL arrays, Osnabrück, Der Andere Verlag, 2002, ISBN 3-936231-00-1
[2] M. Grabherr, R. King, R. Jäger, D. Wiedenmann, P. Gerlach, D. Duckeck, C. Wimmer, Volume production of polarization controlled single-mode VCSELs, 2008
