Ultrafast Laser Laboratory

Vcsel in optical data link polarization noise reduction

Vcsels in Optical Data Link

Present particle detectors are exclusively electronics, from detector signal identification to signal processing. With the growth on the physical size of modern detectors and the added complexity of various monitor and control channels, the number of necessary signal channel outputs from a detector is almost unmanageable. Furthermore, cross talks and EM interference between channels often compounded the difficulty of signal processing. As a first approach, by replacing bulky copper signal cables with light-weight fiber-optics components, one would drastically reduce the mass-volume of particle detectors. We explore an optoelectronic analog system using arrays of novel vertical-cavity surface-emitting lasers (Vcsel) for the transfer of data between detectors and remote data acquisition electronics. Vcsel has a planar structure that is relatively inexpensive to fabricate and mass produce with high reliability. Unlike edge-emitting laser diodes, where varies steps of post optical processing are needed to form the cavity mirrors, a set of Bragg reflecting cavity mirror can easily be deposited on the Vcsel's planar structure, leading to strong optical confinement and lower lasing threshold to below 1 mA. Such sub-mA thresholds can be driven directly by most front-end electronics and contributes little thermal budget to the detector. The large GHz bandwidth of Vcsel is more than adequate for the highest ~100 MHz collision rate planned today. The 850 nm Vcsel family has good radiation resistance owing to their much confined small active area. Furthermore, the planar nature of Vcsel and its low beam divergence ease the fabrication alignment and facilitate the coupling to a ribbon fiber to yield high quality of multichannel optical data link.

Typical 850 nm Vcsels have approximately 1-mA threshold and emitting 2 mW of optical power at the maximum operating DC current of 10-mA. We investigate several lasing characteristics of the Vcsels including the linearity, spatial profile, polarization effects, spectral analysis, and the relative-intensity noise, all affecting the performance of an optical data link. One undesirable intrinsic characteristic of Vcsel is its undefined output polarization due to its azimuthal symmetric birefringence, resulting in relatively large polarization switching noise. We investigate a method to tailor the birefringence of a Vcsel using a high-peak-power laser. By locally altering the birefringence of a Vcsel, one can possibly tailor the desired polarization output thereby suppress the polarization switching noise and provide a lower bit-error-rate for a digital system and better sensitivity, therefore a larger dynamic range, for an analog system.

The intensity noise, current-voltage-power characteristics, and the output polarization are shown here before and after laser tailoring. Laser-induced thermal stress forced the output polarization to lase at a preferred direction by changing the local birefringence, the intensity noise (top 2 figures) are thus reduced.

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For more information or preprint request contact Thomas Y. F. Tsang