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
Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST