New Concept: the EOM BR VCSEL
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In contrast to the conventional direct modulation
approach, light intensity modulation in EOM BR VCSEL is based on
electro-optic modulation of the light output of the laser while
keeping the drive current constant. As the current and power density
in the active zone of the device can and should be kept at low level,
the degradation robustness of the device will be similar to that of
1Gb/s 850 nm VCSEL which provided the expected operation lifetimes in
excess of 106 h at 85°C.
The modulator section in this concept (opposite to the case of external Mach-Zehnder modulators) is monolithically integrated into the device and the size, the power consumption, the epitaxial wafer and processing costs are identical to those of conventional VCSELs. There are no additional costs for submodule assembly. The EOM BR VCSEL concept achieves modulation of the output light by changing the top mirror transparency via applying an electric field. The electric field changes the refractive index of certain layers of the top mirror, leading to a change in the Bragg mirror reflectivity. Since light modulation is achieved by varying the refractive index by changing the reverse bias voltage and the modulator mesa size is very small, the power consumption is minimal easing the driver circuitry. The driver for the gain section operates in CW-mode and is of low cost. The noise problems characteristic for direct modulation and temperature sensitivity are not relevant for the EOM BR VCSEL concept, so that the same bit error rate in the detector can be achieved at much lower light power levels in the fiber. On the other hand, opposite to the case of electroabsorption modulation, no light absorption and heat dissipation are taking place in the modulator section and no carrier depletion- and time-of-flight- related bandwidth limitations characteristic to electroabsorption modulators and pin photodiodes are relevant in this case. The undoped modulator section can be made arbitrarily thick further reducing the differential capacitance to negligible levels. Theoretically Tb/s transmission rates can be achieved. Thus, the concept might enable future-proof high speed error free operation with very low power consumption and thus become a driver of European photonics technology. The necessary processing technology and the fiber-coupling technology of the EOM BR VCSEL are based on prior-art VCSEL devices and therefore broadly used and well-known. This allows using the cost-effective production of VCSELs. Hence, the EOM BR VCSEL is both fast and inexpensive.