Mode control of surface-emitting lasers using fib er Bragg grantings and its application to high-speed local area networks

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A vertical-cavity surface-emitting laser (VCSEL) is a laser that emits light vertically to a semiconductor substrate. As the output beam has a circular profile, a VCSEL is advantageous to coupling with optical fibers. A VCSEL is suitable also for high-capacity parallel communication if they are arranged in a grid. On the other hand, a fiber Bragg grating (FBG) is a diffraction grating in an optical fiber formed by two intersecting ultraviolet laser beams. FBGs reflect light at Bragg wavelengths, and thus they have been applied for wavelength control of laser diodes. In this study, we couple an FBG with a VCSEL and demonstrate control of longitudinal modes and spectral narrowing. We studied optical fiber transmission using the VCSEL modulated with digital signals. In the experiment, we fabricated FBGs that have Bragg wavelengths in agreement with oscillation wavelengths of VCSELs. The Bragg wavelength was about 840 nm, the reflectivity was 30-40%, and the spectral width was 6-7 nm FWHM. Then, we coupled the FBGs to the VCSELs. Spectral widths ofVCSELs have decreased to half of those without FBGs. Such a narrowband laser has an application to optical communication or optical measurement.Next, we performed measurement on digital transmission characteristics with multimode optical fiber. For a fiber length of 1 km, the bit error rate decreased by using an FBG at a data rate of700 Mb/s, and decrease in the power penalty by 2dB was obtained. For an increased fiber length of 2 km, the bit error rate somewhat decreased at a data rate of 300-400 Mb/s by using an FBG. It is concluded that use of an FBG is effective for relatively short distance high-bit rate communication such as local area networks. The mechanism of the improvement is considered to be decrease in jitter, wavelength dispersion, and modal dispersion.