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Semiduplex, high-data-rate transmission over a single optical fiber using GaAs/GaAlAs channeled substrate planar (CSP) structure lasers both as light source and photodetector has been demonstrated. The responsivity in the receiving mode at the operating wavelength (830 nm) was 0.15 A/W. This device is suitable for semiduplex transmission, since the emissive spot and the photosensitive area of the laser coincide. The impulse response was measured to be 2.5 ns full width at half maximum (FWHM) at zero bias voltage. A 500-m single-mode fiber was used, and by using microlenses the input coupling loss was reduced to less than 4.5 dB. Measurements at a data rate of 100 Mbit/s return to zero (RZ) shows an optical power margin of 14 dB for a bit error rate (BER) of 10-9. Using lasers instead of LED's as transceivers would improve the transmission capability and the system margin of a semiduplex communication link.