We have developed a radiation -to-coherent light converter (RCLC) with a monolithically integrated semiconductor chip that consists of a chromium -doped GaAs photoconductor detector and a GaA1As laser diode. When irradiated, the electric pulse generated by the photoconductor detector modulates the laser diode, which has been biased above the lasing threshold, thus converting a radiation pulse to an electric pulse and then to a light pulse. The laser pulse is then transmitted to a fast recorder through a high-bandwidth optical fiber. In the absence of a single -step x -ray pumped laser, our converter appears to be the first integrated device that can efficiently convert x -ray flux into coherent light. This device has been tested successfully with the 50 -ps electron beams of a 17 -MeV linear accelerator and with 50 -ns x -ray pulses from a Z -pinch plasma source.