We have developed a radiation to coherent light converter with a monolithically integrated semiconductor chip that consists of a chromium‐doped GaAs photoconductor detector and a GaAlAs 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 17 MeV linear accelerator and with 50 ns x‐ray pulses from aZ‐pinch plasma source.