New technique to shift lightwave frequency for distributed fiber optic sensing

Frequency translation or shifting is an important technique in many kinds of optical-fiber reflectometry. For example, high-linearity frequency sweep against time is necessary in coherent-OFDR for achieving a high spatial resolution, and changes in the frequency over a wide range is essential for fading noise reduction in coherent-OTDR. In addition, two mutually coherent lightwaves with a frequency difference of about 10 GHz are required in Brillouin-OTDR to realize coherent detection of Brillouin scattered lightwaves. However, conventional techniques are insufficient for these purposes. In this paper, a new technique for the external frequency translation of lightwaves is proposed. This technique enables the high-linearity sweeping of an optical frequency quasi-continuously over a wide range of about 100 GHz. The frequency translator is composed of an optical pulse modulator and an optical ring circuit containing an acousto-optic frequency shifter, an optical amplifier, and a narrow band filter. Here, the acousto-optic frequency shifter and the pulse modulator are synchronously controlled. The pulse launched into the ring circuit experiences a frequency shift at every circulation around the ring circuit. Therefore, the frequency of the output lightwave from the ring circuit is largely translated from that of the original pulse. Its intensity can be kept nearly constant because of balance between gain and loss in the ring circuit. We also demonstrate Brillouin-OTDR as an application.