We present an active optical synthetic aperture-imaging system. A phase-step digital holographic setup is used as a wavefront sensor in the far field. The overlap of the holograms enables the estimation and compensation of their relative positions and phase with a speckle cross-correlation algorithm. Experimental results on a short-range synthetic aperture setup at 633 nm are presented that are based on 128 x 128 holograms. The synthesis is executed in one direction by means of rotation of the object. Test images show a significant gain of resolution in the synthesis direction. Processing errors are estimated through experiment. Random processing errors of a synthetic pupil composed of 33 merged holograms are negligible, but biases induced by unknown optical aberrations ofthe reference wave induce defocusing and astigmatism.

No abstract

We present an original technique for coherent beam combining of an array of fiber amplifiers based on self-adaptive digital holography. In this method, the phase errors between the fibers of the array are compensated by using the diffracted phase-conjugated -1 order of a digital hologram. The proposed method is compatible with a large number of fibers and simply implemented with a CCD detector matrix and a spatial light modulator. This concept is analyzed and experimentally demonstrated with three polarization-maintaining passive fibers at 1.06 microm.

No abstract

Fiber lasers provide an attractive means of reaching high output laser power because of their advantages in terms of compactness, reliability, efficiency and beam quality. In order to obtain much higher output power than it is possible from a single fiber, beam-combining techniques have been investigated. In this communication, we present a new technique of coherent fiber combining, based on self adaptive digital holography that does not require any phase error measurement. A low power plane reference beam is first launched into the fiber amplifier array. The interference pattern between the beams with phase φ (x,y) issued from the fiber array and a plane reference beam is recorded on a digital camera and directly transferred to a Spatial Light Modulator (SLM) which acts as a programmable digital hologram. This hologram is read out simultaneously and a phase conjugate beam with phase -φ (x,y) is generated in order -1 of the diffraction pattern. This beam is then injected in the fiber amplifier array. At the output of the fiber amplifier array, the phase of each elementary beam are locked. Experimental demonstration of coherent beam combining by digital holography is demonstrated with polarization maintaining fibers operating at 1µm. Digital holography is realized thanks to a CCD/CMOS camera and a liquid crystal SLM. Owing to the high resolution of existing SLMs and cameras, this technique could be applied to phase lock a large number of fiber amplifiers.