Optical EngineeringLow-loss wavelength-multiplexed optical scanners using volume Bragg gratings for transmit-receive lasercom systems
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Low-loss, no moving parts, free-space wavelength-multiplexed optical scanner (W-MOS) modules for transmit-receive lasercom systems are proposed and experimentally demonstrated. The proposed scanners are realized using volume Bragg gratings stored in dichromated gelatin (DCG) coupled with high-speed wavelength selection, such as by a fast tunable laser. The potential speed of these scanners is in the gigahertz range using present-day state of the art nanosecond tuning speed lasers. A 940-lines/mm volume Bragg grating stored in dichromated gelatin is used to demonstrate the scanners. Angular dispersion and diffraction efficiency of the volume Bragg grating used for demonstration are studied, versus wavelength and angle of incidence to determine the free-space W-MOS angular scan and insertion loss, respectively. Experimental results show that a tunable bandwidth of 80 nm, centered at 1560 nm, delivers an angular scan of 6.25 deg. The study also indicates that an in-line scanner design realized using two similar Bragg gratings in DCG delivers a 13.42-deg angular scan, which is more than double the angular scan available from the free-space W-MOS using a single volume Bragg grating. Furthermore, a free-space W-MOS using a single Dickson grating features low (<0.15 dB) polarization-dependent loss and an average scanner insertion loss of only 0.4 dB over the 70-nm wavelength band around 1550 nm.