Q-switched laser sources with exceptional frequency stability are required for many remote sensing applications. Airborne systems often require operation under significant vibration which can pose a challenge for injection seeded bulk lasers. An attractive option is the utilisation of a passively Q-switched Non-Planar Ring Oscillator (NPRO), which has the potential to offer highly stable single longitudinal mode output due in part to its monolithic construction. This paper describes measurement of the frequency stability of a passively Q-switched NPRO under simulated aircraft vibration, which was achieved using a custom designed Fabry-Perot etalon to interrogate the optical frequency of every laser pulse. With no vibration, the long term drift was found to be approximately 4MHz/min. With the drift removed, the laser frequency jitter demonstrated a standard deviation of better than 200kHz over 15 minutes, which was the measurement system noise limit. Under vibration, the worst case measurement had a drift of 8MHz/min and the jitter had a standard deviation of 2.09MHz.
There is a growing requirement to transfer large volumes of data between underwater platforms. As seawater is transmissive in the visible band, underwater optical communications is an active area of interest since it offers the potential for power efficient, covert and high bandwidth datalinks at short to medium ranges. Short range systems have been successfully demonstrated using sources with low directionality. To realise higher data rates and/or longer ranges, the use of more efficient directional beams is required; by necessity, these must be sufficiently aligned to achieve the required link margin. For mobile platforms, the acquisition and tracking of each node is therefore critical in order to establish and maintain an optical datalink. This paper describes work undertaken to demonstrate acquisition and tracking in a 3D underwater environment. A range of optical sources, beam steering technologies, and tracking sensors have been assessed for suitability. A novel scanning strategy exploiting variable beam divergence was developed to provide robust acquisition whilst minimising acquisition time. A prototype system was assembled and demonstrated in a large water tank. This utilised custom quadrant detectors based on Silicon PhotoMultiplier (SiPM) arrays for fine tracking, and a Wide Field of View (WFoV) sCMOS camera for link acquisition. Fluidic lenses provided dynamic control of beam divergence, and AC modulation/filtering enabled background rejection. The system successfully demonstrated robust optical acquisition and tracking between two nodes with only nanowatt received optical powers. The acquisition time was shown to be dependent on the initial conditions and the transmitted optical power.
Conference Committee Involvement (1)
Advanced Free-Space Optical Communication Techniques and Applications
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