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.
We describe the development of flexi-circuit addressed, high resolution small aperture Bimorph Deformable Mirror
(BDM) suitable for a satellite to ground optical communications system. In the first phase, the base of an existing BDM
was modified to enable integration onto a commercially available tip-tilt stage. Evaluation demonstrated that there was
no reduction in performance as a result of the integration. In the current phase, a 100 element BDM with a 10mm optical
aperture suitable for integration onto a bespoke tip-tilt platform has been designed and assembled. The results of initial
tests are presented.
We present the results of a study on Dual-Stage Deformable Mirrors using Zonal Bimorph Deformable Mirror (ZBDM)
technology. A high density 'tweeter' DM has been assembled onto a lower density, high dynamic range 'woofer' DM to
generate an integrated mirror which offers both high resolution and dynamic range simultaneously. Such a device has the
potential to significantly simplify the design of astronomical Adaptive Optics (AO) systems. The latest developments are
presented, including the fabrication of a small scale demonstrator.
KEYWORDS: Mirrors, Sensors, Adaptive optics, Actuators, Wavefront sensors, Deformable mirrors, Turbulence, Signal to noise ratio, Wavefronts, Control systems design
We present a novel method for integrating a wavefront sensor into a deformable mirror. This development should
simplify the design of laser and electro-optic systems, and lead to smart mirrors which need no external control systems.
In operation, a small fraction of the incident light is transmitted through the mirror coating and is sampled by a Hartmann
Mask. Options include open loop, traditional closed loop or fully integrated operation whereby the wavefront sensor is
used to provide direct feedback to the mirror actuators, enabling automatic alignment or phase conjugation.
The Zonal Bimorph Deformable Mirror (ZBDM) is a new concept of adaptive mirror. It exploits the benefits normally
associated with bimorph mirrors, namely simple rugged construction, low capacitance, and cost effectiveness, but in a
significant departure from classical, edge supported bimorphs each element is supported from underneath. This results in
a localized (zonal) response that should enable the device to be scaled up to large aperture, multi-1000 element devices.
Crucially, the combination of continuous support coupled with the use of flexi-circuit interconnect promotes the
assembly of a high density 'tweeter' onto a lower density, high dynamic range 'woofer' to generate an integrated, dualstage
deformable mirror which can deliver both high resolution and high dynamic range simultaneously. Such a device
has the potential to significantly simplify the design of AO systems. We present the progress made on the development
of the ZBDM as part of a collaborative programme funded by the UK Science and Technology Facilities Council.
The Zonal Bimorph Deformable Mirror (ZBDM) is a new concept of adaptive mirror. It exploits the benefits normally
associated with bimorph mirrors, namely simple rugged construction, low capacitance, and cost effectiveness, but in a
significant departure from classical, edge supported bimorphs each element is supported from underneath. This results in
a localised (zonal) response and enables the device to be scalable up to large aperture, multi-1000 element devices.
Crucially, the combination of continuous support coupled with the use of flexi-circuit interconnect promotes the
assembly of a high density 'tweeter' deformable mirror (DM) onto a lower density, high dynamic range 'woofer' DM to
generate an integrated, dual-stage deformable mirror which can deliver both high resolution and high dynamic range
simultaneously. Such a device has the potential to significantly simplify the design of astronomical adaptive optics (AO)
systems. We present the progress made on the development of the ZBDM as part of a collaborative project funded by the
newly formed UK Science and Technology Facilities Council.
Adaptive Optics (AO) is a critical underpinning technology for future optical countermeasures, laser delivery, target illumination and imaging systems. It measures and compensates for optical distortion caused by transmission through the atmosphere, resulting in the ability to deploy smaller lasers and identify targets at greater ranges. AO is also well established in ground based astronomy, and is finding applications in free space optical communications and ophthalmology.
One of the key components in an AO system is the wavefront modifier, which acts on the incoming or outgoing beam to counter the effects of the atmosphere. BAE SYSTEMS ATC is developing multi-element Deformable Bimorph Mirrors (DBMs) for such applications.
A traditional bimorph deformable mirror uses a set of edge electrodes outside the active area in order to meet the required boundary conditions for the active aperture. This inflicts a significant penalty in terms of bandwidth, which is inversely proportional to the square of the full mirror diameter. We have devised a number of novel mounting arrangements that reduce dead space and thus provide a much improved trade-off between bandwidth and stroke. These schemes include a novel method for providing vertical displacement at the periphery of the aperture, a method for providing a continuous compliant support underneath the bimorph mirror, and a method for providing a three point support underneath the bimorph. In all three cases, there is no requirement for edge electrodes to provide the boundary conditions, resulting in devices of much higher bandwidth. The target is to broaden the use of these types of mirror beyond the current limits of either low order/low bandwidth, to address the high order, high bandwidth systems required by long range, horizontal path applications. This paper will discuss the different mirror designs, and present experimental results for the most recently assembled mirrors.
Adaptive Optics (AO) is a critical underpinning technology for future laser delivery (including free-space optical communications), target illumination and imaging systems. It measures and compensates for optical distortion caused by transmission through the atmosphere, resulting in the ability to deploy smaller lasers and identify targets at greater ranges.
One of the key components in an AO system is the wavefront modifier, which acts on the incoming or outgoing beam to counter the effects of the atmosphere. BAE SYSTEMS Advanced Technology Centre is developing multi-element bimorph deformable mirrors for such an applications.
Our initial designs were based on a standard construction and exhibited a resonant frequency of 1kHz with a maximum stroke of ±20μm for an active aperture of 50mm. These devices were limited by the necessity to have a 'dead space' between the inner active area and the mirror boundary; this ensured that both the requirements for the stroke and the fixed boundary conditions could be met simultaneously. However, there was a significant penalty to pay in terms of bandwidth, which is inversely proportional to the square of the full mirror diameter. In a series of iteration steps, we have created novel mounting arrangements that reduce dead space and thus provide the optimum trade-off between bandwidth and stroke. These schemes include supporting the mirror from underneath, rather than at its edge. As a result, models of 60mm active diameter mirrors predict a resonance in excess of 5kHz, combined with a maximum stroke greater than ±40μm. This paper will discuss a number of different mirror designs and present experimental results for recently assembled devices.
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