Galvanometric x, y scanning mirrors with a clear aperture of 20 mm at 45 ° were integrated into the system.
The advantage of this type of mirrors from the optical point of view is that they can rotate about a central axis in the plane of the mirror and can be located at the pupil of an optical system allowing it to remain unchanged as the mirrors rotate [17].
The galvanometer scanners provide a convenient and reliable way to perform x, y scanning of the laser beam and the FoV. However, the two mirrors have to be separated from each other [17]. This means that the optical system has to incorporate a pair of additional lenses which relay the image of the entrance pupil from one mirror to the other. This requires more complex optical design and imposes an additional cost. Galvanometric scanning mirrors can produce a steady-state deflection and follow a scan trajectory with considerable fidelity using random computer-controlled waveforms [17].
The scanner used in the design contains a moving magnet actuator which produces a rotary deflection in response to an electric current [8]. The mirrors offer an angular excursion of ± 20 ° and provide a step response of 350 µs, limiting the rate to 2.86 kHz [8]. The minimum scan step was 200 ± 8 µrad inside of the optical system which was equivalent to ~ 17 ± 0.7 µrad in the object plane. This exceeds the requirement of a step size of 28 µrad, providing an error in positioning accuracy of approximately 4 %.
The scanner is controlled by a field-programmable gate array (FPGA) which sends command transistor-transistor logic (TTL) inputs to the servo drivers of the scanning
128 mirrors controlled by LabVIEW control software developed by a systems engineer from Selex ES.
The procedure for calibrating scanning mirrors is described in Chapter 4.
3.15 Acknowledgements
The optical design and tolerance analysis of the receiver were performed in ZEMAX under supervision of Chris Davies of Selex ES. The mechanical design was performed in CAD by Michael Dugan of Selex ES according to the requirements specified by the author and to the tolerances based on the outcome of the tolerance analysis. The safety enclosure was designed by Michael Dugan according to the safety requirements specified by Dr. Stephen Harding of Selex ES. The laser interlock for the safety enclosure was designed and assembled by Naomi Mitchison of Selex ES.
3.16 Conclusions
A re-configurable lidar system was designed which provided a matching FoV per detector pixel for either the single-element detector operating in a scanning mode or the Gm-array operating in a non-scanning mode. The design process included the development of specifications for a long-range lidar and optimisation of the optical design such that the Airy disk diameter is 66 % of a detector pixel for rays arriving at the system from the entire FoV. It also included detailed tolerance analysis and development of the mechanical assembly required to maintain diffraction limited performance after assembly and alignment. Custom aspherical lenses were manufactured according to the outcome of the optimisation and the tolerance analysis. In addition, the system layout and the integration between different components such as the scanning mirrors, the detector, the laser and timing electronics were briefly discussed.
The transmission path of the laser beam for the system operating in a mono-static configuration has been modelled and confirmed that it is possible to transfer the beam through the reflective telescope with a central obstruction while avoiding clipping from the optical components.
129 The transceiver design and assembly had to satisfy on-site health and safety regulations for handling, electrical hazard and eye-safety before being certified for use on Selex ES’ premises.
130
References
[1] http:// www.selex-es.com documents/737448/17645869/body_mm07928_Titan _385ES_HD_LQ_.pdf
[2] A. McCarthy, X. Ren, A.D. Frera, N. R. Gemmel, N.J. Kirchel, C. Scarcella, A. Ruggeri, A. Tosi, G.S. Buller, “Kilometer-Range Depth Imaging at 1550 nm Wavelength Using an InGaAs/InP Single-Photon Avalanche Diode Detector”, Opt. Express, 21, 19, pp. 22098-22113 (2013)
[3] R.G. Driggers, P. Cox, T. Edwards, “Introduction to Infrared and Electro- Optical Systems”, Artech House Inc. (1999)
[4] D. Malacara, Z. Malacara, “Handbook of Optical Design”, Marcel Dekker, Inc. (2003)
[5] M.A. Itzler, “Geiger-mode APD Cameras for LADAR Imaging”, Oral Presentation, Heriot-Watt University, October (2013)
[6] R.E. Fisher, B. Tadic-Galeb, P.R. Yoder, “Optical System Design”, SPIE Press (2008)
[7] Celestron C-GEM Series, Instruction Manual
[8] Cambridge Technology, Galvanometer Optical Scanner Instruction Manual [9] D.C. O’Shea, “Elements of Modern Optical Design”, John Wiley & Sons (1985) [10] R. Kingslake, “Optical System Design”, Academic Press (1983)
[11] W.J. Smith, “Modern Optical Engineering”, McGraw-Hill (2000) [12] ZEMAX, User’s Manual (2011)
[13] G. Hallock-Smith, “Practical Computer-Aided Lens Design”, Willmann-Bell Inc. (2010)
[14] J.M. Geary, “Introduction to Optical Design with Practical ZEMAX Examples”, Willmann-Bell Inc. (2009)
[15] E. Hecht, “Optics”, Adison Wesley (2005)
[16] M. Born, E. Wolf, “Principles of Optics”, 7th Edition, Cambridge University Press (1999)
[17] G.E. Marshall, “Optical Scanning”, Marcel Dekker (1991)
References
131
Chapter 4
Evaluation of the Transceiver in Bi-Static
Configuration with a Scanned Single-Element SPAD
Detector
4.1 Introduction
This chapter describes experimental results acquired with the lidar system operating in a bi-static configuration with a single-element InGaAs/InP SPAD (i.e. described as mode A in previous chapters). Details on the transmitter design, scanning mirrors calibration, the laser source, the InGaAs/InP single-element detector, timing electronics scheme, the alignment and setup of the system used in experiments are described.
Theoretical modelling of the system losses was performed and evaluated against the real-life measurement. Instrumental response function of the system was measured and three-dimensional ToF data was acquired. The chapter presents results which demonstrate the ability of the system to produce three-dimensional images of targets at ranges from 2.8 km to 4.2 km at spatial and depth resolution of < 30 cm. In addition, imaging through a semi-transparent sheet of netting was presented.