Schematic of multiple wavelength time-of-flight sensor illustrating the integration of the telescope, optical routing module, multiwavelength sequencer, fiber translation stage, the enabling circuit, the router, and the SPC-600 (time to amplitude converter, analog to digital converter, and multichannel analyzer).
Schematic drawing of the adapted Meade Schmidt–Cassegrain telescope showing the position of the mounting ring for the laser diodes and an outline of the collection arrangement.
Schematic diagram showing a plan view of the optical routing module indicating the surfaces included in the ray trace model for each wavelength channel, and the positions of the alignment pinholes (PHxx). The fiber-coupled lens assembly labeled Collection No. 6 is replicated for each of the other five output channels.
Timing diagrams illustrating the removal of unwanted background noise with no loss of signal. In (a) we have the laser sequencer clock pulse (shown here for all six lasers), with a different laser pulsing sequentially, for each period, The period is proportional to the reciprocal of the laser repetition rate, (typically, and ) which provides a timing reference enabler clock pulse, seen in (b). The output of the SPAD, including noise signals, is seen in (c) before going to the enabling circuit. In (d) the enabling circuit provides an adjustable time window (jumper selectable from 5 to 50 ns in 5 ns steps), which must lie within the period of the clock, and is active for one clock period in every six. In (e) the enabler, upon receiving the raw output signal of the SPAD, ignores timing events outside the enabler time window, and hence the reference timing window of the laser sequencer clock period, therefore resulting in the removal of the unwanted background photocounts, the result being that the laser photon events remain intact, with all random background photocounts (outside the enabler time window) removed giving a higher signal to noise ratio overall. The random background light contribution has effectively been reduced by .
Comparison between photocounts taken with and without the enabler circuit and the improved signal to background ratio on a semilogarithmic plot. It can be seen that the background contribution of the counts is collected with and without the enabling circuit. The background without enabling is counts, and the solar background with enabling gives , for the same experimental conditions.
Example of sensor response for each of the six wavelength channels. In this case, six simultaneous measurements at each laser wavelength were made for a target at a distance of 2 km. Note the background level is different in each case.
Two measurement responses taken using 3# in. corner cube at a range distance of 17 km which demonstrate the effect of scintillation on the return signal. The integration time used was 100 ms, each set of measurements were taken for ten runs at a particular integration time, and the count rates vary significantly for shorter integration times than they do at longer integration times.
(a) and (b) Two examples of returns fitted to scintillation data at 17 km with a collection time of 0.1 s; (c) two well-defined modes at the minimum separation of the corner cube measurement at 330 m; (d) variation of laser against gauge measured corner cube separation with respect to an offset of 9.1 cm; (e) 630 nm wavelength; and (f) 780 nm wavelength. Two examples of returns from matt targets at a distance of 330 m.
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