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OptiSystem applications:
LIDAR systems design

Introduction
The following four example designs demonstrate how to simulate light detection and ranging systems(LIDAR) using OptiSystem, specifically:
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Light detection and ranging (LIDAR)

Laser pulse time of flight range measurement
Phase-shift range measurement
Frequency Modulation Continuous Wave (FMCW) range measurement with direct detection
FMCW range measurement with coherent detection

The reference project for this application note is: LIDAR system designs Version 1_0.osd.
Example view of OptiSystem model for FMCW LIDAR
using direct detection2

Range measurement (Time of flight) (1)LIDAR design

Using laser pulses, the time of flight range measurement methodmeasures the time it takes for a transmitted pulse to travel fromthe transmit device to the target and back to the receiver. The
range is then calculated from [1]𝑅=𝑇𝑖𝑚𝑒𝑜𝑓𝐹𝑙𝑖𝑔𝑕𝑡∙𝑐2

where c is the speed of light.
The received signal power is determined based on an extendedtarget model and is calculated as follows [2]𝑃𝑟 = 𝑃𝑡𝜌𝐷2 𝜏𝑜𝑝𝑡 𝜏𝑎𝑡𝑚 24𝑅 2

where Pt is transmitted optical power, D is the receiver aperturediameter, ρ is the reflectivity of the target, atm is the atmosphericloss factor, opt is the optical transmission system loss factor andR is the target range
To reliably determine the time trigger for the arriving pulse, theConstant Fraction Time Measurement [3] method is used
(implemented with our Cpp Component).

Layout: Range finder (TofF)

REF 1: Laser ranging: a critical review of usual techniques for distancemeasurement, Optical Engineering, Vol. 40, No. 1. (2001), pp. 10-19 by Markus
C. Amann, Thierry Bosch, Marc Lescure, Risto Myllylä, Marc Rioux
REF 2: Ahmed H. Elghandour; Chen D. Ren; Modeling and comparative studyof various detection techniques for FMCW LIDAR using OptiSystem, Proc.
SPIE 8905, International Symposium on Photoelectronic Detection and Imaging
2013: Laser Sensing and Imaging and Applications, 890529 (September 19,2013)
REF 3: Fast-Timing Discriminator Introduction, Ortec Inc., http://www.orteconline.com/-/media/ametekortec/other/fast-timing-discriminatorintroduction.pdf?la=en (accessed 10 Feb 2017)3

Range measurement (Time of flight) (2)

LIDAR design

In the example below, a Gaussian pulse (Peak pulse time = 1 us) is transmitted optically and reflected from a virtual target(defined by the Free space channel model (Extended target)). After being subjected to attenuation and delay, the received
signal is detected and post-processed by the Cpp Component Constant Fraction Time Measurement.
The received pulse is time triggered at the time sample 6.02e-06 sec and in turn the range is found to be 751.27 m(compared to the global parameter range setting of 750 m). The sensitivity of the Constant Fraction Discriminator can be
modified by changing the input parameters CFTDelay, CFTFraction, CFTNoiseThreshold.
Global parameter settings for LIDAR system.

The Constant Fraction Discriminator works by first making a copy of the received signal,
inverting it, and applying a delay (CFTDelay). The original signal is then reduced inamplitude by the factor CFTFraction and summed with the copied signal waveform. An
example of the output waveform is shown here. The zero-crossing point (from negative topositive) is then found and acts as the time trigger to determine the received pulse time.

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Range measurement (Phase shift)Another way to measure the range of an object/target is with a phase-shift range finder. With this method, the opticalsource is modulated at specific frequency Rf and transmitted towards the target. The reflected signal is then detected witha PIN photodiode followed by a heterodyne receiver. The phase shift resulting from the transmission of the optical signal(), relative to the original reference signal, is then measured and used to calculate the range as follows [1]:𝑅=LIDAR design𝑐∆𝜑𝑅𝑓4𝜋To improve the accuracy of this system, the reference and received modulated signals can be mixed with a local
oscillator RLO to down-convert the received waveform to a lower frequency (RLO – Rf ). These signals are then band-passfiltered (to reduce noise) and processed by a Phase Meter (using our programmable Cpp Component)

REF 1: Laser ranging: a critical review of usual techniques for distance measurement, Optical Engineering, Vol. 40, No. 1. (2001), pp. 10-19 by
Markus C. Amann, Thierry Bosch, Marc Lescure, Risto Myllylä, Marc Rioux

Layout: Range finder (Phase Shift)

In this example the range has been calculated as 0.2012 m(compared to the global parameter setting of 0.2 m)

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Range measurement (FMCW)The last method to be presented is the frequency modulated continuous wave (FMCW) LIDAR. Two models have beendeveloped: FMCW LIDAR with Direct Detection and FMCW LIDAR with Coherent Detection. Both models work on the sameprinciple. A frequency modulated optical transmitter is sent out towards the target and the reflected signal is detected (by aphotodetector) and mixed with the original linear frequency modulated (LFM) signal. As the received signal is time-delayed,
an intermediate frequency signal is produced. Using a frequency counter (implemented with our Cpp Component) thedetected Rf signal is measured and then used to calculate the range as follows [1]:𝑅=LIDAR design

𝑅𝑓 ∙𝑐∙𝑅𝑎𝑚𝑝𝑃𝑒𝑟𝑖𝑜𝑑2∙𝐷𝑒𝑙𝑡𝑎𝐹𝑟𝑒𝑞

where the RampPeriod is equal to the global parameter Time window and the DeltaFreq is equal to the parameter
RFSweepBandwidth (set within the component parameters of the Subsystem RF LFM Waveform Generator)
The only difference between both detection systems is that one uses square law detection whereas the other uses acoherent homodyne detector to recover the incoming optical signal before mixing (the latter thus provides a higher
sensitivity as the detection process is shot-noise limited)

REF: Ahmed H. Elghandour; Chen D. Ren; Modeling and comparative study of various detection techniques for FMCW LIDAR using OptiSystem,
Proc. SPIE 8905, International Symposium on Photoelectronic Detection and Imaging 2013: Laser Sensing and Imaging and Applications, 890529(September 19, 2013)

Layout: FMCW Coherent Detection

Received electrical spectrum. The peak is the intermediate
frequency signal resulting from the mixing between the reference
and received FM waveforms
In this example the range has been calculated as 749 m(compared to the global parameter setting of 750 m)

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