Modeling and Comparative Study of Various Detection Techniques for

FMCW LIDAR using Optisystem

Ahmed H. Elghandoura and Chen D. Rena
a
School of Electronics and Information Engineering,
Changchun University of Science and Technology, Changchun, China

ABSTRACT
In this paper we investigated the different detection techniques especially direct detection, coherent heterodyne detection
and coherent homodyne detection on FMCW LIDAR system using Optisystem package. A model for target, propagation
channel and various detection techniques were developed using Optisystem package and then a comparative study
among various detection techniques for FMCW LIDAR systems is done analytically and simulated using the developed
model. Performance of direct detection, heterodyne detection and homodyne detection for FMCW LIDAR system was
calculated and simulated using Optisystem package. The output simulated performance was checked using simulated
results of MATLAB simulator. The results shows that direct detection is sensitive to the intensity of the received
electromagnetic signal and has low complexity system advantage over the others detection architectures at the expense of
the thermal noise is the dominant noise source and the sensitivity is relatively poor. In addition to much higher detection
sensitivity can be achieved using coherent optical mixing which is performed by heterodyne and homodyne detection.

Key words: FMCW LIDAR, channel modeling, LIDAR detection techniques, Coherent detection, Optisystem

1. INTRODUCTION
Lidar systems have been widely used for measuring range, velocity, vibration, and air turbulence 1. Lidar can provide
finer range resolution and smaller beam size than conventional microwave radar systems. The range accuracy of a lidar
system depends on signal bandwidth and the receiver signal-to-noise ratio 1, 2. To achieve acceptable range accuracy and
detection sensitivity, many long range lidar systems use short pulse lasers with low pulse repetition rate and extremely
high pulse peak power. In these systems, photon damage has been a concern because peak power in the megawatt range
gradually degrades the optics, shortening the lifetime of the system. The alternative approach based on an energy
equivalence principle whereby the high peak power, short-pulse duration, low duty-cycle of present usual systems is
exchanged against continuous wave (CW) operation with a small average power and a comparatively long observation
time. Therefore, FMCW lidar systems have been developed to achieve acceptable range accuracy and fine range
resolution3-6. From sensitivity point of view, FMCW lidar systems have different detection techniques especially direct
detection, coherent heterodyne detection and coherent homodyne detection. Direct detectors are essentially square law
devices that are sensitive to the intensity of the received electromagnetic signal. In contrast, Coherent detection is a linear
process that is sensitive to the amplitude, phase, and polarization of the received signal. Coherent detection has become a
major detection mechanism in lidar systems because of the much improved receiver sensitivity compared with direct
detection 2. Present work is devoted to analytic comparison of some features of different detection techniques applying
them on FMCW lidar system. This paper is organized as follows. In Section 2, we present system description and
mathematical model for direct detection configuration. In Section 3 and section 4, we introduce the two coherent
configurations especially heterodyne detection and homodyne detection. In Section 5, we introduce Optisystem model
for three detection configurations and simulate the developed models. Finally the conclusion and comparative study
between different detection configurations are outlined in Section 6.

2. FMCW LIDAR SYSTEM WITH DIRECT DETECTION

Figure 1shows block diagram of FMCW lidar system with direct detection. In transmitting section, a narrow linewidth
CW laser with wavelength and output power is modulated by linear frequency modulated (LFM) signal via Mach-

International Symposium on Photoelectronic Detection and Imaging 2013: Laser Sensing and Imaging and Applications,
edited by Farzin Amzajerdian, Astrid Aksnes, Weibiao Chen, Chunqing Gao, Yongchao Zheng, Cheng Wang,
Proc. of SPIE Vol. 8905, 890529 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2034878

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 Zehender intensity (MZI) modulator. The modulation process is performed by MZI modulator which has the transfer
function 7-9:

= cos + (1)
2
Where and are the input and output optical fields respectively, is the voltage required to change the optical
power transfer function from the minimum to the maximum8, is the initial phase controlled by DC biasing to
modulator and is RF-LFM signal which can be expressed as:

= cos 2 + (2)

Where is amplitude of LFM signal, is the start frequency, B is modulation bandwidth and is duration time.
The output modulated signal depends on the operating point of MZI modulator if it operates at quadrature point or at null
transmission point 2, 10. For noncoherent systems, MZI modulator operates at quadrature point, then the output modulated
signal will be expressed as:

= 1+ cos 2 + (3)
2 2

Where is angular frequency of transmitted optical signal, is random varying transmitted phase component and
= is modulation index such that ≪ 1 to avoid signal distortion 8. The output modulated signal is directed to
the moving target by telescope.
Laser MZI Optical Optical Telescope
41--
Source Modulator Amplifier Circulator

Photodetector

Frequency RF LFM
Waveform
Generator DSP
Time

Figure 1 Block diagram of FMCW LIDAR system with direct detection 2

There are more factors affect on the reflected signal from target to the receiving section of lidar system such that laser
transmitter models, atmospheric transmission, target reflectivity and angular dispersion 11-13 such that the reflected signal
power is calculated as:

for extended target

4 (4)
=
for any target area
4
Where is optic transmission loss, is atmosphere loss factor, D is circular receiver aperture diameter, ρ is target
reflectivity, is the target area, A is illumination area of transmitted signal at the target and R is target distance. The
reflected signal from the moving target with velocity at distance can be captured by telescope after delay time
= and with Doppler frequency shift = and with reflected signal power such that: (where is light
velocity c = 3 Χ 10 m/s )

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 = 1+ cos 2 − + − (5)
2
In reception section, the direct detection scheme is relatively simple in that no optical mixing is used such that this
method depends on square-law photo-detection. The reflected optical signal is detected by a photodiode with
responsivity ℜ, such that the output photocurrent is expressed as 14:

= ℜ. 1+ cos 2 − + −
2 (6)
Then the photocurrent signal is filtered to get the baseband signal as:

= + t ≈ℜ . 1+ cos 2 − + − (7)

Where and are dc photodetected current signal and ac photodetected current signal respectively as MZI
modulator operates at quadreature point. This output signal is mixed with local oscillator RF-LFM signal to get the beat
signal after LPF as:

= ℜ 2 − +2 (8)

Where is RF-LFM signal amplitude and is the range frequency and it can be calculated as:
2
= (9)

The performance of lidar system with direct detection configuration can be measured by the output signal-to-noise ratio
SNR at photodetector output. There are more noise sources applied on the detected signal 9 such as thermal noise, shot
noise, dark current noise, surface current noise and relative intensity noise RIN. Thermal noise and shot noise effects are
taken in our calculation such that SNR can be calculated as:
ℜ ⁄2
= (10)
2 ℜ +4 ⁄
Where is receiver bandwidth, kb is Boltzmann’s constant equal to 1.38 10 / , Tr is the receiver noise
temperature and is load resistor.

3. FMCW LIDAR SYSTEM WITH COHERENT HERTERODYNE DETECTION

FMCW LIDAR with simplified heterodyne detection is shown in Figure 2. In this configuration, the optical mixing is
performed by using balanced dual photodetector cooperated with 3dB optical coupler.
LFM signal
Generator

T
Optical _ MZI Optical Optical
H Telescope Co /from targe>
Laser
Source Li splitter
Modulator Amplifier Circulator

4
04 <4
3 dB
optical 4--
coupler

T
( DSP I

Figure 2 Block diagram of FMCW LIDAR system with simplified heterodyne detection 1

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 The generated optical signal in transmitting section is divided into two parts; one part is modulated by RF-LFM signal
using MZI modulator which operates at null transmission point for coherent detection systems and transmitted to the
target and the other part is used as LO optical signal which is combined with reflected signal via an ideal 3dB
optical coupler. The outputs of 3dB optical coupler are found to be:
= t + t & = t + t (11)
√ √

Where and are incident optical electric fields on photodetector 1 and photodetector 2 respectively, E t is
LO optical signal which can be expressed as:
t = (12)
Where is randomly varying phase component for optical LO signal and is optical power of LO signal and the
reflected signal can be expressed as:

= cos 2 − + − (13)

By using balanced photodetector and BPF, we can subtract the DC component such that:

= 2ℜ . cos 2 − + − sin + − (14)

Where the removed DC component can be expressed as:

ℜ ℜ ℜ
= + ≈ (15)
2 4 2

In the absence of Doppler shift effect, the simplified heterodyne detection is vulnerable to carrier fading which is
occurred in last term − . The beat signal is produced by mixing the photodetected signal with echo RF
signal and filtering the mixing output using LPF to get range frequency and Doppler frequency :

= ℜ. cos 2 − +2 sin + − (16)

The signal-to-noise ratio SNR can be calculated as:

ℜ ℜ
= = (17)
2 ℜ +4 ⁄ 2

4. FMCW LIDAR SYSTEM WITH COHERENT HOMODYNE DETECTION

The homodyne detection configuration is shown in Figure 3. In this configuration the output modulated signal is split
into two parts; one part is transmitted to the target and the other is used as local oscillator signal for receiving section.
Then the optical local oscillator signal can be expressed as:

= cos 2 + (18)

The reflected signal is mixed with optical LO signal by using balanced dual photodetector cooperated
with an ideal 3 dB optical coupler. The output photodeteted signal will be expressed as:

= 2ℜ . cos 2 + cos 2 − + − sin + − (19)

Such that the removed DC component can be expressed as:

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 ℜ ℜ ℜ
= + ≈ (20)
4 4 4

The beat signal is produced by filtering the photodetected through LPF to get range frequency and Doppler frequency
as:

= ℜ. cos 2 − +2 sin + − (21)

Optical Optical
Laser MZI Optical Telescope To /from target

Source
_, Modulator
_,
splitter
Amptióet Circulator

1.
3dB Optical
LFM signal Couplet
Gene 'ator

DSP

Figure 3 Block diagram of FMCW LIDAR system with coherent homodyne detection 2
The signal-to-noise ratio SNR can be calculated as:
ℜ ⁄2 ℜ
= = (22)
ℜ +4 ⁄ 2

5. DIFFERENT DETECTION MODELS FOR FMCW LIDAR USING OPTISYSTEM

PACKAGE AND MEASUREEMNTS
In this section we model three different detection configurations by using Optisystem package. All developed subsystems
in our modeling are created in Matlab package for co-simulation with Optisystem package. The simulation parameters
are shown in Table 1. These simulation parameters are used in three detection configuration models. We test the
developed models to show the operation and the performance for each detection configuration. Lastly; we collect all
results output from the developed models and do comparison between the three different detection configurations.

Table 1 Simulation Parameters

Parameter Simulated value

LFM signal
= 300 , = 300 , = 10 , =1
parameters
Laser source = 1550 , = 40 , Δ ≤ 50
MZI Modulator = 5.5 , = 20
Channel and target
Extended targets , = 1, = 15 , =1, =1
model
Optical amplifier EDFA , = 1, = 20
Photodetector PIN photodetector, ℜ = 1 / , = 50 Ω , = 410 , = 290

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 5.1 Direct detection model using Optisystem:
FMCW LIDAR system with direct detection technique is simulated by Optisystem package as shown in Figure 4. The
simulated block diagram consists of three subsystems as shown in Figure 4. LFM signal generator subsystem, MZI
modulator subsystem and channel and target model subsystem created in Matlab using the mathematical model of each
one for co-simulation with optisystem.

O tisystem RF Spectrum Anayzer_t

Electrical Amplifier
Gain = 20 dB Electrical lAUhip
LFM signal

RF Spectrum Analyzer
Generator
Lox Pass Rectangle Filer
Cutoff frequency = 200 MHz
Spetta 1x2

Optisystem
Channel Model
Optisa stem & Target Model
PratoCetector PIN

MZI Modulator DG. Subsystem

Optical Amplifier
Gain = 20 dB
CW Laser Measured Subsyste,2 Noise figure = 5.5 dB
Frequency = 1550 nm
Power = 40 mW
Linewidth = 0.05 MHz

Figure 4 Simulated FMCW LIDAR system with direct detection using Optisystem package

Figure 5 shows stationary target at = 750 detected by direct detection configuration. In this simulation, the effect
of thermal noise and shot noise are taken in our consideration. This simulation is done at modulation index = 0.4039
and the transmitted signal power towards the target is = 2 then the reflected signal power from extended target
will be = 41.25 nW, the detected signal power is = 3 × 10 W, the thermal noise power
= 2.6214 × 10 W and shot noise power = 5.41 × 10 W . The calculated signal to noise ratio at
photodetector output will be SNR = -29.5 dB. As shown in Figure 5. The range frequency of the detected signal is at
= 150 which is related to target distance R = 750 m.

E = 150 MHz (R = 750 m)

100 M 200 M 300 M 400 M

Frequency (Hz)

Figure 5 Stationary target at = 750 detected by direct detection technique in FMCW LIDAR system

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 5.2 Coherent heterodyne detection model using Optisystem:
FMCW LIDAR system with simplified heterodyne detection is simulated by optisystem package as shown in Figure 6.
This detection configuration is capable to detect target distance and target velocity simultaneously. MZI modulator
subsystem built by Matlab package and adjusted to operate at null transmission point to achieve modulation index
= 0.1277. We simulated the developed model of simplified heterodyne detection to detect stationary target at distance
750 m ( = 150 MHz) and moving target with velocity 50 km/h ( ≈ 18 MHz) at distance 750 m as shown in Figure 7.
As shown Figure 7, the output spectrum of detected signal is as DSB-SC signal spectrum with bandwidth equals to twice
Doppler frequency shift such that the upper sideband has frequency equals to + = 168 MHz and the lower sideband
frequency equals to − = 132 MHz.

Figure 6 Simulated FMCW LIDAR system with simplified heterodyne detection using Optisystem package

G= 150 MHz (R = 750 m) L -fa = 132 MHz L +fa =168 MHz

X

i,o
( }

100 M M 100 3.4 MOM 300M MOM

Frequency (Hz) Frtneacy(Hi)

(a) Stationary target at = 750 (b) Moving target at = 750 with = 50 /ℎ

Figure 7 Detected target by simplified heterodyne detection in FMCW LIDAR system

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 5.3 Coherent homodyne detection model using Optisystem:
FMCW LIDAR system with homodyne detection is simulated by optisystem package as shown in Figure 8. This
detection configuration is capable to detect target distance and target velocity simultaneously. MZI modulator subsystem
built by Matlab package and adjusted to operate at null transmission point to achieve modulation index β = 0.1277. We
simulated the developed model of homodyne detection to detect stationary target at distance 750 m (f = 150 MHz) and
moving target with velocity 50 km/h (f ≈ 18 MHz) at distance 750 m as shown in
Figure 9. As shown
Figure 9, the output spectrum of detected signal is as DSB-SC signal spectrum with bandwidth equals to twice Doppler
frequency shift such that the upper sideband has frequency equals to f + f = 168 MHz and the lower sideband
frequency equals to − = 132 MHz.

Optisystem
Optisystem
LFM signal

Channel Model
Generator
RF S tram AnaWer_3

Optisystem
Optisystem Low Pass Rectal* Mr
Caton I mammy 200 Wiz
Balanced
ME Modulator Po., Spatter lx2 013 Opt.

Photodetector
i;;;;;;_2
Subsystam_4
CI
RF Spew= Analyaer

Figure 8 Simulated FMCW LIDAR system with homodyne detection using Optisystem package

L -fa= 132 MHz E. +fe= 168 MHz

150 MHz (R=750 m)

Wkipt1/44pPv
V 41'016,mlligw4IN
100M 200M 300M 400M
100 200 M 300 M OW M Freeing aft)
nreqee.cy(ei)

(a) Stationary target at R = 750 m (b) Moving target at = 750 with = 50 /ℎ

Figure 9 Detected target by homodyne detection in FMCW LIDAR system

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 6. CONCLUSION
In this paper we introduced a mathematical analysis of different detection configurations especially direct detection,
simplified heterodyne detection and homodyne detection using Optisystem package. We modeled these detection
configurations by using Optisystem package. All subsystems in the developed models are built by Matlab package for
co-simulation with Optisystem package. Table 2 shows a comparative summary for different detection configuration
used with FMCW lidar system.
Table 2 Comparison table among different detection techniques used with FMCW lidar system

Direct detection Heterodyne detection Homodyne detection

Operation Susceptible to incident optical intensity Susceptible to incident optical intensity and phase

4 ℜ ℜ
SNR = ℜ ⁄2 = =
2 2
application Distance detection Distance and velocity detection
Long distance and High Long distance and High
Advantages Low complexity system
sensitivity sensitivity
No need to phase diversity
Needs phase diversity circuit
disadvantage Low sensitivity and Short distance circuit
High complexity
High complexity

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