Research on Multi-wavelength and Multi-beam Illumination for

Improving Object Illumination Uniformity

Sun Quan, Ing Yu, Yang Yi, Yan Baozhu and Du Shaojun

College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China

Keywords: Illumination Uniformity, Multi-wavelength, Multi-beam, Atmospheric Propagation.

Abstract: The imaging and tracking to the far dim objects in the air are usually difficult for detecting system to get their

clear images and accurate positions, due to the objects neither emitting light nor reflecting sunlight. Using

active laser illumination technology, the target will be illuminated and the echo intensity of target can be

increased. The laser intensity spatial distribution on the target plane fluctuates usually remarkably due to the

atmospheric turbulence and other causes, which will decrease image resolution and tracking accuracy in

optical system. In this paper, we presented a method of using illumination with the supercontinuum spectrum

laser beam combination to improve the effect of target illumination uniformity. As the atmosphere

transmittance ratios differs with wavelength, we set up a multi-wavelength and multi-beam target illumination

simulation system to represent the supercontinuum spectrum laser illumination effect. As the previous

simulation results and the experimental results, which both reflect an improving effect of illumination

uniformity significantly by increasing of the beam number. The illumination uniformity of single beam with

multi-wavelength spectrum components was analyzed, and the simulation results of one single beam, three

beams and six beams with 9 wavelength spectrum were compared, which showed that the illumination

uniformity in target plane was improved with both the number of wavelength spectrum components and the

number of beams increasing. It showed that multi-spectral multi-beam illumination had great advantages in

improving illumination uniformity and the supercontinuum laser can be a good active illumination source.

1 INTRODUCTION

The imaging of and tracking to the far dim objects in

the air are usually difficult for detecting system to get

their clear images and accurate positions, due to the

objects not emitting light or reflecting sunlight. Using

active laser illumination technology, the target will be

illuminated and the echo intensity of target can be

increased. Active illumination with multi-beam has

been proved to be an effective technology to improve

the object illumination uniformity (Pavel et al., 2007)

(Quan et al., 2013) which can increase object image

resolution and tracking accuracy in optical system.

Supercontinuum (SC) laser sources have advantages

of wide spectrum range and good spatial coherence,

high brightness, better in direction. With the

increasing of the average output power (Song et al,

2013) (Xia et al, 2009), they become practicable in

many application areas, such as laser communication,

laser remote sensing, hyperspectral LiDAR, and laser

ranging (Alexander et al, 2012) (Hakala et al, 2012).

These applications all require SC laser sources

propagate through the atmosphere, meaning that the

beam will suffer atmospheric absorption, refraction,

extinction and turbulence effect (Kang and Wenyue,

2015) (Wuming et al, 2014). The wide spectrum of

SC laser propagating through atmosphere will be

separated into several main wavelength bands due to

the transmittance of different wavelengths. We use

SC laser as active illumination source means only the

wavelength bands which have high transmittance can

be valid for illuminating object. In this paper,

we presented a method of using the SC laser beams

combination as active illumination source to improve

the effect of target illumination uniformity. We set up

a multi-wavelength and multi-beam atmospheric

propagation and target illumination simulation

system. The simulation results showed that multi-

spectral multi-beam illumination had great

advantages in improving illumination uniformity.

120

Quan, S., Yu, I., Yi, Y., Baozhu, Y. and Shaojun, D.

Research on Multi-wavelength and Multi-beam Illumination for Improving Object Illumination Uniformity.

DOI: 10.5220/0007356301200124

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 120-124

ISBN: 978-989-758-364-3

2 ESTABLISHMENT OF

SIMULATION SYSTEM

As a numerical example, we chose the SC laser

spectrum rang from 600nm to 1700nm (Chen et al,

2011) and the power spectrum density with average

output power. The transmittance of the atmosphere is

calculated by MODTRAN software (Berk et al,

2014). The calculation was carried out under the

typical atmosphere situations: mid-latitude summer,

rural aerosol model with 23km visibility and CO2

mixing ratio 360ppm. The distance of laser

propagation path is 100km. The transmittance

(λ, z) for different wavelengths is depicted in

Figure 1.

Figure 1: Transmittance in different wavelengths.

The simulation system is built in SeeLight, a

software tool for high fidelity modeling of advanced

optical systems such as laser active illumination and

object detection systems which is developed by

National University of Defense Technology and

Institute of software, Chinese academy of Sciences.

The principle of the software is based on wave optics

theory with performing propagation by the angular

spectrum theory and fast Fourier transform. Figure 2

shows the multi-beam (taking six beams for instance)

and multi-wavelength active illumination simulation

system schematic. The number of beams can be

changed in simulation system by adding or deleting

laser source modals. And the beam spatial

arrangement is cycle type. Each beam contains

several wavelengths. We chose 9 typical wavelengths

which had high transmittance ratios marked with red

lines in Fig.1 for each beam. The data of coherence

lengths and transmittance ratios was shown in Table

1. In atmosphere propagation path modal, turbulence

layer was represented by a phase-changing screen

with the associated transverse wind velocity V. The

turbulence strength was decided by refractive index

fluctuation function Cn

, we used HV57 modal with

average Cn

value of 10

-16

to present moderate

turbulence intensity. At the same turbulence intensity,

different wavelength beam experienced different

coherence length R0 as shown in Table 1. With the

same transmitter diameter of each beam (20cm),

different wavelength beam would undergo different

turbulence influences.

Figure 2: The simulation model schematic of Multi-beam

active illumination was built in SeeLight software.

Table 1: The coherence lengths of 9 typical wavelengths

with high transmittance ratios.

Wavelength(nm)

Transmittance

R0(cm)

670

75.5%

4.4

710

75.9%

4.7

750

79.1%

5.0

810

79.5%

5.5

880

85.9%

6.1

1060

88.7%

7.7

1250

91.8%

9.4

1290

89.3%

9.7

1600

91.2%

12.6

3 SIMULATION RESULTS

Each multi-wavelength beam propagated through

different turbulence area as they distributed over

different situation at the exit of transmitter and their

propagation paths differ to each other. Each

wavelength in same beam propagated through the

same atmosphere path and interacted with the same

turbulence, but they experienced different

transmittances and coherence lengths. Fig.3 showed

the 9 different wavelength beams far field intensity

distributions after propagation through moderate

turbulence. From Figure 3 (a) to (i), the far field spot

turned to be more concentrated and bigger size, which

meant longer wavelength beam suffered less

turbulence distorted.

Research on Multi-wavelength and Multi-beam Illumination for Improving Object Illumination Uniformity

121

(a) 670nm

(b) 710nm

(d) 810nm

(e) 880nm

(f) 1060nm

(g) 1250nm

(h) 1290nm

(i) 1600nm

Figure 3: The different wavelength beams far field intensity distributions.

Figure 4 showed the long exposure far field spot

of one multi-wavelength beam with 9 typical

wavelength components propagating to the object

plane. Compared with each wavelength far field spot

in Figure 3, we can draw the conclusion that the

multi-wavelength SC laser illumination can make

better illumination uniformity than single wavelength

laser illumination.

In our previous work (Quan et al, 2013), we have

already proved that multi-beam illumination could

make better illumination uniformity than one beam

and the larger number of beams could produce the

better uniformity. In this paper, we also simulated

multi-beam lasers with multi wavelength components

illumination system as shown in Figure 2. Three

beams and six beams separately combined together as

illumination sources to simulate the illumination

Figure 4: The long exposure far field spot of one multi-

wavelength beam.

effect. The simulation results presented in Figure 5

show the calculated far field spots intensity of the

86.5% power in bucket of one beam, three beams and

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

122

(a) one beam

(b) three beams

Figure 5: The far field spots intensity of the 86.5% power in bucket.

six beams. The spot intensity RMS of one beam, three

beams and six beams were 39, 31 and 19 separately.

The illumination uniformity showed a good

improvement with the beam number increasing.

We also got the spot intensity RMS of one beam,

three beams and six beams simulated at other

atmosphere conditions with different Cn

values.

Table 2 showed the calculated RMS of far field spots

were all reduced at different turbulence intensity from

average Cn

of 10

-15

, 10

-16

to 10

-17

Table 2: The far field spots RMS data of one beam, three

beams and six beams at different turbulence intensities.

=10

-15

=10

-16

=10

-17

1 beam

3 beams

6 beams

4 CONCLUSIONS

In this paper, we have presented a method of using

the super-continuum spectrum laser beam

combination to improve the illumination uniformity

in atmosphere propagation. As the atmosphere

transmittance ratios differs with wavelength, we set

up a multi-wavelength and multi-beam laser

illumination simulation system to represent the SC

laser illumination effect. The simulation of multi-

beam illumination was carried out to analysis the

number of the illumination beams on the influence of

illumination uniformity under different atmospheric

conditions. The illumination uniformity of one single

beam, three beams and six beams with 9 high

transmittance ratio wavelength spectrum were

compared, which showed that the illumination

uniformity in target plane was improved with both the

number of wavelength spectrum components and the

number of beams increasing. The spot intensity RMS

of one beam, three beams and six beams were all

reduced at different turbulence intensities. The

simulation results showed that multi-spectral multi-

beam illumination had great advantages in improving

illumination uniformity. With high illumination

uniformity at target end, the echo intensity of the

objects will be more uniformity, which will improve

the profile definition of targets. The detecting system

could get their clear images and accurate positions

with SC laser illumination.

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