ENG352 Manual Final

LIDAR MANUAL ENG 352-457

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DESIGNING, DEVELOPING AND OPERATING A LIDAR:

A MANUAL FOR FLEDGLING ATMOSPHERIC SCIENTISTS

SALMAN HAIDER NAQVI

ENG-352-457

PROF. PATRICK BEATON

FINAL DRAFT MANUAL ASSIGNMENT

October 11, 2009

Figure 1 – Basic lidar system Figure 2 – lidar at Soderstrom, Greenland


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CONTENTS

1. Overview…………………………………………………………………………… 3

2. Scientific Background……………………………………………………………… 3

3. Lidar Application…………………………………………………………………... 4

4. Measurement Range……………………………………………………………….. 6

5. Purpose and Importance of NJIT Lidar……………………………………………. 6

6. NJIT Lidar Setup and Schematics…………………………………………………. 7

7. Organization of Lidar Assembly…………………………………………………… 8

8. Procedure of Setting up the Lidar………………………………………………….. 10

9. Transmitter System………………………………………………………………… 11

a. Laser………………………………………………………………………... 11

b. Laser Cooler and Power Supply…………………………………………… 13

c. Mirror………………………………………………………………………. 13

d. Function Generator………………………………………………………… 14

10. Receiver System…………………………………………………………………….14

a. Light Acquisition at a Glance……………………………………………… 15

b. PMT, PMT Housing, Power Supply and Cooler…………………………... 16

c. Signal Preamplifier………………………………………………………… 17

11. Data Acquisition System……………………………………………………………18

a. SR-430 and the Science behind Data Acquisition…………………………. 18

b. Signal Oscilloscope………………………………………………………… 20

c. Computer Operated Data Acquisition……………………………………… 21

d. Data Analysis………………………………………………………………. 22

12. Conclusion…………………………………………………………………………. 22

13. References………..………………………………………………………………… 24


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Overview:

Let us attempt to prepare and operate a LIDAR (Light Detection And Ranging) System.

This manual is prepared to teach you all the technical details involved in beginning to

understand each component of the LIDAR (hereafter written in lower case letters, lidar)

system, its individual working and its specific role within the lidar assembly. So if you feel

that you have accomplished these simple objectives by the end of this manual then cheer up

because you will become really productive for the „Remote Sensing‟ community.

This manual would also be helpful for

- The naïve lidar operators who wish to act as a research assistant with Dr. Andrew

Gerrard by learning to use lidar

- The experienced users who wish to recall some important technical minute details about

the working of any of the tens of individual electrical/optical components constituting a

lidar.

- Anyone who wish to be guided throughout the process of designing a lidar, developing it

(talking about material and financial constraints involved), operating it for meaningful

data collection and then interpreting the data using appropriate computer software.

Scientific Background:

I will begin by answering a simple question which should be really bothering you by

now: what in the earth is a Light Detection And Ranging System and the Remote Sensing? Lidar

is similar to Radar, however it uses laser light in the higher frequency spectrum of

electromagnetic waves (Ultraviolet, UV, to Infrared) to remotely acquire data of a physical


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parameter. Moreover, remote sensing is the acquisition of information of an object or

phenomenon by the use of real-time sensing devices which are not in physical contact with the

object (like satellites, aircraft, radar etc). Lidar performs the remote sensing, of the atmosphere in

our case, by receiving the reflected laser energy from the air molecules at different altitude

levels.

This allows the lidar to generate synoptic profiles of the atmosphere within a certain

altitude range for visually examining the change in atmospheric density between that range. The

figure 2 shows how a laser and telescope are used to acquire the reflected light pulses from the

air molecules (aerosols in this case). It then plots the amount of power received versus the height

from which it is received; the greater the molecules the greater the reflected power. We will

explain this in detail with the help of more pictorials under the data analysis section of our

manual.

Lidar Applications:

So what are the many applications of lidars. Lidar is used for various atmospheric,

landscape, oceanic modeling and geo-sensing applications. They are sometimes installed on

Figure 3 – Lidar for Atmospheric Density Measurements


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board the satellites or aircrafts and are operated remotely from ground and the acquired data is

transmitted to the ground stations.

The lidar at NJIT is being used for atmospheric sensing purposes. Atmospheric scientists

and physicists often resort to this active remote sensing technique using lidars if they are

interested in learning about the:

- atmospheric molecular composition,

- wind waves‟ structure,

- air density variations and

- cyclic trends in atmospheric waves

to interpret various atmospheric and climatic phenomena and to account for climate change. This

essentially provides them with means to get real-time information about our atmosphere which

can only be remotely analyzed. Therefore atmospheric research agencies like National Oceanic

and Atmospheric Administration (NOAA), National Center for Atmospheric Research (NCAR),

Intergovernmental Panel on Climate Change (IPCC) and others are chief users and fans of lidar

technique.

Figure 4 – Satellite and Airborne Lidar Remote Sensing


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Measurement Range:

All lidars also have a measurement range for least-error readings, which depends chiefly

on the laser power and the receiving instruments‟ (telescope‟s) efficiency. Therefore the

specification of each lidar will vary depending on your application. For example, data acquisition

from mesosphere, middle-atmosphere (above 35km altitude), would require over 5 watt laser and

at least half a meter diameter telescope. Moreover lidar‟s range can only be up to 20km with a

2.2 watt laser and a 2.2 inch telescope which is the case with NJIT lidar. However, using the

same lidar with the 1.2m diameter telescope (contrast shown in figure 4) will extend the

measurement range above 40km. The error percentage is dependent on the laser power and the

telescope lens‟ diameter (found from the Rayleigh scattering equation.)

Now that you are conversant with the many usages of lidars, let us look at the ground-

based lidar located at NJIT, which you will learn to operate through this manual.

Purpose and Importance of NJIT lidar:

It is important for you to know the research purpose of this lidar and the significance of

this research. The NJIT lidar has been designed with a specific lower and middle atmospheric

research in mind. It would collect the atmospheric density data between 0 and 25 km altitude

Figure 5 – Measurement Range Variation with Telescope Usage


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range, which can then be analyzed using a three dimensional plot of time, altitude and density for

tracking the particular type of atmospheric waves, stationary gravity waves, above the New

York-Newark metropolitan area. The background research suggests that this heavily urbanized

region of North-East USA generates this special type of waves in the lower-middle atmosphere

due to the significant urban-rural temperature difference effect caused by urbanization.

Therefore, tracking these waves will have implications for aviation safety control by

helping to detect atmospheric turbulence patches, for climate modelers by incorporating the

effect of gravity waves in the global climate models and for future atmospheric researchers. This

lidar is currently placed in North-West NJ National Forest Park due to the relatively clear and

pollutant-free atmosphere in that area.

NJIT Lidar Setup and Schematics:

The NJIT lidar is designed to fit in a 4 ft by 4 ft breadboard table so that it could be

moved easily from place to place. It would be assembled inside the laboratory and then taken

outside to operate the laser and receive the reflected light energy. It is shown in the picture below

and is also labeled for your information.

Figure 6 – Gravity Waves Generation due to Urbanization


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The laser and its associated power supply and cooling equipments are missing in this picture.

However the laser would be fitted on the mount right in front of the mirror placed behind the

computer to shoot the laser vertically into the atmosphere.

Organization of Lidar Assembly:

Now, we would begin talking about each of the components of the lidar in a systematic

fashion. We can start by dividing the entire lidar assembly into three major units: Receiver

System, Transmitter System and the Data Acquisition and Analysis System. Each of the units

requires equal consideration to details since each play a vital role in generating the ultimate

product, our meaningful data which we could relate to at the end. A schematic of our NJIT lidar

would be helpful in giving you a bird‟s eye-view of the intelligent and compact lidar assembly. It

is given in figure 8 below.

Telescope

PMT Cooler

PMT Power Supply

PMT Housing

w/ PMT Mirror

Laser Holder

4x4 Breadboard

Data Analysis PC

SR-430

Function Generator

Oscilloscope

Pre-Amplifier

Figure 7 – NJIT Lidar System Snapshot Labeled


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Turning

Mirror

Transmitter SystemData Acquisition System

Interference

Filter

PMT Cooler

Pulse/Function

Generator

Preamplifier

5X

1.6 W

Tripled Nd:YAG

Laser

Refractor

Telescope

2-inch

f/14

Turning

Mirror

IrisPMT Housing

SR430

High Voltage

Power Supply

(1500V)

Receiver System

NJIT LIDAR SCHEMATICSNJIT LIDAR SCHEMATICS

5X 5X

PMT

Signal Oscilloscope

Iris

Computer

Interface

Tri

gg

er Turning

Mirror

Transmitter SystemData Acquisition System

Interference

Filter

PMT Cooler

Pulse/Function

Generator

Preamplifier

5X

1.6 W

Tripled Nd:YAG

Laser

Refractor

Telescope

2-inch

f/14

Refractor

Telescope

2-inch

f/14

Turning

Mirror

IrisPMT Housing

SR430SR430

High Voltage

Power Supply

(1500V)

Receiver System

NJIT LIDAR SCHEMATICSNJIT LIDAR SCHEMATICS

5X 5X

PMT

Signal Oscilloscope

Iris

Computer

Interface

Tri

gg

er

THE REAL CHALLENGE:

Just putting all the equipments together, connecting them with wires and powering up the

system is a no-brainer but to actually design a compact, transportable, easily debug-able lidar

assembly, having good signal statistics is the real challenging task. As you can see in the picture

the NJIT lidar has been designed so that the transmission, acquisition and analysis of the light

signal are all done on a single compact lidar table. Moreover, each of the equipments has been

selected to keep the error percentage of the acquired data at the minimum level.

Now, I would give you a step by step instruction set for setting up the lidar system so that

you get the taste of lidar assembly.

Figure 8 – NJIT Lidar Schematics & System Breakup


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Procedure of Setting up the Lidar:

After purchasing all the required instruments, the biggest task is to effectively connect

them and set them up on the lidar table. The following steps would lead you to successfully

completing the setup:

1. The telescope should be the first thing to setup on the table with respect to which rest of

the optics would be aligned. Pick a table’s corner to position the telescope.

2. Afterwards, place the optical assembly for directing the incoming light signal from the

telescope towards the photomultiplier tube (PMT) housing.

3. The optics assembly should have a ray-expanding lens, followed by the interference

filter lens and the converging lens afterwards which would focus the light at the PMT

aperture to maximize the energy entering the PMT.

4. All of these instruments should be aligned in a straight line on the longer side of the

table. The other side should have the laser stationed at one end.

5. You should place a first-surface reflective mirror right in front of the laser to transmit

the laser beam vertically towards the atmosphere.

6. The side with the laser beam on it would be covered with metal sheets so that it is

isolated from rest of the work station and does not pose any radiation threats to the users.

7. The computer accessories, monitor, keyboard and mouse should be squeezed in on rest

of the middle space on the table


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8. The bottom shelf of the table would occupy the PMT housing‟s power supply and

cooler, the pre-amplifier, the function generator and the multichannel scalar.

9. The multi-channel scalar is the one that you would need most access to, so it should be

placed at the bottom where it is readily accessible from. While pre-amplifier and the

function generator, which are very light, can be stacked on top of the multichannel scalar.

10. The PC‟s CPU, PMT Housing cooler, and power supply can be stacked on the other side

of the bottom shelf. While the laser power supply and cooler would stay on wheels next

to the table.

Congratulations, you have the lidar system all setup on one table, allowing the ease of movement

from the observatory inside to the open.

Since you should learn how each unit of lidar system works, let us now talk about each of

these units in due detail. We will start with the Transmitter System.

Transmitter System:

The transmitter system consists of the laser, laser power supply and cooling system, first-

surface mirror, and a function generator. Each equipment helps in making the process of laser

transmission to the atmosphere successful and to ensure the as-expected data acquisition.

LASER:

For our application of monitoring the lower atmosphere (0-25 km), we would need a laser

that is capable of generating 2.2 watt power laser pulses with a 355 nm wavelength in the

ultraviolet spectrum. This laser power was chosen because its combination with the 2 inch


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diameter telescope in the receiver side would give less than 1% error in atmospheric density

measurements. The error percentage is dependent on the laser power and the telescope lens‟

diameter (found from the Rayleigh scattering equation.) Leaving all the technical details aside,

the important things to remember are laser power, its operational safety measures and the

purchasing procedure.

One of the reliable manufacturers for such high-powered lasers for scientific purposes is

Quantel. And their “Brilliant b” model satisfies the power and wavelength requirements while

ensuring compactness and user-friendliness of the equipment. The price of this laser as of the

2008 personalized quote is $45k.

When operating the laser, be sure that you are wearing safety goggles (sold separately by

Thor Labs: optical instrumentation firm) the lidar table is covered with laser-obstructing metallic

sheets, and that the beam dump is in place when laser is not in use for data acquisition purposes.

Remember: the 2.2W laser is strong enough to pulverize your eyesight if even a reflective

contact is made with the naked eye. But you can still work with your second eye, right.

Figure 9 – Quantel Brilliant b 2.2 watt Laser Generator


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LASER POWER AND COOLER:

Laser cooler and the power supply are integral to a laser. Laser can easily become

overheated, compromising its own ability to maintain a fixed power, if the cooler does not keep

the laser circuitry well-cooled. The cooling mechanism for the Brilliant b solid state laser is

much efficient than the ruby and other old lasers. The box shown in the figure houses both the

power supply and the cooling cage for the laser.

The cooling cage needs to be filled with the distilled water by unscrewing the left

compartment and filling up the fluid-storage tank. A hose connects the cooler to the laser which

ensures the supply of cooled water to the laser interior circuitry and also the drainage of the hot

water supply from the laser. The cooler weighs a ton which is why the wheels underneath are a

literal blessing.

MIRROR:

The first-surface mirror is used to direct the laser beam into its vertical pathway towards

its atmospheric destination. It also is a unique mirror specifically made to lower the energy losses

Figure 10 – Quantel Brilliant b Laser Power Supply and Cooling Unit


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in the process of reflection from the mirror‟s surface. It can be bought for a couple hundred

bucks from an optics firm.

FUNCTION GENERATOR:

The function generator is used to trigger the laser to send out the pulse. It determines the

laser pulse rate and the receiver acceptance rate. It therefore performs the job of keeping the

receiver and transmitter systems in sync with each other. The moment laser fires, the

multichannel scalar samples the reflected pulse. Therefore, you need to set up the frequency at

which the laser needs to be fired. The typical frequency is 20 Hz, i.e. 0.05 seconds per pulse or

1200 laser pulses are transmitted in a second and subsequently the receiver is prompted to read

1200 times in a second.

Receiver System:

Let us talk now about the lidar‟s Receiver system. The receiver mechanism consists of:

- Telescope

- Optics Assembly: Converging lenses, Interference Filters

Figure 11 – 2MHz Function Generator


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- Photomultiplier tube (PMT) inside its Housing

- PMT Housing‟s Cooler and Power Supply

- Signal Preamplifier.

I would give you a description of how the reflected laser makes its way to the computer through

this receiver system.

LIGHT ACQUISITION AT A GLANCE:

1. The telescope receives the reflected light and directs it to the receiving optics, where

2.the light is diverged with the help of diverging lens,

3.passed through the interference filter to

polarize the light in one direction, 4.

converged using the converging lens so that it is focused on

to the aperture of the PMT housing and 5.

thus sent into the PMT.

The main job of the PMT is to generate the current pulses corresponding to the number

of light photons and finally to produce the voltage (potential difference across a resistor) which

is amplified by the preamplifier to be read by the multichannel scalar in the data acquisition

system. This process of generating voltage pulses is done inside the PMT and is the backbone of

Figure 12 – Meade 2” inch diameter Telescope


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the entire lidar operation. The amount of voltage detected is directly proportional to the

amount of photons in the reflected beam. The energy in the reflected laser beam is directly

proportional to the atmospheric density at any altitude level. The more the air molecules at a

particular altitude level, the more is the reflectance of the laser beam and thus a higher voltage is

produced.

Great, now you know the principles of science behind the lidar‟s working. Now you can

answer the elementary question: how does a lidar generate a snapshot of the atmospheric density

within a given altitude range?

PMT, PMT HOUSING, POWER SUPPLY AND COOLER:

PMT and PMT Housing are shown in the figure along with the power supply and the

cooler. The PMT cooler acts in the same way as the Laser cooler. The important thing to note

here is the placement of each of the equipments on the lidar table. The PMT should be placed in

the line of sight of the telescope‟s outward aperture and all the receiving optics. As far as the

connections are concerned, you should connect the power supply to the cathode of the PMT

housing. The signal output of the PMT is taken from the Housing‟s Anode terminal. The cooler

on the other hand is connected to the 16-pin cooler pin on the top-left of the PMT housing.

Figure 13 – Photomultiplier Tube Housing (Front and Back)


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Now with the output voltage pulses from the PMT housing in hand, connect the output of

the housing to the pre-amplifier‟s „signal-in‟ port. This is critical because many a times the

reflection from higher altitudes is so small that the voltage signal is barely noticeable.

The PMT Housing, Cooler, and the Power Supply are all sold by the „Products for

Research‟ and can be purchased for a total of $15k. The PMT on the other hand ranges from $1k

to $1.5k. It should be specific to the pin slot available in the PMT housing. The PMT suitable

for our application of 377nm wavelength light is sold by Electron Tubes Ltd. for $1.2k and has a

20-pin connector.

SIGNAL PREAMPLIFIER:

The signal coming in from the oscilloscope needs also to be amplified in order to

maximize the detection of photons. The noise can later be eliminated by setting the discriminator

level of the voltage pulses generated at a higher value so that low voltage noisy pulses are

rejected. The preamplifier, SR445A from the Stanford Research Systems, does a perfect job in

this regard and can be easily bought for $1.1k.

Figure 14 – PMT Cooler (on top) and PMT High Voltage Power Supply

Figure 15 – Sample PMT


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The figure above shows the front and back panels of the preamplifier. Any one of the channels

can be used for our purpose and the signal from the PMT would flow into the SR445A through

the IN node, which would then be amplified and the output signal from the OUT node can then

flow into the SR-430.

Data Acquisition System:

We are almost done with understanding the working of the lidar system. Hang in there for

a few more steps. The Data Acquisition system consists of:

- Multichannel Scalar and Averager, SR430

- Signal Oscilloscope

- Computer with a Data Acquisition and Data Analysis Software

SR430 AND THE SCIENCE BEHIND DATA ACQUISITION:

Now we have the output from the pre-amplifier which will go into the multi-channel

scalar, produced and named by Stanford Research Systems as the SR430. It can be bought for

$8k. It is used to sample the voltage pulses against time and is started when the laser is triggered.

The function generator is thus connected to both the laser and the SR430 and triggers both at the

same time.

Figure 16 – Signal Preamplifier


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The SR430 generates profiles of atmospheric density data where each time corresponds

to an altitude level. This time-altitude relationship is explained by the elementary distance-time-

velocity relationship in physics of mechanics. The distance a laser beam covers from when it is

reflected back from an air molecule at a certain altitude level to the lidar receiver is directly

proportional to the speed of light times half the time it takes from leaving the laser transmitting

system to arriving back at the receiver. Thus the altitude versus the photon-counts and hence the

atmospheric density plots are drawn by the SR-430. And it generates these plots for the entire

operational time of the lidar system. Therefore, if lidar is operational for 30 minutes then 30

atmospheric densities vs. altitude profiles are generated by the SR-430.

Figure 17 – Multichannel Scalar and Averager SR430

Figure 18 – SR430 CRT Screen showing sample signal


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SIGNAL OSCILLOSCOPE:

You can use the oscilloscope to visualize the voltage signal coming out of the pre-

amplifier or the function generator to retrieve information about various parameters of the signal.

The LCD screen for the Tektronix TD2022B in its operational mode is displayed below.

The bottom node is for the input signal and the knob above it is used to adjust the vertical

voltage scale on the oscilloscope. The knob above the third node from left adjusts the horizontal

time axis. The important parameters for a signal are its strength, reflected by the peak-to-peak

voltage, and its frequency. The frequency and the voltage level of the function generator are also

set to a precise value with the help of an oscilloscope. So in order to set the frequency to 20 Hz,

the time period of the wave should be 0.05 sec or 50 msec (1/20). Thus set up the horizontal

scale to 10 msec per division and adjust the input signal from the Function generator so that the

sinusoidal wave covers exactly 5 divisions. Similarly trigger voltage level could be set to the

required value by first setting up the scale to appropriate volts per division and then adjusting the

signal voltage from the generator, so that the wave covers exactly the right no. of divisions.

Figure 19 – Signal Oscilloscope and LCD Screen showing sample signal


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Oscilloscope also allows you to visualize the output signal from the PMT in real-time

through which any disturbances in the system or irregularities in the signal can be detected.

COMPUTER OPERATED DATA ACQUISITION:

However, this can be done automatically by the computer. The LabView, graphical

programming language, can be used to create a graphical interface enabling the computer to

communicate with the multichannel scalar using a GPIB (General Purpose Interface Bus) port.

The interface of this communication software is shown below.

Set up the trigger lever, laser pulse rate, operational time and other parameters on the

screen. Afterwards hit Initialize and Go button. The SR-430 would be initialized and the laser

Figure 20 – SR430-PC Communication Interface


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would be prompted at the same time to start firing the beam. The data profiles obtained are

plotted in the graphic plane and the data is stored in a file on the computer.

DATA ANALYSIS:

That is it. We have all the data we need to conduct the extensive data analysis procedure

and to arrive at the scientific conclusions to be described in research papers. I have included the

following graph for you, which shows the three-dimensional plot of atmospheric densities vs.

altitude vs. lidar operational time data in a two-dimensional manner.

The color coding described on the right accounts for the three-dimensional nature of the

plot: the low to high atmospheric densities are shown on a scale of blue to red colors.

This data analysis is done using programming software which is widely used by

physicists, called IDL (Interactive Data Language). The lidar generates and sends the data for

atmospheric density vs. the altitude profiles for every full cycle during the operational period of

the lidar. Thus the atmospheric density index and its corresponding altitude index is plotted

against the time index using IDL.

Figure 21 – Atmospheric Density vs. Altitude vs. Time Plots (Synoptic Change)


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Conclusion:

You are now ready to start building your own lidar system. If you are thinking that it is

too expensive to purchase the $75k worth of instruments then you are not wrong. But you can

instead begin your career as an atmospheric scientist research assistant. Scientists already have

research grants to support such expenses and they are looking for skilled students to operate

these equipments and to carry out data acquisition and elementary data analyses which itself

takes a lot of time.

We have now covered the steps involved in setting us a compact lidar system, the

working of all the important constituents of the lidar assembly, the science behind remote

sensing of lower atmosphere using a lidar, procedure to successfully acquire and store the data

on the computer and to display the meaningful atmospheric density information using interactive

plots.

Now you can go ahead and safely brag about your atmospheric learning achievement.

Figure 22 – Me and My Lidar


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REFERENCES

1. “ARCLITE* System.” The Sondrestrom Research Facility. 11 Oct 2009, 22:00 EST.

<http://isr.sri.com/instruments/data/arclite/arclite.html>

2. “ETX Premier Edition.” Meade Instruments Corporation. 10 Oct 2009, 20:00 EST.

<http://www.meade.com/etx_premier/index.html>

3. “LIDAR: Light Detection and Ranging.” Lambda Photometrics. 11 Oct 2009, 22:00

EST. <http://www.lambdaphoto.co.uk/applications/100.210>

4. “ Lidar at Oltica University.” University of Nova Gorica. 11 Oct 2009, 21:00 EST.

<http://www.ung.si/en/research/atmospheric-research/otlica/lidar-otlica/>

5. “Monitoring Climate Change.” Daily KOS. 11 Oct 2009, 21:30 EST.

<http://www.dailykos.com/story/2009/5/2/727130/-Monitoring-Climate-Change.->

6. “Multichannel Scalar.” Stanford Research Systems. 11 Oct 2009, 22:35 EST.

<http://www.thinksrs.com/products/SR430.htm>

7. “Preamplifier.” Stanford Research Systems. 11 Oct 2009, 20:30 EST.

<http://www.thinksrs.com/products/SR445A.htm >

8. “Subtask: Definition of High Resolution (Lidar) Northern Gulf Coast Geomorphology.”

U.S. Geological Survey. 11 Oct 2009, 21:30 EST.

<http://ngom.usgs.gov/task4_2/>

http://isr.sri.com/instruments/data/arclite/arclite.html

http://www.meade.com/etx_premier/index.html

http://www.lambdaphoto.co.uk/applications/100.210

http://www.ung.si/en/research/atmospheric-research/otlica/lidar-otlica/

http://www.dailykos.com/story/2009/5/2/727130/-Monitoring-Climate-Change.-

http://www.thinksrs.com/products/SR430.htm

http://www.thinksrs.com/products/SR445A.htm

http://ngom.usgs.gov/task4_2/

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ENG352 Manual Final