UNITED STATES BUREAU OF MINES

UNITED STATES DEPARTMENT OF THE INTERIOR

RI 9558 REPORT OF INVESTIGATIONS/1995

Optically Powered Remote Gas Monitor

By T. H. Dubaniewicz, Jr., and J. E. Chilton
U.S. Department of the Interior

Mission Statement

As the Nation’s principal conservation agency, the Department of the

Interior has responsibility for most of our nationally-owned public

lands and natural resources. This includes fostering sound use of our

land and water resources; protecting our fish, wildlife, and biological

diversity; preserving the environmental and cultural values of our

national parks and historical places; and providing for the enjoyment

of life through outdoor recreation. The Department assesses our

energy and mineral resources and works to ensure that their

development is in the best interests of all our people by encouraging

stewardship and citizen participation in their care. The Department

also has a major responsibility for American Indian reservation

communities and for people who live in island territories under U.S.

administration.
Report of Investigations 9558

Optically Powered Remote Gas Monitor

By T. H. Dubaniewicz, Jr., and J. E. Chilton 

UNITED STATES DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

BUREAU OF MINES

Rhea Lydia Graham, Director
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ...................................................................................... 1

Introduction .................................................................................... 2

System components ............................................................................... 3

Laser ...................................................................................... 3

Fiber-optic link ............................................................................... 3

Power converters .............................................................................. 3

Sensors ..................................................................................... 5

Control and alarm ............................................................................. 5

Results ...................................................................................... 6

Calibration test ................................................................................ 6

Field trial .................................................................................... 6

Telemetry design analysis .......................................................................... 7

Optical power budget: Power and data-down link ...................................................... 7

Optical power budget: Data-back link ............................................................... 7

Laser safety issues ............................................................................... 8

Human exposure .............................................................................. 8

Explosion hazard .............................................................................. 8

Summary ...................................................................................... 9

References ..................................................................................... 9

ILLUSTRATIONS

1. Communication and power to remote sensors provided by FO ............................................ 3

2. FOREWARNS prototype ....................................................................... 4

3. General schematic of FOREWARNS .............................................................. 4

4. Optical power supply .......................................................................... 5

5. Underground test of CO remote sensing unit ......................................................... 6

6. Nominal ocular and skin hazard distances for laser characteristics .......................................... 8

TABLES

1. Monitor response to CO calibration gas ............................................................. 7

2. Optical losses in power and data-down link .......................................................... 7

3. Optical losses in data-back link .................................................................. 7

4. Laser operational characteristics .................................................................. 8
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter mm millimeter

cm /min cubic centimeter per minute mV millivolt 3

dB decibel mV/ppm millivolt per part per million

dB/km decibel per kilometer mW milliwatt

dBm decibel referenced to 1 milliwatt nm nanometer

h hour ppm part per million

kHz kilohertz s second

km kilometer V volt

m meter W watt

mA milliampere μA microampere

MHz megahertz μA/ppm microampere per part per million

MHz@km megahertz kilometer μm micrometer

min minute

Reference to specific products does not implyu endorsement by the U.S. Bureau of Mines.
Electrical engineer. 1

Research chemist. 2

 Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh PA.

Optically Powered Remote Gas Monitor

By T. H. Dubaniewicz, Jr., and J. E. Chilton 1 2

ABSTRACT

Many mines rely on toxic gas sensors to help maintain a safe and healthy work environment. This report

describes a prototype monitoring system developed by the U.S. Bureau of Mines (USBM) that uses light to power

and communicate with several remote toxic gas sensors. The design is based on state-of-the-art optical-to-electrical

power converters, solid-state diode lasers, and fiber optics. This design overcomes several problems associated with

conventional wire-based systems by providing complete electrical isolation between the remote sensors and the

central monitor. The prototype performed well during a 2-week field trial in the USBM Pittsburgh Research Center

Safety Research Coal Mine.
2

INTRODUCTION

Canaries alerted the miner to dangerous atmospheric communicate. Distance, coupled with high-power electrical

conditions in the early days of mining. Today, many underground mines rely on computerized atmospheric monitoring

systems (AMS) to help maintain a safe and healthy working

environment. Atmospheric monitoring systems collect data from

various types of gas sensors located throughout the mine, and

alert mine personnel when a particular gas exceeds allowable

levels. These remotely located sensors must have some way of

communicating with the central computer, often over very long

distances. Reliable communication over these long distances is

most important. When properly installed and maintained, AMS

operate reliably. However, current trends in the mining industry

will make maintaining reliable communications among the

remote sensors and central monitor even more difficult.

Recognizing this, the U.S. Bureau of Mines (USBM) initiated a

program to investigate the application of fiber optics (FO) to the

special problems encountered in the mine environment. Most

recently, a fiber-optic remote environmental warning system

(FOREWARNS) has been developed that uses FO for

communications and to deliver power to several remote toxic gas

sensors located in the mine.

The prototype system was designed to accommodate sensors

for three toxic gases of concern in underground mining: nitrogen

dioxide (NO ), sulfur dioxide (SO ), and carbon monoxide (CO). 2 2

Nitrogen dioxide emissions are present in exhaust from dieselpowered machinery; dry atmospheres, such as found in salt

mines, exacerbate this problem. Nitrogen dioxide is also a byproduct of the detonation of explosives used in underground

mines. Sulfur dioxide may be found in the exhaust of diesel

machinery using high-sulfur-content fuel. Sources of CO include

diesel exhaust, explosive fumes, fires, and air oxidation of

pyrophoric coals. Federal mining regulations set exposure limits

for these and other noxious gases in underground coal mines (1)

3

and underground metal and nonmetal mines (2).

Carbon monoxide sensors are also often used for fire detection

along conveyor belts because of their susceptibility to fires (coal

spilled onto seized rollers, defective motors, etc.). These sensors

work well for this purpose because combustible materials in

mines such as coal, wood, brattice cloth, conveyor belting, and

fuels produce CO gas in the initial stages of fires (3). Depending

on ventilation rates and other factors, CO sensors may be spaced

several hundred meters apart along the belt, often extending many

kilometers underground. Highly efficient longwall mining

methods, along with depleting reserves, are extending these

haulage routes and, therefore, the distance over which the CO

telemetry systems must

Italic numbers in parentheses refer to items in the list of references at the 3

end of this report.

systems of some modern longwall machines, allow telemetry

links to be more susceptible to electromagnetic interference

(EMI). 

Another concern involves the use of conveyer belt air

to ventilate working sections of the mine. According to current

mining regulations, working sections of a mine may not be

ventilated by belt air (4). Therefore, an additional entry must be

developed to provide fresh air to the working sections. However,

a variance to this regulation may be granted allowing the use of

belt air for ventilation, provided adequate safety precautions are

taken (5). One of these precautions is using early-warning fire

detection systems like remote CO monitors. If the fire detection

system should fail, a fire could quickly send toxic fumes to the

working area and block off the remaining escape routes. For this

reason, every effort must be made to ensure that early-warning

fire detection systems are reliable.

Fiber optics provides reliable long-distance telemetry. The

EMI problem associated with wire-based telemetry systems is

virtually eliminated in fiber-optic telemetry systems. Fiber optics

also eliminates ground loops. Ground potential may vary

throughout a mine, which could adversely affect electrical signals.

The fragility of optical fiber is a reliability issue with many in the

mining industry. However, properly cabled fiber has proven to

be surprisingly rugged, as evidenced by the thousands of

kilometers of fiber placed on the ocean floor for intercontinental

communication systems.

Despite these advantages, FO has not yet made a big impact

in industrial applications. One reason is the popularity of the 4-

to 20-mA current-loop transmitter. The current-loop transmitters

provide a cost-effective solution to grounding problems

associated with electrical telemetry systems. This situation will

change with the emergence of a fieldbus standard that is intended

to facilitate the use of smart sensors in industrial applications (6).

The fieldbus standard will be based on an all-digital

communications protocol to convey information, as opposed to

the analog electrical current signal used by the current-loop transmitters. An all-digital communications standard should finally

allow the widespread use of fiber-optic cable in applications that

are now dominated by current-loop transmitters.

One of the remaining obstacles to widespread use of FO in the

mining industry is the need for electrical power at remote sensing

locations. Electrical telemetry systems often use the same

conductor for power and communication, reducing the number

of connections to remote sensors. For this reason, rather than

replacing another technology, FO becomes an expensive add-on.

However, recent technological
3

Figure 1

developments allow FO to provide communication and power to Development of FOREWARNS was intended to prove this

remote locations in a networked environment as well. concept and to familiarize designers with this technology.

SYSTEM COMPONENTS

A concept drawing of FOREWARNS is shown in figure 1. power. The electrical-to-optical conversion efficiency at this

A central monitoring station (CMS) communicates with and power level is 22%. The output of the laser is intensity

provides power to three remote sensing units (RSU's) via a large- modulated for simultaneous power and data transmission. The

core fiber-optic cable. The CMS consists of a control box with laser diode array couples up to 5 W into a dense, random fibera liquid crystal display, a laser housing, and a laser power supply. optic bundle pigtail terminating in a SMA-type connector. The

The optical signal is distributed to each RSU by a fiber-optic bundle diameter at the connector is 400 μm, and beam divergence

splitter. Each RSU contains a gas sensor, an optical-to-electrical is about 50E full width at half-maximum power.

power converter, and telemetry circuitry. The RSU responds to

the CMS when polled via standard communications grade, FIBER-OPTIC LINK

multimode fiber-optic cable and splitters (figure 2).

LASER three RSU's, and provides a return path for communication with

A key component of this system is a high-powered, solid-state information back to the CMS. Fiber-optic connections are shown

diode laser. Diode lasers that can launch several watts of power in figure 3. The fiber bundle coupled to the laser diode array

into fiber-optic cable are now available. These solid-state lasers connects directly to a single 400-μm-core-diameter, step index,

are reliable and require low maintenance compared with other hard-clad silica, fiber-optic cable. An SMA-type connector

types of lasers. The operating wavelength of the laser should be is used at this interface; ST-type connectors are used throughout

in the 800-to 850-nm range to efficiently match the power the rest of the link. The numerical aperture (NA) of the single

converters at the remote end. The laser selected for this 400-μm-core-diameter fiber is rather large (0.37) to increase

application was a SDL Inc. (formerly Spectra Diode Labs) model coupling efficiency. The laser power is distributed among the

SDL-3450-P5 with a center wavelength of 814 nm and a spectral three RSU's via two 400-μm-core fiber-optic splitters, with

width of 3 nm. The laser consists of an array of diode lasers splitting ratios of 3:1 and 1:1. The return path consists of a 62.5-

capable of producing more than 5 W of optical μm-core fiber-optic cable and two splitters with 1:1 splitting

Communication and power to remote sensors provided by

FO.

A fiber-optic link distributes the modulated laser power to

the central monitor. A light emitting diode in each RSU transmits

ratios. Fiber-optic connectors are used at all interfaces between

fiber-optic components for convenience. A substantial savings in

the optical power budget can be realized by replacing connectors

at the cable-splitter interfaces with fusion splices. One-hundredmeter lengths of cable separate each of the RSU's and the CMS.

POWER CONVERTERS

An optical-to-electrical power converter in each RSU supplies

enough electrical power to operate the sensor and telemetry

circuitry (figure 4). These power converters represent another

recent technological development. Originally developed by

Varian Associates, Inc., and now licensed to Photonic Power

Systems, the optical-to-electrical power converters are made of a

monolithic gallium arsenide (GaAs) semiconductor device. They

convert light into electrical current at voltages appropriate

forpowering integrated circuits and
4

Figure 2

Figure 3

FOREWARNS prototype.

General schematic of FOREWARNS. (Courtesy of Photonic Power Systems)
5

Figure 4

and pressure).

Optical power supply consisting of fiber-coupled solid-state

laser and power converter.

sensors. The illuminated area of the power converter is circular

with a diameter of about 3 mm, making it ideal for "power-downa-fiber" applications. The optical-to-electrical conversion

efficiency is approximately 50%, exceeding the efficiency of

silicon solar cells. The standard GaAs converter's maximum electrical output power is about 2 W. It can also receive data signals

at rates up to 250 kHz for data transmission. Each RSU contains

one 6-V power converter.

SENSORS

All the sensors used in FOREWARNS contain electrochemical sensing elements. Electrochemical-type sensors work

well for this application because of their extremely low power

consumption. Besides the CO, NO , and SO 2 2 sensors used in the

current system, there are numerous other gases that can be

monitored with electrochemical sensors suitable for use with

FOREWARNS.

The USBM tested a total of three Giner, Inc., model

WMCO100 CO monitors (7). Each CO sensor has a cell

containing a solid membrane electrolyte (Nafion TM DuPont).

Three electrode structures (working, counter, and reference

electrodes) are pressed onto the solid electrolyte. Carbon

monoxide concentrations are determined by measuring the

working-to-counter electrode current. Oxidation of CO at the

working electrode and reduction of oxygen at the counter

electrode produces thiscurrent. The cell generates about

0.4 μA/ppm of CO detected, and requires only 64 μA of bias

current at 6 V. The sensor provides a 10-mV/ppm output

voltage. In addition to low power consumption, the sensor also

exhibits unusually long life (similar units have operated for 9

years in the laboratory). The CO cells require periodic water

replenishment. The 0- to 1-V output of the sensor corresponds

to a range of 0 to 100 ppm of CO.

The USBM also tested a City Technology Ltd. NO 2 sensor

(model 3MNDH) and SO sensor (model 3MSH). These sensors 2

also use a three-electrode structure similar to the CO sensors;

however, the electrodes are contained within an aqueous solution

cell as opposed to being pressed onto a solid membrane

electrolyte. The NO and SO sensors also produce about 2 2

0.4 μA/ppm internally, and provide a 10-mV/ppm output

voltage. Power requirements are 250 μA at 9 V. A voltage

multiplier circuit increases the voltage available from the 6-V

power converter to ensure proper sensor operation. The NO and 2

SO sensors do not require water replenishment and are less costly 2

than the CO sensors; however, they do need to be replaced more

often (expected operating life is 2 years at standard temperature

CONTROL AND ALARM

Photonic Power System's Isolated Power and Data Module

System controls communication between the RSU's and the

CMS. The CMS sequentially polls and displays the identity of

each RSU, sensor output, and alarm threshold. The parallel

configuration of FOREWARNS is fault tolerant to a malfunction

or disconnection of any of the RSU's. Each RSU can be set to

alarm at any interval from 1 to 99 ppm, in 1-ppm intervals, by

switches located on the remote telemetry board. An 8-bit analogto-digital converter (ADC) converts the analog sensor signal to a

digital signal. When the sensor reading is greater than the set

point for that sensor, the remote station generates an alarm signal.

Since the alarm signal is generated at the remote location, it could

be used to activate a nearby battery-powered personnel alarm.

Each RSU has a remote audio alarm powered by a 9-V transistor

battery to demonstrate this feature. Long battery life can be expected as the battery is used intermittently, only during an alarm.

The battery could also conceivably act as a backup power source

for the remote unit and sound the alarm in the event the fiberoptic link was severed. A local alarm at the CMS sounds only

when a sensor in alarm status is polled. At the sensor end, the

remote alarm sounds continuously when the threshold levels are

exceeded.
6

Figure 5

RESULTS

FOREWARNS was calibrated in the laboratory prior to Table 1 summarizes the results recorded after 90 s of exposure.

undergoing a field test in the USBM Pittsburgh Research Center

Safety Research Coal Mine (PRC-SRCM). The PRC-SRCM,

operational since the early 1970's, is a room-and-pillar operation

approximately the size of a working section in a commercial coal

mine. It is used for testing new equipment and technology before

transferring them to industry.

CALIBRATION TEST

One of the CO sensors was installed in a RSU and tested to

determine how accurately FOREWARNS tracked the sensor

output. The accuracy of FOREWARNS largely depends on three

factors: accuracy of the sensor itself, resolution of the ADC, and

resolution of the display. The sensor was exposed to several CO

calibration gases at a rate of 95 cm /min. In these tests neither the 3

laboratory temperature nor atmospheric pressure were controlled.

Column 1 corresponds to the value indicated on the calibration

gas bottle, column 2 shows the voltage output of the sensor, and

column 3 is the gas concentration reported by FOREWARNS.

Combining the resolution of the 8-bit ADC over the 0- to 1-V

output span of the sensors and the display resolution (1 ppm), the

displayed measurement should be within ±0.5 ppm of the reading

indicated by the sensor (column 2). The results in column 3 fall

within this range.

FIELD TRIAL

FOREWARNS was tested for 2 weeks in the PRC-SRCM.

Carbon monoxide, nitrogen dioxide, and sulfur dioxide RSU's

were placed inside the mine, each separated by 100 m of cable

(figure 5). All RSU's and cable were placed in intake air. The

CMS was located in the mine office just outside

Underground test of CO remote sensing unit.
7

Table 1.—Monitor response to CO calibration gas

CO concentration, Sensor output, Monitor response,

ppm mV ppm 

................. 01 8 1

24 . . . . . . . . . . . . . . . . . 249 25

60 . . . . . . . . . . . . . . . . . 620 62

100 . . . . . . . . . . . . . . . . 988 99

air. Pure 1

Table 2.—Optical losses in power and data-down link

Component Loss, dB

400-μm core fiber, per kilometer . . . . . . . . . . . . . . . . . . . . . . 6.0

ST-ST connections (typical) . . . . . . . . . . . . . . . . . . . . . . . . . 0.5

Fiber bundle to single fiber connection . . . . . . . . . . . . . . . . . 2.3

1 by 2 coupler, 3:1 split ratio:

 A2-R1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7

 A2-B1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5

1 by 2 coupler, 1:1 split ratio:

 B2-R2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8

 B2-C1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4

Table 3.—Optical losses in data-back link

Component Loss, dB

62.5-μm core fiber, per kilometer . . . . . . . . . . . . . . . . . . . . 3.0

ST-ST connections (typical) . . . . . . . . . . . . . . . . . . . . . . . . 0.6

1 by 2 coupler, 1:1 split ratio:

 R1-X2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8

 Y1-X2 ........................................ 3.55

1 by 2 coupler, 1:1 split ratio:

 R2-Y2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7

 Z1-Y2 ........................................ 3.54

mine the portal. A 100-m length of cable was also used to

separate the CMS and the NO RSU. The sensors were 2 challenged with a calibration gas twice a day during the first week,

and once a day the second week. All sensors responded when

exposed to the calibration gas. The RSU's were set to alarm when

the sensor reading reached the calibration gas concentration; the

longest alarm response time observed was just under 3 min.

TELEMETRY DESIGN ANALYSIS

The two parameters that tend to be the dominant design at cutoff is about half of this value, as the laser output fell below

criteria for fiber-optic telemetry systems are the transmission rate the lasing threshold at minimum signal modulation. The losses

(how fast) and link length (how far) (8). The transmission rate of associated with the fiber-optic components in the power and datathis system was chosen to be 5 kHz for two reasons: First, at 5 down link are listed in table 2 (refer to figure 3).

kHz the sensors can be easily interrogated about once every 10 s,

which is sufficient for many applications; second, modulating

several amperes of laser-diode current becomes increasingly difficult at higher transmission rates. This transmission rate is

sufficiently low that bandwidth limitations on the maximum

allowable length of fiber-optic cable are negligible (the

bandwidth-length product of the 400-μm fiber-optic cable is 13

MHz km), leaving only the optical power budget to be consid- .

ered.

The optical power budget of a fiber-optic communication

system is usually defined in terms of optical power levels needed

to maintain an acceptable bit error rate (digital systems) or signalto-noise ratio (analog systems). For this system, however, the

optical power budget must also be defined in terms of the amount

of optical power needed to maintain acceptable voltage and

current at the remote sensor. This is the only consideration on the

power and data-down link because the sensor will cease to

function properly before light levels approach the signal detection

limit of the GaAs power converter. Receiver sensitivity on the

data-back link must still be considered.

OPTICAL POWER BUDGET: POWER

AND DATA-DOWN LINK

Researchers determined the minimum laser power needed to

operate the remote sensors experimentally. First, each RSU was

exposed to a test gas, then the laser power was decreased gradually until the unit failed to respond. The peak power measured at

each RSU input just prior to sensor failure was about 70 mW for

each. The average optical power

OPTICAL POWER BUDGET: DATA-BACK LINK

The overall power margin for a single transmitter-receiver pair

is about 10 dB. The losses associated with the fiber-optic

components in the data-back link are listed in table 3 (refer to

figure 3).
8

Figure 6

LASER SAFETY ISSUES

HUMAN EXPOSURE

All lasers should be operated in compliance with appropriate

safety standards. The American National Standards Institute Another safety concern involves fiber-coupled optical ignition

(ANSI) Standard Z136.1 provides guidance for the safe use of of combustible atmospheres (10-20). According to

lasers and laser systems in terms of human exposure (9). The Federal mining regulations, components of AMS installed where

standard defines control measures for each of four laser classifications. The laser used in FOREWARNS is an ANSI class-4 laser

emitting an invisible infrared beam of high power contained

within a fiber-optic cable. Under normal operating conditions,

the laser beam is enclosed within the fiber-optic cable and

 terminated in a RSU so as not to pose a skin or eye hazard.

Another control measure not defined in the Z136.1 standard can

also help reduce the risk of injury due to exposure: The system

can be designed to automatically shut off if communication is

interrupted due to cable disconnection or breakage.

One characteristic of the system is the large divergence of the

beam exiting the cable at the cleaved and polished connector

interface. The angle subtended by the diverging beam (1) can be

determined from the NA of the step index optical fiber by the

equation 1'2sin (NA). -1

The NA of the 400-μm cable is 0.37; therefore, 1 is about

43E. With this large divergence, the intensity of the beam

weakens dramatically with increasing distance from the emitting

surface as compared with other types of lasers. The implications

in terms of human exposure are best illustrated by a laser hazard

evaluation conducted by Rockwell Laser Industries. Rockwell's

LAZAN hazard analysis software calculated the Maximum

Permissible Exposure (MPE) limits for various exposure conditions based on the laser operational characteristics listed in table 4.

The MPE is defined as the radiant exposure that personnel may

receive without adverse biological effects. The MPE was then

used to determine the Nominal Ocular Hazard Distance/Nominal

Hazard Zone (NOHD/NHZ). The NOHD/NHZ is defined as the

distance from a laser at which the radiant exposure is equal to the

MPE. Figure 6 shows the NOHD/NHZ for various exposure

conditions as determined by LAZAN.

EXPLOSION HAZARD

permissible equipment is required shall be intrinsically safe (21).

Currently, there are no standards or guidelines on the safe use of

fiber-optic systems in hazardous (classified) locations in the

United States. Without these standards, approval agencies are not

likely to approve laser-coupled fiber-optic instrumentation as

permissible equipment. The International Society for Measurement and Control (formerly the Instrument Society of America)

has formed the SP12.21 Fiber Optics subcommittee to establish

guidelines and to be a source of information on this subject. The

subcommittee is currently working with international organizations with similar interests to establish an international standard

for the safe operation of fiber-optic systems in hazardous locations.

Nominal ocular and skin hazard distances for laser

characteristics (listed in table 4).
9

SUMMARY

A toxic gas monitoring system that powers and communicates current places a practical limit on the maximum transmission rate

with three separate remote sensors over fiber-optic cable has been of this system. The 5-kHz rate chosen is sufficient for many

demonstrated. The system performed well during a 2-week trial applications. Remote sensor stations required about 35-mW

in the USBM PRC-SRCM. The primary advantage for this average optical power. Control measures defined by existing

particular application is enhanced reliability afforded by fiber- laser safety standards can reduce the risk of injury resulting from

optic telemetry in locations where electrical power may not be physical exposure; however, no such standards exist in the United

readily available. The design is based on state-of-the-art optical- States regarding explosion hazards. Approval agencies are not

to-electrical power converters, solid-state diode lasers, and FO. likely to approve laser-coupled fiber-optic instrumentation as

The digital approach taken is in line with emerging industry permissible equipment in hazardous (classified) locations until

telemetry standards. such standards are established.

Laboratory and field tests led to several observations.

Difficulties in modulating several amperes of laser-diode

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