NIST

PUBUCAJJONil

REFEflEMCE

NIST Special Publication 250-87

Calibration Service for Instruments

that Measure Laser Beam Diameter

Shao Yang

1^ ll^^^^n National Institute of Standards and Technology

I ^1 ^Hilr I United States Department of Commerce

QC

100

.U57

#250-87

2010

NAT'L INST. OF STAND & TECH

AlllDb lOMMba
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'At Boulder, CO 80305 Some elements at Boulder. CO
NIST Special Publication 250-87

Calibration Service for Instruments

that Measure Laser Beam Diameter

Shao Yang

Optoelectronics Division

National Institute of Standards and Technology

325 Broadway

Boulder, CO 80305-3337

July 2010

U.S. DEPARTMENT OF COMMERCE

Gary Locke, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Patrick D. Gallagher, Director
Certain commercial entities, equipment, or materials may be indentified in this document in order to describe an

experimental procedural or concept adequatetly. Such identification is not intended to imply recommention or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities,

materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Technol. Spec. Publ. 250-87, 20 pages (July 2010)

CODENiNTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2010

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: Bookstore.gpo.gov - Phone (202) 512-1800 - Fax: (202) 512-2250

Mail Stop SSOP, Washington, DC 20402-0001
CONTENTS

1. Introduction 1 2. Calibration Service Summary 1 3 . Theory of Measurement 2

3.1 System Basics and Measurement Principle 2

3.2 Measurement Equations 4

3.3 Data Analysis 7

4. Measurement System 7

4.1 Test Beam 7

4.2 Scanning Slit Measurement System 8

4.3 Data Acquisition 8

5. Standard operating Procedure 8

5.1 System Startup 8

5.2 Optical Alignment 8

5.3 Data Acquisition 8

5.4 Data Analysis 9

6. Uncertainty Evaluation 9

6.1 Test Beam Uncertainties 10

6.1.1 Laser Power Stability (Type A) 10

6. 1 .2 Interference Pattern (Type A) 11

6. 1 .3 Wavelength Stability (Type B) 11

6.2 Measurement System Uncertainties 11 6.2.1 Detection Noise (Type A) 11 6.2.2 Measurement Area (Type B) 11

6.2.3 Optical Axis Tih (Type B) 11

6.2.4 Positioning of the Measurement Plane (Type B) 11

6.2.5 Detector Nonhnearity (Type B) 12

6.2.6 Slit Tih (Type B) 12

6.2.7 Off-Centroid Slit Scan (Type B) 12

7 Quality Control 13

8 Summary 14

9 References 15

Calibration Service for Instruments that Measure Laser Beam

Diameter

Shao Yang

National Institute of Standards and technology

Optoelectronics Division

325 Broadway

Boulder, CO 80305

This document describes the calibration service for instruments that measure laser beam

diameter. An overview of the measurement procedures, measurement system, and

uncertainty analysis is presented. A sample measurement report is included in this

document that is similar to what is provided to the customer. The measurement report

contains an absolute correction factor and a summary of the uncertainty assessment for

the device under test.

Key words: laser beam diameter; laser beam diameter measurement; laser beam profile;

slit scan measurement;

1. Introduction

This measurement service compares laser beam diameter measurements made with the

NIST beam diameter measurement system with those made by a test meter (or device

under test, DUT) (usually a laser beam profiler). The measured beam diameter is the

second-order moment (or 4a) beam diameter defined by the International Organization

for Standardization (ISO) [1 - 3]. A test laser beam, generated with a single-mode optical

fiber and an optical fiber collimator at a wavelength of 830 nm, is used as a stable source

for laser beam diameter measurements. The nominal measured beam diameter is 2.2 mm

with a typical expanded (k=2) uncertainty of 0.3 %. The service identification (ID) for the

measurement service in this document is 42220C.

2. Calibration Service Summary

The provision of calibration services (or measurement services in this document) is an

essential element of the work of the Sources, Detectors, and Displays Group (as part of

the Electronics and Electrical Engineering Laboratory). In the conduct of this work, as in

all its efforts, the group is committed to excellence that is characteristic of a global leader

in measurements and standards. Our goal is to provide measurement services that meet

the needs of our customers and, through continuous improvement, to anticipate their

needs, exceed their expectations, and deliver outstanding value to the nation.

This calibration service is performed under conditions determined by the system setup

and the test beam generated therewith. For these measurements, the customer's meter, or device under test (DUT), is sent to NIST, where it is used to measure the test beam

diameter. The test beam diameter has been characterized by the NIST beam diameter

measurement setup. The DUT should be able to measure the second-order moment (or

1
4o) beam diameter defined by ISO, but may use different measurement methods other

than the NIST measurement. At the completion of the cahbration measurements, the

DUT and a cahbration report are sent to the customer. The cahbration report summarizes

the resuhs of the measurements and provides a hst of the associated measurement

uncertainties. (See Table 6.1.) The NIST beam diameter measurement system for this

calibration service was designed and built at NIST.

More information can be found in NIST Optoelectronics Division Quality Manual (QMII) for descriptions of NIST, the NIST Optoelectronics Division, and their quality

systems.

3. Theory of Measurement

3.1 System Basics and Measurement Principle

This service involves a direct comparison between measurements of the beam diameter of

a test laser beam made with the DUT and those of the NIST beam diameter measurement

system. The test beam is generated with a semiconductor diode laser, coupled into a single-mode fiber and expanded with a fiber collimator (as shown in Figure 3.1). The

laser beam is stable in profile, power, and wavelength. The test beam profile is determined by the fiber-collimator, constant over a wide power range controlled with a

fiber attenuator to meet the measurement power requirements for different test meters.

Because the beam diameter varies along the propagation axis, the DUT and NIST

measurements are made at the same position with reasonable tolerance. The NIST

measurement system employs the slit scan method, where a narrow slit with a finite

length scans across the beam in two orthogonal directions. The slit length and the scan

distance are selected according to conditions stipulated in the ISO standard [1-3] for

laser beam diameter measurements. The circularity of the test beam is sufficiently good

that the identification of the major and minor axes of the test beam is not necessary

within our quoted uncertainties.

2
Current source & TE controller

FC/PC

connectors

Monitor

detector

LD LD mount

0

Variable fiber

attenuator

0

Fiber splitter

B.

FC/PC

connectors

Test beam

Collimator

Detection plane

Slit & detector

NIST beam diameter

measurement system

Figure 5.1. Schematic diagram of the NIST beam diameter measurement system. The gray box is the test beam source. The fiber-coupled 830 nm laser diode (LD) is mounted on a temperature-controlled platform.

All the fiber components are single-mode at 830 nm and linked with FC/PC connectors. The single-mode

fiber (dotted line) and the collimator determine the diameter of the test beam. The slit and detector are placed on a rotation indexing mount and moved together during the scan. The DUT and the slit are in the same measurement planefrom the collimator.

3
2 J-oo J-cO O O (Ty= \ \ • -3.2

3.2 Measurement Equations

ISO defines laser beam diameter by use of the second order moments of the beam power

density distribution I{x, [1 - 3]:

r r {x-xf I{x,y)dxdy _2 J-OD J-ao 1 CT = , 3.1

I{x,y)dxdy

J-ao J-QO and

r r (y-y)- I(x,y)dxdy

.J J-OO J-CO I(x,y)dxdy

X) J-oO X and y are the centroid coordinates of the power density distribution:

xlix, y)dxdy x=^^ ^ 33

r r I{x,y)dxdy

J-CO J-CO f

r yl{x,y)dxdy y^±^ 34

I(x,y)dxdy

J-CC J-CO The generahzed beam diameter, used to represent the beam width and also called the 4(7

beam diameter, is defined as d^4J ' ^

' ^4a. 3.5

This expression defines the beam diameter uniquely for all laser beam power

distributions. For beams with a perfectly Gaussian profile, the 4a beam diameter is exactly the width between the two \/e points of the peak of the profile.

The integrations in Equations 3.1 to 3 .4, P(x)= r Iix,y)dy, 3.6

J-Xl and

Q(y)^ r I{x,y)dx, ' 3.7

J-CO are power intensities of the beam along the x and y directions and represent powers in an

infinitely long and narrow slit, P(x) and Q(y), at positions x and y, respectively. We may

rewrite Equations 3.1 to 3.4 as

r (x -x)' P{x)dx ^2^X« , 3.8

r P{x)dx

J-CO

, [{y-yfQ(y)dy ^1=^ , 3.9

[Qiy)dy
f xP{x)dx

x = — , 3.10

r P{x)dx

J-00

y = — . 3.11

f

Q^y)dy

J-oO

Equations 3.5 through 3.11 are the basis of slit scan measurement of the 4f7 laser beam

diameter. The integration limits in Equations 3.1 to 3.4 and 3.6 to 3.1 1 are from negative

to positive infinity. This is impossible to realize in practice. Finite integration limits must

be used in all practical measurements. The ISO standard stipulates that a square

integration area whose width is three times the beam diameter in both the major and

minor axes for a general elliptical beam be used for the evaluation of beam diameter. This

uniquely defines the beam diameter for almost all practical beam profiles as shown in

Figure 3.2 for an ideal Gaussian beam and the test beam in this measurement system. The

ideal Gaussian beam has a theoretical beam diameter of 2.19 mm, and the measured test beam diameter is 2.194 mm. (For an ideal Gaussian beam, its measured beam diameter

within this area differs from the true diameter by less than 5 millionth of a percent.) It is also impractical in many real measurements to limit the widths of measurement area to exactly three times the beam diameter. A reasonable deviation must be tolerated. Figure

3.3 shows the percentage deviation of the measured beam diameter with different

integration areas from that measured with the exact integration area stipulated in ISO

standard for the NIST test beam. The integration limits of the NIST slit scan measurement are from 0 mm to 7 mm. This measurement area is within the range of

integration area for the 0.1 % uncertainty limit for the test beam, as shown in Figure 3.3. For test beam profilers, the measurement areas are always finite but may be different,

depending on the measurement principle and the design of the meter.

5
3

Width of Integration Area Wg {mm)

Figure 3.2 An integration area with a width of three times the beam diameter defines measured beam

diameterfor the test beam and the bestfit Gaussian beam. The solid and dotted curves show the measured

beam diameter, Df,, of the test beam and the Gaussian beam as a function of the width of the integration

area, Wg. The dashed straight line is Wa=2^Di,. The intersecting point defines the beam diameter and the corresponding width ofthe measurement area. The theoretical beam diameter ofthe Gaussian beam is 2.19 mm. The measured test beam diameter is 2. 194 mm.

0.6 1

4 5 6 7 8 9

Width of Integration Area W^ {mm

)

Figure 3.3: Percentage deviation ofthe test beam diameter measured within an area ofwidth Wafrom that measured within the ISO defined area. The solid vertical line indicates the ISO defined measurement area. The two dashed vertical lines define the allowed range of measurement area that keeps the related Type B

uncertainty contribution of measured beam diameter below 0. 1 %. Due to a small side lope peaked at the position ofabout 8 mm, the curve does not level offas expectedfor a more well-behaved beam profile.

6
3.3 Data Analysis

In the NIST slit scan measurement system, the width of the sht is 10 |im and the length is 7 mm. For one single complete measurement, the slit scans in both the horizontal (x) and

vertical (y) directions at steps of 25 [im for a range of 8 mm. Using iterations in the data

processing software, the centroid and a 7 mm range around it in each direction are found

and used in the calculation. The integration area is a square of 7 mm x 7 mm. The data

analysis of the DUT is performed with the software provided by the customer. The final

result is displayed in the meter and recorded.

4. Measurement System

The NIST calibration service for instruments that measure laser beam diameter consists of a system to create a stable laser beam with a nominal diameter of 2.2 mm, referred to as the

test beam, and a system characterizing the test beam by use of the slit scan method. A

schematic of the beam creation-characterization system is shown in Figure 3.1

.

4.1 Test Beam

A laser diode operating at a nominal wavelength of 830 nm and driven in constant current

mode, and mounted on a temperature controlled platform serves as the source for the test beam. The laser is pigtailed with a single-mode fiber connected to a variable fiber attenuator. The fiber attenuator can be used to adjust the test beam intensity for different

measurement conditions. The far end of the attenuator is connected to a 98/2 fiber

splitter. The minor leg of the fiber splitter is coupled to a monitor detector, providing the

means to correct for power drifts of the laser. The main leg of the fiber splitter is connected to a two-meter long fiber coupled to a beam collimator. The nominal focal

length of the collimator is 1 1 mm.

The test beam exiting the collimator serves as the working beam diameter standard for

the measurements. The laser beam has a nearly Gaussian profile with a long tail. The

beam has an ellipticity ratio better than 0.995, as based on results of two-dimensional

pinhole scan, slit-scan, and camera-based measurements. The beam width should ideally

be measured at the beam waist, which is the conjugate point of the fiber end face from the

collimator lens (the back focal point) for an ideal Gaussian beam with perfect alignment

of the collimator. However, the actual test beam waist is at a distance much longer than

this theoretical position, due to the non-ideal Gaussian beam and imperfect alignment of

the collimator. A series of beam width measurements shows that at convenient working

distances between 50 mm and 100 mm from the collimator, the beam width changes by

approximately 1 \im for a path length change of 5 mm, corresponding to a full converging

angle of 0.01°, allowing for a large positioning tolerance with very low uncertainty. The

beam width varies slightly as a ftinction of the laser wavelength [4, 5]. We have observed

that the laser wavelength varies by as much as 0.11 nm or 0.011 % during a four-hour

period (typical duration of an entire beam width calibration), which leads to a change in

beam width of 0.24 [im. The beam width also varies slightly as a function of the

temperature of the fiber [4, 5]. For example, a change of 2 °C in temperature results in a

7
change of 0.002 % in the fiber mode field diameter, or a change of 0.05 [im in the test beam width.

4.2 Scanning Slit Measurement System

For the characterization of the test beam diameter, the NIST measurement system uses

the scanning slit method. The width of the slit is 10 |im (±1 |j.m) and the length is 7 mm

(±25 |im). The long axis of the slit is perpendicular to the scan axis. For one complete

measurement, the slit is scanned in both the horizontal (x) and vertical (y) directions at incremental steps of 25 \xm, for a full range of 8 mm. The resolution of the scanning

linear stages is 0.05 |im. The slit is mounted on an indexed rotation stage in order to change the orientation of the slit for the horizontal and vertical scan measurements.

4.3 Data Acquisition

The data acquisition of DUT measurements follows the instructions of the DUT. For each

data point of the NIST slit scan measurement, the slit integrated beam power intensity is acquired simultaneously with a reading of the monitor detector. The scan data are corrected for the laser power drift by applying a normalization factor using the monitor

readings and recorded in the scan data file.

5. Standard Operating Procedures

5.7 System Startup

All electronics and other support systems are powered up at least one hour prior to the

start of the measurement. All test equipment is allowed to equilibrate with room

temperature. The laser is powered at the pre-selected drive current for at least one hour

before the measurement in order to allow for the output power to stabilize.

5.2 Optical Alignment

The DUT measurement plane and the NIST scanning slit are aligned in positions at the same distance from the collimator surface within a tolerance of 5 mm. The beam pointing

direction is aligned perpendicular to the measurement surface within a tolerance of 2.5°,

which is determined from the shift of the centroid of the scanned data at two different

distances of the scan position from the collimator. For NIST measurement, the beam is centered within the system scan range with enough scan distance on each side from the

center. For DUT measurement, the beam is aligned according to the instructions of the

DUT.

5.3 Data Acquisition

Data acquisition consists of several alternations of NIST measurement runs and DUT

measurement runs. The number and order of alternations depends on measurement

8
specifics of the DUT. Each measurement run may also consist of several measurements.

Each NIST measurement includes one scan in each of the x and y directions. The scan

takes 25 |im in each step and covers a range of 8 mm. Each scan datum is acquired

simultaneously with a monitor detector reading, which is used to correct the slow drift of

the test beam power. Before the first NIST measurement, in the middle of the entire

measurement, and after the last NIST measurement, the system baseline is scanned with

the beam blocked. All the scans are performed with the collimator-scanning-detector

covered in a black box to shield off the stray light from the environment.

Other measurement conditions recorded include test beam wavelength, environment

temperature, distance of measurement plane from the collimator, DUT model, serial

number, and NIST ID number.

5.4 Data Analysis

For the NIST measurement, the average of the system baseline is obtained first from the

baseline measurement data. For each scan in each direction (x or y), a software algorithm

determines the centroid, a 7 mm integration area around the centroid, and the secondorder moment from the corrected scan data with baseline subtraction. The beam diameter

is obtained for every two consecutive scan results in the two, x and y, directions for each

NIST measurement. The average of all the measured beam diameters is the NIST

measurement result.

The DUT measurement result is obtained according to the instrument instructions. The

average of all the DUT measurements is the DUT measurement result.

The ratio of DUT measurement result to the NIST measurement result is the correction

factor supplied to the customer.

6. Uncertainty Evaluation

The uncertainty estimates for the NIST laser beam diameter measurements are assessed

following guidelines given in NIST Technical Note 1297 [6]. To establish the uncertainty

limits, the error sources are separated into Type B errors, whose magnitudes are determined by subjective judgment or other non-statistical methods, and Type A errors, whose magnitudes are obtained statistically from a series of measurements.

AH Type B error components are assumed independent and to have rectangular or uniform distributions (that is, each has an equal probability of being within the region,

±5i, and zero probability of being outside that region). If the distribution is rectangular,

the standard deviation, as, for each Type B error component is equal to 5i/3

^ and the total standard uncertainty is (Has ) where the summation is performed over all Type B error components.

Type A errors are assumed to be independent and normally distributed and, consequently,

the standard deviation, Sr, for each component is

9
where the x values represent the individual measurements and N is the number ofx values

used for a particular Type A error component. The standard deviation of the mean is Sr/N'^, and the total standard uncertainty of the mean is [I(Sr^/N)] where the

summation is carried out for all the Type A error components.

The expanded uncertainty is determined by combining the Type A and Type B standard

uncertainties in quadrature (the combined uncertainty) and multiplying this result by an

expansion factor of 2. The expanded uncertainty, U, is then

U = 2^E^Ts^ + S| . 6.2

The number of decimal places used in reporting the mean value of the calibration factor

listed in the calibration report is determined by expressing the expanded uncertainty to two significant digits.

Sources of uncertainties are identified and their contributions to the beam width

measurement result are determined by test, or theoretical calculation, or computer

simulation, or a combination of these methods. Generalized beam diameter, d, defined in

Equation 3.5 is a function of the second order moment diameters and Aoy measured

independently in the slit scan method. The uncertainty contributions w((7x) and u{Gy) of Ox

and Oy to the uncertainty u{d) of d follow the law of propagation of uncertainty. Some

uncertainty sources, especially the Type A uncertainties, affect Ox and cr,, independently,

and the uncertainties u{ax) and u{ay) are uncorrelated. Because the test beam has very

good circularity, and these uncertainty sources affect the measurements equally in the

two measurement directions, we have A<j^ « 4<Tj = 4cr = J and

u{A(j ^) u{A(7 = u{A(t) , and therefore u{d) =

^^"^^^ Some Type B uncertainty

V2

sources directly impact the measured beam diameter d, so their influences on w(4o>) and

w(4(7y) are completely correlated, resulting in u(d) = u{4(7) . Some other Type B sources

also have a direct impact on beam width d, but their effect on u{4ctx) and u(4ay) are neither completely correlated nor completely uncorrelated. Because it is difficult to estimate the degree of correlation, we use the worst-case estimate in their evaluations,

that is, assuming they are completely correlated. In the following description of the major

uncertainty sources, the values of uncertainties are all their final contributions to the

uncertainty of u(d).

6.1 Test Beam Uncertainties

6.1.1 Laser Power Stability (Type A)

Laser power output exhibited fast fluctuations and slow drift. The combined standard

deviation of laser power in four hours was 1 %. The standard deviation of the measured

laser power was reduced to 0.1 % by applying a power drift correction using input from

10
the monitor detector. Simulation results indicated that this resulted in an uncertainty of

0.1 |im in the beam width measurement.

6.1.2 Interference Pattern (Type A)

Because of the narrow spectral width of the laser beam, interference due to reflections

between the fiber end face and the collimator lens resulted in fluctuations in the scanned

beam profile. From the raw data, it was estimated that this effect caused a RMS

fluctuation of about 0.5 % in the measured signal, resulting in a standard uncertainty of

0.4 |j.m in the final result, according to simulations.

6.1.3 Wavelength Stability (Type B)

The single-mode fiber beam width is dependent on the laser beam wavelength.

Measurements showed that the laser beam wavelength exhibited fast fluctuation and slow

drift. Fast fluctuation will cause noise-like effects on the measurement. However, the

magnitude of wavelength fast fluctuation is less than 0.003 %, which caused a similar

change in beam width magnitude that can be ignored. The slow drift of the wavelength

will cause a difference between results of NIST and DUT measurements and a difference

between NIST measurements before and after the test meter measurements due to the

time difference of the measurements. Test results showed that wavelength drift could be

as high as 0.01 % when the laser is operated in constant current mode, which converts to an estimated standard uncertainty in beam width measurement of 0.07 )im, or 0.003 %,

which can be ignored.

6.2 Measurement System Uncertainties

6.2.1 Detection Noise (Type A)

Detection noise is the combined result of detector noise (dark current noise, shot noise,

etc.) and the readout electronics noise. The detection signal to noise ratio was 5xlO"\

resulting in a standard uncertainty of 0.2 |j,m in the beam width.

6.2.2 Measurement Area (Type B)

The uncertainty limit of the measured beam width due to the difference between the

actual measurement area and the ISO defined area is 0.1 %, as described earlier. The

standard uncertainty is then 0.06 %.

6.2.3 Optical Axis Tilt (Type B)

Results of multiple realignments and measurements showed that the tilt angle between

the optical axis of the beam and the beam profile plane can be controlled within 2.5°. The

standard uncertainty of the measured beam width due to this tilt is 0.05 %.

6.2.4 Positioning of the Measurement Plane (Type B)

For a maximum possible difference of 5 mm in the relative longitudinal positions of slit- scan and DUT measurements, a standard uncertainty of 0.6 p-m, or 0.03 %, is incurred in

the measured beam width.

11
6.2.5 Detector Nonlinearity (Type B)

Nonlinearity measurement of the NIST detector showed a positive response nonHnearity

of up to 2 % occurred at the lower end of the measurement range. Simulations indicated

that this results in a standard uncertainty of 0.4 |im or 0.02 % in the beam width.

6.2.6 Slit Tilt (Type B)

The tilt of the slit can be kept within 1 ° relative to the scan axis. Simulation indicated that

this results in a standard uncertainty of 0.4 \xm or 0.02 %.

6.2.7 Off-Centroid Slit Scan (Type B)

The scanning slit should ideally be centered at the centroid in the direction orthogonal to the scanning direction. Because the slit cannot be perfectly positioned at the center of the

rotation stage, the slit may not be centered at the centroid during the scans. Test results

showed that a maximum position error of 200 |j,m off-center may occur, which may result

in a standard uncertainty of 0.6 [im or 0.03 % uncertainty in the measured beam diameter.

Type A uncertainties discussed in 6.1.1, 6.1.2, and 6.2.1 cannot be separately estimated in

each and every particular measurement. However, their combined effect contributes to the measurement standard deviation of the NIST and DUT measurements, which are

listed in the test report. Other sources of uncertainty we have considered include the finite

slit width, scan resolution, effect of ellipticity, and motion accuracy. Their contributions

to the overall uncertainty of the beam width measurement are all less than 0.1 )j,m and

deemed negligible. Major uncertainty contributions are listed in Table 6.1.

Source Uncertainty

Type A Standard

Deviation (%)

N

NIST measurement 0.02 6

DUT measurement 0.06 30

Type B Standard Uncertainty (%)

Measurement area 0.06

Optical axis (z direction) tilt 0.05

Relative positioning of detection plane 0.03

Detector linearity 0.02

Slit tilt 0.02

Slit off-centroid 0.03

Combined standard uncertainty Uc 0.09

Expanded uncertainty (^2) U=k Uc 0.19

Table 6. 1 Example summary ofmeasurement uncertainties associated with NIST and DUTmeasurements.

12
7. Quality Control

The measurement services of the Optoelectronic Division make use of quality assurance

practices to ensure the validity of measurement results and their uncertainties. Such

practices include:

• Repeated measurements/calibrations compared over many time intervals

• Comparison of previous results obtained using multiple measurement methods, if available.

• Routine, periodic measurements with different methods.

For this service, we periodically measure the test beam diameter using three different

methods, slit scan, two-dimensional pin-hole scan, and camera-based beam profiling.

Figure 7.2 shows the history of the test beam measurements during the period from June

2008 to April 2009.

2.200

2.198

2.196

2.194

2.192

2.190

X

Slit Camera

X Pinhole

Slit average — — Camera average

- - - Pinhole average

X

3> c$> cS> cv% c^* c^* fs^ cv^ c^i <v<?> CN<?> (N*?* CN«?> ce>

Date

Figure 7.1. Test beam measurements using three different methods, slit scan, two-dimensional pinhole

scan, and camera-based beam profilingfrom June 2008 to April 2009. The average and standard deviation

of these measurements are 2.1949 ± 0.0015 mm for slit scan, 2.1951 ± 0.0021 mm for camera beam

profiling, and 2.1948 ± 0.0020 mmfor pinhole scan.

In the Optoelectronics Division, all calibration, MAP, and remote measurement (RM)

services maintain check standards and control charts for periodic test of the measurement

13
service. When available, historic data from previous measurements of the test beam shall

be placed into the test folder by the Measurement Services Coordinator after the

preparation of the calibration report. The Calibration Leader and the Group Leader shall

review this data before signing the calibration reports. If a significant variance from

previous results is observed, the Group Leader may require another measurement of the

calibration item as a test of measurement system conformance.

8. Summary

NIST provides a calibration service for instruments that measure laser beam diameters. In

this document, we have summarized the basic measurement equations, the measurement

procedure, and described the quantities that contribute to the relative standard

uncertainty.

14
References

[1] ISO 1 1 146-1 :2005, ''Lasers and laser-related equipment - Test methods for laser

beam widths, divergence angles and beam propagation ratios - Part 1 : Stigmatic

and simple astigmatic beams," 2005.

[2] ISO 1 1 146-2:2005, "Lasers and laser-related equipment - Test methods for laser

beam widths, divergence angles and beam propagation ratios - Part 2: General

astigmatic beams," 2005.

[3] ISO/TR 11146-3:2004, "Lasers and laser-related equipment - Test methods for

laser beam widths, divergence angles and beam propagation ratios - Part 3: Intrinsic and geometrical laser beam classification, propagation and details of test methods." 2004.

[4] "Single-Mode Fiber Optics: Principles and Applications," Luc B. Jeunhomme, 2"^*

Edition, Marcel Dekker, Inc., New York, 1990.

[5] "Single-Mode Fibers: Fundamentals," Emst-Georg Neumann, Springer-Verlag

Berlin Heidelberg, 1988.

[6] "Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement

Resuhs," NIST Technical Note 1297, Barry N. Taylor and Chris E. Kuyatt, 1993.

15

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