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NBS TECHNICAL NOTE 1248

U.S. DEPARTMENT OF COMMERGE/National Bureau of Standards

Traceability of Laser Interferometric

Length Measurements

Howard P. Layer and W. Tyler Estler

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rheNadonal Bureau of Standards^ was established by an act of Congress on March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their

effective application for public benefit To this end, the Bureau conducts research to assure international competitiveness and leadership of U.S. industry, science and technology. NBS work involves development and transfer of

measurements, standards and related science and technology, in support of continually improving U.S. productivity,

product quality and reliability, innovation and underlying science and engineering. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials Science and Engineering.

The National Measurement Laboratory

Provides the national system of physical and chemical measurement;

coordinates the system with measurement systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific, community, industry, and commerce; provides advisory and research

services to other Government agencies; conducts physical and chemical

research; develops, produces, and distributes Standard Reference

Materials; provides calibration services; and manages the National

Standard Reference Data System. The Laboratory consists of the following centers: The National Engineering Laboratory

Basic Standards^

Radiation Research

Chemical Physics

Analytical Chemistry

Provides technology and technical services to the public and private

sectors to address national needs and to solve national problems;

conducts research in engineering and applied science in support of these

efforts; builds and maintains competence in the necessary disciplines

required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measure- ment traceability services; develops test methods and proposes engineering standards and code changes; develops and proposes new

engineering practices; and develops and improves mechanisms to

transfer results of its research to the ultimate user. The Laboratory

consists of the following centers: The Institute for Computer Sciences and Technology

Applied Mathematics

Electronics and Electrical Engineering^

Manufacturing Engineering

Building Technology

Fire Research

Chemical Engineering^

Conducts research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of

computer technology to improve effectiveness and economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission

by managing the Federal Information Processing Standards Program,

developing Federal ADP standards guidelines, and managing Federal

participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal

agencies; and provides the technical foundation for computer-related

policies of the Federal Government The Institute consists of the following divisions: The Institute for Materials Science and Engineering

Information Systems

Engineering

Systems and Software

Technology

Computer Security

Systems and Network

Architecture

Advanced Systems

Conducts research and provides measurements, data, standards, refer- ence materials, quantitative understanding and other technical information fundamental to the processing, structure, properties and perfor- mance of materials; addresses the scientific basis for new advanced

materials technologies; plans research aroimd cross-cutting scientific themes such as nondestructive evaluation and phase diagram development; oversees Bureau-wide technical programs in nuclear reactor

radiation research and nondestructive evaluation; and broadly disseminates generic technical information resulting from its programs. The

Institute consists of the following divisions:

Ceramics

Fracture and Deformation^

Polymers

Metallurgy Reactor Radiation

'Headquarters and Laboratories at Gaithenburg, MD, unlen otherwue noted; maiUng addreas Gaithcnburg, MD 20899. %ome divisions within the center are located at Boulder, CO 80303.

^ Located at Bouktef, CO, with some elements at Gattheraburg, MO
Traceability of Laser Interferometric

Length Measurements

Howard P. Layer

Center for Basic Standards

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

W. Tyler Estler

Center for Manufacturing Engineering

National Engineering Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

Issued June 1988

U.S. DEPARTMENT OF COMMERCE, C. William Verity. Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
National Bureau of Standards Technical Note 1248

Natl. Bur. Stand. (U.S.), Tech. Note 1248. 5 pages (June 1988)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington DC 20402
TRACEABILITY OF LASER INTERFEROMETRIC LENGTH MEASUREMENTS

Howard P. Layer

Center for Basic Standards

National Measurement Laboratory

W. Tyler Estler

Center for Manufacturing Engineering

National Engineering Laboratory

National Bureau of Standards, Gaithersburg, MD 20899

The National Bureau of Standards maintains the standard of length in accordance

with the definition of the meter adopted by the General Conference of Weights

and Measures (1983). This standard is used within NBS to form a consistent

system of units which are used in NBS calibration services.

LENGTH STANDARD : The meter is defined as "the length of the path travelled by

light in a vacuum during the time interval of 1/c = 1/299 792 458 of a second."^ This definition establishes a fixed value for the speed of light, c, in a vacuum. Neither this definition nor the document which describes it

discusses length measurements when the index of refraction is other than unity.

Such information must come from other sources.

The realization of the meter for industrial and scientific measurements relies

on several types of stabilized lasers operating under specified conditions, for

which vacuum wavelengths have been recommended by the International Bureau of

Weights and Measures.^ The wavelengths of these lasers are intrinsic

standards. The iodine stabilized Helium-Neon laser operating at 633 nm is the

instrument most suitable for the practical realization of the meter because of

its high accuracy (about 2.3 parts in lO-"-^) , its portability, and the ease with

which it can be used to calibrate working lasers.^ The calibration of a working laser yields its vacuum wavelength and is accomplished by measuring the

frequency difference between the working and the standard lasers using a heterodyne photodiode and a frequency counter. This measurement is independent

of the index of refraction of air.

LENGTH MEASUREMENTS : For length measurements in air, the wavelength of the

calibrated working laser must be adjusted for the difference in the index of

refraction, n, between air and vacuum.^ The wavelength of laser light in air

is equal to its vacuum wavelength divided by the index of refraction of air and

is smaller by about 2.7 parts in 10^ for standard atmospheric conditions. For

accurate length measurements, the value of the index of refraction must either

be measured directly or calculated from an empirically derived formula based on

such measurements. The variables which most strongly affect the value of n

are temperature, relative humidity, barometric pressure, and CO2 abundance.

The accuracy of the calculation using the empirical formula is limited to about

1 part in 10' when state-of-the-art technology is used for measuring these four

parameters

.

LASER INTERFEROMETER SYSTEMS : Relating practical laser interferometric

translation measurements to the length standard is difficult because the meter

is defined in terms of a vacuum wavelength whereas length and translation
measurements in the laboratory and shop are made in air. To account for this

difference, laser interferometer systems incorporate transducers for measuring

atmospheric conditions and the algorithm for computing the wavelength in air,

all of which must be correct in order for measurements to be accurate to within

stated uncertainties. Consequently, meaningful traceability of displacement

measurements using laser interferometer systems to the NBS must be concerned

with the total accuracy of the length measurement and not with laser

calibration alone.

TRACEABILITY : The National Bureau of Standards supports laser interferometer

system manufacturers to ensure the traceability of length measurements by

providing technical information and assistance. In particular, NBS has

developed a portable Iodine Stabilized HeNe Laser (633 nm) to ensure that

manufacturers have access to the length standard at an accuracy of 2 . 3 parts in

10-'^^. In addition, NBS provides technical support and information to

interferometer users when they encounter unusual problems which cannot be

solved by using commercial equipment or services.

Calibration of individual laser interferometer systems at NBS is insufficient

to ensure that subsequent measurements made using that system in other

environments will be correct. Unlike gage blocks and other simple artifacts,

the operating characteristics of laser interferometer systems are known to

change after calibration, especially when the systems are transported and are turned on and off frequently. The long term accuracy of laser interferometer

systems is determined by the manufacturer based on experience gained by

comparing their system with one of higher accuracy over extended periods of

time. The laser wavelength accuracy and stability can be measured by

comparison with an iodine stabilized laser and the index of refraction

correction accuracy by comparison to laboratory grade temperature, pressure,

relative humidity, and CO2 measuring instruments. This information should be

incorporated into the specifications provided with the instrument.

In consideration of the above information, NBS believes that the traceability

of stabilized lasers and laser interferometer systems to the National Bureau of

Standards must proceed from the user through the manufacturer to NBS. We know

that the calibration of individual lasers or laser systems does not

characterize their long term accuracy and, thus, this service is not provided

by NBS.

1. NBS Special Publication 330 (1986).

2. Comptes Rendus des Sceances de la 17e CGPM, 1983.

3. Metrologia 19, 163-177 (1984).

4. H.P. Layer, IEEE Trans. Instrum. and Meas .

, IM-39, 358 (1980).

5. F.E. Jones, J. Res. Nat. Bur. Stand. (U.S), 86, 27-32 (1981).

6. W.T. Estler, Appl. Opt., 24, 808-815 (1985).

7. Hewlett-Packard Jol. , 34, No. 4 (Apr. 1983).
NBS-n4A (REV. 2.ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/TN-1248

2. Performint Organ. Report No 3. Publication Date

June 1988

4. TITLE AND SUBTITLE

Traceability of Laser Interferometric Length Measurements

5. AUTHOR(S)

Howard P. Layer and W. Tyler Estler

6. PERFORMING ORGANIZATION (If joint or other thon NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

I. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

SAME AS ITEM //6 ABOVE.

10. SUPPLEMENTARY NOTES

[^21 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual sunf)mary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The National Bureau of Standards maintains the standard of length in accordance

with the definition of the meter, adopted by the General Conference of

Weights and Measures (1983). This standard is used within NBS to form a consistent system of units which are used in NBS calibration services.

12. KEY WORDS (S/x to twelve entries; a/phobeti'co/ order; capitalize only proper names; and separate key words by semicolons)

calibration; length; measurements; meter; standards; traceability

13. AVAILABILITY

[]X] Unlimited

Q] For Official Distribution, Do Not Release to NTIS

[J J Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

j^x) Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

15. Price

USCOMM-DC 0043-P80

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