IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASURF.MENT, VOL. IM-23, NO. 4, DlE,CFMBlER 1974

TABLE II

Mean difference per centFrequency

GHz 1971 1972 1973

8.2 -0.86 -0.06 -0.07

9.0 -0.42 -0.08 -0.01

10.0 -0.66 -0.06 -0.04

12.5 +0.03 -0.08

of the thermistor mount has been proved to be at leastcomparable with that of the best attenuation measurement.

CONCLUSION

A new reference standard-waveguide power-meterdesign, using fluid-flow calorimetry, has been describedwhich is eminently suitable for top level calibrationlaboratories. It has proved to be very stable and con-venient in use and its measurements have been in close

agreement with the U.K. national standards for severalyears. These results indicate that the estimated uncer-tainties are conservative and to realize the full potentialof the design, further work to evaluate more preciselythe residual systematic errors may be justified.

ACKNOWLEDGAiENTThe authors acknowledge the considerable contribution

nade by the staff of the drawing office and the mainworkshops at EQD in manufacturing this equipment anidthe help given by the staff of the Radio Frequency Stand-ards Laboratory with the measurements on them.

REFERENCES[1] J. A. Lane, "Microwave power measurement," IEE Mono-

graph, no. 12, pp. 24-27, 1900.[2] M. L. Crawford and P. A. Hudson," A dual flow calorimeter

for RF power measurement to 4 GHz," NBS Special Publication300, vol. 4, pp. 47-53, 1900.

[3] K. M. Kerns, "Determination of efficiency of microwave bolo-meter mounts from impedance data," J. Res. NBS., vol. 42,pp. 579-585, June 1949.

[4] A. C. Macpherson and D. M. Kerns, "A microwave micro-calorimeter," Rev. Sci. Instr., vol. 26, pp. 27-33, Jan. 1955.

Long-Term Efficiency Mleasurements on Bolometer Mounts

GIOVANNI RIETTO

Abstract-Measurements of effective efficiency on standard wave-guide bolometer mounts at microwave frequencies performed at theNational Electrical Institute (IEN), Turin, Italy, during internationalcomparisons and national calibration activities over a period ofseveral years show a very good repeatability and prove the reliabilityof these mounts as traveling standards and reference for microwavepower calibrations.

INTRODUCTION

S OME RESULTS of efficiency measurements obtainedduring several years from a collection of data on

X-band bolometer mounts are presented. The measure-ments were performed in the Department of Electromag-netic Metrology, National Electrical Institute (IEN),Turin, Italy, where the Italian standards for electric,photometric, and acoustic quantities are maintained.The IEN work on power measurements at microwave

frequencies started in 1964 when microwave calibrationsin Italy became a primary need due to national andEuropean research and development programs. At thattime a calibration service for the main quantities (i.e.,power, attenuation, and impedance) was initiated and

Manuscript received July 2, 1974; revised August 14, 1974.The author is with the National Electrical Institute, Turin, Italy.

contact was established with the U. S. National Bureau ofStandards (NBS) to obtain information and assistance.Details of IEN facilities for microwave calibrations aregiven in [1].

BOLOMETER MOUNTSA number of waveguide bolometer mounts were pro-

duced for use as power standards at X band (8.2-12.4GHz). They were of the type described by Engen for usein his waveguide microcalorimeter [2], different lengthsbeing chosen in order to cover the whole band. The mountswere made from copper in two halves, hard-soldered to-gether. The possibility of electroforming in one piece wasalso investigated with good results. The mounts were usedwith barretters of commercial type, having rated currentsof about 8.5 mA and nominal resistance of 200 Q. For someof them, thermistors can also be used.

Fig. 1 shows two mounts with appropriate code numbers:the first digit indicates a single specimen in a series builtfor one frequency range, which is denoted by the secondnumeral.

MEASURING TECHNIQUESTogether with the bolometer mounts, two waveguide

microcalorimeters of Engen's type were constructed. The

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RIETTO: EFFICIENCY MEASUREAfENTS

NBSRIND (USA) PTB

(SWEDEN) (FRG)

1IEN IERNCEIENRIND IEN 4-1 DPRMI

iEN lEZ/ (USSR)1 EN IE INW s

(ITALY) LCIE

1st international comparison DIENmicrowave power 10 GHz

1966 1967 1968 1969 1970 1971 1972 1973

Fig. 3. IEN participation in international comparisons.Fig. 1. Standard bolometer mounts.

Fig. 2. Setup for effective efficiency measurementS.

setup shown in Fig. 2 is part of the power measuringsystem that provides the national power standard at Xband. The maximum estimated uncertainty for efficiencymeasurements using the microcalorimeter is about ±0.3percent. In order to transfer the calibration from standardbolometer mounts to other types of mounts or to powermeters, a comparison bench was built, giving an additionaluncertainty of about 40.2 percent.

All microwave measurement systems are located in ashielded room, provided with air-conditioning (tempera-ture control ±1lC, humidity control ±5 percent).

INTERNATIONAL POWER COMPARISONS

For the first time in 1966, a direct comparison was madewith another European standards laboratory. Two bolom-eter mounts of different types (one with a film bolometer)were sent by the Swedish Research Institute for NationalDefence (RIND) to IEN, where they were calibratedagainst standard mounts.

In 1967 a standard mount was sent by IEN to NBS forcalibration at 9.375 GHz. The efficiency measured byNBS was 98.38 percent, which deviated by 0.05 percentfrom the IEN value.

In the meantime, the first international comparison oflow power measurements at 10 GHz, sponsored by theBureau International des Poids et Mesures (BIPM), hadbegun, with the Japanese Electrotechnical Laboratory(ETL) acting as a pilot laboratory. The IEN joined in1967, without submitting any transfer standard, andreceived the standards in 1971.

TABLE IMEASURED VALUES FOR MOUNTS

EffectiveEfficiency

Mount Date (percent) Remarks

1/2 11/1967 98.641/1968 98.647/1968 98.666/1969 98.683/1970 98.691/1971 98.586/1971 98.5912/1971 98.62

3/2 2/1967 98.8011/1967 98.801/1968 98.81 measured at RIND2/1968 98.85 measured at LCIE5/1968 98.765/1969 98.761/1971 98.6610/1972 98.704/1973 98.65

6/2 5/1967 98.3711/1967 98.293/1968 98.33 measured at PTB5/1968 98.335/1969 98.087/1971 97.9611/1972 97.972/1973 98.01

9/2 5/1967 98.7412/1967 98.693/1968 98.74 measured at PTB5/1968 98.67

1/1969 98.676/1969 98.6510/1969 98.67

1/3 10/1969 99.1212/1971 99.055/1973 99.03

2/3 3/1970 98.032/1971 97.861/1972 97.80

Other exchanges with foreign laboratories were made in1968, when cooperation with the Physikalisch-TechnischeBundesanstalt (PTB), Federal Republic of Germany,started and a limited cycle of comparison between France[Laboratoire Central des Industries Electriques (LCIE) ],Sweden (RIND), and Italy was completed under thesponsorship of the BIPM.In 1971, as a result of a program for scientific and tech-

nical cooperation between Italy and the USSR, exchangesof transfer standards were made with the Institute of

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-23, NO. 4, DlE,CE,MBE',R 1974

-_ 1/3 8.5GHzLCIE

RID- 3/2 10GHzRI N D-D-PTB 912 10GHz

1/2 10GHz fmeasurementuncertainty

/2 10 GHz

2/39.375 GHz

1967 1968 1969 1970 1971 1972 1973

Fig. 4. Efficiency repeatability of bolometer mounts.

Physical-Technical and Radio-Technical Measurements(PRMI).A diagram of the abovementioned international com-

parisons is shown in Fig. 3. The arrows indicate the direc-tion of circulation of the transfer standards.

RESULTS

The participation in international comparisons and thenational calibration activity over a period of more than 7years have produced a collection of efficiency data whichis felt to give useful information on the stability of stand-ard bolometer mounts with time.Among a group of eight mounts for the frequency range

around 10 GHz, four have shown a very good repeatability

for a long period of years. The others, after a short periodof satisfactory behavior, have become unstable, possiblydue to defective barretters, and are no longer used asstandards. Mounts built for other frequency ranges withinX band, although less used, have also given good results.

Fig. 4 shows the efficiency repeatability of severalmounts; the filled circles indicate results obtained byforeign laboratories. The individual curves have beenshifted vertically to avoid superpositions.

Table I gives the measured values (mean values of aseries of results) for the mounts shown.

CONCLUSIONThe results show that the bolometer mounts of the type

indicated are usually very stable and reliable over longperiods of time and prove very suitable as travelingstandards during international comparison cycles, beingapparently sturdy enough to endure the risks of longjourneys. It is interesting to note that for one of them(3/2) the efficiency remained constant within 0.2 percentover a period of seven years, despite the fact that it wasused in several international comparisons.

REFERENCES[11 G. Giachinio and G. Rietto, "Calibrazioni in microonide presso

l'Istituto Elettrotecnico Nazionale," A lta Freq., vol. 37, July1968.

[2] G. F. Engen, "A refined X-band microwave microcalorimeter,"J. Res. Nat. Bur. Stand., sec. C, vol. 63, pp. 77-82, July-Sept.1959.

Current Status of NBS Low-Power Laser Energy Measurement

E. DALE WEST AND WILLIAM E. CASE, MEMBER, IEEE

Abstract-A set of four electrically calibrated calorimeters is cur-rently in use at the Boulder Laboratories of the National Bureau ofStandards to calibrate and test devices for measuring average laserpower in the range from 100 AW to 1 W and energies in the range0.03 to 10 J. Laser sources used with these calorimeters are argon,krypton, helium-neon and neodymium-doped YAG, all CW, andpulsed YAG with pulse energies about 0.1 J and pulsewidths of about200 ,s and 30 ns.The calorimeters have been intercompared as a check on sys-

tematic errors. The national standard is taken as the average forthree of the calorimeters of the current C4 design, because thesewere designed to permit better measurements of the absorptanceand window transmittance. Deviations from the group average are-0.12, -0.01, and +0.13 percent. The earlier standard C3-1 differsfrom the group average by +0.21 percent.

Manuscript received July 3, 1974.The authors are with the National Bureau of Standards, Boulder,

Colo.

INTRODUCTION

(ALIBRATION and test services at the NationalBureau of Standards for laser power and energy

devices are based on electrically calibrated calorimeters,which relate optical power to the SI base units throughthe accurately known electrical standards. For low powerand energy measurements, a set of four calorimeters ismaintained. Their operating ranges are from 0.4 to 2 ,um inwavelength, from 10-4 to 2 W in power, from 0.03 to 10 Jin energy, and up to 0.1 J/cm2 in pulse energy density.The calorimeters have apertures (maximum beam size)of 2 cm. They were designed to be accurate to 1 percentwhich is deemed sufficient for present requirements oflaser power and energy measurements. The calorimetersare used with a variety of lasers including argon, krypton,

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