/

.*•' OF C0„

\

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* NBS TECHNICAL NOTE 1023

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National 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 and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by .the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

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2 Some divisions within the center are located at Boulder, CO 80303.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402
A System for Measuring the Characteristics of

High Peak Power Detectors

of Pulsed CO2 Radiation

NATIONAL BLTtEAU Of 8TAHOAKDS

UBRA1T

NOV ? 4 1980

Philip A. Simpson

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

v^^<9

^•tAU 0« *

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued September 1980
SsAS

NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1023

Nat. Bur. Stand. (U.S.), Tech. Note 1023, 44 pages (Sept. 1980)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

For sale by the Superintendent ot Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $2.25 (Add 25 percent additional for other than U.S. mailing)
CONTENTS

Page

List of Figures iv

1. Introduction 1

2The Measurement Process 2 2.1 Characteristics to be Measured 2 2.2 Responsivity Measurements 3 2.3 Insertion Loss of Feed-Through Detectors 5 2.4 Determination of Beam Diameter 8 2.5 Time Response Measurements 11 2.6 Description of the System 13 2.6.1 C0 2 TEA Laser 13 2.6.2 Arrangement of Optical Components 13 2.6.3 Data Acquisition System 16 2.7 Measurements at High Peak Powers above the

Region of Detector Linearity 17

3

Tests on Sel ected Detectors 19 3.1 General Remarks Concerning Detector Tests 19 3.2 Pyroelectric Detectors 19 3.3 Photon Drag Detectors 21 3.4 Photon Drag Monitors 23

4. Error Analysis 25

4.1 General Discussion of Errors 25

4.2 Energy Measurement Errors 25

4.3 Beam Splitter Ratio Errors 26

4.4 Waveform Measurement Errors 26

4 .

5

Beam Di ameter Measurement Errors 26 4.6 Time Response Measurement Errors 27 4.7 Summary of Error Analysis 27

5. Conclusions 36

6. Appendix: Transmittance of a Gaussian Beam

Through a Square Aperture 36

7

Acknowl edgments 38

8

References 39

m
List of Figures

Figure Page

1. Arrangement of equipment for measuring the input responsivity of

termi nati ng type detectors 4

2. Arrangement of components for measuring the output responsivity

of feed-through detectors 4

3. Arrangement of equipment for measuring insertion loss of feedthrough detectors 4

4. Tandem arrangement of feed-through detectors for measuring input

and output responsivities and insertion loss 7 5. Arrangement of components for making beam diameter measurements 7 6. Plot of beam diameter vs. aperture diameter for a TEMoo beam 10

7. Plot of beam diameter as a function of distance along the

opti cal axis 12

8. Pulsed CO2 detector characterizing system 14

9. Arrangement of components for measuring detector characteristics 15

10. Block diagram of data acquisition system 15

11. Arrangement of components for producing a calibrated, high

peak power beam 18

12. Arrangement of components for measuring detector responsivity

in the region of nonlinear operation 18

13. Output response of a commercially available pyroelectric detector 20

14. Output response of a laboratory model pyroelectric detector 20

15. Output response of a photon drag detector to an SLM TEA laser pulse 22

16. Photon drag detector response to a 10 ns sample extracted from

an SLM TEA 1 aser pul se 22

17. Output responsivity of a feed-through photon drag detector at various locations on the surface 24

18. Maximum possible error in the transmittance of a circular aperture

contributed by the uncertainty of the beam diameter measurement.

The percent value shown by each curve identifies the uncertainty

in the beam diameter measurement. The ordinate is the maximum

percent error i n the transmi ttance 28

19. Maximum possible error in the transmittance of a square aperture

contributed by the uncertainties of the beam diameter measurement.

The percent value shown by each curve identifies the uncertainty

in the beam diameter measurement. The ordinate is the maximum

percent error i n the transmi ttance 29

20. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse of 1 ns duration (FDHM). The peak value of the

resultant waveform is 1.000 V 30

21. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse of 3 ns duration (FDHM). The peak value of the

resul tant waveform is 1 . 000 V 31

22. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse of 4 ns duration (FDHM). The peak value of the

resultant waveform is 0.997 V 32

23. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse of 5 ns duration (FDHM). The peak value of the

resul tant waveform is 0. 984 V 33

24. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse of 7 ns duration (FDHM). The peak value of the

resultant waveform is 0.915 V 34

25. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse of 10 ns duration (FDHM). The peak value of the

resul tant waveform is 0. 773 V 35

IV
A System for Measuring the Characteristics of High

Peak Power Detectors of Pulsed CO2 Radiation

Philip A. Simpson

A system is described for determining the responsivity of

detectors for high peak power CO2 laser pulses. The insertion loss

of feed- through detectors can also be measured. The basic approach

involves a measurement of the excitation energy and detector output

waveform, and is applicable only for pulse amplitudes where the

detector output voltage is a linear function of the optical power.

However, a method is described for extending measurements to power

levels where the responsivity becomes nonlinear. The time response

of the detectors is checked using short duration (10 ns) rectangular

pulses to simulate an impulse. A method for measuring beam diameter

is also detailed. An error analysis of the system is given.

Key words: Beam diameter; insertion loss; optical detectors; photon

drag detector; pyroelectric detector; responsivity; single longitudinal mode pulse; TEA laser.

1. Introduction

High peak power pulsed CO2 laser radiation is finding increased use in today's high

technology fields. Two of the more prominent programs that employ this type of radiation

are the laser fusion [1] and laser rangefinder 1 programs. Pulses of this type are also a useful tool in property of materials studies [2] since they are capable of producing high

values of energy and electric field in periods of less than 1 us. The need for well

characterized detectors for this type of radiation is therefore essential for attaining

accurate measurement results in these fields.

The National Bureau of Standards Boulder Laboratories have recently developed a system

for characterizing power measuring detectors of CO2 TEA laser pulses with a 10 ns time

resolution. The type of detectors measured with this system are room temperature operating

ones, capable of measuring relatively high peak powers (> 100 kW) with little or no attenuation. These devices generally include such types as photon drag and pyroelectric

detectors. However, the techniques and principles used in this system are readily adaptable

to the characterization of low peak power detectors with modifications mainly to adjust the

amplitude of the optical signal. Also the system can be extended to other wavelengths by

the replacement of certain wavelength-sensitive elements.

x This work was supported by the Calibration Coordination Group of the

Department of Defense, the Air Force Avionics Laboratory, and the Office of

Naval Research.
2. The Measurement Process

2.1 Characteristics to be Measured

The parameter in which we are primarily interested is the responsivity. This quantity

relates the output electrical signal from a detector to the activating optical power. It is

defined as R, the proportionality factor, in the following expression

V(t) = R / G(t-t')P(t')dt (1)

where

V(t) = the time function of detector output voltage, in volts,

R = the responsivity in V/W,

G(t-t') = the detector impulse response in Hz, and

P(t') = the time function of optical power in watts.

Another parameter we shall measure is the insertion loss of feed-through type

detectors. This quantity tells how much a beam is attenuated when passed through the device

and is defined as the ratio of the input power (or energy) to the output power (or

energy). We are also interested in the maximum irradiance to which the detector is subjected in order to avoid possibly inducing such unwanted effects as saturation or damage.

Irradiance, sometimes referred to as power density, usually is expressed in units of watts

per square centimeter.

In making these optical measurements it is imperative that we use a spatially wellcharacterized beam. We need to know the beam size and profile so that we can determine the

peak irradiance and also the percent of the total incident beam being captured by the

detector. The kind of beam employed in this system, unless otherwise noted, is a TEMoo

Gaussian beam since the mathematics for this simple type of beam is straightforward. Its diameter is defined as the diameter of the beam where the intensity falls to 1/e 2 that of

the maximum.

One other characteristic we will measure is the detector time response. For purposes

in this paper, this is defined as the minimum optical pulse transition time that will be

faithfully reproduced within certain prescribed limits of error. Knowledge of the time

response of the detector is required when measuring fast pulses to ensure the detector

output response is not being degraded due to inability of the detector to respond to the

optical signals.
2.2 Responsivity Measurements

For terminating type detectors the responsivity is determined by measuring the energy

incident upon the detector and the voltage output waveform from the detector. The

responsivity is then calculated by dividing the area under the voltage waveform by the

energy. The incident energy is measured using a calibrated beam splitter and calorimeter as shown in figure 1, while the output voltage waveform is measured with a transient

digitizer. Thus,

A

R- = — (2) BE

c where

RLj = the input responsivity of the detector in V/W,

A

v

= the area under the voltage waveform in V*S,

B = the beam splitter ratio defined as the transmitted power

divided by the reflected power, and

E

c

= the energy measured by the calorimeter in joules.

For feed- through type detectors the responsivity associated with the output is also

measured using an arrangement as shown in figure 2. Here

A

c where

R

Q

= the output responsivity of the detector in V/W,

A

y = the same as in eq (1) above, and

E

c = the energy measured by the calorimeter in joules.

These relatively straightforward procedures can contain subtle sources of error. First

it must be established that the optical detector is operating in a region where its voltage

output is a linear function of the optical power. This can be ascertained by inserting

attenuators in the laser beam ahead of the beam splitter and verifying that the responsivity

remains constant for pulses of different peak powers.

Another possible source of error arises from the fact that the calorimeter and/or

detector may not be capturing all of the energy in their respective beams. Their apertures

may not be big enough to contain all the energy in the beam. Reflectance losses in the

detector are not considered separately but are included as part of the responsivity

measurement. For calorimeters with apertures of 25 mm or more, beam capture errors are rather easy to avoid. However, detectors having apertures with dimensions of 1 to 5 mm pose

a problem. To be able to fully capture "all" (99.97%) of the energy of a TEMoo Gaussian

beam, the detector aperture should have twice the diameter of the beam at the 1/e 2 intensity
BEAM FROM

LASFj

CONCAVE MIRROR

CALORIMETER

DETECTOR

UNDER TEST

Fiqure 1. Arrangement of equipment for measuring the input responsivity of terminating

type detectors.

BEAM FROM

LASER

DETECTOR CALORIMETER

I

Figure 2. Arrangement of components for measuring the output responsivity of feed-through

detectors.

BEAM

SPLITTER

CALORIMETER C R

CALORIMETER Cn

BEAM FROM

LASER

Figure 3. Arrangement of equipment for measuring insertion loss of feed-through detectors,

4
points. This means that some sort of focusing is usually required to reduce beam dimensions

to the appropriate size.

Focusing results in increased irradiance. This fact may produce undesirable effects,

such as nonlinear operation due to saturation or actual damage. The peak irradiance of a TEMoo beam is given by

2P

t

P o

= —" (4)

o ttW z

where

P

Q = the peak irradiance in W/cm 2

,

P t

= the power in total beam in W, w = the radius in cm of the beam at the 1/e 2 intensity point.

Thus, remembering the stipulation of aperture diameter equal to twice the beam

diameter, we find a 1 MW beam totally contained just within a circular area of 1 cm 2 (W=0.28

cm) has a peak irradiance of 8 MW/cm2

. If the beam is focused to fit totally within a 5 mm

aperture the peak irradiance increases to 40 MW/cm 2 and for a 1 mm aperture the irradiance

is 1 GW/cm 2

. Obviously, these peak irradiances can cause problems.

2.3 Insertion Loss of Feed-Through Detectors

The insertion loss of feed-through detectors can be measured by several different

methods. The most straightforward method involves measuring the input and output energies

(powers) with a setup as in figure 3. We can say

I.L.

E . out

E B

= -5- (5)

E T where

I.L. = the insertion loss,

E^ = the energy measured by the calorimeter C

R

,

B = the beam splitter ratio, and

Ej = the energy measured by the calorimeter Cj.

For linear devices it can also be shown that the insertion loss can be derived from a knowledge of the input and output responsivities as shown in eq (6) below

I.L. = -5- (6)

R.

i

F
where

R

Q

= the output responsivity in V/W, and

R- = the input responsivity in V/W.

The insertion loss can also be determined with a tandem arrangement of feed- through

detectors as shown in figure 4. The two detectors, "a" and "b", have previously had their

R's determined. The energy is then measured with the calorimeter and the area under the

waveform from detector "a" determined. We can then say

I.L.. =!^. (7)

b ob

Now

V#S

a

Roa where

R oa

= the output responsivity of detector "a" in V/W or V*S/J.

V*S

a

= area under the waveform from detector "a", and

Also,

E ob

= J (9)

where

J = the energy in joules measured by the calorimeter.

Substituting eqs (8) and (9) in eq (7) and simplifying yields

V * S

a I.L. = 5_ , (10)

J(R

oa

)

and if we define V-S

g /J = R' oa

as the "apparent" output responsivity of "a" with "b" inserted in the output beam, then

R

1 i.l = —5i . (in

b

RRoa From a knowledge of the insertion loss and output responsivity the input responsivity

can be determined from eq (6).

When using this tandem method for characterizing feed-through detectors it should be

verified that reflections from detector "b" are low enough to cause negligible effect on the

output waveform of detector "a". This is not an interference effect in photon drag detectors but is an actual subtraction to the output voltage caused by photons traveling in a direction opposite to the main beam [3].
BEAM FROM DETECTOR a DETECTOR b

1 1 CALORIMETER

LASER 1 ^>^S V^^sV\\\\N mi llilt ^^^ ^S5S^

1 II 1 1

Figure 4. Tandem arrangement of feed-through detectors for measuring input and output

responsivities and insertion loss.

BEAM

SPLITTER

CALORIMETER C R CALORIMETE R CT

BEAM FROM

LASER

ADJUSTABLE

APERTURE

Figure 5. Arrangement of components for making beam diameter measurements.
2.4 Determination of Beam Diameter

Knowledge of the diameter of the TEMoo Gaussian beam is required for two purposes

a. calculation of the peak irradiance, and

b. verification that all of the energy is incident upon the surface of the detector or determination of the fraction of energy that is incident.

The procedure for measuring the diameter involves first adjusting the laser output

mirror and an intercavity iris so that the laser produces a circular glowing spot on a carbon paddle [4]. This type of spot usually indicates the laser is operating in the TEMoo

mode but it is possible to confuse this pattern with that produced by higher orders such as the TEMn, TEM22, etc. The construction details of some lasers may render it impossible to attain a TEMoo beam. In this case the size of the multimode beam should be determined by some other method, e.g., scanning.

To confirm that the mode structure is totally TEMoo, we insert an adjustable aperture

in the beam and fix its diameter so it intercepts part of the energy in the periphery of the

beam. Figure 5 shows an arrangement of components for making a beam diameter measurement.

The transmittance of a circular aperture centered on a TEMoo Gaussian beam is given by

-2a 2

T = 1-e

w2

(12)

where

T = the transmittance, i.e., the fraction of energy in the total beam transmitted

through the aperture,

a = radius of aperture, and w = radius of the total beam at the 1/e 2 intensity point.

From figure 5 we can say — (131

E

i where

Ej = the energy in the transmitted beam, and

E.j - the total energy of incident beams.

But

E

i =BE

R (14)
where

B = the beam splitter ratio, and

E R

= the energy measured by C R

. Combining eqs (13) and (14) with eq (12) and solving for w, we get the following expression

for beam radius,

-2 w = a / £n(l-E

T /BER

) (15)

In the experimental setup in figure 5, Ey and E

R are measured by Cj and C

R

, respectively. B is determined by Ej/ER with the aperture wide open.

To ascertain that the beam is truly TEMoo, measurements of Ej and E

R are made for

several different aperture sizes. If the value of w remains constant for various values of

a, this confirms beam is TEMoo. Figure 6 shows a sample plot of beam diameter vs. aperture

size at one specific point in an experimental setup.

If we wish to know the beam diameter over an extended distance along the optical axis,

a series of measurements of beam diameter are made at various points over the region of

interest and the results connected by some sort of smooth curve. For focused beams which

converge rather rapidly (half angle ^ 0.01 radians) measurements should be made on both

sides of the beam waist and fitted to the equation for the propagation of a TEMoo Gaussian

beam [5]

1 + x(z-z ) (16)

where

w

z = the radius of the beam (1/e 2

) at point z along the optical axis,

wo = the radius of the beam at the waist,

X = the wavelength of the radiation,

z = the distance from the point of measurement to some arbitrary

reference point on the optical axis (position of a beam splitter,

mirror, etc.), and

zo = the distance from the position of the beam waist to the reference

point.

Since the precise position of the beam waist is difficult to fix by visual observation,

eq (16) has two unknowns, wo and zo- Therefore, when fittinq data from a focused beam to eq

(16) by a least squares method, some of the measurements of beam diameter should be made far

enough from the waist so that the curve appears linear. This permits estimation of the

slope of the asymptotes, the intersection of which yields zo. Various values of wo can then

be tried in eq (16) and the square of the residuals from each of the least square fits

plotted against wo. The minimum of this curve yields the best estimate of wo. For most
APERTURE DIAMETER, cm

Figure 6. Plot of beam diameter vs. aperture diameter for a TEMoo beam.

in
precise results this process can be repeated for other values of zo close to the originally

assumed value, and the wo and zo associated with the smallest value of square of the

residuals used. This iterative process need only be carried on until the uncertainty in wo and zo is reduced to an acceptable value. Fiqure 7 is a graph showing the results of one

such least squares fit.

2.5 Time Response Measurements

The time response of a detector is determined by examining its output voltage

waveform. Three factors determine this function--the impulse response of the detector, the

impulse response of the measuring system, and the time function of intensity of the

excitation optical signal. The output waveform from the detector is a convolution of these

three factors. The detector impulse response can be measured by simulating an impulse with

a short optical pulse generated by some method (e.g., free induction decay [6], switching

out a single pulse from a modelocked train [7]), then illuminating the detector with the

impulse, and deconvolving the impulse response of the measuring system from the observed

detector output.

Because the original requirement for this particular system was a verification that the

detectors have a 10 ns time response, the sophisticated techniques described above were

relaxed. A step function with a 10 ns risetime is degraded by a measuring system with a 1 ns risetime by only 0.5 percent assuming Gaussian transitions. Such measuring systems are readily available. Also, the need for a short impulse is obviated. A rectangular optical

pulse of 10 ns duration is sufficient for the fastest excitations required and still

contains enough energy to be conveniently measured. In this system such a pulse can be

generated from a single longitudinal mode (SLM) TEA laser pulse with an electrooptic

shutter.

Since the duration of the optical pulse is fixed, the peak power can be determined from

the energy. For instance, if the rectangular pulse has a duration of 10 ns and an energy of

10 mJ, then its peak power is 100 kW. Also from the waveform data the peak voltage can be

ascertained. If we divide the peak voltage by the derived peak power (100 kW in the

instance above), we can arrive at a calculated responsivity. If this calculated

responsivity for the short pulse is equal to that associated with longer pulses, then the

time response is fast enough to cause no error; if not an error is being introduced.

11
DISTANCE FROM REFERENCE POINT, cm

Figure 7. Plot of beam diameter as a function of distance along the optical axis,

12
2.6 Description of the System

There are three major parts to the system. They are:

1. the laser for generating the optical excitation pulse,

2. an arrangement of optical components for dimensioning and time-shaping the optical

pulse and applying it to the detector, and

3. a computer controlled data acquisition system for running the experiment.

Figure 8 is a view of the setup of one version of the system. Often the various components

are rearranged to achieve certain selected results. The three parts are described in the

next three sections.

2.6.1 C0 2 TEA Laser

The laser is contained in an aluminum shielded enclosure seen in the central background

of figure 8. Its power supply is located next to it. The cavity is 2.16 m long. The cell

containing the gain medium is one that is comrnercially available. It is located within a second shield so that the emi generating components are doubly shielded. The gain medium is

a mixture of 78 percent He, 12 percent CO2 and 10 percent N2 and is pressurized to 101 to 104 kPa (760 to 780 mm). Atmospheric pressure at Boulder is approximately 84 kPa (630 mm).

Also contained within the optical cavity is a low pressure gain cell for operating the

laser at a single longitudinal mode (SLM). This cell uses the same gas mixture mentioned

but at a pressure of 133 Pa (1 mm). Since this unit runs CW, a water jacket is used for

cooling.

An adjustable iris is located within the cavity near the output mirror for achieving

TEMoo operation. Output of the laser in the SLM TEMoo mode is approximately 200 kW and

40 mJ. With the iris wide open an SLM pulse operating with many spatial modes of nominally

1 MW and 300 mJ can be achieved.

2.6.2 Arrangement of Optical Components

The optical components are on the metal table in the foreground of figure 8. The

electrooptic switch (E-0 switch) is located on the mounting rail on the right side of the

table and its pulse forming components are housed in the nearby screened enclosure for

suppression of emi

.

Figure 9 is a diagram of an alternate arrangement of optical components that was designed for adjusting the dimensional and time parameters of the optical pulse. The two objectives of this arrangement are the extraction of as little energy as possible to fire

13
14
FROM LASER

2.44 METER COAXIAL CABLE

(12 ns DELAY)

Figure 9. Arrangement of components for measurino detector characteristics.

BEAM SPLITTER

CALORIMETER

LAitK Vt ItLlUK

#2

1

' 1

'

DC AMP TRANS. DIGITIZER

DC AMP

' 1

» SCANNER

PULSER ~[

•

i TIMER

DVM

\

'

I DATA INPUTS \ GRAPHIC COMPUTER

i \

CONTROL OUTPUTS

Figure 10. Block diagram of data acguisition system.

15
the laser triggered spark gap (LTSG), and the synchronization of the arrival times of the

optical and electrical pulses at the E-0 switch. The telescope formed by mirrors Ml and M2

helps achieve the first objective by reducing the laser beam diameter so that it totally

fits within the entrance aperture of the LTSG. The second objective is accomplished using

the optical delay line formed over the path BS2-M3-M4-M5-E0 switch. This path provides the

necessary 50 ns delay to allow for the LTSG to break down and for the electrical pulse to travel the coaxial line. Fine adjustment on the delay for precise synchronization is

accomplished by varying the pressure on the LTSG over the range 760 to 970 kPa (110 to 140

psi) gauge pressures. M6 is used to adjust the size of the beam to fit the entrance

aperture of the detector under test (DUT). Finally, the beam splitter, BS3, allows the

simultaneous measurement of pulse energy and waveform. It is vital that this component be

located outside the region of high irradiance of the focused beam so as to avoid damage and

permit linear operation.

2.6.3 Data Acquisition System

The data acquisition system is shown to the left of the metal table in figure 8. AC power is supplied to it through an electronic regulator to reduce emi conducted over the

power line as well as to enhance the precision of the measurements. Figure 10 is a block

diagram of the various pieces of equipment. The arrangement shown is for feed-through

detectors; for terminating detectors, calorimeter 2 and its associated amplifier can be

omitted.

The graphic computer controls the experiment. It furnishes a signal through the scanner to fire the laser, and acquires and processes the data from the calorimeters and

transient digitizer. Precise time intervals for making calorimeter output measurements are supplied to the computer by the timer. Final results are outputted on the screen and can be

stored as hard copy or on cassette tape.

The calorimeters are a commercial, volume absorbing model with calibration factors of

approximately 100 mV/J. The exact value is determined by calibrating them with the NBS K series of standards. The input aperture of these instruments is 25 mm in diameter.

The amplifiers are used to increase the output of the calorimeters for low energy

signals. They are required in cases such as measuring signals in the 10 to 20 kW range or

10 ns rectangular pulses. The gain is adjustable in steps of factors of 10 over the range

of 100 to 100,000. Maximum output is 10 V.

The DVM is a 6-1/2 digit instrument capable of 1 uV resolution on the most sensitive

range. When controlled by the computer, readings of calorimeter output can be taken at intervals of 0.25 s, although 1 s intervals are normally sufficient.

16
The transient digitizer is a high-speed A to D converter with a bandwidth of 350 MHz.

It is capable of resolving 1 part in 512 of full scale of the time window and the voltage

range. Fastest time window is 5 ns; most sensitive voltage range is 40 mV full scale.

2.7 Measurements at High Peak Powers Above

the Region of Detector Linearity

The responsivity measurement techniques in section 2.2 are limited to power levels for

which the detector output voltage is a linear function of the input power. Thus, the power

range is restricted to some maximum value. This maximum power value can be increased with

the use of a beam splitter as shown in figure 11. The peak value in the transmitted beam is

B times the peak value measured by the detector, where B is the beam splitter ratio. The

only restrictions placed on this technique are that the detector continue to be operated in its linear region and that the beam splitter ratio remain unchanged at the high levels of

energy.

Note that this technique delivers a calibrated beam of known peak power at a high

level. This provides a vehicle for calibrating detectors in the region where their output

is no longer a linear function of the input power (but is still below the damage threshold

of the device). If we insert the detector in output beam as shown in figure 12 and measure

the peak voltage from both detectors with two transient digitizers we can calculate the peak

power on the detector experiencing the high peak power and calculate its responsivity. From

figure 12 we can say

V

P p = _a x B (17)

K b R

a where

P

P = the peak power in W incident upon detector b, b

B = the beamsplitter ratio,

V p

= the peak voltage developed by the calibrated detector a, and

a

R

a = the responsivity in V/W of detector a. From this knowledge of the incident peak power on b we can say

Vp

b

R n

= —5. (18)

B

P P

b where

R

B

= the responsivity of detector b in V/W at the high power level,

Vp = the measured peak voltage developed by detector b, and

Pp = is as in eq (17) above.

This technique determines the nonlinear effects tested for in section 2.2. It may be

necessary also to perform further time response tests at high power levels.

17
BEAM

SPLITTER

CALIBRATED

DETECTOR

CALIBRATED

HIGH PEAK POWER

BEAM

BEAM FROM

LASER

Figure 11. Arrangement of components for producing a calibrated, high peak power beam.

BEAM

SPLITTER

DETECTOR a DETECTOR b

BEAM FROM

LASER

Figure 12. Arrangement of components for measuring detector responsivity in the region of

nonlinear operation.

18
3. Tests on Selected Detectors

3.1 General Remarks Concerning Detector Tests

The tests described in the following sections of this chapter were performed as part of

a program to identify a room temperature operating device that was a promising candidate for

a transfer standard for CO2 TEA laser peak power. Not all the previously mentioned tests were performed on each of the units as some were eliminated early because of various

drawbacks for this specific application. However, the following sections do give specific

examples of the measurements discussed in chapter 2.

3.2 Pyroelectric Detectors

Pyroelectric detectors listed as stock items from two manufacturers plus one laboratory

model were studied. The dimensions of these detectors were of the order of 1 mm square.

All units were capable of resolving model ocking spikes but the commercial models exhibited a mechanical ringing resulting from piezoelectric effects. This undesirable ringing, readily

apparent in figure 13, persisted for several microseconds beyond the cessation of the

optical pulse and caused large distortions in the output waveform. The laboratory model

appears to be adequately damped as there is no ringing visible in its output which is shown

in figure 14.

Pyroelectric detectors of the above size are restricted to only moderate pulse

energies, their upper limit being approximately 10 mJ. Thus, for pulses with an equivalent

duration 2 of 200 ns, the maximum peak power is 50 kW. To measure higher powers some form of

a calibrated attenuator must be employed ahead of the device.

The small aperture size, typically 1 mm square, also accentuates the problems

associated with total beam capture discussed in section 2.2. Because of these shortcomings

extensive measurements on this type of detector were not performed.

Equivalent duration is defined as duration of a rectangular pulse having the same peak power and energy as the pulse in question.

19
itf

16

P

U

T

TINE, uS

Figure 13. Output response of a commercially available pyroelectric detector.

180

80

66

46

2y

TINE, uS

Figure 14. Output response of a laboratory model pyroelectric detector.

20
3.3 Photon Drag Detectors

The photon drag detectors discussed in this section are terminating (beam absorbing)

type units as opposed to the feed-through units discussed in the next section. All had

apertures of 20 mm 2

, two being circular with a 5 mm diameter and one square, 4.5 mm on a side.

The theoretical time response of this type of detector is extremely fast, being less

than 1 ns and approximately equal to the time of flight of photons through the material

[8]. However, less than ideal mounting configurations and poor impedance matching to external circuitry can degrade these fast transition times.

All the detectors resolved modelocking spikes and exhibited good waveform

reproduction. Figure 15 shows the response of one of the detectors to an SLM pulse. The

time response of this detector was measured by the method discussed in sections 2.5 and

2.6.2. Figure 16 shows the detector response to a 10 ns "rectangular" pulse. The noise

along the baseline results from electrical reflections caused by impedance discontinuities

in the coaxial transmission line from the LTSG and in the E-0 switch head. These electrical

reflections prevent the E-0 switch from turning off in a clean manner. (Subsequent

improvements have significantly reduced these perturbations.) The responsivity derived from

the 10 ns pulses was

7.72 x 10 _t+ V/kW ± 6.1%.

This value compares quite favorably with a responsivity of

7.61 x 10

_1+ V/kW ± 2.1%,

which was measured for pulses similar in shape to that in figure 15. The ± values quoted in

this section and also 3.4 are estimates of imprecision based on the standard deviation of

the mean [9]. They have had Student's t distribution applied and are the 95% confidence

values.

The greater imprecision associated with the 10 ns pulses resulted from two sources: misalignment of the detector and decrease in signal to noise in the energy measurement. The

beam misalignment resulted from a slight deviation in the beam position when the polarizeranalyzer was rotated to the extinction position. The small energy in the 10 ns pulse was insufficient to give a visible indication of beam location, so the detector was repositioned

to obtain maximum output as a means of correcting the alignment. By far the greatest

imprecision was caused by the 20-fold reduction in energy. For the total pulse the energy

was typically 10 mJ, while the 10 ns pulse contained 0.5 mJ. The actual measured values of

these energies are one-fourth the above values since they are attenuated by the beam

21
100

60

46

26

TIME, ui

Figure 15. Output response of a photon drag detector to an SLM TEA laser pulse.

100

80

66

40

20

TIME, r.S

Figure 16. Photon drag detector response to a 10 ns sample extracted from an SLM TEA laser

pul se.

22
splitter ratio. The measurement of this low energy by the calorimeter was somewhat

sensitive to random changes in ambient conditions occurring during the measurement period.

3.4 Photon Drag Monitors

These units are similar to the photon drag detectors in section 3.3 but have the added

property of being feed- through devices. They also have larger apertures than the detectors

in 3.3. Three monitors were studied in this section, one having input and output apertures

12.7 mm in diameter and the other two detectors 25.4 mm in diameter. These large apertures

greatly simplify the beam capture problems described earlier but do introduce a new complication, namely a spatial dependence of responsivity.

This variation of responsivity over various portions of the detector surface is readily

apparent to the observer. As he moves the laser beam from the center towards the edge of

the monitor the voltage output increases an appreciable amount. This phenomenon can produce

confusing results if pulsed beams of equal peak power impinge on different areas of the

detector, or are coaxial but of differing diameters, or are coaxial and of the same diameter

but possess different spatial mode structure. This effect can be minimized by limiting the

beam to a region near the center of the detector where spatial variations are small.

The output responsivity of the two 25.4 mm monitors at several positions was measured. A collimated TEMoo SLM beam, 4.8 mm in diameter, served as a probe. A calorimeter placed close to the output aperture measured the output energy for the

responsivity determination. No beam splitter was required.

Figure 17 shows the results of measurements on one detector. The other detector

possessed similar characteristics. The circles represent areas containing 99% of the power

in the beam; the numbers are the measured values of output responsivity in V/MW. The peak

power in the exit beam was in the range 150 kW to 220 kW for this series of measurements.

The imprecision of the measurements at the center is ± 5% while at the edges it increases to

a maximum of ± 11%. Systematic error in both cases is 4.5%. (See chapter 4, Error

Analysis, for a discussion of systematic errors.)

The insertion loss of the two monitors was measured using the tandem method described

in section 2.3 and depicted in figure 4. As previously mentioned the polarity of the output

of photon drag detectors is dependent on the direction of photon flow through the device so

a check was made to verify that any reflected light from the "b" device in figure 4 did not reduce the voltage output of "a". A sheet of teflon, which is a good volume absorber for

10.6 pm radiation [10], was inserted between the two monitors. No change in output waveform

from monitor "a" could be detected, which is consistent with the fact that the devices are

AR coated.

23
Figure 17. Output responsivity of a feed-through photon drag detector at various locations

on the surface.

24
The insertion loss as determined from eq (11) was 1.26, which from eq (5) qives an input responsivity, S-, of 0.289 V/MW. The imprecision for these measurements is 10%.

A power linearity check was made on the monitor with the 12.7 mm apertures. The output

responsivity was measured at various output power levels over the range 37 kW to 185 kW.

The lower levels were generated by attenuating the beam by using the front surface

reflection off a wedged zinc selenide beamsplitter. Over this range the output responsivity

was 0.208 V/MW with an imprecision of 3.9%.

4. Error Analysis

4.1 General Discussion of Errors

The sources of error can be determined by considering the basic principle of the system

as discussed in chapter 2. We need to determine the errors associated with measurement of

the optical energy and those associated with waveform voltaqes. The error contributed by

each measuring device will be considered separately and then the reader can derive the total error for a particular measurement configuration by considering the effect of just those

components used. The uncertainties quoted in this section are based on calibrations made

with random errors estimated at the 95% confidence level.

4.2 Enerqy Measurement Errors

The energy is measured with commercially available, volume absorbing calorimeters.

These units were calibrated against the NBS K series calorimetric standard [11]. The

uncertainty associated with the calibrations is + 3%.

The calorimeters were subjected to fluences of approximately 5 J/cnT and no departure

from linear operation could be detected. Since the units are usually placed where the beam

diameter is 5 mm or greater and since the incident energy very seldom exceeds 180 mJ for a TEMoo beam, calorimeter linearity is not a problem.

The energy measurement is also dependent upon the gain accuracy of the dc amplifier

(when it is in the system) and the DVM. Calibrations of these two instruments show they are

well within their manufacturer's quoted specifications of 0.01% and 0.002%, respectively,

and therefore have a negligible effect on the uncertainty.

In summary, the error in the energy measurement is ± 3%.

25
4.3 Beam Splitter Ratio Errors

A beam splitter is required in this system when measuring energy incident upon a detector (except in the tandem monitor configuration of section 3.4). The units used in

this system are made of zinc selenide. They have uncoated surfaces which form a wedge

having an apex angle of 1°. Danielson and Beers [12] have shown that this type of device

serves as an accurate, calculable energy or power sampling device, the exact ratio of the

sample energy to that in the main beam being dependent upon the index of refraction of the

material, the wedge angle, the order of the reflection, and the angle of incidence of the

laser beam. The ratio of transmitted energy to that from the front surface reflection was measured by a method outlined in [13] to confirm that no serious defects were present in the

units to cause departures from the theoretical ratios. The uncertainty associated with the

beamsplitter ratio is + 0.5%.

The beam splitters were checked for linear operation by observing the ratio while

subjecting them to various values of fluence. At fluences up to approximately 5 J/cm2

(25 MW/cm 2

) no departures from linear operation could be detected. Above that value erratic

operation was noted and at times a faint flashing on the surface noted. At 20 J/cm2 (100

MW/cm 2

) surface damage was produced. As a result the maximum fluence to which they are subjected is kept below the 5 J/cm2 value.

4.4 Waveform Measurement Errors

The time function of output voltage from the detector is measured by the transient

digitizer. This instrument is capable of resolving each time and voltage range with a precision of 1 part in 512. The bandwidth of the instrument is 350 MHz. This instrument

was calibrated by Young and Lawton [14] with an uncertainty of ± 1.5%.

4.5 Beam Diameter Measurement Errors

Beam diameter measurements are made with an adjustable iris as discussed in section

2.4. Errors in this measurement can arise from various sources. When determining the

diameter at one location along the optical axis these sources can include such factors as misalignment of the beam through the center of the aperture, inaccuracy in setting the

aperture diameter, nonrepeatability of laser beam characteristics and noise in the

measurement system. If the aperture size and position are reset prior to each measurement,

the errors will tend to be randomized and the standard deviation will give an idea of the

uncertainty of the measurement. For measurements of beam diameter along an extended

distance of the optical axis, the closeness of fit of the individual points to the

theoretically predicted equation gives an idea of the uncertainties involved.

26
The contribution to the total uncertainty of the measurement of detector responsivity

by errors in the beam diameter measurement will vary dependinq on the ratio of the aperture

size to beam diameter. Obviously, if the detector aperture diameter is much greater than

the beam diameter larger errors can be tolerated. Figure 18 shows the uncertainty

introduced into the transmittance of a circular aperture as a function of the relative sizes

of the aperture and the TEMoo beam. Curves are shown for three values of uncertainty in the

beam diameter measurement. Figure 19 shows the same information for square apertures. A derivation of an expression for the transmittance of a square aperture for a TEMoo Gaussian

beam is given in the Appendix. From the curves presented one can see that errors in beam

diameter measurements can result in uncertainties ranqing from negligible for apertures

twice as big as the beam diameter to nearly 10% when the aperture and beam diameter are

equal

.

4.6 Time Response Measurement Errors

In this section we shall present data to give the reader some feel for the risetimes

that can be measured using a 10 ns rectangular optical pulse to excite the detector as described in section 2.5. From this it will be seen that the waveform shown in fiqure 16 results from a detector with a very fast time response—much faster than 10 ns.

Figure 20 through 25 are computer generated plots showing the convolution of a 1 V, 10

ns rectangular pulse with various duration Gaussian impulse responses. This type of impulse

response was chosen as an example because many mechanisms in nature exhibit, to a first

approximation, a Gaussian response. From the curves we can see that detectors having a Gaussian impulse response with duration greater than 3 ns will exhibit a lowering of the

output when excited with a 10 ns rectangular pulse. When the impulse response duration is 7

ns, the loss in output is nearly 10%. Also, the nearly straight and parallel sides in

figure 20 are quite similar to those in figure 16, indicating a detector time response of

the order of magnitude of 1 ns.

4.7 Summary of Error Analysis

The errors associated with the various sources of systematic error are as follows:

Energy Measurement 3.0%

Beamsplitter Ratio 0.5%

Waveform Measurement 1.5%

For each experiment, the error in the measurement of beam diameter must be determined from

figure 18 or 19. It will generally lie in the ranqe 0.0 to ± 10% depending on the precision

of the measurements and the size of the detector aperture compared to the beam diameter.

27
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

RATIO OF APERTURE DIAMETER TO ASSUMED BEAM DIAMETER

Figure 18. Maximum possible error in the transmittance of a circular aperture contributed

by the uncertainty of the beam diameter measurement. The percent value shown by

each curve identifies the uncertainty in the beam diameter measurement. The

ordinate is the maximum percent error in the transmittance.

28
1.8 LI 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.8

RATIO OF APERTURE SIDE TO ASSUMED BEAM DIAMETER

Figure 19. Maximum possible error in the transmittance of a square aperture contributed by

the uncertainties of the beam diameter measurement. The percent value shown by

each curve identifies the uncertainty in the beam diameter measurement. The

ordinate is the maximum percent error in the transmittance.

29
0)

h

J

>

h

D

Q.

h

D

/ \

1.2

i.e.

0.8.

0.6.

0.4.

0.2.

0.0LL.

PEAK OUTPUT: I .800 VOLTS

l_1 L j. a. !8 20 30 40 58 68 70 80 90

TIME, ns

Figure 20. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse

of 1 ns duration (FDHM). The peak value of the resultant waveform is 1,000 V.

30
h

J

>

h

D

Q.h

D

1.2

1.8

0.8.

8.6.

0.4.

0.2.

0.0

PEAK OUTPUT: 1.000 VOLTS

J L I I 10 20 30 40 50 70 80 m 100

TIME, ns

Figure 21. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse

of 3 ns duration (FDHM). The peak value of the resultant waveform is 1.000 V.

31
CDh

J

>

h

D

Q.

h

D

1.2

1.8

0.8

0.6.

0.4.

0.2.

0.01

PEAK OUTPUT: 0.997 VOLTS

y 1 ij- l i—,—

i

__a_ 10 20 30 40 50 60 70 90 100

TIME, ns

Figure 22. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse

of 4 ns duration (FDHM). The peak value of the resultant waveform is 0.997 V.

32
w

\-

J

>

h

3

0.h

D

Q

18 20 30 40 50 60 70 80 98 li

TIME, ns

Figure 23. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse

of 5 ns duration (FDHM). The peak value of the resultant waveform is 0.984 V.

33
0)h

J

>

h

D

Q.

h

D

18 20 30 40 50 60 70

TIME, ns

Figure 24. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse

of 7 ns duration (FDHM). The peak value of the resultant waveform is 0.915 V.

34
(0h

J

>

h

3

Q.

h

D

10 20 30 40 50 60 70 90 100

TIME, ns

Figure 25. Convolution of a 1 volt 10 ns rectangular pulse with a unit area Gaussian pulse

of 10 ns duration (FDHM). The peak value of the resultant waveform is 0.773 V.

35
To these systematic errors summarized in this section must be added the random errors associated with the individual determinations of energy to arrive at the total uncertainty.

5. Conclusions

A system for measuring the responsivity of detectors of high peak power pulsed 10.6 ym radiation has been developed. By using calibrated beam splitters the methods used are extendable into power ranges where the detectors no longer respond to power in a linear

manner. The insertion loss of feed-through detectors can also be measured. A method has

also been developed to test detectors for response times faster than 10 ns. A technique is

also explained for measuring the diameter of TEMoo Gaussian laser beams.

6. Appendix: Transmittance of a Gaussian

Beam Through a Square Aperture

We assume the irradiance in the beam is qiven by

2 [(x-x ) 2 + (y-y ) 2

l I-Ioe

^

(A-l)

where

I = the irradiance at point x,y,

Io = the maximum irradiance,

a - the "beam size" parameter,

x,y = the coordinates of the point in question,

xo,yo = the center coordinates of the aperture relative to the center

of the laser beam.

We also postulate the aperture is square with dimensions a x a. The power, P, accepted

by the aperture is given as

P = I

Xo+7 J /a * / x o-£

y +£

a

dy e

(x 2+y 2

) (A-2)

We have the following integral in "x" plus a like one in "y"

J(a,x ,a)

If we let t = x/2/a, then

/

x c

a x o~2"

72 X dx (A-3)

36
— (x +a)

J(o,x ,a) = — e" t2

rr I

(A-4)

n

T (x ° +7

} e

L dt + - (-XQ+y)

e _tdt (A-5)

/7

/tt erf

a-^ (x +J)

+ erf ^(-x„- (A-6)

where erf denotes the error function.

A similar expression can be derived in terms of y and the final expression for the

power transmitted is the product of eq (A-6) and its y counterpart. Thus, power transmitted

through the aperture is

P = I ircr erf ^ (x +|) + erf ^ (-x +|)

erf ^

(y 0+f)

+ erf ^ (-y +|) (A-7)

Eq (A-7) gives the power transmitted if the peak irradiance is known. The

transmittance of the aperture can be derived by recalling the total power in the beam is

Pt

= lo

TTcr (A-8)

and dividing eq (A-7) by eq (A-8) yields

-j /2 /2 erf l±- (x n +°) + erf 1± (-x Q +°) o

xy 0^J>

, /? , a, , /Z , a, erf — (yo^ + erf — (-y +^) (A-9)

37
where To = the transmittance.

A computer program can be developed for a solution to eqs (A-7) and (A-9) by recalling

that erf(z) = -erf(-z) and employing the following approximation from reference 15.

erf(z) = 1 (A-10)

(l+a 1 z+a 2 z

2+a 3 z3 +a (+ z

lt +a5Z 5 +a 6 z

6

) 16

where

a

: = .07052 30784

a

2 = .04228 20123

a

3 = .00927 05272

a, = .00015 20143

a

5 = .00027 65672

a c = .00004 30638

6 7 This approximation has an error no greater than 3x10" •

7. Acknowledgments

The author wishes to thank Eric Johnson for invaluable help in deriving many of the

mathematical solutions that form the basis for this work. Also special thanks to Doug

Franzen for his helpful suggestions while the laser was being built. Finally, thanks to Aaron Sanders, Bruce Daniel son, Bill Case, Leonard Schmidt, and Joe Skudler for their

assistance at various times during this endeavor.

38
8. References

1. Jacobs, S. F., et al . , Laser induced fusion and x-ray laser studies, Addison Wesley

Publishing Co., Inc., Reading, MA (1976).

2. Glass, Alexander J., and Guenther, Arthur H., Laser induced damage in optical

materials: 1978, NBS Special Publication 541 (Dec. 1978).

3. Danishevskii , A. M., et al . , Dragging of free carriers in direct interband transitions

in semiconductors, Soviet Physics JETP, 11_, No. 2, pp. 292-295 (Aug. 1970).

4. Kogelnik, H., and Li, T., Laser beams and resonators, Proc. IEEE, j>4_, No. 10, p. 1317

(Oct. 1966).

5. Kogelnik, H., and Li, T., Laser beams and resonators, Proc. IEEE,

J54_, No. 10, p. 1315

(Oct. 1966).

6. Yablonovitch, Eli, Short CO2 laser pulse generation by optical free induction decay,

Applied Physics Lett., 25_, No. 10, p. 580 (15 November 1974).

7. Champagne, L. F., et al .

, A large aperture CdTe Pockels cell for CO2 lasers, Optics

Communications, K3., No. 3, pp. 282-285 (Mar. 1975).

8. Kimmitt, M. F., et al . , Photon drag radiation monitors for use with pulsed CO2 lasers,

J. Phys. E.: Scientific Instruments, 5, p. 239 (1972).

9. Brownlee, K. A., Statistical theory and methodology in science and engineering, John

Wiley & Sons, p. 80 (1965).

10. Gunn, Stuart R., Studies of calorimeter absorbers for CW and pulsed CO2 lasers,

Lawrence Livermore Lab., Rept. UCRL 51854 (1975).

11. West, E. D. , and Schmidt, L. B., A system for calibrating laser power meters for the

range 5-1000 W, NBS Technical Note 685 (May 1977).

12. Danielson, B. L. , and Beers, Y. , Laser attenuator for the production of low power beams

in the visible and 1.06 ym regions, NBS Technical Note 677 (Jan. 1976).

13. Case, W. E., Quality assurance for the NBS C, K, and Q laser systems, NBSIR-1619 (Aug.

1979).

14. Young, M. , and Lawton, R. A., Measurement of pulsed laser power, NBS Technical Note

1010 (Feb. 1979).

15. Abramowitz, M., and Stegun, I., Handbook of mathematical functions, NBS Applied

Mathematics Series 55, p. 299.

39
NBS-1*4A (REV. 9-78)

U.S. DEPT. OF COMM. 2. Gov't Aefcftssiea No, BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-1023

4. TITLE AND SUBTITLE

A System for Measuring the Characteristics of High

Peak Power Detectors of Pulsed C0

? Radiation

3. Recif>*eflf s Accession No,

5. Publication Date September 1980

%1%:ttrtffif,Orsanteati0n ': Ctak

7. AUTHOR(S)

Philip A. Simpson

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

1C Pre k/Worfc Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP)

Calibration Coord Group of the Department of Defense

Air Force Avionics Laboratory

Office of Naval Research

13. Type of Report & Period Covered

1BC^^^^^^c?C^liT

15. SUPPLEMENTARY NOTES

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

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.) A system is described for determining the responsivity of detectors for high

peak power CO,-, laser pulses. The insertion loss of feed-through detectors can also

be measured. The basic approach involves a measurement of the excitation energy anc detector output waveform, and is applicable only for pulse amplitudes where the

detector output voltage is a linear function of the optical power. However, a methc

is described for extending measurements to power levels where the responsivity becomes nonlinear. The time response of the detectors is checked using short duratioi

(10 ns) rectangular pulses to simulate an impulse. A method for measuring beam

diameter is also detailed. An error analysis of the system is given.

t

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Beam diameter; insertion loss; optical detectors; photon drag detector; pyroelectric

detector; responsivity; single longitudinal mode pulse; TEA laser.

18. AVAILABILITY Q^Jnl imited

I | For Official Distribution. Do Not Release to NTIS

[XX Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402

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

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NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's technological marketplace. Order the above NBS publications from: Superintendent of Documents. Government Printing Office, Washington, DC 20402. Order the following NBS publications—FIPS and NBSIR's—from

the National Technical Information Services, Springfield, VA 22161

.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 1 1717 (38 FR 12315, dated May II, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A literature sur- \e\ issued biweekly. Annual subscription: domestic $35; foreign $45.

Liquefied Natural Gas. A literature survey issued quarterly. Annual

subscription: S30.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $45. Please send subscription or- ders and remittances for the preceding bibliographic services to the National Bureau of Standards, Cryogenic Data Center (736) Boulder, CO 80303.
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