^atFonaJ Bureau of library, E-01 Admi

W 2 4 1971

NBS TECHNICAL NOTE 398

U.S.

AENT

OF

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Power and Energy Measurement

of Repetitively Pulsed Lasers
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Nat. Bur. Stand. (U.S.), Tech. Note 398, 10 pages (March 1971)

CODEN: NBTNA

Power and Energy Measurement

of Repetitively Pulsed Lasers

D. A. Jennings

Quantum Electronics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

^ 0F c°*

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Power and Energy Measurement of Repetitively Pulsed Lasers

D. A. Jennings

The problem of measuring average

power, energy per pulse, and peak power

of some of the more common repetitively

pulsed lasers is discussed. The techniques

which have been used at the National Bureau

of Standards are mentioned along with some

of the accuracies obtained. Accuracies of

3 to 1 percent can be achieved, depending

on the laser source and the parameter of

interest.

Key Words: Average power; energy per

pulse; laser; peak power;

repetitively pulsed lasers.

As is often said, the measurement of laser power is a straightforward problem with straightforward solutions. The purpose of these

words of wisdom is to point out some techniques that we have found

useful and some precautions that are important to observe.

The problem of power measurement is overwhelming, to say

the least, in the sense of the ranges to be covered. For example, wavelengths from 300nm to 300g.m, average power levels from microwatts

to tens of kilowatts, peak powers from milliwatts to megawatts,

repetition rates from 1 to 5000 pulses per second and pulse widths of

100(js to 10ns are typical ranges one must contend with. However,

one can do a reasonable job for some lasers at some power levels;

diode lasers around 900nm, Nd: YAG lasers at 1060nm and CO lasers

at 10. 6um, to name a few.
Some of the things we would like to know about laser pulses are

(1) the average power, (2) the peak power, and (3) the energy per pulse.

Several publications deal with the subject of laser energy and power

r* t r ~i p ~i measurements. -I L J yj e j^-ye found two methods particularly

useful for average power levels of 0. 1 to 30 watts: (1) the timed

shutter and liquid cell calorimeter method which can give an accuracy

r3"l of around ±4% ; and (2) the disk calorimeter which is very good for

large diameter beams, and can give an accuracy of around ± 3%.L4J

The peak power of the laser pulse must be less than that which will

damage the measuring device. Typical safe magnitudes for incident

8 , 2

power densities for 30ns laser pulses, are 10 watts/ cm for the

5 , 2

liquid cell calorimeter, and 10 watts/ cm for the disk calorimeter.

The measurement of the energy per pulse is no doubt the easiest

to measure since _ , , Average Power

Energy/pulse = zr—;

-, ;

. By/ v Pulses/second

The accuracy of this technique depends on (1) the energy/pulse

stability of the particular laser system and (2) the knowledge of the repetition rate. The accuracy of the energy per pulse is certainly not

better than the average-power measurement.

The peak power in the pulse is by far the most difficult thing to

measure, and, as in every kind of measurement, it depends on how

much detail one insists on knowing. The peak power is obtained from

measurements of the total energy and the temporal distribution of the

pulse. For example, consider the two pulse shapes as shown in figures

1 and 2. The peak power of a laser pulse, as shown in figure 1, can be

measured though the measurement requires care. A laser pulse with a shape as shown in figure 2 is not so nice, and in some cases, the sub-

-12 pulse width may be as small as 10 s. The question is "What can we
measure?". On the assumption we have a well behaved pulse, i. e. , no substructure, we can measure the energy per pulse. "We also know

that

I

p dt ,

where E is the energy in the pulse and p is the instantaneous power.

So if we have a linear power detector, in principle we can work back

to obtain the peak powers. In what follows, we describe two methods

for measuring the peak power.

As our first example, assume (1) we have a setup whereby we

can monitor the laser pulse with a suitable fast detector to obtain the

temporal behavior of the instantaneous power and (2) that we have

recorded the temporal behavior of the laser pulse intensity and the

energy. (It is assumed the reader is familiar with pulse circuitry).

We will have a photograph similar to figure 3. Let the area under the

laser pulse be A, and the area of one of the grids be a. We know that:

A = B E

A

or 3 = —E

where Bis the constant of proportionality.

We also know that a = Bp t

a or B = -7-

P t

where t is the time per division and p is the instantaneous power/per

division.
tp

Figure 1. Temporal distribution of a well behaved

Q- switched laser pulse.

P

Figure 2. Temporal distribution of a Q- switched laser

pulse which contains substructure that may

not be seen by the detector.

4
P a

t

A

t

Figure 3. A representation of a typical oscilloscope

photograph of the temporal behavior of a laser pulse, from which we may obtain the

peak power.

LASER BEAM SPLITTER CALORIMETER

Figure 4. Schematic and apparatus setup for calibration

of a peak power measurement.
So

a A

or P = Ea

At

and the peak power, P is the number of divisions times p . The ratio

of a /A can be found by the use of a planimeter or one may actually

cut out the grid and the laser pulse from the photograph and weigh

them.

As our second example, consider the experimental arrangement

as shown in figure 4. Again the detector is sufficiently fast and we can

measure the energy per pulse. A simple circuit analysis will tell us

how the circuit works.

We assume a linear detector, i. e. , the instantaneous current is

proportional to the incident power, and we write:

1 = ap

and i dt = a I p dt

or

and

Now

Q = aE

1 =

l =

Q

P . V.— , Q = CV

R c

combining we have

V CV

__R

= c R E

or

V

P = R

o RCV

E.
We note here that E/RCV is a constant, namely—— . Therefore, we

c Rcc

need to measure it only once, and if E is the energy as measured by the

calorimeter, then the power that is measured is that which is transmitted through the beam splitter; that is, we really have a power

monitor.

Several words of caution are due. Since the energy measuring

device measures the total beam, one must assure himself that the

monitor detects a fraction of the total beam. Since some detectors

are not uniform in response over their surface, we must also be

assured that this effect is suitably averaged out. If a beam splitter is

used care must be taken to eliminate interference effects, possibly by

using a wedge-type splitter. Care must be exercised to take into ac- count the polarization properties of beam splitters. Diffuse attenuators

must be used carefully because of "speckle patterns" and the nonLambertian nature of diffusers.

After taking all of these precautions, and more, the accuracy

of the above methods is probably no better than ± 10%.

We have tried to describe some useful techniques for the

measurement of average power, energy per pulse, and peak power for

repetitively pulsed lasers, plus some things to avoid. The techniques

for peak power measurements are good only for the response time of

the detector. The measurement of subnanosecond pulses is another

story.

The author would like to thank Drs. E. D. West and R. L. Smith

for their very helpful comments.
References

1. H. S. Heard, "Laser Parameter Measurements Handbook", John

Wiley and Sons, New York, 1968.

2. D. A. Jennings, IEEE Trans, on Instr. & Meas. IM-15, No. 4,

161 (Dec. 1966).

3. D. A. Jennings, E. D. West, K. M. Evenson, A. L. Rasmussen,

and W. R. Simmons, Technical Note 382, National Bureau of

Standards, Boulder, Colorado, Oct. 1969.

4. D. A. Jennings and E. D. West, Rev. Sci. Inst., 41, 565-567,

(April 197 0).

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