LITERATURE CITED 2 .

2. 3. 4. 5.

H. G. Heard, Laser Parameter Measurement Handbook, Wiley, New York (1968). State Standard (GOST) 25093-75. State Standard (GOST) 8-275-78. State Standard (GOST) 8-276-78. K. P. Aver'yanov, V. V. Bacherikov, V. N. Malnovkin, et al~ Abstracts, 2st All-Union Scientific--Technical Conference on Photometric Measurements and Their Instrumentation, VNIIOFI, Moscow (1974), p. 145.

FLOW-THROUGH METER FOR THE AVERAGE POWER AND ENERGY OF INDUSTRIAL LASER INSTALLATIONS V. K. Volkov, A. P. Palivoda, V. I. Toporov, and B. F. Usachev

UDC 535.3+681.142

The principle underlying the construction of a flow-through meter for average laser power and energy, its characteristic features, and the basis for choosing the principal system elements is described. The results of experimental investigations of the metrologic characteristics, the prospects of using the instrument, and possible ways of its upgrading are reported. In view of the vigorous growth of the use of laser techniques in machine building and instrumentation, much attention is paid to the improvement, development, and manufacture of monitoring and measuring apparatus needed to supervise the control of the radiation parameters of industrial laser system (ILS). The standardized parameters of ILS radiation are the average power and energy. lists the cormnercially produced ILS (up to 1977) and their parameters [I].

The table

It can be seen from the table that the ranges of the standardized parameters are the following: energy, 0.I-250 J; average power, 0.2-20 W; duration, 0.03-7000 ~sec. The commercial instruments developed and manufactured at present (!MO-2, IKT-IM, FN) are incapable of measuring the total ranges of these ILS radiation parameters [2, 3, 4]; the urgency of producing means of measuring these parameters is thus obvious. One of the basic requirements is to develop a meter of the flow-through type, since t h e radiation parameters are frequently insufficiently reproducible, and the processes occurring when the radiation interacts with matter during the ILS operation have not been sufficiently studied. We report here the results of the construction and investigation of a flow-through device for measuring the average power and energy of laser radiation. The operating principle of this device is based on photoelectric conversion of the radiation energy of each pulse and summing the energies of all the pulses that enter the receiver

Translated from Voprosy Metrologicheskogo Obespecheniya Izmereniya Parametrov Tekhnologicheskikh Laserov, pp. 36-45, 1984.

0270-2010/86/0701-0019512.50

9 1986 Plenum Publishing Corporation

19

TABLE

|

Type of ILS :vant - 9 :vant --9M :vant--- I 0 :vant -I 1

J~rad m 1,06

1,06

Erad ~ J

"['0,5, ~ see

1,0 3,0 1,0 I00,0

500 500 500 200

10,0 I0,0 x

r~ep , Hz

Note.

Pav

=

=20 W :rant --I 2 :rant - - I 6 :rant--17 :ristall - - 6 ~istall --7 :rlstall

w -~--~-~--'N

-- 8

:orund

LS --10-1 %K-348 kK-378

-~-

1,06 " -'-"-

] :izil

8,0

30,0 3,0 250,0

_ w..

100- 200 50-76

-

0,I - 0,5

2000-4000

8

0,5 1,5

-'-

;kra - - 8

vet - - 3 0 ~ptin - - 4 8 1 Dptin-~482 :oraU --I

>i 3 , 0 1500-4000 30,0 4000-7000 4,0 1500-4000 0,5- 4 0,I-- 5,0

N

8

20,0 I0,0 0,5-20 4-10 20

0,5-10 0,5 2,8 5,0

0,03 1000-7000

1,o

500-1600 1600 180

2,0

30 3 250

0,4

unit of the meter within a fixed time interval here as

0,5-1,0

(I or I0 sec).

The average power is defined

n

p av

= ,,~ ~L t

,

count

where EE i is the total radiation energy entering the meter within a fixed time tcount. The advantages of the principle underlying the construction of this measurement device are: high sensitivity, fast operation, effective gathering of the information, and measurement in the gas-flow regime. With appropriate improvements it will permit measurement of the space--time characteristics of slowly varying processes. Recognizing that devices used to measure the energy characteristics of ILS radiation must satisfy the requirements of high information content and high speed, it is expedient to use photoelectric radiation receivers as the primary converters. Analysis of the spectral and energy characteristics of ILS shows that the primary converters can be silicon light-emitting diodes, which are widely used for these purposes. Since the ILS radiation can in general be arbitrarily polarized, the measuring devices must be designed to exclude the possibility of making the output radiation of the meter independent of the degree and direction of the radiation polarization. The adopted block diagram of the measurement device is shown in Fig. The optical receiver pickup

I.

] performs the following functions:

a) Attenuation of the measured average power of pulse-modulated radiation to a value that permits the photoreceiver to operate on the linear section of its optical characteristic. b) The entrance pupil of the photoreceiver of the measurement channel must be uniformly illuminated regardless of the structure of the measured-radiation light flux. c) The radiation energy must be converted into electric pulses that can be lengthened and have a voltage amplitude proportional to the radiation energy. d) The transmission (attenuation) state of input radiation.

20

coefficient must be independent of the polarization

i Lr I

i=l

-1!

I, !

Fig. I. Block diagram of meter: I) receiver unit; II) converter; III) electronic pulse counter; 4) printer. I) Synchronization-channel photoreceiver; 2) optical system; 3) measuring-channel photoreceiver; 4) amplitude--time converter; 5) coincidence circuit; 6) strobing-pulse generator; 7) counting-pulse generator; 8) power supply. In addition, a second photoreceiver is installed in the receiver optical pickup for the purpose of generating and shaping electric signals for the synchronization and strobing system. The amplitude--time (A--T) converter transforms the maximum v a l u e s o f the voltage pulses from the measuring photoreceiver into time intervals which are subsequently filled with the counting pulses from the counting-pulse generator. The electronic pulse counter (Ch3-54) stores and sums the pulses counted in a definite time interval (counting time) and feeds the results to a digital display. The printing unit documents the measurement results. A characteristic feature of the construction of this meter is the use, in the optical system, of two two-step attenuators, each based on reflection from two glass plates with a 45 ~ angle between their planes (Fig. I). The distinguishing feature of this system is that while decreasing the power of the incident light it does not alter its polarization, so t h a t the attenuation at the exit from the optical pickup and at the entry into the photometric sphere in which the measurement-channel photoreceiver is placed does not depend on the polarization of the entering radiation. Such a scheme has been considered in the literature a number of times [4, 5]. The photoreceiver of the measurement channel is an FD-7K silicon photodiode, which has a large receiving area. Photodiodes are capable of receiving in the linear operating regime a current pulse imax exceeding I A without exhibiting the fatigue possessed by vacuum photocells and photomultipliers; this is particularly important when working with pulse-modulated radiation of lasers operating in a wide range pulse-repetition frequencies. The photoreceiver of the synchronization channel generates the electric pulses for the synchronization and strobing system. The photoreceiver is an FDIOK photodiode having good temporal characteristics. The converter unit transforms the input signals from the receiving unit into a sequence of pulse packets of frequency equal to the frequency of the optical pulses. The number of count pulses in each packet is proportional to the optical-signal pulse energy. The recording unit stores and sums the pulses counted in a definite time interval, produces a digital display of the measurement results, and feeds the information to the printer. The recording unit is the industrial Ch3-54 frequency meter operating in frequencymeasurement regime based on counting the periods of the measured signal within a definite time interval (counting time). The counting time is chosen in accord with the repetition frequency of the count pulses (I or I0 sec). The development of the meter included investigations of the optical receiving pickup, of the characteristics of the electronic circuitry, including a determination of the linearity of the transfer coefficient of the scaling amplifier (the A--T converter). To exclude effects of the uneven band characteristic of the measurement-channel radiation receiver, as well as to attenuate further the signal in the receiving pickup, an integrating

21

To measuring photoreceiv~r To synchronization photoreceiver

Output radiation

"<..L O,

~"

Fig. 2. sphere was used. glass.

Input radiation

Ray paths in the optical system of the meter.

For better integration,

the entrance to the sphere was covered with ground

The uneven band characteristic was investigated experimentally by scanning the surface of the ground glass, with a laser beam of ~l-mm diamter, in two mutually perpendicular directions. The results of the measurement of the band characteristic of the receiving pickup are shown in Fig. 3. The maximum deviation from the mean value does not exceed due to the measurement errors.

1.2%.

This error is in fact

To permit measurements of the average power in the range 10-2-102 W, the receiving pick i up was provided with a turret of calibrated diaphragms that covered the appropriate band. The experimental investigation of the measurement characteristics accordance with the diagram shown in Fig. 4.

f...

.

IQ

9

9

9

of the meter was in

9

0,s

J * l , l . n l l i . , , , * . J , , . l l n , , I , . l l l l l s s l l l l i l

@ Fig. 3.

22

fO

~

~ mm

Band characteristic of receiving pickup of meter.

%

S

7

Fig. 4. Average power measurement (APM) checking system. I) Adjusting laser; 2) rotary prisms ; 3) set of attenuators; 4) diaphragm; 5) APM receiver unit; 6) converter with recording device; 7) receiver of 0SI-33 APM unit; 8) recorder for OSI-33 APM unit; 9, 10) 0.69- and ].06-pm radiators. The measurement errors consist of random and systematic components, as well as of the errors of the devices used to check the measuring devices, which are also sums of random and systematic errors. The systematic and random errors are summed separately. The systematic error is estimated from the expression Osyst

= I,.I

+8 ~ 2+...+

8

,

where 01, 0a, etc. are the relative values of the components of the systematic error (the coefficient |. | determines the range of the measured quantity with a fiducial accuracy P = 95%). The random component of the error is estimated from the expression D =

y o 2I

2 +D2 +

"'~

+D2

.

n).

where DI, D2 etc. is the ratio of the fiducial interval for each of the random components of the error, determined a probability P = 95%, to the m e a n value of the measured quantity. The resultant APM error is defined as the sum of systematic and random components

The conversion function for the APM can be written in the form

p av

~"/ ~count

where E i is the energy per pulse, joules and tcount is the counting time. The summation is over n pulses entering the meter in an interval of ] or I0 sec. relative error in the measurement of the average power can then be defined as

The

The time-interval error is determined by the accuracy of the frequency meter used for this purpose and is 10-4-10-3% for the Ch3-54 frequency meter (at a ;-sec formation interval). This error component can be disregarded hereafter.

The error ~(EEi) has both systematic and random components. Among the systematic components are: a) the error due to the discrete character of the measured information on the radiationpulse energy fed to the electron counter (01); b) the error due to the nonlinearity of the characteristic of the 03, 0~ registration channel. The random errors include those connected with the method of determining the averagepower conversion coefficient. 23

The error component due to the nonlinearity of the flow-through characteristic of the meter is determined in the course of the experimental check of the conversion coefficient in the entire range of the recorded energies. The error component due to the discrete character of the measured information fed to the electron counter is due to the lack of synchronism, on the one hand, between the counted quartz-generator pulses and the start of the packet formation, and on the other between the time that the recording devices begin to count and the incidence of the light pulses. As already noted, the random components of the error are governed by the method used to define the conversion by the instrument. The conversion coefficient was determined by the scheme illustrated in Fig. 4. The conversion-coefficient

error was determined from the equation

~,

_

gt. /

Z"(JKni ~'~

n (n-t,

An experimental investigation of the error components has shown that the total error does not exceed I0%. The instrument is provided by checking devices since, in fact, it measures the energy of a single laser pulse, it can be checked by the same scheme as used to in the laser-energy measurement system. The result of our research is a flow-throughaverage-power nical specifications :

mete= with the following tech-

Spectral range, pm ...................................................

0.5-I.l

Measured average power range, W ......................................

IO-~-i02

Measured energy range, J .............................................

0.0|-2

Range of single radiation pulse duration (at half maximum), see ......

(5-200)" 10-9

Input aperture of meter, mm ............. , .... ; .......................

40

Main relative measurement error, %, not more than ....................

10

Pulse repetition frequency, Hz .......................................

2-100

The main advantage of this instrument is its effectiveness and ease of calibration of the photometric instrument (information on the e n e r g y and average power at the input of the calibrated instrument is obtained almost immediately). The absence of a monitoring channel in the checking system increases the measurement accuracy appreciably. Further improvements of the instrument should be made along the following lines: 1) lower the measurement error; 2) use the instrument to measure cw-laser power; 3) expand the spectral range of the recorded radiation; 4) expand the range of durations of the recorded radiation. LITERATURE CITED |. 2. 3. 4. 5. 6.

24

K . I . Krylov et al., Use of Lasers in Machinery and Instrument Manufacture [in Russian], Mashinostroenie, Leningrad (1978). N . N . Gurevich, Introduction to Photometry [in Russian], EnerF,iya, Leningrad (1968). Ch. Fabry, General Introduction to Photometry [Russian translation], ONTI, Moscow--Leningrad (1934). State Standard (GOST) 8.207-7v. H . G . Heard, Laser Parameter Measurement Handbook, Wiley, New York (1968). V . S . Belkin and Yu. G. Kriger, Prib. Tekh. Eksp., No. 6, 147 (1975).