JPRS ID: 10137 EAST EUROPE REPORT SCIENTIFIC AFFAIRS

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• Collection: JPRS
• Document Number (FOIA) /ESDN (CREST): CIA-RDP82-00850R000400070052-3
• Release Decision: RIF
• Original Classification: U
• Document Page Count: 10
• Document Creation Date: November 1, 2016
• Sequence Number: 52
• Content Type: REPORTS

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JPRS L/ 10137
~4 November 1981
,
_ E ast E u ro e Re ort
_ p p
SCIENTIFIC AFFAIRS
- cFOUO , o~R 1;
- FBIS FOREIGN BROADCAST INFORMATION SERVICE
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_ JPRS Z/1013'1
24 November 1981
. EAST EI~ROPE REPORT
$CIENTIFIC AFFAIRS
' (FQUO 10/81~
: ~ONTENTS
i
~ CZECHOSLOVAKIA
Photodiode Laser Radiation Uetector Described ~
(Frantisek Lindner, et al.; CESKOSLOVENSKY CASOPIS
PRO FIZIKU, Jun 81) 1
- a - [III - EE - 65 FOUO]
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CZECHASLOVAKIA
PHOTUDIODE LASER RADIATiON DETECTOR DESCRIBED
Prague CESKOSLOVENSKY CASOPIS PRO FIZIKU in Slovak No 3, Jun 81 pp ?.72-276
[Article by Frantisek Lindner, BA2 National Enterprise, Bratislava; and Anton
_ Strba and Pavel Vo~tek, Department of E:.-p~rimental Physics, PFTJK, Bratislava:
"Detection of Laser Radiation With a Semiconductor Photodiode"]
[Text] Laser Rad:ation De�ection by the Help of SEmiconductar
Photodiode
This article analyzes the possibilities for use of the 1PP75
silicon semiconductor diode for detection of helium-neon (He-Ne)
~ and ruby laser radiation. The spectral sensitivity of the
- diode and its range of linear response to radiation ilux and
pulse energy are determined.
1. Introduction
Alongside thermal detectors such as calorimetric and modern pyroelectric detec-
tors, photoelectric radiation detectors continue to be a main topic of interest.
These devices incl~de semiconductor photodetectors, an important feature of
which is their integrating capability, and particularly semiconductor diodes.
The magnitude Id of the current which an illuminated photodiode passes into an
external circuit can be determined from the well-known Shockley equation [1]
qU
(1) 1~ - I~ - (eXP AkT ~ l
where If is the photoelectric current, I~ is the saturation current of the
diode in the forward direction at temperature T, q is the magnitude of the
elementary charge, U is the voltage across the diodp, A is a coefficient char-
acterizing the p-n junction, and k is the Boltzmann constant. The photoelec-
tric current If is a linear functi4n of the radiation flux ~ which prod~ices it:
_ ~2~ 1r = s~~
1
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where S~ is the spectral sensitivity of the diode. Its magnitude for ~ given
wavelength S~ depends on the material ar~d design of the diode.
2. A Detector for Continuous Laser Radiation
In practice, two ways of connecting the diode to the external circuit are nost ~
frequently encountered, the gate connection (Fig. la) and tl:e resistive connec-
tion (Fig. lb). In both circuits, the resistance R represents the load resis-
. tance and the internal resistance of the diode connected in parallel.
In the gate circuit, the voltage U in Equation 1 is equal to the voltage UR
across resistance R. The incr~ase in the current Id resulting from illumina-
tion of the diode changes the voltage across resistance R, which in turn
affects the magnitude of the current Id. In this case the diode current Id and
the voltage UR will not be linear functions of the radiation flux ~ according
~ to Equation 1. Linearity is maintained only for a small voltage UR, such that '
qUR � AkT. Under these conditions the second term of Equation 1 can be ignored
and tne current Id will be equivalent to the photoelectric current If.
Id Id
R UR R UR
Ub
a b
Figure 1. Connection of the Photodiode to the External Circuit: a) gate;
b) resistance
I
In the resistive connection (Fig. lb) the voltage U is equal to the difference
between the power supply voltage Ub and the voltage UR. A suitable selection
of this bias voltage Ub and the resistance R will satiafy the condition
(3) exp qU : I
AkT
and according to Equation l, the diode current Id will be a linear function of
ttie radiation flux ~ with the form
(4) ~d=Sa~~+/�
Accordingly the current Ia can be corisidered a measure of the radiation flux
Because of the small. magnitude of the current, it is more convenient to measure
the voltage UR across resistance R:
_ ~
2
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~5~ UR = S,R~ + !oR
Th~ photoelectric voltage Uf = S~ R~ can be separatea from the dark satura-
tion voitage UOR - IoR by inter~ittent ill~ination or ane-time illumination
of the diode. The photoeZectric voltage Uf will be the pulse component of the
voltage UR and can easily be measured through a buffer capacitor using an
oscilloscope or some other measuring device. TY~e sensitivity S~ of such a
detector, which depends on Uf, is
~6)
S~=S,R
The magnitude of this quantity can be varied linear].y by changing the resis-
tance R. The spectral dependence of the sensitivity S~ is govemed by the
nature of S~ as a function of the wavelength. Because of their spectral sensi-
tivity, commonly used silicon diodes are suitable for use in this manner to
detect the radiation ~lux of He-Ne lasers.
The maximum current which can be recorded by a detector with linear response
= depends on the maximum value of the voltage UR at which condition (3) can still
be met. The condition for the maximum photoelectric voltage Uf = UR - YOR fol-
lows from Equation S. It is assumed that there is no saturation of the photo-
electric effect in the vicinity of the p-n 3unction. The threshold flux for
registration of radiation depends on the noise characteristics of the diode.
The properties of such a radiation flux detect~~r using a 1PP75 diode can be
seen �rom the experimental curves for photoelectric voltage Uf as a function
� of the radiation flux from an He-Ne laser which are presented in Fig. 2. In
this case, continuous He-Ne laser radiation interrupted by a mechanical modu-
lator was scattered by a frosted screen located dirPCtly in �ront of the
diode. The curves for Uf(~) given in Fig. 2 also shows the change in sensi-
tivity S~ as a function of R. These curves can be used to determine the spec-
tral sensitivity of the 1PP75 diode and frosted screen to He-Ne laser light
(a = 632.8 nm), i.e., S~ = 25 �l mA/W. Thus, by a suitable choice of R, it
_ is possible to measure radiation flux in a range of from 10'3 to 10~ MW by this
method, which exceeds the measuring range of the TKGM-203 Czechoslovak-produced
detector now available.
to'
u
, ---~y_~~Z
- tv~ u,-uo;
~o� ~
~
,
3
~p' Figure 2. Amplitude Characteris-
Z' tics of the He-Ne Laser Radiation
~ Flux Detector Using a 1PP75 Diode
10~ With Different Values f~or Resis-
_ o~~ tance R: 1- R= 4k7; 2: R= 43 k;
� " 3: R = M27.
. ,
103 ~
10_' tO.Z 1q, ~~rti~V~ ~po
~ 3
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3. A Laser Pulse Er.argy Detector
The detector's ability to register single radiation pulses and the spectral
sensitivity range of the 1PP75 photodiode (360 to 1,100 nauometers) [2J make.
it possible to use the detector for detection of ruby laser pulses as well
(a = 694.3 nm). But because of the large radiation flux, an attenuator must
be placed before the photodiode. This prevents saturation or auppreseion o_`
the photoeffect in th~ diode and assures linear reaponse to the incident
radiation.
One suitable type of attenuator is a cavity attenuator, which is shown schema-
tically in Fig. 3. It consists of a hollow cylinder with circular openings in
the ends. The openings are covered by frosted screens M. One opening is used
for the radiation input and the other for the output. The radiation is prevented
from passing directly through the attenuator by movable disk D which is the
same size as tiie input opening. The transmissivity T of the attenuator can be
changed by changing the interior coating or by varying the distance L between
the disk and the input opening (Table 1).
L
I M D M I
Figure 3. Arrangement of Cavity Attenuator
Tatrle 1. Transmissivity of Cavity Attenuator WiCh Different Inner Surface
Coatings
Cylinder surface Ends Disk L(mm) T
- White white 3.6 � 10-2
White white white 40 8.8 ' 10-3
- Black white white 40 4.2 � 10-4

fi ~
In detecting ruby laser pulses with a typical width of 10-e sec, the intrinsic
capacitance of the diode and of the leads in Fig. 1 cannot be ignored. These
- can be represented in the form of a capacitor with capacitance C connecte~l in
parallel with resistance R. The time constant RC of the resulting in..tegrating
c~.rcu{*_ :nakes it practically impossible to monitor changes in the radiation
flux ~(t) during the pulse. The time dependence of the photoelectric vol~tage
Uf(t) will be different from that of the incident light pul~e ~(t). Fig. 4
shows the response Uf(t) of the detector to a single radiation pulse for differ-
ence values of the ratio RC/T. The pulse has the form ;
I
- 1
4 '
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(7) _ ~w cosz a r t`
T )
where ~,q is the maximum flux in the pulse and T is the puls~ width at the
~r,/2 level. The pulse duration is 2T. The magnitude Uf(2T) c~f the photo-
electric voltage across resistance R after the end of the light pulse can be
expressed in the form
. UM ~
(g) Ur(?rl = ~ : - exp(-2t/RC))
i +t
rzRC
- where UM = S~ ~,qR. For small values of T/RC, i.e. when RC � T, Equation.8
- can be simplified as
~9 ) UM 2T ~Mz
U