JPRS ID: 9734 VIETNAM REPORT

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• Collection: JPRS
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• Sequence Number: 30
• Content Type: REPORTS

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JPRS L/9980
14 Septeimber 1981
Worldwid~ Re ort
p
TELECOMMUNICATIONS POIICY,
RESEARCH AND DEVEIOPMENT
CFOUO 13/81)
Fg~$ FOREICN BROADCAST INFORMATION SERVICE
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NOTE
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Unfamiliar names rendered phonetically or transliterated are
enclosed in parentheses. Words or names preceded by a ques-
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The contents of this publication in no way represent the poli-
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JPRS L/9980
' 14 September 1981
WORLDWIDE REPORT
TELECOMMUNICATIONS POLICY, RESEARCH AND DEVELOPMENT
(FOUO 13/81)
CONTENTS
WORLDWIDE AFFAIRS
Briefs
Japan, Libya Satellite Station 1
WEST EUROPE
~ ITALY
I Light Powered Optical Telephone Receiver
(Alberto Broaio, et al.; ELETTRONICA E TELECOMUNICAZIONI,
~ Mar-Apr 81) 2
~
i
I
~
!
i
~
;
, _ a _ [III - WW - 140 FOUO]
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WORLDWIDE AFFAIRS
BRIEFS
JAPAN, LIBYA SATELLITE STATION--Nippon Electric Company (NEC) said Saturday the
compan~ has clinched a 7.9 billion yen satellite communication earth station deal
with the Libyan Government. The campany said the contract awarded by the Libyan
_ Department of Communications and Maritime Transport calls for supplying 14 earth
stations ror domestic satellite com~unication on a full turnkey basis. NEC said
it is the largest single earth station export dea~ arranged by the company in
~ recent years. The company has exported earth stdtions mainly to the United States,
, Canada and Thailand. Under the contra~ct, two stations will be completed by
October this yeaz and the remaining 12 stations by September next year. The company
said technical guidance would be p~ovided to the Libyans in operating and maintaining
the stations. The deal is on a yen-payment basis, and 15 percent of the price was
paid when the contract was concluded with the balance payab].e at the time of shig-
ment, NEC said. [Text] [Tokyo THE DAILY YOI~IIliRI in English 24 Aug 81 p 4]
CSO: 5500/2296
I
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ITALY
bIGHT POWERED (1PTICAL TELEPHONE RECEIVER
Rome ELETTRONICA E TELECOMUNICAZIONI, in Italian Mar-Apr 81 pp 55-64
[Article by Dr of Engineering Alberto Brosio, Mauro Perino and Paolo Solina of
the CSELT (Telecommunications Research and Study Center) ilf Turin: "Optical-
Fiber Telephone with Remote Powering of Receiver"]
[Textj Summary--A low-consumption light-powered telephone on optical fiber: Optic-
al fibers utilization is extending to short, medium and long distance links, and
their introduction in the loop netw~~rk is foreseeable. In this view, it will be im-
portant to provide the telephone service over the fiber link; however, service con-
tinuity, even during power-line failures, should be preserved. At CSELT laborator-
ies, a bidirectional speech-transmission system has been implemented in which the
i subscriber set has been light-powered through the optical fiber by the local elec-
trically powered exchange. Link length of about 550 m has at present been reached;
however, a 2-km link length is considered easily achievable by using ad-hoc compon-
~ ents that are at present being investigated.
I. Introduction
The beginning of optical-fiber telecommunications can be seen as going back to 1970,
the year in which two fundamental events came to fruition: the obtaining, by Corning
Glass Works (United States), of the first optical fibers with attenuation less than
20 dB/km, which was judged the limit for use of the fibers themselves in the field
of telecommunications; and the achieveme*.:t, in the Bell Laboratories, of the first
semiconductor laser functioning continuously at ambient temperature. Starting in
that year, the laboratories of the most highly industrialized countries, with the
CSELT among them, began to work on communications in optical fibers.
I Ten years after those events, it can be seen that the progress in this sector has
' been so rapid as to exceed the most optimistic forecasts. Optical fibers are now
becoming a reality in the field of connections between uLban exchanges and short-
distance interurban connections: worldwide, there are now some dozens of installa-
tions in public service, with industrial-type transmission systems, so that it can
be said that for this type of connection, we are now in a transition from the re-
search phase to the industrial and operational phase.
The research-laboratory efforts are now being concentrated mainly in two other sec-
tors: large-capacity and long-distance connections, and th~ connections between ex-
clnanges and subscribers.
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As regards the long-distance connections, it is essentially a matter of exploiting
the possibilities offered by transmission at the wavelengths around 1.3 um and
1.5 um, called also the "second window" and "third window" of transmission, which
make it possible to obtain regeneration pitches on the order of 50-100 km and even
more; in this way it is possible to avoid introducing into the optical cables con--
ductors for remote powering of the regenerators, inasmuch as it is always possible,
with such pitches, to design around a local power supply.
For the distribution network, the outlooks are hazier for now, but in the principal
- laboratories the conviction prevails that optical fibers will be able, in the not too
distant future, to offer great advantages in this area also. Indeed, their great
band width, low attenuation and insensitivity to interference will make it possible
to offer the subscriber a whole series of new services with higher quality at lower
costs than those obtainable with the traditional copper cables, in addition to nor-
mal telephone service.
On the basis of these considerations, the CSELT has undertaken a feasibility study
for development of an "optical telephone"--that is, a system for bidirectional
transmission of telephone conversation between exchange and subscriber in a single
optical fiber. ~
The objective set with regard to the sizing of the system was to maintain a funda- ~ '
mental characteristic of the telephone connection--that is, remote powering of the
subscriber'a set--that guarantees continuity of service even at times when power is
not available in the public network. To achieve this, it is necessary for the power :
for 3ctuation of all the functions of the set (transmission, reception, signaling of
microtelephone position, selection and excitation of ringing mechanism) to be pro-
vided optically from the exchange through the fiber itself.
The solution approaches so far proposed in the literature follow two main lines.
Systems based on direct optoacoustical conversion, by means of nonconventional
transducers developed for the purpose, have been proposed by Bell Laboratories (Bib-
liography l) and by Siemens AG (Bibliography 2); an experimental development of
this type of approach using, on the exchange side, two laser sources at different
wavelengths, is reported in Bibliography 3. The indirect (that is, optoelectrical- ~
electroacoustical) conversion technique, which uses a photovoltaic cell, has also '
been taken under consider,stion by Bell, which has obtained interesting results by ,
using special optoelectronic components developed for the purpose in its own labora-
tories (Bibliography 4, 5).
In the experimental prototype developed in the CSELT laboratories, a solution based
on indirect conversion has heen pursued: this approach, together with work to opti-
mize both the choices for the sizing of the system and the efficiency of the circuit
solutions~ has made it possible to construct the Ctetotype with the use of optical
and optoelectronic components obtainable on the market.
2. Description of the Receivers and CriCeria for Sizing
In the sizing of the receivers for a subscriber-connection network based on optical-
_ fiber carrier, two essential requirements were kept under considesation: remote pow-
ering, and the possibility of exciting~the ringing mechanism from the exchange. In
both cases, the difficulties are related to the limited quantity of power that can
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be transferred from the exchange to the subscriber in a fiber connection, by compar-
ison with what is obtainable with the conventiona]. loop.
The first requirement has conditioned the choice o~ the optical and optoelectronic
components and has necessitated optimization of the subscriber-set's circuits from
the point of view of output and dissipati.on.
F:,r the ringing mechanism, since solutions of conventionaY type cannot be used, a
higher-output transducer of piezoelectri~ type, developed in the CSELT laboratories,
has been employed.
Below are described the various elements that make up the system, with the considera-
tions that have conditioned the design choices. Reference is made to the simplified
block diagram of Figure 1.
~ ' ~ ~ ATTACCU O~UTENTE ( 7 )
/
! j'~ ~ -i ~ c\~ 1 ~6~ I~ElMRATORE~.~ ~
~1 % (1 ~E otti~T~ \ ~ CAVO OTTICO ~ onico ~ 1~pTORIVEU~one~
_ a+ r - (
~ ~ f ~?'J SUONER. ~ I 111C8V. I
I~ ~ p~F ` ~ CENTRALE
I 1 (
II I RICEVR. ~ 1 I IASEII TRASM. I `1~~
i rl a3v ~o~ ~ I 4arm ~5) ~
CONV. VOLTAICA ~~~7Nm 1 I
~ 1 ` ~ CC CC ~ 8 ~ I ~ ~ ~ ~ ~
~ 11ii~ 1 ~
~ ~ ~ TRASM. ~
/ 1` ~i
~ ^ ` \ \ ~ ~ ~ ~ ~ ~ ~
I~
`1 / ~
~ ~ 8641
Figure 1- Simplified block dia,ram of the optical-fiber telephone system
Key:
1. Optical separator 6. Optical cable
~ 2. Ringing mechanism 7. Subscriber-set connection
3. Receiver 8. Photovoltaic cell
4. DC-DC converter 9. Photodetector
' 5. Transmitter 10. Exchange
2.1 Optical Sources and Modulation Technique
The optical source on the exchange side is composed of a multiheterostructure (Ga-
A1-As) semiconductor laser, with emission peak at a wavelength of 820 nm, selected
by the supplier for more than 12 mW of optical power emitted in air.
The source is kept turned on for about 98 percent of the carrier period (50 micro-
seconds), so as to ensure high transfer of inean optical power to the remote-powered
set.
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a) sENZa Moax.azior~ (1)
-~i ~ ~ i-
ACC~lO
~2~
- ,a ~w
s~iNTO '
- r---~s----~+ ( 3 ) I
ww I
I
b~ CUN MODULAZIONE, 4, OURATA '
~
~
un
Figure 2- Laser source in the exchange-end connection: pulse-width-modulation (PWM)
technique
Key:
1. without modulation 3. Off ~
2. On 4. With pulse-width modulation
r
imw~
tZ
t0
8
e ~
~
4
~ ~
2
1~� 173 mA
0
0 60 1C0 1 1d0 ImA1100
Figure 3- Laser source in exchange-end connection: current-optical power in air
characteristic
~e duration of the dark intervals is modulated by the voice signals (Figure 2).
The modulation technique used (pulse-width modulation, PWM) pruves advantageous in
this case inasmuch as it makes it possible to eliminate any eventual nonlinearity
effects introduced by the source because of the presence of discontinuity in the op-
tical power P-current I characteristic (Figure 3);.this type of modulation also
makes it possible to transmit a power with a mean value very close to the peak value.
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a) s~za Moa~~azioNE ~ 1
r~~r~
1
AC~Gp4
~ r'~
!R[MITO
43>
I~--,-,~�~-'I
b~ COM MOOULAZIONE IN AMP(~,A
I
ra
Figure 4- LED source in the subscriber set: pulse-amplitude~odulation (PAM)
te~hnique
Key:
1. Without modulation 3. Off
2. On With pulse-amplitude modulation
16 /
P
(~W) ~
,2 /
_ ~
~
8
~
4
i ~ '
~ ~
0
� 0 2 I 4(mA) 6
Figure 5- LED source in the subscriber set: current-optical power in �iber
chasac[eristic
Since the fr~equency of repetition of the pulses (per Figure 2,'f~ = 106/50 =
20,000 Hz) rs two times greater the sound band, the useful component of the modulat-
ed signal is easily separable, at reception, with a low-pass filter [as published].
6 ~ .
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The question of the optical source to be used at the subscriber end led to the use
of a high-radiance light-emitting diode (LED), which makes it possible to have a re-
latively high value of emitted optical power with low modulation-current values.
The device is activated by a succession of current pulses, fuYnished by the local
oscillator (which is part of the TRASM [expansion unknown) block in the subscriber
set shown in Figure 1) and having a repetition frequency fu of about 10 KHz with a
full-vacuum ratio of ~1 percent.
The choice of the transmission technique, consisting in amplitude modulat;.on of the
aforesaid pulses (Figure 4) by the voice signal (pulse-amplitude mo~ulation, PAM),
makes it possible to use the device in a zone of the optical power-current charac-
teristic (Figure 5) in which the quantic efficiency is higher than at the origin, so
that for equal power consumption, a higher output is obtained than is obtainable
with baseband analog transmission.
- A double-heterosplice (In-Ga-As-P/In-P) LED has been used in the experimental proto-
type, with light-emission peak at the wavelength of 1.27 um; the peak power thrown
into the fiber, in the ahsence of a modulating signal, is 8 uW.
The choice of the two different wavelengths is related to the problem of separation
of transmission directions in the subscriber-end connection; this will be discussed i
more fully below.
~
2.2 Photodetectors ~
The photodetector used in the development of the exchange-end connection is an APD
(Avalanche Photo Diode) of germanium, on the basis of the choice made for the _ :
remote-powered terminal source--that is, the 1.27-um LED.
The choice of the subscriber-side photodetector has to be made on the basis of the
double necessity of providing maximum mean optical-power conversion efficiency so
as ensure correct power supply to the terminal, and of guaranteeing an appropriate
frequency for detection of the voice signal.
In view of the fundamental importance to the system of the problem of remote power- ;
ing of the subscriber terminal, special attention has been devoted to the search for
high-efficiency devices for optical-electrical conversion and to the characteriza-
tion of them.
In the laboratory tests done, several types of silicon PIN1 photodetectors available
on the market have been considered. The one that provided a higher output was a PIN
of the UDT [expansion unknown], which, with an incident optical power of 2 mW, fur-
nished an output equal to 10.7 percent.
Consideration was then given to a photodetector designed by SGS-ATES [expansion un-
known] for use as a solar cell and optimized to function in solar-concentration sys-
' tem~ (Bibliography 6). This component, illuminated with the same optical power of
2 mW, furnished an output close to 16 percent with a notable advantag.e for the spe-
- cific use considered.
, 1. PIN = p-i-n (donor-doped silicon - intrinsic silicon - acceptor-doped silicon)-
silicon detector.
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T?~e I- V characteristics of the two componernts, [TD'T an~ SGS, are compared in
Figure 6.
i2o
~ ? o~~A c ~ y
~~A) 1,235 mM0
100
~,s ~~e 91 A
~
80
~
. 60
40
~ ~
~
0
0 0,5 t +.,5 ~ ?,5 3
Figure 6- V- I characteris~~tbcs r~f c::4 silicon cells (UDT and SGS-Ates)
Key:
1. Optical power
~zoo
~ CELLA SGS-O:fES POTEN2p
a) t
k~ 07Titn l 3
2mW
7COo
i5,8�~ 0325V /
Z � 975 ~+A
tl00
(2)
CELLA u0T
600
~
~ _ ~Q.~~o 0,37Y /
590 ~+A
400
200
~
, 0
0 0,1 02 Q3 Op QS
Figure 7- V- I characteristics of the galZium-arsenide cell (four elements con-
nected in series)
Key:
1. SGS-ATES cell 2. UDT cell 3. Optical power
i
; By optimization of this component for the specific functi~n r.c:: considered, its out-
' put can be further increased. .
From Figure 6 it is seen that the voltage corresponding to maximum output is on the
order of 325 mV.
~
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In view of the fact that the circuits present in the teiephone te~inal in the lab-
oratory experiment a supply voltage of 2V, it would be possible to fuinish this
voltage directly with the conversion device by the use of several photovolt3ic ele-
menCs in series. But since each of the elements constitutes a current generatoi'
whose value is proportionate to the incident optical power, it is necessary, so as
not to reduce the output, to ensure that all the photodetectors are illuminated by
the same optical power; the resultant problems of alignment, together with the
- losses due to the inert zones of separation between the elements and to the drop in
the output of the individual elements with decrease of the incident power, have led
to rejection of the structure with several cells in series when the numbEr of ele-
ments is higher than four.
In the terminal built, the 2V voltage for powering the circuits is consequently ob-
tained with a DC-DC current converter (cc-cc in Figure 1) starting from the ~-0.3 V
available from the photodetector, with a conversion efficiency on the order oi
50 percent.
Nonetheless, the testing toward defining of the optimal configuration of this ele-
ment of fundamental importance for the functioning of the system is being devel-
oped. In particular, a solution that uses gallium-arsenide cells connected in ser-
ies is under examination.
The higher voltage in vacuum, together with the better output of the individual ele-
ments, has made it possible to obtain, in the laboratory tests with four elements
connected in series, a continuous voltage of 2 V with a conversion efficiency of
16.8 percent (Figure 7), eliminating the problem of intrinsic power loss inthe:.DC-DC
converter. Enhanced performance characteristics are anticipated from optimization
of the efficiency, form and gridding of the individual elements.
2.3 Choice of Fi3er and Coupling Problems
For optimization of the transmissien medium, fibers should be chosen that have high ~
core-diameter and numerical-aperture values, so as to facilitate coupling with the
sources and therefore inject higher power levels.
The lowest possible fiber-introduced attenuation values will provide for a greater
subscriber-connection length, while a relatively high band width will make it pos- ;
sible to expand the auxiliary services planned on the subscriber hookups. ~
Several things must be considered for correct sizing of the system as regards the
couplin~ between sources and fibers. The parameters related to the source that in-
fluence the coupling of it with the fiber are, on the one hand, the geometric dimen-
sions of the emitting surface, and on the other, the form of the radiation patterns
relative to the planes parallel and perpendicular to the connection (Figure 8).
For correct laser-to-fiber coupling ~exchange side), diameter values for the core o�
the fiber used that are higher than the maximum image dimension (25.5 um) are suffi-
cient, as is .~.obvious from Figure 9 and Table 1, which show the attenuations of to-
tal power emitted by the source in the opticat coupling with a fiber of numerical
aperture NA = 0.22 for various numerical-aperture and focal-aperture values of the
objective, considering a source having typical dimensions of 0.254 X 12.7 um.
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~ ~ .
: ti
~ ;
~ ,
_ ~ ~
- o,e
- ~ ~
` o) anocowwe
~ ob ~ �
r ~
~ i ~
i ~
's /
~ 04 ,
F i a) vAiIAlLE10`
= 4~
(1)��2 . .
:
. .
�eo ~o ~o o ~o eo
~'Z ~ GRA01 Mw
_ Figure 8- Laser radiation patters: a) relative to the parallel plane at the con-
nection; b) relative to the orthogonal plane
Key:
1. Relative intensity 3. Orthogonal
.2. Degrees 4. Parallel
Table 1- Qptical Coupling between LASER or LED and Optical Fiber
The second objective has the following characteristics: f2 = 18 mm, magnification 7X,
NA = 0.2. For various paramters of the first objective, the dimensions of the
source image on the optical fiber and the coupling losses, with both laser and LED,
are given. For the fiber, NA = 0.22
First Objective LASER LED
Magnifi- Image Di- Attenua- Image Attenua-
fl (mm) cation NA mension (um) tion* (dB) dimension tion* (dB)
- diameter ~
~ 18 7X 0.2 0.254 X 12.7 6.4 50 14
- 16 lOX 0.3 0.286 X 14.3 4.9 56 10.4
- 10 18X 0.45 0.457 X 22.85 3.7 90 6.8
9 20X 0.5 0.508 X 25.4 3.4 100 5.9
* The losses introduced by the objectives are comprised in the values given.
In the same Table 1 are presented, for various values of the numerical aperture and
the focal length of the objectives previously considered for coupling with the las-
er, the values for attenuation of the total optical power emitted by the source in
coupling with a fiber having again a numerical aperture of 0.22, still with refer-
ence to the diagram of Figure 9.
The calculations were carried out considering a source radiation pattern of spheric-
, al type (Figure 10) and assuming the typical value of 50 ~tm as the diameter of the
emitting area of the LED source.
~ On the basis of these considerations, use of fibers with core diameter greater than
100 um proves advisable.
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N(1R OFNiCIAL [1SF. ON1.Y
oe., oa. s
i
~~s[n ~
---------i-------
a~ -I- ~
~
~z
h
~ ai
~
. INI
Figure 9- Diagram of optical coupling between source and fiber
Key: 1. Fiber
~
>
<
~ 0.6
~
2
\ ~ /
7
0
-90 -b7 -70 0 ~ ~ ` ]0~~ EO YO
~
Figure 10 - Radiation pattern of the LED
Key: 1. Relative intensity 2. Degrees
In the system built, a step-index type of fiber Was chosen, with NA = 0.22, core of
200 um, and attenuation o~ 5'dB/tim at 820 nm.
2.4 Optical Couplers
A telephone cottnectian for 5icf~.rec.tional conver~sation with.the use o� a single fiber
must employ opCical couplers tha~ fulfill the functiori of. tI~e trad~itional telephone
forks.
The optical couplers conventionally used are of the beam-splitter type or [as pub-
lished] obtained with Y-fused fibers. These couplers introduce an insertion attenu-
ation not less than 3 dB in both.directions and in addition, if the plane surface of
fiher interfacing with the coupler is not suitably treated, it reflects a part of
the incident signal, which, besides causing a power loss that can be calculated at
2 to 4 percent of the incident power, can be a cause of disturbance, as will be seen
below.
But other techniques can be used, insofar as they permit better output of the cou-
plers used at the phone-set terminal and in the exchange-end connection.
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In the subscriber set, the optoelectronic components coupled to the fiber are the
LED and the photodetector.
~ The structure provided for the experimentation is diagrammed in Figure 11.2
LEO 127 pm
\ lASER 0,82 ~m
~ 1) CELLA fOTO_ LENT)E LENTE FlBRA ~ 4~
LED VOLTAICA SEIFOC
~ R
t ~
~i
~ ~
r~`~ i
~
~i
l651
Figure 11 - Optical coupler in the subscriber terminal
Key:
1. Photovoltaic cell 3. Lens
2. Selfoc lens 4. Fiber
At the center of the photodetector is inserted a Selfoc microlens3 of 2 mm diameter
with a numerical aperture of 0.5, so as to permit focusing of the light beam emitted
by the LED into the fiber.
In view of the reduced longitudinal dimensions (~10 mm) of the cnicrolens, the atten-
uation that occurs in the coupling between the LED and the fiber is due exclusively
to the insertion of the objective with numerical aperture of 0.3, and corresponds to
a value of about 1 dB.
� As regards the attenuation that the signal undergoes at the receiving end, in addi-
tion to the losses due to the objective there is the further loss due to the diminu-
tion of the useful surface of the photodetector.
Assuming for the diameter of the light beam incident on the photodetector a typical
value of 7 mm, and 2 mm for the diameter of the microlens, the power loss due to the
presence of the latter can be evaluated at about 0.4 dB.
The components to be coupled to the fiber at the subscriber-end connection are the
semiconductor laser and the APD photodiode used in detecting the voice signal:
The criterion followed in the designing of this coupler consists in reduction to the
minimum of the signal generated by the ref_lection onto the fiber of the light trans-
- mitted by the laser, inasmuch as the laser disturbs the reception; therefore, inter-
ferential filters were used, per the structure shown in Figure 12. The attenuations
2. Patent 67939-A/80, filed 17 June 1980.
~ 3. The Selfoc microlens is composed of a glass cylinder characterized by a refrac-
tion-index profile that varies from the axis toward the periphery by a parabolic
law.
12
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~iven in the figure relate to the transmission path (wavelength ~1), the reception
path (wavelength ~2), and the diaphony associated with the optical power emitted by
. the laser source and reflected toward the APD photodetector.
� s~aHro ~ 1)
. \
I
061. NA Q2 ( pg,~, NA G6
Ptla~l '
~2) - _ - u.s~n
F~M~ .,o,y i Pr~,,
- FI1T1101NTERRERENZ~ALE
"LrONG ?ASS' ~
I ~~'NA~ fILTIlO1NTERfE11ENZULE
'SMORT ?133'
huN i?~a~i ~4)
I I
a.,uo
- - - - - rE~eo~ao ornco w~ ?urr~~an ( ~nun ~~�a~z,.~, ) ~ 5 )
rFNCO1K0 OTTICO IN ARRIVO ( L[D Ar t,7/ Mn~ ~ b~
AT7lNUKIONE OTT1G 0lLU VIA DI IIKfS10N! � 10 LoY ~~u~ ~ ~ 7~ i
?117~~
ATT@NJAZIONt OTTIW OtLLA vU 01 TIIA~AISLIpiE ~ t0 Loy ~11~~)
\
. r21~11
.rn?w,woae o~ o~~oi+u, � ~o uo �e~ as ~ 9 )
?r~i~
ust '
Figure 12 - Coupler at the exchange-end connection
Key:
1. Mirror 6. Optical route coming
2. Fiber 7. Optical attenuation of recept=on path
_3. "Long pass" interferential filter 8. Optical attenuation of transmission.
4. "Short pass" interferential filter path
5. Optical route going 9. Diaphony attenuation
2 1 I 8 N ,
i
5
I
I
3 I 4
1 Mwnbrur 4 Contenitore
2 Dtra pN:odnmko S Tubo
3 Gvitl paaran 8 AnNlo Nartieo di
bloonWio nu
Figure 13 - Structure of electroacoustic transducer
Key: ,
1. Membrane 4. Case
2. Piezoceramic disc 5. Tube
3. Rear cavity 6. Elastic stop ring
13
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Use of a second-window (1,270 nm) LED as the emitter in the subscriber terminal ob-
viously facilitated solution of the problem of separation of the two paths, with the
interferential filters that were available.
The possibility is being considered of using, with the employment of filters chosen
ad hoc, two closer wavelengths for the two transmission paths (820 and 900 nm, for
Qxample), reserving the longer wavelengths for transmission of auxiliary services.
- 2.5 Ringing Mechanism
The low available power imposes the use of a ringing-mechanism transducer that has
the highest possible efficiency (understood as the ratio between the acoustical pow-
er and the electrical power furnished). In addition, the transducer has to be of re-
duced dimensions so as to fit easily inside the telephone set.
; These two kinds of considerations have led to the development of a piezoelectric
; transducer whose functioning makes use of the resonant technique ~13ibliography 7),
with resonance frequency in the neighborhood of 1,350 Hz.
The basic diagram of the transducer is presented in Figure 13.
It is compcsed essentially of the aluminum membrane (1) onto which is soldered a:_
disc of piezoceramic material metalized on its opposite side (2); the cavity (3),.
which, with the tube (S), constitutes a Helmholtz resonator tuned to the excitation-
sigr.al frequency; the case (4); and the elastic stop rings (5).
The specifications presently in force for the ringing signal stipulate that in an
anechoic chamber, in a free field at a distance of 1 meter, there be a sound-pressure
level equal to 65 dBSpL4; the effective value that must be furnished to the trans-
ducer in order to meet the specifications is 1 V, at the resonance frequency of
1,350 Hz.
I 2.6 Microtelephone Units
As has been seen, the high efficiency required has necessitated the development of
- an ad-hoc ringing-mechanism transducer.
I But for the microtelephone transducers, the products available in the market were
i examined, models of various types and sources being considered.
In this case too, fundamental consideratic:i was given mainly to efficiency criteria
~ calling for a transducer which, in functioning as a microphone, would ;furnish max-
imum electrical power at a given acoustical excitation, and in the inverse function,
maximum acoustical output at a given level of electrical excitation.
The choice fell to a transducer of electrodynamic type--that is, with movable coil--
with plastic membrane and impedances equal to 150 ohm, almost resistive and there-
fore constant throughout the telephone band (Bibliography 8).
2.7 Circuit Particulars
Remote powering from the exchange by means of a light signal, with the related prob-
lems of limited power available at the subscriber end, entails the making of circuifs
4. Sound-pressure level, at 20 uPa (in air).
14
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- functioning at low voltages so as to keep dissipated power as low as possible. Un-
der these :.onditions, realization of the various functions requiYed in the subscrib-
er set could lead to development of "custom" circuits using appropriate technology.
- 1 (5)
' oisco ( 9 )
OMBINATORE 03CILLATORE tO~M: gEpARATORE
FORMA?ORE DI IMPUIS~
OTTICO
(6)
~ MAPLIFICATORE MODULATORE PAM
TRASMITTENTE i PILOTA ~E~ r
r fIBRA
~2 ~ ~ ~10~
a3 ~ ,
2 VCe CONVERTITORE
/ FILTRO
CC CC ALtMENTAZ ~E~~ ~ 11 ~
~ ~ ~ 12 ~ FOTOVOITAICA
1
FILTRO OFA
AMPLIFICAT011E PA~ eA~ ~
RICEVEN7E
~ 3 \ SUpNERIA
J pN
(13)
~.y
( 4 ) 'scw?~a ,,s~ ~m ~ ~F ~6EONAL[ o,�a ~m
~u = 10 kMt ~c kN= l664
Figure 14 - Block diagram of subscriber terminal ,
Key:
1. Dial 8. Low-pass filter
2. Transmitting amplifier 9. Optical separator
3. Receiving amplifier 10. Fiber
4. Signal 11. Photovoltaic cell
5. 10-kHz oscillator, pulse shaper 12. Power-supply filter
6. PAM modulator & pilot 13. Ringing mechanism
7. DC-DC converter
In the present phase of experimentation, the circuits in question have been made by
traditional separate-component techniques, inasmuch as this type of solution does
not preclude the adoption of other, more advanced approaches, it has made greater
f~.exibility possible in the solutions adopted and tested out from time to time, and
ithas made it possible to carry out, in short time-spans, checks of the system's
feasability.
For the subscriber terminal, the block pattern shown in Figure 14 has been adopted. '
~
When the microtelephone is lowered (ON position), the photovoltaic cell is connect-
ed, through a transformer, to the ringing mechanism, thus permitting reception of
the call signal from the exchange, composed of a square wave of frequency equal to
the resonance frequency of the ringing mechanism (1.35 kHz).
In this condition, the subscriber set does not require remote powering, inasmuch as
all the blocks related to the various functions are deactivated. It is nevertheless
possible for the subscriber to call for the line, and therefore for remote powering
from the exchange, by lifting the handset ~OFF position).
15
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In this way, the oscillator and the pilot circuit of the LED are connected with the
power-supply filter through the DC-DC converter. The energy stored in the filter's
condenser is sufficient to activate the pulsed 10-kHz oscillator (Figure 14), where-
by the sequence of unmodulated pulses going from the subscriber end to the exchange
is identified by the exchange as a request for use of the line. This produces an
increase in the power put out by the laser, the polarization of which is automatic-
ally varied.
, With remote powering thus achieved, the subscriber can make his call, which is done
by interrupting the oscillator with the pulses coming from the dial. Alternative
approaches, suitable for use with numerical keyboards, are presently under study.
In the block diagram of the subscriber connection in the exchange (Figure 15) there
are shown, in addition to the circuits for transmission and reception of the voice
signal, those for recognition of switch-hook position and of number called, based on
~ idQntification of the PAM carrier, and the circuit--already mentioned--for automatic
~ variation of the laser-polarization current.
I~ -
~
; ~2) ~
i c~RCUrro a
~ ~ 1 ~ AUTOMIIT
I SEPARATORE p~~~~�qTORE OSCILLATORE
- OTTICO
1.36 kMt 20 kMZ ,
u~A (3) W
J
Q
I ~ ~~i~ PILOTA MODULATORE AMPLIFICATOR ~
I FIBRA I ~ TRASMITTENTE u ? J
i (4) j 10 (11) ~ ~
~ Z
~ W
U
FILTRO AMPIIFIG4TOR �
, PASSAB0.Sf0 RIC�VENTE Z
G~APD ~
\ S ~ RIYEU1T~bRB ( 6 ) ~ ~ (14 ) 12 (13
RICONOSCIMENTO SELEZIONE �
E POSiZIONE DEL GANCIO
/"~f~? .~r'-~_~
~ H~ lEGNALE 1,27 ~+m ~ H~ lEGNALE O.t2 ~m
~=20kM:
~ f~ ? IOkM:
- ebb5
~
i -
Figure 15 Block diagram of subscriber connection in the exchange
Key:
1. Optical separator 9. Oscillator
2. Automatic polarization circuit 10. PWM modulator
3. Pilot 11. Transmitting ampli�ier
4. Fiber 12. Exchange interface
5. Photodetector 13. Exchange
6. Low-pass filter 14. Recognition of number called ar~d of
7. Receiving amplifier switch-hook position
8. Signal
16
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hl)It UH'h'li'IAL USE UNLY
F�- ~ -
tL>
urENTE A
LASEN
2 IDENTIfICAZ10NE VORTANTE ?MA ~ 3~ OO Tp0.SM~SS~O~E
INVIO M~J~E Y~M~M `~~~E A ~ ~O ~
~ L}
~ ~ 100 ~s /
r--~ r--.~ '
u(elr~ A ~ ~ j I I '
LED L _J L �
~ Q~'L ~ T O AVPARES ~ 10 A~ PORjL ~ E YAM 1 300 WIPUELS~uON/OFF) ~ DAASMISStO EEA
~ (v
S OECOOIRICADEI
NUA1E1105ELEZIONATO ~rrSO N~
~1~
UTENTE B
l.oSER ~ ~
e INVIODELTONO e IDENTIFICAZIONE OINVIOPORTANTEiWAA _
~ DI p11AMATA 11.35 kHs) OELLA PORTANTt TRASMISSIONE VER50 ,
. ALL'VTENTE SElEZI0NAT0 VAM ~ 12 ~ L'UTENTE B ~ 13 ~
~8~
~ ~ ~ ~
~1~
UTENTE B
lED
Bl ~ ~ TRASMISSIONE PMA ~
7 OF~'~ / ~ . OALL'UTENTE B
A~IMENTAZIO/IE ` 14 ~
GE~L'AriANECCMiO 8
~ 9 ~ ~656
Figure 16 - Functional time diagram (not to scale)
Key:
1. Subscriber 8. Sending of call ring to subscriber
2. Switch hook called
3. Identification of PAM carrier 9. Powering of set B
4. Sending of PWM carrier 10. Transmission to subscriber A
5. Powering of set A 11. PAM transmission by subscriber A
6. Number-dialing by interruption of 12. Identification of PAM carrier
PAM carrier (~300 pulses ON/OFF) 13. Sending of PWM transmission carrier
7. Decoding of number dialed to subscriber B
14. PAM transmission by subscriber B
The optical power emitted by the laser source remains reduced until either a se-
quence of pufses (line-use request) is identified or a call arrives for the sub-
scriber; the useful life of the device is thus considerably increased.
At rest, the laser is polarized at a level slightly above the threshold, so as to
avoid discharge of the power-supply filter condenser into the subscriber set, thus
recovering its discharge current.
17
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3. Description of the Fiinctioning of the System
To simplify description of the functioning of the system, it is possible to follow a
sequence that corresponds to the various phases of a telephone communication;for tlais
purpose, we refer to the functional time diagram of Figure 16.
With the telephone set at rest, the switch hook is in the ON position (Figure 14),
and the electroni~ circuits are without power supply. The output of the photodetec-
tor (the p~~otavoltaic cell in Figures 1 and 14) is AC-coupled, through a transform-
er, to the ringing mechanism, so as to permit reception of any call coming from the
exchange.
When calling subscriber A lifts the receiver, the switch-hook contacts go into the
OFF position, thus making it possible for the circuits piesent in the subscriber's
terminal set to be powered. In particular, the 10-kHz oscillator is activated that
furnishes the carrier to the PAM modulator connected to the pilot circuit of the
LED, which send a light ~ignal to the exchange (Figure 16, point 1).
This light signal that arrives in the exchange (Figure 15) is initially constituted
by the unmadulated 10-kHz carrier. It is detected by the GeAPD photodiode and ar-
rives both at the number-dialed and switch-hook-position recognition circuit and at
the low-pass filter, and therefore at the receiving amplifier.
Through the first channel, by use of the 10-kHz component (Figure 16, point 2), it
can take the line by closing the relay positioned in the interface circuit in the
direction of the exchange. The automatic exchange-end-connection polarization cir-
cuit corresponding to subscriber A causes the laser to work above the threshold,
with emitted power of 12 mW. Arriving at the calling subscriber's set, this light
flow, constituted by the unmodulated PWM carrier, permits remote powering of the
terminal proper (Figure 16, point 3).
At this point, subscriber A dials his number, interrupting the 10-kHz oscillator
with the pulses from the dial (point 4).
In the exchange, the number dialed is decoded (point 5), and the subscriber-B ex-
change-end connection corresponding to that number is activated and sends the call
ring (point 6). This signal, composed of a square wave at 1,350 Hz (the resonance
frequency of the piezoelectric transducer), excites subscriber B's ringing mechan-
i ism, which is connected to the photovoltaic cell through the switch hook (in ON po-
; sition).
When the receiver of the set called is raised, the switch hook goes into OFF posi-
r.ion (point 7) and the LED send into the fiber the 10-kHz carrier, which, as de-
scribed above, takes the line (point 8). The call ring is thus shut off, while the
circuits for sending the PWM carrier toward subscriber B are activated.
Thus is the phonic connection between the two subscribers involved made and main-
tained for so long as both receivers are off the hook ~point 9).
When the receiver is hung up, there is no further transmission of the 10-kHz carri-
er, and the line-engagement circuit of the subscriber-end connection ir:volved there-
fore frees the line, reducing the optical power emitted by the laser.
18
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4. Results and Outlooks
The system's present capacity makes it possible to reach sections up to 550 m of
- ~'step-index" fiber with core diameter of 200 um (NA = 0.22; attenuation = 5 dB/km at
820 tim) .
Optim~zation of the circuits in the telephone set from the power-consumption pointi
of view has resulted in reduction of electric-power dissipation in the termmnal i.t-
self to only 300 uW, including the efficiency of the DC-DC booster that raises the
voltage from 0.3 to 2 V(about 50 percent). This means that the photovoltaic cell
has to put out, with a voltage of 0.3 V, a current of 1 mA, corresponding to a light
power of about 2 mW incident on the surface of the cell.� In these conditions, sat-
isfactory bidirectional transmission of conversations and,a good ringing level in
the phone set have been achieved.
In subjective tests, the quality has been considered good.
The experimental tests in progress are aimed at increasing the connection length ob-
tainable; in particular, as has been mentioned, it is possible, by the use of galli-
um-arsenide (GaAs) photovoltaic cells, to make connections up to about 2 km.
Table 2 summarizes the characteristics of the system built and the performance char-
acteristics obtained.
TaSle 2- Characteristics of the Optical-Fiber Telephone
a) Subscriber connection in exchange
source GaAlAs I~I LASER = 0.82 um)
mean optical power emitted 12 mW
frequency of pulses f~ 20 kHz
modulation technique PWM (full-vacuum ratio of pulses: 98 perceut)
- photodetector Ge APD
mean optical power received 15 nW
b) Subscriber telephone set
source In-Ga-As-P/In-P-DH LED (1 = 1.27 um)
peak optical power emi.tted 8 uW
frequency of pulses fu 10 kHz
modulation technique PAM (full-vacuum ratio of pul"ses: 1 percent~
photodetector photovoltaic cell (Si)
mean optical power received 2 mW
electric power necessary for
remote powering 300 uW
acoustical power put out by
ringing mechanism 66 dBSPL
c) Connection..length achievable
step-index fiber; core diameter 200 um; NA: 0.22; attenuation: 5 dB/km at
0.82 um
with Si photovoltaic cell (ver-
sion currently made) 500 m
with GaAs photovoltaic cell ~2 km
19
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5. Conclusions
An optical-fiber bidirectional telephone-conversation transmission system has been
built in which the subscriber set is remote-powered optically, through the fiber it-
self, from the exchange. The possibility of optical remote actuation of the ringing
mechanism of the subscriber set has been verified. The connection length presently
obtainable is 550 m, but developments presently being implemerited indicate the pos-
sibility of achieving lengths up to 2 km. The achievement of subscriber-terminal
circuits with very low dissipation has permitted realization of the system with op-
, tical and optoelectronic components available on the market.
Acknowledgements
The authors wish to thank B. Catania for his guidance and G. Modena for having git~~n
us t'ne idea for this piece. R. Ceruti and R. Ravaglia developed the piezoelectric
ringing mechanism and collaborated notably on the electroacoustical problems. Fin-
ally, we men~ion the assistance lent by E. Vezzoni and B. Sordo in evaluation of the
optical coupYing techniques.
BIBLIOGRAPHY
1. Kleinman, D.A. et al., "The Photophone--an Optical Telephone Receiver," J
, ACOUST SOC AM, June 1976.
' 2. Gosch, J., "Light Powered Phone Operates without an Internal Light Source,"
ELECTRONICS, May 1979.
3. Fromm, I., "Optophone--an Optical Transmission System for Sound," FREQUENZ, 1978.
4. De Loach, B.C., et al., "Sound Alerter Powered over Optical Fiber," BSTJ, Novem-
ber 1978.
5. Miller, R.C., et al., "Optically Powered Speech Communication over a Fiber
Lightguide," BSTJ, 5eptember 1979.
6. Conti, M., "Problems Relative to Fabrication of Silicon Cells for Concentrat-
ors," Study Days on "Photovoltaic Conversion of Solar Energy," Microelectronics
Group, Milan, 18-19 October 1979.
I
~ 7. Ceruti, R., and others, ""Piezoelectric Transducer for Electronic Telephone
i
; Ringing Mechanisms," AIA Annual Conference, Siena, October 1979.
8. Brosio, A., et al., "System Study and Experimental Verification of a Telephonic
Connection in Fiber with Optical Remote Powering of the Subscriber Set," unpub-
- lished internal document.
COPYRIGHT: 1981 by ERI-EDIZIONI RAI RADIOTELEVISIONE ITALIANA
11267
CSO: 5500/2244 END
~
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