are terminated by loads and both paths have gain and there- fore relatively low noise figures.

Design of wideband duplexerlcirculators: An unsymmetrical distributed amplifier can be designed so that it has the useful property of wideband S31 isolation.

Computer optimisation using the well-known Touchstone (C) CAD program was used to optimise the S,, characteristic of the constant K design to bandstop form covering the S-band.

The distortion of the conventional regular ladder shaped layout to the meandering layout shown in Fig. 3 occurs as the range of the series element values becomes too great for a regularly shaped realisation. Unpredictable broadside coup- ling has been minimised by meandering the gate and drain lines away from each other.

Measured results: Useful gain of about 6.0dB in the S,, direc- tion and 7.5 dB in the S , , direction was measured. Agreement between measured and predicted gain is good with a maximum of about 2.0dB difference in the S,, direction. The responses have about 2.0 dB ripple in the S-band.

-30 01 U -35 0 L ~Ip-.- ~ - I

1 OGHz 0 OGHz

Fig. 4 I S,, I measured and predicted __ measured response

There is some error resulting in upward tuning and lower than predicted I S,, 1 isolation (Fig. 4). Analysis has indicated that the component value sensitivity is most acute in this path. The causes of the lumped component inaccuracies (which are probably systematic) are being investigated.

The noise figures of the module in both S , , and S,, direc- tions have been measured using the HP 8970A noise figure meter. The noise figure in the S 2 , direction is about 5 dB with 8 dB associated gain, while the S,, direction has a 4 dB noise figure with 6dB associated gain.

Conclusion: This letter has shown the design feasibility of a distributed amplifier that can be used in various duplexer/ circulator roles across the S-band. In four-port connection this module may also be seen as providing amplification in two directions and with low crosstalk between them.

-+- predicted response

0. P. LEISTEN R. J. COLLIER

11th January 1988

Electronic Engineering Laboratories The Unioersity Canterbury, Kent C T 2 7 N T , United Kingdom

R. N. BATES Philips Research Laboratories Cross Oak Lane Redhill, Surrey RH1 5HA, United Kingdom

Reference 1 NICLAS, K. B., WILSER, w. T., KRITZER, T. R., and PEREIRA, R. R.: 'On

theory and performance. of solid-state microwave distributed amplifiers', IEEE Trans., 1984, MTT-31, pp. 447-456

ELECTRONICS LETTERS 3rd March 1988 Vol. 24 No. 5

MEASUREMENT METHOD FOR DETERMINATION OF OPTICAL PHASE SHIFTS IN 3 x 3 FIBRE COUPLERS

indexing terms: Optical fibres, Optical couplers & connectors, Optical measurement

The phase characteristics of 3 x 3 fibre couplers are investi- gated with a self-homodyne experiment at 1.3 and 1.5bm. A 2 x 2 fibre coupler, a fibre delay line and the 3 x 3 fibre coupler under test form a Mach-Zehnder interferometer. The measurement results are compared with values from S-matrix theory for lossy couplers.

Introduction: Optical hybrids are an essential part of many coherent receivers : 180" hybrids in balanced receivers, 120" and 90" hybrids in phase diversity receivers and 90" hybrids in homodyne systems. 180" and 120" hybrids are readily avail- able as 2 x 2 fibre couplers and 3 x 3 fibre couplers, respec- tively. However, fibre-optic 90" hybrids are not easily realised at present. Moreover, in contrast to the 120" hybrid for three-port phase diversity receivers, the 90" hybrid neces- sary for two-port phase diversity receivers and for homodyne receivers requires tight phase tolerances. A practical solution is a 120" hybrid (3 x 3 fibre couplers) with three photodiodes and using the difference photocurrent of two of the three

A high degree of symmetry of the 3 x 3 fibre coupler is needed for a true 90" hybrid. Real couplers have losses and are rarely perfectly symmetric. We therefore mea- sured fused 3 x 3 fibre couplers with an interferometric method and compared the results with theoretical predictions of S-matrix theory for lossy 3 x 3 coupler^.^

Exper iment: Light of a single mode laser is fed into a 2 x 2 fibre coupler whose outputs are connected with two of the three inputs of the 3 x 3 fibre coupler under test. The signal in one arm is delayed by an additional fibre. The three output fibres of the resulting interferometer are connected to Ge photodiodes (Fig. 1) whose currents are monitored by a digital oscilloscope. The amplitude transfer coefficients of the coupler are obtained by carefully measuring the direct currents with one arm of the interferometer open. In contrast to a previous method' we modulated the fibre length instead of the fre- quency of the laser. The frequency excursion together with the time delay results in a modulation of the input phases at the 3 x 3 coupler. The phase shifts of the coupler are determined by attenuating and adding the three output signals in such a way that the sum is time independent. Then the amplitudes of the signals are measured by the computer controlled oscillo- scope. The phase shifts are easily obtained by numerical manipulation of the measured amplitudes. Exchanging the input fibres of the 3 x 3 coupler gives nine angles. Since only five angles are independent, the redundant measurements can be used for an accuracy check.

T h e o r y : The theory of lossless 3 x 3 couplers' can be extended to account for the 10sses.~ As a result, one obtains upper and lower bounds for the phase shifts in the coupler. For low losses and reasonable symmetry of the coupler the allowed ranges for the phase angles are rather small. We

Mach-Zehnder delay line (100m/2m) ' Interferometer

laser ~

1523 nm 1300 nm h

, coupler

A c9

modulation

- ' I I I Ge photodiode tra n stmpedance

amplifiers

Fig. 1 Measurement set-up

265

assume that the coupler is reciprocal and free of reflections. The reduced scattering matrix S is then defined by

jJ = si (1)

where i denotes the 3 amplitudes of the incoming electric field at the coupler input and j the output amplitudes.

The complex coefficients Si , = si, exp (jq5ik) are normalised by the convention

4ii = 412 = 4 2 1 4 2 3 = 4 3 2 (2)'

leaving only four independent angles. The power dissipated in the coupler is given by

P = ? + ( E - S+s)i (3)

with E denoting the unit matrix and + denoting Hermitian. Eqn. 3 together with P > 0 (passive device) allows the deter- mination of the possible ranges for the angles 412, d13, 423,

The relation between the measured angles and 4 i k is given in Reference 4.

$ 2 3 @I2

120' - 120c

1 0 2 5 0 2 9 0401 / ;' T ' j j

0 27 0 39 0261 ' 90 l;no 15(

'2' $31 , 150' I

at 1523 n m

0 2 9 0 2 5 OL1

Fig. 2 Phases of the complex coefficients of the S-matrix of two 3 x 3 couplers

Long arrows, short arrows and shaded sectors are measured data and values of simple and extended theory, respectively. Loss and power transfer matrices are shown on the left of each diagram

Resul t s : Measurement results are shown in Figs. 2 and 3 where the power transfer matrix (si) and the average loss are also given for each coupler. In the phase diagram the shaded sectors indicate the allowed range of the phases calculated with S-matrix theory based on the measured amplitude coeffi- cients sik. Long arrows are measured phases and short arrows are phases calculated with simple theory for lossless couplers. No dependence of the coupler characteristics on the polarisa- tion of the input light could be observed.

Discussion: Two couplers (121 at 1.52pm, 154 at 1.3pm) are rather symmetrical exhibiting phases of 120" 6". Coupler 105 shows large deviations of 431 from 120" at 1.52pm owing to the very small coefficient s23 ( s ; ~ = 0.08). The measured phases and the theoretical predictions usually show good accordance with the simple theory for lossless couplers. This holds even for coupler 121 which has the highest loss of 0.9 dB at 1.5pm where the extended theory allows a large range for the phases.

The properties of coupler 121 at 1.5pm and 1.3pm are compared in Fig. 3 indicating the relatively weak wavelength

266

dependence of the coupler characteristics. Coupler 121 can be used as a 90" hybrid at 1300nm without further electronic processing when inputs 1 & 2 and outputs 1 & 3 are used. The resulting phase difference between both outputs 413 + 412 - 423 amounts to 93". The accuracy of the measurement has

been tested by a check of the redundant angles. The phase error for at least moderately symmetrical couplers was below 1".

126 @31 156 i 90" ,

at

6-

12 1

1523 n

0 9dE

0 33 0 26 0 2 4

0 0 0

at 1300 n m

Fig. 3 Properties o f a 3 x 3 coupler at 1.5 pm and 1.3 pm

Acknowledgments: We thank Dr. M. Raab for the fabrication of the fibre couplers. This work has been supported by the German Ministry of Research and Technology. The authors alone are responsible for the contents.

E. GOTTWALD J. PIETZSCH

11th January 1988

Telecommunication Networks & Security Systems Central Laboratories Siemens AG PO Box 70 00 76,0-8000 Muenchen 70, Fed. Rep. of Germany

References

1

2

PRIEST, R. G . : 'Analysis of fiber interferometer utilizing 3 x 3 fiber coupler', J . Quantum Electron., 1982, QE-18, pp. 160-1603 Koo, K. P., et a/.: 'Passive stabilization scheme for fiber interfero- meters using (3 x 3) fiber directional couplers', Appl. Phys. Lett., 1982,41, pp. 6 1 M 1 8 STEPHENS, T. D., and NICHOLSON, G . : 'Optical homodyne receiver with a six-port fibre coupler', Electron. Lett., 1987, 23, pp. 1106 1108 PIETZSCH, I . : 'Scattering matrix analysis of 3 x 3 fibre couplers', submitted to Journal of Lightwave Technology

3

4

ENDLESS POLARISATION CONTROL USING AN INTEGRATED OPTIC LITHIUM NIOBATE DEVICE

Indexing terms: Integrated optics, Polarisation, Optical com- munications

A recently proposed lithium niobate polarisation convertor is shown to provide endless control between two time-varying polarisation states. Experimental results are presented.

Introduction: There is current interest in 'endless' polarisation control for coherent optical fibre transmission systems and

ELECTRONICS LETTERS 3rd March 1988 Vol. 24 No. 5