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SAPIA, A., SPANO, P., and
B.: 'Polarization switching in semiconductor lasers driven via injection from an external radiation',
Appl. Phys. Lett., 1987, 50, pp. 57-59
T.: 'Optical polarization
bistability with high switching speed in a TM wave injected buried
heterostructure laser', Appl. Phys. Lett., 1987,51, pp. 1971-1973
LOH, w. H., SCHREMER, A. T., and TANG,
c. L.' 'Polarization selfmodulation at multigigahertz frequencies in an external cavity
semiconductor laser', IEEE Photonics Technol. Lett., 1990, 2, (7),
pp. 467469
AMANN. M.-c.. and S T E G ~ L L E R . B.: 'Low-threshold InaaAsP-InP
metal-clad ridge waveguide (MCRW) lasers for 1.ypm wavelength, Appl. Phys. Lett., 1986,48, p. 1027
Y., and TANG, r.L . : 'Polarization
self-modulation in external-cavity semiconductor lasers'. CLEO
1990, PDP 32
designed to provide a good match to the fields in the focal
plane of the p a r a b ~ l o i d The
. ~ TE,, mode is launched in the
feeding waveguide of diameter 0.72,,,,, and a taper section
expands this mode into the central radiating aperture of diaA narrow coaxial waveguide section B regumeter 0~9111420.
lates the coupling between the central waveguide A and an
outer waveguide cavity C whose diameter is 2~611,,,,. Aperture A matches the central lobe of the field in the focal region,
and aperture C matches the first ringlohe of the field, which is
in antiphase with the central lobe. The dimensions of the
coupling section B are critical in achieving the correct phase
and amplitude relationship between fields in apertures A and
C. Coupling to the first and second lobe of the focal plane
fields leads to an efficient aperture illumination.
It is possible to excite 408MHz fields in the outer waveguide cavity C with minimal effect on 1420MHz operation.
The diameter of cavity C is 0,751,,,, and it is able to support
the TE, mode at 408 MHz in its coaxial section and in its
circular section. However, the cavity is very short, and it is
difficult to control the modes in it at 408MHz. A narrow
choke section G in Fig. 1, is fitted around cavity C. Its principal effect is to reduce wide-angle radiation from the feed at
408MHz, and it also has a similar, but smaller, effect at
1420 MHz.
In the initial version of this feedZ the TE,, mode was
excited in cavity C using radial probes located within C ; this
design proved to have poor polarisation performance at
408 MHz. In the present design, the radial probes have been
replaced with slots in the tangential direction (D in Fig. 1); the
slots are themselves energised by fields in a cavity E which is
fed by four radial probes H . Because cavity C is too small to
accommodate slots of length 0.51,,,, a dielectric slab (F in
Fig. 1) is placed behind the slots inside the exciting cavity. The
material used is polyethylene with dielectric constant E, = 2.3,
and the slots can be made to resonate where their length I is
roughly 0.51,,,/~(~,).
The feed at 408MHz can be considered to be a coaxial
waveguide feed. The plate which separates cavity E from the
radiating cavity C is slotted and acts as a resonant iris. At
408MHz the plate is 'transparent' and the feed acts as a
simple coaxial waveguide feed of much greater length. In this
way it has been possible to achieve satisfactory 408 MHz performance in a feed which is only 0.551,,, in depth. The back
wall of cavity C is a bandpass dichroic structure, transparent
at 408 MHz but effectively a reflector at 1420 MHz. There is
some transparency a t 1420MHz, and the depth of cavity E
was chosen to minimise the 1420MHz energy passing through
the slots. The level of 1420MHz signal arriving at the
408MHz output is 16dB below the signal level at the
1420MHz output. This is good isolation, but must obviously
be supplemented by filtering in other system components.
The four probes H are connected to a phasing network5
which combines their outputs to produce right- and leftcircularly polarised radiation.
Indexina terms: Antennas. Antenna feeders
A compact dual-frequency feed operating at 1420 and
408 MHz has been developed for a pnme focus paraboloid.
EKicient dual-mode performance at I420 MHz results from
the combination of modes in a circular waveguide of diameter 0~911,,,, and a surrounding cavity of diameter
2.611,,,,,. A single TE,, mode is excited in the larger cavity
at 408MHz by four slots in its hack wall, which acts as a
dichroic structure, allowing the passage of 408 MHz energy
while reflecting 1420MHz fields. Satisfactory 408 MHz performance is obtained in a feed of depth only 0.551,,,,
without significant effect on 1420 MHz performance.
Introduction: This Letter describes a dual-frequency feed
designed for use at the prime focus of seven 8.5 m paraboloids
U/d = 0.4) used in the synthesis telescope at the Dominion
Radio Astrophysical Observatory. In its initial form' the telescope operated at 1420MHz only; receivng equipment for
408MHz was subsequently added.2 The design of the
408 MHz channel was constrained not to compromise performance at 1420 MHz.
F e e d : The feed is shown in Fig. I. The 1420MHz feed is based
on one of the designs of S ~ h e f f e r .It~ is a multimode feed,
17 5
- 3 6 7 d
Fig. 1 Dualqrequencyfeed
Dunensions in centimetres
Performance: Fig. 2 shows the radiation patterns of the feed.
The appropriate superposition of the 1420 MHz waveguide
modes in apertures A and C (Fig. 1) results in an efficient flat
top aperture illumination for 0 5 30" with a rapid decline of
power levels beyond the reflector rim (0 = 62"). Low backlobes are also evident.
The feed patterns at 408 MHz are broader than the patterns
at 1420 MHz because of the smaller feed size in terms of wavelengths. The feedback can support only one mode F E , , ) at
408 MHz; efficient multimode operation is not possible at that
frequency. The 408MHz slots have no measurable effect on
the 1420 MHz performance.
The radiation patterns were measured on an outdoor
antenna range (408MHz patterns were actually measured
with a scaled feed at 730 MHz.). The small asymmetries in the
patterns probably arise from reflections and other defects in
the range. At angles greater than 590" from boresight, amplitude errors are about 5 3 dB. Within k 90", the errors are less
than k 1 dB at 408 MHz and k0.5dB at 1420 MHz.
Precise crosspolarisation measurements were not attempted, but the performance with slot excitation is definitely
superior to that obtained using probes in cavity C. With the
new design, crosspolarisation lobes are below 34dB, but the
29th August 1991
Vol. 27
No. 18
performance of the feed tested was probably limited by construction inaccuracy. However, the original probe-excited feed
niques, and to E. Danallanko and R. Lipiecki for construction
of the feeds.
22nd July 1991
Electrical Engineering Department
University of Alberta
Edmonton, Alberta, T6G ZC7, Canada
‘Dominion Radio Astrophysical Observatory
National Research Council
Penticton, British Columbia, V Z A 6 K 3 , Canada
S., COSTAIN, C. H., LACEY, J . D., LANDECKEU, T. L., and
BOWERS, F. K . : ‘A supersynthesis radio telescope for neutral hydrogen spectroscopy at the Dominion Radio Astrophysical Observatory’, IEEE Proc., 1973, 61, pp. 127C-1276
P. E., and
ROUTLEDGE, 0.:‘A 408 MHz aperture synthesis radio telescope’,
Radio Sci., 1985,20, pp. 1118-1128
SCHEFFER, H . : ‘Improvements in the development of coaxial feeds
for paraboloidal reflector antennas’. Paper B1/2, 5th European
Microwave Conf,, Hamburg, Federal Republic of Germany, Sept.
MINNETT, H. c., and THOMAS, B. M.: ‘Fields in the image space of
symmetrical focusing reflectors’, IEE Proc., 1968, 115, pp. 14191430
SALEH, A. A. H . : ‘Planar multi-port quadrature-likepower dividers/
combiners’, IEEE Trans., 1981, MTT-29, pp. 332-337
Fig. 2 Measured radiation patterns of feed at 4 0 8 M H r (top) and
1 4 2 0 M H z (bottom),displaced by lOdB
__ E plane
H plane
was far more susceptible to such errors, and on-axis crosspolarisation levels were about 20dB.
At 408MHz, the feed cannot be matched over a broad
band. A VSWR of <2.3 is achieved over 20MHz whereas
over the 4 M H z operating band of the telescope, the
VSWR 5 1.11. The new design has not increased the bandwidth.
In the slot-excited feed, the sensitivity of tuning to changes
in length of probes H is 2MHz/mm. This is greatly superior
to the performance of the probe-excited feed, where the equivalent sensitivity was 8 MHz/mm; this extreme sensitivity made
it diffcult to adjust the feed for good polarisation performance.
Alternative designs: Several alternative methods of exciting
circularly polarised waves in the outer cavity at 408 MHz were
tested. They are
Indexing terms: Intearated oofics, Lasers,Waveouides
A simple approach to the butt coupling between a 1.5flm
BRS type DFB laser and a passive waveguide, based on the
(a) the original design, in which four radial probes are located
approximately midway between the back and front of the
outer coaxial cavity C
vapour phase growth properties on a nonplanar substrate, is
reported. Stable single longitudinal mode emission is
achieved for devices with CW threshold currents as low as
17mA. An output power of 2mW is measured from a 1.1mm
long passive waveguide.
(h) four printed patch antennas placed a t the back of cavity C
( c ) four rectangular slots backed by individual rectangular
waveguide cavities filled with a material of high dielectric constant
(4 a
narrow annular slot backed by a narrow coaxial waveguide (with a ratio of outer to inner diameter close to 1)
All of these alternatives provide adequate illumination of the
reflector at 408 MHz, very similar to that shown in Fig. 2, but
each suffers from one or more drawbacks. Solution (a) is the
original feed at 408 MHz. The tuning sensitivity of the probes
is high, and high coupling exists between orthogonal probes,
leading to poor polarisation performance. Solution (b) is
narrowband and dificult to tune unless the dielectric layer is
thick, in which case the 1420MHz radiation pattern is
affected, and 408 MHz coupling between adjacent patches
becomes unacceptably high, leading to poor polarisation performance. Solution ( c ) has similar problems, and the dielectric
material is heavy and expensive. Solution ( d ) is diffcult to
match because the impedance of the coaxial waveguide is very
Acknowledgments: The DRAO synthesis telescope is operated
as a national facility by the National Research Council of
Canada. The authors have been supported by the Natural
Sciences and Engineering Research Council of Canada. We
are grateful to R. Smegal for advice on measurement tech-
Vol. 27
Introduction: The monolithic integration of lasers, passive
waveguides, and photodiodes, avoiding complicated technological processes, is a key issue in the realisation of low cost
photonic integrated circuits. Various pertinent integration
schemes have been reported, e.g. selective area MOCVD,’
liftoff epitaxy.’ We have also proposed a new butt-coupling
approach, based on the vapour phase growth properties on a
nonplanar ~ u b s t r a t e .In
~ this technique, only one MOCVD
epitaxial growth is necessary to butt join two different devices,
instead of the two which are normally required. We report on
the realisation and the characterisation of a buried ridge stripe
(BRS) distributed feedback (DFB) laser monolithically buttjointed to a passive waveguide by this technique and operating in the CW regime.
Monolithic device realisation and performance: T o define the
active region of the device, grooves of 300pm width are
chemically etched using HCI/H,PO4 solution, in an n+-InP
(100) substrate, along the [Oli] direction. The layers of the
passive and active double heterostructures are grown by
MOCVD at atmospheric pressure, as shown in Fig. 1. For a
groove height of 1.2pmm,
the structure is composed of a l p m
thick n-InP (buffer layer), a 0.5pm thick n-InGaAsP
(waveguide layer, I,,, = 1.3pm), a 0.8pm thick n-InP (first
confining layer), a 0.15pm thick undoped InGaAsP (active
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