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COUPLING OF AN OPTICAL FIBRE TO A
SUBSTRATE ENTRY PHOTODIODE
Indexing terms: Optical fibres, Photodiodes, Optical communication, Coupled circuits
The construction of a quartz block containing an optical fibre
for mounting a substrate illuminated InGaAs/InP photodiode is described. The block allows accurate alignment of the
fibre to the photodiode, which can then be fully characterised
before assembly into a circuit.
Introduction: In a fibre optic communication link, photodiodes
are used as detectors for converting the received optical power
into an electrical output signal. For operation in the 10—1-6
pm wavelength region, diodes fabricated from Ino.53Gao.47As,
lattice matched to an InP substrate, are used.1 Substrate illuminated photodiodes have several advantages over conventional top illuminated photodiodes, including higher quantum
efficiency and lower capacitance,2 since no additional area is
required for the bonding pad. A novel method of coupling the
light from an optical fibre to such a diode chip, and the assembly of the composite into a package, are described.
Three separate areas of assembly are considered: first, the
incorporation of the optical fibre into a suitable mounting
block; secondly, the accurate alignment and fixing of the
photodiode over the fibre end; thirdly connection of
the photodiode mount into a package.
Fibre block assembly: One way of coupling an optical fibre to a
substrate entry photodiode is to mount the diode on a ceramic
block which has a fine hole through it to accept thefibre.After
positioning the diode over the hole, the fibre is inserted into the
block. The disadvantages of this method are the difficulty of
producing a fine hole (~ 150 fim diameter) through a block
about 1 mm thick, and the risk of damage to the end of the
fibre when inserting it into the hole. Other workers3 have used
a mount with a relatively large diameter hole, but it is not clear
how the fibre is aligned. A new method has, therefore, been
adopted where a block is assembled from two parts around the
fibre before the photodiode is mounted on it.
Two strips of quartz, 2 mm x 1 mm, are cut to a suitable
length, and fine grooves machined into one surface of each
(Fig. 1). These grooves are used as locations for thefibres.One
of the strips also has a 0-5 mm x 0-5 mm step cut into one of
the corners, at the insersection of faces A and B, to give a recess
into which nichrome and gold are sputtered through a mask to
form bonding islands. A specially machined tungsten mask is
used to give the required dimensional accuracy to these
islands, and to ensure that they are precisely positioned with
respect to the grooves. The face opposite the grooves (face C)
on the nonstepped block is also sputtered with nichrome and
gold to enable the finished assembly to be soldered into a
package.
Optical fibres are prepared by stripping the protective coating off the end millimetre and locating them in the grooves on
one quartz strip, held in position with epoxy adhesive. Alternatively, it might be possible to use a low melting point glass. The
ends of the fibres just protrude from one end of the grooves. A
thin film of epoxy is applied to the grooved face of the other
quartz strip and the two halves are clamped together so that
the fibres are held securely in position. The epoxy is now cured
to give maximum strength and face A of the assembly, from
which the fibres just protrude, is polished to give a suitable
finish to the ends of the fibres. It is this polishing operation
which dictates the material from which the block is made, since
the block must be of approximately the same hardness as the
fibre if preferential erosion of either thefibreor the block is not
to occur during polishing. The polishing operation would also
remove any metal deposited on the face of the quartz, but
having the metal islands located in the step recess overcomes
this problem. The quartz strips are finally separated into individual blocks by diamond sawing along the planes indicated in
Fig. 1. These planes pass through the middle of the metalised
islands, and so each block has a metallised area at each end of
the step recess. After sawing, the blocks are cleaned to remove
any residues from the polishing and sawing operations, and
inspected for damage to the end of the fibre.
Photodiode assembly: A specially designed assembly jig attached to an X-Y movable stage under a low power stereo
microscope is used for the assembly of the photodiode to the
mounting block.
The active area of the diode is 100 fim diameter, and the chip
is 500 ^m square. However, the optical fibre has an outer
diameter of 125 /an and a core diameter of 50 fim; therefore
alignment of the diode is critical. The microscope has a device
attached to it which superimposes a cross on to the image. The
fibre is aligned so that the centre of the polished end coincides
with the centre of the cross, and a small amount of an optically
transparent epoxy is applied using a fine probe. The photodiode is picked up in the correct orientation and then gently
positioned into the epoxy. A probe is used to manipulate the
chip so that the centre of the active area is positioned under
the cross in the microscope field of view. It is obviously very
important that there is no movement of the block or the jig at
this stage, as this would result in misalignment of the diode.
Once the diode is in position, the assembly jig with the block
still in it is carefully placed in an oven to cure the epoxy.
Wire bonded connections are made between the photodiode
and the gold areas on face A' of the block using 17 [im gold
wire. A pulsed tip thermocompression ball bonder is used, with
a low substrate temperature of 100°C. This prevents the epoxy
substrate entry
photodiode
00007"
gold bonding
polished
surface
gold bond pads
Fig. 2 Completed diode mount
block separation cutting
planes
faceA
back face nichrome/gdd
metallised
faceC
nichrome/gold
metallisation
[6AZ7J1
faceB face B
Fig. 1 Prepared quartz strips before fibre insertion
832
Fig. 3 Diode mount in PIN/FET hybrid receiver
ELECTRONICS LETTERS 29th October 1981
Vol.17
No. 22
softening which would allow the photodiode to move during
bonding. A finished assembly is shown in Fig. 2.
It is possible at this stage to functionally test the photodiode
and to actively check the accuracy of alignment between fibre
and photodiode before final assembly into the receiver circuit.
Receiver assembly: The photodiode block assembly has been
incorporated into a PIN/FET receiver module of the type
previously used with conventional top illuminated diodes4
(Fig. 3). This consists of a thick film hybrid circuit plus the
photodiode mounted in a solid-sidewall metal dual-in-line
package. The fibre exits horizontally from the package and so
the mounting block is attached to the gold plating with the
metallised face (C) down, using either an epoxy adhesive or a
low melting point solder. Wire bond connections are finally
made from the block to the hybrid circuit. As the photodiode is
now in the vertical plane, this final bonding is done to the gold
on the face B' of the step perpendicular to the area to which the
wires from the diode were attached.
over the main part of the passband, the first step is to determine a nonminimum-phase lowpass prototype network from
which various microwave structures, such as, for example,
direct-coupled cavity waveguide filters, can be obtained. The
prototype network is a folded ladder of admittance inverters
and shunt capacitors with additional crosscoupling by further
inverters used to realise the finite transmission zeros, see Fig. 1.
Fig. 1 Lowpass prototype filter
Conclusion: A quartz block containing an optical fibre has
been used as a mount for a substrate illuminated photodiode.
The method of construction of the block has been described,
and also the method by which the diode is aligned and connected to metallised areas on the block, to produce a testable
diode-to-fibre assembly. Photodiodes mounted on such blocks
have been successfully incorporated into PIN/FET receivers
for 1-3" jun optical communications systems.
Acknowledgments: Acknowledgment is made to the Director of
Research, British Telecom, for permission to publish this letter
and to J. Ancell, B. Clark and R. Taylor for their assistance in
preparing the blocks.
B. M. MACDONALD
11th August 1981
A. G. SAUNDERS
British Telecom Research Laboratories
Martlesham Heath, Ipswich, Suffolk IP5 7RE, England
References
1
SMITH, D . R., CHATTERJEE, A. K., REJMAN, M. A. Z., WAKE, D . , a n d
WHITE, B. R.: 'p-i-n/FET hybrid optical receiver for 11-1-6 //m optical communication systems', Electron. Lett., 1980, 16, (19), pp.
750-751
2
LEE, T. p., BURRUS, c. A., DENTAI, A. c , and OGAWA, K.: 'Small area
InGaAs/InP p-i-n photodiodes: Fabrication, characteristics and
performance of devices in 274 Mb/s and 45 Mb/s lightware
receivers at 1-31 /mi wavelength1, ibid., 1980, 16, (4), pp. 155-156
3
LEE, T. p., BURRUS, c. A., and DENTAI, A. G.: 'InGaAs/InP p-i-n
photodiodes for lightwave communications at the 0-95-1-65 /an
wavelength', IEEE J. Quantum Electron., 1981, QE-17, pp. 232-238
4
Several methods for the construction of the nonminimumphase lowpass functions are available from which the element
values of the prototype networks can be determined, but they
are not always easy to apply and require considerable computation. Even more important, however, is the fact that filters
with several crosscouplings tend to be difficult to tune.
Recently,1 a class of lowpass prototype functions with equiripple passband magnitude response has been introduced
having only one pair of finite transmission zeros, which are
located on the real axis of the complex frequency plane symmetrically about the origin at the distance ±ay. It has been
shown that for the values of ox close to unity a very flat delay
characteristic over the central part of the passband can be
obtained, while the skirt attenuation of the resulting filter is
almost identical to that for the all-pole Chebyshev filter of
order n — 1. Since only one extra crosscoupling is required to
realise one pair of real-axis zeros, the practical tuning of the
resulting network is greatly facilitated. The magnitude-squared
function of these prototypes has the form
SMITH, D . R., HOOPER, R. C , AHMAD, K., JENKINS, D., MABBITT, A. W.,
and NICKLIN, R.: 'p-/-n/FET hybrid optical receiver for longer wavelength optical communication systems', Electron. Lett., 1980, 16,
(2), pp. 69-71
0013-5194/81/220832-02$!.50/0
LEAST-SQUARES MAGNITUDE FILTERS
WITH SELF-EQUALISED DELAY
CHARACTERISTIC
A2(co) =
l)]2a>rn-3(co) - 2o\ Tn-2(co)
(2)
2{o\ + co2)
Uco) =
In the above expression, Tn(a>) is the Chebyshev polynomial of
the first kind and e2 is a constant which determines the inband
loss.
The aim of this letter is to demonstrate that further increase
in the bandwidth coverage of delay approximation can be obtained if, instead of providing an equiripple passband magnitude response, the prototype function is determined by
minimising the ratio of the reflected power to the transmitted
power over the normalised passband in terms of a weighted
least-mean-squares norm. Since in all cases of practical interest
the reflected power must be small, this corresponds very nearly
to the minimum of return loss. The weight function associated
with the Chebyshev polynomial of the first kind Tn(co) is used,
i.e. w(to) = (1 — co2)~' 2, and denoting the characteristic function by fn((o) = Pn(co)/(o)2 + a2), the error integral to be minimised takes the form
E=
P2{o>) dto
(3)
(1 - to2)- ' V + a\f
This minimisation problem can be solved explicitly. It has been
proved in a recent paper2 that the error integral reaches its
minimum value if, and only if, the polynomial Pn(co) is orthogonal on the interval [0, 1] with respect to the weight function
In the design of narrow bandpassfiltersat microwave frequencies with stringent magnitude and group delay specifications
ELECTRONICS LETTERS 29th October 1981
(i)
where the characteristic function fn(co) is defined by the rational Chebyshev function
Indexing terms: Filters, Microwave filters, Transfer functions
A new class of nonminimum-phase transfer functions with
one pair of real-axis transmission zeros is described. These
functions are suitable for determining the lowpass prototype
networks in the design of narrow bandpass filters at microwave frequencies with small passband loss and very flat delay
response over the central part of the passband.
J
Vol. 17 No. 22
= {\ -o)2
w(a>)
(I-co2)12
(co2 + a2)2
(4)
833
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