U.H.P. Fischer earned
a degree in physics from the Free University of Berlin, Germany in
1983. In 1988 he made his PhD in physics of condensed matter. In
1988 he joined the Heinrich-Hertz-Institute in Berlin for the
development of broadband optical communication systems. He has served as head of the optical packaging group at the Institute since 1994. In April 2001 he became professor of optical packaging and optical networks at the Unversity of Applied Sciences, Wernigerode, Germany.
Future optical communication systems will use the high bandwidth
of optical fiber in the optical frequency domain. Fast transmitter
and receiver modules are basic elements of these systems, which
should be able to transmit terabits/s of information via the fiber.
Experiments with optoelectronic integrated circuits (OEICs) in
laboratory test beds and field tests require a special packaging
that respects system requirements such as high RF data rate and low
insertion loss. Several concepts for fiber-chip coupling schemes
had been proposed, including laser-micro welding, which is
now becoming the standard for industrial high-volume manufacture.
The investment costs for this laser welding equipment are
considerably high. For laboratory use and rapid prototyping a
flexible design is needed which is able to adapt different OEICs
with changing dimensions to an existing module type.
Figure 1: Future optical network scenario
Future telecommunication networks will be done optically. An example
view of such a network is shown in Figure 1, first presented by Nortel in 1999. A high-speed central-core network acts as a backbone for the
regional and office networks. There will be a very high data speed of
more than 40 Gbits/s and a moderate number of wavelength channels,
Figure 1 shows 32. In the access area of the network, the data rate will
be lower, with 10 Gbits/s, but with a very high number of wavelength
channels. You can consider three customer classes:
- Companies using a high data speed of 10 Gbits/s
- Big companies using smaller business networks connected with moderate
data rates of around 2 Gbit/s.
- Home connections with the smallest data rates at 100 Mbit/s.
At the end of the network connections there will be optical sources such
as laser diode modules or photo receivers. These devices should meet the
specifications of the different network layers, including cost and
performance. At the lowest level, a very high number of customers will
use optical front-ends which must be produced in volume to minimize
cost. At the core network region are high-end devices where function and
performance are critical.
The Optical Mode Field
The behavior of the optical beam can normally be described by
classical ray optical functions for lenses with focus length and
focus point. If the dimensions of the optical beam come close to
1.5µm, which is the wavelength used in optical communications
networks, the behavior of the beam must be described by wave
Here, the optical field within a wave-guide can be described
nearly perfectly by a Gaussian intensity distribution, called p(r),
shown in Equation 1. If the wave travels
within the waveguide, the mode field diameter is constant due to
the combining function of the waveguide itself. At the end of the
waveguide the optical field is not constrained and the field expands
with increasing distance to the output facet of the waveguide. The expansion of the
field can be calculated by Equation 2. The point at which the
intensity has fallen down to 1/e² or 13.5% of the maximum
intensity in the radial direction, shown in Figure 2, defines
the mode field diameter.
After leaving the waveguide, the optical-mode field radius,
which is half the spot size, expands with increasing distance to
the facet. For distances of less than 200 µm the field
distribution is called "near field" and, for larger
distances, "far field". The angle where the intensity falls to 1/e², or 13.5%, of the maximum intensity is the "far field angle", which corresponds to the near-field
radius. These parameters are normally shown in the data sheets of
laser diodes or LEDs.
Figure 2: Intensity distribution of the optical
For an efficient transfer of optical energy from the SMF and a
laser diode wave guide, the mode profiles should
"overlap" as much as possible, as described by
Saruwatari and coworkers and is depicted in Equation 3. You can express the coupling
efficiency h between two Gaussian beams by means
of the mode fields of laser diode wld, and fiber
wSMF and also as a function of lateral, angular and
longitudinal misalignment between the two wave guides:
Equation 5 gives the loss L of the waveguide:
A comparison of the optical mode fields of the optical standard monomode
fiber, SMF, with a typical laser diode is shown in Figure 3, which also
shows the mode field diameter of a standard monomode fiber. The
International Telecommunication Union defines standards for SMF
Figure 3: Far field of an optical fiber in
comparison to the field of a laser diode
The far-field angle of the fiber is defined to a small value of
11.5°. A typical laser diode shows different values for
lateral and vertical axes of 20-to-30° and 30-to-40°,
Comparing the field parameters of fiber and laser diodes, you find a
great mismatch between them. Consequently, the optical coupling
efficiency between these two devices is very low. You can calculate the
coupling loss (in db) between the two fields, without additional
mechanical misalignments, with Saruwatari's formula from Equation 3,
which you can express with the formula of Equation 6:
Equation 5a gives the mode field mismatches between the single mode
components and the corresponding mismatch loss with all lateral and
angular misalignments of the fiber axis relative to the incident beam of
the laser waveguide set to zero.
Adaptation of Mode Fields
You can use "mode field transformers" to couple different mode fields.
Examples of these devices are lenses or lens systems, shown in Figure 4
With these devices, a nearly perfect coupling between the two
wave-guides is possible. Driven by cost constraints, several lens
configurations had been proposed for optical coupling of laser diodes to
Figure 4: Mode field adaptation by an optical lens
or lens system
Comparing the optical fields of a butt-ended SMF of an edge-emitting laser
diode shows a great mismatch. This mismatch is the reason for the very
low coupling efficiency of approximately 15% for a butt-ended fiber.
You can overcome this low efficiency using lenses to better couple the
two optical mode fieldsa coupling efficiency of more than 90% has been
shown. A disadvantage is critical part handling. There are several parts
including one or two lenses, the fiber, and the chip, which must be
handled for optical alignment. This handling results in the rather high
cost of optoelectronic packaging.
We used lenses made at the end of the fiber by
melting the glass fiber and pulling it. This kind of fiber end is
called fiber taper and works like a lens with diameters from
20 µm to 50 µm. Figure 5 shows the chip facet and its waveguide on the left, and the melted fiber taper in front of the waveguide on the right. The fiber is
additionally fixed into a metal cannula. You can achieve a coupling efficiency greater than 50% with these tapered fibers. Unfortunately, a high precision of better than 0.5 µm is
necessary to mount the tapered fiber in front of the OEIC without
additional losses. Therefore, the mechanical resolution of the
coupling mechanism must be better than this value. The procedure to correct misalignment after coupling should not introduce additional displacements and must be stable enough to fix the
coupling mechanism, which is important for good long-term stability. The
short working distance of 10 µm between fiber taper and laser, shown in
Figure 5, is also a problem for the life of the laser diode in case it
comes in contact with the end of the fiber. However, there is only one
device to handle at low cost, which makes this device very good for use
in small and reasonably priced modules.
Figure 5: Sketch of a melted fiber
taper in front of an OEIC (left), along with a photograph of the fiber taper (right)
Figures 6 and 7 show the tolerances for lateral and longitudinal
placement of the fiber taper in front of the OEIC. Both graphs show the distance in micrometers at
the x-axis and a relative intensity of the coupling efficiency
between the tapered fiber and the OEIC. Figure 6 shows that within 2 µm the intensity will be not be lower than 0.5 dB of the maximum intensity. For the longitudinal direction, the
tolerance is much greaterin this case 8 µmshown in Figure 7. It is
four times easier to perform the optical coupling in a longitudinal
direction than in a lateral direction.
Figure 6: X and Y-axis: 2µm tolerance for 0.5
dB additional coupling loss
Figure 7: Z-axis with higher tolerance of
Patented Fiber-Chip Coupling
We have developed a fiber-chip coupling unit for double-sided
coupling of waveguide-fed InP and GaAs OEICs, as shown in Figure
. The OEIC is placed in the center of a miniature optical bench. A
Peltier cooler controls the temperature of the bench. The optical
fiber is fed into a metal tube and fixed by glue or solder. One end
of the tube is clamped in one end of the bench while the other end
can now be adjusted in x- and y-directions by two adjusting pins
driven by Piezo manipulators. The fiber tube also acts as a spring
which assures that the end of the tube is always located in good
contact with the adjusting pins for x and y direction.
Figure 8: Patented fixation mechanism of the
fiber-chip coupling device which allows a correctable mechanical
fixing of the SMF in front of a OEIC within 0.2 µm.
The z-direction is adjusted by longitudinal shifting of the
metal tube. The tapered fiber can be rotated additionally around
the z-axis to adjust the polarization direction of polarization
After optimal adjustment of the tapered fiber in front of the
OEIC facet, a screw fixes the adjusting pins. The fixation process
forces a small mechanical shift of the fiber end due to deforming
pressure. This shift is more than 1 µm in both lateral
positions, which induces an additional optical loss of 50%. The
additional loss in the z direction is more than 5 µm, which
implies a total power loss. Therefore, the fixation must be
corrected in a time-consuming procedure. To overcome this problem, we
developed an automated coupling procedure, which we will explain later in this article.
Figure 9: Manipulator tool for combining optical modules
A manipulator tool, shown in Figure 9, was designed for easy adjustment
of all axes of the fiber in front of the OEIC. The coupling unit and the
manipulator were developed and patented at the Heinrich-Hertz-Institute. The manipulator was designed for adjusting several types of
fiber-chip coupling modules. The fiber can be adjusted in three
Cartesian axes (x, y, z) by piezoelectric drive-assisted micrometer
screws with sub-micrometer resolution. Additionally, the fiber can be
rotated in an axial direction with a resolution of better than 0.5°.
Automated Coupling Procedure
We developed a computerized automated coupling procedure for the
patented mechanism. Figure 10
shows a typical peak search routine
sequence for the best optical coupling. At the top left,
the optical intensity of the laser and at the top right the actual
coupled intensity are shown. The coupling starts in the center
of the window. You see that the search algorithm will search in
concentrically circles for the light. After detecting the laser
light, the optimization will finish within seconds.
Figure 10: Search algorithm of the automated coupling
The laser module and the semiconductor optical amplifier module
consist of a miniature optical bench, which contains the OEIC, the
tapered fiber and the adjusting pins. The bench is located on top
of two Peltier coolers for the thermal stabilization of the OEIC.
The laser bias, Peltier currents, and the temperature sensor are
fed via a standard 10-pin plug and a thick film circuit. The high
frequency for data modulation of the emitted light is fed via two
semi-rigid cables from the V-type connectors directly to the OEIC
without a glass bead. This guaranties a good high-frequency
response. One type of module is especially designed for
double-sided access to semiconductor amplifiers (SOAs) as
shown in the photograph of Figure 11a
, while another
variant is for single-sided laser diode (LD) coupling.
Figure 11: (a) Standardized optoelectronic module
for double-sided fiber-chip coupling with 2.5 GHz bandwidth. (b) Photodiode module with up to 45 GHz bandwidth.
For operating the waveguide integrated photodiodes, fabricated at the Heinrich-Hertz Institute, you do not need any temperature control. We developed a special package suitable for waveguide photodiodes with RF
response up to 45 GHz (Figure 11b).
We performed long-term temperature tests with several modules
between +15°C and +40°C and between –20°C and
+70°C. In all cases we used photodiode chips in the modules
and a stabilized laser source by Agilent to monitor the coupling
efficiency. The receiver photodiode is not temperature dependent
and less polarization sensitive, here ±2 dB, than laser diodes.
The temperature behavior of the laser module, shown in Figure 12
, shows a maximum output variation of ±0.15 dB with temperature. Three cycles within four hours of measuring time were performed with temperature controlling of the OEIC.
Figure 12: Temperature test of the laser module
between 15°C and 40°C
After thermal cycling we completed the test with mechanical
shock and vibration stress. First the modules were dropped from
10 cm to 30 cm onto a metal plate with a 0.5 mm
thick rubber foil. We performed the mechanical shock tests in all
three Cartesian directions.
The measured accelerations for a 30 cm fall amounted to more
than 200 g within 3 ms. The vibration excitation of the
module was measured with an acceleration sensor and a digital
oscilloscope. We found the acceleration to be stronger than
16 g within a broad spectral bandwidth of 50-5000 Hz. The
values vary somewhat due to the polarization dependence of the waveguide-fed photodiodes. We have not detected significant degradation of the coupling efficiency after these tests.
We performed the mechanical shock test until the tested device
showed mechanical damage after dropping it from a height of more
than 70 cm, which is comparable to an 800 g