A Reusable Fiber-Chip Coupling Method for Optical-Communication Transmitter Modules

	ABOUT THE AUTHORS 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:

1. Companies using a high data speed of 10 Gbits/s
2. Big companies using smaller business networks connected with moderate data rates of around 2 Gbit/s.
3. 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.

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 mode field.

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 w_ld, and fiber w_SMF 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 properties.

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°, respectively.

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.

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 fields—a 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 greater—in this case 8 µm—shown 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 8µm

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 dependent fibers.

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°.

Figure 10:  Search algorithm of the automated coupling program

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).

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 acceleration.