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Advancing Microelectronics • Volume 29, No. 1 • January/February, 2002
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Optical Alignment in Optoelectronic Components
W. Jeffrey Shakespeare, T-Networks, Inc.
Abstract
Packaging optoelectronic components presents a number of challenges specifically related to the optical aspects of the components. The typical opto system costs are greatly influenced by the cost of the packaging for many of the components. The packaging cost in turn is often driven by the need to align the optics to extremely close tolerances, of order less than 1 micron to several microns depending on the device. The approach to optical alignment can have a great influence on the final cost of the device as well as the cost of manufacturing capital equipment and overall quality, yield, and manufacturability.
This paper will briefly explore the current alignment technologies beginning with a consideration of the optical requirements and attempt to evaluate qualitatively the merits of the various techniques. There is a discussion of process technologies and equipment with a review of manufacturability and a rough order of magnitude of cost associated with each.
Introduction
In general there are two broad categories of optoelectronic devices from the standpoint of optical alignment.
• “In fiber” type devices that contain the light-modifying component within the fiber itself. These include fiber Bragg gratings, fiber lasers, fused fiber devices such as couplers, taps, wavelength division multiplexors, and filters, polymer modulators, and Erbium doped fiber amplifiers (EDFAs). There is no need to do fiber alignment on these devices since the light is already in the fibers. The only requirement is to be able to couple the fibers themselves together by means of connectors or fiber splicing. This in turn requires that the fiber cores be aligned to one another. There are a large number of standard fiber optic connectors on the market that do this function with order 0.05 dB to 0.5 dB loss in the connection. Direct fiber-to-fiber splicing can be achieved with several commercially available splicing machines that use an optical core alignment technique and achieve 0.01 dB to 0.1 dB optical losses in the connection.
• Discrete devices that must be coupled to the fiber for use in the system. These include the greater of number of components both passive and active such as bulk optical elements, e.g., polarization sensitive and insensitive isolators, thin film filters, wavelength division multiplexors, taps, polarization combiners, in line polarizers, and silica waveguide devices. Active devices include lasers, modulators, photodetectors, semiconductor optical amplifiers, variable optical attenuators, polarization controllers, and many others. This second category will be the main subject of this paper.
Considerations in the Choice of Optical Coupling
Several things influence the choice of optical coupling technology:
• Coupling efficiency required
• Cost — of the device being coupled as well as final assembly cost
• Process platform and manufacturability
• Volume production requirements
• Reworkability of the component and yield at various stages of assembly
• Type of device being coupled — optical properties, size, number of fibers to be coupled, optical output only or both input and output required, wavelength and tenability over a range of wavelengths, need for optical isolation
• Type of fiber — single mode, multimode, polarization maintaining, glass, plastic, fluoride, Erbium or rare earth doped fiber
• Fiber pigtail or connectorization to the component
• Return loss requirements
• Environmental — device thermal dissipation, cooler or uncooled, shock and vibration, hermeticity, central office or outside plant application
The Optics of Alignment
Figure 1 shows a diagram of the optical beam emanating from or coupling into an optical fiber. Light leaving the fiber generally has a Gaussian intensity shape which increases in size but not shape as the beam is measured farther from the fiber face. The angle of beam expansion is characterized by the so-called Numerical Aperture of the beam defined as:
Where theta is the angle of the Gaussian beam intensity point of choice i.e. 1/e2 point or the full width half max point. Full width half max is where the beam intensity is one half the maximum. The 1/e2 point contains over 98% of the light energy in the beam and this is what will be used for discussion.
In general, maximum coupling is obtained by matching the shape and Numerical Aperture of the beam to the fiber. This implies that for each device, an optical scheme must be devised that will shape and focus the beam from the device being coupled to the fiber in order to achieve maximum coupling. Of course each of the considerations listed above must be balanced to obtain the optimum solution to the problem.
Optical Alignment Techniques
There are two fundamental types of alignment schemes:
• Passive optical alignment where the coupling efficiency is determined by the mechanical or vision system tolerances of the piece parts or assembly equipment.
• Active optical alignment where the light is emitted or passed through the device and some type of high accuracy alignment equipment is used obtain a maximum coupling into the optical fiber either manually or automatically.
Table 1 compares the two approaches.
Methods of Passive Optical Alignment
There are several excellent methods of passive alignment including:
• Fiber V-groove in an etched Silicon substrate — (Figure 2). This approach uses the highly accurate photolithographic etching process to produce a relatively tight tolerance submount for a device with a fiber that can be passively positioned and soldered or epoxied into place.
• Optic in V-groove in an etched Silicon substrate — (Figure 3). This approach is the same as above but substitutes the grin lens or other low cost optic for the fiber itself. This also allows for a hybrid passive — active approach with external fiber active alignment.
• High accuracy die bonder, usually with vision system, to assemble device and optic or fiber to a ceramic or metallic substrate. This approach is highly automatable and with latest advances in vision placement technology can achieve accuracy close to the active alignment tolerances.
• High accuracy ceramic or metal fiber ferrule with integrated lens in photonic device. This approach is especially good for broad area detectors or Light Emitting Diodes. This is also the method used in optical connectors along with a high accuracy spring sleeve to align the fibers to each other.
This list is by no means exhaustive but serves to illustrate some of the common methods of passive alignment.
Methods of Active Optical Alignment
Active device alignment is achieved by passing light through or from the device and into the fiber. The end of the fiber is connected to an optical power meter and the fiber or lens system is moved about to get the highest coupling as indicated by the optical power in the fiber. Submicron accuracy mechanical alignment stages such as the ones manufactured by Newport or Melles Griot are used to position the optics. These stages come in several levels of accuracy. Single micrometer stages are capable of being positioned to within a few microns and cost a few thousand dollars depending on the manufacturer. Some of these stages can be equipped with differential micrometers that allow within a micron of accuracy, and for 50 to 100 nanometer capability, piezoelectric micrometers stages can be used. These submicron positioners cost upwards of $15,000 each for a 3 axis alignment system. Alignment of components to tenths of a micron is not only expensive in the cost of equipment but also can take upwards of 30 to 60 minutes to perform manually. A number of manufacturers offer automated alignment algorithms that operate on a PC and use motorized positioners with feedback from the optical power meter to search, find and maximize the received light. These systems can be obtained in 1 to 6 axis alignment and cost from $30,000 to $60,000 depending on the number of axes and the speed and accuracy of the system. A good quality automatic alignment system can align in 3-5 minutes.
Some example methods of active optical alignment:
• One popular method for single mode devices uses a fiber with a lens machined onto the end, held in a metal or ceramic ferrule and moved using high accuracy alignment stages to obtain a maximum coupled power into the fiber, Figure 4. An example of alignment equipment manufactured by Newport Corporation that also laser welds the fiber ferrule to the package is shown in Figure 4a. Of course the fiber ferrule can also be epoxy or solder attached to the package.
• Another method uses a lens train that is actively aligned using micro-positioning stages and then either laser welded or epoxy attached in place, Figure 5.
• It is possible to combine the passive alignment with an active external fiber align in an axi-symmetric configuration. Figure 5a shows equipment manufactured by Newport Corporation that performs the auto-align and laser weld operation in one station.
• Devices such as bulk optic filters and waveguides may be direct butt coupled using a polished fiber ferrule made of glass or ceramic and epoxied directly to the end of the device. This approach puts epoxy in the light path, which can be a problem especially for high power devices, but can be very cost effective. The fiber ferrule is gripped and manipulated with micropositioners to align the device with the fiber core, again looking for maximum received power into the output fiber. This approach can be used for a single fiber or a group of fibers, Figure 6.
Summary and Conclusions
Optical alignment in photonic devices is a significant cost driver that has implications on the fundamental physical architecture of the product as well as the manufacturing processes. As optoelectronic products become more of a commodity, the need for cost effective optical alignment technology will increase, driving both the photonic component and the packaging. This is probably the most significant packaging differentiator and represents opportunity for innovation in product as well as manufacturing process technology. The goal of ubiquitous high bandwidth optical communication is only attainable with the right cost structure and this in turn is dependent on inexpensive, high quality optical coupling technology.
Acknowledgments
The author would like to thank Jason Iceman and Kathy Franklin for help in preparing the illustrations and Steve OíBrien for consultation on the physics of optical coupling.
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