Photonic components are key elements for the information technology (IT). Photonics technology covers the generation of information (cameras, sensors), its transportation (optical communication), storage (CD, DVD) and display (CRT, LCD, others).
The high bandwidth and low attenuation of silica optical fiber enables long-distance phone calls and high-speed Internet access with almost no limits at very low cost. In fact, downloading data from the Internet is basically free, because the optical fiber infrastructure is efficient and requires minimal maintenance.
Optoelectronic devices cover a wide range from High-power laser diodes, PV Cells, Optical Interconnects, DFB’s, SOA’s, Mach-Zender Interferometers, Modulators, Photodetectors, and TOSA/ROSA. A vital function of an optoelectronic device is to manipulate light. As a result, many optoelectronic components’ packaging and assembly processes can be significantly more complex than traditional microelectronic devices. Packaging and assembly technologies are fundamental to making available and practically usable the results of advanced photonic research.
Optoelectronic or Photonics packaging requires highly accurate placement of components to ensure appropriate suitable alignment and coupling of light into and out of devices. In many cases the generation or detection of light results in significant heat being generated, which if not suitably dissipated through appropriate joining technology and material selection, can adversely affect the device operation. Additionally, assembly processes can be complicated by the requirement that fluxes and other organic materials can’t be used as they can degrade the performance of facets in optical devices.
Some of the technologies include Laser Diode Packaging
For the highest performance systems, single-mode optical fibers are used and require high precision and mechanical stability of the packaging system. Therefore metal packages and welding and soldering processes are mainly employed. Butterfly packages are most common in telecom applications. For datacom, TO-cans are mostly used.
Multimode fibers for intermediate range distances in datacom applications relax the alignment tolerances and thereby can reduce cost. The standard transmitter is based on a TO-can with a VCSEL light source inside. Bitcom components are based on low-cost lead-frame packages. They are nonhermetic and offer wide alignment tolerances. Due to their low-cost structure, they can move the application of photonics into the very short distance range and directly compete with electrical solutions based on copper wire cables.
Butterfly assembly process
The butterfly package features a window in the package wall for light output. A lens can be integrated in the window for improved coupling of the light into the fiber pigtail outside the package. In other package designs, the fiber end is inserted into the package through the package wall. In this case, the silica fiber is first metallized and then soldered to a metal feedthrough tube in the package wall. The solder process provides the necessary hermetic sealing.
The butterfly laser assembly process uses mostly soldering processes. First, the laser chip is soldered on the carrier, then the carrier on the thermoelectric cooler. Finally, everything is soldered to the base plate of the butterfly package. The solder processes follow a certain hierachy where solders of higher melting temperature are used first to avoid remelting of solder joints.
The mounting of the first optical lens requires active optical alignment. The lens is positioned in X-Y-Z directions with micropositioning units and fixed in place by laser welding. The butterfly package is closed with a lid. The lid attachment is a seam-seal electrical welding process. Prior to seam-seal, small amounts of helium are added to the atmosphere inside the butterfly package. Any leakage of the package can be detected in a helium leak tester. The optical fiber pigtail is also actively aligned and fixed in position by laser welding.
Epoxy material is seldom used in butterfly packages because of outgassing concerns. If they are used they have to be selected carefully in order to meet the stringent requirements for components used in telecom systems. Outgassing material can potentially condense on elements in the optical path and thereby change output power or cause other reliability issues.
TO-can assembly process
Datacom products are typically sold as transceiver modules including the transmit and receive elements plus some electronic circuits to provide a standard electrical interface. If a multiplexer function is included, the modules are called transponders. 10 Gb/s modules are mostly transponders; at 2.5 Gb/s or below they are primarily transceivers
The transistor outline can (TO-can) is the standard package for datacom components. It was originally developed for transistor packaging and is widely used for optoelectronic chips. Most CD and DVD lasers are packaged in TO-cans. The TO-can provides hermetic sealing of the chip and is relatively inexpensive because of the huge manufacturing volume. The speed of TO-cans is normally sufficient for applications up to 2.5 Gb/s. However, TO-cans with an improved impedance matching at the electrical feedthroughs can operate at 10 Gb/s.
The manufacturing process starts with mounting the VCSEL, monitor photodiode and submount on the TO-header. The window cap is attached in an electrowelding process. With the cap, the TO-can is hermetically sealed. Most diode lasers are shipped in this format. Afterward the barrel is optically aligned in X-Y-Z direction relative to the TO-can using active alignment. The VCSEL operates at some nominal current and output power. The barrel is fixed with UV epoxy and later cured. This component including the TO-can and the barrel is called the transmitter optical subassembly (TOSA).
On the receiver side, the components inside the TO-can are the photodetector and the first stage transimpedance amplifier. Sometimes additional capacitors are also included. The assembly process is equivalent to the transmitter side. The TO-can with the attached barrel is called the receiver optical subassembly (ROSA).
For single-mode applications, the alignment tolerances are much tighter compared with the multimode system. Since the fiber core diameter is approximately 9 µm, the mechanical precision needs to be within 1 µm. Therefore the barrel is typically made of stainless steel. The coupling lens is made of glass or sapphire and inserted into the barrel. The optical alignment is fixed using a laser welding process for the X-Y-Z direction.
The TO-can contains a vertical stem for the mounting of the edge emitting laser. The back facet of the laser chip looks toward the bottom of the package. The monitor photodiode is mounted at this position. The emission from the front facet is coupled to the single-mode fiber. The mounting of the laser chip requires wire bonding at a 90° angle. In the case of DFB lasers, an optical isolator is also inserted into the metal barrel.
Bitcom components are for applications using large-core optical fibers. The emphasis is on low cost to compete with electrical wiring solutions. To reduce cost, bitcom systems are designed for large mechanical alignment tolerances. Polymer-clad silica (PCS or HCS) fibers or plastic optical fibers (POF) with large core diameters of 200 µm to 1 mm are used. These fibers relax the alignment and manufacturing tolerances of optical connectors compared with multimode (50 µm core diameter) or single-mode (9 µm core diameter) fibers. In addition to their high speed potential, bitcom optical links beat electrical links in respect to electromagnetic interference (EMI) noise generation and immunity to EMI. They are also smaller, more flexible and lighter weight compared with high-speed copper cables such as shielded twisted pair (STP) or coaxial. This makes optical fibers the preferred communication media in automotive applications even at data rates of only 25 Mb/s.
Integrated Photonics Packaging
Recent decades have seen the growth of integrated photonics supported by the ever-growing information communication technology needs, as well as lidar systems, biomedical and other industrial sensing applications. The silicon Photonic Integrated Circuit (PIC) platform was created on the foundation of established CMOS (complementary metal–oxide–semiconductor) fabrication technology for silicon electronics. This native compatibility with CMOS technology, together with the ability to build compact, highly integrated photonic subsystems are the main driving forces behind Si-photonics. In recent years, the field has further grown out of the Silicon to include other integrated photonics platforms such as SOI, InP , Si3N4 and Ge.
Hundreds of the unique PIC designs, together containing thousands of Photonics elements can be laid out in a single wafer. Researchers, graduate students, and Small & Medium Enterprises (SMEs) now routinely design PICs through these Multi-Project Wafer (MPW) runs which enable them to develop new devices and share costs without having to individually invest in expensive dedicated wafer runs. These PICs, a few millimeters wide, can easily contain hundreds of electrical and optical connections on a very small footprint, all of which require interfacing with other elements of the system. The challenge for integrated photonics is no longer in fabricating photonic chips, but in fabricating photonic devices, i.e. the coupling of light and electrical signals between the 2D PIC and the 3D world outside.
Developing an operational PIC and demonstrating its functionality in a laboratory environment requires building a first prototype that allows accessing its inputs and outputs. PICs – being tightly integrated devices and at most a few millimeters long – need to be electrically and optically interfaced.
They also need to be placed in a dedicated mechanical housing, that often needs to be thermally cooled. These four domains – electrical, optical, mechanical and thermal – make the core of what is known as photonic packaging. The challenges associated with photonic packaging are often underestimated and remain technically challenging.
The optical alignment tolerances can be sub-micron and must be carefully considered to account for the polarization requirements and thermal expansion. Small misalignments can cause large optical losses which necessitates photonics packages to be well made and accounting for all of these issues already at a lab stage. However, bridging the gap from laboratory to an industrial product commonly brings in significant other issues related to reliability, which reflect in high financial cost of packaging which, at small volumes, can easily amount to about 50% of overall product cost.
Photonic Packaging and Test Technologies
PhotonicLEAP, a European Horizon 2020 collaborative research project based in Ireland, has been awarded over €5 million Euros in funding from the European Commission to develop disruptive technologies that will drive down the cost of integrated photonic packaging and test processes.
Photonic Integrated Circuit (PIC) technologies are the light-based equivalent of electronic circuits – a technology that is becoming increasingly important for existing markets in communications, medical devices, sensors and for emerging markets such as quantum computing and security. However, existing PIC manufacturing processes, in particular packaging and test processes, are difficult to automate, with limited manufacturing capacity, and are costly, with packaging and testing typically accounting for over 75% of the total manufacturing cost. As a result, existing PIC manufacturing processes greatly impede the uptake of PIC technologies across many mass markets.
To address this challenge, PhotonicLEAP will develop disruptive wafer-level PIC module integration, packaging and test technologies which will reduce the costs of PIC production by over 10 times, revolutionising existing applications and creating completely new markets. PhotonicLEAP will use these disruptive technologies to produce a revolutionary Surface Mount Technology (SMT) PIC package, which for the first time incorporates multiple optical and electrical connections. The project will validate these technologies through state-of-the-art demonstrators, including a high-speed optical communication module and a medical device for cardio-vascular diagnostics. Furthermore, the technologies will be implemented by the flagship European PIC Packaging Pilot Line, PIXAPP, for future commercialisation. PIXAPP has an extensive and growing user-base across multiple markets.