Abstract: In this paper, optical interconnect applications in new consumer electronics market, such as high definition multimedia interface (HDMI), universal serial bus (USB), DisplayPort, Customized Video & Audio System, TV market are introduced. Since 2018, consumer electronics active optical cable (AOC) products, which is based on chip on boar D (COB) technology, have acquired consumer market approvation due to low cost, small packaging and p Arallel optics solutions. The detail specification and application of HDMI, USB and TV AOC have been further discussed. Finally, the roadmap and development prospects of optical interconnect in consumer electronics market T have been concluded at the end of this paper. It is believed that a large number of new optoelectronic products will appear in the consumer electr Onics market.
Conventional optical modules, such as small pluggable (SFP), four-channel SFP interface (QSFP), sixteen-channel pluggable optical module (CXP), Gibit passive optical network (GPON) and other optical modules for data centers and telecommunications networks, due to its optical transmission submodule (TOSA)/optical reception Components (ROSA) (optical chip packaging) have high production costs, large volume in mass production, and cannot meet the needs of multi-channel optical channel transmission and other factors, which cannot meet the customer demand standards of the consumer electronics market, so it has not been widely applied in new fields.
With its advantages of low cost and small packaging volume, the on-board chip packaging (COB) process has been used as an optical packaging solution for active optical cables (AOC) in the field of traditional optical modules. Since 2018, in the consumer market, such as high-definition multimedia interface (HDMI), universal serial bus (USB), digital video interface (DisplayPort), professional audio and video field, and split TV market, more and more new COB schemes based on COB schemes have appeared. Type AOC solution. This is a new application of optical interconnection in the field of new consumer electronics.
1.COB process
COB, also known as chip direct pasting, is the process of directly combining and pasting unencapsulated bare chips (including optical chips and electrical chips) to the printed circuit board (PCB), then conducting lead bonding, and finally protecting the packaging.
The technical core of COB consists of two parts: Die Bond and Wire Bond, the former is a patch and the latter is a wire bond. COB is a mature process widely used in electrical chips and luminous secondary tube (LED) packaging. It has been used in some products based on low packaging performance requirements before.
The traditional optical communication industry (specifically refers to the optical communication industry applied in data center networks and telecommunications networks) encounters bottlenecks in multiple parallel packaging such as 40 G/100 G. The previous TOSA/ROSA volume was too large and could not meet the volume requirements of multiple parallel packaging, so COB technology in 2010 It has become a hot topic in the field of optical communication [1].
The COB process has two significantly different characteristics when used in optical communication: (1) Its packaging accuracy requirements have been greatly improved. Typical fixed crystal accuracy requirements develop from hundreds of microns in the original electrical chip and LED packaging to within 5 μm in optical communication packaging; therefore, COB packaging at this time is also known as Sub-Micron precision packaging technology. ( 2) Due to the introduction of optical chips (vertical cavity surface emission laser (VCSEL) and photodiode (PIN)), the optical path part must transmit the optical signal to the optical fiber through optical devices (lens, reflectors or other optical devices); therefore, the protection of the packaging chip cannot adopt the electrical chip packaging processing. At the same time, the optical devices here also need to achieve submicron-level packaging (coupling) accuracy.
In traditional optical communication module markets such as data centers and telecommunications networks, the COB process is used as AOC’s scheme, such as SFP, QSFP, Mini Serial Small Computer System Interface (MiniSAS) and other interface types based on the Electrical and Electronic Engineers Association (IEEE) Ethernet or Inf AOC of iniBand protocol. Compared with the previous optical module scheme, AOC has obvious advantages in terms of cost, multi-channel integration and optical signal detection finished product rate; its only disadvantage is that the length is fixed, and customers cannot freely change the length like the optical module.
Since 2018, a new optical module scheme based on the COB process has emerged in the consumer optical communication market [2], and the AOC production process of the traditional optical communication market has been divided into two parts: the front-end module supplier first provides the semi-finished product of the optical module based on the COB process, and the back-end processing factory produces AO. C finished product.
The introduction of this new technology has greatly promoted the development of optical interconnection application in the consumer electronics market. Samsung took the lead in adopting this new COB process AOC scheme in the split-body TV market. In its new split-body large-size TV, the high-speed video signal, low-speed control signal and power signal between the host box and the TV screen all use its customized hybrid cable AOC. 2018 is the year of the rapid rise of the HDMI AOC market. Many new AOC products have begun to pour into the consumer electronics market, and optical interconnection applications have begun to realize applications and mass production in this new field.
1.1 Low cost
The consumer electronics market is very sensitive to costs. Although some traditional optical communication manufacturers have tried to enter this field before 2018, most of them are expensive due to immature production technology, which cannot really drive the application of optical module products in the consumer electronics market.
The COB process of a low-cost production technology began to appear in the consumer electronic optical interconnection market in 2018. Its main technical characteristics are reflected in 2 points: optical devices suitable for mass production and high-efficiency optical coupling methods in batches.
The optical device guides the light sent in VCSEL to the multimode optical fiber at the transmitting end, and at the same time guides the light from the multimode optical fiber to the PIN at the receiving end, as shown in Figure 1. Because the luminous surface of VCSEL and the photoelectric detection surface of PIN are both perpendicular to the printed circuit PCB board.
The coupling direction of multimode optical fiber is parallel to the PCB board, so in the COB process, the optical device needs to rotate the light by 90°. An efficient method conducive to mass production of optical devices is to use Ultem plastic materials for injection molding and design 3 optical planes respectively.
Take the emission end as an example, the first concentrated surface aggregates the light emitted by VCSEL in a Gausssian energy distribution into a collimated light; the second light plane is a 45° reflective surface, and the collimated beam forms a full reflection when it is 45° incident (the refractive index of Ultem material is about 1.65), and at this time, through this second light After the plane, a collimated beam parallel to the PCB board is formed; the third optical plane is another concentrated surface, and the collimated parallel beam converges through the concentrated surface to form an incident focused beam into the fiber core of the multimode optical fiber.
The optical path at the receiving end is similar to the optical path at the transmitting end, but the direction of the light is opposite. New optical devices are suitable for mass production. On the one hand, the materials suitable for injection molding greatly reduce the cost of a single optical device, and on the other hand, it is reflected in the optimization of optical parameter design.
Optical parameters, such as the optimization of aspherical curvature parameters of two concentrated surfaces, can greatly improve the tolerance of optical devices in their own production and tolerances of optical coupling errors, thus achieving a balance between cost design and the final light sensitivity requirements. To achieve this, optical designers need to have a deep understanding of the optical path principle and have rich experience in production.
Optical coupling can generally be divided into active and passive. Active coupling, that is, continuously adjusting the coupling position through feedback in the coupling process until the optimal value is reached. It is characterized by the need to put the optical chip into the working state during the coupling process, so it is called active; passive coupling, that is, in the coupling process, the optical chip is There are many methods of passive coupling that do not work or are not energized.
The typical one is to fix the optical device precisely to the designated place directly by taking photos and aligning through a precision crystal fixing machine. The error range is generally about 5 μm.
The mainstream AOC optical coupling mode in the market has experienced a development process from passive to active. In the original AOC market, manufacturers used the passive coupling method of large-scale precision crystal fixing machines, which could control the accuracy below 5 μm to meet the performance requirements.
However, according to the comprehensive calculation of the production cost, due to the large equipment investment of the precision crystal fixing machine, the coupling speed of a single equipment cannot really meet the needs of mass production.
Improving the efficiency of the coupling method in batches ultimately requires the use of active coupling method. The active coupling equipment can be customized according to the characteristics of the COB process. For example, according to the characteristics of the small PCB board in the COB process, the customized jig can operate multiple coupling worksits at the same time on 20 or 40 consecutive wrenches.
In addition, the investment of active coupling equipment is small, generally less than one-tenth of the price of precision crystal fixing machine. At the same time, the production capacity of a single equipment is large (the best can reach more than 5 times that of passive coupling equipment), thus greatly reducing production costs.
1.2 Small volume
Compared with the data center and telecommunications network market, the number of product types and functions in the consumer electronics market has greatly increased. At present, the interfaces of optical modules in the traditional data center and telecommunications network market are mostly concentrated in a few types of standards, such as SFP, QSFP, MiniSAS, CXP, etc.
The number of interfaces of consumer electronics far exceeds the number of interfaces in the data center, among which the standard interfaces include HDMI Type A, HDMI Type D, digital video interface (DVI), DisplayPort, Mini DP, USB Typ. e C, Mipi, etc., data interfaces include USB Standard A, USB Micro B, USB Standard B, Mini USB, Thunderbolt, USB Type C, etc.
In addition, more interfaces are customer-customized interfaces, such as an Occulink interface used on Facebook and Microsoft’s virtual reality (VR) helmet, and a customized interface for Samsung split-body TVs. Among the interface types of these new consumer electronics, the small-volume COB process optical module is conducive to the promotion of optical interconnection in the application of different products and different functions.
Since the COB process is to directly combine and paste unencapsulated bare chips (including optical chips and electrical chips) to the PCB circuit board, compared with the encapsulated TOSA and ROSA used in traditional optical modules, the volume required by the bare chip and the designed optical devices is much smaller.
Take the size of the single-channel optical chip as an example, the length and width of a single VCSEL or PIN are 0.25 mm, and the height is generally 0.15 mm; and the customized optical device design, the minimum size can be less than 1 mm. Considering the coupling jig and the structural design of the connecting optical fiber, the length and width can be within 5 mm. It can be seen that the volume of optical modules based on COB technology can adapt to the structural needs of the vast majority of consumer electronics markets.
1.3 Support parallel multi-channel optical path integration technology
The COB process can not only support the design of small-volume and small-sized optical devices and optical modules, but also support parallel multi-channel optical path integration technology. Due to the direct processing of bare chips, 4-way, 8-way or 12-way parallel arrays can be customized when cutting optical chip elements, and optical devices can also be customized to be customized into 4-way, 8-way or 12-way optical channels. 4-way, 8-way and 12-way are common array number combinations.
The corresponding interface types in traditional data center networks are: QSFP, MiniSAS, CXP, etc. In addition, multi-mode optical fiber arrays generally take 12 channels as the common number of channels, and optical fiber plug-ins such as multi-optic propulsion (MPO) optical fiber connectors are also designed with 12 channels as the standard number.
In recent years, there have also been 24-way MPO, which is generally used in some special occasions or optical waveguide system design. The parallel multi-channel optical path integration technology is well implemented in the COB process, which cannot be done by traditional TOSA and ROSA.
Consumer electronics also have a very strong demand for parallel optical channel processing. For example, HDMI, DisplayPort and DVI are all 4 one-way channels, USB Type C is a two-way dual-channel, and special applications such as VR and split-body TV generally require data transmission and video. Transmission in parallel, generally requires 6 channels or more.
The parallel multi-channel optical path greatly simplifies the design of module electrical processing at the optical module end, and solves the problems that high-speed electrical signals cannot be exceeded in transmission, such as electromagnetic interference (EMI), transmission length, cable outer diameter is too thick, excessive volume, etc., providing simple, feasible, reliable and low-cost light for new applications. Interconnection solutions.
2.New consumer electronic optical interconnection applications
2.1 HDMI AOC.
2018 is the year of rapid rise of the HDMI AOC market. Figure 2 is a schematic diagram of the HDMI AOC, in which there are 4 pairs of high-speed signal difference pairs, corresponding to the red, green and blue (RGB) and a set of clock signal pairs in the video signal respectively. Optical interconnection converts these four pairs of high-speed signal pairs to photoelectricity at both ends of the cable and converts them into optical signal transmission, so as to achieve ultra-long-distance transmission that could not be satisfied by previous copper wire technology.
Except for the difference of these four pairs of high-speed signals, other signals are still transmitted according to the original copper wire. ( Note: There are also some technical schemes that turn all signals into optical fibers. However, due to its compatibility and the cost price of the module, large-scale mass production has not been formed. The introduction here is the hybrid cable HDMI AOC that has been mass-produced.)
HDMI AOC is divided into HDMI 1.4, 2.0 and 2.1 versions. The corresponding transmission rates are 3.4 Gbit/s, 6 Gbit/s and 12 Gbit/s per channel, which can transmit 4 K 30 Hz, 4 K 60 Hz and 8 respectively. K 30 Hz HD video resolution. Its interface types are divided into HDMI Type A and Type D.
The main application scenarios of HDMI AOC are large conference rooms, electronic classrooms, home theaters, outdoor shooting, video monitoring and control, engineering wiring and other places where long-distance and high-definition video transmission is needed.
2.2 USB AOC.
USB is the most common interface on the computer side. The principle diagram of USB 3.0 AOC is shown in Figure 3. USB 3.0 AOC has a pair of two-way high-speed signal differential pairs to transmit USB 3.0 high-speed data signals.
Optical interconnection converts this pair of two-way high-speed signals at both ends of the cable and converts them into optical signal transmission, so as to realize long-distance data transmission.
There are many interface types of USB, such as Standard A, Micro B, Mini, Type C, etc., which are divided into USB 2.0, USB 3.0, USB 3.1, etc. according to protocols and rates. Typical application scenarios of USB AOC include industrial cameras, PC data transmission, and various large-scale conference data wiring.
2.3 Split TV AOC
In 2018, Samsung pioneered the transmission of its split TV using optical interconnection scheme, and sold millions of the split TVs that year, opening the way for the mass production application of optical interconnection in the consumer electronics market, especially the TV market. In 2019, Skyworth’s split-body Q80 [3], which applied the optical module scheme of Chinese manufacturers, was released and sold in China for the first time.
The function diagram of split-body TV is shown in Figure 4. The main feature of split TV is the separation of display function and multimedia control function, so that the display only plays the role of display, so the design of the display part can be ultra-thin, ultra-high definition and large size.
Because the control part is separated from the display part, it has the characteristics of upgrade and replacement, and the multimedia external interface is complete, and it can also be used as an ecological center control access point for the Internet of Things plus artificial intelligence (AIoT).
Regarding the design and concept of split TV, it has become a hot topic of discussion on global TV technology since 2018. From the perspective of the essential characteristics of smart home and display, split TV will be a comprehensive processing center and solution for the Internet of Things, artificial intelligence and home entertainment in the future.
In such a specific product form, optical interconnection is one of the indispensable design parts. Due to the ultra-thin characteristics of the display, the transmission of high-definition video signals requires a high-speed and thin data cable, so optical fiber is the only choice.
The whole screen is driven by only one transmission line, so this is a hybrid AOC, which also includes a power supply cable for the screen and a two-way low-speed data control signal. The ultra-high-definition video signal is converted from the host box to the screen through photoelectric conversion.
The two-way low-frequency data control signal connects the communication between the screen and the host box. High-voltage and high-current power signals are supplied to the TV from the host box through this hybrid AOC.
3.The development route of optical interconnection technology and the fit of the development of optical interconnection in the consumer electronics market
The technical development route of optical interconnection [4] is shown in Figure 5, which has experienced transmission from thousands of kilometers to tens of kilometers to hundreds of kilometers of telecommunications network “Telecom”, to transmission within hundreds of meters to 100 meters of data network “Datacom”, and then to the computing network “Computer-c The transmission of om” is less than 1 meter or even a few millimeters.
The telecommunications network is the transmission of data between devices, the data network is the transmission between the card edge server and the server or switch, and the computing network is the optical transmission between the board, between chips and chips, and even between the chips and chips.
The customers of the telecommunications network are operators, such as Telecom, Mobile and Unicom; the customers of the data network are major data centers, such as Tencent Cloud, Alibaba Cloud, Baidu Cloud, etc.; the telecommunications network and data network are the application scenarios of the traditional optical communication modules mentioned in this article. Their common interface types are such as SFP, QSFP, CXP, M iniSAS, etc., the protocols followed include Ethernet, Infiniband, etc.
When it comes to the computing network, there will be a large number of short-distance optical interconnections, such as On-board Optics technology, which has become a research hotspot of optical interconnection in recent years. For the first time in 2016, the Ethernet Association IEEE[5] wrote concepts such as “optical module miniaturization” and “optical integration” into the Ethernet technology development roadmap, and put forward the concept and market development direction of “photoelectric conversion embedded system”.
In the future, silicon optical communication is expected to achieve a shorter distance of optical guardrails, such as communication between chips and chips, and even communication within the chip. Silicon optical communication refers to transmitters (lasers), receivers (photoelectric detectors), modems (if indirectly modulation) and waveguides (silicon waveguides are molded) made of silicon materials.
From a technical perspective, the optical interconnection application in the consumer electronics market belongs to a kind of computational network optical interconnection application: unlike the traditional telecommunications network and data network market, its demand for optical interconnection is not only satisfied with board-edge interconnection (such as split-body TV, medical and other applications), but has begun to make optical modules into the board;
consumption The technical requirements of the market for optical interconnection with low production cost, small volume and parallel multiple optical channels have promoted the development of optical interconnection towards the performance standard requirements of computing network optical modules.
From the perspective of the market, with the rise of the 4 K/8 K high-definition video concept, the total shipment of global HDMI interface products in 2019 is expected to be nearly 1 billion units [6]; under various global USB models in 2019, the shipment of Type-C models alone is expected to exceed 2 billion units [ 7];
The global sales volume of TV in 2018 was about 225 million units [8], and the above are only representative areas of the consumer electronics market. Under the policy of “light in and copper in out”, people’s demand for data transmission is also increasing and faster.
It can be predicted that the growth of optical interconnection applications in the consumer electronics market is an inevitable market trend. In the future field of optical communication, the consumer electronics market will occupy an increasingly important position. Because it covers enough fields and applications, its market share will inevitably far exceed the traditional optical communication market.
Therefore, whether from the perspective of technology or market development, a large number of new optoelectronic products will appear in the consumer electronics market, which is an inevitable trend of optical interconnection development.
4.Conclusion
From the perspective of data transmission, there are only three transmission media widely used by human beings from ancient times to the present: copper, light and wireless, and they all transmit information in the form of electromagnetic waves.
2019 has begun to enter the era of a sharp increase in 5G network, 4K/8K ultra-high-definition video and data volume, and the demand for transmission media will also increase. It can be expected that the optical interconnection market will soon usher in great growth, and due to the introduction of new technologies and new solutions, optical interconnection will be more widely applied in more markets and fields.
When it comes to optical communication, people will not only think of the optical modules applied in their data centers and operators, but also begin to think of optical communication in daily consumer electronic products such as TVs, mobile phones, cars, projectors, electronic classrooms and conference rooms at home. I believe that such a future will come soon!