Ultrashort pulse lasers pave the way for 5G

5G antenna modules are currently being added to cell towers. Within these modules are 64 active antennas whose radiation overlaps to ensure optimum spatial coverage. In the trade, this is known as massive multiple-input multiple-output. Massive MIMO also shapes the picture on the opposite side: MIMO-compatible antennas are needed so that smartphones, autonomous vehicles, and Industry 4.0 systems can work in the 5G network. These are highly complex modules with 16 and more individual antennas. The modules enable mobile devices and systems to use the signals from various transmitters simultaneously, thus increasing the transmitted data rates considerably with no difference in transmission performance.

But there are challenges: 5G networks work with different gigahertz millimeter wave (GHz mmWave) frequencies. The spectrum ranges from 3.3 - 77 GHZ. The lower the frequency, the larger the antenna must be. Conversely, tiny, extremely precise antenna modules are required for the highest frequencies. However, with the combination of tiny antennas and GHz mmWave frequencies, problems are inevitable. For example, the fingers and hands of smartphone users can block data transfer.

Three and more antenna modules per smartphone

To overcome these challenges – in other words, ensuring robust MIMO data transfer in various frequencies—smartphone manufacturers will install at least three antenna modules in various areas of their 5G devices. This significantly increases demand for antenna modules. There are also other challenges involved in manufacturing the very complex, in some cases, 3-dimensional 5G antennas. “The higher the frequency, the finer the structures of the antennas must be,” is how LPKF Laser & Electronics AG describes the fundamental situation. The company offers a laser direct structuring process with which it is possible to attach any shape of antenna, conducting paths, or insulation channels in resolutions to 25 micrometers (µm) directly onto three-dimensional plastic components.

Coherent also addressed the demand for production solutions for 5G antennas at an early stage. In a current scientific paper, Coherent experts Hatim Haloui and Dirk Müller explain why ultrashort pulse lasers can also demonstrate their strengths in this area: “The antennas are manufactured from laminated substrates consisting of a copper layer, an insulator such as LCP (liquid crystal polymer) or modified polyimides, and a connection layer.” In the production process the antenna structures are then cut to size and exposed with a laser. But because copper and polymers have very different ablation thresholds, extreme care is necessary. If the electromagnetic antenna material is affected negatively by heat that is applied, this will shorten its life or even cause short circuits. To avoid these risks, Coherent uses a method involving new picosecond lasers with up to 30 watts of power that work in the ultraviolet wavelength range of 355 nanometers. “This allows scan speeds of several meters per second, with typically around 10 stages that are required for the latest antenna designs,” say the authors.

Highly developed USP processing strategy

To minimize thermal influences on the sensitive antenna material, Coherent has a “Pulse EQ” processing strategy. The pulse frequency is adjusted in real time depending on whether the laser is working in a straight line or with narrow curves. An integrated active pulse control system ensures stable pulse energies. This results in extremely homogeneous sectional images with no thermal effects on the copper-LCP laminate.

Antennas in displays

A recent research report from Professor Wonbin Hong and his team from the Pohang University of Science and Technology in South Korea met with a positive response throughout the world. Similarly to the integration of keyboards in touch displays, the scientists are working on integrating non-visible 5G antennas into displays. In a consortium with industrial partners, such as SK Telecom and LG Electronics, the team was able to implement an antenna-on-display (AOD) in the ultra-high resolution touch display of a 28 GHz 5G smartphone for the first time. By doing this, the scientists overcame the difficulty of integrating three to four modules with dozens of antennas into the installation space of smartphones, which is already very small. The team achieved the invisibility of the AOD by using nanostructures. According to their paper, with this method it is possible to integrate antennas in the complete area of OLED or LCD displays, even if they are foldable or incorporated in wearables.

No matter whether the future of 5G lies in laser direct structuring, antenna production with USP lasers, or antenna-on-display technology—one thing is certain: Photonics is sure to play a key role.

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