By Bengt O. Hallberg

Welcome to my first blog! The theme for this one and the ones to come is Sweden's forgotten WDM Breakthrough in the 1970s and 1980s. It's about how my team and I developed what was probably the world's first pre-production prototype for three-channel fiber optic multimode WDM transmission in 1985, as well as a second singlemode system for drop/insert networks in 1987.

Holographic sinus grating 900 lines/mm in oblique daylight manufactured at the Institute of Optical Research at KTH, Stockholm, Sweden. Size 50 x 50 mm.

Institute of Optical Research 1976 - 1981

In 1978, I was employed as a scientist at the Institute of Optical Research (IOF), affiliated with the Royal Institute of Technology in Stockholm, Sweden. I had joined the group two years earlier while working on my engineering physics thesis. It became part of the newly launched project in fiber optic communication.

The Swedish telecommunications company Ericsson was part of the project. Tech.Lic. Leif Stensland, the Scientific Project Manager at IOF, tasked me with conducting a preliminary study on wavelength division multiplexing (WDM). Not much had been published about it, but I found an interesting paper from Bell Labs in New Jersey. I understood that this technology could play a significant role in future fiber optic communication systems.

Another important group at IOF, led by Professor of Optics Klaus Biedermann, was a world leader in research on holographic gratings. In addition to being an important technology in itself in photonics, it could be used as part of my WDM research.

I received approval to conduct my Ph.D. studies on WDM and worked on both the theoretical and applied aspects. This involved designing and testing various combinations of multiplexers (MUX) and demultiplexers (DEMUX). The gratings in those were manufactured according to my specifications in our laboratory.

A MUX combines multiple wavelengths from multiple transmitters (TX) into a single fiber. A DEMUX splits and distributes the wavelengths to multiple receivers (RX).

A laser diode is a semiconductor structure that produces a coherent, amplified light beam within a cavity. Coherence means stable wavelength and phase. These diodes were intended to serve as light sources in future transmitter (TX) units. For the system to work, the laser diodes in the transmitter (TX) had to be finely tuned to exact wavelengths. I developed a method for this process. It was based on influencing the optical cavity with an external device.

The situation in 1981

By 1981, I had figured out the entire theory and how all the parts worked through laboratory experiments. In other words, I learned how multiple incoming electronic signals are converted into optical signals, combined on an optical fiber, split up at the other end, and sent to multiple receivers, where they're converted back to the original electronic signals. At that time, NEC in Japan had begun producing MUX and DEMUX devices on a small scale.

I became one of the world's leading experts in WDM technology and system design. Perhaps I was the world leader without even knowing it. To my disappointment, however, Ericsson had lost interest in the technology. This meant that the institute could no longer support my project. Although the Board of Technical Development (STU) financed fiber optic research at KTH, they required an industrial stakeholder to contribute funds.

Was all my work over the past three years in vain? I had a family and children, and I needed my income as a scientist. At that time, the political climate in Sweden was against collaboration between universities and industry. We kept a low profile about what we were doing. I had achieved my dream of becoming a physicist and didn't want to give up my position in WDM research.

Apart from lectures and similar activities, I did not publish any of my work in scientific journals. Writing a doctoral thesis would have taken another one or two years of full-time work. Since I was employed at an industrial research institute rather than at KTH, I was given new tasks. At IOF, we collaborated with KTH employees, so my daily work didn't change. Our shared office was on the second floor of the physics building at Lindstedtsvägen 24.

The most important thing for me was finding a solution to the dilemma that other people's decisions had put me in.

Next blog

Read my blog post on February 17, 2026, to find out what happened next!

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