Some reflections of his career at STL
Takis Hadjifotiou joined Standard Telecommunication Laboratories (STL) in Harlow in the early 1970’s. Originally established by Standard Telephones and Cables (STC) in Enfield, the laboratories moved to Harlow in 1959. While much of the Labs’ work directly supported STC’s telephone and telecommunications equipment and component businesses, the range of activities was very broad, encompassing materials science and chemistry in addition to electrical and electronic engineering. Ownership of STC and the laboratories transferred to the Canadian company Northern Telecom in 1991, but it maintained its status as a world class telecommunications research establishment until closure of most of its facilities at the end of 2004.
Takis graduated with a BSc in Electronic Science from the University of Southampton in 1969, and was awarded an MSc in Automatic Control Systems by the University of Manchester in 1970. He returned to Southampton to research Digital Signal Processing for radar target acquisition, earning a PhD in 1973.
I suspect Takis joined STL shortly after his PhD. He was interviewed by Peter Radley, who still remembers Takis’ ability to think on his feet during that encounter. I have not uncovered the earliest projects he worked on, but by the early 1980’s his responsibilities encompassed optical technology.
Following publication of Charles Kao and George Hockham’s pioneering work on optical fibre transmission in 1966, STL was at the forefront of optical fibre research. A key milestone was the Hitchin-Stevenage field trial in 1977, which established the practicality of high speed (140 Mbit/s) transmission using existing Post Office ducts and infrastructure. Takis was involved in some early explorations of optical transmission and played a leading role in the subsequent commercial exploitation of this work, supporting STC Basildon’s SOLS product (Standard Optical Line System) for British Telecom, initially operating at 140 Mbit/s, and later 565 Mbit/s.
Another area where optical technology had substantial impact was in submarine cable systems. As Peter Radley, then head of the Transmission group recalls: “STC was the lead supplier of undersea cable systems and the Chief Engineer at Sub Systems at the time, John Tilley, saw the opportunity based on the STL ground breaking work. What was really impressive was that all the key elements needed were under one roof – lasers, photodetectors, fibres, cables and repeaters where Takis was heavily involved. John Tilley put one his best engineers to work at STL on the repeater design in the same team as Takis. The importance of ‘under one roof’ was the canteen! – at lunchtime you would often see people gathering together to compare notes and share problems – Paul Kirkby, Richard Epworth, Phil Black, Takis, and lots of others. Takis was definitely part of the team culture, listening carefully to others and contributing forcefully”.
Five years after a 1980 deep water cable trial in Loch Fyne in Scotland, September 1985 saw STC commission UK-Belgium, the first international undersea system, followed in 1988 by TAT 8, the world’s first transoceanic undersea optical fibre system.
An important constraint on optical system performance is the sensitivity of the receiver. It limits how far a signal can be sent before it must be amplified or regenerated. In the early 1980’s it was not feasible to amplify optical signals directly, and available detectors such as avalanche photodiodes were relatively noisy and insensitive, compared with radio receivers. A common radio technique mixes an incoming signal with a second local oscillator frequency to produce a slower beat frequency which can be amplified and measured more effectively than the original high frequency signal. Applied to optics, this is known as coherent detection. Takis would have been very familiar with this procedure from his doctorate work on signal processing. In 1982, Nigel Baker, then a recent recruit, was asked to assess the potential of coherent detection for optical transmission in military networks, and was directed to Takis as “someone who knew about Radar”. Nigel said: “I can’t remember what I learned about coherent from that encounter but I did discover that Takis and I shared an interest in maths and philosophy.”
In 1984 Takis was a principal engineer in a group under Steve Wright, investigating Coherent System Technology. At first sight, optical frequency instability in available diode laser sources made this idea impractical, but the potential benefits were evident. Laser frequency fluctuations were rapid, but the output could be made stable enough to beat with the diode’s own delayed output, emitted less than a thousandth of a second earlier. This became the basis for a coherent optical time domain reflectometer (OTDR) developed on behalf of STC Leeds. Used to diagnose breaks and other defects in optical cables by measuring reflected optical pulses, it offered greater range and sensitivity than competing instruments at that time.
In 1986, with the demise of STC Leeds, Takis moved on to investigate coherent detection more broadly. Following Steve Wright’s departure in 1987, Takis took on leadership of the group, now called Lightwave Technology. Laser stability remained a problem, so that precise phase locking of a local oscillator was not possible. To circumvent signal fading, a technique was proposed employing a 3-way fibre coupler to simultaneously mix using 3 different phase offsets, and merge the results into a single usable signal. As Jeff Farrington, Takis’ manager at the time recalls: “This seemed a bit hairy, but appeared to offer a useful improvement in sensitivity compared to a conventional detector. Takis was convinced that he could make it work given a dedicated team and no distraction, so he made a pitch to the management at Greenwich and Bob Williamson was obviously convinced and gave him the money. About 6 months later a working system was delivered, and Takis often mentioned it to me, so I think he was very proud of it.”
Following the lab trial, and assisted by Ian Hardcastle, Takis very nearly secured a contract for a coherent unrepeatered (300 km?) link in Singapore. In retrospect, it was not (yet) the time for widespread adoption of coherent optical detection for telecommunications. Another technology, based on erbium-doped fibre amplifiers (EDFA), became sufficiently mature for commercial exploitation. Fortunately, the significance of EDFAs had been recognized, and Martin Pettitt and others were already working under Takis to explore this technology from a systems perspective.
EDFAs brought to long-distance transmission an ability to boost optical signals without first converting to an electrical signal, amplifying the waveform, then driving a laser to generate a clean copy of the optical signal. A single fibre could carry many optical signals at different wavelengths, and provided they were in the range 1530-1560 nm (later extended to beyond 1600 nm) they could all be amplified by the same amplifier. This avoided the need for expensive electro-optic regeneration on each channel at every repeater site. Capacity could be increased by launching additional wavelengths, or by raising the data rate on existing wavelengths, requiring only a simple upgrade to the terminal equipment. No need to lay new cables, or replace repeaters sitting in damp holes in the ground. Coherent detection retained a theoretical advantage for un-repeatered systems, but EDFA preamplifiers approached this theoretical performance, and EDFA power amplifiers provided increased transmitter powers whatever the detection technology.
In other parts of the Harlow labs, Steve Wilson in Phil Black’s optical fibre group, explored the design and manufacture of erbium-doped optical fibre, and Peter Selway’s laser group were developing pump lasers for the amplifiers. Takis and the Harlow labs delivered essential support to STC submarine systems as they adopted EDFA technology for a series of high profile commercial systems. Arguably, the most prominent project was TAT 12, the world’s first optically-amplified transoceanic cable, although by the time it was commissioned in 1996, STC Submarine systems had been sold to Alcatel.
In 1991 STC was bought by the Canadian company, Northern Telecom, subsequently re-named Nortel following their acquisition of Bay Networks. As Bell Northern Research, Northern Telecom had their own highly competent and respected research and development organisation. Takis was particularly active in forging good working relationships with our new colleagues in Ottawa, and establishing an appropriate role and focus for the Harlow laboratories. Essentially Ottawa would continue to support existing product lines, and apply their extensive resource and expertise to the development of next generation products. Harlow would enhance its role as a centre of excellence in optical transmission, exploring cutting-edge technologies, and assess technology risks and benefits through analysis and laboratory experiments.
This was an environment in which Takis’ deep technical knowledge, broad understanding of the industry and contacts with academia were widely appreciated, and put to good effect. He successfully engaged with Nortel’s customers, as when supporting a demonstration of optical switching at MCI. More speculative activities included work with colleagues in the components group in Harlow, for example assessing developments in academia and industry on novel materials for high speed modulators and non-linear optical processing. Other topics included wavelength conversion, and aspects of all-optical networks. Within the Harlow laboratories, diverse interests included all-optical regeneration, pump lasers for erbium-doped fibre amplifiers, chromatic dispersion compensation using chirped fibre Bragg gratings or etalon devices, Raman amplification and compensation of polarisation mode dispersion.
Throughout the 1990’s Takis led Thomas Lee and others to demonstrate the potential of high bit rate time-domain multiplexing (OTDM), first at 10 Gbit/s, and subsequently 20 Gbit/s, 40 Gbit/s and higher rates. With Mark Kimber he supported the first 10 Gbit/s field trial with British Telecom. In the BNR/Nortel era this expertise became a valuable resource, as the Ottawa product development product team moved first from OC48 (2.5 Gbit/s) to OC192 (10 Gbit/s) and then to develop a 40 Gbit/s system.
In the later 1990’s the “Telecom bubble” accelerated deployment of fibre systems, contributing to a rapid rise in the Nortel share price. Takis’ team were tasked with a series of high profile system demonstrators, successfully delivering non-trivial technical achievements to promote the Nortel brand. Examples included a 100 Gbit/s OTDM dispersion managed soliton system in collaboration with Corning (who providing the dispersion managed fibre), and presented at ECOC ’99. An 80 Gbit/s OTDM system was constructed in Harlow, then transported and displayed at Telecom 99. At OFC 2002, 160 Gbit/s OTDM propagation over 480 km of standard fibre was reported, but by now the telecom bubble had burst.
High bit rate soliton (or “soliton-like”) systems relied on a delicate balance between optical non-linearity in the fibre, and fibre chromatic dispersion. They required carefully tailored (dispersion managed) custom fibre, but there was now a great deal of unused conventional fibre which had been installed during the bubble. Few providers were installing new cables. The need was to deliver higher capacity using existing fibre infrastructure. Meanwhile, digital electronics had become faster, cheaper and more capable, leading Ottawa to scrap plans for 40 Gbit/s OTDM, and introduce a new 10 Gbit/s product using electronic dispersion compensation in the transmitter.
Studies of alternative modulation formats by Takis’ team through the period up to the closure of the Harlow laboratories in 2004, included “return to zero” variants, phase duobinary, quadrature phase shift modulation (QPSK) and differential QPSK. When Ottawa moved to develop a coherent 40 Gbit/s product, Takis and Richard Epworth were able to dust off their reports, publications and experience of coherent transmission from the 1980s.
Takis engaged with academia throughout his career, starting in 1984 with his MSc lectures at the University of Essex. He supervised external PhDs, gave external lectures and was a visiting Professor at several universities. Takis was elected Fellow of the Royal Academy of Engineering, and as an auditor and rapporteur for the European Union Framework programmes, was well known and highly regarded in Brussels. After the lab closure, Takis continued to work with academia and industry, for example supporting Martin Agnew, then at Airbus, on laser links between satellites in low earth orbit. In the year of Covid-19 he was starting to wind down, but unable to lecture in person, still forwarded copies of his lecture notes to Heriot-Watt and to Bangor.
In the many reactions to his passing, a recurring theme is the open-handed support and encouragement given to those he worked with, and their respect for his judgement and knowledge.
Alan Robinson
20 July 2020