@ Mark Ollig
In July 1966, Charles K. Kao and George A. Hockham published a paper showing that impurities in glass were causing severe signal loss in experimental fibers.
They stated that if losses could be reduced to about 32 decibels (dB) per mile, glass fibers could be used for telephone voice transmission.
In 1970, Corning Glass Works scientists Robert Maurer, Donald Keck, and Peter Schultz demonstrated a fused-silica optical fiber with a loss of about 27 to 28 dB per mile.
They used a helium-neon laser as the light source, surpassing Kao’s threshold and proving that glass strands could be used for telecommunications.
By the early 1970s, Corning had reduced fiber losses to about 6 dB per mile. Within a few more years, single-mode fiber designs were approaching losses of less than about 1.6 dB per mile.
AT&T, Illinois Bell, and Bell Labs tested a 1.5-mile fiber-optic telephone link in Chicago May 11, 1977.
The cable, laid in underground telephone ducts, carried voice, data, and video signals encoded as laser light pulses, linking one office building with two of its exchanges.
By the mid-1980s, telephone companies were replacing their copper cabling with fiber-optic cables for interoffice trunk connections.
In 1988, Winsted Telephone Company installed a single-mode fiber-optic cable linking its Class 5 DMS-10 local exchange to the US West Class 4 tandem office in Buffalo.
Class 4 tandem offices connected local Class 5 exchanges, such as Winsted, and routed their long-distance calls through the public switched telephone network (PSTN).
During a conversation with my brother Mike, who worked with me at the telephone company, he recalled those days as if they were yesterday.
Winsted Telephone Company buried a fiber-optic cable from 171 Second Street S., Winsted, to the US West tandem office boundary at the edge of Buffalo.
US West buried its segment from 97 Second Street NE., Buffalo, to that boundary, where the two cables were spliced together at a shared meet point.
I compare that fiber splice to the 1869 ceremonial golden spike that joined the first transcontinental railroad.
The completed interoffice single-mode fiber span measured about 23.5 miles.
It replaced a buried 19-gauge, 25-pair toll cable from the early 1960s that carried Winsted’s long-distance traffic to a US West tandem office in Howard Lake.
From there, calls were handed off to the US West Class 4 tandem in Wayzata and then routed to the AT&T Long Lines Minneapolis Downtown Class 4 tandem at 200 S. Fifth Street.
Some readers may remember that United Telephone Company had a telephone exchange in Howard Lake. The US West (formerly Northwestern Bell) building was a couple of blocks east of it.
But I digress.
The Winsted fiber span was accessed from the company’s Nortel DMS-10 (Digital Multiplex System-10) through an NEC (Nippon Electric Company) RC-28D digital multiplexer.
Technicians installed the single-mode fiber to the RC-28D’s optical modules using FC (Ferrule Connector) or ST (Straight Tip) connectors.
The RC-28D combined 28 T1 (DS1) circuits, each with 24 channels, into a single DS3 signal operating at 44.736 Mbps, enough to carry 672 voice channels (28 T1s × 24).
A High-Speed Transmit Optical (HS XMT OPT) module in the RC-28D converted the channelized DS3 signal into light and sent it over the fiber.
A High-Speed Receive Optical (HS RCV OPT) module in the RC-28D converted the light back into a DS3 signal at the other end.
The DS3 was tested with a T-Berd DS3/DS1 analyzer, which checked for bit errors and verified continuity while monitoring for conditions such as signal loss, frame loss and alarm indications.
Using the formula Loss (dB) = Attenuation per mile (dB/mile) × Distance (miles), the fiber showed an average loss of about 0.5 to 0.75 dB per mile, totaling 12 to 18 dB over 23.5 miles, which was within the DS3 loss margin.
Each optical module in the RC-28D shelf was checked, along with its alarm indicators.
Technicians verified the laser transmitter’s performance by measuring its bias, which is the small, steady current that keeps the laser active.
They checked this at the RC-28D’s LD BIAS MON test point to ensure the transmitter was operating within its specified range.
At the Buffalo tandem office, the DS3 signal transmitted over the shared fiber span from Winsted was received and provisioned for interoffice connections.
That DS3 also carried DS1 voice channels to the Winsted DMS-10 through its digital trunk cards for two-way long-distance calling.
Long-distance calls from the DMS-10 sent dialing information to the tandem using in-band multi-frequency (MF) tones, and the tandem processed the digits and routed the calls through the public switched telephone network.
By the fall of 1988, Winsted Telephone Company had a state-of-the-art fiber-optic link that connected its digital exchange to the public switched telephone network.
It was a “giant leap” into cutting-edge technology for Winsted Telephone Company’s subscribers and for those of us who worked there.
Charles K. Kao, who was awarded the 2009 Nobel Prize in Physics for his work in fiber optics, died Sept. 23, 2018, at age 84.
The 1966 paper “Dielectric-Fibre Surface Waveguides for Optical Frequencies” is available at https://bit.ly/3VlHGDt.
