Richard Hamming: Chapter 21. Fiber Optics

“The goal of this course is to prepare you for your technical future.”

Hi, Habr. Remember the awesome article "You and your work" (+219, 2394 bookmarks, 380k reads)?

So Hamming (yes, yes, self-monitoring and self-correcting Hamming codes ) has a whole book based on his lectures. We translate it, because the man is talking.

This book is not just about IT, it is a book about the thinking style of incredibly cool people. “This is not just a charge of positive thinking; it describes the conditions that increase the chances of doing a great job. ”

We have already translated 20 (out of 30) chapters. And we are working on the publication "in paper."

Chapter 21. Fiber Optics

(Thanks for the translation thanks to Mikhail Gostev, who responded to my call in the “previous chapter.”) Who wants to help with the translation, layout and publishing of the book - write in a personal or mail magisterludi2016@yandex.ru

I touch on the topic of fiber optics because its development covers to a large extent the time of my scientific life, which means I can testify how it looked to me while it was taking shape. Let this serve as an example of the style with which I approached new areas of immense potential value. In fact, fiber optics is a completely separate section, by itself. And finally, this is a topic that you will have to deal with in so far as it still has time to develop already in your time.

When I first heard about a seminar on fiber optics at Bell Labs, I wondered if I should go to it at all — after all, everyone had to do their work, and not hang around their lectures all day.

First of all, I recalled that the optical frequencies are much higher than the electrical frequencies that were in use at that time, which means that the fiber optic connection has a much wider frequency range, and the frequency range is essentially the data transfer rate (in bits per second ...), that is, the name and essence of the game that the telephone company plays, my employer.

Secondly, I knew that Alexander Graham Bell had once passed a telephone conversation using a light beam - he generally had a penchant for technical tricks all his life. So it was possible, and done a long time ago.

Thirdly, I knew about internal reflection when moving from a medium with high refraction to a medium with low refraction - this can be seen in calm water, if you look into it from below: there are angles at which the light fully reflects back into the water,



Figure 21.I.

So I understood more or less what the optical fiber would be - although in general they were a rather fresh idea at that time. In any case, I had enough experience from the university laboratory in drawing glass to understand how to use the surface tension effect to make fibers with a fairly constant diameter, and, to some extent, the associated role of the surface tension of liquid glass. So I took the time to go and explore a promising new venture.

Somewhere at the beginning of the report, the speaker said: "God loved sand, He made so much of it." Inside myself, I heard that we are now forced to develop low-grade copper ores and we can only expect a rise in prices for high-quality copper in the future, and glass material is everywhere full and there is no shortage of expected.

Whether at the lecture itself, or shortly after it, I heard the observation: “The free space for laying telephone wires in Manhattan (New York) ends and if the city continues to grow as it grows now, we will have to lay new cables, that is, dig streets and sidewalks, but if we use fiberglass, then due to the small diameter of the wires we will be able to extract copper and lay the fiber instead. ” I realized that this is enough for the Laboratory to do everything possible to develop a fiber-optic transmission as soon as possible and that it will be a constant source of computational problems, which means that I should be at the forefront.

Long before that, when I decided to stay at the Laboratory, I realized the lack of my knowledge in practical electronics, in this regard, I acquired a couple of sets of amateur radio "Heathkit" and collected their experience for the sake of. However, the resulting devices were quite efficient. Thus, I imagined the amount of messing around with the wires and immediately identified the problem point: how do they propose to splice these thin, thick hair, glass fibers and at the same time maintain a decent signal flow? Fiberglass can not just be soldered to each other and hope for a passable transfer.

By the way, why were such small diameters offered? This will become apparent if you look at an illustration of how the optical fiber works in Figure 21.II.



Figure 21.II

The thinner the diameter, the more you can bend the fiber, without loss to the light beam. This is the main justification for reducing the diameter, and not at all the cost or weight of the cable. In addition, for many forms of transmission, a smaller diameter means a reduction in signal distortion at a given distance.

Soon I realized another significant plus. Optical fiber transmitted the signal so efficiently, that is, it lost so few photons in transmission that the "wiretapping of the line" became an extremely serious achievement. Not that impossible, but very difficult. At about the same time, I realized (in connection with the calculations we made with a group of chemists) that the optical fiber is well protected from electromagnetic interference - especially when a nuclear bomb exploded in the upper atmosphere or on the battlefield, and even from lightning. Yes, large sums from military budgets will be devoted to the study of fibers, as well as directly from the laboratory budget.

Soon there was a problem, the appearance of which I expected - it turned out that the winding of fine fibers could locally affect the refractive index and some of the light could be lost. Of course, adding an additional mirror surface will solve the problem. But soon they came up with covering the core of high-refracting glass with a shell of low-refracting, and on objects whose size is available for human manipulation, and then stretching the resulting structure into fibers of the desired thickness.

Much later, I heard about multi-layer assemblies, in which a soft transition of the refractive index was carried out, and I realized that this is an analogue of _ strong focusing_, which was invented a few years before for cyclotrons. The refractive gradient was achieved by chemical or radiation exposure. Instead of working with hard reflections, it was possible to return the beam back to the center of the fiber due to the gradual curvature as it moves away from the middle,



Figure 21.III.

I did not try to thoroughly understand the dispute between multimodal and unimodal methods of signal transmission, but I managed to carry out several series of calculations on a computer for both rival camps and began to lean towards a unimodal transmission for the same reasons that we preferred the binary system to calculating with higher ground. In general, it is the technical details related to the characteristics of receivers and transmitters, and not a fundamental feature of optical signal transmission.

All this time I constantly waited, when they begin to splice the fibers. As time went on, someone had to offer and try out a few witty tricks, and the spread of alternatives convinced me that it was possible with this problem that caught my attention that they would cope relatively easily - at least it would not be fatal in the field, where it will have to be solved by technicians, and not by experts in controlled laboratory conditions. I understood the difference quite well because I watched various projects (mostly from other companies) that rested on the sad fact that things that are consistently obtained in laboratories are not quite the same as the field results of technicians who work in a hurry and in conditions that can be called at least unfriendly.

As I recall, the first test of fiber-optic communication took place between central offices in Atlanta, GA. The tests were successful (experimental period lasted three years). In the meantime, outsiders in the glass business began to produce extremely transparent glass, and just at those frequencies that we wanted to use - we are talking about the range in which lasers can work reliably. They argued that if the oceans were as transparent as some of their glasses, the bottom of the Pacific could be viewed with the naked eye!

Soon I noticed that in fiber optics we: (1) receive optical signals, (2) transform them into electronic form, (3) amplify them, (4) transform them back into optical form. It is difficult to imagine the worst system architecture. In the Laboratory, it became obvious to me, like many others, that I had to seriously work on the amplification of the optical signal. After some time it became clear that there are several candidates for the role of optical amplifiers, which means that it is likely that one or more of them will become a standard type of equipment in the field. One of the advantages of solitons is the ability to amplify without changing the shape (it does not degrade as it passes through the fiber), while the pulses have to be regenerated (this changes their shape, and in general, it looks more complicated operation than simple amplification).

All the practical aspects of the problem lined up pretty well. As you know, now we widely use fiber. I tried to show my approach to the new technology, what I was looking at, what I was waiting for, what I ignored, what I was tracking, what I was seriously considering. I had no desire to become an expert in this field; I had enough computers and their rapid development in the hardware and software, this is in addition to the continuously expanding area of ​​applications. Each new area that appears in your future will pose similar questions to you, and you will, in effect, answer them with your actions.

The current field of application of fiber optics is extremely wide. As time went by, I managed to figure out that the story of satellite communications, for example, is fraught with many problems. Stationary communications satellites should be located along the equator; there were no other positions for them then. From the first days many equatorial countries have declared that we are invading their aerospace and have to pay for its use. Until now, they have not been able to confirm their claims by force, advanced countries just continued to use the space for free. I give you the opportunity to judge the situation: (1) blatant disregard of the requirements of countries, (2) regardless of the validity of their requirements, and (3) given that they are not all at the moment can use their space, everyone else must wait until ( and if at all) they reach the desired level! This is a non-trivial question of international relations, and the truth is with each of the parties.

Now the satellites are located approximately at every 4th degree and although we can arrange them through 2 °, we will have to use much more accurate antennas of terrestrial transmitters (increase the dish radius?) To transmit a signal to them so that it does not touch the neighboring satellites. We can even expand the signal transmission frequency band and multiply the volume of transmitted data, but the need for passing the atmosphere imposes restrictions. On the other hand, fiber can be laid on the ground in such a density that we ourselves desire. Fiber cables are easy to make and aggregate bandwidth just doesn't fit in your head. The use of satellites means a wide signal _ signal, while the cables provide a certain level of privacy and the ability to force the user to pay, not “ride a hare.

Satellites and optical fiber have advantages and disadvantages. Now satellites are used for what is essentially confidential communications, not broadcasting. I think time will redistribute matter in such a way that each of the methods is applied in the best possible way according to its strengths.

Where are we now? Trans-oceanic cables with optical fiber instead of coaxial waveguides have already appeared, for a much lower price and much higher bandwidth. Now (1993) the question of switching to the newly developed soliton signal transmission system instead of the classical impulse, when connecting across the Pacific with Japan, is being resolved. In my opinion, this is a matter of engineering consideration - in the long run solitons will take up above the pulses. I recommend to follow major technological changes - if the transfer of information on solitons wins the current pulse system, then fundamentally new methods of signal analysis will appear, and you should be aware of when this happens not to be behind, along with many other people.

I read that in the Navy, as well as, obviously, in the Air Force and commercial air travel, losing weight means a huge savings in resources that can be spent on other things. During a visit to the aircraft carrier Enterprise about 14 years ago, being already quite well aware of the trend in optical fiber, I looked at the wiring with particular attention and decided that the fiber would replace all the wires _ associated with the transfer of information_. Energy transfer is a completely different topic. And yet, the centralized power network will remain the main method, or in connection with the combat conditions, will decentralized power networks be preferred? It would be good for them to connect with clearly redundant fiber optic systems, which will undoubtedly establish at least their security concerns. But warships are not so different from office skyscrapers, such as the World Trade Center.

We already have at our disposal an optical fiber so strong that trucks can drive through it, so light that you can launch rockets with an unwinding wire attached throughout the flight - and this means two-way communication for controlling the missile and targeting it , and to obtain data from the rocket - what she sees during the flight.

Being connected with computers, I naturally asked myself how all this can affect and affect the design of computers. You probably know that now (1993) we often connect large elements of computing systems using fiber. In my opinion, replacing most of the internal wiring with fiber is just a matter of time. Can't someone in time make “motherboards” in which the integrated circuit boards are connected by optical fiber? This does not seem unreasonable at all, given the current state of the materials sciences. When will fiber optics get to individual chips? In the end, the frequency range of optics means higher switching speeds! Then, can we not in time make optical chips and put a common light source above the photocell in the board (as on some hand-held calculators) to power the whole circuit and move away from the wires to distribute electricity in the system?



Figure 21.IV.

And can we even replace the wiring with light rays? Light rays can pass through each other without interference (if the intensity is not too high), this alone puts them above the wires,



Figure 21.V.

This brings us to the problem of switching. Is it possible to make a matrix switch optical instead of electronic? Does Bell Labs Phone Laboratory and others not have to actively engage in their development? If it succeeds, will true commutation result, and what has traditionally been the most expensive part of the computer will not be the cheapest? First, memory was the most expensive part of the computer, then magnetic disks appeared, then electronic storage systems at fantastically low prices and the design of computers changed noticeably. With a significant reduction in the price of the switching unit, how will you _v_ construct computers? Will von Neumann's basic design survive the whole thing? What will be the design of computers with the new cost structure of the elements?

You can, as I indicated earlier, keep more or less on par with events, if you actively anticipate the ways in which things and ideas can evolve, and then compare your expectations with what happened in reality. Active anticipation means that you are much, much better prepared to accept new things, than if you passively sit or sluggishly drag after progress. "Good luck accompanies the prepared mind."

The meaning of this report is to show how someone tried to prepare for transient technological changes that will affect his research and work. It is impossible to be on the edge at once in all areas of our high-tech society, but we cannot allow ourselves to be left behind, behind new developments - what happened in practice with many people.

Time after time, I reiterate in this book that my duty as a professor is to increase the likelihood that you will make a significant contribution to our society, and I cannot think of a better way than to develop a habit in you to anticipate events, rather than passively follow. It seems to me that, in order to fulfill my obligations to you and to our institution, I must transfer as much of you as possible from passivity to an actively awaiting, anticipatory position.

In today's chapter, as you can see, I don’t talk about the significance of my contribution, but at least I was prepared and helped others, much more deeply immersed in the topic, by making correct calculations, and not those slightly inappropriate, which are so often done. I believe that I often provided this kind of service to Bell Telephone Lab for the 30 years that passed before my retirement. In the field of fiber optics, I gave you some details and how I came to them.

Now let me go to the forecasts for the near future. It is quite obvious that the “post to house” layout (in English it is called a “hanging” line, even if it is buried in the ground) will become fiber-optic. After installing the optical fiber, you have the potential to access all the information you might want, including television, radio, and possibly newspaper articles chosen according to your profile of interest (you pay a subscription on an individual bill that is delivered to your home). Need for individual channels of information for the most part will not be at all. On your side of the fiber optic cable is one or more digital filters. The channel that you need, telephone, radio or television is chosen just like now - by introducing the corresponding numbers into the filter - and it becomes universal. You will need a filter for each channel that you intend to use per unit of time (perhaps it will be one filter, time-divided), and each filter will be of standard design. Or vice versa, the filters will be supplied directly with the equipment upon purchase.

Will all this happen? It is necessary to assess the political, economic and social conditions before saying that what is possible from a technical point of view will actually happen. Would the state want to allow a single company to be in charge of distributing this amount of information? Will modern cable providers want to share with telephone and, possibly, lose some of their earnings, while they will inevitably face increased state regulation? And in general, do we, as a society, want everything to become that way?

One of the topics that often emerges in this book are restrictions that impose political, legal, social, or economic conditions on something technologically possible, and even economically viable. If something can be done for economic reasons, it does not mean that it should be done. And if you do not have a clear understanding of these aspects, then as a practicing visionary of the future in your specialty, you will make many inaccurate predictions and you will have to painfully dodge explaining why it all went wrong.

To be continued...

Who wants to help with the translation, layout and publication of the book - write in a personal or mail magisterludi2016@yandex.ru

By the way, we also launched another translation of the coolest book - “The Dream Machine: The History of Computer Revolution” )