Richard Hamming: Chapter 21. Fiber Optics

Original author: Richard Hamming
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“The goal of this course is to prepare you for your technical future.”

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

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

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


(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 publication of the book - write in a personal or mail magisterludi2016@yandex.ru

I touch on the topic of fiber optics because the development largely covers the time of my scientific life, which means that I can testify about how it looked to me as it developed. Let this serve as an example of the style with which I approached new areas of great potential importance. 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 in your time.

When I first heard about a seminar on fiber optics at Bell Labs Telephone Laboratory, I wondered if I should go to it at all - after all, everyone should do their work, and not hang around the 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 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 the 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,

image


Figure 21.I.

So I understood more or less what the optical fiber would be like - 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, I heard myself 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 an 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 was enough for the Laboratory to do everything possible to develop a fiber-optic transmission as soon as possible, and that it would 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 of 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, hair-thick, 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.

image


Figure 21.II

The thinner the diameter, the more the fiber can be bent, without loss for the light beam. This is the main rationale 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 during transmission that “wiretapping of the line” became an extremely serious achievement. Not that impossible, but very complex. Around the same time, I realized (in connection with the calculations that we carried out 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 strikes. 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,

image


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 computer calculations for both rival camps and began to lean toward a unimodal transmission for the same reasons that we preferred the binary system to calculus with higher ground. In general, it is the technical details related to the characteristics of receivers and transmitters, and not the fundamental feature of optical signal transmission.

All this time I was constantly waiting for the fibers to be spliced. Time passed, someone had to offer and try out a few witty tricks, and the very 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 quite well the difference because I observed various projects (mostly from other companies), which 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 far as I remember, the first test of fiber-optic communication passed between the central offices in Atlanta, Georgia. The tests were successful (experimental period lasted three years). Meanwhile, the outsiders of 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 glass types, 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 several of them will become a standard type of equipment in the field. One of the advantages of solitons is the possibility of amplification 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 generally looks more complicated operation than simple amplification).

All the practical aspects of the problem lined up pretty well. As you know, we now make extensive use of 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 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 stated 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 a much higher bandwidth. Now (1993) the question of the transition to the newly developed soliton signal transmission system instead of the classical impulse, with communication across the Pacific with Japan, is being solved. 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 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 transmission 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 obviously redundant fiber-optic systems, which will undoubtedly establish at least their security concerns. But warships are not so different from office skyscrapers,

We already have an optical fiber that is so strong that trucks can drive through it, so light that you can launch rockets with an unwinding wire attached throughout the flight - and that means two-way communication for controlling the missile and targeting it , and to get data from the rocket - what it 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,

image


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,

image


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 Telephone Labs and others not have to actively develop them? 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, the most expensive part of the computer was memory, 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 a 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 sit passively or drag about progress inertly. "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 pre-prepared and helped others, much more deeply immersed in the topic, by making correct calculations, and not those slightly inappropriate, which they so often do. I believe that I have 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). The 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 you need, the phone, The radio or television is chosen like it is now - by inserting the corresponding numbers in 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 evaluate 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 part of their earnings on this, while they will inevitably be faced with increased regulation by the state? 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 started the translation of another cool book -"The Dream Machine: The History of the Computer Revolution" )

Book content and translated chapters
Foreword
  1. Intro to The Art of Doing Science and Engineering: Learning to Learn (March 28, 1995) Перевод: Глава 1
  2. «Foundations of the Digital (Discrete) Revolution» (March 30, 1995) Глава 2. Основы цифровой (дискретной) революции
  3. «History of Computers — Hardware» (March 31, 1995) Глава 3. История компьютеров — железо
  4. «History of Computers — Software» (April 4, 1995) Глава 4. История компьютеров — Софт
  5. «History of Computers — Applications» (April 6, 1995) Глава 5. История компьютеров — практическое применение
  6. «Artificial Intelligence — Part I» (April 7, 1995) Глава 6. Искусственный интеллект — 1
  7. «Artificial Intelligence — Part II» (April 11, 1995) (готово)
  8. «Artificial Intelligence III» (April 13, 1995) Глава 8. Искуственный интеллект-III
  9. «n-Dimensional Space» (April 14, 1995) Глава 9. N-мерное пространство
  10. «Coding Theory — The Representation of Information, Part I» (April 18, 1995) (пропал переводчик :((( )
  11. «Coding Theory — The Representation of Information, Part II» (April 20, 1995)
  12. «Error-Correcting Codes» (April 21, 1995) (готово)
  13. «Information Theory» (April 25, 1995) (пропал переводчик :((( )
  14. «Digital Filters, Part I» (April 27, 1995) Глава 14. Цифровые фильтры — 1
  15. «Digital Filters, Part II» (April 28, 1995) Глава 15. Цифровые фильтры — 2
  16. «Digital Filters, Part III» (May 2, 1995) Глава 16. Цифровые фильтры — 3
  17. «Digital Filters, Part IV» (May 4, 1995) готово
  18. «Simulation, Part I» (May 5, 1995) (в работе)
  19. «Simulation, Part II» (May 9, 1995) готово
  20. «Simulation, Part III» (May 11, 1995)
  21. «Fiber Optics» (May 12, 1995) Глава 21. Волоконная оптика
  22. «Computer Aided Instruction» (May 16, 1995) (пропал переводчик :((( )
  23. «Mathematics» (May 18, 1995) Глава 23. Математика
  24. «Quantum Mechanics» (May 19, 1995) Глава 24. Квантовая механика
  25. «Creativity» (May 23, 1995). Перевод: Глава 25. Креативность
  26. «Experts» (May 25, 1995) Глава 26. Эксперты
  27. «Unreliable Data» (May 26, 1995) (готово)
  28. «Systems Engineering» (May 30, 1995) Глава 28. Системная Инженерия
  29. «You Get What You Measure» (June 1, 1995) Глава 29. Вы получаете то, что вы измеряете
  30. «How Do We Know What We Know» (June 2, 1995) пропал переводчик :(((
  31. Hamming, «You and Your Research» (June 6, 1995). Перевод: Вы и ваша работа

Кто хочет помочь с переводом, версткой и изданием книги — пишите в личку или на почту magisterludi2016@yandex.ru

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