Optical Fiber Communications 101: Key Concepts and Technologies

The Power of the Sun in Optical Communication

In 1880, Alexander Graham Bell conducted an experiment where he made a phone call using natural light (sunlight) to convert his voice into light via a “photophone.” This light was transmitted approximately 700 ft. away, converted back to voice for the recipient to hear, and is now believed to be the first instance of wireless transmission of speech. Not surprisingly, this method was initially too difficult to use over longer distances due to the transmission path being space, which caused the light to spread and disperse and was easily further complicated by obstacles like precipitation and physical objects.

Figure 1. Principles of Bell's photophone design

 

It was almost a century later before optical-based communication was put to practical use, thanks in large part to the invention of optical fiber and lasers. A laser’s stable, highly directional beam of light (emitted from tiny semiconductor windows that measure just a few hundred thousandths of a square millimeter) can carry enormous amounts of information. This combination of this plus optical fiber (a high-performance transmission medium made of glass as thin as a human hair capable of trapping optical signals and transmitting them over long distances without significant attenuation) were game changers and set the stage for optical-based telecommunication innovation.
 

How Optical Fiber Communication Works

The most important elements of optical communication are a transmission medium with extremely low optical attenuation and a highly stable, long-life light source that operates with a small current. With the advent of optical fiber as a transmission medium and semiconductor laser as a light source widespread use of optical communications became practical.

The process of optical communication breaks down into a few simple steps:

• An electrical signal is sent from a terminal (e.g., telephone, computer)
• It converts into an optical signal (digital signals "0" and "1" convert into flashing light) via an electrical-to-optical (E/O) converter
• This transmits along the optical fiber, propagates, and converts back into an electrical signal via the communication partner’s optical-to-electrical (O/E) converter at the receiving terminal

E/O converters use light-emitting elements such as semiconductor lasers, O/E converters use light-receiving elements such as photodiodes, and optical elements such as lenses are used at the input and output of optical fiber. It’s important to note that the size of the light-emitting part of a semiconductor laser is on the order of μm, the light-receiving part of a photodiode is on the order of 100 μm, and the core diameter of an optical fiber is around 10 μm, which necessitates advanced technology to align them accurately.

Optical Fiber Advantages
Compared to conventional metallic cables, optical fiber provides an advantage of low loss (~ 0.2dB/km) and wide bandwidth (several hundred MHz to THz) to enable long-distance, high-capacity communication. Additionally, optical fiber is lightweight and less susceptible to noise (no electromagnetic induction).

Optical Fiber Structure
Optical fiber consists of a cylindrical core that propagates light and a concentric cladding that surrounds it. The cladding’s refractive index is slightly smaller than that of the core, which confines light within the core and propagates by repeated total reflection at the boundary with the cladding. Optical fiber is as thin as a hair (diameter of 125 μm, core of 9 μm – 62.5 μm), with most optical fibers used for communication being made from quartz glass.

Optical Fiber Types
When light travels through an optical fiber, only reflections at a certain angle are reflected repeatedly due to the relationship between the difference in refractive index (between the core and cladding of the optical fiber) and the thickness of the core. This special angle is referred to as propagation mode. When there are many such angles, this is referred to as multimode, and when there is only one, this is called single mode. Among multi-mode optical fibers, there is a graded index (GI) optical fiber that has a gradual change in the refractive index distribution of the core. Fibers commonly used in optical communication are single mode and GI.

Figure: Examples of light transmission through different optical fiber types

Optical Fiber Loss
Optical signal rate attenuation as it passes through quartz fiber varies depending on a light’s wavelength. The example in Figure 5 shows optical fiber loss by wavelength. The second and third bands (1.3 μm, 1.55 μm) have small losses and are the wavelength bands primarily used today. With improvements in optical fiber, the distance over which light intensity is halved has extended to approximately 10 km for light with a wavelength of 1.3 μm and approximately 20 km for light with a wavelength of 1.55 μm. In coaxial cables that transmit electrical signals, intensity is halved after approximately 1 km.

Figure: Loss of optical fiber

Optical Fiber Transmission Bandwidth
Optical fiber communication speed is expressed as the number of signals that can be sent per second (bps); the higher the communication speed, the more information that can be sent. In data communication, a high communication speed is known as a wide transmission bandwidth. In the case of coaxial cables, there is a practical limit to how fast communication speed can be increased (because signal attenuation increases). In optical fiber, there is no attenuation even when communication speed increases, which makes it possible to send large amounts of information.

Semiconductor Lasers
Semiconductor lasers convert electrical “0” and “1” signals into blinking optical signals (intensity modulation) and are suitable for high-speed data communications because of their ability to be modulated at high speeds, and photodiodes convert the light into electricity. High-speed devices can blink and detect approximately 25 billion times per second (i.e., communication speed of 25 Gbps).

DFB Lasers and FP Lasers
Most semiconductor lasers in optical communications are distributed-feedback (DFB) lasers and Fabry-Pérot (FP) lasers. Each has the characteristics shown in Table 2. The difference between the two lasers is in their optical spectrums. Unlike FP lasers that emit multiple wavelengths, which results in a broader spectrum, DFB lasers emit a single, narrow wavelength, which makes them ideal for long-distance, large-capacity optical fiber communications.

DFB laser waveforms can be received with almost no change, whereas FP laser waveforms spread over time and can result in digital code errors. This occurs because of speed variations that are dependent on a light’s wavelength and is known as chromatic dispersion. The longer the transmission distance, the greater the waveform degradation between transmitter and receiver, which ultimately limits transmission distance. To achieve long-distance, high-capacity optical fiber communication, less is more, so the fewer the emission wavelengths of a laser, the better.

Optical Fiber Amplifier
No matter how low loss an optical fiber is, all light eventually attenuates and becomes weaker. This is not a problem if transmission distance is only a few tens of kilometers, but for transmissions over hundreds or thousands of kilometers, weakened light must somehow be restored to sufficient strength. To solve this issue, an optical fiber amplifier (sometimes called an optical fiber pump) amplifies light directly without converting it to electricity. One such example is erbium-doped optical fiber, which has the rare-earth element erbium incorporated (aka doped) into its glass core matrix. Erbium absorbs light from an excitation light source and outputs the absorbed light energy in the 1.5 um band used in optical communication, so when a weak optical signal is passed through an erbium-doped fiber, it amplifies and outputs as a stronger optical signal.

WDM Transmission
There are two ways to transmit large amounts of data at high speeds: increase transmission speed or increase the number of transmission paths. Transmission speed increases are relatively cost-effective up to a certain point, as there is an upper limit imposed by electronic circuits, etc. One could increase the number of fibers by 100 times, however, legacy fiber optic cables already in place only have a few core pairs. To combat this, wave division multiplexing (WDM) is incorporated to transmit large amounts of data at higher speeds using fewer fibers.

Earlier optical communication employed sending signals via a single light, through a single fiber, and blinking this light. With WDM, multiple lights are sent simultaneously through the same fiber. It’s important to note that if the wavelengths are the same, they cannot be distinguished. This is helped through the introduction of a light with a different wavelength. Red, purple, cyan, and green lights (which do not actually exist as they are not visible light) flash individually and transmit through a single optical fiber. This groundbreaking technology allows capacity increases by several or even dozens of times without increasing the number of fibers. When combined with an optical fiber pump, WDM enables larger-capacity, higher-speed data communications.

Optical MUX/DEMUX
When multiple light wavelengths need to travel via a single fiber, an optical multiplexer (MUX) is employed that merges, or multiplexes, them into a single beam for transmission. For the opposite function, when the multiplexed signal needs to be separated back into individual signals, each with their own wavelength, an optical demultiplexer (DEMUX) is used. There are two types of these components: one that uses a thin film filter and one that uses an arrayed waveguide grating. Both have excellent features and selection is often determined by the system configuration.

Network Configuration
Optical fiber communications use access lines known as fiber-to-the-home (FTTH), fiber-to-the-premises (FTTP), and fiber-to-the-room (FTTR). These access lines are connected via a network, called a backbone, and these are all connected via a metro network. Dense wavelength division multiplexing (DWDM) is a system that can use hundreds of light wavelengths to multiplex at dense wavelength intervals, while coarse wavelength division multiplexing (CWDM) multiplexes light wavelengths at coarse intervals (about 20 nm).

Figure: Optical fiber communications network configuration concept

 

Optical Fiber Communication Measurement Instruments

Because light is a wave, amplitude and wavelength frequency are important to monitor and measure. The number of waves per unit of time (frequency) is called a wavenumber, and amplitude is a quantity related to light intensity and measures as optical power. Generally, an optical power meter is equivalent to a voltmeter used for electrical measurements, an optical wavelength meter (OWM) is equivalent to a frequency counter, and an optical spectrum analyzer (OSA) is equivalent to an electrical spectrum analyzer. In an OSA, the frequency sweep circuit of a conventional (electrical signal) spectrum analyzer is replaced with a device that uses an optical prism, and the detection circuit is replaced with a photodiode (O/E converter).

Figure: Light wavelength graph
 

Optical Spectrum Evaluation

When light passes through a prism it produces a rainbow-like effect and the resulting bands of colors are known as optical spectra. The optical spectrum evaluated in optical fiber communication is a graph in which the components of light are broken down into wavelengths and the horizontal axis represents the wavelength and the vertical axis represents the level (optical power). The light used in optical fiber communication is not natural light like sunlight, but artificially created light like lasers. Figure 13 shows examples of optical spectra of sunlight and lasers. The spectrum of artificially created light is important in optical fiber communication, so accurate evaluation is necessary. In addition, the spread of WDM transmission methods, which multiplex signals in the wavelength domain, has made optical spectrum analysis a key measurement technology for building communication networks.

Optical Spectrum Analysis and Measurement
An OSA measures the spectrum of light-emitting elements (active components) such as lasers and light-emitting diodes. OSAs enable users to determine emission wavelength and spectrum width, measure the relationship between input and output of passive components, and observe attenuation characteristics, transmission characteristics, cutoff wavelength, etc. Performance requirements for spectrum measurement include:

• Wide bandwidth
• High sensitivity
• High speed (wavelength sweep)
• Wavelength measurement accuracy (reading)
• High resolution (to distinguish up to two small Δλ as two spectral lines of two close wavelengths λ and λ+Δλ as two spectral lines)
• Optical dynamic range (sharpness of filter characteristics, to suppress and measure optical power of adjacent wavelengths when separating light to be measured into wavelengths)

Diffraction Grating
Diffraction grating is an optical element that disperses (separates) constituent wavelengths from polychromatic light and works in similar fashion to a prism. Using very fine grooves (40 to 1200 grooves per mm) cut on a mirror, the incident on the grating causes diffraction (light that reinforces each other due to optical interference) to occur at angles determined by each wavelength. In other words, if the direction (angle) at which light is diffracted by a diffraction grating is known, the wavelength of that light can be identified.

Figure: Diffraction grating

What is a Monochromator?
A monochromator is equivalent to a bandpass filter that divides the input light into narrow wavelength slots and tunes the light’s central frequency through the use of a dispersive element (e.g., diffraction grating). A typical monochromator configuration (shown in Figure 17) consists of concave mirrors (collimating mirror and focusing mirror). Light entering from an optical fiber is collimated by the collimating mirror and directed to the grating. Diffracted light from the grating is then focused via the focusing mirror into a spectrum in the dispersion direction (left and right in Figure 17) with the center slit. Only light of the spectrum wavelength that is focused on the slit is emitted. By rotating the grating, the direction of diffraction of the light changes, and so the wavelength of the emitted light can be changed.

Optical Dynamic Range Improvement Using Bandpass Filters
As explained earlier, a monochromator performs as an optical bandpass filter. Figure 18 shows an example where the filter characteristic light passes through a single-stage bandpass filter, with the difference between the passband and the stopband (optical dynamic range) showing as approximately 40 dB. By connecting another stage bandpass filter after this optical bandpass filter, the dynamic range can be improved dramatically. In an actual monochromator, instead of preparing two types of bandpass filters, the light that passed through the first stage bandpass filter folds back (returns) to the same bandpass filter again to achieve a two-stage filter. This type of monochromator is known as a double-pass type monochromator. For evaluation of the monochromaticity of DFB lasers, which will be discussed later, the OSA in use should have sufficient optical dynamic range.

To obtain good measurement wavelength accuracy, it is important to precisely control the grating rotation angle. As explained previously discussed, if the direction (angle) of light diffracted by the diffraction grating is known, the wavelength of that light can be determined. In an actual monochromator, the wavelength is swept by rotating the diffraction grating, with the rotation angle corresponding to the measured wavelength. One way to rotate the diffraction grating and control its rotation angle is through the use of a stepping motor (see Figure 19). If the wavelength axis sample resolution is not sufficient with the rotation resolution of the stepping motor, the resolution is increased through a reducer. However, if the reducer has errors due to mechanical precision, these may appear as errors in the wavelength measurement.

Figure 20 shows another method of grating control that uses a servo motor and encoder in combination. Using an encoder to detect the rotation angle in combination enables configuration of an angle control servo circuit. Additionally, by using a high-resolution optical encoder, the rotation resolution can be increased, which makes it possible to rotate the grating at high speed without the need for a reducer.

By correcting the wavelength sensitivity characteristics of the monochromator, it is possible to measure the optical spectrum without level deviation. The diffraction efficiency of the grating installed inside the monochromator and the sensitivity of the photodiode have wavelength characteristics. Figure 21 shows the wavelength sensitivity characteristics of the photodiode used in the monochromator. Indium gallium aresenide (InGaAs) photodiodes are used in OSAs for optical fiber communications. If the measurement sensitivity inside the monochromator changes depending on the wavelength, the correct optical power for each wavelength cannot be measured. To solve this problem, OSAs store in advance (via internal memory) a correction value that corrects for the sensitivity difference due to wavelength inside the monochromator. During measurement, measured values obtained are automatically corrected and displays an optical spectrum without level deviation.
 

Optical Spectrum Measurement Examples in Optical Fiber Networks

While OSAs are the standard measuring instruments used in optical fiber communications and are used in applications relating to research and manufacturing of optical fiber communications devices (e.g., transceivers, amplifiers). Although access lines such as coaxial cable and wireless have different forms, if the original line is traced it is connected to an optical fiber network.

Spectrum Comparison of Lasers in Short- and Long-Distance Communication
Optical transmitting and receiving modules (called optical transceivers) are essential for constructing fiber networks. A laser is used in the transmitting module inside an optical transceiver and an OSA evaluates its optical spectrum. Figures 22 and 23 show the results of measuring typical spectra of optical transceiver lasers. The laser in Figure 22 is used for short-distance optical fiber communications, while the one in Figure 23 is used for long-distance communications. Comparing the two, it is clear the shapes of the spectra are different.

In optical fiber communication systems, a digital signal’s pulse spread is a function of the light source spectrum. In other words, the smaller the spread of the laser spectrum, the smaller the pulse spread of the digital signal, which enables faster and longer distance communication. For short-range lasers (like in Figure 22), an OSA measures optical spectrum spread width. For long-range lasers (like in Figure 23), the optical spectrum has excellent uniformity and the evaluation index is the degree to which the second peak is suppressed relative to the peak level of the optical spectrum.

Gain Measurement of an Optical Fiber Amplifier
All optical fiber cables have some aspect of loss which causes attenuation when transmitted over long distances. Gain evaluation for optical fiber pumps mitigate this issue through the amplification of WDM signals all at once. Optical spectrum measurement of input and output signals from an optical fiber pump provides its gain. Gain G (times) is calculated using the following formula:

G = (P OUT - P ASE)/P IN

Evaluating WDM Signal Quality
In a WDM transmission system, an OSA evaluates signal quality through measurement of wavelength, optical power, and signal-to-noise ratio (SNR) of each multiplexed signal. Only wavelengths specified by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) are used for signal wavelengths in a WDM transmission system. However, since a laser’s wavelength may fluctuate depending on its surroundings, it is important to measure the signal wavelength to maintain the communication system’s stability. SNR deteriorates when a signal is repeatedly amplified and transmitted by optical fiber amplifier. As a result, measuring SNR is important for maintaining the a communication system’s quality. OSAs analyze these and other crucial parameters that have a direct impact on a communication system.
 

Case Example

The following images show actual use cases of OSAs employed in the research, development, and manufacture of optical fiber communication devices. Photo 1 shows a simulated measurement of a WDM signal used in trunk communication networks between major cities, with an eight-channel optical signals (DFB laser) multiplexed by an optical multiplexer and its optical fiber amplifier connected to an OSA. The measurement display in Figure 26 shows the eight-channel optical signals multiplexed, with wavelength intervals approximately 1.6 nm and the SNR of each channel measured to be approximately 33 dB.

Figure: WDM signal measurement using an optical spectrum analyzer

The OSA in use is equipped with a high-performance monochromator with a wavelength resolution of 0.01 nm, wavelength accuracy of ±0.01 nm, and optical dynamic range of 70 dB or more. The monochromator has a multi-stage optical bandpass filter structure for sharp filtering characteristics to evaluate high-performance, highly functional optical devices and transmission systems that realize long-distance, high-capacity optical fiber communications. In addition, it has a fast sweep speed, is equipped with a wide range of analysis functions, and supports various external interfaces such as LAN and GP-IB, making it suitable for a wide range of applications from research and development to evaluation and manufacturing lines.

 

References

• Kenichi Nishimura and Hidetoshi Shirakawa; Easy Optical Fiber Communication, September 1993, Ohmsha Co., Ltd.
• Hiroshi Kurosawa and Mitsuhiro Yokota, Fundamentals of Fiber Optics, January 2003, Optronics Co., Ltd.
• Yoshinobu Shimada, Ichiro Yamashita, Norihisa Ohta; Multimedia Network Series Optical Access Method, October 1993 , Ohmsha Co., Ltd.
• Yoshio Kashima: Introduction to Optical Communication Networks, April 2001, Optronics Co., Ltd., edited by Toshiharu Tako and Tatsunori Honda
• Optical Measuring Instrument Guide for User Engineers, May 1998, Optronics Co., Ltd.
• ANDO Technical Report Vol.71 (Special Issue on Optical Technology) July 2001
• Hideo Doumeki: How to Use a Stepping Motor, June 1993, Kogyo Chosakai Co., Ltd.
• Basic knowledge of optical fiber. http://www4.plala.or.jp/infobell/kiso-hika/hika1.html
• Fujitsu Laboratories website: http://www.labs.fujitsu.com/jp/gijutsu/laser/