1. How Optical Fibers Guide Light With a Core and Cladding
Optical fibers used for communication consist of two layers: a central core and an outer cladding. The core has a higher refractive index than the cladding. This difference causes light to reflect completely at the boundary, trapping it inside the core [1].
In single-mode fibers, which are commonly used, the core diameter is about 10 micrometers (many standard products are 9 micrometers), and the outer diameter of the cladding is 125 micrometers. The light travels through only one path [2]. A diagram from the National Institute of Information and Communications Technology (NICT) shows that when a laser light of just 10 milliwatts enters the core, the light intensity per cross-sectional area of the core is equivalent to the surface of the Sun (15.7 kilowatts per square centimeter) [3]. This figure is not a comparison of brightness but a number showing how concentrated the light is in the thin core.
Even with this concentration, light weakens as the distance increases. The approximately 8,300 kilometers (about 5,157 miles) between Tokyo and San Francisco cannot be transmitted in a single shot [5]. So, why does the light weaken?
2. Why Light Weakens Inside Glass Fibers
In the silica glass used for optical fibers, more than 80% of transmission loss is due to Rayleigh scattering. This happens because slight fluctuations in the density of the glass cause the light to scatter in all directions [4].
The loss was 20 dB per kilometer in the 1970s. In just about 30 years, it dropped to about 0.2 dB in the wavelength bands used for communication [4]. Since 20 dB means the strength drops to 1/100, the distance for the light to drop to 1/100 extended from about 1 kilometer to about 100 kilometers. This is a calculated value from dB conversion, not a measured distance.
So, how was the color (wavelength) of light that produces this "about 0.2 dB" loss chosen?
3. Why Infrared Light Is Used for Optical Communication
According to the description of the Japan Prize winning achievements, current optical communication uses the 1.5-micrometer band, where the attenuation per kilometer is lowest at about 0.2 dB [5]. A micrometer is 1/1,000 of a millimeter. This wavelength is in the infrared region and is invisible to the human eye.
Scattering tends to decrease as the wavelength gets longer. However, if the wavelength becomes even longer, the glass itself absorbs more light. There is a band in the middle where the loss is smallest.
Even with the smallest loss, the light drops to 1/100 of its strength after 100 kilometers. For 8,300 kilometers, the light must be amplified many times by optical amplifiers.
4. How Optical Amplifiers Boost Weak Light Without Converting It into Electricity
When single-mode optical communication became practical in the 1980s, weakened light was once converted into electrical signals and then amplified. These electrical amplifiers were large and required large power [5].
Masataka Nakazawa proposed the first optical amplifier in the world by combining a fiber doped with erbium and pump light from a 1.48-micrometer semiconductor laser [5]. The pump light gives energy to the electrons in the erbium, pushing them to a higher state. The energy released by these electrons through stimulated emission is received by the 1.55-micrometer communication light, amplifying it [5]. In experiments at the time, the gain was 12.5 dB (about 18 times). This was an experimental value at the time, not the value of current undersea devices.
Kazuo Hagimoto and others succeeded in an experiment using a small amplifier of about 10 cm square to carry a signal of 1.8 gigabits per second over 212 kilometers. They showed for the first time in the world that optical amplifiers could be put into practical use [5]. About 5 years after development, they were adopted in trunk lines connecting the world [5].
For crossing the Pacific, optical amplifiers are placed in undersea repeaters every few tens to 100 kilometers to amplify the weakened light repeatedly [5]. A diagram in the materials shows more than 100 repeaters for a crossing of about 10,000 kilometers [2] (this is not the number for a specific cable). Can we send more signals through this path of light?
5. How Many Colors of Light Share One Fiber
Optical amplifiers can amplify light of multiple wavelengths together. This property, combined with Wavelength Division Multiplexing (WDM), led to the expansion of capacity in optical communication from the mid-1990s [5]. WDM is a method where signals are placed on light of different wavelengths for each channel at the transmitter and overlapped, then separated by wavelength at the receiver [6].
NTT succeeded for the first time in the world in 2005 in an experiment sending 1,000 wavelengths on one optical fiber (round trip between Keihanna in Kyoto and Dojima in Osaka, 126 kilometers) [6]. This is the number of wavelengths in the experiment, not the number in current undersea cables.
FASTER, which connects Japan and the United States (operational since 2016, total length about 9,000 kilometers), has a design capacity of about 60 terabits per second (Tb/s) total, using 6 pairs of fibers, with 100 wavelengths of 100 gigabits per second (Gb/s) signals per pair (10 Tb/s per pair) [2]. The design capacity per pair of trans-Pacific cables increased by about 2,000 times in about 20 years (from TPC-5 to FASTER) [2]. These are design figures, not actual communication volumes.
By the way, as we layer more colors, a question arises. Does the speed differ depending on the color of the light?
6. Why Different Colors Arrive at Different Times and How to Fix It
When light travels through an optical fiber, there is a difference in arrival time between long-wavelength light and short-wavelength light. This causes the signal to shift and distort, known as chromatic dispersion [2].
In traditional undersea cables, the distortion was corrected by repeating the process of using fibers with opposite properties more than 100 times before crossing the Pacific. In FASTER, digital signal processing (correction by calculation) is used, so dispersion compensation fibers are no longer needed [2].
Another technique is multi-level modulation. Instead of just blinking the light, it uses the position of the light wave (phase) or a combination of position and brightness to send 2, 3, or 6 bits at once. In an example from the same materials, intensity modulation of 10 Gb/s x 40 wavelengths = 0.4 Tb/s became 3.2 Tb/s with QPSK (40 Gb/s x 80 wavelengths), an 8-fold increase [2].
7. Calculating How Much Light Is Lost Over Distance
Let’s try using a calculator. In 1970s fibers, the light dropped to 1/100 after 1 kilometer. If it travels 2 kilometers, it drops to 1/100 of that, which is 1/10,000. At 3 kilometers, it is 1/1,000,000.
With today’s about 0.2 dB/km, traveling 300 kilometers results in a 60 dB loss, which is also 1/1,000,000. Both are values converted from dB. By lining up other distances—20 dB is 1/100, 40 dB is 1/10,000—you can get a feel for the size of the numbers.
For safety, do not look into the end of the cable of a home optical line. If you want to read the materials, NTT news releases [6] and the Japan Prize achievement descriptions [5] are available in Japanese.