The basics of optical fiber you must know

1. Classification of optical fiber

Optical fibers are divided into single-mode fibers (Single-mode) and multi-mode fibers (Multi-mode) according to the transmission mode, as shown in the figure below.

Single-mode fiber: a single optical path that transmits only one mode of light (i.e., only a beam of light entering the optical fiber from a specific angle). Since mode dispersion is completely avoided, the transmission bandwidth of single-mode fiber is very wide, making it suitable for large-capacity, long-distance transmission systems. It uses light-emitting diodes or lasers as light sources and adopts two bands of 1310nm and 1550nm.

Multimode optical fiber: multiple optical paths can transmit multiple modes of light in one optical fiber at the same time. Due to dispersion and phase difference, its transmission performance is poor, the frequency band is narrow, the capacity is small, and the distance is short. It uses laser as the light source and adopts two bands of 850nm and 1300nm.

1. Fiber specifications (commonly used)

• Single mode: 8/125μm, 9/125μm, 10/125μm
• Multimode: 50/125μm European standard; 62.5/125μm American standard
• Industrial, medical and low-speed networks: 100/140μm, 200/230μm
• Plastic: 98/1000μm For automotive controls.

(Note: Fiber inner diameter/fiber outer diameter)

2. Fiber loss

2. Explanation of terms related to transmission technology

1. Digital differential multiplexing

Refers to a method of transmitting multiple digitized data, voice, and video signals simultaneously on the same channel by interleaving bit pulses in different channels or time slots.

2. Wavelength Division Multiplexing Technology

Wavelength division multiplexing (WDM) is a technology that combines two or more optical carrier signals of different wavelengths (carrying various information) at the transmitting end through a multiplexer (also called a combiner, Multiplexer) and couples them to the same optical fiber of the optical line for transmission; at the receiving end, the optical carriers of various wavelengths are separated by a demultiplexer (also called a wavelength splitter or a demultiplexer), and then further processed by the optical receiver to restore the original signal. This technology of simultaneously transmitting two or more optical signals of different wavelengths in the same optical fiber is called wavelength division multiplexing.

3. CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing)

The design of the communication system is different, and the spacing width between each wavelength is also different. According to the difference in channel spacing, WDM can be subdivided into CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing).

• CWDM: wavelength interval ≥ 20nm, usually using eight bands from 1470 to 1610nm (one band at every 20nm interval).
• DWDM: The wavelength interval is <10nm, and the window is also around 1550nm. The optical components are expensive, and optical terminals are not commonly used.

There are two main differences between CWDM and DWDM: First, the CWDM carrier channel spacing is wider, so about 10 wavelengths of light waves can be multiplexed on the same optical fiber, while DWDM can multiplex more than 10 wavelengths of light waves; second, CWDM modulated lasers use uncooled lasers, while DWDM uses cooled lasers. Cooled lasers use temperature tuning, and uncooled lasers use electronic tuning. Since the temperature distribution is very uneven within a very wide wavelength range, temperature tuning is very difficult to achieve and the cost is also very high. CWDM avoids this difficulty, thus greatly reducing costs. The cost of the entire CWDM system is only 30% of that of DWDM.

4. Relay

Fiber optic transmission also has losses, so the transmitted signal power will gradually decay. When it decays to a certain extent, it will cause signal distortion, thus causing reception errors. The maximum transmission distance of an optical terminal is about 120km. When the distance exceeds this, we use relays to complete it. Currently, there are three types of relays:

• Analog relay: Convert the attenuated optical signal from a distance into an analog carrier signal, and then convert it into an optical signal after processing and send it out using a new optical wave. Problem: The analog relay process itself will cause signal attenuation and distortion, making it difficult to ensure signal quality and unable to be relayed multiple times.
• Digital relay: Convert the attenuated optical signal from a distance into a digital electrical signal, and then convert it into an optical signal and send it out using a new light wave. Since digital signals can correct errors and regenerate, there will be no signal attenuation and distortion due to multiple conversions, and the signal transmission quality can be guaranteed, and the relay can be unlimited.
• Optical relay: directly amplifies the attenuated optical signal from a distant location and then transmits it forward. This is the simplest method, and the signal does not need to be converted in any way, but optical amplifiers are very expensive and difficult to use.

3. Interface related knowledge

1. BNC interface

The BNC interface refers to the coaxial cable interface. The BNC interface is used for 75-ohm coaxial cable connection. It provides two channels: receive (RX) and transmit (TX). It is used to connect unbalanced signals.

2. Fiber optic interface

The fiber optic interface is a physical interface used to connect fiber optic cables. There are usually several types such as SC, ST, and FC, which were developed by Japan's NTT Corporation. FC is the abbreviation of Ferrule Connector, and its external reinforcement method is a metal sleeve, and the fastening method is a screw buckle. The ST interface is usually used for 10Base-F, and the SC interface is usually used for 100Base-FX.

3. RJ-45 interface

The RJ-45 interface is the most commonly used interface for Ethernet. RJ-45 is a common name that refers to an 8-position (8-pin) modular jack or plug standardized by IEC (60) 603-7 and defined by international connector standards.

4. Basic knowledge of optical transceiver

1. What is an optical transceiver?

Optical transceiver is a device used in optical fiber communication. It is essentially a converter from electrical signal to optical signal and from optical signal to electrical signal. Optical transceiver can be divided into transmitting optical transceiver, receiving optical transceiver and transmitting optical transceiver.

2. Analog optical transceiver

At the transmitting end, the signal to be transmitted is amplitude or frequency modulated and then the modulated electrical signal is converted into an optical signal and transmitted through the optical fiber. At the receiving end, the optical signal is restored to an electrical signal, and then the signal is demodulated to restore the image, voice or data signal.

3. Digital optical transceiver

At the transmitting end, the image, voice and data signals to be transmitted are digitized and then multiplexed to convert multiple low-speed digital signals into one high-speed signal, which is then converted into an optical signal and transmitted through the optical fiber. At the receiving end, the optical signal is restored to an electrical signal, and the restored high-speed digital signal is decomposed into the original multiple low-speed digital signals, which are then restored to analog signals such as images and voices, as well as data signals.