Basic knowledge of optical fiber and basic knowledge of Optical Fiber

Basic knowledge of optical fiber and basic knowledge of Optical Fiber

I. optical fiber classification

Optical fibers are divided into Single-mode and Multi-mode Optical Fibers by transmission mode, as shown in.

Single-Mode Optical Fiber: a single optical path transmits only one mode of light (that is, only one bundle of light transmitted from a specific angle to the optical fiber), because the mode dispersion is completely avoided, the transmission frequency of a single-mode optical fiber is very wide. Therefore, it is suitable for large-capacity and long-distance transmission systems. The light-emitting diode or laser is used as the light source and the 1310nm and 1550nm bands are used.

Multi-Mode Optical Fiber: multiple optical paths can transmit multiple modes of optical fiber at the same time. Due to the dispersion and difference, its transmission performance is poor, the band is narrow, the capacity is small, and the distance is also short. The laser is used as the light source and the 850nm and 1300nm bands are used.

1. Optical Fiber specifications (commonly used)

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

(Note: Optical Fiber internal diameter/optical fiber outer diameter)

2. optical fiber loss

2. Glossary of Transmission Technology

1. Digital Differential multiplexing

A cross-bit pulse that transmits multiple digital data, voice, and video signals over the same channel through different channels or time slots.

2. Wavelength Division Multiplexing Technology

Wavelength division multiplexing (WDM) combines two or more optical carrier signals of different wavelengths (carrying various information) at the sending end by a Multiplexer, and is coupled to the same optical fiber for transmission. at the receiving end, Demultiplexer separates optical carriers of various wavelengths, the optical receiver then performs further processing to restore the original signal. This technology, known as wavelength division multiplexing, transmits two or more optical signals of different wavelengths simultaneously in the same optical fiber.

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

The design of the communication system varies with the interval width of each wavelength. Based on the channel interval, WDM can be subdivided into CWDM (coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing ).

• CWDM: wavelength interval ≥ 20 nm, usually 1470 ~ Eight 16 nm bands (one at every 20 nm interval ).
• DWDM: The wavelength interval is less than 10 nm, and the window is around 1550nm. The optical device is expensive and the optical device is not commonly used.

There are two main differences between CWDM and DWDM: first, the CWDM Carrier Channel has a wide spacing. Therefore, the same optical fiber can reuse 10 optical waves at the left or right wavelength, DWDM can reuse 10 or more wavelengths of light. Second, the CWDM modulation laser uses a non-cooling Laser, while DWDM uses a cooling Laser. Cooling Laser adopts temperature tuning and non-cooling Laser adopts electronic tuning. Due to the uneven temperature distribution in a wide wavelength segment, temperature tuning is difficult and costly. CWDM greatly reduces costs because it avoids this difficulty. The cost of the entire CWDM system is only 30% of that of DWDM.

4. Relay

Optical Fiber Transmission also suffers losses. Therefore, the signal power transmitted will gradually decrease, which will cause signal distortion to a certain extent, resulting in receiving errors. The maximum transmission distance of the optical terminal is about 120 km. When the distance is exceeded, we use relay to complete the process. Currently, relay is divided into three types:

• Analog relay: converts the attenuation optical signal from a distance into a simulated carrier signal, and then converts the signal into a light signal to use the new light wave. Problem: The process of simulating relay will lead to signal attenuation and distortion, and it is difficult to ensure the signal quality, and it is impossible to relay multiple times.
• Digital relay: converts the attenuation optical signal from a distance into a digital electrical signal, and then converts the signal into a light signal to use the new light wave. Because digital signals can be corrected and regenerated, signal attenuation and distortion will not occur due to multiple conversions, which can ensure signal transmission quality and provide unlimited relay.
• Optical Relay: directly amplifying the attenuation optical signal sent from a distance to continue forward transmission. This is the simplest method, and the signal does not need to be converted in any way, but the optical amplifier is very expensive and difficult to use.

Iii. Interface knowledge

1. BNC Interface

BNC interface refers to the coaxial cable interface. BNC interface is used for 75 Euro coaxial cable connection. It provides two channels: RX and TX. It is used for non-balanced signal connection.

2. Optical Fiber Interface

The optical fiber interface is a physical interface used to connect the optical fiber cable. Generally, there are several types such as SC, ST, and FC, which are developed by Japanese NTT companies. FC is the abbreviation of Ferrule Connector. The external reinforcing mode is a metal sleeve, and the fastening mode is a screw. The ST interface is usually used for 10Base-F, and the SC interface is usually used for 100Base-FX.

3. RJ-45 Interface

RJ-45 interface is the most commonly used Ethernet interface, RJ-45 is a common name, refers to the IEC (60) 603-7 standard, use a modular Jack or plug with 8 locations (8-pin) as defined by international connector standards.

Iv. Basic knowledge of optical terminal

1. What is an optical terminal?

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

2. analog optical terminal

At the transmitting end, the amplitude or frequency of the signal to be transmitted is 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, digital processing is performed on the images, voice and data signals to be transmitted, and then the digital signals are reused to convert multiple low-speed digital signals into one high-speed signal, this signal is then converted to an optical signal and transmitted through 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 multiple low-speed digital signals, finally, the digital signals are restored to analog signals and data signals such as images and speech.