Analysis on the Mode and Characteristics of Optical Fiber Transmission

fiber optic transmission

First, the characteristics of optical fiber transmission

(I) Low transmission loss

Loss is an important characteristic of the transmission medium, it only determines the distance of the relay required to transmit the signal. Optical fiber has the characteristics of low loss as the transmission medium of optical signal. If 62.5/125μm multimode fiber is used, the attenuation at 850nm wavelength is about 3.0dB/km, and the 1300nm wavelength is lower, about 1.0ddB/km. If 9/25μm single-mode fiber is used, the attenuation of 1300nm wavelength is only 0.4dB/km, and the attenuation of 1550nm wavelength is 0.3dB/km, so the general LD light source can transmit 15 to 20km. At present, products that transmit 100 kilometers have appeared.

(II) Transmission frequency bandwidth

The bandwidth of optical fiber can reach more than 1GHz. Generally, the bandwidth of the image is about 6MHz, so it is more than enough to transmit the image of one channel with one core fiber. The advantages of high-bandwidth optical fibers can not only transmit multi-channel images at the same time, but also transmit voice, control signals or contact signals, and some can even use a core optical fiber to achieve bidirectional transmission through special optical passive components.

(iii) Strong anti-interference

The carrier wave in optical fiber transmission is light wave, which is an electromagnetic wave with extremely high frequency, which is much higher than the frequency used in general radio wave communication, so it is not subject to interference, especially strong electrical interference. At the same time, because the light wave is bundled in the optical fiber, there is no radiation, no pollution to the environment, no leakage of the transmitted signal, and strong confidentiality.

(iv) High safety performance

The glass material used in optical fiber is non-conductive and lightning-proof; optical fiber transmission does not produce sparks due to short circuit or poor contact in traditional circuits, so it is especially suitable for flammable and explosive occasions. Optical fiber cannot be tapped like a cable, and once the optical cable is damaged, it will be discovered immediately, so it is more secure.

(v) Light weight and good mechanical properties

Optical fibers are as small as silk, and their weight is quite light. Even for multi-core optical cables, the weight will not double as the number of cores increases, and the weight of the cable is generally proportional to the outer diameter.

2. Optical fiber structure and transmission mechanism

Optical fiber is a medium for light wave transmission. It is a cylinder composed of dielectric materials and is divided into two parts: core and cladding. Light waves travel along the core. In practical engineering applications, an optical fiber refers to a fiber core in which filaments are drawn from a preform and simply clad, and the core is then clad, strengthened and protected to become an optical cable that can adapt to various engineering applications.

(1) Optical fiber transmission mechanism

The propagation process of light waves in optical fibers is a complex boundary problem of electromagnetic fields. Generally speaking, the diameter of the optical fiber core is several tens of times higher than the wavelength of the propagating light, so the qualitative analysis using geometric optics is sufficient, and The understanding of the problem is also very concise and intuitive.

Refraction and reflection occur when a bundle of optical fibers is projected onto the interface of two media with different refractive indices. For a series of interfaces formed by a multilayer medium, if the refractive index n1>n2>n3...>nm, the incident angle of the incident light at each interface gradually increases until total reflection is formed. Due to the change in refractive index, the incident light is deflected and the direction of propagation changes.

An optical fiber consists of a core, a cladding and a jacket. The function of the jacket is to protect the optical fiber and has no effect on the propagation of light. The refractive index of the core and the cladding are different, and the distribution of the refractive index mainly has two forms: continuous distribution (also known as gradient distribution) and discontinuous distribution (also known as step distribution).

When the incident light enters the fiber after being refracted by the end face of the fiber, except for the light that is consistent with the axial direction and propagates in a straight line, the rest of the light is projected to the interface between the core and the cladding: a kind of total reflection at the interface, these light rays The angle with the optical axis remains unchanged, and it propagates forward in the fiber core in a saw-toothed shape without loss, which is called propagating light; the other one is only partially reflected at the interface, and another part is refracted into the cladding, Finally, it is absorbed by the jacket, and the reflected light will be lost when it reaches the interface again, so it cannot propagate, which is called non-propagating light.

In fact, most of the incoming light rays are not the axial light as mentioned above, so there is also a type of leakage light. If the interface between the core and the cladding is very flat, these rays will form total reflection and propagate, but in fact only Partial reflection, although less lossy than non-propagating light, does not propagate well. Only propagating light makes sense for long distance transmission.

When the light entering the fiber propagates to the interface of the core and the cladding, because the refractive index of the core gradually decreases, it is subjected to a centripetal deflection, and the optical fiber whose angle θ with the axis is less than a certain value cannot reach the interface or reach the interface to form total reflection. , so it is beamed in the core and propagates forward without loss in a wave-like shape, becoming propagating light. The rest of the light cannot propagate because a part of it is refracted at the interface and enters the cladding, where it is gradually absorbed.

Therefore, the refractive index and the distribution of the refractive index of the fiber core and cladding are closely related to the propagation characteristics of the fiber.

(II) Classification of optical fibers

Optical fibers can be classified from different perspectives, such as the materials that make up the optical fiber, the manufacturing method, the distribution of the refractive index of the fiber core and the cladding, and the number of modes that the optical fiber can propagate light.

The materials that constitute the fiber core and cladding mainly include: multi-combination glass, high-purity quartz glass and low-loss halide materials. The preparation of the preform and the drawing method of the optical fiber are also different for different materials. At present, the most widely used is high-purity silica glass fiber (silica fiber), which has comprehensive advantages in terms of material preparation technology, transmission characteristics and strength of the fiber.

The refractive index distribution of the fiber core and cladding is related to the fiber material, drawing method and fiber structure. In addition to the gradient distribution type and step distribution type mentioned above, there are single-material fibers, ring fibers, W-type fibers, etc. All belong to step-distributed optical fibers, and each has its own characteristics in structure.

It can also be distinguished according to the modulus of the propagating light. We can understand a ray as representing one mode, or different modes representing different angles of incident light, and the principle of light fluctuation states that optical fibers can only allow light (or modes) of limited discrete trees to propagate. The number that can propagate in the fiber is proportional to the cross-sectional area of ​​the core and the refractive index difference between the center of the core and the cladding. A single-mode fiber is a fiber that allows only one mode of light to propagate. The single-mode fiber has no modal dispersion because only the propagation axis is off, and has a large information carrying capacity. Multimode fibers typically have several hundred and low loss propagating modes. Easy coupling to light sources and large area detectors.

According to the manufacturing method, it can also be divided into CVD (chemical vapor deposition method), MCVD (modified chemical vapor deposition method) and the like.

(iii) Characteristics of optical fibers

The properties of optical fibers include basic properties such as propagation characteristics, geometric parameters, and refractive index differences between core and cladding. The transmission characteristics are mainly reflected in the loss and bandwidth of the optical fiber.

⒈ Numerical aperture NA

It represents the refractive index difference between the fiber core and the cladding and is one of the most important basic properties of the fiber. NA is a parameter that reflects the refractive index relationship between the core and sub-cladding of the fiber. The greater the refractive index difference, the greater the NA, and the more light the fiber can receive and propagate, which is proportional to the number of modes that the fiber can propagate. So in a sense the numerical aperture represents the ability of the fiber to collect light.

⒉ Transmission loss

This is an important optical characteristic of the optical fiber, which largely determines the distance of the relay required to transmit the signal, and is also related to the system economy. The causes of fiber loss include material absorption, scattering loss and structural defects.

Material absorption is a loss mechanism. Since the fiber cannot be a perfect cylinder, some parameters will change periodically along the length direction. These parameters can be either the refractive index distribution or geometric parameters, that is, the change along the length direction, or the axis Deviation from a straight line. This causes a partial transfer of optical power from one propagating mode to the other mode, which is called scattering.

Scattering loss occurs if the transferred mode is a non-propagating mode. The scattering loss is formed at a ratio of 1/λ4, so it is advantageous to choose long wavelength operation. Some small parameter changes, such as material composition, stress, etc., can be reduced by improving the manufacturing technology, but some small refractive index changes are formed by thermal disturbance during the fiber drawing process and cannot be completely eliminated, which determines the fiber scattering loss. the lowest limit.

Fiber defects, such as unsmooth core-clad interface, air bubbles, stress, diameter changes, and axis bending, can cause fiber transmission loss. Therefore, improving the perfection and consistency of fiber structure is an important task in the fiber drawing process.

Fiber loss is measured in decibels per kilometer (dB/km). Silica fibers have three low-loss wavelength regions—0.85μm, 1.3μm, and 1.55μm. Fluoride fibers have lower losses.

3. Transmission bandwidth

It represents the transmission rate of the fiber and is mainly limited by the fiber dispersion. When the optical pulse propagates along the fiber, each pulse will be broadened as the distance increases, and finally the adjacent pulses overlap, which limits the rate of information transmission by the optical fiber and the transmission bandwidth of the optical fiber, resulting in the widening of the optical pulse. The mechanism is the dispersion of optical fibers, including material dispersion, waveguide dispersion and modal dispersion.

The physical meaning of material dispersion is that the propagation speed of light in the medium is inversely proportional to the refractive index, and the refractive index of the fiber material varies with the wavelength, so the light of different wavelengths propagates at different speeds in the fiber. The shorter the wavelength, the more severe the dispersion.

Waveguide dispersion is caused by different trajectories and different transit times of light with different wavelengths in the fiber. For the same mode, light of different wavelengths will follow different trajectories in the fiber and have different transit times, causing waveguide dispersion. Contrary to material dispersion, the longer the wavelength is, the more serious the waveguide dispersion is, and the smaller the diameter of the fiber core is, the more serious the waveguide dispersion is.

Modal dispersion is also called intermodal dispersion. For incident light of the same wavelength, fibers with different incident angles represent different modes, and different modes travel in different paths in the fiber and have different transit times, thereby forming modal dispersion. The modal dispersion decreases as the diameter of the fiber core decreases. When the diameter is small to a certain extent, the fiber becomes a single-mode fiber that allows only one mode to be transmitted, and there is no modal dispersion.

At a wavelength of 1.3 μm, the waveguide dispersion of the fiber cancels out the material dispersion, so theoretically, a 1.3 μm zero-dispersion single-mode fiber can be fabricated. If the zero-dispersion wavelength of the silica single-mode fiber is shifted from 1.3 μm to the lowest loss wavelength of 1.55 μm , a dispersion-shifted (DS) single-mode fiber can be fabricated. If the two zero-dispersion wavelengths in the long wavelength range can make the optical fiber have low dispersion in a wide range, the dispersion-flattened single-mode fiber can be made.

The dispersion of the fiber is related to the length of the fiber or the transmission distance of the signal, so the transmission bandwidth of the fiber is a function of the transmission distance. The bandwidth-distance product is often used to measure the transmission bandwidth of the fiber, while for single-mode fiber, the dispersion value is often used to represent the transmission characteristics.

3. Optical cable

Optical cable is a transmission medium with practical value after protecting and strengthening the optical fiber.

(1) The design goal of the optical cable

The following points should be considered in the design of optical cables: avoid micro-bending loss of the core; avoid damage to the surface of the core; ensure that the optical cable has sufficient mechanical strength, good sealing and moisture resistance; Root core; reasonable weight, volume and core space distribution.

(II) The structure of the optical cable

Commonly used optical cables are divided into layer twist type and skeleton type, and other types include unit type, cord type, ribbon type and so on.

The layered type is a limit-strengthened plastic or steel wire that forms a central reinforcing member and surrounds a buffer layer. Multiple fiber cores are evenly distributed outside the buffer layer and spirally surround the central reinforcing member. A buffer layer is formed, and finally a waterproof cover, usually a polyethylene aluminum cover, is used.

Skeleton cables use a specially shaped plastic skeleton containing a central steel wire, and the core is loosely placed in a cavity around the skeleton. The fiber core is also helically wrapped around the central wire, which ensures that the fiber core is not subjected to additional stress when the cable is bent. The outermost layer of the optical cable is also waterproof. In order to improve the moisture-proof performance of the optical cable, some optical cables are filled with moisture-proof sealant in the cavity of the skeleton, and the fiber core is floating in the sealant, so it has excellent moisture-proof sealing performance.

The multi-core unit structure is to loosely install several cores in a sheath to form a unit, and several units surround the center reinforcement.

The central strength member (central steel wire) bears most of the traction force during construction, so it determines the tensile strength of the optical cable, and the aluminum sleeve and the skeleton improve the lateral compression strength of the optical cable.

According to the number of fiber cores contained in the optical cable, it is divided into single-core and multi-core optical cables. Most of the trunk line applications are multi-core optical cables, and when each point is split, it is mostly single-core optical cables.

4. Light source

(1) Light source for optical fiber transmission

The light source is one of the important components in the optical fiber transmission system. Light sources for fiber optic transmission have completely different requirements than light sources for other applications. The sound itself does not need a lot of power, but it has good stability and enough life. Its geometric size and structure should match the optical fiber, and ensure enough optical power to enter the optical fiber. In order to obtain good transmission effect, the light source should be output at the low loss and low dispersion wavelength of the fiber.

At the same time, the signal modulation capacity is required to be large, and the modulation frequency can be as low as audio frequency and as high as several GHz. According to such requirements, the light sources that can be applied to optical fiber transmission are limited to a few solid-state devices that are small in size, low in price and easy to modulate.

⒈ Light Emitting Diode (LED)

Light-emitting diodes with emission wavelengths of 0.8 to 0.9 μm or 1.1 to 1.5 μm are the simplest solid-state light sources and are widely used in optical fiber transmission. It can provide sufficient output regularity and moderate spectral width, can be easily modulated directly, has a long working life, and is relatively inexpensive.

One of the design requirements of LEDs is to have a structure that can output its radiation, obtain effective external optical power, and facilitate coupling with optical fibers to generate higher input fiber power. There are two structural types of LEDs: surface light-emitting diodes and end-face light-emitting diodes.

Surface light-emitting diodes emit light in a small area of ​​the active area, and the light is output through a thin or transparent semiconductor layer above the active area in a direction perpendicular to the junction plane. The small area of ​​the active area is conducive to the heat dissipation of the higher current density. The active area is made into a small circular surface, the diameter is usually 75-100μm, and the semiconductor layer above is very thin (10-15μm), so that the end face of the fiber can be very Close to the active area, good coupling is obtained. Heat dissipation of LEDs is a very important issue, and the rise in junction temperature will cause a drop in output power. The heterojunction type has higher luminous efficiency and output light efficiency than the homojunction type LED, but the heat dissipation performance is not the same as that of the homojunction type.

An end-face light-emitting diode outputs radiation directly from one end face of the exposed active region. The light emitted by the high-efficiency end-face light-emitting diode forms a relatively directional beam, so it is beneficial to couple the emitted light into the fiber, especially for small-diameter fibers.

Since the refractive index of the active layer is higher than the two sides, a waveguide effect is formed, and the emitted light is confined in the active layer. The reflective film will make the light more concentrated and emitted from one end face. Since the light is emitted from a very small end face, the effective brightness of the end face is very high. When coupling with an optical fiber, it is very effective to place one on the end face, because the light emitting surface of the device is smaller than the cross-sectional area of ​​the optical fiber.

⒉ Semiconductor Laser (LD)

The spectral width emitted by semiconductor lasers is much narrower than that of light-emitting diodes, generally less than 1 nm. It is very advantageous when material dispersion is the main factor limiting the transmission bandwidth. Even with several modes oscillating simultaneously, lasers are more than 50% efficient when coupled to multimode fibers, much higher than LEDs.

Therefore, a laser that emits the same output power as an LED will have 15 to 20 dB higher optical power coupled into the fiber than an LED. At the same time, under normal bias conditions, its modulation frequency can be as high as 1GHz or more, so it is very suitable for long-distance, high-speed transmission systems.

(II) Characteristics of the light source

⒈ Spectral characteristics

This is the basic characteristic of the light source, which is usually expressed by the wavelength λ of the light source and the spectral width Δλ (the width of the optical power of 3dB, also known as the half-value width). Both the transmission loss and the transmission rate limited by material dispersion are related to the wavelength and spectral width of the light source.

⒉ Power efficiency

For the working efficiency of light sources, especially LEDs, it is moot to measure the total output power, because not all the electrical power applied to the diode can be converted into output optical power, and not all output optical power can be coupled into the fiber Get practical applications. Therefore, the index of useful power efficiency is generally used, which represents the ratio of the actual received optical power to the electrical power added to the diode.

The actual received optical power is related to the structure of the light source and the coupling method of the optical fiber. For example, the surface light-emitting diode adopts a large numerical aperture optical fiber to directly couple at the mouth of the diode to increase the useful power, which also improves the useful power efficiency.

Coupling efficiency is also a very useful indicator, which is the ratio of the optical power injected into the fiber to the optical power output from the light source. Generally, the coupling efficiency of LED is a few percent, and the coupling efficiency of LD can reach 50%. In addition, fiber extraction rate is also a frequently used indicator.

3. Output characteristics

The output characteristics of the light source represent the relationship between the operating current and the output optical power (or fiber extraction rate). The output characteristics of the LED have good linearity in a wide range, and when the injection current reaches a certain value, it will be saturated.

The output characteristic curve of LD has an inflection point, which corresponds to the threshold of stimulated light. When the injection current is lower than the threshold, there is only a very low emission output, and the device is in the LED state; when the injection current exceeds the threshold, stimulated emission begins to occur. , which produces a high-power light output with a nice linear region.

The output characteristic of the light source is an important basis for selecting the static operating point and determining the modulation amplitude of the electrical signal when designing the optical transmitter. Ensuring that the light source works in a good linear segment is the key to ensuring the linearity of the transmission system, especially for analog signals, such as video signals, it is very important to reduce the nonlinear clock.

⒋ Efficiency and modulation bandwidth

The output power and power efficiency of the light source are related to the injection current, as well as the geometric dimensions of the active layer of the light source, the doping concentration of materials and other factors. The modulation speed of the light source (modulated directly by the information-carrying signal current) is also related to these factors.

The current technology can properly adjust the parameters affecting efficiency and modulation speed, but the two restrict each other, and it is impossible to obtain large output power and high modulation speed at the same time. The modulation bandwidth corresponds to the direct modulation speed and is defined according to the bandwidth of the electrical signal, that is to say, a device with high output can only be directly modulated at a low rate and has a lower modulation bandwidth. To obtain high modulation speed, output power must be sacrificed.

5. Longevity

The working life represents the working time when the output power of the light source is reduced to half the initial value. LEDs generally reach 107, which is much longer than LDs.

(iii) Modulation of the light source

As with radio communication, information must be loaded onto light waves, that is, modulated. Modulation can be either analog or digital, and the method used depends on system requirements, comprehensive fiber transmission characteristics, detector characteristics, and the characteristics of the light source itself. The analog device is simple and has an advantage in price, although its required high signal-to-noise ratio limits its application to narrow bandwidths and short distances. Digital is ideal for broadband long-haul systems.

For light-emitting diodes, the output light power of the LED can be changed by changing the injection current, which also realizes the light intensity modulation. For semiconductor lasers, direct modulation is performed by changing the drive current.

Common modulation methods include: IM (light intensity modulation), PCM (pulse code modulation), FM (frequency modulation), AM (amplitude modulation), and PFM (pulse frequency modulation).

5. Detector

Like the light source, the detector is also another main component in the optical fiber transmission system. Contrary to the light source, the detector demodulates the optical signal and converts the change of the optical signal into the change of the electrical signal. The main requirements for the detector are: sufficient sensitivity and bandwidth in the working band; low noise introduced and good working stability; easy to couple with optical fiber and combine with processing circuit in structure. Commonly used detectors are semiconductor photodiodes and avalanche diodes (ADPs).

6. Optical fiber transmission system

(1) The structure of the optical fiber transmission system

The optical fiber transmits optical signals, so the optical transmitter completes the E/O conversion and the core device is the light source, while the optical receiver completes the O/E conversion, and the core device is the detector. Therefore, the three elements of the optical fiber transmission system are the light source, the optical fiber, and the detector.

⒈ Selection of optical carrier wavelength

Two aspects should be considered, one is that the detector can work well at this wavelength, and the other is that the fiber has good loss and dispersion performance at this wavelength. Systems with short transmission distances are not very demanding on fiber loss and dispersion, and the cost of light sources and detectors should be considered in wavelength selection.

⒉The choice of light source

The choice of the light source is not only related to the wavelength, but also involves the modulation method, transmission bandwidth (transmission rate) and cost factors of the system. The price of LD is higher than that of LED, the driving circuit is also more complicated than that of LED, and the lifespan is shorter than that of LED. therefore. LEDs are a practical, inexpensive light source device that is sufficient for most applications below 5km.

The input power of LD is 10 to 25 dB higher than that of LED. In applications where noise is the main limiting factor, LED is obviously very disadvantageous. Moreover, LD is also very beneficial in avoiding material dispersion. Therefore, in high-speed, long-distance systems LD is better than LED.

⒊The choice of detector

Compared with PIN diodes, avalanche diodes can improve the sensitivity of the receiver, but they are expensive, sensitive to temperature, and require a complex circuit to ensure stable operation.

⒋The choice of optical fiber

The selection of optical fiber mainly examines the choice between single-mode and multi-mode, including refractive index difference and refractive index distribution.

Using LED as the light source, in order to transmit as much optical power as possible, a multimode fiber must be selected, and a large refractive index difference is desired. Gradient optical centers have certain benefits for reducing intermodal dispersion.

Using LD as the light source, either single-mode fiber or multi-mode fiber can be used. Single-mode fiber has a small cross-sectional area (5 to 10 μm), and fiber splicing is more difficult than multi-mode fiber. The LD is optimally coupled to single-mode fiber in high-speed systems.

(II) Video transmission system

Broadband is the characteristic of video signal, and analog baseband mode and PFM mode are mainly used for transmission in application TV.

⒈ Characteristics of transmission

Signal-to-noise ratio (S/N): S/N affects image resolution and is related to subjective test results. The main factors affecting S/N are the loss of the optical path (fiber), the power of the light source and the sensitivity of the detector. In addition to the reasonable selection of the light source and detector, the coupling between the light source and the optical fiber, the loss of the optical connector, the loss of the optical fiber joint, and of course the length loss of the optical fiber should be included in the calculation of the optical path loss. First, the received optical power of the detector is proposed , and then convert the light source power according to the optical path loss. Using the PFM method can reduce the effect of loss on S/N, but there will be triangular noise, which can be alleviated by pre-emphasizing the video signal.

Amplitude-frequency characteristics: It is easy to form an optical fiber transmission system with a transmission bandwidth of 10MHz, so there is no big problem for baseband transmission. The amplitude-frequency characteristic determines the resolution of the image, and the flatness of the amplitude-frequency characteristic affects the color saturation and hue of the color image. Generally speaking, the optical system has little influence on the amplitude-frequency characteristics, but for multi-channel video transmission, the modulation bandwidth of the light source and the response speed of the detector will cause linear distortion of the image.

Linear distortion: pulse and sliver signals are usually used to test. The main factor affecting the linear distortion is the dispersion of the optical fiber, and the spectral width of the light source also has a certain relationship. The coupling between the laser and the single-mode fiber has minimal linear distortion.

Nonlinear distortion: mainly refers to DG and DP, mainly caused by the nonlinearity of the light source, which needs to be compensated by the optical transceiver circuit

⒉ Typical video transmission system

The video analog baseband transmission is the simplest system, usually can get more than 10MHz bandwidth, and the transmission distance can reach several kilometers. It is the most widely used way of TV signal transmission. These systems have multiple LED light sources.

The PFM transmission method has the advantages of both analog and digital modulation. Compared with the analog method, it is more suitable for long-distance transmission and facilitates relay amplification. It is not as expensive as the digital method. It is an economical and practical method for video signal transmission. The key of PFM lies in modulation and demodulation, light source driving and light receiving preamplifier, etc. are the same as other types of systems. Using LD light source and APD detector PFM can achieve high-quality relayless transmission of tens of kilometers.

(iii) Common multiplex transmission methods

The realization of multiplexing is beneficial to reduce system cost and improve resource utilization. Common methods include frequency division multiplexing (FDM) and wavelength division multiplexing (WDM) methods. The former uses the frequency division multiplexing technology of electrical signals to form a broadband carrier frequency signal from multiple channels, which is then transmitted by one optical fiber, and then split and demodulated at the receiving end to form multiple outputs. The latter uses the multiplexing of optical wavelengths to transmit multiple optical carriers in one optical fiber.

(iv) The establishment of the light path

⒈ Optical fiber connection

There are two ways to connect optical fibers: one is fusion splicing, and the other is to use connectors. The former is the method used in permanent systems and where low loss and high reliability are required. The latter is a detachable way, suitable for short-term systems.

Most of the optical fiber connectors adopt the method of precise geometric positioning, so that the good optical fiber end face can be accurately centered and ensure that the optical power can enter the other optical fiber from one optical fiber to the maximum extent. The main types of connectors are cone positioning type and different groove positioning type. According to the optical coupling method, there are direct coupling and lens coupling.

The core diameter of the single-mode fiber is very small, and the precision of the connector is higher. The deviation of the optical fiber splicing caused by the connector is the same as that of the fusion splicing. It requires high-precision processing to reduce the axial, lateral and angular differences of the optical fiber to ensure good optical coupling. with high precision.

⒉ Combiner and splitter

Optical combiners and optical splitters are optical devices that realize wavelength division multiplexing. The structural types of the beam splitter are prism, interference film filter and diffraction mode. Light splitting and light combining are reciprocal devices, as long as the incident and output directions are changed, the light splitter becomes a light combiner.

For the case of less multiplexing wavelengths, it is easy to use an interference film, and the process is stable. The coupling of optical fibers and interference films often adopts plano-convex rod lenses and self-focusing lenses. The latter is an economical and convenient way. Since the self-focusing lens (focusing rod) has the function of collimating light and concentrating light when the pitch is 1/4λ, the coupling efficiency is high and it is easy to adjust.

Wavelength division multiplexing is an effective method for optical fiber transmission to improve the transmission information capacity. It can reduce costs, especially when fiber optic cables are already laid, and double the transmission capacity.

⒊ Protective device for optical cable connector

Since the spliced ​​fiber is bare, it must be protected. The main functions of the protective device are:

(1) Ensure the tightness of the joint part and prevent moisture from entering the protective cavity. Because moisture is the main cause of increased fiber loss and reduced life. In addition, it also prevents the internal mechanical parts from rusting and losing their original functions.

(2) The remaining optical fibers can be placed well. The fusion splicing of optical fibers must have a certain margin, and there may be different margin lengths, so these fibers must be placed, which requires sufficient size to allow them to be smoothly coiled above the minimum bending limit, and very well fixed.

(3) Reliably fix the optical cable connector to ensure that the optical cable still has a certain mechanical strength after adding the protective device.

⑷ To facilitate on-site operation and use. The protection of optical cable joints is mainly outdoors, and the protection device can be installed without special tools and methods, and has good engineering performance.

Commonly used protective devices are generally that some mechanical structural parts are cavity structures, and there are spaces inside to prevent remaining optical fibers and structures for fixing optical cable ends and optical fibers. The entrance and exit of the optical cable should adopt a certain sealing technology, and the joint of the cavity should also adopt a sealing device.