General

Q1. What are the characteristics of optical fiber cables compared to metallic cables?

Optical fiber has the characteristics of low loss, wide bandwidth, small diameter, and electromagnetic immunity, so it is possible to transmit large capacity and long distance without using a repeater, and the cable can be thin and light. Also, it does not emit electromagnetic radiation and is not affected by electromagnetic radiation.

Q2. How many cores does the optical fiber cable have?

The standard maximum is 1000 fibers. However, we have a track record of up to 2000 hearts in the field and up to 4000 hearts at the research level. In both cases, a thin, lightweight multi-core cable is realized by storing the tape core wires in the slots at high density.

Q3. How are optical and electrical signals connected?

In order to systematically connect optical and electrical signals, a transmitter/receiver equipped with an electrical/optical conversion function is required. The electrical signal is converted to an optical signal by the transmitter and sent out to the optical fiber. The optical signal transmitted through the optical fiber is then converted into an electrical signal by the receiver. A device that converts electrical signals into optical signals is called a light-emitting device, and includes light-emitting diodes (LEDs) and laser diodes (LDs). Photodetectors that convert optical signals into electrical signals include PIN photodiodes (PIN-PD) and avalanche photodiodes (APD).

Fiber (general)

Q4. What is dispersion?

An optical pulse is a collection of slightly different wavelengths, and the propagation time differs depending on the wavelength. Therefore, the phenomenon in which the pulse width spreads over time during propagation is called chromatic dispersion. Chromatic dispersion is classified into material dispersion and structural dispersion, and in single-mode (SM) fibers, the dispersion characteristic is expressed as the sum of these dispersions.
In multimode (MM) fibers, on the other hand, the pulse propagates in several modes with different propagation velocities, and the output light pulse has a wider width than the input light pulse even though the light source has a single wavelength. This is called modal dispersion.
Generally, the magnitude of each dispersion is modal dispersion > material dispersion > structural dispersion.

Q5. What is the cutoff wavelength?

Even the same optical fiber can be SM or MM depending on the wavelength of the light used, but the wavelength at the boundary between MM and SM is called the cutoff wavelength. In other words, it is the shortest wavelength at which only one mode can propagate in SM fiber. The number of modes that an optical fiber can propagate is uniquely determined by the normalized frequency V, and only one mode can propagate when V < 2.405.
V=2π・(core radius)・√((core refractive index)2-(cladding refractive index)2)/(wavelength)

Q6. What is NA?

Abbreviation for Numerical Aperture, originally a measure of optical lens performance. For optical fibers, the NA of the optical fiber is defined as the sine value of the maximum angle at which light can enter from the end face. NA=sinθmax Therefore, a fiber with a larger NA can receive light from a wider range, making it easier to couple with the light source.

Q7. What is the difference between SM and GI?

An optical fiber that propagates in many propagation modes is called a multimode (MM) optical fiber, and an optical fiber that propagates only the lowest fundamental mode is called a single mode (SM) optical fiber. Among MM fibers, the graded-index (GI) fiber has a refractive index profile that increases continuously toward the center. The SM type is used for long-distance, large-capacity transmission, and the GI type is used for medium-to-long-distance, medium-capacity transmission.

Fiber (SM)

Q8. What is the difference between the core diameter and mode field diameter of SM optical fiber?

The core diameter indicates the core region physically defined by the refractive index of the core and cladding, while the mode field diameter indicates the spread of the intensity distribution of the propagation mode when light is actually incident on the fiber. increase. In SM type optical fiber, it is difficult to measure directly due to the small core diameter, and it is not an effective parameter because the refractive index distribution varies depending on the manufacturing method. is used.

Q9. Is it okay to use a general SM fiber (for 1.31μm) at a wavelength of 1.55μm?

Even 1.31 μm zero-dispersion SM fiber can communicate at 1.55 μm due to fiber transmission characteristics. However, when used at 1.55 μm, mechanical characteristics such as bending and lateral pressure are generally severe, so it is necessary to match and confirm the overall system (transmission equipment, line components, construction method, etc.). In principle, it is recommended to operate at the operating wavelength described in the manufacturer's specifications.

Q10. WDM can transmit in many wavelengths, but SM fiber can only transmit in one mode, right?

In SM fiber, one wavelength is transmitted in one mode, so in WDM (Wavelength Division Multiplexing), which transmits over multiple wavelengths, there is one mode for each wavelength. At this time, since the individual modes have different wavelengths, they can be transmitted without interfering with each other.

Fiber (GI)

Q11. What is transmission band?

Transmission bandwidth is a measure of how much information an optical fiber can transmit in one second. In general, the amplitude of the optical input signal is attenuated as the frequency increases. The rate of this attenuation differs depending on the optical fiber, and the transmission band of that optical fiber is defined as the frequency at which the intensity of the incident light is attenuated by half at the output end (3 dB in intensity amplitude and 6 dB in voltage amplitude).

Q12. There are several transmission band standards for GI fiber. How should they be used?

It depends on the type of network used and the length of the cable. When trying to wire 500m with Gigabit Ethernet, it is stipulated that 50/125GI cable should be 400MHz·km (850nm and 1300nm), and 62.5/125GI cable should be 500MHz·km (1300nm) or more.

Q13. How do you calculate the bandwidth for GI fiber distances?

Bandwidth and distance are inversely proportional. So if the length of the same fiber is halved, the bandwidth roughly doubles.

Q14. I think SM fiber is cheaper, but why is MM fiber used for LAN?

Considering only optical cables, SM fibers are generally cheaper. However, transmission/reception equipment is more expensive for SM than for MM. For this reason, when considering the system, it is cheaper to configure with MM, so MM is the mainstream for LANs and the like.

Q15. There are two types of GI fiber, 50/125 and 62.5/125. How should they be used?

50/125 is Japanese specification, 62.5/125 is North American specification GI fiber. Therefore, 62.5/125 is used when connecting to North American equipment (system) via LAN, etc., and 50/125 is used when connecting to domestic equipment (system).

Q16. What are the splicing characteristics of dissimilar fibers? Is fusion splicing possible?
・SM and GI
・50/125GI and 62.5/125GI

It is possible to connect. When connecting fibers with different core diameters, there is no big problem if the core diameter is changed from small to large, but if the core diameter is changed from large to small, the greater the difference in core diameter, the greater the loss. The manufacturer does not guarantee the characteristics when different types of fibers are connected.

Reliability

Q17. How many years does the optical cable last? What is your evaluation method?

During the manufacturing process, optical fibers are subject to probabilistic flaws along their entire length, causing a decrease in strength. In order to remove these scratches and guarantee the long life of the optical fiber, an accelerated deterioration test, usually called a screening test, is performed. In the screening test, a constant tension is applied for a short time during the drawing process of the optical fiber, and weak parts due to scratches are cut and removed in advance, thereby guaranteeing strength.

Q18. Will there be any problems if the optical fiber core wire, cord, and cable are used in a humid place or submerged in water?

We use optical fiber strands that have passed a screening test of 0.5% or more to guarantee the life of the optical fiber for 20 years. Optical fiber cores, cords, and cables do not pose a particular problem in high humidity. If it is immersed in water, there is no problem if it is for a short period of time, but if it is for a long period of time, the effect of water will shorten the rupture life of the optical fiber, or increase the loss due to hydrogen generated by corrosion of the metal inside the cable. A problem arises.

Q19. What is the operating temperature range and storage temperature range for optical fiber cables?

General optical fiber cables are designed to satisfy the characteristics in the temperature range of -20℃ to 60℃ for outdoor use and -10℃ to 40℃ for indoor use.

Q20. How much can optical fiber core wires, cords and cables be bent? Is it okay if a small bending that is less than the allowable bending diameter is applied momentarily during installation work?

In general, there is no problem if the radius of the optical fiber core wire and cord is up to 30 mm, and the bending radius of the optical fiber cable is 10 times or more of the outer diameter of the cable. If a small bending radius that is less than the allowable bending radius is applied, problems such as an increase in optical loss due to bending and an increase in the size of fine scratches on the optical fiber will occur, resulting in a shortened rupture life.

Q21. Why is there a tension member in the optical fiber cable?

Optical fibers are less stretchable than copper, and if the core wire of the optical fiber is stretched, damage to the optical fiber will be accelerated and the rupture life will be shortened. Therefore, in order to prevent the optical fiber core from being stretched more than necessary, a tensile member called a tension member is inserted into the cable. The allowable tension of the optical cable is calculated from the tension when the tension member is stretched by 0.2%.

Q22. Do optical fiber cables require anti-induction measures?

Since the optical fiber itself is made of an insulator, it is not affected by electromagnetic induction. However, if the optical cable uses metal objects such as steel wire tension members, intervening pairs, and aluminum sheaths, anti-induction measures are required. . However, non-metallic type optical cables do not require such measures.

Construction

Q23. Why is optical fiber connection difficult?

When connecting optical fibers, it is necessary to precisely match the cores of the two optical fibers to be connected. In particular, SM fiber has a small core diameter of 10 μm, and even a slight misalignment causes connection loss. In addition, it is necessary to connect optical fibers in consideration of bending, spacing, end surface inclination, end surface roughness, etc., and special equipment is required.

Q24. How should I connect the optical fiber?

There are two types of optical fiber connections: 1) a permanent connection method called fusion splicing, and 2) a detachable connection method called connector connection.

1) Fusion splicing: A method of splicing by melting the end faces of optical fibers with an arc discharge or the like. A fusion splicer is used for fusion splicing.

2) Connector connection: Glue the optical fiber to the ferrule, polish the end face, assemble the connector, and connect.

Q25. What precautions should be taken when installing optical cables?

Pay attention to the following matters when laying optical cables.
1) Laying tension should not exceed the allowable tension of the cable
2) Secure the allowable bending radius of the cable.
3) Avoid twisting, kink, etc. of the cable.
4) Consideration should be given to prevent the cable from being subjected to side pressure.

Q26. How should I calculate the installation tension?

Laying tension is generally calculated by the following method.
1) Straight horizontal: T=μWL
2) Straight slope: (up) T=WL (μcosθ1+sinθ1) (down) T=WL (μcosθ1-sinθ1)
3) Horizontal curve: T2=T1eμθ2
T: Laying tension T1: Tension before bending T2 : Tension after bending W: Cable mass per unit length L: Cable length μ: Friction coefficient (usually calculated as 0.5) θ1: Tilt angle θ2: Bending angle

Q27. How should optical fiber loss be measured?

1) Measurement by transmission method (light source, power meter): By cutting the fiber under test near the input end, measure the optical power Po at the output end and the optical power Pi at the input end. Fiber loss = -10logPo/Pi(dB ).

2) Measurement by the backscattering method (OTDR): A light pulse is injected into the fiber under test, and the Rayleigh backscattered light reflected at the input end is detected and measured.

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