More specifically, we can say that it is a waveguide that has the ability to transmit electromagnetic waves (in the form of light) at optical frequencies.

Structure of optical fiber

An optical fiber is basically a combination of core and cladding. Here, the core is a cylindrical dielectric composed of glass, through which light propagates and it is enclosed in a plastic cladding.

The figure below shows the structure of an optical fiber:

The whole assembly is surrounded by an elastic jacket in order to protect the fiber.

It is noteworthy in the case of optical fibers that cladding does not participate in lightwave transmission as light only propagates through the core. But, in order to reduce the signal losses due to scattering, an optical fiber is necessarily a combination of the core as well as cladding.

The optical fiber is composed in such a way that the core refractive index must be more as compared to the cladding.

Propagation of light ray through an Optical Fiber

As we know that an optical fiber allows propagation of the signal in the form of light (i.e., photons). Now the question arises how light propagates through an optical fiber?

The answer to the above question is total internal reflection.

When light is allowed to transmit through an optical fiber then it travels through the core only, by experiencing continuous reflections from the cladding. These reflections are nothing but total internal reflections.

As we have already discussed in Total Internal Reflection, that it only occurs when light incident from a denser to a rarer medium with the angle of incidence more than the critical angle.

With such an angle of incidence, light propagates through the core by making successive reflections rather being refracted at the cladding.

As the core is a cylinder with a smaller diameter thus, an only small reflection of the light ray will take place and hence this causes the incident angle to be definitely greater than the critical angle.

Modes of Propagation in an optical fiber

When light travels along the fiber then it is obvious that will transmit down the core by taking either single or multiple paths. So basically modes are nothing but the number of paths a light ray takes during propagation along the fiber.

There exist basically two modes of propagation through an optical fiber:

• Single-mode fiber: In single mode fiber the light ray is propagated along the fiber only by taking single (one) path.

So, due to the transmission of the wave due to only one path, the level of signal distortion at the time of transmission is also reduced. This is so because light rays do not transmit through multiple paths hence fidelity of signal at a longer distance is also maintained.

As the diameter of the core is small thus it requires a sharp light beam for which majorly laser light source is utilized.

• Multimode fiber: In multimode fiber, the core diameter is comparatively large as compared to that of the single mode fiber. As here, the fiber allows light ray propagation through multiple paths inside the core.

Here, the chances of signal dispersion and attenuation are more than that of single-mode fiber. At the same time, the broader core diameter permits several propagating paths for the light waves.

What are glass fibers?

Glass is basically an amorphous solid that is generally hard, transparent and brittle in nature. It is composed by melting and quenching several materials. Glass is a material that does not hold a well-defined molecular structure rather it has an arbitrary arrangement of molecules.

Let us have a look at the tabular representation of glass forming substance:

Glass exhibits a special characteristic, that if the composition of the material is changed, then the properties of the material also varies.

Advantages of Optical fiber

1. Lightwave transmission through an optical fiber enables distortion immune signal propagation.
2. Optical fibers permits secured as well as long distance transmission.
3. These cables serve for longer durations as compared to any other transmission cable.

Disadvantages of Optical fiber

1. The installation, as well as maintenance cost of optical fibers, are somewhat large.
2. Due to their fragile nature, it needs more protection from environmental conditions.
3. Though less distortion can transmit a signal to large distances, that requires the use of repeaters at the time of signal transmission.

Usually, fibers are made from silica as it provides better operating characteristics.  Also, silica is chemically stable material and can be employed for use in the harsh environment also. The use of silica is more appropriate in case of optical communication.

The post Optical Fiber appeared first on Circuit Globe.

The refractive index of the core is somewhat more as compared to that of the cladding. However, it is to be noted here that at the core-cladding interface the refractive index shows sudden variation.

The figure given below shows the step index fiber with its refractive index profile:

When light ray propagates along the step index optical fiber then it follows a zigzag path. However, these zigzag patterns are totally composed of straight lines due to total internal reflection.

Mathematically, the refractive index of step index fibre is given as:

: a is the core radius

r is the radial distance

Modes of Step index fiber

Step index single mode fiber

In step index single mode fiber, the core diameter is extremely small, that it allows only one mode to propagate through it. This means that only single light ray propagates through the step index fiber. Due to this the transmitted ray does not experience distortion due to delay differences.

The figure given below shows the propagation of light ray through step index single mode optical fiber:

Here we can clearly see that the diameter of the core is extremely narrow. Due to this, only a single mode propagates down the fiber. Here, the core size lies somewhere between 2 to 15 micrometre.

Step index multimode fiber

In step-index multimode fiber, the diameter of the core is sufficiently large that it allows multiple modes to propagate through it. This simply means that several light rays can propagate down the fiber in case of step-index multimode fiber.

Here, due to the propagation of multiple light rays simultaneously, the fiber experiences distortion due to delay in propagation time.

The figure given below shows the propagation of light rays through step-index multimode optical fiber:

Here, the above figure clearly shows that the diameter of the core is large enough that it allows multiple propagating paths. In this case, the core size lies nearly between 50 to 1000 micrometre.

It is to be noted in case of the step index fiber that the variation in index is given as:

Majorly the light source used in these fibers is light emitting diodes.

Advantages of Step Index Fiber

1. The manufacturing of these fibers is quite easy.
2. It is inexpensive.
3. The propagation takes place through total internal reflection.

Disadvantages of Step Index Fiber

1. In these fibers, as only one light ray propagates through the fiber at a time then it somewhat leads to a drawback in terms of its capacity to carry information signal.
2. Also due to the small diameter of the core, coupling the light inside it is somewhat difficult.

Applications of Step Index Fiber

These optical fibers majorly find its applications in local area network connections. The reason for this is that it has less information transmitting capacity as compared to graded index fiber.

The post Step Index Fiber appeared first on Circuit Globe.

Basically, when a light ray is allowed to pass through the fiber then there are some chances that it may get reflected back. So, to avoid backward reflection of transmitted signal optical isolators are used.

It is also known as optoisolator, photocoupler etc. These are majorly used in laser applications. Due to this, the coherence of light does not get affected.

Components of Optical Isolator

The device consists of mainly 3 components namely, input and output polarizer it can be 2 input and output birefringent plate that acts as a polarizer and a Faraday rotator.

The whole operation of the isolator relies on these three components. Now, let us move further and understand the major types of the optoisolator.

Types of Optical Isolator

An optical isolator is majorly classified into the following categories:

These are mainly categorized on the basis of their polarization characteristics.

Polarization dependent isolator: A polarization dependent isolator is composed of mainly input and output polarizer along with Faraday rotator. These are mainly used in free space optical system as the system maintains the polarization of the source.

Polarization independent isolator: A polarization independent isolator is composed of input and output birefringent wedges along with Faraday rotator. It basically consists of 2 collimators. As firstly, at the time of transmission the beam gets splits and then merged afterwards thereby focusing at the other collimator.

But in the reverse direction, the ray gets split after which it gets diverge and hence not get focused at the collimator on the other side.

Working principle of Optical Isolator

As we have already discussed that it is basically used to direct the transmitted radiation in a single direction by avoiding the chances of backward reflection.

So, let us understand how this happens:

The figure below shows the design of the isolator consisting of 3 components:

Let us now understand how it operates:

As we can see in the figure shown above that the first component is input polarizer. So, light ray on passing through this polarizer gets vertically polarized. This vertically polarized light then fed to a Faraday rotator.

Faraday rotator is nothing but an optical device that rotates the polarization of light because of Faraday effect. It is to be noteworthy here that this effect is non-reciprocal and it is based on magneto-optic effect.

In the case of the optical isolator, the rotation angle of Faraday rotator is 45°. As we have already mentioned in the previous paragraph that it is non-reciprocal that means it simply rotates in a single direction.

After the light ray undergoes a rotation of 45°, then it is allowed to leave through the polarizer placed after the rotator i.e., the output polarizer. The polarizer at the output either absorbs or reflects the light but that relies on the type of polarizer.

Now, let us understand how the light ray does not get reflected by using the same arrangement.

The light absorbed at the output polarizer is allowed to fall at the Faraday rotator. The figure below represents the arrangement when the light is again reflected towards the rotator.

The rotator now rotates the light ray again by 45° in a non-reciprocal manner i.e., in a clockwise direction. After this rotation, the light ray gets horizontally polarized as we can see in the figure shown above.

But, the input polarizer is basically a vertical polarizer. So, the horizontally polarized light will get rejected by the vertical polarizer.

Hence, in this way, no any light ray will get scattered or reflected back at the input by using these 3 components.

This is an optical isolator works.

Applications of Optical Isolator

1. In the optical communication system: These finds wide applications in optical fiber communication system. As at the time of signal transmission, these devices reduce the chances of signal losses due to reflection.
2. In the optical amplification unit: An amplifier boosts the signal level so if we use an isolator along with an amplifier then it will result in better amplification of the transmitted signal.
3. In laser diodes: The laser light source is used to provide a highly coherent light wave. So, when it will be used with an isolator then the chances of having highly coherent radiation will become more.

So, from the above discussion, we can conclude that an optical isolator is one of the major devices used at the time of optical communication.

The post Optical Isolator appeared first on Circuit Globe.

More specifically, we can say that it is the process by which an electrical signal that contains message is converted into a light signal.

Basically, there exist two different methods of modulating the optical signal. These two methods are classified as:

Direct Modulation

As the name itself is indicating that it is a modulation technique in which the information that has to be transmitted is directly placed over a light stream emitted by the source.

In this method, simply the driving current of the light source i.e., the laser is changed directly with the electrical information signal in order to generate a changing optical power signal. So, it does not require individual optical modulators for the modulation of the optical signal.

In this modulation technique, the major drawback is associated with the carrier lifetimes of spontaneous and stimulated emission along with photon lifetime of the source.

While performing direct modulation with the laser transmitter, the laser turns on and off according to the electrical signal or the driving current. But, in this case, the laser linewidth somehow gets widened. This widening of laser linewidth is known as chirp. Due to this reason, direct modulation technique becomes unsuitable for data rates above 2.5 Gbps.

External Modulation

In external modulation, separate optical modulators are used that performs the modification of optical signals in order to change the signal characteristics.

It is basically used to modulate the signals having data rates of more than 10 Gbps. However, there is no any compulsion to use this method only for high data rate signals.

The figure below shows the operational technique of external modulator:

Here, the first block represents the light source which is basically a laser diode. After the diode, an optical modulator circuit is present that modulates the light wave emitted by the source according to the electrical signal.

The laser diode produces an optical signal of constant amplitude. So, in this case, the electrical signal instead of changing the amplitude of the optical signal varies the power level of the optical signal. Hence, at the output of the modulator, a time-varying optical signal is generated.

It is to be noted here that the circuitry of the external modulator can be integrated combinely with the optical source or it can be considered as an individual device.

Basically, optical modulators are of two types:

Electro-optical phase modulator

It is also known as Mach Zehnder Modulator and is composed of lithium niobate as its basic material. The figure below shows the operational method of electro-optical external modulator:

Here, a beam splitter and beam combiner are used to split and combine the light waves.

In this method, when an optical signal enters the modulator then the beam splitter splits the light beam into two halves and allows it to transmit through two different paths. Then the applied electric signal changes the phase of the light beam in one of the paths.

The light waves after transmitting through two different paths reach the beam combiner in order to recombine. This recombination of the two light beams can be constructive or destructive in manner.

When constructive recombination of the beams takes place then at the output of the modulator a bright light wave is achieved. This is shown by pulse 1 at the output. As against, when destructive recombination of the light beams takes place then the two halves cancel each other and thus no light signal is achieved at the output. This is shown by pulse 0 at the output.

Electro-absorption modulator

This modulator is basically composed of indium phosphide. In this type of modulator, the electrical signal containing information varies the properties of the material of light propagation. So, according to the variation in the property either pulse 1 or 0 is achieved at the output.

It is to be noted in electro-absorption modulator that, one can integrate this modulator along with a laser diode and enclose it in a standard butterfly package.

This reduces the space, power and voltage requirements of the device instead of using separate laser source and modulator circuit.

The post Optical Modulation appeared first on Circuit Globe.

Optical line terminal is abbreviated as OLT.

OLT do so by converting an electrical signal into an optical signal or vice versa in order to ensure proper flow of the information.

Usually by the use of OLT information can be transmitted and received up to a distance of 20 Km.

Working of Optical Line Terminal

OLT functions in such a way that it receives data or signal from long-haul or metro network and provides it to the users present in the PON. Also known as the flow of information in the downstream direction.

Likewise, the data from the long distant users are also accepted and distributed by the OLT.

It can manage one or more PON simultaneously.

The figure below shows an OLT that is handling 4 separate passive optical networks at the same time:

Here, each PON has individually 32 connections. Thus we can say a single OLT can provide the data or information to 128 users.

As we have already discussed that OLT offers a downstream and upstream flow of information. So, in case of downstream transmission, the network uses a wavelength of 1490 nm for both data and voice signal and 1550 nm wavelength of video signal distribution.

While, in the case of upstream flow, the wavelength of nearly around 1330 nm is used by PON for both voice and data signal.

Usually the Optical line terminal equipment operates at 155 Mb/s, 622 Mb/s, 1.25 Gb/s or 2.5 Gb/s.

Optical Network Termination

As we have already discussed that an optical line terminal equipment is present at the central office. But optical network terminal, abbreviated as ONT is situated at the user’s zone itself.

Its function is to establish optical connectivity between the network and end user. ONT has the ability to provide combined telecommunication services according to the needs of users at the endpoint.

An ONT can be a simple box present outside the house of the end user or it can be attached to an indoor electronic rack. When mounted in an electronic rack, it determines the receiving end of each of the multiplexed data. Thereby providing the originally transmitted information to the desired location.

Both OLT and ONT are the modules of the passive optical network. And are present in the network to ensure smooth transmission and reception of data or information.

An optical network unit is of similar use but, despite being placing it inside the user’s area like ONT, it is placed inside a shelter, near to the area of the user. More specifically we can say, it is placed in some centralized area within an office building.

As it is placed in an open region, thus it must be rugged enough so as to withstand environmental conditions such as temperature variations etc.

Also, the cabinet in which it is to be housed must be water resistant, windproof etc. in order to ensure proper protection.

Along with all this, a source of power must be provided to it for the proper functioning.

To establish a connection between ONU and customer area, coaxial cable or optical fiber link can be used.

The post Optical Line Terminal (OLT) appeared first on Circuit Globe.

It results in some variation of the actually transmitted wave through an optical fiber. Due to the dispersion of light waves, various adverse effects are noticed on the signal being transmitted.

Information or signal through an optical fiber is transmitted in the form of digital pulses that are based on binary coding.

Now, the question arises why dispersion occurs?

As we know that light through an optical fiber travels by taking multiple paths within it. Due to propagation through several paths, the light waves reach the destination in different time duration. Resultantly, that will cause dispersion of light wave through the fiber.

It is noteworthy here that light propagates inside the optical fiber through total internal reflection.

Types of Dispersion in Optical Fiber

• Intermodal dispersion

This type of dispersion in optical fibers occurs because different light rays that propagate through a multimode fiber have different propagation delays. So, light shows dispersion due to early reaching and sometimes delay in reaching the other end of the fiber.

It is sometimes also known as multipath time dispersion.

Graded index optical fibers efficiently make use of total internal reflection. So, we can achieve light ray without much dispersion in propagation through graded index optical fiber.

• Chromatic dispersion

It is also known as material dispersion. Refractive index of the material varies when the light of different wavelength propagates through the fiber. As we know a light ray is composed of components of a different wavelength.

Chromatic dispersion is a combination of material and waveguide dispersion.

As the wavelength increases, the refractive index of the material decreases and vice-versa. Due to this if light rays of different wavelength propagate through the fiber, then it will make propagation through a different angle of refraction of light. This is known as Chromatic dispersion.

It is generally discussed in case of single mode optical fiber. But, it also occurs in multimode optical fibers. Effect of chromatic dispersion is somewhat smaller as compared to intermodal dispersion.

Let us now discuss how dispersion effects data transmission.

Effect of dispersion in data transmission

1. Dispersion corrupts the transmitted signal: Suppose optical data in form of digital pulses are transmitted. But after propagation at the destination, a spreaded signal is achieved. Due to this broadening, there are chances that these light pulses will get merged and hence the actual information will not be obtained.
   This is clearly represented in the figure shown below:
   
2. Limits the information carrying capacity: The transmission capacity of a system used for transmitting signal is an important factor.  When a large number of light pulses are transmitted through the fiber and if at the other end all the pulses get fairly recovered. Then it is said to a high transmission capacity optical fiber.
   So, when the dispersion rate of consecutive light rays is higher, then it will lead to fewer chances of proper signal recovery at the other end.
   For this reason, sufficient time interval must be provided between the consecutive light pulses. So that spreading will not result in the mixing of information. But this will surely limit the number of pulses transmitted through the fiber.
   Therefore, the information carrying capacity of the signal gets reduced. Hence, the smaller the dispersion, the larger will be the current carrying capacity of the system.

So, we can say that there are various factors that are responsible for the dispersion of light through an optical fiber. At the same time if the spreading will become severe then it can cause deterioration of actually transmitted information.

The post Dispersion in Optical Fiber appeared first on Circuit Globe.

Now the question arises why we are calling it a total internal reflection?

The answer is that the complete light ray gets reflected towards the same medium without any losses, thus it is called so. We will discuss this concept in more detail but before that let us understand how any light ray shows reflection and refraction.

Refraction and Reflection of light ray

Consider two mediums, the first m₁, is the rarer medium and the second m₂ is the denser medium.

We know when a light ray moves from a denser medium to a rarer medium then it holds the tendency to move away from the normal.

In this case, the angle of the refracted ray will definitely be more than the incident angle.

Let us have a look at the figure shown below that represents the refraction of light:

In the above figure, ray 1 represents some specific angle of incidence. Let, this angle be i₁. So, after striking the surface the ray gets refracted and reaches a rarer medium. Here, the angle of the refracted ray is represented by r₁. Further, we can see, that ray 2, incidents the surface with some greater angle than the previous. Consider it to be i₂. So, after striking the surface, ray 2 gets refracted by making an angle r₂ in the rarer medium.

Consider another case when the angle of incidence is increased further, this is represented by ray 3. So, in this case, the refracted ray moves along the surface. Thus, this case shows the refraction angle is 90°.

The angle of incidence for which the refracted ray makes an angle of 90° and starts moving along the surface. Then that specific angle of incidence is known as the critical angle. So, refraction is nothing but the bending of light rays while moving from a medium to another.

Now, let us have a look at the advancement of the figure that is shown above:

The advancement is nothing but it is simply that now the ray strikes the surface with an angle more than the critical angle. This is represented by ray 4 in the above figure. So, we can see, that ray 4 on striking the medium that is making an angle i₄, now gets reflected towards the same medium through which it was incident. Thus, this shows that when the value of incidence angle is raised above the critical value then the ray that was refracting earlier to a different medium will now get reflected and come back to the same medium.

This is known as an internal reflection of light. However, the complete ray was getting reflected without any refraction or loss in the other medium. Therefore, it is known as total internal reflection.

As we have already discussed that the angle of refraction is always greater than the incident angle. So, it is noteworthy that the value of the critical angle can never be 90°.

Conditions for Total Internal Reflection

Basically, there are two conditions that must be necessarily fulfilled in order to achieve total internal reflection, which is as follows:

1. It is mandatory that the light must move from a denser medium to a rarer medium for the TIR to take place.
2. The angle of incidence must be essentially larger than the critical angle.

Applications of Total Internal Reflection

• Optical Fiber Communication: The phenomenon of TIR is utilized in communication through optical fibers.

As when a light ray propagates through an optical fiber, then the small diameter of the core permits only a small reflection of light. So, in this case, the incident angle will definitely be larger as compared with the critical angle. And the refractive index of the core is more as compared to the cladding.

Thus, light can propagate through TIR inside an optical fiber.

• Mirage: It is also a phenomenon generated due to TIR. In a mirage, due to refraction of light water illusion is noticed because of the non-uniform medium.

This phenomenon is majorly experienced on sunny days. From several meters of distance (around 100 m), one can imagine a puddle (a sort of water pond) but on reaching that particular place no such puddle exists and it simply proves as an illusion.

• Extreme brightness in diamond: The extreme brightness of a diamond is also a result of total internal reflection. As diamond has a lower value of critical angle and has a higher refractive index. Due to which complete reflection is noticed in it without major losses.

Critical angle plays a crucial role in TIR. Now, the question arises why it was named so?

The answer to this is that all those rays that incident with an angle less than this angle gets refracted and reach another medium. And all those rays that incident at an angle greater than this particular value will get reflected. Thus this extreme value is known as critical value and that angle is known as the critical angle.

The post Total Internal Reflection (TIR) appeared first on Circuit Globe.

However, the refractive index of the cladding is constant in the case of graded index fiber. Hence the nature of the refractive index of the core is somewhat parabolic.

Unlike graded index optical fiber, the step index fiber has a constant refractive index at the core as well as cladding.

In the figure shown below you can see graded index fiber with its index profile:

In this type of fiber, the light ray experiences refraction thus gets bent towards the core. Thereby allowing propagation of ray in a curved path.

The refractive index of graded index fiber in the mathematical term is expressed as:

: a is the radius of the core

r is the radial distance from the core axis

α shows characteristic of the refractive index profile

n₁ and n₂ are the refractive indices of core and cladding respectively.

Let us have a look at the curve shown below that represents the variation in the profile of the refractive index with various values of α:

Graded-Index Multimode fiber

The diameter of core in graded-index multimode fiber is somewhat between 50 to 100 micrometer. The large diameter of the core allows multiple rays to propagate through the fiber.

The light wave that travels inside the fiber changes its behaviour with time while travelling inside it. As we have already discussed that the refractive index of the core at the axis is comparatively larger than at the other part inside it.

Thus when light is allowed is propagate inside the fiber, then it travels from less dense medium to more dense medium. However, we are aware of the fact that for TIR to take place the light must travel from denser to rarer medium. So, the light ray despite being reflected gets refracted inside the core.

Hence, the light on travelling gets continuously refracted and bends. Thus in case of graded-index multimode fiber, the light rays do not propagate by following a straight line, rather they follow parabolic path due to non-uniformity in the refractive index of the core.

However, some of the modes travel in a straight path or possess less parabolic nature. So, these light rays due to movement in high refractive index region propagate slower than those following a highly parabolic path.

The rays that propagate through the region that is away from the axis travels through the lower refractive index region, and travels longer path but propagates fastly.

So, this somewhat reduces the time to propagate at the other end of the fiber. Hence all the rays reach at the same time despite travelling through different paths.

This eliminates the chances of dispersion inside the core.

Advantages of Graded-Index Fiber

• It can transmit a large amount of information.
• The distortion is comparatively small than step index fiber.

Disadvantages of Graded-Index fiber

• These fibers possess low light coupling efficiency.
• It is somewhat expensive than step index fiber.

Thus from the above discussion, we can say that in the case of graded index fiber the transmitted information signal can be propagated efficiently and the chances of dispersion are also less in this case.

The post Graded Index Fiber appeared first on Circuit Globe.

Numerical aperture is abbreviated as NA and shows the efficiency with which light is collected inside the fiber in order to get propagated.

We know light through an optical fiber is propagated through total internal reflection. Or we can say multiple TIR takes place inside the optical fiber for the light ray to get transmitted from an end to another through an optical fiber.

Basically when the light is emitted from an optical source, then the fiber must be highly efficient so as to collect the maximal emitted radiation inside it.

Thus we can say that the light gathering efficiency of an optical fiber is the key characteristic while transmitting a signal through an optical fiber.

NA is related to acceptance angle. As acceptance angle is that max angle through which light enters the fiber. Hence the acceptance angle and numerical aperture are related to each other.

Propagation through Optical fiber

As we have already discussed that light through an optical fiber is propagated by several continuous total internal reflections.

As we know that an optical fiber is composed of a core which is made up of a very pure form of glass silica and is surrounded by a glass cladding. So, the light is propagated inside the fiber by performing continuous reflections from the cladding.

But the condition of total internal reflection for the propagation of light ray comes into action only when most of the light is collected inside the fiber. So, let us now understand the numerical aperture for optical fiber in detail.

Derivation for Numerical Aperture of Optical Fiber

Consider a light ray XA, that incident inside the optical fiber. The refractive index of the core is ƞ₁ and that of cladding is ƞ₂.

The figure below shows an optical fiber inside which light ray is focused.

So, the ray XA is launched from denser medium to rarer medium by making an angle α with the fiber axis. This angle α is known as the acceptance angle of the fiber.

This incident ray propagates inside the fiber and gets reflected completely by the core-cladding interface.

But for this, the angle of the incident should be more as compared to the critical angle. Otherwise, if the incident angle is less the critical angle then rather being reflected, the ray gets refracted.

According to Snell’s law, the incident and refracted ray propagate in the same plane. Hence, on applying Snell’s law at medium 1 (usually air) and core interface. Then

From the above figure, we can write

On putting the value of θ from the above equation in equation 1, we get,

Since we know

Applying Snell’s law at core-cladding interface, we get

Substituting the above value in equation 4

Substituting the above value in equation 3, we get

As we have already discussed that medium 1 is air, thus refractive index i.e., ƞ will be 1.

So more specifically we can say

This is the expression for the numerical aperture of an optical fiber, having ƞ₁ as the refractive index of core and ƞ₂ as the refractive index of the cladding.

So we can conclude that as the numerical aperture shows the light collecting ability of the fiber thus its value must be high. As higher the value of NA, better will be the optical fiber.

However, the greater value of NA will be achieved only when the difference between the two refractive indices is high and for this either, ƞ₁ is to be high or ƞ₂ to be low.

But no such material exists that has lower refractive index than 1. So, an option stands that if we remove the cladding present over the core then greater NA can be achieved.

While, for optical signal propagation, the only motive is not to have high accepting range but also to propagate the accepted signal with minimal attenuation.

This is so because an optical fiber that has the greatest light gathering efficiency but does not allow light propagation through it properly, is not of any use.

Thus several parameters must be taken into consideration, for selecting the appropriate optical fiber for signal propagation.

The post Numerical Aperture of Optical Fiber appeared first on Circuit Globe.

Nd ion is rare earth metal and it is doped with solid state host crystal like yttrium aluminium garnet (YAG – Y₃Al₅O₁₂) to form Nd:YAG laser. Due to doping, yttrium ions get replaced by the Nd³⁺ ions. Also, the doping concentration is around 0.725% by weight.

Its working principle is such that when optical pumping is provided to the device. Then the Nd ions get raised to higher energy levels and their transition produces a laser beam.

This laser generally emits light of wavelength of nearly 1.064 μm.

Construction of Nd:YAG laser

Nd:YAG laser is basically categorized into 3 domains that are the active medium, pumping source and the optical resonator.

The figure below shows the road like the structure of Yd:YAG laser:

Active medium: This is also known as the laser medium and is the middle portion of the structure i.e., Nd:YAG crystal. Basically when the external energy source is provided then the electrons from lower energy state moves to higher energy state thereby causing lasing action to take place.

External Energy source: Due to the difference in the energy levels, the electrons need some external pumping in order to perform a transition from one state to another. So, for lasing action to take place an external pump source is required.

Basically, as a source of optical pumping, xenon or krypton flash tube is taken in its case.

The Nd:YAG rod and the flash tube are placed inside an elliptical cavity so, that maximum produced light can reach the rod.

Optical resonator: The two ends of the Nd:YAG rod is coated with silver. However, one end is completely coated with silver so as to achieve maximum light reflection.

While the other end is partially coated in order to provide a path for the light ray from an external source to reach the active medium. Thereby forming an optical cavity.

Working of Nd:YAG laser

It is a 4 level systems i.e.; it contains 4 energy levels. So, in this section, we will discuss the working of Nd:YAG laser with the help of the energy level diagram.

The figure below shows the 4 state energy level diagram of Nd:YAG laser:

Here, E₁ is the lowest energy state while E₄ is the highest energy level. However, initially, electrons in E₁ is very much higher as compared to E₄.

So, when external energy is provided in the active medium of the laser. Then the electrons present in the energy state E₁ gains energy and moves to energy state E₄. However, as E₄ is an unstable state and it exhibits short lifespan.

Therefore, electrons that were excited to this state by the application external pumping will not stay at this state for much longer duration and comes to lower energy state E₃ very fastly but without radiating any photon.

The energy state E₃ is the metastable state and exhibits longer lifespan. So, the electrons in this particular state will last for a longer duration. Due to this more number of electrons will be present at the metastable state E₃. Thereby attaining population inversion.

But once the lifetime of the electrons at the metastable state gets exhausted then these electrons by releasing photons come to lower energy state E₂.

E₂ also exhibit shorter lifespan like E₄. Thus, electrons present in E₂ state will come to E₁ without radiating energy in the form of a photon.

So, in this way electrons by gaining single photon of energy releases the energy of 2 photons. Also, as the system is equipped with optical resonators so, more number of photons will get generated as the pumped energy will get reflected inside the active medium.

In this way, several electrons on stimulation produce photons thereby generating a coherent laser beam of 1.064 µm.

Applications of Nd:YAG Laser

These are used in military applications to find the desired target. This type of laser also finds its application in medical field for the surgical purpose. These are also used in welding and cutting of steel and in communication system also.

The post Nd:YAG Laser appeared first on Circuit Globe.