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How Does Polarization Maintaining Fiber Work?

Polarization-maintaining fiber is a fundamental component in fiber optics. It is often abbreviated as PMF or PM fiber. In simple terms, it is a single-mode optical fiber that allows the linearly polarized light to preserve its polarization state while propagating across it, given that the linearly polarized light is properly launched into the fiber. Consequently, there is negligent or no cross-coupling of optical power between two polarization modes. This type of optical fiber is essential for applications where polarization preservation is important.

Now that you know what polarization-maintaining optical fiber is, let’s dive into how it works.

How Polarization-Maintaining Optical Fiber Works

To make polarization-maintaining optical fiber work, a systematic linear birefringence is intentionally introduced. It allows two well-defined polarization modes to travel across the fiber with very different phase velocities.

PM fiber is often characterized by its beat length for a particular wavelength, represented as Lb. The beat length is the distance over which the wave in one mode experiences an additional delay of one wavelength when compared to the other polarization mode. Hence, PM optical fiber of the length Lb /2 is considered equivalent to the half-wave plate.

Now consider the fact that random coupling might occur between two polarization states over a significant length of PM fiber. In such a scenario, at the very initial point 0 along with the fiber, the wave in polarization mode 1 induces amplitude into mode 2 at some phase.

However, at point ½ Lb along with the fiber, the same coupling coefficient between the two polarization modes induces an amplitude into mode 2 which now becomes 180 degrees out of phase with the wave coupled at point 0 and hence results in cancellation.

Now, at point Lb along with the fiber, the coupling is again in its original phase but then, at 3/2 Lb it will get again out of phase. This phenomenon will keep occurring so on.

Hence, the chances of coherent addition of wave amplitudes through cross-coupling over distances that are larger than Lb get eliminated. Most of the power of the wave remains in its original polarization mode and therefore, exits the fiber in the same mode as well.

Thus, optical fiber connectors that are used for PM fibers are specifically used in a way so that the two polarization modes remain aligned with each other and exit in a specific orientation.

Remember that polarization-maintaining fiber doesn’t function like a polarizer as it doesn’t polarize the light. It simply maintains the linear polarization of the launched linearly polarized light that aligns with one of the fiber’s polarization modes. If a linearly polarized light is launched into the fiber at a different angle, it will excite both polarization modes and will allow the light wave to propagate at slightly different phase velocities.

DK Photonics is a top-notch China-based manufacturer of polarization-maintaining fiber in a variety of specifications. If you don’t see PM fiber with the specifications that you need, don’t worry. We also offer customized orders. So, please feel free to contact us to discuss your custom specifications for PM fibers.   

Do Fiber Optic Cables Need Amplifiers?

Fiber optic cables are playing an essential role in creating highly reliable and high-performing optical communication systems and networks. When a signal propagates in a cable for a long distance, it becomes essential to install amplifiers in the network to prevent any distortion or weakening of the signal. In the same way, when light (or optical) signals travel in a fiber optic cable over a long distance, it also needs a fiber in-line amplifier to restore the strength of the light signal. Let’s learn about fiber optical amplifiers in detail.

What is a fiber optical amplifier?

A fiber optical amplifier is a special device that is specifically designed to boost (amplify) light-wave signals traveling across fiber optic cables without changing these signals into electrical form.

The use of optical amplifiers in optical fiber communication applications allows you to retain the optical integrity of the whole system.

What is the need for a fiber optical amplifier?

Wherever data is transmitted in the form of optical signals through a fiber cable, you need a fiber optical amplifier to preserve the strength of optical signals. Typically, when signals are sent from one end to another, then the quality and strength of the signal degrade due to various factors. If a fiber optical amplifier is not used and a degraded signal is sent to the other end, it becomes difficult or sometimes even impossible to regain the original information sent via that particular signal.

Since optical signals carry information in an optical communication network, the use of an electronic amplifier is not preferred; otherwise, some additional units will be needed to convert optical signals into electrical signals and then again need to convert the electrical signals into optical signals after amplification. This process is not only time-consuming but also makes the entire system more expensive and labor-intensive. Plus, data transmission will also be slower.

Therefore, fiber optical amplifiers are installed to amplify optical signals so that data signals can be transmitted at a faster rate and the integrity of data and information carried by signals remains intact.

In short, optical amplifiers allow you to transmit signals over long distances and at a faster rate without any attenuation or distortion.

Different Types of Optical Amplifiers

Optical amplifiers are usually categorized into three categories, namely:

  • Semiconductor amplifiers
  • Doped fiber amplifiers
  • Raman amplifiers

Among all these three amplifiers, doped fiber amplifiers are more commonly used in fiber communications, fiber laser, and fiber sensor applications. Abbreviated as DFA, the active medium of a doped fiber amplifier is created by doping silica core slightly with rare earth elements, usually erbium. Hence, doped fiber amplifiers are popularly known as erbium-doped fiber amplifiers (EDFA).

Sometimes, optical amplifiers are also used in long-distance optical communication networks, optical fiber distributed sensing, and fiber laser applications. These amplifiers are based on the principle of Raman scattering and don’t need the population inversion mechanism for amplification purposes. To amplify signals, these amplifiers utilize standard transmission fiber cables.

DK Photonics is a world-class optical passive component manufacturer based in China and its offerings also include high-quality and reliable fiber optical amplifiers that are widely acclaimed for achieving high gain and low noise amplification. For any queries related to fiber amplifier orders, please get in touch with us.

The Role of Optical Fibers in Fiber Optics Applications and PM Components

In this post, we will first explain what optical fiber is and its types in brief and then discuss the role of optical fibers in fiber optics applications and in PM components.

What is optical fiber?

An optical fiber is a thin, hollow, flexible, and transparent tube-like wire that is either made of glass or plastic and is designed to transmit light signals. It facilitates the transmission of optical signals over long distances and at higher bandwidth levels.

Types of Optical Fibers

  • Polarization-maintaining (PM) fibers
  • Single-mode fibers
  • Multi-mode fibers
  • Rare-earth-doped fibers
  • Highly nonlinear fibers
  • Hollow-core fibers
  • Multi-core fibers, and more

The Role of Optical Fibers and Their Importance

Fiber optics technology is based on the optical fibers that act as waveguides for light. Think of optical fibers as the fundamental part of fiber optics systems and fiber optics communication applications. Optical fibers carry light signals over long distances which enable engineers to transmit information faster, quicker, and in a more reliable way.

They are intrinsically safe as no electrical signals are involved in transferring data (data is transferred via light). When it comes to bandwidth, no current technology is better than optical fibers as they provide more bandwidth and carry more data than copper cables of the same diameter.

Their performance is not restricted by the cable itself but by the electronic components that constitute the system. The use of optical fibers also leads to a decrease in latency, makes data interception incredibly difficult, and can withstand water and temperature fluctuations. What’s more, they don’t produce any electromagnetic interference.

Thus, the use of optical fibers has literally improved the quality, performance, security, and handling capability of data transmission. That’s what makes optical fibers perfect for fiber optics communication.

If we talk particularly about PM optical fibers, they are used in special applications, such as fiber optic sensing, slab dielectric waveguides, interferometry, fiber optics communication, polarization-sensitive systems, and more where it is a requirement to maintain the polarization state of the incoming signal.

Did you know that optical fibers are also used to connect many fiber-optic elements?

While some fiber optic elements are typically made of fibers, other elements are made of different materials but are coupled to fibers. Here are a few examples of PM components:

  • PM couplers that are used to combine light coming from different sources into one fiber
  • Fiber Bragg gratings that are used as wavelength-selective reflectors in telecom and WDM applications
  • Fiber collimators that can launch a collimated beam into a fiber
  • Optical isolators, rotators, and circulators that are used for manipulations depending on beam polarization

Most PM components use PM optical fibers in one way or another; they are either made of PM optical fibers or are connected using PM fibers.

By now, we hope you understand the need for optical fibers in the industry and their importance.

PM Isolator, Coupler, and Circulator- The 3 Important Polarization Maintaining Components

With time, the need for Polarization Maintaining Components is increasing in telecommunications and other related industries. And it’s because Polarization Maintaining Components make the process easy for these industries and deliver quality results. 

Years ago, only a few components were available. Manufacturers were not so aware and even the industry owners were not so keen. Things were going in the right direction with the limited Polarization Maintaining Components. But, today, things have changed and there are many Polarization Maintaining Components in the market, making the functioning and process easier for the industry owners. 

Products are many but some are commonly needed by the industry owners. A few names in this list are PM isolator, PM Coupler, and PM Circulator. In this post, we will discuss these Polarization Maintaining Components. 

PM isolator

In every PM isolator, you will find an optical fiber inside. The fiber inside is a thin strand made of pure glass. The isolator or the optical fiber inside it works on the “total internal reflection” principle. It acts as a guide for the light wave over long distances. The working of a PM isolator is very effective when the light waves try to pass between varying media. 

A PM isolator is used in different applications but majorly, it is used in communication systems, instrumentation applications, and polarization-maintaining fiber-optic amplifiers. Other than this, the PM isolator is used in fiberoptic system testing and fiber-optic LAN system, and CATV fiberoptic links. 

PM coupler

It’s a device used for combining or coupling light from two or more input fibers into one output fiber. The PM coupler consists of an input section at one end and an output section at another end. Simple to understand, a PM coupler converts input light from different fibers to a single output fiber. The process becomes simple. 

The biggest advantage of the PM coupler is that its output section comprises an optical fiber, which can be made in appropriate lengths and easily tapered. Due to this, this component can be used in two separate functional units. Other than this, a PM coupler has higher longevity and is affordable to all users. 

PM circulator

The role of a PM circulator is to separate optical power traveling in opposite directions in one optical fiber. Also, it is used to achieve bi-directional transmission over a single fiber. The PM circulator is highly suitable for use in advanced communication systems and fiber-optical sensor systems because of the high isolation between the input and reflected optical power and low insertion loss. 

Other than this, a PM circulator provides high reliability and excellent optical performance. This is why it is a Polarization Maintaining Component in telecommunications, fiber optic sensing, bio-medical, and photonics research. 

Contact DK Photonics to buy Polarization Maintaining Component

We are one of the leading manufacturers of Polarization Maintaining Components globally. We follow a strict manufacturing princess with advanced production equipment in an excellent production environment. This is why you will get good quality products with quick delivery. The best part is we customize the components on demand. 

What are the differences between Circular, Isolator, & Rotator?

Today, we will discuss three different optical passive components, namely circulator & isolator & rotator. We will first talk about what these components exactly are and then share what makes them different from each other. So, if you are curious to know about these little yet important optical passive components, read the blog till the end.

Circulator & Isolator & Rotator

As we are discussing specifically optical passive components, you will learn here about optical circulators, optical isolators, and optical rotators rather than their electronic counterparts.

What is an optical circulator?

An optical circulator is a high-performance light-wave component that is designed to route the incoming light signals from Port 1 to Port 2 and the incoming light signals from Port 2 to Port 3. In short, it is designed such that the light coming from one port exits from the next port. While some circulators are three-port devices, there are also four-port circulators.

What is an optical isolator?

Also known as an optical diode, an optical isolator is an optical passive component that allows the light to travel in only one direction. Its main component is the Faraday rotator which ensures non-reciprocal rotation while maintaining linear polarization.

The polarization rotation caused by the Faraday rotator always remains in the same relative direction. It means that the rotation is positive 45 degrees in the forward direction and negative 45 degrees in the reverse direction. It happens because of the change in the relative magnetic field direction, positive one way, and negative the other way. Hence, it adds to the total of 90 degrees when light travels in the forward direction and then the same in the backward direction. This is what makes it possible to achieve higher isolation. 

What is an optical rotator?

An optical rotator is typically an in-line Faraday rotator that is designed to rotate the polarization of the input light by 45 degrees. This rotator is used for amplitude modulation of light and is an integral part of optical isolators and optical circulators.

Circulators vs. Isolators vs. Rotators

Difference between an Optical Circulator & Isolator & Rotator

An optical circulator is used to route the incoming light signals from port 1 to port 2 in a way that if some of the emitted light is reflected back to the circulator, it doesn’t exit from port 1 but from port 3. Thus, it wouldn’t be wrong to say that its function is analogous to electronic circulators.

In other words, fiber optic circulators are highly desirable where there is a need to separate optical signals that travel in opposite directions in an optical fiber.

On the contrary, an optical isolator is widely used in all those fiber optic applications where there is a need to prevent unwanted feedback into an optical oscillator, such as a laser cavity.

On the other hand,the main purpose of using an optical rotator is to achieve higher isolation, low insertion loss, high extinction ratio, and high return loss in optical devices such as optical circulators and isolators. As mentioned, they also help ensure non-reciprocal rotation while maintaining linear polarization.

DK Photonics is the leading China-based manufacturer of optical passive components, including regular and high-power optical circulators, isolators, & rotators. If you need optical passive components for your projects and want some guidance, please feel free to connect with us. 

A Short Yet Informative Post On Optical Fused Coupler

To make the work easy for the IT and telecommunication sectors, an optical fused coupler was introduced. And truly speaking, this technology proved its worth in both large and small scale requirements. 

An optical fused coupler works on a wavelength with the help of some scientific formulas. It transmits light waves in multiple paths with the help of two or more inputs. The primary role of the coupler is to complete the task simultaneously for more than one place using waves. 

The light waves are available in the form of either active or passive devices. Users often get confused between these two devices and end up evaluating things wrongly. Simply explained, the passive devices redistribute the signal without optical-to-electrical conversion and active devices split or combine the signals electrically. 

How does an optical fiber coupler work?

The working of an optical fused coupler is very simple. You just need to understand it carefully. Even amateur technicians can use the coupler and improve their telecommunication and IT jobs. 

An optical fiber coupler contains N input ports and M output ports. The value of both input and output ports typically ranges from 1 to 64. It means you will find different categories of optical fused couplers with different numbers of ports. Generally, the couplers with four ports are available and used by the technicians. 

In the optical fused coupler, the light enters from one of the input ports and splits between two output ports. Similarly, remaining or other input ports function in the same way. In some cases, you will find that one input port remains unused. This is referred to as a T or Y type optical fused coupler. 

Types of optical fused couplers 

In the last para, we mentioned T and Y couplers. These are the different types of optical fused couplers. There are types as well. Here, we will explain T and Y couplers along with other types in brief. 

Y coupler– Also known as the optical tap coupler, the Y coupler resembles the letter Y. Just like the structure of the Y alphabet, the light waves split. In simple terms, the signal entering from the input port splits into two output ports. In this, you can control the power distribution ratio precisely. The sense is that it’s easy to meet your specific requirements.

T coupler– The structure seems very similar to the Y coupler, but the working is different from the T coupler. In this, the power distribution is uneven. The signal enters from the one input port and gets distributed into two output ports. The power distribution difference is that one output signal is greater than another output signal. 

X coupler– Though the name is a coupler, it carries out the function of both a splitter and a combiner. It means you will get two things in one package. In this, the coupler combines and divides the optical power from two input ports between the two output ports. Technically, the X coupler is also referred to as a 2×2 coupler. 

An optical fused coupler is an important technical development. It makes the work easier. The only thing is you should connect with the right manufacturer or supplier to get it for your job. 

The Need for Pump Combiners in Fiber Laser & Amplifier Applications

Pump Combiners, also called pump couplers, are optical passive components designed to send pump and signal light into a laser fiber or an optical amplifier. Basically, high-power fiber amplifiers and lasers are designed using rare-earth-doped fibers with double cladding.

Theoretically, one can inject both pump and signal light into the rare-earth-doped double-clad fibers. However, this technique is limited to research stages only. When it comes to industrial fiber laser applications, one needs an all-fiber setup where fiber pump laser diodes can be interfaced with the active fiber through some passive multimode (MM) fibers.

In such cases, one needs to use pump combiners for interfacing. The use of pump combiners helps achieve higher stability and better robustness in the devices. Some of the pump combiners are high-power pump combiners that are specifically made to safely handle power levels of several kW.

In the market, you will find two types of pump combiners. While the first type includes pure pump combiners, the latter includes pump & signal combiners that have an additional signal output.

Typically, pump combiners are denoted as:

Here, N represents the number of pump inputs.

For instance:

  • A 4 x 1 pump combiner means there are four pump inputs.
  • An (18+1) x 1 pump & signal combiner means there are 18 pump inputs and one additional signal input.

In general, there is no problem in not using all pump input ports except that there will be a loss in terms of pump brightness.

In addition, pump combiners are also available in the PM (polarization-maintaining) version.

Common Uses of Pump Combiners in the Industry

  • Pump combiners, including the ones with additional input, are extensively used for industrial high-power lasers. As mentioned above, they are needed to transmit the pump and signal light combined into the laser fiber.
  • Pump signal combiners are also widely used for erbium-doped fiber amplifiers (EDFA) that play a key role in optic fiber communications, such as cable-TV power amplifiers. While low-power EDFAs use dichroic fiber couplers that are based on single-mode (SM) fibers, high-power EDFAs based on double-clad fibers use multimode pumps & signal combiners.
  • Besides, pump combiners are also utilized in direct-diode applications where an output fiber is often a single-clad multimode fiber.
  • Other applications of pump combiners include fiber laser, fiber laser combination, kW class fiber lasers, and industrial research.

DK Photonics is a widely renowned pump combiner manufacturer based in China, offering standard and custom pump combiners, including N x 1 pump combiners, (N+1)X1 pump and signal combiners, and PM (N+1)X1 pump and signal combiners along with cladding power strippers (CPS). For any queries, please write to us at sales@dkphotonics.com.   

Define PM Optical Isolator & How It Differs from an Optical Isolator

Before understanding PM optical isolators, it’s important to know what optical isolators are. So, we will start by discussing the optical isolators and then help you find the answers you are looking for.

Definition of an Optical Isolator

An optical isolator is a passive component based on fiber-optics technology and allows the light signals to propagate in only one direction while blocking the reflections. An optical isolator consists of a Faraday rotator sandwiched between two polarizers.

Definition of a PM Optical Isolator

Here, PM stands for polarization-maintaining. A polarization-maintaining (PM) isolator is an optical isolator that guides optical light in only one direction while preserving its polarization state and eliminating the back reflections.

It blocks and isolates the system from the scattering of reflections in the reverse direction and helps improve the overall performance and efficiency of the optical and light-wave systems.

Why does polarization matter?

When dealing with applications and systems that utilize fiber optics technology, it’s critical to consider the polarization of the light. Even though the priority is given to the wavelength and intensity of the light in most optical systems, polarization is a crucial property of light that affects even those optical systems that don’t measure it explicitly.

Polarization of light has the capability to influence the focus of laser beams and the cut-off wavelength of filters. Plus, it also plays a substantial role in preventing unwanted back reflections.

Thus, for applications where you cannot allow the reflections of polarized light to impact the efficiency and performance of the systems and you need to maintain the polarization state of the light, what you need is a PM optical isolator.

What if PM optical isolators are not used in certain optical applications?

With the absence of PM optical isolators, the light source that emits the light wave or light signal gets exposed to back reflections and scattered signals that ultimately lead to intensity noise and optical damage. By using PM optical isolators, you can achieve greater isolation and higher return loss. Hence, they make the perfect choice for applications that are highly sensitive to optical feedback and reflections.

How are optical isolators different from PM optical isolators?

When it comes to differentiating optical isolators and PM optical isolators, the main difference is that the latter helps retain the polarization state of the incoming light while isolating the light source from any damage.

What You Should Know About Optical Isolators before Purchasing

When searching for optical isolators online, you will notice the term “TGG -based”. TGG (Terbium Gallium Garnet) is a crystal that is widely used as magneto-optic material in Faraday rotators that are the basic part of optical isolators and PM optical isolators.

The primary reason behind using TGG is that this material has excellent transparency properties, and it is highly resistant to laser damage.

When it comes to improving the performance of systems that use light waves, an optical isolator or a PM optical isolator is often a desirable component. However, sometimes, the expensive cost of optical isolators becomes a restriction. Thus, while buying optical isolators, people also look for affordable prices, in addition to their features and specifications.

DK Photonics manufactures PM optical isolators with a broad range of specifications and also caters to custom orders. For any queries related to optical isolators, please connect with us.

Essential Things to Know About Optical Circulators

In sophisticated optical communication systems, the optical circulator has become one of the most critical components. It’s used to split optical signals in an optical cable that is traveling in different directions.

Optical circulators have been widely used in a variety of disciplines, including telecommunications, medicine, and imaging. We’ll learn more about the optical circulator in this article.

What Is an Optical Circulator?

An optical circulator is a device that allows light to travel from one optical cable to the next. It’s a non-reciprocal device that routes light dependent on the propagation direction. Light can be moved forward using both an optical circulator and an optical isolator. In contrast to the optical circulator, the optical isolator often loses more light energy.

Optical circulators typically have three ports, two of which are utilized as input ports and one as an output port. A signal is sent from port 1 to port 2, followed by another signal from port 2 to port 3. Lastly, the third signal can be sent from port 3 to port 1. Because many applications only need two, they can be designed to prevent any light that enters the third port.

Optical Circulator Components Technologies

The following components make up an optical circulator:

Faraday Rotator

Faraday rotators use the Faraday effect, which is the rotation of the polarization plane of electromagnetic waves in a material subjected to a magnetic field parallel to the wave’s propagation direction.

Birefringent Crystal (Birefringent Crystal):

The polarization state of the light beam and the relative orientation of the crystal affect light propagation in the birefringent crystal. The beam’s polarization can be adjusted, or the beam can be split into two orthogonal polarization states.

Beam Displacer and Waveplate

Birefringent crystals come in two varieties: waveplate and beam displacer. A waveplate is formed by cutting a birefringent crystal to a particular orientation in which the crystal’s optic axis is parallel to the crystal border and in the incident plane. An entering beam is separated into two beams with orthogonal polarization states using a beam displacer.

Optical Circulator Classifications

According to the concept of polarization:

Polarization-dependent optical circulators and polarization-independent optical circulators are two types of optical circulators. The former is employed for light with a specific polarization state, whereas the latter is not limited to a light’s polarization state.

The vast majority of optical circulators used in fiber optic communications are polarization-independent.

In terms of functionality:

There are two types of optical circulators: full circulator and quasi-circulator. In a complete cycle, a full circulator makes use of all ports. Light travels from port 1 to port 2, then from port 2 to port 3, and finally from port 3 to port 1.

Light travels through all ports sequentially in a quasi-circulator, but the light from the last port is lost and cannot be sent back to the first port. A quasi-circulator is sufficient for most purposes.

Conclusion

You may now have a general idea of what an optical circulator is. Using an optical circulator to route light signals with minimal loss is a cost-effective and efficient approach.

Let’s Talk About Polarization Maintaining Fibers!!

When it comes to optical fibers always reveal some degrees of birefringence, regardless of having a circular symmetric design. This is because, in practice, there is usually some percentage of stress and other impacts that breaks down the symmetry. As an outcome, the polarization of light disseminating in the Fiber moderately changes in an unmanageable way which also bent the fiber and its temperature.

Principle Of polarization-maintaining Fibers

The mentioned issues can be repaired by utilizing a polarization-maintaining fiber component, which ain’t a fiber without birefringence; however, on the other hand, it is a specialty fiber with a powerful built-in birefringence. Considering that the polarization of light set in motion into the fiber is lined up to one of the axes of birefringent, no matter what comes in, this polarization phase will get properly preserved even if the fiber is in a bent state. Not to mention, the principle behind this can be comprehended as a mode coupling.

To your knowledge, the transmission sustains of the two polarization modes will always be varied because of the powerful birefringence. In this way, the relative phase of co-spreading modes will quickly bob away. Hence, any sort of hurdle along the fibers can efficiently couple all the two modes if only it has a dimensional Fourier component with wave digits that precisely go with the difference of the propagation constants of both the polarization modes. Just in case, if the variation is enormous enough, the usual disturbances, the poking in the fiber will do efficient mode coupling. To put it simply, the polarization beat length must be shorter than the typical length scale over which the parasitic birefringence varies.

Ways Of Realizing Polarization-maintaining Fibers Components

One of the most commonly utilized methods for introducing strong birefringence is to incorporate stress rods of altered glass composition (generally boron-doped glass, with a varied degree of thermal expansion) in the preform on different and opposite sides of the core. When a fiber is pinched from such kind of perform, it wouldn’t be wrong to say that the stress components lead to some mechanical stress with an accurate orientation. By making use of other methods, it is possible to make bow-tie fibers (where the stress elements have gotten a varied shape and go nearby to the fiber core) so that a stronger birefringence can be easily achieved. 

Single-mode and Few-mode Fibers: 

There is nothing wrong with stating that when it comes to polarization-maintaining fiber components, they are usually single-mode fibers. Having said that, however, only in seldom cases do polarization-maintaining components fibers come in few-mode fibers. The main reason behind this is- that it is arduous to manufacture strong and uniform birefringence in the fiberglass in comparison to the enormous core area where plenty of modes can be guided.

Applications: 

Polarization-maintaining fibers components are executed in devices where the polarization state isn’t permitted to drift, for example, as an outcome of temperature changes. Some examples are- fiber interferometers, fiber-optic gyroscopes, and certain fiber lasers.