Why should I calibrate fiber optic test and splicing equipment?

Would you drive a car with a speedometer that gives you faulty readings? How can you tell how fast you were driving? Optical testing equipment that is out of calibration will also cause faulty test readings. A fusion splicer that is out of calibration will produce inferior splices. False readings from an OTDR and a poor connector splice joining cable will cost you time and money. Not to mention customers and network owners who would question your fiber optic installation work. How do you expect to evaluate your installation or repair with equipment that has not been calibrated?

As demand keeps growing, more and more of today’s fiber optic network owners are demanding that their networks handle the increased speed needed to keep up with those demands. This means that your splice equipment and cleaver need to be up to the job. With this increased need for speed, today’s loss budgets are lower than ever. These budgets need to be met. Test equipment must be more accurate than ever.

Items that need to be calibrated

You need to remember your OTDR is an important piece of diagnostic equipment. It must be calibrated at specific intervals to ensure correct diagnostics. A power meter & light source is another important piece of testing equipment in your arsenal. This tool consists of transmitter and receiver. It measures the power of an optical signal that is passed through the fiber cable. When two ends of optical fiber are permanently welded together by an electrical arc, this is known as a fusion splice. Arc calibration is a must for the proper splice to take place. Do not forget the optical fiber cleaver. Cleaving is the process of breaking or cutting of the fiber. A fusion splice requires the use of a highly accurate cleaver. As you can see the each piece of equipment mentioned has a specific job. Not calibrating a cleaver or a fusion splicer can mean a poor splice. Without calibration, optic test equipment such as the OTDR and power meter & light source are somewhat useless in determining things like the quality of connectors and splices.

What is a loss budget?

This calculation is the total optical power loss that the system is allowed to have. This amount is determined by the power losses resulting from the total amount of equipment that the system has. A loss budget for fiber optic networks is derived from installation of items such as patch cords; couplers, adaptors, splices, cable and any additional optical components installed in the system. This is determined when the network is designed. After it is installed this loss must be tested to see if the budget has been met. Is the splice that has been made to extend the cable acceptable? How about a connector? Was it installed properly? Another equally important reason for OTDR testing, is once the system is active, later on if a problem presents itself, you can go back to the original test. You could then compare the new test to the original test and determine the problem quickly and easily. This is why accurate OTDR testing equipment must be maintained. In order for that piece of equipment to be accurate it must be calibrated on a regular basis.

Calibration is not an option. It is a must.

Put calibration off and it could cost you more than the cost of the calibration itself. Incorrect readings could have technicians thinking the installation is better than it really is or just the opposite. Your company name depends on quality and accuracy. It is not worth saving a few bucks on calibration. During the year your equipment such as splicing equipment is subjected to all sorts of events that can cause it to go out of calibration. If you are in the south heat can be your enemy. Up north freezing temperatures are not your friend. Have you left your equipment in your truck only to be bounced around? All those bumps, drops and bangs add up to inaccurate readings. Dirty conditions are no help either. In many instances in order to get paid you need up to date certified testing equipment. If you are certified for ISO 9001 you need your equipment calibrated. ISO clause 7/6 reads in part as; Control of monitoring and measuring equipment. The organization shall determine the monitoring and measurement to be undertaken and the monitoring and measuring equipment needed to provide evidence of conformity of product to determined requirements. The organization shall establish processes to ensure that monitoring and measurement can be carried out and are carried out in a manner that is consistent with the monitoring and measurement requirements. Remember, calibration is always a must when the measurements from your equipment are critical – It’s that simple.

What exactly is calibration?

When you calibrate any piece of equipment the unit to be calibrated is compared to a unit of a known value. This known value comes from another similar device of known accuracy and precision. Equipment that has a laser which is being calibrated means that laser must fall within a specific acceptable range. Should the equipment being tested be found to be “out of calibration” and produces faulty readings, the equipment must be repaired or adjusted so it falls within the acceptable specified range of measurement.

What is NIST Calibration?

NIST stands for National Institute of Standards and Technology. They provide services to make sure the equipment being calibrated is measured up to a particular piece of equipment similar to that of the equipment being calibrated. NIST certifies that that the lab testing to equipment uses a method that meets the standards of the NIST and must match the NIST measurement standard for a particular piece of equipment. For fiber optic purposes, that would be equipment such as an OTDR, a fusion splicer, cleavers, power meters and lights sources.

In simple terms when using the NIST method you need an unbroken chain of documents; your piece of equipment and components are compared to our piece of equipment which in turn was compared to a piece of equipment from the NIST which is within a stated tolerance. NIST sets the tolerance and it is correct. Our equipment was compared to the NIST equipment so we know ours is correct. Finally yours is compared to ours and found to be correct. That is an unbroken chain. This unbroken chain which is traced back to NIST standards for accurate measurement is how uniformity is maintained. Once your equipment has been tested and meets NIST standards you will receive a calibration certificate paper work stating the results and the date. This means your equipment has met the highest test standards. A big plus would be getting that certification from an ISO compliant calibration company.

What is ISO?

The International Organization for Standardization (ISO) is the world’s largest non-governmental organization developer of standards. ISO 9001 is the most sought-after and internationally acclaimed management system standard. They have created over 22,808 International standards and goals. Their standards are voluntary. Companies who seek out this standard are ensuring that their customer requirements are met accurately and consistently. When it comes to calibration a company is working to meet a set of regulatory requirements which in turn will improve company performance, which will improve product and service quality. This method in the end will benefit the customer by assuring them that the ISO certified company has met the exacting ISO standards to bring them a better product.

Conclusion

Over time even a well cared for piece of test equipment can lose its’ accuracy. You must have your equipment calibrated as suggested by the manufacturer. However, in many instances you may need to get it done sooner, as many conditions that the equipment is subjected to may alter or falsify your test results. As networks need to increase their efficiencies loss budgets are becoming smaller and smaller. Only calibrated equipment can assure you are correctly within that budget. Calibration is not really an option. It is a must. Always use a lab that will test to NIST standards and if possible use an ISO certified test lab. Accurate results will always save you time, money and your company reputation.

What is the Purpose of a Power Meter & Light Source?

A Power Meter & Light Source is a low cost way to certify optical fiber. These two pieces of test equipment are used to measure fiber optic light continuity, loss and lastly the actual strength of the optical signal.

Signal Loss

In fiber optics when a beam of light which carries a signal goes through the optical fiber the strength of that beam of light will diminish over distance. This means the signal strength becomes weaker. This loss of light power will affect the fiber optic network in a negative way. The loss of light power or attenuation of the optical fiber is caused by two issues, scattering and absorption of the light source. If the degradation is too great then performance of the network will be affected.

The following can be the cause of signal loss:

• Tight Bends in the Cable

• Dirty or Improperly Cleaned Connectors

• Too much Stress on the Cable During Installation

• Poorly Installed Connectors

• Improper Splicing Technique

• Poor Cable Quality

What Equipment is Needed to Conduct a Power Meter & Light Source Test?

What Training Does an Installer Need?

A Power Meter and Light Source are a pretty simple piece of test equipment to use. The actual connection of the fiber to the test equipment is fairly straightforward. If you are familiar with handling fiber optics the test is very easy. If you are new to fiber optics this test should not present any issues. A simple short video explaining the test should be all you need.

Why use an OTDR in Place of a Power Meter & Light Source?

The Power Meter and Light Source are more limited than an OTDR. A Power Meter can only measure the received optical power. The OTDR can not only tell you there is a break is in the fiber, it can also measure the distance between the test point and the break. In addition, it is able to give you reflectance for each connector. Even though the OTDR can reveal additional information, the Power Meter and Light Source are still an important piece of optical fiber test equipment and their importance should not be underestimated when testing an optical fiber network.

How Does A Power Meter & Light Source Work?

By attaching a reference cable to the light source, power can be measured at the opposite end of the fiber optic cable. The signal is sent from the light source down the fiber. These two pieces of test equipment are used to measure fiber optic light continuity, loss, and lastly the actual quality of the signal. In short, it measures the power of the optical signal that has passed through the fiber cable from the light source.

Steps to Using a Power Meter and Light Source

Using the Power Meter & Light Source to test a fiber optic cable is relatively easy.

• First take the reference cord end face and clean it with 99% reagent grade isopropyl alcohol and lint free fiber optic wipes.

• Next plug the reference cord into the light source and select the wavelength you are testing. When testing a multimode cord attach a mandrel wrap to strip out the higher modes of light that can interfere with the test results. A mandrel wrap is not necessary for singlemode.

• Clean the other end of the reference cord and insert that end into the Power Meter. Now zero out the reference cord by hitting the “zero” button. After zeroing out, do not unplug the reference cord from the Light Source. Take the cord to be tested and clean one end, then attach the connector adapter. Clean the other end of the patch cord.

• Remove the reference cord from the power meter and attach to the test cord adapter, insert the other end of the test cord into the power meter. The reading on the power meter will give you the loss on the connector mated to the reference cord only. To get the loss reading on the other end simply unplug the test cord from the reference cord and switch the connectors. You have now completed the one cord reference test.

• For a two cord reference test attach a connector adapter to the reference cord and insert the other end to the power meter. Zero out the power meter. You are now ready to get a loss reading for the entire cord being tested.

• Take the test cord and clean both ends with the cleaning alcohol and wipes. Connect the test cord in between the two reference cords. The power meter will show a full cord reading for total power loss. Record your loss as needed.

What to Look for when Purchasing a Power Meter and Light Source

The Power Meter and Light Source or Optical Loss Test Set are must have tools for the fiber installer. While they are fairly simple tools to operate, care should be taken in choosing the Power Meter and Light Source as there are many models to choose from.

• Is the equipment easy to use or does it require a huge manual?

• Operation of this piece of equipment should almost be intuitive.

• Appearance is important. Is it easy to hold?

• There should be a minimal amount of buttons on the unit.

• Are screens easy to read? Is it backlit?

• Is the Power Meter and Light source calibrated?

• Does the manufacturer calibrate?

• Can they provide a calibration certificate traceable to NIST standards?

• Does the unit come with a protective carry case?

• What about battery life?

• Are adapter caps included?

• Does the kit include a dual wavelength multimode or single mode light source?

• Does it come with interchangeable adapters allow flexibility with reference cords? As with any fiber optic test equipment, know the manufacturer. Find a reputable company that will stand behind their equipment. If you have questions about your choice, call or email the company and talk with a technical person that can help you decide which piece of test equipment best suits your needs. Remember, there are many manufacturers out there in the marketplace. Consider only those with reputable firms that have a good track record. One that can service and maintain your equipment if needed.

The Different Types of Fiber Optic Fusion Splicers?

You’ve probably heard the term fusion splicer before, but in case you haven’t – an optical fiber fusion splicer is used to “splice” or fuse two separate pieces of glass optical fibers together – whether the optical fiber type is single mode fiber or multimode fiber. The goal is to join the two pieces of bare fiber seamlessly. They are connected to each other by an electric arc. You may need to fusion splice for a variety of reasons – the fiber may have been broken or damaged, or you could be performing a termination of the fiber using a pigtail or a splice on connector (SOC), or you may need to extend the length of a fiber optic cable run to reach an end point in your long haul network. Fusion splicing ensures that the light will pass from one end of the fiber to the other without interruption, making sure there is the least amount of back reflection from the splice. Fusion splicing can be used instead of mechanical splices, and it is actually usually preferred because of its benefits, which we will talk about in the passages below.

Fusion splicing ensures optimal performance, the lowest loss, and the lowest amount of reflectance when compared to a mechanical splice. The price of fusion splicers varies depending on the type you choose, core alignment and ribbon or mass fusion splicers are more expensive than cladding alignment fusion splicers. Most standard fusion splicer features include a large color screen, built-in splice sleeve ovens, and many come with high precision cleavers when purchased as a kit. As technology progresses we are seeing Bluetooth options, fully automated processes, and Wi-Fi capabilities being developed.

Typical applications where fusion splicers are used include fiber to the home applications, applications where splice on connectors are being used, maintenance in data center locations, and in research and development facilities. When you have a fusion splicer you can do repairs on the fly, whether it is a broken fiber or a bad connector, a fusion splicer can be used to make the repair and get your system back up and running in no time. There are many manufacturers of fusion splicers in the marketplace, and each has its own perks, features, and benefits, but there are two main types of splicers that you could potentially purchase.

Core Alignment Splicer

In core alignment units, the cores of the fiber are aligned prior to the splice being performed, not the cladding of the fiber that you are trying to splice. These units work using a system of magnifiers, cameras, and motorized movable fiber holders or clamps to see the fiber. These parts will move the fiber in any way necessary to achieve the proper alignment of the cores of the fiber. After the alignment is achieved according to the parameters set in the software, it will then perform the splice. The operator of the splicer does not have to worry about manually moving the fiber to get the proper alignment.

These splices are performed in mere seconds after alignment is achieved. In the case of core alignment units, specific splice recipes or parameters can be set to achieve the specific results for your application. These attributes contribute to making core alignment splicers more expensive than cladding alignment units, but they are also what make core alignment units so easy to use.

Something to consider if you are working on or doing maintenance in older, established networks is that when you are splicing legacy fibers, core alignment splicers are preferred because the concentricity of the core within the fiber was not as consistent as it is in new fibers, and in this case, you will want to be sure the fiber cores are aligned, not just the claddings.

Ribbon Splicers

A ribbon splicer or mass fusion splicer is exactly what it sounds like; it is a splicer that is made to splice ribbon fiber together. In this case, instead of splicing a single fiber in a splicing cycle, the machine splices up to 12 fibers together, all at the same time. These units are typically more expensive than their single fiber counterparts. They use a cladding alignment system to line up the fibers prior to performing the splice. Almost all ribbon splicers can accommodate up to 12 fiber ribbons in their holders, but many can accommodate as few as 2 fibers. Specific ribbon fiber holders are used to splice fiber ribbons of various fiber counts. Ribbon splicers can splice single fibers with the proper holders, but it would not be cost effective to purchase a ribbon splicer if ribbon fiber is not something that you work with on a regular basis.

Cladding Alignment Splicers

Cladding alignment units are different than a core alignment fusion splicer as they only use a fixed V-Groove to align the fibers based on the claddings of the fibers being spliced. These types of splicers are more basic units and they lack some of the bells and whistles that are commonly seen on the core alignment units. In cladding alignment splicers, the alignment of the fibers being spliced is not as perfect as a core alignment, because this type of splicer only matches up the outer cladding of the fiber, and they move only a single axis. After the splice has been performed, the core of the fiber may be slightly offset if the core concentricity of the fibers is not on dead center. These types of splicers are preferred when cost is an issue because they are more affordable, and when higher loss rates are acceptable. These units are usually handheld and normally much smaller than the core alignment units, so if space is an issue, such as in the case of being up in a bucket truck or a tight telecom closet or handhole it may be beneficial to use a smaller cladding alignment splicer.

After each splice is performed on either a cladding or core alignment unit, the splicer will give an estimated loss reading for the splice and, it will also perform a ‘proof test’ to make sure that the tensile strength the splice is stable and that it will not break apart with any minuscule mishandling. Both of these units will splice your fiber, and get the job done. So, in conclusion, whether you are looking for the premium fusion splicer or something a little more affordable, there are many options that are on the market. It all depends on the options you are looking for and the features you need to complete your job. There are super simple units, and then there are fancy high tech units, but each fiber optic technician has his or her own set of preferences and needs.

Brief Introduction to Polarization Maintaining Isolators

Polarization maintaining isolator which ‘at times’ is also called fiber optic isolator and polarization maintaining optical isolator. It allows and keeps light to travel in one direction only. Its prime job is to prevent back reflection and backscattering in the reverse direction, for all states of polarization. In technical terms, the device is a two-port micro-optic isolator built with PM panda fiber. The isolator is commonly used in lasers, fiber optic systems, and amplifier systems. It actually prevents feedback which is not at all required in an optical oscillator.

Some devices in which this isolator is used

PM isolator is utilized all over the world majorly in communication systems, instrumentation applications, and polarization maintaining fiber-optic amplifiers. The isolator is also used in fiber optic system testing and fiber-optic LAN system and CATV fiber optic links.

Some of the many great features of these isolators

High isolation capacity

High Extinction Ratio

High Return Loss

Low Insertion Loss

Every fiber optic isolator has an optical fiber inside of it which is the most important component. Let’s now discuss how it works.

Optical fiber inside such isolators is a thin strand made of pure glass. It acts as a guide for the light wave over long distances by following the principle of ‘total internal reflection’. These are very effective when the light waves try to pass between two varying media.

The fiber inside these devices including polarization maintaining optical isolator is composed of two layers of glass – the core and the cladding. The core typically carries the actual signal of light and the glass layer surrounding the core is called cladding. In comparison to the core, the cladding has a lower refractive index. All of this causes total internal reflection successfully within the core.

What is transmitted over fiber?

Most fibers work in pairs where digital signals are encoded in light’s analog pulses preferably via the NRZ modulation – Non-Return to Zero. Since they operate in pairs, one is used to transmit while the other to receive, however, both signals can also be sent over a single stand.

Basic yet most used fiber types

SMF – Single Mode Fiber

MMF – Multi-Mode Fiber

The actual difference basically lies in the size of the core. SMF has an in-depth narrow core not more than 9µm which allows the propagation to just a single mode of light, whereas, MMF has a greatly wider core somewhere around 50µm and 62.5µm is also available on the market. MMF allows multiple modes of light to propagate. They both have their different characteristics along with their own pros & cons.

What is the Purpose of a Power Meter & Light Source?

What is a Power Meter & Light Source?

A Power Meter & Light Source is a low cost way to certify optical fiber. These two pieces of test equipment are used to measure fiber optic light continuity, loss and lastly the actual strength of the optical signal.

Signal Loss

In fiber optics when a beam of light which carries a signal goes through the optical fiber the strength of that beam of light will diminish over distance. This means the signal strength becomes weaker. This loss of light power will affect the fiber optic network in a negative way. The loss of light power or attenuation of the optical fiber is caused by two issues, scattering and absorption of the fiber light source. If the degradation is too great, then performance of the network will be affected.

The following can be the cause of signal loss:

• Tight Bends in the Cable

• Dirty or Improperly Cleaned Connectors

• Too much Stress on the Cable During Installation

• Poorly Installed Connectors

• Improper Splicing Technique

• Poor Cable Quality

What Equipment is Needed to Conduct a Power Meter & Light Source Test?

Fiber Optic Cable to Test

Power Meter

Light Source

What Training Does an Installer Need?

A Power Meter and Light Source are a pretty simple piece of test equipment to use. The actual connection of the fiber to the test equipment is fairly straightforward. If you are familiar with handling fiber optics the test is very easy. If you are new to fiber optics this test should not present any issues. A simple short video explaining the test should be all you need.

Why use an OTDR in Place of a Power Meter & Light Source?

The Power Meter and Light Source are more limited than an OTDR. A Power Meter can only measure the received optical power. The OTDR can not only tell you there is a break is in the fiber, it can also measure the distance between the test point and the break. In addition, it is able to give you reflectance for each connector. Even though the OTDR can reveal additional information, the Power Meter and Light Source are still an important piece of optical fiber test equipment and their importance should not be underestimated when testing an optical fiber network.

How Does A Power Meter & Light Source Work?

By attaching a reference cable to the light source, power can be measured at the opposite end of the fiber optic cable. The signal is sent from the light source down the fiber. These two pieces of test equipment are used to measure fiber optic light continuity, loss, and lastly the actual quality of the signal. In short, it measures the power of the optical signal that has passed through the fiber cable from the light source.

Steps to Using a Power Meter and Light Source

Using the Power Meter & Light Source to test a fiber optic cable is relatively easy.

• First take the reference cord end face and clean it with 99% reagent grade isopropyl alcohol and lint free fiber optic wipes.

• Next plug the reference cord into the light source and select the wavelength you are testing. When testing a multimode cord attach a mandrel wrap to strip out the higher modes of light that can interfere with the test results. A mandrel wrap is not necessary for singlemode.

• Clean the other end of the reference cord and insert that end into the Power Meter. Now zero out the reference cord by hitting the “zero” button. After zeroing out, do not unplug the reference cord from the Light Source. Take the cord to be tested and clean one end, then attach the connector adapter. Clean the other end of the patch cord.

• Remove the reference cord from the power meter and attach to the test cord adapter, insert the other end of the test cord into the power meter. The reading on the power meter will give you the loss on the connector mated to the reference cord only. To get the loss reading on the other end simply unplug the test cord from the reference cord and switch the connectors. You have now completed the one cord reference test.

• For a two cord reference test attach a connector adapter to the reference cord and insert the other end to the power meter. Zero out the power meter. You are now ready to get a loss reading for the entire cord being tested.

• Take the test cord and clean both ends with the cleaning alcohol and wipes. Connect the test cord in between the two reference cords. The power meter will show a full cord reading for total power loss. Record your loss as needed.

What to Look for when Purchasing a Power Meter and Light Source

The Power Meter and Light Source or Optical Loss Test Set are must have tools for the fiber installer. While they are fairly simple tools to operate, care should be taken in choosing the Power Meter and Light Source as there are many models to choose from.

• Is the equipment easy to use or does it require a huge manual?

• Operation of this piece of equipment should almost be intuitive.

• Appearance is important. Is it easy to hold?

• There should be a minimal amount of buttons on the unit.

• Are screens easy to read? Is it backlit?

• Is the Power Meter and Light source calibrated?

• Does the manufacturer calibrate?

• Can they provide a calibration certificate traceable to NIST standards?

• Does the unit come with a protective carry case?

• What about battery life?

• Are adapter caps included?

• Does the kit include a dual wavelength multimode or single mode light source?

• Does it come with interchangeable adapters allow flexibility with reference cords? As with any fiber optic test equipment, know the manufacturer. Find a reputable company that will stand behind their equipment. If you have questions about your choice, call or email the company and talk with a technical person that can help you decide which piece of test equipment best suits your needs. Remember, there are many manufacturers out there in the marketplace. Consider only those with reputable firms that have a good track record. One that can service and maintain your equipment if needed.

WHAT IS OPTICAL CIRCULATOR AND ITS APPLICATIONS?

An optical circulator is a multi-port (minimum three ports) nonreciprocal passive component.

The function of an optical circulator is similar to that of a microwave circulator—to transmit a lightwave from one port to the next sequential port with a maximum intensity, but at the same time to block any light transmission from one port to the previous port. Optical circulators are based on the nonreciprocal polarization rotation of the Faraday effect.

Starting from the 1990s optical circulators has become one of the indispensable elements in advanced optical communication systems, especially WDM systems. The applications of the optical circulator expanded within the telecommunications industry (together with erbium-doped fiber amplifiers and fiber Bragg gratings), but also expanded into the medical and imaging fields.

Since optical circulators are based on several components, including Faraday rotator, birefringent crystal, waveplate, and beam displacer, we will have to explain these technologies before jumping into the detail of circulator.

1. Faraday Effect

The Faraday effect is a magneto-optic effect discovered by Michael Faraday in 1845. It is a phenomenon in which the polarization plane of an electromagnetic (light) wave is rotated in a material under a magnetic field applied parallel to the propagation direction of the lightwave. A unique feature of the Faraday effect is that the direction of the rotation is independent of the propagation direction of the light, that is, the rotation is nonreciprocal.

The Verdet constant is a measure of the strength of the Faraday effect in a particular material, and a large Verdet constant indicates that the material has a strong Faraday effect. The Verdet constant normally varies with wavelength and temperature. Therefore, an optical circulator is typically only functional within a specific wavelength band and its performance typically varies with temperature. Depending on the operating wavelength range, different Faraday materials are used in the optical circulator.

Rare-earth-doped glasses and garnet crystals are the common Faraday materials used in optical circulators for optical communication applications due to their large Verdet constant at 1310 nm and 1550 nm wavelength windows. Yttrium Iron Garnet and Bismuth-substituted Iron Garnets are the most common materials.

The Verdet constant of the BIG is typically more than 5 times larger the YIG, so a compact device can be made using the BIG crystals. All these materials usually need an external magnet to be functional as a Faraday rotator. Recently, however, a pre-magnetized garnet (also call latching garnet) crystal has been developed that eliminates the use of an external magnet, providing further potential benefit in reducing overall size.

Faraday rotators in optical circulators are mostly used under a saturated magnetic field, and the rotation angle increases almost linearly with the thickness of the rotator in a given wavelength (typically 40 nm) range. The temperature and wavelength dependence of the Faraday rotation angle of the typical BIG crystals at wavelength of 1550 nm is 0.04-0.07 deg/°C and 0.04-0.06 deg/nm, respectively.

Another common material used in the construction of optical circulators is the birefringent crystal. Birefringent crystals used in optical circulators are typically anisotropic uniaxial crystals (having two refractive indices with one optical axis). In an anisotropic medium, the phase velocity of the light depends on the direction of the propagation in the medium and the polarization state of the light. Therefore, depending on the polarization state of the light beam and the relative orientation of the crystal, the polarization of the beam can be changed or the beam can be split into two beams with orthogonal polarization states.

The refractive index ellipsoid for a uniaxial crystal is shown in the above figure. When the direction of the propagation is along the z-axis (optic axis), the intersection of the plane through the origin and normal to the propagation direction So is a circle; therefore, the refractive index is a constant and independent of the polarization of the light. When the direction of the propagation S forms an angle θ with the optic axis, the intersection of the plane through the origin and normal to S becomes an ellipse. In this case, for the light with the polarization direction perpendicular to the plane defined by the optic axis and S, the refractive index, is called the ordinary refractive index no, is given by the radius ro and independent of the angle θ. This light is called ordinary ray and it propagates in the birefringent material as if in an isotropic medium and follows the Snell’s law at the boundary.

On the other hand, for light with the polarization direction along the plane defined by the optic axis and S, the refractive index is determined by the radius re and varies with the angle θ. This light is called the extraordinary ray and the corresponding refractive index is called the extraordinary refractive index ne. In this case ne is a function of θ and can be expressed as

The ne varies from no to ne depending on the direction of propagation. A birefringent crystal with no < ne is called a positive crystal, and one with no > ne is called a negative crystal.

Therefore, the function of a birefringent crystal depends on its optic axis orientation (crystal cutting) and the direction of the propagation of a light. Birefringent crystals commonly used in optical circulators are quartz, rutile, calcite, and YVO4.

HOW OPTICAL CIRCULATOR WORKS

Optical circulators can be divided into two categories.

polarization-dependent optical circulator, which is only functional for a light with a particular polarization state. The polarization-dependent circulators are only used in limited applications such as free-space communications between satellites, and optical sensing.

polarization-independent optical circulator, which is functional independent of the polarization state of a light. It is known that the state of polarization of a light is not maintained and varies during the propagation in a standard optical fiber due to the birefringence caused by the imperfection of the fiber. Therefore, the majority of optical circulators used in fiber optic communication systems are designed for polarization-independent operation.

Optical circulators can be divided into two groups based on their functionality.

Full circulator, in which light passes through all ports in a complete circle (i.e., light from the last port is transmitted back to the first port). In the case of a full three-port circulator, light passes through from port 1 to port 2, port 2 to port 3, and port 3 back to port 1.

Quasi-circulator, in which light passes through all ports sequentially but light from the last port is lost and cannot be transmitted back to the first port. In a quasi-three-port circulator, light passes through from port 1 to port 2 and port 2 to port 3, but any light from port 3 is lost and cannot be propagated back to port 1. In most applications only a quasi-circulator is required.

The operation of optical circulators is based on two main principles.

Polarization splitting and recombining together with nonreciprocal polarization rotation.

Asymmetric field conversion with nonreciprocal phase shift.