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Why Do You Need a PON Power Meter?

The requirements for testing fiber optic networks will vary depending on the specific type of network as well as the network designer’s overall test requirements. Regardless of type, there are two basic or generic pieces of Optical Test Equipment that will be used; an Optical Time Domain Reflectomer or OTDR, and a pair of optical test equipment pieces that are referred to as a Power Meter & Light Source. These tests are typically measured in “dB”. The term dB is the expression of attenuation or power loss over an optical fiber as it travels from a termination end to a point along the fiber’s path. Once an optical fiber is connected to a piece of active equipment then all tests are in dBm. The active equipment will be transmitting an actual or real optical power at a specific wavelength and referenced at 1mW.

A PON power meter is essential for field technicians installing or maintaining any type of PON network. PON Power Meters are able to simultaneously test upstream and downstream through optical fibers, at 1490nm, 1550nm, and 1310nm wavelengths, as well as estimate signals of the voice, data, and video streams.

The term PON stands for “passive optical network”. A PON is a fiber optic telecommunications network that delivers broadband to transmit data over fiber optic cables to the customer premises. Its architecture implements a point-to-point or a point-to-multipoint arrangement of nodes in a communications network. A Point-to-Multipoint network uses a single fiber to serve multiple endpoints by using unpowered or passive fiber optic splitters. A splitter is used to divide the fiber bandwidth among multiple access points. Passive optical networks are often referred to as the “last mile” between an internet service provider (ISP) and its customers.

Point-To-Point or P-T-P type optical network

A P-T-P network is a network that has two termination points and nothing in-between. As with all fiber optic networks, when it is being constructed the fibers must be terminated to allow for any tests to be performed. So one end of the network is terminated and an OTDR test is performed on each fiber to ensure that the termination and length of fiber beyond does not have any issues. That test result will be stored for future needs and noted in “dB”. If the network requires splicing then after the fibers are spliced, the OTDR again is used to ensure that the splice and added fiber length again meets requirements. The testing with the OTDR continues and is completed after the end of each fiber is terminated. At this point, another set of tests is required which is commonly referred to as an End-To-End test. This test requires the use of a light source and a power meter and again all test results are stored. The optical power meter will be set to “dB” and referenced to a light source which is typically called “Zeroing”. The units are then moved to opposing ends and the field technicians will send and receive wavelengths specified by the designer. Again this is a measurement that will be used against the designer’s overall Link Loss Budget. The P-T-P network will have its termination ends referred to as “A” and “B” and at least two unique wavelengths will be sent and received over each fiber. This is typically required to ensure that any wavelength used by a transmitter can be used between the two that are specified. The network designer will define these wavelengths as well as provide a label for these ends. The technician that is performing the tests will reference these labels in any reporting back to the designer.

Point-To-Multipoint PON type network

Now when a Point-To-Multipoint network is constructed such as a Passive Optical Network or PON many of the tests and test equipment remain the same but will require a few special features. The OTDR testing during construction remains the same, with tests performed each time a fiber is terminated or spliced. Again, this continues to the far ends of the fibers after they are terminated. Once all fibers are terminated, again they will be tested using a power meter and light source. The need for special equipment is required for the activation phase of a PON network and that’s where the similarity between a P-T-P and a P-T-Multipoint ends.

The PON network activation phase begins by connecting a power meter up to an active piece of equipment called an Optical Line Terminal or OLT and set to the appropriate wavelength and the unit is set to “dBm” and this becomes the referenced power.

NOTE: There are several generations of PON network OLTs which use different wavelengths so the optical power meter must have the capability to be set to those wavelengths. GPON is 1490nm, XGPON is 1577nm and NGPON has multiple wavelengths ranging from 1596nm to 1602nm.

As the links are connected out to the far end, the technician repeats the test and ensures that there are no issues. This testing continues out to the far end, which in a PON network there is a piece of active equipment called an Optical Network Terminal or ONT or at times called an Optical Network Unit or ONU. Regardless, that piece of equipment receives light from the OLT transmitter and communicates back to the OLT with its own transmitter. The ONT cannot communicate back to an OLT without first receiving the OLT’s transmitter’s wavelength. At this time, there is an absolute need to use a specialized optical power meter which can measure the OLT’s power and allow that power to pass through and provide the signal to the ONT/ONU so it can send back a signal.

The PON meter has two test ports; one is named DROP and the other ONT or ONU. The technician connects the drop, which is connected via a fiber all the way back to the OLT into the port named DROP and then connects the ONT/ONU connectorized fiber pigtail into the ONT/ONU port. Now the PON meter is in-line between the OLT and ONT/ONU and allows for the OLT to communicate with the ONT/ONU. The technician again will be observing the incoming OLT power level, as well as the outgoing ONT/ONU power level. If all is well, the drop is connected to the ONT/ONU and service activation tasks can continue.

Characteristics of PM Fiber Patch Cords

If the polarization of the input light is not aligned with the stress direction in the fiber, the output light will vary between linear and circular polarization (and generally will be elliptically polarized). And the exact polarization will also be sensitive to variations in temperature and stress in the fiber. The light shall be coupled at the fiber entrance parallel to the slow axis or to the fast axis, then the maintaining of the polarization is therefore possible. It is important to make sure that the polarization of the input light is maintained. PM fiber patch cords maintain the existing polarization of linearly-polarized light that is launched into the fiber with the correct orientation. PM fiber patch cords also feature low insertion loss, high extinction ratio, high return loss, excellent changeability over a wide wavelength range and excellent environmental stability and reliability.

Types of PM Fiber Patch Cords

There are a wide variety of PM fiber patch cords available that support different data rates and suit various connector types. According to different criteria, PM fiber patch cords can be categorized into various types. The following is some detailed information about types of PM fiber patch cords based on 4 different criteria:

Classification by connector type―PM fiber patch cords are capped at both ends with fiber connectors. FC, SC, LC and ST are the commonly used connector types for PM fiber termination. According to the connectors on the both ends, there are many different kinds of PM fiber patch cords, such as LC-FC, SC-FC, or FC-FC PM fiber patch cords.

Classification by fiber type―PM fiber patch cords are built with polarization maintaining fiber. To ensure the polarization of both the input and output light in a PM fiber, several different shapes of rod are used, and the resulting fiber is sold under brand names such as “Panda” and “Bow-tie”. With different PM fiber, there are corresponding PM fiber patch cords, such as Panda PM fiber patch cords and Bow-tie PM fiber patch cords.

Classification by cable type―PM fiber patch cords can also be categorized according to the cable types. There are mainly three kinds of cable types, 250um bare fiber, 900um loose tube jacket and 3mm loose tube jacket. So based on the cable types, there are 3 kinds of PM fiber patch cords.

Classification by fiber length―The standard length is 1 meter. It can vary for special requirements. The length of PM fiber patch cords can be custom made.

Applications of PM Fiber Patch Cords

PM fiber patch cords are often used in polarization sensitive fiber optical systems for transmission of light that requires the PM state to be maintained. PM optical patch cord is a special optical component using the properties of optical fibers specially manufactured so that its transmission parameters can support a particular application. They have a large number of uses, including high-data-rate communications systems, polarization sensitive components, and interferometric sensors. They are also widely used in PM fiber amplifiers, fiber lasers, high speed communication systems, testing equipment and instrumentation applications. Area of use of PM fiber patch cords is very broad and includes equipment such as instrumentation, spectroscopy, aerospace, medical diagnostics and many other industrial applications.

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.

What you should know before buy fiber polishing machine?

THE POLISHING MACHINE

When it is time to purchase a mechanical polishing machine there are a number of questions that should be asked:

1. Are the operating functions simple to use?

2. Does the unit offer easy connector interchangeability?

3. Are the polishing platens easy to access?

4. Is there a pressure-setting feature?

5. Does the polishing motion attack the connectors from all sides equally?

6. Can the machine perform angle polishes?

7. Does the manufacturer have the capability to supply custom fixturing if needed?

8. Are the end results meeting and/or exceeding current end-face standards?

A quality production polisher will answer “yes” to all of these questions.

In detail, a fiber polishing machine will have:

1. Timer–a settable timer allows a pre-defined timed sequence of operations techniques to be used. Timing has proven to be critical in obtaining connector performance specifications. A timer should have time settings ranging from 0 to 60 seconds.

2. Pressure setting device–a polishing machine must have adjustable pressure loading capability. Pressure combined with the hardness of the polishing surface will allow the machine to produce the connectors’ required end-face geometry. This device should have a setting tool that has clearly marked divisions of measurement.

3. Inter-changeability of connector holders–connector holders that can be removed quickly and easily offer increased output, less downtime and improved production. A machine that offers connector holders for all connector types adds flexibility to production.

4. Availability of connector holders–In evaluating the equipment, it is important to consider the available connector holders. It is important that the manufacturer has available holders for the standard connectors used around the world–SC, FC, ST–for both PC and APC configurations.

Also, the manufacturer should have the capability to provide a range of connector holders beyond the “standards” used–versatility in this area will minimize lost opportunities and maximize the ability to meet potential customer requests.

5. Removable Polishing Platens–polishing platens carry the polishing films that act upon the connector end-face. These should be easily removed and replaced. This minimizes contamination, increases connector output and maximizes polishing film life.

6. Polishing Motion–A key element of a high quality polishing system is the motion of the surface that performs the polishing. If the polishing action is not balanced evenly from all sides, connector performance will suffer and costs will increase because of rejected material and excessively rapid wear of the polishing films. To obtain consistent high quality results, the machine must provide an orbital polishing motion–a circular oscillation.

7. Can the Machine perform Angle Polishes– Though new polishing techniques, such as MPC (Maximum Physical Contact), allow PC finished connectors to achieve APC (Angled Physical Contact) results, the need to perform angle polishing is a must. Angle polishing (typically polished to 8°) is necessary when Backrelection readings of <–65dB are demanded.

A polisher should offer the option to polish connectors Flat, with a PC finish, or an APC finish. Different machines should not be purchased for different types of polishes. A quality polisher will have the capability to perform all types of polishing.

8. A ‘Recipe’ for meeting the standards–Standards for today’s connectors are stringent. It is important that the machine manufacturer provides, along with a good, preferably illustrated operation manual, specific polishing “recipes” for obtaining the connector specifications (described in the section below)–and, that you have open lines of communication with the manufacturer to keep you up to date in this developing technology.

Why A Good Fiber Optic Cleaver Helps Cut Out Costly Mistakes?

What Is Fiber Optic Cleaver?

A cleave in an optical fiber is a deliberate, controlled break, intended to create a perfectly flat end face, perpendicular to the longitudinal axis of the fiber. A fiber optic cleaver is a tool that holds the fiber under low tension, scores the surface at the proper location, then applies greater tension until the fiber breaks. Usually, after the fiber has been scored, the technician will use a cleaver either bend or pull the fiber end, stressing the fiber. This stress will cause the fiber to break at the score mark, leaving a 90-degree flat end if all goes well. So the cleaver doesn’t cut the fiber. In fact, it just breaks the fiber at a specific length.

Two Types of Fiber Optic Cleavers

We know that the closer to 90 degrees the cleave is, the more success you will have with matching it to another cleaved fiber to be spliced or mated by a connector. So it’s important to use the proper tool with good technique to consistently achieve a 90-degree flat end. Good cleavers are automatic and produce consistent results, irrespective of the operator. The user need only clamp the fiber into the cleaver and operate its controls. Some cleavers are less automated, making them more dependent on operator technique and therefore less predictable. There are two broad categories of fiber optic cleavers, scribe cleavers and precision cleavers.

Scribe Cleavers

A traditional cleaving method, typically used to remove excess fiber from the end of a connector before polishing, uses a simple hand tool called a scribe. Scribe cleavers are usually shaped like ballpoint pens with diamond tipped wedges or come in the form of tile squares. The scribe has a hard, sharp tip, generally carbide or diamond, that is used to scratch the fiber manually. Then the operator pulls the fiber to break it. Since both the scribing and breaking process are under manual control, this method varies greatly in repeatability. Most field and lab technicians shy away from these cleavers as they are not accurate. However, if in skilled hands, this scribe cleaver offer significantly less investment for repairs, installation, and training classes.

Precision Cleavers

Precision cleavers are the most commonly used cleavers in the industry. They use a diamond or tungsten wheel/blade to provide the nick in the fiber. Tension is then applied to the fiber to create the cleaved end face. The advantage to these cleavers is that they can produce repeatable results through thousands of cleaves by simply just rotating the wheel/blade accordingly. Although more costly than scribe cleavers, precision cleavers can cut multiple fibers while increasing speed, efficiency, and accuracy. In the past, many cleavers were scribes, but over time, as fusion splicers became available and a good cleave is the key to low splice loss, precision cleavers were developed to support various applications and multiple fiber cleaving with blades that have a much longer life span.

Which One to Use: Scribe Cleaver or Precision Cleaver?

While both types perform the functions above, the difference between the two categories of cleavers is the percentage yield of good cleaves. An experienced fiber optic technician will achieve approximately 90% good cleaves with a scribe cleaver, while the precision cleaver will produce 99% good cleaves. The difference doesn’t seem like much so you may hardly to make a specific decision. My suggestion is to buy precision cleavers if you plan to use a lot of mechanical splices or pre-polished splice/connectors. It will pay for itself in no time. If you decide to use the inexpensive scribe cleavers, you must learn how to use it properly. Follow directions, but also do what comes naturally to you when using the device, as they are sensitive to individual technique. Inspect the fibers you cleave to see how good they are and keep practicing until you can make consistently good cleaves.

To find pricing, information and more information on the different fiber optic cleavers currently available, please visit http://www.fiber-mart.com.

Brief Introduction of PM Fiber Patch Cords

Polarization maintaining (PM) fiber optic patch cord is a kind of special fiber patch cord. It can be used in many areas. Here’s what you need to know about PM fiber patch cords if your designs require them.

What is a PM Fiber Patch Cord?

A PM optical fiber is a single mode optical fiber in which linearly polarized light, if properly launched into the fiber, maintains a linear polarization during propagation, exiting the fiber in a specific linear polarization state. PM fiber patch cord is a fiber optic cable made with PM fiber and terminated on both ends with high-quality ceramic fiber connectors. PM fiber patch cord is a base device of optical passive component.

Characteristics of PM Fiber Patch Cords

If the polarization of the input light is not aligned with the stress direction in the fiber, the output light will vary between linear and circular polarization (and generally will be elliptically polarized). And the exact polarization will also be sensitive to variations in temperature and stress in the fiber. The light shall be coupled at the fiber entrance parallel to the slow axis or to the fast axis, then the maintaining of the polarization is therefore possible. It is important to make sure that the polarization of the input light is maintained. PM fiber patch cords maintain the existing polarization of linearly-polarized light that is launched into the fiber with the correct orientation. PM fiber patch cords also feature low insertion loss, high extinction ratio, high return loss, excellent changeability over a wide wavelength range and excellent environmental stability and reliability.

Types of PM Fiber Patch Cords

There are a wide variety of PM fiber patch cords available that support different data rates and suit various connector types. According to different criteria, PM fiber patch cords can be categorized into various types. The following is some detailed information about types of PM fiber patch cords based on 4 different criteria:

Classification by connector type―PM fiber patch cords are capped at both ends with fiber connectors. FC, SC, LC and ST are the commonly used connector types for PM fiber termination. According to the connectors on the both ends, there are many different kinds of PM fiber patch cords, such as LC-FC, SC-FC, or FC-FC PM fiber patch cords.

Classification by fiber type―PM fiber patch cords are built with polarization maintaining fiber. To ensure the polarization of both the input and output light in a PM fiber, several different shapes of rod are used, and the resulting fiber is sold under brand names such as “Panda” and “Bow-tie”. With different PM fiber, there are corresponding PM fiber patch cords, such as Panda PM fiber patch cords and Bow-tie PM fiber patch cords.

Classification by cable type―PM fiber patch cords can also be categorized according to the cable types. There are mainly three kinds of cable types, 250um bare fiber, 900um loose tube jacket and 3mm loose tube jacket. So based on the cable types, there are 3 kinds of PM fiber patch cords.

Classification by fiber length―The standard length is 1 meter. It can vary for special requirements. The length of PM fiber patch cords can be custom made.

Applications of PM Fiber Patch Cords

PM fiber patch cords are often used in polarization sensitive fiber optical systems for transmission of light that requires the PM state to be maintained. PM optical patch cord is a special optical component using the properties of optical fibers specially manufactured so that its transmission parameters can support a particular application. They have a large number of uses, including high-data-rate communications systems, polarization sensitive components, and interferometric sensors. They are also widely used in PM fiber amplifiers, fiber lasers, high speed communication systems, testing equipment and instrumentation applications. Area of use of PM fiber patch cords is very broad and includes equipment such as instrumentation, spectroscopy, aerospace, medical diagnostics and many other industrial applications.