What is VCSEL?

VCSEL stands for Vertical-Cavity Surface-Emitting Laser. It is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers which emit from surfaces formed by cleaving the individual chip out of a wafer.

Advantages of VCSEL laser 

A typical VCSEL consists of two oppositely-doped distributed Bragg reflectors (DBR) with a cavity layer between. In the center of the cavity layer resides an active region, consisting of multiple quantum wells. Current is injected into the active region via a current guiding structure either provided by an oxide aperture or proton-implanted surroundings. As the entire cavity can be grown with one-step epitaxy, these lasers can be manufactured and test on a wafer scale. This presents a significant manufacturing advantage.

The VCSEL cavity is very short, 100-1000 times shorter than that of a typical edge-emitting laser. There is typically only one Fabry-Perot(FP) wavelength within the gain spectrum; hence the FP wavelength(and not the gain peak) determines the lasing wavelength. The optical thickness variation of the layers in a VCSEL changes the lasing wavelength. The position of the layers with thickness variation to the center of the cavity is crucial for the resulting wavelength variation; the closer they are to the cavity center, the larger the wavelength change. This property lends to simple designs of wavelength-tunable VCSEL and multiple wavelength VCSEL arrays.

The VCSEL is, today, an established light source for data transmission in short-distance links, interconnects, and local networks (LANs, SANS, etc.). In these applications, the VCSEL is on-off modulated for the transmission of digital signals. Recent work on analog modulation of VCSELs indicates that VCSELs are suitable light sources also for the transmission of RF and microwave signals in, e.g., radio-over-fiber (RoF) networks used in antenna remoting in cellular systems for mobile communication.

Common to all these applications is that they put certain requirements on the high-speed modulation performance. With higher data rates and modulation frequencies, the requirements become more demanding and it becomes more difficult to identify VCSEL designs that fulfill the requirements.

Basic elements of an EDFA are shown in the figure above. The gain medium in the amplifier is a specially fabricated optical fiber with its core doped with erbium(Er). The erbium-doped fiber (EDF) is pumped by a semiconductor laser, which is coupled by using a wavelength selective coupler, also known as a WDM coupler, that combines the pump laser light with the signal light. The pump light propagates either in the same direction as the signal (co-propagation) or in the opposite direction (counter-propagation). Optical isolators are used to prevent oscillations and excess noise due to unwanted reflection in the assembly. More advanced architecture of an amplifier consists of multiple stages designed to optimize the output power and noise characteristics while incorporating additional loss elements in the mid-stage.

The energy level scheme of the erbium ion and the associated spontaneous lifetime in the glass host are are shown above. The atomic levels of Er-ions are broadened by local field variations at the microscopic level in the glass host. The light emission due to optical transitions from the first excited state to the ground level are perfectly matched with the transmission window of silica transmission fiber (1525-1610nm).

The erbium ions can be excited to the upper energy levels by 980nm or 1480nm pumps. In both cases, it is the first excited state, that is responsible for the amplification of optical signals. The amplification is achieved by the signal photons causing stimulated emission from the first excited state.

A three-level model can be used to describe the population of energy levels in the case of 980nm pumping, while a two-level model usually suffices for the 1480nm pumping case. Nearly complete inversion of Er ions can be achieved with 980nm pumping, whereas due to the stimulated emission at teh pump wavelength the inversion level is usually lower in the case of 1480nm pumping.

A higher degree of inversion leads to lower noise level generated from the spontaneous emission process and is therefore highly desirable for the pre-amplifier stage. The quantum efficiency of the amplifier is higher for 1480 pumping due to better closer match between the signal and pump energies.

The spontaneous lifetime of the metastable energy level is about 10ms, which is much slower than the signal bit rates of practical interest. These slow dynamics are responsible for the key advantage of EDFAs because of their negligible inter-symbol distortion and inter-channel crosstalk.

Dispersion in optical fibers

In an optical medium, such as fiber, there are three types of dispersion, chromatic, modal, and material.

Chromatic Dispersion

Chromatic dispersion results from the spectral width of the emitter. The spectral width determines the number of different wavelengths that are emitted from the LED or laser. The smaller the spectral width, the fewer the number of wavelengths that are emitted. Because longer wavelengths travel faster than shorter wavelengths (higher frequencies) these longer wavelengths will arrive at the end of the fiber ahead of the shorter ones, spreading out the signal.

One way to decrease chromatic dispersion is to narrow the spectral width of the transmitter. Lasers, for example, have a more narrow spectral width than LEDs. A monochromatic laser emits only one wavelength and therefore, does not contribute to chromatic dispersion.

Modal Dispersion

Modal dispersion deals with the path (mode) of each light ray. As mentioned above, most transmitters emit many different modes. Some of these light rays will
travel straight through the center of the fiber (axial mode) while others will repeatedly bounce off the cladding/core boundary to zigzag their way along the waveguide, as illustrated below.

The modes that enter at sharp angles are called high-order modes. These modes take much longer to travel through the fiber than the low-order modes and therefore contribute to modal dispersion. One way to reduce modal dispersion is to use graded-index fiber. Unlike the two distinct materials in a step-index fiber, the graded-index fiber’s cladding is doped so that the refractive index gradually decreases over many layers. The corresponding cross-sections of the fiber types are shown below.

With a graded-index fiber, the light follows a more curved path. The high-order modes spend most of the time traveling in the lower-index cladding layers near
the outside of the fiber. These lower-index core layers allow the light to travel faster than in the higher-index center layers. Therefore, their higher velocity compensates for the longer paths of these high-order modes. A good waveguide design appreciably reduces modal dispersion.

Modal dispersion can be completely eliminated by using a single-mode fiber. As its name implies, single mode fiber transmits only one mode of light so there is no
spreading of the signal due to modal dispersion. A monochromatic laser with single-mode fiber completely eliminates dispersion in an optical waveguide but is usually used in very long distance applications because of its complexity and expense.

Material Dispersion

Material dispersion is caused by the wavelength dependence of the refractive index on the fiber core material, while the waveguide dispersion occurs due to dependence of the mode propagation constant on the fiber parameters (core radius, and difference between refractive indexes in fiber core and fiber cladding) and signal wavelength.

Material dispersion contributes to group delay distortion, along with waveguide delay distortion, differential mode delay, and multimode group delay spread.

Fiber Optic Dispersion Compensation Devices

Dispersion management is the process to design the fiber and compensating elements in the transmission path to keep the total dispersion to a small number. Typically, dispersion compensating elements are placed every 100 km or so.

The figure below shows the performance of a fiber path that has alternating lengths of (+D) NZ-DSF and (-d) NZ-DSF every 20 km. The first 20 km length of fiber is (+D) NZ-DSF, so the dispersion increases over that length to 60 ps/nm. The next 20 km length of fiber is (-D) NZ-DSF type, so the dispersion gradually decreases back to zero. This pattern repeats two more times. At the end of the 120 km fiber path, the dispersion has returned to near zero.

But in most reality applications, fiber is already in place and odds are that the fiber is NDSF type. More than 80% of all single-mode fiber worldwide is NDSF type. In these cases, a more common means of controlling dispersion is the use of DCM (Dispersion Compensating Modules) placed at periodic intervals.

DCM’s are usually one of two types. The first type is DCF or Dispersion Compensating Fiber. This is simply a spool of a special type of fiber that has very large negative dispersion. Typically DCF dispersion can be in the range of -80 ps/(nm∙km), so a 20 km length of DCF can compensate for the dispersion in a 100 km length of NDSF.

The second type of DCM is a FBG (Fiber Bragg Grating) type. Here, a series of FBG’s or one very long FBG is written into a tens of meter length of fiber to perform the dispersion compensation.

Both of these types of DCM’s have relatively high insertion loss. A 60 km compensator may exhibit 6 dB of loss or more. Because of this, DCM’s are usually co-located with EDFA’s.

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Buy Fiber Optic Connector Cleaning Supplies Here

It is hard to conceive of the size of a fiber optic connector core. Single mode fibers have cores that are only 8~9um in diameter. As a point of reference, a typical human air is 50~75um in diameter, approximately 6-9 times larger.

Dust particles can be 20um or larger in diameter. Dust particles smaller than 1um can be suspended almost indefinitely in the air. A 1um dust particle landing on the core of single mode fiber can cause up to 1dB of loss. Larger dust particles, 9um or larger, can completely block the core of a single mode fiber.

Such that fiber optic connectors have to be cleaned each time they are mated. It is essential that fiber optics users develop the necessary discipline to always clean the connectors before they are mated.

The other important thing to keep in mind is that you need to cover a fiber optic connector when it is not in use. Unprotected connector ends are most often damaged by impact, such as hitting the floor. Most connector manufacturers provide some sort of protection boot. The best protectors cover the entire connector end, but they are generally simple closed-end plastic tubes that fit snugly over the ferrule only.

These boot will protect the connector’s polished ferrule end from impact damage that might crack or chip the polished surface. Many of the tight fitting plastic tubes contain jelly-like contamination (most likely mold release) that adheres to the sides of the ferrule. A blast of cleaning air or a quick dunk in a alcohol will not remove this residue.

This jelly-like residue can combine with common dirt to form a sticky mess that causes the connector ferrule to stick in the mating adapter. Often, the stuck ferrule will break off as one attempts to remove it, so always thoroughly clean the connector before mating, even if it was cleaned previously before the protection boot was installed.

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The fiber optic power meter is a special light meter that measures how much light is coming out of the end of the fiber optic cable. The power meter needs to be able to measure the light at the proper wavelength and over the appropriate power range. Most power meters used in datacom networks are designed to work at 850nm and 1300nn. Power levels are modest, in the range of –15 to –35dBm for multimode links, 0 to –40dBm for single mode links. Power meters generally can be adapted to a variety of connector styles such as SC, ST, FC, SMA, LC, MU, etc.

Optical power is usually measured in dBm, or decibels referenced to one milliwatt. This log scale is used because of the large dynamic range of fiber optic links, a range of 1000 or more. Some power levels may be given in microwatts, which many meters measure directly.

Power meters measure average optical power, not peak power, so they are sensitive to the duty cycle of the data being transmitted. It is important to specify the test conditions for measuring the optical power of a transmitter or at a receiver in terms of the data transmitted. Most networks have a diagnostic test signal for just this purpose.

Optical Power Meter Types

Optical power meters, like Digital Multimeters, come in a variety of types. The measurement uncertainty of practically all fiber optic power meters is the same, limited by the physical constrains of transferring standards with optical connectors. Most meters have an uncertainty of +/-5% or approximately 0.2dB, no matter what the resolution of the display may be.

Lower cost field optical power meters usually have a resolution of 0.1dB, laboratory meters display 0.01dB, and a resolution of 0.001dB is available on a few specialized fiber optic power meters.

The appropriate resolution for a measurement should be chosen according to the test. Laboratory measurements of low-loss patch cables, connectors, and fiber splices can be made to 0.01dB resolution and an uncertainty of 0.05dB or less if great care is used in controlling the test conditions.

Field measurements of absolute power are no better than the absolute calibration uncertainty, but relative power measurements can be made to 0.1dB. Cable plant loss is affected by not only the meter uncertainty, but also the characteristics of the test source, and may have an uncertainty of 0.5dB or more.

Test Source for Optical Power Testing

The test source is a portable version of the source that is used in the communication network attached to the fiber. It simulates the signal in the fiber for loss testing with a power meter, so the test source should match the wavelength and source type of the system source.

Generally, multimode fiber is tested with LEDs at both 850nm and 1300nm and single mode fiber is tested with lasers at 1310nm and 1550nm. The test source will typically be a LED for multimode fiber unless the fiber is being used for Gigabit Ethernet or other high-speed networks that use laser sources. LEDs can be used to test single mode fibers less than 5000 meters long, while a laser should be used for long single mode fibers.

You should always match the test light source wavelength to the network requirements. While one refers to 850, 1300, or 1550nm routinely, actual source wavelength may vary. One long runs of fiber typical of WANs or long-distance networks, the difference becomes important.

The fiber attenuation coefficient is a function of wavelength, so source spectral characteristics can be important. When testing long lengths of fiber, you may need to make corrections to nominal source wavelength losses.

A dirty ultra polish connector with a normal return loss of >55dB can easily have >45dB reflectance if it is not cleaned properly. Similar comparisons can be made with angled polish connectors. This can greatly affect system performance, especially in CATV applications where carrier-to-noise ratios (CNR) are directly related to signal quality.

In order to ensure that both connectors are properly cleaned, the termination panel must allow them both to be easily accessed. This easy access has to be for both the patch cord connector and the equipment or OSP connector on the back side of the termination panel. Accessing these connectors should not cause any significant loss in adjacent fibers.

A system that allows uncomplicated access to these connectors has much lower operating costs and improved reliability. Without easy access to connectors, technicians will take more time to perform their work, delaying implementation of new services or redeployment of existing services.

Dirty connectors can also jeopardize the long-term reliability of the network, because dirt and debris can be embedded into the endface of the connector, causing permanent, performance-affecting damage.

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Fiber Optic Cable Routing Paths

The first aspect of fiber cable management is cable routing paths. This aspect is related to fiber cable’s minimum bending radius as improper routing of fibers by technicians is one of the major causes of bend radius violations.

Routing paths should be clearly defined and easy to follow. In fact, these paths should be designed so that the technician has no other option than to route the cables properly. Leaving cable routing to the technician’s imagination leads to an inconsistently routed, difficult-to-manage fiber network.

Improper cable routing also causes increased congestion in the termination panel and the cableways, increasing the possibility of bend radius violations and long-term failure. Well-defined routing paths, on the other hand, reduce the training time required for technicians and increase the uniformity of the work done. The routing paths also ensure that bend radius requirements are maintained at all points, improving network reliability.

Additionally, having defined routing paths makes accessing individual fibers easier, quicker and safer, reducing the time required for reconfigurations. Uniform routing paths reduce the twisting of fibers and make tracing a fiber for rerouting much easier.

Well-defined cable routing paths also greatly reduce the time required to route and reroute patch cords. This has a direct effect on network operating costs and the time required to turn-up or restore service.

Fiber Optic Cable Access

The second aspect of fiber cable management is the accessibility of the installed fibers. Allowing easy access to installed fibers is critical in maintaining proper bend radius protection. This accessibility should ensure that any fiber can be installed or removed without inducing a macrobend on an adjacent fiber.

The accessibility of the fibers in the fiber cable management system can mean the difference between a network reconfiguration time of 20 minutes per fiber and one of over 90 minutes per fiber. Accessibility is most critical during network reconfiguration operations and directly impacts operation costs and network reliability.

Optical Fiber Protection

The third aspect of fiber cable management is the physical protection of the installed fibers. All fibers should be protected throughout the network from accidental damage by technicians and equipment.

Fibers routed between pieces of equipment without proper protection are susceptible to damage, which can critically affect network reliability. The fiber cable management system should therefore ensure that every fiber is protected from physical damage.

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There are two basic types of bends in fiber—microbends and macrobends. As the names indicate, microbends are very small bends or deformities in the fiber, while macrobends are larger bends (see the figure below).

The fiber’s radius around bends impacts the fiber network’s long-term reliability and performance. Simply put, fibers bent beyond the specified minimum bend diameters can break, causing service failures and increasing network operations costs. Cable manufacturers, Internet and telecommunications service providers, and others specify a minimum bend radius for fibers and fiber cables.

The minimum bend radius will vary depending on the specific fiber cable. However, in general, the minimum bend radius should not be less than ten times the outer diameter (OD) of the fiber cable. Thus a 3mm cable should not have any bends less than 30mm in radius. Telcordia recommends a minimum 38mm bend radius for 3mm patch cords (Generic Requirements and Design Considerations for Fiber Distributing Frames, GR-449-CORE, Issue 1, March 1995, Section 3.8.14.4). This radius is for a fiber cable that is not under any load or tension. If a tensile load is applied to the cable, as in the weight of a cable in a long vertical run or a cable that is pulled tightly between two points, the minimum bend radius is increased, due to the added stress.

There are two reasons for maintaining minimum bend radius protection: enhancing the fiber’s long-term reliability; and reducing signal attenuation. Bends with less than the specified minimum radius will exhibit a higher probability of long-term failure as the amount of stress put on the fiber grows. As the bend radius becomes even smaller, the stress and probability of failure increase.

The other effect of minimum bend radius violations is more immediate; the amount of attenuation through a bend in a fiber increases as the radius of the bend decreases. The attenuation due to bending is greater at 1550nm than it is at 1310nm—and even greater at 1625nm. An attenuation level of up to 0,5dB can be seen in a bend with a radius of 16mm. Both fiber breakage and added attenuation have dramatic effects on long-term network reliability, network operations costs, and the ability to maintain and grow a customer base.

In general, bend radius problems will not be seen during the initial installation of a fiber distribution system (FDS), where an outside plant fiber cable meets the cable that runs inside a central office or headend. During initial installation, the number of fibers routed to the optical distribution frame (ODF) is usually small. The small number of fibers, combined with their natural stiffness, ensures that the bend radius is larger than the minimum.

If a tensile load is applied to the fiber, the possibility of a bend radius violation increases. The problems grow when more fibers are added to the system. As fibers are added on top of installed fibers, macrobends can be induced on the installed fibers if they are routed over an unprotected bend (see the figure below). A fiber that had been working fine for years can suddenly have an increased level of attenuation, as well as a potentially shorter service life.

The fiber used for analogue video CATV systems presents a special case. Here, receiver power level is critical to cost-effective operation and service quality, and bend radius violations can have different but equally dramatic effects.

Analogue CATV systems are generally designed to optimize transmitter output power. Due to carrier-to-noise-ratio (CNR) requirements, the receiver signal power level is controlled, normally to within a 2dB range. The goal is for the signal to have enough attenuation through the fiber network, including cable lengths, connectors, splices and splitters, so that no attenuators are needed at the receiver.

Having to attenuate the signal a large amount at the receiver means that the power is not being efficiently distributed to the nodes, and possibly more transmitters are being used than are necessary. Since the power level at the receiver is more critical, any additional attenuation caused by bending effects can be detrimental to picture quality, potentially causing customers to be dissatisfied and switch to other vendors.

Since any unprotected bends are a potential point of failure, the fiber cable management system should provide bend radius protection at all points where a fiber cable makes a bend. Having proper bend radius protection throughout the fiber network helps ensure the network’s long-term reliability, thus helping maintain and grow the customer base. Reduced network down time due to fiber failures also reduces the operating cost of the network.

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Three methods are used today to fabricate moderate-to-low loss waveguide fibers: modified chemical vapor deposition (MCVD), outside vapor deposition(OVD), and vapor axial deposition (VAD).

Modified Chemical Vapor Deposition (MCVD)

In MCVD a hollow glass tube, approximately 3 feet long and 1 inch in diameter (1 m long by 2.5 cm diameter), is placed in a horizontal or vertical lathe and spun rapidly. A computer-controlled mixture of gases is passed through the inside of the tube. On the outside of the tube, a heat source (oxygen/hydrogen torch) passes up and down as illustrated in the following figure.

Each pass of the heat source fuses a small amount of the precipitated gas mixture to the surface of the tube. Most of the gas is vaporized silicon dioxide (glass), but there are carefully controlled remounts of impurities (dopants) that cause changes in the index of refraction of the glass. As the torch moves and the preform spins, a layer of glass is formed inside the hollow preform. The dopant (mixture of gases) can be changed for each layer so that the index may be varied across the diameter.

After sufficient layers are built up, the tube is collapsed into a solid glass rod referred to as a preform. It is now a scale model of the desired fiber, but much
shorter and thicker. The preform is then taken to the drawing tower, where it is pulled into a length of fiber up to 10 kilometers long.

Outside Vapor Deposition (OVD)

The OVD method utilizes a glass target rod that is placed in a chamber and spun rapidly on a lathe. A computer-controlled mixture of gases is then passed between the target rod and the heat source as illustrated in the figure below. On each pass of the heat source, a small amount of the gas reacts and fuses to the outer surface of the rod. After enough layers are built up, the target rod is removed and the remaining soot preform is collapsed into a solid rod. The preform is then taken to the tower and pulled into fiber.

Vapor Axial Deposition (VAD)

The VAD process utilizes a very short glass target rod suspended by one end. A computer-controlled mixture of gases is applied between the end of the rod and the heat source as shown in the figure below. The heat source is slowly backed off as the preform lengthens due to tile soot buildup caused by gases reacting to the heat and fusing to the end of the rod. After sufficient length is formed, the target rod is removed from the end, leaving the soot preform. The preform is then taken to the drawing tower to be heated and pulled into the required fiber length.

Coating the Fiber for Protection

After the fiber is pulled from the preform, a protective coating is applied very quickly after the formation of the hair-thin fiber as shown below. The coating is necessary to provide mechanical protection and prevent the ingress of water into any fiber surface cracks. The coating typically is made up of two parts, a soft inner coating and a harder outer coating. The overall thickness of the coating varies between 62.5 and 187.5 μm, depending on fiber applications.

The most basic design of demountable connector comprises a ferrule of hole diameter 128–129 μm into which the fiber is bonded using an appropriate adhesive as in the figure below. In this most fundamental form the ferrules can be rotated through 360° within an alignment tube or adaptor.

This rotation allows any eccentricity to be fully explored as is discussed below.

In addition a gap of perhaps 5–10 μm is incorporated between the ferrule faces. This gap is intended to prevent damage to the fiber ends but has the disadvantage that it incurs both Fresnel loss and loss due to separation effects as discussed in Chapter 4. Also the gap creates reflections, which limits the achievable return loss to approximately 11 dB.

It is worth while to review the basic joint losses observed in practice as opposed to those provided in manufacturers’ data, which may have been generated using the interference-fit model or similar distortions. Obviously these losses operate in tandem with losses caused by basic parametric mismatches.

The ferrule hole diameter (FHD) is of considerable importance since the cladding misalignment achieved when jointing a fiber of cladding diameter d1, to another with cladding diameter d2 in an FHD of X will be seen from equation (5.2), in the worst case:

cladding misalignment = X – (d1 + d2)/2

For a fixed FHD the impact of using fibers with diameters at the lower end of the specified tolerance can be quite severe. For instance, for FHD of 129 μm the misalignment for two 122 μm fibers can be as much as 7 μm, which could result in a power loss of 0.85 dB for 50 μm core fibers and total loss for single mode fibers.

Two factors work against such losses being encountered. First the specification for cladding diameter is rarely fully explored, with 97% of all fiber lying in the range 123.5–126.5 μm. Taking due account of these factors limits the misalignment loss to approximately 3 μm, corresponding to a loss of some 0.30 dB for a 50 μm core fiber. Although this figure is considerably lower than the 1.85 dB shown above it is nevertheless much greater than would be predicted using the interference-fit model (0 dB).

Second, the professional termination of the connectors involves filling the ferrule with an adhesive, normally some type of epoxy resin.The fiber is guided through the adhesive and passes through the hole in the ferrule end face. For smaller fibers the adhesive serves to assist in the centralization of the fiber in the hole, thereby reducing the eccentricity.

One of the first optical connectors to be used in data communications was the SMA. This basic demountable connector exhibits random mated worst case insertion losses between 1.3 dB and 2.0 dB dependent upon the quality of the connector components themselves.

Keyed Connectors

The fact that the ferrules within basic demountable connectors can be rotated through a full 360° implies that the impact of core eccentricity (due to fiber manufacturing tolerance) and cladding-based misalignment (due to fixed FHD) will inevitably surface in the form of rotational variations in insertion loss. This results in poor repeatability.

This undesirable rotational degree of freedom has since led to the introduction of keyed connectors, e.g. keyed or bayonet-mount connectors such as the ST and FC, which define the orientation of the ferrule face against another. This prevents rotation and ensures good repeatability. However, this repeatability is only guaranteed in a given joint since the inherent cladding-based misalignments are still present.

An example would be that ferrule A against ferrule B may achieve 0.5 dB insertion loss in a highly repeatable and stable fashion. Also ferrule C against ferrule D may achieve the same performance; however, the difficulty arises when ferrule A is measured against ferrule C or B against D. In these circumstances the insertion loss is unpredictable (but repeatable) although it will lie with the bounds of the limits of the random mated worst case calculation for the joint design.

As a result keyed connectors do not necessarily give better results for insertion loss but are perceived to perform in a more repeatable way; however, this is only true for a given joint.