Introduction of the Transients in Optical WDM Networks

A systems analysis continues to be completed to consider dynamical transient effects in the physical layer of an Optical WDM Network. The physical layer dynamics include effects on different time scales. Dynamics from the transmission signal impulses possess a scale of picoseconds. The timing recovery loops in the receivers be employed in the nanoseconds time scale. Optical packet switching in the future networks will have microsecond time scale. Growth and development of such optical networks is yet continuing. Most of the advanced development work in optical WDM networks is presently focused on circuit switching networks, where lightpath change events (for example wavelength add/drop or cross-connect configuration changes) happen on the time scale of seconds.

It is focused on the dynamics from the average transmission power associated with the gain dynamics in Optical Line Amplifiers (OLA). These dynamics may be triggered by the circuit switching events and have millisecond time scale primarily defined by the Amplified Spontaneous Emission (ASE) kinetics in Erbium-Doped Fiber Amplifiers (EDFAs). The transmission power dynamics will also be influenced by other active components of optical network, for example automatically tunable 100GHz DWDM, spectral power equalizers, or other light processing components. When it comes to these dynamics, a typical power of the lightpath transmission signal is recognized as. High bandwidth modulation from the signal, which actually consists of separate information carrying pulses, is mostly ignored.

14_nodes Ring WDMRing WDM networks implementing communication between two fixed points are very well established technology, in particular, for carrying SONET over the WDM. Such simple networks with fixed WDM lighpaths happen to be analyzed in many detail. Fairly detailed first principle models for transmission power dynamics exist for such networks. These models are implemented in industrial software allowing engineering design calculations and dynamical simulation of these networks. Such models could possibly have very high fidelity, but their setup, tuning (model parameter identification) and exhaustive simulations covering a variety of transmission regimes are potentially very labor intensive. Adding description of new network components to such model could need a major effort.

14_nodes Mesh WDMThe problems with detailed first principle models is going to be greatly exacerbated for future Mesh WDM networks. The near future core optical networks will be transparent to wavelength signals on a physical layer. In such network, each wavelength signal travels through the optical core between electronic IP routers around the optical network edge using the information contents unchanged. The signal power is attenuated in the passive network elements and boosted by the optical amplifiers. The lightpaths is going to be dynamically provisioned by Optical Cross-Connects (OXCs), routers, or switches independently on the underlying protocol for data transmission. Such network is basically a circuit switched network. It might experience complex transient processes of the average transmission power for every wavelength signal at the event of the lightpath add, drop, or re-routing. A mix of the signal propagation delay and channel cross-coupling might result in the transmission power disturbances propagating across the network in closed loops and causing stamina oscillations. Such oscillations were observed experimentally. Additionally, the transmission power and amplifier gain transients could be excited by changes in the average signal power because of the network traffic burstliness. If for some period of time the wavelength channel bandwidth is not fully utilized, this could result in a loss of the average power (average temporal density of the transmitted information pulses).

First circuit switched optical networks are already being designed and deployed. Fraxel treatments develops rapidly for metro area and long term networks. Engineering design of circuit switched networks is complicated because performance has to be guaranteed for all possible combinations of the lightpaths. Further, as such networks develop and grow, they potentially need to combine heterogenous equipment from a variety of vendors. A system integrator (e.g., DK Photonics) of such network might be different from subsystems or component manufacturer. This creates a necessity of developing adequate means of transmission power dynamics calculations which are suitable for the circuit switched network business. Ideally, these methods should be modular, independent on the network complexity, and use specifications on the component/subsystem level.

DK Photonics has technical approach to systems analysis that’s to linearize the nonlinear system around a fixed regime, describe the nonlinearity like a model uncertainty, and apply robust analysis that guarantees stability and gratifaction conditions within the presence of the uncertainty. For a user of the approach, there is no need to understand the derivation and system analysis technicalities. The obtained results are very simple and relate performance to basic specifications of the network components. These specifications are somewhat not the same as those widely used in the industry, but could be defined from simple experimentation using the components and subsystems. The obtained specification requirements may be used in growth and development of optical amplifiers, equalizers, optical attenuators, other transmission signal conditioning devices, OADM Modules, OXCs, and any other optical network devices and subsystems influencing the transmission power.

DK Photonics – www.dkphotonics.com  specializes in designing and manufacturing of high quality optical passive components mainly for telecommunication, fiber sensor and fiber laser applications,such as WDM, FWDM, CWDM, DWDM, OADM,Optical Circulator, Isolator, PM Circulator, PM Isolator, Fused Coupler, Fused WDM, Collimator, Optical Switch and Polarization Maintaining Components, Pump Combiner, High power isolator, Patch Cord and all kinds of connectors.

Large Mode Area Fibers

Optical fibers with relatively large mode areas and a single transverse mode or only a few modes.

For some applications, it is desirable to use optical fibers with a large mode area (LMA fibers) – often with single-mode guidance. Due to the reduced optical intensities, such fibers effectively have lower nonlinearities and a higher damage threshold, which makes them suitable for example for the Amplification of intense Pulses or single-frequency signals in Fiber amplifiers, or in case of passive fibers for delivery of such light. While standard single-mode fibers have an Effective Mode Area below 100 μm2, large mode area fibers reach values of hundreds or even thousands of μm2.

DK Photonics – www.dkphotonics.com  specializes in designing and manufacturing of high quality optical passive components mainly for fiber laser applications such as 1064nm high power isolator, Cladding Power Stripper, Multimode High Power Isolator, pump combiner,1064nm Band-pass Filter,(6+1)X1 Pump and Signal Combiner, PM Circulator, PM Isolator, optical Coupler. More information, please contact us.

How To Test Fiber Optic Splitters Or Other Passive Devices

A fiber optic splitter is a device that splits the fiber optic light into several parts by a certain ratio. For example, when a beam of fiber optic light transmitted from a 1X4 equal ratio splitter, it will be divided into 4-fiber optic light by equal ratio that is each beam is 1/4 or 25% of the original source one. A Optical Splitter is different from WDM. WDM can divide the different wavelength fiber optic light into different channels. fiber optic splitter divide the light power and send it to different channels.

Most Splitters available in 900µm loose tube and 250µm bare fiber. 1×2 and 2×2 couplers come standard with a protective metal sleeve to cover the split. Higher output counts are built with a box to protect the splitting components.

Testing a coupler or splitter (both names are used for the same device) or other passive fiber optic devices like switches is little different from testing a patchcord or cable plant using the two industry standard tests, OFSTP-14 for double-ended loss (connectors on both ends) or FOTP-171 for single-ended testing.

First we should define what these passive devices are. An optical coupler is a passive device that can split or combine signals in optical fibers. They are named by the number of inputs and outputs, so a splitter with one input and 2 outputs is a 1×2 fiber splitter, and a PON splitter with one input and 32 outputs is 1×32 splitter. Some PON splitters have two inputs so it would be a 2X32. Here is a table of typical losses for splitters.

Important Note! Mode Conditioning can be very important to testing couplers. Some of the ways they are manufactured make them very sensitive to mode conditioning, especially multimode but even singlemode couplers. Singlemode couplers should always be tested with a small loop in the launch cable (tied down so it does not change and set the 0dB reference with the loop.) Multimode couplers should be mode conditioned by a mandrel wrap or similar to ensure consistency.

Let’s start with the simplest type. Shown below is a simple 1X2 splitter with one input and two outputs. Basically, in one direction it splits the signal into 2 parts to couple to two fibers. If the split is equal, each fiber will carry a signal that is 3dB less than the input (3dB being a factor of two) plus some excess loss in the coupler and perhaps the connectors on the splitter module. Going the other direction, signals in either fiber will be combined into the one fiber on the other side. The loss is this direction is a function of how the coupler is made. Some couplers are made by twisting two fibers together and fusing them in high heat, so the coupler is really a 2X2 coupler in which case the loss is the same (3dB plus excess loss) in either direction. Some splitters use optical integrated components, so they can be true splitters and the loss in each direction may different.

So for this simple 1X2 splitter, how do we test it? Simply follow the same directions for a double-ended loss test. Attach a launch reference cable to the test source of the proper wavelength (some splitters are wavelength dependent), calibrate the output of the launch cable with the meter to set the 0dB reference, attach to the source launch to the splitter, attach a receive launch cable to the output and the meter and measure loss. What you are measuring is the loss of the splitter due to the split ratio, excess loss from the manufacturing process used to make the splitter and the input and output connectors. So the loss you measure is the loss you can expect when you plug the splitter into a cable plant.

To test the loss to the second port, simply move the receive cable to the other port and read the loss from the meter. This same method works with typical PON splitters that are 1 input and 32 outputs. Set the source up on the input and use the meter and reference cable to test each output port in turn.

What about the other direction from all the output ports? (In PON terms, we call that upstream and the other way from the 1 to 32 ports direction downstream.) Simply reverse the direction of the test. If you are tesing a 1X2 splitter, there is just one other port to test, but with a 1X32, you have to move the source 32 times and record the results on the meter.

What about multiple input and outputs, for example a 2X2 coupler? You would need to test from one input port to the two outputs, then from the other input port to each of the two outputs. This involves a lot of data sometimes but it needs to be tested.

There are other tests that can be performed, including wavelength variations (test at several wavelengths), variations among outputs (compare outputs) and even crosstalk (put a signal on one output and look for signal on other outputs.)

Once installed, the splitter simply becomes one source of loss in the cable plant and is tested as part of that cable plant loss for insertion loss testing. Testing splitters with an OTDR is not the same in each direction.

Other Passive Devices

There are other passive devices that require testing, but the test methods are similar.

Fiber optic switches are devices that can switch an input to one of several outputs under electronic control. Test as you would the splitter as shown above. Switches may be designed for use in only one direction, so check the device specifications to ensure you test in the proper direction. Switches may also need testing for consistency after multiple switch cycles and crosstalk.

Attenuators are used to reduce signal levels at the receiver to prevent overloading the receiver. There is a page on using attenuators that you should read. If you need to test an attenuator alone, not part of a system, use the test for splitters above by using the attenuator to connect the launch and receive cables to see if the loss is as expected.

Wavelength-division multiplexers can be tricky to test because they require sources at a precise wavelenth and spectral width, but otherwise the test procedures are similar to other passive components.

Fiber optic couplers or splitters are available in a wide range of styles and sizes to split or combine light with minimal loss. All couplers are manufactured using a very simple proprietary process that produces reliable, low-cost devices. They are physically rugged and insensitive to operating temperatures. Couplers can be fabricated in custom fiber lengths and/or with terminations of any type.

DK Photonics – www.dkphotonics.com  specializes in designing and manufacturing of high quality optical passive components mainly for telecommunication, fiber sensor and fiber laser applications,such as PLC Splitter, WDM, FWDM, CWDM, DWDM, OADM,Optical Circulator, Isolator, PM Circulator, PM Isolator, Fused Coupler, Fused WDM, Collimator and Polarization Maintaining Components, Pump Combiner, High power isolator, Patch Cord and all kinds of connectors.

China laser innovation awards program now accepting nominations

The 2015 Ringier Technology Innovation Awards - Laser Industry is now open for applications up until January 4, 2015.

Now in its second year, the Awards consist of six categories: Lasers, Laser Systems for Production Engineering (Laser Cutting Systems, Laser Engraving Systems, Laser Marking Systems, and Laser Welding Systems), Laser System Components, System Peripherals of Laser Production Engineering, Optical Materials and Components, and 3D Printing. Only new products and solutions launched during 2013-2015 in the China market are eligible to enter the Awards selection process.

Following the entry and nomination stage, online peer voting and expert judging will take place January 9-25, 2015. Industry experts, including Dichen Li, Ph.D., Changjiang Professor, Xi'an Jiaotong University; Xiahui Tang, Professor, National Engineering Research Center For Laser Processing; Youliang Wang, Chairman, Laser Processing Committee of China Optical Society; Xiao Zhu, Chairman, Wuhan Laser Association of Optics Valley of China; and Qingmao Zhang, Vice Chairman, Laser Processing Committee of China Optical Society, will be among the independent panel of judges. Winners will be announced at the Awards ceremony on March 18, 2015, to coincide with Laser World of PHOTONICS 2015 in Shanghai, when all the winners, judges, industry professionals, and media will be present.

Organized by Industrial Laser Solutions China, Ringier Trade Media, and supported by Industrial Laser Solutions, these Awards are presented to a select group of innovators each year in China. The purpose of the Awards is to encourage, acknowledge, and reward those individuals and companies who have introduced and developed a new idea, a new methodology, a new product or a new technology for manufacturing production efficiency, cost-effectiveness, and user convenience, which might result in energy saving and more responsible clean manufacturing in the laser industry.

The Ringier Technology Industry Awards Series have been established since 2006 and cover nine different industries. They are recognized as being the most honest, transparent, and fairest of such Industry Awards in China. Nominations are open to all and the final selections are made by the panel of independent judges based solely on merit.

DK Photonics – www.dkphotonics.com  specializes in designing and manufacturing of high quality optical passive components mainly for fiber laser applications such as 1064nm high power isolator, Cladding Power Stripper, Multimode High Power Isolator, pump combiner,1064nm Band-pass Filter,(6+1)X1 Pump and Signal Combiner, PM Circulator, PM Isolator, optical Coupler. More information, please contact us.

Optical Filters: Filter stacks transmit wide-angle incident light without shifting wavelength(3)

To avoid the problem of color change versus incidence angle in an optical system, thin-film-coated filter elements can be replaced by a filter consisting of a stack of different filter glasses.

JASON KECK

Rugged, no coating degradation

Advantages of using a filter stack rather than a thin-film-coated optical element include wide-angle performance (see Fig. 2) and high durability. Because the glass itself performs the blocking, there is no concern of coating degradation due to extreme environmental shifts, contamination, or mishandling. Filter stacks are as durable as the glass they are made from, surviving aggressive cleaning methods, severe abrasion, salt/fog testing, humidity, and temperature cycling per durability standards of MIL-PRF-13830B, MIL-C-48497A, and MIL-C-675C.

Because all filter glass types have approximately the same index of refraction, there is no Fresnel loss as light propagates from one internal layer to another. However, as with any glass, the air-to-substrate interfaces will incur an ~8% total Fresnel loss for the component.

The addition of a broadband antireflection (BBAR) coating on each air-to-substrate surface can mostly eliminate this loss. The spectral range of the BBAR is designed to be much wider than the active spectral region of the 100G DWDM filter, so the stability of the transmission band will not be affected by changes in the angle of the filter. Blocking coatings can also be added if it is necessary to create steeper edges for in-band performance; however, doing so can affect the wide-angle performance at the edge wavelengths.

ColorLock filter stacks can be designed for spectral ranges from ultraviolet to near-infrared, with transmission exceeding 60% at the specified design wavelength. This transmission may not be as high as with dielectric filters, but is sufficient for applications with controlled and stable illumination, such as for machine vision, in which the consistency of wavelengths from wider incident angles is more important than transmission.

Having overcome considerable design challenges, we believe that these filter stacks can be used as an innovative solution in applications that demand consistent wavelengths from incident angles that are wide enough that dielectric filters would not be sufficient, and where the higher transmission that is afforded by dielectric filters is less important.

DK Photonics – www.dkphotonics.com  specializes in designing and manufacturing of high quality optical passive components mainly for fiber laser applications such as 1064nm high power isolator, Cladding Power Stripper, Multimode High Power Isolator, pump combiner,1064nm Band-pass Filter,(6+1)X1 Pump and Signal Combiner, PM Circulator, PM Isolator, optical Coupler. More information, please contact us.

Optical Filters: Filter stacks transmit wide-angle incident light without shifting wavelength(2)

To avoid the problem of color change versus incidence angle in an optical system, thin-film-coated filter elements can be replaced by a filter consisting of a stack of different filter glasses.

JASON KECK

Wide-angle filter stack apps

There is a multitude of applications for this type of filter. In the field of digital imaging, colorimeters-which take wideband spectral energy readings-are used to profile and calibrate display devices, verifying that pixel color and intensity at the edge of a display matches the performance of pixels in the center of the display.

In astronomy, biomedical or fluorescence imaging, and mineralogy, hyperspectral imaging has many important applications. It is essential that the incident light undergo as little iridescence as possible. Also, when precision imaging instruments are expensively launched into orbit, the filters must be robust enough to withstand extreme environmental operating conditions.

In agriculture, the color of crops or food products reveals vital information. The use of Earth-observing satellites to measure the "vegetation index" of crops (a measurement of green hue) is nothing new, but the affordability of aerial drones has brought new possibilities. A drone can be programmed with GPS data to fly on a fixed pattern over a designated crop area and take wide-angle images at regular intervals, building up a picture of the vegetation index of crops. If the images used in such applications provide accurate spectral data that is as free as possible from iridescent distortion, it can give farmers precise control over fertilizer application rates and greatly improve efficiency and productivity. This is a considerable cost saving over low-resolution, narrowband satellite imagery and conventional aerial photography using manned aircraft.

Design hurdles

There are three complicating factors in the design of such filter stacks. The first is the limited choice in filter glass, limited not only by manufacturer availability but also by physics. Filter glass with an ideal edge cut-on or cut-off wavelength for an application is not always easy to find, or may be impossible to precisely manufacture. Where it is available, the designer is then limited by what the manufacturer can deliver in a reasonable time, as melts may be scheduled as infrequently as once every several years, depending on demand.

The second factor is that, while the perfect filter glass for a particular application may not exist, there are hundreds of other glass types from numerous vendors that can be combined to achieve a close approximation of the requirement.

The third complicating factor is that the design of ColorLock filters is a massively multidimensional, nonsmooth optimization challenge. Physical manufacturing requirements restrict the thickness of all combined individual layers to not exceed the overall thickness requirement of the resulting optical component, further putting restrictions on the selection of specific CWDM filter glass types.

Reynard streamlined this complex design process by developing in-house software into which all of the system requirements are fed. The software produces a manufacturable design for a filter in which the necessary materials are combined at the correct thickness in each layer. The design is then manufactured and validated for performance.

About DK Photonics

DK Photonics – www.dkphotonics.com  specializes in designing and manufacturing of high quality optical passive components such as 8CH CWDM Module,100GHz 8CH DWDM,200GHz DWDM,Mini-size CWDM,compact CWDM,Athermal AWG DWDM Module,100GHz AWG,Thermal AWG DWDM Module,1310/1490/1550nm FWDM, PLC Splitter, Optical Circulator,Optical Isolator,Fused Coupler,Mini Size Fused WDM.

Optical Filters: Filter stacks transmit wide-angle incident light without shifting wavelength(1)

To avoid the problem of color change versus incidence angle in an optical system, thin-film-coated filter elements can be replaced by a filter consisting of a stack of different filter glasses.

JASON KECK

Wide-angle imaging systems have to overcome numerous problems. Distortion of the shape of objects in the scene is the predominant issue, recognizable as the "fish-eye lens" look that is often corrected in software. However, lens distortion is not the only problem.

Iridescence, or the change in transmitted or reflected color of light viewed from different angles, is a phenomenon that can be found both in nature and in artificial light-detecting systems with precise color requirements, where it can cause many problems.

Wide-angle color-sensing applications commonly require that a CWDM wavelength must be detectable regardless of the incident angle. Iridescence through a thin-film-coated optical element can cause problems in this situation by distorting the spectral transmission of light coming from peripheral objects.

Maximizing light transmission in a thin-film WDM coating's passband while blocking out-of-band light is a requirement for coated optical components such as dielectric filters; however, the wavelength's transition commonly only remains steady within relatively narrow cone angles. Beyond angles of 5°, such filters are susceptible to iridescence, observable as a change of color, or "blueshift." As the angle of light entering the filter increases, the light propagates through more of each thin-film stack layer, altering the apparent overall thickness of the optical-filter stack and affecting the performance of the original intended design. This can make such filters unsuitable for wide-angle imaging applications with bright illumination and where higher standards of consistency are required of the wavelength of all incident light.

One of the more convoluted wide-angle imaging solutions is the use of a cluster of cameras or a polycamera, pointing in various directions like the compound eye of an insect; the resulting multiple pictures are then assembled into one image in software. Although the light entering each camera thus fills only a narrow cone angle, the complexity and resultant high expense of such a system is obvious.

Engineers at Reynard have addressed this problem in a single optical device with a system in which two or more layers of filter glass are combined into a stacked configuration. These ColorLock filter stacks eliminate the wavelength shift as incident angle increases and are customized to meet specific system needs.

Software is used to determine the exact composition and thickness of the layers in these filters; the software determines a merit function that best estimates the filter requirements and allows filter stacks to be designed for band pass, short-wave pass, long-wave pass, or user-specified functions. Incident angles can be as high as 50° without any shift in the transmitted wavelength, while more traditional coated filters with the same conditions would see a significant shift toward shorter wavelengths.

 

About DK Photonics

DK Photonics – www.dkphotonics.com  specializes in designing and manufacturing of high quality optical passive components such as 8CH CWDM Module,100GHz 8CH DWDM,200GHz DWDM,Mini-size CWDM,compact CWDM,Athermal AWG DWDM Module,100GHz AWG,Thermal AWG DWDM Module,1310/1490/1550nm FWDM, PLC Splitter, Optical Circulator,Optical Isolator,Fused Coupler,Mini Size Fused WDM.