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Understanding Dwdm

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Understanding DWDM
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The ever growing demand on bandwidth has far exceeded previous expections
Rapidly growing demand for data communication makes it neces. To upgarde
Fundamental change in the requirements for future network structures.
When time comes to expand exsisting links, simple and cost effective solutions
Mainly two ways to upgrade: to install new fibers or to use WDM technology
Installing new fibers is very time -consuming and expensive due to the high costs of laying fibers
With data rates of 2.5 Gib/s and short fiber runs, the physical properties of the fiber can be neglected (more or less). At higher data rates and longer paths the chromatic dispersion becomes the limiting factor.
( e.g: A 4-fold increase in the bit rate requires a 16-fold decrease in the optical path length due to spurios puls expansion.)
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Traffic - The Future!
slide gives an overview of data traffic in the future.
Enormous increase of internet applications till 2002
hardly increase of voice traffic.
A linear rise of data traffic
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The Optical Layer as the new mux fabric
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shows the development of data rates over the last years.
Accelerating increase at Line and network speeds
With TDM only we achieve 10 Gbit/s signals
TDM combined with WDM we achieve up to 1 Tbit/s
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new layer is born: the Optical Layer
WDM multiplexing technology:
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New virtual layer is implemented --> The WDM optical network layer
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Future Broadband Network Layers
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Geographical Distribution of
DWDM Technology 1996/1997
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slide shows the geograpical distribution of DWDM technology in the year 96/97.
Nearly 75% of the market in the USA
Only 18% in Europe
8%
ROW
0%
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Geographical Distribution of
DWDM Technology 2001/2002
USA
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4%
Europe
25%
14%
ROW
7%
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1310 nm/1550 nm TDM point-to-point connections
with opto-electrical regenerators
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In recent years, fiber optics has developed almost exclusively towards higher bit rates, with each fiber transporting a signal from a single laser. Wavelengths of 1310 nm and 1550 nm are used in tele-communications applications, corresponding to the second and third optical window.
The slide shows a typical point to point connection with a electrical regeneration unit
certain limits of TDM:
tor transmission systems of more than 10 Gbit/s, the requirements to the electronics are extremely demanding and the required measurement technique is quite expensive.
Therefore starting from 2.5 Gbit/s combinations of TDM and WDM are used.
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Classic WDM point-to-point connection
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Transmission of multiple signals at different wavelengths (WDM = Wavelength Division Multiplexing) was exploited(ausgenutzt) in a few isolated cases, but there was no major breakthrough. As a general rule, the signals were in different optical windows (broadband-WDM). One reason for this is that each time signals are regenerated, they must be separated with optical filters prior to processing in separate repeaters and retransmitted at the proper wavelength with separate laser diodes. As a result, few systems currently use this technique to double transmission capacity.
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TDM point-to-point connections
OFA basics
Structure and properties of optical fiber amplifiers
An optical fiber amplifier (OFA) is essentially a piece of erbium-doped optical fiber. Radiation outside the data wavelength range from a powerful pump laser is coupled into this fiber, in addition to the signals being transmitted.
With today's in-line amplifiers, data signals in a wavelength range from 1530 nm to 1565 nm can be amplified approx. 20 to 30 dB in order to compensate for fiber loss on paths around 100 km. On a transmission link fitted with fiber amplifiers instead of conventional repeaters, it is possible to increase the transmission capacity by adding additional wavelengths without spending a lot of money.
Besides amplifying the data signals, spontaneous emission of photons also occurs in fiber amplifiers, and these photons are amplified as well. The resultant spurious signal, known as an amplified spontaneous emission (ASE), is rather large in magnitude, having a power of several milliwatts. Its spectral distribution clearly exhibits the wavelength dependency of optical amplification. The overall signal at the output of a WDM system thus comprises the superimposed data signals and the spurious spectrum of the ASE.
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Transmission of multiple channels using WDM systems
with 8, 16, 32, ... channels (multiplexing of 2.5 Gbit/s signals)
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Basics of optical DWDM systems
At the termination, we feed four 2.5 Gbit/s signals to four optical transmission modules. The optical output signals are converted if necessary to defined wavelengths in the 1550 nm window using wavelength transponders. This makes it possible to use existing standard transmission modules with wavelengths in the 1310 nm or 1550 nm band. Using an optical WDM coupler, the four optical signals are bunched together and forwarded to an optical fiber amplifier (OFA). Depending on the path length, one or more fiber amplifiers boost the optical signal, which is attenuated due to the fiber loss. In many cases, a booster is also used after the WDM coupler. At the termination on the receiving end, it is common to preamplify the optical signals and then separate them using optical fibers and convert them to electrical signals in the receiver modules. This entire arrangement must be duplicated in the opposite direction to carry the signals in that direction.
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DWDM test parameters
to ensure even power distribution over the entire bandwidth
to notice immediately whether any channels have dropped out
Channel wavelength / Channel spacing
Signal-to-noise ratio
to ensure that error-free transmission is possible in each data channel
Overall power
Crosstalk
to provide a quality indication for the system components (couplers, ...)
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DWDM spectrum and corresponding test parameters
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- Wave: Channel wavelength, 0.1 nm resolution
- Power: Channel level in dBm
- Power stability: Power fluctuations in the channel. Difference between minimum and maximum power level occurring during measurement interval.
- S/N ratio: Signal-to-noise ratio for channel. Difference between channel power level and noise (within specified scan bandwidth)
- S/N ratio stability: Fluctuations in the S/N ratio
Reading out results
All power level and S/N results for the WDM channels can be read out under remote control and further processed. The stability is represented using the Max/Min values. The transmitted results are given as floating point values separated by commas.
Storing and managing results
Measurement results can be stored with their setup parameters and recalled for later use. Six memory locations are provided. The table values can also be checked with a View function.
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Channel center wavelength and allowable deviation for a 100 GHz channel spacing
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As specified in ITU-T Rec. G.692, all data channels in a DWDMsystem must fall in a specified channel grid. The WDMSystem mode is capable of generating all of the ITU-T channel assignments. Based on a user-selectable reference wavelength or frequency, the channels to be analyzed can be adapted to any DWDM system using the appropriate channel spacing. Channel center wavelength and allowable deviation for a 100 GHz channel spacing
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DWDM measurements with an optical spectrum analyzer
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As transmission quality expectations grow, network equipment manufacturers and carriers are experiencing an increasing need for future-proof techniques for quality monitoring. In today's SDH/ SONET networks, transmission quality is monitored by computing the bit interleaved parity (BIP) for a data block of fixed size. The BIP is transmitted with the signal via an overhead channel and compared with the value computed from the transmitted data. This is a means of monitoring each regenerator section, entire multiplex sections and complete SDH/SONET paths. This same technique is used in ATM with suitable modifications. In a purely optical DWDM network, bits are not defined due to the transparency with respect to different transmission technologies. This makes the above described technique not usable. However, even with limited network transparency, some kind of marker is needed to designate the start of a data block. Such a marker must be derived from the actual bit stream, e.g. the start of a frame or a cell, thus requiring an awareness of the transmission format. Without any additional means, this technique is incompatible with any type of purely optical network.
S/N measurements
Since optical networks are analog crea- tures, the signal-to-noise (S/N) ratio is an obvious choice when it comes to assessing the transmission quality. However, the S/N is a necessary quality criterion but not a sufficient one. On the one hand, there is no clear relationship between the S/N ratio and bit error ratio (BER). On the other hand, in an all optical network (AON), only the optical S/N ratio can be determined and not the S/N ratio at the decider input since the latter is a function of the properties of the receiver and is not clearly related to the optical S/N ratio. Moreover, the S/N ratio alone does not reflect any of the dispersion-related delay distortion, which is the limiting effect at very high speeds. Taking into account the statically measured dispersion does not offer any significant improvement since the polarization mode dispersion fluctuates over time.
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OFA monitor output: 8 data signals and ASE spurious spectrum
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DWDM test equipment for field applications
In terms of test equipment, such a system has very different needs than existing single-channel systems. To operate single-channel systems, the exact wavelength is less important, and we can just measure the power of the single signal with a broadband power meter.
The new and fundamental requirement for fiber test equipment used to install and maintain DWDM systems is to characterize components and network elements as a function of wavelength. If we measure using a conventional power meter, we obtain the sum of the power for all data signals plus the ASE. If one data carrier drops out, the noise power and the power of the other carriers increase. Accordingly, such a measurement does not reliably indicate whether one carrier has significantly dropped in power or fallen out entirely.
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DWDM system with the recommended reference test points
from ITU-T Rec. G.692
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In order to plan and implement flexibleand future-proof DWDM systems and components, basic standards are necessary. This is the only way to ensure that highly variable components and modules from different manufacturers comply with defined interface parameters and interact properly. Since current recommendations are limited to 4- to 8-channel systems, a variety of possible combinations are imaginable for the system wavelengths in use. Accordingly, the individual manufacturers must agree on specific focus wavelengths in order to ensure problem-free interplay of the various system elements. It is common to divide the wavelength range between 1530 and 1565 nm into two bands. The band below 1545 nm is known as the short band and above 1545 nm is the long band.
System reference points
According to ITU-T Rec. G.692, reference test points are to be provided in DWDM systems:
- S1 to Sn are reference points directly at the output of the individual optical transmitters 1 to n of the DWDM system.
- RM1 to RMn are reference points for the individual fibers directly before the input of the WDM multiplexer. S' is the test point directly at the output of the WDM multiplexer and R' a further test point directly at the input of the demultiplexer.
- SD1 to SDn are the corresponding reference points directly at the output of the demultiplexer and R1 to Rn are the reference points at the input of the individual receiver modules of the DWDM system.
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100 GHz channel grid in the range 1530 to 1565 nm from ITU-T Rec. G.692
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Current Standards
ITU-T Recommendation G.692 (ªOptical Interfaces for Multichannel Systems withOptical Amplifiersº) defines interface parameters for optical DWDM systems with 4 and 8 data channels at bit rates up to the OC-48/STM-16 standard and path lengths between fiber amplifiers of 80, 120, 160 km with regenerator spacings up to 640 km. The revised Rec. G.692 will also define 16- and 32-channel systems up to the
OC-1 92/STM-64 standard as well as bidirectional systems. According to ITU-T Rec. G.692, all data channels in a DWDMsystem should fall in a specified 100 GHz channel grid. The ITU-T grid is based on a reference center frequency of 193.1 THz, which corresponds to an optical wavelength of 1552.52 nm. Conversion between center frequency f and focus wavelength l is based on the relation c = l6f using the exact value for the speed of light c = 299792458 m/s.
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Division of transmission links into three categories
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The individual transmission links are divided into three main categories according to length and are labelled L, V and U:
- L: Long haul
- V: Very long haul
- U: Ultra long haul
Whereas ultra long haul links do not have in-line amplifiers, 640 km can be bridged in the long haul type with up to 7 in-line amplifiers and 600 km with up to 4 in-line amplifiers in the very long haul type.
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Possible system wavelengths in the ITU-T 100 GHz channel grid
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Future standards
So far, ITU-T Rec. G.692 specifies a100 GHz channel grid in the 1550 nm range. Smaller channel spacings are theoretically possible. It remains to be seen whether smaller channel spacings will become widespread. It is certain that the currently standardized frequencies will be retained, but they might be supplemented with additional frequencies. In April 1997, the ITU-T began standardization work on all optical networks (AON). A group of working topics and problem areas (ªliving listº) was compiled. However, results are not expected prior to 1999, especially for the difficult areas of the physical layer and the components & subsystems.
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Optical add/drop multiplexing principleand monitoring of system parameters
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Telecommunications systems of the future will be based on photonic networks in which the extraordinarily wide bandwidth of the optical fibers is exploited with high- performance wavelength multiplex equipment (DWDM). In these networks, messages will be transmitted exclusively in the optical spectrum. If messages are transmitted optically and also switched optically, we speak of an optically transparent photonic network. Optically transparent networks are non-service-dependent and form the basis for powerful data highways. The optical transparency of a message network is referred to the individual transmission channel which can be used between the network terminations with any bit rate and any modulation format. This channel does not cross any opto-electrical interfaces, the signal remains in the optical range when changing to other optical carrier frequencies, and the signal content is not regenerated. To some extent, the optical transparency of a channel is generally understood as invariance with respect to bit rate, code, transmission mode and protocol if the signal is transferred as an amplitude-keyed carrier. This is conventional, particularly at high bit rates. Here as well, the channel does not cross any opto-electrical interfaces and is not regenerated, but there can be frequency converters in the signal path that are only partially modulation-transparent. We can arrange several channels of this sort next to one another on the frequency axis to obtain a DWDM system in which each channel has a different optical frequency or wavelength. The transmission bandwidth of the fiber is thus more fully exploited.
Add/drop multiplexers in DWDM systems
For reasons of network configuration, communications systems with high channel capacities should offer facilities for adding and dropping subbands (add/ drop function) at various points along a path. Since not all of the channels have the same source and destination site, this is the only way to operate networks in a cost-effective manner in many cases. In this aspect, systems based on WDM technology are nearly ideal if individual parallel subbundles are easily added and dropped.
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Optical add/drop multiplexing principle and monitoring of system parameters
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Analysis of the system parameters
When commissioning complex systems and when monitoring them, we have to make sure that the optical add/drop multiplexers work properly. A listing of all data channels present in the system along with the major parameters such as optical wavelength, signal-to- noise (S/N) ratio and their stability vs. Time simplifies the test work. When it comes to qualifying a system, more in-depth bit error analysis is required. Since bit error measurements are directly possible due to the multichannel transmission technique used in DWDM systems, selective prefiltering of the data channels of interest is necessary.
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Optical switching in photonic networks
using optical cross-connects
Basic structure of photonic networks
A photonic network is divided into a regional level with moderate data rates and the long- distance level in which the signal carriers are transported at high data rates over long distances. A gateway lies at the interface between the two areas. Here, multiple low-speed DWDM chan-nels are linked to a high-speed long-distance channel. Optically transparent linkage of the data streams occurs in the regional network mainly through wavelength routing and in the long-distance network using cross-connects. The fu- ture portends huge long-distance networks in which all major cities are interconnected via multichannel fiber links and cross-connects, all optically transparent. These large long-distance networks themselves are connected to the intercontinental connections.
All optical networks (AON) will be a major part of future communications networks, and their performance will be determined significantly by the use of optical wavelength multiplexing and system components such as optical add/drop multiplexers and cross-connects. One major feature of these photonic networks is the described non-service-dependent transparency length of the individual optical signal paths.
Applications of optical cross-connects
Optical cross-connects can connect signals from any input line to any output line. The structure of a cross-connect thus corresponds almost exactly to that of a switching system. A cross-connect basically comprises a coupling field, a controller and interfaces to the line connections. It differs from a switching system in the way that the switching/connections are handled. In cross-connect systems, the connections are not set up based on signalling information from the connected subscribers or switches. Instead, special control information provided by the network operator to a telecommunication management network (TMN) is used. This allows the network operator to optimally route the user data streams through the network, to switch in spare circuits if links fail and to adapt the routing of the information streams to the current requirements of the traffic.
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Optical cross-connect with OADM
Functional design of cross-connects
Data streams are routed in the nodes of a long-distance optical network by WDM cross-connects. An optical cross-connect contains a switching matrix for selecting the output fibers and transponders for converting to a specific optical carrier frequency. Messages must be switched to specific output fibers and output frequencies. To control and monitor the individual components in the cross-connect, opto-electrical interfaces are used. In the optically transparent cross-connect itself, the data path lies exclusively in the optical range, i.e. it does not cross any opto-electrical interfaces. In the actual switching matrix of a transparent cross-connect, the switches are actuated mechanically, thermo-optically or purely optically. The fiber switch meets the stringent crosstalk requirements but diverges for large coupling fields. Here, integration of theswitching matrices is desirable, e.g. reconfigurable switching matrices withpolymer-based directional coupler switches. The main components of the transponders are optical filters and optical frequency converters. Filters are built to reroute optical carriers froma fiber bundle and suppress spurious mixing products that arise during frequency conversion. Examples include the thermally tunable Mach-Zehnder fiber filter, the fiber ring filter and the grating filter based on fiber technology or planar silicon and polymer integration, not to mention indium phos- phide based designs. The optical frequency converter is designed to convert an input signal from one optical carrier frequency to another carrier frequency. The choice of the new carrier frequency should be as flexible as possible. Examples of frequency converters include dispersion-shifted fibers, injection- coupled lasers and optical semiconductor amplifiers in which the effects of gain compression or four-wave mixing are used for frequency conversion.
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4-channel DWDM signal with characteristic noise floors
and ASE spurious spectrum
Determining the bit error ratio (BER)
All in all, the decisive parameter in determining the transmission quality of a system is the bit error ratio. The S/N ratio or the signal-to-noise-power ratio is only a rough guide. However, a low S/N ratio is always a direct indication of a system error.
The opposite is not necessarily true, for the reasons mentioned above. To make a qualitative assessment requires a more in-depth check of the actual signal using a state-of-the-art bit error analyzer.
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Superimposition of the actual data signal and accumulated noise
and incorrect interpretation of S/N measurements
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Wavelength-selective bit error measurement
Due to the multichannel transmission technique used in DWDM systems, the actual information, i.e. the individual data channels in the multiplex signal, must first be selected and made available for bit error measurement. Since most modern bit error test sets do not have any wavelength selectivity, a tunable prefilter is always required for the range 1530 to 1565 nm. Moreover, the only way to detect the loss of one or all channels is through wavelength-selective power measurement. However, there is a problem in that a signal loss must be distinguished from a source-dependent low power level (e.g. long sequence of logical zeroes). This problem is heightened by the presence of amplified spontaneous emissions from fiber amplifiers, which can give the appearance of a data signal even when none is present. Moreover, the individual noise components tend to accumulate over the transmission path in case of intermeshed amplifier paths. Noise floors, products of the filter characteristics of all involved WDM components, become superimposed at the locations of the individual carrier signals with the actual data signal and thus corrupt a pure signal-to-noise measurement.
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Quality monitoring in DWDM systems
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Test solutions for quality monitoring
Together, an opticl spectrum analyzer and a bit error test set are ideal for comprehensive quality monitoring on transparent DWDM systems. Conventional spectrum analyzers can precisely determine all of the optical parameters, but do not provide external filter functions for further bit error analysis. The OSP-102A Optical Spectrum Analyzer is fitted with an external monitor output for this application. The internal filter in the OSP-102A can be automatically tuned to the desired wavelength in order to directly evaluate the appropriate channel in a DWDM system using a bit error analyzer (e.g. ANT-20). This is the quick and easy way to do selective error analysis work in a single channel of a multiple carrier system at the bit, frame and alarm levels. Jitter measurements are just as simple.
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Cyclical recording and stability measurement of
Channel-No.
Actual
Add/drop technology
Optical add/drop multiplexers (OADM) usually have two optical interfaces which are used to integrate them into a system. They also have optical interfaces to simple multiplexers, switches or terminations. The OADM can add or drop individual wavelengths between the interfaces. Several OADMs can be interconnected via their two main interfaces to form a fiber ring.
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Spectrum analysis and power leveladjustment
in DWDM systems
wavelength response of
todays EDFAs cascading
DWDM channel power
Spectrum analysis and power level adjustment
The power level balance of a DWDM system is an important issue when it comes to system configuration. In systems with optical fiber amplifiers, the channel-based power level fluctuates if no countermeasures are taken as a function of the current channel usage since the total available power of the amplifier is distributed among the cur- rently used channels. This can significantly restrict the dynamic range of the optical receivers and produce saturation effects in case of channel drop-outs. This makes it necessary to record the signal spectrum at the output of the fiber amplifiers and cross- onnects using a spectrum analyzer and adjust the output levels as necessary. Variable attenuators directly integrated into the optical crossconnect are useful for adjusting the levels. Particularly with cascaded fiber ampliiers, a gain tilt arises if no counter- measures are taken, i.e. the signal spectrum begins to tilt as the number of amplifiers increases. Here as well, selective level measurement and appropriate adjustment of the different channel levels is very important.
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Quality monitoring through wavelength-selective bit error analysis
(automatic evaluation of bit error histograms)
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Gain tilt effect due to cascaded optical fiber amplifiers
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Graphical display of an 8-channel DWDM signal
Analysis over the full
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The OSP-102A Optical Spectrum Analyzer is designed primarily for measurements on modern dense WDMsystems in the wavelength range around 1550 nm. With ist compact size and low weight, the instrument is well suited to on-site installation, maintenance and repair applications. Spectrum and power meter functions In OSP- 102A's ªSpectrum Analysisºmode, a tunable optical filter is swept across the selected wavelength range, the level of the filtered signal is measured and the result is displayed in graphical format. By inserting marker lines, the displayed result can be evaluated in terms of absolute and relative wavelength and power level. A Storage Normalizer function enables a precise comparison of two spectra. In order to display the main parameters for a WDM spectrum quickly and clearly, there is another mode known as ªWDM Systemº. Here, a Scan function determines all of the main parameters from the spectral trace and complies them into a table. Determination of parameters such as the channel wavelength and signal-to-noise (S/N) ratio of a channel is based on an algorithm in which the selectivity curve of the filter is incorporated into the result. The OSP-102A's software enables evaluation of WDM signals with up to 40 channels. In the ªPower Level Measurementº mode, the filter is fixed to one wavelength and the power is displayed numerically for this wavelength. Alternatively, the power at this wavelength can be monitored vs. time using the embedded ªStability Measurementº software, which is very helpful when troubleshooting intermittent drop-outs.
Test procedure
After connecting the instrument and setting the test parameters, sweeping can be controlled with the Run and Holdfunction. A counter records the number of sweep passes. To display test traces, two memories are provided. The main memory is for the current results and the background memory is for saving traces or for comparison measurements (reference trace).
Result evaluation Using the Storage Normalizer function, the difference between two traces can be displayed on the screen. The result is always visible about the zero line. This enables highly accurate evaluation of very small deviations from a specific de- sired value. Using the OSP-102A's markers, the power level and wavelength properties of traces are easy to evaluate. Vertical marker lines are displayed in the graphics window and can be shifted as desired using the rotary control. In a marker display above the graphics window, the power level and wavelength values can be read off with a resolution of 0.01 dB or 0.01 nm. The absolute wavelength marker is he main marker and appears as a continuous line. The wavelength value is displayed as an absolute value, like the power level, which results from the intersection with the test trace. The relative wavelength marker is an auxiliary marker and appears as a dashed line in addition to the absolute marker. The wavelength difference with respect to he absolute marker is displayed, as is the power level difference resulting from the intersections with the test trace, in dB. The Peak Search function makes settings easier if only the wavelength at the maximum trace level is of interest. A keypress sends the absolute marker directly to the maximum trace level and the focus wavelength is displayed.
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New portable DWDM System Analyzer for 50 GHz/0.4 nm and higher spaced optical DWDM systems with up to 256 channels
DWDM System Analyzer
OSA-155
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OSA-155
WDM-channel-selector
for