Non-linear control
3.3 DWDM Components
In a DWDM system, DWDM components are classified into two types: multiplexer and de-multiplexer, as shown in Figure 3-14. The main function of the multiplexer is to combine multiple signal wavelengths into a single optical fiber for transmission. The main function of the de-multiplexer is to separate the multiple signal wavelengths transmitted in a single optical fiber. The key to the performance of a DWDM system is DWDM component whose requirements are enough multiplexing channels, low insertion loss, large crosstalk attenuation, wide pass-band, etc. Multiplexer and de-multiplexer are the same in principle and only need to change the input and output directions. The performances of the DWDM components used in DWDM systems should meet the requirements defined in ITU-T G.671 and other related recommendations.
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Figure 3-14 DWDM components
There are many methods to manufacture DWDM components each of which has its own features. At present, there are four types of widespread commercial DWDM components: interference light filter type, optical fiber coupler type, optical grating type and arrayed waveguide grating (AWG) type.
3.3.1 Optical Grating Type DWDM Component
Optical grating type DWDM component, a kind of angular dispersion type component, employs the angular dispersion component to separate and combine optical signals of different wavelengths. The most prevalent diffraction grating is made by depositing epoxy resin on a glass substrate and then fabricating grating lines on the epoxy resin to form a so-called reflective-type blazed diffraction grating. When the incident light reaches the optical grating, the optical signals with different wavelengths are reflected in different angles due to the angular dispersion function of the grating. Then these signals are converged to different output optical fibers via lenses in order to implement wavelength selection function. The inverse process is also right, as shown in Figure 3-15. The advantage of the blazed diffraction grating is high-resolution wavelength selection function which can
WDM 1 2 n
1 2
n
(a) Multiplexer
1 2 n WDM
1 2
n
(b) De-multiplexer
Figure 3-15 Principle of blazed optical grating type DWDM component
The blazed grating type filter has excellent wavelength selectivity and can reduce the wavelength spacing to about 0.5nm. Moreover, the grating type component is parallel operated and its insertion loss doesn't increase with the number of wavelengths multiplexed. Hence large number of multiplexing channels can be achieved. At present, multiplexing of 131 wavelengths with a spacing of 0.5nm is implemented and the isolation is good. For a wavelength spacing of 1nm, the isolation is up to 5dB. The disadvantage of blazed grating is relatively large insertion loss, generally 3~8dB. Moreover, it is very sensitive to polarization and its optical channel bandwidth-to-spacing ratio isn't ideal. So the optical spectrum utilization ratio isn't high enough. And the wavelength fault-tolerance requirement for the laser and DWDM component is relatively high. Additionally, its temperature drift varies with the thermal expansion coefficient and refractive index of the material. Typically, the component temperature shift is relatively high, approximately 0.012nm / . If temperature control measures are adopted, the temperature shift can be reduced to 0.0004nm / . So temperature control measures are feasible and necessary.
This optical grating requires high manufacturing accuracy and is not suitable for mass production. Hence, it is generally applied in experimental scientific research.
Except for the above-mentioned optical fiber component, the manufacturing technology for optical fiber Bragg grating filter is gradually maturing. It is manufactured employing the interference of high power ultraviolet light beams to form periodic variation of refractive index at the optical fiber core. The accuracy can be up to 10000 lines per centimeter, as shown in Figure 3-16. Fiber Bragg grating can be feasibly designed and manufactured with low cost. It has very low
1 2 3 4 5 n output (in)
1 2 n input (out) Diffraction
grating
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insertion loss and stable temperature characteristic. Its intraband filtering characteristic is flat and out-of-band is very steep ( rolling slope is better than 150dB/nm and out-of-band suppress ratio is up to 50dB). This component can be directly melted with the optical fiber of the system. So it can be fabricated into band-pass or band-stop filter with small channel spacing. At present, it is extensively applied in DWDM system. However this kind of optical fiber grating has relatively narrow wavelength range, only applicable to single wavelength. The benefit it brings in is that the filters can be added or removed according to the number of wavelength used. So the application is flexible.
Figure 3-16 Optical fiber Bragg grating filter
3.3.2 Dielectric Film Type DWDM Component
Dielectric film filter type DWDM component is a kind of interactive DWDM component consists of dielectric films (DTF). DTF interference filter is composed of tens of dielectric films of different material, different refractive index and different thickness combined according to design requirements. Each layer is 1/4 wavelength in thickness. Layers of high refractive index and low refractive index are alternatively overlapped. When the light incidents on the high refractive layer, the reflected light has no phase shift. However, when the light incidents on the low refractive layer, the reflected light undergoes a 1800 phase shift. Since the layer thickness is 1/4 wavelength (900), the light reflected by the low refractive layer
Periodic variation of the refractive index (grating) Ultraviole light interference
1 2 3 2
interference type filter can be made to band pass certain wavelength range and band stop the other wavelength range, forming the required filter characteristics.
The structural principle of the film interference type filter is shown in Figure 3-17.
The main features of dielectric film filter DWDM component are as follows:
miniaturization and structural stability of the component can be implemented via design, the signal pass-band is flat and polarization-independent, and its insertion loss is low and channel isolation is good. The disadvantage is that the number of channels can't be large. The specific characteristics are related to its structure. For instance, if the film filter type DWDM component utilizes soft material, its wavelength may be changed under the environmental influence because the filter can easily absorb moisture. When employing hard dielectric film material, the temperature stability is better than 0.0005nm/ . Additionally, this kind of component has relatively long design and manufacturing process and low volume of production. And if epoxy resin is used along the light path, it is not easy to achieve high isolation and narrow bandwidth.
In DWDM systems, when only 4 to 16 wavelengths are involved, this type of DWDM component is relatively ideal.
Figure 3-17 Principle of film interference filter type de-multiplexer
3.3.3 Fused Conical Type DWDM Component
There are two types of optical fiber coupler. The extensively used one is fused biconical tapered coupler, i.e. drawing multiple fibers under hot-melt condition to
1-4
4 2
3 Self-focusing lens
1 filter
3 filter
Glass
1
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form a cone and slightly twisting them to fuse them together. Because the cores of different fibers are extremely close, the required coupling power can be obtained via evanescent wave coupling on the conical region. The second type of coupler employs grinding and polishing methods to remove part of cladding of the optical fiber so that only a thin cladding layer is left. Then two optical fibers processed via the same method are butt jointed and coated a layer of index matched solution between them. Thus the two fibers can couple via the evanescent wave in the cladding and obtain the demanded coupling power. Fused conical type DWDM component is simple to manufacture and is extensively applied.
3.3.4 Integrated Optical Waveguide Type DWDM Component
Integrated optical waveguide type DWDM component is a plane waveguide component based on optical integration technology. The typical manufacturing process is to deposit a thin layer of silica glass on the silicon substrate, form the demanded pattern by utilizing photetch and etch. This component supports integration manufacture and has great application prospective in future access networks. Moreover, except for DWDM component, it can be fabricated into matrix structure to add/drop optical signal channels (OADM). This is a preferred scheme for implementing optical switching in future optical transport networks.
A typical component which uses integrated optical waveguide DWDM is arrayed waveguide grating (AWG) optical multiplexer/de-multiplexer manufactured by NTT Company, Japan. It has many advantages, including small wavelength spacing, large number of channels and flat pass-band. So it is especially suitable for ultrahigh-speed and large capacity DWDM systems. Its structural diagram is shown in Figure 3-18.
1 2
Waveguide grating
Free space
Figure 3-18 Principle of AWG DWDM component
3.3.5 Performances of DWDM Components
Table 3-1 Comparison of various DWDM components Componen
Crosstalk (dB) Insertion loss (dB)
DWDM Networking Section 4 DWDM Networking Design