The implementation of WDM technology for fiber-optic communication systems requires several new optical components. Among them are multiplexers that combine the output of several transmitters and launch it into an optical fiber; demultiplexers which split the received multichannel signal into individual channels destined to different receivers; star couplers that mix the output of several transmitters and broadcast the mixed signal to multiple receivers; tunable optical filters which filter out one channel at a specific wavelength that can be changed by tuning the passband of the optical fiber; multiwavelength optical transmitters whose wavelength can be tuned over a few nanometers; add-drop multiplexers and optical routers that can distribute a WDM signal to different ports.
1. Tunable Optical Filters
It is instructive to consider optical filters first since they are often the building blocks of more complex WDM components. The role of a tunable optical filter in a WDM system is to select a desired channel at the receiver. The following figure shows the selection mechanism schematically. The filter bandwidth must be large enough to transmit the desired channel but, at the same time, small enough to block the neighboring channels.
All optical filters require a wavelength-selective mechanism and can be classified into two broad categories depending on whether optical interference or diffraction is the underlying physical mechanism. Each category can be further subdivided according to the scheme adopted. In this section we consider four kinds of optical filters; The following figure shows an example of each kind.
The desirable properties of a tunable optical filter include: (1) wide tuning range to maximize the number of channels that can be selected, (2) negligible crosstalk to avoid interference from adjacent channels, (3) fast tuning speed to minimize the access time, (4) small insertion loss, (5) polarization insensitivity, (6) stability against environmental changes (humidity, temperature, vibrations, etc.), and (7) last but not least, low cost.
A Fabry-Perot (FP) interferometer - a cavity formed by using two mirrors - can act as a tunable optical filter if its length is controlled electronically by using a piezoelectric transducer [see figure (a) above]. The transmissivity of a FP filter peaks at wavelengths that correspond to the longitudinal-mode frequencies. Hence, the frequency spacing between two successive transmission peaks, known as the free spectral range, is given by
ΔνL = c / (2ngL)
where ng is the group index of the intracavity material for a FP filter of length L.
If the filter is designed to pass a single channel, the combined bandwidth of the multichannel signal, Δνsig = NΔνch = NB/ηs, must be less than ΔνL, where N is the number of channels, ηs is the spectral efficiency, and B is the bit rate. At the same time, the filter bandwidth ΔνFP (the width of the transmission peak) should be large enough to pass the entire frequency contents of the selected channel. Typically, ΔνFP ~ B. The number of channels is thus limited by
N < ηs(ΔνL/ΔνFP) = ηsF
where F = ΔνL/ΔνFP is the finesse of the FP filter. The concept of finesse is well known in the theory of FP interferometers. If internal losses are neglected, the finesse is given by
F is determined solely by the mirror reflectivity R, assumed to be the same for both mirrors.
The equation of N above provides a remarkably simple condition for the number of channels that a FP filter can resolve. As an example, if ηs = 1/3, a FP filter with 99% reflecting mirrors can select up to 104 channels. Channel selection is made by changing the filter length L electronically. The length needs to be changed by only a fraction of the wavelength to tune the filter. The filter length L itself is determined from ΔνL = c / (2ngL) together with the condition ΔνL > Δνsig. As an example, for a 10-channel WDM signal with 0.8-nm channel spacing, Δνsig ≈ 1 THz. If ng = 1.5 is used for the group index, L should be smaller than 100 μm. Such a short length together with the requirement of high mirror reflectivities underscores the complexity of the design of FP filters for WDM applications.
A practical all-fiber design of FP filters uses the air gap between the two optical fibers [see figure (a) above]. The two fiber ends forming the gap are coated to act as high-reflectivity mirrors. The entire structure is enclosed in a piezoelectric chamber so that the gap length can be changed electronically for tuning and selecting a specific channel. The advantage of fiber FP filter is that they can be integrated within the system without incurring coupling losses. Such filters were used in commercial WDM fiber links starting in 1996. The number of channels is typically limited to below 100 (F ≈ 155 for the 98% mirror reflectivity) but can be increased using two FP filters in tandem. Although tuning is relatively slow because of the mechanical nature of the tuning mechanism, it suffices for some applications.
Tunable FP filters can also be made using several other materials such as liquid crystals and semiconductor waveguides. Liquid-crystal-based filters make sue of the anisotropic nature of liquid crystals that makes it possible to change the refractive index electronically. A FP cavity is still formed by enclosing the liquid crystal material within two high-reflectivity mirrors, but the tuning is done by changing the refractive index rather than the cavity length. Such FP filters can provide high finesse (F ~ 300) with a bandwidth of about 0.2 nm. They can be tuned electrically over 50 nm, but switching time is typically ~ 1 ms or more when nematic liquid crystals are used. It can be reduced to below 10 μs by using smectic liquid crystals.
Thin dielectric films are commonly used for making narrow-band interference filters. The basic idea is quite simple. A stack of suitably designed thin films acts as a high-reflectivity mirror. If two such mirrors are separated by a spacer dielectric layer, a FP cavity is formed that acts as an optical filter. The bandpass response can be tailored for a multicavity filter formed by using multiple thin-film mirrors separated by several spacer layers. Tuning can be realized in several different ways. In one approach, an InGaAsP/InP waveguide permits electronic tuning. Silicon-based FP filters can be tuning using thermo-optic tuning. Micromechanical tuning has also been used for InAlGaAs-based FP filters. Such filters exhibited a tuning range of 40 nm with < 0.35 nm bandwidth in the 1.55-μm region.
A chain of Mach-Zehnder (MZ) interferometers can also be used for making a tunable optical filter. A MZ interferometer can be constructed simply by connecting the two output ports of a 3-dB coupler to the two input ports of another 3-dB coupler [see figure (b) above]. The first coupler splits the input signal equally into two parts, which acquire different phase shifts (if the arm lengths are made different) before they interfere at the second coupler. Since the relative phase shift is wavelength dependent, the transmittivity T(ν) is also wavelength dependent. In fact, we find that T(ν) = |H(ν)|2 = cos2(πντ), where ν = ω/2π is the frequency and τ is the relative delay in the two arms of the MZ interferometer. A cascaded chain of such MZ interferometers with relative delays adjusted suitably acts as an optical filter that can be tuned by changing the arm length slightly. Mathematically, the transmittivity of a chain of M such interferometers is given by
where τm is the relative delay in the mth member of the chain.
A commonly used method implements the relative delays τm such that each MZ stage blocks the alternate channels successively. This scheme requires τm = (2mΔνch)-1 for a channel spacing of Δνch. The resulting transmittivity of a 10-stage MZ chain has channel selectivity as good as that offered by a FP filter having a finesse of 1600. Moreover, such a filter is capable of selecting closely spaced channels. The MZ chain can be built by using fiber couplers or by using silica waveguides on a silicon substrate. The silica-on-silicon technology was exploited extensively during the 1990s to make many WDM components. Such devices are referred to as planar lightwave circuits (PLC) because they use planar optical waveguides formed on a silicon substrate. Tuning in MZ filters is realized through a chromium heater deposited on one arm of each MZ interferometer. Since the tuning mechanism is thermal, its use results in a slow response with a switching time of about 1 ms.
A separate class of tunable optical filters makes use of the wavelength selectivity provided by a Bragg grating. Fiber Bragg gratings provide a simple example of grating-based optical filters. In its simplest form, a fiber grating acts as a reflection filter whose central wavelength can be controlled by changing the grating period, and whose bandwidth can be tailored by changing the grating strength or by chirping the grating period slightly. The reflective nature of fiber gratings is often a a limitation in practice and requires the use of an optical circulator. A phase shift in the middle of the grating can convert a fiber grating into a narrowband transmission filter. Many other schemes can be used to make transmission filters based on fiber gratings. In one approach, fiber gratings are used as mirrors of a FP filter, resulting in transmission filters whose free spectral range can vary over a wide range 0.1-10 nm. In another design, a grating is inserted in each arm of a MZ interferometer to provide a transmission filter. Other kinds of interferometers, such as the Sagnac and Michelson interferometers, can also be used to realize transmission filters. Figure (c) above shows an example of the Michelson interferometer made by using a 3-dB fiber coupler and two fiber gratings acting as mirrors for the two arms of the Michelson interferometer. Most of these schemes can also be implemented in the form of a planar lightwave circuit by forming silica waveguides on a silicon substrate.
Many other grating-based filters have been developed for WDM systems. In one scheme, borrowed from the DFB-laser technology, the InGaAsP/InP material system is used to form planar waveguides functioning near 1.55 μm. The wavelength selectivity is provided by a built-in grating whose Bragg wavelength is tuned electrically through electrorefraction. A phase-control section, similar to that used for multisegment DFB lasers, have also been used to tune distributed Bragg reflector (DBR) filters. Multiple gratings, each tunable independently, can also be used to make tunable filters. Such filters can be tuned quickly (in a few nanoseconds) and can be designed to provide net gain since one or more amplifiers can be integrated with the filter. They can also be integrated with the receiver, as they use the same semiconductor material. These two properties of InGaAsP/InP filters make them quite attractive for WDM applications.
The spectral response of a fiber grating can be tuned either by heating or by compressing the grating so that either the effective mode index or the physical grating period changes in a prescribed manner. Tuning over a range of 40 nm was realized in 2002 by using the compression technique. Another problem with grating filters is that they do not exhibit periodic filtering characteristics because a grating has a single stop band centered at the Bragg wavelength. This property can be changed by making a superstructure or sampled grating. Such gratings contain multiple subgratings, separated by sections with a uniform refractive index, and are called superstructure gratings because of their doubly periodic nature. They are discussed in a later tutorial in the context of dispersion compensation.
In another class of tunable filters, the grating is formed dynamically by using acoustic waves. Such filters, called acousto-optic filters, exhibit a wide tuning range (> 100 nm) and are quite suitable for WDM applications. The physical mechanism behind the operation of acousto-optic filters is the photoelastic effect through which an acoustic wave propagating through an acousto-optic material creates periodic changes in the refractive index (corresponding to the regions of local compression and rarefaction). In effect, the acoustic wave creates a periodic index grating that can diffract an optical beam. Wavelength selectivity stems from this acoustically induced grating. When a transverse electric (TE) wave with the propagation vector k is diffracted from this grating, its polarization can be changed from TE to transverse magnetic (TM) if the phase-matching condition k' = k ± Ka is satisfied, where k' and Ka are the wave vectors associated with the TM and acoustic waves, respectively.
Acousto-optic tunable filters can be made by using bulk components as well as waveguides, and both kinds are available commercially. For WDM applications, the LiNbO3 waveguide technology is often used since it can produce compact, polarization-independent, acousto-optical filters with a bandwidth of about 1 nm and a tuning range over 100 nm. The basic design, shown schematically in figure (d) above, uses two polarization beam splitters, two LiNbO3 waveguide, a surface-acoustic-wave transducer, all integrated on the same substrate. The incident WDM signal is split into its orthogonally polarized components by the first beam splitter. The channel whose wavelength λ satisfies the Bragg condition λ = (Δn)Λa is directed to a different output port by the second beam splitter because of an acoustically induced change in its polarization direction; all other channels go to the other port. The TE-TM index difference Δn is about 0.07 in LiNbO3. Near λ = 1.55 μm, the acoustic wavelength Λa should be about 22 μm. This value corresponds to a frequency of about 170 MHz if we use the acoustic velocity of 3.75 km/s for LiNbO3. Such a frquency can be easily applied. Moreover, its exact value can be changed electronically to change the wavelength that satisfies the Bragg condition. Tuning is relatively fast because of its electronic nature and can be accomplished in a switching time of less than 10 μs. Acousto-optic tunable filters are also suitable for wavelength routing and optical cross-connect applications in dense WDM systems.
Another category of tunable optical filters operates on the principle of amplification of a selected channel. Any amplifier with a gain bandwidth smaller than the channel spacing can e used as an optical filter. Tuning is realized by changing the wavelength at which the gain peak occurs. Stimulated Brillouin scatting (SBS), occurring naturally in silica fibers, can be used for selective amplification of one channel, but the gain bandwidth is quite small (< 100 MHz). The SBS phenomenon involves interaction between the optical and acoustic waves and is governed by a phase-matching condition similar to that found for acousto-optic filters. SBS occurs only in the backward direction and results in a frequency ship of about 10 GHz in the 1.55-μm region.
To use the SBS amplification as a tunable optical filter, a continuous-wave (CW) pump beam is launched at the receiver end of the optical fiber in a direction opposite to that of the multichannel signal, and the pump wavelength is tuned to select the channel. The pump beam transfers a part of its energy to a channel down-shifted from the pump frequency by exactly the Brillouin shift. A tunable pump laser is a prerequisite for this scheme. The bit rate of each channel is even then limited to 100 MHz or so. In a 1989 experiment in which a 128-channel WDM network was simulated by using two 8 x 8 star couplers, a 150-Mb/s channel could be selected with a channel spacing as small as 1.5 GHz.
Semiconductor optical amplifiers (SOAs) can also be used for channel selection provided that a DFB structure is used to narrow the gain bandwidth. A built-in grating can easily provide a filter bandwidth below 1 nm. Tuning is achieved using a phase-control section in combination with a shift of Bragg wavelength through electrorefraction. In fact, such amplifiers are nothing but multisection semiconductor lasers with antireflection coatings. In one experimental demonstration, two channels operating at 1 Gb/s and separated by 0.23 nm could be separated by selective amplification (> 10 dB) of one channel. Four-wave mixing in an SOA can also be used to form a tunable filter whose center wavelength is determined by the pump laser.
2. Multiplexers and Demultiplexers
Multiplexers and demultiplexers are the essential components of any WDM system. Similar to the case of optical filters, demultiplexers require a wavelength-selective mechanism and can be classified into two broad categories. Diffraction-based demultiplexers employ an angularly dispersive element, such as a diffraction grating, which disperses incident light spatially into various wavelength components. Interference-based demultiplexers make use of devices such as optical filters and directional couplers. In both cases, the same device can be used as a multiplexer or a demultiplexer, depending on the direction of propagation, because of the inherent reciprocity of optical waves in dielectric media.
Grating-based demultiplexers use the phenomenon of Bragg diffraction from an optical grating. The following figure shows the design of two such demultiplexers.
The input WDM signal is focused onto a reflection grating, which separates various wavelength components spatially, and a lens focuses them onto individual fibers. Use of a graded-index lens simplifies alignment and provides a relatively compact device. The focusing lens can be eliminated altogether by using a concave grating. For a compact design, the concave grating can be integrated within a silicon slab waveguide. In a different approach, multiple elliptical Bragg gratings are etched using the silicon technology. The idea behind this approach is simple. If the input and output fibers are placed at the two foci of the elliptical grating, and the grating period Λ is adjusted to a specific wavelength λ0 by using the Bragg condition 2Λneff = λ0, where neff is the effective index of the waveguide mode, the grating would selectively reflect that wavelength and focus it onto the output fiber. Multiple gratings need to be etched, as each grating reflects only one wavelength. Because of the complexity of such a device, a single concave grating etched directly onto a silica waveguide is more practical. Such a grating can be designed to demultiplex up to 120 channels with a wavelength spacing of 0.3 nm.
A problem with grating demultiplexers is that their bandpass characteristics depend on the dimensions of the input and output fibers. In particular, the core size of output fibers must be large to ensure a flat passband and low insertion losses. For this reason, most early designs of multiplexers used multimode fibers. In a 1991 design, a microlens array was used to solve this problem and to demonstrate a 32-channel multiplexer for single-mode fiber applications. The fiber array was produced by fixing single-mode fibers in V-shaped grooves etched into a silicon wafer. The microlens transforms the relatively small mode diameter of fibers (~ 10 μm) into a much wider diameter (about 80 μm) just beyond the lens. This scheme provides a multiplexer that can work with channels spaced by only 1 nm in the wavelength region near 1.55 μm while accommodating a channel bandwidth of 0.7 nm.
Filter-based demultiplexers use the phenomenon of optical interference to select the wavelength. Demultiplexers based on the MZ filter have attracted the most attention. Similar to the case of a tunable optical filter, several MZ interferometers are combined to form a WDM demultiplexer. A 128-channel multiplexer fabricated with the silica-waveguide technology was fabricated by 1989. The following figure illustrates the basic concept by showing the layout of a four-channel multiplexer. It consists of three MZ interferometers. One arm of each MZ interferometer is made longer than the other to provide a wavelength-dependent phase shift between the two arms. The path-length difference is chosen such that the total input power from two input ports at different wavelengths appears at only one output port. The whole structure can be fabricated on a silicone substrate using SiO2 waveguides in the form of a planar lightwave circuit.
Fiber Bragg gratings can also be used for making all-fiber demultiplexers. In one approach, a 1 x N fiber coupler is converted into a demultiplexer by forming a phase-shifted grating at the end of each output port, opening a narrowband transmission window (~ 0.1 nm) within the stop band. The position of this window is varied by changing the amount of phase shift so that each arm of the 1 x N fiber coupler transmits only one channel. The fiber-grating technology can be applied to form Bragg gratings directly on a planar silica waveguide. This approach has attracted attention since it permits integration of Bragg gratings within planar lightwave circuits. Such gratings were incorporated in an asymmetric MZ interferometer (unequal arm lengths) resulting in a compact multiplexer.
It is possible to construct multiplexers by using multiple directional couplers. The basic scheme is similar to that shown in the figure above but simpler since MZ interferometers are not used. Furthermore, an all-fiber multiplexer made by using fiber couplers avoids coupling losses that occur whenever light is coupled into or out of an optical fiber. A fused biconical taper can also be used for making fiber couplers. Multiplexers based on fiber couplers can be used only when channel spacing is relatively large (> 10 nm) and are thus suitable mostly for coarse WDM applications.
From the standpoint of system design, integrated demultiplexers with low insertion losses are preferred. An interesting approach uses a phased array of optical waveguides that acts as a grating. Such gratings are called arrayed waveguide gratings (AWGs) and have attracted considerable attention because they can be fabricated using the silicon, InP, or LiNbO3 technology. In the case of silica-on-silicon technology, they are useful for making planar lightwave circuits. AWGs can be used for a variety of WDM applications and are discussed later in the context of WDM routers.
The following figure shows the design of a waveguide-grating demultiplexer, also known as a phased-array demultiplexer. The incoming WDM signal is coupled into an array of planar waveguides after passing through a free-propagation region in the form of a lens. In each waveguide, the WDM signal experiences a different phase shift because of different lengths of waveguides. Moreover, the phase shifts are wavelength dependent because of the frequency dependence of the mode-propagation constant. As a result, different channels focus to different output waveguides when the light exiting from the array diffracts in another free-propagation region. The net result is that the WDM signal is demultiplexed into individual channels. Such demultiplexers were developed during the 1990s and became available commercially by 1999. They are able to resolve up to 256 channels with spacing as small as 0.2 nm. A combination of several suitably designed AWGs can increase the number of channels to more than 1000 while maintaining a 10-GHz resolution.
The performance of multiplexers is judged mainly by the amount of insertion loss for each channel. The performance criterion for demultiplexers is more stringent. First, the performance of a demultiplexer should be insensitive to the polarization of the incident WDM signal. Second, a demultiplexer should separate each channel without any leakage from the neighboring channels. In practice, some power leakage is likely to occur, especially in the case of dense WDM systems with small interchannel spacing. Such power leakage is referred to as crosstalk and should be quite small (< -20 dB) for a satisfactory system performance.
3. Add-Drop Multiplexers and Filters
Add-drop multiplexers are needed for wide-area and metro-area networks in which one or more channels need to be dropped or added, while preserving the integrity of other channels. Figure (a) below shows a reconfigurable optical add-drop multiplexer (ROADM) schematically; it houses a bank of optical switches between a demultiplexer-multiplexer pair. The demultiplexer separates all channels, optical switches drop, add, or pass individual channels, and the multiplexer combines the entire signal back again. Any demultiplexer design discussed in the preceding subsection can be used to make such ROADMs. It is even possible to amplify the WDM signal and equalize the channel powers at the add-drop multiplexer since each channel can be individually controlled. The new component in such multiplexers is the optical switch, which can be made using a variety of technologies including LiNbO3 and InGaAsP waveguides.
If a single channel needs to be demultiplexed, and no active control of individual channels is required, one can use a much simpler multiport device designed to send a single channel to one port while all other channels are transferred to another port. Such devices avoid the need for demultiplexing all channels and are called add-drop filters because they filter out a specific channel without affecting the WDM signal. If only a small portion of the channel power is filtered out, such a device acts as an "optical tap" as it leaves the contents of the WDM signal intact.
Several kinds of add-drop filters have been developed since the advent of WDM technology. The simplest scheme uses a series of interconnected directional couplers, forming a MZ chain similar to that of a MZ filter discussed earlier. However, in contrast with the MZ filter, the relative delay τm is made the same for each MZ interferometer. Such a device is sometimes referred to as a resonant coupler because it resonantly couples out a specific wavelength channel to one output port while the remainder of the channels appear at the other output port. Its performance can be optimized by controlling the coupling ratios of various directional couplers. Although resonant couplers can be implemented in an all-fiber configuration using fiber couplers, the silica-on-silicon waveguide technology provides a compact alternative for designing such add-drop filters.
The wavelength selectivity of Bragg gratings can also be used to make add-drop filters. In one approach, referred to as the grating-assisted directional coupler, a Bragg grating is fabricated in the middle of a directional coupler. Such devices can be made in a compact form using InGaAsP/InP or silica waveguides. However, an all-fiber device is often preferred for avoiding coupling losses. In a common approach, two identical Bragg gratings are formed on the two arms of a MZ interferometer made using two 3-dB fiber couplers. The operation of such an add-drop filter can be understood from figure (b) above. Assume that the WDM signal is incident on port 1 of the filter. The channel, whose wavelength λg falls within the stop band of the two identical Bragg gratings, is totally reflected and appears at port 2. The remaining channels are not affected by the gratings and appear at port 4. The same device can add a channel at the wavelength λg if the signal at that wavelength is injected from port 3. If the add and drop operations are performed simultaneously, it is important to make the gratings highly reflecting (close to 100%) to minimize the crosstalk. As early as 1995, such an all-fiber, add-drop filter exhibited the drop-off efficiency of more than 99%, while keeping the crosstalk level below 1%. The crosstalk can be reduced below -50 dB by cascading several such devices.
Several other schemes use gratings to make add-drop filters. In one scheme, a waveguide with a built-in, phase-shifted grating is used to add or drop one channel from a WDM signal propagating in a neighboring waveguide. In another, two identical AWGs are connected in series such that an optical amplifier connects each output port of one with the corresponding input port of the other. The gain of amplifiers is adjusted such that only the channel to be dropped experiences amplification when passing through the device. Such a device is close to the generic add-drop multiplexer shown in Figure (a) above with the only difference that optical switches are replaced with optical amplifiers.
In another category of add-drop filters, optical circulators are used in combination with a fiber grating. Such a device is simple in design and can be made by connecting two ends of a fiber grating to two 3-port optical circulators. The channel reflected by the grating appears at the unused port of the input-end circulator. The same-wavelength channel can be added by injecting it from the output-end circulator. The device can also be made by using only one circulator provided it has more than three ports. The figure below shows two such schemes.
Scheme (a) uses a six-port circulator. The WDM signal entering from port 1 exits from port 2 and passes through a Bragg grating. The dropped channel appears at port 3 while the remaining channels re-enter the circulator at port 5 and leave the device from port 6. The channel to be added enters from port 4. Scheme (b) works in a similar way but uses two identical gratings to reduce the crosstalk level. Many other variants are possible.
4. Star Couplers
The role of a star coupler, as seen in the figure below, is to combine the optical signals entering from its multiple input ports and divide it equally among its output ports.
In contrast with demultiplexers, star couplers do not contain wavelength-selective elements, as they do not attempt to separate individual channels. The number of input and output ports need not be the same. For example, in the case of video distribution, a relatively small number of video channels (say 100) may be sent to thousands of subscribers. The number of input and output ports is generally the same for the broadcast-and-select LANs in which each user wishes to receive all channels. Such a passive star coupler is referred to as an N x N broadcast star, where N is the number of input (or output) ports. A reflection star is sometimes used for LAN applications by reflecting the combined signal back to its input ports. Such a geometry saves considerable fiber when users are distributed over a large geographical area.
Several kinds of star couplers have been developed for LAN applications. An early approach made use of multiple 3-dB fiber couplers. A 3-dB fiber coupler divides two input signals between its two output ports, the same functionality needed for a 2 x 2 star coupler. Higher-order N x N stars can be formed by combining several 2 x 2 couplers as long as N is a multiple of 2. The figure below shows an 8 x 8 star formed by interconnecting 12 fiber couplers. The complexity of such star couplers grows enormously with the number of ports.
Fused biconical-taper couplers can be used to make compact, monolithic, star couplers. The figure below shows schematically a star coupler formed using this technique.
The idea is to fuse together a large number of fibers and elongate the fused portion to form a biconically tapered structure. In the tapered portion, signals from each fiber mix together and are shared almost equally among its output ports. Such a scheme works relatively well for multimode fibers but is limited to only a few ports in the case of single-mode fibers. Fused 2 x 2 couplers were made as early as 1981 using single-mode fibers; they can also be designed to operate over a wide wavelength range. Higher-order stars can be made using a combinatorial scheme.
A common approach for fabricating a compact broadcast star makes use of the silica-on-silicon technology in which two arrays of planar SiO2 waveguides, separated by a central slab region, are formed on a silicon substrate. Such a star coupler was first demonstrated in 1989 in a 19 x 19 configuration. The SiO2 channel waveguides were 200 μm apart at the input end, but the final spacing near the central region was only 8 μm. The 3-cm-long star coupler had an efficiency of about 55%. A fiber amplifier can be integrated with the star coupler to amplify the output signals before broadcasting. The silicon-on-insulator technology has been used for making star couplers. A 5 x 9 star made by using silicon rib waveguides exhibited low losses (1.3 dB) with relatively uniform coupling.
5. Wavelength Routers
An important WDM component is an N x N wavelength router, a device that combines the functionality of a star coupler with multiplexing and demultiplexing operations. The figure (a) below shows the operation of such a wavelength router schematically for N = 5. The WDM signals entering from N input ports are demultiplexed into individual channels and directed toward the N output ports of the router in such a way that the WDM signal at each port is composed of channels entering at different input ports. This operation results in a cyclic form of demultiplexing. Such a device is an example of a passive router since its use does not involve any active element requiring electrical power. It is also called a static router since the routing topology is not dynamically reconfigurable. Despite its static nature, such a WDM device has many potential applications in WDM networks.
The most common design of a wavelength router uses a AWG demultiplexer shown below modified to provide multiple input ports. Such a device, called the waveguide-grating router (WGR), is shown schematically in the figure (b) above.
It consists of two N x M star couplers such that M output ports of one star coupler are connected with M input ports of another star coupler through an array of M waveguides that acts as an AWG. Such a device is a generalization of the MZ interferometer in the sense that a single input is divided coherently into M parts (rather than two), which acquire different phase shifts and interfere in the second free-propagation region to come out of N different ports depending on their wavelengths. The symmetric nature of the WGR permits to launch N WDM signals containing N different wavelengths simultaneously, and each WDM signal is demultiplexed to N output ports in a periodic fashion.
The physics behind the operation of a WGR requires a careful consideration of the phase changes as different wavelength signals diffract through the free-propagation region inside star couplers and propagate through the waveguide array. The most important part of a WGR is the waveguide array designed such that the length difference ΔL between two neighboring waveguides remains constant from one waveguide to next. The phase difference for a signal of wavelength λ, traveling from the pth input port to the qth output port through the mth waveguide (compared to the path connecting central ports) can be written as
where n1 and n2 are the refractive indices in the regions occupied by the star couplers and waveguides, respectively. The lengths δp and δ'q depend on the location of the input and output ports. When the condition
n1(δp + δ'q) + n2ΔL = Qλ
is satisfied for some integer Q, the channel at the wavelength λ acquires phase shifts that are multiples of 2π while passing through different waveguides. As a result, all fields coming out of the M waveguides will interfere constructively at the qth port. Other wavelengths entering from the pth port will be directed to other output ports determined by the condition from this equation. Clearly, the device acts as a demultiplexer since a WDM signal entering from the pth port is distributed to different output ports depending on the channel wavelengths.
The routing function of a WGR results fro the periodicity of the transmission spectrum. This property is also easily understood from the equation above. The phase condition for constructive interference can be satisfied for many integer values of Q. Thus, if Q is changed to Q + 1, a different wavelength will satisfy the equation and will be directed toward the same port. The frequency difference between these two wavelengths is the free spectral range (FSR), analogous to that of FP filters. For a WGR, it is given by
Strictly speaking, FSR is not the same for all ports, an undesirable feature from a practical standpoint. However, when δp and δ'q are designed to be relatively small compared with ΔL, FSR becomes nearly constant for all ports. In that case, a WGR can be viewed as N demultiplexers working in parallel with the following property. If the WDM signal from the first input port is distributed to N output ports in the order λ1, λ2, ..., λN, the WDM signal from the second input port will be distributed as λN, λ1, λ2, ..., λN-1, and the same cyclic pattern is followed for other input ports.
The optimization of a WGR requires precise control of many design parameters for reducing the crosstalk and maximizing the coupling efficiency. Despite the complexity of the design, WGRs are routinely fabricated in the form of a compact commercial device (each dimension ~ 1 cm) using either silica-on-silicon technology or InGaAsP/InP technology. WGRs with 128 input and output ports were available by 1996 in the form of a planar lightwave circuit and were able to operate on WDM signals with a channel spacing as small as 0.2 nm while maintaining crosstalk below 16 dB. The number of channels could be increased to 256 by employing silica waveguides with a relatively large core-cladding index difference of 1.5% while maintaining the 25-GHz channel spacing. A combination of several suitably designed AWGs can increase the number of channels to more than 1,000 while maintaining the 10-GHz resolution. The only negative aspect of such devices is that insertion losses for AWG demultiplexers can exceed 10 dB.
6. WDM Transmitters and Receivers
Most WDM systems use a large number of DFB lasers whose frequencies are chosen to match the ITU frequency grid precisely. This approach becomes impractical when the number of channels becomes large. Two solutions are possible. In one approach, single-mode narrowband lasers with a tuning range of 10 nm or more are employed. The use of such lasers reduces the inventory and maintenance problems. Alternatively, multiwavelength transmitters which generate light at 8 or more fixed wavelengths simultaneously can be used. Although such WDM transmitters attracted attention in the 1990s, it was only after 2001 that monolithically integrated WDM transmitters, operating near 1.55 μm with a channel spacing of 1 nm or less, were developed and commercialized using the InP-based photonic integrated-circuit (PIC) technology.
Several different techniques have been pursued for designing WDM transmitters. In one approach, the output of several DFB or DBR semiconductor lasers, independently tunable through Bragg gratings, is combined by using passive waveguides. A built-in amplifier boosts the power of the multiplexed signal to increase the transmitted power. In a 1996 device, 16 gain-coupled DFB lasers were integrated, and their wavelengths were controlled by changing the width of the ridge waveguides and by tuning over a 1-nm range using a thin-film resistor. In a different approach, sampled gratings with different periods are used to tune the wavelengths precisely of an array of DBR lasers. The complexity of such devices makes it difficult to integrate more than 16 lasers on the same chip.
In a different approach, a waveguide grating is integrated within the laser cavity to provide lasing at several wavelengths simultaneously. An AWG is often used for multiplexing the output of several optical amplifiers or DBR lasers. In a 1996 demonstration of the basic idea, simultaneous operation at 18 wavelengths (spaced apart by 0.8 nm) was realized using an intracavity AWG. The figure below shows the laser design schematically.
Spontaneous emission of the amplifier located on the left side is demultiplexed into 18 spectral bands by the AWG through the technique of spectral slicing. The amplifier array on the right side selectively amplifies the set of 18 wavelengths, resulting in a laser emitting at all wavelengths simultaneously. A 16-wavelength transmitter with 50-GHz channel spacing was built in 1998 by this technique. In another approach, the AWG was not a part of the laser cavity but was used to multiplex the output of 10 DBR lasers, all produced on the same chip in an integrated fashion. AWGs fabricated with the silica-on-silicon technology can also be used although they cannot be integrated on the InP substrate.
The PIC approach was followed intensively after 2001. In a 2002 transmitter, 12 tunable DFB lasers were integrated on the same InP chip and their outputs were combined using a micro-electro-mechanical system (MEMS) within a butterfly module. Such a transmitter provided up to 20 mW of fiber-coupled power at ITU wavelengths within the C band with spacing of 50 GHz set precisely by a wavelength locker. Such a device is not fully integrated because it employs discrete lenses to couple light from lasers to the MEMS. By 2005, fully integrated large-scale PIC transmitter chips had been developed and commercialized. The figure below shows the architecture of such a 10-channel transmitter incorporating more than 50 functions on a single chip.
The outputs of a tunable DFB laser array pass through electroabsorption modulators (EAMa) (driven by 10-Gb/s electrical bit streams) and variable optical attenuators before they are combined by an AWG multiplexer. An optical power-monitor (OPM) array is also integrated within the chip to ensure power. All laser wavelengths, separated in frequency by 200 GHz, fall on the ITU grid within the C band. A 10-channel receiver PIC chip was also developed to match this WDM transmitter. By 2006, this approach was extended to fabricate 40-channel WDM transmitters with 40-Gb/s bit rate per channel. More recently, transmitters suitable for phase-encoded (DQPSK) bit streams have been developed by integrating multiple MZ interferometers within the InP chip.
Fiber lasers can also be designed to provide multiwavelength output and therefore act as a CW WDM source. A ring-cavity fiber laser containing a frequency shifter (e.g., an acousto-optic device) and an optical filter with periodic transmission peaks (such as a FP filter, a sampled grating, or an AWG) can provide its output at a comb of frequencies set to coincide with the ITU grid. Up to 16 wavelengths were obtained by 2000 by this technique, although the power is not generally uniform across them. By 2009, 50-wavelength fiber lasers covering the entire C band with 100-GHz channel spacing were developed. The output at all wavelengths exhibited the same polarization with a narrow spectral bandwidth (< 0.2 MHz). The main disadvantage of fiber lasers is that a demultiplexer is needed to separate the channels before data is imposed on them using individual modulators.
A unique approach to WDM sources exploits the technique of spectral slicing for realizing WDM transmitters and is capable of providing more than 1000 channels. The output of a coherent, wide-bandwidth source is sliced spectrally using a multipeak optical filter such as an AWG. In one implementation of this idea, picosecond pulses from a mode-locked fiber laser are first broadened spectrally to bandwidth as large as 200 nm through supercontinuum generation by exploiting the nonlinear effects in an optical fiber. Spectral slicing of the output by an AWG then produces many WDM channels with a channel spacing of 1 nm or less. In a 2000 experiment, this technique produced 1000 channels with 12.5-GHz channel spacing. In another experiment, 150 channels with 25-GHz channel spacing were realized within the C band covering the range 1530-1560 nm. The SNR of each channel exceeded 28 dB, indicating that the source was suitable for dense WDM applications.
The generation of supercontinuum is not necessary if a mode-locked laser producing femtosecond pulses is employed. The spectral width of such pulses is quite large to begin with and can be enlarged to 50 nm or more by chirping them using 10-15 km of standard telecommunication fiber. Spectral slicing of the output by a demultiplexer can again provide many channels, each of which can be modulated independently. This technique also permits simultaneous modulation of all channels using a single modulator before the demultiplexer if the modulator is driven by a suitable electrical bit stream composed through TDM. A 32-channel WDM source was demonstrated in 1996 by using this method. Since then, this technique has been used to provide sources with more than 1000 channels.
On the receiver end, multichannel WDM receivers have been developed because their use can simplify the system design and reduce the overall cost. Monolithic receivers integrate a photodiode array with a demultiplexer on the same chip. Typically, a planar concave-grating demultiplexer or a WGR is integrated with the photodiode array. Even electronic amplifiers can be integrated within the same chip. The design of such monolithic receivers is similar to the transmitter shown earlier except that no cavity is formed and the amplifier array is replaced with a photodiode array. Such a WDM receiver was first fabricated in 1995 by integrating an eight-channel WGR (with 0.8-nm channel spacing), eight p-i-n photodiodes, and eight preamplifiers using heterojunction bipolar transistor technology. By 2007, PIC receivers with a large number of photodiodes became available.