Optical Components for WDM
`Lightwave Networks
`
`MICHAEL S. BORELLA, MEMBER, IEEE, JASON P. JUE, DHRITIMAN BANERJEE, BYRAV
`RAMAMURTHY, STUDENT MEMBER, IEEE, AND BISWANATH MUKHERJEE, MEMBER, IEEE
`
`Recently, there has been growing interest in developing optical
`fiber networks to support
`the increasing bandwidth demands
`of multimedia applications, such as video conferencing and
`World Wide Web browsing. One technique for accessing the huge
`bandwidth available in an optical fiber is wavelength-division
`multiplexing (WDM). Under WDM, the optical fiber bandwidth
`is divided into a number of nonoverlapping wavelength bands,
`each of which may be accessed at peak electronic rates by
`an end user. By utilizing WDM in optical networks, we can
`achieve link capacities on the order of 50 THz. The success of
`WDM networks depends heavily on the available optical device
`technology. This paper is intended as a tutorial on some of the
`optical device issues in WDM networks. It discusses the basic
`principles of optical transmission in fiber and reviews the current
`state of the art in optical device technology. It introduces some
`of the basic components in WDM networks, discusses various
`implementations of these components, and provides insights into
`their capabilities and limitations. Then, this paper demonstrates
`how various optical components can be incorporated into WDM
`optical networks for both local and wide-area applications. Last,
`the paper provides a brief review of experimental WDM networks
`that have been implemented.
`Keywords—Device issues, experimental systems, lightwave net-
`work, optical amplifier, optical fiber, switching elements, tunable
`receiver, tunable transmitter, wavelength converter, wavelength-
`division multiplexing.
`
`I.
`
`INTRODUCTION
`the field of computer and
`Over the past few years,
`telecommunications networking has experienced tremen-
`dous growth. With the rapidly growing popularity of the
`Internet and the World Wide Web and with the recent
`Manuscript received December 9, 1996; revised May 7, 1997. This work
`was supported in part by the Defense Advanced Research Projects Agency
`under Contracts DABT63-92-C-0031 and DAAH04-95-1-0487; in part by
`NSF under Grants NCR-9205755, NCR-9508239, and ECS-9521249; in
`part by Pacific Bell; and in part by UC MICRO Program.
`M. S. Borella is with the School of Computer Science, DePaul Univer-
`sity, Chicago, IL 60604 USA (e-mail mborella@cs.depaul.edu).
`J. P. Jue is with the Department of Electrical and Computer En-
`gineering, University of California, Davis, CA 95616 USA (e-mail:
`jue@ece.ucdavis.edu).
`D. Banerjee is with Hewlett-Packard Company, Roseville, CA 95747-
`5557 USA (e-mail:banerjee@rosemail.rose.hp.com).
`B. Ramamurthy and B. Mukherjee are with the Department of Com-
`puter Science, University of California, Davis, CA 95616 USA (e-mail
`byrav@cs.ucdavis.edu; mukherje@cs.ucdavis.edu).
`Publisher Item Identifier S 0018-9219(97)05722-8.
`
`deregulation of the telecommunications industry in the
`United States, this growth can be expected to continue
`in the foreseeable future. The next decade may bring
`to the home and office multiple connections of high-
`definition television, video mail, and digital audio, as well
`as full Internet connections via user-friendly graphic user
`interfaces. As more users start to use data networks, and
`as their usage patterns evolve to include more bandwidth-
`intensive networking applications, there emerges an acute
`need for very high bandwidth transport network facilities
`whose capabilities greatly exceed those of current high-
`speed networks, such as asynchronous transfer mode (ATM)
`networks.
`The key to the future of networks rests in the relatively
`young field of fiber optics. Optical fiber provides the huge
`bandwidth, low loss rate, and cost effectiveness to enable
`the vision of a “global village.” Given that fiber has
`a potential bandwidth of approximately 50 Tb/s—nearly
`four orders of magnitude higher than peak electronic data
`rates—every effort should be made to tap into the capabil-
`ities of fiber-optic networks.
`Wavelength-division multiplexing (WDM) is one promis-
`ing approach that can be used to exploit the huge bandwidth
`of optical fiber. In WDM, the optical transmission spectrum
`is divided into a number of nonoverlapping wavelength (or
`frequency) bands, with each wavelength supporting a single
`communication channel operating at peak electronic speed.
`Thus, by allowing multiple WDM channels to coexist on
`a single fiber, we can tap into the huge fiber bandwidth,
`with the corresponding challenges being the design and de-
`velopment of appropriate network architectures, protocols,
`and algorithms.
`Research and development on optical WDM networks
`have matured considerably over the past few years, and
`a number of experimental prototypes have been and are
`currently being deployed and tested in the United States,
`Europe, and Japan. It is anticipated that the next generation
`of the Internet will employ WDM-based optical backbones.
`The success of WDM networks relies heavily upon
`the available optical components. A block diagram of a
`
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`0018–9219/97$10.00 © 1997 IEEE
`
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`

`
`Fig. 1. Block diagram of a WDM transmission system.
`
`WDM communication system is shown in Fig. 1. The
`network medium may be a simple fiber link, a passive
`star coupler (PSC) (for a broadcast and select network),
`or a network of optical or electronic switches and fiber
`links. The transmitter block consists of one or more op-
`tical transmitters, which may be either fixed to a single
`wavelength or tunable across a range of wavelengths.
`Each optical transmitter consists of a laser and a laser
`modulator and may also include an optical filter for tuning
`purposes. If multiple optical transmitters are used, then a
`multiplexer or coupler is needed to combine the signals
`from different laser transmitters onto a single fiber. The
`receiver block may consist of a tunable filter followed
`by a photodetector receiver or a demultiplexer followed
`by an array of photodetectors. Examples of some WDM
`transmitters and receivers are shown in Fig. 2. Amplifiers
`may be required in various locations throughout the network
`to maintain the strength of optical signals.
`Designers of next-generation lightwave networks must
`be aware of the properties and limitations of optical fiber
`and devices in order for their corresponding protocols
`and algorithms to take advantage of the full potential of
`WDM. Often, a network designer may approach the WDM
`architectures and protocols from an overly simplified, ideal,
`or traditional-networking point of view. Unfortunately, this
`may lead an individual to make unrealistic assumptions
`about the properties of fiber and optical components, and
`hence may result in an unrealizable or impractical design.
`This paper serves as an introduction to WDM device
`issues. No background in optics or advanced physics is
`needed. For a more advanced and/or detailed discussion of
`WDM devices, we refer the interested reader to [1]–[6].
`This paper presents an overview of optical fiber and
`devices such as couplers, optical transmitters, optical re-
`ceivers and filters, optical amplifiers, optical routers, and
`switches. It paper attempts to condense the physics behind
`the principles of optical transmission in fiber in order to
`provide some background for the novice reader. WDM
`network-design issues are then discussed in relation to the
`advantages and limits of optical devices. Last, this paper
`demonstrates how these optical components can be used to
`create broadcast networks for local networking applications
`and wavelength-routed networks for wide-area deployment.
`The paper concludes with a note on the current status
`of optical technology and how test networks have used
`some of the optical devices described in this paper with
`a reasonable amount of success.
`
`II. OPTICAL FIBER
`Fiber possesses many characteristics that make it an ex-
`cellent physical medium for high-speed networking. Fig. 3
`shows the two low-attenuation regions of optical fiber [1].
`
`Centered at approximately 1300 nm is a range of 200 nm
`in which attenuation is less than 0.5 dB per kilometer. The
`total bandwidth in this region is about 25 THz. Centered
`at 1550 nm is a region of similar size with attenuation as
`low as 0.2 dB per kilometer. Combined, these two regions
`provide a theoretical upper bound of 50 THz of bandwidth.1
`The dominant loss mechanism in good fibers is Rayleigh
`scattering, while the peak in loss in the 1400-nm region is
`due to hydroxyl-ion (OH ) impurities in the fiber. Other
`sources of loss include material absorption and radiative
`loss.
`By using these large low-attenuation areas for data trans-
`mission, the signal loss for a set of one or more wavelengths
`can be made very small,
`thus reducing the number of
`amplifiers and repeaters needed. In single-channel long-
`distance experiments, optical signals have been sent over
`hundreds of kilometers without amplification. Besides its
`enormous bandwidth and low attenuation, fiber also offers
`low error rates. Fiber-optic systems typically operate at bit
`error rates (BER’s) of less than 10
`.
`The small size and thickness of fiber allows more fiber
`to occupy the same physical space as copper, a prop-
`erty that is desirable when installing local networks in
`buildings. Fiber is flexible, difficult to break, reliable in
`corrosive environments, and deployable at short notice
`(which makes it particularly favorable for military com-
`munications systems). Also, fiber transmission is immune
`to electromagnetic interference and does not cause interfer-
`ence. Last, fiber is made from one of the cheapest and most
`readily available substances on earth, sand. This makes fiber
`environmentally sound; and unlike copper, its use will not
`deplete natural resources.
`
`A. Optical Transmission in Fiber
`Before discussing optical components, it is essential to
`understand the characteristics of the optical fiber itself.
`Fiber is essentially a thin filament of glass that acts as
`a waveguide. A waveguide is a physical medium or path
`that allows the propagation of electromagnetic waves, such
`as light. Due to the physical phenomenon of total internal
`reflection, light can propagate the length of a fiber with little
`loss. Fig. 4 shows the cross section of the two types of fiber
`most commonly used: multimode and single mode. In order
`to understand the concept of a mode and to distinguish
`between these two types of fiber, a diversion into basic
`optics is needed.
`Light travels through vacuum at a speed of
`m/s. Light can also travel through any transparent material,
`but the speed of light will be slower in the material than
`in a vacuum. Let
`be the speed of light for a given
`material. The ratio of the speed of light in a vacuum to that
`in a material is known as the material’s refractive index ( )
`and is given by
`.
`When light travels from material of a given refractive
`index to material of a different refractive index (i.e., when
`
`1Usable bandwidth, however, is limited by fiber nonlinearities (see
`Section II-E).
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`

`
`Fig. 2. Transmitter and receiver structures.
`
`Fig. 3. The low-attenuation regions of an optical fiber.
`
`refraction occurs), the angle at which the light is transmitted
`in the second material depends on the refractive indexes of
`the two materials as well as the angle at which light strikes
`the interface between the two materials. Due to Snell’s law,
`we have
`, where
`and
`are the
`refractive indexes of the first substance and the second
`is the angle of incidence, or
`substance, respectively;
`the angle with respect to normal that light hits the surface
`between the two materials; and
`is the angle of light in
`the second material. However, if
`and
`is greater
`than some critical value and the rays are reflected back into
`substance
`from its boundary with substance .
`Looking again at Fig. 4, we see that the fiber consists of
`a core completely surrounded by a cladding (both of which
`consist of glass of different refractive indexes). Let us first
`consider a step-index fiber, in which the change of refractive
`index at the core-cladding boundary is a step function. If the
`refractive index of the cladding is less than that of the core,
`then total internal reflection can occur in the core and light
`can propagate through the fiber (as shown in Fig. 5). The
`angle above which total internal reflection will take place
`is known as the critical angle and is given by
`, which
`corresponds to
`90 . From Snell’s law, we have
`
`The critical angle is then
`
`So, for total internal reflection, we require
`
`(1)
`
`In other words, for light to travel down a fiber, the light
`must be incident on the core-cladding surface at an angle
`greater than
`.
`In some cases, the fiber may have a graded index, in
`which the interface between the core and the cladding
`undergoes a gradual change in refractive index with
`(Fig. 6). A graded-index fiber reduces the minimum
`required for total internal reflection and also helps to
`reduce the intermodal dispersion in the fiber. Intermodal
`dispersion will be discussed in the following sections.
`For light to enter a fiber, the incoming light should be at
`an angle such that the refraction at the air-core boundary
`results in the transmitted light’s being at an angle for which
`total internal reflection can take place at the core-cladding
`boundary. As shown in Fig. 7, the maximum value of
`can be derived from
`
`From (1), since
`(2) as
`
`(2)
`
`, we can rewrite
`
`(3)
`
`is referred to as the numerical
`The quantity
`aperture of the fiber (NA) and
`is the maximum angle
`with respect to the normal at the air-core boundary, so that
`the incident light that enters the core will experience total
`internal reflection inside the fiber.
`
`B. Multimode Versus Single-Mode Fiber
`A mode in an optical fiber corresponds to one of possibly
`multiple ways in which a wave may propagate through
`the fiber. It can also be viewed as a standing wave in
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`
`Fig. 4. Multimode and single-mode optical fibers.
`
`(a)
`
`(b)
`
`Fig. 5. Light traveling via total internal reflection within a fiber.
`
`the transverse plane of the fiber. More formally, a mode
`corresponds to a solution of the wave equation that is
`derived from Maxwell’s equations and subject to boundary
`conditions imposed by the optical fiber waveguide.
`An electromagnetic wave propagating along an optical
`fiber consists of an electric field vector
`and a magnetic
`field vector
`. Each field can be broken down into three
`components. In the cylindrical coordinate system, these
`components are
`and
`, where
`is
`the component of the field that is normal to the wall (core-
`cladding boundary) of the fiber,
`is the component of
`the field that is tangential to the wall of the fiber, and
`is the component of the field that is in the direction of
`propagation. Fiber modes typically are referred to using
`the notation
`(if
`), or
`(if
`),
`where
`and
`are both integers. For the case
`, the
`modes are also referred to as transverse-electric (TE), in
`, or transverse-magnetic (TM), in which
`which case
`case
`.
`Although total internal reflection may occur for any angle
`that is greater than
`, light will not necessarily
`propagate for all of these angles. For some of these angles,
`light will not propagate due to destructive interference
`between the incident light and the reflected light at the
`core-cladding interface within the fiber. For other angles
`of incidence, the incident wave and the reflected wave at
`the core-cladding interface constructively interfere in order
`to maintain the propagation of the wave. The angles for
`which waves do propagate correspond to modes in a fiber.
`If more than one mode may propagate through a fiber, the
`fiber is called multimode. In general, a larger core diameter
`or high operating frequency allows a greater number of
`modes to propagate.
`The number of modes supported by a multimode optical
`fiber is related to the normalized frequency
`which is
`
`Fig. 6. Graded-index fiber.
`
`Fig. 7. Numerical aperture of a fiber.
`
`defined as
`
`(4)
`
`is the radius of the core, and
`,
`where
`is the wavelength of the propagating light
`in vacuum.
`In multimode fiber,
`the number of modes
`is given
`approximately by
`
`(5)
`
`The advantage of multimode fiber is that its core diameter
`is relatively large; as a result, injection of light into the
`fiber with low coupling loss2 can be accomplished by using
`inexpensive, large-area light sources, such as light-emitting
`diodes (LED’s).
`The disadvantage of multimode fiber is that it introduces
`the phenomenon of intermodal dispersion. In multimode
`fiber, each mode propagates at a different velocity due to
`different angles of incidence at the core-cladding boundary.
`This effect causes different rays of light from the same
`source to arrive at the other end of the fiber at different
`times, resulting in a pulse that is spread out in the time
`domain. Intermodal dispersion increases with the distance
`2Coupling loss measures the power loss experienced when attempting
`to direct light into a fiber.
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`
`of propagation. The effect of intermodal dispersion may be
`reduced through the use of graded-index fiber, in which
`the region between the cladding and the core of the fiber
`consists of a series of gradual changes in the index of
`refraction (see Fig. 6). Even with graded-index multimode
`fiber, however, intermodal dispersion may still limit the bit
`rate of the transmitted signal and may limit the distance
`that the signal can travel.
`One way to limit intermodal dispersion is to reduce the
`number of modes. From (4) and (5), we observe that this
`reduction in the number of modes can be accomplished by
`reducing the core diameter, reducing the numerical aperture,
`or increasing the wavelength of the light.
`By reducing the fiber core to a sufficiently small diameter
`and reducing the numerical aperture, it is possible to capture
`only a single mode in the fiber. This single mode is the
`mode, also known as the fundamental mode. Single-
`mode fiber usually has a core size of about 10 m, while
`multimode fiber typically has a core size of 50–100 m
`(see Fig. 4). A step-index fiber will support a single mode
`if
`in (4) is less than 2.4048 [7].
`Thus, single-mode fiber eliminates intermodal dispersion
`and can hence support transmission over much longer dis-
`tances. However, it introduces the problem of concentrating
`enough power into a very small core. LED’s cannot couple
`enough light into a single-mode fiber to facilitate long-
`distance communications. Such a high concentration of
`light energy may be provided by a semiconductor laser,
`which can generate a narrow beam of light.
`
`C. Attenuation in Fiber
`Attenuation in optical fiber leads to a reduction of the
`signal power as the signal propagates over some distance.
`When determining the maximum distance that a signal
`can propagate for a given transmitter power and receiver
`sensitivity, one must consider attenuation. Let
`be
`km from the
`the power of the optical pulse at distance
`transmitter and
`be the attenuation constant of the fiber
`(in dB/km). Attenuation is characterized by [2]
`
`(6)
`
`where
`is the optical power at the transmitter. For a
`link length of
`km,
`must be greater than or equal
`to
`, the receiver sensitivity. From (6), we get
`
`(7)
`
`The maximum distance between the transmitter and the
`receiver (or the distance between amplifiers)3 depends more
`heavily on the constant
`than on the optical power
`launched by the transmitter. Referring back to Fig. 3, we
`note that the lowest attenuation occurs at approximately
`1550 nm.
`
`D. Dispersion in Fiber
`Dispersion is the widening of a pulse duration as it travels
`through a fiber. As a pulse widens, it can broaden enough to
`interfere with neighboring pulses (bits) on the fiber, leading
`to intersymbol interference. Dispersion thus limits the bit
`spacing and the maximum transmission rate on a fiber-optic
`channel.
`As mentioned earlier, one form of dispersion is inter-
`modal dispersion. This is caused when multiple modes of
`the same signal propagate at different velocities along the
`fiber. Intermodal dispersion does not occur in a single-mode
`fiber.
`Another form of dispersion is material or chromatic dis-
`persion. In a dispersive medium, the index of refraction is a
`function of the wavelength. Thus, if the transmitted signal
`consists of more than one wavelength, certain wavelengths
`will propagate faster than other wavelengths. Since no laser
`can create a signal consisting of an exact single wavelength,
`material dispersion will occur in most systems.4
`A third type of dispersion is waveguide dispersion. Wave-
`guide dispersion is caused because the propagation of
`different wavelengths depends on waveguide characteristics
`such as the indexes and shape of the fiber core and cladding.
`At 1300 nm, material dispersion in a conventional single-
`mode fiber is near zero. Luckily,
`this is also a low-
`attenuation window (although loss is lower at 1550 nm).
`Through advanced techniques such as dispersion shift-
`ing, fibers with zero dispersion at a wavelength between
`1300–1700 nm can be manufactured [8]. In a dispersion-
`shifted fiber, the core and cladding are designed such that
`the waveguide dispersion is negative with respect to the
`material dispersion,
`thus canceling the total dispersion.
`The dispersion will only be zero, however, for a single
`wavelength.
`
`E. Nonlinearities in Fiber
`Nonlinear effects in fiber may potentially have a signif-
`icant impact on the performance of WDM optical com-
`munications systems. Nonlinearities in fiber may lead to
`attenuation, distortion, and cross-channel interference. In a
`WDM system, these effects place constraints on the spacing
`between adjacent wavelength channels, limit the maximum
`power on any channel, and may also limit the maximum
`bit rate.
`the index of
`1) Nonlinear Refraction: In optical fiber,
`refraction depends on the optical intensity of signals prop-
`agating through the fiber [9]. Thus, the phase of the light
`at the receiver will depend on the phase of the light sent
`by the transmitter, the length of the fiber, and the optical
`intensity. Two types of nonlinear effects caused by this
`phenomenon are self-phase modulation (SPM) and cross-
`phase modulation (XPM).
`SPM is caused by variations in the power of an optical
`signal and results in variations in the phase of the signal.
`
`3The amplifier sensitivity is usually equal to the receiver sensitivity,
`while the amplifier output is usually equal to optical power at a transmitter.
`
`4Even if an unmodulated source consisted of a single wavelength, the
`process of modulation would cause a spread of wavelengths.
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`
`The amount of phase shift introduced by SPM is given by
`(8)
`where
`is the nonlinear coefficient for the index of
`refraction,
`,
`is the length of the fiber,
`and
`is the optical
`intensity. In phase-shift-keying
`(PSK) systems, SPM may lead to a degradation of the
`system performance since the receiver relies on the phase
`information. SPM also leads to the spectral broadening
`of pulses, as explained below. Instantaneous variations
`in a signal’s phase caused by changes in the signal’s
`intensity will result in instantaneous variations of frequency
`around the signal’s central frequency. For very short pulses,
`the additional frequency components generated by SPM
`combined with the effects of material dispersion will also
`lead to spreading or compression of the pulse in the time
`domain, affecting the maximum bit rate and the BER.
`XPM is a shift in the phase of a signal caused by the
`change in intensity of a signal propagating at a different
`wavelength. XPM can lead to asymmetric spectral broad-
`ening, and combined with SPM and dispersion may also
`affect the pulse shape in the time domain.
`Although XPM may limit the performance of fiber-optic
`systems, it may also have advantageous applications. XPM
`can be used to modulate a pump signal at one wavelength
`from a modulated signal on a different wavelength. Such
`techniques can be used in wavelength conversion devices
`and are discussed in Section VII.
`2) Stimulated Raman Scattering (SRS): SRS is caused by
`the interaction of light with molecular vibrations. Light
`incident on the molecules creates scattered light at a longer
`wavelength than that of the incident light. A portion of the
`light traveling at each frequency in a Raman-active fiber
`is downshifted across a region of lower frequencies. The
`light generated at the lower frequencies is called the Stokes
`wave. The range of frequencies occupied by the Stokes
`wave is determined by the Raman gain spectrum, which
`covers a range of around 40 THz below the frequency of the
`input light. In silica fiber, the Stokes wave has a maximum
`gain at a frequency of around 13.2 THz less than the input
`signal.
`The fraction of power transferred to the Stokes wave
`grows rapidly as the power of the input signal is increased.
`Under very high input power, SRS will cause almost all
`of the power in the input signal to be transferred to the
`Stokes wave.
`the channels of shorter
`In multiwavelength systems,
`wavelength will lose some power to each of the higher-
`wavelength channels within the Raman gain spectrum. To
`reduce the amount of loss, the power on each channel needs
`to be below a certain level. In [10], it is shown that in a
`ten-channel system with 10-nm channel spacing, the power
`on each channel should be kept below 3 mW to minimize
`the effects of SRS.
`3) Stimulated Brillouin Scattering (SBS): SBS is similar
`to SRS except that the frequency shift is cause by sound
`waves rather than molecular vibrations [9]. Other charac-
`teristics of SBS are that the Stokes wave propagates in the
`
`opposite direction of the input light, and SBS occurs at
`relatively low input powers for wide pulses (greater than 1
`s) but has negligible effect for short pulses (less than 10
`ns) [11]. The intensity of the scattered light is much greater
`in SBS than in SRS but the frequency range of SBS, on the
`order of 10 GHz, is much lower than that of SRS. Also, the
`gain bandwidth of SBS is only on the order of 100 MHz.
`To counter the effects of SBS, one must ensure that
`the input power is below a certain threshold. Also,
`in
`multiwavelength systems, SBS may induce cross talk be-
`tween channels. Cross talk will occur when two counter-
`propagating channels differ in frequency by the Brillouin
`shift, which is around 11 GHz for wavelengths at 1550 nm.
`The narrow gain bandwidth of SBS, however, makes SBS
`cross talk fairly easy to avoid.
`4) Four-Wave Mixing (FWM): FWM occurs when two
`wavelengths operating at frequencies
`and
`, respec-
`tively, mix to cause signals at
`and
`. These
`extra signals, called sidebands, can cause interference if
`they overlap with frequencies used for data transmission.
`Likewise, mixing can occur between combinations of three
`or more wavelengths. The effect of FWM in WDM systems
`can be reduced by using unequally spaced channels [12].
`FWM can be used to provide wavelength conversion, as
`will be shown in Section VII.
`5) Summary: Nonlinear effects in optical fibers may po-
`tentially limit the performance of WDM optical networks.
`Such nonlinearities may limit the optical power on each
`channel,
`limit
`the maximum number of channels,
`limit
`the maximum transmission rate, and constrain the spacing
`between different channels.
`It is shown that in a WDM system using channels spaced
`10 GHz apart and a transmitter power of 0.1 mW per
`channel, a maximum of about 100 channels can be obtained
`in the 1550-nm low-attenuation region [9].
`The details of optical nonlinearities are very complex
`and beyond the scope of this article. They are a major
`limiting factor in the available number channels in a WDM
`system, however, especially those operating over distances
`greater than 30 km [9]. The existence of these nonlinearities
`suggests that WDM protocols that limit the number of nodes
`to the number of channels do not scale well. For further
`details on fiber nonlinearities, the reader is referred to [11].
`
`F. Couplers
`A coupler is a general term that covers all devices that
`combine light into or split light out of a fiber. A splitter
`is a coupler that divides the optical signal on one fiber to
`two or more fibers. The most common splitter is a 1
`2 splitter, as shown in Fig. 8(a). The splitting ratio
`is
`the amount of power that goes to each output. For a two-
`port splitter, the most common splitting ratio is 50
`50,
`though splitters with any ratio can be manufactured [8].
`Combiners [see Fig. 8(b)] are the reverse of splitters, and
`when turned around, a combiner can be used as a splitter.
`An input signal to the combiner suffers a power loss of
`about 3 dB. A 2
`2 coupler [see Fig. 8(c)], in general, is
`a 2
`1 combiner followed immediately by a 1
`2 splitter,
`
`BORELLA et al.: WDM LIGHTWAVE NETWORKS
`
`1279
`
`Exhibit 1032, Page 6
`
`

`
`(a)
`Fig. 8. Splitter, combiner, and coupler.
`
`(b)
`
`(c)
`
`Fig. 9. A 16  16 PSC.
`
`which has the effect of broadcasting the signals from two
`input fibers onto two output fibers. One implementation of a
`2 coupler is the fused biconical tapered coupler, which
`2
`basically consists of two fibers fused together. In addition
`to the 50
`50 power split incurred in a coupler, a signal
`also experiences return loss. If the signal enters an input
`of the coupler, roughly half of the signal’s power goes to
`each output of the coupler. However, a small amount of
`power is reflected in the opposite direction and is directed
`back to the inputs of the coupler. Typically, the amount of
`power returned by a coupler is 40–50 dB below the input
`power. Another type of loss is insertion loss. One source of
`insertion loss is the loss incurred when directing the light
`from a fiber into the coupler device; ideally, the axes of
`the fiber core and the coupler input port must be perfectly
`aligned, but full perfection may not be achievable due to
`the very small dimensions.
`The PSC is a multiport device in which light coming
`into any input port is broadcast to every output port. The
`PSC is attractive because the optical power that each output
`receives
`equals
`
`(9)
`
`is the optical power introduced into the star
`where
`by a single node and
`is the number of output ports of
`the star. Note that this expression ignores the excess loss,
`caused by flaws introduced in the manufacturing process,
`that
`the signal experiences when passing through each
`coupling element. One way to implement the PSC is to
`use a combination of splitters, combiners, and couplers as
`shown in Fig. 9. Another implementation of the star coupler
`is the integrated-optics planar star coupler, in which the star
`coupler and waveguides are fabricated on a semiconductor,
`glass (silica), or polymer substrate. A 19
`19 star coupler
`on silicon has been demonstrated with excess loss of around
`8
`3.5 dB at a wavelength of 1300 nm [13]. In [14], an 8
`star coupler with an excess loss of 1.6 dB at a wavelength
`of 1550 nm was demonstrated.
`
`III. OPTICAL TRANSMITTERS
`To understand how a tunable optical transmitter works,
`we must first understand some of the fundamental principles
`of lasers and how they work. Then we will discuss various
`implementations of tunable lasers and their properties.
`Good references on tunable laser technology include [1],
`[2], [15].
`
`A. How a Laser Works
`The word “laser” is an acronym for light amplification
`by stimulated emission of radiation. The key words are
`stimulated emission, which is what allows a laser to produce
`intense high-powered beams of coherent light (light that
`contains one or more distinct frequencies).
`To understand stimulated emission, we must first acquaint
`ourselves with the energy levels of atoms. Atoms that are
`stable (in the ground state) have electrons that are in the
`lowest possible energy levels. In each atom, there are a
`number of discrete levels of energy that an electron can
`have; thus, we refer to them as states. To change the level of
`an atom in the ground state, that atom must absorb energy.
`This energy can be in many forms but for our purposes
`here, it can be either light or electrical energy. When an
`atom absorbs energy, it becomes excited and moves to a
`higher energy level. At this point, the atom is unstable and
`usually moves quickly back to the ground state by releasing
`a photon, a particle of light.
`There are certain substances, however, whose states are
`quasistable, which means that the substances are likely to
`stay in the excited state for longer periods of time without
`constant excitation. By applying enough energy (in the
`form of either an optical pump or an electrical current)
`to a substance with quasi-stable states for a long enough
`period of time, population inversion occurs, which means
`that there are more electrons in the excited state than in
`the ground state. As we shall see, thi