Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
Photonics and Optical Communication (Course Number 300352) Spring 2007 Waveguides Dr. Dietmar Knipp Assistant Professor of Electrical Engineering
http://www.faculty.iu-bremen.de/dknipp/
Waveguides 1 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
Photonics and Optical Communication
3 Waveguides 3.1 Introduction 3.2 Reflection and Refraction at the Boundary between two Media 3.3 Total internal reflection 3.3.1 Light propagation in an optical fiber 3.3.2 Acceptance angle 3.4 Planar Waveguide 3.4.1 Planar Mirror Waveguide 3.4.2 Planar Dielectric Waveguide 3.5 Modes in Waveguides 3.5.1 Transverse Electric Waves 3.5.2 Transverse Magnetic Waves 3.5.3 Transverse Electro Magnetic Waves 3.5.4 Calculating Modes in a planar wave guide 3.5.5 The effective refractive index 3.5.6 The Mode chart 3.5.7 Designing a planar wave guide
Waveguides 2 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.8 TE versus TM Modes 3.5.9 Types of Modes 3.5.10 Numbering of modes 3.6 Coupling between Waveguide
References
Waveguides 3 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.1 Introduction Light can be confined by an optical waveguide. The waveguide is formed by a medium which is embedded by an another medium of lower refractive index. The medium of higher refractive index acts as a “light trap”. Light is confined in the waveguide by multiple total internal reflections. By doing so light can be transported from one location to another location. Waveguides can be distinguished in terms of slabs, strips and fibers. The most widely applied waveguide structure is the optical fiber, which is made out of two concentrically cylinders of low-loss glass with slightly different refractive index.
Waveguides can be distinguished in terms of a slab, a strip or a fiber. Ref: Saleh & Teich, Fundamentals of Photonics
Waveguides 4 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.1 Introduction If a lot of optical component like wave guides, light sources and light receivers are integrated together on a substrate (chip) we speak about integrated optics. The goal is to miniaturize optics like electronics to improve performance and reduce cost.
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3.1 Introduction The optical fiber in its existing form (the fiber consists of a core and a cladding) was invented 40 years ago. The first fibers were used in the near infrared wavelength region at around 800nm-900nm. As technology of fibers and light sources evolved the optical transmission window was shifted to 1310nm in the mid 1980‘s and 1550nm in the 1990‘s. Internal reflection is a requirement for the guidance or confinement of waves in a waveguide. Total internal reflection can only be achieved if the refractive index of the core is larger than the refractive index of the cladding. In the following, we will briefly repeat the related ray optics.
Waveguides 6 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.2 Reflection and Refraction at the Boundary between two Media The reflection and refraction of light at an interface can be described by Snell‘s
law. The angle of incidence is given by θ1 which is related to the angle of refraction θ2.
n1 ⋅sinθ1 = n2 ⋅sinθ2
Snell‘s law.
Reflection of rays at an interface. (a) From a high to a low refractive medium, (b) The critical angle, (c) Total internal reflection. Ref: J.M. Senior, Optical Fiber Communication
Waveguides 7 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.3 Total internal reflection
With increasing angle of incidence θ1 the angle of refraction θ2 also increases. If n1 > n2, there comes a point when θ2 =π/2 radians. This happens when -1 θ1=sin (n2 / n1). For larger values of θ1, there is no refracted ray, and all the energy from the incident ray is reflected. This phenomena is called total internal reflection. The smallest angle for which we get total internal reflection
is called the critical angle and θ2 equals π/2 radians. The total internal reflection is an requirement for the guidance of light in an optical fiber.
n2 Critical angle sinθc = n1
Waveguides 8 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.3.1 Light propagation in an optical fiber An optical fiber can be described by an cylindrical core surrounded by a cladding. Usually (at least for optical communication) the fiber core and the
cladding are made of silica (SiO2). The refractive index of the core is slightly higher than the refractive index of the cladding so that the light is guided in the fiber.
Transmission of a light ray in a perfect optical fiber. Ref: J.M. Senior, Optical Fiber Communication
Waveguides 9 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.3.2 Acceptance angle Total internal reflection is required to guide light in an optical fiber. We know that only light under sufficient shallow angles (angle greater than the critical angle) can propagate in the fiber. The question is now under what angle a ray can enter a fiber? It is clear that not all rays entering the fiber core will continue to be propagated along the fiber. Only rays that enter the fiber within a acceptance cone (acceptance angle) will propagate along the fiber, whereas rays outside of the cone will not be guided.
Coupling of a ray into a fiber. The ray can only be coupled into the fiber when the angle of incident is within the acceptance cone. Ref: J.M. Senior, Optical Fiber Communication
Waveguides 10 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.3.2 Acceptance angle In the following we will derive an expression for the the acceptance angle from the refractive indices of the three media involved, namely the core of the
fiber (n1), the cladding of the fiber (n2) and the air (n0). In order to enter the fiber Snell‘s law has to be fulfilled.
n0 ⋅sinθ1 = n1 ⋅sinθ2
The angle θ2 can now be described by π θ = −φ 2 2 So that the Snell‘s law can Coupling of a ray into a fiber. The ray can only be modified to be coupled in the fiber when the angle of incident is within the acceptance cone. n0 ⋅sinθ1 = n1 ⋅ cosφ Ref: J.M. Senior, Optical Fiber Communication
Waveguides 11 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.3.2 Acceptance angle If we considerφ now the trigonometrically relationship sin2 ()+ cos2 φ ()=1 θ The expression can be modified to 2 n0 ⋅sin 1 = n1 1− sin φ Now the equation can be combined with the equation for the critical angle −1 φ = sin ()n2 n1 Leading to the relationship for the numerical aperture
2 2 NA = n0 ⋅sinθ1 = n1 − n2 Numerical aperture
The acceptance angle can now be calculated by
θ n2 − n2 <θ = sin−1 1 2 Acceptance angle a 1 n 0
Waveguides 12 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.4 Planar waveguide The planar waveguide is the simplest form of an optical waveguide. The waveguide can be realized by a simple sandwich structure which consists of a slab embedded between two mirrors or a two regions of lower refractive index. Depending on the waveguide structure it can be distinguished between planar mirror waveguides, where the core of the waveguide is embedded between two mirrors and planar dielectric waveguides, where the core of the waveguide is embedded between regions of lower refractive index. Types of waveguides: - Mirror waveguides - Dielectric Waveguides Choosing different claddings of the waveguide has an important influence on the propagation of waves in the waveguide.
Waveguides 13 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.4.1 Planar Mirror waveguide A planar mirror waveguide is shown on this slide. The cladding of the waveguide is formed by a conducting material, which can be a mirror. As a consequence of the conducting cladding the tangential components of the electric and the magnetic field is zero at the interface between the core and the cladding.
ET = 0 BT = 0
Therefore, the waves can not extend in the Cross section of a mirror cladding of the waveguide. Subsequently the waveguide modes of propagation are defined by the Ref: Back to Basics in Optical dimensions of the core of the waveguide. Communications, Tutorial Agilent Technologies
Waveguides 14 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.4.2 Planar Dielectric waveguide A planar dielectric waveguide is shown on this slide. The cladding of the waveguide is formed by a dielectric medium of lower refractive index. The waves extend in the cladding of the waveguides. The wave propagating the waveguide can be described by ω E(r,t)= E(r)⋅exp()j()t − βz
The complex amplitude of the wave corresponds to the transverse standing Cross section of a dielectric wave perpendicular to the direction of waveguide. propagation. Due to the fact that waves Ref: Back to Basics in Optical extend in the cladding the wavelengths Communications, Tutorial Agilent that can propagate are larger than 2 Technologies times the diameter of the core. β is a propagation constant.
Waveguides 15 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5 Modes in Waveguides The planar waveguide is the simplest form of an optical waveguide. The waveguide can be realized by a simple sandwich structure which consists of a slab embedded between two regions of lower refractive index. The optical ray within the waveguide can be described by a transverse electromagnetic wave, which can be a TE, TM or TEM wave.
Propagation of a wave in a planar waveguide. We can impose self- consistency condition which requires that the wave reproduces itself. Fields that satisfy this conditions are called Eigenmodes (modes) of the waveguide. Ref: Saleh and Teich, Fundamentals of Photonics
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3.5 Modes in a Waveguides As a consequence of the superposition of planar waves we get an interference pattern, which is formed in the waveguide (the z-direction is the propagation direction of the wave). If the total phase change upon two successive reflections is equal to 2πm constructive interference is observed, where m is a positive integer. The phase shift has two contributions. The reflection of the plane wave at the interface leads to a phase shift, which depends on the angle θ and the distance traveled. Later on we will derive an expression for the modes of propagation in such a structure. Formation of modes in a planar dielectric waveguide. (a) plane wave propagation in a waveguide and corresponding electric field distribution in the optical fiber. The interference of the plane waves in the waveguide is forming the lowest order mode (m=0). Ref: J.M. Senior, Optical Fiber Communication
Waveguides 17 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5 Modes in a Waveguides In the figure on the previous slide the lowest order mode (m=0) is shown. A mode of propagation is only observed when the angle between the propagation vector and the interface (boundary of the cladding and the core) has particular values. For all modes of propagation a standing wave is formed in the waveguide. Depending on the mode of propagation an electric field distribution is formed. For the lowest order mode the electric field is maximized in the center of the core. The electric field decays towards the boundaries. For all modes of propagation the self-consistency condition has to be satisfied which means that the wave in the waveguide reproducing itself. Before discussing the modes of propagation mathematically we will discuss the propagation of waves in a waveguide phenomenologically .
Waveguides 18 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5 Modes in Waveguides Again we assume a plane wave which propagates in the z-direction. We observe constructive interference across the waveguide as a consequence of the superposition of the propagating waves. In the examples shown on this slide the propagation modes are m=1, 2 and 3. The number of modes corresponds to the number of zeros in the transverse electric field pattern. How do we determine the self- consistency conditions. In order to Propagation of waves in a waveguide achieve total internal reflection the and the corresponding transverse angle of incidence has to be smaller electric (TE) field pattern of three than lower order models m=1, 2, 3. θ π n n −1 2 −1 2 Ref: J.M. Senior, Optical Fiber C < − sin = cos 2 n1 n1 Communication
Waveguides 19 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5 Modes in Waveguides For self-consistency the wave reproduces itself and the phase shift between the two waves has to be zero or a multiple of 2π. We can assume that the field in the slab is in the form of a monochromatic plane wave bouncing back and forth at an angle θ smaller than the critical
angle θC. A round trip can be described by:
AC − AB = 2d sinθ Planar dielectric waveguide. For self-consistency the phase Ref: Saleh and Teich, Fundamentals of shift between the two waves has Photonics to be zero or a multiple of 2π.
Waveguides 20 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5 Modes inWaveguides λ θ 2π ϕ 2d sin − r = 2πm for m = 0,1,2, Mode Equation
There is a phase ϕr introduced by each internal reflection at the boundary. The reflection phase is a function of the angle θ and it depends on the polarization of the incidence wave (TE, TM or TEM wave), which is described by the complex reflection and transmission coefficients (see Review of optics). The complex reflection and transmission coefficients can be separated in an amplitude and a phase, where the phase depends on the angle of incidence.
Reflection coefficient and phase shift on reflection for a transverse electric wave as a function of the angle of incidence for a glass/air interface. Ref: J.M. Senior, Optical Fiber Communication
Waveguides 21 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.1 Transverse Electric Waves In the case of a transverse electric field (TE mode) the electric field is perpendicular to the direction of propagation of the wave (z-direction). As we are dealing with electro-magnetic waves each wave consists of a periodically varying electric and magnetic field which is again perpendicular to each other. In the case of a TE transverse wave the electric is perpendicular to the
direction of propagation (Ez=0) and the magnetic field has a (small) component to the direction of propagation. This is due to the fact that the traveling wave is not propagating in a straight line in the wave guide, meaning the ray is propagation on a zigzag path.
x Propagation of a TE wave in a slab waveguide.
B Ref: J.C. Palais, Fiber Optic Communication z
Waveguides 22 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.2 Transverse Magnetic Waves In the case of transverse TM modes the magnetic field is perpendicular to the direction of propagation and the electric field has a (small) component to the z- direction of propagation. Again the traveling wave is propagating on a zigzag path rather than a straight line in the wave guide.
x Propagation of a TM wave in a slab waveguide. E Ref: J.C. Palais, Fiber Optic Communication z
Waveguides 23 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.3 Transverse Electro Magnetic Waves In the case of a TEM transverse wave (TEM modes) both the electric and the magnetic field are perpendicular to the direction of propagation, which means that the rays propagate straight in the fiber. Such cases occurs only for single mode fibers.
3.5.4 Calculating Modes in a planar wave guide The mode equation and the equation for the phase shift have to be merged, which leads to a transcendental equation. The transcendental equation has to be solved to get the modes which propagate in a given waveguide structure. A detailed mathematical description is given by Saleh and Teich in their book Fundamentals of Photonics.
Waveguides 24 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.5 The effective refractive index
The effective refractive index is defined by: neff = n1 sinθ The effective refractive index is a key parameter for waveguides like the refractive index is a key parameter for the free space propagation of waves. The effective refractive index changes the wavelength in the same way that a bulk refractive index does. The idea of the effective refractive index gets clear by simply looking at the structure of a waveguide. The effective refractive index is a corrected refractive index which simply assumes that the wave propagates in a straight line the media (in our case in the core of the waveguide structure.)
Plane wave propagating in a x waveguide. The effective refractive index considers that the plane wave propagates by following a zigzag path. Ref: J.C. Palais, Fiber Optic z Communication
Waveguides 25 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.6 The Mode chart The mode chart for a waveguide structure is an absolutely essential graph to study the propagation of modes in a given waveguide structure. The thickness/diameter (d) of the core of the waveguide is usually normalized by the wavelength of the incident light. The different modes of propagation are plotted for the propagation angle and the effective refractive index.
Mode chart for a symmetric slab. The following refractive indices were assumed for the core
n1=3.6 and the cladding n2=3.55 (AlGaAs structure). Ref: J.C. Palais, Fiber Optic Communication
Waveguides 26 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.6 The Mode chart For the particular mode chart shown on the previous slide an AlGaAs waveguide structure (used for a laser diode) is assumed The critical angle for the structure −1 is θc = sin ()n2 n1 = 80.4° Therefore, the range of angles for which the ray is trapped in the waveguide is then 80.4° – 90°. As a consequence the effective refractive index ranges from 3.55 to 3.6. From the mode chart we can draw the following conclusions: When the core thickness is very small in comparison to the wavelength of the propagating light the wave travels very close to the critical angle and the effective index is close to the refractive index of the refractive index of the cladding. The wave penetrates deeply into the outer layers, because the rays are near the critical angle. With increasing thickness of the core the ray travels at larger angles. The ray travels more parallel to the waveguide axis. For thick films (thickness is large in comparison with the wavelength of the propagating light) the effective index is very close to the refractive index of the film itself.
Waveguides 27 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.7 Designing a planar wave guide For any given propagation angle there is a set of film thicknesses that will allow rays to propagate. The following equation has to be satisfied for the higher modes,λ where m is a positive integer. λ d d m = + m 0 2n1 cosθ
In orderλ to change the mode the normalized thickness has to change by: 1 ∆()d = 2n1 cosθ
Table of TEm modes in a gallium arsenide waveguide. Ref: J.C. Palais, Fiber Optic Communication
Waveguides 28 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.7 Designing a planar wave guide The following equation can be applied to calculate the number of TE modes supported by the dielectric waveguide, where m is increased to the nearest integer. d m = 2 NA Number of TE Modes λ0 1 2 2 2 NA = ()n1 − n2 Numerical Aperture
3.5.8 TE versus TM Modes So far we discussed only the propagation of TE modes. However, TM modes exhibit almost identical propagation behavior. This is why we will not distinguish between TM and TE modes. Therefore, the curve in the mode chart were labeled as both TE and TM modes. This is mostly true since the difference in the refractive index for the core and the cladding are very small (in the range of a few percent). Even with increasing difference the cutoff modes are identical. For each TE mode there will be always a TM mode. The number of total modes is therefore twice the number of TE modes. The electric field distribution for the different mode is shown in the following.
Waveguides 29 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.9 Types of Modes In the case of a transverse TE (TE modes) wave the electric field is perpendicular to the direction of propagation and the magnetic field has a (small) component that is in the direction of the propagation. In the case of a transverse TM (TM modes) wave the magnetic field is perpendicular to the direction of propagation and the electric field has a (small) component that is in the direction of the propagation. In the case of a TEM transverse wave (TEM modes) both the electric and the magnetic field are perpendicular to the direction of propagation, which means that the rays propagate straight in the fiber. Such cases occurs only for single mode fibers. Furthermore, helical modes (HE or EH) modes exist. Under such conditions the ray travels in a circular path in the fiber and electric and magnetic field have components in the z-direction. These modes can be realized either as a HE or a EH mode depending on which field contributes most to the z-direction.
Waveguides 30 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.10 Numbering of Modes It turns out that the difference in refractive index between the cladding and the core is usually very small. The modes for TE, TM, HE and EH modes are very similar. Therefore, we can simplify the way we look at modes in waveguides and fibers. The listed modes can be summarized and explained using only a single set of LP (linear polarized) modes. The TE and the TM modes were numbered based on the number of zeros in their
electric field pattern across the waveguide. Therefore a TE0 mode would be a continuous distribution with only a single maxima but no zeros. A TE00 mode would be a mode for a 2-dimensional waveguide structure and the electric field distribution would correspond to a single spot. Obliviously a waveguide structure does not have
to be symmetric. A TE21 would be now a pattern with 2 zeros in one direction and a third zero in the perpendicular direction.
Electric field distribution for some symmetric and asymmetric slab waveguides. The numbering for TE, TM and TEM mode is identical. Ref: H. Dutton, Understanding Optical Communication
Waveguides 31 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.5.10 Numbering of modes Numbering of linear polarized modes is different from numbering TE and TM modes, but LP numbers are only used for fibers (circular waveguides). LP
modes are described by LPlm where m is the number of maxima along the radius of the fiber and l is half of the number of maxima around the circumference.
Correspondence between the linear polarized modes and the traditional exact modes in a cylindrical fiber. Ref: J.M. Senior, Optical Fiber Communication
Waveguides 32 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.6 Coupling between waveguides If waveguides are sufficiently close to each other light can couple from one waveguide to the other. Coupling occurs if the electric fields of the two waveguides overlap. This effect can be used to build couplers and switches. The two waveguides are formed by two slabs of higher refractive index similar to single waveguide structures. Maxwell equations can be used to describe the coupling of modes from one waveguide to the other waveguide. The problem can be described by two coupled differential equations (Coupled-Mode Equations).
Coupling between two parallel planar waveguides.
At z1 most of the light is guided in waveguide 1,
at z2 the light is equally divided amongst the two waveguides,
at z3 most of the light is guided in waveguide 2. Ref.: B.E.A. Saleh, M.C Teich, Fundamentals of Photonics
Waveguides 33 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.6 Coupling between waveguides Let‘s assume that light propagates in waveguide 1. No light (at least at this point) propagates in waveguide 2. If the waveguides are close enough to each other and the electric fields of the waveguides overlap, the wave (in waveguide 1) couples in the waveguide 2. Based on intuition it could be expected that half of the light is coupled in the waveguide 2. However, this is not the case. Almost all the light is coupled from waveguide 1 in waveguide 2. The length after which all the light is coupled form waveguide 1 to waveguide 2 is called the coupling
length L0. If the two waveguides are close to each other for more than the coupling length the light starts to couple back in the waveguide 1. Depending on the length of the coupling structure the waves couple back and forth between the waveguides.
Waveguides 34 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.6 Coupling between waveguides This effect can be used to build switches and 3dB couplers. If the two waveguides are close to each other for a distance which is equal to the coupling length the entire optical power is coupled from one waveguide to the other waveguide. The structure can be used as switch. If the two waveguides are close to each other for only half of the coupling length the incoming optical power is divided into two equal intensities. Such a structure can be used as an 3dB coupler. In discrete optics a beam splitter would be used to separate a beam into two equal beams.
Waveguide based optical coupler, (left) switching of the power from one waveguide to the other, (right) a 3dB coupler. Ref.: B.E.A. Saleh, M.C Teich, Fundamentals of Photonics
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3.6 Coupling between waveguides Similar coupling behavior is observed for single mode fibers. Two single-mode fibers have to be placed close and parallel to each other to accomplish coupling. In this case we are of course speaking about the cores of the single- mode fibers, which have to be placed close to each other. Like already described for the planar waveguides the electric field of the two fiber cores has to overlap so that waves can couple from one fiber in the other. Again, we can define a coupling length. If the fiber cores are close for longer than the coupling length we observe an oscillation of the intensity from one fiber to the other fiber.
Coupling between single mode fibers. Ref.: H. J.R. Dutton, Understanding optical communications
Waveguides 36 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
3.6 Coupling between waveguides The coupling length strongly dependent on the separation of the two single mode fiber cores. The further apart they are the greater the coupling length. Furthermore, the coupling length is strongly wavelength dependent. For different wavelengths the coupling length changes.
Waveguides 37 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp
References: Stamatios V. Kartalopoulos, DWDM, Networks, Devices and Technology, IEEE press and Wiley Interscience, 2003. Eugene Hecht, Optics, Addison Wesly, 4th edition, 2002 Fawwalz T. Ulaby, Fundamentals of Applied Electromagnetics, Prentice Hall, 2001. John M. Senior, Optical Fiber Communications, Prentice Hall Series in Optoelectonics, 2nd edition, 1992. Bahaa E.A. Saleh, Malvin Carl Teich, Fundamentals of Photonics, Wiley-Interscience (1991) Harry J. R. Dutton, Understanding Optical Communications, Prentice Hall Series in Networking, 1998. (Formerly freely available as a red book on the IBM red book server. Joseph C. Palais, Fiber Optic Communications, Prentice Hall Series, 1998. 4th edition.
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