Department of Physics

Seminar Ia - 1st year, 2nd Cycle

SILICON PHOTONICS

Author: Katja MIHORKOAdvisor: prof. dr. Igor MUSEVIC

Ljubljana, January 2016

Abstract

Silicon photonics is the optical analogue for silicon microelectronics. Integrated electronic circuits canno longer follow the demands for speed and information densities that we have today. By using photons todetect, process and transmit information, not only on great distances but also on scale of integrated circuits,we can further boost the amount of information that we can transfer in a short amount of time. Besidesspeed, signal quality is improved as well. Silicon photonics is a very wide, fast growing field and this seminaronly cowers a small part of it. In this seminar I shall address the most important advantages and somedisadvantages of waveguides, ring microresonators and lasers used in integrated silicon photonics.

Contents

1 Introduction 2

2 Silicon waveguides 2

3 Silicon microring resonators 53.1 All-pass ring resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2 Add-drop resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Applications of ring resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 LASER-s 10

5 Conclusion 12

1 Introduction

Integrated electronic circuits can no longer follow the demands for speed and amount of information thatwe have today. Due to so called skin effect the transfer of data is severally compromised[1]. This effectcauses that the electric current flows mainly at the surface or skin of the conductor, causing the effectiveresistance of the conductor to increase at higher frequencies where the skin depth is smaller. This meanshigher the frequencies, smaller the effective cross-section of the conductor, which leads to lower speed andless information we can transfer in a short amount of time.

Luckily, there is at least one answer to this problem - light. For a long time now, light has proven to be agreat solution when transfering signals across long distances. Optic cables have first been used as waveguidesacross the oceans, but nowadays they have replaced copper wires on short distances as well. The next step isto use light to transfer information on an even smaller scale - making integrated photonic circuits. There aremany advantages to using photons instead of electrons as transferrers. Optical waveguides enable us to havehigher speeds of information transfer, the losses and dispersion of the signal are greatly smaller comparedto electronic waveguides[1]. On top of that optical waveguides enable us to transfer great densities of infor-mation, because we can use the same waveguide to transmit several light signals with different wavelengthsat the same time.

Using photons to detect, process and transmit information seems to be more efficient than using electriccurrent. The only obstacle that integrated photonics is facing is that the costs of manufacturing and devel-oping new processes are, at present, great. That is why the goal is to find methods for designing and fabri-cating high-performance, single-mode waveguides in silicon using complementary metal-oxide-semiconductor(CMOS) production, which is already well known from producing electronic integrated circuits [2]. Thisis where silicon photonics, which is the optical analogue of silicon microelectronics comes in place. In thisseminar I will present three of the most important parts needed for a photonic circuit: waveguide, microringresonator and laser.

2 Silicon waveguides

If we want to talk about integrated photonic circuits than we need to find a way how to guide light anywhereon the circuit we want, using simple but low-loss waveguides, that use total internal reflection. One of thepossibilities is to fabricate silicon waveguides (the spectral transmission range of silicon is between 1.2 and 7micrometers [3], as seen in Fig.1(a)) in a silicon-on-isolator (SOI) wafer in the telecom fibre-optic wavelengthrange 1.2 - 1.6 micrometers [4]. SOI provides a planar thin-film structure, which can guide light in the surfacesilicon layer with much higher refractive index compared to silicon-dioxide. The problem that emerges isthat this planar structure becomes multimodal for silicon films thicker than 0.2 micrometers (for telecomwavelength regime). Problematic effects kick in when light is guided in a waveguide whose dimensions areclose to or below the wavelength of the light. In particular, light beams travelling in the two polarizations ofthe guided mode will travel at different speeds and very tiny variations in the overall or local structure willsignificantly alter the waveguide properties [4]. This are all unwanted side effects that deform our signal. A

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generally useful waveguide for photonics needs to be single mode and polarization independent. Scientistshave discovered a way to make multi-micrometer ridge waveguide in SOI single mode by only using certaindimensions of the waveguide. The idea is that all higher-order modes would couple into the lowest-ordermode of the surrounding slab waveguide and be lost.

(a) (b) (c)

Figure 1: (a) Spectral transmission region of silicon [5]. (b) A simple SOI ridge waweguide with adjustableparameters r, w and e. The refractive index of silicon is n = 3.47 and of silicon-dioxide n = 1.44. (c)Theoretical single-multimode boundary for a wavelength of 1.5 µm and r equals 3.2 µm. To fulfill the single-mode condition, the cross section dimensions of a waveguide should typically be submicrometer in size [4].

SOI waveguides channel light through transverse and lateral confinement in a silicon core, surroundedby a silicon oxide bottom cladding and a low index top cladding (oxide or air) as we can see in Fig.1(b).Electromagnetic waveguides are analyzed by solving Maxwell’s equations, or their reduced form, the elec-tromagnetic wave equation, with boundary conditions determined by the properties of the materials andtheir interfaces. These equations have multiple solutions or modes. Each mode is characterized by a cut-offfrequency below which the mode cannot exist in the guide. Waveguide propagation modes depend on theoperation wavelength and polarization and the shape and size of the guide. The longitudinal mode of awaveguide is a particular standing wave pattern formed by waves confined in the cavity. The transversemodes are classified into different types: TE modes (transverse electric) have no electric field in the directionof propagation, TM modes (transverse magnetic) have no magnetic field in the direction of propagation andTEM modes (transverse electromagnetic) have no electric nor magnetic field in the direction of propagation.In hollow waveguides TEM waves are not possible, since Maxwell’s equations will give that the electric fieldmust then have zero divergence and zero curl and be zero at boundaries, resulting in a zero field. The modewith the lowest cut-off frequency, the ground mode, is the only possible mode in a single mode waveguide.

Figure 2: TE, TM and TEM modes, where k is pointing in the direction of propagation of the electromagneticfield[6].

As I have mentioned before, a single mode waveguide can become multimodal if the dimensions of thecross section are not right as seen in Fig.1(c). Fig.3 illustrates that not only dimensions w and e have limita-tions, but we also need to be careful which size of r we use. Waveguides that do meet the above mentionedcriteria can in fact be manufactured. Furthermore, they can easily be tapered in size so that they can bemade compatible with other photonics elements, such as for example single-mode optical fibres [7].

Now, let us take a closer look at a typical silicon waveguide. The most commonly used dimensions arebetween 400 nm and 500 nm in width, and between 200 nm and 250 nm in height [8]. The index contrastbetween core and cladding is very high, which means that the confinement of light is very strong. This hasa very useful advantage. It enables light guiding in bends with very small radii without radiation losses. I

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Figure 3: Waveguide effective index (the ratio of the propagation constant in the waveguide to the free spacepropagation constant) as a function of SOI waveguide dimension r. We can see that for dimensions above 2.5µm the manufacturability is ensured whilst below this dimension, the effective index becomes highly sensitiveto dimensional variations, and below 1 µm the behaviours of the two polarizations (TE and TM) diverge [4].

will talk more about this advantage in the next section, when I will present the use of microring resonators.

If we take a closer look at the boundary conditions at the core/cladding interface, we can see that thenormal component of electric displacement field D = Eε must be continuous. Therefore the field amplitudeat the cladding side of the interface will be stronger for a mode with the dominant E-field polarized normalto the interface. If the waveguide width is larger than its height (the most commonly used geometry), theground mode will have quasi TE polarization, with a strong discontinuity on the top and bottom surface [8].

Figure 4: (a) Illustration of the mode spreading for standard waveguide for TE polarized light [8]. (b) Reducedinteraction with vertical sidewalls and lower confinement - waveguide cross section for the TE polarization.We can also notice the effect of the deviation between effective indices [8].

Propagation losses in silicon waveguides originate from multiple sources (2-3dB/cm with air claddingand less than 2dB/cm with oxide cladding) [8]. Light scattering at sidewall roughness, that is shown in Fig.5, is considered to be the strongest loss contribution. Due to the nature of fabrication process, roughnesson the vertical sidewalls of the waveguide is unavoidable. There is some scattering at top surface roughnesspresent but it is much less severe. If we want to minimize the scattering, we can optimize the fabrication toreduce the surface-roughness or we can adjust the cross section of the waveguide for a certain polarization.In general TE-light is the preferred polarization, as TE is the ground mode of the waveguide. A surface-roughness optimized waveguide cross section for TE-polarized light measures only 100 nm in height and 600nm in width [8]. This cross section is still single mode and has 7x less scattering loss than a standard designof width 220 nm and height 450 nm as shown in Fig. 4. This improvement is the result of lower overlap ofthe mode with the vertical sidewalls.

Because of the high index contrast, the dispersion of silicon waveguides is strong. This means thatthe effective index of the propagating mode is wavelength dependent, so monochromatic waves of differ-ent wavelengths will travel through the waveguide with different velocities. This causes propagating pulses(that comprise a sum of monochromatic waves) to broaden and to be delayed (more than due to ordinarywaveguide dispersion). Also, because of the high index contrast and small core size, photonic waveguides areexceptionally sensitive to dimensional variations. The effect of width and thickness changes on the effectiveindex are relevant to ring resonators as the effective index determines the optical roundtrip length.

By taking advantage of the high refractive index contrast between silicon and silicon-dioxide and usingsilicon-on-isolator wafers, scientists can now construct micrometer-scale integrated optical circuits. This was

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Figure 5: SEM view of a photonic wire where the sidewall roughness is clearly visible [8].

the foundation for serious commercial development of silicon photonics.

3 Silicon microring resonators

As I have mentioned in the previous section, high refractive index contrast between silicon and silicon-dioxideenables us to guide light in bends with very small radii without radiation losses. This is possible becausethe angle at which the total internal reflection occurs is less than 25. Ring resonators are one of the greatinventions using this feature and consequently, they play an important role in the success of silicon photonics.

A generic ring resonator consists of an optical waveguide which is looped back on itself, such that aresonance occurs when the optical path length of the resonator is exactly a whole number of wavelengthsL = nλ. Ring resonators therefore support multiple resonances, and the spacing between these resonances,the ’free spectral range (FSR)’, depends on the resonator length. For many applications it is preferred tohave a relatively large FSR (several nm), and this implies the use of small rings. This can be achievedonly by using high-contrast waveguides with strong confinement. Because of the very high refractive indexcontrast between silicon and its oxide (or air), single-mode strip waveguides (so-called photonic wires) canhave bend radii below 5 micrometers. This enables us to produce extremely compact rings, that have FSRover 20 nanometers at telecom wavelengths around 1550 nanometers [8].

A ring resonator as a stand-alone device only becomes useful when there is a coupling to the outsideworld. The most common coupling mechanism is using codirectional evanescent coupling between the ringand an adjacent bus waveguide, as you can see in Fig. 6. If the optical ring resonator and the waveguide areclose enough, the light in the waveguide will be transmitted into the ring because of the transmission effect.There are three aspects that affect the optical coupling: the distance, the coupling length and the refractiveindices between the waveguide and the optical ring resonator. The closer the distance and the longer thecoupling length, the easier the optical coupling happens.

Figure 6: Examples of silicon ring resonators used in different devices [8].

The transmission spectrum of the bus waveguide with a single ring resonator shows dips around the ringresonances. This way, the ring resonator behaves as a spectral filter, which can be used for applications inoptical communication, especially ’wavelength division multiplexing (WDM)’. WDM is a technology whichmultiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of

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laser light. Alternatively, these ring spectra can be used for sensing - the position and the shape of theresonance dips are very sensitive to a variety of effects, which can be detrimental (stability of a filter) oradvantageous (as a sensor, or for tuning) [8]. Now let us first take a look at two ways we can couple aring resonator to a waveguide and later we will discus some of the applications that use these kinds of ringresonators.

3.1 All-pass ring resonators

The most simple form of constructing a ring resonator is by feeding one output of a directional coupler backinto its input, the so-called all-pass filter (APF), which we can see in Fig. 7.

Figure 7: All-pass ring resonator, where r is the self-coupling coefficient, k is the cross-coupling coefficientand a is the single-pass amplitude transmission of the electric field. [8].

The interaction between an optical ring resonator and a waveguide can be described by the relations:

Et1 = rEi1 + kEi2 (1)

Et2 = −k∗Ei1 + r∗Ei2, (2)

where Ei1 is the complex amplitude of the incoming electric field (Einput) in the waveguide, Et1 is thecomplex amplitude of the transmitted electric field (Epass) in the waveguide, Et2 is the complex amplitudeof the electric field that comes into the ring and and Ei2 is the complex amplitude leaving the ring and r andk are the coupler parameters and they satisfy equation r2 + k2 = 1, which means there are approximatelyno losses in the coupling section.

In order to further simplify the model, Ei1 is chosen to be equal to 1. Then the round trip in the ringis given by:

Ei2 = αEt2eiφ, (3)

where φ = Lβ is the single-pass phase shift, with L the round trip length and β the propagation con-stant of the circulating mode. α is the power attenuation coefficient.

From these equations we obtain the exact form of Et1, Et2 and Ei2, which further leads to the belowstated equations for transmittance.

If we assume that continuous wave operation and matching fields are valid, than the basic spectralproperties of an APF ring resonator can easily be derived. Furthermore if we also assume that reflectionsback into the bus waveguide are negligible, than we can write ratio of the transmitted and incident field inthe bus waveguide as

EpassEinput

= e(π+φ)a− re(−iφ)

1− rae(iφ). (4)

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a is the single-pass amplitude transmission, including both propagation loss in the ring and loss in thecouplers. It relates to the power attenuation coefficient α [1/cm] as a2 = e−Lα. By squaring Eq. 4, weobtain the intensity transmission Tn with r being the self-coupling coefficient:

Tn =IpassIinput

=a2 − 2ra cos(φ) + r2

1− 2ra cos(φ) + r2a2. (5)

Similarly, we can define k as the cross-coupling coefficient, and so r2 and k2 are the power splittingratios of the coupler.

We find the ring to be on resonance when the phase φ is a multiple of 2π, or when the wavelength ofthe light fits a whole number of times inside the optical length of the ring:

λres =neffL

m, m = 1, 2, 3... (6)

Figure 8: (a) Effective phase delay of an all-pass filter with a = 1 and various values of the self-couplingr [8]. (b) Effective phase delay of an all-pass filter for r = 0.85 and different values of a. Note that forcritical coupling r = a = 0.85 we get an abrupt φ phase shift. For over and undercoupling, the phase shift iscontinuous, but in opposite direction [8].

Now let us consider an ideal cavity that has zero attenuation, a = 1. This predisposition leads totransmission being unity for all values of detuning φ. Under critical coupling, when the coupled power isequal to the power loss in the ring 1− a2 = k2 or r = a, the transmission at resonance drops to zero, as wecan see in Fig.10. The phase argument of the field transmission varies periodically with frequency. All-passresonators delay incoming signals by temporarily storage the optical energy within the resonator [8]. FromEq. 4 we can also calculate the effective phase shift by:

φ = π + φ+ arctan(r sin(φ)

a− r cos(φ)) + arctan(

ra sin(φ)

a− ra cos(φ)). (7)

The phase response for a ring with no intrinsic loses (a = 1) is plotted in Fig. 8 for different values ofthe self-coupling coefficient r.

We see that for strong self-coupling (good resonances) the phase response becomes very steep near theresonance. A ring resonator can thus add a significant wavelength dependent phase shift, which can be usedto slow down light in an optical buffer.

In the Fig. 8 we see the phase response for different values of intrinsic ring loss a and a fixed r = 0.85.We see that for critical coupling, when r = a, the transmission experiences sudden π phase shift at theresonance wavelength. However, for overcoupling, where r < a, the transmission experiences continuouspositive phase delay, while for undercoupling, when r > a the phase shift near the resonance shows strongdecrease, which is plotted as a 2π phase shift [8].

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Figure 9: Add-drop ring resonator [8].

3.2 Add-drop resonators

When the ring resonator is coupled to two waveguides, the incident field is partly transmitted to the dropport. The transmission to the pass and the drop port can also be derived from using the same limitationsas before:

Tp =IpassIinput

=r22a

2 − 2r1r2a cos(φ) + r211− 2r1r2a cos(φ) + (r1r2)2

, (8)

Td =IdropIinput

=a(1− r21)(1− r22)

1− 2r1r2a cos(φ) + (ar1r2)2. (9)

If we again take a look at an ideal cavity, where the attenuation is negligible (a = 1), critical couplingoccurs at symmetric coupling (k1 = k2). For a lossy resonator, critical coupling occurs when the losses matchthe coupling as ar2 = r1.

Figure 10: Transmission spectrum of an all-pass ring and the two outputs of add-drop ring with the importantspectral features indicated. a = 0.85, r = r1 = r2 = 0.9. Because of the additional losses introduced bythe second coupling section, the add-drop rings have a broader peak. Also coupling is further from criticalcoupling, resulting in a smaller extinction ratio [8].

The characteristic parameters are indicated in Fig. 10. They depend on the losses and coupling coeffi-cients and can be extracted directly from the formulas for transmission Eq. 5, Eq. 8 and Eq. 9. From thesame equations we can derive the full width at half maximum (FWHM) of the resonance spectrum for anall-pass ring resonator [8]:

FWHM =(1− ra)λ2resngLπ

√ra

, (10)

and for an add-drop ring resonator configuration:

FWHM =(1− r1r2a)λ2resngLπ

√r1r2a

. (11)

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Within a first order approximation of the dispersion, the wavelength range between two resonances orFSR in function of wavelength equals:

FSR =λ2

Lng. (12)

In both FWHM and FSR we find the group index, which takes into account the dispersion of thewaveguide and is defined by

ng = neff − λ0dneffdλ

. (13)

3.3 Applications of ring resonators

Now, that we understand how simple ring resonators work, we can quickly take a look at some most interestingapplications. Because their geometry and concept is very simple, they are usable for a variety of applications.As a resonator, they can serve as spectral filters, which can be very useful for communication purposes. Theycould also be employed in optical delay lines, since they can store optical energy in a resonance. Becauseof their sensitivity, they are suitable for a variety of sensing microring applications [8]. Besides all thesepassive applications, silicon microrings are also usable as active ring resonators, where an active section isincorporated in the ring. This could be a fast phase shifter to shift the resonance, to make a modulator, orit could be a gain or absorption spectrum, to make a ring laser or resonant detector.

Figure 11: (a and b) Principle of a wavelength-selective two-ring switch concept and a response in dropoperation [8]. (c and d) Wavelength tuning by driving the tuner in the same direction and switching to passoperation by driving the tuners in opposite direction [8].

Using a double ring resonator as a wavelength drop filter has an additional advantage: when both ringscan be tuned individually, it becomes possible not only to move the resonance wavelength around (by tuningboth rings in the same direction) but also switch the resonance on or off (by detuning the rings with respectto one another). This is illustrated in Fig. 11. This turns the rings into a wavelength-selective switch, whichcan drop a wavelength off a bus waveguide without disturbing the adjacent wavelength channels.

Compact ring resonators can be used to realize optical delay lines or buffers in photonic integratedcircuits. Near resonance, the ring resonators will have a strong dispersion, and therefore a large group delay,storing the optical signal before releasing it. This large group delay, combined with a relatively large band-width provides an ideal combination for optical buffers. While the group delays generated by a single ringresonators are too small for practical applications, high order ring resonators can be used to increase groupdelay. Fig. 12 shows the two most widely used delay line configurations: a SCISSOR consisting of all-passfilter, and a coupled resonator optical waveguide (CROW) [8].

As I have mentioned before, apart from rings being used as passive devices (where they filter opticalsignals or act as a sensor), they can also be used as an electrically activated device. In a ring modulator, theresonator is tuned in such way that the operating wavelength is on the slope of the resonance peak. If wethen modulate the optical length of the ring (by changing the effective index in the ring), the resonance peakis shifted and the transmission/reflection of the cavity is changed. When using ring resonators, the most

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Figure 12: SEM of multiple APFs (left) and coupled resonator optical waveguide (right) [8].

commonly used configuration is a single all-pass filter close to critical coupling. In this configuration, thereis a strong dip in the transmission spectrum of the bus waveguide, which means that a large modulationdepth ERmod can be achieved with a relatively small shift of the rings resonance frequency [8].

Figure 13: (a) Top view of a schematic ring modulator: The ring has an active section. (b) A diode isembedded in the cross section of the active region. (c) When biased, the transmission drops: The modulatoris operated on the slope of the ring [8].

The modulation of the effective index in the ring can be done in many different ways. The mostcommonly used technique relies on manipulating the carrier density in the ring. If we put a p-n diode inthe core, which can be reverse-biased to increase or decrease the depletion zone in the junction, we canmanipulate the concentration of electrons and holes thus changing the refractive index and absorption ofsilicon as shown in Fig. 13. Manipulating the carrier density in the ring will result in a modulation of theeffective index.

4 LASER-s

If we want to use light (photons) to transfer information we first need a special light-source. The easiest way toproduce light signal that has a specific wavelength is by using lasers. Laser stands for Light Amplification byStimulated Emission of Radiation. This means that lasers emit light through a process of optical amplificationbased on the stimulated emission of electromagnetic radiation.

Figure 14: Stimulated emission [9].

The main difference from other sources of light isthat lasers emit light coherently. Besides spatialcoherence laser beams also have high temporal co-herence, which means lasers can emit light with avery narrow spectrum. Temporal coherence can beused to produce pulses of light as short as a fem-tosecond which is of great importance when talkingabout producing light signal in order to transfer in-formation.Basic lasers are build from a gain medium (to am-plify light), a mechanism to supply energy to thegain medium (electric current or light at a different

wavelength) and an optical cavity, which consist of a pair of mirrors (one of the two being partially trans-

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parent). Light bounces back and forth between the mirrors, passing through the gain medium and beingamplified each time.

The gain medium has to be excited by an external source of energy into an excited state in order for theamplification process to be successful. The gain medium absorbs pump energy, which raises some electronsinto excited quantum states. Particles can interact with light by either absorbing or emitting photons. Weknow two kinds of emission - spontaneous (without external influence) or stimulated (caused by externalinfluence). In the process of stimulated emission another photon is emitted in the same direction as thelight that is passing by ass seen in Fig. 14. In the end we get two identical coherent photons that havethe same wavelength phase and direction of propagation. When the number of particles in one excited stateexceeds the number of particles in some lower-energy state, population inversion is achieved and the amountof stimulated emission due to light that passes through is larger than the amount of absorption. This processis called amplification.

Now that I have explained how a laser works, we can take a look at some lasers we use in silicon photonics.It is very important that lasers in any communication system operate within the proper parameters for theoptical fibers in the system. The lasers must send out a signal within the optimum transmission windows ofthe fibers with enough power to reach the receiver or repeater with a sufficient signal-to-noise ratio, but notso much as to create undesirable nonlinearities.

Figure 15: (a and b) An example of a laser diode [10],[11]

Most used lasers in integrated photonics are semiconductor laser diods [12]. A simple example of suchlaser is shown in Fig. 15. The forward-biasing voltage, V, causes electrons and holes to enter the depletionregion and recombine. Alternatively, we can say that the external energy provided by V excites electrons atthe conduction band. From there, they fall to the valence band and recombine with holes.

The most commonly used laser is so called VCSEL, which stands for vertical-cavity surface-emittinglaser. The first VCSELs produced 850-nm radiation with a beam width on the order of 10 micrometers.Because these lasers produced multiple transverse modes, they could be used only with multi-mode fiber.By 2002, 1.5- to 1.6-micrometers VCSELs had evolved into tunable components for single-mode fiber [13].

Figure 16: An illustrated figure of a simple VCSEL[13].

A VCSEL is a type of semiconductor laserdiode with laser beam emission perpendicular fromthe top surface, contrary to conventional edge-emitting semiconductor lasers which emit from sur-faces formed by cleaving the individual chip out ofa wafer. The laser resonator consists of two dis-tributed Bragg reflector mirrors (DBR) parallel tothe wafer surface. The planar DBR-mirrors consistof layers with alternating high and low refractive in-dices as shown in Fig. 16. Each layer has a thicknessof a quarter of the laser wavelength in the material,yielding intensity reflectivities above 99%. This kindof mirrors are required in VCSELs to balance theshort length of the gain region. DBR-mirrors are

doped as p-type and n-type materials, forming a diode junction [13].

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5 Conclusion

Optical communication, the core of broadband networks, has superseded copper wires in long-haul datacommunication systems, significantly improving their speed, bandwidth and signal quality. The integrationof optical cable in short-distance communication systems (such as those within or between computer clusters,within a motherboard or within a microprocessor chip) will not only further boost internet speeds, it will alsoenable the development of extremely powerful computers. Silicon photonics can provide broad bandwidthswith very low optical absorption; it has been successfully implemented in low-loss optical waveguides andother passive components, such as microring resonators.

In this seminar I have covered three optical devices that play a very important role in the realizationof silicon photonic integrated circuits. Waveguides are much like roads that guide light from one point tothe other, thus connecting every point in a circuit. Without lasers there would be no photons to transferinformation and without microring resonators it would be a lot harder to detect and process optical signals. Iwould like to point out that integrated photonic circuits are still far away from electronic integrated circuits.In this seminar I have covered only three basic elements that I found very interesting and can be used tobuild several different optical devices that are similar to the electronic devices.

References

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[4] A. Rickman, The commercialization of silicon photonics, Nature photonics, 2014, 8, 579

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[7] K. Okamoto, Progress and technical challenge for planar waveguide devices: silica and silicon waveguides,Laser & photonics reviews, 2012, 6, 14

[8] W. Bogaerts, P. De Heyn, T. Van Vaerenbergh, K. De Vos, S. Kumar Selvaraja, T. Claes, P. Dumon, P.Bienstman, D. Van Thourhout and R. Baets, Silicon microring resonators, Laser & photonics reviews,2012, 6, 47

[9] https : //en.wikipedia.org/wiki/Stimulated emission (Figure 14)(7.12.2015)

[10] https : //creativentechno.wordpress.com/2012/01/07/how − led− works/ (Figure 15, a)(7.12.2015)

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[12] J. Justice, C. Bower, M. Meitl, M. B. Mooney, M. A. Gubbins and B. Corbett, Wafer-scale integrationof group III-V lasers on silicon using transfer printing of epitaxial layers, Nature photonics, 2012, 6,610

[13] P. Daukantas, Lasers in communications, Optics & photonics news, 2010, 21, 28

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