MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI

Transcription

1 MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI DEPARTMENT : ECE SUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS SUBJECT CODE : EC 2402 UNIT III: SOURCES AND DETECTORS PART -A (2 Marks) 1. What are the three requirements of Laser action? 1. Absorption 2. Spontaneous emission 3. stimulated emission. 2. What are the three types of Laser diode structures? 1. Gain indexed guide 2. Positive indexed guide 3. Negative indexed guide 3. What are the fundamental structures of Index guided lasers? 1. buried hetero structure. 2. Selectively diffused construction 3. Varying thickness structure 4. Bent layer configuration. 4. What is the difference between LED and LASER diode? [AUC NOV 2013] LED Incoherent No optical resonant cavity Output radiation has broad spectral width No spatial and temporal coherence LASER Coherent Optical energy from optical resonant cavity Highly monochromatic Spatial and temporal coherence 5. What is the significance of intrinsic layer in PIN diodes? [AUC NOV 2012] PIN photo detector structure consists of P and N regions separated by a very lightly n-doped intrinsic region. When the photodiode is reverse biased, the intrinsic region of diode is fully depleted of carriers. 6. what are the advantages of LED? [AUC MAY 2012] Simpler fabrication Low cost Reliability Generally less temperature dependence Linearity Simpler drive circuitry

2 7. what are the characteristics that a semiconductor laser diode should possess? [AUC NOV 2011] For fiber optical systems, semiconductor laser diodes are used as an optical source. Because its output radiation is highly monochromatic and the light beam is very directional. 8. What are the properties of photo detector? [AUC NOV 2007] High response or sensitivity Minimum addition of noise Fast response speed Long operating life Reasonable cost 9. What is avalanche multiplication noise? [AUC NOV 2008] The shot noise and bulk dark current noise in a photo detector are multiplied by avalanche gain mechanism. These multiplied noise are referred to avalanche multiplication noise. 10. What is dark current noise? [AUC April 2004,MAY2010] The dark current noise arises from electrons or holes which are thermally generated in the PN junction of the photo diode. This current continues to flow through the bias circuit of the device when no light is incident on the photodiode. 11. Define detector response time? [AUC MAY2013] The response time of a photodiode together with its output circuit depends on following three factors. Transit time of a photo-carriers in the depletion region The diffusion current of the photo-carriers generated outside the depletion region. The RC time constant of a photo diode and its associated circuit. 12. What are the merits of index guided lasers? [AUC MAY 2010] Index guided laser supporting only fundamental transverse mode and longitudinal mode is known as single mode laser. Such a device emits a single well collimated beam of light having bell shaped Gaussian intensity profile. 13. Define longitudinal modes. [AUC NOV 20O9] Longitudinal modes are associated with the length of the cavity and determine the typical spectrum of the emitted radiation. 14. Define lateral modes. [AUC NOV 2010] These modes lie in the plane of the pn junction. They depend on the sidewall preparation and the width of the cavity. It determines the shape of the lateral profile of the laser beam.

3 PART (B) 1. Draw the schematic diagram of high radiance surface emitting LED and explain the working in detail? [AUC NOV 2011] Ans: High radiance is obtained by restricting the emission to a small active region within the device. A well is etched in a substrate (GA As) to avoid the heavy absorption of the emitter radiation and to accommodate the fiber. These structures have a low thermal impedance in the active region and hence radiance emission into the fiber. Double hetero structures are used to get increased efficiency and less optical absorption. The structure of a high radiance etched well DH (Double Hetero structure) surface emitter which is also known as burros type LED is as shown in figure (4.5). This structure emits light in band of 0.8 to 0.9 um wavelength. The plane of the active light emitting region is made perpendicular to the fiber axis. The fiber is cemented in a well matched through the substrate of the fiber so that maximum emitted light is coupled to the fiber. Due to large band gap conjoining area, the internal absorption is less and the reflection coefficient at the back crystal face is high, hence forward radiance is good. The active area in circle is of 50μm in diameter and up to 2.5μm thick. The emission from this active area is isotropic with

4 120 half power beam width is used for practical purpose. Isotropic pattern from a surface emitter is lambert Ian pattern. The source is equally bright when viewed from any direction but power diminishes as cosφ where $ is the angle between viewing direction and to the normal to the surface. Power is down to 50%, when Φ = 60, so that the total half power beam width is 120. The power coupled into a multimode step index fiber may be estimated from the relationship. PC =П(1-r)ARD (NA)2..(1) Where, PC = Power coupled into fiber r = Fresnel reflection coefficient A = Emission area of source RD = Radiance of the source NA = Numerical aperture Power coupled into the fiber depends on (i) Distance and alignment between emission area and the fiber. (ii) Medium between the emitting area and the fiber. (iii) Emission pattern of SLED Addition of Epoxy resin in the etched well reduces the refractive index mismatch and increases the external power efficiency of the device. Hence the power coupled in the double hetero structure surface emitters are more than Pc(optical power) that is given by equation (1), For graded index fiber-direct coupling requires the source diameter of about one half the fiber core diameter. 2. Draw the schematic of edge emitting double hetero junction LED and explain its working in detail? [AUC MAY 2012] Ans: Double Hetero junction Laser If a single p-n junction diode is fabricated from suitable single crystal semiconductor material it exhibits photo emissive properties. It is known as 'homo junction' p-n diode. However the emissive properties of a junction diode can be improved considerably by the use of 'hetero junction'. A hetero junction is an interface between two adjoining crystal semiconductors having different values of band gap energies. Devices are fabricated with hetero junction are said to have hetero structures. They are of two types, (i) Isotopes such as n-n or p-p type (ii) Anisotope such as p-n type. The isotope p-p junction has a potential barrier within the structure. The structure is capable of confining min carriers to small active region called cavity. It effectively reduces the diffusion

5 length of the carrier and thus the volume of the structure where radioactive recombination may occur. Figures show the schematic layer structure, energy band diagram and refractive index profile, for a double hetero junction injection laser diode with biasing. The laser oscillations take place in the central p-type GaAs region which is known as active layer. There is hetero junction at the both sides of the active layer. A forward bias voltage is applied by connecting the positive electrode of the power supply voltage to the P-side of the structure and negative electrode to the n-side when a voltage which is almost equal to the band gap energy. The hetero junctions are used to provide potential barrier in the injection laser. In this structure it is possible to obtain both carrier and optical containment to the active layer.

6 The above figure represents the layer structure of a broad area DH. The GaAs layers acts as active layer which is sandwiched between p-type AZGaAs and n-a/gaas layer and these two layers act as the confinement layers. Light is emitted from the central GaAs active layer through the front and back side of the device. In the case of the DH broad area laser structure, the optical confinement in the vertical direction is achieved by the refractive index at the hetero junction interfaces between the active layer and the containment layers, but the laser.action takes places across the whole width of the device. In a broad area laser the sides of the cavity are formed by simple roughening the ends of the device to reduce the unwanted emission and limit the horizontal transverse modes. Stripe Geometry Laser In order to overcome the difficulties in a broad area laser structure, the stripe geometry laser structure has developed and in this structure the active area does not enter upto the edges of the device. A common method is used to introduce the stripe geometry to the structure which provides the\ optical contaminant in the horizontal plane as shown in the figure above. The stripe geometry is usually formed by creating a high resistance area on either side of the stripe by 'Proton bombardment' technique or by oxide oscillation. The stripe usually acts as a guiding mechanism which avoids all major difficulties encountered in the case of a broad area laser. The contact stripe provides the balance of guiding single transverse mode operation in a direction parallel to the junction plane, whereas broad area devices allow multiple mode operation in this horizontal plane. The width of the stripe generally ranges from 2.0 to 65 mm and stripe laser find wide application in fiber communications.

7 3. Obtain the expression for the 3dB modulation bandwidth of LED and discuss the importance of radiative recombination life time? [AUC NOV 2010] Ans: The expression for the 3 db modulation bandwidth of LED in optical communication may be obtained in either electrical and optical terms.if we consider the associated electrical circuitry in an optical fiber communication system to use the electrical definition, where the electrical signal power has dropped to half of its constant value due to the modulated portion of the optical signal. Hence, this corresponds to the electrical 3 db frequency at which the output electrical power is reduced by 3 db with respect to the input electrical power. We can also consider the high frequency 3 db point, when the optical source operates down to D.C. The expression for the electrical bandwidth can be obtained from the ratio of the electrical output power to the electrical input power in decibels and is given as

8 Therefore In optical regime the bandwidth is defined by the frequency at which the output current has dropped to 0.5 of the input current to the system. The Modulation bandwidth of LED is generally determined by three methods, They are 1. The doping level in the active layer, 2. Due to the injected carriers, the reduction in radiative lifetime. 3. The parasitic capacitance of the device. If we assume that the parasitic capacitance is negligible, then the speed at which an LED can be directly current modulated is fundamentally limited by the recombination lifetime of the carriers,

9 . 4. Discuss the major requirements of an optical source for use in optical communication systems? [AUC NOV 2010] Ans: The development of efficient semiconductor optical sources along with low-loss optical fibers, led to substantial improvements in fiber optic communications. Semiconductor optical sources have the physical characteristics and performance properties necessary for successful implementations of fiber optic systems. It is desirable that optical sources must be, 1. Compatible in size to low-loss optical fibers by having a small light emitting which are capable of launching light into fiber. 2. Launch sufficient optical power into the optical fiber to overcome fiber attenuation and connection losses allowing for signal detection at the receiver. 3. Emit light at wavelengths that minimize optical fiber loss and dispersion. 4. Optical sources should have a narrow spectral width to minimize dispersion. 5. Allow for direct modulation of optical output power. Maintain stable operation in changing environmental conditions (such as temperature). Cost less and be more reliable than electrical devices, permitting fiber optic communication systems to compete with conventional systems. Semiconductor optical sources suitable for fiber optic systems range from inexpensive Light Emitting Diodes (LEDs) to more expensive

10 semiconductor lasers. Semiconductor LEDs and laser diodes are the principle light sources used in fiber optics. Semiconductor sources are designed to operate at wavelengths (i.e., 850 nm, 1300 nm and 1500 nm) that minimize optical fiber absorption and maximum system bandwidth. By designing an optical source to operate at specific wavelengths, absorption from impurities in the optical fiber such as hydroxyl ions (OH-) can be minimized. Maximizing system bandwidth involves designing fibers and sources that minimize chromatic and inter modal dispersion at the intended operational wavelength. 5. Compare the advantageous and disadvantageous of LED and Explain the key process involved in the LASER operation? [AUC NOV 2013] Ans: 1. Simple Fabrication: There are no mirror facets and is some structures no striped geometry 2. Cost: The simpler construction of LED leas to much reduced cost which is always likely to be maintained. 3. Reliability The LED does not exhibit catastrophic degradation and has proved to be less sensitive to gradual degradation than the injection laser. 4. Simpler Drive Circuitry This is due to lower drive currents and reduced temperature dependence which makes temperature compensation circuits unnecessary. 5. Linearity Ideally, the LED has a linear light output against current characteristics unlike the injection laser. Disadvantage An LED radiates rather dispersed light, which makes coupling this light into an optical fiber a problem. The key processes involved in laser action are as given below. (i) Absorption. (ii) Spontaneous emission. (iii) Stimulated emission.

11 These three key processes are represented by 2-energy level diagrams. Where, E1= Energy of ground state. E2 = Energy of excited state.

12 6. Explain the structure and operation of injection laser diode. [AUC NOV 2011] INJECTION LASER DIODE: Stimulated emission by the recombination of the injected carriers is encouraged in the semiconductor injection laser by the provision of an optical cavity in the crystal structure in order to provide feedback of photons. The injection laser has many advantages over other semiconductor sources. They are i) High radiance due to amplifying effect of stimulated emission. ii) Narrow line width of the order of 1nm or less. iii) Modulation capability in the range of gigahertz. iv) Relative temporal coherence which allows heterodyne detection. v) Good spatial coherence, which allows the output to be focused by a lens into a spot which has greater intensity than the dispersed unfocussed emission. Early injection lasers had the form of Fabry Perot cavity often fabricated in gallium arsenide. The basic structure of this homojunction device is shown below. Fig: Schematic diagram of a GaAs homojunction injection laser with a Fabry Perot cavity. If a PN junction is formed by a single crystal semiconductor material, it is said to be homojunction laser diode. Generally the homojunction lasers are not used in the communication applications due to their higher divergence and high threshold current. Further they produce high dispersion and have low optical confinement. A heterojunction is an interface between two adjoining single crystal semiconductor with different band gap energies. Improved carrier confinement and lower threshold current densities were achieved using heterojunction structures.

13 Efficiency The differential external quantum efficiency η D which is the ratio of the increase in photon output rate for a given increase in the number of injected electrons. If P e is the optical power emitted from the device, I is the current, e is the charge of an electron and hf is the photon energy, then Where E g is the band gap energy expressed in electron volts. η D is sometimes referred to as the slope quantum efficiency. For a continuous wave semiconductor laser it ranges between 40-60%. The internal quantum efficiency of the semiconductor laser η i, which is defined by Its value ranges between %. It is related to differential external quantum efficiency by the expression Where is the loss coefficient of the laser cavity, L is the length of the laser cavity and r1 and r2 are the cleaved mirror reflectives. Another parameter is the total efficiency η r which is defined as As the power emitted P e changes linearly when the injection current I is greater than the threshold current I th, then

14 For high injection current (e.g. I= 5 I th ) then η T η D, where as for lower currents, I= 2 I th the total efficiency is lower and around %. The external power efficiency of the device η ep in converting electrical input to optical output is given by Where P = IV is the dc electrical input power. Using eqn 6.29 for the total efficiency, we find 7. Explain single mode laser structure. [AUC MAY 2009] Single mode laser structures: i) Buried Heterostructure (BH) Laser: The BH laser is obtained by etching a narrow metal stripe in DH semiconductor material and effectively burying it in high resistivity, lattice matched n type material with an appropriate bandgap and low refractive index. This rocess involves growth of double liquid phase eitaxial (LPE) to give the structure as shown below. Fig: schematic representation of the structure of GaAs/ Al Ga As buried heterostructure laser

15 These devices may have very small active regions to allow single mode operation. The small active region also gives low threshold currents of 10 ma or less and good linearity without kinks. Wide modulation bandwidths of 2 GHz have also been obtained. The major drawback is the beam divergence in the junction plane (40-50 ) from the small active region. ii) Transverse Junction Stripe (TJS) layer: This structure is shown below which consists of a DH multilayer in n type semiconductor material. The lateral waveguide is achieved by two consecutive zinc diffusions in order that the structure is confined by p+ -n and p-n gallium arsenide homojunctions. Carrier injection is obtained laterally across the homojunction in the central layer and the DH structure confines the carriers to the central active region. The device has good characteristics for single longitudinal and transverse mode operation with typical threshold currents of the order of 20 ma giving continuous wave optical output power of around 3mW. Fig: structure of a GaAs/ AlGaAs transverse junction stripe laser. iii) Distributed Feedback (DFB) Lasers: These devices consist of a complex structure which determines the wavelength of the longitudinal mode emission. An optical grating is incorporated into the heterosructure waveguide to provide periodic variations in refractive index along the direction of wave propagation so that feedback of optical energy is obtained through Bragg scattering. The corrugated grating may be applied over the whole active length of the device where it gives what is known as distributed feedback and eliminates the need for end mirrors. An improved mesa stripe geometry DFB laser structure fabricated with the GaAs/AlGaAs system which employs a separate confinement heterosructure is shown below

16 In this device the corrugations are separated from the active layer and formed in an Al 0.07 Ga 0.93.As layer on the p side of the junction. The n type Al 0.3 Ga 0.7.As and ptype Al 0.17 Ga 0.83 As layers confine the optical field to the p type GaAs active layer. However sufficient optical power leaks across the thin p type Al 0.17 Ga 0.83 As buffered layer into the corrugated region such that distributed feedback is obtained. The emission frequency from the structure is determined by the corrugation period. Hence the DFB structure can provide superior longitudinal mode discrimination over conventional Fabry Perot structures where single longitudinal mode operation is dependent on the gain spectrum of the optical cavity. When suitably fabricated and operating with a single longitudinal and transverse mode such DFB lasers can give narrow emission line widths of less than 1nm in comparison with 1-2 nm for conventional DH injection lasers. Another advantage of DFB laser over Fabry perot cavity design is that it exhibits reduced wavelength sensitivity to changes in temperature and injection current. The emission from a Fabry Perot injection laser follows the temperature dependence of the energy gap whereas the lasing from the DFB structure follows the smaller temperature dependence of the refractive index. The typical wavelength shift with temperature for a DFB laser is nmk -1 while the ordinary DH structure gives a typical shift of nmk -1.

17 8. Distinguish between LED and ILD. [AUC MAY 2012] Comparison of LED and ILD: S.No Parameters LED LASER 1 Principle of operation Spontaneous emission Stimulated emission 2 Output beam Non coherent Coherent 3 Spectral width Broad spectrum(20-100nm) Much narrower (1-5nm) 4 Data rate Low Very high 5 Transmission distance Smaller Greater 6 Temperature sensitivity Less sensitive More temperature sensitive 7 Coupling efficiency Very low High 8 Compatible fibers Multimode step index multimode GRIN Single mode SI Multimode GRIN 9 Circuit complexity Simple Complex 10. Life time 10 5 hours 10 4 hours 11 Cost Low High 12 Output power Linearly proportional to drive current Proportional to current above threshold 13 Current required Drive current 50 to 100 ma peak Threshold current 5 to 40 ma 14 Wavelengths available 0.66 to 1.65 µm 0.78 to 1.65µm 15 Applications Moderate distance low data rate Long distance high data rates

18 9. Explain detail about PIN photo detector. [AUC MAY 2008] PIN PHOTODETECTOR: The most common semiconductor photo detector is the pin photodiode shown below Fig: pin photodiode with applied reverse bias The device structure consists of p and n region separated by a very lightly n doped intrinsic (i) region. In normal operation a sufficiently large reverse bias voltage is applied across the device so that the intrinsic region is fully depleted of carriers. That is, the intrinsic n and p carrier concentrations are negligibly small in comparison with the impurity concentration in this region. When an incident photon has an energy greater than or equal to the bandgap energy of the semiconductor material, the photon can give up its energy and excite an electron from the valence band to the conduction band. This process generates free electron from the valence band to the conduction band. This process generates free electron hole pairs which are known as photocarriers, since they are photon generated carriers as shown below. Fig: Energy band diagram of a photodetector

19 The photodetector is normally designed so that these carriers are generated mainly in the depletion region where most of the incident light is absorbed. The high electric field present in the depletion region causes the carriers to separate and be collected across the reverse bias junction. This gives rise to a current flow in an external circuit, with electron flowing for every carrier pair generated. This current flow is known as photocurrent. On the average charge carriers move a distance L n or L p for electrons and holes respectively. This distance is known as the diffusion length. The time it takes for an electron or hole to recombine is known as carrier life time and is replaced by τ m and τ p respectively. The life times and diffusion lengths are related by the expressions. Where D n and D p are the electron and hole diffusion coefficients which are expressed in units of centimeters squared per second. Optical radation is absorbed in the semiconductor material according to the exponential law Here α s (λ) is the absorption coefficient at a wavelength λ, P 0 is the incident optical power level, and P(x) is the optical power absorbed in a distance x. The upper wavelength cutoff λ c is determined by the band gap energy E g of the material. It is expressed in units of electron volts (ev), then λ c is given in units of micrometers by The cutoff wavelength is about 1.06 µm for Si and 1.6 µm for Ge. If the depletion region has a width w, then the total power absorbed in the distance w is If we take into account a reflectivity R f at the entrance face of the photodiode then the primary photocurrent I p is given by Where P 0 is the optical power incident on the photodetector, q is the electron charge and hv is the photon energy. The important characteristics of a photodetector are its quantum efficiency and its response speed. These parameters depend on the material bandgap, the operating wavelength and the doping and thickness of the p,i and regions of the device. The quantum efficiency η is the number of electron hole carrier pairs generated per incident photon of energy hv and is given by

20 Here I p is the average photocurrent generated by a steady state average optical power P 0 incident on the photodetector. The performance of a photodiode is often characterized by the responsivity R. this is related to the quantum efficiency by The responsivity falls off rapidly beyond the cutoff wavelength which is shown in following figure. Fig: comparison of responsivity and quantum efficiency as a function of wavelength for pin photodiode. 10. Explain detail about Avalanche photo diode. [AUC NOV 2008] AVALANCHE PHOTODIODES: Avalanche photodiodes (APDs) internally multiply the primary signal photocurrent before it enters the input circuitry of the following amplifier. This increases receiver sensitivity, since the photocurrent is multiplied before encountering the thermal noise associated with the receiver circuit. In order for a carrier multiplication to take place, the photogenerated carriers must traverse a region where very high electricfield is present. In the high field region a photogenerated electron or hole can gain enough energy so that it ionizes bound electrons in the valence band upon colliding with them. This carrier multiplication mechanism is known as impact ionization. The newly created carriers are also accelerated in the high electric field, thus gaining enough energy to cause further impact ionization. This phenomenon is the avalanche effect.

21 A commonly used structure for achieving carrier multiplication with very little excess noise is the reach through construction shown in the below figure. Fig: Reach through avalanche photodiode structure The reach through avalanche photodiode (RAPD) is composed of a high resistivity p type material deposited as an epitaxial layer on a p + substrate. A p-type diffusion or ion implant is then made in the high resistivity material followed by the construction of an n + layer. This configuration is referred to as p + πpn + reach through structure. The π layer is basically an intrinsic material that inadvertently has some p doping because of imperfect purification. The term reach through arises from the photodiode operation. When low reverse bias voltage is applied most of the potential drop is across the pn junction. The depletion layer widens with increasing bias until a certain voltage is reached at which peak electric field at the pn junction about 5 to 10 percent below that is needed to cause avalanche breakdown. At this point the depletion layer just reaches through to the nearly intrinsic π region. The average number of electron hole pairs created by a carrier per unit distance travelled is called the ionization rate. Most materials exhibit different electron ionization rates α and hole ionization rates β. experimentally obtained values of α and β for five different semiconductor material are shown in following figure.

22 The ratio k= β/ α of two ionization rates is a measure of photodetector performance. The multiplication M for all carriers generated in the photodiode is defined by Where I M is the average value of the total multiplied output current and I p is the primary unmultiplied photocurrent. The performance of an APD is characterized by its responsivity R APD, which is given by Where R o is the unity gain responsivity. 11. Comparison between PIN and AVALANCHE photo detector. [AUC MAY 2010] Comparison of PIN and APD: S.No Parameters PIN APD 1 Sensitivity Less sensitive (0-12 db) More sensitive (5-15 db) 2 Biasing Low reverse biased voltage High reverse biased voltage (5 to 10 V) ( volts) 3 Wavelength region nm nm 4 Gain No Internal gain Internal gain 5 S/N ratio Poor Better 6 Detector circuit Simple More complex 7 Conversion efficiency 0.5 to 1 Amps/ watt 0.5 to 100 Amps/ watt 8 Cost Cheaper More expensive