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General requirements for a light source for use in optical communications 1. The emission wavelength compatible with the loss spectrum of glass fibers, 820nm, 1300nm & 1550nm. 2. The sources should be capable of modulation at rates in excess of 1GHz for high data rate transmission. 3. The spectral width of the sources should be narrow in order to minimize the bandwidth limiting pulse dispersion in the fibers. 4. The average emitted power of the source that is needed is typically few milliwatts, although higher power values are needed for very long continuous fiber links or if high loss fibers are used. 5. The radiance of the source should be as high as possible for effective coupling into the low-loss fiber with small NA ( ~0.2). This means that the beam spread of the sources must be minimized. 6. The sources must have long lifetime and it must be possible to operate the device continuously at room temperature. 7. The sources must be highly reliable. 8. The sources should be reasonably low cost. The principal light sources used for fiber optic communications applications are heterojunction-structured semiconductor laser diodes or injection laser diodes (ILDs) and light-emitting diodes (LEDs). TYPES OF OPTICAL SOURCES Wideband continuous spectra sources (Incandescent Lamps). Monochromatic incoherent sources (Light Emitting Diodes - LED). Monochromatic coherent sources (Light Amplification by Stimulated Emission of Radiation - LASER) SPECTRAL LINEWIDTHS Transmitter The starting point of the optical communication system is the transmitter where the electrical signal convert to the optical signal by modulate the optical source. Converting the electrical signal into the optical signal is using an electronic circuit. The circuit is a driving circuit. The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD). These devices are mounted in a package that enables an optical fiber to be placed in very close proximity to the light emitting region in order to couple as much light as possible into the fiber. Transmitter (Cont’ 1) In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus “every last drop” of light onto the fiber and in other cases, a fiber is “pigtailed” directly onto the actual surface of the emitter. The most important part on this circuit is the light source, because the design of the circuit is depending on the source. There are few types of semiconductor sources in communication system. The most commonly use in the communication system is Light Emitting Diode (LED) and Laser Diode (LD). These two types of sources has different characteristic and the driving circuit for each type of sources should be different. The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light. OPTICAL TRANSMITTERS On-OFF Modulation Linear Modulation Figure 1. Basic Optical Modulation Methods Intensity The basic optical transmitter converts electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels, as shown in Figure 1. Component of Transmitter Light source Driving Circuit Protection and voltage supply Light Source and Semiconductor Laser The sources for transmitter is semiconductor laser Most common semiconductor laser in the transmitter are Light Emitting Diode (LED) and Laser Diode (LD) Semiconductor Laser Semiconductor laser is developed by two of semiconductor materials that are p-type and ntype materials in which n-type material contains more electrons and p-type material contains more holes. The materials producing the energy gap between the two materials that is called band gap. Semiconductor Laser (Cont’1) Semiconductor material have conduction properties between insulator and metals The conduction properties creates the energyband that call band gap There two energy bands: valance – lower, meaning less energy and conduction – upper, meaning higher energy There are separate by energy gap as shown in figure 1 below Semiconductor Laser (Cont’2) Figure 1: Electron-hole recombination process Semiconductor Laser (Cont’3) This band gap is the place where the recombination and excitation process occur. The electron can be either at the valance band or conduction band When some external energy – either through temperature or by an external electric – is provided to the electron at the valance band, some electron acquire enough energy to leap over energy gap and occupy energy level at the conduction band This process call excitation process where electron leaves holes (positive charge carries) at the valance band Semiconductor Laser (Cont’4) As a result, the electron-hole recombination process releases the quantum of energy-a photon that called laser (Mynbaev, 2001). The concentration of electron and holes is known as the intrinsic carries concentration ni Doping can increase the conduction and the doped semiconductor is called an intrinsic material Doping can increases the conduction by two method: first, doping to adding electron concentration and create donor level as shown in figure 2 below Semiconductor Laser (Cont’5) Electron energy Electrons concentration distribution Conduction band Ec No. of electron states Ep Donor level No. of hole states Ev Valance band Figure 2: Donor level and electron concentration at conduction band Semiconductor Laser (Cont’6) Donor level will provide an electron to the conduction band, thus increase the electron concentration at the conduction band Second by increase the hole concentration at valence band by creating the acceptor level as shown on figure 3 Accepter level will receive the electron from the valance band, thus increase the conduction and leave high hole concentration at valance band The energy produce by the semiconductor laser is depend on the surface of the material where there are indirect band gap and direct band gap. Band Gaps Semiconductor material have two types of band gaps: Direct band gap and Indirect band gap Direct band gap, where the electron and hole have the same momentum value as shown on figure 4 Indirect band needs a thirds particle to conserve momentum where the conduction band has minimum energy and valance band has maximum energy level as shown on figure 5 Band Gaps (Cont’ 1) Conduction band Direct band gap energy Edir λ3 λ2 Electron trasnsition λ1 Photon energy hv = Edir Valance band Figure 4: Direct band gap material Band Gaps (Cont’ 2) Conduction band Photon of energy Eph λ2 Electron trasnsition λ1 Photon energy hv = Eind + Eph Indirect band gap energy Eind Valance band Figure 5: Indirect band gap material Direct and Indirect Bandgap Semiconductors Number of photons generated Number of e - crossing the junction Rate of Rad. Rec. Rate of radiative recomb. Rate of Non - Rad. Rec. 1 / rr 1 / rr 1 / nr int nr rr / rr nr rr 1 Think of rr or nr as the ave. time e-h recombines 1 rr / nr But 1/nr varies drastically for direct or indirect bandgap material. 1 injected into LED is I then, If current But varies for different material. Rr Rnr I / q Very different for direct bandgap/indirect rr Rr is totalmaterial no. of photon generated bandgap / sec, thus optical power generated internally to LED I hcI Pint int hv int q q By assuming outside medium is air, total power emitted by LED Pint P n(n 1) 2 Example How semiconductor laser works The pn junction on semiconductor material create the depletion region This region create when electron from n material filled holes at p material. When the equilibrium state achieved its prevent from net movement of charges The junction or the depletion now has no mobile carrier, since electron and holes locked into covalent structure. If external battery is connected to the junction in reversed biased the depletion region will increase and the minority carrier flow across the junction How semiconductor laser works (Cont’ 1) Minority carrier flow p side n side Widened depletion region + - Figure 6: Reverse biased widen the depletion region How semiconductor laser works (Cont’ 2) This will act as photodiode as shown on figure 6 If the external battery supply connected to the junction in forward biased, then the magnitude of the barrier potential reduced and allow the electron to diffuse Conduction band electron on the n side material across the and recombine with the opposite charge Once across, they significantly increase the minority carriers concentration and the access carrier then recombine with the opposite charge majority carriers The recombination process of excess minority carriers is the mechanism which optical radiation is generated How semiconductor laser works (Cont’ 3) p side n side narrowed depletion region - + Figure 7: Forward biased lowering the barrier potential Light Emitting Diodes (LED) LED is a semiconductor diode; the construction of the LED is same as other diode but the other regular diode is loss the recombination energy in the thermal. LED is used the recombination energy into radiation spectrum of light. LEDs have relatively large emitting areas and as a result are not as good light sources as LDs. However, they are widely used for short to moderate transmission distances because they are much more economical, quite linear in terms of light output versus electrical current input and stable in terms of light output versus ambient operating temperature. Light Emitting Diodes (LED) (Cont’1) LEDs are of interest for fiber optics because of five inherent characteristics: They are small They possess high radiance (i.e., They emit lots of light in a small area). The emitting area is small, comparable to the dimensions of optical fibers. They have a very long life, offering high reliability. They can be modulated (turned off and on) at high speeds. LED Structures and Configuration There are two possible structure in LED: Homostructure and heterostructure Homostructure configuration have drawback where the active region is too defuse which makes the device’s efficiency very low Homostructure makes the device radiates a broad light beam and make coupling light into fiber inefficient. LED Structures and Configuration (Cont’1) Most LED is design using heterostructure because its gives good confinement of recombination process Two type LED configurations Edge Emitting LED (ELED) Surface Emitting LED (SLED) Energy Gaps in LEDs Eg=hc/l = 1240eV-nm/l (1) Equation 1 defines the bandgap energy Eg: Where: h = Plank's Constant = 4.13 x 10-15 eV•s c = speed of light = 2.998 x 108 m/s l = wavelength in nm LED Configuration (a) (b) Figure 8: LED structures a) Edge emitters b) Surface emitter LED Configuration (Cont’1) The Full Half Wave Maximum (FHWM) of LED is depending on the emitting surface of LED where surface emitting will give more FWHM compare to the edge emitting. In the optical communication system, LED source is using by slicing into small spectrum width by using the grating. Edge Emitter More complex and expensive devices, offer high output power levels and high speed performance The output power is high because the emitting spot is very small, typically 30-50 µm, allowing good coupling efficiency to similarly sized optical fibers. Relatively narrow emission spectra. The fullwidth, half-maximum (FWHM) is typically about 7% of the central wavelength Edge Emitter (Cont’1) The super-radiant LED: a cross between a conventional LED and a laser. Have a very high power density and possess some internal optical gain like a laser, but the optical output is still incoherent, unlike a laser. Very narrow emission spectra, typically 1-2% of the central wavelength and offer power levels rivaling a laser diode. These devices are popular for fiber optic gyroscope applications. Surface Emitter Comparatively simple structure, relatively inexpensive, offer low-to-moderate output power levels, and are capable of low-to-moderate operating speeds. Optical output power is as high or higher than the edgeemitting LED, since the emitting area is large, causing poor coupling efficiency to the optical fiber. Surface-emitting LEDs are almost perfect Lambertian emitters. This means that they emit light in all directions. Surface Emitter (Cont’1) The radiant intensity is maximum normal to the surface and decreases in proportion to the cosine of the angle from the normal. N = N0cosA (Lambert’s cosine law ) Thus very little of the total light goes in the required direction for injection into an optical fiber. LED Materials There are many material in construct LED, for example GaAlAs (gallium aluminum arsenide) for short-wavelength devices. Long-wavelength devices generally incorporate InGaAsP (indium gallium arsenide phosphide). These material gives different energy gap as shown in table 1 below Different material also will gives different wavelength for different application LED Materials (Cont’1) Material Energy Gap Eg (eV) Wavelength (nm) Si 1.17 1067 Ge 0.775 1610 GaAs 1.424 876 InP 1.35 924 InGaAs 0.75-1.24 1664-1006 AlGaAs 1.42-1.92 879-650 InGaAsP 0.75-1.35 1664-924 Table 1: Energy gap and wavelengths LED Materials (Cont’2) The first materials, GaP and AlAs, are used to make emitters in the visible portions of the spectrum. The next three materials, GaAs, InP, and AlGaAs, are used to make emitters in the near infrared portion spectrum generally referred to as the “first window” in optical fiber. The last material, InGaAsP is used to make emitters in the infrared portion spectrum referred to as the “second and third windows” in optical fibers. The energy gap corresponds to the energy of the emitted photons and also is indicative of the voltage drop associated with a forward biased LED. LED Optical Spectrum Figure 9: LED Spectrum Light Emitting Diode For optical communication systems requiring bit rates less than approximately 100200 Mb/s together with multimode fiber-coupled optical power in the tens of microwatts, semiconductor light emitting diodes (LEDs) are usually the best light source choice. These LEDs require less complex drive circuitry than laser diodes since no thermal or optical stabilization circuits are needed, and they can be fabricated less expensively with higher yields. To be useful in fiber transmission applications an LED must have 1. A high radiance (or brightness)- a measure in watts, of the power radiated into a unit solid angle per unit area of the emitting surface. 2. A fast response time - the time delay between the application of a current pulse and the onset of optical emission. 3. A high quantum efficiency - the fraction of injected electron-hole pairs that recombine radiatively. In a heterostructure LED, carrier confinement is used to achieve a high level of radiative recombination in the active region of the device, which yields a high quantum efficiency. Optical confinement is of important for preventing absorption of the emitted radiation by the material surrounding the pn junction. This dual confinement leads to both high efficiency and high radiance. LED structures The two basic LED configuration being used for fiber optics are surface emitters (also called Burrus or front emitters) and edge emitters. Surface Emitter LED (SLED) In the surface emitter, the plane of the active light-emitting region is oriented perpendicularly to the axis of the fiber. Normally, a well is etched through the substrate of the device, into which a fiber is then cemented in order to accept the emitted light. The circular active area in practical surface emitters is nominally 50μm in diameter and up to 2.5mm thick. The emission pattern is essentially isotropic with a 1200 half-power beam width and is called a lambertian pattern. In this pattern, the source is equally bright when viewed from any direction, but the power diminishes as cosθ, θ is the angle between the viewing direction and the normal to the surface. The power coupled Pc into a multimode step index fiber may be estimated from the relationship , where r is the Fresnel reflection coefficient at the fiber surface, A is the smaller of the fiber core cross section or the emission area of the source and RD is the radiance of the source. The power coupled into the fiber is also dependent on other factors such as the distance and alignment between the emission area and the fiber, the SLED emission pattern and the medium between the emitting area and the fiber. Edge emitter LED (ELED) The high radiance ELED used in optical communications is similar to a conventional contact stripe injection laser. This structure forms a waveguide channel that directs the optical radiation toward the fiber core. Most of the propagation light is emitted at one end face only due to a reflector on the other end face and an antireflection coating on the emitting end face. The emission pattern of this ELED is more directional than that of the SLED. In the plane parallel to the junction, where there is no waveguide effect, the emitted beam is lambertian with half-power width of . In the plane perpendicular to the junction, the half-power beam width has been made as small as 25-350 by a proper choice of the waveguide thickness. LED Optical output power Intrinsically the LED is a very linear device in comparison with the majority of injection lasers and hence it tends to be more suitable for analog transmission. The surface emitter radiates significantly more optical power into air than the edge emitter, and that both devices are reasonably linear at moderate drive currents. Output spectrum The spectral linewidth of an LED operating at room temperature in the 0.8 to 0.9 mm wavelength band is usually between 25 and 40nm at the half maximum intensity points (FWHP). For materials with smaller bandgap energies operating in the 1.1 to 1.7mm wavelength region the linewidth tends to increase to around 50 to 160nm. Typical spectral output characteristics for InGaAsP devices. The output spectral widths of SLEDs tend to be broader than those of edge-emitting LEDs because of different internal-absorption effects. The output spectra tends to broaden at a rate of between 0.1 and 0.3 nm0C-1 with increase in temperature due to the greater energy spread in carrier distributions at higher temperatures. The peak emission wavelength is also shifted by +0.3 to 0.4nm0C-1 for AlGaAs devices and by +0.6nm0C-1 for InGaAsP devices. Analog LED Driver Circuits Figure 12 below is the simplest of the three configurations. It uses a transistor, Q1, and a limited amount of resistors to convert an analog input voltage into a proportional current flowing through the LED, D1. Also referred to as a transconductance amplifier, this configuration converts a voltage into a current. In LEDs, the light output equates proportionally to the drive current, not the drive voltage. LEDs exhibit a peak drive current at about 100 mA, and the voltage drop is typically 1.5 Volts. Analog LED Driver Circuits (Cont’1) Figure 12: Simple LED circuit Analog LED Driver Circuits (Cont’2) Figure 13 eliminates most of the nonlinearities associated with Q1. In this case, U1 forms a feedback loop that drives the base of Q1 in such a way that assures that VR2 equals VIN. The circuit still experiences some lesser nonlinearities associated with Q1 and also is limited by the delay associated with the feedback signal in the servo loop formed by U1. Thus allowing the circuit to only achieve a bandwidth of about 10-100 MHz. The circuit in Figure 3b work well in application transmitting DC coupled analog signals Analog LED Driver Circuits (Cont’3) Figure 13:Linear, Low Frequency LED driver circuits Digital LED Drive Circuit The main concern on the circuit is where the circuit can support the maximum speed. The current input to the circuit or the signal current is not the main concern since the circuit can detect the current Digital circuit and the analog circuit is does not have much different where the only data signal is in digital where the threshold level is fixed Digital LED Drive Circuit (Cont’1) Figure 14: Digital LED drive circuits. Digital LED Drive Circuit (Cont’2) Figure 14 is a simple series driver circuit. The input voltage is applied to the base of transistor Q1 through resistor R1. The transistor will either be off or on. When transistor Q1 is off, no current will flow through the LED, and no light will be emitted. When transistor Q1 is on, the cathode (bottom) of the LED will be pulled low. Transistor Q1 will pull its collector down to about 0.25 Volts. The current is equal to the voltage across resistor R2 divided by the resistance of R2. The voltage across R2 is equal to the power supply voltage less the LED forward voltage drop and the saturation voltage of the drive transistor. The key advantage of the series driver is its low average power supply current. This type of driver circuit is rarely used at data rates above 30-50 Mb/s. Laser Diode (LD) Laser diode is different to LED, even though the material used in construction lased diode is similar to the LED, but the radiate light is came from the other process, stimulated emission process. Laser diode light is monochromatic and the spectral width of the light is small. LD is a semiconductor that emits coherent light when forward biased Laser Diode (Cont’1) Laser diode also produces coherent light where all oscillations are in phase and provide better detection for receiver of an information signal. Since the laser diode is monochromatic, the light is easy directed especially directed into the fiber. Compare to the LED, laser diode is highly intense and power efficient. LED need 150mA of current to achieve power radiate at 1mW but laser diode only need 10mA current to achieve same power level. Laser Diode (Cont’2) Five inherent properties make lasers attractive for use in fiber optics: They are small. They possess high radiance (i.e., They emit lots of light in a small area). The emitting area is small, comparable to the dimensions of optical fibers. They have a very long life, offering high reliability. They can be modulated (turned off and on) at high speeds. Laser Diode Working Principle Laser from LD is create from the stimulated emission radiation Stimulated radiation has four properties External photon forces a photon with similar energy (Ep) to be emitted, where the external photon stimulates radiation with the same frequency. This will ensure the spectral width of the light is radiated narrowed Since all photon propagate in the same radiation, all of them contribute to output light. Current to light conversion occur at the high efficiency and produce high power laser output. Then, ensure the efficiency of power output where only 10mA current for 1mW power Laser Diode Working Principle (Cont’1) The stimulated photon propagates in the same direction as the photon that stimulated it; hence, the stimulated light will be well directed. Since a stimulated photon is radiated only when external triggers and both photon are said to be synchronized. This make photon are in phase and so the stimulated radiation is coherent Laser Diode Working Principle (Cont’2) To achieve stimulate emission, the number of photon should be large enough To get the large number of photon, two mirror is used to double up the number of photon This two mirror provide positive optical feedback Population inversion will provide the number of electron to excites at much higher rate Laser Diode Working Principle (Cont’3) Population inversion is occur when the high density forward current is passed through the small area active area For laser action (lasing) we need to have more electron at the higher-energy conduction band than at the lowerenergy valance band Thus we need population inversion where provide necessary condition for lasing effect because the greater the number of excited electrons, the greater the number of photon stimulated Laser Diode Working Principle (Cont’4) The number of excited electron also determine the gain of semiconductor diode The loss of diode is at the photons’ absorb by semiconductor material before they can escape to create radiation Other loss is at the mirror, where mirror do not reflect 100% of incident photon The gain is increasing by increasing the forward current The loss and the gain become equal at threshold level where the current at this level called threshold current. Laser Diode Working Principle (Cont’5) At this threshold condition, laser diode start to act as laser, where when the forward current increase the number of emitted stimulated photon increase which means the intensity of radiated light also increase Then we will get the well directed, highly intense, monochromatic and coherent light Laser Diode Working Principle (Cont’6) Lasing effect and input-output characteristic occur when below process met: Population inversion Stimulated emission Positive feedback Output Power (mW) Laser Diode Characteristic Ith Forward current (mA) Figure 15: input-output characteristic of LD Laser Diode Characteristic (Cont’1) Laser diode light that can characterized as below Monochromatic : The spectral width of the radiated light is very narrow. The line width of a laser diode can be in tenth or hundred of nanometer Well directed: A laser diode radiates narrow , well directed beam that can be easily launched into optical fiber Laser Diode Characteristic (Cont’2) Highly intense and power efficient: A laser diode can radiate hundreds of milliwatts of output power. LD making the current to light conversion 10 times more efficient than it is in the best LEDs. Coherent: Light radiates by a laser diode is coherent; where all oscillation are in phase. Laser Diode Characteristic (Cont’3) Threshold current of laser diode depend on the temperature The threshold current will increase when the temperature increase as shown in figure 16 below There are two types of laser diode design that provide solution for the temperature: Cooled laser diode and uncooled laser diode Laser Diode Characteristic (Cont’4) Uncooled laser diode means, laser diode does not require any cooling This means also laser diode consumes less overall power and heat generated at nonradiative transition of exicited electrons is small. Cooled laser diode means, laser diode needs a heat pump to transfer heat from the one place to another Cooled laser diode always include a thermoelectric cooler (TEC) which function to keep laser diode at operating temperature Laser Diode Characteristic (Cont’5) Figure 16: The threshold current at different temperature Laser Diode Characteristic (Cont’6) Laser diode is linear in terms of light output versus electrical current input, but unlike LEDs, they are not stable over wide operating temperature ranges and require more elaborate circuitry to achieve acceptable stability The other effect of temperature is changes in the slope efficiency. The slope efficiency is the number of milliwatts or microwatts of light output per milliampere of increased drive current above threshold. Most lasers show a drop in slope efficiency as temperature increases. Laser Diode Characteristic (Cont’6) Emission Pattern: The pattern of emitted light affects the amount of light that can be coupled into the optical fiber. The size of the emitting region should be similar to the diameter of the fiber core. Figure 17: Laser emission pattern and fiber core Structure of Laser Diode Laser diode structure is quite similar to the construction of the edge emitting LED Two ends surface are cleaved to make them work as mirror for positive feedback The thickness of active region in laser diode is very small Latest technology using quantum well technique to reduce the current threshold Types of Laser Diode There ere two types of laser diode commonly user in communication system Fabry-Perot laser diode Distributed feedback (DFB) laser diode Fabry-Perot laser diode The arrangement for Fabry-Perot laser diode is where the active medium is place between two mirror Two mirror in the arrangement to provide positive feedback for laser diode The wave or the photon travel from light hand mirror into the right hand mirror where each wave reflected produce 180° phase shift than continue to propagate Thus, will produce the standing wave for Fabry-Perot laser diode as shown in figure 16 For lasing action, the medium must reach the stage of population inversion. Although the population inversion is necessary to achieve lasing, it is not the only required condition. In addition, a minimum threshold gain within the amplifying medium must be also attained so that laser oscillations are initiated and sustained. This threshold gain may be determined by considering the change in energy of a light beam as it passes through the amplifying medium. The light gets amplified by the process of optical feed back inside the active medium, i.e. the medium with population inversion. The active medium has two mirrors – ideally one of the mirrors should have 100% reflectivity, and the other Amplifying medium should be partially reflecting so that the laser light can be extracted from the active medium. The structure essentially acts as a Fabry-Perot resonator. A stable output is obtained when the optical gain is exactly matched by the losses in the amplifying medium. However, there are some factors that contribute to the losses inside the active medium; these are as follows: • Absorption and scattering in the amplifying medium. • Absorption, scattering and diffraction at the mirrors. • Non-useful transmission through the mirrors. The cavity gain should be sufficient to overcome the losses, and finally to amplify the light. Gain / loss Fabry-Perot laser diode (Cont’1) λN+2 λN+1 λN λN-1 λN-2 λ (nm) Figure 18: Gain-loss curve for possible longitudinal modes Fabry-Perot laser diode (Cont’2) Fabry-Perot lasers are the most economical, but they are generally noisy, slower devices. Fabry-Perot lasers further break down into: Buried Hetero (BH) and Multi-Quantum Well (MQW) types BH and related styles ruled for many years, but now MQW types are becoming very widespread. Fabry-Perot laser diode (Cont’3) MQW lasers offer significant advantages over all former types of Fabry-Perot lasers. They offer lower threshold current, higher slope efficiency, lower noise, better linearity, and much greater stability over temperature. The performance margins of MQW lasers are so great, laser manufacturers get better yields, so laser cost is reduced. One disadvantage of MQW lasers is their tendency to be more susceptible to backreflections. Output power, mW Fabry-Perot laser diode (Cont’4) Δλ λ (nm) Figure 19: Fabry-Perot laser diode and standing wave Distributed Feedback Laser Diode In distributed feedback laser diode, the Bragg grating is used to reduce the spectral width of the laser spectrum The Bragg grating only allow selective wavelength propagate This Bragg grating also acts as mirror Output power, mW Distributed Feedback Laser Diode (Cont’1) Gain λ Δλ λ (nm) Figure 20: The DFB laser diode spectrum Distributed Feedback Laser Diode (Cont’2) The radiated wavelength is depend on the Bragg condition given by below formula 2 Λ neff = λB The spectral width of the laser spectrum is extremely narrow and suitable for communication system especially in WDM system DFB laser were proposed in early 1960s but were not developed commercially until the 1980s Distributed Feedback Laser Diode (Cont’3) DFB lasers are quieter devices (e.g., high signal-tonoise), have narrower spectral widths, and are usually faster devices. DFB lasers offer the highest performance levels and also the highest cost of the two types. They are nearly monochromatic (i.e. they emit a very pure single color of light.) while FP lasers emit light at a number of discrete wavelengths. DFB lasers tend to be used for the highest speed digital applications and for most analog applications because of their faster speed, lower noise, and superior linearity. Distributed Feedback Laser Diode (Cont’4) Figure 21: An injection laser diode which has a Bragg reflection grating in the active region in order to suppress multiple longitudinal modes and enhance a single longitudinal mode Laser Diode Material The material inside laser diode will present the wavelength of laser diode Different material will present different wavelength Different material also provide different energy gap Material in laser diode will provide how long the transmitter can support and the wavelength mode and type Laser Diode Material (Cont’1) InGaAs:Abbreviation for indium gallium arsenide. Generally used to make high-performance longwavelength detectors InGaAsP: Abbreviation for indium gallium arsenide phosphide. Generally used for long-wavelength light emitters Injection Laser Diode (ILD): A laser employing a forwardbiased semiconductor junction as the active medium. Stimulated emission of coherent light occurs at a PIN junction where electrons and holes are driven into the junction Laser Diode Material (Cont’3) The material construction in laser diode are include quantum well, graded index confinement layer, contact layer, cladding layer and substrate These material are combined and put in together with different type of construction such as shown in figure 21. The other types of construction are include: Basic structure of broad-area Gain-guided Ridge-waveguide (RWG) Laser Diode Material (Cont’4) Different configuration of arrangement of material will give different band gap energy The arrangement that available in constructing the laser diode are: Single-quantum well LD Multiple quantum well LD Graded-index separate-confinement hetrostructure Laser Diode Driver Circuit The design of laser diode driver circuit should be suitable to the characteristic of laser diode The laser diode has certain threshold level when its start in linear region The laser diode driver should make the laser operate in linear region where the output power will be linear to the input current Laser Diode Driver Circuit (Cont’1) Therefore, the circuit of laser diode should provide certain level of dc current when the system not operate This bias current for the threshold level should suitable to the laser diode used because different laser diode has different threshold current level Laser Diode Driver Circuit (Cont’2) The modulation of the laser signal is the done by the driving circuit The driving circuit will modulate the electrical signal into optical source. The driving circuit will make the laser flash according to the data modulation The rise time and fall time of the signal is depend on the driving circuit and the source itself Laser Diode Driver Circuit (Cont’3) The driving circuit is design according to the source (as mention before) and also the signal receive. If the signal is in the form of analog the analog driving circuit is to modulate used and if the signal is in digital the digital driving circuit is used to modulate Analog Driving Circuit Figure 22: (a) Analog Laser Diode Drive Circuit Analog Driving Circuit (Cont’1) Figure 22a, shows Laser drive circuit with moderate linearity and good performance in frequencies up to 500 MHz. The analog signal path only involves C1, R1, Q1, R2, and D1, the laser diode. Q1 acts as a transconductant stage in which voltage flows in and current flows out. C1 passes only the AC portion of the analog input signal. R1, usually only a few tens of Ohms, squelches any possible oscillations in Q1. Analog Driving Circuit (Cont’2) The AC portion of analog input voltage VIN appears at the base of Q1 and also at the emitter of Q1. VIN, the AC voltage at the emitter of Q1, imposes across R2 to create a modulation current VIN/R2. U1 supplies DC current to the laser through R3 and R1. U1 creates a servo loop that maintains a constant photodiode current through the rear facet monitor PIN diode. Analog Driving Circuit (Cont’3) Figure 22: (b) Analog Laser Diode Drive Circuit Analog Driving Circuit (Cont’4) Figure 22b, shows a more advanced analog laser circuit, offering excellent linearity at very high frequencies (GHz). The signal path of this circuit only involves U2, Z1, C1, and the laser diode, D1. Amplifier U2 provides input matching, gain and isolates the laser from outside conditions. The block labeled Z1 can take on many functions. Analog Driving Circuit (Cont’5) At a minimum, it interfaces the output of the amplifier U2, usually 50 or 75Ohms, to the laser that has an impedance ranging from 5 Ohms to 25 Ohms. Sometimes the laser package incorporates this impedance matching. Digital Laser Drive Circuit Figure 23: Digital Laser Diode Drive Circuit Digital Laser Drive Circuit (Cont’1) Figure 12, illustrates common discrete component circuit configurations that function to drive lasers for digital applications. However, a wide variety of highly integrated ICs exist because of the high demand for digital laser drivers. The discrete component circuit configurations illustrate the most commonly used principles in commercially available laser driver ICs. Vertical Cavity Surface Emitting Laser (VCSEL) VCSEL: are a new laser structure that emits laser light vertically from its surface and has vertical laser cavity. Figure 24 illustrates the structure of a VCSEL. VCSEL: Lasers that emit light perpendicular to the plane of the wafer they are grown on. They have very small dimensions compared to conventional lasers and are very efficient. Vertical Cavity Surface Emitting Laser (VCSEL) (Cont’1) Figure 24: Digital Laser Diode Drive Circuit Vertical Cavity Surface Emitting Laser (VCSEL) (Cont’2) The vertical cavity is design vertically, where the laser feedback is arranged in the vertical direction Several quantum well are made in the arrangement of the VCSEL within the active region to enhance the light gain The region is placed between Bragg reflector which work as highly refractive mirrors Vertical Cavity Surface Emitting Laser (VCSEL) (Cont’3) Closely resembles those of conventional edgeemitting semiconductor lasers. The heart of the VCSEL is an electrically pumped gain region, also called the active region, emits light. VCESEL operate when layers of varying semiconductor materials above and below the gain region create mirrors. Each mirror reflects a narrow range of wavelengths back into the cavity causing light emission at a single wavelength Vertical Cavity Surface Emitting Laser (VCSEL) (Cont’4) The principle operation is same as other laser diode where the emission of photon will produce after stimulated emission process and the photon will experience positive feedback cause by the mirror. VCSELs are typically multi-quantum well (MQW), with lasing occurring in layers only 2030 atoms thick. Bragg-reflectors with as many as 120 mirror layers form the laser reflectors. Vertical Cavity Surface Emitting Laser (VCSEL) (Cont’5) The advantages of the VCSEL are: The VCSEL is operate in single mode regime where the size of the cavity is very small that allow only one regime The VCSEL diode has very small dimension which allow manufacturer to fabricate many diodes on one subtrate. Vertical Cavity Surface Emitting Laser (VCSEL) (Cont’6) The size of the VCSEL also provide low power consumption VCSEL can radiate 3mW output power for 10 mA current. Since the VCSEL is small the active area is also small which bring the VCSEL high switching speed and make VCSEL can support high modulation rate up to 200GHz Vertical Cavity Surface Emitting Laser (VCSEL) (Cont’7) A VCSEL diode radiates a circular output beam in contrast to that radiated by edgeemitting laser The VCSEL fabrication can be said as the merger between optic and electronic. The fabrication of the VCSEL is very similar to the electronic chips where gives VCSEL enormous range of advantage that chips have. The difficulties in DH laser are overcome by a further development of a large-optical cavity (LOC) laser and uses regions of AlGaAs of varying composition. Some common material systems used in fabrication of heterojunction lasers. Each of the advances described has lowered the operating threshold of GaAs lasers. The typical current densities necessary to achieve the lasing threshold of the various junction types at 300° K. Homojunction Single heterojunction Double heterojunction Double heterojunction, large optical cavity 40,000 A/cm2 10,000 A/cm2 1,300 A/cm2 600 A/cm2 Laser Lifetime The operating lifetime of a laser diode is reduced significantly by operation at elevated temperature. The lifetime is reduced by a factor that varies with absolute temperature T as exp(Ea/kT), where Ea is an activation energy, typically around 0.5 to 0.7 eV, and k is Boltzmann’s constant. According to this dependence, an increase in operating temperature of 40 Celsius degrees will decrease the lifetime by a factor around 30. function of operating time, for various operating temperatures. At 20° C, the mean time The percentage of a typical 5-mW Al1–xGaxAs laser diodes laser that have failed as a before failure is 770,000 hours, but at 70° C, it has fallen to 27,000 hours. Vertical Cavity Surface Emitting Lasers (VCSELs) Advantages over the edge-emitting lasers: Its design allows the chips to be manufactured and tested on a single wafer. Large arrays of devices can be created exploiting methods such as 'flip' chip optical interconnects optical neural network applications to become possible. In the telecommunications industry, the VCSEL's uniform, single mode beam profile is desirable for coupling into optical fibres. The cavity length of VCSELs is very short typically 1-3 wavelengths of the emitted light. As a result, in a single pass of the cavity, a photon has a small chance of a triggering a stimulated emission event at low carrier densities. Therefore, VCSELs require highly reflective mirrors to be efficient. For VCSELs, the reflectivity required for low threshold currents is greater than 99.9%. Such a high reflectivtiy can not be acheived by the use of metalic mirrors. VCSELs make use Distributed Bragg Reflectors. (DBRs). These are formed by laying down alternating layers of semiconductor or dielectric materials with a difference in refractive index. R