LECTURE ON “OPTICAL SOURCES” BY MukulMittal Lecturer E.C.E G.P.C.G, Patiala(9463376309) firstname.lastname@example.org PUNJAB EDUCATION SOCIETY
What is optical sources and why it is used? • Optical sources are used to produce light pulses, so that data can be transmitted in light pulses inside the optical fiber. OPTICAL SOURCES LASERS LED (Light Amplification (Light Emitting by Stimulated Emission Diode) of Radiation)
LED(Light Emitting Diode) • LED is just a forward biased p-n junction. When free electrons from the “conduction band” recombine with holes, they enter the (lower energy) “valence” band and light is emitted. • The wavelength of light emitted by the LED is inversely proportional to the band gap energy. The higher the energy the shorter the wavelength.
Heterojunctions • A heterojunction is a junction between two different semiconductors with different bandgapenergies. Charge carriers (electrons or holes) are attracted over the barrier from the material of higher bandgapenergy to the one of lower bandgap energy. • When a layer of material with a particular bandgap energy is sandwiched between layers of material with a higher energy bandgap a double heterojunction is formed.
Design Challenges Getting the Light into a Fibre Confining and Guiding the Light within the Device Getting Power to the Active Region Getting Rid of the Heat
Surface Emitting LED (SLED) • It emit the light on the surface. • Operates at 850 nm wavelength • Surface emitting LEDs are often called “Burrus” LEDs because they were first described by Burrus and Miller.
Edge Emitting LED ( ELED) • It emits the light by directing the light out the side of the device • Operate in the 1310 nm region
Coupling light Inside the fiber Anti-Reflection Coatings :This is usually a 1/4 wavelength thick coating of magnesium fluoride (MgF²). • Make sure that the surface is relatively rough or cutting the surface at an angle will also reduce immediate back-reflections.
Characteristics of LEDs • Low Cost • Low Power • Relatively Wide Spectrum Produced: LEDs do not produce a single light wavelength but rather a band of wavelengths called the “spectral width” and is typically about .05 of the wavelength (50 to 100 nm). Power POWER SLD SLED • Incoherent Light ELED • Digital Modulation current temperature • Reliability
LASERS“Light Amplification by the Stimulated Emission of Radiation” • Ideal laser light is single-wavelength only. It is formed in parallel beams and is in a single phase. That is, it is “coherent”. • Lasers can produce relatively high power. Some types of laser can produce kilowatts of power. • Because laser light is produced in parallel beams, a high percentage (50% to 80%)can be transferred into the fibre.
Disadvantages • Lasers have been quite expensive by comparison with LEDs. • Temperature control and output power control is needed. • Lasers have to be individually designed for each wavelength they are going to use. • Amplitude modulation using an analogue signal is difficult with most lasers because laser output signal power is generally non-linear with input signal power.
Remember • The critical characteristic here is that when a new photon is emitted it has identical wavelength, phase and direction characteristics as the exciting photon. • The photon that triggered (stimulated) the emission itself is not absorbed and continues along its original path accompanied by the newly emitted photon.
Spontaneous Emission • Excited atoms decay and emit photons randomly in all directions. • Spontaneous emission is not lasing! Stimulated Emission • Materials capable of stimulated emission are distinguished by the fact that they have a high energy state that is “metastable”. It can hold its high energy state for some length of time before decaying spontaneously.
Population Inversion(Necessary Condition) • A population Inversion occurs when there are more electrons in the higher energy state than there are in the lower energy state. • The amount of light bouncing between the mirrors will increase very quickly. • Other materials (dopants for example) should not absorb light of the required wavelength.
To make a laser you need • A material that can enter a high energy metastable state. It should have a bandgap energy of the right magnitude to produce light of the required wavelength. • A way of supplying energy to the material. • A suitable method of confinement of the material and of the emitted light. • A pair of parallel mirrors at each end of the cavity. • The wavelength of light produced is really determined by the characteristics of the lasing material.
Terminologies • Semiconductor lasers produce a range of wavelengths called the “spectral width” of the laser. spectral width is around 6 to 8 nm. • Semiconductor lasers produce a series of “lines” at a number of discrete wavelengths called“linewidth” • The signal is attenuated as it travels on the fibre and thus the higher the signal power you use the further you can go without needing to regenerate it. • Operating Wavelength, Frequency (Wavelength) Stability, Tuning Range and Speed are considered for laser action.
Fabry-Perot Lasers • The Fabry-Perot laser is conceptually just an LED with a pair of end mirrors. • In this cavity acts as a Fabry-Perot resonator and light will bounce between the two mirrors. • The distance between the mirrors is an integral multiple of half wavelengths. • Wavelengths that are not resonant undergo destructive interference with themselves and are reflected away.
Distributed FeedBack (DFB) • This is just a periodic variation in the RI of the gain region along its length. The presence of the grating causes small reflections to occur at each RI change(corrugation). When the period of the corrugations is a multiple of the wavelength of the incident light, constructive interference between reflections occurs and a proportion of the light is reflected. Other wavelengths destructively interfere and therefore cannot be reflected. The effect is strongest when the period of the Bragg grating is equal to the wavelength of light used (first order grating).
Disadvantages Of DFB • Due to chirp the refractive index changes which changes the resonant wavelength of the grating and the wavelength of the laser output changes. • During lasing the cavity heats up and Chirp occurs , due to change in RI. Phase Shifted grating DFB
IntegratedAbsorption Modulators • External modulators are used at high speed modulation. Distributed Bragg Reflector (DBR) Lasers • DBR lasers have a partitioned cavity with the grating in a region that is not active (amplifying) by this there is a lot less wavelength variation from the causes.
Sampled-Grating Tunable DBR Lasers • A sampled grating consists of a number of short grating sections with periodic blanking between them. Two sampled gratings are used in such a way that the interaction between them causes a small change in RI to produce a large change in wavelength. • External Cavity DBR
Quantum Wells • DBR and DFB lasers are often built using a “Quantum Well” structure. • When light is confined into a cavity smaller than its wavelength it behaves as a particle (quantum) rather than as a wave. • It has higher gain characteristic but a lower maximum output power than non-QW devices.
Vertical Cavity Surface Emitting Lasers (VCSELs) • VCSELs emit from the surface. They are constructed by laying down a very large number (perhaps 500) of relatively thin layers of semiconductor material. • The device emits light vertically through the stack of material layers.
Fibre Ring Lasers • It is used to make a very narrow linewidth laser. • The structure is very similar to that of a fibre ring resonator .However, in this case the wavelength is controlled by the tuneable FP filter and not by the length of the fibre loop. • Very efficient operation. • Stable wavelength produced • The device is tuneable over a range of up to 40 nm.
Laser Operation Hole Burning: the path taken by the dominant mode Chirp: abrupt change in the carrier (electron and hole) flux density
SUMMARY • Lasers are used than LEDS • Operation of LED is based on combination of holes and electrons. • Principle of Operation of LASERS • Various types of LASER. • Advantages and Disadvantages of LASERS.
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