NEW YORK CITY COLLEGE of TECHNOLOGY

Transcription

1 NEW YORK CITY COLLEGE of TECHNOLOGY THE CITY UNIVERSITY OF NEW YORK DEPARTMENT OF ELECTRICAL AND TELECOMMUNICATIONS ENGINEERING TECHNOLOGY Course : TCET 4102 (TC 700) Fiber-optic communications Module 6-2 Prepared by: Professor Djafar K. Mynbaev Spring 2008 D. Mynbaev TCET 4102,Module 6-2,Spring

2 Module 6-1: Light sources laser diodes (LDs) Light sources review LEDs review Laser diodes Acronym laser Spontaneous and stimulated emission Positive feedback Population inversion Gain and loss Threshold condition The LD s input-output characteristic Light radiated by LD Types of laser diodes Textbook: Djafar K. Mynbaev and Lowell L. Scheiner, Fiber-Optic Communications Technology, Prentice Hall, 2001, ISBN Notes: The figure numbers in these modules are the same as in the textbook. New figures are not numbered. Always see examples in the textbook. Key words Laser Spontaneous and stimulated emission Optical positive feedback Population inversion Optical gain and loss Threshold condition Vertical cavity surface emitting laser (VCSEL) diode Distributed feedback (DFB) laser diode Fabry-Perot laser diode D. Mynbaev TCET 4102,Module 6-2,Spring

3 Light sources - review Info Electronics Light source (Laser diode, LD) Optical fiber Transmitter (Tx) Photodiode (PD) Detailed block diagram of a fiberoptic communications system: The function of a light source is to convert electrical information signal into optical information signal. Electronics Receiver (Rx) Info D. Mynbaev TCET 4102,Module 6-2,Spring

4 Light sources: semiconductors (review) Introduction A transmitter consists of a light source, coupling optics, and electronics. Only miniature semiconductor light sources -- light emitting diodes, LEDs, and laser diodes, LDs -- are used in fiber-optic communications technology. LEDs and LDs are the heart and soul of transmitters. This is why we ll concentrate on their principle of operation and key features. Though LEDs practically disappear from the current scene of optical communications, we will study them as examples of semiconductor light sources. Progress in fiber-optic communications technology cannot be achieved without progress in both light sources and photodetectors has accompanied the progress made in optical fiber. At the same time, integrated electronics became part and parcel of transmitter/receiver specifications. LEDs have been around for more than thirty years. They have found application in nearly every consumer electronic device: TV sets, VCRs, telephones, car electronics, and many others. They were used in fiber-optics communications, mostly because of their small size and long life. However, their low intensity, poor beam focus, low modulation bandwidth, and incoherent radiation -- in comparison with laser diodes, that is and appearance inexpensive lase4r diodes, VCSELs, result in their retirement form the mainstream of optical communciations. Light radiation by a semiconductor Energy band diagram First, you ll recall that all materials consist of atoms, which are nuclei surrounded by electrons rotating at stationary orbits. Each orbit corresponds to a certain energy value; thus, these atoms may possess only discrete energy values. We represent this idea through an energylevel diagram. Semiconductors are solid-state materials consisting of tightly packed atoms. Atoms, in turn, are bonded by interatomic forces into a lattice structure. Each atom includes many electrons, but a material s properties are determined by its outermost electrons. The important fact is that in semiconductors (and in solids in general) the possible energy levels of an electron are still discrete, but they are so close to one another that we depict them as an energy band rather than a set of separate levels. We think of an energy band as a wide, continuous region of energy, but if you had a magic magnifier to look at this band closely, you would see the discrete energy levels that make up the band. Fig. 9.1a shows this.. It should be noted that the vertical axis in Fig. 9.1 represents an electron s energy, while the horizontal axis serves merely as a visual aid. D. Mynbaev TCET 4102,Module 6-2,Spring

5 Light sources - review In semiconductors we distinguish two energy bands: valence (lower, meaning less energy) and conduction (upper, meaning higher energy). They are separated by an energy gap, Eg, where no energy levels (that is, no electrons) are allowed. In other words, electrons can be either at the valence band or at the conduction band but cannot be in between -- at the energy gap. An energy band consists of allowed, or possible, energy levels, which means the electrons may occupy them. Figure 9.1 D. Mynbaev TCET 4102,Module 6-2,Spring

6 Light sources - review Discuss the concept of spectral width! Figure 9.2 D. Mynbaev TCET 4102,Module 6-2,Spring

7 Light sources: p-n junction (review) Figure 9.3 D. Mynbaev TCET 4102,Module 6-2,Spring

8 Light sources LEDs (review) The above reasoning can be quantified as follows: Light power is energy per second, that is, the number of photons times the energy of an individual photon, Ep. The number of photons is equal to the number of excited (injected) electrons, N, times the internal quantum efficiency, ηint. Thus, P = (N ηint Ep)/t (9.1) On the other hand, the number of electrons times the electron charge per second constitutes current: I = Ne/t (9.2) and N = It/e. Hence, the radiated light power is: P = (It/e) (ηint Ep)/t = [(ηint Ep)/e] I (9.3) Here, Ep is measured in joules. If you measure Ep in electron volts and I in ma, then P (mw) = [ηint Ep(eV)] I (ma). (9.3a) In sum, an LED s light power is proportional to the forward current, as Fig 9.4b shows. Figure 9.4 D. Mynbaev TCET 4102,Module 6-2,Spring

9 Light sources LEDs: Reading the data sheet (review) First page of specifications sheet of IF-E97 LED (see the lab manual). Discussion of all parameters given in table Characteristics follows. Pay attention to specification T A = 25 0 C. As any semiconductor device, an LED changes its parameters when ambient temperature changes. D. Mynbaev TCET 4102,Module 6-2,Spring

10 Light sources LEDs: Reading the data sheet (review) Pcoupled (µw) IF-E97 (Red) IF-E91D (IR) IF-92 (Blue) IF-E93 (Green) I F (ma) Input-output characteristic of various LEDs. The slope of such a graph is the measure of LED s efficiency; in other words, this slope shows how efficiently an LED converts current into light. Output and coupled power An LED, couples only a fraction of its output power into a fiber. In the simplest approach, we can relate these powers by using formula, P0 = Pin / (NA) 2. The values of coupled power are given in the table of specifications and shown in the graph Coupled Power vs. Forward Current. Coupled power, obviously, depends on the type of fiber and on the LED s package. LED output and coupled power depends on wavelength. See data sheets of the LEDs in your lab manual. D. Mynbaev TCET 4102,Module 6-2,Spring

11 Light sources LEDs: Reading the data sheet (review) Observe nonlinearity (saturation) in the graph Coupled power versus forward current (Figure 1). Compare spectral width shown in Figure 2 with that given in optical specifications. Excerpts from data sheet of IF-E97 (super-bright red LED) are shown here. Observe typical inputoutput characteristic (Figure 1) and graphical presentation of the LED s spectral width (Figure 2). D. Mynbaev TCET 4102,Module 6-2,Spring

12 Light sources LEDs Reading the data sheet (review) Rise/fall time, tr, is defined as 10 to 90 percent of the maximum value of the pulse, as Fig. 9.9b shows. For an LED, this characteristic shows how an output light pulse follows the input electrical-modulating pulse. (See Fig. 9.9c.) An ideal step pulse is shown as a double-dotted line in Fig. 9.9b. This enables you to visualize the pulse distortion caused by the rise/fall time. Rise/fall time is essentially determined by an LED s capacitance, C, and the total recombination lifetime, τ. Modulation bandwidth, BW, is the range of modulating frequencies within which detected electric power declines at -3 DB. In electronics, the general relationship between bandwidth and rise time is given by the well-known formula BW = 0.35/tr (9.9) This formula stems from the exponential response of an RC circuit to a step-input pulse. Power-bandwidth product is another important characteristic of an LED. It appears that the product of an LED s output optical power and its modulation bandwidth is constant: BW x P = constant (9.12) In other words, you can increase an LED s bandwidth but only at the expense of its output power. Alternatively, you can increase output power but then bandwidth decreases. Figure 9.9 D. Mynbaev TCET 4102,Module 6-2,Spring

13 Light sources LEDs (review) Summary In LEDs, an information-containing electrical signal pumps electrons at the conduction band; they then fall to the valence band and radiate light. This is how an electrical signal is converted into an optical signal. Thus, on/off electrical pulses are converted into on/off optical flashes, which are transmitted down the optical fiber. Since an LED is a semiconductor diode, a radiating mechanism can be explained in terms of the p-n junction model. When an external electrical signal is applied, electrons and holes enter the depletion region and recombine, resulting in the release of a quanta of energy, that is, photons. In other words, electron-hole recombinations produce light. Again, this light radiation occurs if, and only if the LED is forward-biased, a phenomenon that forces electrons and holes to penetrate in active region and recombine. An LED radiates light at a wavelength not less than that dictated by the energy gap. The spectral width of this light is rather wide (on the order of tens of nanometers) because electron transitions from many levels of the conduction and valence bands contribute to this light. The power of the radiated light is proportional to the forward current, as an LED s principle of operation suggests. An LED radiates rather dispersed light, which makes coupling this light into an optical fiber a problem. Special coupling techniques, including lens-coupling, improve coupling efficiency. There are two types of LEDs: surface-emitting (SLED) and edge-emitting (ELED). The latter type radiates less divergent light, which, along with good coupling technique, allows the manufacturer to even couple an ELED with a singlemode fiber. At 100 ma of forward current, an SLED couples into an MM fiber at about 50 μw and an ELED couples into an SM fiber at about 10 μw. D. Mynbaev TCET 4102,Module 6-2,Spring

14 Light sources laser diodes (LDs) Laser The acronym laser means light amplification by the stimulated emission of radiation. The first working ruby laser was developed in 1960 by the American scientist Theodore Meiman. The theoretical and practical foundations for this development were made by the American Charles Townes and the Russians Alexander Prokhorov and Nikolay Basov, who shared the Nobel Prize for Physics in 1964 for their work. The laser is a device that amplifies (or, as we now know, generates ) light by means of the stimulated emission of radiation. How a laser produces light amplification and what the words stimulated emission of radiation mean is our next consideration. In optical communications we use semiconductor lasers, which are small diodes. The laser chip placed on a needle to show its dimension. (Wikipedia.) The laser diode assembly placed near a penny to show its dimension. D. Mynbaev TCET 4102,Module 6-2,Spring 2008 (Wikipedia.) 14

15 Light sources laser diodes (LDs) Spontaneous and stimulated radiation We distinguish between two types of radiation: spontaneous and stimulated. Spontaneous means that radiation occurs without external cause. That s exactly what happens in an LED: Excited electrons from the conduction band fall, without any external inducement, into the holes at the valence band, which results in spontaneous radiation. The properties of spontaneous radiation follow naturally from the way it occurs:. First, the transition of electrons from many energy levels of conductance and valence bands contributes to the radiation produced, thus making the spectral width of such a source very wide. (See Fig. 9.2.). This is why a typical LED s Δλ is about 60 nm at an operating wavelength of 850 nm and about 170 nm at an operating wavelength of 1300 nm. Second, since photons are radiated in arbitrary directions, very few of them create light in the desired direction, a factor that reduces the output power of an LED. This means that current-light conversion occurs with low efficiency and an LED has relatively low output power. Third, even those photons that contribute to output power do not move strictly in one direction; thus, they propagate within a wide cone, yielding widespread radiated light. For this reason, we model an LED with a Lambertian source. Fourth, this transition, and therefore photon radiation, occurs at any time; in other words, photons radiate independently of one another. Hence, no phase correlation between different photons exists and the total light radiated is called incoherent. These four main properties of spontaneous radiation -- wide spectral width, low intensity, poor directiveness, and incoherence -- make it impossible to use LEDs as light sources for long-distance communication links. However, if you let an external photon hit an excited electron, as Fig. 9.10a shows, their interaction results in the electron jumping at the valence band and radiating a new photon. The difference between spontaneous and induced emission is that the latter is stimulated by an external photon. Thus, this radiation is said to be stimulated because it is caused by an external source -- the external photon. Stimulated radiation also has four main properties. First, an external photon forces a photon with similar energy (Ep) to be emitted. In other words, the external photon stimulates radiation with the same frequency (wavelength) it has. (Remember, Ep = hf = hc/λ.) This property ensures that the spectral width of the light radiated will be narrow. In fact, it is quite common for a laser diode s Δλ to be about 1 nm at both 1300 nm and 1550 nm. Second, since all photons propagate in the same direction, all of them contribute to output light. Thus, current-light conversion occurs with high efficiency and a laser diode has high output power. (In comparison, to make an LED radiate 1 mw output power requires up to 150 ma of forward current; a laser diode, on the other hand, can radiate 1 mw at 10 ma.) Third, the stimulated photon propagates in the same direction as the photon that stimulated it; hence, the stimulated light will be well directed. If you compare the beam of a laser pointer -- available in any stationery store -- with any type of lamp, you ll appreciate the difference between spontaneous and stimulated radiation in terms of the way each directs light. Fourth, since a stimulated photon is radiated only when an external photon triggers this action, both photons are said to be synchronized, that is, time-aligned. This means that both photons are in phase and so the stimulated radiation is coherent. Thus, D. in Mynbaev contrast to spontaneous radiation, stimulated TCET 4102,Module radiation has narrow 6-2,Spring spectral 2008 width, high intensity (power), a high degree 15of directiveness, and coherence. This is why laser diodes, which radiate stimulated light, find use in long-distance communication links.

16 Light sources laser diodes (LDs) Figure 9.10 D. Mynbaev TCET 4102,Module 6-2,Spring

17 Light sources laser diodes (LDs) Positive feedback To radiate stimulated light with essential power, we need not one photon but millions and millions. Here is how we can stimulate such radiation: We place a mirror at one end of an active layer, as Fig. 9.10b shows.. Two photons -- one external and one stimulated -- are then reflected back and directed to the active layer again. Fig. 9.10b illustrates how mirrors help to generate more and more photons. Thus, the two mirrors provide positive optical feedback. These two mirrors, then, constitute a resonator. More about the acronym laser Observe in Fig. 9.10b that the number of stimulated photons increases,; hence, this system amplifies light. The first external photon comes one of the spontaneous photons, which will head in the right direction (from mirror 1 to mirror 2). This photon triggers the entire process. Keep in mind, though, that our explanations are oversimplified. The important thing for you to remember is that we are discussing a dynamic and random process. A countless number of photons and electron-hole pairs are involved in the process; therefore, when we are describing the action of one photon or one electron, we are presenting only a bird s eye view of the event, not a close-up account of this intricate, complex phenomenon in action.. Excitation and radiation are governed by statistical laws. Population inversion Refer to Fig. 9.10b and notice how fast the number of stimulated photons rises. To sustain this dynamic process, we need an incalculable number of excited electrons available at the conduction band. For laser action (lasing) we need to have more electrons at the higher-energy conduction band than at the lower-energy valence band. This situation is called population inversion. To create this population inversion, high-density forward current is passed through the small active area. Population inversion is a necessary condition to create a lasing effect. The number of excited electrons determines the gain of a semiconductor diode. On the other hand, a laser diode introduces some loss. Look again at Fig. 9.10b. It seems that the number of stimulated photons continues to grow to infinity, as does the gain but, in fact, this is not true. This figure does not show the loss of these photons. Loss is a constant for a given diode, but gain can be changed by increasing the forward current, as Figure 9.11a shows. Increasing gain by increasing forward current eventually reaches the point where gain equals loss, a situation called the threshold condition. (The corresponding forward current is called the threshold current. At this threshold condition, a semiconductor diode starts to act like a laser. As we continue to increase the forward current (that is, the gain), the number of stimulated photons emitted continues to increase, which means the intensity of the output light also continues to increase. What we have, then, is a semiconductor diode that radiates monochromatic, well-directed, highly intense, coherent light. The point to remember? To make a laser diode generate light, gain must exceed loss. D. Mynbaev TCET 4102,Module 6-2,Spring

18 Light sources laser diodes (LDs) Figure 9.11 D. Mynbaev TCET 4102,Module 6-2,Spring

19 Light sources laser diodes (LDs) Lasing effect and input-output characteristic Taking all the above considerations into account, we conclude that a semiconductor diode functions like a laser (where gain exceeds loss) if the following conditions are met: Population inversion. Stimulated emission. Positive feedback. Fortunately, we know how to achieve these conditions. Let s try to build an input-output characteristic of a laser diode (Fig. 9.11b). Since input here is forward current I and output is light power P, this graph is also called the P-I characteristic. When a small forward current is applied, a number of electrons are excited and the diode radiates like an LED. Hence, one can expect to see the same line that Fig. 9.4 shows. But when the current density becomes sufficient enough to create population inversion and the threshold condition is reached (where gain equals loss), the diode starts to work like a laser. You will then see a much more intense, color-saturated, well-directed beam. This change is reflected by the graph in Fig. 9.11b, which shows that the laser diode emits much more power. After the threshold current, Ith, is passed, increasing output power requires much less current to flow than before it was passed. In other words, the slope of the input-output characteristic, ΔP/ΔI, becomes much steeper than that for an LED. A common laser diode with 1 mw output power has about a 30-mA threshold current and a 60-mA driving current. One can easily build the input-output characteristic by measuring the laser diode s output power while varying the forward current. The arrangement for this measurement is shown in Fig. 9.11c and in figure below. R1 V1 12 V 200 Ohm R2 Key = A 1kOhm 50% Optical fiber (jumper) Power meter Laser diode D. Mynbaev TCET 4102,Module 6-2,Spring

20 Light sources laser diodes (LDs) Laser-diode light: an analysis A laser diode radiates light that can be characterized as follows: (1) Monochromatic. The spectral width of the radiated light is very narrow. Indeed, the width for a laser diode can be in tenths or even hundredths of a nanometer. (2) Well directed. A laser diode radiates a narrow, well-directed beam that can be easily launched into an optical fiber. (3) Highly intense and power efficient. A laser diode can radiate hundreds of milliwatts of output power. A new type of laser diode, the VCSEL, radiates 1 mw at 10 ma of forward current, making it 10 times more efficient than the light-conversion efficiency currently achieved in LEDs. (4) Coherent. Light radiated by a laser diode is coherent; that is, all oscillations are in phase. This property is important for the transmission and detection of an information signal. As you can see, these characteristics are very similar to those of stimulated emission. But remember: Only the combination of an active medium and a resonator, which together form a laser, produces light with these remarkable properties LED Laser diode Light radiated by an LED and LD D. Mynbaev TCET 4102,Module 6-2,Spring

21 Light sources laser diodes (LDs) p-n junction The above discussion of the principle of action of a laser was sufficiently general to apply to any solid-state laser. But in fiber-optic communications technology we use only laser diodes, that is, semiconductor devices. So let s now consider laser-diode action from the standpoint of a p-n junction. We know by now that electrons and holes are injected by forward current into an active area and recombine and that each recombination results in the radiation of a photon. (See Fig. 9.3,) We know, too, that charges are carried out by the current, thus sustaining this dynamic process. In laser diode, its active area is much smaller than that in an LED. Its small size (thickness) results in a much higher current density and, thus, a much more intensive recombination process. A huge number of electrons injected into a small area leads to population inversion, to stimulated emission and, when gain exceeds loss, to laser action --the generation of monochromatic, coherent, powerful light. The main point to remember is this: An active region of a laser diode is very small, ranging from a few micrometers to a few nanometers. This fact requires very high precision in the fabrication of a laser diode and accounts for its high price in comparison with that of LEDs. Again, the laser chip placed on a needle to show its dimension. (Wikipedia.) D. Mynbaev TCET 4102,Module 6-2,Spring

22 Light sources laser diodes (LDs) Figure 9.12 D. Mynbaev TCET 4102,Module 6-2,Spring

23 Light sources laser diodes (LDs) Basic structures and types of laser diodes The basic construction of a laser diode is shown in Fig. 9.12a. If it looks similar to the edgeemitting LED shown in Fig. 9.5b, it in fact is except for two major differences: First, the thickness of an active region in a laser diode is very small, on the order of 0.1 μm = 100 nm. Second, a laser diode s two end surfaces are cleaved to make them work as mirrors. Since the refractive index of GaAs -- the material making up the active region -- is about 3.6, more than 30 percent of incident light will be reflected back into the active region at the GaAs-air interface. Thus, no special mirrors are required and these surfaces, called laser facets, provide positive feedback. This basic type of laser diode is called a broad-area LD. To confine charge carriers -- electrons and holes -- even more securely within the laser diode s small active region, a strip contact is used. (See Fig. 9.12b.) This construction restricts the current flow within this narrow region. Since current flow produces gain in an active region, this type of laser diode is called gain-guided. A means to even further circumscribe the active region is to surround it with a material having a lower refractive index. Such an LD is called index-guided. Its structure is very similar to the core-cladding arrangement in an optical fiber. These surrounding layers are called cladding layers and the term sandwich is usually used to describe this structure. The most popular construction of an index-guided LD, one where the cladding layer s thickness varies, is known as a ridge waveguide, RWG (Fig. 9.12c). In an index-guided laser diode, the small active region is buried between several layers having a lower refractive index. Such a structure is called a buried heterostructure (BH). D. Mynbaev TCET 4102,Module 6-2,Spring

24 Light sources laser diodes (LDs) Basic structure of a vertical-cavity surface-emitting laser diode (VCSEL). Source: www/mtmi.vu.lt See also Slide 24 and Figure 9.16 in Slide 25. D. Mynbaev TCET 4102,Module 6-2,Spring

25 Light sources laser diodes (LDs) There are several types of commercially available laser diodes: Quantum-well LDs, Fabry-Perot LDs, Distributed Feedack LDs, and others. Please see your textbook for discussion of each type of LD. We concentrate here on vertical-cavity surface-emitting lasers -- VCSEL Recent developments, however, have led to the fabrication of a new type of laser diode: a vertical-cavity surface-emitting laser (VCSEL). Since the space within a resonator is called the cavity, the words vertical cavity mean that the structure providing laser feedback is arranged in the vertical direction. The words surface emitting mean, in this context, that the laser s beam is emitted perpendicular to the wafer. (To recall the meanings of the terms edge emitting and surface emitting, see Fig. 9.5.) A basic arrangement of a VCSEL is shown in Fig. 9.16a. A semiconductor heterostructure (not shown) forms an active region. Several quantum wells are made within this active region to enhance light gain. This region is placed between Brag reflectors -- the stacks of layers with alternate high and low refractive-index material. (See Fig. 9.16b.) Each of these layers is λ/4 thick and is made from GaAs (n = 3.6) and AlAs (n = 2.9). These layers work like high-reflective mirrors, providing positive feedback. Several significant advantages of VCSEL diodes make them among the hottest areas of activity in transmitter technology today: The size of the resonant cavity is very small, on the order of 2 μm. This results in huge spacing between two adjacent longitudinal modes, λn - λn+1. Indeed, using Formula 9.14 for λ = 850 nm = 0.85 μm and L = 2 μm, we compute λn - λn+1 = μm = nm. The spectral width of a gain curve is only a few nanometers; therefore, not more than one mode can be within the gain curve. Thus, a VCSEL diode operates in a singlemode regime. This is shown in Fig. 9.16c, where the numbers give you an idea of the order of magnitude. The sketch is given without scale. The mode shown in black is not generated. (Note: Strictly speaking, Formula 9.14 is true only for Fabry-Perot lasers and should not be applied to VCSELs. However, it gives the order of magnitude correctly because, in reality, mode spacing in a VCSEL is about 100 nm.) VCSEL diodes have very small dimensions: A typical resonant cavity and diameter of the active region are about 1-5 μm, and the thickness of the active layer is about 25 nm = μm. This allows manufacturers to fabricate many diodes on one substrate, thereby making onedimensional and two-dimensional (matrix) arrays of diodes, precisely the constructions we need in multichannel systems. The small size of a VCSEL s resonant cavity leads to a concomitant key advantage: low power consumption and high switching speed. Thus, a VCSEL can radiate 3 mw output power at 10 ma forward current and it has an intrinsic modulation bandwidth up to 200 GHz. The first advantage stems from the fact that high current density is reached at low current value because of the small active area. (And don t forget the high quantum efficiency of this device.) The second advantage results from the short distance that electrons and holes have to travel within the active region before they recombine and the short distance a radiated photon has to travel before it escapes from the laser. This spawns a short lifetime, leading to the high modulation bandwidth of this device. A VCSEL diode radiates a circular output beam in contrast to that radiated by edge-emitting lasers. The fabrication technology for VCSELs is very similar to that for electronic chips, a fact that gives them the enormous range of advantages that chips have. VCSELs therefore manifest a useful new twist in the marriage of optics and electronics. Most commercially available VCSELs operate at 850 nm. D. Mynbaev TCET 4102,Module 6-2,Spring

26 Light sources laser diodes (LDs) Figure 9.16 D. Mynbaev TCET 4102,Module 6-2,Spring

27 Light sources laser diodes (LDs) Laser diodes - Summary A transmitter is the component of a fiber-optic communications link that converts an electrical information signal into an optical signal. A transmitter consists of a light source, coupling optics, and electronics. Only miniature semiconductor devices -a laser diode (LD) -- is used as the light sources in fiber-optic transmitters. However, we study LED operation to better understand the whole picture. To better understand the physics behind LED and LD operations, review the energy-level concept introduced in Chapter 2, page XX. In doing so, it is important for you to recall that external electrical energy is used to pump electrons at the upper (higher) energy level. These electrons then cross to the lower energy level and radiate photons. This basic mechanism of light radiation holds true for semiconductor material also, but the energy levels are so close to each other that they create a bundle of energy called an energy band. The upper and lower bands are called conduction and valence bands, respectively. These bands are separated by a so-called prohibited region, where no energy levels exist. This region is known as an energy gap, or bandgap. A laser diode is the most more efficient light source for fiber-optic communications. The term laser stands for light amplification by stimulated emission of radiation. The key words here are stimulated emission. When electrons are pumped at the conduction band, they can exist there for a while. If at that time, however, external photons enter the medium, they stimulate the electrons, which now excited, drop to the valence band and emit other photons. Thus, these new photons (light) are stimulated by the external photons. We therefore now have created sufficient photons to set in motion light amplification. This process takes place in the so-called active region, or area, of the semiconductor material. Such a material is called an active medium. To make light amplification by the stimulated emission of radiation work, we must do two more things. First, we have to put two parallel mirrors at the ends of the active medium so that the stimulated photons return to the active region and stimulate new photons to be radiated. Thus, these two mirrors provide positive optical feedback. Second, since more and more stimulated transitions occur, the upper (conduction) band is depleted very quickly; hence, we need to pump more and more electrons at this band; in fact, we need to have more electrons at the conduction band than at the valence (lower) band. This situation is referred to as population inversion because, normally, the lower band contains more electrons than the upper one. To sum up, population inversion, stimulated emission, and positive optical feedback are the three conditions necessary to achieve lasing action. The input to the laser diode is forward (driving) current (IF) and the output is light power (P). While the forward current is small, a laser diode works like a regular LED. But when the forward current reaches the threshold current, population inversion is then created and lasing action begins. Threshold current (Ith ) is one of the critical characteristics of a laser diode. It ranges from approximately 3 ma to 40 ma. As with many other characteristics of an LD, threshold current depends on temperature. (Ith increases with a rise in temperature.) This threshold effect can also be described in terms of a gain-loss relationship: When the optical gain of an active medium (determined by the value of population inversion) equals the loss of this medium (determined by the transmission of photons through the mirrors and absorption of these photons within the medium), the threshold condition is achieved and lasing action starts. The input-output characteristic of a laser diode is often called P-I or L-I graph, that is, a graph of power (light) vs. current. The slope of this graph is another critical characteristic of an LD. It is called slope efficiency or differential (quantum) efficiency, S = ΔP/ΔIF. This slope shows how efficiently a laser diode converts input current into output light. For the best LDS, this efficiency can reach a value of 0.1 mw/ma, while the efficiency of the best LEDs is 100 times less. Unfortunately, the slope efficiency of a laser diode also depends on temperature (it decreases when temperature rises). To control a laser diode s characteristics during a temperature change, two methods are used: A thermoelectric cooler keeps the LD s temperature approximately constant and a monitor photodiode and control circuit stabilize the average output power. A laser diode radiates monochromatic, well-directed, highly intense, coherent light. These properties make a laser diode the light source of choice for long- and intermediate-distance fiber-optic networks. To improve the characteristics of a laser diode, laser scientists have made substantial innovations in two areas: First, they have made the active region as small as possible so that electron-hole recombinations take place in a small, well-controlled area. This allows the laser operator to decrease the driving current (or, more precisely, the current density), which results in an increase in diode efficiency. Secondly, laser scientists have developed several types of optical feedback systems -- optical resonators -- that allow them to make a spectral width of radiated light extremely small--on the order of tenths of a nanometer. The distributed feedback (DFB) laser has such a line width and this is why a DFB laser is the most popular light source for long-haul optical links. The vertical-cavity surface emitting laser (VCSEL), the latest development in laser-diode technology, combines both ideas-- quantum-well and DFB -- to introduce the best possible characteristics attainable today into a laser diode. D. Mynbaev TCET 4102,Module 6-2,Spring

28 Module 6 (light sources) - assignments See reading assignment and homework problems in the course s outline. After study this module you must be able to: Explain where a light source reside in the fiber-optics communications system and what is its function. Describe what kind of light sources are in use in fiber-optics communications system. Explain radiation process in semiconductors based on energy-band model and on p-n junction model. Explain the principle of LED operation and describe the LED circuit schematics. Describe the input-output characteristic of an LED based on the principle of its operation. Describe the problem with coupling light from an LED into an optical fiber and explain the difference between coupled power and radiated power. Explain all data given in an LED manufacturer s specifications. Explain what the acronym laser stands for. Describe the difference between spontaneous emission and stimulated emission. Describe the principle of operation of a laser diode based on energy-band and p-n junction models and demonstrate the properties of laser light. Explain basic structure and main types of laser diodes. Explain the concept of gain and loss and threshold condition in a laser diode. Describe input-output characteristic of a laser diode. Describe basic structure and main properties of vertical-cavity surface-emitting laser (VCSEL) diode. D. Mynbaev TCET 4102,Module 6-2,Spring