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This paper describes the directly modulated optical transmission which is used to the current optical communications networks. Specifically, it has been given emphasis to laser and led optical transmitters, modulation formats and a short report for the optical fibers.
Optical transmitters are devices which convert the electrical input signal into an optical signal. Then they modulated it with the optical carrier and transmit it via an optical fiber which is the communication channel.
Main component of the optical transmitters are the optical sources, which produce the optical carrier. As we will describe in the following sections, there are two types of optical sources: a) Light Emitting Diodes (LEDs), b) Lasers.
We use these two types because the benefits are very important for the modern optical communications. On the other hand, they have many differences between them.
This paper is organized as follows. Section 2 is a short describe of the optical fibers which is the transmission medium of the modern optical communications systems. Section 3 is an important part of this paper, because it describe the two common types of optical sources, LED and semiconductor laser. A completed comparison between LED and laser diode is described in the Section 4. Direct modulation which is also important is described in section 5. Finally, Section 6 presents the conclusion of this paper.
2. Optical Fibers
We use optical fibers, to transmit pulses of light with the lowest attenuation. The two general types of the optical fibers which are common used today are, the multi-mode optical fibers and the single- mode optical fibers .
The multimode fiber was the first fiber which was available in the market place. It has larger core diameter than the single mode fiber. The name (multimode) derives from the fact that it "propagates more than one mode of light" .These light rays cover different distances until they arrive in the final destination. For a multimode fiber the numbers of modes can be over 1000.
Modal dispersion occurs when the signal is spreading in time, because the propagation velocity of the optical signal is not the same for all modes. This phenomenon has a negative impact on the quality of the signal. This has as a result, the reduction of the distance that can cover. For this reason we use multimode fibers for short distances.
The second type of optical fiber is single-mode fibers which have much smaller core diameter. That allows the propagation of only one light mode  each time. As a result, the fidelity of the signal remains stable, for long distances and the dispersal modes greatly reduced. Due to these properties, the single-mode fibers have bigger bandwidth capacity than to multimode. The ability of transferring large amounts of information over long distances with low losses, has introduced the single-mode fiber in high bandwidth and distances applications, like the WDM (Wavelength Division Multiplexing) optical networks . The typical diameter of a single-mode fiber is 9-10 micrometers, whereas the diameter of the multi-mode fiber is 50 micrometer.
Finally the main advantages of the optical fiber are:
Higher bandwidth and more capacity
Low attenuation and long distances between repeaters.
Small size and low weight.
3. Optical sources
Optical transmitters use as sources, Light Emitting Diodes (LEDs) or Light Amplification by Stimulated Emission of Radiation (LASERs) which are made from semiconductor materials. Both types of these sources are based their operation on the same principle. Particularly, the extra holes in the p region and the extra electrons of n region, recombine and release energy in the form of optical light.
The main difference between laser and led is that lasers, operate on the mechanism of stimulated emission and produce coherent radiation, in contrast to LED, where the emission of photons are spontaneous and the producing optical signal is incoherent. For this reason, laser achieves higher performances than LED and it has better characteristics. Some of these characteristics are monochromatic light, transmitted power and better modulation in high rate signals. 
3.1 Light Emitting Diodes - LED
A light emitting diode  is a semiconductor junction with two regions, p-n, which emits incoherent optical radiation in forward bias. The light emitting diode uses a p-n junction to introduce electrons and holes in the same region of a semiconductor. When the electrons fill the holes, it is produced light through the phenomenon of spontaneous emission.
The light emitting diodes are commonly used as light sources for multimode fibers, with low transmission rates in short distances. Are not affected by the temperature and they have lower cost from the semiconductor lasers. On the other hand, semiconductor lasers can be used for longer distances and higher transmission rates. Lasers can be also used for single mode and multimode fibers. However, are more sensitive in temperature and they have higher cost than LEDs.
The light which emitted from a LED, is proportional to the voltage which is applied to the diode in the forward bias (or forward direction). They are devices which operate with low energy consumption and they have longer lifetime. They operate in low data rate speeds and they suitable for transmission rates less than 1Gbps (approximately 100 to 200 Mb/s), while they have wide spectrum range. It used in optical communications with multimode fibers.
There are two main types of LEDs: a) Surface-Emitting LEDs and b) Edge-Emitting LEDs .
The Surface Emitting led has a limited radiant power so its use, is limited in optical fiber communications. The Edge Emitting led achieves "higher radiances at the emitting facet, making it easier to lunch the light into the optical fiber".
In the end of the optical fiber (side of the receiver), a photodiode converts the falling light into electrical pulses. With the current technology the response time of a photodiode is about 1ns.This puts an upper limit of 1 Gbps.
Light emitting diodes emitting in the red or the infrared band and frequently used as optical sources for systems with short distance fibers.
Generally, a diode consists of a P region and an N region, which have impurities that give them the desired electrical characteristics. The wavelength which will emit a semiconductor diode depends on the internal energy levels. In other words, the exact choice of the semiconductor, determines the wavelength of peal emission of photons. The gaps between the energy levels depend on the composition of the material in the contact region of the diode. Usually, the wavelength for a light emitting diode and for a glass fiber is 820nm and 850nm. Light emitting diodes for glass fibers are made from Gallium-Arsenide (GaAs) or from Gallium-Aluminun-Arsenide (GaAlAs). The pure GaAs diodes emits near to 910nm. The aluminum is added, to reduce the current drive requirements. That helps to increase the lifetime of the diode. Moreover it helps the reducing of the energy gap. So the light emission is in the region from 750nm to 900nm. The light emitting diodes can also be made from Gallium-Arsenide-Phosphorus (GaAsP). These types of diodes emit in different wavelengths in the visible band, near to 650nm. Commonly used in combination with plastic fibers and they haven't good performance in the wavelength, like that which produced by GaAs diodes. The GaAs diodes have worse performance than the GaAlAs diodes but are cheaper and suitable for low speed links of plastic fibers (in short distances).
The following table 1  shows as the wavelength of different types of LED.
Wavelengths for different types of semiconductor material
3.2 Light Amplification by Stimulated
Emission of Radiation - LASER
The term Laser, is the acronym of the terms Light Amplification by Stimulated Emission of Radiation. In a few words, it is the light amplification that caused from the stimulated emission of radiation.
Lasers are devices that produce coherent beams of optical radiation by stimulated, electronic, ionic or molecular transitions of the higher energy levels. When they return to the lower energy levels emits energy. The active mean of diode laser is a semiconductor diode which is forward biased (forward direction). It should be noted that the radiation of the laser can be either in time consistent, either locally consistent or both. The value of the coherent grade may be 0.9. To a coherent beam of an electromagnetic energy, the waves have the same frequency and phase.
Most of lasers have a special substance that can increase the intensity of the light which passes through it. This substance is an amplifier mean and can be in form of solid, liquid or gas. Independently of the natural state of the substance, the mean should have atoms, molecules or ions, which have the abilities to store energy and then they can release it, in the form of light.
The cavity of a single laser is designed to reflect internally, the waves which are in the infrared band (IR) or in the ultraviolet band (UV) in order to reinforce each other. The cavity can contain gases, liquids or solids elements. The materials which have been chosen for the cavity, determine the emission wavelength. At each edge of the cavity there is a mirror. The first mirror reflects 100% and does not allow the transit of energy. The other mirror allows approximately the 5% of the energy to pass through it. An external power source, supplies the cavity with energy. This is called pumping. Because of pumping, it is appearing an electromagnetic field. This field has the resonance frequency of the material which is in the cavity.
The waves reflected from one mirror to the other. The length of the cavity has been chosen such that, the reflected waves reinforces each other at the resonance frequency of the material which is in the cavity. The electromagnetic waves at the resonance frequency, leaves the cavity from the mirror, which does not reflect 100%. The output looks like a continuous beam or a sequence of light pulses with short duration and high intense.
There are different types of lasers which are:
He-Ne laser, b) Ruby lasers, c) Excimer laser d) Organic Dye laser and e) Carbon Dioxide laser .
The most popular type of lasers, are the semiconductor lasers which are described below.
3.3 The Semiconductor Laser
The semiconductor laser , which referred as laser diode is an electronic device, which has small size and low energy requirements. It has a p-n junction diode in the same way as the LED. Electrons and holes reconnected and create light photons. This happens when the diode is forward biased. The difference between the diode is that the depletion layer is narrow enough. This layer concentrates the carriers. At the end of the region there is a reflective layer which acts like a mirror. This "mirror" is triggering the production of more photons so it generates more coherent light energy. Two main forms of laser diode have been used until now, the horizontal and the vertical type.
There are different types of semiconductor lasers, such as: a) Homostructure laser, b) Single heterostructure laser, c) Double heterstructure laser, d) GRINSCH ( Graded-index separate confinement heterostructure) laser, e) Mirror laser: FP, DFB, DBR.
The semiconductor laser is suitable for single-mode fiber applications because of its excellent performance. The desirable characteristics of laser include the precise wavelength, the good output power, and the control of the chirp (the phenomenon of changing the frequency over time). The semiconductor laser has the first two characteristics. However, the chirp phenomenon can be affected by the signal modulation techniques.
The two types of the semiconductor lasers which widely used are: a) The monolithic Fabry-Perot laser (Fig.1)  and b) DFB (Distributed Feedback) laser (Fig.2) . The second type of laser is suitable for WDM (Wavelength Division Multiplexing) optical applications , because it emits almost monochromatic light. Also has a good signal to noise ratio (SNR), good linearity and it can be used for high transmission rates. The central frequencies of the DFB laser are in the band around 1310 nm and from 1520 up to 1565 nm. The second frequency range is compatible with the fiber amplifiers which have addition of Erbium ( EDFA).
Fabry-Perot cavity for a laser diode. The crystal function as a mirror . The dielectric reflecting layer reduces the optical sources in the cavity.
Structure of a distributed-feedback (DFB) laser.
The threshold current (Ith) is the most important parameter a laser diode because lasing occurs only at current levels above this value Ith.
Other important parameters which usually used for laser diode is: a) The peak lasing wavelength and b) The beam divergence angle.
The first parameter is given in nanometers. The second is defined as the angle away from the beam axis before the light intensity drops to 50%.
Finally, the rated light output, is another important parameter. This parameter show us which should be the best light output level for reliable operation (continuous). The output level measured in milliwatts..
Another thing that should have in mind is the laser safety. Today most of the laser diodes devices have low optical levels. Although this output power (â‰ˆ 5 mW), can cause damages to a human eye. Lasers, that is not visible, like the Infrared (IR) can cause damages to human eye.
So it is necessary to take protection care when we use laser devices, like laser pointers.
4. Comparing LEDs and Laser diodes
The main difference between LED and Laser diode is that Leds are based on spontaneous emission, whereas Laser diodes are based on stimulated emission.
LEDs used in multimode transmission systems. They have broad output beam and broad spectral width. That means, first that is hard to capture and focus the beam, second it adds the dispersion phenomenon which is undesirable. Laser diodes are used in high data rate communications. They have narrow emission spectrum and they used in systems which require coherent radiation. They used in single mode systems. LEDs are less expensive, they consume low power and they have longer lifetime than lasers. On the other hand, they are moderate to high speed rates and difficult to be driven. Lasers and LEDs are sensitive in the temperature. Due to this sensitivity in temperature, lasers diode stops to operate at temperature higher than 100Â°C.  
The following table 2  depicts the parameters of LEDs and Laser diodes.
10^5 to 10^8
Increase wavelength 0.6 nm/Â°C
Increase wavelength 0.25/Â°C
Parameters of LED and LASER diodes.
Modulation of the optical carrier
The optical carrier C(t) which produced by an optical source is written in the form,
C(t) = A0cos(2Ï€f0 + Ï†0) (1)
which show us, that the parameters which can be configured (modified), is the width A0, the frequency f0 and the phase Ï†0. Consequently, the carrier can be modulated either through the technical modulations AM, FM, PM, PolM (for analog information signal) or at ASK, FSK, PSK, PolSK (for digital information signal).
From the above types of modulation, the fiber optical systems use, almost exclusively the amplitude modulation (analog amplitude modulation-AM or amplitude signal keying-ASK). Considering that optical links is now almost digital, the analysis will focus on ASK modulation. The introduction of the amplitude modulation in optical communications mainly ought to easy implementation and the low cost.
This technique provides that if m(t) is the electrical information signal which was applied to the transmitter, then the transmitted (modulated) signal is of the form,
S(t) = m(t).c(t) = m(t).A0cos(2Ï€.f0+Ï†0) (2)
According to equation (2), for digital information signal m(t), the optical source will transmit (ON) or will be in Â«silenceÂ» (OFF) depending on, if it is applied to the transmitter electric pulses (state Â«1Â»), or not (state Â«0Â»). In practise the source emits optical power in both situations (Â«1Â» and Â«0Â»), so the quality of the modulation is determined by the Extinction Ratio (equation 3). The Extinction Ratio or EX should be high, more than 8 db. This ratio is defined as,
where P(1) and P(0), the optical power which emitted from the transmitter and correspond to digital pulses Â«1Â» and Â«0Â».
Generally, there are two basic ways to implement the modulation of the output signal of an optical source. In the first way, the signal directly alter, the injection current of the source (direct modulation), while in the second, the modulation takes place after the exit of the optical carrier from the source (external modulation). This paper describes the direct modulation which is generally used for 2.5Gbps speed, for Local Area Networks (LANs).
In the direct modulation (Fig. 3) , the digital signal (electrical pulses), directly alters the injection current of the optical source. Then the optical source operates in Â«ONÂ» or in Â«OFFÂ» mode, depending on the fact that, it is applied electrical pulse in the source (state Â«1Â») or not (state Â«0Â»).
Direct modulation. The m(t) signal modulate the injection current of the optical source. If the m(t) signal is in digital form, the source toggle in ON or OFF mode, according to m(t) = Â«1Â» or Â«0Â».
Although the direct modulation is easily implemented in lasers sources, causes instability in the phase of the modulated signal. This instability is known as Â«chirpÂ» and get worse when the transmission rates are high. The Â«chirpÂ» is generally an undesirable phenomenon because it has as a result, the loss of power and interference with the adjacent channels (Fig. 4) . For the semiconductor lasers which use direct modulation, the "chirp" causes limitations to high speed rates (over 10 Gbps).
Chirp phenomenon in ASK direct modulation.
Restriction of the Chirp phenomenon
To avoid the Â«chirpÂ» phenomenon, the laser should operate in the linear region of their characteristics (over the current threshold Ith).
One way to avoid the appearance of the Â«chirpÂ», is the separation of the modulation process, from the process of producing the optical carrier. The separation can be achieved if the modulator is placed after the laser, so the output signal from the laser can be modulated without changes of the current power. This technique is known as an external modulation and provides the ability of phase modulation techniques. Phase modulation is extremely difficult to be implementing with the direct modulation.
Today the optical communications systems take the place of the conventional communication systems, which use copper cables. Technology like FTP (Fiber to Home) becomes real. Also the FSO (Free Space Optical) which is a line of sight technology using light beams, to provide optical communication already has many applications. It is a new technology without fiber optic cables but wireless products that can transmit voice and data.
In this paper the reader has the ability to learn more about the LEDs and lasers diodes which commonly used in the current optical systems. Also it has been described the direct modulation which finds implementation in laser sources.
As the technology continuous to expand, new communications systems have been developed the last years. Researchers tried and find out new implementations for the optical communication system. The "chirp" phenomenon which causes fluctuation in the frequency of the modulated signal was faced with the use of external modulators. This invention gave us the ability for higher transmission rates.
It is true that the last ten years the optical transmission systems have been developed so much. Now remains to see how far optical communications can go and how much, this will improve the quality of our lives.