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Optical Communication Power

Abstract

This senior design project accomplished the design and testing of a pair of transceivers which use lasers to communicate over an optical channel. These laser modems connect to a computer's COM port, initialize quickly, and can communicate at speeds between 110 and 115200 bits per second. This report explains the relevant areas of serial communication, modulation, laser diode characteristics, circuit design, and optics.The important functions of both integrated circuits used to construct the transceivers will also be discussed.

The possibility of visible red light laser being used as signal light source for Free-Space Optical (FSO) communication is proposed. Based on analysis of transmission in atmospheric channel concerning 650 nm laser beams, performance of wireless laser communication link utilizing a low power red laser diode was evaluated. The proposed system can achieve a maximum range of 300 m at data rate 100 Mb/s theoretically. An experimental short-range link at data rate 10 Mb/s covering 300 m has been implemented in our university. It is feasible to enhance the system performance such as link range and data rate by increasing transmitting power and decreasing laser beam divergence angle or through other approaches.

INTRODUCTION

A growing and seemingly insatiable bandwidth demand in the marketplace is bring Free-Space Optical (FSO) technology into the carrier space as a means of broadband access for closing the “last mile” connectivity gap throughout metropolitan networks. Terrestrial short-range FSO systems which are generally point-to-point links between buildings on the campus or different buildings of a company can be used to setup wireless communication networks or to supplement radio and optical fiber networks. Generally medium for short-range links is infrared radiation designed to operate in the windows of 780~850 and 1520~1600 nm using resonant cavity light emitting diodes and laser diodes. This report proposes a short-range FSO system using visible light ray (650 nm red laser beams) as the communication medium. Visible light communications comprise a technology for transmission of information

Using light that is visible to the human eye. The possibility is based on tremendous technical advances of available components at low cost and efficient modulation techniques.

We build a system made up of high brightness visible LEDs (Laser) which reaches over 100 m outdoors and can provide the function of open space, wireless broadcasting of audio or data signal. High power stable and continuous operation semiconductor LD (Laser) at 650 nm has been developed for a long time, and is commonly used in optical access and storage system, laser printer or as indicators. A new feature of frequency modulation has been utilized as well due to their low cost and small size. In order to realize this system, study of atmospheric Channel concerning red light laser ray is required. It is concluded through numerical analyses that visible laser beam can be used for terrestrial short-range FSO links. Performance parameters of the proposed system were evaluated in detail and experimental system was described finally.

An initial hurdle faced by early means of laser communication was the enormous heat generated by pumped laser action. However, in the late 1960's, semiconductor laser was developed and ever since, the possibilities for laser communication have grown. Though developed for carriers, new laser technologies are finding a place in private networks. Recent breakthroughs in wireless technology and the need for a wireless extension of the Internet have increased the demand for faster, higher bandwidth wireless access networks. The two wireless options nowadays are either radio or optical networks. Radio frequency has been the primary medium of communication for a long period of time. However, in this day and age, the RF spectrum has become congested and may no longer be sufficient for broadband high-speed applications. In addition to this radio communication requires the leasing of frequencies in order to be legally permitted to use them. On the other hand, optical communication is the key to supply the ever-increasing demand for higher bandwidth, without the associated hassles or interference experienced with radio communication.

Entrepreneurs and technologists who know of this are borrowing many of the technologies initially designed for fiber-optics systems and applying them to what is now called Free Space Optical (FSO) communication as shown in Figure

FSO systems run in the infrared (IR) spectrum, which is at the bottom of the light spectrum. Specifically, the optical signal is in the range of 1 THz (1Terahertz = 1trillion Hz =1,000,000,000,000 cycles per second) in terms of wavelength. FSO is a free space (wireless) technology, meaning that the signal travels in the free space between transmitter and receiver, rather than through a conductor such as a wire or fiber, or through a waveguide of some sort. Another important feature of FSO is that it is unaffected by electromagnetic interference and radio frequency interference, which increasingly plague radio based communication systems. FSO systems are used in disaster recovery applications and for temporary connectivity while cabled networks are being deployed.

What is Free Space Optical Communications?

Free space optical communications is a line-of-sight (LOS) technology that transmits a modulated beam of visible or infrared light through the atmosphere for broadband communications. In a manner similar to fiber optical communications, free space optics uses a light emitting diode (LED) or laser (light amplification by stimulated emission of radiation) point source for data transmission. However, in free space optics, an energy beam is collimated and transmitted through space rather than being guided through an optical cable. These beams of light, operating in the Terahertz portion of the spectrum, are focused on a receiving lens connected to a high sensitivity receiver through an optical fiber.

Unlike radio and microwave systems, free space optical communications requires no spectrum licensing and interference to and from other systems is not a concern. In addition, the point-to-point laser signal is extremely difficult to intercept, making it ideal for covert communications. Free space optical communications offer data rates comparable to fiber optical communications at a fraction of the deployment cost while extremely narrow laser beam widths provide no limit to the number of free space optical links that may be installed in a given location.

The fundamental limitation of free space optical communications arises from the environment through which it propagates. Although relatively unaffected by rain and snow, free space optical communication systems can be severely affected by fog and atmospheric turbulence. The main design challenges in free space optical communications are as follows:

Fog: Fog is vapor composed of water droplets, which are only a few hundred microns in diameter but can modify light characteristics or completely hinder the passage of light through a combination of absorption, scattering, and reflection. This can lead to a decrease in the power density of the transmitted beam, decreasing the effective distance of a free space optical link.

Scintillation: Scintillation is the temporal and spatial variation in light intensity caused by atmospheric turbulence. Such turbulence is caused by wind and temperature gradients that create pockets of air with rapidly varying densities and, therefore, fast-changing indices of optical reflection. These air pockets act like lenses with time-varying properties and can lead to sharp increases in the bit-error-rates of free space optical communication systems, particularly in the presence of direct sunlight.

Beam Wander: Beam wander arises when turbulent wind current (eddies) larger than the diameter of the transmitted optical beam cause a slow, but significant, displacement of the transmitted beam. Beam wander may also be the result of seismic activity that causes a relative displacement between the position of the transmitting laser and the receiving photo detector.

Absorption and Scattering: Absorption is caused primarily by the water vapour (H2O) and carbon dioxide (CO2) in the air along the transmission path. Gases in the atmosphere have many resonant bands, called transmission windows, which allow specific frequencies of light to pass through. These windows occur at various wavelengths. Absorption is not generally a big concern in an infrared laser transmission system.

Scattering has a greater effect than absorption. The atmospheric scattering of light is a function of its wavelength and the number and size of scattering elements in the air. The most common scattering elements in the air that affect laser beam transmission are fog and smog, rain, and snow.

Weather: The biggest issue for free space laser communication is fog. While the laser beam transmits through fog, the moisture particles are so small and dense and like many tiny prisms to distort and dissipate the signal. This problem makes the light with information distort and produces an error bit. Although the liquid content of a heavy shower is 10 times that of a dense fog, the radius of a raindrop is about 1000 times that of a fog droplet. This is the primary reason that attenuation via rain is 100 times less than that of fog. The effects of snow on a laser transmission fall somewhere in between those of fog and rain, depending on the degree of water particles in the snow.

Shimmer: This is the direct result of a combination of factors, including atmospheric turbulence, air density, light refraction, cloud cover, and wind. The combination of factors will cause a similar disturbance when a laser beam is transmitted through the atmosphere.

Moving building: This is because of the natural movements of buildings. Although we are not aware of the movement, buildings often sway from side to side even settle into the ground. This problem can offset the laser beam and make the receiver laser receive less power.

ADVANTAGES

The main benefit of point-to-point laser connection is that bandwidth is dedicated between the points, so this can make high-speed communication possible between different places on earth. Small beam divergence, small size, and large information bandwidth due to operation at a higher frequency are all advantages of a laser system. Additionally, the advantages of light weight, small volume, and lower power consumption provide laser communication a potential edge over RF communication. The laser communication also has no FC requirement, is easy to install and can be safer

Light source and eye safety

The majority of Free-Space Optical communication systems were designed to operate in the infrared bands (780~850 nm and 1520~1600 nm) which already provided higher data rates and over longer distances. For short-range FSO system, using cheap light source which provides sufficient data rates in the range of tens of Mbps is a well suited solution. Low cost FSO systems using LEDs were developed by Leitgeb et al. (2002; 2003a) for data rates of 10 and 100 Mbps. For reliable, inexpensive, high-performance lasers around 650 nm are readily available, visible red laser instead of infrared ray was utilized here. Transmitter with visible light and receiver are easy to install and can provide high data rates without tracking system therefore have smaller size and cost less. Although visible light is appealing, eye safety is still a concern among laser communication systems. In terms of wavelength, the IEC60825 rulings permit longer wavelength devices to output much more power than shorter ones. The longer wave length system is thus readily designed to be eye safe. By the United States Center for Devices & Radiological Health (CDRH), visible laser light between 1.0 and 5.0 mW is considered eye-safe with caution if viewed for less than 0.25 s. Laser beam at 650 nm is supposed to be easily noticed by human eye while possible harm can be avoided promptly.

UNDERSTANDING OF WIRELESS LASER COMMUNICATION GROUNDS

Wireless Laser Communication is very similar to fiber communication in which the air acts as some kind of fiber. Optical pulses deliver digital

information in both of them. Moreover, Laser Communication System (LCS) is usually a part of optical fiber line. The transmitter (TX) and receiver (RX) parts of LCS are very similar to TX and RX parts of Fiber

Communication System. The difference is only in optical media for communication data delivery. An “atmosphere line” is not sometimes

such clear and stable as a perfect optical fiber line. A clear atmosphere

transparency is similar to fiber one, but in fog, snow and rain conditions its transparency becomes thousands times lower. An “air line” can not also keep a laser beam within strongly defined boundaries (light pipe) as it holds an optical fiber. A strong light scattering appears in bad weather conditions (fog, snow, rain, and dust for example). A worm air also has fast changeable areas with a little different temperature. Those areas have a different index of refraction and act as moving gas lenses that change directions of laser beams. The laser beam that reaches RX lens has therefore very strong difference in its power depending on weather condition. It means that RX electronic of LCS must to work within a wide dynamic range of signal. The minimal available for RX level of optical signal defines the sensitivity of LCS, the maximal available level defines the saturation level of RX. The difference within both levels is a dynamic

range of LCS. Those optical power levels are defined usually in decibels (dB). This is a relative value and has sometimes several different definitions if estimating power level is compared with a specified “reference level”. The “dBm” is the mostly used value in LCS specifications. In that definition the estimating power level in mW is compared with 1mW reference power level (1mW = 0.001W):

Optical signal level (in dBm) = 10 log (estimating power in mW /1mW)

For example:

1W = 1000mW power (in dBm) is equal to:

10 log (1000 mW/ 1mW) = 10 log 1000 = 30dBm

1mW power (in dBm) is:

10 log (1 / 1) = 0dBm

1μW power = 10-6W = 10-3mW in dBm is:

10 log (10-3 / 1) = − 30 dBm

For example, if in Lacom System LCS specification the sensitivity value is noted as -34dBm, it means that the minimal available level of

laser beam power is:

log-1 (-34 / 10) = log-1 (-3.4) =0.0004 mW = 0.4 μW

That optical power must be delivered to RX lens aperture for good communication.

The “air line” can not hold the high level of light power for a long distance because of laser beam spreading, its scattering and absorption in air. Laser beam spreading in clear air depends on a laser beam divergence. The narrow beam divergence could increase a working distance of LCS. That is preferable of course, but firstly TX optics has some limitation of focusing. Secondly, it is difficult to keep a pointing of narrow laser beam exactly within RX lens aperture. Laser beam wanders along RX lens aperture after mechanical vibration of LCS, building swing, air no stability, etc. That is why the beam divergence narrower than 1mrad is used only for space communication systems. An auto-align option of LCS can help in an exact pointing.

The laser beam (signal beam) intercepts by RX lens together with

optical and electrical noises. The relation between signal and noise limits mainly LCS sensitivity. Spectral filters are used for reducing a

background optical light that can make a strong optical noise. It is

preferable to pass to photo detector only a laser beam light. The bandwidth of TX and RX electronics must be also limited according to the spectrum of transmitting data electrical signal. It means that the transmitted and received signals could not be distorted by LCS electronics.

CONCLUSION

The 650 nm wavelength laser diodes were developed a long time ago and cost less. Atmospheric transmission of 650 nm laser beam was analyzed. Performance parameters of a short-range FSO system utilizing red light LD were calculated. We also describe our fundamental experiment based on a 650 nm wavelength laser diode.

As a medium for short-range FSO systems, 650 nm laser has both advantages and disadvantages when compared with infrared media. On the one hand, visible red laser sources capable of high-speed operation are available at low cost. Like the infrared, the Visible spectral region is unregulated worldwide and FCC licenses are not necessary. On the other hand, it is suitable only for short-range communications, as the output power of red laser is restricted due to eye safety problem.

The following conclusions were made:

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