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Of masers, lasers and optical fiber.

Of Masers, Lasers and Optical Fiber

Introduction

Two modern inventions, satellite communications and lasers, have had enormous impact on communications. Satellite communications with the use of microwave signals provides mankind with real-time radio communications anywhere on earth. With the use of fiber optic (FO) waveguides, the laser, an offspring of the maser, transmits point-to-point signals connected with FO cable in real time. Each has its advantage. The advantage of satellite communications is that there is no need for cables, while the advantage of FO cable is that it is a direct, reasonably secure communication medium that is nearly impervious to weather conditions.

Those Amazing Masers

When the paper "Infrared and Optical Masers"[1] was published in 1958, the authors may have suspected that they had hit upon something profound, but they could not have foreseen the tremendous technological impact their discovery would have on society 30 years later. Prior to the introduction of the maser, various other electronic devices had used moving electric charges to obtain electronic amplification and oscillation. With the use of masers, amplification and oscillation are obtained through the use of atoms or the transitions between quantum energy levels in atoms. The term maser is an acronym for microwave amplification by stimulated emission of radiation and optical masers (masers operating at optical frequencies) are know as lasers, an acronym for light amplification by stimulated emission of radiation. More recently the m in masers stands for molecular rather than microwave because maser action is attainable at various frequencies, not just microwave frequencies, using different atoms, ions and molecules.

Masers and lasers are based on quantum mechanical principles in which energy is transferred from a pump source to a paramagnetic material. The pump source supplies electrons at a higher energy state. Paramagnetic materials, such as ruby crystal, rutile crystal, liquid oxygen and gallium nickel ferrite (Ni[Ga.sub.2-x][O.sub.4]), are magnetized like iron, but to a much lower degree. The energy stored in the paramagnetic material is released from the crystal when triggered by a resonant signal. It is important to cool the active crystal and the maser cavity to -270 [degrees] C in order to reduce the ambient thermal noise energy that would mask the amplified signal.

Masers are classified by the number of elevated energy levels of the electrons within the crystal before they radiate their pumped-up energy. In the rest state, lower energy levels have a higher population density than do higher levels. Therefore, matter absorbs a net amount of energy as a result of induced transitions from lower to higher energy levels. Maser activation produces greater electron occupation in the higher levels, as shown in Figure 1.

One type of laser is a ruby crystal laser, shown in Figure 2, in which the pumped-up energy is concentrated on a ruby rod through the use of mirrors rather than waveguides. The magnetic field between the two pole faces are adjustable in order to promote the energy level split within the paramagnetic crystal. The resulting energy is [E.sub.3] - [E.sub.2] = [hf.sub.s,] where [f.sub.s] is the signal frequency and hfp is the pump frequency signal. The pump frequency signal is introduced from an outside source and corresponds to the energy split [E.sub.3] - [E.sub.1]. It is usually twice the signal frequency.[2]

Lasers

Lasers are masers operating at optical frequencies. The laser is used for the amplification or generation of coherent light waves by means of stimulated emission of radiation. A coherent light beam does not diffuse but concentrates its energy on a spot on a target that is not much larger than the laser's aperture. A coherent beam is generated when the laser organizes the energy waves emitted by a stimulated atom so that they travel in the same direction at the same frequency in phase with the stimulated radiation.

The laser, invented by Shawlow and Townes,[1] is the first coherent light source ever used. It consists of an active material (solid dielectric, fluid or gas) between two parallel plates of an interferometer that is excited by an external energy source to invert the population of two energy levels of energy difference [DELTA]E, with an electromagnetic wave having a frequency of f = [DELTA] E/h, where h is the Planck's constant.

The interferometer is tuned to the resonant frequency f. The interferometer then acts as a resonant cavity, allowing energy to be built up as the reflected signals add (constructively) in phase. When the rate at which energy emitted into the cavity exceeds that at which it was lost by absorption of transmission through the interferometer plates, the laser will oscillate.

In 1960, the first working laser, built by T.H. Maiman,[3] used a single synthetic ruby crystal ([Cr.sup.3+]:[AI.sub.2][O.sub.3]). The aluminum oxide crystal contained a small concentration of chromium ions. The interferometer was made by silver-plating the opposite parallel flat faces of the crystal. Radiation in the green absorption band of the ruby crystal elevated the chromium ions to the excited energy level [E.sub.3]. The energy dropped rapidly from this level to the lower levels, [E.sub.2] and [E.sub.1]. The ions emit red light during this transition in the form of a pulse of energy. To pump energy pulses into the ruby crystal, the ruby crystal was placed in one of the foci of an elliptical mirror and the cylindrical pump lamp was placed in the other focus line.

Not all solid-state lasers require single crystals as the active material. Many lasers operate with amorphous glasses as dielectrics, while still others use gases and liquids as their active materials. Lasers made of various materials for numerous applications provide efficiencies from 0.1 to 10 percent and output energy levels ranging from 10mJ for an [F.sub.2] laser to 100 for a [CO.sub.2] gas laser.

Lasers are grouped by the wavelength of their emitted signals. The smallest wavelength is emitted by the Fe laser (157 [Angstrong]). One of the longest wavelenghts is produced by the [CH.sub.3]F laser (496.7 [mu]m). The most powerful [CO.sub.2] lasers operate at wavelengths of 9.4 [Angstrong] and at 10.6 [mu]m. GaAs semiconductor diode lasers produce wavelengths from 864 to 904 [Angstrong].

Gas Lasers

Gas lasers are excited by passing an electrical discharge through a gas or a gas mixture. The most common gas lasers are the helium neon laser, the argon ion laser and the [CO.sub.2] laser. The helium neon laser contains a mixture of five parts of He to one part of Ne at a pressure of 2 to 5 torr. The discharge tube has a bore of 1 to 2 mm and is 20 to 100 cm long. Power output is in the range from 0.5 to 50 mW.

Argon lasers work on the principle of energy transition between levels of singly ionized Ar ions. High current densities are required (> 100A/[cm.sup.2]) to achieve the lasing threshold. BeO plasma tubes are used for efficient heat conduction. Argon lasers produce output power levels of up to 15 W in the blue and green light region of the frequency spectrum. Efficiency of the argon laser is on the order of 0.1 percent.

The [CO.sub.2] laser is the most powerful and most efficient laser in use. It produces RF power of up to 40 kW at wavelengths between 9.6 and 10.6 [mu]m with efficiencies of 30 percent or higher. High power [CO.sub.2] lasers require discharge tubes several meters long. Folded [CO.sub.2] lasers with gas re-circulation have been developed to reduce the unwieldly design of single tube without loss of laser power or efficiency.

Liquid Lasers (Dye Lasers)

The most widely used liquid laser is the dye laser. It is called dye laser because the organic solvent rhodamine 6G is used to suppress the triple-excited state. A flowing dye solution is used to remove molecules in the triple-state from the lasing region. Dye lasers produce a broadband 10 nm signal of a few watts of power at an efficiency of 20 percent. Argon lasers are used to pump CW die lasers. The various dyes used determine the laser's output wavelengths.

Semiconductor Lasers

Semiconductor lasers are optically pumped either with an xenon-filled flash tube, a crypton-filled arc tube or a tungsten-iodine lamp. The first tested solid-state laser was the three-level ruby laser, which used chromium-doped aluminum oxide ([Al.sub.2][O.sub.3]) as the active layer. Various glasses (Nd-glass) and the four-level yttrium aluminum garnet (YAG) also have been used.

Semiconducter lasers can involve one material, homojunctions, or a different material on one side, a single heterostructure, or different materials on both sides, a double heterostructure. Single heterostructure and double heterostructure laser diodes can provide powers to 1000 W when arranged in stacks. These lasers operate at very high efficiencies (80 percent). A typical double heterostructure is shown in Figure 3, in which there exists a different bandgap and index of refraction than that of the active materal, which is typical of these structures. GaAs, InAs, [Ga.sub.x][Al.sub.1-x]As and similar compounds produce radiation in the near infrared spectrum (800 to 3000 nm). The p-and n-regions of the p-n junctions are highly doped and population inversion between electron and hole concentrations occurs in the junction region. GaAs has a high reflective index, such that surfaces cleaved perpendicular to the junction plane will act as mirrors, enhancing lasing action.

Semiconductor lasers have their advantages. They do not require high voltages, flash tubes or elliptical mirrors. They operate at room temperature, which eliminates the requirement for cryogenic temperature equipment.

Other Lasers

CW lasers are used in commercial and consumer products. Commercial applications include the ubiquitous office copiers, laser printers, compact disc players and FAX machines. For these applications, 633 nm He+Ne lasers and 750 nm GaAs diode lasers are used. Detailed tables that list the wavelength, active material, material form, power and comments for both common CW lasers and high and medium power pulsed lasers have been previously published[6] and serve as a valuable reference to anyone working in the field.

Optical Fibers

Optical fibers are generally used as a transmission medium, but have found other applications in the form of passive devices, such as couplers and power splitters, and in active devices, such as lasers and amplifiers. Optical fiber is a cylindrical structure made of a dielectric material. It consists of a core, which is the inner region where light propagates, and the cladding, which serves as a mechanical protection of the core. The cladding must have a smaller refractive index than the core to allow for light confinement.[5] Optical fiber can be made from various materials. There are three basic optical fiber configurations in use. They include single mode, step-index multimode and graded-index multimode.

Fiber Materials

Common materials used in the fabrication of optical fibers are silica, glass and plastic. Silica ([SiO.sub.2]) is the main component in silica fibers. Rutile ([TiO.sub.2]), phosphorus pentoxide ([P.sub.2][O.sub.5]) or aluminum oxide ([Al.sub.2][O.sub.3]) are used as dopants in the fiber core to increase the refractive index of [SiO.sub.2].

Combinations of glasses are used for the core and cladding materials in some optical fibers. It is important that these materials have similar coefficients of thermal expansion, similar viscosity, as well as long term chemical stability, high purity and low density fluctuations.

Plastics, including PCP and acrylic, are used in the fabrication of some optical fibers. These fibers are used in environments where corrosion and stress problems exist. They are often used in short distance computer applications. Using plastic has the advantage over silica and glass because the fibers can be manufactured at a much lower cost.

Optical Fiber Configurations

The main difference in the three configurations is the size of the core and refractive index, which is the ratio of light velocity in a vacuum to its velocity in the transmitting medium. Each configuration has its advantages and disadvantages, which dictate its applications.

Singlemode Fiber

Because of its small core of 8 to 10 [mu]m, the singlemode fiber only permits a fundamental wave to propagate, as shown in Figure 4. These fibers require an intense light source. They can carry information over 100 km before the signal requires regeneration. Fiber optic communication systems that require long haul and high capacity use singlemode optical fibers and GaAsInP laser diodes at a wavelength of 1.3 [mu]m.

Step-Index Multimode Fibers

The refractive index changes between the core and the cladding along the diameter of a step-index multimode fiber. The core refractive index is higher than the cladding refractive index. In multimode fibers, light rays from the core strike the cladding at various angles of incidence, shown in Figure 4. These fibers are used for short distance because of their lower operating speed and large dispersion.

Graded-Index Multimode Fibers

The refractive index of the graded-index multimode fiber is not the same. It is highest at the center of the core and decreases gradially in a radial direction toward the outer edge. The graded-index rays follow a sinusodial path compared to the zig-zag path of the step-index fiber, shown in Figure 5.

Semiconductor Technology

Fiber optics use light impulses traveling through fibers to transmit information. Various optical fibers are used as the medium for this transmission.

Transmitters

GalnAsP/InP lasers produce up to 10 mW of power at wavelengths of 1.55 [mu]m. The launch optics that transmit the light energy from the laser into the monomode fiber are of great importance since they determine the overall efficiency of the transmission system. Table 1 shows the launch efficiency of various approaches.

Table : TABLE I
         LAUNCHERS FROM LASERS
         TO FIBER OPTIC CABLE
Launch Type             Efficiency (%)
Butt launch               10 to 15
Cylindrical lens          30 to 50
Shaped fiber optic end    60 to 65
Optical microdevice       80 to 90


Photo Detectors and Receivers

The optoelectronic effect can be easily understood by considering electromagnetic radiation as a stream of emitted photons. Each photon contains a certain amount of energy, called the quantum. The energy of each photon equals

E = h[nu]

where

h = Planck's constant

[nu] = frequency of radiation

= 6.62 X [10.sup.-34] J

Two energy levels or bands are of interest to semiconductors; they are the conduction band, where the electrons are free to move and may be used as charge carriers, and the valence band, where the electrons are bound to the parent atom.

Depletion Layer Photodiodes

The depletion layer diodes operate in 0.4 to 1.8[mu]m wavelength range. These diodes have a carrier transit velocity of [10.sup.5]m/s.

Avalanche Photodiodes

The avalanche photodiodes are reverse biased. As the reverse voltage is increased, the reverse current remains constant until the breakdown voltage is reached. The increase in carriers is caused by the avalanche effect due to the ionization of the semiconductor material. Avalanche diode sensitivity is noise limited. This noise originates in the load circuit and the preamplifiers. The gain of an avalanche photo diode (APD) is given by,

[Mathematical Expression Omitted]

where

[I.sub.m] = the excitation current

[I.sub.p] = current generated by

photon absorption

Silicon Photodiodes

Silicon photodiodes operate well in the 0.8 to 0.9 [mu]m wavelength range. They have low leakage currents in Si P-N junctions and have a large difference in the carrier ionization rates, producing low noise, high gain, wide bandwidth, avalanche photodetectors (APDs).

Germanium APDs

and III-V Photodiodes

For wavelengths exceeding 1 [mu]m, GaAs and Ge APDs have to be used. Ge APDs have been in use since 1965. These diodes have a high excess noise factor and have been used mainly at wavelengths of 1.3 [mu]m. They have been superseded by compound semiconductors, in particular by III-V photodiodes. III-V compounds have a band gap suitable for detection of electron beams in the 1.3 to 1.55 [mu]m wavelengths.

Applications

Fiber Optic Links

The output from a satellite earth station antenna can be transmitted to a remote receiver over a distance of 15 miles by means of a single fiber optic cable. The Ortel model 10005A TVRO fiber optic link is such a system. With this system, the satellite receiving antenna may be placed far from urban interference signals, feeding the earth station facility that is close to the community it serves, by means of a fiber optic cable. The system can transmit 12 channels from a single polarization. This system, which operates at a modulation frequency from 950 to 1450 MHz, has a typical signal to noise ratio at a cable length of 15 miles of 60 dB.

Undersea Fiber Optic

Communications Links

Lightwave communication systems have proven themselves in land communications. The successful manufacture of long cable lengths has sparked the laying of fiber optic undersea cables. The important factor in undersea cables is reliability and durability. Undersea fiber cables must be able to withstand high water pressures, as well as the exposure to sea life in its various forms. Undersea cables are an important part of worldwide communications, and it is very likely that most copper cables will be replaced with fiber optic cables within the next 10 years.

The first submarine telegraph cable between North America and Great Britain was laid in 1858. Almost 100 years later, in 1956, voice transmission by cable between the US and Great Britain was achieved. Alexander Graham Bell had invented the telephone in 1876, 80 years earlier, and overseas telephone communications had been introduced in 1927.

Many of the technical innovations that were required for copper cable undersea transmission have counterparts in fiber optic undersea cable communications and include unattended repeater stations and cable laying equipment, such as sea plows that dig a continuous trench into the sea bed to bury the cable, wherever possible, and fault locating systems for cable maintenance and replacement. One of the difficulties in initiating worldwide fiber optic communication systems was the lack of standards in cable and coupler design and establishment of standard wavelengths. AT&T Bell Laboratories became the industry leader in the establishment of these standards, while the National Bureau of Standards and Technology provided the US government with guidance.

An Optical Submarine System

The S280 is a digital undersea transmission system that operates at a bit rate of 280 Mb/s at a wavelength of 1.3 [mu]m.[4] It has a capacity of 11,520 channels. The cable for the system was laid at sea depths of up to 6500 m. Repeating stations are spaced 45 km apart. The FO losses are 0.38 dB/km at 1.31 [mu]m wavelength and 0.27 dB/km at 1.6 [mu]m wavelength. Like most FO cable installations, a lifetime of 25 years is expected with no more than two ship repairs required during this time span. The repeaters also are submerged and are housed in a polyethylene covered steel housing.

An Optical Fiber

Submarine Cable System

The OS-280M optical fiber submarine cable system operates at a wavelength of 1.3 [mu]m and 300 Mb/s for sea depths of up to 1300 m. Fiber optic cable loss changes of less than [+ or -] 0.01 dB/km were observed after a one-and-a-half year period of submerged operation. Repeaters of an 8000 km long haul system are placed at 35 to 50 km intervals; transmission bit rate is 280 Mb/s; and transmission capacity is 3780 channels.

Many other fiber optic undersea cables have been laid across the Atlantic Ocean and for inter-island communication system in the Pacific Ocean. Recent technological innovations have improved the efficiency and reliability of this information transmission mode, such as SAW transveral filters for timing recovery and ultra-high speed silicon ICs.

Time Delay Compensation

for Phased-Array Antennas

Distances from the common feed line of a phased array's radiating elements vary. In the case of large arrays, they may introduce a time delay on the order of 0.5 [mu]s between the center element and the edge elements. Since these time delays are constant, they can be compensated for with delay lines installed between the source and the phase shifters.

FO delay lines require less space and are much lighter than coils of coaxial cable. The signals coming from the modulated source are transformed into optical signals by means of solid-state laser diodes, such as GaAs laser diodes. From the laser diodes, the signals are delayed corresponding to their antenna element location, and converted to RF signals by the avalanche photo diodes ([APD.sub.s]). The RF signals are fed through phase shifters to amplifiers and then to the antenna elements, as shown in Figure 6.

Medical Applications

Opto-electronic instruments readily have found applications in the medical field in diagnostics and surgery, while lasers have been used in cardiology, specializing in angioplasty. Recently, Arye Rosen of Drexel University has shown that mm-wave angioplasty may also be successful in removing arterial obstructions.

The He-Ne laser is used in the laser nephelometer to measure the rheumatoid factor of various organisms to different antibiotics. Luminescence spectroscopy is used for the diagnosis of malignant tissue and of tooth decay diagnosis. Endoscopy is used to inspect interior organs without the need of invasive -urgery. Table 2 lists various laser uses in medicine. [Tabular Data Omitted]

One intriguing medical application proposed by oncologists was to isolate the virus that causes cancer (if indeed it is a virus), grow the virus into a crystal and use this crystal as a laser to radiate a laser beam on the malignant tissue. The doctors suspect that the virus crystal's resonant wavelength should be most destructive to the malignant tissue.

Conclusion

Fiber optics is a new technology that will continue to grow with the growth of semiconductor and optical waveguide technologies. Considerable overlap of traditional microwave technology and optics can be seen. Fiber optics have already found use in voice, data and video transmission. Currently, their applications are moving into the commercial arena, where they have become solutions to the problems of high capacity, size, weight and reliability.

References

[1.] A. L. Shawlow and C. H. Townes, "Infrared

and Optical Masers," Phys. Rev.,

Vol. 12, December, 1958. [2.] T.K. Ishii, Microwave Engineering, Second

Edition, Harcourt, Brace & Jovanovich,

1989. [3.] T.H. Maiman, "Stimulated Optical Radiation

in Ruby," Nature, Vol. 187, August,

1960. [4.] Peter K. Runge and Patrick R. Trischitta,

Undersea Lightwave Communications,

IEEE Press, 1986. [5.] Frederico Tosco, (ed.), Fiber Optic Communications

Handbook, Second Edition,

TAB Books, June, 1991. [6.] Edward C. Jordan, (ed.), Reference Data

for Engineers: Radio, Electronics, Computer,

and Communications, Seventh

Edition, Howard W. Sams & Co. Inc., Indianapolis,

IN, 1985. [7.] G.H.B. Thompson, Physics of Semiconductor

Lasers, John Wiley & Sons Inc.,

New York, 1980. [8.] A.E. Siegman, An Introduction to Lasers

and Masers, McGraw-Hill Book Co.,

1971.

PHOTO : Fig. 1 Energy levels and population in Cr-doped ruby maser crystals; (a) before and (b) after pumping, that is, coupling of additional external RF energy to the signal.

PHOTO : Fig. 2 A schematic diagram of a ruby crystal laser. The optically parallel finished ends of the ruby rod constitute a Fabry-Perot resonator and create optical standing waves between them.

PHOTO : Fig. 3 A schematic diagram of a (a) double heterostructure and its distribution perpendicular to the junction of (b) energy of conduction and valence bands, (c) refractive index and (d) light intensity.[7]

PHOTO : Fig. 4 Multimode fibers; (a) a step index fiber and (b) a graded index fiber.

PHOTO : Fig. 5 A comparison of (a) multimode fiber and (b) singlemode fiber propagation.

PHOTO : Fig. 6 Microwave phased array with fiber optic delay line compensation.
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Author:Stiglitz, Martin R.; Blanchard, Christine
Publication:Microwave Journal
Date:Jul 1, 1991
Words:3985
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