Teachers Supplements

Section I

Laser Characteristics
General
Power
Beam Diameter
Divergence
Modulation
Mode
Irradiance
Coherence Length

What to Look for in Selecting a Laser
Introduction
Importance of Output Power
Beam Diameter
Beam Divergence
Polarization
Ruggedness and Longevity
Modulation

Section II

Laser Theory, Operation, and Technology
Theory of Laser Action Introduction
Characteristics of the Laser Beam
   Small Divergence
   Monochromatic
   Coherent
   High Density
Operation of the Helium-Neon Laser
Energy Absorption and Collision Transfer
Spontaneous Emission
Function of Helium
Stimulated Emission
Population Inversion
Multiple Reflections
Polarization
Summarizing the Theory of the Helium-Neon Laser
Operation of the Solid State VLD Laser
Photon Production
Index-Guided Photon Enhancement
Laser Beam Characteristics
Beam Shape
Beam Visibility
Beam Coherence Length

Section I

LASER CHARACTERISTICS

General
Metrologic manufactures both He-Ne (helium-neon) gas lasers and the newer VLD (visible laser diode) solid state lasers. The He-Ne lasers produce a red-orange beam at a wavelength of 633nm (nanometers) (632.8x10-9 meter). They are of hard seal construction with mirrors permanently fused to the ends of the laser tube body. This prevents entry of moisture which can shorten laser life. The solid state VLD lasers produce a red beam at a wavelength somewhere in the range of 635 to 680 nm. The wavelength depends on the geometry of the individual laser crystal and also upon the operating temperature to some extent. They are mounted on metallic heat sinks for thermal stability and are surrounded by metal shielding to resist damage from rough handling and static electricity. Back to Top

Power
Power is related to the intensity of the laser beam and is measured in milliwatts. All of the Metrologic lasers are low-power, ranging from 0.3 to 8.0mW(milliwatts).
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Beam Diameter
Beam diameter is the diameter of the laser beam cross section. It is measured between points near the outer edge of the beam where its intensity is only about 86% (1-1/e2) of the intensity at the beam center.
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Divergence
The edges of a laser beam are never exactly parallel. The beam spreads out (diverges) forming a small angle measured in milliradians. To determine divergence, measure the beam diameter at a distance of several meters from the laser. Dividing the arc (beam diameter) by the radius (laser distance) gives the angle in radians. Multiply this by 1,000 for the angle in milliradians. Thus, if the beam divergence is given as 1.2mRad, the diameter of the laser beam at a distance of 10 meters should be 0.012 meter.
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Modulation
Modulated lasers contain modulation driver circuitry. This circuitry varies the beam intensity whenever audio or video signals are input to the laser. Metrologic has three different modulated lasers, the ML868, ML869, and ML268.

The ML868 and ML869 are He-Ne lasers that can change the beam intensity up to 15% at rates of 1MHz. The ML268 is a VLD laser that can change the beam intensity as much as 17% at rates up to 6MHz. Such lasers are especially useful for communications and special experiments. Their modulated beams are able to carry audio or video information through any transparent media.
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Mode
Mode refers to the distribution of light in the beam. The mode preferred for holography and for research is the TEMoo mode which is a true Gaussian distribution of light. All of the lasers listed herein have only the TEMoo mode.
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Irradiance
Many laser safety measurements are made in terms of irradiance, which is the power density of the laser beam. Irradiance is usually quoted as power per area (i.e. mW/cm2).
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Coherence Length
This is the distance that the laser beam can travel while its photons remain in phase with each other. For He-Ne lasers, the coherence length is about 10 to 20 centimeters; for VLD lasers it is only several centimeters. A long coherence length simplifies alignment for interferometry and holography set-ups.
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WHAT TO LOOK FOR IN SELECTING A LASER

Introduction
The lasers listed herein can all be used for teaching and other applications. However, the laser model that might be best suited for the student laboratory might not be powerful enough for demonstrations in a large lecture hall, bright enough to illuminate a large object for holography, or have the modulation capabilities for video and audio communications. Since individual needs differ, it might be helpful to consider the following factors when planning your purchases. Back to Top

Importance of Output Power
The intensity of the laser beam is a function of the output power, which ranges from 0.3mW to 8.0mW for the lasers Metrologic produces. Those having the lowest powers are ideal for general classroom use and for most investigations in the student laboratory. They are excellent for viewing holograms and can also be used for making them. However, long exposure times of several seconds are required to make a hologram and special care must be exercised to be certain that there is no movement during the exposure interval.

The brighter beams of the higher power lasers are better for large lecture hall demonstrations, for long-distance communications, and for research and lab work where detailed observation and measurements of diffraction patterns and interference fringes are desired. Brighter beams also mean better holograms because of the shorter exposure intervals.
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Beam Diameter
Although the diameter of a helium-neon laser beam is less than a millimeter as is emerges from the aperture, it maybe expanded, collimated into a parallel-edge beam, or focused into a tiny point with various lenses and lens combinations. If the laser is to be used for a research project that involves beam shaping, knowing the beam diameter specifications will be helpful in planning for accessories. For example, the larger the original diameter of the beam, the less expansion is required to illuminate a given area, and the greater will be the intensity when it is focused to a fine point.
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Beam Divergence
An ideal laser has a fine beam with perfectly parallel edges. Actual lasers come close to this ideal but even the best have a small amount of divergence that makes the beam spread out and lose their power density (irradiance) with distance. If the laser beam is to be used in a laboratory without external collimating optics, it is important to choose a laser with the smallest divergence that is available.
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Polarization
All He-Ne lasers with fixed internal mirrors produce a beam of light that is elliptically polarized. That is, the plane of polarization might be vertical during a particular instant and then gradually shift to a horizontal orientation a few minutes later. This effect is most pronounced in the short laser tubes. If the laser is to be used for research or for demonstrations of the polarizing plane rotations that occur whenever the a beam is transmitted through certain liquids or gases, choose the longest lasers because they offer the best stability.
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Ruggedness and Longevity
Regardless of the model that is chosen, the purchaser can be assured that all Metrologic lasers will have long shelf lives, will be resistant to shocks and will withstand hard usage from several generations of students.

All of our He-Ne lasers use hard-sealed tubes and solid state circuit boards for a long trouble-free life. Our solid state VLD lasers are even more rugged and can be expected to operate trouble-free for at least 10,000 hours.
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Modulation
It is possible , using a variety of techniques and set-ups, to modulate any Metrologic laser to carry digital audio, or video information over the laser beam. However, the Metrologic modulated lasers ML268, ML868, and ML869 have built-in modulation driver circuits that can save a great deal of time, trouble, and expense.

Metrologic He-Ne modulated lasers ML868 and ML869 can vary the intensity of their beams up to 15% of the nominal output power. This is adequate for most voice communication, speed of light, and video experiments that require only small bandwidths.

For the most precise speed of light experiments, superb audio and color TV pictures, and freedom from over-modulation distortions, the solid-state VLD ML268 is recommended. Its beam intensity can be varied as much as 17% with a bandwidth of 4.5MHz or more.

If a school can afford to purchase only one laser for general use and communications, it should be a modulated solid-state VLD laser like Metrologic’s ML268.

If the laser is needed for general use and communications, as well as for holography and demonstrations in a brightly lit room, the one laser to purchase is a modulated He-Ne laser like Metrologic’s ML868 or ML869.
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Section II

LASER THEORY, OPERATION, AND TECHNOLOGY

Theory of Laser Action Introduction
From Genesis, Chapter 1:
"In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. And the spirit of God moved upon the face of the waters. And God said, Let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness."

Considering man’s eternal long-term fascination with light, it is somewhat amazing that the laser was not discovered before 1960. Although Arthur Schawlow and Charles Townes accurately predicted the laser in 1958, one can read more than the seeds of that invention in a classic paper by Einstein in 1917:
"When a molecule during a transition from one quantum-theoretically possible state to another absorbs or emits energy in the form of radiation, such an elementary process can be thought of as being a symmetrical (non-directional) process. It now turns out that we arrive at a consistent theory only, if we assume each elementary process to be completely directional."

An excellent reprint source of the Einstein paper and other classical laser papers is "Laser Theory", published by the institute of Electrical & Electronic Engineers, N.Y.C.1972.

Another group who came close to the laser was Lord Rayleigh’s team of experimenters, in the United Kingdom, who were working with gaseous discharges. It is surprising to us that none of those brilliant physicists noticed lasing action while playing with long tubes of gaseous discharge.

All of which leads us to the exciting conclusion that makes teaching and learning worthwhile: that is, there is plenty of life in the old science discovery game yet. Back to Top

Characteristics of the Laser Beam
Light from a laser differs from ordinary light in four important respects that make the laser valuable.
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Small Divergence.
The beam from a laser does not spread out much (diverge) after it leaves the laser. Thus, instead of being dissipated rapidly, the energy is concentrated in a narrow beam. A typical Metrologic laser beam has a diverging angle of 1 milliradian, i.e., it diverges 1mm for every 1000mm of travel.
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Monochromatic.
The laser light is said to be monochromatic because it is mostly one color, or one wavelength.
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Coherent.
Ordinary light is incoherent with crests and troughs being emitted at random from different parts of the light source. Laser light, however, is coherent with almost all crests and troughs in phase regardless of where they are generated in the laser tube.
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High Intensity.
Laser light is very intense because all its energy is concentrated. Although light from a powerful ruby laser or a carbon dioxide laser can be made to burn through concrete or steel, the light from the typical classroom laser is relatively safe and even if focused on the hands cannot be felt.
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Operation of the Helium-Neon Laser
Laser action, which produces the red laser beam, is generated in the laser tube. The tube consists of a long glass capillary surrounded by a hermetically sealed glass jacket, one inch in diameter. It contains a mixture of 85% helium and 15% neon gases, at a pressure of about 1/300 atmosphere.
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Energy Absorption and Collision Transfer
To produce a laser beam, there must be a continuous supply of energy that can be converted into electromagnetic radiations. To supply this energy efficiently, a high voltage from the laser power supply creates a strong electric field along the length of the laser tube. In this field, free electrons of the laser medium gain kinetic energy as they are accelerated toward the positive terminal.

Before reaching the positive terminal, there is a high probability that an energetic electron will collide with one of the many helium atoms and give it additional energy. This puts the helium in an excited state as its electrons "jump" to higher energy levels. Think of these levels as irregularly spaced landings on a staircase where the electrons can rest momentarily before falling back toward the ground level.

Since the helium atoms has only two electrons, and a limited number of excited levels that each of its electrons can occupy, there is a good possibility that collisions will raise a substantial number of helium atoms to the desired 2S energy level. Each atom of excited helium at this level happens to have the exact amount of energy that neon requires before it will emit its red light.

The thermal motions of excited helium atoms result in collisions with neon atoms in the laser tube. During such a collision, a second energy exchange occurs. The helium atom reverts to its ground state and the neon atom absorbs just enough energy to raise one of its electrons to its 3S2 level. At this particular landing, called a metastable level, a neon electron can rest for a comparatively long time before "jumping" back down toward ground level.

Of course there is always the possibility that an energized electron will supply the correct amount of energy directly to the neon without requiring the helium as an intermediary. However, this does not happen very often because neon has 10 electrons and many different excited levels for each. Thus, there is a rather low probability that an energetic electron can excite a neon atom directly to the desired 3S2 lever where laser action can occur.
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Spontaneous Emission
A neon atom can remain in the excited state at the 3S2 level for a time that can last typically several microseconds. Then, for no apparent reason and without any external stimulation, the atom de-excites itself by spontaneously falling toward the ground energy level either in one large step or in a series of several smaller steps.

Each time the neon electron undergoes a transition and falls a step to a lower energy level, it releases energy in the form of an electromagnetic wave burst, or a photon of light. Recall that a photon is a short electromagnetic wave and, depending on its energy, its wavelength can be as long as that of infrared light or as short as an x-ray.

When an excited neon electron makes a transition from the 3S2 down to the 2P4 level, 1.96 eV of energy is released from the neon and a photon of red light with a wavelength of 633nm is created. By itself, this photon does not create much light, but, together with the combined energy of many others of the same wavelength, an intense laser beam can be produced.
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Function of Helium
Helium is included in the laser tube because it enhances the neon’s output of red light by several orders of magnitude in a highly efficient energy exchange process. Although neon gas alone can provide some laser action, the laser effect is many times greater when it is mixed with a large amount of helium in proportions of about 1 to 6.
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Stimulated Emission
In 1917, Einstein postulated that a photon released spontaneously from an excited atom could interact with another excited atom, and stimulate it to de-excite and produce another photon. The new photon would have the same wavelength, energy, and phase as the original photon and the two would join and continue traveling in the same direction. These two photons could then proceed to stimulate other excited atoms to produce additional photons in a cascade effect. The light that they produce is called coherent light and it consists of many photons of the same frequency and phase that combine their energies by constructive interference.
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Population Inversion
For stimulated emission to be effective, it is essential that a large number of excited neon atoms be present in the laser tube at any given time. In the normal state of matter, most of the electrons are in the ground state or lowest energy levels. Without a substantial number of excited neon atoms present, photons will pass through the laser medium without encountering a sufficient number of excited atoms to have a noticeable effect. However, when the majority of neon atoms are excited by helium collisions and remain excited for a comparatively long time at a metastable level, "population inversion" is said to exist. In this condition, any photon traversing the laser tube has a high probability of producing many stimulations along the way and an amplified beam of light is produced.
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Multiple Reflections
To further strengthen the output of coherent light in a laser, a high reflective mirror is placed at each end of the tube. Any photons that are traveling along the tube axis will be reflected back into the laser medium and receive further reinforcement as they stimulate additional excited atoms to produce 633nm light. These mirrors reflect most, but not all, of the photons back into the laser medium. The mirrors transmit a small percentage of the incident photons at each reflection but they are sufficient to form the bright laser beam that comes out of the laser aperture.

Unlike ordinary mirrors, which have a coating of silver on the back surface, the laser mirrors are front-surface dielectric mirrors consisting of alternate layers of silicon oxide and titanium oxide each about a quarter wavelength (160nm) thick.

The mirror at the cathode end of the tube reflects as much as 99.9+% of the incident photons back into the laser tube. But, the mirror at the opposite (anode) end is not as good a reflector because it has a smaller number of coatings on its surface. It reflects only 99% of the light and transmits the remaining 1% to form the laser beam. The thicknesses of the mirror’s dielectric coatings have been designed to resonate the 633Nm waves at the expense of other emissions produced by the neon gas.

The mirrors in Metrologic lasers are mounted in a "semi-confocal" arrangement, i.e., a flat mirror at the rear (cathode) end of the laser tube and a concave mirror at the front end where the beam comes out. If both of the mirrors were flat, the output power of the laser would be greater but it is difficult to align flat mirrors. It is even more difficult to maintain their alignment with thermal stresses as the laser is heated and cooled, as well as the additional vibrations and shocks that can always be expected in the student laboratory.

At the sacrifice of some power, the semi-confocal arrangement features a non critical alignment that provides greater stability.

The concave mirror focuses light into a cone with the apex slightly beyond the plane of the distant flat mirror. As the reflected light returns to the front of the laser tube, it retraces the cone diverging as it approaches the curves mirror again. To compensate for the divergence of the light that is transmitted through the front mirror, the exterior surface of the mirror has a curve similar to that of a converging lens. This produces a laser beam whose edges are almost parallel.

Mirrors mounted on the laser tube with epoxy cement have been replaced with hard-seal mirrors that are fused directly to the laser tube without using any cement. This hard-seal mounting avoids seepage of moisture, which was the leading cause of failures in older models. When moisture seeped in through the epoxy cement at the ends of the laser tube, the oxygen from H2O collided with the helium and took away energy that would have otherwise been available to the neon. Without this source of energy, laser action ceased.

The mirror coatings are made of materials such as titanium oxide and silicon oxide layers. Because these coatings do not break down at temperatures as high as about 500(C, the mirrors can be heated and hard-sealed directly to either glass or metal without the need for any epoxy.

Coatings of TiOx and SiO2 have another important advantage. They can withstand the ultraviolet radiations of the energized helium and neon gases that constantly bombard the mirrors.
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Polarization
Because of variations in the mirrors of individual laser tubes, the beam will vibrate more strongly in a particular polarization plane, producing a polarized output. Also there will be a second favored plane of vibration that is at right angles to the first one. In a shorter laser tube, it is common to observe that the strongest light vibrations shift back and forth between these two favored planes. This interesting effect is called elliptical polarization and it may be observed by passing the laser beam through a polaroid filter and watching the changes in beam intensity over a period of several minutes.
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Summarizing the Theory of the Helium-Neon Laser
Free electrons, accelerated by an electric field, collide with helium atoms and transfer their energy to the helium. Excited helium atoms at the 2S level then collide with neon atoms, producing a population inversion in the neon with most atoms at their 3S2 level. When stimulated by photons that are reflected back and forth inside the laser tube, additional photons are emitted and join the original photon to form a beam of coherent light.
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Operation of the Solid State VLD Laser
Laser action, which produced the red beam of the VLD laser, is generated in a thin active layer sandwiched between p-doped and n-doped semi-conductors. Typical dimensions of the laser chip are on the order of one or two millimeters. The active layer where light is emitted is only a few microns thick.

The laser chips are built on a slab of gallium arsenide crystal that is about 0.5mm thick. This gallium arsenide is an n-type semi-conductor which has surplus of mobile electrons within the crystal lattice.

Impurities are added in sequence to the surface and diffuse into the gallium arsenide crystal to form p-type semi-conductor layers. These impurities capture electrons and leave holes in their place. We may think of the holes as carriers of positive charge because they move in the same direction as positive charges under the influence of an electric field. The schematic structure diagram shows the arrangement of the semi-conductor layers which comprise the laser.
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Photon Production
Applying a forward bias of about 2.5V dc to the laser electrodes cause any mobile electrons to move towards the positive electrode. Holes move toward the negative electrode. With sufficient energy both electrons and holes are injected into the active layer. When an electron-hole pair meet, they annihilate each other and release their energy to produce a photon of red light. By itself, a photon does not create much light. But, together with combined energy of many others of the same wavelength, an intense laser beam is produced.
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Index-Guided Photon Enhancement
Photons, created in the active area of the laser, stimulate other electron-hole pairs to meet, annihilate, and produce additional photons of the same wavelength, phase, and direction. Upon reaching the ends of the crystal, most of the photons emerge to form the laser beam. However, about 36% of the photons are reflected back into the active layer by a pair of flat cleaved ends at opposite sides of the laser diode. They enhance further stimulation of electron-hole pairs.

Other photons may travel toward the sides of the diode crystal instead of going in the desired direction toward the laser aperture. To minimize this loss, the semi-conductors that comprise the laser are arranged so their indices of refraction decrease with distance from the main channel. As the index of refraction becomes lower, stray photons are bent away from the normal and many are guided back into the main channel.
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Laser Beam Characteristics
The beam emitted from the aperture of a visible laser diode differs from that of a He-Ne laser in three significant ways; the shape, visibility, and coherence length.
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Beam Shape
Because the beam comes from an end of a rectangular slab rather than a round capillary tube, the beam is emitted with an elliptical cross section. In the Metrologic VLD lasers this is corrected by mounting an astigmatic lens over the laser aperture.
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BeamVisibility
Red light consists of electromagnetic waves that can be as short as 600nm or as long as 700nm. The human eye is most sensitive to the shorter wavelengths in this part of the spectrum. Therefore even if laser beams are equally intense, those from the 633nm He-Ne lasers and the 630nm VLD lasers appear to be about four times as bright as those from the 670nm VLD’s. However, for silicon diode photocell, the reverse is true. They are much more sensitive to the longer wavelengths of red light.
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Beam Coherence Length
The beams from the earlier gain-guided VLD lasers were unable to maintain coherence for distances exceeding 2mm. Because the coherence distance was so short, it was almost impossible to perform holography or interferometry experiments with these lasers. Metrologic is now using the newer index-guided laser diodes for all of its solid state lasers. Because these can maintain beam coherency for distances up to 5cm, holography and interferometry are now possible using either our VLD lasers or our He-Ne lasers.
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