Lasers in Medicine
By Karl Jalbert, Bishop & Associates Inc.

Many UFO watchers and conspiracy theorists believe that lasers, optical fibers, Kevlar^®, and even the common transistor are all products that were reverse-engineered from recovered wreckage of an alleged alien spacecraft that crashed on a cattle ranch near Roswell, New Mexico, in 1947. As a man of science, I certainly do agree that it would be most arrogant of mankind to assume that we are alone in this expansive universe—nevertheless, from my understanding of quantum physics (and from watching the Discovery Channel), the constraints of time and distance undoubtedly prevent interstellar travel as we understand it; and so in my humble opinion, these products were most likely nothing more than the brainchildren of a few clever individuals laboring in a more terrestrial world.

A Less Glamorous Beginning
In 1917, Albert Einstein first theorized about the process which makes lasers possible. It was called "Stimulated Emission.” By 1954, Charles Townes and Arthur Schawlow invented the MASER (Microwave Amplification by Stimulated Emission of Radiation), using ammonia gas and microwave radiation. It was used to amplify radio signals, and also as an ultra-sensitive detector for space research. The MASER was the precursor to the contemporary optical laser; it used similar technology, but operated in the RF/microwave frequency, not in the optical or “light” spectrum. It wasn’t until May 1960, while working together at Columbia University, that Charles Townes and Arthur Schawlow published papers about a visible-light laser. The first successful optical laser (a ruby laser) was demonstrated by Theodore Maiman in December 1960. Despite the fact that many historians claim Maiman invented the first optical laser, there is some controversy that Gordon Gould, a doctoral student under Charles Townes at Columbia University, was the first. Gould failed to file for a patent for his invention until 1959, and as a result, Gould's patent was refused, and his technology was exploited by others. It took until 1977 for Gould to finally win his patent-war, and receive his first patent for the laser. The name LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation. Since its development in 1960, lasers have become powerful and indispensable tools, used in almost every technology segment. Laser applications in medicine and surgery have similarly evolved, and while medical lasers have never become the "magic ray" that some had hoped, they have become indispensable tools in clinical practice.

There are many medical laser systems available today, but they all use the principal of selective photothermolysis, which means, getting the right amount of the right wavelength of laser energy, to the right tissue, to damage or destroy only that tissue, and nothing else.

Wavelength vs. Frequency
In many of my tutorials on fiber optics through the years, I have been asked this question: “At what frequency does light operate?” If you are new to optics, it would be helpful to understand the difference between wavelength and frequency. Though wavelength and frequency are basically the same—that is, the measurement of a propagating sine wave through either air or through a conductor—in electronics we measure the analog (sinusoidal wave) or digital (square wave) in frequency. Frequency is defined as the measurement of the number of peaks and valleys (oscillations) of a carrier wave passing a given point per second. In the early days of radio, it was measured in cycles-per-second (cps). In the SI system, we use Hertz (named after the German physicist Heinrich Hertz). For example, one megahertz (1MHz) is equal to one million cycles-per-second (1MHz/s) passing a theoretical point of measurement in one second. Frequency (ƒ), is calculated by dividing the number of cycles by T (time). A more accurate measurement takes many cycles into account and averages the period between each.

In optics we use wavelength, which is the distance between the crests (cycles) of a propagating wave. It is represented by the Greek letter lambda (λ). Light is also electromagnetic energy, but it operates at a much more minuscule wavelength than RF and microwave. The “frequency” is actually so high that counting it is almost impossible. An infinitesimally small unit of measurement, called a nanometer (nm), is used to measure the wavelength instead. A nanometer is an SI-based unit equivalent to one billionth of a meter (or one millionth of a millimeter), expressed as 1×10^-9 m. The light spectrum was originally measured in angstroms. An angstrom (Å) is a non-SI unit of length that is internationally recognized, equal to 0.1 nanometer (nm). It is still often used in expressing the sizes of atoms, lengths of chemical bonds, and the visible-light spectrum. It can be written in scientific notations as 1×10⁻¹⁰ m. There are 10,000 angstroms in a micron (μm) and 10 angstroms in a nanometer (nm). For example 7,000 angstroms is equal to 700 nanometers.

The Electromagnetic Spectrum
Electromagnetic (EM) radiation can be described in terms of a stream of photons (the smallest quantum unit of light or electromagnetic energy). Photons are generally regarded as particles with zero mass and no electric charge, each traveling in a wave-like pattern similar to ripples generated on a calm pond after a pebble is dropped into the water. But, these “ripples” are moving at the speed of light (@299,792,458 meters per second) and carrying some amount of energy. The only difference between radio waves, visible light, and gamma-rays is the energy of the photons. Radio waves have photons with low energies, microwaves have a little more, and infrared has still more, and then comes visible, ultraviolet, X-ray, and gamma rays.

The amount of energy a photon holds makes it sometimes behave more like a wave and sometimes more like a particle. This is called the “wave-particle duality” of light. It is important to understand that we are not talking about a difference in what light is, but only in how it behaves. Low energy photons (such as radio) behave more like waves, while higher energy photons (such as X-rays) behave more like particles. The truth is, the electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. Each way of thinking about the EM spectrum is related to the others in a precise mathematical way. The relationship is: The wavelength equals the speed of light divided by the frequency.

λ= c

   n(nu)

The Visible Light Spectrum
The visible light spectrum is the section of the EM spectrum that is visible to the human eye. It ranges in wavelength from approximately 400 nanometers (nm) to 700 nm (Figure 3). It is also known as the optical spectrum of light. The wavelength (which is related to frequency and energy) of the light determines the perceived color. The ranges of these different colors are listed in the table below. Some sources vary these ranges fairly drastically and the boundaries of them are somewhat approximate, as they blend into each other. The edges of the visible light spectrum blend into the ultraviolet and infrared levels of radiation. Medical lasers operate in this spectrum. Most light that we interact with is in the form of “white light,” which contains many or all of the visible wavelengths. Launching white light through a prism (Figure 4) causes the wavelengths to bend at slightly different angles due to optical refraction. The resulting light is therefore split across the visible color spectrum like a rainbow (in which airborne water particles are acting as the refractive medium to sunlight). The order of wavelengths shown (Figure 4), is in order of wavelength, which can be remembered by the mnemonic “Roy G. Biv” for Red, Orange, Yellow, Green, Blue, Indigo (the blue/violet border), and Violet. By using special sources, refractors, and filters you can get a narrow band of about 10 nm in wavelength. This narrow band is called “monochromatic” light. Lasers are the most consistent source of narrow, monochromatic light that we can achieve.

Medical Lasers
Laser light generally differs from other light in that it is focused in a narrow beam, limited to a narrow range of wavelengths (monochromatic), and consists of waves that are in phase with each other, or “coherent.” These properties arise from interactions between the process of stimulated emission, the resonant cavity, and the laser media used.

Laser media can be either solid, liquid, gas, or electronic. Laser gases are contained in cylindrical tubes and are excited by an electric current or external light source, which is said to “pump” the laser. In physics, pumping is the use of light energy to raise the atoms of a system from one energy level to another. For example, a system may consist of atoms having a random orientation of their individual magnetic fields. When optically pumped, the atoms will undergo a realignment of individual magnetic fields with respect to the direction of the light beam; that is, they will become coherent. Medical lasers have three types of excitation mechanisms. In most gas lasers, high voltage DC electricity is used. With some CO₂ lasers, radio frequency electricity excites the gas. This type of excitation is needed to produce an ultra-pulsed output, which is the delivery of very fast, extremely powerful bursts of light. Media that do not conduct electricity, such as solid and liquid media, are excited with light produced by flash lamps or other lasers (i.e., argon or nitrogen). Lasers are named for the media that is used to produce the light. Some solid-media lasers commonly used in medical applications are: erbium-yttrium-aluminum-garnet (Er: YAG); holium-yttrium-aluminum garnet (Ho: YAG); neodymium-yttrium-aluminum-garnet (Nd: YAG); and alexandrite, ruby, and potassium-titanyl-phosphate (KTP). Carbon dioxide (CO₂), argon, copper-vapor, and excimer lasers are examples of medical lasers with gas media. Dye lasers have liquid media, and diode lasers have electronic media.

The combination of laser media and resonant cavity fabricates what often is called simply a laser, but technically it is a laser oscillator. Oscillation determines many laser properties, and it means that the device generates light internally. Without mirrors and a resonant cavity, a laser would just be an optical amplifier, which can amplify light from an external source, but not generate a beam internally. Elias Snitzer, a researcher at American Optical Corporation, demonstrated the first optical amplifier in 1961, but such devices were seldom used until the spread of communications based on fiber optics.

Lasers can generate pulsed or continuous beams, with average powers ranging from microwatts to over a million watts (in the most powerful experimental lasers). A laser is called continuous-wave if its output is nominally constant over an interval of seconds or longer; one example is the steady red beam from a laser-pointer. Pulsed lasers concentrate their output energy into brief high-power bursts. These lasers can fire single pulses or a series of pulses at regular intervals. Instantaneous power can be extremely high at the peak of a very short pulse. Laboratory lasers have generated peak power exceeding 10¹⁵ watts for intervals of about 10^-12 seconds.

Although a visible laser produces what looks like a point of light on the opposite wall of a room, the alignment, or collimation of the beam is not perfect. The extent of beam spreading depends on both the distance between the laser mirrors and diffraction, which scatters light at the edge of an aperture. Diffraction is proportional to the laser wavelength, divided by the size of the emitting aperture—the larger the aperture, the more slowly the beam spreads. For example, a red (633 nm) helium-neon laser with a one-millimeter aperture produces a beam that diverges at an angle of about .057° (one milliradian). Such a small angle of divergence will produce a one-meter spot, at a distance of one kilometer. In contrast, a typical flashlight beam produces a similar one-meter spot within just a few meters. However, not all lasers produce tight beams. Semiconductor lasers emit light near the 1,000-nonameter wavelength from an aperture of comparable size, so their divergence is 20 degrees or more, and external optics are needed to focus their beams.

Laser Types
In general, there are two types of medical laser systems, contact and non-contact. Contact systems work by sending laser light through a fiber or sapphire crystal tip. The tip absorbs the radiant energy and becomes hot. Direct contact between the tissue and the heated tip causes conduction of the heat energy from the tip to the tissue, resulting in the vaporization of the target cells. In contrast, non-contact laser systems do not directly touch the tissue. Instead, the laser light transfers radiant energy to the tissue. Heat results when the cell absorbs the radiant energy and the molecules in the tissue begin to move. In both types of systems, the laser light itself is not hot. Heat is created only after the laser's radiant energy is absorbed, either by the tip or by the tissue.

Courtesy Shore Laser Center, Albert Poet MD

The Pulsed Dye laser (left) is used as the treatment of choice for Port Wine Stains, especially in infants and children, and for laser treatment of thick, red scars. The ruby laser (right) is used for the treatment of tattoos (Q-Switched mode), treatment of pigmented lesions, including freckles, liver spots, Nevus of Ota, cafe-au-lait spots (Q-Switched mode), and also for laser hair removal (free-running mode).

Through the use of optical fibers, laser light can be delivered to places within the body that the beams could not otherwise reach. One example involves threading a fiber through the urethra and into the kidney so that the end of the fiber can deliver intense laser pulses to kidney stones. The laser energy splits the stones into fragments small enough to pass through the urethra without requiring surgical incisions. Fibers also can be inserted through small incisions to deliver laser energy to precise spots in the knee joint during arthroscopic surgery. Lasers can kill malignant cells, and vaporize small growths in inaccessible parts of the body, such as the bowel and vocal cords, avoiding the need for surgery. As the machines become cheaper, more manageable, and user-friendly, small hospitals and clinics are able to launch their own laser centers. The rapidly expanding repertoire of laser devices brings significant benefits, in some cases almost replacing conventional therapy. The laser hasn't yet replaced the surgeon's knife, but if present trends continue, the surgical scalpel may ultimately retire to medical museums, along with leech jars and cupping basins.

Karl Jalbert Director of Military & Industrial Technology, Bishop & Associates, Inc. Applications engineer, product manager, and marketing manager, Karl Jalbert’s tenure in the connector industry has spanned over 20 years. In 1985, Jalbert joined the applications engineering team and later became western regional sales manager for Optical Fiber Technologies Inc. (OFTI), which became part of the AMP Inc. product line in 1992. Throughout the 1990s, Jalbert was a senior applications engineer for M/A-Com in Boston, and senior product management specialist for AMP in Harrisburg, PA. Prior to joining Bishop & Associates, Jalbert was a market development director for FCI Electronics USA, focusing on the industrial and embedded computing market sectors.   Jalbert holds a BSME from Central New England College.