Lasers-Operation

Laser Operation

Energy and Power: Force is defined as that which changes the state of a mass. Energy is force exerted over a distance, the ability or capablity to do work. Energy can be transferred from one part of a system to another in one form or another, but it cannot be created or destroyed. The total amount of energy is a given system remains unchanged.
There are many different forms of energy (kinetic, potential, electromagnetic, chemical, etc.), and a number of different units used to express energy, including BTU, calories, etc. The international unit of energy is the Joule, named after English physicist James Prescott Joule, the first to quantify the relationship between heat transfer and mechanical work.

One Joule of Energy equals:

The kinetic energy released by a dropped cell phone
The heat energy generated i by a human at rest in 0.01 second
The food energy contained in 1/4 oz. of beer
The energy needed for 10,000,000 mosquitoes to take flight

Power is the rate at which energy is transferred. One Joule expended over the course of 1 second equals 1 Watt, the international unit of power, although power is commonly expressed in other units, including Horsepower, BTU/hour, Volt Amperes (VA), etc. Optical power is the average rate of energy transfer by electromagnetic radiation.

Watt equivalents:

Car engine = 200 kilowatts (kW)
Incandescent light bulb= 60 W
Home furnace = 35 kW
Resting human = 100 W
Maximal activity, human = 4 kW

The terms energy and power are often used interchangeably in the common lexicon, but to illustrate the difference, consider two people on a one mile long jogging path, one running at 6 mph, the other walking at 3 mph. Both expend the same amount of energy, but the runner will complete the mile in half the time of the walker, generating twice as much power.

To further illustrate the difference between energy and power, note how the power of a 1 Joule laser pulse increases as the duration of the pulse decreases:

Laser Operation: Lasers can be operated in Continuous Wave (CW) or Pulsed mode.

CW lasers are pumped continously, inducing a steady state population inversion and a true continous output, The rate of energy production is expressed in units of power, orwatts.

The power output per unit area is referred to as the Power density, irradiance, or optical intensity, expressed in Watts/cm²

In pulsed operation, pumping causes a population inversion and stimulated emission of laser light. Light output ceases as the populaton inversion becomes depleted, but continued pumping restores the population inversion and laser output. Because the output is a series of energy pulses, it's more conveniently expressed in terms of energy in joules (the average power can be calcuated by multiplying the pulse energy by the repetition rate).

The energy output per unit area is referred to as the energy density, or more commonly, the Fluence, or flow of energy, expressed in Joules/cm².

The units used to express the output of pulsed lasers can be a source of confusion, especially in medical applications. Although the fluence used in a given treatment is commonly referred to as "joules", fluence is denominated by the area over which the energy is applied.

The pulse energy multiplied by the repetition rate, or average power, is the best way to measure a given laser's ability to generate photons.

Most medical lasers and light based devices operate in pulsed mode. In a typical Solid State "bulk" laser, pump energy is supplied by a flashlamp or lamps mounted at one focus of an tuned elliptical cavity, with a crystalline rod doped with rare earth or transition metal ions at the other focus (the "pump chamber"). This arrangement maximizes absorption of the pump energy. Laser photons from the gain medium resonate between the highly reflective (HR) mirror and the partially reflective mirror (output coupler, or OC), and exit the resonator into a delivery device.

A Pulse Forming Network (PFN) stores energy from a Capacitor Charging Power Supply, which triggers the flashlamp to fire. An ionization arc is maintained by a Simmer Supply, to decrease the load and prolong the life of the flashlamp. Although flashlamps are much more efficient than an light bulb (7% vs. 50%), the enormous amounts of heat generated are managed by a cooling system, typically deionized water circulated directly through the pump chamber, then on to a heat exchanger.

CW operation is similar, although an arclamp capable of continuous operation, rather than a flashlamp, is used as a pump source. CW systems have a much higher duty cycles compared to pulsed systems, with correspondingly heavier duty components and more demanding cooling requirements. Gain media is typically optimized for CW operation as well.

Lasers can also be operate in Quasi CW mode, in which pulses are repeated at a very high (kHz) rate (typically corresponding to the lifetime of the population inversion), simulating CW operation, but with a much lower duty cycle.

Q-Switching can be used to increase the power (not the energy!) of a laser pulse. By degrading the Q, or Quality of the laser resonator, pump energy is stored until the population inversion is complete and the gain medium is saturated with energy. When the the Q is restored, laser action begins very rapidly and depletes the population inversion, producing a very short (nanosecond domain) but very powerful pulse of laser light. Power = Energy/time, so although the pulses are powerful, the average energy output of a Q-Switched laser is actually lower than a "free running" pulsed laser. Q-Switching with very high (kHz) repetition rates can be operated in Quasi CW mode to simulate the operation of a much higher power CW or pulsed laser.

Modulating the quality of a resonator can be active or passive. In active q-switching, the switch is typically an externally controlled electro-optical device, in which voltage is used to change the refractive index of certain crystals (Pockels cell), blocking or transmitting light in the resonant path.

In passive q-switching, the "switch" is typically a saturable absorber which attenuates light transmission below a given threshold. As light intensity increases, the absorber becomes saturated and passes light in the resonant path, enabling laser action. The absorber desaturates after the laser pulse, blocking transmission and allowing the process to repeat. The saturable absorber can be a crystal, dye, or semiconductor , and because the process is passive, very high repetition rates are possible and design is simplified compared to active q-switching, but with much less control and lower pulse energies.

Frequency Doubling, or Second Harmonic Generation, is a process in which the frequency of photons passing through certain crystalline media at specific angles can be doubled, halving the wavelength. The process can be thought of as two lower energy energy combined into a single high-energy photon. The most common application of frequency doubling is for generating 532 nm (green) laser light from 1064 nm (infrared) laser light, as in the familiar green laser pointer.

Potassium Titanyl Phosphate (KTP) is the most common non-linear crystal used for frequency doubling in medical laser systems, hence the name KTP laser.

Diode Lasers: Similar to the familar LED, diode lasers are numerically the most common laser and are arguably the most versatile. Pump energy is supplied by low voltage direct current flowing through a P/N junction composed of exotic materials such as Gallium Arsenide (GaAs) or Indium Phosphide (InP), whose atomic structure allows photon emission. The P/N junction is assembled to form an optically resonant cavity.

Most laser diodes operate in CW mode in the red and Near Infrared (NIR) wavelength range, but modifications to the material and construction allow a variety of wavelengths, including blue (as in Blu-Ray Disc players), and power options. Disadvantages of diode lasers include fastidious power requirements, generally poor beam quality and low power output compared to "traditional" lasers. Diode lasers may be "stacked" for high power output, and are increasingl being used as pump lasers for other media. Current medical applications include laser tracking beams, low power cutting and tissue coagulation, laser hair removal, and pump sources for DPSS and fiber lasers

Diode Pumped Solid State (DPSS) laser: Diode lasers make a convenient pump source for various gain media, typically those solid state media utilizing Neodymium as the dopant (Neodymium:Yttrium Aluminum Garnet or Nd:YAG, Nd:Yttrium Vanadate or Nd:YVO₄). The 1064 nm output is typically frequency doubled to 532nm. The familiar green laser pointer is a DPSS laser.

Diode lasers are compact, efficient, and reliable pump sources, but their high cost compared to traditional lamp pumping limits their application.

Fiber Lasers: Fiber lasers employ a specialized optical fiber doped with a rare earth ion as the gain medium. Fiber lasers are typically pumped with diode lasers, and instead of conventional HR and OC mirrors, may use Fiber Bragg Gratings (Distributed Bragg Reflectors) that can be integrated into the fiber. Fibers may be meters or even kilometers long, providing excellent thermal dissipation for high power operation. Coiling the fiber allows a rugged, compact maintenance-free design capable of high beam quality and small spot size.

Thin Disk Lasers ("Laser Mirrors"): A novel variation of DPSS Lasers, thin disk lasers utilize a very thin layer of gain medium (1-300 microns) mounted on a heat sink, front pumped by diode lasers. Specialized coatings allow the the pump energy to be reflected back onto the disk, extracting maximum efficiency from the pump, with thermal dissipation provided by the integral heat sink. Mirror lasers are capable of very high output with excellent beam quality.

VCSEL (Vertical Cavity Surface Emitting Lasers): In contrast to conventional diode lasers, VCSELS use a monolithic resonator with integrated Distributed Bragg Reflectors in place of conventional mirrors, integrated with a "quantum well" gain medium. Emission is from the facing surface, as opposed to the edge emission of conventional diode lasers. Advantages include good beam quality, relative ease of fabrication using conventional semiconductor techniques, and their ability to be manufactured on a substrate in a monolithic array.

Free Electron Laser: Currently a research tool, the FEL utilizes free electrons produced in a klystron or accelerator traveling at relativistic speed as the gain medium. "Wiggler" magnets induce oscillations in the electron stream with emission of photons. Output wavelength is tunable over a range of wavelengths. Free electron lasers are enormously bulky and expensive, but are capable of producing wavelengths in the surgically applicable 3000-6000 nm range. Practical alternatives are currently under development.

Intense Pulsed Light (IPL): Although not true lasers, IPL devices are extensively used for medical applications, including treatment of vascular lesions, hair removal, and skin rejuvenation. Broadband light from 500-1200 nm output from a flashlamp is passed through cutoff filters appropriate for the intendend application. Contruction and operation is similar to solid state bulk lasers, but without the a gain medium or resonator. IPL devices are capable of large spot sizes, but in many clinical situation, the broadband output of IPL lacks the specificity for optimal treatment.

Next: Light-Tissue Interactions

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Page last updated February 26 2008

© 2008 Albert Poet MD

Artwork/Illustrations © 2008 Albert Poet MD

There are many different forms of energy (kinetic, potential, electromagnetic, chemical, etc.), and a number of different units used to express energy, including BTU, calories, etc. The international unit of energy is the Joule, named after English physicist James Prescott Joule, the first to quantify the relationship between heat transfer and mechanical work.

Power is the rate at which energy is transferred. One Joule expended over the course of 1 second equals 1 Watt, the international unit of power, although power is commonly expressed in other units, including Horsepower, BTU/hour, Volt Amperes (VA), etc. Optical power is the average rate of energy transfer by electromagnetic radiation.

Laser Operation: Lasers can be operated in Continuous Wave (CW) or Pulsed mode.

CW lasers are pumped continously, inducing a steady state population inversion and a true continous output, The rate of energy production is expressed in units of power, orwatts.

The power output per unit area is referred to as the Power density, irradiance, or optical intensity, expressed in Watts/cm²

In pulsed operation, pumping causes a population inversion and stimulated emission of laser light. Light output ceases as the populaton inversion becomes depleted, but continued pumping restores the population inversion and laser output. Because the output is a series of energy pulses, it's more conveniently expressed in terms of energy in joules (the average power can be calcuated by multiplying the pulse energy by the repetition rate).

The energy output per unit area is referred to as the energy density, or more commonly, the Fluence, or flow of energy, expressed in Joules/cm².

The units used to express the output of pulsed lasers can be a source of confusion, especially in medical applications. Although the fluence used in a given treatment is commonly referred to as "joules", fluence is denominated by the area over which the energy is applied.

The pulse energy multiplied by the repetition rate, or average power, is the best way to measure a given laser's ability to generate photons.

Most medical lasers and light based devices operate in pulsed mode. In a typical Solid State "bulk" laser, pump energy is supplied by a flashlamp or lamps mounted at one focus of an tuned elliptical cavity, with a crystalline rod doped with rare earth or transition metal ions at the other focus (the "pump chamber"). This arrangement maximizes absorption of the pump energy. Laser photons from the gain medium resonate between the highly reflective (HR) mirror and the partially reflective mirror (output coupler, or OC), and exit the resonator into a delivery device.

A Pulse Forming Network (PFN) stores energy from a Capacitor Charging Power Supply, which triggers the flashlamp to fire. An ionization arc is maintained by a Simmer Supply, to decrease the load and prolong the life of the flashlamp. Although flashlamps are much more efficient than an light bulb (7% vs. 50%), the enormous amounts of heat generated are managed by a cooling system, typically deionized water circulated directly through the pump chamber, then on to a heat exchanger.

CW operation is similar, although an arclamp capable of continuous operation, rather than a flashlamp, is used as a pump source. CW systems have a much higher duty cycles compared to pulsed systems, with correspondingly heavier duty components and more demanding cooling requirements. Gain media is typically optimized for CW operation as well.

Lasers can also be operate in Quasi CW mode, in which pulses are repeated at a very high (kHz) rate (typically corresponding to the lifetime of the population inversion), simulating CW operation, but with a much lower duty cycle.

Q-Switching can be used to increase the power (not the energy!) of a laser pulse. By degrading the Q, or Quality of the laser resonator, pump energy is stored until the population inversion is complete and the gain medium is saturated with energy. When the the Q is restored, laser action

begins very rapidly and depletes the population inversion, producing a very short (nanosecond domain) but very powerful pulse of laser light. Power = Energy/time, so although the pulses are powerful, the average energy output of a Q-Switched laser is actually lower than a "free running" pulsed laser. Q-Switching with very high (kHz) repetition rates can be operated in Quasi CW mode to simulate the operation of a much higher power CW or pulsed laser.

Modulating the quality of a resonator can be active or passive. In active q-switching, the switch is typically an externally controlled electro-optical device, in which voltage is used to change the refractive index of certain crystals (Pockels cell), blocking or transmitting light in the resonant path.

In passive q-switching, the "switch" is typically a saturable absorber which attenuates light transmission below a given threshold. As light intensity increases, the absorber becomes saturated and passes light in the resonant path, enabling laser action. The absorber desaturates after the laser pulse, blocking transmission and allowing the process to repeat. The saturable absorber can be a crystal, dye, or semiconductor , and because the process is passive, very high repetition rates are possible and design is simplified compared to active q-switching, but with much less control and lower pulse energies.

Frequency Doubling, or Second Harmonic Generation, is a process in which the frequency of photons passing through certain crystalline media at specific angles can be doubled, halving the wavelength. The process can be thought of as two lower energy energy combined into a single high-energy photon. The most common application of frequency doubling is for generating 532 nm (green) laser light from 1064 nm (infrared) laser light, as in the familiar green laser pointer.

Potassium Titanyl Phosphate (KTP) is the most common non-linear crystal used for frequency doubling in medical laser systems, hence the name KTP laser.

Diode Lasers: Similar to the familar LED, diode lasers are numerically the most common laser and are arguably the most versatile. Pump energy is supplied by low voltage direct current flowing through a P/N junction composed of exotic materials such as Gallium Arsenide (GaAs) or Indium Phosphide (InP), whose atomic structure allows photon emission. The P/N junction is assembled to form an optically resonant cavity.

Most laser diodes operate in CW mode in the red and Near Infrared (NIR) wavelength range, but modifications to the material and construction allow a variety of wavelengths, including blue (as in Blu-Ray Disc players), and power options. Disadvantages of diode lasers include fastidious power requirements, generally poor beam quality and low power output compared to "traditional" lasers. Diode lasers may be "stacked" for high power output, and are increasingl being used as pump lasers for other media. Current medical applications include laser tracking beams, low power cutting and tissue coagulation, laser hair removal, and pump sources for DPSS and fiber lasers

Diode Pumped Solid State (DPSS) laser: Diode lasers make a convenient pump source for various gain media, typically those solid state media utilizing Neodymium as the dopant (Neodymium:Yttrium Aluminum Garnet or Nd:YAG, Nd:Yttrium Vanadate or Nd:YVO₄). The 1064 nm output is typically frequency doubled to 532nm. The familiar green laser pointer is a DPSS laser.

Diode lasers are compact, efficient, and reliable pump sources, but their high cost compared to traditional lamp pumping limits their application.

Fiber Lasers: Fiber lasers employ a specialized optical fiber doped with a rare earth ion as the gain medium. Fiber lasers are typically pumped with diode lasers, and instead of conventional HR and OC mirrors, may use Fiber Bragg Gratings (Distributed Bragg Reflectors) that can be integrated into the fiber. Fibers may be meters or even kilometers long, providing excellent thermal dissipation for high power operation. Coiling the fiber allows a rugged, compact maintenance-free design capable of high beam quality and small spot size.

Thin Disk Lasers ("Laser Mirrors"): A novel variation of DPSS Lasers, thin disk lasers utilize a very thin layer of gain medium (1-300 microns) mounted on a heat sink, front pumped by diode lasers. Specialized coatings allow the the pump energy to be reflected back onto the disk, extracting maximum efficiency from the pump, with thermal dissipation provided by the integral heat sink. Mirror lasers are capable of very high output with excellent beam quality.

VCSEL (Vertical Cavity Surface Emitting Lasers): In contrast to conventional diode lasers, VCSELS use a monolithic resonator with integrated Distributed Bragg Reflectors in place of conventional mirrors, integrated with a "quantum well" gain medium. Emission is from the facing surface, as opposed to the edge emission of conventional diode lasers. Advantages include good beam quality, relative ease of fabrication using conventional semiconductor techniques, and their ability to be manufactured on a substrate in a monolithic array.

Free Electron Laser: Currently a research tool, the FEL utilizes free electrons produced in a klystron or accelerator traveling at relativistic speed as the gain medium. "Wiggler" magnets induce oscillations in the electron stream with emission of photons. Output wavelength is tunable over a range of wavelengths. Free electron lasers are enormously bulky and expensive, but are capable of producing wavelengths in the surgically applicable 3000-6000 nm range. Practical alternatives are currently under development.

Intense Pulsed Light (IPL): Although not true lasers, IPL devices are extensively used for medical applications, including treatment of vascular lesions, hair removal, and skin rejuvenation. Broadband light from 500-1200 nm output from a flashlamp is passed through cutoff filters appropriate for the intendend application. Contruction and operation is similar to solid state bulk lasers, but without the a gain medium or resonator. IPL devices are capable of large spot sizes, but in many clinical situation, the broadband output of IPL lacks the specificity for optimal treatment.

Page last updated February 26 2008