- Laser Modes of Operation
- Continuous Wave Operation
- Pulsed Operation
- Pulsing and Super Pulsing: A Commentary
Laser Modes of Operation
A laser can be classified as operating in either continuous or pulsed mode, depending on whether the power output is essentially continuous over time or whether its output takes the form of pulses of light on one or another time scale. (Figure 1)
- Continuous Wave
- Modulated Continuous Wave
- Q Switched (nanosecond pulses)
- Long pulsed (micro or millisecond pulses)
Graph A:Continuous wave (CW) is like a light that is constantly “on”
Graph B and C:Modulated lasers produce light that is “on” half the time and “off” half the time. The duty cycle is simply 50 percent, because the light is “on” half the time and “off” half the time. An advanced definition of duty cycle is the duration of the pulse divided by the period, or the time from the beginning of one pulse to the beginning of the next.
Graph D: The laser represented has a duty cycle of 25 percent; it is “on” one-fourth of the time and “off” three-fourths of the time.
Continuous Wave Operation
Continuous wave emission has no interruption during the delivery of their energy
Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as continuous wave (CW).
Many types of lasers can be made to operate in continuous wave mode to satisfy such an application. Many of these lasers actually lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will in fact produce amplitude variations on time scales shorter than the round-trip time (the reciprocal of the frequency spacing between modes), typically a few nanoseconds or less.
In most cases these lasers are still termed “continuous wave” as their output power is steady when averaged over any longer time periods, with the very high frequency power variations having little or no impact in the intended application. (However the term is not applied to mode-locked lasers, where the intention is to create very short pulses at the rate of the round-trip time).
For continuous wave operation it is required for the population inversion of the gain medium to be continually replenished by a steady pump source. In some lasing media this is impossible. In some other lasers it would require pumping the laser at a very high continuous power level which would be impractical or destroy the laser by producing excessive heat. Such lasers cannot be run in CW mode.
Pulsed operation of lasers refers to any laser not classified as continuous wave, so that the optical power appears in pulses of some duration at some repetition rate. This encompasses a wide range of technologies addressing a number of different motivations.
Some lasers are pulsed simply because they cannot be run in continuous mode.
In other cases the application requires the production of pulses having as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this goal can sometimes be satisfied by lowering the rate of pulses so that more energy can be built up in between pulses
In laser ablation for example, a small volume of material at the surface of a work piece can be evaporated if it is heated in a very short time, whereas supplying the energy gradually would allow for the heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature at a particular point.
Other applications rely on the peak pulse power (rather than the energy in the pulse), especially in order to obtain nonlinear optical effects. For a given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as Q-switching.
Pulsing and Frequency
The words pulsing and frequency are used interchangeably to describe the same concept. The interruption of energy flow on a predetermined basis. Some manufacturers have laid claim to special frequencies, claiming that they produce better clinical outcomes.
Pulsing and Super Pulsing: A Commentary
By Jan Tunér, DDS and Lars Hode, DrSci (Swedish Laser Medical Society)
There are principally two types of pulsing in laser photo therapy
- chopped (switched) or
A chopped beam is a continuous beam that is electronically (or mechanically) switched between on and off.
During the moments when it is on it has typically the same output power as in continuous mode, but as it is not on all the time, the average output power is less than when it is continuous.
The average power is a function of the continuous wave power and the duty cycle (the ratio of the “on” time of the beam to the total emission (“on” + “off”) time, usually expressed as a percentage).
Typical laser types are most of the gas lasers (such as the HeNe laser) and all semiconductor (diode) lasers (except the GaAs laser). The GaAs laser was the first semiconductor laser in the world. In order to generate laser light, the current density in the GaAs semiconductor crystal had to be extremely high. As a consequence of the high electric current the output power of this semiconductor laser is very high.
Typical peak power is in the order of many watts. However, when an electric current is conducted through a material heat is generated, and with the necessary high current in this laser the crystal will burn up immediately unless the time of current conduction is extremely short, i.e., super-pulsed GaAs lasers cannot work continuously.
The maximal pulse time for this laser is in the order of 100 to 200nanoseconds and, after each such pulse, a long cooling time is needed, usually about a thousand times longer than said pulse time.
This form of pulsing is called super pulsing and, although the peak power is very high, the average output of super-pulsed lasers is comparatively low. Typically the GaAs laser produces its maximum emission at 904 nm.
What About Average Power Output
Restating the above, even though the peak power of the super-pulsed GaAs laser may be very high,it lasts for an extremely short time compared to the pulse cycle, resulting in an average output power that is usually a thousand times lower than the peak power. For clinical use, it is the average power that counts.
The energy (dose) delivered from pulsed lasers is always the average output power multiplied by the exposure time. The average power is the important output of the laser.
Some manufacturers prefer to label these lasers as “very strong” and state only the peak power which then can be in the order of 100 watts.
This sounds impressive, but typically these lasers emit10-100 mW average power, and this is what counts for the treatment.
The GaAs lasers are quite useful in physiotherapy, but care has to be taken.
In some super-pulsed lasers the average output changes with the set pulse frequency, so that low pulse repetition rates deliver very low average outputs.
This means that with such lasers, with low frequency settings, the treatment time may be impractically long in order to deliver a reasonable dose.
A Serious Problem
One manufacturer, for example, promotes its super-pulsed lasers as having 25,000 mW or50,000 mW of power, and offers the user a small number of preset ‘programs’ which, essentially,only adjust the pulse frequency and, therefore, the average output power. One of these programs sets a frequency of 5 Hz. To calculate the average power one must only know the Peak Power, the Pulse Frequency and the Pulse Duration.
As mentioned previously, the pulse duration (i.e., the‘width’ of each pulse of energy) of most GaAs devices is 100-200 nanoseconds (0.0000001 –0.0000002 sec). If we use the manufacturer’s ‘highest’ power option (50,000 mW), select their 5 Hz program, and assume the longest possible pulse duration (0.0000002 sec) for our calculation, we arrive at an Average Output Power of only 0.050 mW, or fifty millionths of one Watt.
With this very low average power it will take twenty thousand seconds (5.6 hours) for this manufacturer’s laser to deliver one Joule. Impractically long, perhaps?
Other super-pulsed lasers employ “pulse trains”, which enable the average output to be maintained at a constant level over all frequencies. The importance of checking upon this is obvious when it comes to acquiring a GaAs laser.
One manufacturer claims that its dual-wavelength (800 nm and 970 nm) high-powered Class IV laser has better penetration due to its ‘Intense Super Pulse’ emission. However, these diode lasers are not super pulsed, they are “chopped”, and chopping does not offer increased penetration. In this case chopping the output simply reduces the tissue-heating effect of the high power laser by both reducing the average power and also allowing time for the tissue to thermally relax (i.e., dissipate heat) between each pulse of light.
The biological differences between super-pulsed and chopped emissions are likely to be
fundamental. Is pulsing then of interest? The in vitro studies by e.g. TiinaKaru clearly show that the type of pulsing is of importance. However, in these situations one type of cell and one type of reaction is studied.
In the clinical situation, many types of cells are irradiated and a multitude of events happen. So is pulsing then of any clinical importance?
The answer is that we do not know.This is well presented in the recent literature review by Hashmi et al, http://www.ncbi.nlm.nih.gov/pubmed/20662021.
Some lasers are pulsed to allow for heat dissipation, but that has nothing to do with bio stimulation.
Chopping is an option in some continuous lasers and users should be aware of the fact that suggested pulse repetition rates are only setting options; we do not know if the different pulse repetition rates provide different biological results.
Many “recommended” frequencies employed in therapeutic lasers are, in fact, carried over from other fields and modalities, especially electrical stimulation.
Nogier’s frequencies, for example, are often incorporated into laser therapy protocols for both humans and animals; yet their original application was in humans only, specifically auricular therapy delivered by electrical stimulation.
Due largely to the impact of pulse frequency upon the average power of the first therapeutic diode laser, the GaAs, Nogier’s original frequencies (there are seven, ranging from 1.14Hz to 146 Hz) are even presented at a higher “harmonic” so as to achieve a higher average output power, further increasing the disparity between their original intended application and their current use.
Despite this, and the fact that there have been no studies undertaken to compare or confirm the efficiency of the original or higher-harmonic laser-delivered frequencies in humans or animals, these and other frequencies are provided as an integral part of many different therapeutic laser devices and their pre-programmed protocols.
There are many variations of so called pre-programmed lasers on the market. Some offer starter protocols that employ simple variations of power, frequency and time, making these parameters known to the user and even affording them the option of changing them as their knowledge and experience improves. Others, however, provide the user with nothing more than a choice of letters or numbers that represent different “proprietary programs”, ensuring that the user is kept completely in the dark as to what they’re actually doing.
Such programs may consist of various frequencies and exposure times, often in automatically-changing combinations of such; for instance, 20 seconds of500 Hz + 40 second of 120 Hz + 10 second of 1500 Hz.The user is informed only that that“program” is supposed to be the best for e.g. headache, and that another program and time/frequency combination is the best for arthritis, etc.
The buyer of such an instrument trusts that the constructor of the instrument knows that this is a fact. However, there are no such optimal time/frequency combinations scientifically proved to be better than others.
Also – how can a setting for “arthritis”, for example, be the same for a finger joint as well as for a knee?
Who can verify the pulse repetition rates recommended? Such preset protocols will generate nothing more than vaguely satisfactory outcomes, at best; neither what your patients expect of you, nor what you should expect of a clinical tool that has, most likely, cost you thousands of dollars.
One particular manufacturer has corrupted the use of the terms ‘Optical Window’ and ‘Therapeutic Window’, well-known to many within the photo therapy field, to label their preset programs as so called‘Therapeutic Optical Windows’ that, supposedly, deliver optimal combinations of the many different parameters that influence clinical outcomes.
As an exercise, let’s consider the various device and treatment parameters and patient characteristics that affect variations in photo therapy outcomes, and determine how many iterations of these must be clinically tested and validated before one could claim, with even a hint of honesty, to have determined the optimal “Therapeutic Optical Windows” for even a handful of indications.
First we take the various parameters of, say, a switched continuous wave device (e.g., output power, spot size, wavelength, pulse frequency, duty cycle).
Then we add the irradiation duration, treatment technique, number of points to be treated or the area of affected tissue, and the target tissue depth.
Next, toss in a handful of such patient characteristics as skin color and tissue type and whether their condition is acute, sub-acute and chronic.
Finally, consider some desirable clinical outcomes such as analgesia, reduction of inflammation, enhanced tissue repair and/or nerve tissue regeneration. Although this gives us a very simplified set of factors, we are still left with potentially billions of combinations of variables that must be subjected to clinical testing in order to support this manufacturer’s claims.
In forty something years of research into photo therapy, by hundreds of researchers, we have barely even scratched the surface in terms of determining upper and lower activity thresholds of irradiation duration and intensity, and yet we’re now supposed to believe that one company only has considered and tested every possible iteration and distilled them into nine optimal “Optical Therapeutic Windows”? Even the most credulous among us must baulk at that …We recommend, instead, availing yourself of high-quality research published peer-reviewed journals, informative manuals and qualified seminars, rather than automatic settings. Use palpation,your own physiologic knowledge, your patients’ feedback and your experience guide you in your choice of parameters.