Thermal and Physical Implications of Pulsed Versus Continuous Emissions
Energy is the ability to work and is measured in Joules. The measuring unit for most dental laser applications is the millijoule (mJ), and this is a parameter that is controlled by the operator. Power is the rate of doing work, or energy used over a period of time and is measured in Watts (W). One Watt equals one Joule per second. These are typically the watts displayed on the machines' control panels. A laser beam can be emitted either as a continuous beam or in a pulsed fashion.
The pulse rate per second is measured in Hertz (Hz). The wattage displayed on the laser unit is a product of the Millijoule (energy) per pulse times the number of pulses per second (hertz). Peak Power refers to the power level in each individual laser pulse and this measurement is not visible to the operator on most devices, yet it is the most important variable dictating how the laser beam will interact with the target tissue.
Continuous wave emission mode means the laser is on the whole time it is turned on. In these lasers peak power equals the wattage output displayed. There are two basic forms of pulsed laser modes: gated wave and free running pulsed. A gated wave pulse is usually created with a shutter that blocks the laser beam from reaching the handpiece and target tissue at varying speeds. This pulse form is sometimes referred to as "chopped." The laser is on constantly, but the shutter device blocks the light from transmitting. "Superpulsed" lasers are a form of gated lasers with extremely short pulse durations.
Free running pulsed lasers are not on constantly but emit photons in powerful bursts of energy measured in millionths of seconds. To better understand these concepts one can use an example of a flashlight. When a flashlight is turned on it is in continuous wave mode, moving one's hand back and forth across the beam is gated wave mode, and turning it on and off repeatedly is free running pulsed mode. Each of these temporal emission modes has important characteristics when the laser energy interacts with tissues that need to be understood well.
Peak power can be a difficult concept to understand but its importance cannot be overstated. Each pulse has a fixed amount of energy, usually the millijoules displayed on the unit. As shorter pulses are used this same energy is effectively squeezed into a smaller space, which increases the peak power of the pulse. One can think of a rubber band around a wrist as an analogy. If the rubber band is lifted three inches, it will have a fixed amount of energy in it. Now if the rubber band is slowly returned to the wrist, it will not hurt a bit. If it is let go, the energy is released in a much shorter time and it will sting and make a popping sound. Yet the actual energy expended is identical. Hard tissue dental lasers can have peak powers in the thousands of watts, and these short bursts of extreme power allow for the efficient cutting of enamel, dentin, and bone.
Figure 3. Peak power equals average power.
A laser running in continuous mode has equal peak power and average power. Average power is what is displayed on the control panel. Peak power is not shown but can be calculated.
Figure 4. Average power is half of peak power.
In a simple gated wave laser average power is half peak power. Here a blinking shutter blocks the laser beam half the time.
Figure 5. High peak power/low average power.
Average power is a tiny fraction of peak power in free running pulsed mode. These lasers flash off and on like a strobe light in short bursts of energy from 50 to 1000 millionths of a second (microseconds). In this example a laser displaying 5 watts running at ten hertz with a fifty microsecond pulse will have peak power of 10,000 watts per pulse. This powerful burst of energy allows for efficient tooth and bone ablation with minimal thermal effects.
The thermal implications of the three pulse modes are profound. Thermal relaxation refers to various tissues’ inherent ability to dissipate heat. In continuous mode there is no thermal relaxation at all, and potentially damaging heat can build in the tissue quickly. Gated wave mode presents basically a half on/half off exposure to laser energy and the ability of the tissue to absorb the heat is limited. Superpulsed lasers improve on the standard gated mode's thermal effects through sophisticated pulse time control. Thermal relaxation occurs the most when free running pulsed lasers are used. Each pulse is temporally very short, anywhere from 50 to one thousand millionths of a second depending on which device and settings are used. There is adequate time between each pulse to allow the tissue to absorb and dissipate the heat to minimize thermal damage.
This lack of tissue heating results in the lowered post-operative discomfort and predictable healing seen after many laser procedures. It also contributes to the ability to perform many operative dentistry procedures and even some soft tissue ones without local anesthesia. An excellent analogy for free running pulsed lasers is if a finger is moved rapidly back and forth through a candle flame it will not burn as the tissue can absorb and dissipate the momentary high heat exposure. If the same "pulsing" finger is moved slowly through the flame it will get burned eventually (gated mode). Placing the finger in the flame and holding it there will cause rapid thermal damage (continuous mode).