Conclusions
Despite their many advantages, ultrashort laser systems suffer from several drawbacks. Some of these may be overcome as the technology gets more refined, others may be solved as we learn more about the physics of ultrashort pulses.
Cost
Chiefly among the downsides of these laser systems is their prohibitively high cost, especially when compared to existing conventional systems. A mechanical drill for example is available for about 10% of the cost of a femtosecond system, and while refined production techniques would bring down the costs if these laser systems were produced and sold in larger quantities, the fact remains that a laser is much more complex and requires more exotic materials than a comparatively simple mechanical drill. As such, treatment costs would rise if these systems become more wide-spread, and insurance companies would most likely be reluctant to pay for these procedures if the conventional systems achieve satisfactory results.
Group Velocity Dispersion
Figure 1. Group velocity dispersion is compensated for through prisms or gratings that cause higher-velocity wavelengths to travel longer paths than the slower wavelengths, thereby compressing the pulse.
Financial issues aside, there are several technical problems with ultrafast laser systems. One of these problems is related to the effect of Group Velocity Dispersion (GVD). Recall from the Basics of Lasers that a laser pulse consists of light of many different wavelengths. Because the refractive index of any material is a function of the wavelength, the various modes that make up a laser pulse experience different indices of refraction as they travel through various media such as air, optical systems, fibers and the laser cavity itself. Since the velocity of a wave is depended on the optical density or the index of refraction of a medium, the various modes will travel through the medium at different speeds, causing the pulse to widen and therefore to become longer. Thus, short pulses eventually become long pulses that are no longer yield the expected results when they interact with tissue.
To counteract this undesired effect, the group velocity dispersion has to be compensated inside the cavity. If the expected dispersion of the entire system is known, it can be compensated for. This is usually achieved with a pair of prisms or gratings arranged in a way so that the faster modes have to travel longer distances than the slower ones as illustrated in Figure 1. The pulse thus gets compressed before leaving the laser and traveling through the optics of the system. Additionally, there are some optical fibers that prevent GVD which can be used to guide the pulse train onto the tooth without danger to the patient's eyes while maintaining a tight pulse profile.
Outlook
Future developments are hard to predict, but there are several areas that are currently undergoing heavy research. Obviously, as technology advances and the systems evolve and mature we can expect to see more compact, more efficient and more affordable systems to appear on the market. Beyond these enhancements to existing systems however there is a whole area of nearly untapped potential left. Femtosecond pulses, while short, are not the end-all of ultrashort pulses. Research into attosecond pulses is already underway1. Due to the inverse relationship between pulse length and LIOB threshold, these even shorter pulses require even less energy to achieve plasma formation.
Potential other uses for lasers include brain surgery, specifically the removal of tumors2. By drilling a small hole in the skull and passing a fiberoptic probe through it, surgeons can ablate tumors with extremely high precision. Moreover, the beam itself can potentially be used to image the surgery in vivo and in real-time, making it easier for the surgeon to avoid vessels. On account of the coagulating properties of certain types of lasers, bleeds could be rapidly dealt with before major brain injury occurs.
Outside of surgery, multiphoton microscopy is moving towards exploiting three-photon absorption and third harmonic generation to get highly specific signals from tissue without staining and with low laser intensities.