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Научни трудове на Съюза на учените в България-Пловдив, серия Б. Естествени и хуманитарни науки, т. XVIII, ISSN 1311-9192 (Print), ISSN 2534-9376 (On-line), 2018. Scientific researches of the Union of Scientists in Bulgaria-Plovdiv, series B. Natural Sciences and the Humanities, Vol. XVIII, ISSN 1311-9192 (Print), ISSN 2534-9376 (On-line), 2018.
HIGH ENERGY OUTPUT TWO-CHANNEL ND: GLASS LASER FOR
DEN TAL APPLICATIONS Margarita Deneva, Valko Kazakov, Pepa Uzunova, Marin Nenchev Technical University-Sofia and Plovdiv Branch, Scientific QOE -Laboratory and Department of Optoelectaonics and Laser Engineering and
Medical Academy -Sofia
Pbstrast. On the base of our previous experiment in specialized laser sources and in progress of our solution for laser treatment of high dimension teeth lesions, we present an approach for development of appropriate for the purpose, high-energy two channel laser sources. The application of the developed special laser is related mainly to the problem of enamel cracking when the high dimension caries lesion is treated by needed for the purpose high energy light. In the solution, which can be also applied in different laser (and for different applications), the single active medium is divided optically in two parallel parts that each part generates in proper, independently controlled resonators. The two outputs from the two channels can be suitably superimposed on the treated object using appropriate simple optics. The essential advantages of the solution, except the use of single laser at the place of two ones and simplicity of realization, is the capability of the laser to provide the two light, each being with completely independent energetically, spectral and spatial control. The study and practical realization of the laser solution is on the example of high output energy Nd:Glass laser (output in each channel up to 5 J). In addition, the developed laser can operate at different spectral lines of Nd:Glass active medium that assures the advantage of smoothly tunable selected spectrum in each line. This is realized using structures of Interference Wedged type.
Key wards: two-channel laser, high laser power, hard dental tissue treatment, enamel protection
I. Introduction.
The lasers have useful applications in the dental medicine, and their potential for teeth problem solution is under continuous development [Beena, 1992]. The application of the high power/energy laser radiation for treatment of caries lesions or other defects in hard tooth tissues is considered as a perspective procedure [Beena, 1992; Deneva at al., 2015]. Increasing the energy parameters of the operating laser light, on one hand, facilitates and in many cases - allows effective treatment of the lesions on the hard dental tissue. However, in the same time, by general reasons, especially due to the produced strong temperature difference between the treated lesion and surroundings, the needed application of high energy for effective treatment can also lead to tooth enamel cracking. This problem becomes essential when the treated lesions are with high dimension and light with high energy density is necessary. The aim of the present work is to give some contribution to the solution of this problem, especially in instrumental (laser) direction. On the base of our experience in the field of multi-channel lasers [Deneva at al., 2015, Deneva at al., 2007; Louyer at al., 2003, Kisov at al., 2013] we have developed and described here specialized two-channel laser sources, suitable for application in solution of the problem discussed up.
II. Discussion of the principle of the solution of the problem far cracking the teeth tissue, approach for realization and development of suitable lasers.
From acceptable physical reasons, the discussed above crack of enamel is result of strong temperature difference between the treated part of the teeth and its environment (i.e. ~ 700oC
against ~ 37oC), especially when the treatment concerns the high dimension lesions and needed high energy application. This difference leads to strong mechanical tension in the treated part and cracking of the tooth enamel, respectively. In our previous work we have discussed this problem and demonstrate treatment by an original laser for eliminating or decreasing the possibility of such damage [Deneva at al., 2015].
The proposed and demonstrated solution in our work [Deneva at al., 2015] and its development, reported here, is based on the lesion treatment with combined pulse laser light, composed by the suitably low energy illumination and heating the surrounding lesion part before the illuminating with high energy density operating part. The initial illumination by suitable energy density light heats up the enamel to acceptable, non-producing crack temperature. Thus, the difference from the reached temperature of the lesion by strong heating with operating light and surrounding is strongly reduced. Respectively, the mechanical tension essentially decreases and the enamel cracking is avoided.
Fig.1. Treated by laser light lesions on the tooth surfaces: (a) small dimension lesion treated via low energy laser pulses; (b) - relatively high dimension lesions and treatment by high energy laser pulses; and (c) - treatment of the lesion with dimension as this one in Fig. (b).
The problem with the frequently appearance of the enamel crack is illustrated by the photographs in Fig.1, taken in our experimental investigations. In the left is shown treatment of low dimensions ~ 1 mm caries lesion and in the right - with high dimension of ~ 2% mm. The treatment is by focalized on the lesion pulsed laser light with dimensions, corresponding to the lesions, energy of 0.7 J and 1.5 J respectively and pulse duration of 1.5 ms. The crack trace in the case of high dimension teeth, illuminated by high energy light is clearly evident. Note that, in real situation (tooth in the mouth) the crack in the enamel with the time progressively increases and finally leads to damages of the tooth.
In the cited previous our work [Deneva at al., 2015] for this aim we have developed a special laser source where one laser, in a single active medium, generates the needed two lights in a single pulse. The proposed solution is based on our patented approach to create two-coaxial channel laser by generate the two lights in optically separated external ring part of the active medium and the second light - in the internal cylindrical part, each part generates in its own resonator with independent spatial, temporal and spectral control. As we have shown in Ref. 1, the suitable heating of the surrounding enamel by the ring external part creates necessary protection against the high temperature gradient appearance between the surrounding enamel and treated lesion with suitable high energy density light. The effect is definitively positive and the enamel crack is avoided.
The developed for the purpose coaxial two channel laser is very convenient, however with the drawback that it is with relatively complex architecture and difficult variation of the working volumes ratio (the change of the constructive element is needed). This laser has general and essential advantages to be applied for generation at different possible lines of the active medium, equalizing without losses the output energy for the strong and weak lines [Deneva at al., 2015].
In the present work, in development of the discussed principle we have created very simple two-channel laser solution that is suitable for application in the discussed dental laser treatment problem. The main its advantages are the simplicity of its realization and very easy control of the output energy in each channel.
III. Schematic of the two-channel specialized laser system - description.
The development of the two-channel laser is on the example of flash-lamp pumped Nd:Glass laser, which can easily generate high output multi-joules energy. The Nd:Glass cylindrical rod with diameter of 1 cm and length of 25 cm is pumped in cylindrical reflector by high power pump
light emission flash-lamp type IFP 5 000 (up to 5 000 J pump electric energy) in standard electric power supply. In a single channel arrangement we use optical Fabry-Perot type resonator with length of ~50 cm. In standard flat-flat mirror resonator (80% output mirror and of 99% end mirror) the laser emits 10 J energy in free lasing pulse with a time length of ~3 ms (pump electrical power of 1 000 J).
IV.Two-channel laser system arrangement and operation - experimental test.
In Fig.2 is given the schematic of the two-channel laser arrangement, realized on the base of modification of the described laser in point III. The active medium is optically separated in two parallel parts along the laser rod. One part (Channel 1) operates in the flat-flat two mirrors (Me1, Mout -part of this mirror) resonator. To form the second channel, a rectangular prism Pr is used to reflect perpendicularly a cross section part of the active medium as it is shown in Fig.2. This part is retro reflected by the mirror Me2. The mirror Me2 and the part of output mirror Mout are adjusted such way that with the prism Pr forms the second lasing channel. Thus we obtain the desired two
Electrical supply and control
rÄ'oi treated Channel 1 light spot area i
teeth
Pump flash lamp
Q-switch
Channel 2
IW
(variable ■ filter)
Fig.2. Schematic of the two channel high energy output NdGlass laser.
parallel channels laser using a single active medium. The generated light is emitted in parallel beams trough the output mirror Mout. For 1000 J pump electrical energy and ~5 ms pump pulse, the laser emits in each channel light pulse with energy of 2 J and length of ms. Output energy is measured by the home-made energy-meter using thermo-resistors in a bridge scheme as sensors suitable for the emitted long-term pulses. The energy density distribution in the illuminated by the beam areas was obtained using the developed by us method [Kazakov at al., 2016], which is based on computer treated spots, marked by the laser light on the Xerox-treated tracing paper. The typical output spots by the two emissions of the developed laser are shown in the photographs on Fig.3 (a). In Fig.3(b) and Fig.3(c) are shown 2D and 3D diagram of the energy density distribution of the spot. _
(b) y,u"
Fig.3. The typical formed spots by the two emissions (actual photograph) and the energy density distribution for the two-channel beams - 2D and 3D.
The variation of the output energy in the channel is obtained by varying the part of separation of the rod changing the position of the prism Pr. The control can be done also by introducing a Brewster glass plate in each resonator. The variation of the losses allows varying the starting moment of laser generation independently in each resonator. Other way to obtain variable losses is by inserting an Interference Wedge (IW) [Deneva at al., 2007; Stoykova and Nenchev, 2010] and varying its transmission by simple its sliding around the resonance transmission maximum. The
temporal delay between the two emissions is controlled by the length of the resonators or using noted controlling losses in each resonator.
The question is also for appropriate illumination by the two emissions of the surrounding area and the treated lesion avoiding the enamel damage. A suitable system [Deneva and, Nenchev, to be published] of two partial lenses (Li and L2, Fig.2) allows to obtain precisely controlled and focalized superposition of both beams on the treated tooth area.
In Fig.3(c) is shown the typical photograph of the region of the tooth lesion after treatment with a suitable pair of laser light pulses. The working light is with energy parameters, approximately as these ones for the Fig.2.(b) —1.7 J. The pre-heating pulse is with energy of 1 J. The delay of working pulse with respect to the pre-pulse is approximately 0.5 ms. No traces of cracking can be seen.
V. Theoretical description and differential equations system.
To investigate theoretically the emissions in both channels when vary the generating volumes, first we need to obtain this portion of the total pumping energy, which corresponds to the partial active volumes (defined by their diameters d1 and d2).
Let's the line AB (Fig. 4) divides the cross section of the crystal in two segments, defined by d1 and d2. In consideration we accept that the generated parts of the rod have circular form (Fig. 3a). The precise circular forms can be obtained by introducing a suitable diaphragm in front of the output mirror with holes having diameters of the expected diameter of each generation. The searched portion is proportional to the ratio between the areas of the corresponding segment and the containing there circle. Let's consider the case when we define the diameter d1 of the active medium, generating in first channel. The area of the segment AMSB (S) is the difference between the area of the sector AOB and the area of the triangle AOB. If a is the central angle, we use well known formulas for area of a sector (S1) and triangle
inside of circle (S2): S1 = (n-r2 - a)/360 ; S2 = (r2 • sinaj/2. Using simple trig°nometric transformations, we find expression for S as function of the parameters of the crystal cross section: S = S1 - S2 = {n-r2 -arccos [(r - d1 )/r]j/180d1 -(2- r - d1)-(r - d1). Thus defining different
values for d1, we can estimate the pumping energy for this volume. In a similar way we find the pumping energy for the second volume defined by the diameter d2.
These evaluations we use in a properly adapted system of differential equations [O. Svelto, 2008] that describes the process of laser generation in the two-channel resonator. In the considerations we use the values of laser system discussed in the previous points.
Here q1,2 are the generated photon number for the corresponding channel; Poutl2 is the corresponding output power for the channel 1 and 2, which integration in the time (from 0 to the length pulse) gives the output energy. In the system, with N is noted the population of the upper laser level per unit volume in the Nd:Glass active medium. The term^^ = (^1,2 ^ i _ c)lV^ - L )[s-1],
where cr^is the emission cross-sections for the
Fig. 4. Cross section of the laser rod separated at two generating parts.
dN
N
d = Rp (t) " % ^1,2 - N "V
^ = VP B1,2 ^1,2-N-
«1,2
1,2
with
P„
1 out1,2(t) = 0vcl2L)-hvql2(t)
wavelength in the channel; JV^2 is the working volume (varying in the calculations); c = 3x1010 cm/s is the light velocity; L = L + (n-1)-1 - the optical length of resonator, where l=25 cm is the length of the active medium, n is the refractive index and L is of order of 50 cm. The time term V of 0.23 ms is the lifetime of the upper laser level for the Nd:Glass AM. The dumping
Epump, Ei 12.0
0.5 di, cm 0.5 d2, cm
(a)
time of a photon in the resonator is TCl2=L'/(c.y1,2), where y1,2 [Svelto, 2008 ] describes the loss into the resonator for the wavelength in the channel. The pump pulse is presented as trapezium type with rise front of 0.3 ms, plato 2.5 ms and fall front 0.9 ms. The system was solved numerically by Runge-Kutta-4 method. From the solution we obtain qx2(t) and the respective output power for each channel; y1 characterises the output of the laser resonator. Combined results for two cases of pumping with obtained output energies for both channels are presented in Fig. 5.
IV. Conclusion.
In the work we have discussed the approach to eliminate the enamel cracking as result of dental treatment of the teeth lesions by high energy laser light pulses. We have presented the simple solution of appropriate, developed by us, high energy laser that is very suitable for application of this purpose. The reported investigations can be considered as contribution in the needed accumulation of knowledge and development of suitable apparatus for advancing of the laser treatment of hard tissue tooth problems as a positive potential for dental practice. Acknowledgement
The authors thank to NSF-Bulgaria for partial financial support of the work by contract DN 08/13 (2016). References
Beena V. S., Laser application in dentistry, Journal of Dentistry, Indian Dental Associaton, vol.63, Issue 3 (b) (1992) 1-7
Deneva M., St. Rabadgiiska, N. Kaimakanova, V. Kazakov, P. Uzunova, M. Nenchev, Proc. X Intern. Confer. On Communications, Electromagnetics and Medical Applications, (CEMA'15) Sofia, BG, (2015), pp. 42-46; Uzunova P., St. Rabadgiiska, Tz. Uzunov, H. Kisov, N. Kaimakanova, M. Deneva, E. Dinkov, M. Nenchev, Proc. SPIE 8770, 87701A (2013) USA; doi: 10.1117/12.2014113
Deneva M., M Nenchev, E Wintner, S Topcu, Opt.
a E Epump 20 J
^ / □ / Eout E
□ Eout1
-J _*[
0.0 1.0
0.1
0.9
0.2 0.8
0.3
0.7
0.4 0.6
0 5 d1, cm 0.5 d2, cm
Fig. 5. The output energy in each channel (Eouti and Eout2) as function of the rod cross section separation - x axis, given by di and d2 (Fig.4). In the graphs are marked the corresponding pump energies and output energy. Calculations are given for two total pumping of the system: total pump energy of 11 J (a) and 20 J (b).
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Kisov H.; M Deneva; M Nenchev, Proc. SPIE 8770, 87701Q (2013); doi: 10.1117/12.2013695 Louyer Y., J.-P. Wallerand, M. Himbert, M. Deneva, M. Nenchev, Applied Optics Vol 42, (2003) 5463-5476,
Svelto O., Principles of lasers, 5th ed. Springer Science-Business Media, 2008.
