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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.
RING LONG FIBER RESONATOR LASER SYSTEM BASED ON INTRACAVITY SPECTROSCOPY FOR ECOLOGICAL AND INDUSTRIE I, MONITORING Margarita Deneva Technical University-Sofia and Plovdiv Branch, Scientific QOE -Laboratory and Department of Optoelectronics and Laser Engineering
Abstract. On the base of our experience in specialized laser devices and as further progress of our previous work on the development of applied system for laser monitoring of ecological pollutions, we propose and develop, as theory and experimental test, a new solution of such system for laser monitoring of pollutions appearance in sequences of high number of separated indoor places -e.g. the rooms in manufactures. The principle of the proposal is to use special laser system that represents a single suitable ring laser source operating in long (20-50 m) fiber resonator. The ring fiber resonator is composed by series of short free space parts, coupled by long optical fibers. To increase many times the sensibility of the indication, we realize suitably the laser for registration of the pollutants by intra-cavity laser spectroscopy method. The solutions, based on ring waveguide fiber resonator, as the developed in the work, allow to very suitable laser spectrum for intra-cavity spectroscopy and increasing more than two-times the controlled objects (two times higher usefulness for monitored length of the resonator). The theoretical treatment of the laser operation is carried out, including comparison with standard monitoring by the common control via extraresonator monitoring and with standard standing wave laser. The laser spectral control in our proposal includes also original solution, using wedged interference structures, for producing suitable controlled multi-bands spectrum, permitting simultaneous monitoring in different spectral bands. A comparison with experimental test, on the base of detecting appearance of Na- atoms in controlled space is given. The laser system proposed is very suitable and competitive for control in multiple rooms where the appearance of specific gases impurities with possibility to blasting is possible - in laboratories, enterprises, related with military type production. The use of any electric instrumentation for monitoring represent due to a danger by possibility to provide an electric spark (including the smallest one). The proposed fully optical system has a significant advantage in order to be completely free to create such a problem.
Key words: intra-cavity spectroscopy, ring laser system, long fibers resonator, multi registration parts, theoretical treatment, experiment with Sodium atoms.
1. Introduction
Specialized laser systems are established at present as important tools for distant monitoring of air pollutions, in ecology, in industrial enterprises producing different type chemical materials, in military production, etc. [Burakov at al., 1994; Deneva at al., 2007; Deneva at al., 2010; Stoykova and Nenchev, 2001]. Here we propose high-sensitive, long-distance scanning reliable fiber optics laser system for such ecological control, especially for monitoring of close working places (multiple separated rooms) and where the formation of a smallest electric spark is absolutely non-acceptable (military production). The principle of the proposal is to use laser with long (50 m and more) ring resonator, composed by series of short free space parts in each
monitored place (room), coupled by long optical fibers. The laser has specific resonator construction with spectral characteristics of the generation (smooth tunable spectrum) that are appropriate for intra-cavity laser spectroscopy registration. The registration on the base of the intra-cavity laser spectroscopy method [Demtroder, 2003; Burakov at al., 1994) makes the laser developed extremely sensitive for atoms or molecules presence in investigated free-space resonator region, in principle down to single atoms. On the base of the theoretical description of the work of such systems, we show the optimal conditions of operation and the achieved extremely high sensitivity, compared with the standard extra-cavity laser techniques (mainly of the LIDAR type). We present also the experimental laboratory test of developed such system by registration of Sodium atoms presence in the air in different places.
2. Details of the principle of the proposed system
2.1. General description
In the proposed system, the main elements are the laser source and the fiber build special ring resonator. For the laser, essential requirement is to be suitable for intra-cavity spectroscopy -as is known, with a homogeneously broadened active medium, with controlled wideband generation, and the emitted spectrum is smooth, structure-less [Demtroder, 2003]. As additional advantage in our case is that the spectrum is tunable in the laser gain. The principle of the extremely high sensitive intra-cavity registration consists of this that the spectral absorption lines of the investigated atoms or molecules present in the laser cavity and fall in the laser generation spectrum. Thus the absorption at this line introduces the selective additional losses for the generation. The decreasing of the generation intensity at this lines is result of strong competition between the generation at different lines in the wide spectrum in the homogeneous broadened active laser medium [Demtroder, 2003; Burakov, 1994; Meyer, Nenchev, 1982]. Thus the formed holes in the generated spectrum at the absorption lines is of order of magnitude and more than in the case of standard extra-resonator spectroscopy where the laser light simply passes through the volume of the investigated atomic particles. No any competition in this case presents and the formed holes in the spectrum are resulted only by absorbed energy from passed laser light. The additional important factor to increase the competition and respectively - the sensibility is to use temporally long generation - long pump pulses (flashlamp-pumping with long exciting pulses or cw diode pumping) or cw operation [Demtroder, 2003; Burakov, 1994; Meyer, Nenchev, 1982]. The condition to avoid the typical structure in the wide-band laser spectrum is that in the resonator are eliminated any parallel reflective surfaces (including low reflection), forming the resonance structure of Fabry-Perot type. As very convenient laser for intra-cavity laser spectroscopy, which is applied here, following our earlier work [Meyer and Nenchev, 1982] is a waveguide type laser that naturally enables to exclude the parallel surfaces into the cavity and to obtain smooth spectrum. Also, in the used ring fiber resonator there is no presence of parallel reflecting surfaces. In other hands, the use of fiber resonator, build with low losses fiber, introduces extremely small losses. The use of ring resonator permits to increase two times monitored objects (rooms) in comparison with the linear resonator in the same laser operating condition. The discussed basic physical and technical reasons are motivated our choice and investigations of such laser system as very suitable for the ecological and industrial control in the indoor work places.
The schematic of the system proposed, taking into account the given general description, is presented in Fig. 1.
2.2 Details of the principle of the proposed system and apparatus for measurement.
The principle of the system is shown in Fig. 1. We will provide our development -theoretical modeling and investigations, on the example of the Rh6G dye active medium that corresponds very well to the presented above physical requirements for realization of such type of systems [Demtroder, 2003]. The results and conclusion are applicable also for other laser active mediums of this type - e.g. semiconductor lasers. In our system, the laser is composed by a small diameter (internal 1 mm; external 8 mm) thick glass capillary tube, closed by Brewster glass
windows and filed by Rh6G dye (3.10-4 mol/l) in ethanol as active medium (AM). The dye active solution flows through the cell, which is cylindrical glass capillary-tube with internal diameter of 1 mm, external diameter of 10 mm and length of 100 mm. The pumping is by xenon flash lamp in elliptical type reflector thus assuring the exciting light pump energy of 0.1 J to 1 J (electrical energy of ~ 100 J) in 13 ^s pulse with rise front 4 ^s, plato 4 ^s and fall front 14 ^s). The focal length of the lens L02 is 10 cm and for the lenses L0i and L03 it is 2.5 cm, respectively. The special ring resonator is formed by output coupling and spectral controlling system (OCSCS), optical waveguide type AM and the series (BRi; i=1 -8) representing short free space parts in between two long optical fibers (OFi), coupled by lenses (Li). The OCSCS subsystem consists of lenses (L01, L02) and low resolution (~ 5 nm) Interference Wedge (IW [Stoykova and Nenchev, 1993] working in transmission and reflecting mode (IW + RIW [Stoykova, Nenchev, 1993; Stoykova and Nenchev, 2001]) and thus serving in once as element assuring unidirectional operation, spectral selection-tuning with transmission of ~ 60% and output coupling by the reflected light. The direction of the laser operation is shown in the picture by the small arrows. The advantage of the application of IW is its tunability with simple its translation in its plane. The low selective IW permits to generate the needed wide-band spectrum, however with the tunability. Note that the important condition in functioning of the system is that there is no need of high energy output, only a few mili-joules are sufficient for reliable registration of the output spectrum. Thus, as output, can be used the natural small reflection from IW (10%). Note also that all lenses in the system are AR coated. The introduced non-selective losses in the laser resonator are sum by the losses in the coupling parts (lenses + losses by the light introduction in the fiber) and losses in the fibers, transformed in logarithmic losses [Svelto, 2008] are calculated to be y = 0.5574.
Fig.1. The schematic of the proposed laser control system - bottom and top-left the apparatus for spectrum analyses (description in the text)
The subsystem for spectral analyses, in one variant, is based on diffraction grating DG in near grazing incident angle, where the resolution is highest, and photograph or CCD matrix R with corresponding construction arrangement (left inset in Fig.1, details are not shown). In equal manner, the spectrum is registered after expanding telescope (LT1-LT2) and using convenient Interference Wedge IW [Stoykova, Nenchev, 1993; Stoykova and Nenchev, 2001] (or combination of IW' s-CIWS), right inset in Fig.1.
3. Theoretical modeling and analysis of the system action.
For generalizing of the analysis for the action of the system, find the optimal conditions for operation and the expected advantages, we have provided detailed comparative study simultaneously by comparing the obtained results with obtained ones for simple standard extraresonator registration system. Below, as an essential point of the presentation, we give general detailed description of the characteristics of the system, which lead to the reality of its expected advantageous practical functioning.
The modeling is on the base of properly adapted system of differential equations, which describes the process of laser generation in a dye active medium in the ring resonator. The adapted system is the following. In the consideration we will use the values of laser system discussed in the previous point. The differential rate equations system [Svelto, 2008], adapted for description of the case under investigation - described Rh6G dye solution, and pumping with the system parameters described above, is:
dN
dt
= Rp (t) -
Z Bi
N
Here qi (t) are the generated photon number for the
N2--2 corresponding wavelength in the considered laser
spectrum; Pout, is the corresponding output power, which
dqi qi integration in the time (from 0 to the length pulse) gives — = Va ■ Bi ■ qi ■ n2--
dt Tc, the output energy. In the systems, with N2 is noted the
with p (t) = { / 2L') h (t) population of the upper laser level per unit volume in the
out,() = Y-c/ )■ () WAM. The term Bt = {a'e-l-c)/{va-r) [s-1], where is
the emission cross-sections for the given wavelength in the laser spectrum; Va is the working volume (0.05 cm3); c = 3x1010 cm/s is the light velocity; L=L + (n-1)-1 - the optical length of
resonator, where l=10 cm is the length of the active medium, n is the refractive index and L is 25 m. The time term T of 3 ns is the lifetime of the upper laser level for the Rh6G WAM. The dumping time of a photon in the resonator is T = L' /{c- ), where y, [Svelto, 2008] describes the
sum of the losses into the resonator for the wavelength in the considered laser gain spectrum (depending on the reflectivity of the IW1), the constant losses y (described above) and the losses in the AM. The system was solved numerically by Runge-Kutta-4 method. From the solution we obtain qi(t) and the respective output power for each wavelength in the spectrum; y1 characterises the output of the laser resonator. The calculations are prepared for the spectral range (588 -591) nm, in which the Sodium D-lines are also comprised.
The main points in the numerical study are the comparison of the registration of extra cavities and internal cavities for the presence of sodium vapor outside and within the described special resonator. In parallel, the investigation includes the dependence of the Na-atoms concentration - related with the corresponding absorption, of the line position in the generated spectrum and the dependence of pump energy. We will study the behavior of the formed by the Sodium atoms absorption holes in the spectrum at the Sodium D-lines (doublet D2 =589.0 nm and D1=589.6 nm). The presented series of investigations will be used for evaluation of the sensitivity of the system and especially when the intra-cavity detection is applied. Conditions for optimization for maximum sensitivity are discussed.
As a preliminary point, we have provided study for optimization of the absorption line position in respect to the maximum of the spectrum of generation. The obtained results show that the maximum depth of the holes for the equal laser operation conditions is obtained for the holes spectrally disposed at the maximum of the generated spectrum, similarly of the obtained results for linear resonator [Deneva and Nenchev, 2017].
The suitably selected collection of results with corresponding discussion from systematic comparative study of the depth of the formed holes as a function of the main influencing factors are plotted in the series of graphs in Fig.2 and Fig. 3. Fig. 2 is for conventional extra-cavity case
and Fig.3 is for the intra-cavity case. The study is based on varying the absorption of Sodium D-lines (proportional to the sodium atoms concentration) and the generated energy, accepted to be proportional to the pump energy. In the investigations we have accepted that the same Sodium atoms concentration and length - path of the laser beam through the volumes, is placed one time outside the resonator and after this - inside the resonator. The presence of Sodium atoms and its concentration will be generalized with the absorption in percent's.
(80%-40%), Ep=0.4 J; (a),(b) (80% - 40%)
(30% -15%), Ep=0.4 J (30%-15%), Ep=0.5 J
Fig.2. Suitably selected collections of results from systematic comparative study of the depth of the formed holes for extra-cavity spectroscopy case as a function of the absorption and the pump energy Ep-(noted under the figures). In Fig.2-(b),(c),(d), is shown the laser spectrum in extended scale. In normal scale - Fig.2(a) the holes are no evident. In brackets are shown the percentage of absorption of the D-lines, first - the strong, the second - the weak one.
I 1
1
/ 1
/ 1
J.
M
F—M 1IM IH \ 1 -M zz (J)
1
J
"■i L> ' T+ "H ' u*
( 0.5%-0.25%), Ep=0.9J Ep=0.7 J, ( 0.3%-0.15%, b,c), Ep = 0.4 J (0.1%-0.05%), Ep=0.9J
Fig.3. Suitably selected collections of results from systematic comparative study of the depth of the formed holes for intra-cavity spectroscopy case as a function of the absorption and the pump energy Ep (noted under the figures). The comparison of Fig. 3(d) with Fig. 2(c) shows the increasing of two order of magnitude of the sensitivity for intra-cavity registration - reliable intra-cavity registration to (0.2- 0.3) % absorption - Fig.3(d) at the place of 30 % for extra-cavity case - Fig.2(d) .
The conclusions by the given selected graphs in Fig.2 and Fig.3, in one hand, and the experimental test, from the other hand, made with laboratory arrangement of the proposed system, are in good agreement. In Fig.4 are shown the registered spectra by the laboratory mating system with wave-guide Rh6G flash-lamp pumped laser equipped with the described fiber resonator. Firstly, such type of laser configuration permits naturally and easily to avoid the parallel surfaces that form resonant structures and to generate very suitable for the laser spectroscopy smooth (structure-less) spectrum, as can be seen in Fig.4. Secondly, as can be marked by the comparison of Fig.4 (a) and Fig.4 (b) and the corresponding their traces of the blacking, the realization, based on intra-cavity laser spectroscopy, assures extremely higher sensitivity comparing to the extra-cavity registration. For the concentration of the Sodium atoms, obtained in the flame of a match, the hole in the spectrum by working of the
Fig. 4. Registered spectra by the laboratory mated system as described system with wave-guide Rh6G flash-lamp pumped laser equipped with the described (see the text) type fiber resonator.
system using intra-cavity spectroscopy registration is very well evident and not observed extra-cavity spectroscopy as working mode.
Conclusion.
In the present work we have proposed a suitable laser-based long fiber-resonator system for monitoring of gas pollutions in the closed series of premises where the appearance of such problems is expected. From the theoretical analysis we have shown the feasibility and the advantageous for extremely high sensitivity of such system. The analysis shows of approximately two-order of magnitude low concentration registration by the proposed system in comparison with system, based on standard extra cavity spectroscopy technique. Using specially designed fiber resonator permits application for tracking of many premises in factory, especially for military production, where the use of any electric instrumentation hides danger of electric spark (including the smallest one). The proposed system, as essential advantage, is completely free of such problem. The experimental test confirms the conclusion of the theory for the useful properties and possibility of the system.
Acknowledgements
The author thanks to NSF-Bulgaria for partial financial support of the work by contract DN 08/13 (2016). References
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