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Optical resonator for high-power transverse flow CO2 lasers
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Optics & Laser Technology 35 (2003) 105 – 113 www.elsevier.com/locate/optlastec
Optical resonator for high-power transverse "ow CO2 lasers I. Gutua , C. Petrea , I. Ivanovb , I.N. Mihailescua;∗ a National
Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-54, RO, Bucharest V 76900, Magurele, Romania b Politehnica University, Bucharest, Romania Received 23 July 2002; accepted 14 October 2002
Abstract A new design of the U-type resonator is described. In this way, a laser beam with symmetrical intensity pro3le (regarding to a symmetry plane) can be extracted from an active medium that exhibits gain asymmetry along one of the transverse directions. The whole area of the active medium cross-section can be used, and consequently the laser e6ciency will be increased. This resonator structure was applied for e6ciency power extraction (as a low order TEM modes laser beam) from a DC excited transverse "ow CO2 laser with cylindrical geometry. Although the cross-section area of the discharge was entirely used (including the cathode fall region), a symmetrical intensity pro3le of the laser beam (regarding to the two orthogonal symmetry planes) was obtained in the near 3eld as well as in the far 3eld; the gain asymmetry along the "ow direction was compensated by the gas circulation "uidodynamical circuit with two counter"owing discharge channels. A double-U optical resonator was introduced in order to provide a laser beam with axial symmetry. For the practical construction of these two types of optical resonators we have developed two new types of 90◦ de"ection elements: the 3rst one, which does not reverse the image (and which has the properties of the pentaprism), and the second one, which rotates the image with 90◦ angle. Both elements exhibit good focusability if they are equipped with two concave mirrors. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Optical resonator; High e6ciency; Transverse "ow CO2 laser
1. Introduction In the high power laser range (¿ 3 kW), the fast "ow CO2 laser (in the transverse or axial "ow technique) is practically the unique laser available for industrial applications at the present time. The low dimensions and weight, a simple design, low investments and operating costs, reliability, are the basic requirements for an industrial machine tool. The fast axial "ow lasers (FAFL) provide a laser beam of a moderate diameter (∼ 25 mm) with good optical quality, but from the constructive point of view, they are not a compact device, are expensive, have large dimensions and high operating cost [1]. On the other hand, the transverse laser "ow (TFL) with "uidodynamical circuit designed in cylindrical geometry [2] is a very compact and simple device, more cheap, has low dimensions and operating cost [1]; nevertheless, when working at maximum e6ciency, the emitted laser beam is multi-mode of a large diameter (∼40 mm) with asymmetric intensity pro3le that depends upon the gas mixture excitation level [3]. Ideally, a multi-kW ∗
Corresponding author. Tel.: +40-17805385; fax: +40-14231791. E-mail address: [email protected] (I.N. Mihailescu).
CO2 industrial laser should have the valuable constructive features of TFL and the laser beam characteristics similar to those of the FAFL. The above-mentioned drawback of TFL concerning the laser beam quality, is a consequence of the active medium characteristics [3]: • The discharge cross-section has not a circular shape and has large dimensions in comparison with the laser beam diameter (as estimated for low value of the resonator Fresnel number, generally NF ¡ 3)—Fig. 1a. Moreover, because of the dissimilar shape of the electrodes or the DC excitation, active medium exhibits an asymmetry of the gain and refractive index pro3le along the anode–cathode direction. A folded resonator is suitable for laser power extraction in order to ensure a good optical quality laser beam. The asymmetry in the cathode–anode direction is practically eliminated, but the matching coe6cient m has a low value. Consequently, the laser e6ciency is poor. Also, the electrodes assembly is a voluminous unit and the optical resonator structure requires a complex design. • The gas mixture "ows perpendicularly to the optical axis. Accordingly, in the cross-section of the laser beam, the
0030-3992/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 0 - 3 9 9 2 ( 0 2 ) 0 0 1 5 4 - 8
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Fig. 1. Typical shape of the electrical discharge in a DC excited transverse "ow CO2 lasers; Da —diameter of the mode control aperture; m — matching coe6cient; V —gas "ow velocity: (a) Large area discharge suitable for folded optical resonator; m has a low value and the asymmetry of the gain and refractive index pro3le along the "ow direction is kept; (b) small area discharge suitable for single path optical resonator; m has a high value but the gain and refractive index pro3le exhibit very strong asymmetries in the laser beam cross-section.
gas mixture temperature increases along the "ow direction. Therefore gain and refractive index pro3le will exhibit an asymmetry along the "ow direction. Furthermore, it depends upon the excitation level of the active medium. To improve the laser performances (e6ciency and optical quality of the laser beam) some methods have been developed for the compensation of the active medium asymmetry along the "ow [4–7] and across the "ow direction [7,8]; if the RF excitation is used and the electrodes have the same dimensions, the active medium asymmetry across the "ow is not present [3,5,6]. On the other hand, we succeeded to introduce the simultaneous compensation of the both asymmetries types in a DC excitation transverse "ow laser with cylindrical geometry [9]. Also, for increasing the output power, two parallel discharge channels optically connected by a U-type resonator were used [10,11]; but, under these conditions, the overall symmetry characteristics of the active medium were not aJected. In our standard transverse "ow CO2 laser with cylindrical geometry we used two counter"owing discharge channels [12]; a U-type optical resonator combines the output power of the channels and provides the full asymmetry compensation along the "ow direction. Nevertheless, the active medium asymmetry across the "ow is kept. In this paper, a design procedure for the DC excited transverse "ow CO2 lasers with good optical quality laser beam and high e6ciency is presented. Essentially, it consists in a new design for: • The geometry of the discharge cross-section, by decreasing the discharge cross-section dimensions to values comparable with the diameter of the laser beam with good optical quality and by using the whole area of the dis-
charge for the active medium, Fig. 1b. This way, a very high value of the matching coe6cient could be obtained (m → 1). Nevertheless, the gain and refractive index pro3le become very asymmetric with the respect to the optical axis. • The optical resonator; we developed a single pass U-type optical resonator by which a symmetrical intensity pro3le of the laser beam is obtained using the whole area of the discharged cross-section. We tried to clarify if under above-mentioned conditions it is possible to extract the laser power from the active medium as a good quality laser beam. Also, by this design, some facilities of the practical order can be obtained: • The voluminous discharge units and complex structures of the multifolded resonator [13], speci3c to the transverse "ow lasers, are ruled out. • A reduced value of the blower speed is valid in case of using many discharge channels placed in a "uidodynamical circuit with cylindrical geometry, in comparison with the situation when the same total electrical power is delivered in one large volume discharge unit [14]. With respect to the decrease of the active medium volume, we previously emphasized in our investigation concerning the building of an anode–cathode unit (as in Ref. [12]) that the amount of the electrical power delivered to the discharge is practically unaJected by the reduction of the cross-section area of the discharge. This keeps valid for a large range of the discharge cross-section dimensions [la = (15–45) mm, ha–c = (20–40) mm] if the reduction of the anode length la is performed simultaneously with the decrease of the anode– cathode distance ha–c and the increase of the gas mixture pressure.
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2. Design procedure
3. Experimental set-up
In order to design the geometry of the discharge cross-section it is primarily necessary to establish the value of two parameters:
The experimental set-up for this investigation was based on a transverse "ow CO2 laser with cylindrical geometry (Fig. 2a) equipped with two discharge channels of 1:2 m length each one [12]. A U-type optical resonator (with optical path length Lr = 4:2 m) combines the output power of the two channels and ensures the compensation of the asymmetry along the "ow direction. Now we follow the design procedure from the previous chapter. For the Fresnel number of the optical resonator we have chosen the value of NF =2:8; this corresponds to a multimode laser beam (low-order transverse modes) suitable for welding and surface heat treatment. From the value of these parameters (Lr = 4:2 m and NF = 2:8), we determined the diameter of the mode control aperture Da = 22:5 mm. Based on this dimension we have designed the geometry of the discharge cross-section, Fig. 3. From this 3gure we can estimate: the anode length on a gas "ow direction la = 22 mm; the anode–cathode distance ha–c = 24 mm; the position of the optical axis (l0 = 14 mm; h0 = 12 mm). Therefore, a two electrode assemblies for which la = 22 mm; ha–c = 24 mm; c = 10 mm, have been built (as in Ref. [12]) for this experiment. We used an optical resonator similar to that presented in Fig. 2b. We notice that the peculiar properties of this resonator derive from the characteristics of the 90◦ deviation element composed of two "at mirrors (Mc ; Mc ). We emphasized that this element designed as in Fig. 4, has remarkable characteristics of the pentaprism:
• Lr is the optical path length inside the optical resonator; this one depends on the length of the optical discharge, which in turn is a function of the maximum laser power level. • NF is the Fresnel number of the optical resonator; this parameter is in direct connection with the transverse mode number (TEMp1 ) that can oscillate inside optical resonator. Roughly, we can select the mode TEM00 or TEM01 by choosing NF ¡ 2 or NF ¡ 3, respectively. By knowing these parameters, we can determine the diameter of the mode control aperture Da , from the de3nition relation of the Fresnel number NF = Da2 =4 Lr . From Fig. 1b, we can appreciate that a matching coe6cient of maximum value (m ∼ 0:85%), could be reached for: ha–c ∼ Da ;
la ∼ Da ;
h0 ∼ Da =2;
l0 ∼ (2=3)la :
A practical solution for compensating the both types of the active medium asymmetries is presented in Fig. 2. We used a U-type optical resonator—Fig. 2b, having one of the corner mirrors replaced by a new 90◦ de"ection element (composed of two "at mirrors-Mc , Mc ), in association with a cylindrical "uidodynamical circuit (equipped with two counter-"owing discharge channels), Fig. 2a, which ensures the total compensation of the both types of the active medium asymmetries [7,12].
• The transmitted image is not reversed; this advantage has been exploited to compensate the anode–cathode asymmetry.
Fig. 2. Practical solution for compensation of the both types active medium asymmetries: (a) Gas circulation "uidodynamical circuit with two counter"owing discharge channels, that provides conditions for compensation of the asymmetry along the "ow direction. 1—metallic cylinder; 2—heat exchanger; 3—blower; 4—electrical discharge; 5—laser beam; (b) U-type resonator with anode–cathode asymmetry compensation element (Mc ; Mc ; ), that combines the output power of the two channels and provides the compensation of the both types of active medium asymmetries.
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Fig. 3. Design of the discharge region geometry for obtaining a high value of the matching parameter m ; a U-type optical resonator with anode–cathode asymmetry compensation element was used.
Fig. 5. Beam patterns (front view) of the output beam in the near 3eld and far 3eld (in the focus of a 9:5 m concave mirror), respectively, for diJerent laser power levels PL ; burning pro3le on a plexiglas plate, 2:5 s irradiation time. Fig. 4. One piece element for 90◦ de"ection of the laser beam used in U-type optical resonator; it exhibits properties alike pentaprism.
• Any incident ray onto entrance aperture is de"ected by precisely 90◦ , irrespective of its angle to the optical axis. This last property ensures a high stability to misalignment of the optical resonator. We have measured the deviation angle of the transmitted ray versus element of misalignment angle . A linear relation connects these two angles: = 0:015, for ∈ [(−5◦ ) − (+5◦ )]. As compared to the 90◦ deviation by means of a mirror tilted at 45◦ angle (when = 2), this new 90◦ deviation element has a stability by two orders of magnitude larger. Consequently, the alignment of the resonator cannot be achieved by adjusting the entire element presented in Fig. 4, but only by means of one of the mirrors (Mc in our case). 4. Experimental results We further present experimental results concerning the laser beam quality and e6ciency. Near and far-3eld mode patterns of the laser beam (measured at 1:5 m distance from
the output mirror and in the focus of a 9:5 m concave mirror, respectively) have been analyzed for three laser power levels. According to the evidence in Fig. 3, a mode control aperture (at end mirror) of maximum diameter Da = 23 mm was used. A multimode laser beam (low-order transverse modes) was expected to be generated. From Fig. 5 we note that although in the near future the cross-section of the laser beam has a circular shape, in the far 3eld it presents an elliptical one. This feature denotes that the beam divergence has not the same values along the two orthogonal directions; we can also observe that the beam pattern is rotated under a certain degree. Figs. 6 and 7 show the near 3eld and the far-3eld intensity pro3les, respectively, along the two main directions: perpendicular and parallel to "ow. We observe that although the area of the discharge was entirely used, a symmetrical intensity distribution was achieved on these main directions. From the presented beam patterns, one evaluates some beam parameters: the beam diameter (16 mm), the divergence on "ow direction ( ∼ 2:5 mrad), the divergence on perpendicular to "ow direction (⊥ ∼ 3:5 mrad). One can also observe that the characteristics of the laser beam do not depend of the laser power level PL .
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Fig. 6. Near 3eld intensity pro3le of the output beam for 3 output power levels, along two main directions: perpendicular and parallel to the "ow direction—up and down, respectively; burning pro3le on a plexiglas plate, 2:5 s irradiation time.
Fig. 9. Laser power dependence on the discharge current for diJerent Fresnel number NF of the optical resonator; gas mixture pressure CO2 : N2 : He = 3:18:39 Torr.
Fig. 7. Far 3eld intensity pro3le (in the focus of a 9:5 m concave mirror) of the output beam for 3 output power levels, along two main directions: perpendicular and parallel to the "ow direction— up and down, respectively; burning pro3le on a plexiglas plate, 2:5 s irradiation time.
Fig. 8. Near and far 3eld (in the focus of a 9:5 m concave mirror) intensity pro3le of the output beam for small values of the Fresnel number NF of the optical resonator; along "ow direction (left), and perpendicular to "ow direction (right); 700 W laser power, 2:5 s irradiation time; ZnSe output coupler with 0.58 transmission and (−38 m) curvature radius.
It is important to emphasize that though we got a symmetrical laser beam intensity pro3le, the optical quality of the beam is not identical along the two main orthogonal directions. Indeed, the divergence on perpendicular to "ow direction is by 1.4 times higher than the divergence on parallel "ow direction ⊥ ∼ 1:40 . In order to verify if this divergence behavior is due to the use of the whole discharge area, a laser beam characterized by small Fresnel number NF = 1:57 (that corresponds to approx. TEM00 intensity pro3le) has been analyzed (Fig. 8). To this purpose, we used only the small central part of the discharge; the diameter
of the mode control aperture was of only Da = 16:7 mm. We obtained the same evolution of the divergence: ∼1:4 mrad; ⊥ ∼1:9 mrad and ⊥ ∼1:40 . The occurrence of the two peaks in the intensity pro3le perpendicular to "ow direction, for a slight increase of the Fresnel number NF = 1:68 con3rms the fact that laser beam has a larger divergence along this direction. The above-mentioned characteristics of the laser beam lead to the conclusion that two orthogonal symmetry planes ˜ and ⊥ V ˜ ) for gain g0 and refractive index n0 (their (V crossing being optical axis) are active in the whole medium; but gain g0 and refractive index n0 pro3le referring to these two symmetry planes are not identical. This means that on the laser beam cross-section, a preferential direction—gas "ow direction—can still be identi3ed. We succeeded to extract a laser power of 2:5 kW with 14% e6ciency (Fig. 9). It is noticeable result for a DC excitation transverse "ow laser in which the "owing speed was only 20 m=s; also, the transmission of the output coupler (50% in our case) was not optimized. 5. Design of a new optical resonator for transverse ow laser This section is devoted to the development of a new optical resonator for TFL, which the eJect of the transverse "ow over the laser beam quality is indistinguishable. We present in Fig. 10 a setup of four discharge channels and four gas "ow directions, which can generate a laser beam without a preferential direction in its cross-section. The particularity of this arrangement consists in the fact that the gas "ow direction of the 3rst pair of channels (D1 ; D2 ) is perpendicular to gas "ow direction of second pair of channels (D3 ; D4 ). Also, the active medium asymmetry along the "ow direction is compensated in each pair of the discharge
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Fig. 10. Transverse "ow laser with four electrical discharge channels D1; 2; 3; 4 and an adequate "ow direction arrangement V , in which there is not a preferential direction on the laser beam cross-section; Mo —output mirror, Me — end mirror.
Fig. 11. Schematic representation of a 90◦ de"ection element that rotates the wave front through a 90◦ angle (R type element); M1 ; M2 "at mirrors.
channels. Because the compensation direction in those two elementary units—(D1 ; D2 ) and (D3 ; D4 )—is at 90◦ relative angle, it is expected that one preferential direction on the laser beam cross-section be not present. Practically, the four discharge channels with an adequate gas "ow direction can be obtained as a compact device, by using a gas "owing "uidodynamical circuit in a cylindrical geometry with four discharge channels [7] (similar as in Fig. 2a). In order to bend the laser beam inside the cylindrical laser chamber, under the requirement of keeping the same orientation of the wave front, it is necessary to introduce a new 90◦ de"ection element, which rotates the image by 90◦ (Fig. 11). It consist of two 45◦ tilted "at mirrors (on the propagation direction) which have the incidence planes perpendicular to each other. We further call this 90◦ de"ection element as “R-type element”. For compensating the anode–cathode asymmetry and to improve the resonator stability, we used the optical
element showed in Fig. 4. We called this 90◦ de"ection element as a “P-type element”. The 90◦ de"ection elements used in the synthesis of this resonator are schematically represented in Fig. 12. The patterns of the laser beam after 90◦ de"ection by these elements (Fig. 13b–d) are in good accordance with the characteristics presented in Fig. 12. The pattern of the input laser beam—Fig. 13a— is rotated by 180◦ , unchanged or rotated by 90◦ in the case of the 45◦ tilted "at mirror, the P and R element, respectively. We note that by using the 90◦ deviation elements presented in Fig. 12, we can synthesize the resonator from Fig. 10. This includes four counter"owing discharge channels on the circumference of a cylinder [7] (Fig. 14). In the case of the 3rst two arms (Me − Mr and M2 − P1 ) of ˜ (that represents the wave front the resonator, the vector W ˜ , while for the orientation) is parallel to the "owing speed V ˜ is two other arms (M3 − M4 and M0 − P2 ) the vector W perpendicular to the "owing speed. This way we eliminated any preferential direction on the cross-section of the laser beam. In addition, in each pair of resonator arms, the both ˜ and ⊥ V ˜) types of the active medium asymmetries (V are compensated; if RF excitation is used, the 90◦ de"ection elements P1; 2 can be replaced by simple 45◦ tilted "at mirrors. Although the element of type P and R was developed as a component of the optical resonator, both elements can be used for the laser beam focusing if these are equipped with spherical mirrors (Fig. 15). We observe for these elements a strong astigmatism. However, the compact form of these elements (Fig. 16), the possibility of inclining the laser beam and even the astigmatism do recommend their use in the 3eld of the laser surface treatments [15]. These focusing elements can be also used for the laser beam transport at large distance (Fig. 17). In comparison with free propagation (middle of Fig. 17), the P(R) focusing element provides
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Fig. 12. Schematic representation of a 90◦ de"ection element that can be used in the resonator of the TFL with good optical quality and high e6ciency: (a) mirror at 45◦ angle where the image is rotated by 180◦ (or reversed); (b) element of type P (with properties like of a pentaprism) where the image is not reversed; (c) element of type R, where the image is rotated through a 90◦ angle.
Fig. 13. Beam pattern of the input (a) and the output laser beam after 90◦ de"ection by: 45◦ tilted "at mirror (b), element of type P (c) and element of type R (d); burning patterns on wooden plate; the asymmetric shape of the input laser beam was obtained by partial observation of a circular shape laser beam.
Fig. 14. Optical resonator in combination with four counter"owing discharge channels D1; 2; 3; 4 , in which besides the symmetrization of the gain and refractive index pro3les along and perpendicular to the "ow direction, can provide the same qualities of the laser beam along these main directions. ˜ —gas "ow direction; W ˜ —a vector attached to the wavefront. M1; 2; :::; 7 —"at mirror; P1 ; P2 —element type P; R1 —element type R; A—anode; C—cathode; V
a constant diameter for the laser beam along relatively long distance (6 m instead of 3 m). Also, the P element can be used in optical systems at 90◦ de"ection element (generally when it is used as moving device) with high stability to misalignment.
6. Conclusions and additional applications We have demonstrated that using the whole cross-section area of an active medium with asymmetric gain pro3le, a laser beam with symmetrical intensity pro3le could be
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Fig. 15. The cross-dimensions of the laser beam (burning pattern on a moving 45◦ tilted wooden plate) focused by lens, element of type P and R, on two orthogonal planes (emergency plane of the focusing element and a plane perpendicular to it, up and down, respectively); F1; F2—focus position on the emergency plane of the focusing element and on a plane perpendicular to it, respectively; element of type P or R is composed by two 600 mm curvature radius concave copper mirrors; 120 mm focal length of the lens.
Fig. 16. Photograph of the element of type P (left) and type R (right).
Fig. 17. Intensity pro3le of the free propagation laser beam on two main orthogonal directions after 90◦ de"ection by: 45◦ "at mirror and element of type P or R, each equipped with 2 × 22 m concave mirrors.
I. Gutu et al. / Optics & Laser Technology 35 (2003) 105 – 113
extracted. These features were achieved by a new design of the U-type optical resonator; one of the corner mirrors has been replaced by a new 90◦ de"ection element having the properties of the pentaprism: • The transmitted image is not reversed, this advantage has been exploited to compensate the gain asymmetry in the plane of the optical axes of the resonator arms. • Any incident ray onto the entrance aperture is de"ected by precisely 90◦ , irrespective of its angle to the optical axis; this last property provides a high stability to misalignment of the optical resonator. This new design of the optical resonator has been experimented for e6cient power extraction (as a laser beam of the low order TEM modes) from the active medium of the DC excited transverse "ow CO2 laser with cylindrical geometry. Using the whole area of the discharge cross-section and a single pass of the beam through the discharge region, we obtained: • A laser beam of 16 mm diameter and a power of 2:5 kW, at 14% electro-optical e6ciency and 2.8 Fresnel number of the optical resonator; the intensity distribution pro3le is independent of the laser power level and it is symmetrical with respect to the two orthogonal symmetry planes. • Advantages of the practical order (the small diameter of laser beam, easy to be used in applications; voluminous electrodes unit for large area discharge and the complex structure of the folded resonator, speci3c to the actual transverse "ow lasers, were ruled out). Nevertheless, the divergence along the two reference directions (along and across the "ow) have diJerent values, of 2:5 and 3:5 mrad, respectively. This behavior is valid irrespective of the value of the Fresnel number of the optical resonator. This means that the eJect of the transverse gas "owing over the optical quality of the laser beam is still present, though the both types of the active medium asymmetries are compensated. In order to provide axial symmetry of the laser beam, we have introduced a new con3guration for the double-U optical resonator. For the synthesis of this resonator, another type of 90◦ de"ection element (which rotates the image by 90◦ angle) has been developed. We consider that high power and high e6ciency CO2 lasers, with laser beam characteristics similar to those of the FAFL, can be obtained using by the design procedure presented in this paper; the transverse "ow technique being used, this CO2 laser will be a compact device of small dimensions with the investment and operating cost much lower than the FAFL with equivalent power. Also, the U-type optical resonator developed in this work can be used for e6cient laser power extraction from other laser active
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mediums that exhibits gain asymmetry along one of the transverse directions. Both 90◦ de"ection elements can be used for laser beam focusing (when a strong astigmatism is present) if they are equipped with spherical mirrors of the same curvature radius; these features recommend their use in the laser surface treatment processes. Also, the P element can be used in optical systems as 90◦ de"ection element (generally when it is used as moving device) with high stability to misalignment. References [1] ROFIN-SINAR Laser GmbH. “CO2 Lasers”, Chap. 2.2 and “Economic aspects of laser material processing”. In: Introduction to industrial laser material processing, Hamburg, 2000. p. 17–22, 161– 6 [Chapter 6]. [2] Gutu I, Comaniciu N, Draganescu V, Axinte C, Farcas I. Gas transport CO2 laser with cylindrical geometry. Rev Roum Phys 1978;23(5):447–56. [3] Sona A. RF and DC excitated high power CO2 lasers. Proc SPIE 1987;801:23–31. [4] HoJman P. The start of a new generation of CO2 lasers for industry. Proceedings of the LIM-2 Conference, Birmingham, 1985; p. 201– 6. [5] Shock W, Hall Th, Wildermuth E, Wessel K, Gehringer E, Schnee P, Hadinger T. Compact transverse "ow CO2 laser with RF excitation. Proc SPIE 1988;1031:76–81. [6] Wildermuth E, Walz B, Wessel K, Shock W. Characteristics of a compact 12 kW transverse "ow CO2 laser with rf-excitation. Proc SPIE 1990;1397:367–71. [7] Gutu I, Pascu ML. A new design for high-power transverse-"ow CO2 lasers. Laser Optoelectronik 1992;24(4):37–43. [8] Triebel W, Ose E. Comparison between electrical discharge and optical parameters of a transverse "ow cw CO2 laser. Proc SPIE 1987;801:62–7. [9] Gutu I, Medianu R, Petre C. High quality beam transversal gas transport cw CO2 laser”. Proc SPIE 1997;3405:99–105. [10] Fantini V, Incerti G, Cerri W, Donati V, Garifo L. A 5 kW cw CO2 laser for industrial applications. Fifth International Symposium on Gas Flow and Chemical Lasers 1984, Institute of Physics Conference Series No. 72. Bristol and Baston: Adam Hilger Ltd. p. 17–19. [11] Wang Run-wen. Research and development of high power CO2 laser in SIOFM. Proc SPIE 1995;2502:51–6. [12] Gutu I, Julea T, Draganescu V, Dumitras D, Mihailescu IN. Development of a high power gas transport CO2 laser with cylindrical geometry. Opt Laser Technol 1986;18(6):308–12. [13] Gukelberger A. New developments of CO2 high power lasers in the multi-kilowatt range and their use in industrial production. International Symposium on Industrial Applications of High Power Lasers—Linz 1983; Proc SPIE 1983;455:24 –8. [14] Gutu I, Pascu ML. New design of high power CO2 lasers for industrial applications. Technical Report-LOP-83-1992. Institute of Atomic Physics, Bucharest, 1992. [15] Taca M, Alexandrescu E, Fantini V, Serri L, Ivanov I, Petre C, Mihailescu IN, Gutu I. Laser surface treatment of materials by using oblique incidence of the CO2 laser beam. Presented at ROMOPTO 2000—Sixth Conference on Optics, Bucharest, 4 –7 September 2000; p. 11.O.4, Proc SPIE 2001;4430:253–60.
Optics & Laser Technology 35 (2003) 105 – 113 www.elsevier.com/locate/optlastec
Optical resonator for high-power transverse "ow CO2 lasers I. Gutua , C. Petrea , I. Ivanovb , I.N. Mihailescua;∗ a National
Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-54, RO, Bucharest V 76900, Magurele, Romania b Politehnica University, Bucharest, Romania Received 23 July 2002; accepted 14 October 2002
Abstract A new design of the U-type resonator is described. In this way, a laser beam with symmetrical intensity pro3le (regarding to a symmetry plane) can be extracted from an active medium that exhibits gain asymmetry along one of the transverse directions. The whole area of the active medium cross-section can be used, and consequently the laser e6ciency will be increased. This resonator structure was applied for e6ciency power extraction (as a low order TEM modes laser beam) from a DC excited transverse "ow CO2 laser with cylindrical geometry. Although the cross-section area of the discharge was entirely used (including the cathode fall region), a symmetrical intensity pro3le of the laser beam (regarding to the two orthogonal symmetry planes) was obtained in the near 3eld as well as in the far 3eld; the gain asymmetry along the "ow direction was compensated by the gas circulation "uidodynamical circuit with two counter"owing discharge channels. A double-U optical resonator was introduced in order to provide a laser beam with axial symmetry. For the practical construction of these two types of optical resonators we have developed two new types of 90◦ de"ection elements: the 3rst one, which does not reverse the image (and which has the properties of the pentaprism), and the second one, which rotates the image with 90◦ angle. Both elements exhibit good focusability if they are equipped with two concave mirrors. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Optical resonator; High e6ciency; Transverse "ow CO2 laser
1. Introduction In the high power laser range (¿ 3 kW), the fast "ow CO2 laser (in the transverse or axial "ow technique) is practically the unique laser available for industrial applications at the present time. The low dimensions and weight, a simple design, low investments and operating costs, reliability, are the basic requirements for an industrial machine tool. The fast axial "ow lasers (FAFL) provide a laser beam of a moderate diameter (∼ 25 mm) with good optical quality, but from the constructive point of view, they are not a compact device, are expensive, have large dimensions and high operating cost [1]. On the other hand, the transverse laser "ow (TFL) with "uidodynamical circuit designed in cylindrical geometry [2] is a very compact and simple device, more cheap, has low dimensions and operating cost [1]; nevertheless, when working at maximum e6ciency, the emitted laser beam is multi-mode of a large diameter (∼40 mm) with asymmetric intensity pro3le that depends upon the gas mixture excitation level [3]. Ideally, a multi-kW ∗
Corresponding author. Tel.: +40-17805385; fax: +40-14231791. E-mail address: [email protected] (I.N. Mihailescu).
CO2 industrial laser should have the valuable constructive features of TFL and the laser beam characteristics similar to those of the FAFL. The above-mentioned drawback of TFL concerning the laser beam quality, is a consequence of the active medium characteristics [3]: • The discharge cross-section has not a circular shape and has large dimensions in comparison with the laser beam diameter (as estimated for low value of the resonator Fresnel number, generally NF ¡ 3)—Fig. 1a. Moreover, because of the dissimilar shape of the electrodes or the DC excitation, active medium exhibits an asymmetry of the gain and refractive index pro3le along the anode–cathode direction. A folded resonator is suitable for laser power extraction in order to ensure a good optical quality laser beam. The asymmetry in the cathode–anode direction is practically eliminated, but the matching coe6cient m has a low value. Consequently, the laser e6ciency is poor. Also, the electrodes assembly is a voluminous unit and the optical resonator structure requires a complex design. • The gas mixture "ows perpendicularly to the optical axis. Accordingly, in the cross-section of the laser beam, the
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Fig. 1. Typical shape of the electrical discharge in a DC excited transverse "ow CO2 lasers; Da —diameter of the mode control aperture; m — matching coe6cient; V —gas "ow velocity: (a) Large area discharge suitable for folded optical resonator; m has a low value and the asymmetry of the gain and refractive index pro3le along the "ow direction is kept; (b) small area discharge suitable for single path optical resonator; m has a high value but the gain and refractive index pro3le exhibit very strong asymmetries in the laser beam cross-section.
gas mixture temperature increases along the "ow direction. Therefore gain and refractive index pro3le will exhibit an asymmetry along the "ow direction. Furthermore, it depends upon the excitation level of the active medium. To improve the laser performances (e6ciency and optical quality of the laser beam) some methods have been developed for the compensation of the active medium asymmetry along the "ow [4–7] and across the "ow direction [7,8]; if the RF excitation is used and the electrodes have the same dimensions, the active medium asymmetry across the "ow is not present [3,5,6]. On the other hand, we succeeded to introduce the simultaneous compensation of the both asymmetries types in a DC excitation transverse "ow laser with cylindrical geometry [9]. Also, for increasing the output power, two parallel discharge channels optically connected by a U-type resonator were used [10,11]; but, under these conditions, the overall symmetry characteristics of the active medium were not aJected. In our standard transverse "ow CO2 laser with cylindrical geometry we used two counter"owing discharge channels [12]; a U-type optical resonator combines the output power of the channels and provides the full asymmetry compensation along the "ow direction. Nevertheless, the active medium asymmetry across the "ow is kept. In this paper, a design procedure for the DC excited transverse "ow CO2 lasers with good optical quality laser beam and high e6ciency is presented. Essentially, it consists in a new design for: • The geometry of the discharge cross-section, by decreasing the discharge cross-section dimensions to values comparable with the diameter of the laser beam with good optical quality and by using the whole area of the dis-
charge for the active medium, Fig. 1b. This way, a very high value of the matching coe6cient could be obtained (m → 1). Nevertheless, the gain and refractive index pro3le become very asymmetric with the respect to the optical axis. • The optical resonator; we developed a single pass U-type optical resonator by which a symmetrical intensity pro3le of the laser beam is obtained using the whole area of the discharged cross-section. We tried to clarify if under above-mentioned conditions it is possible to extract the laser power from the active medium as a good quality laser beam. Also, by this design, some facilities of the practical order can be obtained: • The voluminous discharge units and complex structures of the multifolded resonator [13], speci3c to the transverse "ow lasers, are ruled out. • A reduced value of the blower speed is valid in case of using many discharge channels placed in a "uidodynamical circuit with cylindrical geometry, in comparison with the situation when the same total electrical power is delivered in one large volume discharge unit [14]. With respect to the decrease of the active medium volume, we previously emphasized in our investigation concerning the building of an anode–cathode unit (as in Ref. [12]) that the amount of the electrical power delivered to the discharge is practically unaJected by the reduction of the cross-section area of the discharge. This keeps valid for a large range of the discharge cross-section dimensions [la = (15–45) mm, ha–c = (20–40) mm] if the reduction of the anode length la is performed simultaneously with the decrease of the anode– cathode distance ha–c and the increase of the gas mixture pressure.
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2. Design procedure
3. Experimental set-up
In order to design the geometry of the discharge cross-section it is primarily necessary to establish the value of two parameters:
The experimental set-up for this investigation was based on a transverse "ow CO2 laser with cylindrical geometry (Fig. 2a) equipped with two discharge channels of 1:2 m length each one [12]. A U-type optical resonator (with optical path length Lr = 4:2 m) combines the output power of the two channels and ensures the compensation of the asymmetry along the "ow direction. Now we follow the design procedure from the previous chapter. For the Fresnel number of the optical resonator we have chosen the value of NF =2:8; this corresponds to a multimode laser beam (low-order transverse modes) suitable for welding and surface heat treatment. From the value of these parameters (Lr = 4:2 m and NF = 2:8), we determined the diameter of the mode control aperture Da = 22:5 mm. Based on this dimension we have designed the geometry of the discharge cross-section, Fig. 3. From this 3gure we can estimate: the anode length on a gas "ow direction la = 22 mm; the anode–cathode distance ha–c = 24 mm; the position of the optical axis (l0 = 14 mm; h0 = 12 mm). Therefore, a two electrode assemblies for which la = 22 mm; ha–c = 24 mm; c = 10 mm, have been built (as in Ref. [12]) for this experiment. We used an optical resonator similar to that presented in Fig. 2b. We notice that the peculiar properties of this resonator derive from the characteristics of the 90◦ deviation element composed of two "at mirrors (Mc ; Mc ). We emphasized that this element designed as in Fig. 4, has remarkable characteristics of the pentaprism:
• Lr is the optical path length inside the optical resonator; this one depends on the length of the optical discharge, which in turn is a function of the maximum laser power level. • NF is the Fresnel number of the optical resonator; this parameter is in direct connection with the transverse mode number (TEMp1 ) that can oscillate inside optical resonator. Roughly, we can select the mode TEM00 or TEM01 by choosing NF ¡ 2 or NF ¡ 3, respectively. By knowing these parameters, we can determine the diameter of the mode control aperture Da , from the de3nition relation of the Fresnel number NF = Da2 =4 Lr . From Fig. 1b, we can appreciate that a matching coe6cient of maximum value (m ∼ 0:85%), could be reached for: ha–c ∼ Da ;
la ∼ Da ;
h0 ∼ Da =2;
l0 ∼ (2=3)la :
A practical solution for compensating the both types of the active medium asymmetries is presented in Fig. 2. We used a U-type optical resonator—Fig. 2b, having one of the corner mirrors replaced by a new 90◦ de"ection element (composed of two "at mirrors-Mc , Mc ), in association with a cylindrical "uidodynamical circuit (equipped with two counter-"owing discharge channels), Fig. 2a, which ensures the total compensation of the both types of the active medium asymmetries [7,12].
• The transmitted image is not reversed; this advantage has been exploited to compensate the anode–cathode asymmetry.
Fig. 2. Practical solution for compensation of the both types active medium asymmetries: (a) Gas circulation "uidodynamical circuit with two counter"owing discharge channels, that provides conditions for compensation of the asymmetry along the "ow direction. 1—metallic cylinder; 2—heat exchanger; 3—blower; 4—electrical discharge; 5—laser beam; (b) U-type resonator with anode–cathode asymmetry compensation element (Mc ; Mc ; ), that combines the output power of the two channels and provides the compensation of the both types of active medium asymmetries.
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Fig. 3. Design of the discharge region geometry for obtaining a high value of the matching parameter m ; a U-type optical resonator with anode–cathode asymmetry compensation element was used.
Fig. 5. Beam patterns (front view) of the output beam in the near 3eld and far 3eld (in the focus of a 9:5 m concave mirror), respectively, for diJerent laser power levels PL ; burning pro3le on a plexiglas plate, 2:5 s irradiation time. Fig. 4. One piece element for 90◦ de"ection of the laser beam used in U-type optical resonator; it exhibits properties alike pentaprism.
• Any incident ray onto entrance aperture is de"ected by precisely 90◦ , irrespective of its angle to the optical axis. This last property ensures a high stability to misalignment of the optical resonator. We have measured the deviation angle of the transmitted ray versus element of misalignment angle . A linear relation connects these two angles: = 0:015, for ∈ [(−5◦ ) − (+5◦ )]. As compared to the 90◦ deviation by means of a mirror tilted at 45◦ angle (when = 2), this new 90◦ deviation element has a stability by two orders of magnitude larger. Consequently, the alignment of the resonator cannot be achieved by adjusting the entire element presented in Fig. 4, but only by means of one of the mirrors (Mc in our case). 4. Experimental results We further present experimental results concerning the laser beam quality and e6ciency. Near and far-3eld mode patterns of the laser beam (measured at 1:5 m distance from
the output mirror and in the focus of a 9:5 m concave mirror, respectively) have been analyzed for three laser power levels. According to the evidence in Fig. 3, a mode control aperture (at end mirror) of maximum diameter Da = 23 mm was used. A multimode laser beam (low-order transverse modes) was expected to be generated. From Fig. 5 we note that although in the near future the cross-section of the laser beam has a circular shape, in the far 3eld it presents an elliptical one. This feature denotes that the beam divergence has not the same values along the two orthogonal directions; we can also observe that the beam pattern is rotated under a certain degree. Figs. 6 and 7 show the near 3eld and the far-3eld intensity pro3les, respectively, along the two main directions: perpendicular and parallel to "ow. We observe that although the area of the discharge was entirely used, a symmetrical intensity distribution was achieved on these main directions. From the presented beam patterns, one evaluates some beam parameters: the beam diameter (16 mm), the divergence on "ow direction ( ∼ 2:5 mrad), the divergence on perpendicular to "ow direction (⊥ ∼ 3:5 mrad). One can also observe that the characteristics of the laser beam do not depend of the laser power level PL .
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Fig. 6. Near 3eld intensity pro3le of the output beam for 3 output power levels, along two main directions: perpendicular and parallel to the "ow direction—up and down, respectively; burning pro3le on a plexiglas plate, 2:5 s irradiation time.
Fig. 9. Laser power dependence on the discharge current for diJerent Fresnel number NF of the optical resonator; gas mixture pressure CO2 : N2 : He = 3:18:39 Torr.
Fig. 7. Far 3eld intensity pro3le (in the focus of a 9:5 m concave mirror) of the output beam for 3 output power levels, along two main directions: perpendicular and parallel to the "ow direction— up and down, respectively; burning pro3le on a plexiglas plate, 2:5 s irradiation time.
Fig. 8. Near and far 3eld (in the focus of a 9:5 m concave mirror) intensity pro3le of the output beam for small values of the Fresnel number NF of the optical resonator; along "ow direction (left), and perpendicular to "ow direction (right); 700 W laser power, 2:5 s irradiation time; ZnSe output coupler with 0.58 transmission and (−38 m) curvature radius.
It is important to emphasize that though we got a symmetrical laser beam intensity pro3le, the optical quality of the beam is not identical along the two main orthogonal directions. Indeed, the divergence on perpendicular to "ow direction is by 1.4 times higher than the divergence on parallel "ow direction ⊥ ∼ 1:40 . In order to verify if this divergence behavior is due to the use of the whole discharge area, a laser beam characterized by small Fresnel number NF = 1:57 (that corresponds to approx. TEM00 intensity pro3le) has been analyzed (Fig. 8). To this purpose, we used only the small central part of the discharge; the diameter
of the mode control aperture was of only Da = 16:7 mm. We obtained the same evolution of the divergence: ∼1:4 mrad; ⊥ ∼1:9 mrad and ⊥ ∼1:40 . The occurrence of the two peaks in the intensity pro3le perpendicular to "ow direction, for a slight increase of the Fresnel number NF = 1:68 con3rms the fact that laser beam has a larger divergence along this direction. The above-mentioned characteristics of the laser beam lead to the conclusion that two orthogonal symmetry planes ˜ and ⊥ V ˜ ) for gain g0 and refractive index n0 (their (V crossing being optical axis) are active in the whole medium; but gain g0 and refractive index n0 pro3le referring to these two symmetry planes are not identical. This means that on the laser beam cross-section, a preferential direction—gas "ow direction—can still be identi3ed. We succeeded to extract a laser power of 2:5 kW with 14% e6ciency (Fig. 9). It is noticeable result for a DC excitation transverse "ow laser in which the "owing speed was only 20 m=s; also, the transmission of the output coupler (50% in our case) was not optimized. 5. Design of a new optical resonator for transverse ow laser This section is devoted to the development of a new optical resonator for TFL, which the eJect of the transverse "ow over the laser beam quality is indistinguishable. We present in Fig. 10 a setup of four discharge channels and four gas "ow directions, which can generate a laser beam without a preferential direction in its cross-section. The particularity of this arrangement consists in the fact that the gas "ow direction of the 3rst pair of channels (D1 ; D2 ) is perpendicular to gas "ow direction of second pair of channels (D3 ; D4 ). Also, the active medium asymmetry along the "ow direction is compensated in each pair of the discharge
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Fig. 10. Transverse "ow laser with four electrical discharge channels D1; 2; 3; 4 and an adequate "ow direction arrangement V , in which there is not a preferential direction on the laser beam cross-section; Mo —output mirror, Me — end mirror.
Fig. 11. Schematic representation of a 90◦ de"ection element that rotates the wave front through a 90◦ angle (R type element); M1 ; M2 "at mirrors.
channels. Because the compensation direction in those two elementary units—(D1 ; D2 ) and (D3 ; D4 )—is at 90◦ relative angle, it is expected that one preferential direction on the laser beam cross-section be not present. Practically, the four discharge channels with an adequate gas "ow direction can be obtained as a compact device, by using a gas "owing "uidodynamical circuit in a cylindrical geometry with four discharge channels [7] (similar as in Fig. 2a). In order to bend the laser beam inside the cylindrical laser chamber, under the requirement of keeping the same orientation of the wave front, it is necessary to introduce a new 90◦ de"ection element, which rotates the image by 90◦ (Fig. 11). It consist of two 45◦ tilted "at mirrors (on the propagation direction) which have the incidence planes perpendicular to each other. We further call this 90◦ de"ection element as “R-type element”. For compensating the anode–cathode asymmetry and to improve the resonator stability, we used the optical
element showed in Fig. 4. We called this 90◦ de"ection element as a “P-type element”. The 90◦ de"ection elements used in the synthesis of this resonator are schematically represented in Fig. 12. The patterns of the laser beam after 90◦ de"ection by these elements (Fig. 13b–d) are in good accordance with the characteristics presented in Fig. 12. The pattern of the input laser beam—Fig. 13a— is rotated by 180◦ , unchanged or rotated by 90◦ in the case of the 45◦ tilted "at mirror, the P and R element, respectively. We note that by using the 90◦ deviation elements presented in Fig. 12, we can synthesize the resonator from Fig. 10. This includes four counter"owing discharge channels on the circumference of a cylinder [7] (Fig. 14). In the case of the 3rst two arms (Me − Mr and M2 − P1 ) of ˜ (that represents the wave front the resonator, the vector W ˜ , while for the orientation) is parallel to the "owing speed V ˜ is two other arms (M3 − M4 and M0 − P2 ) the vector W perpendicular to the "owing speed. This way we eliminated any preferential direction on the cross-section of the laser beam. In addition, in each pair of resonator arms, the both ˜ and ⊥ V ˜) types of the active medium asymmetries (V are compensated; if RF excitation is used, the 90◦ de"ection elements P1; 2 can be replaced by simple 45◦ tilted "at mirrors. Although the element of type P and R was developed as a component of the optical resonator, both elements can be used for the laser beam focusing if these are equipped with spherical mirrors (Fig. 15). We observe for these elements a strong astigmatism. However, the compact form of these elements (Fig. 16), the possibility of inclining the laser beam and even the astigmatism do recommend their use in the 3eld of the laser surface treatments [15]. These focusing elements can be also used for the laser beam transport at large distance (Fig. 17). In comparison with free propagation (middle of Fig. 17), the P(R) focusing element provides
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Fig. 12. Schematic representation of a 90◦ de"ection element that can be used in the resonator of the TFL with good optical quality and high e6ciency: (a) mirror at 45◦ angle where the image is rotated by 180◦ (or reversed); (b) element of type P (with properties like of a pentaprism) where the image is not reversed; (c) element of type R, where the image is rotated through a 90◦ angle.
Fig. 13. Beam pattern of the input (a) and the output laser beam after 90◦ de"ection by: 45◦ tilted "at mirror (b), element of type P (c) and element of type R (d); burning patterns on wooden plate; the asymmetric shape of the input laser beam was obtained by partial observation of a circular shape laser beam.
Fig. 14. Optical resonator in combination with four counter"owing discharge channels D1; 2; 3; 4 , in which besides the symmetrization of the gain and refractive index pro3les along and perpendicular to the "ow direction, can provide the same qualities of the laser beam along these main directions. ˜ —gas "ow direction; W ˜ —a vector attached to the wavefront. M1; 2; :::; 7 —"at mirror; P1 ; P2 —element type P; R1 —element type R; A—anode; C—cathode; V
a constant diameter for the laser beam along relatively long distance (6 m instead of 3 m). Also, the P element can be used in optical systems at 90◦ de"ection element (generally when it is used as moving device) with high stability to misalignment.
6. Conclusions and additional applications We have demonstrated that using the whole cross-section area of an active medium with asymmetric gain pro3le, a laser beam with symmetrical intensity pro3le could be
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Fig. 15. The cross-dimensions of the laser beam (burning pattern on a moving 45◦ tilted wooden plate) focused by lens, element of type P and R, on two orthogonal planes (emergency plane of the focusing element and a plane perpendicular to it, up and down, respectively); F1; F2—focus position on the emergency plane of the focusing element and on a plane perpendicular to it, respectively; element of type P or R is composed by two 600 mm curvature radius concave copper mirrors; 120 mm focal length of the lens.
Fig. 16. Photograph of the element of type P (left) and type R (right).
Fig. 17. Intensity pro3le of the free propagation laser beam on two main orthogonal directions after 90◦ de"ection by: 45◦ "at mirror and element of type P or R, each equipped with 2 × 22 m concave mirrors.
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extracted. These features were achieved by a new design of the U-type optical resonator; one of the corner mirrors has been replaced by a new 90◦ de"ection element having the properties of the pentaprism: • The transmitted image is not reversed, this advantage has been exploited to compensate the gain asymmetry in the plane of the optical axes of the resonator arms. • Any incident ray onto the entrance aperture is de"ected by precisely 90◦ , irrespective of its angle to the optical axis; this last property provides a high stability to misalignment of the optical resonator. This new design of the optical resonator has been experimented for e6cient power extraction (as a laser beam of the low order TEM modes) from the active medium of the DC excited transverse "ow CO2 laser with cylindrical geometry. Using the whole area of the discharge cross-section and a single pass of the beam through the discharge region, we obtained: • A laser beam of 16 mm diameter and a power of 2:5 kW, at 14% electro-optical e6ciency and 2.8 Fresnel number of the optical resonator; the intensity distribution pro3le is independent of the laser power level and it is symmetrical with respect to the two orthogonal symmetry planes. • Advantages of the practical order (the small diameter of laser beam, easy to be used in applications; voluminous electrodes unit for large area discharge and the complex structure of the folded resonator, speci3c to the actual transverse "ow lasers, were ruled out). Nevertheless, the divergence along the two reference directions (along and across the "ow) have diJerent values, of 2:5 and 3:5 mrad, respectively. This behavior is valid irrespective of the value of the Fresnel number of the optical resonator. This means that the eJect of the transverse gas "owing over the optical quality of the laser beam is still present, though the both types of the active medium asymmetries are compensated. In order to provide axial symmetry of the laser beam, we have introduced a new con3guration for the double-U optical resonator. For the synthesis of this resonator, another type of 90◦ de"ection element (which rotates the image by 90◦ angle) has been developed. We consider that high power and high e6ciency CO2 lasers, with laser beam characteristics similar to those of the FAFL, can be obtained using by the design procedure presented in this paper; the transverse "ow technique being used, this CO2 laser will be a compact device of small dimensions with the investment and operating cost much lower than the FAFL with equivalent power. Also, the U-type optical resonator developed in this work can be used for e6cient laser power extraction from other laser active
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mediums that exhibits gain asymmetry along one of the transverse directions. Both 90◦ de"ection elements can be used for laser beam focusing (when a strong astigmatism is present) if they are equipped with spherical mirrors of the same curvature radius; these features recommend their use in the laser surface treatment processes. Also, the P element can be used in optical systems as 90◦ de"ection element (generally when it is used as moving device) with high stability to misalignment. References [1] ROFIN-SINAR Laser GmbH. “CO2 Lasers”, Chap. 2.2 and “Economic aspects of laser material processing”. In: Introduction to industrial laser material processing, Hamburg, 2000. p. 17–22, 161– 6 [Chapter 6]. [2] Gutu I, Comaniciu N, Draganescu V, Axinte C, Farcas I. Gas transport CO2 laser with cylindrical geometry. Rev Roum Phys 1978;23(5):447–56. [3] Sona A. RF and DC excitated high power CO2 lasers. Proc SPIE 1987;801:23–31. [4] HoJman P. The start of a new generation of CO2 lasers for industry. Proceedings of the LIM-2 Conference, Birmingham, 1985; p. 201– 6. [5] Shock W, Hall Th, Wildermuth E, Wessel K, Gehringer E, Schnee P, Hadinger T. Compact transverse "ow CO2 laser with RF excitation. Proc SPIE 1988;1031:76–81. [6] Wildermuth E, Walz B, Wessel K, Shock W. Characteristics of a compact 12 kW transverse "ow CO2 laser with rf-excitation. Proc SPIE 1990;1397:367–71. [7] Gutu I, Pascu ML. A new design for high-power transverse-"ow CO2 lasers. Laser Optoelectronik 1992;24(4):37–43. [8] Triebel W, Ose E. Comparison between electrical discharge and optical parameters of a transverse "ow cw CO2 laser. Proc SPIE 1987;801:62–7. [9] Gutu I, Medianu R, Petre C. High quality beam transversal gas transport cw CO2 laser”. Proc SPIE 1997;3405:99–105. [10] Fantini V, Incerti G, Cerri W, Donati V, Garifo L. A 5 kW cw CO2 laser for industrial applications. Fifth International Symposium on Gas Flow and Chemical Lasers 1984, Institute of Physics Conference Series No. 72. Bristol and Baston: Adam Hilger Ltd. p. 17–19. [11] Wang Run-wen. Research and development of high power CO2 laser in SIOFM. Proc SPIE 1995;2502:51–6. [12] Gutu I, Julea T, Draganescu V, Dumitras D, Mihailescu IN. Development of a high power gas transport CO2 laser with cylindrical geometry. Opt Laser Technol 1986;18(6):308–12. [13] Gukelberger A. New developments of CO2 high power lasers in the multi-kilowatt range and their use in industrial production. International Symposium on Industrial Applications of High Power Lasers—Linz 1983; Proc SPIE 1983;455:24 –8. [14] Gutu I, Pascu ML. New design of high power CO2 lasers for industrial applications. Technical Report-LOP-83-1992. Institute of Atomic Physics, Bucharest, 1992. [15] Taca M, Alexandrescu E, Fantini V, Serri L, Ivanov I, Petre C, Mihailescu IN, Gutu I. Laser surface treatment of materials by using oblique incidence of the CO2 laser beam. Presented at ROMOPTO 2000—Sixth Conference on Optics, Bucharest, 4 –7 September 2000; p. 11.O.4, Proc SPIE 2001;4430:253–60.