LASER DEVICE

Abstract: Laser device having a monolithic structure to be employed in telecommunications, including a first laser (3) capable to emit radiations at a first wavelength lambda 1 pumped by a second laser (1) capable to emit radiations at a second wavelength lambda 2 through a first portion of focusing optical guide (2) and a first surface (4) essentially transparent to radiation lambda 2 and reflecting the radiation lambda 1. The first laser (3) is coupled, through a second surface (5) essentially transparent to said radiation lambda 1 and essentially reflecting said radiation lambda 2, to the first end of a second portion of focusing optical guide, whose second end is coated by a mirror (11). The presence of an air gap (8) having adjustable width is foreseen between said first surface (4) and the mirror (11).

Description: DESCRIPTION "LASER DEVICE" Technical Field The present invention refers to a laser device to be employed as continuous source of a single mode, monochromatic radiation in telecommunications, of the type including an active medium capable to emit a radiation at a first wavelength λl, pumped by a laser capable to emit a radiation at a second wavelength λ2 < λl. Industrial Applicability The device according to the invention has application in telecommunication systems and particularly it can be profitably employed, but not only, as continuous source at the frequency of 1.54 μm (the so-called third window) for coherent transmission in optical fiber. For optical fiber telecommunications future developments it is foreseen the use of a radiation having 1,54 μ (1540 nm) wavelength corresponding to the so-called third window to which optical fibers for telecommunications present a reduced attenuation. A coherent light source for this use has to supply a sufficiently high output power, in the range of some mW to have practical . applications, with single mode type emissions and with a width of the emitted spectrum line to be as reduced as possible. As an example, current sources of this kind are realized with single mode, DBF semiconductor laser, having typical optical powers of 1-4 mW in single mode fiber and 1-30 HZ linewidth. Background Art To obtain sources capable to produce the required wavelength, it was proposed as active medium the so-called glass lasers, that is including an Erbium (Er) doped glass. Two additional laser devices are employed as pump source, and particularly a semiconductor laser having 808 nm wavelength pumping ' a neodymium laser, which in its turn emits a 1060 nm radiation pumping the glass laser capable to emit the laser radiation at the desired wavelength of 1.54 μ . Said glass lasers contain also a given ytterbium (Yb) percentage having the scope to increase the pumping yield since Ytterbium has an absorption band wider than the Erbium one, then transferring to this last the power absorbed. This device requires a double conversion and also the neodymium emission yield is very low (in the range- of 1- 2%), so as to render critical the operation of the device, therefore even minimum losses of the laser cavity can cause a poor output power, so that use has to be made of very accurate and expensive processing of the most critical components. In the paper "Diode-pumped CW Bulk Er:Yb: glass laser" by P. aporta et. al., OPTICS LETTER, Vol. 16, no. 24, December 15, 1991, it was proposed to pump directly the glass laser through a semiconductor laser emitting at 980 nm, focusing the pumping radiation on the glass laser with a traditional optical system. The optical resonator cavity is defined between a surface coating of the glass element, which is essentially .transparent to the 980 nm radiation and highly reflecting to the 1.54 μm radiation and a 1.54 μ reflecting curve mirror. Since the light emitted by the glass laser is a multimodal one, a so-called etalon, that is a glass sheet having adequate thickness determining the elimination of undesired propagation modes, as to obtain a single mode laser having narrow emission line, is placed between the glass laser and the mirror. This device, even if effective from the conceptual point of view, ' has no practical application in telecommunication systems due to its large dimensions, associated costs, difficult setting for the etalon calibration, so there is a need to realize a laser source having reduced dimensions, limited cost, high output power and very narrow spectrum line. The scope of the present invention is to overcome the above mentioned drawbacks and limits, and particularly to realize a device having an output linewidth lower than one hundred kHz (compared to the values of 1-30 MHz of known DBF single mode lasers at this wavelength). Disclosure of Invention According to the invention, theεe scopes are attained with a laser device to be employed as continuous source of a single mode, monochromatic radiation in telecommunications, of the type including an active medium capable of emitting a radiation at a first wavelength λl, pumped by a laser able to emit a radiation at a second wavelength λ2 < λl, characterized by the fact that it consists of a continued structure made up of the following parts: - a first portion of focusing optical guide whose first face is optically coupled to said pumping laser; - an active portion (laser) capable of emitting a laser radiation at the desired wavelength λl, between the second face of said first portion of focusing optical guide and the first face of said active portion being placed a first filter essentially transparent to said radiation λ2 and essentially reflecting said radiation λl, to the second face of said active ' portion being applied a second filter essentially transparent to said radiation λl and essentially reflecting said radiation λ2; - a second portion of focusing optical guide, whose first end is optically coupled to said second filter and whose second end is coated with a mirror partially reflecting said radiation λl; - an air gap of adjustable width placed between said first filter and said partially reflecting mirror; - a first portion of optical fiber optically coupled to said partially reflecting mirror; - support means of said parts including a member to vary the width of said air gap. Additional favourable characteristics are object of the depending claims. The invention shall now be illustrated making reference to the enclosed drawings, showing a preferred, but not limiting, realization form of the invention, where: figures 1A and IB schematically show a first and a second realization form of the device realized according to the present invention; figure 2 shows a construction drawing of the device according to this invention, carried out according to said second realization form (figure IB); figures 3A and 3B schematically show a third and a fourth realization of the device according to this invention; figure 4 shows a construction drawing of the device according to the present invention,__realized according to said fourth realization form (figure 3B); figures 5 to 8 schematically show four realization forms of the device according to the present invention. With reference to figure 1A, the device according to the invention includes a first semiconductor laser 1 which, according to a preferred realization form, emits radiations at a wavelength between 960 and 980 nm and is optically coupled to a monolithic structure having continued form and preferably with circular cross section. The structure includes a focusing guide 2, made according to a preferred realization form of a so-called SELFOC lens that is essentially a segment of optical guide of the graded index type, having length equal to P/2, such to focus a radiation beam incoming from the front face on a spot of the opposite end surface. P is the so-called focusing pitch, that is the distance between the two contiguous plans or the formation of identical images. An active portion 3 which according to the realization form here described is preferably, but not limitedly, made of Ytterbium and Erbium doped glass, butt coupled to this focusing surface. The said surface of this active portion 3 is prefereably rounded with a concave side towards the active material (as shown in figures 1, 2 and 3A) , to focus radiations emitted by the pumping laser 1 and to facilitate the excitation of the fundamental mode. However, according to the invention, this surface can have a flat configuration as shown in the remaining figures. This surface is coated with a layer 4 which is highly transparent to the pumping radiation λ2 = 960 - 980 nm and highly reflecting to laser radiation λl = 1.54 μm emitted by the active material 3. The opposite surface of the active material 3 is also coated by a layer 5 highly reflecting the pumping radiation λ2 and transparent for the λl radiation emitted by the same active meadium 3. Layers with these characteristics are obtained, in the known manner, with the depositing of layers of appropriate oxides or metals. The laser radiation λl = 1.54 μm emitted by the active portion (laser) 3 passes through the first part or segment of a pair of additional SELFOC lenses 6 and 7a, axially aligned and separated by an air gap 8 having an adjustable axial width d. The opposing faces of said pair of lenses 6 and 7a show, according to a preferred realization form, the surfaces 9 and 10 coated by layers reflecting the radiation λl emitted by the active medium. The second part or segment 7a, in the realization form shown in Fig. 1, has a curved end surface, in particular that of a half-sphere and this surface has a coating or mirror 11a highly reflecting to laser radiation λl = 1.54 μm emitted by the active medium 3. The total length of the pair of lenses 6, 7a plus the air gap d is approximately equal to 1/2 P and therefore is such as to focus the laser radiation 3 in a limited axial area matching the mirror 11a. The optical radiation coming out from the active medium (laser) 3 is therefore partially reflected backwards by the reflecting surface 11a and refocused by this one, with an essentially similar path, on the second face 5 of the active portion (laser) 3, while the transmitted portion of the optical power crossing the layer 11a (poorly transparent) forms the useful laser radiation which is coupled through a single mode optical fiber 12, to an external modulator or other guide and modulation device (not showny. The system of lenses 6 and 7a, of the active portion 3 and of the reflecting layers 4 and 11 forms the resonator cavity of the glass laser which is therefore limited by said surface 4 and by said mirror 11. Furthermore, according to the - invention, the two portions 6 and 7a are separated by an air gap 8, representing a discontinuity of the refraction index in the axial direction (becoming an approximately unit value if air is present in the cavity) and acts as an etalon. This etalon, whose surfaces 9 and 10 are preferably coated with layers reflecting the λl =1.54 μm radiation, has the function to suppress the unwanted modes so as to obtain a single mode laser having a narrow output line. The air space 8, according to additional realization forms described hereafter, can be foreseen in whichever position included in said surface 4 and said mirror 11. To select one mode a piezoelectric element 13 is foreseen, driven by an adjustable voltage supply through a variable resistence R allowing the distance d to be changed and thus obtain the desired shifts (in the range of one tenth of a micron) to form an etalon with the desired features. According to a further realization form shown in figure IB, the total length of the pair of lenses 6 and 7b plus the air gap d is equal to approximately 1/4 P and the end surface of the end portion 7B is flat and coated by a flat mirror lib surrounding the resonator at 1.54 μm, while the operation is essentially equivalent to that shown in figure 1A. In particular, since the layer lib is weakly transparent, radiation passing through this layer is focused by an additional optical guide segment 14a, approximately 1/4 P long, whose end face is coated by an anti-reflecting surface 15 to the radiation λl = 1.54 μm. A length of optical fiber 12 is coupled to said further optical guide segment 14a, similar to that shown in figure IB. Figure 2 shows a construction drawing of the device according to the present invention and in particular it shows the above mentioned monolithic structure, positioned on a submount 21, sectioned to better highlight the positioning of said structure versus the laser diode 1 and the fiber 12. In particular the submount 21 consists of a rectangular section steel guide having at one end a lifted portion 22 on which -the laser 1 is welded and having at the other end a "V" shaped housing 23 where the fiber 12 is installed and fixed with glue. Between the lifted portion 22 and the housing 23, housings are foreseen, separated by cavities 24, showing a form complementary to that of said first focusing element 2, active portion (laser) 3, second focusing element 6 and 7b. We can see that between selfoc lenses 6 and 7b an air gap forming the etalon 8 showing the reflecting surfaces (9 and 10) the radiation λl emitted by the active portion (laser) 3, is interposed. The piezoelectric element 13 is not shown here in order not to unusefully complicate the drawing. Figure 3A shows a third realization form of the device according to the invention, differing from the previous ones due to the fact that the first segment of optical guide 2 is replaced by a length of optic fiber 30 suitable to couple the laser 1 to said active portion (laser) 3. The remaining portion of the monolithic structure is entirely similar to the one shown in figure 1A. Figure 3B shows a fourth realization form of the device according to the invention employing the same length of optical fiber 30 previously shown in figure 3A and differing from said third realization form due to the fact that the first face of the active portion is flat and to the fact that the selfoe lens 7a - having a curved end surface is replaced by a lens 7b - having a flat end portion and coupled to an additional optical guide segment (selfoc lens) 14b, similar to that shown in figure IB. Figure 4 shows a construction drawing of said fourth realization form (see figure 3B) of the device according to the invention where it can be see that said monolithic structure is enclosed inside the female portion 40 of a connector for optical fiber systems. Similar to as shown in figure 3B the active portion (laser) 3 is coupled to the remaining part of the resonator made up of the portions of optical guide 6 and 7b in cascade to which, a further focusing optical guide portion 14 b is foreseen: a pig-tail 12 is coupled to said portion 14b. The optical fiber 30 is on the contrary inclosed in the male portion 41 of the connector for optical fiber systems, and in this way one end of fiber 30 can be coupled to said active portion (laser) 3. The second end of fiber 30 is connected to a box body 42 inside which a submount 43 capable of supporting the pumping laser 1 is housed. Figure 5 shows a fifth realization form of the device according to the invention repeating the same arrangement of elements previously described in figure 1A with the sole exception that the second face of the active medium 3 is angled with respect to the first face at an angle θR whose value is approximately 1°. This fifth realization form has an advantage over the realization forms previously described when it is deemed useful to eliminate reflections of radiation generated by the second face of said active portion. Obviously it is possible to use an active portion 3, shaped as shown in figure 5, combined with the element shown in figures 3A and 3B (the selfoc lens 2 is replaced by an optical fiber 30) as well as being combined with the elements shown in figure IB (that is combined with a group of lenses 6 and 7b plus etalon 8 having a total length equal to approximately 1/4 P plus an additional length of focusing optical fiber 14b having total length equal to approximately 1/4P)." Figure 6 shows a sixth realization form of the device according to the invention differing from said fifth realization form due to the fact that the second face of the active portion 3 is angled with respect to the first face by an angle ΘB having a value equal to the Brewester angle (equal approx. to 34°). The slope of said second face of the active portion 3 according to the above mentioned angle ΘB allows linearly polarized radiation to be obtained, that is radiation along one polarization plane. This characteristic is advantageous when the radiation guided in the optical fiber 12 is modulated. In fact, if this radiation is linearly polarized, the modulator is capable of using 100% of the associated power. The slope of the second face of the active medium 3 determines- a deviation ΘD of optical signals with respect to the optical axis A and consequently said radiations are incident on the first face of said second segment of optical guides 6 and 7 in a position deviating by a predetermined quantity 1. with respect to the optical axis. As a consequence, these radiations are focused on the second face of said.segment of optical fiber in a position shifted by the same quantity and in the opposite direction to the previous one with respect to the optical axis. Fiber 12 is consequently positioned of the focusing point at said distance 1 of the optical axis. In this case the length of lenses 6, 7 and etalon 8, equals approximately 1/2 P. To prevent difficulties arising from the search of a focusing point not coinciding with the optical axis A, it could be convenient to insert a wedge shaped body 60, made of glass or active material, between said active portion and the second focusing optical guide, which determines the offset of said angle θD and consequently radiation having λl wavelength focuses on points coinciding with the optical axis. Fiber 12 has therefore to be positioned on the device optical axis. Figure 7 shows a realization form of the device, shaped according to figure 6, that is with the wedge shaped body 60 housed in its position, but with etalon 8 placed between the active portion 3 and the second focusing optical guide 6-7. This solution has the advantage of allowing linearly polarized radiation to be obtained which can be coupled to an optical fiber 12 placed on the optical axis of the device and not requiring, at the same time, the cutting of selfoc 6 or 7 to realize the etalon 8 since this one is realized interposing an air gap between the wedge shaped body 60 and the selfoc lens 6 or 7. An additional realization form allows a linearly polarizedλl radiation to be obtained, which can be coupled to an optical fiber 12 arranged on the optical axis of the resonator portion is shown in figure 8. According to this solution, the resonator axis is at an angle ΘD with respect to the optical axis of elements 2 and 3. Of course, the particular configuration of the second face of the resonator 3 (angle ΘB) as shown in figures 6 to 8 can be profitably applied to the laser device configuration previously shown in figures 3A and 3B (the optical guide portion 2 is replaced by an optical fiber 30), as well as making reference to the laser device configuration previously shown in figure IB (the round mirror 11a is replaced by a flat mirror lib). The device according to the invention is of compact construction, with a size of about 4 cm 3 compatible with the requirements of optical fiber telecommunication systems, and emitting a highly monochromatic radiation. Also it is simple to realize and of limited cost, complying with the above mentioned scope. In the present description the invention has been shown making reference to the construction of a laser device inclusing an active glass medium 3 - employing Erbium (Er) as a dppant and Ytterbium (Yb) as codopant, pumped by laser 1 operating between 960 - 980 nm approx. to obtain a device able to operate in the so-called third window. However, the invention may be applied in other situations and particularly when combined with an active medium consisting of a glass matrix realized with different elements such as silicate, phosphate and fluorozirconate, doped with elements different from the above mentioned ones. In particular, the invention can also be profitably employed when combined with an active medium, consisting of a crystalline matrix, doped with elements differing from the above mentioned ones. Said glass matrix can for instance employ: A) Ytterbium as codoping element and Erbium or Praseodymium as. doping elements, Yb3 + (Er, Pr); B) Aluminium as codoping element and Erbium as doping elements, A13 + (Er); C) Lanthanium as codoping element and Erbium as doping elements, La3 + (Er); D) Chromium as codoping element and Erbium, Thulium or Holmium as doping elements, Cr3 + (Er, Tm, Ho); E) Mercury as codoping element and Neodymium as doping element, Mg2 + (Nd); F) Thulium as codoping element and Holmium as doping element, Tm3 + (Ho); G) Germanium as codoping element and Erbium or Praseodymium as doping elements, Ge4 + (Er, Pr). We draw attention to the fact that for the specific field of telecommunication the following is of particular interest: - the use of doping and codoping elements mentioned under A, particularly Praseodymium and Neodymium, for the realization of a laser capable of emitting radiation at the 1.3 μm wavelength, that is in the so-called second window; - the use of doping and codoping elements mentioned under D, particularly with Tm and Ho doping elements, for the realization of a laser capable of emitting radiation at approx. 2 μm wavelength, destined to be used combined with fibers for MIR applications, that is combined with fibers operating in the medium infrared and showing therefore an attenuation of about two orders of magnitude less than current fibers. The crystalline matrix can on the contrary employ for instance the following doping and codoping elements: - Er:CaF2 or Er:LiYF4 for the realization of a laser capable of emitting radiation at the 1.54 μm wavelength, that is in the so-called third window; Nd:YAG or ND:YLF for the realization of a laser capable of emitting radiation at the 1.3 μm wavelength, that is in the so-called second window; Though the invention has been shown making reference to the preferred realization forms, it is generally subject to additions and modifications, as will be evident to a technician in this field.

Claims: CLAIMS 1. Laser device to be employed as continuous source of a single mode, monochromatic radiation in telecommunications, of the type including an active medium capable of emitting a radiation at a first wavelength λl, pumped by a laser capable of emitting a radiation at a second wavelength λ2 < λl, characterized by the fact of consisting of a single structure made of the following parts: - a first portion of focusing optical guide (2) whose first face is optically coupled to said pumping laser (1) ; - an active portion (laser 3) capable to emitting a laser radiation at the desired wavelength λl, between the second face of said first portion of focusing optical guide (2) and the first face of said active portion (3) being placed a first filter (4) essentially transparent to said radiation λ2 and essentially reflecting said radiation λl, to the second face of said active portion (3) being applied a second filter (5) essentially transparent to said radiation λl and essentially reflecting said radiation λ2; - a second portion of focusing optical guide (6 - 7) , whose first end is optically coupled to said second filter (5) and whose second end is coated by a mirror- (11) partially reflecting said radiation λl; - an air gap (8) of adjustable width, placed between said first filter (4) and said mirror (11); - a first portion of optical fiber (12) optically coupled to said partially reflecting mirror (11); - support means of said parts including a member to vary the width of said air gap (8). 2. Device according to claim 1, characterized by the fact that a third and a fourth optical filter (9 and 10 respectively) reflecting said radiation λl are applied to the opposite faces of the elements forming said air gap (8). 3. Device according to claims 1 and 2, characterized by the fact that the second face of said active portion (3) is angled with respect to the first face by an angle (ΘR) whose value is approximately 1°. (figure 5) 4. Device according to claims 1 and 2, characterized by the fact that the second face of said active portion (3) is angled with respect to the first face by an angle (ΘB) whose value is equal to the Brewester angle, (figure 6) 5. Device according to claim 1, characterized by the fact that said first face of the active portion (3) is rounded, with the concave part towards the active material. 6. Device according to claim 1, characterized by the fact that said first face of the active portion (3) is flat. 7. Device according to claims 1 to 6, characterized by the fact that said second portion of focusing optical guide is made up of at least one optical guide segment with graded refraction index whose length is approximately 1/2 P, being P the focusing pitch of the guide segment, and by the fact that the second end of this segment is coated by a curved mirror (11a) partially reflecting the radiation λl. (figure 1A) 8. Device according to claims 1 to 6, characterized by the fact that said second portion of ^ focusing optical guide is made up of at least one optical guide segment with graded refraction index (6 or 7), whose length is approximately 1/4 P, being P the focusing pitch of the guide segment, and by the fact that the second end of this segment is coated by a flat mirror (lib) and between this one and said first fiber portion (12), a second optical guide segment with graded refraction index (14a) having length equal to approximately 1/4 P and an interposed anti- reflecting layer (15). (figure IB) 9. Device according to claims 1 to 6, characterized by the fact that said first portion of focusing optical guide (2) is made of one optical guide portion with graded refraction index, whose length is approximately P/2, where P is the focusing pitch of the guide segment. 10. Device according to claims 1 to 6, characterized by the fact that said first portion of focusing optical guide (2) is made up of a second portion of optical fiber (30) . (figure 3) 11. Device according to claims 1 to 10, characterized by the fact that said member to adjust the width of said air gap (d) is combined to said second portion of optical guide (6 and 7) and includes a piezoelectric element (13), driven by an adjustable supply voltage (R, V) . 12. Device according to claim 4, characterized by the fact that the axis of said second focusing optical guide portion (6 - 7) matches the axis of said active portion (3) and said first portion of optical fiber (12) deviates from the axis of said second portion of focusing optical guides (6 - 7) of a quantity (1) which is function of the angle (ΘD) of which optical signals deviate in consequence of the angle of the second face of said active portion (3) according to the Brewester angle and by the fact that said second portion of focusing optical guides (6 - 7) is nearly 1/2 P long, where P is the focusing pitch, and the second face is flat (figure 6) . 13. Device according to claim 4_, characterized by the fact that the axis of said second focusing optical guide portion (6 - 7) is angled with respect to the axis of said active portion (3) by an angle equal to the angle (ΘD) deviating the optical signals in consequence of the slope of the second face of said active portion according to the Brewester angle and said first portion of optical fiber (12) is positioned on the same axis of said second portion of focusing optical guide (6 - 7) (figure 8) . 14. Device according to claim 4 characterized by the fact that a glass made, wedge shaped portion (60) is put between said active portion (3) and said second portion of focusing optical guide (6 - 7) . 15. Device according to claim 14 characterized by the fact that said wedge shaped portion (60) is made of active material, (figure 7) ' 16. Device as in claim 1, characterized by the fact that said second portion of focusing optical guides consists of two segments of graded refraction index optical guide (6 and 7), and said air gap (8) is put between said two segments. 17. Device as in claim 1, characterized by the fact that said second portion of focusing optical guide includes a sole element of graded refraction index optical guide (6 or 7), the air gap (8) being put between said active portion (3) and said graded refraction index optical guide element (6 or 7). (figure 7) 18. Device according to claims 1 to 17, characterized by the fact that said active medium is made of a glass matrix doped with Er:Yb and that λ2 is between 960 and 980 nm. 19. Device according to claims 1 to 17, characterized by the fact that said active medium consists of! a glass matrix doped with Yb3 + (Pr) and that λ2 is between 960 and 980 nm. 20. Device according to claims 1 to 17, characterized by the fact that said active medium consists of a glass matrix doped with Cr3 + (Tm, Ho) and that λ2 is nearly 780 nm. 21. Device according to claims 1 to 17, characterized by the fact that said active medium consists of a crystalline matrix doped with Erbium, Er:CaF2. 22. Device according to claims 1 to 17, characterized by the fact that said active medium consists of a crystalline matrix doped with Erbium, Er:LiYF4. 23. Device according to claims 1 to 17, characterized by the fact that said active medium consists of a crystalline matrix doped with Neodymium, NdrYAG. 24. Device according to claims 1 to 17, characterized by the fact that said active medium consists of a crystalline matrix doped with Neodymium, Nd:YLF. 25. Device according to claims 7 and 8, characterized by the fact that said single structure it is supported by a submount (21) consisting of a single body at one end of which a raised portion (22) is foreseen on which the pumping laser (1) is welded, and at the other end a "V" shaped housing is foreseen inside of which is arranged and fixed said first portion of optical fiber (12) , between said raised portion (22) and said "V" shaped housing (23), being foreseen a number of housings suitable to house said first portion of optical fiber (2), said active portion and said second portion of optical guide (6 - 7) . 26. Device according to claim 10, characterized by the fact that said single structure is positioned inside the female portion (40) of a connector for optical systems while one end of said second portion of optical fiber (30) is arranged inside the male portion (41) of said connector and the other end is optically coupled to the pumping laser (1) which is installed on a " submount (43) housed inside a box body (42) .

Inventors: CHIARETTI GUIDO, DI ROCCO DANIELE, LAPORTA PAOLO, SVELTO ORAZIO, CHIARETTI, GUIDO, DI ROCCO, DANIELE, LAPORTA, PAOLO, SVELTO, ORAZIO
Applicants: CHIARETTI GUIDO, ITALTEL SPA, LAPORTA PAOLO, ROCCO DANIELE DI, SVELTO ORAZIO, ITALTEL SOCIETA ITALIANA TELECOMUNICAZIONI S.P.A.
Application year: 1993
Main classification: H01S3
IPC codes: G02B6, H01S3, H01S5