SINGLE-MODE LASER DEVICE

Abstract: Laser device to be used as a continuous wave source of a single-mode and monochromatic radiation in telecommunications of the type including a pump laser (1) suitable to emit a radiation with a wavelength of lambda 1, a resonating cavity (2) and an optical fibre rod (8). The resonating cavity includes at least a first and second mirror (6 and 7), and at least a portion of active material (3, 4; 30) suitable to emit a radiation with a wavelength of lambda 2. The first mirror (6) is highly transparent to the radiation lambda 1 and highly reflecting the radiation lambda 2, and the second mirror (7) is highly reflecting the radiation lambda 2.

Description: "SINGLE-MODE LASER DEVICE" Technical Field The present invention relates to a laser device to be used as a continuous wave source of a single-mode and monochromatic radiation in telecommunications of the type including a pump laser suitable to emit a radiation with a first wavelength λl, and an active medium suitable to emit a radiation with a second wavelength λ2 > λl. The device according to the invention finds application in telecommunication systems, and in particular it can be advantageously used, but not exclusively, as a continuous wave source with the wavelength of 1.54 μm (the so called third window) for transmission on optical fibre. For the future telecommunication developments in optical fibre the use of a radiation with a wavelength of 1.54 μm (1540 nm) is foreseen corresponding to the so called third window for which the optical fibres present a reduced loss. A source of coherent light for this use must supply a sufficiently high output power (of the order of some W to be practically applicable) with emission of the single- mode, longitudinal and transversal type, and with an emitted spectral line width reduced as much as possible. Background Art As an indication, the current sources of this type are realized by semiconductor lasers of the single-mode DFB type with the typical optical power coupled to single-mode fibre of 1 - 4 W and with a line width of 1 - 30 MHz. In order to obtain sources suitable to produce the requested wavelength the use of the so called glass laser has been proposed, that is including - as the active medium - an element of glass doped with ions of rare earths as for example Erbium (Er, etc.). Two further laser devices are used as pump source, and more precisely a semiconductor laser with a wavelength of 808 nm pumping a Neodymium laser emitting at its turn a radiation at 1060 nm pumping the glass laser suitable to emit the laser radiation at the desired wavelength of 1.54 μm. These glass lasers include moreover a certain percentage of Ytterbium (Yb) with the aim to increase the pumping efficiency as the Ytterbium has a larger and more intensive absorption band than that of the Erbium, and transfers to the later the absorbed energy. This device requires a double conversion and moreover the emission efficiency of the Neodymium is very low (about 1 - 2%), so to make the functioning of the device critical, therefore even very low losses of the laser cavity may determine a too modest emitted power, so that it becomes necessary to apply a very accurate and expensive processing of the most critical elements. In the article "Diode-pumped CW Bulk Er:Yb: glass laser" by P. Laporta et al., OPTICS LETTER, Vol. 16, nr. 24, December 15, 1991 is proposed to pump the glass laser directly by a semiconductor laser emitting at 980 nm focusing the pumping radiation on the glass laser by means of a traditional optical system. The resonant optical cavity is defined between a superficial coating of the glass element being substantially transparent to the radiation at 980 nm and strongly reflecting to the radiation at 1.54 μm, and a curved mirror reflecting at 1.54 μm. As the light emitted by the glass laser is multi-mode there is a so called etalon put between the active medium and the mirror, that is a glass slab of an appropriate thickness determining the elimination of the longitudinal modes of undesired propagation in order to get a single- mode laser with narrow line emission. This device, even if valid in a conceptual way, is still not suitable to be used in telecommunication systems due to its large dimensions, the linked costs, the complexity of the focussing of the etalon, therefore there is the necessity to realize a laser source of reduced dimensions as well as limited costs with high emission power and a very narrow spectral line. Objects of the invention It is the object of the present invention to overcome the limitation and inconveniences of the known illustrated solutions, and in particular to realize a laser device characterized by a compact structure and easy construction. Disclosure of the invention These objects are achieved by the invention consisting of a laser device for the use as a continuous wave source of single-mode and monochromatic radiation of the type including: a pump laser suitable to emit a radiation at a first wavelength of λl; - a resonating cavity with an end optically coupled to the pump laser; an optical fibre rod connected to the other end of the resonating cavity; characterized in that said resonating cavity is composed by: two portions of active medium suitable to emit a radiation at a second wavelength λ2, pumped by said pump laser and separated by an interspace of adjustable length or etalon, the active medium surfaces defining said interspace being plane and parallel to each other but not right angled to the longitudinal axis of the cavity and partially reflecting said second wavelength λ2; a first mirror highly transparent at the wavelength λl and highly reflecting at the wavelength λ2 set up on the surfaces of the active medium turned towards the pump laser, and a second mirror highly reflecting at the wavelength λ2 set up on the other surface of the active medium turned towards the optical fibre rod. Moreover the invention consists of a laser device to be used as a continuous single-mode and monochromatic radiation source in telecommunications, of the type including: - a pump laser suitable to emit a radiation at a first wavelength λl; - a resonating cavity one end of which being optically connected to the pump laser; an optical fibre rod connected to the other end of the resonating cavity; characterized in that said resonating cavity is composed by: a portion of active medium suitable to emit a radiation at a second wavelength λ2; a first mirror highly transparent at the wavelength λl and highly reflecting at the wavelength λ2; - a second mirror highly reflecting at the wavelength λ2. Brief Description of the Drawings The invention will now be described with reference to the enclosed drawings illustrating a preferred but not limiting embodiment of the invention, wherein: - figure 1 shows schematically a first embodiment of the device realized according to this invention; - figure 2 shows schematically a realization variation of the device shown in fig. 1; figure 3 shows schematically a second embodiment of the invention; figure 4 shows schematically a realization variation of the device shown in figure 3; figure 5 shows in a schematically way a realization variation of the device shown in fig. 4. Detailed Description of the Preferred Embodiments With reference to figure 1 the device according to the invention includes a first semiconductor laser 1 (pump laser) emitting according to a preferred embodiment of the invention radiations at a wavelength λl between 960 and 980 nm and which is optically connected to a resonating cavity 2 set up by a monolithic lengthened structure, preferably with circular section. The resonating cavity includes: a first 3 and a second 4 portion of active medium suitable to emit a radiation at a wavelength λ2 equal to 1.54 μ (1540 nm) ; an interspace 5 (etalon) with adjustable length put between said first and second portion of active material, and more in general an interspace set up by any element with a different refractive index from that of the active medium; a first coating or mirror 6 highly transparent at the wavelength λl of 960 - 980 nm of the pump laser and highly reflecting at the wavelength λ2 = 1540 nm emitted by the active medium. a second coating or mirror 7 partially reflecting (for example about 99%) at the wavelength λ2 = 1540 nm emitted by the active medium. Coating 6 as well as coating 7 are obtained in a known way by means of a device of an oxide or metal layer on the curved surface - with a concavity turned towards the active medium - of said first 3 and said second 4 portion of active material, respectively. This concavity has the function of favouring the setting up of the fundamental propagation made of the radiation at 1540 nm. An optical fibre rod 8 is connected to the second face of coating 7 which is slightly transparent at the wavelength λ2 emitted by the active medium. According to a preferred embodiment the dimensions of the etalon 5 may be modified by means of a piezoelectric device 9 modifying the dimensions according to the variations of the voltage V applied to it through a variable resistance R. Always according a preferred embodiment, the etalon 5 must be positioned in the focusing zone of the two coatings or mirrors 6 and 7. The functioning of the above described structure may be explained as follows. The pump diode laser 1 emits a beam of slightly diverging radiation at a wavelength λl passing through coating 6 and exiting the two portions of active material 3 and 4 emitting in its own turn a radiation at the wavelength λ2 propagating within the resonating cavity. The different refractive index of the area (or of the material) existing in said interspace 5 with respect to the refractive index of the active medium carries out a filtering operation, and cancels therefore the unwanted longitudinal propagation modes favouring in this way the propagation of a single-mode and monochromatic radiation. This radiation is reflected by the mirrors 6 and 7 and focuses in the centre of the resonating cavity preferably in correspondence with etalon 5. As specified above the mirror 7 is slightly transparent, a portion of the radiation λ2 (about 1%) leaves of the resonating cavity 2 and forms the a useful signal which, coupled with the optical fibre, is sent to the not illustrated external modulation devices. Figure 2 shows a variation of the device according to the invention. In particular this type of realization foresees the presence of a first optical guide portion 20 of graded refractive index with a length between P/4 and P/2, which converges a divergent radiation beam entering from the front side on a zone of the opposite end surface. P is the so-called pitch or the distance lying between two contiguous planes of perfect imaging. In particular such first portion of focusing optical guide is put between the pump laser 1 and the resonating cavity defined as specified before with reference to figure 1. This type of realization foresees moreover the existence of a second focusing optical guide portion 21 also with a length between P/4 and P/2 put between said cavity and the optical fibre 8. This second portion focuses the divergent radiation entering from the front side 2 on a zone of sufficiently reduce dimensions of the opposite end surface. The functioning of this embodiment is mainly analogous to that described before regarding figure 1 making said first optical guide portion converge the divergent radiation λl emitted by the pump laser, a radiation defining the emission of a radiation λ2 by the active medium 3 and 4 refocusing to the centre of the resonating cavity 2 (connected with etalon 5) to diverge again and to be refocused by said second portion of focusing optical guide 21. Analogously to that described with reference to figure 1 this radiation λ2 is coupled to the optical fibre 8 and is then sent to external modulation devices (not shown). Figure 3 shows a further embodiment of the device realized according to the present invention. Referring to figure 3 the device according to the invention includes a first semiconductor laser 1 (pump laser) which according to a preferred embodiment emits radiation at a wavelength λl included between 960 and 980 nm and is optically connected to a resonating cavity 2 including a first and second opposite mirrors 6 and 7 between which a laminar portion 30 of active material is placed with a thickness much smaller than the longitudinal distance between the mirrors. Between the plate 30 and each one of the mirrors 6, 7 an interspace is placed. The position of the active medium 30 may be modified inside the cavity by means of a piezoelectric device 9 which is driven by the voltage V through said variable resistance R. The first coating or mirror 6 is highly transparent at the wavelength λl of 960 - 980 nm of the pump laser and highly reflecting at the wavelength λ2 = 1540 nm emitted by the active medium, while the second coating or mirror 7 is partially reflecting (for example about 99%) at the wavelength λ2 = 1540 nm emitted by the active medium. Coating 6 as well as coating 7 are realised in the known way by means of an oxide or metal layer on concave bodies with the concavity turned towards the active material. This concavity has the function of favouring the setting up of the fundamental propagation mode of the radiation at 1540 nm. At the second side of coating 7, slightly transparent at the wavelength λ2 emitted by the active medium, is optically connected to the optical fibre rod 8. The functioning of the device illustrated in figure 3 is the following. The pump laser diode 1 emits a slightly divergent radiation beam at a wavelength λl passing through the first coating 6 and focussing within the active medium 30, exiting it to emit in its own turn a radiation at the wavelength λ2 propagating inside the resonating cavity 2. The different refractive indexes between the air and the active medium 30 has a filtering action and cancels therefore the unwanted propagation modes favouring in this way the propagation of a single-mode and monochromatic radiation. This radiation is reflected by the mirrors 6 and 7 and focuses in the resonating cavity 2 connected to the active element 30. Since mirror 7 as specified before is slightly transparent, one portion of the radiation λ2 (about 1%) leaves the resonating cavity 2 and forms a useful signal which is coupled to the optical fibre 8 and sent to external modulation devices which are not shown. In other words since on the left as well as on the right side of the active medium 30 there is an element (air) with a refractive index different to that of the active medium this element fulfills the etalon function making the radiation propagating inside the cavity monochromatic. Studies carried out by the Applicant show that, in order to be able to act as a single-mode longitudinal selector and guarantee at the same time the necessary optical amplification for the functioning of the laser, the above mentioned portion of active material 30 must be dimensioned in a way respecting the following conditions: given B the oscillating band of the laser transition, L of the length of the active medium equal to the thickness of the disc, c the speed of the light in the medium and k a suitable constant, the oscillating longitudinal single-mode condition within the band B may be expressed as: L < c/(2kB), (1) given N the atomic concentration of the active ion in the laser mean, g the total logarithmic losses for the passage through the resonator, s the emission cross section stimulated by the laser transition, the laser emission condition (gain higher than losses) results that it can be expressed as: N > g/(sL), (2) Figure 4 shows a variation of the device according to the invention. In particular this embodiment foresees the presence of a first optical guide portion of the graded refractive index type 40 with a length between P/4 and P/2 so to converge a divergent radiation beam entering from the front side on a zone of the opposite end surface. In particular this first focusing optical guide portion is put between the pump laser 1 and the resonating cavity defined as illustrated in figure 3. This embodiment provides moreover for the presence of a second portion of the focusing optical guide 41, also this with a length included between P/4 and P/2 placed between said cavity and the optical fibre 8. This second portion focuses in this way the divergent radiation entering from the front side on a space of reduced dimension of the opposite end surface. The functioning of this embodiment is substantially analogous to that illustrated before with regard to figure 3 making said first portion of optical guide 40 converge the divergent radiation λl emitted by the pump laser, a radiation determining the emission of a radiation λ2 by the active medium 30 refocusing in the centre of the resonating cavity 2 (connected to the active medium) to diverge again and to be refocused by said second portion of the focusing optical guide 41. Analogously to the above described referring to figure 3 the radiation λ2 is optically coupled to the optical fibre 8 through the guide 41 and is then sent to external modulation devices (not shown). Figure 5 shows a further embodiment of the type of realization illustrated in figure 4 differing from this one by the fact that the mirrors or coatings 6 and 7 are deposited directly on a portion of active material 30. In this way etalon, active medium and resonating cavity are coincident. In other words, since the active medium 30 has the dimensions equal to those of an etalon the resonating cavity 2 has dimensions reduced in such a way that it is not necessary to have inside the resonating cavity an element with a different refractive index compared to that of the active medium to realize the etalon function. The device according to the invention is constructed in a compact way with dimensions of about 4 cm^ compatible with the needs of optical fibre telecommunication systems, and it emits a highly monochromatic line. Moreover the realization is simple and the cost reasonable according to the specified objects. According to a preferred type of realization the laser device according to the invention includes an active glass medium (3 - 4 or 30) using Erbium as a dopant and Ytterbium as a co-dopant pumped by a laser 1 at about 960 - 980 nm - in order to get a device suitable to operate in the third window. However the invention may be applied in other situations and especially in combination with an active medium set up by a vitreous matrix realized with elements different from glass such as silicates, phosphates and fluozirconates doped with elements different from those specified before. Moreover the invention may be advantageously used in combination with an active medium consisting of a crystal matrix also doped with elements different from those specified before. In particular the abovesaid vitreous matrix may use for example: A) Ytterbium as co-dopant element and Erbium or Praseodymium as dopant element, Yb3 + (ER, Pr); B) Aluminium as co-dopant element and Erbium as dopant element, A13 + (Er); C) Lanthanum as co-dopant and Erbium as dopant, La3 + (Er); D) Chromium as co-dopant and Erbium, Thulium or Holmium as dopant, Cr3 + (Er, Tm, Ho); E) Mercury as co-dopant and Neodymium as dopant, Mg2 + (Nd); F) Thulium as co-dopant and Holmium as dopant, Tm3 + (Ho); G) Germanium as co-dopant and Erbium or Praseodymium as dopant, Ge4 + (Er, Pr). We want to attract attention to the fact that the following points are of particular interest for the specific telecommunication sector: the use of the dopants and co-dopants according to point A, especially the Praseodymium and the Neodymium for the realization of a laser suitable to emit radiations at a wavelength of 1.3 μm (the so-called second window); - the use of the dopants and co-dopants as under point D, especially with the dopants Tm and Ho, for the realization of a laser suitable to emit radiation at a wavelength of about 2 μm destined to be used in combination with fibres for MIR applications, that is in combination with fibres operating in the infrared medium and having therefore an attenuation of about two orders smaller with respect to the current fibres. The crystal matrix may instead use for example as dopant and co-dopant elements: - Er:CaF2 or Er:LiYF4 for the realization of a laser suitable to emit radiations at a wavelength of 1.54 μn, that is in the so-called third window; Nd:YAG or Nd:YLF for the realization of a laser suitable to emit radiations at a wavelength of 1.3 μm that is in the so-called second window. Although the invention has been illustrated referring to a preferred type of realization it will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departing from the scope and spirit thereof.

Claims: CLAIMS 1. Laser device to be used as a continuous wave source of single-mode and monochromatic radiation in telecommunications, of the type including: - a pump laser (1) suitable to emit a radiation at a first wavelength λl; a resonating cavity (2) with one end optically connected to the pump laser (1) ; an optical fibre rod (8) connected to the other end of the resonating cavity (2); characterized in that said resonating cavity (2) is set up by: two portions of active material (3, 4) suitable to emit a radiation at a second wavelength λ2 pumped by said pump laser (1) and separated by an interspace (5) of an adjustable width or etalon, the surfaces of the active material (3, 4) defining said interspace (5) being flat and parallel to each other, but not right-angled to the longitudinal axis of the cavity, and partially reflecting at said second wavelength λ2; a first mirror (6) highly transparent at the wavelength λl and highly reflecting at the wavelength λ2 set up on the surface of the active medium turned towards the pump laser (1) ; and - a second mirror highly reflecting at the wavelength λ2 set up on the surface of the active medium turned towards the optical fibre rod (8) (Fig. 1). 2) Device according to claim 1, characterized in that between said pump laser (1) and the cavity a first optical guide segment with graded refractive index (20) is placed. (Fig. 2) . 3. Device according to claim 2, characterized in that between said resonating cavity and the optical fibre (8) a second focusing optical guide segment (21) is placed. (Fig. 2). 4. Device according to the claims 2 and 3, characterized in that said first and second focusing optical guide segments (20, 21) have a length between P/4 and P/2, P being the focusing step of the focusing optical guide segment. 5. Laser device to be used as a continuous wave source of single-mode and monochromatic radiation in telecommunications of the type including: a pump laser (1) suitable to emit a radiation at a first wavelength λl; - a resonating cavity (2) of which one end is optically connected to the pump laser (1) ; an optical fibre rod (8) connected to the other end of the resonating cavity (2); characterized in that said resonating cavity (2) includes: - a portion of active material (30) suitable to emit a radiation at a second wavelength λ2; a first mirror (6) highly transparent at the wavelength λl and highly reflecting at the wavelength λ2; - a second mirror (7) highly reflecting at the wavelength λ2. 6. Device according to claim 5, characterized in that said first and second mirror (6 and 7) are realized through the deposition of metals or oxides on the same sides of portions of active material (3). (Fig. 5). 7. Device according to the claims 5 or 6, characterized in that the length L of said active material portion (30) and the atomic concentration N of the active ions in the laser medium is dimensioned in such a way to respect the following conditions: L< c/(2kB) being c the speed of the light in the medium and K a suitable constant, and N> g/(sL) being g the total logarithmic losses for the passage of the resonator by the light and s the emission cross section stimulated by the laser transition. 8. Device according to the claims 1 to 7, characterized in that said active medium consists of a vitreous Er:Yb doped matrix and that λ2 is between 960 and 980 nm. 9. Device according to the claims 1 to 7, characterized in that said active medium is set up by a vitreous Yb3 + (Pr) doped matrix and that λ2 is between 960 and 980 nm. 10. Device according to the claims 1 to 7, characterized in that said active medium consists of a vitreous Cr3 + (Tm, Ho) doped matrix and that λ2 is about 780 nm. 11. Device according to the claims 1 to 7, characterized in that said active medium consists of an Erbium doped crystal matrix, Er:CaF2. 12. Device according to the claims 1 to 7, characterized in that said active medium consists of an Erbium doped crystal matrix, Er:LiYF4. 13. Device according to the claims 1 to 7, characterized in that said active medium is consists of a Neodymium doped crystal matrix, Nd:YAG. 14. Device according to the clams 1 to 7, characterized in that said active medium consists of a Neodymium doped crystal matrix, Nd:YLF.

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