Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/0627—Construction or shape of active medium the resonator being monolithic, e.g. microlaser
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4201—Packages, e.g. shape, construction, internal or external details
  • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
  • G02B6/4206—Optical features
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/07—Construction or shape of active medium consisting of a plurality of parts, e.g. segments
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S2302/00—Amplification / lasing wavelength
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/0602—Crystal lasers or glass lasers
  • H01S3/0604—Crystal lasers or glass lasers in the form of a plate or disc
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/0602—Crystal lasers or glass lasers
  • H01S3/0615—Shape of end-face
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/08004—Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection
  • H01S3/08009—Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection using a diffraction grating
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/08018—Mode suppression
  • H01S3/08022—Longitudinal modes
  • H01S3/08031—Single-mode emission
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
  • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
  • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/1062—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using a controlled passive interferometer, e.g. a Fabry-Perot etalon
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/16—Solid materials
  • H01S3/1601—Solid materials characterised by an active (lasing) ion
  • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
  • H01S3/1608—Solid materials characterised by an active (lasing) ion rare earth erbium
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/16—Solid materials
  • H01S3/1601—Solid materials characterised by an active (lasing) ion
  • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
  • H01S3/1618—Solid materials characterised by an active (lasing) ion rare earth ytterbium
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/16—Solid materials
  • H01S3/17—Solid materials amorphous, e.g. glass

Definitions

• 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 ⁇ l, and an active medium suitable to emit a radiation with a second wavelength ⁇ 2 > ⁇ l.
• 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 ⁇ m (the so called third window) for transmission on optical fibre.
• 1.54 ⁇ m (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.
• 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.
• 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 ⁇ m.
• 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.
• Yb Ytterbium
• 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.
• the 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.
• 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 ⁇ l; - 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 ⁇ 2, 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 ⁇ 2; a first mirror highly transparent at the wavelength ⁇ l and highly reflecting at the wavelength ⁇ 2 set up on the surfaces of the active medium turned towards the pump laser, and a second mirror highly reflecting at the wavelength ⁇ 2 set up on the other surface of the active medium turned towards the optical fibre rod.
• 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 ⁇ l
• 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 ⁇ 2; a first mirror highly transparent at the wavelength ⁇ l and highly reflecting at the wavelength ⁇ 2; - a second mirror highly reflecting at the wavelength ⁇ 2.
• FIG. 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.
• 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 ⁇ l 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.
• a first semiconductor laser 1 pump laser
• a resonating cavity 2 set up by a monolithic lengthened structure, preferably with circular section.
• 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 ⁇ 2 emitted by the active medium.
• 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.
• the etalon 5 must be positioned in the focusing zone of the two coatings or mirrors 6 and 7.
• the pump diode laser 1 emits a beam of slightly diverging radiation at a wavelength ⁇ l 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 ⁇ 2 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.
• the mirror 7 is slightly transparent, a portion of the radiation ⁇ 2 (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.
• 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.
• 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.
• this radiation ⁇ 2 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.
• 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 ⁇ l 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.
• a piezoelectric device 9 which is driven by the voltage V through said variable resistance R.
• 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.
• the pump laser diode 1 emits a slightly divergent radiation beam at a wavelength ⁇ l 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 ⁇ 2 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.
• mirror 7 as specified before is slightly transparent, one portion of the radiation ⁇ 2 (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.
• 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:
• Figure 4 shows a variation of the device according to the invention.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• an active medium consisting of a crystal matrix also doped with elements different from those specified before.
• vitreous matrix may use for example:
• Ytterbium as co-dopant element and Erbium or Praseodymium as dopant element, Yb3 + (ER, Pr);
• Thulium as co-dopant and Holmium as dopant Tm3 + (Ho);
• Germanium as co-dopant and Erbium or Praseodymium as dopant Ge4 + (Er, Pr).
• 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 ⁇ m (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 ⁇ m 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 ⁇ n, 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 ⁇ m that is in the so-called second window.

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• Physics & Mathematics (AREA)
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• Optics & Photonics (AREA)
• Engineering & Computer Science (AREA)
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• General Physics & Mathematics (AREA)
• Lasers (AREA)

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• 1992
  • 1992-12-10 IT ITMI922797A patent/IT1256472B/it active IP Right Grant
• 1993
  • 1993-12-03 EP EP94902683A patent/EP0729658A1/en not_active Withdrawn
  • 1993-12-03 WO PCT/EP1993/003419 patent/WO1994014215A2/en not_active Application Discontinuation

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