EP0674815A1

EP0674815A1 - Multi-mode laser device

Info

Publication number: EP0674815A1
Application number: EP94903848A
Authority: EP
Inventors: Guido Chiaretti; Daniele Di Rocco; Paolo Laporta; Orazio Svelto
Original assignee: Italtel SpA; Italtel Societa Italiana Telecomunicazioni SpA
Current assignee: Italtel SpA
Priority date: 1992-12-18
Filing date: 1993-12-15
Publication date: 1995-10-04
Also published as: IT1256738B; ITMI922879A0; WO1994015385A1; ITMI922879A1

Abstract

Laser device of a lengthened and monolithic structure to be used in telecommunications of the type including a pumping laser (1) suitable to emit a radiation at a wavelength μ1, a resonating cavity (2) and an optical fibre rod (6). The resonating cavity comprises a first and second mirror (4 and 5) and a portion of active material (3) suitable to emit a radiation at the wavelength μ2 ⊃ μ1. The first mirror (4) is set up on the surface of the portion of active material (3) facing the pumping laser (1), and is highly transparent at the radiation μ1 and highly reflecting at the radiation μ2. The second mirror (5) is set up on the surface of the portion of active material (3) facing the optical fibre rod (6) and highly reflecting at the radiation μ2.

Description

MULTI-MODE LASER DEVICE

Technical Field

The present invention relates a multi-mode lase device for the use in telecommunication systems, and i particular it can be advantageously but not exclusivel used as a laser source suitable to emit a plurality o equi-distributed frequencies and substantially stable fo the optical fibre transmission. For a better exploitation of the transmissio capacities of optical fibres the use of several radiation at different wavelengths is foreseen, each one suitable t transport informations (such as for example televisio channels, radio channels, data, etc.) able to propagate i the optical fibre without appreciable interference with th others, and which can be easily separated through filters, heterodyne detectors or similar devices for the extractio of the individual transported informations. It is evident that in systems of this type the stability o the different frequencies must be guaranteed as an frequency drift or creep causes interference phenomenons able to limit the transmission capacity of the system o even to endanger it in a considerable way. More precisely, when the distance between the various modes are no constant there is the risk that the channels will overla each other, and as a consequence the informations transmitted by them will be lost.

Therefore is the necessity to realize a device able t generate a plurality of different frequencies wit sufficient stability to maintain a constant distance (i frequency) among them.

Background Art

In applications of the known type for obtaining th above-said plurality of different frequencies, a pluralit of single-mode lasers is normally used each one syntonize on one of the requested wavelengths. In order to avoid th drift problems mentioned before these known solution provide for the stabilization of all these emissions in way that the relative distances remain constant enough fo the use in telecommunication systems.

In these known solutions a frequency stabilizatio circuit is required, detecting the various wavelengths and, through as many feedback circuits, it drives th individual lasers in a way to maintain the relativ distances constant with respect to the others. This circui turns out to be quite complicated, bulky and expensive.

Object of the Invention

The object of the present invention is to eliminate o to reduce substantially the above mentioned deficiences, and in particular to generate the above-said plurality o different frequencies through a single laser device.

Disclosure of the Invention According to the invention these objects are realize through a multi-mode laser device for the emission of plurality of equi-distributed and substantially stabl frequencies for the transmission in optical fibre of the type comprising: - a pumping laser able to emit a radiation at a first wavelength λl; a resonating cavity having one end optically coupled to the pumping laser; an optical fibre rod optically coupled to the other end of the resonating cavity; characterized in that said resonating cavity is set up by: a portion of active material able to emit a radiation at a second wavelength λ2 pumped by said pumping laser; a first mirror highly transparent at the wavelength λl and highly reflecting at the wavelength λ2 set up on the surface of the active medium facing the pumping laser, and a second mirror highly reflecting the wavelength λ set up on the other surface of the active medium facing th optical fibre rod; means suitable to vary the dimensions of sai resonating cavity.

Further advantageous characteristics are the object o the appended claims.

Brief Disclosure of the Drawings The invention will now be illustrated with referenc to the enclosed drawings showing a preferred but no limiting embodiment of the invention, wherein: figure 1 shows schematically a first embodiment of th present invention; figure 2 shows an emission diagram of the multi-mod laser device subject of the present invention; figure 3 shows a second embodiment of the invention; figure 4 shows a third embodiment of the presen invention; figure 5 illustrates a constructive drawing of th device according to the present invention realize according to said third embodiment (figure 4); figure 6 shows a fourth embodiment of the present invention; figure 7 illustrates a constructive drawing of th device according to the present invention realized i accordance with said fourth embodiment (figure 6) ; figures 8 to 10 illustrate schematically as man further embodiments of the present invention.

Detailed Description of Preferred Embodiment

With reference to figure 1 the device according to th invention includes a first semiconductor laser 1 (pumpin laser) which in accordance with a preferred embodimen emits radiations at a wavelength λl between 940 and 980 nm, and it is optically coupled to a resonating cavity 2 consisting of a monolithic lengthened structure havin preferably a circular cross section. The resonating cavity includes:

- a portion of active material 3 suitable to emit radiation at a wavelength λ2 equal to 1.535 μm (1535 nm) ; a first filter of mirror 4 highly transparent at th wavelength λl of 940 - 980 nm of the pumping laser, an highly reflecting at the wavelength λ2 = 1535 nm emitted b the active medium; - a second filter or mirror 5 partially reflecting (fo example about 99%) at the wavelength λ2 = 1535 nm emitte by the active medium.

The filter 4 as well as the filter 5 are obtained in known way by putting down an oxide or metal layer on th curved surface - with the concavity facing the active medium - of said portion of active material 3. This concavity has the function of favouring the setting up of the fundamental propagation mode of the radiation at 1535 nm. On the second side of the filter 5 which is slightly transparent at the wavelength λ2 emitted by the active medium an optical fibre rod 6 is coupled.

The functioning of the previously described structure is the following. The pumping laser diode 1 emits a beam of slightly diverging radiations at a wavelength λl which pass through the filter 4 and excite the portion of active material 3 emitting at its turn a radiation at the wavelength λ2 propagating inside the resonating cavity. Since the mirror 5, as specified before, is slightly transparent a portion of the radiation λ2 (about 1%) gets out of the resonating cavity 2 and constitutes a useful signal which, coupled to the optical fibre 6 is sent to an external modulation device not shown. According to the invention a tuning device 7 is foreseen, fed by a variable voltage indicated schematically by V, to vary axially the position of the mirrors 4 and 5 in order to modify the length of the resonating cavity. This tuning device 7 may be constituted by a warming u element (or by a piezoelectric device) able to define variation of the geometric dimensions of the portion of active material, and as a consequence of the dimensions of the resonating cavity. Even if not explicitly illustrate in the following figures this device is foreseen or ca however be applied to all embodiments of the invention. From the Erbium laser device illustrated before a frequency emission (Power P in function of the wavelength λ) of the type schematically illustrated in figure 2 is obtained or a comb of various longitudinal frequency modes fl, f2, f3, ...., which allocation in frequency/ wavelength - and in a less sensitive way its distance d between the modes - depends on the geometric dimensions of the cavity which, as specified before, may be modified acting on the tuning device 7. In other words where for some specific applications it turns out to be necessary to obtain the longitudinal modes fl - f4 allocated in a range of wavelengths λ for example superior with respect to the other applications it is possible to obtain said modes fl - f4 acting on the tuning device as specified above.

Figure 3 shows a second embodiment of the device according to the invention.

This embodiment foresees in particular the presence of a first optical guide portion 30 of the type with grade refraction index at a length comprised between P/4 and P/2, thus to converge a beam of diverging radiation entering by the front side on a zone of the opposite terminal surface. P is the so-called pitch, or the distance existing betwee two contiguous planes of perfect imaging. In particular this first focusing optical guide portion is put betwee the pumping laser 1 and the resonating cavity 2. This embodiment foresees moreover the presence of second focusing optical guide portion 31, also this one of a length comprised between P/4 and P/2 put between th cavity 2 and the optical fibre 6. This second portio focuses in this way the diverging radiation entering by th front side on a zone of a sufficiently reduced dimension o the opposite terminal surface.

The functioning of this embodiment is substantiall analogous to that illustrated before with respect to figur 1, with the said first optical guide portion 30 tha converges the diverging radiation λl emitted by the pumpin laser, a radiation which defines the emission of radiation λ2 by the active medium 3 that re-focuses at th centre of the resonating cavity 2, diverges again and i re-focused by said second focusing optical guide portio 31. Analogously to what described before with concernin figure 1 this radiation λ2 is coupled to the optical fibr 6 and is then sent to external modulation devices no shown.

Figure 4 illustrates a third embodiment of th invention differing from the embodiment shown in figure 3 by the fact that it foresees the use of a further focusin optical guide portion 40 allocated inside the resonatin cavity, and it is especially put between said portion o active material 3 and said mirror 5.

The opposite sides of said portion of active material 3 and of said portion of focusing optical guide 31 turn out to be separated by an interspace, and they are provide with antireflecting coatings 41 and 42, respectively.

The other side of said portion of focusing optical guide 40 turns out to be flat and provided with said secon mirror 5 to which said portion of optical guide 31 is optically coupled, suitable to forward the radiation i output of the resonating cavity defined by the mirrors 4 and 5 to the optical fibre 6.

To said portion of active material 3 and/or to sai portion of focusing optical guide 31 said tuning devices 7 are connected - in the specific case preferentially set up by a piezoelectric element - which act on the variable resistance R making it possible to modify the dimension of said interspace, and as a consequence the dimensions of said resonating cavity delimited by the mirrors 4 and 5. Therefore the optical guide 40 is called hereafter optical tuning guide.

The aiitireflecting coatings 41 and 42 prevent said interspace from introducing a variation in the refraction index, and that means in other words that this interface exerts an etalon function with consequent propagation inside the cavity of a single longitudinal mode. The antireflex mirrors 41 and 42 prevent in this way the interspace from exerting the etalon function making so the propagation of a plurality of longitudinal modes possible. According to a preferred embodiment the optical tuning guide 40 presents lengths comprised between 3/4 P and 1/2 P and the coupling optical guide in fibre 31 presents an approximate length equal to 3/4 p.

Figure 5 shows a constructive drawing of the device of figure 4 according to the present invention, and it shows in particular the above-said monolithic structure positioned on one submount element 51 sectioned in order to better evidence the allocation of this structure with respect to the laser diode 1 and the fibre 6. In particular the submount 51 consists of a metallic guide with rectangular cross section presenting at one end a raised portion 52 on which the laser 1 is welded, and at the other end it presents a "V-Groove" where the fibre 6 is positioned and blocked by adhesives. Between the raised portion 52 and the seat 53 seats are foreseen, separated by notches 54 presenting a complementary form to that of the above-mentioned first focusing optical guide 30, the active portion (laser) 3, the optical tuning guide 40, and the coupling optical guide in fibre 31. The piezoelectric element 7 has not been illustrate here thus not to complicate uselessly the drawing.

Figure 6 shows a fourth embodiment of the inventio which differs from the previous ones by the fact that th first optical guide segment 30 is replaced by an optica fibre rod 60 suitable to connect laser 1 to said activ portion (laser) 3. The remaining portion of the monolithi structure is absolutely analogous to that illustrated wit reference to figure 4. Figure 7 shows a constructive drawing of said fourt embodiment (see figure 6) of the device according to th invention in which it is possible to see that sai monolithic structure is inserted in the interior of th female portion 70 of a connector for optical fibre systems. Analogously to what illustrated in figure 6 this monolithi structure consists of the active portion (laser) 3 and o the portions of optical tuning guide 40 and of the couplin in fibre 31.

The optical fibre rod 60 coupling with the diode 1 i on the other hand inserted inside the male portion 71 o the connector for optical fibre systems , and in this wa one end of the fibre 60 turns out to be connected to sai active portion (laser) 3. The second end of the fibre 60 i instead connected to a box-shaped body 72 wherein submount 73 is allocated suitable to support the pumpin laser 1.

Figure 8 shows a fifth embodiment of the inventio which repeats the same element disposition previousl illustrated with reference to figure 4 with the exceptio that the second face of the active medium 3 is incline compared to the first face with an angle ΘR with a value o about 1°. This fifth embodiment turns out to b advantageous compared to the forms of realizatio illustrated before when it is thought to be useful t eliminate the reflections of the radiations generated b the second face of said active portion. It is obviously understood that it is possible to us an active portion 3 shaped as illustrated in figure combined with the element disposition illustrated in figur

6 (the selfoc lens 30 is replaced by an optical fibre ro 60) .

Figure 9 illustrates a sixth embodiment of the devic realized according to the invention differing from th fifth embodiment by the fact that the second face of th active portion 3 is inclined with respect to the first fac with an entity angle ΘB equal to the Brewester angle (equa to about 34° ) . The inclination of said second face of th active portion 3 according to the above-said angle ΘB make it possible to obtain a linearly polarized radiation or radiation pending on a unique plane of polarization: thi characteristic turns out to be advantageous when th radiation corresponding on the fibre 6 is submitted to modulation procedure. In fact if this radiation turns ou to be linearly polarized the modulator is able to use 100 of the power associated to it. The inclination of the second face of the activ medium 3 defines a deviation ΘD of the optical signal compared to the optical axis A, and therefore the above said radiations arrive on the first face of said optica tuning guide segment 40 in a position moving by a prese entity with respect to the optical axis A. As consequence of this, these radiations turn out to b focused on the second face of the optical guide segment i a position moving from the same entity and in a directio opposite to the previous one always compared to the optica axis. The fibre 6 is consequently positioned in connectio to the focusing point at said distance 1 of the optica axis A.

In order to avoid the difficulties deriving from th research of a focusing point not coinciding with th optical axis A it may be convenient to insert a wedge shaped body 90 made of glass or active material, betwee said active portion and the second focusing optical guide, defining the cancellation of said angle ΘD, and therefor the radiations at the wavelength λl focus in points coinciding with the optical axis. The fibre 6 - when th wedge-shaped body 90 is used - will therefore be positione on the optical axis of the device.

Figure 10 illustrates a further embodiment making i possible to obtain in any case a radiation λl linearl polarized and to be conveyed on a fibre 6 disposed on th optical axis of the coupling lens in fibre 31. According t this solution the axis of the portion of active material 3 is inclined by an angle form of realization ΘD with respect to the optical axis of the elements 40 and 31.

Obviously the particular shape of the second face of the resonating portion 3 (angle ΘB) as illustrated in the figure 9 and 10 may be advantageously employed also wit reference to the shape of the laser device previousl illustrated with reference to figure 6.

The device according to the invention is constructe in a compact way with dimensions of about 4 cm^ compatibl with the needs of optical fibre telecommunication systems. Moreover its realization is simple and of reasonable cost according to the mentioned objects of the invention.

In the present description the invention has bee illustrated with reference to the construction of a laser device including an active glass medium 3 - using Erbiu (Er) as a dopant and Ytterbium (Yb) as a co-dopant pumpe by a laser 1 at about 940 - 980 nm - in order to obtain device suitable to operate in connection with the so-calle third window.

However the invention may be applied in othe situations and especially in combination with an activ medium consisting of a vitreous matrix realized wit silicates, phosphates and fluozirconates doped wit elements different from those specified above. Moreover th invention may be used in combination with an active mediu consisting of a crystal matrix this one also doped with elements different from those specified before.

In particular the above-said vitreous matrix may use for example: A) Ytterbium as co-dopant and Erbium or Praseodymium as dopants, Yb3 + (Er, Pr);

B) Aluminium as co-dopant and Erbium as dopant, A13 +

(Er);

C) Lanthanium as co-dopant and Erbium as dopant, La3 + (Er);

D) Chrome as co-dopant and Erbium, Thullium or Holmium as dopants, Cr3 + (Er, Tm, Ho);

E) Mercury as co-dopant and Neodymium as dopant, Mg2 +(Nd); F) Thullium as co-dopant and Holmium as dopant, Tm3 + (Ho); G) Germanium as co-dopant and Erbium or Praseodymium as dopants, Ge4 + (Er, Pr).

Attention is pointed out to the fact that the following points are of particular interest for the telecommunication sector:

- the use of the dopants and co-dopants as shown at point A, especially Praseodymium and Neodymium , for the realization of a laser suitable to emit radiations at a wavelength of 1.3 μ , the so-called second window;

- the use of dopants and co-dopants as mentioned at point D, especially the dopants Tm and Ho, for the realization of a laser suitable to emit radiations at a wavelength of about 2 μ , destined to be used in combination with fibres for the MIR applications, that is in combination with fibres operating in the infrared mediu and therefore presenting an attenuation which is about two orders smaller compared to the current fibres.

The crystal matrix instead may for example use as dopants and co-dopant: Er:CaF2 or Er:LiYF4 for the realization of a lase suitable to emit radiations at a wavelength of 1.54 μ , th so-called third window;

Nd:YAG or Nd:YLF for the realization of a lase suitable to emit radiations at a wavelength of 1.3 μm, the so-called second window.

Although the invention has been illustrated referrin to a preferred type of realization it will be understood by those skilled in the art that various modifications an changes may be made to the present invention without deporting from the scope and spirit thereof.

Claims

1. Multi-mode laser device for the emission of a plurality of equi-distributed and substantially stable frequencies for the transmission in optical fibre of the type including: a pump laser (1) suitable to emit a radiation at a first wavelength λl; a resonating cavity (2) having one end optically connected to the pump laser (1) ; an optical fibre rod (6) coupled to the other end of the resonating cavity (2); characterized in that said resonating cavity (2) consists of a lengthened structure including: - a portion of active medium (3) suitable to emit a radiation at a second wavelength λ2, pumped by said pumping laser (1) ; a first mirror (4) highly transparent at the wavelength λl and highly reflecting at the wavelength λ2 set up on the surface of the active medium facing the pump laser (1) ; a second mirror (5) highly transparent at the wavelength λ2 set up on the other surface of the active medium (3) facing the optical fibre rod (6) ; - means (7, V, R) suitable to vary the dimensions of said resonating cavity (2). (Figure 1).

2) Device according to claim 1, characterized in that between said pump laser (1) and the cavity (2) is put a first optical guide segment (3). (Figure 3). 3) Device according to claim 2, characterized in that between said resonating cavity (2) and the optical fibre (6) a second focusing optical guide segment (31) is put. (Figure 3) .

4) Device according to the claims 2 and 3, characterized in that said first and second optical guide segments consist of optical guides with graded refraction index (30, 31) and they present lengths comprised between P/4 and P/2, P being the pitch of the focusing optical guide segment.

5) Device according to the claims 1 to 4, characterized in that between said portion of active medium

(3) and said second mirror (5) an optical tuning guide (40) is put, and in that the opposite faces of said portion of active medium (3) and said portion of optical tuning guide turn out to be separated by an interspace and that they are provided with antireflection coatings (41 and 42, respectively). (Figure 4).

6) Device according to claim 5, characterized in that the second face of said optical tuning guide (40) is flat and presenting said second mirror (5) set up on this surface.

7) Device according to claims 5 and 6, characterized in that said portion of active material (3) and /or to said optical tuning guide (40) means are connected suitable to modify the dimensions of said interspace and consequently the dimensions of said resonating cavity. (Figure 4).

8) Device according to claim 5, characterized in that the second face of said active portion (3) is inclined compared to the optical axis (A) by an angle (ΘB) with a value equal to the Brewester angle. (Figure 9). 9) Device according to claim 5, characterized in that the second face of said optical portion (3) is inclined with respect to the optical axis (A) by angle (ΘR) with a value equal to about 1°. (Figure 8).

10) Device according to the previous claims, characterized in that said first face of the active medium

(3) is curved with its concavity facing the active medium.

11) Device according to the previous claims, characterized in that said first face of the active medium (3) is flat, and also in that said first portion of optical guide (3) consists of a further portion of optical fibre (6) . (Figure 6) . 12) Device according to claim 7, characterized in that said means to vary the dimensions of said interspace comprise a piezoelectric element (7) fed with an adjustable voltage (R, V). 13) Device according to claim 8, characterized in that the axis of said optical tuning guide (40) coincides with the axis of said active portion (3) , and in that said optical fibre rod (6) moves with respect to the axis of said second portion of focusing optical guide (31) of one entity (1) which is function of the angle (ΘD) of which deviate the optical signals because of the inclination of the second face of said active portion (3) according to the Brewester angle. (Figure 9).

14) Device according to claim 8, characterized in that the axis of said optical tuning guide (40) and of said second portion of the focusing optical guide (31) is inclined with respect to the axis of said active portion (3) by an angle equal to the angle (ΘD) of which deviate the optical signals because of the inclination of the second face of said active portion according to the Brewester angle, and said optical fibre rod (6) is set up on the prolongation of the axis of said second focusing optical guide portion (31). (Figure 10).

15) Device according to claim 13, characterized in that between said active portion (3) and said optical tuning guide a wedge-shaped portion (90) of glass is inserted. (Figure 9).

16) Device according to claim 15, characterized in that said wedge-shaped portion (90) consists of active material. (Figure 9).

17) Device according to the claims 1 - 16, characterized in that said active medium consists of a vitreous Er:Yb doped matrix and that λ2 is comprised between 940 and 980 nm. 18) Device according to the claims 1 to 16, characterized in that said active medium consists of vitreous Yb3 + (Pr) doped matrix and that λ2 is comprise between 940 and 980 nm.

19) Device according to the claims 1 to 16, characterized in that said active medium consists of a vitreous Cr3 + (Tm, Ho) doped matrix and that λ2 is about 780 nm.

20) Device according to the claims 1 to 16, characterized in that said active medium consists of a crystal Erbium doped matrix, Er:CaF2. 21) Device according to claims 1 to 16, characterized in that said active medium consists of a crystal Erbium doped matrix, Er:LiYF4.

22) Device according to claims 1 to 16, characterized in that said active medium consists of a crystal Neodymium doped matrix, Nd:YAG.

23) Device according to claims 1 to 16, characterized in that said active medium consists of a crystal Neodymium doped matrix, Nd:YLF.

24) Device according to claims 1 to 7, characterized in that it is supported by a submount (51) set up by a body of lengthened shape providing at its end for a "V" groove in which said first portion of optical fibre (6) is allocated and blocked, and between said raised portion (52) and said "V" groove it is provided with a plurality of seat suitable to receive said first portion of optical guide (30), said active portion (3) and said second portion of optical guide (31). (Figure 5).

25) Device according to claim 11, characterized in that said lengthened structure is allocated inside the female portion (70) of a connector for optical systems while one end of said portion of optical fibre (60) is allocated inside the male portion (71) of said connector and the other end is optically connected to the pumping laser (1) being positioned on a submount (73) allocated inside a box-shaped body (72). (Figure 7).