WO2008119363A1

WO2008119363A1 - External cavity laser module comprising a multi-functional optical element

Info

Publication number: WO2008119363A1
Application number: PCT/EP2007/002941
Authority: WO
Inventors: Paolo Zago; Alessio Pirastu
Original assignee: Pirelli & C. S.P.A.
Priority date: 2007-04-02
Filing date: 2007-04-02
Publication date: 2008-10-09
Also published as: CA2732912C; EP2186171A1; CA2732912A1

Abstract

An external cavity laser module (1) configured to emit an output optical beam (32) along a first output optical axis (X) is described. The laser module (1) comprises a gain medium (13) including a front facet (24) serving as an end mirror of the external cavity, a back facet (25) opposite to said front facet (24) and a bent waveguide. The gain medium (13) is configured to emit an intra-cavity optical beam into the external cavity along an intra-cavity optical axis (26) and to couple a front facet optical beam out of the external cavity from sthe front facet (24) along a second output optical axis (31 ), the intra-cavity optical axis (26) the said second output optical axis (31) forming a gain medium angle α not smaller than 10°. The laser module (1) also includes a multi-functional optical element (27) arranged outside the external cavity along the second output optical axis (31), the multi-functional optical element having an input facet (40) partially reflective so as to reflect a first portion of the front facet optical beam and an exit facet (41) through which the remaining portion of the front facet optical beam is transmitted, the transmitted portion giving rise to the output optical beam (32) along the first output optical axis (X), wherein the input (40) and exit facets (41) form an optical element angle γ comprised between 5° and 45° and wherein an incidence angle β between the second output optical axis (31) and an axis (N) normal to the input facet (40) is comprised between 30° and 70°.

Description

External cavity laser module comprising a multi-functional optical element Technical field

The present invention relates to an external cavity laser module including, as gain medium, a semiconductor gain chip with a bent chip waveguide, and a multi-functional optical element which splits, deviates and shapes the beam emitted by the gain chip to obtain an output laser beam with reduced ellipticity substantially parallel to the optical bench and a monitoring test beam. The configuration of the multi-functional optical element enhances the degree of freedom in the gain chip's positioning. Technological background The use of lasers as tuneable light source can greatly improve the reconfigurability of wavelength division multiplexed (WDM) systems or of the newly evolved dense WDM (DWDM). For example, different channels can be assigned to a node by simply tuning the wavelength. Also, tuneable lasers can be used to form virtual private networks based on wavelength routing. Different approaches can be used to provide tuneable lasers, distributed Bragg reflector lasers, Vertical Cavity Surface Emitting Lasers (VCSELs) with a mobile top mirror, or external cavity diode lasers. External-cavity tuneable lasers offer several advantages, such as high output power, wide tuning range, good side mode suppression and narrow linewidth. In telecommunication applications, external cavity lasers generally include, as gain medium, a semiconductor gain chip, such as an InP Fabry-Perot diode laser, which represents a good compromise between the frequency bandwidth of interest, efficiency and costs.

Additionally, generally the semiconductor gain chip comprises a bent waveguide, i.e. a waveguide defining a curved path for the transmitted light, so that the optical beam exits at an angle with respect to the front facet of the gain chip, in order to minimize back reflections. An example of an external cavity laser is given in the International application WO 2005/041372 in the name of the Applicant.

In "Automated Optical Packaging Technology for 10 Gb/s Transceivers and its Application to a Low-Cost Full C-Band Tunable Transmitter", written by Marc Finot et al., and published in the Intel Technology Journal, Volume 8, Issue 2, 2004, pages 101-114, a temperature-tuned external cavity laser is disclosed. The laser includes a bent gain chip and an optical etalon composed by two thermally tuned Si filters. In addition, the laser includes collimating lenses and a prism that sends a small fraction of the output beam power to a monitor photodiode. External cavity lasers generally comprise a beam splitter in order to measure the beam power of the output beam, for example by means of a photodetector.

In the Japanese Patent Application JP 5157910, a beam splitter is described for application in an optical recorder. The beam splitter decreases the number of parts constituting the optical pickup of an optical recorder, can be miniaturized and facilitates assembly at the time of production. The beam splitter construction is the following: a polarized light separating film for transmitting and reflecting the P polarized light and S polarized components of a laser beam respectively at prescribed ratios is formed by vapour deposition, etc., on the incident surface of a beam shaping prism for converting the luminous flux having an elliptical sectional shape entering from a laser diode into a flux having circular shape. Summary of the invention The present invention relates to an external cavity laser module, in particular a laser for telecommunication applications.

The laser of the invention comprises a gain medium, such as a semiconductor gain chip, emitting an optical beam. The gain medium comprises a back facet and a partially reflective front facet, opposite to the back facet and defining a first end mirror of the laser external cavity. The gain medium comprises a bent waveguide so that in the gain medium the light follows a curved waveguide path in order to reduce unwanted back-reflections in the output beam. In more detail, the gain medium emits an optical beam from the back facet into the external cavity towards a second end mirror arranged along an intra-cavity optical axis, corresponding substantially to the propagating direction of the beam emitted from the back facet. The external laser cavity is thus delineated between the partially reflective front facet of the gain medium and the second end mirror. The distance between the front facet of the gain medium and the second end mirror defines the length of the external cavity. To minimize light that is internally reflected, the back facet of the gain medium is preferably treated by an antireflection (AR) coating.

Given the partial reflectivity of the front facet of the gain medium, a second optical beam is emitted from the front facet along an output optical axis, which is the axis of the optical beam coupled out the laser cavity from the gain medium, in the following referred also to as the front facet output optical axis.

Due to the bent waveguide structure of the gain medium, the optical axis of the beam emerging from the back facet, i.e., the intra-cavity optical axis, forms an angle with the beam exiting the front facet, called in the following (gain medium) angle α. The value of α is generally fixed, within a given fabrication tolerance, by the gain chip manufacturer. In particular, the selection of the angle α between the two beams depends on the desired operating wavelength range of the gain medium, such as the C-band or the L-band, which are of interest for telecommunication applications.

The optical elements forming the external cavity have to be carefully positioned and aligned, for example through active optical alignment, due to minimization of optical losses and to mechanical constraints. The cavity length cannot be freely varied because any variation of the physical length of the laser cavity changes the free spectral range of the laser external cavity.

According to the invention, the external cavity laser module comprises a multi-functional optical element, which is arranged outside the external cavity so as to receive the laser beam emitted from the front facet of the gain medium, i.e., it is arranged along the front facet output optical axis. The multi-functional optical element shapes, deflects and splits the beam outputted by the gain medium as detailed below.

The laser beam exiting the multi-functional optical element defines the output laser beam of the external cavity laser along a laser output optical axis. The output laser beam is then preferably coupled to an optical waveguide, typically an optical fibre, to which the laser power is transferred.

Preferably, the laser output optical axis coincides with the optical fibre longitudinal axis in order to minimize coupling losses. In the gain medium comprised in the external-cavity laser module of the invention, the gain medium angle α is larger than, or equal to, 10° since smaller angles would not generally guarantee the expected chip performances in terms of reflectivity reduction. Preferably, the external cavity laser is a tuneable laser comprising an element tuneable in wavelength across a wavelength range, such as the C-band (1520-1570 nm). According to a preferred embodiment, the tuneable element is a tuneable mirror serving as second end mirror of the external cavity.

Although a preferred embodiment of the present invention is an external cavity laser comprising a tuneable mirror serving as end mirror, the present invention also envisages, among others, external cavity lasers comprising an intra-cavity tuneable element, such as a wedge shaped etalon, and an end mirror.

According to a preferred embodiment, the optical elements comprised within the external cavity laser, e.g. the gain medium and the end mirror, are mounted on a common platform, also referred to as the optical bench. More preferably, also the multi-functional optical element is placed on the same platform. Preferably, all the described beams, i.e. the beams emitted by the front and back facet of the gain medium and the output laser beam that has been transmitted through the multi-functional optical element lie on planes substantially parallel to the plane defined by the optical bench. Preferably, the external cavity laser is housed in a package and the laser output optical axis corresponds to the main longitudinal axis of the package itself, which is preferably parallel to or coincides with the main optical axis of the optical fibre optically coupled to the package. Preferably, the external cavity laser module further comprises a photodetector, preferably placed outside the external cavity. In an external cavity laser, and even more in a tuneable laser, it is desired to monitor the output power of the beam emerging from the gain medium in order to control the laser stability and/or to maintain the phase of the laser cavity by means of feedback algorithms. The laser output power can be measured by means of the photodetector. In order to perform the monitoring of the laser output power, the photodetector has to be placed in proximity to the gain medium front facet. To this end, the multi-functional optical element is configured to reflect a portion of optical beam exiting the front facet of the gain medium. The Applicant has found that the angle between the front facet output optical axis and the axis of the deflected beam toward the photodetector should be properly selected, as better outlined below. It is preferred that only a small portion of the beam intensity, i.e., not larger than 4% of the total beam intensity, is spilled from the output beam so as not to penalise the output power.

The Applicant has observed that semiconductor gain chips generally used for external cavity lasers for optical communication systems emit a divergent laser beam, i.e., a beam having an elliptical cross-section. The ellipticity of a laser beam can be defined by the ratio between two orthogonal axes defining the cross-section of the optical beam emitted by the gain chip. The first axis is the lateral mode field diameter, MFD(X), of the beam cross-section along a direction lying on a plane parallel to the surface of the substrate, e.g., the optical bench, on which the laser chip is placed. The second axis is the transverse mode field diameter, MFD(Y), of the beam cross-section along a direction perpendicular to the first axis and perpendicular to the substrate. Typically, depending on the gain chip, the ellipticity, i.e., MFD(X)/MFD(Y), can vary between 0.4 and 1. The value of 1 corresponds of course to a circular beam cross-section. The two mode field diameters depend on the divergence angles along the lateral and transversal directions. The range of the two divergence angles (or the range of the lateral and transversal mode field diameters) is furnished by the manufacturer of the gain chip: a minimum and maximum transverse divergence angle and a minimum and maximum lateral divergence angle are generally given, between which lie the values of the divergence angles of the gain chip used.

In the technical field of optical telecommunication systems, it is often required to build optoelecronic components as small as possible so that they fit in standard packages, such as butterfly packages. This requirement affects the laser design: the number of optical elements used in a laser module and an appropriate choice of their location within the package are selected in such a way that the whole available space is optimally occupied while keeping assembly costs and complexity relatively low. Applicant has noted that, in order to obtain a relatively short external cavity laser, e.g., 8-13 mm of physical cavity length, without decreasing the aligning tolerances, the gain medium and the other optical elements comprised within the laser cavity are preferably arranged along an intra- cavity optical axis that is inclined, i.e. it forms an acute angle e, with respect to output laser beam coupled into the optical fibre, the latter coinciding - as mentioned above - in most cases with the package main longitudinal axis. The Applicant has noted that this layout realizes a good space occupation.

At the same time it is desired not to introduce several additional optical elements, in order to reduce the physical encumbrance of the laser module. According to the invention, the multi-functional optical element, onto which the beam emitted by the gain medium front facet impinges, serves as a prism since it comprises two facets forming an angle γ therebetween and deflects the incident optical beam from the initial beam direction (i.e., the front facet output optical axis) by a given deflection angle. Two distinct planes are defined by the two facets, which are both perpendicular to the optical bench, which is taken as a reference substrate on which the multi-functional optical element is arranged. The angle of deflection is advantageously selected so as the optical beam exiting the multi-functional optical element propagates substantially along the main optical axis of the package. The deflection angle is determined by the angle γ between the two facets of the optical element, by the incidence angle of the beam onto the multi-functional optical element facet and by the material in which the multi-functional optical element is realized.

The first facet of the multifunctional optical element is at least partially reflective, and its reflectivity preferably ranges between 2% to 4%, more preferably is of about 3%. The second facet is anti-reflective, for example by means of an anti-reflection (AR) coating, and has preferably a residual reflectivity of the order of 0.1. The first facet of the multi-functional optical element intercepts the front facet output beam and, due to its reflectivity, is apt to reflect part of the optical beam towards a photodetector. Due to the physical dimensions of the photodetector and to the optical characteristics of the beam, an angle β between the front facet output optical axis and the normal to the first facet of the multi- functional optical element should be properly selected. The angle β should be comprised between 30° and 70°.

The Applicant has found that the angle β should be preferably selected to be comprised between 42° and 50°. A wider angle would require a relatively large first facet in order to obtain an incidence of the whole beam on the first facet itself (the beam has a finite size) and a narrower angle would require a location of the photodetector within the external cavity or too close to the gain medium. The multi-functional optical element thus acts as a beam splitter reflecting and dividing a part of the beam towards a photodetector. According to the invention, a degree of freedom is available for the external cavity laser design: the positioning of the gain medium, and therefore the angle e of the intra-cavity optical axis with respect to the output beam main axis, can be suitable selected within a certain range of values. Depending on the desired angle e, the front facet output optical axis, which depends on a that is fixed for a given gain chip, is then determined and thus the angle y is then also determined in such a way that the beam emerging from the multi-functional element is aligned with the main longitudinal axis of the package.

The Applicant has observed that in order to achieve a good coupling efficiency between the laser output beam and the output optics, such as an optical fibre, along which the optical laser power should be transmitted, ellipticity of the laser output beam should be minimized. The Applicant has found that a re-shaping of the output laser beam aimed to a decrease in beam ellipticity can produce a decrease in optical coupling losses up to 0.5 dB. In other words, the optical power coupled from the laser to the optical fibre (or an optical waveguide) can gain up to 0.5 dB when the beam cross-section is nearly circular. It is to be observed that the increase in optical coupling efficiency could also allow employing a transmitter in optical communication systems with output power of for instance 12.5 dB instead of 13 dB, thereby increasing the yield of external-cavity lasers and thus decreasing manufacturing costs.

Applicant has examined some of the manufactured gain chips available on the market and considered their range of divergence angles. Applicant has found that the angle y between the two facets of the multi-functional optical element should be comprised between 5° and 45°, preferably 5° and 20°, more preferably between 7° and 20°, in order to obtain a correct realignment of the output laser beam and to increase the optical coupling efficiency between the laser output beam and the optical fibre.

With 5° ≤y <20°, the ellipticity of the laser beam emerging from the gain chip is modified in such a way that the coupling losses are reduced for the majority of the possible ellipticity values (within the manufacturing tolerance) for gain chips typically used in external-cavity lasers for telecommunications and for the particular laser designs of interest. Even for a circular beam emerging from the gain chip, the modification induced by the multi-functional optical element would not significantly affect the coupling efficiency. The multi-functional optical element acts as a beam shaper modifying the ellipticity of the beam. More in detail, the multi-functional optical element modifies the beam dimensions along the two orthogonal axes defined by a plane perpendicular to the optical bench intersecting the beam. Preferably, the multi-functional optical element leaves the lateral dimension unaffected and modifies only the transverse one. The multi-functional optical element is preferably made of a low dispersion material, i.e., a material having a refractive index that changes of a relatively small amount within the wavelength range of interest. A dispersion of 0.05% within the wavelength range of interest is preferred. As an example, a suitable material is the E-F2 glass as defined in the Hoya catalogue. When the angles a, β, y have been selected, the angle δ between the front facet output optical axis and the main output optical axis is given by

δ *

where n is the refraction index of the material in which the multi-functional optical element is realized and the angle e between the intra-cavity optical axis and the main output optical axis is given by e = δ + a - n. The multi-functional optical element also allows a relaxation in the alignment tolerances of the optical elements forming the external cavity laser. Brief description of the drawings

Further features and advantages of an external cavity laser module comprising a multi- functional optical element according to the present invention will become more clear from the following detailed description thereof, given with reference to the accompanying drawings, where:

- fig. 1 is a schematic lateral view of a preferred embodiment of the external cavity laser module of the present invention; - fig. 2 is a schematic top view of the external cavity laser module of fig. 1 ;

- figs. 3a and 3b represent a perspective view and a schematic lateral view, respectively, of a multi-functional optical element included in the external cavity laser module of figs. 1 and 2;

- fig. 4 is a graph showing the relationship between the angle y between the two facets of the multi-functional optical element of fig. 3a as a function of the angle β between the gain medium front facet output optical axis and the axis normal to the first facet of the multi-functional optical element of fig. 3a, for a fixed δ (solid curve). For each angles pair, the corresponding modification in the beam dimension is visualized on the same graph (dotted curve); - fig. 5 is a graph showing the losses "saved" modifying the beam shape of the front facet emerging beam using the multi-functional optical element of fig. 3a as a function of the angle β. For the selected β value, the multi-functional optical element selected to modify the beam shape has an angle y corresponding to the selected β as given by the graph of fig. 4; - fig. 6 is a graph showing the variations in an angle δ between the front facet output optical axis and the main output laser beam axis as a function of the variations of the angle β; fig. 7 is a graph showing the variation in the refractive index of a preferred embodiment of the material forming the multi-functional optical element as a function of the wavelength of the incident light. Preferred embodiments of the invention

Laser assemblies are typically housed in a package that protects the laser components and other electronic or thermoelectric components associated to the laser assembly from the external environment. External cavity lasers for telecommunications are generally housed in hermetically sealed packages so as to allow the laser assembly to be sealed within an inert atmosphere to prevent contamination/degradation of the optical surfaces of the various components of the laser. According to a preferred embodiment of the present invention, the external cavity laser module is tuneable in wavelength across an operating wavelength range, such as the C-band (1520- 1570 nm).

A side view of a tuneable external-cavity laser module according to a preferred embodiment of the present invention is schematically depicted in Fig. 1 (not to scale). The laser module 1 comprises an external cavity laser assembly housed in a package, e.g., a butterfly package, which defines an enclosure 7. The package includes a boot 17 for the insertion of an optical fiber, i.e., fiber pigtail 22. A glass window 23 closes up hermetically the laser assembly from the boot for fiber insertion. The laser assembly includes a gain medium 13, a collimating lens 3, a grid generator 4, a deflector 6 and a tuneable mirror 8. The gain medium 13 is based on a semiconductor laser chip, for example an InGaAs/lnP multiple quantum well Fabry-Perot (FP) gain chip especially designed for external-cavity laser applications. The diode comprises a back facet 25 and a front facet 24. The diode's back facet 25 is an intracavity facet and is treated with an anti-reflection (AR) coating. The gain chip comprises an active layer with a waveguide that is bent (shown schematically in fig. 2) so that it has an angled incidence on the front facet in order to further reduce back reflections. The front facet 24 is partially reflective and serves as one of the end mirrors of the external cavity. The reflectivity of the front facet can range for instance of about 10% in order to allow a relatively high laser output power. The tuneable mirror 8 functions as end mirror of the laser external cavity, which has a physical length defined by the partially reflecting front facet 24 of the laser chip 13 and the tuneable mirror 8.

The external cavity laser assembly is placed on an optical bench or platform 10, which functions also as mechanical reference base for the optical elements. The use of a common optical bench is preferred because it minimises the design complexity and simplifies the alignment between the components of the tuneable laser. The platform 10 is made of a thermally conductive material, such as aluminium nitride (AIN), silicon carbide (SiC) and copper-tungsten (CuW). Optical bench 10 is placed on a TEC 11 , e.g. it is glued or soldered on the (upper) thermally stabilised surface of the TEC. The grid generator 4 is preferably a Fabry-Perot (FP) etalon, which is structured and configured to define a plurality of equally spaced transmission peaks. In applications for WDM or DWDM telecommunication systems, transmission peak spacing, i.e., the free spectral range (FSR) of the grid element, corresponds to the ITU channel grid, e.g., 50 or 25 GHz. The laser can be designed in such a way that the operating wavelengths are aligned with the ITU channel grid. In order to stabilise its temperature, the FP etalon 4 is preferably housed in a thermally conductive housing 5 to promote thermal contact with the platform 10.

Temperature monitoring of the platform 10 is provided by a thermal sensor device 12, such as a thermistor, which is placed on the platform and is operatively coupled to the TEC 11 so as to provide control signals to cool or heat the surface of the TEC in thermal contact with the platform 10, and thus to heat or cool platform 10 in order to maintain an approximately constant temperature, T₁, e.g., T₁=SO⁰CiCI ⁰C. In the embodiment of Fig. 1 , the thermal sensor device is placed in proximity of the FP etalon 4, for control of its thermal stability.

The gain chip 13 is preferably placed, e.g., by bonding, on a thermally conductive submount 21 so as to position the emitted beam at a convenient height with respect to the other optical elements and to further improve heat dissipation. The thermally conductive submount 21 , made for instance of SiC, is placed on the optical bench 10.

Within the laser cavity, the beam emerging from the laser chip back facet 25, which propagates along an intra-cavity optical axis 26, is collimated by collimating lens 3. After the FP etalon 4, the laser beam strikes a deflector 6 that deflects the beam propagating along the intra-cavity axis 26 onto the tuneable mirror 8 along optical path 29. The tuneable mirror 8 reflects the light signal back to the deflector 6, which in turn deflects the light signal back to the gain medium 13. The deflector 6 is in this embodiment a planar mirror, for instance a gold-coated silicon slab.

According to the embodiment illustrated in Fig. 1 , the external laser cavity is a folded resonant cavity having an optical path length, which is the sum of the optical path 26 between the partially reflective front facet 24 of the gain medium 13 and the deflector 6 and the optical path 29 between the deflector and the tuneable mirror 8. Although not shown in Fig. 1 for sake of clarity, the deflector can be secured in the cavity for instance by means of a support structure that is fixed to the platform 10. Examples of supporting structures for the deflector are described for instance in WO patent application No. 2006/002663. Preferably, the deflector is aligned to the laser beam by means of active optical alignment techniques. The tuneable mirror 8 is preferably an electro-optic element, in which tuneability is achieved by using a material with voltage-dependent refractive index, preferably a liquid crystal (LC) material. For example, the tuneable mirror is that described in WO patent application No. 2005/064365. The tuneable mirror is driven with an alternating voltage to prevent deterioration of the liquid crystal due to dc stress. The tuneable mirror serves as the coarse tuning element that discriminates between the peaks of the FP etalon. The FWHM bandwidth of the tuneable element is not smaller than the FWHM bandwidth of the grid etalon. For longitudinal single-mode operation, the transmission peak of the FP etalon corresponding to a particular channel frequency should select, i.e., transmit, a single cavity mode. Therefore, the FP etalon should have a finesse, which is defined as the FSR divided by the FWHM, which suppresses the neighbouring modes of the cavity between each channel. For single-mode laser emission, a longitudinal cavity mode should be positioned over the maximum of one of the etalon transmission peaks (the one selected by the tuneable element). In this way, only the specified frequency will pass through the etalon and the other competing neighbouring cavity modes will be suppressed.

In the preferred embodiments, the laser assembly is designed to produce substantially single longitudinal and, preferably, single-transversal mode radiation. Longitudinal modes refer to the simultaneous lasing at several discrete frequencies within the laser cavity. Transversal modes correspond to the spatial variation in the beam intensity cross section in the transverse direction of the lasing radiation. Generally, an appropriate choice of the gain medium, e.g., a commercially available semiconductor laser diode including a waveguide, guarantees single spatial, or single transversal, mode operation. The laser is operative to emit a single longitudinal mode output, which depends on the spectral response of the optical elements within the cavity and on the phase of the cavity.

The tuneable mirror 8 lays substantially horizontally with respect to the principal surface plane of the thermally conductive platform 10 (e.g., it can be glued or soldered to the upper surface of the platform) in order to maximise thermal contact with the TEC. The FP etalon 4 and the tuneable mirror 8 are mounted on the surface region of the optical bench 10 placed above the TEC 11 in order to minimise the thermal resistance of the heat flow path. It is to be understood that the present invention envisages also an external cavity laser wherein the tuneable mirror or in general an end mirror is mounted vertically with respect to the optical beam. The laser beam is coupled out of the external cavity by the front facet 24 of the laser chip 13. Preferably, a collimating lens 14 can be placed along the optical path 31 of the laser beam output from the front facet 24 of the gain medium 13.

The laser 1 further comprises output coupling optics including a fiber focus lens 16 which directs the output light 32, which has passed through an optical isolator 15, into fibre pigtail 22. The direction of the light 32 going into the fiber pigtail 22 defines the output axis X of the output beam 32. Optical isolator 15 is employed to prevent back-reflected light being passed back into the external laser cavity and is generally an optional element. The output coupling optics further comprises a multi-functional optical element 27 which is placed after lens 14. The optical element 27, shown in detail and in an enlarged scale on figures 3a and 3b, has a prism-like shape and comprises a first and a second facet 40, 41 , forming an angle y between them. The two facets 40 and 41 lie on two distinct planes perpendicular to the optical bench 10. The first facet 40, on which the laser beam 31 emitted from the gain chip front facet 24 is incident, is partially reflective with a reflectivity of 2 % ± 0.1 and it picks off a portion of the output light propagating along optical path 31 from the gain chip 13 as a test beam (in fig. 2 the axis 33 along which the test beam propagates is shown), which is directed to a photodiode 28 for power control. The output power is monitored by monitoring the photodiode current which is proportional to the laser beam power. The optical element 27 and the photodiode 28 can be placed on a common submount 43 shown in figs. 2, and 3a or on two different submounts (not shown) that are placed on platform 10.

With now reference to fig. 2 where the external cavity laser module 1 of fig.1 is depicted in a simplified top view and the beam directions within the laser module are schematically shown. For sake of clarity, the package enclosure and fibre pigtail 22 are not shown. The incident angle formed between an axis N normal to the first facet 40 surface and the incident beam propagating along the front facet output optical axis 31 is indicated with β. The Applicant has found that the angle β should be preferably comprised between 42° and 50° by taking into account the finite size of the incident beam, the dimensions of the facet itself and the encumbrance of the photodiode.

The second facet 41 of the optical element 27 has an anti-reflective coating and the reflectivity is < 0.2 % across the wavelengths range of interest. Preferably, said wavelengths range of interest is the C-band (1525 nm to 1565 nm) or the L- band (1570 nm to 1610 nm). The incident beam propagating along the front facet output optical axis 31 and impinging on the multi-functional optical element 27 is then transmitted through the second facet 41 and deflected by given angle, which is a function on the angle y between the first 40 and the second facet 41 , the refractive index n of the optical element and the angle of incidence of the beam. The beam 32 transmitted through the optic element 27 defines the output beam of the laser system, which propagates along the main longitudinal axis X of the optical bench 10. As shown in fig. 2, the main output optical axis X of the output beam forms a first angle δ with respect to the direction of the beam propagating along the front facet output optical axis 31. A second angle ε is formed between the main optical output axis X and the beam emitted by the back facet 25 of the gain medium 13 and propagating within the external cavity along the intra- cavity optical axis 26. The Applicant has noticed that, if the intra-cavity optical axis 26 is tilted with respect to the optical output axis X, an optimal space occupation within the package 7 can be achieved. The angles ε and δ are determined by the incidence angle β on the multi-functional element and the angle γ between the two facets 40, 41 of the optical element 27.

The material in which the multi-functional optical element 27 is realized is preferably non dispersive, to avoid a variation in the refractive index and thus a dependence of the output beam direction on the wavelength of the incident beam. Preferably, the material in which the multi-functional optical element is made is an E-F2 glass, as described in the Hoya catalogue. The refraction index n of this material varies as a function of the wavelength of the incident light as plotted in the graph of fig. 7. As shown, the refractive index n changes from 1.5945 to 1.5938 in the operative range of 1520-1570 nm. An angle α is defined as the angle formed between the two beams emerging from the front and back facet of the gain medium 13. This angle is generally fixed for a given type. According to a preferred embodiment of the invention, the gain medium is a semiconductor gain chip having an angle α of 19.5°. Any chip angle, as long as a ≥ 10°, can however be used in the gain medium 13 comprised in the laser module 1 of the invention. Preferably, the aηgle α is smaller than 35°. The front facet output beam propagating along the front facet output optical axis 31 is a divergent beam. Applicant has considered the ranges of divergence angles of the emitted beams of some of the commercially available semiconductor laser chips, as indicated in table 1. Thus, the front facet output beam 31 generally exhibits a cross-sectional elliptic shape. The mode field diameter (MFD) of the beam in cross section has a first value (MFD(Y)) along the direction parallel to the main direction of the optical bench 10 (Y direction) and a second mode field diameter (MFD(Z)) along a direction orthogonal to the bench (Z direction).

Table 1

The multi-functional optical element 27 reshapes the beam propagating along the front facet output optical axis 31 by modifying the ratio between the two diameters. Preferably, the element is configured to modify the diameter of the beam along the Y axis while leaving the MFD along the Z axis substantially unaffected.

The variation in MFD(Y) induced by the multi-functional optical element 27 depends on the angle γ between the two facets 40, 41 and on β.

For a selected angle δ (in the example δ = 167.5°), the graph of fig. 4 reports computer simulations showing the angle γ as a function of the incidence angle β (left scale; filled diamonds connected with a dashed line).

From the curve plotted in fig. 4, the range of interest for the incidence angle β results to correspond to a range of the multi-functional optical element apertures, γ , from about 7° to 20°.

For different δ values, curves that are qualitatively similar to the one of fig. 4 can be obtained.

For values of angle δ ranging from 150° to 170°, the Applicants have found that the prism aperture γ should be selected between about 5° and 45°. In particular, if δ=150°, a suitable range of angle γ is comprised between about 30° and 45°, whereas for δ=170°, a suitable range of angle γ is comprised between 5° and 15°. On the same graph of fig. 4, the curve with filled squares connected by a solid line (right scale) represents the corresponding increase of the MDF(Y) for each selected pair of β and γ angles. In particular, the MDF(Y) percentage variation with respect to the mean MDF(Y) of the chip under consideration (chip 1 of table 1 ) is calculated. All simulations are performed considering a laser module 1 comprising a collimator (schematically depicted in fig. 1 with reference number 17) accepting optical beams having MFD of 450um +/- 30 μm. An isolator 15 can be part of the collimator (not shown in the figures) or can be placed within the package, as illustrated in the embodiment of fig. 2. The coupling efficiency between the output laser beam 32 and the fiber 22 is maximised for a circular cross-section of the beam. Therefore, an elliptic beam introduces coupling losses in the system. In fig. 5 several curves are plotted, each for a different value of the β angle, showing the "saved" losses, as detailed below. For each curve, a different multi-functional optical element 27 is considered in the calculations, i.e. in each curve, the multi-functional optical element has an angle γ between the facets given by the graph of fig. 4 corresponding to the selected value of β. Each curve represents the calculation, for different MFD(Y) of the input beam incident on the multi-functional optical element 27, of the losses "saved" due to the presence of the multifunctional optical element. In other words, the ordinate of the graph represents the coupling losses that should be added if the multi-functional optical element 27 were not modifying the incident beam cross section. The curves clearly shows that for the angles of interest in the preferred configuration of laser module, which is for 42° < β < 50°, and for MDF(Y) values within the range of interest (which is the range declared by the chip manufacturers, the chip used in the simulation is chip n. 1 of table 1 ), there is always a losses reduction due to the presence of the multi-functional optical element which modifies the MDF(Y) of the incident beampropagating along the front facet output optical axis 31. In fig. 6 a graph is shown representing the variations in the angle δ due to variations of the angle β. More in detail, the abscissa represents the variation from a selected β = 48° (thus β = 48° is the zero in the abscissa). It is clear from the graph that a misalignment in the optical element positioning, i.e. an erroneous positioning which results in a different β angle with respect to the selected one, results in a smaller variation in the angle δ which is relevant for the coupling losses. The multi-functional optical element thus relaxes the tolorance requirements for the alignment of the optical elements comprised in the laser cavity and in the output optics.

Claims

1. An external cavity laser module (1 ) configured to emit an output optical beam (32) along a first output optical axis (X), said laser module (1 ) comprising a gain medium (13) comprising a front facet (24) serving as an end mirror of said external cavity, a back facet (25) opposite to said front facet (24) and a bent waveguide; said gain medium (13) being configured to emit an intra-cavity optical beam into said external cavity along an intra-cavity optical axis (26) and to couple a front facet optical beam out of the external cavity from said front facet (24) along a second output optical axis (31 ), said intra-cavity optical axis (26) and said second output optical axis (31 ) forming a gain medium angle α,

- a multi-functional optical element (27) arranged outside the external cavity along said second output optical axis (31 ), said multi-functional optical element comprising an input facet (40) partially reflective so as to reflect a first portion of the front facet optical beam and an exit facet (41 ) through which the remaining portion of said front facet optical beam is transmitted, said transmitted portion giving rise to the output optical beam (32) along said first output optical axis (X), wherein said input (40) and exit facets (41 ) form an optical element angle γ comprised between 5° and 45° and wherein the multifunctional optical element is arranged so that an incidence angle β between said second output optical axis (31 ) and an axis (N) normal to said input facet (40) is comprised between 30° and 70°.

2. The external cavity laser module (1 ) of claim 1 , wherein the incidence angle β is comprised between 42° and 50°.

3. The external cavity laser module (1 ) of claims 1 or 2, wherein the optical element angle γ is comprised between 5° and 20°.

4. The external cavity laser module (1 ) of claim 2, wherein the optical element angle γ is comprised between 7° and 20°.

5. The external cavity laser module (1 ) according to any of the preceding claims, further comprising a photodetector (28) arranged so as to monitor the first beam portion that is reflected by the multi-functional optical element (27) and propagates along a test axis (33).

6. The external laser module (1 ) according to any of the preceding claims, wherein said laser module (1 ) further includes a tuneable element (8).

7. The external cavity laser module (1 ) according to claim 6, further including a channel allocation grid element (4) arranged in the external cavity to define a plurality of pass bands substantially aligned with corresponding channels of a selected wavelength grid.

8. The eternal cavity laser module (1 ) according to claim 7, wherein said channel allocation grid element (4) is a Fabry-Perot etalon.

9. The external cavity laser module (1) according to one of claims 6-8, wherein said tuneable element (8) is a tuneable mirror.

10. The external cavity laser module (1 ) of claim 9, wherein said tuneable mirror (8) is an external cavity end mirror.

11. The external cavity laser module (1 ) according to claim 1 , wherein the gain medium (13) and the multi-functional optical element (27) are placed on a common platform (10).

12. The external cavity laser module (1 ) of claim 11 , wherein said platform (10) is a thermally conductive platform.

13. The external cavity laser module (1 ) according to any of the preceding claims, wherein said multi-functional optical element (27) is realized in a material with a dispersion smaller than or equal to 0.05 % across a predetermined wavelength range.

14. The external cavity laser module (1 ) according to claim 13, wherein said predetermined wavelength range is comprised between 1525 nm and 1565 nm.

15. The external cavity laser module (1 ) according to claim 13, wherein said predetermined wavelength range is comprised between 1570 nm and 1610 nm.

16. The external cavity laser module (1 ) according to any of the preceding claims, wherein said exit facet (41) comprises an anti-reflective (AR) coating with a residual reflectivity smaller than or equal to 0.1 %.

17. The external cavity laser module (1 ) according to any of the preceding claims, wherein said partially reflective input facet (40) has a reflectivity comprised between 2% and 4%.

18. The external cavity laser module (1 ) according to any of the preceding claims, wherein said output optical beam (32) is optically coupled to an optical fibre (22) having a longitudinal optical axis (X) substantially coinciding with the first output optical axis (X).

19. The external cavity laser module (1 ) according to any of the preceding claims, wherein an angle δ between the second output optical axis (31 ) and the first output optical axis

(X) is given by

σ = π + γ - p + a si •n ( n • si ^■n ( a si ■n (^ή*(β —)λ - γ ^N

where n is the refraction index of the material in which the multi-functional optical element is realized, and an angle e between the intra-cavity optical axis (26) and the main output optical axis (X) is given by e = δ + a — π.

20. The external cavity laser module (1 ) according to claim 19, wherein the angle δ is comprised between 150° < δ < 170°.

21. The external cavity laser module (1 ) according to claim 19 or 20, wherein the angle ε is comprised between -20°< ε <20°.

22. The esternal cavity laser module (1 ) according to any of the preceding claims, wherein the gain medium angle is not smaller than 10°.