External cavity laser device, corresponding system and method

Abstract

A laser device (100) comprising: a source (S) of electromagnetic radiation comprising at least one reflecting surface (RS), said source (S) being configured to generate a light beam following an optical path (OPa; OP) outside said source (S); a dispersion table (6) being located outside said source (S) along said optical path (OP) of said light beam generated by said source (S), said dispersion table (6) comprising at least one reflection axis forming an angle (θ; φ) with said optical path (OPa; OP) of said light beam and being configured to reflect at least a first portion of said light beam generated by said source (S) towards said source (S) a spectral portion; and a second spectral portion of the light generated by a source (S) reflected along the reflection axis, wherein the at least one reflecting surface (RS) and the dispersion table (6) form at least one variable-length external optical cavity (RS, L, 6); at least one collimating lens (C) positioned along the optical path (OPa; OP) and configured to collimate the light beam from the source (S); a collimator module (3) in which the source (S) and the at least one collimating lens (C) are mounted; and an actuator (24) configured to change the length (L) of the variable-length external optical cavity (RS, L, 6). In the device: the actuator (24) is mechanically coupled to the collimator module (3); and the actuator (24) is configured to change the length (L) of the at least one external optical cavity of the variable-length gain medium (RS, L, 6) by moving the collimator (3).

Description

Technical Field

[0001] The present disclosure relates to external cavity laser devices. Examples of such devices may be, for example, external cavity lasers (ECLs) or long external cavity lasers (LECLs).

[0002] One or more embodiments may be applied in the context of atomic and / or molecular spectroscopy as well as in the context of metrology. Background Art

[0003] External cavity lasers (ECLs)—particularly those with diode lasers (called external cavity diode lasers (ECDLs))—are one type of coherent radiation source that can be stabilized in frequency.

[0004] For numerous and / or diverse applications, such as atomic and / or molecular spectroscopy and metrology, the above-mentioned lasers are required to:

[0005] - present mechanical strength characteristics that will facilitate its long-term use without requiring any intervention on the part of an external operator; and / or

[0006] - is tunable in frequency relative to a reference.

[0007] Radiation sources (also called gain media or light sources) used in this type of device include diode lasers, gain chips, as described above, but also other types of gain media, such as superluminescent LEDs (SLEDs), which are gain media configured to generate light via amplified spontaneous emission.

[0008] Both diode lasers and gain chips, as well as SLEDs, comprise, for example, a semiconductor gain medium provided with at least one reflective region, in particular, for example, a reflective surface on one of the two faces of the gain medium. This reflective region can be obtained, for example, by applying a reflective process or otherwise by (full or partial) internal reflection.

[0009] External cavity laser devices and corresponding tunable systems typically include:

[0010] - a source of electromagnetic radiation configured to generate and / or emit a light beam (referred to as "light source" or "gain medium" for short); as mentioned above, this source comprises, for example, a diode laser, an SLED or a so-called "gain chip"; in particular, these are configured to generate light mainly in the visible spectrum, the near ultraviolet, and the near and mid-infrared. Such a source is included in an external optical cavity marked by at least one reflective area, in particular a reflective surface, of

[0011] - a lens for collimating the light emitted by the source;

[0012] - spectrally selective optical elements that facilitate frequency selection of the light beam, such as filters or diffraction gratings, in order to obtain laser emission at a target frequency; and

[0013] - an output coupler comprising at least one reflecting element providing the light beam at the output; this element may also coincide with the spectrally selective optical element.

[0014] Thus, between such a source of electromagnetic radiation and the spectrally selective optical element, in particular between the reflecting region of the source and the spectrally selective optical element, an external optical cavity of variable length is defined, for simplicity, referred to as an "external cavity". The resonant modes of the external optical cavity are currently known as the longitudinal modes of the cavity.

[0015] Conventionally, there is at least one actuator, for example a piezoelectric actuator, which can be electrically controlled in order to enable the length of the external cavity to be varied. As discussed below, the emission frequency of the light of the laser is varied accordingly.

[0016] Various types of external cavity lasers are known and commonly used, which differ from one another, for example, with respect to different choices of output couplers and spectrally selective elements.

[0017] A first type of configuration (referred to as an "etalon-like" configuration) uses an interference filter as the spectrally selective device, while the output coupler comprises a mirror coupled to an actuator.

[0018] The use of a mirror makes the system mechanically stable and facilitates low sensitivity to misalignment, particularly if an additional lens is included to focus the beam onto the mirror. An interference filter (e.g., an etalon filter) is placed within the external cavity, ensuring that only a limited frequency range of incident radiation is transmitted. This range can be altered by varying the angle between the normal (i.e., perpendicular to the direction of the etalon interference filter) and the wave vector of the incident radiation, or by changing the temperature of the filter itself.

[0019] There are some disadvantages to using filters as spectrally selective (i.e., frequency selective) elements, among which are:

[0020] - Lack of temperature stability: in order to obtain emission in the same longitudinal mode over a long period of time, it is practically required to precisely control the temperature of the interference filter; and

[0021] - High absorption losses of the filter itself: Such losses can be offset by a higher reflectivity of the output coupler mirror, at the expense of a lower output power of the laser.

[0022] A second type of known configuration, known as the Littman-Metcalf configuration, uses one system as both spectral selection device and output coupler, consisting of:

[0023] - a mirror mounted on a purposely provided support, in which a piezoelectric actuator is provided; and

[0024] -Diffraction grating.

[0025] Compared to using interference filters, using a Littman-Metcalf configuration provides a reduced temperature dependence of the output beam's spectrum. In this type of configuration, the first diffraction order of the beam is sent to a mirror, which again reflects the radiation toward a grating, which in turn reflects it toward the source, thus forming an external optical cavity. However, due to the dependence of the grating's reflection angle on the wavelength of the radiation, only a limited range of spectral components (i.e., frequencies) participate in the lasing.

[0026] To change this range—and therefore the lasing wavelength—the angle between the diffraction grating and the wave vector of the incident radiation must be varied. This condition is achieved by rotating a rotatable arm of the mirror support, for example, about an external pivot point, to change the angle between the mirror and the diffraction grating. Output coupling is achieved via radiation from the laser reflected by the diffraction grating in the zeroth order.

[0027] In addition, in the Littman-Metcalf configuration, there are significant losses in the external cavity, mainly due to radiation reflected by the mirror and, in turn, by the grating in the zeroth order. Such losses, again as in the etalon-like configuration, lead to a reduction in the emitted power of the laser.

[0028] In a third type of configuration, known as the Littrow configuration, a diffraction grating is used as both an output coupler and a spectrally selective element, as described below with respect to Figure 1 discussed.

[0029] In particular, Figure 1 In part a) of FIG. 1 , a first example of a configuration of a Littrow-type laser device 100 a comprises a light source S, a collimating lens C, an output coupler module 6 ab comprising a spectrally selective element, in particular obtained via a diffraction grating 6 a, and an actuator A. In a second example of a configuration of a Littrow-type laser device 100 b , the output-coupler module 6 ab may comprise, in addition to the elements discussed above with reference to the structure 100 a , a diffraction grating 6 a and a mirror 6 b attached, for example, to the same support on which the diffraction grating 6 a is mounted.

[0030] Mirror 6b is arranged to receive the beam reflected by the diffraction grating at orientation angle θ and reflect it along an axis parallel to the optical axis of emission of source S. The subsequent discussion regarding Littrow configuration 100a applies in the same manner to similar elements in Littrow configuration 100b.

[0031] The gain medium / source S (eg a diode laser) has at least one reflective region RS, such reflective region RS (eg a spatially connected region such as a surface) obtained in a manner known per se. The source S is configured to emit radiation (or visible light).

[0032] For example, the gain medium S may be housed in a diode support.

[0033] The light beam generated / emitted by the source S follows an optical path OPa (100a) or OPb (100b), indicated by a dashed line in the figure, outside the source S. It is pointed out that by "source S" is meant, for example, a diode laser, and that the optical paths OPa, OPb are outside the module identifying the source S, in particular outside the semiconductor junction and, if present, outside the internal Fabry-Pérot cavity, otherwise outside the packaging of the source S.

[0034] Along this optical path, OPa and OPb are positioned as follows:

[0035] - collimating lens C; and

[0036] - an output coupler module 6ab comprising a spectrally selective element, in particular a diffraction grating 6a.

[0037] Once the light beam emitted by the source S is collimated by the lens C, it will impinge on the diffraction grating 6a, so the diffraction grating 6a is also outside the light source S.

[0038] An actuator A (eg, a piezoelectric actuator) is coupled to the grating 6a in the output coupler module 6ab.

[0039] The reflecting area / surface RS of the source S and the diffraction grating 6a in the output coupler module 6ab (which is eg located at the end of the optical axis passing through the collimating lens C) form at least one external cavity with a variable length L.

[0040] According to the formula , the length L of the external cavity determines the wavelength λ of the light beam emitted by the source S,

[0041] where n is an integer.

[0042] The grating 6a operating as a spectrally selective element in the output coupler module 6ab reflects the incident beam in zero order according to an axis which forms an orientation angle θ with the optical paths OPa, OPb of the light.

[0043] The horizontal and vertical orientation angles of the output coupler 6ab are such as to align the grating 6a in the module 6ab along the optical paths OPa and Opb in such a manner that a spectral portion of the light beam incident thereon (e.g., the first diffraction order) is back-reflected toward the light source S. In other words, the spectral portion of the light beam incident on the grating 6a propagates at an angle of 180° relative to the direction of incidence. The remaining portion of the light beam at the output (e.g., the zeroth-order reflected beam) proceeds along the optical paths OPa and OPb.

[0044] A portion of the light reflected toward the gain medium / source S has a spectral component at at least one frequency equal to the frequency of the longitudinal resonant mode of the external optical cavity RS,L,6a of length L. This condition creates a feedback network on the source itself, which facilitates adjusting the properties of the emission based solely on the properties of the external cavity and no longer solely on the emission characteristics of the source S. It can be said that the operation of the laser device 100a, 100b is in some way "dominated" or controlled by the properties of the external cavity and not by the properties of the source S.

[0045] The orientation angle θ is conventionally adjusted:

[0046] - via one or more adjustment screws of a support (not visible) on which the output coupler 6ab including the grating 6a is mounted; and / or

[0047] - via an electrical signal, for example processed by spectral analysis of the portion of light at the output, sent to a piezoelectric actuator A mechanically coupled to the module 6ab.

[0048] like Figure 1 As illustrated in both parts a) and b) of FIG, actuator A (by which the tilt angle θ of grating 6a relative to the direction of light propagation (i.e., optical paths OPa, OPb) is varied) is mechanically coupled to grating 6a. This actuator is disposed within output coupler module 6ab, which includes an adjustable base fixed relative to grating 6a. The mechanical coupling of piezoelectric actuator A to module 6ab or grating 6a facilitates varying the angle θ between the normal to diffraction grating 6a and the incident radiation. By varying the orientation angle θ of diffraction grating 6a, the wavelength λ of the emitted output beam can be adjusted.

[0049] Thus, the Littrow configuration 100a, 100b presents the advantage of no dispersion of the optical power and provides a spectrally selective device 6a with low dependence on temperature.

[0050] In some cases, actuator A is disposed within a mechanical support supporting the diffraction grating, in a purposely provided "fork-shaped" slot, formed, for example, by engraving a groove in the material from which the support is made to accommodate actuator A. Other types of mechanical supports are also known in the prior art. The use of Littrow configurations 100a, 100b requires that the support included in output coupler module 6ab be designed with specific elastic properties to enable actuator A to have a sufficiently wide travel range, while facilitating high resolution of orientation, i.e., high resolution of changes in angle θ. Consequently, a disadvantage of Littrow configurations 100a, 100b is the mechanical complexity required for production.

[0051] Instead, a disadvantage of adjusting the emitted wavelength λ as discussed previously is that, in fact, during wavelength variation, a so-called "competition between modes" is provided, e.g. a systematic mode hopping between the various resonance frequencies of the external cavity formed by the reflecting region RS and the spectrally selective element 6a.

[0052] The value of the total gain, given by the combined product of the gain of the source S, the dispersion of the grating 6a, and the resonant mode of the external cavity, can be comparable for adjacent / close modes (i.e., frequencies) of the external cavity. A small change in the orientation angle θ of the grating 6a, achieved by the actuator A, can result in a higher total gain for modes with frequencies close to the current oscillation mode, thus causing the laser device 100a, 100b to jump to a mode with higher gain and lose stability in the emitted frequency.

[0053] The configuration of the Littrow type 100a, 100b therefore imposes significant constraints on the properties of the actuator A, requiring the provision of significant fine adjustments and presenting a complex electromechanical setup.

[0054] A further disadvantage of the Littrow configurations 100a, 100b is that variations in the grating angle θ also cause variations in the frequency range diffracted by the diffraction grating 6a that contributes to the laser radiation at the output of the device. The fact that the movement of the piezoelectric actuator A has a dual effect on the laser—namely, changing the length L of the external cavity and changing the spectral selectivity of the diffraction grating 6a—can lead to instabilities in the emitted spectrum due to mode hopping.

[0055] This effect can be overcome by appropriately choosing the pivot point / axis about which the diffraction grating 6a is rotated, such that this point / axis is located in the emission plane of the optical radiation of the source S. Such a pivot point of the spectrally selective element 6a of the output module 6ab is constrained to be positioned in the emission plane of the source S. In this configuration 100a and 100b, the grating 6a must therefore be mounted on a sufficiently rigid mechanical system—so as not to jeopardize the stability of the laser emission frequency due to coupling of low-frequency mechanical noise—and can be finely rotated by θ in order to be able to control the laser emission frequency.

[0056] From an optomechanical point of view, the presence of two concurrent technical parameters makes the system complex to produce and the laser sensitive to misalignment. Consequently, these devices 100a, 100b and the corresponding optomechanical systems are characterized by increased mechanical complexity, which results in increased laser production costs and reduced possibilities for integrating the laser, for example, into more complex or portable devices.

[0057] Once again, another disadvantage of performing wavelength tuning in Littrow configuration 100a is the fact that by changing the orientation angle θ of grating 6a, the direction of the output beam also changes, thereby limiting its application, for example, to environments that do not require pointing stability. In particular, as long as changes in wavelength result in changes in the emission angle of the laser, Littrow configuration 100a does not exhibit pointing stability as the emission frequency changes.

[0058] Such effects can be reduced by reflecting the radiation emitted by the laser via a mirror fixed relative to the support of the diffraction grating, however, making the mechanical implementation of the laser increasingly complex.

[0059] In an embodiment of structure 100b, the mirror 6b coupled in a manner attached to the grating 6a in module 6ab facilitates obtaining an output light beam with a fixed emission axis as the orientation angle θ of the diffraction grating 6a changes, however, at the same time presents a disadvantage - a rigid displacement of the emission direction as a function of the orientation angle θ.

[0060] About Figure 1 Solutions similar to the discussed solution, in particular solution 100a, are known, for example, from document US 797 00 24 B2.

[0061] Despite the large amount of activity in this area, as discussed previously, further improved solutions are of interest. Summary of the Invention

[0062] It is an aim of one or more embodiments to help provide such an improved solution.

[0063] According to one or more embodiments, the above objects may be achieved by means of a device having the characteristics specified in the appended claims.An external cavity laser device configured to actuate at a source may provide an example of such a device.

[0064] One or more embodiments may be directed to corresponding systems.A tunable optical laser system may provide an example of such a system.

[0065] One or more embodiments may be directed to a corresponding method.

[0066] One or more embodiments present advantages that facilitate improved performance, such as high mechanical / optical stability, along with reduced assembly complexity.

[0067] The claims form an essential part of the technical teaching provided herein with reference to the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] One or more embodiments will now be discussed, purely by way of example, with reference to the accompanying drawings, in which:

[0069] - Figure 1 This has been discussed previously;

[0070] - Figure 2 is an exemplary optical diagram of one or more embodiments of the external cavity laser apparatus discussed herein;

[0071] - Figure 3 is an exemplary exploded view of one or more embodiments of the apparatus discussed herein;

[0072] - Figure 4 is an exemplary diagram of a frequency spectrum in one or more embodiments of the apparatus discussed herein;

[0073] - Figure 5 is an exemplary graph of an emission spectrum of one or more embodiments of the devices discussed herein;

[0074] - Figure 6 is an exemplary diagram of a tunable optical laser system according to one or more embodiments; and

[0075] - Figure 7 is a diagram provided as an example of a laser tuning method according to one or more embodiments. DETAILED DESCRIPTION

[0076] In the following description, one or more specific details are provided to provide a thorough understanding of the examples of the disclosed embodiments. The embodiments may be obtained without one or more of the specific details or with other methods, components, materials, etc. In other cases, well-known operations, materials, or structures are not illustrated or described in detail so that certain aspects of the embodiments are not obscured.

[0077] References to "an embodiment" or "one embodiment" in the framework of this specification are intended to indicate that a particular configuration, structure, or characteristic discussed with reference to the embodiment is included in at least one embodiment. Therefore, phrases such as "in an embodiment" or "in one embodiment" that may appear in one or more points of this specification do not necessarily refer to the same embodiment.

[0078] Furthermore, the particular configurations, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0079] The references used herein are provided for convenience only and therefore do not limit the scope of protection or the scope of the embodiments.

[0080] In the solutions discussed previously, it has been noted how it is not feasible to continuously vary the emission frequency of the laser via an actuator without changing the pointing direction of the beam emitted by the device, unless losses are introduced in the optical path.

[0081] One or more embodiments facilitate decoupling the mechanical stability of an arrangement in order to decouple the pointing of a laser from its frequency stability.

[0082] At this point, Figure 2 An optical diagram of an embodiment of an external cavity laser device 100 provided as an example is shown, which is driven / actuated at a source S by a linear actuator mechanically coupled thereto, facilitating:

[0083] - Reduce the complexity of manufacturing mechanical components; and / or

[0084] - Frequency and pointing stability.

[0085] Again, for simplicity, similar reference numerals are used hereinafter to designate components similar to those described herein, unless otherwise indicated. Figure 1 those similar parts.

[0086] External cavity laser device 100 (e.g., Figure 2 Examples include:

[0087] a linear actuator 24 comprising a variable-dimension element 2, i.e. an element configured to change its dimensions along at least one axis upon receiving a command variable, and in particular a transducer element that converts the command variable into a linear movement; the variable-dimension element 2 is, in the example, an element made of piezoelectric material; the linear actuator 24 further preferably comprises an elastic return element 4, for example an O-ring or a mechanical spring made of polymer material, i.e. an elastic element arranged on the element on which the actuator operates in order to exert an elastic reaction force in a direction opposite to the direction of the force exerted by the displacement of the variable-dimension element 2, thus facilitating its stability;

[0088] a collimator module 3 comprising a source of electromagnetic radiation (also referred to simply as "light source") S and at least one collimating lens C mounted in the module 3; and

[0089] - a dispersive stage 6, for example an optical diffraction grating, which can act both as a spectral selection device and as an output coupler.

[0090] The collimating lens C may comprise, for example, an achromatic doublet or any lens group designed to collimate the optical radiation emitted by the gain medium S.

[0091] The light source S and the collimating lens C are for example assembled together in a collimator module 3, wherein the lens C is arranged along an optical path OP of light at the output of the light source S in order to collimate such light.

[0092] From an optical point of view, a light source S generates a light beam and sends it to a collimating lens C in a collimator 3, by which it is collimated onto / towards a diffraction grating 6; from there, it is reflected in the zeroth order at a static orientation angle φ. In this case, in practice, the grating 6 is not associated with an actuator that changes its orientation, as in the case of device 100a.

[0093] A linear actuator 24 is arranged upstream of the collimator module 3; in particular, arranged upstream of the collimator module 3 is a variable dimension element 2, which converts an electrical command signal into a linear displacement.

[0094] Instead, in Figure 3 1 is an exploded view of the device 100 , which facilitates understanding of the mechanical structure of the device 100 .

[0095] exist Figure 2 and Figure 3 In the embodiment shown in FIG, at least one external optical cavity RS,L,6 of variable length is defined between the electromagnetic radiation source S and the dispersive stage 6.

[0096] The linear actuator 24 is mechanically coupled to the collimation module 3, for example to the light source S integrated therein, so as to move the source S and the lens C in an axial direction, in particular in the direction of the emission axis of the source S, for example along the axis of the external cavity RS,L,6.

[0097] In the example, collimation module 3 comprises a cylindrical collimator tube. In linear actuator 24, variable dimension element 2 (i.e., a piezoelectric actuator) is coupled to a first surface (or facet) of collimation module 3, for example, on the side of gain medium / source S, while in the illustrated example, restoring element 4 is coupled to a second surface of collimation module 3 and is located on the opposite side of variable dimension element 2 from source S, for example, on the side of lens C. It should be noted that the above arrangement of the elements of linear actuator 24 is provided purely by way of example. Other examples of arrangements may envision restoring element 4 coupled to a first facet of module 3 and variable dimension element 2 coupled to a second facet of module 3.

[0098] The elastic return element 4 (O-ring) may not even be present in the actuator 24, as long as its absence does not jeopardize the implementation, operation or tunability via the variable dimension element 2. The function of the return element 4 is to increase the stability and reduce the movement hysteresis of the variable dimension element 2.

[0099] In one embodiment, for example, when the variable dimension element 2 is coupled to the second facet of the module 3, the variable dimension element 2 may have a shape that allows light radiation to pass freely therethrough; for example, it may be shaped like a hollow cylinder.

[0100] Alternatively, multiple actuators can be used, for example formed by three variable length elements 2 having longitudinal axes arranged at an angle of for example 120° relative to each other, which can be coupled to the collimator module 3, thus facilitating the passage of cables without the need for holes in the actuator itself.

[0101] The return element 4 suitable for this purpose may be any cylindrical element comprising an elastic material, for example an O-ring made of a polymeric material.

[0102] The elastic return element 4 cooperates with the extension or compression of the variable size element 2 and the consequent displacement of the collimator module 3. Specifically, the return element 4 applies a force opposite to the force applied by the variable size element 2 (i.e., it acts in the opposite direction), thereby facilitating adjustment of its stroke.

[0103] It should be noted that the linear actuator 24 can also be obtained using other technologies known per se with similar dynamic and resolution conditions, and is not limited to linear actuators comprising piezoelectric materials, as will be appreciated by those skilled in the art. For example, an electromagnetic actuator can be used, which comprises a magnet and a coil operating along the displacement axis.

[0104] Therefore, with particular reference to Figure 2 1 , in an optical diagram similar to that discussed above with respect to optical paths OPa, OPb, light emitted by a light source S in the device 100 propagates along an optical path OP external to the source S and impinges on the surface of the dispersion table, specifically on the surface of the diffraction grating 6. As expected, the grating 6 has at least one optical axis, e.g., a reflection axis, which forms a static angle φ with the propagation direction of the light from the source.

[0105] Note that because the device 100 no longer requires dynamic variation of the static angle φ, for example, via fine adjustment, the structure can have different properties than the Littrow-type 100a, 100b structures. For example, the mechanical support for the grating 6 used in one or more embodiments discussed herein does not require the presence of a specific housing or specific flexible material for the piezoelectric actuator. For example, a conventional mirror mount can be used.

[0106] External cavity laser device 100 facilitates obtaining a beam with a stable pointing direction. The dispersion table (grating 6 in this example) is aligned to select a target frequency and an angle φ over the entire range of frequencies or wavelengths selected by the grating. This angle can maintain the same value (i.e., a constant angle) during changes in the wavelength of the emission via linear actuator 24, for example.

[0107] In particular, the pivot point of the rotation element of the dispersion table 6 is no longer constrained to lie in the emission plane of the gain medium / light source S. This further leads to the possibility of reducing the overall size of the laser device itself.

[0108] Therefore, the static angle φ formed by the reflection axis of the grating and the propagation direction of the light beam (for example with the optical path OP) is chosen so as to determine the emission frequency of the device 100 even only roughly, while fine adjustment of the frequency (or wavelength) is achieved via the linear actuator 24 that moves the collimator module 3.

[0109] With Figure 3 The embodiments of the mechanical structure illustrated in the exploded view of facilitate obtaining, for example, a light beam (at the output from the device 100) that is spatially and temporally coherent, having a constant direction (i.e., pointing stability) over a wide frequency range. Figure 3 As can be seen in FIG, this embodiment comprises, as at least partly mentioned above, a collimator module 3, a dispersion table 6 comprising a diffraction grating, a linear actuator 24 and a support module 7, which are assembled on an adjustable base 9 provided with one or more adjustment screws 10, for example micrometer screws. The support module is provided with one or more adjustment screws 8, for example micrometer screws.

[0110] Once again, as mentioned, the collimator module 3 comprises in the example a light source S, for example a source of electromagnetic radiation, and at least one collimating lens C at its two ends.

[0111] As discussed above, a light source / gain medium S (e.g., a diode laser, a diode laser with an antireflection coating (gain chip), or an SLED) exhibits at least one substantially reflective region RS. In particular, the light source can have regions (e.g., areas, surfaces, corners, or side surfaces) coated with an antireflection coating in a manner known per se. In principle, the source S can have one, two, or more substantially reflective regions RS.

[0112] In particular, in case of a light source S (such as a diode laser), the region RS is located near the outer end of the gain medium / source S (ie the end opposite to the emission surface of the light beam).

[0113] In a variant embodiment, such a reflection region RS corresponds to at least one of the inner surfaces of the optical waveguide.

[0114] The source S is assembled with its collimating lens C inside a collimator module 3 , such collimator module being arranged between, for example, a piezoelectric transducer operating as variable dimension element 2 and an O-ring made of polymer material operating as elastic return element 4 inside a mounting support 5 .

[0115] For example, moving the piezoelectric transducer 2 of the collimator module 3 along its axis of symmetry causes a small change in the length L of the external cavity RS,L,6 of the device 100 (i.e., the ECL device). This facilitates precise variation and correction of the wavelength of the radiation (e.g., laser radiation) by changing the mode of the external cavity RS,L,6. The purpose of the O-ring 4 is to facilitate the movement of the variable-size element 2 of the linear actuator 24 or to ensure its stability.

[0116] Therefore, the collimator module 3 has the function of facilitating the movement of one end on one side of the source S (ie, for example, the reflection region RS of the external cavity RS, L, 6) while maintaining the target focal distance between the source and the collimator. Figure 3 As can be seen in FIG, the linear actuator 24 and the collimator module 3 are shaped like cylinders and fit into corresponding hollow housings of the diode support 5. Figure 3 Designated by reference numeral 1 is a sleeve, for example, connected to the threaded end of diode support 5, containing inside it the linear actuator 24 and collimator module 3, which facilitates holding the entire assembly together and, for example, preloading linear actuator 24. An elastic return element 4, associated with the second face of collimator 3, presses against the end wall of the cylindrical cavity of support 5, exerting a spring force opposing the force exerted by element 2. Element 4 (such as a spring) can thus be preloaded in a manner that facilitates adjusting the range of expansion variations of piezoelectric element 2.

[0117] The support 5 or the piezoelectric actuator 2 may be perforated, for example with a hole in the shape of a cylinder, so that the power supply cable of the diode can exit from the support 5 .

[0118] In one embodiment, the light source S may include a semiconductor diode laser with an anti-reflection coating (gain chip) on the output surface, which emits 790 nm (1 nm = 10 -9 m) light, was produced at Eagleyard Photonics in Berlin, Germany. It is worth noting that in various embodiments, the laser device 100 can be operated with gain media and optical materials that enable it to operate in different spectral ranges, for example, from the near ultraviolet to the mid-infrared, provided that a suitable source and suitable optical materials are available.

[0119] As mentioned, the collimating lens C may comprise an achromatic lens composed of, for example, a doublet lens.

[0120] The linear actuator 24 is coupled to a light source S, which may be configured to emit coherent light. As mentioned, the source may be of the diode laser type or of the SLED type, for example.

[0121] In the example in question, the diffraction grating implementing the dispersion stage 6 operates both as a spectrally selective element and as an output coupler. By way of example only, such a grating 6 may be a grating manufactured by Thorlabs and known under the trade name GH13-18V, having a pitch of 100 nm per mm (1 mm = 10 nm). -3 m) Holographic dispersion grating with 1800 grooves.

[0122] As mentioned, the linear actuator 24 is configured to move the collimator module 3 , which may include an electromagnetic radiation source S and a collimator lens C.

[0123] The linear actuator 24 may comprise, for example, an annular piezoelectric actuator of the type Piezosystem Jena HPSt 500 / 15-5 / 7.

[0124] Figure 3 , a support module 7 for the grating 6 is shown, which is of a type similar to the mechanical supports with fork-shaped grooves that are used in the prior art to support the actuators of the gratings in the Littrow configurations 100a, 100b. This is an example of a possible mechanical support. In a variant embodiment, the support module 7 can be, for example, a simple mirror holder. In general, in further embodiments, the support module 7 can also be obtained in different ways. In one or more embodiments, the gain medium S and the lens C, as well as the collimator 3, can be integrated into the linear actuator 24, for example, to form a single / integrated module.

[0125] In one or more embodiments, by way of example only, the collimator module 3 may include a collimator tube of the type known under the trade name Thorlabs LT230P-B.

[0126] like Figure 3 As can be further seen in the exploded view of FIG, the grating 6 is attached to a support module 7 having a rotatable arm in such a way that the reflection axis of the grating 6 forms a static angle φ with the propagation direction of the light. This angle φ can be selected by moving a micrometer screw 8. This facilitates the selection of wavelengths with spectral resolution based on the characteristics of the grating, as discussed previously.

[0127] The source S (e.g. a diode) and the supports 5, 7 of the grating 6 are mounted on a base 9, which is optionally adjustable, for example via micrometer screws 10. In this variant, the portion of the base 9 to which the support 7 of the grating 6 is attached can be bent via two screws 10. Vertical alignment of the beam reflected by the grating (e.g. towards the source S) can be obtained by adjusting the bending of the portion of the base 9 obtained via at least two screws 10, which work oppositely to push and pull two opposite sides of the base.

[0128] A Peltier cell 11, mounted on a block 12, is made of aluminum, for example, and acts as a heat sink. It can be used to stabilize the temperature of the base 9 and the head of the gain medium / source S using a control unit or control circuit.

[0129] For example, in this embodiment, the beam emitted by laser device 100 has low temperature dependence, and the emission frequency can be locked to a reference by adjusting the length L of external optical cavity RS,L,6 itself, maintaining the orientation of diffraction grating 6 (e.g., static angle φ) stable and thus not changing the emission angle. Once diffraction grating 6 is aligned to select the emission frequency of device 100, laser 100 thus exhibits a stable pointing direction φ.

[0130] In a variant embodiment, the size of the external cavity RS,L,6 is greater than or equal to 10 cm (1 cm = 10 -2 m); that is, it is a LECL configuration.

[0131] In LECL, the use of Figure 2 The embodiment discussed is particularly advantageous because it facilitates extending the frequency range over which the device can operate without mode hopping. In fact, in the Littrow configuration, the dynamic range of the variability of the actuator A of the grating 6a required to ensure that the pivot point will be located in the emission plane can be complex and cumbersome to provide both mechanically and optically.

[0132] In a variant embodiment, the device 100 may be mounted in a protective case P, for example made of PVC or plexiglass.

[0133] The external cavity device 100 may have Figure 4 The frequency behavior is shown in the diagram.

[0134] The horizontal axis f of the abscissa indicates the range of frequencies f, while the vertical axis S(f) indicates the magnitude of the spectrum. The first curve gm represents the spectrum of the dispersion table 6 (grating mode distribution), while the second curve ecm represents the spectrum of the (resonant) modes of the external cavity RS,L,6 (external cavity modes), as a function of the length L of the cavity itself. In Littrow-type devices 100a and 100b, both the corresponding grating mode distribution and the corresponding external cavity mode are shifted (e.g., displaced) at frequency f by the action of a piezoelectric actuator A coupled to module 6ab. In one or more embodiments of the long-cavity laser device 100, only the external cavity mode is shifted / changed by the action of a linear actuator 24 (e.g., variable-length element 2), while the grating mode distribution is manually varied based on the selection of a static angle φ, for example, via an adjustment screw provided on the grating holder 7.

[0135] Therefore, the dispersion spectrum gm of such a diffraction grating of the dispersion table 6 of the exemplary device 100 is independent of variations in the length L of the variable-length external optical cavity RS,L,6, within the limit that the dispersion spectrum depends on the static angle φ formed by one of the reflection axes of the grating 6 and the direction of propagation of the light beam. In addition, at the same time, the resonance spectrum ecm varies as a function of a single geometrical parameter of the external cavity RS,L,6, namely as a function of the length L of the variable-length external optical cavity RS,L,6.

[0136] Figure 5 Shown as Figure 2 and Figure 3 Graphs of emission spectra of one or more embodiments of the external cavity laser device 100 illustrated in FIG.

[0137] Figure 5 The frequency range is represented in arbitrary units on the horizontal axis of the abscissa f, and the light intensity is represented on the axis of the ordinate when: a Littrow configuration device and an external cavity laser device (like reference Figure 2 and Figure 3 The device in question) is frequency-locked to the saturation spectrum signal from the rubidium cell in an independent manner via slow feedback to a piezoelectric transducer (whose frequency band is < 15 Hz). The feedback counter slows down the frequency changes of the two lasers due to temperature changes or changes in some other properties. Figure 5 As shown, the spectrum of the signal representing the beat frequency between the two (independent) lasers results in a linewidth of a single laser that is comparable to the linewidth achieved (again as an example) with the frequency-locked ECL design in the previous Littrow devices 100a, 100b.

[0138] One or more embodiments include a corresponding system 500, such as Figure 6 exemplified in .

[0139] The device 100 can be arranged in a laser optical system 500 that can be tuned in an automated manner, wherein the emission wavelength (or frequency) of the device 100 can be locked to a reference in a feedback network, which facilitates variations in the dimensions (e.g., length L) of the external cavity RS,L,6 itself, maintaining the orientation φ of the grating 6 constant, as described below (e.g., with respect to Figure 6 ) discussed.

[0140] System 500 (e.g. Figure 6 ) can include:

[0141] - a laser device, such as, for example, the external cavity laser device 100 discussed above;

[0142] a power supply 540 , which may be coupled to said optical device 100 and is configured to supply a light source S present in the optical device 100 ;

[0143] a control unit 520 coupled to the actuator 24 and configured to act on a feedback signal (e.g. for processing it and / or providing it and / or using it as a driver), the feedback signal being, for example, an error signal regarding a target behavior of the device 100, e.g. a mismatch in the frequency value provided at the output; optionally, the control unit 520 may be coupled both to the actuator 24 and to a power supply 540 for powering the light source S;

[0144] a configuration interface 510 , for example a graphical interface of a software, which is configured to provide information on how to perform the powering of the gain medium S and how to drive the linear actuator 24 of the device 100 via the control unit 520 ; and

[0145] - Optionally, a safety interlock configured to decouple the power supply 540 from the device 100 and / or drive the control unit 520 to decouple the power supply 540 from the device 100; as an example, the control unit 540 may comprise a controller known under the trade name Thorlabs LDC8002 manufactured by Thorlabs.

[0146] In one or more embodiments, for a system including an ECL (and / or LECL) device 100, the power supply 540 can be driven, for example, via the control unit 520, so as to operate as an additional regulator on the device 100, for example, by performing additional current regulation, which can thus regulate, for example, the supply current provided by the power supply 540 of the light source S, so as to facilitate maintaining the stability of the output frequency of the light emitted by the external cavity RS,L,6 of the device 100. In addition to this, in alternative embodiments, the control unit 520 can also be configured to drive the power supply 540 so that changing the current of the source S during a frequency sweep can be performed simultaneously with changes in the linear actuator 24, as discussed below.

[0147] The linear actuator 24 can receive a feedback signal from the control unit 520 and convert a portion of the spectrum of the feedback signal provided by the control unit 520 into a movement, for example, based on the value of its own cutoff frequency, in such a way that the actuator 24 is activated at / responsive to frequencies below this cutoff value. The power supply 540 that powers the radiation source S can alternatively be driven via a signal having a frequency band that includes frequencies higher than the frequency of the linear actuator 24. As can be seen from the foregoing, the feedback signal provided by the control unit 520 can be used to drive the power supply 540 to correct or adjust the behavior of the device 100, effectively performing faster adjustments than those achieved via the linear actuator 24. In one or more embodiments, the linear actuator 24 has a sufficient cutoff frequency to respond to the spectral components of the feedback signal from the control unit 520, which enables compensation for temperature changes and / or low-frequency acoustic and mechanical noise (e.g., where the frequency is less than 10 kHz), while the power supply 540 that powers the radiation source S facilitates fine correction of errors or correction of additional error sources in addition to the error sources already discussed. Taking into account the cutoff frequency of the actuator, the feedback signal can therefore be shared between the actuator and the power supply according to various strategies (e.g., according to different spectral partitioning).

[0148] The purpose of such an arrangement is to reduce the spectral line width of the radiation emitted by the external cavity laser device 100 .

[0149] Figure 7 A diagram is shown representing the operation of the method 1000 implemented via the system 500 and via the device 100 in question. Referring to the diagram, it includes:

[0150] - providing a tunable optical laser system 500 as previously discussed;

[0151] - choosing the static angle φ formed by the reflection axis of the dispersion table 6 and the optical path OP of the light propagation from the source S;

[0152] providing 1520 a frequency reference in a manner known per se, for example a frequency reference from a stable optical cavity, a stable laser radiation, or a spectral signal from an atomic or molecular sample (e.g. obtained via an absorption spectroscopy system) which provides a signal representing a certain reference spectrum;

[0153] - generating 1530 a light beam via such a system 500;

[0154] - performing a comparison 1540 of the light beam with such a reference signal;

[0155] - providing 1550 a signal representative of such performed comparison to such control unit 520 of such system 500; and

[0156] - varying 1560 a dimension (eg length L) of the external cavity RS,L,6 as a function of such signal indicative of the comparison, eg in an iterative manner.

[0157] In a variant embodiment, the method may also include, together with or as an alternative to the method 1000 , carrying out a change in the supply current 540 of the source S in the system 500 .

[0158] Note that, Figure 7 The order of the actions illustrated in the diagram of is provided purely as an example, but may also be different. For example, the action 1520 of providing a frequency reference and the action 1530 of generating a beam may be performed at the beginning of the method or before the action 1510.

[0159] Therefore, in this framework, one or more embodiments of the laser tuning method 1000 include, for example, the following operations:

[0160] - using 1500 such a tunable optical laser system 500;

[0161] - selecting 1510 an angle φ formed by said at least one reflection axis in the dispersive table 6 and said optical path OP of said light beam coming from the electromagnetic radiation source S;

[0162] - carrying out a comparison 1540 of a signal indicative of such light beam 530 with a signal indicative of such frequency reference 1520 provided to the system 500 ;

[0163] - providing 1550 a signal indicating such performed comparison 1540 to such control unit 520 of such system 500; and

[0164] - iteratively varying 1560 such length L of such at least one external cavity RS,L,6 as a function of such signal representing such comparison 1550 via moving 24 such source of electromagnetic radiation S as a function of said signal indicative of such comparison 1550 .

[0165] Without prejudice to the underlying principle, the details and embodiments may vary even considerably with respect to what has been discussed purely as an example, without thereby departing from the scope of protection of the present invention as defined by the appended claims.

Claims

1. A laser device (100), comprising: - a source (S) of electromagnetic radiation configured to generate a light beam, said light beam following an optical path external to said source (S); a dispersion table located outside said source (S) along the optical path of said light beam generated by said source (S), said dispersion table comprising at least one reflection axis forming an angle (θ; φ) with said optical path of said light beam and configured to reflect: - reflecting towards said source (S) at least a first spectral portion of said light beam generated by said source (S), and - reflecting a second spectral portion of said light generated by a source (S) along said reflection axis, wherein at least one external optical cavity of variable length is defined between said electromagnetic radiation source (S) and said dispersion stage; - at least one collimating lens (C) arranged along said optical path and configured to collimate said light beam coming from said source (S); and - a collimator (3) in which said source (S) and said at least one collimating lens (C) are mounted; and - an actuator configured to vary the length (L) of the at least one variable-length external optical cavity, in: - the actuator (24) is mechanically coupled to the collimator (3); - the actuator (24) is configured to change the length (L) of the at least one variable-length external optical cavity by moving the collimator (3); and - said angle (θ; φ) formed by at least one reflection axis of said dispersion table with said optical path of said light beam is constant during a variation of the length (L) of said at least one variable-length external optical cavity via said actuator, wherein said second spectral portion of said light generated by said source (S) is reflected by said dispersive table in zeroth order.

2. The device (100) according to claim 1, characterized in that The source (S) comprises at least one reflecting region (RS), the at least one external optical cavity of variable length being defined between the at least one reflecting region (RS) and the dispersive table (6).

3. The device (100) according to claim 1 or claim 2, characterized in that The actuator (24) comprises a variable length element (2) associated with one of the ends of the collimator (3).

4. The device (100) according to claim 3, characterized in that The variable length element (2) comprises a piezoelectric actuator.

5. The device (100) according to claim 3 or claim 4, characterized in that The variable length element (2) is geometrically configured to allow the passage of a light beam from a source (S), in particular being shaped like a hollow cylinder.

6. The device (100) according to any one of the preceding claims, characterized in that The actuator (24) comprises an elastic return element (4) associated with the collimator (3) so as to apply a spring force to the collimator (3) which opposes the force applied by the variable length element (2).

7. The device (100) according to any one of the preceding claims, characterized in that The actuator (24) comprises a set of linear piezoelectric actuators.

8. The device (100) according to any one of the preceding claims, characterized in that The dispersion table (6) is configured to directly deflect the second spectral portion of the light generated by the source (S) at the output from the device (100) in a constant direction (φ) as the frequency of the first spectral portion and / or the second spectral portion varies.

9. The device (100) according to any one of the preceding claims, characterized in that The dispersion table (6) is coupled to an adjustable support (7), which: - configured to be orientable in rotational and / or translational axes; and - provided with at least one adjustable screw (8) configured to align the adjustable support (7) of the dispersion table (6) with the source (S).

10. The device (100) according to any one of the preceding claims, characterized in that It includes at least one of the following: - a base (9), a heat sink (12) and a Peltier element (11); and - Protective shell (P), And among them: - the base (9) comprises a support (5) and / or one or more adjusting screws (10) and is configured to be coupled to at least one of the source (S), the actuator (24) and the one or more adjusting screws (10); and - The Peltier element (11) is configured to be arranged between the base (9) and the heat sink (12).

11. The device (100) according to any one of the preceding claims, characterized in that The source(s) comprises, alternatively: - a diode laser; or - Gain chip; or -Super Luminous LED – SLED.

12. A tunable optical laser system (500), comprising: - The external cavity laser device (100) according to any one of claims 1 to 11; - a power supply (540) which is coupleable to the external cavity laser device (100) and is configured to supply the electromagnetic radiation source (S) located in the external cavity laser device (100); as well as - a control unit (520) coupled to at least said actuator (24).

13. A laser tuning method (1000), comprising: - using (1500) the tunable optical laser system (500) according to claim 12; - selecting (1510) an angle (θ; φ) formed by said at least one reflection axis in a dispersion table and said optical path of said light beam from a source of electromagnetic radiation (S); - performing a comparison (1540) of a signal representative of said optical beam (1530) with a signal representative of a frequency reference (1520) provided to the system (500); - providing (1550) a feedback signal indicative of said comparison (1540) to said control unit (520) of said system (500); as well as - iteratively changing (1560) the length (L) of the at least one external cavity as a function of the feedback signal indicative of the comparison (1550) by moving (24) the electromagnetic radiation source (S) as a function of the feedback signal indicative of the comparison (1550).

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