LASER DIODE ARRANGEMENT WITH AN EXTERNAL RESONATOR

Description

[0001] The invention relates to a laser diode arrangement with an external resonator according to the preamble of claim 1.

[0002] Laser diodes are characterized by their small size and reliability. However, the wavelength of a laser beam emitted by a laser diode can only be tuned with relative inaccuracy by influencing the laser diode temperature and injection current. Furthermore, the linewidth, i.e., the bandwidth of the emission wavelength, is relatively large. Coupling a laser diode with an external resonator is particularly widespread in optical metrology and allows for more precise tuning of the desired optical properties.

[0003] An external resonator typically features wavelength-selective elements such as gratings and filters that feed only light of specific wavelengths—the resonator modes—back into the laser diode. This leads to an amplification of the stimulated emission at the selected wavelength. At the same time, the wavelength-selective element allows the emission wavelength to be tuned across the laser diode's gain range.

[0004] In particular, two arrangements consisting of a laser diode and such an external resonator have become established, known as the Littrow and Littman arrangements.

[0005] The Littrow arrangement is based on feedback via an optical diffraction grating, particularly a blazed grating, which is oriented such that the first order of diffraction is reflected back into the laser diode. The direct reflection (0th order of diffraction) forms the laser's output beam. If the alignment of the grating to the emitted laser beam of the laser diode is influenced, the wavelength can be changed within a certain range by amplifying a different longitudinal resonator mode in the laser diode. A mode hop can be achieved during detuning by appropriately choosing the pivot point of the diffraction grating.

[0006] In the Littman arrangement, the laser beam emitted by the laser diode strikes an optical diffraction grating at a shallow angle, with the first-order diffraction being directed onto a resonator mirror. This reflects the light back into the laser diode via the grating, while the zero-order diffraction is coupled out as a useful beam via the diffraction grating. The laser frequency is tuned by rotating the mirror. By appropriately selecting the pivot point of the resonator mirror, the tuning can be performed continuously and mode-hop-free over a wide frequency range.

[0007] Diode lasers with an external resonator in a Littman configuration according to Lolon are also known. However, these require relatively complex tuning devices, in which a MEMS actuator moves the aforementioned resonator mirror according to a correctly selected pivot point, thus enabling mode-hop-free wavelength tuning. In addition to the cost-intensive manufacturing process, another disadvantage is the low resonance frequency of the MEMS actuator, which makes the device very sensitive to interference.

[0008] A laser diode array with an external resonator in a Schenk Littrow arrangement also uses a tuning device with a MEMS actuator. A movable mirror is structured with lines similar to a diffraction grating and replaces the grating in the classic Littrow arrangement. However, mode-hop-free wavelength tuning is not possible.

[0009] For example, DE 10 2014 201 701 A1 discloses a micromechanically manufactured optical device in which a plate can be moved rotationally and translationally relative to a support for the device via a deflection device, with the plate carrying a diffraction grating. The combination of these two types of movement by a deflection device can lead to the device being susceptible to interference. Furthermore, translational motion requires a movement of several millimeters, which is not feasible on a MEMS scale.

[0010] Furthermore, US 5 956 355 A discloses a laser diode arrangement for generating single-mode, tunable laser radiation according to the preamble of claim 1.

[0011] The aim of the invention is to overcome these and other disadvantages of the prior art and to create a laser diode array with an external resonator that is tunable without mode hopping, yet has a significantly simpler structure than a MEMS actuator from lolon and a significantly higher resonant frequency, thus being less susceptible to external interference. Furthermore, the aim is to generate single-mode laser light whose wavelength is continuously tunable and free of mode hopping. The design should be simple, cost-effective, and easy to handle.

[0012] Main features of the invention are set out in the characterising part of claim 1.

[0013] Embodiments are the subject of claims 2 to 12.

[0014] A laser diode arrangement for generating single-mode, tunable laser radiation is proposed, comprising a laser diode and an external resonator coupled thereto, which has a resonator mirror and at least one wavelength-selective optical reflection element with a diffractive surface, wherein the resonator mirror is rotatably mounted about a pivot axis and can be selectively rotated about the pivot axis by means of a deflection device.The laser diode arrangement is characterized in that the external resonator is designed such that a laser beam emitted by the laser diode into the external resonator strikes a mirror surface of the resonator mirror at a first angle of incidence dependent on the rotation of the resonator mirror and is reflected by the latter onto the at least one wavelength-selective optical reflection element at a second angle of incidence dependent on the first angle of incidence, wherein an optical path length between the resonator mirror and the diffracting surface is influenced by the first angle of incidence, and that the wavelength-selective optical reflection element reflects the laser beam back onto the resonator mirror so that it is guided by the resonator mirror back into the laser diode, wherein the wavelength of the laser beam reflected by the wavelength-selective reflection element is influenced by the second angle of incidence.

[0015] This design makes it possible to significantly reduce manufacturing costs and, at the same time, achieve lower susceptibility to interference, while still enabling mode-hop-free tuning over a wide frequency range. This is made possible in particular by the arrangement of a resonator mirror at an end of the laser diode facing the resonator, which reflects a laser beam emitted by the laser diode to the wavelength-selective reflection element. Compared to a diffraction grating that can be moved rotationally and translationally using a deflection device, the moving parts of the deflection device and the resonator mirror can have a lower mass and thus a significantly higher resonant frequency. Furthermore, the rotary bearing is mechanically much simpler to implement and allows for improved positional stability, particularly on the MEMS scale.This significantly contributes to reducing the susceptibility of the laser diode array to external influences. Furthermore, the design of the wavelength-selective optical reflection element can be realized largely independently of mechanical constraints that must be considered for rotationally and translationally movable gratings.

[0016] The laser diode can preferably be equipped with collimating optics. For example, an optical transmission component in the form of a collimating lens is arranged in the area of ​​the front and / or rear facet. This ensures that divergent laser light emerging from the front and / or rear facet of the laser diode is collimated. However, depending on the application of the laser light, other optics can also be used.

[0017] The emission wavelength achieved by the laser diode array is determined by two conditions. The first condition is referred to as the grating-determined wavelength λ g . This can be expressed by the equation n ⋅ λ g = g ⋅ sin φ with λ g wavelength determined by the diffracting surface, g lattice constant of the diffracting surface, ϕ angle of incidence on the diffracting surface and n diffraction order, here first order n=1 The wavelength determined by the diffracting surface is therefore directly dependent on the angle of incidence of the laser beam reflected by the resonator mirror and incident on the diffracting surface.

[0018] The second condition is given by the resonator-determined wavelength λ R to λ R = 2 ⋅ L R / m with λ R Wavelength determined by an optical resonator length LR LR optical length of the laser resonator mNumber of wave nodes in the laser resonator The optical length L R is composed of the partial distances of an optical length of the laser diode, a length of the laser beam between the laser diode and the resonator mirror, and a length of the laser beam between the resonator mirror and the diffracting surface. Since this optical length is traversed twice, the factor 2 is included in the second condition above.

[0019] When the resonator mirror is rotated around the pivot axis, the angle of incidence and the angle of reflection of the emitted laser beam on the resonator mirror change according to the law of reflection. This results in a changing angle of incidence ϕ on the diffracting surface and, simultaneously, a changing length of the laser beam traveling between the resonator mirror and the diffracting surface. Using only a single control variable in the form of the first angle of incidence, i.e., the rotational position of the resonator mirror, the optical length of the resonator and the wavelength of the laser beam returned to the laser diode are simultaneously influenced.

[0020] If the number of wave nodes m changes when tuning the laser wavelength, this is referred to as a mode hop. The goal of the laser diode arrangement is to tune the wavelength without changing the number of wavelength nodes. This is referred to as mode hop-free wavelength tuning. The formal requirement for this is 2 ⋅ L R / λ g − m < 1 2

[0021] Ultimately, this condition is met by adjusting the resonator length and the wavelength determined by the diffracting surface. According to the invention, this can be achieved by rotating the resonator mirror. With a suitable choice of the lattice constant of the diffracting surface and the resonator length LR, the requirement for mode-hop-free tuning is met across the laser's tuning range, according to the above equation.

[0022] Preferably, the optical path length between the resonator mirror and the diffractive surface increases with increasing first angle of incidence. The resonator mirror and the diffractive surface are therefore arranged such that, as the first angle of incidence increases, the point of impact of the laser beam reflected onto the diffractive surface is increasingly farther away from the resonator mirror. Increasing the optical path length is thus very simple and is achieved solely by rotating the resonator mirror.

[0023] Furthermore, the wavelength of the laser beam reflected by the wavelength-selective reflection element preferably increases with increasing first angle of incidence. The wavelength-selective reflection element can be arranged relative to the resonator mirror such that the second angle of incidence also increases with increasing first angle of incidence. The diffractive surface is then to be adapted such that the wavelength increases with increasing second angle of reflection.

[0024] Preferably, the wavelength-selective reflection element and the resonator mirror are positioned relative to one another such that the change in the optical path distance between the resonator mirror and the diffracting surface caused by rotation of the resonator mirror and the change in the wavelength of the laser beam reflected by the wavelength-selective reflection element are adapted to one another such that the condition in the above-mentioned formula 3 is always fulfilled.

[0025] The laser diode preferably has a rear facet and a front facet. To enable the use of the external resonator, the front facet facing the external resonator is preferably coated with an anti-reflection layer, so that the eigenmodes of the laser diode, whose space delimited by its facets also forms a resonator, are suppressed. Consequently, in an advantageous embodiment, the laser diode has a rear facet and a front facet facing the external resonator and provided with an anti-reflection layer.

[0026] In an advantageous embodiment, the at least one wavelength-selective optical reflection element is an optical diffraction grating. Generally, the diffraction grating can be a grooved reflection grating designed to diffractically reflect rays. The diffraction grating is preferably designed such that incident rays impinge perpendicularly on the grooves. Incident and reflected light rays are then always aligned parallel to each other. The optical diffraction grating can be scribed or holographic. In particular, a scribed diffraction grating can be designed as a blaze grating.

[0027] The pivot axis is positioned on a side of the resonator mirror facing away from the laser diode, at a distance from the mirror surface. The mirror surface is thus pivoted along a circular path such that the mirror surface is directed radially outward along this circular path. However, the distance between the mirror surface and the pivot axis could preferably be as small as possible, so that the range of motion of the incident laser beam and thus the mirror surface can be minimized. Such an arrangement allows for a simple and flat implementation of the resonator as a micromechanical element.

[0028] The distance between a line on the mirror surface closest to the pivot axis of the resonator mirror and a point of impact of the entire laser beam on the mirror surface is greater than 0. In real diode lasers with an external resonator, dispersion can occur across the tuning range of the laser diode. This changes the refractive index of the laser diode material and thus the optical length of the laser diode. The influence of dispersion can be compensated for by the distance between a line on the mirror surface closest to the pivot axis of the resonator mirror and the point of impact of the laser beam on the mirror surface. This distance introduces an independent higher-order contribution, which is used to compensate for the dispersion of the laser material by adjusting a change in the resonator length that depends on the rotation of the resonator mirror.

[0029] An advantageous embodiment further comprises at least one optical transmission component between the resonator mirror and the laser diode. This makes it possible, in particular, to influence the coupling quality of the external resonator to the laser diode.

[0030] It may be appropriate for the at least one optical transmission component to include a collimator. This can generate a parallel beam from divergent laser light, which impinges on the resonator mirror.

[0031] Furthermore, to obtain a desired bundle of parallel light beams, a collimator can also be arranged on a side of the rear facet facing away from the laser diode. Preferably, the at least one wavelength-selective reflection element is arranged obliquely to a radiation direction of the laser beam emitted by the laser diode. In particular, the diffractive surface can face away from the laser diode and always be non-parallel to the radiation direction.

[0032] It might also be advisable to arrange the resonator mirror at an angle to a radiation direction of the laser beam emitted by the laser diode, with the mirror surface facing the laser diode.

[0033] The resonator mirror could be movable such that, at least in one position of the resonator mirror and the wavelength-selective reflection element, the resonator mirror is aligned parallel to the wavelength-selective reflection element. The resonator mirror can thus cover a large area of ​​the wavelength-selective reflection element by reflecting laser beams through pivoting.

[0034] The laser diode array is preferably manufactured as a micromechanical system. The respective deflection devices can be designed, in particular, as electrostatic actuators.

[0035] Further features, details, and advantages of the invention will become apparent from the wording of the claims and from the following description of exemplary embodiments with reference to the drawings. They show: Fig. 1 is a schematic representation of a laser diode array, Fig. 2 is a schematic representation of a micromechanical resonator mirror, Fig. 3a to 3c are schematic representations of the laser diode array with different rotation angles of the resonator mirror with a central pivot axis of the resonator mirror, Fig. 4a to 4c are schematic representations of the laser diode array with different rotation angles of the resonator mirror with an offset pivot axis of the resonator mirror to compensate for dispersion effects, and Fig. 5 is an exemplary micromechanical structure of the laser diode array.

[0036] Fig. 1 schematically shows a laser diode arrangement 2 with a diode laser 4, which has a front facet 6 preferably coated with an anti-reflection layer and a rear facet 8 acting as an end mirror. The front facet 6 faces an external resonator 10, which has a resonator mirror 12 and a wavelength-selective optical reflection element in the form of a diffraction grating 14. The resonator mirror 12 is pivotally mounted about a pivot axis 16. A laser beam 18 emitted by the laser diode 4 strikes the resonator mirror 12 and is reflected by it to the diffraction grating 14. A diffractive surface 20 diffracts and reflects the incoming light, so that a first diffraction order 22 strikes the resonator mirror 12 and is reflected back into the laser diode 4. A laser beam 24 is coupled out via the rear facet 8.For parallel alignment of the laser beams 18 and 24, a collimation optics with a first collimator in the form of a first collimation lens 26 and a second collimator in the form of a second collimation lens 28 is used.

[0037] The optical length resulting from this arrangement 2, which is also referred to as LR below, is composed of the sum of the optical length of the laser diode 4 and the optical length experienced by the laser beams 18 and 22 together. The diffractive surface 20 can be designed in the form of a blazed grating or another grating shape and is arranged obliquely to the emitted laser beam 18. The diffraction grating 14 preferably has grating lines arranged parallel to the pivot axis 16. By rotating the resonator mirror 12, the direction of the laser beam 22 passing between the resonator mirror 12 and the diffraction grating 14 can be influenced. In addition to the wavelength determined by the diffractive surface 20 and dependent on the direction of incidence, the optical length LR of the resonator is also influenced. This will be discussed in more detail in the description of the Figuren 3a bis 4c discussed in more detail.

[0038] Fig. 2 shows a micromechanical design of the resonator mirror 12. A mirror surface 30 is formed on a circular surface 32, which is coupled to two actuator sections 34 and 36. These could be based on an electrostatic operating principle and perform a quasi-static position dependent on an input voltage in the form of a rotation around the pivot axis 16. The resonator mirror 12 can, in particular, be manufactured in one piece and from silicon.

[0039] Fig. 3a bis 3c show the influence of a rotation of the resonator mirror 12 on the tuning of the arrangement 2.

[0040] The laser beam 18 emitted by the laser diode 4 hits Fig. 3a onto the mirror surface 30 at an angle of incidence γ -1 , which, due to the reflection of the laser beam 18, leads to a reflected laser beam 22 which strikes the diffraction grating 14 at an angle of incidence ϕ -1 . The wavelength generated by the diffraction grating 14 at first order of diffraction is therefore calculated as λ g = g sin (ϕ -1 ).

[0041] The alignment of the diffraction grating 14 relative to the resonator mirror 12 results in an optical path or distance d -1 to be bridged by the laser beam 22. For example, in Fig. 3a A region of the diffraction grating 14 close to the laser diode 4 is affected, having a short optical path length d -1 . The optical length LR of the laser resonator 10 is therefore calculated as the sum of the optical length of the laser diode 4, the length of the laser beam 18 passing between the laser diode 4 and the resonator mirror 12, and the optical path length d -1 caused by the rotation of the resonator mirror 12.

[0042] In Fig. 3b the resonator mirror 12 is rotated slightly further clockwise in the plane of the drawing, resulting in an angle of incidence γ 0 of approximately 45°. As a result, the diffraction grating 14 is hit centrally, resulting in an angle of incidence ϕ 0. In the exemplary configuration, the optical path length or distance d 0 to be bridged by the laser beam 22 is greater than d -1. Due to the changed angle of incidence γ 0 and the greater optical path length, a standing wave with a higher wavelength can be generated between the rear facet 8 and the diffraction grating 14, so that the output laser beam 24 consequently has a lower frequency than the one in Fig. 3a .

[0043] This applies analogously to the representation in Fig. 3c There, the resonator mirror 12 is rotated slightly further clockwise in the plane of the drawing, so that the angle of incidence γ +1 is increased even further. The diffraction-dependent wavelength is influenced by the equally larger angle of incidence ϕ +1 at the diffraction grating 14 and the optical path or distance d +1 , which is greater than d 0 and d -1 , leads to an even higher wavelength of the standing wave between the rear facet 8 and the diffraction grating 14 with the same oscillation modes than in Fig. 3a and 3b . The coupled-out laser beam 24 can therefore have an even lower frequency.

[0044] The laser diode 4 can exhibit dispersion effects over the tuning range, in which the refractive index of the material of the laser diode 4 and thus the optical length of the laser diode 4 change. As shown in the Fig. 4a bis 4c As shown, these effects can be compensated by clever positioning of the pivot axis 16.

[0045] The pivot axis 16 can be arranged centrally on the resonator mirror 12 on a side of the resonator mirror 12 facing away from the laser diode 4. For example, the pivot axis 16 is arranged in a Fig. 1 and 3a bis 3c offset such that a measure of the distance between a line on the mirror surface 30 closest to the pivot axis 16 of the resonator mirror and a meeting point 40 of the laser beam 18 on the mirror surface 30 is increased. The pivot axis 16 is arranged in the drawing plane for this purpose approximately significantly above the line formed by the laser beam 18. The laser beam 18 then mainly strikes a radially outer section of the resonator mirror 12 directed towards the laser diode 4. This offset of the pivot axis 16 results in a change in the optical length of the laser beam 18 between the front facet 6 and the resonator mirror 12 by an amount δ -2 or δ +2 , when assuming the same rotational positions as in Fig. 3a bis 3c Simultaneously, the angles of incidence of the laser beam 18 onto the resonator mirror 12 can change from γ -1 to γ ​​-2 and from γ +1 to γ ​​+2, resulting in changed angles of incidence at the diffraction grating 14 from ϕ -1 to ϕ -2 and from ϕ +1 to ϕ +2. By skillfully adjusting the position of the pivot axis 16, the dispersion effect of the laser diode 4 can be compensated for by combined influence of diffraction parameters and the optical length LR between the back facet 8 and the diffraction grating 14.

[0046] Finally, Fig. 5 exemplary realization of the laser diode arrangement 2 as a micromechanical system which is arranged on a carrier 42.

[0047] The invention is not limited to one of the embodiments described above, but can be modified in many ways.

[0048] All features and advantages arising from the claims, the description and the drawings, including design details, spatial arrangements and method steps, can be essential to the invention both individually and in a wide variety of combinations. Bezugszeichenliste

[0049] 2Laser diode array 4Laser diode 6Front facet 8Rear facet 10External resonator 12Resonator mirror 14Wavelength-selective optical reflection element / diffraction grating 16Pivot axis 18Laser beam between front facet and resonator mirror 20Diffracting surface 22First diffraction order / laser beam 24Outcoupled laser beam 26First collimation lens 28Second collimation lens 30Mirror surface 32Circular surface 34Actuator section 36Actuator section 40Point of impact of laser beam on mirror surface 42Carrier γ, γ -2 , γ -1 , γ 0 , γ +1 , γ +2 First angle of incidence / angle of incidence of the emitted laser beam on the mirror surface ϕ, ϕ -2 , ϕ -1 , ϕ 0 , ϕ +1 , ϕ +2 second angle of incidence / angle of incidence of the laser beam reflected from the mirror surface onto the diffracting surface a -2 , a 0 , a +2 distance between the line closest to the first pivot axis and the point of impact of the laser beam onto the mirror surface d -2 , d -1 , d 0 , d +1 ,d +2 Optical path length between mirror surface and diffracting surface δ -2 , δ +2 Change in the optical length of the laser beam between front facet and resonator mirror when changing the angle of incidence on the mirror surface,

Claims

1. Laser diode arrangement (2) for generating single-mode tunable laser radiation, having: ▪ a laser diode (4) and ▪ an external resonator (10) coupled thereto, said external resonator having a resonator mirror (12) and at least one wavelength-selective optical reflection element (14) having a diffractive surface (20), ▪ wherein the resonator mirror (12) is mounted so as to be able to rotate about a swivel axis (16) and is able to be turned selectively about the swivel axis (16) by way of a deflection device, wherein a) the external resonator (10) is designed such that a laser beam (18) emitted by the laser diode (4) into the external resonator (10) impinges on a mirror surface (30) of the resonator mirror (12) at a first angle of incidence (γ) dependent on the rotation of the resonator mirror (12) and is reflected thereby onto the at least one wavelength-selective optical reflection element (14) at a second angle of incidence (ϕ) dependent on the first angle of incidence (γ), b) wherein an optical path (d) between the resonator mirror (12) and the diffractive surface (20) is influenced by the first angle of incidence (γ), and c) wherein the wavelength-selective optical reflection element (14) reflects the laser beam in diffracted form back onto the resonator mirror (12) such that it is guided from the resonator mirror (12) back into the laser diode (4), d) wherein the wavelength of the laser beam reflected by the wavelength-selective reflection element (14) is influenced by the second angle of incidence (ϕ), e) wherein the swivel axis (16) is positioned on a side of the resonator mirror (12) that faces away from the laser diode (4) at a distance from the mirror surface (30), characterized in that f) a distance (a-2, a0, a+2) between a line, closest to the swivel axis (16) of the resonator mirror (12), on the mirror surface (30) and a point of incidence (40) of the overall laser beam (18) on the mirror surface (30) is always greater than 0.

2. Laser diode arrangement (2) according to Claim 1, wherein the optical path (d) between the resonator mirror (12) and the diffractive surface (20) increases as the first angle of incidence (γ) increases.

3. Laser diode arrangement (2) according to Claim 1 or 2, wherein the wavelength of the laser beam reflected by the wavelength-selective reflection element (14) increases as the first angle of incidence (γ) increases.

4. Laser diode arrangement (2) according to one of the preceding claims, wherein the laser diode (4) has a rear facet (8) and a front facet (6) that faces the external resonator (10) and that is provided with an antireflection layer.

5. Laser diode arrangement (2) according to one of the preceding claims, wherein the at least one wavelength-selective optical reflection element (14) is an optical diffraction grating.

6. Laser diode arrangement (2) according to one of the preceding claims, furthermore having at least one optical transmission component (26) between the resonator mirror (12) and the laser diode (4).

7. Laser diode arrangement (2) according to Claim 6, wherein the at least one optical transmission component (26) comprises a collimator (26).

8. Laser diode arrangement (2) according to one of the preceding claims, wherein a collimator (28) is arranged on a side of the rear facet (8) that faces away from the laser diode (4).

9. Laser diode arrangement (2) according to one of the preceding claims, wherein the at least one wavelength-selective reflection element (14) is arranged obliquely with respect to an emission direction of the laser beam (18) emitted by the laser diode (4), wherein the diffractive surface (20) faces away from the laser diode (4).

10. Laser diode arrangement (2) according to one of the preceding claims, wherein the resonator mirror (12) is arranged obliquely with respect to an emission direction of the laser beam (18) emitted by the laser diode (4), wherein the mirror surface (30) faces the laser diode (4).

11. Laser diode arrangement (2) according to one of the preceding claims, wherein the resonator mirror (12) is able to be moved such that, at least in one position of the resonator mirror (12) and of the wavelength-selective reflection element (14), the resonator mirror (12) is aligned parallel with the wavelength-selective reflection element (14).

12. Laser diode arrangement (2) according to one of the preceding claims, wherein the laser diode arrangement (2) is produced as a micromechanical system.