Tuning arrangement

0001The present invention relates to a tuning device for semiconductor diode lasers with an external resonator according to the preamble of claims 1, 13 and 14.

0002A semiconductor laser diode operated in the direction of flow can be used to generate coherent light by means of stimulated emission and to emit it in a directed manner. The wavelength of the emitted laser light is determined by the respective stoichiometry and the microscopic structure of the semiconductor laser material. Typical emission wavelengths of semiconductor laser diodes are between 630nm and 1550nm.

0003For some applications of semiconductor lasers, it is necessary to introduce optical elements into the laser resonator, for which an external resonator is used. The light emitted by a laser facet is collimated and fed back into the semiconductor laser with a separate (external) resonator end mirror. The laser facet facing the external resonator is generally antireflection-coated in order to ensure better coupling of the external resonator to the semiconductor laser.

0004With an external resonator, which contains a wavelength-selective element, for example an optical diffraction grating, the emission wavelength can be tuned over the amplification range of the laser. Typical bandwidths are between 12nm and 120nm, depending on whether semiconductor laser diodes with an emission wavelength of 630nm or those with an emission wavelength of 1550nm are used.

0005Two typical arrangements for laser resonators that contain wavelength selective elements are the Littrow and Littman arrangements. A Littrow arrangement is a resonator which contains an optical diffraction grating as the resonator end mirror, the directions of the incident light and the positively interfering light scattered back at the grating grooves coincide. The Littman arrangement is a folded resonator that contains a diffraction grating between the resonator end mirrors. In this arrangement the grating is positioned within the resonator so that the first order of diffraction of the grating meets the resonator end mirror. The zeroth diffraction order of the grating can then be used as the useful beam of the laser. The grating thus has a double function as a wavelength-selective element and as a coupling-out element.

0006The advantage of the Littman arrangement is that the illuminated grating surface is 4 to 7 times larger than that of the Littrow arrangement. As a result, the spectral selectivity of the grating is increased by the same factor, so that single-mode laser emission is ensured with the Littman arrangement with longer resonator lengths, and consequently very small line widths are achieved. Another advantage is that commercially available semiconductor lasers are often only supplied in housing designs that do not allow access to the rear laser facet, which is required in the Littrow arrangement as an output mirror.

0007If the resonator length is kept constant when tuning the emission wavelength of the laser system, the number changes <i>m</i> the node of the standing light wave in the laser resonator, which is referred to as a mode jump. It follows from this that the wavelength cannot be tuned continuously but jumps in discrete steps. In addition to the difficulty of setting a desired wavelength, this has the consequence that considerable fluctuations in the output power of the laser can occur. Mode jumps can be avoided by tuning the optical resonator length when adjusting the wavelength<i>L</i>_<i>opt</i> varies so that the resulting wavelength λ_<i>R</i> the wavelength λ determined by the grating_<i>G</i>, is tracked. Formally, the condition must<maths id="math0001" num="(1)"><math display="block"><mrow><mfenced open="|" close="|"><mrow><mfrac><mrow><mtext>2·</mtext><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext mathvariant="italic">opt</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext>λ</mtext></mrow><mrow><mtext mathvariant="italic">G</mtext></mrow></msub></mrow></mfrac><mtext> - </mtext><mtext mathvariant="italic">m</mtext></mrow></mfenced><mtext> &lt; </mtext><mfrac><mrow><mtext>1</mtext></mrow><mrow><mtext>2</mtext></mrow></mfrac></mrow></math><img file="EP0801451A2_D0001.tif" /></maths> remain fulfilled.

0008For mode-free wavelength tuning of a laser that does not contain any dispersive media, a simple rotation of a resonator mirror has been proposed, the pivot point being in the common intersection of the mirror planes of the resonator end mirrors with the plane of the diffraction grating. Now semiconductor lasers show considerable chromatic dispersion due to their light amplification mechanism. As a result, the geometric length<i>L</i>_<i>geo</i> different from the optical length <i>L</i>_<i>opt</i>. The formal context is:<maths id="math0002" num="(2)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext mathvariant="italic">opt</mtext></mrow></msub><mtext> = </mtext><mtext mathvariant="italic">n</mtext><mtext>(λ) </mtext><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext mathvariant="italic">geo</mtext></mrow></msub></mrow></math><img file="EP0801451A2_D0002.tif" /></maths> Here is <i>n</i>(λ) is the refractive index of the laser material, the value of which depends on the emission wavelength of the semiconductor laser. For easier handling, Eq. (2) in a power series:<maths id="math0003" num="(3)"><math display="block"><mrow><mtable><mtr><mtd><mrow><mtable><mtr><mtd><mrow><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext mathvariant="italic">opt</mtext></mrow></msub><mtext> = </mtext><mfenced open="(" close=")"><mrow><apply><sum /><lowlimit><mtext mathvariant="italic">i</mtext><mtext>=0</mtext></lowlimit><uplimit><mtext>∞</mtext></uplimit><mrow><msub><mrow><mtext mathvariant="italic">n</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msub><mtext mathvariant="italic"> ·</mtext><msup><mrow><mtext> Δλ</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msup></mrow></apply></mrow></mfenced><mtext></mtext><msub><mrow><mtext mathvariant="italic">· L</mtext></mrow><mrow><mtext mathvariant="italic">geo</mtext></mrow></msub></mrow></mtd></mtr><mtr><mtd><mrow><msub><mrow><mtext mathvariant="italic">n</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msub><mtext> = </mtext><mfrac><mrow><msup><mrow><mtext mathvariant="italic">d</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msup><mtext mathvariant="italic">n</mtext><msub><mrow><mtext>(λ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext>)</mtext></mrow><mrow><mtext mathvariant="italic">d</mtext><msup><mrow><mtext>λ</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msup></mrow></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><msub><mrow><mtext>Δλ = λ - λ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></mtd></mtr><mtr><mtd><mrow><msub><mrow><mtext>λ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> = Development point</mtext></mrow></mtd></mtr></mtable></mrow></mtd></mtr></mtable></mrow></math><img file="EP0801451A2_D0003.tif" /></maths> Mode-shift-free wavelength tuning of semiconductor lasers with an external resonator over large wavelength ranges is only possible if the chromatic dispersion is taken into account. The number is a measure of the quality of compensation<i>i</i>up to which the development coefficients <i>n</i>_<i>i</i> can be taken into account. It is called "compensation of dispersion<i>j</i>-th order "if the terms <i>n</i>_<i>i</i> up to and including the term <i>n</i>_<i>j</i> are taken into account exactly. Mode-shift-free wavelength tuning of a semiconductor laser with an external resonator over ranges greater than 60 nm - 80 nm is generally only possible if the second-order dispersion and the third-order dispersion are taken into account at least approximately.

0009An example of the compensation of the chromatic dispersion of all components of a semiconductor laser system - consisting of semiconductor laser, collimation optics, air-filled external resonator - is in the publication by Favre. Le Guen, "82 nm of continuous tunability for an external-cavity semiconductor laser", Electronics Letters, 27 (2), pp. 183-184, Jan. 17, 1991. Favre presents a semiconductor laser assembly in a Littrow arrangement, in which the global chromatic dispersion of the semiconductor laser, the imaging optics and the air in the external resonator can be compensated for using an adjusting screw, but only the first-order dispersion. Furthermore, mode-shift-free wavelength tuning is not possible over the entire amplification range of the laser; rather, mode jumps occur, which is due to insufficient compensation of the contributions of the second and higher order of the chromatic dispersion of the laser system.

0010A second, later example for the compensation of the chromatic dispersion of all components of a semiconductor laser system consisting of semiconductor laser, collimation optics, and external resonator filled with air is described in WO 94/08371. In the case of a semiconductor laser structure in a Littman arrangement, the pivot point of the mirror arm is then selected such that the first order chromatic dispersion can be exactly compensated, while a non-correctable presetting is selected for the second order dispersion. The consequence of this is that mode-free wavelength tuning does not succeed over the entire tuning range of the laser system.

0011The general purpose of the invention is to provide means for generating coherent light with a continuously and quickly tunable wavelength of narrow spectral line width. In particular, a simple tuning device for a semiconductor laser with an external resonator is to be realized, which enables mode-jump-free wavelength tuning over the entire amplification range of semiconductor lasers in a Littman arrangement.

0012Important features of the invention are given in the characterizing part of claims 1, 13 and 14. Refinements are the subject of claims 2 to 12.

0013In a tuning device for a semiconductor diode laser provided with an anti-reflection coating with an external resonator in a Littman arrangement, consisting of at least one optical transmission component, for example a collimator, furthermore an optical diffraction grating, a tuning arm rotatable about an axis, on which a resonator end mirror is attached, and adjusting devices for the position of the tuning arm and for the distance between the mirror plane and the axis of rotation to compensate for the first-order chromatic dispersion of all in the laser contain components (e.g. Semiconductor diode laser, collimator optics and air in the resonator), by mechanically linking the changes in the geometric resonator length and the angular relationship between the grating and the mirror, the invention provides according to the characterizing part of claim 1 that for the targeted adjustment and compensation of the chromatic dispersion of higher orders Grid is arranged variable in angle and variable in relation to the axis of rotation of the tuning arm. If the tuning arm is now rotated, this causes a translation and a rotation of the resonator end mirror at the same time. The geometric resonator length changes due to its displacement<i>L</i>_<i>geo</i>, consequently also the optical resonator length according to equations (2) and (3) <i>L</i>_<i>opt</i> and thus the wavelength λ determined by the resonator_<i>R</i>. The rotation of the resonator end mirror causes a change in the wavelength λ determined by the grating_<i>G</i>. This is done according to the invention in such a way that equation (1) remains fulfilled at all times by changing the wavelength λ determined by the grating_<i>G</i> is synchronized.

0014What is important is the measure of claim 2, according to which an adjusting means is provided in each case to compensate for the chromatic dispersion of one order (1st; 2nd; 3rd). This means that every single setting can be optimized in an easy-to-use fine-tuning.

0015The tuning device according to the invention means a considerable advance over the publication WO 94/08371. Only a single device for compensation is provided there, so that only the first order chromatic dispersion can be compensated for. A non-correctable default setting is selected for 2nd order chromatic dispersion. However, since the refractive index of semiconductor lasers is a complicated function of the wavelength, at least the 3rd order contribution of the chromatic dispersion (<i>n</i>₃) also take into account. If this is not done, there is an upper limit of the mode jump-free tuning range at approx. 60nm to 80nm for 1.55µm laser diodes. In contrast, according to the invention, mode-jump-free wavelength tuning is possible over the entire amplification range of semiconductor laser diodes, in that the second-order dispersion can be compensated exactly and the third-order dispersion can be compensated for almost exactly.

0016According to claim 3, an adjusting means for translating the resonator end mirror in the direction of the mirror normal is available for adjusting the distance between the axis of rotation and the mirror plane. In particular, this can be a large-caliber fine-thread screw, on which the mirror is fastened so that its normal runs parallel to the screw axis. It is structurally favorable if the resonator end mirror is a right-angled prism in accordance with claim 5 such that the light beam coming from the grating penetrates into the prism via the hypotenuse surface, experiences total reflection one after the other on the two catheter surfaces and then out of the prism via the hypotenuse surface emerges, the line of intersection of the prism catheter surfaces perpendicular to the furrows of the grid. Thanks to this alignment of the prism, the optimal quality of the laser resonator is guaranteed without impairing the wavelength selectivity of the grating. This makes the resonator largely insensitive to adjustment errors, which means that the laser can be used under harsh everyday conditions.

0017An alternative to claim 5 is formulated in claim 6. If a cylindrical lens is inserted between the grating and the resonator end mirror in such a way that the longitudinal axis of the cylindrical lens is approximately perpendicular to the grating furrows and that the mirror plane of the resonator end mirror is in the focal plane of the cylindrical lens, the resonator is also largely insensitive to adjustment errors.

0018The feature of claim 7, according to which the grille is fastened on a grille holder with two degrees of freedom of adjustment, is very advantageous. The grid holder can in particular be rotatable in two directions with the aid of adjusting means, the axes of rotation being perpendicular to one another. According to claim 9, a first direction of rotation runs along an axis of the grid holder, which allows the adjustment of the resonator quality. According to claim 10, a second direction of rotation runs along an axis of the grating holder, as a result of which the angle of incidence of the light emitted by the semiconductor diode laser on the grating can be adjusted. Because the first axis of rotation is parallel to the lattice furrows and the second axis of rotation is perpendicular to the latter, the two axes of rotation span a plane that runs parallel to the lattice plane. This arrangement contributes greatly to the convenient and reliable adjustability.

0019For changing the distance between the grating plane and the axis of rotation of the tuning arm, claim 11 provides a sliding table on which the grating holder is fastened together with the grating. This arrangement is very stable. According to claim 12, a fine-thread screw is provided as an adjusting means for the translation of the grating and the grating holder, preferably in the direction of the grating normal, so that the distance adjustment can be carried out very precisely.

0020In a device of the type mentioned at the outset, the invention provides, according to independent claim 13, a combination which has a two-dimensional arrangement for the targeted adjustment and compensation of the chromatic dispersion of higher orders such that the grating is mounted on a grating holder in such a way that it can be varied in angle and that it is mounted on a Sliding table with which the grille is variable in relation to the pivot point of the tuning arm. In this way, a working level is defined in which the required compensation settings can be made quickly and effectively.

0021Also of fundamental importance is the tuning device according to the further independent claim 14, which is characterized in that the grating is mounted on a grating holder so that the angle of incidence of the light emitted by semiconductor diode lasers can be adjusted onto the grating for the targeted adjustment and compensation of the higher order chromatic dispersion is changeable while the grid holder is mounted on a sliding table in such a way that the distance between the grating plane and the pivot point of the tuning arm is adjustable. The three relevant setting options are combined in this combination in a simple and clear manner.

0022Further features, details and advantages of the invention emerge from the wording of the claims and from the following description of exemplary embodiments with reference to the drawings. Show in it<dl id="dl0001" compact="compact"><dt><b>Fig. 1A:</b></dt><dd>1 shows a schematic illustration of the basic arrangement of a tuning device,</dd><dt><b>Fig. 1B:</b></dt><dd>a prism beam path,</dd><dt><b>1C:</b></dt><dd>Beam path with cylindrical lens and mirror,</dd><dt><b>Fig. 2:</b></dt><dd>1 shows a schematic illustration of a preferred design of a tuning device,</dd><dt><b>Fig. 3A:</b></dt><dd>2 shows a schematic side / sectional view of a grid holder with adjusting means,</dd><dt><b>3B:</b></dt><dd>a schematic side / sectional view of an angle adjustment device,</dd><dt><b>4A, 4B, 4C:</b></dt><dd>one side view and top view of rotary axis designs,</dd><dt><b>Fig. 5:</b></dt><dd>A schematic representation of the optical conditions of the overall arrangement and</dd><dt><b>Fig. 6:</b></dt><dd>a wavelength tuning diagram.</dd></dl>

00231A shows a semiconductor laser diode with an external resonator in a Littman arrangement. A semiconductor laser 1 has a mirror facet which is provided with an anti-reflection coating 2 and which faces the external resonator. A lens 3 collimates the laser light emitted by the mirror facet that at an angle of incidence θ₁ hits the surface of a diffraction grating 4. Light of the zeroth diffraction order 5 is coupled out as a useful beam from the laser. Light of the first diffraction order 6 hits the external resonator end mirror 7, is reflected there and, after it has been diffracted a second time by the grating 4, is fed back into the laser 1. The mirror 7 is fastened on a tuning arm 8 and this is rotatably mounted about an axis of rotation 9 parallel to the mirror plane 701. If the tuning arm 8 is rotated about the axis of rotation 9, the wavelength λ determined by the grating 4 changes_G and at the same time the wavelength λ determined by the resonator length_<i>R</i>.

00241B shows a main section of a prism 15. The section plane is spanned by two intersecting normals of the catheter surfaces 152 and 153. A beam coming from the grating 4 and running in this plane penetrates into the prism 15 via the hypotenuse surface 151, is successively totally reflected on the catheter surfaces 152, 152 and leaves the prism 15 again via the hypotenuse surface 151. Incoming beam 61 and outgoing beam 62 are anti-parallel to each other. When the emerging beam 62 hits the collimator lens 3 after the diffraction at the grating 4, it is fed back into the semiconductor laser 1, largely independently of the angle to the hypotenuse surface 151.

00251C shows a section through the cylindrical lens 16 and the resonator end mirror 7, such that the sectional plane is perpendicular to the longitudinal axis 162 of the cylindrical lens 16. A beam coming from the grating and running in this plane and parallel to the optical axis of the cylindrical lens is refracted in the direction of the optical axis of the cylindrical lens 16, reflected at the mirror plane 701 and then refracted at the cylindrical lens 16 a second time. If the distance between the cylindrical lens 16 and the mirror plane 701 is chosen equal to the focal length of the cylindrical lens 16 on the mirror side, then the incident beam 61 and the emerging beam 62 are antiparallel to one another. When the incident beam 62 hits the collimator lens 3 after the diffraction at the grating 4, it is fed back into the semiconductor laser 1, largely independently of the angle of the mirror plane 701 to the incident 61 and incident beam 62.

00262 shows a top view of a preferred embodiment of the tuning device according to the invention. The one-sided anti-reflection coated semiconductor diode laser 1 is firmly mounted together with the collimator lens 3 on a base plate 10 on which the grating 4 is also fastened together with the grating holder 11. The tuning arm 8, which carries the external resonator end mirror 7, is connected to the base plate 10 in such a way that it can only rotate along the axis of rotation 9. A return spring 12 presses the tuning arm 8 against an adjusting unit 13, which preferably consists of a micrometer screw and a piezotranslator, and controls the precise setting of the tuning arm 8. If the micrometer screw is turned or the electrical voltage applied to the piezotranslator is changed, this causes rotation and translation of the resonator end mirror 7 at the same time. The rotation changes the angle of reflection θ₂ (Fig. 5) of the light of the first diffraction order, ie the tuning steel 6 and therefore the wavelength λ determined by the grating 4_<i>G</i>. The translation changes the resonator length<i>L</i>_<i>opt</i> and thus the wavelength λ determined thereby_<i>R</i>.

0027FIG. 2 also shows a sliding table 14, the components of which can be seen more clearly from FIGS. 3A and 3B. It can be seen that the grid holder 11 consists of an upper part 111 and a lower part 112 which are screwed together and fastened on a sliding table 14. The lower part 112 of the grid holder 11 is a bending element which is pressed apart by a fine-thread screw 113. As a result of their rotation, the grid 4 is rotated about an axis 114 which is at least approximately parallel to the grid plane 401 and is perpendicular to the grid furrows 403. This setting option allows the resonator quality to be adjusted.

0028The displacement table 14 also has a fine-thread screw 141, the rotation of which enables the grid holder 11 and the grid 4 to be translated in a direction which corresponds at least approximately to the direction of the grid normal 402. This degree of freedom of adjustment is used to determine the mode jump-free wavelength tuning range.

0029FIG. 3B shows a section through the upper part 111 of the lattice holder 11 (cf. FIG. 3A). The upper part 111 is also a bending element which is pressed apart by a fine-thread screw 115, the rotation of which causes the grid 4 to rotate about an axis 116 parallel to the grid furrows 403. This also contributes to the setting of the maximum mode jump-free wavelength tuning range. Once the settings of the lattice holder 11 and the sliding table 14 have been made, they can remain unchanged during the rest of the tuning process.

00302 and FIGS. 4A, 4B, 4C show possible implementations of the tuning arm 8. It carries a screw 801 to which the return spring 12 can be fastened. The resonator end mirror 7 is seated on a large-caliber fine-thread screw 802, which is screwed into the tuning arm and, by its rotation, allows the mirror 7 to be translated in the direction of the mirror normal 702. Once the tuning arm (8) has been set, it can remain unchanged during the rest of the tuning process.

0031A constructive possibility is illustrated in Fig. 4A. Here the axis of rotation 9 is a metal rod 901 which is rotatably connected to the tuning arm 8 via bearings 902 and to the base plate 10. In the variant according to FIG. 4B, the tuning arm 8 is also firmly seated on the base plate 10, but it has a constriction 903 which acts as an axis of rotation 9. Fig. 4C illustrates a tuning arm 8, which is connected via two balls 904 and a spring 905 to a rod 906, which in turn is fastened on the base plate 10. The axis of rotation 9 lies in the connecting line 907 of the center points of the balls 904. It can be seen that all of these variants allow a highly precise, practically play-free rotary movement of the tuning arm 8, as is required for mode-free wavelength tuning of the laser 1. As from Fig. 6 where the laser beam intensity as a function of the wavelength is shown in a diagram, there is a mode jump-free course over a wavelength range of 130 nm in the measurement example, which essentially equates to a doubling of the range compared to conventional technology.

0032In the following the function of the tuning device according to the invention is further explained with reference to FIG. 5. You can see the distance<i>x</i>₂ is adjustable adjustable between the axis of rotation 9 and the mirror plane 701. The latter contains straight lines that run parallel to the axis of rotation 9, as a result of which the geometric definition of the term for the distance between point and straight line can be used in the usual way. By translating the resonator end mirror 7 with the aid of the screw 802 in the direction of its normal 702, the resonator length changes<i>L</i>_<i>geo</i>without the wavelength λ determined by the grating 4_<i>G</i> would be changed. This is an exclusive change in the number of nodes<i>m</i> of the standing wave in the laser resonator possible. Rotation and translation of the resonator end mirror 7 are synchronized so that the wavelength λ determined by the grating 4_<i>G</i> and that of the resonator length <i>L</i>_<i>opt</i> certain wavelength λ_<i>R</i> differ by less than a quarter of the wavelength (see. Eq. (1)). Because the refractive index of semiconductor lasers, in contrast to, for example, a gas laser or dye laser, has a considerable chromatic dispersion, the geometric length differs<i>L</i>_<i>geo</i> according to Eq. (3) Considerable in optical length<i>L</i>_<i>opt</i>. The adjustment means present in the tuning device according to the invention now enable compensation in a surprisingly simple and reliable manner. The adjusting screw 802 serves to compensate for the 1st order dispersion of all components contained in the laser resonator (semiconductor laser 1, anti-reflection coating 2, collimator optics 3, air). The second-order dispersion is exactly compensated by means of the fine-thread screw 141. The fine-thread screw 115 permits the compensation of the 3rd order dispersion and a preselection of the compensation of the 4th order dispersion.

0033The relevant optical-geometric variables are shown schematically in FIG. θ₁ denotes the angle of incidence of the light emitted by the laser 1 onto the grating 4; it can be adjusted using the adjusting screw 115. With<i>x</i>₁ the distance between the grating plane 401 and the pivot point 9 of the tuning arm 8 is designated. This distance<i>x</i>₁ is adjusted by translating the grating 4 in the direction of the grating normal 403, specifically by turning the adjusting screw 141 of the sliding table 14, which causes the grating holder 11 to be translated together with the grating 4. Small angle differences between the translation direction and grating normals 402, which could be caused by rotation of the grating 4 about the axes of rotation 115 and 116, interfere with the adjustment of the distance<i>x</i>₁ Not.

0034<i>x</i>₂ denotes the distance between the mirror plane 701 and the pivot point 9 of the tuning arm 8; this can be set with the Scharube 802. The geometric resonator length<i>L</i>_<i>geo</i> consists of two partial lengths <i>L</i>₁ and <i>L</i>₂ together. Partial length<i>L</i>₁ is the geometric distance between the resonator end surface 101 and the center 404 of the illuminated surface of the grating 4. The partial length <i>L</i>₂ is given by the geometric distance between the mirror plane 701 and the center 404 of the illuminated surface of the grating 4. According to equations (2) and (3), there is a connection<maths id="math0004" num="(4)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext mathvariant="italic">opt</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic"> = n (λ) · (L</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext></mtext><msub><mrow><mtext mathvariant="italic">+ L</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mtext mathvariant="italic">).</mtext></mrow></math><img file="EP0801451A2_D0004.tif" /></maths> This description of the dispersion of the semiconductor laser material can easily be extended to the dispersion of all media contained in the resonator, for example by weighting and adding up their refractive index functions according to their length. The global dispersion of all media contained in the resonator causes a difference between the resonator wavelength λ_<i>R</i> and the grating wavelength λ_<i>G</i>which can be described in a series development as follows:<maths id="math0005" num="(5)"><math display="block"><mrow><mtable><mtr><mtd><mrow><mtable><mtr><mtd><mrow><msub><mrow><mtext>λ</mtext></mrow><mrow><mtext mathvariant="italic">R</mtext></mrow></msub><mtext mathvariant="italic"> -</mtext><msub><mrow><mtext> λ</mtext></mrow><mrow><mtext mathvariant="italic">G</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic"> = f</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext> · Δλ + </mtext><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msup><mrow><mtext> · Δλ</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext> + </mtext><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>3</mtext></mrow></msub><msup><mrow><mtext> · Δλ</mtext></mrow><mrow><mtext>3</mtext></mrow></msup><mtext> + ...</mtext></mrow></mtd></mtr><mtr><mtd><mrow><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msub><mtext> = Development coefficient</mtext></mrow></mtd></mtr><mtr><mtd><mrow><msub><mrow><mtext>Δλ = λ</mtext></mrow><mrow><mtext mathvariant="italic">G</mtext></mrow></msub><msub><mrow><mtext> - λ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></mtd></mtr><mtr><mtd><mrow><msub><mrow><mtext>λ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> = Development point</mtext></mrow></mtd></mtr></mtable></mrow></mtd></mtr></mtable></mrow></math><img file="EP0801451A2_D0005.tif" /></maths> Are special <i>f</i>₁, <i>f</i>₂, <i>f</i>₃ Functions of the setting variables θ₁, <i>x</i>_i and <i>x</i>₂. With their help, the value of the development coefficients can be selected, which is given below in the order of the adjustment process on the laser resonator.<dl id="dl0002" compact="compact"><dt><i>x</i>₂:</dt><dd>The value of <i>x</i>₂, which is selected by means of the adjusting screw 802 of the tuning arm 8, serves for the exact compensation of the development coefficient <i>f</i>₁.</dd><dt><i>x</i>₁:</dt><dd>The value of <i>x</i>₁, which is set by means of the adjusting screw 141 of the sliding table 14 - on which the grid holder 11 is fastened together with the grid 4 - leads to the exact compensation of the development coefficient<i>nf</i>₂.</dd><dt>θ₁:</dt><dd>The value of the angle of incidence θ₁ the grating 4 can be adjusted in a sufficiently large interval by means of the adjusting screw 115. His choice ensures the exact compensation of the development coefficient<i>f</i>₃.</dd></dl>

0035It can be seen that the tuning device according to the invention is not only constructed in a relatively robust manner, but is also extremely sensitive to use and makes it possible for the first time to tune the emission wavelength of semiconductor diode lasers over the entire amplification range without a jump in mode. Computed values ​​for the tuning range are many times above the amplification range of semiconductor lasers. Therefore, there is even a large tolerance range for the selection of the aforementioned adjustment means (802, 141, 115) with which the adjustment process can be carried out quickly and safely. The resonator quality is largely insensitive to external disturbances that can cause adjustment errors. The wavelength tuning behavior when rotating the tuning arm 8 about the axis of rotation 9 is retained without the spectral selectivity of the grating resonator being influenced.

0036The invention is not limited to the examples shown and described, but can be modified in many ways. It can be seen, however, that in a preferred embodiment of a semiconductor diode laser 1 with an anti-reflective coating 2, collimator optics 3, reflection diffraction grating 4 and resonator end mirror 7 in a Littman arrangement, angle of reflection θ₂ of the grating and the geometric resonator length <i>L</i>_<i>geo</i> are mechanically linked to compensate for the chromatic dispersion of all media contained in the resonator (semiconductor diode laser 1, anti-reflection coating 2, collimator optics 3, air) so precisely when the mirror 7 is rotated about the axis of rotation 9 that the compensation for the dispersion first, second and third order with the help of Justiermittls 802, 141, 115 is effected and thus a mode shift-free wavelength tuning is guaranteed over the entire amplification range of the semiconductor diode laser 1.

0037All of the features and advantages arising from the claims, the description and the drawing, including structural details, spatial arrangements and method steps, can be essential to the invention both individually and in the most varied of combinations.

List of reference numbers

0038<dl id="dl0003" compact="compact"><dt>1</dt><dd>Semiconductor laser</dd><dt>2</dt><dd>Anti-reflective coating</dd><dt>3</dt><dd>Lens / collimator optics</dd><dt>4</dt><dd>(Diffraction) grating</dd><dt>5</dt><dd>Useful beam</dd><dt>6</dt><dd>Tuning beam</dd><dt>7</dt><dd>Resonator end mirror</dd><dt>8</dt><dd>Arm</dd><dt>9</dt><dd>Axis of rotation</dd><dt>10</dt><dd>Base plate</dd><dt>11</dt><dd>Lattice holder</dd><dt>12</dt><dd>Return spring</dd><dt>13</dt><dd>Adjustment unit</dd><dt>14</dt><dd>Sliding table</dd><dt>15</dt><dd>prism</dd><dt>16</dt><dd>Cylindrical lens</dd><dt>61</dt><dd>Incident beam</dd><dt>62</dt><dd>Drop beam</dd><dt>101</dt><dd>Resonator end face</dd><dt>111</dt><dd>upper part</dd><dt>112</dt><dd>lower part</dd><dt>113</dt><dd>Fine thread screw</dd><dt>114</dt><dd>Axis of rotation</dd><dt>115</dt><dd>Fine thread screw</dd><dt>116</dt><dd>Axis of rotation</dd><dt>141</dt><dd>Fine thread screw</dd><dt>151</dt><dd>Hypotenuse area</dd><dt>152</dt><dd>first catheter surface</dd><dt>153</dt><dd>second catheter surface</dd><dt>154</dt><dd>Straight line</dd><dt>161</dt><dd>Longitudinal axis of the cylindrical lens</dd><dt>162</dt><dd>Focal plane of the cylindrical lens</dd><dt>401</dt><dd>Grid plane</dd><dt>402</dt><dd>Grid standards</dd><dt>403</dt><dd>Furrows</dd><dt>404</dt><dd>Beam center</dd><dt>701</dt><dd>Mirror plane</dd><dt>702</dt><dd>Mirror standards</dd><dt>801</dt><dd>screw</dd><dt>802</dt><dd>(Adjusting) screw</dd><dt>901</dt><dd>Metal rod</dd><dt>902</dt><dd>camp</dd><dt>903</dt><dd>Constriction</dd><dt>904</dt><dd>Bullets</dd><dt>906</dt><dd>Rod</dd><dt>907</dt><dd>Connecting line</dd></dl>

Reference sybollist

0039<dl id="dl0004" compact="compact"><dt>λ</dt><dd>Emission wavelength of the semiconductor laser with external resonator</dd><dt>λ₀</dt><dd>Development point</dd><dt>λ_<i>G</i></dt><dd>Wavelength determined by the diffraction grating</dd><dt>λ_<i>R</i></dt><dd>Wavelength determined by the resonator</dd><dt><i>L</i>_<i>geo</i></dt><dd>Geometric length of the laser resonator</dd><dt><i>L</i>_<i>opt</i></dt><dd>Optical length of the laser resonator</dd><dt><i>L</i>₁</dt><dd>First part length of the resonator</dd><dt><i>L</i>₂</dt><dd>Second part length of the resonator</dd><dt><i>m</i></dt><dd>Number of wave nodes in the resonator</dd><dt><i>n</i>(λ)</dt><dd>Wavelength-dependent refractive index of the laser diode 1</dd><dt><i>n</i>_<i>i</i></dt><dd><i>i</i>-th development coefficient of the refractive index at the development point λ₀</dd><dt><i>f</i>_<i>i</i></dt><dd><i>i</i>-th development coefficient of the deviation between λ_<i>G</i> and λ_<i>R</i> at the development point λ₀</dd><dt>θ₁</dt><dd>Angle of incidence of the grating 4</dd><dt>θ₂</dt><dd>Angle of reflection of the first diffraction order of the grating 4</dd><dt><i>x</i>₀</dt><dd>Distance between pivot point 9 and resonator end mirror 101</dd><dt><i>x</i>₁</dt><dd>Distance between pivot point 9 and grid plane 401</dd><dt><i>x</i>₂</dt><dd>Distance between pivot point 9 and resonator end face 7</dd></dl>