HK1083042B - Resonator - Google Patents

Description

Resonator having a dielectric layer

Technical Field

The present invention relates to a wavelength tunable light source.

Technical Field

Wavelength tunable light sources, mainly wavelength tunable laser sources, are currently growing rapidly and the use of such light sources takes place in fields such as telecommunications. Thus, a variety of different wavelength tunable light sources are well known in the art.

JP-06021549 discloses such a light source. This document discloses a tunable semiconductor laser comprising: a semiconductor gain element having a mirrored end and an outer second mirrored end, both said mirrored ends defining an optical cavity. Two condenser lenses are arranged in the optical cavity, and a rotatable filter is arranged between the two condenser lenses. By rotating the filter, the wavelength of the main lasing mode can be changed. However, this structure contains multiple elements, and therefore it is quite sensitive to misalignment and expensive to manufacture. It is difficult to make the resonator mechanically and optically stable over a long period of time. Furthermore, it is complicated to implement wavelength tuning without mode hops.

A second prior art device utilizing the same basic principles is disclosed in "OFC' 98/Wednesday potter/124 WQ27, Interference-filtering of a semiconductor laser in a simulation-tall generator external facility, P.Zorabedian, W.R. Trutna Jr. Even if this structure is insensitive to angular misalignment of the external mirrors, this type of wavelength tunable light sources has the disadvantage that their structure is complicated and that the structure is sensitive to the individual positions of the individual elements, and therefore, misalignment of the elements can cause a reduction in the functionality of the light source. As with the above structure, this structure also contains a plurality of elements, and therefore, it is not only sensitive to positional misalignment of the elements, i.e., misalignment due to relative movement of the elements along the optical axis of the system, but also expensive to manufacture. Furthermore, it is complicated to implement wavelength tuning without mode hops.

A resonator structure having a simpler structure is disclosed in "connected-centralized laser resonator, R.V.Pole, Journal of Optical Society of America, Vol.55, No.3, pages 254-. This document discloses a laser resonator which is a spherical resonator in which an active gain medium is placed in the center of an optical cavity, which acts like a lens, thus realizing a conjugate concentric resonator. However, such light sources are not tunable, thereby limiting its use in contemporary applications. Furthermore, since the lens element is identical to the optical gain element, this structure cannot be used for a semiconductor laser.

Other examples of tunable external cavity structures are the so-called Littman and littrow structures well known to those skilled in the art. However, both structures are sensitive to detuning, so that a more stable structure is required. Furthermore, many prior art structures contain multiple elements and therefore have disadvantages related to offset losses. Furthermore, the different optical cavities described above suffer from large resonator losses due to the presence of several component surfaces within the optical cavity, resulting in reduced optical power output.

Disclosure of Invention

It is therefore an object of the present invention to provide a wavelength tunable light source which overcomes at least some of the above-mentioned disadvantages of the prior art and which provides a light source having a simple and stable construction and which, in addition, can be realized in a cost-effective manner.

The above object is achieved with a wavelength tunable light source comprising: a primary resonator having a first mirrored end and a second mirrored end, the two ends defining an effective cavity length, i.e., a beam path length of a lasing mode of the cavity; an optical gain element having opposing first and second end faces, said second end face being positioned within said primary resonator; a mirror element constituting the second mirrored end; and a dispersive focusing resonator element positioned between said second end face along the beam path and said mirror element, wherein said effective cavity length of said primary resonator is variable. Thus, a stable tuneable resonator insensitive to detuning can be achieved, while at the same time having a simple structure. Furthermore, tuning can be achieved without mode hopping. Furthermore, the resonator has low losses. Furthermore, due to the structure of the resonator, a resonator can be realized which is less sensitive to the quality of the anti-reflection coating on the inner surface of the resonator, thereby realizing a resonator suitable for low-cost mass production.

According to a particularly preferred embodiment of the invention, the optical gain element is a surface emitting element. In this case, the resonator is substantially self-tuning, for example, which facilitates ease of manufacture and ensures stable operation. More preferably, there is a second mirrored end on the core end of the optical fiber. In this case, the resonator structure facilitates self-alignment, thereby enabling the outgoing laser beam to be automatically coupled into the fiber.

Other preferred embodiments of the invention and its advantages will be apparent to the skilled person from the claims and the following description.

Drawings

The presently preferred embodiments of the present invention are described below with reference to the accompanying drawings.

Figure 1 is a schematic view of a first version of a first main embodiment of the invention.

Fig. 2 is a schematic diagram of the working principle of the present invention.

Figure 3 is a schematic representation of a second version of the first main embodiment of the invention.

Figure 4 is a schematic view of a third alternative of the first main embodiment of the invention.

Figure 5 is a schematic view of a first version of a second main embodiment of the invention.

Figure 6 is a schematic representation of a second version of the second main embodiment of the invention.

Figure 7 is a schematic view of a third main embodiment of the invention.

Fig. 8a is a schematic diagram of the round-trip path of a light beam circulating multiple times in a prior art resonator.

Figure 8b is a schematic diagram of the round-trip path of a light beam circulating multiple times in a resonator of the first main embodiment of the present invention.

Figure 8c is a schematic diagram of the round-trip path of a light beam circulating multiple times in a resonator according to a second main embodiment of the invention.

Detailed Description

A first version of the first main embodiment of the invention is disclosed in fig. 1. Fig. 1 discloses a laser source comprising: a main cavity M defined by the first mirrored end 1 and the second mirrored end 2. Within the main resonator is arranged an optical gain element 3, here an edge emitting semiconductor element. The term "edge-emitting semiconductor element" is to be interpreted as an optical gain element in which light propagates substantially in a direction perpendicular to the normal of the surface of the epitaxial layers constituting the active material of the optical gain element. The optical gain element 3 has a first end face 3a and a second end face 3b opposite to each other, wherein said first end face 3a in this case constitutes a first mirror end face 1 of said resonator. The second end face 3b is within the cavity, with or without an anti-reflection coating, as described below. In this case the second mirrored end 2 of the cavity has a partially transmissive broadband reflective coating on the mirror element 4, the reflectivity of which is such that a desired predetermined proportion of the power between the two mirrored ends of the main resonator M is emitted from said mirror element 4 at all relevant wavelengths.

According to the invention, a dispersive, focusing element 5 is also arranged in the resonator between said second end face 3b of the edge-emitting semiconductor element 3 and the mirror element 4. The structure of the dispersive, focusing element is described below. The dispersive, focusing element exhibits a wavelength dependent focal length which produces a wavelength dependent position of the image plane of the second end face of the optical gain element, as indicated in fig. 2. FIG. 2 shows a first wavelength λ₁Is focused to a first plane z₃₁Having a second wavelength λ₂Is focused to a second plane z₃₂And has a third wavelength λ₃Is focused to a third plane z₃₃. Thus, moving the mirror element 4 between different longitudinal positions, i.e. along the beam path of the resonant mode, changes the effective length of the resonant cavity, thereby causing the light source to emit laser light at different wavelengths. The dispersive, focusing element enables a high Q-value of the resonator, i.e. a small loss in a narrow wavelength interval for each chosen position of the mirror element 4. Thus, only the z-axis direction of the resonator in the case of fig. 1 or 2 is providedMoving only the mirror element 4 allows the resonator to be tuned to resonance at different wavelengths. Furthermore, this movement can be done without changing the total phase shift within the main resonator, i.e. without mode hops.

Thus, when the laser light is emitted at the selected wavelength, the mirror element 4 is moved to a position which is the image plane position of the dispersive, focusing element 5 at the given wavelength. Thus, while the mirror element 4 remains in a fixed position, other elements in the resonator may also be moved to achieve the same effect, e.g. the movement of the optical gain element 3 itself relative to the dispersive focusing element. The optical gain element and the dispersive focusing element may also be moved relative to the mirror element 4 to achieve the desired effect.

According to a variant of the first version of the first main embodiment of the invention, the mirror elements 4 of the above-described embodiment can be exchanged for reflecting elements having reflecting surfaces, the reflectivity of which in cross-section is changed in such a way that the main lowest order mode of the resonator is highly reflective and the higher order mode has a low reflectivity. Thus, the resonator Q-value is reduced for all wavelengths in the cavity, except for the desired laser wavelength. The highest reflectivity is in the central area of the mirror element and outside this area there is a lower specular reflectivity. The area of the highly reflective surface may be chosen such that it substantially corresponds to the extension of the main mode on the mirror. In this way, two axial modes having resonant frequencies different from the resonant frequencies of the primary and higher spatial modes can be suppressed. The mirror element may be designed in various ways as long as the primary mode has a higher specular reflectivity than light of a wavelength other than the primary mode. This can be achieved by applying an anti-reflection coating, an absorbing layer, an extended light scattering layer or a reflective or transmissive diffractive surface relief with respect to the mirror region outside the highly reflective region of the master mode. It should also be noted that mirror elements with such a reflectivity, which in cross section is otherwise variable, may be used. In general, it may be that R ═ R (x, y), where R is the reflectivity of the mirror element, and x and y determine the coordinate system describing the reflective surfaces of the mirror element. By properly designing the reflectivity pattern of the mirror elements, any mode can be selected within the cavity, which is not necessarily the primary mode. In this case, however, a gain element is chosen that allows the desired mode of propagation, otherwise the losses are quite severe.

According to a second alternative of the first main embodiment of the invention, as shown in fig. 3, the mirror element may be arranged directly on one end face of the optical fiber. Thus, the light output by the light source can be directly coupled into an optical fiber of the single-mode or multi-mode type.

According to a third aspect of the first main embodiment of the present invention, as shown in fig. 4, the second end face 3b of the semiconductor gain element 3 may have a high reflectivity, e.g. between 20-40%, typically about 30%, which is typical of the reflectivity of the interface between the semiconductor material and air, i.e. without anti-reflection coatings or the like. This embodiment further comprises means 6 for changing the optical path length of the gain element, here heating means, e.g. a thermo element or a resistor, of the gain element 3, thereby changing its optical path length. The primary resonator here comprises two coupled resonators, one defined by the two end faces 3a, 3b of the optical gain element 3 and the other defined by the second end face 3b of the optical gain element 3 and the mirror element 4. In this embodiment, it is desirable to control the phase relationship between the two resonators, wherein the heating means 6 is used to change the waveguide optical length of the optical gain element as a function of the desired laser wavelength. It should be noted that in this embodiment the mirror element 4 may be any of the types of elements described above. It should also be noted that mode phase control in an optical gain element according to this scheme may also be utilized in case the reflectivity of the second end face 3b of the semiconductor gain element 3 is outside the above 20-40% interval. For example, when the reflectivity of the second end face 3b of the semiconductor gain element 3 is 10^-3-10^-2Can be used in another preferred embodiment in order to be able to tune the light source over a wide wavelength range. In this context, it should be noted that other methods of varying the effective optical cavity length are possible,this is achieved by changing the refractive index of an element in the optical cavity, e.g. a separate optical plate in the optical cavity, by means of temperature changes or changing the electric field, the refractive index of the optical plate can be changed. Another possibility is to use a gain element that is divided into a gain part and a phase part, which parts are arranged one after the other along the optical path in the optical cavity. The gain section is thus arranged to generate gain and can be controlled by means of a gain current, the phase section not providing gain but only changing the refractive index of the phase section, which is controlled by means of a phase current. Therefore, by changing the optical path of the phase section, the optical path of the gain element can be changed.

According to another version of the first main embodiment of the invention, the laser beam may be coupled out through a first mirrored end of the main resonator, i.e. in this case the first end of the optical gain element. In this embodiment the mirror element 4 is arranged as a reflective and partially transmissive broadband mirror, while the first end face 3a is arranged to emit part of the power of the laser beam. This scheme can also be used in any of the other embodiments described above.

A first version of the second main embodiment of the invention is described below with reference to figure 5. Fig. 5 discloses a laser source comprising: the first mirrored end 11 and the second mirrored end 12 define a main cavity M. Within the main cavity M, an optical gain element 13, here a surface emitting semiconductor gain element, is arranged. The term "surface emitting semiconductor element" is to be interpreted as an optical gain element in which the optical beam propagates substantially in a direction parallel to the normal of the surface of the epitaxial layers constituting the active material of the optical gain element. The optical gain element has opposite first and second end face portions 13a, 13b, wherein said first end face portion 13a in this case constitutes the first mirror end face 11 of said resonator. The second end face portion 13b is within the cavity and may or may not be anti-reflection coated. The second mirrored end 12 of the cavity has a partially transmissive broadband reflective coating on a mirror element 14, the reflectivity of which is selected such that a predetermined proportion of the total optical power generated in the resonator at all wavelengths is emitted from the second mirrored end. According toIn the present invention, the dispersive focusing element is arranged in the optical cavity between said second end surface of the surface emitting semiconductor gain element 13 and the mirror element 14. The structure of the dispersive, focusing element is described below. The dispersive, focusing element has a wavelength dependent distance to its image plane as shown in figure 2. FIG. 2 shows that light having a first wavelength λ 1 is focused onto a first plane z₃₁The light having the second wavelength lambda 2 is focused onto a second plane z₃₂And light having a third wavelength λ 3 is focused onto a third plane z₃₃Schematic representation of (a). Thus, as described above, moving the mirror element 14 between different positions along the beam path of the resonator may change the effective length of the optical cavity, and the light source may be arranged to emit laser light at different wavelengths. The dispersive, focusing element enables a high Q value of the resonator, i.e. a small loss at a narrow wavelength interval for each chosen position of the mirror element 14. Thus, in the case shown in fig. 5, the resonator can be tuned to resonance at different wavelengths by simply moving the mirror element 14 along the z-axis of the resonator. Furthermore, this shift can be done without changing the total phase shift within the main resonator, i.e. without mode hops.

Therefore, when the laser light is emitted at the selected wavelength, the mirror element 14 is moved to the image plane position of the dispersive, focusing element 15 at the selected wavelength. In addition, other elements in the optical cavity may also be moved to achieve the same effect.

According to another variant of the second embodiment, the mirror element 12 may have a reflecting surface whose reflectivity varies in its cross-section in the manner described above. In this case, tuning of the resonator wavelength is possible without misalignment of the resonator when the mirror element is moved in the lateral and/or longitudinal direction, i.e. when the mirror element is moved in a beam path perpendicular to and/or along the resonance mode.

According to a second aspect of the second main embodiment of the present invention, as shown in fig. 6, the mirror elements of the above second main embodiment may be arranged on the end face of the core region of an optical fiber, so that the light output from the light source can be directly coupled into the optical fiber, which may be a single mode or multimode optical fiber. This configuration enables self-aligning laser light into the fiber. Since the lasing mode is allowed to move laterally in the surface emitting gain element and said second mirror element is arranged to the core as described above, the lasing mode is automatically adjusted to the position of the core, since this is the position where the resonator losses are minimal.

According to a further version of said second main embodiment the laser beam may be coupled out through the first mirrored end of the main resonator, i.e. in this case through the first end of the optical gain element. In this embodiment the mirror element 14 is preferably a highly reflective broadband mirror, and the first end face 13a is arranged to emit a laser beam of a predetermined partial power as required.

It is noted that in the device according to the first main embodiment of the invention the second end surface of the optical gain element is imaged onto the second mirrored end of the resonator, whereas in the second main embodiment the first mirrored end is imaged onto the second mirrored end of the resonator, said imaging making the resonator device very stable. Thus, any object point on one mirror plane is imaged onto the other mirror plane. Fig. 8b shows the working principle of the first main embodiment, in which the mirror element 4 is angularly misaligned (fig. 8a discloses a reference example of a prior art resonator). The beam marked 1 in fig. 8 is incident on the mirror element 4 and is returned to the resonator along the beam path marked 2. When the light beam, labelled 2, reaches the surface 3b, this mode winds up with the waveguide mode of the optical gain element and proceeds to the first mirrored end 3a and is reflected within the optical gain element, again striking the surface 3b, after which the above process is repeated. This can be seen as the ring resonator mode being spatially filtered in the waveguide/gain element, once per round trip of the resonator. In the case of the second main embodiment of the arrangement according to the invention, fig. 8c shows the stability of the resonator with an angularly misaligned mirror element 4. The basic principle is substantially the same as described above in fig. 8 b.

The first mirror element 1 may also be a reflective chirped grating, i.e. a reflective stack with alternating high and low reflectivities, wherein the thickness of the layers is different throughout the stack. With such a chirped mirror element, a broadband wavelength characteristic can be obtained. The use of such a chirped grating at the end of the optical gain element waveguide has the further advantage of allowing the waveguide length of the optical gain element to be wavelength dependent. For example, a chirped grating mirror can be designed whose waveguide length increases/decreases by an appropriate amount with wavelength, so that the total effective phase shift of the optical gain element becomes a constant value independent of wavelength. This may be viewed as an alternative to the embodiment shown in figure 4.

The dispersive, focusing element in the above embodiments is described in detail below. In the direct method, the dispersive, focusing element is made up of a single positive refractive lens element having both dispersive and focusing characteristics. The refractive index of the refractive element is related to the wavelength of the incident light, thereby creating a dispersive effect. Alternatively, the dispersive, focusing element may be constituted by a combination of refractive lenses, which function as positive lenses, even if one or more of the lenses in the combination is a negative lens. In this case, the focal length of the lens combination varies with the wavelength of the incident light. By selecting a suitable combination of materials in the lens combination, the amount of dispersion can be adjusted to compensate for changes in the optical path length of the resonator due to wavelength changes, thereby providing a resonator without mode hops. Further, the dispersive, focusing element may be composed of one refractive element and one diffractive element. This can be formed, for example, by means of a lens having one refractive surface and an opposite refractive surface with a diffractive surface relief. Such an element may also be a refractive lens element and a separate diffractive element, which may for example be arranged to diffract the surface relief on an element having a second mirrored end (e.g. a glass plate on an element having a diffractive surface relief and a mirror on the opposite surface). Such a separate diffractive element may also be separate from the second mirrored end. It should be noted, however, that there must be some distance between the mirror and the diffractive surface relief to achieve the desired function. In the case of a non-linear cavity geometry, such as a folded cavity, the dispersive, focusing element may be a focusing, reflective surface with a diffractive surface relief. As an alternative, the dispersive, focusing element may be a so-called GRIN element (gradient index element), preferably with a diffractive surface relief on one of its surfaces. As an alternative, the dispersive, focusing element may be constituted by a diffractive surface relief on a plane or curved surface, which is part of the body with the mirror element 2. However, a holographic optical element may be utilized as the dispersive, focusing element, either alone or in combination with any of the above.

With respect to the dispersive focusing element described above, it should be noted that as long as the focusing element has the appropriate dispersion, the laser becomes tunable, regardless of the sign of the dispersion. However, in some cases, mode hopping may be a problem. By compensating for the fact that the Q-value of the resonator is maximal over the wavelength-dependent resonator length, a laser without mode hopping can be realized, the resonator length corresponding to a constant phase shift within the resonator. The dispersion is chosen so that the mode waist moves at the same speed as the node pattern extends along the optical axis of the cavity. The speed of movement of the lower die waist can be controlled for different wavelengths, for example, to obtain a certain lens dispersion, the distance between the optical gain element and the dispersive focusing element can be adjusted.

As noted above, the present invention is not limited to a straight cavity geometry, and in fact, it may be implemented in any curved or folded optical cavity. Fig. 7 shows an example of such an optical cavity. In this case, the two dispersive, focusing elements 5a, 5b are arranged in optical cavities at a distance from each other. Each dispersive, focusing element has a tilted dispersion with respect to the original beam path direction, i.e. the element provides an output beam with a tilted propagation direction, the tilt angle of which is related to the wavelength of the incident light. Between the two dispersive focusing elements a central movable mirror 9 is arranged, the output beam from one dispersive focusing element being reflected into the other dispersive focusing element, and the position of the mirror determining which wavelength is reflected, thereby providing a tunable light source with a variable effective cavity length. Preferably, the arrangement is such that any object point of the first mirrored end is imaged not only on the second mirrored end, but also on the central movable mirror. Thus, a very stable resonator can be realized. In addition, other folded cavity configurations that utilize the concepts of the present invention can be readily devised by those skilled in the art. The oblique dispersion can be achieved by means of diffraction grating elements or refractive elements, e.g. prisms. Further, tilted dispersion can also be used in non-folded cavities, where the dispersive element (e.g., a rotationally symmetric lens) is used "off-axis", i.e., the optical mode entering the dispersive element is at a position different from the position of the optical axis of the element.

According to another aspect of the invention, a second dispersive, focusing element may be arranged within the optical cavity, wherein the spatial filter unit is placed between the first focusing element and the second focusing element. For example, the spatial filter element may be a pinhole element with a central aperture that allows the primary mode, i.e. the lowest order mode, to pass through the pinhole element while the higher order modes have higher losses. By varying the position of the pinhole element tunability can be achieved.

Thus, according to the invention, a tunable laser source can be realized, the tunable spacing of which is limited only by the spectral width of the optical gain element. In contrast, without the use of a dispersive focusing element, a tunable laser source is achieved that can be tuned only within a narrow, finite wavelength interval (of the order of about one free spectral width of the resonator). Such a laser source has an inherently simple structure and can be manufactured to be cost-effective and compact. In its simplest embodiment, the tunable light source comprises a single diffractive element, which may have the various designs described above, and which is placed between a first reflective surface and a second reflective surface, which together form a resonator. Furthermore, since any object point on the first mirrored end or the second end of the optical gain element can be imaged onto the second mirrored end (or vice versa), the resonator is very stable and insensitive to misalignments. In the case of surface emitting elements, the optical cavity is even self-aligning, which facilitates manufacture and makes the optical cavity insensitive to changes over time.

Furthermore, it should be noted that throughout the description, for simplicity, the terms "reflective and transmissive" are used to describe features of the mirror elements. The skilled person will appreciate that such elements are generally partially reflective and partially transmissive in nature, such partial characteristics being included in the above definitions of "reflective" and "transmissive". Furthermore, the skilled person will readily understand that the above described embodiments are only examples of how the invention may be implemented. It should be particularly noted that the light sources described above typically have one or two light outlets, but multiple outlet light cavities are also possible within the scope of the invention. However, the optical source resonator typically has two optical outputs, one output outputting a laser beam and the other output outputting a monitoring beam, which is used to monitor and control characteristics of the laser, e.g., power and wavelength.

It should also be noted that the choice of Numerical Aperture (NA) within the resonator, i.e. the sine of the mode divergence angle (θ) as it propagates to the second mirror element, must be carefully considered. If the NA value is low, the mode selectivity of the resonator is also low; if the NA number is high, the mode selectivity is also high. Preferably, the NA value (close to the second mirror element) is chosen to be in the range of 0.1-0.6, which may be applicable in some applications even if the NA value is outside this range.

Furthermore, it is noted that the first end face and the second end face of the optical gain element may be parallel or non-parallel, and that the term "opposite" may cover all these cases, e.g. the optical gain element is somewhat curved and the planes at its ends form an angle. It is also contemplated that the end face of the optical gain element may be a surface that is internally reflective or a surface that allows modes to be output from the optical gain element.

With respect to the optical gain element, it should be noted that various materials may be utilized, such as semiconductor materials, doped waveguide materials, such as erbium doped fibers, or doped solid state crystals. However, other materials may also be utilized. It should furthermore be noted that the method of pumping the optical gain material may be electrical or optical, which is not important for the content of the present invention.

The light source according to the invention can be used in a number of fields of application, for example telecommunications, but also in applications such as information generation and control, or for component testing and measurement. In the latter case, the light source may be used to test and measure wavelength characteristics of components, subsystems and larger systems in the field of telecommunications. The light source is particularly useful in the field of WDM where the light source can be placed on one of the hundreds of standardized wavelength channels according to the ITU grid standard. Furthermore, the light source according to the invention can also be used in the field of spectroscopy, for example for detecting gases or for measuring the composition of gases. The light source may also be used in the field of metrology, for example for measuring distance and/or velocity.

Finally, it should be noted that many modifications and variations of the present invention will be apparent to those skilled in the art after studying this disclosure and the appended claims.

Claims (21)

1. A wavelength tunable light source comprising:

a primary resonator having a first mirrored end and a second mirrored end, the two ends defining an effective optical cavity length, i.e., a beam path length of a resonant mode of the optical cavity;

an optical gain element having opposing first and second end faces, said second end face being positioned within said primary resonator;

a mirror element constituting the second mirrored end; and

a dispersive focusing resonator element located between said second end face along the beam path and said mirror element;

wherein the effective optical cavity length of the primary resonator is variable.

2. The light source according to claim 1, wherein said first mirrored end of said primary resonator is formed by said first end of the optical gain element.

3. A light source according to claim 1 or 2, wherein the effective cavity length is variable by varying the distance between said first and second mirrored ends along the beam path of said cavity.

4. The light source according to claim 3, wherein the distance of said dispersive focusing element from its image plane is wavelength dependent.

5. The light source according to claim 4, wherein said mirror element forming said second mirrored end is positioned in an image plane of the dispersive focusing element at the wavelength of the laser light emitted by the laser.

6. A light source according to claim 1 or 2, comprising: a second dispersive focusing element, wherein the spatial filtering unit is placed between the first focusing element and the second focusing element.

7. A light source according to claim 1 or 2, wherein the dispersive, focusing element has a wavelength dependent tilt dispersion.

8. The light source according to claim 7, further comprising: a second dispersive focusing element, wherein a deflection mirror is arranged to reflect the beam output from one of said dispersive focusing elements before entering the other of said dispersive focusing elements, said deflection mirror being movable to vary said effective optical cavity length.

9. The light source according to claim 1, wherein said optical gain element is comprised of a semiconductor gain element.

10. The light source according to claim 9, wherein said semiconductor gain element is comprised of a surface emitting semiconductor gain element.

11. The light source according to claim 10 having an automatically self-adjusting optical cavity, whereby the emission of the laser light is independent of the individual positions of the individual intrinsic elements in the light source.

12. The light source according to claim 9, wherein said semiconductor gain element is formed by an edge emitting semiconductor element.

13. The light source according to claim 12, wherein said light source comprises: means for varying the optical length of the optical gain element to control the phase of the lasing mode.

14. The light source according to claim 13, wherein the optical length of said optical gain element is changeable by adjusting the temperature of the gain element.

15. The light source according to claim 13, wherein the mirror element 1 comprises: a chirped axial grating that may be designed to alter the optical path length of the optical gain element.

16. The light source according to claim 13, wherein the optical gain element is divided into a gain section and a phase section, which are arranged successively along the beam path of the optical cavity, wherein the optical length of the gain element is changeable by changing the optical length of the phase section.

17. The light source according to claim 1, wherein the optical length of the main resonator is variable such that the total phase shift of the main resonator remains constant regardless of the wavelength of the emitted laser light.

18. The light source according to claim 1, wherein the second surface of the optical gain element and the second mirrored end define two light conjugate planes.

19. The light source according to claim 1, wherein said second mirrored end is on one end face of the optical fiber or on a core end face of the optical fiber in case of a surface emitting semiconductor element.

20. The light source according to claim 1, wherein the dispersive, focusing element is formed by a single imaging optical element.

21. The light source according to claim 1, wherein the dispersive, focusing element is formed by a combination of a plurality of optical elements.