DE102009026449B4 - Optical thin film structure with a distributed cavity - Google Patents

Abstract

Optical thin-film structure for transmitting light of a wavelength λ along an optical axis (1) with a periodic variation of the refractive index along the optical axis (1), - wherein the variation of the refractive index comprises a plurality of periods, an optical length of the periods within the Thin layer structure (2) is increased compared to an initial length of the periods of λ / 2, - the majority of the elevations forming an optical cavity distributed over the optical thin layer structure (2) and the sum of the elevations (2m-1) λ / 4 with m as is a positive integer, - the increases in the optical length for forming the distributed optical cavity being distributed over at least 5 periods of variation in the refractive index, - in which the increases in the optical length of the periods in the variation of the refractive index have an enveloping function along the optical axis , - with the enveloping function has its maximum in the interior of the thin-film structure (2), is continuous and fades towards or in front of the two ends of the optical thin-film structure (2).

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to an optical thin-film structure for transmitting light of a wavelength λ along an optical axis with a periodic variation of the refractive index along the optical axis. Such an optical thin-film structure is also referred to as vertically layered, because the individual layers are applied to one another when they are produced on a substrate, and can be used in particular as an optical filter. It can also be used as an optical grating to the z. B. couple selected modes in an active material of a laser. The present invention relates to the optical thin film structure as such, but also has specific applications of the optical thin film structure.

By the optical length of a period of the variation of the refractive index is meant the relative length of the period with respect to the wavelength of light passing through the optical thin film structure along the optical axis. In this relative length of the refractive index is in addition to the geometric length.

If in the independent claim 1 of a periodic variation of the refractive index is mentioned, this statement, unless otherwise stated here, does not mean that the individual periods of the variation must be identical or the individual periods symmetrical with respect to their optical length course must have the refractive index. However, the variation of the refractive index within the periods of the optical thin film structure is always symmetrical, that is, contiguous regions of relatively high refractive index and relatively low refractive index each make up about half of the optical length of the periods.

STATE OF THE ART

As narrow-band optical filters Fabry-Pérot structures are often used which consist of two mirrors enclosing a cavity. The length of the cavity determines the wavelength λ of the transmitted filter line with the condition L _cav = mλ / 2. Because of the high achievable reflectivities and the low absorption, multi-layer structures, so-called distributed Bragg reflectors (DBR), which have an alternating sequence of individual layers with different refractive indices and optical thicknesses of λ / 4 are often used as mirrors. Thus, the optical length of the variation of the refractive index here is λ / 2. Such Fabry-Pérot structures are described, for example, by Yeh, P., Optical Waves in Layered Media, Wiley (2005) and MacLeod, HA, Thin Film Optical Filters, Adam Hilger Ltd. (1969).

From the US 6,301,042 B1 and the DE 103 18 767 A1 In addition, Fabry-Pérot filters are known in which the distributed Bragg reflectors, which include the optical cavity with the condition L _cav = mλ / 2, have varying layer thicknesses.

Distributed Bragg reflectors with widely varying single layers are also used as broadband, highly dispersive mirror elements for influencing the pulse shape of fs laser systems (see Szipöcs, R. et al., Chirped Multilayer Coatings for Broadband Dispersion Control in Femtosecond Lasers, Optics Letters, Vol 19 No. 3 (1994)) as well as spectrally selecting components, eg. In demultiplexers (see Gerken, M. et al., Multilayer Thin-Film Structures with High Spatial Dispersion, Applied Optics, Vol. 42 No. 7 (2003)).

Optical filters and reflectors in which, instead of discrete thin layers, there is a continuous transition following a harmonic progression between the different refractive indices are known as "rugate structures" (see Bovard, BG, Rugate Filter Theory: An Overview, Applied Optics, Vol 32 No. 28 (1993)). Rugate structures are characterized by an increased damage threshold due to the absence of abrupt interfaces. Narrow-band rugate filters likewise have a cavity which is produced by a larger period length in the middle region of the refractive index profile. Alternatively, a narrow-band, but inverse, characteristic can also be achieved by only a very small difference in refractive indices (see Bovard, BG, Rugate Filter Design: The Modified Fourier Transform Technique, Applied Optics, Vol. 29 No. 1 (1990)), As a result, the stop band width decreases sharply.

In both rugate and multilayer structures, there are possibilities for influencing the filter characteristic, especially the suppression of side modes, by superimposing the refractive index profile on the enveloping function (see Abu-Safia, HA et al., Rugate Filter Sidelobe Suppression Using Half-Apodization, Applied Optics, Vol 32 No. 25 (1993) and Wang, X. et al., Helicon Plasma Deposition of a TiO ₂ / SiO ₂ Multilayer Optical Filter with Graded Refractive Index Profile, Appl. Phys. Lett., Vol 25 (1998)).

The meaning of vertical emitting laser devices with Fabry-Pérot construction, hereafter also abbreviated as VCSEL according to their English term "vertical-cavity surface-emitting laser", has both since the 1980s in the scientific as well as in the economic area increased strongly. The significant advantages over edge-emitting laser structures are a lower threshold for laser emission, the ability to create round beam profiles, and the ability to test and characterize the structures on the substrate during and after processing. Disadvantages of known VCSELs are above all the concentration of the laser-active material within the cavity, which may only have a small thickness in the range of the emission wavelength in order to comply with the condition for single-mode. The small laser-active volume limits the maximum achievable power due to the limited number of charge carriers. The laser-active material is generally positioned in the maximum of the cavity mode and can be optimally utilized, but this also leads to strong local field strengths and a concentration of heat loss in this area. The associated undesirable effects on the optical, electrical and mechanical properties of the laser device can be problematic, especially with regard to dynamic behavior (see Wilmsen, C. et al., Vertical Cavity Surface-Emitting Lasers, Cambridge, UP (2001)). ,

For distributed feedback edge emitting laser devices through a grating etched into a surface of the laser structure, the effectiveness of a distributed phase offset has been demonstrated. In particular, the effect of spectral hole burning is efficiently suppressed (see Chen, N. et al., Analysis, Fabrication and Characterization of Tunable DFB Lasers with Chirped Gratings, IEEE JSTQE, Vol. 3 No. 2 (1997)).

From the US Pat. No. 4,756,602 For example, a thin film optical structure having a periodic refractive index variation along an optical axis is known in which an optical cavity between distributed Bragg reflectors is divided into a plurality of thin films and, correspondingly, a plurality of refractive index variations along the optical axis such that each is one of the cavity Thin film has an optical thickness of less than λ / 4.

Furthermore, it is the US Pat. No. 4,756,602 as is known, to provide a plurality of spaced-apart optical cavities in an optical thin-film structure in order to optimize their optical properties. Each of these cavities alone satisfies the requirement L _cav = mλ / 2.

T. Kusserow et al .: 'Widely tunable Fabry-Pérot filters with InP / air levels', Photonik 6/2007, pages 64-66, T. Kusserow et al .:' Tunable Fabry-Pérot filters based on InP / Air DBR levels "; DGaO Proceedings 2007; ISSN 1614-8436 and the DE 103 18 767 A1 describe tunable Fabry-Pérot filters.

The DE 103 31 586 A1 describes a microlaser device with a vertical resonator structure (VCSEL) with a thin-film structure whose layers have optical thicknesses of (2m-1) λ / 4 with m as a positive integer.

The US 2006/0133437 A1 describes an organic injection laser with a resonator formed between two distributed Bragg reflectors. The distributed Bragg reflectors each have an optical thickness of their layers of λ / 4. The resonator arranged between the distributed Bragg reflectors has an optical length of a whole number of half wavelengths.

Also from the US Pat. No. 6,115,180 A. known optical filters have distributed Bragg reflectors with layers whose optical thicknesses λ / 4, wherein an intermediate cavity has an optical length of an integer multiple of λ / 2. This also applies to the US 2007/103785 A1 ,

From the US 2002/131176 A1 a temperature-stable layer system is known which has a further optical mirror made of a thin-film structure with layers whose optical thicknesses are λ / 4.

From the DE 102 43 839 A1 For example, a thin film structure with variable thickness dielectric layers is known for forming a mirror that utilizes the Brewster angle of the material of the thin film structure.

The US 2007/0103785 A1 describes an optical filter. This is a modified distributed Bragg reflector, which has an approximately 100% transmission outside its reflection band. A thin-film structure of the known optical filter has no cavity defining a narrow-band transmission within the reflection band.

The US 2002/0131176 A1 defines an optical filter with multiple discrete cavities between distributed Bragg reflectors. In this case, instead of a third optical material having a refractive index between the refractive indices of two existing optical materials, a layer thickness in the region of a distributed Bragg reflector is formed smaller than λ / 4 and the adjacent layer thickness slightly larger than λ / 4, but of three successive layers a total of a thickness of 3/4 λ is maintained.

From the DE 693 31 533 T2 For example, a distributed reflector and a tunable wavelength semiconductor laser are known. The DE 602 02 683 T2 also describes a tunable laser.

OBJECT OF THE INVENTION

The invention has for its object to provide a thin film optical structure for transmitting light of a wavelength λ along an optical axis with a periodic variation of the refractive index along the optical axis, in which the electromagnetic field strength as in a conventional Fabry-Pérot structure on a single central, the optical cavity discretely forming layer is concentrated.

SOLUTION

The object of the invention is achieved by an optical thin-film structure with the features of independent claim 1. The dependent claims 2 to 13 relate to preferred embodiments of the new optical thin film structure. The claim 14 is directed to an optical filter with the new optical thin-film structure for a wavelength λ. Claim 15 relates to a laser with active material and the new optical thin-film structure. The subclaims 16 and 17 relate to preferred embodiments of this laser.

DESCRIPTION OF THE INVENTION

An optical cavity is also formed in the novel thin film optical structure, but is not discrete between distributed Bragg reflectors, but extends over several periods of refractive index variation along the optical axis, by increasing the optical length to form the optical fiber optical cavity are distributed over these multiple periods of variation of the refractive index. Thus, in the new optical thin-film structure, there is no region of increased or reduced refractive index which has a significantly longer optical length than the lower and higher refractive index regions adjacent thereto and in which the electromagnetic field strength of the injected light concentrates. Rather, many adjacent regions of increased and / or decreased refractive index have excess dimensions, i. H. Increases in their optical length, these excesses add up to the typical excess of optical cavity in a Fabry-Pérot structure according to the prior art by the sum of the increases (2m - 1) λ / 4 with m as positive whole Number is.

That in an optical structure with the features of the preamble of independent claim 1 actually dispensed with a discrete cavity and this cavity over a large number of periods of variation of the refractive index can be distributed, so that there is a favorable distribution of the electromagnetic field strength, without losing the wavelength selectivity present by the dimensions of the cavity, it is surprising.

The increases in the optical length of the periods of refractive index variation for forming the optical cavity are located along the optical axis within the thin film structure within its thin film structure. Ie. it extends only over a portion of the thin film structure and is not normally concentrated at one of the ends of the thin film structure along the optical axis. An enveloping function indicating the increases in optical length of the periods of refractive index variation along the optical axis thus has its maximum in the interior of the thin film structure. In addition, this function is continuous, so that the increase in the optical length has no major jumps and in particular does not start abruptly or ends.

Accordingly, it is preferable if the elevations of the optical length in the new thin-film structure are distributed over a plurality of directly adjacent periods of the variation of the refractive index. However, if individual periods of refractive index variation between those of increased optical length do not have an increased optical length, this is usually harmless. This is especially true if the number of periods of refractive index variation over which the increase in optical length is distributed in the new thin-film structure is comparatively large, so that the increase in the optical length of a single period of the refractive index variation is comparatively small.

The number of periods of refractive index variation over which the optical length increases in the new thin-film structure are distributed is at least 5 and typically at least 10. However, it may be significantly greater.

With respect to the increases in the optical length of the individual periods of the variation of the refractive index over which the cavity is distributed, the implementation of the present invention can take place in different ways. Thus, to increase the optical length of the individual periods, the geometric length of these periods can be increased. Alternatively or additionally, to increase the optical length of the periods, the amplitude and / or the duty cycle of the variation of the refractive index may be increased over the periods. In the latter case, the expansion of the regions of the higher refractive index periods results in an increasing asymmetry in the variation of the refractive index over the periods.

The variation of the refractive index can follow a rectangular function. To form this rectangular function, it is possible, in particular, to use a layer structure consisting of two materials with different refractive indices alternating in layers, in which case the layer thickness of at least one of the materials is increased over the distributed cavity in order to increase the optical length of the periods of the refractive index variation. Whether only the layer thickness of one of the two materials is increased over the distributed cavity, or else the layer thicknesses of both materials in this region of the optical thin-film structure is increased, is fundamentally unimportant for the function of the optical cavity and can therefore be distinguished from other criteria such as the simple one Making the new thin-film structure dependent.

The thin layers of a layer structure for forming the new optical thin film structure may be made of dielectric materials, semiconductor materials or organic materials. Individual thin films may also be wholly or partially formed as an air gap or as a gas or liquid-filled gap.

However, the variation of the refractive index in the new optical thin-film structure can also be continuous; in particular, a harmonic variation in the manner of a rugate structure is also possible.

It is preferred if the distribution of the elevations of the optical length over the periods of the variation of the refractive index is continuous and harmonious and in particular also mirror-symmetrical to a center of the distributed cavity. However, the distributed cavity may not only be disposed in the center of the optical thin film structure along the optical axis, but may also have a targeted offset therewith so as to be closer to an end of the optical thin film structure along the optical axis. As a result, a mode defined by the optical cavity is coupled out of the thin-film structure predominantly in the same direction along the optical axis.

Preferably, the increase in the optical length distributed in the new optical thin-film structure over several periods is the same as half the output length of the periods. Starting from a period length of λ / 2, so that the regions with increased or reduced refractive index each have an optical length of λ / 4, the distributed increase in the optical length is thus a further λ / 4.

As already mentioned, the new optical thin-film structure can be used, in particular, as an optical filter, and, when tuned to a wavelength λ, has a high filtering efficiency for the wavelength despite the optical cavity distributed over several periods of refractive index variation λ on. In principle, the cavity with selectivity for the wavelength λ can be achieved not only by increasing the optical length by λ / 4, but also by an odd number multiple of λ / 4. The efficiency as an optical filter for the wavelength λ increases in a cavity with the increase in the optical length of the periods of variation of the refractive index by (2m + 1) λ / 4 even with increasing m; However, such a cavity is always tuned at least also to the mode with the wavelength (2m + 1) λ, which can be disadvantageous.

In another preferred specific embodiment of the new optical thin-film structure, this is part of a laser. In this case, the new optical thin-film structure can be used quite generally as an optical grating, be coupled to the desired modes in a laser-active material and thereby selected.

In a particularly preferred embodiment of a laser with the new optical thin-film structure, the laser is designed as a VCSEL, its laser-active material being enclosed by the optical thin-film structure in the form of a hollow cylinder. In this case, the inner diameter of the hollow cylinder can be alternately varied according to the sequence of layers of materials with different refractive index, in order to achieve a complex coupling of the laser emission and the cavity mode of the new optical thin-film structure. In a specific embodiment of the laser according to the invention, in which the laser-active material and the new optical thin-film structure intermesh, the inner diameter of layers of one of two materials having different refractive indices over the region of increasing the optical length to form the optical cavity is also one provided continuous variation.

The hollow cylinder may in turn be surrounded by air or embedded in another surrounding material, wherein at the outer boundary of the hollow cylinder, a jump of the refractive index takes place, which causes an index guide of the light in the hollow cylinder with sufficient size. The light guidance in the hollow cylinder can otherwise be effected by means of known measures such as ARROW structures or Bragg fiber guiding. The base of the hollow cylinder may have any polygonal or curve-based shape. The same applies to the base area of the inner volume of the hollow cylinder.

Ideally, the dimensions of the hollow cylinder and of the inner volume filled by the laser-active material on the one hand and the lateral refractive index jump on the other hand are dimensioned such that only a single transverse mode is capable of propagation. By tuning the distribution function, but also selectively several propagatable modes can be selected by number and type.

• – Deponieren der Schichten der optischen Dünnschichtstruktur auf einem Substrat;
• – Strukturieren des Hohlzylinders;
• – selektives chemisches Entfernen eines der Materialien mit unterschiedlichem Brechungsindex zur Gestaltung des Innenvolumens des Hohlzylinders;
• – Füllen des Innenvolumens mit einem laseraktiven Material;
• – Verschließen des Hohlzylinders mit einem Deckglas, einer Abschlussschicht oder mittels Wafer-Bonding. Das Pumpen des erfindungsgemäßen Lasers kann in für VCSEL typischer Weise optisch oder elektrisch erfolgen.

A method for producing a laser according to the invention typically comprises the main steps:

• Depositing the layers of the optical thin-film structure on a substrate;
• - structuring of the hollow cylinder;
• Selective chemical removal of one of the different refractive index materials to form the internal volume of the hollow cylinder;
• - filling the internal volume with a laser-active material;
• - Closing the hollow cylinder with a coverslip, a final layer or by means of wafer bonding. The pumping of the laser according to the invention can take place optically or electrically in VCSEL-typical way.

Advantageous developments of the invention will become apparent from the claims, the description and the drawings. The advantages of features and of combinations of several features mentioned in the introduction to the description are merely exemplary and can come into effect alternatively or cumulatively, without the advantages having to be achieved by embodiments according to the invention. Further features are the drawings - in particular the illustrated geometries and the relative dimensions of several components to each other and their relative arrangement and operative connection - refer. The combination of features of different embodiments of the invention or of features of different claims is also possible deviating from the chosen relationships of the claims and is hereby stimulated. This also applies to those features which are shown in separate drawings or are mentioned in their description. These features can also be combined with features of different claims.

BRIEF DESCRIPTION OF THE FIGURES

The invention is explained in more detail below with reference to various embodiments with reference to the accompanying drawings and described. In this case, the representation of the optical thin-film structure according to the invention in the drawings is not to scale, but the inventive aspects of the optical thin-film structure are - so that they can be seen in the drawings - shown to scale.

1 shows a schematic longitudinal section through a first embodiment of the new optical thin film structure along its optical axis.

2 is a plot of refractive index to one 1 largely corresponding second embodiment of the new optical thin-film structure along its optical axis, wherein the refractive index follows a rectangular function here.

3 is a plot of the refractive index along the optical axis to another embodiment of the new optical thin film structure, the refractive index following a harmonic function.

4 is a plot of the refractive index along the optical axis to another embodiment of the new optical thin film structure, the refractive index following a rectangular function with a duty cycle increased over several periods.

5 is a plot of the refractive index along the optical axis to another embodiment of the new optical thin film structure, the refractive index following a rectangular function with pitches of equal length over several periods of increased pitch.

6 FIG. 12 is a plot of the refractive index along the optical axis to another embodiment of the novel optical thin film structure, the refractive index following a rectangular function with amplitude decaying to both ends of the thin film structure.

7 FIG. 13 is a plot of the refractive index along the optical axis to another embodiment of the novel optical thin film structure, the refractive index following a harmonic function with amplitude decaying toward both ends of the thin film structure.

8th FIG. 12 is a plot of the refractive index along the optical axis to another embodiment of the novel optical thin film structure, the refractive index following a harmonic function with amplitude and geometric period length decaying toward both ends of the thin film structure.

9 shows a schematic longitudinal section along the optical axis through a Embodiment of the thin film optical structure having a thin film structure in which a part of the thin films is formed by gas filled gaps.

10 to 12 show a longitudinal section through a formed as a hollow cylinder new optical thin film structure along its optical axis, and a plan view of and a perspective view of this embodiment of the new optical thin film structure.

13 to 15 show the 10 to 12 corresponding views of another, also designed as a hollow cylinder embodiment of the new optical thin-film structure; and

16 to 18 also show the 10 to 12 corresponding views of yet another designed as a hollow cylinder embodiment of the new optical thin-film structure.

DESCRIPTION OF THE FIGURES

In the 1 in a longitudinal section along an optical axis 1 sketched optical thin-film structure 2 has a layer structure 3 from alternating layers 4 and 5 on, once made of a material 6 with high refractive index and once a material 7 exist with low refractive index. The refractive index is above all layers 4 and the refractive index over all layers 5 constant. The thickness of the layers 4 and 5 has a base thickness λ / 4, where λ is the wavelength for which the optical thin film structure 2 is provided as an optical filter. In the middle of the optical thin-film structure 2 along the optical axis 1 is the thickness of the layers 4 and 5 Increased beyond λ / 4, in such a way that the sum of the increases in turn also λ / 4. In this way, one over a plurality of layers 4 and 5 realized distributed optical cavity of effective length λ / 2, so that the optical thin-film structure 2 along the optical axis 1 only transmitted light of wavelength λ. At the same time, the electromagnetic field strength is within the optical thin-film structure 2 not concentrated on a single central layer discretely forming the optical cavity as in a conventional Fabry-Pérot structure, but the electromagnetic field strength is over all layers 4 and 5 distributed with their layer thickness increase relative to λ / 4 contribute to the formation of the optical cavity.

The new optical thin-film structure is functional not only in a tuning starting from λ / 4 and with distributed increases in the layer thickness by a total of further λ / 4. The distributed increases in layer thickness may also be (2m-1) λ / 4, where m is a positive integer. This also results in the selectivity for light of wavelength λ. At higher values of m, however, higher harmonics of this wavelength can also be achieved by the optical filter than by the optical thin-film structure 2 is formed, pass through.

2 outlines the principal course of the refractive index along the optical axis 1 for the embodiment of the optical thin film structure 2 according to 1 However, the course of the refractive index for a larger number of layers 4 and 5 is reproduced as being in 1 is shown. It can be seen that the refractive index here follows a rectangular function which is symmetric within each period, that is, the refractive index over equally long regions along the optical axis 1 every period is high and low. In the middle of the optical thin-film structure 2 For example, the optical lengths of the periods of the variation of the refractive index are increased, as well as the increase of the length of the periods to the center of the optical thin-film structure 2 concentrated and dies to the ends of the optical thin-film structure before or before this. In this embodiment of the optical thin film structure 2 are both the optical thicknesses of the layers 4 as well as the layers 5 increased in the middle of the layer structure. The type of distribution function of the optical length increases of the periods of refractive index variation determines the distribution of the cavity mode along the optical axis of the optical thin film structure 2 , As a distribution function, arbitrary mathematical function types (eg, linear, polynomial, exponential, etc.) can be used and adjusted to achieve a desired filter characteristic. The size of the distributed increases in the optical length of the period of the variation of the refractive index may also deviate slightly from λ / 4 or (2m-1) λ / 4, depending on the chosen objective. The thin films from which the optical thin film structure according to 1 and 2 is constructed, may consist of any inorganic or organic materials.

3 outlines the progression of the refractive index over the optical axis for a new optical thin-film structure 2 in which a harmonic refractive index profile is provided in the manner of a rugate structure. Again, the optical length of the periods of refractive index variation in the center of the optical thin film structure along the optical axis is increased.

While according to the previous figures, the geometric length of the periods of the variation of the refractive index in the middle of the new optical thin-film structure has always been increased 4 the course of the refractive index over the optical axis for an embodiment of the new optical thin film structure in which the geometric length of the periods of the variation of the refractive index is constant, but the duty cycle has a change. That is, the relative proportion of the regions long of the high refractive index optical axis is increased in the middle of the thin film structure, which is equivalent to an increase in the optical length of these periods of refractive index variation. It is understood that a variation of the duty cycle is also possible with a design of the new optical thin-film structure starting from a rugate structure.

5 outlines the progression of the refractive index over the optical axis for an embodiment of the new optical thin-film structure in which the geometric length of the periods of the variation of the refractive index in the middle is increased, but only by an increase of the layer thicknesses of the layers 5 with the lower refractive index, while the layer thicknesses of the layers 4 with the higher refractive index remains constant along the entire optical axis. The reverse case, ie only increased layer thicknesses in the layers with the higher refractive index is possible. Both of these variants can also be realized on the basis of a rugate structure.

In the embodiment of the new optical thin-film structure, the course of the refractive index over the optical axis in 6 is shown, the geometric length of the variation of the refractive index along the optical axis is constant. Here, in order to increase the optical length of the periods, the amplitude of the variation of the refractive index in the center of the optical thin film structure is increased. For the enveloping function of the refractive index increase, all conceivable possibilities are again available which provide the function with a maximum in the middle of the thin-film structure. However, this maximum does not have to be exactly in the middle of the thin-film structure. Rather, the distributed through the distributed increase in the layer thicknesses optical cavity in the new device may also be closer to one of its ends along the optical axis.

7 FIG. 12 shows the progression of the refractive index versus the optical axis for an embodiment of the novel optical thin film structure. FIG 6 except that it is not based on a layered structure of discrete layers but on a rugate structure.

In 8th the development of the refractive index over the optical axis is sketched for an embodiment of the new optical thin-film structure which starts from a rugate structure and in which the increased optical length of the periods of the variation of the refractive index in the middle of the thin-film structure by an increased geometric length the periods and on the other by a refractive index profile superimposed enveloping function is reached.

To the region of the optical thin-film structure in which the optical length of the periods of refractive index variation for the formation of the optical cavity is increased, regions of the optical thin-film structure without these ridges may join on one or both sides to form conventional distributed Bragg reflectors there , However, the reflectors can also be designed as chirped DBR in the edge regions of the elevations.

The main advantage of the new optical thin-film structure is the distribution of the cavity mode over a larger area of the thin-film structure along its optical axis. As a result, high field strengths of the electromagnetic field, which are strongly localized in the region of the optical cavity, can be avoided, and thus higher destruction thresholds of, for example, an optical filter can be achieved. In particular, in this regard, the implementation of the new optical thin-film structure based on a rugate structure is advantageous, since in this case additionally discrete interfaces between individual discrete layers with different refractive index are avoided. Further advantages of the new optical thin-film structure are that, when used as an optical filter, the filter characteristic is to be influenced by means of the various parameters available for configuring the increase of the optical length of the periods of the variation of the refractive index.

9 sketched in one 1 corresponding representation the possibility of a layer structure 3 the layers 5 with low refractive index than with gas 8th as a material 7 filled column 9 provided. The layers can 4 through spacers 10 be mutually supported to define the height of the column. Instead of the spacers 10 but can also be z. B. active material within the layer structure 3 the new optical thin-film structure 2 be provided.

In the following figures, embodiments of the new optical thin film structure will be described 2 explained, which are provided as components of vertically emitting laser structures. This is the optical thin-film structure 2 as a hollow cylinder 11 educated. The inner volume 12 of the hollow cylinder 11 is filled with an organic or inorganic laser active material that can be stimulated by supplying energy for stimulated emission. Due to the index coupling of the laser emission with the surrounding optical thin film structure 2 only the wavelength λ is amplified, to which the optical thin-film structure 2 is tuned, and thus achieves a longitudinal monomode laser characteristic.

The 10 to 12 sketch such a hollow cylinder 11 with constant inside diameter 13 and constant outside diameter 14 ,

The 13 to 15 In contrast, sketch a variant of the hollow cylinder 11 in which the inner diameter 13 a rectangular function follows by the inner diameter 13 ' in the region of the layers of the one material with the one refractive index is smaller and the inner diameter 13 '' is larger in the region of the layers of the other material with the other refractive index. The spatially periodic structure of the internal volume generated in this way 12 of the hollow cylinder 11 is then also filled with the organic or inorganic laser active material. Due to the spatially periodic sequence of reinforcing material layers becomes a complex coupling of the laser emission with the surrounding filtering optical thin film structure 2 reached.

In another, in the 16 to 18 sketched variant of the hollow cylinder 11 is the larger inside diameter 13 '' along the optical axis 1 z. B: varied by varying widely. The base of the hollow cylinder and the inner volume may have any polygon-based or curve-based geometric shapes to z. B. influence the emission properties or the beam profile of the laser. The outer boundary of the hollow cylinder 11 can be done by removing the surrounding material or by changing the refractive index at the interface to the surrounding material. The dimensions and material properties of hollow cylinders 11 and internal volume 12 can be dimensioned such that tranversale Monomodigkeit the laser structure is achieved. Through a targeted design of the distribution function as well as the dimensions and material properties of hollow cylinders 11 and internal volume 12 Also, a desired number and type of multiple laser modes can be achieved. The transverse guidance of the waves in the active medium is preferably by indexing, which, as known by waveguides, is based on a refractive index difference between waveguide (high refractive index) and surrounding material (low refractive index). It should be noted here that the laser-active material has a higher refractive index than the surrounding multi-layer structure of the hollow cylinder 11 should have. For a particular combination of materials for which this condition can not be met, the following alternative methods are possible to control the waveguide in the internal volume of the hollow cylinder 11 to reach. The wall thickness of the hollow cylinder may be small compared to the diameter of the internal volume filled by the laser active material. The multi-layer structure can be used partly as an air gap structure (cf. 9 ), thereby increasing the effective average refractive index of the optical thin film structure 2 reduced. It is possible to use so-called ARROW structures for waveguiding. If indexing of the waves is not possible, the guidance can be achieved by periodic structuring of the surrounding material. The waveguide takes place here by interference in a concentric annular Distributed Bragg reflector, the hollow cylinder 11 surrounds, or by a photonic crystal structure in which the hollow cylinder 11 surrounding material.

To produce the new optical thin-film structure, thin films can be deposited on a substrate. Subsequently, the structuring of the hollow cylinder 11 , z. B. by dry etching or removal with focused ion radiation. The internal volume of the hollow cylinder 11 can be structured by selective undercut. Subsequently, the internal volume 12 filled with a suitable laser active material. On the resulting thin-film structure, a finishing layer is then applied, which can be done on the one hand as an antireflection coating, on the other hand also serves as a protective layer for liquid and other sensitive materials. The conclusion of the structure can alternatively be done by a cover slip or by wafer bonding.

The main advantage of the embodiment of the invention in the form of a hollow-cylinder-based laser structure is the possibility of being able to exploit a larger laser-active volume compared to conventional VCSEL structures. The cavity mode is not only located in the region of a thin cavity layer with an optical thickness of λ / 2, but can be distributed over the total height of the multilayer structure, this also reduces unwanted optical thermal, electrical and mechanical effects, such. As the spectral hole burning or thermal lenses that can occur due to the high field strengths and intensities in a conventional discrete cavity. The distribution of increases in the optical length of the periods of refractive index variation makes it possible to distribute the cavity mode over a larger laser active volume, thereby providing vertically emitting laser structures with higher output powers and improved emission characteristics, especially in the dynamic range. In addition, the advanced parameters of the distribution methods and functions can be exploited in the design of the optical thin film structure to reduce the emission characteristics of the laser, e.g. As sideband suppression to influence.

LIST OF REFERENCE NUMBERS

1

optical axis

2

optical thin-film structure

3

layer structure

4

layer

5

layer

6

material

7

material

8th

gas

9

gap

10

spacer

11

hollow cylinder

12

internal volume

13

Inner diameter

14

outer diameter

Claims (17)

Optical thin-film structure for transmitting light of a wavelength λ along an optical axis ( 1 ) with a periodic variation of the refractive index along the optical axis ( 1 ), Wherein the variation of the refractive index comprises a plurality of periods, wherein an optical length of the periods within the thin-film structure ( 2 ) is increased with respect to an output length of the periods of λ / 2, wherein the majority of the elevations exceed the optical thin-film structure (FIG. 2 ) and the sum of the elevations (2m - 1) is λ / 4 with m as a positive integer, the elevations of the optical length being distributed over at least 5 periods of the variation of the refractive index to form the distributed optical cavity, - wherein the increases in the optical length of the periods of the variation of the refractive index along the optical axis have an enveloping function, - wherein the enveloping function is at its maximum in the interior of the thin-film structure ( 2 ) is continuous and to the two ends of the optical thin film structure ( 2 ) ends or before. The thin film optical structure of claim 1, wherein the increase in the optical length is distributed over a plurality of directly adjacent periods of refractive index variation. The thin film optical structure according to any one of claims 1 to 2, wherein the increase in the optical length is distributed over at least 10 periods of the refractive index variation. An optical thin film structure according to any one of claims 1 to 3, wherein, to increase the optical length, the geometric length of the periods of refractive index variation is increased. An optical thin film structure according to any one of claims 1 to 4, wherein, to increase the optical length, the amplitude of the variation of the refractive index over the periods is increased. The optical thin film structure according to any one of claims 1 to 5, wherein, to increase the optical length, the duty ratio of the variation of the refractive index over the periods is increased. An optical thin film structure according to any one of claims 1 to 6, wherein the variation of the refractive index along the optical axis ( 1 ) follows a rectangular function. An optical thin film structure according to claim 7, wherein, to increase the optical length, the layer thickness of at least one of two materials having different refractive index in successive layers is increased. An optical thin film structure according to any one of claims 1 to 6, wherein the variation of the refractive index is continuous. An optical thin film structure according to claim 9, wherein the variation of the refractive index follows a harmonic function. An optical thin film structure according to any one of claims 1 to 10, wherein the enveloping function has a symmetric distribution function along the optical axis (Fig. 1 ). An optical thin film structure according to any one of claims 1 to 11, wherein the maximum of the enveloping function along the optical axis ( 1 ) off-center in the optical thin film structure ( 2 ) is arranged. An optical thin film structure according to any one of claims 1 to 12, wherein the sum of the increases is λ / 4. An optical filter for a wavelength λ having an optical thin film structure according to any one of claims 1 to 13. A laser emitting at a wavelength λ having a thin film optical structure according to any one of claims 1 to 13, wherein the optical thin film structure ( 2 ) in the form of a hollow cylinder ( 11 ) is formed, which encloses laser active material. The laser of claim 15 having a thin film optical structure according to claim 8, wherein said layers of one of said two materials have a smaller inner diameter ( 13 ' ) than the layers of the other of the two materials. The laser of claim 16, wherein the inner diameter of the layers of the one material over the region of increasing the optical length of the periods of refractive index variation in turn has a continuous variation along the optical axis. 1 ) having.

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