OPTICAL COMPONENT FOR MODULATING A LIGHT FIELD AND APPLICATIONS THEREOF

Abstract

Optical component (10) for modulating light field (1) incident thereon, particularly amplitude and/or phase in dependency on intensity (I) thereof, includes stack (11) of refractive layers (12, 13) on substrate (14), made of materials having third-order nonlinearity, and having alternatingly varying refractive indices (n), including linear contribution (n.sub.0) and non-linear contribution (n.sub.2), and determining reflectance and transmittance spectra of the optical component, wherein refractive layers (12, 13) are configured such that reflectance and transmittance of the optical component have a Kerr effect based dependency on intensity (I) of the incident light field with n=n.sub.0+I.Math.n.sub.2, and refractive layers (12, 13) are made of at least one of dielectric and semiconductor layers, wherein non-linear contribution (n.sub.2) is below 10.sup.−12 cm.sup.2/W. A resonator device including the optical component, a method of modulating a light field using the optical component and a method of manufacturing the optical component are described.

Claims

1. An optical component, being configured for modulating a light field incident on the optical component, wherein the light field has a predetermined center wavelength, comprising a stack of refractive layers being arranged on a substrate, being made of materials having a third-order nonlinearity, and having alternatingly varying refractive indices (n), including a linear contribution (n.sub.0) and a non-linear contribution (n.sub.2), and determining reflectance and transmittance spectra of the optical component, wherein the refractive layers are configured such that reflectance and transmittance of the optical component have a Kerr effect based dependency on the intensity (I) of the incident light field with n=n.sub.0+I.Math.n.sub.2, wherein the refractive layers are made of at least one of dielectric and semiconductor layers, and the non-linear contribution (n.sub.2) is below 10.sup.−12 cm.sup.2/W.

2. The optical component according to claim 1, wherein the refractive layers are configured such that the reflectance and transmittance spectra of the optical component have a slope section including the center wavelength of the light field.

3. The optical component according to claim 2, wherein the slope section has a slope steepness of at least 5.

4. The optical component according to claim 2, wherein the slope section has a slope steepness of at least 7.

5. The optical component according to claim 2, wherein the refractive layers are configured such that the slope section has decreasing reflectance and increasing transmittance at increasing wavelengths, and the reflectance and the transmittance of the optical component are increasing and decreasing, respectively, at increasing amplitudes of the incident light field.

6. The optical component according to claim 2, wherein the refractive layers are configured such that the slope section has increasing reflectance and decreasing transmittance at increasing wavelengths, and the reflectance and the transmittance of the optical component are decreasing and increasing, respectively, at increasing amplitudes of the incident light field.

7. The optical component according to claim 2, wherein the slope section of the reflectance and transmittance spectra is at least one of an edge section and a resonance section of the reflectance and transmittance spectra.

8. The optical component according to claim 1, wherein the refractive layers are configured such that the reflectance and transmittance spectra of the optical component have a plateau section including the center wavelength of the light field.

9. The optical component according to claim 1, having at least one of the features the dielectric layers are made of at least one of Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, HfO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3 and SiO.sub.2, and the semiconductor layers are made of at least one of Ge, Si and ZnSe.

10. The optical component according to claim 1, wherein the refractive layers have at least one of the features a number of the refractive layers is at least one of at least 2 and at most 1000, and a thickness of the stack of refractive layers is at least 1 nm and at most 1000 μm.

11. The optical component according to claim 1, wherein the refractive layers have at least one of the features a number of the refractive layers is at least one of at least 10 and at most 100, and a thickness of the stack of refractive layers is at least 5 nm and at most 15 μm.

12. The optical component according to claim 1, wherein the substrate has a thickness below 500 μm.

13. The optical component according to claim 1, wherein the substrate has a thickness below 300 μm.

14. The optical component according to claim 1, wherein the stack of refractive layers includes at least one full-wave cavity being resonant at least at the center wavelength of the light field.

15. The optical component according to claim 1, wherein the refractive layers are configured such that the reflectance and transmittance spectra include wavelengths in a range from 500 nm to 10 μm.

16. An optical resonator device, including a plurality of cavity mirrors spanning a resonator beam path, and at least one optical component according to claim 1.

17. The optical resonator device according to claim 16, wherein the at least one optical component is arranged as a reflective or as a transmissive component.

18. A method of modulating a light field, comprising the steps of directing the light field on an optical component comprising a stack of refractive layers being made of materials having a third-order nonlinearity and having alternatingly varying refractive indices (n), including a linear contribution (n.sub.0) and a non-linear contribution (n.sub.2), and determining reflectance and transmittance spectra of the optical component, wherein the refractive layers are configured such that reflectance and transmittance of the optical component have a Kerr effect based dependency on the intensity (I) of the incident light field with n=n.sub.0+I.Math.n.sub.2, and providing the modulated light field in at least one of reflection and transmission relative to the optical component, wherein the refractive layers are made of at least one of dielectric and semiconductor layers, and the non-linear contribution (n.sub.2) is below 10.sup.−12 cm.sup.2/W.

19. A method of manufacturing an optical component, being configured for modulating a light field incident on the optical component, wherein the light field has a predetermined center wavelength, comprising depositing a stack of refractive layers on a substrate, wherein the refractive layers are made of materials having a third-order nonlinearity and having alternatingly varying refractive indices (n), including a linear contribution (n.sub.0) and a non-linear contribution (n.sub.2), and determining reflectance and transmittance spectra of the optical component, wherein the refractive layers are configured such reflectance and transmittance of the optical component have a Kerr effect based dependency on the intensity (I) of the incident light field with n=n.sub.0+I.Math.n.sub.2, the refractive layers are made of at least one of dielectric and semiconductor layers, and the refractive layers are configured such that the non-linear contribution (n.sub.2) is below 10.sup.−12 cm.sup.2/W.

20. The method according to claim 19, wherein the optical component is configured for modulating at least one of an amplitude and a phase of the light field in dependency on the intensity (I) thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Further details and advantages of the invention are described in the following with reference to the attached drawings, which show in:

[0046] FIGS. 1 to 3: schematic illustrations of stacks of refractive layers according to preferred embodiments of the invention;

[0047] FIG. 4: graphical representations of changing reflectance spectra with the inventive method of modulating an amplitude of a light field;

[0048] FIG. 5: graphical representations of further features of the optical component according to preferred embodiments of the invention;

[0049] FIG. 6: a schematic illustration of a test set-up for characterizing inventive optical components;

[0050] FIG. 7: graphical representations of further features of the optical component according to preferred embodiments of the invention; and

[0051] FIGS. 8 and 9: schematic illustrations of an optical resonator device according to preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] Features of preferred embodiments of the invention are described in the following with reference to the design of the optical components as illustrated in FIGS. 1 to 3 and the applications of the optical components as illustrated in FIGS. 8 and 9. It is noted that the implementation of the invention is not restricted to the optical components illustrated in an exemplary manner. In particular, embodiments of the invention can be modified with regard to the number, thickness and material of the refractive layers. Furthermore, the inventive optical components can be used not only in optical resonators, but also in other optical set-ups, e.g., laser amplifiers, or as laser spatial filters.

[0053] According to the invention, it is possible to exploit the optical Kerr effect to provide components working in transmission and components working in reflection. Exemplary reference is made to optical components working in reflection. Optical components working in transmission are configured in an analogue way.

[0054] The practical configuration of an inventive optical component (MAM) is designed using a simulation software calculating the number of the refractive layers, the refractive indices thereof and the thicknesses thereof in dependency on the reflectance and/or transmittance of the layer stack and the reflectance change to be obtained. This calculation includes solving an optimization problem which is based on introducing a merit function (F(X), see e.g., according to [7]), estimating the proximity of designed spectral characteristics to target ones. The software solving the optimization problem utilizes approaches such as needle optimization and gradual evolution algorithms [8, 9], avoiding problems of convergence at a local minimum. Subsequently, the optical component is manufactured using the result of the calculation. As an example, the simulation software comprises a commercial OptiLayer software package (www.optilayer.com).

[0055] The inventive optical component has an intensity dependent reflectance, transmittance and/or phase. The intensity of the incident light field, which is required for obtaining this optically non-linear behavior, is provided for each practical implementation by numerical simulations on the basis of the features of an optical component practically used, or by test measurements with the optical component.

[0056] FIG. 1 schematically shows an embodiment of an optical component 10, comprising a stack 11 of refractive layers 12, 13 with alternating refractive indices, being arranged on a substrate 14. The refractive layers 12 (drawn with white lines, comprising e.g., semiconductors) have a higher refractive index compared with the refractive layers 13 (drawn with black lines, comprising e.g., dielectrics). Alternatively, other materials can be combined, e.g., exclusively dielectric materials or exclusively semiconductor materials, as well as dielectric/semiconductor structures.

[0057] The optical component 10 is arranged to be irradiated with incident light 1 having a center wavelength and for reflecting (2) and/or transmitting (3) the incident light. It is noted that the reflected/transmitted light (2/3) is schematically illustrated only. In practice, all of the refractive layers 12, 13 contribute to the reflected light, i.e., the incident light partially propagates through the stack 11. The reflected light 2 is a superposition of all contribution from the refractive layers 12, 13.

[0058] With a practical example, the optical component 10 is produced using magnetron sputtering technique (Helios, Leybold Optics). For the optical component 10 working at a center wavelength 1030 nm, the dielectric materials used for magnetron sputtering manufacturing comprise Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, and SiO.sub.2 (e.g., 69 layers with a thickness of 9300 nm), while the semiconductor materials comprise Ge and ZnSe (e.g., 70 layers with a thickness of 8000 nm). Semiconductor materials such as Ge or Si in combination with ZnSe can be used when creating optics for longer wavelengths (around 2 μm). The produced samples were characterized with the help of an industrial spectrophotometer (Perkin Elmer Lambda 950) and a home-built white-light interferometer (WLI) (see FIG. 5, below).

[0059] FIGS. 2 and 3 schematically illustrate the optical thicknesses (geometrical thickness multiplied with refractive index) within the stack of refractive layers. According to FIG. 2, the refractive layers have varying optical thicknesses, while FIG. 3 shows an example, wherein a full-wave cavity 15 is formed within the stack of refractive layers having identical optical thickness.

[0060] The optical component 10 is designed on the basis of the following theoretical considerations. The optical Kerr effect is a non-linear optical effect which occurs when intense light propagates in media possessing third-order non-linearity. Using the Kerr effect has particular advantages as it predicts an increase of reflectance for higher intensities and ultrashort (several fs) time response—two incredibly important parameters for modern ultrafast solid-state lasers. The Kerr effect physically originates from a non-linear polarization generated in the medium, which itself influences the propagation properties of the light. It is the effect of an instantaneously occurring non-linear response, which is described by the following formula:

n=n.sub.0+I.Math.n.sub.2  (1)

where n.sub.0 is the linear refractive index, n.sub.2 is the second-order non-linear refractive index of the medium, and I is intensity of the light. Put simply, the refractive index growth consequently leads to higher reflectance values. Therefore, non-linear effects are observed before approaching the damage threshold of the coating.

[0061] The non-linear refractive index depends on the proximity of a test wavelength to the bandgap, and scales in proportion to the inverse fourth power of the bandgap energy E.sub.g:

n.sub.2˜E.sub.g.sup.−4  (2)

For transparent crystals and glasses, n.sub.2 is normally of the order of 10.sup.−16 cm.sup.2/W to 10.sup.−14 cm.sup.2/W. Thorough studies of non-linear refractive indices of the dielectric materials preferably used for the inventive optical component have not been presented in the available literature. The bandgap energies of high-index dielectric materials Ta.sub.2O.sub.5, Nb.sub.2O.sub.5 and HfO.sub.2 are known to be 3.8, 3.9 and 5.1 eV respectively, which allows one to conclude that the typical values of n.sub.2 do lie between 10.sup.−16 cm.sup.2/W and 10.sup.−14 cm.sup.2/W.

[0062] Semiconductor materials possess rather high non-linear index values. Si and Ge, for instance, are known to have strong third-order non-linear optical coefficients with reduced TPA in the IR spectral range. The non-linear refractive index of a single-crystal ZnSe in the 1200-1950 nm region is known to be between about 1.6×10.sup.−14 and about 0.9×10.sup.−14 cm.sup.2/W.

[0063] Depending on the relationship of the reflectance spectrum of the optical component 10 and the center wavelength of the incident light 1 (see FIG. 1), several types of optical components operating in reflection can be provided, in particular optical components working in the middle of the reflectance spectrum (type I), and optical components working on a steep slope of the reflectance spectrum (type II). The Kerr effect of types I and II is illustrated in FIG. 4, which presents numerical simulations of intensity dependent changes of reflectance spectra of a MAM, e.g., as shown in FIG. 1. The reflectance spectra are calculated with the commercial OptiLayer software package (www.optilayer.com).

[0064] FIG. 4(a) shows an example of a broadband MAM working in the middle (plateau section) of the reflectance zone (type I), while FIGS. 4(b) and 4(c) show examples of a MAM working at an edge structure (long wavelength end of reflectance spectrum, type II) and a resonant structure (within the reflectance spectrum, type II). The type II design, in particular if the slope is at least 5, is extremely sensitive to the change of refractive index. The GDD of such structures is carefully tailored and remains nearly constant in the desired wavelength range, e.g., if used for mode-locking of lasers (see FIG. 5(b)). Additionally or alternatively, it is possible to create an edge structure with an opposite slope resulting in decreasing reflectance (increasing transmittance) for increasing laser intensity. As an example, the MAM can work at an edge structure at the short wavelength begin of reflectance spectrum.

[0065] An important advantage of the type I MAM (FIG. 4(a)) designed for reflection operation is its broad bandwidth, which can result in the generation of pulses in the range of several fs, although an absolute increase of reflectance can hardly exceed several percent. The type II (FIGS. 4(b) and (c)) of MAM, on the other hand, is extremely responsive to the intensity increase and can reach 20% in particular cases (if the initial reflectance is 50%). Another benefit of these sensitive designs is the possibility to work at the edge itself, the position of which can be easily adjusted by varying the angle of incidence and thus influencing the modulation depth of the device. However, the estimated bandwidth of the example “edge structure” design (FIG. 4(b)) is 10 nm, which would limit the pulse durations down to 100-200 fs.

[0066] A basic example of a resonant structure (FIG. 4(c)) consists of two stacks of refractive layers enclosing a full-wave cavity 15 in between (Fabry-Perot filter, see FIG. 3). The advantage of the resonant structure is the electric field enhancement obtained in the cavity 15. The latter fact means that much lower intensities are required for approaching non-linear regime.

[0067] It is noted, that another slope of a resonant structure can be used to achieve the decreasing reflectance (increasing transmittance). Furthermore, more complicated multiresonant structure embodiments, including multiple cavities, can be designed in order to further amplify the optical Kerr effect.

[0068] In analogy to FIG. 4, intensity dependent negative GDD values of the optical component, in particular the stack of refractive layers thereof, can be obtained. Accordingly, the phase of the incident light can be controlled in dependency on the intensity of the incident light.

[0069] As an example, the comparison of the designed and measured data for the optical component of type II for reflection operation, extremely responsive to the intensity increase and possessing controlled negative GDD, is presented in FIG. 5. Corresponding results can be obtained with the optical component of type I. FIG. 5(a) shows the designed transmittance (black dashed curve) and the measured data (via spectrophotometer, grey circles) for 0° angle of incidence. FIG. 5(b) shows the designed GDD (black dashed curve) and the measurement performed with a WLI (grey circles) for 20° angle of incidence and p-polarized light. Control of GDD characteristics additionally provides the control over the phase of the light. Despite the challenging for fabrication design (sensitive to manufacturing errors), the agreement between theory and experiment is quite good. Some deviation can be noticed in FIG. 5, however, this is not important as it can easily be compensated for by tuning the angle of incidence, adjusting the position of the slope.

[0070] Increasing and controlled non-linear change of reflectance is one of the main features employed with the inventive optical components. A measurement setup 20 used for characterization of sample components and for modulation depth measurements is shown in FIG. 6. The measurement setup 20 includes a laser source 21 with an Yb:YAG regenerative amplifier (p-polarization, central wavelength=1030 nm, repetition rate=50 kHz and pulse duration=1 ps). The laser beam is focused on an optical component 10 in order to provide intensities up to slightly below the damage threshold thereof. The pulse intensity was varied with a half-wave plate followed by a polarization cube placed in front of the focusing lens. The reflected and transmitted average power values were measured for each incident power value via a powermeter placed in two positions 22, 23. The temperature of the sample was monitored during the measurements as well (via IR camera 24). The sample was placed on a rotational stage in order to adjust the angle of incidence and achieve different initial reflectance-to-transmittance ratios.

[0071] An example characterization of a sample component of type II (edge structure), using the measurement setup 20 described is shown in FIG. 7. The graphs reveal the evidence that the reflectance (R) of the sample component increases by 7±2%, the transmittance (T) decreases by 9.3%±0.4%, the relative temperature grows by 113±2° C., and the total losses are on a scale of the experimental uncertainty for the case R.sub.0/T.sub.0=80%/20% (right graph). In fact, the temperature effect is estimated to be almost a half of the observed reflectance increase. Overall, the experimentally observed pre-damage behavior of the studied optical components occurs to be an interplay of the temperature-induced and nonlinear effects in the multilayer structures. Nevertheless, it is possible to almost fully circumvent the temperature influence by introducing an optical modulator.

[0072] The increase of reflectance and decrease of transmittance depend on the angle of incidence, which determines the initial proportion R to T (R.sub.0/T.sub.0). As one can see (FIG. 7), the absolute modifications of reflectance and transmittance are almost twice higher for the case of R.sub.0/T.sub.0=50%/50% compared to those of R.sub.0/T.sub.0=80%/20% for the same component of type II.

[0073] Additionally, with the measurement setup 20 described, it has been found that the inventive optical components do not show hysteresis phenomena with cycling increasing and decreasing intensities of incident light. This represents a further advantage over the structures described in [6].

[0074] Pump-probe measurements have shown the fast time response of the inventive optical component. A conventional degenerative pump-probe setup has been used with the laser source 21 of FIG. 6. Measuring time-resolved recovery curves of the optical component of type II (edge structure) for different initial R.sub.0/T.sub.0 at 85% of damage threshold fluence provided evidence that the reflectance change disappears very quickly after the pulse for all the initial conditions. A response time of the optical component shorter than 1 ps was obtained.

[0075] Applications of the inventive optical component for oscillator mode-locking are illustrated in FIGS. 8 and 9, which show optical resonator devices 30 with a plurality of cavity mirrors 31, 32 spanning a resonator beam path (linear resonators). Alternatively, ring resonators could be provided in analogue way. A mode-locked oscillator can be operated in the regime of either negative or positive dispersion. Dispersion compensation can be provided by a set of dispersive mirrors 32. The inventive optical component 10 and a gain medium 33 (laser crystal) are included in the resonator beam path. The laser gain medium can be chosen as a bulk crystal medium, as a slab crystal and as a thin-disk crystal. In case of thin-disk crystal medium this elements serves as one of the folding mirrors (in reflection) previously known from state of the art. In the optical resonator device 30, the optical component 10 is used as a mode-locking device.

[0076] FIGS. 8 and 9 show typical mode-locked oscillator configurations consisting of the laser gain medium 33, the linear or ring resonator beam path, the highly reflective mirrors 31 (curved mirrors of convex and concave shape) and dispersive mirrors 32. According to FIG. 8 the optical component 10 is operated in reflection, while a transmission configuration is shown in FIG. 9.

[0077] Preferably, the optical component 10 is positioned inside of the resonator cavity where the beam size is small enough to provide necessary intensity and thus cause the reflectance change. For instance, the optical component 10 can be located in the beam focus formed by two concave mirrors 31 as shown in FIG. 8 and simultaneously serve as an output coupler.

[0078] Many other cavity configurations can be considered providing the small beam size inside of the cavity. For instance, the small beam size can be achieved closer to one of the end mirrors 32. In this case, the optical component 10 can be used as an end-mirror or positioned closer to the end mirror as shown in FIG. 9.

[0079] The optical component 10 can be used as a mode-locker in the combination with other mode-locking techniques, for instance in combination with KLM. Furthermore, more than one optical component 10 can be used inside of the laser cavity. Additional optical components 10 can enhance the effect of self-amplitude modulation. Moreover, complimentary MAMs can be inserted inside of the cavity such that the overall bandwidth (or dispersion curve) provided by two or more optical components 10 is better than that provided by the single optical component 10.

[0080] The features of the invention disclosed in the above description, the drawings and the claims can be of significance both individually as well as in combination or sub-combination for the realization of the invention in its various embodiments.