ANGULAR DISPERSION

The invention relates to a device and a method for performing broadband nonlinear optical processes. In particular, the present invention is an optical device with achromatic phase matching that enables broadband optical parametric amplification or frequency conversion to be used, especially, in combination with light sources emitting coherent light, as well as a method to accomplish said processes.

To perform nonlinear optical processes, particularly for coherent amplification or frequency conversion of light emitted by broadband or tunable coherent light sources, media with non-linear response can be made use of. To ensure an output of desired bandwidth or tuning range, the bandwidth of amplification or frequency conversion has to be large enough. In numerous optical processes the bandwidth is determined by a phase-matching condition that depends on the dispersion (wavelength-dependence) of the index of refraction of the nonlinear me- dium used and the directions of propagation (defined by the wave vectors) of the spectral components involved in the nonlinear interaction. In certain cases, the bandwidth limited by the dispersion of the refractive index of the nonlinear medium can be significantly increased through the application of non-collinear geometry and/or angular dispersion.

It is well-known that the bandwidth of a nonlinear optical process can be considerably increased in many cases if a proper amount of angular dispersion is introduced into one or more of the interacting beams in the nonlinear optical process. The angular dispersion is, however, accompanied by an unavoidable spatial beam spreading, i.e. the beam which was initially collimated becomes di- vergent. From a practical point of view, usually this is unfavorable and should be avoided as it results in a decrease in e.g. the beam intensity even for small propagation distances due to the increasing spot size or impairs beam focusability due to the elliptic beam cross section. Consequently, to eliminate beam spreading, an appropriate imaging optics is inserted between the element introducing the angular dispersion (or the angular dispersive element) and the nonlinear medium/ wherein said imaging optics images the exit surface of the former to the entrance surface of the latter.

Such a solution is discussed by a publication of G. Szabo et al. [Appl. Phys. B50 (1990) 51] for frequency doubling or U.S. Patent No. 6,741 ,388 B2 [Jovanovic et al.] in relation to parametric optical amplification, wherein an optical lens and a telescope built up of lenses, respectively, are utilized as the imaging optics concerned.

Such imaging optics can be used only to a limited extent in systems of high intensity. The reason for this is, on the one hand, the decrease in beam quality when the beam passes through the lens materials and, on the other hand, the chromatic aberration of imaging for broadband light (e.g. ultrashort light pulses), as well as the beam distortion arising as a consequence the latter. Moreover, to avoid destruction of the optical components in laser systems of high intensity, the application of large beam sizes is required. Thus, the aberrations due to the imaging optics utilized become even greater and may considerably limit the useful beam size.

CN Patent No. 1400487 [Xu Zuyuan et al.] discloses an arrangement with diffraction gratings provided on the entrance and exit surfaces of a nonlinear crystal for the introduction of monochromatic laser light, i.e. having a single spectral component, into the nonlinear crystal in a case wherein said introduction is not possible via a direct coupling-in so that the laser light propagate within the crystal along the phase-matching direction. This solution is based on the known property of diffraction gratings that the gratings alter the propagation direction of the light passing therethrough.

In light of the above the object of the present invention is to provide an optical device, as well as a method, to perform broadband nonlinear optical processes that, on the one hand, allow an attainment of higher bandwidth via employing angular dispersion and, on the other hand, are deprived of disadvantages due to the application of an imaging optics. In particular, the object of the present invention is to provide such a compact optical device, as well as a method, for allowing broadband and/or tunable optical parametric amplification or frequency conversion that utilize the phenomenon of angular dispersion in order to increase the bandwidth, but do not exhibit the above disadvantages due to the application of imaging optics.

A yet further object of the present invention is to provide an optical device which is compact and also exhibits a relatively high bandwidth to perform broadband nonlinear optical processes, and thus ensures phase matching required for such processes in general over a wide wavelength range.

It should also be noted that the phase matching of a coupled-in light beam with multispectral composition might be accomplished preferably at a certain (i.e. the central) frequency with a small bandwidth even without the presence of an angular dispersive element. However, making use of an angular dispersive element ensures that, through introducing angular dispersion into the coupled-in light beam, the condition for being phase-matched is fulfilled over a wide wavelength range since said element directs each frequency component of coherent light pulses emitted by e.g. a broadband laser source into the respective corresponding phase-matching direction. To this end, it is actually required that a groove density be provided that differs from the one being sufficient for a monochromatic laser beam to be simply coupled in or other angular dispersive element be utilized.

Owing to the solutions in accordance with the invention, broadband amplifi- ers and frequency converters being more compact than existing similar devices can be constructed which can be highly preferentially utilized in combination with e.g. light sources that emit coherent light (e.g. laser oscillators).

The object related to the provision of a compact optical device to perform broadband nonlinear optical processes is achieved by accomplishing an optical device as set forth in Claim . Further preferred embodiments of said optical device are defined by Claims 2 to 8. The object related to the provision of a method for performing broadband nonlinear optical processes is accomplished by providing a method as defined in Claim 9. Possible further preferred variants of the method according to the invention are set forth in Claims 10 to 13.

From now on, the inventive solutions are discussed in detail in relation to preferred embodiments thereof with reference to the attached drawings, wherein - Figure 1 is a schematic representation of a first embodiment of the optical device according to the invention that exhibits an angular dispersive element and an element for optical coupling on its input side;

- Figure 2 is a schematic representation of a second embodiment of the optical device according to the invention that is provided with an angular dispersive element on its input side and an element with angular dispersion compensating property on its output side;

- Figure 3 is a schematic representation of a third embodiment of the optical device according to the invention, wherein a cover medium of appropriate optical properties covers an angular dispersive element located on the input side;

- Figure 4A is a schematic representation of an embodiment of the inventive optical device illustrated in Figure 1 , wherein the angular dispersive element located on the input side consists of a first optical grating formed on a planoparallel substrate and a second optical grating formed on the exit surface of a wedge arranged close to said substrate, wherein both gratings have a grating constant of 3.333 μιη, the angle between the two gratings is 6.97°, the apex angle of the wedge made of material BK7 is 2.96° and the angle of incidence onto the first grating is 80.43°;

- Figure 4B shows calculated signal wave angular dispersion curves for optical parametric amplification [dashed line: values of angle required by the broadband phase matching; continuous line: values of angle obtained for the angle dispersive element of the embodiment of the optical device according to the invention illustrated in Figure 4A] at a pump wavelength of .064 μπι and a signal wavelength of about 1.5 μΐη (pump of extraordinary polarization, with phase matching of the ordinary signal and the idler) at an angle of a = 3.4° between the pump and the signal in a non-collinear amplifying geometry containing an LiNb0 ₃ crystal as the nonlinear optical medium;

- Figure 4C shows calculated amplification curves for optical parametric amplification for coupling-in at various amounts of angular dispersion [dashed line: without angular dispersion of the signal; continuous line: with angular dispersion of the signal according to the continuous line of Figure 4B] when assuming a crystal length of L = 5 mm. In what follows, some exemplary embodiments of the compact optical device according to the invention are discussed in detail with reference to Figures 1 to 3.

The optical device 100 according to the invention comprises a nonlinear medium 107 having entrance and exit surfaces 107a, 107b and an angular dispersive element 105 in optical coupling with the entrance surface 107a through an optical contact 106. In the optical device 100, the angular dispersive element 105 is coupled to the nonlinear medium 107 directly with no imaging optics inserted therebetween, apart from said optical contact 106. The angular dispersive element 105 is provided by e.g. at least one diffraction-based element (preferentially one or more optical gratings), or at least one refraction-based element (preferentially one or more prisms) or any combinations thereof (that is, e.g. a so-called grism or grating prism which is a prism combined with a grating). The optical contact 106 is provided by e.g. a short propagation distance in vacuum or in air, or a dielectric material (e.g. glass or an adhesive), or a composition of various dielectric materials (e.g. optical thin films) or any combinations thereof. When applied, the optical contact 106 is chosen/formed so as to obtain such a propagation distance to be travelled by a beam with angular dispersion through said optical contact 106 which is sufficiently small in order that any further incidental beam expansion effects due to the optical contact 106 be negligible from the point of view of the application concerned.

In a possible further embodiment, the angular dispersive element 105 is formed directly on/in the surface of the nonlinear medium 107; in such a case there is no optical contact 106. In a possible yet further embodiment, the angular dispersive element 105 is formed within the bulk nonlinear medium 107 itself at a given depth from the entrance surface 107a along the beam propagation direction; there is no need for the optical contact 106 in this case either.

The optical device 100 serves for performing nonlinear optical processes. In what follows, the operation of said optical device 100 according to the invention is outlined in brief. To this end, optical parametric amplification and/or frequency conversion are/is considered as the nonlinear optical process. An input signal 102 coupled into the optical device 100 through the entrance surface 07a is generated by a broadband and/or tunable signal emission source 101. When said signal 102 is coupled into the nonlinear medium 107 through the optical contact 106, said angular dispersive element 105 introduces an adequate amount of angular dispersion into the signal 102. Here, the signal 108 travelling within the nonlinear medium 107 interacts with a pump beam 104 generated by a pump source 103 and coupled also into the nonlinear medium 107. An output signal 109 amplified or frequency converted via the nonlinear optical interaction that takes place within the nonlinear medium 107 exits said nonlinear medium 107 through its exit surface 107b.

A possible further embodiment of the optical device according to the invention is illustrated in Figure 2. An input signal 202 generated by a broadband and/or tunable signal emission source 201 is coupled into this optical device 200. When said signal 202 is coupled directly into the nonlinear medium 206, an adequate amount of angular dispersion is induced in said signal 202 by an angular dispersive element 205. The signal 208 travelling in the nonlinear medium 206 interacts with a pump beam 204 generated by a pump source 203. An output signal 209 amplified or frequency converted via the nonlinear optical interaction that takes place within the nonlinear medium 206 exits said nonlinear medium 206, in this case through an angular dispersion compensating element 207 that is optically coupled with the exit surface of said nonlinear medium 206. Thus, the output signal 209 no longer contains angular dispersion. It is noted that, similarly to the arrangement shown in Figure 1 , said angular dispersion compensating element 207 can be equally formed/arranged along the beam propagation direction down- stream of said angular dispersive element 205 within the nonlinear medium 206 itself and/or on its exit surface.

A possible yet further embodiment of the optical device according to the invention is shown in Figure 3. An input signal 302 generated by a broadband and/or tunable signal emission source 301 is coupled into this optical device 300. In this embodiment, the input signal 302 reaches an angular dispersive element 306 when passing through a cover medium 305. An adequate amount of angular dispersion is introduced into said signal 302 by said angular dispersive element 306 when said signal 302 enters the nonlinear medium 307. The signal 308 travelling within said nonlinear medium 307 interacts with a pump beam 304 generated by a pump source 303. An output signal 309 amplified or frequency converted via the nonlinear optical interaction exits said nonlinear medium 307.

A practical embodiment of the optical device according to the invention is shown in Figure 4A. An input signal 402 generated by a broadband and/or tunable signal emission source 401 is coupled into this optical device 400. In this embodiment, an adequate amount of angular dispersion is introduced into said signal 402 by passing it through an angular dispersive element 405 comprising an optical grating 411 , a wedge 410 and a further optical grating 406 provided on the exit surface of said wedge 410. After entering the nonlinear medium 407, the signal 408 travelling within the nonlinear medium 407 interacts with a pump beam 404 generated by a pump source 403. An output signal 409 amplified or frequency converted via the nonlinear optical interaction exits said nonlinear medium 407.

Reverting now to the optical device 100 shown in Figure 1 , in the case of broadband parametric amplification, the input signal 102 is typically a low-energy ultrashort light pulse. The pump source 103 emits typically a short light pulse, the duration of which falls typically into the femtosecond, pikosecond or nanosecond range. The nonlinear medium 107 is, for example, a birefringent nonlinear crystal (in particular, e.g. barium borate (BBO), lithium borate (LBO), potassium dihydro- gen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP), potassium titanyl phosphate (KTP), potassium titanyl arsenate (KTA), lithium niobate (LiNbOs) or any other suitable material) arranged in the light path with appropriate axis orientation, in which phase matching can be obtained at the central wave- length of the pump and the signal. Depending on the wavelengths employed and the material of the nonlinear crystal, said phase matching can be either collinear or non-collinear; specifically, for the optical device sketched in Figure 1 , non-collinear phase matching is attained. The bandwidth of a parametric amplifier is determined by the index of refraction of the nonlinear medium, besides beam direction. In many cases, the bandwidth can be remarkably increased by utilizing angular dispersion, as it is disclosed for the signal beam in U.S. Patent No. 6,741 ,388 B2 already cited. It is noted that in certain cases an adequate angular dispersion of the pump may result in an increase of the bandwidth if the pump beam is broadband enough (when e.g. the pulse duration falls into the femtosecond range). As it was already discussed, the devices suitable for introducing angular dispersion can be devices based on diffraction (e.g. optical grating(s)), devices based on refraction (e.g. prism(s)) or the combinations thereof. Due to angular dispersion, an initially collimated beam becomes divergent which is disadvantageous for most applications because as a consequence of e.g. an increased spot size the beam intensity decreases and the elliptic beam cross section impairs focusability. To eliminate these difficulties such an imaging optics has been applied earlier that imaged the output of the angular dispersive element to the input of the nonlinear crystal. Various image defects, however, led to beam distortions that limited the useful beam size. This limitation was particularly significant in systems of high power.

The operation of the embodiments of the optical device according to the invention illustrated in Figures 1 to 3 and Figure 4A can be described in relation to further examples of broadband nonlinear optical processes, in particular for frequency conversion processes, similarly to the case of optical parametric amplification considered earlier. Such processes can be e.g. the sum and difference frequency generation, wherein the output signals 109, 209, 309, 409 are actually the frequency converted signals, as it is apparent to a person skilled in the art. In such processes, as in the case of optical parametric amplification, it is quite frequent that one of the input beams (typically the pump beam 104, 204, 304, 404) is a narrowband beam, the angular dispersion of which is negligible. In certain cases - such might be the case of e.g. the second or third harmonic generation, optical rectification, etc. - there is only one input beam.

The optical device according to the invention allows the construction of compact parametric amplification or frequency conversion devices. Contrary to the setups employing optical imaging elements (imaging optics), a further advantage of the optical device according to the invention is the lack of image defects. Due to this, larger beam sizes can be used that allows the inventive optical device to be used in laser systems of large power and the construction of such systems. Moreover, the optical device according to the invention can be used as a separate stage in multi-stage or hybrid optical parametric amplification systems (further de- tails of such systems can be found e.g. in U.S. Patent No. 6,870,664 B2, U.S. Patent No. 6,873,454 B2, U.S. Patent No. 6,791 ,743 B2 and U.S. Patent No. 6,775,053).

The optical device according to the invention can also be used in tunable and/or high power and/or ultrafast laser systems. By means of that, light sources suitable for e.g. laser electron acceleration, materials examination, spectroscopy can be constructed. A yet further field of utilization for such light sources is hadron therapy in the filed of medical sciences which is based on high energy ion beams created by high intensity lasers. Furthermore, the optical device according to the invention allows the development of such high power laser systems, especially in the infrared wavelength range (e.g. about the wavelength of 1.5 μΐτι), the pulse duration of which is shorter than what is achievable in this wavelength range by other techniques nowadays. It is an effective solution to apply diode-pumped solid-state lasers in a direct way, that is, without frequency conversion that decreases efficiency in said wavelength range for pumping the optical parametric amplifier. In high power systems, due to large beam sizes, only those crystals can be used from among the crystals suitable for optical parametric amplification or frequency conversion, that can be manufactured with sufficiently large dimensions and good optical quality. Up to now, KTA and/or KTP crystals have been and are used for broadband optical parametric amplification in the near infrared range, e.g. in the vicinity of the wavelength of 1.5 μΐη (see e.g. the publication of O. D. Mucke et al. [Advances in Solid State Lasers Development and Applications, edited by M. Grishin, In-Tech, 2010]). Said crystals, however, are not available at present with large dimensions (i.e. with an aperture of 5 to 10 cm), and also exhibit relatively low nonlinear coefficients.

Contrary to this, LiNb0 ₃ crystals that are also available and/or can be manufactured with relatively large dimensions (~ 10 cm) can be used in the optical device according to the invention advantageously, as it is shown in Figures 4A, 4B and 4C. In particular, in the case of a LiNb0 ₃ crystal with a length of L = 5 mm, and for a pump wavelength of e.g. about 1 μηι and a signal having a wavelength of about 1.5 urn, and for a case wherein the effective nonlinearity is much higher than the nonlinear coefficients of KTA/KTP crystals (pump with extraordinary polarization, ordinary signal and idler, i.e. exploiting the phase matching of p(e) -> s(o) + i(o)), a bandwidth of about 5 nm can be obtained at zero angular dispersion (see the dashed line of Figure 4C). If, however, the angular dispersion of the input sig- nal is chosen in accordance with the solid line of Figure 4B, the available bandwidth for the amplification can be increased above 200 nm, as it is represented by the amplification curve, indicated by a solid line, of Figure 4C. Thus, Figure 4C clearly shows a disadvantage of the present invention: it provides about a forty- fold increase in the bandwidth.

Taking the above into consideration, an optical parametric amplifier that is efficient, compact and scalable for high powers can be constructed by means of the optical device according to the invention, in particular, for applications in the near infrared range if a LiNb0 ₃ crystal is used as the nonlinear optical medium therein.