DE102016107499A1 - Frequency doubling and method for generating electromagnetic radiation - Google Patents

Abstract

The present invention relates to a frequency doubler for electromagnetic radiation with a non-linear optical doubler crystal, which is designed and arranged so that it in an operation of the frequency doubling of electromagnetic radiation with a fundamental frequency, which is einstrahlbar in the doubling crystal, electromagnetic radiation with respect to the fundamental frequency generates a doubled frequency, and a resonator for the electromagnetic radiation with the fundamental frequency, wherein the doubler crystal is arranged in the resonator. In contrast, it is an object of the present invention to provide a frequency doubler having an increased efficiency of conversion of the electromagnetic radiation having the fundamental frequency into the electromagnetic radiation having the frequency doubled from the fundamental frequency. To achieve this object, the invention proposes to provide in the resonator in addition to the doubler crystal a filter with a frequency-dependent attenuation for electromagnetic radiation, wherein the attenuation at a center frequency is lower than at frequencies above or below the center frequency and wherein the filter configured and arranged is that the center frequency is equal to the fundamental frequency.

Description

The present invention relates to a frequency doubler for electromagnetic radiation with a non-linear optical doubler crystal, which is designed and arranged so that it in an operation of the frequency doubling of electromagnetic radiation with a fundamental frequency, which is einstrahlbar in the doubling crystal, electromagnetic radiation with respect to the fundamental frequency generates a doubled frequency, and a resonator for the electromagnetic radiation with the fundamental frequency, wherein the doubler crystal is arranged in the resonator.

The present invention also relates to a method for generating electromagnetic radiation having a frequency doubled from a fundamental frequency, comprising the steps of: placing a nonlinear optical doubler crystal in a resonator for electromagnetic radiation having a fundamental frequency, irradiating the electromagnetic radiation having the fundamental frequency into the resonator, and generating electromagnetic Radiation having a frequency doubled from the fundamental frequency in the doubler crystal.

From the prior art it is known to use non-linear optical processes for frequency conversion of electromagnetic radiation. Applications for such a non-linear optical frequency conversion result in particular from the fact that not all spectral ranges of the electromagnetic radiation can be generated directly in the form of laser radiation with solid-state lasers or semiconductor lasers. Therefore, laser oscillators are often used to generate electromagnetic radiation having a fundamental frequency, and the electromagnetic radiation having the fundamental frequency is then converted or converted by means of a nonlinear optical process into a frequency range which is actually to be achieved.

Widely used for frequency conversion is the frequency doubling to produce the second harmonic of the electromagnetic radiation at the fundamental frequency. This process is also called "second harmonic generation" (SHG). In order to make the conversion of the electromagnetic radiation with the fundamental frequency into the electromagnetic radiation with the frequency doubled compared to the fundamental frequency as efficient as possible frequency doubling are known from the prior art, in which the doubling crystal is arranged within a resonator for the electromagnetic radiation with the fundamental frequency ,

Compared to this prior art, it is an object of the present invention to provide a frequency doubling for electromagnetic radiation and a method for generating electromagnetic radiation with a frequency doubled compared to a fundamental frequency, which increased efficiency of the conversion of the electromagnetic radiation with the fundamental frequency in the electromagnetic radiation allow the frequency doubled compared to the fundamental frequency.

To achieve this object, a frequency doubler for electromagnetic radiation is proposed according to the invention with a nonlinear optical doubler crystal, which is designed and arranged so that it in an operation of the frequency doubler of electromagnetic radiation with a fundamental frequency, which is einstrahlbar in the doubling crystal, electromagnetic radiation with a generated frequency doubled compared to the fundamental frequency, and a resonator for the electromagnetic radiation with the fundamental frequency, wherein the doubler crystal is arranged in the resonator, wherein in the resonator in addition to the doubler crystal a filter with a frequency-dependent attenuation for electromagnetic radiation is provided, wherein the attenuation is lower at a center frequency than at frequencies above or below the center frequency, and wherein the filter is configured and arranged such that the center frequency Glei ch is the fundamental frequency.

The idea underlying the invention is to prevent reverse conversion of electromagnetic radiation having the doubled frequency to be generated back into electromagnetic radiation at the fundamental frequency or at a frequency similar to the fundamental frequency. For this purpose, a frequency-selective filter according to the invention is arranged in addition to the doubler crystal in the resonator. The frequency selectivity of the filter is expressed by the fact that it introduces an attenuation for electromagnetic radiation which is lower at a center frequency which is substantially equal to the fundamental frequency than at frequencies above and / or below the center frequency. For the purpose to be achieved, this filter must be chosen such that it transmits electromagnetic radiation with the fundamental frequency with the lowest possible attenuation. In contrast, it is necessary to attenuate the frequencies between the fundamental frequency and the doubled frequency more, since these possible target frequencies of a reverse conversion form.

In particular, in one embodiment of the invention, the damping, which is the Frequency-selective filter has, for the fundamental frequency lower than for frequencies between the fundamental frequency and the doubled frequency.

In one embodiment, if the frequency-selective filter is arranged in the beam direction of the electromagnetic radiation having the fundamental frequency behind the nonlinear optical doubler crystal in the resonator, then the frequency-selective filter must also carry the electromagnetic radiation with the doubled frequency which is to be generated in the nonlinear crystal transmit the least possible attenuation. In other words, then the doubled frequency must be outside the bandwidth of the frequency-selective filter.

In an embodiment, it is sufficient for the design of the filter if this reduces electromagnetic radiation, which is generated by an OPO back conversion process, either at the frequency of the signal radiation or at the frequency of the idler radiation. Due to the fact that the OPO process is doubly resonant, the disturbance of the process at one of the two frequencies is sufficient. Therefore, it is sufficient for the filter to have attenuation greater than the center frequency attenuation either for frequencies above the center frequency or below the center frequency.

Basically, it is desirable to provide a high power density of the fundamental frequency radiation in the doubler crystal because the conversion efficiency scales with the power density in the crystal. It has been found that the arrangement of the non-linear optical doubling crystal within a resonator, which is resonant for the electromagnetic radiation with the fundamental frequency, in this way leads to an increase in the conversion efficiency and thus to increase the output power of the electromagnetic radiation with the doubled frequency.

However, it turns out that when certain boundary conditions are met, the non-linear optical crystal not only acts as a doubler crystal for the fundamental frequency radiation, but at the same time an optical parametric oscillator (OPO) is formed by the doubling crystal and the resonator. Such a pumped OPO frequency of radiation at the frequency doubled from the fundamental frequency is also referred to as subharmonically pumped OPO. The OPO is driven by the generated electromagnetic radiation at twice the frequency as pump radiation and generates signal and idler radiation at frequencies in the vicinity of the fundamental frequency. In this case, because of the phase matching conditions to be met, a back conversion to the exact fundamental frequency is physically precluded so that the frequencies of the signal and idler radiation of the back conversion process are adjacent to the line of the fundamental frequency.

If the described back-conversion process generates signal and idler radiation with a non-negligible power, this considerably reduces the output power of the frequency doubler at the frequency doubled compared to the fundamental frequency. In addition, since the reconversion process is two-fold resonant, i. For both the signal radiation and the idler radiation resonating conditions of the resonator must be met, this process is sensitive to thermal and mechanical influences on the resonator. The back conversion process is thus unstable, which in turn leads to increased intensity noise of the electromagnetic radiation generated at the doubled frequency.

In one embodiment of the invention, therefore, the attenuation of the filter at the signal or idler frequencies generated by the subharmonic OPO process is just so great that the power threshold of the subharmonic OPO process is undershot.

The back conversion to frequencies in the environment of the fundamental frequency occurs in embodiments when the power density of the electromagnetic radiation with the fundamental frequency in the doubler crystal exceeds a threshold at which the OPO back conversion process starts.

The required power densities are achieved in embodiments of the present invention, in particular, when the focusing of the electromagnetic radiation with the fundamental frequency into the doubler crystal results in a very small beam waist within the doubling crystal. Lowering the power density in the doubling crystal below the threshold critical for the back conversion process could be achieved by widening the beam waist in the crystal. However, such an enlargement of the beam waist is limited if, in one embodiment, the doubler crystal in a direction perpendicular to a beam direction of the electromagnetic radiation in the resonator has a dimension which is very small. This is provided that focusing of the electromagnetic radiation at the fundamental frequency within the resonator in the doubler crystal is selected such that over the entire length of the crystal the beam diameter of the electromagnetic radiation having the fundamental frequency is less than the width and height of the crystal , In one embodiment, this means Invention very small in the sense of the present application, that a dimension of the doubler crystal in a direction perpendicular to a beam direction 2 mm or less and preferably 1 mm or less. This dimension of the doubling crystal is then greater over the entire length of the crystal than the diameter of the focused radiation at the fundamental frequency or, in one embodiment, at least about three times greater than the beam diameter. For the purposes of the present application, the diameter of a circle in which the intensity of the electromagnetic radiation has dropped to 1 / e ² is referred to as the beam diameter. Such focusing typically exceeds the performance threshold for the OPO process in the crystal.

Although it may be possible to choose crystals which have a cross-section perpendicular to the propagation direction of the electromagnetic radiation with the fundamental frequency which is large enough so that a lower focus is possible to the power density below the threshold value for the OPO process to reduce. However, in one embodiment of the invention, the non-linear optical doubling crystal is a periodically poled crystal, for example a periodically poled lithium niobate crystal. With such periodic polarities of a non-linear optical crystal, crystals suitable for doubling a selected fundamental frequency or a selected range of fundamental frequencies can be tailored.

In one embodiment of the invention, the doubler crystal is a periodically poled nonlinear optical crystal. This has, in one embodiment, a poling period considered to be small, namely a poling period of 30 μm or less, preferably a poling period in a range of 4 μm to 20 μm.

However, such periodically poled nonlinear optical crystals can currently only be made with a dimension in a direction perpendicular to the beam direction, which is typically about one hundred times the poling period.

A tunability of the frequency doubler is realized in one embodiment of the invention such that the doubled frequency is tuned by tuning the radiated in the operation of the frequency doubler in the doubler crystal electromagnetic radiation with the fundamental frequency. An adjustment of the phase matching of the doubler crystal for a desired fundamental frequency can be done by adjusting a temperature of the crystal or the poling period of the crystal.

If the frequency doubler is designed to be tunable, so that the frequency of the electromagnetic radiation with the doubled frequency can be set, then, in addition to the configuration of the nonlinear optical doubling crystal, the resonator condition must also be satisfied for a whole bandwidth of fundamental frequencies. However, this need for the tunability of the frequency doubler also means that the resonance condition of the resonator is also fulfilled for signal and idler radiation from the back conversion process around the fundamental frequency. This in turn favors the reconversion process.

In one embodiment of the invention, therefore, a bandwidth of the filter over which it introduces an attenuation greater than the attenuation at the center frequency is smaller than a frequency range in which mirrors of the resonator are reflective to the fundamental frequency electromagnetic radiation.

In one embodiment of the invention, the mirrors for the electromagnetic radiation having the fundamental frequency are highly reflective over a bandwidth of at least 100 nm, preferably of at least 200 nm and particularly preferably of at least 400 nm.

However, parasitic back-conversion processes already occur at levels which are highly reflective over a bandwidth of at least 1 nm or at least 5 nm or of at least 10 nm around the wavelength of the fundamental frequency.

In one embodiment of the invention, the electromagnetic radiation levels at the fundamental frequency are reflective in a wavelength range of 1,000 nm to 1,200 nm, preferably 900 nm to 1,300 nm.

In one embodiment of the invention, the resonator is a linear resonator with at least two mirrors. However, the concept of the present invention is independent of the concrete shape of the resonator. Therefore, other forms of resonators, in particular a ring resonator or a Bow-Tie resonator, are suitable for implementing the present invention.

If the frequency doubler, i. the frequency of the electromagnetic radiation in the output of the frequency doubler with the doubled frequency, tunable, so it is expedient in one embodiment of the invention, when the filter is designed such that the center frequency of the filter is also tuned.

In one embodiment of the invention, either a Fabry-Perot etalon or a Lyot filter is suitable as a filter. These are also tunable in embodiments so that the center frequency of the filter in the resonator can be adjusted. If the filter is a Fabry-Perot etalon, the frequency tuning can be done, for example, by tilting the etalon in the beam path within the resonator. If the e filter is a Lyot filter, the center frequency of the filter can be tuned in particular by turning or pivoting the Lyot filter.

If the filter is a Fabry-Perot etalon, then in one embodiment of the invention it has a free spectral range (FSR) with a value in a range of 100 GHz to 3 THz.

In one embodiment of the invention, the filter is a Fabry-Perot etalon and has a finesse in a range of 0.5 to 10.

In a further embodiment of the invention, the filter has a holder, wherein the holder is designed such that the filter is movable into or out of a beam path of the electromagnetic radiation in the resonator. In one embodiment, this movement may be motor driven.

Such an embodiment has the advantage that it allows an adaptation of the frequency doubler in operation to the pump power of the electromagnetic radiation with the fundamental frequency. Since appreciable reverse conversion due to the underlying OPO process begins only after a certain threshold of power density in the doubler crystal, the filter in the resonator can be omitted so long as the pump power at the fundamental frequency remains below the threshold. Only then must the filter be moved into the beam path to stabilize the frequency doubling process.

In addition, a filter that can be introduced into and removed from the beam path makes it easier to adjust the frequency doubler in the beam path of the electromagnetic radiation with the fundamental frequency. The filter may in such an embodiment after adjustment, i. optimizing the output power of the electromagnetic radiation with the doubled frequency, are introduced into the beam path.

In one embodiment of the invention, the filter is pivotable in and out of the beam path in the resonator.

The above object is further achieved by a system comprising a laser and / or an optical parametric oscillator for generating the electromagnetic radiation with the fundamental frequency and a frequency doubler, as previously described in embodiments of the present invention, the frequency doubler in a beam path is arranged in the operation of the laser or optical parametric oscillator and radiated electromagnetic radiation at the fundamental frequency, so that the frequency doubler generates and emits electromagnetic radiation at a frequency doubled relative to the fundamental frequency. While in one embodiment of the invention the system consists of a laser or an optical parametric oscillator and the frequency doubler, there are embodiments in which the system comprises a laser, an optical parametric oscillator and the frequency doubler, in which case the laser is the optically parametric one Oscillator pumps and the optical parametric oscillator the frequency doubler.

Furthermore, the above object is also achieved by a method of generating electromagnetic radiation having a frequency doubled from a fundamental frequency, the method comprising the steps of: arranging a nonlinear optical doubler crystal in a resonator for electromagnetic radiation having a fundamental frequency, irradiating the electromagnetic radiation introducing the fundamental frequency into the resonator, generating electromagnetic radiation having a frequency doubled from the fundamental frequency in the doubler crystal, and introducing an attenuation for electromagnetic radiation in the resonator, wherein the attenuation for electromagnetic radiation having the fundamental frequency is smaller than for frequencies above or below the fundamental frequency, so that a reverse conversion of the electromagnetic radiation with the doubled frequency into electromagnetic radiation with a frequency between the Fund Amentalfrequenz and the doubled frequency is reduced.

As far as aspects of the invention with respect to the frequency doubler have been described before, they also apply to the method for generating electromagnetic radiation having a frequency doubled relative to a fundamental frequency. In this case, the method comprises those steps which have been described above on the basis of the corresponding devices of the frequency doubler. In particular, embodiments of the frequency doubler are also suitable for carrying out the different method.

Further advantages, features and applications of the present invention will become apparent from the following description of an embodiment and the associated figure.

1 shows a schematic sectional view of an embodiment of the frequency doubler according to the invention.

For easier understanding are in the 1 the beam paths of the electromagnetic radiation with the fundamental frequency 1 inside and outside the resonator and the electromagnetic radiation at the frequency doubled from the fundamental frequency 2 shown offset parallel to each other and not collinear as factually correct. Here is the electromagnetic radiation with the fundamental frequency 1 shown as a solid line, while the electromagnetic radiation with the doubled frequency 2 is shown with a dashed line.

The frequency doubler 3 In the embodiment shown, there are four components, namely two mirrors 4 . 5 , a doubling crystal 6 and a Fabry-Perot Etalon 7 , The two mirrors 4 . 5 together form the resonator of the frequency doubler 3 , This resonator 4 . 5 is resonant for the fundamental frequency of the pump radiation 1 , To the resonance condition for efficiently operating the frequency doubling crystal 6 to meet over a wide Abstimmbandbreite are the two mirrors 4 . 5 of the resonator over a wavelength range of the fundamental of 900 nm to 1,300 nm, ie over a bandwidth of about 400 nm, highly reflective.

To achieve efficient frequency doubling of the selected fundamental wavelength range, the doubler crystal is 6 around a periodically poled lithium niobate crystal with a small poling period. However, such poled crystals with a small poling period can currently only be realized as very thin crystals. Therefore, the crystal points 6 in a direction perpendicular to the beam path 1 the electromagnetic radiation with the fundamental frequency only a dimension of less than 1 mm.

Consequently, in the embodiment shown (not visible in the figure) the mirrors 4 . 5 focussing, wherein the beam waist within the doubling crystal 6 and the beam diameter over the entire length of the crystal is less than the dimensions of the crystal in directions perpendicular to the beam path 1 the electromagnetic radiation with the fundamental frequency. So be within the crystal 6 high power densities of the pump radiation 1 reached. This, in turn, interacts with the broadband reflecting mirrors 4 . 5 a comparatively efficient back conversion through an optically parametric oscillator process which is pumped by the doubled frequency radiation generated by the frequency doubling process. In order to suppress this reverse conversion is in the resonator, ie in the beam path between the two mirrors 4 . 5 a Fabry-Perot Etalon 7 arranged. This etalon has a free spectral length of about 820 GHz in the illustrated embodiment. The etalon itself is made of a YAG crystal with a thickness of 100 μm using the Fresnel reflections on the surfaces 8th . 9 of the crystal 7 educated. The etalon has a comparatively low finesse.

The etalon 7 is tiltable in the beam path, ie the angle which the surfaces 8th . 9 with the beam 1 Include is adjustable. In this way, the center frequency of the filter can be tuned.

In addition, in the illustrated embodiment, the etalon 7 in a direction perpendicular to the beam path 1 in the beam path and out of this movable. In this way, the etalon can be removed from the beam path when the pump power of the electromagnetic radiation with the fundamental frequency 1 is so small that an effective reconversion is not to be feared anyway. In this case, the output power of the electromagnetic radiation with the doubled frequency can be 2 increase when the etalon 7 not in the beam path.

For purposes of the original disclosure, it is to be understood that all such features as will become apparent to those skilled in the art from the present description, the drawings, and the claims, while concretely described only in connection with certain other features, both individually and separately any combination with other of the features or feature groups disclosed herein are combinable, unless this has been expressly excluded or technical conditions make such combinations impossible or pointless. On the comprehensive, explicit representation of all conceivable combinations of features is omitted here only for the sake of brevity and readability of the description.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description is exemplary only and is not intended to limit the scope of the protection as defined by the claims. The invention is not limited to the disclosed embodiments.

Variations of the disclosed embodiments will be apparent to those skilled in the art from the drawings, the description and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude their combination. Reference signs in the claims are not intended to limit the scope of protection.

LIST OF REFERENCE NUMBERS

1

 electromagnetic radiation with the fundamental frequency

2

 electromagnetic radiation with twice the frequency

3

 frequency doubler

4

 mirror

5

 mirror

6

 doubler

7

 Fabry-Perot etalon

8, 8

Surface of the etalon 7

Claims (14)

Frequency doubler ( 3 ) for electromagnetic radiation with a nonlinear optical doubler crystal ( 6 ) configured and arranged to operate in an operation of the frequency doubler ( 3 ) of electromagnetic radiation having a fundamental frequency ( 1 ) in the doubling crystal ( 6 ), electromagnetic radiation having a frequency doubled from the fundamental frequency ( 2 ) and a resonator for the electromagnetic radiation with the fundamental frequency ( 1 ), wherein the doubler crystal ( 6 ) is arranged in the resonator, characterized in that in the resonator in addition to the doubler crystal ( 6 ) a filter with a frequency-dependent attenuation for electromagnetic radiation, wherein the attenuation at a center frequency is lower than at frequencies above or below the center frequency, is provided, wherein the filter is configured and arranged such that the center frequency is equal to the fundamental frequency. Frequency doubler ( 3 ) according to the preceding claim, characterized in that the frequency doubler ( 3 ) is configured such that the doubled frequency ( 2 ) by tuning in the operation of the frequency doubler ( 3 ) in the doubling crystal ( 6 ) radiated electromagnetic radiation with the fundamental frequency ( 1 ) is tunable. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the resonator is mirror ( 4 . 5 ), which for the electromagnetic radiation with the fundamental frequency ( 1 ) in a frequency range greater than a bandwidth over which the filter effects attenuation of electromagnetic radiation that is greater than the attenuation at the center frequency, are reflective. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the resonator is mirror ( 4 . 5 ), which for the electromagnetic radiation with the fundamental frequency ( 1 ) are reflective over a bandwidth of at least 1 nm, preferably of at least 5 nm and more preferably of at least 100 nm. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the resonator is mirror ( 4 . 5 ), which for the electromagnetic radiation with the fundamental frequency ( 1 ) are reflective in a wavelength range from 1000 nm to 1200 nm, preferably from 900 nm to 1300 nm. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the filter is designed such that the center frequency of the filter is tunable. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the filter is a Fabry-Perot Etalon ( 7 ) or a Lyot filter. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the filter is a Fabry-Perot Etalon ( 7 ) having a free spectral range with a value in a range of 100 GHz to 3 THz. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the filter is a Fabry-Perot Etalon ( 7 ) having a finesse in a range of 0.5 to 10, and preferably 2 to 8. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the doubler crystal ( 6 ) in a direction perpendicular to a beam direction of the electromagnetic radiation in the resonator has a dimension which is 2 mm or less, and preferably 1 mm or less. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the doubler crystal ( 6 ) a periodic is a polarized nonlinear optical crystal having a poling period of 30 μm or less, preferably 20 μm or less, and more preferably 10 μm or less. Frequency doubler ( 3 ) according to one of the preceding claims, characterized in that the filter has a holder, wherein the holder is configured such that the filter is movable in a beam path of the electromagnetic radiation or out of this. System comprising a laser and / or an optical parametric oscillator for generating the electromagnetic radiation with the fundamental frequency ( 1 ) and a frequency doubler ( 3 ) according to one of the preceding claims, wherein the frequency doubler ( 3 ) in a beam path of the electromagnetic radiation with the fundamental frequency (generated and radiated by the laser or the optical parametric oscillator in operation). 1 ) is arranged so that the frequency doubler ( 3 ) electromagnetic radiation having a frequency doubled from the fundamental frequency ( 2 ) generates and radiates. Method for generating electromagnetic radiation having a frequency doubled compared to a fundamental frequency ( 2 comprising the steps of arranging a non-linear optical doubling crystal ( 6 ) in a resonator for electromagnetic radiation having a fundamental frequency ( 1 ), Irradiation of the electromagnetic radiation with the fundamental frequency ( 1 ) in the resonator and generating electromagnetic radiation having a frequency that is twice as great as the fundamental frequency ( 2 ) in the doubling crystal ( 6 ), characterized in that the method additionally comprises the step of introducing an attenuation for electromagnetic radiation in the resonator, wherein an attenuation for electromagnetic radiation with a frequency above or below the fundamental frequency is greater than an attenuation of the electromagnetic radiation with the fundamental frequency ( 1 ), so that a reverse conversion of the electromagnetic radiation with the doubled frequency ( 2 ) into electromagnetic radiation having a frequency between the fundamental frequency and the doubled frequency ( 2 ) is reduced.

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