US-12620763-B2 - Apparatus and method for generating electromagnetic useful radiation

US12620763B2US 12620763 B2US12620763 B2US 12620763B2US-12620763-B2

Abstract

A method for generating an electromagnetic useful radiation having a useful frequency is provided and includes generating and radiating an electromagnetic pump radiation with a pump frequency, coupling the pump radiation into an external optical resonator having a resonance frequency. The resonance frequency is at least initially substantially equal to the pump frequency, such that resonator electromagnetic radiation oscillates in the resonator at the resonance frequency. The method further includes temporally, after coupling the pump radiation, changing the resonance frequency of the resonator so that the resonance frequency of the resonator radiation oscillating in the resonator is changed over a tuning bandwidth, wherein the pump frequency does not follow the change in resonance frequency, decoupling of the resonator radiation as useful radiation with the useful frequency different from the pump frequency from the resonator, pumping of an amplifying medium arranged in the resonator; and amplifying the resonator radiation oscillating in the resonator in the amplifying medium. The amplification is smaller than a threshold amplification required by the resonator and the amplifying medium for a laser action of the resonator.

Inventors

Simon J. HERR

Assignees

Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.

Dates

Publication Date: 20260505
Application Date: 20211013
Priority Date: 20201014

Claims (14)

1 . A method for generating an electromagnetic useful radiation comprising a useful frequency, the method comprising the steps of: a) generating and emitting an electromagnetic pump radiation comprising a pump frequency; b) coupling the pump radiation into an external optical resonator, which external optical resonator comprises a resonance frequency, wherein the resonance frequency is at least initially substantially equal to the pump frequency, such that an electromagnetic resonator radiation oscillates in the resonator at the resonance frequency; c) temporally after step b) changing the resonance frequency of the resonator so that the resonance frequency of the resonator radiation oscillating in the resonator is changed over a tuning bandwidth, wherein the pump frequency does not follow the change in resonance frequency; d) coupling out the resonator radiation out of the resonator as the useful radiation ( 2 ) comprising the useful frequency different from the pump frequency; e) pumping of an amplifying medium arranged in the resonator; and f) amplifying the resonator radiation oscillating in the resonator in the amplifying medium, wherein an amplification is smaller than a threshold amplification required by the resonator and the amplifying medium for a laser action of the resonator.
2 . The method according to claim 1 , wherein the resonator is a microresonator.
3 . The method according to claim 1 , wherein the amplifying medium is a four-level laser medium.
4 . The method according to claim 1 , wherein the resonator comprises an electro-optic material, wherein the resonator radiation propagates through the electro-optic material, and wherein changing the resonance frequency comprises the steps of applying a voltage to the electro-optic material and changing the voltage.
5 . The method according to claim 1 , wherein the amplifying medium and the electro-optic material is a rare earth ion doped lithium niobate or a rare earth ion doped lithium tantalate.
6 . The method according to claim 1 , wherein steps c) and d) are carried out simultaneously or step d) is carried out temporally after step c).
7 . The method according to claim 1 , wherein in step c) the tuning bandwidth is larger than a pump frequency bandwidth of the pump radiation.
8 . The method according to claim 1 , wherein during step c) the coupling of the pump radiation into the resonator is interrupted.
9 . The method according to claim 1 , wherein pumping the amplifying medium comprises the steps of generating and emitting an electromagnetic amplifying pump radiation comprising an amplifying pump power, and illuminating the amplifying medium with the amplifying pump radiation, wherein the amplifying pump power is less than a threshold pump power required for laser operation of the resonator.
10 . A device for generating an electromagnetic useful radiation comprising a useful frequency, wherein the device comprises: a first radiation source configured such that the first radiation source generates and emits electromagnetic pump radiation at a pump frequency in an operation of the device; an external optical resonator; a controller; a pumping means; and an amplifying medium, wherein the resonator is arranged and configured: so that the resonator has a tunable resonance frequency, so that, in the operation of the device, the pump radiation emitted by the first radiation source is coupled into the resonator, and so that in the operation of the device the useful radiation is coupled out of the resonator, wherein the controller is operatively coupled to at least one of the first electromagnetic radiation source and the resonator and is arranged so that, in the operation of the device, the controller sets at least the pump frequency or the resonance frequency such that the resonance frequency is, at least initially, essentially equal to the pump frequency, such that, in the resonator, an electromagnetic resonator radiation oscillates at least initially at the resonance frequency, and then changes the resonance frequency of the resonator and thus the useful frequency of the useful radiation over a tuning bandwidth, wherein the device is configured such that the pump frequency does not follow a change in the resonance frequency, wherein the amplifying medium is arranged in the resonator so that in the operation of the device the amplifying medium is pumped by the pumping means and the resonator radiation is amplified in the amplifying medium, wherein the amplification is smaller than a threshold amplification required by the resonator and the amplifying medium for a laser action of the resonator.
11 . The device according to claim 10 , wherein the resonator comprises an electro-optic element, which electro-optic element comprises an electro-optic material such that an optical resonator length of the resonator and thus the resonance frequency is changeable depending on a voltage applied to the electro-optic material, and wherein the controller is coupled to the electro-optic element such that the controller controls the voltage applied to the electro-optic material and thus the optical resonator length in the operation of the device.
12 . The device of claim 11 , wherein the electro-optic material is the amplifying medium.
13 . The device according to claim 10 , wherein the resonator comprises a waveguide, preferably an integrated waveguide.
14 . The device according to claim 13 , wherein a material of the waveguide is the electro-optic material, wherein the electro-optic element comprises a plurality of electrodes for applying the voltage to the electro-optic material, wherein the electrodes are spaced 100 μm or less from the waveguide.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is a National Stage of International Application No. PCT/EP2021/078259 filed Oct. 13, 2021, which claims benefit of European Patent Application No. 20 201 837.0 filed Oct. 14, 2020, both of which are herein incorporated by reference in their entirety. The present invention relates to a method for generating an electromagnetic useful radiation comprising a useful frequency, the method comprising the steps of: a) generating and emitting an electromagnetic pump radiation comprising a pump frequency, b) coupling the pump radiation into an external optical resonator, which external optical resonator comprises a resonance frequency, wherein the resonance frequency is at least initially substantially equal to the pump frequency, such that an electromagnetic resonator radiation oscillates in the resonator at the resonance frequency, c) temporally after step b) changing the resonance frequency of the resonator such that the resonance frequency of the resonator radiation oscillating in the resonator is changed over a tuning bandwidth, wherein the pump frequency does not follow the change of the resonance frequency, and d) coupling the resonator radiation out of the resonator as the useful radiation comprising the useful frequency different from the pump frequency. Optical resonators of this type are used in a variety of photonic devices. It proves to be problematic, especially for resonators which are pumped once, i.e. over a fixed time interval, with the pump radiation, that the possible period of use of the useful radiation coupled out of the resonator is determined by a decay time τ of the resonator. Here, the decay time τ is given as τ=Qλ/(2 πc), wherein Q is the Q factor of the resonator, λ is the vacuum wavelength of the resonator radiation in the resonator, and c is the vacuum light velocity. In lithium niobate microresonators a few millimeters in size, Q factors on the order of 108 are typically achieved, corresponding to decay times, i.e. periods of use in the range of about 100 ns. For chip-integrated microresonators, the achievable Q factors are currently significantly lower. Although the optical resonator can be pumped again, the useful radiation then oscillating in the resonator again only has a period of use of about 100 ns. In contrast, the present invention is directed to providing a method and an apparatus for generating an electromagnetic useful radiation with an increased period of use. To solve this task, a method is proposed according to the invention, comprising the steps: a) generating and emitting an electromagnetic pump radiation comprising a pump frequency,b) coupling the pump radiation into an external optical resonator, which external optical resonator comprises a resonance frequency, the resonance frequency being at least initially substantially equal to or equal to the pump frequency, such that resonator electromagnetic radiation oscillates in the resonator at the resonance frequency,c) temporally after step b) changing the resonance frequency of the resonator so that the resonance frequency of the resonator radiation oscillating in the resonator is changed over a tuning bandwidth, wherein the pump frequency does not follow the change in resonance frequency, andd) coupling of the resonator radiation out of the resonator as useful radiation comprising a useful frequency different from the pump frequency,e) pumping of an amplifying medium arranged in the resonator, andf) amplifying the resonator radiation oscillating in the resonator in the amplifying medium, wherein an amplification is smaller than a threshold amplification required by the resonator and the amplifying medium for a laser action of the resonator. According to the present invention, electromagnetic tunable useful radiation or useful radiation tunable with respect to the useful frequency is provided. For this purpose, the resonance frequency of the resonator and thus the frequency of the resonator radiation oscillating in the resonator as well as the useful frequency of the useful radiation coupled out of the resonator are adiabatically changed during the period of use. In this way, a spectrally tunable source is provided. Therefore, the optical resonator comprises a tunable resonance frequency. The useful radiation provided in this way can be reliably tuned over a wide frequency range without the need for tuning of the pump radiation. Therefore, no mode jumps occur during tuning, which would otherwise lead to uncontrolled generation of useful radiation. In an embodiment, the tuning duration is substantially equal to the period of use of the useful radiation. Here, the tuning duration is the time required to tune the resonance frequency starting from the pump frequency over the entire tuning bandwidth provided by the resonator to a maximum useful frequency spaced from the pump frequency. In this context, the period of use of the useful radiation is the time period durin