Device for generating a controllable electromagnetic frequency signal at the end of an electro-optical path

Abstract

The invention relates to a device for generating a controllable electromagnetic frequency signal at the end of an electro-optic path, wherein the device comprises a continuous-wave signal which generates a coherent electromagnetic frequency signal, a Doppler compensation unit, and a controllable reference oscillator which generates a controllable frequency signal. The Doppler compensation unit is used to minimize the relative optical noise and to control the phase / frequency of the electromagnetic frequency signal.

Description

[0001] The invention relates to a device for generating a controllable electromagnetic frequency signal at the end of an electro-optic path and a method for controlling an electromagnetic frequency signal at the end of an electro-optic path.

[0002] It is known from the prior art that optical frequency references, or continuous-wave laser signals, typically provide electromagnetic frequency signals of several hundred terahertz (optical band). The sources of these continuous-wave laser signals can be narrowband laser light sources, cavity-stabilized lasers with sub-hertz linewidths, or lasers stabilized on atomic or molecular electronic or nuclear transitions. These continuous-wave laser signals are characterized by high frequency stability and very low phase noise.

[0003] Because the electromagnetic frequency signals of continuous-wave laser signals are usually transmitted from the source to the application via optical fibers or free-space paths, the electromagnetic frequency signal can be unintentionally altered. While optical fibers facilitate the handling of laser light (especially with regard to laser safety) and reduce the effects of, for example, mechanical instabilities, a disadvantage of handling fibers is their acoustic and thermal sensitivity. This leads to a change in the optical path of the light traveling through the fiber and thus to a corresponding shift in the transmitted frequency and / or phase due to the Doppler effect (Doppler shift). It should be noted that the same effect is also responsible for frequency changes in free-space paths, and the principle described below is also used in this context.

[0004] A method for compensating phase noise during the transmission of an optical signal on an optical waveguide is known from the following publication: YANG, Jun [et al.]: Noise compensation methods for optical fiber frequency sweeping interferometry: a review. In: Journal of lightwave technology, Vol. 41 2023, No. 13, pp. 4035-4050. ISSN 0733-8724. https: / / ieeexplore.ieee.org / stamp / stamp.jsp?tp=&arnumber=9973336

[0005] The object of the invention is to provide a device for generating an electromagnetic frequency signal that provides a virtually noise-free and controllable electromagnetic frequency signal at the end of an electro-optical path. Furthermore, a corresponding method for controlling an electromagnetic frequency signal at the end of an electro-optical path is to be provided. With a controllable electromagnetic frequency signal at the end of an electro-optical path, the frequency and / or phase of the electromagnetic frequency signal can be controlled and / or changed, preferably without significantly altering the signal-to-noise ratio at the end of the electro-optical path.

[0006] The problem is solved according to the invention by claims 1, 11 and 12.

[0007] The device according to the invention for generating a controllable electromagnetic frequency signal at the end of an electro-optic path comprises a continuous-wave signal that generates a coherent electromagnetic frequency signal. These continuous-wave signals can be narrowband laser light sources, cavity-stabilized lasers with sub-Hertz linewidths, or lasers stabilized on atomic or molecular electronic or nuclear transitions. These continuous-wave laser signals are preferably characterized by high fractional frequency stability and very low phase noise.

[0008] The device further comprises a Doppler compensation unit. At the end of an electro-optic path, the Doppler compensation unit measures the noise of an electromagnetic frequency signal and modulates the electromagnetic frequency signal before it propagates into the optical path. This modulation causes the noise coupled into the electromagnetic frequency signal along the electro-optic path and the modulation of the Doppler compensation unit to destructively interfere, resulting in a virtually noise-free electromagnetic frequency signal at the end of the optical path. A preferred electro-optic path can be between 0.1 and 100 km long.

[0009] The device further comprises a controllable reference oscillator which generates a controllable frequency signal. A change in the frequency signal of the controllable reference oscillator is preferably denoted by Δ. A reference oscillator can, for example, be configured as an RF oscillator (OCXO, synthesizer, etc.), or as an optical oscillator (laser with a photodiode), or as a combination of the two. Other preferred embodiments for the reference oscillator are: a quartz oscillator, a phase-locked loop circuit, a frequency generator, or a combination of the aforementioned reference oscillators. Another preferred reference oscillator can also be purely optical, e.g., a tunable laser that generates an RF signal by means of a photodiode. Preferably, the reference oscillator provides a frequency in the range of 5–400 MHz, with the controllable frequency having a precision in the range of 10 -6 - 10 -2Hertz. It is also particularly preferred that the reference oscillator provides a carrier frequency of 100 kHz to 100 GHz.

[0010] The Doppler compensation unit also features an input for a signal from the reference oscillator, which is used to shift the electromagnetic frequency signal in phase and / or frequency. In other words, the Doppler compensation unit is used to minimize relative optical noise and to control the frequency of the electromagnetic signal by providing an additional input for a controllable frequency signal from the reference oscillator and shifting the electromagnetic frequency signal proportionally to the controllable frequency signal of the reference oscillator.

[0011] Preferably, the Doppler compensation unit comprises an asymmetric beam splitter which directs a large portion of the coherent electromagnetic frequency signal into path "A" and a smaller portion into path "B". An asymmetric beam splitter divides the quanta of an electromagnetic frequency signal unevenly, directing a larger proportion of the quanta in one direction and a smaller proportion in another. "Large portion" preferably refers to more than 50% of the quanta. In an optional embodiment, however, it can also be 98% to 2%, or 99% to 1%, or 95% to 5%, or 90% to 10%.

[0012] Preferably, the Doppler compensation unit includes a signal deflection unit along path "A". The larger portion of the split coherent electromagnetic frequency signal propagates from the asymmetric beam splitter towards the signal deflection unit. A signal deflection unit deflects the coherent electromagnetic frequency signal depending on its propagation direction. For example, a signal deflection unit can transmit a coherent electromagnetic frequency signal propagating through the input of the signal deflection unit almost undisturbed, while a coherent electromagnetic frequency signal propagating into the output of the signal deflection unit is deflected in one direction. The deflection direction can, for example, be 90° to the axis of the incident coherent electromagnetic frequency signal.A preferred signal deflection unit can be configured, for example, as a circulator, beam splitter, or polarizing beam splitter cube (PBS) in conjunction with a lambda / 4 plate. A preferred circulator can couple out the output direction depending on three incidence directions 1-3 in a precise, cyclic sequence (e.g., 1->2, 2->3, and 3->1). Preferred incidence and / or emission directions of a cube-shaped circulator are, for example, two opposing faces and a further face, preferably located at a 90° angle to the two opposing faces. A preferred circulator includes several beam splitters, Faraday rotators, and mirrors internally.

[0013] Preferably, the Doppler compensation unit has one or more phase-shifting and / or frequency-shifting elements along path "A". These phase-shifting and / or frequency-shifting elements are used to modulate the electromagnetic frequency signal and can preferably be configured as acousto-optic modulators, electro-optic modulators, and / or magneto-optic modulators. They have an input for one or more control signals. The control signal is preferably a frequency signal in which the frequency and / or amplitude can be controlled. A change in the frequency and / or amplitude of the control signal preferably results in a shift in the frequency and / or phase of the incident electromagnetic frequency signal. Preferably, the control signal comprises two superimposed frequency signals. The first signal could preferably be proportional to the noise of the electro-optic path.The second signal could be a controllable frequency signal, for example from a reference oscillator, which controllably shifts the frequency and / or phase of the electromagnetic frequency signal. It is further preferred that each signal is fed into a separate input for a control signal of the phase and / or frequency shifting element, and that the two frequencies are preferably superimposed internally within the phase and / or frequency shifting element.

[0014] Preferably, the Doppler compensation unit has an electro-optic path along path "A". The electro-optic path is preferably designed as a free-space path and / or optical fiber. Noise coupling can occur, particularly along the electro-optic path. This noise can preferably be phase and / or frequency noise.

[0015] Preferably, the Doppler compensation unit has a partially transparent reflector along path "A", preferably at the end of path "A". The partially transparent reflector transmits a large portion, preferably over 50%, of the quanta of the electromagnetic frequency signal undisturbed. A small portion, preferably less than 50%, of the quanta of the electromagnetic frequency signal is reflected. The path of the reflected electromagnetic frequency signal follows path "C". The path of the majority of the electromagnetic frequency signal continues to follow path "A". Path "C" is preferably equivalent to path "A" between the partially transparent reflector and the signal deflection unit, wherein the quanta of the electromagnetic frequency signal propagate in the opposite direction to the quanta from path "A".The end of the electro-optic path, where the controllable electromagnetic frequency signal can be used for further experiments, is located behind the partially transparent reflector. Preferably, the division of the electromagnetic frequency signal at the partially transparent reflector can also be 98% to 2%, or 99% to 1%, or 95% to 5%, or 90% to 10%. The partially transparent reflector can preferably be configured as a PM mirror or fiber-coupled Faraday rotator mirror, a normal mirror with a special coating, or a free-beam Faraday rotator followed by a mirror.

[0016] In a preferred embodiment, an arrangement "behind" one component is considered to be an arrangement "behind" another component along an optical path. If an electromagnetic frequency signal passes first component A and then component B, then component B is arranged "behind" component A.

[0017] Preferably, path "C" initially runs along path "A" in the opposite direction, back through the optical path, and then back through one or more phase and / or frequency-shifting elements into the output of the signal redirection unit. Due to the two passages through the optical path and the phase and / or frequency-shifting elements, noise is coupled into the electromagnetic frequency signal along path "C" twice, and it is also shifted twice by the phase and / or frequency-shifting elements. The signal redirection unit directs the electromagnetic frequency signal arriving at the output in a different direction than path "A". Thus, path "A" and path "C" diverge at this point.

[0018] Preferably, the Doppler compensation unit in the beam path of path "B" and path "C" includes a coupler element which superimposes a portion of the electromagnetic frequency signal coming from the asymmetric beam splitter and the reflected portion of the electromagnetic frequency signal coming from the signal deflection unit, and sends the superimposed electromagnetic frequency signal to a photodetector, wherein the photodetector converts the superimposed electromagnetic frequency signal into an electrical signal. Preferably, the two electromagnetic frequency signals are frequency-shifted, and the superimposed signal is a beat frequency, optionally with amplitude modulation.

[0019] Preferably, the Doppler compensation unit includes a mixer that demodulates the electrical signal of the superimposed electromagnetic frequency signal, the photodetector, with the reference oscillator frequency signal. The mixer signal serves to control a control electronics for a local oscillator, which controls the phase-shifting and / or frequency-shifting elements in beam path "A". In the unshifted case, the local oscillator frequency corresponds to half the frequency of the reference oscillator. The unshifted case is defined as the frequency of the controllable electromagnetic frequency signal at the end of the electro-optical path being unshifted from the frequency of the continuous wave source.The mixer's output is thus proportional to the cosine of the phase difference between the electromagnetic frequency signal of the continuous wave signal and the forward and reverse beams along path "C", which have passed twice through the electro-optical path and the phase and / or frequency-shifting elements. The mixer's output serves as an error signal for an electrical control loop. This control electronics regulates the local oscillator, which generates the signal for the phase and / or frequency-shifting elements and, in the feedback loop, cancels out the phase and frequency changes in the optical signal transmitted through the fiber.

[0020] Preferably, the Doppler compensation unit has exactly one phase and / or frequency shift element.

[0021] Preferably, the continuous wave source emits coherent electromagnetic waves with a frequency in the range between 190 THz and 300 THz, or 100-520 THz, or 999 THz - 166 THz, wherein the Doppler compensation unit has the following properties: The frequency shift of the continuous wave signal at the end of the electro-optic path is relative to the coherent electromagnetic frequency signal of the unshifted continuous wave source. A preferred typical incremental frequency shift is 10 -15 , relative to the continuous wave source. Preferably, the signal of the unshifted continuous wave source is, for example, at a frequency of 300 THz when the reference oscillator frequency Δ changes by 10° - 10 2 Hz range around 10 -15 Hz shifted. Another preferred incremental shift of 10 -17 Hz is possible. It is still preferred that the frequency shift of the continuous wave signal be 5 * 10 -15Hz at 200 THz coherent electromagnetic frequency signal of the continuous wave source with a shift of the reference oscillator frequency of 1 Hz.

[0022] Preferably, the highly stable frequency signal is converted into an electrical radio frequency signal in the range of 10 at the end of an electro-optical path, preferably behind the partially transparent reflector. 7 - 10 10 Hz converted. This conversion allows the device to be used, for example, in clock ensembles together with preferably highly stable atomic clocks and frequency references, particularly preferably AHM, rubidium, or cesium atomic clocks.

[0023] Preferably, the Doppler compensation unit, the signal deflection unit, the phase and / or frequency-shifting elements, and the device for generating a highly stable frequency signal can be configured as part of a photonic integrated circuit (PIC). The signal deflection unit, e.g., a circulator, can be replaced by PIC-compatible components, e.g., a beam splitter, and the phase and / or frequency-shifting element(s) can be configured internally or externally.

[0024] The invention further relates to a method for controlling an electromagnetic frequency signal at the end of an electro-optical path, in particular using a device according to one of the preceding claims. The method comprises the following steps: a. Electromagnetic frequency signal is divided asymmetrically, a large part of the quanta propagates along path “A”, a small part of the quanta propagates along path “B”. b. An electromagnetic frequency signal along path “A” passes through a signal deflection unit c. The phase and / or frequency of the split electromagnetic frequency signal is shifted a first time by ω along path “A” by one or more phase and / or frequency shifting elements. d. Noise is coupled into the electromagnetic frequency signal along path “A” along an optical path. e. A portion of the electromagnetic frequency signal along path “A” is transmitted along path “A” by a partially transparent reflector, or reflected along path “C”. f. Noise is coupled into the electromagnetic frequency signal along the path “C” for the second time when it passes the electro-optical path. g. The phase and / or frequency of the reflected electromagnetic frequency signal is shifted a second time by ω along path “C” by one or more phase and / or frequency-shifting elements. The total frequency offset is now the original frequency ± 2ω. h. Electromagnetic frequency signal along path “C” passes through a signal deflection unit and is superimposed with signal from path “B” in a coupler. i. The signal generated by the coupler element is sent to a photodetector and converted into an electrical signal. j. The electrical signal of the photodetector is demodulated at the frequency of the reference oscillator 2ω. k. The demodulated signal is used to control the phase and / or frequency shift element(s). l. A change in the reference oscillator frequency Δ preferably leads to a shift in the phase and / or frequency of the local oscillator and thus to a change in the frequency of the controllable electromagnetic frequency signal at the end of an electro-optic path, preferably by Δ / 2. Preferably, the change in the reference oscillator frequency Δ is permanent, i.e., preferably of longer duration than the changes in the frequency of the controllable electromagnetic frequency signal at the end of an electro-optic path due to coupled-in noise.

[0025] Preferably, the above order of the described components corresponds to the order of the components along the optical paths.

[0026] Preferably, the path “C” runs (anti)parallel to path A. It is further preferred that path C runs (anti)parallel to path “A” at least until it passes through the phase and / or frequency shift element.

[0027] The invention further relates to the use of a Doppler compensation unit to minimize the relative optical noise, as well as to control the frequency of the electromagnetic frequency signal.

[0028] Preferred embodiments of the invention are explained below with reference to figures.

[0029] They show: Fig. 1 Doppler compensation to influence the phase / frequency of an optical signal (continuous wave laser).

[0030] The device for generating a controllable electromagnetic frequency signal according to Fig. 1 features a continuous wave signal DS, a Doppler compensation unit, and a controllable reference oscillator RO.

[0031] The continuous wave signal DS according to Fig. 1 sends an electromagnetic frequency signal towards the Doppler compensation unit. The Doppler compensation unit according to Fig. 1 indicates: a. An asymmetric beam splitter AST, b. A signal redirection unit (SUE), c. One or more phase and / or frequency shift elements (PFS), d. An electro-optical path OS, e. A semi-transparent reflector TR, f. A coupler element, COPPER g. A photodetector PD, h. A mixer X, i. A control unit, in Fig. 1 featuring controller R1 and local oscillator LO

[0032] The Doppler compensation unit according to Fig. Module 1 has an input for the electromagnetic frequency signal of the continuous wave signal DS. The received electromagnetic frequency signal is first split into two paths (path A and path B) in the asymmetric beam splitter AST, with a large portion of the electromagnetic frequency signal being routed to the signal deflection unit SUE (path A). A small portion of the electromagnetic frequency signal is routed directly to the coupler element KOPPLER (path B).

[0033] The signal diversion unit SUE in section A according to Fig. 1 transmits the electromagnetic frequency signal, coming from the asymmetric beam splitter AST, to the phase and / or frequency shifting element(s) PFS. Here, the frequency of the electromagnetic frequency signal is preferably shifted slightly. Slightly shifted can preferably mean a few Hertz (10 -6 - 10 -2Hertz). The frequency shift is proportional to the frequency of the local oscillator LO. From here, it is routed to the electro-optic path OS, where phase and / or frequency noise can couple in. Of course, phase and / or frequency noise can also couple in at any other point in the device. At the end of the optical path OS is a partially transparent reflector TR, which transmits a large portion of the electromagnetic frequency signal. A small portion of the electromagnetic frequency signal is reflected (path C), back through the electro-optic path OS, and back through the phase and / or frequency shift element(s) PFS, where the frequency of the electromagnetic frequency signal is shifted again by the same frequency as before. From here, the electromagnetic frequency signal propagates back to the signal deflection unit SUE, where it is then routed towards the coupler element KOPPLER.

[0034] The coupler element KOPPLER according to Fig. Step 1 superimposes the electromagnetic frequency signals from the asymmetric beam splitter AST (section B), specifically the smaller portion of the beam splitter signal, with the portion of the electromagnetic frequency signal reflected by the partially transparent reflector TR, originating from the signal deflection unit SUE (section C). Due to the slight frequency difference, the two signals are superimposed to form a beat frequency and are converted into an electrical signal by the photodetector PD.

[0035] The electrical signal of the photodetector PD according to Fig. Signal 1 is demodulated by mixer X, together with the electrical signal of the reference oscillator RO, to form the PFS control signal, so that the coupled phase and / or frequency noise is included in the PFS control signal. This PFS control signal serves as the basis for the controller unit R1 and / or local oscillator LO to control the phase and / or frequency shift element(s) PFS, thus compensating for the phase and / or frequency noise of the optical path OS. The phase and / or frequency shift element(s) PFS can therefore compensate for the noise of the optical path OS based on this signal.

[0036] The controllable electromagnetic frequency signal at the end of an electro-optical path according to Fig.1 is controllable by shifting the reference oscillator RO frequency. This reference oscillator RO frequency is controllable by an external control element EX. An external control element can preferably be a computer, computing unit, microcontroller, microprocessor, or FPGA. Controlling the electromagnetic frequency signal at the end of an electro-optical path can be helpful, for example, when using the electromagnetic frequency signal in a clock ensemble, where the frequency and phase of clock signals must be set very precisely and without jumps. This type of control allows frequency shifts in the sub-Hertz range and the synchronization of the frequencies of multiple clocks in a clock ensemble.

Claims

[1] Device for generating a controllable electromagnetic frequency signal at the end of an electro-optical path, the device comprising: a. a continuous wave signal that generates a coherent electromagnetic frequency signal b. a Doppler compensation unit c. a controllable reference oscillator which generates a controllable frequency signal d. characterized by , that the Doppler compensation unit is used to minimize the relative optical noise, as well as to control the frequency of the electromagnetic frequency signal, by having an additional input for a controllable frequency signal of the reference oscillator and shifting the electromagnetic frequency signal proportionally to the controllable frequency signal of the reference oscillator. [2] Device according to one of the preceding claims, wherein the Doppler compensation comprises: a. An asymmetric beam splitter which directs a large part of the coherent electromagnetic frequency signal to the end of an electro-optic path, wherein the path to the end of the electro-optic path has: i. A signal redirection unit, ii. One or more phase and / or frequency shift elements, iii. An electro-optical path, iv. A semi-transparent reflector, v. In which part of the electromagnetic frequency signal at the end of the electro-optic path is propagated back through the electro-optic path by the reflector, and the phase and / or frequency shifting element propagates it to the signal deflection unit, wherein the signal deflection unit diverts the reflected electromagnetic frequency signal and feeds it into a coupler element, b. A coupler element which superimposes a part of the electromagnetic frequency signal coming from the asymmetric beam splitter and the reflected part of the electromagnetic frequency signal coming from the signal deflection unit and sends the superimposed electromagnetic frequency signal to a photodetector, wherein the photodetector converts the superimposed electromagnetic frequency signal into an electrical signal, c. A mixer that demodulates the electrical signal of the superimposed electromagnetic frequency signal with the reference oscillator frequency signal, d. A control element that uses the demodulated electrical signal of the superimposed electromagnetic frequency signal to control one or more phase and / or frequency shifting elements in the optical path. [3] Device according to claim 2, characterized by, that the partially transparent reflector reflects the coherent electromagnetic frequency signal in the ratio 90 / 10, or 95 / 5, or 98 / 2 or 99 / 1. [4] Device according to claim 2 or 3, characterized by , that the asymmetric beam splitter divides the coherent electromagnetic frequency signal in the ratio 98 / 2, or 99 / 1, or 95 / 5 or 90 / 10. [5] Device according to one of the preceding claims , characterized by that the device for generating a controllable electromagnetic frequency signal at the end of an electro-optical path has exactly one phase and / or frequency shifting element. [6] Device according to any one of the preceding claims, characterized by , that the signal redirection unit is designed as: a. Circulator and / or b. Beam splitter [7] Device according to any one of the preceding claims, characterized by , that the phase and / or frequency shift element is designed as: a. Acousto-optic modulator b. And / or electro-optical modulator c. And / or magneto-optical modulator [8] Device according to any one of the preceding claims, characterized by , that the continuous wave source emits coherent electromagnetic waves with a frequency in the range between 190 THz and 300 THz or 100-520 THz, or 999 THz - 166 THz, and the Doppler compensation unit has the following properties: a. Frequency shift of the continuous wave signal by 5 * 10 -15 Hz at 200 THz with a shift of the reference oscillator frequency of 1 Hz. [9] Device according to any one of the preceding claims, characterized by , that the highly stable frequency signal at the end of an electro-optical path is converted into an electrical radio frequency signal in the range of 10 7 - 10 10 Hz is converted. [10] Device according to any one of the preceding claims, characterized by, that the Doppler compensation unit has a signal deflection unit and a phase and / or frequency shift element and the device for generating a highly stable frequency signal is designed as a photonic integrated circuit (PIC), wherein the signal deflection unit, in particular a circulator, can be replaced by PIC-compatible components, in particular a beam splitter, and phase and / or frequency shift element(s) can be designed externally. [11] Method for controlling an electromagnetic frequency signal at the end of an electro-optical path using a device according to one of the preceding claims. [12] Use of a device according to one of claims 1-10 for minimizing the relative optical noise and for controlling the frequency of the electromagnetic frequency signal.

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