DESCRIPTION OF THE INVENTION

1. Technical field

The present invention relates to method with adequate apparatus for tuning the laser frequency implemented in a rotating device with an optical waveguide covered by the technical field falling under Classes under G01 , G02 and H01 of the International Patent Classification.

2. Technical problem

The problem to be solved by the present invention is to engineer a method and adequate apparatus for tuning the frequency of a laser. The method should be applicable for versatile types of lasers, which are used to couple light into an optical waveguide for further propagation and processing.

Our aim with this invention is to establish a robust method for tuning the laser frequency, which is applicable for various types of lasers (i.e. , the method is not laser specific). Our approach by its design allows for low-loss and non-diffracting coherent light propagation, which results with tunable frequency shift at the output.

3. State of the art

Tunable laser sources have a broad range of applications in various fields including physics, chemistry, biology, and medicine. Lasers are used for excitations of atoms, molecules, or solid-state systems with a specific purpose. In most applications, laser frequency must correspond to an electronic transition that is given by the system that the laser should excite. For example, to resonantly excite a specific electronic transition in a given molecule, laser frequency must correspond exactly to the energy difference between two molecular levels involved in this transition. However, the frequency of a laser is determined by its construction and design. Therefore, it is highly desirable to have robust methods for tuning the laser frequency.

The concept of a tunable laser implies that the frequency of the emitted light can be changed to some degree by employing a certain specific physical mechanism, usually related to the way in which the laser functions. There are many types of tunable lasers including dispersive tunable laser oscillators, excimer lasers, gas lasers (e.g., CO2 and He-Ne lasers), dye lasers (liquid and solid state), transition metal solid-state lasers, semiconductor crystal and diode lasers, and free electron lasers, see Ref. [1] for a review. However, the physical mechanisms that are used in excimer lasers, gas lasers, dye lasers, transition metal solid-state lasers, semiconductor crystal and diode lasers, and free electron lasers for tuning the laser frequency are laser specific: in simple words, they depend on the way that a particular one of these lasers is constructed and designed.

There are schemes for tuning laser frequencies that circumvent this problem and that are applicable for different types of lasers (in other words they are not laser specific). More specifically, by using the concepts from acousto-optics, which studies interaction of light and sound waves, one can achieve manipulation of light waves, including modulation of wave amplitude and phase, shift of its frequency or beam deflection.

The field of acousto-optics has developed starting with the seminal papers on diffraction of light long time ago [2, 3], Subsequent progress in research has been spurred particularly with the development of lasers. More general and pedagogical introduction into the field and into numerous applications can be found in the literature (see, for example, Refs. [4, 5, 6]).

Let us describe frequency shifting of laser beams in acousto-optic devices, as it is based on the Doppler effect, which is the physical mechanism used in our device as well. It is well known that the frequency of a wave can be changed by Doppler shift. If a source of electromagnetic radiation of frequency & and wavevector k in vacuum, moves to the observer with some velocity v, the observer will see radiation at frequency w + k ■ v.

One laser-frequency tuning device, which can be interpreted via the Doppler shift, is an acoustic optical frequency shifter. In this device, sound waves which travel inside a material generate periodic variation of the index of refraction, that is, they produce a moving grating. Laser light that impinges on this device is diffracted at such a travelling refractive index grating. The positions of the intensity maxima of diffracted light can be deduced from the Bragg condition (which assures the constructive interference of light waves). The intensity of a light beam is distributed into corresponding diffracting orders, and every intensity maximum corresponds to one diffracting order. Due to the Doppler effect, this diffracted light experiences a shift of the optical frequency. The shift can be positive or negative depending on the acoustic frequency [4, 6]. The frequency of the drive is fixed in most devices, which means that the frequency is shifted by a fixed amount(s) in the positive or negative direction. However, there are drivers for variable frequency. A variable frequency driver can be based on a voltage-controlled oscillator (VCO), that adjusts the drive frequency to make the frequency shift continuous [7],

Acousto-optic frequency shifters, which we discussed so far, operate in the bulk media. Subsequently, similar shifters in optical fibers have also been developed (for first implementation see Refs. [8, 9, 10]). In [8], a frequency shifter was constructed by using a birefringent single-mode fiber placed in a specific environment, which under application of standing pressure wave enabled coupling of incident wave in one polarization state to a frequency-shifted wave in orthogonal state. Alternatively, in [9,10] an idea to frequency shifting is to launch an acoustic wave for propagation along the length of the optical fiber. More specifically, in Ref. [10] a frequency shifter has consisted of an optical fiber with two propagation modes and an acoustic transducer for generating an acoustic wave. Due to the coupling of optical and acoustic waves, the incoming light wave in one mode can be partly transferred into a second optical mode with different (shifted) frequency at the output.

Importantly, in all these inventions based on acousto-optics, there is inevitable loss of the frequency shifted light that occurs, either due to scattering on a grating lattice or (in case of shifters implemented in optical fibers) due to coupling to other optical modes. The maximal efficiency that can be achieved is up to 90% (this does not include losses due to coupling of the light from the laser to the fiber but exclusively the losses due to acoustic-optical frequency shift). This method, albeit commonly used, is not effective whenever an experiment or device demands precise frequency tuning simultaneously with a high conversion efficiency (i.e., low-loss of intensity during frequency conversion). A single laser frequency tuning device that is not laser specific, and that does not diminish the intensity of the emitted shifted frequency is yet not existent.

Previous inventions contained in patent and scientific literature have often used the Doppler effect in order to achieve the frequency shift of the light beam. For example, if the light is reflected from some moving mirror, its frequency will be Doppler shifted. One can imagine various types of experimental setups, that is, systems of mirrors which can enable this frequency shift. The distinction between every such invention is the architecture of the device and the system of mirrors that are using the Doppler effect. This architecture is manifested in the characteristics and in the functionality of the device.

As a side note, we note that the Doppler shift is also used to tune the frequencies in devices related to y ray spectroscopy in atomic nuclei, in Mbssbauer spectroscopy experiments (see, for example, Ref. [11]). However, these rotating devices and schemes are not related to lasers, and do not produce coherent light that is coupled into an optical waveguide (fiber), but they are incoherent sources of electromagnetic radiation. 4 Essence of the invention

We invent a rotating device utilizing an optical waveguide and a method for tuning the frequency of versatile types of lasers by using the Doppler shift effect. Let the frequency of a laser standing still (e.g., mounted on a standing optical table) be &>. By mounting this laser on our rotating device, its frequency can be tuned in the range [w - £i _max Rk,< _f j + n _maK Rk], where is the maximal frequency of rotation of the device, is the radius at which the laser is mounted, and k = M/C, where is the speed of light. The method can be used on any laser source that can be mounted on the rotating device. The light from the laser is coupled into an optical fiber that rotates with the laser on the device, and the light is finally launched along the axis of rotation for further propagation and processing. The method is thus applicable for all compact laser sources such as semiconductor lasers. This makes the tuning method applicable for versatile types of lasers.

Let us describe our device in detail. The laser (1.1) is mounted on a disc (1.2) that can rotate around its axis of rotation (2.2); the laser is located at radius R (2.1) from the axis of rotation, and it emits light into an optical waveguide (1 .4) in the direction Si xR (1.5) that is perpendicular to the radial vector (1.6) and the rotation frequency vector n (1.7). The direction fi x /? (1.5) for the emission of light is optimal for functionality of the device, however, the device is functional with smaller efficiency if the emitted light deviates for some angle (smaller than 30 degrees) from the optimal direction fi x R (1.5). The top view of the device is illustrated in Fig. 1 and the side view in Fig. 2.

The disk can rotate around its axis of rotation (2.2) at frequency n e here positive frequencies correspond to rotation in counter-clockwise direction, whereas negative frequencies correspond to clockwise direction. The counterclockwise direction of rotation is illustrated with an arrow (1.3) in Fig. 1. Rotation of the disc can be most efficiently enabled by using an electric engine, however, it is not relevant for the essence of this invention how is the rotation achieved. The exact shape of the rotating disc is also not essential as the method would be functional if a disc is replaced by a rotating square table or some other shape; the disc shape is an optimal configuration for rotation due to axial symmetry, but it is not essential. An essential ingredient is the maximal frequency of rotation that can be achieved D _mat , because the maximal frequency shift depends upon it.

Laser light is coupled into an optical waveguide that has a helical spiraling shape (1.4), which ends aligned exactly at the rotation axis (2.2), see Fig. 2. The exact shape of the spiraling optical waveguide is not essential for this invention. The optical waveguide can be suspended at waveguide holders (2.3). However, because the functionality of the device does not depend on the exact shape of the waveguide, the exact implementation of waveguide holders is not essential. The frequency of light inside this rotating waveguide is shifted due to the Doppler effect. The frequency shift depends on the rotation frequency a, that is, it is equal to tiRk.

The optical mode in the helical rotating waveguide (1.4) is emitted exactly at the axis of rotation (1.4). By emitting light exactly along the axis of rotation (2.2), one obtains a laser beam that is stationary (not moving) and that can be aligned for further processing.

As an example, the laser beam emitted along the axis of rotation (2.2) from the rotating optical waveguide (1.4) can be coupled at the connection point (3.3) into another optical waveguide (3.1) that is aligned at the axis of rotation (2.2), but that is fixed and non-rotating, as illustrated in Fig. 3. In this setup, the optical mode excited with the laser in the rotating waveguide (1.4) is axially symmetric. The fixed optical waveguide (3.1) can then guide the light of frequency w + flRfc towards an observer (3.2) i.e., towards the place where shifted frequency is needed for further experiments, spectroscopy, sensing, space mission applications, or any other usage (see Fig. 3).

As the next example, the laser beam (4.2) emitted along the axis of rotation (2.2) from the rotating optical waveguide (1.4) can be launched into free space at the point (3.3) towards a mirror (4.1), from which it can be reflected towards an observer (3.2) for further processing (see Fig. 4). In this setup, the optical mode excited with the laser in the rotating waveguide (1.4) does not have to be axially symmetric.

If the optical waveguide would not be aligned exactly at the rotation axis (2.2), the beam would still be laser shifted and useful for applications where alignment of the optical setup is of lesser importance.

This device can be used in conjunction with tunable lasers, in the sense that it can enhance the degree of tunability of an already tunable laser. More specifically, if the laser itself already has an internal mechanism that can tune its frequency in the interval by mounting this laser onto the presented rotating device, its range of tunability can be enhanced to the interval + n _v.ar Rk],

This device operates for any laser source (1.1) that can be mounted on a rotating device (1.2), independently on how the laser functions. For the operation of the device, it is not essential how is the rotation of the device obtained (via electric engine or in any other way), and the exact shape of the spiraling optical waveguide

(1.4) is also not essential. That is to say that the operation of the device is robust, and it is not laser specific. The intensity loss depends only on the coupling losses from the laser into the waveguide, and losses due to propagation in the waveguide

(1.4). Losses due to propagation in the waveguide are negligible, that is, frequency shift obtained via our method experiences low-losses.

Let us further elaborate on the details of the invention. More specifically, the light traveling in our setup is guided by the waveguide geometry to the desired point in space where it can be outcoupled along the axis of rotation for further processing or manipulation. We do not employ any reflecting surfaces/mirrors for the Doppler shift. Reflection of light from surfaces (such as glass) which are either at rest or moving, can be well described by means of geometrical optics. In contrast, propagation of light in a waveguide is fundamentally explained by the wave nature of light (since the size of the waveguide cross section is comparable to the wavelength of light). In our setup, light propagating within the waveguide experiences low losses and does not suffer from beam diffraction, which is a drawback for systems using moving mirrors.

Optical waveguides offer a more robust way of light propagation than a system of mirrors. Geometry of the waveguide can be adjusted to the needs of specific problem, even at smaller length scales. In the system with mirrors, any small deflection of a mirror will result in unwanted change in the beam propagation direction and consequently affect the light shifts. In contrast, in our system the frequency shift is not dependent on the exact shape of a waveguide as long as the initial and final point and direction of the waveguide are fixed (the middle of the waveguide can be moved without affecting the result). In the system with mirrors, they have to be kept clean and protected from external effects (like dust), whereas there is no dust accumulation inside the waveguide with time. One of the key differences is diffraction: Light propagating in the system with mirrors propagates in air and therefore diffracts, light beam broadens, and wave front evolves. In our setup, in the waveguide, there is no diffraction. Power loss of light in the waveguide can be relatively low when compared to the light freely propagating and reflecting from surfaces.

5. Brief description of the figures in the drawings

Figure 1 : The rotating device, TOP VIEW

Position 1.1: Laser;

Position 1.2: Rotating disc;

Position 1.3: Arrow pointing in the counter-clockwise rotation direction;

Position 1.4: Rotating optical waveguide;

Position 1.5: Vector fi x fl (its direction is into the plane in Figs. 2-4);

Position 1.6: Radial vector R;

Position 1.7: Rotation frequency vector n (its direction is out of the plane in

Fig. 1); Figure 2: The rotating device, SIDE VIEW

Position 2.1: R, radius of the rotating disc;

Position 2.2: Axis of rotation;

Position 2.3: Waveguide holders;

Figure 3: Side view of the rotating device and outcoupling of light to a fixed waveguide,

Position 3.1 : Fixed optical waveguide;

Position 3.2: Observer;

Position 3.3: Connection point;

Figure 4: Side view of the rotating device and outcoupling of light to a fixed mirror,

Position 4.1 : Fixed mirror;

Position 4.2: Laser beam emitted from (1.4) and reflected from (4.1).

6. Detailed description of at least one way of carrying out invention, with providing an example where appropriate and with reference to the drawings, if any.

This invention can be carried out as follows. First, we choose a laser source (1.1) whose light frequency will be shifted. Since the mechanism operates for versatile types of lasers (it is not laser specific), the choice of laser sources is broad. For example, one can choose a solid-state laser that has small weight (on the order of 1 kg). Next, this laser is mounted on a disc (1.2) that can be made of metal, plastic, wood, or any material that is strong enough to withhold rotation without inducing strong vibrations. The laser (1.1) is then mounted on this disc (1.2) as illustrated in Fig. 1. The disc can rotate around its axis of rotation, and the rotation is driven by an electric engine. The laser source (1.1) is coupled to an optical waveguide (1.4), which is also mounted on a disc and rotates together with the laser and the disc (see Figs. 1 and 2). The direction of the laser beam at the position where it is coupled into the waveguide is x R (1.5), see Fig. 1. The other end of the rotating optical waveguide (1.4) is aligned with the axis of rotation (2.2), and emits light along the axis of rotation in direction fi. The rotating optical waveguide can be outcoupled to a fixed optical waveguide (3.1) which guides the light towards the detector/observer (3.2) as illustrated in Fig. 3.

7. The way in which the invention is capable of industrial or any other application, where it is not obvious from the description or nature of the invention.

The invention is meant to provide a robust and efficient way of changing the frequency of coherent light by using the Doppler effect. It is not restricted to a specific application. Practically all optical and industrial laboratories around the world use schemes for changing the frequency of the light. Our device can find applications in any of these laboratories where robustness and low-losses are essential for performing the desired tasks, that is where current schemes are inadequate. Moreover, they can utilize our device in conjunction with other tunable laser sources to enhance the window in which frequency can be tuned.

References

[1] F. J. Duarte (Ed.), Tunable Lasers Handbook (Academic, New York, 1995).

[2] L. Brillouin, Diffusion of Light and X-rays by a Transparent Homogeneous Body, Annales de Physique 17, p. 88-122 (1922).

[3] P. Debye and F. W. Sears, On the scattering of light by supersonic waves, Proc. Natl. Acad. Sci. 18, p. 409 (1932).

[4] R. Adler, Interaction between light and sound, IEEE Spectrum 4, p. 42 (1967).

[5] J. Lekavich, Basics of acousto-optic devices, Lasers and Applications, p. 59- 64 (1986).

[6] A. Yariv, Quantum Electronics (Wiley, New York, 1989).

[7] R. Paschotta, article on 'acousto-optic frequency shifters' in the RP Photonics Encyclopedia, <https://www.rp-photonics.com/acousto_optic_frequency_shi fters.html>, accessed on 2022-02-14.

[8] K. Nosu et al., Acoustic Optic Frequency Shifter for Single Mode Fibers, Electronics Letters 19, p. 816 - 818 (1983).

[9] H. J. Shaw et al., Single mode fiber optic single sideband modulator and method of frequency, US Patent Number 4,684,215 (1987).

[10] H. J. Shaw et al., Acousto-optic frequency shifter using optical fiber and method of manufacturing same, US Patent Number 4,735,485 (1988).

[11] Y. Yoshida (Ed.) and G. Langouche (Ed.), Modern Mdssbauer Spectroscopy New Challenges Based on Cutting-Edge Techniques: New Challenges Based on Cutting-Edge Techniques (Springer, Singapore, 2021).