RU2710002C1 - Compact device with lasers with multiple longitudinal modes, stabilized high-quality micro-resonators with generation of optical frequency combs - Google Patents

Abstract

FIELD: laser engineering.

SUBSTANCE: invention relates to laser equipment. Laser device comprises at least one laser (L) with a plurality of longitudinal modes for generating laser radiation having a spectrum of a plurality of longitudinal modes; at least one high-quality micro-resonator (M), connected by optical feedback to said at least one laser (L) with multiple longitudinal modes; tuning unit (TU) for tuning spectrum of multiple longitudinal modes of laser radiation. Tuning unit is designed so that to match each of at least one frequency of separate longitudinal mode of said laser with corresponding resonance frequency of micro resonator, so as to obtain at least one matched frequency. Laser device is configured to output laser radiation having an output spectrum with at least one dominant longitudinal laser mode, each of which is on one of said at least one matched frequency and with reduced width of dominant longitudinal laser mode.

EFFECT: technical result consists in enabling increase in laser generation power with narrow line width without additional amplification, while maintaining compact size of device.

32 cl, 12 dwg

Description

FIELD OF THE INVENTION

The invention relates to methods and compact instruments providing stabilization of the laser frequency and a significant narrowing of the laser spectrum through its energy-efficient conversion from a spectrum containing many relatively wide longitudinal modes to a narrow single-mode spectrum or to a spectrum with several narrow spectral modes.

The described effect is provided by two mechanisms. The first mechanism is resonant optical feedback from a high-Q optical microresonator as a result of injection of laser radiation scattered in a high-Q microresonator back into the laser. The second mechanism is the competition of longitudinal laser modes under optical feedback conditions, which leads to an effective redistribution of laser power in favor of one or several narrow laser lines. In the case of a microcavity made of a material having an intensity-dependent refractive index, the generation of spectrally narrow laser lines at a power level above the pump threshold can provide a parametrically generated one or more optical frequency combs having many narrow spectral modes spaced at fixed frequency values. In addition, the present invention relates to compact sources of highly coherent high-power radiation, sources of optical frequency combs or compact multi-frequency lasers providing simultaneous stable generation at several wavelengths (frequencies), which is very important for many practical applications. The present invention may find application in metrology using the heterodyne effect, compact spectroscopic sensors, including wearable devices, coherent lidars, optical data transmission, etc.

BACKGROUND OF THE INVENTION

There are various approaches to stabilizing the laser frequency and narrowing the laser spectrum, including approaches that use the laser frequency pulling by a high-Q microcavity, which can be considered known analogues of the present invention. In addition, specialists will be able to get an idea of the physical principles and / or specific material and technical means underlying the present invention from a number of information sources presented below.

A general approach to passively stabilizing the laser frequency and narrowing the laser spectrum is to use resonant optical feedback from an external optical element. Various disturbances, such as changes in temperature and vibration, can cause fluctuations in the wavelength, power level, and optical phase of the laser. A more stable external optical element providing spectrally selective optical feedback, such as a diffraction grating [1], a high-quality Fabry-Perot resonator (FP) [2], or a combination thereof, allows for narrowing of the laser spectrum and a stable laser frequency. When using a high-Q cavity, the stabilized laser emits much higher than the generation threshold, and optimal frequency stabilization is achieved with weak optical feedback (<10 ^-4 ). The method of optical feedback from an external high-Q resonator allows to obtain a higher output power and better short-term stability than stabilization using a diffraction grating. However, high-Q FP resonators, successfully used for many laser stabilization applications, are relatively bulky and are used only in laboratories. In addition, high-precision mirror coatings are designed for one narrow wavelength range.

Another approach to stabilizing the laser frequency and narrowing the spectrum is the introduction of compact high-Q crystalline WGM microcavities [3]. Compact ring resonators have a characteristic size of 0.1-10 mm and can have an ultra-high Q factor from the ultraviolet region of the spectrum (UV) to the middle infrared region (SIR). The optical feedback from a high-Q crystalline WGM microcavity is based on Rayleigh resonance scattering as a result of the inhomogeneities of the resonator surface: a certain part of the radiation is reflected back to the laser when the frequency of the laser radiation is consistent with the frequency of the resonator mode. In other words, the spectrally selective part of the laser radiation, self-injected back into the laser, draws in its lasing frequency. This process, called "pulling the laser generation frequency", provides fast optical feedback and leads to a significant narrowing of the laser spectrum. To date, there have been reports of narrowing the line width and stabilizing the frequency of various types of lasers, including quantum cascade lasers [4] and distributed feedback lasers (POS) [5], by delaying the generation frequency of the WGM laser by a microcavity, and a decrease has actually been demonstrated line widths up to ~ 1 Hz.

Tightening the laser generation frequency using WGM microresonators is mainly used to stabilize the frequency and narrow the spectrum of single-mode laser diodes, previously stabilized by a grating placed in the active medium of a laser, for example, POC lasers [6], or using any type of Bragg reflector. The main reason is the best stability of the laser frequency, when there is only one microcavity mode within the laser gain bandwidth, since undesired multimode generation and mode competition can occur when several modes create the same optical feedback. Usually WGM microcavities with ultra-high Q factor have a high spectral density of resonance modes, and the aforementioned lasers with preliminary stabilization have a relatively narrow line width, ~ 1-10 MHz, and a limited radiation power of the order of tens of milliwatts. It is also known that crystalline MgF ₂ optical microcavity pumped by a compact distributed feedback laser (POC), the generation frequency of which is pulled by an optical resonator, provides a compact source of optical frequency combs [7–9]. When the POC laser is connected to the optical WGM microcavity through a prism of total internal reflection, part of the radiation is scattered back to the POC laser as a result of resonant Rayleigh scattering, causing the laser frequency to be pulled at the frequency of the selected mode of the optical microcavity. This effect of delaying the laser frequency leads to a narrowing of the laser line width by 3-4 orders of magnitude compared to the line width of the POC laser in the free-running mode, which is necessary for efficient transmission of radiation from the laser to the optical microcavity and generation of the optical frequency comb. However, more powerful lasers with a narrow line width, which are necessary for many applications, including coherent lidars and spectroscopy, require subsequent amplification, which leads to a loss of compactness. In addition, POC lasers are not available at all required wavelengths corresponding to specific atomic-optical transitions. To solve this problem, semiconductor optical amplifiers (POAs), available in a much wider range of wavelengths, are used for stabilization in microcavities with a preliminary narrowing of the generation spectrum by means of a resonator with a diffraction grating [10]. Dual resonator configurations typically result in complex devices that cannot be compact.

Other important prototypes of the present invention include dual-frequency lasers necessary for use in spectroscopy [11], lidars [12], holographic interferometry [13], terahertz optical sources [14] and other fields. Firstly, a two-frequency device with a wide diversity (hundreds of nm) can be realized using, for example, two-frequency generation at two different ions in one solid-state laser material or simultaneous generation at two wavelengths in one ion. Traditionally, two-frequency generation with a small difference in wavelengths (up to tens of nm) was carried out using electro-optical or acousto-optical modulation of single-frequency laser radiation. However, such devices are not very compact. At present, various methods have been developed to ensure operation of the compact diode laser system at two wavelengths, and they can be divided into two main categories: (1) monolithic two-frequency diode lasers and (2) diode laser systems based on various feedback methods from an external resonator. Monolithic dual-frequency diode lasers exhibit stable operation at two wavelengths, however, they have a limited tuning range for the difference in frequencies between the two wavelengths, and the output power is usually less than 500 mW. In feedback methods from an external resonator, various frequency-selective elements were used, for example, volume diffraction gratings for methods with external resonators with a configuration of the “double Littmann” and “double Litrov” type, a two-fiber Bragg grating, a two-period holographic element, and single-frequency volume Bragg gratings or monolithic multiplex Bragg grating. The amplifying medium in a two-frequency diode laser system with an external cavity is usually a single-mode diode laser with a scallop waveguide, so the output power of these laser systems is usually several hundred milliwatts.

SUMMARY OF THE INVENTION

Given the above technical problems, the invention will be further described by way of example and not limitation, with reference to the description and drawings presented below.

This summary of the invention precedes the detailed description of specific exemplary embodiments to give an overview of aspects of the claimed invention, which will be further explained below, and is in no way intended to define or limit the scope of the present invention in any way.

The present invention is the provision of a compact laser-based device with many longitudinal modes, stabilized by a high-Q microcavity, to provide one or more powerful narrow laser lines, usually having a line width of about 1 kHz or less, and in some cases one or more parametrically generated optical frequency comb.

The technical result of the present invention is to increase the laser radiation power with a narrow line width without additional amplification, while maintaining the compact size of the device with a limited number of optical elements. The increase in radiation power is the result of the use of lasers with many longitudinal modes of a wide spectrum, which are much more powerful than traditionally used ROS lasers stabilized by WGM microcavity for narrowing the spectrum and generating optical frequency combs. In this case, unlike the laser device mentioned in [6], the competition of the longitudinal modes under the condition of resonant optical feedback leads to an effective redistribution of the laser power in favor of one or several narrow laser lines at the resonant frequencies of the high-Q microcavity. In addition, the present invention allows to obtain a more compact and powerful dual-frequency or even multi-frequency laser device with the generation of narrow lines, having at least one external high-quality resonator. Also, the present invention allows to obtain a compact powerful source of one or more parametrically generated optical frequency combs through the use of the aforementioned at least one high-quality microcavity.

In one aspect of the present invention, the laser device comprises at least one laser with a plurality of longitudinal modes for generating laser radiation having a spectrum of a plurality of longitudinal modes; at least one high-Q microcavity coupled by optical feedback to said at least one laser with a plurality of longitudinal modes; a tuning unit for adjusting the spectrum of the plurality of longitudinal modes of the laser radiation so as to match each of at least one frequency of the individual longitudinal modes of the at least one laser with the plurality of longitudinal modes with the corresponding resonant frequency of the at least one high-Q microcavity, so as to obtain at least at least one agreed frequency; moreover, the laser device is configured to output the output laser radiation having an output spectrum with at least one dominant longitudinal laser mode, each of which at one of the at least one matched frequency, and with a reduced width of the dominant longitudinal laser mode.

According to one embodiment, said at least one high-Q microcavity is made of a material having an intensity-dependent refractive index, wherein the laser device is further configured to produce at least one optical frequency comb, and each of said at least one optical frequency comb parametrically generated by one of said at least one dominant longitudinal laser mode, if said at least one a dominant longitudinal laser mode has a higher pumping capacity threshold generating optical frequency comb.

According to one embodiment, the tuning unit is further configured to both vary the diversity of the longitudinal modes in the frequency domain and change the frequency of each of the individual longitudinal modes of the at least one laser with a plurality of longitudinal modes.

According to one embodiment, said at least one high-Q microcavity is configured to provide optical feedback to said at least one laser with a plurality of longitudinal modes by generating at least one counterpropagating mode, each of which is at one of said at least one matched frequency.

According to one embodiment, said at least one multi-longitudinal mode laser is an electrically powered semiconductor laser diode.

According to one embodiment, said at least one multi-longitudinal-mode laser and said at least one high-Q microcavity are made on a single chip using microlithography.

According to another embodiment, the tuning unit is further configured to control the injection current of said at least one laser with a plurality of longitudinal modes and / or the temperature of the active medium of said at least one laser with a plurality of longitudinal modes, preferably each of said at least one laser with many longitudinal modes separately.

According to yet another embodiment, said at least one laser with a plurality of longitudinal modes is coupled by optical feedback to said at least one high-Q microcavity by transmitting radiation scattered in said at least one high-Q microcavity back to said at least one laser with a plurality longitudinal mod.

According to one embodiment, one or more of said at least one laser with a plurality of longitudinal modes is coupled / connected by optical feedback to one or more of said at least one high-Q microcavity through a communication element.

According to one embodiment, the coupling element is a prism of total internal reflection or a conical optical fiber or waveguide.

According to another embodiment, one or more of the at least one high-Q microcavity is provided with an additional coupling element.

According to one embodiment, the additional communication element is configured to output one or more dominant longitudinal laser modes generated in said one or more of said at least one high-Q microcavity, or said one or more dominant longitudinal laser modes and one or more optical frequency combs generated in said one or more of said at least one high-Q microcavity to filter Rowan non-resonant portion of the output spectrum.

According to another embodiment, said at least one optical frequency comb is a dissipative soliton Kerr optical comb.

According to yet another embodiment, the tuning unit is further configured to tune the resonant frequencies of the high-Q modes of said at least one high-Q microcavity by changing the diversity of high-Q modes in the frequency domain and changing the resonant frequency of each of the high-Q modes.

According to another embodiment, the tuning unit is further configured to control the temperature of said at least one high-Q microcavity and / or the external pressure applied to said at least one high-Q microcavity, preferably to each of said at least one high-Q microcavity individually.

According to yet another embodiment, said at least one high-Q microresonator is made of electro-optical material that changes its refractive index in response to an electromagnetic field applied to said at least one high-Q microcavity, and wherein the tuning unit is further configured to control the electromagnetic field, applied to said at least one high-Q microcavity, preferably to each of mentioned at least one high-quality microcavity separately.

According to yet another embodiment, said at least one high-Q microcavity is a WGM microcavity.

In a second aspect of the invention, a method for converting a laser spectrum comprising: generating at least one laser with a plurality of longitudinal modes of laser radiation having a spectrum of a plurality of longitudinal modes; adjusting the spectrum of the plurality of longitudinal modes of laser radiation to match each of at least one frequency of a separate longitudinal mode of said at least one laser with a plurality of longitudinal modes with a corresponding resonant frequency of at least one high-Q microcavity coupled by optical feedback with said at least one laser with many longitudinal modes, so as to obtain at least one matched frequency; the output of the output laser radiation having an output spectrum with at least one dominant longitudinal laser mode, each of which at one of said at least one matched frequency and with a reduced width of the dominant longitudinal laser mode.

According to one embodiment, said at least one high-Q microcavity is made of a material having an intensity-dependent refractive index, and the method further comprises parametric generation of at least one optical frequency comb of each of said at least one dominant longitudinal laser mode, if said at least one dominant longitudinal laser mode has a power above the pump threshold of the generation of an optical frequency comb.

According to one embodiment, the spectrum of the plurality of longitudinal modes is adjusted both by varying the spacing of the longitudinal modes in the frequency domain, and by changing the frequency of each of the individual longitudinal modes of said at least one laser with a plurality of longitudinal modes.

According to another embodiment, said at least one high-Q microcavity is configured to provide optical feedback to said at least one laser with a plurality of longitudinal modes by generating at least one counterpropagating mode, each of which at one of said at least one matched frequency.

According to another embodiment, the spectrum of the plurality of longitudinal modes is adjusted by controlling the injection current of said at least one laser with a plurality of longitudinal modes and / or the temperature of the active medium of said at least one laser with a plurality of longitudinal modes, preferably each of said at least one laser with many longitudinal modes separately.

According to yet another embodiment, said at least one counterpropagating mode transmitted back to said at least one laser with a plurality of longitudinal modes is generated by resonant Rayleigh scattering in said at least one high-Q microcavity.

According to one embodiment, one or more of said at least one laser with a plurality of longitudinal modes is coupled / connected by optical feedback to one or more of said at least one high-Q microcavity through a communication element.

According to one embodiment, the coupling element is a prism of total internal reflection or a conical optical fiber or waveguide.

According to another embodiment, one or more of the at least one high-Q microcavity is provided with an additional coupling element.

According to one embodiment, one or more dominant longitudinal laser modes or said one or more dominant longitudinal laser modes and one or more optical frequency gratings are removed from one or more of the at least one high-Q microcavity through an additional coupling element to filter out the non-resonant part of the output spectrum.

According to another embodiment, the resonant frequencies of the high-Q modes of said at least one high-Q microresonator are adjusted both by changing the diversity of the high Q modes in the frequency domain and by changing the resonant frequency of each of the high Q modes.

According to yet another embodiment, the method further comprises controlling the temperature of said at least one high-Q microcavity and / or the external pressure applied to said at least one high-Q microcavity, preferably each of said at least one high-Q microcavity individually.

According to another embodiment, said at least one high-Q microcavity is made of electro-optical material that changes its refractive index in response to an electromagnetic field applied to said at least one high-Q microcavity, and the method further comprises controlling an electromagnetic field applied to said at least one high-Q microcavity, preferably to each of the at least one high-Q microcavity separately.

According to another embodiment, said at least one high-Q microcavity is an WGM microcavity.

In a third aspect of the invention, a method of operating a laser device according to any one of claims 1 to 17, comprising adjusting a spectrum of a plurality of longitudinal modes of laser radiation to match each of at least one frequency of an individual longitudinal mode of at least one laser with a plurality of longitudinal modes with the corresponding resonant frequency of at least one high-Q microcavity, so as to obtain at least one matched frequency; the output of the output laser radiation having an output spectrum with at least one dominant longitudinal laser mode, each of which at one of said at least one matched frequency and with a reduced width of the dominant longitudinal laser mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Based on the above objectives of the present invention and its aspects, specific exemplary embodiments of the claimed invention will be described in detail below, which should be considered together with the accompanying drawings and which are in no way intended to define or limit the scope of the invention, but only disclose specific examples of it implementation. Other embodiments, modifications or equivalent substitutions based on this description will be apparent to those skilled in the art, and all such embodiments, modifications and equivalent substitutions are considered to be included in the present invention.

The drawings are provided for the purpose of illustration only, as an aid to reading and to understand the description, and should not be construed in any way as defining or limiting the scope of the invention. The drawings show the following:

FIG. 1a shows a single laser L with a plurality of longitudinal modes coupled to one crystalline optical high-Q microcavity M, which propagates high-Q modes using a coupling element such as prism C of total internal reflection.

FIG. 1b shows a single laser L with a plurality of longitudinal modes coupled to one optical high-Q microcavity M using a coupling element C. An additional communication element C2 can be used to filter out the non-resonant part of the generated laser radiation.

FIG. 2 schematically shows an example of a single L ₁ laser with many longitudinal modes stabilized simultaneously by several parallel high-Q microcavities M ₁ ... M _N through several coupling elements C ₁ ... C _N.

FIG. 3 schematically shows another example in which one L ₁ laser with a plurality of longitudinal modes is simultaneously stabilized by several successive high-Q microcavities M ₁ ... M _N through several C ₁ ... C _N coupling elements.

FIG. 4 schematically shows another example in which several lasers L ₁ ... L _m with many longitudinal modes are simultaneously stabilized by one high-Q microcavity M ₁ .

FIG. 5 shows an example of a circuit with several lasers L ₁ ... L _m with many longitudinal modes stabilized simultaneously by several parallel and serial N˟P high-quality microcavities M _PN .

FIG. 6a shows a configuration option of the tuning unit.

FIG. 6b shows an example of a measurement equipment circuit used to control spectrum transforms.

FIG. 7 shows a waveform of the photodetector signal D of FIG. 1 (solid curve), where the frequency of a laser L with many longitudinal modes oscillates along the abscissa axis, and the power of resonant Rayleigh backscattering (dashed curve).

FIG. 8 shows an example of converting a spectrum of a plurality of longitudinal modes of a longitudinally-mode laser into an output spectrum of output laser radiation emitted by a laser device, the spectrum having one dominant longitudinal laser mode when one frequency of a separate longitudinal mode is consistent with one resonant frequency of a high-Q microcavity.

FIG. 9a shows the result of converting a spectrum of at least one laser with a plurality of longitudinal modes stabilized by at least one high-Q microcavity when one frequency of a separate longitudinal mode is consistent with one resonant frequency of at least one high-Q microcavity.

FIG. 9b shows the result of spectrum conversion of said at least one laser with a plurality of longitudinal modes stabilized by at least one high-Q microcavity when two frequencies of individual longitudinal modes are consistent with two resonant frequencies of at least one high-Q microcavity.

FIG. 9c shows the result of spectrum conversion of said at least one laser with a plurality of longitudinal modes stabilized by at least one high-Q microcavity when the four frequencies of the individual longitudinal modes are consistent with the four resonant frequencies of the at least one high-Q microcavity.

FIG. 10a shows the result of converting a spectrum of a laser with many longitudinal modes stabilized by a high-quality microcavity of magnesium fluoride with the generation of one dominant longitudinal laser mode.

FIG. 10b shows an example of an optical spectrum of a parametrically generated optical frequency comb in the case shown in FIG. 9a.

FIG. 11a shows the optical spectrum of two simultaneously generated optical frequency combs (blue and red curves in Fig. 11a) in the case when a laser with many longitudinal modes is stabilized by a high-quality microcavity of magnesium fluoride with the generation of two dominant longitudinal laser modes pumping parametrically generated frequency combs.

FIG. 11b shows a radio frequency beat spectrum of narrow comb modes.

FIG. 12a schematically shows a top view of a laser device with at least one laser with a plurality of longitudinal modes and at least one high-Q microcavity made on a single crystal using microlithography.

FIG. 12b schematically shows a side view of the laser device of FIG. 12a.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention was created taking into account a number of known solutions shown above, and is aimed, in particular, to eliminate and / or mitigate at least some of the disadvantages of these known solutions. In particular, the embodiments of the claimed invention listed above provide a laser device comprising at least one laser with a plurality of longitudinal modes for generating laser radiation having a spectrum of a plurality of longitudinal modes; at least one optical high-Q microcavity that propagates high-Q modes and is coupled by optical feedback to said at least one laser with a plurality of longitudinal modes, and a tuning unit for adjusting the spectrum of the plurality of longitudinal modes of laser radiation to match each of at least one frequency individual modes of said at least one laser with a plurality of longitudinal modes with a corresponding resonant frequency of said at least one is high soundly microcavity, so as to obtain at least one coherent frequency. The laser device is configured to provide output laser radiation having a transformed spectrum with at least one dominant longitudinal laser mode, each of which is at one of said at least one matched frequency and with a reduced width of the dominant longitudinal mode. In addition, embodiments of the claimed invention provide a method for operating a laser device.

Those skilled in the art will understand that the various exemplary embodiments should not be construed in any way as defining or limiting the scope of the claimed invention, and that other material and technical means equivalent or explicitly similar to those listed below may be provided by those skilled in the art to perform various operations, functions, method steps, etc. described below. The present detailed description is not intended to determine or limit the scope of the claimed invention, which should be determined only by reference to the attached claims.

The term "high-Q microcavity" as used here means a microcavity that supports wave propagation, containing radiation and having a characteristic size of 0.01-10 mm, often with about a micrometer size of localization of the light field in one direction. The radiation is reflected internally at the edges of a high-Q microcavity in the form of a loop, which creates a series of optical high-Q modes of a standing wave or high-Q resonances, similar to those that can exist on a vibrating guitar string. In a high-Q microcavity, the Q factor of the high-Q modes exceeds 10 ⁵ and usually is about 10 ⁶ -10 ⁹ , preferably 10 ⁸ -10 ⁹ and higher. In particular, high-Q microresonators can be spherical, toroidal, disk, or microresonators with any other form of high-Q whispering gallery (WGM) modes with a closed radiation path in the cavity of the resonator, which ensure the propagation of whispering gallery modes, which are a type of light waves that can propagate along concave surface. Whispering gallery mods, originally discovered for sound waves in the whispering gallery of St Paul’s Cathedral in London, may exist for light and other waves. In another particular case, a high-Q microcavity can be an integrated resonator of various configurations with a closed light path along the waveguide, which allows the propagation of high-Q specific waveguide modes. Alternatively, high-Q microcavities can be a linear Fabry-Perot resonator with a closed segment of the light path equal to twice the length of the longitudinal resonator times the refractive index of the resonator materials. As used herein, the terms “high-Q microcavity” and “high-Q microcavity” are used in the same sense and can be used interchangeably depending on the context. The terms “high-Q mode”, “high-Q mode” are used in the same meaning for a high-Q mode, which is distributed in a high-Q microcavity, and can also be used interchangeably depending on the context.

In the present invention, the first improvement of the prior art is to increase the power of generating radiation with a narrow line without additional amplification, which allows to obtain a compact device with a limited number of optical elements. The increase in power is the result of the use of semiconductor lasers with a wide spectrum of radiation and with many longitudinal modes, which are much more powerful than the conventionally used semiconductor lasers with distributed feedback (ROS), stabilized by an WGM microcavity. It is known that a single-mode POC laser with a characteristic spectral width of 1-10 MHz coupled to a WGM microcavity made of magnesium fluoride provides laser generation with a narrow (<1 kHz) line width due to resonant optical feedback generated by Rayleigh scattering in the microcavity, output back to the POC laser [5]. A compact optical frequency comb source is also well known, in which crystalline MgF ₂ optical WGM microcavity is pumped by a compact distributed feedback laser (ROS), which is stabilized by an optical resonator [7–9]. When the POC laser is coupled to the optical WGM microcavity through a prism of total internal reflection, part of the light is scattered back to the POC laser as a result of resonant Rayleigh scattering, causing the laser frequency to be pulled at the frequency of the selected optical mode of the WGM microcavity. This effect of delaying the laser generation frequency leads to a narrowing of the laser line width by 3–4 orders of magnitude compared to the line width of the POC laser in the free-running mode (1–10 MHz), which is necessary for efficient light transmission from the laser to the optical WGM microcavity and generating an optical frequency comb.

Existing POC lasers have a radiation power limited to 50 mW and a limited working spectral range. In some embodiments of the present invention, a conventional, more powerful laser with many longitudinal modes, also known as a Fabry-Perot laser coupled to a high-Q microcavity, similarly provides generation of a narrow emission line, but with increased power. Generation of a narrow high power emission line can provide a parametrically generated high power optical frequency comb. A laser with many longitudinal modes contains a Fabry-Perot optical resonator, usually containing two plane-parallel mirrors or two concave mirrors and an active medium enclosed inside the resonator. The output spectrum of the laser has a width determined by the gain band of the active medium of the laser, filled with spectral lines spaced at a fixed frequency, inversely proportional to the length of the Fabry-Perot resonator. The width of the spectral lines depends on the reflectivity of the mirrors of the Fabry-Perot resonator. As used herein, the terms “multiple longitudinal modes” or “laser spectral lines” have the same meaning and can be used interchangeably depending on the context. In the context of this application, the term "laser spectral line" refers to the laser line of a separate longitudinal mode.

The authors found that to obtain stable single-frequency narrow-line laser generation in laser diodes, initial preliminary mode selection and preliminary stabilization are not required, and a high-Q microcavity can also effectively cope with these problems. Therefore, it is possible to use simpler and cheaper Fabry-Perot (FP) laser diodes of higher power. For example, the spectrum of a conventional near-infrared semiconductor laser with edge radiation has spectral lines 20-40 MHz wide spaced 15-40 GHz in the spectral range 10-20 nm wide. In one aspect of the embodiments of the present invention, if such a laser is connected to a high-Q microcavity, and the frequency of one of the individual longitudinal modes of the laser is matched to one of the resonant frequencies of the high-Q microcavity, resonant optical feedback from the high-Q microcavity occurs at this matched frequency. Further, the competition of longitudinal modes under conditions of resonant optical feedback leads to an effective redistribution of laser power in favor of one dominant longitudinal laser mode at a consistent frequency and a decrease in its line width up to the line width of the resonant mode of the microresonator. Used in this context, the terms “dominant longitudinal laser modes” and “spectrally narrow laser lines” have the same meaning and can be used interchangeably depending on the context. In the context of the present application, the term “spectrally narrow laser line” means a generation line of a dominant longitudinal laser mode, the line width of which is reduced to the width of the resonant mode of the microcavity due to interaction with a high-quality microcavity.

In the context of this application, the terms “matching”, “agreed frequency” mean the following. Typically, the width of a single longitudinal mode (20–40 MHz) is much wider than the width of the resonant mode of a microresonator (100 kHz – 1 MHz). Therefore, when tuning the laser spectrum, it is necessary to obtain the overlap region of the spectral curve of a separate longitudinal mode with the spectral curve of the resonance mode of the microresonator in the frequency domain. In this case, to provide optical feedback from the microcavity, a part of the laser power of a separate longitudinal mode corresponding to the overlapping region will be sufficient. In this case, the agreed-upon frequency corresponds to the resonant frequency of the microresonator, and the center of the spectral curve of a separate longitudinal mode and the center of the spectral curve of the resonant mode of the microresonator may not coincide in the frequency domain and may simply be close to each other. Usually, “matching” can be achieved when the resonant frequency of the microresonator is on the slope of the spectral curve of a separate longitudinal mode. Obviously, other relative positions of the spectral curves of individual longitudinal modes and the spectral curves of the resonance modes of the microresonator are possible, corresponding to their matching and providing various degrees of optical feedback from the microresonator. The levels of feedback degrees of several simultaneously stabilized individual longitudinal modes significantly affect their competition and the redistribution of laser power in favor of one or more dominant longitudinal laser modes. Dominant longitudinal laser modes correspond to longitudinal laser modes at matched frequencies, where their power amplitudes are several times higher than the amplitudes of the nearest neighboring modes or side modes as a result of mode competition. In addition, side modes are suppressed as a result of redistribution of laser power in favor of dominant longitudinal modes. The degree of mode dominance usually characterizes the side mode suppression coefficient (CBPM), which is the ratio of the amplitude of the dominant mode to the amplitudes of the nearest side modes. In accordance with the present invention, the CBMP for stabilized modes is typically in the range of 30-60 dB, preferably in the range of 40-60 dB. For example, 40 dB often provides a sufficient degree of mode dominance for the purposes of this invention.

In the present invention, since conventional lasers with many longitudinal modes are much more powerful (up to 500 mW in continuous mode) than POC lasers, the resulting narrow-line generation power is increased, and the power of a potentially generated optical frequency comb is also increased. Since Fabry-Perot semiconductor lasers cover a wider spectral range than POC lasers, narrow line generation can be obtained by using a coupling with a high-Q microcavity at any optical frequency necessary for a particular application.

A second improvement of the prior art with a laser with a plurality of longitudinal modes stabilized by a high-Q microcavity is the possibility of simultaneously producing several dominant longitudinal laser modes when several frequencies of several longitudinal laser modes with many longitudinal modes are simultaneously matched with several resonant frequencies of a high-Q microcavity. This makes it possible to obtain a compact-sized device containing only one laser with many longitudinal modes and one high-Q microcavity acting as an external resonator, in particular, it allows to create the most compact and powerful two-frequency or even multi-frequency laser device with stabilized frequencies and reduced line width.

In one embodiment of the present invention shown in FIG. 1a, a laser L with a plurality of longitudinal modes is coupled optical feedback with a crystalline optical high-Q microcavity M through a communication element C, such as a prism of communication with total internal reflection. High-Q microcavity M provides the distribution of circulating high-Q modes. In a particular case, a high-Q microcavity M can be a high-Q WGM microcavity, which ensures the propagation of circulating whispering gallery modes (WGM). The laser generates laser radiation 1 having a spectrum of a plurality of longitudinal modes. The laser device has a tuning unit TU for tuning the spectrum of a plurality of longitudinal modes of laser radiation 1 generated by a laser L with a plurality of longitudinal modes. The tuning unit TU is configured to both vary the longitudinal mode spacing in the frequency domain and change the frequency of each of the individual longitudinal laser modes L to match each of at least one frequency of a separate longitudinal laser mode L with the corresponding (in the frequency domain) high-Q resonant frequency microcavity M, so as to obtain at least one matched frequency. For example, if “at least one” used in the phrase “each of at least one frequency of a separate longitudinal mode” is equal to unity, this means that each of one frequency of one separate longitudinal mode of a laser L with many longitudinal modes is matched with one the corresponding resonant frequency of a high-Q microcavity. In this case, one matched frequency is obtained for one pair of frequencies. If “at least one” used in the phrase “each of at least one frequency of a separate longitudinal mode” is equal to three, this means that each of the three frequencies of three separate longitudinal modes of the laser L with many longitudinal modes is consistent with the corresponding resonant frequency high-quality microcavity. In this case, three matched frequencies are obtained for three pairs of frequencies, where the first frequency of the first individual longitudinal mode is matched with the first resonant frequency, the second frequency of the second individual longitudinal mode is matched with the second resonant frequency, and the third frequency of the third individual longitudinal mode is matched with the third resonant frequency. If “at least one” used in the phrase “each of at least one frequency of a separate longitudinal mode” is N, this means that N frequencies N of individual longitudinal laser modes L with many longitudinal modes are consistent with N corresponding high-quality resonant frequencies microcavity, where N is a number equal to or greater than 1. In this case, N matched frequencies are obtained for N frequency pairs, where the 1st frequency of the 1st separate longitudinal mode is matched with the 1st resonant frequency, the 2nd frequency is the 2nd separate longitudinal mode matched with the 2nd resonance frequency, ..., Nth frequency of the Nth separate longitudinal mode is consistent with the Nth resonant frequency.

A high-Q microcavity M provides optical feedback to said at least one laser with a plurality of longitudinal modes by generating at least one counterpropagating mode (see "mode-s" in Fig. 1a), each of which is at least one at least one agreed frequency. In some embodiments, when the high-quality microcavity M is a high-quality WGM microcavity, the high-quality microcavity M provides optical feedback to said at least one laser L with a plurality of longitudinal modes by generating at least one counterpropagating whispering gallery mode ( WGM), each on one of at least one matched frequency. Said at least one counterpropagating mode (mode s) is transmitted back to the laser L with a plurality of longitudinal modes. Laser radiation 2 supplied to photodetector D combines the radiation received from the high-Q microcavity M through the prism C of coupling with total internal reflection and laser radiation 1 not transmitted to the high-Q microcavity M. Mentioned at least one counterpropagating mode (mode-s ), transmitted back to the laser L with many longitudinal modes in the form of a light wave (s) S, is provided due to resonant Rayleigh scattering in a high-Q microcavity M. Under the condition of resonant optical second feedback laser power L with a plurality of longitudinal modes effectively redistributed by competing longitudinal modes in favor of at least one dominant longitudinal laser modes, each one of the at least one coherent frequency. In this case, the laser device is configured to emit output laser radiation having an output spectrum with said at least one dominant longitudinal laser mode with a reduced width of this at least one dominant longitudinal mode (as compared to the corresponding individual longitudinal laser mode). For example, if five matched frequencies were obtained, five dominant longitudinal laser modes will be present in the output spectrum. In general, instead of a prism of total internal reflection, another type of coupling element C, such as a conical optical fiber or waveguide, as schematically shown in FIG. 1a. A variant of a conical optical fiber or waveguide connected on one side to one or more lasers with many longitudinal modes also allows communication with one or more microresonators at the same time. For example, a conical optical fiber can be connected by its narrow part simultaneously with two toroidal microresonators from opposite sides. A waveguide mounted on the same semiconductor chip with microresonators can be connected simultaneously with three or more microresonators located at a distance less than the wavelength from it.

In FIG. 2 schematically shows an example of a single L ₁ laser with many longitudinal modes, simultaneously stabilized by several parallel high-Q microcavities M ₁ ... M _N , preferably WGM microcavities, through several C ₁ ... C _N coupling elements. Laser radiation 1 having a spectrum of a plurality of longitudinal modes is divided such that each high-Q microcavity receives laser radiation 1. A tuning unit TU (not shown) adjusts the spectrum of a plurality of longitudinal modes of laser radiation 1 to match each of at least one frequency of a separate longitudinal laser mode L ₁ with many longitudinal modes with resonant frequencies of several high-Q microcavities M ₁ ... M _N to obtain at least one matched frequency. For example, you can get four matched frequencies for one laser L ₁ and three high-Q microcavities M ₁ , M ₂ , M ₃ , while the first matched frequency can be obtained for a laser L ₁ and high Q microcavities M ₁ , the second and third matched frequencies for a laser L ₁ and high-Q microcavity M ₂ , the fourth matched frequency for the laser L ₁ and high-Q microcavity M ₃ . Other combinations are also possible. Each high-Q microcavity M ₁ ... M _N provides the generation of one or more counterpropagating modes, each at one of at least one matched frequency. In particular, as noted above for the example with one laser L ₁ and three high-Q microcavities M ₁ , M ₂ , M ₃ , each of the high-Q microcavities M ₁ and M ₃ generates one counterpropagating mode, and the high-Q microcavity M ₂ generates two counterpropagating modes at corresponding matched frequencies. The aforementioned one or more counterpropagating modes (mode s) are transmitted back to the L ₁ laser with many longitudinal modes in the form of light wave (s) S ₁ ... S _N , which provides optical feedback and redistribution of the power of the L ₁ laser with many longitudinal modes in favor of at least one dominant longitudinal laser mode, each at a consistent frequency and with a reduced width of the dominant longitudinal laser mode. In particular, as noted above for the example with one L ₁ laser and three high-Q microcavities M ₁ , M ₂ , M ₃ , the power of a L ₁ laser with many longitudinal modes can be redistributed in favor of four dominant longitudinal laser modes with reduced widths.

In FIG. Figure 3 shows a schematic diagram of another example in which a L ₁ laser with multiple longitudinal modes is simultaneously stabilized by several successive high-Q microcavities M ₁ ... M _N , preferably WGM microcavities, through several C ₁ ... C _N coupling elements. Each high-Q microcavity M ₁ ... M _N is connected to a bus waveguide and receives laser radiation 1 having a spectrum of many longitudinal modes. The tuning unit TU (not shown) adjusts the spectrum of a plurality of longitudinal modes of laser radiation 1 to match each of at least one frequency of a separate longitudinal mode with the corresponding resonant frequency of a high-Q mode of several high-Q microcavities M ₁ ... M _N to obtain at least one matched frequency. Each high-Q microcavity M ₁ ... M _N provides the generation of one or more counterpropagating modes, each of which at one of the aforementioned at least one matched frequency. The light wave (s) S ₁ ... S _N , resonantly scattered back from the high-Q microcavity M ₁ ... M _N , provides optical feedback and redistribution of the laser power L ₁ with many longitudinal modes in favor of one or more dominant longitudinal laser modes, each of which at one of said at least one matched frequency and with a reduced width of the dominant longitudinal laser mode. Each light wave (s) S ₁ ... S _N can be in resonance with only one high-Q microcavity to exclude their interaction.

In FIG. 4 schematically shows another embodiment in which several lasers L₁... L_m with many longitudinal modes simultaneously stabilized by one high-Q microcavity M₁preferably one WGM microresonator. Laser radiation 1 is combined and transmitted to a high-Q microcavity M₁ through element C₁ communication. High Q microcavity M₁ receives laser radiation 1 having a spectrum of a plurality of longitudinal modes. The tuning unit TU (not shown) adjusts the spectrum of the plurality of longitudinal modes of laser radiation 1 to match each of at least one frequency of a separate longitudinal mode with the corresponding resonant frequency of a high-Q microcavity M₁so as to obtain at least one matched frequency. For example, you can get three matched frequencies for three lasers L₁, L₂, L₃ and one high-quality microcavity M₁while the first matched frequency can be obtained for the laser L₁ and high-Q microcavity M₁, the second matched frequency can be obtained for laser L₂ and high-Q microcavity M₁ and the third matched frequency can be obtained for laser L₃ and high-Q microcavity M₁. Light wave (s) S1 resonantly scattered back from a high-Q microcavity M₁ at matched frequencies, provides optical feedback with each of the lasers and the redistribution of power of each of the lasers L₁, L₂, L₃ with many longitudinal modes in favor of the corresponding dominant longitudinal laser modes at matched frequencies and with a reduced width.

In FIG. 5 shows a diagram of a more general case of another embodiment of the invention in which lasers L ₁ ... L _m with many longitudinal modes are simultaneously stabilized by several parallel and sequential high-Q microcavities M _PN , preferably WGM microcavities, via C _PN coupling elements. As in the previous embodiments, each of the high-Q microcavities M _PN receives laser radiation 1 from lasers L ₁ ... L _m with many longitudinal modes having a spectrum of many longitudinal modes. The tuning unit TU (not shown) adjusts the spectrum of the plurality of longitudinal modes of laser radiation 1 to match each of at least one frequency of a separate longitudinal mode with the corresponding resonant frequency of the high-Q microcavity M _PN , so as to obtain at least one matched frequency. Resonant backscattering S _PN from a high-quality microcavity M _PN at the aforementioned at least one matched frequency provides optical feedback and power redistribution of each of the lasers L ₁ ... L _m with many longitudinal modes in favor of the corresponding dominant longitudinal laser modes, each of which is on one of said at least one matched frequency and with a reduced width of the dominant longitudinal laser mode.

The present invention is not limited to the embodiments shown in FIG. 1-5. Other configurations are also possible based on combinations of one or more of the embodiments of FIG. 2-4. For example, a laser device may include several parallel components, as shown in FIG. 2, each of which contains a laser L ₁ with many longitudinal modes, simultaneously stabilized by several consecutive high-Q microcavities M ₁ ... M _N through several communication elements C ₁ ... C _N , as shown in FIG. 3. In addition, the coupling element C may be made in the form of a conical optical fiber or waveguide. In this case, as explained above, one or more of the aforementioned at least one laser with a plurality of longitudinal modes can be connected by optical feedback to one or more of the aforementioned at least one high-Q microcavity through one of the aforementioned communication elements.

In one embodiment, the tuning unit TU (see FIG. 1a) may include several TL and TM subnodes, each of which may be associated with each of said at least one laser (L) and said at least one high-Q microcavity (M) or can be made as a single unit associated with each of said at least one laser and said at least one high-Q microcavity. Other configurations of the TU tuning node are also possible. The adjustment unit may be configured to control the parameters of the at least one laser, in particular, the injection current and / or the temperature of the active medium of the said at least one laser. Preferably, the tuning unit may be configured to control these parameters of each of said at least one laser with a plurality of longitudinal modes individually. In addition, the tuning unit can be configured to control the parameters of the at least one high-Q microcavity, in particular its temperature, the external pressure applied to the at least one high-Q microcavity, an electromagnetic field applied to the at least one high-Q microcavity in case the latter is made of electro-optical material. Preferably, the tuning unit may be configured to control these parameters of each of the at least one high-Q microcavity separately. The tuning unit can be equipped with or connected to such measuring equipment as temperature sensors, pressure sensors, electromagnetic field sensors, current regulators, which are used for spectrum adjustment. The tuning unit can additionally communicate with measuring equipment for controlling spectrum transformations, such as photodetectors, optical spectrum analyzers, electric spectrum analyzers, oscilloscopes and their analogs, lasers with a reference narrow line width for measuring the width of dominant longitudinal modes, and so on. The tuning unit TU can also be connected or contain control equipment, such as heating elements, current regulators, clamp piezoceramic elements, radio frequency antennas or capacitor plates and waveguides connected to said at least one high-Q microresonator and supplying an external electromagnetic field to it to change its resonant frequencies. This measuring and control equipment can be configured to measure and change the aforementioned parameters of said at least one laser and / or said at least one high-Q microcavity. In one embodiment, the tuning unit may further comprise a processor configured to provide analytical feedback between the measurement and control equipment for adjusting the spectrum of the plurality of longitudinal modes of said at least one laser and / or for adjusting the resonance frequency spectrum of high-quality modes of said at least one high-quality microcavity to obtain consistent frequencies. The processor can be functionally connected to a computer control system, which can be programmed to control the tuning unit in a predetermined manner in order to optimize its operation.

In one embodiment, shown in FIG. 1a, in which the laser device comprises one laser L and one high-Q microcavity M, the tuning unit TU can be made as shown in FIG. 6a. This tuning unit TU may contain two sub-nodes TL, TM connected to the laser L and high-Q microcavity M, respectively, for controlling their parameters, as described above.

In FIG. 6b shows a specific example of a portion of a measuring equipment circuit used to control spectrum transforms. The laser radiation from an indium phosphide (InP) diode laser L with many longitudinal modes is collimated using a “Lens” lens (the first along the laser beam in Fig. 6b) and transmitted to a high-quality microcavity M from magnesium fluoride (MgF ₂ ) through an element C communications, such as a glass prism of communication with total internal reflection. The radiation subjected to resonant Rayleigh backscattering returns to the diode laser L with many longitudinal modes and causes the laser frequency to be pulled by the resonant frequency of the high-Q microcavity M. The output laser radiation having the output spectrum is collimated in a single-mode fiber using two lenses (the second Lens and the third The "lens" along the laser beam in Fig. 6b) and is analyzed by the optical spectrum OSA analyzer on a PD photodiode with an OSC oscilloscope and a fast FPD photodiode with an ESA analyzer ctric characteristics. A 50/50 beam splitter is used to evenly split the laser output between the OSA and ESA. Sensitive Oscilloscope OSC accepts 1% of the laser energy for OSA using a 99/1 beam splitter. The repetition rate of soliton pulses is controlled by a fast photodiode FPD and an ESA analyzer of the electrical spectrum. The detuning of the laser frequency from the optical resonance is monitored on a PD photodiode using an OSC oscilloscope. To measure the width of the heterodyne line, a tunable narrow-wavelength FL laser is used.

In one specific embodiment of the present invention, the sub-node TL of the tuning unit TU (shown in FIGS. 1a, 6a, b) receives the data measured by the current sensor 1 and / or the temperature sensor 2 and sends them to the processor P. The TM sub-node of the tuning unit TU receives the data measured by the pressure sensor 8, the temperature sensor 9 and / or the electromagnetic field sensor 10, and sends them to the processor P. The processor P can also be configured to communicate with the optical spectrum OSA analyzer, PD photodiode with OSC oscilloscope, FPD fast photodiodeESA electric spectrum analyzer and FL reference laser line with a narrow generation spectrum management transformations, for example, to measure the width of the dominant mode in the longitudinal mode lasing frequency pulling. Processor P provides analytical feedback between the measurement and control equipment. For example, the processor P sends a control signal to the current controller 3 to change the injection current of the associated laser L with a plurality of longitudinal modes, and / or the processor P sends a control signal to the heating element 4 to change the temperature of the laser active medium of the associated laser L with many longitudinal modes, in order to adjust the spectrum of the L laser with many longitudinal modes to obtain one or more consistent frequencies. The processor P sends a control signal to the pressure-sensitive piezoceramic element 5 to change the pressure applied to the associated high-Q microcavity M, and / or the processor P sends a control signal to the heating element 6 to change the temperature of the associated high-Q microcavity M in order to adjust the spectrum of high-Q modes at least one high-quality microcavity M to obtain consistent frequencies. In addition, if the high-Q microcavity M is made of electro-optical material, the processor P sends a control signal to the electromagnetic field regulator 7 to change the electromagnetic field applied to the associated high-Q microcavity M in order to adjust the spectrum of high-Q modes of at least one high-Q microcavity M to obtain one or more matched frequencies. The CCS computer control system can be programmed to control the tuning unit TU in a predetermined manner in order to optimize its operation.

In the embodiment shown in FIG. 2, the tuning unit can be made as comprising a single TL subnode coupled to a L ₁ laser with a plurality of longitudinal modes, and N TM subnodes associated with each of the high-Q microcavities M ₁ ... M _N. In the embodiment of FIG. 5, the tuning unit can be made as containing M subnodes TL associated with each of the lasers L ₁ ... L _m with many longitudinal modes, and P * N subnodes TM associated with each of the high-quality microcavities M _PN . However, other configurations are possible. For example, in the embodiment of FIG. 2, the tuning unit can contain two TL and TM subnodes, while the TM subnode is connected to all high-Q microcavities M ₁ ... M _N.

In one particular embodiment, the spectrum of the multiple longitudinal modes of laser radiation can be slightly tuned TL in the frequency domain by adjusting the injection current of the laser to achieve matching each of one or more frequencies of the individual longitudinal modes of at least one laser L with many longitudinal modes, preferably each from said at least one laser with a plurality of longitudinal modes individually, with a corresponding resonant frequency of a high-Q microcavity M through a node TU tuning (see Fig. 1). In FIG. 7, the spectrum tuning of a plurality of longitudinal modes of laser radiation is shown as a signal (solid curve) from the photodetector D (see Fig. 1a). The transmittance drops shown on the signal are resonances of a high-Q microcavity. The sharp edges of the resonances are due to the fact that the laser frequency is pulled out of the tuning frequency when the laser is stabilized by the resonance frequency modes of the microresonator. The power of resonant Rayleigh backscattering propagating in the form of counterpropagating modes (mode s) (as shown in Fig. 1a), which is responsible for optical feedback and the narrowing of the laser spectrum when it is transmitted from a high-Q microcavity, is shown in the same Fig. 7 (dashed line), where the power of Rayleigh backscattering increases 100 times compared to the power LD on the same scale. The power peak of the resonant Rayleigh backscattering corresponds to the resonant frequency of a high-Q microcavity when the laser frequency is controlled by the laser injection current. When the laser injection current does not tune and the frequency of one of the longitudinal modes of the laser is adjusted to match the resonant frequency of the high-Q microcavity, the process of competition of the longitudinal modes of the laser is started under conditions of resonant optical feedback.

The result of mode competition under optical feedback conditions is illustrated in FIG. 8, which shows an example of the conversion of a near-infrared laser spectrum with many longitudinal modes in the free-running mode (shown on the left) containing several longitudinal modes in the range from 1535 nm to 1550 nm with a spacing interval of ~ 0.14 nm (which corresponds to ~ 17 GHz in the frequency domain) to the output spectrum having one dominant longitudinal mode (shown on the right). When one frequency of one separate longitudinal mode of the spectrum of many longitudinal modes is consistent with one resonant frequency of a high-Q microcavity, the power of many longitudinal modes is redistributed in favor of one dominant longitudinal laser mode, further narrowed by a high-Q microcavity several thousand times, the power of which increases significantly (Fig. 8) .

In one embodiment of the invention, when the laser injection current does not fluctuate, the spectrum of the plurality of longitudinal modes of laser radiation can be tuned by the tuning unit TU by changing the temperature of the active medium of at least one laser with a plurality of longitudinal modes, preferably each of said at least one laser with a plurality of longitudinal modes individually, which leads to a change in its refractive index and, consequently, to a change in the interval of separation of longitudinal modes in the frequency domain and changing the frequency of each of the individual longitudinal modes.

In yet another embodiment, when the laser injection current does not fluctuate, and the temperature of the active medium of the laser is stable, the tuning unit TU can be configured to adjust the resonant frequencies of high-quality modes of at least one high-quality microcavity M, which allows you to further fine-tune at least one matched frequency under optical feedback. The tuning unit is capable of controlling both the spacing interval of high-Q modes in the frequency domain and the frequency of each of the individual high-Q modes of the at least one high-Q microcavity by controlling the temperature and / or external pressure applied to the high-Q microcavity. Both options lead to a change in the refractive index and / or dimensions of the at least one high-Q microcavity, preferably each of the at least one high-Q microcavity individually, and therefore to a change in its resonant frequencies and / or diversity of the high Q modes.

In another embodiment, at least one high-Q microcavity may be made of electro-optical material that changes its refractive index in response to an applied electromagnetic field, and the tuning unit TU is further configured to control an electromagnetic field applied to said at least one high-Q microcavity, preferably each of said at least one high-Q microcavity individually. A change in the refractive index of said at least one high-Q microcavity leads to a change in its resonant frequencies, wherein the tuning unit is capable of adjusting the resonance frequencies of said at least one high-Q microcavity, which makes it possible to further fine-tune said at least one matched frequency under optical feedback conditions. As specific examples of electro-optical materials for the manufacture of a high-Q microcavity, lithium niobate and indium phosphide can be mentioned.

In FIG. 9a-9c show the results of spectrum conversion of at least one laser with a plurality of longitudinal modes stabilized by at least one high-Q microcavity when each of one or several frequencies of separate longitudinal modes of said at least one laser is matched with a corresponding resonant frequency of said at least at least one high-quality microcavity. In the cases shown in FIG. 9b and 9c, the competition of longitudinal modes under optical feedback provides the simultaneous generation of several narrow laser lines.

In FIG. 9a shows the generation of one frequency when one frequency of a separate longitudinal mode of said at least one laser L with a plurality of longitudinal modes is matched with one resonant frequency of said at least one high-Q microcavity M. In this case, the laser device generates output laser radiation having an output spectrum with one dominant longitudinal laser mode. In FIG. 9b shows two-frequency generation when the two frequencies of the individual longitudinal modes of the at least one laser with a plurality of longitudinal modes are consistent with the two resonant frequencies of the at least one high-Q microresonator M. In this case, the laser device generates output laser radiation having an output spectrum with two dominant longitudinal laser modes. FIG. 9c shows an example of four-frequency laser generation in which the four frequencies of the individual longitudinal modes of the at least one laser L with the plurality of longitudinal modes are consistent with the four resonant frequencies of the at least one high-Q microresonator M. In this case, the laser device emits laser output having output spectrum with four dominant longitudinal laser modes. As explained above, the power of the at least one laser with many longitudinal modes is redistributed in favor of one or more narrow laser lines as a result of mode competition in resonant optical feedback.

In the embodiments of the present invention illustrated in FIG. 9b-9c, several individual longitudinal modes of the spectrum of a plurality of longitudinal modes are converted to several dominant longitudinal modes with a reduced width of dominant longitudinal laser modes as a result of optical feedback of said at least one laser with a plurality of longitudinal modes with said at least one high-Q microcavity and tuning spectrum of a set of longitudinal modes for matching each of several frequencies of individual longitudinal modes with the corresponding resonant frequency of the aforementioned at least one high-quality microcavity, so as to obtain several consistent frequencies. Such embodiments of the present invention provide greater flexibility for multiple narrow-line oscillations at different frequencies required for specific applications, since the generation of several narrow laser lines can relate to different lasers with many longitudinal modes and different high-Q microcavities, as shown above in FIG. 2-5. This allows you to design the output spectrum, consisting of narrow laser lines at arbitrary frequencies required for specific applications, by appropriate selection of lasers with many longitudinal modes and high-quality microresonators with specific frequencies for tuning.

In one embodiment, one or more dominant longitudinal laser modes can be derived from a high-Q microcavity M via an additional coupling element C2 (see Fig. 1b), which is a prism of total internal reflection. In other embodiments, one or more of the at least one high-Q microcavity M may be provided with one additional coupling element C2 in the form of a conical optical fiber or waveguide used in place of the total internal reflection prism shown schematically in FIG. 1b. The use of an additional coupling element can significantly improve the spectral purity of the output spectrum of laser radiation generated by a laser device and containing dominant longitudinal laser modes by filtering out the non-resonant part of the output spectrum. Spectral purity of the output spectrum is often required for many microphotonic applications. In FIG. 1b, laser radiation 1 and laser radiation 2 having an output spectrum are output from the high-Q microcavity M through the coupling element C (prism of total internal reflection), as shown on the right; these laser radiation 1 and 2 contain weak nonresonant laser longitudinal modes. An additional prism C2 coupling provides the output from the high-Q microcavity M of laser radiation 3 having a clean spectrum consisting only of dominant longitudinal laser modes without the non-resonant part of the output laser radiation (shown on the left). In other words, laser radiation 3 has dominant longitudinal laser modes with a higher lateral mode suppression coefficient (CPBM) parameter than was explained above compared to laser radiation 2. As a rule, in this case, the CPBM can be in the range of 40-70 dB, preferably in the range of 50-70 dB. For example, 60 dB often provides a sufficient lateral mode suppression factor for this application.

Another group of embodiments of the present invention includes variants in which said at least one high-Q microcavity is made of a material having an intensity-dependent refractive index, and in which the laser device is further configured to generate at least one optical frequency comb, each of which of said at least one optical frequency comb is parametrically generated by one of said at least one dominant product laser mode if said at least one dominant longitudinal laser mode has a power above the pump threshold for generating an optical frequency comb. There are many materials with an intensity-dependent refractive index such as fluorides that are transparent over a wide spectral range: calcium fluoride, barium fluoride and magnesium fluoride. Also worth mentioning are quartz, silicon, silicon nitride, diamond and others.

In FIG. 10a shows an example of a spectrum conversion of a laser with many longitudinal modes stabilized by a high-quality WGM microresonator from magnesium fluoride with the generation of one dominant longitudinal laser mode having a reduced width, as explained above, since only one frequency of a separate longitudinal mode is consistent with one resonant frequency of a high-quality WGM Magnesium fluoride microcavity for one consistent frequency. Mode competition under optical feedback conditions, as in the optical circuit of FIG. 1a, led to a redistribution of laser power in favor of a single dominant longitudinal laser mode at a consistent frequency. The side mode suppression ratio is about 30 dB. Further, in FIG. 10b, output laser radiation with an output spectrum having one dominant longitudinal laser mode, efficiently transmitted to a high-Q microcavity with a power level above the pump threshold, parametrically generates a dissipative soliton Kerr comb (DKG). A Kerr soliton comb is an optical frequency comb having a plurality of spectral modes spaced by a fixed frequency value other than spacing of the longitudinal laser modes. In FIG. 10b shows the signal immediately after the coupling prism C shown in the optical circuit of FIG. 1a. This signal combines laser radiation 2 emanating from a high-Q microcavity M and non-resonant laser radiation 2 transmitted from a laser with many longitudinal modes L through a prism C of coupling. In a specific embodiment, when a cleaned optical frequency comb is required, an additional coupling element C2 (FIG. 1b) can be used to filter out the non-resonant part of the output spectrum.

The specific measured data shown in FIG. 10a, b were obtained using a high-quality WGM microcavity made of magnesium fluoride (MgF ₂ ) with a diameter of 5.5 mm and a radius of curvature of the edge of 500 μm, which corresponds to a free spectral range (SDS) of about 12.5 GHz (inversely proportional to the pulse propagation time in the microcavity in the forward and reverse direction). The refractive index of magnesium fluoride depends on the laser intensity, which allows you to generate DKG. A high-Q microcavity was made by precision turning of a massive crystal with a single-chip tool. An ultra-high characteristic quality factor exceeding 10 ⁹ was achieved by polishing with diamond suspensions. The laser diode used to obtain the specific measurement data of FIG. 10a, b, has an optical spectrum consisting of dozens of longitudinal modes spanning ~ 10 nm, with a longitudinal mode spacing f = 17.68 GHz around a central wavelength of 1535 nm and with a maximum output power of 200 mW. The intensity in the laser gain region is approximately evenly distributed between the lines. By gradually changing the injection current, one of the longitudinal laser modes was matched with the resonant mode of the microresonator to obtain optical feedback and one dominant longitudinal mode with a reduced width.

Then, gradually adjusting the frequency of the diode laser on the low-frequency side using the injection current, but remaining in the tightening mode, the authors were able to smoothly switch the system to the soliton comb mode (mainly single-soliton comb) with a very characteristic envelope sech2 (x) (Fig. 10b). The soliton comb has a span of about 30 nm with a line spacing of 12.5 GHz corresponding to the free spectral range of the microresonator. In the optical spectrum, additional residual laser lines with a spacing of 17.68 GHz are visible, although they are weak and can be filtered using the drop-port configuration with an additional communication element (see Fig. 1b).

Other embodiments of the present invention include those in which several optical frequency combs can be generated by several dominant longitudinal laser modes. In FIG. 11a shows the optical spectrum of two simultaneously generated optical frequency combs (blue and red curves in FIG. 11a) in the case of a diode laser with many longitudinal modes stabilized a high-quality WGM microcavity made of magnesium fluoride with the generation of dominant longitudinal laser modes pumping parametrically generated frequency combs. As before, laser radiation having a spectrum of a plurality of longitudinal modes is first tuned in order to match two frequencies of the longitudinal modes with two resonant frequencies of a high-Q microcavity so as to obtain two consistent frequencies. A high-Q microcavity accepts individual longitudinal modes of laser radiation at matched frequencies corresponding to its resonant frequencies to provide optical feedback due to Rayleigh backscattering. Competition of modes under optical feedback leads to a redistribution of laser power in favor of two dominant longitudinal modes at two matched frequencies. When the power of the dominant modes exceeds the pump threshold, an optical frequency comb is generated. Each of the combs represents a plurality of narrow spectral modes spaced by a fixed frequency value, different from the spacing between longitudinal laser modes. In FIG. 11b shows the radio frequency beat spectrum of narrow comb modes measured using a fast photodiode and an electric spectrum analyzer. There are two beats corresponding to the combs. The width of the beat lines is less than 1 kHz, which demonstrates the narrowness of the modes of these frequency combs. The narrow spectral modes of the two combs are spaced apart at different frequencies with a difference of about 10 MHz.

In one embodiment, the at least one high-Q microcavity can be fabricated on a semiconductor chip (silicon nitride or silicon-on-insulator) together with at least one laser with many longitudinal modes. An example is known in which such a hybrid integration of materials of group III-V for the near infrared range with silicon photonic resonators was successful. Thus, in one embodiment, at least one laser with many longitudinal modes and at least one high-Q microcavity can be performed on a single chip using microlithography. In FIG. 12 a, b schematically shows a crystal containing one laser L with a plurality of longitudinal modes having mirrors M1, M2, and one high-Q microcavity M, preferably one WGM microcavity. In the top view shown in FIG. 12a, a laser L with a plurality of longitudinal modes integrated in a material of group III-V 11, for example indium phosphide (InP), is connected to a silicon / silicon nitride waveguide 13 (Silicon / Si ₃ N ₄ ) through a special connector 12. Connector 12 connects a layer of material 11 III-V with a bonded layer 13 of silicon / silicon nitride (see side view in Fig. 12b), which contains a bus waveguide and a high-quality microcavity M. Connector 12 can be either in the form of a vertical connection of conical waveguides (see option I in Fig. 12b, which shows an enlarged view of the cone a 14), or in the form of a vertical connection with a Bragg grating (see option II in Fig. 12b, which shows an enlarged view of the grating 15) of waveguides made of different materials. Otherwise, a laser L with a plurality of longitudinal modes and a high-Q microcavity M made on a chip can operate in the same way as the laser device of FIG. 1a, including a tuning unit TU (not shown in FIG. 12a, b). In some embodiments, said at least one laser with a plurality of longitudinal modes may be coupled by optical feedback to one or more of said at least one high-Q microcavity through a coupling element C made in the form of a waveguide located at a subwavelength from said at least one high-quality microcavity. The additional communication element C2 shown by the dashed border in FIG. 12a may be optionally used and may be configured to output one or more dominant longitudinal laser modes generated in one or more of the aforementioned at least one high-Q microcavity in order to filter out the non-resonant part of the output spectrum. For high-Q microcavities and lasers with many longitudinal modes made on a single semiconductor chip, some individual elements of the TL and TM subnodes associated with lasers and microcavities can be appropriately integrated into the same crystal. Such elements can be metal microheaters for adjusting the temperature of the active medium of microresonators or lasers, photodetectors for measuring output signals, etc.

Next, a method for converting a laser spectrum with respect to at least one laser L with a plurality of longitudinal modes for generating laser radiation having a spectrum of a plurality of longitudinal modes and at least one high-Q microcavity M coupled by optical feedback to said at least one laser will be described. L with the plurality of longitudinal modes depicted in FIG. 1-5.

In a first step of the method, at least one laser L with a plurality of longitudinal modes generates laser radiation having a spectrum of a plurality of longitudinal modes.

In a second step of the method, a spectrum of a plurality of longitudinal modes of laser radiation is adjusted to match each of at least one frequency of a separate longitudinal mode of said at least one laser L with a plurality of longitudinal modes with a corresponding resonant frequency of at least one high-Q microresonator M coupled by optical feedback with said at least one laser L with a plurality of longitudinal modes, so as to obtain at least one matched frequency.

In a third step of the method, output laser radiation is output having an output spectrum with at least one dominant longitudinal laser mode, each of which is at one of said at least one consistent frequency and with a reduced width of the dominant longitudinal laser mode.

It should be noted that this method may include additional steps.

For example, in the case when said at least one high-Q microresonator M is made of a material having an intensity-dependent refractive index, the method may comprise an additional step of parametric generation of at least one optical frequency comb of each of said at least one dominant longitudinal laser modes if said at least one dominant longitudinal laser mode has a power above the pump threshold of the generation of an optical frequency comb.

The method may include an additional step of adjusting the spectrum of the plurality of longitudinal modes, both by changing the spacing of the longitudinal modes in the frequency domain, and by changing the frequency of each of the individual longitudinal modes of the at least one laser L with a plurality of longitudinal modes.

The method may include an additional step of providing optical feedback to said at least one laser L with a plurality of longitudinal modes by generating at least one counterpropagating mode, each of which is at one of said at least one matched frequency.

The method may include an additional step of adjusting the spectrum of the plurality of longitudinal modes by controlling the injection current of said at least one laser with a plurality of longitudinal modes and / or the temperature of the active medium of each of said at least one laser L with a plurality of longitudinal modes, preferably each of said at least one laser with many longitudinal modes separately.

The method may include an additional step for adjusting the resonance frequencies of each of the at least one high-Q microresonator M, both by changing the spacing of high-Q modes in the frequency domain and by changing the resonant frequency of each of the high Q modes.

The method may include an additional step of controlling the temperature of said at least one high-Q microcavity and / or the external pressure applied to said at least one high-Q microcavity M, preferably to each of said at least one high-Q microcavity separately.

In the case where one or more of the at least one high-Q microresonator M is provided with an additional coupling element C2, the method may include an additional step of outputting one or more dominant longitudinal laser modes or one or more dominant longitudinal laser modes and one or more optical frequency comb from one or more of the aforementioned at least one high-Q microcavity through an additional communication element to filter the cut the back part of the output spectrum.

In the case where said at least one high-Q microcavity M is made of electro-optical material that changes its refractive index in response to the influence of an electromagnetic field applied to said at least one high-Q microcavity, the method may include an additional step of controlling the electromagnetic field applied to said at least one high-Q microcavity M, preferably to each of the at least one high-Q brotnogo microcavity separately.

Next, a method for operating a laser device with respect to at least one laser L with a plurality of longitudinal modes for generating laser radiation having a spectrum of a plurality of longitudinal modes, at least one high-Q microcavity M coupled by optical feedback to at least one laser L c will be described. a plurality of longitudinal modes, and a tuning unit TU for tuning a spectrum of a plurality of longitudinal modes of laser radiation, as shown in FIG. 1-6.

In the first step of the method, the spectrum of a plurality of longitudinal modes of laser radiation is tuned by a tuning unit TU for matching each of at least one frequency of a separate longitudinal mode of said at least one laser with a plurality of longitudinal modes with a corresponding resonant frequency of at least one high-Q microcavity M, to get at least one matched frequency.

In a second step of the method, output laser radiation is output from the device having an output spectrum with said at least one dominant longitudinal laser mode, each of which is at one of said at least one consistent frequency and with a reduced width of the dominant longitudinal laser mode.

It should be noted that embodiments of the invention are described with reference to various objects. However, one skilled in the art will understand from the above and the following description that, unless otherwise indicated, in addition to any combination of features related to one type of object, any combination of features related to different objects is also considered disclosed in this application. However, all features can be combined with synergistic effects, which are more than a simple summation of the features.

Although the invention has been illustrated in the drawings and described in detail in the foregoing description, these drawings and description should be considered as illustrative or exemplary, but not restrictive. The invention is not limited to the disclosed embodiments. Those skilled in the art will be able to envision and practice other variations of the disclosed embodiments as a result of a study of the drawings, disclosure, and dependent claims.

In the claims, the word “comprising” does not exclude other elements or components, and the singular does not exclude the plural. The mere fact that certain measures are mentioned in mutually different dependent dependent claims does not mean that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting its scope.

LITERATURE

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[3] D.V. Strekalov et al. Nonlinear and quantum optics with whispering gallery resonators \\ J. Opt. 18, 123002 (2016);

[4] S. Borri et al. Whispering gallery mode stabilization of quantum cascade lasers for infrared sensing and spectroscopy \\ Proc. SPIE 10090, Laser Resonators, Microresonators, and Beam Control XIX, 1009008, February 20, 2017;

[5] W. Liang et al. Whispering-gallery-mode-resonator-based ultranarrow linewidth external-cavity semiconductor laser \\ Opt. Lett. 35, p. 2822, 2010;

[6] US7991025B2, Tunable lasers locked to whispering gallery mode resonators, GM Global Technology Operations LLC;

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[8] W. Liang et al. Spectrally Pure RF Photonics Source Based on a Resonant Optical Hyper-Parametric Oscillator \\ Proc. of SPIE, Vol. 8960, 896010, 2010;

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Claims (51)

1. A laser device comprising: at least one laser with many longitudinal modes for generating laser radiation having a spectrum of many longitudinal modes; at least one high-Q microcavity coupled by optical feedback to said at least one laser with a plurality of longitudinal modes; a tuning unit for adjusting the spectrum of a plurality of longitudinal modes of laser radiation to match each of at least one frequency of a separate longitudinal mode of said at least one laser with a plurality of longitudinal modes with a corresponding resonant frequency of said at least one high-Q microcavity, so as to obtain at least at least one agreed frequency moreover, the laser device is configured to output laser radiation having an output spectrum with at least one dominant longitudinal laser mode, each of which is at one of the at least one consistent frequency and with a reduced width of the dominant longitudinal laser mode. 2. The laser device according to claim 1, wherein said at least one high-Q microcavity is made of a material having an intensity-dependent refractive index, moreover, the laser device is additionally configured to receive at least one optical frequency comb, and wherein each of said at least one optical frequency comb is parametrically generated by one of said at least one dominant longitudinal laser mode if said at least one dominant longitudinal laser mode has a power above the pump threshold of generation of the optical frequency comb. 3. The laser device according to any one of paragraphs. 1 and 2, wherein the tuning unit is further configured to both vary the longitudinal mode spacing in the frequency domain and change the frequency of each of the individual longitudinal modes of the at least one laser with a plurality of longitudinal modes. 4. The laser device according to any one of paragraphs. 1 and 2, wherein said at least one high-Q microcavity is configured to provide optical feedback to said at least one laser with a plurality of longitudinal modes by generating at least one counterpropagating mode, each of which is on one of said at least one harmonized frequency. 5. The laser device according to any one of paragraphs. 1 and 2, wherein said at least one laser with a plurality of longitudinal modes is an electrically powered semiconductor laser diode. 6. The laser device according to any one of paragraphs. 1 and 2, wherein said at least one laser with a plurality of longitudinal modes and said at least one high-Q microcavity are made on a single crystal using microlithography. 7. The laser device according to any one of paragraphs. 1 and 2, wherein the tuning unit is further configured to control the injection current of said at least one laser with a plurality of longitudinal modes and / or the temperature of the active medium of said at least one laser with a plurality of longitudinal modes, preferably each of said at least one laser with many longitudinal modes separately. 8. The laser device according to any one of paragraphs. 1 and 2, wherein said at least one laser with a plurality of longitudinal modes is connected by optical feedback to said at least one high-Q microcavity by transmitting radiation scattered in said at least one high-Q microcavity back to said at least one laser with many longitudinal modes. 9. The laser device according to any one of paragraphs. 1 and 2, wherein one or more of said at least one laser with a plurality of longitudinal modes is coupled / connected by optical feedback to one or more of said at least one high-Q microcavity through a communication element. 10. The laser device according to claim 9, wherein the communication element is a prism of total internal reflection or a conical optical fiber or waveguide. 11. The laser device according to any one of paragraphs. 1 and 2, wherein one or more of said at least one high-Q microcavity is provided with an additional communication element. 12. The laser device according to claim 11, wherein the additional communication element is configured to output one or more dominant longitudinal laser modes generated in said one or more of the at least one high-Q microresonator, or said one or more dominant longitudinal laser modes, and one or more optical frequency combs generated in said one or more of said at least one high-Q microcavity to filter non-resonant part of the output spectrum. 13. The laser device according to claim 2, wherein said at least one optical frequency comb is a dissipative soliton Kerr optical comb. 14. The laser device according to any one of paragraphs. 1 and 2, wherein the tuning unit is further configured to adjust the resonant frequencies of the high-Q modes of the at least one high-Q microresonator by changing the diversity of the high Q modes in the frequency domain and changing the resonant frequency of each of the high Q modes. 15. The laser device according to any one of paragraphs. 1 and 2, wherein the adjustment unit is further configured to control the temperature of said at least one high-Q microcavity and / or the external pressure applied to said at least one high-Q microcavity, preferably to each of said at least one high-Q microcavity individually. 16. The laser device according to any one of paragraphs. 1 and 2, wherein said at least one high-Q microcavity is made of electro-optical material that changes its refractive index in response to an electromagnetic field applied to said at least one high-Q microcavity, and moreover, the tuning unit is additionally configured to control an electromagnetic field applied to said at least one high-Q microcavity, preferably to each of said at least one high-Q microcavity separately. 17. The laser device according to any one of paragraphs. 1 and 2, wherein said at least one high-Q microcavity is an WGM microcavity. 18. A method for converting a laser spectrum, comprising: generating at least one laser with a plurality of longitudinal modes of laser radiation having a spectrum of a plurality of longitudinal modes; adjusting the spectrum of the plurality of longitudinal modes of laser radiation to match each of at least one frequency of a separate longitudinal mode of said at least one laser with a plurality of longitudinal modes with a corresponding resonant frequency of at least one high-Q microcavity coupled by optical feedback with said at least one laser with many longitudinal modes, so as to obtain at least one matched frequency; the output of the output laser radiation having an output spectrum with at least one dominant longitudinal laser mode, each of which at one of said at least one matched frequency and with a reduced width of the dominant longitudinal laser mode. 19. The method according to p. 18, moreover, said at least one high-Q microcavity is made of a material having an intensity-dependent refractive index, and moreover, the method further comprises parametric generation of at least one optical frequency comb of each of said at least one dominant longitudinal laser mode, if said at least one dominant longitudinal laser mode has a power above the pump threshold of generation of the optical frequency comb. 20. The method according to any one of paragraphs. 18 and 19, the spectrum of the plurality of longitudinal modes being adjusted both by varying the spacing of the longitudinal modes in the frequency domain, and by changing the frequency of each of the individual longitudinal modes of said at least one laser with a plurality of longitudinal modes. 21. The method according to any one of paragraphs. 18 and 19, wherein said at least one high-Q microcavity is configured to provide optical feedback to said at least one laser with a plurality of longitudinal modes by generating at least one counterpropagating mode, each of which is on one of said at least one harmonized frequency. 22. The method according to any one of paragraphs. 18 and 19, wherein the spectrum of the plurality of longitudinal modes is adjusted by controlling the injection current of said at least one laser with plurality of longitudinal modes and / or the temperature of the active medium of said at least one laser with plurality of longitudinal modes, preferably each of said at least one lasers with many longitudinal modes separately. 23. The method of claim 21, wherein said at least one counterpropagating mode transmitted back to said at least one laser with a plurality of longitudinal modes is generated by resonant Rayleigh scattering in said at least one high-Q microcavity. 24. The method according to any one of paragraphs. 18 and 19, wherein one or more of said at least one laser with a plurality of longitudinal modes is coupled / connected by optical feedback to one or more of said at least one high-Q microcavity through a communication element. 25. The method according to p. 24, and the communication element is a prism of total internal reflection or a conical optical fiber or waveguide. 26. The method according to any one of paragraphs. 18 and 19 moreover, one or more of the aforementioned at least one high-Q microcavity is provided with an additional communication element. 27. The method according to p. 26, wherein one or more dominant longitudinal laser modes or said one or more dominant longitudinal laser modes and one or more optical frequency gratings are removed from one or more of the at least one high-Q microcavity through an additional communication element, to filter out the non-resonant part of the output spectrum. 28. The method according to any one of paragraphs. 18 and 19, wherein the resonant frequencies of the high-Q modes of the at least one high-Q microresonator are adjusted both by changing the diversity of the high Q modes in the frequency domain and by changing the resonant frequency of each of the high Q modes. 29. The method according to any one of paragraphs. 18 and 19, the method further comprising controlling the temperature of said at least one high-Q microcavity and / or the external pressure applied to said at least one high-Q microcavity, preferably each of said at least one high-Q microcavity individually. 30. The method according to any one of paragraphs. 18 and 19 wherein said at least one high-Q microcavity is made of electro-optical material that changes its refractive index in response to an electromagnetic field applied to said at least one high-Q microcavity, and moreover, the method further comprises controlling an electromagnetic field applied to said at least one high-Q microcavity, preferably to each of said at least one high-Q microcavity individually. 31. The method according to any one of paragraphs. 18 and 19, wherein said at least one high-Q microcavity is a WGM microcavity. 32. The method of operation of the laser device according to any one of claims 1 to 17, comprising: adjusting the spectrum of the plurality of longitudinal modes of the laser radiation to match each of the at least one frequency of the individual longitudinal modes of the at least one laser with the plurality of longitudinal modes with the corresponding resonant frequency of the at least one high-Q microcavity so as to obtain at least one matched frequency; the output of the output laser radiation having an output spectrum with at least one dominant longitudinal laser mode, each of which at one of said at least one matched frequency and with a reduced width of the dominant longitudinal laser mode.

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