RU2295184C2 - Solid-state laser - Google Patents

Abstract

FIELD: quantum electronics; laser power installations, laser chemistry, laser medicine, metallurgy.

SUBSTANCE: proposed fully independent continuous-wave fiber-optic solid-state laser distinguished by steady characteristics has active element, resonator, and optical pump source. Active element is made in the form of fiber-optic element or stack of fiber-optic elements. Pump source is made of radio-luminescent material whose radio radiation is free from gamma-ray components. Pump source is disposed on each fiber-optic element throughout its entire length.

EFFECT: enhanced service life and generation power range of laser.

7 cl, 5 dwg

Description

The invention relates to quantum electronics, and more particularly to solid-state lasers with continuous radiation, and can be used in energy laser systems, laser chemistry, laser medicine, metallurgy and other laser technological processes on Earth and in outer space.

A well-known solar-pumped laser (RF Patent No. 1701082, H 01 S 3/09, 1994) containing a system of two confocal concave multi-aperture mirrors and an active element, characterized in that, in order to increase the efficiency, a second system is introduced into it of two confocal different-aperture parabolic cylindrical mirrors and two roof-shaped return reflectors, the mirrors of the first system are made parabolic-cylindrical, in the smaller mirror of the first system there is a slit parallel to the focal line of the system with a width equal to the aperture of the smaller mirror of the second c themes, the aperture of the larger mirror of the second system is equal to the aperture of the smaller mirror of the first system, the focal lines of both systems are parallel to each other, the smaller mirror of the second system is optically connected along the pump radiation with its large mirror through two return reflectors arranged in series and the gap in the smaller mirror of the first system, and the active element is located between these reflectors, while one of the roof-shaped return reflectors is placed between the focal line and a large mirror of the second system, while formed therein a longitudinal slot parallel to the focal line of the second system with a width equal to the smaller aperture mirror of the first system.

A disadvantage of the known laser is the low pumping efficiency, since the spectrum of solar radiation is close to the spectrum of thermal radiation of a black body, in which the intensity of the lines of the visible range is the same everywhere.

Therefore, in addition to lines coinciding with the absorption band of the activator, the entire remaining region of the solar spectrum only heats the matrix of the active element. The pump power is limited by the constancy of solar radiation, the operation of the laser is completely dependent on the weather, a constant re-orientation of the laser along the Sun is necessary. More efficiently, a solar-pumped laser works in space even periodically (it does not work in the shadow of the Earth).

A laser is known (US Patent 5,313,485, H 01 S 3/0915, 1994), in which spectral conversion of lamp light into a photoluminescent glow of a phosphor is used to improve the matching of the pump and absorption spectra of the activator.

The disadvantage of this laser is the presence of a pump lamp, hence the low laser resource, the need for power, the size of the active element is limited by the interelectrode distance of the lamp. The generation power is limited by the capabilities of the pump lamp and losses during spectral conversion; the laser operates only in a pulsed mode on artificial crystals.

Under vacuum pumping, the spectra are weakly matched, and besides, there is strong absorption in the AIG matrix. This is especially harmful in the ultraviolet region.

A continuous solid-state laser is known (Japanese Patents No. 57-177584, 57-177585, H 01 S 3/091, 1982) with electroluminescent pumping. An active element of Nd: YAG (neodymium - yttrium-aluminum garnet) is coated with conductive films with output electrodes. A layer of phosphor is placed between the films. The inner film is made transparent, and the outer one is reflective. The resonator mirrors are applied to the ends of the active element.

The disadvantage of the laser is the need for power supply, the use of an artificial Nd: YAG crystal and low optical pump power, which is limited by the fact that it is impossible to supply large electric power to the electrodes due to the probability of breakdown of the luminescent layer. The luminescence of the phosphor is shielded by adjacent layers in thickness, so only one inner layer effectively works on the active element. Inevitable loss of pump radiation in the electrically conductive inner film.

Closest to the technical nature of the present invention is a continuous fiber-optic erbium laser ELD-15 produced by NTO "IRE-Polyus" (Russia, 141120, Moscow region, Fryazino, Vvedensky square, 1; t.526-90 -83, F. 702-95-73). In the ELD series lasers, the active element is erbium-doped optical fiber, and laser diodes operating at a wavelength of 970 nm are used for pumping. The IRE-Polyus NTO uses similar physical principles in high-power and super-powerful (up to 6 kW) industrial neodymium fiber-optic lasers, which today are the most efficient in converting electrical energy into optical.

However, all types of lasers developed by the NTE "IRE-Polyus", like other manufacturers, cannot work without power supply. Even with a theoretically acceptable efficiency of 20%, which is reported in the advertising information of this company, at least 30 kW of electric power is required for a 6-kilowatt output power. Obviously, the use of such a laser is limited by the possibility of obtaining large electric power for pumping and the need to remove heat from the remaining 80% of this power. The design flaws also include the inevitable high divergence of the beam, which limits its range.

This invention solves the problem of creating a continuous solid state fully autonomous laser with a long service life and a wide range of lasing power with stable characteristics. The essence of the invention lies in the fact that a solid-state fiber-optic laser contains an active element, a resonator and an optical pump source, while the pump source is made of radioluminescent material and is placed on each fiber-optic element along its entire length.

The active element of the laser can be made of optical fiber doped with Nd ³⁺ . In addition, the active element can be made in the form of a packet of optical fibers assembled into a bundle, a bay from this bundle, and the final sections of the fiber-optic elements can be in contact with each other.

The end surfaces of the optical fiber elements are coated with reflective interference coatings, while the reflective coating on one of the end surfaces of the optical fiber elements is made with a reflection coefficient of 100%, and on the other with a reflection coefficient of 93-97%.

A brief description of the drawings.

The invention is illustrated by the example of a preferred embodiment of the inventive fiber optic laser.

Figure 1 is a diagram of a fiber optic laser.

Figure 2 is a sectional optical fiber.

Figure 3 is a diagram of an optical fiber laser assembled in a packet.

4 is a diagram of an optical fiber laser assembled in the form of a bay.

5 is a diagram of a fiber optic laser with adjacent end portions of the fiber optic elements.

The pump source 1 proposed in this invention is practically dimensionless and does not complicate the design, uniformly excites an active element of fiberglass 2 along its entire length at any length, does not require power supply and special maintenance, works continuously with a huge and unparalleled resource. Active element 2 is made of optical fiber doped with Nd ³⁺ .

The package of fiber-optic elements can be made in the form of a bundle 3, which in turn can be assembled, for example, in the form of a bay 4. Interference mirrors with reflection coefficients of 100% 5 and 97% 6 are applied to the end surfaces of the fiber-optic elements.

Tightly packed fibers form a bundle of arbitrary shape and length. The number of fibers in the bundle and its length directly proportionally determine the laser power. For convenience in operation and reliability of the structure, the tourniquet is laid in a bay and closed with a durable outer casing.

Alpha / beta-radioluminescent luminescence, falling through the side surface into the fiber, propagates in it in two opposite directions, reflected from the interface. In this case, the pump light excites Nd ³⁺ activators and an avalanche-like amplification process occurs until a stationary state appears, in which the losses and amplification are in a constant ratio to each other. As in any laser, the excess of gain over loss is the amount of lasing power.

For such a design to work as a laser emitter, positive feedback in the form of a resonator is necessary. To do this, all the fibers at the end are fused into a monolithic glass mass, and both ends are polished in a plane perpendicular to the axis of each fiber. A multilayer interference mirror with a 100% reflection coefficient at a wavelength of 1.064 microns is applied to one of the planes. An antireflection coating is applied to the second end plane at the same wavelength. Another interference mirror of the resonator is applied to the glass substrate separately. This mirror is the output for the generation beam and has a reflection coefficient of 93-97%. The mirror moves in a plane perpendicular to the output beam and allows you to turn the laser on and off mechanically. The installation of the output mirror in the working position should accurately reproduce the alignment of the resonator, so that there are no additional losses due to misalignment. However, such a device in a fiber optic laser is still ineffective. This is due to the fact that the luminescence from the ends of the active element (bundle) in each fiber occurs in the form of scattered rays with a large range of angles at the exit, that is, in the form of a wide cone. The interference effect in reflection mirrors takes place only for parallel rays incident normally to the plane of the mirror. Only under such a condition mirrors provide the necessary reflection coefficient. Therefore, before approaching the ends, all zigzag beams in the fiber must be straightened, parallel and normal to the planes of the mirrors. This can be achieved by a smooth, but significant change in the refractive index over the cross section of the fiber from the side surface to the center. Moreover, the refractive index at the side surface should be greater, and less in the center. The fiber itself should be straightforward and rigid, not bent over a section of 50-80 centimeters to the end. Then the oscillations of the rays passing through the fiber in this section gradually damp, the rays do not reach the lateral surface, but bend closer to the center and completely straighten, passing along the geometric axis of the fiber.

Another embodiment of a resonator for a laser with a flexible active element may be a design with one output (93-97%) mirror, if both ends of the bundle are combined into one common bundle and polished its end 7. There is no other end because the bundle is bent and turned back to to the beginning. Further, everything is the same as in the embodiment described above.

This device is a multicomponent laser, that is, many lasers are arranged in one design, the number of which is equal to the number of fibers in the bundle. Depending on the purpose of such a laser, the radiation received at the output can be used in different ways.

If you install a collecting lens, then, up to aberrations, all the rays will be collected at one point, beyond which they again disperse. Then at this point almost all the laser power will be concentrated, and it can be used for high-temperature effects on materials (welding, cutting, hardening, engraving, and so on).

In addition, all parallel beams can be reduced to a single beam of small diameter with high power density and minimal divergence. Such a beam has a wider range of applications (location, communications, long-range measurements, surgery, surveying and surveying, construction work, beam weapons, energy transmission from a distance, meteorology, astronomy, navigation, and much more). In order to reduce all the beams into one at the laser output, it is necessary to use an optical mixer. Its principle of operation is the same as when rectifying a zigzag beam in a fiber, that is, using a gradient refractive index over the cross section of the mixer, which allows all the input beams to be reduced at the output into one rectilinear with the smallest possible diameter. Moreover, the output mirror of the resonator can be located on the input end of the mixer.

There is no standing wave in the cavity of such a laser; instead, a traveling wave. Therefore, there is no “burnout” effect in the volume of the active element, there is no thermal lensism and thermal misalignment, there is a much more uniform removal of power in volume, diffraction effects are averaged, the mode structure of the radiation in the output cross section is also strongly averaged in space and time. Thus, there is a highly efficient coordinated operation of the resonator with the active element, which allows to obtain stable uniform radiation.

In order to reduce losses due to the reflection of rays inside the light guide fiber due to immersion at the interface of the glass, a radioluminescent luminous paint (powder, capsules with gas), the side surface of the fiber is coated with a thin film transparent to pumping light with a refractive index much larger than in the rest of the volume. Then the pumping light entering the fiber begins to experience the light guide effect, already being reflected from this film, not reaching the interface and not being absorbed there.

The relatively low specific power of the alpha / beta-radioluminescent glow of the pump in the proposed design is compensated by its effective distribution over the volume of the active element. As can be seen from the design description, the fiber bundle is penetrated by a pump glow from the outside and from the inside, while in an ordinary solid-state laser, the active element is excited only through the side surface. In addition, a sufficiently high light amplification and a large generation power makes it possible to obtain a huge length of the active element, which is unattainable in other designs, with continuous pumping over the entire length. And finally, a significant factor in increasing the laser efficiency is the effective spectral matching of the pump radiation with the absorption of the Nd ³⁺ activator.

For these purposes, a phosphor is selected with a narrow spectral range of radiation and maximally coinciding with the most intense lines of the absorption spectrum of Nd ³⁺ . In particular, this applies to the three main lines 810; 750 and 580 nanometers. In addition to narrow ranges, these luminosity lines should not be at all other wavelengths. For example, alpha / beta-radioluminescent self-luminous paint consists of a mixture of three components: a phosphor (an optical radiation source for pumping an active laser element), an activator (an alpha / beta radiation source for exciting a phosphor) and an adhesive composition that holds components together and has good adhesion to fiberglass (type SiO ₂ ).

As a phosphor, for example, doped crystals of cadmium sulfide (CdS), cadmium telluride (CdTe) and zinc selenide (ZnSe) can be used. The article by V. P. Makhniy, Ya. N. Barasyuk “Integrated ionizing radiation detector based on the cadmium sulfide-telluride heterojunction”, published in “Letters to the ZhTF”, 1997, volume 23, p. 14, reports on their effective use for indication (detection) of ionizing radiation. They have a high energy output, that is, a strong glow under the influence of radiation, speed and high transparency for radioluminescent radiation, resistance to moisture, temperature and radiation. In addition, the alpha-radioluminescence spectra of these phosphors have maxima that almost coincide with two of the three main lines in the absorption spectrum of neodymium.

Sources of alpha radiation can use the stable long-lived isotopes 238Pu, 239Pu, 241 Am or 244Cm. As such, they are offered as products of ZAO Ritverts FSUE NPO Radium Institute named after V.G. Khlopin (194021, Russia, St. Petersburg, 2nd Michurinsky Prospect, Building 28).

As an activator, you can also use other materials with similar properties, for example Tritium (ZN), as a source of beta-radioluminescence (http://www.gpi.ru: http: //betabatt.com: http://boysstuff.co .uk; http://uk.gizmodo.com; http://armytechnology.com).

Waste from the nuclear industry that can be disposed of or stored in repositories can also be used as an activator. This should be an extremely cheap material with special properties: a half-life of up to hundreds of years or more, and there should be no gamma component in its radio emission, only alpha and / or beta radiation should be used. Radioluminescent (self-luminous) paint (or powder) is initially checked for radiation safety in accordance with the requirements of regulatory acts regulating the safety of such materials (standards, necessary certification, etc.). The phosphor should not reduce the brightness of the glow within the half-life of the activator. The working mixture should be as finely dispersed so that when applied to the surface of the fiber, the film thickness should be no more than 0.1 millimeters with good adhesion to the material of the fiber (it is also possible to solder capsules with gas into the surface of the fiber). All of these requirements should not depend on the influence of temperature in the range of at least plus or minus 50 degrees Celsius.

For the first experimental sample of a fiber optic laser, glass fiber with a diameter of 0.2-0.3 mm can be used. With activation of Nd ³⁺ in 1 atomic%. The number of fibers in the bundle is at least 10, the length of the bundle is 100 meters. Further detailed studies of the principles of operation of such a laser and optimization of its design may be the subject of a separate dissertation.

High reliability and stability of generation, a huge resource and complete autonomy (lack of power supply and water cooling), almost unlimited (only by the size of the laser) possibilities of increasing the output power and its density distinguish the proposed design from the existing industrial and military lasers in the fields of applications, cost price, qualifications and volumes of service, efficiency and prospects for further developments.

The lack of power for pumping makes the laser completely independent and autonomous, which can dramatically expand its field of application.

The industry has mastered the production of phosphors, the radiation of which lies in narrow ranges and collectively covers the spectral region from ultraviolet to infrared. Therefore, one can choose as a source of optical pumping such a radioluminescent material, the spectral emission maximum of which coincides, for example, with the main absorption lines of the Nd ³⁺ activator. It is realistically to obtain a very effective agreement between the pump and absorption spectra of the activator, and to minimize the losses in the absorption region of the matrix. Thus, the optical pump efficiency is significantly increased.

The temperature balance that exists when pumping a solid-state active element by the radioluminescent method compares favorably with known technical solutions. There is no high-temperature plasma, and the spectrum of the pump radiation is optimally matched with the absorption lines of the activator. Therefore, the internal and external heating of the active element is many times less. There is no need for water cooling. The laser design is simplified, its full autonomy is achieved.

Without water cooling, a small amount of heat is evenly distributed over the volume of the solid-state active element, therefore, it does not have large internal stresses. It becomes possible to use glass with Nd ³⁺ to obtain a continuous generation mode. The optically pure glass matrix of the active element significantly reduces harmful absorption losses. Glass is many times cheaper than artificially grown AIG crystals. Fiberglass active elements can be made in large sizes without sacrificing quality. The activator Nd ³⁺ more evenly enters the glass, it is possible to widely vary its concentration without loss of matrix quality.

For many applications of laser radiation, the decisive factor is the stability of the generation power (communication, control of chemical reactions, etc.), which mainly depends on the stability of the pump. During lamp pumping, a gas discharge lamp is not stable in luminosity due to the physical state of the discharge plasma. The situation is exacerbated by the turbulence of the cooling flow along the active element, which actually leads to noise modulation of losses in the laser cavity due to thermal instability of the active element.

With diode pumping, its stability is much higher, but it also completely depends on the stability of the power supply. Radioluminescent pumping is completely free from these shortcomings and is ideally stable. No other methods can surpass such stability. Obviously, the field of application of highly stable lasers is much wider.

Since activators have huge half-lives (hundreds of years), it means that the very fact of radioluminescence and the presence of pump energy are equal to the same periods. Unreachable by any other means, the resource of such a pump source makes the laser almost eternal. There are no elements that degrade over time.

The high purity and uniformity of the glass in the optical fiber provide its high radiation resistance to the effects of the activator of the radioluminescent phosphor. Moreover, the immunity of the glass matrix to radiation is due to the composition of radiation, since it does not have a γ component, and β and α radiation are corpuscular in nature with very low penetrating power (for example, α radiation delays a sheet of paper). In addition, radioluminescent (self-luminous) paints, powders, gas capsules, for example, based on nuclear waste, are initially radiation-safe in accordance with the requirements of the standard for such materials.

Thus, the use of the radioluminescent fiber-optic pumping method allows you to create a device that does not require maintenance completely autonomously and independently, has a huge resource and produces continuous laser radiation.

Claims (7)

1. A solid-state fiber-optic laser containing an active element, a resonator and an optical pump source, characterized in that the active element is made of an optical fiber element or a packet of optical fiber elements, the pump source is made of radio-luminescent material, in the radio emission of which there is no gamma component, and placed on each fiber optic element along its entire length. 2. The solid-state optical fiber laser according to claim 1, characterized in that the optical fiber elements are doped with Nd ³⁺ . 3. The solid state laser according to claim 1, characterized in that the packet of fiber optic elements is assembled in the form of a bundle. 4. The solid-state laser according to claim 1, characterized in that the packet of fiber optic elements is assembled, for example, in the form of a bay. 5. The solid-state laser according to claim 1, characterized in that the final sections of the fiber-optic elements are in contact with each other. 6. The solid state laser according to claim 1, characterized in that the end surfaces of the fiber optic elements are coated with reflective interference coatings. 7. The solid-state laser according to claim 7, characterized in that the reflective coating on one of the end surfaces of the optical fiber elements is made with a reflection coefficient of 100%, and on the other with a reflection coefficient of 93-97% at the generation wavelength.

Images