CA2314316A1

CA2314316A1 - An upconversion active gain medium and a micro-laser on the basis thereof

Info

Publication number: CA2314316A1
Application number: CA 2314316
Authority: CA
Inventors: A. Faouzi Zerrouk; Alexander Zagoumenny; Iouri Zavartsev; Pavel Studenikin
Original assignee: UTAR LABORATORIES CANADA INC.
Current assignee: UTAR LABORATORIES CANADA Inc
Priority date: 2000-07-21
Filing date: 2000-07-21
Publication date: 2002-01-21
Also published as: AU2001276221A1; WO2002009245A3; WO2002009245A2

Abstract

An upconversion active gain medium including a crystal host doped with two groups of active ions capable of producing either blue or ultraviolet lasing radiation at room temperature from single band infrared pumping radiation that is continuous or quasi-continuous is disclosed. An upconversion micro-laser incorporating a chip of said upconversion active gain medium, an optical cavity for resonating at least said lasing radiation and a small size pumping source is disclosed too. The pumping radiation energy is applied to the crystal host by means of a beam or a set of beams of said pumping radiation to be absorbed by sensitizer ions and provide the energy transfer to the activator ions.
The crystal host is made of a new class of materials the composition thereof being arranged to be compatible with the preferable upconversion mechanism for increasing efficiency and providing stability and reproducibility of the micro-laser parameters. Said crystal host may be arranged to produce said lasing radiation at one of the wavelengths preferably using additionally a red or an infrared exciting radiation having been produced also by said crystal host itself during the upconversion process. Besides, a structure of the crystal host may be arranged to produce said lasing radiation being polarized.

Description

AN UPCONVERSION ACTIVE GAIN MEDIUM
AND A MICRO-LASER ON THE BASIS THEREOF
FIELD
The present invention relates generally to upconversion solid-state lasers for producing blue or ultraviolet lasing radiation from infrared pumping radiation, and more specifically to an upconversion active gain medium being made of a rare earth doped crystal host capable of producing the blue or ultraviolet lasing radiation under normal ambient temperature operating conditions as a result of absorbing continuous or quasi-continuous single band infrared pumping radiation, and to an upconversion micro-laser on the basis of such active gain medium.
The present invention can be used for a variety of applications in the fields of optical data storage and computer CDROM (Compact Disk Read Only Memory), red-green-blue multicolored display systems and optical information processing. In these applications the higher resolutions afforded by short optical wavelengths are important, the continuous wave operation of visible or ultraviolet lasers at room temperature and small sizes that the lasers can achieve are crucial, and the simplicity of alignment or any other adjustment of the packaged arrangement are highly desirable.

BACKGROUND
Upconversion or excited state absorption based conversion of pumping radiation to radiation that has a shorter wavelength than the wavelength of the pumping radiation has been known for many years.
The alternative conversion mechanism - nonlinear wavelength conversion (frequency doubling, for example) that employs nonlinear frequency conversion crystals requires critical alignment and high maintenance of the lasers realizing this mechanism. In order for doubling (or any nonlinear conversion process) to be effected in each laser, the optical axes of the laser's nonlinear crystal must be precisely oriented at the phase-matching angle determined with respect to the axis of the radiation beam to be converted and polarization directions. These restrictions hinder development of the nonlinear wavelength conversion mechanism for applications such as optical data storage and computer CDROM (Compact Disk Read Only Memory), red-green-blue multicolored display systems and optical information processing (the Applications). Upconversion lasers have no similar restrictions.
Since the first demonstration of an upconversion laser more than 25 years ago, a variety of rare earth-doped crystal and glass hosts have been proposed to produce lasing radiation in the red, green, blue and ultraviolet wavelengths.

The peculiarities in producing blue or ultraviolet lasing radiation are connected with the greater photon energy in these wavelength ranges. This production makes use of a rare earth ion ground state as a low state in a lasing transition or a rare earth ion excited state as an upper state in the transition. In both cases problems arise because, for the specified host and specified pumping scheme, a population inversion in the rare earth states can only be achieved in continuous operation by increasing pumping radiation intensities or by cooling the host.
Hitherto, the above problems were solved for a glass host, having low thermal conductivity and unordered rare earth ions in the host, by using a long length optical fiber with low doping levels. The small core radius of the optical fiber allows high pumping radiation intensities with modest pumping powers. The guiding properties of the optical fiber allow a good confinement of the pumping radiation giving efficient excitation over its entire length and, therefore, minimizing thermal problems. Thus, the continuous wave upconversion lasing operation at one or more wavelengths in the blue or ultraviolet regions has been achieved on a practical basis at room temperature, using glass fiber.
The optical fibers appropriate for upconversion lasers are usually fabricated from glasses with low phonon energy such as fluoride glasses. Booth et al. in his publication entitled "Operation of Diode Pumped Tm3~ ZBLAN
Upconversion Fiber Laser at 482 nm"(IEEE Journal of Quantum Electronics, Vo1.32, N 1, January 1996, p.118-123) disclosed a continuous wave room-temperature operation at 482 nm of a Tm''-doped fluorozirconate (ZBLAN) glass fiber laser. This laser pumped with single-mode semiconductor diode lasers operated in the 1110-1150 nm wavelength range. The fiber had a core diameter of 3 Vim. The slope efficiency (versus launched pumping power) of 13.8 was obtained in a 38 cm length of fiber laser with 2,500 ppm-doping. The measurements indicated that shorter, more heavily doped fiber has a higher output because of the reduced effect of the background loss. From the practical viewpoint this means that less fiber is required for a more heavily doped fiber laser to achieve a desired output power.
Fiber length is still the main restriction in developing compact rare earth doped glass fiber lasers.
Further reducing fiber length by means of increasing its doping levels is limited due to the increased influence of rare earth pair interactions. The latter effect, which is called photo-darkening, reduces the population of rare earth ions excited states associated with lasing transition and, therefore, the fiber laser output.

In contrast to the glass fibers, crystal hosts are much smaller in size, typically no more than several millimeters in the direction of the lasing radiation. This size reduction of crystal hosts is due to their higher thermal conductivity and their capability of accepting more rare earth ions in their crystal structures. The latter properties facilitate the large separation of the rare earth active ions in order to reduce non-radiative relaxation of their excited states via the rare earth pair interactions. Crystal hosts also have the advantages of high melting points (about 1000 to 1600 centigrade degrees for fluorides) and stability in an ambient environment that is very important for commercial Applications.
However, when rare earth crystals are used, there is a problem that the temperatures of the upconversion laser operation are much lower than room temperature. MacFarlane et al. (Applied Physics Letters, vo1.52, No. l6, pp. 1300-1302,1988) disclosed an upconversion laser operation at 380 nm at temperatures up to 90°K in an Nd3'doped LaF3crystal pumped simultaneously at 591nm and 788 nm. YLiF4:Tm''pumped at 649 nm and 781 nm has produced a pulsed output at 450 nm. A fluoride erbium doped bulk crystal has produced blue continuous laser operation at 470 nm when mounted in a helium cryostat with operating temperature variation between 15°K and 120°K (US
Patent No. 5,008,890 and Canadian Patent No. 2,040,557).

Thrash et al. described rare earth ion upconversion laser systems in patents US 5,299,215 and US 5,488,624, 1996.
Barium yttrium fluoride Tm:Yb codoped crystal hosts have produced a quasi-continuous blue lasing radiation at a wavelength of approximately 455 nm utilizing the upconversion laser transition 1D2 -~ 3H4 . The output of the pumping source, typically comprising one or more laser diodes, was made quasi-continuous, such as with a chopper wheel, to prevent crystal hosts from overheating. Thrash et al, in his article entitled "Upconversion laser emission from Yb3'- sensitized Tm3' in BaY2Fe" (Journal of the Optical Society of America Bulletin, Vo1.11, N5, May 1994, pp.881-885) demonstrated a quasi-continuous operation of the 456 nm upconversion laser transition 1Dz -~ 'H9 at a temperature of 215°K in the ytterbium-sensitized BaYlYbo_9gTmo.oZFe crystal. Using two 100 mW diode lasers, polarization coupled to form a single pumping beam, pulsed operation of the 456 nm laser transition was achieved with a threshold of 190 mW at room temperature. At a temperature of liquid-nitrogen this fluoride crystal displayed a threshold of approximately 20 mW at the 482 nm for the upconversion laser transition 1G4 ~ 3H6.
Reduced temperature operation and the lack of availability of the corresponding equipment for supporting such low temperatures are formidable obstacles in developing compact lasers for the purposes of the Applications. The alternative variant of using pulsed operation of blue or ultraviolet lasers at room temperature to prevent crystal hosts from overheating seems to be of value only for researching fluorescence spectra and upconversion excitation mechanisms in various crystal hosts or for other similar optical measurements. But this variant is entirely unacceptable for the purposes of the Applications, because of continuous or other specific operation of associated equipment in such Applications not being suited to pulsed laser operation.
Further developing compact blue or ultraviolet lasers to achieve their continuous wave operation at room temperature, therefore, requires a new approach in selecting a more suitable crystal host as an active gain medium and a related pumping scheme.
Until now, fluoride and fluoride based materials have been the most preferred materials having been used as crystal hosts for upconversion lasers. The same choice has been made as a rule in respect of glass hosts for fiber upconversion lasers. The only reason for such choice has been the low phonon energies in fluorides despite the different affect that crystal and glass hosts have on the upconversion processes in rare earth ions contained in these hosts.

In fact, the effect of the crystal host is determined by a number of factors. So using only the values of phonon energies as a basis for the choice of crystal host for blue or ultraviolet lasers providing continuous wave operation at room temperature has proven to be unacceptable for the purposes of the Applications. Low ground state splitting in a fluoride crystal host results in high population of the low state in lasing transitions at room temperature according to the Boltzmann distribution. This high low state population reduces the population inversion for the lasing transition and increases the lasing threshold. On the other hand, the low thermal conductivity of fluorides hinders the increase of pumping radiation intensities to increase population inversion and is, in essence, the main reason for fluoride-based crystal host overheating at the required pumping radiation intensities. It is necessary, therefore, to take into account both the values of the thermal conductivity and energy splitting of the low state of the lasing transition as well as the values of phonon energies in order to make a better choice of the crystal host.
Another factor to be considered in selecting a crystal host is related to the effect on the crystal host of the photon energy of lasing radiation produced by the blue or ultraviolet lasers having energies high enough to change the structure of their crystal hosts themselves by creating in the latter the color centers and vacancies or defect centers during the upconversion lasing operation. This leads to increased loss at the wavelengths of lasing radiation and correspondingly higher lasing thresholds that can reduce laser outputs and even prevent their continuous wave operation. The stability of the crystal host must therefore be taken into account.
On the other hand, the presence of strong ultraviolet radiation together with blue radiation in the laser output could be absolutely unacceptable when only blue radiation is required. Apart from this circumstance, lasing operation in several wavelength ranges simultaneously reduces the efficiency at the required wavelengths and stimulates a decrease in stability and reproducibility of laser parameters because of competition between the corresponding upconversion mechanisms. This problem affects blue wavelength radiation to the greatest extent. For these reasons, crystal hosts have to be arranged to produce lasing radiation in only one of the wavelength ranges, namely, either in the blue wavelength range or in ultraviolet one. So a capability of providing spectral purity of produced radiation (in visible radiation - color purity) has to be taken into account as a further factor in making a careful choice of a crystal host.
In view of the last factor, a crystal host selected to study an upconversion mechanism which is in the specified transitions of the crystal host or with a structure of energy levels near the specified state of rare earth active ions could not be used as the active gain medium. In these or other similar cases the crystal hosts are arranged (in their compositions, rare earth doping levels and pumping schemes) to produce a wide-ranging fluorescent emission that often covers several wavelength ranges or for changing parameters of pumping schemes over a wide range such as, for example, changing the pumping radiation wavelengths.
An upconversion laser material comprising a micro-sphere that is doped with an ion of a rare earth element and made of crystal or glass was disclosed in U.S. Patent No.

5,684,815 issued to Kiolaka Miora et al. In particular, the micro-spheres can be made of fluoride single crystals, chloride single crystals, bromide single crystals or iodide single crystals. But using a crystal that is small in phonon energy is considered to be preferable. The rare earth element can be erbium, holmium, praseodymium, thulium, neodymium or dysprosium. When Tm3'-doped LiYFqsingle crystal was pumped with a dye laser at 650 nm the occurrence of upconversion lasing emission at 480 nm was ascertained. The upconversion lasing radiation rotates within the micro-sphere repeatedly totally reflecting on its outer surface. Furthermore, when the thus totally reflected radiation has the same phase, this radiation resonates within the micro-sphere. In other words, the micro-sphere serves as a resonator. A series of total internal reflections forms the radiation into a thin ring at the equator of the dielectric micro-sphere having a diameter within a range 50 to 2,000 microns. The latter, however, imposes the limitations on such laser output power at room temperature, because the micro-sphere cannot be contacted with a holder or cooler for extracting the heat. A fundamental disadvantage of the micro-sphere resonator is the absence of the spectral selectivity or spectral purity in lasing emission. The upconversion red, green and blue radiation can resonate within the micro-sphere and, as a result, lasing operation in red, green and blue wavelengths begins simultaneously with low efficiency at the blue wavelength. In addition, such a scheme of direct excitation of rare earth ions is not an efficient one because of wavelength sensitivity that requires the pumping source to have a changeable wavelength output. Such a pumping source is not acceptable for the purposes of the Applications, where the stability and reproducibility of optical parameters as well as the simplicity of adjustment of the packaged arrangement are highly desirable.
Zhang et al. reported a more complicated scheme of the rare earth ion excitation in the article "Blue Upconversion with Excitation into Tm Ions at 780 nm in Yb- and Tm-Codoped Fluoride Crystals" (Physical Review B, Vo1.51, N
14, 1995, p.9298-9301). When only Tm'' ions are used, two pumping sources (e.g. 780 nm and 650 nm) are required to achieve upconversion blue lasing operation. Apart from such a pumping scheme suffering from wavelength sensitivity, there is a definite inconvenience in having to align beams of both pumping sources so as to couple these beams to the small-sized upconversion crystal. This makes the adjustment of the packaged arrangement using such blue laser more difficult.
The fluoride crystals, such as LiYFq, BaY2Fe and KYFQ doped only with Tm3' ions having been excited at only 780 nm did not produce any detectable blue emission. But the same crystals codoped with Tm3~ and Yb'' produced the strong blue emission at 455 and 480 nm when excited into the Tm'' 'FQ state at 780 nm.
It is necessary to note that the efficient energy transfer of Yb''~ Tm3', yb3'-~ Er3', Yb3'~ Ho'', etc. in laser crystal and glass hosts has been known for more than 22 years (see publication of A.M. Prokhorov entitled "Handbook of laser", vol.l, Moscow, 1978, p.291). Among them rare-earth Tm3+ and Pr'~ ions are particularly appropriate for upconversion blue lasers due to their numerous long-lived metastable levels, which store population during the upconversion process. Pumping radiation energy may be applied to the crystals codoped with two groups of rare earth active ions by different ways. In laser systems (see U.S. Patent Nos.
5,299,215 and 5,488,624 supra.) fluoride Tm:Yb codoped crystal hosts the Yb3'ions functioned as a sensitizer, at a wavelength in the range of 830 nm to 1100 nm to stimulate upconversion lasing transitions 1D2 --~ 3H4 in Tm'' ions, which functioned as an activator. The substantial interest in this pumping scheme is motivated by the extremely broad and strong Yb absorption band in the operating range of readily available high-power semiconductor diode lasers. Thus, ytterbium-sensitized upconversion lasers have the advantage of using a single infrared pumping source (e. g., at 960 nm). Such diode lasers proved to be the most practical pumping source taking into account low cost, small size and simplicity of alignment of the packaged arrangement.
In an alternative pumping scheme fluoride crystals codoped with Tm3' and Yb'~ were pumped into Tm3' ions at 780 nm (see Zhang's article above). Another pumping scheme was described by A. Knupfer et al. in the article entitled "Two step pumped YLF:Tm blue upconversion laser"(Journal de PHYSIQUE IV Colloque C4, supplement au Journal de Physique III, Vol. 4, 1994, C4-403). Several oxide and fluoride crystals doped with Tm'' or Tm'' and Yb'' ions, such as Y3A15012, YA103, YLiFa, GdLiFQ, were tested to obtain blue lasing emission at the 1D2 ~ jFq transition at room temperature by two step pulsed pumping into Tm3'ions at 780 nm and 650 nm. The proper choice of the delay time between pulses of the dye laser and the Ti: sapphire laser would be very crucial for such a pumping process. Pulsed lasing operation at 453 nm could be obtained in the fluoride crystals only, while the oxide Y3A15012 and YA103 crystals were destroyed before the lasing threshold was reached.

Therefore, the function of ytterbium ions in a specified crystal host doped with two groups of rare earth active ions and the effectiveness of the energy transfer between the ions of these two groups could be changed considerably depending on the composition and the structure of the crystal host as well as the pumping scheme used. The effectiveness of the energy transfer is determined also by the concentration of the active ions due to the phenomena of "concentration quenching" where the upconversion laser output reduces as the concentration of the active ions exceeds a definite limit for each of the two groups of rare earth active ions.
Meanwhile, the choice of specific pumping scheme should not be considered to be predetermined in advance regardless of the choice of the crystal host or the specific concentration level of rare earth active ions in the latter.
All of these schemes are participants in the complicated upconversion process, which could occur in different ways because of the presence of several mechanisms at the same time affecting populating or depopulating the desired energy levels of rare earth activator ions in competition with each other.
The constituents of the crystal host and its structure form an environment for the activator ions affecting the position and structure of the energy levels. The pumping scheme provides that the pumping radiation should be used at specific wavelengths to realize conditions compatible with an energy transfer from the sensitizer ions to the activator ions in the crystal host by means of one of the preferred mechanisms and to reach, as a result, the population inversion between the desired energy levels. The energy transfer could be more efficient in a crystal host where the phonon energies are closer to the energy mismatch between the related transitions in the sensitizer and activator ions. The latter presupposes that phonon energies of the crystal host selected due to the new approach could not be as low as in a fluoride-based one.
Also it is clear that these energies should not to be too high so as to favor radiative lasing transitions as compared with non-radiative ones.
Therefore, according to the new approach the choice of the crystal host doped with two groups of rare earth active ions, takes into account factors such as the choice of concentration levels of the two groups of active ions, and the choice of the related pumping scheme, provided the energy transfer between the active ions of the two groups is coordinated. This is, in essence, the basis of the identity and purpose of the new approach and provides an opportunity to achieve the purposes of the Applications. This approach could allow the preferred upconversion mechanism that is the most compatible with the pumping conditions to be realized in providing spectral purity of produced lasing radiation and its higher efficiency. Further, this approach enables crystal hosts other than fluorides to be tested and selected as active gain media for compact blue or ultraviolet upconversion lasers.
Until now, there were some crystal hosts selected in the relevant art using a low phonon energy principle, namely, fluoride single crystals, chloride single crystals, bromide single crystals or iodide single crystals (see U.S. No.
5,684,815, supra.). As a rule, there were no oxide single crystals among them. On the one hand, oxide single crystals having been used as indicated in publications for only fluorescence in several wavelength ranges to study the physical aspects of the upconversion process or achieving purposes other than those in the Applications. So these oxide crystals could not be used as active gain media for producing a specified radiation in the blue or ultraviolet ranges. On the other hand, active gain media using oxide crystals are well known. But they have been employed to produce lasing radiation other than blue or ultraviolet (see, for example, U.S. patent No. 5,682,397). Besides, pumping schemes such as pumping into activator ions and/or operating conditions such as impulse operation, low working temperature, etc. (see, for example, A. Knupfer's article above or Silversmith's article in Appl. Phys. Lett, 1987, v.51, N 24, pp. 1977-1979, U.S.
Patent No. 5,682,397) oxide crystals have not been considered as suitable for use in upconversion lasers that have to be used in the Applications.

r CA 02314316~2000-07-21 Therefore, such oxide single crystals (their compositions, rare earth doping levels and pumping schemes) could not be applicable as active gain media in compact upconversion lasers for the purposes of the Applications.
SUN~1A,RY OF THE INVENTION
It is, therefore, an object of the present invention to provide an improved upconversion active gain medium.
Another object of the present invention is to provide an upconversion active gain medium being made of a rare earth doped crystal host capable of producing either blue or ultraviolet lasing radiation at room temperature (under normal ambient temperature operating conditions) from substantially single band infrared pumping radiation that is continuous or quasi-continuous. The active gain medium can be employed in an upconversion micro-laser, where a scheme of pumping into rare earth sensitizer ions is realized, thus, giving an opportunity for using a small size infrared pumping source having simplicity in alignment or any adjustment in the packaged arrangement.
A still other object of the present invention is to provide a rare earth doped crystal host being made of a new class of materials, the composition thereof being arranged to be compatible with the preferred upconversion mechanism so as to produce blue or ultraviolet lasing radiation at the required wavelengths efficiently. The crystal host being employed in an upconversion micro-laser provides for the stability and reproducibility of output parameters.
A further object of the present invention is to provide a rare earth doped crystal host having a structure arranged to produce the blue or ultraviolet lasing radiation being polarized.
A still further object of the present invention is to provide a rare earth doped crystal host being arranged to produce the blue or ultraviolet lasing radiation at one of the wavelengths, preferably, using, in addition, a red or an infrared exciting radiation having also been produced by the crystal host itself during the upconversion process.
In accordance with the present invention, there is provided an upconversion active gain medium capable of producing blue or ultraviolet lasing radiation from substantially single band infrared pumping radiation that is continuous or quasi-continuous. The active gain medium consists of an oxide crystal host doped with two groups of active ions, one of the groups being ytterbium ions which function as a sensitizer, the second of the groups being selected from other than ytterbium rare earth ions which function as an activator. The sensitizer ions are capable of absorbing the pumping radiation energy in the wavelength range of 915 nm to 1080 nm and transferring at least part of this pumping radiation energy to the activator ions. The activator ions are excited to at least one of their upper states so as to produce blue or ultraviolet lasing radiation when these excited activator ions relax from the upper state into one of their related low states.
The active gain medium may be employed in an upconversion micro-laser comprising a chip being made of an upconversion active gain medium having an oxide crystal host doped with two groups of active ions, one of the groups being ytterbium ions which function as a sensitizer, the second of the groups being selected from other than ytterbium rare earth ions which function as an activator. The sensitizer ions may be capable of absorbing the pumping radiation energy in the wavelength range of 915 nm to 1080 nm and transferring at least part of this pumping radiation energy to the activator ions having been excited to at least one of their upper states. As a result blue or ultraviolet lasing radiation is emitted when these excited activator ions relax from the upper state into one of their related lower states.
The micro-laser further includes a pumping source for generating the continuous or quasi-continuous single band infrared pumping radiation having at least one wavelength in the range of 915 nm to 1080 nm, the pumping source being optically coupled to the chip of upconversion active gain medium for applying the pumping radiation energy to the chip by means of a beam or a set of beams of the pumping radiation.
The micro-laser may have an optical cavity consisting of the chip of upconversion active gain medium for resonating at least the blue or ultraviolet lasing radiation produced by the chip.
The present invention is based on the results of the careful choice of a crystal host being more suitable as an upconversion active gain medium of the upconversion micro-laser for achieving the purposes of the Applications. The choice of oxide crystal host is most unexpected based upon the relevant art, where oxides have not been considered as a prospective upconversion material for producing blue or ultraviolet lasing radiation up to now.
It is only this new approach, which is responsible for revealing all the properties of an oxide crystal host that are the most essential and preferable to accomplish the foregoing objects. With respect to fluorides, oxides have higher thermal conductivity and its (greater affecting of) constituents (atoms or ions) more greatly affect the position and structure of the activator and sensitizer ions energy levels, the constituents forming an environment for the two groups of rare earth active ions. Higher thermal conductivity of an oxide crystal host makes for increasing pumping radiation intensities to increase population inversion between the desired activator ions energy levels without overheating such a host. So in all senses, proposed oxide crystal hosts proved to be of interest as a new class of materials providing realization of the preferable upconversion mechanism in producing the blue or ultraviolet lasing radiation at the desired wavelengths.
The greater environmental effect of an oxide crystal host on said sensitizer ions results in the greater overlap of their energy levels. On the one hand, this considerably reduces wavelength sensitivity when pumping into sensitizer ions as compared with pumping into activator ions and provides stability and reproducibility of said micro-laser parameters that are very important for commercial arrangements using such lasers in mentioned Applications. On the other hand, this provides more success probability in selecting related transitions in the activator and sensitizer ions that are most compatible with each other for increasing the effectiveness of energy transfer between them. The selecting of such related transitions is carried out by choosing the specific wavelengths in the wavelength range of the pumping radiation.
The greater environmental effect of an oxide crystal host on said activator ions results in a greater splitting of their states such as, for example, the ground state. As a result, some of the energy levels having a higher energy and so lower population at room temperature could be used as a low state for the lasing transition providing the population inversion in the case when the low state could not be reached from the levels having a lower energy. Therefore, using an oxide crystal host provides more beneficial conditions for increasing the population inversion for desired lasing transition and reducing the lasing threshold.
The increased effect an oxide crystal host has on the upconversion process is attributable to its higher phonon energies that may be sufficient to cover possible energy mismatch between the selected transitions in the sensitizer and activator ions. These higher. phonon energies could sustain continuous lasing operation either in blue or ultraviolet wavelength ranges at room temperature due to one of the preferred upconversion mechanisms. Therefore, the possibility of providing spectral purity of produced radiation arises not only due to resonating either blue or ultraviolet radiation in an upconversion laser optical cavity, but also due to using such oxide crystal host as an upconversion active gain medium. A chip being made of such active gain medium may be employed in an upconversion micro-laser where a scheme of pumping into rare earth sensitizer ions is realized. This provides stability and reproducibility of said micro-laser parameters as a result of reducing the influence of other upconversion mechanisms on upconversion process at the required wavelengths in said blue or ultraviolet range.
The peculiarities of affecting an oxide crystal host really display that the choice of the pumping scheme have to be made in a favor of pumping into rare earth sensitizer ions as being the most coordinated with the energy transfer from the sensitizer ions to the activator ions for increasing the effectiveness of the desired lasing transition and sustaining continuous either blue or ultraviolet lasing operation. It is very important to know that the concentration levels of ytterbium ions may be far higher in the oxide crystal host than for any activator ions without concentration quenching.
Besides, the extremely broad and strong ytterbium absorption band in infrared wavelength range and the availability of high-power semiconductor diode lasers that can operate in this range were taken into account in making the choice.
Thereupon, pumping source operating in the wavelength range of 915 nm to 980 nm could comprise a semiconductor infrared laser diode for generating a beam of the pumping radiation or a laser diode array for generating a set of beams of the pumping radiation. A small size and simplicity in alignment or any adjustment of the packaged arrangement inherent to such a pumping source are the desirable characteristics for using this source in the upconversion micro-laser to reach the objectives of the Applications. Pumping with diode lasers directly is the simplest and most convenient way to realize the pumping scheme. The scheme is implemented by applying the pumping radiation energy to the chip by means of the beam or the set of beams of pumping radiation. However, it is possible to use other pumping sources, for example, a diode pumped solid state laser for generating a beam of the pumping radiation.
In a preferred embodiment, a gallium-based oxide crystal host displays the properties of an oxide crystal host to the utmost.
The gallium-based oxide crystal host may be a garnet structure single crystal or a perovskite structure single crystal and the rare earth activator ion may be thulium. The garnet single crystal structure, or the perovskite single crystal structure is capable of producing the blue or ultraviolet lasing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in the wavelength range of 915 nm to 980 nm.
The gallium-based oxide crystal host may be a garnet single crystal structure or a perovskite single crystal structure and the rare earth activator ion may be praseodymium. Thus, the garnet single crystal structure, or the perovskite single crystal structure is capable of producing the blue lasing radiation at a wavelength in the range 480 nm to 495 nm when pumped in the wavelength range of 915 nm to 980 nm.
According to the present invention, the chip made of gallium-based oxide single crystal having a perovskite structure has the advantage of being able to produce the blue or ultraviolet lasing radiation to be polarized. In this case the chip of such a uniaxial single crystal should be oriented so its axis is transverse (perpendicular) to the upconversion micro-laser axis.
Use of an oxide crystal host also provides an opportunity to vary its composition in a wide range. This provides greater effectiveness of the energy transfer between the sensitizer and the activator ions by changing the constituents of the oxide crystal host (and their concentrations) forming the environment for the rare earth active ions. Among these constituents may be one or more elements selected from:
- the group consisting of Y, La, Gd, Lu and Yb, occupying dodecahedral sites of said garnet structure;
- the group consisting of Ga, Sc and A1, occupying octahedral sites of said garnet structure;
- the group consisting of Y, La, Gd, Lu and Yb, occupying non-octahedral sites of said perovskite structure;

- the group consisting of Ga, Sc and Al, occupying octahedral sites of said perovskite structure.
So greater effectiveness for a specified lasing transition could be achieved by producing the blue or ultraviolet lasing radiation at a required wavelength preferably by selecting the following constituents: the main elements forming along with oxygen the oxide crystal host, and their contents in the latter.
Moreover, there is also an opportunity to finely regulate the spectral purity of an oriented upconversion process of an oxide crystal host by introducing a small amount of the following additional elements into the composition of the oxide crystal host:
- one or more metal elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, T1, Pb and/or auxiliary elements selected from Ce, Er, Eu, Sc, Tb and Bi on dodecahedral sites of said garnet structure;
- one or more metal elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W and/or said subsidiary elements selected from B and Be on octahedral sites of said garnet structure;
- one or more metal elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, T1, Pb and/or ancillary elements selected from Ce, Er, Eu, Sc, Tb and Bi on non-octahedral sites of said perovskite structure;
- one or more metal elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W and/or said subsidiary elements selected from B and Be on octahedral sites of said perovskite structure.
This fine regulation could allow continuous wave operation of either blue or ultraviolet micro-lasers at room temperature to be realized in the case when it seems to be impossible using only the constituents mentioned above.
Besides, some of said additional metal elements or fluorine could be used to provide a stability of the crystal host structure to the effect of its own radiation and, therefore, a continuous lasing operation in the blue or ultraviolet wavelength range.
In the absence of these elements the oxygen vacancy or the color center defects could be created in the oxide crystal host by the absorption of ultraviolet photons producing by the excited activator ions of this host. The oxygen vacancy causes the radiation absorption in the range of 320 nm to 500 nm to be increased and, therefore, the losses at the wavelengths of lasing radiation to be increased. This leads to higher lasing thresholds that could be the reason for the falling off in the micro-laser parameters (for example, efficiency) during the upconversion process or even preventing their continuous wave operation. Such a phenomenon was also called photo-darkening. The experiments performed by the authors showed that a micro-laser having gallium-based oxide crystal host of perovskite structure with these color center defects demonstrates low blue or ultraviolet lasing emission when pumped in the range of 915 nm to 980 nm. The same experiments performed for gallium-based oxide crystal host of garnet structure with these color center defects demonstrated no emission in blue or ultraviolet range (i.e. laser threshold was not reached) until a pumping power of about 1.6 W at 960 nm.
Apart from this, said gallium-based oxide crystal host of garnet or perovskite structure co-doped with other metal elements, such as Mn, Fe, Co, showed yellow color centers after ultraviolet irradiation and demonstrated low blue or ultraviolet lacing emission when pumped in the range of 915 nm to 980 nm.
On the other hand, the availability some of said additional metal elements or fluorine in a composition of said gallium-based oxide crystal host of the garnet or perovskite structure showed no additional absorption (or coloration) after ultraviolet irradiation and demonstrated strong blue or ultraviolet lacing radiation when pumped in the range of 915 nm to 980 nm.

Therefore, by selecting constituents and additional metal elements or fluorine for a composition of said gallium-based oxide crystal host of the garnet or perovskite structure, all its properties for obtaining an upconversion could be realized to obtain said upconversion active gain medium can be accomplished and hence the foregoing objects can be achieved. Its full compositions are described as follows.
According to the one embodiment of the present invention, said oxide crystal host may be a garnet structure single crystal selected from the groups of such crystals having in each of these groups one of the following general formulae:
~ A 3_x_yH XYbY } f B ~Ga2_Z l Ga,~l2 , {A' 3_X_y_tH' xYbYMet} ~B ~ zGa2-Z_"Me' ~~ Ga3012 , {A' 3_x_y_tH' xYbyMet} ( B' sGaz_Z_~Me' "] Ga301z_qFq, where H' is said rare earth activator ion;
A' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Y, La, Gd, Lu and auxiliary elements, said auxiliary elements being capable of occupying a portion not exceeding 0.05 of all dodecahedral sites of said garnet structure; and 3~10'<x<0.45, 1~10-2<y<2.94, 0~t<0.01;
B' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Al, Sc and subsidiary elements, said subsidiary elements being capable of occupying a portion not exceeding 0.05 of all octahedral sites of said garnet structure; and 0~z<2;
Me is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0~v<0.004; 0~q<0.95.
The rare earth activator ion may be thulium. The garnet single crystal structure is capable of producing blue or ultraviolet lasing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in the wavelength range of 915 nm to 980 nm.
The rare earth activator ion may also be praseodymium and 3~10-3<x<0.09, thereby the garnet single crystal structure is capable of producing blue lasing radiation at a wavelength in the range 480 nm to 495 nm when pumped in the wavelength range of 915 nm to 980 nm.
According to the other embodiment of the present invention, said oxide crystal host may be a perovskite structure single crystal selected from the groups of such crystals having in each of these groups one of the following general formulae:
~ A ~_x_YH xYbY } Ga~_zB ~03 {A' 1_x_y-tH' XYbyMet} Gal_Z_~B' zMe' ~~3 {A' 1_X_Y_tH' xYbyMet}Gal_z_~B' ZMe' ~O3_qFq, where H' is said rare earth activator ion;
A' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Y, La, Gd, Lu and ancillary elements; said ancillary elements being capable of occupying a portion not exceeding 0.05 of all non-octahedral sites of said perovskite structure and 1~103<x<0.15, 1~102<y<0.95, 0~t<0.005;
B' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Al, Sc and subsidiary elements, said subsidiary elements being capable of occupying a portion not exceeding 0.05 of all octahedral sites of said perovskite structure; and 0~z<0.5;
Me is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0~v<0.003; 0<q<1.5.
The rare earth activator ion may be thulium. The perovskite single crystal structure is capable of producing the blue or ultraviolet lasing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in the wavelength range of 915 nm to 980 nm.

The rare earth activator ion may also be praseodymium and 1~10-3<x<0.03. The perovskite single crystal structure is capable of producing the blue lasing radiation at a wavelength in the range 480 nm to 495 nm when pumped in the wavelength range of 915 nm to 980 nm.
In particular, the selection of constituents and additional elements or fluorine for a specified composition of said oxide crystal host doped with Tm and Yb ions makes it possible achieving continuous lasing operation only at the 1G4 3H6 transition (blue radiation at a wavelength of about 485 nm) or at the 1D2 -~ 3F4 transition (blue radiation at a wavelength of about 455 nm) at room temperature. Besides, it is possible to realize continuous lasing operation only at the 1D2 ~ 3H6 transition (ultraviolet radiation at a wavelength of about 365 nm) at room temperature. Such specified compositions are discovered beneath in the detailed description of the preferred embodiment. Lasers producing said blue or ultraviolet radiation in said conditions for the purposes of applications in mentioned Applications are not known in the relevant art. In said A. Knupfer's article lasing at the 1D2 ~ 3FQ transition in the fluoride crystal host at room temperature was described but only for pulsed operation.

Further capabilities of said gallium-based oxide crystal host employed in said upconversion micro-laser according to the present invention are related with producing said blue or ultraviolet lacing radiation at one of the wavelengths preferably using an additional red or an infrared exciting radiation having been produced by said crystal host itself during the upconversion process. This exciting radiation used at the specified wavelengths creates conditions affecting the upconversion process (on the activator ions) to realize the preferred upconversion mechanism in producing the blue or ultraviolet lacing radiation. Such external conditions could be very useful in the case when the environmental affect on the oxide crystal host activator ions proves to be insufficient to sustain continuous wave operation at room temperature. It is particularly remarkable that this exciting radiation represents itself as in-cavity radiation only and doe not form any essential part of the upconversion micro-laser output radiation.
For realizing these capabilities according to a further embodiment of the present invention, the optical cavity may be made for resonating both the blue or ultraviolet lacing radiation in the wavelength ranges of 450 nm to 460 nm or 350 nm to 380 nm and a red exciting radiation in the wavelength range of 630 nm to 695 nm produced also by the chip. The red exciting radiation is associated with relaxation of relatively low energy upper states into their related intermediate states, in order to provide an additional excitation of a high upper state using the red exciting radiation and therefor to produce the blue or ultraviolet lacing radiation preferably at one of the wavelengths associated with relaxation of said relatively high upper state of the activator ions into its related low state.
According to another version of the present invention, the optical cavity may be made for resonating both the blue or ultraviolet lacing radiation in the wavelength ranges of 450 nm to 460 nm or 350 nm to 380 nm and an infrared exciting radiation in the wavelength range of 1850 nm to 2150 nm produced also by the chip, the infrared exciting radiation being associated with relaxation of one of the low states into its related ground state of the activator ions, in order to provide an additional depletion of such a low state and thereby to produce said blue or ultraviolet lacing radiation preferably at one of the wavelengths associated with relaxation of a relatively higher one of an upper state of the activator ions into the depleted low state.
In particular, the selection of said optical cavity resonating both the ultraviolet lacing radiation and the red exciting radiation makes it possible to achieve continuous lacing operation at the 1Dz ~ 3H6 transition (ultraviolet radiation at a wavelength of about 365 nm only) at room temperature. Accordingly, it is possible to realize continuous lasing operation at the 1D2 --~ 3F4 transition (blue radiation at a wavelength of about 453 nm only) at room temperature by selecting said optical cavity resonating both said blue lasing radiation and said infrared exciting radiation. Lasers producing said blue or ultraviolet radiation in said conditions for the purposes of applications in mentioned Applications are not known in the relevant art.

The objects, advantages and features of the present invention will become more apparent from the following detailed description of the preferred embodiment thereof in connection with the accompanying drawings, in which:
FIG.1 is a simplified schematic block diagram of an upconversion micro-laser using a chip of an upconversion active gain medium embodying the present invention;
FIGS.2A and 2B show energy state diagrams of Tm, Yb ions and FIG. 2C show energy state diagrams of Pr and Yb ions [and accordingly schematic views of different mechanisms of the energy transfer there between in an upconversion active gain medium according to the present invention];
FIG.3A is the room temperature fluorescence spectra of {Y2.695Pro.ooSYbo.3} [SW.89,Gao.o6Alo.o4Tlo.oo3~Ga301z crystal host when pumped into Yb ions in the wavelength range of 920 nm to 958 nm;
FIG.3B shows room temperature spectral curves describing the dependence of the upconversion fluorescence intensity of blue (488 nm) and green (541 nm) radiation from the crystal host of FIG.3A as a function of the pumping radiation wavelength;
FIG.4A is the room temperature blue fluorescence spectra of {Y1.82Luo,~Tmo.o3Ybo.4s~ ~Sc~.9Gao.osAlo.oa]Ga3G~z crystal host when pumped in the wavelength range of 918 nm to 946 nm;
FIG.4B is the room temperature ultraviolet fluorescence spectra of {Y1.3,6Tmo.lzYb,.SCao.oo4} ~SW.9Gao.~]Ga3011.992F0.008 crystal host when pumped in the wavelength range of 918 nm to 946 nm;
FIGS.5A, 5B, and 5C depict simplified schematic block diagrams of upconversion micro-lasers embodying the present invention and using a semiconductor infrared laser diode provided with a lens at its output and different optical cavity configuration;
FIGS.6A and 6B depict simplified schematic block diagrams of upconversion micro-lasers embodying the present invention and using the chip of an upconversion active gain medium in a form of crystal ball of different sizes;
FIG.7 is a simplified schematic block diagram of an upconversion micro-laser according to the present invention using the chip of an upconversion active gain medium in a form of double convex lens;
FIG.8 is a simplified schematic block diagram of an upconversion micro-laser according to the present invention using the chip of an upconversion active gain medium in a form of optical waveguide having a core with a greater refractive index and pumping with a semiconductor infrared laser diode;
FIGS.9A, and 9B depict simplified schematic block diagrams of upconversion micro-lasers according to the present invention using the chip of an upconversion active gain medium in a form of optical waveguide pumping with a laser diode array.
DETAILED DESCRIPTION OF THE PREFERRED EN~ODIMENT
With reference to FIG. 1, a simplified schematic block diagram of an upconversion micro-laser 1 used, according to the present invention, for producing blue or ultraviolet lasing radiation from single band infrared pumping radiation will be described in the following. The micro-laser 1 has a chip 10 being made of an upconversion active gain medium, an optical cavity 20 comprises chip 10 for resonating at least the blue or ultraviolet lasing radiation produced by the chip and a pumping source 30 for generating said single band infrared pumping radiation that is continuous or quasi-continuous.
The pumping source 30 should be optically coupled to the chip 10 for applying to the chip 10 the pumping radiation energy by means of a beam 31 of the pumping radiation.
Accordingly, the chip 10 has an input surface 11 optically coupled with the pumping source 30 to pass the pumping radiation through the input surface 11 into the chip 10 and an output surface 12.
The optical cavity 20 is defined by a first mirror 21 and a second mirror 22 opposing each other on a common laser axis 23 along the axis of the beam 31. The chip 10 is arranged in optical cavity 20 on the common laser axis 23 so its input surface 11 and output surface 12 are disposed adjacent the first mirror 21 and the second mirror 22, respectively. The size of the chip 10 along the laser axis 23 may be within the range of 0.1 mm to 20 mm while its size in the transverse (perpendicular) direction to this axis 23 may be of about 0.4 - 2 mm. The first mirror 21 is designed to be transmitting the pumping radiation while reflecting the blue or ultraviolet lasing radiation. The second mirror 22 is designed to be reflecting the pumping radiation and also the blue or ultraviolet lasing radiation while transmitting a portion of the blue or ultraviolet lasing radiation from the micro-laser 1. In other words, the second mirror 22 serves as an output coupler.
It is expedient that the pumping source 30 be provided with a lens 32 at its output to focus the beam 31 into a pumping region 13 of the chip 10 and to match the beam 31 size within the chip 10 with a required size of the pumping region 13 in the transverse direction to the axis 23. This provides increasing the pumping radiation intensity in pumping region 13 and optimizing the micro-laser 1 output efficiency.
This pumping region's form and size in the transverse direction to the axis 23 are determined by a desired mode (in particular, TEMoo mode) of lasing radiation being produced by micro-laser 1. Usually, matching the size of the beam 31 provides for its overlapping the desired mode. The lens 32 may be a single cylindrical or spherical lens (with a focal length of about 30-90 mm), a pair of lenses having a possibility of changing the distance between them or a combination of more than two lenses, if it is necessary to realize such a matching more precisely. When the overlapping is too much, other transverse modes of the lasing radiation could be excited. The latter may be unacceptable if the single mode operation of the micro-laser 1 is required.

The pumping source 30 operating in the wavelength range of 915 nm to 980 nm could comprise a semiconductor infrared laser diode 33 (see FIG.5A, for example) for generating the beam 31. The laser diode 33 has usually a substrate 34 for heat abstracting and making connections to a power supply unit (not shown). The other variant of the pumping source 30 may comprise a diode pumped solid state laser such as a Nd:YV04 laser operating at 1.064 ~.lzn and pumping with a diode (diode-808 nm) or similar for one for generating a beam 31 of said pumping radiation in the far part of said wavelength range of 915 nm to 1080 nm.
Said chip 10 of an upconversion active gain medium has preferably a gallium-based oxide crystal host doped with ytterbium ions which function as a sensitizer and thulium (or praseodymium) ions which function as an activator.
FIGS.2A, 2B, 2C show energy state diagrams of Tm, Yb ions (FIGS.2A, 2B) and Pr, Yb ions (FIGS.2C) and accordingly schematic views of different mechanisms of the energy transfer there between in an upconversion active gain medium according to the present invention.
The pumping radiation energy from the pumping source 30 in the wavelength range of 918 nm to 946 nm preferably (FIGS.2A, 2B) is absorbed by ytterbium ions to be excited, as a result, to their 2F5,2 state. Upconversion mechanisms depicted in FIGS.2A, 2B include transferring at least part of this pumping radiation energy to the 3H5 state of thulium ions.
The relaxation of the latter to their 'F4 state is followed by the first Yb-Tm upconversion energy transfer, where the 2F5,2 -j zF~,z transitions in Yb ions cause the Tm ions to be excited to their 3Fz,3 states . Then relaxation of Tm ions to their 3H4 state is followed by the second Yb-Tm upconversion energy transfer that populates one of Tm ions upper states - 1G4 due to the zFs,2 --~ ZF~,z transitions in Yb ions .
When Tm ions relax from their lGQupper state into one of their relating low states - 'H6, blue lasing radiation at a wavelength of about 485 nm is produced (FIG.2A). Continuous room temperature lasing operation only at the 1G4 ~ 3H6 transition could be achieved in the following conditions:
selecting a specified composition for said gallium-based oxide crystal host of the chip 10, using small thulium concentration (because of concentration quenching) and low Yb concentration, resonating only blue lasing radiation in the wavelength range of 480 nm to 495 nm by means of optical cavity 20.
When Tm ions relax from their lGQupper state into their relating intermediate 3F4state, said red exciting radiation at a wavelength of about 650 nm is produced (FIG.2B). Said red exciting radiation stimulates increasing efficiency of a cross-relaxation process involving two thulium ions in their 'F3state, one Tm ion being excited to a relatively higher 1D2 state of its upper states while the other Tm ion relaxing to its ground state - 'H6. The energy of both transitions in Tm ions conforms to photon energy at the wavelength of about 695 nm. Besides, the 1G4 ~ 3H6 transition in Tm ions is effectively quenched, providing single-line lasing operation in blue (or ultraviolet) range at the wavelength of nearby 455 nm (or nearby 365 nm accordingly).
Continuous room temperature lasing operation only at the 'F4 (or 1D2 -~ 3H6 accordingly) transition could be achieved in the following conditions: selecting a specified composition for said gallium-based oxide crystal host of the chip 10, using higher thulium concentration and higher Yb concentration (as compared with the case of FIG.2A), resonating both said blue (or ultraviolet) lasing radiation in the wavelength range of 450 nm to 460 nm (or 350 nm to 380 nm accordingly) and said red exciting radiation in the wavelength range of 630 nm to 695 nm by means of optical cavity 20.
The further capabilities in producing blue lasing radiation preferably at the wavelength of nearby 455 nm arise with an additional depletion of its relating low state - 3F4 of Tm ions using an infrared exciting radiation in the wavelength range of 1850 nm to 2150 nm (not shown in FIG.2B) produced also by the chip 10. Said infrared exciting radiation is associated with relaxation of said 3F4 state into the 'H6 state of Tm ions. Continuous room temperature lasing operation only at the 'D2 -~ 'F4 transition could be achieved in the following conditions: selecting a specified composition for said gallium-based oxide crystal host of the chip 10, resonating both said blue lasing radiation in the wavelength range of 450 nm to 460 nm and said infrared exciting radiation in the wavelength range of 1850 nm to 2150 nm by means of optical cavity 20.
An upconversion mechanism illustrated in FIG.2C
includes exciting ytterbium ions to their zF5,2 state by absorbing the pumping radiation energy from the pumping source 30 in the wavelength range of 918 nm to 958 nm preferably and transferring at least part of this pumping radiation energy to 1G4 state of praseodymium ions. The latter stimulates Yb-Pr upconversion energy transfer, where the zFs,2 -~ ZF"2 transitions in Yb ions cause the Pr ions to be excited to their lI6state.
Then relaxation of Pr ions to their 'Po state populates this upper state. Tn~hen Pr ions relax from their 'Poupper state into one of their relating low states - 3H4, blue lasing radiation at a wavelength of about 488 nm is produced. The 3Po -~ 'H5 transitions of Pr ions are also possible producing the green emission at the wavelength of nearby 541 nm.

The fluorescence spectra from the chip 10 of gallium-based oxide crystal host -Yz.s9sPro.oosYbo.s} ~SW .as~C'ao.osAlo.o9Tl.p.oo3]Ga3Glz selected according to the new approach is depicted in FIG.3A. It shows, in particular, that continuous blue lasing radiation at a wavelength of nearby 488 nm could be produced by said chip 10 at room temperature when pumped in the wavelength range of 920 nm to 958 nm. The peaks at 488 nm and at 541 nm were identified as being related to the 'Po -~ 3HQ transition and the 3Po ~ 3H5 transition of the Pr3~ ions correspondingly (see FIG.2C).
FIG.3B shows room temperature spectral curves describing the dependence of the upconversion fluorescence intensity of blue (488 nm) and green (541 nm) radiation from said crystal host as a function of the pumping radiation wavelength. A 1 Watt Ti:sapphire laser tunable over the range of 880 nm to 980 nm was used as a pumping source 30 in this case. Its output radiation was focused into the chip 10 with the lens 32 of focal length 50 mm. FIG.3B demonstrates also the possibility of choosing the specific wavelengths in said range of pumping radiation to increase the effectiveness of Yb-Pr energy transfer in a favor of blue (488 nm) upconversion mechanism.
Meanwhile, taking this into account, it is worth paying attention that more successful possibilities of providing spectral purity of radiation in the blue or ultraviolet range are related with a more careful choice of composition for the gallium-based oxide crystal host.

Apart from this, it is important to note that spectroscopic characteristics of Yb:Pr fluoride crystals and Yb:Pr oxide crystals are different. Accordingly, the optimal pumping of the Yb:Pr YLiF4 fluoride crystal host occurred at a wavelength of about 850 nm whereas for said Yb:Pr gallium-based oxide crystal host - at a wavelength of about 925 nm (see FIG.3B). This is what could be expected due to results of direct comparing the properties of the YLFQ fluoride crystal host and a gallium-based oxide crystal host with a composition closed to that of pointed out above with reference to FIG.3A.
This oxide crystal host displayed the greater ground state spl i t t ing ( DEYLiF = 415 cm 1, DEoX;de = 5 9 5 cm 1 ) and phonon energy (560 ciriland 640 crrilaccordingly). So these facts, on the one hand, illustrate the practical importance of the new approach in choosing composition of crystal hosts. On the other hand, these facts demonstrate the difficulties in this way because the known data obtained in the relevant art for other crystal hosts couldn't be often used without special researching. The latter is explained by existence of numerous effects of the crystal on the position and structure of the energy levels of rare earth active ions doping this crystal host.
It is noteworthy that the variation of composition of said gallium-based oxide crystal host in a wide range provides also an opportunity of further optimization of crystal host's properties when using in said micro-laser according to the present invention. This optimization may be related with providing spectral purity of produced radiation, reaching continuous wave lacing operation at one of the wavelength preferably or increasing effectiveness in the wavelength range (blue or ultraviolet) as a whole and so on.
The specified composition of the gallium-based oxide crystal host may include the various combinations of said constituents and additional elements as well as fluorine depending on the crystal structure and the purposes of applications in mentioned Applications.
The garnet structure single crystal may consist of {A' 3_X_yTmXYbY} [B' zGa2_z] Ga3012, where A' is Y or Lu, or Y+Lu in the ratio being about 4:1, or Y+Lu+La+Ce in the ratio being about 20:5:5:1; B' is Sc, or Sc+Al in the ratio being about 5:1, or Sc+A1+Be in the ratio being about 30:9:1; and the particular values of subscript parameters are at nearby:
0.03 for x; 0.45 for y; 1.8 for z.
The garnet structure single crystal may also consist of {Y3_X_y_tTmxYbyMet} (SczGa2_~_~Me' "] Ga3012, where Me is Ca and Ma' is Ti, or Me is Ca+Mg in the ratio being about 3:1 and Me' is Ti+Zr in the ratio being about 3:1, or Me is Ca+K in the ratio being about 3:1 and Me' is Hf+Ta in the ratio being about 3:1, or Ma is Ca+Mg+K in the ratio being about 3:1:1 and Ma' is Ti+Nb+W in the ratio being about 5:2:1; and the particular values of subscript parameters are at nearby: 0.06 for x; 0.3 for y; 0.0005 for t; 1.6995 for z; 0.0005 for v.
The garnet structure single crystal may also consist of {A' 3_X_Y_tPrXYbYCat} [B' ZGa2_ZTi~] Ga3012, where A' is Y or Lu, or Y+Lu in the ratio being about 5:1, or Y+Lu+Gd+Sc in the ratio being about 30:15:14:1; B' is Sc, or Sc+A1 in the ratio being about 2:1, or Sc+Al+B in the ratio being about 1:1:1; and the particular values of subscript parameters are at nearby: 0.005 for x; 0.1 for y; 0.002 for t; 0.06 for z; 0.002 for v.
The garnet structure single crystal may also consist of {A' 3_X_y_tTmXYbYCat} [B' ZGa2_z_~Me' ~~ Ga3012_qFQ, where A' is Y or Lu, or Y+Lu in the ratio being about 1:1, or Y+Ce+Tb in the ratio being about 116:3:1; or Y+Lu+Gd+Er+Bi in the ratio being about 40:24:24:1:1; B' is Sc, or Sc+Al in the ratio being about 19:1, or Sc+Al+Be in the ratio being about 38:1:1; and the particular values of subscript parameters are at nearby: 0.12 for x; 1.5 for y; 0.004 for t; 1.0 for z; 0 for v; 0.008 for q.
The perovskite structure single crystal may consist of {A' 1_X_YTmxYbY}Gal_zB' Z~3, where A' is La or Lu, or La+Lu in the ratio being about 4:1, or La+Lu+Y+Ce in the ratio being about 20:5:1:1; B' is Sc, or Sc+Al in the ratio being about 5:1, or Sc+Al+Be in the ratio being about 30:9:1; and the particular values of subscript parameters are at nearby: 0.01 for x; 0.15 for y; 0.28 for z.
The perovskite structure single crystal may also consist of {Lal_x_Y_tTmXYbYMet}Gal_Z_~Sc2Me'~03, where Me is Ca and Me' is Ti, or Me is Ca+Mg in the ratio being about 3:1 and Ma' is Ti+Zr in the ratio being about 3:1, or Me is Ca+K in the ratio being about 3:1 and Me' is Hf+Ta in the ratio being about 3:1, or Me is Ca+Mg+K in the ratio being about 3:1:1 and Me' is Ti+Nb+W in the ratio being about 5:2:1; and the particular values of subscript parameters are at nearby: 0.02 for x; 0.1 for y; 0.00017 for t; 0.34 for z; 0.00017 for v.
The perovskite structure single crystal may also consist of {A' 1_x-y-tPrxYbYCat}Gal_z-"B' zTl 03, where A' is La or Lu, or La+Lu in the ratio being about 5:1, or La+Lu+Gd+Sc in the ratio being about 30:15:14:1; B' is Sc, or Sc+Al in the ratio being about 2:1, or Sc+A1+B in the ratio being about 1:1:1;
and the particular values of subscript parameters are at nearby: 0.0017 for x; 0.1 for y; 0.0006 for t; 0.02 for z;
0.0006 for v.
The perovskite structure single crystal may also consist of {A' 1_x_Y_tTmxYbYCat}Gal_z_~B' ZMe' 03_qF''q, where A' is La, or La+Y in the ratio being about 15:1, or La+Ce+Tb in the ratio being about 116:3:1; or La+Lu+Gd+Er+Bi in the ratio being about 40:4:4:1:1; B' is Sc, or Sc+A1 in the ratio being about 19:1, or Sc+A1+Be in the ratio being about 38:1:1; and the particular values of subscript parameters are at nearby: 0.04 for x; 0.5 for y; 0.0003 for t; 0.2 for z; 0 for v; 0.0006 for q.
The results of a direct realization of the purposes of applications in mentioned Applications in respect of providing spectral purity of produced radiation are demonstrated in FIGS.4A, 4B.
FIG.4A shows the room temperature blue fluorescence spectra of {Yl,ezLuo.~Tmo,~jYb0.45~ ~SCi.sGao_osAlo.oa~Ga3G~2 crystal host when pumped in the wavelength range of 918 nm to 946 nm. There was examined that the peak at 455 nm is caused by the 1D2~ 3F4 lacing transition while the peak at 485 nm is related to the 1G4 -~ 3H6 lacing transition ( see thulium energy level diagram in FIG.2B).
FIG.4B shows the room temperature ultraviolet fluorescence spectra of {Y1.3~6Tmo.izYbi.sCao.ooa} ~SW
.9~'ao.nC'a30m.99aFo.oos crystal host when pumped in the wavelength range of 918 nm -946 nm. The peak at 365 nm was identified as being related to the 1Dz --~ 'H6 lacing transition of Tm3~ ions (see FIG.2B) . This crystal host revealed no additional coloration and stability of its composition in respect of its own ultraviolet radiation produced.

Well-known techniques can be used to obtain the specified composition of gallium-based oxide crystal host. In one technique fluoride crystals are prepared starting from the highest purity oxides commercially available. The host oxide is doped by diffusion at an elevated temperature (700°C) to give a host/dopant oxide mixture for subsequent conversion to fluorides (see, for example, Canadian patent Ca 2,040,557 and references in its description). It is clear, of course, that for preparing oxide crystals the last conversion is not required except for, perhaps, the case of obtaining the specific composition of gallium-based oxide crystal host using fluorine (described above in relation with FIG.4B, in particular).
The raw materials were high-purity powders characterized by the level of 99,995 for the oxide constituents (Gaz03, Scz03, Yz03, La20,, GdZ03, Luz03) , additional elements (Ce02, Erz03, Tb203, Biz03 etc. ) and dopants (Yb203, Tmz03). The purity levels of other oxide and fluoride chemical materials were of 99,95 and 99,995. All of the oxide crystals used may be grown by the Czochralski technique. The required raw materials are contained in an iridium crucible 40 mm in diameter and 40 mm high in a furnace for optimizing the doping levels of rare earth ions. An iridium crucible 80 mm in diameter and 80 mm high is used for growing crystals. A
furnace atmosphere of about 2 percent oxygen in dry nitrogen is maintained for crystal growth. The temperature of the charge is raised to the melting temperature over a period of several hours. A crystal boule pulling rate was about 1.5 mm/h using a pull boule rotation rate of 10-35 rpm to obtain crystal boules up to 40 mm in diameter and 100 - 120 mm in length. The melt fraction crystallized was between 10 and 30 percent. Such a boule could be used for preparing many chips for micro-lasers 1. It is possible to develop the technology of growing gallium-based oxide crystals up to 100 mm in diameter by using a crucible 20 cm in diameter and 20 cm high. The large size crystals are the basis of a low price commercial production of said upconversion blue or ultraviolet micro-lasers.
The Czochralski technique allows continuous viewing of the crystal growing, and gives the opportunity of aborting this process and restarting it again if the problems of changing parameters or developing a polycrystalline structure are arisen. For the Czochralski growing the gallium-based oxide crystals of specific composition using fluorine, the raw materials have to contain additionally HF as the source of F-.
The alternative variant using only oxide raw materials in the melt but a reactive atmosphere containing anhydrous HF, as the source of F-, and CO with helium as a carrier gas is possible too. This variant was also discovered in detail in mentioned patent Ca 2,040,557.

It is particularly remarkable that the chip 10 made of said gallium-based oxide crystal may be shaped most diversely and used in micro-lasers 1 having various optical cavity configurations. This allows reaching the specific purposes of applications in mentioned Applications and displays the great practical value of the present invention.
In one embodiment of the present invention the chip may be made in a form of parallelepiped (FIG.5A), polished facets of which that are transversal (perpendicular) to said laser axis 23 being said input 11 and output 12 surfaces of the chip 10.
In other embodiment of the present invention the chip 10 may be made in a form extended along said laser axis and provided with a flat and a convex polished end surfaces as said its input 11 and output 12 surfaces respectively (FIG.5B).
For both embodiments the first 21 and the second 22 mirrors may comprise dielectric coatings applied directly to the input 11 and output 12 surfaces of the chip 10 respectively therefore forming a monolithic chip-optical cavity structure. The latter improves the stability and robustness of the optical cavity 20, which would be important in a commercial micro-laser. The dielectric coating as the first mirror 21 may be made to have a transmissivity in the range of 70~ to 95~ for the pumping radiation (in the wavelength range of 915 nm to 980 nm) and a reflectivity in the range of 99,8 to 99,99 for the blue or ultraviolet lacing radiation (at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm respectively).
Accordingly, the dielectric coating as the second mirror 22 may be made to have a reflectivity in the range of 70g to 95~
for said pumping radiation (in the wavelength range of 915 nm to 980 nm) and in the range of 60~ to 99,8 for said blue or ultraviolet lacing radiation (at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm respectively). Such dielectric coatings are well known in the art and commercially available.
Such a monolithic chip-optical cavity structure is a simplest resonator configuration providing an efficient lasing operation for said pumping radiation power of about 0.5 - 2 W
being applied to the chip 10 by means of the beam 31 of said pumping radiation from the pumping source 30. The latter comprises a semiconductor infrared laser diode 33 on the substrate 34 and is provided with the lens 32.
In case of the plane-plane cavity structure shown in FIG.5A a generated mode of lacing radiation is determined by the thermal lens induced in the chip 10 of said upconversion active gain medium. When the chip 10 is made of {Y3_X_ yTmxYbY} [SczGa2_z]Ga301z single crystal, this thermal-induced lens acts as a mirror with a radius of about 30-35 mm for pumping radiation power of 2 W. Such a thermal lens provides a stable resonator configuration with good overlapping the pumping region 13 of said generated mode by the beam 31 within the chip 10. For a generated mode waist of 70 ~m and the pumping radiation power of 1W the thermal lens gives a constant beam diameter for a cavity length up to 6 mm. So an optimum size of the chip 10 along the laser axis 23 in this variant may be within the range of about 0.1 mm to 6 mm. The chip 10 may be longer if it is made doped with said active ions only in its middle region to be pumped by said infrared pumping radiation remaining undoped the rest parts of the chip 10, between which said middle region being disposed along said laser axis 23.
This chip 10 configuration may be useful for the micro-laser of a higher power where the greater heat abstraction from the chip 10 is required.
The plane-concave cavity structure shown in FIG.5B
has a stable resonator configuration and differs from that of in FIG.5A by the forms of the chip 10 and the second mirror 22 used. The convex output end surface 12 of the chip 10 has the radius of curvature of 30 mm to 40 mm.
A still further embodiment of the present invention comprises, on the contrary, a concave-plane cavity structure shown in FIG.5C. The first 21 and the second 22 mirrors are configured to have a spherical and a flat surfaces respectively therefore forming a stable resonator (optical cavity) configuration. The chip 10 is made with flat polished end surfaces being said its input 11 and output 12 surfaces and disposed on the laser axis 23 separately with respect to the mirrors 21 and 22. The better heat abstraction may have that variant, where the chip 10 is disposed between two undoped crystals 14 and 15 along the laser axis 23. Each undoped crystal 14 (or 15) has a flat polished end surface to provide a good thermal and optical contact with the input surface 11 (output surface 12) of the chip 10. A lens cement may be used to improve said optical contact by reducing the unwanted reflections at the interfaces between them and to integrate the chip 10 with undoped crystals 14 and 15 into a single construction. The other polished end surface of undoped crystal 14 (or 15) is made spherical (flat) for applying thereto directly a dielectric coating as said first mirror 21 (second mirror 22). The latter has the same parameters as the corresponding mirror in FIG.5B mentioned above.
In other variant the chip 10 may be disposed between such undoped crystals 14 and 15 along the laser axis 23 but separately from them. The same mirrors 21 and 22 (as in the previous variant) are applied to said undoped crystals 14 and 15 respectively, at least one of which serving as a polarizer.

This allows producing said blue or ultraviolet lasing radiation to be polarized.
Yet another embodiment of the present invention comprises a monolithic chip-optical cavity structure with the stable resonator (optical cavity) configuration shown in FIGS.6A, 6B. The chip 10 is made in a form of polished crystal ball disposed on the laser axis 23. Such crystal ball may have a diameter within the range of 0.1 mm to 6 mm. The first 21 and the second 22 mirrors comprise dielectric coatings applied directly to the parts of said crystal ball surface opposing each other on the laser axis 23 along the beam 31 axis, said parts being the input 11 and output 12 surfaces of the chip 10 respectively. The first mirror 21 is designed to be transmitting said pumping radiation while reflecting said blue or ultraviolet lasing radiation as well as said red exciting radiation. The second mirror 22 is designed to be reflecting said pumping radiation and said blue or ultraviolet lasing radiation as well as said red exciting radiation while transmitting a portion of said blue or ultraviolet lasing radiation, i.e. the second mirror 22 serves as an output coupler.
The variant of FIG.6B is structurally and functionally similar to that of FIG.6A except for the fewer size of the chip 10. So the semiconductor infrared laser diode 33 may be directly coupled to the chip 10 without focusing lens 32 due to the greater surface curvature of said crystal ball. The emission facet of the laser diode 33 may be as close as 30-50 ~m from the crystal ball in this case. On the other hand, a suitable heat sink 16 may be used for the small ball 10, if desired, to provide abstracting the excessive heat.
This embodiment differs from the upconversion laser material described in US patent No 5,684,815 in using:
- other crystal host - gallium-based oxide single crystal of a garnet structure;
- the composition of {A' 3_X-Y-~TmxYbyMet} [B' ZGa2_Z] Ga301z_qFq being arranged to be compatible with the upconversion mechanism to produce said blue or ultraviolet lasing radiation at one of the wavelengths preferably;
- other pumping scheme with pumping into Yb sensitizer ions as being the most coordinated with said energy transfer instead of pumping into Tm ions;
- the semiconductor infrared laser diode as the pumping source operating in the wavelength range of 915 nm to 980 nm instead of a dye laser at 650 nm;
- the outer resonator (optical cavity) defined by the first mirror 21 and the second mirror 22 opposing each other on the laser axis 23 along the beam 31 axis;
- the first mirror 21 that has a transmissivity in the range of 70~ to 95~ for said pumping radiation (in the wavelength range of 915 nm to 980 nm) and a reflectivity in the range of 99,8 to 99,99 for said blue or ultraviolet lasing radiation (at one of the wavelengths in the ranges of 450 nm to 460 nm or 350 nm to 380 nm respectively) as well as in the range of 99,8 to 99,99 for said red exciting radiation (in the wavelength range of 630 nm to 695 nm); and - the second mirror 22 that has a reflectivity in the range of 70~ to 95~ for said pumping radiation (in the wavelength range of 915 nm to 980 nm), in the range of 60~ to 99,8 for said blue or ultraviolet lasing radiation (at one of the wavelengths in the ranges of 450 nm to 460 nm or 350 nm to 380 nm respectively) and in the range of 99,8 to 99,99 for said red exciting radiation (in the wavelength range of 630 nm to 695 nm).
All of this allows providing spectral purity of produced radiation in contrast to the micro-sphere resonator construction used in US patent No 5,684,815.
A further embodiment of the present invention shown in FIG.7 uses the same laser diode 33 - chip 10 configuration as that of FIG.6B. At the same time, the chip 10 having polished spherical surfaces as said input 11 and output 12 surfaces are made in a form of double convex lens. The chip is disposed on the laser axis 23 separately with respect to the first 21 and second 22 mirrors that are configured both to have spherical surfaces, therefore forming a stable optical cavity configuration. The chip 10 being arranged in the optical cavity 20 so its input 11 and output 12 surfaces to be disposed adjacent the first 21 and second 22 mirrors respectively. The latter has the same parameters as the corresponding mirrors in FIG.6A mentioned above. The chip 10 has a size along the laser axis 23 within the range of 0.3 mm to 6 mm. The first 21 and second 22 mirrors have the same radius of curvature as the corresponding surfaces of the chip in the range of 26 mm to 46 mm.
A still further embodiment of the present invention shown in FIG. 8 uses the same laser diode 33 - chip 10 configuration as that of FIG.6B. Meanwhile, the chip 10 is made in a form of an optical waveguide extended along the laser axis 23 and has polished flat end faces being the input 11 and output 12 surfaces of said chip. The first 21 and second 22 mirrors comprise dielectric coatings applied directly to the input 11 and output 12 surfaces of the chip 10 respectively therefore forming a monolithic chip-optical cavity structure. The first 21 and second 22 mirrors have the same parameters as the respective mirrors in FIG.5A mentioned above. The optical waveguide 10 may be extended and has a core (not shown in FIG.8) with a greater refractive index than that of an outer part of the optical waveguide 10 surrounding said core. The small core radius allows high pumping radiation intensities with modest pumping powers from the semiconductor infrared laser diode 33. A heat sink 16 may be used, if desired, to provide abstracting the excessive heat.
A yet further embodiment of the present invention is shown in FIGS.9A, 9B where the optical cavity 20 is defined by the first mirror 21 and the second mirror 22 opposing each other on the common laser axis 23. The chip 10 of an upconversion active gain medium is made in a form of an optical waveguide extended along the laser axis 23 and has a side face optically coupled with the pumping source 30 to serve as an input surface 11 through which said pumping radiation pass into the chip 10. The latter has also the first polished flat end face 17 and the second polished flat end face being the output surface 12 of the chip 10. The chip 10 is arranged in the optical cavity 20 so its first 17 and second 12 end faces to be disposed adjacent the first 21 and second 22 mirrors respectively.
The pumping source 30 for generating a set of beams of said pumping radiation in the wavelength range of 915 nm to 980 nm comprises a linear infrared laser diode array 35 arranged along the side face 11 of the chip 10 at a specified distance from this face 11. The emission facet of each laser diode 33 of the array 35 may be as close as 30-100 Elm from the input surface 11. So each laser diode 33 may be optically coupled with the specified part of the input surface 11 by means of the corresponding beam 31, the axis of which being transversal or almost transversal (within the range of several degrees) to the laser axis 23. All of said parts of the input surface 11 are disposed along the laser axis 23 with overlapping each other, if necessary. The laser diode array 35 has usually a single substrate 36 for heat abstracting and making connections to a power supply unit (not shown in FIGS.9A, 9B). A heat sink 16 may be used, if desired, to provide abstracting the excessive heat.
In the variant of FIG.9A of this embodiment the first 21 and second 22 mirrors comprise dielectric coatings applied directly to the first 17 and second 12 end faces of the chip 10 respectively therefore forming a monolithic chip-optical cavity structure.
In the variant of FIG.9B of this embodiment the first mirror 21 comprises dielectric coating applied directly to the first end face 17 of the chip 10, while the second mirror 22 is disposed separately with respect to the chip 10 and configured to have a spherical surface therefore forming a stable optical cavity configuration.
For both variants of this embodiment the reflectivity of the first 21 and second 22 mirrors for said blue or ultraviolet lasing radiation (at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm respectively) is in the range of 99,8 to 99,99 and in the range of 60~ to 99,8 respectively.
The additional dielectric coatings 18 and 19 may be applied directly to the input surface 11 and an opposite side face of the chip 10. These coatings 18 and 19 have a transmissivity in the range of 70~ to 95~ and a reflectivity in the range of 70~ to 95~ respectively for said pumping radiation (in the wavelength range of 915 nm to 980 nm) to increase its absorbing by the chip 10. At the same time both coatings 18 and 19 may have a reflectivity in the range of 99,8 to 99,99 for the blue or ultraviolet lacing radiation (at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm respectively) for improving efficiency in continuous wave lacing operation.
One more embodiment of the present invention differs from that of shown in FIG.9A only in the form of the chip 10 and the properties of the mirror 21 and 22. The references concerning this embodiment may be made also with respect to this FIG.9A.
The chip 10 of an upconversion active gain medium is made in a form extended along the laser axis 23. The chip 10 has a side face optically coupled with the pumping source 30 to serve as an input surface 11, a first polished flat end face 17 and a second polished flat end face 12 being an output surface of the chip 10.
The first mirror 21 has a reflectivity in the range of 99,8 to 99,99 for said blue or ultraviolet lasing radiation (at one of the wavelengths in the ranges of 450 nm to 460 nm or 350 nm to 380 nm respectively) as well as in the range of 99,8 to 99,99 for said infrared exciting radiation (in the wavelength range of 1850 nm to 2150 nm). The second mirror 22 has a reflectivity in the range of 60~ to 99,8 for said blue or ultraviolet lasing radiation (at one of the wavelengths in the ranges of 450 nm to 460 nm or 350 nm to 380 nm respectively) and in the range of 99,8 to 99,99 for said infrared exciting radiation (in the wavelength range of 1850 nm to 2150 nm).
The herein-proposed upconversion micro-laser 1 functions as follows. Single band infrared pumping radiation having at least one wavelength in the range of 915 nm to 1080 nm (preferably in the range of 915 nm to 980 nm) is generated by the pumping source 30 in the form of a beam 31 or a set of beams 31. When only one beam 31 is used, said pumping radiation passes through the focusing lens 32 (for micro-laser's embodiments of FIGS.1, 5A-6A) or directly (for micro-laser's embodiments of FIGS.68, 7,8) along the laser axis 23 and penetrates the optical cavity 20 through the first mirror 21 for applying to the input surface 11 of the chip 10. When a set of beams 31 is used (for micro-laser's embodiments of FIGS.9A, 9B), said pumping radiation passes through the coating 18 (FIG.9B) or directly (FIG.9A) into the optical cavity 20 for applying to the input surface 11 of the chip 10.
Then, the pumping radiation that has transmitted through the input surface 11 into the chip 10 is absorbed to a greatest extent by ytterbium ions, which function as a sensitizer, to provide the energy transfer to the activator ions as was described above in connection with FIGS.2A-3B, for example.
With this, said blue or ultraviolet lacing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm produced by the chip 10 appears from the second mirror 22 being an output coupler of the optical cavity 20, when the lacing threshold is exceeded.
The foregoing and furher variants of the embodiments of the the present invention stated before are by no means exhaustive. Modifications and changes, without, doubt are possible. Thus, for instance, there are practicable embodiments similar to that of FIG.5A except the composition of the chip's single crystal and the properties of the mirror 21 and 22. The garnet structure single crystal doped additionally with fluorine to have the composition of the general formula {A' 3_X_y_tTmXYbYMet} [B' zGa2_Z] Ga3012_qFq has demonstrated the lowest room temperature threshold when producing said blue or ultraviolet lacing radiation at one wavelength preferably using said red or infrared exciting radiation. The latter permits achieving continuous wave upconversion lasing operation and high efficiency at the desired wavelength without self-terminating for the following examples describing properties of the mirror 21 and the mirror 22.
Example 1 The first mirror 21 has a transmissivity in the range of 70~ to 95~ for the pumping radiation (in the wavelength range of 915 nm to 980 nm) and a reflectivity in the range of 99,8 to 99,99 for the ultraviolet lasing radiation (in the wavelength range of 350 nm to 380 nm) as well as in the range of 99,8 to 99,99 for said red exciting radiation (in the wavelength range of 630 nm to 695 nm). The second mirror 22 has a reflectivity in the range of 70~ to 95~
for the pumping radiation (in the wavelength range of 915 nm to 980 nm), in the range of 60~ to 99,8 for the ultraviolet lasing radiation (in the wavelength range of 350 nm to 380 nm) and in the range of 99,8 to 99,99 for said red exciting radiation (in the wavelength range of 630 nm to 695 nm). In this case the ultraviolet lasing radiation at 366 nm can be obtained with the room temperature lasing threshold of about 140 mW.
Example 2 Both mirrors' properties are similar to that of example 1 with respect to the pumping radiation and said red exciting radiation. The first mirror 21 has a transmissivity in the range of 70~ to 95~ for the pumping radiation and a reflectivity in the range of 99,8 to 99,99 for the blue lacing radiation (in the wavelength range of 450 nm to 460 nm) as well as in the range of 99,8 to 99,99 for said red exciting radiation. The second mirror 22 has a reflectivity in the range of 70~ to 95~ for the pumping radiation, in the range of 60~ to 99,8 for the blue lacing radiation (in the wavelength range of 450 nm to 460 nm) and in the range of 99,8 to 99,99 for said red exciting radiation. Then, the blue lacing radiation at 455 nm can be obtained with the room temperature lacing threshold of about 50 mW.
Examp 1 a 3 Both mirrors' properties are similar to that of example 1 with respect to the pumping radiation and blue lacing radiation.
The first mirror 21 has a transmissivity in the range of 70~ to 95~ for the pumping radiation and a reflectivity in the range of 99,8 to 99,99 for the blue lacing radiation as well as in the range of 99,8 to 99,99 for said infrared exciting radiation (in the wavelength range of 1850 nm to 2150 nm). The second mirror 22 has a reflectivity in the range of 70~ to 95~ for the pumping radiation, in the range of 60~ to 99,8 for the blue lacing radiation and in the range of 99,8 to 99,99 for said infrared exciting radiation (in the wavelength range of 1850 nm to 2150 nm). Then, the blue lasing radiation at 455 nm can be obtained with the room temperature lasing threshold of about 70 mW.
It is understood that the above-described embodiments of the proposed invention can by no means be regarded as limiting the present invention, but are to be interpreted as illustrative to promote understanding of its essence, and that various changes and improvements may be effected therein by those skilled in the art without departing from the scope or spirit of this invention as defined in the appended claims.

Claims

1. ~An upconversion active gain medium capable of producing blue or ultraviolet lasing radiation from substantially single band infrared pumping radiation that is continuous or quasi-continuous comprising:
an oxide crystal host doped with two groups of active ions, one of said groups being ytterbium ions which function as sensitizer, the second of said groups being selected from other than ytterbium rare earth ions which function as an activator, said sensitizer ions being capable of absorbing said pumping radiation energy in the wavelength range of 915 nm to 1080 nm and transferring at least part of this pumping radiation energy to said activator ions having been excited to at least one of their upper states so as to produce said blue or ultraviolet lasing radiation when these excited activator ions relax from said upper state into one of their relating low states.

2. ~An upconversion active gain medium according to claim 1, wherein said oxide crystal host is a gallium-based oxide crystal host.

3. ~An upconversion active gain medium according to claim 2, wherein said gallium-based oxide crystal host is a garnet structure single crystal and said rare earth activator ion is thulium, thereby said garnet structure single crystal is capable of producing said blue or ultraviolet lasing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in the wavelength range of 915 nm to 980 nm.

4. ~An upconversion active gain medium according to claim 2, wherein said gallium-based oxide crystal host is a garnet structure single crystal and said rare earth activator ion is praseodymium, thereby said garnet structure single crystal is capable of producing said blue lasing radiation at a wavelength in the range 480 nm to 495 nm when pumped in the wavelength range of 915 nm to 980 nm.

5. ~An upconversion active gain medium according to claim 1, wherein said oxide crystal host is a garnet structure single crystal selected from the groups of such crystals having in each of these groups one of the following general formulae:
{A' 3-x-y H' x Yb y} [B' z Ga2-z]Ga3O12, {A' 3-x-y-t H' x Yb y Me t} [B' =Ga2-z-v Me' v] Ga3O12 , {A' 3-x-y-t H' x Yb y Me t} [B' z Ga2-z-v Me' v] Ga3O12-q F q, where H' is said rare earth activator ion;

A' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Y, La, Gd, Lu and auxiliary elements, said auxiliary elements being capable of occupying a portion not exceeding 0.05 of all dodecahedral sites of said garnet structure; and 3.cndot.10 -3 < x < 0.45, 1.cndot.10 -2 < y < 2.94, 0.cndot.t < 0.01;
B' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Al, Sc and subsidiary elements, said subsidiary elements being capable of occupying a portion not exceeding 0.05 of all octahedral sites of said garnet structure; and 0 .cndot. z < 2;
Me is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0 .cndot. v < 0.004; 0 .cndot. q <
0.95.

6. ~An upconversion active gain medium according to claim 5, wherein said auxiliary elements are Ce, Er, Eu, Sc, Tb and Bi and said subsidiary elements are B and Be.

7. ~An upconversion active gain medium according to claim 5, wherein said garnet structure single crystal consists of {A' 3-x-y Tm x Yb y} [B' z Ga 2-z] Ga3O12, where A' is Y or Lu, or Y+Lu in the ratio being about 4:1, or Y+Lu+La+Ce in the ratio being about 20:5:5:1; B' is Sc, or Sc+Al in the ratio being about 5:1, or Sc+Al+Be in the ratio being about 30:9:1; and the particular values of subscript parameters are at nearby:
0.03 for x; 0.45 for y; 1.8 for z.

8. ~An upconversion active gain medium according to claim 5, wherein said garnet structure single crystal consists of {Y 3-x-y-t Tm x Yb y Me t} [Sc z Ga2-z-v Me'v] Ga3O12, where Me is Ca and Me' is Ti, or Me is Ca+Mg in the ratio being about 3:1 and Me' is Ti+Zr in the ratio being about 3:1, or Ma is Ca+K in the ratio being about 3:1 and Me' is Hf+Ta in the ratio being about 3:1, or Me is Ca+Mg+K in the ratio being about 3:1:1 and Me' is Ti+Nb+W in the ratio being about 5:2:1; and the particular values of subscript parameters are at nearby: 0.06 for x; 0.3 for y; 0.0005 for t; 1.6995 for z; 0.0005 for v.

9. ~An upconversion active gain medium according to claim 5, wherein said garnet structure single crystal consists of {A'3-x-y-t Pr x Yb y Ca t} [B'z Ga 2-x Ti v] Ga3O12, where A' is Y or Lu, or Y+Lu in the ratio being about 5:1, or Y+Lu+Gd+Sc in the ratio being about 30:15:14:1; B' is Sc, or Sc+Al in the ratio being about 2:1, or Sc+Al+B in the ratio being about 1:1:1; and the particular values of subscript parameters are at nearby:
0.005 for x; 0.1 for y; 0.002 for t; 0.06 for z; 0.002 for v.

10. An upconversion active gain medium according to claim 5, wherein said garnet structure single crystal consists of {A' 3-x-y-t Tm x Yb y Ca t} [B' z Ga2-z-v Me' v] Ga3O12-q F q, where A' is Y or Lu, or Y+Lu in the ratio being about 1:1, or Y+Ce+Tb in the ratio being about 116:3:1;
or Y+Lu+Gd+Er+Bi in the ratio being about 40:24:24:1:1;

B' is Sc, or Sc+Al in the ratio being about 19:1, or Sc+Al+Be in the ratio being about 38:1:1; and the particular values of subscript parameters are at nearby:
0.12 for x; 1.5 for y; 0.004 for t; 1.0 for z; 0 for v; 0.008 for q.

11. An upconversion active gain medium according to claim 5, wherein said garnet structure single crystal capable of producing said blue lasing radiation at one or more wavelengths in the range of 450 nm to 495 nm at room temperature when pumped in the wavelength range of 918 nm to 946 nm consists of {Y3-a-x-y Lu a Tm x Yb y} [Sc z-b Al b Ga2-z] Ga3O12, where the particular values of subscript parameters are at nearby:
0.7 for a; 0.03 for x; 0.45 for y; 0.04 for b; 1.94 for z.

12. An upconversion active gain medium according to claim 5, wherein said garnet structure single crystal capable of producing said blue lasing radiation at a wavelength of nearby 488 nm at room when pumped in the wavelength range of 920 nm to 958 nm consists of {Y3-x-y-t Pr x Yb y Me t} [Sc z-b Al b Ga2-z~
v Ti v] Ga3O12, where the particular values of subscript parameters are at nearby: 0.005 for x; 0.3 for y; 0.00 for t; 0.04 for b;
1.937 for z; 0.003 for v.

13. An upconversion active gain medium according to claim 5, wherein said garnet structure single crystal capable of producing said ultraviolet lasing radiation at a wavelength in a range of 350 nm to 380 nm at room temperature when pumped in the wavelength range of 918 nm to 946 nm consists of {Y3-x-y-t Tm x Yb y Ca t} [Sc z Ga2_z-v Me'v]Ga3O12-q F q, where the particular values of subscript parameters are correspondingly at nearby: 0.12 for x; 1.50 for y; 0.004 for t; 1.90 for z; 0.00 for v; 0.008 for q.

14. An upconversion active gain medium according to claim 2, wherein said gallium-based oxide crystal host is a perovskite structure single crystal and said rare earth activator ion is thulium, thereby said perovskite structure single crystal is capable of producing said blue or ultraviolet lasing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in the wavelength range of 915 nm to 980 nm.

15. An upconversion active gain medium according to claim 2, wherein said gallium-based oxide crystal host is a perovskite structure single crystal and said rare earth activator ion is praseodymium, thereby said perovskite structure single crystal is capable of producing said blue lasing radiation at a wavelength in the range 480 nm to 495 nm when pumped in the wavelength range of 915 nm to 980 nm.

16. An upconversion active gain medium according to claim 1, wherein said oxide crystal host is a perovskite structure single crystal selected from the groups of such crystals having in each of these groups one of the following general formulae:

{A'1-x-y H' x Yb y}Ga1-z B' z O3 {A'1-x-y-t H' x Yb y Me t}Ga1-z-v B' z Me' v O3 {A'1-x-y-t H' x Yb y Me t}Ga1-z-v B' z Me' v O3-q Fq, where H' is said rare earth activator ion;
A' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Y, La, Gd, Lu and ancillary elements; said ancillary elements being capable of occupying a portion not exceeding 0.05 of all non-octahedral sites of said perovskite structure and 1~10 -3<x<0.15, 1~10 -2<y<0.95, 0~t<0.005;
B ' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Al, Sc and subsidiary elements, said subsidiary elements being capable of occupying a portion not exceeding 0.05 of all octahedral sites of said perovskite structure; and 0~z<0.5;
Me is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0~v<0.003; 0<q<1.5.

17. An upconversion active gain medium according to claim 16, wherein said ancillary elements are Ce, Er, Eu, Sc, Tb and Bi and said subsidiary elements are B and Be.

18. An upconversion active gain medium according to claim 16, wherein said perovskite structure single crystal consists of {A'1-x-y Tm x Yb y} Ga1-z B' z O3, where A' is La or Lu, or La+Lu in the ratio being about 4:1, or La+Lu+Y+Ce in the ratio being about 20:5:1:1;
B' is Sc, or Sc+Al in the ratio being about 5:1, or Sc+Al+Be in the ratio being about 30:9:1; and the particular values of subscript parameters are at nearby:
0.01 for x; 0.15 for y; 0.28 for z.

19. An upconversion active gain medium according to claim 16, wherein said perovskite structure single crystal consists of {La1-x-y-t Tm x Yb y Me t}Ga1-z-v Sc z Me' v O3, where Me is Ca and Me' is Ti, or Me is Ca+Mg in the ratio being about 3:1 and Me' is Ti+Zr in the ratio being about 3:1, or Me is Ca+K in the ratio being about 3:1 and Me' is Hf+Ta in the ratio being about 3:1, or Me is Ca+Mg+K in the ratio being about 3:1:1 and Me' is Ti+Nb+W in the ratio being about 5:2:1;
and the particular values of subscript parameters are at nearby: 0.02 for x; 0.1 for y; 0.00017 for t; 0.34 for z;
0.00017 for v.

20. An upconversion active gain medium according to claim 16, wherein said perovskite structure single crystal consists of {A'1-x-y-t Pr x Yb y Ca t}Ga1-z-v B' z Ti v O3, where A' is La or Lu, or La+Lu in the ratio being about 5:1, or La+Lu+Gd+Sc in the ratio being about 30:15:14:1;
B' is Sc, or Sc+Al in the ratio being about 2:1, or Sc+Al+B in the ratio being about 1:1:1; and the particular values of subscript parameters are at nearby: 0.0017 for x;
0.1 for y; 0.0006 for t; 0.02 for z; 0.0006 for v.

21. An upconversion active gain medium according to claim 16, wherein said perovskite structure single crystal consists of {A'1-x-y-t Tm x Yb y Ca t}Ga1-z-v B' z Me' v O3-q F q, where A' is La, or La+Y in the ratio being about 15:1, or La+Ce+Tb in the ratio being about 116:3:1;
or La+Lu+Gd+Er+Bi in the ratio being about 40:4:4:1:1;
B' is Sc, or Sc+Al in the ratio being about 19:1, or Sc+Al+Be in the ratio being about 38:1:1; and the particular values of subscript parameters are at nearby:
0.04 for x; 0.5 for y; 0.0003 for t; 0.2 for z; 0 for v;
0.0006 for q.

22. An upconversion micro-laser for producing blue or ultraviolet lasing radiation from substantially single band infrared pumping radiation that is continuous or quasi-continuous comprising:
~ a chip being made of an upconversion active gain medium having an oxide crystal host doped with two groups of active ions, one of said groups being ytterbium ions which function as a sensitizer, the second of said groups being selected from other than ytterbium rare earth ions which function as an activator, said sensitizer ions being capable of absorbing said pumping radiation energy in the wavelength range of 915 nm to 1080 nm and transferring at least part of this pumping radiation energy to said activator ions having been excited to at least one of their upper states so as to produce said blue or ultraviolet lasing radiation when these excited activator ions relax from said upper state into one of their relating low states.
.cndot. a pumping source for generating said continuous or quasi-continuous single band infrared pumping radiation having at least one wavelength in said range of 915 nm to 1080 nm, said pumping source being optically coupled to said chip of upconversion active gain medium for applying said pumping radiation energy to said chip by means of a beam or a set of beams of said pumping radiation;
~ an optical cavity comprising said chip of upconversion active gain medium for resonating at least said blue or ultraviolet lasing radiation produced by said chip.

23. An upconversion active gain medium according to claim 22, wherein said oxide crystal host is a gallium-based oxide crystal host.

24. An upconversion micro-laser according to claim 23, wherein said gallium-based oxide crystal host is a garnet structure single crystal selected from the groups of such crystals having in each of these groups one of the following general formulae:
{A'3-x-y H' x Yb y} [B' z Ga2-z] Ga3O12, {A'3-x-y-t H' x Yb y Me t} [B' z Ga2-z-v Me' v] Ga3O12, {A'3-x-y-t H' x Yb y Me t} [B' z Ga2-z-v Me' v] Ga3O12-q F q, where H' is said rare earth activator ion;
A' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Y, La, Gd, Lu and auxiliary elements, said auxiliary elements being capable of occupying a portion not exceeding 0.05 of all dodecahedral sites of said garnet structure; and 3~10 -3<x<0.45, 1~10 -2<y<2.94, 0~t<0.01;
B' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of A1, Sc and subsidiary elements, said subsidiary elements being capable of occupying a portion not exceeding 0.05 of all octahedral sites of said garnet structure; and 0~z<2;
Me is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0~v<0.004; 0~q<0.95.

25. An upconversion micro-laser according to claim 24, wherein said auxiliary elements are Ce, Er, Eu, Sc, Tb and Bi and said subsidiary elements are B and Be.

26. An upconversion micro-laser according to claim 24, wherein said rare earth activator ion is thulium, thereby said garnet structure single crystal is capable of producing said blue or ultraviolet lasing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in the wavelength range of 915 nm to 980 nm.

27. An upconversion micro-laser according to claim 24, wherein said rare earth activator ion is praseodymium and 3~10~
3<x<0.09, thereby said garnet structure single crystal is capable of producing said blue lasing radiation at a wavelength in the range 480 nm to 495 nm when pumped in the wavelength range of 915 nm to 980 nm.

28. An upconversion micro-laser according to claim 23, wherein said gallium-based oxide crystal host is a perovskite structure single crystal selected from the groups of such crystals having in each of these groups one of the following general formulae:

{A'1-x-y H' x Yb y} Ga1-z B z O3 {A'1-x-y-t H' x Yb y Me t}Ga1-z-v B' z Me' v O3 {A'1-x-y-t H' x Yb y Me t}Ga1-z-v B' z Me' v O3-q F9, where H' is said rare earth activator ion;
A' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Y, La, Gd, Lu and ancillary elements; said ancillary elements being capable of occupying a portion not exceeding 0.05 of all non-octahedral sites of said perovskite structure and 1~10 -3<x<0.15, 1~10 -2<y<0.95, 0~t<0.005;
B' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Al, Sc and subsidiary elements, said subsidiary elements being capable of occupying a portion not exceeding 0.05 of all octahedral sites of said perovskite structure; and 0~z<0.5;
Me is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of more than two elements selected from the group consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0~v<0.003; 0<q <1.5.

29. An upconversion micro-laser according to claim 28, wherein said ancillary elements are Ce, Er, Eu, Sc, Tb and Bi and said subsidiary elements are B and Be.

30. An upconversion micro-laser according to claim 28, wherein said rare earth activator ion is thulium, thereby said perovskite structure single crystal is capable of producing said blue or ultraviolet lasing radiation at one or more wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in the wavelength range of 915 nm to 980 nm.

31. An upconversion micro-laser according to claim 28, wherein said rare earth activator ion is praseodymium and 1~10~
3<x<0.03, thereby said perovskite structure single crystal is capable of producing said blue lasing radiation at a wavelength in the range 480 nm to 495 nm when pumped in the wavelength range of 915 nm to 980 nm.

32. An upconversion micro-laser according to claim 22, wherein said pumping source operating in the wavelength range of 915 nm to 980 nm comprises a semiconductor infrared laser diode for generating a beam of said pumping radiation or a laser diode array for generating a set of beams of said pumping radiation.

33. An upconversion micro-laser according to claim 22, wherein said pumping source is provided with a lens at its output to focus said pumping radiation beam into a pumping region of said chip of upconversion active gain medium and to match said pumping radiation beam size with a required size of said chip pumping region.

34. An upconversion micro-laser according to claim 22, wherein said optical cavity is defined by a first mirror and a second mirror opposing each other on a common laser axis along said pumping radiation beam axis, and said chip of an upconversion active gain medium has an input surface optically coupled with said pumping source to pass said pumping radiation through said input surface into said chip and an output surface, said chip being arranged in said optical cavity so said its input and output surfaces to be disposed adjacent said first and second mirrors respectively.

35. An upconversion micro-laser according to claim 34, wherein said chip has a size along said laser axis within the range of 0.1 mm to 20 mm.

36. An upconversion micro-laser according to claim 34, wherein for producing said blue or ultraviolet lasing radiation to be polarized said chip is an uniaxial single crystal being oriented so its axis to be transversal to said laser axis.

37. An upconversion micro-laser according to claim 34, wherein said first and second mirrors comprise dielectric coatings applied directly to said input and output surfaces of said chip respectively therefore forming a monolithic chip-optical cavity structure.

38. An upconversion micro-laser according to claim 37, wherein said chip is made in a form extended along said laser axis and provided with a flat and a convex polished end surfaces as said its input and output surfaces respectively.

39. An upconversion micro-laser according to claim 37, wherein said chip is made in a form of parallelepiped, polished facets of which that are transversal to said laser axis being said input and output surfaces of said chip.

40. An upconversion micro-laser according to claim 37, wherein said chip is made in a form of an optical waveguide extended along said laser axis and has polished flat end faces being said input and output surfaces of said chip.

41. An upconversion micro-laser according to claim 40, wherein said optical waveguide is extended and has a core with a greater refractive index than that of an outer part of said optical waveguide surrounding said core.

42. An upconversion micro-laser according to claim 37, wherein said chip is made in a form of polished crystal ball disposed on said laser axis.

43. An upconversion micro-laser according to claim 42, wherein said crystal ball has a diameter within the range of 0.1 mm to 6 mm.

44. An upconversion micro-laser according to claim 37, wherein said chip is made doped with said active ions only in its middle region to be pumped by said infrared pumping radiation remaining undoped the rest parts of said chip between which said middle region being disposed along said laser axis.

45. An upconversion micro-laser according to claim 34, wherein said chip is made with flat polished end surfaces being said its input and output surfaces and disposed on said laser axis separately with respect to said mirrors, while said first and second mirrors are configured to have a spherical and a flat surfaces respectively therefore forming a stable optical cavity configuration.

46. An upconversion micro-laser according to claim 45, wherein for producing said blue or ultraviolet lasing radiation to be polarized said chip is disposed between two undoped crystals along the laser axis, at least one of which serving as a polarizer.

47. An upconversion micro-laser according to claim 34, wherein said chip having polished spherical surfaces as said its input and output surfaces is made in a form of double convex lens disposed on said laser axis separately with respect to said first and second mirrors that are configured both to have spherical surfaces.

48. An upconversion micro-laser according to claim 47, wherein said chip has a size along said laser axis within the range of 0.3 mm to 6 mm, and said first and second mirrors have the same radius of curvature as the corresponding chip surfaces in the range of 26 mm to 46 mm.

49. An upconversion micro-laser according to claim 34, wherein said first mirror is designed to be transmitting said pumping radiation while reflecting said blue or ultraviolet lasing radiation, and said second mirror is designed to be reflecting said pumping radiation and also said blue or ultraviolet lasing radiation while transmitting a portion of said blue or ultraviolet lasing radiation.

50. An upconversion micro-laser according to claim 49, wherein said first mirror has a transmissivity in the range of 70% to 95% for said pumping radiation and a reflectivity in the range of 99,8% to 99,99% for said blue or ultraviolet lasing radiation, and said second mirror has a reflectivity in the range of 70% to 95% for said pumping radiation and in the range of 60% to 99,8% for said blue or ultraviolet lasing radiation.

51. An upconversion micro-laser according to claim 22, wherein said optical cavity is defined by a first mirror and a second mirror opposing each other on a common laser axis, and said chip of an upconversion active gain medium is made in a form of an optical waveguide extended along said laser axis and has a side face optically coupled with said pumping source to serve as an input surface through which said pumping radiation pass into said chip, a first polished flat end face and a second polished flat end face being an output surface of said chip, said chip being arranged in said optical cavity so said its first and second end faces to be disposed adjacent said first and second mirrors respectively.

52. An upconversion micro-laser according to claim 51, wherein said pumping source for generating a set of beams of said pumping radiation in the wavelength range of 915 nm to 980 nm comprises a linear infrared laser diode array arranged along said side face of said chip at a specified distance from it.

53. An upconversion micro-laser according to claim 51, wherein said first and second mirrors comprise dielectric coatings applied directly to said first and second end faces of said chip respectively therefore forming a monolithic chip-optical cavity structure.

54. An upconversion micro-laser according to claim 51, wherein said first mirror comprises dielectric coating applied directly to said first end face of said chip, while said second mirror is disposed separately with respect to said chip and configured to have a spherical surface therefore forming a stable optical cavity configuration.

55. An upconversion micro-laser according to claim 51, wherein a reflectivity of said first and second mirrors for said blue or ultraviolet lasing radiation is in the range of 99,8% to 99,99% and in the range of 60% to 99,8% respectively.

56. An upconversion micro-laser according to claim 22, wherein said optical cavity is made for resonating both said blue or ultraviolet lasing radiation in the wavelength ranges of 450 nm to 460 nm or 350 nm to 380 nm and a red exciting radiation in the wavelength range of 630 nm to 695 nm produced also by said chip, said red exciting radiation being associated with relaxation of relatively lower ones of said upper states into their relating intermediate states, in order to provide an additional excitation of a relatively higher one of said upper states using said red exciting radiation and thereby to produce said blue or ultraviolet lasing radiation preferably at one of the wavelengths associated with relaxation of said relatively higher upper state of said activator ions into its relating low state.

57. An upconversion micro-laser according to claim 56 wherein said optical cavity is defined by a first mirror and a second mirror opposing each other on a common laser axis along said pumping radiation beam axis, and said chip of an upconversion active gain medium has an input surface optically coupled with said pumping source to pass said pumping radiation through said input surface into said chip and an output surface, said chip being arranged in said optical cavity so its input and output surfaces to be disposed adjacent said first and second mirrors respectively.

58. An upconversion micro-laser according to claim 57 wherein said first and second mirrors comprise dielectric coatings applied directly to said input and output surfaces of said chip respectively, therefore forming a monolithic chip-optical cavity structure with the stable optical cavity configuration, said first mirror is designed to be transmitting said pumping radiation while reflecting said blue or ultraviolet lasing radiation as well as said red exciting radiation, and said second mirror is designed to 3be reflecting said pumping radiation and said blue or ultraviolet lasing radiation as well as said red exciting radiation while transmitting a portion of said blue or ultraviolet lasing radiation.

59. An upconversion micro-laser according to claim 58 wherein said first mirror has a transmissivity in the range of 70% to 95% for said pumping radiation and a reflectivity in the range of 99,8% to 99,99% for said blue or ultraviolet lasing radiation as well as in the range of 99,8% to 99,99%
for said red exciting radiation, and said second mirror has a reflectivity in the range of 70% to 95% for said pumping radiation, in the range of 60% to 99,8% for said blue or ultraviolet lasing radiation and in the range of 99,8% to 99,99% for said red exciting radiation.

60. An upconversion micro-laser according to claim 22 wherein said optical cavity is made for resonating both said blue or ultraviolet lasing radiation in the wavelength ranges of 450 nm to 460 nm or 350 nm to 380 nm and an infrared exciting radiation in the wavelength range of 1850 nm to 2150 nm produced also by said chip, said infrared exciting radiation being associated with relaxation one of said low statesinto its relating ground state of said activator ions, in order to provide an additional depletion of such a low state and thereby to produce said blue or ultraviolet lasing radiation preferably at one of the wavelengths associated with relaxation of a relatively higher one of said upper states of said activator ions into said depleted low state.

61. An upconversion micro-laser according to claim 60 wherein said optical cavity is defined by a first mirror and a second mirror opposing each other on a common laser axis, said chip of an upconversion active gain medium is made in a form extended along said laser axis and has a side face optically coupled with said pumping source to serve as an input surface through which said pumping radiation beams pass into said chip, a first polished flat end face and a second polished flat end face being an output surface of said chip, said first and second mirrors being made of dielectric coatings applied directly to said first and second end faces of said chip respectively therefore forming a monolithic chip-optical cavity structure, and said pumping source comprises a linear infrared laser diode array arranged along said side face of said chip at a specified distance from it.

62. An upconversion micro-laser according to claim 61 wherein said first mirror has a reflectivity in the range of 99,8% to 99,99% for said blue or ultraviolet lasing radiation as well as in the range of 99,8% to 99,99% for said infrared exciting radiation, and said second mirror has a reflectivity in the range of 60% to 99,8% for said blue or ultraviolet lasing radiation and in the range of 99,8% to 99,99% for said infrared exciting radiation.