WO2010058315A1 - A secondary laser source and a laser device - Google Patents

A secondary laser source and a laser device Download PDF

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Publication number
WO2010058315A1
WO2010058315A1 PCT/IB2009/054933 IB2009054933W WO2010058315A1 WO 2010058315 A1 WO2010058315 A1 WO 2010058315A1 IB 2009054933 W IB2009054933 W IB 2009054933W WO 2010058315 A1 WO2010058315 A1 WO 2010058315A1
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laser
predisposed
crystal
secondary
laser source
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PCT/IB2009/054933
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French (fr)
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Gian Paolo Meloni
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Laserwave S.R.L.
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/02Constructional details
    • H01S3/04Cooling arrangements
    • H01S3/042Cooling arrangements for solid state lasers
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/09Processes or apparatus for excitation, e.g. pumping
    • H01S3/091Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/0941Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
    • H01S3/09415Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling a device placed within the cavity
    • H01S3/108Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling a device placed within the cavity using a non-linear optical device, e.g. exhibiting Brillouin- or Raman-scattering
    • H01S3/109Frequency multiplying, e.g. harmonic generation
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11Pulse generation, e.g. Q-switching, mode locking
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/005Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/02Constructional details
    • H01S3/04Cooling arrangements
    • H01S3/0401Cooling arrangements of optical elements being part of laser resonator, e.g. windows, mirrors, lenses
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/02Constructional details
    • H01S3/04Cooling arrangements
    • H01S3/0405Conductive cooling, e.g. by heat sinks or thermo-electric elements
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06Construction or shape of active medium
    • H01S3/0602Crystal lasers or glass lasers
    • H01S3/0612Non-homogeneous structure
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08Construction or shape of optical resonators or components thereof
    • H01S3/08072Thermal lensing or thermally induced birefringence; Compensation thereof
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/09Processes or apparatus for excitation, e.g. pumping
    • H01S3/091Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094049Guiding of the pump light
    • H01S3/094053Fibre coupled pump, e.g. delivering pump light using a fibre or a fibre bundle
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S3/00Lasers, i.e. devices for generation, amplification, modulation, demodulation, or frequency-changing, using stimulated emission, of infra-red, visible, or ultra-violet waves
    • H01S3/23Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
    • H01S3/2383Parallel arrangements

Abstract

The secondary laser source of the invention comprises: at least a laser crystal (307), predisposed to receive in input a laser radiation and to emit in output a laser radiation having determined characteristics; an output optical unit (305), predisposed to receive the laser radiation emitted by the laser crystal (307) and to transmit to outside at least a laser radiation which exhibits a same wavelength as the laser radiation emitted by the laser crystal (307); a support (315) predisposed for supporting the laser crystal (307), which support (315) is made of a highly heat-conducting material; at least an active cooling element, located in contact with the support (315); a heat exchanger (318), struck by a cooling fluid, which is located in contact with the active cooling element (317). The invention also concerns a laser device comprising the secondary laser source.

Description

Description A Secondary Laser Source and a Laser Device.

Technical Field

The invention relates to a power laser device for use in the sectors of medicine and industry. In more detail, the invention relates to a first semiconductor power laser source (diode), which via an optic fibre excites or "pumps" a second laser device in the solid state, preferably a crystal type, which is externally positioned on the apparatus and can be replaced by a further laser device, preferably a crystal type, but with different optic characteristics, in particular in terms of wavelength, power, divergence, quality of the laser beam (TEMOO and M2), continuous, repetitive or Q- switched pulse modes. The device of the invention can be used in surgical, therapeutic and aesthetic treatments. In the industrial field the device can be used for ablation, welding, piercing, marking and heat treatment, of metals, semiconductors, glassy materials, ceramic and plastic materials and wood. Background Art The present technology enables realisation of many laser devices of the gas, liquid, solid state (crystal) and semiconductor (laser diodes) types, which generate directional radiations that are coherent and monochromatic over a wide range of wavelengths. In particular, in the dermatological and medical aesthetic field, lasers are much-used for reduction and elimination of defects and disorders due to various pathologies, such as pigmented lesions, vascular lesions, acne and skin lesions in general. Further, they are used for activities of a cosmetic type, such as the removal of tattoos, depilation, skin rejuvenation and the reduction of wrinkles.

Lasers used in dermatology are mainly based on the concept of selective absorption of laser radiation in the skin at the target, which in jargon is known as the chromofore. The use of an appropriate wavelength together with the laser energy dosing density in the human tissue (power, energy density and exposure times), determine the correct resolution of the medical and/or aesthetic problem. In order to obtain the best clinical result in any situation, it is necessary to use different laser wavelengths. The specific wavelength enables maximum absorption of the laser radiation by the chromofore target, together with the limitation of the peripheral heat damage and the consequently non-invasiveness of the treatment. With the high power and the small dimensions of the laser beam, or with high energy density, even surgical treatment is possible. The vaporisation and/or coagulation of the target strongly depend on the wavelength of the laser beam. In particular, the infrared optical radiation exhibits very high peaks of absorption by the water at the following spectral bands: 1.7, 2-2, 2.8-3.2 and for values of above 5 micron. The depth of penetration into the skin of a laser beam at these frequencies is substantially limited to a few tenths or hundredth of micron. The laser- human tissue interaction at these wavelengths produces ablative or vaporisation-type effects.

In the industrial field, too, the possibility of having available a plurality of wavelengths enables ablation, fusion, welding, marking and heat treatment of widely-varying materials. In order to obtain the desired result, substantially every type of material requires an appropriate wavelength, i.e. for the correct interaction of the laser radiation with the target material. The working process also strongly depends on the power characteristics, energy density and radiation time. The final result is thus linked to the above- mentioned characteristics.

There exist today a wide range of power laser beams, but in all cases, each device generates a single determined wavelength. Tunable or multi- frequency lasers are available, i.e. lasers able to produce a simultaneous emission of various wavelengths, but these systems are extremely complex, produce low-to-medium level power and for this reason are usually dedicated to scientific research. Since a laser device is able to emit a luminous radiation in order to perform various treatments or operations, it is necessary to avail of several laser apparatus with different ideal wavelengths. Alternatively there exist very complexe apparatus which are massive and high-cost, which integrate two or more laser sources. A recent introduction in the medical field has been an apparatus which uses a crystal laser source, excited by means of a lamp, which in turn excites a remote laser source, also a crystal, for obtaining several wavelengths (Patent WO 2008/042854 Al).

Crystal laser devices normally comprise a discharge lamp for exciting crystals. Laser devices of this type are always complicated and massive because of their intrinsic nature and low efficiency (maximum 5%). They generally require liquid cooling systems and expensive and frequent maintenance (for example replacement of the discharge lamps, cooling fluid, filters etc). Further, they exhibit poor laser beam optical characteristics (TEM and M2) , especially at high power. A valid and innovative alternative for the generation of laser radiation with different wavelengths is one which uses semiconductor laser devices. The semiconductor laser (laser diode) is at present the best device with respect to all the other laser types, in terms of efficiency (60%), duration, reliability and compactness. It can be used directly or for solid-state (crystal) and optic fibre laser excitation. A disadvantage is that the semiconductor laser exhibits a high optical divergence, which limits its use and increases the complexity and cost of commercial devices.

Over recent years the excitation of crystal laser systems by laser diodes, also known as DPSSL (Diode Pumped Solid State Laser), have undergone huge development. Today laser sources of this type with power potential in the order of kW are available.

The possibility of using laser diodes to optically excite different doped crystals with different elements means that various wavelengths can be obtained, which are intrinsic characteristics of the crystal.

For example, Nd-doped crystals (Neodymium) exhibit some spectral absorption lines in the near infrared range (NIR), such as 808 and 880 nm.

The possibility of providing laser diodes generating this wavelength enables the use thereof as optical excitation sources. The high efficiency of diodes (60%) and the low total losses due to the excitation process (about 30-50%) enable very compact, even air-cooled units to be made available.

There also exists the possibility of using special crystals having a wide emission band (e.g. Titanium: sapphire), which can be made to oscillate on a determined frequency within the possible emission band.

Further, other frequencies can be generated using the principle of the

Raman spectral shift.

Thus SPSSL sources can be made which generate different wavelengths.:

1053, 1064, 1330 1440 nm for Nd: YAG5Nd: YAP5Nd: YLF and Nd:YVO4, Nd:Cr:GSGG;

1030 nm for Yb: YAG;

2.0-2.1 μm for Ho:YAG;

1.7-2.2 μm (tunable) for Tm: YAG5Tm: YAP5TM: YLF; 1.54 μm for EπGlass;

2.71-2.94 μm for Er: YAG, Er:YALO3,Er:YLF and EnCr: YSGG. The further use of additional crystals known as "non-linear" internally of the crystal laser source enables generation of double, triple, quadruple, quintuple frequencies, and intermediate radiations for adding or subtracting frequencies. By way of example, KTP and LBO crystals are used for duplicating the fundamental frequency of Nd:YAG, Nd:YAP, Nd:YLF and Nd:YVO4, obtaining laser emissions in the visible field, 532 mm, and crystals BBO and LBO for generating frequencies in the ultraviolet field, at 335 nm and 266 nm.

With a succession of crystals a laser system can be devised which generates a determined wavelength, with functioning modes that can be both continuous and pulsed. Further, crystals can be inserted in the optic cavity which function as saturable absorbers, both active and passive, or other acoustic or electro- optic devices. These devices enable pulse-type laser emission, exploiting the principle of a brusque change in the quality coefficient (Q) of the optic cavity. This characteristic generates the emission of optic pulses, known as Q-switched pulses, usually having very high peaks (MW) and pulse times comprised between hundreds of ps and tens of ns.

Some saturable absorbers are Cr4+:YAG, V:YAG and KD*P, which are passive, and others are Quartz and LiNbO3 for active devices of the acoustic-optic type. Crystals such as ADA, ADP, CDA, KDA, RDP are used for Q-switches of the electro-optic type. At present small and medium laser sources are available on the market, and have good optic properties.

The limitation of the above-described sources is constituted by the fact the laser beam emitted is one only. Further, the described sources have to be placed internally of a single apparatus for medical or industrial use, which leads to the need to duplicate some electrical and mechanical parts, all of which further complicates the entire device.

The complexity of the apparatus, caused by the presence of several laser sources, leads to the apparatus being unwieldy and consequently poorly portable. A not final consideration is the very high cost due to the integration of two or more distinct laser beams.

Also known are devices relating to optically pumped crystal lasers which by optic fibres and external sources (US Patent US4,723,257). The laser emission power is in the region of a few mW. These devices are generally for scientific use or for pointing, and therefore cannot be used for medical- surgical, aesthetic or industrial applications. The mechanical structure of these known devices, the type of optic components cited and the lack of active cooling considerably limit the performance of the laser source. Further, if the pumped laser sources are of the crystal type, the excitation lamps have to replaced with a certain frequency. Also, there has to be a water cooling circuit, and this has to be kept under control and maintained by frequent replacement of the coolant liquid and the various filters. Already buildable and available on the market are integrated power laser sources, including small ones (a cubic decimetre) which generate a single laser emission in the optic band going from the ultraviolet to the infrared. These integrated laser sources, in order to be able to operate correctly and stably, internally exhibit both the laser diode primary source (or pump) and the secondary crystal laser system. These two known-type devices must be precisely controlled and properly cooled in order to operate efficiently and continuously. To this end, a cooling device must be provided, for example an air-air or air-water heat exchanger, either active or passive, which considerably increases the dimensions of the devices.

The total dimensions of the device and the weight thereof make integration of the laser sources in a moderate-size casing impossible. Further, owing to the intrinsic structure of the optical system, replacement of the integrate laser source with another having different optical characteristics

(wavelength, potential), implies replacement of the whole laser source, including the pump laser diode.

The aim of the present invention is to provide a laser device enabling emission of laser beams of different wavelengths with a sole primary source. A further aim of the present invention is to provide a primary source and a secondary source which are couplable to define a laser device.

Disclosure of Invention

Characteristics and advantages of the invention will better emerge from the detailed description that follows, made with reference to the accompanying figures of the drawings, given by way of non-limiting example, in which: figure 1 is a schematic representation of a laser device of the present invention; figure 2 is a schematic representation of a primary source of the present invention; figure 3 is a schematic representation of a first example of a secondary source of the present invention; figures 4A, 4B, 4C are three examples of laser crystals predisposed for use in a secondary source of the present invention; figures 5 and 6 are schematic representations of two embodiments of the secondary source of the present invention, complete with a cooling device.

Figure 1 schematically illustrates a laser device 100 of the present invention. The device includes a main laser unit 101 which comprises a primary laser source 102, preferably a semiconductor type and in particular a laser diode. The main laser unit 101 further comprises a supply unit 103, predisposed for supplying energy to the primary laser source. A control unit 104, provided with a control unit 105 including a hardware and/or software user interface, for example a display and a keyboard, is predisposed for controlling the primary laser source 102. The control unit 104 comprises, for example, a PC, a PLC, a microprocessor or more simply a discrete electronic card, to enable activation and control of the electric current polarising the semiconductor laser device. The main laser unit also comprises a cooling device 106, predisposed for cooling the primary laser source 102.

Figure 2 schematically illustrates an embodiment of the primary laser source 102, in which the primary source 102 is a diode source. The primary laser source comprises one or more laser diode emitters 107, 108, 109, each predisposed for emitting a laser radiation of a determined wavelength and power. The number of laser diodes shown in the diagram of figure 2 is purely an example.

The laser diodes 107, 108, 109 are optically combined with one another by a combiner 110 and are focussed by a lens 111 such as to produce a suitable laser radiation for exciting a secondary laser source 300. Collimation and focussing of the laser radiation produced by the laser diodes 107, 108, 109 can be obtained in general using lenses and mirrors, or bundles of optic fibres. The lens 111 focalises the laser radiation produced by the diodes 107, 108, 109 on an optic connector 112 by means of which the laser radiation is transmitted to an optic fibre 200.

The power emitted by the primary laser source 102 depends on the number of the laser diodes used. The primary laser source 102 could comprise a single laser diode emitter or, preferably, a plurality of emitters arranged in a linear array, with the aim of increasing the total power of the radiation emitted.

The different wavelengths produced by the single laser diodes depend on the chemical-physical properties of the semiconductors used. The choice of a particular semiconductor leads to obtaining laser radiation emissions of predetermined wavelengths, both in the visible region of the spectrum and in the near infrared and medium ranges. Some examples of obtainable wavelengths are (in nm): 390, 410, 430, 450, 630, 645, 650, 660, 670 ,690, 750, 790, 808, 830, 880, 910, 940, 980, 1000, 1060, 1450, 1550 and 2000 nm. The semiconductors are preferably realised using epitaxial growth technology based on the MOCVD method. The basic constituents are As and Ga with Al or In.

Optic potential can reach the order of thousands of Watts either in continuous or pulsed mode. The cooling device 106 can be realised with convection or forced systems using air or water, heat pump, or with active systems such as Peltier cells. A further possibility is an air/water or water/water exchanger cooling plant integrated into the main laser unit 101 or external thereof. Preferably, for reasons of system compactness, and compatibly with the power and times of laser emission, the cooling device 106 is a Peltier cell type. The possibility of controlling the temperature of the laser diodes is fundamental so as to be able to tune the emission frequency thereof in combination. Typically the emission frequency variation is 0.3 nm/°C. The perfect centring of the pump laser radiation with the absorption band of the crystal secondary laser source determines the maximum power and the stability of laser emission. Further, the regulation of the temperature enables the frequency of the laser radiation emitted by the laser diodes to be tuned with the absorption frequency of the laser crystals 307 of the secondary laser source 301 which will be illustrated in the following part of the present description.

The primary laser source 102 obtained can reach high power levels and is also able to emit laser radiations of various wavelengths. The source is also compact, efficient, low-consumption, reliable and economical.

The laser device of the present invention comprises at least an optic fibre 200, predisposed to transport the laser radiation from the primary source 102 to a second laser unit 300. The optic fibre 200 preferably has a core diameter, i.e. the conductive part, comprised between 50 and 2000 μm. Generally the optic fibres used for taking the laser radiation from the primary laser source to the second laser source are made of glass or quartz. For high powers a quartz optic fibre is preferable. The optic fibre 200 conveying the laser radiation of the primary laser source 102 can be provided with an optic connector 310 for connection to the secondary laser source 301, or it can be directly coupled to the source 301 itself. An electrical connection is also interposed between the main laser unit 101 and the secondary laser unit 300, which can also be used for supplying the sensors and components present in the second laser unit 300 and, in particular, also the Peltier cells, as will be further described herein below, which are present internally of the second laser unit 300.

The second laser unit 300 comprises a secondary laser source 301, preferably a crystal type (solid state), and a cooling device 302 predisposed to cool the secondary laser source. The cooling device 302 can be selected from among those described for cooling the primary laser source 102. The cooling of the two primary and secondary laser sources might however be done by a single cooling device, which in this case too can be selected from among those described for cooling the primary laser source 102. The output laser radiation 303 of the device of the present invention is emitted by the secondary laser source 301.

Figure 3 illustrates an embodiment of a crystal (solid state) secondary laser source 301. The configuration of this diagram is an end-pumping one. The efficiency conversion value of the configuration is among the highest, together with the superior optical characteristics of the laser beam (TEM and M2).

The secondary laser beam 301 of figure 3 comprises at least a pair of optics or mirrors 306 which form the optical group focussing the laser radiation coming from the primary laser source 102. The secondary laser source 301 further comprises a laser crystal 307 and a "pumping" optic or lens 304 which is interposed between the focalising optical group 306 and the laser crystal 307. The pumping optic 304 is transparent only to laser radiation generated by the primary laser source 102, while it totally reflects the fundamental radiation generated by the laser crystal 307 and the other frequencies generated internally of the resonator by any non-linear crystals 308, 309 which will be described herein below. The pumping optic 304 could also not be present as an independent element, but could be directly realised on the laser crystal 307. The secondary laser source 301 further comprises an output optic 305, located downstream of the laser crystal 307, which partially transmits at the wavelength for which the secondary laser source 301 is designed. The output optic 305 and the pump optic 304 form the optic resonator. If only the laser crystal 307 is present, the optic transmission of the laser beam with be relative to the fundamental wavelength of the crystal, for example 1064 nm for Nd. YAG. The transmission percentage can vary between 50% and 1%, according to the intrinsic gain characteristics of the secondary laser source. The value of the transmission percentage is strongly determined by the type of laser crystal 307, by the internal losses of the optical components, by the specific optical characteristics of the resonator and by the operative conditions of the system, continuous or pulsating. By way of example, in the case of continuous excitation, for Nd:YVO4 lasers the transmission percentage can vary between 10% and 30%, while for the Tm: YAG crystal it is between 1% and 5%.

One or more non-linear crystals 308, 309 can be interposed between the laser crystal 307 and the output optic 305, which crystals 308, 309 are predisposed to multiply the wavelength of the laser radiation by a determined factor. In this case the transmission percentage of the laser radiation of the output optic 305 will have to be maximum, as close as possible to 100%, in comparison to the wavelength multiplied by the nonlinear crystals. Consequently, the reflectivity of the output optic 305 must be maximum (100%) in comparison to the fundamental wavelength of the laser crystal 307. In a case of emission of a laser radiation exhibiting a contemporary oscillation of several laser frequencies, appropriate percentages of reflectivity for each specific wavelength will have to be considered. The laser crystal 307 can be realised in various geometrical forms: cylindrical, parallelepiped, discoid, cubic or spherical. The laser crystal is commonly made of an optically transparent material such as YAG, YAP, YLF, YVO4,YSGG, GSGG, GDVO4, FAP, Kgd(WO4)2, SFAP, glass, ceramic and of any combination thereof. The dope generally belongs to the rare earth group and is constituted by the following elements: Ce, Cr, Er, Ho, Nd, Th, Tm, Sm, Yb and any combination thereof. The laser crystal 307 preferably should be of a composite type, realised by diffusion bonding, with non-dopes end cups (figure 4A), discrete doping (figure 4B), or variable doping (figure 4C), the last being obtainable with ceramic crystals. The use of these crystals considerably reduces the formation of the thermal lens, i.e. the local deformation of the crystals due to the temperature, which is responsible for the reduction of the optical performance (power and quality of the beam) of the entire secondary laser source. The laser crystal 307 with non-doped end cups of figure 4A is made up of a constant-doped central part 400, while the two end parts 401 are not doped. This leads to a smaller thermal lens, as the point of absorption of the pumped excitant laser radiation is internal of the crystal 402 and not on the air-crystal interface 403, as happens in the classic uniform-doped crystals. In the illustration of figure 4B, the crystal 307 comprises, apart from the two non-doped zones 401, several discrete layers with different doping layers 404, 405, 406, 407. In this way the thermal lens is sub-divided into several smaller thermal lenses distributed on several interfaces 408, thus further improving the behaviour of the crystal. The crystal of figure 4C exhibits a continuous distribution, according to a determined profile 409, of the doping level, and offers better performance with respect to the two preceding crystals.

For double, triplicate, quadruplicate and other harmonic frequency emissions, and by adding or subtracting frequencies, and for Raman effect emissions, it is necessary to insert at least a non-linear crystal or means 308, 309 between the laser crystal 307 and the output optic 305. Some examples of suitable non-linear crystals are: ADA, ADP, APDA, Banana (Ba2NaNb5O15), BBO, CBO, CDA, CdSe, DADA, DADP, DCDA, DKB5, DKDA, DKDP, DRDA, DRDP, KB5, KCN, KDP, KLN, KNbO3, KTA, KTP, LBO, LFM, LiIO3, LiNbO3, MgOiLiNbO3, MHBA, RDA, RDP, Urea, ZnGeP2.

To obtain triplicate and quadruplicate effects and beyond, two or more crystals are necessary. Each crystal can be singly temperature-controlled, with passive or active means (e.g. Peltier cells) which determine the operating stability thereof. By way of example, KTP and LBO crystals are commonly used for the duplication of the fundamental frequency of the Nd-doped crystals. The classic fundamental wavelength of ND:YAG and ND: YVO4 is 1064 nm. In this case the cited non-linear crystals duplicate the fundamental frequency of oscillation. Thus the fundamental infrared emission is then converted into visible laser radiation in the green region of the spectrum, 532 nm. Using the same procedure doubling, triplication, quadruplication and beyond of a specific laser radiation can be done by using an appropriate combination of non-linear crystals.

By way of example the fundamental radiation of 1064 nm, with insertion of a KTP crystal (BBO) in the cavity doubles the frequency which was already previously doubled (532 nm). The result is a 226 nm emission (ultraviolet region). The dimensions of these crystals can vary from tenths of a millimetre to several centimetres in all directions.

In a case of insertion of a Q-switching device, either passive or active, one or more non-linear crystals can be located externally of the optical resonator, downstream of the output optic 305. The optical focussing group 306 comprises at least a lens, but is usually made up of a pair or a triplet of lenses. The function of the optical group 306 is to collect the laser radiations coming from the optic fibre, collimate and/or focus them internally or in proximity of the laser crystal 307. The optic properties of the optic focussing group 306 are important for optimally illuminating the laser crystal 307. The diameter of focussed laser beam (spot size) and the depth of focus are crucial for a correct optic coupling.

The optic group 306 could also be absent. In this case the laser radiation coming from the optic fibre 200 enters directly into the pump optic 304. The secondary laser source 301 could be provided with a Q-switching (Q- S) element 311. This component, either active or passive, enables generation of very powerful and short optic pulses (from hundreds of ps to hundreds of ns). Some examples of Q-switching elements are some saturable absorbers such as Cr4+:YAG, V:YAG and KD*P as passive materials, Quartz, LiNbO3, TeO2 for acoustic-optical Q-S devices, and crystals ADA, ADP, CDA, KDA, KDP, RDP for electro-optical Q-S devices. The possibility of generating very short and extremely powerful pulses is used in the medical field for some dermatological pathologies, and in the cosmetic field for removal of tattoos. In the industrial field they are used for marking or micro-perforating of components and parts of very varied materials (metal, ceramic, glass, plastic).

Figure 3 illustrates a further device that the secondary laser source 301 can be provided with, i.e. a diode or DPSS pointing laser 313, with emission in the visible field, red, green, blue. The function of the pointing laser 313 is to identify and precisely define the target for the output laser radiation 303. In the medical-aesthetic field, the pointing laser 313 is indispensable for a priori evaluation of the region to be worked in. The output laser radiation 303 of the secondary source is superposed on the visible optical radiation of the pointing laser. To perform the superposing of the two laser beams, an optical combiner 312 is required downstream of the output optic 305. Preferably a dichroic mirror is used, which reflects the laser radiation coming from the pointing laser 313 and transmits, with the least possible leakage, the laser radiation coming from the laser crystal 307 or from the non-linear crystals 308, 309. Led illuminators can be used alternatively to the pointing laser 313.

The arrangements of the laser crystal 307, the non-linear crystals 308, 309 and the Q-switching device 311 can be swapped in various ways, according to particular constructional needs. Alternatively to a linear alignment of the type illustrated in figure 3, the various components might be aligned on several axes, for example two axes, with a V-array, or on three axes, with a Z-array. The cooling device 302 of the secondary laser source 301 is very important for guaranteeing high emission power, functional stability and operativity in continuous mode. An insufficient cooling can generate, in active crystals, onset of high thermal gradients (thermal lens and optic distortion), which limit the obtainable power, the emission time and which degrade the optical properties of the laser radiation (TEM and M2).

The cooling modes are mainly a function of the excitation power of the laser diode, the excitation time, the pumping modes (continuous or pulsed) and the thermal properties of the secondary laser source (crystal). The secondary laser source 301 should preferably be cooled actively, via forced air or water convection, heat pump or by Peltier cells.

Figure 5 schematically illustrates a cooling device which is especially suitable for continuous functioning that is not very prolonged (a few minutes). The cooling device shown in figure 6 comprises one or more Peltier cells 317 and an air exchanger 318. The crystals 307, 308, 309 are supported by a support 315 made of a material exhibiting a high heat conductivity, for example silver, copper, SiC or synthetic diamond. The Peltier cells 317 are in intimate contact with the support material 315 in order to cool the support material 315 itself and therefore the crystals 397, 308, 309. The air exchanger or radiator 318 is preferably made of a highly- conductive material, for example aluminium or copper, and is predisposed to dissipate the heat absorbed by the Peltier cells 317, preferably with the aid of a fan 319. Figure 6 schematically shows a particularly suitable cooling device for prolonged emission times. The device of figure 7 differs essentially from the device of figure 6 essentially in that it comprises a water exchanger 318 rather than an air exchanger. Also in the device of figure 7, the support 315 for the crystals is realised in a material exhibiting a high heat conductivity, for example silver copper, SiC or synthetic diamond. The Peltier cells 317 are in intimate contact with the support material 315 in order to cool the support material 315 itself and the crystals 307, 308, 309. The heat exchanger 318, which is crossed by a water circuit 320, is predisposed to dissipate the heat absorbed by the Peltier cells 317.

Depending on the destination of use of the laser device, whether medical- surgical or industrial, it is possible to use a focussing device which is variable along the optic axis X. Figure 7 illustrates an optical regulating device, comprising at least a lens 500, preferably a pair of lenses 500, which modify the diameter of the focal point.

By varying the position of the lens 500 along the optic axis X, the dimension of the focal point of the laser radiation or output beam 303 can be regulated on a predetermined plane 502. The focussing device, coupled with the secondary laser source 301, enables the user to modify the energy density for the predetermined aim (ablation, vaporisation, fusion, coagulation and heating).

The laser device can also be provided with a deflection device for displacing the output laser beam 303 along two perpendicular axes on a predetermined plane 502. Figure 8 illustrates a deflection device of this type.

The deflection device comprises a lens 500 predisposed to focus and/or collimate the output laser beam 303 on a pair of mirrors 503, 504. Each mirror is controlled by an electro-mechanical device which regulates the angular position thereof. A first mirror 503 is rotatable about an axis S which is perpendicular to the optic axis X of the output laser beam 303. A second mirror 504 is rotatable about an axis T which is perpendicular to the rotation axis S of the first mirror 503. The output laser beam 303 is deflected by the first mirror 503 towards the second mirror 504 and reflected by the second mirror towards the predetermined plane 502. The output laser beam 505 from the second mirror 504 will have a direction and consequently a position on the predetermined plane 502 which is determined by the angular position of each mirror 503, 504. A lens 506 can be predisposed to focus and/or collimate the deflected laser beam before it reaches the predetermined plane 502.

The secondary laser source of the present invention offers important advantages. It enables emission of a high-power laser radiation having excellent optical characteristics. It can be configured in various different ways, each being predisposed for emission of a laser radiation having different characteristics. The source of the present invention further exhibits modest dimensions and is thus easily transportable and connectable to the primary laser source by mean of an optic fibre. This enables, with only one primary laser, obtaining emission of a desired laser radiation by connecting a suitable secondary laser source to the primary laser source, which secondary laser source can be easily replaced with a different secondary laser source for emitting a different laser radiation. The laser device of the present invention is extremely functionally flexible, as thanks to the possibility of replacing the secondary laser source it is able to emit laser radiations having different characteristics. Further, the primary laser source can be arranged in a remote position with respect to the secondary laser source, thus offering the possibility of varying the layout of the laser device with considerable freedom. A further important advantage of the laser device is that it require much less frequent maintenance, if any at all, with respect to known-type laser devices.

Claims

Claims.
1. A secondary laser source, comprising: at least a laser crystal (30 predisposed to receive in input a laser radiation and to emit in output a Ia; radiation having determined characteristics; an output optical unit (30 predisposed to receive the laser radiation emitted by the laser crystal (3 C and to transmit to outside at least a laser radiation which exhibits a sai wavelength as the laser radiation emitted by the laser crystal (30' characterised in that it comprises a cooling device (302) which comprises support (315) predisposed for supporting the laser crystal (307), whi support (315) is made of a highly heat-conducting material; at least active cooling element, located in contact with the support (315); a h< exchanger (318), struck by a cooling fluid, which is located in contact w the active cooling element (317).
2. The secondary laser source of claim 1, wherein the active cooli element (317) is a Peltier cell.
3. The secondary laser source of one of the preceding claims, comprisi an optical focussing group (306), predisposed to receive a laser radiation input and to focus the laser radiation on the laser crystal (307).
4. The secondary laser source of one of the preceding claims, comprising pump optical unit or lens (304) interposed between the optical focussi group (306) and the laser crystal (307).
5. The secondary laser source of one of the preceding claims, comprising least a non-linear crystal (308) predisposed to be crossed by a las radiation and to multiply by a determined factor a wave length of the las raHiatinn it is r.rπ«i<jpH hv
6. The secondary laser source of one of the preceding claims, comprising a Q-switching element (311), predisposed to enable emission of a pulse laser radiation.
7. The secondary laser source of one of the preceding claims, comprising: a pointing device (313), predisposed to emit a laser radiation in the visible field; an optical combiner (312), predisposed to superpose the laser radiation emitted by the output optical unit (305) on the laser radiation emitted by the pointing device (313).
8. The secondary laser source of one of the preceding claims, wherein the laser crystal (307) exhibits at least a central portion (400), containing a determined percentage of doping materials, and at least two terminal portions (401), located at ends of the central portion (400), which are undoped.
9. The secondary laser source of claim 8, wherein the central portion (400) of the laser crystal (307) is subdivided into a plurality of portions (404, 405, 406, 407) each of which contains a different percentage of doping materials.
10. The secondary laser source of claim 9, wherein the percentage of doping materials contained in the laser crystal (307) varies with continuity in various portions of the laser crystal (307).
11. The secondary laser source of claims 1, 2, 5 and 6, wherein the laser crystal (307), at least a non-linear crystal (308) and the Q-switching element (311) are aligned in any sequence along an optical axis (X).
12. The secondary laser source of claims 1, 2, 5 and 6, wherein the laser crystal (307), at least a non-linear crystal (308) and the Q-switching element (311) are distributed and aligned along two axes which have a reciprocal V-intersection.
13. The secondary laser source of claims 1, 2, 5 and 6, wherein the laser crystal (307), at least a non-linear crystal (308) and the Q-switching element (311) are distributed and aligned along three axes which intersect one another at Z.
14. The secondary laser source of claims from 3 to 13, wherein the optical focusing group (306) comprises one, two or three lenses.
15. The secondary laser source of one of the preceding claims, comprising a focusing device which comprises at least a lens (500), arranged downstream of the output optical unit (305), which is mobile with respect to the output optical unit (305) along the optical axis (X) such as to vary a diameter of a focal point of radiation or an output laser beam (303) on a predetermined plane (502).
16. The secondary laser source of one of the preceding claims, comprising a deflecting device, predisposed to displace the output laser beam (303) on a predetermined plane (502), which comprises a lens (500) predisposed to focus and/or collimate the output laser beam (303) on a first mirror (503), rotatable about a perpendicular axis (S) to the optical axis (X) of the output laser beam (303), which is predisposed to direct the output laser beam (303) onto a second mirror (504) which is rotatable about a perpendicular axis (T) to the rotation axis (S) of the first mirror (503).
17. A laser device, comprising: a primary laser beam (102), predisposed to emit a laser radiation; a secondary laser source (301) according to at least one of the preceding claims; at least an optic fibre (200) which extends between the primary laser source (102) and the secondary laser source (301) and is predisposed to guide the laser radiation emitted from the primary laser source (102) to the secondary laser source (301) and to excite the laser crystal (307).
18. The laser device of claim 17, wherein the primary laser source (102) comprises at least a laser diode emitter (107), predisposed to emit a laser radiation of a determined wavelength and power.
19. The laser device of claim 18, comprising at least a second laser diode emitter (108), predisposed to emit a laser radiation of a determined wavelength and power.
20. The laser device of claim 19, comprising an optical combiner (100) predisposed to combine the laser radiations emitted by the laser diode emitters (107, 108) with one another.
21. The laser device of claim 17, wherein the primary laser source (102) and the secondary laser source (301) are each provided with optical connectors (112, 310) between which the optic fibre (200) extends.
22. The laser device of one of claims from 18 to 21, wherein the primary laser source (102) comprises a focusing element (111) predisposed to focus and collimate the laser radiation emitted by at least a laser diode emitter (107).
23. The laser device of one of claims from 17 to 22, comprising a cooling device (106) predisposed to cool the primary laser source (102).
24. The laser device of one of claims from 17 to 23, comprising: a supply unit (103), predisposed to supply energy to the primary laser source (102); a control unit (104), provided with a command unit (105), predisposed to control the primary laser source (102).
PCT/IB2009/054933 2008-11-18 2009-11-06 A secondary laser source and a laser device WO2010058315A1 (en)

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CN106785824A (en) * 2016-12-16 2017-05-31 湖北工业大学 High-efficiency all-solid-state thulium laser packaging module

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US4805177A (en) * 1988-04-25 1989-02-14 Laser Diode Products, Inc. Laser construction
US4901324A (en) * 1988-12-19 1990-02-13 Laser Diode Products, Inc. Heat transfer device for cooling and transferring heat from a laser diode device and associated heat generating elements
US5084886A (en) * 1990-10-01 1992-01-28 Laser Diode, Inc. Side-pumped laser system with independent heat controls
US6130902A (en) * 1998-05-26 2000-10-10 Shimoji; Yutaka Solid state laser chip

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Publication number Priority date Publication date Assignee Title
US4805177A (en) * 1988-04-25 1989-02-14 Laser Diode Products, Inc. Laser construction
US4901324A (en) * 1988-12-19 1990-02-13 Laser Diode Products, Inc. Heat transfer device for cooling and transferring heat from a laser diode device and associated heat generating elements
US5084886A (en) * 1990-10-01 1992-01-28 Laser Diode, Inc. Side-pumped laser system with independent heat controls
US6130902A (en) * 1998-05-26 2000-10-10 Shimoji; Yutaka Solid state laser chip

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN106785824A (en) * 2016-12-16 2017-05-31 湖北工业大学 High-efficiency all-solid-state thulium laser packaging module

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