SE519930C2

SE519930C2 - Method and apparatus for monitoring that the power density does not get too high in an optical fiber.

Info

Publication number: SE519930C2
Application number: SE0103582A
Authority: SE
Inventors: Joakim Frid; Sverker Haard; Sheila Galt; Manuel Jose Lopez-Qui Teijeiro; Mats Sjoeberg; Peter Thorsell
Original assignee: Permanova Lasersystem Ab
Priority date: 2001-10-29
Filing date: 2001-10-29
Publication date: 2003-04-29
Also published as: SE0103582L; WO2003041906A1; SE0103582D0

Abstract

The invention relates to a method and a device for monitoring the maximum power density in an optical fibre (3) made for transmitting high optical power, specifically power exceeding 1 kW, and where the optical radiation is focused to a very small diameter by means of focusing optics (2). An intermediate focus is provided for the incident optical beam (1) in a gas-filled cell (5), before the beam (1) has reached the focusing optics (2). The gas-filled cell (5) is arranged to absorb the beam (1) if the power density in the intermediate focus spot exceeds a certain value. The gas if preferably an argon gas and the focusing angle ( alpha 1) of the focusing optics (2) and the focusing angle ( alpha 2) of the intermediate focusing optics (4) are arranged in such a way that ( alpha 1/ alpha 2)<2>>Eth(Ar)/Eth(SiO2), where Eth is the limit value for discharge in argon (Ar) and quartz (SiO2), respectively, which means that a discharge is obtained in the argon gas before there is a damage of the fibre (3) quartz material.

Description

519 930 _ 2 _ Uppfinningen kännetecknas av att laserstrålen är anordnad att fokuseras i en gasfylld cell innan den når fokuse- ringsoptiken och om effekttätheten överstiger en viss nivå sker ett genombrott i gasen och ett plasma bildas som ab- sorberar strålningen. Den gasfyllda cellen bildar på så vis en form av säkring som "löses ut" då effekttätheten blir för hög. 519 930 _ 2 _ The invention is characterized in that the laser beam is arranged to be focused in a gas-filled cell before it reaches the focusing optics and if the power density exceeds a certain level, a breakthrough occurs in the gas and a plasma is formed which absorbs the radiation. The gas-filled cell thus forms a form of fuse which is "triggered" when the power density becomes too high.

Enligt en föredragen utföringsform utgöres gasen av argon under några atmosfärers övertryck.According to a preferred embodiment, the gas is argon under the overpressure of a few atmospheres.

I det följande skall uppfinningen närmare beskrivas i an- slutning till bifogade ritning varvid figur 1 schematiskt visar ett optiskt fibersystem där strålning fokuseras mot fiberns kärna, och figur 2 det optiska fibersystemet med en säkring anordnad i enlighet med uppfinningen.In the following, the invention will be described in more detail in connection with the accompanying drawing, in which Figure 1 schematically shows an optical fiber system where radiation is focused towards the core of the fiber, and Figure 2 the optical fiber system with a fuse arranged in accordance with the invention.

I figur 1 visas en laserstråle 1 som med hjälp av fokuse- ringsoptik i form av en eller flera linser eller speglar 2 fokuseras mot ändytan hos en optisk fiber 3 vilken på känt sätt har en kärna av kvartsglas och en omgivande cladding med lägre brytningsindex, exempelvis av glas eller någon polymer med lämpligt brytningsindex. Claddingens funktion är att stänga inne strålningen i kärnan så att strålningen leds genom fibern i dess utbredningsriktning tills den lämnar fibern.Figure 1 shows a laser beam 1 which by means of focusing optics in the form of one or more lenses or mirrors 2 is focused towards the end surface of an optical fiber 3 which in a known manner has a core of quartz glass and a surrounding cladding with a lower refractive index, e.g. of glass or any polymer with a suitable refractive index. The function of the cladding is to trap the radiation in the core so that the radiation is conducted through the fiber in its direction of propagation until it leaves the fiber.

Utanför fiberns cladding är i allmänhet ytterligare lager anordnade för att bl.a. öka den mekaniska hållfastheten hos fibern. Dessa lager visas ej här eftersom de kan vara av i och för sig känt slag och ej har någon funktion för att förklara uppfinningstanken.Outside the cladding of the fiber, additional layers are generally arranged to e.g. increase the mechanical strength of the fiber. These layers are not shown here because they may be of a per se known type and have no function for explaining the inventive concept.

Laserstrålen 1 som fokuseras mot fiberns ändyta kan vara av i och för sig känt slag. Den laserkälla som är mest an- 10 15 20 25 30 35 519 930 _ 3 _ vänd i högeffektlasrar är Nd-YAG lasern, vars våglängd är 1,06 pm. Denna våglängd lämpar sig bra för överföring i optiska fibrer. Andra i och för sig kända lasrar som kan vara aktuella är exempelvis diodlasrar, CO-lasrar och an- dra typer av YAG-lasrar.The laser beam 1 which is focused towards the end surface of the fiber can be of a type known per se. The laser source most commonly used in high power lasers is the Nd-YAG laser, whose wavelength is 1.06 μm. This wavelength is well suited for transmission in optical fibers. Other lasers known per se which may be relevant are, for example, diode lasers, CO lasers and other types of YAG lasers.

Gemensamt för dessa typer av lasrar är att de höga effek- terna kan förorsaka personskador såväl som skador i det optiska systemet om inte strålningen leds på korrekt sätt genom fibern. Även en liten skada hos fibern kan förorsaka svåra materiella skador.Common to these types of lasers is that the high effects can cause personal injury as well as damage to the optical system if the radiation is not conducted correctly through the fiber. Even a small damage to the fiber can cause severe property damage.

Som nämnts inledningsvis är de lasrar som används idag potenta nog att fokusera strålningen till mycket små dia- metrar, samtidigt som effekten hos lasrarna kan vara myck- et hög. Därför finns det även i en oskadad fiber en risk för skador i kvartsmaterialet för dessa typer av lasrar om effekttätheten blir för hög.As mentioned at the outset, the lasers used today are potent enough to focus the radiation to very small diameters, at the same time as the power of the lasers can be very high. Therefore, even in an undamaged fiber, there is a risk of damage to the quartz material for these types of lasers if the power density becomes too high.

För en given strålkvalitet hos en laser ges fokusfläckens storlek av den vinkel al varmed laserstrålen fokuseras av fokuseringsoptiken. Effekttätheten mot fiberns 3 ändyta kommer för en given lasereffekt att vara proportionell mot mn*- I figur 2 visas ett arrangemang som förskonar kvartsfibern från strålningsskador. Innan laserstrålen når fokuserings- optiken 2 får den passera en "säkring" i form av en gas- fylld cell 5 i vilken skapas ett mellanfokus för laser- strålen med hjälp av minst en fokuserande lins 4, vars fo- kuseringsvinkel är az. Effekttätheten för detta mellanfokus är proportionell mot (a2)2.For a given beam quality of a laser, the size of the focus spot is given by the angle at which the laser beam is focused by the focusing optics. The power density against the end surface of the fiber 3 will for a given laser power be proportional to mn * - Figure 2 shows an arrangement which spares the quartz fiber from radiation damage. Before the laser beam reaches the focusing optics 2, it must pass a "fuse" in the form of a gas-filled cell 5 in which an intermediate focus is created for the laser beam by means of at least one focusing lens 4, the focusing angle of which is az. The power density of this intermediate focus is proportional to (a2) 2.

Då mellanfokus(fläcken) ligger inne i en gasfylld cell 5 kommer man vid en given effekttäthetsnivå att åstadkomma ett genombrott i gasen och ett plasma bildas som absorbe- 10 15 20 25 30 35 519 930 _ 4.. rar lasereffekten. Denna process är snabb, nästan full- ständig absorption inträder inom några få nanosekunder, (104 s). Vid vilken nivå gasen får sitt genombrott bestäms bl.a. av gastyp och vid vilket tryck man arbetar. Om man t.ex. använder argon under några atmosfärers övertryck kommer effekttätheten för genombrott i argon att vara ca 2-3 gånger högre jämfört med nivån för skador i kvartsma- terialet.When the intermediate focus (spot) is inside a gas-filled cell 5, at a given power density level, a breakthrough in the gas will be achieved and a plasma is formed which absorbs the laser power. This process is fast, almost complete absorption occurs within a few nanoseconds, (104 s). At what level does the gas get its breakthrough is determined i.a. of gas type and at what pressure you work. If one e.g. If argon is used under the overpressure of a few atmospheres, the power density for breakthroughs in argon will be about 2-3 times higher compared with the level of damage to the quartz material.

Genom att anpassa vinklarna az och al så att 2 (afaﬂ >Emmn/gmwmm där Em är gränsvärdet för genombrott i argon (Ar) respek- tive kvarts (SiO2), fås genombrott i argongasen innan kvartsmaterialet skadas. Eftersom strålningen då kommer att absorberas i det bildade plasmat, kommer kvartsfibern att skonas från strålningsskador. När strålningen upphör kommer plasmat att rekombinera, och gasen blir åter trans- parent. Rekombinationstiden för Argongas under några atmo- sfärers övertryck är ca 100 nanosekunder.By adjusting the angles az and al so that 2 (afa ﬂ> Emmn / gmwmm where Em is the limit value for breakthroughs in argon (Ar) and quartz (SiO2) respectively, breakthroughs in the argon gas are obtained before the quartz material is damaged. the formed plasma, the quartz fiber will be spared from radiation damage.When the radiation ceases, the plasma will recombine, and the gas will become transparent again.The recombination time of Argongas under the overpressure of some atmospheres is about 100 nanoseconds.

Som framgår av figuren 2 kan den gasfyllda cellen rent principiellt vara uppbyggd av den fokuserande linsen (4) (inträdeslins) och en motsvarande utträdeslins och ett om- givande hölje som innsluter gasen.As can be seen from Figure 2, the gas-filled cell can in principle be composed of the focusing lens (4) (entry lens) and a corresponding exit lens and a surrounding housing which encloses the gas.

Uppfinningen är inte begränsad till det exempel som visas ovan utan kan varieras inom ramen för de efterföljande pa- tentkraven.The invention is not limited to the example shown above but can be varied within the scope of the appended claims.

Claims

10 15 20 25 30 35 519 930 _5- PATENT REQUIREMENTS

A method for monitoring that the power density does not become too high in an optical fiber (3) intended for the transmission of high optical effects, preferably effects exceeding 1 kW, and where the optical radiation is focused to very small diameters of a focusing optics ( 2) characterized in that an intermediate focus is created by the incident optical radiation (1) before it reaches the focusing optics (2) by means of at least one focusing lens (4) and absorbs the radiation if the power density of the intermediate focus spot exceeds a certain value.

Method according to claim 1, characterized in that intermediate focus is created in a gas-filled cell (5) before the radiation reaches the focusing optics (2) and if the power density exceeds a certain level a breakthrough occurs in the gas and a plasma is formed which absorbs the radiation.

Method according to claim 2, characterized in that when the gas consists of argon and the fiber of quartz material, the focusing angle (a1) of the focusing optics (2) and the focusing angle (az) of the intermediate focus optics (4) are adjusted so that / Ethwioz), where Éth is the limit value for breakthrough in argon (Ar) and quartz (SiO2), respectively, whereby breakthrough in the argon gas is obtained before the quartz material in the fibers (3) is damaged.

Device for monitoring that the power density does not become too high in an optical fiber (3) intended for the transmission of high optical effects, preferably effects exceeding 1 kw, and where the optical radiation is focused to very small diameters of a focusing optics (2) - drawing a device (5) for creating an intermediate field of the incident optical radiation (1) before it reaches the focusing optics (Z) which device (Z) 5) 'comprises at least one focusing lens (4) and is arranged to absorb the radiation. (1) if the power density nos between the focus spot exceeds a certain value.

Device according to claim 4, characterized in that the device comprises a gas-filled cell (5) so arranged that if the power density in the intermediate focus spot exceeds a certain level, a breakthrough occurs in the gas and a plasma is formed which absorbs the radiation.

Device according to claim 5, characterized in that the gas consists of argon and the fiber (3) of quartz material.

Device according to claim 6, characterized in that the focusing angle (a1) of the focusing optics (2) and the focusing angle (n) of the intermediate focusing optics (4) are so adjusted * that (az / ulf> Ethwlqfïšthßioz), where Eth is the limit value for breakthrough in argon (Ar) and quartz (SiO2), respectively, whereby breakthrough in the argon gas is obtained before the quartz material in the fiber (3) is damaged.

Device according to claim 7, characterized in that the gas has an overpressure of some atmospheres.