GB2239733A - A laser oscillator - Google Patents

A laser oscillator Download PDF

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Publication number
GB2239733A
GB2239733A GB9000079A GB9000079A GB2239733A GB 2239733 A GB2239733 A GB 2239733A GB 9000079 A GB9000079 A GB 9000079A GB 9000079 A GB9000079 A GB 9000079A GB 2239733 A GB2239733 A GB 2239733A
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GB
United Kingdom
Prior art keywords
pump
wavelength
optical device
cavity
fibre
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB9000079A
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GB9000079D0 (en
Inventor
Jonathan Richard Armitage
Richard Wyatt
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British Telecommunications PLC
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British Telecommunications PLC
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Filing date
Publication date
Application filed by British Telecommunications PLC filed Critical British Telecommunications PLC
Priority to GB9000079A priority Critical patent/GB2239733A/en
Publication of GB9000079D0 publication Critical patent/GB9000079D0/en
Publication of GB2239733A publication Critical patent/GB2239733A/en
Application status is Withdrawn legal-status Critical

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Classifications

    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • 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
    • H01S3/094003Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre

Abstract

A laser oscillator 4 comprises an optical fibre 5 contained within a resonant cavity formed by two dielectric mirrors 2, 3 (or fibre grating reflectors for example), a pump signal 6 being coupled into the fibre 5 by coupler 8 and the resonant cavity having a resonant mode at a wavelength substantially equal to the pump wavelength. The pump signal is caused to make multiple passes through fibre 5. <IMAGE>

Description

AN OPTICAL DEVICE This invention relates to an optical device of the type comprising a laslng medium, and particularly, bt not exçlusively to a device wherein the lasing medium comprises an optical fibre.

Such devices may be used as lasers, in which a pump signal input into the lasing medium produces a population inversion and this allows lasing to occur at a lasing wavelength. In order for lasing to take place, the pump power absorbed by the lasing medium must exceed its lasing threshold. The pump pcwer absorbed will depend upon the power of the pump signal and on the nature and the length of the lasing medium. For example , if the lasing medium comprises a rare-earth doped fibre, the absorption will depend1 amorost other things, upon the concentration of the dopant ions in the fibre.

It is known in the art that laser act Ion can be caused by passing a pump signal once through the lasing medium. The length of the lasing medium is ideally sufficiert to absorb a large fraction of the pump, for if only a small fraction of the pump power is absorbed by the medium, the device will be inefficient.

One known way of increasing the efficiency of a lasing device is to cause the pump signal to pass twice through the lasing medium by means of a mirror placed at one end of the lasing medium. This results in a doubling of the effective absorption length of the lasing medium.

Another known way of increasing the efficiency of a lasing device which contains a fixed length of lasing medium is to increase the concentration of active species in the lasing medium. For instance, if the lasing device is a rare earth doped optical fibre, the efficiency of the lasing medium may be increased by increasing the concentration of the rare earth ions. A problem with this method of increasing the efficiency of the lasing device, is that high concentrations of dopant ions may result in problems of concentration quenching and ion-io interactions. The donut concentration may only be increased to a limited extent, therefore.

AccordIng to a first aspect of the Invention, there is provided an optical device for producing an output signal having an output wavelength comprising a lasing medium.

pumpable at a pump wavelength1 and first and second reflectIng means defining a resonant cavity having a resonant mode at a wavelength substantially equal to the pump wavelength, the lasing medium being positioned within the cavity.

By arranging that the resonant cavity has a resonant mode substantially equal to the pump Wavelength the pump signal will resonate within the cavity and as a result it will make multiple passes through the lasing medium. This will result in the effective pump absorption length of the lasing medium being substantially increased.

Preferably, the optical device further comprises pump source for producing a pump signal at the pump wavelength, and coupling means for coupling the pump signal into the cavity.

The pump source may be any type of coherent light-source but preferably it is a semiconductor laser, such as a GaAlAs laser. Certain types of OaAlAs lasers operate substantIally on a single longitudinal ode with line widths of only a few tens of WF when running well above threshold. The absolute wavelength of the pump source is not particularly important as long as it falls within the absorption band of the lasing medium. The spectral width of the pump signal must be narrower than the resonant mode of the cavity.

Advantageously, the cavity is a Fabry-Perot etalon. It could also be a ring cavity.

The reflecting means may be fibre grating reflectors, but preferably they each comprise a dielectric mirror. Each reflecting means may be made to have different reflectivies at different wavelengths. In other words, each mirror may reflect the pump signal and the output signal to dIfferent degrees.

Advantageously, the first reflection ineans is substantially 100% reflectIve at the output wavelength, and the second reflective means is substantially 1000/o reflectIve at the pump wavelength.

Conveniently, the iasing medium is a rare earth doped optical fibre. The length of the fibre may be chosen to be substantially equal to the length of the cavity, and the reflecting means may be formed on the ends of the fibre.

According to a second aspect of the present invention there is provided a method of pumping a lasing medium at a pump wavelength comprising the steps of: positioning first and second reflecting es such that they form a resonant cavity around the lasing medium the resonant cavity having a resonant mode at a wavelength substantially equal to the p wavelength; causIng a pump source to produce a pump signal at the pump wavelength; and coupling the pump signal into the cavity.

The invention is applicable to all types of optically pumped lasers and amplifiers and is particularly useful when it is desired to form a laser or amplifier having a short lasing medium.

The invention may be used to form a single longitudinal mode fibre laser. It is well known that the longitudinal mode spacing in a laser is inversely proportional to the cavity length. In order to produce a single longitudinal mode fibre laser, it is necessary to reduce the number of modes in the cavity to one. This may be achieved by using a very short length of fibre. By forming the laser according to the invention, the pump signal will make multiple passes through the fibre, and an efficient laser will result.

The invention may be used to form a resonant amplifier. It is known tat, in order that an input signal is efficiently coupled into the amplifier, a short device is required so that the linewidth of the cavity modes Ls sufficiently broader than the linewidth of the input signal.

The invention may be used to form a Yb3+ laser oscillating at 980nm, which would form a very attractive pump source for erbium doped fibre amplifiers. In order to force the Yb3+ laser to oscillate at 980nm rather than beyond l m, a short length of fibre st be used.

The in-;entlon ray be used to form a bulk 3-ievel laser. In a bulk laser it is necessary to focus the pump signal into the lasing medium. If the focussed spot is too small it will diverse sgnl-icantly so that the pump spot size is large at the ends of the laser medium. The present invention by providing a sort lasing medium whlch absorbs all the pump power, and ths allows all pump spots to be achieved, overcomes tis problem.

in this specIfication the term "optical" is intended to refer to tat part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.

The invention will now be described by way of example only with reference to the accompanying drawIngs in which Figure 1 is a schematic diagram of a symmetric lossless Fabry-Perot etalon; and Figure 2 is a schematic diagram of a fibre laser according to the present invent ion; In all known optically pumped lasers, for example rare-earth doped fibre lasers and amplifiers, the pump signal has been launched into the laser medium and allowed to propagate either once or twice only down the fibre. It is well known that the signal produced by the pump signal will resonate within the cavity to produce an output signal at a lasing wavelength.

Referring to figure 1, a symmetric lossless Fabry-Perot etalon is shown comprising two dielectric mirrors 2, 3 spaced apart by a distance d and each having a reflectivity of t?, at a particular wavelength #.

Simple theory shcws that the power transmission and reflection coefficients T and R respectively of the etalon are given by the followIng relationships; <img class="EMIRef" id="027058268-00050001" />

where F = 4R1 and @ =2nd (1-R1)2 # If 8 = 2m #, T = 100% and R = 0% Supposing that mirror 3 is replaced by a perfect mirror and that the medium inside the etalon is lossy.

If a is the absorption coefficient of the tedium, and mirror 3 is a 100% reflector at the pump waveiength then the power reflection coefficient of the is etalon given:- <img class="EMIRef" id="027058268-00050002" />

where <img class="EMIRef" id="027058268-00060001" />

If R, is chosen to be equal to e-2ad, then the expression for the reflectivity (3) simplifies to <img class="EMIRef" id="027058268-00060002" />

which is equal to equation (2).

For 6 = 2m#, R will be 0% but because the reflectivity of mirror 3 is 1000/o, there is no transmission through the etalon. Therefore, on resonance, all the input power is absorbed by the laser medium within the cavity.

Furthermorer the intracavity intensity on resonance is given approximately by Iintra = Iin ad A typical value of ad would be around 0.1 (i.e. 10% loss per pass) so that an etalon with a finesse of around 30 would be produced.

Referring to Figure 2, a laser 4 according to the invention is shown. The laser 4 comprises a length of Ytterbium doped optical fibre 5 positioned within a Fabry-Perot etalon of the type shown in Figure 1 and comprising dielectric mirrors 2 and 3.

Laser 4 is pumpable at a pl=p wavelength, may be caused to laser to a lasing wavelength, ad has an absorption coefficient. In this example, laser 4 is pumpable at 860nm and will lase at 980nm. The absorption coefficient a at 860 nm is 3.6% per cm and the dopant concentration of Ytterbium ions is 1020 ions/cm.

The reflectivity of mirror 3 at 860nm is 1000/o and tat of mirror 2 at 860nm is 92.8%, that is e-2ad. As explained with reference to figure 2, there will thus be no transmission of the pump signal, and on resonance all the pump power is absorbed in the cavity. When the lasing threshold has been reached, the laser 4 will lase at a lasing wavelength of 980nm. The reflectivity of the mirror 2 is 1000/o at 980nm and that of mirror 3 is less tn 400/0 at 980nm. This ensures that the laser oscillates at 980nm rather than 1.03 m.

The laser 4 ray be nanufactured by chosing the length of the fibre 5 appropriately. The dielectric mirrors 2,3 are then formed on a respective end of the fibre 5.

The pump signal 6 is produced from an index guided GaAlAs laser 7 which operates essentially on a single longitudinal mode with narrow linewidths. The pump signal 6 is coupled into the laser 4 by means of a coupler 8. Alternatively the pump signal 6 may be coupled into the laser by means of a microscope objective.

Claims (13)

1. An optical device for producIng an output signal having an output wavelength comprising a lasing medium plimpable at a pump wavelength, and first and second reflecting means defining a resonant cavity having a resonant mode at a wavelength substantially equal to the pump wavelength, the lasing medium being position within the cavity.
2. An optical device as claimed in claim 1 further comprising a pump source for producing a pump signal at the pump wavelength, and coupling means for coupling the pp signal into the cavity.
3. An optical device as claimed in claim 1 or claim 2 2 to 6 wherein the pump source is a semiconductor laser.
4. An optical device as claimed in any one of the preceding claims wherein the cavity is a Fabry-Perot etalon.
5. An optical device as claimed in any of the preceding claIms wherein the reflecting means each comprise a dieletric mirror.
6. An optical device as claimed in any one of the preceding claIms wherein the first reflective means is substantially 1000/o reflective at the output wavelength.
7. An optical device as claimed in anyone of the preceding claims wherein the second reflective means is substantially 1000/o reflective at the pump wavelength.
8. An optical device as claimed in anyone of the preceding claims wherein the lasing medium is a rare-earth ion doped optical fibre.
9. A method of pumping a lasing medium at a pump wavelength comprising the steps of: positioning first and second reflecti g means, such that they form a resonant cavity around the lasing medium the resonant cavity having a resonant mode at a wavelength substantially equal to the pump wavelength; causing a pump source to produce a pump s'ar.al at the pump wavelength; and put on same line as; and couplIng the pump signal into the cavity.
10. a method according to claim 9 wherein the lasing medium is a rare-earth ion doped fibre.
11. A method according to claim 10 wherein the length of the fibre is arranged to be substantially equal to the length of a cavity, and the reflecting means are formed on either end of the fibre.
12. An optical device substantially as hereinbefore described with reference to the accompanying drawing.
13. A method substantially as hereinbefore described th reference to the accompanying drawings.
GB9000079A 1990-01-03 1990-01-03 A laser oscillator Withdrawn GB2239733A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB9000079A GB2239733A (en) 1990-01-03 1990-01-03 A laser oscillator

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB9000079A GB2239733A (en) 1990-01-03 1990-01-03 A laser oscillator

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Publication Number Publication Date
GB9000079D0 GB9000079D0 (en) 1990-03-07
GB2239733A true GB2239733A (en) 1991-07-10

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0743722A2 (en) * 1995-05-15 1996-11-20 AT&amp;T IPM Corp. High-power pumping of three-level optical fiber laser amplifier
DE19723269A1 (en) * 1997-06-03 1998-12-10 Heidelberger Druckmasch Ag Solid-state lasers with one or more pumping light sources
US8556511B2 (en) 2010-09-08 2013-10-15 Abbott Cardiovascular Systems, Inc. Fluid bearing to support stent tubing during laser cutting

Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3403352A (en) * 1967-04-10 1968-09-24 Westinghouse Electric Corp Laser having efficient coupling between a phosphor pump source and the laser medium
US3766488A (en) * 1971-06-17 1973-10-16 Bell Telephone Labor Inc Dye laser with pump cavity mode matched to laser resonator
GB1565614A (en) * 1977-03-31 1980-04-23 Hughes Aircraft Co Lasers
EP0123198A1 (en) * 1983-04-13 1984-10-31 COMPAGNIE GENERALE D'ELECTRICITE Société anonyme dite: Gas laser device for emitting pulses of a unique frequency
US4513424A (en) * 1982-09-21 1985-04-23 Waynant Ronald W Laser pumped by X-band microwaves
US4710940A (en) * 1985-10-01 1987-12-01 California Institute Of Technology Method and apparatus for efficient operation of optically pumped laser
WO1988006358A1 (en) * 1987-02-11 1988-08-25 Macquarie University Wavelength locked laser light source
US4837771A (en) * 1985-05-01 1989-06-06 Spectra-Physics, Inc. High-efficiency mode-matched solid-state laser with transverse pumping and cascaded amplifier stages
EP0327310A2 (en) * 1988-02-02 1989-08-09 Massachusetts Institute Of Technology Solid state microlaser

Patent Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3403352A (en) * 1967-04-10 1968-09-24 Westinghouse Electric Corp Laser having efficient coupling between a phosphor pump source and the laser medium
US3766488A (en) * 1971-06-17 1973-10-16 Bell Telephone Labor Inc Dye laser with pump cavity mode matched to laser resonator
GB1565614A (en) * 1977-03-31 1980-04-23 Hughes Aircraft Co Lasers
US4513424A (en) * 1982-09-21 1985-04-23 Waynant Ronald W Laser pumped by X-band microwaves
EP0123198A1 (en) * 1983-04-13 1984-10-31 COMPAGNIE GENERALE D'ELECTRICITE Société anonyme dite: Gas laser device for emitting pulses of a unique frequency
US4837771A (en) * 1985-05-01 1989-06-06 Spectra-Physics, Inc. High-efficiency mode-matched solid-state laser with transverse pumping and cascaded amplifier stages
US4710940A (en) * 1985-10-01 1987-12-01 California Institute Of Technology Method and apparatus for efficient operation of optically pumped laser
WO1988006358A1 (en) * 1987-02-11 1988-08-25 Macquarie University Wavelength locked laser light source
EP0327310A2 (en) * 1988-02-02 1989-08-09 Massachusetts Institute Of Technology Solid state microlaser

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0743722A2 (en) * 1995-05-15 1996-11-20 AT&amp;T IPM Corp. High-power pumping of three-level optical fiber laser amplifier
EP0743722A3 (en) * 1995-05-15 1996-11-27 At & T Corp
DE19723269A1 (en) * 1997-06-03 1998-12-10 Heidelberger Druckmasch Ag Solid-state lasers with one or more pumping light sources
US6434177B1 (en) 1997-06-03 2002-08-13 Heidelberger Druckmaschinen Ag Solid laser with one or several pump light sources
US8556511B2 (en) 2010-09-08 2013-10-15 Abbott Cardiovascular Systems, Inc. Fluid bearing to support stent tubing during laser cutting

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Publication number Publication date
GB9000079D0 (en) 1990-03-07

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