CA2189568C - Optical fibre laser - Google Patents
Optical fibre laser Download PDFInfo
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- CA2189568C CA2189568C CA002189568A CA2189568A CA2189568C CA 2189568 C CA2189568 C CA 2189568C CA 002189568 A CA002189568 A CA 002189568A CA 2189568 A CA2189568 A CA 2189568A CA 2189568 C CA2189568 C CA 2189568C
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES 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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
- H01S3/0675—Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES 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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/0632—Thin film lasers in which light propagates in the plane of the thin film
- H01S3/0635—Thin film lasers in which light propagates in the plane of the thin film provided with a periodic structure, e.g. using distributed feed-back, grating couplers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES 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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
- H01S3/1055—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length one of the reflectors being constituted by a diffraction grating
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES 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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/1601—Solid materials characterised by an active (lasing) ion
- H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
- H01S3/1608—Solid materials characterised by an active (lasing) ion rare earth erbium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES 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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/1601—Solid materials characterised by an active (lasing) ion
- H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
- H01S3/1618—Solid materials characterised by an active (lasing) ion rare earth ytterbium
Abstract
An optical fibre distributed feedback laser comprises an amplifying optical fibre (50) operable to provide optical gain at a lasing wavelength, in which a diffraction grating (30) is disposed on at least a po rtion of the amplifying optical fibre to provide distrib uted optical feedback for sustaining lasing action at the lasing wavelength.
Description
WO 95!31020 21 ~s ~ ~ U ~ PCT/GB95/01026 OPTICAL FIBRE LASER
This :invention relates to optical fibre lasers.
Many light sources for optical fibre communications are electrically pumped Fabry-Perot (F-P) type semiconductor lasers, where the optical feedback is provided by reflections from the laser cavity end-surfaces (mirrors).
A lasing mode in a F-P type laser has to meet two conditions:
1) the cavity gain must be equal to the cavity losses, and !O 2) the cavity round trip phase shift must be equal to a multiple of 2n.
The thresho:Ld gain is the same for all modes satisfying the phase condition. Since the gain difference between the adjacent longitudinal modes is general7_y relatively small and there are many modes which satisfy the phase condition, these lasers may exhibit mufti-mode 1'i operation.
In applications such as high speed optical communication where dynamic single-made operation under modulation is required, it is common to use semiconductor distributed Bragg reflector (DBR) lasers.
A DBR laser is a F-P-type laser where the end-reflectors are 2O Bragg gratings which have a wavelength dependent reflection owing to their periodic spatial variation of the refractive index.
The reflection from a Bragg grating is strongest at the Bragg wavelength i~B whic:h fits the grating period A (~,B = 2nA, where n is the average refractive= index of the medium). The lasing conditions for a 25 DBR laser are the same as for a F-P laser with mirrors, but since the gratings only reflect within a narrow spectral band around the Bragg wavelength the mode closest to the Bragg wavelength will experience stronger feedback than the other longitudinal modes. If the difference in reflectivity (feedback) between adjacent longitudinal modes is 30 sufficiently large, only a single longitudinal mode will tend to oscillate under modulation conditions.
Semiconductor distributed feedback lasers have been proposed. In these devices a ~;rating extends across the lasing region and causes distributed feedback of the laser radiation.
3~i DBR lasers .in which the gratings and the gain medium are formed from optical fit~re have been reported. These devices have the advantage that they are compatible with optical fibre communication kl~,C.t'ik:l~IF'C) / c)11'1 t-'C:'1' :'?:.i- :)-:aE: : 11:l:3 : +fJ ~:a '_'a~3.3~'I-~l(~5-~ Geneva CfI:Idl1 . .'-.
' ..' links, they are simple in construction, an~.3 can offer low intensity noise and narrow linewidths.
However, in order to achieve single frequency operation from a DEAR fibre laser. Cwo apparently conf'licring requirements must be Gi ad~3ressad .
Firstly, the length of the Fibre cavity {the fibre between the two end-reflectar,sj must oe made as short as possible. The reason far this is that the spe,~tr&1 spacing between resonant Cavity mocfe5 increases wi.tn a decreasir:g cavity length. Where the resonant cavity iC) modes are more widely sapaz~ated, the differences in the round-trip cavit;~ lus5 between adjacent modes coos=:r3 ~,y the wave~:ength dQpendent refle~;tit~rl of the gratings are more exaggerated, so .it is more likely that a single oscillarian mode will dominate.
However, a second requirement results from the fart that fibre 1i lss~rs are iongitudinal.ly pumped opti,:c~tly, i:~ contrast to the elPCtrical pumpiu~; c~F semiconductor lasers. This feature means thFlt the ':'fibre cavity should be made long enough to allow sufficient energy at the pump wavelength to be abac:rbed.
The first rEquirrament listed above leads to a desired cavity 2l1 1 e.~.gth of about a few c;entiolatres or less . However, in erbium {
Er3'~
doped fibres commonly used as the gain med.i.um, the pump absorption in such short laser lengths is normally only a few percent., and hence the 'iO~CLi~.1(?d slope effWienuy of these lasers is very low (~.ess than 1:J
even with high Erg°-concentration. If the pump absorption is increased %~; by doping the hare more heavi:~y, so-called concerttcation quenching occurs leading to a loss of slope efficiet:Gy arid the lasers r~rc also hen prom to strong self-pulsation. Although a "MOPA" (Master Oscillator i:'ower Amplifier) confi~::ation tms been proposed, where the residual pump faower is used r,r~ pump a fibre amplifier, the output from '3() a MOPA is mare noisy than the direct output from a laser, olio it usually requires an c:~pt.ical isolator between the laser and the amplifier to eliminate r_oupling between the laser and the amplifier and to improve the noise.
US-A-3 958 18$ discloses a side-pumped DD le.se>" m2tking use of a 3'i grating formed by micromachining grooves into an optical fibre. The artic~.e, "Photoser~sitiwzty arid self-organisation in opt:.cal fibres and waveg~sides", pros SPIE, val ~'04~~ pp 6y-~5 di.scloses an ultraviolet AfiA~~NDEO Si~EET
Kt~c..t'filr:~llt'u / <Wlt't t'C.I '?al- :l-.lti : ll:l:i : +L.) fi.3 '?~ia;34$65-~ Geneva Cil:#O?
?1 ~~5~~ - .. :,, side~pumped transient fibre Uf8 lt~ser with pulsed operation. ZEFE
,Tournal of Quantu~a, Electronics. 25 (1~$~r) " pp $71-$7~ azd Optical Engineering, 31(199 0 3, pp 440-445 disclose Nd:YAG crystal fihrc~ 11FB
lasers.
This invention provides tzn optical fibre di:;trbuted feedback laser comprising ~m amplifying optical Fibre doped with e>~bium and ytterbium and oper~gble t:o provide optict~.?, gain at a losing ~rr~velength, in which a diFfract;ion grating is disposed on at least a portion of trhP
&raplifying optical fibre en provide distributed optical feedback for sustaining la:;ing action within the portion at tae losing wavelength.
In a UFH (distribuCed feedback) fi.bra laser according to tee i.nv~ntion, the optical feedback required f or the laser eperaCi,.on is provided by a periodic spatial variation of the reFractive index of the actual gain medium ar of other fibrF~ properties suc!~ that an optical '> wave travelling in the fcrward di.-ectton is successively reflected by i.Y:e grating into a wave: travelling in the backward direction rend vice versa. Zn other words, the gain mediu>z, and the feedback grating are co-located.
Fibre DHB la.lers will have the same advantages over semiconductor 2U lasers as the fibre DHR lasers described above, but will also tend to have a better single-frequency sc.ability than the fibre D6R lasers.
This feature ari9e;3 because the laser wavelength of a fibra DfD laser can be uniguely cleter~nined by the Dragg wavelength of cne single seating. Also, single mode cperltion is pro>r,oted because the threshold 27 gain increases with wavelength away from the 8ragg wavclongth. In contrast. in a fitare nGll .laser, which is am r-P-type laser, the laser wavelength i.s the wavelength wirh lowest round trip loss. This normally corresponds to the longitudinal mode closest to (but in general different from) the Bragg wavelengr~l. In other words, the ~0 requirement for st:ort :length in nrc3er to achieve mode stability is relaxed.
In additi~~n, since the lowest loss mode in an F-P type laser depends on the product oaf the ref.lecti,an spectra of the two gratings, art accurate determinat:ic~n of the laser wavelengrh this requires 3j matching of the Dr~~gg wavelengths of two gratings, either through the writing of the gr,3t;ings, or aFuer the writing through tuning of one of the gratings {eithf;r by heating/eoclittg czr by stretching the grating).
~~NaEa sH~~t kh:C'.1';\k:vlh';) ~ U~1!'1 1'C'T .,-.,~i_ ,~i_:it~ : 1l:i:i : ~~t.j ti;) :.';39~14~1fi5~ Gan~~:a CH:#1:3 ~ ~., 2~ ~~=~6~ : ..: ;
.. ., A fibre DFS laser can operate at a wavelength defined by on~,y cane grating.
Accordingly, a laser ae~::ording to the invention Ca.n avoid the problems de9cribed abo~re by allowing robust single mpde operation without necessari:.y comprermi,sing the laser power output or noise perfox-mance.
The amplifying optical fibre is doped with rare earth dopants.
In particular, the smFlifying optical fib~~ is doped with erbium, and.
as cc~-dopant cc ir,.crease the absorption of the pimp wavelength, with 1C~ ytt',Prbium (Yb3~) .
Preferably tJze amplifying opt.i4a1 fibre is optically pumped. Xrt this case it iv pre:ferred that the laser courpri,ses means far. generating optical pump radiation to pump the amplifying optical fibre.
A part of the amplifying fibre can advantageously be used try 7~ amp7.ify the laser output, being pumped by residual optical pwnping rr~di.ation emerging from the laser.
In one preferred e~mbndiaront the laser comp:~ises a mirror disposed a.t one end of the c!iffra.ct,.ion gratin; for I'eflocting at least a part. aY
optical radiation propagating along the diffraction grating. "'he 2C~ rri.rrar can int.rodL;ce a phase shaft to promote the propagation within the laser of r:~da.atiau at ti single mode either side of the Eragg wavelengt~i of the diffraction grating.
Preferably, the l.angitudinal separation between the mirror and the diffraction grating i5 less than 1 sicromptre (about hxl.P a Ly wavelength in thp fibre medium). Zt is preferrod that this separation can be adjusted by suitable adjustment metros.
As an alternative approach, it is preferred that the ,grating itself comprises at least one phase irregularity. This can promote the propagation of radiation at the Bragg wavelength.
;CI The phase irr~gut,arity could be a discontinuity or a substantially continuous phase variation along the grating.
One prefsrr~ed way of generating cs grating with a phrAse irregularity is tree so-ca112d "hloire" method, in which the grating is fabricated by g~ene:cating an inLerf erence pattern betsvean two superposed 3~i diffraction: gratings. The use of this method to form gratings havinv a phase irrpgula:~~ty, and the potent~.a1 use of such grat:.ngs ::n srmiconductor DfI3 lasers, is described in the paper "Phase Shifted A~R~~VGEJ SH~Ef Ktl.l'HK YIIt'(J i Uvlt'! t'O;l' :'W3- :t-:Jt~ : tl.'lf ~ +49 89 .':39,3=t-lEio- Gemvs CH:#14 ~1 a956~3 .
:~
Moire Grating Fibre Reacnators", Clectronics Lette~s, ~'oi 26, Yo 1, pp l0-11 (~ggo;.
Although th~a pha~~ irregularity could be perniancr~tly FabricatEd into the grating, the phase irr~gu.larity could inste,~d be gerera;;ed only during oper,atioci or i;~ fabri.catian, far example by heating a pardon of ,~.he grafting delring operation.
Viewed from a see:ond aspect, this inventicn provides a method of Fxuricating ari ,~f~cical fibre distributwd feedback laser, the method cataprising thp step pF fabricr~ti.ng a diffraction gracing on at least a nortinn of an arnp7_ifyir.g optical fibre doped with erbium and ytterbium and aperabl~~ to provide optical gain at a losing wavelength, thG
diffraction grating providing distributed optical fnedbaak for sustaining lacing action w:i,thin the portion at tree losing wavelength.
The invenCion well now be described by way of example with rcferenrc~. to the aeccmPanying drawings, throughout which like parts are referred to by like re:'erences , and n which:
Figure 1 is a sc:hernatic diagra..~r of a doped opiiea.l. fi'ore DF3 la.~~r employing a. uri:.fc;rm grating with no and-reflecGars;
Figure ~ i5 a s:~hem~,tic diagram of a doped optical fibre DFB
2~ laser emplny~.ng a phassy--shifted grating with cio end-reflectors;
figure 3 zs a sc:hemn,tic diagram of a doped optical fibre DFB
lr~s~~ employing a uniform grating with one end-reflector;
Figure 4 is a graph illustrating the theoretical laser threshold gait; of fundsment:al mode of a DF13 laser as G fmiction of grating ?'a strength with (i) a uniFortn grt~ting, (ii) a uniform grating with ant substantially lOCi> end-reflector, ~~d (iii) a a/2 phase shifted grating;
Figure 5 i:. a schematic diagram of tu>. Er3' : Yb''' dcped Fibre DFF3 aser employing =x uniform grating with a substant~:ally I.OOx ~~3 end-reflecrnr;
Figure 6 is a schematic diagram of an fir'' ; Yb3' 3cped fibre D~'H
laser employing a (temperature) phase-shifted grating.
Figure 7 is a graph illustrating empirical laser characteristics for a DFB fibre laser employing (i) a uniform grating with 3y substantially 1U0: end-reflector, and (ii) a (temperature) phase shiftad grating;
~1~1E~J~~~ ~Ilr.' ..
KW . t'AK:vvIr'l) / toll'1 YC1' :'W3- 'v-;~t~ : I 1 : l~1 . +E9 E39 '?~3,1,i4~65-> i:en~,va CH:fiilS
5a Figure 8 is a graph illustrating the optical spectrum of a uniform grating fibre Flfb laser with a stustalltially 10(~x end-rPfl.ector in single-mode and double-mode operation;
figure 9 is a graph iilustrating trl.e opticttJ, spectrum of a uniform grating ffibre aF~ lasEr with a substantially 100y end-reflector against mirror displacement;
Figure 10 is a graph illustrating laser linewidth of a aingle-mode uniform grating fibre DfB laser with a subs tantiesliy 100x et:d-reFZector and a phssc~-,shifted grating fibre Jf8 laser;
AMENDE,a SHEET
Figure 11 is a graph illustrating the optical spectrum of a temperature phase-shifted grating fibre DFB laser at maximum power; and Figure 12 is a graph illustrating the optical spectrum of a temperature phas e-shifted grating fibre DFB laser against wire-temperature.
Figures 1. 2 and =i are schematic diagrams of doped optical fibre DFB lasers. In pa.rticu.lar, Figure 1 illustrates a laser employing a uniform grating with no end-reflectors; Figure 2 illustrates a laser employing a phase-shifted grating with no end-reflectors; and Figure 3 illustrates a laser employing a uniform grating with one end-reflector (e. g. a dichroic e:ad reflector).
The DFB lasers shown in Figures 1 to 3 comprise optical fibres containing one or more rare-earths, in which the feedback (a Bragg grating) and the gain (the doped fibre) are substantially co-located.
In particular, in Figure 1 a uniform grating 10 is imposed on a doped optical fibre 20; in Figure 2 a grating 30 having a phase discontinuity 40 is imposed on a doped optical fibre 50; and in Figure 3 a doped optical fibre 60 halving a uniform grating 70 is disposed adjacent to a reflector 80.
The lasers of Figures 1 to 3 are optically end-pumped (e.g. by a semiconductor pump laser).
In a DFB fibre laser such as the lasers illustrated in Figures 1 to 3, the optical feedback required for the laser operation is provided by a periodic spatial variation of the refractive index of the gain medium or of other fibre properties such that an optical wave travelling in the forward direction is successively reflected by the grating into a wave travelling in the backward direction and vice versa.
The strengtl-~ of the feedback is normally given in terms of the product xL, where x = rcOn/1B is the coupling coefficient and L is the length of tree grating. ~n is the refractive index modulation. The reflectivity at th.e Bra.gg wavelength is given as Tanhz(xL). The width of the grating stop band is proportional to x. The higher x is, the bigger is the fre~3uenc;y separation between the resonant modes of the 3~ DFB structure.
While for a F-P-type laser the threshold gain is the same for all the longitudinal modes, the DFB laser threshold gain increases the WO 95/31020 ~ ~ ~ ~ ~ ~ ~ PCT/GB95/01026 further away the resonant modes are from the Bragg wavelength.
However, with a uniform grating along the whole cavity length and no end-reflectors (i.e. th,e laser of Figure 1) no real mode can exist in the centre of the stop-band, ie. at the Bragg wavelength. At this wavelength the round trip gain has a real value, but with a negative sign. In such a device two modes of equal threshold occur symmetrically on each side of the stop-band, where the phase condition can be satisfied. Acco:cd:ingly a uniform DFB laser of the type shown in Figure 1 will oscillate at two different frequencies separated by the width of the stop-band (which is proportional to x).
Figures 2 and 3 illustrate two possible ways of obtaining single frequency operation of a DFB laser. In Figure 2 a single-pass phase shift (or irregularity) of n is introduced in the grating. This corresponds to a r'/2 optical phase shift so that the round trip phase condition is satisfied at the Bragg wavelength.
The phase shift can be introduced into the grating either through a phase shift in the grating itself or through a phase-adjustment region. In a phase shifted DFB laser the field of the mode decays exponentially as Fin evanescent field from the phase shift region into the DFB structures on the two sides and is coupled out. A phase shifted DFB laser also has the lowest possible laser threshold gain since the feedback is strongest at the Bragg wavelength, and so is suitable for sing7.e mode operation. The threshold discrimination and frequency separation between the fundamental mode and the first higher order modes is greater than that of the uniform DFB laser of Figure 1, and much greater than F-P-type lasers, including DBR lasers.
An alternative arrangement is shown in Figure 3 in which an end reflector is used to change the round trip phase shift in the laser.
. With one end-reflector one of the two modes will be enhanced, while the other will be suppressed, depending on the phase of the mirror relative to the grating. The end-reflector will effectively increase the grating length and reduce the threshold gain. Also the threshold will depend on the mirror phase. It is therefore preferred that the solution of Figure 2 i:a adopted.
Fibre DFB lc,sers will have the same advantages over semiconductor DBR/DFB lasers as the fibre DBR lasers described above, but will also tend to have a better single-frequency stability than the fibre DBR
WO 95/31020 ~ PCT/GB95/01026 lasers. This feature arises because the laser wavelength of a phase-shifted fibre DFB laser is unicuely determined by the Bragg wavelength of one single grating, while in a fibre DBR laser, which is an F-P-type laser, the laser wavelength is the wavelength with lowest round trip loss. This normally corresponds to the longitudinal mode closest to (but in general different frcw) the Bragg wavelength. However, since the lowest loss mode in an L'-P laser depends on the product of the reflection spectra of the two, gratings, an accurate determination of the laser wavelength requires r~atching of the Bragg wavelengths of two gratings, either through the writing of the gratings, or after the writing through tuning of one of the gratings (either by heating/cooling or by stretchi-~; the grating). With one mirror and one grating end-reflector matchin;~ is not a problem, but this type of F-P
laser is no longer a true all-fibre laser.
The gratings 10, 30 an: 40 can be fabricated as intra-core gratings or gratings at the ccre/cladding boundary.
Intra-core fibre Bragg gratings can be produced non-invasively in germanosilicate-based and ether photosensitive fibres. In germanosilicate fibres the gratings can be written holographically through the cladding using a coherent UV (ultraviolet) two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244nm (nanometres). The Bragg wavelength of the grating can easily be adjusted by changing the angle between the two interfering begs.
Fibre gratings can also re written by the use of a silica phase mask illuminated by a single U~ laser beam, as described in the paper "Phase-mask method for volume manufacturing of fibre phase gratings", Conference on Optical Fibre Communications, 1993, post deadline paper PD16.
Figure 4 is a graph illustrating the theoretical laser threshold gain of the fundamental mode of a DFB laser as a function of grating strength xL with (i) a uniform grating, (ii) a uniform grating with one substantially 100 end-reflector, and (iii) a n/2 phase shifted grating. Figure 4 shows that a phase-shifted grating DFB laser has a significantly lower threshold gain than uniform grating DFB lasers (with or without end-reflector). In the case of a uniform grating DFB
laser with one end-reflector the threshold gain is significantly lower than without any end-reflectors, but it depends on the phase relationship between th.e reflector and the grating.
A phase-shifted fibre DFB laser can be robustly single mode for much longer cavity lengths than a fibre DBR laser (because of the differential threshold gain between the fundamental mode at the Bragg wavelength and higher order modes) provided that one can write sufficiently long fibre gratings.
Two specific prototype examples of DFB optical fibre lasers will now be described with reference to Figures 5 to 12. These are (a) a uniform grating fibre DFB laser with one end-reflector with substantially 100%. reflectivity at the lasing wavelength (Figure 5);
and (b) a phase-shifted grating fibre DFB laser with no end-reflectors (Figure 6).
Figure 5 is a schematic diagram of an Er3':Yb3' doped fibre DFB
laser employing a uniform grating 90 written into an Er3':Yb3' doped fibre 100 with an end-reflector 110 (e. g. a dichroic reflector) having a reflectivity of substantially 100% at the lasing wavelength.
Similarly, F'igure~ 6 is a schematic diagram of an Er3':Yb3' doped fibre DFB laser in which a uniform grating 120 is written into an Er3':Yb3' doped optical fibre 130. In this case, a temporary phase irregularity is introduced into the grating in operation by heating a small region of the grating by passing current through an electrical resistance wire 140 wound around the fibre. This generates a temperature gradient 1!j0 along the fibre, altering the phase of the grating impressed on the fibre. This arrangement will be referred to as a (temperature) pha~~e-shifted grating.
The fibres ace doped with both Er3' and Yb3', the latter having the effect of increasing th~~ absorption at the pump wavelength by more than two orders of magnitude. The 980nm pump excites the Yb3' ions, and energy is efficiently transferred to the Er3' ions by resonant coupling.
In each of the two cases the grating was a 2cm (centimetres) long uniform fibre Bragg grating with a refractive index modulation of about 2.1"10-'' (xL=8.5) ~~rritten in an alumino-phosphosilicate Er3':Yb3'-doped fibre. The Er3':Yb3'-doped fibre was made photosensitive using the so-called "hydrogen :loading" technique, described in the article, "High pressure HZ loading as a technique for achieving ultrahigh UV
WO 95/31020 ~ ~ PCT/GB95/01026 photosensitivity and thermal sensitivity in Ge02 doped optical fibres,"
Electronics Letters, vol. 29, pp. 1191-1193, 1993. The fibre was soaked in 140 atmospheres of H2 at 80 degrees Celsius over 15 hours.
The grating was then written with a Krypton-fluoride (KrF) excimer 5 laser at 248nm using a side-writing technique described in the article "Formation of Bragg gratings in optical fibres by a transverse holographic method," Optics Letters, vol. 14, pp. 823-825, 1989. The laser was fired at 20Hz with a pulse fluence of about 0.5J/cm2, exposing the 20mm fibre section for over 3 hours.
10 The phosphorus oxide (P20~) and aluminium oxide (A1203) concentrations in the Er3':Yb3'-doped fibre are 18 and 2 wt% (percentage by weight) respectively. The Er3' concentration is approximately 1000ppm (parts per million) (0.26wt%). The fibre has an Yb3':Er3' concentration ratio of 12.5:1, a Numerical Aperture of 0.2 and a second mode cut-off wavelength of 1130nm. The diameter of the fibre was 125um. The peak absorption at 1535nm (which was also the fibre grating Bragg wavelength) was 0.5dB/cm. This is approximately the maximum available gain (with complete population inversion). Hence, with a 2cm long DFB structure the maximum available single pass gain is ldB.
Referring to Figure 5, the total length 160 of the Er3':Yb3'-doped fibre was 3cm, and the length 170 of the grating at the reflector end was 2cm. The fibre was embedded in a glass capillary with a diameter of 3mm. An output end was angle-polished to avoid reflections.
The laser was pumped by a 100mW 980nm diode laser through the reflector which transmitted about 97% of light at the pump wavelength.
The laser characteristics of the uniform grating DFB laser with an end-reflector having a substantially 100% transmission at the pump wavelength is shown as a curve 200 in the graph of Figure 7. The laser had a threshold incident pump power of about l5mW and an output power of about 2mW for 90mW incident pump power. The output power varied with changing lateral position of the mirror relative to the fibre grating, which will change the mirror reflectivity (both phase and amplitude) and the launching conditions. The slope efficiency was about 5%. Lasing also occurred with an end-reflector with 4%
reflectivity, ie. without the mirror. The threshold pump power in this case was 34mW.
The optical spectrum for the laser of Figure 5 is shown in Figure WO 95/31020 ~ ~ r PCT/GB95/01026 8. The basic spectrum ;?20 has two peaks as expected for a uniform DFB
laser, but the relative power in each of the two modes depends on the mirror phase relat_we to t:he fibre grating. The separation between the two modes is about 0.25nm, as expected from the theory assuming a grating strength of xL=8.5. Figure 9 shows the laser spectrum for increasing mirror displacement (oz) and hence changing phase.
Adjustment means (such as a micrometer screw) can be provided to allow the longitu~3inal mirror-grating separation to be adjusted to improve the laser's performance.
The optical :_inewidth when the laser was single-mode was measured with a self-heterodyne delay-line to be about 60kHz, as shown by a curve 240 in the graph of Figure 10.
During performance testing of the phase-shifted grating fibre DFB
laser as shown in Figure 6, the prototype laser was the same as the uniform grating fibre DFB laser of Figure 5, but with the end-reflector removed and replaced with a glass microscope slide with index-matching fluid to remove any reflections (having the same effect as angle-polishing of the end-face).
The phase shift was introduced by having the electrical resistance wire 140 wrapped around the glass capillary, substantially at the centre of the grating. Alternatively the wire could contact the fibre directly. The wire was 2cm long and 175um in diameter, and its resistance was 0.46 Ohm;cm. By running an electric current through the wire the fibre grating was heated more at the centre than at the ends, causing a falling temperature gradient 150 at each side of the wire.
The refractive index and hence the Bragg wavelength is dependent on the temperature, and is thus changed more at the centre than the ends of the grating, cau:>ing an optical phase-shift for the forward and backward propagating waves in the DFB structure. This caused a gradual phase-shift which can be shown to yield a more uniform intensity distribution along' the gain medium than an abrupt phase shift at the centre of the DFEt structure. This will give a more uniform gain saturation and better use of the available gain.
With this method the position of the phase-shift can easily be changed by chang:_ng t:he position of the wire. With increasing temperature the output power varied periodically, indicating that the phase-shift also varied periodically (as expected). In the present WO 95/31020 ~ ~ r ' PCT/GB95/01026 C_ ! 895f.~8 embodiment the maximum output power occurred with a wire current of 320mA and voltage of 0.6V. The output power from one end of the phase shifted fibre DFB with an optimum phase shift is plotted as a curve 210 in Figure ~.
The pump source was a 150mW 980nm diode laser. The threshold incident pump power is about 32mW, and the slope efficiency is about 5%. The maximum fibre laser output power at the output (angle-polished) end is l.9mW with 12'7mW incident pump power. The laser output power from the input end was not measured, but would be expected to be similar to that at the output end. The gradually increasing slope at low pump powers is probably due to the lcm long piece of Er3':Yb3~-doped fibre at the output end which at low pump power acts as an absorber (with maximum 0.5dB absorption), while at higher pump powers it acts as an amplifier (with maximum 0.5dB gain). Hence, this laser is an example of a MOPA (Master Oscillator Power Amplifier) configuration.
The laser linewidth in this case was measured to be about 300kHz, as shown by a curve 250 in Figure 10. This linewidth was found to be substantially independent of the output power.
The optical spectrum of the phase-shifted fibre DFB laser is shown in Figure 11. The laser is single-mode, with no side-modes.
Figure 12 shows variations in the spectrum with increasing wire temperature. The amplitude increases from zero to a maximum and then decreases again. The wavelength increases with increasing temperature as expected.
There are several other possible ways to induce a phase-shift in the fibre grating, other than the "wire-method" shown in Figure 6. One possibility is to write two gratings with slightly different Bragg wavelengths on top of each other, to form a so-called Moire grating.
This technique is described in the article "Phase Shifted Moire Grating Fibre Resonators", Electronics Letters. Vol 26, No 1, pp 10-11 (1990).
The Moire grating normally has a permanent n phase shift which corresponds to an optical phase shift of n/2, but can also be made with smaller phase shifts (using different exposure times for the two gratings). By having several small phase shifts along the grating, adding up to rc, a gradual phase shift, and hence a uniform intensity distribution can be realised.
An alternative technique is to apply a temperature gradient along the fibre when ttie grating is being written or to induce a variation of refractive index: by means of a un~tform W beam before writing the grating, in order- to generate a phase irregularity or discontinuity at around the centre of the grating. Alternatively. a small section in the middle of the grating could be stretched before, during or after the writing of 'the fixating. It is also possible to leave a small section in the midd7.e of the grating unexposed, causing a phase-adjustment region. This can also be done by either erasing the grating 1t) or changing the ~werage refractive index at the centre of the grating by focusing a single W beam at this point. A phase shifted grating could also be made by using a special phase-mask (with phase shift;.
Whether a discrete phase discontinuity is achieved through modification of the grating or through the use of a mirror, it is iii important to notes that; the phase discontinuity does not tend to act as a resonant cavity (ae~ in a F-p laser) . In fact, the length of the phase irregularity or the mirror-grating separation is preferably less than about 1 micxnmetx~e (1 x 10-6 m) (or about half a wavelength in the fibre medium).
2C1 Although tt~e embodiments described above have related to optical fibres. it will of course be appreciated that the techniques are equally apFlicable to planar waveguides, and in particular, glass planar waveguides:.
This :invention relates to optical fibre lasers.
Many light sources for optical fibre communications are electrically pumped Fabry-Perot (F-P) type semiconductor lasers, where the optical feedback is provided by reflections from the laser cavity end-surfaces (mirrors).
A lasing mode in a F-P type laser has to meet two conditions:
1) the cavity gain must be equal to the cavity losses, and !O 2) the cavity round trip phase shift must be equal to a multiple of 2n.
The thresho:Ld gain is the same for all modes satisfying the phase condition. Since the gain difference between the adjacent longitudinal modes is general7_y relatively small and there are many modes which satisfy the phase condition, these lasers may exhibit mufti-mode 1'i operation.
In applications such as high speed optical communication where dynamic single-made operation under modulation is required, it is common to use semiconductor distributed Bragg reflector (DBR) lasers.
A DBR laser is a F-P-type laser where the end-reflectors are 2O Bragg gratings which have a wavelength dependent reflection owing to their periodic spatial variation of the refractive index.
The reflection from a Bragg grating is strongest at the Bragg wavelength i~B whic:h fits the grating period A (~,B = 2nA, where n is the average refractive= index of the medium). The lasing conditions for a 25 DBR laser are the same as for a F-P laser with mirrors, but since the gratings only reflect within a narrow spectral band around the Bragg wavelength the mode closest to the Bragg wavelength will experience stronger feedback than the other longitudinal modes. If the difference in reflectivity (feedback) between adjacent longitudinal modes is 30 sufficiently large, only a single longitudinal mode will tend to oscillate under modulation conditions.
Semiconductor distributed feedback lasers have been proposed. In these devices a ~;rating extends across the lasing region and causes distributed feedback of the laser radiation.
3~i DBR lasers .in which the gratings and the gain medium are formed from optical fit~re have been reported. These devices have the advantage that they are compatible with optical fibre communication kl~,C.t'ik:l~IF'C) / c)11'1 t-'C:'1' :'?:.i- :)-:aE: : 11:l:3 : +fJ ~:a '_'a~3.3~'I-~l(~5-~ Geneva CfI:Idl1 . .'-.
' ..' links, they are simple in construction, an~.3 can offer low intensity noise and narrow linewidths.
However, in order to achieve single frequency operation from a DEAR fibre laser. Cwo apparently conf'licring requirements must be Gi ad~3ressad .
Firstly, the length of the Fibre cavity {the fibre between the two end-reflectar,sj must oe made as short as possible. The reason far this is that the spe,~tr&1 spacing between resonant Cavity mocfe5 increases wi.tn a decreasir:g cavity length. Where the resonant cavity iC) modes are more widely sapaz~ated, the differences in the round-trip cavit;~ lus5 between adjacent modes coos=:r3 ~,y the wave~:ength dQpendent refle~;tit~rl of the gratings are more exaggerated, so .it is more likely that a single oscillarian mode will dominate.
However, a second requirement results from the fart that fibre 1i lss~rs are iongitudinal.ly pumped opti,:c~tly, i:~ contrast to the elPCtrical pumpiu~; c~F semiconductor lasers. This feature means thFlt the ':'fibre cavity should be made long enough to allow sufficient energy at the pump wavelength to be abac:rbed.
The first rEquirrament listed above leads to a desired cavity 2l1 1 e.~.gth of about a few c;entiolatres or less . However, in erbium {
Er3'~
doped fibres commonly used as the gain med.i.um, the pump absorption in such short laser lengths is normally only a few percent., and hence the 'iO~CLi~.1(?d slope effWienuy of these lasers is very low (~.ess than 1:J
even with high Erg°-concentration. If the pump absorption is increased %~; by doping the hare more heavi:~y, so-called concerttcation quenching occurs leading to a loss of slope efficiet:Gy arid the lasers r~rc also hen prom to strong self-pulsation. Although a "MOPA" (Master Oscillator i:'ower Amplifier) confi~::ation tms been proposed, where the residual pump faower is used r,r~ pump a fibre amplifier, the output from '3() a MOPA is mare noisy than the direct output from a laser, olio it usually requires an c:~pt.ical isolator between the laser and the amplifier to eliminate r_oupling between the laser and the amplifier and to improve the noise.
US-A-3 958 18$ discloses a side-pumped DD le.se>" m2tking use of a 3'i grating formed by micromachining grooves into an optical fibre. The artic~.e, "Photoser~sitiwzty arid self-organisation in opt:.cal fibres and waveg~sides", pros SPIE, val ~'04~~ pp 6y-~5 di.scloses an ultraviolet AfiA~~NDEO Si~EET
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?1 ~~5~~ - .. :,, side~pumped transient fibre Uf8 lt~ser with pulsed operation. ZEFE
,Tournal of Quantu~a, Electronics. 25 (1~$~r) " pp $71-$7~ azd Optical Engineering, 31(199 0 3, pp 440-445 disclose Nd:YAG crystal fihrc~ 11FB
lasers.
This invention provides tzn optical fibre di:;trbuted feedback laser comprising ~m amplifying optical Fibre doped with e>~bium and ytterbium and oper~gble t:o provide optict~.?, gain at a losing ~rr~velength, in which a diFfract;ion grating is disposed on at least a portion of trhP
&raplifying optical fibre en provide distributed optical feedback for sustaining la:;ing action within the portion at tae losing wavelength.
In a UFH (distribuCed feedback) fi.bra laser according to tee i.nv~ntion, the optical feedback required f or the laser eperaCi,.on is provided by a periodic spatial variation of the reFractive index of the actual gain medium ar of other fibrF~ properties suc!~ that an optical '> wave travelling in the fcrward di.-ectton is successively reflected by i.Y:e grating into a wave: travelling in the backward direction rend vice versa. Zn other words, the gain mediu>z, and the feedback grating are co-located.
Fibre DHB la.lers will have the same advantages over semiconductor 2U lasers as the fibre DHR lasers described above, but will also tend to have a better single-frequency sc.ability than the fibre D6R lasers.
This feature ari9e;3 because the laser wavelength of a fibra DfD laser can be uniguely cleter~nined by the Dragg wavelength of cne single seating. Also, single mode cperltion is pro>r,oted because the threshold 27 gain increases with wavelength away from the 8ragg wavclongth. In contrast. in a fitare nGll .laser, which is am r-P-type laser, the laser wavelength i.s the wavelength wirh lowest round trip loss. This normally corresponds to the longitudinal mode closest to (but in general different from) the Bragg wavelengr~l. In other words, the ~0 requirement for st:ort :length in nrc3er to achieve mode stability is relaxed.
In additi~~n, since the lowest loss mode in an F-P type laser depends on the product oaf the ref.lecti,an spectra of the two gratings, art accurate determinat:ic~n of the laser wavelengrh this requires 3j matching of the Dr~~gg wavelengths of two gratings, either through the writing of the gr,3t;ings, or aFuer the writing through tuning of one of the gratings {eithf;r by heating/eoclittg czr by stretching the grating).
~~NaEa sH~~t kh:C'.1';\k:vlh';) ~ U~1!'1 1'C'T .,-.,~i_ ,~i_:it~ : 1l:i:i : ~~t.j ti;) :.';39~14~1fi5~ Gan~~:a CH:#1:3 ~ ~., 2~ ~~=~6~ : ..: ;
.. ., A fibre DFS laser can operate at a wavelength defined by on~,y cane grating.
Accordingly, a laser ae~::ording to the invention Ca.n avoid the problems de9cribed abo~re by allowing robust single mpde operation without necessari:.y comprermi,sing the laser power output or noise perfox-mance.
The amplifying optical fibre is doped with rare earth dopants.
In particular, the smFlifying optical fib~~ is doped with erbium, and.
as cc~-dopant cc ir,.crease the absorption of the pimp wavelength, with 1C~ ytt',Prbium (Yb3~) .
Preferably tJze amplifying opt.i4a1 fibre is optically pumped. Xrt this case it iv pre:ferred that the laser courpri,ses means far. generating optical pump radiation to pump the amplifying optical fibre.
A part of the amplifying fibre can advantageously be used try 7~ amp7.ify the laser output, being pumped by residual optical pwnping rr~di.ation emerging from the laser.
In one preferred e~mbndiaront the laser comp:~ises a mirror disposed a.t one end of the c!iffra.ct,.ion gratin; for I'eflocting at least a part. aY
optical radiation propagating along the diffraction grating. "'he 2C~ rri.rrar can int.rodL;ce a phase shaft to promote the propagation within the laser of r:~da.atiau at ti single mode either side of the Eragg wavelengt~i of the diffraction grating.
Preferably, the l.angitudinal separation between the mirror and the diffraction grating i5 less than 1 sicromptre (about hxl.P a Ly wavelength in thp fibre medium). Zt is preferrod that this separation can be adjusted by suitable adjustment metros.
As an alternative approach, it is preferred that the ,grating itself comprises at least one phase irregularity. This can promote the propagation of radiation at the Bragg wavelength.
;CI The phase irr~gut,arity could be a discontinuity or a substantially continuous phase variation along the grating.
One prefsrr~ed way of generating cs grating with a phrAse irregularity is tree so-ca112d "hloire" method, in which the grating is fabricated by g~ene:cating an inLerf erence pattern betsvean two superposed 3~i diffraction: gratings. The use of this method to form gratings havinv a phase irrpgula:~~ty, and the potent~.a1 use of such grat:.ngs ::n srmiconductor DfI3 lasers, is described in the paper "Phase Shifted A~R~~VGEJ SH~Ef Ktl.l'HK YIIt'(J i Uvlt'! t'O;l' :'W3- :t-:Jt~ : tl.'lf ~ +49 89 .':39,3=t-lEio- Gemvs CH:#14 ~1 a956~3 .
:~
Moire Grating Fibre Reacnators", Clectronics Lette~s, ~'oi 26, Yo 1, pp l0-11 (~ggo;.
Although th~a pha~~ irregularity could be perniancr~tly FabricatEd into the grating, the phase irr~gu.larity could inste,~d be gerera;;ed only during oper,atioci or i;~ fabri.catian, far example by heating a pardon of ,~.he grafting delring operation.
Viewed from a see:ond aspect, this inventicn provides a method of Fxuricating ari ,~f~cical fibre distributwd feedback laser, the method cataprising thp step pF fabricr~ti.ng a diffraction gracing on at least a nortinn of an arnp7_ifyir.g optical fibre doped with erbium and ytterbium and aperabl~~ to provide optical gain at a losing wavelength, thG
diffraction grating providing distributed optical fnedbaak for sustaining lacing action w:i,thin the portion at tree losing wavelength.
The invenCion well now be described by way of example with rcferenrc~. to the aeccmPanying drawings, throughout which like parts are referred to by like re:'erences , and n which:
Figure 1 is a sc:hernatic diagra..~r of a doped opiiea.l. fi'ore DF3 la.~~r employing a. uri:.fc;rm grating with no and-reflecGars;
Figure ~ i5 a s:~hem~,tic diagram of a doped optical fibre DFB
2~ laser emplny~.ng a phassy--shifted grating with cio end-reflectors;
figure 3 zs a sc:hemn,tic diagram of a doped optical fibre DFB
lr~s~~ employing a uniform grating with one end-reflector;
Figure 4 is a graph illustrating the theoretical laser threshold gait; of fundsment:al mode of a DF13 laser as G fmiction of grating ?'a strength with (i) a uniFortn grt~ting, (ii) a uniform grating with ant substantially lOCi> end-reflector, ~~d (iii) a a/2 phase shifted grating;
Figure 5 i:. a schematic diagram of tu>. Er3' : Yb''' dcped Fibre DFF3 aser employing =x uniform grating with a substant~:ally I.OOx ~~3 end-reflecrnr;
Figure 6 is a schematic diagram of an fir'' ; Yb3' 3cped fibre D~'H
laser employing a (temperature) phase-shifted grating.
Figure 7 is a graph illustrating empirical laser characteristics for a DFB fibre laser employing (i) a uniform grating with 3y substantially 1U0: end-reflector, and (ii) a (temperature) phase shiftad grating;
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5a Figure 8 is a graph illustrating the optical spectrum of a uniform grating fibre Flfb laser with a stustalltially 10(~x end-rPfl.ector in single-mode and double-mode operation;
figure 9 is a graph iilustrating trl.e opticttJ, spectrum of a uniform grating ffibre aF~ lasEr with a substantially 100y end-reflector against mirror displacement;
Figure 10 is a graph illustrating laser linewidth of a aingle-mode uniform grating fibre DfB laser with a subs tantiesliy 100x et:d-reFZector and a phssc~-,shifted grating fibre Jf8 laser;
AMENDE,a SHEET
Figure 11 is a graph illustrating the optical spectrum of a temperature phase-shifted grating fibre DFB laser at maximum power; and Figure 12 is a graph illustrating the optical spectrum of a temperature phas e-shifted grating fibre DFB laser against wire-temperature.
Figures 1. 2 and =i are schematic diagrams of doped optical fibre DFB lasers. In pa.rticu.lar, Figure 1 illustrates a laser employing a uniform grating with no end-reflectors; Figure 2 illustrates a laser employing a phase-shifted grating with no end-reflectors; and Figure 3 illustrates a laser employing a uniform grating with one end-reflector (e. g. a dichroic e:ad reflector).
The DFB lasers shown in Figures 1 to 3 comprise optical fibres containing one or more rare-earths, in which the feedback (a Bragg grating) and the gain (the doped fibre) are substantially co-located.
In particular, in Figure 1 a uniform grating 10 is imposed on a doped optical fibre 20; in Figure 2 a grating 30 having a phase discontinuity 40 is imposed on a doped optical fibre 50; and in Figure 3 a doped optical fibre 60 halving a uniform grating 70 is disposed adjacent to a reflector 80.
The lasers of Figures 1 to 3 are optically end-pumped (e.g. by a semiconductor pump laser).
In a DFB fibre laser such as the lasers illustrated in Figures 1 to 3, the optical feedback required for the laser operation is provided by a periodic spatial variation of the refractive index of the gain medium or of other fibre properties such that an optical wave travelling in the forward direction is successively reflected by the grating into a wave travelling in the backward direction and vice versa.
The strengtl-~ of the feedback is normally given in terms of the product xL, where x = rcOn/1B is the coupling coefficient and L is the length of tree grating. ~n is the refractive index modulation. The reflectivity at th.e Bra.gg wavelength is given as Tanhz(xL). The width of the grating stop band is proportional to x. The higher x is, the bigger is the fre~3uenc;y separation between the resonant modes of the 3~ DFB structure.
While for a F-P-type laser the threshold gain is the same for all the longitudinal modes, the DFB laser threshold gain increases the WO 95/31020 ~ ~ ~ ~ ~ ~ ~ PCT/GB95/01026 further away the resonant modes are from the Bragg wavelength.
However, with a uniform grating along the whole cavity length and no end-reflectors (i.e. th,e laser of Figure 1) no real mode can exist in the centre of the stop-band, ie. at the Bragg wavelength. At this wavelength the round trip gain has a real value, but with a negative sign. In such a device two modes of equal threshold occur symmetrically on each side of the stop-band, where the phase condition can be satisfied. Acco:cd:ingly a uniform DFB laser of the type shown in Figure 1 will oscillate at two different frequencies separated by the width of the stop-band (which is proportional to x).
Figures 2 and 3 illustrate two possible ways of obtaining single frequency operation of a DFB laser. In Figure 2 a single-pass phase shift (or irregularity) of n is introduced in the grating. This corresponds to a r'/2 optical phase shift so that the round trip phase condition is satisfied at the Bragg wavelength.
The phase shift can be introduced into the grating either through a phase shift in the grating itself or through a phase-adjustment region. In a phase shifted DFB laser the field of the mode decays exponentially as Fin evanescent field from the phase shift region into the DFB structures on the two sides and is coupled out. A phase shifted DFB laser also has the lowest possible laser threshold gain since the feedback is strongest at the Bragg wavelength, and so is suitable for sing7.e mode operation. The threshold discrimination and frequency separation between the fundamental mode and the first higher order modes is greater than that of the uniform DFB laser of Figure 1, and much greater than F-P-type lasers, including DBR lasers.
An alternative arrangement is shown in Figure 3 in which an end reflector is used to change the round trip phase shift in the laser.
. With one end-reflector one of the two modes will be enhanced, while the other will be suppressed, depending on the phase of the mirror relative to the grating. The end-reflector will effectively increase the grating length and reduce the threshold gain. Also the threshold will depend on the mirror phase. It is therefore preferred that the solution of Figure 2 i:a adopted.
Fibre DFB lc,sers will have the same advantages over semiconductor DBR/DFB lasers as the fibre DBR lasers described above, but will also tend to have a better single-frequency stability than the fibre DBR
WO 95/31020 ~ PCT/GB95/01026 lasers. This feature arises because the laser wavelength of a phase-shifted fibre DFB laser is unicuely determined by the Bragg wavelength of one single grating, while in a fibre DBR laser, which is an F-P-type laser, the laser wavelength is the wavelength with lowest round trip loss. This normally corresponds to the longitudinal mode closest to (but in general different frcw) the Bragg wavelength. However, since the lowest loss mode in an L'-P laser depends on the product of the reflection spectra of the two, gratings, an accurate determination of the laser wavelength requires r~atching of the Bragg wavelengths of two gratings, either through the writing of the gratings, or after the writing through tuning of one of the gratings (either by heating/cooling or by stretchi-~; the grating). With one mirror and one grating end-reflector matchin;~ is not a problem, but this type of F-P
laser is no longer a true all-fibre laser.
The gratings 10, 30 an: 40 can be fabricated as intra-core gratings or gratings at the ccre/cladding boundary.
Intra-core fibre Bragg gratings can be produced non-invasively in germanosilicate-based and ether photosensitive fibres. In germanosilicate fibres the gratings can be written holographically through the cladding using a coherent UV (ultraviolet) two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244nm (nanometres). The Bragg wavelength of the grating can easily be adjusted by changing the angle between the two interfering begs.
Fibre gratings can also re written by the use of a silica phase mask illuminated by a single U~ laser beam, as described in the paper "Phase-mask method for volume manufacturing of fibre phase gratings", Conference on Optical Fibre Communications, 1993, post deadline paper PD16.
Figure 4 is a graph illustrating the theoretical laser threshold gain of the fundamental mode of a DFB laser as a function of grating strength xL with (i) a uniform grating, (ii) a uniform grating with one substantially 100 end-reflector, and (iii) a n/2 phase shifted grating. Figure 4 shows that a phase-shifted grating DFB laser has a significantly lower threshold gain than uniform grating DFB lasers (with or without end-reflector). In the case of a uniform grating DFB
laser with one end-reflector the threshold gain is significantly lower than without any end-reflectors, but it depends on the phase relationship between th.e reflector and the grating.
A phase-shifted fibre DFB laser can be robustly single mode for much longer cavity lengths than a fibre DBR laser (because of the differential threshold gain between the fundamental mode at the Bragg wavelength and higher order modes) provided that one can write sufficiently long fibre gratings.
Two specific prototype examples of DFB optical fibre lasers will now be described with reference to Figures 5 to 12. These are (a) a uniform grating fibre DFB laser with one end-reflector with substantially 100%. reflectivity at the lasing wavelength (Figure 5);
and (b) a phase-shifted grating fibre DFB laser with no end-reflectors (Figure 6).
Figure 5 is a schematic diagram of an Er3':Yb3' doped fibre DFB
laser employing a uniform grating 90 written into an Er3':Yb3' doped fibre 100 with an end-reflector 110 (e. g. a dichroic reflector) having a reflectivity of substantially 100% at the lasing wavelength.
Similarly, F'igure~ 6 is a schematic diagram of an Er3':Yb3' doped fibre DFB laser in which a uniform grating 120 is written into an Er3':Yb3' doped optical fibre 130. In this case, a temporary phase irregularity is introduced into the grating in operation by heating a small region of the grating by passing current through an electrical resistance wire 140 wound around the fibre. This generates a temperature gradient 1!j0 along the fibre, altering the phase of the grating impressed on the fibre. This arrangement will be referred to as a (temperature) pha~~e-shifted grating.
The fibres ace doped with both Er3' and Yb3', the latter having the effect of increasing th~~ absorption at the pump wavelength by more than two orders of magnitude. The 980nm pump excites the Yb3' ions, and energy is efficiently transferred to the Er3' ions by resonant coupling.
In each of the two cases the grating was a 2cm (centimetres) long uniform fibre Bragg grating with a refractive index modulation of about 2.1"10-'' (xL=8.5) ~~rritten in an alumino-phosphosilicate Er3':Yb3'-doped fibre. The Er3':Yb3'-doped fibre was made photosensitive using the so-called "hydrogen :loading" technique, described in the article, "High pressure HZ loading as a technique for achieving ultrahigh UV
WO 95/31020 ~ ~ PCT/GB95/01026 photosensitivity and thermal sensitivity in Ge02 doped optical fibres,"
Electronics Letters, vol. 29, pp. 1191-1193, 1993. The fibre was soaked in 140 atmospheres of H2 at 80 degrees Celsius over 15 hours.
The grating was then written with a Krypton-fluoride (KrF) excimer 5 laser at 248nm using a side-writing technique described in the article "Formation of Bragg gratings in optical fibres by a transverse holographic method," Optics Letters, vol. 14, pp. 823-825, 1989. The laser was fired at 20Hz with a pulse fluence of about 0.5J/cm2, exposing the 20mm fibre section for over 3 hours.
10 The phosphorus oxide (P20~) and aluminium oxide (A1203) concentrations in the Er3':Yb3'-doped fibre are 18 and 2 wt% (percentage by weight) respectively. The Er3' concentration is approximately 1000ppm (parts per million) (0.26wt%). The fibre has an Yb3':Er3' concentration ratio of 12.5:1, a Numerical Aperture of 0.2 and a second mode cut-off wavelength of 1130nm. The diameter of the fibre was 125um. The peak absorption at 1535nm (which was also the fibre grating Bragg wavelength) was 0.5dB/cm. This is approximately the maximum available gain (with complete population inversion). Hence, with a 2cm long DFB structure the maximum available single pass gain is ldB.
Referring to Figure 5, the total length 160 of the Er3':Yb3'-doped fibre was 3cm, and the length 170 of the grating at the reflector end was 2cm. The fibre was embedded in a glass capillary with a diameter of 3mm. An output end was angle-polished to avoid reflections.
The laser was pumped by a 100mW 980nm diode laser through the reflector which transmitted about 97% of light at the pump wavelength.
The laser characteristics of the uniform grating DFB laser with an end-reflector having a substantially 100% transmission at the pump wavelength is shown as a curve 200 in the graph of Figure 7. The laser had a threshold incident pump power of about l5mW and an output power of about 2mW for 90mW incident pump power. The output power varied with changing lateral position of the mirror relative to the fibre grating, which will change the mirror reflectivity (both phase and amplitude) and the launching conditions. The slope efficiency was about 5%. Lasing also occurred with an end-reflector with 4%
reflectivity, ie. without the mirror. The threshold pump power in this case was 34mW.
The optical spectrum for the laser of Figure 5 is shown in Figure WO 95/31020 ~ ~ r PCT/GB95/01026 8. The basic spectrum ;?20 has two peaks as expected for a uniform DFB
laser, but the relative power in each of the two modes depends on the mirror phase relat_we to t:he fibre grating. The separation between the two modes is about 0.25nm, as expected from the theory assuming a grating strength of xL=8.5. Figure 9 shows the laser spectrum for increasing mirror displacement (oz) and hence changing phase.
Adjustment means (such as a micrometer screw) can be provided to allow the longitu~3inal mirror-grating separation to be adjusted to improve the laser's performance.
The optical :_inewidth when the laser was single-mode was measured with a self-heterodyne delay-line to be about 60kHz, as shown by a curve 240 in the graph of Figure 10.
During performance testing of the phase-shifted grating fibre DFB
laser as shown in Figure 6, the prototype laser was the same as the uniform grating fibre DFB laser of Figure 5, but with the end-reflector removed and replaced with a glass microscope slide with index-matching fluid to remove any reflections (having the same effect as angle-polishing of the end-face).
The phase shift was introduced by having the electrical resistance wire 140 wrapped around the glass capillary, substantially at the centre of the grating. Alternatively the wire could contact the fibre directly. The wire was 2cm long and 175um in diameter, and its resistance was 0.46 Ohm;cm. By running an electric current through the wire the fibre grating was heated more at the centre than at the ends, causing a falling temperature gradient 150 at each side of the wire.
The refractive index and hence the Bragg wavelength is dependent on the temperature, and is thus changed more at the centre than the ends of the grating, cau:>ing an optical phase-shift for the forward and backward propagating waves in the DFB structure. This caused a gradual phase-shift which can be shown to yield a more uniform intensity distribution along' the gain medium than an abrupt phase shift at the centre of the DFEt structure. This will give a more uniform gain saturation and better use of the available gain.
With this method the position of the phase-shift can easily be changed by chang:_ng t:he position of the wire. With increasing temperature the output power varied periodically, indicating that the phase-shift also varied periodically (as expected). In the present WO 95/31020 ~ ~ r ' PCT/GB95/01026 C_ ! 895f.~8 embodiment the maximum output power occurred with a wire current of 320mA and voltage of 0.6V. The output power from one end of the phase shifted fibre DFB with an optimum phase shift is plotted as a curve 210 in Figure ~.
The pump source was a 150mW 980nm diode laser. The threshold incident pump power is about 32mW, and the slope efficiency is about 5%. The maximum fibre laser output power at the output (angle-polished) end is l.9mW with 12'7mW incident pump power. The laser output power from the input end was not measured, but would be expected to be similar to that at the output end. The gradually increasing slope at low pump powers is probably due to the lcm long piece of Er3':Yb3~-doped fibre at the output end which at low pump power acts as an absorber (with maximum 0.5dB absorption), while at higher pump powers it acts as an amplifier (with maximum 0.5dB gain). Hence, this laser is an example of a MOPA (Master Oscillator Power Amplifier) configuration.
The laser linewidth in this case was measured to be about 300kHz, as shown by a curve 250 in Figure 10. This linewidth was found to be substantially independent of the output power.
The optical spectrum of the phase-shifted fibre DFB laser is shown in Figure 11. The laser is single-mode, with no side-modes.
Figure 12 shows variations in the spectrum with increasing wire temperature. The amplitude increases from zero to a maximum and then decreases again. The wavelength increases with increasing temperature as expected.
There are several other possible ways to induce a phase-shift in the fibre grating, other than the "wire-method" shown in Figure 6. One possibility is to write two gratings with slightly different Bragg wavelengths on top of each other, to form a so-called Moire grating.
This technique is described in the article "Phase Shifted Moire Grating Fibre Resonators", Electronics Letters. Vol 26, No 1, pp 10-11 (1990).
The Moire grating normally has a permanent n phase shift which corresponds to an optical phase shift of n/2, but can also be made with smaller phase shifts (using different exposure times for the two gratings). By having several small phase shifts along the grating, adding up to rc, a gradual phase shift, and hence a uniform intensity distribution can be realised.
An alternative technique is to apply a temperature gradient along the fibre when ttie grating is being written or to induce a variation of refractive index: by means of a un~tform W beam before writing the grating, in order- to generate a phase irregularity or discontinuity at around the centre of the grating. Alternatively. a small section in the middle of the grating could be stretched before, during or after the writing of 'the fixating. It is also possible to leave a small section in the midd7.e of the grating unexposed, causing a phase-adjustment region. This can also be done by either erasing the grating 1t) or changing the ~werage refractive index at the centre of the grating by focusing a single W beam at this point. A phase shifted grating could also be made by using a special phase-mask (with phase shift;.
Whether a discrete phase discontinuity is achieved through modification of the grating or through the use of a mirror, it is iii important to notes that; the phase discontinuity does not tend to act as a resonant cavity (ae~ in a F-p laser) . In fact, the length of the phase irregularity or the mirror-grating separation is preferably less than about 1 micxnmetx~e (1 x 10-6 m) (or about half a wavelength in the fibre medium).
2C1 Although tt~e embodiments described above have related to optical fibres. it will of course be appreciated that the techniques are equally apFlicable to planar waveguides, and in particular, glass planar waveguides:.
Claims (16)
1. An optical fibre distributed feedback laser comprising an amplifying optical fibre (20) doped with erbium and ytterbium and operable to provide optical gain at a lasing wavelength in which a diffraction grating (10) is disposed on at least a portion of the amplifying optical fibre to provide distributed optical feedback for sustaining lasing action within the portion at the lasing wavelength.
2. A laser according to claim 1, in which the amplifying optical fibre is optically pumped.
3. A laser according to claim 2, in which the amplifying optical fibre is optically end-pumped.
4. A-laser according to claim 2, comprising means for generating optical pump radiation to pump the amplifying optical fibre.
5. A laser according to any one of the preceding claims, in which the amplifying optical fibre is formed of alumino-phosphosilicate glass.
6. A laser according to any one of the preceding claims, comprising a mirror (80) disposed at one end of the diffraction grating for reflecting at least a part of optical radiation propagating along the diffraction grating.
7. A laser according to claim 6, in which the longitudinal separation between the mirror and the diffraction grating is less than about 10 -6 metres.
8. A laser according to Claim 6 or claim 7, comprising means for adjusting the longitudinal separation between the mirror and the diffraction grating.
9. A laser according to any one of claims 1 to 5, in which the diffraction grating comprises at least one phase irregularity (40).
10. A laser according to claim 9, in which the grating is formed as an interference pattern between two diffraction gratings superposed on the amplifying optical fibre during fabrication of the laser.
11. A laser according to claim 9, comprising means for heating a portion of the grating during operation of the laser, to generate the phase irregularity.
12. A laser according to claim 9, in which the grating comprises a portion having a phase irregularity formed by heating the portion during fabrication of the laser.
13. A method of fabricating an optical fibre distributed feedback laser, the method comprising the step of fabricating a diffraction grating on at least a portion of an amplifying optical fibre (20) doped with erbium and ytterbium and operable to provide optical gain at a lasing wavelength, the diffraction grating providing distributed optical feedback for sustaining lasing action within the portion at the lasing wavelength.
14. An optical waveguide distributed feedback laser comprising an amplifying optical waveguide (20) doped with erbium and ytterbium and operable to provides optical gain at a lasing wavelength, in which a diffraction grating is disposed on at least a portion of the amplifying optical waveguide to provide distributed optical feedback for sustaining lasing action within the portion at the lasing wavelength.
15. A laser according to claim 14, in which the waveguide is a planar waveguide.
16. A laser according to claim l4 and 15, in which the waveguide is formed of glass doped with erbium and/or ytterbium,
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9409033.9 | 1994-05-06 | ||
| GB9409033A GB9409033D0 (en) | 1994-05-06 | 1994-05-06 | Optical fibre laser |
| PCT/GB1995/001026 WO1995031020A1 (en) | 1994-05-06 | 1995-05-05 | Optical fibre laser |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2189568A1 CA2189568A1 (en) | 1995-11-16 |
| CA2189568C true CA2189568C (en) | 2002-10-22 |
Family
ID=10754683
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002189568A Expired - Lifetime CA2189568C (en) | 1994-05-06 | 1995-05-05 | Optical fibre laser |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US5771251A (en) |
| EP (1) | EP0759216B1 (en) |
| CA (1) | CA2189568C (en) |
| DE (1) | DE69515475T2 (en) |
| GB (1) | GB9409033D0 (en) |
| WO (1) | WO1995031020A1 (en) |
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-
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-
1995
- 1995-05-05 CA CA002189568A patent/CA2189568C/en not_active Expired - Lifetime
- 1995-05-05 WO PCT/GB1995/001026 patent/WO1995031020A1/en active IP Right Grant
- 1995-05-05 EP EP95918058A patent/EP0759216B1/en not_active Expired - Lifetime
- 1995-05-05 US US08/737,181 patent/US5771251A/en not_active Expired - Lifetime
- 1995-05-05 DE DE69515475T patent/DE69515475T2/en not_active Expired - Lifetime
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| US11658452B2 (en) | 2021-02-11 | 2023-05-23 | Ciena Corporation | Powering up an optical amplifier in an optical line system |
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Also Published As
| Publication number | Publication date |
|---|---|
| WO1995031020A1 (en) | 1995-11-16 |
| DE69515475T2 (en) | 2000-11-23 |
| EP0759216B1 (en) | 2000-03-08 |
| CA2189568A1 (en) | 1995-11-16 |
| DE69515475D1 (en) | 2000-04-13 |
| EP0759216A1 (en) | 1997-02-26 |
| GB9409033D0 (en) | 1994-06-29 |
| US5771251A (en) | 1998-06-23 |
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