CA2245531A1 - External cavity semiconductor laser with monolithic prism assembly - Google Patents
External cavity semiconductor laser with monolithic prism assembly Download PDFInfo
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- CA2245531A1 CA2245531A1 CA 2245531 CA2245531A CA2245531A1 CA 2245531 A1 CA2245531 A1 CA 2245531A1 CA 2245531 CA2245531 CA 2245531 CA 2245531 A CA2245531 A CA 2245531A CA 2245531 A1 CA2245531 A1 CA 2245531A1
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Abstract
A miniature, external cavity, filter-locked laser has a semiconductor optical amplifier, such as a diode laser, and a monolithic prism assembly positioned in the external resonant cavity. The monolithic prism assembly includes a transparent substrate carrying a thin film Fabry-Perot interference filter which is tilted, that is, oriented not normal to the path of the travel of the laser light in the external cavity. Such optical devices can be economically mass produced in advantageously small size, having reproducible spectral performance properties held within tight tolerances. Significantly advantageous applications include dense wavelength division multiplexing systems requiring tightly spaced wavelength subranges for each of the multiple channels. High wavelength stability against temperature and humidity changes, etc., can be achieved.
Description
~;Xl~;KNAL CAVIlrY SEMICONDUCTOR LASER
WITH MONOLIlrHIC PRISM A~ sFMRLy FIE LD OF I~IE INV~NT~ON
The present ,~ is directed to optical deYices having an external ca~rity s~u~ laser. such as an ~Atl~ cavity diode las_r, in which an e~ te~l opti~
cavity extends t~ ,n one facet of an e~c~ g se ~;rQr~ J~ Io~ optical ~ . and an e~cternal ~ . Mor.~ particularly, the present invention is directed to external cavity o~ optical ~ having st~bili7~o~ erni~ n wa~elP!ng~hc - RA,CKGROUND
P~tem~l cavity sf~ ru~ o( lasers are known and have uul--L.uu~ uses and ;t ~l ir)l~, inr ~lriin~ fiber-optic cu~ c . l ir)nc~ In extemal cavity diode lasers which are typical of such optical devices, an optical cavity extends between one facet of an edge-5~ .";.~r~ ol diode laser and an e~tPrn~l high ~~ne~lul. A second facet of the ge~--~i~g sG~ ur laser. bu.-. C~ the high r~ne lor and the first ~acet, typically carries an anti-reflpction coating to allow light to escape the laser chip with ~
Semicol.~luctu~ diode lasers have been used c~ ,ly as l~ ;lt~ ~ for fiber-optic C~-.. ;.~l;~nc In one common and low cost imp' ~inn, edges of two opposing end facets of the laser chip are cleaved to form ~n.~" reflective ~--- ri.nPc and provide the ~ nL t~peL~ y for laser ope~ti~ n Such Fabry-Perot (FP) lasers typically emit in multiple Inngitn~iîn~l modes and have large ou~put banLlwidll,s. for e~ r~p~ 3 nm to 10 nm. In another common; ~ ~ ~rlP ~ l io~ with slightly "l.,lcased cn. . . ~IP ~ ;I y, a Bragg grating is etched in the active region of the Fabry-Perot laser cavity to forrrl a ~ u~f d f~P~h7 -~ laser (DFB). D;~ Pcl ~D ,~,cL lasers have the advant-age of single Ic)ngimriin7~l " . . .
mode emission which provides very narrow bandwid~hs typically, for P~s~mple, less than 0.01 nm. In a third 7trplirs~tion, the .1;i~ d Bragg reflector (DBR) laser ~ a wavelength-selective Bragg grating for one of the cleaved facets of the Fabry-Perot laser.
The wavelength-se}ective Bragg grating has the effect of Lllo~ci~!g a laser with single 1~,..~;1.,.l;..~l mode output.
- Ap~ of these and other diode.lasers has been ;---~i~ due to inadequate.
stability and accuracy in the particular wa\~lf-l~g~ ...Lt~ed In particular, for e,~i,r"rl~, such diffi~ tif~c have been ~L~.;...-c~d in the ayL)li~ n of diode lasers in Dense Wa~ nglllDivision~ DWDM). Inthisadvancedfiber-opticcf,,.. n.. ;.~1;~", tP~hrlolf)gy, many closely spaced wa~.le.~ s or ~ ....f.lc are t~nL....;Il~ .d cimnlt7~n~ "~ly down a single fiber or fiber bundle. Typical spacing of channels in DWDM systems can .
~range from S.nm to 71S little as l.nm or less between ch7tnnPlc~ To 7~rc~rn~1ich crÇ~,cli.~_ DWDM systerns, stable and accurate U~tt~ of prede~ hled wavelengths are needed fore~ lb/idualch~nnel. Ins~ tTrln~stableand arc~ t~,~àv~leuglh-sele~ ,lece;~
are needed to scle.,li~.,ly remove or receive the illdi~idual channel wavP.iP.n~thc with low or no cross talk from other ,;11" l .. .~ .ki. For â DWDM syslem to operate r.rfi.-;- .1 Iy, lh~.cr. lc;
the ll~u~ ~- and ll:;Cc;iV~l device for a given channel must be tuned with great accuracy to the same wa~ h band.
U,lrol~ t~,ly, the wàvele~lglh band emitted by l,~seillly known se~nico~ r diode lasers, ;..~ g the above rnentic)rtPd FP lasers, DFB lasers and DBR lasers, vary to an ulla,ccipLably large degree with ~lu~c.alulc and other factors. Center wa~,length p~nl~ c~ e of an FP laser, for ~ , 'e, is typically as much 0.4 nm per degree centiEr~A~ change in operating ~r.l--~ at room tcr2pc.dtu~c~ The comparable variance for D~B lasers is typically as much as 0.1 nm ~er degree centigrade. P~,.,c.~ly known l- -ctor diode lasers also suffer the disaLlvanta~e of poor m~nllf~t--rin~ r~pe~t~hiliry That is. an int~nrled or specified emiccinn wavel.,"~Lh is not achieved with ~le~
a~_~,u.d~;y when such lasers are y~ u~cd in large commercial qU~ntit;r c, These ~fi~iPnCi~5 rende,r present s~ .r diode lasers difficult and costly to imriement into ~ ".i;..
arrli~ nc such as DWDM systems, and in many cases entirely unsuitable.
ItisKnownthatthe~.. lf.. i.l.. erif.~lf.. ~rif.. nf of an~div~lu~llasercanhemitg . by cnnh~llin~ the ~ t~ . . e of the laser to w, .ithi~l an e~ r.l~r s.mall te "l~ G range using, for PY~mplP thf rmopl~ctric coolers with closed loop feerlhaclr from a t~mpP~hlrr~
sensor. Such controls are complex and costly. The even more difficult problem ofco... L,u~ ,g lot-to-lot wavelength variation in commprcial m~nllf~ctllring of ~GsG.~ly ~nown s~ .. lit on.l~.c~or diode lasers, which can be as great as l S nm and even 1 10 nm. has been partially addressed by culling through pro~t~tion batches for lasers having the desired wavel~n~th Thi~ lf e~ -e of wavelength testing of individual lasers has ci~n;r;. ~
adverse impact on m~mlf~ctllring yield, with correspondingly increased costs and c-~mrl~Yjty.
It has also been ~.u~os~ to use an ~ItPrn~tive type of semicon.lllrtor diode laser, s~- ;r.~ y, extern~l cavity tunable lasers. FYtPrn~l cavity tunable lasers are suggested. for e~mplP., in Widely Tunable External Cavity Diode Lasers. Day et al. SPIE, Vol. 2378, P.
3541. In the diode laser devices sl~ggpct~l by Day et al, an anti-reflective coating is placed on one facet of a diode laser chip. The emitted light is captured within a collim~ting lens~
and a rliffr~tion grating is used to select or tune the wavelength of the laser. Laser action occurs, generally, provided that the grating is selPcting a wavelength within the diode's spectral gain region. A diode laser device employing a diffraction grating disposed in an extemal cavity also is suggested in U.S. patent 5,172,390 to Mooradian. Unfor~unately, ~ tion gra~ings disposed within the external cavity of a diode laser causes a .cignifir~nt increase in the overall size or bulk of the device. The diffrac~ion ~ratinP and the complexity of the required ~rating ~lignmPnt system can also si~-lificartly increase the cost of the device. As to the size or bulk of the device, the cavity length for a diode laser having a diffraction gratin~ disposed in an external r~;son~rll cavity, in acco~dance with known devices, is txpically from.25 rnm to over 100 mm, in con ast to the ~much s~nalIer 1 mm size.
or smaller of FP lasers and DFB la~ers t~ ;} above. T~ie ~; rr ~ ;on grating and grating mount also have been found to exhibit tr ~ k~ Since the ~l~rr~cl;~
grating sets .the wavelength of the laser, such t~mperatllre de~ e of the grating and grating mount cause unwanted inct~hility in the emitted wavelength of the laser. In z~ 1itinn, long term w~vcle,~ Il drift problems have been e~pP - ;~ nce(l due, it is belieYed, to the mech~niral compl~ity of the diffraction grating and grating mount aspec~s of such devices.
It is an object of the present invçntion to provide s~miron~ ctnr laser.devices having .
good wavelength stability and accuracy. In particular, it is an object to provide such devices having acceptable m~mlfactlrin~ costs, c~ e ity~ and size o~ buL~c c~ s iti~n~l obiects of the invention will be ~ GIIL from the following disclosure and from the detailed description of certain preferred embo lim~.ntc SIJMMARY
In acco,.lancc with a first aspect, an external cavity laser is provided, having an optical ~mlllif;~r optically coupled to an external resonant cavity, with a mnnolithic prism assembly in the external resonant cavity. In accordance with certain pl~fell.,d r.lllc, the optical ~mplifi~:r is a sellliconductor optical amplif;er, such as those used in diode lasers, e.g., those for-m--ed of InGaAsP. Other suitable gain el~ments inc}ude, for mpiç erbium doped silica, ge~ ania or other optical m~Pri~l formed as fiber, etc. In CA 0224~31 1998-07-30 wo 97/3049S PCT/US97/02190 cenain emb~limcn-c the optical ~mrlifj~r is a direc~ band~ap opucal emiuer. The monolithic prism assembly comprises a transparent substrate. that is. a suhstrate which s substantially optically transparent to the laser light, and incorporates a Fabry-Perot r~ e filter c~....... l";~ multiple thsn film reflectors sandwichin~ aL least one thin film cavity between thern... The thin film ~abry-Perot intelr~l~nce. filter is disposed in the path of travel of the laser light in the external cavity. More 5~ 1y, it is oriented at a slight.
. angle to a ~ ~ plane. That is. it i~ cd on a surface of the ~ a~ an .
internal or external surface, as ~is~-uc.ced below) which is disposed at a non-zero angle to a transverse plane. As used here, a "transverse piane" is an im~8in~ry plane normal or orthogonal to the path of laser light in the external cavity. Cull~yoluilllgly, a transverse surface of the tral~l)a,c~nl snhstr~te is one which lies in (or a~ llately inj a transverse plane. Thus, the thin film Fabry-Perot interference filter is in a non-transverse plane; it is not norrnal to the path of travel of the !aser light passing through.it. Rather, it is at an acute angle, typically less than 45~ and more than 0C, preferably about 1~ to 5~, more preferably about 1~ to 2~ to a transverse plane.
The thin film Fabry-Perot in~erference filter, in accordance with one aspec~, is a stable, l~luwl~d i"le.r~lel~ce filter provided as a coating on a surface of the llan~yalelll op~ical s~ -I o~ . of the m~nrl!ithi~ prism assembly. While the interference filter preferably has at least one thin film cavity layer sandwiched between thi~ film reflector layers, more ~ fe- ai-ly~ it is a multi-cavity filter of two to five cavities, most preferably having two or three cavities. Each such cavity has an optical thirl~n~cc (c~ t.od as its actual physical thi~kn~ss times the refractive index of the cavity material~ equal to an even number of - quarter wavelengths or QWOTs. The wavelength referred to is typically about the center of ~.he wavelength band of laser light to be tr~ncmitt~d through the filter. Each cavity layer preferably is formed of one to three dielectric films, each such f1lm having an optical CA 0224~31 1998-07-30 WO 97/3049s PC~/US97/02190 ~i.i. L ,.. ~ ec}ual to an even number of quar~er wavelen,~ths preferably totaling 1 to 15. more preferably 5 to 10 half wavel~ Lhs optical thickness for each such dielectric film. The reflector layers sandwiching a cavi~y layer between them each preferably is formed of quar~erwavelength optical 11.;. L~ flms, as describ:d further below. Most preferably, the fi!ter is a multi~avity thin film Fabry-Perot na.,~wballd il~leLrt l. nce filter wherein the in~ lual reflector cavities fi'~ UCiu~i, are coh~,..,hlly coupled to each other using a ~ufi~le.w~ n~7L ~ C~ optical CO~ layer. The use of a ~t~ . cavity Fabry-Perot interference filter in the monolithi~ prism assembly yields a filter with increased slope of the spectral skirts, along with a wider tr~ncmicsion zone, as compared to a single cavity filter. Both of these effects yield highly ~nPfici~l improvements in the ~r(~ e c,l".~ s of the extemal cavity lasers disclosed here in comparison with prior known fltering devices, such as etalons and diffraction gratings, as ~lieru~ed further below.
It will be further.nn~l~r~tood from the rliC~ elow, that the ~m~)nnlithic prisTn assembly need not provide any light diffraction function in the classic sense of a prism.
Light from the opticil emitter is acted upon, such that in~band light is ~ led and out-of-band light is re~ected away. Thus, in-band l~ght is s~p~r~te~1 from out-of-band light. The mon-)lithiC prism assembly could also be referred to as a filter monolith, a1~ain me~ning a ~lall5~d~ substrale assembly incorporatin" a thin film Fabry-Perot interference f~ter .
Cu~ uliS~lg at least one thin film cavity sdndwicl1ed between thin film reflectors.
The ~ J~US~ ;.' prism assembly in accol~ldllce with certain preferred emb~~
carries a reflective coating, preferably a high r~fl~ction coating, on a transverse surface defining one end of the extemal resonant cavity of the optical amplifier. and the aforesaid Fabry-Perot i,l~lÇt;lence filter is carried on a second surface ~hich is spaced from, and at an acute angle to the firs~ surface. Optical devices in accordance with this aspect adv~nt~eo--s1y comprise an external cavi~y, edge emittin~ semiconductor diode laser CA 0224~31 1998-07-30 wo 97/30495 PCT/US97/02190 having an anti-renection coatin. on a first emi~ter facet optic'ally coupled to the external resonant cavity. An output coupler reflective coating is provided on a second emitter facet of the diode laser. The monolithic prism assembly is position~d in the external resonant eavity, c~ a lldl~S~dlGi~ optical substrate carrying a cavity-reflective coating on an 5exurnal surfaee to define one end of the extemal resonant cavity. A thin film Fabry-Perot e filteris provided on a seeond surface of the optieal ~ul,~l-dle, being p~
within the external ~ cavity l~h. ~.~ ~he refl~ctive coatirlg and the f~t emitte~ facet.
One or more ct llim~tin~ means, sueh as gradient index lenses or bulk lenses, etc., also are p~;l;....~A in the externai resonant cavity, e.g.. for focussing light between the Pabry-Perot il~t~ .. I; .. ~ n. ~ filter and the first emitter fa~et of the diode laser. T~ol~t~ ~ can be used outside the cavity in the usual way.
Those who are s~illed in the art, or knowledgeable in this area of t~chnology, will ~,c~g",.:e that the optical devices ~licclos~d here are a ci~ 1 technolog-~i advance.
External cavity lasers employing semic~ l optical amplifiers in accordance with this 15iicrlosll~ can be ~lùced with accurate and'reproducible emicsion wavç~ thc, exçell~nt Pe~ G stability, and eyrell~nt ,~ n~e to wavcle~ }i drift. Moreover. it is highly ci.~ that such lasers. especially in accordance with plGfé~ ,d embo~im~.ntc. can be produced in I~ dlu~G size and in large commercial qu~ntiti.oc. havin~ manufacturing costs c r ~ i to kno-,vn FiP lasers, DFB lasers or DBR lasers. As .liccnccf -1 below, plGÇ~ ,d c.l.bo~1;.. ~ .. l~ of the laser devices disclosed here may be referred to as filter-locked laser~
for their use of thin film Fabry-Perot interference filters in the external laser cavity to st~hili7e or lock the emitted wavelength or band. Preferred embo iimentc of the lasers ~iicrlt~ced here. and optical devices incol~oldLhlg them. can be reproducibly m~nnf~ctllred in commercial ql~ntitips with ~miccic.~ wavelen~th held to within + 0.1 nm. and with temperature depen~lenre of 0.005 nm/~C or less, preferably .001 nm or less. Long teml wavelength drih can be held to less than + 0.l nm. Moreover. the morlnlithic prism assembly incorporating the thin film Fabry-Perot interference filter enables overall cavity length of the diode laser to be less than S mm in certain preferred embo limPnt~. Those ski~led in the art will recognize that these pa~ ging and perforn~nre ch~r~cteri~tirs render S l"~r~"rd ~ hof~ f ~ of the lasers ~ nl~lse~1 here, and optical devices incoll-oldLiilg them, suitable and co~ ";idlly pr~rti~l for ay~ such as, most notably, dense w~ h.~ divisionmultiplexingfiber-optic~.. 1.. ;.. ~;nnsystems. As~ s~ab~ve, previously known diode lasers, such as those incorporating iiffra~tiorl gratings or the like for wavelength control, were too complex, bulky, unreliable andfor costly to satisfy the 10- exacting ~ui~ ~ of such appli~tinn~
A~ ti-)n~l aspects and advantages of the present invention will become ap~a,Gl,l or more readily nn~ ;tood from the following detailed description of certain ~ r~ ,d embo~iim~nt~ .
BRIEF DESCRIPI~ON OF TH~ DRAVVrNGS
Certain t~ ~ f~ . ~1 L.l-bo~ of the invention are ~iic,~ ed below with le~ e to the accolll~dllyillg drawings in which:
Fig. I is a s~ . "~ str~tion of a first preferred embodiment of an optical device incorporating an external cavity laser in acc-"ddllce with the foregoing ii~closl1re;
Fig. 2 is a sch~m~tic illustration of an external cavity laser in acco,dance with a second preferred emb~~
Figs. 3 - 5 are s(;h~ ." ~l;c ilh1ctr~tiorlc of the thin film Fabry-Perot interference filter of the monolithic prism assembly suitable to he employed in the semicnn-ll1ctor optical ~nplifiers of Figs. l and 2;
wo 97/30495 PCT/US97/02190 Fig. 6 is a ~raph showing the theoretical perforrnance of a hiBh quality three-cavity Fabry-Perot interference filter in accor~lance with Figs. 3 - 5. along with the cO.l~,s~Jollding perforrnance of coll~p~t~h1P one and two-cavity thin film Fabry-Perot interference fiiters;
Flg. 7 is a schPm~tir illnctr~tif~n of an optical device cu~ lg an external cavity S laser in accordance with another p,efel-~d embo~impnt; and Fig. 8 is a sel-~ . ,.,.un ill. ,~(".l ;h- - of a dense wa~le~ h division ml~ de~ice utili~ a number of external cavity lasers in accù~ c~ with the present i~ ,.,~io, It should be untlPrstood that the optical devices illustrated in the drawings are not n~eecc,.. ;Iy to scale, either in their various ~iimpnci(7n.c or angular relationships. It will be well within the ability of those skilled in the art, with the aid of the foregoinP ~liccloc-~lre and the following detailed description of preferred emb~imen~cl to select suitable ;""~ and an~ular rel~tinn.chiins for such devices int~nfl~(l for a particular appiir~tir,n DETAILED DESCRIP'IION OF CERTAIN PREFERRED EMBODIMENTS
Those who are s~illed in ehis area of eechnology will lucû~ e from the above ~iiC~nccit~n that the external cavity lasers ~icclosed here have IlUm~,lUUS ~p~
;"( 1."1;"~ use in fiber-optic telecomml-nir~tionc systems~ especially in systems employing dense wavelength division m~ wherein ex~remely narrow and preclsely controlled ;Qn wav~ n~thc are required. Additional applications include. for example, use in test c~ and the like, as well as laboratory instrnmlon-~tion In cûntrast to previously known laser devices. such as FP lasers, DFB lasers andDBR lasers, in the dev~ces disclosed here a thin film Fabry-Perot illLc.f~lcnce filter carried on a L~ ,..l substrate is used in an exterrral laser cavity to lock the emitted wavcle~ h or band of the external cavity laser within a narrow gain re~ion. The laser can even be limited to a sin~le ~omic~ n mode by employinP a suitable thin film Fabry-Perol narrowband , interference filter. Thus. the ~asers disclosed and discussed here can be said to be filter-.
Iocked. Muud~u~ (e.~.. having an external resonant cavity of overal} len th. inC~ n~ the ~..u~n~ ;r prism ~,llbly, of less than 5 nm) thin film filter-locked external cavity lasers, or "FL lasers" as they will be referred to in some ;,,~ c below, have especiallyadv~nt?~ol-c app1i~Rtion in DWDM fiber.optic telecom,~ alion systems. Precise and stable ~va~ele,~lg~ll laser emitters are called for in such a~ ;cd~ s~ so that closely ~ e d c1~ , are reliably ~p~~ and distinct. Those slcilled in the-art will recognize. that there are various other applications for the FI~ lasers ~1icc}t)sed here, especially applir~tionc calling for a stable and accurate narrowband laser source.
In the FL laser sch~mRti~R11y ilh1ct~t.ocl in Fig. 1, an external cavity diode laser bly is seen to include a laser diode chip 10 having a finst emitter facet 12 and a second, opposite emitter facet 14: Emitter facet 14 carries a coating, specifically, an output coupler mirror, that is, an output coupler reflective coating 16. Light emitted through coating 16 is received into c~-1}imRtin~ lens 18 which preferably is a gradient index lens or the like.
t~ mRt~(l light from gradient index lens 18 is passed to fiber-optic pigtail 20, wl,~ ,. bj it enters a fiber-oetic c~ l r~ r system. Grin lens 18 could opti~Rlly be deleted in favor of a butt c~..l.l;..~ between emitter facet 14 (with anti-reflective coating 16~ and pigtail 20. ~dAiti()nRl}y, an isolator can be employed outside the cavity following a _radient index lens or other col}imRting lens. passing light to pigtail 20. Isolators are generally well known and their use and the use of other optional col,lpo~ents in the FL lasers ~licc}osed here will be apparent to those skilled in the art in view of the present disclosure. Emitter facet 12 carries coating 22, preferably an anti-reflective coating. Light passing through anti-reflective coating 22 is received and collimRted by a second co}lim~tin~ means 24 which.
again, p,efe.~l)ly is a gradien~ index lens or the like. Light is passed from collim~rin~ means 24 into a monoli~hic prism assembly 26 co"~p,i~hlg a transparent optical substrate 28.
Substrate 28 preferably is optical glass. such as BK7 or B270. both available from Schott Glaswerke (Mainz. Germany). or the like. Outside surface 30 of ~ub~lrdle 28 carries high ref~ector end mirror 32, such that the external cavity of the diode laser is defined between output coupler mirror 16 and high reflector end mirror 32. Preferably, anti-reflective S coating 34 is carried on exterior surface 36 of substrate 28 to fz~rilitz~t~ passing of c~-llimsttPd light from lens means 24 into the monnlithir prism assembly 26.
'rhe prism stc~f....hlj~ further c~ e~ thin film Fabry-Perot ;11~ e filter 38 on internal surface 40 of substrate 28. An "intemal surface" as that term is used here. most typically is a surface-to-surface contact interface between two parts or pieces of ~ Ja~ L
substrate which have been cement~d or otherwise integrz~t.o~ to each other to form the mcnolithic prism z~ e.l~hiy An optical coating on an intemal surface. for eYz mpie the Fabry-Perot il.~"c.~.~.ce filter 38, is advantageously protected and st ~hili7ed by the bonded pie~es of the ~ between :which it is sandwiched. In that regard, i~ should be recognized that the gap shown between first piece 42 of substrate 28 and second piece 44 is highly ~ ~,.g~ t~ in Fig. 1 for l)ul~es of illustration. It should also be r~cog~d that Fabry-Perot i.l~.Ç~,.e.~ce filter 38 could be formed on interfacial surface 46 of part 42, as well as on interfacial surface 40 of part 44. An extemal surface. correspondingly, is a surface of the substrate which does not form a surface-to-surface contact int~t~ce with another part or portion of the substrate. It maj, therefore, be exposed to atmosphere or in ~ with another optical el~ment, such as a coilimz~tinE means, monntin~ structure or the like. An eYt~t~tzti surface may be coated, as in the case of the ~ù~ la~e in the .llbOdilllent of Fig. I wherein external surface 30 carries high reflector end mirror coating 32 and extemal surface 36 carries anti-reflection coating 34. A round hole or other aperture can be placed in or on surface 36 or other suitable location to limit beam angles impinginE~
on the filter. Optionally, part 42 of the transparent substrate 28 of the monolithic prism WO 97/30495 PCT/US97/0219~
assembly could be deleted in favor of.'e.g.. an air gap. Since air has a lower index of refraction. this would yield an advanta~e in reduced optical distance. It should be recognized that components of the optical device illllstrat.-d in Fig. 1 may be spaced from or in abull~ with ~ f.. ~t cle.. ,l~ as required by the performance and p~ck~ing S ~l~c;~ ionc of a given app!ication.
It can be seen in Fig. 1 that high reflector end mirror 32 on external surface 30 of t~ 28, L;es in a l,~ "~ plane, that is, in a plane which is S~ clhl~ y no~nal to ~he path of travel of collim~tlo~ light in the ex~ernal cavity. The Fabry-Perot illt,_~lt~ ,cc fslter 38 is carried at an acute an,~le to a transverse plane. The angle ~ (see Figs. l and 7) be~.,el.
coating 32 and i.ll~re~lce filter 38 at their im~gin~ry ;~ e(.;liOII point (upward in the plane of the paper as viewed in Figs. l and 7) is larger than zero degrees. That is, the filter is not normal or orthogonal to the path of light from the emitter facet. The angle ~ may be e".l~)~ i;.. l~i as large as 45 ~ . Typically, it is l ~ to S ~, prefera~ly from 1~ to 2P, More generally, the hlle~rt.~il,ce filter is position~l at a slight angle between the emitter facet of the laser chip and the high IGrle~;lol coating defining one end of the external cavity, suGh that the Fabry-Perot filter is slightly tilted beyond the. IlUlllt~ al aperture of the laser chip. Wavelengths which do not fall within the p~csb~n-l of the filter and which are, therefore, not tr~ncmitte~l through the filter, are ~iup~nGssed. That is, the band pass hlt~.r~ .lce filter ~u,~)l,lGsses spectral modes which fall outside the ~ l of the filter 2Q primarily by r~flectin~ such out-of-band wavelengths at an angle away from the emitter facet of the laser chip, due to its aforesaid tilt angle.
In operation, light emitted from the anti-reflection coated facet 12 of laser chip lO
is collected and cnllim~ d by lens means 24 and directed by it into mon-~lithir prism ~se..ll)ly 26. Light which is in-band of Fabry-Perot inte~ nce filter 38 ~ through the filter with low loss along a path n;~lt;sen~ed by arrow 27, while light which is out-of--WO 97/30495 PCT/US97~02~90 band is reflected out of the resonant ravity away from faceL 12 alonY a path ~ se.lLed by arrow 29. Out-of-band wavelengths are reflected by flter 38. therefore, not back to the emit~er facet of diode chip 10. but rather along a path whereby it is intPntir~n~lly lost. The tr~n~mittPc~ light7 after passing through Fabry-Perot interference filter 38 strikes high S reflector end mirror 32 from which it is rPflPcs~i back toward facet 12 of laser chip 10 along the' path of arrow 27, but in reverse di*ction. ' The in-band light, the.t_ro~ passes again ~ through filter 38 ~s it returns to 'the emitter facet 12. Advant~geo -cly, the mn- .r~ .r. prism assembly 26, inrlllrling reflector coating 32 and Fabry-Perot i~ .,ce filter 38, can be assembled into a simple back-to-back miniature prism assembly having a ~limPn~ion between anti-reflective coating 34 and reflector coating 32 as small as 2 mm or less. The optical device illncss~sPd in Fig. 1 can be pack~ged sufficiently compactly, therefore. unlike prior known devices, to meet string~n~ size cun~L~ or limit~is)nc of various com--,e,cial appliç~ticrl~, inrl-l~ing ce~tain fiber-optic ~nn~ e~tinnc applir~tionc~ such as DWDM
applir~ti~ nc - ' As noted above, light refiected from high reflector end mirror 32 passes back again through Fabry-Perot i"l~.Ç~ ,.~e filter 38, thereby further removin~ u~w~ ed out-of-band light while Sr~n~mitting in-band light. The'trancmistpd in-band light passes again through lens means 24 wh~ by it is ,~ r~ into the laser diode 10, ~mpiihPd and directed to the output facet 14. As in~ zlsPsl above, outpu~ facet 14 carries partially reflective output coupler mirror coating 16, such that a portion of the light striking coating 16 is reflected back into the laser chip to continue the occi~ ion function, and the reln~in~lPr is l l ~m~ ed out of the laser to fiber-optic pi~tail 20. Secùnd collim~ter 18 can be used to collimate the - light from output facet 14 into' fiber-optic pigtail 20, preferably having anti-reflection coatings 48 and 50 on its exterior surfaces in the path of travel of the light. Typically, an anti-reflective coating 52 also will be provided at the input surface of fiber-optic pigtail 20.
CA 0224553l l998-07-30 Similarly, surfaee 25 of ct3liim ntn~ means 24 carries anti-reflection coating 35. Generally, it will also ~e preferred to provide a very slight an_ le ~o surfaces carrving anti-r.-fl,oction c~ tingc., such as co~tingc 16, 22, 34, 35, 48, 50. By angling or tilting these surfaces away from a transverse plane, such that they are not precisely normal to the path of travel of the .
S light, uudesilable back reflection due to imperfect anti-rt flPctiQn is reduced or eliminQt.o-1 The ove~all length of the external cavity of the lacer is equal to the length of the laser diode itsel~ plus the length of the in-cavity Cn~ Q~ E Içns 24. plus the focus distance ~i~.. .,~al the laser end facet 12 and the in-cavity col~im~ tinE lens, plus the length of the mrnrlithir prism assembly. Those skilled in the art will recognize, however, that the total optical cavity length is the produc~ of each such individual length cwllpO~ m~-l-irliP.d by the average refractive index of that cc",-pone.~l. The length of the laser cavity defines.the wavelength spacing of the lonEit~ n~l modes that the laser can support. To provide a single m,ode output in a~ c~ re. with certain preferred embo~lim~ntc~ the bandwidth of the Fabry-Perot u~ r~ ce filter of the mwtolithic prism assembly is made ndllu~r than the a~jacent spectral mode spacing. In those embol-mPrts in which the Fabry-Perot ila.~ ce filter is wider than the mode sparin~ more than one mode may be emitted, whichrnay be disadv~nt~gPouc for certain long haul Icl~cu..~ sionC applir~t~. nc, but which has ~lcPfillnPcc in other a~ l;r-.~iol ,~ The spectral mode spacing, referred to as Delta-T ~mlj~l~ can be c~lrul~tpil in accordance with the formula:
Delta-l ~mhtl~ = ~ambda-/ 12 x cavity length x refractive index~
In a preferred e.llb.3~ en~ in accordance with Fig. 1, the colllpon~,.l~ have the size and optical plup._~lles shown in Table 1 below:
TABI,E 1 Col-lpol-enL Typical Length Av. N Product - LaserChip 0.5 mm 3.6 }.8 c TrH~ilont index 1.0 mm 1.5 1.5 Focus distance 0.~ mm 1.0 0.2 Fillel/~ ,l 2.0 mm 1.5 3.0 Total 3.7 6.5 Using the formula given above and the .. f ;~'HI values of Table 1, where "Av. N."
is the spacing of possible l~ngit~l~lin~l lasing modes for the preferred embodiment in acco~ .ce with Fig. 1 Delta-Lambda will be a~yluAill,ately 0.17 nm. For single mode O~./H~ n the Fabry-Perot il~le~r~.~,.-cc filter of the monolithic prism assembly is plGfe~ably a b~n.l~Ac~ filter having bandwidth less than twice sueh vaiue, that is, less than 0.34 nrn.
~l~fc.ably, the f~lter has a bandwidth of less than 0.3 nm. As used here, the bandwidLl! of the filter is its 3dB bandwidth that is, the width in nAllol~.ctl ~ over which at least S0% of total l~cc;v~d ~ ~c are 1- H-~n;l1~ through the filter. Employing a multi-cavity Fabry-Perot i~ Ç~ .,c filter, specif~n~lly, a two-cavity ultra~narrow bAn~iracc filter CC~ d at 1550 nrn with l)anliw;(llll of 0.25 nm, in accordance with preferred embo iimrn~c .l;~ ed further below. a spectral mode at 1550 nrn is ~ .cl while other modes will be rejeeted.
In particular, in each pass through the filter the nearest speetral modes at 1550. li nm and 1548.83 nm will be re~eeted by ay~ ly 8 dB. Thus, effectively total l~,je.,liull of these a~ rent spectral modes can be achieved, as the light must travel twice through the Fabry-Perot interference filter, once passing from the laser chip to the high reflector end mirror 32. and a second time retlected baclc from the end mirror 32 to the laser chip.
Spectral modes farther from the 1550 nm tr~n~mitt~nre mode are rejected in even greater degree.
W O 97/3049~ PCTrUS97/02190 In this regar~ the effect of using mulliple-cavity interference filters is illustrated in the graph of Fig. 6. It can be seen in Fig. 6 that the tI ~n~ re ~o~,lies at 1550 nm are eY~PIIP.r~t for one~cavity, two-cavity and three-cavity filters. The spectral skirts of the two-cavity and three-cavity filters have h~t,IL,asil-gly greater slope. along with a wider S ~ ;ol ~ zone, as ~,ull-pa-ed to a single cavity filter. That is, out-of-band spectral modes are reflected in greater degree by a tu~o-cavity filter than a one-cavity filter, and the effect iSsllhst~nt~ y;l~ for a three-cavity. f~ter over a two-cavity f~ter. Both of these effecOE are adv~nt~g~ollc to the pe- r~ e of external caviey lasers in accordance with the pItre.l~d emb~1;~.~c.~lc .~ here, providing advantages over prior known filtering devices, such as etalons and diffraction gratings. Thus, the optical perforrnance of an e~ternal cavity diode laser as ~liccloseci here is achieved by controlling the Fabry-Perot r~ e filter of the monnlithi~ prism assembly. As ~liccllCce~l further below, ~xcellent teçll~ u-~s are available for l~tJ-u~lucibly producing Fabry-Perot interference filters with bulk density near unity to prevent water absorption induced filter shifting, etc. This is P~ lly true in those pl.,f.,.. ~d emb~limentc wh.,re;n the Fabry-Perot i .Lc;,Ç~.. ,.~ce filter is provided on an internal surface of the ~n()nnlithir prism ~cc-~mhly It can be seen that the optical device of Fig. 2 has aspects in common with Fig. 1, and it will be nnrlerst~od to function in correspondingly similar fashion. The reference lllb~ of Fig. 1 are used for common elemerlt~ or features in Fig. 2. Output coupler mirror coating 16 of the e.... bodi".e.. L of Fig. 1 is replaced ~y high reflector end mirror coating 17 in the embo~limen~ of Fig. 2. End mirror 17 defimes the right-hand side (as viewed in Fig. 2) of the resonant cavity. A~ ition~lly; high reflector end mirror 32 in the of Fig. 1 is replaced in the embotliment of Fig. 2 with optical coupler coating 33. Light is emitted through coating 33 to optical receiver device 56. for example, a fiber optic pig tail, light sensor, etc. More specifically, Iight emitted through coating 33 passes CA 0224~31 1998-07-30 Wo 97/30495 PCT/US97/Q2190 to optical isolator 54 and then to gradient index lens 55 before reachmg pigtail 56. Anti-reflect ve coatings 57 are provided in acco-~lance with known tPchniflul~s In certain applications the embodiment of Fig. 2 advantageously avoids amplifying "noise" as light passes back through the laser diode. Those skilled in the art will recognize that an optical S coupler such as coating 33, specifically, a coating in the nature of a beam splitter, could be used in the c.nbodi~ of Fig. 1 in place of high reflector end mirror 32 to provide a signal to an optieal l~,c~ device. The optical receiver device may co~ , for ~ , ~e a diode sensor for a power r~r. ll,~. ~ loop or simply an output signal carrying optical fiber, etc.
The thin film Fabry-Perot hll~L~Lcllct filter of the mf)nolithie prism assembly used in the optical devices disclosed here can be produced in accordance with col.lll~c.~,-ially known techniques~ whose applicability will be readily al)pa.~,-t in view of the present fii~l~sme; In p~rt;f ~ r, high~uality i..~ . r~ e, filters c~ stacked layers of metal oxide m~tf-fi~lc, such as niobia and silica, can be pro~luced by commercially known plasma ~si~ nec, such as ion assisted electron beam e vayol~lion, ion beam s~ g, and reacti~re m~gnlot~on s~ ,. ;.. g, for eY~mplç, as rlic~ lose~i in U.S. patent No. 4,851,09S
to Scobey et al. Such coating methods can produce hl~ ,n. e cavity filters formed of stacked ~1 1F~ ;C optlcal coatings which are advantageously dense and stable, with low film scatter and low absorp~ion, as well as low sensitivity to telll~ld~ changes and ambi~.rt llul~lidily. The spectral profile of such coatings is suitable to meet S~ g,f--~ ir..~
~ e~ ;r.f ~ m~ In particular, multi-cavity narrow b~n~ filters can be ~ duced using such tf chnirl~lr~, which are tran~!,dlt;n~ to a wavelength range s~_tJd~dted from an ~rij~rr...l wavelength range (e.g., from the wavelength range of an ~ rent channel in a dense wavelength division multiplexing fiber optic system) by as little as two n~nomr-ter~ or less.
One suitable deposition techni~ue is low pll.,;~:~Ult magnetron s~ ;..g in which the CA 0224553l l998-07-30 vacuum cl~ her of a magnetron sputtering system which can be o~h~-wi~e convP.nti-n~l is e~l-ippe-l with high speed vacuurn pumping. A gas manifold around the magnetron and target m~tPri~l confines the inert working gas, typically argon, in the vicinity of the magnetron. As the gas diffuses and expands from the area of the magnetron, the unlls~l~lly S high ~ n~p;~g speed vacuum removes the eYr~nrlin~ gas from the cl~ .her at a high speed.
The inert gas ,~ UlG in the el~ hl r, yl~fe~bly in 1i~ ~ of 5 x 10-5 Torr to 1.5 x 10 Torr, is then a function of the p~n~ g speed.of the vacuum pump and the c~ p~
e.~ iP l~ y of the magnetron baMe. Reactive gas enters the ch~mber through an ion gun which ionizes the gas and directs it toward the ~ n,-t~ This has the effect of reducing the amount of. gas required to provide the film with proper st~ hiomPtry as well a~s reducing .
the reactive gas at the magnetron. Throw distance of 16 inch and longer can be achieved As noted aoove, the flter preferably colllpli3es a multi-cavity coating in which two ~iPlP~trir thin f~lm stackc which by themselves forrn a IGne~;~ol for the unw~,l~d wavelengths are sep~ ecl by a cavity layer. This ssructure is then repeated one or more times to produce the ~ l multi-cavity filters with e.. ~ n~ ed blocking. and il"~"o~,d in-band t.,...~ ;on fl~tnPc~. The net effect is to produce a n~ wballd tr~ncmi~cive filter where in-band light is n~ .rl and out-of-band light is rPflPctt-~l In p,efel,.,d three-cavity embo~;...r,.,l~ produced by the deposiuon t~chnirluec m~n~ion~cl above, with dense, stable metal oxide film stacks, ey~ellpnt therrnal stability has been achieved, for ~.,;....ple, O.a~4 nm per degree c~ or better at 1550 l~ f-tC ~ ~, and ultra-narrow band widths 5ep~ tl ~~ by as lit~le as 2 nm or even as little as 1 nm.
In accu,~aluc with the above m-ontionPc~ preferred emb~~ , the inte.ren,nce filter typically is formed of two m~t~ori~lc, the first being a high refractive index m~t-o.ri~
such ac niobium pent~nci~ nill.~ dioxide, t~nt~lnm pen~ ito and/or mixtures thereof, for ~ P" I~ LUIe~ of niobia and titania. etc. At 1.5 microns wavelength. the refractive CA 0224~31 1998-07-30 wo 97/30495 PCT/US97/02190 index for these m~t~ C iS roughly 2.1 to 2.3. The low refractive index material is typically silica, having a refractive index of about 1.43. An imc-rc.~ ce filter has an "optical L "~ " which is the n~ 1i product of its physical ~ times its ~Gfid~.liYG index.The optical lh;. I~.ec~ of the Fabry-Perot interference filter used in the monolithir prism assembly of the optical devices ~1;c~1Oced here varies, of course, with the physical thi~,knf~cc of the filter and with the ~fia~ /e index of the n~t~ri~l sel~cte~ It will be we~ withirl the ab~ity of those skilled in the art, in view-of this ~l icf l~ . to select suitable m~t~ri~lc and film thirknecces to achieve spectral tr~ncmift~nçe ~ lu~,li~s suitable to meet the l~uilGulents of a given aprlir~inn The monnlithir prism assembly col"p~ a thin film Fabry-Perot il~L~.G.,ce filter in the optical devices ~licclosed here has ci~nifir~nt advaMa~es over prior known devices used for such optical devices. Especially when produced with durable m~reri~lc to .form dense layers of near unity packin~ density, the illt~ ,lce filter of the mon~lithir prism assembly is highly stable over time and with respect to humidity and other ~mki~nt cu,,~ iol,c. E~ llc.ll,ore, a large number of optical ~lilri~ blocks can be coated cimnll~ne.,usly with the ~t~,~re.Gn~,c filters in a single coating run, thereby ~ lly reducing m~m.~ costs. They are readily manllf~rtllred colllp.i~i.-g multiple cavities coherently coupled usin" quarter wave ~h;~ layers in accordance with known lr~l.ll;.l~lPS, yielding i"cleased slope of the spectral skirts along with a wider u,.l.~
zone. As ~ticcncsed above, all of these effects, plus the lllhliaLule size in which the l;ll.;r prism a~",bly can be readily r;~ ,t~ ~1, offer ci~..;l~r(~ advantages over other types of filtering devices, such as etalons and diffraction gr~tin,oC Moreover. the stability of tlie ih.tc.~.ence filter is ~nh~nre~l since it is- fomned on an optical ~lb~ lr, e.cpeçi~llly when carried on an intemal surface of the monolithir prism assembly, as discu~ed above.
Such interference filters can be produced in extremely small sizes, for eY~m~ , less than CA 0224553l l998-07-30 WO 97/30495 PCT/US97tO2190 0.5 mm thick and only a few miilimPterc in ~ mprer As such. they can be readily ~?~r~ e d into tiny, relatively low-cost laser devices. They can be readily m~nl~f~ctl-red using cu~ ially available tPchni(lmPc to tr~ncmit an intent1Pd or s~e~ ed wavelength within plus or minus 0.1 nm, with ~ ,..cly narrow bandw;.lths of, for er~mple 0.3 nm or less.
SAs noted above, tr~ inn of the in-band wavclell~Lh range can be extremely high.Preferred filin stack aLIu~;lul~,S for the multi-cavity ..lt~.r~h..lce filter 38 in the f'~ ~T; ~ of ~e FL laster ill~ IrA in Figs. 1 and 2 are ~ ct~ted in Pigs. 3 -5. Preferably. the ~I.i. L ..~ of each alternating layer (for eY~ml~lP of niobinm p,..l.,. ;,-i~
and silicon dioxide), as well as the total ~h;- lC ... SS of the film stack, is precise~y controlled, 10e.g. within 0.01% or 0.2 nm over several square inches of area. In ~lrii-inn, film stacks deposited with very low film absorption and scatter and with packing density near unity have low water-induced filter shifting. Such ultra-narrow, multi-cavity ban~ip~cs filters have.P.x~PIlPnt perfo.. ~n~e ci~ c~ s~ inr!--rlin~ tel~.r~ r and en~i.u.. ~ ul stability; narrow bandwidth; high tr~n~mitt~nre of the desired optical signal and high 15refl.oct~nre of other wavrl .~g;l.~, steep edges, that is, h;ghly selective L~ y ~particularly in designs employing three cavities or more); and relatively low cost and simple cr.n~ .n A three-cavity filter is shown in Fig. 3, sand~iclled bel~. ~,cn parts 42 and 44 of a ~ optical substrate. (See Figs. I and 2.) The first cavity assembly 85 is ;~ e~ irly ~ cpnt ~Jb~lr~lP part 44. A second cavity assembly 86 immr~ r lj overlies the first cavity and a third cavity assel.,bly 87 ~ .. rdi~tely overlies the second ca~Jity ~csembly and forms a surface-to-surface interface with substrate part 42. In Fig. 4 the ~l~u~.;Lul~ of the "first cavity" 85 is further illustrated. A sequenre of stacked films, efL.ably about 5 to 15 films of ~ltP~ting high and low refractive index m~tPri~lc, are ~le~o~;lP~ to forrn a first reflector. Preferably, the first film ilT~mP~ tPly ~ Pnt the 25~ r surface is a layer of high index ~ o.ri~l, followed by a layer of low index material, CA 0224~3l l998-07-30 wo 97/3049~ PCT/US97/02190 e~c. Each of ~he high tndex layers 90 is an odd integer of quarter wavel~.ngthc optical thir~nPcc (QWOT) ~ dbly one or three quarter waVplpnethlc or other odd number of QWOTs. The low refractive index layers 92 which are i~ .leaved with the high ~la~;Li~, index layers 90 are similarly one quarter wavelength optical thi~T~nP~cs or other odd number S of QWOTs in ~hi-~L-nPcc There may be, for ~."-llr abbut six sets of high and low ~iaCTi~'~ index layers forming the bo~tom-most ~ e~ n~ l 94- Cavity spacer 96, ,h shown 5nl.~ lly as asingle layer, typically cr.~ c one to four ~ e films of high and low index materials, wherein each of the films is an even number of QWOTs in ~ c, that is, an integral number of half wave~n~thc optical ~1.;. L ~;~ci The second ~TiPl~ctric rP.fi~ctor.98 ~ felably is s~b~;.. l ;~lly identir~i to AiP~ectrir reflector 94 ~L ~ A above. The second and third cavities are deposited, in turn, i~--"-f~ y upon the fs~st cavity and ~ lc;felal~ly are snbs~ lly jrlP.ntir~l in forsn.
One ~It~ , film stack is ~ ctr~ecl in Fig. 6, wherein the upper and lower .
reflPctc rs 94, 98 are as dcs-;.ibed above for the embo~T;~ of Figs. 4 and 5. The cavity spacer 97 is shown to be formed of four fslms, two high index films 97a alT~~laLulg with two low index fslms 97b. Each film is 2 Q~!OTs thick or one half wavelength. Various other alt.,..l~live suitable f~sm stack :jl.u~lu-~,~ are possible, and will be a~a.~,..L to those skilled in .he zrt in view of this Ticr,Tr~ylre ~n ~co,.l~ e with a further ~-t;fc-,~d aspect. the Fabry-Perot i~ ,Ç~,.G,lce filter of the monolithir prism assembly of the optical devices .l;c~ ed here may be further -..r 5l;1l.;l;,~.d or made otherwise tunable through the use of tilt ~jllctmenr means.
That is, means can be ~ ;ded, most ~1~ felabl~ ~ccociP~t~.d with the mounting means for the m~n~lithi~ prism assernbly, for altering the tilt angle of the Fabry-Perot h.Lt~rc.~"1ce filter, either independently or not of the pl~ s~ Qn angle of any other coating carsied by the mnnolithir prism assembly. In typical preferred embo~limentc, the angle of the fslter to the .
cQIlim~t~l light is i~ ,dse~s ac ~ ure increases. and cor~Fs~olldillgly decreased as the tc~ u~; of the filter drops. In addition. similar ~echn;~lneC can be used to tune the wavck.ngll.~ by ~t~ ;ng the tilt angle of the filter.
The altc.-lalivt; ~ .f~ . ~c~l ~,.-~b~-l~nl of an FL laser s~ -- A~ lly illustrated in Fig.
7 will be un-lPrstQod from the following description to operate in accoldallcc with the pri~lcisples ~l;C~ I above. A first diode laser 58 carries a high reflector end mirror 60 at a fsrst emitter facet 62, and anti-re-fl~octinn coating 64 at the opposite emitter facet 65 at its .
in~.race with collimAtin~ means 66. A second diode laser 68 carries optical coupling coating 70 at emitter facet 72. such that a l~onalll cavity is established b~ e.l coating 70 and coating 60. Light which passes through optical coupler coating 70 is r~cei .r~d by fiber optic pigtail 73 after passing through collim~ting means 74. Second emitter facet 76 of.the second diode laser 68 carries anti-reflection coating 77 through which light is passed to c~ g means 78~ P~ A between co~ ting means 78 ~c~ori~tPd with the second emitter ~acet 76 of diode laser 68 and the coll;m~Ating means 66 associatecs with first diode laser 58 is a monolithic prism assemhly 79 carrying a thin film Fabry-Perot nall~wl/alld filter 80 on irsternal surface 81. ~ tinn~l anti-re-fl~ction coatings 82 are used .at the various surfaces of ~lem~ntc of the optical device. It will be recognized that high reflector end mirror ~0 could be ,~ PA by an optical couplin,~ coatin~, such that li~ht could be emitted from the laser device to a fiber optic pigtail, diode sensor of a power feeflhack loop. etc.
Various col-,polle.l~ of Fig. 7 shown spaced apart can adv~nt~ously be butt coupled to seduce the overall size of t~se device and its optical length.
A dense channel wavclc,.~lh division multipl~:Ying device is illllctra~eci in Fig. 8, , ' .,~lg an FL 'saser as <3iccl()se(l above at each of eight separate ports or ch~Ann~lc on an optical block 100. Thss m~ iplPYin~ device has the ability to mllltirl~lc individual, s~al~te wavelength si~nals in~o a cornmon fsber optic carrier line and/or to rlernl~ Py such signals.
Wo 97/30495 PCT/US97/02190 Typical specific~-iollc for an optical m~ iplPY-ng device in acccldd~-ce with the preferred embodimen~ illustrated in Fig. 8 inciude those provided in Table 2.
Number of Ch~nn~lc 8 Channel ~avelen~th 1544-1560 Channel spacing 2 nm + 0.2 nm ~;.. ;,,~.. , rcnl~tinn 20 dB to 35 dB
Insertion loss ~total) less than 6 dB
1~ Fiber type single mode, 1 meter pigtail O~alillg tc.llpefalulc range -2()~C to ~50~C
The optical mnl~ device of Fig. 8 meeting the s~ ionc of Table 2, in addition to optical block 10Q which, preferably, is a stable glass ~ .u~e, is seen to include means for receiving coll;....~t~ light, such as a fiber optic gradient index lens collim~tnr 112 or the liIce, receives highly coilim~tf~d light 114 from optical port 118 of the optical block at a slight angle through a hole or facet in surface 116 of the optical block. In ~c~
with one preferred c~ f l~, the optical block has a ll ,;rl~ ~ ....CC "a" of S mm, a length "b"
of 14.1 mm or more, and a refractive index of about 1;5. The Col~im~ light ~ .~ly has a di~ , of not more than about 0.15~ and the tilt angle "c" at which the collim~t~
light exits the optical block is about 15 ~ . Multi-wavelength light bounces within the optical block b_L~ .. the high reflector coating 134 and o~iLe surface 120. A channel (or m--ltir~ c~ are added (or l~,.llu~,d) at each (or every other, etc.) bounce by a ~L~, filter which l,a,~",i~s a next wavelength inw~.,.l..~. Alt~ ative to such series of filters, a graded wavelength, ~--,r~.ably all~ le~,L-ic, narrowband b~ filter 122 is carried on surface 120 of the optical block. Such filter can be made in acco,.lance with the t~ ;..g~ of co-pending, commonly owned U.S. patent a~ ion Serial No. 08/490,829, filed June 15, 1995 and entitled "Optical Mnl~ ;pl,~, ;.,g Device," the ~iicclosllre of which is incorporated herein by reference. Specifically, filter 122 in such embo~ llp~ is a -cnntimlo~lC, variable thil-L ..~, mulli-cavity inlerference filter, and. mosI preferably, is a c~ntin~ c linearly variable filter. Such filter 122 is ll~l~y~re~ll at port 124 to a sub-range of the w~ e.~Il.s in~hl(lFrl in the cr~llim~t-F~d light 114. Speçific~lly~ light 126 passes through port 124 of the optical block from a collim~tir ~ lens means 128 ~Cc~x;AI,3d with a first signal channel. The optical signal passed by pon 124 is generated by an external cavity semicon(l~ tt r diode laser 129 in accoldaIlce with any of the p-tf~ d ~F ~b~;",~"l.;
above,meeting ~ ;-~'pF-~I spectral perf~ n~P ChA~ ina~,cu,~l~,cewith Table 2, for a first channel of the mn' . ' ~ device.
T~he c~..~;....~...~ filter l~ at port 124 is reflective of wavrlF .~lhc which are not ~in-band" of the filter at that l~catir\n Light 132is reflected between filter 122 on surface 120 of the optical block and high reflector film or coating 134 on surface 116. High reflector .
film 134 does not cover optical port 118, so as to avoid interfering with the passage of Light 114. Tlhus, light 132 is reflected by refIector film 134 to strike surface 120 of the optical block at port 124, where it is reflected to pass through port 118. At the location of port 136 next A~ to port 124,-the c~ o~, variable ~ L"~C" multi~avity intc--r~ cc filter 122 is ~lal~yal~ to a dirr~ L wavel~ ll or sub-range of wavelengths than it is at port 124. E;or dense channel wavelcl.gLII division mllltiplF xinp ~ppli~tinnc, the wavelF n~;ll, separation ~c;L~ ,C~l each of the m~lltirl~ ports linearly spaced along surface 120 of the opticai block is ~I~,r~,.al~ly about 2 nm or less. Thus, at port 136 an optical sigr~al eu~.~.. ~ll li.~g to a second channel is llAI ~ d through the filter 122 from a collim~tin~
lens 138, g. ..~ ~i.t. ~1 by external cavity s~ u-~ or diode laser 139 in accu-dance with the ~fe.l~ emb~limF ntc above. As at the first port 124, the i"~.r.,.~nce filter 122 at port 136 reflects light which is not in-band at that location. Thus, the portion 142 of the light 11~ which first entered the optical block prior to this point (i.e.. Iight having wav~ IF "g,~
of othersofthe-~h~nnF~Ic,oriein~ingatlaserdiodedevices 149, 159, 169, 179, 189Or 199) ~=
CA 0224~31 1998-07-30 wo 97/3049s PCT/USg7/02190 is rPflPctPd on toward port 118 from port 136. In similar fashion. the reflected wave~Pn~thc at earlier pointc in the optical block cascade in a zigzag or "multiple-bounce" path in the optieal block with the optical signal for each individual channel being added at sn~cPc~;
bounces at surface 120 of the optical block.
SIt is a tPchnologically and cu"".... e-~ially ~ignifi~nt advantage of p-efe.l~d f ...~lx~ i of the deviees llicclrlced here that mnltir~ ch~nnPlc can be so tightly spaeed in a narrow wav~ l~n~ range, with reliable and preeise g~n~r~tion of desired wavrl~
sub-ranges by reliable and co,~ ,c.~;ially feasible eYt~rn~l cavity diode laser deviees. With Fl lasers sueh DWDM mnhir~l-oYin~ dev*es ean now be mass produeed with suitable wa~ hcontrolandcf~ ylt~ -spectral~lrol~ re~ such that fiber-optie DWDM mllhirleY~(l systems are rendered eo--.~ ;ially feasible.
It will be ~ from the above ~ n~ of the invention and detailed r~
of cenain plt;fe.l. d ~ bo~ n~...lc that various arlditir n~ and mo~lifir ~tionC can be made to the embodiments ~ osed without depar~ing from ~he uue scope and spirit of the invention.
All such mOIl;r.. ~l;"l.~ and ~.l;l;.. ,c are int~ntl~Pcl to be covered by the following claims.
WITH MONOLIlrHIC PRISM A~ sFMRLy FIE LD OF I~IE INV~NT~ON
The present ,~ is directed to optical deYices having an external ca~rity s~u~ laser. such as an ~Atl~ cavity diode las_r, in which an e~ te~l opti~
cavity extends t~ ,n one facet of an e~c~ g se ~;rQr~ J~ Io~ optical ~ . and an e~cternal ~ . Mor.~ particularly, the present invention is directed to external cavity o~ optical ~ having st~bili7~o~ erni~ n wa~elP!ng~hc - RA,CKGROUND
P~tem~l cavity sf~ ru~ o( lasers are known and have uul--L.uu~ uses and ;t ~l ir)l~, inr ~lriin~ fiber-optic cu~ c . l ir)nc~ In extemal cavity diode lasers which are typical of such optical devices, an optical cavity extends between one facet of an edge-5~ .";.~r~ ol diode laser and an e~tPrn~l high ~~ne~lul. A second facet of the ge~--~i~g sG~ ur laser. bu.-. C~ the high r~ne lor and the first ~acet, typically carries an anti-reflpction coating to allow light to escape the laser chip with ~
Semicol.~luctu~ diode lasers have been used c~ ,ly as l~ ;lt~ ~ for fiber-optic C~-.. ;.~l;~nc In one common and low cost imp' ~inn, edges of two opposing end facets of the laser chip are cleaved to form ~n.~" reflective ~--- ri.nPc and provide the ~ nL t~peL~ y for laser ope~ti~ n Such Fabry-Perot (FP) lasers typically emit in multiple Inngitn~iîn~l modes and have large ou~put banLlwidll,s. for e~ r~p~ 3 nm to 10 nm. In another common; ~ ~ ~rlP ~ l io~ with slightly "l.,lcased cn. . . ~IP ~ ;I y, a Bragg grating is etched in the active region of the Fabry-Perot laser cavity to forrrl a ~ u~f d f~P~h7 -~ laser (DFB). D;~ Pcl ~D ,~,cL lasers have the advant-age of single Ic)ngimriin7~l " . . .
mode emission which provides very narrow bandwid~hs typically, for P~s~mple, less than 0.01 nm. In a third 7trplirs~tion, the .1;i~ d Bragg reflector (DBR) laser ~ a wavelength-selective Bragg grating for one of the cleaved facets of the Fabry-Perot laser.
The wavelength-se}ective Bragg grating has the effect of Lllo~ci~!g a laser with single 1~,..~;1.,.l;..~l mode output.
- Ap~ of these and other diode.lasers has been ;---~i~ due to inadequate.
stability and accuracy in the particular wa\~lf-l~g~ ...Lt~ed In particular, for e,~i,r"rl~, such diffi~ tif~c have been ~L~.;...-c~d in the ayL)li~ n of diode lasers in Dense Wa~ nglllDivision~ DWDM). Inthisadvancedfiber-opticcf,,.. n.. ;.~1;~", tP~hrlolf)gy, many closely spaced wa~.le.~ s or ~ ....f.lc are t~nL....;Il~ .d cimnlt7~n~ "~ly down a single fiber or fiber bundle. Typical spacing of channels in DWDM systems can .
~range from S.nm to 71S little as l.nm or less between ch7tnnPlc~ To 7~rc~rn~1ich crÇ~,cli.~_ DWDM systerns, stable and accurate U~tt~ of prede~ hled wavelengths are needed fore~ lb/idualch~nnel. Ins~ tTrln~stableand arc~ t~,~àv~leuglh-sele~ ,lece;~
are needed to scle.,li~.,ly remove or receive the illdi~idual channel wavP.iP.n~thc with low or no cross talk from other ,;11" l .. .~ .ki. For â DWDM syslem to operate r.rfi.-;- .1 Iy, lh~.cr. lc;
the ll~u~ ~- and ll:;Cc;iV~l device for a given channel must be tuned with great accuracy to the same wa~ h band.
U,lrol~ t~,ly, the wàvele~lglh band emitted by l,~seillly known se~nico~ r diode lasers, ;..~ g the above rnentic)rtPd FP lasers, DFB lasers and DBR lasers, vary to an ulla,ccipLably large degree with ~lu~c.alulc and other factors. Center wa~,length p~nl~ c~ e of an FP laser, for ~ , 'e, is typically as much 0.4 nm per degree centiEr~A~ change in operating ~r.l--~ at room tcr2pc.dtu~c~ The comparable variance for D~B lasers is typically as much as 0.1 nm ~er degree centigrade. P~,.,c.~ly known l- -ctor diode lasers also suffer the disaLlvanta~e of poor m~nllf~t--rin~ r~pe~t~hiliry That is. an int~nrled or specified emiccinn wavel.,"~Lh is not achieved with ~le~
a~_~,u.d~;y when such lasers are y~ u~cd in large commercial qU~ntit;r c, These ~fi~iPnCi~5 rende,r present s~ .r diode lasers difficult and costly to imriement into ~ ".i;..
arrli~ nc such as DWDM systems, and in many cases entirely unsuitable.
ItisKnownthatthe~.. lf.. i.l.. erif.~lf.. ~rif.. nf of an~div~lu~llasercanhemitg . by cnnh~llin~ the ~ t~ . . e of the laser to w, .ithi~l an e~ r.l~r s.mall te "l~ G range using, for PY~mplP thf rmopl~ctric coolers with closed loop feerlhaclr from a t~mpP~hlrr~
sensor. Such controls are complex and costly. The even more difficult problem ofco... L,u~ ,g lot-to-lot wavelength variation in commprcial m~nllf~ctllring of ~GsG.~ly ~nown s~ .. lit on.l~.c~or diode lasers, which can be as great as l S nm and even 1 10 nm. has been partially addressed by culling through pro~t~tion batches for lasers having the desired wavel~n~th Thi~ lf e~ -e of wavelength testing of individual lasers has ci~n;r;. ~
adverse impact on m~mlf~ctllring yield, with correspondingly increased costs and c-~mrl~Yjty.
It has also been ~.u~os~ to use an ~ItPrn~tive type of semicon.lllrtor diode laser, s~- ;r.~ y, extern~l cavity tunable lasers. FYtPrn~l cavity tunable lasers are suggested. for e~mplP., in Widely Tunable External Cavity Diode Lasers. Day et al. SPIE, Vol. 2378, P.
3541. In the diode laser devices sl~ggpct~l by Day et al, an anti-reflective coating is placed on one facet of a diode laser chip. The emitted light is captured within a collim~ting lens~
and a rliffr~tion grating is used to select or tune the wavelength of the laser. Laser action occurs, generally, provided that the grating is selPcting a wavelength within the diode's spectral gain region. A diode laser device employing a diffraction grating disposed in an extemal cavity also is suggested in U.S. patent 5,172,390 to Mooradian. Unfor~unately, ~ tion gra~ings disposed within the external cavity of a diode laser causes a .cignifir~nt increase in the overall size or bulk of the device. The diffrac~ion ~ratinP and the complexity of the required ~rating ~lignmPnt system can also si~-lificartly increase the cost of the device. As to the size or bulk of the device, the cavity length for a diode laser having a diffraction gratin~ disposed in an external r~;son~rll cavity, in acco~dance with known devices, is txpically from.25 rnm to over 100 mm, in con ast to the ~much s~nalIer 1 mm size.
or smaller of FP lasers and DFB la~ers t~ ;} above. T~ie ~; rr ~ ;on grating and grating mount also have been found to exhibit tr ~ k~ Since the ~l~rr~cl;~
grating sets .the wavelength of the laser, such t~mperatllre de~ e of the grating and grating mount cause unwanted inct~hility in the emitted wavelength of the laser. In z~ 1itinn, long term w~vcle,~ Il drift problems have been e~pP - ;~ nce(l due, it is belieYed, to the mech~niral compl~ity of the diffraction grating and grating mount aspec~s of such devices.
It is an object of the present invçntion to provide s~miron~ ctnr laser.devices having .
good wavelength stability and accuracy. In particular, it is an object to provide such devices having acceptable m~mlfactlrin~ costs, c~ e ity~ and size o~ buL~c c~ s iti~n~l obiects of the invention will be ~ GIIL from the following disclosure and from the detailed description of certain preferred embo lim~.ntc SIJMMARY
In acco,.lancc with a first aspect, an external cavity laser is provided, having an optical ~mlllif;~r optically coupled to an external resonant cavity, with a mnnolithic prism assembly in the external resonant cavity. In accordance with certain pl~fell.,d r.lllc, the optical ~mplifi~:r is a sellliconductor optical amplif;er, such as those used in diode lasers, e.g., those for-m--ed of InGaAsP. Other suitable gain el~ments inc}ude, for mpiç erbium doped silica, ge~ ania or other optical m~Pri~l formed as fiber, etc. In CA 0224~31 1998-07-30 wo 97/3049S PCT/US97/02190 cenain emb~limcn-c the optical ~mrlifj~r is a direc~ band~ap opucal emiuer. The monolithic prism assembly comprises a transparent substrate. that is. a suhstrate which s substantially optically transparent to the laser light, and incorporates a Fabry-Perot r~ e filter c~....... l";~ multiple thsn film reflectors sandwichin~ aL least one thin film cavity between thern... The thin film ~abry-Perot intelr~l~nce. filter is disposed in the path of travel of the laser light in the external cavity. More 5~ 1y, it is oriented at a slight.
. angle to a ~ ~ plane. That is. it i~ cd on a surface of the ~ a~ an .
internal or external surface, as ~is~-uc.ced below) which is disposed at a non-zero angle to a transverse plane. As used here, a "transverse piane" is an im~8in~ry plane normal or orthogonal to the path of laser light in the external cavity. Cull~yoluilllgly, a transverse surface of the tral~l)a,c~nl snhstr~te is one which lies in (or a~ llately inj a transverse plane. Thus, the thin film Fabry-Perot interference filter is in a non-transverse plane; it is not norrnal to the path of travel of the !aser light passing through.it. Rather, it is at an acute angle, typically less than 45~ and more than 0C, preferably about 1~ to 5~, more preferably about 1~ to 2~ to a transverse plane.
The thin film Fabry-Perot in~erference filter, in accordance with one aspec~, is a stable, l~luwl~d i"le.r~lel~ce filter provided as a coating on a surface of the llan~yalelll op~ical s~ -I o~ . of the m~nrl!ithi~ prism assembly. While the interference filter preferably has at least one thin film cavity layer sandwiched between thi~ film reflector layers, more ~ fe- ai-ly~ it is a multi-cavity filter of two to five cavities, most preferably having two or three cavities. Each such cavity has an optical thirl~n~cc (c~ t.od as its actual physical thi~kn~ss times the refractive index of the cavity material~ equal to an even number of - quarter wavelengths or QWOTs. The wavelength referred to is typically about the center of ~.he wavelength band of laser light to be tr~ncmitt~d through the filter. Each cavity layer preferably is formed of one to three dielectric films, each such f1lm having an optical CA 0224~31 1998-07-30 WO 97/3049s PC~/US97/02190 ~i.i. L ,.. ~ ec}ual to an even number of quar~er wavelen,~ths preferably totaling 1 to 15. more preferably 5 to 10 half wavel~ Lhs optical thickness for each such dielectric film. The reflector layers sandwiching a cavi~y layer between them each preferably is formed of quar~erwavelength optical 11.;. L~ flms, as describ:d further below. Most preferably, the fi!ter is a multi~avity thin film Fabry-Perot na.,~wballd il~leLrt l. nce filter wherein the in~ lual reflector cavities fi'~ UCiu~i, are coh~,..,hlly coupled to each other using a ~ufi~le.w~ n~7L ~ C~ optical CO~ layer. The use of a ~t~ . cavity Fabry-Perot interference filter in the monolithi~ prism assembly yields a filter with increased slope of the spectral skirts, along with a wider tr~ncmicsion zone, as compared to a single cavity filter. Both of these effects yield highly ~nPfici~l improvements in the ~r(~ e c,l".~ s of the extemal cavity lasers disclosed here in comparison with prior known fltering devices, such as etalons and diffraction gratings, as ~lieru~ed further below.
It will be further.nn~l~r~tood from the rliC~ elow, that the ~m~)nnlithic prisTn assembly need not provide any light diffraction function in the classic sense of a prism.
Light from the opticil emitter is acted upon, such that in~band light is ~ led and out-of-band light is re~ected away. Thus, in-band l~ght is s~p~r~te~1 from out-of-band light. The mon-)lithiC prism assembly could also be referred to as a filter monolith, a1~ain me~ning a ~lall5~d~ substrale assembly incorporatin" a thin film Fabry-Perot interference f~ter .
Cu~ uliS~lg at least one thin film cavity sdndwicl1ed between thin film reflectors.
The ~ J~US~ ;.' prism assembly in accol~ldllce with certain preferred emb~~
carries a reflective coating, preferably a high r~fl~ction coating, on a transverse surface defining one end of the extemal resonant cavity of the optical amplifier. and the aforesaid Fabry-Perot i,l~lÇt;lence filter is carried on a second surface ~hich is spaced from, and at an acute angle to the firs~ surface. Optical devices in accordance with this aspect adv~nt~eo--s1y comprise an external cavi~y, edge emittin~ semiconductor diode laser CA 0224~31 1998-07-30 wo 97/30495 PCT/US97/02190 having an anti-renection coatin. on a first emi~ter facet optic'ally coupled to the external resonant cavity. An output coupler reflective coating is provided on a second emitter facet of the diode laser. The monolithic prism assembly is position~d in the external resonant eavity, c~ a lldl~S~dlGi~ optical substrate carrying a cavity-reflective coating on an 5exurnal surfaee to define one end of the extemal resonant cavity. A thin film Fabry-Perot e filteris provided on a seeond surface of the optieal ~ul,~l-dle, being p~
within the external ~ cavity l~h. ~.~ ~he refl~ctive coatirlg and the f~t emitte~ facet.
One or more ct llim~tin~ means, sueh as gradient index lenses or bulk lenses, etc., also are p~;l;....~A in the externai resonant cavity, e.g.. for focussing light between the Pabry-Perot il~t~ .. I; .. ~ n. ~ filter and the first emitter fa~et of the diode laser. T~ol~t~ ~ can be used outside the cavity in the usual way.
Those who are s~illed in the art, or knowledgeable in this area of t~chnology, will ~,c~g",.:e that the optical devices ~licclos~d here are a ci~ 1 technolog-~i advance.
External cavity lasers employing semic~ l optical amplifiers in accordance with this 15iicrlosll~ can be ~lùced with accurate and'reproducible emicsion wavç~ thc, exçell~nt Pe~ G stability, and eyrell~nt ,~ n~e to wavcle~ }i drift. Moreover. it is highly ci.~ that such lasers. especially in accordance with plGfé~ ,d embo~im~.ntc. can be produced in I~ dlu~G size and in large commercial qu~ntiti.oc. havin~ manufacturing costs c r ~ i to kno-,vn FiP lasers, DFB lasers or DBR lasers. As .liccnccf -1 below, plGÇ~ ,d c.l.bo~1;.. ~ .. l~ of the laser devices disclosed here may be referred to as filter-locked laser~
for their use of thin film Fabry-Perot interference filters in the external laser cavity to st~hili7e or lock the emitted wavelength or band. Preferred embo iimentc of the lasers ~iicrlt~ced here. and optical devices incol~oldLhlg them. can be reproducibly m~nnf~ctllred in commercial ql~ntitips with ~miccic.~ wavelen~th held to within + 0.1 nm. and with temperature depen~lenre of 0.005 nm/~C or less, preferably .001 nm or less. Long teml wavelength drih can be held to less than + 0.l nm. Moreover. the morlnlithic prism assembly incorporating the thin film Fabry-Perot interference filter enables overall cavity length of the diode laser to be less than S mm in certain preferred embo limPnt~. Those ski~led in the art will recognize that these pa~ ging and perforn~nre ch~r~cteri~tirs render S l"~r~"rd ~ hof~ f ~ of the lasers ~ nl~lse~1 here, and optical devices incoll-oldLiilg them, suitable and co~ ";idlly pr~rti~l for ay~ such as, most notably, dense w~ h.~ divisionmultiplexingfiber-optic~.. 1.. ;.. ~;nnsystems. As~ s~ab~ve, previously known diode lasers, such as those incorporating iiffra~tiorl gratings or the like for wavelength control, were too complex, bulky, unreliable andfor costly to satisfy the 10- exacting ~ui~ ~ of such appli~tinn~
A~ ti-)n~l aspects and advantages of the present invention will become ap~a,Gl,l or more readily nn~ ;tood from the following detailed description of certain ~ r~ ,d embo~iim~nt~ .
BRIEF DESCRIPI~ON OF TH~ DRAVVrNGS
Certain t~ ~ f~ . ~1 L.l-bo~ of the invention are ~iic,~ ed below with le~ e to the accolll~dllyillg drawings in which:
Fig. I is a s~ . "~ str~tion of a first preferred embodiment of an optical device incorporating an external cavity laser in acc-"ddllce with the foregoing ii~closl1re;
Fig. 2 is a sch~m~tic illustration of an external cavity laser in acco,dance with a second preferred emb~~
Figs. 3 - 5 are s(;h~ ." ~l;c ilh1ctr~tiorlc of the thin film Fabry-Perot interference filter of the monolithic prism assembly suitable to he employed in the semicnn-ll1ctor optical ~nplifiers of Figs. l and 2;
wo 97/30495 PCT/US97/02190 Fig. 6 is a ~raph showing the theoretical perforrnance of a hiBh quality three-cavity Fabry-Perot interference filter in accor~lance with Figs. 3 - 5. along with the cO.l~,s~Jollding perforrnance of coll~p~t~h1P one and two-cavity thin film Fabry-Perot interference fiiters;
Flg. 7 is a schPm~tir illnctr~tif~n of an optical device cu~ lg an external cavity S laser in accordance with another p,efel-~d embo~impnt; and Fig. 8 is a sel-~ . ,.,.un ill. ,~(".l ;h- - of a dense wa~le~ h division ml~ de~ice utili~ a number of external cavity lasers in accù~ c~ with the present i~ ,.,~io, It should be untlPrstood that the optical devices illustrated in the drawings are not n~eecc,.. ;Iy to scale, either in their various ~iimpnci(7n.c or angular relationships. It will be well within the ability of those skilled in the art, with the aid of the foregoinP ~liccloc-~lre and the following detailed description of preferred emb~imen~cl to select suitable ;""~ and an~ular rel~tinn.chiins for such devices int~nfl~(l for a particular appiir~tir,n DETAILED DESCRIP'IION OF CERTAIN PREFERRED EMBODIMENTS
Those who are s~illed in ehis area of eechnology will lucû~ e from the above ~iiC~nccit~n that the external cavity lasers ~icclosed here have IlUm~,lUUS ~p~
;"( 1."1;"~ use in fiber-optic telecomml-nir~tionc systems~ especially in systems employing dense wavelength division m~ wherein ex~remely narrow and preclsely controlled ;Qn wav~ n~thc are required. Additional applications include. for example, use in test c~ and the like, as well as laboratory instrnmlon-~tion In cûntrast to previously known laser devices. such as FP lasers, DFB lasers andDBR lasers, in the dev~ces disclosed here a thin film Fabry-Perot illLc.f~lcnce filter carried on a L~ ,..l substrate is used in an exterrral laser cavity to lock the emitted wavcle~ h or band of the external cavity laser within a narrow gain re~ion. The laser can even be limited to a sin~le ~omic~ n mode by employinP a suitable thin film Fabry-Perol narrowband , interference filter. Thus. the ~asers disclosed and discussed here can be said to be filter-.
Iocked. Muud~u~ (e.~.. having an external resonant cavity of overal} len th. inC~ n~ the ~..u~n~ ;r prism ~,llbly, of less than 5 nm) thin film filter-locked external cavity lasers, or "FL lasers" as they will be referred to in some ;,,~ c below, have especiallyadv~nt?~ol-c app1i~Rtion in DWDM fiber.optic telecom,~ alion systems. Precise and stable ~va~ele,~lg~ll laser emitters are called for in such a~ ;cd~ s~ so that closely ~ e d c1~ , are reliably ~p~~ and distinct. Those slcilled in the-art will recognize. that there are various other applications for the FI~ lasers ~1icc}t)sed here, especially applir~tionc calling for a stable and accurate narrowband laser source.
In the FL laser sch~mRti~R11y ilh1ct~t.ocl in Fig. 1, an external cavity diode laser bly is seen to include a laser diode chip 10 having a finst emitter facet 12 and a second, opposite emitter facet 14: Emitter facet 14 carries a coating, specifically, an output coupler mirror, that is, an output coupler reflective coating 16. Light emitted through coating 16 is received into c~-1}imRtin~ lens 18 which preferably is a gradient index lens or the like.
t~ mRt~(l light from gradient index lens 18 is passed to fiber-optic pigtail 20, wl,~ ,. bj it enters a fiber-oetic c~ l r~ r system. Grin lens 18 could opti~Rlly be deleted in favor of a butt c~..l.l;..~ between emitter facet 14 (with anti-reflective coating 16~ and pigtail 20. ~dAiti()nRl}y, an isolator can be employed outside the cavity following a _radient index lens or other col}imRting lens. passing light to pigtail 20. Isolators are generally well known and their use and the use of other optional col,lpo~ents in the FL lasers ~licc}osed here will be apparent to those skilled in the art in view of the present disclosure. Emitter facet 12 carries coating 22, preferably an anti-reflective coating. Light passing through anti-reflective coating 22 is received and collimRted by a second co}lim~tin~ means 24 which.
again, p,efe.~l)ly is a gradien~ index lens or the like. Light is passed from collim~rin~ means 24 into a monoli~hic prism assembly 26 co"~p,i~hlg a transparent optical substrate 28.
Substrate 28 preferably is optical glass. such as BK7 or B270. both available from Schott Glaswerke (Mainz. Germany). or the like. Outside surface 30 of ~ub~lrdle 28 carries high ref~ector end mirror 32, such that the external cavity of the diode laser is defined between output coupler mirror 16 and high reflector end mirror 32. Preferably, anti-reflective S coating 34 is carried on exterior surface 36 of substrate 28 to fz~rilitz~t~ passing of c~-llimsttPd light from lens means 24 into the monnlithir prism assembly 26.
'rhe prism stc~f....hlj~ further c~ e~ thin film Fabry-Perot ;11~ e filter 38 on internal surface 40 of substrate 28. An "intemal surface" as that term is used here. most typically is a surface-to-surface contact interface between two parts or pieces of ~ Ja~ L
substrate which have been cement~d or otherwise integrz~t.o~ to each other to form the mcnolithic prism z~ e.l~hiy An optical coating on an intemal surface. for eYz mpie the Fabry-Perot il.~"c.~.~.ce filter 38, is advantageously protected and st ~hili7ed by the bonded pie~es of the ~ between :which it is sandwiched. In that regard, i~ should be recognized that the gap shown between first piece 42 of substrate 28 and second piece 44 is highly ~ ~,.g~ t~ in Fig. 1 for l)ul~es of illustration. It should also be r~cog~d that Fabry-Perot i.l~.Ç~,.e.~ce filter 38 could be formed on interfacial surface 46 of part 42, as well as on interfacial surface 40 of part 44. An extemal surface. correspondingly, is a surface of the substrate which does not form a surface-to-surface contact int~t~ce with another part or portion of the substrate. It maj, therefore, be exposed to atmosphere or in ~ with another optical el~ment, such as a coilimz~tinE means, monntin~ structure or the like. An eYt~t~tzti surface may be coated, as in the case of the ~ù~ la~e in the .llbOdilllent of Fig. I wherein external surface 30 carries high reflector end mirror coating 32 and extemal surface 36 carries anti-reflection coating 34. A round hole or other aperture can be placed in or on surface 36 or other suitable location to limit beam angles impinginE~
on the filter. Optionally, part 42 of the transparent substrate 28 of the monolithic prism WO 97/30495 PCT/US97/0219~
assembly could be deleted in favor of.'e.g.. an air gap. Since air has a lower index of refraction. this would yield an advanta~e in reduced optical distance. It should be recognized that components of the optical device illllstrat.-d in Fig. 1 may be spaced from or in abull~ with ~ f.. ~t cle.. ,l~ as required by the performance and p~ck~ing S ~l~c;~ ionc of a given app!ication.
It can be seen in Fig. 1 that high reflector end mirror 32 on external surface 30 of t~ 28, L;es in a l,~ "~ plane, that is, in a plane which is S~ clhl~ y no~nal to ~he path of travel of collim~tlo~ light in the ex~ernal cavity. The Fabry-Perot illt,_~lt~ ,cc fslter 38 is carried at an acute an,~le to a transverse plane. The angle ~ (see Figs. l and 7) be~.,el.
coating 32 and i.ll~re~lce filter 38 at their im~gin~ry ;~ e(.;liOII point (upward in the plane of the paper as viewed in Figs. l and 7) is larger than zero degrees. That is, the filter is not normal or orthogonal to the path of light from the emitter facet. The angle ~ may be e".l~)~ i;.. l~i as large as 45 ~ . Typically, it is l ~ to S ~, prefera~ly from 1~ to 2P, More generally, the hlle~rt.~il,ce filter is position~l at a slight angle between the emitter facet of the laser chip and the high IGrle~;lol coating defining one end of the external cavity, suGh that the Fabry-Perot filter is slightly tilted beyond the. IlUlllt~ al aperture of the laser chip. Wavelengths which do not fall within the p~csb~n-l of the filter and which are, therefore, not tr~ncmitte~l through the filter, are ~iup~nGssed. That is, the band pass hlt~.r~ .lce filter ~u,~)l,lGsses spectral modes which fall outside the ~ l of the filter 2Q primarily by r~flectin~ such out-of-band wavelengths at an angle away from the emitter facet of the laser chip, due to its aforesaid tilt angle.
In operation, light emitted from the anti-reflection coated facet 12 of laser chip lO
is collected and cnllim~ d by lens means 24 and directed by it into mon-~lithir prism ~se..ll)ly 26. Light which is in-band of Fabry-Perot inte~ nce filter 38 ~ through the filter with low loss along a path n;~lt;sen~ed by arrow 27, while light which is out-of--WO 97/30495 PCT/US97~02~90 band is reflected out of the resonant ravity away from faceL 12 alonY a path ~ se.lLed by arrow 29. Out-of-band wavelengths are reflected by flter 38. therefore, not back to the emit~er facet of diode chip 10. but rather along a path whereby it is intPntir~n~lly lost. The tr~n~mittPc~ light7 after passing through Fabry-Perot interference filter 38 strikes high S reflector end mirror 32 from which it is rPflPcs~i back toward facet 12 of laser chip 10 along the' path of arrow 27, but in reverse di*ction. ' The in-band light, the.t_ro~ passes again ~ through filter 38 ~s it returns to 'the emitter facet 12. Advant~geo -cly, the mn- .r~ .r. prism assembly 26, inrlllrling reflector coating 32 and Fabry-Perot i~ .,ce filter 38, can be assembled into a simple back-to-back miniature prism assembly having a ~limPn~ion between anti-reflective coating 34 and reflector coating 32 as small as 2 mm or less. The optical device illncss~sPd in Fig. 1 can be pack~ged sufficiently compactly, therefore. unlike prior known devices, to meet string~n~ size cun~L~ or limit~is)nc of various com--,e,cial appliç~ticrl~, inrl-l~ing ce~tain fiber-optic ~nn~ e~tinnc applir~tionc~ such as DWDM
applir~ti~ nc - ' As noted above, light refiected from high reflector end mirror 32 passes back again through Fabry-Perot i"l~.Ç~ ,.~e filter 38, thereby further removin~ u~w~ ed out-of-band light while Sr~n~mitting in-band light. The'trancmistpd in-band light passes again through lens means 24 wh~ by it is ,~ r~ into the laser diode 10, ~mpiihPd and directed to the output facet 14. As in~ zlsPsl above, outpu~ facet 14 carries partially reflective output coupler mirror coating 16, such that a portion of the light striking coating 16 is reflected back into the laser chip to continue the occi~ ion function, and the reln~in~lPr is l l ~m~ ed out of the laser to fiber-optic pi~tail 20. Secùnd collim~ter 18 can be used to collimate the - light from output facet 14 into' fiber-optic pigtail 20, preferably having anti-reflection coatings 48 and 50 on its exterior surfaces in the path of travel of the light. Typically, an anti-reflective coating 52 also will be provided at the input surface of fiber-optic pigtail 20.
CA 0224553l l998-07-30 Similarly, surfaee 25 of ct3liim ntn~ means 24 carries anti-reflection coating 35. Generally, it will also ~e preferred to provide a very slight an_ le ~o surfaces carrving anti-r.-fl,oction c~ tingc., such as co~tingc 16, 22, 34, 35, 48, 50. By angling or tilting these surfaces away from a transverse plane, such that they are not precisely normal to the path of travel of the .
S light, uudesilable back reflection due to imperfect anti-rt flPctiQn is reduced or eliminQt.o-1 The ove~all length of the external cavity of the lacer is equal to the length of the laser diode itsel~ plus the length of the in-cavity Cn~ Q~ E Içns 24. plus the focus distance ~i~.. .,~al the laser end facet 12 and the in-cavity col~im~ tinE lens, plus the length of the mrnrlithir prism assembly. Those skilled in the art will recognize, however, that the total optical cavity length is the produc~ of each such individual length cwllpO~ m~-l-irliP.d by the average refractive index of that cc",-pone.~l. The length of the laser cavity defines.the wavelength spacing of the lonEit~ n~l modes that the laser can support. To provide a single m,ode output in a~ c~ re. with certain preferred embo~lim~ntc~ the bandwidth of the Fabry-Perot u~ r~ ce filter of the mwtolithic prism assembly is made ndllu~r than the a~jacent spectral mode spacing. In those embol-mPrts in which the Fabry-Perot ila.~ ce filter is wider than the mode sparin~ more than one mode may be emitted, whichrnay be disadv~nt~gPouc for certain long haul Icl~cu..~ sionC applir~t~. nc, but which has ~lcPfillnPcc in other a~ l;r-.~iol ,~ The spectral mode spacing, referred to as Delta-T ~mlj~l~ can be c~lrul~tpil in accordance with the formula:
Delta-l ~mhtl~ = ~ambda-/ 12 x cavity length x refractive index~
In a preferred e.llb.3~ en~ in accordance with Fig. 1, the colllpon~,.l~ have the size and optical plup._~lles shown in Table 1 below:
TABI,E 1 Col-lpol-enL Typical Length Av. N Product - LaserChip 0.5 mm 3.6 }.8 c TrH~ilont index 1.0 mm 1.5 1.5 Focus distance 0.~ mm 1.0 0.2 Fillel/~ ,l 2.0 mm 1.5 3.0 Total 3.7 6.5 Using the formula given above and the .. f ;~'HI values of Table 1, where "Av. N."
is the spacing of possible l~ngit~l~lin~l lasing modes for the preferred embodiment in acco~ .ce with Fig. 1 Delta-Lambda will be a~yluAill,ately 0.17 nm. For single mode O~./H~ n the Fabry-Perot il~le~r~.~,.-cc filter of the monolithic prism assembly is plGfe~ably a b~n.l~Ac~ filter having bandwidth less than twice sueh vaiue, that is, less than 0.34 nrn.
~l~fc.ably, the f~lter has a bandwidth of less than 0.3 nm. As used here, the bandwidLl! of the filter is its 3dB bandwidth that is, the width in nAllol~.ctl ~ over which at least S0% of total l~cc;v~d ~ ~c are 1- H-~n;l1~ through the filter. Employing a multi-cavity Fabry-Perot i~ Ç~ .,c filter, specif~n~lly, a two-cavity ultra~narrow bAn~iracc filter CC~ d at 1550 nrn with l)anliw;(llll of 0.25 nm, in accordance with preferred embo iimrn~c .l;~ ed further below. a spectral mode at 1550 nrn is ~ .cl while other modes will be rejeeted.
In particular, in each pass through the filter the nearest speetral modes at 1550. li nm and 1548.83 nm will be re~eeted by ay~ ly 8 dB. Thus, effectively total l~,je.,liull of these a~ rent spectral modes can be achieved, as the light must travel twice through the Fabry-Perot interference filter, once passing from the laser chip to the high reflector end mirror 32. and a second time retlected baclc from the end mirror 32 to the laser chip.
Spectral modes farther from the 1550 nm tr~n~mitt~nre mode are rejected in even greater degree.
W O 97/3049~ PCTrUS97/02190 In this regar~ the effect of using mulliple-cavity interference filters is illustrated in the graph of Fig. 6. It can be seen in Fig. 6 that the tI ~n~ re ~o~,lies at 1550 nm are eY~PIIP.r~t for one~cavity, two-cavity and three-cavity filters. The spectral skirts of the two-cavity and three-cavity filters have h~t,IL,asil-gly greater slope. along with a wider S ~ ;ol ~ zone, as ~,ull-pa-ed to a single cavity filter. That is, out-of-band spectral modes are reflected in greater degree by a tu~o-cavity filter than a one-cavity filter, and the effect iSsllhst~nt~ y;l~ for a three-cavity. f~ter over a two-cavity f~ter. Both of these effecOE are adv~nt~g~ollc to the pe- r~ e of external caviey lasers in accordance with the pItre.l~d emb~1;~.~c.~lc .~ here, providing advantages over prior known filtering devices, such as etalons and diffraction gratings. Thus, the optical perforrnance of an e~ternal cavity diode laser as ~liccloseci here is achieved by controlling the Fabry-Perot r~ e filter of the monnlithi~ prism assembly. As ~liccllCce~l further below, ~xcellent teçll~ u-~s are available for l~tJ-u~lucibly producing Fabry-Perot interference filters with bulk density near unity to prevent water absorption induced filter shifting, etc. This is P~ lly true in those pl.,f.,.. ~d emb~limentc wh.,re;n the Fabry-Perot i .Lc;,Ç~.. ,.~ce filter is provided on an internal surface of the ~n()nnlithir prism ~cc-~mhly It can be seen that the optical device of Fig. 2 has aspects in common with Fig. 1, and it will be nnrlerst~od to function in correspondingly similar fashion. The reference lllb~ of Fig. 1 are used for common elemerlt~ or features in Fig. 2. Output coupler mirror coating 16 of the e.... bodi".e.. L of Fig. 1 is replaced ~y high reflector end mirror coating 17 in the embo~limen~ of Fig. 2. End mirror 17 defimes the right-hand side (as viewed in Fig. 2) of the resonant cavity. A~ ition~lly; high reflector end mirror 32 in the of Fig. 1 is replaced in the embotliment of Fig. 2 with optical coupler coating 33. Light is emitted through coating 33 to optical receiver device 56. for example, a fiber optic pig tail, light sensor, etc. More specifically, Iight emitted through coating 33 passes CA 0224~31 1998-07-30 Wo 97/30495 PCT/US97/Q2190 to optical isolator 54 and then to gradient index lens 55 before reachmg pigtail 56. Anti-reflect ve coatings 57 are provided in acco-~lance with known tPchniflul~s In certain applications the embodiment of Fig. 2 advantageously avoids amplifying "noise" as light passes back through the laser diode. Those skilled in the art will recognize that an optical S coupler such as coating 33, specifically, a coating in the nature of a beam splitter, could be used in the c.nbodi~ of Fig. 1 in place of high reflector end mirror 32 to provide a signal to an optieal l~,c~ device. The optical receiver device may co~ , for ~ , ~e a diode sensor for a power r~r. ll,~. ~ loop or simply an output signal carrying optical fiber, etc.
The thin film Fabry-Perot hll~L~Lcllct filter of the mf)nolithie prism assembly used in the optical devices disclosed here can be produced in accordance with col.lll~c.~,-ially known techniques~ whose applicability will be readily al)pa.~,-t in view of the present fii~l~sme; In p~rt;f ~ r, high~uality i..~ . r~ e, filters c~ stacked layers of metal oxide m~tf-fi~lc, such as niobia and silica, can be pro~luced by commercially known plasma ~si~ nec, such as ion assisted electron beam e vayol~lion, ion beam s~ g, and reacti~re m~gnlot~on s~ ,. ;.. g, for eY~mplç, as rlic~ lose~i in U.S. patent No. 4,851,09S
to Scobey et al. Such coating methods can produce hl~ ,n. e cavity filters formed of stacked ~1 1F~ ;C optlcal coatings which are advantageously dense and stable, with low film scatter and low absorp~ion, as well as low sensitivity to telll~ld~ changes and ambi~.rt llul~lidily. The spectral profile of such coatings is suitable to meet S~ g,f--~ ir..~
~ e~ ;r.f ~ m~ In particular, multi-cavity narrow b~n~ filters can be ~ duced using such tf chnirl~lr~, which are tran~!,dlt;n~ to a wavelength range s~_tJd~dted from an ~rij~rr...l wavelength range (e.g., from the wavelength range of an ~ rent channel in a dense wavelength division multiplexing fiber optic system) by as little as two n~nomr-ter~ or less.
One suitable deposition techni~ue is low pll.,;~:~Ult magnetron s~ ;..g in which the CA 0224553l l998-07-30 vacuum cl~ her of a magnetron sputtering system which can be o~h~-wi~e convP.nti-n~l is e~l-ippe-l with high speed vacuurn pumping. A gas manifold around the magnetron and target m~tPri~l confines the inert working gas, typically argon, in the vicinity of the magnetron. As the gas diffuses and expands from the area of the magnetron, the unlls~l~lly S high ~ n~p;~g speed vacuum removes the eYr~nrlin~ gas from the cl~ .her at a high speed.
The inert gas ,~ UlG in the el~ hl r, yl~fe~bly in 1i~ ~ of 5 x 10-5 Torr to 1.5 x 10 Torr, is then a function of the p~n~ g speed.of the vacuum pump and the c~ p~
e.~ iP l~ y of the magnetron baMe. Reactive gas enters the ch~mber through an ion gun which ionizes the gas and directs it toward the ~ n,-t~ This has the effect of reducing the amount of. gas required to provide the film with proper st~ hiomPtry as well a~s reducing .
the reactive gas at the magnetron. Throw distance of 16 inch and longer can be achieved As noted aoove, the flter preferably colllpli3es a multi-cavity coating in which two ~iPlP~trir thin f~lm stackc which by themselves forrn a IGne~;~ol for the unw~,l~d wavelengths are sep~ ecl by a cavity layer. This ssructure is then repeated one or more times to produce the ~ l multi-cavity filters with e.. ~ n~ ed blocking. and il"~"o~,d in-band t.,...~ ;on fl~tnPc~. The net effect is to produce a n~ wballd tr~ncmi~cive filter where in-band light is n~ .rl and out-of-band light is rPflPctt-~l In p,efel,.,d three-cavity embo~;...r,.,l~ produced by the deposiuon t~chnirluec m~n~ion~cl above, with dense, stable metal oxide film stacks, ey~ellpnt therrnal stability has been achieved, for ~.,;....ple, O.a~4 nm per degree c~ or better at 1550 l~ f-tC ~ ~, and ultra-narrow band widths 5ep~ tl ~~ by as lit~le as 2 nm or even as little as 1 nm.
In accu,~aluc with the above m-ontionPc~ preferred emb~~ , the inte.ren,nce filter typically is formed of two m~t~ori~lc, the first being a high refractive index m~t-o.ri~
such ac niobium pent~nci~ nill.~ dioxide, t~nt~lnm pen~ ito and/or mixtures thereof, for ~ P" I~ LUIe~ of niobia and titania. etc. At 1.5 microns wavelength. the refractive CA 0224~31 1998-07-30 wo 97/30495 PCT/US97/02190 index for these m~t~ C iS roughly 2.1 to 2.3. The low refractive index material is typically silica, having a refractive index of about 1.43. An imc-rc.~ ce filter has an "optical L "~ " which is the n~ 1i product of its physical ~ times its ~Gfid~.liYG index.The optical lh;. I~.ec~ of the Fabry-Perot interference filter used in the monolithir prism assembly of the optical devices ~1;c~1Oced here varies, of course, with the physical thi~,knf~cc of the filter and with the ~fia~ /e index of the n~t~ri~l sel~cte~ It will be we~ withirl the ab~ity of those skilled in the art, in view-of this ~l icf l~ . to select suitable m~t~ri~lc and film thirknecces to achieve spectral tr~ncmift~nçe ~ lu~,li~s suitable to meet the l~uilGulents of a given aprlir~inn The monnlithir prism assembly col"p~ a thin film Fabry-Perot il~L~.G.,ce filter in the optical devices ~licclosed here has ci~nifir~nt advaMa~es over prior known devices used for such optical devices. Especially when produced with durable m~reri~lc to .form dense layers of near unity packin~ density, the illt~ ,lce filter of the mon~lithir prism assembly is highly stable over time and with respect to humidity and other ~mki~nt cu,,~ iol,c. E~ llc.ll,ore, a large number of optical ~lilri~ blocks can be coated cimnll~ne.,usly with the ~t~,~re.Gn~,c filters in a single coating run, thereby ~ lly reducing m~m.~ costs. They are readily manllf~rtllred colllp.i~i.-g multiple cavities coherently coupled usin" quarter wave ~h;~ layers in accordance with known lr~l.ll;.l~lPS, yielding i"cleased slope of the spectral skirts along with a wider u,.l.~
zone. As ~ticcncsed above, all of these effects, plus the lllhliaLule size in which the l;ll.;r prism a~",bly can be readily r;~ ,t~ ~1, offer ci~..;l~r(~ advantages over other types of filtering devices, such as etalons and diffraction gr~tin,oC Moreover. the stability of tlie ih.tc.~.ence filter is ~nh~nre~l since it is- fomned on an optical ~lb~ lr, e.cpeçi~llly when carried on an intemal surface of the monolithir prism assembly, as discu~ed above.
Such interference filters can be produced in extremely small sizes, for eY~m~ , less than CA 0224553l l998-07-30 WO 97/30495 PCT/US97tO2190 0.5 mm thick and only a few miilimPterc in ~ mprer As such. they can be readily ~?~r~ e d into tiny, relatively low-cost laser devices. They can be readily m~nl~f~ctl-red using cu~ ially available tPchni(lmPc to tr~ncmit an intent1Pd or s~e~ ed wavelength within plus or minus 0.1 nm, with ~ ,..cly narrow bandw;.lths of, for er~mple 0.3 nm or less.
SAs noted above, tr~ inn of the in-band wavclell~Lh range can be extremely high.Preferred filin stack aLIu~;lul~,S for the multi-cavity ..lt~.r~h..lce filter 38 in the f'~ ~T; ~ of ~e FL laster ill~ IrA in Figs. 1 and 2 are ~ ct~ted in Pigs. 3 -5. Preferably. the ~I.i. L ..~ of each alternating layer (for eY~ml~lP of niobinm p,..l.,. ;,-i~
and silicon dioxide), as well as the total ~h;- lC ... SS of the film stack, is precise~y controlled, 10e.g. within 0.01% or 0.2 nm over several square inches of area. In ~lrii-inn, film stacks deposited with very low film absorption and scatter and with packing density near unity have low water-induced filter shifting. Such ultra-narrow, multi-cavity ban~ip~cs filters have.P.x~PIlPnt perfo.. ~n~e ci~ c~ s~ inr!--rlin~ tel~.r~ r and en~i.u.. ~ ul stability; narrow bandwidth; high tr~n~mitt~nre of the desired optical signal and high 15refl.oct~nre of other wavrl .~g;l.~, steep edges, that is, h;ghly selective L~ y ~particularly in designs employing three cavities or more); and relatively low cost and simple cr.n~ .n A three-cavity filter is shown in Fig. 3, sand~iclled bel~. ~,cn parts 42 and 44 of a ~ optical substrate. (See Figs. I and 2.) The first cavity assembly 85 is ;~ e~ irly ~ cpnt ~Jb~lr~lP part 44. A second cavity assembly 86 immr~ r lj overlies the first cavity and a third cavity assel.,bly 87 ~ .. rdi~tely overlies the second ca~Jity ~csembly and forms a surface-to-surface interface with substrate part 42. In Fig. 4 the ~l~u~.;Lul~ of the "first cavity" 85 is further illustrated. A sequenre of stacked films, efL.ably about 5 to 15 films of ~ltP~ting high and low refractive index m~tPri~lc, are ~le~o~;lP~ to forrn a first reflector. Preferably, the first film ilT~mP~ tPly ~ Pnt the 25~ r surface is a layer of high index ~ o.ri~l, followed by a layer of low index material, CA 0224~3l l998-07-30 wo 97/3049~ PCT/US97/02190 e~c. Each of ~he high tndex layers 90 is an odd integer of quarter wavel~.ngthc optical thir~nPcc (QWOT) ~ dbly one or three quarter waVplpnethlc or other odd number of QWOTs. The low refractive index layers 92 which are i~ .leaved with the high ~la~;Li~, index layers 90 are similarly one quarter wavelength optical thi~T~nP~cs or other odd number S of QWOTs in ~hi-~L-nPcc There may be, for ~."-llr abbut six sets of high and low ~iaCTi~'~ index layers forming the bo~tom-most ~ e~ n~ l 94- Cavity spacer 96, ,h shown 5nl.~ lly as asingle layer, typically cr.~ c one to four ~ e films of high and low index materials, wherein each of the films is an even number of QWOTs in ~ c, that is, an integral number of half wave~n~thc optical ~1.;. L ~;~ci The second ~TiPl~ctric rP.fi~ctor.98 ~ felably is s~b~;.. l ;~lly identir~i to AiP~ectrir reflector 94 ~L ~ A above. The second and third cavities are deposited, in turn, i~--"-f~ y upon the fs~st cavity and ~ lc;felal~ly are snbs~ lly jrlP.ntir~l in forsn.
One ~It~ , film stack is ~ ctr~ecl in Fig. 6, wherein the upper and lower .
reflPctc rs 94, 98 are as dcs-;.ibed above for the embo~T;~ of Figs. 4 and 5. The cavity spacer 97 is shown to be formed of four fslms, two high index films 97a alT~~laLulg with two low index fslms 97b. Each film is 2 Q~!OTs thick or one half wavelength. Various other alt.,..l~live suitable f~sm stack :jl.u~lu-~,~ are possible, and will be a~a.~,..L to those skilled in .he zrt in view of this Ticr,Tr~ylre ~n ~co,.l~ e with a further ~-t;fc-,~d aspect. the Fabry-Perot i~ ,Ç~,.G,lce filter of the monolithir prism assembly of the optical devices .l;c~ ed here may be further -..r 5l;1l.;l;,~.d or made otherwise tunable through the use of tilt ~jllctmenr means.
That is, means can be ~ ;ded, most ~1~ felabl~ ~ccociP~t~.d with the mounting means for the m~n~lithi~ prism assernbly, for altering the tilt angle of the Fabry-Perot h.Lt~rc.~"1ce filter, either independently or not of the pl~ s~ Qn angle of any other coating carsied by the mnnolithir prism assembly. In typical preferred embo~limentc, the angle of the fslter to the .
cQIlim~t~l light is i~ ,dse~s ac ~ ure increases. and cor~Fs~olldillgly decreased as the tc~ u~; of the filter drops. In addition. similar ~echn;~lneC can be used to tune the wavck.ngll.~ by ~t~ ;ng the tilt angle of the filter.
The altc.-lalivt; ~ .f~ . ~c~l ~,.-~b~-l~nl of an FL laser s~ -- A~ lly illustrated in Fig.
7 will be un-lPrstQod from the following description to operate in accoldallcc with the pri~lcisples ~l;C~ I above. A first diode laser 58 carries a high reflector end mirror 60 at a fsrst emitter facet 62, and anti-re-fl~octinn coating 64 at the opposite emitter facet 65 at its .
in~.race with collimAtin~ means 66. A second diode laser 68 carries optical coupling coating 70 at emitter facet 72. such that a l~onalll cavity is established b~ e.l coating 70 and coating 60. Light which passes through optical coupler coating 70 is r~cei .r~d by fiber optic pigtail 73 after passing through collim~ting means 74. Second emitter facet 76 of.the second diode laser 68 carries anti-reflection coating 77 through which light is passed to c~ g means 78~ P~ A between co~ ting means 78 ~c~ori~tPd with the second emitter ~acet 76 of diode laser 68 and the coll;m~Ating means 66 associatecs with first diode laser 58 is a monolithic prism assemhly 79 carrying a thin film Fabry-Perot nall~wl/alld filter 80 on irsternal surface 81. ~ tinn~l anti-re-fl~ction coatings 82 are used .at the various surfaces of ~lem~ntc of the optical device. It will be recognized that high reflector end mirror ~0 could be ,~ PA by an optical couplin,~ coatin~, such that li~ht could be emitted from the laser device to a fiber optic pigtail, diode sensor of a power feeflhack loop. etc.
Various col-,polle.l~ of Fig. 7 shown spaced apart can adv~nt~ously be butt coupled to seduce the overall size of t~se device and its optical length.
A dense channel wavclc,.~lh division multipl~:Ying device is illllctra~eci in Fig. 8, , ' .,~lg an FL 'saser as <3iccl()se(l above at each of eight separate ports or ch~Ann~lc on an optical block 100. Thss m~ iplPYin~ device has the ability to mllltirl~lc individual, s~al~te wavelength si~nals in~o a cornmon fsber optic carrier line and/or to rlernl~ Py such signals.
Wo 97/30495 PCT/US97/02190 Typical specific~-iollc for an optical m~ iplPY-ng device in acccldd~-ce with the preferred embodimen~ illustrated in Fig. 8 inciude those provided in Table 2.
Number of Ch~nn~lc 8 Channel ~avelen~th 1544-1560 Channel spacing 2 nm + 0.2 nm ~;.. ;,,~.. , rcnl~tinn 20 dB to 35 dB
Insertion loss ~total) less than 6 dB
1~ Fiber type single mode, 1 meter pigtail O~alillg tc.llpefalulc range -2()~C to ~50~C
The optical mnl~ device of Fig. 8 meeting the s~ ionc of Table 2, in addition to optical block 10Q which, preferably, is a stable glass ~ .u~e, is seen to include means for receiving coll;....~t~ light, such as a fiber optic gradient index lens collim~tnr 112 or the liIce, receives highly coilim~tf~d light 114 from optical port 118 of the optical block at a slight angle through a hole or facet in surface 116 of the optical block. In ~c~
with one preferred c~ f l~, the optical block has a ll ,;rl~ ~ ....CC "a" of S mm, a length "b"
of 14.1 mm or more, and a refractive index of about 1;5. The Col~im~ light ~ .~ly has a di~ , of not more than about 0.15~ and the tilt angle "c" at which the collim~t~
light exits the optical block is about 15 ~ . Multi-wavelength light bounces within the optical block b_L~ .. the high reflector coating 134 and o~iLe surface 120. A channel (or m--ltir~ c~ are added (or l~,.llu~,d) at each (or every other, etc.) bounce by a ~L~, filter which l,a,~",i~s a next wavelength inw~.,.l..~. Alt~ ative to such series of filters, a graded wavelength, ~--,r~.ably all~ le~,L-ic, narrowband b~ filter 122 is carried on surface 120 of the optical block. Such filter can be made in acco,.lance with the t~ ;..g~ of co-pending, commonly owned U.S. patent a~ ion Serial No. 08/490,829, filed June 15, 1995 and entitled "Optical Mnl~ ;pl,~, ;.,g Device," the ~iicclosllre of which is incorporated herein by reference. Specifically, filter 122 in such embo~ llp~ is a -cnntimlo~lC, variable thil-L ..~, mulli-cavity inlerference filter, and. mosI preferably, is a c~ntin~ c linearly variable filter. Such filter 122 is ll~l~y~re~ll at port 124 to a sub-range of the w~ e.~Il.s in~hl(lFrl in the cr~llim~t-F~d light 114. Speçific~lly~ light 126 passes through port 124 of the optical block from a collim~tir ~ lens means 128 ~Cc~x;AI,3d with a first signal channel. The optical signal passed by pon 124 is generated by an external cavity semicon(l~ tt r diode laser 129 in accoldaIlce with any of the p-tf~ d ~F ~b~;",~"l.;
above,meeting ~ ;-~'pF-~I spectral perf~ n~P ChA~ ina~,cu,~l~,cewith Table 2, for a first channel of the mn' . ' ~ device.
T~he c~..~;....~...~ filter l~ at port 124 is reflective of wavrlF .~lhc which are not ~in-band" of the filter at that l~catir\n Light 132is reflected between filter 122 on surface 120 of the optical block and high reflector film or coating 134 on surface 116. High reflector .
film 134 does not cover optical port 118, so as to avoid interfering with the passage of Light 114. Tlhus, light 132 is reflected by refIector film 134 to strike surface 120 of the optical block at port 124, where it is reflected to pass through port 118. At the location of port 136 next A~ to port 124,-the c~ o~, variable ~ L"~C" multi~avity intc--r~ cc filter 122 is ~lal~yal~ to a dirr~ L wavel~ ll or sub-range of wavelengths than it is at port 124. E;or dense channel wavelcl.gLII division mllltiplF xinp ~ppli~tinnc, the wavelF n~;ll, separation ~c;L~ ,C~l each of the m~lltirl~ ports linearly spaced along surface 120 of the opticai block is ~I~,r~,.al~ly about 2 nm or less. Thus, at port 136 an optical sigr~al eu~.~.. ~ll li.~g to a second channel is llAI ~ d through the filter 122 from a collim~tin~
lens 138, g. ..~ ~i.t. ~1 by external cavity s~ u-~ or diode laser 139 in accu-dance with the ~fe.l~ emb~limF ntc above. As at the first port 124, the i"~.r.,.~nce filter 122 at port 136 reflects light which is not in-band at that location. Thus, the portion 142 of the light 11~ which first entered the optical block prior to this point (i.e.. Iight having wav~ IF "g,~
of othersofthe-~h~nnF~Ic,oriein~ingatlaserdiodedevices 149, 159, 169, 179, 189Or 199) ~=
CA 0224~31 1998-07-30 wo 97/3049s PCT/USg7/02190 is rPflPctPd on toward port 118 from port 136. In similar fashion. the reflected wave~Pn~thc at earlier pointc in the optical block cascade in a zigzag or "multiple-bounce" path in the optieal block with the optical signal for each individual channel being added at sn~cPc~;
bounces at surface 120 of the optical block.
SIt is a tPchnologically and cu"".... e-~ially ~ignifi~nt advantage of p-efe.l~d f ...~lx~ i of the deviees llicclrlced here that mnltir~ ch~nnPlc can be so tightly spaeed in a narrow wav~ l~n~ range, with reliable and preeise g~n~r~tion of desired wavrl~
sub-ranges by reliable and co,~ ,c.~;ially feasible eYt~rn~l cavity diode laser deviees. With Fl lasers sueh DWDM mnhir~l-oYin~ dev*es ean now be mass produeed with suitable wa~ hcontrolandcf~ ylt~ -spectral~lrol~ re~ such that fiber-optie DWDM mllhirleY~(l systems are rendered eo--.~ ;ially feasible.
It will be ~ from the above ~ n~ of the invention and detailed r~
of cenain plt;fe.l. d ~ bo~ n~...lc that various arlditir n~ and mo~lifir ~tionC can be made to the embodiments ~ osed without depar~ing from ~he uue scope and spirit of the invention.
All such mOIl;r.. ~l;"l.~ and ~.l;l;.. ,c are int~ntl~Pcl to be covered by the following claims.
Claims (37)
1. An external cavity laser comprising, in combination, a semiconductor optical amplifier optically coupled to an external resonant cavity, and a monolithic prism assembly in the external resonant cavity comprising a transparent substrate having a thin film Fabry-Perot narrowband interference filter at an acute angle to a transverse plane of the external resonant cavity.
2. The external cavity laser in accordance with Claim 1 wherein the acute angle of the thin film Fabry-Perot interference filter of the monolithic prism assembly to a transverse plane of the external resonant cavity is greater than zero degrees and less than 45°.
3. The external cavity laser in accordance with claim 1 wherein the thin film Fabry-Perot interference filter has a bandwidth of less than 0.3 nm.
4. The external cavity laser in accordance with claim 1 wherein the thin film Fabry-Perot interference filter comprises thin film reflector layer sandwiching between them at least one thin film cavity layer.
5. The external cavity laser in accordance with claim 4 wherein each cavity layer is formed of one to four dielectric films of alternating high and low refractive index, each having an optical thickness equal to an integral number of half wavelengths, and the reflector layers each is formed of two to twelve dielectric films of alternating high and low refractive index, each having an optical thickness equal to an odd number of quarter wavelengths.
6. The external cavity laser in accordance with claim 1 wherein the thin film Fabry-Perot interference filter is a multi-cavity narrowband filter.
7. The external cavity laser in accordance with claim 6 wherein the Fabry-Perot interference filter has a bandwidth of less than 0.3 nm.
8. The external cavity laser in accordance with claim 7 wherein the external resonant cavity has an overall length of less than 5 nm.
9. The external cavity laser in accordance with claim 1 wherein the semiconductor optical amplifier comprises a direct bandgap semiconductor optical emitter.
10. An external cavity laser comprising, in combination, a semiconductor optical amplifier optically coupled to an external resonant cavity, and a monolithic prism assembly in the external resonant cavity comprising a transparent substrate having a transverse first surface and a Fabry-Perot interference filter on a second surface at an acute angle to the first surface, the Fabry-Perot interference filter comprising multiple thin film reflector layers sandwiching between them at least one thin film cavity layer.
11. The external cavity laser in accordance with claim 1 wherein the acute angle is from 1° to 5°.
12. An external cavity laser comprising, in combination, a semiconductor optical amplifier optically coupled to an external resonant cavity, and a monolithic prism assembly in the external resonant cavity comprising a transparent substrate carrying a Fabry-Perot interference filter at an acute angle to a transverse plane of the external resonant cavity, the Fabry-Perot interference filter comprising at least one cavity layer formed of half wavelength optical thickness dielectric films sandwiched between reflector layers each formed of quarter wavelength optical thickness dielectric films.
13. The external cavity laser in accordance with claim 12 wherein the Fabry-Perot interference filter is a three-cavity filter.
14. A monolithic prism assembly comprising a transparent optical substrate carrying a reflective coating on a first surface of the optical substrate in a first plane and a thin film Fabry-Perot narrowband interference filter on a second surface of the optical substrate in a second plane spaced from and at an acute angle to the first plane.
15. The monolithic prism assembly of claim 14 wherein the Fabry-Perot interference filter is a multi-cavity narrowband filter.
16. The monolithic prism assembly of claim 15 wherein the Fabry-Perot interference filter has a bandwidth of less than 0.2 nm.
17. The monolithic prism assembly of claim 14 wherein the reflective coating is a high-reflective coating on an external surface of the substrate, and the Fabry-Perot interference filter is on an internal surface of the substrate.
18. The monolithic prism assembly of claim 14 further comprising a second reflective coating on a third surface of the optical substrate in a third plane which is parallel to the first plane and on an opposite side of the second plane.
19. An optical device comprising, in combination:
an external cavity laser comprising a semiconductor diode laser having an anti-reflection coating on a first emitter facet optically coupled to an external resonant cavity, a monolithic prism assembly in the external resonant cavity comprising a transparent optical substrate carrying a thin film Fabry-Perot narrowband interference filter at an acute angle to a transverse plane of the external resonant cavity, and collimating means for focusing light between the emitter facet and the Fabry-Perot interference filter.
an external cavity laser comprising a semiconductor diode laser having an anti-reflection coating on a first emitter facet optically coupled to an external resonant cavity, a monolithic prism assembly in the external resonant cavity comprising a transparent optical substrate carrying a thin film Fabry-Perot narrowband interference filter at an acute angle to a transverse plane of the external resonant cavity, and collimating means for focusing light between the emitter facet and the Fabry-Perot interference filter.
20. The optical device in accordance with claim 19 wherein the diode laser is an edge emitting diode laser.
21. The optical device in accordance with claim 20 further comprising an output coupler reflective coating on a second facet of the diode laser.
22. The optical device in accordance with claim 21 wherein the monolithic prism assembly further comprises a reflective coating on a transverse first surface of the transparent optical substrate at one end of the external resonant cavity, and the thin film Fabry-Perot narrowband interference filter is on a second surface of the optical substrate at an acute angle to the first surface.
23. The optical device in accordance with claim 22 wherein the thin film Fabry-Perot narrowband interference filter is a multi-cavity filter.
24. The optical device in accordance with claim 23 wherein the Fabry-Perot interference filter has a bandwidth of less than 0.3 nm.
25. The optical device in accordance with claim 24 wherein the external resonant cavity has an overall length of less than 5 mm.
26. The optical device in accordance with claim 22 wherein the reflective coating is a high- reflective coating on an external surface of the substrate, and the Fabry-Perot interference filter is on an internal surface of the substrate.
27. An optical device comprising, in combination, an external cavity semiconductor diode laser, an anti-reflection coating on a first emitter facet of the diode laser optically coupled to an external resonant cavity, an output coupler reflective coating on a second emitter facet of the diode laser, a monolithic prism assembly in the external resonant cavity comprising a transparent optical substrate carrying a cavity-reflective coating at one end of the external resonant cavity and a thin film Fabry-Perot interference filter within the external resonant cavity between the reflective coating and the first emitter facet at an acute angle to the reflective coating, the thin film Fabry-Perot interference filter comprising multiple thin film reflectors sandwiching between them at least one thin film cavity layer.
28. The optical device in accordance with claim 27 wherein the cavity reflective costing is a high-reflective coating.
29. The optical device in accordance with claim 27 wherein the cavity-reflective coating is partially transparent and optically couples the diode laser to an output element.
30. The optical device in accordance with claim 29 wherein the output element is a diode sensor of a power feedback loop.
31. The optical device in accordance with claim 27 wherein the output coupler reflective coating is optically coupled through a second collimating means to a fiber-optic pigtail.
32. An optical device comprising, in combination:
an external cavity semiconductor diode laser comprising first and second edge emitting lasers at opposite ends of a common external resonant cavity, the first edge emitting laser having a first reflective coating on a first facet defining a first end of the external resonant cavity, and the second edge emitting laser having an output coupler reflective coating on a second facet defining a second end of the external resonant cavity and optically coupling the diode laser to an output element;
a monolithic prism assembly in the external resonant cavity between the first and second edge emitting lasers, comprising a transparent optical substrate carrying a thin film Fabry-Perot interference filter having at lest one thin film cavity sandwiched between thin film reflectors; and collimating means focusing light between the first and second edge emitting lasers.
an external cavity semiconductor diode laser comprising first and second edge emitting lasers at opposite ends of a common external resonant cavity, the first edge emitting laser having a first reflective coating on a first facet defining a first end of the external resonant cavity, and the second edge emitting laser having an output coupler reflective coating on a second facet defining a second end of the external resonant cavity and optically coupling the diode laser to an output element;
a monolithic prism assembly in the external resonant cavity between the first and second edge emitting lasers, comprising a transparent optical substrate carrying a thin film Fabry-Perot interference filter having at lest one thin film cavity sandwiched between thin film reflectors; and collimating means focusing light between the first and second edge emitting lasers.
33. The optical device in accordance with claim 32 wherein the monolithic prism assembly has first and second anti-reflection coatings on parallel external surfaces, the thin film Fabry-Perot interference filter being on an internal surface of the monolithic prism assembly between the first and second anti-reflection coatings.
34. The optical device in accordance with claim 32 wherein the output element is a second collimating means optically coupled to a fiber-optic pigtail.
35. The optical device in accordance with claim 32 wherein the first reflective coating is a high-reflective coating.
36. The optical device in accordance with claim 32 wherein the first reflective coating is partially transparent and optically couples the diode laser to a second output element.
37. A dense wavelength division multiplexing fiber-optic communication system comprising, in combination, a semiconductor diode laser having a monolithic prism assembly in an external resonant cavity of overall length less than 5 mm, the monolithic prism assembly comprising a transparent optical substrate carrying on an internal surface a multi-cavity thin film Fabry-Perot interference filter having a bandwidth of less than 0.3 nm, angled to reflect out-of-band wavelengths away from the diode laser and comprising multiple thin film reflectors sandwiching between them at least one thin film cavity.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US60084896A | 1996-02-13 | 1996-02-13 | |
| US08/600,848 | 1996-02-13 | ||
| PCT/US1997/002190 WO1997030495A1 (en) | 1996-02-13 | 1997-02-12 | External cavity semiconductor laser with monolithic prism assembly |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2245531A1 true CA2245531A1 (en) | 1997-08-21 |
Family
ID=29422905
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2245531 Abandoned CA2245531A1 (en) | 1996-02-13 | 1997-02-12 | External cavity semiconductor laser with monolithic prism assembly |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2245531A1 (en) |
-
1997
- 1997-02-12 CA CA 2245531 patent/CA2245531A1/en not_active Abandoned
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