APPLICATION NOTE. 1No. Test and Characterization of Laser Diodes: Determination of Principal Parameters

Transcription

1 APPLICATION NOTE No. Tst and Caractrization of Lasr Diods: Dtrmination of Principal Paramtrs

2 Tst and Caractrization of Lasr Diods: Dtrmination of Principal Paramtrs By Kamran S. Mobaran, P.D. Introduction: It is oftn ncssary to quantitativly assss t quality, prformanc, and caractristics of lasr diods. Tis is don troug prforming a sris of xprimnts and obtaining crtain significant paramtrs from wic w can dtrmin ow wll t lasr diod is prforming. It is tn possibl to stablis wtr or not t lasr diod mts t dsird spcifications. T following is a brif dscription of t common paramtrs tat can b xprimntally dtrmind and t tcniqus involvd in t analysis of t raw data tat lad to maningful and asy-to-intrprt rsults. Output Ligt vs. Input Currnt Curv and Trsold Currnt: Praps t most important caractristic of a lasr diod to b masurd is t amount of ligt it mits as currnt is injctd into t dvic. Tis gnrats t Output Ligt vs. Input Currnt curv, mor commonly rfrrd to as t L.I. curv (sown in Figur ). As t injctd currnt is incrasd, t lasr first dmonstrats spontanous mission wic incrass vry gradually until it bgins to mit stimulatd radiation, wic is t Output Ligt pr Mirror Fact (W) L = mm W = µm Ligt 3 I t Injction Currnt ( A ) Figur. A typical Ligt vs. Currnt (L.I.) curv associatd wit a igpowr lasr diod. I t rprsnts t trsold currnt at wic t dvic bgins to las. T fficincy of t lasr in convrting lctrical powr to ligt powr is dtrmind by t slop of t L.I. curv, dnotd by t cang in output powr ovr t cang in currnt ( P/ I). T inst scmatically sows a broad ara ( µm wid strip) lasr diod mitting radiation from bot its front and back mirror facts. I P onst of lasr action. T first paramtr of intrst is t xact currnt valu at wic tis pnomnon taks plac. Tis is typically rfrrd to as t trsold currnt and is dnotd by t symbol I t. It is gnrally dsirabl tat t trsold currnt b as low as possibl, rsulting in a mor fficint dvic. Tus, trsold currnt is on masur usd to quantify t prformanc of a lasr diod. Trsold Currnt Dnsity: Trsold currnt dpnds upon t quality of t smiconductor matrial from wic t dvic is fabricatd and t gnral dsign of t wavguid structur. Howvr, trsold currnt also dpnds upon t siz and ara of t lasr dvic. On lasr diod could dmonstrat a muc igr trsold currnt tan anotr dvic and yt b considrd a muc bttr lasr. Tis is bcaus t ara of t dvic can b larg. A lasr tat is widr or longr obviously rquirs mor lctric currnt to rac t onst of lasr action tan a lasr of smallr ara. As a rsult, wn comparing t trsold currnt valus of diffrnt dvics, it is mor appropriat to rfr to trsold currnt dnsity ratr tan trsold currnt. Trsold currnt dnsity is dnotd by t symbol J t and is dtrmind by dividing t xprimntally obtaind trsold currnt valu I t by t ara of t lasr. It is always dsirabl for a lasr to av a low trsold currnt dnsity valu. Trsold currnt dnsity is on of t paramtrs tat is a dirct indication of t quality of t smiconductor matrial from wic t dvic is fabricatd. In comparing t prformanc of various lasr dvics on must compar t trsold currnt dnsity valus ratr tan t trsold currnt valus. In calculating t currnt dnsity of t lasr it is ncssary to accuratly masur t ara of t lasr troug wic currnt is bing injctd. Tis is only possibl in broad ara typ lasrs wit strip widts on t ordr of µm or mor. In suc cass t ara troug wic t currnt is flowing is vry muc t sam as t ara of t mtallic contact of t lasr. In cass of ridg lasrs (Figur ) t widt of t ridg is only a fw microns, wil, du to currnt sprading, t actual widt of t cannl troug wic currnt is flowing could b considrably mor. Tis maks it impractical to accuratly dtrmin currnt dnsity valus in cass of narrow strip ridg lasrs.

3 Gold Pad for Wir Bonding Omic Contact Mtallization Ridg Widt 3 µm µm Elctric Currnt Injction L = mm (Cavity Lngt) Figur. Scmatic rprsntation of an indx guidd ridg structur lasr diod and its divrging output bam profil. Slop of t L.I. Curv: Gold Wir Bonds Ligt Just as it is dsirabl to rac lasr action at as low a trsold currnt as possibl, it is also dsirabl to gt mor and mor ligt out of t dvic wit t xpnditur of as littl currnt as possibl. In otr words, you want to b abl to incras t input currnt slowly and yt av rapid incras in t output ligt mission. A lasr diod, wic as a good convrsion rat of input lctric powr to output ligt powr, is obviously a dvic tat prforms wll. A dirct masur of t ability of t dvic to do tis is t slop of t L.I. curv abov t trsold currnt point. Tis slop is dnotd as P/ I and as t units of Watts pr Amprs (W/A), or in t cas of low powr lasrs (mw/ma). P/ I, t slop of t L.I. curv abov t trsold currnt I t, tlls us dirctly ow many Watts of powr t lasr outputs for vry Amp incras in its input currnt. Otr important paramtrs ar typically xtractd from masurmnt of tis P/ I slop fficincy. Extrnal Diffrntial Quantum Efficincy: Rsulting dirctly from t xprimntal masurmnt of t slop of t L.I. curv is anotr paramtr rfrrd to as Extrnal Diffrntial Quantum Efficincy, η d. Tis is a figur of mrit, masurd in prcntag, wic indicats t fficincy of a lasr dvic in convrting t injctd lctron ol pairs (input lctric cargs) to potons mittd from t dvic (output ligt). A prfct ypottical dvic tat convrts prcnt of t injctd currnt into output ligt wit no wast in t form of at gnration would tortically av an η d valu qual to %. Of cours suc a dvic dos not xist in rality. W can dtrmin t Extrnal Diffrntial Quantum Efficincy valu of a ral lasr diod by masuring its slop of t L.I. curv, P/ I, abov trsold currnt. W tn nd to compar tat to t slop of t L.I. curv of t prfct, % fficint, tortical dvic. Tis is don as xplaind blow. θ θ On singl lctron as an lctric carg of q, wr q =.6x 9 Coulombs. On Coulomb, dnotd by C, is an lctric carg qual to t lctricity transfrrd by a stady currnt of on Amp in on scond. Tis mans tat on C/sc is on Amp. Now on t otr and, on poton, of wavlngt λ, as an nrgy of E = (c/λ), wr is Planck s constant, wic rlats t nrgy and t wavlngt of t poton. Rmmbr tat t unit of nrgy is Joul, on Joul pr scond is quivalnt to on Watt of powr. In an idal prfct lasr, t rcombination of ac lctron-ol pair rsults in t gnration of on poton, and additionally t poton survivs its travl troug t lasr wavguid structur and is mittd from t dvic to contribut to t output ligt powr. In a ral lasr, som lctron-ol pair rcombinations rsult in t gnration of potons, wil otrs rsult in t gnration of otr, undsirabl, forms of nrgy, suc as at. In addition, not all t potons gnratd insid t lasr ar mittd from t dvic. Som of tm ar rabsorbd by t structur of t lasr (Figur 3). As a rsult, in t cas of an idal prfct lasr an lctric carg of q Coulombs rsults in (c/λ) Jouls of output optical nrgy. Tis mans tat a currnt of q Coulombs pr scond (Amps) rsults in a ligt powr of (c/λ) Jouls pr scond (Watts). Tus, t slop of t L.I. curv of an idal, prfct, lasr mitting at a wavlngt of λ would tortically b (c)/(λq), wr as mntiond, is Planck s constant, λ is t wavlngt of gnratd ligt, and q is t lctric carg of a singl lctron. Not tat wavlngt λ of ligt is rlatd to its frquncy υ troug t rlationsip υ = c/λ, wr c is t spd of ligt. Output Ligt potons Mirror Fact p-sid n-sid Injction Currnt Figur 3. Scmatic rprsntation of t combination procss of lctron-ol pairs witin a lasr diod structur and t gnration of potons. Now, to compar t fficincy of a ral lasr, bing tstd in t laboratory, to t idal prfct lasr, w can compar t slops of tir L.I. curvs. Tis mans, tat t fficincy of a ral lasr, in convrting input currnt to output ligt, is t ratio of its associatd P/ I paramtr to c/qλ. Sinc bot trms P/ I and c/qλ av t ols Activ Rgion lctrons Mirror Fact Output Ligt

4 sam units of Watt/Amp, t rsultant trm: ( P/ I) / (c/qλ) is a unit-lss paramtr givn in a prcntag form. Tis is wat is rfrrd to as t Extrnal Diffrntial Quantum Efficincy, η d, and is quivalnt to ( P/ I) / (c/qλ). To calculat η d, simply masur t slop of t L.I. curv in units of Watt/Amp and multiply it by qλ/c. Not tat in doing tis, w took into account t fact tat potons of diffrnt wavlngt av diffrnt nrgis. For xampl on blu poton (t poton of blu color radiation, wit wavlngt in t 4 nm rang of t spctrum) as mor nrgy tan a rd poton (t poton of rd color radiation, wit wavlngt in t 6 nm rang of t spctrum). In addition, not tat, as sown in Figurs and 3, a lasr diod could itr mit ligt from bot its front and back mirror facts, or in cass wr t back fact is coatd wit ig rflctivity coating, only mits from its front fact. Tis mans tat wn spaking about t slop of t L.I. curv or t Extrnal Diffrntial Quantum Efficincy w must always mak it clar wtr P/ I and η d ar pr on mirror fact or pr two mirror facts. Cavity Lngt Dpndnc of t Trsold Currnt Dnsity and t Extrnal Diffrntial Quantum Efficincy: Bot t trsold currnt dnsity J t and t Extrnal Diffrntial Quantum Efficincy η d valus ar paramtrs tat ar dpndnt on t cavity lngt of t lasr diod. As a rsult of tis, in comparing t caractristics of various lasr diods or batcs of dvics, it is igly dsirabl to xtract, from t xprimntally obtaind data, paramtrs wic ar indpndnt of t gomtry and dimnsions of t dvic structur, and ar dirct indicators of t quality of t smiconductor crystal matrial from wic t lasr dvic is fabricatd. In ordr to do tis, it is ncssary to xprimntally masur t trsold currnt and t slop of t L.I. curv of not just on particular lasr, but ratr dvics of various cavity lngts. T data obtaind can tn b tabulatd, and plottd in ordr to dtrmin som important paramtrs of intrst associatd wit lasr diods. Typically, broad ara lasr diods, wit clarly dfind aras, sould b usd in prforming ts masurmnts. As is t cas wit various products in t smiconductor industry, t quality of t matrial from wic any dvic is constructd from is t most significant factor in dtrmining t rliability, robustnss, and ovr-all lif xpctancy of t dvic. In cass of ig-powr smiconductor lasrs, wic oprat at ig optical powr dnsitis and tmpratur lvls, rapid dgradations will occur if t smiconductor matrial is of low quality. You can also s t significanc of tis issu in rgard to tlcommunication lasrs usd in undr watr transocanic fibr lins. Som of ts basic proprtis ar dscribd blow. T data sown in t tabl in Figur 4 can b usd to xtract ts paramtrs. Cavity Lngt P L η d I η d µm η d = Slop Efficincy ( mirrors).4 W/A P I qλ c Extrnal Diffrntial Quantum Efficincy 8 % Figur 4. Tabl wit typical Slop ( P/ I) and Extrnal Diffrntial Quantum Efficincy (DQE) valus for lasrs wit various cavity lngts. T data tabulatd r ar plottd in Figur 5. P/ I as bn multiplid by bcaus tis particular st of data is rlatd to a lasr diod wit as clavd mirror facts and mitting qual amounts of ligt from bot its front and back mirror facts (suc as sown in t inst of Figur ). But only t ligt mittd from its front mirror fact is dtctd and masurd. For accurat rsults, on sould tak masurmnts of or mor lasrs of xactly t sam cavity lngt and input t avrag valu of ts masurmnts in t tabl. Intrnal Quantum Efficincy: Tis paramtr is a masur of t fficincy of a lasr in convrting lctron-ol pairs (injctd currnt) into potons (ligt) witin t lasr diod structur. For xampl, if t intrnal quantum fficincy is dtrmind to b 75 prcnt, tn 75 prcnt of lctron-ol pairs ar convrtd into potons and t rmaining 5 prcnt into otr forms of nrgy suc as at. T symbol η i is usd to rprsnt tis fundamntal proprty. Unlik t xtrnal diffrntial quantum fficincy, t intrnal quantum fficincy is indpndnt of t gomtrical proprtis of t lasr dvic, suc as t cavity lngt or t strip widt. As a rsult, it is t propr paramtr for comparison of t matrial quality of various lasrs mad from various smiconductor wafrs. In ordr to do tis, it is ncssary to xprimntally masur t xtrnal diffrntial quantum fficincy of lasrs of various cavity lngts, wit t rsults tabulatd as sown in Figur 4. T intrnal quantum fficincy is tn dtrmind by plotting t curv of invrs xtrnal diffrntial quantum fficincy vrsus t cavity lngt as sown in Figur 5. T invrs of t intrcpt point of t linar fit lin of t st of data points wit t vrtical axis is t intrnal quantum fficincy paramtr, indicatd in prcntag form. Intrnal quantum fficincy is on of t main figurs of mrit tat sould b usd in assssing t quality of t smiconductor wafr from wic t lasr diod is manufacturd from. Intrnal quantum fficincy is rlatd to xtrnal quantum fficincy troug t rlationsip sown in t inst of Figur = 6.66x -34 J.sc q =.6x -9 C c =.99x 8 m/sc λ =.8x -6 m (in tis cas) 3

5 Invrs xtrnal Diffrntial Quantum Efficincy (/ηd) 3 η i η d = + η i Cavity Lngt, L (µm) η i = 85 % α i = 7.7 cm - α i ln ( /R ) Figur 5. A typical cavity lngt dpndnc of t invrs xtrnal diffrntial quantum fficincy. Intrnal quantum fficincy, η i is dtrmind from t vrtical axis intrcpt point (/η i ) of t linar fit lin. Intrnal loss, α i is qual to t slop of t lin multiplid by t (η i )ln(/r) paramtr, wr L is t lasr cavity lngt in units of cm and R is t rflctivity of t mirror facts of t lasr. For a typical GaAs/AlGaAs basd lasr R=.3. Not tat tr is a diffrnc btwn intrnal quantum fficincy, η i and xtrnal diffrntial quantum fficincy, η d. T intrnal quantum fficincy is a dirct indication of t fficincy of a lasr in convrting lctron-ol pairs (injctd currnt) into potons (ligt) witin t lasr diod structur. But rmmbr tat not all of t potons tat ar gnratd find tir way out of t dvic; som of tm ar rabsorbd du to various intrnal loss mcanisms. As a rsult, t xtrnal diffrntial quantum fficincy is an indication of t fficincy of a lasr in convrting lctron-ol pairs (injctd currnt) into potons mittd from t lasr dvic (output ligt). T valu of t xtrnal diffrntial quantum fficincy is always smallr tan t intrnal quantum fficincy. (η d ) / (η i ) is t ratio of numbr of potons mittd from t lasr to numbr of potons gnratd witin t lasr. Intrnal Loss: As mntiond abov, t intrnal quantum fficincy paramtr is a masur of t fraction of lctron-ol rcombinations tat rsult in optical potons insid t lasr cavity (tus t trm intrnal). Howvr, partially du to intrnal losss (α i ) of t lasr wavguid, not all t potons gnratd insid t lasr cavity find tir way to t outsid to contribut to t output ligt mittd from t mirror fact(s) of t lasr diod. Tus t Extrnal Diffrntial Quantum Efficincy valu of any lasr diod is always smallr tan its intrnal quantum fficincy. T ligt tat propagats troug t lasr diod cavity suffrs from losss as is t cas of ligt propagation in any optical wavguid. T intrnal loss is t paramtr wic corrsponds to t loss of t optical wav. Its valu is dtrmind xprimntally troug masuring t slop of t linar fit lin to t invrs of xtrnal diffrntial quantum fficincy vrsus cavity lngt data points as sown in Figur 5. L Transparncy Trsold Currnt Dnsity: Anotr significant paramtr tat could b xtractd from t data, xprimntally masurd and tabulatd, in Figur 4, is t transparncy trsold currnt dnsity dnotd by t symbol J o. Trsold currnt dnsity dpnds upon t cavity lngt of t lasr diod. As a rsult it would not b accurat to compar t quality of on st of smiconductor wafrs wit t otr using just t trsold currnt dnsity paramtr. Using t data tabulatd in Figur 6 it is possibl to xtract a paramtr tat is indpndnt of t dvic gomtry. Tis is don troug plotting t curv of trsold currnt dnsity vrsus t invrs cavity lngt as sown in Figur 7. T intrcpt of t linar fit lin of t data plottd in tis curv wit t vrtical axis provids us wit t transparncy trsold currnt dnsity valu. Tis is t appropriat paramtr tat sould b usd in comparing Cavity Lngt L I t Ara J t L µm Trsold Currnt.48 A Dvic Ara (W= µm). cm Trsold Currnt Dnsity 4 A/cm Invrs Cavity Lngt 5. cm Figur 6. Tabl wit typical I t and J t valus for lasrs wit various cavity lngts ranging from µm to 4 µm. For accurat rsults, on sould tak masurmnts of about or mor lasrs of xactly t sam cavity lngt and input t avrag valu of ts masurmnts in t tabl. T data tabulatd r ar plottd in Figur 7. Not tat it is gnrally bttr tat t rsults of t two tabls sown in Figurs 4 and 6 ar compild togtr in on tabl. Trsold Currnt Dnsity, J t (A/cm ) J o Invrs Cavity Lngt, /L ( cm - ) Figur 7. Typical rprsntation of trsold currnt dnsity, J t vrsus t invrs cavity lngt, /L for lasrs of diffrnt cavity lngts. T intrcpt of t linar fit lin of t data points wit t vrtical axis dtrmins t valu of t transparncy currnt dnsity J o. 4

6 t quality of various smiconductor wafrs from wic diffrnt lasrs diods ar fabricatd. J o could b tougt of as t trsold currnt dnsity of a tortical lasr wic as an infinitly long optical cavity wit no loss of optical wav at its mirror facts. Caractristic Tmpratur: In most applications t ability of t lasr diod to prform wll at lvatd tmpraturs is of grat intrst. Tis is spcially of concrn in t cas of ig-powr lasr diods wr t amount of at gnratd causs t dvic tmpratur to ris significantly. As a rsult it is of utmost importanc for t smiconductor crystal to b robust noug so as not to dgrad du to dvic opration at ig tmpraturs. T caractristic tmpratur of t lasr diod, wic is commonly rfrrd to as T o (pronouncd T-zro) is a masur of t tmpratur snsitivity of t dvic. Higr valus of T o imply tat t trsold currnt dnsity and t xtrnal diffrntial quantum fficincy of t dvic incras lss rapidly wit incrasing tmpraturs. Tis translats into t lasr bing mor trmally stabl. In ordr to masur t caractristic tmpratur of a lasr diod it is ncssary to xprimntally masur t L.I. curv of a lasr at various tmpraturs. T rsults ar tn tabulatd and t T o dtrmind. Typically popl prform ts masurmnts at tmpraturs ranging from 5 dgrs Clsius up to about 8 dgrs Clsius, and at 5 or dgr incrmnts. (Not tat oprating a lasr tat is not rmtically sald, at tmpraturs significantly coolr tan t room tmpratur, will rsult in watr condnsation on t dvic. Tis will caus damag to t lasr diod du to lctrical sorts.) Convntional AlGaAs lasrs usually av T o valus abov dgrs. From ts xprimntally masurd L.I. curvs t caractristic tmpratur of t dvic is dtrmind by plotting t J t data points (or t I t points) vrsus t tmpratur on a logaritmic scal and tn masuring t slop of t linar fit lin as sown in Figurs 8, 9,, and. Output Ligt pr Mirror Fact (W) Tmpratur Figur 8. Ligt vrsus currnt caractristic curvs for a lasr diod oprating at various tmpraturs. Ligt.5..5 Injction Currnt ( A ) o C Tmp ( o C) I t (A) J t (A/cm ) ln(j t ) Cavity Lngt, L = mm Strip Widt, W = µm Figur 9. Tabulatd trsold currnt I t, and trsold currnt dnsity J t valus dtrmind for t L.I. curvs at diffrnt tmpraturs. T last column lists t natural logaritm (ln) valus of J t. J t = J o xp[t/t o ] J t ln = J o ln(j t ) - ln(j o ) = T T o Figur. Equations outlining t procss of finding t caractristic tmpratur. T st quation indicats tat J t incrass xponntially wit T. T o and J o (not to b confusd wit transparncy currnt dnsity paramtr) ar t two constants. Taking t natural logaritm of bot sids and rarranging t trms rsult in t nd and 3 rd quations. Taking t drivativ of bot sids of t 3 rd quation (wit t drivativ of ln(j o ) bing zro) and rarranging t trms rsult in t 5 t quation from wic T o can b dtrmind. Not tat is an indication of cang. Natural Logaritm of Trsold Currnt Dnsity In (Jt) T T o ln(j t ) = T o T T T o = ln(j t ) T T o = ln(j t ) = Tmpratur ( o C ) Figur. Grap sowing t variations of t trsold currnt dnsity J t wit incrasing tmpratur. T invrs of t slop of t linar fit to tis st of data points is t caractristic tmpratur T o valu. Instad of plotting t J t points on can altrnativly us I t, data points, t outcom will b t sam. 5

7 Spctrum and Pak Wavlngt of Emission: T optical spctrum of lasr diods dpnds on t particular caractristics of t lasr s optical cavity. Most convntional gain or indx guidd dvics av a spctrum wit multipl paks (Figur ). powr. As t oprating currnt is incrasd on mod bgins to dominat until, byond a crtain oprating powr lvl, a singl narrow linwidt spctrum appars. Multi longitudinal mod (gain guidd dvic) Singl longitudinal mod (indx guidd dvic) λ p Rlativ Intnsity (a.u.) λ Ligt L = mm Rlativ Intnsity (a.u.) P = 5 mw P = 3 mw P = mw Rlativ Intnsity (a.u.) P = 5 mw P = 3 mw P = mw P =.5 mw P =.5 mw Wavlngt (nm) Figur. Typical spctrum of a mm cavity lngt lasr diod oprating just abov trsold. T pak wavlngt of mission (λ p ) is at 8 nm. T sparation btwn adjacnt paks ( λ) is about.9 nm. T numbr of spctral lins wic a lasr is capabl of supporting is a function of t cavity structur, as wll as t oprating currnt. T rsult is tat multimod lasr diods xibit spctral outputs aving many paks around tir cntr wavlngt. T optical wav propagating troug t lasr cavity forms a standing wav btwn t two mirror facts of t lasr. T priod of oscillation of tis curv is dtrmind by t distanc L btwn t two mirrors. Tis standing optical wav rsonats only wn t cavity lngt L is an intgr numbr m of alf wavlngts xisting btwn t two mirrors. In otr words, tr must xist a nod at ac nd of t cavity. T only way tis can tak plac is for L to b xactly a wol numbr multipl of alf wavlngts λ/. Tis mans tat L = m(λ/), wr λ is t wavlngt of ligt in t smiconductor mattr and is rlatd to t wavlngt of ligt in fr spac troug t indx of rfraction n by t rlationsip λ = λ o /n. As a rsult of tis situation tr can xist many longitudinal mods in t cavity of t lasr diod ac rsonating at its distinct wavlngt of λ m = L/m. From tis you can not tat two adjacnt longitudinal lasr mods ar sparatd by a wavlngt of λ = (λ o ) /nl. For a typical GaAs/AlGaAs basd lasr n=3.5. Unlik convntional lasr diods tat display multi-mod spctra, singl frquncy lasr diods suc as distributd fdback (DFB) and distributd bragg rflctor (DBR) typ of dvics display a singl wll dfind spctral pak. Figur 3 sows a comparison btwn ts two spctral baviors at various output powr lvls. Evn singl mod dvics can support multipl mods at low output Wavlngt λ (nm) Wavlngt λ (nm) Figur 3. Effct of incrasing oprating currnt (and powr) lvl on t output spctra of multimod gain guidd dvic and singl mod indx guidd dvic. Cntr Wavlngt Cangs wit Tmpratur: T cntr wavlngt of a lasr diod is dirctly proportional to its oprating tmpratur. Tr is a linar rlationsip btwn tmpratur and cntr wavlngt as sown in Figur 4. As tmpratur incrass, so dos t cntr wavlngt of t lasr diod. Tis caractristic is usful in spctroscopic applications, lasr diod pumping of solid stat lasrs and rbium-dopd fibr amplifirs, wr t wavlngt of mission of t lasr diod can b accuratly tmpratur tund to t spcific proprtis of t matrial wit wic t lasr diod is intracting. Pak Wavlngt, λp (nm) Cas Tmpratur ( C) Figur 4. Effcts of tmpratur on cntr wavlngt. Po = 3 mw 6

8 Mod Hopping: Singl mod lasrs xibit a pnomnon calld mod opping (Figur 5), in wic t cntr frquncy of t lasr diod ops ovr discrt wavlngt bands and dos not sow continuous tuning ovr a broad rang. On can cang t wavlngt wr t discontinuitis tak plac by making small adjustmnts to t driv currnt. Wn slcting a spcific lasr diod for an application wic rquirs a spcific wavlngt, suc as spctroscopy, mod opping must b takn into account wn tmpratur tuning t dvic. Pak Wavlngt, λp (nm) Po = 7 mw omic contacts dpositd on t two sids of t dvic. As a rsult, masurmnt of t sris rsistanc valu can b a mans of assssing t quality of t mtallic contacts dpositd on t lasr. Voltag (V) V vs. I curv (dv/di) vs. I curv 3 Injction Currnt ( A ) Figur 6. Typical lasr diod Voltag vs. Currnt curv and t corrsponding Sris Rsistanc vs. Currnt curv Rsistanc, dv/dl (Ω) Cas Tmpratur ( C) Figur 5. Mod opping obsrvd wil tmpratur tuning a singl mod lasr diod. Dynamic Sris Rsistanc: T sris rsistanc of t lasr diod is typically dtrmind troug calculating t drivativ of t voltag vrsus injction currnt caractristic curv of t dvic. On way of doing tis is to us a computr program to dtrmin t first drivativ of t voltag vrsus currnt caractristic curv of t dvic tat is obtaind xprimntally (Figur 6). Hig sris rsistanc valus for a lasr diod could b t rsult of low quality mtal Analysis Rsults Tabl: In summary, t caractrization of lasr diods typically involvs dtrmination of at last t following principal paramtrs: Trsold Currnt, Trsold Currnt Dnsity, Transparncy Trsold Currnt Dnsity, Extrnal Diffrntial Quantum Efficincy, Intrnal Quantum Efficincy, Intrnal Loss, Spctrum and t Pak Wavlngt of Emission, Caractristic Tmpratur, and t Sris Rsistanc. It is good practic to collct and prsnt all ts rsults, associatd wit ac group of lasrs tstd, in an organizd and asy to undrstand tabular form. On possibl xampl of suc form is sown on nxt pag. It rprsnts all t significant paramtrs of intrst in t tsting and caractrization of lasr diods in on singl pag and tus making it asy for intrprtation and comparison purposs. Plas rfr to otr Nwport Application Nots for discussions rgarding t Exprimntal Laboratory Stups and Tcniqus for masurmnt of t paramtrs mntiond r. 7

9 Srial Numbr Lasr Structur Manufacturing Dat Caractrization Dat Lasr Diod Analysis Rsults Form Otr Commnts & Info. PARAMETER SYMBOL UNIT VALUES Currnt Puls Widt Currnt Puls Frq. Currnt Duty Cycl nsc Hz % Lasr Strip Widt W µm Cavity Lngts L µm Trsold Currnt Trsold Currnt Dnsity Slop of L.I. Curv Extrnal Diffrntial Quantum Efficincy Pak Wavlngt I t J t P/ I η d λ p A A/cm W/A % nm Caractristic Tmpratur Sris Rsistanc Intrnal Quantum Efficincy Intrnal Loss Transparncy Currnt Dnsity T o R s η i α i J o K Ω % cm - A/cm

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