Ene gy es o an e icien andom lase emission
collec ing sys em
Ignacio Ipa agui e,a,*Jon Azka go a ,aJoaquin Fe nández,b
Sa a Ga cía-Re illa ,aand Rolindes Baldaa,b,c
aUni e sidad del País Vasco UPV/EHU, Dp o. de Física Aplicada I,
Escuela de Ingenie ía de Bilbao, Bilbao, Spain
bDonos ia In e na ional Physics Cen e DIPC, San Sebas ián, Spain
cMa e ials Physics Cen e CSIC-UPV/EHU, San Sebas ián, Spain
Abs ac . The p oblem o ligh collec ion in andom lase s (RLs) is add essed. As he adia ion
emi ed by his sys em is Lambe ian due o i s spa ial incohe ence, a de ice based on an ellip-
soidal e olu ion mi o is designed, de eloped, and es ed in o de o op imize he ha es ing o
he adia ion emi ed by he RL. The sys em p o ides a simple injec ion p ocedu e o he emi ed
ene gy a he en ance o a mul imode op ical ibe . The esul s ob ained show ha he de ice has
a ne ene gy e iciency o 35%, close o he heo e ically expec ed one. ©The Au ho s. Published by
SPIE unde a C ea i e Commons A ibu ion 4.0 Unpo ed License. Dis ibu ion o ep oduc ion o his
wo k in whole o in pa equi es ull a ibu ion o he o iginal publica ion, including i s DOI. [DOI: 10
.1117/1.OE.60.1.010502]
Keywo ds: op ics; ligh ; lase s; sca e ing.
Pape 20201173L ecei ed Oc . 6, 2020; accep ed o publica ion Jan. 15, 2021; published
online Jan. 29, 2021.
1 In oduc ion
Random lase s (RLs) a e a ac i e adia ion sou ces o se e al applica ions, such as op ical
senso s, high-de ini ion speckle- ee imaging, li hog aphy, holog aphic lase displays, e c.1–3
Many ma e ials, such as colloidal dye solu ions, solid-s a e dyes, polyme s, c ys al powde s,
e c., ha e been es ed as RL sou ces,4–9showing e y a ied ene gy e iciencies, pulse wid hs,
and emission spec a anges.
In pa icula , RLs based on Nd3þions ha e been es ed as sou ces o speckle- ee imaging in
nea -in a ed and nanosecond pulse egime.10 The ene gy e iciency o all hese sys ems is e i-
den ly gi en by he physical p ope ies o he ac i e medium i sel and he pumping condi ions,
bu he abili y o cap u e he ene gy emi ed by he RL, which is a e y impo an issue o imple-
men applica ions, has no been ho oughly discussed. The main ouble comes om he
omnidi ec ionali y o he emission. I is wo h no ing ha he adia ion emi ed by an RL is
Lambe ian due o he e y low ans e se cohe ence o he emission.1,2
The aim o his wo k is o design, de elop, and es a de ice ha cap u es as much adia ion as
possible emi ed in all di ec ions by he RL. Fo his pu pose, an ellipsoidal e olu ion mi o is
employed. The ex e nal su ace o he powde ed ma e ial (lase ac i e medium) is placed and
cen e ed on a ocus o he ellipsoidal mi o , whe eas on he o he one, he op ical head o
a mul imode op ical ibe is loca ed o collec he emi ed lase adia ion once he pumping adi-
a ion is elimina ed by il e ing. The de ice has been es ed and i s e iciency measu ed, u ning
ou o be e y close o ou heo e ical p edic ion.
2 Design and Manu ac u ing o he Collec ing De ice
The scheme o he de ice is shown in Fig. 1. The pumping beam is add essed o he mi o hole
by means o an o ien able mi o (a), and i is concen a ed on he ac i e medium by a mobile
lens (b) o achie e he desi ed pumping size on i s su ace. The ac i e medium consis s o
*Add ess all co espondence o Ignacio Ipa agui e, i.ip[email p o ec ed]
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a qua z cell 6 mm in diame e and abou 2 mm deep. The cell is illed wi h powde o he limi o
he cell walls so ha he emi ed adia ion has no obs acle o each he e lec i e su ace o he
ellipsoid. I is a ached o he ellipsoidal mi o (c) in a ixed posi ion by a hin od. To acili a e
ocusing ope a ions, he ellipsoidal mi o has been designed so ha he ocus F is in he unca -
ing plane o he ellipsoid, as can be seen in Fig. 1. The ou e su ace o he powde is placed and
cen e ed on F. A ine adjus men o he o ien a ion o he mi o (a) is equi ed o achie e he
pump beam jus on F, which is impo an o minimize geome ic abe a ions. Thus, wi hin his
simple scheme, i he pumping beam alls on F, all he adia ion emi ed is di ec ed a e e lec-
ion o he o he ocus o he ellipsoid F′whe e a il e (d) o emo e he pumping adia ion is
placed. The head o a mul imode op ical ibe (e) wi h XYZ adjus men cap u es he adia ion
and ca ies i o an absolu e ene gy me e ( ) Ophi PE9. E en ually, a cha ge-coupled de ice
(CCD) came a (g) (Spi icon SP503U om Ophi ) is placed on F′ o analyze he beam quali y.
The dis ance be ween bo h oci o he ellipsoid is 40 mm and he pe iapsidal dis ance is 20 mm,
so he diame e o he mi o is 60 mm. The diame e o he in e nal hole h ough which he
pumping adia ion passes is 10 mm. I should be bo ne in mind ha his conca e mi o , as a
pa axial elemen , gi es an image a F′(−60 mm) o an objec placed a F (−20 mm). The e o e,
i s ocal dis ance is −15 mm, and he pa axial la e al magni ica ion o he mi o in ou wo king
condi ions is −3.
The speci ic pa ame e s we men ioned abo e led us o each a comp omise be ween he
maximum image size in F′and he nume ical ape u e (NA) o he op ical ibe . The minimum
diame e (a 1/e) o he emission ci cle ha we can ge is close o 300 μm,11,12 which becomes
abou 1 mm a he ibe head F′. Taking in o accoun ha he ela ionship be ween he inciden
ene gy wi hin he ibe a ea and he o al inciden ene gy is gi en by 1−exp½−ð ∕ oÞ2, whe e
is he co e adius o he ibe and ois he beam adius a 1/e. Thus, o a ibe o 1.5 mm in
diame e and a beam o 1 mm, coupling losses o app oxima ely 10% a e ob ained. On he o he
hand, he NA o he ibe is 0.5, which means an admi ance hal angle o app oxima ely 30 deg,
so he maximum angle o emission o he RL wi h espec o he semi-majo axis o he ellipse is
app oxima ely 77 deg. The geome ic losses due o he pumping hole o o he ou e mos a ea o
he mi o a e gi en by he ac o cos2θ1−cos2θ2, wi h angles o 14 deg (pumping hole) and
75 deg (ou e mos a ea), which gi es a loss close o 15%. As an inc ease o he la e al augmen-
a ion would include mo e adia ion wi hin he NA o he ibe , whe eas i s dec ease would
p oduce he opposi e e ec , a comp omise abou conc e e dimensions and dis ances was eached
in o de o ha e minimal coupling losses. Ano he way would be o make he mi o diame e
smalle , bu in addi ion o some new losses, i gene a es o he mechanical incon eniences. On he
Fig. 1 Pumping and lase adia ion collec ion sys em scheme: (a) o ien able olde mi o ,
(b) mobile lens, (c) ellipsoidal mi o and ac i e ma e ial jus a ocus F, (d) il e o pumping
emo al, (e) op ical ibe wi h i s head (wi h XYZ adjus men ) jus a ocus F′, ( ) absolu e ene gy
me e , and (g) CCD came a.
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o he hand, i should be no ed ha he pumping a eas we mus use (abou 0.1 mm2) limi he
pumping ene gy o no much mo e han 2 mJ, depending on he ma e ial, o a oid damage.11
As such a mi o is no comme cially a ailable, we designed i and con ac ed i s mecha-
niza ion. The chosen ma e ial was b ass due o i s high e lec i i y and a o able p ope ies o
machining and polishing. Once machined, we polished i by using 0.06-μmsize powde (Eposil
M11 om ATM) un il he equi ed op ical quali y was achie ed. The op ical es o he uncoa ed
mi o , pe o med by measu ing he e lec ion o a He-Ne lase beam, con i ms a sui able op ical
quali y. Likewise, we obse ed he image gi en by a ligh -emi ing diode (LED) sou ce a i s oci
posi ions o e i y ha he design is ul illed and o quali a i ely de ec geome ic abe a ions, as
well as he pa axial ocal poin lying 5 mm away om F. As a cu iosi y, Fig. 2shows he images
ob ained wi h an LED sou ce se a F ( ed) and wi h he LED se a F′(blue).
To es ima e he ne yield, all he losses should be aken in o accoun . Fi s , hose on he
e lec i e uncoa ed b ass su ace o he ellipsoid. We ha e measu ed i s abso p ion and di usion
losses a ound 1064 nm, which gi e a alue o abou 15% (yield η¼0.85). Second, he pu ely
geome ic losses due o he inle po o he pumping and he limi o he ibe NA (η¼0.85).
Thi d, losses due o he cell clamping sys em, which pa ially obs uc s he adia ion e lec ed by
he mi o (η¼0.90). Fou h, beam- ibe geome ical coupling yield (η¼0.90). Finally, losses
due o pump emo al il e and ibe inpu and ou pu (η¼0.80). F om hese pa ial yields, a ne
yield o abou 45% is ob ained.
3 Tes o he Collec ing Sys em
To es he de ice, we ha e i s ob ained images o he ocused beam o a ious powde ed
ma e ials loca ing he CCD came a jus a F′. Ou aim was o obse e he quali y and size o
he ocused beam and he possible e ec s due o he geome ic abe a ions o he mi o . Figu e 3
shows wo images ob ained wi h wo di e en ma e ials. In Fig. 3(a), we p esen he image on F′
o he adia ion emi ed by a nea ly s oichiome ic powde ed ma e ial, Nd0.9Y0.1Al3ðBO3Þ4,
pumped a 810 nm and wi h emission a 1063 nm. In hese powde s, he di usi e abso p ion
leng h is e y sho (abou 10 μm),11–13 which explains he well-concen a ed image ob ained a
F′, wi h a diame e a 1/e sligh ly smalle han 1 mm. Figu e 3(b) shows he image on F′o he
adia ion emi ed by a ce amized oxisul ide, La2O2S∶Nd 9%, pumped a 819 nm and wi h emis-
sion a 1076 nm whose di usi e abso p ion leng h is e y long compa ed o he i s case. As a
consequence, we ind a much wide ocused beam and a much mo e widesp ead adia ion. I is
wo h no ing ha his di e ence is only due o he di usi e p ope ies o he ac i e medium,
because he pump beam size is he same in bo h cases.
The absolu e ou pu ene gy o bo h powde ed c ys als ha e been di ec ly measu ed wi h
he ene gy me e by using ou collec ing de ice. The inciden pump ene gy is measu ed
Fig. 2 Images o a isible LED om ellipsoidal mi o . Red, LED in F, image in F′; la e al aug-
men a ion −3. Blue, LED in F′, image in F; la e al augmen a ion −1∕3.
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Fig. 4 Ou pu ene gy ( ed poin s) and lase slope line o s imula ed emission o he
Nd0.9Y0.1Al3ðBO3Þ4powde sample measu ed as unc ion o he pump ene gy wi h he collec ing
sys em used a e exci a ion a 810-nm wa eleng h.
Fig. 3 CCD images o he op imal ocused beam. (a) Nd0.9Y0.1Al3ðBO3Þ4, pumped a 810-nm
wa eleng h, emission a 1063 nm. (b) Ce amics oxisul ide La2O2S∶Nd 9%, pumped a 819-nm
wa eleng h, emission a 1076 nm.
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simul aneously using he di usion om a olde mi o wi h a ixed pho odiode (Newpo 818-
BB-21), p e iously calib a ed by loca ing he ene gy me e abo e he pumping en ance on he
ellipsoidal mi o .11–13 To ge he de ice pe o mance, he ob ained lase slopes ha e been com-
pa ed wi h hei p e iously ob ained12–14 o al absolu e slopes, and hey a e abou 20%. In he
case o he ce amized oxisul ide, he absolu e slope ob ained is less han 1%, showing a lousy
beam- ibe coupling. Fo he nea s oichiome ic bo a e, he esul is shown in Fig. 4. An absolu e
slope o 7% has been ob ained. I is wo h no ing ha i he e iciency o he collec ing sys em
was 1, he slope would be 0.20. The e o e, he ene ge ic yield o he de ice in ou wo king
condi ions is 35%, which is close o ou es ima ion om Sec. 2.
4 Summa y and Conclusions
In his wo k, a de ice o collec as much ene gy as possible emi ed om an RL has been
designed, de eloped, and es ed. The de ice is based on an ellipsoidal e olu ion mi o whose
geome y is mos sui able o cap u e he emi ed adia ion due o i s Lambe ian p o ile.
Images o he ocused beam ha e been ob ained o di e en ac i e ma e ials in o de o
obse e he ocus quali y and he e ec o geome ic abe a ions. These measu emen s e eal
ha ac i e ma e ials wi h he sho es possible di usi e abso p ion leng h a e desi ed. The e o e,
highly abso ben and di usi e ma e ials, ha is, s oichiome ic (o almos ) powde s, a e he mos
sui able ones.
The ene gy collec ing e iciency o ou sys em is 35%, which is close o he p e ious heo-
e ical es ima ion. This pe o mance could be imp o ed by applying a p o ec ed gold coa ing on
he mi o su ace, which would allow o each a ne yield no a om 50%. An al e na i e
op ion could be o ha e a mo e concen a ed pump beam in o de o imp o e he coupling
be ween he ocused beam and ibe head. Un o una ely, he minimum diame e (a 1/e) o he
emission ci cle ha we can ge is close o 300 μm, which becomes abou 1 mm a he ibe
head (F′).
The de ice is alid o use wi h o he RLs, such as dyes RLs, and e en o collec any
Lambe ian o simila emission ype om a no oo ex ensi e sou ce.
Acknowledgmen s
This wo k was suppo ed by he Basque Go e nmen PIBA2018-24, Spanish Go e nmen
MINECO unde P ojec No. MAT2017-87035-C2-2-P (AEI/FEDER, UE), and Basque
Coun y Uni e si y (UPV/EHU) PPG17/07 and GIU17/014. We wish o hank J. E. Ruiz,
esea che o he Lase Ma e ial P ocessing G oup o he UPV/EHU, and J. M. La añaga, labo-
a o y specialis echnician (Mechanical Enginee ing Depa men ), o hei help in he design
and polish o ellipsoidal mi o s, and R. Hueso and T. del Río o Plane a y Science G oup o
he UPV/EHU o he pho og aphs o he mi o images. The au ho s decla e no con lic s o
in e es .
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