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Photoluminescence and lasing in whispering gallery mode glass microspherical resonators
Journal of Luminescence 170 (2016) 755–760
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Photoluminescence and lasing in whispering gallery mode glass microspherical resonators D. Ristić a,b, S. Berneschi c, M. Camerini c, D. Farnesi c,d, S. Pelli c,d, C. Trono c, A. Chiappini e, A. Chiasera e, M. Ferrari e, A. Lukowiak f, Y. Dumeige g, P. Féron g, G.C. Righini c,d, S. Soria c,n, G. Nunzi Conti c,d Ruđer Bošković Institute, Division of Materials Physics, Laboratory for Molecular Physics, Bijenička c. 54, Zagreb, Croatia Center of Excellence for Advanced Materials and Sensing Devices, Research unit New Functional Materials, Bijenička c. 54, Zagreb, Croatia c IFAC-CNR Istituto di Fisica Applicata, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy d Centro Studi e Ricerche "E. Fermi", Piazza del Viminale 2, 00184 Roma, Italy e CSMFO Group, Istituto di Fotonica e Nanotecnologie, IFN-CNR, Via alla Cascata 56/C, 38050 Povo-Trento, Italy f Institute of Low Temperature and Structure Research, PAS, ul. Okolna 2, Wroclaw 50-950, Poland g Laboratoire d'Optronique, (CNRS-UMR 6082-Foton), ENSSAT, 6 rue de Kérampont, 22300 Lannion, France a
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art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2015 Received in revised form 29 September 2015 Accepted 20 October 2015 Available online 28 October 2015
We report experimental results regarding the development of Er3 þ -doped glass microspherical cavities for the fabrication of compact sources at 1.55 μm. We investigate several different approaches in order to fabricate the microspheres including direct melting of Er3 þ -doped glass powders, synthesis of Er3 þ doped monolithic microspheres by drawing Er3 þ -doped glass, and coating of silica microspheres with an Er3 þ -doped sol–gel layer. Details of the different fabrication processes are presented together with the photoluminescence characterization in free space configuration of the microspheres and of the glass precursor. We have analyzed the photoluminescence spectra of the whispering gallery modes of the microspheres excited using evanescent coupling and we demonstrate tunable laser action in a wide range of wavelengths around 1.55 μm. As much as 90 μW of laser output power was measured in Er3 þ doped glass microspheres. & 2015 Elsevier B.V. All rights reserved.
Keywords: Whispering gallery modes Microspherical lasers Erbium doped glass Sol–gel Silica–hafnia
1. Introduction Optical resonators play a ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are totally internally reflected. Due to minimal reflection losses and potentially very low material absorption, these guided modes, known as whispering gallery modes, 10 4 can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonantwavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited for the investigation of fundamental processes in quantum or non-linear optics [1–4] as well as for applications in photonics [5,6] but also as very sensitive sensors [7,8]. WGM microspherical resonators play an important role in the pursuit of compact and efficient laser sources because of their intrinsic potential for low lasing threshold and narrow spectral n
Corresponding author. E-mail address: [email protected] (S. Soria).
http://dx.doi.org/10.1016/j.jlumin.2015.10.050 0022-2313/& 2015 Elsevier B.V. All rights reserved.
characteristics [9–12]. Since the pioneering works of Garret et al. [13] on Sm2 þ :CaF2 spheres and works on Morphology-Dependent Resonances (MDRs) and laser effects in droplets during the 80's [4] rare earth-doped glass microspherical lasers became subject of numerous studies, significant progress was achieved in the past decade as described in recent reviews [14,15]. Up to now, WGM microspherical lasers have been obtained by melting rare-earth doped optical fiber tips using fusion techniques [11,16–18], by microwave plasma torch fusion of grounded powers [19,9] or by electric tube furnace [20] by sol–gel [21–23] or glass [24] coating of silica microspheres and by rare-earth ion implantation [25]. The literature about WGM microresonators (WGMR) laser is vast and deep; there are many papers based on WGMR with different geometrical shapes (microspheres, microdisks, toroids, etc.), made of various glasses (silica, telluride, phosphate, ZBLAN, etc.) and various dopants (Er, Er:Yb, Nd, Tm, Er:Yb:Tm, etc.), which cannot be all cited here but have been described in several reviews [14,26] and specially in [15]. In this paper we will present the results obtained in our laboratories studying up to three different types of Er3 þ -doped glass microspherical cavities for the implementation of compact
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and low threshold laser sources at 1.55 μm. For light coupling into the WGMs we used tapered silica fibers. We first report experimental results on the characterization of microspherical cavities obtained by melting powders of Er3 þ doped glasses. After a brief description of the fabrication process and characterization setup, we investigate the effects that our glass fusion process has on the spectroscopic properties of different Er3 þ -doped commercial oxide glasses, i.e. two phosphate glasses and one silicate glass from Schott. This study is important especially considering the idea that in principle, as a very small amount of glass is necessary to fabricate a microspherical laser, by studying its properties, it will be easier to optimize the glass composition before making larger quantities for systems such as fiber or integrated lasers and amplifiers. Our experiments have been focused on the 4I13/2-4I15/2 transition of Er3 þ ions. We also measured the shift of the laser line by UV irradiation of microspheres fabricated in photorefractive glass. A similar characterization was performed on monolithic microspheres obtained by drawing one type of commercial Er3 þ doped phosphate glass (IOG1). We then present a simple method based on the sol–gel technology that has been developed to coat passive microspheres with an Er3 þ -doped layer. This technique is quite flexible as the sol–gel process permits a precise control of the dopant concentration and of the thickness and index of the active medium. This technique offers the possibility to prepare tailored materials with an attractive improvement in their physical and chemical properties [27,28], such as selecting the radial order n of the ‘active’ WGM [29]. The microspheres were prepared by fusion of a standard telecom fiber and they were then dipped in a silica–hafnia sol activated with Er3 þ ions. We first report on the luminescence properties of this latter coating that was selected because of the high quantum efficiency previously measured in Er3 þ -doped silica–hafnia waveguides [30]. Then WGMs spectra were analyzed for different sphere diameters, coating thicknesses and Er3 þ concentrations. In all types of glasses we demonstrate whispering gallery mode laser action at various wavelengths around 1550 nm by using a 1480 nm pump laser in order to avoid the strong thermal effects typical of 980 nm pumping mechanism [31].
2. Microspheres fabrication
Er2O3 and 3 wt% of Yb2O3 and a potassium–barium–alumino phosphate glass (Schott IOG2) which contains 2 wt% of Er2O3 and 3 wt% of Yb2O3; and one silicate glass (Schott IOG10), which contains 1 wt% of Er2O3 and 8 wt% of Yb2O3. In a first approach pieces of each bulk glass were ground first, and microspheres were then fabricated using a plasma torch. The plasma is generated using a microwave supply with a nominal oscillator frequency of 2.4 GHz and a maximum power of 2 kW. Argon is used as plasma gas and oxygen or nitrogen as sheath gas. The glass powders are axially injected and melt when passing through the plasma flame while the surface tension forces give them their spherical shape. Free quenched spheres are collected a few ten centimeters lower. No additional annealing was performed on them. The diameter of the spheres depends mainly on the powder size and may vary from 10 to 200 mm. Microspheres with diameters in the range between 50 μm and 100 μm were selected and then glued to the tip of a taper optical fiber with diameter below 20 μm for ease of handling (Fig. 1a). In the second approach, 5 cm long rods of 1.5 mm square cross section were cut from the commercial IOG1 wafers. The rods are then drawn in a home–made drawing device in a controlled environment; the obtained glass thread is then introduced for a certain length in a glass capillary in order to confer mechanical stability to the whole structure and an easy handling. The drawn glass threads have a nominal diameter of 10 μm and a length of 2 cm. The entire microsphere fabrication system was encased in a glass enclosure with an N2 saturated atmosphere. An oxygenbutane torch was carefully aligned and slowly approached to the tip of the thread till the glass started to melt, and the microsphere was formed because of surface tension, remaining attached to the fiber stem (that is why we would refer to them as monolithic microspheres). The exposition time lasted less than 2 s, and the whole process was monitored with a CCD camera. After retracting the torch, the microspheres were stored under vacuum conditions. We obtained microspheres with diameters in the range between 60 and 120 μm, depending on the glass thread diameter and the exposition time. Over-heating would collapse the structure while under-heating would not form a microsphere. It is worth noticing that by using this approach, useful post-processing steps like, for instance, thermal annealing or ion-exchange, could be implemented (as no polymer glue is used in this case [9]) Fig. 1b shows the optical image of a monolithic microsphere of 105 μm of diameter.
2.1. Microspheres from bulk glass 2.2. Sol–gel coated microspheres Microspheres were produced from different type of glasses: three commercial glasses: two phosphate glasses, i.e. a sodium– alumino–phosphate glass (Schott IOG1) which contains 1.5 wt% of
Silica microspheres were made by melting the end of a stripped standard telecommunication fiber (SMF 28) using both a fiber
Fig. 1. Optical image of: (a) a microsphere of about 80 μm fabricated by direct melting of glass powder and glued to the tip of a fiber; (b) a ‘monolithic’ microsphere fabricated directly from the tip of an Er3 þ doped phosphate glass thread.
D. Ristić et al. / Journal of Luminescence 170 (2016) 755–760
fusion splicer [32] and an oxygen gas torch. The cleaved tip of the fiber is partially fused and surface tension forces produce the spheroidal shape. The sphere diameter can be controlled in a range between a few and several hundreds microns. In order to activate the surface, a thin film was deposited using a sol–gel method. We used a silica–hafnia film with a ratio of 70:30 (70SiO2–30HfO2) doped with 0.15%, 0.3% and 1% mol of Er3 þ ions. Microsphere M1 has the largest concentration of Er3 þ ions. (1% mol), M2 the smallest (0.1% mol) and M3 has a concentration of about 0.3% mol. In a previous study we demonstrated high quantum efficiency in this type of film [30]. The starting solution, obtained by mixing tetraethylorthosilicate (TEOS), ethanol, deionized water and hydrochloric acid as a catalyst, was prehydrolyzed for 1 h at 65 °C. The molar ratio of H2O:TEOS:HCl was 200:100:1. An ethanolic colloidal suspension was prepared using as a precursor HfOCl2 [28], and then added to the TEOS solution with a Si/ Hf molar ratio of 70/30. The overall amount of Si and Hf in the sol was controlled so that c(Si(OC2 H5 )4 )þc(HfOCl2 )¼4.48 10– 4 mol/mL in order to have a dipping rate of about 30 nm/dip. Er3 þ was added as Er(NO3)3 5H2O with a molar concentration of 0.1%, 0.3% and 1%. The Er3 þ -doped silica–hafnia film was deposited on the silica microspheres by dip coating in multiple steps. The dipping speed was 40 nm/min and the dipping time 20 s [33]. Before adding a new layer, after each dip the film was annealed in air for 50 s at 900 °C. Final films were stabilized by a last treatment for 2 min in air at 900 °C. As a result of this procedure, crack free films were obtained, keeping a high quality factor [34] (Fig. 2).
3. Experimental setup 3.1. Photoluminescence spectroscopy Photoluminescence (PL) measurements in the region of the I13/2- 4I15/2 transition of the Er3 þ ions were performed using as excitation source the 980 (or 981) nm line of a Ti:sapphire laser or a stabilized fiber pigtailed semiconductor laser diode operating at 976 nm or the 514.5 nm line of an Ar þ ion laser. The luminescence was dispersed using a single-grating monochromator with a resolution of 1 nm. The light was detected using an InGaAs photodiode and standard lock-in technique. Lifetime decay curves were obtained recording the signal with a digital oscilloscope. For both the precursor bulk samples and the microspheres the standard bulk excitation in free space configuration (without coupling to the WGMs) was employed. All the measurements were performed at room temperature. 4
3.2. Laser characterization
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Fig. 2. Optical image of an 0.3% Er3 þ -doped sol–gel coated silica sphere of about 130 mm in diameter, and the taper fiber in the background.
Fig. 3. Sketch of the experimental set-up: DBR or TLD (distributed Bragg reflector or tunable diode laser), Iso (optical isolator), att (attenunator), pol (polarization control), WDM (wavelength division multiplexer), PM (photomultiplier), OSA (optical spectrum analyzer).
Tunic Reference ES). In what follows we refer to the pump power as the difference between the launched one (measured after the input WDM) and the one collected after the resonator (see Fig. 3). The system also included an optical isolator to prevent feedback into the laser diode, an attenuator, and a 1.48/1.55 mm multiplexer which is finally connected to the tapered fiber coupler. The co-propagating signal from the microsphere and the fluorescence were collected and sent to the OSA. In order to control the position of the microsphere relative to the taper, the samples were mounted on a nanotranslation stage with piezoelectric actuators with 20 nm resolution. By varying the contact position of the tapered fiber coupler relative to the sphere and increasing the pump power the lasing spectrum moved towards WGMs of shorter wavelength. This effect can be explained with reference to a similar shift that occurs in the Er3 þ gain spectrum when the inversion rate increases [35].
4. Results and discussion 3þ
The experimental setup for laser characterization of Er doped microspheres is sketched in Fig. 3. Among the different pumping wavelengths which can be used to excite Er3 þ ions in oxide glasses (810 nm, 980 nm and 1480 nm) we chose 1480 nm so that phase and mode matching condition between the fiber coupler and the microsphere can be better fulfilled at both the pump and the laser wavelengths [9]. Moreover, by pumping at 1480 nm instead of 980 nm, we strongly reduce the heating effects due to the multiphonon non radiative decay from the 4I11/2 to the 4 I13/2 level, which can lead the microsphere temperature to reach the melting point in some glasses [31]. We used a home-made biconical tapered fiber to couple the pump light into the WGMs of the microsphere and-at the same time-to couple the fluorescence or laser light out of it [32]. For the optical characterization and lasing measurements we used as a pump either a fiber stabilized laser (Corning Lasertron, 1.5 nm linewidth) or a narrow-line tunable laser (300 kHz linewidth Anritsu
4.1. Microspheres from bulk glass 4.1.1. Photoluminescence The photoluminescence spectra of the 4I13/2- 4I15/2 transition of the Er3 þ ion for the bulk samples and for the respective microspheres were obtained after pumping at 976 nm by using a semiconductor laser operating with 30 mW of output power. Typically we did not observe a significant broadening of the bandwidth while we observed a reduction in the lifetime (in the case of IOG2 glass, for instance this reduction was of about 20% from a value of 6.870.2 ms in the bulk [36]). The lifetime measurements give information about the site-to-site inhomogeneity. In fact, the faster relaxation of the 4I13/2-4I15/2 decay curves of Er3 þ ion in the microspheres, as compared to the bulk, in both the silica and the phosphate glasses, indicates that energy transfer mechanisms among active ions are effective due to the microsphere fabrication
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process. Table 1 summarizes the luminescence values for the three glasses considered in our study. 4.1.2. WGM lasers In Fig. 4a we collect WGMs laser spectra from an IOG2 glass microsphere with a diameter of 70 μm. When increasing the pump intensity above a minimum threshold of 2.5 mW, we obtained laser oscillation at 1569.4 nm. Fig. 4b shows the laser emission of a 50 μm IOG10 microsphere at 1543.89 nm. The minimum threshold for IOG10 is 400 μW. Higher concentration of Yb2O3 in this latter glass and consequent stronger pump absorption may be responsible of the lower pump threshold. Being IOG1 a photorefractive glass, laser emission could be trimmed by UV irradiation. We have irradiated the IOG1 microlaser with an excimer laser (KrF, 248 nm) and measured the shift of the laser line by increasing the cumulative UV dose. We have observed a blue-shift that saturates at about 13 pm for doses higher than 1 kJ/cm2. The result is shown in Fig. 5. This blue-shift indicates that the induced change in the index of refraction is negative and rather small ( 10 5), in agreement with the data published in literature [37]. The resonator Q values were obtained by measuring the resonance linewidth of the WGM modes at 1.6 μm, which is almost outside the absorption band of the 4I13/2 level (σabs ¼0.35 10 21 cm2). For a microsphere 75 μm in diameter a typical resonance in the undercoupling regime [38] is shown in the inset of Fig. 6. The corresponding Q value of 0.8 106 is in agreement with that calculated taking into account only the residual absorption at that wavelength. When increasing the pump intensity above a minimum threshold of less than 1 mW, lasing occurred both in single mode and multimode condition and reached a maximum taper-coupled output power of 90 mW at 1568.3 nm with a pump power of 6.1 mW, as shown in Fig. 6. This represent the highest value obtained so far in phosphate glasses when pumping at 1480 nm almost an order of magnitude larger than the highest power emitted (10 mW) by an Er-doped silica microsphere laser [39]. With the same microsphere, when changing
the coupling condition, we also obtained lasing action at lower wavelengths, down to 1534.7 nm (almost 34 nm shift), but with lower efficiency. For a monolithic microsphere of about 105 mm of diameter, we measured single mode laser emission at a maximum wavelength of 1569.3 nm (with output power of 4.8 mW), and at minimum λ of 1541.2 nm (5.3 mW), obtained with a pump power of 7 mW and 4.2 mW, respectively. The corresponding spectra are shown in Fig. 7. A reason for the lower efficiency of this type of microsphere is that a diameter in excess of 100 μm is too big for an ideal matching with the silica fiber taper. On the other hand, as the monolithic microspheres are obtained starting from a glass thread
Fig. 5. Shift of the emission laser line by UV irradiation (KrF, 248 nm) of photorefractive IOG1 microspheres.
Table 1 Bulk lifetime and microsphere lifetime, peak emission wavelengths, fluorescence effective bandwidth for the 4I13/2-4I15/2 transition, and peak absorption crosssections of Er3 þ ions, measured for different experimental silicate and phosphate glasses. Glass
τmeas ( 7 0.2 ms)
IOG10 10.2 IOG2 6.8 IOG1 10.7
τmeas ( 7 0.2 ms) sphere
λ (nm) Δλeff fl. ( 7 1 nm)
σε ( 7 0.01 cm2)
6.2 5.8 8.7
1535 1533 1534
5.8 10 21 8 10 21 7 10 21
32 50 30
Fig. 6. WGMs laser spectrum from a 75 μm microsphere in IOG1 glass with a power peak of 90 μW at 1568.3 nm obtained using a pump power of 6.1 mW at 1480 nm. The inset shows a resonance around 1600 nm of the same sphere corresponding to a Q factor of 0.8 106.
Fig. 4. (a) Laser effect at 1569.4 nm in a 70 μm IOG2 microsphere. Maximum output power of about 70 nW. (b) Lasing action of a 50 μm IOG10 microsphere at 1543.89 nm. Maximum output power of 240 nW.
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with a square section, a strong and detrimental eccentricity, with a large number of competing modes [14] is always present in microsphere with a diameter below 100 μm.
4.2. Microspheres from sol–gel coated microspheres 4.2.1. Photoluminescence Fig. 8a shows the photoluminescence spectra relative to the 4 I13/2 -4I15/2 transition of the Er3 þ ions obtained upon excitation at 514 nm for microsphere doped with 1 mol% (M1) and 0.1 mol% (M2). The PL spectra are quite similar, according to a constant siteto-site inhomogeneity, both exhibiting an emission peak at 1.53 mm with a shoulder at about 1.55 mm and a spectral full width at half maximum (FWHM) of about 48 nm. Upon excitation at 980 nm, we observed that the shape and the bandwidth of the emission spectrum do not change, indicating negligible site selection for erbium ions. Fig. 8b shows the decay curve of the 4I13/2 metastable state obtained upon 514 nm excitation. The curves can be fitted by a single exponential function and a lifetime of 4.4 ms and 6.2 ms is respectively obtained for the microsphere doped with 1 mol% (M1) and 0.1 mol% (M2). The same lifetimes were measured upon 980 nm excitation. The decrease of the lifetime with the increase of Er3 þ concentration suggests that energy transfer processes are effective.
1400
1450
1500
1550
4.2.2. WGM lasers If the coating is thick enough so that the number of erbium ions interacting with the electric field of the WGMs is high enough, the stimulated emission gain is greater that the cavity round trip loss and the sphere will start to lase, with a launched power of about 10 mW. Fig. 9 shows the laser spectrum of a microsphere coated by 300 nm of 70 SiO2–30HfO2 film activated with 1% mol of Er3 þ (M1). The maximum peak power is 325 nW at 1564 nm. For a 130 μm microsphere coated with about 700 nm of 0.3 mol% Er3 þ doped 70SiO2–30HfO2 (M3), the maximum wavelength at which lasing was possible was found to be 1560 nm and minimum 1530 nm as shown in Fig. 10.
Fig. 9. WGMs laser spectrum of the microsphere M1 corresponding to the maximum peak power 350 nW.
Fig. 10. WGMs laser spectra corresponding to the maximum (1560.4 nm, red line) and minimum (1530.6 nm, black line) wavelength peak values obtained from a 130 μm microsphere (M3) coated with about 700 nm of 0.3 mol% Er3 þ doped 70SiO2–30HfO2.
M2 FWHM=48 nm
Μ2 τ=6.2 ms
M1 FWHM=48 nm
Μ1 τ=4.4 ms
1600
Wavelength (nm)
1650
1700
Intensity (a. u.)
Intensity (a. u.)
Fig. 7. WGMs laser spectra corresponding to the maximum (1541.2 nm, black line, output power 5.3 mW) and the minimum (1569.3 nm, blue line, output power 4.8 mW) wavelength peak values obtained from a 105 μm monolithic microsphere in IOG1 glass. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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0.00
0.02
Time (s)
Fig. 8. (a) PL spectra around 1.55 μm of the M1 and M2 sol–gel coated microspheres under excitation at 514 nm; (b) luminescence decay curve from the 4I13/2 state of Er3 þ ions in the M1 and M2 sol–gel coated microspheres under excitation at 514 nm.
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5. Conclusions We presented recent results obtained in our laboratories studying Er3 þ -doped glass microspherical cavities for the fabrication of compact and low threshold laser sources at 1.55 μm. We developed different procedures in order to fabricate the microcavities. We first consider microspheres fabricated by melting powders of Er3 þ -doped silica and phosphate glasses. Photoluminescence and lifetime measurements performed both on the bulk precursors and on the microspheres have revealed a shortening of the lifetime in the microcavities as compared to the corresponding bulks. This effect was related to inhomogeneous change in the local environment of Er3 þ ions induced by the microsphere fabrication process. We have also developed a new fabrication process to produce monolithic microspheres, which would allow various types of post-processing treatments. Laser action was obtained in all microspheres and different wavelengths could be excited within a broad range, depending on pump coupling conditions. A record peak power of 90 μW was obtained by pumping with 6.1 mW at 1480 nm. We then presented a simple technique to obtain Er3 þ doped microcavities by coating silica microsphere with a thin film of Er3 þ -doped sol–gel. The aim of having a thin layer around the base silica sphere is the tailoring of the WGM resonator, in order to select the radial order mode of the ‘active’ WGMs. We observed a broad PL spectrum typical of Er3 þ doped silica–hafnia system and analyzed the WGMs laser spectrum of the cavity.
Acknowledgments The authors are grateful to F. Cosi and M. Brenci for helpful discussions. M. Camerini is now at Selex ES. D. Farnesi is a PhD student at the University of Parma. Centro Studi e Ricerche “E. Fermi” is gratefully acknowledged for funding. The research was partially performed in the framework of the CNR-PAS joint Project (2014–2016) and of the CNES R&T Project SHYRO (2011– 2014). Cost Action MP1401 is gratefully acknowledged S. Berneschi thanks the Italian Ministry of Education, University and Research (MIUR) for its support through PRIN 2010–2011 Project ARTEMIDE (Ref. 20108ZSRTR).
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Photoluminescence and lasing in whispering gallery mode glass microspherical resonators D. Ristić a,b, S. Berneschi c, M. Camerini c, D. Farnesi c,d, S. Pelli c,d, C. Trono c, A. Chiappini e, A. Chiasera e, M. Ferrari e, A. Lukowiak f, Y. Dumeige g, P. Féron g, G.C. Righini c,d, S. Soria c,n, G. Nunzi Conti c,d Ruđer Bošković Institute, Division of Materials Physics, Laboratory for Molecular Physics, Bijenička c. 54, Zagreb, Croatia Center of Excellence for Advanced Materials and Sensing Devices, Research unit New Functional Materials, Bijenička c. 54, Zagreb, Croatia c IFAC-CNR Istituto di Fisica Applicata, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy d Centro Studi e Ricerche "E. Fermi", Piazza del Viminale 2, 00184 Roma, Italy e CSMFO Group, Istituto di Fotonica e Nanotecnologie, IFN-CNR, Via alla Cascata 56/C, 38050 Povo-Trento, Italy f Institute of Low Temperature and Structure Research, PAS, ul. Okolna 2, Wroclaw 50-950, Poland g Laboratoire d'Optronique, (CNRS-UMR 6082-Foton), ENSSAT, 6 rue de Kérampont, 22300 Lannion, France a
b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2015 Received in revised form 29 September 2015 Accepted 20 October 2015 Available online 28 October 2015
We report experimental results regarding the development of Er3 þ -doped glass microspherical cavities for the fabrication of compact sources at 1.55 μm. We investigate several different approaches in order to fabricate the microspheres including direct melting of Er3 þ -doped glass powders, synthesis of Er3 þ doped monolithic microspheres by drawing Er3 þ -doped glass, and coating of silica microspheres with an Er3 þ -doped sol–gel layer. Details of the different fabrication processes are presented together with the photoluminescence characterization in free space configuration of the microspheres and of the glass precursor. We have analyzed the photoluminescence spectra of the whispering gallery modes of the microspheres excited using evanescent coupling and we demonstrate tunable laser action in a wide range of wavelengths around 1.55 μm. As much as 90 μW of laser output power was measured in Er3 þ doped glass microspheres. & 2015 Elsevier B.V. All rights reserved.
Keywords: Whispering gallery modes Microspherical lasers Erbium doped glass Sol–gel Silica–hafnia
1. Introduction Optical resonators play a ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are totally internally reflected. Due to minimal reflection losses and potentially very low material absorption, these guided modes, known as whispering gallery modes, 10 4 can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonantwavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited for the investigation of fundamental processes in quantum or non-linear optics [1–4] as well as for applications in photonics [5,6] but also as very sensitive sensors [7,8]. WGM microspherical resonators play an important role in the pursuit of compact and efficient laser sources because of their intrinsic potential for low lasing threshold and narrow spectral n
Corresponding author. E-mail address: [email protected] (S. Soria).
http://dx.doi.org/10.1016/j.jlumin.2015.10.050 0022-2313/& 2015 Elsevier B.V. All rights reserved.
characteristics [9–12]. Since the pioneering works of Garret et al. [13] on Sm2 þ :CaF2 spheres and works on Morphology-Dependent Resonances (MDRs) and laser effects in droplets during the 80's [4] rare earth-doped glass microspherical lasers became subject of numerous studies, significant progress was achieved in the past decade as described in recent reviews [14,15]. Up to now, WGM microspherical lasers have been obtained by melting rare-earth doped optical fiber tips using fusion techniques [11,16–18], by microwave plasma torch fusion of grounded powers [19,9] or by electric tube furnace [20] by sol–gel [21–23] or glass [24] coating of silica microspheres and by rare-earth ion implantation [25]. The literature about WGM microresonators (WGMR) laser is vast and deep; there are many papers based on WGMR with different geometrical shapes (microspheres, microdisks, toroids, etc.), made of various glasses (silica, telluride, phosphate, ZBLAN, etc.) and various dopants (Er, Er:Yb, Nd, Tm, Er:Yb:Tm, etc.), which cannot be all cited here but have been described in several reviews [14,26] and specially in [15]. In this paper we will present the results obtained in our laboratories studying up to three different types of Er3 þ -doped glass microspherical cavities for the implementation of compact
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and low threshold laser sources at 1.55 μm. For light coupling into the WGMs we used tapered silica fibers. We first report experimental results on the characterization of microspherical cavities obtained by melting powders of Er3 þ doped glasses. After a brief description of the fabrication process and characterization setup, we investigate the effects that our glass fusion process has on the spectroscopic properties of different Er3 þ -doped commercial oxide glasses, i.e. two phosphate glasses and one silicate glass from Schott. This study is important especially considering the idea that in principle, as a very small amount of glass is necessary to fabricate a microspherical laser, by studying its properties, it will be easier to optimize the glass composition before making larger quantities for systems such as fiber or integrated lasers and amplifiers. Our experiments have been focused on the 4I13/2-4I15/2 transition of Er3 þ ions. We also measured the shift of the laser line by UV irradiation of microspheres fabricated in photorefractive glass. A similar characterization was performed on monolithic microspheres obtained by drawing one type of commercial Er3 þ doped phosphate glass (IOG1). We then present a simple method based on the sol–gel technology that has been developed to coat passive microspheres with an Er3 þ -doped layer. This technique is quite flexible as the sol–gel process permits a precise control of the dopant concentration and of the thickness and index of the active medium. This technique offers the possibility to prepare tailored materials with an attractive improvement in their physical and chemical properties [27,28], such as selecting the radial order n of the ‘active’ WGM [29]. The microspheres were prepared by fusion of a standard telecom fiber and they were then dipped in a silica–hafnia sol activated with Er3 þ ions. We first report on the luminescence properties of this latter coating that was selected because of the high quantum efficiency previously measured in Er3 þ -doped silica–hafnia waveguides [30]. Then WGMs spectra were analyzed for different sphere diameters, coating thicknesses and Er3 þ concentrations. In all types of glasses we demonstrate whispering gallery mode laser action at various wavelengths around 1550 nm by using a 1480 nm pump laser in order to avoid the strong thermal effects typical of 980 nm pumping mechanism [31].
2. Microspheres fabrication
Er2O3 and 3 wt% of Yb2O3 and a potassium–barium–alumino phosphate glass (Schott IOG2) which contains 2 wt% of Er2O3 and 3 wt% of Yb2O3; and one silicate glass (Schott IOG10), which contains 1 wt% of Er2O3 and 8 wt% of Yb2O3. In a first approach pieces of each bulk glass were ground first, and microspheres were then fabricated using a plasma torch. The plasma is generated using a microwave supply with a nominal oscillator frequency of 2.4 GHz and a maximum power of 2 kW. Argon is used as plasma gas and oxygen or nitrogen as sheath gas. The glass powders are axially injected and melt when passing through the plasma flame while the surface tension forces give them their spherical shape. Free quenched spheres are collected a few ten centimeters lower. No additional annealing was performed on them. The diameter of the spheres depends mainly on the powder size and may vary from 10 to 200 mm. Microspheres with diameters in the range between 50 μm and 100 μm were selected and then glued to the tip of a taper optical fiber with diameter below 20 μm for ease of handling (Fig. 1a). In the second approach, 5 cm long rods of 1.5 mm square cross section were cut from the commercial IOG1 wafers. The rods are then drawn in a home–made drawing device in a controlled environment; the obtained glass thread is then introduced for a certain length in a glass capillary in order to confer mechanical stability to the whole structure and an easy handling. The drawn glass threads have a nominal diameter of 10 μm and a length of 2 cm. The entire microsphere fabrication system was encased in a glass enclosure with an N2 saturated atmosphere. An oxygenbutane torch was carefully aligned and slowly approached to the tip of the thread till the glass started to melt, and the microsphere was formed because of surface tension, remaining attached to the fiber stem (that is why we would refer to them as monolithic microspheres). The exposition time lasted less than 2 s, and the whole process was monitored with a CCD camera. After retracting the torch, the microspheres were stored under vacuum conditions. We obtained microspheres with diameters in the range between 60 and 120 μm, depending on the glass thread diameter and the exposition time. Over-heating would collapse the structure while under-heating would not form a microsphere. It is worth noticing that by using this approach, useful post-processing steps like, for instance, thermal annealing or ion-exchange, could be implemented (as no polymer glue is used in this case [9]) Fig. 1b shows the optical image of a monolithic microsphere of 105 μm of diameter.
2.1. Microspheres from bulk glass 2.2. Sol–gel coated microspheres Microspheres were produced from different type of glasses: three commercial glasses: two phosphate glasses, i.e. a sodium– alumino–phosphate glass (Schott IOG1) which contains 1.5 wt% of
Silica microspheres were made by melting the end of a stripped standard telecommunication fiber (SMF 28) using both a fiber
Fig. 1. Optical image of: (a) a microsphere of about 80 μm fabricated by direct melting of glass powder and glued to the tip of a fiber; (b) a ‘monolithic’ microsphere fabricated directly from the tip of an Er3 þ doped phosphate glass thread.
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fusion splicer [32] and an oxygen gas torch. The cleaved tip of the fiber is partially fused and surface tension forces produce the spheroidal shape. The sphere diameter can be controlled in a range between a few and several hundreds microns. In order to activate the surface, a thin film was deposited using a sol–gel method. We used a silica–hafnia film with a ratio of 70:30 (70SiO2–30HfO2) doped with 0.15%, 0.3% and 1% mol of Er3 þ ions. Microsphere M1 has the largest concentration of Er3 þ ions. (1% mol), M2 the smallest (0.1% mol) and M3 has a concentration of about 0.3% mol. In a previous study we demonstrated high quantum efficiency in this type of film [30]. The starting solution, obtained by mixing tetraethylorthosilicate (TEOS), ethanol, deionized water and hydrochloric acid as a catalyst, was prehydrolyzed for 1 h at 65 °C. The molar ratio of H2O:TEOS:HCl was 200:100:1. An ethanolic colloidal suspension was prepared using as a precursor HfOCl2 [28], and then added to the TEOS solution with a Si/ Hf molar ratio of 70/30. The overall amount of Si and Hf in the sol was controlled so that c(Si(OC2 H5 )4 )þc(HfOCl2 )¼4.48 10– 4 mol/mL in order to have a dipping rate of about 30 nm/dip. Er3 þ was added as Er(NO3)3 5H2O with a molar concentration of 0.1%, 0.3% and 1%. The Er3 þ -doped silica–hafnia film was deposited on the silica microspheres by dip coating in multiple steps. The dipping speed was 40 nm/min and the dipping time 20 s [33]. Before adding a new layer, after each dip the film was annealed in air for 50 s at 900 °C. Final films were stabilized by a last treatment for 2 min in air at 900 °C. As a result of this procedure, crack free films were obtained, keeping a high quality factor [34] (Fig. 2).
3. Experimental setup 3.1. Photoluminescence spectroscopy Photoluminescence (PL) measurements in the region of the I13/2- 4I15/2 transition of the Er3 þ ions were performed using as excitation source the 980 (or 981) nm line of a Ti:sapphire laser or a stabilized fiber pigtailed semiconductor laser diode operating at 976 nm or the 514.5 nm line of an Ar þ ion laser. The luminescence was dispersed using a single-grating monochromator with a resolution of 1 nm. The light was detected using an InGaAs photodiode and standard lock-in technique. Lifetime decay curves were obtained recording the signal with a digital oscilloscope. For both the precursor bulk samples and the microspheres the standard bulk excitation in free space configuration (without coupling to the WGMs) was employed. All the measurements were performed at room temperature. 4
3.2. Laser characterization
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Fig. 2. Optical image of an 0.3% Er3 þ -doped sol–gel coated silica sphere of about 130 mm in diameter, and the taper fiber in the background.
Fig. 3. Sketch of the experimental set-up: DBR or TLD (distributed Bragg reflector or tunable diode laser), Iso (optical isolator), att (attenunator), pol (polarization control), WDM (wavelength division multiplexer), PM (photomultiplier), OSA (optical spectrum analyzer).
Tunic Reference ES). In what follows we refer to the pump power as the difference between the launched one (measured after the input WDM) and the one collected after the resonator (see Fig. 3). The system also included an optical isolator to prevent feedback into the laser diode, an attenuator, and a 1.48/1.55 mm multiplexer which is finally connected to the tapered fiber coupler. The co-propagating signal from the microsphere and the fluorescence were collected and sent to the OSA. In order to control the position of the microsphere relative to the taper, the samples were mounted on a nanotranslation stage with piezoelectric actuators with 20 nm resolution. By varying the contact position of the tapered fiber coupler relative to the sphere and increasing the pump power the lasing spectrum moved towards WGMs of shorter wavelength. This effect can be explained with reference to a similar shift that occurs in the Er3 þ gain spectrum when the inversion rate increases [35].
4. Results and discussion 3þ
The experimental setup for laser characterization of Er doped microspheres is sketched in Fig. 3. Among the different pumping wavelengths which can be used to excite Er3 þ ions in oxide glasses (810 nm, 980 nm and 1480 nm) we chose 1480 nm so that phase and mode matching condition between the fiber coupler and the microsphere can be better fulfilled at both the pump and the laser wavelengths [9]. Moreover, by pumping at 1480 nm instead of 980 nm, we strongly reduce the heating effects due to the multiphonon non radiative decay from the 4I11/2 to the 4 I13/2 level, which can lead the microsphere temperature to reach the melting point in some glasses [31]. We used a home-made biconical tapered fiber to couple the pump light into the WGMs of the microsphere and-at the same time-to couple the fluorescence or laser light out of it [32]. For the optical characterization and lasing measurements we used as a pump either a fiber stabilized laser (Corning Lasertron, 1.5 nm linewidth) or a narrow-line tunable laser (300 kHz linewidth Anritsu
4.1. Microspheres from bulk glass 4.1.1. Photoluminescence The photoluminescence spectra of the 4I13/2- 4I15/2 transition of the Er3 þ ion for the bulk samples and for the respective microspheres were obtained after pumping at 976 nm by using a semiconductor laser operating with 30 mW of output power. Typically we did not observe a significant broadening of the bandwidth while we observed a reduction in the lifetime (in the case of IOG2 glass, for instance this reduction was of about 20% from a value of 6.870.2 ms in the bulk [36]). The lifetime measurements give information about the site-to-site inhomogeneity. In fact, the faster relaxation of the 4I13/2-4I15/2 decay curves of Er3 þ ion in the microspheres, as compared to the bulk, in both the silica and the phosphate glasses, indicates that energy transfer mechanisms among active ions are effective due to the microsphere fabrication
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process. Table 1 summarizes the luminescence values for the three glasses considered in our study. 4.1.2. WGM lasers In Fig. 4a we collect WGMs laser spectra from an IOG2 glass microsphere with a diameter of 70 μm. When increasing the pump intensity above a minimum threshold of 2.5 mW, we obtained laser oscillation at 1569.4 nm. Fig. 4b shows the laser emission of a 50 μm IOG10 microsphere at 1543.89 nm. The minimum threshold for IOG10 is 400 μW. Higher concentration of Yb2O3 in this latter glass and consequent stronger pump absorption may be responsible of the lower pump threshold. Being IOG1 a photorefractive glass, laser emission could be trimmed by UV irradiation. We have irradiated the IOG1 microlaser with an excimer laser (KrF, 248 nm) and measured the shift of the laser line by increasing the cumulative UV dose. We have observed a blue-shift that saturates at about 13 pm for doses higher than 1 kJ/cm2. The result is shown in Fig. 5. This blue-shift indicates that the induced change in the index of refraction is negative and rather small ( 10 5), in agreement with the data published in literature [37]. The resonator Q values were obtained by measuring the resonance linewidth of the WGM modes at 1.6 μm, which is almost outside the absorption band of the 4I13/2 level (σabs ¼0.35 10 21 cm2). For a microsphere 75 μm in diameter a typical resonance in the undercoupling regime [38] is shown in the inset of Fig. 6. The corresponding Q value of 0.8 106 is in agreement with that calculated taking into account only the residual absorption at that wavelength. When increasing the pump intensity above a minimum threshold of less than 1 mW, lasing occurred both in single mode and multimode condition and reached a maximum taper-coupled output power of 90 mW at 1568.3 nm with a pump power of 6.1 mW, as shown in Fig. 6. This represent the highest value obtained so far in phosphate glasses when pumping at 1480 nm almost an order of magnitude larger than the highest power emitted (10 mW) by an Er-doped silica microsphere laser [39]. With the same microsphere, when changing
the coupling condition, we also obtained lasing action at lower wavelengths, down to 1534.7 nm (almost 34 nm shift), but with lower efficiency. For a monolithic microsphere of about 105 mm of diameter, we measured single mode laser emission at a maximum wavelength of 1569.3 nm (with output power of 4.8 mW), and at minimum λ of 1541.2 nm (5.3 mW), obtained with a pump power of 7 mW and 4.2 mW, respectively. The corresponding spectra are shown in Fig. 7. A reason for the lower efficiency of this type of microsphere is that a diameter in excess of 100 μm is too big for an ideal matching with the silica fiber taper. On the other hand, as the monolithic microspheres are obtained starting from a glass thread
Fig. 5. Shift of the emission laser line by UV irradiation (KrF, 248 nm) of photorefractive IOG1 microspheres.
Table 1 Bulk lifetime and microsphere lifetime, peak emission wavelengths, fluorescence effective bandwidth for the 4I13/2-4I15/2 transition, and peak absorption crosssections of Er3 þ ions, measured for different experimental silicate and phosphate glasses. Glass
τmeas ( 7 0.2 ms)
IOG10 10.2 IOG2 6.8 IOG1 10.7
τmeas ( 7 0.2 ms) sphere
λ (nm) Δλeff fl. ( 7 1 nm)
σε ( 7 0.01 cm2)
6.2 5.8 8.7
1535 1533 1534
5.8 10 21 8 10 21 7 10 21
32 50 30
Fig. 6. WGMs laser spectrum from a 75 μm microsphere in IOG1 glass with a power peak of 90 μW at 1568.3 nm obtained using a pump power of 6.1 mW at 1480 nm. The inset shows a resonance around 1600 nm of the same sphere corresponding to a Q factor of 0.8 106.
Fig. 4. (a) Laser effect at 1569.4 nm in a 70 μm IOG2 microsphere. Maximum output power of about 70 nW. (b) Lasing action of a 50 μm IOG10 microsphere at 1543.89 nm. Maximum output power of 240 nW.
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with a square section, a strong and detrimental eccentricity, with a large number of competing modes [14] is always present in microsphere with a diameter below 100 μm.
4.2. Microspheres from sol–gel coated microspheres 4.2.1. Photoluminescence Fig. 8a shows the photoluminescence spectra relative to the 4 I13/2 -4I15/2 transition of the Er3 þ ions obtained upon excitation at 514 nm for microsphere doped with 1 mol% (M1) and 0.1 mol% (M2). The PL spectra are quite similar, according to a constant siteto-site inhomogeneity, both exhibiting an emission peak at 1.53 mm with a shoulder at about 1.55 mm and a spectral full width at half maximum (FWHM) of about 48 nm. Upon excitation at 980 nm, we observed that the shape and the bandwidth of the emission spectrum do not change, indicating negligible site selection for erbium ions. Fig. 8b shows the decay curve of the 4I13/2 metastable state obtained upon 514 nm excitation. The curves can be fitted by a single exponential function and a lifetime of 4.4 ms and 6.2 ms is respectively obtained for the microsphere doped with 1 mol% (M1) and 0.1 mol% (M2). The same lifetimes were measured upon 980 nm excitation. The decrease of the lifetime with the increase of Er3 þ concentration suggests that energy transfer processes are effective.
1400
1450
1500
1550
4.2.2. WGM lasers If the coating is thick enough so that the number of erbium ions interacting with the electric field of the WGMs is high enough, the stimulated emission gain is greater that the cavity round trip loss and the sphere will start to lase, with a launched power of about 10 mW. Fig. 9 shows the laser spectrum of a microsphere coated by 300 nm of 70 SiO2–30HfO2 film activated with 1% mol of Er3 þ (M1). The maximum peak power is 325 nW at 1564 nm. For a 130 μm microsphere coated with about 700 nm of 0.3 mol% Er3 þ doped 70SiO2–30HfO2 (M3), the maximum wavelength at which lasing was possible was found to be 1560 nm and minimum 1530 nm as shown in Fig. 10.
Fig. 9. WGMs laser spectrum of the microsphere M1 corresponding to the maximum peak power 350 nW.
Fig. 10. WGMs laser spectra corresponding to the maximum (1560.4 nm, red line) and minimum (1530.6 nm, black line) wavelength peak values obtained from a 130 μm microsphere (M3) coated with about 700 nm of 0.3 mol% Er3 þ doped 70SiO2–30HfO2.
M2 FWHM=48 nm
Μ2 τ=6.2 ms
M1 FWHM=48 nm
Μ1 τ=4.4 ms
1600
Wavelength (nm)
1650
1700
Intensity (a. u.)
Intensity (a. u.)
Fig. 7. WGMs laser spectra corresponding to the maximum (1541.2 nm, black line, output power 5.3 mW) and the minimum (1569.3 nm, blue line, output power 4.8 mW) wavelength peak values obtained from a 105 μm monolithic microsphere in IOG1 glass. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
759
0.00
0.02
Time (s)
Fig. 8. (a) PL spectra around 1.55 μm of the M1 and M2 sol–gel coated microspheres under excitation at 514 nm; (b) luminescence decay curve from the 4I13/2 state of Er3 þ ions in the M1 and M2 sol–gel coated microspheres under excitation at 514 nm.
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5. Conclusions We presented recent results obtained in our laboratories studying Er3 þ -doped glass microspherical cavities for the fabrication of compact and low threshold laser sources at 1.55 μm. We developed different procedures in order to fabricate the microcavities. We first consider microspheres fabricated by melting powders of Er3 þ -doped silica and phosphate glasses. Photoluminescence and lifetime measurements performed both on the bulk precursors and on the microspheres have revealed a shortening of the lifetime in the microcavities as compared to the corresponding bulks. This effect was related to inhomogeneous change in the local environment of Er3 þ ions induced by the microsphere fabrication process. We have also developed a new fabrication process to produce monolithic microspheres, which would allow various types of post-processing treatments. Laser action was obtained in all microspheres and different wavelengths could be excited within a broad range, depending on pump coupling conditions. A record peak power of 90 μW was obtained by pumping with 6.1 mW at 1480 nm. We then presented a simple technique to obtain Er3 þ doped microcavities by coating silica microsphere with a thin film of Er3 þ -doped sol–gel. The aim of having a thin layer around the base silica sphere is the tailoring of the WGM resonator, in order to select the radial order mode of the ‘active’ WGMs. We observed a broad PL spectrum typical of Er3 þ doped silica–hafnia system and analyzed the WGMs laser spectrum of the cavity.
Acknowledgments The authors are grateful to F. Cosi and M. Brenci for helpful discussions. M. Camerini is now at Selex ES. D. Farnesi is a PhD student at the University of Parma. Centro Studi e Ricerche “E. Fermi” is gratefully acknowledged for funding. The research was partially performed in the framework of the CNR-PAS joint Project (2014–2016) and of the CNES R&T Project SHYRO (2011– 2014). Cost Action MP1401 is gratefully acknowledged S. Berneschi thanks the Italian Ministry of Education, University and Research (MIUR) for its support through PRIN 2010–2011 Project ARTEMIDE (Ref. 20108ZSRTR).
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