Observation of 2:7 μm emission from diode-pumpedEr3þ=Pr3þ-codoped fluorophosphate glass

Ying Tian,1,2 Rongrong Xu,1,2 Liyan Zhang,1 Lili Hu,1 and Junjie Zhang1,*1Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics,

Chinese Academy of Sciences, Shanghai 201800, China2Graduate School of Chinese Academy of Sciences, Beijing 100039, China

*Corresponding author: jjzhang@mail.siom.ac.cn

Received November 17, 2010; accepted November 29, 2010;posted December 6, 2010 (Doc. ID 138350); published January 5, 2011

This work reports the intense emission at 2:7 μm in a Er3þ=Pr3þ-codoped fluorophosphate (FP) host glass. This FPglass shows good thermal stability and high transmittance around 3 μm. The emission characteristic and energytransfer upon excitation of a conventional 980 nm laser diode are investigated. The prepared glass possesses higherspontaneous transition probability (22:16 s−1) along with a larger calculated emission cross section ð6:57� 0:11Þ ×10−21 cm2 corresponding to the laser transition 4I11=2 → 4I13=2. In addition, the effect of Pr3þ codoping on the 2:7 μmphotoluminescence in FP glass is demonstrated. Hence, the advantageous spectroscopic characteristics ofEr3þ=Pr3þ-codoped FP glass together with the outstanding thermal property indicate that this kind of glass maybecome an attractive host for developing solid-state lasers at around 2:7 μm. © 2011 Optical Society of AmericaOCIS codes: 160.5690, 300.6280, 160.4670, 160.2750, 160.3130.

In recent years, owing to various useful applications, in-cluding military, remote sensing, atmosphere pollutionmonitoring, eye-safe laser radar, and medical surgery,much interest has been devoted to searching for newmaterials to use as hosts for mid-IR lasers operating inthe wavelength region around 2:7 μm [1]. It is known thattwo important factors, including the glass host and theactive ions, must be considered in developing more effi-cient optical devices based on rare-earth ions [2]. Thehost glass material for mid-IR lasers is expected to pos-sess a minimal absorption coefficient in the typical H2Oabsorption band at 3 μm, low nonradiative decay ratesand high radiative emission rates, and compatibility withwaveguide fabrication processes for the majority of ap-plications [3]. Among many alternatives, fluoride glasseshave emerged as natural candidates for such rare-earth-doped optical devices [4]. However, fluoride glasses re-quire a more complex fabrication route and have inferiorthermal stability.Fluorophosphate (FP) glasses with more significant

advantages compared to fluoride and phosphate glassserve as a useful host for many applications, such asplanar waveguide amplifiers, short wavelength lasersources, optical amplifiers, and solid-state frequency up-converters [5–7]. They exhibit good moisture resistance,thermal behavior, high solubility for rare-earth ions withbroad absorption and emission bands, long fluorescencelifetime, and tailorable properties by varying the fluorideto phosphate ratio, and possess a less complex fabrica-tion route than that of fluoride glasses [8]. These charac-teristics render this class of glasses as extremely suitableto be used as a host to mid-IR solid-state lasers. More-over, based on our prepared FP glasses doped with Yb3þ,Tm3þ, and Ho3þ, the 2 μm spectroscopic properties havealready been investigated according to their certain char-acteristic transitions, and these glasses show some po-tential application for mid-IR laser media with highgain [9–11].Er is a well-known ion with transitions in the IR

region around 1550 nm (4I13=2 → 4I15=2) and 2:7 μm

(4I11=2 → 4I13=2), as well as in the green region around550 nm (4S3=2 →

4I15=2) [12]. However, the 2:7 μm emis-sion cannot be obtained efficiently because the fluores-cence lifetime of the upper 4I11=2 level is considerablyshorter than that of the lower level 4I13=2 [13,14]. Fortu-nately, codoping of Pr3þ, Tm3þ, Yb3þ, Nd3þ, or Ho3þ,respectively, with Er3þ has been used as a feasible alter-native to quench the lower level [1,14–18].

To our knowledge, the observation of emission around2:7 μm in Er3þ=Pr3þ-codoped FP glasses has not beensuccessfully obtained. In this Letter, we present 2:7 μmemission in a Er3þ=Pr3þ-doped fluorophosphate glass.Besides, the spectroscopic investigation along with ana-lysis of radiative properties and emission cross sectionhas been made for future applications in mid-IR lasers.

The investigated glass has the following molar compo-sitions: 20AlðPO3Þ3-80RF2-3ErF3-0:6PrF3 (R ¼ Mg, Ca,Sr, Ba). The fabrication methods, as well as thermaland optical property measurements of glass samples,were described in our previous work [9–11].

After the thermal analysis of the prepared glass, it isobtained that Tg is 438:8 °C and Tx is 677:5 °C. TheΔT (Tx − Tg) has been frequently used as a rough esti-mate of the glass formation ability or glass stabilityagainst crystallization, and large ΔT is beneficial tothe fiber drawing [19]. It is clear that ΔT (238:7 °C) ofour present FP glass is significantly higher than that ofother kinds of rare-earth-ion-doped FP and fluorideglasses reported by several researchers [11,20]. Thus,the Er3þ=Pr3þ-codoped FP glass possesses a better ther-mal stability, which can achieve a larger working rangeduring optical fiber drawing.

Figure 1(a) indicates the absorption spectra of thePr3þ- and Er3þ=Pr3þ-doped sample at room temperaturein the wavelength region of 370–2100 nm, and the ab-sorption bands of Er3þ and Pr3þ corresponding to thetransitions starting from the ground state to the higherlevels are labeled. The strong absorption around980 nm of the codoped sample indicates that this glasscan be excited efficiently by a 980 nm laser diode

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(LD). Figure 1(b) presents the IR transmittance spectrumof the FP sample. As it can be seen, the maximum trans-mittance reaches as high as 90%, and there is no typicalH2O absorption at the 3 μm band. Therefore, the excel-lent IR transmission property provides potential applica-tion for 2:7 μm laser material.The intensity parameters Ωt (t ¼ 2, 4, 6) of Judd–Ofelt

theory [21,22] were calculated using the absorption dataaccording to the procedure described elsewhere [23].The Ω4 ð1:02� 0:08Þ × 10−20 cm2 and Ω6 ð0:91� 0:06Þ ×10−20 cm2 of Er3þ in our prepared glass are comparablewith those reported from similar systems, while the Ω2ð5:14� 0:10Þ × 10−20 cm2 of FP is higher than that offluoride glasses and smaller than that of phosphateglasses [24,25]. Theoretically, an O2− ion possesses high-er polarizability than an F− ion, resulting in increasingcovalency of the ligand from fluoride to phosphate glass[26]. The rms error deviation of intensity parameters is0:17 × 10−6, which indicates the validity of the Judd–Ofelttheory for predicting the spectral intensities of Er3þ andthe reliable calculations. Further calculation about theradiative transition probabilities and branching ratio ofthe 4I11=2 → 4I13=2 transition results in 22:16 s−1 and17.63%, respectively, which are comparable with the re-sults of Er3þ-doped ZBLAN glass (30 s−1 and 18%) [27].The IR photoluminescence (PL) spectra of Er3þ- and

Er3þ=Pr3þ-doped FP glasses were measured as shownin Fig. 2. In Er3þ singly doped FP glass, 2:7 μm emissionfrom the excited 4I11=2 state is almost absent, whereasintense photoluminescence from 4I13=2 is observed. Thisdemonstrates a high efficiency of the nonradiative re-laxation of 4I11=2 to 4I13=2 in the Er3þ singly doped sam-ple, while 2:7 μm emission can be observed in theEr3þ=Pr3þ-codoped sample. Moreover, as is suggestedby the PL spectra shown in Fig. 2(b), a significant reduc-tion in the emission intensity of the 4I13=2 level is ob-served between the Er3þ singly doped and theEr3þ=Pr3þ-doped sample, which justifies that Pr3þ ionscan be used effectively to depopulate the Er3þ: 4I13=2 le-vel. According to the Fuchtbauer–Ladenburg theory andemission spectra, the 2:7 μm emission cross section σemcan be calculated [28,29]. It is worth noting that the peakof the calculated emission cross section in theEr3þ=Pr3þ-doped FP sample at 2708 nm achieves

ð6:57� 0:11Þ × 10−21 cm2, which is higher than theresult of Er3þ-doped oxyfluoride transparent glassceramics (4:3 × 10−21 cm2) [29] and ZBLAN glass(5:7 × 10−21 cm2) [30].

In Fig. 3, the upconversion spectra of Er3þ- andEr3þ=Pr3þ-doped FP glasses are demonstrated. Thegreen emission signal is weaker in Er3þ=Pr3þ-codopedFP glass than in the Er3þ singly doped one. Based onthe experimental phenomenon and theoretical data,the involved energy transfer mechanisms are indicatedin the inset of Fig. 3. Ions of the 4I15=2 state are excitedto the 4I11=2 state by ground state absorption when theprepared sample is pumped by a 980 nm LD. On onehand, some ions in the 4I11=2 level undergo the energytransfer upconversion (ETU1) process, which makes acontribution to the population of 4F7=2 level. After that,the excited energy that is stored in the 4F7=2 level decaynonradiatively to the next-lower 2H11=2 and 4S3=2 levels.Hence, the green emission can be attributed to theEr:2H11=2 →

4I15=2, Er:4S3=2 →4I15=2, and Pr:3P0 →

3H5transitions. For the extremely weak green emission inEr3þ=Pr3þ-codoped FP glass, it can be inferred that, after

Fig. 1. (a) Absorption spectra of Pr3þ- and Er3þ=Pr3þ-dopedFP glasses. (b) IR transmission spectrum of Er3þ=Pr3þ-dopedFP glass.

Fig. 2. IR PL spectra in the (a) 2:7 μm and (b) 1:5 μm regionsof Er3þ and Er3þ=Pr3þ FP glasses.

Fig. 3. Upconversion spectra of Er3þ and Er3þ=Pr3þFP glasses. Inset, energy transfer sketch of Er3þ=Pr3þ-codopedFP glass when pumped at 980 nm.

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the Pr3þ addition into the system, the ETU1 process isweaker, which is favorable to the energy storage ofthe 4I11=2 level and subsequent 2:7 μm emission. Onthe other hand, ions in the 4I11=2 level decay radiativelyto 4I13=2 with 2:7 μm emission or nonradiatively to the4I13=2 level. Then the 1:55 μm emission happens owingto the Er3þ:4I13=2 → 4I15=2 transition. Besides the obviousdrop of 1:55 μm emission in the codoped sample, the life-time of the 4I13=2 level in the Er3þ=Pr3þ-codoped sample(760 μs) is 1 order of magnitude smaller than that in theEr3þ singly doped sample (8:6 ms). Therefore, ions in the4I13=2 level are largely depopulated by energy transfer(ET1) to Pr:3F3;4 and fast decay to the ground state bymultiphonon relaxation. This ET1 process is much moreefficient than the Er3þ:4I13=2 → Pr3þ:1G4 (ET2) process[31]. It is noted that the 4I13=2 þ 4I13=2 → 4I15=2 þ 4I9=2(ETU2) process is beneficial to 2:7 μm emission, becausethe 4I11=2 level is populated by a fast multiphonon decayfrom the 4I9=2 level [1,31].In conclusion, intense emission at 2:7 μm is observed

in the Er3þ=Pr3þ-codoped FP glass with better thermalstability. The absence of strong OH− absorption at3 μm guarantees the observation of intense photolumi-nescence at 2:7 μm. For Er3þ=Pr3þ-codoped FP glass, thehigher spontaneous transition probability (22:16 s−1) andemission cross section ð6:57� 0:11Þ × 10−21 cm2 give evi-dence of intense 2:7 μm emission. The results and ana-lyzed energy transfer indicate codoping relatively smallamounts of Pr3þ with Er3þ in FP glass plays an importantrole in 2:7 μm emission when the sample is pumped by acommon 980 nm LD. Thus, Er3þ=Pr3þ-codoped FP glasswith desirable thermal resistance properties and spectro-scopic characteristics will be a promising material for2:7 μm lasers.

The authors are thankful to Chinese National 863 Pro-ject (no. 2007AA03Z441) for financial support of thiswork.

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