Continuous-wave 1.5-μm-band laser is demonstrated in thulium doped tellurite glass microsphere by single laser pumping. 1.9-μm laser from the lower transition (3F4→3H6) of Tm3+ is generated to depopulate the lower level (3F4) of 1.5-μm transition (3H4→3F4) in order to achieve the population inversion. Laser wavelength of 3H4→3F4 transition is shifted by 30 nm from the emission peak. Slope efficiency of 1.9-μm laser is improved after the1.5-μm laser starts to lase.
© 2005 Optical Society of America
The requirement of carrying higher bit-rate through one commercial silica fiber which has a broad low-loss band (1460-1650 nm < 0.25 dB/Km) has drawn attention of research to thulium oxide doped material whose 3H4→3F4 transition at 1.5μm covers the S-band (1460nm-1530nm) of optical communication (see Fig. 1). However, such transition is a self-terminating transition that is hard to achieve population inversion because its lower level lifetime is significantly longer than its upper level lifetime. Several special techniques have been developed to depopulate the lower level of the transition at 1.5μm. Trivalent rare-earth ions, such as Ho3+, Tb3+, and Nd3+, are codoped with Tm3+ to reduce the population of the long-lived 3F4 state [1–3] through resonant energy transfer process. Some studies, in order to achieve population inversion, used dual-wavelength pump method to depopulate the 3F4 state by exciting thulium ions at 3F4 state to a higher energy level by excited state absorption . In this paper we demonstrate a 1.5-μm laser from the self-terminating transition of Tm3+ in a microsphere resonator, and study the condition of CW lasing in such a laser system.
Among oxide glasses, tellurite glass is an attractive host for thulium ions due to its low phonon energy that helps Tm3+ to stay at the upper level of the 3H4→3F4 transition by decreasing the multi-phonon decay rate from 3H4 to 3H5. Tm-doped tellurite glass samples with different concentrations are fabricated and characterized in order to choose the optimum concentration for the 1.5 μm microsphere laser. Figure 1 shows the absorption coefficient of the 0.5 wt% Tm2O3 doped tellurite glass and the corresponding energy level diagram of the electronic states of Tm3+. Spontaneous emission rates of different transitions are calculated by Judd-Oflet theory. Lifetimes of both 3F4 and 3H4 states are measured for glasses with different concentrations. We find that when concentration is larger than 0.5wt%, the lifetime of 3H4 state drop from 180 μs to 120 μs or less. This is mainly because of the cross relaxation that occurs at high thulium doping concentrations and dramatically depopulates the energy level of 3H4, the upper lasing level of 1.5 μm transition. The emission spectrum also shows that when the doping concentration is higher than 0.5 wt%, the intensity ratio between 1.9 μm transition and 1.5 μm transition increases nonlinearly . Hence, thulium doping concentration of 0.5 wt% was chosen in the experiment in order to eliminate the cross-relaxation energy transfer that decrease the emission intensity of 1.5-μm-band transition and to achieve efficient pump absorption.
In microsphere light can propagate around the edge with very low loss due to total internal reflection. The propagation mode, called whispering-gallery mode, applies to all wavelengths generally, which means microsphere can be used as a laser resonator working at most wavelengths within the gain band. In our research microsphere is used as a laser cavity for both 1.9 μm and 1.5 μm lasers. Microspheres are fabricated by the spin method . The diameter of the microsphere ranges from the tens of to the hundreds of micron. A fiber taper made of Corning SMF-28 fiber by the typical heat-and-stretch method was used to couple the pump light into and signal laser out of the microsphere. A Ti-sapphire laser at 793 nm is employed to directly excite the 3H4 state of Tm3+.
3. Lasing condition on self-terminating transition
The condition for CW lasing on a self-terminating transition (level 3 -> level 1) was described by ,
where in the case of thulium, level 1, 2, and 3 represent 3F4, 3H5, and 3H4 states. A1 is the sum of spontaneous emission rate and nonradiative decay rate of the state 3F4, lower level of laser transition, and 1 is the total decay rate from state 3H4 to state 3F4. The criterion derived under the condition that no stimulated emission and absorption were considered in the steady state rate equations. It states that in order to get population inversion, the requirement for CW lasing, the total decay rate of the lower level of laser transition should be greater than the total relaxation rate from the upper level to the lower level. In the case of Tm3+, the calculated spontaneous emission rates are A10=540 s-1, A32=98 s-1, A31=260.5 s-1, A30=2863 s-1, where the subscript number i represents the corresponding energy level shown in Fig. 1. All ions reaching the 3H5 state will decay to the 3F4 state immediately through multiphonon relaxation, because 3H5 and 3F4 states are so close in energy that only 2 or 3 phonons needs to be generated to accomplish the multi-phonon decay. Therefore, to simplify the analysis, a 3-level model in which the population of 3H5 is zero is adopted. The total decay rate from 3H4 state to 3F4 state () can be written as,
where Aijnr and Aij are the nonradiative and radiative decay rate from level i to level j respectively.
For Tm3+ doped in the material with low phonon energy, the inequality (1) is satisfied. For example, 1.5 μm cascade laser of thulium ions has been demonstrated in fluoride glass [8–9]. In that case, the threshold of 1.5 μm is lower than that of the 1.9 μm laser, because the upper lasing level (3H4) of the 1.5μm laser transition was directly excited by the pump laser, while the upper lasing level (3F4) of the 1.9 μm laser transition was populated by the ions relaxed from 3H4. According to the above spontaneous emission rates of different transitions, only 11% of the population excited to 3H4 state, upper level of 1.5-μm transition, reaches the 3F4 state, the upper level of the 1.9 μm transition. Due to the low pump efficiency, the 1.9 μm transition started to lase after 1.5 μm transition did.
In our experiment, the phonon energy of the tellurite glass was measured as 929cm-1 which is higher than that of the fluoride glass. The nonradiative decay rate A32nr is 2302 s-1 calculated by A32nr=1/τmeas-(A31+A32+A30). Therefore, the inequality (1) is not satisfied. However, we found the population inversion of the upper transition can be reached by making the lower transition laser first, which will depopulate the lower level of the upper transition. Under such situation, the steady state rate equations then can be written as,
where N i is the population density of ions in level i, W 10 and W 01 are the stimulated emission rate and the stimulated absorption rate respectively. W 03 is the pumping rate. Stimulated emission of 1.5 μm transition is not considered here, as we just need to find the condition for population inversion of the upper transition. Rate equations are solved for the population inversion ratio N 3/N 1. To simplify the result, only terms with the highest order of W 03, W 01 or W 10 is considered in both denominator and numerator, because usually the pumping rate, stimulated emission rate and stimulated absorption rate are much larger than the spontaneous emission rate. Therefore, the moderated condition for population inversion is,
Inequality (2) states that if the pumping rate W 03 and stimulated emission rate W 10 are large enough, the population inversion of the upper transition will be built up. Furthermore, if the lower laser transition is a 4-level system, in which W 10 ≫ W 01, the criterion can be simplified as W 10> A 31 *. It’s quite similar to the inequality (1), except that A 10 is replaced by W 10.
4. Results and discussion
The laser performances of both 1.5μm and 1.9μm laser are shown in Fig. 2. The threshold of 1.9 μm laser is lower than that of the 1.5μm laser, which is the opposite of the phenomenon observed in fluoride glass. When the lower transition starts to lase, its intensity is not strong enough to make the stimulated emission rate satisfy the inequality (2) of the cascade laser. With the increase of pump and signal laser, the inequality (2) is then satisfied and the upper transition starts to lase. Careful inspection of Fig. 2 also shows that as soon as 1.5 μm transition start to laser, the slope of the l.9 μm laser is increased because of the high pumping efficiency resulting from the high stimulated emission rate of upper laser transition. In the experiment we also found that the intensity ratio between the upper transition laser and lower transition laser increased, when the lower transition laser operated at shorter wavelength at which the lower transition had larger emission cross-section.
Figure 3 shows the laser spectrum of microsphere. The emission spectrum of the glass sample with the same doping concentration is also illustrated in the inset of Fig. 2. The laser wavelengths are around 1.9 μm and 1.5 μm. Both are red shift from the emission peak of the respective transition bands. For 1.9 μm laser which is a 3-level system, the red shift of the laser wavelength results from the mode mismatch between pump and signal laser. The radial distribution of the whispering-gallery modes of pump and signal laser did not overlap well . Part of the 1.9 μm laser WG mode area is not covered by the pump laser. Therefore, the reabsorption from those unpumped thulium ions make the lower transition laser operate at long wavelength. For 1.5μm laser, the red shift is caused by the reabsorption from the thulium ions at the lower laser level (3F4). When the 1.9 μm laser is working in steady state, there are certain thulium ions at the upper level (3F4) which is also the lower level of the 1.5 μm transition. The absorption from those ions at 3F4 states will push the upper transition laser toward long wavelength.
In summary, we demonstrate CW 1.5-μm-band laser from thulium doped tellurite glass microsphere. The condition for the CW laser in a self-terminating system has been studied. The 1.9 μm laser is achieved first in order to depopulate the lower level (3F4) of the 1.5 μm laser transition that is a self-terminating transition. The improvement in slope efficiency of 1.9 μm laser is observed, after 1.5 μm transition starts to lase.
The authors would like to acknowledge the support from NSF grant #DMS-0335101 and state of Arizona TRIF photonic program.
2 . G.H. Rosenblatt , R.J. Ginther , R.C. Stoneman , and L. Esterowitz , Advanced Solid state Lasers, “ Laser emission at 1.47 μm from fluorozirconate glass doped with Tm 3+ and Tb 3+ ,” Proceedings of the OSA Topical Meeting (5 vols), 1988 , p 373 .
3 . S. Tanabe , X. Feng , and T. Hanada , “ Improved emission of Tm 3+ -doped glass for a 1.4-μm amplifier by radiative energy transfer between Tm 3+ and Nd 3+ ,” Opt. Lett. 25 , 817 ( 2000 ). [CrossRef]
4 . T. Kasamatsu , Y. Yano , and T. Ono , “ Gain-shifted dual-wavelength-pumped thulium-doped fiber amplifier for WDM signals in the 1.48-1.51 mm ,” IEEE Photon. Technol. Lett. 13 , 31 ( 2001 ). [CrossRef]
5 . Jianfeng Wu , Shibin Jiang , Tiequn Qiu , and N. Peyghambarian , “ Cross relaxation energy transfer of thuliumin tellurite glass ” Submitted to J. Opt. Soc. Am. B .
6 . X. Peng , F. Song , S. Jiang , N. Peyghambarian , M. Kuwata-Gonokami , and L. Xu , “ Fiber-taper-coupled L-band Er 3+ -doped tellurite glass microsphere laser ,” Appl. Phys. Lett. 82 , 1497 ( 2003 ). [CrossRef]
8 . R.M Allen , L. Esterowitz , and I. Aggarwal , “ An efficient 1.46μm thulium fiber laser via a cascade process ,” IEEE J. Quantum Electron. 29 , 303 , ( 1993 ). [CrossRef]
9 . R.M. Percival , D. Szebesta , and S.T. Davey , “ Highly efficient CW cascade operation of 1.47 and 1.82 mm transitions in Tm-doped fluoride fiber laser ,” Electron. Lett. 28 , 1866 ( 1992 ). [CrossRef]