1. Introduction

Due to the strong vibrational absorption of water molecules at approximately 3 μm, high-power fibre lasers operating near 3 μm have strong interest for medical applications [1,2] and also for sensing [3]. Three rare earth dopants have generated laser emission near 3 μm: Er³⁺ [1,2,4], Ho³⁺ [5,6], and Dy³⁺ [7]. The Er³⁺ ion, doped into various glasses, has been the most extensively studied and provides lasing near 3 μm in both crystal [8] and fibre [4] laser configurations. Due to pump excited state absorption (ESA) and the lower Stokes limit (of ~35%), the slope efficiency achieved in the ~970-980 nm diode pumped Er³⁺-doped ZBLAN fibre laser is limited to ~25% [4]. The recent development of the 2.86 μm Ho³⁺, Pr³⁺-co-doped ZBLAN fibre laser has shown that the slope efficiency generated when pumped with the 1.1 μm Yb³⁺-doped silica fibre laser could be augmented to 29% [5]. Both the Er³⁺- and Ho³⁺, Pr³⁺-doped ZBLAN fibre lasers are four-level laser systems, however, the quasi-three-level 3 μm fibre laser system based on Dy³⁺-doped ZBLAN [7] offers the possibility of broadly tunable and highly efficient mid-infrared light generation.

 

Fig. 1. Energy levels of the Dy³⁺ ion. The laser transition and pump GSA and pump ESA for two different pumping schemes at 1.1 μm and 1.3 μm are also shown.

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Fig. 2. Measured absorption spectrum for a 40,000 ppm molar Dy³⁺-doped ZBLAN bulk sample in the range 1- 1.5 μm. Peak pump wavelengths are shown for a 1.1 μm pump source from a Yb-fibre laser and two lines from a ~1.3 μm Nd:YAG laser.

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The laser emission at 3 μm in Dy³⁺ was first demonstrated in BaY₂F₈ host material at 77 K [9]. The measured ⁶H_13/2 → ⁶H_15/2 emission spectrum from Dy³⁺in BaY₂F₈ at 77 K ranged from ~2.78 μm to ~3.55 μm [9]. Further investigation was made of other Dy³⁺-doped crystals [10,11] and a room-temperature laser based on the ⁶H_13/2 → ⁶H_15/2 transition of Dy³⁺ in BaYb₂F₈ at 3.4 μm was demonstrated in 1997 [12]. Laser emission at 2.9 μm from the Dy-ZBLA has been predicted at liquid nitrogen temperature and at room temperature [13]. The generation of continuous wave 2.9 μm laser output from a Dy³⁺-doped ZBLAN fibre has been previously demonstrated using optical pumping with a 1.1 μm Yb³⁺-silica fibre laser [7]. The measured laser slope efficiency was approximately 5% and the pump power level at lasing threshold was ~1.78 W. The Dy³⁺ ion has a number of longer wavelength absorption bands [13]: the ⁶H_15/2 → ⁶H_9/2 , ⁶F_11/2 transition centred at 1.3 μm, the ⁶H_15/2 → ⁶H_11/2 transition centred at 1.7 μm and the ⁶H_15/2 - ⁶H_13/2 transition centred at 2.84 μm; see Figs 1 and Fig. 2. Longer wavelengths can be used to pump the Dy³⁺-doped ZBLAN fibre with the expectation of higher efficiency.

In this investigation, we extend the work presented in Ref [7] by way of pumping Dy³⁺-doped ZBLAN fibres at the longer wavelength absorption band centred at 1.3 μm. We produce a four-fold increase in the slope efficiency with the current pumping scheme as compared with pumping at the shorter 1.1 μm wavelength. We understand the better performance of the current fibre laser is largely a result of reduced levels of pump ESA.

2. Experimental Set-up

 

Fig. 3. Experimental arrangement for the 2.96 μm Dy-doped ZBLAN fibre laser pumped by a ~1.3 μm Nd:YAG crystal laser.

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A schematic of the experimental set-up is shown in Fig. 3. An arc lamp pumped cw Nd:YAG (Quantronix Model 416) laser operating at a wavelength of ~1.3 μm in the fundamental transverse TEM₀₀ mode was used as the pump source. Fig. 4 shows the spectrum of the 1.3 μm Nd:YAG pump laser to the Dy doped fibre.

 

Fig. 4. Normalised continuous ~1.3μm Nd:YAG pump laser spectrum before and after transmitted through the Dy-ZBLAN fibre.

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In Fig. 4 the two peaks in the spectrum of the pump laser are centred at 1325 nm and 1344 nm, and with FWHM of ~20 nm. The maximum intensities stable in time and are of the two peaks are about equal before being launched into the Dy³⁺-doped ZBLAN fibre. The measured 1325 nm peak spectrum after transmission through the Dy³⁺-ZBLAN fibre has lower intensity than the 1344 nm peak, which is expected given that it is more strongly absorbed; see Fig. 2 which shows clearly that the 1325 nm peak is closer to the absorption peak of the (⁶H_15/2 → ⁶H_9/2 , ⁶F_11/2) absorption band.

The pump laser power was varied by the use of glass filters, while the lamp current of the Nd:YAG laser was kept constant to maintain laser beam quality. The beam was steered and focused into the Dy³⁺-doped ZBLAN fibre with a 10×, 0.25-numerical aperture IR microscope objective lens. Both ends of the doped fibre were carefully cleaned and perpendicularly cleaved (cleaver, PK Technology Inc. FK12-STD). The Dy³⁺-doped ZBLAN fibre (Le Verre Fluore, France), which is identical to the fibre used for the previous experiments in Ref. [7], had a Dy³⁺ concentration of 1000 ppm (or 1.7×10²⁵ m^-3), a core diameter of 15 μm, and a NA of 0.13. The energy level diagram of the Dy³⁺ ion and absorption bands near 1.3 μm is shown in Figs 1 and 2, respectively. The fibre supported single transverse mode operation down to 2.55 μm. The fibre had a measured intrinsic loss of 20 dB/km at 2.1 μm. The coupling of the pump laser and the transmitted pump power through the doped fibre were optimised. The pumped end mirror, which was ~90% transmitting at ~1.3 μm and ~99% reflecting at ~3 μm, was carefully placed and butted against the facet of the fibre at the pumped end. An output coupler with a 50% reflection at ~3 μm was butted against the output end to the fibre. The fibre laser output was measured with a thermo-electric power meter or a liquid nitrogen cooled InSb photo-detector and the spectra measured by a computer controlled auto-scan monochromator. A Ge filter, which has very low transmission at wavelengths shorter than 1.8 μm and transmitted about 57% at wavelengths longer than 2 μm, was used to block out the transmitted pump power for the power and spectral measurements. The measured output power was corrected for the reflection losses of the Ge filter.

A silica transmission fibre with a core diameter of 17.6 μm, and a NA of 0.13 was used to check the pump launch efficiency into the Dy³⁺-doped ZBLAN fibre. By measuring the transmitted 1.3 μm laser from the silica fibre, the launch efficiency into this fibre was calculated. The launch efficiency into the Dy³⁺-doped fibre is expected to be close to or slightly lower than the launch efficiency measured for the silica fibre. Thus, the measured slope efficiencies quoted in this investigation may be slightly lower than the actual values.

3. Results

The Dy³⁺-doped ZBLAN fibre laser output power as a function of the absorbed pump power for fibre lengths of 60 cm and 77 cm are shown in the Fig. 5.

 

Fig. 5. Laser output power of the 2.96 μm Dy³⁺-doped ZBLAN fibre laser with variation of the absorbed pump power for two fibre lengths.

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From the measured absorption spectrum for the Dy³⁺-doped ZBLAN fibre, Fig. 2, the absorption coefficient for pump wavelengths in the region 1.325 to 1.350 μm ranged from ~5 to ~2.7 m^-1. Consequently, the overall percentage absorption of the 1.3μm pump laser in the 77 cm and 60 cm fibres was about 92% and 84%, respectively. This gives the average measured absorption coefficient of the ~1.3 μm pump laser as approximately 3 m^-1. The laser performance with respect to the incident pump power, absorbed pump power and launched pump power for the two different fibre lengths are given in Table 1. The calculated M² for the laser is about 1.04. The output power dependencies shown in Fig. 5 show no evidence of saturation with pump power. The laser may be able to be pumped up to the facet damage limit of the fluoride fibre. This gives a limit on the end pumping of the fluoride fibre as the glass transition temperature of the fibre is expected to be low at just above 300 °C.

Table 1. Performance of the Dy³⁺- doped ZBLAN fibre laser for two different fibre lengths.

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Fig. 6. Measured spectrum of the output from the Dy³⁺-doped ZBLAN fibre laser for a ~60 cm fibre length. The output power is ~30mW

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The emission spectrum from the Dy³⁺-doped ZBLAN fibre laser is shown in Fig. 6. The optical spectra of the laser output for the two different fibre lengths were similar. The central wavelength of the output is located at 2.96 μm as shown in Fig. 6. The linewidth (i.e., FWHM) of the output was ~14 nm. Compared to the Dy³⁺-doped ZBLAN fibre laser pumped with the 1.1 μm Yb³⁺ fibre laser [7], the lasing wavelength in the present case is slightly longer. The main reason for the longer lasing wavelength arises from the comparatively higher reflectivity of the output coupler (50%) used in the current investigation. For the present cavity, the lower output coupling forces a lower value of the pump power at threshold, therefore, there are comparatively more ground state Dy³⁺ ions present in the fibre at threshold as compared to the higher output coupling case [7]. The current lasing wavelength is consequently shifted to longer wavelength because of the larger signal absorption at shorter wavelengths arising from the absorption band (⁶H_15/2 → ⁶H_13/2) centred at 2.84 μm.

The time dependence of the output has also been investigated, and the results are shown in Fig. 7. As for other rare earth doped fibre lasers, particularly those operating with the ground state as the lower laser level, the output power from the Dy³⁺-doped ZBLAN fibre laser exhibited a high degree of modulation. Fig. 7 shows that the output from the Dy³⁺-doped ZBLAN fibre laser is not pure cw even when pumped by a stable cw laser. The output power is far below the threshold for stimulated Brillouin scattering so it is understood that the self-modulations are due to saturable absorber effects caused by the under-pumped section at the distal end of the fibre. This saturable effect can be stabilised by using uniform pumping, for example by a double end-pumping scheme [14,15]. This self-modulation effect has been shown to depend on the output coupler reflectivity and also the fibre length [15].

 

Fig. 7. Typical output intensity fluctuations of the ~70 cm Dy-ZBLAN fibre for ~1.8 W launched pump power and generating an average power of ~150 mW at ~3 μm for two different time scales over (a) 250 ms and (b) 1 ms.

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4. Discussion

The absorption coefficient for the Dy³⁺-doped ZBLAN fibre in the range 1- 1.5 μm shown in Fig. 2 was calculated from the measured absorption cross-section of Dy³⁺-doped ZBLAN. Generally the absorption coefficient at the 1.1 μm pump wavelength of the Yb³⁺-fibre laser (~6 m^-1) is higher as compared to the absorption coefficient relevant to the ~1.3 μm Nd:YAG laser (which was between 3-5 m^-1). Additionally the absorption peak of the ⁶H_15/2 → ⁶H_9/2 , ⁶F_11/2 absorption transition is closer to 1.29 μm and would have an absorption coefficient of ~10 m^-1. Thus, by pumping the Dy³⁺-doped ZBLAN at the 1.29 μm absorption peak, e.g. with a Raman fibre laser, the optimal length could be further reduced which could lead to perhaps better performance from the 3 μm fibre laser. The present pump arrangement is more efficient than previous work in terms of launched power, however, one must be aware that the current 1.3 μm Nd:YAG laser is considerably less efficient than the 1.1 μm fibre laser used in the original experiment. As a result, a Raman fibre laser may be of significant benefit.

The possible pump ESA and laser transitions for the two different pumping schemes, i.e. at 1.1 μm and 1.3 μm, are shown in Fig. 1. The 1.1 μm Yb³⁺-fibre laser pumped Dy³⁺-doped ZBLAN fibre laser producing output near 3 μm output gives a Stokes limit of ~37%. Pump radiation at a wavelength of 1.1 μm is first absorbed into the ⁶H_7/2 , ⁶F_9/2 levels after excitation from the ⁶H_15/2 ground state. Since the energies of the excited state absorption transitions ⁶H_13/2 → ⁶F_3/2, ⁶F_5/2 → ⁴F_9/2 or ⁶F_3/2 → ⁴I_15/2 are nearly equivalent to a photon with a wavelength of about 1.18 μm, the 1.1 μm Yb³⁺ fibre laser pumping scheme suffers from strong pump ESA. Initially, the ⁶F_5/2 or ⁶F_3/2 energy levels are populated from pump ESA starting from the ⁶H_13/2 energy level. Then the Dy³⁺ ion can be further excited to ⁶F_9/2 or ⁴I_15/2 energy levels after absorbing a second 1.1 μm pump photon and emission of a yellow photon (at a wavelength of ~575 nm) results. The observation of a yellow fluorescence (~575 nm) due to the transition between ⁴F_9/2 → ⁶H_13/2 energy levels from the Dy³⁺-ZBLAN fibre when strongly pumped by 1.1 μm laser indicates that excitation to the ⁴F_9/2 multiplet exists [7]. In comparison, the ~1.3 μm laser pumped 3 μm Dy³⁺-doped ZBLAN fibre laser has a higher Stokes limit of ~44% and the only possible pump ESA processes are ⁶H_11/2 → ⁶F_3/2 and ⁶H_13/2 → ⁶F_7/2 as shown in Fig. 1. The reduced pump energy losses resulting from reduced pump ESA is understood to be the reason why the 1.3 μm pump wavelength is more useful for 3 μm Dy³⁺-doped ZBLAN fibre lasers than the 1.1 μm pump wavelength. Note that no yellow fluorescence was observed from the fibre laser in the present experiments. The development of a high power fibre laser near 1.3 μm to replace the lamp pumped Nd:YAG laser would be useful.

5. Conclusions

A practical quasi-continuous wave Dy³⁺-doped ZBLAN fibre laser operating at 2.96 μm has been demonstrated using a cw ~1.3 μm Nd:YAG laser as the pump source. The best performance achieved with the fibre laser configuration displayed a threshold of 0.5 W and a slope efficiency of ~20% with respect to the absorbed pump power. The maximum output power achieved is about 0.18W. The current slope efficiency is ~45% of the Stokes efficiency limit. The four-fold improvement in the slope efficiency and threshold over past demonstrations is attributed to reduced levels of pump ESA and optimisation of the output coupling.