1. Introduction

The development of application-oriented mid-infrared lasers is an important and hot research direction in laser community. Located in the atmospheric window, the strong absorption band of water and the safety zone of human eyes, 2 µm waveband lasers have important applications in the fields of transparent plastic material processing, atmospheric environment monitoring, clinical medical treatment, free space optical communication and so on. The trivalent thulium ion (Tm³⁺) has an abundant energy level structure, enabling thulium lasers to output emissions ranging from 1.8 µm to 2.3 µm with different laser transitions involved, as shown in Fig. 1(a). The popular ³F₄→³H₆ transition operates in the 2 µm region, and the ³H₄→³H₅ transition emits in the 2.3 µm region [1–4]. The ³F₄→³H₆ laser transition at 2 µm has been extensively studied and realized in a wide variety of host materials. Tm³⁺ ions can be excited around 800 nm from the ground level of ³H₆ to the ³H₄ level using the commercial AlGaAs laser diodes (LDs). Then the doping-concentration-dependent cross relaxation (CR) process of ³H₆ + ³H₄ →³F₄ + ³F₄ can support a high slope efficiency well beyond stokes limit for the ³F₄→³H₆ laser transition. However, the high power laser operation at 2.3 µm starting from the ³H₄ level remains to be a challenge [5,6]. Several strategies have been developed to achieve lasing from the upper laser level of ³H₄, for example, using the fluorozirconate (ZBLAN) fibers or tellurite glass to minimize non-radiative decay rates [7,8], or Yb-Tm co-doping to depopulate the ³F₄ level [9,10].

 

Fig. 1. The energy levels of Tm³⁺ ions (a); the diagrammatic sketch for the experimental setup (b).

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The co-lasing on the ³F₄→³H₆ transition may open a new avenue to boost the transition of ³H₄→³H₅ in Tm-singly-doped bulk material. The laser level of ³F₄ can be depopulated by 2 µm laser emission to the lower-lying energy level of ³H₆. This can weaken the population trapping in ³F₄ level, and recycle the energy back into the upper laser level of ³H₄. Furthermore, simultaneous dual-wavelength laser operation also has the effect of decreasing the overall quantum defect. In 1991, R. G. Smart et al. realized the 1.9 and 2.3 µm dual-wavelength laser operation in Tm-doped ZBLAN fiber pumped by a Ti:sapphire laser [11]. In 1992, R. M. Percival et al. also obtained simultaneous 1.9 and 2.3 µm laser output in Tm-doped ZBLAN fiber laser, with the maximum output power of 90 and 45 mW, respectively [12]. Recently, S. V. Muravyev et al. realized the dual-wavelength continuous-wave (CW) lasing at 1.95 and 2.3 µm in Tm-doped tellurite fiber with total maximum powers of ∼43 mW and 1.7 mW, respectively [13]. C. L. Jia et al. reported a passively Q-switched Tm³⁺:ZBLAN all fiber laser at 1895nm and 2315 nm using graphene as saturable absorber, generating pulses with 4.5 µs pulse width, 1.625 mW average output power at 1895nm and 4.9 µs pulse width, 0.77 mW average output power at 2315 nm [14].

In this paper, the first dual-wavelength laser operation in Tm-singly-doped bulk material at 1.9 and 2.3 µm was reported. With a compact configuration of CW 785 nm LD pumping, simultaneous CW laser oscillation of Tm:YLF around 1908 and 2305 nm were successfully realized at room temperature by using three-mirror cavity. The two lasing waves share the same input mirror but use different output couplers in the three-mirror coupled resonator, enabling the dual-wavelength laser performance to be independently adjusted. Under an absorbed pump power of 15.2 W, the maximum CW output powers of 5.49 W around 1908nm and 1.12 W around 2305 nm were simultaneously obtained, corresponding to a total optical-to-optical conversion efficiency of 43.5%. A Cr²⁺:ZnSe was further used as saturable absorber to realize the dual-wavelength passive Q-switching, generating pulses with pulse widths of 554 ns at 1.9 µm and 4 µs at 2.3 µm. Due to the quite different absorption characteristics of water at two wavebands, this simultaneous dual-wavelength Tm-doped solid-state laser at 1.9 and 2.3 µm can be used to explore more potential in the medical application.

2. Experimental setup

The experimental arrangement of the dual-wavelength laser with a three-mirror configuration is shown schematically in Fig. 1(b). The pump source was a fiber coupled CW 785 nm LD with a core diameter of 400 µm and a numerical aperture of 0.22. Its radiation was coupled into the laser crystal by a focusing optical system, generating a 200 µm radius of pump beam within the laser crystal. The laser material with low Tm³⁺ doping concentration and low phonon energy is the normal condition for achieving lasing from the ³H₄ level. Low doping concentration can weaken the Tm³⁺ ion-pair cross relaxation under ∼800 nm pumping, and low-phonon-energy host material can ensure a relatively longer lifetime of the ³H₄ level [15–17]. In the experiment, a 1.5 at.%, 4 × 4 × 8 mm³ and a-cut Tm:YLF crystal was wrapped with indium foil and mounted in copper block cooled by water at a temperature of 16°C. The three-mirror laser cavity was formed by an input mirror M1 and two output couplers M2 and M3. M1 was a concave mirror with a 300 mm radius of curvature, anti-reflection coated at 785 nm on one surface, high-reflection coated at 1850-2050nm and 2250-2350 nm and high-transmission coated at 785 nm on the other surface. One side of the plano-plane output coupler M2 was coated to be partially reflecting at 2250-2350 nm (R = 97%) and highly transmitting at 1850-2050nm. The remaining side of M2 was anti-reflective at both lasing wavelengths. On the other hand, one side of the plano-plane output coupler M3 was coated to be partially reflecting at 1850-2050nm (R = 95%) and highly transmitting at 2250-2350 nm. The remaining side of M3 was anti-reflective at both lasing wavelengths. The cavity length for 2.3 µm oscillation was 2 cm and the total cavity length was 5 cm. The 1.9 and 2.3 µm laser beams were separated by a filter that was high-reflection coated at 1850-2050nm (R > 99.8%) and high-transmission coated at 2250-2350 nm (T = 98.6%). A three-mirror configuration is favorable to achieve simultaneous dual-wavelength lasing with a new degree of freedom.

3. Experimental results and discussions

To begin with, the feasibility of achieving dual-wavelength lasing in Tm:YLF using the three-mirror cavity was studied. Under a given pump power, two spectral lines with central wavelength of 1908 and 2305 nm were observed, as shown in Fig. 2. The lasing spectrum was measured by an optical spectrum analyzer (Yokogawa, AQ6375). The two wavelengths respectively correspond to the ³F₄→³H₆ and ³H₄→³H₅ transition, demonstrating the success of achieving simultaneous dual-wavelength laser actions. Taking advantage of three-mirror laser cavity, the interaction between the two laser transition processes were further studied. Under the simultaneous dual-wavelength lasing condition, one output coupler can be appropriately misaligned but the other output coupler remained unchanged. So the cavity-misalignment for one laser transition was introduced as a new degree of freedom for observing and adjusting laser operation at the other transition. In the first case, the three-mirror resonator was changed in such a way that the 2.3 µm output coupler was fixed and the 1.9 µm output coupler was misaligned. The dependence of output powers at the two wavelengths on the cavity-misalignment was shown in Fig. 3(a). When the 1.9 µm output coupler was misaligned from the optimized position, the output power for 1.9 µm was decreased from 4.69 to 0 W. Then it could return to the initial power level by realigning the output coupler. In this adjustment process, the 2.3 µm output power presented a similar tendency as that of 1.9 µm power. But the difference is 2.3 µm laser kept oscillating all the time. In the second case with the 1.9 µm output coupler fixed, the output powers at the two wavelengths were also synchronously varied with cavity-misalignment for 2.3 µm laser, as shown in Fig. 3(b). The interaction mechanism between the two laser transitions lies in the competition effect of population in ³H₄ and ³F₄ level. The introduction of cavity-misalignment for 1.9 µm laser will result in the accumulation of population in the ³F₄ level. The population in ³H₄ level was forced to be reduced, decreasing the output power accordingly. The other case is vice versa.

 

Fig. 2. The lasing spectrum of dual-wavelength Tm:YLF laser in the CW and PQS mode.

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Fig. 3. The dependence of CW output powers at the two wavelengths on the cavity-misalignment, with 2.3 µm output coupler fixed (a); with 1.9 µm output coupler fixed (b).

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Figure 4 shows the comparison of laser performance of Tm:YLF at 1.9 and 2.3 µm under dual-wavelength and single-wavelength scheme. The dual-wavelength case denotes that the 1.9 and 2.3 µm output couplers were used at the same time to achieve two-wavelengths output, while single output coupler was separately used in the single-wavelength case. Compared to the 2.3 µm laser performance in the single-wavelength case, higher output powers were obtained with increasing the pump power in the dual-wavelength case. A maximum output power of 1.12 W at 2.3 µm was achieved, increased by 14.3% than the single-lasing case. With regard to the 1.9 µm laser performance, a slight decrease of output power in the dual-wavelength case was observed. Based on the experimental results, several comparative advantages of dual-wavelength can be summarized as follows. Firstly, the trapped population in ³F₄ level can be depopulated by self-based stimulated emission of ³F₄→³H₆ at 1.9 µm. This will in turn increase the population at ³H₄ level, boosting the transition of ³H₄→³H₅ at 2.3 µm. Secondly, it was found that the output powers for the two wavelengths were all nearly increased linearly with the pump power in the dual-wavelength case. This is because the lower and upper energy levels corresponding to the ³H₄→³H₅ and ³F₄→³H₆ transition are not directly related. So the competition effect in simultaneous dual-wavelength laser operation can be greatly reduced. Thirdly, a total optical-to-optical conversion efficiency of 43.5% was obtained in the dual-wavelength case under an absorbed pump power of 15.2 W, greatly larger than the 36.3% and 6.45% obtained in the single wavelength laser at 1.9 and 2.3 µm, respectively. Furthermore, from the perspective of application, the simultaneous dual-wavelength Tm:YLF laser can be used to explore more potential in the medical application. Indeed the biological tissue absorption at 2.3 µm is much weaker in comparison with 1.9 µm absorption [18]. So the coagulation depth in the tissue under the 2.3 µm radiation is stronger. On the other hands, the 1.9 µm wave provides the good ablation effect required for the surgery [19]. Then the simultaneous dual-wavelength Tm-doped laser can be developed for precise surgery with good coagulation (in urology, genecology and so on) as a replacement of combination of Er- and Tm-lasers.

 

Fig. 4. The comparison of CW laser performance of Tm:YLF at 1.9 and 2.3 µm under dual-wavelength- and single-lasing scheme.

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A proof-of-principle experiment on simultaneous dual-wavelength passive Q-switching of Tm:YLF using Cr²⁺:ZnSe as saturable absorber was further demonstrated. Cr²⁺:ZnSe has wide absorption spectra ranging from 1.5 µm to 2.4 µm, large saturation cross-section, small saturation energy and good physical characteristics, enabling Cr²⁺:ZnSe to act as ideal saturable absorbers for passive Q-switched (PQS) Tm-doped lasers operating in the spectral range of 1.9-2.3 µm [20–22]. In the experiment, the Cr²⁺:ZnSe absorber with initial transmission of 88.2% at 1908nm and 94.1% at 2305 nm was placed before the output mirror M2 to realize the dual-wavelength Q-switching operation. An aperture was also placed between Tm:YLF and Cr²⁺:ZnSe to eliminate the influence of unabsorbed pumping light on Cr²⁺:ZnSe. When the corresponding single output coupler was used, both the 1.9 and 2.3 µm laser can achieve the single wavelength PQS operation, generating conventional Q-switched pulses in both cases. The pulses were recorded by a Tektronix DPO7104C digital oscilloscope (1-GHz bandwidth, 5-Gs/s sampling rate) and a fast InGaAs photodiode (Thorlabs, DET10D/M).

As shown in Fig. 5, the oscillating thresholds for the dual-wavelength PQS laser were increased greatly, and the maximum average output powers of 202 and 77 mW were achieved at 1.9 and 2.3 µm, respectively. It was also found that the PQS lasing spectrum for the ³F₄→³H₆ transition was shifted from 1908nm to 1880nm. The ³H₄→³H₅ transition exhibited little change in comparison with the CW mode. In this dual-wavelength passive Q-switching, the 2.3 µm transition had a higher oscillating threshold than that of 1.9 µm. Before the 2.3 µm laser started to oscillate, the waveform of 1.9 µm laser presented typical single-pulse oscillogram, as shown in Fig. 6(a). When the laser threshold of 2.3 µm was reached (according to the optical spectrum), sporadic weak 2.3 µm pulse-group appeared in two adjacent 1.9 µm pulse, as shown in Fig. 6(b). With the increase of pump power, the 2.3 µm pulse-group evenly appeared in every two adjacent 1.9 µm pulses. Figure 6(c) showed the simultaneous dual-wavelength pulse train and single pulse at the two wavelengths obtained at the maximum average output power. The corresponding single pulse waveforms were shown in Figs. 6(d) and 6(e), respectively. It was interested that a group of 2.3 µm pulses were formed between two adjacent 1.9 µm pulses. A similar phenomenon has also been reported in the doubly passively Q-switched Nd:YVO₄ laser at 1064 and 1342 nm by our group [23]. The 1.9 µm pulses and 2.3 µm pulse-group had a similar repetition rate, varying from 0.97 kHz to 1.6 kHz with the pump power, as shown in Figs. 7(a) and 7(b). But the internal repetition rate of 2.3 µm pulse-group had a higher value of 13.8-24.8 kHz. The pulse widths for 1.9 and 2.3 µm laser pulses presented the variations of 657-554 ns and 4.9-4 µs, respectively. In the simultaneous dual-wavelength PQS operation, it was found that both 1.9 and 2.3 µm laser was more sensitive to the misalignment of output coupler at the other wavelength. It was hard to adjust the PQS laser performance by misaligning the other output coupler.

 

Fig. 5. Average output powers of dual-wavelength PQS laser.

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Fig. 6. The pulse train of the dual-wavelength PQS laser obtained near the threshold of 2.3 µm laser, slightly below threshold (a) and slightly above threshold (b); Typical pulse train obtained at maximum average output power (c), the corresponding single pulse at 1.9 µm (d), and the single pulse at 2.3 µm (e).

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Fig. 7. Pulse characteristics of the dual-wavelength PQS laser, 1.9 µm (a) and 2.3 µm (b).

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The physical explanations for the intermittent oscillation as well as the formation of 2.3 µm pulse-group can be understood as follows. The upper state lifetime of 2.3 µm transition is ∼1 ms, one order of magnitude shorter than the lifetime of the ³F₄ level (∼10 ms). Furthermore, the modulation depth of Cr²⁺:ZnSe at 2.3 µm is smaller than that at 1.9 µm. So the 2.3 µm transition acted on by the Cr²⁺:ZnSe crystal would be opened up earlier than that of the 1.9 µm transition. The population would be accumulated in the upper level ³F₄ for the 1.9 µm due to the close of Q-switch. Then the inversion populations related to 2.3 µm would be partially supplemented by the enhanced up conversion processes of (³F₄, ³F₄ → ³H₄, ³H₆). This would make the populations at ³H₄ level quickly achieve such a degree that made the 2.3 µm transition oscillate again, resulting in the generation of a group of pulses.

An interesting question is that the Tm:YLF laser can give the gain inside Cr²⁺:ZnSe intracavity pumped by the 1.9 µm pulses. Is there additional contribution to the 2.3 µm pulse oscillation induced by the gain switching of Cr²⁺:ZnSe? From the perspective of gain switching, the laser emission needs to be amplified in a number of resonator round trips. So the gain-switched laser pulse sets in with a certain delay with the pumping pulse. However, as shown in Fig. 6, the 2.3 µm pulse started to oscillate ∼500 µs in advance with regard to the 1.9 µm pulse. Moreover, the 2.3 µm pulse width is far greater than that of 1.9 µm. From the perspective of Q-switching, the pulse parameters (pulse repetition rate and pulse width) obtained in the dual-wavelength PQS laser were similar with that generated in the single wavelength PQS operation. Therefore, the 2.3 µm pulses obtained in this dual-wavelength laser should not be caused by the gain-switched Cr:Znse laser. However, a novel Cr²⁺:ZnSe-based 2.3 µm amplifier pumped by the dual-wavelength Tm:YLF laser at 1.9 and 2.3 µm may be further designed with appropriate Cr²⁺:ZnSe parameters. This will enable the integration of the 2.3 µm seed injection and 1.9 µm self-pumping in one pass.

4. Conclusion

In conclusion, a 785 nm LD pumped dual-wavelength Tm:YLF laser emitting at 1.9 and 2.3 µm was demonstrated. Co-lasing on the ³F₄→³H₆ transition was used to depopulate the trapped population in ³F₄ level. By exploiting the three-mirror configuration, the dual-wavelength CW and passive Q-switched operation was obtained. In the CW mode, the maximum output powers of 5.49 W at 1.9 µm and 1.12 W at 2.3 µm were simultaneously obtained, corresponding to a total optical-to-optical conversion efficiency of 43.5%. In the passive Q-switched operation, the intermittent oscillation of 1.9 and 2.3 µm was reported in the Cr²⁺:ZnSe-based passively Q-switched laser. A group of 2.3 µm pulses between the two adjacent 1.9 µm pulses was also observed in the dual-wavelength pulsed laser. The corresponding physical explanations for the intermittent oscillation as well as the related phenomena were given. Further research activities should be concentrated on the power scaling and pulse synchronization of the dual-wavelength Q-switched laser.