1. Introduction

Due to the features of eye-safe and high atmospheric transmission, nanosecond laser sources operating at the long-wavelength region (>2050 nm) of 2 μm wavelength range can find many applications in free space optical communication, spectroscopy and gas detection [1]. In surgical applications, the penetration depths in soft tissue distinguish between two close wavelengths. The 2.1 μm laser has a penetration depth of 300 μm, while the 2 μm laser penetrates only 100 μm [2]. Furthermore, 3-5 μm ZnGeP₂ optical parametric oscillators can ideally be pumped with lasers around 2.1 μm since ZnGeP₂ typically suffers high optical loss induced by growth defects that becomes significant at wavelengths below ~2.1 μm [3]. The trivalent thulium ion (Tm³⁺) have the most interesting transition around 2 µm that starts in the ³F₄ manifold of Tm³⁺ and ends in a thermally populated Stark level of the ³H₆ ground state. Tm-doped laser materials have broadband absorption around 800 nm which allow direct pumping of Tm³⁺ ions by AlGaAs laser diode (LD). Furthermore, the doping-concentration-dependent cross-relaxation process in Tm³⁺ ions can support a high-slope-efficiency well beyond stokes limit. However, a problem for most of the Tm-doped crystals is that the laser emissions of Tm³⁺ usually cover a short-wavelength range of ∼1840-2030 nm in common host materials of vanadates, lithium yttrium fluoride (YLF), and yttrium aluminum garnet (YAG) [4–7]. The wavelengths above 2.05 μm can be addressed by holmium (Ho)-doped or Tm/Ho-codoped gain medium. However, Tm/Ho-codoped laser involves complex energy transfer processes between Tm³⁺ and Ho³⁺ ions, and the Ho-based laser is a typical complex system that involves the use of expensive 1.9 μm LD or Tm-based lasers as the pumping source.

Sesquioxides offer great properties as host media for Tm³⁺ doping, such as high thermal conductivities and low phonon energies [8]. A distinguishing feature of Tm-doped sesquioxides is the strong ground-level Stark splitting, resulting in a broadband and structured emission spectra that extend beyond 2.1 μm. Due to the high melting-point (~2450$℃$) of sesquioxides, it is a challenge to grow high-quality large size crystals with conventional crystal growth techniques_. Transparent sesquioxides ceramics have beening attracted much attention because they can be fabricated at a much lower temperature with high optical quality, large size and high dopant concentration compared to single crystals. Up to now, efficient and high-power laser operations have been demonstrated in Tm:Lu₂O₃ and Tm:Y₂O₃ ceramic [9–13]. However, these lasers always operate at random multi-wavelengths emissions around gain peaks in the free running mode. To make Tm:Lu₂O₃ or Tm:Y₂O₃ laser emits at 2.1 μm, a wavelength selector is required to suppress strong lasing around ~2060 nm. Due to the wavelength-dependent diffraction efficiency, low insertion loss, and excellent thermal stability, volume Bragg gratings (VBGs) offer an efficient way to select a specific wavelength with linewidth on sub-nanometer scale [14, 15]. Therefore, a VBG-based 2.1 μm narrow-linewidth short-pulsed Tm-doped sesquioxide laser pumped by the commercially available ~800 nm LD become more attractive, since this laser can be a cost-effective alternative to Ho-doped lasers, and have promising applications in Doppler wind lidar and high resolution gas spectroscopy [16].

For the short-pulsed operation, passive Q-switching has advantages over the active Q-switching in terms of compactness, simplicity and low cost in design. Conventional “Slow”bulk SAs, such as Cr:ZnSe and Cr:ZnS, enable the generation of high-energy pulses at relatively low repetition rates (~10kHz) [17, 18]. Promising two-dimensional materials, represented by graphene, topological insulators, transition metal dichalcogenides, and black phosphorus, have aroused considerable attention as “fast” SAs for short pulse generation at 2 µm waveband with high pulse repetition rates [19–21]. Benefitting from the broadband surface plasmon resonance (SPR) tunability and ultrafast nonlinear response, recently gold nanorods (GNRs) have been successfully exploited as SAs for the generation of short pulses [22, 23]. Our group have experimentally realized the GNRs-based nanosecond passive Q-switching of a LD end-pumped Tm:YAG laser emitting multi-wavelengths around 2000 nm [24]. The potential of GNRs SAs for generating short pulses at the longer wavelength have not been exploited.

In this paper, a 785 nm LD-pumped nanosecond Tm:Y₂O₃ ceramic laser at 2.1 μm was realized by using VBG to provide a spectrally controlled feedback and using GNRs as the passive Q-switcher. In the continuous-wave (CW) mode, a compact laser based on Tm:Y₂O₃ ceramic generated 1.1 W with a slope efficiency of 14%. The central wavelength of the Tm:Y₂O₃ ceramic laser was locked at 2101.5 nm with a linewidth (FWHM) of 0.4 nm. In the Q-switched mode, pulses with a maximum average output power of 455 mW, a minimum pulse width of 609 ns, and a pulse repetition rate of 79 kHz were achieved under the absorbed pump power of 12 W. Our work provides a cost-effective method for developing a miniaturized 2.1 μm Tm-based laser operating in short-pulse state.

2. Experimental setup

For the laser experiments, a very simple and compact setup was chosen, as shown schematically in Fig. 1. A linear cavity was employed for both CW and Q-switching laser experiments. Due to the fact that the diffraction efficiency of VBG strongly depends on incident wavelength, even the closely spaced axial laser modes can be reflected with different efficiency. Therefore, a reflecting-VBG was placed in the laser resonator as a wavelength selector and a pump input mirror simultaneously to achieve the wavelength-locking and narrow-linewidth operation. In order to efficiently integrate the GNRs SA with the laser resonator, the GNRs was directly deposited on one surface of the plano-plane output mirror, forming a compact configuration of saturable output coupler (SOC) to reduce the insertion losses. The above two design allows for a more compact and efficient laser in short-pulse operation.

 

Fig. 1 Schematic of the VBG-locked Tm:Y₂O₃ ceramic laser with the inset showing the TEM image of GNRs.

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A fiber-coupled 785 nm LD was used as the pump source. The output-coupling fiber of the LD had a core diameter of 400 μm and a numerical aperture of 0.22. The laser beam of the pump was reimaged into the ceramic with a spot radius of 100 μm by a lens assembly. The 12 mm long laser resonator comprised a VBG, a Tm:Y₂O₃ ceramic, and a GNRs-based SOC. A short cavity roundtrip time can lead to a significant reduction of the pulse duration in Q-switching operation. The reflecting-VBG (OptiGrate Corporation) was designed to have a central wavelength of 2101 nm and >99% diffraction efficiency with a spectral width (FWHM) ~1 nm. It was mounted in a copper heat sink to ensure good thermal contact. The transmittance of 4.6 mm-thick VBG at the pump wavelength was measured to be 67%. The 2 at.% 4 × 4 × 8.5 mm³ Tm:Y₂O₃ ceramic was optically polished, and both ends of the ceramic were antireflection (AR) coated at 760-810 nm and 1950-2150 nm. The Q-switched laser pulses were detected by using a fast InGaAs photodiode (DET10D/M, Thorlabs) and recorded with an oscilloscope with 1 GHz bandwidth, and 5 Gs/s sampling rate (DPO7104C, Tektronix). Spectrum of the Tm:Y₂O₃ ceramic laser was analyzed by an optical spectrum analyzer (AQ6375, Yokogawa).

3. Experimental results and discussions

The VBG-locked CW laser operation was investigated first. Three available plano-plane mirrors with the transmittance of 2%, 5% and 10% at 1950-2150 nm were used as output couplers, respectively. However, laser oscillation could not be obtained with the 5% and 10% output coupler. High output coupling would result in an increased inversion. Then the increased upconversion losses that start from the upper laser level ³F₄ of Tm³⁺ ions would be introduced. So the low gain at 2.1 μm for Tm:Y₂O₃ failed to support the laser oscillation. For the 2% output coupler, a maximum output power of 1.1 W was obtained with a slope efficiency of 14%, as shown in Fig. 2. The linearly increase of output power indicated that further power scaling should be achieved by increasing the pump power. The absorbed pump power used in Fig. 2 was measured under lasing conditions as follows. The pump power transmitted through the VBG was first measured, which was denoted as the incident pump power. Then the residual pump light passing through the output coupler was separated from the laser radiation by a bandpass filter. Further considering the partial reflection by the output coupler, the pump absorption efficiency under lasing conditions was determined to be 45% with respect to the incident pump power. The central wavelength of the VBG-locked Tm:Y₂O₃ ceramic laser was locked at 2101.5 nm with a linewidth of 0.4 nm, as shown in Fig. 3. Further optimization of the laser performance at 2100 nm should be possible by using other output couplers with the transmittance around 2%.

 

Fig. 2 Laser performance of VBG-locked 2.1μm Tm:Y₂O₃ ceramic in CW and passively Q-switching operation.

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Fig. 3 Lasing spectrum Tm:Y₂O₃ ceramic in free-running and VBG-locked operation.

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For comparison, the laser performance of Tm:Y₂O₃ ceramic laser was investigated in free-running mode (without VBG to lock the operating wavelength). A plano-plane dichroic mirror with high reflectivity at 1950-2150 nm (R > 99.8%) and high transmittance at 785 nm (T > 98%) was used as the pump input mirror. Better experimental results were achieved with the 5% output coupler. An output power of 3.2 W was achieved at an absorbed pump power of 12 W, corresponding to a slope efficiency of 35%. The comparatively low slope efficiency at 2100 nm could be attributed to the smaller emission cross section of Tm:Y₂O₃ at 2100 nm (~1 × 10⁻²¹ cm²) than that at 2050 nm (~4 × 10⁻²¹ cm²) [13]. Owing to the broadband gain around 2050 nm for Tm:Y₂O₃, typical multi-wavelengths emissions appear in the free running mode, as shown in Fig. 3.

For the passive Q-switching, relatively small emission cross section at lasing wavelength can enhance the energy storage capacity, resulting in an improvement of passive Q-switching performance in terms of increased pulse energy and decreased pulse width [25]. The GNRs sample was provided by Nanjing XFNANO Materials Tech Co., Ltd. The transmission electron microscopy (TEM) image of GNRs was measured with a Tecnai G2 F20 TEM (FEI), as shown in Fig. 1. The GNRs had an average length of 261.1 nm and an average diameter of 17.4 nm, resulting in an average aspect ratio of 15 (corresponding to the longitudinal SPR wavelength of 1845 nm in aqueous solution). The GNRs-based SOC was formed by drop-casting the solution onto an output coupler with 2%-transmissivity at 2.1 μm, followed by a slow drying at room temperature. The used output coupler was a plano-plane JGS1 quartz glass substrate with partial-reflection coatings on one surface and anti-reflection coatings on the other surface. The GNRs were deposited on the surface with partial-reflection coatings. Benefitting from the near-field SPR coupling effect when more plasmonic GNRs are close to one another, the SPR response wavelength range can be extended beyond 2.1 μm, as demonstrated in [24]. The initial transmission of SOC deposited with GNRs was measured to be about 1% at 2100 nm. So it is suitable to serve as the passive Q-switcher for a 2.1 μm laser.

By inserting the GNRs-based SOC into the cavity, typical passive Q-switching operation was realized under a wide range of pump powers. The dependence of the average output power on the absorbed pump power is shown in Fig. 2. The maximum average output power of 455 mW was achieved under an absorbed pump power of 12 W, corresponding to a CW to Q-switching conversion efficiency of 41.4%. With the increase in pump power, the pulse repetition rates increased from 22 to 79 kHz (Fig. 4(a)). This increase was accompanied by a decrease of the pulse width (determined as full width at half-maximum (FWHM)] from 968 to 609 ns (Fig. 4(b)). Figure 4(c) and 4(d) also depict the pulse peak power and pulse energy versus the absorbed pump powers. Figure 5 shows the temporal waveform of the 609 ns pulse and corresponding pulse train. The single pulses had symmetric shape, indicating the output coupling used in our experiment was suitable for balancing the rise and fall time of pulses [26]. The pulse-to-pulse intensity instability in the pulse trains was <6%. The instability should be mainly attributed to heating of the SA by the non-absorbed pump radiation. The obtained pulse width is much smaller than that obtained in the 2.1 μm passively Q-switched Ho-doped bulk lasers with graphene as SAs [27–29]. Compared with the conventional bulk SAs (such as Cr:ZnS and Cr:ZnSe), the feasibility of GNRs-based SAs for generation of pulses at high repetitions is also interesting for medical and material processing applications [30].

 

Fig. 4 Pulse repetition rates (a), pulse width (b), pulse peak power (c) and pulse energy (d) versus the absorbed pump power.

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Fig. 5 Typical pulse train and single pulse with the pulse width of 609 ns at the repetition rate of 79 kHz.

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4. Conclusions

A 2.1 μm CW and passively Q-switched Tm:Y₂O₃ ceramic laser utilizing a VBG as wavelength selector and GNRs as SA, have been reported. The VBG acted as a wavelength selector and a pump input mirror to obtain wavelength locking at 2.1 μm. In the CW mode, the VBG-locked Tm:Y₂O₃ ceramic laser generated 1.1 W at 2101.5 nm with a linewidth (FWHM) of 0.4 nm. In the passively Q-switched operation, pulses with a maximum average output power of 455 mW, a minimum pulse width of 609 ns, and a pulse repetition rate of 79 kHz were achieved. Our work provides a cost-effective method for developing a miniaturized 2.1 μm nanosecond Tm-based laser, having promising applications in Doppler wind lidar and high resolution gas spectroscopy.