Passively mode-locked Nd:LuAG laser at 1338 nm

1. Introduction

Picosecond solid-state lasers operating in 1.3 μm spectral regions have been found to have wide applications in the fields of remote sensing, information storage, medicine as well as atmospheric pollution monitoring [1, 2]. Especially, due to the weak dispersion effect and low intrinsic loss of the silica fiber around 1.3 μm, such kinds of lasers are also considered as ideal light sources for optical fiber communications. Passive mode-locking lasers based on the ⁴F_3/2→⁴I_13/2 transitions in Nd³⁺ doped crystals have provided a convenient approach to generate 1.3 μm picosecond laser pulses. Nowadays, SESAM [3], SWCNT [4] and Graphene [5] have become popular saturable absorbers suitable for different wavelength regions. By using the above mentioned materials and other kinds of saturable absorption mode-locking like PPLN nonlinear mirror, picosecond lasers around 1.3 μm have been realized with various Nd³⁺ doped crystals, such as Nd:YAG [6], Nd:YVO₄ [2,7], Nd:GGG [8], Nd:LGGG [9] Nd:GdVO₄ [10] and Nd:YLF [11] et al. With Graphene or SWCNT as saturable absorbers, the obtained pulse durations were 11 ps from Nd:GdVO₄ laser [12] and 16.5 ps from Nd:YVO₄ laser [13], respectively. Based on nonlinear saturable absorber lens, the achieved pulse duration was varied from 6 ps (Nd:YVO₄ laser) [7] to 9.2 ps (Nd:GdVO₄ laser) [10]. By using the PPLN nonlinear saturable absorber mirror for mode-locking Nd:YVO₄ laser, the generated shortest pulse duration was 9.5 ps [14]. While, much shorter pulse could be generated by employing SESAMs, e.g., pulse durations of 5.36 ps from Nd:GGG laser [8], 4.55 ps from Nd:LGGG laser [9], 4.6 ps from Nd:YVO₄ laser and 5.7 ps from Nd:YLF laser [11]. Besides the short pulse generation ability, SESAM is superior to the other saturable absorbers with the advantage of free designable characteristics, that is, the non-saturable losses, absorption band, modulation depth, as well as saturation fluence could be exactly controlled to satisfy the specific requirements of lasers, making it becomes an effective and widely used tool for realizing 1.3 μm mode-locking lasers.

With respect to the Nd³⁺ doped gain medium for 1.3 μm lasers, besides the above mentioned laser crystals, lutetium aluminum garnet (Lu₃Al₅O₁₂, LuAG), a kind of garnet crystals isomorphic with YAG, is another potential host candidate for generating high performance laser radiations at 1.3 μm. Similar with YAG crystal, LuAG also possesses fantastic mechanical and physical properties such as large thermal conductivity, small thermal expansion as well as high fracture resistance [15–17]. In the case of Nd³⁺ ions doping, a large absorption cross section of 1.52 × 10⁻²⁰ cm² was found at the absorption peak of 809 nm with a broad FWHM of 5 nm, indicating Nd:LuAG crystal is suitable for high-brightness AlGaAs diode-laser pumping [18]. Besides, the significant radii difference between Nd³⁺ ions and Lu³⁺ ions leads to a strong lattice distortion and broad emission spectrum in Nd:LuAG crystal, which would further lead to the generation of relatively shorter pulses. For example, in the spectral region of$1.06\text{μm}$, the passive mode-locking Nd:LuAG laser yielded a 5.4 ps pulse which was much shorter than 19 ps obtained from passive mode-locking Nd:YAG laser [19]. Moreover, in combination with a large emission-cross section of 5 × 10⁻²⁰ cm² [20], the Nd:LuAG is considered to be a promising crystal for realizing 1.3 μm picosecond lasers. Retrospect the previous research work on Nd:LuAG crystal, both the CW and highly stable passive Q-switching Nd:LuAG lasers at 1.3 μm were demonstrated [21], however, to our best knowledge, report on mode-locking Nd:LuAG laser at 1.3 μm has not been found yet. In addition, only a small long-term pulse amplitude instability for the Q-switched Nd:LuAG laser at 1.3 μm has been observed in our previous work [21], which indicates the promising thermal properties of Nd:LuAG crystal, so the long-term mode-locking operation stability is also highly expected.

In this paper, a stable passive mode-locking Nd:LuAG laser in 1.3 μm spectral region with a semiconductor saturable absorber mirror is presented for the first time. The mode-locking Nd:LuAG laser yielded a maximum average power of 0.54 W, giving a slope efficiency of 10.2%. The shortest pulse duration was 18.3 ps under a repetition rate of 46.9 MHz, corresponding to a single pulse energy of 11.5 nJ and peak power of 628.4 W. This work has indicated the promising potential of Nd:LuAG crystal for realizing 1.3 μm mode-locking lasers.

2. Experimental setup

The mode-locking laser characteristics were investigated by using a 3.2 m resonator cavity, as schematically shown in Fig. 1. A fiber-coupled laser diode was used as the pump source with a maximum output power of 30 W. Its emission wavelength was located at 808 nm which was very close to the absorption peak of 809 nm for Nd:LuAG crystal. The Nd:LuAG crystal had a dimension of $3\times 3\times 10{\text{mm}}^3$and was grown by the Czochralski method according to the formula (Nd_0.01Lu_0.99)₃Al₅O₁₂, which structure and spectral properties had been demonstrated in Ref [22]. The pump light was focused into the 1% doped Nd:LuAG laser crystal through a 1:1 imaging module and with a pump spot diameter of 400 μm. The laser crystal was wrapped in indium foil and mounted in a copper block cooled to 18°C by water. In order to suppress the etalon effect and improve the stability of the mode-locking operation, the laser crystal was tilted with a small incidence angle with respect to the cavity axis [23]. M₁ was a flat mirror (R = ∞) working as input mirror with AR coated at 808 nm and HR coated around 1340 nm. Both M₂ (R = 800 mm) and M₃(R = 200 mm) were concave mirror and also HR coated around 1340 nm. The output coupler (OC, R = ∞) employed in the experiment had a transmission of 3.8%. A piece of commercial semiconductor saturable absorber mirror (SESAM) with a modulation depth of 1.2% and a relaxation time of 1 ps was used for starting and stabilizing the mode-locking lasers. The non-saturable loss and absorbance of SESAM at 1340 nm were 0.8% and 2%, respectively. By using the ABCD matrix propagation theory and considering the thermal lens effect, the beam waist radii were calculated to be about 155 μm inside the Nd:LuAG crystal and about 45 μm on SESAM.

 

Fig. 1 Experimental setup of the passive mode-locking Nd:LuAG laser.

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3. Experimental results and discussions

First, the continuous-wave (CW) operation of Nd:LuAG laser was realized for comparison by replacing the SESAM with a plane HR mirror. The average output power was measured by using a laser power meter (MAX 500AD, Coherent, USA). In the experiment, the Nd:LuAG crystal could absorb about 80% of the incident pump power. The average output powers versus absorbed pump powers are shown in Fig. 2. As the absorbed pump power augmented from 0.34 W to 6.72 W, the average output power almost increased linearly to a maximum average output power of 1.14 W, giving a slope efficiency of 18.6%.

 

Fig. 2 The power performance of Nd:LuAG lasers in CW and CWML regimes.

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With a SESAM employed in the resonator cavity and aligning the cavity carefully, the laser could easily run into metastable Q-switched and mode-locking (QML) regime while the absorbed pump power was increased higher than 3.74 W, corresponding to the average output power of 0.23 W (see in Fig. 2). When the absorbed pump power was increased to 5.44 W, stable continuous-wave mode-locking (CWML) operation could be realized. For a passive mode-locking laser with a SESAM, the minimum average output power P for sustaining CWML operation must satisfy: $P\ge {\left(F_{sat,L}{F}_{sat,A}{A}_{eff,L}{A}_{eff,A}\Delta R\right)}^{1/2}{T}_{oc}/{\tau }_R$ [24]. Here, $F_{sat,L}=hv/m{\sigma }_e$ is the saturation fluence of the gain medium and $A_{eff,L}=\pi {\omega }_L^2$ is the effective laser mode area inside the gain medium; $m$ is the number of passes through the gain medium per cavity round trip; ${\sigma }_e$ is the emission cross-section of the laser crystal; ${\omega }_L^{}$ is the beam radius inside the gain medium; $F_{sat,A}$、$\Delta R$ are the saturation fluence and the modulation depth of SESAM, respectively;$A_{eff,A}=\pi {\omega }_A^2$ is the effective laser mode area on SESAM; ${\tau }_R$ is the cavity round-trip time. Considering the parameters in our experiment: ${\omega }_L^{}$ = 155 μm, ${\omega }_A^{}$ = 45 μm, m = 2, $\Delta R$ = 1.2%, T_oc = 3.8%, $F_{sat,A}$ = 70 μJ/cm², ${\sigma }_e$ = 5 × 10⁻²⁰ cm², ${\tau }_R$ = 3.2 × 10⁻⁸ s, the minimum average output power P for stable CWML operation was calculated to be 0.43 W, which is much similar with that of 0.44 W obtained in the experiment. With the absorbed pump power further enhanced to reach 6.72 W, a maximum output power of 0.54 W was obtained. Although no power saturation effect was observed from Fig. 2, the pump power was not further increased since the SESAM tended to be damaged when the absorbed pump power exceeded 6.72 W. Due to the nonsaturable loss of 0.8% introduced by the SESAM, the slope efficiency in CWML regime was only 10.2% which was lower than that of 18.6% in CW regime. Since the Nd:LuAG crystal has an emission-cross section of 5 × 10⁻²⁰ cm² at 1.3 μm which is much smaller than that of 9.67 × 10⁻²⁰ cm² at 1.06 μm, the slope efficiency and average output power level obtained here are lower than those for 1.06 μm, where a slope efficiency of 60.2% and a maximum average output power of 1.05 W have been achieved [19]. Under the maximum output power, the M² factor of the mode-locked laser beam was measured by using a 10.0/90.0 knife-edge method. It is best fitted to be 1.13 in tangential plane and 1.16 in sagittal plane, respectively.

The pulse profiles of the passive mode-locking Nd:LuAG laser were monitored by a fast PIN photodiode with rise time of 400 ps and recorded by a digital oscilloscope (1 GHz bandwidth, Tektronix DPO 7102, USA). Figure 3 shows the temporal profiles of pulse trains under different time scales at the maximum absorbed pump power of 6.72 W. In the short time span of 200 ns, the pulse repetition rate was observed to be 46.9 MHz, which matched well with the cavity length of 3.2 m. And in the long time span of 100 ms, the stable pulse train demonstrated a high amplitude stability. In the experiment, a long-term stable mode-locking operation in hours was observed under no extra perturbation.

 

Fig. 3 The pulse trains of passive mode-locking Nd:LuAG laser under 200 ns (upper) and 100 ms (lower) time spans.

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To further evaluate the stability of the CWML operation, a spectrum analyzer (N9913A, Agilent Inc.) was employed for measuring the radio frequency (RF) spectrum of the CWML laser. The corresponding RF spectrum is shown in Fig. 4, a sharp peak with a signal-to-noise ratio up to 63 dB at the fundamental beat note around 46.9 MHz was observed. The absence of any side peaks revealed a stable continuous wave mode-locked operation, lacking of Q-switching instabilities. Besides, the RF spectrum in a wide span of 1 GHz (see inset of the Fig. 4) implied the single-pulse mode-locking operation.

 

Fig. 4 The RF spectrum of the passive mode-locking Nd:LuAG laser (resolution bandwidth (RBW:100 Hz), Inset: 1 GHz wide-span spectrum (RBW: 10 MHz).

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The pulse duration was measured with a commercial autocorrelator (APE GmbH, Pulse Check 150). The autocorrelation trace of the passive mode-locking Nd:LuAG laser is shown in Fig. 5. By assuming a sech² pulse shape, the pulse duration was measured to be 18.3 ps. The corresponding pulse energy and peak power were calculated to be 11.5 nJ and 628.4 W, respectively. The inset part in Fig. 5 is the output laser spectrum recorded by an optical spectrum analyzer with a resolution of 0.05 nm. The emission wavelength of the mode-locking laser was located at 1338.07 nm with a spectral FWHM of 0.144 nm. The corresponding time bandwidth product was calculated to be 0.441, which was 1.40 times of the Fourier transform limit value (0.315) for the sech² shaped pulse. In comparison with the $1.3\text{μm}$ passive mode-locking Nd:YAG laser with a spectral FWHM of 0.122 nm and pulse duration of 22.4 ps [6], the Nd:LuAG laser possesses a broader spectral bandwidth and shorter pulse duration.

 

Fig. 5 Autocorrelation trace of the passive mode-locking Nd:LuAG laser. Inset: the corresponding spectrum centered at 1338.07 nm with a FWHM of 0.144 nm.

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4. Conclusion

In conclusion, a stable passive mode-locking Nd:LuAG laser around 1.3 μm with a semiconductor saturable absorber mirror is demonstrated for the first time. A maximum average output power of $0.54\text{W}$at absorbed pump power of 6.72 W was obtained with a slope efficiency of 10.2%. A shortest pulse duration of 18.3 ps with high signal-to-noise ratio up to 63 dB under a pulse repetition rate of $47\text{MHz}$was achieved, corresponding to a single pulse energy of 11.3 nJ and peak power of 628.4 W. This work indicates that the Nd:LuAG crystals which possess excellent mechanical and physical properties are superior to Nd:YAG crystals in realizing mode-locked lasers at 1.3 μm.