Tunable and Passively Mode-Locking Nd_0.01:Gd_0.89La_0.1NbO₄ Picosecond Laser

Abstract

A high-quality Nd_0.01:Gd_0.89La_0.1NbO₄ (Nd:GLNO) crystal is grown by the Czochralski method, demonstrating wide absorption and fluorescence spectra and advantage for producing ultrafast laser pulses. In this paper, the tunable and passively mode-locking Nd:GLNO lasers are characterized for the first time. The tuning coverage is 34.87 nm ranging from 1058.05 to 1092.92 nm with a maximum output power of 4.6 W at 1065.29 nm. A stable continuous-wave (CW) passively mode-locking Nd:GLNO laser is achieved at 1065.26 nm, delivering a pulse width of 9.1 ps and a maximum CW mode-locking output power of 0.27 W.

1. Introduction

Ultrafast lasers have been applied in various fields, such as high-precision micro machining, aerospace, and medical diagnostics [1,2]. Benefiting from their low quantum defects, wide gain bandwidth, and simple three-level electronic structure, Yb³⁺-doped laser mediums attract widespread attention in the 1 μm band [3,4,5]. However, the overlap of absorption and emission bands can bring re-absorption loss, resulting in high laser threshold. Compared with Yb³⁺-doped gain mediums, Nd³⁺-doped crystals have no re-absorption loss and are used in low-threshold and high-efficiency ultrafast laser. As is known, the typical gain bandwidth of the Nd³⁺-doped laser materials is narrow, e.g., the gain bandwidth of the Nd:YVO₄ and Nd:YAG crystals were measured to be only 0.96 and 0.80 nm, respectively [6,7]. For this reason, considerable efforts have been made to explore novel Nd³⁺-doped laser materials with a broad gain bandwidth. The pulse duration of 19.2 ps at 1064 nm was achieved in a passively mode-locked Nd:YVO₄ laser in 2008 [8]. Mohammad et al. [9]. reported pulse duration of 16 ps generation in a Nd:GdVO₄ crystal in 2017. He et al. [10]. obtained 3.8 ps pulse duration at a repetition rate of 112 MHz in a Nd:GdYVO₄ crystal. Previously, theoretical and experimental results have demonstrated that Nd³⁺-doped disordered crystals possess broad emission spectra and are suitable for generating ultrashort lasers [11,12,13].

In the last decade, researchers have invested tremendous enthusiasm into extending Nd³⁺-doped disordered crystals family and exploring their excellent properties. In 2017, a novel disordered crystal Nd_0.01:Gd_0.89La_0.1NbO₄ (Nd:GLNO) was successfully grown by Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences [14]. Owing to La³⁺ having a relatively large ionic radius in the lanthanide system, the La³⁺-doped disordered crystals exhibit a wider fluorescence bandwidth [15]. Moreover, the difference in ionic radii between La³⁺ and Gd³⁺ ions is small, denoting the Nd:GLNO crystal possesses excellent lattice matching and thermal property [16,17]. The fluorescence lifetime and the radiative lifetime of Nd:GLNO crystal was obtained to be 176.1 μs and 184.5 μs, respectively. The luminescent quantum efficiency of the ⁴F_3/2 level was estimated to be 95.4% [18]. Ma et al. presented the CW and passively Q-switched Nd:GLNO lasers with Cr⁴⁺:YAG crystal and PdSe₂ as saturable absorbers (SAs), respectively, in 2018 and 2020 [15,19]. Unfortunately, the tunable and CW mode-locking Nd:GLNO crystal lasers have not been studied to date.

In this paper, the absorption and florescence spectra of the Nd:GLNO crystal were systematically investigated demonstrating a wide absorption and emission band. A tunable operation Nd:GLNO crystal laser was realized with a tuning range of 34.87 nm from 1058.05 to 1092.92 nm. By employing a semiconductor saturable absorber mirror (SESAM) as SA, a stable CW mode-locking Nd:GLNO crystal laser was achieved, generating the shortest pulse duration of 9.1 ps and the maximum mode-locking output power of 0.27 W.

2. Experimental Setup

Figure 1 demonstrates schematic setups of the Nd:GLNO lasers. The 808 nm laser diode was chosen as a pump source with a core diameter of 400 μm and a numerical aperture (NA) of 0.22. The size of the c-cut Nd:GLNO crystal was 2 × 2 × 5 mm³. To effectively reduce the influence of thermal effects, the laser crystal was covered with indium and embedded into a copper block. The cooling temperature of the copper block was controlled at 15.5 °C. The total laser cavity length of the mode-locking and tunable lasers was 1.94 m and 0.33 m, respectively. Mirrors M₁, M₂, M₄, M₅ and M₆ were all processed with anti-reflection (AR) coating around 808 nm and high-reflection coating (HR, R > 99.9%) at 1030–1100 nm. The curvature radii were R = ∞, R = 200, R = ∞, R = 300 and R = 150 mm, respectively. The output mirror M₃ was partial transmittances (T) coated at 1030–1100 nm (T = 1, 10, 15%, 25% are available). A quartz birefringent filter (BF) was employed in tunable laser cavity to achieve laser tuning operation. The parameters of the SESAM are as follows: saturable fluence is 90 μJ/cm², absorptance is 1.5%, a modulation depth is 0.8%, damage threshold is 30 mJ/cm², and a relaxation time is 1 ps. A laser power meter (Fieldmax-II, PM10) was used for measuring laser power. The laser output spectra and pulse width of mode-locked Nd:GLNO laser were measured by a spectrometer (Avantes, AcaSpec-3468-NIR256-2.2) and a commercial autocorrelator (APE Pulse Check, 150), respectively. The typical pulse profile and pulse train were recorded by a digital oscilloscope (R&S, RTO 2012) together with a fast InGaAs photon detector (New Focus, 1611).

3. Results and Discussion

Figure 2 presents the absorption and fluorescence spectra of the c-cut Nd:GLNO crystal at room temperature. As shown in Figure 2a, the absorption peak is at 808 nm and FWHM is 13 nm. Based on the equation σ = α(λ)/N_c, where α is the absorption coefficient (8.97 cm⁻¹) and N_c is the concentration of Nd³⁺, the maximum absorption cross-section of the Nd:GLNO crystal was calculated to be 10.49 × 10⁻²⁰ cm². Moreover, the stimulated emission cross-section (${\sigma }_{\mathrm{em}}$) can be estimated from the fluorescence spectra using the Füchtbauer–Ladenburg equation: ${\sigma }_{\mathrm{em}}\left(\lambda \right)=\frac{{\lambda }^{5}I\left(\lambda \right)}{8\pi n^{2}c{\tau }_{\mathrm{m}}\int \lambda I\left(\lambda \right)\mathrm{d}\lambda }$ [19,20], where τ_m, c, n, I(λ) are the fluorescence lifetime, velocity of light, reflective index and fluorescence intensity, the calculated stimulated emission cross-section of 18 × 10⁻²⁰cm² was relatively large, which was suitable for generating ultrafast laser pulse.

A V-type laser cavity was designed to investigate the CW laser output properties of the Nd:GLNO crystals. Figure 3 displays the relationship between output power and absorbed pump power at different transmittances of output couplers. The maximum CW output power of 4.60 W was achieved with the output mirror of T = 15%, corresponding to an optical-to-optical efficiency of 37.90% and a slope efficiency of 49.67%. Furthermore, the laser output wavelength could be flexibly tuned by carefully varying the angle of the BF.

Table 1. Output parameters of the tunable Nd:GLNO crystal laser.

T (%)	Wavelength (nm)	Output Power (W)
1	1058.05	0.83
	1065.29	1.11
	1065.61	0.86
	1091.98	0.58
	1092.29	1.00
	1092.61	0.98
	1092.92	0.79
10	1065.29	4.13
	1092.29	3.03
	1092.61	2.83
15	1065.29	4.60
	1092.29	0.43
	1092.61	0.31

records the tuning wavelength and the corresponding output power with the output couplers of T= 1%, 10% and 15%, respectively. As the transmittance increased, the longitudinal mode oscillation in the cavity was suppressed. Therefore, the tuning range was further reduced. The total tuning coverage of the Nd:GLNO crystal laser was 34.87 nm ranging from 1058.05 to 1092.92 nm. Figure 4 demonstrates the typical single wavelength and multi-wavelength spectra of the Nd:GLNO crystal tunable laser.

To realize the CWML Nd:GLNO laser operation, a Z-type laser cavity was employed as shown in Figure 1b. Ultrafast laser pulse output was achieved using a SESAM. To reduce the intracavity loss and make the SESAM easily saturated, the CWML laser output characteristics were obtained experimentally at the output mirror of T_oc = 1%. As shown in Figure 5, the minimum absorbed pump power to suppress Q-switched mode-locking laser was 3.05 W. The maximum CWML laser output power 0.27 W was achieved. The CWML pulse train was measured using a detector and 1 GHz bandwidth oscilloscope. Figure 6 presents the stable mode-locking pulses recorded at nanosecond and microsecond time scales, respectively. The pulse repetition rate (PRR) is 51.6 MHz corresponding to the cavity length of 1.94 m. Figure 7 demonstrates the signal-to-noise ratio of the first beat. The radio frequency spectrum was clean and stable, indicating excellent stability of the mode-locking ultrafast laser. The signal-to-noise ratio was up to 72.3 dB at the fundamental frequency of 51.6 MHz. The FWHM bandwidth of the autocorrelation trace was about 14.0 ps, corresponding to a pulse duration of 9.1 ps by a sech²-shape pulse fitting. The mode-locking pulse spectrum was shown in the inset of Figure 8. The central wavelength of the measured pulse was located at 1065.26 nm with a FWHM of 0.9 nm.

4. Conclusions

In conclusion, the Nd:GLNO crystal was grown by the Czochralski method and the spectral characteristics at room temperature were discussed. The maximum CW output power of 4.60 W was obtained with the output mirror of T = 15%, corresponding to an optical-to-optical efficiency of 37.90% and a slope efficiency of 49.67%. The tuning coverage of the tunable Nd:GLNO laser was 34.87 nm at T = 1% ranging from 1058.05 to 1092.92 nm. To the best of our knowledge, a picosecond CWML Nd:GLNO laser at 1065.26 nm was experimentally demonstrated using a SESAM as saturable absorber for the first time. The maximum CWML laser output power of 0.27 W was achieved. The Nd:GLNO crystal ultrafast laser produced 9.1 ps mode-locked pulses with pulse repetition rate of 51.6 MHz and a signal-to-noise ratio of 72.3 dB. The results indicated that the Nd:GLNO crystal is a promising Nd³⁺-doped gain medium for generating ultrafast laser pulses.