Introduction

Dual-wavelength synchronously mode-locked lasers are attractive for applications like terahertz (THz) radiation generation, pump-probe measurement. They have been experimentally widely investigated [1–13]. Previous experimental studies on the dual-wavelength synchronously mode locked lasers were mainly focused on the Ti:sapphire laser [2–13], taking advantage of the large smooth gain bandwidth of the laser. Various techniques have been developed for achieving dual-wavelength synchronous mode locking in the laser. These include the cross-phase coupling method [5] and the gain spectrum splitting method [10]. Recent studies have further shown that some of the rare-earth doped solid-state lasers could also exhibit dual or multiple wavelength mode locking [11–13]. Xie et al have experimentally demonstrated the dual-wavelength synchronous mode locking of a Nd-doped CNGG laser [11]; Agnesi et al have shown multi-wavelength mode locking in a diode pumped Nd:LGGG laser [12]; Yoshioka et al have observed dual-wavelength mode locking in a Yb:YAG ceramic laser [13]. A common feature of these rare-earth doped solid state lasers is that their gain spectra have multiple closely spaced, but well defined spectral bands. Under appropriate experimental conditions, one or more of these gain spectral bands can be mode locked, leading to the multi-wavelength mode locking of the lasers.

In this paper, we report on the passive mode locking of a new diode pumped solid state laser: the Nd:LuYSiO₅ (Nd:LYSO) laser. We show that due to the mixed crystal nature of the LYSO, its gain spectrum exhibits similar features as those of the Nd:CNGG and Nd:LGGG lasers. When mode locked with a semiconductor saturable absorber mirror (SESAM), two gain spectral bands of the laser, centered at the wavelength of 1075.8 nm and 1078.1nm, respectively, can be simultaneously mode locked. The mode locked pulses were found to be synchronous in the laser cavity. Consequently they beat with each other and form a stationary interference pattern on top of the mode locked pulse of the laser. The mode locked pulses of the laser has a pulse width of 8.9 ps, while the interference pattern on top of them has a temporal pattern width of 997 fs and a pattern repetition rate of 0.59 THz. When 12.7 W of pump power was absorbed, 1.7 W of the mode-locked pulses was obtained, with a slope efficiency of 24.3%. To our knowledge, this is the first report of a mode locked Nd:LYSO laser.

LYSO is an alloy of the YSO and LSO single crystals. YSO and LSO are oxyorthosilicate biaxial monoclinic crystals. Both of them are well-known scintillator materials widely used in the medical industry. Although oxyorthosilicate crystals have been used as laser gain host since 1970’s, it was recently demonstrated that the Yb-doped LSO, YSO and GSO could have high efficient CW and mode locked laser operation. LYSO was introduced to combine the advantages of both LSO and YSO crystals. The laser performance of the Yb-doped LYSO and GYSO was experimentally investigated [14–18]. However, so far less attention has been paid on the Nd:LYSO lasers [19].

We note that Sorokin et al have studied the spectroscopic features of the Nd-doped mixed scandium garnets. It was found that due to the lack of precise translational symmetry of the crystal, Nd ions doped in the mixed crystal could have a large inhomogeneous broadening fluorescence spectrum, which is significantly broader than those of Nd ions in single crystals. Using an Nd-doped mixed scandium garget, they have generated mode locked pulses as short as 260 fs in the laser [20].

Encourged by their result, we have grown the Nd-doped LYSO mixed crystals. In a previous paper we have reported the fluorescence spectrum of the Nd:LYSO crystal [19]. Like other Nd-doped mixed crystals, it shows a broad emission range from 1050 nm to 1082 nm, with several clearly identifiable spectral bands, centered at 1060 nm, 1068 nm, 1074 nm, 1078 nm, 1086 nm, 1092 nm and 1108 nm, respectively. Comparing with the Nd:YAG crystal, the emission peak of the Nd:LYSO is shifted to the 1078.1 nm.

Experimental setup

Figure 1 shows a schematic of our experimental setup. The pump source was a fiber-coupled laser diode whose central emission wavelength was temperature-tuned to 811nm. The coupling fiber has a core diameter of 100 μm and an NA of 0.22. M1 was a plane mirror that is coated high reflection in the wavelength range of 1000-1100 nm (R>99.5%) and high transmission (HT) at 811 nm. M2 was a concave mirror with a radius of curvature (ROC) of 500 mm. It has 6% transmission at 1080 nm. A V-shaped cavity was used in the experiment where L1 and L2 were 472 mm and 520 mm, respectively. The Nd:LYSO crystals used was grown with the Czochralski method. It has 0.5 at. % Nd-doping concentration, and was cut into dimensions of 3 × 3 × 5.5 mm³. The crystal was HT coated at the pump and laser wavelengths. The crystal were wrapped with indium foil and mounted in a water-cooled copper block whose temperature was maintained at 19°C throughout the experiment. A piece of commercial SESAM (BATOP GmbH) was used to initiate and maintain mode-locking of the laser. The SESAM was designed to operate at 1060 nm and had a modulation depth of 1.6%, a non-saturable loss of 1.4% and a relaxation time of 500 fs. The pulse trains were detected with a fast photo-detector (New Focus 1611, 1 GHz bandwidth) together with a digital oscilloscope (Tektronix DPO 7104, 1 GHz bandwidth). The optical spectrum was measured using an optical spectrum analyzer (Ando AQ 6315B) and the pulse width was measured with a commercial autocorrelator (APE GmbH, Pulsecheck)

 

Fig. 1 Experimental arrangement of the diode-pumped Nd:LYSO mode-locked laser.

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Experimental results and discussion

The CW performance of the laser was first studied. For the CW operation the SESAM shown in Fig. 1 was replaced with a plane mirror (HR at 1076nm). Figure 2 shows the relation of the CW output power versus the absorbed pump power of the laser. The CW operation of the laser has a pump threshold of 2.5 W. A maximum output power of 3.83 W was obtained under 12.7 W absorbed pump power. The slope efficiency of the laser was 37.5%. Throughout the experiment about 70% of the pump power were absorbed by the crystal.

 

Fig. 2 The output power of the Nd:LYSO CW laser and mode-locked laser with respect to the 811 nm pump power.

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Mode locking of the laser could be easily achieved. Figure 2 shows the output power of the mode-locked laser versus the absorbed pump power. Initially the Q-switched mode locking was observed. It has a threshold pump power of 6.6 W. The stable CW mode-locking occurred as the absorbed pump power increased to 11W. A maximum output power of 1.7 W was obtained under 12.7 W absorbed pump power. The slope efficiency was 24.3%.

Figures 3(a) and 3(b) show typical mode-locked pulse trains measured under the maximum available pumping of 12.7 W. Figure 3(a) was recorded with a time scale of 10 ns/div while Fig. 3(b) was recorded with a time scale of 1 ms/div. It can be seen that the pulses trains were fully modulated with good pulse stability. The pulse repetition rate is 151.2 MHz, which is in agreement with the cavity length.

 

Fig. 3 The pulses temporal behavior of the 1080.2 nm mode-locked laser. (a) with time span of 100 ns (b) with time span of 10 ms.

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Figure 4 shows the optical spectrum of the mode locked pulses. Two spectral bands of the laser with center wavelength located at 1075.8 nm and 1078.1nm, respectively, were simultaneously mode locked. The spectral band centered at 1075.8nm has a mode locked FWHM spectral bandwidth of 0.67 nm, while that centered at wavelength 1078.1nm has a FWHM of 0.48 nm. The center frequency difference between the two bands is 0.59 THz. In addition, the spectral intensity ratio of them is about 1:0.2. Figure 4 was simultaneously measured with the mode locking state shown in Fig. 3. However, only one mode-locked pulse was observed in the oscilloscope trace, indicating that the two mode-locked pulses are synchronized and temporally overlapped.

 

Fig. 4 Optical spectrum for the Nd:LYSO mode-locked laser at 1075.8 nm and 1078.1 nm.

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Figures 5(a) and 5(b) further show the measured autocorrelation traces of the mode-locked pulses. Figure 5a was obtained in a large scan range of the autocorrelator. The FWHM of the autocorrelation trace is about 12.6 ps. If a Gaussian-pulse shape is assumed, the mode-locked pulse duration is 8.9 ps. On top of the measured autocorrelation trace a certain fine structure is visible. Figure 5b shows the zoom-in measurement of the fine structure. The autocorrelation trace shown in Fig. 5b displayed a similar structure as that obtained by Xie et al on a diode pumped Nd:CNGG laser [11]. In agreement of the spectral measurement, it suggests that the laser is in a dual-wavelength synchronous mode locking state. In particular, within the mode locked pulses there are stationary intensity modulation patterns. The temporal pattern separation is about 1.68 ps, which corresponds to the beat frequency between the 1074.8 nm and 1078.1 nm, and the pattern has a temporal width of about 997 fs.

 

Fig. 5 Autocorrelation trace of the Nd:LYSO mode-locked pulses. (a) in a long time range (b) in a short time range.

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To better understand the measured autocorrelation trace, we have also numerically simulated the beating between two collinear and temporally overlapped pulses with different center wavelengths. The interference pattern of the pulses is given by $I=I_1+I_2+2\sqrt{I_1{I}_2}\cos (2\pi \nu t)$ [21], where I₁ and I₂ are the intensity of each of the pulses. ν is the difference frequency of the pulses. As a result of the beating the pulse intensity becomes modulated by a cosine function, and the modulation frequency is ν. The modulation depth depends on the intensity ratio $I_1/I_2$ and the pulse width ratio $W_1/W_2$ of the two pulses. Assuming that the pulses have a Gaussian shape, we numerically calculated the interference pattern and its corresponding autocorrelation trace under different $I_1/I_2$ and $W_1/W_2$ ratios, as shown in Fig. 6 . The intensity modulation depth decreased with the increase of$I_1/I_2$, while the measured pulse width decreased with the decrease of $W_1/W_2$. In our experiment, based on the mode-locking spectra shown in Fig. 4, it is estimated that the intensity ratio between the pulses is about 1:0.2, and the pulse width ratio is 1:0.4. Using the values, the calculated autocorrelation trace is given in Fig. 6(b3), which is in a fair agreement with that shown in Fig. 5(b). Therefore, the fine structure measured on the autocorrelation traces were formed due to the beating of the two mode-locked pulses.

 

Fig. 6 The calculated beat pulse traces (a1-a3) and corresponding autocorrelation traces (b1-b3) with different intensity ratio $I_1/I_2$ and pulse width ratio $W_1/W_2$

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We note that different from Xie’s experiment, no dispersion compensation was conducted in the current laser. Synchronization between the two different wavelength mode locked pulses could still be achieved in the laser. We believe this could become possible because the effect of the SESAM. The saturable absorption of SESAM could impose an attraction force between the pulses [22]. Although the effect of cavity dispersion is to separate the pulses, the effect of the SESAM is to tarp them together. Eventually a balance between them could be reached and consequently a stable pulse evolution is formed in the cavity.

Conclusions

In conclusion, we have first experimentally investigated the passive mode-locking of a diode pumped Nd:LYSO laser. It was found that the laser can be simultaneously mode locked at its two closely spaced gain spectral bands, one centered at 1075.8 nm and the other at 1078.1 nm, with a SESAM. Moreover, the two different wavelength mode locked pulses were found to be synchronized in the laser and their temporal beating generated a stationary interference pattern on top of the mode locked pulse. The interference pattern has a pattern repetition rate of 0.59 THz and temporal pattern width of about 997fs. 1.7 W average output power was obtained at 12.7 W absorbed pump power. The slope efficiency of the mode locked laser was 23.2%. To our knowledge, this is the first report on the mode locking of a Nd:LYSO laser.