1. Introduction

Nd-doped ferroelectric multi-functional materials have attracted a great surge of interest in recent years because of the combination of the laser, the electro-optical and the nonlinear optical properties. As a typical ferroelectric material, the first observation of the laser oscillator was reported in Nd-doped lithium niobate (Nd:LiNbO₃) crystal in 1967 [1]. Subsequently, several groups investigated the Nd:LiNbO₃ lasers [2–4]. However, the photorefractive effect in LiNbO₃ can reduce greatly the output power. One breakthrough has been made by adding magnesium oxide (MgO) into LiNbO₃ to remarkably reduce the photorefractive effect. The discovery stimulated the investigation of the Nd,MgO:LiNbO₃ laser [5–8]. With respect to the laser transition ⁴F_3/2 → ⁴I_11/2, most reports focused on the high-gain polarization (π-polarization) at 1084 nm [6–8], while the output powers were very low (<50 mW). In 1986, T. Y. Fan et al. reported the first low-gain polarized (σ-polarized) Nd,MgO:LiNbO₃ laser pumped by a dye laser at 598 nm, but the maximum output power was only about 4 mW with a slope efficiency of approximately 9% [5]. In addition, with a laser diode as pump source, it is feasible to arrange much simpler and more compact solid-state lasers with higher efficiency [9]. The first σ-polarized Nd,MgO:LiNbO₃ laser pumped by a laser diode at 823 nm was demonstrated in 1988, although the output power was very small owing to the low diode pump power [6]. The initial absorption spectra show that the absorption coefficient at 808 nm is much higher than that at 823 nm [6]. However, until now, there is no Nd,MgO:LiNbO₃ laser with σ-polarized operation at 1094 nm pumped by a laser diode at 808 nm.

Coherent sources in the spectral range around 1.6 μm have a lot of applications in the fields of range-finding, surgery, communications and environmental monitoring. In order to obtain 1.6 μm spectral sources, lasers with Er-doped crystal play an important role [10–17]. However, for the Q-switching operation, the pulse duration is still relative wide, often on the scale of several tens of nanoseconds. Another common method to generate the radiation at 1.6 μm is stimulated Raman scattering (SRS) [18, 19]. Based on a chemical vapor deposited (CVD) diamond, a Raman laser at 1.63 μm pumped by a Nd:YAG/V:YAG laser at 1.34 μm was reported with a pulse duration of 6 ns and a peak power of about 8 kW [18]. Recently, an AOM Q-switched Nd:YAG at 1.44 μm driven BaWO₄ Raman laser at 1.67 μm was realized. The output peak power was about 9 kW with a pulse duration of 27 ns [19]. There is also an approach based on the second nonlinearity, i.e. OPO, which is rarely reported. Most recently, Duan et al. reported an AOM Q-switched Nd:YVO₄/RbTiOPO₄ (RTP) IOPO operating at 1619 nm [20]. However, the high repetition rate (60 kHz) and the broad pulse duration (6.5 ns) would result in a low peak power (3.64 kW).

In addition, the combination of the OPO and SRS nonlinear processes impregnates new activities for the 1.6 μm generation. The first demonstration was the enhanced Raman convention at 1.64 μm from a periodically poled KTP (PPKTP) OPO, although it required the proper idler wave (in the absorption band beyond 3 μm in KTP) and the opportune temperature [21]. Also, the conventional 1064 nm Nd:YAG laser could be first converted by a KTiOAsO₄ (KTA) crystal to the first Stokes radiation at 1091 nm, and then the KTP IOPO with the signal wave at 1625 nm was generated [22]. However, the precise coatings and accurate adjustment, which should suppress the oscillation of the signal waves at 1534 and 1572 nm, make the operation at 1625 nm much more difficult.

In this paper, an efficient, 808-nm-laser-diode-pumped, σ-polarized Nd,MgO:LiNbO₃ laser at 1094 nm was realized for the first time. A maximum output power of 1.49 W was obtained at an absorbed diode pump power of 6.48 W. The corresponding optical-to-optical conversion efficiency was approximately 23%. For an application, a KTP IOPO pumped by an actively Q-switched Nd,MgO:LiNbO₃ laser with an AOM was first demonstrated. The maximum output power of the signal wavelength at 1630 nm was about 96 mW with a minimum pulse duration of 1.69 ns at an AOM repetition rate of 5 kHz, corresponding to a peak power of 11.4 kW.

2. Experimental setup

The experimental setup of the KTP IOPO pumped by an AOM Q-switched Nd,MgO:LiNbO₃ laser is shown in Fig. 1.The pump source was a fiber-coupled laser-diode (FAP-I system, Coherent Inc., USA) with a central wavelength at 808 nm. The core diameter and the numerical aperture were 400 μm and 0.22, respectively. The pump beam was collimated and focused into the c-cut Nd,MgO:LiNbO₃ crystal with a spot of 400 μm in diameter by a focusing system (1:1 imaging module). The absorption coefficient at 808 nm was measured to be about 1.85 cm⁻¹. The polished Nd,MgO:LiNbO₃ crystal (3 mm × 3 mm × 10 mm, 1 at.% Nd³⁺, 5 at.% MgO) and KTP (5 mm × 5 mm × 20 mm) were wrapped with a thin layer of indium foil and mounted in copper holders to maintain at 13 °C. The KTP crystal was X-cut (θ = 90°, φ = 0°) for Type-II NCPM configuration for the maximum effective nonlinear coefficient and acceptance angle. The input surface of the KTP crystal was high-reflectivity (HR) coated at the spectral range from 1500 to 1650 nm (R>99.8%) and anti-reflectivity (AR) coated ranging from 1000 to 1200 nm (R<0.2%), while the other surface was both AR-coated at 1094 and 1630 nm (R<0.2%). M₁ was a concave mirror (radius of curvature: 500 mm) with HR-coating at 1000 - 1200 nm (R>99%) and high transmission (HT) coating at 808 nm (T>95%). The output coupler (OC) was HR-coated at 1000 - 1100 nm and partial-reflectivity (PR) coated at 1630 nm (T = 4% at 1630 nm). An AOM (The 26th institute, CETC, China) with AR-coating at 1094 nm (R<0.2%) had an effective length of 47 mm. The OPO and the fundamental wave cavity lengths were 24 and 92 mm, respectively. The average output power was measured by a PM100D power meter with a S314C power head (Thorlabs Inc., USA). The pulse characteristics of the fundamental and signal waves were detected and recorded by a fast InGaAs photo-detector with a rise time of 400 ps (New Focus model 1611, Newport Inc. USA) and a DPO 7104C digital phosphor oscilloscope (1 GHz bandwidth and 20 GS/s sampling rate, Tektronix Inc., USA). The spectra were measured by a Wavescan Laser spectrometer (Resolution: 0.4 nm, APE GmbH, Germany).

 

Fig. 1 The schematic setup for an AOM Q-switched Nd,MgO:LiNbO₃/KTP IOPO. AOM: acousto-optic modulator.

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3. Experimental results and discussions

First, the CW σ-polarization operation of the diode-end-pumped Nd,MgO:LiNbO₃ laser at 1094 nm was realized. In this case, the KTP crystal and the AOM were removed and an OC with transmission of T = 4% at 1094 nm was employed. The physical length of the laser resonator was approximately 18 mm. The threshold power was found to be about 0.55 W. To prevent the laser crystal from damaging, the absorbed diode pump power did not exceed 6.48 W (corresponding to the incident pump power of 7.69 W) in the experiment. Figure 2 shows the average output power versus the absorbed diode pump power. Inset is the corresponding lasing spectra near 1093.5 nm at the absorbed diode pump power of 6.48 W. As shown in Fig. 2, a maximum output power of 1.49 W is obtained under an absorbed diode pump power of 6.48 W, corresponding to an optical-to-optical efficiency of about 23% and a slope efficiency of 25.7%.

 

Fig. 2 Average output power versus the absorbed pump power. Inset, lasing spectra at absorbed power of 6.48 W.

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As a common nonlinear crystal for OPO, an X-cut KTP for Type-II NCPM configuration also has the advantages of low sensitivity on the temperature and no waking-off effect. For the pump wavelength range from 1 to 1.1 μm, the calculated signal wavelength is shown in Fig. 3 based on the phase matching condition by using the Sellmeier equations of KTP. The signal wavelength theoretically varied from 1.45 to 1.65 μm. For a conventional Nd:YVO₄/KTP OPO, as shown in Fig. 3, the signal wavelength is about 1570 nm. As calculated, for a pump wavelength beyond 1080 nm, signal wavelength in 1.6 μm spectral bands could be generated. What’s more, for X-cut KTP Type-II NCPM OPO operation, the propagations of all three waves are along X-axis of the KTP crystal, while the polarization direction of the pump wave as well as the signal wave should be along Y-axis of KTP, because the optical axes lie in the X-Z plane (at 18° to the Z-axis). In our experiment, the polarization direction of the fundamental laser at 1094 nm is perpendicular to the c-axis of the Nd,MgO:LiNbO₃ crystal and the X-axis of the KTP crystal, which is shown in Fig. 1. In order to make the Y-axis of KTP parallel to the polarization direction of the fundamental wave, the KTP crystal can be rotated and the signal wave at 1630 nm based on the AOM Q-switched Nd,MgO:LiNbO₃/KTP IOPO could be expected.

 

Fig. 3 Signal wavelength of an X-cut KTP NCPM OPO versus the pump wavelength.

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Subsequently, an AOM and a KTP crystal were inserted into the laser resonator with a physical length of about 92 mm. The OC was substituted by a mirror with transmission of T = 4% at 1630 nm and HR coating at 1000 - 1100 nm. Then the KTP IOPO with a signal wavelength at 1630 nm could be realized. In order to prevent the damage and obtain relative high fundamental pulse energy, the AOM repetition rate was first set as 5 kHz. A dichroic mirror with HT coating at 1600 nm and HR coating at 1100 nm was put behind the OC in order to separate the output waves. The IOPO with much higher repetition rate of 20 kHz was also realized, but the output power at the signal wavelength was very low. Therefore, the AOM repetition rate was fixed as 5 kHz in the experiment. The variation of the average output power versus the absorbed diode pump power at an AOM repetition rate of 5 kHz is shown in Fig. 4.The obtained maximum output power was 96 mW at the signal wavelength of 1630 nm, corresponding to a diode-to-signal conversion efficiency of 1.48%. Inset in Fig. 4(b) is the typical spectra of the signal wave as well as the fundamental wave at an absorbed diode pump power of 6.48 W. The signal wavelength fitted well with the calculated value shown in Fig. 3. Owing to the limits of our laser spectrometer, the idler wavelength at 3327 nm (calculated wavelength) was not observed. Furthermore, the mirrors of the IOPO (made by BK7 glass) and the KTP crystal can strongly absorb the idler wave, which also made it very difficult to be measured in the experiment. The temporal pulse shapes of the depleted fundamental wave and the signal wave were recorded by a D7104C digital phosphor oscilloscope via a fast InGaAs photodetector with a rise time of 400 ps. Figure 4(c) shows the corresponding pulse train of the signal wave under the maximum output power of the signal wave. According to the pulse train, the peak-to-peak fluctuation and the time jitter were calculated to be 1.9% and 3%, respectively.

 

Fig. 4 (a) average output power of the signal wave versus the absorbed diode pump power. (b) typical spectra and (c) pulse train of the single wave at absorbed pump power of 6.48 W.

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Figure 5 displays the temporal pulse shapes under an absorbed diode pump power of 6.48 W with an AOM repetition rate of 5 kHz. As shown in Fig. 5, the measured minimum pulse duration for the signal wave is 1.69 ns, which is much shorter than that of the fundamental wave (about 6.9 ns). This is a well-known phenomenon for an IOPO operation. The highest peak power of 11.4 kW at an absorbed diode pump power of 6.48 W was obtained. Although the maximum average output power was low, the peak power of 11.4 kW was still higher than the reported in Ref [19]. The M² factor of the signal beam was measured to be about 1.3 by using the 90/10 knife-edge method. The experimental results show that Nd,MgO:LiNbO₃ crystal can be a promising candidate for high efficient laser and good pump source for 1.6 μm signal wave generation.

 

Fig. 5 Temporal pulse shape at an absorbed diode pump power of 6.48 W and an AOM repetition rate of 5 kHz.

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4. Conclusions

In conclusion, a laser-diode-pumped high efficient σ-polarized Nd,MgO:LiNbO₃ laser emitting at 1094 nm was demonstrated. The maximum output power was 1.49 W, corresponding to the slope and optical-to-optical efficiencies of 25.7% and 23%, respectively. For applications, a KTP IOPO pumped by an actively Q-switched Nd,MgO:LiNbO₃ laser with an AOM was demonstrated. The signal wave at 1630 nm with a maximum output power of 96 mW and a minimum pulse duration of 1.69 ns was realized under an absorbed diode pump power of 6.48 W with an AOM repetition rate of 5 kHz. The peak power was calculated to be 11.4 kW.