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A diode-pumped passively mode-locked Nd,La:CaF2 laser has been investigated by using a semiconductor saturable absorber mirror (SESAM) for the first time. With Nd,La:CaF2 disordered crystal as gain medium, the mode-locked laser generated pulses with pulse duration of 11 ps and repetition rate of 87.5 MHz at the central wavelength of 1065.8 nm. The maximum average output power was 110 mW corresponding to pulse energy of 1.26 nJ and peak power of 115 W, respectively. Additionally, continuous wave (CW) laser output performance of Nd,La:CaF2 was investigated, and the maximum average output power was as high as 488 mW.
© 2015 Optical Society of America
1. Introduction
Among trivalent rare-earth ion laser materials, neodymium (Nd3+) doped gain materials have been attracted tremendous attention due to their excellent optical properties, especially for driving the development of the high-power and ultra-short lasers. Generally, high-power pulses can be easily generated in Nd:YAG or Nd:YVO4 crystals on account of their large emission cross sections and good mechanical properties. However, their narrow gain linewidths are fatal flaws for achieving femtosecond pulses [1–5]. Although, Nd:glass materials successfully generate femtosecond pulses because of broad gain linewidth as a result of inhomogeneous broadening effects, Nd:glass materials have low thermal conductivity and small emission cross section, which limits the average output power and the repetitive operation [6–9]. In the face of this situation, MeF2-MF3-NdF3 disordered crystal materials (Me = Ca, Sr, Ba, etc, M = Y, Sc, La, Gd, Lu, etc), a combination of remarkable optical properties and mechanical properties, have generated widespread interest in producing the high-power and ultra-short pulses.
The Nd3+-doped fluoride crystals are suitable for obtaining the high-power pulses because of a relatively large emission cross section and a long upper level lifetime. Meanwhile, fluorides, which have the advantages of high damage threshold, large size growth, low nonlinear refractive coefficient and better thermal conductivity, were chosen as the substrate, better laser performance can be expected within Nd3+-doped fluoride crystals in comparison with Nd:glasses [10–14]. But the singly doped Nd:MeF2 materials have been abandoned for a long time due to a very detrimental concentration quenching effect which results from the clustering of the rare earth ions and some cross-relaxation type energy transfers which kill their emission quantum efficiency [15]. With the development of crystal materials, the solution is to codope the buffer ions M3+ with the purpose of breaking the clusters.
The M3+ ions break the [Nd3+-Nd3+] clusters and modulate spectral characteristics of Nd3+ ions on account of the formation of new [Nd3+-M3+] clusters, which weakens the very detrimental concentration quenching effect and enhances the emission quantum efficiency [16]. Moreover, the fluorescence linewidth and the emission cross section of crystals can be adjusted by varying the M-doping concentration [17]. At the same time, the disordered structure of crystals further broaden the fluorescence linewidth [18–21]. So, the family MeF2-MF3-NdF3 disordered crystals are a kind of suitable gain medium to generate the high-power and ultra-short pulses, repetitively. Up to now, the Nd,Y:CaF2 of this family have been overall researched about the properties of spectrum, CW, Q-switching and mode-locking [16, 17, 22–24]. In the [17], the broad fluorescence linewidth of 31 nm and the 103 fs pulses have been reported, which proves that the family MeF2-MF3-NdF3 disordered crystals have the great developing potential. The Nd,Y:SrF2 and the Nd,Lu:CaF2 have also been researched [23,25–28]. But the characteristics of Nd,La:CaF2 laser have never been successfully studied.
To Nd,La:CaF2 disordered crystals, La3+ ions were the buffer ions to break the [Nd3+-Nd3+] clusters. In the crystals, the Nd3+ and the La3+ ions are randomly distributed in the lattice sites to form multiple types of substitutional sites, which provides strong inhomogeneous lattice field for rare earth dopants, and correspondingly leads to large ground-state stark splitting and broad emission spectra. Relative to the mature 0.5 at.%Nd, 10 at.%Y:CaF2 [17], the Nd,La:CaF2 crystals have a unique advantage doped in the low La concentration that was 8%. In addition, to 0.5 at.%Nd, 10 at.%Y:CaF2 [17], the fluorescent lifetime and the emission cross section were 361.27 μs and 2.05 × 10−20 cm2, respectively. The 0.5 at.%Nd, 8 at.%La:CaF2 have a long fluorescent lifetime and a large emission cross section that of 481 μs and 2.22 × 10−20 cm2. These characteristics mean that the 0.5 at.%Nd, 8 at.%La:CaF2 will have a better laser characteristics.
In this paper, 0.5 at.%Nd, 8 at.%La:CaF2 was elected as gain mediums. The continuous wave (CW) and continuous wave mode-locked (CWML) characteristics of diode-pumped Nd,La:CaF2 laser were successfully researched. The maximum average output power of the CW laser was 488 mW. Without extra negative dispersion elements and using a SESAM as saturable absorber, the CWML pulse duration of ~11 ps and the pulse repetition rate of 87.5 MHz were obtained at the central wavelength of 1065.8 nm with the maximum average output power of 110 mW. The results proved that Nd,La:CaF2 disordered crystals were potential efficiency laser materials for generating the high-power and ultra-short pulses.
2. The spectral property and CW operation of diode-pumped Nd,La:CaF2 laser
The Nd,La:CaF2 disordered crystals (0.5 at.%Nd, 8 at.%La) were grown using the traditional and mature temperature gradient technique (TGT). Figure 1(a) is the room-temperature absorption spectra of the disordered crystals. As follow, the crystals have several strong absorption peaks including 736 nm, 791 nm and 796 nm. At the same time, each absorption band is broad. The 791 nm and the 796 nm correspond to commercialized LD pumping.
Fig. 1 The room-temperature spectra of 0.5 at.%Nd, 8 at.%La:CaF2. (a) The absorption spectra. (b) The emission spectra.
The room-temperature emission spectra of the disordered crystals are shown in Fig. 1(b). The crystals have a broad emission band ranging from 1030 to 1100 nm, which is in favour of the amplification and the generation of the ultra-short laser pulses. The emission peaks are at 1050 nm and 1065 nm. So, the crystals also suit to study dual-wavelength lasers.
The CW laser properties were studied firstly. The uncoated Nd,La:CaF2 disordered crystal with the dimension of 3 mm × 3 mm × 5 mm was pumped by a fiber coupled semiconductor laser at 790 nm with a fiber core diameter of 100 μm and numerical aperture of 0.22. A optics coupling system of 1:0.8 was used to focus the pump laser on the crystal. In order to remove the heat and reduce the thermal effect, the crystal was mounted in a Cu holder whose temperature was stabilized at 12 °C by cooling water. The experiment employed a simple plane-concave resonator whose length was 190 mm for reducing the loss and obtaining high average output power. The input plane mirror M1 was high reflection at 1.06 μm. The output concave mirror M2 had a radius of 200 mm with the different transmission including 1%, 2% and 3%. The average output power was measured by the power meter (30A-SH-V1, made in Israel).
The relationships between the average output power and the absorbed pump power with different transmission mirrors are shown in Fig. 2. As is shown, with different output mirrors, the absorbed pump power and the average output power were a nearly linear function. When the transmission of M2 was 2%, the maximum average output power was 488 mW at the absorbed pump power of 1.89 W, giving the slope efficiency of 26.9%. The maximum slope efficiency was 27%, when the transmission of M2 was 3%. Because the laser threshold power was very low. When the absorbed pump power was 58 mW, the CW laser was already obtained at the 1% transmission of output mirror. In order to protect the crystal from destroying, the experiment researches were done in a low incident pump power level. Moreover, if the crystal was coated for antireflection at the lasing wavelength and the pump wavelength, the average output power was expected to enhance.
3. Diode-pumped passively mode-locked Nd,La:CaF2 laser
The diode-pumped passively CW mode-locked Nd,La:CaF2 laser was mainly studied within a ~1.7 m long cavity. Figure 3 shows the experiment equipment. M1 was the same one as mentioned earlier. The output concave mirror M2 had radius of 200 mm with the transmission of 2% at 1030-1080 nm. The concave mirrors M3, M4 were high reflection at 1.06 μm whose radius were 800 mm and 100 mm, respectively. To acquire the CWML laser, the intracavity energy density not only need reach the saturation fluence of SESAM but also need less than the damage threshold of SESAM for protecting SESAM from damage. So, firstly used a plane mirror M5 as the cavity mirror, the cavity mirrors and the distance between M1-M2 and M4-M5 were adjusted carefully to obtain the appropriate intracavity energy density and the maximum average output power of CW laser. Simultaneously, for reducing losses, each mirror was compactly placed.
Figure 4 shows a black line function, which was the average output power of CW laser as a function of the absorbed pump power. The slope efficiency of CW laser was 17.6% and the maximum average output power was 300 mW, when the absorbed pump power was 1.89 W. The laser mode was in TEM00.
At the appropriate intracavity energy density, the CWML laser was successfully obtained by using a SESAM to instead of the plane mirror M5. The SESAM had relaxation time of 5 ~10 ps, modulation depth of 2.4%, non-saturable absorption of 20% of the total absorption and saturation fluence of 50 uJ/cm2. Figure 4 shows a approximate red line function, which was the average output power of CWML laser as a function of the absorbed pump power. The CWML laser had a slope efficiency of 7.8% and a maximum average output power of 110 mW at the absorbed pump power of 1.89 W. Compared to the two functions, the slope efficiency and the maximum average output power of CWML laser were less than that of CW laser, mainly because of the non-saturable loss of SESAM. The loss was from the mismatch between the quantum well material and its substrate. With the development of fabrication of SESAM, the slope efficiency and the maximum average output power of CWML laser were expected to be as high as that of CW laser.
Pulse trains, which were recorded by a fast photodiode (New Focus 1611) recorded with a rising time of 400 ps and with a 1 GHz digital oscilloscope (Tektronix DPO4104, USA), are showed in Fig. 5. From the short time span of 100 ns, the pulse repetition rate was 87.5 MHz which was very coincident with the theory that was f = c/2L (f was the pulse repetition rate, c was the speed of light, L was the laser cavity length). The pulse trains had a superb amplitude stability from the long time span of 200 ms.
Figure 6 is the radio frequency (RF) spectrum which was measured by a spectrum analyzer (Rohde & Schwarz FSC 3). From the RF spectrum, it was obvious that the CWML laser was quite obtained. The fundamental beat note was one peak at the repetition rate of 87.5 MHz with a resolution bandwidth (RBW) of 1 kHz in the span of 380 kHz. In addition, the high signal-to-noise ratio was 40 dB explaining a pure CWML laser. From the span of 880 MHz and the RBW of 10 kHz, the CWML laser was stable operation.
Fig. 6 The RF spectrum of CWML laser with a span of 380 kHz and a RBW of l kHz. (inset) The RF spectrum with a span of 880 MHz and a RBW of 10 kHz.
The autocorrelation trace of the CWML laser was collected by the autocorrelator (Femtochrome Research FR-103XL). It is showed in Fig. 7(a). The pulse duration was ~11 ps, which was assumed Gauss fitting. The pulse energy and the peak power were estimated to be 1.26 nJ and 115 W by average output power of 110 mW, pulse repetition rate of 87.5 MHz and pulse duration of 11 ps. The spectrum was measured by an optical spectrum analyzer (Avaspec-3648-USB2). As the Fig. 7(b), the CWML laser was a full width at half maximum of 2.1 nm at the central wavelength of 1065.8 nm.
In more than two hours, the stable CWML laser was always presenting. The time-bandwidth product was ~6.1. Because the average output power of CWML laser was a little low, the intracavity dispersion was not compensated. But the time-bandwidth product of 6.1 showed that the pulse duration could be further compressed.
The 0.5 at.%Nd, 8 at.%La:CaF2 was a novel crystal, so the crystal properties need further research, including thermal effect, mechanical effect and so on. Then making use of the effective data, the laser characteristics can be further studied to acquire a laser of higher power, more narrow pulse width and accurate polarization.
4. Conclusions
In conclusion, the novel Nd,La:CaF2 disordered crystals with high optical quality were obtained by TGT method. Their broad absorption and emission bands were suitable for generating ultra-short lasers by LD pumped. We have demonstrated, for the first time to our knowledge, a diode-pumped Nd,La:CaF2 laser at picosecond passively mode-locked operation with a SESAM. Laser pulse duration of 11 ps was obtained with an average output power of 110 mW and a repetition rate of 87.5 MHz. The research results show that Nd,La:CaF2 disordered crystals are excellent bulk media for picosecond pulse generation at 1 μm wavelength. If optimizing the crystal growth and experimental design, higher average output power will be obtained. With the compensating intracavity dispersion, femtosecond laser can be expected.
Acknowledgments
The authors acknowledge support from the National Natural Science Foundation of China (Nos. 61475089, 61178056, 61422511 and 51432007).
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