1. Introduction

High power, high efficient laser emissions around 1.3 μm and their frequency-doubled red lasers have important applications in spectroscopy, fiber optics, laser machining, medicine, optics communication, military and laser display. These laser emissions can be achieved from ⁴ F _3/2-⁴ I _13/2 transition of Nd³⁺ doped materials. Besides the commonly used Nd:YAG crystal (operation at 1319 nm [1] and 1356 nm [2]), Nd:YVO₄ [3] and Nd:GdVO₄ [4] can also be used to obtain laser source around 1.3 μm, in which main laser emission from ⁴ F _3/2 to ⁴ I _13/2 transition is 1342 nm line. As a new laser source, laser operaton at 1386 nm not only has many potential applications in the fields that we have mentioned above, but also has its own application foreground. For example, compared with other red laser sources (660 nm and 671 nm), the frequency-doubled red laser (793 nm) has much more saturation in the field of laser display. Compared with other commonly used laser transition (1.06 μm and 1.3 μm), it has better performance in some regions of laser machining for its long wavelength.

Diode-pumped solid-state lasers have some important advantages compared to the conventional flash-lamp pumped lasers, such as high efficient, compact, and reliable. Diode-end-pumped configuration can provide much stronger pump power density than transversely pump structure. Therefore it is possible for cw operation to be achieved at some weak transitions by diode-end-pumped configuration. Nd:YVO₄ crystal has been identified as one of the most promising materials for diode-end-pumped solid-state lasers because of its high absorption over a wide pumping wavelength bandwidth and its large stimulated-emission cross-section. Efficient, high power laser operation has been report by many researchers, but these reports are mainly focusing on the traditional 1064 nm [5] transition of ⁴ F _3/2-⁴ I _11/2, 1342 nm [3] transition of ⁴ F _3/2-⁴ I _13/2 and 914 nm [6] transition of ⁴ F _3/2-⁴ I _9/2. Except the 1342 nm line, cw operation of other ⁴ F _3/2-⁴ I _13/2 transition is very hard to be achieved, due to its small stimulated-emission cross-section and strong parasitical oscillations. Besides Nd:YVO₄ crystal, there are some Nd doped crystals and one Ho doped crystal [7] which can produce about 1386 nm laser. For example, laser at 1387 nm can be obtained using Nd:Mgo:LiNbO₃ crystal. Compared with these crystals, laser at 1386 nm based on Nd:YVO₄ crystal has the advantages of simpleness, more efficient and more powerful. In addition, the price of Nd:YVO₄ crystal is very cheap.

In this paper, we report a cw diode-end-pumped Nd:YVO4 laser which is operating at 1386 nm. A maximum output power of 305 mW at an incident pump power of 4.24 W with a slope efficiency of 13.9% has been achieved. As far as we know, this is the first time that cw operation at 1386 nm transition of Nd:YVO₄ crystal is reported. According to the pump power threshold and experimental configuration, we calculate the stimulated-emission cross-section of Nd:YVO₄ at this transition, which is determined to be about 3×10^-19cm². In addition, cw simultaneous operation at 1342 nm and 1386 nm is also observed.

2. Transitions from ⁴F_3/2 to ⁴I_13/2 manifold in Nd:YVO₄ crystal

 

Fig. 1. Transitions from 4 F _3/2 to 4 I _13/2 manifold and their energy.

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Stimulated-emission cross-section and fluorescence branching ratios of Nd:YVO₄ crystal have been reported by several researchers [8–13]. However all of these researchers fix their attentions on 914 nm, 1064 nm or 1342 nm transitions. To our knowledge, there is no report on the stimulation-emission cross-section and fluorescence branching ratio of 1386 nm transition. The ⁴ I _13/2 manifold energy levels and corresponding transitions involved in our work from 4 F _3/2to ⁴ I _13/2 energy levels in the Nd:YVO₄ crystal are shown in Fig. 1. The 1386-nm laser emission is coherent radiation from R₂ sublevel (11383.5 cm^-1) of ⁴ F _3/2 manifold to X₇ sublevel (4167.4 cm^-1) of ⁴ I _13/2 manifold (the exact wavelength is 1385.8 nm). Except the well-known 1342 nm line, the fluorescence of other transitions is very weak, which are neglected by many researchers. In our experiment, an Agilent optical spectrum analyzer (model 86142B) is used to explore the fluorescence spectrum of Nd:YVO₄ crystal from 1200 nm to 1500 nm, which is shown in Fig. 2.

 

Fig. 2. Fluorescence spectrum of Nd:YVO₄ crystal from 1200 nm to 1500 nm

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3. Experiments

 

Fig. 3. The schematic of the experimental setup.

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A schematic diagram of our experimental set-up is depicted in Fig. 3. A simple plane-plane cavity configuration is investigated. A high-brightness fiber-coupled diode laser source emitting at 808nm with a fiber core diameter of 400 μm and a numerical aperture of 0.22 is employed. The multi-lens optical coupler has about 90% transmission at 808 nm, and can focus the pump radiation into gain medium with a spot size of about 440 μm in diameter. The gain medium is a plane-parallel polished Nd:YVO₄ crystal (3×3×3 mm), with 0.7at.% Nd³⁺ doping level. Because 1386 nm laser transition is only 44 nm apart from the 1342 nm line, it is difficult for Nd:YVO₄ crystal to obtain the coatings which simultaneously have high transmission at 1342 nm and high reflectivity of more than 99.8% at 1386 nm. The pumping facet of the Nd:YVO₄ crystal is coated with high reflectivity (R>99.8%) at wavelengths of both 1342 nm and 1386 nm, high transmission at 808 nm, and 1064 nm. The opposite side has high transmission coatings at wavelength of 1386 nm. A flat mirror is used as the output coupler with partial transmission (T=0.73%) at 1386 nm to provide an output, with high transmission at 1064 nm (T>90%) and partial transmission at 1342 nm (T=6.94%) to suppress the strong 1064 nm and 1342 nm transitions. The round-trip reflectivity at 1064 nm and 1342 nm are less than 0.1% and 93%, respectively. According to the experimental phenomenon observed, we find that the round-trip reflectivity at 1342 nm is too high to suppress the parasitic oscillation at 1342 nm.

 

Fig. 4. Output power at 1386 nm Vs incident pump power

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When incident pump power was increased to 1.15 W, laser emission at 1386 nm began to oscillate. As we had indicated above that the transmission of our output coupler at 1342 nm was not high enough, so laser emission at 1342nm appeared also when incident pump power was increased to 1.5 W. Thus simultaneous dual-wavelength operation at 1342 nm and 1386 nm was observed.

Chen [14] indicated that in the condition of cw dual-wavelength operation, the relative output powers of each wavelength were very sensitive to the alignment of the output coupler, for the relative cavity losses L_i were adjusted in the alignment procedure. In our experiment, the same phenomenon was also observed; even the laser emission at 1342 nm could be eliminated. By this means, we successfully acquired single wavelength operation of diode-end-pumped Nd:YVO₄ laser at 1386 nm. The disadvantage of this method is higher pump threshold, which is increased to 1.94 W, and low slope efficiency due to the large round-trip cavity loss. At an incident pump power of 4.24 W, a maximum laser output of 305 mW at 1386 nm is obtained with a slope efficiency of 13.9%. To the best of our knowledge, this is the first time that cw laser emission at 1386 nm in Nd: YVO₄ crystal is reported. The curve of output power at 1386 nm versus incident pump power and the spectrum of laser emission are depicted in Fig. 4 and Fig. 5, respectively. If the incident pump power was continuously increased, the wavelength of laser emission would shift to 1342 nm, and laser output power would be increased to 900 mW. This phenomenon can be explained as follows. When incident pump power is higher than 4.24 W, the gain of 1342 nm transition will be much larger than that of 1386 nm line, so the laser emission transition must be shifted to the 1342 nm line for its larger gain, due to their competitive interaction, though we have misaligned the cavity to increase the relative round-trip cavity loss at 1342 nm. We believe that if a better output coupler, which has enough transmission at 1342 nm so as to suppress the strong parasitical oscillation at this transition, is used, an output power of several watts at 1386 nm should be obtained.

 

Fig. 5. Spectrum of 1386-nm laser emission

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4. Discussion

The stimulated-emission cross-section σe can be determined through three methods as follows: spectroscopic method, threshold formula method and Tucker’s method of comparison of laser efficiencies [11]. Spectroscopic method has the highest precision, but the whole system is expensive and complicated in which a large number of spectroscopic measurements are required. Tucker’s method is rather simple, flexible and also has reasonable precision. It is advantageous to measure an unknown cross-section of another crystal relative to the “standard”. In addition, it does not require precise and absolute calibration of radiation detectors, and does not require precise measurement of the pumped volume (diameter). However, after we did experiments with all the output coupler we had, we found that only one output coupler could be used to achieve 1386 nm laser output. So we have to use the threshold formula method to get an approximate value of stimulated-emission cross-section of Nd: YVO₄ crystal at 1386 nm.

Stimulated emission cross-section oe can be obtained from the formula as follows:

Where ω_p and ω_c are the pump and the cavity beam spot-size, respectively, hv_p is the pump photon energy, T is the output coupler transmission at 1386 nm, L is the round-trip cavity loss, P_th is the absorbed threshold pump power, τ_f is the fluorescence lifetime, and η_p is the pumping efficiency. For the plane-parallel resonator we used (cavity length is 20 mm, crystal length is 3 mm, and refractive index of the crystal is 2.16), the cavity beam waist ω_c is calculated to be about 170±10 μm. In this calculation the focal length of pump-induced thermal lens is determined to be about 200 mm at threshold incident pump power, and the cavity spot-size should have no significant variation inside the crystal. In the case of the pump beam, the spot-size can also be considered as constant in the 3 mm long crystal. The pumping efficiency η_p represents the fraction of the absorbed pump photons producing population inversion, which can be estimated by calculating the average mode overlap between the pump beam and the cavity beam over the crystal length l according to [15]

The basic parameters used in calculation are ω_p=220 μm, τ_f =74 μs [10], T=0.73%, L=0.0259, and λ=1386 nm. The value of round-trip cavity loss L is measured at 1342 nm by the Findlay-Clay method [16]. Thus the stimulated-emission cross-section σ_e is determined to be 3 × 10^-19cm², which is about half of that at the 1342 nm transition.

As we have mentioned above, we do not have enough optical couplers operating at 1386 nm, so, when we used the Findlay-Clay method to measured the round-trip cavity loss L, we have to do these measurements at 1342 nm. For the wavelength discrepancy between 1386 nm and 1342 nm is only 44 nm, we believe that error of L owing to this substitute should less than 5%. We think that parameters, which maybe result in comparative errors, are the pump and the cavity beam spot-size, ω_p and ω_c. The pump beam spot-size is measured as about 220±20 μm. In addition the cavity beam spot-size also has a few errors. So the stimulated-emission cross-section σ_e should be (3±1)× 10^-19cm².

5. Conclusions

In conclusion, the cw power performance of an diode-end-pumped Nd:YVO₄ laser at 1386 nm has been reported. A maximum output power of 305 mW with a slope efficiency of 13.9% is achieved. To the best of our knowledge, this is the first time that cw laser emission at 1386 nm in Nd:YVO₄ crystal is reported. And the stimulated-emission cross-section σ_e is determined to be 3×10^-19cm², according to experimental threshold data. We believe that if better output couplers are used, an output power of several watts should be obtained. In addition, simultaneous cw operation at 1342 nm and 1386 nm is also observed.