1. Introduction

Diode-pumped Nd-laser systems have been widely used because of the compactness and high conversion efficiency. Three fundamental wavelengths, 0.9, 1 and 1.3 μm, from the transition of ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, and ⁴F_3/2→⁴I_13/2, have unique applications in laser spectroscopy, medical treatment, and two-photon microscopy. Through the frequency-doubling process into the visible region, the blue, green, and red lasers are useful for display, hologram, and fluorescence spectroscopy. However, the huge emission cross-section differences between each transition increase the difficulty of simultaneously generating multi wavelengths. For an Nd:GdVO₄ laser crystal, the ratio of the emission cross section between the ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, and ⁴F_3/2→⁴I_13/2 is approximately 1:18.9:5.1 [1]. To balance the line competition of resonated waves, adjusted cavity-mode matching [2], varied output coupler [3], and multiple gain medium [4] are adopted. Herault [4] reported dual-wavelength laser operation between ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I_11/2. The 912 nm only existed when the intracavity power of 1063 nm remained below a few watts. The current study reports a novel configuration to actively modulate the loss of ⁴F_3/2→⁴I_11/2 (1 μm) and ⁴F_3/2→⁴I_13/2 (1.3 μm). The following discussion shows a first wavelength-switchable Nd-laser among three fundamental wavelengths by a cascaded electro-optic periodic poled lithium niobate Bragg modulators (EPBM). Dual or triple-wavelength emission can be also achieved by manipulating the loss of 1 and 1.3 μm.

Periodically poled lithium niobate (PPLN) crystals [5] have attracted considerable attention because their collinear process uses the largest nonlinear coefficient, and changes the phase-matching condition by the lithography process. In addition to using the PPLN crystal as a frequency converter, Lin et al. [6] demonstrated the Q-switching process in an EPBM. When a laser passes through the EPBM at Bragg incident angle θ_B,m = sin⁻¹(mλ₀/(2nΛ)), the laser is deflected to a certain angle matching momentum conservation, where m is the diffraction order, λ₀ is the laser wavelength, n is the average refractive index of the grating, and Λ is the grating period. Because the Bragg angle condition means another phase-matching condition for deflecting the certain wavelength, cascaded PPLN gratings can be designed for several wavelengths at the same Bragg angle in a single device. Table 1 shows the design of PPLN grating periods for diffracting 1.063 and 1.342 μm at the same Bragg incident angle (0.7 degree). Since the EPBM is insensitive to the temperature [6], multi-function device including frequency converter, Bragg deflector and phase modulator can be integrated monolithically [7]. In the current design, the grating period of 20.3 μm is also matched the 3rd order second harmonic generation (SHG) for 1/1063 + 1/1063 →1/531.5.

Table 1. Calculated PPLN grating periods for matching the same Bragg incident angle (0.7°) at 1.063 and 1.342 μm.

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2. Experimental setup

Figure 1 shows a schematic of a wavelength switchable through EPBM in a diode-pumped Nd:GdVO₄ laser. The PPLN adopted in this paper was a 2-mm-thick 5 mol.% MgO:PPLN (made by HC Photonics, Taiwan). The dimensions of EPBM were 10 mm (width in x) x 15.5 mm (length in y) x 2 mm (thickness in z) and were separated in two sections, EPBM1 and EPBM2, for diffracting 1064 and 1342 nm at the same Bragg angle θ_B,1(1064 nm) = θ_B,1(1342 nm) = 0.7°. The grating period of EPBM1 was 20.3 μm, and the dimensions were 10 mm (width in x) x 8 mm (length in y). The grating period of EPBM2 was 25.7 μm, and the dimensions were 10 mm (width in x) x 5.5 mm (length in y). Using a cw laser with approximately 200 mW at 1063 nm, the half-wave voltages of the 20.3 and 25.74 μm gratings were measured at 740 and 1020 V, respectively. The input faces of the EPBM are optically polished and have anti-reflection coating at 1063 and 1342 nm (R<1%). The NiCr electrodes were coated on ± z surfaces. To independently apply the electric field, an uncoated electrode of 2 mm was placed between EPBM1 and EPBM2. This study used a 25 W fiber-pigtailed diode laser at 808 nm pumped an a-cut 0.1-at.% 5-mm-long Nd-doped GdVO₄ crystal through a set of 1 to 2 coupling lenses. The diameter of the fiber core was 200 μm. The reduced Nd doping level helped diminish the reabsorption loss in a 912 nm cavity [9] and absorbed only 55% diode power. The Nd:GdVO₄ crystal was wrapped by indium foil and kept in a copper housing for water cooling at 18 °C. The two end surfaces of the Nd:GdVO₄ crystal are optically polished and have anti-reflection coating (R<1%) at 808, 912, 1063, and 1342 nm. The 912 nm laser cavity was formed by a flat high reflection (HR) (R>99.8%) coated mirror M₁ and mirror M₂ with 300 mm radical of curvature (ROC). Mirror M₂ has a 3% output coupling at 912 nm and a high transmission coating (HT) (T>99%) at 1063 and 1342 nm. The 1063 and 1342 nm lasers share the same z-folded cavity constructed by M₁, M₃, M₄, and M₅. M₃ and M₄ are concave mirrors with ROC 300 and 100 mm, respectively, and both have an HR coating (R>99.8%) at 1063 and 1342 nm. To efficiently couple out the 912 nm, M₃ was also coated with HT coating (T>95%) at 912 nm. The 1063 and 1342 nm lasers were coupled out through the flat end mirror M₅ with 35 and 7% output coupling, respectively. The distance between M₁ and M₃ is 130 mm, and that between M₁ and M₄ was 420 mm. The total z-folded cavity length was approximately 500 mm. The thermal focal length in the gain medium was estimated to be 30-40 cm at a pump power of 25 W. When the thermal focal length of the Nd:GdVO₄ crystal varied, the beam radius inside the EPBM was calculated between 280 and 300 μm at 1063 nm, contributing to more than a 70% loss of modulation at a half-wave voltage [7].

 

Fig. 1 Schematic of the wavelength switchable Nd:GdVO₄ laser pumped by a 808 nm diode laser. The EPBM1 and EPBM2 function as a loss modulator for 1063 and 1342 nm, respectively. The 912 nm laser cavity was formed by the M₁ and output coupler M₂. The 1063 and 1342 laser cavity were formed by M₁, M₃, M₄, and output coupler M₅.

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The performance of the 1063 nm laser was first characterized. At a pump power of 10 W, Fig. 2 shows the output power of 1063 nm through applied voltages of EPBM1 and EPBM2. We first turned off the applied voltage on EPBM2 and recorded the output power as a function of applied voltage on EPBM1 (filled circle). Because of the strong stress-induced refractive index change [10] in MgO:PPLN, the highest output power (approximately 0.83 W) of the 1063 nm laser was measured at −300 V, and the modulated loss was higher than the gain at 300 V. Then we fixed the EPBM1 = −300 V and varied the applied voltage of EPBM2 (open circle). The maximum output power of 1063 nm was also appeared at EPBM2 = −300 V. Although EPBM2 (Λ = 25.74 μm) did not match the Bragg angle at 1063 nm, the high order Bragg grating and side-band coupling effects still affected the performance at 1063 nm. When both EPBM sections were operated at 300 V, the 1063 nm did not lase until reaching the maximum diode power. The stress-induced refractive index change was calculated as 2x10⁻⁵, and then found that it can be eliminated through an annealing process. In this study, we still adopted EPBM chips without the annealing process.

 

Fig. 2 Output power of 1063 nm versus applied voltage on EPBM sections at 10 W pump power. The filled and open circles represent the output power when the EPBM2 and EPBM1 are fixed at 0 and −300 V, respectively.

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3. Experimental result

After characterizing the performances of EPBM, Fig. 3 shows the measured output power of three wavelengths at different applied voltages. To ensure the lasing of 912 nm cavity, a 300 dc voltage was applied at both EPBM1 and EPBM2 to provide high loss at 1063, and 1342 nm cavity. The open triangle shows the measured 912 nm output power versus pump power. After overcoming the threshold at 10.5 W, the 912 nm output power reached 2 W at a 25 W pump power. The slope efficiency was calculated at approximately 13.25%. Until the pump power reached 25 W, there was no measured output of 1063 or 1342 nm. The generation of 1342 nm was realized when EPBM1 and EPBM2 were operated at 20 V and −300 V, respectively. The threshold was found at 5 W, and the maximum output power was measured ~1.4 W at 21 W pump power. Moderate loss on EPBM1 (1063 nm) was applied because the output power of 1342 nm was found to decrease effectively while the applied loss of 1063 nm was increased. Dual-wavelength lasing between 1063 and 1342 nm occurred when the pump power exceeded 21 W. This dual-wavelength laser provides the possibility to use in the application of two-color spectroscopy, to process sum-frequency mixing into the sodium region and to utilize in the pump-probe system. The generation of 1063 nm was achieved by removing the stress-induced refractive index change through the applied −300 V on both EPBM sections. The threshold appeared at 2.3 W and reached a 5 W output at a 25 W pump power. The 912 and 1342 nm were completely suppressed during the line competition. Therefore, the laser wavelength was switchable by applying the voltage on EPBM. This is the first demonstration of a triple-wavelength switchable laser system.

 

Fig. 3 Measured output power of 912, 1063, and 1342 nm when switching dc voltages on EPBM sections. 912 nm: EPBM1 = EPBM2 = 300 V, 1063 nm: EPBM1 = EPBM2 = −300 V, 1342 nm: EPBM1 = 20 V, EPBM2 = −300 V

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Figure 4 shows the spacial profile of three wavelengths at the maximum output power. The transverse mode with Gaussian distribution shows the beam quality of this switchable laser system. Further beam quality characterization was conducted by the standard knife-edge measurement. The M² values of 912, 1063 and 1342 nm are measured to be 1.3, 1.8, and 1.4, respectively.

 

Fig. 4 Far-field beam profiles at maximum output power: (a)912 nm, (b) 1063 nm, (c)1342 nm.

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Because EPBM has the ability to actively control the loss of 1063 and 1342 nm, we were searching for a balanced point to generate three wavelengths simultaneously. When the pump power was 25 W, triple-wavelength lasing among 912, 1063, and 1342 nm was produced at biased voltages of EPBM1 = 180 and EPBM2 = −50 V. The output power of all three wavelengths was approximately 100 mW. Two dichroic mirrors were used to combine 912 and 1063/1342 nm into an optical spectrum analyzer, as shown in Fig. 5 . This is the first report of triple-wavelength lasing at a single-laser cavity. Because of the strong line competition among three wavelengths at the ⁴F_3/2 energy level, the power fluctuation was increased effectively. To trace the short term stability, three photodiodes were used to monitor each wavelength, as shown in Fig. 6 . At the full pump power and single-wavelength lasing scheme, the 10 second rms power variation of 912, 1063, and 1342 nm was 1%, 1.6%, and 6.2%, respectively. However the 10 second rms power variation of 912, 1063, and 1342 nm was increased to 14%, 20.2%, and 10.5%, respectively, in the triple-wavelength scheme. The power fluctuation of 912 and 1063 nm was severe because of the huge cross section difference. During the gain competition, 912 nm was in a disadvantage position compared with 1063 nm, as shown in Fig. 6(a) and 6(b). Since the triple-wavelength scheme belongs to the critical equilibrium, any perturbations including pump power variation, thermal issue or vibration will unbalance the equilibrium. Without any feedback control, triple-wavelength scheme can only sustain in 10 minutes and switch to the single-wavelength scheme. Because ⁴F_3/2→⁴I_11/2 has the largest emission cross section, 1063 nm was usually survived.

 

Fig. 5 Measured optical spectrum at triple-wavelength lasing scheme. At 25 W pump power, three fundamental wavelengths of Nd:GdVO₄ laser can be generated simultaneously with output power ~100 mW for each wavelength. The applied voltages on EPBM1 and EPBM2 are 180 and −50 V, respectively.

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Fig. 6 10 seconds power fluctuation of three wavelengths at single and triple wavelength scheme: (a)912 nm, (b)1063 nm, (c)1342 nm.

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4. Conclusion

In conclusion, we have demonstrated for the first time a cw triple-wavelength-selectable scheme involving cascaded electro-optic periodically poled lithium niobate Bragg modulator in a diode-pumped Nd:GdVO₄ laser. At 25 W pump power, the wavelength can be switched among 912, 1063, and 1342 nm with output power of 2, 5, and 1.4 W, respectively. Simultaneously triple fundamental wavelengths generation can be also realized by controlling the loss of ⁴F_3/2→⁴I_11/2 (EPBM1 = 180 V), and ⁴F_3/2→⁴I_13/2 (EPBM2 = −50 V). The performance of the EPBM allows to choose the output wavelength of Nd:GdVO₄ laser by switching the dc voltages and integrate monolithically with frequency conversion process which is useful in practical applications.