We report on a tunable intracavity optical parametric oscillator (IOPO) achieved using a two-dimensional (2D) periodically poled lithium niobate (PPLN) as simultaneously an electro-optic (EO) Bragg Q-switch and an optical frequency mixer (OFM) in a diode-pumped Nd:YVO4 laser. The 2D periodic domain inversion structure is designed to provide two orthogonal reciprocal vectors to respectively satisfy the phase-matching conditions required by the two quasi-phase-matching devices (i.e., the PPLN EO Bragg deflector and the PPLN OFM). At a ~140-V Q-switching voltage and a 1-kHz switching rate, we obtained a signal wave at 1550 nm with a pulse energy of 9.7 μJ (corresponding to a peak power of ~2.4 kW) from the IOPO at 9.1-W diode pump power. Simultaneously we also observed multi-wavelength generation from the system originating in the single-pass parametric conversions in the 2D nonlinear photonic crystal structure. Temperature tuning of the IOPO signal wavelength in the eye-safe region was also demonstrated.
© 2011 OSA
Compact, high peak-power coherent light sources tunable in the eye-safe wavelength range (1.5-1.6 μm) are of particular interest for many applications such as optical ranging and remote sensing. The use of all-solid-state Q-switched laser pumped optical parametric generation/oscillation technique has been one of the most popular means of attaining such light sources. Optical parametric oscillators (OPOs) either external or internal to a Q-switched laser have been demonstrated [1,2]. Intracavity nonlinear frequency conversion takes advantage of the high pump power in the cavity to attain high nonlinear conversion efficiency. However, the development of intracavity OPOs (IOPOs) since 1970s  did not seem to draw much attention until recently when compact and efficient Q-switched lasers and advanced nonlinear optical materials were made available. Besides, Taira et al. has first integrated an electro-optic (EO) Q-switch and a nonlinear wavelength converter in a monolithic KTP crystal in a laser system to achieve a highly compact, pulsed intracavity frequency doubler . Such an intracavity-element integration technique has played an essential role in the development of internally Q-switched lasers  and self-frequency converters .
Quasi-phase-matching (QPM) materials, fabricated in a ferroelectric crystal like LiNbO3 (known as the periodically poled lithium niobate (PPLN)) whose domain polarity is periodically reversed in space, have been widely used to demonstrate efficient optical frequency mixers (OFMs) including OPOs [1,7]. In addition to the nonlinear coefficients, the periodic domain reversal structure modulates also the sign of the EO coefficients. Recent studies on the EO effects of QPM materials have led to the development of several interesting photonic devices including a Bragg deflector  and a polarization mode converter (PMC)  at the modulation of the diagonal and off-diagonal dielectric tensor elements of the materials, respectively. Interestingly, both the PPLN EO Bragg deflector and PMC have been exploited to implement laser Q switches [10,11]. Since the crystal domains of a QPM material can be engineered with high flexibility, the fabrication of multiple QPM devices of different domain structures in a single crystal is viable. The integration of a PPLN EO PMC and a PPLN optical parametric gain medium (OPGM) in a monolithic LiNbO3 to accomplish a pulsed IOPO in a Nd:YVO4 laser has been demonstrated . Though such a monolithic design has facilitated the construction of a compact and efficient system, it hindered the attainment of wide wavelength tuning of the OPO with temperature, as the working bandwidth of the PPLN EO PMC is as narrow as ~2.2°C-cm at the 1064-nm pump wavelength . A similar integration idea has been conducted to achieve a pulsed eye-safe optical parametric generator (OPG) pumped by a Nd:YVO4 laser Q-switched by a PPLN EO Bragg deflector in the same substrate with the OPGM . In contrast to a typical QPM device, a PPLN EO Bragg deflector is characterized by a wide operational bandwidth due to its non-critically longitudinal phase-matching process at the small-angle Bragg diffraction scheme. Derived from the coupled-mode theory , we obtained the ratio (rΔT) of the temperature bandwidth of a PPLN EO Bragg deflector over that of a PPLN EO PMC, given byEq. (1) has almost a constant value (~0.8-0.9) according to the temperature dispersion of LiNbO3 over a wide spectral range (0.4-4.5 μm) , the bandwidth ratio rΔT depends on θB predominantly. The ΔTBragg can then be several orders of magnitude larger than the ΔTPMC at a small θB. The wide temperature acceptance bandwidth of a PPLN EO Bragg deflector has facilitated the system in Ref.  to achieve a wavelength tunable light source via temperature tuning. However, the monolithic PPLN design in Ref.  requires the pump laser beam to fold twice to accomplish the eye-safe laser generation, making it difficult to be integrated in an intracavity scheme (and therefore in an IOPO) for the efficient wavelength conversion. It is thus desirable to construct a new integrated QPM device to advance the realization of a wide wavelength-tunable, high peak-power IOPO in a simple system configuration. In this work, we have constructed a two-dimensional (2D) PPLN device where a temperature-insensitive EO Bragg Q-switch has been integrated collinearly with an OPGM, with which we demonstrated a compact, tunable pulsed IOPO in the eye-safe region in a diode-pumped Nd:YVO4 laser.
2. System design and construction
A PPLN EO Bragg deflector is an efficient and fast (nanosecond response time for a bulk application) optical beam deflector, driven by an appropriate external electric field Ez applied along the PPLN crystallographic z axis to access its largest EO coefficient r33 (~31 pm/V at 633 nm) . In contrast to the collinearly phase-matching scheme employed in a PPLN OFM, the PPLN EO Bragg deflector generally works in a scheme where the interacting wave vectors are almost orthogonal to the PPLN grating vector. For example, for a 20-μm period PPLN, the 1st-order Bragg angle θB is as small as 0.7° (which is 89.3° with respect to the PPLN grating vector) for a 1064-nm wave according to the well-known Bragg diffraction formula,1,15], has a dimension of 40 mm in length (along crystallographic x axis), 8 mm in width (along crystallographic y axis), and 0.78 mm in thickness. Figure 1 shows the schematic arrangement of the pulsed IOPO constructed in a 1064-nm Nd:YVO4 laser using the fabricated 2D PPLN. The laser gain medium is an a-cut 0.3-at. % Nd:YVO4 crystal, having a 3 mm × 3 mm clear aperture and 9 mm length. The pump laser is a fiber coupled diode laser at 809 nm. The 1064-nm laser oscillates in a cavity formed by two high-reflection mirrors, designated M1 and M2. M1 is a 15-cm radius-of-curvature meniscus dielectric mirror having ~93% transmittance at 809 nm and ~99.8% and >99.3% reflectance at 1064 and 1530-1580 nm, respectively. M2 is a plane-plane dielectric mirror having ~99.6% reflectance at 1064 nm and ~99.4% transmittance at 1550 nm (>96% transmittance at 1500-1800 nm). A singly resonant oscillator (SRO), pumped by the 1064-nm laser intracavity power, resonates at the optical parametric signal generated from the 2D PPLN in a resonator formed by mirror M1 and an output coupling mirror (designated M3). M3 is a plane-plane dielectric (BK7) mirror having ~60% reflectance at 1550 nm. The use of a relatively high output coupling ratio (40%) in mirror M3 is to suppress the emergence of unwanted satellite signal pulses usually observed in a high-gain IOPO [7,16]. Such a three-mirror configuration is probably the simplest cavity scheme that an IOPO can be constructed in a linear fashion. The arrangement of the cavity mirrors in the present scheme (refer to Fig. 1) also allows a great freedom of alignment for the mirror M3 during the system optimization without interfering the lasing of the 1064-nm pump wave. The inset of Fig. 1 shows a microscopic image of an HF-etched z surface of the 2D PPLN, revealing a tetragonal domain structure whose periodicities along the crystallographic x and y axes form a QPM grating of 30-μm period to phase-match the 1064 nm pumped 1550 and 3393 nm optical parametric generation process at 40°C and an EO Bragg grating of 20-μm period to phase-match the 1064-nm Bragg diffraction process at θB~0.7°, respectively. A 20-μm-period Bragg grating was used due to the limit of our current electric-field poling technique for 2D domain engineering in LiNbO3 (mainly limited by the lithographic definition process for such a 2D pattern), though a Bragg deflector with a shorter Bragg grating period and therefore a larger Bragg angle (refer to Eq. (2)) might be advantageous to its operation as a laser Q-switch since the laser diffraction loss (and therefore the hold-off capacity) introduced at the low-Q state increases with the diffraction angle. The two z surfaces of the 2D PPLN crystal were sputtered with NiCr alloy (in an area of 40×6 mm2) as the electrodes to apply the Ez in the crystal. Both of the end (x) faces of the crystal were optically polished and had an anti-reflection (AR) coating for wavelengths at 1064 and 1500-1600 nm.
Figure 2 shows the transmittance of a 1064-nm wave in the 2D PPLN when working as an EO Bragg deflector (monitored at the zeroth-order direction) as a function of the applied voltage. The result was characterized by a 1064-nm fiber laser of beam radius ~110 μm and M2~1.1 incident at the Bragg angle (~0.7°) in the crystal. The shape of the transmission curve has resembled to that obtained in a typical 1D PPLN EO Bragg deflector . It can be found from Fig. 2 that the half-wave voltage  and the corresponding diffraction efficiency of the 2D PPLN EO Bragg deflector were ~140 V and ~50%, respectively. Since a 1D PPLN EO Bragg deflector with a device length of ~1.9 cm has a theoretical half-wave voltage of 140 V , the 1.9-cm length can then be regarded as the effective coupling length with the present device (whose physical length is 4 cm). The reduction on the coupling length can be attributable to the discrete Bragg grating configuration formed by the 2D domain inversion structure. A further study of this effect is underway.
3. Output performance characterization and analysis
In operation, we drove the 2D PPLN with a 140-V, 300-ns voltage-pulse train at 1 kHz to activate its functionality as an EO Bragg Q switch. The 300-ns pulse duration corresponds to the Q-switch open time or the high cavity Q state, during which the 2D PPLN is pumped for parametric oscillation. Figure 3(a) shows the measured output pulse energy and pulse width of the IOPO signal at 1550 nm as a function of the diode pump power when the 2D PPLN was temperature controlled at 41.5°C. At the diode pump power of 9.1 W, we obtained the signal pulse energy of ~9.7 μJ, corresponding to a peak power of ~2.4 kW from the system. The energy fluence of the intracavity 1064-nm laser at this diode pump power was estimated to be on the order of 0.1 J/cm2. The damage threshold of PPLN has been confirmed to be identical to that of a single-domain lithium niobate, which is 3 J/cm2 for a 10-ns pulsed laser at 1064 nm . Figure 3(b) shows the measured 1550-nm signal pulse (~4-ns pulse width) and the corresponding 1064-nm pump pulse (depleted, green line) at the diode power of 9.1 W. A train of the output 1550-nm signal pulses was also recorded, as shown in the inset of Fig. 3(b). A peak-to-peak intensity fluctuation of ~5% was found. Since the 2D PPLN used in the present system is equivalently a 2D nonlinear photonic crystal (NPC) , multiple wavelength conversions via the optical parametric generation processes phase-matched in such a 2D reciprocal lattice are possible according to the QPM condition, given byFigure 4(a) shows the calculated signal (solid lines; refer to the left vertical axis) and the corresponding idler (dashed lines; refer to the right vertical axis) wavelength tuning curves versus their phase-matching angles with respect to the crystallographic x axis (symbolized by θs and θi for the signals and idlers, respectively) for the 1064-nm pump wave incident at near the Bragg angle (θp~0.77°) in the 2D PPLN at 41.5°C. In this pump configuration and crystal temperature, the signal generations predicted in the calculation agree with the measured spectral data (see below). The system remains in good operation since the 0.07° angle difference between the Bragg and pump incident angles is wellwithin the characterized beam divergence angle (~ ±0.2°) of the 1064-nm pump laser mode. In Fig. 4(a), only those conversion processes associated with some lowest orders of Kmn like K1,0, K1,1, K1,-1, and K1,2 are considered because they not only obtain relatively high nonlinear gains but also produce signal waves in the spectral range of interest (i.e., in the eye-safe spectral region). Though one can find at least one phase-matching signal (idler) wavelength for every θs (θi) in such a 2D NPC, enhanced parametric generations can be expected on those θs (θi) where, Case I, the signal and idler are collinearly coupled (as those degenerate points of the wavelength tuning curves, or points A, B, C, and D labeled in Fig. 4(a)) and, Case II, two reciprocal vectors contribute to the same signal (or idler) wavelength generation (as those intersection points of the signal curves: D, E, and F and of the idler curves: G, H, I, and J labeled in Fig. 4(a)) according to the study in Ref. . The wave-vector matching diagramsof some representative QPM schemes (A-D and G) were plotted in Fig. 4(b) for reference. The increase of optical parametric gain is expected in Case I due to the least beam walk-off problem between the signal and idler couplings, while it is due to the enhancement of the effective nonlinear coefficient in Case II . It deserves to note that point D in Fig. 4(a) presents a special scheme in which the parametric signal is commonly generated from two QPM processes involving both Cases I and II of gain enhancement. Figure 4(c) shows the measured output signal spectrum of the IOPO at 9.1-W diode power with a grating (600 grooves/mm) spectrometer of 0.8-nm spectral resolution. In this measurement a cylindrical lens was used to facilitate the collection of those off-angle signals as well as the 1550-nm resonant signal. We indeed observed the emergence of multiple signal wavelengths over the spectral range of our measurement (1525-1800 nm), as shown in Fig. 4(c). The letters (A to J) labeled aside each measured spectral peak denote the QPM schemes (refer to Fig. 4(a)) that contribute to that wavelength generation. The letters in parentheses denote the QPM schemes that contribute to signal generations whose wavelengths are located within the spectral band of an observed peak and might jointly contribute to the formation of that spectral peak. The inset shows the enlargement of a lower portion of the obtained spectrum to present a more detailed spectral content. The 1550-nm signal oscillates mainly with the QPM scheme A using K1,0 (see Figs. 4(a) and 4(b)). The linewidth of this resonant signal was ~5.4 nm. A relatively broad signal spectrum measured in this work (when compared to a typical pulsed OPO system using 1D PPLN as the OPGM ) can be attributable to the characteristics of the nonlinear conversion in a 2D PPLN where multiple QPM schemes are possible to jointly contribute to a signal generation. In our system, the divergence angle of the 1550-nm resonant beam in the cavity was characterized to be ±0.27° (with a beam quality factor of M2~1.6) with respect to the cavity axis (or ± 0.27° with respect to the θs corresponding to the point A labeled in Fig. 4(a)). In this signal beam divergence, a span of signal wavelengths from 1550 to 1555 nm phase-matched using K1,0 and importantly one of the signals (~1553 nm; see the point S labeled in Fig. 4(a)) originating from the QPM scheme G (see also Fig. 4(b)) are within the resonant bandwidth and might result in the forming of a broader signal linewidth. Compared to the 1550-nm resonant wave, other simultaneously measured signals were with much less intensity due to their non-resonant (off-angle), single-pass generation process. A broadly continuous wavelength generation originating from the off-angle parametric conversions in the 2D PPLN could be the main constituent of the background spectrum, resulting in the presence of a certain noise level in the output spectrum. Nevertheless, some wavelengths generated with the gain-enhanced QPM schemes B-J can be identified, as indicated in Fig. 4(c). In particular, the gain enhancement effect with the QPM scheme D is indeed obvious. Besides, the relatively long intersection of the K1,1 and K1,2 idler curves along the vertical (wavelength) axis at scheme J (see Fig. 4(a)) has resulted in the formation of a signal band around 1790 nm (see Fig. 4(c)).
Continuous signal wavelength tuning in the present IOPO system was also demonstrated in the eye-safe region via the temperature control of the 2D PPLN, as shown in Fig. 5 (the corresponding idler wavelengths were calculated according to the energy conservation law). The experimental data (green squares) agree well with the theoretical fit (red line) based on the Sellmeier equation of the crystal .
We have designed and fabricated a 2D PPLN crystal for simultaneously being an EO Bragg Q-switch and an OPGM in a diode-pumped, 1064-nm Nd:YVO4 laser to realize a compact, tunable pulsed IOPO. The 2D PPLN, building in a z-cut, 40-mm long (along crystallographic x axis), 8-mm wide (along crystallographic y axis), and 0.78-mm thick LiNbO3, has a tetragonal domain structure whose periodicities along the crystallographic x and y axes form QPM gratings of 30- and 20-μm periods, respectively, phase-matched to the nonlinear optical conversion processes in the PPLN OPGM and the PPLN EO Bragg deflector. When driving the 2D PPLN with 140-V pulses at 1 kHz, we obtained a signal at 1550 nm from the IOPO system with a pulse energy of ~9.7 μJ, corresponding to a peak power of ~2.4 kW, at 9.1-W diode pump power. We also observed multi-wavelength parametric generation from the system, as predicted from the characteristic phase-matching schemes in the 2D NPC structure. Continuous wavelength tuning of the IOPO signal in the eye-safe region via the temperature control of the 2D PPLN was also achieved.
This work was supported by the National Science Council (NSC) of Taiwan under Contract Nos. 98-2221-E-008-013-MY3 and 100-2623-E-008-007-D. The authors thank the Thin Film Technology Center (TFTC) at National Central University, Taiwan, for offering the service of the optical coatings.
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