We report a design and demonstration of an electro-optically Q-switched intracavity optical parametric oscillator (IOPO) based on a unique ramped duty-cycle periodically poled lithium niobate (PPLN) in a diode-pumped 1064-nm Nd:YVO4 laser. The PPLN crystal, having a double-prism domain (DPD) structure with a domain period of 30 μm, can work simultaneously as an electro-optic (EO) beam deflector (and therefore an EO Q-switch in the laser cavity) and an optical parametric down converter. The characterized deflection sensitivity of the DPD PPLN device was 1.15°/kV-cm. At a 180-V Q-switching voltage and a 1-kHz switching rate, we measured a down-converted signal at 1550 nm with pulse energy of >8.1 μJ (or peak power of >2.3 kW) from the constructed IOPO at 7.5-W diode pump power. Continuous wavelength tuning of the IOPO signal was also demonstrated.
©2013 Optical Society of America
Electro-optic (EO) Q-switches are popular laser intracavity elements for producing fast laser pulses with high stability and repeatability. EO Q-switches are usually a Pockels cell made of birefringent crystals such as LiNbO3, LiTaO3, KDP, and KTP which are characterized by robustness and low insertion loss (during the high cavity-Q state). An EO beam deflector or diffractor based on these crystals has also been demonstrated to be a high-quality laser Q-switch [1, 2]. Those EO crystals are also important nonlinear-optic (NLO) materials and have been widely used in laser systems to realize efficient intracavity-pumped wavelength converters. It is thus possible to integrate the functionalities of an EO Q-switch and a NLO wavelength converter in a common substrate to be an element of a laser system to build an advantageous pulsed intracavity wavelength converter. Such integration is always desirable to further increase the compactness and robustness and reduce the element insertion loss of the laser systems working with multiple intracavity elements. Such an idea has been first realized by Taira et al. in an intracavity second harmonic generation (SHG) system with KTP crystal . However, in that system a special and careful arrangement of the crystal has to be made to achieve a proper crystal birefringence to meet the simultaneous requirement for the EO induced polarization-state switching and SHG phase-matching conditions. Quasi-phase-matching (QPM) technology  is known to solve the difficulty with the use of the critically birefringence phase-matched NLO wavelength conversion method. Besides its application in wavelength conversion, a QPM material has recently been exploited to realize several novel EO devices including EO laser Q-switches based on its unique EO effects [2, 5]. Furthermore, such EO Q-switches have been successfully integrated with a QPM wavelength converter in a single QPM crystal thanks to the highly engineerable characteristics of the QPM technology. The integration can be made in a single QPM domain structure and has been proven to be advantageous over those in a cascade/separate configuration; for example, a pulsed intracavity SHG system based on a single aperiodically poled lithium niobate (APPLN) crystal integrating the functionalities of a periodically poled lithium niobate (PPLN) Šolc-type Bragg Q-switch (with the access of the EO coefficient r51)  and a PPLN second harmonic generator has shown an efficiency higher than that of a system with a LiNbO3 crystal simply cascading the two PPLN devices . Another success was demonstrated in a pulsed intracavity optical parametric oscillator (IOPO) where a single 2-dimensional (2D) PPLN crystal was used to integrate the functionalities of a PPLN Bragg Q-switch (with the access of the EO coefficient r33)  and a PPLN optical parametric down converter (OPDC) to make possible the oscillation in a compact system , or otherwise an inconvenient folded extra-cavity optical parametric generation (OPG) system has been used for accommodating a cascading PPLN device . In this work, we have further proposed a unique double-prism domain (DPD) PPLN crystal that can integrate the functionalities of an EO beam deflector (and therefore an EO Q-switch when working in a laser cavity) and a wavelength converter, by which we have demonstrated a compact and efficient pulsed IOPO tunable in the telecom/eye-safe band (1.5-1.6 μm). Though EO prism devices have been studied and widely used as an optical scanner or switch [9, 10], this work presents a novel device scheme and demonstrates, to the best of our knowledge, the first application of such a device in a laser system to work as an EO Q-switch (and as a NLO QPM OPDC simultaneously).
2. Device design, fabrication, and characterization
An EO beam deflector, when working in a laser system, can be an efficient active Q-switch whose hold-off capacity (the loss difference between the low- and high-Q states) increases with increasing deflection angle (θ) in the small-angle deflection regime (sinθ~θ<<1). A double-prism EO beam deflector consisting of a pair of crystallographic z-axis reversed, right-angle EO-crystal prisms with their hypotenuse surfaces closely bonded could be one of the simplest configurations among various EO beam deflectors . The double-prism EO beam deflector can also be implemented in a monolithic ferroelectric crystal where the domain of one of the prism sections is poled to have an opposite polarity with respect to the other one . Figure 1(a) shows the schematic of such a DPD structure. When an external electric field, Ez, is applied along the z-axis of the crystal, the change of the EO induced refractive index in this DPD structure will cause the optical path length experienced by the beam traversing the structure to vary linearly along the transverse direction (i.e., crystallographic y-axis direction) of the beam, leading to the change of the beam propagation direction. Inspired by the QPM technique based on a ferroelectric crystal, we have worked out a unique QPM device in a monolithic LiNbO3 that cascades a number of the DPD structures in series along the crystallographic x-axis, forming a ramped duty-cycle PPLN, as schematically shown in Fig. 1(b). In our design, each DPD structure has a length of 30 μm (corresponding to the domain period (Λ) of the PPLN) in order to satisfy the QPM condition of the 1064 nm pumped 1550 nm and 3393 nm OPG process at 40°C. Besides being a PPLN OPDC, this DPD PPLN can function as an EO beam deflector as well and can behave the same as the single DPD EO beam deflector (see Fig. 1(a)) of the same device length. It can be shown that the deflection angle of a double-prism EO beam deflector is proportional to the magnitude of the gradient of the EO induced refractive index along the y-axis . Accordingly we can readily arrive at an expression for the deflection angle of a DPD PPLN device under the small index change and plane wave approximations asFig. 1(b)), N is the number of the DPD structures composing the PPLN device, Δne is the extraordinary refractive index change due to the application of Ez, Leff is the total effective working length, D and L are the device width and length, respectively, ne is the extraordinary refractive index, and r33 is the relevant EO coefficient. The rightmost expression in Eq. (1) is exactly the same as that derived for a single DPD EO beam deflector . The most efficient operation of this DPD PPLN EO beam deflector occurs when the structure width D is made equal to the beam size w (see Fig. 1(b)), since in this case the Leff is maximized and equal to the total length of the device L. In our design, the dimension of D is made to be 320 μm according to the laser mode size which is ~300 μm as estimated from the cavity configuration built for the IOPO (see below). Here we let the structure dimension (D) be slightly larger than the estimated beam dimension (w) to gain more assurance that the whole beam can be within the domain structure for a complete beam deflection. Calculation shows that the deflection angle of our DPD PPLN EO beam deflector (which was made in a half-mm-thick z-cut LiNbO3, see below) is 1.15°/kV-cm, or ~1° for a 3-cm long device with 300 V applied voltage according to Eq. (1).
Since the ramped duty-cycle PPLN OPDC and the DPD PPLN EO beam deflector (as an EO Q-switch) are to be co-operated in a common substrate in a Nd:YVO4 laser to make a compact pulsed IOPO tunable over a broad spectral band, it is thus essential for the DPD PPLN EO beam deflector to function insensitively to the temperature so that the Q-switching can operate effectively during the wavelength tuning of the IOPO via the temperature control of the PPLN OPDC in the same substrate. The temperature sensitivity of a DPD PPLN EO beam deflector can be obtained from Eq. (1), given by13, 14]. Considering that temperature tuning of an OPO system usually performs in a convenient range between room temperature and less than 200°C, the maximum angle deviation from the initial deflection angle of the device in this temperature tuning range will be less than 0.09°, implying such a DPD PPLN EO Q-switch can operate over a wide temperature range with almost the same hold-off capacity.
Another possible monolithic scheme of integrating the two device functionalities could be based on a substrate cascading a single DPD EO beam deflector and an ordinary (50%-duty-cycle) 30-μm period PPLN OPDC (of equal length assumed), as schematically shown in Fig. 1(c). It is interesting to compare the present DPD (or ramped duty-cycle) PPLN device with such a cascadingly integrated device. In terms of the nonlinear parametric gain, the effective nonlinear coefficient of a ramped duty-cycle PPLN can be obtained from the Fourier analysis of its unique periodic DPD structure via the formulaFigure 2 shows the 2D Fourier spectrum of the domain structure shown in Fig. 1(b), where a dominant spatial frequency at (fx, fy) = (~0.033, 0) μm−1 which corresponds to the reciprocal vector required for quasi-phase-matching the designated OPG process (i.e., for satisfying K = kp-ks-ki, where kp, ks, and ki are the wave vectors of the pump, signal, and idler waves along the crystallographic x axis, respectively) in LiNbO3 is resolved. The Fourier coefficient, being the reduced nonlinear factor (dr) obtained in Eq. (3), is found to be ~0.3185 (or 1/π) at this frequency, and is exactly half of that of a 50%-duty-cycle PPLN (which is 2/π). Since the nonlinear parametric gain of a QPM device is a function of the parametric gain coefficient (which is proportional to the effective nonlinear coefficient, deff = dr × d33, where d33 is the relevant nonlinear coefficient) multiplied by the device length, this implies that the two devices have the same parametric gain since the 50%-duty-cycle PPLN OPDC in the device shown in Fig. 1(c) uses only half of the device length. Nevertheless, since the single DPD EO beam deflector in the device shown in Fig. 1(c) takes up also half of the device length, the DPD PPLN device shown in Fig. 1(b), when working as a beam deflector, can have an important advantage of having only half of the operating voltage of the single DPD device shown in Fig. 1(c) according to Eq. (1). Besides, an interesting comparison can also be made with a 2D PPLN structure that works to integrate the dual functionalities of an EO Bragg diffractor and an OPDC , where we found the structure has the same reduced nonlinear factor (again obtained by using Eq. (3)) as the DPD PPLN.
We fabricated an array of DPD PPLNs each having a dimension of 320 μm × 27 mm and spaced by 300 μm from each other along the crystallographic y axis in a 30-mm-long (along the crystallographic x axis), 3-mm-wide (along the crystallographic y axis), and 0.5-mm-thick LiNbO3 using the standard electric-field poling technique . The array of DPD PPLNs was fabricated in the crystal in order to facilitate the search and access of the fine domain structure (with 320-μm width dimension) of the device during the optical alignment process. Figure 1(d) shows a typical microscopic image of an HF-etched z surface of our fabricated DPD/ramped duty-cycle PPLN. The two z surfaces of the DPD PPLN crystal were sputtered with NiCr alloy to form the surface electrodes for the Ez application. Both of the end (x) faces of this crystal were optically polished and had anti-reflection (AR) coatings for wavelengths at 1064 and 1500-1600 nm. The performance of this fabricated device when functioning as an EO beam deflector was first characterized by a M2~1.1, cw 1064-nm fiber laser. The DPD PPLN device was mounted on a crystal holder which was equipped with the temperature controlling and voltage applying mechanisms. A beam profiler, placed 3 cm downstream from the DPD PPLN crystal, was used to monitor the deflected laser mode and derive the deflection angle during the application of Ez. Figure 3(a) shows the measured deflection angle of the device as a function of the applied voltage for several different beam sizes (w~300, 200, and 100 μm) at 40°C. The laser beam size was changed via the use of different lens sets. An auxiliary alignment optics has also been employed to ensure that each measurement the beam was incident to the DPD PPLN at the same angle (at normal incidence). The black line represents the theoretical fit, indicating the deflection sensitivity (defined here as dθ/dV or theslope of the data shown in Fig. 3(a)) of the present device is independent of the incident beam size, as predicted by Eq. (1). However, the measurement results showed the deflection sensitivity obtained with a beam of a larger beam size (i.e., a less divergent beam) exhibited relatively better agreement with the theoretical fit which was made based on the plane wave approximation. The effect of the Gaussian beam divergence on the deflection sensitivity of the present device requires a further study. The inset of Fig. 3(a) shows the measured deflection angle of the device as a function of the applied voltage for several different working temperatures for a beam of beam size w~300 μm. The results have been in good agreement with the theoretical fits (in solid lines). Figure 3(b) shows the measured deflection efficiency and the corresponding deflection angle of the device as a function of the applied voltage for a beam of beam size w~300 μm. The deflection efficiency of the present DPD PPLN device can be >94% in this voltage range. It is also found that the efficiency exhibits a slow decline with increasing voltage, which can be attributable to the simple rectangular working-area design of the present device (the DPD PPLN has a constant width D), causing some outer portion of the beam to be deflected out of the structure earlier without making use of full length of the device. This implies more portions of the beam will experience shorter and therefore different working lengths with the increasing applied voltage, leading to the form of an elongated/extended beam mode at a higher working voltage. For the comparison, the inset of Fig. 3(b) shows the captured mode profiles of the beam after traversing the device when 0, 300, and 600 volts were applied, respectively. It is expected that the deflection efficiency of the present device can be further enhanced via the use of a trapezoidal (a horn-shaped) working-area design . Nevertheless, the >94% deflection efficiency from the present DPD PPLN EO device has been far superior than that with a PPLN EO Bragg device (~50% diffraction efficiency) at a proper working voltage (say, <300 V for fast EO switching)  and is sufficient for producing a high loss during the low cavity-Q state for achieving a high-quality Q-switched and high peak-power laser operation.
3. IOPO system construction and output performance characterization
The DPD/ramped duty-cycle PPLN crystal was then installed in a diode-pumped 1064-nm Nd:YVO4 laser to construct a compact, tunable pulsed IOPO. Figure 4 shows the schematic arrangement of such a novel system. The laser gain medium, pumped by a fiber-coupled 809-nm diode laser, is a 9-mm-long, a-cut 0.3-at. % Nd:YVO4 crystal with a 3 × 3 mm2 clear aperture. The 1064-nm laser oscillates in a linear 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 >99.5% reflectance at 1064 and 1520-1600 nm, respectively. M2 is a plane-plane dielectric mirror having ~99.6% reflectance at 1064 nm and ~99.4% transmittance at 1550 nm (>99.2% transmittance at 1520-1600 nm). The parametric down converted signal, generated via the ramped-duty-cycle, 30-μm period PPLN crystal intracavity pumped by the 1064-nm laser, builds up in a singly resonant oscillator formed by the mirror M1 and a plane-plane dielectric output coupling mirror (designated M3) having 82-83.5% reflectance at 1520-1600 nm.
In operation, we drove the DPD PPLN device with a ~300-ns-voltage-pulse train at a 1-kHz pulse rate. The 300-ns pulse duration corresponds to the Q-switch open time, during which the 1064-nm laser pulses are formed and depleted via the efficient intracavity optical parametric conversion process performed in the ramped duty-cycle PPLN. We found that a Q-switch voltage of ~180 V was sufficient to make the system functioning for at least a pump power up to 7.5 W that is maximum available in this experiment. The M2 factor of the output1550-nm beam was characterized to be ~1.5 at this maximum pump power. Figure 5(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 temperature of the ramped duty-cycle PPLN was controlled at 40°C. We obtained signal pulse energy of >8.1 μJ, corresponding to peak power of >2.3 kW from the system at diode pump power of 7.5 W. The inset shows a typical trace of the measured signal pulse train at this pump power, from which a peak-to-peak intensity fluctuation of <5% was found. Figure 5(b) shows the measured output signal spectrum of the IOPO at 7.5-W diode power at 40°C. The signal has a spectral width (FWHM) of ~1 nm which is ~3 times narrower than that of a single-pass (i.e., OPG) signal (dashed blue line, Fig. 5(b)) measured from the same system without the installation of the mirror M3. The results suggest that it took ~9 round trips for the OPO signal to build up . Figure 6 shows the temperature tuning of the IOPO signal wavelength (green dots) in a range from 1540 to 1590 nm. The corresponding idler wavelengths (blue squares), deduced according to the energy conservation law, were also displayed. These signal/idler data agree well with the theoretical temperature tuning curve (red line) plotted according to the crystal thermo-optic data .
We have successfully designed and constructed a unique double-prism domain (or ramped duty-cycle) PPLN crystal that integrates the functionalities of an EO beam deflector and an OPDC to work in a compact diode-pumped, 1064-nm Nd:YVO4 laser to realize an efficient tunable pulsed IOPO. The DPD PPLN crystal, having a structure width of 320 μm and a domain period of 30 μm, can work as an EO beam deflector with a normalized deflection sensitivity of 1.15°/kV-cm and can simultaneously work as a QPM wavelength converter for performing the 1064 nm pumped 1550 and 3393 nm OPG process at 40°C. When driving the DPD PPLN with 180-V pulses at 1 kHz to Q-switch the 1064-nm laser, we obtained a signal at 1550 nm from the IOPO system with a pulse energy of >8.1 μJ, corresponding to a peak power of >2.3 kW, at 7.5-W diode pump power. The signal wavelength tuning of the IOPO via the temperature control of the ramped duty-cycle PPLN was also demonstrated in the telecom/eye-safe band.
This work was supported by the National Science Council (NSC) of Taiwan under Contract No. 102-2221-E-008-099-MY2. The authors acknowledge the High-Energy Optics and Electronics (HOPE) Laboratory at National Tsing-Hua University, Taiwan, for providing the measurement instruments. The authors also thank the Thin Film Technology Center (TFTC) at National Central University, Taiwan, for offering the service of the optical coatings.
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