July 1, 2006 / Vol. 31, No. 13 / OPTICS LETTERS
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Cascaded optical parametric oscillation with a dual-grating periodically poled lithium niobate crystal Pavel V. Gorelik and Franco N. C. Wong Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Dmitri Kolker and Jean-Jacques Zondy Institut National de Métrologie, Conservatoire National des Arts et Métiers, 61 rue du Landy, 93210 La Plaine St. Denis, France Received March 8, 2006; accepted April 11, 2006; posted April 19, 2006 (Doc. ID 68809) We demonstrate continuous-wave cascaded optical parametric oscillation in which the signal field of the primary parametric oscillator internally pumps the secondary parametric oscillator. Wavelength tuning is achieved with temperature tuning and a fan-out grating structure of a dual-grating periodically poled lithium niobate crystal. Above the secondary threshold the primary signal power is clamped, and all the other output powers increase linearly with the input pump power, in accordance with theory. Cascaded parametric oscillation offers a convenient and efficient way to generate multiple tunable outputs. © 2006 Optical Society of America OCIS codes: 190.4970, 190.2620, 190.4180.
Multiple interactions in a 共2兲 nonlinear crystal such as periodically poled lithium niobate (PPLN) is a convenient method to provide enhanced functionality of a nonlinear optical device. Schneider and Schiller1 used a single MgO: LiNbO3 crystal in an external resonant cavity for second-harmonic generation of 532 nm light that served as an internal pump for a nearly degenerate doubly resonant optical parametric oscillator (OPO) using the same crystal and cavity. Additional frequencies were also generated by sum-frequency mixing of the OPO signal and idler outputs with the input light at 1064 nm. However, tuning was limited because all the nonlinear interactions must take place within the narrow phasematching bandwidth of the single crystal. The technique of quasi-phase-matching and the availability of multiple gratings in a single PPLN crystal offer flexibility in achieving multiple interactions by allowing each nonlinear process to take place in its own grating section. Back-to-back differencefrequency generation in a dual-grating PPLN chip has been used to achieve optical frequency division by 3.2 More recently, we have realized self-phase locking in a divide-by-3 OPO by using a dual-grating PPLN chip in a triply resonant cavity.3 In this Letter we take the concept of multiple interactions to a new level of flexibility and tunability in a convenient setup. First, we incorporate a fan-out grating section4 in a dual-grating PPLN chip. The phase matching of the uniform-grating first section is tunable by temperature. The second section consists of a fan-out grating that we tune by a simple translation of the chip across the variable-period grating without affecting the first-section phase matching. We utilize this dual-grating design to demonstrate a tunable cascaded OPO in a cavity that is resonant at all generated frequencies. Pumped at 532 nm, the first grating generates the primary OPO (P-OPO) 0146-9592/06/132039-3/$15.00
while the secondary OPO (S-OPO) is internally pumped by the primary signal field. We show a wide tunability of ⬃200 nm by translating across a portion of the fan-out section. Compared with the square-root dependence of a conventional OPO, the power transfer from the pump to the S-OPO outputs is more efficient with a linear proportionality. Moreover, we observe optical limiting for the primary signal output while the pump is no longer clamped above the S-OPO threshold. Consider an input pump power ep2 (in units of photons per second), positive nonlinear couplings 1 and 2 for the two grating sections, and a round-trip field loss rate j, where the subscripts j = p, s, i, +, and − correspond to the pump, primary signal, primary idler, secondary signal, and secondary idler, respectively. The zero-detuning equations for the internal modes Aj are ˙ = − A − A A + 冑2 e , A p p p 1 s i p p
共1兲
˙ = − A + A A* − A A , A s s s 1 p i 2 + −
共2兲
˙ = − A + A A* , A i i i 1 p s
共3兲
˙ = − A + A A* , A + + + 2 s −
共4兲
˙ = − A + A A* . A − − − 2 s +
共5兲
˙ = 0, we obtain Writing Aj = rj exp共ij兲 and setting A j the steady-state solutions for two regimes: above threshold for the P-OPO but not for the S-OPO, and above threshold for both. For only the P-OPO, 2 = 0 and we recover the standard zero-detuning solution
p = s + i,
rp2 = si/12 ,
© 2006 Optical Society of America
共6兲
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OPTICS LETTERS / Vol. 31, No. 13 / July 1, 2006 2 2sr2s = 2iri2 = 4ep1 共ep/ep1 − 1兲,
共7兲
2 ep1 = psi/212 ,
共8兲
showing that the internal pump field rp is clamped. Note in Eq. (7) that the slope efficiency of the outputs has a square-root power dependence on the pump. At higher pump powers the signal is strong enough to drive the S-OPO with nonzero outputs A+ and A−. The steady-state solution to the full set of equations is
p = s + i,
s = + + − ,
共9兲
2 2sr2s = 2s+−/22 ⬅ 4ep2 ,
共10兲
2 ˜ p2 − 1兲, 共ep2/e 2+r+2 = 2−r−2 = 4ep2
共11兲
2 2 2 ˜ p, ep/e 2prp2 = 4ep1
2 2 2 ˜p , 2iri2 = 4ep2 ep/e
2 2 2 2 ˜ep2 = ep1 共1 + ep2 /ep1 兲 .
共12兲 共13兲
In the case in which the pump is nonresonant, with 2 2 p much larger than the other loss rates, ep1 ep2 and 2 the S-OPO threshold ˜ep is only slightly higher than 2 . As expected, the internal the P-OPO threshold ep1 pump field rp is no longer clamped owing to the presence of the S-OPO. On the other hand, Eq. (10) shows that the primary signal field rs is clamped, and the cascaded OPO serves as an optical limiter for As.1,5 Let us note that when A± are tuned to degeneracy with A+ = A− = Ai, we retrieve the case of the selfphase-locked 3-to-1 OPO3 in which the static and dynamic behaviors are quite different.6 In this latter device, neither the pump nor the signal undergoes clamping, and the signal and idler powers are lower than those of a conventional 共2 = 0兲 OPO.3,6 Most interestingly, the cascaded OPO is more efficient in transferring the input pump power to the other outputs with a linear slope efficiency. Figure 1 shows the experimental setup. The pump was a cw single-frequency 532 nm Coherent Verdi-8 laser. We used a 20 mm long PPLN crystal (HC Photonics) with a 13 mm first section of a 7.2 m grating period that could be phase matched for nondegener-
Fig. 1. Schematic of experimental setup. Cascaded OPO output powers are monitored and output wavelengths are measured. LP1, 1 m long-pass filter; LP2, 0.7 m longpass filter; BP, 532 nm bandpass filter.
Fig. 2. Typical traces for pump, signal, and 1.6 m outputs versus cavity PZT scan for input pump power of (a) 107 mW and (b) 321 mW. Dashed line in (a) indicates clamping level of primary signal under S-OPO operation.
ate OPO outputs at ⬃0.8 and ⬃1.6 m. The PPLN temperature was typically set between 150°C and 220°C to allow P-OPO tuning. The PPLN chip had a 7 mm second section with a fan-out grating structure whose periods varied from 19.45 to 19.85 m over the 10 mm width of the crystal. At a fixed temperature, the PPLN chip could be translated along its width to access different grating periods of the fan-out section without affecting the phase-matching condition for the P-OPO. The fan-out section was designed for pumping by the primary signal at 0.8 m with tunable outputs centered at 1.6 m. The crystal was antireflection coated at the pump, signal, and idler wavelengths with ⬃0.25% losses at each facet. The optical cavity was doubly resonant at the signal and idler wavelengths of the P-OPO with an input mirror M1 (60 mm radius) that was highly reflecting at 0.8 and 1.6 m and ⬃96% transmitting for the pump. The output mirror M2 (25 mm radius) had output coupling coefficients of 0.44% (0.37%) for the 0.8 m signal (1.6 m idler) and ⬃0.26% for the pump. The M1 was mounted on a piezoelectric transducer (PZT) for scanning and stabilizing the cavity length. The mirrors were separated by ⬃8.5 cm and were attached to an aluminum spacer block without the use of any spring-loaded mirror mounts for improved mechanical stability. We aligned the cavity by translating the mirror mounting plates that were then tightly secured. Final cavity alignment was accomplished by pump beam adjustments. 2 兲 of We measured a typical P-OPO threshold 共ep1 43 mW (the minimum observed was 35 mW) and a
July 1, 2006 / Vol. 31, No. 13 / OPTICS LETTERS
˜ p2兲 of 70 mW, confirming that the S-OPO threshold 共e secondary threshold was not much higher than the primary one. Figure 2 shows typical traces of the pump, signal, and idler, and the S-OPO outputs at 1.6 m under PZT scanning at two different pump levels. In Fig. 2(a) at a pump level of 107 mW, S-OPO operation is evident at a number of locations at which the signal output drops from a higher conventional OPO output level to the S-OPO clamping level, as indicated by the dashed line. At these locations we observe a corresponding increase in power for the pump and the 1.6-m outputs. In Fig. 2(b) the pump level is 321 mW, which is substantially above the S-OPO threshold, and the S-OPO operation can be sustained over a longer period of PZT scan time (cavity detuning). Note also that the 1.6 m output power is significantly higher under S-OPO operation than that when the system is below the S-OPO threshold. Figure 3 plots the output powers for the pump, primary signal, and total 1.6 m outputs (consisting of the primary idler and the secondary signal and idler) as a function of the input pump power. The measurements are extracted from traces similar to Fig. 2 near the peaks of the envelopes (approximating zero cavity detuning) at different input pump powers from 64 to 321 mW. The solid lines represent the linear fits to the data but exclude those taken at the lowest pump power of 64 mW at which the S-OPO was below threshold. From the output signal power we infer that the internal signal power of the cascaded OPO was ⬃56 mW. Figure 3 clearly shows that the output pump (through M2) and the 1.6 m outputs are proportional to the input pump power. The observed cascaded OPO behavior in Figs. 2 and 3 are in good agreement with our simple theoretical model. We demonstrate the tunability of the S-OPO without affecting the P-OPO by accessing the different
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Fig. 4. Plot of P-OPO idler (filled squares), S-OPO signal (filled triangles), and idler (filled circles) wavelengths at different locations along the fan-out grating section.
grating periods of the fan-out section of the dualgrating PPLN chip. At a fixed temperature of 204°C and with the OPO cavity servo locked for cw operation, we monitored the primary signal wavelength with a wavelength meter and measured the 1.6 m outputs with an optical spectrum analyzer. Figure 4 plots the 1.6 m output wavelengths as functions of the relative beam location at the fan-out grating, showing a tuning range of 200 nm centered around the primary idler output at ⬃1596 nm. In summary, we have demonstrated a cascaded OPO in which the primary signal serves as an internal pump for the secondary OPO that can be tuned independently. An OPO is often used as a tunable pump source for another OPO. In a cascaded OPO, this pumping can now be internal to the optical cavity, resulting in more efficient power transfer with a linear slope efficiency. We have verified this power efficiency and have shown that this internal pump is clamped. The wide tunability of the outputs is facilitated by a fan-out grating structure showing a tuning range of ⬃200 nm, making the cascaded OPO a simple and efficient tunable source. This work was supported by the Department of Defense MURI program administered by ONR under grant N00014-02-1-0717 and by the MIT-France Seed Fund program. F. Wong’s e-mail address is
[email protected]. *Present address, Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02420. References
Fig. 3. Plot of (a) pump (filled circles), (b) signal (filled squares), and (c) 1.6 m output (filled triangles) powers versus input pump power. The S-OPO was above threshold except for data taken at 64 mW which are excluded for the linear fits (solid lines), showing clamping of the primary signal, and linear output powers for the pump, primary idler, and S-OPO outputs.
1. K. Schneider and S. Schiller, Opt. Lett. 22, 363 (1997). 2. P. T. Nee and N. C. Wong, Opt. Lett. 23, 46 (1998). 3. J.-J. Zondy, D. Kolker, and F. N. C. Wong, Phys. Rev. Lett. 93, 043902 (2004). 4. P. E. Powers, T. J. Kulp, and S. E. Bisson, Opt. Lett. 23, 159 (1998). 5. A. E. Siegman, Appl. Opt. 1, 739 (1962). 6. J.-J. Zondy, A. Douillet, A. Tallet, E. Ressayre, and M. Le Berre, Phys. Rev. A 63, 023814 (2001).
