1. Introduction

Continuous-wave (cw) optical parametric oscillators (OPOs) combine a large single-frequency tuning range from the visible to the near infrared with narrow linewidth and are therefore important light sources for spectroscopy and other applications. Using lithium niobate as the nonlinear optical material, they have been realized with idler outputs up to 4.7 μm [1]. At higher pump levels, additional spectral components are reported and commonly assigned to Raman scattering processes [2, 3]. To investigate the origin of this feature we present a SROPO based on lithium niobate crystals with QPM periods from 24.4 to 31.5 μm generating idler wavelengths from 2.33 to 5.32 μm. With experimental data from this setup we provide clear evidence, that most of the observed peaks originate from cascaded phase-matched optical parametric processes.

2. Experimental setup

Our experimental setup, sketched in Fig. 1, comprises a singly-resonant OPO in a bow-tie configuration with two concave mirrors (curvature radius 100 mm) and two plane ones. The nonlinear medium is a 5 % MgO-doped congruent PPLN crystal (HC Photonics Corp.), measuring 50 × 8.2 × 0.5 mm³. To cover a large tuning range, this setup is implemented in two different configurations: for a crystal with 7 QPM periods, 24.4, 24.8, 25.0, 25.3, 25.6, 26.0, and 26.4 μm, we use a Peltier driven oven with a temperature range up to 150 °C and a mirror set with reflectivities higher than 99.9 % in a wavelength range from 1200 to 1400 nm. The second crystal, with period lengths from 28.5 to 31.5 μm in 0.5 μm increments, is positioned on an oven with a temperature range of 60 to 200 °C. In this configuration, a mirror set reflecting more than 99.9 % for signal wavelengths from 1400 to 1800 nm is used. All crystal surfaces are antireflection coated with a residual reflectivity smaller than 1 % at pump and signal wavelengths. The pump source is a single-frequency 20 W cw Yb:YAG disk laser at 1030 nm (VersaDisk, ELS). Pump, signal, and idler waves are polarized along the optical axis of the nonlinear crystal. We record the spectra of the signal field either with a Burleigh WA-650 spectrum analyzer in combination with a Burleigh WA-1500 wavemeter which provides a spectral resolution of 4 GHz or with an Agilent 86140B spectrum analyzer, having 10 GHz resolution. The idler and residual pump powers are measured after being separated by a dichroic mirror.

 

Fig. 1. Schematic illustration of the experimental setup: P _p, P _p ^*, P _s, and P _i represent powers of the single-frequency pump wave, its transmitted portion, the signal wave, and the idler wave. The signal wavelength is denoted by λ _s.

Download Full Size | PDF

 

Fig. 2. Measured signal wavelengths () and calculated idler wavelengths () for the QPM period lengths around 30 μm (left) and around 25 μm (right). The theoretical values () are calculated with a temperature dependent Sellmeier equation [4] considering the thermal expansion of the crystal [5].

Download Full Size | PDF

3. Results and discussion

3.1. Tuning range

One main aspect of optical parametric processes in nonlinear crystals is their wide tuning range. The measured signal and idler tunability reachable with the above mentioned SROPO system is depicted in Fig. 2, covering a signal wavelength range of 1.28 to 1.33 μm and a corresponding idler range of 4.5 to 5.3 μm for QPM periods around 25 μm. The crystal with period lengths centered around 30 μm provides a tuning range of 1.40 to 1.85 μm for the signal and 2.33 to 3.90 μm for the idler wavelength, respectively. Thereby, the single-frequency idler output power P _i ranges from several W at 2.8 μm to a few mW around 5 μm, while the pump threshold increases from 0.5 to 3 W. At this point it should be mentioned that the upper limit of 5.32 μm is set simply by the available minimum QPM period length of 24.4 μm. Thus, generation of idler waves with longer wavelengths is feasible. However, the 5.32 μm wavelength obtained is to our knowledge the largest reported idler wavelength reached with a cw SROPO.

3.2. Spectral features

At higher pump levels, additional spectral components appear with frequency shifts larger than the bandwidth of the optical parametric gain profile. A typical signal spectrum can be seen in Fig. 3. Besides the expected signal wave of the initial parametric oscillation at λ _s ¹ = 1304 nm with a gain bandwidth of 0.1 THz, one obtains four further wavelengths λ _s ², λ _s ³, λ _s ⁴, and λ _r with frequency shifts Δν₁, Δν₂, Δν₃, and Δν_r, respectively. In order to elucidate the origin of these additional peaks, Fig. 4 shows the measured dependence of the relative peak positions on the QPM period for a fixed crystal temperature of 60 °C. Spectral lines that are caused by stimulated Raman scattering will exhibit a constant shift. Indeed, we find that the relative offset of λ _r with respect to the initial signal wavelength λ _s ¹ is independent of the QPM period for the line at 7.6 THz (254 cm^-1). This is consistent with a prominent phonon absorption in MgO-doped PPLN crystals [6] as well as spontaneous Raman scattering spectra [7]. Under favorable conditions, we can also observe another Raman mode at 630 cm^-1. However, the other recorded frequency shifts cannot be explained this way.

 

Fig. 3. Measured spectrum for a crystal at 60 °C with a QPM period length of 25.6 μm and a pump power of P _p = 4.7 W. The expected signal frequency at 1304 nm is denoted as λ _s ¹ (other symbols: see text).

Download Full Size | PDF

3.3. Cascaded optical parametric processes

The relative positions of the components λ _s ², λ _s3, and λ_s ⁴ depend monotonously on the QPM period length, indicating that a phase-matching process is involved. They can, in fact, be explained by cascaded optical parametric processes sketched in Fig. 5: The initial phase-matched parametric oscillation a) converts the pump wave λ _p at 1030 nm to an idler wave λ _i ¹ and a resonant signal wave λ _s ¹. The key is: The latter one itself acts as a pump wave for further forward and backward optical parametric processes shown in Fig. 5b) and c). They are both phase-matched via the same QPM period of the nonlinear crystal satisfying the phase matching condition

or

respectively. Both of them can be continued to any higher order whereas the signal wave of the preceeding oscillation acts as a pump wave for the succeeding one, which can be seen in Fig. 4, where Δν₂ corresponds to two consecutive backward processes shown in Fig. 5b). The frequency offsets Δν₁ and Δν₃ in Fig. 4 correspond to the idler frequencies generated in the crystal and are measured to be tunable from 1.34 to 1.70 THz and 3.06 and 3.59 THz, respectively by changing the OPM period length from 31.0 to 24.4 μm. In contrast to the initial singly-resonant optical parametric oscillation, the cascaded processes are resonant for their pump and signal waves. The direct forward and backward terahertz generation with the above mentioned processes has already been shown in pulsed systems [8].

The theoretical values presented in Fig. 4 are calculated using a temperature dependent Sell-meier equation for 5 % MgO-doped congruent lithium niobate [4] which is valid for the near infrared pump and signal waves. To calculate the refractive index of the idler waves in the terahertz range, we use a Sellmeier equation for 6.1 % MgO-doped congruent lithium niobate at 300 K ranging from 1 to 5 THz [9].

4. Conclusions

We have demonstrated a SROPO delivering cw idler waves up to 5.32 μm wavelength. Within this singly-resonant system further cascaded optical parametric oscillations generating idler waves in the terahertz regime have been detected. The theoretical model allows to reassign many spectral components with frequency offsets less than 250 cm^-1, which are commonly observed in OPO signal spectra, to optical parametric processes. Furthermore, the prominent phonon absorption around 250 cm^-1 in MgO-doped lithium niobate is confirmed. Our model predicts the generation of tunable cw terahertz generation in the nonlinear crystal so that cascaded optical parametric oscillations may serve as a useful cw THz source.

 

Fig. 4. Measured frequency shifts Δν₁ (), ∆ν₂ (), Δν₃ (), and Δν_r () for the additional spectral components with respect to the initial signal wavelength λ _s ¹ (see Fig. 3). The theoretical values () are calculated using a temperature independent Sellmeier equation in the terahertz regime [9]. The error bars are of the order of the symbol size and therefore not drawn. Details are given in the text.

Download Full Size | PDF

 

Fig. 5. Schemes of the cascaded optical processes (top) and their wave vector diagrams (bottom). The initial parametric oscillation a) converts the pump wave λ _p at 1030 nm to the resonant signal wave λ _s ¹ which acts as a pump source itself for the subsequent parametric backward oscillation b) and the parametric forward oscillation c). Forward and backward denotes here the propagation direction of the idler wave with respect to the pump wave.

Download Full Size | PDF