1. Introduction

Optical parametric oscillators (OPOs) are valuable for continuous-wave (cw) infrared spectroscopy [1, 2]. They convert pump light to signal and idler light with larger wavelengths. Light emitted by OPOs exhibits a narrow linewidth and can be tuned continuously over a spectral range of several micrometers. The standard setup for OPO-based infrared spectroscopy includes a mirror resonator and a pump laser with a wavelength of about 1 µm [2]. A miniaturization of the setup can be facilitated using whispering gallery resonators (WGRs). They exhibit a size of at most a few millimeters and a chip-integrable geometry [3]. Additionally, the monolithic design makes antireflective coatings obsolete. WGR-OPOs show narrow linewidths and low power thresholds of optical parametric oscillation [4]. Efficiencies of more than 50 % with optical powers in the milliwatt regime as well as a cw output have been demonstrated, recently [5]. However, for broadband spectroscopy with WGR-OPOs pumped at around 1 µm wavelength, two main challenges have to be addressed. First, the nonlinear-optical process needs to be quasi-phase-matched using periodic poling of the crystal [6]. Second, single-mode operation and controlled adjustment of the OPO wavelengths over a wide range is essential. Quasi-phase-matching (QPM) of OPOs in mirror resonators is realized by linear periodic structures. Convenient methods for linear domain inversion are based on photolithographic masks and homogeneously applied voltages [7]. Most intuitively, nonlinear-optical processes in WGRs are quasi-phase-matched using periodic poling along the circumference, i.e. radial poling [8]. Mask poling of radial structures, however, shows a non-uniform structure because x- and y-axes pole differently strong [9]. Therefore, we apply calligraphic poling [10].

In WGR-OPOs, the wavelengths of the generated light can be adjusted by the temperature of the crystal. Generally, the temperature is changed and simultaneously the pump resonance is tracked by tuning the pump wavelength. Using this technique, mode-hop-free tuning (mode numbers of pump, signal and idler light are fixed) over 0.5 GHz [5] as well as controlled tuning over 100 nm (mode numbers of the pump light are fixed) could be observed [11] and high-resolution spectroscopy was shown [12]. However, the wavelength scanning range covered by laser diodes used for pumping the OPO is limited to several GHz. OPO tuning that exceeds the scanning range of the laser has only been demonstrated with uncontrolled mode-hops and jumps of the output wavelengths [13, 14]. This originates from a lack of mode control as each whispering gallery mode has a different effective refractive index [15]. Our approach makes wavelength selection for WGR-OPOs over a spectral range of several hundred nanometers feasible: We keep the radial and polar pump mode numbers q and p as well as pump laser frequency fixed. The temperature is increased such that the azimuthal pump mode number m becomes m + 1 for each temperature step. In order to recognize the pump modes, the laser is tuned over about 2 GHz. The range of selectable wavelengths can be further expanded by making use of pump modes with different q and p, which is similar to a multi-grating design [16]. This technique enables us to demonstrate spectroscopy of polystyrene.

2. Fabrication of a radially-poled whispering gallery resonator

Our fabrication process of a radially structured resonator needed for infrared spectroscopy starts with calligraphic poling. Analogous to [10], the poling setup consists of a needle which is connected to a high voltage source and linearly pulled across a z-cut wafer of lithium niobate on a grounded rotation stage. In this experiment, we used a tungsten needle with 5 µm tip diameter, a writing velocity of 200 µm/s and a voltage of −350 V applied to the z⁻-side. Additionally, the rotation stage was heated to 100 °C. For the fabrication, a 200-µm-thick wafer of z-cut stoichiometric lithium niobate doped with 1.3 % MgO was used. To ensure good conductivity, the z⁺-side of the wafer was coated with a 100-nm-thick gold layer by vapor deposition. In order to support the switching behavior of the domains, glycerol was evaporated on the heated rotation stage to condense on the wafer and at the tungsten tip. A similar method using water vapor shows that the drops enhance the contact between needle and wafer [17]. While pulling the needle across the wafer, charges are deposited on the z⁻-side of the wafer. These carriers cause stress-induced birefringence. This effect was used for in-situ visualization of the poling process by including two polarizers in the microscope. By poling of domains along the x-axes once and repeating it along the y-axes up to three times, dependence of the domain size on the orientation of the crystal lattice was suppressed.

Like for a linear crystal employed in mirror resonators [18], a grating period of the domains of about 30 µm is needed to phase match the Type-0 (all three light waves are extraordinarily polarized) parametric process pumped by 1040 nm wavelength. By considering the dispersion relation for WGRs [19] and Gayer’s Sellmeier equation [20], the exact number of domains needed for a resonator with 1 mm radius was calculated to be 225. In order to permanently visualize the domains after the poling [21] (see Fig. 1(a)), the wafer was submerged in hydrofluoric acid (50 %) for 15 min. The blue circle in Fig. 1(a) indicates the position of the resonator rim, for which the poling structure was optimized. Along this rim, almost all domains were successfully inscribed. Comparing the microscope images of the top and bottom side of the wafer (see Figs. 1(b) and (c)) reveals that all domains are grown through the sample. White-light interferometer images were taken with a lateral resolution of 5 µm to estimate the quality of the structure by determining its Fourier coefficients S_j [13]. From this, the effective nonlinear-optical coefficient can be derived as d_eff = |S_j|d₃₃, where d₃₃ is the nonlinear-optical coefficient. In case of a perfect QPM structure, a maximum value of |S_j| = 0.64 can be reached. The Fourier transform was performed along the future resonator rim, indicated by the blue line in Fig. 1(a). Figure 1(d) shows a maximum around j = 225. Variations of the future position of the resonator rim yield Fourier coefficients |S₂₂₅| between 0.16 and 0.18. Comparing this value to that of previous experiments [13], this is a 1.8-times improvement in the effective nonlinear-optical coefficient.

 

Fig. 1 a) Radial structure with 225 switched domains in a z-cut lithium niobate wafer fabricated using calligraphic poling. The blue circle indicates the resonator rim (≈1 mm radius). The black lines mark the x- and y-axes. A zoomed part of the structure (rectangle in (a)) is shown in (b) and the bottom side in (c). d) Along the blue circle, the Fourier coefficients of the structure with a maximum at j = 225 were determined.

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The wafer with the characterized radial domain structure was used to manufacture a WGR. In order to be able to polish the resonator if surface roughness dominates intrinsic absorption, it was fabricated with a major radius larger than 1 mm. White-light interferometry was used to determine the major and minor radius to be 1.09 mm and 0.19 mm, respectively. The center of symmetry of the resonator coincides with the one of the domain structure within 0.02 mm.

3. Experimental setup and characterization of the optical parametric oscillation

In the next step, Type-0 quasi-phase-matched optical parametric oscillation was studied in the periodically poled WGR. The resonator was placed next to a rutile prism in a temperature-stabilized oven, depicted in Fig. 2(a). A cylindrical PMMA housing with NBK7 window ensured a millikelvin temperature stability. The distance between resonator and prism could be adjusted with a piezoelectric actuator. Light of an external-cavity diode laser with 1040 nm wavelength, a linewidth of 100 kHz, and a mode-hop-free tuning range of 50 GHz was focused into the back of the prism and was evanescently coupled to the WGR. The outcoupled pump light was separated from the parametric beams using a low pass filter and monitored on a detector. A maximum coupling efficiency of 42 % was reached. The parametric signal and idler beam were simultaneously monitored with a second detector and a spectrometer with a wavelength resolution of 3 nm.

 

Fig. 2 a) Sketch of the experimental setup used for optical parametric oscillation in the radially-poled WGR. b) Part of the pump spectrum (blue, detected at D_p) and corresponding signal plus idler spectrum (red, detected at D_s+i) obtained by tuning the pump laser. 12 GHz of the total free-spectral-range FSR ≈20 GHz are shown. The pump modes used for wavelength selection in Fig. 4 are marked with black symbols.

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An intrinsic quality factor of 1.4 × 10⁸ of the radially-poled resonator was deduced from a linewidth of 2 MHz. The quality factor of bulk lithium niobate measured at this wavelength is 1.5 × 10⁸ [22]. This provides evidence that the surface roughness of the resonator plays a negligible role. Above threshold, a multitude of pump light resonances in the frequency spectrum is quasi-phase-matched for optical parametric oscillation (see Fig. 2(b)). With the pump mode marked with a square in this figure, a minimal OPO threshold of 21 µW was observed in under-coupling, whereas a maximum conversion efficiency of 45 % was attained in overcoupling.

4. Broadband wavelength selection and spectroscopy

In order to demonstrate spectroscopy using whispering gallery resonators, a method of broadband wavelength selection by mode control had to be developed. The pump resonance frequencies (as shown in Fig. 2(b)) are shifted by heating and cooling the WGR due to the thermal expansion and the thermo-optic effect. A pump resonance with the same radial and polar modes numbers q and p, but different azimuthal mode number m, exists every free-spectral-range (FSR). In Fig. 3, the shift of the spectrum is schematically shown with the help of three simplified FSRs. In principle, this method allows a fixed pump laser frequency. Here, it is swept over about 0.1 GHz to compensate for temperature fluctuations. Wider tunability (approximately 2 GHz) of the pump laser is needed to enable orientation in the pump spectrum and recognition of certain modes. One can select a specific pump mode, and thereby the OPO wavelengths, by changing the temperature until the resonance frequency of this mode matches the fixed laser frequency. The OPO branch of each pump mode can be pursued by increasing the temperature such that the spectrum is shifted by one FSR (m becomes m + 1). This can be repeated within the whole temperature range available for the setup.

 

Fig. 3 Schematic graph of the pump mode shift due to increase of temperature (from T₁ to T₂). Each simplified FSR contains three pump modes. Tuning of one specific mode is realized by keeping the pump laser frequency fixed and increasing the temperature such that light is coupled into the pump mode with same q and p in the next FSR.

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For this WGR, the required temperature change for the shift of the pump spectrum by one FSR (≈20 GHz) was approximately 2 °C. Wavelength selection characteristics of three mode triplets measured with a pump laser frequency within a range of 0.6 GHz are shown in Fig. 4(a). For each branch another pump mode, indicated by the black symbol in Fig. 2(b), was tracked over the whole temperature range. Thereby, controlled wavelength selection over more than 100 nm of both, signal and idler light, was possible. As every pump light resonance has a different effective refractive index, each pump mode induces optical parametric oscillation with a different pair of signal and idler wavelengths. This flexibility in the choice of the pump mode is comparable to a multi-grating design in standard OPOs [16].

 

Fig. 4 a) OPO branches of three mode triplets. This shows controlled wavelength selection over the spectral ranges 1.8 – 1.9 µm and 2.2 – 2.5 µm. b) Transmission spectrum of polystyrene taken using the middle WGR-OPO branch (circles) extended by controlled tuning (filled circles). The solid line represents the spectrum measured with a grating spectrophotometer (Cary 500).

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The idler branch of the pump mode, indicated by circles in Figs. 2(b) and 4(a), was used to demonstrate spectroscopy of a 0.8-mm-thick sample of polystyrene. For this purpose, a dielectric mirror that reflects signal light up to 1.85 µm and transmits idler light was included in the setup shown in Fig. 2(a). To normalize the transmission, a reference detector for the idler light was added. The spectroscopic resolution in Fig. 4(a) is about 20 nm. This can be increased by combining wavelength selection with controlled tuning, i. e. frequency tuning of the pump laser [11]. If the laser is tunable over one FSR of the pump light (≈20 GHz for our resonator), the spectral spacing of the measurement points can be reduced down to the FSR of the generated light (here ≈ 21 GHz for our resonator). Figure 4(b) shows the transmission of polystyrene between 2.28 and 2.46 µm measured with our WGR-OPO setup. Additional measurement points, taken by controlled tuning, are indicated by filled circles. The spectrum of the polystyrene sample was also taken with the Varian Cary 500. Both measurements show good agreement within 2 %. Only for wavelengths close to the OPO degeneracy, the WGR measurement shows systematically up to 5 % higher transmission values, due to leakage of signal light into the idler path. Alternatively, the spectroscopic resolution of wavelength selection can be increased using larger WGRs, with smaller FSR.

5. Conclusion

We addressed the challenge of using optical parametric oscillation for broadband spectroscopy if the standard mirror resonator is replaced by a compact whispering gallery resonator. Quasi-phase-matching for the radial geometry was achieved by calligraphic poling. Our approach of pump mode control enabled us to demonstrate broadband wavelength selection for the WGR light source. In combination with controlled tuning, this source was successfully used for a proof-of-principle experiment of broadband spectroscopy. In the next step, the light source capabilities can be extended by mode-hop-free tuning to fill up the gaps in the scanning range. Furthermore, locking the laser to the pump mode at each temperature would result in a tunable continuous-wave light source. In summary, we are now one significant step closer to a small system containing a laser diode and a whispering gallery resonator that can be used for broadband infrared laser spectroscopy.