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We generate broad bandwidth visible light ranging from 498 to 611 nm via third-harmonic generation in a silica toroid microcavity. The silica toroid microcavity is fed with a continuous-wave input at a telecom wavelength, where third-harmonic generation follows the generation of an infrared Kerr comb via cascaded four-wave-mixing and stimulated Raman scattering effects. Thanks to these cascaded effects (four-wave mixing, stimulated Raman scattering, and third-harmonic generation) in an ultrahigh-Q microcavity, a broad bandwidth visible light is obtained. The visible light couples with the whispering gallery mode of the cavity by demonstrating the evanescent coupling of the generated visible light with a tapered fiber based on an add-drop configuration.
© 2016 Optical Society of America
1. Introduction
Optical frequency combs at visible wavelengths are expected to be used for frequency metrology, as optical clocks, and for sensing applications [1–3]. For example, the wavelength of a visible comb overlaps such atomic transitions as strontium (698 nm) and ytterbium (578 nm) that are used in optical clock applications [4]. On the other hand, visible lasers play an important role in medical sciences. For instance, green light is used for sensing hemoglobin [5], and near infrared (IR) light is used for optical coherence tomography [6].
Optical frequency combs have been demonstrated using Ti:sapphire lasers and fiber lasers [7–10]. On the other hand, the recent development of ultrahigh quality factor (Q) whispering gallery mode (WGM) microcavities [11–13] has enabled us to generate a broad bandwidth Kerr comb on a chip [14,15]. A Kerr comb is generated via cascaded four-wave mixing (FWM), and research on the temporal domain behavior of Kerr combs [16–18] is under way. In these studies, the Kerr combs are generated in the IR region and there have been few reports on the generation of visible combs. This is because of the dispersion of microcavity devices. To have a stable output, the system must exhibit a phase-locked state, where pulses circulate in the cavity. To obtain a phase-locked state, an anomalous dispersion is usually needed, which is possible in the IR regime by carefully designing the structural dispersion [19]. However, SiO2 exhibits normal material dispersion in the visible region, which makes it difficult to generate Kerr combs at visible wavelengths. A recently reported alternative method shows that a phase-locked state is possible even in a normal dispersion system by generating a dark soliton [20] by the use of local anomalous dispersion, or by pumping the cavity at a high intensity with effectively red detuning [21–23].
The generation of visible light has been demonstrated using ultrahigh-Q crystalline microcavity via optical parametric oscillation [24], ultrahigh-Q silica toroid microcavity and microsphere resonator via third harmonic generation (THG) [25,26]. Efficient wavelength conversion is achieved even though the cavity is continuous-wave (CW) pumped. A conversion efficiency of 10−6 has been reported in a microsphere resonator [26]. Since the photon density scales with Q/V, where V is the mode volume of the cavity, the THG and sum frequency-mixing generation (SFG) efficiencies are usually proportional to (Q/V)3. Therefore, the use of an ultrahigh Q microcavity is attractive for generating visible light via χ(3) effects.
THG generation can be used for visible light generation, and by using IR pulses circulating in the cavity we should be able to obtain a reasonably high conversion efficiency and a broad bandwidth at visible wavelengths. In fact, attempts have been made to generate visible light in microcavities by converting Kerr combs in the IR region via optical nonlinearities to avoid the problem of dispersion that occurs when using SiN, AlN microrings [27,28]. Although these devices generated broad bandwidth light, we may be able to obtain an even broader bandwidth by using SiO2 since it exhibits broad stimulated Raman gain.
While a WGM cavity system is attractive thanks to its high (Q/V)3, we need to consider another important factor regarding the realization of THG and SFG, namely energy and momentum conservation. The conservation laws for THG are satisfied when there is a resonant mode at a frequency three times higher than the frequency of the fundamental excitation mode. However, this condition is not automatically satisfied due to the dispersion of the cavity system. The presence of the dispersion changes the free-spectral range (FSR) and shifts the frequency of the cavity resonance. As a result, the TH mode is not usually at a frequency exactly three times higher than the IR mode, unless we carefully design and employ different azimuthal modes. Previous studies revealed that these conservation laws are satisfied by using different azimuthal modes in the toroid microcavity [25–27]. Since a toroid microcavity is multimode, there are a number of azimuthal modes, and this will increase the possibility of a mode overlap at THG or SFG frequencies.
The aim of this study is to develop a broad-bandwidth on-chip visible light source. In particular, we are interested in using a silica toroid microcavity, since silica has broad stimulated Raman scattering (SRS) gain [29]. This may allow us to obtain a broad bandwidth IR comb and ultrashort optical pulses [30,31]. The use of silica toroid microcavities is also beneficial because we can lower the SRS threshold and enable the efficient generation of cascaded Raman scattering [32,33] due to the high Q and small V. Indeed, previous study has demonstrated visible light at various wavelength by the use of THG and SFG with the help of SRS generation [26]. Further optimization should allow us to obtain as cascaded SRS [34] and comb spectrum. Then we should be able to use a broad IR comb generated in a toroid microcavity as an internal light source for the excitation of THG and generate an efficient comb-like visible broad bandwidth light.
Since the visible light will be converted from an IR comb via THG and SFG, the bandwidth of the visible light is limited by the bandwidth of the optical frequency comb in the IR region. Therefore, we expect the use of a broad bandwidth optical Kerr comb in the IR region assisted by the cascaded Raman effect to enable us to have visible light with a broader bandwidth than that obtained without the SRS effect.
The paper is organized as follows. Section 2 describes the experimental setup and the basic property of the Kerr comb that we used for the following experiments. We describe a time domain measurement with which we show that the IR comb is in a pulsed state. The formation of pulses in the IR region is important in terms of obtaining high THG conversion efficiency. In Section 3, we show the generation of broad bandwidth visible light using an IR comb with and without Raman gain. In particular the cascaded SRS will enable us to obtain broad bandwidth visible light. Finally, we discuss the THG conversion efficiency. Section 4 provides the conclusion to this paper.
2. Experimental setup
Figure 1(a) shows the TE01 mode of a silica toroid microcavity. It exhibits an anomalous dispersion in the IR region, which is needed to generate FWM and obtain a phase-locked state. The dispersion of this mode is calculated by the finite element method (COMSOL Multiphysics), and the calculated β(2) values are shown in Figs. 1(b) and 1(c). As shown in Fig. 1(c) the dispersion at visible light wavelengths is normal, which shows that a THG conversion process is needed to generate a comb.
Fig. 1 (a) The cross-sectional mode profile of the silica toroid microcavity. The diameter of the microcavity is 70 μm and the minor diameter is 6.2 μm. It shows the TE01 mode. (b) and (c) show dispersions of the mode shown in (a) calculated using the finite element method.
Figure 2(a) shows a schematic illustration of the experimental setup. The CW laser light is coupled with a toroid microcavity by using a tapered fiber with a diameter of ~600 nm. The THG spectrum is measured in two different ways; by collecting the scattered TH light with a free space telescope, and by using an evanescently coupled tapered fiber by constructing an add-drop configuration. To simplify the experiments, we usually use free-space coupling based on the telescope setup. The latter approach was used to measure the efficiency.
Fig. 2 (a) Experimental setup. An add-drop configuration is used to collect visible light. A continuous-wave tunable laser diode (TLD) with a linewidth of 100 kHz (Santec TSL-710) is amplified to ~1 W with an erbium doped fiber optical amplifier (EDFA) (Pritel LNHP30) and is then evanescently coupled to the microcavity via a tapered fiber made from a single mode fiber at a telecom wavelength. An optical spectrum analyzer (OSA) is for the IR region (Yokogawa AQ6375) and is connected at the through port of the setup. A spectrometer (SPEC) is used for the visible region (Ocean Optics USB2000 + ) and is connected to the drop port for the first experiment and aligned to collect light with a telescope for subsequent experiments. The resolution of the SPEC for visible light is 430 GHz. (b) Kerr comb with 1-FSR (0.97 THz). The pump wavelength is 1547.5 nm and the pump power is 0.5 W. (c) Second-harmonic generation autocorrelation signal for (a). The pulse interval is 1 ps, which matches the 1-FSR spacing (0.97 THz).
Figure 2(b) shows the measured spectrum of a phase-locked state in the IR wavelength region of a silica toroid microcavity with a diameter of ~90 μm and a Q of 8.5 × 106 obtained by optimizing the detuning and the power. The cascaded FWM occurs with a 1-FSR spacing, which results in a comb spacing of 0.97 THz. Figure 2(c) is a background-free second-harmonic autocorrelation (SHG-AC) measurement trace. The red dots are the measured data and the black solid line is the calculated autocorrelation trace for Fourier-transform limited pulses obtained from the spectrum in Fig. 2(b). These two sets of data agree well, which indicates that the cavity is in a phase-locked state. By using this condition, we expect to obtain efficient and broad-bandwidth THG thanks to the high peak power of the pulses circulating in the WGM microcavity.
3. Third-harmonic generation
3.1. THG via FWM
First, we demonstrate the THG generation via CW pumping. With 1538.6 nm pumping, we observed a green light emitting from the toroid microcavity, and the measured spectrum is shown in Fig. 3(a). The inset in Fig. 3(a) shows a CMOS camera image. The wavelength confirms that this light is the result of THG. The TH spectrum in Fig. 3(a) was measured through an add-drop configured tapered fiber, and this confirms that the generated THG couples with the WGM, since only the WGM will couple out with the evanescently coupled tapered fiber. Although this demonstrates THG, it is CW based. So, next we tried to achieve a phase-locked state at an IR wavelength to enable the generation of a visible light with a broad bandwidth.
Fig. 3 (a) The spectra of the pump and generated visible light. The visible light is collected through the drop port. The pump laser emits at 1538.6 nm and at 0.73 W. The TH is measured at 512.9 nm. The inset is a photograph taken from top of the sample during the measurement. (b) IR and visible spectra when the pump laser operates at 1560.9 nm and 1.14 W. The toroid microcavity used for this experiment has major and minor diameters of 80 and 5 μm, respectively. The longitudinal mode spacing of the Kerr comb is 1-FSR, which is a spacing of 690 GHz. The longitudinal modes are not resolved in the visible spectrum due to the limited wavelength resolution of the spectrometer. (c) IR and visible spectra when the pump laser operates at 1545.9 nm and 0.94 W. The same cavity is used as in (b). The longitudinal mode spacing of the Kerr comb is 5-FSR. The thick linewidth of each longitudinal mode is due to the limited wavelength resolution of the spectrometer.
To generate a visible light based on SFG and THG in a WGM microcavity, we pumped this cavity with a 1.14-W CW laser light. Then the generation of an IR region comb with 1-FSR spacing is observed as shown in Fig. 3(b). And we obtained visible wavelength light with a bandwidth of 18 THz. This spectrum is obtained with the telescope setup, but we have confirmed that the spectrum obtained through a fiber and that obtained through a microscope setup are identical except as regards the collection efficiency, which indicates that the generated THG components all couple with WGM. Although the shape of the IR spectrum suggests that the cavity is not in a soliton state, it is clearly in a pulsed state (either an unstable modulation or chaotic state). We checked using a numerical calculation that the peak power of the pulse in the cavity is about the same and will realize a similar wavelength conversion efficiency even in the Turing or chaotic states. Indeed, we can see a broad visible light in Fig. 3(b).
To investigate the generation of the THG in more detail we changed the pumping condition. Figure 3(c) shows the comb spectra in the IR region when we pumped the cavity at 1545.9 nm and 0.94 W. The obtained comb had a 5-FSR spacing, which corresponds to a frequency spacing of 4.4 THz. Then the generated TGH exhibited an even broader bandwidth as shown in Fig. 3(c). The FSR spacing of the modes in the visible region is 4.3 THz. This agrees well with the spacing in the fundamental mode, which indicates that the THG process is taking place. Importantly the generated light has a very broad 23 THz bandwidth. Since the FSR of the generated THG signal is identical to the FSR of the IR comb, we learned that no degenerate FWM is occurring at the THG wavelength. This is due to the normal dispersion in the visible wavelength regime [Fig. 1(c)] and the much higher nonlinear threshold required for THG than for FWM.
3. 2. SRS assisted broadband THG
Finally, we describe SRS assisted broadband visible light generation. The SRS signal can be generated by optimizing the modulation instability gain by changing the pumping condition. When we control the pump power, we can adjust the maximum frequency of the modulation instability gain in between the longitudinal modes of the WGM microcavity. Then, FWM is suppressed. On the other hand, the SRS gain is broad. As a result, the SRS gain will overcome the modulation instability gain, the FWM comb is suppressed and the SRS comb dominates. If we control the pump carefully, we can even obtain a hybrid state. A more detailed discussion can be found elsewhere [35].
Figure 4(a) shows the measured IR spectrum. By pumping the cavity at a frequency ωp (1542.2 nm) we obtained a single Stokes signal at a frequency of ωRS (1630 nm). When we measured the spectrum at a visible wavelength we observed four lines (3ωp, 2ωp + ωRS, ωp + 2ωRS, and 3ωRS) corresponding to the THG and SFG signals between the pump and the Stokes signal. This is direct evidence that the Stokes light can interact coherently with the pump light and generate an SFG signal.
Fig. 4 Generated IR comb and visible spectra for a silica toroid microcavity with a Q of 1.2 × 107. The major and minor diameters were 50 and 13 µm, respectively. (a) Spectra when we pump the cavity with 1542.2 nm at 0.5 W. A single Stokes signal at 1630 nm is obtained. (b) Measured spectra when the pump laser operates at 1542.56 nm at 0.5 W. High order SRS at an 11 FSR interval is observed at the IR wavelength. The generated visible light has a bandwidth of 50 THz with a frequency spacing of 11 FSR. The resolution of the spectrometer used to measure visible light is insufficient to resolve each FSR line, but each spectral component has a frequency spacing of 11 FSR. (c) Measured spectra when the pump laser operates at 1551.6 nm and 1 W. The bandwidth of the generated light is very large at 110 THz. Inset is a CCD camera picture taken from the top. It shows that the light also couples toward the counter-clockwise direction, which is due to the clockwise and counter clockwise coupling via scattering [36].
When we change the pumping condition, we observed strong peaks at a spacing of 11 FSR as shown in Fig. 4(b). This is the cascaded generation of SRS. The generation of the cascaded SRS signals allows us to have a very broad spectrum in the IR regime. Indeed, we observed an even broader bandwidth of 50 THz at the visible wavelength. The spectrum peaks at the visible wavelength are due to the THG and SFG of the cascaded SRS signals at the IR wavelength. Although the resolution of our spectrometer used to measure visible light is insufficient to resolve each FSR line, the frequency spacing of each line corresponds to an FSR spacing of 11 whose value is the same as the frequency interval of the strong peaks in the IR spectrum.
By pumping the cavity even more strongly, the spectrum at the IR wavelength becomes even broader as shown in Fig. 4(c), because each SRS component exhibits cascaded FWM. Under such a condition, we observed that the cavity radiated bright visible light as shown in the inset of Fig. 4(c). Indeed, thanks to this broad bandwidth light at the IR wavelength, we obtained a visible light with a 110 THz bandwidth ranging from 498 to 611 nm, as shown in Fig. 4(c).
As has been discussed multiple times in previous studies [24–26], we believe that the phase matching condition between the pump and THG is satisfied because the pump is exciting different azimuthal modes for the THG. Since a toroid microcavity is multimode, there are a number of azimuthal modes, and this has increased the possibility of the phase matching condition. The generation of broad bandwidth visible light in Fig. 4 shows that the phase matching condition will satisfy at broad wavelength regime, and this suggest us that it should be possible to have some degree of wavelength tunability by changing the wavelength of the pump.
3.3 Efficiency and discussion
Finally, we measured the conversion efficiency through the add-drop tapered fiber setup. Fig. 5 shows the THG light power as a function of the input power. We used the same cavity that we used for the measurement in Fig. 4. The input wavelength was set at 1538.6 nm and the input power was changed. The experimental setup is as shown in Fig. 2(a), and the data are all measured with a fiber measurement method.As shown in the inset of Fig. 5(a), we observed only one mode generating at a visible wavelength (512.9 nm).When we increased the pump, we observed a clear transition of the input power dependence as shown with the red and blue solid lines in Fig. 5(a).When the power is below 0.73 W, the dependence can be fitted to the red solid line, which is a cubic function.The blue solid line is also a cubic function but with a different coefficient.The IR spectra of these two regions are shown in Figs. 5(b) and 5(c), where we observe a clear comb spectrum in Fig. 5(b) while there is only a CW light in Fig. 5(c).To obtain the conversion efficiency, we independently measured a reference source with known power and acquired a reference table between power and measurement counts of our visible spectrometer.Then we are able to calculate the conversion efficiency, where we obtained a maximum value of about > 1.1 × 10−9.This value is obtained from the output power of the drop-port single mode fiber.Considering the coupling efficiencies, the internal conversion efficiency is estimated to be about > 10−6, which is a high value considering the low efficiency of the THG process.We would like to note that these conversion efficiencies are underestimated because our setup exhibit large propagation loss at the tapered fiber due to the bending required to approach it to the cavity surface and an additional coupling loss at the interface between the single-mode fiber and the spectrometer, both of which are able to be solved by optimization.
Fig. 5 (a) Measured input power dependence of light generated by the χ(3) process. In this experiment, we only generate one line in the visible region (512.9 nm), and the Kerr comb in the IR region disappears when the input power exceeds 0.73 W. The red solid line is a cubic function fitted to the data from 0.55 to 0.73 W, and the blue solid line is the data above 0.73 W. (b) (c) The Kerr comb spectra at each point shown in (a).
When the IR spectrum is comb-like, the IR light exhibits a pulse like behavior while it circulates in the WGM modes. Since the peak power is high, the conversion efficiency is also high. On the other hand, the conversion efficiency is low in a regime where the pump power exceeds 0.73 W, because the IR light circulating in a WGM cavity is CW. We believe this explains the clear transition of the conversion efficiency between the red and blue fitted lines. In addition, the bistable nature of the nonlinear cavity changes the resonant wavelength of the cavity when we change the input power. This also contributes to the transition from the red to blue lines in Fig. 5(a).
We would like to note that we do not yet know the origin of the existence of the offsets of the two fitted cubic functions. The origin of the smooth transition from the red to the blue line is also not completely clear. But we think that the Kerr effect plays an important role in this behavior, because the resonant wavelength modulation due to the Kerr effect changes the coupling with the input fiber. Although some behavioral aspects are yet to be revealed, Fig. 5 shows that the conversion efficiencies of the states differ significantly when the fundamental light is in a CW or a pulsed mode.
4. Summary
In this study, we generated comb-like visible light with a broad bandwidth by converting Kerr combs in the IR region via the χ(3) optical nonlinearity in a silica toroid microcavity. We measured visible light with a 110 THz bandwidth, which was converted from an SRS assisted Kerr comb. We also measured the input power dependence of the TH light and confirmed that phase locking in the IR wavelength region is the key to achieving higher conversion efficiency.
Funding
Part of this work was supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan, KAKEN #15H05429 and Japan for the Photon Frontier Network Program.
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