A tunable Raman laser in the hollow bottle-like microresonator is demonstrated. By controlling the pump laser frequency, we have demonstrated continuous Raman laser frequency tuning. We also have studied the interesting transient mode evolution with Raman gain by sweeping the pump and probe laser, and verified the thermal tuning mechanism by theoretical simulations. By mechanically stretching the resonator, we have achieved the large range frequency tuning of the Raman laser, with the tuning range of 132 GHz with the resolution about 85 MHz. The demonstrated tunable Raman laser can be used as a source for future optical applications.
© 2017 Optical Society of America
In the past decades, whispering gallery mode (WGM) optical microcavity becomes an emerging research field due to their high quality factors (Q) and small mode volumes . The optical fields circulating in such WGM microcavities can greatly enhance the interaction between the photons and matter. A number of research fields have been boosted by the WGMs, such as laser sources [2,3], sensors [4–7], cavity quantum electrodynamics (QED) [8–10], optomechanics [11–13], and nonlinear optics [14–19]. Among those applications, the WGM-based Raman laser holds the great potential for integrated ultra-low-threshold laser sources. The Raman gain is originated from the interaction between photon and optical phonon of the material, where the phase matching is automatically satisfied. Thus, it is easy to realize the Raman lasing in the ultra-high Q WGM resonators, such as the microspheres, microtoroids and microbubble resonators [3–5, 20, 21]. These microresonators can be fabricated from various materials, including the silica, titanium sensitized silica, As2S3, poly(dimethylsiloxane) (PDMS), CaF2, BaF2 and diamond [22–28]. The cascaded Raman lasing has also been reported in the WGM microresonators [5,29]. Benefiting from the very narrow linewidth of the Raman laser, the single nanoparticle detection by a microtoroid cavity has been realized recently [4,5].
However, for the conventional microcavity, such as microspheres, microdisks, and microtoroids, the tuning range of the optical modes is very small due to the structure and its potential applications are limited [30–32]. The hollow bottle-like microresonators (BLMRs) are used for improving the tunability because of the thin wall [33–37]. Besides, the bottle-like resonator with an empty channel inside is a promising candidate for optofluidics sensing [38–40].
In this paper, we have demonstrated the tuning of Raman laser in a BLMR. We have fabricated the BLMRs with a high-Q factor of 2.2 × 108, along with the ringing phenomenon by different frequency scanning speeds [41, 42]. We demonstrate the fine and coarse tuning of the Raman lasing frequency by two different approaches. At first, we study the transient mode evolution of Raman gain and estimate Raman mode tuning at GHz level by pump injection thermal tuning continuously. Then, by mechanically stretching the BLMRs, we realize the coarse Raman mode tuning with resolution of about 85 MHz, with tuning range of 132 GHz. These approaches are promising for future applications of WGM microresonators, such as tunable Raman lasing sources and sensors [4,5].
2. Experimental setup
The BLMR was fabricated by melting two parts along a capillary at a distance of about 300 μm, with more fabrication details presented in . The photograph of a typical BLMR, with a diameter of 76 μm and a wall thickness of 18.7 μm, was shown as the inset of Fig. 1. One side of the BLMR was glued on a piezoelectric transducer (PZT) stage and a three-dimensional nanopositioner was used to control the coupling condition, as indicated in Fig. 1. A tapered fiber was used to evanescently couple light from a tunable diode laser into and out the microcavity. The coupling efficiency was optimized by changing the distance between the microbottle and the fiber taper. The input laser (New Focus, TLB-6728-P) wavelength is 1550nm. A fiber polarization controller (FPC) was used to control the polarization of the input laser. A fraction of the output light was monitored with a 125 MHz low noise photo-detector (PD) and digital oscilloscope (DSO). The rest of output light was measured by the optical spectrum analyzer (OSA) or by another photo-detector after the Bandpass filter of 1620 nm band. The whole system was placed in a clean chamber to reduce the influence of the contaminants and the perturbation of the air flow. Figure 2 shows the transmission spectra of typical optical mode in the BLMR with ringing phenomenon [41–44].
In the high-Q WGM resonators, the lifetime of photon , where the total decay rate of the optical field amplitude κ = κ0 + κex with the intrinsic decay rate κ0 and coupling decay rate κex, is very high. And τ ≫ τc, which is the round trip time of photons traveling around the perimeter of the microbottle. We define the characteristic frequency sweeping speed Vc = 4/τ2, which corresponds to the speed that the laser sweeps through the cavity resonance within the photon lifetime of the microcavity . When the scanning speed Vs ≪ Vc, the transient response of the WGM can be neglected, and the transmission is a symmetric Lorentz-shaped dip in spectrum . When the scanning speed comparable with the characteristic speed Vs ≥ Vc, the transmission shows the interference between cavity emission from the stored light and the directly transmitted signal light, which leads to the ringing phenomenon, as shown in Fig. 2. The transient transmission of the cavity can be calculated by the dynamic evolution equation from the coupled mode theory, as shown by the red lines in Fig. 2. Through the calculation, the intrinsic Q factor (Q 0) and external Q factor (Qex) of this mode are estimated to be as high as 2.2 × 108 and 1.2 × 109, respectively.
3. Transient mode evolution with Raman gain
First of all, we study the Raman gain in the BLMRs by the pump laser. By sweeping the pump laser through the resonance, we found that the transmission of the pump laser was significantly different for low and high pump powers. Such effect can be attributed to the thermal effect of silica resonator that the heating of the microcavity by strong pump laser shifts the pump cavity resonance. When the direction of laser frequency sweeping is the same as the thermal induced resonance frequency shift (red shift for silica), it takes longer time for the laser to catch the resonance frequency of the cavity, and leads to the asymmetric and broad transmission dip on the spectrum (Fig. 3(b)) . Since the thermal induced refractive index change and geometry expansion are independent of the mode wavelength, the frequency of the mode with Raman gain is also shifted with pump laser sweeping. During the thermal shift, the energy in the cavity is accumulated, and the Raman gain becomes higher, and the Raman lasing is generated when threshold condition is satisfied. Therefore, it is beneficial to study the transient dynamics of the Raman gain, and we would expect interesting effects for the modes with Raman gain associated with the pump thermal effect, which explores the possible way to realize a stable and tunable Raman laser.
To study the transient evolution of the mode with Raman gain and thermal effect due to pump laser, a tunable diode laser (New Focus, TLB-6730-P) was introduced to probe the Raman gain around 1620nm. The probe light was separated from the pump laser and detected with the assistance of a bandpass filter around 1620nm. The probe laser frequency was swept across the Raman gain resonance when the pump laser was also sweeping, by such method we can estimate the thermal frequency drift compared to the probe laser sweeping. By adjusting the initial detuning of the probe laser, the resonance would show significant gain effect with the pump laser on-resonance when the probe laser swept across the mode. As shown in Fig. 3(c), there is a dip in the transmission spectrum, corresponding to the probe laser on-resonance with the mode. As the time goes on, the pump laser chases the pump mode resonance, while the probe laser is off-resonance and shows a transmission of unity. When pump laser catches the resonance, the intracavity pump field increases and the strong gain induces the Raman lasing around 1620nm. As shown by the peak in Fig. 3(c), the transmitted power exceeds unity, and approximately linearly increases with the pump power absorbed by the cavity (i.e. the extinction shown in Fig. 3(b)).
By adjusting the initial detuning of probe laser, we can measure the 1620nm resonance for different intracavity pump power. As indicated by the arrows in Fig. 3(c), the probe laser is on-resonance with the mode with Raman gain at different times for repeated experiments shown in Fig. 3(b) and Fig. 3(c) with different initial probe laser frequency. We should note that the laser frequency sweeping speeds for both pump and probe laser are fixed in all measurements. The detailed resonance transmission spectrum is summarized in Fig. 3(d). Below the Raman threshold, we find that the dip depth is increased and the ringing phenomenon is more obvious. This is because the Raman gain compensates the cavity loss, which leads to a reduced effective cavity loss or an increased quality factor. For a fiber-taper coupled resonator, the effect intrinsic loss rate κeff is κeff = κ0 − ξ(t) = ωb/Qeff, where ξ is the Raman gain factor. With the increase of ξ, κeff decreased, the system in the under-coupling regime will enter into the over-coupling regime for a fixed external decay rate κex. Then the probe laser extinction becomes larger with increased ξ. In addition, the transient interference between cavity field and probe laser field becomes more significant for smaller κeff , which is discussed below quantitatively by solving the cavity field evolution dynamic equations. If the gain exceeds the threshold, there are two lasers output at 1620nm band – one is probe laser and the other is the Raman laser, and they show strong beating signals as shown in Fig. 3(d5), which has been observed in a similar system .
The mode evolution with gain in the steady state has been studied in [47,48]. Here, we study our system with similar technique but with the thermal effect of the pump laser and probe laser, which is because of the broaden mode coming from the transient temperature of pump laser and probe laser. Combined with the thermal dynamics [45, 49], we numerically solve the coupled-mode equations to study the transient mode behavior. In ultra-high Q microresonators, the cavity field is very sensitive to the scattering centers in the mode volume of the resonator , which can induce the mode splitting. The intracavity fields are divided into clockwise (CW) modes and counter-clockwise (CCW) modes. Therefore, the time evolution of the intracavity fields (both CW and CCW modes for both pump and probe) are given by [45,47–49]45]. κa,i, κb,i are the intrinsic energy decay rates, κa,e, κb,e are external coupling energy decay rates, and ga and gb are the coupling strengths between CW modes and CCW modes of pump and probe laser.
Since the temperature change ΔT is due to the cavity photon absorption, the relation of ΔT and the intracavity field of pump should satisfyFig. 3(d). The deviation of simulation may be from the high-order interaction in the intracavity field of 1520nm and 1620nm or other nonlinear effects.
Comparing the fitting and experiment results in Fig. 3(d), our theoretical model is consistent with experiment results. We set γ1 = γ2 = −6 × 10−6(1/°C), and from the fitting we obtain K/Cp = 1.4 × 103(1/s), αa/Cp = 2.58 × 1011(°C/J), ga/2π = 0.6 MHz, with other parameters κa,i/2π = 2.82 MHz, κa,e/2π = 0.394 MHz, κb,i/2π = 1.09 MHz, κb,e/2π = 0.15 MHz, gb/2π = 1.6 MHz, va/2π = 3.7 MHz/μs, vb/2π = 1.2 MHz/μs from measurements. All the fittings are based on the same parameters, and Raman gain is adjusted within 20% for Fig. 3(d.1)–(d.4) because of the deviation of the pump extinction for experiment and fitted results in Fig. 3(b). In the above experiments, the range of the pump mode frequency tuning is about 2 GHz, the corresponding Raman mode tuning range is also in the GHz level. In other word, we can control the Raman mode tuning continuously by slightly varying the pump laser frequency or power. Actually, such thermal Raman frequency tuning has the advantage that the temperature and cavity resonance are thermally locked to the pump laser and can be very stable against environment perturbations .
4. Tunable Raman laser
For our BLMRs, not only it holds the advantages of fine tuning through thermal effect by pump laser , but also it has the great potential for large range frequency tuning by mechanically stretching . Therefore, we also study the coarse tuning of Raman lasing by stretching the resonator with the PZT. A nano-positioner was used to improve the exciting intensity of Raman lasing. Here, the Raman lasing was chosen at the 1650 nm band through the optical spectrum analyzer, as shown in Fig. 4. The typical spectrum shows the Raman laser (right peak) power of −14 dBm with the pump power (left peak) of −4.5 dBm. The inset of Fig. 4 shows the threshold of Raman lasing, which was measured by varying the input pump power through the variable optical attenuator. It is noted that the final threshold was about 0.3 mW. The slop of the threshold curve reduced when pump power was stronger than 0.75 mW, which might be induced by Raman modes competition.
Since the geometry of the resonator could be easily changed with the hollow bottle-like structure, the resonance wavelength could be tuned in a large range due to the geometry boundary condition variation and the strain-induced refractive index change of the BLMRs [10,33]. As shown in Fig. 5, the wavelength of Raman laser was tuned by stretching the hollow bottle-like microresonators. Using a 10:90 fiber splitter, the optical mode of pump laser and the corresponding Raman laser can be monitored through the DSO and OSA. By increasing the voltage of PZT, the PZT position is tuned from 0 to 30 μm, and the wavelength of Raman lasing is tuned from 1651.9 nm to 1650.7 nm. The tuning range is about 1.2 nm, which is measured from the OSA, where resolution is set to 0.02 nm. In the wide-range of Raman tuning, the Raman lasing is stimulated steadily and persistently in the BLMR. The slope of the laser frequency versus the position of PZT is 4.23 MHz/nm with the PZT resolution of 20 nm, which is consistent with the precious work .
5. Discussion and conclusions
Here, we demonstrate two different methods to tune the Raman laser frequency, i.e. thermal tuning and strain tuning. For the thermal tuning, we can achieve a tuning range for the Raman mode at GHz level. We can control the Raman mode tuning continuously by varying the pump laser frequency or power through thermal effect. For the strain tuning, it is a coarse tuning by a PZT with the resolution of 20 nm, which is measured by an OSA with resolution of 0.02 nm. For this approach, we observe the Raman mode wavelength tuning range of 1.2 nm, corresponding to a frequency range of 132 GHz, with a minimum tuning step of about 85 MHz.
In summary, we have fabricated hollow bottle-like microresonators with high Q optical mode of 2.2 × 108, along with ringing phenomenon by different scanning speeds. We also observe a low threshold Raman lasing in BLMR, and experimentally demonstrate the fine and coarse tuning methods. The fine tuning is studied by a transient mode evolution approach and showing continuous Raman laser frequency tuning, while the coarse tuning is implemented by the strain method and can get the tuning range of 132 GHz. These tuning methods are promising for potential applications, such as tunable Raman lasing sources and sensors [4,5,51].
National Key Research and Development Program of China (Grant No.2016YFA0301300, 2016YFA0301700); the National Natural Science Foundation of China (Grant No.61575184); Anhui Provincial Natural Science Foundation (Grant No. 1508085QA08); the Fundamental Research Funds for the Central Universities.
The authors thank Xiao Xiong and Fang-Jie Shu for discussion about the numerically simulation.
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