1. Introduction

Due to unique advantages in sensor field [1], medical treatment [2], resonance fluorescence lidar detection [3] and terahertz source [4], dual-longitudinal-mode/dual-wavelength dye lasers are widely studied [5–7]. At present, dye lasers operating at dual wavelengths are mainly achieved by two methods, such as pumping two kinds of laser dyes in parallel [8–10]. With this method, a single-resonator dye laser with dual-wavelength output can be designed and achieved through the energy transfer effect between two dyes. Furthermore, the lasing wavelength can be switched by exciting different dye molecules selectively. There are also dual-wavelength lasers obtained by adopting grating and retroreflective tuning mirrors [4,11,12]. The grating as a dispersion component is used to generate diffraction light, which forms laser oscillation in the resonator composed of mirrors. By rotating the retroreflective mirror, the dual-wavelength output is realized. However, by this way, sophisticated design and fine adjustment are needed to obtain a dual-wavelength laser.

Compared with the dye lasers mentioned above, optofluidic microcavity dye lasers that integrate microcavity, microfluidic channel and liquid gain medium are of great interest because of the advantages of compact structure, low energy consumption, tunable emission wavelength and other unique optical features [13–17]. They have a wide range of applications in novel integrated photonic devices, such as tunable light sources [15] and bio-controlled lasers [17], and in biomedical analysis with high sensitivity [14]. Especially in the field of biochemical analysis, they have been developed to be a powerful tool because of the enhanced sensitivity with the resonant cavity. However, to the best of our knowledge, due to the unreasonable cavity length and serious mode competition, most optofluidic dye lasers have unstable multi-longitudinal-mode lasing emission [13–22], which affects the accuracy of biochemical sensing. Stable optofluidic lasers with dual longitudinal modes, which is compared to the single-longitudinal-mode optofluidic laser, can provide more parameters for sensing signals, including wavelength interval, lasing wavelength and lasing intensity, to realize multi-dimensional and multi-parameter biochemical sensing and laser-based dual-wavelength ratiometric biochemical sensors with high sensitivity. But the stable dual-longitudinal-mode optofluidic lasers have not been reported. The main challenges of realizing the stable dual-wavelength lasing are to select an appropriate microcavity and effectively suppress the mode competition. Optofluidic ring resonators (OFRRs) that support the circulating waveguide mode or whispering gallery mode take advantage of compact size and high Q-factors, which contribute to achieve optical device integration and low lasing threshold [16–18]. Thanks to the resonator and microfluidic channel being integrated in the hollow-core region of the fiber and the essence of light guide in the fiber core, optofluidic microring dye lasers based on hollow-core microstructured optical fibers (HCMOFs) have the advantages of compact and robust structure, and the high-efficiency interaction between light and gain medium [23–26]. Based on the excellent optical properties of the HCMOF, a multi-wavelength optofluidic dye laser with an ultra-low threshold of 15.14 nJ/mm² has been achieved [27]. Especially, stable single-longitudinal-mode lasing has also been achieved in the HCMOF-based optofluidic microring laser [28].

In this paper, we propose and experimentally demonstrate a stable dual-longitudinal-mode optofluidic microcavity laser based on a HCMOF, in which a hexagonal silica ring inside a simplified hollow-core microstructured optical fiber acts as the only resonator. A donor-acceptor dye solution of mixing a small amount of Rhodamine B into 1 mM Rhodamine 6G solution is filled into the resonator as gain medium, which can make the laser emission co-excited by different kinds of dye molecules, so that the mode competition in the laser is diminished. Experimental results show that the optofluidic laser obtained by the method of lateral pumping can generate dual longitudinal modes under energy transfer effect, with a threshold of 90 nJ/mm², same polarization states at two lasing wavelengths, controllable lasing wavelength and power ratio by changing the concentration of Rhodamine B. Moreover, the dual-longitudinal-mode laser can operate stably with a maximum lasing intensity fluctuation of 3.2% within 30 minutes.

2. Experimental setup and operation principle

Figure 1 shows the schematic diagram of the experimental setup. The interfaces at the black dashed rectangles are sealed with melted wax. Orange parts between interfaces are different gain media composed of dual-dye mixtures of 1 mM Rhodamine 6G and different concentrations of Rhodamine B in a volume ratio of 1:1. The gain media are filled in the microfluidic channel of the optofluidic laser at equal intervals to quickly obtain lowest concentration of Rhodamine B required for the laser to generate dual longitudinal modes under energy transfer effect. The inner diameter of the microfluidic channel is 0.3 mm. The concentration interval of Rhodamine B is 0.01 mM. Driven by a syringe pump, each gain medium flows into a hexagonal silica microring resonator inside a simplified hollow-core microstructured optical fiber (SHMOF) successively. By the method of lateral pumping, the lasing generated by different gain media is continuously recorded by an optical spectrometer (OS, Ocean Optics HR4000CG-UV-NIR) with a highest wavelength resolution of 0.75 nm. A 532 nm Q-switched pulsed laser (Changchun New Industries Optoelectronics Technology Co., Ltd., MPL-N-532) with a pulse duration of 5 ns and a repetition rate of 2 kHz is used as pumping laser and focused on the SHMOF through a convex lens (Thorlabs, LB1844-A-ML). The power of the pumping laser can be adjusted by rotating a tunable attenuator (Thorlabs, NDC-50C-2M-A). The SHMOF, whose one end is connected to the microfluidic channel, is clamped on a fiber rotator (Thorlabs, HFR007) fixed on a three-dimensional translation stage, so the fiber can be rotated from 0° to 360°. Detector probe (DP) of the OS clamped by a three-dimensional translation stage is placed on the fiber lateral to collect the lasing spectrum. A laptop records and saves the collected spectrum. The collection numerical aperture of the DP is about 0.22. A long-pass filter (Thorlabs, FEL0550) is placed between the SHMOF and the DP to eliminate the scattered light of the pumping laser.

 

Fig. 1. Schematic diagram of detecting the concentration range of the acceptor to make the dye laser operate at dual wavelengths.

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Schematic diagram of the SHMOF-based lasing emission and the cross section of the SHMOF taken by a scanning electron microscope are shown in Fig. 2. The hexagonal silica walls around the hollow core, having a side length of ∼16 μm and a thickness of ∼400 nm, form a quasi-circular acting as a microring resonator. The cladding diameter of the SHMOF is ∼170 μm. As shown in Fig. 2(a), by the method of lateral pumping, the focused pumping laser is launched into the central hole and absorbed by the liquid gain medium flowing in from one end of the fiber and out from another end. Then the fluorescence is stimulated and coupled into the hexagonal silica microring. Similar to microfiber knot dye laser based on the evanescent-wave-coupled gain [29], the fluorescence is trapped in the silica walls and form mode distribution due to the optical confinement effect of the submicron waveguide. By the evanescent wave-coupled gain, the fluorescence is amplified by stimulated emission, and continually coupled into the silica microring and form oscillations in the microcavity. When the gain of the resonant mode in the microring cavity is larger than the loss, laser emission will form and can be detected in the radial direction of the fiber. Due to the strong laser emission in the radial direction of the fiber, the lasing spectrum can be collected by using the multi-mode fiber DP with the collection numerical aperture of 0.22. In addition, the lasing intensity is nearly uniformly distributed in the radial direction of the optical fiber [13], so the change of the lasing intensity collected by the DP of the spectrometer placed on the side of the optical fiber can represent the lasing intensity change of the optofluidic laser. The lasing wavelengths can be determined by the following standing wave condition within the ring cavity

 

Fig. 2. (a) SHMOF-based optofluidic microcavity laser. (b)-(d) Cross section of the SHMOF taken by scanning electron microscope.

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Different gain media have different gain spectra. Changing the gain spectrum of the optofluidic microcavity laser by introducing different concentrations of acceptor dye into donor dye is the principle of achieving stable dual-longitudinal-mode emission. Rhodamine 6G and Rhodamine B, with high fluorescent efficiency, have been widely used in optofluidic microcavity lasers [26,27] and energy transfer dye lasers [30]. Using dual-dye mixture composed of Rhodamine 6G and Rhodamine B as gain medium, the change of gain spectrum and process generating dual wavelengths with the concentration increase of acceptor are qualitatively shown in Fig. 3. When the content of Rhodamine B in the dual-dye mixture is small, only the gain of λ₁ at shorter wavelength is greater than the lasing threshold, and the optofluidic microcavity laser only excites laser emission at λ₁, as shown in Fig. 3(a). As the concentration of Rhodamine B increases, energy transfer occurs between Rhodamine 6G and Rhodamine B, which reduces the gain at λ₁ and makes the gain at λ₂ reach the threshold, causing the laser to start laser emission at λ₂. Continuing to increase the concentration of Rhodamine B, the energy transfer efficiency gradually becomes stronger, so that the gain at λ₂ gradually increases and the gain at λ₁ gradually decreases until only the gain at λ₂ is greater than the threshold, as shown in Fig. 3(b)-(d). This shows that the laser can simultaneously generate laser emission at λ₁ and λ₂ within a certain concentration range of the acceptor. And due to the overlap of the emission spectra of Rhodamine 6G and Rhodamine B around 570 nm [16,24], the laser emission around 570 nm is co-produced by the dye molecules of Rhodamine 6G and Rhodamine B, which diminishes the mode competition between λ₁ and λ₂, so that the laser can stably operate at dual wavelengths. Continuing to increase the concentration of Rhodamine B, the gain at λ₂ gradually decreases and the gain at λ₃ at longer wavelength reaches the threshold, which means the laser can operate at λ₂ and λ₃ at the same time, as shown in Fig. 3(e)-(f). It can be concluded that a stable dual-wavelength optofluidic microcavity laser can be obtained by using dual-dye mixture as the gain medium, and the dual lasing wavelengths of the laser can be regulated by changing the concentration of the acceptor.

 

Fig. 3. (a)-(f) gain spectra of dual-dye mixture with gradually increasing the concentration of acceptor.

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3. Results and discussion

In order to quickly obtain lowest concentration of Rhodamine B required for the laser to generate dual longitudinal modes, the spatial resolution technology is introduced into the microfluidic channel of the optofluidic laser, shown in Fig. 4(a). From one end of the microfluidic channel, each gain media is filled into the microfluidic channel with an air interval by a syringe in a spatial order, and then the other end of the microfluidic channel is connected to the hollow-core optical fiber and sealed with melted wax for experiment. The more detailed steps are as follows. Firstly, the gain medium is sucked into a syringe, and a section of microfluidic channel with inner diameter of 0.3 mm is taken for experiment. The ends of the microfluidic channel are respectively marked as port 1 and port 2; Secondly, the needle tip of the syringe is inserted into port 1 of the microfluidic channel, and the syringe is pushed by hand to make the gain medium flow into the microfluidic channel. After a length of gain medium flows into the channel, the syringe is pulled out to make the needle tip separate from the microfluidic channel; Then, the microfluidic channel is swung gently by hand to make the liquid gain medium flow to port 2 and form a length of air gap at port 1; After that, the step 2 is repeated until the microfluidic channel contains six liquid columns; Finally, the prepared hollow-core microstructured optical fiber is inserted into port 2 of the microfluidic channel. After sealing the interfaces at two ends with paraffin, the whole device is put on the syringe pump for experimental research on the optofluidic laser.

 

Fig. 4. (a) Liquid gain media are arranged in the microfluidic channel with spatial resolution. (b) Lasing spectrum with the dual-dye mixture as gain medium. (c)-(d) Lasing spectra with each gain medium lengths of 0.4 cm and 1 cm. (e) Lasing intensity change of λ₂ with time at each gain medium length of 0.4 cm and 1 cm.

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We conduct detection and analysis experiments on the same gain media by setting different spatial resolution parameters in microfluidic channel, including the length of air gap between gain media (L₁) and the length of each gain medium (L₂). If L₁ is too large, it will take a long time to complete the experiment, making the experiment low-efficiency. If L₁ is too small, the gain medium will be mixed together due to air compression while flowing in the microfluidic channel, thereby affecting the accuracy of the experimental results. Here, the same gain media with an interval of 0.4 cm (L₁ = 0.4 cm) are arranged in the microfluidic channel and flow into the hollow core of the fiber in order. For the length of each gain medium, firstly, L₂ is equal to L₁. The lasing spectra are shown in Fig. 4(c). Six sections of liquid gain medium are filled into the microfluidic channel. Under the action of the syringe pump at a speed of 0.3 μL/min, the obtained laser is discontinuous because of an air gap between gain media. Due to the viscosity of the liquid, non-planar surfaces shown in the inset of Fig. 4(c), are formed at the interface of air and liquid. So that the lasing spectra cannot be output stably in the form of Fig. 4(b) and the lasing intensity and lasing wavelength change dynamically. Especially during the period when the liquid gain medium just flows into and is about to flow out of the resonator, as shown in the white dashed ellipse of Fig. 4(c), there are other lasing wavelengths besides λ₁ and λ₂. To improve this situation, L₂ is set to ∼ 1 cm. The lasing spectra are shown in Fig. 4(d) and more stable than the results in Fig. 4(c). As shown in Fig. 4(e), we analyze the changes of lasing intensity of λ₂ with time from the beginning of a section of gain medium into the fiber to the half of the next section of gain medium out of the fiber. When L₂ = 1 cm, the lasing intensity of λ₂ fluctuates slightly and is more stable. The stable time can last for 300 s, which makes more data available for spectral analysis. So, we use the spatial resolution parameters with a length of 1 cm and an air interval of 0.4 cm for experiments.

According to the above spatial resolution parameters, with the dual-dye mixture of 1 mM Rhodamine 6G and different concentrations of Rhodamine B as gain medium and arranging in the microfluidic channel, the measured lasing spectra of the optofluidic microring laser are shown in Fig. 5. Due to the formation of non-planar surfaces, the lasing spectrum changes greatly within 15 s when the liquid gain medium just flows into the fiber and will flow out of the fiber, while the lasing spectrum is excited stably during other times. We select the stable lasing spectra of each gain medium for spectral analysis. Figure 5(a) shows that when the concentration of Rhodamine B is low, such as from 0.01 mM to 0.08 mM, the distance between the donor molecule and the acceptor molecule is too large to form energy transfer, so the optofluidic microring laser has only laser emission at the wavelength of 564.67 nm (λ₁). At the concentration of 0.09 mM, a new lasing wavelength is excited under the effect of energy transfer, as shown by the arrow in Fig. 5(a). Increasing the concentration of Rhodamine B to 0.15 mM, the generation of energy transfer effect enables the realization of a dual-wavelength laser with the lasing wavelengths of 564.67 nm and 570.08 nm (λ₂) as shown in Fig. 5(b). Moreover, in the process of increasing the concentration of Rhodamine B, the lasing intensity at 564.67 nm gradually decreases and the lasing intensity at 570.08 nm gradually increases. The dual-longitudinal-mode output with adjustable power ratio can be achieved by changing the concentration of Rhodamine B. Continuing to increase the concentration of Rhodamine B, as shown in Fig. 5(c), at the concentration of 0.17-0.23 mM, the lasing wavelengths of laser are changed to 570.08 nm and 575.49 nm (λ₃). Thus, we draw a conclusion that a dual-wavelength laser can be obtained as well as the wavelengths and power ratio of the two lasing wavelengths can be controlled by adjusting the concentration of Rhodamine B of the dual-dye gain medium. By substituting the cavity length, L_C = 96 μm, and the calculated effective RI, n_eff = 1.3967, 1.3954 and 1.3940 by the finite element analysis method, into Eq. (1), 564.67 nm, 570.08 nm, and 575.49 nm are the lasing wavelengths approximately corresponding to the 237^th, 235^th and 233^th longitudinal mode, respectively. Figures 5(d)–5(e) present the change of lasing intensity with the concentration of Rhodamine B, which implies the lasing intensity changes with a good linearity of greater than 96.2% as a function of the Rhodamine B concentration. This dual-wavelength laser allows a laser-based Rhodamine B concentration sensor with a high sensitivity of 240 a.u./μM, and in future work, the gain medium can be replaced by biomaterials to achieve high-sensitivity biosensing.

 

Fig. 5. (a)-(c) Lasing spectra with dual-dye mixture as the gain medium at different concentrations of Rhodamine B. (d)-(e) Lasing intensity at different concentrations of Rhodamine B.

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The relations between dual-longitudinal-mode output intensity and pumping energy at lasing wavelengths of 564.67 nm (λ₁) and 570.08 nm (λ₂) between 0.09 mM Rhodamine B and 0.15 mM Rhodamine B and the lasing spectra with different pumping powers at three concentrations are presented in Fig. 6. Figure 6(a) shows that the growth curves of lasing intensity with pumping power at λ₁ and λ₂ gradually approach firstly and then move away as the concentration increases. When the concentrations are 0.09 mM and 0.10 mM, respectively, the lasing intensity at λ₁ is always greater than that at λ₂ with the increase of pump energy density. Increasing to 0.13 mM, 0.14 mM and 0.15 mM, the laser intensity at λ₂ is always greater than that at λ₁. At the concentration of 0.11 mM and 0.12 mM, the lasing intensity at λ₁ is approximately equal to that at λ₂ at some pumping powers, which indicates that the power-balanced dual-longitudinal-mode optofluidic microring laser with the wavelength separation of 5.41 nm can be obtained at these two concentrations. Lasing spectra with power-unbalanced dual longitudinal modes at different pumping powers are shown in Fig. 6(b) and Fig. 6(d), in which the difference of lasing intensity at λ₁ and λ₂ is greater than 12000. As shown in Fig. 6(c), using the dual-dye mixture of 1 mM Rhodamine 6G and 0.11 mM Rhodamine B as gain medium, the threshold of the power-balanced dual-longitudinal-mode optofluidic lasers is about 87.7 nJ/mm². Compared with the dual-wavelength dye lasers reported in Ref. [4] and Ref. [8–12], the proposed optofluidic microcavity dye lasers with dual longitudinal modes in this work have a lower threshold.

 

Fig. 6. Lasing intensity as a function of the pump energy density at different concentrations from 0.09 mM to 0.15 mM of Rhodamine B. (b)-(d) Lasing spectra with different pumping powers at the concentration of 0.09 mM, 0.11 mM and 0.15 mM.

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As one of the important parameters for evaluating laser performance, the dual-wavelength lasing stability is investigated. When the optofluidic lasers are used for biosensing, the detection of biomarkers is achieved by monitoring the drift of lasing wavelength or the change of lasing intensity in lasing spectra information. Therefore, the lasing spectrum stability of the optofluidic lasers is very important in the biosensing applications and needs to be studied. According to the above conclusions, we use the dual-dye mixture of 1 mM Rhodamine 6G and 0.11 mM Rhodamine B as gain medium, and obtain the power-balanced dual-longitudinal-mode output at pump energy density of 118 nJ/mm² (112 μW), shown at the top of Fig. 7(a). By analyzing the collected lasing spectra shown in Fig. 7(b), we get that the average values of lasing intensity at λ₁ and λ₂ are 7833 and 7949 respectively, which are approximately the same. In order to more intuitively and clearly see the lasing spectrum change of the obtained power-balanced dual-longitudinal-mode laser with time, we analyze the lasing intensity changes at two lasing wavelengths within 30 minutes, shown in Fig. 7(c). Calculating the standard deviation coefficients of the lasing intensity at two lasing wavelengths, we know that the lasing intensity variations at the peak wavelengths of 564.67 nm and 570.08 nm are less than 3.2% and less than 2.1%, respectively. The small lasing intensity fluctuations of the obtained power-balanced dual-wavelength laser are mainly due to the instability of the 532 nm Q-switched pulsed laser. As shown in Fig. 7(d), the standard deviation coefficients of the 532 nm pulsed laser at 50 μW, 90 μW, 110 μW, and 150 μW are respectively 2.01%, 2.13%, 2.19% and 2.77% within 35 minutes, which means that the pumping laser has power fluctuation with less than 3%. Still using the dual-dye mixture of 1 mM Rhodamine 6G and 0.11 mM Rhodamine B as gain medium, at the pump energy density of 136 nJ/mm² (131 μW), a stable dual-longitudinal-mode optofluidic laser with a power ratio of 44.7% is received and shown in the middle picture of Fig. 7(a). As shown in Fig. 7(e), calculating the standard deviation coefficients of the lasing intensity at peak wavelengths, within 30 minutes, the lasing intensity fluctuations of λ₁ and λ₂ are less than 1.0% and less than 3.2%. Changing the gain medium to the dual-dye mixture of 1 mM Rhodamine 6G and 0.12 mM Rhodamine B, at the pump energy density of 141 nJ/mm² (136 μW), we achieve another dual-longitudinal-mode optofluidic laser with a power ratio of 83.2%, as shown at the bottom of Fig. 7(a). It can be seen from Fig. 7(f) that the lasing intensity fluctuation is less than 1.6% within 30 minutes. The main reason of the excellent stability characteristics is because the laser emission around 570 nm is co-excited by different kinds of dye molecules and the mode competition between λ₁ and λ₂ is greatly diminished.

 

Fig. 7. Lasing spectra of dual-longitudinal-mode output with power-balanced, power ratios of 44.7% and 83.2%. (b) Lasing spectra of power-balanced dual-longitudinal-mode output with time at pump energy density of 118 nJ/mm². (c) Lasing intensity changes of power-balanced dual-longitudinal-mode output at two lasing wavelengths within 30 minutes. (d) Power changes of 532 nm Q-switched pulsed laser within 35 minutes. (e)-(f) Lasing intensity changes of dual-longitudinal-mode output with a power ratio of 44.7% and 83.2% at two lasing wavelengths within 30 minutes.

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In addition to the above-mentioned power-balanced dual-longitudinal-mode output achieved only at a certain pumping power of specific concentration, we also realize power-balanced dual-longitudinal-mode outputs at different pumping powers, as shown in Fig. 8(a). In another experiment, with dual-dye mixture of 1 mM Rhodamine 6G and 0.12 mM Rhodamine B as gain medium, the wavelengths of laser emission are 563.12 nm and 568.53 nm, which are slightly different from the above excitation wavelengths because of the replacement of optical fiber with another section of the same microstructure. Figure 8(b) gives the change of lasing intensity with pumping power at the lasing wavelengths. A power-balanced dual-longitudinal-mode laser, having the same intensity change with the pump energy density at two lasing wavelengths, is realized. As shown in Fig. 8(c), at pump energy density of 272 nJ/mm², the lasing intensity variations at λ₁ and λ₂ are respectively less than 1.4% and less than 1.1%. For comparison, we also replace the dual-dye mixture with the single dye of 1.16 mM Rhodamine 6G as the gain medium in the same lasing cavity to trying to obtain a power-balanced dual-wavelength laser. However, the stability of the obtained dual-wavelength laser is very poor, and the lasing intensity changes of λ₁ and λ₂ at pump energy density of 287 nJ/mm² are shown in Fig. 8(d). Compared with the results in Fig. 8(c), firstly, the lasing intensities of two lasing wavelengths are not always balanced. At the 5^th minute and the 25^th minute, the power ratios of the two peak wavelengths are respectively 71.4% and 56.8%, which are power-unbalanced output. Secondly, the lasing intensities of the two peak wavelengths fluctuate greatly. The standard deviation coefficients of the lasing intensity at λ₁ and λ₂ reach 12.6% and 7.3%, respectively. Therefore, it is difficult to realize stable dual-longitudinal-mode lasing based on a single dye gain medium because of the serious mode competition. The introduction of a small amount of another dye can effectively diminish the mode completion due to the laser emission co-excited by different kinds of dye molecules.

 

Fig. 8. (a) Lasing spectra of power-balanced dual-longitudinal-mode output having the same intensity change with the pump energy density. (b) Lasing intensity as a function of the pump energy density at two lasing wavelengths. (c) Lasing intensity changes of power-balanced dual-longitudinal-mode output at two lasing wavelengths within 30 minutes at pump energy density of 272 nJ/mm². (d) Lasing intensity changes of power-balanced dual-longitudinal-mode output with 1.16 mM Rhodamine 6G as gain medium at two lasing wavelengths within 30 minutes.

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As shown in the inset of Fig. 9(a), by adding an analyzer placed between the SHMOF and the DP, we also study and analyze the polarization characteristics of the dual longitudinal modes. When the spindle of the analyzer is perpendicular to the fiber, the lasing intensities of 564.67 nm (λ₁) and 570.08 nm (λ₂) are the maximum values, and the position of the analyzer at this moment is set as the starting point (recorded as 0°). As shown in Fig. 9(a), rotating the analyzer clockwise to 90°and making the spindle parallel to the fiber, the lasing intensities of the two peak wavelengths decrease synchronously from the maxima to 0, which indicates that the polarization states of the dual longitudinal modes are parallel to the cross-sectional plane of the fiber and the dual longitudinal modes are both linear polarization modes. Analyzing the normalized curves of the lasing intensities of two lasing wavelengths, shown in Fig. 9(b), the curves are approximately the same from 0° to 360°, which illustrates that the dual longitudinal modes have the same linear polarization state. Therefore, it can be concluded that a stable dual-wavelength laser can also be obtained even if the polarization states of the modes are the same, which can be used to generate microwave signals through beat frequency.

 

Fig. 9. (a) Lasing spectra of dual-longitudinal-mode output obtained by rotating an analyzer. (b) Polarization detection of dual-longitudinal-mode laser: the normalized curve of the transmitted light intensity varying with the angle of the analyzer.

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4. Conclusion

In summary, a stable dual-longitudinal-mode fiber optofluidic microcavity laser, with a hexagonal silica ring inside a hollow-core microstructured optical fiber as the only resonator and a dual-dye mixture as the gain medium, has been realized. The threshold of the dual-longitudinal-mode laser is only about 90 nJ/mm², and the lasing wavelengths and power ratio of the dual-wavelength laser can be controlled by adjusting the concentration of Rhodamine B. By introducing a small amount of Rhodamine B into Rhodamine 6G solution, the dual-longitudinal-mode lasers have good stability with a maximum lasing intensity fluctuation of 3.2% within 30 minutes even if the dual longitudinal modes have the same linear polarization states. As a result of the resonator and microfluidic channel being integrated in the hollow-core region of the fiber and the essence of light guide in the fiber core, the laser also has the advantages of compact and robust structure and the high-efficiency interaction between light and gain medium. This study provides possibilities for the improvement of controllable dual-wavelength laser and extends more applications in biosensing, medical treatment, resonance fluorescence lidar detection and terahertz source.