Tuning of laser modes in an Er3+-doped whispering-gallery-mode microresonator

Qi Yu; Qi Miao; Zhenxiang Xu; Zhiwei Huang; Jialiang Lü; Hongtao Li; Liang Lü

doi:10.3788/COL202422.121406

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Abstract

In this paper, we experimentally investigate the tuning characteristics of laser modes for an ultrahigh-Q Er³⁺-doped microbottle resonator (MBR) based on the whispering gallery mode (WGM). Thanks to the optimized Er³⁺ doping technique, the laser threshold could be as low as 70 µW. Benefiting from the abundant axial modes and radial modes of the MBR, our experiments demonstrate that the number of laser modes can be flexibly controlled by varying the pump power, adjusting the coupling positions along the axis of the MBR, as well as modifying the coupling diameter of the tapered fiber. The laser mode switching is performed from single-mode to multimodes. Furthermore, different from the traditional external tuning method, we propose a simple and stable approach for continuous wavelength tuning of the laser mode based on the thermal effect associated with the high Q MBR. By precisely adjusting the pump laser wavelength, the emitted laser wavelength can be tuned over a range of 0.102 nm with a high linearity of 99.96%. The engineering of laser mode switching and precise wavelength tuning of the Er³⁺-doped MBR is expected to have promising applications in miniature tunable single-mode lasers, laser precision measurement, and so on.

Keywords

fiber laser microresonator whispering gallery mode

1. Introduction

Characterized by the high-Q factor and small mode volume, whispering-gallery-mode (WGM) microcavity lasers offer advantages such as a low threshold, narrow linewidth, compact size, and high integration^[1]. These properties make them widely applicable in fields such as optical sensing^[2,3], biological detection^[4], and gas detection^[5]. Typically, WGM microcavity lasers are fabricated from solid materials, including semiconductors^[6] and composite glass materials^[7]. Due to their ease of preparation and processing, glass materials doped with rare-earth ions have been extensively studied^[8,9]. It was first reported that ${Er}^{3 +}$ -doped sol-gel films can be applied to the surface of silica microspheres to create low-threshold microcavity lasers^[10]. Recently, both flexible switching of laser modes and fine-tuning of laser wavelength have attracted significant attention due to their importance in the practical optical applications in laser precision measurement^[11], nanoparticle detection^[12], and so on.

For the realization of laser mode switching, it has been reported that the mode selection excitation could be achieved by using spatial light modulation and loss engineering. However, this approach introduces scattering loss, which affects laser conversion efficiency^[13]. An add-drop configuration is also available for the flexible manipulation of laser modes, though it involves complex operations^[14]. In addition, the wavelength tunability of the WGM microcavity laser is of importance for tunable single-mode lasers. Traditional tuning methods usually utilize the aerostatic pressure method^[15], mechanical stretching^[16], and thermal tuning methods^[17,18]. However, these methods may induce jitter in the microcavity system, inevitably degenerating the $Q$ factors and thereby compromising the performance of the microlaser. In contrast, the all-optical tuning method has the advantages of easy operation and good stability. Zhu et al. proposed an all-optical tunable microlaser based on iron-oxide-coated thermal tuning with a large tuning range^[19]. However, the nonlinear tuning is constrained by the thermal control of the iron oxide nanoparticles, which impacts the practical applications in light detection and ranging (LiDAR).

In this paper, we systematically investigated the laser mode switching and precise wavelength tuning characteristics of an ultrahigh-Q ${Er}^{3 +}$ -doped microbottle resonator laser (MBL). Based on the optimized sol-gel method, the $Q$ factor of the microbottle resonator (MBR) reaches approximately $\sim 4.5 \times 10^{7}$ , which contributes to an exceptionally low laser threshold of $\sim 70 μW$ for the MBL. Benefiting from the diverse distributions of azimuthal, radial, and axial WGMs for the MBL, experiments demonstrate that laser mode switching between the single mode and multimodes can be achieved by adjusting the pump power, varying the coupling positions along the axis of the MBR, and changing the coupling diameter of the tapered fiber. Furthermore, by precisely adjusting the wavelength of the pump laser, the all-optical tuning of the output laser wavelength can be continuously tuned over a range of 0.102 nm with a high linearity of 99.96%. Our work offers a systematic control technique for the engineering of laser modes based on WGMs, enhancing the practical applications of the MBLs in miniature tunable single-mode lasers and laser sensing.

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2. Experimental Results and Discussions

In experiments, the ${Er}^{3 +}$ ion-doped MBR was fabricated using the optimized sol-gel method^[10], which could effectively prevent defects on the surface of the MBR, thereby enhancing the $Q$ factor of the resonator. The erbium doping concentration is estimated as ∼8 × 10¹⁸ cm^–3. Here, the axial length of the fabricated MBR is $\sim 280 μm$ , and the equatorial diameter is $\sim 180 μm$ , as shown in the inset of Fig. 1. The profile of the MBR can be fitted with a truncated harmonic oscillator profile expressed by $R (z) = R (0) [1 - {(Δ k z)}^{2} / 2],$ (1)where $R (z)$ represents the radius variations as a function of $z$ . $Δ k$ denotes the curvature of the bottle profile. According to Eq. (1), the parameters are determined as $R (0) = 90 μm$ , $z_{\max} = 140 μm$ , and $Δ k = 0.0056 {μm}^{- 1}$ .

Figure 1.Experimental setup. ISO, isolator; PC, polarization controller; OSC, oscilloscope; OSA, optical spectrum analyzer; SG, signal generator; OC, optical coupler; PD, photodetector. The inset shows the microscope of an MBR.

The experimental setup, schematically shown in Fig. 1, is used to measure the spectral characteristics of the prepared ${Er}^{3 +}$ ion-doped MBR. A tunable narrow linewidth external-cavity laser (Toptica, 1420–1530 nm) was utilized as the pump source. The pump laser passed through an isolator (ISO) and a polarization controller (PC) to adjust the polarization state. Subsequently, the pump light was coupled into the MBR by a tapered fiber with a diameter of $\sim 2 μm$ . The output of the tapered fiber was also used to collect the emitted laser. A 20/80 optical coupler (OC) was implemented to separate the output light into two paths, of which the 20% port went into the optical spectral analyzer (OSA) for observing the laser spectrum. The light from the 80% port was fed to the photodetector (PD) followed by an oscilloscope (OSC) to monitor the dynamic behaviors of the laser in the time domain. Additionally, a triangular-wave signal generator (20 Hz, 8 V_pp) was used to externally modulate the pump laser, precisely sweeping the laser frequency in a range of 42 GHz.

First, the $Q$ factor of the ${Er}^{3 +}$ ion-doped MBR was measured in the 1480 nm band and 1550 nm band, respectively. Figures 2(a) and 2(b) demonstrate that the obtained $Q$ factors were $Q_{1480} = 2.3 \times 10^{7}$ and $Q_{1550} = 4.5 \times 10^{7}$ , respectively, which ensures a low threshold and narrow linewidth for the emitted laser mode. Notably, the $Q$ factor in the 1480 nm band is lower than that in the 1550 nm band, which can be attributed to strong light absorption by the ${Er}^{3 +}$ ion at 1480 nm^[20].

Figure 2.Transmission spectrum of the Er³⁺ ion-doped MBR measured in (a) 1480 nm and (b) 1550 nm bands, respectively. (c) Typical spectrum of laser emission at 1553.21 nm as the pump wavelength is located at 1464.19 nm. The inset shows the energy level diagram of Er³⁺ ions. (d) Transmission of the pump light (blue curve) and laser light (red curve) as the pump wavelength is finely scanned near the resonant mode at 1470.68 nm.

To obtain lasing in the 1550 nm band, ${Er}^{3 +}$ ions are usually pumped in either the 980 nm or 1480 nm band. Due to the experimental conditions, a narrow linewidth tunable laser in the 1480 nm band was chosen to pump the ${Er}^{3 +}$ ion-doped MBR. As shown in the inset of Fig. 2(c), when pumped in the 1480 nm band, ${Er}^{3 +}$ ions are excited from the ${{}^{4}I}_{15 / 2}$ to the ${{}^{4}I}_{13 / 2}$ level. Here, they undergo nonradiative intersubband transitions, and subsequently radiatively transition back to the ${{}^{4}I}_{15 / 2}$ level, emitting photons in the 1550 nm band. A population inversion between the first excited state ${{}^{4}I}_{13 / 2}$ and the ground state ${{}^{4}I}_{15 / 2}$ is achieved at a relatively low pump power, due to the long lifetime of state ${{}^{4}I}_{13 / 2}$ with the order of milliseconds^[21].

To investigate the process of laser emission for the fabricated ${Er}^{3 +}$ ion-doped MBR, the pump wavelength was finely scanned in the time domain. Figure 2(d) shows the transmission of the pump light (blue curve) and emitted laser light (red curve) as the pump wavelength was finely scanned near the resonant mode at 1470.68 nm. When the pump wavelength is upscanned from shorter to longer wavelengths until it reaches the resonant wavelength, light is coupled into and circulates within the resonator, forming a resonant dip in the transmission spectrum. Due to the high $Q$ and strong pump power, the resonant mode is broadened, which is attributed to a resonant wavelength shift caused by the thermal effect from light absorption in silica^[22,23]. Consequently, a distinctive thermal broadening, resembling a sawtooth waveform, is evident in the 1450 nm band. In addition, the high intensity of pump light inside the resonator enhances the gain in the laser emission band. As the laser gain surpasses the cavity losses, lasing is achieved. The red curve in Fig. 2(d) characterizes the increase in the power of the lasing light in the 1550 nm band as the pump power coupled into the resonator increases. The typical spectrum of laser emission at 1553.21 nm was observed when the pump wavelength was located at 1464.19 nm, as shown in Fig. 2(c).

Figure 3(a) depicts the evolution of the emitted laser spectrum at $z = 0 μm$ measured at different pump powers. It is seen that no laser is emitted under low pump power. As the pump power increases to 70 µW, the gain of the emitted laser mode surpasses the total loss, and the laser mode near 1552.8 nm becomes the dominant lasing mode. Meanwhile, redshifts of a few picometers for both pump mode and emitted laser mode were observed with the increase of the pump power. Such a resonance shift is attributed to the photorefractive effect mentioned above. We suggest that this mechanism provides an available approach for laser output wavelength fine-tuning. Further increase of the pump power ( $\sim 100 μW$ ) not only increases the emitted laser power but also transitions the dominant laser mode into multimode operation (i.e., laser modes located at $\sim 1552.31 nm$ and $\sim 1553.96 nm$ ). Such a mode switching is attributed to thermal effects and photorefractive changes under high pump power, which leads to the competitive mode gain. These observations demonstrate that our MBL possesses the ability to control and manipulate lasing modes by varying the pump power.

Figure 3.(a) Evolution of the spectra at z = 0 µm in the range of 1552 to 1555 nm measured at different pump powers; (b) relationship between the pump power and the emitted laser power at different axial positions of the MBR, z = 0 µm, 15 µm, 30 µm, 45 µm, respectively.

Compared with other microresonators, the MBR with an elongated shape supports highly nondegenerate WGMs. These modes are well separated along the axial direction of the MBR, forming a series of equally spaced and dense axial modes. Theoretically, it is possible to selectively excite lasing modes by adjusting the pump positions along the axial direction of the MBR. In experiments, a tapered fiber with a diameter of 2.5 µm was positioned at various axial positions of the MBR. Figure 3(b) demonstrates the linear change of the output emitted laser power with the increasing of the pump power when the taper is positioned at 0, 15, 30, and 45 µm away from the MBR center, respectively. The pump laser threshold P_th and conversion efficiency $η$ measured at different axial positions are 70 µW ( $1.6 \times 10^{- 3} %$ ), 120 µW ( $1.28 \times 10^{- 3} %$ ), 219 µW ( $1 \times 10^{- 3} %$ ), and 336 µW ( $1.29 \times 10^{- 3} %$ ), respectively. Herein, the pump threshold power P_th of the MBL can be approximately expressed as^[24] $P_{th} \approx \frac{2 π n h c V_{p}}{λ_{l} λ_{p} Q η σ_{em} τ},$ (2)where $n$ and $Q$ refer to the effective refractive index and quality factor of the MBR, respectively. $λ_{l}$ and $λ_{p}$ indicate the wavelength of the emitted laser mode and pump laser mode, respectively. $V_{p}$ is the mode volume of the pump mode. $h$ and $c$ refer to the Planck constant and the speed of light in a vacuum, respectively. $σ_{em}$ and $τ$ correspond to the stimulated emission cross section and fluorescence lifetime of the gain ions, respectively. It is seen from Eq. (2) that the high $Q$ factor and small mode volume are critical for a low laser threshold in ${Er}^{3 +}$ ion-doped MBL. Due to the high $Q$ factor of the excited fundamental axial mode, the minimum laser threshold of 70 µW is obtained at the axial position of $z = 0 μm$ . Additionally, as the coupling position is gradually shifted away from the center of the MBR, the laser threshold increases due to the larger mode volume of higher axial modes. However, despite these variations in the threshold, the conversion efficiency measured at different axial positions remains relatively close, thanks to the uniform concentration distributions of ${Er}^{3 +}$ ions through the optimized sol-gel method.

Furthermore, a tapered fiber was coupled at axial positions of $z = 0$ , 10, 30, 50 µm of the MBR using the pump wavelength of 1483.63 nm with a pump power of 5 mW, respectively. The simulated WGM field distributions of axial orders at different positions (e.g., $z = 0$ , 10, 30, and 50 µm) are shown in Figs. 4(a)–4(d). The evolutions of emitted laser spectra at different axial contacted positions are depicted in Figs. 4(e) and 4(f). It indicates that both the fundamental axial mode and other lower-order modes are selectively excited at the contacted position $z = 0$ , where large overlaps of their mode distributions contribute to laser emission. When the tapered fiber is coupled at $z = 0$ [see Fig. 4(e)], multimode lasing occurs at the wavelength of $\sim 1553.216$ , $\sim 1552.26$ , and $\sim 1551.26 nm$ , corresponding to the axial quantum numbers $q = 0$ , 1, 2, which is theoretically calculated using the formula in Ref. [25]. When the tapered fiber is moved further by 10 µm away from the center area, lasing with a richer spectrum shown in Fig. 4(f) is obtained because of the large field overlap of higher-order WGMs^[26]. However, as the contacted position is located at 30 and 50 µm away from the center area, the number of lasing modes gradually decreases [see Figs. 4(g) and 4(h)], which may be caused by the smaller concentration of ${Er}^{3 +}$ ions, and large mode volumes of higher-order modes^[27], thus increasing the laser threshold according to Eq. (1). Consequently, precise control of the axial coupling position of the tapered fiber with the MBR enables the flexible engineering of the number of laser modes, facilitating the switch between single-mode and multimode laser operations.

Figure 4.(a)–(d) WGM field distributions with axial order q = 0, 2, 18, and 49, corresponding to contacted positions of z = 0, 10, 30, and 50 µm, respectively; (e)–(h) evolution of emitted laser spectra at different axial contacted positions.

Considering the phase-matching conditions between the tapered fiber and the MBR, we have experimentally demonstrated that the selectively excited radial modes also help to engineer the number of emitted laser modes. Figures 5(a)–5(c) investigate the evolution of emitted laser spectra when the MBR is coupled with the tapered fiber at the diameter of $d = 2.9 μm$ , 3.2 µm, and 3.4 µm, respectively. The axial coupling position of the MBR was kept at $z = 0 μm$ . It is seen that several laser modes are generated when the MBR is in contact with the tapered fiber at a diameter of 2.9 µm. As the diameter of the contacted tapered fiber increases, the number of emitted laser modes gradually reduces until a single-mode laser is achieved [see Fig. 5(c)]. This is coincident with the theoretically calculated results shown in Fig. 5(d) using the formula in Ref. [28]. That is, when the MBR is coupled with the tapered fiber at a large diameter (e.g., $d = 3.4 μm$ ), the phase matching with a large propagation constant only supports the excitation of the fundamental radial mode, resulting in a single laser mode occurrence. As the coupled diameter of the tapered fiber decreases to less than 3.2 µm, not only the fundamental radial mode but also the low-order radial modes (i.e., $l = 2$ ) are excited, which results in the multimode laser. The corresponding field distributions of different orders of radial modes are shown in the inset of Fig. 5(d). Therefore, the number of emitted laser modes of the MBR can be flexibly controlled by adjusting the coupling diameter of the tapered fiber, enabling the laser mode to switch between the single mode and multiple modes.

Figure 5.(a)–(c) Evolution of emitted laser spectra when the MBR is coupled with the tapered fiber at different diameters (d = 2.9, 3.2, and 3.4 µm); (d) calculated propagation constant as a function of the microfiber radius and the radial order of the MBR. Insets show the field distributions of different orders of radial modes.

In addition to engineering the number of laser modes, precise tuning of the emitted laser wavelength is crucial for applications such as frequency-modulated continuous-wave laser detection and ranging (FMCW LiDAR) systems^[11] and single-particle detection^[12]. Different from traditional external tuning methods, continuous wavelength tuning is experimentally achieved by continuously changing pump wavelength, employing the thermal effects of a high $Q$ MBR. For instance, when the pump wavelength was set as $\sim 1480 nm$ with the power of 20 mW, the emitted laser had a wavelength of 1556.5 nm. By continuously changing the pump wavelength from 1480.042 to 1480.141 nm, the emitted laser could be continuously tuned from 1556.516 to 1556.618 nm, as shown in Fig. 6(a). Meanwhile, the laser power exhibited slight fluctuations. When the pump wavelength increases from 1480.042 to 1480.091 nm, the emitted laser power rises from $- 33.39$ to $- 29.8 dBm$ , and subsequently decreases from $- 29.8$ to $- 38.6 dBm$ , with the pump wavelength increasing from 1480.091 to 1480.141 nm.

Figure 6.(a) Evolution of emitted laser spectra as the wavelength of the pump laser increases; (b) wavelength shifts of the emitted laser mode with the increasing wavelength of the pump laser. The fitting line shows a linearity of 99.96% with a tuning range of 0.102 nm.

Because of that, the cavity mode significantly broadens as pumped with high power. When the initial pump wavelength is located at the blue detuning region [see the blue curve in Fig. 2(d)], the energy accumulation is insufficient to support strong emitted laser power. However, as the pump wavelength gradually increases with the step of 1 pm, the detuning decreases, leading to an increase in laser power. The emitted laser power reaches the maximum value of $- 29.8 dBm$ , when the pump wavelength is adjusted to 1480.091 nm, which corresponds to the optimal matching of the pump mode with the resonant mode. The linear relationship between the pump wavelength and the emitted laser wavelength is depicted in Fig. 6(b). The linearity of the continuous tuning can be up to 99.96% with a range of 0.102 nm. Notably, the minimum tuning accuracy is $\sim 1 pm$ , which is constrained by the fine-tuning resolution of the pump light source.

3. Conclusion

In summary, we systematically investigated the laser mode switching and precise wavelength tuning characteristics of an ultrahigh-Q ${Er}^{3 +}$ -doped MBL. Benefiting from the diverse distributions of azimuthal, radial, and axial WGMs for the MBL, experiments demonstrate that laser modes between the single mode and multimodes can be flexibly switched by adjusting the pump power, varying the coupling positions along the axis of the MBR, and changing the coupling diameter of the tapered fiber. Meanwhile, by precisely adjusting the wavelength of the pump laser, the all-optical tuning of the output laser wavelength can be continuously tuned over a range of 0.102 nm with a high linearity of 99.96%. By optimizing the coupling state and improving the $Q$ factor of the MBR, it is expected that the tuning range could be further improved by utilizing much-broadened thermal effects. Future work could focus on the optimization of the rare-earth doping process for improving the intrinsic $Q$ of the microcavity, as well as the flexible design of the microlaser via upconversion or downshifting processes^[8]. Our work offers a systematic control technique for engineering laser modes based on WGMs, enhancing the practical applications of the MBLs in miniature tunable single-mode lasers, laser precision measurement, and cavity–matter–light interaction investigations.

References

[1] Y. Xiao, Q. Gong. Optical microcavity: from fundamental physics to functional photonics devices. Sci. Bull., 61, 3(2016).

[2] L. He, S. K. Oezdemir, J. Zhu et al. Detecting single viruses and nanoparticles using whispering gallery microlasers. Nat. Nanotechnol., 6, 428(2011).

[3] X. Jiang, A. J. Qavi, S. H. Huang et al. Whispering-gallery sensors. Matter, 3, 371(2020).

[4] N. Toropov, G. Cabello, M. P. Serrano et al. Review of biosensing with whispering-gallery mode lasers. Light Sci. Appl., 10, 42(2021).

[5] Y. Guo, Z. Li, N. An et al. A monolithic graphene-functionalized microlaser for multispecies gas detection. Adv. Mater., 34, 2207777(2022).

[6] C. Xu, J. Dai, G. Zhu et al. Whispering-gallery mode lasing in ZnO microcavities. Laser Photonics Rev., 8, 469(2014).

[7] E. A. Anashkina, A. Andrianov. Erbium-doped tellurite glass microlaser in C-band and L-band. J. Lightwave Technol., 39, 3568(2021).

[8] B. Jiang, S. Zhu, L. Ren et al. Simultaneous ultraviolet, visible, and near-infrared continuous-wave lasing in a rare-earth-doped microcavity. Adv. Photonics, 4, 046003(2022).

[9] Y. Hu, X. Mao, H. Yang et al. Demonstration of Yb3+-doped and Er3+/Yb3+-co-doped on-chip microsphere lasers. Opt. Express, 29, 25663(2021).

[10] L. Yang, K. J. Vahala. Gain functionalization of silica microresonators. Opt. Lett., 28, 592(2003).

[11] M. Li, M. Tian, C. Lin et al. Microcavity Raman laser-based FMCW LiDAR with enhanced echo sensitivity. ACS Photonics, 11, 801(2024).