Passively Mode-Locked Fiber Laser Based on a TiO₂/SiO₂-Assisted Microsphere Resonator

Abstract

A composite dual-cavity passively mode-locked fiber laser based on a functionalized microsphere resonator is proposed and experimentally demonstrated. The nonlinear response of the resonator is enhanced by depositing TiO₂ film on a SiO₂ microsphere, which leads to improved mode-locking performance. The wavelength selectivity and optical field confinement of the microsphere resonator are exploited, allowing it to simultaneously serve as an intracavity narrowband filter and a nonlinear modulation element. The threshold of the mode-locked laser was measured to be as low as 34 mW, and stable mode-locked operation was achieved at a pump power of 105.7 mW, with a pulse duration of 2.8 ns, a repetition rate of 13.88 MHz, and a signal-to-noise ratio of 74.86 dB. The output spectrum exhibited a central wavelength of 1560.12 nm, a 3 dB linewidth of 0.06 nm, and a side-mode suppression ratio of 55.13 dB. This straightforward design provides an effective approach for the miniaturization of passively mode-locked fiber lasers.

1. Introduction

With the continuous development of laser technology towards miniaturization, high energy efficiency, and simple structure, fiber lasers are gradually replacing the previous bulky solid-state pulsed sources. To satisfy the increasing demands for high stability and high coherence in applications such as medical treatment [1], optical communication [2], and manufacturing [3], various pulsed laser technologies have been developed and widely investigated. Among them, Q-switched lasers typically operate in a regime of high pulse energy and low repetition rate, whereas passively mode-locked lasers (MLLs) generate phase-coherent ultrashort pulse trains at high repetition rates and can serve as seed sources for frequency combs and chirped pulse amplification systems [4,5,6]. In passive MLLs, the high optical power circulating in the cavity accumulates significant nonlinear phase modulation, thereby establishing a fixed phase relationship among the frequency components and enabling stable mode-locking. Traditional mode-locking schemes based on saturable absorbers, nonlinear polarization rotation (NPR), nonlinear amplifying loop mirrors (NALMs), and nonlinear multi-mode interference [7,8,9,10] typically achieve nonlinear accumulation by increasing the cavity length, raising the pump power, or introducing an amplification section. However, these approaches are inherently limited by constraints on cavity length, optical loss, and noise accumulation, which ultimately restrict the properties of the pulse laser.

Whispering-gallery-mode (WGM) microcavities [11,12,13], distinguished for their capability to enhance light–matter interactions within extremely small mode volumes, have found widespread applications in micro-lasers [14,15], sensing [16,17], nonlinear optics [18,19], cavity quantum electrodynamics [20], and non-Hermitian optics [21]. In particular, micro-lasers serve as key light sources in integrated photonics and optoelectronic systems. As a result, micro-resonator-assisted fiber lasers have been widely investigated for pulsed laser generation owing to their strong field confinement and comb-like spectral filtering properties. Peccianti et al. demonstrated a pulsed laser based on a CMOS-compatible high-Q micro-ring resonator exhibiting an ultrahigh repetition rate and narrow spectral linewidth [22]. Wang et al. reported a high-repetition-rate comb laser source using a micro-ring resonator, and the repetition rate can be precisely controlled by adjusting the cavity length [23]. Huang et al. utilized a microfiber resonator combined with high-birefringence fibers to achieve terahertz-repetition-rate pulses. The repetition rate can be tuned by adjusting the birefringence of the fiber using polarization controllers [24]. These approaches primarily focus on generating ultrafast pulses at extremely high repetition rates, which are attractive for applications such as biological imaging and quantum technologies. In contrast to high-repetition-rate schemes, micro-resonator-based fiber lasers have also been explored for stable nanosecond pulse generation, where system robustness, low threshold power, and compactness are of primary concern. Kues et al. employed a micro-ring resonator as the nonlinear element in an NALM-based ring cavity to achieve a stable nanosecond pulsed laser [25]. Nevertheless, most previously reported schemes rely on complex instrumentation or precision coupling schemes, resulting in system complexity and elevated costs. This has driven considerable interest in developing more compact and easily integrated strategies to enhance the nonlinear response of microcavities. In previous studies, other high-nonlinearity materials have been explored for passive mode-locking. For example, γ-MnO₂ was incorporated into a dual-core, pair-hole fiber (DCPHF) carrier, enabling ultrafast pulse generation [26]. The results highlight that the integration of functional nonlinear materials into fiber or microcavity platforms can effectively control pulse lasers. Inspired by this approach, coating high-nonlinearity materials onto the surface of WGM microcavities has been regarded as a promising approach. Such configurations effectively strengthen light–matter interactions and facilitate nonlinear optical processes. TiO₂ is a material with high refractive index, broadband transparency, and an optical nonlinearity approximately 30 times that of SiO₂ [27]. By improving mode field confinement, TiO₂ significantly enhances the instantaneous nonlinearity, thereby facilitating mode-locking formation and optimizing pulse properties. Compared with magnetron sputtering, bottom-up growth, and atomic layer deposition techniques [28,29,30], the sol–gel method is favored due to the simple processing, low cost, and excellent film uniformity, offering significant advantages for fabricating composite films [31]. Therefore, integrating a functional TiO₂ film coating with a WGM microsphere resonator (MR) represents an effective approach to realize passively MLLs.

In this work, we propose and experimentally demonstrate a composite dual-cavity system based on a TiO₂/SiO₂ functionalized microsphere resonator (TiO₂/SiO₂ FMR). This system enables the stable generation of passively mode-locked lasers (MLLs), which is a performance enhancement strategy that has rarely been investigated for FMR-based mode-locking. The TiO₂/SiO₂ FMR is fabricated by coating a TiO₂ film onto the surface of a SiO₂ MR. A key feature of this design is that the TiO₂/SiO₂ FMR simultaneously functions as an ultra-narrowband spectral filter and a high nonlinear modulation element. Moreover, the strong optical field confinement promotes the formation of mode-locked pulses. The influence of the TiO₂ film on the output characteristics of the laser is also investigated. Experimentally, stable MLL is achieved without auxiliary components, producing pulses with a pulse duration of 2.8 ns, a repetition rate of 13.88 MHz, and a signal-to-noise ratio (SNR) of 74.86 dB. The wavelength optical laser emits at 1560.12 nm with a 3 dB bandwidth of 0.06 nm and a side-mode suppression ratio (SMSR) of 55.13 dB. Moreover, the mode-locking threshold is 34 mW. This approach offers an effective route toward compact and miniaturized pulsed laser sources and provides new insights into mode locking enabled by material and microcavity-assisted field manipulation.

2. Experiment Setup

The configuration of the composite dual-cavity passive MLL based on TiO₂/SiO₂ FMR is shown in Figure 1. The laser employs a ring-cavity structure. A 980/1550 nm wavelength-division multiplexer (WDM) is used for pump coupling, and a 2.3 m single-mode erbium-doped fiber (EDF, Nufern, EDFL-980-HP, East Granby, CT, USA) serves as the gain medium. A polarization-independent isolator (ISO) is inserted to isolate the reflected light from the TiO₂/SiO₂ FMR and ensure the unidirectional propagation of the light. A polarization controller (PC) is inserted to adjust the intra-cavity polarization state. The 980 nm pump is provided by a laser diode. The TiO₂/SiO₂ FMR is incorporated into the main ring cavity via a fiber taper (FT), where it functions simultaneously as a nonlinear modulation element and a narrowband spectral filter. The output laser is extracted from the 10% port of the OC, and the 90% port is used as the cavity feedback to maintain the operation of the laser. The experimental results are measured by an oscilloscope (OSC, Agilent, DSO9254A, Santa Clara, CA, USA), an optical fiber coupling power meter (Thorlabs, S145C, Newton, MA, USA), a spectrum analyzer (OSA, Yokogawa, AQ6370D, Tokyo, Japan), and a radio frequency analyzer (ESA, Agilent, N9030A, Santa Clara, CA, USA).

3. Fabrication and Properties of TiO₂/SiO₂ FMR

3.1. Fabrication of the TiO₂/SiO₂ FMR

A TiO₂ film is synthesized by the sol–gel method and coated onto the surface of the SiO₂ MR, forming the TiO₂/SiO₂ FMR with enhanced optical nonlinearity. The SiO₂ MR is fabricated by producing a fiber-stem integrated microsphere through a localized arc-melting technique. A standard single-mode fiber is tapered to form a single-ended FT with a length of 4–6 cm. The tapered tip is melted using a modified fusion splicer, and a microsphere is formed under surface tension.

The TiO₂ sol is prepared by mixing titanium butoxide (8.5 mL, Shanghai Meryer Biochemical Technology Co., Ltd., Shanghai, China) with anhydrous ethanol (20 mL, Chengdu Jinshan Chemical Reagent Co., Ltd., Chengdu, China) to form Solution A, and glacial acetic acid (5 ml, Tianjin Beilian Fine Chemicals Development Co., Ltd., Tianjin, China) with 95% ethanol (22.5 ml, Chengdu Jinshan Chemical Reagent Co., Ltd., Chengdu, China) to form Solution B. Solutions A and B are combined at room temperature, magnetically stirred for 4 h, and aged for 12 h to obtain a homogeneous colloid. The cleaned SiO₂ MR is then immersed in the TiO₂ sol for 5–10 min and baked at 160 °C for 10–15 min for preliminary curing. A subsequent densification process using electrode discharge produces a TiO₂/SiO₂ FMR with a smooth surface.

Finally, a 2–3 cm section of coating on a single-mode fiber is removed and cleaned, and an FT with the diameter of 1–4 μm is fabricated using a precision tapering system (XQ7170-B04, Shenzhen OSCOM Technology Co., Ltd., Shenzhen, China) based on a single-point heating and pulling technique.

3.2. Properties of TiO₂/SiO₂ FMR

The performances characterization of TiO₂/SiO₂ FMR is presented in Figure 2. Figure 2a shows the structure of the TiO₂/SiO₂ FMR. Figure 2b indicates that the diameter of the microsphere is about 128 µm, and the layered structure of the TiO₂ film can be clearly seen. To further confirm the characteristics of TiO₂ films, X-ray diffraction (XRD) spectrum is measured, as shown in Figure 2c. To gain deeper insight into the influence of the TiO₂ film on the optical-field distribution, finite-element simulations based on COMSOL Multiphysics 6.2 are performed. Figure 2d,e show the simulated optical field distributions of SiO₂ MR and TiO₂/SiO₂ FMR (a wavelength of 1550 nm, a refractive index of 2.19, and a TiO₂ film thickness at 100 nm), respectively. The results indicate that the optical field in the TiO₂/SiO₂ FMR is more strongly confined near the equatorial region within the TiO₂ layer, thus resulting in enhanced nonlinearity and improved modulation capability of the laser.

Additionally, the optical properties of the TiO₂/SiO₂ FMR are measured. The transmission spectrum of 128 μm TiO₂/SiO₂ FMR is recorded using a broadband light source. In Figure 2f, a broadband light source (Hoyatec, SLED, BLS, Shenzhen Hoyatek Co. Ltd., Shenzhen, China) is used as pump source. The coupling system between the FT and the MR is monitored in real time by a microscope (CCD, 1200×). The transmitted light signal is adjusted using a variable optical attenuator (VOA, Shenzhen MC Fiber Optics Co., Ltd., Shenzhen, China) and subsequently analyzed by an optical spectrum analyzer (OSA, AQ6370D, Tokyo, Japan). The results show that the free spectral range (FSR) is 4.05 nm, and a periodic WGM distribution is observed, exhibiting a typical comb filter characteristic, as shown in Figure 2g. Figure 2h presents the characterization of the Q factor, which is measured by scanning the frequency of a tunable laser at 1550 nm. The observation process is shown in the red line of Figure 2f. The broadband light source (TOPTICA CTL 1550, Graefelfing, Germany) serves as the pump source, which is coupled through a MR-FT system and connected to an oscilloscope (OSC, Yokogawa, DLM2034, Tokyo, Japan) via a photodetector (PD, Newport, RI, USA). According to the formula $Q=\omega /\Delta \omega$ (where $\omega$ is the resonance frequency and $\Delta \omega$ is the Lorentz linewidth), the measured resonance linewidth of 224 MHz corresponds to a Q factor of approximately 8.6 × 10⁵.

4. Results and Discussion

4.1. Output Performance of the MLL

A TiO₂/SiO₂ FMR with a diameter of 128 μm and a resonance linewidth of 224 MHz is inserted into the main ring cavity, which has a total length of 14.4 m, corresponding to a repetition rate of 13.88 MHz. Therefore, a maximum of 16 longitudinal modes of the main ring cavity can oscillate within a single TiO₂/SiO₂ FMR resonance. By finely adjusting the PC and gradually increasing the pump power, both continuous-wave (CW) and pulsed states are achieved. Figure 3 illustrates the evolution of the laser with increasing pump power. Figure 3a depicts the influence of pump power on the output spectrum. As the pump power increases from 3.2 mW to 105.7 mW, the spectral center remained at approximately 1560.12 nm, while the output intensity increased. The inset illustrates the spectral evolution at different pump powers, showing a clear increase in peak intensity accompanied by a slight redshift of the central wavelength. These results indicate that the intracavity mode selection is stable and there is no observable mode hopping. In the future, methods such as optimizing the microcavity materials, introducing a temperature control system, and applying active frequency locking techniques can be employed in real time to compensate for frequency deviations, thereby further enhancing the stability of the system [32,33,34].

The evolution of the laser states with increasing pump power is shown in Figure 3b. As the pump power increases, CW lasing is first observed with a threshold of 3.2 mW. For pump powers below 34 mW, only continuous noise is recorded on the OSC, indicating that the system remained in CW operation. When the frequency difference between the main-cavity longitudinal modes and the WGMs is less than half of the resonance linewidth of the MR, mode resonance in the composite dual-cavity system is established. The main-cavity modes effectively fell within the resonance window of the MR and are cyclically amplified, achieving laser emission. However, the intracavity energy is initially too low to excite the nonlinear effects, and the system remains in the CW state. As the pump power is increased to 34 mW, the intracavity energy reaches the threshold for the Kerr effect, triggering the transition from CW to pulsed operation. Within the pump range of 34 to 105.7 mW, highly stable and periodic pulse trains are observed on the OSC, indicating the system remained in a stable mode-locked state. In this regime, further increases the pump power enable Kerr-induced dynamic compensation of small mode detuning between the two cavities. This compensation allows coherent superposition of multiple longitudinal modes and leads to the formation of low-loss mode-locked pulses. When the pump power exceeds 105.7 mW, the compensation is insufficient to overcome the mode detuning, and mode-locking is disrupted.

By finely adjusting the PC, the laser output of the MLL at the pump power of 105 mW is characterized in terms of the optical spectrum, pulse train, single pulse, and radio-frequency (RF) spectrum, as summarized in Figure 4. Figure 4a shows the output spectrum of the MLL at with a resolution of 0.02 nm. The wavelength of the mode-locking operation at 1560.12 nm is observed, and is associated with an SMSR of 55.13 dB and a 3 dB linewidth of approximately 0.06 nm. These results show that single-wavelength lasing is achieved, with mode hopping effectively suppressed due to the pronounced wavelength-selective property of the TiO₂/SiO₂ FMR. Figure 4b presents the pulse train recorded by the OSC, showing a pulse interval of 72 ns, corresponding to a fundamental pulse repetition rate of 13.88 MHz. It is worth noting that, during the experiment, no pulse splitting is observed. The corresponding single-pulse train is shown in Figure 4c, and the temporal profile fitted by a Gaussian function, yielding a full width at half maximum (FWHM) of 2.8 ns. To evaluate the stability of the MLL, the RF spectrum is measured over a span of 20 MHz with a resolution of 2 kHz, as presented in Figure 4d. A pronounced peak is located at 13.88 MHz with a high SNR of 74.86 dB, indicating excellent good stability and temporal coherence. This RF peak corresponds to the pulse repetition rate of the 14.4 m main ring cavity, confirming that the MLL operates at the fundamental rate. During the mode-locking process, the TiO₂/SiO₂ FMR serves as a nonlinear modulator, promoting pulse formation. In addition, the high SNR is attributed to the narrow-linewidth filtering capability, which effectively suppresses spurious frequency components, and further enhances the stability of the pulse train [35].

In the experiment, the TiO₂/SiO₂ FMR simultaneously provides narrowband spectral filtering and nonlinear modulation. On the one hand, the MR supports WGMs, the transmission spectrum exhibits a comb-like distribution, which enables only selected longitudinal modes to oscillate. This typical frequency selectivity effectively suppresses wavelength hopping, thereby significantly enhancing wavelength stability. On the other hand, the strong nonlinearity of the TiO₂/SiO₂ FMR efficiently induces the Kerr effect within localized regions. As a result, low-threshold, highly stable, and highly coherent MLL can be obtained based on the TiO₂/SiO₂ FMR.

To evaluate the long-term stability of the mode-locking state, the output spectrum and the power of the MLL are continuously monitored at the pump power of 105.7 mW, as shown in Figure 5.

The output spectrum is recorded at 5 min intervals over a period of 50 min, as illustrated in Figure 5a. The wavelength remains highly stable throughout the measurement, with a maximum shift of only 0.04 nm (the inset), demonstrating excellent spectrum stability and coherence of the MLL. The stability of the output power is also measured, as shown in Figure 5b. The maximum average output power of MLL is 1.34 mW, with an RMS fluctuation of 0.95% over 60 min, further exhibiting long-term stability and robustness of the system. Notably, the composite dual-cavity system is encapsulated in a self-made protective box, which effectively isolates external disturbances. In addition, the narrowband filtering property of the TiO₂/SiO₂ FMR limits longitudinal mode competition, thereby preventing pulse splitting and frequency shifts. To further reduce power fluctuations and improve system performance, feasible optimization strategies include maintaining a stable polarization state, minimizing external disturbances such as mechanical vibrations and air turbulence, and precisely tuning the coupling conditions between the MR and the FT.

4.2. Comparison of MLL Performance in TiO₂/SiO₂ FMR vs. SiO₂ MR

In order to further investigate the influence of functional coating on the output characteristics of MLL, a comparative study between TiO₂/SiO₂ FMR vs. SiO₂ MR is performed. Figure 6 shows the dependence of the pulse duration, output power, single-pulse energy, and peak power on the pump power for MLL.

As shown in Figure 6a, the pulse duration gradually decreases with increasing pump power. Comparative results indicate that the narrowest pulse duration obtained with the TiO₂/SiO₂ FMR is 2.8 ns, representing a 57% reduction relative to 6.6 ns achieved with the SiO₂ MR. Figure 6b,c show that both the average output power and single-pulse energy increase linearly with pump power. At the pump power of 105.7 mW, the output power and single-pulse energy of the SiO₂ MR reach 1.41 mW and 101.6 pJ, respectively, which are slightly higher than those of the TiO₂/SiO₂ FMR (1.34 mW and 96.3 pJ). Figure 6d shows that the peak power increased with pump power, and the TiO₂/SiO₂ FMR achieved a maximum peak power of 34.6 mW, representing a 126% enhancement relative to 15.3 mW for the SiO₂ MR.

The experimental results indicate that the observed improvement in mode-locking performance originates from the introduction of TiO₂ film onto of the MR. The coated TiO₂ film enables significant pulse compression and an increase in peak power, which is attributed to the enhanced Kerr nonlinearity. The functional film strengthens local phase modulation and promotes coherent longitudinal mode locking. Although the coated MR improves pulse performance, the mode-locking threshold does not show a noticeable decrease. The additional scattering and absorption losses introduced by the TiO₂ film result in a lower Q factor of the MR. The intensity-dependent behavior of the passive MLL has also been demonstrated. At low pump power, the intracavity intensity is insufficient to induce the nonlinear phase modulation necessary for pulse formation. Conversely, excessive pump power increases nonlinear accumulation in the cavity, disrupting the phase synchronization between the main ring cavity and the MR, thereby causing the mode-locking state to disappear. Based on the experimental results, the output characteristics of the MLLs can be further optimized by modifying the microcavity structure, material properties, and device parameters.

In addition, as shown in

Table 1. Comparison of passively MLLs with various methods.

Structure	Threshold (mW)	Pulse Duration (ns)	SNR	Reference
gold nanoparticles-SA	157	436	65	[36]
CuNWs-SA	104.62	173	61	[37]
FePc-SA	97.62	356	71.6	[38]
GIMF-SA	480	4	51.2	[39]
FMR-dual-cavity system	34	2.8	74.8	This work

, the performance of nanosecond pulsed lasers realized by different approaches is compared. Lower threshold power and higher SNR are achieved in this work, which confirms the feasibility of the proposed dual-cavity configuration. These improvements are enabled by the incorporation of the FMR, whose inherent mode-selective property provides effective regulation for stable nanosecond mode-locked operation. Therefore, the compact FMR-assisted dual-cavity architecture is a promising solution for miniaturized, high-performance pulsed laser sources.

5. Conclusions

In summary, a composite dual-cavity passively MLL based on a TiO₂/SiO₂ FMR has been proposed and experimentally demonstrated. The TiO₂/SiO₂ FMR coated with a high nonlinear TiO₂ film on the surface of SiO₂ MR, provides both narrow-band spectral filtering and nonlinear modulation, playing a key role in the generation of MLLs. The MLL exhibits a low threshold of 34 mW. At the pump power of 105 mW, stable pulsed output is achieved with a pulse duration of 2.8 ns, a repetition frequency of 13.88 MHz, and an SNR of 74.86 dB. The central wavelength of the spectrum is located at 1560.12 nm, with an SMSR of 55.13 dB and a 3 dB linewidth of approximately 0.06 nm. Compared with an uncoated SiO₂ MR, the pulse duration can be compressed by approximately 60%, while the peak power increased by nearly 126%. These results demonstrate that the TiO₂/SiO₂ FMR not only enables low-threshold, stable MLL operation, but also provides insight into its influence on pulse characteristics. The results indicate that functional coating of MR provides an effective degree of freedom for mode-locking, which is beyond simple cavity optimization and offers a new route for microcavity-assisted ultrafast fiber lasers.