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Abstract
A novel fiber laser structure, to the best of our knowledge, based on an erbium-doped fiber ring (EDFR) and a silica microbottle resonator (MBR) is proposed and investigated experimentally. Two fiber laser samples based on MBRs with different geometries and diameters of 200 and 150 µm are fabricated, and their performance is studied experimentally. Periodic whispering gallery mode spectra of the MBRs are dependent on the position of the fiber taper used for coupling of light into the MBR, and this dependence is explored to achieve lasing at different wavelengths by moving the light coupling point along the axis of the microbottle incorporated into the proposed EDFR-MBR system. The influences of the pump laser power and light polarization on the system performance and laser stability are also investigated. Single-mode lasing with a maximum optical signal-to-noise ratio of 32 dB is demonstrated.
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1. INTRODUCTION
Whispering gallery mode (WGM) resonators of various geometries offer the advantages of miniature size, small mode volumes and high quality ($Q$) factors. As a result, they have attracted significant interest for applications in diverse areas, including sensing and optical communications [1,2]. Numerous low-threshold micro-lasers have been created based on active WGM resonators, where the material of the resonator itself serves as the active lasing medium. Examples of such micro-lasers include liquid droplets infused with organic dyes or quantum dots, sol-gel infused silica spheres, and rare-earth-doped glass microspheres [3]. On the other hand, passive WGM resonators have the potential to be used as etalons for laser frequency stabilization, which is useful for telecommunications and frequency metrology [4]. An advantage of frequency selection by a passive resonator is typically very low absorption, and several experiments have been conducted using such passive WGM resonators for laser stabilization. For example, Kieu and Mansuripur [5] proposed an erbium-doped fiber laser (EDFL) whose cavity was formed by a fiber Bragg grating mirror at one end, and by a silica microsphere resonator acting as a wavelength-selective feedback mirror for the fiber laser at the other end of the cavity. Sprenger et al. [6] also reported a very narrow-linewidth lasing from a fiber ring laser stabilized with a high-Q WGM microsphere resonator, coupled into the microsphere using a fiber taper (FT) and coupled out using an angle-polished fiber.
Even more importantly, when the spectrum of the wavelength-selective WGM element in such a system can be tuned, it is possible to tune the output spectrum of the EDFL. This was demonstrated by several researchers, including Wei and Krishnaswamy [7], who reported a polymer micro-ring resonator integrated with a fiber ring laser for ultrasound detection. In their experiment, acoustic waves caused strain or deformation of the micro-ring resonator, leading to shifts of the WGM resonance wavelength and thereby shifts in the EDFL spectrum. Ma et al. [8] experimentally demonstrated a tunable sub-kilohertz single-mode fiber (SMF) laser based on an iron-oxide-nanoparticle-coated silica microbottle resonator (MBR) acting as the feedback element of the fiber ring laser. The proposed laser was linearly tuned within a range of 2.7 nm using the photothermal effect of iron oxide nanoparticles.
MBRs were studied by Sumetsky [9] who introduced the idea of thickening the radius of a WGM cylinder to fully confine light. The distribution of WGMs along the axis of an MBR has a characteristic variation length greatly exceeding those of microsphere and microtoroid resonators which not only allows better coupling of light into the microcavity, but also offers a richer resonant spectrum [10]. Although MBRs have lower $Q$ factors compared to microspheres, the external access to their WGMs is significantly simplified which makes them suitable for applications as tunable or switchable wavelength-selective elements.
Recently, we proposed and experimentally demonstrated a novel laser structure based on an erbium-doped fiber ring (EDFR) and a WGM silica MBR as the wavelength-selective element and investigated the tunability of the laser by moving the light coupling point along the microbottle’s long axis [11]. In this work, we further expand this research by fabricating two MBR samples with different shapes and diameters (200 and 150 µm), incorporating them in the proposed EDFL system to study the influence of the MBR’s geometry on the lasing performance and to explore the possibility of laser tuning, mode switching, and multi-wavelength operation. We also investigate the influence of the pump laser power and light polarization within the fiber ring on the system’s output spectrum.
2. LASER STRUCTURE AND EXPERIMENTAL SETUP
A schematic diagram of the proposed fiber ring laser structure is illustrated in Fig. 1. Laser diode (LD) emitting at 980 nm is used as a pump light source, along with a current driver as its output power control. A wavelength division multiplexer (WDM) is used to launch the pump laser signal into the fiber ring laser cavity. A section of the EDFL with the length of 13 m absorbs the pump light and produces amplified spontaneous emissions (ASEs) in the range from 1500 to 1600 nm. The EDFL is then transmitted through one of the ports of a three-port optical fiber circulator (OFC) (port 3 in the diagram) which forms a fiber ring loop. Then the port of the OFC (port 1) is connected to a FT with a uniform waist diameter of ${\sim}{1}\;{\unicode{x00B5}{\rm m}}$ for excitation of the WGMs in the silica MBR, which is placed perpendicularly and in direct contact with the FT. The ASE spectrum launched into the FT and coupled into the MBR, excites WGMs in the microresonator.
Fig. 1. Schematic diagram of the proposed fiber laser: LD, laser diode; WDM, wavelength- selective coupler; EDF, erbium-doped fiber; OFC, optical fiber circulator; FT, fiber taper; PC, polarization controller; MBR, microbottle resonator; OSA, optical spectrum analyzer.
The back-reflection from the MBR signal is then returned into the ring cavity through port 2 of the OFC. The role of the MBR in the present system is to act as the wavelength-selective filter providing external feedback into the fiber laser ring. The WGM spectrum excited within the MBR due to coupling in and out of the resonator is weakly reflected and fed back into the ring cavity giving rise to a single- or multi-mode lasing emission, at the wavelengths corresponding to a particular WGM spectrum and light polarization state. A polarization controller is introduced into the ring cavity in order to study the influence of light polarization on the performance of the system and to improve the system’s stability. The output laser spectrum is observed using an optical spectrum analyzer (OSA) with a spectral resolution of 0.01 nm.
3. FABRICATION OF THE SILICA MICROBOTTLES
A commercial optical fusion splicer (Sumitomo Electric industries, Type-36) in a manual mode with a constant arc power of 35 W was used to fabricate two different MBRs by splicing two cleaved ends of standard SMFs. Figure 2 illustrates the main MBR fabrication steps.
Fig. 2. Fabrication of a MBR using fusion splicer: (a) alignment of the fiber ends; (b) partially melted fiber ends after application of one arc; (c) fabricated MBR with the largest diameter in the middle section.
Figure 2(a) shows two coating-stripped, cleaned, and cleaved SMF ends aligned within the splicer. In the second step, shown in Fig. 2(b), each end of the fiber is heated and partially melted by applying a single electric arc discharge. Then, by controlling the fusion splicer motors, both partially melted fiber ends are brought in contact with each other, and a single arc discharge of a 5 s duration is applied to the contact region to form a solid MBR, as shown in Fig. 2(c), with the largest diameter at the middle region. Figure 3(a) shows a microbottle sample (MBR-1) with the diameter of the middle section of ${\sim}{200}\;{\unicode{x00B5}{\rm m}}$.
Fig. 3. Microscopic images: (a) MBR-1 with a diameter of 200 µm and (b) MBR-2 with a middle diameter of 150 µm.
The number of applied arc discharges can alter the shape of the MBR and its WGM field distribution [12]. The second microbottle sample for our experiments (MBR-2) was fabricated by firstly creating two microspheres at the ends of the SMFs by applying two consecutive electrical arc discharges. Then both microspheres were aligned and spliced to construct a MBR with a 150 µm diameter in its middle region, shown in Fig. 3(b). WGM spectra of both the MBRs had $Q$ factors circa ${{10}^4}$.
Because of the influence of external vibrations and possible contamination, the FT may become fragile and unstable [13]. To improve the mechanical stability in our experiments the FT-MBR system was stabilized on a microscope glass slide using an ultraviolet (UV) adhesive. The packaging process may be divided into three steps. The initial step involves affixing two glass spacers onto the main microscope slide to ensure that the central part of the FT does not come in contact with the microscope slide. Then the opposite taper ends are fixed to the spacers with the UV glue to keep the taper firmly in its position (step 2). During the third step, an MBR sample was fixed to a pair of micro-translational stages with its ends, and the distance between the MBR and the FT was carefully adjusted under a microscope to achieve the maximum coupling efficiency.
Subsequent changes in the coupling positions of the FT along the long axis of the fabricated MBRs were realized using the same method involving the translation stages and optical microscope.
4. EXPERIMENTAL RESULTS AND ANALYSIS
Figure 4 shows the experimental transmission spectra obtained before and after coupling the FT with MBR-1. As can be seen from the figure, initially, the transmission of the FT displays a broad spontaneous emission spectrum consistent with that of the EDFL pumped by a 980 nm laser with a driving current of 200 mW with a broad peak around 1532 nm. After the MBR is brought in contact with the taper close to the point D2 (see the inset in Fig. 5), a single narrow laser line appears at a wavelength of 1531.75 nm with a linewidth of 0.3 nm (38.3 GHz) and a signal-to-noise ratio of ${\sim}{32}\;{\rm dB}$. It should be noted that the linewidth measurement in our experiment was limited by the resolution of the OSA (10 pm), which corresponds to ${\sim}\;{10}\;{\rm GHz}$ level. It is expected that in reality the linewidth should be narrower, given that a relatively high $Q$ factor of the MBR spectrum can be achieved.
Fig. 4. Transmission spectra of the FT recorded by the OSA before (black line) and after (red line) coupling to the MBR at the position D2 illustrated in the inset of Fig. 5.
Fig. 5. (a) FT transmission spectra at different coupling points along the MBR-1 axis; (b) FT transmission spectra at different coupling points along the MBR-2 axis. The coupling points are illustrated schematically in the insets of the graphs.
The appearance of the lasing peak in Fig. 4 (red line) is due to the presence of the MBR, which provides external feedback into the fiber ring cavity. It is known that, in practical WGM resonators, surface inhomogeneities can cause a power transfer between different modes within the resonator. As a result, the excitation of WGMs leads to dips in the spectrum of light transmitted through the FT, and to the corresponding peaks in the spectrum of the backward-propagating light trough the taper. These back-reflected spectral peaks determine the final output positions and peak power of the lasing modes, depending on the ratio of gain and loss for each of the peak wavelengths in the ring cavity.
In comparison with other types of WGM resonators, MBRs support whispering gallery (WG) modes in a unique manner [9]. Light coupled into the cavity through the FT will propagate in a spiral shape and rotate back to the previous point of incidence after propagating many times around the cavity axis. Light will also oscillate back and forth between two turning points, forming an axial mode because MBRs can also be considered as Fabry–Perot cavities, where “skew” rays are totally reflected at the two turning points close to the bottle necks. Through the combination of the WG-ring and WG-bouncing ball principles, MBRs can support true three-dimensional WGM light confinement resulting in complex spectra with a large number of resonances. As shown previously by many researchers, the WGM spectrum of a MBR is strongly dependent on its shape and diameter. Moreover, different spectral characteristics are observed when the bottle is evanescently excited at different points along its length [14].
To investigate the possibility of tuning of the proposed EDFL-MBR spectrum, we carried out a series of experiments in which the position of the FT on the surface of the MBR-1 and MBR-2 (light coupling point) was moved along the axis of the microbottle. Figure 5 illustrates the results of the experiments, where the colored lines correspond to the transmission spectra of the FT corresponding to coupling points D1-D5 with their positions illustrated schematically in the inset of the graph. The pump laser power was set constant at 200 mW.
As can be seen from Fig. 5(a), changes in the coupling position of the FT along the long axis of the MBR-1 result in the excitation of the different WGMs corresponding to different diameters of the MBR at the corresponding coupling points, which leads to changes of the lasing wavelength. From Fig. 5(a), it can be seen that the emission wavelengths corresponding to the FT positions D1 and D5, located close to the opposite stems of the microbottle, are 1545.8 and 1539.9 nm, respectively. Although the fabricated MBR was not perfectly symmetrical, the diameters of the microbottle at the positions close to the opposite stems were ${\sim}{100}\;{\unicode{x00B5}{\rm m}}$ and led to similar lasing wavelengths. Similarly, the midway positions (D2 and D4) with corresponding diameters of ${\sim}{150}\;{\unicode{x00B5}{\rm m}}$ show lasing outputs at 1531.5 and 1534 nm. The peak associated with the coupling point at the middle section of the MBR, where its diameter is circa 200 µm, leads to emission at 1563.8 nm.
Similarly, the variation of the TF position along the long axis of the fabricated MBR-2 excites different WGMs corresponding to different diameters of the MBR. The emission wavelengths corresponding to TF sites D1 and D5, positioned adjacent to the opposing stems of the microbottle, are 1531.1 and 1531.6 nm, as shown in Fig. 5(b). Although, once again, the fabricated MBR was not ideally symmetrical, the sizes of the microbottles at the opposing stems were ${\sim}{75}\;{\unicode{x00B5}{\rm m}}$, resulting in similar lasing wavelengths. Similarly, the halfway sites (D2 and D4) with 100 µm diameters produce lasing outputs at 1531.8 and 1532.5 nm. The peak associated with the coupling point at the middle section of the MBR where its diameter is circa 150 µm causes emission at 1529.3 nm.
Fig. 6. Transmission spectra of a stand-alone MBR sample (a) when the TF is coupled to the center of the bottle and (b) when the TF is coupled to a point close to the bottle’s neck; (c) output lasing spectrum corresponding to the TF position (a), and (d) output lasing spectrum corresponding to the TF position (b).
As can be seen from the above results, it is possible to tune the spectral positions of the lasing modes by moving the TF along the MBR axis. It should be noted, however, that the tuning laws for both MBR samples differ, and the spectral shifts appear to be non-monotonic. This can be explained by the differences in modal density of the transmission spectra of the MBRs when the TF is moved along the axis of the microbottle, reported by Murugan et al. in [14]. The study showed that as the tapered fiber is moved from the bottle center to the neck, significant changes in the modal density of the transmission spectrum occur due to differences in modal excitation and selectivity. This, in turn, changes the filtering properties of the MBR spectrum in the proposed setup giving rise to various lasing modes in the fiber ring cavity. This is also illustrated in Fig. 6 below, where graphs (a) and (b) show the transmission WGM spectra of a different MBR sample when the TF is coupled to its central point (a) and a point close to the bottle’s neck (b).
Figures 6(c) and 6(d) illustrate the corresponding lasing spectra after the coupled MBR was connected to the fiber ring. It can be seen that the different coupling positions of the taper lead to different modal densities of the spectra [(a) and (b)]. Additionally, some dips in the WGM spectra of the MBR can be seen as the lasing modes when the MBR is incorporated into the fiber ring cavity. For example, spectral dips at 1531.5 and 1532.5 nm give rise to the lasing modes in Fig. 6(c).
5. INFLUENCE OF THE PUMP POWER ON THE OUTPUT SPECTRUM
As the next step, we investigated the influence of the pump laser power on the output transmission of the FT-coupled MBR-1 and MBR-2, and the results are illustrated in Fig. 7(a) and 7(b), respectively. As one can see from the graphs in Fig. 7(a), the lasing wavelength remains unchanged until the pump power is increased from 190 to 205 mW. At the power level corresponding to 205 mW, a new peak at a wavelength of 1563.8 nm is observed which dominates the first peak as the pump power is gradually increased up to 250 mW. The signal-to-noise ratio of this peak increases with the increase of the pump power.
Fig. 7. Output lasing spectra at different pump power levels for the systems with (a) MBR-1 and (b) MBR-2.
In the case of the system with MBR-2, shown in Fig. 7(b), while the pump power is in the region from 160 to 240 mW, the lasing wavelength stays constant at 1532.5 nm. A new peak at 1531.5 nm is found at a higher power level of 260 mW, which also increases in power as the pump power increases to 280 mW.
Plots in Fig. 8 summarize the results presented in Fig. 7, showing the output power of the different lasing peaks as a function of the pump laser power for MBR-1 (a) and MBR-2 (b).
Fig. 8. Output optical power versus laser pump power for the two laser peaks: (a) system with MBR-1 and (b) system with MBR-2.
An increase in the pump laser power leads to a change in the dominant laser mode. A shift to the lasing peak at 1563.8 nm is observed at pump powers above 200 mW for the system based on MBR-1 while, for the system based on MBR-2, the mode shifts from 1532.52 to ${1531.82}\;{\rm nm}$ as the pump power rises above 250 mW. Further increase in the pump power above 300 mW typically leads to the increase of the number of lasing peaks that correspond to the back-reflected WGM resonances.
6. INFLUENCE OF LIGHT POLARIZATION AND LASER STABILITY STUDIES
In the next series of experiments, the polarization effects were studied by using a three-paddle polarization controller (FPC030, Thorlabs). The light polarization state within the fiber ring was varied using the middle half-wave plate puddle by changing its initial vertical position from 0° to 45°, 90°, ${-}{45}^\circ$, and ${-}{90}^\circ$ angles, while the quarter-wave plates were kept at their vertical positions. The twisting areas of the fiber caused by turning the paddle by an angle $\tau$ will cause the polarization to rotate by an angle:
where $\alpha = - {n^2}\;{p_{44}}$ [15], ${P_{44}}$ is the fiber’s elasto-optic coefficient, and $n$ is the core’s refractive index. The elasto-optic coefficient of fused silica is given by ${P_{44}} = \frac{{{P_{11}} - {P_{12}}}}{2}$, where the numerical values of ${P_{11}}$ and ${P_{12}}$ are 0.121 and 0.270, respectively [16]. Equation (1) allows us to calculate the rotation of polarization from the original position as 0°, 7.22°, 14.44°, ${-}{7.22}^\circ$, and ${-}{14.44}^\circ$, respectively.A series of experiments was carried out for both MBR-1 and MBR-2 based systems at five different polarization states as calculated above and at a fixed level of the pump source power. In each case, the experiment was run for up to one hour to investigate the stability of the laser output. As an example, Fig. 9(a) shows the experimental output spectrum for the MBR-2 based laser with the polarization controller set to 0° polarization state and the pump power set to 205 mW. Lasing at three wavelengths, 1533.1, 1533.84, and 1548.88 nm, with extinction ratios of ${\sim}{30}$, ${\sim}{31}$, and ${\sim}{38}\;{\rm dB}$, respectively, is observed. The experiment was run for 50 min, and the output spectra were recorded with 5 min intervals. Figure 9(b) illustrates the fluctuations in the laser peak wavelengths (left) and output power of each of the peaks (right) as functions of time. While the lasing peak wavelengths remain very stable, maximum power fluctuation within the duration of the experiment was significant, reaching up to 5 dB, demonstrating poor stability.
Fig. 9. (a) Output spectrum of the MBR-2 based system at 0° polarization state and laser pump power of 205 mW; (b) wavelength drift and power fluctuations during the 50 min experiment for the three laser peaks.
It should be noted that tweaking the paddles of the polarization controller to modify the polarization state of light in the ring cavity can make a significant impact on the output spectrum of the proposed system. A change in the light polarization within the laser cavity leads to changes in the WGM spectrum of the coupled MBR, thus causing various filtering effects. Since the interrelationship between the gain and losses at a specific laser wavelength is determined by the filter function, it is possible to control the number of lasing peaks and/or improve their stability. For example, Fig. 10(a) shows the single-mode (at 1532 nm) spectral output of the system based on MBR-1 with light polarization fixed at 14.44°. As can be seen from Fig. 10(b), the laser had excellent stability throughout the 40 min experiment.
Fig. 10. (a) Output spectrum for MBR-1 at 14.44° polarization; (b) peak wavelength drift and power fluctuations over the 40 min experiment.
7. CONCLUSION
In conclusion, here we proposed and investigated experimentally a novel fiber laser structure composing an EDFR and a silica MBR. We experimentally demonstrated that the output spectrum of the proposed EDFR-MBR system can be tuned by means of moving the light coupling site along the long axis of the MBR. The MBR in the proposed structure acts as the wavelength-selective filter providing external feedback into the fiber laser ring cavity. The WGM spectrum excited within the MBR due to coupling in and out of the resonator is weakly reflected and fed back into the ring cavity giving rise to a single or multi- mode lasing emissions, at the wavelengths corresponding to a particular WGM spectrum and light polarization state. Single- and multi-line laser spectra with different peak wavelengths were demonstrated for the systems based on two MBRs with $Q$ factors circa ${{10}^4}$, different geometries, and diameters of 200 and 150 µm. The maximum optical signal-to-noise ratio of the proposed EDFL system of up to 32 dB is demonstrated. The influences of the pump laser power and light polarization within the fiber ring on the system performance were also investigated. It was found that an increase of the pump laser power may lead to mode competition in the laser cavity, resulting in changes in the number of peaks in the output spectrum, their central wavelengths, and their signal-to-noise ratios. Finally, changes in the light polarization state in the fiber ring can have a significant impact on the output spectrum of the proposed system. A change in the light polarization within the laser cavity leads to changes in the WGM spectrum of the coupled MBR, thus causing various filtering effects. Since the interrelationship between the gain and losses at a specific laser wavelength is determined by the filter function, it is possible to control the number of lasing peaks and/or improve their stability. The proposed laser structure has many potential applications as a low-cost source in optical communications and in fiber sensing.
Funding
Science Foundation Ireland (18/CRT/6222); Irish Research eLibrary.
Acknowledgment
This publication has emanated from research conducted with the financial support of the Science Foundation of Ireland. Open access funding provided by Irish Research eLibrary.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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