We demonstrate an all-fiber single-longitudinal-mode (SLM) narrow-linewidth ring laser stabilized by a microsphere resonator and fiber Bragg gratings (FBGs) with a large continuous wavelength tuning range from 1540 nm to 1570 nm. In the experiment, stable lasing with a linewidth smaller than 5 kHz was obtained. The laser wavelength was linearly and continuously tuned in the range of 0.15 nm by increasing the pump power and discretely tuning with a step of 0.1 nm by stress-controlled FBGs. The tuning range of the proposed laser configuration was determined by the FBG, and the SLM state was ensured by the coupling between the few-mode tapered fiber and the microsphere, which was simpler in alignment than other ring lasers utilizing microresonators.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Single-longitudinal-mode (SLM) narrow-linewidth lasers have been widely used in the coherent communication, metrology and sensing [1–3]. Besides SLM and narrow linewidth, mode-hop-free tuning and large tuning range are also expected in applications. Typically, such a laser commercially available usually utilizes a Fabry-Pérot (F-P) interferometer as its wavelength selector, and the typical linewidth is around hundreds of kilohertz. As the development of whispering-gallery-mode (WGM) microcavities [4–6], they have been used as wavelength selectors and stabilizers in SLM narrow-linewidth lasers due to their high-Q factors, which could be 105 times larger than that of a typical F-P interferometer  and potentially provide a better performance in SLM lasers. In 1998, a prism coupled microsphere was used in a narrow-linewidth diode laser . In 2014, the laser linewidth of su-kHz was obtained by stabilizing a ring laser with a prism coupled CaF2 WGM resonator . Various configurations of all-fiber narrow-linewidth lasers were proposed after tapered fiber (TF) was demonstrated as an effective and simple method to couple light into the WGM microcavity [9–12]. However, the tuning range of the proposed configurations was usually very small based on the thermal effect of the cavity [10,13]. To obtain a large continuous tuning range, a large number of resonance peaks in a WGM microcavity are required, which would cause the serious problem of the strong mode competition and mode hop between different WGM resonance peaks within the laser gain band. Thus, an additional wavelength selector with a suitable bandwidth is necessary to select the WGM resonance peaks to expand the tuning range and stabilize the longitudinal mode. In an all-fiber laser configuration, fiber Bragg grating (FBG) could be a good choice, and the WGM resonance peak selectivity and laser wavelength fine tunability within the passband of the FBG need to be explored systematically. It was noticed that a linear laser cavity with a WGM cavity as one of its reflectors was reported in 2007 . However, such a configuration could not provide mode-hop-free tuning. Due to the high-Q factor of the WGM microcavity, several unexpected nonlinearities such as the stimulated Raman scattering (SRS)  and stimulated Brillouin scattering (SBS)  would occur, the latter of which would lead to mode hop in the linear laser cavity. Thus we introduce an FBG to a ring laser with an in-cavity isolator and try to realize a narrow-linewidth, mode-hop-free SLM laser with a large tuning range.
In this paper, we demonstrated a stable all-fiber SLM ring laser with a linewidth narrower than 5 kHz based on a microsphere resonator and FBGs, which could be continuously tuned from 1540 nm to 1570 nm. The SLM state was ensured by the constructed narrow-band add/drop filter (ADF) based on the coupling between a few-mode TF and a travelling-wave microsphere WGM. The microresonator coupled with only one TF, which made the WGM coupling configuration much easier and stabler than the previous structure, and the laser frequency drift was smaller than 100 MHz within 1 hour. With the help of FBGs to pre-select the discrete WGM resonant wavelengths, such a narrow-linewidth laser could realize continuous tuning through the thermal effect without mode hop. The FBGs used in the experiment can be replaced by a single specially designed FBG with a tuning range up to 50 nm [16, 17] to avoid changing FBGs during the tuning, which will improve the performance in the practical applications.
2. Experiment and discussion
It is well known that the thermal effect could provide the tunability of the resonance wavelength in the WGM microcavity, which could be applied in a tunable narrow-linewidth laser. However, the tuning range due to thermal effect is usually small, and the high light power in the microcavity may cause unexpected nonlinear effect. To introduce a large tuning range, an additional tunable wavelength selector of large tuning range is necessary, and we combine a microsphere resonator with FBGs as the wavelength selector as shown in Fig. 1.
In the proposed configuration, the wavelength selector (WS) of the ring laser was composed of FBGs with a fiber circulator (FC) and an add/drop filter (ADF). The ADF consisted of a fused silica microsphere resonator (MSR) and two tapered fibers (TFs) as shown in the dotted-line box, which has been reported in our previous work . We introduced a MSR to replace the microdisk in the experiment to decrease free spectral range (FSR) of the ADF. Such a structure is more stable than typical ADFs because only one TF coupled to the microcavity. In the ADF, the MSR was of 264 μm diameter and fabricated from a fiber tip with a fiber fusion splicer. The TF coupling to the MSR was of 2.5 μm diameter which was large enough to support the LP11 mode, and its transition-zone length was about 20 mm, which was long enough to satisfy the adiabatic propagation condition for both LP01 and LP11 modes . When the LP01 mode at the resonance wavelength of the microsphere was input in Port 1 (Port A of the WS), the WGM in the MSR could be excited by the LP01 fiber mode. Meanwhile, the excited WGM could be coupled out to the high-order fiber modes in the TF, for example, the LP11 mode . By another TF, the LP11 mode excited by the WGM could be led out to Port 3 . Port 3 of the ADF was used as a narrow bandpass filter to ensure the SLM operation of a ring laser, and Port 2 was used as the laser output coupler. The extracted light at Port 3 was further sent to an FBG with an FC, which was used as the additional resonance wavelength selector of the MSR and could provide an extended tuning range for the ring laser.
With a tunable laser sweeping from 1550 nm to 1560 nm connected to Port 1, the transmission spectra of the ADF at Port 2 and Port 3 were measured by a pair of photodetectors and an oscilloscope. The laser output power was set to be 0.01 mW to avoid the thermal effect during the scanning process . The microsphere was placed on a 3D nano-positioning system to precisely locate the gap and position relative to the TF. To utilize the ADF with a high-Q state and an adequate output ratio of Port 2, the microsphere was tuned to under-coupling with the TF. With the gap between the microsphere and the TF tuned to be about 300 nm, the transmission spectra of the ADF with wavelength between 1550 nm and 1560 nm were obtained, as shown in Fig. 2(a) with part of it shown in Fig. 2(b) as samples (the spectra from 1550 nm to 1551 nm). The blue and red curves denote the transmission spectra of Port 2 and Port 3, respectively. In the range from 1550 nm to 1560 nm, there were 425 resonance peaks in the transmission spectrum of Port 2 (only considering peaks with resonant transmissions smaller than 0.9), while there were 303 resonance peaks in the transmission spectrum of Port 3 (only considering peaks with resonant transmission larger than 0.05), indicating that at least 71.3% of the WGMs excited by the LP01 fiber mode could be efficiently coupled to the LP11 fiber mode, which were applicable for the laser. The average free spectral range (FSR) of Port 3 was 0.033 nm (4.0 GHz) in the experiment, which was much smaller than the theoretical FSR of about 2 nm at the wavelength of 1550 nm for a silica microsphere (n =1.45) with a diameter of 264 μm, as result of the excitation of high-order radial modes and the ellipticity-induced azimuthal mode split for an imperfect experimental microsphere . A typical resonance peak is shown in Fig. 2(c). The typical full width at half maximum (FWHM) of 0.086 pm (10 MHz) and quality factor of 2 × 107 were obtained. Note that the efficiency of the ADF was not tuned to approach 100% in order to maintain the laser output at Port 2. 100 peaks of Port 3 were selected, and the calculated Q-factor quantity distribution is shown in Fig. 2(d). The average quality factor was about 5 × 107 and the corresponding average bandwidth was 4 MHz.
When we connected the FBG and the circulator to Port 3 of the ADF, the transmission spectrum at Port C would be the combination of the spectra from the ADF and the FBG. The total transmission spectrum would highlight only a few resonance peaks of the ADF, for example, the green peak with a Q-factor of 2 × 107 shown in Fig. 3(a). The transmission spectra of Port 3 (ADF) and FBG are shown with red and blue curves, respectively. The total fiber length of the ring laser cavity was about 50 m in the experiment, and the corresponding longitudinal mode interval was about 4 MHz. Figure 3(b) shows the longitudinal mode selection with the ADF resonance peak. The emitted longitudinal mode selected by the ADF resonance peak is shown as the red line, and the inhibited ones are shown as the black lines.
With the wavelength selector in Fig. 1, a typical ring laser was constructed as shown in Fig. 4. An erbium doped fiber amplifier (EDFA) was used as the gain medium, which was pumped with a 980 nm semiconductor laser. Laser unidirectional emission was ensured by a fiber isolator (FI) to avoid the spatial hole burning, which also prevented the nonlinear backscattering from the microsphere. A polarization controller (PC) was used to optimize the laser output. Port 3 and Port 2 of the ADF were used to select laser wavelength and output laser, respectively. The laser output was split by a 50:50 coupler and launched into the optical spectrum analyzer (OSA) and a delayed self-heterodyne interferometer (DSHI), respectively.
The linewidth of the output laser was measured with the DSHI method [24,25]. A frequency shift of 0.9 MHz in the DSHI was obtained by an acousto-optic tunable add/drop coupler which was the same as our previous work [21,26]. The delayed fiber length was 40 km with a delayed time of τd = 200 μs, which indicated a linewidth measurement resolution of about 5 kHz . The beat signal of the DSHI was recorded by a photodetector (PD) and an oscilloscope. Figure 5(a) shows the optical spectrum of laser output at the wavelength of 1550.5 nm measured by the OSA, and the corresponding spectrum of the DSHI beat signal is shown in Fig. 5(b) with a sampling rate of 50 MHz and a total record time of 2 ms. The SLM output state was confirmed with the side mode suppression ratio of about −70 dB. Figure 5(c) shows the detailed spectrum around the shifted frequency with a 3-dB bandwidth of 6.6 kHz. Based on the DSHI spectrum, the laser noise spectrum is shown in Fig. 5(d). The noise background was fitted in a Lorentzian profile with a 3-dB bandwidth of 300 Hz. Note that the low-frequency noise deviated from the Lorentzian shape with an experimental 3-dB bandwidth of 3.3 kHz, which was induced by the environment instability such as the vibration and temperature fluctuation which usually broadened the laser linewidth [28,29]. The measurement results showed that the fabricated laser had a linewidth smaller than 5 kHz at the wavelength of 1550.5 nm.
The laser tunability was also tested. By stretching the FBG around different centre wavelengths, corresponding WGM resonance wavelengths could be selected to stabilize the laser, and the tuning result is shown in Fig. 6(a) and 6(b). In Fig. 6(a), wavelength tuning from 1540 nm to 1570 nm was realized by selecting FBGs. There were ten FBGs applied in the experiment, each with a tuning range of 3 nm. The discrete 0.1 nm tuning step of the output laser wavelength could be achieved by choosing WGM resonant wavelength with each FBG, and the tuning result from 1550 nm to 1551 nm is shown as an example in Fig. 6(b). The step tuning by selecting WGM resonance peaks with FBGs caused laser output power fluctuation of about 4 dB, which was induced by the difference in the resonance transmissions of the WGM-based ADF, as shown in Fig. 2(a). The above tuning range could also be achieved with a single specially designed FBG whose tuning range could be up to 50 nm . Within the discrete tuning step of each FBG, the laser wavelength could be further continuously tuned by increasing the pump power because of the thermo-optic effect of the fused silica microsphere [10,30,31], and the tuning result is shown in Fig. 6(c). The laser power, wavelength and signal-to-noise ratio (SNR) were all increasing as the increasing of the pump power. Figure 6(d) shows the tuning relationship of the output laser wavelength and power with the pump power, which are denoted by the black squares and blue triangles, respectively, and both of them were tuned almost linearly and continuously with the pump power. The laser wavelength could be tuned in a range of 0.15 nm (18 GHz) without mode hop, which was restrained by the bandwidth of the FBG. Meanwhile, the mode competition between the adjacent ADF transmission peaks within the FBG passband could be eliminated by fine tuning the resonant wavelength of the FBG in the experiment. Consequently, the SLM laser output state was maintained at the WGM resonance peak with a higher efficiency compared with the other peaks. With the average step tuning of about 0.1 nm from the FBG to select WGM resonance peaks, and the continuous tuning span larger than 0.1 nm by the WGM thermo-optic effect, the laser output wavelength could be further continuously tuned to cover the whole range from 1540 nm to 1570 nm. To eliminate the laser power fluctuation caused by the ADF efficiency fluctuation and pump power tuning in the laser wavelength tuning process, a variable optical attenuator (VOA) could be considered to connect with the laser output port in the practical applications. In the wavelength tuning range from 1540 nm to 1570 nm, the laser linewidths were measured every 2 nm by DSHI with delaying fiber length of 40 km, as shown in Fig. 7. The measured spectra are presented in Fig. 7(a) and the linewidths accordingly are shown in Fig. 7(b). The measured laser linewidths fluctuated from 2.7 kHz to 3.8 kHz, demonstrating that the laser linewidth could remain relatively stable and narrower than the measurement resolution of 5 kHz when switching wavelengths. Meanwhile, the laser stability was measured by OSA. The laser power and wavelength drift in 1 hour were smaller than 0.2 dB and 100 MHz, respectively, as shown in Fig. 8.
In summary, we demonstrated an all-fiber SLM narrow-linewidth ring laser with a large continuous tuning range based on a microsphere resonator and FBGs. It could avoid the influence from the nonlinear back scattering in the microsphere. The SLM state was ensured by the coupling between the travelling-wave WGM and few-mode TF. The laser wavelength was simultaneously stabilized and selected by an FBG. Lasing with a linewidth smaller than 5 kHz was obtained, and the wavelength could be linearly and continuously tuned in a range of 0.15 nm by increasing the pump power and discretely tuned by stretching the FBG with an average tuning step of 0.1 nm. The continuous tuning could be realized from 1540 nm to 1570 nm in the experiment by changing FBGs. The structure could be improved by using a single specially-designed FBG with a tuning range of 50 nm. Furthermore, because only one TF was employed to couple with the microcavity, it not only simplified the alignment of the configuration, but also made the laser more stable. In the the experiment, the laser wavelength was stabilized within 100 MHz in 1 hour.
National Natural Science Foundation of China (NSFC) (61635004, 61705023, 61705024, 61405020, 11574161); Key Research and Development Project of Ministry of Science and Technology (2016YFC0801200); Chongqing Postdoctoral Program for Innovative Talents (CQBX201703); Fundamental Research Funds for the Central Universities (106112017CD-JXY120003, 106112017CDJXY120004); The National Science Fund for Distinguished Young Scholars (61825501).
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