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Abstract
A compound resonant cavity-type single longitudinal mode erbium-doped fiber laser is proposed and verified in this paper. We use a compound four-cavity to expand the longitudinal mode spacing of the laser and a homemade grating F-P filter and a saturable absorber to narrow the gain spectral bandwidth, enabling the laser to operate in a single-frequency state. Through theoretical analysis and experimental verification of the important roles of the main constructed fiber laser parts, the center wavelength of the laser output of 1550.06 nm is obtained, and the optical signal-to-noise ratio is 65 dB. In a 50 minute period, the wavelength fluctuation is exhibited along with the maximum power fluctuation of 0.28 dB. The value of the laser output linewidth is ∼250 Hz, measured by using the delayed self-heterodyne method.
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1. Introduction
In recent years, researchers have performed much research on single longitudinal mode, narrow linewidth fiber lasers, which exhibit long coherence length and low phase noise, giving them great potential in fields such as microwave photonics, coherent LIDAR, and free-space optical communications [1,2]. Linear and ring cavities are the two types of fiber laser cavities that can achieve single longitudinal mode output [3,4]. Linear cavities include the distributed Bragg reflection (DBR) [5–7] and distributed feedback laser (DFB) [8,9]. Linear cavities have simple structures, single longitudinal mode outputs and stable power, but their output laser linewidths are wide and their output optical power is low, generally requiring the output power to be amplified. The ring cavity single longitudinal mode fiber laser [10] can incorporate a variety of complex optics in its long cavity structure to suppress the space burning hole effect [11] and flexibly achieve the single longitudinal mode output. However, due to its longer cavity length, the output of the single longitudinal mode is not stable [12]. Suppressing multimode competition and mode hopping is still the main direction of current research.
In general, fiber lasers use a single transverse mode fiber in the cavity, so only the longitudinal mode must be selected. A single longitudinal mode can be achieved by reducing the gain bandwidth or increasing the free spectral range (FSR). These two approaches are often contradictory for ultrashort-cavity structures because increasing the free spectral range usually requires using a shorter resonant cavity and minimizing the optic losses. However, reducing the gain bandwidth usually requires adding filter elements, which increases the resonant cavity length and losses. To achieve a single longitudinal mode in a ring cavity, a combination of both approaches is usually chosen, usually by incorporating saturable absorbers, filters with very narrow bandwidths, ultrashort linear cavities, and compound cavities into the system [13–17]. The compound cavity is added to the main cavity of the ring cavity to effectively increase the longitudinal mode spacing. Combining the structures of narrowband filter pieces and compound cavities is an effective scheme for achieving single longitudinal mode output. A variety of compound cavity structures have been previously reported, but the experimental principles have not been described in detail, and most of these structures were proposed simply to use the Vernier principle [18,19]. In 2020, W. X. Cui et al. analyzed and demonstrated that the passive resonant cavity nested in three rings exhibits good frequency selective characteristics using the Jones matrix, and a stable single longitudinal mode was obtained when the pump power was 159 mW [20]. In 2021, L. Zhang et al. introduced three cascaded subrings combined with a fiber Bragg grating to obtain a stable single-frequency output with a power of 1.11 W and a linewidth of 1.9 kHz [21]. In the same year, W. G. Han et al. proposed a single longitudinal-mode thulium-doped fiber laser by inserting a compound double-ring cavity into the 2 µm band, achieving a stable single longitudinal-mode output with a linewidth of 14.194 kHz at 1941.56 nm [22]. In the following year, they designed a hybrid compound cavity consisting of three homogeneous gratings and two couplers; this cavity had a high optical signal-to-noise ratio of 75.65 dB at 2049.160 nm [23]. In 2022, P. G. Tang et al. reported a laser structure that combines F-P etalon and grating to stably output a single longitudinal mode, with an output linewidth of less than 110kHz and a maximum output power of about 40 mW [24].
In this paper, we propose using a dynamic grating composed of a saturable absorber and an FBG combined with a homemade grating F-P filter for preliminary screening of the laser output longitudinal mode and inserting a compound four resonant cavity composed of three 2 × 2 couplers to expand the free spectral range of the main cavity. Theoretical calculations and experiments verify the roles played by each main part of the constructed fiber laser, and the output characteristics of the laser are tested to verify that the system can achieve stable single longitudinal mode laser output.
2. Experimental setup and rationale
2.1 Optical path structure of lasers
The structure of the proposed compound four-cavity single longitudinal mode erbium-doped fiber laser is depicted in Fig. 1. The 980 nm pump laser (Pump LD) is incidental to the 7.5 m long erbium-doped fiber (EDF, absorption: 7.2-8.4@/1531 nm, NA: 0.22-0.24; I-6(980/125), Fibercore) after the 980/1550 nm wavelength-division multiplexer (WDM), and the optical isolator (ISO) ensures unidirectional transmission of the light in the main resonance cavity. Then, in the homemade grating F-P filter, the grating F-P filter is incidental to the circulator (Circulator), and the circulator and the isolator play the same role. To ensure unidirectional operation of the laser in the cavity, the laser passes through a section of 5 m unpumped EDF (saturated absorber, absorption: 5.0-6.7@/1531 nm, NA: 0.22-0.24; I-4(980/125), Fibercore) and the dynamic grating composed of fiber Bragg grating (FBG) to further inhibit the multimode oscillations caused by the environmental perturbations and mode. The reflected laser enters the compound four-cavity by three 2 × 2 couplers (Coupler) for further filtering and longitudinal mode selection. Finally, 20% of the laser is output from the 20:80 coupler, and 80% of the laser is returned to the main cavity to continue the cycle. A single longitudinal mode laser output is ultimately formed after many cycles.
The proposed compound four-sub resonant cavity consists of three 2 × 2 couplers with a 50:50 coupling ratio. The signal light is divided into two beams after passing through the first coupler and enters into the second coupler through the single-mode fibers with lengths of LA = 128.3 cm and LB = 48 cm; it is then output and passes through the single-mode fibers with lengths of LC = 102.5 cm and LD = 68 cm. After the third coupler, 50% of the output laser is returned to the first coupler via a single-mode fiber of length LE = 110 cm, and 50% of the laser reaches the 20:80 coupler, where 20% of that laser is output, and 80% is returned to the main cavity. The total cavity length of the constructed laser is approximately 29 m, corresponding to a longitudinal mode spacing of ∼7 MHz.
2.2 Characterization of grating F-P filters
The grating F-P filter used is made in house. Its structure is shown in the blue dashed box in Fig. 1, with a central transmission peak wavelength of 1550 nm, a reflectivity (R) of 85%, and a reflection bandwidth of 0.1 nm, due to the grating F-P cavity with a free spectral range as follows:
The grating F-P cavity fineness is defined as follows:
Therefore, the 3 dB bandwidth of the transmission peak of the grating F-P filter is as follows:
where c = 3 × 108 m/s is the speed of light, n = 1.468 is the refractive index of the fiber core, and d = 5 cm is the effective cavity length of the grating F-P filter. The free spectral range of the grating F-P filter is calculated to be ∼2 GHz, the fineness F is 19, and ΔF is ∼100 MHz.Analytical calculations indicate that the grating F-P filter can initially narrow the gain bandwidth and reduce the number of longitudinal modes, but its passband is not yet narrow enough to oscillate in only one longitudinal mode; therefore, it must be combined with saturated absorbers to suppress the excess longitudinal modes and achieve single longitudinal mode operation.
2.3 Saturable absorber characterization
By using a section of 5 m unpumped erbium-doped fiber as a saturable absorber and the FBG to form a dynamic grating, the reflected light and the incident light interference form a standing wave when the signal light through the unpumped EDF is reflected back by the FBG. The reason for this is that the unpumped EDF has less optical loss at the center frequency of the formed dynamic grating and more optical loss at other frequencies. At this time, the saturable absorber formed by the dynamic grating is equivalent to a narrow-band filter, and these selective absorption characteristics can inhibit not only other frequencies of the longitudinal mode but also mode hopping to achieve single mode operation of the fiber laser.
The 3 dB bandwidth of the saturated absorber is:
When the deviation between the optical frequency of the side mode and the main mode is overly large, the standing wave field formed by light of different wavelengths is superimposed. As a result, the refractive index in the fiber no longer undergoes a periodic change and thus loses the narrowband filtering effect, so it the longitudinal mode of the laser must be initially screened through the grating F-P filter so that the number of longitudinal modes of the laser output is restricted to the effective range of the ultranarrow bandwidth filter. The gain bandwidth is effectively narrowed by the combined effect of the grating F-P filter and the saturable absorber.
2.4 Compound four-cavity characterization
The grating F-P filter together with the dynamic grating is not sufficient for outputting a stable single longitudinal mode, so the compound four-cavity is introduced into the main cavity to expand the longitudinal mode spacing. The compound four-cavity structure is shown in Fig. 2. This structure consists of three 50:50 couplers forming four subcavities of varying lengths. 50% of the light remains in the compound ring to form a narrowband interference. Using compound cavities produces a Vernier effect; that is, adding one or more empty ring cavities formed by the compound cavity as a comb filter in the main cavity of the fiber laser causes the FSR sizes of these cavities to vary due to the peaks of the transmission curve of the various cavities interleaved with each other. The total transmission curve of the neighboring peaks of the interval are ultimately enlarged as a result. There are two ways to screen the longitudinal mode using the compound cavity structure: one is to add multiple subcavities with similar values of FSR so that the compound cavity FSR increases and mode competition is suppressed, with the disadvantage that jumping the mode is easy; the other is to insert short subcavities to improve the FSR of the compound cavity, but the cavity length is overly short, leading to fusion bonding difficulties. Therefore, reasonably designing the cavity length difference can effectively increase the longitudinal mode spacing of the fiber laser.
The cavity lengths of the four subresonant cavities in this design are LACE = 3.57 m, LADE = 2.77 m, LBCE = 3.23 m and LBDE = 2.42 m, and the corresponding free spectral ranges are calculated from the free spectral range (R) formula, R = c/nL, where c = 3 × 108 m/s is the speed of light, n = 1.468 is the refractive index of the fiber core, and L is the cavity length. RACE = 57 MHz, RADE = 74 MHz, RBCE = 63 MHz and RBDE = 84 MHz. While the longitudinal mode spacing of the main cavity is ∼7 MHz, according to the Vernier effect calculation, the main cavity frequency can be ignored. Finally, the total FSR of the compound cavity is the least common multiple of the FSR corresponding to the four cavities, i.e., ∼177 GHz.
2.5 Single longitudinal mode laser realization principle
The principle of the single longitudinal mode laser is to press the gain bandwidth to a very narrow point through the grating F-P filter and the saturable absorber, and the longitudinal mode interval can be expanded to a maximum by inserting the composite four-cavity, so as to realize the mode selection and long-term stable operation of the single longitudinal mode laser. The specific realization process is shown in Fig. 3. Figure 3(a) depicts the reflected bandwidth of the FBG, about 37.5 GHz, and the longitudinal mode spacing in the annular cavity in Fig. 3(b) is ∼7 MHz. Figure 3(c) depicts the transmittance peak schematic diagram of the grating F-P filter, with an FSR of ∼2 GHz and a transmittance peak bandwidth of ∼100 MHz. Multiple longitudinal modes are still clearly present in the gain bandwidth at this time. Figure 3(d) shows that the dynamic grating gain bandwidth of the dynamic grating in Fig. 3(d) is 4.26 MHz, which is comparable to the main cavity FSR of 7 MHz. This can achieve the transient single longitudinal mode output, but it is not sufficiently stable. The FSR of the proposed compound four-cavity is calculated to be 177 GHz according to the Vernier effect, as shown in Fig. 3(e). This is because the laser forms oscillations only when the frequency simultaneously satisfies the passband conditions of the main cavity, subloop resonant cavities, grating F-P filter and saturable absorber [26]. The other longitudinal modes are effectively suppressed through multiple cycles, and a stable single longitudinal mode laser output is ultimately achieved, as shown in Fig. 3(f).
3. Experimental results and discussion
3.1 Tests on the roles of the main components
The proposed single longitudinal-mode fiber laser is built on an ordinary bench. The entire system does not take any temperature-controlled vibration isolation measures and is experimented on and evaluated at room temperature. As shown in Fig. 4, the construction of a system for measuring beat frequency and linewidth.
The importance of the saturable absorber, the grating F-P filter and the compound four-cavity are verified. The measured RF beat frequency spectrum is shown in Fig. 5. When the saturable absorber, grating F-P filter and compound four-cavity act together, as shown in Fig. 5(a), in addition to the zero-frequency DC component of the spectrum analyzer itself, only one 100 MHz beat frequency is present. It is continuously stable and does not exhibit mode hopping, which indicates that the intense mode competition in the proposed fiber laser has been effectively suppressed, i.e., at this time, the single-longitudinal-mode operation has reached a stable condition. When the saturable absorber is removed, the beat frequency signal near the 100 MHz signal has reached a stable condition, the longitudinal mode outside 100 MHz near the beat frequency signal is almost completely suppressed, but the side mode still exists within 100 MHz, and a small amount of instantaneous mode hopping is exhibited, as shown in Fig. 5(b), which indicates that the transmission peak of the grating F-P filter reaches ∼100 MHz but is not sufficient for achieving stable single longitudinal-mode lasing. When the grating F-P filter is removed, the side mode is almost completely suppressed, and the gain reaches stable operation without mode hopping, as shown in Fig. 5(c); furthermore, the gain bandwidth is approximately a few megahertz, which is consistent with the theoretical calculation, but mode hopping is exhibited at a certain instant. The saturable absorber can effectively narrow the gain bandwidth to suppress the side mode, but due to the narrow gain bandwidth, it is prone to mode hopping. When the compound four-cavity is removed, as shown in Fig. 5(d), the ability of the system to suppress the side mode is unstable, and the longitudinal mode spacing is not sufficiently wide. Furthermore, more than one longitudinal mode occurs, and the longitudinal mode spacing is 7.4 MHz, which aligns with the longitudinal mode spacing of the main cavity, so the laser cannot achieve stable single longitudinal mode output, indicating that the compound four-cavity can effectively expand the free spectral range.
Fig. 5. RF beat spectrum (a) beat result when saturated absorber, grating F-P filter, and compound four-cavity are acting together; (b) display of instantaneous mode-hopping beat captured by removing the saturated absorber; (c) display of instantaneous mode-hopping beat captured by removing the grating F-P filter; (d) display of instantaneous mode-hopping beat captured by removing the compound four-cavity.
Therefore, the combination of a saturable absorber, grating F-P filter, and compound four-cavity is crucial for stabilizing the output of single longitudinal-mode fiber lasers. The combination of the designed narrow-band filter combined and the compound four-cavity is excellent for model selection, enabling the excess longitudinal modes in the main cavity to be suppressed in large quantities and achieving the stabilized single-longitudinal-mode output.
3.2 Laser output characterization test
The output single longitudinal mode laser is measured using a spectral analyzer model AnritsuMS9740B with a resolution of 0.03 nm. The laser center wavelength is 1550.06 nm with a 3 dB bandwidth of 0.032 nm when the pump power is stabilized. The bandwidth measurement is limited by the spectrometer resolution due to its close proximity to the resolution of the spectral analyzer and the output spectral. As shown in Fig. 6, 10 measurements are stored within 50 min. The output signal-to-noise ratio reaches as high as 65 dB. To confirm the stability of the laser output, a repeat scan of the output spectrum is measured with the spectral analyzer at 5 min intervals within 50 min. As shown in Fig. 7, the laser wavelength does not change within 50 min, and the amount of change in the optical power is less than 0.28 dBm, which demonstrates the stability of the output laser. In addition, the small optical power instability that exists is primarily caused by fluctuations in the ambient temperature, power fluctuations in the pump laser and unavoidable environmental vibrations.
To further analyze the stability of the laser output single longitudinal mode, the beat frequency of the laser is measured using the autodifferential method to analyze whether the output is a stable single longitudinal mode. The measurement spectrum is recorded within 50 min, as shown in Fig. 8. In addition to the zero frequency, only one beat frequency signal is present, and no mode-hopping is exhibited throughout the entire measurement process, indicating that the laser operates stably in single longitudinal mode.
Linewidth is an important parameter for evaluating the performance of single-frequency lasers. In 1980, T. Okoshi proposed a measurement method, the delayed self-heterodyne method, achieving a measurement accuracy on the order of kHz [27]. The delayed self-heterodyne method is used for the measurement in this paper. It enters the Electrical spectrum analyzer (ESA, RIGOLDSA815) after the 40 km delayed fiber, as shown in Fig. 9. The center frequency of the RF hopping signal is 100 MHz, the scanning width is 200 kHz, and the resolution is 100 Hz. Theoretically, the linewidth should be measured by using the longer extension line. Due to the limited laboratory conditions, we fit the measured data to the Lorentz curve to obtain the linewidth of the laser, thus reducing the measurement error. Currently, using a 20 dB bandwidth to calculate the linewidth of a fiber laser is more accurate than using a 3 dB bandwidth. The 20 dB bandwidth obtained from the Lorentz-fitted curve is 5 kHz, so the actual laser linewidth of the proposed laser is ∼250 Hz.
4. Conclusion
In this paper, a single longitudinal-mode erbium-doped fiber laser combining a compound four-cavity type, a grating F-P filter, and a saturable absorber in the 1550-nm band is proposed, and the principle of its single longitudinal-mode realization is analyzed in detail. The grating F-P filter and saturable absorber compress the gain bandwidth to a very narrow bandwidth, theoretically up to 4.26 MHz, and the compound four-cavity expands the longitudinal mode spacing up to 177 GHz, contributing to the mode selection of the laser output and enhancing the long-term stability of the single-frequency output. The experiments confirm the important roles of each major portion of the device. The excellent model selection of the system ensures the stable laser output of the single longitudinal mode and achieves a center wavelength of 1550.06 nm. The optical signal-to-noise ratio is as high as 65 dB. Within a 50 min measurement time, The output wavelength fluctuates less than 0.03 nm, the fluctuation of the optical power is less than 0.28 dB, and no mode-hopping is exhibited in the testing process. The laser output linewidth of approximately 250 Hz is measured by the delayed self-heterodyne method.
Funding
Central Guiding Local Science and Technology Development Fund Projects (2023ZYZX4004); Guangxi Normal University Major Scientific and Technological Achievements Transformation and Cultivation Project (2020PY002).
Disclosures
No conflict of interest exits in the submission of this paper.
Data availability
Data underlying the results presented in this paper can be obtained from the authors upon reasonable request.
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