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Abstract
In optical communications systems, the used filter and/or demultiplexer needs to have a broad free spectral range (FSR) in order to accommodate more channels and have acceptable interchannel crosstalk. The Vernier effect applied to fiber filters is a recent effective tool to enlarge the FSR. Here, by harnessing the Vernier effect of a hybrid interferometer consisting of a Mach–Zehnder interferometer (MZI) and Sagnac interferometer (SI), we proposed and experimentally demonstrated a new kind of comb filter for a switchable and interval adjustable multi-wavelength C-band erbium-doped fiber laser (EDFL) application. In the designed comb filter, the MZI is composed of bi-tapered polarization-maintaining fibers (PMFs) fabricated by fusion splicing and has the function of achieving the switchability of the proposed dual-wavelength EDFL. The SI configured by nesting tapered PMF is employed as a switchable and wavelength-spacing tuning component of triple-wavelength EDFL. In this experiment, the FSR of the MZI and the SI is designed to be close but not equal, which could be achieved by properly adjusting the length of the employed PMF, so the Vernier effect can exist and a comb spectrum with an obvious envelope is obtained. Through the adjustment of the polarization controller (PC1) and (PC2) inside the cavity, a switchable and interval-adjustable multi-wavelength EDFL was achieved. To the best of the authors’ knowledge, this is the first time that an all-fiber hybrid filter based on the Vernier effect has been used to manipulate the spectral output characteristic of an EDFL and achieve a switchable multi-wavelength fiber laser.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Since the pioneering application of the optical Vernier effect to narrow the laser linewidth of fiber laser and the innovative work on comb filters by Nobel Laureate Prof. Theodor Hänsch in 2005, the Vernier effect has been applied intensively in the field of optical fiber sensors [1]. The extension of the application to fiber laser and optical communications systems is still very limited. Comb filters are indispensable components in many optical systems, particularly in fiber laser systems. All-fiber comb filters, such as Mach–Zehnder interferometers (MZIs) [2], Fabry–Perot interferometers (FPIs) [3–5], and Sagnac interferometers (SIs) [6–8] are commonly utilized as wavelength selectors in lasers assisting a multi-wavelength operation. Recently, because of their outstanding inherent merits of as flexible wavelength tunability, narrow linewidth, high signal-to-noise (SNR), and ratio long service life [9,10] multi-wavelength erbium-doped fiber lasers (EDFLs) have drawn extensive research interest. They have great potential in wavelength division multiplexing (WDM) optical communication systems, optical instrument testing, and optical fiber sensors [11–14]. Moreover, multi-wavelength EDFL may provide switchable or adjustable spacing continuous wave laser illumination with several lasing lines, in contrast to other types of lasers including narrow linewidth lasers [15,16], mode-locked lasers [17], and Q-switched lasers [18]. Therefore, multi-wavelength EDFLs are more valuable in practical uses than fixed-wavelength fiber lasers due to their varied output wavelengths [19]. Nevertheless, an unstable multi-wavelength lasing output may be produced because of the strong-mode competition which is a consequence of the homogenous gain broadening in the erbium-doped fiber (EDF) at room temperature [20]. Therefore, to suppress the mode competition induced by the EDF, a compound filter implanted in the laser cavity is highly required [21]. One of the most crucial key components for achieving a wavelength-switchable multi-wavelength EDFL output and influencing its flexibility and stability is the comb filter [22]. In this sense, several comb-filter structures have been utilized in some works such as cascaded fiber Bragg gratings (FBGs) [23], Sagnac loop [24], and all-fiber MZI structure [25]. However, when only one filtering method is used, the interferometer effect is weak. Therefore, it is advantageous to design an efficient filtering method in order to achieve a stable fiber laser [26]. Therefore, external non-fiber components or unique structures are required to achieve switchable multi-wavelength lasing. As a result, the development of technologies for creating lasers with high SNR, low power fluctuation, and flexible tunability is of great importance. W. He et al. implemented a multi-wavelength EDFL based on a hybrid interferometers-based comb filter which is composed of a Sagnac loop and MZI [27]. Y. Chang et al. demonstrated a switchable multi-wavelength fiber laser based on a hybrid structure fiber filter, which is made up of two ellipsoidal structures and a core-offset splicing point. However, this filter has a larger insertion loss and is not suitable for laser robustness. Another frequent way for achieving flexible fiber laser output is to employ polarization-maintaining fibers (PMFs) in a fiber laser cavity. W. He et al. used a double polarization-maintaining Sagnac loop comb filter in the laser cavity and achieved switchable output [28]. However, this filter has an optical signal-to-noise ratio (OSNR) of less than 20 dB. Recently, two cascaded or parallel interferometers using the Vernier effect based on PANDA-type PMF have been reported for various applications [29]. On the other hand, recently Vernier effect has been proposed in many types for measuring various parameters. Generally, in photonics, devices based on the Vernier effect are made up of two interferometers that serve as the fixed and sliding parts, respectively of the Vernier caliper. The fixed component corresponds to the fixed main scale of the Vernier caliper, and the sliding section corresponds to the sensing part. Two interferometers with approximate but uneven interferometric periods are required to achieve the Vernier effect in a fiber sensor. That is, their free spectral ranges must be slightly detuned (FSRs). The wavelength interval between two adjacent interference dips is defined as the FSR. It can be modified by varying the optical path difference (OPD), which can be achieved by changing the refractive index and/or physical length of the fiber interferometer. The envelope of the spectrum of the photonic device based on the Vernier effect is the product of the individual interferometers [30].
In this paper, we report a switchable and interval-adjustable multi-wavelength erbium-doped fiber laser based on the Vernier effect of a hybrid comb filter. The hybrid filter consists of a Sagnac loop and an MZI. In this experiment, a switchable and interval-adjustable dual-wavelength EDFL was achieved by adjusting PC1 incorporates inside the ring cavity. While tunable single-, dual-, and triple-wavelength lasers can be simultaneously achieved at room temperature by adjusting PC2. The proposed system offers the advantages of flexibility in dual-, and triple-wavelength EDFL generation in addition to switchability and wavelength spacing adjustment application.
2. Fabrication of the all-fiber hybrid filter
The schematic diagram of the proposed filter based on the hybrid configuration is shown in Fig. 1. The hybrid configuration consists of two interferometers, the first one is an in-line MZI structure and the other is a Sagnac loop interferometer. The MZI is used to realize the switchability of the dual laser state as a narrowband comb filter and the Sagnac loop interferometer is utilized to achieve the switchable triple wavelength EDFL as a wideband filter.
The MZI was constructed using a panda-type polarization-maintaining optical fiber (PMF, Thorlabs: PM1550-XP-1440-1625 nm), which is fabricated by splicing a segment of PMF between SMFs (Corning SMF-28) and using a fusion splicer (Fujikura 60s) as illustrated in Fig. 2. The fabrication process of the structure can be described as follows: a segment of PMF was spliced between two segments of SMFs, and a tapered technique was applied at the spliced joint between the lead-in SMF and the lead-out SMF.
At first, the tapered structure of the two fiber regions is formed by heating and fusion splicing the fiber tips of one end of PMF with one end of lead-in SMF, by using the manual mode at STD + 30-bit Arc1 power, STD + 30-bit Arc2 power 30000 ms Arc time and Re-arc time 1200 ms. The microscopic image of the fabricated tapered fiber tips (Taper 1 and Taper 2) with 10x magnification of the two spliced joints is shown in Fig. 3. The two fabricated tapered structures have symmetric diameters with D = 40 µm. Second, the other fiber end of the PMF was heated and spliced with the one end of lead-out standard SMF and formed the second tapered fiber joints. Both ends of PMFs spliced with SMFs formed two tapered fibers at each structure. To fabricate another tapered fiber based on the SI, the same procedures were used. In this work, to produce the Vernier effect, a hybrid configuration based on two interferometers which are the in-line MZI and SI is proposed and fabricated. In this hybrid configuration, both MZI and SI were fabricated based on a cascaded tapered structure with different PMF lengths (L). Then the fabricated in-line MZI was direct-connected to the cavity and the SI loop was connected by a 50:50 fiber coupler with the cavity set-up.
3. Characterization and principle of the hybrid filter
In this work, the filter structure is a cascade of two interferometers: in-line bi-tapered MZI and the other one is SI with a nested bi-tapered MZI as depicted previously in Fig. 1. To find the optimal length of PMF and to generate a discrete spectrum of the Vernier effect, the performances of the single in-line bi-tapered MZI structure with different PMF lengths are investigated. At first, a series of experiments with varying MZI lengths were achieved to validate the appropriate PMF length. Four samples of in-line tapered fiber structures with PMF lengths of 50, 80, 110, and 140 mm have been fabricated to observe the Vernier effect spectra. Figure 4 illustrated the experimental setup used to characterize the transmission spectra of the proposed filters.
A broadband light source (BBS, Thorlabs: SLD1550S-A1) with an operating wavelength from 1400 to 1600 nm was used to measure the light propagation characteristics of the proposed MZI and SI filters. The transmission spectra were detected by using an optical spectrum analyzer (OSA, Yokogawa, AQ6370C). The transmission spectra of the four samples are given in Fig. 5. From this figure, it is obvious that longer PMFs have a lower extinction ratio and a denser FSR due to rising transmission losses in the interferometer as its length grows. A higher extinction ratio and a denser FSR, on the other hand, enhance the generation of the Vernier effect. As a result, there is a trade-off between selecting a low FSR or a high extinction ratio of the spectrum. For suitable FSR and spectrum contrast, the tapered MZI structures lengths (L1 and L2) were chosen to be 80 mm and 110 mm, MZI-1interferometer was used as a sensing arm and MZI-2 was nested then in the SI interferometer as a reference arm. Then the fabricated hybrid filter structure was series-connected between a 50:50 fiber coupler in a hybrid configuration as shown in Fig. 6
The difference between L1 and L2 in terms of their lengths would cause phase differences in the hybrid filter structure. This led to an overlap in the transmission spectrum induced by the phase differences occurring within the series hybrid filter interferometer. Subsequently, an amplified spontaneous emission (ASE) of EDF was used as the input light to evaluate the transmission characteristics of the proposed hybrid filter after incorporating it inside the proposed ring cavity which was detected by using the optical spectrum analyzer (OSA). First, the transmission characteristics of the individual MZI filter structures (MZI-1 with L1 = 80 mm and MZI-2 with L2 = 110) were investigated. And then, the cascaded interferometer structure-based comb filters composed of MZI-1 and SI with a nested tapered MZI (MZI-2) were analyzed. Figure 7 illustrates the comb-like transmission spectrum of the single in-line MZIs and the hybrid filter structure. The reference FSR of MZI-1 and the sensing FSR of the MZI-2 are measured to be 0.6 nm and 0.5 nm respectively as illustrated in Fig. 7 (a and b). Because of the small difference between the FSRMZI-1 and FSRMZI-2, the final transmission spectrum of the series hybrid structure is the output spectrum of the two series in-line hybrid structures superimposed, where the modulation generated by the MZI is added to the transmission spectrum of the SI (after nested bi-tapered MZI-2), leading to the formation of an envelope as seen in Fig. 7(c).
Fig. 7. Spectrum of (a) sensing interferometer MZI-1, (b) reference interferometer MZI-2, and (c) series hybrid structure MZI-1 and SI nested with MZI-2.
The superposition spectrum is formed in the shape of a comb-like spectrum and a large envelope with large FSR equals 12.5 nm in the upper envelope obtained. In the fabrication process of the hybrid structure interferometer based on cascading two tapered fiber structures, the output spectrum of the cascaded SI and MZI is determined by the transmission spectra of the two interferometers. For SI, its transmission function can be expressed as:
where φ1 = 2πΔnL/λ is the phase shift between the two polarization modes, Δn and L are the birefringence coefficient and length of the PMF, respectively, and λ is the wavelength of the incident light. When φ1 = 2πΔnL/λm = 2πm (m = 0,1,2,3…, λm represents the wavelength corresponding to the m-order trough), the transmission spectrum function will reach the minimum value, which appears as a trough on the waveform. The interval between two adjacent troughs is defined as the free spectral range (FSR), which can be expressed as:For MZI, its transmission function can also be expressed as:
where φ2 = 2πnΔL/λ is the phase difference produced by the interference of the upper and lower arms of the MZI by the incident light, n is the refractive index of the fiber (when the two arms are the same type of fiber), and ΔL is the length difference between the two arms. When φ2 = 2πnΔL/λm = 2πm (m = 0,1,2,3…), The waveform will show a dip as the transmission spectrum function reaches its maximum value, and its FSR can be written as:When two interferometers are cascaded, the FSR of the envelope produced can be expressed as:
4. Application
Switchable multi-wavelength fiber lasers are considered a way to maximize transmission bandwidth and prevent channel collisions in the optical communication system. Therefore, to provide switchable multi-wavelength lasing, the proposed comb filter was integrated into the EDFL ring cavity. The configuration of the EDFL ring cavity is shown in Fig. 8. A 90 cm length EDF (Liekki ER80-8/125) with an absorption of 31 dB/m, doping concentration of 4700 ppm, and numerical aperture (NA) of 0.13 was utilized as a gain medium. A 976 nm laser diode (LD) with a 300-mW maximum output power (BL976-SAG300) was employed to pump the gain medium through a 980/1550 nm wavelength-division multiplexer (WDM). A polarization-independent optical isolator (ISO) is used to ensure the unidirectional operation of the proposed laser cavity. A 90:10 output coupler (OC) was used to output the optical signal. An OSA (YOKOGAWA AQ6370C) was employed to record and analyze the output spectrum. Two PCs are utilized to adjust the polarization condition of the circulating light in the proposed cavity, PC1 is used to control the difference in the MZI structure, and PC2 is used to tune the SI structure.
In this experiment, firstly, without incorporating the proposed filter, the LD power was set at 65 mW, which was the threshold pump power of the EDFL. As shown in Fig. 9, a single-lasing line EDFL with peak power located at 1560.8 nm and a maximum SNR of 52.2 dB was obtained. Multi-wavelength lasing operation is a balance between intensity-dependent loss (IDL) and mode competition of the doped fiber, and the incorporation of the proposed hybrid comb into the ring cavity setup filter will change the IDL. Also, because the emission laser lines were directly dependent on the spectrum transmission peaks generated by the filter, therefore, changes to the lasing properties necessitated a shift in this spectral pattern. This shifting in the proposed cavity was achieved firstly via adjusting the incorporated in-line PC1 which is cascaded with the MZI. The variation of PC1 causes a rotation to achieve in the polarization states and a variation of the birefringence within the ring cavity, which might be suitable to balance the gain and loss of the lasing wavelengths. Whereas, in-line PC1 is made out of a rotating fiber squeezer and two fiber-holding clamps. The fiber squeezer sandwiched the central piece of the fiber strand. By manually compressing the fiber, it induces stress-induced birefringence inside the fiber. This functions as a movable, rotational wave plate. The wave plate's angle and retardance may be modified continuously and independently, allowing any arbitrary input polarization state to be transformed into any needed output polarization state. Because of the minimal inherent loss and back reflections produced by the all-fiber architecture, this PC is an excellent alternative to typical free-space PCs, which consist of two quarter-wave plates and one half-wave plate [31]. Therefore, in this experiment, the proper adjustments of the PC1 with increasing the output power of the pump source to 110 mW permits for operation of a dual-wavelength EDFL located at 1532.8 nm and 1533 nm with specific wavelength spacing of 0.2 dB and optical signal to noise ratio (OSNR) of 30.4 dB as illustrated in Fig. 10 (state a). In Fig. 10, the try-and-error approach was considered to rotate and squeeze the PC1 in order to adjust the polarization state and observe the output lasing wavelengths while doing so.
Three types of dual-wavelength EDFL outputs with different intervals were obtained by carefully rotating the PC1 as illustrated in Fig. 10 (State b to d). Consequently, by appropriately varying the PC1 of the MZI filter to state b, the output laser spectrum with dual-wavelength lasers located at 1531.8 and 1533.4 nm was obtained with OSNR of 30.9 dB illustrated in Fig. 10 (state b). Similarly rotating PC1 to state c, the dual-wavelength lasing lines with peaks powers located at wavelengths of 1531.7 and 1532.7 nm were archived with OSNR of 31.6 dB. Finally, by controlling the in-line PC1 state (state d), the proposed ring cavity realized a dual wavelength in which the first peak power appeared at 1530.1 nm and the second peak located at 1531.4 nm wavelength with OSNR of 31 dB, and the lasing interval is 1.3 nm. Then, by fixing the pump power at 110 mW and the position of PC1 at stated, triple-, dual-, single-, and dual-wavelength lasing outputs can be switched to each other arbitrarily when the PC2 of the SI filter was precisely rotated. Each of the three plates ($\mathrm{\theta }$1, $\mathrm{\theta }$2, and $\theta $3,) of the 3-paddle PC2 was rotated in a range of 0◦ to 180◦ with an increment step of 10◦, and setting the other two plates fixed at 0◦ to the normal of the table in the clockwise direction. This way permits the plate to act as a tuning element. By rotating the plate, multi-wavelength emissions with adjustable wavelength spacing were produced”. The angle’s orientations of the PC are depicted in Fig. 11.The basic operating concept might be based on the polarization hole burning (PHB) effect, the spectral hole burning (SHB) effect, and the balance of gain and loss between wavelengths [32]. The polarization states and birefringence within the laser ring cavity are modified when the PC2 rotates. This causes wavelength-dependent gain and loss in EDF and introduces the PHB effect. Furthermore, due to the saturation effect in EDF, the SHB effect is also produced to mitigate the severe mode competition in the laser cavity. SHB effect can also be influenced by the rotation of PC2 [33]. As a result, multi-wavelength operation and switching between distinct lasing regimes may be obtained by carefully tuning the PC2 to balance the gain and loss at certain wavelengths. By rotating PC2 to state 1, triple wavelengths lasing was obtained with peak power located at 1532.6 nm, 1551.7 nm, and 1560.7 nm, and the two wavelength intervals are 18.9 nm and 9 nm respectively. Carefully adjusting the PC2 to state 2, the lasing line ‘‘2’’ disappeared and two wavelengths of lasing were obtained at 1532.6 nm and 1560.7 nm. By continually rotating the PC2 to state3, only the lasing line marked as ‘‘2’’ show up, and the leasing operation switches from dual-wavelength to single-wavelength operation. Lastly, rotating PC2 to state 4, the laser oscillates up to dual- wavelengths where the two lasing lines marked as ‘‘2’’ and ‘‘3’’ show up. From Fig. 11, the channel spacing of the multi-wavelength lasing is not uniform. This might be attributed to the spectral SHB effect affected by the PC2, which in turn affects the laser gain spectrum [34].
5. Conclusion
In conclusion, the optical Vernier effect is an effective tool to enlarge the FSR values of a comb filter by several folds without compromising signal identification and monitoring. In this work, this is realized by superimposing the spectrum of the MZI and the SI. The obtained envelope of the superimposed spectrum has an enlarged FSR of 12.5 nm. The proof of the Vernier principle was generated with a hybrid comb filter to demonstrate a new technique to select between single- or switchable and interval-adjustable multi-wavelength operations in EDFL. The manipulation of the EDFL operation states can be achieved simply by detuning the PC1 and PC2. Here the comb filter was formed from cascaded two-tapered fiber MZI with tapered fiber SI. The two-tapered fiber MZI structure consisting of SMF-PMF-SMF has the function of realizing the switchability of dual-wavelength EDFL. The SI with a segment of a tapered spliced PMF is used as a switchable and wavelength-interval tuning component of the achieved triple-wavelength EDFL. Adjusting the PC1 connected with the proposed MZI configuration permits for modification of the wavelength intervals between dual-wavelength outputs. Where, dual-wavelength outputs with various wavelength intervals of 0.2, 1.6,1, and 1.3 nm have been obtained. By adjusting the PC2, the laser can operate in single-, dual- and triple-wavelength laser states. The proposed hybrid filter represents a simple and flexible means for switchable and wavelength spacing tuning applications of dual-and triple-wavelength EDFL.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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