Recent progress in multi-wavelength fiber lasers: principles, status, and challenges Download: 1312次
1. INTRODUCTION
Nowadays, the fiber laser has attracted much attention due to its enormous superiorities such as admirable beam quality, cheapness, good compatibility, and simple and compact structure[1
Fig. 1. Applications of MWFL: (a) DWDM technology for an optical communication system, and (b) the multi-wavelength Raman fiber laser for long-distance simultaneous measurement of strain and temperature selected from Ref. [12]. (c) Phased array antenna system selected from Ref. [14]. (d) Microwave signal generation based on a multi-wavelength Brillouin fiber laser selected from Ref. [16].
For MWFLs, when the spacing between the neighbor wavelengths is less than the uniform linewidth of the gain fiber, the intense mode competition and mode hopping, existing in a homogeneously broadening gain medium, are inevitable and serious. The homogeneous gain broadening is the key factor for suppressing the multi-wavelength operation whenever pulsed or continuous wave (CW) operation forms. Accordingly, to obtain stable MWFLs, it is necessary to weaken the homogeneous broadening effect of the gain fiber to restrain mode competition and mode hopping[1719" target="_self" style="display: inline;">–
In the room temperature condition, there are a few methods to obtain multi-wavelength operation: using the frequency shift feedback technique[2224" target="_self" style="display: inline;">–
In the MWFLs, there are two main categories: CW[35
What the MWFL discussed above possesses is a single-gain medium in the cavity, which owns a limited gain spectrum. The single-gain medium fiber laser cannot obtain wide range multi-wavelength operation. We also summarize dual-cavity pulsed fiber lasers owning two gain media, naturally generating two colors, and the two beams independently operate. Dual-wavelength pulsed fiber lasers with broad wavelength range separation are applied in many fields such as nonlinear frequency conversion, the pump–probe technique, chemical sensing, and Raman scattering spectra.
Here, we briefly review the current status of the MWFL, especially passively mode-locked multi-wavelength operation fiber lasers, and then analyze and discuss the principle, challenges, and perspectives of MWFLs.
2. MULTI-WAVELENGTH CONTINUOUS WAVE FIBER LASERS
The multi-wavelength CW fiber laser has attracted the attention of many researchers due to its huge potential in application fields such as optical fiber sensors, millimeter-wave generators, and optical communications systems[35,4951" target="_self" style="display: inline;">–
2.2 A. Multi-Wavelength CW Fiber Laser with the Aid of a Frequency Shifter
By introducing a frequency shifter into the cavity, the optical signal circulates and passes through the frequency shifter to produce the frequency shift[23]. Therefore, one wavelength cannot be continuously amplified on account of frequency shift. As a result, the uniform broadening of gain fiber is suppressed to a great extent, and the frequency shifter prevents the emission from a single wavelength and results in stable MWFL operation at room temperature conditions. Zhou et al. reported a multi-wavelength Er-doped fiber laser (EDFL) with the aid of sinusoidal phase modulation feedback (see Fig.
Fig. 2. Multi-wavelength EDFL based on a phase modulator: (a) the schematic of the experimental setup; the output spectrum characteristics (b) without modulation feedback and (c) with modulation feedback. Selected from Ref. [24].
2.3 B. Multi-Wavelength CW Fiber Laser with the Aid of Filter Structures
Active modulator techniques need electrical devices driven by external power, which increases insertion loss and breaks all-fiber structures. Therefore, the multi-wavelength CW fiber laser based on passive modulation structures has been intensively exploited by researchers around the world. In the multi-wavelength CW fiber laser, an ultra-narrow linewidth mode selecting filter is indispensable[52]. Filter devices pick up designated wavelengths from the net gain spectrum of the fiber laser and effectively suppress homogeneous gain broadening to produce multiple wavelengths in the fiber laser. There are several passive modulation structures utilized to achieve multi-wavelength operation such as the comb filter, Mach–Zehnder interferometer (MZI)[2628" target="_self" style="display: inline;">–
MZI generally is composed of two optical couplers (OCs). The first OC divides light equally into two parts, which interfere with each other in the second OC. Two beams experience different paths with a tunable phase shift to generate the comb filter effect. Luo et al. demonstrated multi-wavelength CW operation in an EDFL with the help of the dual-pass MZI filter[27]. The tunable transmission spectra of the MZI comb filter, resulting in multi-wavelength operation, were accurately measured. The spacing and the position of transmission spectral peaks were changed by only adjusting the polarization controller (PC), as shown in Fig.
Fig. 3. Multi-wavelength operation based on the MZI filter effect: (a) the experimental schematic of an EDFL; (b) the comb filter transmission spectra; (c) the spectral characteristics of 14-wavelengths operation; (d) the spectral characteristics of 29-wavelengths operation. Selected from Ref. [27].
The SMS structures also generate the comb filter effect based on the multi-mode interference (MMI) effect based on the theory of generating multi-wavelength operation as the MZI, which means the MWFL can be produced with the aid of the SMS interferometer.
Zhang et al. demonstrated stable tri-wavelengths in a Tm-doped fiber laser (TDFL) by utilizing the SMS interferometer[57]. The MMI effect is generated in the MMF, which excites all the modes. However, similarly, they only realized multi-wavelength CW operation for lack of a mode locker. Anum et al. reported a compact tunable and switchable single/dual-wavelength EDFL based on the SMS structure, and then carefully adjusted the PC position of the paddle (see Fig.
Fig. 4. MWFL based on the SMS interferometer: (a) the experimental schematic diagram of dual-wavelength EDFL; (b) the output spectral tunable dual-wavelength fiber laser. Selected from Ref. [30].
A fiber grating (FG) has been intensively applied in MWFL and fiber sensors due to their wavelength-pick nature, which has the unique advantage of fiber compatibility. There are various FGs, such as a CFBG[53,64–
He et al. reported a dual-wavelength narrow linewidth single-longitudinal mode (SLM) operation linear-cavity EDFL by applying an FBG-based F-P filter and a narrow-band FBG. Finally, three different cases of tunable dual-wavelength fiber lasers, emitting at 1569.38 and 1569.60 nm, 1568.84 and 1569.38 nm, and 1569.61 and 1569.81 nm, were obtained. Wang et al. demonstrated a dual-wavelength TDFL with the aid of three FBGs, as shown in Figs.
Fig. 5. Multi-wavelength fiber laser and the output characteristics: (a) the schematic diagram of dual-wavelength EDFL; (b) optical spectral evolution with different pump power; (c) the stability measurement of optical spectra. Selected from Ref. [66]. (d) The schematic diagram of multi-wavelength TDFL; (e) the stable tri-wavelength operation. Selected from Ref. [72].
These filter devices are made from fiber and thus assures the all-fiber laser structure, which is good for stable multi-wavelength operation. More interestingly, the spacing and the position of the transmission spectral peak can be altered by adjusting an appropriate position of the PC, and a tunable MWFL was intensively explored.
In addition, the MWFL also can be obtained based on two types of filter combinations. For instance, Zhao et al. demonstrated a switchable MWFL with the aid of a combined filter that is assembled with a phase-shifted fiber Bragg grating (PSFBG) and an MZI[72]. The four stable output cases, single-, dual-, triple-, and quadruple-wavelength emissions, can be realized, as shown in Figs.
2.4 C. Multi-Wavelength CW Fiber Laser Based on Intensity-Dependent Loss Structures
There is another way to achieve a multi-wavelength by applying an intensity-dependent loss structure, including nonlinear polarization rotation (NPR)[73,74] and a nonlinear amplification loop mirror (NALM)[7577" target="_self" style="display: inline;">–
The NPR structure includes a polarization-dependent isolator (PD-ISO) and two PCs. The experimental setup, as shown in Fig.
Fig. 6. MWFL based on two types of intensity-dependent loss structures: (a) schematic of the NPR mode-locked TDFL; (b) working principle of the NPR structure. Selected from Ref. [78]. Two cases of output spectrum of MWFL based on NPR structures: (c) 22-wavelength operation; (d) 28-wavelength operation. Selected from Ref. [73]. (e) The experimental setup of the NALM structure. Output spectrum characteristics of EDFL based on the NALM structure at two different states by adjusting the PCs. Selected from Ref. [75]. (f) 41 wavelengths; (g) 50 wavelengths. Selected from Ref. [76].
Yan et al. presented the transmission equation for the NPR structure and the birefringent fiber, as shown in Fig.
In the schematic diagram,
The transmission equation is a sinusoidal fashion. The transmittance (also meaning loss) of the setup changes with phase delay introduced by the PC and the light intensity. As is well known, the NPR structure can achieve mode-locked operation due to the light transmittance increasing as the light intensity increases. However, if we make the setup transmittance at the state where transmittance decreases with light intensity, the setup can act as a kind of light intensity balancer. The balanced effect can effectively restrain the uniform broadening effect of doped fiber to inhabit mode competition and hopping, and the MWFL operation is finally obtained.
Feng et al. demonstrated stable CW multi-wavelength operation in an EDFL with the aid of the NPR structure, as displayed in Figs.
The NALM cavity contains a Sagnac interferometer and a ring cavity combined with a coupler, which can also be named a figure-of-eight cavity. The working principle of multi-wavelength operation based on NALM is the same as NPR. The Sagnac interferometer also forms the comb filter effect[79], and the transmission equation of the Sagnac loop can be expressed as
Feng et al. demonstrated a multi-wavelength CW EDFL with the aid of an NALM structure behaving as an amplitude equalizer[76]. The number of wavelength operation of lasers launched was up to 50, and the wavelength spacing was 0.8 nm. More interestingly, the spectrum amplitude jitter was slight [see Figs.
NPR and NALM structures introduce intensity-dependent cavity loss and form the comb filter effect in a fiber laser cavity in order to obtain multi-wavelength operation. More interestingly, the two structures also can balance the amplitude of every wavelength, and a uniform amplitude MWFL is obtained, which is favorable for DWDM communication systems.
2.5 D. Multi-Wavelength CW Operation Based on the Nonlinear Effect
With a high-power laser transmitting in fiber, it is easy to produce a remarkable nonlinear effect such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), and stimulated Brillouin scattering (SBS). The FWM effect, a third-order nonlinear response process in medium, can prominently contribute to realizing multi-wavelength operation[8082" target="_self" style="display: inline;">–
The FWM effect makes energy of each wavelength transfer among them, and power is again redistributed through a fast FWM process. The fast FWM effect is achieved when low-power wavelength laser intensity increases and high-power wavelength laser intensity reduces, which can effectively restrict longitudinal mode competition and hopping introduced by uniform broadening of the gain fiber. Accordingly, stable MWFLs can be produced through the FWM effect[18,33,82
Liu et al. experimentally demonstrated and explained in detail that FWM can achieve multi-wavelength operation[33]. Harun et al. reported multi-wavelength EDFL by inserting a piece of bismuth-based Er-doped fiber (Bi-EDF) in the resonator[83]. A fiber laser, having up to 17 wavelengths with a fixed wavelength spacing of 0.41 nm, was realized, as shown in the Figs.
Fig. 7. Multi-wavelength operation in the ring EDFL: (a) the experimental setup of backward pumping; (b) the experimental setup of forward pumping; (c) the output spectrum of forward and backward pumping. Selected from Ref. [83]. The multi-wavelength Brillouin–Raman fiber laser: (d) the experimental setup; (e) and (f) illustrations of multi-wavelength lasing spectra at different DCF lengths. The magnified views are shown in graphs on the right. Selected from Ref. [34].
In addition, the SBS effect continuously generates new-order Stokes frequency when the power exceeds the Brillouin threshold and can stimulate Stokes lines[34,8890" target="_self" style="display: inline;">–
Stimulating the FWM or SBS effect generally needs a high-power laser and high nonlinearity medium. Therefore, the MWFL can be generated by adding a high nonlinearity fiber (HNLF)[18,82,86], photonic crystal fiber (PCF)[84], and dispersion compensation fiber (DCF)[85] to induce high nonlinearity effect.
In conclusion, the different methods have different advantages. For example, the number of multi-wavelengths achieved by the NPR or NALM structure is relatively large, and the amplitude of each wavelength is relatively uniform. The multi-wavelength operation achieved by FBGs that possess a designed reflection bandwidth can precisely control which wavelength emits, and the narrow filter linewidth is suitable to achieve SLM operation. The multi-wavelength operation based on the SMS structure or FWM effect can achieve a simple and compact experimental setup.
3. MULTI-WAVELENGTH PULSED FIBER LASER
Over the past decades, the pulsed fiber laser has been widely studied due to large pulse energy, ultrashort pulse duration, excellent beam quality, low cost, simple structure, and good compatibility[5,91
On the other hand, there are also two main categories of pulsed fiber lasers based on different pulse generation technologies: active modulation techniques[106
In general, there are mainly three ways to achieve a multi-wavelength pulsed fiber laser, including active modulation techniques, adding wavelength or intensity-dependent loss structure to the cavity, and applying the high nonlinearity effect of two-dimensional (2D) materials. We mainly introduce multi-wavelength mode-locked fiber lasers (MWMLFLs) based on passive modulation technology.
3.4 A. Multi-Wavelength Pulsed Fiber Laser Based on NPR or NALM Structure
The pulsed MWFL, especially passively MWMLFL, has been comprehensively demonstrated due to its wide range of practical applications from civilian to military. Nowadays, passively mode-locked devices are roughly divided two types: real and artificial SAs. Artificial SA structures including NPR[78,119
Tang et al. established a theoretical model of NPR mode locking based on the complex nonlinear Schrodinger equation (NLSE) and revealed the evolution process of a pulse[120]. Stable MWMLFLs have been intensively reported, where output wavelengths range from 1 to 2 μm. Xu et al. numerically and experimentally demonstrated an all-normal-dispersion dissipative soliton (DS) mode-locked Yb-doped fiber laser (YDFL)[31] with the tri-wavelength operation. A stable tri-wavelength operation with fixed 16.4 nm spacing between adjacent peaks was obtained. Song et al. demonstrated steady dual-wavelength and tri-wavelength mode-locked operation in an EDFL[122]. The interval between the two adjacent peaks was switchable: 12.67 nm (
Fig. 8. Spectrum characteristic of the dual-wavelength TDFL: (a) the three-states switchable dual-wavelength conventional soliton; (b) the numerical simulation transmission spectrum of the NPR; (c) the comparison between simulative and experimental results. Selected from Ref. [78].
In the NALM structure fiber laser, the Sagnac interferometer also forms a comb filter, which contributes to multi-wavelength operation. However, compared with the MZI and SMS interferometer, the NALM setup can alone achieve multi-wavelength operation without another mode-locking structure.
Plenty of MWMLFLs were also demonstrated based on the NALM structure. He et al. demonstrated a tunable multi-wavelength TDFL-based Sagnac loop filter[131]. They placed a PMF in the Sagnac loop to shorten the interval of adjacent peaks. Furthermore, they deduced the spacing between adjacent channels as
Jin et al. applied the NALM structure in the TDFL to achieve stable tunable four-wavelength mode-locking operation[126], where the separation between adjacent channels maintains 6 nm whether there is mode locking or CW operation, as shown in Fig.
Fig. 9. Schematic and laser characteristics of the NALM fiber laser: (a) the schematic diagram of a mode-locked Tm/Ho-doped fiber laser; (b)–(e) tunable multi-wavelength spectrum (left), corresponding pulse trace (middle), and single pulse (right); (f) and (g) show CW operation characteristics. Selected from Ref. [126].
In addition, researchers combined filter devices and active modulation technology to achieve multi-wavelength mode-locking operation. Jain et al. demonstrated a five-wavelength mode-locked-operation-based active Mach–Zehnder intensity modulator under 10 GHz driving signal frequency[29]. The stable ultrashort pulse was obtained, where the pulse width is 14 ps, and the repetition rate is 10 GHz. The MWFL with an ultrahigh repetition rate is suited to high-speed and large-capacity optical fiber communication systems.
3.5 B. Multi-Wavelength Pulsed Fiber Laser Based on 2D Materials
A real SA[43,45,132,133], possessing excellent nonlinear absorption characteristics depending on light intensity, has been intensively studied to realize a
Fig. 10. 2D materials. (a) The 2D family members. Selected from Ref. [135]. (b) The current dominant SAs for ultrashort-pulse generation. Selected from Ref. [136]. (c) The sketch map of the saturable absorption process in the BP. Selected from Ref. [154].
Nowadays, 2D materials have been extensively applied in the field of photonic devices, especially as mode lockers to generated ultrashort pulses in fiber lasers due to their broadband absorption, fast carrier dynamics, ease of fabrication, ease of integration into the cavity, and highly nonlinear optical saturable property. The SA is the key device for mode-locked fiber lasers, and the current dominant SAs, as shown in Fig.
Martinez et al. summarized the methods of integrating graphene or carbon nanotubes (CNTs) with fiber, as shown in Fig.
Fig. 11. Diverse methods of integration of CNT-/graphene-SAs into the resonant cavity: (a) sandwiched film between two fiber connectors; (b) in-fiber microfluidic channels; (c) PCFs filled by the SA; (d) D-shaped fiber; (e) tapered fiber; (f) fully integrated monolithic fiber laser. Selected from Ref. [177].
There are a few methods of transferring 2D materials on the surface of a fiber such as optical deposition[178180" target="_self" style="display: inline;">–
In past decades, SESAMs have been comprehensively used in fiber lasers for mode-locked[182184" target="_self" style="display: inline;">–
On the other hand, 2D materials process high third-order nonlinear susceptibility, which generates a remarkable nonlinear effect. As expounded upon above, the FWM effect obviously mitigates mode competition and stabilizes the multi-wavelength operation. Hence, 2D materials have double functions: as a mode locker resulting from natural saturable absorption properties and a multi-wavelength generator resulting from a high nonlinear refractive index (
Graphene, a type of one-atom-thick layered graphite, processes a zero bandgap structure, full waveband absorption, and short recovery time characteristics, so it can work as a SA to achieve mode-locked operation[6,178,179,188
Luo et al. experimentally confirmed that graphene can generate FWM and obtained stable five-wavelength
Fig. 12. Characteristics of dual-wavelength YDFL-based graphene SA (GSA): (a) microscopy image of tapered fiber-based GSA; (b) the schematic diagram of dual-wavelength YDFL; (c) the spectrum of dual-wavelength CW operation; (d) the spectrum of mode-locked operation; (e) the oscilloscope trace, inset: single-pulse envelope; (f) the RF spectrum. Selected from Ref. [196].
These carbon-based materials, as the SA applied in an ultrafast fiber laser, are easily damaged under high-power laser exposing, which limits practical applications. Hence, looking for other materials to replace graphene to achieve mode-locked working is a hot issue. TIs, a typical direct bandgap 2D material, have a narrow bandgap (
Guo et al. applied home-made few-layer
Fig. 13. TI-SA and characteristics of MWMLFL: (a) the solution of ; (b) Raman spectrum of , inset: scanning electron microscope (SEM) image; (c) optical deposition process, inset: photo of the end of the fiber; (d) the saturable absorption characteristic of TI-SA; (e) the output spectrum under 116.2 mW pump power; (f) long-time output wavelength stability measurement of the tri-wavelength mode-locking operation over 9 h. Selected from Ref. [212].
TMDs, a novel 2D family member, also possess ultrafast optical nonlinear properties[131,152,153,213
Guo et al. applied a high-power pulsed laser beam to deposit
Fig. 14. Output properties of dual-wavelength EDFL: (a) the spectrum of the dual-wavelength EDFL; (b) the pulse traces; (c) long-term output spectrum stability measurement. Selected from Ref. [224].
Recently, BP, a thermo-dynamically stable allotrope of phosphorus, walks into researchers’ field of vision due to excellent properties[136,155,162,225
Lu et al. applied the wide-band Z-scan measurement technique to demonstrate that BP has broadband and enhanced saturable absorption characteristics, as shown in Fig.
Fig. 15. Characteristics of BP nanoparticles (NPs): (a) the atomic force microscope (AFM) image; (b) height profiles of the sections marked in (a); (c) Raman spectrum; (d) the linear absorption spectrum; (e) the Z-scan measurements of BP-PMMA film; (f) the relation of normalized transmittance and intensity. Selected from Ref. [154].
The same as other 2D materials discussed above, BP also possesses strong saturable absorption and high nonlinear refractive index characteristics. Zhao et al. demonstrated stable tri-wavelength mode-locking operation in the EDFL-based few-layer BP-SA[233]. They removed the BP-SA and observed the output spectrum characteristics [see Figs.
Fig. 16. Output characteristics of tri-wavelength mode-locking based on the BP-SA: (a) the schematic of the EDFL; (b) the characteristics of the pulse trace (up) and spectrum (down); the emission spectrum of the EDF (c) without and (d) with BP-SA. Selected from Ref. [233].
In addition, some researchers combined the real SA and filter devices to realize MWMLFL. Liu et al. demonstrated a tri-wavelength mode-locked fiber laser with the aid of a single-walled CNT (SWCNT) SA and three CFBGs working together[48]. The output three wavelengths operate at about 1540, 1550, and 1560 nm, corresponding to three central wavelengths of CFBGs (see Fig.
Fig. 17. Schematic diagram and laser output characteristics of the fiber laser: (a) the schematic of the tri-wavelength mode-locked fiber laser; (b) the measured reflection spectra of three CFBGs; (c) the normalized absorption characteristic of the SWCNT-SA; (d) linear absorption characteristic of the SWCNT-SA; (e)–(g) the output spectrum and corresponding autocorrelation intensity trace of , , and , respectively. Selected from Ref. [48].
In the 2D materials multi-wavelength mode-locked fiber laser (see Table
3.6 C. Other Optical Phenomena with Multi-Wavelength Pulsed Operation
Table 1. Summary of the Multi-Wavelength Pulsed Lasers Based on a Real SA
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There are a few different optical phenomena accompanying multi-wavelength pulsed operation, such as wavelength tuning[29,122,212], wavelength switching[69,121,123], bright–dark soliton pair[235], and different kinds of soliton pulses[31,132,182]. For example, the tunable or switchable multi-wavelength operation can be obtained in three different operation modes, CW,
Table 2. Summary of the MWFL Based on the Filter Effect in the Cavity
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Table 3. Summary of the Multi-Wavelength Mode-Locked Lasers Based on NPR or NALM
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The spacing between adjacent peaks of transmission
In theory, the spacing between adjacent peaks and the position of the transmission peak change with birefringence of fiber and polarization states. Therefore, tunable or switchable MWMLFL can probably be generated based on the NPR structure.
Yan et al. reported a switchable tri-wavelength mode-locked TDFL with the aid of the NPR technique, as shown in Fig.
Fig. 18. Switchable multi-wavelength mode-locked TDFL: (a) the experimental setup; the spectrum of the switchable tri-wavelength of (b) pair-by-pair and (c) one-by-one. Selected from Ref. [124]. (d) The schematic of the YDFL based on a graphene-oxide (GO)-SA, and spectral characteristics of tunable multi-wavelength DS; (e) the tunable single-wavelength spectra; (f) the wavelength-tunable dual-wavelength DSs; (g) the spectrum of spacing-tunable dual-wavelength DSs; (h) the switchable spectrum dynamics of tri-wavelength DSs by adjusting the orientation of the PC. Selected from Ref. [132].
The tunable or switchable fiber lasers as stated have huge potential applications in optical signal processing, fiber sensors, and WDM communication systems. When it comes to tunable or switchable fiber lasers, no matter whether it is CW or mode-locked operation, the polarization state is inevitably mentioned. When parameters of cavity (e.g.,
In the mode-locked fiber laser, there are generally four kinds of solitons: conventional soliton[236
Huang et al. reported stable multi-wavelength dissipative soliton YDFLs based on a graphene-oxide SA[132]. The fiber laser achieved abundant experimental phenomena of tunable dual-wavelengths, spacing-tunable dual-wavelength, and wavelength switchable DSs, as shown in Figs.
What is discussed above is all bright pulses, but there is another pulse type: the dark pulse, which can also be generated in the fiber laser. The dark pulse, a localized intensity dipping on the CW background, has significant potential application in optical communication due to lower loss. Zhang et al. experimentally achieved stable dark soliton operation from an all-normal-dispersion fiber laser[252]. They also numerically analyzed dark solitons based on the NLSE. Since then, dark solitons are part of numerous experiments successively springing up based on the NPR structure[253] and real SA[72,235,254,255]. Ning et al. demonstrated the bright–dark pulse pair based on the NALM setup[256]. They experimentally found that the pulse pair is formed because of the intensive cross-coupling effect between the bright and dark pulses and is located at two different wavebands (see Fig.
Except for the bright–dark pair achieved by NALM, the bright and dark pair fiber laser also may be obtained with real SA. Zhao et al. experimentally demonstrated a dual-wavelength bright and dark pulse pair based on
Fig. 19. Laser characteristics of a bright–dark soliton pair based on NALM structures: (a) oscilloscope pulse traces and (b) the corresponding optical spectrum. Selected from Ref. [257]. The laser characteristics of the bright–dark pulse based on the : (c) the pulse trace of a bright pulse (up) and dark pulse (down) and (d) corresponding optical spectrum, respectively. Selected from Ref. [72].
In conclusion, abundant experimental phenomena have been obtained in fiber lasers, which indicates that fiber lasers are powerful, versatile, and ideal platforms for studying peculiar nonlinear evolution processes. The tunable and switchable MWFL operating in CW or pulsed forms can achieve different wavelength emission based on different requirements, which greatly extend the application ranges.
4. DUAL-CAVITY DUAL-WAVELENGTH PULSED FIBER LASER
The MWFL discussed above is based on single-gain medium in the resonant cavity, and the multi-wavelength operation principle suppresses mode competition and hopping of the gain fiber. There is other way to achieve two-color pulse operation, which is the dual-cavity structure fiber laser. Every cavity has an independent gain medium and pump source. The wavelength intervals between different gain media are large. There is no gain competition between the two wavelengths, so dual-wavelength operation may be obtained in the dual-cavity fiber laser. Compared with the MWFL based on a single-gain fiber, the dual-cavity structure fiber laser can achieve two-color operation in a wider range. Moreover, the two wavelengths have their own independent tuning range in the gain bandwidth.
Dual-wavelength fiber lasers, especially two-wavelength pulsed operation at the same repetitive frequency, have extensive applications such as nonlinear frequency conversion, pump–probe technique, chemical sensing, and Raman scattering spectra, owing to their compact structure, stable operation, excellent heat dissipation, and excellent beam quality[197,257,258]. In recent years, the passively synchronous dual-wavelength pulsed fiber laser has attracted much attention. There are mainly three methods to synchronize two beams with different wavelengths based on the cross-absorption modulation effect of materials, the XPM effect of fiber, and the gain-switched effect.
4.3 A. Passively Synchronized Mode-Locked Dual-Wavelength Fiber Laser Based on Cross-Absorption Modulation of Materials
As we know, graphene has excellent nonlinear saturable absorption characteristics, which acts as a mode locker applied in the fiber laser. When two-beam lasers with different wavelengths simultaneously illuminate graphene, the transmittance of one beam light is not only affected by its own light intensity, but is also affected by the light intensity of another beam, which is called the cross-absorption modulation effect, as shown in Fig.
Dual-wavelength passively synchronized
Fig. 21. Passively synchronized two-color fiber laser with the aid of SWCNTs: (a) the experimental setup of the fiber laser; (b) linear transmission of SWCNTs; (c) the intensity autocorrelations of the Er laser; (d) the intensity autocorrelations of the Yb laser; (e) the corresponding spectrum of the Er laser; (f) the corresponding spectrum of the Yb laser. Selected from Ref. [257].
In the dual-cavity passively synchronized fiber laser, the SA devices shall have a wide wavelength saturable absorption bandwidth covering gain fiber emission wavelength and be placed on the public area of the dual cavity.
4.4 B. Passively-Synchronized Dual-Wavelength Fiber Laser Based on the XPM Effect
When two or more beams of light with different frequencies transmit in the fiber at the same time, they will interact through the nonlinear effect in the fiber, which is called XPM. From the perspective of physics, the effective refractive index of light waves in a medium is not only related to its own intensity, but also related to the intensity of other waves transmitted at the same time, which is the reason for the generation of XPM. Due to the XPM effect in fiber, a nonlinear phase shift related to the intensity can be obtained, as shown in the formula
On the right-hand side of the equation, the first item stems from the self-phase modulation effect, and the second item originates from XPM due to different frequency lights transmitting in the fiber.
Rusu et al. proposed dual-wavelength synchronized mode-locked fiber lasers based on the XPM effect in a common linear cavity[258]. When the master pulse (1.55 μm) and slave pulse (1 μm) overlap temporally in the public area of the linear cavity, the spectrum of the slave pulse shifted because of intensive phase modulation afforded by the master pulse. The slave pulse’s group velocity alters correspondingly, so the oscilloscope train of the slave pulse actively matches the master pulse. A two-color synchronized mode-locked fiber laser was obtained by the XPM effect, as shown in Fig.
Fig. 22. Passively synchronized two-color fiber laser based on the XPM effect: (a) the schematic diagram of the fiber laser; (b), (c) intensity autocorrelation trace (inset: corresponding spectrum) of the Er laser and Yb laser. Selected from Ref. [258].
In the dual-cavity two-color fiber laser, the XPM effect and cross-absorption modulation effect, all originating from third-order nonlinearity, may work together for passively synchronized mode-locking operation. Sotor et al. reported a two-color mode-locked fiber laser, including two ring cavities (Er- and Tm-doped) based on a common graphene SA placed in the public area of the dual cavity, as shown in Fig.
Fig. 23. Dual-wavelength dual-loop cavity passively synchronized mode-locked fiber laser: (a) the schematic diagram of the experimental setup; the relation between repetition rates of Er- and Tm-doped cavities and Er-cavity length offset (b) with a common GSA in the public area and (c) with two independent in the different loops; (d) the central wavelengths versus the offset of Er-cavity length based on a common GSA; (e) the RF spectrum. Selected from Ref. [197].
4.5 C. Dual-Wavelength Pulsed Fiber Laser Based on Gain-Switched Technology
What is discussed above are two gain fibers, where the two-color fiber laser can also be realized in a single-gain fiber processing two rare earth ions such as an Er/Yb co-doped fiber (EYDF) (see Table
Table 4. Summary of the Dual-Cavity Two-Color Mode-Locked Lasers
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Fig. 24. Synchronized dual-cavity two-color -switched EYDF laser: (a) the schematic of the experimental setup; (b) the energy level diagram of the EYDF; -switched traces under different pumps of (c) 1 μm and (d) 1.5 μm; optical spectra of (e) 1 μm and (f) 1.5 μm; the corresponding RF spectra of (g) 1 μm and (h) 1.5 μm. Selected from Ref. [260].
EYDF exists in the non-radiative energy transferring process between the energy levels of
Fig. 25. Dual-wavelength -doped fluoride fiber laser: (a) the experimental setup; (b) the energy level of the cascade transition process; (c) the illustration of laser upper-level populations of and , respectively, and the temporal domain evolution of pulse intensity; the characteristics of optical and corresponding RF spectra (inserted) at the different pump powers of (d), (e) at 3.76 W and (f), (g) at 6.47 W, respectively. Selected from Ref. [261].
In the passive dual-cavity synchronized mode-locked fiber laser, it is very necessary to ensure exactly equivalent lengths to meet the same repetition rate. If not, two-color
5. CONCLUSION AND PERSPECTIVE
Among the kinds of fiber lasers, the multi-wavelength mode-locked fiber laser has been intensively investigated due to plentiful practical applications from civilian to military. There are a few methods to realize MWMLFLs. Researchers insert a fiber interferometer to form the comb filter effect. However, they cannot obtain multi-wavelength mode-locking operation due to the lack of other mode-locked devices. NPR and NALM structures figure out the question due to simultaneously processing mode-locker and comb filter dual effect. NPR or NALM structures induce wavelength or intensity-dependent loss, which is helpful for multi-wavelength operation. However, the spectral bandwidth of the mode-locked multi-wavelength laser is relatively small, and the pulse duration is relatively large. Stable multi-wavelength mode-locking operation can be achieved based on the 2D materials SA. 2D materials possess peculiar characteristics of broadband saturable absorption and high nonlinearity effect, which are indispensable for multi-wavelength mode-locking operation. Until now, the MWMLFL have been demonstrated with the aid of the multiple 2D materials SA, for example, graphene, TI, TMDs, and BP.
In the future, we predict that the MWMLFL will develop in five directions as follows.
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Article Outline
Hualong Chen, Xiantao Jiang, Shixiang Xu, Han Zhang. Recent progress in multi-wavelength fiber lasers: principles, status, and challenges[J]. Chinese Optics Letters, 2020, 18(4): 041405.