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Abstract
High power 1.5 µm band fiber lasers are of great importance for many practical applications. Generally, the technical targets including high average output power, narrow linewidth, temporally suppressed intensity dynamics, high spectral purity, single transverse mode lasing, and excellent robustness are the major concerns when constructing a high-performance laser source. Here, we demonstrate the highest output power of a wavelength tunable 1.5 µm band random fiber laser based on the active fiber gain mechanism to the best of our knowledge. A master oscillator power-amplifier (MOPA) configuration is employed to greatly boost the output power to 20 watt-level with a single transverse mode lasing and the same linewidth as the seed, benefiting from the spectral broadening free feature when employing the random fiber laser as the seed. This work not only enriches the progress of random fiber laser, but also provides an attractive alternative in realizing high performance lasing light source at 1.5 µm band.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Random fiber lasers (RFLs) have been investigated extensively in the past decades due to the unique structure and the attractive lasing performance in contrast to conventional lasers. Typically, RFLs in solid core silica fiber can fall into two categories in terms of the optical feedback mechanism: the coherent feedback RFL [1–4] and the incoherent feedback one [5–7]. The former type is generally realized by the inscription of randomly spaced fiber Bragg gratings (FBGs) which shows considerably low lasing threshold in a relatively short fiber length but usually exhibits strong random resonant mode competitions [1,2]. For the incoherent feedback RFLs, random distributed Rayleigh scattering is directly employed to provide the optical feedback, while the nonlinear gain mechanisms, such as the stimulated Raman scattering (SRS) [5] and active gain in rear-earth doped fibers [8], provide the optical amplification. The open cavity structure (full open or half open), i.e., without resonant oscillations, is the major feature of an incoherent feedback RFL. The scope of research on the incoherent feedback RFLs has been significantly broadened in the past decade, ranging from high efficiency/power output [9–11], agile lasing wavelength [12,13], narrow linewidth [14,15] or supercontinuum generation [16–18], to few mode [19] or multi transverse mode lasing [20,21]. Various applications have been also innovated benefiting from the intriguing advantages of incoherent feedback RFL, such as low noise [22] or high spectral purity light source [23–25], speckle-free [20] or temporal ghost imaging [26], and low noise distributed amplification [27].
Among all these research interests, high power operation of RFLs is attracting considerable attention due to their simple and robust structure that alleviates the requirements for high power fiber components [9] and the suppressed temporal dynamics that lead to a low relative intensity noise [22]. Although the 1.5 µm band RFL is the first to be concerned when mentioning the concept of RFL and has been investigated widely ever since, the high power RFL is mostly realized at the ∼1.1 µm band which is initiated by the ytterbium doped fiber (YDF) active gain. The output power record has been continuously refreshed from hundred-watt level to kilowatt level for the SRS gain mechanism [28–30], while several kilowatts RFLs have been achieved through the master oscillator power-amplifier (MOPA) strategy [31–33]. In contrast, the output power of the 1.5 µm band RFL is greatly restricted due to the following limitations. Firstly, the output power of commercial Raman pumps (e.g., 1455 nm, 1480 nm) is generally less than 20 W with an extremely high cost. Secondly, to circumvent the spectral broadening effect near the zero dispersion wavelength (ZDW, ∼1.3 µm for SMF-28e) region which further blocks the successive Raman shifting beyond the ZDW, specialty Raman fibers with longer ZDW [23,24], combined ZDWs [34] or a larger Stokes shift [35] is routinely required if the initiated gain is employed from YDF. This would also increase the cost and structural complexity. Besides, the spectral purity of the 1.5 µm band random lasing is also limited for the high order Raman Stokes approach due to the presence of unavoidable residual lower-order Stokes emissions. To achieve a higher spectral purity, a high power amplified spontaneous emission (ASE) source is generally required [23] at the expense of lowering the compactness of the pumping scheme.
In terms of the gain mechanism, high power RFL is generally realized in two major approaches, the SRS gain one and the active fiber gain one. For the SRS gain approach, to boost the power scaling range as broad as possible, the lasing threshold of the next order Raman Stokes should be increased which is usually obtained by reducing the length of the Raman fiber. Meanwhile, the lasing threshold for the Raman Stokes of interest would be also increased, e.g., tens or hundreds of watts [28–30]. On the other hand, the spectral purity is highly dependent on the intensity noise figure of the Raman pump laser [23]. Therefore, high power ASE light source is usually required to reach a high spectral purity, which obviously increases the complexity of the pumping scheme. Even though an ASE source is employed, the high spectral purity can be obtained only at the highest output power regime which counts against the flexible power regulation across the whole power scaling range. Besides, due to the strong nonlinear effects such as four wave mixing (FWM), self-phase modulation (SPM), the 3 dB bandwidth of the Raman gain based RFL at high power regime would be broadened to several nanometers with the increase of output power.
In comparison, the MOPA configuration based on active fiber can mitigate the above limitations to some extent. The lasing threshold can be greatly reduced in the seed light source part by employing a proper length of the passive fiber, while commercial laser diodes (LDs) can directly act as the effective lasing pump in both stages. High spectral purity RFL can also be obtained across the whole power scaling range due to the pure single-wavelength light amplification in the active fiber. One of the most attractive features for the random fiber lasing seeded MOPA configuration is the spectral broadening free property benefiting from the strongly suppressed temporal dynamics of the random fiber lasing seed light [31]. Therefore, the linewidth of the amplified lasing output keeps the same as that in the seed light source across the whole power scaling range, which is promising for practical applications. Nevertheless, the power boosting of the random fiber lasing seeded MOPA configuration is still mainly realized at the ∼1.1 µm band based on the initiate gain from YDF. Therefore, it is of great interest to maximize the power scaling range of 1.5 µm band random fiber lasing by employing EYDF as the effective gain medium.
Here, we propose a 20 watt-level single transverse mode 1.5 µm band RFL in a MOPA configuration based on erbium/ytterbium doped fiber (EYDF), which is the highest output power from the active fiber gain based 1.5 µm band RFL as far as we know. With the aid of a wavelength selective fiber loop mirror, wavelength tuning range of ∼20 nm is also obtained. Moreover, benefiting from the suppressed temporal dynamics due to the unique open cavity structure, the output spectrum of the amplified random fiber lasing is not broadened and the linewidth remains almost the same across the whole power scaling range, resulting in a narrow linewidth output in the high-power regime. Both the temporal dynamics in short-time scale (e.g., microseconds) and the output power fluctuation in long-time scale (e.g., tens of minutes) show excellent stability. This work provides a reliable configuration in achieving high performance (e.g., single transverse mode, narrow linewidth, high power output, high spectral purity and wavelength tunability) RFL at the 1.5 µm band, which is not only a milestone for the RFL research but also has great potential for practical applications.
2. Experimental results
The schematic diagram of the single mode high power tunable RFL based on the MOPA configuration is shown in Fig. 1. Firstly, the seed light is generated in a conventional active fiber gain based half-open structure which is composed of a fiber loop mirror (1×2 3dB coupler), a piece of single mode EYDF active gain fiber (3 m, Nufern SM-EYDF-10P/125-XP), and a spool of single mode fiber (SMF, 2 km, YOFC G652D). A 976 nm laser diode (LD) is used as the initial pump source, which is coupled into the structure through a (2 + 1)×1 signal/pump combiner. A fiber tunable filter (TF, FWHM bandwidth is 0.11 nm at 1530 nm, wavelength resolution is 0.02 nm, WL Photonics Inc.) is inserted in the fiber loop mirror to provide wavelength selective point feedback, while the SMF provides passive feedback through random distributed Rayleigh scattering. To ensure the power passing through the TL is well below the maximum tolerant power, a 1:99 coupler is inserted between the single mode EYDF and the fiber loop mirror, and the 1% output port is monitored by a power meter (JW3211C, Joinwit). A 1550 nm ISO (SMF-28e) is deployed at the distal end of the SMF to prevent all the feedbacks from the following part and make sure all the backward feedbacks in the seed light part come from the Rayleigh scattering. It is worth noting that by using the EYDF as initiated gain fiber, the standard SMF can be used as the effective medium due to the direct optical conversion from the 976 nm to the 1.5 µm band. This also greatly reduce the requirement of specialty Raman fibers in contrast with that in the YDF initiated gain approach.
Fig. 1. Experimental setup of the high power tunable RFL with narrow linewidth. TL, tunable filter. SM, single mode. EYDF, erbium/ytterbium co-doped fiber. LD, laser diode. SMF, single mode fiber. ISO, isolator. MM, multimode. CPS, cladding power stripper. MFA, mode field adaptor.
The random lasing seed light is then injected into the power amplifier through the signal port of another (2 + 1)×1 signal/pump combiner. To dramatically boost the output power in the power amplifier part, a piece of multimode EYDF (3m, NA of the core is 0.2, Nufern MM-EYDF-12/130-HE) is used here, which is pumped by two additional 976 nm LDs. Another high power ISO (SMF-28e) is connected at the end of the PA part to prevent any parasitic feedback and maintain the single transverse mode output. An integrated cladding power stripper (CPS) and the mode field adaptor (MFA) is inserted between the EYDF and the ISO to strip the unabsorbed pump light and lower the insertion loss. The lasing outputs are monitored by an optical spectrum analyzer (AQ6375B or AQ6370D, Yokogawa), a power meter (919P-250-35, Newport), a photo-detector (DET10N2, Thorlabs, bandwidth 70 MHz), an oscilloscope (MDO3054, Tektronix), and a radio-frequency (RF) spectrum analyzer (N9322C, Keysight). It is worth noting that one of the major obstacles limiting the output power scaling range of the EYDF based lasers or amplifiers resides in the strong thermal loading introduced by the quantum defect. Therefore, the multimode EYDF in the power amplifier part is specially immersed in a water sink to dissipate the generated heat and minimize the thermal effect.
First of all, characteristics of the random lasing seed light are thoroughly investigated. Figure 2(a) gives the wavelength tuning range of the random lasing seed light when the pump power is set to be 3.025 W. It is shown the tuning range of the pure single peak random lasing regime starts from 1532 nm to 1552 nm. When the filter is tuned beyond this range, parasitic lasing at the 1535 nm would appear simultaneously, as shown in the insets of Fig. 2(a). Although the tunable filter allows a much broader wavelength tuning range from 1520 nm to 1580 nm, and the EYDF can indeed provide effective gain from 1530 nm to 1625 nm, the obtained wavelength tuning range of the random lasing is dramatically restricted due to the high insertion loss of the filter and the greatly reduced wavelength selective feedback of the fiber loop mirror. Therefore, the net gain of the selected lasing wavelength beyond the tuning band would be considerably suppressed, which leads to a comparable or an even lower value than that at the maximum cross-section wavelength (i.e., 1535 nm). This could be optimized by using a high reflective wavelength selective mirror such as fiber Bragg grating (FBG) or low insertion loss filter. It is also observed that the spectral intensity peak shows a quasi-periodic variation in Fig. 2(a). Since the lasing is collected into the OSA in free space, to exclude the measuring error, the output power and the 3 dB bandwidth variations versus the tuning wavelength are also considered, as shown in Figs. 2(b) and 2(c). Here, the output power is kept at ∼1 W while the pump power keeps the same. It is shown that the output power is only slightly changed due to the wavelength dependent gain profile of the EYDF. The power variation is not as prominent as that in the spectrum in Fig. 2(a). Besides, the 3 dB bandwidth of each obtained lasing wavelength calculated from Fig. 2(a) concentrates near 0.2 nm and the variation also does not correspond to the quasi-periodic in Fig. 2(a). Both the direct output power and the 3 dB bandwidth variation verify that the lasing operation at different tuning wavelengths is relatively stable.
Fig. 2. (a) The wavelength tuning range of the random lasing seed light. Insets, the spectra when the TF is tuned at 1531 nm and 1553 nm that show notable parasitic lasing at ∼1535 nm. (b) Output power variation versus the tuning wavelength. (c) 3 dB bandwidth variation versus the tuning wavelength.
Four representative lasing wavelengths, i.e., 1535 nm, 1540 nm, 1545 nm and 1550 nm, within the wavelength tuning range are considered for the investigations of the output power and spectral characteristics. The output power versus the launched pump power for the four wavelengths in the seed light generation part is shown in Fig. 3. The lasing threshold for the four wavelengths is ∼0.2 W benefiting from the active fiber based gain mechanism. The random lasing output increases almost linearly and the maximum output power under pump power of 3.815 W reaches 1 W level. Only a slight difference in the slope efficiency is observed in Fig. 3, i.e., 30.7% for the 1535 nm light and 27.3% for the 1550 nm one, which is determined by the wavelength dependent net gain difference of the EYDF. It is worth noting that due to the wavelength selective reflection in 1.5 µm band, the proposed structure for the Raman Stokes light is a complete open cavity which has a much higher lasing threshold. The lasing output here is still dominated by the active amplification in the 1.5 µm band, without the excitation of the Raman Stokes.
The spectral characteristics of all the above four lasing wavelengths are thoroughly investigated, as shown in Fig. 4. Since the overall trends of the spectral evolution for the considered four wavelengths are approximately similar, only the optical spectra (resolution is 0.05 nm) at 1545 nm are provided in Fig. 4(a). The spectral profile stays smooth and stable when the pump power is well above the lasing threshold, while a significant spectral broadening at the pedestal (>20 dB) is observed which is unlike the conventional RFL based on pure Raman gain mechanism. It has been reported that strong and fast intensity fluctuations with the generation of extreme events are observed when filtering the output with a larger spectral detuning with respect to the central lasing wavelength [36]. The extreme events would lead to further spectral broadening due to the modulation instability effect at the anomalous dispersion in combination with the active gain mechanism in the EYDF. To give a complete comparison of the spectral linewidth evolution versus the launched pump power, the 3 dB, 10 dB and 20 dB bandwidth of the four lasing wavelengths are calculated and given in Fig. 4(b). It is shown that the 3 dB bandwidth only undergoes a slightly broadening from ∼0.12 nm to ∼0.25 nm, while the 10 dB bandwidth broadens from ∼0.30 nm to ∼0.60 nm. However, for the 20 dB bandwidth, it shows a remarkable broadening when the pump power is increased above 2.5 W, which is also clearly confirmed in Fig. 4(a) due to the appearance of the strong spectral pedestal. Besides, the spectral broadening effect is more notable for the shorter wavelength, since it is much closer to the zero-dispersion wavelength (∼ 1.3 µm) of the SMF and therefore experiences a more prominent nonlinear effect.
Fig. 4. Spectral characteristics of the seed light. (a) Optical spectra at 1545 nm. (b) 3 dB, 10 dB and 20 dB bandwidth variation versus the pump power.
Furthermore, the temporal dynamics of the seed light are also evaluated in terms of the short time temporal domain traces and the radio frequency spectrum, as shown in Fig. 5. Here, only the 1545 nm seed light is chosen as an example. The short-time temporal domain traces corresponding to three pump power regimes are given in Fig. 5(a), which show typical continuous wave (CW) operation of random fiber lasing. To characterize the temporal dynamics quantitatively, the std/mean value defined by the value of standard deviation divided by the mean is applied here. It is shown the std/mean value is decreased from 0.0943 under pump power of 0.96 W to 0.0542 under pump power of 3.03 W. There are no instantaneously random pulse spikes on the CW background due to the sufficient random distributed feedbacks and the low transmission loss at the 1.5 µm band [37]. The radio frequency spectra of both the 70 MHz (VBW is 300 Hz) and 200 kHz (VBW is 30 Hz) band are measured under pump power of 3.03 W, as shown in Fig. 5(b). Apart from a slight intensity increase of the lasing signal compared with the noise floor of the photo-detector (PD) as shown in the 70 MHz band, there is no resonant frequencies within the whole band range, which further verifies that the structure is a half-open cavity and is free of resonant oscillation.
Fig. 5. Temporal dynamics of the seed light at 1545 nm. (a) Short-time temporal domain traces under different pump power. (b) Radio frequency spectrum.
For the power amplifier part, the output power versus the launched pump power is firstly investigated, as shown in Fig. 6. All the four lasing wavelengths show linear power increase especially within the pump power range of 47 W, corresponding to a slope efficiency of 33.7% for the 1545 nm random lasing. As the pump power increases above 54.6 W, a gradual drop of the slope efficiency is noticed which indicates the active gain in the MM EYDF starts to get saturated. Therefore, the pump power is only added to 61.9 W at first, which shows the output power reaches 17.84 W at 1550 nm and 19.47 W at 1540 nm. After all the following investigations, i.e., the power scaling range, the optical spectrum and the temporal characteristics, we individually fix the lasing wavelength at the 1545 nm and further increase the pump power to a much higher level. The maximum output power obtained is 20.15 W under pump power of 72.5 W, with a slope efficiency of 10.9% for the pump power range from 58.1 W to 72.5 W. The dramatically reduced efficiency indicates the saturation of the MM EYDF and the further power boosting is terminated by the fiber fuse of the MM EYDF which appears at ∼ 30 cm away from the splice point at the pump end. Therefore, the limit of the power scaling range of the proposed configuration and the employed MM EYDF is ∼ 20 watt-level. It is worth noting that although the EYDF in the power amplification is a multimode fiber, the outgoing mode of the 20 watt-level output is a pure single transverse mode. The excellent output beam quality is guaranteed by the integrated CPS and MFA which could strip the mismatched higher order modes that are propagating in the cladding. Besides, the use of SMF at the output port of the integrated CPS and MFA, as well as the ports of the ISO, also warrants the operation of the single fundamental Gaussian mode in the laser system.
The spectral characteristics of the power amplifier light are investigated and given in Fig. 7. From the optical spectra at the 1545 nm as shown in Fig. 7(a), the output keeps narrow linewidth feature even when the output power reaches the 20 watt-level. This is further verified by the statistic bandwidth variation versus the launched pump power for the four wavelengths, as shown in Fig. 7(b). It is worth noting that due to the imposed strong attenuation when measuring the spectrum, the 20 dB bandwidth is not available at the lower pump power regime and therefore we only consider the 20 dB bandwidth above the pump power of 8.39 W. Both the 3 dB and the 10 dB bandwidths stay almost unchanged as those in the seed light part across the whole power scaling range. Although the 20 dB one fluctuates within a ∼0.5 nm band range, there is no significant broadening at this regime. Therefore, the high power tunable random lasing also has the spectral-broadening-free property as that has been extensively verified at the 1.1 µm band. This benefits from the continuous wave operation in the open cavity structure, which suppresses the existence of instantaneous pulses that would further induce spectral broadening through the nonlinear effects. For the cascaded Raman gain based RFL approach or one stage active fiber based one, the 3 dB bandwidth could not maintain at such narrow level once the lasing power is up to more than 10 W. Therefore, the narrow linewidth random lasing seeded MOPA configuration is the most attractive approach to realize both high power and narrow linewidth laser operation.
Fig. 7. Spectral characteristics of the power amplifier light. (a) Optical spectra at 1545 nm. (b) 3 dB, 10 dB and 20 dB bandwidth variation versus the pump power.
The detailed lasing performance of the power amplifier output at 1545 nm is further analyzed, as shown in Fig. 8. Typical continuous wave operation is verified for the short-time temporal dynamics as that in the seed light part, as shown in Fig. 8(a). A suppressed std/mean value is even noticed in contrast with the seed light, which may be attributed to the saturation effect of the active gain mechanism in the power amplifier stage. Besides, to manifest the stability of the whole laser configuration, the long-time output power output is also analyzed within a time window of 30 min (pump power of ∼54 W). As shown in Fig. 8(b), the output power is stabilized at ∼17.5 W with an extremely small std/mean value of 0.0025, which indicates the excellent stable operation capability of the MOPA structure in long-time scale. Both the short-time and long-time domain stabilities are of paramount importance for practical applications. Figure 8(c) gives the radio frequency spectrum of the amplified 1545 nm under pump power of 58.1 W. Similar with that of the seed light shown in Fig. 5(b), there is no resonant frequencies within the whole frequency band for the amplified lasing output. Finally, the spectral purity (the power ratio between the 1.55 µm band and all the other potential bands) under the highest pump power of 72.5 W is shown in Fig. 8(d). None of the next order Raman Stokes, the parasitic lasing at 1535 nm and ∼1 µm are observed, which show 100% spectral purity even for the highest lasing output. However, for previous YDF based 1.55 µm band random fiber lasing approach where cascaded lower Raman Stokes orders are inevitable, not only the final spectral purity is greatly limited, but also the highest spectral purity only occurs at the highest power transfer regime. In contrast, for our proposed approach, the spectral purity remains 100% across the whole power scaling range.
Fig. 8. Characteristics of the power amplifier light at 1545 nm. (a) Short-time temporal domain traces. (b) Long-time output power stability. (c) Radio frequency spectrum. (d) Spectral purity for the whole band ranging from 1000 nm to 1700nm.
3. Conclusions
In conclusion, a high power tunable 1.5 µm band RFL is realized in a EYDF based MOPA configuration. Benefiting from the unique features of random fiber lasing, the MOPA output show high performances such as the record high output power, narrow linewidth lasing due to the spectral broadening free property, high spectral purity across the whole power scaling range and single transverse mode output. Although the wavelength tuning range is limited by the high insertion loss of the tunable filter, this strategy still works beyond this range when a higher wavelength selective point feedback is employed. This work provides an attractive alternative in realizing high performance lasing light source at 1.5 µm band.
Funding
China Postdoctoral Science Foundation (2020M682868); Basic and Applied Basic Research Foundation of Guangdong Province (2020A1515111143); Natural Science Foundation of Guangdong Province (2021A1515011532); National Natural Science Foundation of China (61875132, 62005186); Shenzhen Government’s Plan of Science and Technology (JCYJ20190808143813399, JCYJ20200109105606426, RCYX20210609103157071); Open Foundation of Key Laboratory of High Power Laser and Physics, Chinese Academy of Sciences (SGKF202107).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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