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Abstract
Based on a 1.8-cm-long heavily Tm3+-doped germanate fiber and being in-band-pumped by a 1610 nm single-mode laser, a high-efficiency and high-power single-frequency distribute Bragg reflector (DBR) fiber laser emitting at 1950 nm is demonstrated. The DBR fiber laser has a maximum output power of ~617 mW and a slope efficiency for the absorbed pump power reaches to more than 42.2%. A stable single-longitudinal-mode laser output with a signal-to-noise ratio of greater than 63 dB is realized. The measured relative intensity-noise of the fiber laser reaches to around –150 dB/Hz at frequencies of over 8.4 MHz. It is beneficial to exploit the sub-watt and high-efficiency single-frequency laser from fiber oscillators directly, especially in the application of multiple paths coherent beam combination and optical medical technology.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Continue-wave single-frequency fiber laser (SFFL) operating at 2.0 μm band based on Tm3+-doped silica fiber had been demonstrated firstly in 2004 [1]. Due to the advantages of narrow linewidth, long coherence length, low intensity-noise, tunable wavelength, and compact structure, the SFFLs operating near 2.0 μm, particularly in 1.7-2.1 μm, have a broad application prospect in high-resolution spectroscopy, nonlinear optics, laser lidar, photodynamic therapy, and noninvasive medicine [2–7]. This band not only covers absorption band of atmospheric gases, such as H2O and CO2, but also can be used and integrated as a pump source for Mid-IR distributed feedback (DFB) Raman fiber lasers [8]. In addition, comparing with the SFFL operating around 1.0 μm band, the SFFL at 2.0 μm has a higher threshold of triggering nonlinear effect [9], which facilitates to realize high-power laser operation [10,11].
Besides the ring cavity [12], single-longitudinal-mode lasing at the 2.0 μm band from the linear short cavity structure, including DFB [1,13] and distribute Bragg reflector (DBR) [14–17], has been extensively reported. Meanwhile, to efficiently stimulate laser at 2.0-μm-band, pumps operating at different wavelengths have been utilized. Some researchers used 0.8 μm laser as pump light because of the high absorption cross section of Tm3+-doped fiber in this band. J. Geng has accomplished an SFFL working at 1893 nm had a maximum output power of 50 mW and a slope efficiency of ~30% with respect to the launched pump power of 805 nm laser diodes (LD) in 2007 [18]. However, when being pumped by 0.8 μm laser, the whole active fiber in 2.0 μm DBR laser would suffer from the potential thermal effects because of the quantum defect [19,20]. Even worse, excessive heat can result in performance degradation of the output laser, such as high intensity-noise and frequency noise [21,22]. Others used pump working at 1.5 μm band and attempted to use energy of the pump more efficiently by means of in-band-pumping. In 2011, Z. Zhang has completed single-frequency DBR laser at 1943 nm with a slope efficiency of ~13%, which was pumped by 1565 nm laser [23]. Owing to the limited absorption cross section around 1.5 μm, SFFLs being in-band-pumped get relatively low efficiency. Moreover, low doped concentration of Tm3+ in active fiber further constrains the enhancement of the relevant slope efficiency. For short-cavity SFFL, heavily rare-earth ion doped active fiber can be used to realize high-power and high-efficiency laser output [24]. Different Tm3+-doped glass fibers are employed for SFFL at 2.0 μm, such as silica, silicate, and germanate glass fiber [1,12–16]. A SFFL at 1950 nm has been demonstrated in a DBR laser cavity with a maximum output power of 18 mW and slope efficiency for absorbed pump power of 13.4% by using a 1.9-cm-long commercially available Tm3+-doped silica fiber by S. Fu in 2015 [16]. Based on Tm3+-doped germanate fiber (TGF), Q. Yang has achieved over 100 mW stable continue-wave single transverse and longitudinal mode lasing with slope efficiency for absorbed pump power of ~22% at 1950 nm in 2015 [17]. Hence, in order to realize both high-power and high-efficiency SFFL laser around 2.0 μm, it is feasible to further improve doped concentration and choose more reasonable pump wavelength.
In this article, we report a both high-power and high-efficiency heavily Tm3+-doped DBR SFFL operating at 1950 nm based on a 1.8-cm-long heavily TGF and being in-band-pumped by a high-power single-mode 1610 nm pump laser. The maximum output power of 617 mW and slope efficiency for the absorbed pump power of >42.2% are achieved.
2. Experimental setup
The experimental setup of in-band-pumped 1950 nm DBR SFFL is depicted in Fig. 1. It consists of a short laser cavity with TGF, a 1610/1950 nm wavelength division multiplexer (WDM), an isolator (ISO), and a 1610 nm pump laser. The 1950 nm laser cavity is constructed by cleaving a high-reflection FBG (HR-FBG) and a polarization-maintaining partial-reflection FBG (PM-FBG), which are close to the grating area, and then directly splicing to a 1.8-cm-long TGF with ion concentration of 7.6 × 1020/cm3. The HR-FBG has a 3-dB-bandwidth of 0.43 nm and a reflectivity of >99.8% at signal wavelength. The PM-FBG is written in a section of PM fiber (PM-1550, Nufern) with a 3-dB-bandwidth of 0.12 nm and a reflectivity of 69.95% at 1950 nm. The laser cavity is directly placed in a copper tube that is temperature-controlled by a cooling system with an accuracy of ± 0.1°C. The fiber laser is counter pumped by a high-power 1610 nm pump laser, through a 1610/1950 nm WDM. The in-band-pumping laser source consists of a 1610 nm LD seed and two cascaded all-fiber amplifiers. The 1610 nm seed laser is a LD with power of ~10 mW and the maximum output power is amplified to be ~1.8 W. The pump port and common port of the WDM is fusion spliced to the 1610 nm pump laser and the PM-FBG, respectively. The signal port of the WDM is used to collect the output signal laser, and after that an ISO is utilized to protect the laser cavity from the back reflections laser.
Fig. 1 Experiment setup of 1950 nm DBR SFFL pumped by 1610 nm MOPA system. (HR-FBG: high-reflection fiber Bragg grating; PM-FBG: polarization-maintaining partial reflection fiber Bragg grating; WDM: wavelength division multiplexer; ISO: isolator.)
3. Result and discussion
The single-mode TGF used in this experiment was fabricated by the rod-in-tube technique. Figure 2 shows the absorption spectrum of the core glass in TGF during covering wavelength from 300 to 2100 nm. One can be observed in the figure that there are six absorption peaks centering at 1650, 1210, 790, 684, 472, and 356 nm. Comparing with the absorption peak at 1650 nm, the remaining five ones are fairly sharp. It indicates that 2.0 μm output is more susceptible to the wavelength fluctuation of pump operating near 1210, 790, 684, 472, and 356 nm. Hence, for 1950 nm DBR fiber laser based on TGF, it is an advisable to choose pump wavelength during the band where 1650 nm situates.
Figure 3(a) shows the simulation of the maximum output power at 1950 nm with the maximum pump power of 1.5 W and 1.8-cm-long TGF as a function of the wavelength of pump from 1540 to 1700 nm. It is evident that the maximum output power at 1950 nm grows as the pump wavelength increases from 1540 to 1643 nm. When the pump wavelength is larger than 1643 nm, the maximum output power gets a gradual decline. The simulated results show that 1643 nm pump has an advantage over 1568 and 1610 nm pump for the in-band-pumped 1950 nm SFFL. Considering the profile of emitting cross section of Er3+/Yb3+ co-doped active fiber (EYDF), watt-level master oscillation parameter amplifier (MOPA) systems for both signal wavelength of 1.5 μm and 1610 nm have been well developed [25,26]. However, the 1643 nm fiber laser is difficult to achieve high power because of its low emitting cross section in core glass of EYDF. Hence, pump laser operating at 1568 and 1610 nm are selected in this experiment for comparison. According to the Fig. 2, the absorption cross section of TGF at 1610 nm (0.312 × 10−20 cm2) is more than 3.5 times that at 1568 nm (0.088 × 10−20 cm2). As for the wavelength of pump laser with the same power, it is expected that high-power and high-efficiency output laser at 1950 nm can be realized through being in-band-pumped by laser emitting at 1610 nm as shown in Fig. 3(a). More parameters of this active fiber can be found in our preview work [27].
Fig. 3 Simulation of the maximum output power at 1950 nm as a function of (a) the wavelength of pump from 1540 to 1700 nm; (b) TGF length from 0 to 3.0 cm at 1610 nm, with the maximum pump power of 1.5 W.
Theoretically, the total effective length of the short linear cavity is determined by the length of gain fiber, the effective length and reflectivities of two FBGs, and the refractive index of optical fibers in the linear cavity fiber laser [28]. In order to get single-longitudinal-mode operation in the DBR fiber laser output, the temperature range of single-longitudinal-mode needs to be expanded by keeping the length of linear cavity short [29]. On the other hand, it is essential to ensure that the active fiber is long enough to gain the output power as high as possible. Meanwhile, the self-pulse phenomenon also requires constant attention due to saturable absorption effect [30]. In order to ensure a single-longitudinal-mode and high output power, it is necessary to optimize the resonant cavity basing on simulation of the relationship between the output power and the length of TGF. Figure 3(b) shows the maximum output power at 1950 nm as a function of TGF length from 0 to 3.0 cm with the maximum pump power of 1.5 W at 1610 nm. It can be found that the laser is generated when the length of TGF is longer than 0.25 cm. The maximum output power is improved gradually when the length of TGF is increasing from 0.25 to 2.0 cm. However, a saturation effect occurs with TGF length between 2.0 and 2.28 cm. Hence, from the maximum output power point of view, the length of TGF needs to be less than 2.28 cm.
Based on the simulation results mentioned above, the initial length of TGF is decided to be 2.3 cm. Nevertheless, the TGF is too long to realize a single-longitudinal mode output. Therefore, in this experiment, the appropriate length of active fiber is determined by the cut-back method from 2.3 cm in interval of 0.1 cm. The longitudinal-mode characteristics are measured by a scanning Fabry–Perot interferometer (Thorlabs, SA200-18B) with a free spectral range (FSR) of 10 GHz and fineness of 150. When TGF is longer than 2.0 cm, there are more than one longitudinal mode whatever its temperature is. When the temperature of cavity is controlled to be 20.7°C and the length of TGF is cut to be 2.1-cm-long, where the main longitudinal mode is dominant, there are still other two modes, as shown in Fig. 4(a). Even though the temperature of the short cavity is controlled in a broad range, more than one longitudinal modes is always present. Further decrease in the length of TGF can result in growing temperature range of the single-longitudinal-mode operation. Whereas, there is always a trade-off when one takes the laser output power into account. By trials and errors, a 1.8-cm-long TGF is adopted to ensure wide working temperature range. According to the simulation results mentioned above, it is worth noting that the maximum output power with 1.8-cm-long TGF is only ~10 mW less than that with 2.0-cm-long TGF. In addition, stable single-longitudinal-mode output is produced when the temperature of the short linear cavity is controlled at 18.9°C, as shown in Fig. 4(b). There is no mode-hopping or mode competition being observed with the pump power increasing at such temperature. Hence, 1.8-cm-long TGF is applied to the resonant cavity.
Fig. 4 Longitudinal modes characteristics of the fiber laser with the TGF length and temperature of (a) 2.1 cm and 20.7°C; (b) 1.8 cm and 18.9°C, respectively.
It is noted from Fig. 5 that the experimental output lasing threshold of the launched pump power is around 70 mW when being pumped by 1610 nm laser. The output power is approximately linearly increased with the augment of pump power after the threshold. The maximum output power of >617 mW at 1950 nm is obtained at the maximum absorbed pump power of 1545 mW. Due to the limited pump power, the output power cannot continue to be improved. The slope efficiency with respect to the launched pump power reaches to ~36.2%. If the residual pump power is excluded, the efficiency of >42.2% is obtained, as shown in Fig. 5. However, when the 1.8-cm linear TGF cavity is pumped by 1568 nm fiber laser, the output lasing threshold is 234.95 mW and the slope efficiency is 9.95%. Even though being pumped by both 1568 and 1610 nm laser are in-band-pumping, the output results are different, mainly because these two kinds of wavelengths are distinct in the both absorption and emission section of the TGF as mentioned above.
Fig. 5 Based on 1.8-cm-long TGF, simulated and experimental results of output powers versus pump power as a function of different pump wavelengths.
In order to verify that the laser operating at 1610 nm would provide higher output power and efficiency for 1950 nm SFFL, numerical modeling is conducted based on the described simulation model to analyze the output power of the DBR Tm3+-doped fiber laser [31–33]. Figure 5 also shows the simulated output power versus absorbed pump power as a function of different pump wavelengths. It can be seen that when the 1.8-cm-long TGF is pumped by the laser working at 1610 nm, the slope efficiency gets to be ~44.1%, and the threshold is about 52.8 mW. However, the slope efficiency is only 11.5% and the threshold is about 208.2 mW when 1568 nm fiber laser is used as pump. Comparing simulation and experiment accomplished above, the results are in good agreement, and the main source of difference is resulted from the estimation of the experiment loss.
The output spectrum is recorded by an optical spectrum analyzer (OSA, YOKOGAWA, AQ6375) with a spectrum resolution of 0.02 nm when the output laser is operating at ~500 mW, which is shown in Fig. 6(a). The detected signal needs to be attenuated to 1 mW to protect the experimental apparatus from damage of high power laser. It is noted that the center wavelength of the output signal is 1950.39 nm and there is almost no the amplified spontaneous emission (ASE) around 1.9 μm band because of the employment of FBGs in the short linear cavity. As can be seen from Fig. 6(a), there is little pump light at 1610 nm left, and the SNR is more than 63 dB.
Fig. 6 (a) Output spectrum of the 1950 nm DBR SFFL. Inset: laser spectrum measured by the OSA with wavelength span of 10 nm; (b) RIN of the fiber laser and the shot noise limit are also shown for comparison in the frequency band of 0–15 MHz with different output power. Inset: power stability of the fiber laser for ~2 hours.
The relative intensity-noise (RIN) of the fiber laser is measured by an electrical spectrum analyzer, whose resolution bandwidth is set to be 1 kHz. In each measurement of RIN, the laser powers are attenuated to 0.5 mW before being injected into photoelectric detector in order to ensure consistency of each experiment. Figure 6(b) shows the output RIN under conditions of different output powers in the frequency range of 0-15 MHz and the calculated shot noise limit (SNL) of –153.9 dB/Hz @0.5 mW is also illustrated for comparison. Here, the SNL is calculated by 2hν/P, where h is the Planck constant, ν is the lasing frequency of signal and P is the laser power [34]. It can be observed that the RIN spectra are dominated by peaks at the relaxation oscillation frequency (ROF). The ROFs move toward the higher frequency from 0.675 to 0.945 MHz with the output power increasing. Meanwhile, the highest intensity amplitude of output RIN decreases from −102.5 to −108.5 dB/Hz, indicating that the pump power is one of the factors that affect the ROF for linear all-fiber single-frequency laser [35]. After ROF, the intensity amplitude of output RIN decreases constantly to around −150 dB/Hz until frequency reaches to around 8.4 MHz. The power stability is also measured for two hours when the output power gets around 475 mW by utilizing an optical power meter. The SFFL is stable with output power fluctuation relative to the average power of 1.0% approximately during the entire period, and the tested data is depicted in the inset of Fig. 6(b).
The laser linewidth is measured by a self-heterodyne method. As shown in Fig. 7, the heterodyne signal is stable and after a Lorentzian fitting, it is 251 kHz with −20 dB from the peak, which indicates that the measured full width of half maximum is approximately 12.55 kHz.
Fig. 7 Linewidth of the Tm3+-doped germanate glass fiber laser measured by the self-heterodyne method.
The progress of SFFLs based on heavily Tm3+-doped glass fibers is summarized in Table 1. SFFLs at ~2 μm have been realized in various heavily Tm3+-doped glass fibers with different pump wavelengths. Comparing Ref [12] and [36], it can be seen that more heavily Tm3+-doped concentration is conducive to higher slope efficiency under the condition of the similar pump wavelength. Besides, it is beneficial to achieve high output power through being in-band-pumped. Silicate glasses are not the ideal host glasses for mid-IR lasers since the high phonon energy could lead to fast multiphoton relaxation which decreases the quantum efficiency and also causes thermal damage of the fiber laser [27]. Although fairly high output power (up to 580 mW) can be achieved in silicate host glass [23], further enhancement in slope efficiency is restricted. Thus, researchers resort to germanate glass that is featured by low phonon energy. By virtue of the TGF, hundred-mW output power as well as >20% slope efficiency for 2.0 um SFFLs are realized [36]. Hence, TGF offers an ideal choice due to its low photo energy. Usually, hundred-mW output power for 2.0 μm SFFLs are obtained through TGF. In terms of the wavelength of pump, 0.8 μm single-mode fiber laser cannot offer high energy to pump Tm3+-doped glass fiber because of its limited power. By comparing with the results of other experiments, pump wavelength of 1610 nm is an efficient method to enhance the output power and slope efficiency for SFFL at 2.0 μm.
Table 1. Single-frequency fiber lasers based on heavily Tm3+-doped glass fibers
4. Conclusion
In conclusion, a high-efficiency and high-power single-frequency DBR TGF laser at 1950 nm is developed. By employing the 1.8-cm-long TGF pumped by high-power laser operating at 1610 nm, a stable single-frequency laser with the output power of >617 mW and the slope efficiency with respect to the absorbed pump power of >42.2% is obtained, which provides a SNR of greater than 63 dB. The measured RIN of fiber laser is less than –150 dB/Hz at frequencies of over 8.4 MHz. This SFFL can lay a good foundation for high-power master oscillation parameter amplifier system at 2.0 μm band. Furthermore, it can provide a promising candidate for high-resolution molecular spectroscopy, nonlinear optics and coherent lidar applications in the future.
Funding
National Key Research and Development Program of China (2016YFB0402204).
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