IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 11, JUNE 1, 2008
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High-Power Single-Longitudinal-Mode Fiber Laser With a Ring Fabry–Pérot Resonator and a Saturable Absorber X. X. Yang, L. Zhan, Q. S. Shen, and Y. X Xia
Abstract—We demonstrated a high-power single-longitudinalmode (SLM) all-fiber laser. In the cavity, a fiber Bragg grating is used to select the appropriate wavelength at 1565 nm, and a fiber ring is incorporated to act as a Fabry–Pérot resonator, which greatly reduces the longitudinal-mode density. Meanwhile, an unpumped erbium-doped fiber serves as a saturable absorber to ensure the SLM operation. Finally, a high-power SLM fiber laser producing as high as 867-mW output power has been experimentally demonstrated. Index Terms—Fiber laser, high-power, ring Fabry–Pérot resonator, saturable absorber, single-longitudinal-mode (SLM).
ITH THE rapid growth of the optical communications and fiber sensing systems, single-longitudinal-mode (SLM) fiber lasers, especially high-power SLM lasers have became more and more necessary. In the field of coherent optical communication, community antenna television systems, laser radar, and interferometric sensing [1]–[3], the high-power SLM fiber lasers have been attracting great interest of many applications. Recently, the development of the high-power fiber laser has made great progress on both the output power and operation wavelength performance. To date, kilowatt-class fiber lasers have been reported [4], but they usually worked at the operation of multiple longitudinal-mode. The development of SLM high-power fiber lasers has not been so fast, mostly because the bandwidths of the usual optical filters are much broader than the spacing of the longitudinal modes in fiber lasers. This results in the surplus of the longitudinal modes that accord with the resonance. Finally, strong and stable amplified spontaneous emission (ASE) forms, and beating noise, which has been generated as a result of beating between the lasing mode and ASE, causes intense fluctuations in the output intensity of the laser [5]. In the same way, the output power of the SLM fiber lasers has also encountered bottleneck. Commonly, the SLM fiber lasers
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Manuscript received September 30, 2007; revised December 3, 2007. This work was supported by the National Natural Science Foundation of China under Grant 60577048, by Shanghai Leading Academic Discipline Project of T0104, and by the Program for New Century Excellent Talents in the University of China. The authors are with the Institute of Optics and Photonics, Department of Physics, State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.922332
have a lower scalable output power of about 100 mW, because the general apparatus cannot afford such high power and the configuration is too complex [6]. The direct method to improve the output power is to increase the pump power, but the exorbitant pump power not only results in the complex configuration of the laser, also needs enormous radiator. Up to now, researchers have proposed several techniques to achieve high-power SLM fiber lasers, e.g., using the fiber Bragg grating (FBG)-based fiber laser that utilizes the transmitted light of FBG as self-injection feedback for SLM oscillation [1], using a short unidirectional ring cavity with the heavily doped phosphate fiber as the active medium [7], and employing cladding-pumped extremely high-doped fiber, which allows the high-gain laser cavity to be sufficiently short so that the SLM operation can be gained [8]. In [8], the generation of as high as 1.6 W of SLM output from a cladding pumped Er–Yb-codoped fiber laser has been demonstrated. Although this power is the highest among SLM fiber lasers, the low optical-to-optical slope efficiency is not perfect. The short cavity also increases the complexity of the configuration, and does not benefit to further improve the output power. In [9], the utilization of Mach–Zehnder filters to improve the longitudinal mode selectivity of ring cavity has been demonstrated. With the same Mach–Zehnder structure, an SLM fiber ring laser has been reported, but its output power is only 15 dBm [10]. In this letter, we demonstrate a high-power SLM all-fiber laser. In the cavity, an FBG is used to select the appropriate wavelength at 1565 nm. Meanwhile, owing to the fiber ring effect in the cavity, which acts as the Fabry–Pérot resonator, the spacing of longitudinal modes is expanded and the subsistent longitudinal modes are mostly decreased [11]. At last, an unpumped erbium-doped fiber (EDF) serves as the saturable absorber to eliminate the existent modes and thus ensures SLM operation [12]. It is well known that, the adequately long EDF can be used as a saturable absorber to suppress neighbouring mode lasing, but the output power of the laser sharply descends synchronously. In order to achieve a high-power laser, the EDF length should not be too long. Because of low insertion loss and distinguished mode-selecting effect of the fiber ring, we use the configuration as the Fabry–Pérot resonator to substitute the long EDF. To the best of our knowledge, the use of such a fiber ring inside the cavity to achieve high-power SLM laser is reported for the first time. By applying such a configurations, we elevate the output power and restrain the multilongitudinal-mode operation. Finally, a high-power SLM fiber laser producing as much as 867 mW has been realized.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 11, JUNE 1, 2008
Fig. 1. Schematic diagram of the experimental setup.
Fig. 3. (a) Output power at 1565 nm versus total pump current. (b) Optical spectrum of the laser emission measured by an OSA.
where is the light speed in free space, is the refractive index of the fiber, and is the length of the ring cavity. Here, is 1.45, and is 4.28 m. Thus, the FSR is 49 MHz. The resonate condition of the fiber ring is
Fig. 2. Schematic diagram of the fiber ring setup.
Fig. 1 shows the experimental configuration of the high-power SLM fiber ring laser. In the ring cavity, the laser contains a high-power erbium-doped fiber amplifier (EDFA), whose pump current can be adjusted from 300 to 4800 mA. The high-power EDFA consists of a double-clad Er–Yb-codoped fiber, which is bidirectionally pumped by two high-power multimode 976-nm laser diodes. The multimode laser diode uses a 100- m stripe width chip coupled in a 50/125- m fiber. The pump light and the signal are coupled in the common port of the wavelength-division multiplexer (WDM). The WDM component is equipped with a single-mode fiber on its signal port, and with a 50/125 fiber on its pump port. The common port is a double-clad fiber. The traveling light transits through the 30% port of the 30/70 coupler, and then enters into an optical spectrum analyzer (OSA) and a radio-frequency (RF) analyzer. An attenuator is used following the output signal to ensure the power of the incoming light is lower than 10 mW, which is the maximum allowable input power of the OSA and photodetector. The output power is measured directly by a powermeter. At the 70% port, an FBG reflects 90% of the power at 1565 nm, and the 3-dB reflection bandwidth of the FBG is 0.168 nm. The fiber ring consists of two 10/90 couplers, and the two 10% ends connected together. The whole fiber ring forms a Fabry–Pérot resonator, which is used to reduce the longitudinal mode density. Before the fiber ring, a 3-m unpumped EDF of 240-ppm erbium ion concentration is used as a saturable absorber. At last, by adjusting the PC, the proper polarization of the ring cavity can be obtained. Fig. 2 shows the configuration of the fiber ring. The beam inputs at point 1, and exports at point 2. The two couplers connect together with the 10% ports. Here, the fiber ring as a Fabry–Pérot resonator can look like a short-cavity configuration, and is used to expand the spacing of the longitudinal modes. The two 10/90 couplers performs high-reflection mirrors in Fabry–Pérot resonator, which ensures narrowband resonant peaks. Without considering the loss, we can obtain the free spectral range (FSR) according to the length of the cavity [13]. The FSR is given by (1)
(2) where is an integer, is the wavevector. From (1), we can obtain that, if the length of the fiber ring cavity is shorter, the FSR is larger accordingly. So the spacing of longitudinal modes is enlarged. When it exceeds the range of the spectrum that the laser can resonate, the SLM can be obtained. Furthermore, the bandwidth is also in inverse proportion to . According to (2), only if the spacing of the longitudinal modes is integer times of the FSR, could the longitudinal modes be achieved in the cavity. Owing to the Vernier effect [14], which has effect on the longitudinal modes of the effective fiber ring and the longitudinal modes in the resonator, the mode spacing has been greatly expanded. Theoretically, the Fabry–Pérot resonator has an ultralow insertion loss at the FP resonant wavelengths [15]. In the configuration, we use two 10/90 couplers, and the two 10% ends are connected together. We can regard this fiber ring configuration as a Fabry–Pérot resonator. So we can consider that the reflectivity of the mirror in FP resonator is 90%. The higher coupling ratio of couplers and the longer corresponds to the narrower linewidth of transmission. The output power at 1565 nm versus the combined pump power is shown in Fig. 3(a). The output power preponderates over 867 mW at the pump current of 4800 mA, and limited by the available pump power. Note that the output power maintenances linear to the pump current with an initial slope slightly over 0.176 W/A. According to the relationship of the pump current and the output power of the pump laser diode, the optical-to. As shown in optical slope efficiency may approach Fig. 3(a), the primal pump current should be adjusted above 300 mA, which is restricted by the instrumental requirement of the pump laser diodes. At the maximum output power level, the laser reaches 867 mW, which can be increased by improving the efficiency of the pump scheme. Fig. 3(b) shows the optical spectrum of the output light of the laser after attenuation. The centered laser spectrum is 1565 nm, which is the reflection peak of the FBG. To verify SLM operation, we use a 1-GHz RF analyzer to analyze the output signal after attenuation. Fig. 4(a) shows the RF
YANG et al.: HIGH-POWER SLM FIBER LASER WITH A RING FABRY–PÉROT RESONATOR AND A SATURABLE ABSORBER
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parallel and orthogonal to the optical axis. Certainly, a longer absorber can also ensure SLM and eliminate the mode-hoping, but results in larger cavity loss. Usually, in order to improve the output power of the laser, the overlong EDF is not applied. In summary, we have demonstrated the novel high-power SLM all-fiber ring laser with the use of fiber ring inside the cavity to limit the multilongitudinal-mode operation. In the configuration, an unpumped EDF is also applied to achieve high-power SLM laser. Finally, a laser with output power up to 867 mW is realized. Apart from the inherent advantages of the fiber lasers, the high-power SLM fiber laser may satisfy the applications in many fields. REFERENCES Fig. 4. (a) RF spectrum only using FBG as a wavelength selector with a span of 70 MHz. (b) RF spectrum with a span of 1 GHz using FBG along with a fiber ring resonator. (c) RF spectrum of the laser output adding saturable absorber in the primary configuration. (d) Frequency mode hoping.
spectrum of the laser signal only using the FBG without fiber ring and unpumped EDF in the cavity. The FBG is applied as the wavelength selector, whose reflected spectrum is measured by 0.168 nm. In Fig. 4(a), it is observed that the laser performs multilongitudinal-mode operation. The measured beat frequency is 4.9 MHz, which agrees with the calculated cavity frequency according to 40 m cavity length. When we use fiber ring with the FBG, the RF spectrum is shown in Fig. 4(b). There is only one longitudinal-mode operation except the zero frequency in the 1-GHz range, and locates at 690 MHz, which is 14 times as large as 49 MHz of the FSR. Although the laser still performs the multilongitudinal-mode operation in Fig. 4(b), we find distinctly that the multilongitudinal-mode lasing is extremely restrained by the fiber ring. Especially, the peaks of longitudinal-mode beat frequency do not appear near the zero frequency. As shown in Fig. 4(c), when we add the unpumped EDF that serves as a saturable absorber, only the single peak exists at the zero frequency, and the other remained peaks disappear from the RF analyzer. The result distinctly indicates that the laser is under SLM oscillation, since no correlative beat frequency has been observed. When up to 700-mW output power, the occasional modehoping appears as shown in Fig. 4(d). By adjusting the PC, the polarization of the beam alters to adapt the parts of the apparatus, so that the mode-hoping can be restricted. The oscillation of the modes with effective change of polarization state can be suppressed due to the large different emission for light polarized
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