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All-fiber passively Q-switched low-threshold erbium laser Valery N. Filippov, Andrei N. Starodumov, and Alexander V. Kir’yanov Centro de Investigaciones en Optica, Loma del Bosque 115, Col. Lomas del Campestre, 37150 Leon, GTO, Mexico
Received October 4, 2000 A novel all-fiber passively Q-switched erbium laser with a Co21 :ZnSe crystal as a saturable absorber is demonstrated experimentally. A pump power threshold of 20.5 mW, which the authors believe is the lowest to date, has been measured. Giant pulses with energy of 3.6 nJ and peak power of 0.7 mW have been obtained. © 2001 Optical Society of America OCIS codes: 140.3500, 140.3510, 140.3540.
Q-switched erbium fiber lasers are currently used in communications, ref lectometry, distributed fiber-optical sensing, etc. There are two methods, active and passive, that one can use to force a laser to generate giant pulses. Actively Q-switched f iber lasers are well known, in widespread use, and the most thoroughly investigated kind of laser. Among the active methods of Q switching are the use of an all-fiber intensity modulator,1 current modulation of a pump laser,2 scanning of intracavity Fabry – Perot f ilters,3 and, the most frequently used, acousto-optical modulation inside the laser cavity.4 – 7 All actively Q-switched lasers contain bulk elements, which makes their design rather complicated. That is why much attention has been paid to developing passively Q-switched fiber lasers. An all-fiber configuration, compactness, and simplicity of design are the main advantages of such lasers compared with actively Q-switched lasers. A few methods of achieving a passive Q-switch mode in fiber erbium lasers have been proposed: (i) a laser with distributed backscattering,8 (ii) a laser with a gallium liquefying mirror,9,10 and (iii) a laser with a semiconductor structure acting as a saturable-absorber mirror.11 It was shown that the passively Q-switched ytterbium laser with backward ref lection8 generates rather powerful (as much as 10 kW) and short giant pulses but essentially needs a high pump power 共⬃2.5 W兲. As was noted in Ref. 8, an erbium laser developed with the same principles is highly unstable. High pump powers of 0.9 and 1.2 W have been reported in lasers with gallium9,10 and saturableabsorber11 mirrors, respectively. However, the high pump threshold in these lasers is related to large core fiber and cavity configurations rather than to the saturable absorber. In this Letter we report, for the first time to our knowledge, a novel conf iguration of an erbium-doped fiber laser oscillating in the passive Q-switch mode under cw pumping with a Co21 :ZnSe crystal as the saturable absorber. The laser is characterized by an essentially low threshold 共⬃20.5 mW兲 of giant-pulse generation. Co21 :ZnSe crystals were recently demonstrated to be excellent Q switches for bulk solid-state erbium lasers.12,13 The remarkable advantage of the 0146-9592/01/060343-03$15.00/0
crystals is their extremely low bleaching intensity, ⬃0.8 kW兾cm2 , and a moderate relaxation time of the excited state 共290 ms兲 within the spectral range 1400–1800 nm. In our opinion, the Co21 :ZnSe crystal is a highly promising material for erbium lasers with all-fiber architectures. The experimental setup is shown in Fig. 1. The fiber laser was pumped by a commercial laser diode (wavelength, 976 nm) through a wavelength-division multiplexer. The cavity was formed by two fiber Bragg gratings with ref lectivities of 94.2% and 88.5%, respectively. The maximum ref lectivity of both gratings was at a 1560-nm wavelength with a 2-nm FWHM bandwidth. We used a 20-m erbium-doped fiber with an absorption of 1 dB兾m at a 980-nm wavelength as an active medium. The key element of the scheme for a laser with a passive Q-switch mode was a U-bench unit with the Co21 :ZnSe crystal inside (see Fig. 1, inset). It was especially developed to provide a power density of the order of 1 kW兾cm2 (Ref. 12) at the crystal. The initial transmission of the crystal was 92%, and the final transmission (unbleachable) was 99.5%. The crystal was tuned along the U-bench length (the air gap between lenses was 4 mm) such that the location of the beam waist (diameter, 15 mm) was close to the crystal center. The thickness of the crystal was 0.5 mm; both crystal’s facets were antiref lection coated for a wavelength of 1550 nm. The total optical losses of the U-bench with the crystal were 3 dB. The overall
Fig. 1. Schematic of the passively Q-switched fiber laser: WDM, wavelength-division multiplexer; FBG’s, fiber Bragg gratings. © 2001 Optical Society of America
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Fig. 2. Output spectrum of the laser. The resolution is Dl 苷 0.1 nm, and the pump power is 71.1 mW.
Fig. 3. Typical output pulse train and pulse shape. pump power is 48.53 mW.
principle, be shortened by application of a highly doped erbium f iber of a shorter length as the active medium of the laser. The repetition rate of the giant pulses and the average output power grew linearly with increasing pump power (Fig. 4). Note that, starting from ⬃85 mW of pump power, the regime of passive Q-switching became unstable with a timing jitter and, at a further increase of pump power, was replaced by cw laser oscillation. The latter occurred when a period between the adjacent pulses approached, in the time domain, the pulse duration of a giant pulse, which, in turn, was determined by the cavity length. In the case under study, a stable giant-pulse mode was observed within the range 20– 85 mW of pump power. Let us mention that there was no change in the output power of the laser at its transit from a passive Q-switch mode to a cw mode. The maximum pulse energy was measured to be 3.56 nJ at 60.6 mW of pump power. The dependence of pulse energy on pump power is shown in Fig. 5. Initially, the pulse energy grew monotonically with the pump, reaching a maximum near 60 mW of input power, and then smoothly decreased to 3.16 nJ (at 84.7 mW of pump power). The last observation can
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length of the cavity was 22 m. Output characteristics of the laser were studied with a wideband high-speed germanium photoreceiver, a powermeter, and an oscilloscope. The threshold of cw operation of the laser was 19.3 mW at a wavelength of 1560.12 nm. Just above the threshold of oscillation, when the pump power was increased to 20.5 mW, the laser transition to a passive Q-switch mode. The spectrum of laser oscillation in the passive Q-switch mode is shown in Fig. 2. The laser emitted a narrow line with a FWHM bandwidth of less than 0.1 nm (resolution is limited by a spectrum analyzer). The train of giant pulses (Fig. 3) had a stable repetition rate, with the pulses being well approximated by a Gaussian curve (see Fig. 3, inset). The pulse length in the train was 5.36 ms (pump power, 48.53 mW), with a peak power of 0.7 mW. The rather long pulse duration is explained by the considerable length of the cavity, which is, in turn, chosen to provide total pumping of the whole fiber length. Note that the pulse duration could, in
Fig. 4. Pulse repetition rate (squares) and average output power (crosses) as functions of pump power.
Fig. 5. Pulse energy versus pump power.
March 15, 2001 / Vol. 26, No. 6 / OPTICS LETTERS
be explained by the fact that for high pump powers the repetition rate of pulses in the train 共.20 kHz兲 begins to exceed the characteristic frequency of bleaching decay, which for the Co21 :ZnSe saturable absorber is 3.5 kHz. It is clear that in this case the crystal is bleached only partially, and, as a consequence, the pulse energy decreases (Fig. 5). In conclusion, we have demonstrated, for the first time to our knowledge, the operation of an all-fiber cw-pumped low-threshold erbium laser with a Co21 :ZnSe crystal in the passively Q-switched mode. We believe that this laser scheme with a U-bench unit containing a passive Q switch could find wide application for developing more-powerful f iber lasers with giant oscillating pulses. This research was supported by the Consejo Nacional de Ciencia y Tecnología (Mexico) through projects 32269-E, 32195-E, and 32023-A. V. N. Filippov’s e-mail address is
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