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Transform-limited pulses generated by an actively Q-switched distributed fiber laser C. Cuadrado-Laborde,1,2,* P. Pérez-Millán,1 M. V. Andrés,1 A. Díez,1 J. L. Cruz,1 and Yu. O. Barmenkov3 1
Departamento de Física Aplicada y Electromagnetismo, Instituto de Ciencia de los Materiales de la Universidad de Valencia, Universidad de Valencia, C/Dr. Moliner, 50-Burjassot E-46100, Spain 2 CIOp (CONICET-CIC), P.O. Box 124, La Plata 1900, Argentina 3 Centro de Investigaciones en Óptica, León, Guanajuato 37150, México *Corresponding author:
[email protected] Received August 4, 2008; accepted September 18, 2008; posted October 8, 2008 (Doc. ID 99772); published November 7, 2008 A single-mode, transform-limited, actively Q-switched distributed-feedback fiber laser is presented, based on a new in-line acoustic pulse generator. Our technique permits a continuous adjustment of the repetition rate that modulates the Q factor of the cavity. Optical pulses of 800 mW peak power, 32 ns temporal width, and up to 20 kHz repetition rates were obtained. The measured linewidth demonstrates that these pulses are transform limited: 6 MHz for a train of pulses of 10 kHz repetition rate, 80 ns temporal width, and 60 mW peak power. Efficient excitation of spontaneous Brillouin scattering is demonstrated. © 2008 Optical Society of America OCIS codes: 140.3510, 140.3490, 140.3540, 290.5830.
The development of pulsed light sources with narrow linewidths and high peak powers is crucial in different applications such as distributed Brillouin sensors, coherent lidar, laser Doppler velocimeters, spectroscopy, instrumentation, etc. Distributed-feedback (DFB) lasers, either fiber lasers (FLs) or semiconductor lasers, operate on a single-mode narrow linewidth regime. The relatively simple fabrication of the former involves the writing of a grating structure with ultraviolet light into an active fiber and singlemode pump, leading to an alignment-free resonator with optimum overlap of pump and signal light. For these fiber-Bragg-grating (FBG)-based DFB FLs, the distributed reflection occurs in the grating when an appropriate phase shift has been generated within it [1,2]. A number of techniques have been proposed for this; however, statics phase shifts only allow cw operation [1–3]. Recently, some approaches have been reported to obtain single-frequency pulsed fiber lasers, based on active Q switching of DFB fiber cavities [4,5]. Here we report an acoustically Q-switched DFB FL that provides transform-limited pulses of 800 mW peak power and 32 ns pulse width. Our work includes the development of a new in-line acoustic pulse generator that permits the access of both outputs of the DFB FL; it introduces no direct perturbation in the grating, since it is located several centimeters apart and permits a flexible adjustment of the repetition rate. Figure 1 shows the scheme of the proposed DFB FL. The FBG was 100 mm long, and was written in a 1500 parts per million (ppm) erbium hydrogenloaded fiber (codoped with germanium and aluminum) of the same length using a doubled argon laser and a uniform period mask. The FBG shows more than 30 dB attenuation at the Bragg wavelength 共B = 1532.447 nm兲 and a 3 dB bandwidth of 88 pm. The FBG was pumped through a 980/ 1550 nm wavelength division multiplexer (WDM) with a 976 nm semiconductor laser, providing a maximum pump power of 130 mW. A square-shaped rod of a magneto0146-9592/08/222590-3/$15.00
strictive material (Terfenol-D, 15 mm long and 1 mm2 section) was bonded outside the FBG to a free section of fiber at 88 mm from the center of the grating, and placed inside a small coil; see Fig. 1. When a pulse of electric current is applied to the coil, the small rod lengthens and stretches the section of fiber attached to it [6], generating in this way a longitudinal acoustic pulse that propagates toward the FBG. The pulse propagating along the FBG generates a phase shift opening a transmission peak within the reflection band of the grating; as a consequence a high Q resonance is produced and a laser pulse is emitted [4,5]. Otherwise, if no perturbation is present within the FBG, there is no efficient feedback for the optical signal, and the laser emission is not allowed. The transmission properties of the passive FBG interacting with the acoustic pulses have been further investigated by illuminating the FBG with a tunable laser (Agilent 81940A) at B and detecting the reflected signal. When the coil current is zero the reflectance is maximal; but when a current pulse of 220 mA amplitude and 5 s temporal width is applied to the coil, a transmission peak opens the reflection band, being as narrow as 2 s (FWHM); see Fig. 2. This new in-line acousto-optic modulator allows continuous tuning of the repetition rate plus easy access to both DFB FL extremes, becoming a good alternative to piezoelectric-based devices.
Fig. 1. Q-switched distributed feedback-fiber laser setup. Lengths are in millimeters, whereas B and F stand for backward and forward outputs of the laser, taking as reference the pump direction. © 2008 Optical Society of America
November 15, 2008 / Vol. 33, No. 22 / OPTICS LETTERS
Fig. 2. Reflection at the Bragg wavelength when an acoustic pulse has been launched to the FBG.
Figure 3 exemplifies the laser behavior. Figure 3(a) shows a 10 kHz train of the voltage pulses applied to the current driver, together with the train of optical pulses generated (backward output). The delay between them 共22 s兲 corresponds to the distance that the acoustic pulse has to travel from the magnetostrictive device to the FBG. A detail of the optical pulse is shown in Fig. 3(b), whereas Fig. 3(c) shows a detail of the electrical current pulse applied to the coil. Finally, the spatial position of the acoustic pulse, at the instant when the laser emission is produced, was estimated by dumping the acoustic pulse with a drop of oil displaced longitudinally over the grating. There was no laser emission when the drop was at 123 mm from the magnetostrictive transducer; this indicates that laser emission takes places around the last third of the grating; see Fig. 1. The dependence of the laser peak power and temporal width with the coil current amplitude 共I兲 is shown in Fig. 4 for both optical outputs (backward and forward with respect to the optical pump direction). When the electric current is low (i.e., ⬍150 mA兲, there is no laser emission since the Q value is not high enough. Above this current there is one laser pulse per each launched acoustic pulse. When the electric current increases, the Q value improves, and this results in higher (and shorter) opti-
Fig. 3. DFB FL behavior at 10 kHz repetition rate and 80 mW of pump power. (a) Emitted optical train pulses and voltage signal applied to the current driver (left and right axes, respectively), (b) detail of a single optical pulse of the train, and (c) detail of a single current pulse applied to the coil.
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Fig. 4. Peak power (left ordinate) and temporal width (right ordinate) of the optical pulses as a function of the coil current, for forward and backward outputs, with 55 mW of pump power and 2 kHz repetition rate.
cal pulses. At higher electric currents (i.e., ⬎230 mA), the DFB FL emits two pulses per acoustic pulse. Figure 5 shows the backward optical pulses’s peak power and temporal width, as a function of the pump power, for several repetition rates ranging from 200 Hz to 20 kHz, when the current pulses had 200 mA amplitude. As expected, optical pulses become higher and shorter as the pump power increases. At low repetition rates 共500 Hz兲 optical pulses of 800 mW and 32 ns were obtained in the backward direction, by pumping with 46 mW. As shown in the figure, the pump threshold increases with repetition rate. There is also an upper pump limit for each repetition rate, beyond which the laser emits more than one optical pulse per cycle. Similar results, but with lower peak powers (about a factor of 10), were obtained for the forward output. Temporal widths and peak power jitters were measured to be below 5%. The emission linewidth 共⌬DFB兲 was measured, using a classical heterodyne configuration [7]. A tunable laser was used as local oscillator (LO), with ⌬LO = 100 kHz. The output of this laser was superimposed to the optical pulses of the DFB FL (backward output), through a 1550 nm 20 dB coupler. The beat sig-
Fig. 5. Peak power (dotted curves, left ordinate) and temporal width (solid curve, right ordinate), of the backward optical pulses as a function of the pump power, for different repetition rates.
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nal at the coupler output was detected with a 1 GHz bandwidth photodetector and analyzed with a 500 MHz oscilloscope. The beating between both fields is centered at frequency DFB − LO and has the same spectrum as the laser under test, provided that ⌬LO Ⰶ ⌬DFB [7]. In the example of Fig. 6 the DFB FL frequency rate was 10 kHz and pumped with 80 mW, whereas the LO wavelength was selected close enough to the DFB FL emission wavelength. Figure 6 shows the spectrum of the beat signals, resulting in a DFB FL linewidth of ⌬DFB = 6 MHz (i.e., 43 fm at DFB), measured directly in the spectrum at −3 dB. The inset in Fig. 6 shows the characteristic heterodyne beating. The optical pulses from the DFB FL had a temporal width of 80 ns (see Fig. 5), so according to the time-frequency uncertainty principle [8], its bandwidth cannot be lower than 5.5 MHz. From the comparison between this last value and the linewidth measurement for ⌬DFB, we conclude that the optical pulses of our DFB FL are transform limited. Brillouin as well as coherent scattering-based fiber optic sensors and instruments have a great potential and show increasing scientific and technological uses [8–10]. The light source employed by most of these applications includes an external-cavity diode laser plus an amplitude modulator and an optical power amplifier [10]. Here we propose the use of a relatively simple Q-switched DFB FL as an appropriate source for such applications. Figure 7 (solid curve) shows the backscattered light spectrum after illuminating a 10.5 km length optical fiber spool (Corning SMF-28) with the backward output of our DFB FL (4 kHz repetition rate and 74 mW pump power). The Brillouin spectrum was registered with an optical spectral analyzer (Walics HR 12, NetTest, with a resolution of 20 pm). The extreme of the fiber optic spool was terminated with a matching refractive index liquid 共n = 1.46兲. The central (highest) peak corresponds to the proper laser beam reflections after successive connections and splices together with Rayleigh scattering. Peaks symmetrically positioned at both sides correspond to the Brillouin backscattering by Stokes and anti-Stokes processes. The measured Brillouin shift
Fig. 6. Spectrum corresponding to the heterodyning between the backward DFB FL output and a tunable narrow linewidth laser used as local oscillator. The inset shows the original beating signal in the time domain. The frequency rate was 10 kHz, and the pump was of 80 mW.
Fig. 7. Brillouin spectra at room temperature for 4 kHz repetition rate and two values of pump power 关74 mW (solid curve) and 120 mW (dotted curve)], after backscattering in a 10.5 km length SMF-28 fiber.
results in 88 pm (i.e., 11.24 GHz at 1532.4 nm), which corresponds to the expected value in this fiber [8]. Finally, increasing the pump power up to 120 mW raises the Stokes peak respect to the antiStokes peak [see Fig. 7 dotted curve)], which agrees with previous reported results [10]. In conclusion, we have reported a transformlimited actively Q-switched DFB FL using a novel inline acoustic pulse generator based on a magnetostrictive device. Both outputs of the laser are available and transform-limited optical pulses of up to 800 mW peak power and 32 ns pulse width are generated at variable repetition rates in the kilohertz range. Narrow linewidths as low as 6 MHz were measured at a 10 kHz repetition rate. This work has been financially supported by the Ministerio de Educación y Ciencia of Spain (TEC 2005-07331-CO2-01). C. Cuadrado-Laborde acknowledges the Secretaría de Estado de Universidades e Investigación del Ministerio de Investigación y Ciencia (Spain) and Facultad de Ingeniería, Universidad Nacional de La Plata (UNLP, Argentina). References 1. J. Kringlebotn, J. Archambault, L. Reekie, and D. Payne, Opt. Lett. 19, 2101 (1994). 2. W. H. Loh and R. I. Laming, Electron. Lett. 31, 1440 (1995). 3. C. Matos, P. Torres, L. Valente, W. Margulis, and R. Stubbe, J. Lightwave Technol. 19, 1206 (2001). 4. M. Delgado-Pinar, A. Díez, J. L. Cruz, and M. V. Andrés, Appl. Phys. Lett. 90, 171110 (2007). 5. P. Pérez-Millán, J. L. Cruz, and M. V. Andrés, Appl. Phys. Lett. 87, 011104 (2005). 6. J. L. Cruz, A. Díez, M. V. Andrés, A. Segura, B. Ortega, and L. Dong, Electron. Lett. 33, 235 (1997). 7. A. Galtarossa, E. Nava, and G. Valentini, in SingleMode Optical Fiber Measurement: Characterization and Sensing, G. Cancellieri, ed. (Artech, 1993). 8. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001). 9. Y. Li, F. Zhang, and T. Yoshino, J. Lightwave Technol. 21, 1663 (2003). 10. T. R. Parker, M. Farhadiroushan, R. Feced, V. A. Handerek, and A. J. Rogers, IEEE J. Quantum Electron. 34, 645 (1998).
