980-nm Picosecond Fiber Laser

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 11, NOVEMBER 2003 1519 980-nm Picosecond Fiber Laser O. G. Okhotnikov, L. A. Gomes, N. Xiang, T. Jouhti, A. K. Chin, R. Singh, and A. B. Grudinin Abstract—A mode-locked Yb3+ fiber laser operating at 980 nm is reported. Using a semiconductor saturable-absorber mirror in a laser incorporating a grating-pair dispersive delay line, we obtain reliable self-starting 1.6-ps pulse operation. Index Terms—Fiber laser, mode-locked lasers, quantum wells, semiconductor devices, short pulse generation. F IBER LASERS and fiber amplifiers have commercialapplications in telecommunications. The recent, unprece- dented growth of communications technology has resulted in many cost-effective and reliable devices as well as a better understanding of their physical mechanisms. As a result, the performance of fiber lasers, e.g., output power and pulse energy, have steadily increased. There are recent reports of fiber lasers with several kilowatts of output power and pulse energies as high as several millijoules [1]. The need for compact ultrashort lasers at short wavelengths ( 1 m) makes fiber systems an attractive solution. Contin- uous-wave (CW) tunable operation and mode-locked operation for the three-level transition of a Nd : glass at 910-nm band were reported [2], [3]. Ytterbium (Yb)-doped fiber lasers and fiber amplifiers can operate in the spectral region from 970 to 1150 nm [4]. In this broad wavelength range, there are a number of applications ranging from micromachining at fundamental wavelengths to biomedical applications at frequency-doubled wavelengths. In this letter, we present detailed results on a mode-locked fiber laser operating in the 980-nm spectral band. The 980-nm band is attractive not only because frequency doubling of a master source operating at this wavelength would generate 488 nm, a wavelength widely used in a number of applications, but also because, owing to a high emission cross section at 980 nm, one can achieve very high gain in a short length of optical fiber and, thus, avoid unwanted nonlinear effects. Addi- tionally, this 980-nm mode-locked fiber laser complements the spectral region addressed by mode-locked Ti : sapphire lasers. The schematic of the mode-locked Yb-fiber laser is shown in Fig. 1. The optical pump source is a 915-nm single-mode pig- tailed laser diode capable of delivering up to 220 mW of op- tical power. The fiber laser was pumped through a 915/980-nm fused-fiber wavelength-division-multiplexing coupler. Manuscript received April 8, 2003; revised June 25, 2003. O. G. Okhotnikov, L. A. Gomes, N. Xiang, and T. Jouhti are with the Op- toelectronics Research Centre, Tampere University of Technology, FIN-33101 Tampere, Finland (e-mail: Oleg.Okhotnikov@orc.tut.fi). A. K. Chin is with the Axcel Photonics Inc., Marlborough, MA 01752 USA. R. Singh is with the Matsushita Kotobuki Electronics Peripherals of America, Inc., Shrewsbury, MA 01545 USA. A. B. Grudinin is with the NewOptics Ltd., Southampton SO31 4RA, U.K. Digital Object Identifier 10.1109/LPT.2003.818645 Fig. 1. Cavity configuration for an Yb-fiber laser. (a) (b) Fig. 2. (a) Output power versus absorbed pump power. (b) Spectrum of free-running laser. The doped fiber used in the experiments was an alumo–sil- icate fiber with dopant level of 10 000 ppm (by weight). The fiber was produced by the solution doping technique and had an NA and cut off at 910 nm. The fiber laser was first eval- uated in a CW regime when it was lasing from cleaved ends. With a doped-fiber length of 40 cm the laser was operating at 980 nm without any intracavity wavelength-selective ele- ments. As it can be seen from Fig. 2(a), the slope efficiency in CW regime was as high as 91%. 1041-1135/03$17.00 © 2003 IEEE 1520 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 11, NOVEMBER 2003 Fig. 3. Spectral dependence of relative intensity noise of a free-running 980-nm fiber laser. At 170 mW of launched pump power, the unabsorbed portion of the pump power was approximately 30 mW. For fiber lengths longer than 40 cm, the laser tended to operate at 1030 nm since 980-nm radiation was reabsorbed. It should be noted that unlike cladding pumped Yb-doped fiber lasers operating at 980 nm, where unwanted gain at 1040 nm causes significant problems [5], in our core-pumped case, 980-nm lasing was achieved rel- atively easy as long as pump power was high enough to create population inversion at far end of the doped fiber. The spectrum of this free-running laser, shown in Fig. 2(b), is typical for mul- timode operation where several longitudinal modes lase simul- taneously. Yb-doped fiber lasers generally have poor noise characteris- tics, which limits their applications. The noise characteristics indicate a short photon lifetime, which makes self-starting mode locking particularly difficult. In the present Yb-doped fiber laser, particular attention was paid to the fiber fabrication in order to minimize self-pulsing effects which frequently occur in Yb-doped fiber lasers due to clustering of Yb-ions or structural defects in doped silica glass. Those effects may lead to not only low pump-to-signal conversion efficiency but also, in some cases, act as randomly distributed saturable absorbers with long recovery times. Such saturable absorbers cause self- -switching of the laser that leads to chaotic temporal output. As seen from Fig. 3, the relative intensity noise (RIN) up to 100 MHz of our free-running fiber laser is approximately 140 dB/Hz. The strong noise peaks at frequencies corre- sponding to the round-trip frequency in combination with low noise of the free running laser indicate the laser’s ability to self-start. The high value of material dispersion for silica at wavelengths below 1.1 m causes difficulties in producing self-starting mode-locked operation of silica-based fiber lasers. Waveguide dispersion can generally be used to compensate for the material dispersion at wavelengths longer than 1.3 m. In the region of 1 m, we employed bulk dispersion-compensating elements within the cavity to control the total dispersion of the cavity [6]–[8]. For a laser with a Fabry–Pérot cavity, a semiconductor (a) (b) Fig. 4. (a) Autocorrelation trace of generated pulses. (b) Measured optical spectrum (solid curve) when the distance between gratings is 6.5 cm. Dotted curve shows a Gaussian fit. saturable-absorber mirror (SESAM) was used to self-start the mode locking [9]. In our laser, the group-velocity dispersion of the fiber-laser cavity is compensated by the anomalous dispersion of the grating pair, as shown in Fig. 1. This dispersion compensator comprises of a gold-coated 1600-line/mm diffraction-grating pair. The laser-cavity mirrors consist of a high-reflectivity (HR) mirror and the SESAM structure. The reflectivity of the HR mirror was 97% at 980 nm and the output coupler had a transmission of 7% at 980 nm. The GaInNAs-based SESAM, operating in the 940–1050-nm wavelength range, comprises 26 pairs of AlAs and GaAs quarter-wave layers that form a distributed Bragg reflector with a center wavelength at 1000 nm. The SESAM, similar to the long-wavelength structure described in [10], was grown by solid-source molecular-beam epitaxy on an n-type GaAs (001) substrate. An antiresonant cavity is formed by the uncoated front surface of the epitaxial wafer and the highly reflecting AlAs–GaAs mirror stack [9]. The SESAM has a saturation fluence of 3 J cm and a modulation depth of . OKHOTNIKOV et al.: 980-nm PICOSECOND FIBER LASER 1521 Mode-locked operation was obtained for pump powers above 40 mW. With proper alignment of the laser cavity, the laser was self-starting for pump powers above 50 mW. Fig. 4 shows the autocorrelation and corresponding spectrum for the mode-locked pulse train at 980 nm with an output power of 3 mW, a repetition rate of 30 MHz, pulse duration of 2.3 ps re- sulting in a pulse energy of 0.1 nJ. Absorbed pump power in the mode locked regime was 25-30 mW and the laser threshold was around 20 mW. The measurements in Fig. 4 were performed for a grating separation of 6.5 cm, corresponding to a second-order dispersion of 1.67 ps (double pass). In Fig. 4(a), the mea- sured autocorrelation trace (solid curve) followed a Gaussian profile (dotted curve). The pulse spectrum, shown in Fig. 4(b), exhibits soliton sidebands at all pump powers, indicating that the laser operates in the anomalous-dispersion regime. The time-bandwidth product was equal to 0.47, indicating that the pulses were nearly bandwidth-limited with Gaussian temporal and spectral profiles. The average value of the cavity dispersion near 1 m, estimated from the soliton sidebands, was 1.6 ps . In conclusion, we have demonstrated a mode-locked Yb-fiber laser emitting picosecond pulses in the 980-nm spectral re- gion. Optimal matching of the reflection characteristics of a semiconductor absorber with a highly efficient Yb-doped fiber resulted in stable self-starting mode-locked operation with pulsewidths of 1.6 ps. The 91% slope efficiency of the free-running 980-nm fiber laser was close to the theoretical limit, although the mode-locked laser exhibits a slope efficiency approximately 50% lower. A master oscillator power amplifier configuration, where a power amplifier follows a mode-locked master source, is expected to produce output power in the region of 200 mW with just two single-mode pumps. Low pump-power requirements and high efficiency of Yb-doped fibers make 980-nm mode-locked fiber lasers a very attractive competitor to solid-state lasers. REFERENCES [1] J. Limpert, A. Liem, T. Schreiber, H. Zellmer, and A. Tuennermann, Proc. Photonics West 2003, p. 222. [2] R. Hofer, M. Hofer, G. A. Reider, M. Cernusca, and M. H. Ober, “Mod- elocking of a Nd-fiber laser at 920 nm,” Opt. Commun., vol. 140, pp. 242–244, 1997. [3] O. G. Okhotnikov and J. R. 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