STu3P.4.pdf
CLEO:2016 © OSA 2016
Simplified all-polarization maintaining fiber laser modelocked in the all-normal dispersion regime Weijian Ni 1, Bo Xu1,2, Amos Martinez3 , Sze.Yun. Set1 , Shinji Yamashita1* 1
Research Center of Advanced Science and Technology, The University of Tokyo,4-6-1 Komaba, Meguro-ku, Tokyo, 153-8904, Japan Department of Engineering Science, Graduate School of Informatics, The University of Electro-Communications (UEC), 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan 3 Aston Institute of Photonic Technologies, Aston University, UK. *E-mail address:
[email protected]
2
Abstract: We report a simplified, all-normal dispersion, all polarization maintaining (PM) fiber laser operating at 1030 nm and all-PM fiber pulse compressor capable of generating 700 fs pulses with 0.45 nJ pulse energy. OCIS codes: (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber; (190.4400) Nonlinear optics, materials.
1. Introduction Rapid advances in ultrafast fiber laser technologies have enabled their exploitation in applications in fields extending from optical imaging to pulsed laser machining. When we consider pulsed fiber lasers from the perspective of their commercialization, robustness against environmental fluctuations and turnkey operation are key requirements. In order to ensure the stability of operation, attention is shifting towards mode-locking mechanisms not relaying on free-space optics or moving components and capable of all-PM operating such as nonlinear amplifying loop mirror (NALM) configuration [1-5]. Here, we propose a simple, compact, all-PM fiber, all-normal dispersion (ANDi) pulsed fiber laser emitting at 1030 nm, using a single pump laser diode. The pulse is then compressed using an all-PM fiber compressor to generate 0.4 nJ pulses with 700 fs pulse duration. The compactness and simplicity of operation of this laser makes it suitable for a range of operations outside of the lab extending from optical coherence tomography (OCT) and laser imaging to laser machining. 2. Experimental setup The proposed configuration of the all-PM Ytterbium (Yb) fiber oscillator is illustrated in Fig. 1(a). In this work, the mode-locking element is the NALM which is the left loop of the cavity. A 976 nm diode laser was used to forward pump a lowly-doped Yb fiber (core absorption: 80 dB/m at 975 nm, PM-YSF-LO, Nufern) through a PMwavelength division multiplexer (WDM) 980/1030. A 50/50 splitter coupled the NALM loop to an unidirectional main loop that contained a 80/20 splitter, with 20% as the output of the laser and an optical isolator (ISO) responsible for the unidirectional laser operation. An optical bandpass filter with 2 nm spectral half-width was incorporated to reduce the large positive-chirp and spectral bandwidth of the pulses which are relaunched back into NALM. Additionally, an in-line polarizer was employed to improve the polarization extinction ration as well as enhancing mode-locking stability. For the all-PM fiber compressor, the highly positive chirped pulses directly were compressed with chirped fiber Bragg grating [CFBG, reflection bandwidth of ~20 nm (at 3 dB); chromatic dispersion of 2.85 ps/nm, centered at 1030 nm] through a circulator. The sacrificed power due to the circulator and CFBG was compensated back utilizing the home-made ytterbium doped fiber amplifier (YDFA). The distortion of spectrum was disposed away by carefully adjusting the pump current of YDFA and the length of PM 980 fibers after the YDFA. 3. Experimental results This laser was pumped only with one 976 nm laser diode with a maximum output power of 500 mW. Self-started mode-locking was achieved at pump powers above 230 mW. Stable pulse operation was reliably sustained under a wide tuning range of pump diode driven currents (from 100 mW to 300 mW) once the mode-locking was triggered. The output spectrum of the oscillator was observed as shown in the black trace of Fig. 1(b), exhibiting the typical characteristic features of the pulses generated in highly positive dispersion regime: steep sides. At the pump level of 230 mW, the full-width-half-maximum (FWHM) of the output optical spectrum was~ 3.3 nm centering at 1030 nm, the average output power was about 5mW corresponding to a pulse energy of 0.45 nJ. The laser operates at its fundamental repetition rate at a frequency of 12 MHz with a signal-to-noise ratio (SNR) of approximately 90 dB for the fundamental RF signal and 70 dB for the 20th order RF signal, both measured with a span of 5 kHz, and resolution bandwidth of 30 Hz (Fig. 1 (c)). Meanwhile, single-pulse operation was verified (Fig. 1(d)). Since the pulse directly generated from the oscillator has very strongly and linearly positive chirp properties, the all-PM fiber type compressor was realized subsequently. As the three traces shown in Fig. 1(b), the average power,
STu3P.4.pdf
CLEO:2016 © OSA 2016
3 dB bandwidth and shapes of spectrum were perfectly kept at every monitoring position during the whole compressing, amplifying and re-compressing processes. The pulse duration and chirped properties were measured by an optical analyzer (HR100 Pulse Analyzer, Southern Photonics), and the pre-compression pulse duration was estimated to be approximately 2.62 ps (inset of Fig.1(e)). Finally, the Fourier transform limited pulse with ~700 fs pulse duration was achieved when the chromatic dispersion of CFBG and the length of extra-PM 980 fiber were carefully selected and optimized, Fig.1(e).
Output Power (µW)
80 (b)
60
Output spectrum after oscillator after circulator after extra-cavity PM 980 fibers
40 20 0 1020
1025 1030 1035 Wavelength (nm)
1040
Fig.1: (a) All-PM figure of eight Yb fiber oscillator and compressor. (b) Output spectrum at the different position. (c) Fundamental RF signal (inset: 20th RF signal); (d) Pulse train. (e) Autocorrelator trace after compression (Inset: autocorrelator trace and chirp line of pre-compression)
4. Conclusion A simplified, compact, all-PM fiber, all-normal dispersion ultrafast fiber laser operating at 1030 nm was developed only using a single pump laser diode. Moreover, the pulse is subsequently compressed using an all PM fiber compressor, generating 700 fs pulses with 0.45 nJ pulse energy corresponding to its relatively low fundamental repetition rate of 12 MHz. Since the mode-locker of NALM provide the additional degree of freedom for manipulating the pulse characteristics, higher pulse energy could be expected by carefully optimizing the cavity design, such as the length of PM 980 fiber and the YDF. Simple and compact all-PM fiber systems such as the one presented here provide a good solution for practical uses out of the lab and commercial products. Acknowledgements Amos Martinez acknowledges funding from the H2020 Marie-Sklodowska-Curie Individual Fellowship scheme. Note The co-author of Bo Xu is now working at the second affiliation of UEC, but the experiment was done when Bo Xu worked in the first affiliation. 5. References [1] Aguergaray C, et al., Opt. Express, 20, 10545,(2012) [2] Erkintalo M, et al., Opt. Express,20, 22669, (2012) [3] B. Xu, et al., Laser Physics Letters, 11(2), 025101, (2014). [4] B. Xu, et al., Photonics Technology Letters, IEEE , 26, 2(2014). [5] Patrick B, et al., Optics Communications , 364, (2016).
