a2064_1.pdf CMR4.pdf
Laser-mode Dynamics Measurement and Control of Modelocked Er-fiber Lasers Yohei Kobayashi, Dai Yoshitomi, Youichi Sakakibara, Hiromichi Kataura, Hideyuki Takada, Masayuki Kakehata and Kenji Torizuka National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan Tel: +81-29-861-5073, fax: +81-29-861-3349, e-mail:
[email protected]
Taketo Onuma and Hideki Yokoi Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548 Japan
Takuro Sekiguchi and Shinki Nakamura Ibaraki University, 4-12-1, Nakanarusawa, Hitachi 316-8511, Japan
Abstract: Dynamics of laser mode of femtosecond Er-fiber lasers were investigated by using a beat signal between two mode-locked lasers. The beat linewidth was controlled to 8 mHz. ©2007 Optical Society of America
OCIS codes: (120.5050) Phase measurement, (120.3930) Metrological instrumentation
Introduction Femtosecond frequency comb has become a promising tool to measure an optical frequency or to make an optical atomic clock. Control of femtosecond frequency comb is also a key technology for coherent addition of independent lasers. Recently details of comb noise were studied, and narrow comb lines were realized by improving the feedback bandwidth in Er-fiber laser oscillators [1-4]. However, the origin of relatively broad linewidth of carrier-envelope offset frequency (fceo) in a fiber laser is not fully understood yet because of complicated correlation of mode spacing and fceo move in a cavity. Furthermore, the fceo measurement by use of self-referencing technique includes the amplitude-to-phase noise conversion. In this report, we have investigated the fceo dynamics of mode-locked fiber laser itself directly. We made two Er-doped mode-locked fiber lasers, and their pulse timings were passively locked in a precision of ~3 fs [5]. The longitudinal-mode beat of two lasers was observed by superimposing them. We found the fceo fluctuation in the fiber laser cavity is quiet. The beat linewidth can be suppressed to 8 mHz by feedback. Experiment and result Figure 1 shows a schematic of the experimental setup. Femtosecond mode-locked Cr:forsterite laser (wavelength: 1.3 μm) works as a master laser. The master pulse train is divided into two beams, and injected into Er-doped fiber lasers through 1.3/1.5 μm wavelength division multiplexing mixers. A four-wave mixing process among 1.3 and 1.5 μm pulses inside the fiber locks the pulse timings. EDF LD PC
PMT
jitter measurement
BBO PC
SG
1. 3/1.5 WDM
PLL electronics
OC
Mode-locked Cr:forsterite laser
50% coupler HM
Saturable absorber
OC
Pin PD waveplate Pol
1.3/1.5 WDM
Spectrum analyzer EDF LD
Fig. 1. Experimental setup. HM; beam splitter, PD; photo diode, WDM; wavelength division multiplexer, EDF; Er-doped fiber, OC; 10-% output coupler, Pol; polarizer, PC; polarization controller including a polarizer, SG; signal generator, LD; 980-nm laser diode for Er ion pumping.
a2064_1.pdf CMR4.pdf
We have made two types of Er-fiber lasers. Fiber laser 1 is a nonlinear polarization-rotation mode-locked laser, and fiber laser 2 is a mode-locked laser with a saturable absorber made by carbon nanotubes. Both pulse timings are locked to the master laser, and two outputs are superimposed in time and space. The hetelodyne beat between two fiber lasers is observed by spectrum analyzers. The mode spacings of two fiber laser combs are exactly the same because of the tight timing locking. Then only the difference of the fceo frequency can be observed. Figure 2(a) shows the beat signal in free running with a resolution bandwidth (RBW) of 10 kHz. The linewidth of the beat signal is RBW, and there is no tail. Figure 2(b) shows relative fceo beat dynamics. The beat signal fluctuates relatively slowly (< 200 kHz/ms) even when compared with that in a solid-state laser. This implies that the comb fluctuation due to fceo motion is not large in both types of fiber lasers. 5.0
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Fig. 2. (a): Relative fceo beat in free running measured by an analogue spectrum analyzer, (b): Beat dynamics measured by a digital spectrum analyzer.
The beat signal is locked to a reference frequency by an electronic feedback system. The error signal drives the laser diode current of the fiber laser 1. The locked beat signal is shown in figure 3. The beat linewidth is suppressed into 8 mHz, whereas the 3-dB bandwidth of the fiber laser response is 2 or 3 kHz. This shows the feasibility of laser linewidth narrowing in both lasers into mHz region. The integrated phase error from 25 mHz to 1 MHz is 1.55 rad.. The tight phase locking with narrow bandwidth would be coming from the small fluctuation of fceo beat due to the suppression of an accordion mode by passive timing synchronization.
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Fig. 3. Locked beat note. The span is 1 Hz, and the beat linewidth is 8 mHz. Inset shows an RF signal with 1-MHz span.
References [1] J. McFerran, W. Swann, B. Washburn, and N. Newbury, “Elimination of pump-induced frequency jitter on fiber-laser frequency combs,” Opt. Lett. 31, 1997 (2006) [2] W. Swann, J. McFerran, I. Coddington, N. Newbury, I. Hartl, M. Fermann, P. Westbrook, J. Nicholson, K. Feder, C. Langrock, and M. Fejer, “Fiber-laser frequency combs with subhertz relative linewidths,” Opt. Lett. 31, 3046 (2006) [3] E. Benkler, H. Telle, A. Zach, and F. Tauser, “Circumvention of noise contributions in fiber laser based frequency combs,” Opt. Exp. 13, 5662 (2005) [4] I. Hartl, G. Imeshev, M. Fermann, C. Langrock, and M. Fejer, “Integrated self-referenced frequency-comb laser based on a combination of fiber and waveguide technology,” Opt. Exp. 13. 6490 (2005) [5] D. Yoshitomi, Y. Kobayashi, M. Kakehata, H. Takada, K. Torizuka, T. Onuma, H. Yokoi, T. Sekiguchi, and S. Nakamura, “Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train,” Opt. Lett. 31, 3243 (2006)
