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We report a 10 GHz harmonically and regeneratively mode-locked Yb fiber laser with a phase-locked loop (PLL) technique at 1.1 μm. Stable mode locking was achieved by optimizing the average dispersion of the fiber cavity to an anomalous dispersion to operate as a soliton laser. As a result, a 1.1 ps optical pulse with a timing jitter of 140 fs was successfully generated.
©2011 Optical Society of America
1. Introduction
To meet the growing demand for high capacity optical networks, it has become increasingly important to exploit a new transmission wavelength other than the 1.55 and 1.3 μm bands. The 1.1 μm region is particularly attractive because a wideband ytterbium (Yb)-doped fiber amplifier (YDFA) [1] is available. There have been several reports on 10-40 Gbit/s single-channel and WDM transmissions at 1.1 μm using photonic crystal fiber (PCF) or hole-assisted fiber (HAF) as the transmission medium [2–7]. These fibers offer single-mode, low-dispersion operation even at short wavelengths where single-mode operation is generally difficult with conventional step-index fiber (SIF). Approaches using several different light sources have been employed to generate data signals at 1.1 μm. These approaches include the external modulation of a DFB fiber laser [2,3] or an InGaAs/GaAs mode-locked semiconductor laser [5], a quantum-dot frequency comb laser [6], and the direct modulation of InGaAs VCSEL [7].
With a view to increasing the bit rate beyond 40 Gbit/s, for example by using optical time division multiplexing (OTDM), it is important to generate stable picosecond optical pulses at a high repetition rate. An actively mode-locked Yb fiber laser (Yb MLFL) was first reported as such a pulse source in [8], where a 10 GHz, 2 ps pulse was obtained with harmonic FM mode locking. An Yb MLFL with an all-fiber cavity configuration is demonstrated in [9], in which the repetition rate is stabilized to an external signal with a phase-locked loop (PLL).
In this paper, we present a 10 GHz Yb MLFL that employs a harmonic and regenerative mode-locking technique [10] to realize a picosecond optical pulse with low jitter and long-term stability. By designing the cavity configuration to operate as a soliton laser, we successfully generated a 1.1 ps optical pulse with a timing jitter as low as 140 fs. The pulse width was greatly reduced from 2.5 ps in the previous report [11] by optimizing the fiber cavity dispersion.
2. Configuration of mode-locked Yb fiber laser
Figure 1 shows the configuration of a harmonically and regeneratively Yb MLFL. The fiber ring cavity consists of a 0.98 μm laser diode (LD) as a pumping source, a WDM coupler, an Yb-doped fiber (YDF), a 50% output coupler, an isolator, a LiNbO3 (LN) intensity modulator, a 3 nm optical bandpass filter, and a PCF. We used a 0.9 m long YDF as a gain medium whose absorption was about 240 dB/m, and we obtained a gain of more than 20 dB. All the fibers are polarization maintaining to keep the TE-polarization in the cavity. The total fiber length in the cavity and the longitudinal mode spacing were 122 m and 1.7 MHz, respectively. The total loss in the cavity was 16.6 dB. Part of the optical output was led to a 10 GHz clock extraction circuit, and a 10 GHz harmonic beat signal between the longitudinal modes was obtained, which was fed back to the LN intensity modulator. This regenerative feedback loop makes long-term stable operation possible because the clock signal always follows the cavity length change [10]. Moreover, the repetition rate was stabilized to the external clock from a synthesizer with a PLL technique [12]. The error signal between the repetition rate and the synthesizer was fed back to the fiber cavity, in which the cavity length could be tuned with a piezoelectric transducer (PZT) by applying a voltage signal.
We optimized the average dispersion of the fiber cavity to an anomalous dispersion so that the laser operated as a soliton laser. Since it is difficult to realize an anomalous dispersion in the 1.1 μm band with a conventional SIF, we inserted a polarization-maintaining (PM) PCF with an anomalous dispersion as shown in Fig. 2 and Table 1 . The PCF had 90 holes and the dispersion was + 8.7 ps/nm/km at 1067 nm, which we measured by using the time-of-flight method [13]. We can also use this PCF as a nonlinear medium for spectrum broadening byself phase modulation (SPM). Because of the two large air holes in the vicinity of the silica core, the mode field was ellipsoid with diameters of 3.1 and 6.35 μm along the slow and fast axes, respectively. We spliced the PCF and SIF using a fusion splicer to obtain a polarization extinction ratio of more than 30 dB. However, the insertion loss including the splice loss was 4 dB due to the mode mismatch between the PCF and the SIF. Figure 3 shows the measured average dispersion of the fiber cavity when a 107 m-long PCF was inserted. The average dispersion was + 0.2 ~ + 3 ps/nm/km in the 1055 ~1070 nm wavelength range.
Table 1. Characteristics of PM-PCF
3. Output characteristics of Yb MLFL
Figure 4(a) and 4(b) show the output power, pulse width and time-bandwidth product (TBP) against the pump power. Here, the operating wavelength was 1067 nm, and the average dispersion in the cavity was +2.7 ps/nm/km. The threshold pump power was 90 mW, and a maximum output power of 63 mW was obtained at a pump power of 450 mW. As shown in Fig. 4(b), when the pump power increased, the pulse width decreased and a minimum pulse width of 1.1 ps was obtained with a pump power of 450 mW. On the other hand, the TBP increased at a higher pump power, which was caused by the nonlinear chirp induced at the PCF in the cavity. This nonlinear chirp could not be compensated for completely with linear chirp compensation. Figure 5(a) and 5(b) show the optical spectrum and the autocorrelation waveform, respectively, at a pump power of 300 mW, where the TBP was minimum. The optical spectrum provides a good fit with the sech profile, whose spectral width was 0.85 nm (224.2 GHz). A 1.6 ps pulse was obtained directly from the MLFL, and the pulse width was slightly reduced to 1.5 ps after external chirp compensation with a dispersion of 0.6 ps/nm as shown in Fig. 5(b). The TBP was 0.336, which indicates that the output pulse was a nearly transform limited sech pulse. Figure 6(a) and 6(b) show the electrical spectrum of the 10 GHz clock signal and single sideband (SSB) phase noise spectrum from 10 Hz to 1 MHz, respectively. There was only one clock component at 10 GHz and the supermode noise was suppressed by more than 70 dB, which was realized with the combination of SPM and spectral filtering [14]. We estimated the timing jitter by integrating the SSB phase noise curve from 10 Hz to 1 MHz in Fig. 6(b), and the timing jitter of 140 fs was obtained.
Fig. 5 (a) Optical spectrum and (b) autocorrelation waveform of the laser output pulse (wavelength: 1067 nm, pump power: 300 mW).
Fig. 6 (a) 10 GHz electrical clock signal and (b) SSB phase noise spectrum (wavelength: 1067 nm, pump power: 300 mW).
We also measured the wavelength dependence of the output characteristics within the tuning range of the bandpass filter. The pulse width and TBP as a function of wavelength are shown in Fig. 7 . Here, the pump power was fixed at 300 mW. A pulse width of about 1.5 ps was obtained over a 10 nm regime. However, the TBP increased at wavelengths below 1061nm, and the laser operation was unstable. This was caused by spectral broadening induced by self-phase modulation (SPM) in the PCF, which became large because the average dispersion was close to zero. On the other hand, a stable pulse operation was obtained above 1063 nm.
To confirm that this Yb MLFL operated as a soliton laser, we compared the peak power of the pulse circulating in the cavity, Pp, with the peak power required for a fundamental soliton, Psoliton, which is given by [15,16]
where c is the velocity of light, γ is a nonlinear coefficient, Dave is the average dispersion of the laser cavity and τFWHM is the full width at half maximum (FWHM) of the pulse. By using the following values corresponding to Fig. 5, i.e., γ = 10 W−1km−1, Dave = 2.7 ps/nm/km, and τFWHM = 1.6 ps, Psoliton is calculated to be 198 mW. On the other hand, the peak power of the pulse in the cavity was 313 mW, which is 1.6 times higher than Psoliton. This additional power is attributed to the dispersion management in the cavity. However, since the dispersion management is relatively weak in this cavity configuration, the pulse behaves as an average soliton, which explains the generation of a nearly transform-limited sech pulse from the MLFL as shown in Fig. 5.In addition, we carried out a numerical simulation of the transient waveform evolution and steady-state pulse propagation in the laser cavity. The nonlinear wave propagation in the fiber is described by the following nonlinear Schrödinger equation:
Here, u(z, t) is the complex amplitude of the pulse, d(z) is the dispersion, and γ is a nonlinear coefficient. The signal gain of the YDF for use in the calculation is given by
where go is the small-signal gain, P is the input power and Ps is the saturation power, and the gain bandwidth was 40 nm. Figure 8 shows the dispersion map of the fiber cavity and the numerical result of the variation of the pulse width under steady state propagation. The pulse width variation within a period is relatively small in spite of the large dispersion variation, which confirms the average soliton operation. Figure 9 shows the calculated pulse waveform and the optical spectrum in a steady state, respectively, with a pump power of 300 mW. A 1.5 ps optical pulse with a TBP of 0.345 was obtained, which was very close to the experimental result shown in Fig. 5.Fig. 8 Dispersion map of the fiber laser cavity (blue line) and the numerical result of the change in the pulse width (red line).
4. Conclusion
We have successfully demonstrated a 10 GHz harmonically and regeneratively Yb MLFL with a low jitter and long-term stability. A 1.5 ps, nearly transform limited sech pulse was generated from the MLFL, and a minimum pulse width of 1.1 ps was obtained. This Yb MLFL is expected to provide an attractive light source for ultrahigh-speed OTDM transmission in the 1.1 μm band as well as for supercontinuum generation and optical metrology.
References and links
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