1. Introduction

Among many promising ultra-high bit rate light sources, actively mode-locked Erbium-doped fiber lasers (ML-EDFLs) are very attractive for the generation of high power ultra-short optical pulses at multiple wavelengths (MWs) with multiple Gb/s repetition rate [1]. Compared with the compact, inhomogeneously gain broadened semiconductor-based multiwavelength laser source [2], EDFL is competitive for its all-fiber structure and higher pulse energy. Due to the homogeneously broadened gain property, many MW-ML EDFLs are developed with the wavelength spacing larger than homogenous linewidth (∼3.5 nm) to overcome the gain competition [3, 4]. However, the development of MW-EDFL with narrow channel spacing (100 GHz or 50 GHz) for telecommunications is still a great challenge. Recently, we developed a novel dual-wavelength ring MW-EDFL mode-locked at 10 GHz [5]. Instead of the complex temporal-multiplexing scheme [6], highly nonlinear fibers (HNLFs) are incorporated in the cavity to suppress the strong gain competition using inter-channel multiple four-wave mixing (FWM) process. In this paper, we further extend this novel structure to generate 4 wavelengths mode-locked at 10-GHz simultaneously with 0.8 nm wavelength spacing anchored on ITU-T grids. The fiber nonlinearity is also used to improve the pulse stability. Although the gain medium is shared in the nonpolarization-maintaining cavity, the excellent mode locking performance makes this configuration promising for generating economic high-speed MW-EDFL sources.

2. MW-ML-EDFL configuration

 

Fig. 1. System setup. TOF = tunable optical filter. CWDM is a coarse 1480/1550 nm WDM coupler.

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The setup of the MW-ML-EDFL is shown in Fig. 1. It has a ring configuration containing a LiNbO3 phase modulator driven by a 10 GHz RF synthesizer. The gain medium is a 7.66 m EDF (Er3+ concentration is 2000 ppm) pumped with a 150 mW 1480 nm laser diode. The MW operation is attained by using a pair of 4-channel dense wavelength-division-multiplexers (DWDMs) with 100 GHz spacing. The measured center wavelengths of DWDM are 1556.55, 1557.36, 1558.17, and 1558.98 nm, respectively, and their angular frequencies are assigned as ω₁, ω₂, ω₃, and ω₄. The 3-dB bandwidth is 0.58 nm and the extinction ratio is greater than 30 dB. Each channel is followed by an adjustable optical delay line (ODL), variable optical attenuator (VOA) and polarization controller (PC). ODLs are adjusted to ensure that different wavelength pulses experience the same effective cavity length. The outputs are monitored from a 10/90 fused coupler.

For a stable mode-locking performance, we add 1-km HNLF in the cavity to suppress cross-gain saturations using FWM. The zero-dispersion wavelength (ZDW) of the HNLF is 1559 nm and the dispersion slope is 0.023 ps/nm²/km. Thus the phase-matching condition for FWM is readily satisfied for DWDM channels that are quite close to ZDW. The HNLF has a nonlinear coefficient of 10 W^-1km^-1. The typical loss is less than 0.75 dB/km from 1550 to 1650 nm and the cut-off wavelength is below than 1290 nm.

3. Experimental results and discussions

 

Fig. 2. Optical spectrum of 4-channel EDFL output. (a): CW operation, (b): Mode-locked at 10 GHz..

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We first implement the continuous-wave (CW) operation of the ring EDFL. As shown in Fig. 2(a), 4 channels appear simultaneously by tuning VOAs and PCs appropriately. The lasing spectra are broadened due to the parametric process and two sidebands are stimulated by the phase-matched FWM. With HNLFs, the fundamental repetition rate of the ring laser is around 195 KHz. The laser is then harmonically mode-locked at 9.998 GHz with four-channel output. The optical spectrum of the total 4-wavelength mode-locked output is shown in Fig. 2(b) (Resolution is 0.01 nm). The 10 GHz frequency tones are clearly shown.

 

Fig. 3. Simultaneously output pulse trains of 4 channels.

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Figure 3 demonstrates corresponding time-domain 10 GHz pulse waveforms of four wavelengths that are recorded by a 45 GHz photodetector and displayed on a 50 GHz sampling oscilloscope. Stable pulse trains can be obtained. It is clear that 4 wavelengths well within the homogenous linewidth of EDF can be harmonically mode-locked successfully although pulses overlap during the amplification. No pulse dropouts are observed. The autocorrelation traces of pulses are measured and the pulse widths vary from 17.2 ps to 18.7 ps for 4 wavelengths. The time-bandwidth products are calculated as 0.39 ∼ 0.41, which represent nearly transform limited pulses.

We also measured the stability (timing jitter and amplitude fluctuation characteristics) of this laser. All wavelengths appear to have similar noise performances. The measured RF power spectrum of the first order (10 GHz) harmonic of 1558.17 nm output is shown in Fig. 4. As seen in Fig. 4(a), the supermode noise suppression is more than 50 dB over 1 MHz span and corresponds to a very stable output pulse train. According to the Von der Linde method [7], the amplitude fluctuation is calculated as 0.45% (from Fig. 4(a), span 1MHz and RBW 1 KHz) and the timing jitter is about 9.95×10^-4 (from Fig. 4(b), span 5 KHz and RBW 30 Hz). Since the pulse interval is 100 ps, the timing jitter of pulse train is approximately 100 fs. The same results can be obtained with second order (20 GHz) power spectrum.

 

Fig. 4. RF power spectrum of 1558.17 nm output.

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We have achieved stable 4-wavelength pulse trains and we believe that the gain clamping effect in EDF are compensated by the multiple FWM between four channels. The physical insights can be explained as follows. Since all wavelengths experience amplification in EDF, due to gain competitions, one wavelength (let say ω₂) is dominant and its power is greater than others. However, during the propagation along HNLFs, two degenerative-FWM processes take place between 4 wavelengths (ω₁=2ω₂-ω₃ and ω₄=2ω₃-ω₂). At the same time, a single nondegenerate FWM process occurs between four wavelengths (ω₂+ω₃= ω₁+ω₄). As discussed and proved in Ref. [8], the multiple FWM processes hold an important unique property of self-stability for EDFL. Although the two degenerate FWM processes transfer energy from two central pumps to two sideband waves, there exists a power transfer between two pump waves that is different from the conventional coupled-mode models of nondegenerate FWM. Due to the power conservation, the power exchange between two pumps (ω₂ and ω₃) is three times more than that between two sidebands (ω₁ and ω₄). The energy flow transfers from higher-power wave ω₂ to lower-power wave ω₃ compensate the EDF gain competition in a self-driven manner. The same interactions exist between other wavelength-pairs (ω₁ and ω₂, ω₃ and ω₄). Two more sideband ω₅ and ω₆ [as shown in Fig. 2(a)] are generated and involved in multiple FWM processes and lead to power transfer from high power channels to low power wavelengths. The power transfer will continue during the lightwave propagation in HNLF and result in a stable multiwavelength operation. Unlike the temporal/polarization multiplexing technique, four wavelengths are correlated due to phase matching conditions. The advantage is that four wavelengths can be generated synchronously without careful and tedious operation for each wavelength. They can be further handled independently outside the cavity.

Also, the use of HNLF and the DWDM spectral filtering are crucial to the output pulse stability. The 10-GHz frequency combs generated through actively ML are close to the ZDW of HNLF hence intra-pulse FWM among these longitudinal modes occurs. New sidebands with 10 GHz spacing appear and all tones are phase-locked under nearly phase-matching conditions to achieve a stable mode locking performance [9]. Furthermore, our proposed scheme will be suitable for more wavelength generation by choosing proper pump power, comb filter and fiber dispersion. The compact, highly nonlinear microstructured fiber will be employed to reduce the cavity length.

4. Conclusions

In summary, a novel self-stable MW 10-GHz mode-locked EDFL with 0.8 nm wavelength spacing has been successfully demonstrated. 4-channel (1556.55, 1557.36, 1558.17, and 1558.98 nm) pulses can be obtained simultaneously and synchronously. No pulse dropouts were observed. The gain competition in homogenously broadened EDF is eliminated by multiple FWM processes occurring in HNLFs. Also, the fiber nonlinearity plays an important role to stabilize the output pulse fluctuation. More than 50 dB supermode noise suppression ratio can be achieved. Therefore, with proper combination of pump power, fiber nonlinearity and dispersion map, our scheme is promising for the development of high-speed, dense channel spacing, multiwavelength fiber laser systems with superior performance.