1. Introduction

Multiwavelength mode locked fiber laser sources can find applications such as WDM communication systems, fiber-optic sensors and optical instrumentation, thanks to the numerous advantages including the generation of narrow pulse trains at very high repetition rates at multiple wavelengths, high power, low noise and the compatibility with other fiberoptic components. Different mode-locked multiwavelength fiber lasers have been reported in the last few years. Li et al. [1] reported a mode locked multiwavelength fiber laser using a self-seeded semiconductor Fabry-Perot laser diode with a fiber external cavity incorporating fiber Bragg gratings (FBGs). A multiwavelength mode locked lasing up to 3 wavelengths were realized. To obtain more mode-locked lasing wavelengths, we proposed an actively mode-locked multiwavelength fiber ring laser using a sampled fiber Bragg grating (SFBG) [2]. Since the SFBG has intrinsically identical round-trip time for all the wavelengths, active mode locking of up to 8 wavelengths using an external Mach-Zehnder intensity modulator was realized. To achieve stable multiwavelength mode locking, Hayashi et al. [3] demonstrated recently an actively mode locked fiber laser using polarization-maintaining devices; a stable 13-wavelength mode locked lasing was realized. However, because of the strong homogeneous line broadening of the erbium-doped fiber (EDF) at room temperature, the fiber lasers demonstrated in [2, 3] were cooled in liquid Nitrogen, which makes the system bulky and impractical for real applications. To suppress homogeneous line broadening at room temperature, Bellemare et al. [4] proposed to use a frequency shifter in a cw fiber lasers to prevent steady-state lasing; the effect corresponds to a reduction of homogeneous line broadening. Stable multiwavelength lasing at room temperature was demonstrated. A similar approach was recently proposed by Zhou et al. [5], where the frequency shifter was replaced by a phase modulator. The inclusion of a frequency or phase shifter in the laser cavity would lead to the laser to have a variable cavity length, determined by the modulation signal applied to the frequency or phase shifter. For active mode locking, the round-trip distance or time must be a constant. Therefore, the approach using a frequency or phase shifter may not be suitable for active mode locking. In addition, the frequency or phase shifter in the laser cavity introduces high insertion loss.

For multiwavelength actively mode locked fiber lasers with a high repetition rate, another problem that limits the performance is the high noise generated by the relaxation oscillation and beating between the supermodes due to the long lifetime of the upper-state erbium ions. By using an intra-cavity Fabry-Perot filter or a nonlinear polarization rotator [6, 7], the supermode noise can be suppressed, but at the cost of high insertion loss. It is known that semiconductor materials have weaker homogeneous line broadening. It was demonstrated that the use of semiconductor materials as the gain medium can provide stable multiwavelength lasing at room temperature [8, 9]. However, lasers using semiconductor materials tend to have low output power and high noise figure compared with EDF-based lasers. To achieve stable multiwavelength mode locked lasing at room temperature with low supermode noise, in this paper we propose a novel multiwavelength actively mode locked fiber ring laser with suppressed homogeneous line broadening and reduced supermode noise. In the proposed laser configuration, an EDF pumped by a 980-nm laser diode is used to provide the gain. The suppression of the homogeneous line broadening and the reduction of the supermode noise are realized simultaneously by incorporating a semiconductor optical amplifier (SOA) into the laser cavity, where the SOA is biased to operate just above the transparent point. Since the SOA is operating just above the transparent point, the gain of the laser is determined by the EDF. Experiment show that stable multiwavelength mode locked lasing with reduced supermode noise at room temperature is obtained.

2. Principle of operation

To achieve simultaneous multiwavelength lasing with small wavelength spacing, the homogeneous line broadening of the EDF must be suppressed. The line-shape Γ_h of homogeneous broadening at frequency ν with Lorentzian profile is expressed as

where ν₀ is the center frequency, Δν is the width between the half power point of Lorentzian curve, which is mainly affected by the lifetime of the carrier and the elastic collision among the carriers at upper state and lower state.

Semiconductor material is an inhomogeneous line broadening medium, in which all input carriers at different frequencies are amplified dynamically. The gain coefficient of an SOA at a temporal frequency ν is given by

where A ₂₁ is the spontaneous emission parameter of the level 2 to level 1 transition, c is the speed of light in free space, n_r is the material refractive index, l(ν) is the transition lineshape function, and N₁ and N₂ are the average numbers of atoms at energy level 1 and level 2. Since SOAs have much faster carrier lifetime (a few hundreds of picoseconds) than EDFs, the spectral hole burning of an EDF gain spectrum can be compensated by the faster inhomogeneous broadening of an SOA. The suppression of the homogeneous line broadening is the temporal averaging of the local rate and stimulated emission of the EDF and the SOA ions.

 

Fig. 1. Spectral hole burning of the EDF. (a) Output spectrum of the EDF. (b) Output spectrum of the EDF with an SOA biased at 130 mA.

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The homogeneous linewidth Γ_h is proportional to the burning hole width Δλ_h [10],

where P_s is the signal power and P_sat is the saturation power. Therefore, by measuring the spectral hole burning, the homogeneous broadening linewidth can be obtained. Fig. 1(a) shows the gain spectral hole burning of an 11.2-m long EDF induced by the homogeneous line broadening. Inset A of Fig. 1(a) shows the amplified spontaneous emission (ASE) spectrum of the EDF over the whole C band; Inset B of Fig. 1(a) shows a zoom-in view of the spectrum around 1532 nm. In Fig. 1(a), the gain spectral hole burning is clearly observed at 1532 nm when a -9-dBm laser signal is launched into the EDF. The burned hole width is measured to be 4.2 nm, which indicates that the EDF gain is dominated by the homogeneous line broadening at room temperature. Figure 1(b) shows the gain spectrum of the same EDF with an SOA cascaded; the SOA is biased at 130 mA, 15 mA higher than the transparency current. Again, Inset A of Fig. 1(b) shows the ASE spectrum of the EDF with the SOA over the whole C band, Inset B of Fig. 1(b) is the zoom-in view of the spectrum around 1532 nm. From Fig. 1(b), it can be seen that the spectral hole burning is greatly suppressed when the same input signal is injected into the EDF. Therefore, stable multiwavelength lasing at room temperature may be achieved if an SOA biased just above the transparent point is incorporated into the fiber ring laser.

To obtain high repetition rate and narrow pulses, active harmonic mode locking should be used. As a result of harmonic mode locking, the laser cavity has a large number of competing supermodes. The beating between the supermodes in the laser cavity results in the output intensity fluctuation. The ratio of the gain of power fluctuation between a frequency interval Δν to that of the average power can be derived from the rate equation of the small signal gain by adding a small perturbation from the average power [11],

where g is the gain, E_s is the saturated energy, P_ave is the average power of pulses, and τ_c is the carrier lifetime. For conventional fiber ring lasers with a typical cavity length L of a few tens of meters, Δν is around a few MHz (Δν = c/n(λ)L) and a number of cavity modes compete each other. The beating between the cavity modes produces a large number of supermodes. Since the lifetime of EDF carrier is approximately 10 ms, Δν ≫ 1 /τ_c. So, R approaches to 1, which indicates that a large power fluctuation of the output pulse and hence results in strong supermode beating. However, since an SOA has a very short carrier lifetime, which allows R to approach to e ^-g, resulting in a stable pulse. Therefore, by introducing an SOA, the supermode noise of the EDF laser can be significantly reduced.

3. Experiment

 

Fig. 2. Schematic diagram of the multiwavelength actively mode locked erbium-doped fiber ring laser. WDM: 980-nm pump coupler, IM: intensity modulator, OC: optical coupler, OI: optical isolator, PC: polarization controller, SG: signal generator, OSCI: oscilloscope, OSA: optical spectrum analyzer, LD: laser diode, PD: photodetector.

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The configuration of the proposed multiwavelength actively mode locked fiber ring laser is shown in Fig. 2. A segment of 20-m EDF pumped by 980-nm diode laser with a pumping current of 270 mA is employed to provide the majority of gain for the laser. An SOA is incorporated in the cavity to suppress the homogeneous line broadening of EDF and to reduce the supermode noise. It should be noted that the SOA is biased at 115 mA, just above the transparent point with a very low gain of about 2 dB around 1560 nm, to avoid the increase of the laser linewidth and the SOA-induced noise. The muliwavelength selection is performed by a Lyot-Saganc loop, which consists of a 3-dB coupler, a polarization controller, and two sections of high birefringent (HiBi) fibers. The unidirectional operation is ensured by an isolator. Active mode locking is realized by applying an RF signal to the intensity modulator incorporated in the laser cavity.

 

Fig. 3. Optical spectrum of the multiwavelength actively mode-locked fiber ring laser. (a) With SOA. (b) Without SOA.

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Fig. 4. Pulse intensity of the multiwavelength fiber laser locked at 820.024 MHz.

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Figure 3(a) shows the output of the laser with an SOA incorporated. It is clearly seen that a stable multiwavelength lasing is generated. The wavelength spacing is tunable by adjusting the length difference of the two segments of the PMF. In the experiment, it is found that a stable operation is still maintained while the wavelength spacing is reduced to 0.2 nm, which verifies that the homogeneous line broadening is significantly suppressed. Figure 3(b) shows the output spectrum of the laser without including the SOA. As expected, the laser generates fewer wavelengths with high amplitude fluctuations.

With a cavity length of 64 meters, the fiber laser has a round-trip frequency of 3.2 MHz. Since all the wavelengths share the same cavity, mode locking can be achieved at the same harmonics for all the wavelengths. Applying an RF signal to the electro-optic modulator, a stable pulse train from one branch of the coupler is observed using a Tektronix digital phosphor oscilloscope, while the multiple wavelength lasing is simultaneously monitored from the other branch of the coupler by an optical spectrum analyzer. Figure 4 shows the pulse train when the fiber laser is mode locked at 820.024 MHz, the 256^th harmonic of the fundamental round-trip frequency. The pulse train has a full-width at half-maximum (FWHM) of 500 ps with a repetition rate of 820 MHz. If a high pump power and a longer EDF is applied to compensate for the loss from the electro-optic modulator at the higher frequencies, the fiber laser could be locked at a higher harmonic frequency, and hence enable to achieve a narrower pulse width and a higher repetition rate.

The reduction of supermode noise by using the SOA in the laser cavity is also examined. Figure 5(a) shows the pulse train without the SOA. It can be seen that high intensity fluctuations are presented, which are generated due to the beating of the supermodes. Figure 5(b) shows the pulse train with the SOA included. The supermode noise is significantly suppressed. Note that in the above two situations, the laser are operating at multiwavelength. However, because of the homogeneous line broadening, without the SOA included in the laser cavity, only two wavelength lasing is achieved. With the SOA included in the laser cavity, up to 8-wavelength lasing with a signal-to-noise ratio better than 20 dB is achieved. It is also found that the supermode noise reduction can be achieved as long as the bias current of the SOA is above the transparent point. Moreover, with a higher bias current, the noise reduction is slightly improved, but with a broadened pulse width.

 

Fig. 5. Intensity fluctuations of the multiwavelength actively mode locked fiber ring laser. (a) Without SOA. (b) With SOA.

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4. Conclusion

A novel multiwavelength actively mode-locked EDF ring laser operating at room temperature was demonstrated. In the proposed laser structure, an SOA biased to operate just above the transparent point was incorporated in the laser cavity to suppress the homogeneous line broadening and to reduce the supermode noise. A stable multiwavelength active mode locking at room temperature was demonstrated. The supermode noise generated by the beating of the supermodes was also investigated. It was found that the amplitude fluctuations of the pulse train were significantly reduced when the SOA biased just above the transparent point was included in the laser cavity.