1. Introduction

Wavelength-tunable pulsed sources at high repetition rates are important for applications in optical communications and measurements. Among the different techniques to generate such pulsed source, active mode locking of a fiber ring laser provides a simple solution [1]. To define the output wavelength, the laser usually contains a wavelength selective element such as a fiber Fabry Perot (FP) filter [2], a fiber Bragg grating [3], or a FP laser diode [4]. In general, the wavelength selective elements will have a limited tuning range or gain bandwidth that restricts the tunability of the pulsed source. Hence, the bandwidth of the optical gain medium may not be fully utilized and most of the published works have tuning ranges of about 50 nm [5]. In this work, we demonstrate the generation of wavelength-tunable pulses with the dispersion tuning approach using a birefringence loop mirror filter (LMF) in a mode-locked semiconductor optical amplifier (SOA) ring laser. The LMF [6] is a passive, all-fiber, and polarization-independent device that serves as a comb filter in the laser. Thus, the output wavelengths can be tuned over a wide spectral range while maintaining a fixed grid spacing. In addition, with the controllable transmission ratio of the LMF, the SOA gain can be spectrally shifted to extend the wavelength tuning range of the laser. By applying the technique of dispersion tuning [7, 8], electrical tuning of 10 GHz mode-locked pulses has been achieved across a range of 97nm.

2. Experiment

The experimental setup is shown in Fig. 1(a). A pigtailed SOA with a maximum small-signal gain of 25 dB is used to provide the cavity gain. The SOA is biased at 102.5 mA. The optical output is then connected to a birefringence LMF through a 90:10 coupler and an isolator. The LMF is constructed with a 5-m polarization-maintaining fiber (PMF) connected to the output ports of a 3 dB coupler. A polarization controller is placed in the loop filter and acts as a polarization state rotator for both the clockwise and the counter-clockwise propagating branches. The transmission characteristic of the LMF is shown in Fig. 1(b). An output comb with a spacing of ~1.6nm is obtained near 1550 nm. Another isolator is connected to the output port of the loop filter to block the amplified spontaneous emission (ASE) of the SOA from entering the loop in the opposite direction. The ring cavity is completed by connecting the isolator to the SOA through a dispersion compensating fiber (DCF), a polarization controller, and a LiNbO₃ Mach-Zehnder intensity modulator. The 25-m DCF provides a dispersion of -2 ps/nm at 1550 nm. The modulator is sinusoidally driven by a 27-dBm electrical signal at about 10 GHz. The modulator provides periodic loss modulation with a modulation depth of 94% to 96% throughout the tuning range. The fundamental frequency of the laser is 4.57 MHz and harmonic mode-locking is obtained near 10 GHz.

 

Fig. 1. (a) Experimental setup of the wavelength tunable, harmonically mode-locked SOA ring laser that contains an intracavity birefringence loop mirror filter (LMF). DCF: dispersion compensating fiber; PC: polarization controller; PMF: polarization-maintaining fiber; SOA: semiconductor optical amplifier. (b) Measured transmission function of a 5-m LMF near 1550 nm.

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3. Principle

The key to achieve widely wavelength-tunable pulses using our dispersion tuning approach is the use of a LMF to shift the spectral gain of the SOA. The LMF defines a comb such that the output wavelength of the fiber laser can be tuned across a pre-determined wavelength grid. The working principle of the LMF is described as the following. The input light is split into two counter-propagating branches at the 3 dB coupler. After traveling through the fiber loop, the two branches will recombine again and interfere at the coupler. By controlling the polarization controller (PC) in the fiber loop, the counter-propagating branches are set to have a 90° polarization difference during their transit in the PMF. The light intensity that couples back to the cavity depends on the phase difference between the two interfering branches, and is related to the product of the modal birefringence and the length of the PMF. The transmission function is a wavelength dependent sinusoidal function given by

The PMF defines the comb spacing (∆λ) given by

where ∆n and L are the birefringence and the length of the PMF, respectively. In our work, a 5-m PMF with a birefringence of 3x10^-4 is used, resulting in a wavelength spacing of 1.6nm at around 1550nm.

The gain peak position of the SOA ring laser is dependent on the amount of the optical power in the cavity. A strong optical feedback tends to saturate the SOA, leading to gain compression and a shift of the gain peak towards a longer wavelength [9, 10]. The transmission ratio of the LMF is also tunable through adjustment of the PC. The function of adjusting the PC is to change the transmission ratio of the LMF and hence switch the gain band of the SOA between the short and the long wavelength region. The ring laser will oscillate in short wavelength band if the transmission ratio is low. Hence, the amount of cavity feedback power is controllable and the gain peak of the SOA can be tuned. Figure 2 shows the spectra of a continuous wave output that is generated by the SOA ring laser under different levels of the optical feedback. Figure 2(a) shows that the laser oscillates in the short wavelength region centered at 1513 nm when there is a large cavity loss. By adjusting the PC inside the LMF, a stronger feedback power can be obtained to support oscillation in the long wavelength region centered at 1582 nm, as shown in Fig. 2(b). The study shows that the output wave band can simply be tuned by adjusting the PC inside the LMF.

 

Fig. 2. Output spectra of the SOA cw ring laser with (a) weak optical feedback, laser output centered at 1513 nm. (b) strong optical feedback, laser output centered at 1582 nm.

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Wavelength selection of the output pulses is performed by dispersion tuning [7]. The dispersion inside the cavity is provided by the 25-m DCF. As different wavelengths propagate at different group velocities in the DCF, adjustment of the modulation frequency will result in a corresponding tuning of the center mode-locked wavelength. Electrical tuning of the wavelength can be achieved by slightly varying the frequency applied to the modulator. The tuning relation can be expressed as

where D is the dispersion coefficient of the fiber, F is the mode-locking frequency, L is the length of propagation and N is the harmonic order. Since D is negative for a fiber providing a normal dispersion, a slight increase in the modulation frequency at a given harmonic will favor lasing at a longer wavelength. The pulsed output is coupled out of the cavity via a 90:10 coupler and is monitored using a 40GHz photo-detector and a 40GHz digital sampling oscilloscope. The spectral characteristic is measured with an optical spectrum analyzer with a 0.01 nm resolution.

4. Results and discussion

With the LMF, the output wavelength can be electrically tuned in steps of 1.6-nm. Figure 3(a) and (b) show some samples of the 10-GHz mode-locked pulses obtained at different center wavelengths within the 97-nm tuning range along with the corresponding optical spectrum. It is noted that the ground floor of the optical pulses is raised when the pulses are generated near the tuning edge at the short wavelength side. The observation is mainly caused by the incomplete suppression of the non-lasing modes.

 

Fig. 3. Characteristics of the output at different wavelengths. (a) Pulse trains (Arrows indicate the ground level). (b) Corresponding optical spectra

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In Fig. 3(b), it is interesting to note the appearance of small peaks located at ~50 nm away from the output wavelength. The peaks are more pronounced when the output wavelength is near the edges of the tuning range. The observation is explained by simultaneous mode-locking of two different center wavelengths in the ring laser [11]. The repetition rate of the output pulses is about 10 GHz, corresponding to a period of 100 ps. In principle, in order to obtain mode-locked output at only one center wavelength within the tuning range of 97nm, the amount of cavity dispersion should be less than 1.03ps/nm. However, the 25-m DCF provides a dispersion of ~2.0 ps/nm and thus dual-wavelength operation can be supported in our setup. Fortunately, by adjusting the LMF, the gain peak of the SOA can be arbitrarily shifted to the desired range to suppress one of the output wavelengths. Therefore, a single center mode-locked wavelength can be obtained throughout the whole tuning range.

 

Fig. 4. The superimposed optical spectra of the output pulses throughout the 97-nm tuning range.

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Figure 4 shows the superimposed optical spectra of the output pulses within the 97-nm tuning range from 1491.1 nm to 1587.7 nm. The wavelength spacing of neighboring outputs is 1.6 nm. The variations of the peak power and the pulse width at different output wavelengths are plotted in Fig. 5. The power varies between 0.4 and 2 mW, whereas the deconvolved pulse width spans the range of 18.6-26.4 ps. The time-bandwidth product of the output pulses varies between 0.53 and 1.00. The degradation of the output peak power near the edges of the tuning range is caused by the finite gain bandwidth of the SOA. The SOA in our setup has a gain peak at 1520 nm and a 3-dB bandwidth of ~45 nm. With the use of LMF to shift the gain peak of the SOA, the mode-locked laser can oscillate at 1587.7 nm that is about 68 nm away from the original gain peak. However, near the short wavelength region, the LMF will have no effect in extending the tuning range. In addition, since the intensity modulator cannot operate at the short wavelength side, the degradation of the peak power is more severe near the region.

Figure 6(a) shows the variation of the signal-to-background suppression ratio across the tuning range. The highest value of 34.6 dB is obtained at 1533 nm. Degradation is more severe towards the edge of the tuning range in the short wavelength region, and is believed to be caused by the limited operating bandwidth of the modulator. During the 2-hour stability test, the SMSR varies less than 2.4 dB at both the short-wavelength and the long wavelength edges of the tuning curve. Moreover, less than 5% of pulse width variation is observed over the 1-hour measurement. Figure 6(b) shows the dependence of the output wavelength on the detuning of the modulating frequency. A linear plot with a slope of 10.56 nm/MHz is obtained from our experimental measurement. In our setup, the fundamental frequency is 4.57 MHz and the harmonic order is around the 2333rd. The measured slope of the tuning curve corresponds to a group velocity dispersion of 1.95ps/nm, in close agreement with the dispersion of ~2.0 ps/nm provided by the DCF. Although the frequency-varying tuning scheme has limitations in its application to communications, the source can be applied to high speed testing of the spectral response of optical components. Alternatively, by including a variable air gap or by adoption of the compensated dispersion-tuning scheme [12 ], the modulation frequency can be kept unchanged during the tuning.

 

Fig. 5. The variations of the peak power and the pulse width across the tuning range.

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Fig. 6 (a) The signal-to-background suppression ratio of the output within the tuning range of 1491.1 to 1587.7 nm. (b) The dependence of the output wavelength on the detuning of the modulating frequency.

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5. Conclusion

Widely wavelength-tunable optical pulses have been generated in a harmonically mode-locked SOA fiber laser using the dispersion tuning approach. A birefringence loop mirror filter is used to define the grid spacing of the output wavelengths. By exploiting the controllable transmission ratio of the LMF and the spectral shift in the gain peak of the SOA, the tuning range of the output can be significantly enhanced. A tuning range of 97 nm has been successfully demonstrated. The 10 GHz optical pulses show an output signal-to-background suppression ratio as high as 34 dB.