1. Introduction

The ever increasing demand for greater bandwidth requires optical communication systems that can process data and operate at higher and higher speeds. Therefore, wavelength tunable optical sources capable of generating short pulses at high repetition rates have garnered an increasing amount of interest. Actively mode-locked fiber lasers with erbium-doped fiber amplifiers (EDFAs) or semiconductor optical amplifiers (SOAs) as the gain medium have been widely investigated and are routinely used to create these optical sources. Active mode-locking is achieved typically by employing amplitude or phase modulators driven by a radio-frequency (RF) signal [1,2]. However, the cost of commercially-available electronic components capable of operating at frequencies beyond 40 GHz remains expensive and has limited the repetition rate of mode-locked lasers employing electronically-driven modulators. One way to circumvent this limitation is to use rational harmonic mode-locking to achieve repetition rate multiplication [3–6]. The drawback of rational harmonic mode-locking is that the output pulses exhibit significant variations in amplitude and the complexity of the system increases as additional steps are taken to equalize these variations [7].

It is also possible to perform mode-locking via all-optical techniques, such as cross-gain modulation in an SOA [8], cross-absorption modulation in an electro-absorption modulator [9], or ultrafast switching/gating using a nonlinear optical loop mirror (NOLM) [10,11] or SOA-based Mach-Zehnder interferometer (MZI) [12,13]. The interest in all-optical approaches includes the potential for (1) achieving significantly higher repetition rates compared to what is possible using conventional electronic approaches and (2) performing all-optical signal processing functions. For example, Bigo and Desurvire demonstrated a mode-locked erbium-doped fiber laser synchronized to an external optical signal at 20 GHz and investigated the application for all-optical clock recovery [10] while Schares et al. demonstrated a mode-locked erbium-doped fiber laser synchronized to external optical signals at 40 GHz and 80 GHz for applications in ultrafast all-optical logic operations [12,13] (an output repetition rate of 160 GHz was also achieved using rational harmonic mode-locking). However, this latter approach is complex since the SOA-MZI gate must be driven by two signals—the input (synchronizing) signal and a delayed replica—whose delay must be controlled carefully to optimize the gating response whenever the frequency of the synchronizing signal is changed.

In this paper, we demonstrate a mode-locked semiconductor fiber laser (SFL) that is synchronized to an external optical signal using a NOLM. The advantages of this approach are the following: (1) only one copy of the synchronizing signal is required to drive the NOLM directly so ultrahigh repetition rates can be obtained without resorting to rational harmonic mode-locking and (2) the laser can be synchronized easily to any input frequency by simply adjusting the power of the synchronizing signal launched into the NOLM and a tunable optical delay line (TODL). In [14], we reported preliminary results for operation at 40 GHz; in this paper, we improve the performance of the mode-locked SFL at 40 GHz and we also demonstrate synchronization to 80 GHz and 160 GHz external signals. This is the highest reported repetition rate from an actively mode-locked SFL incorporating a NOLM without the use of frequency multiplication techniques.

2. Experimental setup

The experimental setup is shown in Fig. 1 . The input (synchronizing) pump signal is generated using a commercially available mode-locked fiber laser (UOC, Pritel) operating at OC-768 or 39.813120 GHz and a passive optical multiplexer (OMUX, U²T). The laser generates picosecond pulses that are wavelength tunable over the C-band and the OMUX can multiply the repetition rate by factors of 1, 2, or 4 by controlling the multiplexer stages. A high power EDFA and a variable optical attenuator (VOA) are used to amplify and control the peak power of the pulses from the OMUX before they are injected into the NOLM.

 

Fig. 1 Experimental setup. NOLM: nonlinear optical loop mirror, SFL: semiconductor fiber laser, PC: polarization controller, ISO: isolators, TBPF: tunable bandpass filter, TODL: tunable optical delay, VOA: variable optical attenuator, MLL: mode-locked fiber laser.

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The NOLM is composed of two 50:50 couplers, a polarization controller, and highly nonlinear fiber (HNLF). The HNLF has a length of 1001 m, a nonlinear parameter of 11.5 W⁻¹⋅km⁻¹, and a dispersion of −0.04 ps/(nm⋅km) and a dispersion slope of 0.02 ps/(nm²⋅km) at 1550 nm. The operation of the NOLM has been detailed in [10,11,15]; essentially, it functions as a loss modulator within the SFL cavity that is optically triggered by the pump pulses.

In addition to the NOLM, the SFL includes an SOA (CIP-NL) as the gain medium, a zero-dispersion tunable bandpass filter (TBPF), optical isolators to ensure unidirectional propagation, a 50% output coupler, a TODL, and a polarization controller (PC), all connected in a ring configuration. The TBPF (QTM100C, Peleton) is used to select the output wavelength (within the C band) and to filter out the input pump signal. The filter bandwidth can be adjusted in 100 GHz increments and the overall spectral shape is box-like (the filter edges can be fit to a 4th order Gaussian function). The TODL is adjusted such that the round trip propagation time inside the cavity matches a harmonic of the fundamental frequency of the input pump signal. The PCs are used to optimize the output pulses from the SFL.

The output pulses are characterized using an optical sampling module with an impulse response time of 7.7 ps, an intensity autocorrelator, and an optical spectrum analyzer (OSA) with a resolution bandwidth of 0.06 nm. Root-mean-square (rms) timing jitter measurements at 40 GHz and 80 GHz are made using a digital sampling oscilloscope with a precision time base with a 200 fs noise floor and a persistence time of 100 ms.

3. Experimental results and discussions

The primary advantage of our SFL is that mode-locking at different repetition rates and wavelength tuning can be achieved readily without disconnecting or changing any part of the assembly. Indeed, to change from 40 GHz to 80 GHz to 160 GHz, we simply need to adjust the power of the pump signal launched into the NOLM as well as adjust the TODL.

3.1 40 GHz operation

First, we consider the impact of the TBPF bandwidth on the laser output characteristics. We set the input pump wavelength at 1543 nm. The input pulses are 1.6 ps in duration with a sech² shape and have a 3 dB spectral bandwidth of 2.55 nm; the rms timing jitter is 230 fs. The pulses are amplified to an average power of 10.5 mW before being launched into the NOLM. The TBPF is centered at 1558 nm and the bandwidth is increased in 100 GHz increments up to 500 GHz. The results are shown in Fig. 2(a) . As expected, the output pulse width decreases with increasing filter bandwidth from 6.6 ps at 100 GHz to 2.0 ps at 500 GHz (the pulse widths are extracted from the autocorrelation traces which have are best fitted to sech² pulses). The corresponding rms timing jitter of the output pulses decreases from 520 fs at 100 GHz to an average of 330 fs for larger filter bandwidths. Increasing the filter bandwidth beyond 500 GHz does not cause significant changes in the output pulse widths nor rms timing jitter. Figure 3 shows a sample output spectrum, waveform, and autocorrelation trace of the SFL output for a TBPF bandwidth of 300 GHz. The output spectrum exhibits some asymmetry in part because the box-like response of the filter does not provide additional spectral shaping and from four-wave mixing inside the SOA. Since the laser output is taken after the SOA, amplified spontaneous emission (ASE) noise contributes to a slight DC offset in the output pulses. The time-bandwidth product of the output pulses is 0.42, which suggests that they exhibit a small amount of chirp.

 

Fig. 2 SFL output characteristics at 40 GHz. Pulse width and rms timing jitter as a function of (a) TBPF bandwidth for a central wavelength of 1558 nm and (b) output wavelength for a TBPF bandwidth of 300 GHz.

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Fig. 3 (a) Output spectrum, (b) waveform, and (c) autocorrelation with sech² fit of the SFL output at 40 GHz with the TBPF centered at 1558 nm and with a bandwidth of 300 GHz.

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Next, we investigate the wavelength tunability of the SFL. We now set the input pump wavelength at 1535 nm. In this case, the input pulses are 1.7 ps in duration with a 3 dB spectral bandwidth of 1.75 nm and an rms timing jitter of 210 fs. The average input power to the NOLM is 17.3 mW. The TBPF bandwidth is fixed at 300 GHz and is tuned from 1546 nm to 1562 nm (we do not consider shorter wavelengths in order to limit spectral overlap with the input signal). Figure 2(b) shows the output pulse width and rms timing jitter as a function of wavelength. The output pulse widths and rms timing jitter exhibit very little variation across the tuning range with average values of 2.8 ps and 270 fs, respectively. The average output power varies slightly from 4.29 mW at 1546 nm to 5.14 mW at 1562 nm; this is due in part to the gain profile of the SOA.

3.2 80 GHz Operation

Again, we begin by investigating the impact of the TBPF bandwidth on the output pulse characteristics. The input is set at 1543 nm and is now amplified to an average power of 24.2 mW to maintain a similar peak pump power launched inside the NOLM for proper switching. As before, the TBPF is centered at 1558 nm and the bandwidth is increased from 200 GHz to 500 GHz (a bandwidth of 100 GHz was insufficient for the SFL to operate properly at 80 GHz in part due to an overly restricted spectral content). The results are summarized in Fig. 4 (a): the output pulse width decreases from 2.9 ps for a filter bandwidth of 200 GHz to 1.8 ps at 500 GHz. The output rms timing jitter remains at an average value of 380 fs with very little variation as the filter bandwidth is increased. Figure 5 shows a sample output spectrum, waveform, and autocorrelation trace of the SFL output for a TBPF bandwidth of 500 GHz. In this case, the average output power is 5.0 mW and the time-bandwidth product is 0.82.

 

Fig. 4 SFL output characteristics at 80 GHz. Pulse width and rms timing jitter as a function of (a) TBPF bandwidth for a central wavelength of 1558 nm and (b) output wavelength for a TBPF bandwidth of 500 GHz. The solid points refer to a pump at 1535 nm while the hollow points refer to a pump at 1559 nm.

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Fig. 5 (a) Output spectrum, (b) waveform, and (c) autocorrelation with sech² fit of the SFL output at 80 GHz with the TBPF centered at 1558 nm and with a bandwidth of 500 GHz.

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To investigate the wavelength tunability of the laser, we set the input pump wavelength at 1535 nm and 1559 nm. With the input at 1535 nm, the SFL can be tuned from 1550 to 1562 nm whereas it can be operated from 1530 nm to 1546 nm with the pump at 1559 nm. The input pulses exhibit similar characteristics at both wavelengths: the pulse widths are 1.7 ps and 2.1 ps with corresponding 3 dB bandwidths of 1.56 nm and 1.68 nm, and the rms timing jitters are 330 fs and 260 fs at 1535 nm and 1559 nm, respectively. In this case, the TBPF bandwidth is fixed at 500 GHz. Figure 4(b) shows the output pulse widths and rms timing jitter as a function of wavelength: the solid points refer to a pump at 1535 nm while the hollow points refer to a pump at 1559 nm. As can be seen, regardless of the input pump wavelength, the output pulse width is relatively constant with an average value of 1.9 ps from 1538 nm to 1558 nm and increases to 2.7 ps at the edges of the tuning range (< 1534 nm and 1562 nm). The longer output pulses at the edges of the tuning range are due largely to the increased spectral separation (over 25 nm) between the pump and output signals which causes greater walk-off in the NOLM (and hence a broader switching window). Finally, the average output power of the SFL is 3.4 mW at 1530 nm and increases to 4.6 mW at 1562 nm.

Compared to the relatively uniform rms timing jitter characteristics of the SFL at 40 GHz as a function of wavelength, the SFL is more sensitive at 80 GHz. In particular, the rms timing jitter varies from 320 fs at 1546 nm to 450 fs at 1530 nm when the input is at 1559 nm; similarly, when the input is at 1535 nm, the rms timing jitter increases from 350 fs at 1550 nm to 580 fs at 1562 nm (on the other hand, the rms timing jitter varies only by 40 fs over a similar tuning range at 40 GHz).

3.3 160 GHz Operation

At 160 GHz, we do not have the instruments to measure the rms timing jitter nor temporal waveforms; the laser output was optimized on the basis of observing the OSA and autocorrelation traces.

We first consider an input pump wavelength of 1562 nm. The pulses at the output of the OMUX are 1.8 ps (sech²) with a 3 dB bandwidth of 1.45 nm; they are amplified to an average power of 55.3 mW before being launched into the NOLM, again to ensure a similar peak power launched inside the NOLM for proper switching. The TBPF is centered at 1546 nm and the bandwidth is 700 GHz. Figure 6 shows the spectrum and autocorrelation of the output pulses from the SFL. The pulses have a duration of 1.3 ps and the 3 dB bandwidth is 3.70 nm. The spectral peaks are separated by ~1.27 nm, which confirms operation at 160 GHz. The average output power is 4.62 mW and the time-bandwidth product is 0.60.

 

Fig. 6 (a) Output spectrum and (b) autocorrelation with sech² fit of the SFL output at 160 GHz with the TBPF centered at 1546 nm and with a bandwidth of 700 GHz.

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Operation using a pump wavelength of 1543 nm was also achieved. In this case, the TBPF is fixed at 1559 nm with a bandwidth of 700 GHz. The output pulses from the SFL are 2.0 ps in duration with a 3 dB bandwidth of 1.65 nm and the average output power is 5.22 mW. Although the characterization of the SFL is not as extensive as that performed for lower repetition rates, these measurements demonstrate the capability for synchronizing the SFL at 160 GHz at different wavelengths.

4. Conclusion

We have demonstrated a mode-locked SFL incorporating a NOLM that is capable of synchronizing to an external input optical signal at 40 GHz, 80 GHz, and 160 GHz. By simply adjusting the average power of the pump pulses at the input of the NOLM and the TODL in the cavity, the SFL can be mode-locked (synchronized) to a wide range of input frequencies and is tunable within the C-band. Pulses as short as 1.3 ps and a 3 dB bandwidth of 3.70 nm have been generated directly at 160 GHz. Such a source can be used in all-optical signal processing applications, for example ultrafast logic operations. However, it should be noted that stable operation is an important requirement for mode-locked fibre lasers. While the rms jitter of the laser output pulses is not significantly higher than that of the input synchronizing pulses, a more thorough investigation on long-term stability, including the influence of the synchronizing pump signal characteristics (e.g., duty cyle, pump power, signal-to-noise ratio, and timing jitter transfer) and the possibility of incorporating an active stabilization system is required.