In this work we present for the first time, to the best of our knowledge, a passively synchronized thulium (Tm) and erbium (Er) doped fiber laser mode-locked by a common graphene saturable absorber (GSA). The laser consists of two ring resonators combined with a 90 cm long common fiber branch incorporating the saturable absorber (SA). Such laser generates optical solitons centered at 1558.5 nm and 1938 nm with pulse durations of 915 fs and 1.57 ps, respectively. Both laser loops were passively synchronized at repetition frequency of 20.5025 MHz by nonlinear interaction (cross phase modulation, XPM) in common fiber branch between generated pulses. The maximum cavity mismatch of the Er-laser in synchronization regime was 0.78 mm. The synchronization mechanism was also investigated. We demonstrate that the third order nonlinearities of graphene enhance the synchronization range. In our case the range was increased about 85%. The integrated RMS timing jitter between the synchronized pulses was 67 fs.
© 2014 Optical Society of America
Large third order nonlinearities [1,2], ultra-broadband and flat absorption bandwidth [3,4], fast recovery time [5,6], relatively simple manufacturing technology cause that graphene is an effective SA for ultrashort optical pulse generation [7–25]. Since the first demonstrations [7,8] of mode-locked lasers using graphene as SA, such absorbers have been used to generate laser pulses in the spectral range from 0.8 µm  to 2.5 µm  in mode-locked [7–23] and Q-switched [24,25] regime. Thanks to its very broadband and flat absorption, graphene can be used as a common SA to build two-color mode locked lasers. Recently, we have demonstrated a laser setup consisting of two loop resonators (Er- and Tm-doped) mode-locked by a common graphene-based saturable absorber. Stable mode-locked operation obtained at central wavelengths separated by about 380 nm cause that such laser can be considered as an effective platform to generate synchronized optical pulses.
Two-color mode-locked laser sources generating synchronized optical pulses are required in many applications such as nonlinear frequency conversion [26,27], multi-color pump-probe spectroscopy  and Raman scattering spectroscopy . The multi-wavelength mode-locked laser sources can be obtained by spectral broadening of a stable seed source in highly nonlinear fibers , Raman soliton self-frequency-shift of stable femtosecond laser  or by synchronization of independent mode-locked lasers [30–37]. The repetition rate control of two-color lasers can be achieved either by active synchronization [30,31], that uses electronic feedback to control the cavity length, passive synchronization [32–34], where the nonlinear interaction of two pulses causes a change of the refractive index and couples the repetition rates of synchronized lasers, and hybrid synchronization (active-passive) [35,36] using both of above mechanisms. The repetition rate coupling can be also obtained by the nonlinear coupling effects between the S1 and S2 states of the single-walled carbon nanotube-based common SA .
In this paper we present for the first time, to the best of our knowledge, a passively synchronized Tm- and Er-doped fiber laser mode-locked by a common GSA. The graphene layers deposited onto a fiber ferrule supported mode locked operation at 1558.5 nm and 1938 nm and generation optical pulses with duration of 915 fs and 1.57 ps, respectively. Both laser loops were connected by a 90 cm fiber branch with common GSA. The lasers were synchronized at a repetition frequency of 20.5025 MHz. The maximum Er-cavity mismatch in the synchronization regime was 0.78 mm which corresponded to the repetition frequency change of both cavities of 1.08 kHz. The integrated RMS timing jitter measured using cross-correlation technique was of 67 fs. We have investigated the impact of graphene layers on synchronization holding range. In the laser configuration with common GSA the synchronization range was increased about 85% compared to the laser configuration with two independent cavities with 90 cm long common fiber branch (without common GSA). We believe that third order nonlinearities of common GSA are responsible not only for mode-locked operation, but they also enhance the cross phase modulation (XPM) effect and make the synchronization much more stable.
2. Experimental setup
The setup of the two-color fiber mode locked laser is depicted in Fig. 1. It is similar to previously presented by the authors . The laser consists of two loop resonators with one 90 cm long common branch, which contains the GSA. The 1.55 µm loop is based on a 45 cm long highly Er-doped fiber (nLight Liekki Er80-4/125), pumped by a 980 nm laser diode via a fused 980/1550 nm wavelength division multiplexer (WDM). The light is coupled out from the 1.55 µm cavity through a 20% coupler. The Er-doped cavity incorporates also a delay line formed by two fiber collimators mounted on precise XYZ stages. The 2 µm loop is based on a 1.3 m long piece of Tm-doped fiber (Nufern SM-TSF-9/125), pumped by a 1569 nm laser diode (beforehand amplified in an Er/Yb-doped fiber amplifier, EYDFA). The signal from the 2 µm cavity was also coupled out by a 20% coupler. Fiber-based in-line polarization controllers (PC) placed in both loops allow to adjust the intra-cavity polarization and start the mode-locked operation. Fiber isolators spliced into both cavities force the signal propagation counterclockwise and clockwise for 2 µm and 1.55 µm cavities, respectively. The signals are combined by a 1570/2000 nm WDM and directed to the common GSA. After passing through the SA, the beams are separated using a filter-type WDM, which reflects the 1.55 µm signal and lets the 2 µm signal pass. The lengths of both loops were set to be around 20.5 MHz.
The saturable absorber contained two graphene layers grown on copper foils by CVD technique. In order to simplify the transfer process of graphene layers onto the fiber ferrule, we have used graphene with a supporting layer based on poly(methyl methacrylate) (PMMA). Such composite can be formed into small pieces and can be simply stacked on the fiber ferrule. The fabrication process and full characterization of such graphene/PMMA has been presented by the authors before [17,18].
In order to investigate the impact of graphene on the synchronization mechanism one additional detachable connection (two fiber connectors combined with fiber adapter) in each loop was inserted (elements indicated as GSAXPM in Fig. 1). Such setup configuration allows to investigate the synchronization mechanism between the loops and maintain the repetition frequency in two cases. In the first case, the graphene layers were inserted between the fiber connectors placed in the common branch. Both lasers were mode-locked by the same SA. In the second case the graphene layers were removed from the common GSA and placed between the connectors indicated as GSAXPM in Fig. 1. The laser cavities were mode-locked independently and were synchronized through XPM in the 90 cm long common branch.
The performance of the laser was observed using an optical spectrum analyzer with scanning range up to 2400 nm (Yokogawa AQ6375), 12 GHz digital oscilloscope (Agilent Infiniium DSO91304A), 7 GHz RF spectrum analyzer (Agilent EXA N9010A) coupled with a 12 GHz photodetector (Discovery Semiconductors DSC2-50S), an optical autocorrelator/ crosscorrelator (Femtochrome FR-103XL) and a FFT Analyzer (SR760).
3. Experimental results
3.1 Simultaneous mode-locked of Er- and Tm-doped cavity
In the first step the setup configuration with graphene/PMMA layers placed between fiber connectors spliced in common branch was investigated. Similar to the laser configuration presented in  both laser cavities were mode-locked by the same SA. Stable, fundamental mode-locked operation of the Er- and Tm-doped loops was observed at pump powers in the range from 20 - 30 mW and 140 – 200 mW, respectively. To obtain the mode-locked operation alignment of PCs in each loop was needed. Because the dispersion of both loops was all-anomalous, the generated pulses presented in Fig. 2(a) and Fig. 2(b) have soliton-like shape with characteristic Kelly’s sidebands. The FWHM of such optical solitons centered at 1558.6 nm and 1937.3 nm were of 3.9 nm and 2.63 nm, respectively. The additional CW signal at 1569 nm presented in the inset graph in Fig. 2(a) originates from the pump power unabsorbed by thulium-doped active fiber. The signal fits the reflection band of the filter-type WDM and is redirected to the 1.55 µm output. The spectral dips (Fig. 2(b)) observed at the soliton pulse generated at 2 µm loop results from water absorption lines around 2 µm region . The output powers measured at 2 µm and 1.55 µm outputs were 1.5 mW and 0.5 mW, respectively.
The autocorrelation traces measured at 1.55 µm and 2 µm laser outputs together with a sech2 fitting are depicted in Fig. 3. The pulse durations after deconvolution and time bandwidth products (TBP) are 914 fs and 0.43 for the soliton pulse centered at 1558.5 nm and 1.57 ps and 0.33 for pulse centered at 1937.3 nm. The pulse energies are at the level of 24.3 pJ and 73 pJ at 1.55 µm and 2 µm, respectively. The laser performance of the Er- and Tm-doped cavities were measured at pump power levels of 25 mW and 160 mW, respectively.
3.2 Passive synchronization of mode-locked Er- and Tm-doped cavities
The behaviors of the repetition frequencies change of Er- and Tm-doped loops when the length of the erbium cavity was lengthened is presented in Fig. 4(a). As it is clearly seen, at the beginning both lasers work at different repetition rates independently (labeled as fTm, fEr – for thulium and erbium cavity, respectively). Increasing the distance between fiber collimators causes decreasing of the fEr. When the fEr was close enough to fTm, the synchronization of generated pulses was observed. The synchronization mechanism is based on the XPM effect. The interaction of the optical solitons centered at 1558.5 nm and 1937.3 nm in the common fiber branch cause the change of the nonlinear refractive index n2. Hence, both laser start to operate with a new repetition rate fc (fTm <fc< fEr). Figure 4(d) presents the RF spectrum of synchronized pulses with the signal to noise ratio (S/N) better than 65 dB. Within the synchronization range both lasers worked in the master-slave configuration; the Tm-doped laser follows the repetition rate determined by the Er-doped cavity. When the synchronization between both cavities was lost the fTm returned to its initial value and was independent from the further increase of the distance between fiber collimators. The maximum Er-cavity length change without losing the synchronization was of 0.78 mm. In such cavity mismatch range the fTm, fEr were changed about 1.08 kHz (Fig. 4(a)).
The refractive index change in the synchronization regime also results in the tuning of central wavelengths of generated solitons [32–34]. Depending on which pulse is leading in the common branch the optical spectra can be either blue or red shifted from its initial central wavelengths observed in free-running operation . At the same time, we have also measured the optical spectra of both lasers (Fig. 4(c)). At the beginning the generated optical solitons were centered at 1558.6 nm and 1937.3 nm (the optical spectra are presented in Fig. 2). When the fTm, fEr were close enough to start the synchronization the Er-laser pulse arrived ahead the Tm-laser pulse to the common fiber branch, because the fEr was slightly higher than fTm. The synchronization between both loops was originated from the interaction of the leading and trailing part of pulses generated in Tm- and Er-doped cavities, respectively. Hence, generated solitons at 1558.6 nm and 1937.3 nm were red- and blue-shifted to 1559.4 nm and 1937.05 nm, respectively (Fig. 4(c)). Further increasing of the Er-cavity length caused the master-slave synchronization mechanism. Hence, the direct change (decreasing) of the fEr was slightly faster fTm. It results in the Er-laser pulse propagation behind the Tm-laser pulse. So, the solitons generated in Tm- and Er-doped cavities were continuously shifted to red and blue, respectively. When the synchronization was lost both lasers start to operate at the free-running wavelengths. Similar mechanism was observed previously by Wei et al.  in passive synchronization setup of Ti:sapphire and Cr:forsterite lasers.
The XPM and saturable absorption effects are responsible for passive synchronization between both resonators and mode-locked operation, respectively. Both effects are based on the third order nonlinearities. Taking into account that graphene is characterized by large nonlinearities [1,2], we expected that the common GSA has an additional impact to synchronization mechanism. In order to estimate such impact the laser setup with independent graphene SA in each loop was investigated. The graphene layers were removed from the common GSA thus the repetition frequencies remained unchanged. Similar to the previously investigated setup with common GSA both lasers start to operate in mode-locked regime with different repetition frequencies (Fig. 4(b)). The generated soliton pulses were centered at 1559.5 nm and 1934.5 nm. The laser performances like FWHMs, pulse durations and output powers were at the level of 4.2 nm, 850 fs, 0.5 mW and 3.2 nm, 1.45 ps, 2 mW for Er- and Tm-doped cavities, respectively and were close to that obtained in setup with common GSA. Hence, the intracavity intensities in the common branch were also comparable to previously investigated setup. When the distance between fiber collimators in the Er-doped loop was increasing the synchronization of both laser was observed. The fTm has followed the fEr until the synchronization was lost. As is seen in Fig. 4(b) the maximum Er-cavity length change without losing the synchronization was of 0.42 mm. The range is almost twice smaller than that measured for the setup with common GSA. This confirms that the large third order nonlinearities of graphene enhanced the XPM effect thus the synchronization holding range was increased about 85%. So, the common graphene layers can be used as SA in two-color mode locked laser and additionally stabilize the synchronization between such lasers.
In order to investigate the relative time fluctuations between the two synchronized pulses generated in the laser setup with common GSA, the cross-correlation technique was employed. The ultrafast timing-jitter is indirectly measured in this method. It is proportional to the intensity change of the sum frequency signal of two pulses overlapped in nonlinear crystal. To measure the cross-correlation signal the Femtochrome FR-103XL autocorrelator with cross-correlation option was used. The output pulses from the lasers were amplified in Er- (Liekki, Er80-8/125) and Tm-doped (Nufern SM-TSF-9/125) fibers up to the levels of 6 mW and 10 mW, respectively. Such amplified pulses were focused in the lithium iodate (LiIO3) nonlinear crystal. In order to measure the timing jitter in the linear response region the Tm-doped laser pulse passes beforehand through the time-delay line and was set in the at the half-maximum of the cross-correlation signal. Measured cross-correlation signal with time duration of 1.78 ps is presented in Fig. 5(a). The sum frequency signal detected by the photomultiplier was next recorded by the FFT analyzer in the frequency range from 1 Hz to 10 kHz. On the basis of the FFT signal the jitter spectral density and integrated RMS jitter were calculated  and are presented in Fig. 5(b). The timing jitter saturates at the level of 67 fs.
Concluding, the paper presents for the first time to the best of our knowledge passive synchronization of two fiber lasers mode-locked by the common GSA. The Er- and Tm-doped laser cavities sharing an 90cm long fiber branch with GSA support mode-locked operation and generation of optical pulses at 1558.5 nm and 1938 nm with pulse durations of 915 fs and 1.57 ps, respectively. Both lasers were synchronized at the repetition frequency of 20.5025 MHz. The Er-cavity length can be changed about 0.78 mm without losing the synchronization. The measured integrated RMS timing jitter was of 67 fs. The synchronization mechanism based on a nonlinear interaction between the generated pulses was investigated. We found out that the common GSA enhanced the nonlinear interactions between the optical pulses propagated in common fiber branch. As a result the synchronization holding range was extended about 85% in comparison to laser configuration with the common fiber branch without GSA.
The work presented in this paper was supported by the National Science Centre (NCN, Poland) under the project “Saturable absorption in atomic-layer graphene for ultrashort pulse generation in fiber lasers” (contract no. UMO-2011/03/B/ST7/00208)” and by the Polish Ministry of Science and Higher Education under the project no. POIG.01.01.02-00-015/09-00.
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