A 165-fs all-fiber ring laser is demonstrated with a fundamental repetition rate of 235 MHz based on a 5.7-cm-long Er3+/Yb3+ codoped phosphate glass fiber and a technique of nonlinear polarization evolution. In order to further enhance the fundamental repetition rate and compact the structure of the all-fiber laser, an optical integrated module is designed. By employing this novel optical module, a much more compact 105-fs mode-locking all-fiber ring laser, operating at a 325 MHz fundamental repetition rate, is realized.
© 2012 OSA
Recently, as known for numerous applications such as optical frequency metrology, arbitrary optical waveform generation, high speed optical sampling, and laser ranging, femtosecond (fs) fiber lasers with a repetition rate, particularly higher than 100 MHz, have attracted more attention [1, 2]. Normally, it is quite easy to generate a high repetition rate pulse either with actively or passively mode-locking technique. Taking the passively mode-locking technique for example, it can be conducted on an ultrashort linear cavity benefiting from the semiconductor saturable absorbers . The mechanisms for pulse shaping of the semiconductor saturable absorption limit the pulse duration at best to an order of picosecond. Passively harmonic mode-locking can overcome this limit. However, both its stability and timing jitter are not as good as that of passively fundamental mode-locking . Therefore, nonlinear polarization evolution (NPE) technique has attracted more interests at present because of its capability to create fs pulses with high fundamental repetition rates [4, 5].
The fundamental repetition rate higher than 200 MHz of a pulse fiber ring laser has been realized with NPE technique at ~1.5 μm. For instance, Wilken et al. reported an Er3+ doped fiber ring laser mode-locked by NPE, which operated at a fundamental repetition rate of 250 MHz and a pulse width of 70-fs . Also with this technique, Morse et al. achieved a repetition rate of 301 MHz as well as a pulse duration only 108-fs . Within their fiber laser, a 55 cm Er3+ doped fiber was pumped by two 750 mW/980 nm laser diodes (LD). Almost at the same time, similar results were presented by Peng et al. . Zhao et al. employed a 33.5 cm Er3+ doped bismuthate fiber as gain medium, and a polarization beam splitter as both pump input and laser output coupler. They demonstrated a 234 MHz pulse fiber ring laser in the strong normal dispersion regime . Very recently, Ma et al. reported a 225 MHz repetition rate fiber laser with a 40 cm Er3+ doped fiber. After dechirped by a Brewster prism pair outside the laser cavity, the pulse duration was measured to be 37-fs .
In the schemes of these cavities, mode-locking has to be performed in free space. To realize high fundamental repetition rates in these configurations, long active fibers are required to offer enough gain and thus to initiate mode-locking operation. However, a long laser cavity imposes significant limitations on high fundamental repetition rate that a mode-locking fiber laser can achieve. The substitution of fiber-based elements by free space ones can partly shorten the cavity length. However, it would lead to etalon effects, which eventually impair the operation of mode-locking . Most of all, these schemes do not facilitate the design of laser cavity more compact and better tolerant to environmental vibrations.
Rare-earth-ions doped phosphate glass fiber (PGF) is featured with a high dopant concentration, a large emission cross section and a high gain coefficient [12–15]. This implies a great possibility to build a more compacted ring laser with this fiber. In this letter, an all-fiber ring fs laser by NPE is constructed with 5.7 cm PGF, and it demonstrates a 165-fs pulse with a fundamental repetition rate high up to 235 MHz. In order to compact the configuration of the mode-locking all-fiber ring laser and further enhance the fundamental repetition rate, an integrated optical module is designed and being used to achieve a 105-fs pulse train at a fundamental repetition rate of 325 MHz.
Figure 1 shows the scheme of our high fundamental repetition rate all-fiber ring laser. Single mode PGF is designed with a numerical aperture of 0.206 at 1.5 μm and a core diameter of 5.4 μm and fabricated with traditional rod-in-tube method. The core is doped with 3.0 mol% Er3+ and 5.0 mol% Yb3+. The propagation loss is minimized by thoroughly dehydrate process during glass melting and measured by cut-back method to be 0.04 dB/cm, while the net gain is 5.2 dB/cm at 1535 nm. The quantum efficiency is 93.8% at the laser emission wavelength of 1.5 μm. A 5.7 cm single mode PGF is utilized as the gain medium, which is butt-connected with a 1% output coupler (OC) and a wavelength division multiplexer (WDM). A polarization-sensitive isolator (PS-ISO) fusion-spliced with OC and WDM is used to provide polarization selection and ensure unidirectional operation. In order to reduce the broadening effect induced by the lead fiber of OC, the output port of OC is cut to about 11.2 cm. An in-line polarization controller (PC) is inserted into the fiber loop to adjust the polarization state of the light inside the ring cavity. The whole length of the cavity is 84.7 cm. A 750 mW/980 nm LD (Oclaro Inc.) is launched into the cavity through a 980/1550 nm WDM.
As the cavity loss is optimized by simply adjusting PC , the ring laser starts operating in continuous wave (CW) when the pump power is 150 mW. As the pump power increases to 725 mW, self-started mode-locking operation is attained. Keeping the setting of PC, mode-locking can sustain as the pump power goes down to 553 mW continuously. The optical spectrum of mode-locking pulse is shown in Fig. 2(a) . Kelly sidebands, due to periodic amplification and loss of the soliton, can clearly be distinguished . They can be suppressed and removed by adjusting PC or using a shorter cavity [18, 19]. The radio frequency (RF) spectrum of the corresponding pulse is examined and shown in Fig. 2(b) with a 1 kHz resolution bandwidth. It indicates a 235 MHz fundamental repetition rate and a 55 dB signal-to-background ratio. The second-harmonic generation (SHG) autocorrelation trace of the mode-locking pulse is illustrated in Fig. 2(c). As a sech pulse profile is assumed, the pulse duration is found to be 165-fs, giving a time-bandwidth product of 0.483. The deviation of the time-bandwidth product from that of the transform limited pulse is due to the chirp inside the cavity and the pulse broadening at the output port of OC. Judging from the optical spectrum shown in Fig. 2(a) and the types of the fibers in the cavity, the output pulse carries linear chirp with a negative chirp parameter. By using a fiber with normal dispersion such as dispersion compensation fiber, the pulse can be compressed to the Fourier transform limited pulse . The inset of Fig. 2(c) shows the entire scan range of the autocorrelator. Within the scanning window of 185 ps, there is only one pulse observed in the experiment. Figure 2(d) is the corresponding oscilloscope trace of the pulse train, which reveals a stable operation of the pulse. All the measurements are performed when the pump power is 725 mW.
As shown in Fig. 1, there are three fiber-based components in our cavity: PS-ISO, OC and WDM. Those fiber-based components with physical lengths around 6 cm will become the ultimate limitation of increasing the fundamental repetition rate after the optical fiber inside the cavity has been cut to a minimum length. In order to obtain a higher repetition rate and make the cavity much more compact, an optical integrated module (OIM) is designed to replace these three fiber components. As shown in Fig. 3(a) , the OIM has a physical length of 5.5 cm and an insertion loss of 1.7 dB at 1550 nm. The pump laser is launched into the cavity from 980 nm port, and then reflected into 980/1550 nm port to pump PGF. By using this OIM, we set up a very compact all-fiber laser with a 5.7 cm PGF, shown in Fig. 3(b). The compact fiber laser is pumped by two 750 mW/980 nm laser diodes, which are combined via a polarization beam combiner.
The cavity length of compact mode-locking all-fiber laser is cut to be around 90 cm initially. With an appropriate setting of PC, self-starting mode-locking is achieved when the pump power increases to about 700 mW. The corresponding output power, pulse duration and repetition rate are 54 mW, 196-fs and 215 MHz respectively. Keeping the pump power, bound-soliton operation can be realized by adjusting PC appropriately. As shown in Fig. 4 , the optical spectrum of bound-soliton has a strongly regular modulated structure, whose modulation period is 7.76 nm. The high-contrast interference pattern with a large modulation depth suggests that the pulses bound together keep a fixed and stable phase relationship. The SHG autocorrelation trace of bound-soliton is also shown in the inset of Fig. 4. It should be pointed out that the CW component coexisting with the spectrum of the mode-locking pulse would arise at a pump power higher than 700 mW together with an appropriate setting of the PC, as shown in Fig. 4. This is due to independent lasing in the sub-cavity constructed by the straight cleaved ends of the PGF , which is mechanically connected with other fibers. This phenomenon can be eliminated by using angle cleaving or fusion splicing methods.
In order to further enhance the fundamental repetition rate of the compact all-fiber mode-locking laser, the fiber ring is cut to a shorter length about 60 cm. In addition, the fiber length of the output port is cut to an even shorter length about 5.0 cm to reduce the pulse broadening effect. In this case, we can achieve mode-locking operation with a pump power about 850 mW and an appropriate PC setting. The optical spectrum of mode-locking pulse, centered at 1555 nm, is shown in Fig. 5(a) . At such a high pump power, the CW component appears simultaneously with the mode-locking spectrum due to the sub-cavity effect. With this CW component, the pulse train in Fig. 5(d) is not as stable as that shown in Fig. 2(d). The corresponding RF spectrum is measured by a 12 GHz photodetector together with a 3 GHz spectrum analyzer. The measured RF pulse train and its zoom-in figure of the fundamental mode are shown in Fig. 5(b) and 5(c) respectively. As can be seen in Fig. 5(c), the compact all-fiber ring laser operates at a fundamental repetition rate high up to 325 MHz with a 65 dB signal-to-background ratio, which is just corresponding to the total cavity length. By employing SHG autocorrelation method, the mode-locking pulse duration is measured to be 105-fs, as shown in the inset of Fig. 5(d). Similarly, due to the chirp inside the cavity and broadening by the output lead fiber, the time-bandwidth product is calculated to be 0.485, which is larger than that of the transform limited pulse.
In summary, based on traditional fiber-based components and a 5.7 cm-long highly doped single mode phosphate glass fiber, we demonstrate an all-fiber ring pulse laser with a fundamental repetition rate up to 235 MHz and a pulse duration of 165-fs. To further compact the structure and increase the fundamental repetition rate of the all-fiber NPE mode-locking laser, an optical integrated module is fabricated. With a much more compact cavity configuration, a 325 MHz fundamental repetition rate fiber laser with a pulse duration of 105-fs is demonstrated. This work proves the feasibility of high fundamental repetition rate in a more compacted cavity. The fundamental repetition rate is expected to be scaled up with an appropriate improvement of the cavity parameters.
This research was supported by the National 863 Hi-tech Program (2011AA030203), the Guangdong and Hong Kong Invite Public Bidding Program (TC10BH07-1), the Science and Technology Programs (2009A090100044, 2009B091300127 and 2010B2101230), the Guangdong Education Department Project (2009N9100200 and cgzhzd0903) the Fundamental Research Funds for the Central Universities (2009ZM0219, 2011ZZ0001, 2011ZG005 and 2009ZZ0054) and the NSFC (U0934001, 51072060 and 60977060).
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