Compact high-repetition-rate femtosecond fiber lasers have a lot of applications such as optical frequency metrology [
Photonics Research, Volume. 4, Issue 1, 0027(2016)
Compact 517 MHz soliton mode-locked Er-doped fiber ring laser
We report on a compact passive mode-locked Er:fiber ring laser operated at the fundamental repetition rate of 517 MHz, which we believe is the highest fundamental repetition rate ever reported in a ring cavity fiber laser. The key technique is the employment of two innovative high-power wavelength domain multiplexer collimators with all gain fiber cavity suited for the high power (up to 2 W) pumping. The laser is featured with a direct chirpfree output pulse, which is 97 fs without extracavity compression at an average output power of 90 mW.
1. INTRODUCTION
Compact high-repetition-rate femtosecond fiber lasers have a lot of applications such as optical frequency metrology [
Fiber lasers employing a linear cavity and saturable absorber have reached the GHz level [
A ring cavity adapting nonlinear polarization evolution (NPE) relies on a fast saturation absorption mechanism and takes advantage of very short pulse generation [
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Therefore, we have been trying to increase the pulse repetition rate to take advantage of high output power and short pulses.
To further increase the pulse repetition rate with silica-based Er:fiber, besides using a short fiber cavity, we have to increase the pump power. This requires removing all nongain fibers. We developed a so-called active wavelength domain multiplexer (WDM) collimator that uses implemented gain fiber as the pigtail to reduce the cavity length without losing gain. However, the fiber can be easily damaged at the pump power above 0.8 W. The solution is to enlarge the mode field size of the fiber tip inside the WDM collimator.
In this paper, we report a 517 MHz repetition rate, NPE mode-locked Er:fiber soliton fiber laser. With the anomalous dispersion gain fiber, the laser can be easily mode locked in the soliton regime.
2. DESIGN AND CONSTRUCTION OF THE FIBER LASER
To make the laser mode locked easier at such a high repetition rate, we intend to operate the laser in the soliton regime. The schematic of the cavity is illuminated in Fig.
Figure 1.Schematic of the Er fiber laser. ISO, Faraday isolator; PBS, polarization beam splitter;
3. CHARACTERIZATION OF THE FIBER LASER
The pump power was coupled into the cavity via both WDM collimators. Because of the large difference between the pump and the laser wavelengths, the WDM has a higher loss than the 980/1030 loss for Yb:fiber lasers. The pump source is two pairs of 974 nm laser diode modules, which offer a pump power up to 1100 and 900 mW from each side, respectively. At the maximum pump power, the CW output power was 236 mW, showing an overall efficiency of 11.8%. The mode locking can be easily initialized with the standard NPE at above the pump power of 2 W. When mode-locked, the laser average output power is 90 mW. The fundamental repetition rate of the mode locking is 517 MHz (Fig.
Figure 2.Radio frequency spectrum up to 3 GHz with the resolution bandwidth of 30 kHz.
The pulse spectrum is shown in Fig.
Figure 3.(a) Soliton pulse spectrum measured in linear scale (black) and simulation result (red). (b) Spectrum measured in log scale (black) and simulation result (red).
Figure 4.(a) Fringe-resolved autocorrelation trace of the direct output pulses. (b) Inset is the calculated intensity autocorrelation trace of transform-limited pulse is about 73 fs (blue line) and simulated pulse profile with 82 fs (red dot). In both cases, the pulse profiles are assumed to be secant hyperbolic.
This short pulse will benefit the subsequent power amplification and spectrum expansion for making it an optical frequency comb. However, the direct output power drops from 230 mW at CW to 90 mW at mode lock, which is not yet understood.
Figure
Figure 5.Evolution of pulse duration inside the laser cavity. Gain fiber, CorActive Er-doped gain fiber (Er80-8/125); NPE, saturable absorber.
4. CONCLUSION
We have demonstrated a femtosecond mode-locked Er:fiber ring laser operated at 517 MHz fundamental repetition rate. To the best of our knowledge, this is the highest fundamental repetition rate yet reported in a ring cavity fiber laser. The innovative 980/1550 WDM collimator ensures the laser remains compact and stable. The optical spectrum width is 30 nm, and the direct output pulse duration is 97 fs without extracavity compression. The output power at the pump power of 2 W is 90 mW corresponding wih the pulse energy of 0.18 nJ. Compared with our previous report on 464 MHz Er:fiber lasers [
Further increasing the pulse repetition rate involves a higher pump power and highly doped fiber such as the phosphate glass fiber.
Acknowledgment
Acknowledgment. This work was partially supported by the Major National Basic Research Program of China (2013CB922401) and the National Natural Science Foundation of China (60927010, 10974006, and 11027404).
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Jian Zhang, Ziyun Kong, Yizhou Liu, Aimin Wang, Zhigang Zhang, "Compact 517 MHz soliton mode-locked Er-doped fiber ring laser," Photonics Res. 4, 0027 (2016)
Received: Apr. 29, 2015
Accepted: Jun. 23, 2015
Published Online: Sep. 26, 2016
The Author Email: Zhigang Zhang (zhangjianzj199009@163.com)