Ultrafast fiber lasers have rapidly attracted significant attention in the past decade with the development of optical fiber devices [
Photonics Research, Volume. 3, Issue 4, 129(2015)
Optimization of spectral distortion in a ytterbium-doped mode-locked fiber laser system
A method for optimizing the spectral distortion of an ultrafast pulse in a polarization-maintaining picosecond linear-cavity fiber laser with a one-stage fiber amplifier is proposed and demonstrated. The mechanism of control of the spectral distortion in the fiber system has been investigated. The experimental and theoretical results illustrate that the filtering effect of a fiber Bragg grating can effectively decrease the spectral oscillatory distortion accumulated by self-phase modulation. Injected into a Nd:YAG regenerative amplifier, the ultrafast pulse could produce high pulse energy of 1.2 mJ at a repetition rate of 1 kHz.
1. INTRODUCTION
Ultrafast fiber lasers have rapidly attracted significant attention in the past decade with the development of optical fiber devices [
In some delicate applications, such as laser ranging or frequency doubling, another important feature is the shape of the spectrum. It is our main concern to achieve smooth spectra with reduction of spectral distortion, especially for a multi-longitudinal-mode Yb-doped ultrafast fiber laser with a linear all-fiber cavity. In order to generate a smooth spectrum, the mechanisms of output spectrum broadening should be analyzed and understood. These questions have been explored in only a few papers [
The traditional method of improving spectral oscillatory fringes is to sacrifice the output power or increase the diameter of the fiber. In comparison with previous work, we came up with a novel method, i.e., using the filtering effect of a fiber Bragg grating (FBG) to effectively reduce frequency chirp induced by SPM effects and to optimize the spectral distortion.
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2. EXPERIMENTAL SETUP
The schematic diagram of the fiber laser is shown in Fig.
Figure 1.Picosecond pulse Yb-doped fiber laser source based on a MOPA configuration. ISO, optical isolator.
The oscillator is based on a linear cavity, in which the mode field diameter of the Yb-doped fiber is 6 μm with Yb-doped absorption (at 976 nm) of 219 dB/m. The Yb-doped fiber length is 0.4 m. The gain fiber is core-pumped by a LD via a polarization-maintaining (PM) fiber wavelength division multiplexer (WDM). The back mirror is a homemade fiber-coupled SESAM, which is glued to a fiber optic connector (FC/PC). The front mirror of the oscillator is a high reflectivity (60%) FBG of 0.22 nm bandwidth, which supplies the filtering effect for a mode-locked pulse. The total fiber length of the laser cavity is 2.6 m, radiating a pulse train with repetition rate of 38 MHz. The linear polarization mode-locking pulse propagates along the fast axis of the fiber. The oscillator output fiber is connected to a one-stage PM fiber amplifier. The backward pump is introduced into the fiber amplifier, the purpose of which is to reduce the nonlinear effects.
3. RESULTS AND DISCUSSION
Stable mode-locked self-starting is obtained at a threshold pump power of 80 mW, as shown in Fig.
Figure 2.(a) Typical output power and slope efficiency of an oscillator, and (b) amplified output power and slope efficiency.
The corresponding change in spectrum of the oscillator is shown in Fig.
Figure 3.(a) Output spectra versus different output power. All the curves are measured by an optical spectrum analyzer with 0.02 nm resolution. (b) Intensity autocorrelation trace of an amplified mode-locked pulse.
The spectra of the amplifier are shown in Fig.
Figure 4.(a) Spectra versus laser power from 8 to 71 mW. (b) Spectra versus laser power from 82 to 172 mW. The bandwidth of the FBG in the fiber oscillator is 0.22 nm.
The disadvantage of considerable spectral broadening is that it is not conducive to amplification by a laser crystal. This is because the pulse spectrum is much broader than the bandwidth of the laser crystal, so that only a fraction of the pulse energy will be amplified. To solve this problem, we should know the mechanism of the spectral broadening, which indicates that the origin of the spectral structure can be understood by referring to a time dependence of the SPM-induced frequency chirp. The spectra, shown in Fig.
The pulse evolutions in Fig.
Figure 5.Spectral evolution of the incident spectra introduced by SPM with a nonlinear Schrödinger equation fast Fourier transform algorithm: (a) Spectral bandwidth of 0.18 nm, (b) Spectral bandwidth of 0.12 nm.
The results of spectral evolution in the fiber of Fig.
To solve the above-mentioned spectral distortion for the linear cavity, we decided to adopt the more narrow FBG spectral width of 0.15 nm to decrease the frequency chirp induced by SPM. The reflection spectra of two types of FBG have the same Gaussian shape, which ensures that the unique difference between them is caused by the spectral bandwidth. The spectra of mode-locked trains amplified by the one-stage fiber amplifier are shown in Fig.
Figure 6.(a) Spectra versus laser power from 8 to 71 mW. (b) Spectra versus laser power from 82 to 172 mW. The bandwidth of the FBG in the fiber oscillator is 0.15 nm.
Figure 7.Intensity autocorrelation of amplified mode-locked pulse.
To demonstrate the significance of optimizing spectral distortion, we injected the pulses generated in Fig.
4. CONCLUSION
We have investigated the mechanism of spectral distortion in a picosecond fiber system. Through numerical simulations and experiments, it is shown that the spectral broadening and oscillatory profile with increasing power in the mode-locked fiber laser is attributed to frequency chirp induced by the SPM effect. The use of the filtering effect of a FBG is an effective solution to optimize spectral distortion. By adopting the above method, an ultrafast fiber laser system could be used as the oscillator of a solid regenerative amplifier, and has largely transcended the conventional solid mode-locked laser. The amplified pulse could produce high pulse energy of 1.2 mJ with a repetition rate of 1 kHz by injecting into the Nd:YAG regenerative amplifier.
[6] B. Ortac, M. Plotner, T. Schreiber, J. Limpert, A. Tunnermann. Environmentally-stable wave-breaking-free mode-locked Yb-doped all-fiber laser. Proc. SPIE, 6873, 68731M(2008).
[7] M. A. Lapointe, M. Piche. Linewidth of high-power fiber lasers. Proc. SPIE, 7386, 73860S(2009).
[12] G. P. Agrawal. Nonlinear Fiber Optics(2010).
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Fuqiang Lian, Zhongwei Fan, Zhenao Bai, Xiaohui Li, Qi Jie Wang. Optimization of spectral distortion in a ytterbium-doped mode-locked fiber laser system[J]. Photonics Research, 2015, 3(4): 129
Category: Lasers and Laser Optics
Received: Jan. 6, 2015
Accepted: Apr. 7, 2015
Published Online: Jan. 6, 2016
The Author Email: Zhongwei Fan (aoefiberlaser@126.com)