We report on an all-fiber femtosecond Ytterbium laser without dispersion compensation operating in the similariton pulse regime. The oscillator was mode-locked with a saturable Bragg reflector along with nonlinear polarization evolution. A pulse energy of 0.8 nJ was achieved with a pulse duration of 10 ps. The pulses could be externally dechirped to 627fs which was within 8.7% of the Fourier transform limited pulse duration. The realized oscillator combines compact size, high stability and alignment-free operation.
©2007 Optical Society of America
In the last years passively mode-locked Ytterbium fiber lasers have been intensively investigated owing to their potential in realizing reliable and cost-effective ultrafast light sources around 1 μm, which are highly requested for many applications like frequency metrology or biophotonics. One main goal of the present research is the maximization of the output energy of those laser systems. This effort resulted in the realization of self-similar operation which has commonly been recognized as a promising route to high energy pulses [1, 2]. A second but not less important topic is the realization of compact, high stability and alignment-free complete fiber-based set-ups, similar to current ultrafast Erbium fiber lasers at 1.5 μm . In this wavelength range, gain fibers have a normal group velocity dispersion (GVD) and passive fibers with anomalous GVD above 1.3 μm (zero dispersion point of fused silica) can be used for an efficient dispersion management. This simple implementation is not possible at 1 μm, thus either bulk intracavity grating pairs , photonic crystal fibers [4, 5, 6, 7], higher-order mode fibers  or chirped fiber Bragg gratings  have been used to provide anomalous GVD. Furthermore, instead of dispersion control spectral filtering with an interference filter has successfully been applied . In another approach Adel et al. reported on a mode-locked Ytterbium fiber laser system operating without any anomalous dispersion control . This idea has been continued by Herda and Okhotnikow, who recently published their results on a dispersion compensation-free ultrafast Ytterbium fiber laser, which has been mode-locked by a saturable absorber mirror generating low energy pulses . In this study pulse durations were in excess of several picoseconds due to the highly uncompensated chirp. For these lasers without dispersion compensation a stable mode-locking is possible due to filtering effects by the gain bandwidth and in  by the saturable absorber respectively acting as a effective dispersion compensation. In the similariton pulse regime such filtering effects are more stringent because of the high chirp of the pulses . Therefore it is possible to generate stable pulses trains with a highly linear chirp characteristic for similaritons.
Here, we report to the best of our knowledge for the first time on a mode-locked all-fiber laser at 1 μm without internal dispersion compensation operating in the similariton pulse regime. We have successfully integrated discrete parts by fiber integrated components, spliced to an all-fiber ring cavity providing an alignment free and stable single pulse operation.
2. Experimental setup
The experimental setup of the all-fiber laser is shown in Fig. 1. A 16 cm long highly doped Ytterbium gain fiber (1200 dBm-1 absorption at 976 nm) was core pumped by a single mode laser diode with an output power of 400 mW via a 976 nm / 1026 nm wavelength division multiplexer. For unidirectional operation a fiber coupled polarization dependent isolator was placed 0.46 cm behind the gain fiber, whose rejection port was used as the output port. This isolator was optimized for a wavelength of 1064 nm and therefore caused increased losses at 1028 nm but did not introduce any significant spectral changes in the transmitted or reflected power spectrum, which was ensured with a broadband spectrum before integrating in the laser. A fiber coupled polarization beam splitter (PBS) followed 102 cm of standard fiber behind the isolator. Both the fiber coupled isolator and the fiber coupled PBS were alignment-free micro-optical parts. In combination with a polarization controller (PC), a sigma branch with 94cm standard fiber and a fiber coupled saturable Bragg-reflector (SBR) was realized followed by 2.6 m of standard fiber including a 2 % linear output coupler acting as a monitor port. The commercially available fiber coupled SBR had a low-intensity absorption of 40 %, a modulation depth of 24 %, a relaxation time constant < 500 fs and a saturation fluence of 130μJ cm-2 . It was shown by Herda et. al. that a high value of nonlinear reflectivity of the SBR allows reliable self starting continuous wave mode locking with picosecond pulses for large values of normal net dispersion . For our setup, we calculated a net dispersion of 0.147 ps2. In order to manipulate the polarization state for the nonlinear polarization evolution (NPE) in the fiber section  we used mechanical polarization controller. Although the SBR dominated self-starting operation, the output characteristics were still sensitive to the settings of the polarization controllers. This indicates that the SBR enabled the start-up characteristics of the mode-locking process whereas the pulse shaping in the steady state was caused by NPE . Due to the highly chirped nature of the pulses strong influences of temporal or spectral filtering effects are present. By adjusting the polarization controller we manipulate the state of polarization for the NPE leading to slightly changed transmission characteristics and filtering effects respectively. As a result changes in the spectral width up to 1 nm could be observed. However, it is to mention that the laser always operates in the self similar regime.
3. Experimental results
In Fig. 2(a) the output power in respect to pump power is shown. Quasi-continuous-wave operation started at pump powers of 48 mW and self starting mode-locking operation was achieved for pump powers above 148 mW. For pump power above 260mW multiple pulse operation could be observed due to overdriving of the virtual saturable absorber (NPE). By measuring the power at the 2 % linear coupler we estimated the fluence on the SBR. We operated the SBR a factor of six over the saturation fluence which is a typical value for stable single pulse mode-locking . Therefore we conclude that overdriving of the SBR was not the reason for the multiple pulse operation.
The slope efficiency was 9 % and a maximum output of 28 mW could be achieved for stable single pulse operation, which was verified by using a combination of a long range second harmonic autocorrelator (150 ps scanning range) and a fast InGaAs-photodetector (rise-time <70 ps) with a 1 GHz oscilloscope. A typical power spectrum of the generated similaritons with its characteristic steep edges is shown on a logarithmic and a linear scale in Fig. 2(b). It should be noted that for increasing pump power above the mode-locking threshold a slight increase of the spectral bandwidth could be observed. More important, no significant changes in the spectral shape could be observed, which is an indication for self-similar operation. The measured spectral width was 4.2 nm (FWHM) centered at a wavelength of 1028 nm.
In Fig. 3(a) the measured time signal of the photodiode and the corresponding RF-spectrum (Fig. 3(b)) are shown. As can be seen, the fundamental repetition rate was 34.8MHz corresponding to a maximum pulse energy of 0.8 nJ at maximum average output power. Since the laser operated at a large normal net dispersion without any dispersion compensation, strongly chirped output pulses were expected at the rejection port. In Fig. 4(a), the measured second order autocorrelation trace at the rejection port is shown. The autocorrelation width of 14 ps corresponds to a pulse duration of 10 ps assuming a Gaussian shape for similariton pulses . From the measured power spectrum we calculated the bandwidth limited pulse duration to 548 fs by a fast Fourier transformation assuming a zero phase (Fig.4(b)). This chirp factor of 13 points out that the output pulses were highly chirped.
The output pulses were externally dechirped with a grating compressor with an anomalous dispersion of -0.95 ps2. The dechirped pulses had an autocorrelation width of 873 fs corresponding to a pulse width of 629 fs assuming a Gaussian profile (Fig. 4(b)). Therefore the measured pulse duration was within 8.7 % of the Fourier transform limit pointing out the highly linear chirp of the pulses. The small wings in the autocorrelation traces indicate uncompensated higher order dispersion, which resulted mainly from the positive third order dispersion (TOD) in the cavity (1.4∙105 fs3) and the positive TOD generated by the used external grating compressor (2.2∙106 fs3).
The large positive chirp of the pulses at the output port can not be removed during the round trip as the pulse compression is only caused by filters. It is obvious that neither spectral (gain bandwidth) nor temporal (saturable absorbers) filters can flip the chirp from positive to negative. Therefore we conclude, that the pulses were positively chirped at every position in the cavity being also an evidence for self-similar operation in a laser.
In conclusion we presented to the best of our knowledge the first all-fiber ring cavity operating in the similariton pulse regime without any dispersion compensation. The spectra were centered around 1028 nm with a half width of 4.2 nm. The output pulses had a pulse duration of 10 ps assuming a Gaussian pulse shape and a pulse energy of 0.8 nJ, although the fiber coupled isolator was not optimized for laser operation at 1028 nm. The pulses could be dechirped to 629 fs, which was only a deviation of 8.7 % of the Fourier transform limit. With this all-fiber setup an alignment-free, highly compact and stable femtosecond laser source at 1 μm was realized. Further system integration should be possible by using a fiber pulse compressor based on a hollow core photonic bandgap fiber .
This research was supported by the Deutsche Forschungsgemeinschaft in the frame of SFB 407.
References and links
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