Modelocking in an Yb-doped figure-eight fiber laser is demonstrated utilizing anomalous dispersion from an LP02 higher-order-mode fiber for dispersion management. Outside the laser cavity, the pulses were re-compressed to 95 fs using a second HOM module, the shortest demonstrated pulses to date from an Yb-doped figure-eight fiber laser. Operation of the laser with HOM fiber in the cavity is compared to an Yb figure-eight laser that utilizes all-normal dispersion fibers.
©2007 Optical Society of America
Passively-modelocked, Yb-doped fiber lasers are reliable sources of ultrashort pulses in the 1 μm wavelength region. At these short wavelengths one of the difficulties lies in finding fibers with anomalous dispersion to compensate the normal dispersion of standard single mode fibers (SMF) and create dispersion managed laser cavities. Dispersion management of fiber laser cavities is desirable as proper dispersion compensation produces the broadest modelocked spectra and shortest pulses . Photonic crystal fibers and photonic bandgap fibers with anomalous dispersion have been utilized for dispersion compensation in nonlinear polarization evolution ring lasers [2,3]. However, to date, only relatively long (850 fs) pulses have been demonstrated from an Yb-doped figure eight laser . In contrast to lasers based on nonlinear polarization evolution, figure-eight lasers do not inherently depend on polarization rotation for mode-locking. Therefore, they have the advantage of being compatible with an environmentally stable, polarization maintaining configuration .
Recently, anomalous dispersion at wavelengths shorter than the zero dispersion wavelength of silica was demonstrated in a specially designed fiber in which light propagated in a single higher-order mode (HOM) . Light was coupled into and out of the HOM with the use of UV written, broadband, long period gratings (LPG). In this work, we demonstrate an Yb-doped figure-eight fiber laser producing 95 fs pulses, that utilizes the HOM fiber for dispersion compensation. To the best of our knowledge, these pulses are the shortest produced to date in an Yb-doped figure-eight laser.
Details of the higher order mode module are shown in the inset of Fig. 1(a). Long period gratings are used to couple into and out of the LP02 mode of the fiber from the LP01 mode. The measured transmission spectrum, plotted in Fig. 1(a), had a 1-dB bandwidth of 51 nm. Typical module insertion loss was < 0.5 dB, including the splice loss between the HOM fiber and SMF pigtails. The dispersion of the fiber, plotted in Fig. 1(b), was measured using spectral interferometery. The LP02 mode of this fiber had an anomalous disperison paramater of +60 ps/nm-km at 1080 nm, compared to the normal dispersion of standard SMF at these wavelengths.
A schematic of the figure-eight laser is shown in Fig. 2. Two 975-nm diode lasers were used to bi-directionally pump an 82-cm length of Yb-doped fiber. Total launched pump power was a maximum of 400 mW. A 50/50 splitter coupled the nonlinear amplifying loop to a unidirectional loop that contained a 50/50 splitter as an outcoupler, an isolator and the HOM module for dispersion compensation. A polarization controller was placed in each loop to control the polarization state and initiate modelocking. For appropriate settings of the polarization controllers, self-starting modelocking was observed.
Typical spectra corresponding to different states of operation are plotted in Fig. 3(a). Depending on the relative lengths of the SMF compared to the HOM fiber in the cavity, as well as the alignment of the polarization controllers, the laser operated in a wide variety of regimes typical of passively modelocked fiber lasers. With short lengths of SMF in the cavity the laser operated with soliton pulses, typified by the sidebands visible in the pulse sectrum. When the length of normal dispersion SMF in the cavity was increased the laser operated with a spectrum with steep sides, similar to spectra of recently published fiber lasers with all normal dispersion fiber [7, 8]. In addition, the laser could operate with noise-like pulses , characterized by very broad spectra and corresponding strong amplitude and phase noise on the pulse train as measured with an oscilloscope, electrical spectrum analyzer and autocorrelator. The mode of operation was switched between noise-like operation and clean modelocked pulse train with manipulation of the polarization controllers.
The 10 dB spectral width of the modelocked laser is shown in Fig. 3(b) as a function of the repetition frequency (length of SMF) in the laser cavity. Noise-like spectra are plotted with open circles, and clean modelocked spectra with solid squares. The broadest spectrum was measured at a repetition frequency of approximately 18.3 MHz, corresponding to a laser cavity with 0.82 cm of Yb fiber, 2 m of HOM fiber, and approximately 8.5 m single-mode fiber. The net dispersion parameter of the cavity for the broadest measured, clean modelocked spectrum was D = -.23ps/nm (β 2 = +0.14ps2). While the shortest pulses in passively modelocked fiber lasers typically occur for small values of residual normal cavity dispersion, this value is somewhat higher than reported for other lasers. The reason for this is not fully understand, but may have to do with spectral filtering by intracavity components. Spectral filtering has be recently shown to stabilize modelocking in fiber laser cavities for large amounts of normal dispersion . We are currently develop a nonlinear Scrhödinger equation based model of the laser cavitys to better understand this effect in our setup.
Although the anomalous dispersion was provided by higher order mode rather than conventional fundamental mode fiber, the laser displayed behavior typical of soliton and stretched pulse lasers. For example, when the laser operated with the soliton spectrum shown in Fig. 3(a), the pulses tended to break up into multiple pulses in the laser cavity at high pump powers. For some alignments of the polarization controllers, these multiple pulses tended to bunch together, as reported for previous fiber lasers [10, 11]. A typical example of a measurement of a soliton bunch is shown in Fig. 4. While this regime of operation is interesting, and perhaps useful for producing high repetition rate bursts of pulses, in general we operated the laser with a single pulse in the cavity by turning down the pump power. The maximum power out of the laser for single pulse operation was approximately 2 mW, corresponding to approximately 0.1 nJ pulse energy.
In order to better quantify the effect of the HOM module’s dispersion on the laser operation, the HOM module was removed from the cavity. In this setup, all the fibers in the laser had normal dispersion. Recent results have shown modelocking in all-normal dispersion fiber ring lasers [7,8]. We observed modelocking in the figure eight fiber laser with similar characteristics as the previously reported recently in a Yb ring laser with all normal dispersion fiber. . Specifically, the modelocked spectrum shown in Fig. 5(a) displays steep sides with sharp peaks at the edges at the edges. The spectrum without the HOM in the cavity had a 10-dB width of 13.1 nm.
At the maximum pump power with all normal dispersion fiber, the laser continued to operate with a single pulse in the cavity. The average output power was 50 mW and pulse repetition frequency was 33 MHz, corresponding to 1.5 nJ pulses. The output pulse autocorrelation, shown in Fig. 6(a), had a FWHM of 10 ps, whereas the bandwidth limited pulses for the spectrum shown in Fig. 5(a) is 240 fs. Therefore the output pulses were more than 20 times the bandwidth limit and highly chirped. Again, this result is consistent with reports of all normal dispersion lasers and similariton lasers, that operate with highly stretched pulses in the cavity that are many times the bandwidth limit. We have not attempted to compress these pulses outside the laser cavity at this time.
The broadest measured spectrum occurred with the HOM in the cavity, shown in Fig. 5(a) and had a 10-dB width of 44 nm. The corresponding pulse train is shown in Fig. 5(b). The output pulses were then propagated through a second HOM module in order to re-compress them. The length and dispersion of the second HOM used outside the cavity were approximately the same as the module used inside the laser cavity. Note that the autocorrelation was measured after the pulses had propagated through the HOM module and had been converted back to the LP01 mode. The amount of normal dispersion fiber between the laser cavity and the second HOM module was cut back and the correlation was measured. The bandwidth limited pulses of the spectrum in Fig. 5(a) have a FWHM of 72 fs; the shortest measured correlation had a FWHM of 137 fs, corresponding to 95 fs pulse width, assuming a Gaussian pulse shape. The pump power was reduced and the pulse train was monitored with a long range autocorrelator, a fast sampling scope, and an electrical spectrum analyzer to ensure single-pulse operation with the HOM in the cavity. For single pulse operation, the average power was limited to a maximum of approximately 2 mW.
In conclusion we have demonstrated a passively-modelocked, Yb-doped, figure-eight, fiber laser operating at 1080 nm using a higher-order-mode fiber for intracavity dispersion compensation. Although operating with the LP02 mode in the higher order mode fiber, the laser displayed typical features reported in lasers based on all single mode fiber, such as soliton bunches. We also reported modelocking without the HOM module in the cavity and all normal dispersion fibers. In this configuration the laser operated with highly stretched pulses with pulse energies of 1.5 nJ, 50 mW average power, and 33 MHz repetition frequency. In contrast the laser with the HOM for dispersion compensation operated with single pulse operation with 0.1 nJ pulse energy, 2 mW average power and a 10-dB spectral width of greater than 40 nm, more than 3 times as broad as the spectrum from the all-normal-dispersion-fiber laser. The pulses were compressed using a second HOM module outside the laser to 95 fs, the shortest pulses demonstrated to date for an Yb-doped figure-eight fiber laser operating at 1 micron.
References and links
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