We demonstrate 503MHz fundamental high repetition rate operation in a ring cavity passively mode-locked Yb:fiber laser incorporating a novel wavelength–division-multiplexing collimator and a piece of all-solid photonic bandgap fiber. The Yb doped fiber was directly fabricated as one fiber pigtail into the functional collimator, greatly shortening the cavity length and facilitating the splicing operation. A 5cm long photonic bandgap fiber with abnormal dispersion at the lasing wavelength (centered at 1030nm) decreases the net dispersion for shorter output pulses. The spectral bandwidth of the pulse was 34nm. The direct output pulse was measured to be 156fs and the dechirped pulse was about 76fs. With this innovative Yb:fiber pigtailed WDM collimator, the ring cavity laser has the potential to work at a repetition rate up to GHz.
©2011 Optical Society of America
Compact and easy fabricating high repetition rate femtosecond fiber lasers are required for a variety of applications, such as frequency metrology  and ultrafast sampling . Particularly, the high repetition rate Yb:fiber laser are desirable to achieve sufficiently large comb line spacing for high-precision calibration of astronomical spectrographs [3,4]. Although high repetition rates can be achieved with harmonic mode-locking, the repetition rate is not sufficiently stable for those applications. Linear cavity Yb:fiber lasers, mode-locked by saturable absorbers such as semiconductor saturable absorbers have the potential in running at a fundamental repetition rate up to GHz level [5–8]. It is because the fiber tailed wavelength division multiplexer (WDM) can be placed outside the linear cavity, greatly decreasing the cavity physical size and enhancing the repetition rate. However, the saturable absorber pulse shaping mechanism in linear cavity limits the spectral bandwidth to a few nanometers. The nonlinear polarization evolution (NPE) with large modulation depth and essentially instantaneous response has been proved to be able to produce compressable shorter pulses . 200MHz high repetition rate femtosecond pulses generation was presented by Ilday et al in a stretched-pulse Yb:fiber laser . The repetition rate up to 250-300MHz was predicted by shrinking the free-space region and minimizing the undoped fiber length with a careful engineering effort. The high repetition rate operation up to 570 MHz in Yb:fiber laser with an intracavity transmission grating pair and high pump power (1.4W) was demonstrated by Wilken et al . It should be noticed that in order to shorten the cavity, the deposition of the pump power was made by direct free-space coupling, which degrades the system stability.
All-normal dispersion (ANDi) fiber laser without the intracavity grating pair can save some space for a higher repetition rate [12,13]. However, the large net normal dispersion of ANDi fiber laser can increase the amplitude and frequency noise of the laser, and destroy the fCEO signal in a laser frequency comb [14,15]. Therefore, dispersion compensation is needed to minimize jitter, and to produce a narrower fCEO linewidth in a frequency comb.
Instead of bulky grating pair that needs careful alignment efforts, and may degrade the long-term stability, photonic crystal fibers (PCFs) [16–18] have been developed as in-line fiber components to provide the anomalous dispersion for mode-locked fiber lasers. This kind of fibers are still rare in laboratories, because they have also some drawbacks: for small-core PCF with abnormal dispersion, the combination of large effective nonlinearity (through the small core size) and anomalous GDD (group delay dispersion) in a PCF will restrict its use to low energy (1 nJ or below) lasers ; hollow-core photonic bandgap fibers (PBGFs) are difficult to realize the all-fiber system due to the splicing difficulty . However, all-solid PBGFs  have shown more dispersion engineering flexibilities, transmission bands control and rare-earth doping capabilities at the same time. The functions of dispersion compensation and gain medium can be incorporated in the same piece of Yb-doped all-solid PBGF. This will facilitate the high repetition rate (several GHz) mode-locked fiber lasers, and subsequent astro-combs. On the other hand, it is also not easy to design and fabricate suitable bandgap fiber that can balance its dispersion, nonlinearity, bandgaps, gain and confinement loss. Those parameters greatly affect the pulse evolution and so laser performance. To date, the reported bandwidths of an Yb fiber laser using photonic bandgap fibers for dispersion compensation were normally limited to 20nm, resulting in a pulse width of 100fs [16–18].
Here efforts have been made to obtain a compact and easy-fabricating high repetition rate femtosecond fiber laser. We demonstrate a 503MHz mode-locked Yb:fiber ring laser with an innovative wavelength–division-multiplexing collimator (WDM collimator) and a short piece of all-solid PBGF. The full width at half maximum (FWHM) spectrum was 34nm. The pulse duration of direct output laser is measured to be 156fs, and the dechirped pulse width is 76fs. The direct output power from fiber laser is 160mW with 850mW 975nm pump laser diode.
2. The cavity design and arrangement
The longest fiber component in an Yb:fiber laser is the WDM that is a 7-9cm long tube plus at least 5cm long pigtails for splicing. To shorten the fiber cavity and to avoid the spatial coupling, one must find a fiberized WDM component. We have developed an innovative WDM collimator to replace the traditional three-fiber-port WDM and collimator. The schematic of the WDM collimator is presented in Fig. 1 , one single mode fiber and one Yb doped fiber are hold in a twin-core capillary, followed by a lens. A dichroic filter (the WDM filter) is directly fixed on the other side of the lens. The collimated pump beam (975nm) is reflected by this filter, and coupled back directly into the Yb doped fiber with the total coupling efficiency of 85%. The emission from Yb doped fiber can be collimated by the lens and pass through the WDM filter as output beam. With the above optical elements, this WDM collimator combined the functions of a WDM and a collimator.
Compared with the conventional fiber laser, our new WDM collimator greatly shortens the cavity. In addition, the fiberized coupling has a better stability than the spatial coupling , and relaxes the critical pump alignment. Furthermore, the WDM can keep the leading fiber in reasonable length for comfortable splicing, which makes the engineering of the high repetition rate (>500MHz) laser easier, decreasing the risk of breaking fiber components during the splicing.
The all-solid PBGF used in our laser cavity was pulled from the same preform as in reference . Its transmission band is ranged from 930nm to 1050nm at 3dB with the confinement loss of about 1dB/m at 1030nm, shown in Fig. 1 (Fiber a) of reference . The dispersion of this fiber is estimated to be about −100fs2/mm at 1030nm. The core is lowly doped with Yb ions. The Yb-doped region has a radius of roughly 1μm whereas the waist of the fundamental mode at 977 nm is about 3.46μm. Limited by the low Yb-doping density and short length, in this case, the PBGF mainly acts as the anomalous dispersion segment. Due to its all solid structure, this PBGF can be easily spliced to the standard single mode fiber with the coupling loss of about 2 dB (two splicing points), which can be improved by adjusting the optimizing parameters or redesigned the bandgap fiber.
The cavity configuration is shown in Fig. 2 , which includes a gain fiber, one WDM collimator, one normal collimator and several free-space polarization controlling components. The pump beam was coupled into the Yb doped fiber by the WDM collimator. It is a segment of 17cm long highly doped Yb:fiber (Coractive YB120) with over 1200dB/m absorption at 975nm. The laser beam at 1030nm is collimated and will transmit the dichroic mirror as an output. The calculated group velocity dispersion (GVD) coefficient of Yb doped fiber is + 23fs2/mm, so the total dispersion of gain fiber is + 3910fs2. 5 cm all-solid PBGF was spliced to the end of Yb doped fiber to provide the negative dispersion of about −5000fs2 at 1030nm in the cavity. The other end of PBGF was spliced with one commercial collimator with the single mode pigtailed fiber. The coupling loss between the WDM collimator and standard collimator is about 1dB. Standard single-mode fiber (SMF) has a calculated GVD of +24fs2/mm at the wavelength of 1.03μm, and the dispersion of 7 cm pigtailed SMF is about +1680fs2. The total dispersion of the fiber region is calculated to be +590fs2. The nonlinear phase shift is calculated to be 0.4π. A bulk Faraday rotator, half wavelength plate and two polarization beam splitters (PBS) are used as an isolator to impose unidirectional operation. The laser was pumped by combined two single mode coupled diodes which supply a total power up to 850mW. The mode locking was initialized with the rotation of the waveplates and the birefringent filter. The output pulses are rejected from the PBS and dechirped with a pair of gratings outside the cavity.
3. Characteristics of the output pulses
At the maximum pump power, the CW output was ~200 mW, indicating an efficiency of 24%. This low efficiency is partly due to the high splicing loss between PBGF and other fibers. When mode locked, the laser delivers an average output power of 160 mW at the repetition rate of 503MHz (shown in Fig. 3 ), resulting in a single pulse energy of 0.32 nJ.
The spectrum width (FWHM) of the output pulse was measured to be 34nm, shown in Fig. 4(a) . The calculated Fourier-transform limited pulse width was 67fs, shown in Fig. 4(b). The measured autocorrelation traces of the direct output pulses and the dechirped pulses are shown in Fig. 5(a) and (b) . The direct output pulse width was estimated to be 156fs. The dechirped pulses obtained with the 600 grv/mm grating at a separation of 5 mm was deconvoluted to be 72fs for a Gaussian profile assumed.
Based on the measured spectrum and balanced correlation trace, the pulse intensity and phase of the compressed pulses are obtained with PICASO (phase and intensity from correlation and spectrum only) method , and are shown in Fig. 5(c). The retrieved pulse duration is about 76fs, agreeing with the autocorrelation results. The pre-pulse amplitude is 20% of the main maximum peak. This is mainly caused by the high third-order dispersion of the used PBGF , since the laser spectrum is located near the red edge of PBGF’s transmission band. The output power after grating compressor is about 80mW, and the pulse peak power is estimated to be about 1.8kW.
As discussed in Ref , the major difficulties for high repetition rate operation in a fiber laser are the physical size of the fiber components and the limited pump power.
Our innovative Yb:fiber pigtailed WDM collimator makes the Yb:fiber ring laser easily obtain 503MHz repetition rate. With a higher Yb ions doping density, the gain fiber can be shortened so that to be able to work at a higher repetition rate to >600MHz.
Furthermore, if another Yb doped fiber collimator can be used as the second collimator, the laser cavity can be composed of only gain fibers, and laser cavity length can be further decreased. In addition, with highly-doped gain fiber, the total Yb:fiber length can be <10cm and GHz repetition rate can be expected in all-normal dispersion mode locking regime.
For the given pump power, a shorter fiber cavity length results in a lower pulse energy, so that a smaller nonlinear phase shift to be accumulated. As a result, the pump power threshold for mode-locking increases with the repetition rate. Our simulation based on ANDi fiber laser (similar configuration as Fig. 2 without PBGF) indicates that for the repetition rate of 1GHz, the laser would require over 1.5W of pump power that is out of our availability.
PBGF with anomalous dispersion can compress the pulse inside the cavity to accumulate more nonlinear phase shift, lowering the mode locking threshold. However, a PBGF may introduce a higher splicing loss than a single mode fiber does, which compromises the intracavity shorter pulses. The development of highly Yb-doped all-solid PBGF may be required and can further lift the repetition rate with reasonable pump power.
The use of PBGF in the laser cavity helps to obtain a short femtosecond pulse output without extracavity pulse compression, suitable for some nonlinear applications. In contrast to 1~2 ps pulses directly from ANDi Yb fiber laser at the relatively lower repetition rate (<100MHz) , the output pulse pulse (156fs) of this laser is much shorter, benefited from the short fiber cavity and dispersion compensation of PBGF.
We have demonstrated a compact Yb:fiber laser operating up to 503MHz repetition rate with an innovative Yb:fiber pigtailed WDM collimator. A piece of all-solid PBGF was used to compensate the cavity dispersion to obtain a higher nonlinear phase shift, a shorter pulse output. The optical spectrum bandwidth of pulses is 34nm corresponding a transform limited pulse of 67 fs. The direct output pulse is 156 fs, and was dechirped to 76fs.
The high repetition rate and direct short pulse output promise this Yb:fiber laser to be a possible direct source of octave spanning spectrum for the frequency comb generation. The further higher repetition rate operation is possible by directly incorporating the highly Yb doped core into the all-solid PBGF and fabricating WDM collimator with this fiber as output pigtails.
The authors wish to thank Minglie Hu and Cheng Qian for the discussions on the pulse measurement, and thank G. Bouwmans and L. Bigot for providing the all solid PBGFs. This work was partially supported by the Nature Science Foundation of China (60927010, 10974006, 11027404, and 60907040).
References and links
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