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We demonstrate a high-energy femtosecond laser system that incorporates two rapidly advancing technologies: chirally-coupled-core large-mode-area Yb-fiber to ensure fundamental-mode operation and high-dispersion mirrors to enable loss-free pulse compression while preserving the diffraction-limited beam quality. Mode-locking is initiated by a saturable absorber mirror and further pulse shortening is achieved by nonlinear polarization evolution. Centered at 1045 nm with 39-MHz repetition rate, the laser emits 25-nJ, positively chirped pulses with 970-mW average power. 6 bounces from double-chirped-mirrors compress these pulses down to 80 fs, close to their transform-limited duration. The loss-free compression gives rise to a diffraction-limited optical beam (M2 = 1.05).
©2010 Optical Society of America
1. Introduction
Power/energy scaling of ultrafast Yb-fiber oscillators has experienced rapid progress largely driven by many real-world applications that desire high peak and average power femtosecond pulses. For moderate pulse energies, high power/energy Yb-fiber oscillators may replace current complicated master-oscillator-power-amplifier (MOPA) systems. The bottleneck in constructing high power/energy Yb-fiber oscillators arises from the accumulated nonlinear phase due to tight confinement of ulstrashort pulses circulating inside the fiber. Stable mode-locking limits the nonlinear phase below a certain threshold to avoid the onset of detrimental effects, e.g., multiple pulsing. Two orthogonal approaches are being pursuit to overcome this limitation: (1) temporally elongate the pulse inside the fiber and (2) spatially relax the confinement exerted by the fiber to the optical beam. The first approach is implemented via managing laser cavity dispersion. Depending on the amount of net cavity dispersion, the intra-cavity pulse exhibits a duration ranging from sub-ps to ~10 ps, corresponding to different mode-locking regimes (i.e. soliton, stretched-pulse, similariton, and dissipative soliton) [1]. For example, dissipative soliton lasers possess large positive net cavity dispersion. Its intra-cavity pulse is strongly chirped and stretched which in turn substantially reduces the acquired nonlinear phase and thus facilitates formation of high energy pulses. Recently, a 30-nJ dissipative soliton laser based on double-clad single-mode Yb-doped fiber has been demonstrated [2]. The second scaling approach relies on the use of large-mode-area (LMA) Yb-fibers that have been widely incorporated into high power/energy fiber amplifiers. To retain a diffraction-limited beam quality, one of the most important merits for fiber lasers, special care must be taken to ensure that the LMA Yb-fiber operates at fundamental mode (i.e. LP01 mode) although it also accommodates higher-order modes. Using a photonic crystal LMA Yb-fiber (4000 µm2 mode-area) as the gain medium and a fiber-based mode filter to improve intra-cavity beam quality, pulses with sub-µJ energy and sub-ps (711 fs after compression) duration have been generated [3]. For generation of ultrafast (~100 fs after compression) pulses, pulse energies >100 nJ are demonstrated using the same type of fiber with smaller mode-area [4,5]. Despite remarkable success in implementing high power/energy laser system, photonic crystal LMA fibers suffer from high splicing loss and cannot be tightly coiled for compact packaging, which offset key advantages (e.g. robustness and monolithic integration) offered by conventional single-mode fiber technology.
Yb-fiber oscillators normally emit positively chirped pulses. Dechirping these pulses to their transform-limited duration necessitates external compression. Most systems exploit a pair of dielectric diffraction-gratings working at a double-pass configuration to accomplish the compression. Dielectric gratings outperform their standard metal-coated counterparts in terms of power/energy handling capability. Nonetheless, diffraction-grating based compressors demand critical alignment and lack robustness. Even a slight mis-alignment may introduce noticeable spatial chirp and pulse front tilt, thus degrading the excellent beam quality warranted by the fiber oscillator itself. Furthermore, diffraction-grating based compressors introduce a loss of at least 20%.
To address these drawbacks arising from photonic-crystal LMA Yb-fiber and diffraction-grating based compressors, we adopt two rapidly advancing technologies: chirally-coupled-core (3C) LMA fiber that only transmits pure fundamental-mode and high-dispersion mirror (HDM) that permits loss-free, spatial-distortion-free pulse compression. In this paper, we demonstrate such an Yb-fiber laser system that delivers 25-nJ, 80-fs pulses at 39 MHz repetition rate with diffraction-limited (M2 = 1.05) beam quality.
2. Key elements: 3C LMA fiber and HDMs
In contrast to traditional LMA fibers that require special care to ensure fundamental mode propagation, 3C LMA fibers intrinsically support only the LP01 mode, making them indistinguishable from conventional single-mode fibers [6–9]. A typical 3C fiber consists of a large straight central core and at least one smaller side core that is chirally wound around the main core. Higher-order modes of the central core are efficiently coupled to the side core where they are strongly attenuated by a high propagation loss. Thus 3C LMA fibers are effectively single-mode and do not rely on any external mode management or require coiling in any specific geometry. The fibers allow simple low-loss (~0.05-0.1 dB) splicing for both passive-to-passive and gain-fiber-to-passive without causing modal distortion. 3C LMA Yb-fibers have been incorporated into CW fiber lasers achieving up to 250-W of output power [8] and a MOPA system (as the amplifier) generating 150-W, 10-ns pulses at 250-kHz repetition-rate [9]. However, a mode-locked oscillator employing this new class of fiber has not been demonstrated.
Chirped multilayer coatings, such as HDMs, represent another breakthrough in laser technology and have experienced tremendous developments for more than one decade [10]. They are widely used in solid-state (especially Ti:sapphire) lasers to provide broadband dispersion control of few-cycle pulses. Recently, there is considerable research interest in developing HDMs and incorporating them into ultrafast solid-state lasers [11–13]. A HDM provides negative group-delay dispersion (GDD) in the range between −1000 fs2 and −2000 fs2, about three orders of magnitude less than what can be achieved from a diffraction-grating pair. At first glance, HDMs are by no means a substitute for diffraction-gratings to compress chirped pulses in a fiber laser system. This is definitely true for a fiber MOPA system employing chirped-pulse amplification, in which the pulse is typically pre-stretched to > 100 ps. The chirped pulse directly emitted from an Yb-fiber oscillator, however, is typically at the ps level in duration; in this scenario, a compressor constructed from HDMs is entirely possible. As a proof of concept, we successfully dechirp a sub-ps pulse (generated by a 3C Yb-fiber oscillator) to 80-fs duration using 6 bounces from home-designed HDMs (−2000 fs2 GDD for each bounce).
3. Experimental setup and results
Figure 1 illustrates the schematic setup. The 976-nm pump with 45-W maximum power is delivered by a 400-µm core-diameter fiber with 0.22 NA. A dichroic mirror transmits the pump and reflects the lasing pulse that circulates counter-clockwise inside the ring-cavity. Arbor Photonics provided 3C fiber (4.2 m) with an Yb-doped central core of 33.5-μm diameter (21-μm mode-diameter, 0.059-NA) and a 250-μm 0.46-NA hexagonal-shaped inner cladding (inset of Fig. 1). The coupling efficiency of pump light into the 3C fiber length is 75%. The growth of total forward and backward ASE (i.e. amplified spontaneous emission) has a threshold of 4 W absorbed pump power. Mode-locking is initiated by a commercially available saturable absorber mirror (SAM) that has 35% modulation depth and 500-fs relaxation time. A combination of waveplates (i.e. QWP1, HWP, and QWP2 in Fig. 1) and polarizer (i.e. PBS1 in Fig. 1) provides an additional mode-locking mechanism via nonlinear polarization evolution (NPE) that significantly reshapes the intra-cavity pulse. Rotation of these three waveplates leads to different mode-locking states. A pair of 600-line/mm diffraction-gratings at a double-pass configuration provides negative group-delay dispersion to control the net cavity dispersion. A long-wavelength pass filter with a cutoff wavelength at 980 nm blocks the unabsorbed pump to prevent it from causing damage on the SAM. An optical isolator ensures unidirectional circulation of the intra-cavity pulse. Single-pulse operation is monitored using a long-range autocorrelation and optical/RF spectral measurements.
Fig. 1 Schematic setup of the laser system. DM: dichroic mirror; QWP: quarter-wave plate; HWP: half-wave plate; LPF: long-wavelength pass filter; DG: diffraction grating; PBS: polarization beam splitter; SAM: saturable absorber mirror; HDM: high-dispersion mirror. The fiber’s cross-section shows the central core and the side core.
By changing the net intra-cavity dispersion, the laser may run in the similariton or dissipative soliton regime, leading to output spectra with sharp falling edges and spiky features that impose a large pedestal onto the externally compressed pulse. To avoid compromising pulse quality, the net cavity dispersion in the current setup is slightly negative and the laser operates in the stretched-pulse mode-locking regime. The direct output pulse ejected from PBS1 exhibits positive chirp with sub-ps duration. In our system, we use home-designed HDMs as the compressor. The HDM includes 55 alternating layers of Nb2O5 and SiO2; each bounce provides a dispersion of −2000 fs2 with a reflectivity >99.99%. The optimal structure in terms of layer number and thickness is determined by minimizing the integrated phase distortion ratio (PDR) with uniform weighting, which minimizes the fraction of optical pulse energy pulled into phase distortions in each bounce [14]. While this is not necessarily equivalent to minimizing the resulting pulse width, per se, optimizing the PDR has been empirically shown to yield shorter pulses than optimizing GDD or GD [14]. Figure 2 shows the auto-correlation (AC) trace duration of the compressed pulse versus number of bounces from HDMs. The inset (a) plots the designed and measured group delay as a function of wavelength and the inset (b) indicates the corresponding spectrum of the pulse. The AC trace duration continuously deceases from initial 510 fs to 210 fs after 6 bounces and then starts to increase with more bounces. The measured transmission of the compressor with 6 bounces is >99%. It is noteworthy that each bounce shortens the AC duration by about 60 fs prior to the 6th bounce, which agrees with the fact that the optical spectrum has 10-nm bandwidth (inset (a) in Fig. 2), corresponding to a 120-fs FWHM transform limited pulse having a 170-fs AC duration. The deviation of our best compression (210-fs AC duration, inset (b) in Fig. 2) from the transform-limited duration is mostly due to the discrete nature of the dispersion compensation using HDMs with −2000 fs2 dispersion per bounce.
Fig. 2 Duration of AC trace versus number of HDM bounces. Inset: (a) designed (blue, solid line) and measured (red, dashed line) group delay as a function of wavelength and (b) the optical spectrum of the pulse.
In an Yb-fiber laser system that employs a diffraction-grating compressor, optimization of compression is achieved by varying grating separation which results in a tunable dispersion to match the input chirped pulse. As a contrast, the total dispersion from a HDM compressor cannot be continuously adjusted. We can, however, vary the emitted pulse’s chirp to match the HDM compressor by tweaking laser parameters, such as pump power, separation of diffraction-grating pair, mode-locking state owing to NPE etc. Changing the settings of NPE followed by fine tuning of the pump power can significantly improve the compression and bring the compressed pulse close to its transform-limited duration. Figure 3 presents an optimized compression of the laser pulse resulting from a different mode-locking state with increased intra-cavity power. The spectrum (inset (a) in Fig. 3) has twice the bandwidth of the former mode-locking state. The laser emits 970-mW average power corresponding to 25-nJ pulse energy. After 6 HDM bounces, the measured AC trace (red, solid line) has a duration of 110 fs, only 10-fs longer than its calculated counterpart (blue dashed line) using the transform-limited pulse. The full-width-half-maximum duration of the experimentally compressed pulse is estimated to be 80 fs, slightly longer than its transform-limited duration. The RF spectrum, shown in the same figure as inset (b), exhibits a >60 dB signal to background ratio; and the integrated relative intensity noise is less than 1%.
Fig. 3 Compression of a broadband pulse. Two curves represent the measured AC trace (red, solid line) and calculated AC trace (blue, dashed line) corresponding to the transform-limited pulse, and. Inset: (a) optical spectrum and (b) RF spectrum measured at resolution bandwidth of 300 kHz.
In the laser community, M2 has become a standard measure to quantify the optical beam quality. Measuring M2 is widely performed for CW or nano-second pulsed fiber lasers; it rarely appears in the work of high power/energy ultrafast LMA-fiber lasers. To the best of our knowledge, M2 measurements of compressed pulses from LMA fiber lasers have not been reported. To verify that the HDM compressor does preserve incident beam quality from the 3C Yb-fiber oscillator, we have measured M2 of the optical beam before and after the compressor under a variety of laser operation states: non-lasing (i.e. pump power below threshold or cavity blocked), CW lasing, and mode-locking at different states. All the measured values fall into a range of 1.03-1.07. Figure 4 plots the beam profile and M2 measurement for the compressed pulse shown in Fig. 3. M2 = 1.05 evidently confirms that the loss-free compressor based on HDMs maintains the diffraction-limited beam quality that originates from the 3C Yb-fiber. Note that single-mode operation of the 3C fiber has been tested separately by feeding in a broadband optical source and recording the transmitted optical spectrum (similar to S2 measurement [15]); no noticeable modulation is observed, which indicates that the fiber is truly single-mode (i.e. LP01). Therefore the M2 measurement undoubtedly confirms that the HDM compressor is distortion free and we do achieve diffraction-limited beam quality.
Fig. 4 Beam profile and M2 measurement of the compressed pulses. M2 = 1.05 indicates that the HDM based compressor preserves the diffraction-limited beam quality of the 3C Yb-fiber oscillator.
4. Discussion and conclusion
As discussed, the key to energy scaling of fiber lasers is reducing the fiber-optic nonlinear effects by (1) increasing spatial mode-area inside fiber and (2) increasing intra-cavity pulse duration. As for our laser configuration, these two methods translate to demanding a 3C Yb-fiber with larger mode-area and (2) more dispersion from a HDM compressor to dechirp a longer emitted pulse. Work with significantly larger core 3C fibers is ongoing. In the current setup, we use 6 HDM bounces for optimal compression. Compressing longer emitted pulses requires more bounces. Eventually it is the fabrication error on layer thickness that limits the maximum permitted number of bounces before reaching an unacceptable compression quality. By taking into account fabrication errors, 10 bounces on our HDMs introduce negligible phase error for an optical pulse with 40-nm bandwidth. Further optimization should allow a robust mirror design less susceptible to fabrication errors [14].
In conclusion, we demonstrate a high-energy femtosecond laser system based on two rapidly developing technologies—3C LMA Yb-fiber and (−2000-fs2 GDD) HDMs. Working at 1045 nm with 39-MHz repletion-rate, the resulting laser emits 25-nJ pulses with 970-mW average power. A loss-free, spatial-distortion-free compressor constructed from HDMs compresses these positively chirped pulses down to 80 fs, in a diffraction-limited optical beam (M2 = 1.05).
Note added during revision: we are aware that a dissipative soliton oscillator using 3C Yb-fiber has been demonstrated in Ref. [16].
Acknowledgements
The authors would like to thank Kai-Hsiu Liao for assistance in preparing the 3C Yb-fiber. This work was supported by Massachusetts Institute of Technology (MIT) Lincoln Laboratory under contract 70083323 and the Air Force Office of Scientific Research under contract FA 99550-10-1-0063. University of Michigan acknowledges support of US Army Research Office grant W911NF0510572.
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