A large-mode-area Erbium-doped fiber amplifier (LMA-EDFA) based all-fiber-integrated amplified compressor with ultrashort length of 5.37 m and ultralow pumping power (260 mW) is proposed. The LMA-EDFA suppresses nonlinear soliton-self-frequency-shift effect happened during femtosecond pulse amplification, in which the fiber laser pulse is reshaped to a low-pedestal hyperbolic-second shape with nearly 100% energy confinement. The pre-chirped amplification from 0.96 to 104 mW and the simultaneous compression of a passively mode-locked fiber laser pulse from 300 to 56 fs is demonstrated. The input pulse energy of 24 pJ is amplified up to 2.6 nJ with shortened pulsewidth of 56 fs and peak power as high as 46 kW.
©2007 Optical Society of America
Passively mode-locked Erbium-doped fiber lasers (EDFL) are dependable source for generating sub-100fs pulses at 1550nm, however, such EDFLs typically generate pulses with energy lower than those of solid-state lasers (such as Ti:sapphire laser). Owing to the inherent fiber nonlinearity, the EDFL usually meet difficulty in obtaining multi-nJ pulses. To increase the peak power, the multi-mode fibers were employed suppressing the nonlinear effect, and compression of chirped pulses with bulk-optic components after the EDFL is frequently performed . Alternatively, the chirped pulse amplification (CPA) technique were particular designed to stretch ultrashort pulses prior to amplification and to recompress them back after the amplification is completed, with all three types of dispersive elements [2–4]. A vintage configuration which employed a fiber based pulses stretcher in an EDFA, and a pair of bulk diffraction gratings as the compressor after the EDFA was reported to achieve 420-fs pulses with energy of 3 nJ after the grating pair. Nonetheless, such a bulky diffraction grating compressor makes the whole fs laser system incompact. Subsequently, fiber Bragg gratings were inserted after the EDFA to stretch and compress the pulsewidths to 408 fs  and 900 fs  associated with energies of 3 and 1.6 nJ, respectively. The all-fiber configuration although is alignment-free, which still limits the achievable peak powers due to the fiber nonlinearity. Recently, a photonic bandgap fiber (PBF) based compressor, has been introduced into the CPA system, [5, 6] however, such special structural fibers are difficult to splice with standard optical fibers for high coupling efficiency. In particular, the extremely large GVD of the PBF leads to a strict tolerance on the optimized length, while the large dispersion slope of the PBF also makes the nonlinear chirp hard to be compensated completely. These constrain the peak power of compressed pulses at few kW. More recently, a highly doped EDF [7, 8] was employed, to shorten length of the EDF and to reduce the high-order nonlinear effect during pulse amplification and compression processes in EDFA and single mode fiber (SMF) based soliton compressor, respectively. To date, the ultrashort pulsewidths of 34 fs  and 43 fs  were obtained with peak power of 140 and 43 kW, respectively, at the cost of low confinement ratio with <55% and 39% of the pulse energy remaining in central peaks and the pulse pedestal remains uncompressed.
In this work, we demonstrate a novel large-mode-area (LMA) highly Er-doped fiber based EDFA for concurrently pre-chirping, amplifying, and compressing 300-fs passively mode-locked EDFL pulses, which results in the generation of high-power and low-pedestal, ultrashort hyperbolic-second pulse. To prevent the Kerr or self-phase modulation and stimulated Raman scattering based nonlinear effects induced during the high-power amplification process, our concept is using such a simplified EDFA based all-fiber compressor with a very short but highly doped LMA Er-doped fiber for minimizing the nonlinear process during the amplification of EDFL pulses, and using a pre-chirped SMF segment for controlling the chirp of pulse before launching into the Er-doped fiber and the last compression stage. With the fine adjustment on both the lengths of the SMF segments for per-chirping and soliton compressing within the LMA-EDFA, we primarily report the optimized condition for obtaining the shortest pulsewidth, the highest peak power and the optimum energy confinement of the self-started nonlinear-polarization-rotation mode-locked EDFL pulses after amplified compression in LMA-EDFA.
2. Design and implementation of EDFL and LMA-EDFA
The experimental setup for the generation of high-power ultralow-pedestal femtosecond pulse is configured by a passively mode-locked EDFL system and a specially designed LMA-EDFA with pre-chirping function for input pulses. The passively mode-locked EDFL illustrated in Fig. 1 is ring cavity design , which consists of a single laser diode at 980 nm as the pumping source launched via a 980/1550nm single-mode-fiber (SMF) based wavelength division multiplexing coupler, a segment of highly doped Er-doped fiber (Lucent 97005) with length of only 1.08 m as the gain medium, a polarized isolator sandwiched by two sets of polarization controllers (with λ/4+ λ/2+ λ/4 plates and λ/4+ λ/4 plates, respectively) as the passive mode-locker, and a SMF 1×2 coupler based output port with 10% coupling ratio.
The total SMF length in the EDFL ring cavity is 3.9 m, which leads to a total dispersion of about +0.024±0.011 ps2. The mode-locking of EDFL is self-started by using nonlinear polarization rotation at a threshold pumping power of only 210 mW. In Fig. 2, we schematically show a highly-doped LMA Er-doped fiber based EDFA system, which is a simplified design for pre-chirping, amplifying and compressing the mode-locked EDFL pulses simultaneously. The LMA-EDFA consists of a 1.28m-long SMF segment as a pulse prechirper, a polarization controller (PC) was inserted between the EDFL output port and the prechirped SMF segment to improve the polarization quality of input EDFL pulse, a forward 980/1550nm pumping wavelength-division-multiplexing (WDM) coupler, a LMA Er-doped fiber as the gain medium and a backward pumping WDM coupler. The peak absorption, the mode-field diameter, and length of the EDF are 80±8 dB/m, 9.5±0.8 um, and 1.32 m, respectively. The length of LMA Er-doped fiber is only 30 cm, and the HI-1060 fiber used in the pumping branch of the WDM was cut into 25-cm long. The SMF lengths before and behind the LMA Er-doped fiber are 2.78 m and 1.27 m, respectively.
In experiment, two high-power laser diodes at wavelength of 980 nm are employed to bi-directionally pump the LMA-EDFA through two WDM couplers directly spliced to both ends of the Er-doped fiber. The maximum launching power of the forward and backward pumping lasers are 140 mW and 120 mW, respectively. The LMA-EDFA exhibits a small-signal gain of 20 dB and a maximum average output power of 104 mW. Afterwards, the EDFL pulse is launched into the LMA-EDFA for raising its peak power into the soliton regime. The WDM coupler spliced at the output port of the LMA-EDFA compensates the chirping of amplified EDFA pulse with its anomalous dispersion property, and the amplified EDFL pulse concurrently experiences a low-order soliton-effect compression at this stage. In stead of the LMA Er-doped fiber, all of the other fiber components are made by SMF-28 with a dispersion parameter β2 of -21.4 ps2/km.
3. Pre-chirped amplification and compression
Originally, the 300-fs mode-locked pulse has a strong frequency chirp and a mode-locked spectral width limited by the effective gain bandwidth of EDFL. Such a pulse shortening requires a nonlinear-effect free pre-chirped amplifier with integrated soliton compressor. In principle, the achievable soliton order depends on the peak power of the input pulse, as describe by N2 = LD/LNL = γP0T0 2/ ∣ β2∣ , where N is the soliton order, γ is the nonlinearity coefficient, P0 is the peak power, T0 is the half-width (at 1/e-intensity point) of the incident sech2-like pulse with a FWHM of TFWHM~1.763T0, and β2 is the GVD parameter. The pulse pattern of high-order soliton reaches a minimum pulsewidth periodically at an optimum fiber length Zopt, as given by the relationship of Zo = πLD/2 ≅ Zopt[0.32/N+1.1/N2]-1. However, the inherent drawback of soliton-effect compression is the degrading pulse quality Qc (i.e. the pulse energy confinement ratio, defined as the energy ratio of the central pulse to total pulse). The Qc decreases monotonically from its ideal value of 1 as the soliton order N increases, providing a broadened pulse with a separately large and uncompressed pedestal. The best solution is to remain the pulsewidth short during the amplifying process, which efficiently reduces the required soliton compression order, suppresses the pedestal and raise the energy confinement ratio. These criteria can all be fit with the pre-chirped LMA-EDFA based amplified compressor.
As a result, the autocorrelation traces and corresponding spectra of the original pulse, the dispersed and the compressed pulses before and behind the LMA-EDFA are shown in Fig. 3. The passively mode-locked EDFL yields a Hyperbolic second pulse shape with its pulsewidth and linewidth of 300 fs and 16.6 nm, respectively, at repetition frequency of 40 MHz and central wavelength of 1560 nm. The average power such a passively mode-locked EDFL is 0.96-mW, corresponding to a pulse energy of 24 pJ. After passing through a segment of SMF inserted between the EDFL and the LMA-EDFA with optimized length of 2.78 m, the EDFL pulse is pre-chirped before launching into the LMA-EDFA. At this stage, the EDFL pulsewidth is broadened from 300 fs to 840 fs for keeping sufficiently low peak power during the amplification process. Such a pre-chirped operation also help to avoid the excessive nonlinear effects happened in the LMA-EDFL. After amplification, the maximum average power of the EDFL pulse-train becomes 104 mW, corresponding to pulse energy of 2.6 nJ.The EDFL pulsewidth greatly shortens from 840 fs to 56 fs, corresponding to a maximum pulse compression ratio of up to 15. To date, the compressed pulse shape is exactly fitted by a Hyperbolic second profile. Clearly, a significant spectral broadening has also occurred with a 3-dB linewidth of up to 51 nm due to nonlinearity self-phase modulation at the last SMF based soliton compressing stage. The compressed pulsewidth and pulse energy confinement ratio as a function of the SMF length are illustrated in Fig. 4. The overall pulse compressing ratio of such a pre-chirped LMA-EDFA compressor is 5.5 with respect to the original EDFL pulsewidth of 300 fs. Under a pre-chirping SMF length of 2.78 m, the compression of amplified pulse can be optimized at a compressing SMF length of 1.27 m, whereas the pulsewidth rapidly broadens if the SMF further lengthens. It is also found that a complete energy confinement occurs at the optimized SMF length condition, whereas the pulse quality as well as its energy confinement ratio monotonically decays as the SMF lengthens to >1.8 m.
The logarithmic plot of the auto-correlated pulse shapes obtained at nearly optimized soliton condition are shown in Fig. 5, while the peak powers of the pedestals are sufficiently low although the diagnostic sensitivity of the auto-correlator is limited, such a 100% energy confinement ratio was never observed and reported in previously reported systems to our best knowledge. The fast Fourier transformation of the amplified and compressed EDFL spectrum reveals such a nearly transform-limited shape of the auto-correlated pulse should be somewhat distorted by multi-pedestals. However, these tiny pedestals are unavailable to be resolved due to both the limited sensitivities of the second-harmonic generation based detection and the digitized I/O interfacial data port of the commercial auto-correlator. The average power fluctuation and side-mode frequency spectrum of the fiber laser pulse-train are shown in Fig. 7, which reveal a maximum power drift of <0.8% within 1 hr and a super-mode suppressing ratio of >35 dB (determined by a RF spectrum analyzer with resolution of 100 Hz) after the amplified compression in the LMA-EDFA. These results interpret the comparable high stability of such a compact high-power femtosecond fiber laser system with other works reported previously.
4. Comparisons with conventional approaches
It is interesting to compare the results obtained in this work with those reported before. Recently, Nicholson and co-workers have demonstrated a chirp-pulse amplifier using four laser diodes to offer forward (backward) pumping power of 1.28 W. Such an EDFA generates output power up to 800 mW, which supports to obtain 34-fs pulses with pulse energy of 8.7 nJ from a similar EDFL system at a repetition rate of 46 MHz . Although the peak power of pulse can be 140 kW, the pulse compression ratio and the energy confinement ratio is only 7 and 55%, respectively. On the other hand, Takayanagi et al. have demonstrated an alternative approach to obtain a compressed pulsewidth of 41.3 fs at a repetition rate of 48 MHz, providing the average power and the pulse energy of 215 mW and 4.5 nJ, respectively . However, the pumping power required to obtain similar peak power of the compressed EDFL pulse is still as high as 400 mW, while the compressed pulse quality is even worse than 39% and the pulse compression ratio is only 6. In these cases, the high peak power and ultrashort pulse can be obtained under highly pumped EDFA, but half of the pulse energy is scattered to the pedestal. Later on, the same group further demonstrated the pulse compression in a small-core multimode fiber , which increases the energy confinement ratio to 84% at a cost of broadened pulsewidth and small pulse compressing ratio. In contrast, the pre-chirped LMA-EDFA needs ultralow pumping power (only 260 mW) for simultaneous amplified compression of the passively mode-locked EDFL pulse from 300 fs to 56 fs. Although the average and peak powers of the generated pulses are 104 mW and 46 kW, the energy of 2.6 nJ can be entirely confined within the central portion of the amplified EDFL pulse. That is, the pulse shape is nearly transform-limited with extremely low pedestal power after simultaneous amplifying and compressing in the LMA-EDFA.
By using a newly designed large-mode-field-area Er-doped fiber based pre-chirped EDFA with ultrashort length and ultralow pumping power, we primarily demonstrate the simultaneous amplification and compression of a passively mode-locked Erbium-doped fiber laser (EDFL) pulse from 0.96 mW to 104 mW and from 300 fs to 30 fs, respectively. The mixed large mode-field-area and pre-chirping design in an EDFA greatly suppresses the stimulated Raman scattering induced nonlinearly soliton-self-frequency-shift effect happened during the amplification femtosecond laser pulses in conventional EDFA module. The original hyperbolic-second-shape pulse with energy of 24 pJ is generated via the self-started passive mode-locking of EDFL at repetition frequency of 40 MHz. With the specially designed ultrashort-length pre-chirped LMA-EDFA, the energy of EDFL pulse can be greatly amplified to 2.6 nJ with its pulsewidth being compressed to 56 fs, providing a peak power as high as 46 kW after the pre-chirped amplification/compression procedure. Such a simplified pre-chirped LMA-EDFA compressor is able to reshape the EDFL pulse to an ultralow pedestal shape at a relatively high average output power condition.
The work was financially supported by National Science Council of Taiwan R.O.C. under grant NSC94-2215-E-002-040 and NSC95-2221-E-002-448.
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