A fully fiberized, single-polarization, gain-switched diode-seeded fiber master oscillator power amplifier (MOPA) system is demonstrated delivering 28ps pulses at variable repetition frequencies ranging from 53 MHz up to 858 MHz. An average signal output power of 200 W was achieved with good OSNR for all operating frequencies. A maximum pulse energy of 3.23 μJ at a repetition frequency of 53 MHz was achieved, corresponding to a pulse peak power of 107 kW. The extraction of higher pulse energy was limited primarily by the onset of nonlinear effects such as SRS which lead to compromised pulse quality at higher peak powers.
© 2013 Optical Society of America
High-power lasers operating in the picosecond regime are currently in demand for a number of applications including laser machining, material processing, frequency-doubling and the pumping of optical parametric oscillators [1–3]. Most of these applications require high average powers and high peak power picosecond pulses with pulse repetition frequencies ranging from the kHz to MHz regimes. Titanium sapphire lasers are commonly employed for many of these applications but emerging fiber laser and amplifier systems provide an alternative solution . Ytterbium-doped fiber amplifiers (YDFAs) in particular have been demonstrated to be capable of providing high single-pass gain, very high optical-to-optical efficiencies, diffraction-limited output beams and to be highly robust with regards to thermal effects due to the fiber geometry which allows for the efficient dissipation of heat generated in the lasing process [5–7].
A key approach to generating high average powers from fibers is the implementation of the MOPA architecture. In a MOPA setup, a low-power seed laser (usually a fiber oscillator or semiconductor laser diode (SLD)) is fed into a chain of fiber amplifiers which are used to boost the output signal power to the desired level. The technique has been used successfully in the picosecond regime allowing pulses from either a mode-locked or gain-switched seed laser to be amplified to high optical output powers. Up to 157 W of diffraction-limited output was recently demonstrated using a mode-locked fiber laser seeded MOPA , whilst a gain-switched-SLD seeded MOPA produced 321W of multimode output (M2 = 2.4) .
Although substantial progress has been made in scaling the average output power from a fiberized system to the hundreds of watts level, the maximum peak power in these experiments has often been limited by nonlinear effects such as self-phase-modulation (SPM) and stimulated Raman scattering (SRS)  due to the relatively long active fiber lengths required to absorb the coupled pump power in cladding pumping schemes. These effects can be minimized by using large-mode-area (LMA) fibers which incidentally also helps in shortening the device length due to the higher pump absorption that results from their relatively large core-to-clad area ratio. A peak power of approximately 270 kW has already been demonstrated from an all-fiberized MOPA system incorporating LMA fibers with a core diameter of 30 μm . LMA fiber with an even larger core diameter should in principle allow even further scaling up of the peak power, but this may rapidly lead to higher bending loss, a larger heat load per unit length and a tendency to multi-mode operation [12, 13]. Thus, a compromise needs to be struck when designing the LMA fiber in terms of extractable peak power, ease of fiber handling and the output beam quality under high power operation.
Here we report the amplification of near transform limited 28 ps pulses from a gain-switched SLD in an all-fiberized amplifier chain. The MOPA produces linearly polarized, diffraction-limited output at repetition frequencies ranging from 53 MHz to 858 MHz with a maximum average output power of 200 W. A maximum pulse energy of 3.23 μJ at a repetition frequency of 53 MHz was achieved corresponding to a pulse peak power of 107 kW. The performance of the system at various frequencies is detailed in this paper.
2. Experimental setup
A schematic of the experimental setup is shown in Fig. 1. A commercial Fabry-Perot laser diode with polarization-maintaining (PM) fiber pigtail was gain-switched using a stable train of sinusoidal electrical current pulses superimposed upon a DC bias. The pigtail of the SLD was spliced to a PM fiber Bragg grating (FBG) with a 3 dB bandwidth of 0.24 nm and a reflectivity of 7.2%. The repetition frequency was tuned to 858 MHz to achieve synchronization between the emitted pulses from the diode and reflected pulses from the FBG. This resulted in near-transform-limited pulses (the time-bandwidth product was estimated to be 0.54 assuming Gaussian-shape pulses) with a full width at half maximum (FWHM) of 28 ps and pulse energy of 7 pJ. The pulse stream from the gain-switched SLD was then amplified in a four-stage PM YDFA MOPA chain. It is worth noting that in this system, we use a simple FP diode in conjunction with a simple uniform FBG to get near-transform-limited pulses directly from the laser .
An operating wavelength of 1040 nm was chosen over the more commonly used 1060 nm in order to minimize the overall length of amplifier fiber in the system since the emission cross-section in YDF is much higher at 1040nm than at 1060 nm . The first stage amplifier consisted of a 50 cm long core-pumped PM YDFA (5 μm core and 130 μm cladding diameters) pumped by a 180 mW single-mode 976 nm pigtailed laser diode. The output from this amplification stage was coupled into an in-line electro-optic modulator (EOM), which acted as a pulse picker for reducing the pulse repetition frequency. The EOM had an extinction ratio of 40dB and had an excess loss of 5 dB. A second core-pumped PM YDFA with an active fiber length of 1 m was used after the EOM to ensure adequate seeding of the cladding-pumped third-stage preamplifier.
The third-stage PM YDFA comprised of a 2.5 m long cladding-pumped fiber with a relatively large core diameter of 10 μm, core NA of 0.08 and an inner-cladding diameter of 125 μm with 0.46 NA. The fiber was co-directionally pumped by a 10 W, 975 nm multi-mode (MM) pump diode through a fiberized (2 + 1)x1 MM pump combiner. The output was then taper-spliced to the final-stage amplifier, which comprised a 2.6 m long LMA fiber (Nufern PLMA-YDF-30/250-VIII) with core and cladding diameters/NA of 30 μm/0.06 and 250 μm/0.46, respectively. This fiber had a (cladding) absorption of 6 dB/m at 975 nm corresponding to an estimated total pump absorption of 15 dB in the 2.6 m long fiber. The taper-spliced section was mounted on a water-cooled aluminum plate to dissipate the excess heat caused by the small amount (3%) of unabsorbed pump power. The LMA fiber was also coiled to a diameter of 76 mm to introduce additional bend induced loss to the guided LP11 mode. The combination of taper-splicing and coiling ensured single-mode operation, whilst the use of a fast-axis blocking PM isolator ensured single-polarization seeding into the power amplification stage. The amplifier was free-space counter-pumped by 16 spatially combined MM laser diodes operating at 976 nm. A 19x1 pump combiner with the output fiber having a diameter of 200 μm and NA of 0.46 was used to combine the MM pump diodes. A combination of two aspherical lenses with identical focal lengths (f = 20mm) was used to launch the pump into the LMA fiber resulting in ~95% pump coupling efficiency. A dichroic mirror (DM) was used to separate the pump and signal beams. To avoid damage to the output facet, a short length end cap (1.3 mm) was spliced to the output of the LMA fiber and this was angle polished to avoid coupling of the 4% reflected signal into the fiber core.
3. Results and discussion
The optical pulses from the gain-switched SLD were measured directly with a fast photo-detector (Agilent 83440D) and a digital communication analyzer (Agilent Infiniium 86100C). The inset of Fig. 2(a) shows the impulse response of the detection system measured by injecting a very short (7 ps) optical pulse indicating that the detection system can clearly resolve pulses as short as 16 ps. Figure 2(a) shows the temporal profile of the optical pulses with a FWHM of 28 ps. The measured (spectral) side-mode suppression ratio (SMSR) was ~40 dB as shown in Fig. 2(b) with a polarization extinction ratio (PER) of over 20dB. The 6.0 mW average output power from the seed diode was amplified to 50 mW in the first stage preamplifier. The EOM pulse picker effectively blocked any background continuous wave (CW) signal generated in the form of amplified spontaneous emission (ASE) in the first stage amplifier. The average powers measured after the EOM at various pulse repetition frequencies were 17 mW at 858 MHz, 6.4 mW at 430 MHz, 3.5 mW at 214 MHz, 1.6 mW at 107 MHz and 0.8 mW at 53 MHz whilst the maximum average output powers after the second-stage amplifier were measured to be 43 mW at 858 MHz, 32 mW at 430 MHz, 23 mW at 214 MHz, 17 mW at 107 MHz, and 13 mW at 53 MHz.
A maximum average output power of 2.6 W was obtained after the third stage cladding pumped preamplifier. The shape and duration of the pulse were preserved after the preamplifiers as shown by the red dashed plot in Fig. 2(a). The corresponding low resolution (1.0 nm) optical spectra measured with an ANDO AQ6370B optical spectrum analyzer (OSA) after different preamplifier stages at a reduced repetition frequency of 214 MHz are shown in Fig. 2(b). The seed had an optical signal to noise ratio (OSNR) of 40 dB. The short wavelength ASE around 1030 nm started to build up after the second stage preamplifier (red dash-dot line) due largely to the short length of the gain medium. A moderate gain of 8 dB was extracted from this stage. The length of the gain medium of the third stage amplifier was chosen such that the short wavelength ASE after the second stage gets reabsorbed inside the gain medium whilst providing over 20 dB signal gain. This gain partitioning scheme ensured the maximum optical signal to noise ratio (OSNR) before seeding into the power amplifier stage, as illustrated by the blue dotted line.
The output power as a function of the launched pump power from the final stage amplifier is shown in Fig. 3 with an estimated slope efficiency of 77% with respect to the launched pump power. A maximum output power of 200 W was obtained for a total launched pump power of 270 W at all frequencies except for 53 MHz for which a maximum output power of 171 W was measured. Further power scaling was limited by significant nonlinear distortions experienced by the amplified pulses as well as transfer of energy to the SRS line. The polarization extinction ratio (PER) of the output signal at maximum operating power was measured to be 15 dB and an M2 of ~1.1 based on D4σ method which qualifies that the output beam was diffraction limited.
The maximum peak power extraction of a fiberized picosecond MOPA system is usually limited by the onset of SPM and SRS. Several techniques have been used to reduce these nonlinear effects, such as: (i.) minimizing the total fiber length including both active and passive fibers, (ii.) increasing the fiber core diameter while preserving the single-mode propagation, and (iii.) reducing the pulse peak power. In our system, we managed this by gain partitioning the various amplification stages as well as keeping the passive fiber length down to only 1.5 m after the third pre-amplification stage.
At the maximum operating output power of 200 W, a reasonable OSNR of >30 dB was achieved for repetition frequencies ranging from 214 MHz to 858 MHz as shown in Fig. 4(a) which shows that most of the amplified powers was contained in the signal band. A maximum pulse peak power (pulse energy) of 33.4 kW (0.93 μJ) was achieved whilst the pulse maintained its temporal shape as evident from the black dash-dot line in Fig. 4(b).
When the repetition frequency was reduced to 107 MHz or lower, significant nonlinear distortion was observed and the SRS assisted peak at around 1090 nm started to build-up as shown by the solid pink and dotted brown lines in Fig. 4(a), restricting the maximum pulse peak power extraction from the system to around 73 kW and pulse energy to ~2.0 μJ at 53 MHz repetition frequency (corresponding to a maximum average output power of 108W). The broad SRS peak at around 1090 nm was measured to be 29 dB below the signal peak. One of the factors that led to the relatively low SRS threshold is the seeding of ASE around the SRS region from the third preamplifier stage. Figure 4(b) shows the output spectra from the third preamplifier stage for two different output powers represented by the solid black and red line plots. Approximately 0.08% (2.1 mW) of the ASE power was estimated to be within the SRS region at 2.6 W of total optical output power. In an attempt to increase the SRS threshold, the seeding to the power stage amplifier was reduced from 2.6 W to 1.4 W so as to reduce the total ASE seeding power down to 1.1 mW. Furthermore, reduction in the seeding power increased the amplifier gain which in turn reduced the effective length of the amplifier . We estimated the effective length to be around 0.54m, a 23% reduction compared to when operating at 2.6 W. The combined effects of reduced ASE seeding and effective length of the amplifier helped in increasing the SRS threshold.
Figure 4(b) clearly shows the improvement brought about by the reduction in seeding power to the booster amplifier. At an output power of 113 W, the SRS peak is measured to be approximately 40 dB below the signal peak whilst it is only 29dB at 108 W of output power when 2.6W of seeding power was used. This increased SRS threshold allowed us to scale up the power to 171W and resulted in an approximate 1.6 fold increase in the extractable pulse energy. We estimated that 96% of the total output power was contained in the signal band. The corresponding peak power and pulse energy achieved were 107 kW and 3.23 μJ respectively. We noticed that the output pulses broaden from 28 ps to 30 ps as shown in Fig. 4(c) due to the combined effect of SPM and dispersion within the active fiber.
The influence of SPM on the broadening of the signal line-width is shown in Fig. 5. The level of spectral broadening depends on the pulse peak power. For the same average output power, the pulse peak power increased with a decrease in repetition frequency, resulting in a linear increase in spectral bandwidth with power as shown in Fig. 5(a). However, as the pulse peak power was increased far above the SRS threshold, the intra-band Raman scattering also plays a part in the broadening of the spectral bandwidth causing the lineshape to broaden asymmetrically towards the long wavelength side as illustrated by the solid blue line in Fig. 5(b). The green dash-line plot in Fig. 5(a) shows the nonlinear increase in spectral bandwidth due to the combined effect of SPM and SRS when the pulse peak power exceeds 80kW. The degree of spectral broadening is relatively modest in absolute terms – only a few nm at the lowest repetition frequencies investigated. The nonlinear phase shift accumulated in the final stage amplifier was estimated by fitting the measured output spectrum through the use of an in-house developed numerical modeling tool. We estimate a total nonlinear phase-shift of 28.3 radians at the maximum pulse peak power of 107 kW whilst the seed spectral width broadened by ~25 times from its original value . It is to be noted here that the input linewidth to the final stage amplifier also increased with a decrease in operating frequency. However, the drop in seed power from 2.6 W to 1.4 W helped in keeping the nonlinear broadening factor down as is evident from the solid purple and dashed green lines as long as the pulse peak power does not exceed a critical power for intra-pulse Raman scattering to play a domineering role.
A gain-switched diode-seeded all-fiber MOPA amplifying 28 ps optical pulses up to an average output power of 200 W at 1040 nm with variable repetition frequencies is demonstrated. The ASE was well suppressed at the maximum operating output power across all the operating frequencies with OSNR values ranging between 25 dB to 35 dB. A maximum pulse energy of 3.23 μJ was achieved at the lowest operating frequency of 53 MHz corresponding to a pulse peak power of 107 kW. Further energy scaling was primarily limited by the rapid build-up of the SRS peak at around 1090 nm. It may be possible to enhance the output pulse energy by filtering out the long wavelength signal components after the third pre-amplifier stage using a narrow band-pass filter (<0.5 nm) which otherise acts as a seed for SRS line. On the other hand the linear frequency chirp introduced by SPM can be exploited to compress the pulses and enhance the peak power significantly , thus greatly extending the operational parameter space of the source.
The authors acknowledge NUFERN for providing the sample fibre (NUFERN PLMA-YDF-30/250-VIII) and Peh Siong Teh thanks the Public Service Department (PSD) of Malaysia for the financial support. This work was supported in part by the UK Technology Strategy Board project SMART LASER.
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