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A 1553 nm Er-doped fiber master-oscillator-power-amplifier (MOPA) laser system providing pulses with a 6 kHz repetition rate, 5 ns duration, ~210 μJ energy, ~300 MHz linewidth, and with a near diffraction limited beam quality, was developed. A gain fiber as short as 0.7 m in length was utilized in order to relax the SBS effect. To the best of our knowledge, thus generated peak power of 40 kW is the highest one obtained from a single frequency Er-doped silica fiber laser. The pulse quality was verified by frequency conversion with a periodically poled lithium niobate nonlinear crystal (PPLN) for second harmonic generation. A pulse energy as high as ~100 μJ was achieved at 776.6 nm with a moderate incident energy of 133 μJ, indicating an energy conversion efficiency of 75%.
© 2015 Optical Society of America
1. Introduction
High peak power erbium (Er) doped fiber lasers emitting at a wavelength around 1.55 μm are highly in demand for such applications as remote sensing [1], materials processing [2], medical science [3] and frequency conversion [1,4–6]. There are several studies reporting Er-doped fiber lasers capable of generating 20 kW to ~1.2 MW of peak power, but typically, the pulse linewidths were broad [7–11]. This was due to the seed lasers used, which were amplified spontaneous emission (ASE) sources, directly modulated laser diodes (LDs), or fiber laser oscillators [7–11]. However, some applications, such as frequency conversion or LIDAR, call for single frequency operation.
When it comes to pulsed single frequency operation, fiber lasers are hindered by unwanted nonlinear effects, predominantly stimulated Brillouin scattering (SBS), due to the limited core diameter. Currently, to satisfy the requirement of a narrow linewidth, for applications such as LIDAR for example, where pulse durations longer than 100 ns are needed and the single frequency peak power is generally limited to ~2 kW, even with the use of phosphate fibers [12]. Recently, using a specially designed Yb-free silica based Er-doped fiber with a core diameter of 35 μm, Kotov et al. obtained a record single frequency peak power of 4 kW with a reduced pulse duration of 60 ns [13]. Considering SBS as an interaction between photons and phonons in a fiber and that the threshold of this effect can be increased by using pulse durations shorter than the phonon lifetime or by reducing the length of the fiber itself, NP Photonics Inc. have reported peak powers of 50 kW [14] and >100 kW [15] for a single frequency 1.55 μm fiber laser with a pulse duration of nearly 2~3 ns, using their proprietary high gain phosphate fibers. The fiber lengths used were less than 15 cm and optical-to-optical efficiencies were calculated to be approximately 2.4% in [14] and 1.5% in [15]. However, these special fibers are presently not commercially available, and to the best of our knowledge, there are currently no reports of high peak power (>10 kW) single frequency 1.55 μm fiber lasers based on commercially available silica fibers.
Motivated by our recent development of a high power narrow-linewidth laser at 193 nm, which was designed to seed an ArF excimer laser amplifier [6,16], a single frequency, 1553 nm, ~5 ns duration, Er-doped fiber laser with a repetition rate of 6 kHz, was demanded. Meanwhile, a high pulse energy of ~200 μJ and a narrow linewidth of <500 MHz were requested for this application. The relatively low repetition rate and the short pulse duration were conditions imposed by the ArF excimer laser amplifier itself, and in this letter, we will give a detailed description on how to realize as high a peak power as possible with Er-doped silica fiber systems.
As mentioned above, the SBS threshold can be increased by reducing the fiber length. Yet, shorter fibers lead to weaker pumping absorptions. Therefore, there is a trade-off between the fiber length and the pumping efficiency. Excluding phosphate fibers [14,15] and the above-mentioned special Yb-free Er-doped 35 μm core diameter fiber [13], the shortest reported silica large mode area (LMA) fiber used for single frequency 1.55 μm laser operation is 1.4 meters [17]. The optical-to-optical efficiency was 16%. However, when sufficient pump powers are available and the pumping efficiency is no longer a problem, we would like to bring forward the idea of “trading pump power for peak power”. For a fixed gain fiber type, the absorption coefficient α is fixed. The SBS limited single pulse energy extractable from a fiber can be increased by shortening the length and by increasing the pump power. It should be noted that the increase in extractable single pulse energy from a shortened fiber is not proportional to the increase of pump power, which means the optical-to-optical efficiency is reduced. This is to be expected from the decrease in gain with shorter fibers. To recapitulate, higher pulse energies can be intentionally obtained from shorter fibers at the expense of pump power and the relationship between input pump powers and output pulse energies is not linear. An additional benefit from using short fibers is the induced nonlinear phase accumulation from self-phase modulation (SPM), which gives rise to laser linewidth broadening, is proportional to the fiber length and is accordingly lessened.
Another point worth considering is the seed laser employed to saturate fiber amplifiers for pulsed single frequency operations. Typically, there are two types of laser used to seed fiber amplifiers, current directly modulated laser diodes and externally modulated single frequency CW lasers. Current directly modulated laser diodes are compact, convenient, and cost-effective options, but cannot provide single frequency operation. Externally modulated single frequency CW lasers however, are suitable for single frequency operations. For the generation of narrow-linewidth high peak power pulses, both single frequency distributed feedback LDs (DFB-LDs) and fiber lasers are appropriate seed sources. On the one hand, DFB-LDs are easily integrated in a system; while on the other hand, fiber lasers deliver much higher average power. For obtaining pulse from CW laser, either electro-optic or acousto-optic modulators can be adopted, depending on the required pulse duration and extinction ratio. Semiconductor optical amplifiers (SOAs), are another candidate, as they can be used as fast optical switches while providing considerably small signal gain.
Based on the above-mentioned considerations, we realized a high peak power, single frequency, 5 ns Er-doped fiber MOPA laser system with a repetition rate of 6 kHz, using a 0.7-m long Er/Yb co-doped polarization-maintaining LMA fiber. The maximum energy delivered was ~210 μJ. The linewidth was determined to be 300 MHz, using a scanning Fabry-Pérot (FP) interferometer. This non-Fourier transform limited performance was a result of self-phase-modulation occurring in the main amplifier. The beam quality was measured to be near diffraction limited. Frequency conversion was also demonstrated, via second harmonic generation (SHG) using a periodically poled lithium niobate (PPLN) nonlinear crystal and over 100 μJ of pulse energy was obtained at 776.6 nm with an incident energy of 133 μJ and pulse duration of 5 ns, which indicates an energy conversion efficiency of 75%. Furthermore, 300-mW of output power at 193 nm was generated by two-stage wavelength mixings with the fourth harmonic of an Yb-fiber laser system [16].
2. Experiment setup
The schematic diagram of the Er-doped fiber MOPA laser system is shown in Fig. 1. The experimental configuration consisted of a single-frequency CW DFB-LD as the seed laser, a SOA based pulse generation system, and three stages of fiber amplification. A fiber isolator was directly connected to the DFB-LD using an FC/APC type of fiber connector to prevent possible damages from back-reflections. The DFB-LD was followed by an SOA which worked both as a modulator and as a gain medium that amplified and optically switched the CW laser. The SOA was fixed onto a homemade circuit board and was driven by an electrical signal from a digital delay device that was triggered by a master clock, and which made the synchronization with the Yb-doped fiber laser branch easy to control [16]. As a precaution, another fiber isolator was inserted, where the output end was spliced to the input port of a 976nm/1550nm wavelength division multiplexing (WDM). An Er110-4/125 single mode fiber pumped by a single mode 976 nm laser diode was used as the gain medium for the first stage of amplification. The output from the spliced collimator was directed to the next amplification stage after passing through a bulk isolator and a grating for out-of-band ASE suppression. The second stage amplifier was an Er/Yb double cladding fiber with a core diameter of 10 μm and length of 0.8 m, which was spliced to a pump combiner and pumped by a multi-watt fiber coupled 976 nm laser diode with a fiber diameter of 105 μm. A similar set of free-space optics involved in the beam path between the first and the second amplifier stages were used after the second stage amplifier. The output propagated through a half wave plate, followed by a grating to suppress the out-of-band ASE and a high power bulk isolator. Finally, the third stage amplifier consisted of a polarization maintaining Er/Yb co-doped 25/300 LMA fiber with a small N.A. of 0.09, which enabled to control the transverse modes by coiling the fiber with a diameter of 7.5 cm. The fiber length was only 0.7 m and to prevent possible damages on the output end facet the fiber was terminated with a coreless end cap, which extended the mode field diameter and thus reduced the peak intensity. The length of the end cap was 1.5 mm, with a diameter of 1 mm. The pump source was a multimode laser diode emitting at 976 nm which was coupled to a 105 μm diameter fiber, and had a maximum output power of 30 W. To avoid pump coupling in free-space, a special combiner with mode matched 25/300 LMA gain-less input and output fibers was used. The input end was also terminated with an end cap and a dichroic mirror in the beam path of the amplified output was used to isolate the signal from the residual pump beam.
3. Experiment results
The fiber coupled DFB-LD seed laser had a typical CW output power of 6 mW. The center wavelength had a linewidth of approximately 20 MHz and could be tuned within a narrow range spanning over 1553.35 nm to 1553.55 nm, depending on the pump current and the temperature of operation. By applying a pulsed electric signal to the SOA, which has an extinction ratio of ~60 dB, an average output power of ~3 μW was obtained, corresponding to a pulse energy of 0.5 nJ. This implied the generation of pulses with a peak power of 100 mW, a repetition rate of 6 kHz and a duration of 5 ns. The linewidth was measured with a scanning FP interferometer and a value of 180 MHz was obtained, corresponding to a time-bandwidth-product of 0.9. It was about two times of Fourier transform limited value of the Gaussian pulse. For longer pulse durations (5~50 ns), the time-bandwidth-product increased accordingly. This deviation from the Fourier transform limit condition was as a result of the non-perfect Gaussian pulse shape, as shown in Fig. 2. More suitable electronics could help improve the shape of the pulses.
The pulsed seed power was relatively low, so a two-stage pre-amplifier was necessary before seeding the final amplifier. The first and second pre-amplifier stages increased the average power to ~1 mW and ~50 mW, corresponding to pulse energy of ~167 nJ and ~8.3 μJ, respectively. While two small-sized bulk reflective gratings were used to suppress out-of-band ASE, the one placed in-between the pre-amplifiers limited the bandwidth to approximately 6 nm, due to the distance with respect to the input lens of the second pre-amplifier and its aperture. Figures 3 to 5 collate the results from the final amplifier. Figure 3(a) shows the dependence of the output pulse energy as a function of laser diode pump power for a repetition rate of 6 kHz. The maximum pulse energy of 208 μJ was obtained with a pump power of 26 W, which corresponds to a conversion efficiency of 4.8%. To unambiguously discriminate the signal power in the pulse from the background CW component, the pulse energy was measured directly using a fast Ophir PE9F pyroelectric energy meter that accurate at repetition rates up to 25 kHz and insensitive to CW radiation. To avoid damaging the energy meter sensor at high energy levels, a filter was used to attenuate pulse energies higher than 160 μJ. The CW component ratio, determined by comparing measurement results from the energy and power meters, was found to be approximately 34%. The presence of relatively high levels of the CW component in the final amplified output originated from two aspects of the laser system. One was the presence of the unfiltered out-of-band spectrum in the pre-amplification stages arising from the use of gratings as spectral filters and the other was the presence of the in-band 1553 nm CW component stemming from the limited pulse extinction ratio of the SOA. This problem could be mitigated by using a narrow band (~1 nm) in-line fiber filter and adding another time gate in the amplifier chain. However, if longer pulse durations were to be used, the CW component would significantly decrease since pulses of longer duration would extract more energy than those of shorter duration and would introduce less CW component in the pre-amplification stages.
Fig. 3 Performance characteristics of the main amplifier. (a) Output energy versus pump power, (b) optical spectrum, (c) linewidth and (d) pulse shape measurements.
Fig. 4 Experimental results for second harmonic generation. (a) Output energy at 776.6nm and the corresponded conversion efficiency, (b) spectrum of the 776.6nm, (c) the pulse shape of 776.6nm laser, and (d) temperature bandwidth of the PPLN crystal.
Figure 3(b) shows the spectra of the laser output at pulse energies of ~200 μJ and ~30 μJ recorded by a YOKOGAWA optical spectrum analyzer (AQ6370C). The instrument resolution was set to 0.02 nm. At both pulse energies, the laser peak was at least 25 dB higher than signals at other wavelengths. Increasing pulse energies, led to increasing amounts of the CW component, as evidenced by the broad signal at the base of the laser peak. The spectral bandwidth of the ASE centered at 1535 nm was cut off by the diffraction grating situated between the two pre-amplifiers. This explains the decrease in the pulse-to-CW ratio for increasing pump powers, as shown in Fig. 3(a). The linewidth was measured using a scanning FP interferometer with a free spectral range (FSR) of 4 GHz (TOPTICA, miniScan102) and a fineness of 500, yielding a slightly broadened linewidth of ~300 MHz, as shown in Fig. 3(c). The deviation from the original linewidth of the SOA seed laser was mainly caused by self-phase-modulation (SPM) occurring inside the final amplifier, and the extent to which the linewidth was broadened could be estimated from the value of the maximum nonlinear phase shift ϕNL [18]. φNL = (2π/λ)n2L(P/πr2), where n2 is the nonlinear index coefficient, L is the fiber length, P is the laser peak power, and r is the mode field radius. In our case, given n2 = 2.2 × 10−20 m2/W (for silicon fiber), L = 0.7 m, P = 20 kW (simple average over the fiber), r = 12.5 μm, the nonlinear phase shift ϕNL was calculated to be approximately 1.3, which corresponds to an SPM induced spectral broadening by a factor of ~1.3. This is in agreement with the experimental results (300 MHz / 180 MHz ~1.67). However, the estimated nonlinear phase shift has been underestimated because of the use of core radius (12.5 μm) in the estimation instead of the real mode field radius. SPM induced linewidth broadening could be compensated for by phase modulation of the seed laser with a conjugate signal if strict Fourier transform limited linewidth were pursued. This has been demonstrated with low power systems [19,20].
Figure 3(d) shows the pulse shapes acquired with a fast photodiode, both for low and high pulse energy instances. For low energies, the pulse shape retained its symmetry, but at increasing energies, the pulse shape became distorted and the duration (FWHM) decreased. Such an effect was expected since the leading edge of the pulse, at high energies, experiences higher gain. The polarization extinction ratio (PER) was measured to be >12 dB in the case of the highest energy output, thus making the MOPA laser system suitable for subsequent frequency conversion.
To further characterize the output of the high peak power fiber MOPA system, the laser was used to pump a PPLN crystal with length of 10 mm for second harmonic generation (SHG). As shown in the Fig. 4(a), using pulses with a moderate incident energy of 133 μJ, a 5 ns duration (shown in Fig. 4(c)) and a 6 kHz repetition rate, >100 μJ of output energy was obtained at the frequency doubled wavelength of 776.6 nm (shown in Fig. 4(b)), corresponding to an energy conversion efficiency of 75% and a peak power of 20 kW. Figure 4(d) gives the temperature bandwidth of the PPLN crystal, which is around 6°C. This experiment did not focus on optimizing the SHG process, as the power conversion efficiencies obtained were sufficiently high to confirm the high beam quality of the fundamental pump pulses and their moderate optical bandwidth. To avoid reaching the damage threshold of the PPLN crystal, the highest pump pulse energy was not used.The beam quality of the MOPA system was further verified by measuring the M2 factor. Figure 5 shows the beam profiles in both the near and the far fields. M2 factors of ~1.07 and 1.26 were obtained in the X and the Y directions, respectively. The small discrepancy between the two values was attributed to the coiling of the fiber, which was only carried out in horizontal plane instead of both horizontal and vertical planes. This laser system was also used to generate a deep ultraviolet (DUV) narrow-linewidth laser output at 193 nm [6,16]. An output power of 300 mW was achieved at 193 nm by frequency mixing the output of the fiber MOPA with that of a Yb-doped 1030 nm laser.
While the fiber MOPA system had a free-space configuration, it proved to be stable over a continuous period of a few months without the need for realignment. Figure 6 shows the long term stability test of the output power of the fiber MOPA laser system, which was however not operated at the maximum pulse energy. Within a continuous period of 1.5 hours, the standard deviation of the output power was estimated to represent 1.17% of the average output power. Considering that the environmental temperature varied between 21.5°C and 24.5°C within a period of approximately 20 minutes, this fiber MOPA system not only proved to be stable, but also robust against environmental fluctuations.
4. Summary
In conclusion, we have demonstrated a 1553 nm Er-doped fiber MOPA laser system that generated pulses with a 6 kHz repetition rate, a 5 ns duration, >200 μJ of energy, a ~300 MHz linewidth, and a near diffraction limited beam quality. To achieve such high peak power performances, the fiber was shortened to a length of 0.7 m and the optical-to-optical efficiency was disregarded. The performance of the fiber MOPA system was verified by carrying out SHG, which yielded a pulse with a repetition frequency preserved at 6 kHz and an energy of 100 μJ at 776.6 nm, corresponding to an energy conversion efficiency of 75%. By frequency mixing the fiber MOPA output with that of a 1030-nm Yb-doped fiber laser, 300-mW of output was generated in the DUV at 193 nm. The Er-doped fiber MOPA system was robust, stable and provided pulses with a high peak power, a relatively low repetition frequency and a narrow linewidth.
Acknowledgments
We would like to thank Dr. Alissa Silva for her careful proof reading. This research project was carried out in support of the Photon Frontier Network Program and Photon and Quantum Basic Research Coordinated Development Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work was also supported by the New Energy and Industrial Technology Development Organization (NEDO).
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