We demonstrate an average ultraviolet (UV) power generation of 20 W by harmonically converting the output of an all-fiber master oscillator power amplifier (MOPA) system. The third-harmonic generation (THG) UV output provides a conversion efficiency of 40% from the amplified fundamental output. The seed source, which consists of a 1064-nm-wavelength continuous-wave laser diode and two cascaded intensity modulators, produces chirp-free pulses with tunable durations from 100 ps to 2 ns at arbitrary repetition rates and a high dynamic extinction ratio. The fiber MOPA system generates a maximum average power of 50 W and a maximum peak power of 83 kW.
© 2013 Optical Society of America
High-power pulse lasers are used in numerous industrial applications. Ultraviolet (UV) lasers based on the harmonic generation of Q-switched solid-state lasers are widely used in laser precision processing or micro-processing. Q-switched solid-state lasers typically have pulse widths in the range of from 10 ns to 1 μs. However, generating pulse widths of less than 10 ns using a Q-switched solid-state laser is difficult, and the pulse width depends on the repetition rate . Additionally, the processing quality and throughput are often limited by the pulse width and repetition rate. The all-fiber master-oscillator-power-amplifier (MOPA) architecture can support variable pulse widths and pulse repetition rates and can therefore solve these limitations.
For efficient harmonic generation, the fundamental laser beam should have good beam quality, high polarization extinction, sufficient peak power, and narrow line width. Polarization maintaining large-mode-area (LMA) fiber can support good beam quality with high peak power and high polarization extinction in the fiber MOPA architecture. The spectral property of the fundamental beam, such as the line width, is basically attributed to the seed source. In the fiber MOPA architecture, the spectral property of the seed sources will be maintained by minimizing the effect of optical nonlinearities, such as self-phase modulation (SPM).
Efficient UV generation in nanosecond pulse durations have been demonstrated in previous studies [2–4]. In those studies, the amplifiers use photonic crystal fibers (PCF), which provide single transverse mode operation despite their large (approximately 30 μm) mode diameter. In reference , a cascaded fiber amplifier seeded by a gain-switched laser diode is operated at pulse durations ranging from 2 to 20 ns, and at repetition rates ranging from 200 kHz to 2 MHz. A UV output power of 30 W is demonstrated at a pulse duration of 2 ns and a repetition rate of 400 kHz. Here, all coupling to and from fibers uses free-space optics. Bulk optical isolators and spectral filters are inserted between the amplifier stages for amplified spontaneous emission (ASE) management.
Gain switching of single longitudinal mode laser diodes, such as distributed feedback laser diodes (DFB-LD), has proven to be a simple method of generating nanosecond pulses and is a candidate for seed sources. However, spectral line broadening occurs under the generation of picosecond pulses with single-mode laser diodes due to significant frequency chirp [5,6]. The final line width of a fiber laser system that contains a single-mode laser diode with a pulse duration of picoseconds is much broader than that of a system with a pulse duration of nanoseconds . Fabry-Perot (FP) laser diodes have also been selected for use as seed sources in fiber MOPA systems [8–12]. Since FP lasers, or multi-longitudinal mode lasers, have wide spectra of output under gain switching , they have also been used in conjunction with injection locking in order to ensure single-mode operation in many cases [9–12]. Even in these cases, however, spectral line broadening occurs due to frequency chirp under fast modulation in FP lasers, resulting in widths of approximately 0.1 nm to 0.3 nm. Moreover, in cascaded fiber amplifiers, SPM spreads the spectra, which prevents it from meeting the spectral acceptance of harmonic generation crystals required for UV generation .
Externally modulated lasers have operational flexibility with respect to pulse widths and repetition rate, and require modulators such as electro-optic modulators (EOM) or acoustic optical modulators (AOM). Moreover, such lasers have been selected as the seed sources of fiber MOPA systems that require pulse durations of 2 ns to 20 ns [13–15]. In the present paper, we demonstrate tunable 100 ps to 2-ns pulse widths at an arbitrary repetition rate with seed sources containing a DFB-LD and an x-cut 10-GHz LiNbO3 Mach-Zehnder intensity modulator (MZIM), which has a rise time of 70 ps. X-cut electrode topologies result in chirp-free modulation due to the symmetry of the applied fields in the electrode gaps . The dynamic extinction ratio (DER) of an MZIM for single-stage modulation is low, and typically has a value in the range of from 20 dB to 30 dB. In order to enhance the DER, the MZIM is followed by another MZIM or an AOM. In the latter case (MZIM-AOM), the total seed system was more simplified than in the former case (MZIM-MZIM) because the need for a complex feedback bias control for the second MZIM in the system is avoided.
We have also extended a previous study  on UV generation from an all-fiber MOPA in terms of not only shorter pulses, but also monolithic construction of a MOPA, or an all-fiber coupling system. Unlike in previous studies [2–4], we do not use PCF, which is an obstacle for an all-fiber coupling system. Here, all couplings to and from fibers use fusion splices. In-line multi-function modules (isolators and filters) are inserted between the amplifier stages for ASE management. These modifications permit a maximum average power of 50 W, a maximum peak power of 83 kW, and efficient UV generation by harmonically converting the output of the MOPA system in the sub-nanosecond pulse width range.
2. Seed source operation
Figure 1 shows the two seed configurations used in the comparative study, both of which use a 1064 nm DFB-LD driven under a continuous wave mode. In Fig. 1(a), two LiNbO3 MZIMs have the same 10-GHz modulation bandwidth, or the same rise time of 70 ps. These two MZIMs modulate the laser light in turn, and create laser pulses with synchronized RF input signals that have the same voltage amplitudes and same pulse widths. Optical pulse generation with cascade modulation of MZIMs is shown in Fig. 2. The generated pulses have sharp rising and falling edges, and therefore can be regarded as rectangular waveforms.
Figure 3(a) illustrates the pulse definition. In this figure, we denote the pulse width and the repetition frequency by τ and 1/T, respectively. The static extinction ratio (SER), which is measured without sending any electrical signal to the RF port of the MZIM, and the DER are given as follows:Fig. 3(a) cannot be directly measured, and therefore evaluating the DER is difficult. We can only measure the optical average power pavg, which is expressed as follows:Table 1 is plotted as a function of the duty cycle τ/T in Fig. 3(b), where Pmax was 6.22 mW. The plots were fitted by Eq. (3), and hmin was obtained as hmin = 57.9 nW, which corresponds to the DER of 50.3 dB if The minimum duty cycle in Table 1 is 0.398 × 10−3 (34 dB). If the DER of 44 dB is ensured in cascade modulation, the optical pulse contains 90% of the total energy, even in the case of the minimum duty cycle. It is thought that a 10-dB excess of DER is a pulse quality criterion.
In general, the optical output power P is given as a function of the phase difference φ of the two waveguides in the MZIM byEq. (5) can be given as follows: The phase drift is denoted by the voltage as follows: Here, and ΔV is the voltage value corresponding to the offset from the minimum point. If << 1 and << 1 are satisfied, then the following approximation is applied to Eq. (4):
The second term of Eq. (6) is detectable using a lock-in amplifier (LIA). The reference to input to the LIA is defined as Here, θ is the phase difference between the input signal and the reference to the LIA. The quadrature component of the LIA output is expressed as follows:
The Y component was used for the feedback signal of the DC bias control in the present system. The first term of Eq. (6) is proportional to the square of the dither signal amplitude and provides an extinction limit of the SER or the DER given by
Figure 4 shows the setup of the MZIMs. A dither signal, which is generated by a function generator (FG), and a DC bias voltage are applied to the DC port of a MZIM. A rectangle wave voltage is applied to the RF port. The optical signal is divided by a coupler, converted into an electrical signal in the PD, and then input to the LIA. The Y component of the signal vsis input to a bias controller (BC) for feedback bias control. In Fig. 1(a), the dither signals for MZIM1 and MZIM2 have amplitudes of 0.15 V and 0.10 V and frequencies of 760 Hz and 1040 Hz, respectively. Therefore, when Vπ = 5 V, the extinction limit for cascade connection of MZIMs is 62.6 dB in Fig. 1(a), which corresponds to the dither amplitudes.
The measured SER was 55.4 dB, and the DER was 50.3 dB in the wide operation range of Table 1. The deterioration of extinction from the limit value is significant in the case of cascade modulation of two MZIMs. Each modulator shoulders a 30-dB extinction. In this case, the second term on the right-hand side of Eq. (6) is approximately 1 ppm order of pmax for the MZIM2, and is difficult to detect due to its small size. Therefore, the bias control of MZIM2 causes deterioration from 60 dB to 50 dB. In contrast, in Fig. 1(b), MZIM2 is replaced by an AOM that has a rise time of 25 ns. The gate time for the AOM was set to 50 ns. The optical pulse generation with MZIM1 was conducted under the same conditions as Fig. 2. Since the dither signal for MZIM1 has an amplitude of 0.10 V in Fig. 1(b), the extinction limit of MZIM1 is 33.1 dB.
The measured SER was 32.7 dB, and the DER was 32.3 dB for single-stage modulation. The AOM has a high extinction ratio of 50 dB, but the AOM produces a wide pedestal background (50 ns) below the baseline of pulse waveforms by MZIM1. When MZIM1 is driving at a pulse duration of 200 ps and a repetition rate of 2 MHz, the DER for the cascade connection of MZIM1 and AOM decrease by 6 dB in comparison with the cascade connection of MZIM1 and MZIM2.
3. Three-stage fiber amplification and third harmonic generation
Figure 5 shows a three-stage fiber amplifier system. The first amplifier uses a double-pass configuration based on a 3-m-long, polarization-maintaining double-clad Yb-doped fiber (6 μm/125 μm core cladding diameter with 0.15/0.46 NA) to boost the power of the seed pulses. The double-pass amplifier is composed of a three-port circulator, a combiner, the Yb-doped fiber mentioned above, a 600-mW fiber-pigtailed 976-nm laser diode, and a fiber Bragg grating (FBG). The Yb-doped fiber coupled to the second port of the circulator was pumped by the fiber-pigtailed 976-nm laser diode. The amplified pulses were reflected by the FBG with a bandwidth of 0.5 nm. The FBG acts as a band pass filter, which cuts the ASE during amplification. The reflected pulses were amplified again to an average power of approximately 40 mW in the Yb-doped fiber. The third port of the circulator is coupled to the second amplifier based on a 3-m-long, polarization-maintaining double-clad Yb-doped fiber (12 μm/125 μm core cladding diameter with 0.08/0.46 NA) through an in-house fabricated in-line filter/isolator module, which can manage the mode sizes (6 μm to 12 μm) between amplifiers. Because the input and output fibers of the module have different mode sizes, a mode-field adaptor (6 μm to 12 μm) was omitted from the amplifier system. The second-stage amplifier is pumped by a 5-W fiber pigtailed 976-nm laser diode via an in-house fabricated (1 + 1) × 1 pump signal combiner. The second amplifier amplified the signal to an average power of 2 W. The output end of the second amplifier is spliced to the third amplifier based on a 4-m-long, polarization-maintaining double-clad Yb-doped fiber (25 μm/250 μm core cladding diameter with 0.07/0.46 NA) through another in-house fabricated in-line filter/isolator module, which also can manage the mode sizes (12 μm to 25 μm) between amplifiers.
The filters contained in the multi-function modules have a bandwidth of 5 nm and are used to eliminate ASE. The second and third amplifiers are cladding pumped in the counter-propagating configurations.
The third-stage amplifier is pumped by two 35-W fiber pigtailed 976-nm laser diodes via an in-house fabricated (2 + 1) × 1 pump signal combiner. Since the signal line of the combiner is constructed of Yb-doped fiber, the combiner emits the signal beam outside immediately after amplification in the Yb doped fiber. This minimizes the effect of the fiber’s optical nonlinearity. The fundamental waves were converted to third-harmonic waves by two LBO crystals (type I SHG: 5 mm × 5 mm × 15 mm and type II THG: 5 mm × 5 mm × 20 mm). The third-harmonic waves were separated using two harmonic separators. The beam waist diameters measured in the SHG and THG crystals are 100 μm and 110 μm, respectively.
In order to compare the two seed sources described in the previous session, both were coupled to the amplifier system in turn. Figure 6(a) shows the output average power of the amplifier system for the two seed sources with a pulse duration of 200 ps at 2 MHz. In this fig., all of the output data points follow an identical curve. The slope efficiency was approximately 60%. The maximum output power was 33 W, which corresponds to the maximum peak power of 83 kW. Figure 6(b) shows the spectra when the seed sources are amplified to an average power of 33 W. As can be seen in the figure, there is little difference between the two sources. The spectral band widths (FWHM) are <0.07 nm, which correspond to the resolution of the spectrum analyzer. The THG output powers converted from the amplifier output are shown in Fig. 7. The maximum THG power was 11.9 W, which corresponds to an efficiency of 37.9% for the first seed source (MZIM1 + MZIM2) in Fig. 1(a). The maximum THG power was 11 W, which corresponds to an efficiency of 35% for the second seed source (MZIM1 + AOM) in Fig. 1(b). It is thought that a decrease in the DER for the second seed source (MZIM1 + AOM) reduces the efficiency of THG in Fig. 7. Nevertheless, although there is a slight difference in the results of the two seed sources, they are equivalent from a practical point of view.
One of the 35-W pump laser diodes was replaced by a 60-W fiber pigtailed 976-nm laser diode. Figure 8(a) shows the output average power of the amplifier system for the pulse width/repetition rate condition: 200 ps at 4 MHz. The maximum output power was 50.8 W, which corresponds to a peak power of 63.5 kW. The slope efficiency was approximately 56%. Figure 8(b) shows the spectrum when the seed source in Fig. 1(a) is amplified to an average power of 50.8 W. Neither ASE nor stimulated Raman scattering (SRS) is entirely observed in Fig. 8(b).
Figure 9 shows the THG output powers converted from the over-pumped amplifier output and demonstrates the pulse width flexibility of the fiber MOPA system seeded by the seed source in Fig. 1(a). The maximum THG powers were 21.5 W, 19.5 W, and 16.3 W for the pulse width/repetition rate conditions: 2 ns at 400 kHz, 200 ps at 4 MHz, and 100 ps at 8 MHz, respectively. These conditions maintain a fundamental peak power for the same fiber laser output. The corresponding THG conversion efficiencies were 42.3%, 38.2%, and 32.1%. Possibly as a result of the decrease in the transform limited line width, the THG conversion efficiency increases with the pulse width despite the fact that the SHG efficiencies are around 60% for the all conditions. Another possible explanation for the lower efficiency for 100 ps is the reduction in the DER resulting from the slow response of the MZIMs. As shown in Fig. 2, 150 ps is the minimum pulse width that can maintain the maximum transmission of the transfer function of the MZIMs.
The 50.8 W average output power of the third amplifier was limited only by the pump power in Fig. 8. The peak power of 83 kW in Fig. 6 is close to the threshold of stimulated Brillouin scattering (SBS) at a pulse width of 2 ns. The SBS threshold is dependent on the peak power, the pulse width, the spectral width, the mode field diameter, and the fiber length . The phonon life time of silica fiber is approximately close to 10 ns. In addition, the SBS gain band is extremely narrow and typically around 50 MHz. Therefore, for a pulse width of less than 1 ns, the SBS gain is substantially reduced. The nonlinear interaction length in a Yb-doped fiber is confined to around a value of the effective transmission length of the pump light . In the present study, it corresponds to approximately 0.4 m in the third amplifier, which has a cladding absorption coefficient of 11 dB/m. At a pulse width of 200 ps, even at twice the peak power (150 kW: 200 ps @ 1 MHz) did not induce SBS and SRS in the third amplifier.
In order to estimate the nonlinearity of the fiber MOPA system, the gain-switched DFB-LD shown in Fig. 1 was coupled to the amplifier system. The gain switched DFB-LD is operated at the pulse duration of 50 ps and the repetition rate of 8 MHz. The picosecond frequency chirp induces the dynamic line broadening in the DFB-LD, at which time the spectral change during amplification can be detected by the spectrum analyzer. Figure 10(a) shows the spectrum of the DFB-LD and Fig. 10(b) shows the spectrum when the DFB-LD is amplified to an average power of 33 W. It was found that the 10-dB spectrum bandwidth broadened by SPM from 0.488 nm to 3.655 nm at the average power of 33 W, which corresponds to a maximum peak power of 83 kW. The expansion rate of the spectra was limited to a value less than 8.
We demonstrated an average power of 20 W at 355 nm by harmonically converting the output of a fiber MOPA system in the wide operating ranges of the pulse width and the repetition rate. Seed sources containing a DFB-LD and two cascaded intensity modulators were used to generate seed pulses with tunable pulse widths ranging from 100 ps to 2 ns at arbitrary repetition rates, and were found to support the flexible operation of the fiber MOPA system.
For efficient UV generation, a chirp-free external modulator was used in the seed sources. The DER of the chirp-free modulator was enhanced to a sufficient value of approximately 50 dB. Moreover, for efficient UV generation using the fiber MOPA system, optical fiber nonlinearities, such as SPM, were minimized using multi-function devices, which reduces the total number of devices in the all-fiber architecture, and an in-house fabricated pump-signal combiner, which reduces the interaction length of optical nonlinear effects.
Since the obtained output average power was limited by the available pump power, we believe that further power-scaling is possible with the current all-fiber MOPA system. In the final amplifier, the MOPA system can generate high peak power in excess of 80 kW, without SBS and SRS, in the pulse width range of from 100 ps to 2 ns. Since the SBS threshold is dependent on the pulse width, we also believe that that further peak power scaling should be possible for shorter pulse widths within this range.
The present study was supported by the Japan Science and Technology Agency through A-STEP (Adaptable & Seamless Technology Transfer Program through Target-driven R&D) Grant Number AS2414040J.
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