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Abstract
High peak power, narrow linewidth sources continue to be in high demand. Fiber amplifiers are a compelling option to scale peak power of long 100-ns-pulses because of their compact size and robustness. Unfortunately, stimulated Brillouin scattering (SBS) limits peak power of narrow linewidth fiber sources causing instability. We demonstrate SBS suppression for 130-ns pulses from a 5 MHz linewidth seed laser in a fiber amplifier by using tapered fiber with large 50 µm diameter core in the output. The longitudinal change in the core diameter induces frequency shift in the SBS gain peak and the back-travelling Stokes wave is suppressed towards smaller core. We reach 2.2 kW peak power with 18.7 dB polarization extinction ratio and record breaking 4 kW peak power by exciting both polarization states of the polarization maintaining tapered fiber. The output beam quality equals to single mode fibers with M2 = 1.08.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber based high peak power light sources, with narrow linewidth and nanosecond pulses, are extremely useful for applications including coherent lidar systems [1], remote sensing [2] and frequency conversion [3]. Typical architecture for these light sources follows the master oscillator – power amplifier scheme (MOPA) using fiber amplifiers, making the systems compact and robust when compared to solid-state sources with free space optics.
The peak power scaling of nanosecond pulses in a fiber amplifier is mainly limited by Stimulated Brillouin scattering (SBS). The phenomenon is non-linear and based on electrostriction, where oscillating electric field gives rise to acoustic lattice vibrations (phonons) [4]. The acoustic phonon causes varying refractive index, acting as a grating inside the fiber, leading to a back-reflecting Stokes wave of light. In high power fiber amplifiers this back reflection experiences gain and causes instability to the amplifier.
The bandwidth of SBS gain is in the order of tens of megahertz in single mode silica fibers [5]. Especially, in case of narrow linewidth (∼ 1 MHz) sources the SBS gain and laser spectrum have perfect overlap lowering the SBS threshold when compared to sources with larger linewidth. Furthermore, the phonon lifetime in quartz is about 10 ns [6,7], making the amplification of pulses with a duration shorter and longer than 10 ns essentially different cases. In case of short pulses, the interaction time between the pulse and acoustic wave is shorter than the lifetime of the phonon and the short pulse interacts only with partially developed phonon. In case of long, tens or hundreds of nanoseconds pulses the phonon has time to fully develop and interact causing significantly lower SBS thresholds than with short pulses.
SBS suppression in fiber amplifiers can be achieved with several different strategies that have been introduced in the literature. The basic principle is to alter the acoustic properties of the fiber along the fiber length, shifting the SBS gain peak to a different frequency or artificially broaden the linewidth of the laser for example by frequency chirping. The acoustic properties of the fiber can be manipulated by exploiting a longitudinal temperature or mechanical stress gradient in the gain fiber [8,9,10]. Tapering the fiber has also been shown to be beneficial [11] as the change in core size leads to a shift in the SBS gain spectrum. Furthermore, the SBS threshold can be doubled by using birefringent polarization maintaining fiber and exciting the slow and fast waves equally as first shown by Stolen [12]. The slow and fast waves have slightly different wave number and the momentum conservation law between the laser light and Stokes wave ensures that SBS process is independent for both polarisation states. Moreover, the electric field density can be lowered by using large-mode-area (LMA) fibers enabling higher peak power than single mode fibers. Usually, authors have used only one of abovementioned strategies for SBS mitigation. All these measures might be imposed simultaneously by exploiting a short end pumped tapered active fiber with large mode field diameter.
The SBS threshold for narrow linewidth sources can be in megawatt-level in peak power for nanosecond pulses shorter than 10 ns whereas for pulses longer than 10 ns the threshold is two orders of magnitude lower. Schorstein et al. achieved 50 kW peak power for 10 ns pulses with Fourier limited 44 MHz linewidth by using 55 µm LMA fiber [13]. Lago et al. used a phase modulator leading to 28 GHz linewidth and 5 ns pulse length achieving 280 kW peak power without a sign of SBS in the 40 µm LMA fiber [14]. Palese et al. applied coherent combination of two identical sources using 40 µm tapered fibers reaching 420 kW for 1 ns pulses while being limited by stimulated Raman scattering rather than SBS [2]. Di Teodoro et al. achieved 1.54 MW for 1.55 ns and Fourier limited linewidth of 1.1 GHz with a 100 µm core photonic crystal fiber [7]. By using 200 µm core and allowing a few nanometers wide spectrum and spatial multimode output 2.4 MW peak power has been achieved for 4 ns pulses [15] and 8.5 MW for 10 ns pulses [16]. For long pulses, the SBS threshold is quite close to the threshold of CW lasers and amplifiers [17]. Zhu. et al applied strain gradient method to achieve 500 W peak power with 500 ns pulses in a 25 µm core silica fiber maintaining 2 MHz linewidth [8]. Wang et al. applied temperature gradient for 15 µm core fiber and achieved 2.2 kW peak power with 53 ns pulses, although the linewidth was 230 MHz [18]. Fu et al. have achieved 3.4 kW peak power with Fourier limited linewidth for 140 ns pulses by using very short 34 cm phosphate fiber [6].
In this work, we present amplification of narrow, near Fourier limited, 5 MHz linewidth 130-ns-pulses in the short 1.2 m tapered fiber. We manipulate the SBS gain frequency by changing the core size longitudinally in the birefringent tapered fiber. By end pumping, we have more gain in wide side maintaining low electric field density through the fiber, while the total energy grows more towards the end. The tapered shape suppresses the backwards travelling Stokes wave and preserves excellent beam quality with M2=1.08. We reach peak power of 2.2 kW and pulse energy of 0.29 mJ with linear polarization output and by exciting both slow and fast waves we are able to reach record breaking peak power of 4 kW and pulse energy of 0.52 mJ.
2. Experiment
We have designed an all fiber light source to demonstrate pulse amplification in a tapered fiber. The design is presented in Fig. 1 and contains master oscillator, pre-amplifier and power amplifier stages. The master oscillator forms the initial pulse train and the pre-amplifier is used to increase and control pulse energy entering the power amplifier. The final power scaling is done in the tapered fiber inside the power amplifier.
Fig. 1. Schematic picture of the MOPA-system. The distributed feedback (DFB) laser diode is used as the master oscillator and pulses are created with an acousto-optic modulator (AOM). The shape and duration of the pulse can be fully controlled with the AOM. The pre-amplifier is a dual stage design and a bandpass filter is used between stages to reduce amplified spontaneous emission. Pulse energy entering the power amplifier can be controlled by adjusting the second stage amplification. The power amplifier is constructed from an ytterbium-doped tapered fiber together with pump laser diode and cladding mode stripper.
We constructed the master oscillator and pre-amplifier with Panda type polarization maintaining (PM) single mode fibers. The tapered fiber used for power amplifier has been described in detail in Section 2.2. All fiber connections were spliced accurately together with Vytran GPX 3400 glass processing station.
2.1 Master oscillator and pre-amplifier
The master oscillator has been designed to have full control over the pulse shape and duration by creating pulses from a continuous wave (CW) seed laser with an acousto-optic modulator (AOM). The light from a narrow-linewidth distributed feedback (DFB) laser (DFB-1053-PM-50, Innolume) is coupled into a single mode PM980 fiber having a core diameter of 6 µm. The PM fiber is aligned to have the panda rods (the slow axis) parallel to the linear polarization axis of the laser light. The laser emission wavelength is 1053 nm having the linewidth less than 5 MHz, according to manufacturer’s specification. The seed laser is protected by fiber-pigtailed isolator to block back-reflected light, which can cause the fatal damage of the source.
Acousto-optic modulator (T-M150, Gooch & Housego) forms pulses from CW laser light at 10 kHz repetition rate. The AOM is driven with a 150 MHz radio frequency (RF) signal and the throughput is controlled with the amplitude. Symmetrical output has been shown to be achievable by pre-shaping the input pulse [19,20]. We apply pre-shaping by controlling the RF modulation amplitude with a function $A = \frac{{{t^3}}}{{{{({350\; ns} )}^3}}}$, (t = 0…350 ns). The throughput has a slow growth to maximum amplitude and the pulse is reshaped towards more symmetrical shape after amplifier. The pulse shape driving the AOM and the pulse shape after the pre-amplifier are shown in Fig. 2. The output power after the AOM is 40 µW.
Fig. 2. Pulse shape driving the AOM and after the pre-amplifier. The pulse forefront depletes gain from trailing edge thus experiencing more gain and shifting the peak towards forefront.
The pulses are coupled into a custom-built pre-amplifier stage to raise and accurately control the pulse energy before the power amplifier. We use a dual stage design with a 4 nm bandpass filter in between the stages to reduce amplified spontaneous emission (ASE). The first and the second stage are identical in design, using short 1-m-long Yb3+-doped single mode fibers (PM-YSF-HI-HP, Nufern). Pump lasers (AC1409, Gooch & Housego) are counter-coupled to the gain fiber core with wavelength division multiplexers. The both pump lasers are controlled individually and each stage is isolated.
The first stage of the pre-amplifier is used with a fixed pump power. The output is near the saturation limit with 280 mW pump power and 3.14 mW filtered output power as measured with Ophir 3A power meter. Higher pump power at this stage mainly increases the amount of ASE rather than signal. The optical to optical efficiency is 1.1%.
The second stage of the pre-amplifier is used to control the power entering the tapered power amplifier and capable of reaching the SBS threshold of single mode fiber. We raised the second stage pump power until the shape of consecutive pulses, monitored with a fast photodiode and oscilloscope, became unstable, indicating SBS threshold. The pre-amplifier output power is SBS limited at 16 mW with 74 mW pump power and the efficiency is 22%. The pulse energy was measured to reach 1.4 µJ with 124 ns pulse length leading to 11 W peak power at maximum. The pre-amplifier output power is presented in Fig. 3(a) and the spectrum at maximum output is presented in Fig. 3(b). The reached SBS threshold is fairly typical limit for a single mode fiber as in pre-amplifier we are not trying to mitigate SBS, but rather have sufficient controllable pulse energy as an input for the larger core power amplifier.
Fig. 3. (a) Output of the pre-amplifier. The power is limited by SBS at 16 mW. (b) Spectrum after dual stage pre-amplifier. The bandpass filter between amplifiers can be seen as spectral shape from 1051 nm to 1055 nm. Curve from 1010 nm to 1100 nm is ASE from the second preamplifier.
2.2 Tapered power amplifier
We use an active ytterbium-doped tapered double clad fiber (T-DCF) as the main component of the power amplifier. The tapered fiber is manufactured by a stacking method. The active core rod was surrounded by a set of different sized rods from pure silica (F300) and boron doped silica to arrange a panda-like structure. The assembly of rods was collapsed down to a solid preform and drawn to a tapered fiber [21]. The fiber end face is presented in Fig. 4(a) and the measured cladding size profile in Fig. 4(b). The cladding diameter has 10 times the diameter of the core. The core diameters are 17 µm and 49 µm at the narrow and wide side of the T-DCF, respectively. The active core has an almost perfect step-like refraction index profile with the difference of refractive index Δn = 0.0023 which corresponds to NA = 0.08. The total length was 1.2 m. The ratio between input and output diameters is 2.9.
Fig. 4. (a) End face of active panda type tapered fiber. (b) Measured cladding diameter at different positions of tapered fiber. The Cladding has 10 times the diameter of the core.
The pump absorption was measured for the core and cladding separately. The in-core small signal absorption at 976 nm is 700 dB/m. The counter-propagated cladding pump absorption was measured at 976 nm to be 3.3 dB/m. The presence of borosilicate tensile rods in the cladding leads to an efficient mixing of the cladding modes, and therefore no shaping of the cladding is required for efficient clad pump absorption.
The birefringence of the active T-DCF was measured by using a standard method of polarization mode beating [22]. The broadband polarized light from 1.5µm ASE source was launched into the narrow side of T-DCF to excite both polarization eigenstates equally. The output of the T-DCF was then collimated and passed a through an analyzer and registered by an optical spectrum analyzer. The resulted birefringence value is 0.41 × 10−4 for 1030 nm wavelength and polarization extinction ratio (PER) is about 30 dB for fiber length of 4 m.
The T-DCF core has 17 µm diameter at the narrow side. Thus, the narrow side can support up to 3-4 transversal modes. For selective excitation of the fundamental mode only, we use a 10-cm-long coupling fiber, having a core size of 10 µm, in between the single mode pre-amplifier and the tapered power amplifier.
The power amplifier is constructed by counter pumping the T-DCF from the wide side. The multimode pump, having maximum power of 50W at wavelength of 976 nm, is coupled to the tapered fiber via a dichroic mirror. A cladding mode stripper (CMS) is used at the narrow side to remove the unabsorbed pump radiation. The fiber was mounted as a 35 cm coil on an aluminium plate together with the CMS to maintain the adequate cooling. The wide end of the T-DCF was cleaved at 6-degree angle to prevent lasing from the fiber facet.
The power amplifier pulse energy and SBS threshold was tested with two different polarization configurations: one-mode and two-mode excitation. To maintain linear, one-mode, output configuration the PM-splice was made by aligning the panda rods of the pre-amplifier fiber and power amplifier coupling fiber in 0°-degree angle, maintaining the light only on the slow axis of the T-DCF. To obtain elliptical, two-mode, output polarization state we spliced the pre-amplifier output fiber and the power amplifier coupling fiber at 45° angle to exite two polarization states inside the T-DCF of nearly equal amplitude.
3. Results
The main limitation of the pulse amplification is SBS. We monitor the SBS threshold by comparing consecutive pulse shapes on an oscilloscope. The SBS threshold was found by pumping the T-DCF until the shape started to become unstable. We define this instability of pulse shapes as the critical SBS threshold. Further increase in pump power resulted into very strong and short backward travelling pulses, which is not safe operating state for the fiber amplifier.
We compared the SBS thresholds between the one-mode and two-mode excitation at identical conditions by fixing the pre-amplifier output to 10.8 mW at 10 kHz, corresponding 1 µJ in pulse energy and 8 W in peak power. The results are presented in Fig. 5(a). For one-mode case SBS threshold was reached with pulse energy of 243 µJ and 1.9 kW peak power. For two-mode case the SBS threshold was reached with 319 µJ pulse energy and 2.5 kW peak power.
Fig. 5. (a) The output pulse energy of the power amplifier. The absorbed pump power is approximately 4 dB. For fitted data, the pre-amplifier output power was 10.8 mW. Optimized pre-amplifier powers were 5.4 mW and 9.6 mW for one-mode and two-mode excitation, respectively. (b) Spectrum of the power amplifier output. Measurement was taken with optimized pre-amplifier and maximum output energy. (c) Pulse shape before and after the power amplifier.
The output pulse energy was optimized for both cases separately by tuning the pre-amplifier output together with the power amplifier pump. With one-mode excitation, optimal pre-amplifier output of 5.4 mW and 44 W pump power resulted in pulse energy of 288 µJ and 2.2 kW peak power. The average power was 4.6 W containing 1.7 W of ASE. With two-mode excitation we were able to raise the pre-amplifier output to 9.8 mW and pump the taper with 48.5 W. This resulted in record breaking 524 µJ pulse energy and 4 kW peak power. Average output power was 7 W. We would like to note that amount of ASE was equal to 1.8 W. The measurement of pulse energy was performed directly by an energy meter (Ophir Optronics), whereas the peak power values were calculated based on the measured energy value and pulse duration. We recorded the spectrum in optimized conditions for both cases and they are presented in Fig. 5(b).
The pulse shape before and after the power amplifier, measured with a fast photodiode and oscilloscope, is presented in Fig. 5(c). The pulse duration after the power amplifier output is 130 ns. When compared to pre-amplifier output, used as an input for power amplifier, the pulse length is 4 ns longer and the shape has become more symmetrical.
We characterized the output beam by measuring the M2 beam parameter and the beam transverse profile. M2 parameter was analyzed with Beamscope 5 by focusing the beam and measuring the beam size on both sides of the focal point having in total 80 measurement points. The measurement data is shown in Fig. 6(a). The M2 parameter was then used as a free parameter in the theoretical fit shown as the solid line in the figure. The beam profile picture was taken with Ophir Spiricon LBA 270 beam profile camera from a collimated output beam and is presented in Fig. 6(b).
Fig. 6. (a) Beam quality parameter M2 for both axes. The line fit has been made for 80 data points in both cases. 40 points are presented here for clarity. (b) Beam profile picture. Measurement was made from collimated beam 1 m distance from taper output. One pixel corresponds to 4.4 × 4.4 µm2.
The amplified beam has a round bell-shaped form being almost symmetric Gaussian. Small fluctuations in circular cross-section of the taper affect to the mode shape. The beam has near perfect M2 values of 1.08 and 1.05 for two orthogonal axes measured. The transverse output mode is still strictly single mode and light is not coupled to other modes inside the taper.
The polarization extinction ratio (PER) was measured for one-mode excitation by placing a high extinction polarizer in the output and comparing the pass-through pulse energy with a pulse energy meter. The achieved PER was 18.7 dB.
4. Discussion
Tapered profile in the gain fiber has multiple benefits for high power amplification. The SBS is caused by the high electric field density and as the pulse energy grows along the fiber so does the fiber core size. This leads to electric field density staying below SBS threshold even though the pulse energy grows. Change in core size leads also to an SBS gain peak shift so that the Stokes wave generated in the narrow part of the taper has different frequency than the Stokes wave generated in the wide part. The result is wider SBS gain spectrum with lower peak value and thus increased SBS threshold. Furthermore, the tapered profile suppresses the Stokes waves travelling backwards towards smaller core.
Being able to grow the fiber diameter along the fiber length simplifies the power amplifier design to a single stage. LMA -fiber solutions tend to have multi-stage designs with different core sizes [8]. The discrete change in core size leads coupling losses between separate amplifier stages and potentially increases ASE generation as the gain may be insufficient after the increase in core size. If we compare a possible equivalent LMA – fiber configuration to the used tapered fiber, the power amplifier could have three stage design with 10 µm, 25 µm and 50 µm core sizes. Ideally these stages should be core pumped to preserve beam quality. Pumps should be controllable separately for optimal operation, which leads to a need of three pump lasers and drivers. The tapered power amplifier uses only one pump laser coupled to the cladding with free space optics or robust integrable micro-optical assembly and the slow growth in core size ensures good beam quality in the output.
The tapered fiber is polarization maintaining and birefringent, which enables excitation of two orthogonal polarization states without cross coupling. As shown by Stolen [12] it is possible to double the SBS threshold by exciting both polarization states equally. We achieved to push the SBS threshold from 2.2 kW in one-mode case up to 4 kW with two-mode excitation. Most likely the spliced angle between the fiber axes was not exactly 45 degrees leading to multiplier 1.82 rather than doubling the threshold. Even though the excitation to the fiber is linear, the 0.41 × 10−4 birefringence induces phase difference to the two polarization states. Inside the fiber the phase difference is 90° after 6.4 mm length meaning that the polarization is momentarily circular before turning to elliptical and back to linear and it continues to change until the fiber ends. The output polarization with two-mode excitation is practically elliptical as it depends on the total length of the tapered fiber and is rather sensitive to the length.
The gain experienced by the signal is proportional to electric field density implying to use small core size for better efficiency. This leads to SBS problems as the power increases and the core size has to be increased again lowering the efficiency. In optimal case, the tapered power amplifier is able to maintain the pulse peak power on the edge of SBS threshold along the fiber from narrow 17 µm side up to wide 50 µm output. We control the input power with the pre-amplifier and the power scaling with the power amplifier’s pump laser. In the optimal case, the pulse peak power would grow at the same rate as the fiber cross-section keeping the system on edge of SBS threshold at all the time. Having too low input power leads to a high ASE at the output and the SBS threshold may not be reached due to reduced gain, and the signal power will be low. On the other hand, too high input power leads to early increase of the power, and the SBS threshold may be reached too early in the narrow side of the T-DCF. By controlling the second stage of our pre-amplifier we were able to optimize the one-mode and two-mode cases individually. The result of too high input power can be seen in the amplification results of the non-optimized cases in Fig. 5(a). The SBS threshold is reached with lower pump power, implying too early power increase in the tapered fiber. To optimize the amplification, we are lowering the pre-amplifier power from its maximum, and increasing the pump power in the power amplifier. This yields to a higher ASE at the output and less efficient pumping, but eventually the highest peak output power is achieved when lowering the pre-amplifier power cannot anymore be compensated by increasing the power amplifier pumping.
The linewidth of the amplified output is defined by the seed laser linewidth, pulse length and shape, and possible nonlinear effects in the fiber. The seed laser was specified to have the linewidth of less than 5 MHz, while Fourier transform analysis of the generated 130-ns-long pulses yields to 3–6 MHz linewidth depending on the shape of the pulse. These are typical values of linewidth commonly achieved in coherent lidar applications [23]. Further, the linewidth can be broadened by nonlinear effects like self-phase modulation and four-wave mixing in the fiber. We calculated the nonlinear phase shift to be at maximum of about 0.1 radians using silica fiber parameters and the geometry and peak power levels in our fiber amplifier. The phase shift is small to affect any significant nonlinear broadening for the laser linewidth. Thus, we estimate the laser linewidth at the output of the tapered power amplifier to be less than 10 MHz.
Good beam properties have high impact on application point of view. Especially coherent lidar performance is heavily dependent on beam quality [23]. Typically, fibers with large mode area tend to need extra care to preserve the beam quality. The splicing between the single mode and LMA -fibers has to be very precise and still the sudden change in core size may lead to multimode excitation. Mode cleaning by coiling the fiber can be used to introduce losses for higher order modes and M2 values of 1.2 [8] and 1.06 [18] have been achieved with 10-cm coil diameter. The beam quality parameter M2 = 1.08 achieved in this work is exceptionally good and is consequence from T-DCF properties. The beam from single mode pre-amplifier fiber is coupled to the T-DCF precisely and only the fundamental mode is excited. The slow growth of tapered fiber core size ensures the growth of the beam size at the same rate and no power is lost to other modes.
5. Conclusion
We have demonstrated record-breaking peak power of long 130-ns-pulses with a tapered fiber amplifier. The combination of tapered fiber longitudinal profile and excitation of both polarization states with a large core diameter in the tapered fiber leads to 4 kW peak power and 524 µJ pulse energy. With PM-splice, we achieved the peak power of 2.2 kW and 288 µJ energy. When compared to LMA-fibers, the polarization maintaining tapered fiber offers simple single stage power amplifier solution with high gain and excellent beam quality of M2 = 1.08. These properties are highly valued in applications like coherent lidar and frequency conversion.
Funding
Vaisala Oyj; Academy of Finland, Photonics Research and Innovation (PREIN) (320165).
Disclosures
KP: Vaisala Oyj (F), JR: Ampliconyx Oy (E), TN: Ampliconyx Oy (I,E), RG: Ampliconyx Oy (I), VF: Ampliconyx Oy (I,E,P). YC and JT declare no conflicts of interest.
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