High power light sources emitting at 2 μm are useful for applications in the processing of semiconductors, clear polymers, and water-rich biological tissues and biomaterials
Opto-Electronic Advances, Volume. 3, Issue 6, 190039-1(2020)
Compact pulsed thulium-doped fiber laser for topographical patterning of hydrogels
We report the generation of high energy 2 μm picosecond pulses from a thulium-doped fiber master oscillator power amplifier system. The all-fiber configuration was realized by a flexible large-mode area photonic crystal fiber (LMA-PCF). The amplifier output is a linearly-polarized 1.5 ns, 100 kHz pulse train with a pulse energy of up to 250 μJ. Pulse compression was achieved with (2+2)-pass chirped volume Bragg grating (CVBG) to obtain a 2.8 ps pulse width with a total pulse energy of 46 μJ. The overall system compactness was enabled by the all-fiber amplifier design and the multi-pass CVBG-based compressor. The laser output was then used to demonstrate high-speed direct-writing capability on a temperature-sensitive biomaterial to change its topography (i.e. fabricate microchannels, foams and pores). The topographical modifications of biomaterials are known to influence cell behavior and fate which is potentially useful in many cell and tissue engineering applications.
Introduction
High power light sources emitting at 2 μm are useful for applications in the processing of semiconductors, clear polymers, and water-rich biological tissues and biomaterials
An increase in MFA supports higher peak powers, but amplification of pulse energies to hundreds of microjoule remains difficult due to the intrinsic peak power damage thresholds in silica. For amplification at high pulse energies, the chirped pulse amplification (CPA) technique can be exploited
In this paper, we present a fiber master oscillator power amplifier (MOPA) system which consists of an all-fiber amplifier and a compact multi-pass CVBG-based compressor. The main amplifier exploits CPA in a thulium-doped large-mode area photonic crystal fiber (TD-LMA-PCF) with an MFA of more than 1000 μm
Methodology
Fiber MOPA system
The all-fiber amplifier and the compact multi-pass compressor are depicted in
Figure 1.Schematic of the system with the microscopy image of the thulium-doped large-mode area photonic crystal fiber (TD-LMA-PCF) cross-section and the splice point between the TD-LMA-PCF to a polarization-maintaining 30/250 silica fiber.
A polarization controller (PC) and polarization-dependent (PD) isolator function together as a Lyot filter
Due to the high insertion loss of the AOM (7 dB), the pulse energy after the AOM is about 1 nJ. The 100 kHz pulse train with 1 nJ pulse energy is further amplified in a double-pass pre-amplifier (Pre-amplifier 2) to 200 nJ through a 2.3-metre long cladding-pumped PM TDF and CFBG 2 (D = +8.3 ps/nm). CFBG 2 serves as a reflector with a high reflectivity of 99% with a spectral bandwidth of 30 nm at 1975 nm central wavelength. The dispersion of CFBG 2 is low as compared to CFBG 1, and thus does not affect the pulse duration of the stretched nanosecond pulse. CFBG 2 also reduces the out-of-band amplified spontaneous emission (ASE) from propagating to the next stage by spectral filtering. The 100 kHz pulse train was further amplified through a 2.57-metre long TD-LMA-PCF with a 0.04 NA, 50-μm core diameter (mode field diameter of 37±3 μm), and a 250 μm inner cladding diameter. Its single-mode PM guidance enables better output stability and quality without tight coiling. As tight coiling is not required to induce losses for the higher-order modes, coiling-induced loss and mode area compression effects were eliminated. Furthermore, the multimode pump core with an air cladding has a large NA of > 0.5, which matches well with conventional double-cladding fibers for low-loss coupling of pump light. The TD-LMA-PCF is cladding-pumped by 6 fiber-pigtailed 793 nm laser diodes with a total output of up to 180 W, using a (6+1)×1 PM-combiner. The all-fiber amplifier was realized by splicing the TD-LMA-PCF to the 30/250 PM fiber (0.06 NA) of the combiner, and the splice point is shown in
The compressor consists of a pair of CVBGs (D = +22.8 ps/nm) in a compact multi-pass configuration to re-compress the chirped nanosecond pulses. The CVBG's central Bragg wavelength is 1977.5 nm with a bandwidth of 36 nm. The large aperture, 5 mm (vertical) × 10 mm (horizontal), allows a large beam input diameter and consequently enables high pulse energy handling. The CVBGs were mounted on water-cooled copper blocks continuously kept at 20 ℃. The multi-pass configuration was realized by directing the beam into each CVBG with a small angle of incidence using a series of half waveplate (HWP), quarter waveplates (QWP), thin-film polarizers (TFP), and mirrors as shown in
Topographical engineering of biomaterial
A laser scanner system was set up which consists of an X-Y scanner (scanning speeds of up to 1000 mm/s) and a telecentric f-theta lens (focal length of 28 mm) as shown in
Figure 2.Schematic of laser scanner setup for the topographical engineering of hydrogels.
Two types of samples were prepared for the demonstration of topographical engineering of biomaterials using the fiber laser. The first type was hydrogels (0.4% agarose + 0.25% gelatin), prepared using a protocol used in cell culture studies
Results and discussion
Fiber laser performance and discussion
The maximum output power from the all-fiber amplifier was 25 W which corresponds to a pulse energy of 250 μJ with a 21% slope efficiency at 100 kHz repetition rate as shown in
Figure 3.
The transmission of the high-power free-space isolator before the compressor was measured to be 81.65±1.50% from 1 W to 25 W output powers. After the isolator and undergoing 4 passes through the CVBG pair, the final output had a maximum average power of 4.63 W from the initial main amplifier output of 15 W. The final compressor output of 4.63 W corresponds to a pulse energy of 46.3 μJ at 100 kHz repetition rate. To the best of our knowledge, this is the highest output pulse energy from a CVBG-based compressor at the 2 μm wavelength regime.
The output power from the main amplifier to the compressor stage was limited at 15 W due to excessive losses at the first CVBG. The limited transmission bandwidth of the CVBGs acts as a spectral filter, allowing the out-of-band light to leak from the back of the CVBG as transmission. Thus, the main amplifier output with wavelengths below 1959 nm and above 1995 nm were not reflected by the CVBGs. The leakage from CVBG 1 was measured to be 1.7% of the main amplifier output at 2 W output power, which increased progressively as shown in
Figure 4.
The PER after the 4-pass compressor was at least 26 dB at all powers measured. The second-harmonic autocorrelation measurement of the dechirped pulses at 1.7 W and 4.6 W, corresponding to 17 μJ and 46 μJ pulse energies respectively, are shown in
From the autocorrelation trace, the pulse width is not transform-limited and there are pedestal features even at low pulse energies. Looking at the spectral profile of the chirped seed pulse, there are modulations which correspond to amplitude modulations in the temporal domain. This in turn causes Kerr-induced temporal phase oscillations during amplification in the main amplifier
The beam quality after the 4-pass compressor was checked through an M2 measurement with a beam profiler camera at the highest output pulse energy according to the ISO 11146 compliant standard. The measurement which resulted in an M2 value of 1.5 and the collimated beam profile at the highest pulse energy are shown in
Figure 5.
In this work, the main amplifier slope efficiency was limited to 21%, due to the low seeding energy and repetition rate. Increasing the seeding energy by increasing the gain at the pre-amplifier stages would bring about undesirable nonlinear effects at 100 kHz repetition rate operation. Hence, we did not increase the seeding energy to the main amplifier. At the full 65 MHz repetition rate, the main amplifier slope efficiency improved to 39% at the expense of the pulse energy
We acknowledge the limitations of our work and propose these strategies to improve future work. The pulse energy and consequently, the peak power, were limited in this work due to nonlinear effects. The nonlinear effects in the main amplifier could be better mitigated by using a CFBG with a higher stretching factor, with a matching CVBG compressor. The spectral modulations from the pre-amplifier stages cause Kerr-induced phase modulations in the main amplifier, degrading the recompressed pulse temporal profile. These modulations may have originated from nonlinear effects in the pre-amplifier stages. In this work, the AOM introduced a large insertion loss (7 dB) necessitating high gains in the pre-amplifier stages to provide enough seeding energy to the main amplifier. Besides replacing the AOM, the phase modulations can be reduced by spectral amplitude shaping
Application on topographical engineering of hydrogels
The fiber laser was used to demonstrate biomaterial processing on hydrogels, a commonly used scaffold and substrate in cell culture studies. Hydrogels are cross-linked polymeric biomaterials with human tissue-like water content and physiochemical resemblance. Hence, they have been widely accepted as tissue analogs in cell therapy, tissue engineering and regenerative medicine research
The topography and chemical composition of the substrate affect cell behaviour and proliferation, rendering the manipulation of substrate properties important in cell and tissue engineering applications
Topographical manipulation of biomaterials can be done using various techniques such as 3D-printing, lithography-based methods, and moulding
A wide range of materials can be laser-engineered, but some require modification to be photolabile to enhance laser-material interactions. The use of a 2 μm laser source eliminates the need for photolabile materials as most biomaterials are water-rich and can absorb at this wavelength strongly. This is advantageous as it also minimizes potential sources of contamination. Biomaterials such as gelatin-based gels typically have low melting temperatures of below 40 ℃ depending on concentration
The highest scanning speed of 1000 mm/s and laser output power of 3 W were used to create microchannel structures with channel widths of approximately 25-30 μm on the hydrogels as shown in
Figure 6.Microscopy image of microchannels abricated on the hydrogel.
Figure 7.
The effect of the 2 μm laser source on dried hydrogel films was then investigated. The films were prepared from the same hydrogels as before and were then left to dry in a vacuum oven set at 40 ℃ for 16 hours. Opaque white foam was observed on the film surface after laser processing at scanning speeds of 100 to 150 mm/s and at the laser output power of 4 W. The mechanism of the foam formation is laser-induced expansion due to explosive ablation
It was observed that the photon flux controls the morphology of the foam on the hydrogel films. Photon flux was varied by controlling the scanning speed and line spacing. At a low scanning speed (100 mm/s) and line spacing of 100 μm, the photon flux is higher, and the formation of pores was observed. Decreased photon flux resulted in foam formation without pores and increased smoothness as shown in
Conclusions
In conclusion, we report on the generation of high-energy megawatt-level picosecond pulses at a 100 kHz repetition rate from a compact thulium-based fiber MOPA system. The chirped output from the all-fiber amplifier is a linearly polarized nanosecond pulse train with up to 25 W average power corresponding to pulse energies of 250 μJ with a PER of > 11 dB. The high-energy output was realized using a TD-LMA-PCF with an MFA of more than 1000 μm2 as the main amplifier gain fiber. The all-fiber configuration was enabled by optimizing the splice point between the TD-LMA-PCF to a 250-μm PM silica fiber. We further devised a compact (450 mm by 450 mm footprint) compressor using a 4-pass CVBG configuration. Through the compressor, we obtained a 2.8 ps pulse duration with a total pulse energy of 46 μJ, despite high device losses. To the best of our knowledge, this is the highest output pulse energy from a CVBG-based compressor at 2 μm wavelengths. The high-energy picosecond output was then used for high-speed direct-writing on a temperature-sensitive biomaterial, for topographical engineering to create microchannels, foam structures and pores. The fabricated morphology was controlled by simply steering the beam and varying the laser parameters. The topographical modification of hydrogels influence cell adhesion, cell alignment, facilitate cell-cell interactions and hence directing cell growth which will be useful in cell therapy, tissue engineering, and regenerative medicine applications.
Acknowledgements
This work was supported by Agency for Science, Technology and Research (A*STAR), Singapore through the X-ray Photonics Programme (1426500052) and A*STAR Graduate Academy through the A*STAR Graduate Scholarship. The authors would like to thank Ahmad Amirul and Clarice Loke for their assistance with the scanning electron microscopy images.
Author contributions
All authors reviewed and revised the manuscript with valuable suggestions. E. Lee, B. Sun and J. Luo performed the fiber laser experiments, the measurements and analysis. E. Lee and D. Yong designed and initiated the hydrogel processing project. S. Singh assisted in sample preparations and discussions. X. Yu and Q. J. Wang provided the initial fiber laser project direction, designed and initiated the fiber laser project.
Competing interests
The authors declare no competing financial interests.
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Elizabeth Lee, Biao Sun, Jiaqi Luo, Satnam Singh, Deepak Choudhury, Derrick Yong, Xia Yu, Qijie Wang. Compact pulsed thulium-doped fiber laser for topographical patterning of hydrogels[J]. Opto-Electronic Advances, 2020, 3(6): 190039-1
Category: Original Article
Received: Nov. 4, 2019
Accepted: Mar. 4, 2020
Published Online: Aug. 11, 2020
The Author Email: Yu Xia (qjwang@ntu.edu.sg)