1. Ultra-compact, stabilized fiber lasers and their application to laser spectroscopy
Recently have developed compact, temperature-stabilized, and repetition rate-tunable Er-doped fiber lasers with 125 MHz repetition rate (frep). The laser is thermally stabilized by placing the entire resonator on a metal core printed circuit board (MCPCB) heating plate, with uniformly spread heating traces. We have implemented three different actuators for repetition rate tuning and stabilization: long range piezoelectric transducer (PZT) stretcher (40 µm of travel), small range PZT (3 µm travel) and a gold-coated resistive fiber heater. The PZTs enable frep tuning by 2.6 kHz in total (2.5 kHz and 0.1 kHz) and locking of the oscillator’s frep to an external reference with stability better than 2 mHz over >20 hours of measurement .
Figure 1. Photograph showing the prototype of the Er-doped fiber laser (a), acquired CO2 absorption spectrum recorded with a cavity-enhanced Vernier spectrometer based on the Er-doped laser (b).
2. Novel mode-locking techniques
We are developing Erbium-doped fiber lasers, which generate ultrashort (femtosecond) optical pulses. We are investigating different mode-locking techniques based on saturable absorbers (graphene, SESAM), as well as saturable absorber-free methods (e.g. nonlinear loop mirrors).
We developed a NALM-based Er-doped fiber laser in so-called figure-nine configuration and we investigated its performance in various dispersion regimes. We show that the spectral and temporal phase of the pulses at both figure-nine outputs have clearly different characteristics. One of the laser outputs provides pulses with significantly better quality; nonetheless, the rejection output also offers ultrashort pulses with broad spectra. Pulses as short as 79 fs with an energy of 83 pJ were generated directly from the laser in the near-zero dispersion regime .
Figure 2. Experimental setup of the NALM laser with a non-reciprocal phase shifter . PM WDM: polarization maintaining wavelength division multiplexer; PM EDF: polarization-maintaining erbium-doped fiber; PM SMF: polarization-maintaining single-mode fiber; LD: laser diode; col: collimator; PBC; polarization beam combiner; FR: Faraday rotator; QWP: quarter-wave plate; M: mirror.
3. Mid-infrared frequency combs (6 - 9 μm)
We have developed a prototype of a mid-infrared (mid-IR) frequency comb source which covers the spectral range of 6.5 – 9 μm wavelength . Thanks to its broad spectral coverage, the laser enables targeting of entire molecular bands of air pollutants and greenhouse gases. e.g. methane, nitrous oxide or sulfur dioxide. The “heart” of the mid-IR comb is a femtosecond Erbium-doped fiber laser with 125 MHz repetition frequency and wavelength of 1560 nm.
We have implemented a simple and effective method of output power stabilization via monitoring of the relative intensity noise (RIN) in the mid-IR signal. The details of this technique are presented in our paper published in Optics Express (K. Krzempek et al. ). We managed to build a device at verified technology readiness level (TRL) of 9. The entire system was packed in a compact housing (W×L×H: 43×35×14 cm) and contains the fiber-optic part (mode-locked oscillator, amplifiers, modulators, pump lasers, etc.) and the complete electrical part (power supplies, laser diode drivers, temperature controllers, diagnostics, modulation inputs/outputs, and the main control board with a touchscreen).The prototype is a fully functional, stand-alone device, operated with one button. We have programmed safe starting and shut-down procedures. The box contains the entire fiber-optic and electrical part. The nonlinear crystal which generates the mid-IR beam is placed outside the main box as a plug&play module. The output fiber from the main box delivers two ultrashort, synchronized laser pulses (fixed ~1.55 μm and 1.8 – 2.0 μm wavelength-tunable pulse). The mid-IR module is placed on a 10x15 cm-sized breadboard, easily detachable from the main box and transportable. Figure 3 shows the photograph of the system during tests in the laboratories of Umea Universtity in Sweden.
Figure 3. Photographs showing the mid-IR frequency comb: (a) before shipment, (b) installed at Umea University.
We applied the mid-infrared optical frequency comb for broadband precision spectroscopy of nitrous oxide (14N216O) . We combined our laser with a high-resolution Fourier transform spectrometer at Umeå University and retrieved line center frequencies of the ν1 fundamental band and the ν1 + ν2 – ν2 hot band with up to one order of magnitude improved precision compared to previous studies. The DFG comb source is offset-frequency-free, which implies that an RF lock of the repetition rate is sufficient to achieve absolute stability of the comb mode frequencies. This, in combination with the sub-nominal resolution sampling-interleaving scheme in the FTS, provides spectra with a calibrated frequency axis. This achievement opens up precision measurements of entire bands of various molecules of interest in atmospheric science and astrophysics, such as methane, ammonia, sulfur dioxide, or methanol in this fingerprint spectral region. Figure 4 shows an example of acquired N2O spectrum with 9 MHz optical sampling point spacing.
Figure 4. (a) Broadband absorption spectrum of 3% N2O in N2 (0.02 mbar pressure); (b) zoom into the P(17) line with Voigt fit.
4. Femtosecond fiber lasers for two-photon fluorescence microscopy
We develop compact fiber lasers generating ultrashort pulses at 780 nm wavelength for application to two-photon excited fluorescence (TPEF) microscopy . This task was carried out in the frame of the First TEAM Project extension in collaboration with Prof. Maciej Wojtkowski (TEAM-TECH Project leader). We have developed a laboratory version of a laser which generates sub-60 fs pulses with >1.3 nJ of pulse energy at 780 nm wavelength. The laser was already tested in TPEF imaging of biological samples. Preliminary experiments show excellent fluorescence signal properties obtained with our laser.
Figure 5 shows the optical spectrum generated from the laser and the corresponding pulse. The duration of the output pulse is 56 fs. The source is capable of generating 1 nJ pulses with sub-60 fs duration at frep ranging from 1 to 10 MHz.
Figure 5. (a) Generated optical spectrum and (b) recorded autocorrelation trace of the second harmonic pulse, revealing a pulse duration of 56 fs.
We developed the laser as a compact, plug&play, stand-alone portable device (Fig. 6). The prototype contains the entire fiber-optic part of the setup and driving electronics, power supply, touchscreen, etc. The housing is 19" rack compatible and has a size of 415 x 120 x 280 mm (W×L×H). The second harmonic generation module, responsible for frequency doubling from 1560 nm to 780 nm, is connected as a detachable part and can be easily replaced by any other fiber-coupled module with another crystal. The laser generates ultrashort pulses at 780 nm wavelength with sub-60 fs duration and >1.3 nJ of energy, with a widely tunable pulse repetition rate (1 – 12 MHz) thanks to the integrated, self-made pulse picker . The laser was installed at IChF and integrated with the scanning TPEF microscope (Fig. 6).
Figure 6. Photograph of the 780 nm fiber laser integrated with the TPEF microscope at IChF PAN
We have performed several experiments with imaging of various biological samples. We have shown that reducing the pulse repetition rate allows increasing fluorescence intensity from rat skin samples while maintaining the average power at the same level. The results were published in Biomedical Optics Express . The lower repetition rate of the excitation source reduces the thermal damage probability, especially important in imaging of dermal structures or ophthalmology. A tunable repetition rate provides experimental flexibility, making it suitable for a variety of applications.
Figure 7. Obtained two-photon excited fluorescence images of various biological samples : (a) Ex vivo frog liver cross-section imaged with 380 µW excitation power, (b) Cross-section of Epipremnum scindapsus stalk stained with rhodamine B imaged with 750 µW excitation power, (c) Chamaedorea elegans leaf imaged with 115 µW excitation power, (d) Epipremnum scindapsus leaf imaged with 154 µW excitation power.
 A. Głuszek, F. Senna Vieira, A. Hudzikowski, A. Wąż, J. Sotor, A. Foltynowicz, and G. Soboń, "Compact mode-locked Er-doped fiber laser for broadband cavity-enhanced spectroscopy", Appl. Phys. B 126, 137 (2020)
 Z. Łaszczych and G. Soboń, "Dispersion management of a nonlinear amplifying loop mirror-based erbium-doped fiber laser," Opt. Express 29, 2690-2702 (2021).
 K. Krzempek, D. Tomaszewska, A. Głuszek, T. Martynkien, P. Mergo, J. Sotor, A. Foltynowicz, and G. Soboń, "Stabilized all-fiber source for generation of tunable broadband fCEO-free mid-IR frequency comb in the 7 – 9 µm range," Opt. Express 27, 37435-37445 (2019)
 A. Hjältén, M. Germann, K. Krzempek, A. Hudzikowski, A. Głuszek, D. Tomaszewska, G. Soboń, and A. Foltynowicz, "Optical Frequency Comb Fourier Transform Spectroscopy of 14N216O at 7.8 μm", arXiv:2103.03682 [physics.chem-ph] (2021).
 D. Stachowiak, J. Bogusławski, A. Głuszek, Z. Łaszczych, M. Wojtkowski, G. Soboń, "Frequency-doubled femtosecond Er-doped fiber laser for two-photon excited fluorescence imaging", Biomed. Opt. Express 11, 4431-4442 (2020)
 A.Głuszek, G. Soboń, J. Sotor, "Fast, universal, and fully automatic pulse-picker unit for femtosecond laser systems", Proc. SPIE 10974, Laser Technology 2018: Progress and Applications of Lasers, 1097407 (2018)