1. Ultra-compact, stabilized fiber lasers and their application to laser spectroscopy
We are developing compact and repetition rate-tunable 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).
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. 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 2 shows the photograph of the system during tests in the laboratories of Umea Universtity in Sweden.
Figure 2. Photographs showing the mid-IR frequency comb: (a) before shipment, (b) installed at Umea University.
3. 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 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 3 shows an exemplary 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 3. (a) Generated optical spectrum and (b) recorded autocorrelation trace of the second harmonic pulse, revealing a pulse duration of 56 fs.