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Abstract
We demonstrate a femtosecond all-polarization-maintaining Nd fiber laser working at 920 nm mode locked by a biased non-linear loop mirror. The broadest spectral width of the pulse is 25.2 nm and the output power is 8 mW with 320 mW pump power. The measured pulse width is 109 fs with extra-cavity compression. The laser configuration of all-polarization-maintaining fiber can directly enhance the environmental stability of generated pulses. The seed pulses of the oscillator were amplified over 400 mW, which served as the light source for a two-photon microscope. To the best of our knowledge, this is the first demonstration of a 920 nm femtosecond Nd polarization-maintaining fiber laser based on a non-linear loop mirror.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For the application of fluorescence imaging technology in neuroscience, one of the most ideal tools is two-photon microscopy (TPM). TPM with the laser excitation at 920 nm has played an important role in observing brain activity via calcium imaging. Until now, solid-state femtosecond lasers, such as Ti-sapphire lasers, have dominated the applications of TPM [1]. However, due to the high cost, large size and high environmental requirements, the Ti-sapphire laser cannot meet the practical requirements for extended imaging applications. Femtosecond fiber lasers become more and more attractive, since they have good features of compact, simple structure, high stability and low cost. Instead of traditional Ti-sapphire femtosecond laser, an alternative promising method to obtain ultrafast pulses at 920 nm band is based on Nd femtosecond fiber lasers.
Three major techniques have been used to build the 920 nm femtosecond fiber laser. Jay. R. get it from 1030nm Yb femtosecond laser by SPM effect in a photonic crystal fiber [2]. Bo Li used frequency shift followed by frequency doubling to get 920 nm femtosecond laser from an Er fiber laser [3]. The two approaches require a high-power femtosecond Yb or Er fiber laser [4]. Since the three-level transition (4F3/2 − 4I9/2) of Nd ions falls in the wavelength bands of ∼920 nm, generating 920 nm femtosecond laser from Nd fiber is a more direct way. Compared with the nonlinear frequency conversion technique, the direct generation of 920 nm laser by Nd fiber is preferred due to its robustness and simplicity.
However, the realization of this three-level transition is not easy for Nd fiber oscillators and Nd fiber amplifiers. This is due to the ground-state absorption and the undesired competition with the four-level transition 4F3/2-4I11/2 (1060-1100 nm) when pumped at 808 nm [5]. Suppression of the strong four-level emission peak (4F3/2–4I11/2) at 1060 nm remains to be discussed [6].
The popular mode-locking mechanisms includes the Saturable Absorber (SA) [7,8], Nonlinear Polarization Rotation (NPR) [9–11], and Nonlinear Amplifying Loop Mirror (NALM) [12,13]. However, both SA and NPR mode-locking suffer from drawbacks that limit their suitability for industrial applications [12]. A core-pumped all-normal dispersion mode-locked Nd-doped fiber laser based on nonlinear polarization rotator have been developed at 910 and 935 nm [14]. Even though NPR can achieve high power output, the system requires adjustable control over polarization which increases the system’s environmental sensitivity. Kilian Le Corre presented a compact passively mode-locked fiber laser performing at near 910 nm, which includes a semiconductor saturable absorber mirror (SESAM) [15]. However, SA mode-locking faces the problems of short work-life and cannot afford high power operation. The nonlinear amplifying loop mirror (NALM) based femtosecond fiber lasers [16] have attracted a lot of interest in recent years.
In our previous work, a 920 nm mode-locked Nd fiber laser with a phase biased nonlinear loop mirror was demonstrated [17], where the Nd fiber was non-polarization-maintaining, leading to unpredictable degradation of laser performance [18].
In this paper, we report a 920 nm NALM mode-locked Nd fiber laser with all polarization-maintaining (PM) fiber. In our experiment, the mode-locking state of the laser is more environmentally stable because of the all-polarization-maintaining fiber cavity and NALM mode-locking. It is also more robust and simpler because of the direct generation of 920 nm laser by such a PM Nd fiber. The output power is 8 mW under the pump power of 320 mW at the repetition rate of 43.6 MHz. The pulse energy is 0.18 nJ. After the extra-cavity dispersion compensation, the pulse can be compressed from 2.6 ps to 109 fs. To the best of our knowledge, this is the first demonstration of an all-polarization-maintaining femtosecond Nd fiber laser at 920 nm mode locked by biased NALM.
2. Experimental setup
The schematic of the 920 nm NALM Nd fiber laser is shown in Fig. 1.
Fig. 1. Schematic of Nd fiber laser. Gain fiber, Nd fiber; WDM, wave length division multiplexer; 808 nm LD, laser diode at 808 nm; Gratings, transmission grating pair; l/8, eighth-wave plate; FR, Faraday rotator; PBS, polarization beam splitter; Mirror, total reflection mirror.
The system is constructed of a total reflection mirror, a gain fiber, two-wavelength division multiplexers (WDMs), a 10:90 coupler as output port, two collimators, a 45° Faraday rotator and a λ∕8 wave plate placed between two polarizing beam splitters (PBS), which serves as a nonreciprocal phase shifter to introduce a phase bias, a pair of transmission gratings with the groove density of 1500 lines/mm. The gain fiber in the cavity is customized fiber, whose core absorption and core diameter are 40dB/m @ 805nm and 5μm, respectively. The Nd fiber is pumped from two sides by two 808 nm laser diodes through two WDMs. The cavity contains 135 cm PM Nd fiber and 300 cm PM fiber. The intra-cavity net dispersion is calculated to be +0.0446 ps2 with a grating separation distance of 9.7 mm. P-polarized or s-polarized beams from one collimator obtain π/2 phase delay and enter the other collimator after passing through the phase shifter composed by the λ∕8 wave plate and the 45° Faraday rotator twice. The strong four-level emission peak (4F3/2–4I11/2) at 1060 nm was blocked the by grating pair where the 1060 nm components are not reflected back to the cavity. The grating pair worked also as a dispersion compensation component. Besides, all the optical devices in this cavity work in the 920 nm band that can also suppress the 1060 nm component.
After introducing the phase shifter, the reflectance of the nonlinear loop mirror for weak laser light is enhanced by 50%, and it works in a high-slope range, and it helps the laser to achieve mode-locking, with a lower mode locking threshold [19,20].
The mode-locked pulse profile depends on the intra-cavity dispersion, which can be controlled by the distance of two gratings in the cavity. When the distance of gratings is about 9.7 mm, we get the broadest full-width at half-maximum (FWHM) of the spectrum.
The seed is amplified to be a light source for two photon microscopes. In order to avoid the nonlinear effect, the conventional technique of Chirped Pulse Amplification is applied [21]. The distributed suppression of the transition at 1060 nm is required to obtain the effective gain in the amplified Nd fiber. The bending W-type Nd fiber, whose clad absorption and core diameter are 0.5dB/m @808nm and 4.8μm, respectively, is used to provide the higher loss for the long wavelength at 1060 nm band, without obvious bending loss at the 920 nm band, to suppress the unwanted amplification for the four-level emission [22]. In the experiment, the pulse output from the oscillator is stretched to about 32ps by a 30m long PM980-XP (Nufern) fiber before being amplified. The length of the W-type fiber in amplification is 4.4 m, and the radius of curvature is adjusted to 30 cm. The pulse can be compressed by grating pairs with the groove density of 1500 lines/mm. Due to the use of PM fiber, the mode-locking state of the laser remains stable even the fiber is manually shaken.
3. Experiment results and discussions
The optical spectrum, radio frequency (RF) spectrum and the autocorrelation trace of the oscillator are shown in Fig. 2(a), (b) and (c) respectively. The pulse spectrum is ranged from 905 nm to 937 nm and is centered at 921 nm with an FWHM of 25.2 nm. As shown in Fig. 2(b), the repetition rate is 43.6 MHz, and the RF spectrum is measured under the resolution bandwidth of 300 Hz. Figure 2(c) shows the measured autocorrelation trace (black curve) and calculated autocorrelation trace of transform-limited pulses (red curve). The FWHM of measured autocorrelation trace is 109 fs with an extra-cavity compression (groove density of 1500 lines/mm with the grating separation of 9.0 mm), and the transform-limited pulse width is about 85 fs.
Fig. 2. Experimental results of the oscillator. (a) Measured optical spectra of output pulse from oscillator. (b) RF spectrum of output pulse from oscillator train. (c) Measured autocorrelation trace and calculated transform-limited autocorrelation trace of oscillator pulses.
A π/2 non-reciprocal reflective phase shifter is introduced in the loop to reduce the mode-locking starting threshold. The mode-locking starting threshold is related to the intra-cavity dispersion, which can be controlled by the grating separation in the cavity. Mode-locking is self-starting under 400 mW pump power. After that, we tuned down the pump power to obtain a stable single-pulse output. The maximum pump power for a single pulse is 320 mW, and the average output power is stabilized to 8 mW. When the pump power is adjusted above 320 mW, pulse splitting occurs and is considered as the soliton splitting. The fundamental repetition frequency is 43.6 MHz, indicating the pulse energy of 0.18 nJ.
The results of the amplified pulse are shown in Fig. 3. The output pulse from the oscillator is injected into the amplifier, then boosted to a maximum of 400 mW at a pump power of 14W and the slope efficiency is about 3%, shown in Fig. 3(a). The FWHM of the spectrum of the amplified pulse is shrunk to 9.8 nm, due to the gain narrowing. The pulse width was compressed by grating pairs to 211fs, shown in Fig. 3(c).
Fig. 3. Experimental results of amplifier. (a) Power of the amplified pulse versus pump power. (b) Measured optical spectra of output pulse from compressor. (c) Measured autocorrelation trace of compressed pulses.
4. Two-photon imaging with the 920 nm laser pulses
We used a miniature two-photon microscope (mTPM) probe-head to verify the imaging capability of the 920 nm laser pulses [23]. The experiment setup is shown in Fig. 4. The amplified laser is coupled into a 2 m-long hollow-core photonic bandgap fiber and sent to the mTPM probe-head. The core diameter of the bandgap fiber is about 8 μm, and the cladding is about 152 μm. The laser pulse is broadened to over 1 picosecond due to the dispersion of the bandgap fiber. By enlarging the distance of the compressor grating pairs after the amplifier, the dispersion is compensated, and the pulse is re-compressed to 220 fs. The mTPM probe-head consists of a MEMS-based scanning mirror, a scan lens, a dichroic mirror and an objective, as shown in the inset of Fig. 4. The operation frequency of the MEMS-based scanning mirror is set to 2400 Hz, resulting a frame rate of ∼9 fps with a frame size of 600×512 pixels. The laser is focused at the sample (pollen grains dispersed in water) with an average power of 90 mW. The autofluorescence signal of the pollen grains is collected by the objective, filtered by the dichroic mirror, and then delivered to a photomultiplier tube (PMT) via a supple fiber bundle (SFB). A bandpass filter (520nm±35nm) is put in front of the PMT to further suppress the excitation laser power. The two-photon image of the pollen grains is shown in Fig. 5. The imaging results show that the pollen structure is clearly visible, which proves the feasibility of the oscillator as a multiphoton imaging light source.
Fig. 4. Schematic of the two-photon microscope. L, lens; C, collimator; SL, scan lens; DM, dichroic mirror; OBJ, objective; mTPM, miniature two photon microscope; SFB, supple fiber bundle; PMT, photomultiplier tube.
5. Summary
We have demonstrated, to the best of our knowledge, the first 920 nm femtosecond Nd polarization-maintaining fiber laser based on a non-linear loop mirror. We have proved the feasibility of such a laser pulse to the two-photon imaging, delivered by a hollow core fiber. The stability of the laser can be improved by making the amplifier polarization-maintaining. The amplification slope efficiency can be optimized by using fiber with high clad absorption coefficient. This Nd fiber laser's capability in miniature two-photon microscopy shows great potentials in clinical and biomedical applications.
Funding
National Key Research and Development Program of China (2020YFB1312802); National Natural Science Foundation of China (31830036, 61975002).
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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