Ultrafast fiber lasers, utilizing fibers as the gain medium, offer significant advantages in heat dissipation, integration, and cost-effectiveness. These features make them ideal for applications such as nonlinear optical microscopy, laser micromachining, and biomedical photonics. Traditional ultrafast lasers, based on mode-locked technology, are renowned for their high coherence and low noise. However, their fixed output pulse parameters limit their adaptability for applications requiring variable pulse characteristics. Non-mode-locked techniques, including modulation instability for generating solitons and gain-switching diodes, offer alternative ways to produce ultrafast laser outputs with adjustable pulse parameters. Yet, modulation instability is constrained by the physical properties of the fiber material and system stability, limiting pulse repeatability and energy control. Although gain-switching diodes provide an alternative method, they often exhibit poor pulse coherence and timing accuracy, leading to random fluctuations and incomplete coherence. Time lens technology, leveraging high-bandwidth modulation, offers a superior approach by effectively suppressing continuous optical background. In this paper, we develop an ultrafast laser using time lens technology and demonstrate its effectiveness in two-photon fluorescence microscopy of rhodamine B solid samples.

The setup of the non-mode-locked ultrafast laser system (Fig. 1) utilizes a distributed feedback laser diode as the seed source, emitting continuous laser light with a center wavelength of 1030 nm. This laser beam passes through an isolator to a cascaded phase modulator, which is driven by a sinusoidal signal from an oscillator to achieve spectral broadening. A continuous wave amplifier (CW AMP) compensates for power loss caused by the phase modulator. An intensity modulator (IM) chops the continuous light into a series of optical pulses, propelled by Gaussian-like electrical pulses generated by an arbitrary waveform generator (AWG). A pulse amplifier compensates for the power loss introduced by the IM. The amplified laser beam is collimated and then compressed by a dispersion compensator to generate ultrafast laser pulses. In the two-photon fluorescence (TPF) microscope setup (Fig. 5), the ultrafast laser beam is split into two beams. The weaker reflected beam excites the TPF signal, while the stronger beam is deflected by a galvanometer after being reduced by the 4f optical system, enabling 2D scanning of samples. The 4f optical system, consisting of a scanning lens and a tube lens, expands and directs the beam to the objective lens’s pupil. An objective lens with a numerical aperture (NA) of 0.95 and a magnification of 40 focuses the beam onto the sample surface to excite the TPF signal. The sample is mounted on a three-dimensional stage, which is electronically controlled to adjust the imaging position. The emitted fluorescence passes through a reflector to a band-pass filter that isolates the fluorescent signal from other components. The fluorescence signal is detected by a photomultiplier tube (PMT), converting the optical signal into an electrical signal for amplification and further analysis.

The non-mode-locked laser, constructed using the time lens technique, outputs laser pulses with a full width at half maximum (FWMH) of approximately 3 ps [Fig. 4(a)], a spectral width of 1.59 nm [Fig. 2(b)] , and a repetition rate of 80 MHz. The pulse shape exhibits a good Gaussian-like profile, with a root mean square (RMS) power value of 0.98% [Fig. 4(b)] . The TPF microscopy results [Fig. 6(b)] show stronger signals in regions where the sample aggregates into clusters, which indicates higher concentration and consequently greater fluorescence brightness in these areas compared to the surrounding regions. TPF microscopy of the same region is performed at different power levels while maintaining all other experimental conditions constant (Fig. 7). By fitting the changes in average fluorescence intensity of three target regions using a quadratic function, it becomes evident that the trends are consistent across the regions. As the average power of the excitation laser increases, there is a marked increase in the TPF signals across all three regions. Higher average power leads to a more pronounced increase in TPF signals. Under the same average power of excitation, regions with higher concentration exhibit a strengthened fluorescence intensity and a steeper gradient in the fluorescence intensity curve relative to the excitation power. The data points closely align with the quadratic fitting curve, which illustrates the nonlinear relationship between TPF intensity and average excitation power, thereby confirming the second-order nonlinear optical property of the TPF signal.

We propose a non-mode-locked, high-repetition picosecond fiber laser based on a time lens structure. This setup incorporates a phase modulator and a dispersion compensator to construct a time lens for pulse compression. A 10 GHz sinusoidal signal drives the phase modulator to broaden the spectrum to 1.59 nm. The pulse is compressed to 3 ps by adjusting the vertical distance between grating pairs in the dispersion compensator to 11.6 cm. The pulse width of the ultrafast laser can be fine-tuned by adjusting the amount of broadening and dispersion compensation. To generate Gaussian-like optical pulses with a repetition rate of 80 MHz, we utilize a programmable arbitrary waveform generator to produce Gaussian-like electrical pulses. The optical pulse waveform and repetition rate can be controlled by adjusting the parameters of the electrical pulse. The laser demonstrates good power stability, with an RMS value of 0.98% over one hour of operation. The laser is used to perform TPF microscopy of solid samples with rhodamine B dye, which verifies that the output pulse parameters meet the demands of nonlinear optical microscopy and contribute to the practical advancement of this field.