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Abstract
We demonstrate cascaded nonlinear pulse compression of a Yb-doped fiber laser. The system is based on two pulse compression stages with bare single-mode fiber (SMF) and ultra-high NA (UHNA) fibers combined with two pairs of chirped mirrors. The 10 nJ, 110 fs input pulses are compressed down to 9.1 fs at 90 MHz, revealing a broadband spectrum from 800 nm to 1350 nm. This technique provides a simple approach to sub-10-fs compact Yb-doped fiber lasers for a variety of applications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Low-pulse energy femtosecond light sources are in great demand in various fields, including biomedical imaging [1], ultrafast spectroscopy, and surface functionalization [2]. For example, in biomedical applications, coherent anti-Stokes Raman spectroscopy enables label-free identification of biomolecules with a high signal-to-noise ratio [3,4]. Traditionally, Ti:sapphire lasers have been used as light sources for these applications due to their ability to generate laser pulses shorter than 10 femtoseconds in duration [5,6]. However, the high cost and bulkiness of Ti:sapphire lasers have limited their widespread adoption since Kerr-lens mode locking of Ti:sapphire lasers requires a high-power green pumping source.
Yb fiber laser technology has emerged as a promising alternative. Yb fiber lasers offer several advantages, including lower cost due to direct pumping by laser diodes, inherent robustness and compactness, and easier beam handling. In addition, Yb lasers can be easily amplified to high power thanks to a double-clad fiber (DCF) amplifier configuration with a high-power pump laser diode. However, Yb lasers have the inherent drawback of a narrow bandwidth compared to Ti:sapphire lasers, which makes the direct generation of sub-10 fs laser pulses impossible. To overcome this limitation, several nonlinear pulse compression techniques have been developed. These nonlinear pulse compression techniques rely on the third-order optical nonlinearity of the materials to induce self-phase modulation (SPM), resulting in spectral broadening. Subsequent chirp compensation allows the pulse duration to be compressed by more than 10 times its original duration.
The choice of techniques and materials for spectral broadening depends on the pulse energy, as it necessitates a delicate balance between avoiding laser-induced material degradation, achieving significant spectral broadening, and ensuring a clean phase spectrum [7]. Figure 1 illustrates the relationship between pulse energy and pulse duration for Yb-based amplifiers, both before and after the application of nonlinear compression techniques as used in various studies. The plot also includes data points from Yb-based oscillators [8–10]. When the initial pulse energy exceeds 100 nJ, the most common approach involves employing bulk media such as multi-plate methods [11], multi-pass cells (MPCs) [12–21], and cascaded thick bulk materials [22]. Furthermore, waveguiding structures such as gas-filled hollow-core fibers, photonic crystal fibers (PCF), and solid-core fibers (SCF) have been used to increase the nonlinear interaction length [23–28].
Fig. 1. Pulse energy versus pulse duration for nonlinear compression methods. The hollow circle represents results using a Yb fiber-based amplifier. Pulse energy under 10 uJ is plotted.
At pulse energies below 100 nJ, it is challenging to achieve nonlinear compression down to 10 fs with substantial spectral broadening of low-peak-power pulses while maintaining a flat phase spectrum. Fibers with small core diameters, including SMF, are effective in achieving sufficient spectral broadening [29–31]. N. V. Didenko et al. reported that a pulse duration of 12.1 fs was obtained by nonlinear pulse compression using an SMF [29]. However, long fibers must be used to obtain sufficient spectrum at low pulse energies, which makes it difficult to generate shorter pulse durations due to higher-order dispersions. For example, optical pulse breaking occurs, as a consequence of the interplay between self-phase modulation (SPM) and group velocity dispersion [32,33]. Consequently, the spectral phase is distorted when aiming for the spectral broadening required to achieve 10 fs pulses. As a result, pulse durations shorter than 10 fs have not yet been achieved.
In this paper, two stages of nonlinear pulse compression with short fibers were used to solve the above problems. At each stage, pulse-compressible, moderate spectral broadening occurs. As a result, we demonstrated the generation of sub-10 fs pulses with an initial pulse energy of 10 nJ. This pulse energy range is advantageous for various non-destructive spectroscopic applications, including nonlinear microscopy. In particular, such pulse energy can be easily realized using single-stage fiber amplification, which simplifies the requirements for the initial laser source and leads to a more compact and robust laser system. We achieved pulse compression down to 9.1 fs by using a 50 mm SMF and a 10 mm UHNA fiber with a 110-fs input pulse. This method allows sufficient spectral broadening even at 10 nJ incident pulse energy while mitigating undesirable nonlinear effects for pulse compression.
2. Experimental setup
The experimental setup is illustrated in Fig. 2. The laser was pumped by a 300 mW laser diode (LD) at 976 nm. The pump light was coupled into a fiber-based cavity using a wavelength division multiplexer (WDM). A nonlinear polarization rotation scheme was used for mode locking, which was adjusted by three waveplates. The repetition rate of the oscillator was 90 MHz and the average output power was over 20 mW.
Fig. 2. Experimental setup for sub-10 fs pulses with Yb-based laser system. LD; laser diode, WDM; wave division multiplexer, QWP; quarter-wave plate, HWP; half-wave plate, PBS; polarizing beam splitter, CM; chirped mirror, DCF; double-clad fiber, BPF; band pass filter, ISO; isolator, SHG-FROG; second harmonic generation frequency-resolved optical gating.
The output pulse from the oscillator was amplified by a Yb-doped fiber-based amplifier. A 2 m long Yb-doped DCF (Thorlabs, YB1200-10/125DC) was used for amplification. The clad of the DCF was pumped by a laser diode at 976 nm through a fiber pump combiner. The output pulse was collimated by a plano-convex lens with a focal length of 12 mm. A band pass filter (BPF) was placed behind the collimation lens to filter out the pump light. As the seed pulse passed through the DCF, an output power of 1.7 W at 1030 nm was measured after the BPF. The output pulse was then compressed by a pair of transmission gratings with a groove density of 1000 lines/mm. The absolute efficiency of this compressor was 81%. The group delay dispersion (GDD) was tuned by adjusting the grating spacing. After compression and passing the ISO, the system delivered an average output power of up to 1.2 W with 110 fs.
The first nonlinear compression was performed using a 50 mm SMF (Corning, HI1060). The length of the fiber was optimized to minimize undesirable nonlinear effects. The amplified pulse was coupled to the SMF using an objective lens (20x magnification, NA = 0.40). The coupling efficiency was 66%. An aspherical lens with a focal length of 11 mm was used to collimate the output pulse from the SMF. Half and quarter waveplates were then used to adjust the polarization to p-polarization for the ideal angle of the chirped mirror pair (CM).
A pair of chirped mirrors (Edumund Optics, 12-328) was used to compensate for the additional dispersion caused by the first stage of nonlinear spectral broadening. The number of reflections from the pair of chirped mirrors was adjusted based on the pulse duration obtained by the second harmonic generation frequency-resolved optical gating (SHG-FROG) method. The CM compensated the second-order dispersion by -2000 fs2 with 10 bounces. In addition to the 10 reflections, two additional bounces (-400 fs2) were added to compensate for the dispersion of the objective lens in the second stage. The pulse duration after the first stage is 28 fs. The net energy efficiency of the first stage was 60%.
In this main compression stage, the pre-compressed laser pulse was coupled into the 10-mm long UHNA SMF (Thorlabs, UHNA4) using an objective lens (40x magnification, NA = 0.65). The input power was 0.72 W and the coupling efficiency into the fiber was 55%. A reflective-type objective lens was employed to eliminate the chromatic aberrations for the output beam collimation. Part of the laser beam was clipped by an internal structure of the reflective lens, resulting in a 40% reduction of the average power. Better throughput will be realized by replacing the objective lens with an off-axis parabolic mirror. As a result, the average power of the output was 0.25 W. An ultra-broadband CM (Edmund Optics, 14-674) was used to compensate for the dispersion of the second-stage nonlinear spectral broadening. 16 bounces of this CM gave a GDD of -960 fs2.We used a homemade SHG-FROG system to characterize the pulse shape at each stage of compression. SHG-FROG can reconstruct pulse properties in both the spectral and temporal domains [34]. The colinear beam was focused through a parabolic mirror onto a 5 µm thick barium beta borate (BBO) crystal to minimize chromatic aberrations and phase mismatch.
3. Results
Figure 3 shows the spectrum of the laser pulses at each stage of the nonlinear compression. A spectrum analyzer (Yokogawa, AQ6374) was used to measure the spectrum. The original spectral width of the amplified output is 10 nm at an intensity level of 3 dB (dashed line).
Fig. 3. The spectrum of the input and output pulses. Three spectra are vertically offset. (a) spectrum with a logarithm scale (b) spectrum with a linear scale
After the first spectral broadening, the spectrum ranges from 950 nm to 1100 nm (blue line). The Fourier transform limited (TL) pulse duration corresponding to this spectrum is 20 fs. The second stage of nonlinear compression broadens the spectrum from 800 nm to 1350 nm (orange line), showing a much broader spectrum compared to the first stage result using SMF. The TL pulse duration corresponding to this spectrum is 6.6 fs.
Figure 4 shows both measured and retrieved temporal and spectral results from SHG-FROG. The retrieved traces show good agreement with the measured traces for the first and second nonlinear compressions, as shown in Fig. 4(a),(c). The reconstructed pulse shape at the first stage has a full width at half maximum (FWHM) of 27.8 fs (TL pulse duration is 20 fs), as shown in Fig. 4(b), revealing a good contrast temporal waveform. The pulse compression factor for the first stage was 3.7. The second stage of nonlinear compression resulted in an FWHM of 9.1 fs, as shown in Fig. 4(d), and the corresponding pulse compression factor was 3.3. The total pulse compression factor was 12.1. During the process of optimizing the fiber lengths and chirp compensations, it was observed that the temporal shape control at the input for each fiber is critical in achieving a pedestal-free time profile.
Fig. 4. Measurement results of SHG-FROG. Measured and retrieved FROG traces at (a) the output of the first fiber and (c) the output of the second fiber. (b)(d) Retrieved temporal profile of compressed pulse.
To achieve the TL pulse of 6.6 fs, additional dispersion compensation will be required. According to the FROG measurement, a pulse duration of 7.8 fs can be achieved by compensating for both second-order and third-order dispersion. This can be accomplished by inserting a prism-based compressor or using appropriate dispersion-compensating materials. For further dispersion compensation, a more complex system, such as the 4-f configuration with a spatial light modulator, will be necessary.
Figure 5 shows long-term power stability after the second compression using a thermal power meter. The average power for this observation was 136 mW. The pulse energy fluctuation calculated from the standard deviation was 0.81 mW (0.5%).
4. Conclusion
In conclusion, we have demonstrated the generation of sub-10 fs pulses at 10 nJ initial pulse energy using two pulse compression stages. SMF and UHNA fibers were used for normal dispersion spectral broadening and chirped mirrors for dispersion compensation. A pulse duration of 9.1 fs at a 90 MHz repetition rate was obtained from the SHG FROG measurements. To the best of our knowledge, this result is the first achievement of sub-10 fs in a pulse compression method using SMF with a Yb-doped amplifier. Compared to other studies using SMF, this regime provides a simple approach to achieve sub-10 fs. This technique can then be applied as a potential source for biomedical applications and surface functionalization that require moderate pulse energy.
Funding
Ministry of Education, Culture, Sports, Science and Technology (JPMXS0118067246).
Disclosures
The authors declare 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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