Abstract
Over the past decade, there has been a growing interest in tunable ultrashort optical pulses that are useful for a number of commercial and scientific applications, including ultrafast spectroscopy, material processing, nonlinear optics, and defense security. In most of the cases so far, the pulses have been realized using solid state lasers or dye lasers with many free space optical elements. However, in the systems using those lasers, it is necessary to control the optical elements with very high precision through complicated adjustments in order to get the stable operation and tune the wavelength of the laser pulse, and the whole system is rather large and not easy to control, thus the overall system cost is rather high. Meanwhile, the tuning range remains limited by the bandwidth of the gain medium.
Recently, ultrafast fiber laser technology has progressed remarkably and the performance is comparable to standard solid-state lasers[
At present, the wavelength tunable ultrashort pulse sources are usually used to seed high-power amplifiers or optical parametric amplifiers, etc.[
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Here, we report a simple and compact all-fiber laser system that generates tunable femtosecond optical pulses from 1.6 to 2.32 μm with an Er-doped mode-locked fiber laser based on telecom components and a silica-based highly nonlinear fiber. In the meantime, the use of a single-clad Tm-doped fiber amplifier allows the production of high-power femtosecond pulses around 1.93 μm without an external pulse compressor. We obtain pulse trains at a repetition frequency of 32.9 MHz with an average power of 360 mW, pulse energy of 10.9 nJ, duration of 94 fs, and corresponding peak power of 105 kW.
One should note that the magnitude of the wavelength shift is up to 720 nm. Furthermore, we can obtain femtosecond pulses with a peak power of 105 kW based on single-clad Tm-doped fiber amplifier in an all-fiber system without any free space optical components.
The experimental setup of the all-fiber laser system is shown in Fig.
Figure 1.Optical layout of the experimental setup.
In the HNLF front end, an in-house built all-fiber master oscillator passively mode-locked by using nonlinear polarization rotation (NPR) delivers 490 fs optical pulses with a 6.9 nm spectral bandwidth centered at 1.56 μm at a fundamental frequency of 32.9 MHz. The optical pulses pass through an optical isolator for preventing back reflections and are then amplified in a 3-m-long customized Er-doped fiber with a mode field diameter of 5.7 μm at 1550 nm, an absorption of 16.5 dB/m at 980 nm, and a normal dispersion of
Figure 2.Average output power and pulse width of the amplified pulses as a function of pump power.
Next, the compressed pulses are directly coupled into HNLF with a core diameter of 2 μm for wavelength conversion. The HNLF has the zero dispersion wavelength of 1453 nm and its nonlinear coefficient is much higher than SMF-28 because of a smaller effective mode area. As the coupling power and pulse width vary, the wavelength of the generated optical pulses is shifted arbitrarily and continuously. Figure
Figure 3.Output spectrum measured at the fiber output with different pump power and HNLF length (a) 26 mW, 5 m and (b) 45 mW, 7 m.
Figure
Figure 4.Characteristics of wavelength shift of soliton pulses in function of fiber input power (dots with experimental data) and arbitrarily fit (red curve).
To verify the tunability and stability of the optical pulses, two stages of single-clad Tm-doped fiber amplifiers are used to boost the power level further in the 2 μm region.
First, the coupling power is optimized to pump the 7-m-long HNLF for generating 1.95 μm optical pulses in order to match the Tm-doped fiber gain bandwidth. The seed pulses are then coupled into a 2 m length of single-clad Tm-doped fiber to filter out the long wavelength element and amplified up to 10 mW average power at 1.95 μm. The Tm-doped fiber used in the amplifier has a core/cladding diameter of 9/125 μm, and a numerical aperture (NA) of 0.16 with an absorption coefficient of 13 dB/m at 1550 nm. The group velocity dispersion (GVD) of the fiber is dominated by the negative contribution from the host silica material and has been estimated to be
Finally, the 3-m-long main amplifier is made of single-clad Tm-doped fiber, similar to the previous. An Er-doped fiber laser with a coupled power of 2.0 W at 1560 nm is used to pump the amplifier, which resulted in a maximum output power of 360 mW at around 1931 nm, as shown in Fig.
Figure 5.Average output power as a function of launched 1560 nm pump power (dots with experimental data) and linear fit (red curve).
Before the UHNA fiber, the average power is 10 mW, the full width at half-maximum (FWHM) pulse width at the output of the amplifier is measured at 9 ps with an autocorrelator and the autocorrelation trace with corresponding spectrum are shown in Fig.
Figure 6.Autocorrelation trace (left) and spectrum (right) of uncompressed pulses.
Figure 7.Autocorrelation trace (left) and spectrum (right) of compressed pulses.
The compressed output pulse train observed in a high-speed oscilloscope is shown in Fig.
Figure 8.Recorded output pulse train.
All the fibers are fusion spliced to make the system mechanically stable. Running a few hours at maximum average power, the all-fiber laser system exhibits exceptional stability with overall power fluctuations as low as 0.5%. In order to further verify the stability of the laser system, we record the RF spectrum with 510 Hz resolution bandwidth (RBW) and 1 MHz span. The signal-to-noise ratio (SNR) in the RF spectrum is higher than 70 dB, which indicates that the all-fiber laser system is in a very steady state, as shown in Fig.
Figure 9.RF spectrum of the compressor pulses recorded with 510 Hz RBW.
In conclusion, we demonstrate tunable femtosecond optical pulses from 1.6 to 2.32 μm based on SSFS in a silica-based highly nonlinear fiber with an all-fiber Er-doped laser system. The generated optical pulses can be used for Tm-doped fiber amplifier seeding to replace mode-locked Tm-doped fiber laser oscillators, usually more difficult to realize because of the relatively high anomalous dispersion of silica fibers in this wavelength range. In conjunction with the two stages of single-clad Tm-doped fiber amplifier without an external pulse compressor, we obtain a femtosecond optical pulse at 1.93 μm with a 32.9 MHz repetition frequency and 360 mW of output power correspond- ding to 105 kW peak power, providing an all-fiber laser system with a compact and versatile structure. Simultaneously, we believe that these tunable optical pulses may also be a good source of optically synchronized pump and seed pulses for amplifying systems based on Cr:ZnSe or Cr:ZnS.
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