We demonstrate generation of widely tunable femtosecond pulses by utilizing the soliton self-frequency shift effect in a Tm-doped fiber amplifier, seeded by dispersion managed mode-locked Tm oscillator. The monochromatic soliton pulses with a duration of the order of 100 fs have been obtained and its wavelength can be adjusted continuously in the range of 1.9-2.36 μm by varying the pump power. The efficiency of Raman conversion is as high as 97% with output power up to 1.16 W. The experimental results are in good agreement with numerical simulations of pulse propagation in Tm-doped fiber amplifier.
© 2017 Optical Society of America
High power sources of femtosecond pulses at wavelengths longer than 2 μm are useful for a number of commercial and scientific applications, including eye-safe LIDAR, two-photon microscopy, optical coherence tomography, THz generation, trace gas sensing and micromachining [1–3]. In the meantime, new scientific discoveries and practical applications in this spectral region require versatile laser sources with wide tunable range and high output power. To date, such laser sources with spectral tuning range of several microns have been successfully realized based on optical parametric systems or difference frequency generators [4–6]. However, all these optical systems have large volume and complex structure, which increase the cost of practical applications.
Since the soliton self-frequency shifting (SSFS) effect was originally discovered by Mitschke and Mollenauer in 1986 , tunable femtosecond pulse sources can be produced by using SSFS in fiber with their compactness, high stability, excellent output beam quality, narrow output pulse and wide spectral tunability. Over the past decade, many researchers have conducted relevant works on tunable femtosecond sources involving various optical fibers, such as polarization maintaining fibers , photonic crystal fibers (PCF) [9–11], highly-nonlinear fibers (HNLF) [12,13], germanium-doped core silica-cladding fibers [14,15], and soft glass fibers [16,17], etc. In our previous work, we have demonstrated the generation of tunable femtosecond pulses from 1.6 to 2.3 μm in HNLF pumped by an Er-doped femtosecond fiber laser . Furthermore, femtosecond pulses with broad tunability in the range of 2-3 μm were generated in a germanium-doped core silica-glass cladding fiber with a driving pulse at 2 μm . In order to obtain longer wavelength soliton pulses, soft glasses with low phonon energies were desirable. Impressive results with soliton wavelength shift up to 4.3 μm were realized in a 2 m InF3 fiber using a 1.9 μm fiber laser as the pump source . However, in the field of nonlinear optics and material processing, where femtosecond pulses with high pulse energy and peak power are needed, tunable soliton pulse source based on passive fiber cannot meet its requirements. Moreover, these soliton shifts are generally associated with the generation of dispersive wave or secondary soliton that reduces significantly the energy transfer into soliton pulses.
For the sake of obtaining high-quality femtosecond pulses with high output power and wavelength longer than 2 μm, we focus on the SSFS in the Tm-doped fiber amplifier. For instance, the Tm-Ho power amplifier was demonstrated to deliver femtosecond soliton pulses in the wavelength range from 1.97 to 2.15 μm with an average output power up to 230 mW and the efficiency of Raman conversion ranged from 47% to 62% over the tuning range . In addition, the tunability from 2 to 2.2 μm was reported and 130 fs pulses with pulse energy of 38 nJ and average power of 3 W were obtained . And optically synchronized two-color femtosecond pulses could be generated directly from a Tm/Yb-co-doped amplifier, one pulse at ~2 μm and the second pulse with a tunable wavelength up to 2.3 μm . Nevertheless, the conversion efficiency of Raman soliton pulses was relatively low, which resulted in the reduction of soliton energy. Recently, Luo et al. reported a high-efficiency Raman soliton laser system with the tunable range from 1.98 to 2.31 μm . However, the pulse energy of Raman soliton was relatively low.
In this letter, we report a watt-level fiber-based femtosecond source that produces high quality tunable soliton pulses from 1.9 to 2.36 μm based on SSFS in a Tm-doped fiber amplifier seeded by dispersion managed mode-locked Tm oscillator. The monochromatic soliton pulses with a duration of the order of 100 fs and energy up to 34 nJ have been obtained. Moreover, it is worth noting that the efficiency of Raman conversion is as high as 97%. Numerical simulations of the tunable femtosecond pulses are in good agreement with the experimental results.
2. Experimental setup
The schematic of the tunable femtosecond pulse source was presented in Fig. 1. The all-fiber Tm-doped master oscillator passively mode-locked by SESAM was built with linear cavity configuration. The gain fiber of the oscillator was 0.15-m-long single-clad Tm-doped fiber with 5.5 μm core diameter pumped by a homemade 1560 nm continuous wave Er-doped fiber laser. The Tm-doped fiber has an absorption coefficient of 340 dB/m at 1560 nm with a dispersion value of about −70 ps2/km at 2000 nm. The ultrahigh numerical aperture (UHNA) fiber was use as dispersion compensation fiber for adjusting the amount of dispersion in the oscillator. The UHNA fiber has a core diameter of 2.2 μm, a cladding diameter of 125 μm, and a numerical aperture of 0.36 with a dispersion value of about 90 ps2/km at 2000 nm. The cavity length was around 2.93 m and the net dispersion was slightly positive. The Faraday rotation mirror was used to reflect light into the cavity and a 30% fiber coupler was used to output the signal. The mode-locked was achieved by a SESAM with modulation depth of 25%, relaxation time of 10 ps.
The Raman shifter fiber was based on a 4 m piece of silica double-clad Tm-doped fiber (TDF). The TDF has a core/cladding of 10/130 μm and a numerical aperture of 0.15. At 1950 nm, the second-order dispersion and nonlinear coefficient of the TDF is −78 ps2/km and 1.14 W−1km−1. Two fiber-pigtailed diodes at 793 nm were employed to pump the amplifier with the total output power of 24 W. All the fiber ends were angle cleaved to suppress Fresnel reflections. The fiber polarization controllers (PC) was placed before the amplifier to optimize the performance since the Raman frequency shift exhibited a strong dependence on the input polarization.
3. Experimental results and discussion
When the pump power increased to 900 mW with proper adjustment the reflective coupling between SESAM and the fiber end, the Tm-doped fiber master oscillator produced stable femtosecond optical pulses with a repetition rate of 34.15 MHz and average output power of 2 mW wavelength centered at 1922.8 nm. The mode-locked pulses were monitored by using a 25 GHz real-time oscilloscope and a 7.5 GHz InGaAs photodetector. No signs of pulse splitting or double-pulses were observed in the case of prolonged operation. Due to dispersion management, the 3 dB spectral bandwidth was as wide as 28.4 nm and the pulse width was estimated to be 200 fs, as shown in Fig. 2. The signal-to-noise ratio in the RF spectrum was higher than 60 dB, indicating the good mode-locking stability. The optical pulses passed through an optical isolator for preventing backward reflections and were then coupled into the Tm-doped fiber amplifier.
When the seed pulse propagated in the Tm-doped fiber amplifier with low pump power, the spectrum experienced weak broadening but mainly symmetrical and the pulse duration began to decrease gradually resulting in the increase of pulse peak power. When the pump power approached to about 6 W, a noticeable spectrum broadening due to self-phase modulation occurred and the pulses became narrow under the optical soliton compression effect. Further power increase led to pulse break-up with the formation of new spectrum component named Raman soliton. The soliton escaped from the gain band because of the SSFS. The remaining signal was then amplified to pump soliton pulses for further shifting the pulse to longer wavelengths.
Figure 3 showed output spectra as a function of the incident pump power in the Tm-doped fiber amplifier. The optical spectra were measured with the optical spectrum analyzer. We could see from the picture that the wavelength of Raman soliton pulses could be continuously tunable from 1.9 to 2.36 μm by only changing the pump power. The maximum average output power of soliton pulses reached 1.16 W corresponding pulse energy of 34 nJ at the wavelength of 2.29 μm. When the pump power increased to 14.6 W, the soliton pulses reached a maximum central wavelength of 2.36 μm with the frequency shift up to 460 nm. However, the loss of quartz glass fiber increased sharply when the wavelength of Raman solitons was longer than 2.4 μm, resulting in the limit of energy scaling and tuning range.
The wavelength shift of the monochromatic soliton pulses as a function of 793 nm pump power in the Tm-doped amplifier was shown in Fig. 4. Due to the onset of the infrared absorption edge of silica glass and the change of fiber dispersion and nonlinear coefficient along with the shifted wavelength, the wavelength of soliton pulses saturated at 2.36 μm and the speed of frequency shift reduced gradually. If the pump power was further increased, the output spectrum will generate a second Raman soliton or evolve into a supercontinuum emission.
Figure 5 showed the autocorrelation trace and the optical spectrum of soliton pulses when the pump power was 9.7 W. The pedestal free good trace was observed and this trace was well fitted to the sech2 pulse. The full width at half-maximum (FWHM) of pulse width was 148 fs with the output power of 740 mW. The spectral width of soliton pulses at FWHM was 30 nm centered at 2028 nm. The corresponding time-bandwidth product was 0.324 and this value was almost close to the transform-limited of sech2 pulse. The soliton number at the output was estimated to be ~3.9. If the TDF was extended, the higher order solitons would evolve as same as . Limited by the measurement range of the autocorrelator, the pulse width of longer wavelength soliton pulses could not be measured. But one could see that the spectral width of Raman soliton corresponds to Fourier transform-limited durations of order 100 fs.
The efficiency of Raman conversion was defined as the ratio of the shifted soliton output power to the total power from amplifier. In order to measure the efficiency of Raman shifted soliton, the bandpass filter was utilized to separate soliton pulses and unconverted seed pulses. After measurement and calculation, the output power and conversion efficiency of Raman shifted soliton as a function of their center wavelength were shown as Fig. 6. The conversion efficiency was more than 90% in the tunable range of 1.9-2.23 μm, indicating the almost all of the energy was transferred to the shifted soliton pulses. In particular, the maximum output power of soliton pulses was 1.16 W centered at 2.29 μm with the efficiency of 83.6%. The abrupt decrease of efficiency in longer wavelengths was due to the gain spectrum of the TDF and the high loss of quartz glass fiber beyond 2.4 μm. The emission spectrum of thulium ions ends around 2.2 μm, so the Raman soliton with a wavelength larger than 2.2 μm would not be amplified. Meantime, the limit of tuning range was limited by the background loss of the silica fiber.
4. Numerical simulation
For better understanding of the generation of tunable Raman soliton pulses, we used a generalized nonlinear Schrödinger equation (GNLSE) to model the pulse propagation in the Tm-doped fiber amplifier, which included fiber gain, the higher-order dispersion and the higher-order nonlinear effects. The equation was given as shown below:
In the simulation process, we investigated generation of soliton pulses in a Tm-doped fiber amplifier. Figure 7 showed the simulation results of generated soliton pulses spectrum at different average output power. We could draw the conclusion that the numerical simulation results were in good agreement with the experimental under the same conditions. The energy of Raman soliton occupied most of the output energy, indicating the efficiency of Raman conversion was more than 90% over the tuning range. However, the small simulation errors began to appear about conversion efficiency compared to the results of experiment when the soliton was shifted to a wavelength longer than 2.3 μm. The numerical simulation had achieved significantly better performance than the results reported here. The main reason was that the wavelength dependence of the parameters were not taken into consideration. In particular, the loss of quartz fiber abruptly increased at long wavelength and the nonlinear coefficient of the fiber exhibits strong wavelength dependence because the mode area of the fiber increased with wavelength. Moreover, it was assumed that the gain in frequency domain was only dependent on the emission cross sections of thulium ions.
We paid particular attention to the generated process of the enhanced Raman soliton and the spectral evolution of the maximum frequency shifted soliton pulses in the Tm-doped fiber amplifier was shown in Fig. 8. Here, one should see the spectral broadening at the initial stage of propagation due to self-phase modulation. After a certain propagation distance, the spectrum began to separate and the monochromatic soliton was formed. The generated soliton escaped from the fiber gain band and the overall energy was almost transferred to the shifted soliton pulses, resulting in a very high conversion efficiency. It should be possible to achieve significantly better performance than the results reported here by optimizing Tm-doped fiber amplifier. Even longer wavelength shifts could be expected by using the soft glass fiber to overcome the absorption edge of silica.
In , high-efficiency femtosecond Raman soliton could be realized by optimizing the chirp of input pulses before the amplifier. However, we obtained high-efficiency Raman soliton in the numerical simulation with the transform-limited pulses at input of the amplifier. And we have carried out another experiment that high-efficiency Raman soliton could be realized without any dispersion management in a Tm-doped fiber amplifier seeded by soliton mode-locked fiber laser. The highest efficiency of energy transferred to a Raman soliton was up to 93%, and the tunable range was from 1.95 to 2.3 μm. In conjunction with the simulation results, we can concluded that dispersion management was not a key factor to achieve high conversion efficiency in contrast to the previous study in .
In conclusion, we have reported a high-energy all-fiber femtosecond pulse source that generates soliton pulses tunable from 1.9 to 2.36 μm by simply varying the pump power. The pulses were produced by utilizing the SSFS in the Tm-doped fiber amplifier seeded dispersion managed mode-locked Tm oscillator. We obtained optical soliton with pulse energy of 34 nJ at 2.29 μm and the peak power was estimated in exceed of 100 kW. Moreover, one should note that the overall energy was almost transferred to the shifted soliton pulses with high output power based on conventional fiber. The experimental results were in good agreement with the numerical simulation of pulse propagation in Tm-doped fiber amplifier. Higher pulse energy and wider tuning range can be achieved by using soft glass fibers with optimum fiber designs. A fiber-based tunable femtosecond pulse source should facilitate numerous applications including time-resolved spectroscopy, multiple wavelength pump-probe experiments and multiphoton excitation microscopy.
National Natural Science Foundation of China (NSFC) (61527822, 61235010).
The authors acknowledge funding support from the National Natural Science Foundation of China (NSFC).
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