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We demonstrate a core-pumped Q-switched thulium-doped fiber laser system with fast tunability capability over 100 nm without any movable part. With up to 7 kW peak power in a diffraction-limited beam, this source is well adapted for pumping a frequency agile mid-IR parametric oscillator or amplifier based on Quasi-Phase-Match single-period crystals.
© 2015 Optical Society of America
1. Introduction
There is an increasing demand for laser sources with wavelengths around 2 µm for applications ranging from stand-off detection of bio-aerosols or chemicals, material processing, laser surgery and frequency conversion. For example, frequency agile mid-infrared optical parametric oscillators (OPO) could be obtained with pump laser systems delivering pulses of several kW peak power of wavelength rapidly tunable over a large bandwidth and of linewidth lesser than a pump-acceptance bandwidth of 2 nm [1].
Thulium-doped fibers exhibit a broad spectral emission range and, in particular, single-mode Tm-doped silica fibers have enabled continuous (CW) laser emission from 1700 to 2100 nm depending on the pumping scheme [2] .A CW emission continuously tunable over 250 nm was even achieved in a core-pumped configuration [3] with a maximum output power of 30 mW. A spectral line-width as low as 300 MHz (0.01 cm−1) was measured in a configuration including a tunable Fabry–Perot filter [4]. A high-power single-frequency, single-polarization, thulium-doped all-fiber MOPA system was recently described delivering above 200 W [5].
Yet the continuous emission of these tunable sources is not powerful enough to allow efficient pumping of frequency conversion stages based on nonlinear crystals. Pulsed emission at fixed wavelengths was also demonstrated with an all-fibered gain-switched pumping configuration [6]. A tunable and pulsed Q-switched operation was lately demonstrated [7]. Nevertheless, the speed at which the wavelength of the high-energy-pulses can be changed remains limited as the tunability is based the adjustment of a position of a slit. On the other hand, Acousto-Optic Tunable Filters (AOTF) give a wavelength selection with a delay ultimately limited by the establishment of an acoustic wave over the diameter of the beam to diffract. Response-times of 1 µs per mm beam diameter are now available in free-space configurations where tunability is provided by tuning a RF signal in the range of tens of MHz. The availability of digitally-controlled, programmable digital synthesizers and GHz-bandwidth amplifiers offer rapid switching compared to the position adjustment delays required when tunability is based on moving mechanical parts.
Based on a preliminary set-up [8], we present here experimental results of a Q-switched single-mode thulium-doped silica fiber system providing nanosecond pulses of up to 7 kW peak power. A wide tunability of 100 nm is obtained without any moveable part thanks to a fast Acousto-Optic Tunable Filter allowing wavelength selection in 50 µs.
2. Experimental setups
The architecture described in Fig. 1 is based on a Q-switched single-mode thulium-doped silica fiber laser followed by a single-stage amplifier. In order to prevent nonlinear effects in the fibers, the laser and the amplifier are based on short lengths of highly-doped core-pumped active fibers.
The laser (see Fig. 1) is based on a 20 cm long thulium-doped silica fiber with high core-dopant concentration (2.5% wt. Tm2O3) and a 6 µm core diameter that is single-mode above 1.75 µm and slightly multimode at the 1.54 µm pump wavelength (iXFiber-Tm-O-6-130). This active fiber is spliced to an equivalent length of undoped passive fiber and a SMF28-FC/APC connector added for practical reasons before coupling and collimation to a free-space path. The free-space path allows an external-cavity feedback comprising a lens of focal length 5 mm and NA ~0.4, an Acousto-Optic Modulator (AOM) for cavity Q-switching, an electronically-controlled AOTF to tune the emitted wavelength with a line-width of 2 nm (FWHM) and a broadband highly reflective mirror to close the cavity on the AOTF first diffraction order. The Tm-doped fiber is core-pumped at 1.54 µm through a fibered WDM coupler. A home-made CW Er/Yb fiber laser (up to 6 W) was used as pump source. At the output of the WDM coupler, a simple FC/PC connector provides 4% back-reflection to close the cavity and forms the output coupler. Two cavity configurations were tested: the AOM Q-switch was aligned either on its zeroth or first order of diffraction to compare the influence of losses and diffraction efficiency on laser performance. Both configurations have a total cavity length of 5.7 m.
Two versions were also tested for the amplification stage as depicted in Fig. 2. The general set-up comprises a SMF28-fibered WDM coupler, a short length of double-clad thulium-doped fiber as active fiber and a fibered non-polarized isolator inserted before the WDM to avoid that emission from the amplification stage be coupled back into the laser. The manual adjustment of FC/PC connectors in a sleeve enabled to couple the amplifier to the oscillator while maintaining laser operation, at the expense of 1.8 dB insertion losses averaged over the wavelength range.
The version “A” of the amplifier is based on a 25 cm long double-clad thulium-doped fiber (~2% wt. Tm2O3) of 10 µm core diameter (NUFERN PM-TDF-10P/130) and a 35 cm long SMF28 passive fiber with a FC/APC connector. The active fiber is pumped by a fibered 1.54 µm laser diode amplified in an Er/Yb amplifier delivering up to 4 W output power.
The amplifier version “B” is based on a 48 cm long double-clad Tm-doped fiber (~1% wt. Tm2O3) of 20 µm core diameter (IXF-2CF-Tm-O-20-250) and an equivalent length of corresponding passive fiber whose output facet is cleaved with an angle of 8°. A Mode Field Adaptor is inserted between the output of the WDM coupler and this second Tm-doped fiber to preserve single-mode propagation of the laser signal when passing from the SMF28 fiber of 10 µm core diameter to this active fiber of 20 µm core diameter. This larger core active fiber is pumped by a second home-made CW Er/Yb fiber laser source delivering up to 6.5 W.
3. Comparison of laser oscillator configurations
The laser oscillator sustains both configurations where the optical cavity in the external-cavity feedback is aligned on the zeroth or on the first diffraction-order of the AOM. In both cases, for any AOM repetition rates between 1 kHz and 20 kHz, pumping conditions (i.e. launched power) can be found so that a stable Q-switch operation is obtained over a 100 nm tuning range. For each repetition rate, increasing the pump power makes Amplified Spontaneous Emission (ASE) develop so that it finally clamps the gain available for the tunable Q-switch pulse and reduces the tuning range. A tradeoff is chosen between pump rate (pulse energy) and tuning range. Pulse energies were measured with an Energy-meter. Their temporal profile was observed on a 1 GHz bandwidth oscilloscope with a fast-response photodetector of spectral range 830 to 2100 nm and rise-time / fall-time below 50 ps.
With the external-cavity feedback aligned on the 0th diffraction order of the AOM (and losses induced by diffraction on the 1st diffraction order), the laser is tunable for all the repetition rates from 1869 to 1962 nm with pulse energy in the 10–14 µJ range. For example, a pump power of 0.4 W is sufficient for a 1 kHz operation. Pulse-widths are measured in the 35–45 ns range (See Fig. 3(a)). The exact temporal profiles of the pulses were exploited to estimate the emitted peak power from the signal amplitude and area of the fast-response photodetector and independently of the pulse shape and width. An example of temporal profile at the highest output pulse energy is given Fig. 3(b). Peak powers between 150 and 260 W are obtained with this configuration where the external cavity feedback is aligned on the zeroth diffraction order of the AOM [8]. Estimations based on the Full-Width at Half Maximum of the pulses temporal profiles yield values larger by 10 to 20%.
Fig. 3 Output characteristics of the laser oscillator with the external cavity feedback aligned on the zeroth diffraction order of the AOM. (a) Laser energy and pulse width vs. emitted wavelength. (b) Temporal profile at 1930 nm, rep. rate of 5 kHz and pump of 0.7 W.
Spectral measurements were performed with a resolution of 0.05 nm for all repetition rates. We observed that for repetition rates below 2 kHz, our spectrum analyzer missed pulses along wavelength scanning. Significant dynamic and reliable linewidth estimations were obtained only for repetition rates above 2 kHz. The spectra of the output pulses have a line-width below 0.6 nm (FWHM) over the whole tuning range. For a 5 kHz repetition rate, ASE building up between output pulses could be evaluated at a level 30 dB below the laser one (Fig. 4). Comparison between pulse energy and average power shows that on the whole tuning range of the laser, ASE contributes to less than 5% of the total output average power at 1 kHz repetition rate and to less than 1% for repetition rates of 5 kHz and above.
Fig. 4 Laser output spectrum at a 5 kHz repetition rate with the external cavity feedback aligned on the zeroth diffraction order of the AOM.
With the external cavity feedback aligned on the zeroth diffraction order of the AOM of around 94% transmission, the cavity losses of this configuration mainly come from the 4% back reflection of the output coupler. The additional losses induced during Q-switching when the AOM is driven to diffract to its first diffraction order are limited by a transmission of around a percent remaining on the zeroth order. An increase in Q-switch losses is needed to limit as much as possible the build-up of ASE between two pulses.
To reduce the level of ASE generated at low repetition rates between laser pulses, a second Q-switch configuration was investigated where the external-cavity feedback is aligned on the AOM’s first diffraction order. The laser also offers a wide tunability between 1865 and 1957 nm for repetition rates ranging from 1 to 20 kHz. Again, stable pulsed operations for any repetition rate are achieved by adjusting the pump level. The emitted pulse energies are between 8 and 18 µJ in the whole tuning range (See Fig. 5(a)). An example of temporal profile at maximum output pulse energy is given Fig. 5(b). The reduced transmission of 85% provided on the first diffraction order produces larger cavity losses. A pump power higher than in the previous Q-switch configuration is required to compensate for the increased cavity losses (0.8 W vs 0.4 W at 1 kHz).
Fig. 5 Output characteristics of the laser oscillator with the external cavity feedback aligned on the first diffraction order of the AOM. (a) Laser energy and pulse width vs. emitted wavelength. (b) Temporal profile at 1925 nm, rep. rate of 2 kHz and pump of 0.8 W.
This leads to larger net gain along the thulium-doped fiber. The wavelength gain dependence could explain the larger pulse energy variation with wavelength. The larger gain available provided by the reduction of ASE is also consistent with the shortening of pulse widths to values between 25 and 35 ns. Such a reduction in pulse durations yields pulses with peak powers between 300 and 600 W [8].
Output spectra have a line-width below 0.8 nm (FWHM) over the whole tuning range of the laser emission. Driving the AOM to its zeroth diffraction order between pulse emission enables to increase the additional Q-switch losses compared to the previous configuration. For a 5 kHz repetition rate, time-averaged spectral measurement also showed ASE but at a level 35 dB below the laser (Fig. 6). Depending on the wavelength, ASE contributes from 5% to 14% of the total output average power at 1 kHz repetition rate and below 1% on the whole tuning range for repetition rates of 5 kHz and above. At low repetition rates, ASE appears to profit from the larger net gain available in the active fiber which partially moderates the benefit of the increased Q-switch losses of this configuration. These spectral characteristics are well suited to pump parametric amplifiers. With its higher peak power, this second configuration (external cavity aligned on the first diffraction order of the AOM) is chosen for the laser configuration in the following experiments.
Fig. 6 Laser output spectrum at a 5 kHz repetition rate with the external cavity feedback aligned on the first diffraction order of the AOM.
This small-core, single-mode thulium-doped fiber laser configuration demonstrates a nanosecond-pulsed operation with a 100 nm tunability around 1.9 µm. A collimated beam of diameter less than 1.4 mm in the external-cavity path and an AOTF response-time of 1 µs per mm diameter allow changing wavelengths within a few microseconds. With a cavity lifetime of a few nanoseconds and an AOTF switching time less than 50 µs, the switching speed of the laser wavelength is only limited by the oscillator repetition rate. Our current driver could not provide a 1 µs response-time. Nevertheless, experiments were performed at a repetition rate of interest of 1 kHz for the laser oscillator, synchronized with an alternative changing between wavelengths separated by 25 nm on both sides of the maximum output energy.
Further amplification is thus required as the output peak power achieved is not large enough to provide an efficient parametric gain in non-linear crystals. Moreover, as the solution for coupling the output emission into another single-mode fiber adds 1.8 dB insertion losses on average over the tuning range, only 400 W maximum peak power is available for the following amplification stages.
4. Comparison of amplifier versions
With the external-cavity feedback aligned on the AOM’s first diffraction order, the laser emission is further injected into one of the amplifier stage version.
The output fiber of the version “A” of the amplification stage ensures a monomode spatial emission in the wavelength range covered by the oscillator. It is operated for repetition rates of the oscillator up to 10 kHz and for pump powers up to 2.2 W. For the laser – amplifier system, an optimum repetition rate is found around 2 kHz, maximizing the output peak power for the whole spectral range (Fig. 7).
Fig. 7 Impact of the pulse repetition rate and emission wavelength on the output peak power of the version “A” of the amplification stage.
In this configuration, a CW pump of only 2.2 W in the amplifier gives output peak powers between 1.5 and 2.5 kW (Fig. 8).
Fig. 8 Output characteristics of the version “A” of the amplification stage at the oscillator optimal repetition rate (2 kHz). (a) Peak power and pulse width vs. emitted wavelength. (b) Temporal profile at 1925 nm, oscillator pump of 0.8 W and amplifier pump of 2.2 W
For this repetition rate of the oscillator and pump power in the amplifier, the spectra of the amplified pulses have a line-width of 0.6 to 0.9 nm (FWHM) over the whole tuning range. The development of a tail towards the highest wavelengths when peak powers exceed 2 kW (see Fig. 9) indicates the onset of nonlinear effects. At higher pump level, pulse widths increase slightly by a few nanoseconds. Nonlinear effects such as Modulation Instability may create new higher spectral components at wavelengths above the spectral range of the photodetector and produce this apparent increase of pulse width.
Fig. 9 Output spectrum of the version “A” of amplifier for 2.2 W pump with laser tuned at 1930 nm (blue), or 1960 nm (red), 2 kHz. To avoid superposition of the ASE contribution centered at 1865 nm, the red curve is shifted by −3dB.
Comparison between pulse energy and average power shows that ASE represents 28% to 45% of the average power emitted by the amplifier, depending on the selected wavelength. Increasing the repetition rate to 10 kHz reduces this ratio to a range of 1 to 10%.
The version “B” of the amplification stage relies on a larger core diameter to push the peak power limit at which nonlinear effects develop. It is also operated for repetition rates of the oscillator up to 10 kHz and allows pump powers at least up to 6.2 W. Higher pump power leads to a spectral broadening too large for the targeted application. For the laser – amplifier system, an optimum repetition rate is found around 1 kHz, when the output peak power can be maximized for the whole spectral range (Fig. 10). In this configuration, a CW pump of 6.2 W in the amplifier gives output peak powers between 3.5 and 7.3 kW depending on the wavelength.
Fig. 10 Impact of the pulse repetition rate and emission wavelength on the output peak power of the version “B” of the amplification stage.
Again, our measurement system allowed spectral linewidth estimations for repetition rates down to 2 kHz. For this repetition rate of the oscillator and 6.2 W of pump power in the amplifier, the amplified pulses have a spectral line-width of 0.7 to 1.5 nm (FWHM) over the whole tuning range. The development of a tail towards the highest wavelengths when peak powers exceed 6 kW (see Fig. 11) is also an indication that nonlinear effects develop.
Fig. 11 Output spectrum of the amplifier version “B” for 6.2 W pump with laser tuned at 1900 nm, 2 kHz. Inset a zoom of the main peak in a 20 nm span.
For a 2 kHz repetition rate, ASE contributes from 32% to 42% of the average power emitted by the amplifier, depending on the selected wavelength. This ratio is reduced to a range of 4 to 10% when the repetition rate is increased to 10 kHz. If this fiber laser system pumped a frequency conversion stage such as an OPO, ASE would not participate to the nonlinear effect and would not be detrimental. More important is the fraction of pulse peak power comprised in the 2 nm pump-acceptance bandwidth targeted. A factor of comparison was estimated as follows: first, the average pulse power integrated over a 2 nm span centered on the peak (see P1 in inset of Fig. 11) was compared to the ASE level at the base of the peak (−41 dBm) integrated over the same 2 nm span (see P2 in inset). The contribution of ASE was here estimated at 3% of the pulse power over this 2 nm span and was thus neglected. This average power integrated over a 2 nm bandwidth centered on the peak was then compared to the overall integral across the entire range and was estimated at 53% of total average power.
The beam quality is measured at the output of the amplifier version “B” after collimation with a high NA lens and focusing with a lens of 300 mm focal length. Transverse profile characterizations performed at 10 kHz with a pyro-electric detector behind a rotating slit (NanoScan) indicates that the beam remains nearly diffraction limited with a M2<1.1 in both vertical and horizontal directions. The difference between horizontal and vertical waists values in Fig. 12 may come from astigmatism due to an imperfect alignment of the high NA collimating lens. Measurements made at different wavelengths or different repetition rates perfectly match Fig. 12, making this source appropriate to pump a tunable mid-infrared parametric oscillator or amplifier based on OP-GaAs crystals [9].
Fig. 12 Amplifier version “B” signal beam quality measurements (see text) for 5.4 W amplifier pump and with laser tuned at 1930 nm, 10 kHz (x: horizontal-axis; + : vertical-axis; dashed curves: fits).
The key laser parameters obtained at maximum output peak power for the different repetition rates are reported in Table 1.
Table 1. Output characteristics of amplifier version “B” for 6.2 W pump and maximum output peak power
5. Conclusion
We demonstrate a core-pumped single-mode thulium-doped fiber system that delivers pulses of nanoseconds widths and peak power up to 7 kW, for a few Watts of pump power. A tunability of 100 nm is obtained around the 1.9 µm wavelength and the switching speed should only be limited by the oscillator repetition rate. The expected availability of high performance fibered AOM and AOTF around 2 µm paves the way to successful implementation of all-fibered widely tunable and high peak power sources in this wavelength range. The achieved peak power, spectral linewidth and perfect beam quality now enable us to pump a frequency agile mid-infrared parametric oscillator or amplifier based on a Quasi-Phase-Match single-period crystal such as OP-GaAs.
Acknowledgments
The research leading to these results has received funding from the European Union Seventh Framework Program (FP7/2007 – 2013) under grant agreement n°17884, the collaborative Integrated Project MIRIFISENS.
The authors would like to thank the anonymous reviewers for their valuable comments and suggestions to improve the quality of the paper.
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