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Abstract
We present a high peak power rapidly tunable laser system in the long-wave infrared comprising an external-cavity quantum cascade laser (EC-QCL) broadly tunable from 8 to 10 µm and an optical parametric amplifier (OPA) based on quasi phase-matching in orientation-patterned gallium arsenide (OP-GaAs) of fixed grating period. The nonlinear crystal is pumped by a pulsed fiber laser system to achieve efficient amplification in the OPA. Quasi phase-matching remains satisfied when the EC-QCL wavelength is swept from 8 to 10 µm with a crystal of fixed grating period through tuning the pump laser source around 2 µm. The OPA demonstrates parametric amplification from 8 µm to 10 µm and achieves output peak powers up to 140 W with spectral linewidths below 3.5 cm−1. The beam profile quality (M2) remains below 3.4 in both horizontal and vertical directions. Compared to the EC-QCL, the linewidth broadening is attributed to a coupling with the OPA.
© 2017 Optical Society of America
1. Introduction
There is an increasing demand for laser sources from 8 to 12 µm for stand-off detection of chemical traces. This application requires a fast and large spectral tunability with a narrow linewidth to achieve the spectral resolution of the components to identify (< 5 cm−1 for chemicals deposited on surfaces) within a few seconds. In addition, it requires a good output beam quality and high peak power to achieve long-range detection. On one hand, narrow-linewidth pulsed optical parametric oscillators (OPO) [1-6] can achieve most of these targets, but so far within complex systems with slow tuning means. Vodopyanov [4] demonstrated a pulsed optical parametric oscillator tunable from 4 to 14 µm based on orientation patterned GaAs (OP-GaAs) requiring a complex pump at 3 μm. Robertson [5] reported an intracavity OPO based on zinc germanium phosphate medium (ZGP) but the tunability from 5.5 to 10 µm was provided by angle-tuning of the crystal. Clément [6] demonstrated a narrow linewidth OP-GaAs based OPO but the spectral range provided by tuning the crystal temperature remained limited and slowly addressed. On the other hand, external-cavity quantum cascade lasers (EC-QCL) based on a single-chip electrically pumped semiconductor gain medium have demonstrated a broad spectral tunability (~300 cm-1) with a good output beam quality and a narrow linewidth ≤ 1 cm−1 in the full tuning range [7] These sources are well adapted for detection of chemical traces except that at room temperature, the available short pulses of a few hundreds milliwatts peak power limit the stand-off detection range to several meters at best [8-11]. We propose here to enhance the peak power of an EC-QCL tunable from 8 to 10 µm thanks to optical parametric amplification (OPA).
OPA requires a high peak power pump source and a nonlinear medium enabling frequency conversion from the pump to signal and idler waves. Gallium arsenide (GaAs) is one of the most interesting mid-IR nonlinear materials with a very large second-order nonlinear optical coefficient d14~90 pm/V near 10 µm [12]. An efficient nonlinear conversion is obtained if the relative phases between interacting waves are controlled and maintained constant all along propagation in the crystal. This phase-matching condition is usually achieved in birefringent nonlinear crystals by rotation or temperature tuning of the nonlinear crystal.
Quasi Phase-Matching (QPM) is an alternative technique where the sign of the nonlinear coefficient is periodically reversed to reset the phase relationship between the interacting waves [13-15]. The growth of such orientation-patterned GaAs (OP-GaAs) was achieved [12] and thanks to QPM, several optical parametric oscillators demonstrated an efficient conversion from the pump to the generated signal and idler waves [16,17]. OPA based on OP-GaAs as nonlinear crystal formerly showed high gain at 4.5 µm while preserving the spectral and spatial properties of an amplified DFB-QCL [18]. Using ZGP as nonlinear medium, Clément et al. [19] demonstrated OPA of a single-frequency continuous-wave QCL tunable from 7.8 to 8.4 µm, but again the tunability was achieved by tuning the angle of the medium.
Phase-matching can also be realized with wavelength tuning of both pump and signal beams, avoiding any action on the nonlinear crystal [20-22]. In particular, Klein et al. [20]demonstrated a PPLN-based OPO whose wavelength is tuned from 3.16 to 3.5 µm within 330 µs with a fiber laser all-electronically tunable through an Acousto-Optic Tunable Filter (AOTF).
We present here a pulsed laser system tunable in the long-wave infrared using an EC-QCL that is broadly tunable from 8 to 10 µm and an OPA that does not involve any moveable part. The OPA is based on OP-GaAs of fixed period and a thulium-doped fiber pump system acousto-optically tunable around 2 μm. Quasi phase-matching in the OPA is achieved over the whole tuning range of the EC-QCL by simultaneous wavelength tuning of the pump, thus avoiding any action on the nonlinear crystal.
2. Tunable fiber pump system
The tunable pulsed fiber pump system is described in Fig. 1. It is based on the fiber pump system previously described in [23] and improved in this work using only polarization-maintaining fibers so that the output pulses have a linear polarization, independent of the emitted wavelength. A tunable Q-switched thulium-doped silica fiber laser oscillator is followed by a fiber amplification stage. The laser and the amplifier are based on short lengths of highly-doped core-pumped active fibers to prevent nonlinear effects in the fibers.
Fig. 1 Set-up of the tunable pulsed fiber pump system. O.C.: output coupler. PM: polarization-maintaining component.
The laser is based on a thulium-doped silica fiber with a length of 44 cm, a high core-dopant concentration (2.5% wt. Tm2O3), a core of diameter 6 µm and NA 0.22. This fiber is single-mode above 1.75 µm and slightly multimode at the 1.54 µm pump wavelength. The Tm-doped fiber is core-pumped through a WDM coupler fibered with a NUFERN PM1950 fiber to match the active fiber mode diameter. A home-made CW Er/Yb fiber laser (up to 6 W) was used as pump source. The other port of the WDM coupler is spliced to an equivalent length of passive fiber with a FC/APC connector 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. At the output of the active fiber, a simple FC/PC connector provides 4% back-reflection to close the cavity and forms the output coupler. This connectorized fiber was chosen with a core diameter of 10 µm and NA 0.10 to limit spectral broadening at the laser output and to match the input fiber of the amplifier. The optical cavity in the free-space path is aligned on the first diffraction-order of the AOM to reduce Amplified Spontaneous Emission (ASE) generated at low repetition rates between laser pulses and to shorten pulse-widths [23]. In this work, the WDM coupler is placed opposite the output coupler to reduce ASE at the laser output. Fiber lengths were reduced as much as possible between components and the geometrical length of the cavity is 2.3 m. The laser output is coupled to the amplifier input fiber by means of two collimating lenses of identical focal length (5 mm).
In the AOTF, the access-time to any wavelength is ultimately limited by the delay of establishment of the acoustic wave over the volume of the beam to diffract. With a TeO2 crystal and a configuration where the acoustic wave is quasi-collinear with the optical beam over an interacting length around 20 mm, the acoustic velocity of 4260 m/s let us expect a response-time of a few µs. Tunability is here provided by changing the acoustic wave frequency around 30 MHz by a few percents with a PC-controlled driver. Employing a programmable digital synthesizer, such AOTFs demonstrated rapid tuning of a fiber laser. [20]
The amplifier comprises a WDM coupler, a 30 cm long Tm-doped fiber (estimated doping ~1.4% wt. Tm2O3) with a core diameter of 25 µm and a NA of 0.09 as active fiber and a polarization-sensitive isolator inserted before the WDM to avoid that emission from the amplification stage be coupled back into the laser. The WDM coupler and the isolator are fibered with the 10-µm core diameter fiber chosen for the output coupler of the laser. 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 passive fiber of core diameter 10 µm to the active fiber of core diameter 25 µm. This larger core active fiber is pumped by a second home-made CW Er/Yb fiber laser source delivering up to 12 W. The active fiber output is spliced to an equivalent length of corresponding passive fiber with an angled FC/APC output connector to avoid back reflection and to prevent the amplifier from lasing.
The laser oscillator provides stable Q-switch operation for any AOM repetition rates between 1 kHz and 50 kHz. Characterizations were performed at the optimum repetition rate of 2 kHz that was found for the previous fiber pump system [23]. Pumping conditions (i.e. launched power) can be found so that a stable Q-switch operation is obtained over a 100 nm tuning range. Increasing the pump power makes ASE develop so that it finally clamps the gain available for the tunable Q-switch pulse and reduces the tuning range. A trade-off 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. 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, independently of the pulse shape and width. For a launched power of 1.2 W, the laser is tunable from 1880 to 1980 nm with pulse energy in the 8–17 μJ range. Pulse-widths are measured in the 23–37 ns range and output peak powers range between 300 and 700 W [Fig. 2(a)]. Spectral measurements were performed with a resolution of 0.05 nm. The spectra of the output pulses have a linewidth below 0.7 nm (FWHM) over the whole tuning range after further propagation in 1 meter of passive fiber. ASE building up between output pulses could be evaluated at a level 40 dB below the laser one [Fig. 2(b)]. The output polarization is linear with an extinction ratio larger than 10 dB.
Fig. 2 Characterizations of the laser oscillator at 2 kHz repetition rate with 1.2 W launched power. (a) Laser peak power and pulse width vs. emitted wavelength. (b) Output spectrum for laser tuned at 1933 nm.
Amplification is required as the laser output peak power is not large enough to provide an efficient parametric gain in non-linear crystals. With the laser operated at 2 kHz repetition rate, the amplifier delivers pulses tunable from 1880 to 1980 nm with pulse-widths between 23 and 34 ns. Output energy scales up to 200 µJ when operated with a pump power up to 9 W in the amplifier. This maximum launched pump power was voluntarily limited to keep the amplifier output energy below the damage threshold of the antireflection coating on the OPA nonlinear crystal. At the amplifier output, 15% of the output energy appears to come from the cladding, thus cannot be properly focused and contributes to losses. Nevertheless, after filtering these losses with a diaphragm, the amplifier still provides peak powers between 3 and 6.7 kW depending on the wavelength [Fig. 3(a)].
Fig. 3 Amplifier output characterization for laser pumped with 1.2 W, 2 kHz repetition rate and amplifier pumped with 9 W. (a) Peak power and pulse width vs. emitted wavelength. (b) Spectrum for laser tuned at 1933 nm. Inset a temporal pulse profile for laser tuned at 1933 nm.
The amplified pulses have a spectral line-width of 0.4 to 0.9 nm at −3 dB (FWHM) and below 2.4 nm at −10 dB over the whole tuning range (See example Fig. 3(b)). The amplifier output beam is linearly polarized (Ellipticity 0.7-0.8), with a small +/−5° variation of the polarization axis depending on the wavelength.
The beam quality is measured at the output of the amplifier for 1.2 W launched power in the oscillator operated at 2 kHz repetition rate and 9 W pump power in the amplifier. Transverse profile characterizations were performed with a pyro-electric detector behind a rotating slit after collimation with an achromatic reflective mirror of focal length 7 mm and focusing with a lens of focal length 60 mm. The beam remains nearly diffraction limited with a M2<1.4 both in the horizontal and vertical directions. An important figure for the application is the fraction of pulse peak power comprised in the 2 nm pump-acceptance bandwidth targeted. A factor of comparison is estimated as follows: the average pulse power integrated over a 2 nm span centered on the peak is compared to the overall integral across the entire range. We thus estimate that 70 to 90% of amplified output power stands in a 2 nm bandwidth (corresponding to the spectral acceptance bandwidth of the OPA), making this tunable fiber pump suited to pump an OPA based on OP-GaAs.
Today, the tunable fiber pump system comprises components aligned in a free-space path. Yet, the incoming availability of high performance fibered AOM and AOTF around 2 μm paves the way to successful implementation of all-fiber widely tunable and high peak power pump systems in this wavelength range.
3. Optical parametric amplification of the EC-QCL
The tunable fiber laser system pumps our OPA set-up described in Fig. 4. The OPA is seeded with an EC-QCL based on a two-stack heterocascading-design chip [7]. The EC-QCL emission is tunable from 8 to 10.2 µm within 1 s by means of an intra-cavity grating in a Littrow configuration mounted on a fast rotary stage. Round-trip cavity length is 44 mm. It delivers pulses of 250 ns pulse width and peak power from 100 to 350 mW at a maximum repetition rate of 200 kHz. At the output of the QCL chip, a high NA lens provides a collimated output beam with a quasi-diffraction limited profile (M2<1.4). Spectral linewidths are below 1 cm−1 (FWHM) on the whole tuning range. A MEMS grating based version of this EC-QCL was recently developed providing a scan speed of 1 ms per bandwidth [11,24].
Fig. 4 Description of the EC-QCL and OPA set-up. λ/2: zero-order half-wave plate. D: diaphragm. DM: dichroic mirror. LP: Long-Wave Pass filter above 6600 nm. Blue arrow: pump beam. Red arrow: EC-QCL beam. Green arrow: complementary signal generated in the OPA of wavelength between 2.3 and 2.6 µm. Inset left a single-shot trace of the temporal pulse profile of EC-QCL output tuned at 1060 cm-1. Inset right a single-shot trace of the temporal pulse profile of the OPA output for EC-QCL tuned at 1200 cm−1.Both temporal envelopes slightly change from pulse to pulse.
The OPA is based on an OP-GaAs nonlinear crystal of period grating 66 μm, thickness 500 μm (along the [001] crystallographic axis), 2 mm wide (along [110]) and 32 mm length (along [−110], the beam-propagation direction). The crystal temperature was tuned to 97°C to provide Quasi Phase-Matching from 8 to 10 µm with a pump tunable from 1880 to 1980 nm. Its facets were antireflection coated at the pump, EC-QCL and complementary signal wavelength generated between 2.3 and 2.6 µm through parametric interaction in the crystal. The EC-QCL is linearly polarized along the [110] crystallographic axis of the OP-GaAs crystal. At the fiber amplifier output, the pump beam is collimated with an achromatic reflective mirror of focal length 7 mm and NA = 0.4. A zero-order half-wave plate enables to align the polarization axis of the pump at all wavelengths along the [111] crystallographic axis and thus maximize the parametric gain in the OPA.
Both the fiber pump beam and the EC-QCL beam are superposed with a dichroic mirror (DM) and focused to overlap with respective diameters at 1/e2 of 190 and 200 μm at the center of the OP-GaAs crystal. The crystal is tilted in the horizontal plane compared with the beams direction to avoid facet back reflections to the amplifier and the EC-QCL. A fraction of the EC-QCL emission is reflected by the dichroic mirror and provides a port to control the EC-QCL pulse. At the OPA output, an identical dichroic mirror (DM) reflects the residual pump but partially transmits the complementary signal generated in the OPA of wavelength between 2.3 and 2.6 µm. A Long-Wavelength Pass filter with a transmission of 85% above 6600 nm further discards this complementary signal from the OPA.
The EC-QCL and the fiber pump system are operated at 2 kHz repetition rate and at their maximum set-points. With proper pulse synchronization, tuning the pump wavelength from 1980 to 1887 nm allows parametric amplification of EC-QCL pulses from 8 to 10 µm. The amplified pulse energy remains too limited to be measured with an Energy-meter and was thus estimated with the output average power and the known 2 kHz repetition rate. The exact temporal profile of the pulses was exploited to calculate the emitted peak power from the signal amplitude and area of a photodetector (photovoltaic sensor from Vigo SA) of bandwidth 250 MHz in the range 8-12 µm, independently of the pulse shape and width. The OPA delivers amplified pulses of 10 ns, limited by the pump pulse width, and peak power gain between 20 and 26 dB leading to peak powers from 20 to 140 W [Fig. 5]. Beam profile measurements of the amplified EC-QCL were performed with a micro-bolometer camera of 480x640 frame size and 17 µm x 17 µm pixel dimension. The output beam presents a M2 of 3 in the horizontal plane and an M2 of 3.4 in the vertical plane. An image of the beam profile at the crystal output is given in the inset of Fig. 5.
Fig. 5 Output peak power and OPA gain vs. EC-QCL wavelength with EC-QCL at maximum output peak power, laser pumped with 1.2 W, amplifier pumped with 9 W and EC-QCL and pump synchronized at 2 kHz. Inset shows an image of the beam profile at the output of the crystal when EC-QCL tuned at 1144 cm−1.
After collimation of the output beam and propagation over 2 meters, spectral measurements were performed with a monochromator (Chromex sm250) with a resolution of 4 nm. The fast-response photo-detector was employed and its signal was measured with a lock-in amplifier. A neutral density filter of optical density 3.0 and tilted by 30° was inserted between the monochromator output slit and the photo-detector to avoid saturation of the detector, and Fabry-Perot and back-reflection effects. Starting with an EC-QCL of spectral linewidth ~1 cm−1, spectral broadening occurs as spectra present a linewidth between 2.5 and 3.5 cm−1 (FWHM) over the full tuning range. This spectral broadening may come from the coupling observed between the OPA and EC-QCL as the EC-QCL pulse shape on the control-port changes when the OPA is pumped with a wavelength satisfying QPM.
Figure 6(a) shows several OPA output spectra recorded for different EC-QCL peak powers and constant pump power. The output spectrum of the “unamplified” EC-QCL is reported for its maximum input peak power of 350 mW when the OPA is not pumped. Its linewidth is limited by the monochromator resolution and its shape does not change when the peak power is varied. When the OPA is pumped (fiber laser pumped with 1.2 W, amplifier pumped with 9 W) and the EC-QCL is driven to deliver its lowest peak power (120 mW), the output spectrum already appears broader and its peak is shifted compared to the unamplified EC-QCL. Increasing the EC-QCL peak power increases the peak of the amplified signal and brings it closer to the unamplified one. This behavior does not correspond to spectral gain saturation where the spectral edges of the EC-QCL spectrum would experience larger gain than its peak. Instead, a coupling between the OPA and the EC-QCL seems to develop as soon as the EC-QCL emits, and varies with EC-QCL peak power. An explanation could be that the lack of isolation between the EC-QCL and the OPA may lead to the development of a parasitic oscillation. A very small temporal signal could be observed directly at the OPA output when the OPA was pumped and the EC-QCL output blocked, indicating the onset of optical parametric generation (OPG). Nevertheless, its spectrum (not shown) is indistinguishable from the noise level on Fig. 6(a), showing that OPG does not significantly contribute these results.
Fig. 6 (a) OPA output spectrum of amplified pulses for different output peak power of the EC-QCL, tuned at 1174 cm−1 for laser pumped with 1.2 W, amplifier pumped with 9 W and EC-QCL and pump synchronized at 2 kHz. Blue: EC-QCL delivering 350 mW peak power. Red: EC-QCL with 270 mW peak power. Green: EC-QCL with 190 mW peak power. Cyan: EC-QCL with 120 mW peak power. Black: reference of unamplified EC-QCL with 350 mW peak power. (b). Set-up integrated in a transportable optical head (aluminum housing) and electronic controls combined in a 19-inch rack cabinet for field testing.
Finally, starting with an EC-QCL of 350 mW maximum peak power, the OPA provides an optical gain from 20 to 26 dB. In the same conditions as reported by Morales-Rodríguez et al. [8], and given that the detection sensitivity varies with the inverse of the square of the distance, the stand-off detection distance could be increased by a factor 10–20 to reach 15–30 m.
Performance of an equivalent system will be described in [25]. We briefly indicate here that with a PC-based system controller to tune the EC-QCL and pump wavelengths, this system offers the potentiality of a tuning rate of 2 seconds per full bandwidth 1000-1250 cm−1 (8-10 µm) with tuning steps of 2 cm−1 and several-pulse averaged acquisition for each wavenumber.
A set-up based on an EC-QCL of further extended tunability was finally integrated in a transportable optical head. The electronic controls for the EC-QCL, pump system and OPA (temperature controller) were combined in a 19-inch rack cabinet [Fig. 6(b)]. This system was integrated together with a long-range transmitting optic system and a long-range detection system and is currently being used for field tests [25].
4. Conclusions
In conclusion we demonstrated, for the first time of our knowledge, amplification of a tunable EC-QCL in an OP-GaAs nonlinear crystal of fixed period with a tunable fiber pump system. The fiber pump system comprises a tunable Q-switched thulium-doped silica fiber laser followed by a fiber amplification stage delivering pulses tunable from 1880 to 1980 nm with pulse-widths between 23 and 34 ns and peak powers between 3 and 6.7 kW in a quasi-diffraction-limited beam of M2<1.4 and spectral linewidth <0.9 nm (FWHM).
Starting from an EC-QCL delivering pulses tunable from 8 to 10 µm with peak power between 100 to 350 mW, an OP-GaAs crystal of length 32 mm, and fixed grating period of 66 µm, demonstrates parametric amplification over the full tuning range and achieves output peak powers up to 140 W. The output beam presents a M2 of 3 in the horizontal plane, and of 3.4 in the vertical plane. Spectral broadening occurs and is attributed to a coupling between the OPA and the EC-QCL. Output spectral linewidths remain below 3.5 cm−1 (FWHM) making this source suitable for remote stand-off detection of chemicals deposited on surfaces.
Funding
The European Union Seventh Framework Program (FP7/2007–2013) (17884, ICT project MIRIFISENS); EDA JIP-CBRN (A-1152-RT-GP, AMURFOCAL).
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