We report a novel scheme of generating broadly tunable femtosecond mid-IR pulses based on difference frequency mixing the outputs from dual photonic crystal fibers (PCF). With a 1.3 W, 1035 nm, 300 fs and 40 MHz Yb fiber chirped pulse amplifier as the laser source, a PCF with single zero dispersion wavelength (ZDW) at the laser wavelength is employed to spectrally broaden a portion of the laser pulses. Facilitated by self-phase modulation, its output spectrum possesses two dominant outermost peaks that can be extended to 970 nm and 1092 nm. A different PCF with two closely spaced ZDWs around the laser wavelength is used to generate the intense Stokes pulses between 1240 – 1260 nm. Frequency mixing the dual PCFs outputs in an AgGaS2 crystal results in mid-IR pulses broadly tunable from 4.2 μm to 9 μm with a maximum average power of 640 µW at 4.5 μm, corresponding to 16 pJ of pulse energy.
© 2013 Optical Society of America
Mid-infrared (mid-IR) coherent sources delivering high repetition rate, ultra-short pulses at the “fingerprint region” wavelengths between 2 µm – 20 µm have attracted great interest due to their emerging applications in many scientific and industrial disciplines [1, 2]. Difference frequency generation (DFG) offers a convenient frequency down conversion scheme for mid-IR generation due to its simple geometry. Its inherent advantage of generating carrier-envelope offset (CEO) phase stabilized output without the need of active stabilization is also highly desirable for certain applications . Various DFG schemes have been demonstrated so far for the realization of broadly tunable mid-IR sources, such as mixing the dual wavelengths outputs from an optical parametric oscillator (OPO) synchronously pumped by a high power solid state laser [4, 5] and mixing the outputs from a high power two color Yb:fiber chirped-pulse amplifier , where their spectral tunabilities are facilitated by the wide spectral coverage of an OPO and the broad output spectrum of a high power amplifier, respectively. Another versatile fiber based approach involves pumping a photonic crystal fiber (PCF) with an Erbium or Ytterbium fiber laser and mixing the generated continuum (usually the Raman-shifted soliton) with the residual pump [7–12]. Since the signal wavelength can be conveniently tuned by the soliton self-frequency shift , the Raman-soliton facilitated scheme enables broadly tunable mid-IR sources, with the limitation that the Raman soliton is constrained to its fundamental soliton energy, which imposes an upper limit on the available signal pulse energy for subsequent frequency mixing.
In previous work, we have demonstrated an alternative approach to fiber based mid-IR sources by generating intense Stokes pulses in the normal dispersion region (NDR) of a PCF as the signal wavelength . The PCF that was employed possesses two closely spaced zero dispersion wavelengths (ZDWs) around the pump wavelength which is known to support generation of stable and compressible NDR continuum with narrow spectral bandwidth [15, 16]. Due to the high spectral intensity of the Stokes pulses, the resultant mid-IR source was able to deliver 3 mW average power (75 pJ) of 5.5 μm pulses at 40 MHz. However, this was narrowly tunable due to the fixed spectral region of the Stokes pulses which is determined by the PCF dispersion profile [16–18]. Obviously, a spectral tuning/broadening mechanism needs to be introduced to expand the tuning range.
In this manuscript, we propose and demonstrate a novel self-phase modulation facilitated scheme of generating broadly tunable mid-IR pulses by frequency mixing the outputs from dual PCFs. The PCF with 2ZDWs (PCF-signal) is kept as the signal source to generate the intense Stokes pulses, while another conventional PCF with single zero dispersion wavelength around the pump wavelength (PCF-pump) is employed to spectrally broaden the pump pulses via self-phase modulation. Output spectra of PCF-pump exhibit the typical SPM structure consisting of two outermost peaks and an oscillatory structure in the middle. Spectral tuning is realized by varying the coupled-in power which alters the outermost peaks wavelengths. In our set-up the pump wavelength can be extended to 970 nm and 1092 nm. Mixing it with signal pulses between 1240 – 1260 nm in a type-II AgGaS2 crystal results in mid-IR pulses tunable from 4.2 μm to 9 μm with a maximum average power of 640 μW, corresponding to 16 pJ of pulse energy.
2. Experimental set-up
The experimental set-up is shown in Fig. 1. The pump source is an Yb:fiber chirped pulse amplifier (CPA) which delivers 1.3 W, 1035 nm, 300 fs pulses at 40 MHz. The CPA configuration has been described in detail in . The amplifier output is split into two arms by a polarizing beam splitter paired with a half-wave plate. In the pump arm, ~500 mW of amplifier power is coupled into 10 cm of PCF-pump (SC-1040-5.0, Crystal Fibre) to generate the spectrally broadened pump pulses, whose spectrum is monitored by an optical spectrum analyzer (86142A, Hewlett-Packard). The rest of the amplifier output is directed to the signal arm for coupling into 14 cm of PCF-signal (1050-NL-ZERO-2, Crystal Fibre) to generate the intense Stokes pulses. A variable metallic attenuator is placed in front of PCF-signal to control its coupled-in power while the split ratio between the two arms is being adjusted. Aspheric lenses are used for coupling into/out of the PCFs and typical coupling efficiencies of 60% and 35% are achieved for PCF-pump and PCF-signal, respectively. For both PCFs, the output beam polarizations are carefully adjusted to orthogonal by sets of half wave plates. A beam delay line and a telescope system are implemented in the pump arm for control of timing overlap and mode matching between the pump and signal, which are collinearly combined with a dichroic mirror (DMLP-1180, Thorlabs) and are focused inside a type-II AgGaS2 crystal (AGS-402H, Eksma Optics) by a plano-convex lens with 100 mm of focal length for difference frequency mixing. The focusing spot size is estimated to be ~30 μm in diameter. The generated mid-IR pulses are filtered out by a 3 mm thick Ge filter and then collimated by a parabolic reflector. A calibrated thermopile detector (818P-001-12, Newport) and a grating spectrometer are subsequently used for measuring the mid-IR power and spectrum. The custom built grating spectrometer consists of a 150 l/mm gold coated grating with its blaze wavelength at 6 μm (Baush&Lomb) and is set up in a Czerny-Turner configuration to eliminate wavelength-dependent aberrations of the diffracted wavefronts. Calibration of the spectrometer has been accomplished with up to 15th order diffraction of a He-Ne laser.
3. Theoretical considerations
Dynamics of supercontinuum generation with launching wavelengths at different spectral positions with respect to the fiber zero dispersion wavelength has been studied in detail ; and selection of the launching condition highly depends on the intended application. In our cases, given the intense Stokes pulses between 1240 – 1260 nm, ~100 nm of pump spectral broadening is enough to cover the frequency down conversion wavelength from 4 – 10 μm. So the launching condition where a broadband supercontinuum can be generated (i.e. pumping in the anomalous dispersion region of the fiber ) is not needed. Moreover, we want the spectral intensity of the wavelengths of interest as high as possible, this makes launching near the fiber zero dispersion wavelength highly desirable since in this case pulse evolution at the initial stage is dominated by self-phase modulation where a large portion of the spectral power reside in the outermost part of the spectrum. Bearing this in mind, we choose the PCF with its zero dispersion wavelength at 1040 nm (SC-1040-5.0, Crystal Fibre) as PCF-pump; a typical dispersion map of the fiber can be found at . With a core diameter of 4.8 μm, the PCF exhibits a modest nonlinearity of 11 (W∙km)−1 at 1060 nm. Given these values, the important launching parameters such as dispersion length , nonlinear length and soliton order can be readily estimated from Eq. (1) – Eq. (3) :
For better understanding of the pulse evolution dynamics, we have carried out a rigorous numerical simulation based on the Split-Step Fourier Transform method . Figure 2 shows the pulse spectral and temporal evolution within 20 cm of propagation in fiber assuming ~300 mW of average power.
The simulation confirms that the pulse evolution is indeed governed by self-phase modulation at the initial stage. With a positive up-chirp being created at the central part of the pulse, the leading edge of the pulse experiences spectral red-shift while the trailing edge of the pulse experiences blue-shift. While the spectral broadening is going on, the outermost peaks gradually shift to the region where dispersion of the fiber cannot be neglected. In the anomalous dispersion region, this eventually leads to formation of the Raman soliton after ~10 cm of propagation which results in a spike at the pulse leading edge that is clearly visible in the temporal evolution map. From the pulse leading edge, the soliton “walks through” the pulse after further propagation, creating drastic temporal interference as well as re-distribution of spectral intensities. The occurrence of Raman effects spreads out the power within the red-shifted SPM peak which imposes a limit on the peaks extension. Spectral interference can also be noticed, and we find that by varying the launching parameters this interference phenomenon is strongly influenced by the initial chirp of the pulse. The fact that practical laser sources with an initial chirp may follow quite a different evolution pattern depending on the sign and magnitude of the chirp is consistent with reported studies on self-phase modulation . Creation of the SPM spectral peaks results from the spectral interference between different parts of the pulse possessing the same frequency, and it becomes more complicated when the launching condition deviates from the ideal situation. Due to the limitations imposed by Raman effects, we choose the fiber length to be 10 cm for optimum operation.
4. Results and discussion
The measured spectra of the PCF-pump output with varying coupled-in power are shown in Fig. 3. Spectrum of the laser pulse quickly evolves into the dual peaks structure at low coupled-in powers and the peak intensities scale up rapidly as the coupled-in power is increased from 20 mW to 100 mW. The peaks continue to shift outwards with further increase in power, but their gain in spectral intensities become less obvious since a portion of the power starts to fill in between the peaks, leading to the typical SPM spectrum consisting of two dominant outmost peaks and an oscillatory structure in the middle. The power-dependent spectral tunability of the outermost peaks constitutes the tuning mechanism of the reported mid-IR source.
Within the range of our available pump power, a single pulse structure of the PCF-pump output is expected to be kept and experimental measurement confirms that the red-shifted peak is at the pulse leading edge and the blue-shifted peak is at the pulse trailing edge. We also observe that the temporal separation between them varies in the range of 200 – 400 fs depending on the pump power. Due to the short fiber length as well as the low fiber dispersion, pulse broadening is not significant so that a prism compressor is not required for post-compression. After optimizing the power allocation between the dual PCFs, an optimum 320 mW of average power is coupled into PCF-pump which makes the furthest shifted spectral peaks reaching 970 nm and 1092 nm, respectively. During the experiment, we notice that the peak intensity doesn’t increase monotonically with its spectral shift due to the altered pulse evolution induced by the initial chirp. We also notice that at ~300 mW of coupled-in power the red-shifted peak becomes less stable which is possibly due to the occurrence of soliton formation.
Characterizations of the Stokes pulses from PCF-signal were discussed in detail in . In this set-up, the Stokes pulses wavelengths are kept in the range between 1240 – 1260 nm and their average power vary from 40 – 50 mW. As the signal wavelength, they get combined with the spectrally broadened pump pulses and are focused inside the DFG crystal for difference frequency generation of the mid-IR pulses at the idler wavelengths.
The DFG crystal is a 2 mm thick, type-II AgGaS2 crystal cut at θ = 50° (AGS-402H, Eksma Optics). It has an anti-reflection coating from 1.1 µm to 2.6 µm for the front surface and 2.6 µm to 11 µm for the back surface. Although GaSe has been routinely used for mid-IR generation due to its high nonlinearity and transparency above 12 µm, we chose AgGaS2 as a power-efficient nonlinear medium because it is able to be polished and AR-coated. Its low birefringence and small group velocity mismatch are also favorable for reducing the spatial walk-off between the ordinary (signal) beam and the extraordinary beam (pump/idler) as well as minimizing the temporal walk-off of the mixed pulses (shown in Fig. 4(a)). Due to the high refractive index of the crystal, the external incidence angle can vary as much as 58 degrees over the tuning range (shown in Fig. 4(b)). In the same figure, a comparison between measured and theoretically predicted phase matching angle is also plotted which shows good agreements. Proper control over the phase matching angle, timing delay as well as mode matching is necessary for optimizing the mid-IR output over the tuning range.
The measured mid-IR spectra over the source tuning range are shown in Fig. 4(c). They span a broad tuning range between 4.2 µm to 9 µm with a FMHW bandwidth varying from 100 nm to 415 nm. The maximum measured power is 640 µW at 4.5 µm, corresponding to 16 pJ of pulse energy. The power level remains in excess of 400 µW between 4 – 5 µm, and decreases to ~135 µW at 9 µm. The reduction in DFG efficiency at longer wavelength is due to the increased beams walk-off as is shown in Fig. 4(a). The low intensity SPM peaks around the pump wavelength at low coupled-in power of PCF-pump leads to the reduced mid-IR power between 5 – 6 µm, while the rise in power at 8 – 9 µm is attributed to a stronger 1092 nm peak at the maximum coupled-in power. Due to the nature of the spectral broadening, that intensity, spectral bandwidth as well as temporal width of the SPM peaks vary at each spectral position; it is unlikely that the mid-IR tuning curve can be fitted according to its theoretical DFG efficiency. Nevertheless, with the Stokes pulse wavelength spanning in the range of 1240 – 1260 nm, smooth spectral tuning is able to be achieved between the measured points of wavelengths.
We have not yet performed autocorrelation measurement to directly determine the mid-IR pulse width. We have reported in our previous work  cross-correlation measurement of the Stokes pulse and the un-broadened pump pulse showing a cross-correlation width of 220 fs, to which the reported mid-IR pulse width is on a comparable scale. In this manuscript the Stokes pulse is un-changed and the pump pulse does not experience significant temporal broadening, as was discussed earlier, so that we expect the reported mid-IR source in this manuscript to have a similar pulse width with that in .
In conclusion, we have demonstrated a novel self-phase modulation facilitated approach to generating broadly tunable mid-IR pulses based on frequency mixing the dual photonic crystal fibers outputs. Since the nonlinear frequency conversion processes we employ, such as self-phase modulation and intense Stokes pulse generation are scalable, further improvement of the mid-IR source is possible providing a more powerful pump source. We believe that the reported approach will lead to a promising solution for high performance and compact mid-IR sources.
We thank Per Adamson, Dr. Jianming Dai and Dr. Xi-Cheng Zhang for loaning of the mid-IR detectors and Dr. Qiang Lin and Dr. Govind Agrawal for helpful discussions on nonlinear fiber optics.
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