We have developed a widely tunable mid-infrared difference frequency generation (DFG) source by mixing ∼ 1 W Ti:sapphire laser and 6 W Nd:YAG laser beams in a 50-mm MgO-doped long periodically poled lithium niobate (MgO:PPLN). The power of the DFG source is > 2 mW over the tuning range of 2.66–4.77 μm and its free-running linewidth is about 100 kHz. Combining various frequency stabilisation schemes for the Nd:YAG laser and the Ti:sapphire laser, the DFG frequency can be precisely controlled. Besides, its frequency can be determined better than 12 kHz by measuring the Ti:sapphire laser frequency using an optical frequency comb. Two high resolution spectroscopic studies on 12C16O2 molecule are demonstrated using this DFG source. The saturation spectra of R(18) and R(60) transitions of 0001 ← 0000 fundamental band at 4.2 μm and P(20) transition of [1001, 0201]I ← 0000 band at 2.7 μm have been observed and their absolute transition frequencies are measured with an accuracy better than 30 kHz.
© 2013 Optical Society of America
The mid-infrared (mid-IR) coherent sources have been enthusiastically developed since the early era of lasers. One of the major impetuses is that many molecular rovibrational spectral lines exist in mid-IR spectral region, and appropriate coherent sources are strongly demanded for the relevant studies and applications such as spectroscopy and trace detection. In the past, either the wavelength tunability or power was limited for available sources such as molecular lasers, lead-salt diodes and nonlinear optical devices. Nevertheless, the emergence of the innovative sources such as quasi-phase-matching (QPM) nonlinear optical devices, quantum cascade laser (QCL) and solid state lasers significantly changes the scenario [1–5].
Among these innovative sources, the QPM continuous-wave difference frequency generation (DFG) is rather favorable for studying high precision spectroscopy [6–10]. The frequency of DFG source is solely determined by the difference between two input sources whose wavelengths are usually in either visible or near infrared region, in which frequency control technique is very mature. Moreover, the frequencies of two input sources can be precisely measured or directly locked [6, 7, 9, 11] to an optical frequency comb (OFC) and therefore the DFG frequency. If higher power is desired, several schemes including an optical power amplifier for one input source , cavity-enhanced scheme [13, 14] and waveguide nonlinear crystal [9, 15, 16] can significantly boost the output.
In this paper, we describe the development of a mid-IR DFG source with mW-level power, wide tuning range, narrow linewidth and high frequency accuracy. The DFG is generated by pumping a periodically poled MgO-doped LiNbO3 (MgO:PPLN) crystal with a Ti:sapphire laser and a Nd:YAG laser and its wavelength can cover 2.66–4.77 μm by tuning the Ti:sapphire laser and choosing the QPM PPLN period correspondingly. The frequency accuracy of the DFG source is better than 12 kHz by applying appropriate locking schemes to the pump sources and employing an OFC for measuring the absolute frequencies. Comparing with the very latest high precision spectrometers based on DFG [8–10] and cw optical parametric oscillator (cw OPO) [17–19], the reported spectrometer has a very large tuning range, a decent linewidth and frequency accuracy but a mediocre sensitivity attributed to the lack of absorption enhancement implementation. The characteristics of these spectrometers are smmarised in Table 1. To demonstrate this novel source, we investigate the saturation spectra of 12C16O2 rovibrational transitions: 0001 ← 0000 R(18) and R(60) at 4.2 μm and [1001, 0201]I ← 0000 P(20) at 2.7 μm, and their absolute frequencies are measured with an accuracy better than 30 kHz.
2. Experimental setup
2.1. Difference frequency generation source
The schematic of our DFG source is illustrated in Fig. 1. The mid-IR DFG radiation is generated by simultaneously pumping a MgO:PPLN crystal with a pump laser and a signal laser. The pump beam is from a Ti:sapphire laser (MBR-110, Coherent) with power ∼ 1 W in the wavelength range 760–870 nm, and its linewidth is about 100 kHz in free run. The signal beam is either a 1064 nm “Master” Nd:YAG laser (Model 166, Lightwave Electronics), or “Slave” Nd:YAG laser (Mephisto 1200NE, InnoLight) offset-locked to the former one, depending on different experimental schemes and the linewidths are both about 5 kHz. To boost the power of the idler laser, i.e. the DFG radiation, the signal beam is amplified to 6 W by an Yb-fibre amplifier (AR-10K-1064-LP-SF, IPG Photonics). An optical isolator is attached to the fibre amplifier to prevent the potential damage caused by the optical feedback effect. The pump and signal laser beams are combined by a dichroic mirror (DM) and directed into an antireflection-coated MgO:PPLN crystal. In order to achieve good beam overlapping, these two laser beams are focused into the MgO:PPLN crystal with different lenses. The MgO:PPLN crystal (HC Photonics) is 50 mm long and 1 mm thick and hosts ten different gratings whose periods are from 21.25 to 23.5 μm with 0.25 μm increment. In order to satisfy QPM condition, we tune the crystal temperature, which can be set between 30 and 200 °C, to adapt to the selected PPLN period. A Brewster angle germanium plate is following to block the pump and the signal beams. The output power of the DFG source is initially larger than 15 mW at 3.6 μm as a fresh crystal is installed but deteriorates to about 5 mW after a period of three-month operation due to unknown mechanisms.
2.2. DFG frequency tuning
Two different DFG frequency tuning schemes have been implemented in the experiment. First scheme, called Ti:S tuning, is tuning the Ti:sapphire laser while master Nd:YAG laser frequency is locked to a specific hyperfine transition of 127I2 using wavelength modulation spectroscopy. Apparently, the DFG frequency of this scheme carries the same frequency modulation coming from the master Nd:YAG laser. Second scheme, called YAG tuning, is to tune the slave Nd:YAG laser respect to the master YAG laser via offset-locking while the Ti:sapphire laser is locked to a highly stable optical cavity by typical side-of-fringe locking scheme . The offset-locking is based on the frequency-dependent phase shift experienced by the beat frequency of the master and slave lasers when it propagates along a 10 m long coaxial cable . The beat frequency is precisely tuned by a RF frequency synthesizer and therefore the slave laser frequency as well. The DFG frequency are modulated by directly applying a sinusoidal signal on the piezo of slave Nd:YAG laser. Because of the exceptional linewidth performance of the slave Nd:YAG laser, the bandwidth of the loop filter is set to be pretty low, and the influences coming from master and slave Nd:YAG laser modulations on beat frequency locking are suppressed. The stable optical cavity is constructed by optical contacting the cavity mirrors directly onto a ultra-low thermal expansion Zerodue spacer. The thermal drift of the cavity is further suppressed by placing it into a vacuum chamber which is constantly pumped by an ion pump and enclosed in a temperature stabilised box. By beating against our OFC we found that the frequency jitter of the stabilised Ti:sapphire laser is around 10 kHz and the frequency drift is less than 100 kHz/hr. To avoid the Ti:sapphire frequency fluctuations due to the laser intensity variation, a differencing scheme is employed.
2.3. Absolute frequency measurement
To precisely determine the DFG frequency, we stabilise the master Nd:YAG laser frequency on the a10 component of R(56) 32–0 transition of 127I2 at 532 nm and use a homemade OFC to measure the frequency of Ti:sapphire laser. This specific optical transition has been recommended by the 2001 meeting of consultative committee for length (CCL) as one of the optical frequency standards . The recommended frequency of a frequency-doubled Nd:YAG laser stabilised with an external iodine cell having cold-finger temperature −15 °C is 563 260 223 513 kHz with a relative uncertainty of 8.9 × 10−12. The experimental setup of the master Nd:YAG laser is depicted in Fig. 2. The frequency of the master laser is doubled by a 50-mm-long MgO:PPLN crystal (HC Photonics) with a period of 6.5 μm. A dichroic beam splitter is used to separate the 532 nm beam and the 1064 nm beam. Doppler-free saturated-absorption spectroscopy is done in a 100-mm-long iodine cell whose pressure is around 4 Pa by maintaining the cold finger temperature at 0 °C. The retroreflected 532 nm beam is detected by a photodetector (2001-FS, New Focus). To obtain a proper error signal for frequency locking, we modulate the laser frequency and the third-harmonic demodulated signal of the saturation dip of the transition is obtained using a lock-in amplifier and used to stabilise the laser frequency. The resultant frequency stability is better than 5 kHz and the fundamental frequency of the master Nd:YAG laser is 281 630 111 749 kHz after correcting the iodine pressure shift .
Our OFC is based on a Kerr-lens mode-locked Ti:sapphire laser (Gigajet20, Gigaoptics). Its repetition frequency is about 1 GHz, and the output spectrum of the mode-locked laser is further stretched by coupling the light into a 1 m long photonic crystal fibre (NL-PM-750, NKT Photonics A/S) to generate a supercontinuum covering 500–1450 nm. The repetition frequency fr is detected by a fast silicon photodiode and phase-locked to an RF signal source by controlling the laser cavity length. The carrier envelope offset frequency δ is detected via the f-2f self-reference scheme [24, 25], and phase-locked to another RF signal source by controlling the intracavity optical intensity. To ensure the frequency accuracy of the OFC, all RF equipments including RF synthesizers and universal frequency counters are referenced to a GPS disciplined Rb frequency standard (PRS10, Stanford Research System). The overall accuracy of our OFC is better than 10−12 level for 1000 s measurement time. This OFC has been used on absolute frequency measurements of near infrared hyperfine transitions of I2, 2S–3S transitions of Li , and 6P–6D transitions of Tl .
3. High resolution spectroscopic measurements of 12C16O2
To demonstrate the application of our DFG source on high precision spectroscopy, we proceed the absolute frequency measurement on the 0001 ← 0000 fundamental band at 4.3 μm and [1001, 0201]I ← 0000 band at 2.7 μm of the common carbon dioxide isotopologue (12C16O2).
3.1. 0001 ← 0000 fundamental band at 4.3μm
The saturation spectroscopy of the 0001 ← 0000 fundamental band at 4.3 μm is observed by collimating the DFG beam into a 20-cm long flowing gas cell and being retroreflected by a spherical mirror. The available 4.3 μm DFG power is about 2 mW and the radius of the collimated beam is about 2 mm. The theoretical saturation intensity is estimated about 0.0026 mW/mm2. The reflected beam is partially picked up by a CaF2 plate and fed into a LN2-cooled InSb detector. To reduce the atmospheric absorption of the DFG beam, we enclose the experimental region with a plexiglas cover and continuously purge the region with dry nitrogen. The DFG frequency is tuned by the Ti:S tuning scheme and the 3rd harmonic demodulated saturation dip of CO2 spectral line is obtained using a lock-in amplifier. The observed spectrum 0001 ← 0000 R(18) line shown in Fig. 3 is acquired with modulation frequency 3 kHz and modulation depth 1.1 MHz on the master Nd:YAG laser. The signal-to-noise ratio (SNR) is above 1000 @ 1 Hz bandwidth for CO2 pressure ∼ 2 mTorr. Meanwhile, the FWHM of the transition 1.16 MHz is derived from the dependence of the peak amplitude of the third-derivative signal on the modulation width . To measure the absolute frequency of the transition centre, we lock the Ti:sapphire laser onto the CO2 transition centre and measure its frequency using the OFC. The transition frequency is determined as 70 834 903 061(12) kHz by the difference between the Ti:sapphire and the master Nd:YAG laser frequencies. Although the precision of the measurement is about 8 kHz, the accuracy of this measurement is limited to 12 kHz which includes the uncertainties of the OFC, offsets coming form frequency locking electronics. The self-induced pressure shift is negligible for current experimental condition.
Meanwhile, the absolute frequency of CO2 0001 ← 0000 R(60) line, whose absolute frequency 71 500 327 991.5(8) kHz was measured by D. Mazzotti et al. , is also measured to benchmark our system. Since the absorption of R(60) is ∼ 200 times weaker than R(18), we increase the CO2 pressure to 40 mTorr. For each Ti:sapphire laser frequency measurement we take 2000 counts with gate time 0.1 s. The absolute frequency of R(60) has been measured 22 times over 8 days and the standard deviation of each measurement is between 10 to 17 kHz. The chronological record of our data is shown in Fig. 4 and the mean value is 71 500 327 973.2(100) kHz. The 18 kHz discrepancy between the measurement and the previous result  might be attributed to the pressure shift.
3.2. [1001, 0201]I ← 0000 band at 2.7μm
The saturation spectroscopy and heterodyne frequency measurements of the [1001, 0201]I ← 0000 band were first performed by W. Urbans group . A tunable color centre laser with output power > 10 mW was used for the spectroscopy, and the frequency was measured by heterodyning the color centre laser with two frequency-stabilised CO lasers simultaneously. The absolute frequencies of nine transitions in [1001, 0201]I ← 0000 band were measured with accuracy better than 1.2 MHz (two standard deviations). The small transition dipole moment and high saturation intensity limited the signal-to-noise ratio, and hence the accuracy of frequency measurements.
Instead of the conventional saturation spectroscopy, the saturation dip is detected by saturated fluorescence spectroscopy which greatly increases the SNR for the advantages of zero background and the immunity of interference fringes. The same scheme had been used on the frequency stabilisation of a sequence-band CO2 laser  and the frequency measurement of CO2 laser , and more recently, on the frequency stabilisation of a PPLN optical parametric oscillator . The experimental setup is based on a longitudinal 4.3 μm fluorescence collecting cell (L-cell) which is illustrated in Fig. 5. The L-cell is basically a 20-cm long pyrex tube with gold-coated interior surface to collect the fluorescence. A CaF2 Brewster window is installed at one end and a long wavelength pass filter (LWPF), whose transmission and reflection are > 80% at 4.3 μm and > 98% at 2.7 μm respectively, is at the other end. The DFG light is focused into the L-cell by a CaF2 lens of focal length 300 mm and retroreflected by the LWPF. The available DFG power is about 5 mW and the radius of the focal spot is about 0.12 mm. The theoretical saturation intensity is estimated about 47 mW/mm2. The 4.3 μm fluorescence is detected by a LN2-cooled InSb detector behind LWPF. Contrary to 0001 ← 0000 fundamental band experiment, the YAG tuning scheme is used to observe the saturated spectrum. The second harmonic demodulated saturation spectrum of P(20) line shown in Fig. 6 is acquired by applying a modulation with frequency 400 Hz and depth 1.9 MHz on the slave Nd:YAG laser along with lock-in time constant 300 ms and CO2 pressure 20 mTorr. Meanwhile, the master Nd:YAG laser is modulated by a sinusoidal signal with frequency 30 kHz, depth 1.5 MHz to lock the laser frequency to the 127I2 hyperfine transition. The spectrum is fitted with a mathematical model  (solid curve), and the fitting residual is shown as well. The quality of the experimental data is better than  even though our DFG power is lower. A frequency accuracy of 30 kHz is assigned to this measurement which includes the estimated pressure shift, the uncertainties coming from frequency measurement, contributed by OFC, offset locking and fitting. The absolute frequency of P(20) transition is determined as 110 862 679 166(30) kHz which is 20 times more accurate than the previous result . The FWHM of P(20) transition 1.48 MHz is directly derived from fitting.
We successfully construct a mW-level MgO:PPLN-based DFG mid-IR source whose wavelength covers 2.66–4.77 μm, and demonstrate its applications in high precision molecular spectroscopy and absolute frequency measurement. The sub-Doppler spectroscopies 0001 ← 0000 at 4.3 μm and [1001, 0201]I ← 0000 at 2.7 μm of CO2 have been performed with conventional saturated absorption and saturated fluorescence spectroscopy respectively. Their absolute frequencies can be determined better than 30 kHz by employing OFC and are comparable or even superior than the previous experiments. We have extensively measured the CO2 transition frequencies of 0001 ← 0000 fundamental band at 4.3 μm and [1001, 0201]I ← 0000 band at 2.7 μm and the results will be published elsewhere.
The wavelength span of the DFG source is currently limited by the Ti:sapphire laser. By replacing the output coupler, it can easily cover 700–1000 nm, and then the wavelength of the DFG source can span over 2–16 μm. Different nonlinear optical crystals are definitely switched because of the optical transparency but the frequency accuracy is still the same. The frequency accuracy of the DFG source can be further improved by measuring the frequencies of Nd:YAG and Ti:sapphire lasers simultaneously.
This work is supported by the National Science Council of Taiwan, ROC under grant NSC 96-2112-M007-014-MY3.
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