An oscillation threshold of 780mW has been demonstrated in a singly-resonant, continuous-wave optical parametric oscillator (CW SRO) using a fiber-amplified, distributed feedback (DFB) fiber laser as pump source. A linewidth of 1MHz was measured, and the idler frequency was fine-tuned by up to 130GHz by tuning the pump laser. To our knowledge, this is the first example of a single frequency CW SRO pumped by an all-fiber pump source, a reduction in threshold by a factor of three over previous 1μm-pumped CW SROs, and a reduction by two orders of magnitude in the linewidth of CW SROs pumped by fiber pump sources.
© 2006 Optical Society of America
Singly-resonant continuous-wave optical parametric oscillators (SROs) pumped by continuously tunable, single-frequency pump sources have been shown [Ref. 1] to provide the operating characteristics required to perform high resolution spectroscopy in the important infrared spectral region above 2μm wavelength. In contrast to other types of CW OPO, the singly resonant device allows straightforward, monotonic coarse frequency tuning of the signal and idler wavelengths. For continuous fine tuning of the OPO output frequencies, it is required to implement a means of mode-hop-free scanning. This has been demonstrated using scanning of a tilted etalon synchronized with servo control of cavity length [Ref. 2]. However continuous tuning has been implemented much more simply and robustly by using a continuously tunable pump source to scan the idler frequency while operating the OPO with a fixed signal frequency. This was first demonstrated using a high power diode laser in a master-oscillator power-amplifier (MOPA) configuration as a pump source . The power available from this source was barely enough to exceed oscillation threshold in the singly resonant CW OPO. The capabilities of the tunable pump / SRO combination have recently been extended by taking advantage of the higher power available from diode-pumped solid-state lasers and fiber lasers. From 2000 onwards there have been enormous advances in the capabilities of fiber lasers and amplifiers in general, and in particular in the development of single frequency fiber sources. Single frequency power levels into the hundreds of watts with diffraction-limited beam quality have been demonstrated [Ref. 3]. As with the diode-laser pump source, the best combination of spectral and power characteristics is again provided in a MOPA configuration. However, in fibers, this can be implemented as an all-in-fiber source, with no free space optical beam alignment required. There is no possibility for gradual cavity misalignment as is the case with standard diode-pumped solid-state lasers. Continuous wave SROs have been pumped by fiber-based sources [Ref. 4] with impressive wide and rapid tuning characteristics. However, to date the narrowest linewidth demonstrated in such fiber-pumped OPOs has been above 100MHz. Although this is adequate for spectroscopy at atmospheric pressure, many applications require a higher level of spectral resolution of below 10MHz. We have previously demonstrated single frequency operation of a CW doubly resonant OPO (DRO) using a distributed feedback fiber laser, generating stable single frequency output [Ref. 5]. In this paper, we extend this work, reporting the operation of a CW SRO pumped by an all-fiber pump source, with a measurement-resolution-limited OPO idler linewidth of 1MHz. Using a nonlinear crystal of length 80mm (we believe this to be the longest interaction length of a quasi-phase-matched nonlinear material used to date) we demonstrated a CW SRO oscillation threshold of 780mW, which is by a factor of two the lowest threshold yet measured in any CW SRO pumped by a tunable laser, and a factor of three lower than any previous 1μm-pumped CW SRO.
2. Experimental configuration
The experimental configuration of the OPO and its pump source is shown schematically in Fig. 1. The seed source for the MOPA was a commercially available DFB fiber laser operating with 20mW single frequency, linearly polarized output at 1083nm. The measured linewidth of this source was 50kHz. Slow tuning of this seed could be performed by varying the fiber temperature between room temperature and 50°C, or rapid tuning by applying a voltage to a piezoelectric transducer attached to the fiber. Continuous mode-hop-free tuning of the seed wavelength of around 0.25nm was available by each of these mechanisms. The output fiber from this laser was connectorized into a commercial polarization-maintaining fiber amplifier. With 10mW seed power, up to 3.5 Watts of single frequency, single transverse mode output in a linear polarization was available to pump the OPO. The fiber amplifier did not suffer from the amplitude instabilities reported in Ref.  which result from stimulated brillouin scattering (SBS). As a result, stable output was obtained without the necessity to deliberately broaden the linewidth of the seed. The output from the amplifier was collimated by an aspheric lens, passed through a bulk optical isolator, and then focused into the OPO.
The OPO was configured similarly to other CW SROs [Refs. 1, 2, 4, 6], using a four-mirror ring cavity, consisting of two flat mirrors and two plano-concave curved mirrors as shown in Fig. 1. The cavity was resonant at only the signal wavelength. The nonlinear material used was 5% Magnesium-Oxide doped periodically-poled Lithium Niobate (MgO:PPLN). The crystal used had dimensions 80mm × 7mm × 1mm. The crystal was poled with periods between 31.3 and 32.5μm. Crystals with discrete poling period regions or in a “fan-out design” [Ref. 6] with poling period linearly varying across the width of the crystal were used. The cavity was designed for a focusing parameter [Ref. 7] ξ ~ 1. The front surfaces of the cavity mirrors were highly reflecting (~99.9%) at the signal wavelength and highly transmissive (~99%) at both pump and idler wavelengths. Angle of incidence on the mirrors was 7°. Mirror substrates were undoped YAG and the curved mirrors had radii of curvature 150 mm. The curved mirrors were separated by 250 mm and the flat mirrors by 150 mm. The MgO:PPLN crystals were anti-reflection coated for all three wavelengths. The nonlinear crystal was mounted in a temperature controlled oven for coarse tuning capability. The pump beam was focused to a 1/e2 waist radius of 80μm, positioned at the center of the nonlinear crystal, to be mode-matched with the OPO cavity.
3. Performance characteristics
The OPO was typically operated with the MgO:PPLN crystal temperature-controlled just above room temperature at 30°C. Tuning to a specific wavelength was performed by translating the crystal laterally relative to the pump beam, so that the beam passed through a region of appropriate poling period for the desired phase-matching wavelength. Under optimum alignment conditions, oscillation thresholds as low as 780mW were measured at 2.8μm.
The maximum pump depletion was 85% at an input power of 2.8 Watts (Fig. 2). At this input level 750mW idler power was measured. We were able to exceed oscillation threshold between idler wavelengths 2650nm and 3200nm. The beam quality of the idler output was measured and found to be near-diffraction-limited at a power output level of 500 mW. Using a scanning knife-edge instrument we were able to measure an M2 parameter of 1.04.
4. Spectrum and fine tuning
Spectral measurements of pump and signal wavelengths were made using a scanning Fabry-Perot interferometer. With this device, we confirmed single longitudinal mode oscillation of the pump and signal wavelengths. The pump was observed to tune entirely without mode-hops using both the available tuning mechanisms. It was observed that with or without an intracavity etalon, the OPO signal would operate on a fixed single longitudinal mode. Even when the pump was scanned over a frequency range greater than the free spectral range of the OPO (and up to its maximum PZT-tuning range of 60GHz), the signal did not mode-hop unless the cavity was perturbed. Based on the single frequency operation of the pump and signal waves, we know that by energy conservation, the idler wave must possess a similarly narrow spectrum. As expected, when the pump frequency was varied, the change in photon energy was directly transferred to the frequency of the idler wave. While monitoring the signal spectrum on the Fabry-Perot, we recorded the idler wavelength (Fig. 3) using a wavemeter as a function of the pump tuning parameter. Over a range of 60GHz tuning of the pump frequency (90V applied to the fiber PZT), the change in idler frequency exactly mirrored the tuning of the pump, and no signal mode-hops were observed. Tuning was continued using the temperature control of the DFB fiber laser. This method of tuning causes more perturbation in the fiber laser amplitude than PZT tuning, and this perturbation was sufficient to cause an occasional mode-hop of the signal wavelength to a near-neighbor longitudinal mode. However, no jumps to adjacent orders of the intracavity etalon were observed, and the overall idler tuning observed by pump tuning was equivalent to the maximum pump tuning available. We believe that the tuning capability provided by pump PZT tuning could be significantly extended using a PZT with greater range of strain capability. PZT tuning of similar DFB fiber lasers of several nanometers have been demonstrated by the manufacturers.
We made a direct measurement of the idler spectrum at a wavelength of 3.17μm. This was performed by coupling the OPO idler beam into a static Fabry-Perot cavity consisting of two highly reflecting mirrors designed for cavity ringdown spectroscopy. By linearly scanning the laser frequency using the pump PZT tuning, we were able to monitor the transmission peaks of the Fabry-Perot cavity over multiple 1GHz free spectral ranges. Based on the ratio of the time taken to scan between two transmission orders and the time to scan through an individual peak, we estimated a resolution-limited linewidth of 1.1MHz for the idler (Fig. 4).
5. Spectroscopic measurements
Based on the available OPO idler coarse tuning from 2650 nm to 3200 nm, and the rapid PZT tuning, we performed high resolution spectroscopic measurements of a variety of gases. We measured single pass transmission of the idler wavelength while tuning the pump laser at a rate of 30Hz using a sawtooth waveform applied to the PZT. Continuous mode-hop-free scans of up to 60GHz were recorded in water vapor at 2709 nm, carbon dioxide at 2810 nm, nitrous oxide at 2879 nm, ammonia at 2897 nm and methane at 3167 nm.
We have included two spectra obtained at opposite ends of the tuning range of the OPO. Figure 5 shows a typical recorded spectrum for carbon dioxide measured at 2810nm. A theoretical fit made using HITRAN 2004 data shows strong agreement with the recorded data. By simply translating the MgO:PPLN crystal approximately 3mm we were able to tune the OPO to an operating point at the other end of its range and obtain a similar quality spectrum of methane (Fig. 6).
In summary, we have demonstrated the first single frequency, CW singly resonant OPO pumped by an all-fiber pump source. The device shows greatly improved performance over the previous state-of-the-art in terms of its low 780mW threshold and its 1MHz idler linewidth. Based on the ability to straightforwardly tune anywhere between 2650nm and 3200nm, and the capability for rapid (kHz rate) mode-hop-free fine-tuning over a range of 60GHz, the device provides a new capability as a spectroscopic source for measurements of a variety of molecular species in a wavelength region where many species possess very strong absorption features. With the rapid improvements in the capabilities of single frequency fiber sources, we expect that high-power frequency-converted fiber sources will become an important technology for spectroscopy and sensing in the mid-infrared region.
The support of the Air Force Research laboratories and the National Institute of Standards under SBIR awards (contract nos. F2960103-C-0191 and SB134104C0023 respectively) is gratefully acknowledged. The assistance of professor J. Houston Miller of the department of chemistry at George Washington University in Washington D.C. is also acknowledged in providing theoretical fits to spectral data.
References and links
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