Continuous-wave, multimilliwatt, mid-infrared source tunable across 64–75  μm based on orientation-patterned GaAs

Abstract

We report a continuous-wave (cw) source of tunable mid-infrared radiation providing tens of milliwatt of output power in the 6460-7517 nm spectral range. The source is based on difference-frequency generation (DFG) in orientation-patterned (OP)-GaAs pumped by a Tm-fiber laser at 2010 nm and a 1064 nm-Yb-fiber-pumped cw optical parametric oscillator. Using a 25.7-mm-long OP-GaAs crystal, we have generated up to 51.1 mW of output power at 6790 nm, with >40 mW and >20 mW across 32% and 80% of the mid-infrared tuning range, respectively, which is to the best of our knowledge the highest tunable cw power generated in OP-GaAs in this spectral range. The DFG output at maximum power exhibits passive power stability better than 2.3% rms over more than 1 h and a frequency stability of 1.8 GHz over more than 1 min, in high spatial beam quality. The system and crystal performance at high pump powers have been studied. , with wavelengths up to 5 μm also obtained at the mW power level [7]. However, the intrinsic onset of absorption in MgO:PPLN and other oxide-based ferroelectric materials beyond ∼4 μm presents a fundamental barrier to wavelength generation at practical powers further into the mid-IR. As such, the search for alternative nonlinear materials with high transmission above ∼4 μm and effective techniques for spectral expansion into the longer mid-IR wavelengths must be explored. In this context, orientation-patterned (OP)-GaAs is a highly attractive nonlinear crystal. Its high nonlinear coefficient d eff d 14 ∼ 94 pm∕V [8], wide transparency across 0.9-17 μm, high thermal conductivity (46 W/mK), and high damage threshold [9] make it a primary candidate for deep mid-IR generation. Its nonlinear figure of merit, F d 2 eff ∕n 3 (n is the refractive index), is nearly ten times that of MgO:PPLN. In an early report, a pulsed OPO based on OP-GaAs, tunable in the 5.7-9.1 μm range was demonstrated, using a PPLN OPO pumped by a Q-switched Nd:YAG laser at 1064 nm [10]. Recently, a pulsed OP-GaAs OPO tunable over 8.8-11.5 μm was reported using direct pumping with a Q-switched Tm, Ho:YLF laser [11]. However, the development of cw OPOs at wavelengths >4 μm still remains a major challenge, in a large part due to the high threshold sensitivity to cavity loss in practical singly resonant configuration, combined with the difficulty in achieving relatively complex mirror and crystal coatings of high quality and low loss in the deep mid-IR spectral range to achieve oscillation threshold. In the meantime, one can circumvent this obstacle by deploying the alternative technique of DFG, which is single-pass and does not require the attainment of an oscillation threshold. The technique can provide an effective and robust approach for the generation of nonnegligible output powers at longer wavelengths [2]. In the past few years, DFG based on OP-GaAs, with cw output powers up to several μW has been demonstrated [12]. A few years ago, a cw source tunable across 7.6-8.2 μm with sub-mW output power was reported by using DFG between an Er-doped f...

Frequency down-conversion based on optical cascading process—New effective way for generation of far infrared or THz radiation

“…, we obtain the sets of Eq ( 5) and (6). In turn, the expansion series (30) transforms into the form (4).…”

“…The MgO:LiNbO 3 crystal is used in [5] to obtain femtosecond pulse tunable in the range 3.2 −4.8μm of frequencies with maximal average power of 1.1mW using nonlinear mixing of the pulse 170mW, 65fs at a fixed wavelength of 1.58μm (pump) with the pulse 11.5mW, 40fs tunable in the near-infrared range of frequencies: 1.05 and 1.18μm. In [6], the DFG in GaAs crystal was realized, and as a result, a tunable source of M-IR CW radiation was obtained with maximal power of 51 mW on the frequency λ 1 = 6543 nm. The input power of the pulses was 40 mW at the frequency λ 3 = 2010 nm and at the frequency λ 2 = 2900 nm.…”

Performance characterization of mid-infrared difference-frequency-generation in orientation-patterned gallium phosphide

“…As a result, access to the deep-IR spectral range has been most effectively achieved by exploiting nonlinear frequency conversion techniques based on non-oxide-based nonlinear crystals of oreintation-patterened GaAs (OP-GaAs) or ZnGeP 2 (ZGP) offering a longwavelength absorption edge well beyond 4 μm. On the other hand, the short-wavelength absorption edge in OP-GaAs and ZGP at ~2 μm precludes the direct deployment of wellestablished solid-state or Yb-fiber lasers at ~1.06 μm, thus necessitating the use of cascaded OPO pumping [8], direct OPO pumping using Tm/Ho-doped solid-state lasers [9], or difference-frequency-generation (DFG) between Tm-fiber laser and OPO [10]. The choice of suitable materials offering the required linear and nonlinear optical properties, in addition to the short-wavelength transparency cut-off, are clearly key factors for practical and efficient frequency conversion.…”

Nanosecond difference-frequency generation in orientation-patterned gallium phosphide

“…The ability to tune one of the input beams in the DFG process results in tunable output at longer wavelengths. Such DFG sources based on near-IR nonlinear materials such as -BaB2O4 (BBO) [5], KTiOPO4 (KTP) [6] and MgO-doped periodically poled LiNbO3 (MgO:PPLN) [7], and mid-IR nonlinear materials such as ZnGeP2 (ZGP) [8] and orientation-patterned GaAs (OPGaAs) [9] have been previously demonstrated. Although ZGP and OP-GaAs can provide spectral coverage deep into the mid-IR, they require pumping beyond 2 μm due to two-photon absorption at short-wavelength transparency cut-off [10].…”