Green-Yellow-Orange-Red Spectral Range with Sum-frequency Generation Using BIBO Crystal Pumped with an Optical Parametric Amplifier

: We show broadband sum-frequency generation (SFG) in the Green-Yellow-Orange-Red spectral range using bismuth triborate, BiB 3 O 6 crystal (BIBO) as nonlinear material. We perform a noncollinear phase-matching configuration within BIBO crystal using the remaining light behind the second harmonic generation stage and the infrared idler of an optical parametric amplifier (OPA). The obtained mixing radiation of ultrafast light sources to generate femtosecond pulses across 520.5 to 742.5 nm region is observed. SFG spectrum from single-pass cross-correlation intensity over such visible range is showed. The SFG wavelengths as a function of tunable wavelength idler OPA agrees with the expectations of parametric conversion condition and open the door to practical multi-beam or multi-color sum-frequency generators.


Introduction
The growing field of ultrafast light sources still requires multi-beam or multi-colour systems motivated by scientific, medical and industrial applications [1][2][3][4]. Recently, wavelength tunability and intense pulse are needed for industrial manufacturing [5,6], scientific research of medical device fabrication [7], spectroscopy techniques to analyze surfaces and interfaces [8], biological imaging with multiphoton microscopy methodologies [9], time-resolved experiments [10] and many other fundamental applications of modern science. Multi-beam and access to the new spectral region of femtosecond pulses are possible with parametric frequency conversion, ergo ultrafast optical parametric oscillator (OPO) [11], optical parametric amplifiers(OPA) [12], second and third harmonic generators [13][14][15] or, as we show here, sum-frequency generators.
Novel ultrafast systems constitute powerful devices for generation of widely tunable coherent radiation with the help of nonlinear materials. Progress of birefringent crystals as nonlinear material enables frequency conversion for the generation in the new spectral region [16]. A nonlinear optical material like bismuth triborate, BiB3O6 (BIBO) shows exceptional nonlinear coefficients (deff=3.2 pm/V) for wavelength generation between 290 and more than 2500 nm [17]. Such BIBO crystal has demonstrated the efficient tunable parametric generation of femtosecond pulses compared with other birefringent materials [18,19]. With a single BIBO crystal, femtosecond optical parametric oscillator covers visible range across 480-710 nm at room temperature operation [20]. Sum-frequency generation delivering visible pulses from 412 to 500nm with noncollinear optical parametric oscillator (NOPO) was shown as interesting configuration operated with an output coupler with simultaneous emission of infrared (IR) and visible (VIS) light [21]. Amplified robust systems, like noncollinear and collinear optical parametric amplifiers, are useful as light sources for ultrafast spectroscopy and many other fundamental and real-word applications in modern science [22]. Operation of noncollinear optical parametric amplifiers (NOPA) systems in continuous tuning between 550 and 700nm and near-infrared is demonstrated [23]. In the case of collinear optical parametric amplifiers (OPA) systems, see, e.g. a second harmonic generator (SHG) of 520nm from femtosecond pulses of 1040nm output of Ytterbium-doped Potassium-Gadolinium Tungstate (Yb:KGW) laser (Spirit-8-OPA, Spectra Physics), the SHG stage is used as a pump of such OPA to generates in the infrared range signal (630-1020 nm) and idler (1040-2600nm). It is known that OPA systems require nonlinear conversion accessories to cover in the visible region and the remaining 1040nm light behind the SHG stage can be still used.
In this Letter, we experimentally expose BIBO crystal to generate the sum of the optical frequencies under two synchronously beams. One beam from the remaining 1040nm of SpiritOne-8 Spectra Physics laser and second, from an idler optical parametric amplifier. Such beams were leading to a parametric output with higher frequency in the visible range. This procedure allows nonlinear mixing radiation of ultrafast light sources to generate high-repetition-rate femtosecond pulses across the 520.5 to 742.5 nm region with noncollinear configuration. It is known that access to higher frequencies is possible with harmonic generators. Indeed, the collinear optical parametric amplifier systems require nonlinear optical harmonics to cover UV-visible range [24]. The nonlinear frequency conversion presented here offers several advantages as it achieved with the remaining fundamental 1040nm pump light, allowing single-pass noncollinear phase-matching condition on a broad spectrum, while at the same time providing a viable tool of tunable coherent light.

Materials and Methods
We perform a tunable visible light with noncollinear phase-matching configuration [25][26][27] that allows parametric frequency conversion with the help of BIBO as nonlinear material. BIBO crystal was mounted on a rotation stage (RS) for changing the phase-matching angle. The two beams that are coming from different arms overlap with a fixed angle of 14 within a BIBO crystal. The remaining 1040nm and the IR idler beams are focused and crossed inside a BIBO crystal with orientation condition to the SFG with noncollinear phase-matching. In our case, both focused beam waist radiuses are close to 50 micrometres. Sum frequency generation of the pulses is achieved in a single pass of 500 m crystal. The crystal is a type I interaction in the yz plane (=90º) at an internal angle of 161º at normal incidence.
The set-up for the parametric frequency generation is shown in Figure 1. For our sum-frequency generation experiments, we used a SpiritOne-8 and SpiritOPA systems emitting a 20 kHz train of <400fs pulses.

Results
The present study demonstrates the functionality of BIBO crystal in combination with an optical parametric amplifier (OPA) to generate high-repetition-rate femtosecond pulses across the 520.5 to 742.5 nm region. IR idler source is a computer controller tunable range 1040-2600nm from the automated collinear SpiritOPA. Such IR idler pulse of SpiritOPA is tuned to pump simultaneously with the remaining 1040nm pulse. Both pulses overlap in the crystal with different focal lenses to ensure maximum output power. Light emerged at the focal plane as sum-frequency generation that it is produced in the middle of the two beams. This shows that SFG will be separated from the IR light beams because the proper phase-matching and the intensity generation is measured (Figure 2). The focal plane of this sum frequency generated radiation is imaged in turn by a focal lens onto the input plane of a detector (D). Figure 2 shows the sum frequency generation power measured at different wavelengths as a function of tunable idler OPA source, inset a) (wine line and intensity values), and the remaining 1040nm light, inset b) (red line and intensity values). The following expression gives typical SFG wavelength: where =1040nm and is the tunable IR idler. To acquire the power of maximum in a SFG using pulses with different values of , the crystal was rotated to change the internal angle of the crystal with the gyration of the crystal holder for noncollinear phase-matching. All the tunable internal angle range is from 120 to 152 . Typical sum generation presented in this work for reproducibility, a motorized rotation is used for controlled rotation of the crystal. A second motorized translation stage and a mounted retroreflector prism are performed for a controlled path length of a variable delay line. Such a pump pulse with a variable delay line is used for improving the temporal pulse overlap superimposed in the crystal because of group-velocity matching. Adjustment of the optical delay line is needed because changes in the angle of phase-matching produce small changes in the length of the crystal. Here, we were able to achieve in the wavelength range of 520.5 to 742.5nm of such SFG from single-pass of maximum cross-correlation intensity as shown in the inset c) of Figure 2 (orange line and intensity values).
The normalized spectrum of SFG from single-pass cross-correlation intensity over such a visible range is showed in Figure 3. In this measurement, light emerged as a sum-frequency generation is coupled to a high-resolution spectrometer (HR4000) with an optical diffusor. As typical apparatus commonly used to measure the pulse duration, our generated light can be examined with the help of our adjustable optical delay. The average pulse-width of the idler OPA light beams were 330-fs measured with such 0.5 mm BIBO crystal, data not shown.

Discussion
The nonlinear frequency conversion as sum-frequency generator presented here offers several advantages as it achieved with the remaining fundamental 1040nm pump light, allowing single-pass noncollinear phase-matching condition on a broad spectrum, while at the same time providing a viable tool of tunable coherent light. The obtained mixing radiation of ultrafast light sources to generate femtosecond pulses across 520.5 to 742.5 nm region is observed from Figure 2 with the help of BIBO as nonlinear material. The frequency conversion with noncollinear phase-matching an effective scheme that provide tuning coverage across the full transparency of the BIBO crystal and the corresponding magnitude of the significant nonlinear coefficient as a function of pumped wavelengths [28]. Green-Yellow-Orange-Red spectral range is generated and a tunable wavelength system may be implemented as a conversion accessory of optical parametric amplifier. It can be observed from Figure 3 that the changes in the bandwidth of such spectral range may be caused by a length detuning of the optimal phase matching of angle crystal and adjustment of path length to improve the sum-frequency generation of such overlap. Our nonlinear optical results open the door to high-resolution nonlinear spectro/microscopy applications, as it offers the possibility of selectively wavelength in a single-pass noncollinear configuration.

Conclusions
In conclusion, we have shown broadband sum-frequency generator using the remaining fundamental 1040nm light of a femtosecond laser source and IR idler from the optical parametric amplifier. Sum frequency generation into visible range is possible with noncollinear phase-matching in BIBO crystal. The obtained mixing radiation of ultrafast light sources to generate high-repetitionrate femtosecond pulses. We provide wide tuning across Green-Yellow-Orange-Red spectral range. The direct separation of three interacting waves is demonstrated with a simple cross-correlation configuration. Such configuration might be useful to perform practical multi-beam or multi-color experiments to scientific, medical and industrial applications.
Author Contributions: All authors contributed equally to this work.

Funding:
The work presented here has been supported by the program CATEDRAS-CONACYT grant number 1039. RQT thanks financial support by the program PAPIME grant number PE101319.