1. Introduction

Bismuth triborate, BiB₃O₆ (BIBO), is an interesting and relatively new nonlinear optical crystal belonging to the borate family. Its main advantage is the exceptionally high second order nonlinear susceptibility [1] which is associated with the contribution of the BiO₄ anionic group [2]. The effective nonlinearity of BIBO can be larger than that of KTP and the lower transparency edge extends deeper into the UV (286 nm) [3]. High nonlinear coefficients are an important prerequisite for realization of down-converting optical parametric devices in which the required pump intensities to achieve the threshold are often so high that damage problems occur (optical parametric oscillators, OPOs, travelling-wave type optical parametric generators, OPGs, and optical parametric amplifiers, OPAs) or in which the parametric gain, when using unamplified pump sources, is extremely low, as is the case in, for example, synchronously pumped OPOs (SPOPOs). There exist only very few demonstrations of the down-conversion potential of BIBO. The first report [4] on BIBO-based nanosecond OPO operation with pumping at 532 nm and the subsequent extension of this work with pump sources at different repetition rates [5] revealed greater potential of BIBO in comparison to KTP, BBO, and LBO. OPG/OPA operation of BIBO at 10 Hz in the picosecond regime was demonstrated only very recently, with 35 ps pump pulses also at 532 nm [6]. BIBO was implemented also in a SPOPO operating at 76 MHz although in this case the purpose was to produce femtosecond pulses in the visible and the pumping was at 400 nm [7].

All previous down-conversion experiments confirmed the good damage resistivity of BIBO which is related to its band-gap. In addition, the fact that the band-gap is below 300 nm makes this crystal very suitable for pumping with short pulses in the 800 nm spectral range, using Ti:sapphire laser systems, because two-photon absorption can be avoided. In this work we investigate the potential of BIBO as an OPA for such down-conversion from the 800 nm spectral range into the near-IR, by using amplified femtosecond pulses at 1 kHz as a pump.

2. Phase-matching properties of BIBO for OPA

Although initially several different set-up configurations and also different crystals were tested, the material now in use for near-IR OPGs/OPAs, and in particular in commercial devices, with 800 nm pumping, is type-II BBO [8]. This is due to several important advantages the most important of which is the possibility to tune, even close to degeneracy, with almost constant signal and idler bandwidth. The bandwidth is determined basically by the group velocity mismatch (GVM) between the signal and idler pulses. As a monoclinic crystal with point group 2, BIBO is optically biaxial which offers a greater variety of phase-matching configurations. Nevertheless, for the same reason, it is preferable to use type-II interaction and for a pump wavelength of 800 nm this is possible only in the x-z principal plane where BIBO can be regarded as optically positive (oe-o or eo-o interaction).⁹ For these processes the effective nonlinearity, assuming a signal wavelength of 1400 nm, is d_eff=d₁₂cosθ≈2.4 and ≈2.2 pm/V, respectively [9], see Fig. 1(a). This means that the nonlinear figure of merit, which involves also the index of refraction, of BIBO is roughly two times higher than that of BBO.

As can be seen from Fig. 1(a), the tunability for oe-o interaction is achieved with much smaller angle variation than for eo-o interaction. In addition, the spectral bandwidth defined as the absolute value of the GVM parameter, 1/v_S-1/v_I, is larger and less dependent on the wavelength in the former case. Thus, type oe-o interaction is preferable for BIBO exactly as in the optically negative BBO where eo-e interaction possesses such properties. These two cases are compared in Fig. 1(b). It can be seen that BIBO, similar to BBO, exhibits for a pump wavelength of 800 nm the property that the signal and idler waves travel in opposite directions relative to the pump which ensures exponential growth of the parametric gain even beyond the pulse walk-off length [10]. However, as can be expected from the smaller band-gap, the GVM of BIBO is larger, i.e. the spectral acceptance per unit length is smaller. Thus shorter crystals should be used in the femtosecond regime which compensates for the larger effective nonlinearity. Nevertheless, the use of shorter crystals helps to avoid undesirable higher order nonlinear processes and to better utilize the transparency window of a given crystal. Since in the case of down-conversion the limiting factor is the mid-IR cut-off edge it can be expected that shorter crystals of BIBO will permit wider tunability. Indeed, idler absorption limits the tunability with BBO to less than 3 μm, e.g. max. 2.7 μm in Ref. [11], or even less (2.5 μm) in commercial devices. On the other hand the 3 μm spectral range is very important for molecular spectroscopy. The transmission problem can be circumvented by employing other non-borate crystals such as KTP in a second stage but it is difficult to produce short pulses with them [12].

 

Fig. 1. Angle tunability and effective nonlinearity d_eff of BIBO in the x-z optical plane (oe-o and eo-o interaction) for a pump wavelength of 800 nm (a), and comparison of the GVM parameters 1/v_P-1/v_I and 1/v_S-1/v_I for BIBO (oe-o) and BBO (eo-e), where v_P, v_S, and v_I denote the pump, signal, and idler group velocities.

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For the present experiment, two uncoated samples of BIBO were available, both cut at θ=42° in the x-z optical plane, with an aperture of 7×7 mm² and thicknesses of 3 and 5 mm. The transparency measured for polarization parallel to the y principal optical axis (o-polarization with respect to the x-z plane) is shown in Fig. 2. If compared to the first measurements in Ref.3, it can be seen that the substructure below 2.5 μm is in fact absent in the present crystals. The spectra, in particular the long-wave edge, are very similar to those measured in Ref. [13]. The transparency is indeed better for the thinner crystal. Comparing with BBO [14], one can conclude that although the absolute upper limits for the transmission are similar, for the same thickness, BBO exhibits an absorption feature below 2.5 μm.

 

Fig. 2. Transparency of the used BIBO samples for polarization parallel to the y principal optical axis corresponding to the pump and idler waves.

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3. Experimental results

We compared BIBO and BBO in a modified set-up of a commercial double-pass OPA seeded by white light continuum (Clark-MXR), shown in Fig. 3. The continuum is generated in a 2-mm thick sapphire plate which is simultaneously used to adjust the proper polarization. A polarizer between the two passes through the nonlinear crystal serves to select the signal wavelength for seeding the second pass. The pump energy used to pump the second pass was 300 μJ. Finally, the signal and idler pulses are coupled out using a mirror slightly displaced in the uncritical direction.

 

Fig. 3. Experimental set-up of the seeded OPA: L, lenses, T, telescopes, BS, beam splitters, DM, dichroic mirrors, CM, curved mirror with 30 cm radius of curvature, WLG, white light continuum generation, OPA, optical parametric amplifier.

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Using a standard BBO crystal which was 5 mm thick but antireflection-coated, the tunability extended from 1180 to 2500 nm with a maximum output level of 80 μJ (signal+idler) for a pump energy of 375 μJ. Such overall conversion efficiencies (of the order of 20%) are typical for tunable femtosecond OPGs/OPAs based on BBO type-II crystals [11]. The 5-mm long BIBO crystal provided more output energy than the 3-mm long BIBO crystal, but the temporal and spectral characteristics were not satisfactory: depending on the wavelength, the signal and idler pulses were either structured or longer than 200 fs as a result of the pulse splitting and reconversion. Thus it can be concluded that the larger GVM in BIBO, see Fig. 1(b), requires the use of shorter crystals than in the case of BBO.

 

Fig. 4. Pulse energy (full squares) and pulse duration (FWHM) assuming Gaussian pulse shapes (full circles) for the BIBO based OPA using a 3-mm thick sample. The open triangles show the energy obtained with the standard antireflection-coated 5-mm thick BBO crystal.

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With the 3-mm thick BIBO crystal, we were able to obtain sub-200 fs pulse duration throughout the whole tuning range (Fig. 4). The pulse duration was estimated by fitting cross correlation traces obtained by sum-frequency generation with a reference pulse at 800 nm using a 0.7 mm thick type-I BBO crystal. Gaussian pulse shapes were assumed and the time-bandwidth product was estimated by measuring the pulse spectra with a multichannel analyzer equipped with a 128-element pyroelectric array. In principle the range of the pulse durations obtained with the 3-mm thick BIBO was similar to the results with the standard 5-mm thick BBO crystal, but the dependence on the wavelength was different. The shorter signal and idler pulse durations obtained in the limits of the tunability range of BIBO (Fig. 4) can be explained by the increasing acceptance bandwidth, see Fig. 1(b).

 

Fig. 5. Cross correlation trace of the idler pulses at 3050 nm obtained by sum-frequency generation with a reference pulse at 800 nm (black symbols) and a Gaussian fit (red line). The inset shows the corresponding idler spectrum.

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The main advantage of BIBO seems the possibility to have somewhat larger tuning range extending to slightly above 3 μm, while under the same conditions the tuning with BBO had an upper limit of ≈2.5 μm, Fig. 4. Although the maximum energy level obtained with BIBO was of the same order of magnitude (80 μJ for signal+idler), the internal conversion efficiency was obviously higher (more than 30% in the maximum for the second pass) because this sample was uncoated.

Figure 5 shows a cross-correlation function of the idler pulses at 3050 nm (black symbols) from the FWHM of which (136 fs) a pulse duration of 110 fs is obtained by a fitting procedure (red curve). The time-bandwidth product is 0.55, somewhat above the Fourier limit of 0.44 for Gaussian pulse shapes (it is ≈0.5 for the pump pulses). The pulses at the signal wave (1085 nm) are slightly shorter having a FWHM of 100 fs.

4. Summary and Conclusion

In summary, we have implemented for the first time to our knowledge the BIBO crystal in a 800 nm pumped femtosecond optical parametric amplifier and demonstrated efficient and tunable operation with certain advantages (extension to the 3 μm spectral range) in comparison to BBO. The shortest pulses obtained were of the order of 100–110 fs. Another practical advantage in comparison to BBO, which is at present the standard material for this application, is the non-hygroscopicity of BIBO. Similar results with type-II BIBO pumped near 800 nm can be expected also in the picosecond regime. On the other hand, preliminary analysis indicates that type-I interaction in BIBO can be quite useful for generation and amplification of broad femtosecond continua. Work is in progress to investigate other phase-matching configurations with BIBO for this application and to scale the output energy by using large aperture crystals.