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Lasers & Sources

BiB3O6 offers competitive edge over recognized rivals

The nonlinear optical material, BiB3O6, represents a promising new alternative for wavelength conversion in ultrafast laser sources.
11 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200705.0662

A wide range of applications, including spectroscopy, photochemistry, photomedicine, biophotonics, and nanotechnology, all depend on the ability to generate coherent optical radiation in different spectral regions, from the ultraviolet (UV) to the mid-infrared (mid-IR), and in different temporal domains ranging from ultrafast femtosecond to steady-state continuous-wave (cw) operation. However, conventional lasers can only provide radiation in discrete spectral and temporal regions, determined by the properties of the specific laser gain material. Suitable nonlinear crystals can be used to overcome such limitations since they allow wavelength conversion in existing sources to new spectral and temporal domains. They accordingly represent the key element in any wavelength conversion process, but must fulfil a variety of important requirements and offer a wide range of desirable linear and nonlinear optical characteristics.

Over the past decade, several nonlinear materials have been successfully developed and exploited in variety of wavelength conversion devices and applications. For visible and UV generation, β-BaB2O4 (BBO) and LiB3O5 (LBO) have recently emerged as choice materials. This is because they exhibit deep UV transparency well below 400nm, and high tolerance to optical damage, while being available at low cost. However, their relatively low nonlinear optical gain coefficients (deff∼ 1–2pm/V) have restricted their use mainly to operating regimes involving high pulse intensities or limited crystal geometries.

As for efficient wavelength conversion of cw or low-intensity (high-repetition-rate) pulsed lasers, the nonlinear material requirements become even more stringent. The low intensities available require higher material gain, longer interaction lengths and tighter beam focusing to compensate for the significantly reduced amplification. These constraints also translate in additional requirements from other parameters such as tolerance to laser beam quality degradation, focusing, and double refraction. In this context, bismuth triborate (BiB3O6 or BIBO),1–3 offers major advantages over BBO or LBO, owing to substantially higher nonlinear gain coefficients (deff∼ 3–4pm/V), flexible propagation geometries, small double-refraction and broad wavelength coverage. It is chemically and thermally stable, allows room temperature operation, and is also available at low cost.

To demonstrate the potential of BIBO, we have developed a number of ultrafast wavelength conversion sources for the visible and UV in the low-intensity, high-repetition-rate regimes. Using optical second harmonic generation (SHG), we have achieved efficient wavelength conversion of femtosecond and picosecond Ti:sapphire lasing into the near-UV and blue over the 375-450nm spectral range. Our design provides average optical powers of up to 1W at conversion efficiencies exceeding 50% in a simple single-pass geometry.3,4 Direct experimental comparison of BIBO with BBO in this regime has also confirmed the superior performance of the former, with power enhancements exceeding 30%, despite its shorter crystal length. Figure 1 plots the output power performance and conversion efficiency of the SHG system in picosecond operation and Figure 2 shows a photograph of the blue output beam generated by the device.

Figure 1. Second harmonic average power and conversion efficiency at 410nm versus input Ti:sapphire laser power.

Figure 2. Photograph of the blue output beam generated by second harmonic generation in a Ti:sapphire laser with BiB3O6.

In a further demonstration of the unique promise of BIBO, we have also used it to develop, for the first time, a new wavelength conversion source that can provide high-repetition-rate femtosecond pulses across the entire visible spectrum, as well as the UV.5,6 This uniquely versatile source is a synchronously-pumped optical parametric oscillator (SPOPO) that utilizes the SHG output of a femtosecond Ti:sapphire laser in the blue, also BIBO-generated, as the pump beam (see Figure 2). Subsequent wavelength conversion of the blue beam in the SPOPO using a single BIBO crystal and a single set of mirrors results in wide and continuous wavelength coverage across the entire green-yellow-orange-red region (480–710nm) at average powers of 270mW in 120fs pulses with a a repetition rate of 76MHz. Photographs of the tunable SPOPO output beams generated across the visible spectrum are shown in Figure 3. Moreover, by internal SHG of the visible signal pulses, we have also successfully generated femtosecond pulses in the 240–355nm range at practical average powers of up to 100mW. Combining the Ti:sapphire laser with the BIBO SHG and SPOPO system thus provides a uniquely versatile source of femtosecond pulses across the entire spectral range from 240nm in the UV to 1000nm in the near-IR.

Figure 3. Photographs of the generated output beam from the BiB3O6 synchronously-pumped optical parametric oscillator, covering the entire visible spectrum from the green, through yellow and orange, to the red.

Other recent experiments have also successfully demonstrated the potential of BIBO for the generation of high-energy femtosecond and picosecond pulses in the UV, visible, and near-IR, again confirming its superior performance over BBO and LBO.7–9 These results clearly demonstrate the potential of BIBO as a very attractive nonlinear material for wavelength conversion in both low and high-intensity operating regimes.

Majid Ebrahim-Zadeh
Institute of Photonics Sciences (ICFO)
Barcelona, Spain
Institucio Catalana de Recerca I Estudis Avancats (ICREA)
Barcelona, Spain