Optical Parametric Oscillators

product focus

Optical parametric oscillators Optical parametric oscillators simultaneously generate two beams of coherent light that are widely tunable in wavelength. This flexibility makes them a popular tool in various areas of scientific research, reports Neil Savage.

124

EKSPLA

O

ptical parametric oscillators (OPOs) are convenient sources of coherent light that suit numerous scientific applications, including various types of spectroscopy, multiphoton excitation, and light detection and ranging. In contrast to a laser, which relies on stimulated emission from a gain medium, OPOs use a nonlinear crystal to convert a pump beam into an idler beam and a signal beam. By placing the nonlinear crystal in a cavity, an oscillator is formed and operation commences when the pump power is above a certain threshold value. A unique feature of OPOs is that the output wavelengths of the idler and signal beams can be tuned by changing the phasematching properties of the nonlinear crystal. This is often accomplished by altering the temperature or orientation of the crystal, or by using a crystal that contains a grating. An important benefit of OPOs is that their wide wavelength tunability allows them to produce wavelengths that are inaccessible to conventional lasers. Emission can typically be tuned over hundreds of nanometres, giving researchers selectable access to all wavelengths spanning from the ultraviolet to the near-infrared — and all from a single source. The practicality and ease of tuning of OPOs took a big step forward when the industry started using periodically poled crystals. In such crystals, the nonlinear properties of the medium are reversed on a periodic basis, allowing phase matching that would otherwise be impossible. The approach is often referred to as quasi-phase-matching. “They make the whole system easier to tune,” says Marco Arrigoni, director of marketing in the Scientific Marketing Segment at Coherent. Tuning is achieved simply by using motorized stages to move the end mirror of the OPO cavity. Previous designs used crystals that had to be tuned by varying their orientation or temperature. Fanned periodically poled crystals, in which the period varies within the bulk of the crystal, are a new innovation that has been introduced in the past year. “At the top they might be 5 micrometres wide, and at the bottom maybe 7 micrometres,”

Arrigoni says. “If you translate the crystal perpendicular to the pump beam you can generate a whole series of curves.” The result, he explains, is a quasi-independent choice of pump and OPO wavelengths. This gives researchers greater freedom in designing their experiments. For example, in doing biological research that involves attaching different fluorescing compounds to different molecules, this wavelength tunability could be used to select two beams that will cause different fluorophores to fluoresce. “You have two degrees of freedom rather than one,” Arrigoni says. PRODUCT ROUND-UP The NT242 tunable laser system from EKSPLA (Vilnius, Lithuania) integrates an OPO and a diode-pumped solid-state laser into a compact housing. The OPO has a signal wavelength range of 420–709 nm, an idler range of 710–2,300 nm and a UV range of 210–419 nm. It has a repetition rate of 1 kHz for use in a variety of laboratory applications, including laser-induced fluorescence, photolysis, remote sensing, light detection and ranging, and research on nonlinear optical materials. The OPO provides >45 mW of output power in the UV, and has a separate output for the 355 nm pump beam. Other customized options are available, such as outputs for 266, 532 and 1,064 nm light. The emission has a linewidth of 5 cm–1 and a pulse duration of 4–7 ns. The instrument can be controlled by a PC running supplied LabView drivers and connected via an RS232 interface. There is also a remote control pad that allows simple control of all parameters, with a backlit display that can easily be read through laser safety goggles. www.ekspla.com

The Inspire range of ultrafast OPOs from the Spectra-Physics division of Newport Corporation (Santa Clara, California, USA) allows gap-free tuning in the range of 345–2,500 nm without changing the intracavity optics or crystals. When pumped with the company’s Mai Tai Ti:Sapphire lasers, these OPOs can deliver high power in the UV and visible range and provide adjustable pulse widths of 80–350 fs. Tuning is controlled by a computer. The HF model provides fully automated tuning and cavity alignment to maintain optimal power and pulse duration. The Auto model provides semi-automated tuning and nearly transform-limited pulse duration flexibility, allowing pulse widths to be tailored to match experimental requirements. The device has five output ports: a signal output of >400 mW at 490–750 nm, an idler output of >200 mW at 930–2,500 nm, a fundamental output of >1 W at 690–1,040 nm, a doubled fundamental output of >1 W at 345–520 nm, and an alignment output. Simultaneous output from either two or three ports is possible for applications that require more than one wavelength. www.newport.com The Levante Emerald OPO from APE (Berlin, Germany) uses a frequency-doubled ytterbium laser emitting at 523 nm as a pump source instead of a Ti:Sapphire laser. It typically generates pulses of 300 fs in duration. The signal beam has a tunable range of 690–980 nm, with an output power of >0.5 W at 750–980 nm. The idler beam has a tunable range of 1,120–2,150 nm, with an output power of >0.35 W at 1,150–1,350 nm. The output pulses are nearly transformlimited and have high pointing stability. The signal’s spectral bandwidth is typically 4–5 nm. The typical time-bandwidth product of the generated pulses is 0.6. The pulse repetition rate is 76 MHz, but other values are available if required. The output has a beam quality M2 of 300 mW. An interchangeable pre-aligned wavelength converter module can

is designed for molecular spectroscopy, remote sensing, and imaging applications. The OPO idler beam covers the wavelength range of 2,400–4,700 nm, and the signal beam covers 1,400–1,900 nm. One version provides output powers of >250 mW at 150 kHz, whereas the other provides >60 mW. The OPO provides pulse energies of up to 2 μJ and peak powers of up to 200 W, which increases the signal-to-noise ratio for greater detection sensitivity and allows for greater stand-off distances for the detector. It comes with broadband (