What is an Optical Parametric Oscillator (OPO)
An Optical Parametric Oscillator (OPO) is a source of coherent light with a tunable frequency. An OPO consists of a nonlinear crystal placed inside a cavity which is pumped by a fixed frequency laser. Discover more about Chromacity’s picosecond OPOs which span the near to mid-IR.
How does an Optical Parametric Oscillator work?
In the OPO, the nonlinear crystals split a high-frequency beam into two low-frequency beams. One of these beams reverberates in the cavity, which amplifies the magnitude of the beam splitting effect. By tuning the parameters of the cavity, Chromacity can change the ratio of power, and therefore the frequency of the two beams. A partially reflective mirror at the end of the cavity permits partial transmission of the low energy beam, delivering a source of tunable frequency light.
What are the applications for Optical Parametric Oscillators?
Optical Parametric Oscillators can be used to generate light in frequencies that conventional lasers cannot easily access. Furthermore, most lasers have a fixed wavelength, whereas a tunable OPO can deliver light over very wide ranges, giving it far greater versatility than a laser. Applications for an OPO make use of these two features. Spectroscopy in particular uses frequencies lasers cannot produce, or across a wider range of frequencies than a laser can achieve.
What is the difference between an OPO and OPA?
In an Optical Parametric Oscillator, one of the beams reverberates in the cavity, which amplifies the conversion process and enables the delivery of higher intensity tunable light. In an Optical Parametric Amplifier (OPA) there is no reverberation. Consequently, the intensity of the tunable light delivered is much lower than an OPO.
What is a nonlinear crystal?
A nonlinear crystal has a material response to applied electric fields that is nonlinearly proportionate to the applied field. This material response gives rise to unique phenomena when strong electric fields are applied, as in high-intensity ultrashort-pulses or highly focussed light. One of these phenomena is the conversion process that is important for the parametric generation of light.
What is parametric generation of light?
The parametric generation is a process that makes use of high-intensity, coherent light to generate a range of nonlinear effects in certain optical materials. When a weak, low energy beam and a high energy pump beam interact with enough intensity within these non-linear materials, pump photons can convert to signal and idler photons (where the energy of the idler wavelength is the difference between the signal and pump energies).
Semiconductors like Gallium Phosphide have high transparency in the mid-infrared compared to traditional nonlinear crystals, making them highly suitable for generating light in the mid-infrared. An OPGaP OPO is a source of tunable light that incorporates an OPGaP (orientation patterned gallium phosphide) nonlinear crystal to generate coherent tunable light across the mid-IR, 5-12 µm region. OPGaP is a relatively new nonlinear material that has been shown to display a very non-linear coefficient when optically pumped at 1 µm.
Quantum Cascade Lasers (QCL) emit light in the mid-IR, typically between 5 – 6 µm. These semiconductor-based lasers offer a narrow bandwidth at low pulse energies. An optical parametric oscillator delivers more high average power across a broad tuning range, which is more effective for long-range stand-off detection and identifying complex chemical compounds, much further into the mid-IR, in comparison to using multiple QCLs to achieve similar results.
Fiber lasers have the benefit of being able to generate high optical gain within the active fiber. This is because the pump light can be confined within the fiber. This leads to an efficient lasing process. Short-pulse fiber laser architectures suffer from spectral broadening, where different spectral components travel at different velocities, thus causing the short pulse to broaden.
The design that Chromacity has developed makes use of a part fiber, part free space laser architecture. This allows Chromacity to benefit from creating a high average power from the active fiber section whilst being able to control the intracavity dispersion of the laser pulses in the free space section, where we can insert dispersion compensating optics.
The efficiency of our systems ensures that Chromacity can provide a compact ultrafast source that does not need to be water-cooled and has a very simple user interface. The simple cavity design also lends itself to be stable; and robust and affordable. This is in contrast to Ti:sapphire laser systems, which are complex, bulky, and expensive to operate. While ti:sapphire lasers can suit certain applications that demand tunability, they are much less appealing for fixed wavelength applications.
What is a supercontinuum laser?
Light from a laser source converts into a wide spectral bandwidth where the C wavelength significantly broadens to generate a supercontinuum source. The power can be spread over many hundreds of wavelengths, however, it also means that the power in each nm of bandwidth is typically low.
What are the applications for supercontinuum sources?
Supercontinuum lasers have comparable applications to OPO’s operating in the same range because they can deliver a large range of wavelengths. By applying a bandpass filter, users can select an individual wavelength for multispecies spectroscopy, or remove the filter for illumination across the entire supercontinuum range where a white light source is desired.
What is a mode-locked laser?
A mode-locked laser is a system capable of delivering pulses of light between 10^-10 and 10^-15 seconds in length. A laser cavity of a particular length can support multiple modes, which slip in and out of phase with each other as they travel through the cavity. Where these elements constructively interfere, they form a pulse that reverberates in the laser cavity. By selectively amplifying only this pulse, users can produce high-intensity laser light with pulse lengths much shorter than traditional modulation would permit.
What is the difference between active and passive mode-locking?
Active mode-locking describes the situation where an external drive is applied to the laser cavity to cause mode-locking and this can be done by modulating either the intracavity loss or the intracavity phase. In contrast, passive mode-locking involves no external modulation, but instead, the cavity contains an element whose optical loss decreases as the power incident on it increases (ie. a nonlinear loss). This self-amplitude modulation effect can be considered as being similar to an external amplitude modulation driven at exactly the cavity frequency.
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