Creating an Ultrabroad Tunable Laser Source for Hyperspectral Imaging and Advanced Spectroscopy

Widely tunable filters can be used alongside supercontinuum lasers to give an ultrabroad tunable laser source.

Tunable laser sources (TLS) can be used by researchers to access a wide range of wavelengths. As a result tuneable light sources are used frequently for the determination of wavelength dependent chemical and physical changes and in the field of microscopy.

For a TLS to be effective a supercontinuum laser with an ultrabroad wavelength range should be used. Also, a tuneable filter which is capable of isolating narrow bands is a necessity for precise and accurate wavelength selection. This ability is described by a filters optical density (OD) and out-of-band rejection.

The LLTF Contrast™ from Photon etc. provides a high OD and out-of-band rejection for use in tuneable laser sources.   

The Main Assets of the LLTF Contrast

LLTF Contrast is a sub-nanometric, non-dispersive bandpass optical filter that has up to 65% efficiency, including all coupling losses and polarizations. It has a high optical density of more than OD 6.5 and a high out-of-band rejection of less than -60 dB. The efficiency refers to the signal throughput; which is the ratio of the filtered intensity across the input intensity for a particular wavelength.

The LLTF Contrast™ is based in volume Bragg gratings (VBG) and also serves as a widely tunable optical filter. A particular configuration will enable the resonant filter to choose a narrow part of a laser spectrum, either broadband or monochromatic, depending on the angle of the VBG.

Photon etc’s laser line tunable filter (LLTF) Contrast™

Figure 1. Photon etc’s laser line tunable filter (LLTF) Contrast™

When compared to other filters which are commercially available in the market, the LLTF Contrast filter offers a broad tunability that ranges from 1000 to 2300 nm in the SWIR range and from 400 to 1000 nm in the VIS range, with bandwidths of less than 5 nm and 2.5 nm respectively.

This bandwidth is the spectral Full Width at Half Maximum (FWHM) of the filtered beam, which is at a definite wavelength as well. High resolution LLTF are also available and provide sub-nm FWHM over custom spectral range.

In fact, LLTF Contrast is the only tunable optical filter which has an optical density of greater than OD 4.

To determine these specifications, quality control and precise performance tests need to be carried out on extremely sensitive setups. The instrument is mostly used to generate a widely tunable laser source, which is a special kind of source achieved by integrating a commercial supercontinuum source with the tunable filter.

Additional data on the characteristics of ultrabroad tunable laser sources is discussed in the following section.

Creating an Ultrabroad Tunable Laser Source

When TLS is combined with an appropriate filter, it is capable of delivering a quasi-monochromatic output across the entire supercontinuum spectral range. Such types of sources are employed in a many fields of research such as:

  • Reflection/absorption spectroscopy
  • Photoluminescence excitation
  • Spectrally resolved light beam induced current (LBIC)
  • Detector calibration
  • Hyperspectral imaging
  • Steady state pump-probe experiment
  • Incoherent light transport experiment

The TLS can also be used as a general laboratory light source, besides being a highly versatile tool for many experiments. It delivers a narrowband wavelength which is tunable across a wide range within a user-friendly instrument.

Figure 2 shows an example of the output power of a TLS.

Output from Photon etc’s tunable laser source: the LLTF is coupled with a 4W supercontinuum laser. Visible option covering 400 - 1000 nm and SWIR option covering 1000 - 2500nm.

Figure 2. Output from Photon etc’s tunable laser source: the LLTF is coupled with a 4W supercontinuum laser. Visible option covering 400 - 1000 nm and SWIR option covering 1000 - 2500nm.

This plot arises from the measurements carried out with the SWIR and VIS versions of the LLTF Contrast™ filter and a Fianium WhiteLase SC-400-4 source to span the entire range of the SC laser; a Gentec pyroelectric detector was also used to determine the output power

This information has been sourced, reviewed and adapted from materials provided by Photon etc.

For more information on this source, please visit Photon etc.

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