The term biophotonics denotes a combination of biology and photonics, with photonics being the science and technology of generation, manipulation, and detection of photons, quantum units of light. Photonics is related to electronics in that it is believed that photons will play a similar central role in future information technology as electrons do today.
Biophotonics has therefore become the established general term for all techniques that deal with the interaction between biological items and photons. This refers to emission, detection, absorption, reflection, modification, and creation of radiation from biomolecular, cells, tissues, organisms and biomaterials. Areas of application are life science, medicine, agriculture, and environmental science. Similar to the differentiation between "electric" and "electronics" a difference can be made between applications, which use light mainly to transfer energy via light (like Therapy or surgery) and applications which excite matter via light and transfer information back to the operator (like diagnostics). In most cases the term biophotonics is only referred to the second case.
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Biophotonics can be used to study biological materials or materials with properties similar to biological material, i.e., scattering material, on a microscopic or macroscopic scale. On the microscopic scale common applications include microscopy and optical coherence tomography. On the macroscopic scale, the light is diffuse and applications commonly deal with diffuse optical imaging and tomography (DOI and DOT).
In microscopy, the development and refinement of the confocal microscope, the fluorescence microscope, and the total internal reflection fluorescence microscope all belong to the field of biophotonics.
The specimens that are imaged with microscopic techniques can also be manipulated by optical tweezers and laser micro-scalpels, which are further applications in the field of biophotonics.
DOT is a method used to reconstruct an internal anomaly inside a scattering material.[1] The method is non invasive and only requires the data collected at the boundaries. The typical procedure involves scanning a sample with a light source while collecting light that exits the boundaries. The collected light is then matched with a model, for example, the diffusion model, giving an optimization problem.
The most dominantly used light source are beam lights. However also LEDs, SLEDs or lamps play an important role. Typical wavelengths, which are used in biophotonics are between 600 nm (Visible) and 3000 nm (near IR).
Lasers play a more and more important role in Biophotonics. Their unique intrinsic properties like precise wavelength selection, widest wavelength coverage, highest focussability and thus best spectral resolution, strong power densities and broad spectrum of excitation periods make them the most universal light tool for a wide spectrum of applications. As a consequence a variety of different laser technologies from a broad number of suppliers can be found in the market today.
Major gas laser, which are used for biophotonics applications and their most important wavelengths are:
- Argon Ion laser: 457.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514,5 nm (multi-line operation possible)
- Krypton Ion laser: 350.7 nm, 356.4 nm, 476.2 nm, 482.5 nm, 520.6 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm, 752.5 nm, 799.3 nm
- Helium-Neon laser: 632.8 nm (543.5 nm, 594.1 nm, 611.9 nm)
- HeCd lasers: 325 nm, 442 nm
Other commercial gas lasers like carbon dioxide (CO2), carbon monoxide, nitrogen, oxygen, xenon-ion, excimer or metal vapor lasers have no or only very minor importance in biophotonics. Major advantage of gas lasers in biophotonics is their fixed wavelength, their perfect beam quality and their low linewidth/high coherence. Argon ion lasers can also operate in multi-line mode. Major disadvantage are high power consumption, generation of mechanical noise due to fan cooling and limited laser powers. Key suppliers are Coherent, CVI/Melles Griot, JDSU, Lasos, LTB and Newport/Spectra Physics.
The most commonly integrated laser diodes, which are used for diode lasers in biophotonics are based either on GaN or GaAs semiconductor material. GaN covers a wavelength spectrum from 375 to 488 nm (commercial products at 515 have been announced recently) whereas GaAs covers a wavelength spectrum starting from 635 nm.
Most commoly used wavelengths from diode lasers in biophotonics are: 375, 405, 445, 473, 488, 515, 640, 643, 660, 675, 785 nm.
Laser Diodes are available in 4 classes:
- Single edge emitter/broad stripe/broad area
- Surface emitter/VCSEL
- Edge emitter/Ridge waveguide
- Grating stabilized (FDB, DBR, ECDL)
For biophotonic applications the most commonly used laser diodes are edge emitting/ridge waveguide diodes, which are single transverse mode and can be optimized to an almost perfect TEM00 beam quality. Due to the small size of the resonator, digital modulation can be very fast (up to 500 MHz). Coherence length is low (typically < 1 mm) and the typical linewidth is in the nm-range. Typical power levels are around 100 mW (depending on wavelength and supplier). Key suppliers are: Coherent, Melles Griot, Omicron, Toptica, JDSU, Newport, Oxxius, Power Technology. Grating stabilized diode lasers either have an lithographical incorporated grating (DFB, DBR) or an external grating (ECDL). As a result, the coherence length will raise into the range of several meters, whereas the linewidth will drop well below picometers (pm). Biophotonic applications, which make use of this characteristics are Raman spectroscopy (requires linewidth below cm-1) and spectroscopic gas sensing.
Many advanced applications in biophotonics require individually selectable light at multiple wavelengths. As a consequence a series of new laser technologies has been introduced, which currently looks for precise wording.
The most commonly used terminology are supercontinuum lasers, which emit visible light over a wide spectrum simultaneously. This light is then filtered e.g. via acousto-optic modulators (AOM, AOTF) into 1 or up to 8 different wavelengths. Typical suppliers for this technology are Koheras or Fianium.
In another approach (Toptica/iChrome) the supercontinuum is generated in the infra-red and then converted at a single selectable wavelength into the visible regime. This approach does not require AOTF's and has a background-free spectral purity.
Since both concepts have major importance for biophotonics the umbrella term "ultrachrome lasers" is often used.
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