Using Narrow Linewidth and Single Frequency DPSS Lasers for Your Application


In this quick reference guide we outline the key characteristics of lasers that are required for a variety of applications. While single frequency lasers with continuous-wave operation are well suited to fields requiring ultra-stable performance from an exact wavelength, you may also want to examine specifications for linewidth, spectral stability, coherence length, output power, and management of ongoing consumable maintenance. The optimal specifications change between applications and different laser-based measurement techniques.


This overview covers the following applications - click on each application to navigate to each section:



Raman Spectroscopy & Microscopy


What do lasers do for Raman applications?

  • Lasers allow for the detection of a Raman signal & analysis of materials.

  • Lasers for Raman need to tackle interference, noise, and background fluorescence to facilitate readings.

What laser characteristics are needed for Raman spectroscopy and microscopy applications?

  • Wavelength and wavelength stability

  • UV and NIR are well suited to Raman applications. UV wavelengths excite some materials and cause fluorescent noise. Longer wavelengths result in a weaker Raman signal.

  • The strength of the Raman signal is dependent on the wavelength of the laser source. Lower wavelengths into the UV produce stronger Raman signals and higher spatial resolution. It is important to balance this with the occurrence of background fluorescence and the possibility of sample damage at high energy. Longer wavelengths, such as 532 nm, 785 nm, and 1064 nm, in combination with highly sensitive detectors, allow for the widest range of samples to be measured.

  • Must maintain stability to maximise resolution. Spectral drift should not be more than a few picometers over time.

  • VBGs improve wavelength accuracy and stability. VBGs also improve spectral purity for a better signal-to-noise ratio.

  • Output power and power stability

  • Output power should not fluctuate more than a few percent in varying ambient temperature.

  • Linewidth

  • Sets the limit to the spectral resolution of the recorded Raman signal – i.e. how small a difference in Stokes shift can be detected.

  • Most applications require linewidth that is a few tens of picometers or less. Higher resolution applications require much smaller linewidths – 1 MHz.

  • Laser linewidth should be narrower than the spectral linewidth of the spectrometer.

  • Beam quality

  • For confocal microscopy – Diffraction-limited TEM00 beams are optimal for maximising spatial resolution in Raman applications.

  • Probe based – beam quality is less important.

  • Heatsink & integration

  • Management of excess heat prolongs the lifetime and stability of a laser system.

  • Consideration in design for system integration is important..


Holography & Imaging


How are lasers used in holographic applications?

Holography is the science of generating a 3D image by recording the interference patterns between two light beams: one reference beam and one object incidence beam. Lasers are used to expose an object to light, thus recording its fine details and dimensions into an interference image. The laser is then used to illuminate this image and product a 3D holographic image.


What laser characteristics are needed for holography?

  • Wavelength and wavelength stability

  • Ultra-stable wavelength prevents distortion of the final image.

  • Wavelengths used in holography are selected for their specific application area. Security labels would be ineffective if they were recorded in the IR region, outside the range of the human eye, and many modern holographic images are created using multiple wavelengths - red, green and blue - in order to produce a final image in colour. Holographic applications that do not rely on the eye can be operated outwith the visible spectrum and data storage, for instance, would indeed benefit from shorter UV wavelengths, leading to higher information density.

  • Linewidth

  • The linewidth of the laser is a key characteristic that needs to be considered - where a narrow linewidth improves resolution capability of imaging equipment. Any phase difference between two light paths during the imaging stage of holographic reproduction will reduce the resolution available in the final image.

  • Coherence length

  • The larger the depth of field the longer the coherence length that is needed.

  • Lengths over several metres are sufficient.

  • The requirement for highly accurate phase information is what mandates the use of lasers with excellent spatial and temporal coherence.

  • Continuous-wave operation

  • CW operation ensures there is enough light exposure during image recording to facilitate high resolution holograms.

  • High output power

  • Higher output powers also contribute to a longer exposure time, allowing for higher resolution holograms.


Flow Cytometry


How are lasers used in flow cytometry?

Lasers enable quick, precise, and non-invasive data collection of many different properties of samples simultaneously. Cells, or other particles, are individually passed through a light path at high speed, causing an energy change in the incident light by scattering or fluorescent emission. The resulting light is detected, and these properties can be examined.


What laser characteristics are needed for flow cytometry?

  • Wavelength

  • Ultraviolet lasers are used to excite ultraviolet fluorochromes that act as detection reagents used for the absorption of specific wavelengths.

  • Output power and power stability

  • High output power increases signal strength for scattering effects, though this should be balanced with consideration of avoiding damage to the measured sample.

  • Ultra-stable power output is necessary to avoid inaccurate measurements. The magnitude of light scattering back towards the light source is used to determine the size of the cell - any changes in the incident power level during the measurement will cause inaccuracy.

  • Beam quality

  • High beam quality is required, to allow the laser to be focused to spot size similar to the size of the cells being measured.



Optical Tweezing, Trapping, and Manipulation


What is optical tweezing used for?

  • Used for cold atom physics and facilitating quantum application areas such as atomic clocks and quantum sensors.

  • Can also be used to investigate molecular properties of cells, atoms.


What do lasers do for optical tweezing applications?

Lasers hold and manipulate atoms, molecules, cells and allow for the inspection of certain properties. IR lasers can enable these measurements without sample damage.


What laser characteristics are needed for optical tweezers?

  • Output power stability

  • Ultra-stable power output is necessary to have a “firm grip” on the particles.

  • Beam quality

  • TEM00 Gaussian beam allows for tight focus with high power density.

  • Beam pointing stability

  • High beam stability allows for good, continued focus on required samples.



Interferometry


What is interferometry used for?

  • Interferometry is a versatile inspection technique used across a wide range of applications.

  • Can be used to measure gravitational waves, structural integrity, and surface topography.

What do lasers do for interferometric applications?

Interferometry relies on the superimposition of two coherent light paths, most often split from a single source, to form an interference pattern. Ultra-stable lasers with imperceptible fluctuations in power and wavelength stability are best suited to interferometric techniques as they facilitate consistent and high-resolution measurements.


What laser characteristics are needed for interferometry?

  • Narrow linewidth

  • A narrow linewidth (in combination with beam quality and pointing stability) determines the highest resolution available for sample inspection.

  • Low power noise

  • Reducing power noise from the laser source is conducive to clear analysis of inspected samples.

  • Beam quality

  • A TEM00 Gaussian beam reduces the possible complexities in analysing the measurement result.

  • Beam pointing stability

  • High beam pointing stability ensures a consistent measurement on the selected sample.



Semiconductors


What are semiconductors used for?

Semiconductors are conductive components designed to control the flow of electric currents. The compact, low-cost, and rugged nature of semiconductors have facilitated ongoing advancements in technology development.


What do lasers do for semiconductor applications?

  • Lasers drive the inspection and measurement at nearly every step of semiconductor wafer fabrication.

  • Critical parameters - such as thin film thickness or non-uniformity in deposition, defects, holes and scratches, overall flatness, deviations in the crystal structure or consistency of doping - can be detected and optimised using several interferometric techniques.

What laser characteristics are needed for semiconductors?

  • Wavelength

  • Continuous-wave single frequency DPSS lasers in the UV range can improve error detection performance. Single frequency sources ensure a precise interference pattern during inspection and measurement.

  • Low noise

  • Low noise level, in combination with narrow linewidths, increases the signal to noise ratio and enhances measurement and inspection sensitivity.

  • Ultra-stable wavelength and output power

  • Spectral and power stability eliminate errors in prolonged measurement and ensure stable operation and precise high-resolution measurements.

  • Small footprint

  • Compact laser sources allow for integration into existing systems without augmentation of the existing set-up, as well as reducing bench-space requirements. The ability for modern DPSS lasers to produce excellent beam quality at high power from a small footprint, allows for maximum flexibility in deployment and operation.

  • Low maintenance

  • Lasers are often integrated into semiconductor processing and inspection equipment. Solid state lasers that don’t require consumable maintenance (such as gas and liquid replacement for other types of lasers) help manufacturers avoid unscheduled downtime while conserving production time and costs.


Solar


What is solar used for?

A photovoltaic cell, or solar cell, is a manufactured component composed of semiconducting materials that absorb sunlight to convert photons into electricity. PV cells are usually primarily in solar panels and work as a renewable source of energy.


What do lasers do for solar applications?

Laser-based inspection techniques in photovoltaic applications reveal a variety of material properties. Measurements such as surface reflectance, deep level traps, carrier diffusion, crystalline structure and boundaries, junction type depth and temperature, optical absorption and scattering, and photon degradation all have an influence on the efficiency of solar cells.


What laser characteristics are needed for semiconductors?

  • Wavelength

  • UV light is used to facilitate techniques in photovoltaic cell inspection, where shorter wavelengths, such as 349 nm, allow for analysis on increased surface complexity and high-power UV sources radiate or ablate degraded materials on the substrate barrier.

  • Low noise

  • Low noise level, in combination with narrow linewidths, increases the signal to noise ratio and enhances measurement and inspection sensitivity.

  • Ultra-stable wavelength and output power

  • Spectral and power stability eliminate errors in prolonged measurement and ensure stable operation and precise high-resolution measurements. Maintaining these precise laser characteristics ensure high production yield at lower cost.


This guide gives a brief overview of the laser characteristics you should consider when selecting narrow linewidth and single frequency CW DPSS lasers for your application area. For more in-depth support in laser selection, please contact our technical team at info@uniklasers.com.