Supercontinuum lasers output white light with a spectral width similar to conventional lamps, but with the properties of a laser beam. This combination ("as wide as a lamp and as bright as a laser") makes them a popular alternative to classical broad-spectrum light sources such as xenon and deuterium lamps and halogen lamps. However, it is not that the classical lamps have already died out. While supercontinuum lasers can provide substantial benefits in some applications, there are still many applications for which conventional light sources are irreplaceable.
The choice of the optimal light source for the specific application under consideration is usually preceded by several questions.
1. What wavelengths are important to us?
A light source is usually chosen to have the highest possible output at the wavelengths needed and a weak output at wavelengths that can cause problems (parasitic signal, unwanted sample heating).
2. What kind of light source do we need?
The radiometric quantityradiance tells us that if a power P is allowed to fall at a given solid angle, the light source will appear to be more intense the smaller the effective area of the source.
If we need to let optical radiation pass through some optical components, the radiance of the light source plays a very important role. Light sources with a smaller radiating area are easier to collimate and therefore easier to focus. For example, the radiance of a 75 W Xe lamp is normally more than twice as large as the radiance of a 150 W Xe lamp (the lower wattage lamp has a smaller arc).
3. How large an area are we planning to irradiate?
If we need to let the optical radiation fall on a relatively large area, then the total output power is more important than the radiance of the source used. For example, the radiance of a 75 W Xe lamp may be comparable to that of a 1000 W Xe lamp. However, when incident on an area of a few cm2, a 1000 W lamp can provide roughly 30 times the irradiance. The radiometric quantityirradiance expresses the power P incident on area A.
4. Is the stability of the light source important to us?
For certain experiments, the stability of the light source used may be critical. Laser sources and halogen lamps tend to be more stable than xenon and deuterium lamps.
5. What mode should the light source operate in?
The preferred mode depends on the intended application. For time-resolved measurements (fluorescence and phosphorescence lifetime studies, transient absorption) we need a pulsed light source. In contrast, for ratio measurements (sample transmittance, absorbance) we prefer a continuous mode. Continuous and pulsed xenon and deuterium lamps are now commonplace. Broad-spectrum supercontinuum lasers operate primarily in pulsed mode. At high repetition rates (tens of MHz), however, they provide near-continuous output (quasi-continuous mode).
6. Do we need to tie the output beam into an optical fiber?
Comparison of individual light sources
| Halogen lamp | Xenon lamp | Deuterium lamp | SuperK COMPACT | SuperK EVO HP | |
|---|---|---|---|---|---|
| Spectral output | ~250 (300) to 2700 nm | ~200 (230) to 2500 nm | ~160 (200) to 400 nm | 450 to 2400 nm | 410 to 2400 nm |
| Mode | continuous | continuous, pulsed | continuous, pulsed | pulsed (kHz) | Quasi-continuous (MHz) |
| Source type | Thermal | Arc | Arc | Laser | laser |
| Coherent output | No | no | no | Yes | no |
| Achievable radiance [mW/cm2/sr] | lower | high | lower | very high | very high |
| Typical power spectral density [mW/nm] | >1.5 mW/nm @ 500-700 nm (150 W halogen)* >3 mW/nm @ 500-700 nm (600 W halogen)* | >1 mW/nm @ 400-600 nm (150 W Xe)* >5 mW/nm @ 400-600 nm (450 W Xe)* | >20 µW/nm @ 200-300 nm (30 W D2) | >50 µW/nm @ 600-900 nm | >1 mW/nm @ 600-900 nm |
*typical values with F/1.3 condenser and rear reflector
Main advantages of each light source
Halogen lamps
- Excellent stability
- Continuous and smooth spectrum
- Ideal for radiometric and photometric calibrations
Xenon lamps
- Properties close to a point light source (small concentrated arc)
- Almost smooth spectral profile in the UV and VIS region
- Possibility to simulate the solar spectrum
Deuterium discharge lamps
- Continuous spectrum in the UV
- Negligible output in the VIS and IR region
- Availability of a radiometrically calibrated version
SuperK COMPACT
- Very stable coherent broad spectrum light source
- Price comparable to conventional broad-spectrum light sources
- Efficient fibre output
SuperK EVO HP
- Very stable coherent broad spectrum light source
- Relatively high output
- Efficient fibre output
