Supercontinuum Lasers

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Supercontinuum White Light Lasers

Contribution by Prof. Goery Genty

Director of Flagship for Photonics Research and Innovation

Tampere University, Finland

A supercontinuum laser emits broadband light -white light- that can span from the ultraviolet to the infrared wavelength range that is generated from a cascade of nonlinear effects. These effects are induced by the interaction of high-intensity pump laser pulses with a transparent dielectric nonlinear medium. Optical fibers are the workhorse for supercontinuum generation as light can be confined over long distances leading to efficient nonlinear frequency conversion and spectral broadening, with up to several watts of average power. Because it is produced in a guided mode, the supercontinuum preserves the spatial coherence properties of the incident laser resulting in spatially coherent laser light of arbitrary wavelength. 

Advances in fiber-optics engineering have enabled the development of supercontinuum lasers from a wide range of pump-laser/fiber combinations, resulting in products that are now affordable and as compact as a shoebox. Supercontinuum lasers are available over an extensive range of wavelengths, spanning from the UV to the IR. When silica fibers are used the generating medium, the wavelength range is confined to the transparency window of silica from c.a. 400 nm up to 2400 nm. Longer wavelengths up to 4 μm and even beyond can be reached using soft glass fibers made for example of fluoride. 

Supercontinuum laser nonlinear fiber
Figure 1. Dispersion of a supercontinuum laser generated in a nonlinear fiber.

Supercontinuum Generation

The generation of a supercontinuum involves a series of complex nonlinear processes such as self-phase
modulation, soliton dynamics, four-wave mixing and stimulated Raman scattering. The spectral   characteristics of the resulting supercontinuum dramatically depend on the dispersion at the pump laser which determines the exact nature of the nonlinear processes involved, as illustrated in Figure 2.

Figure 2. Supercontinuum generation for a femtosecond pump laser with central wavelength in (a) the normal dispersion regime and (b) the anomalous dispersion regime of an optical fiber.

Specifically, for a femtosecond pump laser with central wavelength in the normal dispersion regime (Fig. 2, left panel), the supercontinuum develops from self-phase modulation that leads to a relatively flat continuous spectrum. In the anomalous dispersion regime (Fig. 2, right panel), soliton dynamics (higher-order soliton compression and fission, dispersive wave generation and Raman self-frequency shift) leads to a spectrum with significant spectral variations and fine spectral structure.

Besides its broad bandwidth and spatial coherence, another important property of a supercontinuum light source is its intensity and phase stability. The intensity stability is generally characterized in terms of relative-intensity noise, while the phase stability essentially affects the temporal or spectral coherence. Any sufficiently powerful laser source can generate a supercontinuum across a certain bandwidth range, but only a supercontinuum generated from a femtosecond pump laser possesses high intensity and phase stability, as well as the temporal coherence necessary for applications in frequency metrology (see Fig. 3). Although a supercontinuum produced from a laser emitting pulses of picosecond duration or longer is less stable, it has numerous applications relying on time-averaged measurements, for example, in imaging and spectroscopy. The cause of the supercontinuum stability difference between femtosecond and picosecond seed pulses is rooted in the generating nonlinear mechanisms which are coherently seeded from the pump laser spectral components for short pump pulses, while it is the technical laser noise that seeds the broadening dynamics for long pump pulses. As a result, the supercontinuum spectrum generated by femtosecond laser systems are highly stable while those generated from picosecond pulses are inherently ‘noisy’. In the latter case, the supercontinuum could appear smooth and continuous when measured with a spectrometer. But this apparent smoothness is only the result of time-averaging over consecutive pulses while on a single-pulse time scale the spectrum would display very fine spectral structure. This fine spectral structure varies from pulse to pulse and is responsible for the noisy properties of these sources. 

Supercontinuum app
Figure 3. Supercontinuum characteristics as a function of pump laser parameters.

Key Features of Supercontinuum Laser Sources

  • Wide Spectral Range: Supercontinuum sources cover an extensive spectral range, spanning from the ultraviolet to infrared wavelength range, allowing the users to tailor the light source to meet the specific requirements of their applications
  • High Power and Stability: Supercontinuum sources can combine high optical power levels while maintaining excellent stability. This feature can be crucial for applications demanding precision and repeatability.
  • Tunable Output: The output spectrum of supercontinuum sources can be fine-tuned with, for example, tunable filters, providing the flexibility needed for a wide range of studies.
  • Compact and User-Friendly: Supercontinuum sources are generally designed with practicality in mind, compactness and user-friendly interfaces. This simplicity ensures easy integration into existing optical systems and reducing complexities.

Example Applications of Supercontinuum Lasers

Due to their unique characteristics, supercontinuum sources are now routinely used in diverse scientific and industrial domains ranging from frequency metrology to highly sensitive molecular spectroscopy and high-resolution imaging. They are gradually replacing traditional lamps or LED.

Spectroscopy

Supercontinuum sources find a natural fit in spectroscopy applications. Supercontinuum sources provide a broad spectral coverage, allowing to perform absorption and fluorescence spectroscopy across a wide range of wavelengths at the same time. Supercontinuum light is also utilized in multiphoton spectroscopy such as two-photon and three-photon excitation, enabling to study samples with higher spatial resolution and reduced photodamage compared to conventional linear absorption spectroscopy.  The broad bandwidth of supercontinuum sources is also advantageous in time-resolved spectroscopy (e.g. in pump-probe or time-correlated experiments) to study ultrafast processes and dynamics in materials. Supercontinuum light is employed in fluorescence spectroscopy for excitation purposes. 

Microscopy

In confocal microscopy, the broad bandwidth of supercontinuum light can be filtered to precisely match the excitation wavelength of particular fluorophores or cells. Supercontinuum sources can also be applied in stimulated-emission-depletion microscopy using two different wavelengths to inhibit fluorescence in a specific region and activate it in a sub-diffraction-limit focal spot, enabling imaging below the diffraction limit. The broad spectrum can also enhance contrast and resolution in multiphoton microscopy and coherent anti-Stokes Raman scattering.

 

Biomedical Imaging

Supercontinuum sources enhance biomedical imaging technologies. For example, they can be used to selectively excite multiple fluorophores simultaneously, enabling the study of complex biological samples. Applications such as optical coherence tomography (OCT) and photoacoustic imaging also benefit from the superior performance and tunability offered by supercontinuum light sources, yielding improved resolution and sensitivity. They also find applications in biomedical diagnostics, serving as ultrasensitive probes or for detecting specific molecules in a patient’s breath. 

Frequency Metrology

Supercontinuum sources contribute to advancements in precision measurements. When highly stable, supercontinuum sources can be seen as frequency combs — a set of evenly spaced spectral lines with precisely known frequencies — which can be used as reference tools for calibrating and measuring optical frequencies with exceptional accuracy. The broad bandwidth coverage enables precise calibration across a wide spectral range, allowing to improve the accuracy of optical frequency standards and stabilize atomic clocks. Highly stable supercontinuum sources have also significantly improved the stability and accuracy of wavelength calibration in astrophysical spectrographs, allowing for resolving astrophysical Doppler shifts and detecting exoplanets in orbit around a star. 

Axiom Optics now offers a range of Supercontinuum Lasers as well as tunable filters and modules. 

To view these products, click HERE.

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