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Fiber Light Sources

Wideband Ligth Sources

Product description

Nano-Giga supplies broadband light sources for optical fibers. The first is a supercontinuum laser sources with ultra-broadband ranges from 450nm to 2500nm, the second is a broad band ligth source in the range 1260nm-1650nm. These wideband or broadband light sources are designed to enhance analytical spectroscopy, fiber sensing, optical coherence tomography (OCT), and other test & measurement capabilities. Devices benefit from low-cost, field proven telecommunication components to provide rugged, reliable solutions.

Wideband Light Sources

ASE Light Sources

Product description

Amplified Spontaneous Emission

In a laser medium with large gain, the luminescence from spontaneous emission can be amplified to high power levels. This amplified luminescence may be used in applications where light with low temporal coherence but good spatial coherence (see below) is required. It also occurs in lasers, even when operated below the laser threshold.

Whereas luminescence originally goes in all spatial directions, ASE can be strongly directional for gain media with a large aspect ratio. As an extreme case, consider a fiber laser or fiber amplifier, where ASE propagating along the fiber can be much more powerful than the omnidirectional luminescent emission.

In lasers and particularly in high-gain amplifiers, amplified spontaneous emission is usually an unwanted effect. It tends to limit the gain achievable in a single stage of a fiber amplifier to the order of 40–50 dB. Higher gain values are possible e.g. for amplification of pulses, if several amplifier stages are used, which are separated by filters, Faraday isolators, and/or optical modulators (switches). Particularly in some fiber lasers, ASE can prevent lasing at extreme wavelengths, if the gain at other wavelengths is high enough for generating strong ASE. Such problems can often be overcome by optimizing the overall laser design, with special attention to fiber length, doping level and the like, and ASE at unwanted wavelengths may be suppressed with certain fiber designs (e.g. photonic crystal fibers) exhibiting high propagation losses outside the desired spectral region. Similar challenges arise in the context of some bulk lasers, e.g., Nd:YAG lasers operating at 946 nm, where strong ASE at 1064 nm can suppress 946-nm lasing.

Even if amplified spontaneous emission in an amplifier is not strong enough to extract significant power, it can contribute significantly to the noise of the amplified signal. The noise figure of a laser amplifier can be considered to be limited by ASE. Note that for quasi-three-level gain media this ASE effect is stronger than for four-level media.

SLED Light Sources

Product description

Superluminescent Sources

A superluminescent source has a very low temporal coherence, resulting from the large emission bandwidth (compared with that of, e.g., a laser). This greatly reduces the tendency for speckle, as are often observed with laser beams, e.g. from laser diodes. On the other hand, spatial coherence is usually very high: the output of a superluminescent source can be very well focused (similar to a laser beam) and is thus suitable for obtaining by far higher optical intensities than with an incandescent lamp, for example. This makes such devices interesting for applications such as optical coherence tomography (OCT) (e.g. in the medical sector), device characterization (e.g. in optical fiber communications), gyroscopes, and fiber-optic sensors. See the article on superluminescent diodes for more details on applications.

The main kinds of superluminescent sources are superluminescent diodes (SLDs) and fiber amplifiers. Fiber-based sources can provide much higher output powers, whereas SLDs are much more compact and also cheaper. In both cases, the emission bandwidth is at least several nanometers and often tens of nanometers, sometimes even well above 100 nm.



LED Light Sources

Product description

Light-emitting Diodes

A light-emitting diode (LED) is an optoelectronic device which generates light via electroluminescence. It contains a p–n junction, through which an electric current is sent. In the heterojunction, the current generates electrons and holes, which release their energy portions as photons when they recombine. Although the fundamental process of light generation is the same as in laser diodes, light-emitting diodes do not exhibit laser action, i.e., they usually do not exploit stimulated emission. Their optical spectrum is substantially wider.

As LEDs can be quickly modulated, they are suitable for optical fiber communications over short distances. While the poor directionality of their emission requires the use of multimode fibers and thus restricts the transmission distances, the cost is significantly lower than for a system with single-mode fibers and laser diode transmitters. Moderately fast power modulation is also useful, e.g., for application in light barriers, as the modulated LED light is easily distinguished from the ambient light, and for remote controls.


Fiber Lasers

Product description

Fiber Lasers

Fiber lasers are usually meant to be lasers with optical fibers as gain media, although some lasers with a semiconductor gain medium (a semiconductor optical amplifier) and a fiber resonator have also been called fiber lasers (or semiconductor fiber lasers). Also, devices containing some kind of laser (e.g., a fiber-coupled laser diodes) and a fiber amplifier are often called fiber lasers (or fiber laser systems).

High-power Fiber Lasers
Whereas the first fiber lasers could deliver only a few milliwatts of output power, there are now high-power fiber lasers with output powers of hundreds of watts, sometimes even several kilowatts from a single fiber. This potential arises from a high surface-to-volume ratio (avoiding excessive heating) and the guiding effect, which avoids thermo-optical problems even under conditions of significant heating.

 Narrow-linewidth Fiber Lasers
Fiber lasers can be constructed to operate on a single longitudinal mode with a very narrow linewidth of a few kilohertz or even below 1 kHz. In order to achieve long-term stable single-frequency operation without excessive requirements concerning temperature stability, one usually has to keep the laser resonator relatively short (e.g. of the order of 5 cm), even though a longer resonator would in principle allow for even lower phase noise and a correspondingly smaller linewidth. The fiber ends have narrow-bandwidth fiber Bragg gratings, selecting a single resonator mode. Typical output powers are a few milliwatts to some tens of milliwatts, although single-frequency fiber lasers with up to roughly 1 W output power have also been demonstrated.

Q-switched Fiber Lasers
With various methods of active or passive Q switching, fiber lasers can be used for generating pulses with durations which are typically between tens and hundreds of nanoseconds.The pulse energy achievable with large mode area fibers can be several millijoules, in extreme cases tens of millijoules, and is essentially limited by the saturation energy (even for large mode area fibers) and by the damage threshold (the latter particularly for shorter pulses). All-fiber setups (not containing any free-space optics) are quite limited in terms of the achievable pulse energy, as they can normally not be realized with large mode area fibers and effective Q switches.

Raman Fiber Lasers
A special type of fiber lasers are fiber Raman lasers, relying on Raman gain associated with the fiber nonlinearity. Such lasers usually use relatively long fibers, sometimes of a type with increased nonlinearity, and typical pump powers of the order of 1 W. With several nested pairs of fiber Bragg gratings, the Raman conversion can be done in several steps, bridging hundreds of nanometers between the pump and output wavelength. Raman fiber lasers can e.g. be pumped in the 1-μm region and generate 1.4-μm light as required for pumping 1.5-μm erbium-doped fiber amplifiers.