amplified spontaneous emission (original) (raw)

Acronym: ASE

Definition: a process where spontaneously emitted radiation (luminescence) is amplified

Alternative term: superluminescence

Categories: laser devices and laser physics, optical amplifiers, quantum photonics, fluctuations and noise

• optical transitions
  • spontaneous emission
    * amplified spontaneous emission

Related: Tutorial on Fiber Amplifiers Part 4: Amplified Spontaneous EmissionASE in Ytterbium-doped FibersErbium-doped Fiber Amplifier for a Long-wavelength SignalDesigning a Double-clad Fiber AmplifierYtterbium-doped 975-nm Fiber Lasersspontaneous emissionoptical amplifierslaser noiseamplifier noisesuperluminescent sourcessuperluminescent diodessuperluminescencewhite light sourcesFiber Amplifiers: Stronger ASE in Backward DirectionFiber Amplifiers: More ASE for Larger Core with Higher NA?Amplified Spontaneous Emission in Fiber Amplifiers

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DOI: 10.61835/qf7 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

Introduction

In a laser gain medium with large gain, the luminescence (most often fluorescence) 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.

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 the 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 laser gain media, this ASE effect is stronger than for four-level media.

Figure 1: ASE spectra of forward and backward ASE in a forward-pumped ytterbium-doped fiber amplifier, calculated with the RP Fiber Power software.

Particularly in quasi-three-level gain media, ASE spectra can strongly differ from the spectrum of spontaneous emission, and can be substantially dependent on the propagation direction.

As shown in Figure 1, the spectrum of ASE in the output of a rare-earth-doped fiber amplifier can differ strongly from that of the fluorescence leaving the fiber in a transverse direction. This is because of wavelength-dependent amplification and reabsorption. (The latter occurs only for quasi-three-level laser gain media.) Furthermore, ASE powers emitted in forward and backward directions in a fiber amplifier can differ. Usually, ASE is stronger in the direction opposite to that of pumping. Finally, the spectral shape of ASE can depend on the pump intensity level. An example simulation is shown in Figure 2 for an Yb-doped fiber. The higher the average Yb excitation, the shorter is the peak wavelength of ASE because for lower excitation the shorter-wavelength ASE is attenuated strongly by reabsorption. For fluorescence light radiated to the side of the fiber, that effect would not be seen.

Case Studies

ASE in Ytterbium-doped Fibers

We study various aspects of amplified spontaneous emission (ASE) in ytterbium-doped fibers — for example, why it is different in forward and backward directions, how the fiber length can have a crucial impact, and how the fiber core diameter matters.

Figure 2: Spectra of backward ASE in the same fiber amplifier as that of Figure 1, calculated for different pump power levels. With increasing power, the spectrum shifts toward shorter wavelengths (where the gain grows more quickly) and becomes narrower.

Interesting aspects come into play when ASE occurs in waveguide structures such as fibers. For example, the amount of ASE into a single-mode core does not depend on the detailed waveguide parameters (e.g., on the core diameter and numerical aperture). Contrary to a common belief, the ASE power depends only on the number of modes, not on particular mode properties (except for propagation losses). Considerations involving some “capture fraction” for spontaneous emission, which is limited by the condition of total internal reflection, are appropriate only for strongly multimode waveguides, but not for few-mode or single-mode situations. (See the Photonics Spotlight of 2007-08-06 for details.) Some fiber amplifier models are wrong in this respect.

ASE is sometimes also called superluminescence, and correspondingly there is the term superluminescent sources (also ASE sources or white light sources) for light sources emitting superluminescent radiation. Such broadband (but usually spatially coherent) sources are useful for a number of applications.

In lasers, ASE is usually quite weak, thus not extracting substantial amounts of optical power, since the laser gain is typically not as large as the gain in a fiber amplifier, for example. It usually does not have a significant influence on the laser efficiency. However, it contributes to laser noise.

In the context of free-electron lasers, the term self-amplified spontaneous emission (SASE) is common. (The addition of “self” emphasizes that the amplification occurs in the same device which produced the spontaneous emission.) Such devices do not have a laser resonator, but generate only strong ASE; the ASE is considered as the laser output, rather than a phenomenon competing with it.

A phenomenon related to ASE, but with important physical differences, is superfluorescence.

Figure 3: The output spectra of forward and backward ASE of an erbium-doped fiber amplifier. As the gain for long-wave signals (blue marked region) is substantially lower than at shorter wavelengths, short-wavelength ASE is a substantial performance limitation in this case.

Coherence Properties of Amplified Spontaneous Emission

The temporal coherence of ASE can be very low, if its bandwidth is large. In the time domain, the associated electric field exhibits fast random fluctuations, i.e., it has a rather short coherence time and coherence length. In the frequency domain, this corresponds to a large optical bandwidth. That bandwidth can be of the order of the gain bandwidth, but particularly in high-gain situations it is often substantially smaller than that.

The spatial coherence of ASE depends strongly on the circumstances. ASE from a short laser crystal, which is pumped with a large mode area, can have quite low spatial coherence, as the effective laser gain in substantially different propagation directions can be similar. On the other hand, ASE from a rare-earth-doped single-mode fiber can exhibit essentially perfect spatial coherence. This implies that such ASE can be well focused to a very small spot if suitable focusing optics (with low chromatic aberrations) are used.

Is Amplified Spontaneous Emission Unavoidable?

As ASE ultimately arises from a quantum effect (spontaneous emission), it is an unavoidable amplifier noise effect. Any phase-insensitive amplifier must at least add as much excess noise to a signal as corresponds to ASE in an ideal four-level laser amplifier. However, ASE may be above that quantum-mechanical limit, e.g. if one uses a (quasi)-three-level amplifier system and/or there are additional propagation losses (from absorption or scattering) within the amplifier. See the article on amplifier noise for more details.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is amplified spontaneous emission (ASE)?

Amplified spontaneous emission is luminescence from a laser gain medium that is amplified by the medium itself. It can become very powerful in high-gain systems like fiber amplifiers, where it propagates along the fiber.

In what situations is ASE an undesirable effect?

In high-gain amplifiers, ASE is often unwanted as it can extract significant power, limit the achievable gain, and add noise to the amplified signal. In lasers, it can also prevent operation at desired wavelengths by creating strong competing gain at other wavelengths.

Can ASE be useful?

Yes, light sources based on ASE are called superluminescent sources or ASE sources. They are useful in applications requiring light with low temporal coherence (a broad bandwidth) but good spatial coherence.

What are the coherence properties of ASE?

ASE typically has low temporal coherence due to its large optical bandwidth. Its spatial coherence, however, can be very high, for example when generated in a single-mode fiber, which allows the light to be focused to a small spot.

Why does the ASE spectrum often differ from the spontaneous emission spectrum?

The ASE spectrum is shaped by wavelength-dependent amplification and reabsorption within the gain medium. This causes some spectral parts to be amplified more than others, and the final shape can depend on factors like the pump power and propagation direction.

Is it possible to build an optical amplifier without ASE?

No, ASE is an unavoidable effect in phase-insensitive optical amplifiers because it arises from the fundamental quantum process of spontaneous emission. It represents a fundamental source of amplifier noise.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 41 suppliers for superluminescent sources. Among them:

⚙ hardware

Innolume provides superluminescent sources, particularly superluminescent diodes (SLDs) that emit high-intensity, low-coherence light over a broad spectrum (bandwidth more than 100 nm) and reaching high output power (more than 250 mW) through amplified spontaneous emission.

These SLDs have tilted waveguides and anti-reflective coatings (<0.001%) that suppress optical feedback and minimize spectral ripples.

We offer SLDs in flexible packaging options:

• TO cans
• C-mounts
• AlN submounts
• bare dies
• fiber-coupled 14-pin modules (PM/HI fiber, photodiode are optional)

These SLDs find applications in optical coherence tomography, fiber-optic gyroscopes, interferometry, optical sensing and telecommunications.

⚙ hardware

Spectra Quest Lab offers ASE light sources in the 950-nm and 1040-nm bands. They have a very low gain ripple of typically 0.03 dB (RMS) and relatively flat spectral characteristics with a 3 dB bandwidth above 50 nm; PM fiber-coupled output power is 100 mW (typ.) and have come as turnkey bench-top packages. A current modulation port with 1 MHz bandwidth is provided for high-speed modulation.

⚙ hardware

Quantifi Photonics' SLED 1000 Series is a super-luminescent LED light source with high output power, large bandwidth and low spectral ripple. They are available in various wavelengths in compact benchtop or PXI form factors and ideal for test procedures in telecom and datacom applications.

Bibliography

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