Optical amplifier

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An optical amplifier is a device that amplifies an incoming optical signal as it passes through it without converting it to an electrical signal in between.

The amplification is based on stimulated emission triggered by the signal to be amplified. An optical amplifier is a laser in its most original form without an optical resonator .

In contrast to regenerative amplifiers, they work independently of the modulation of the optical signal and are therefore sometimes referred to as "transparent". This property and the large amplifier bandwidth are particularly important when using wavelength division multiplexing ( WDM ). Optical amplifiers have been used for all long-range fiber optic connections, such as the transatlantic routes , since high reliability has been guaranteed .

Most optical amplifiers also spontaneously emit photons , which are then amplified as well. This undesirable effect is called ASE ( amplified spontaneous emission , see also under: superluminescence ). The radiation caused by ASE is incoherent and unpolarized. ASE is one of the factors that limits the number of amplifiers that can be cascaded. The ratio of the total power of a signal to the noise power at the signal frequency in a small bandwidth (usually 0.1 nm) is called the optical signal-to-noise ratio (OSNR) and is given in dB in relation to the bandwidth (e.g. dB / 0.1 nm).

Erbium-doped fiber amplifier (EDFA)

In Erbium doped fiber amplifiers (engl. Erbium-doped fiber amplifier , EDFA ) is a conventional optical fiber over a length of one to about 100 meters erbium doped . This fiber section is then optically pumped with the aid of a semiconductor laser .

Typical values ​​for commercial EDFAs:

  • Operating wavelength
    • C-band (approx. 1530-1560 nm)
    • L-band (approx. 1570-1600 nm).
    • S-band (below 1480 nm) requires other dopants.
  • low noise with a noise figure of 3–6 dB
  • high gain (20–40 dB) and low dependence on the polarization of the light signal.
  • Max. optical output power : 10-35 dBm
  • internal gain: 25-50 dB
  • Gain deviation: ± 0.5 dB
  • Active fiber length: 10–60 m for C-band EDFAs and 50–300 m for L-band EDFAs
  • Number of pump lasers: 1-6
  • Pump wavelength: 980 nm or 1480 nm

EDFA were first demonstrated in 1987 by a group at the University of Southampton led by David N. Payne and at Bell Laboratories ( Emmanuel Desurvire and others). With the EDFA, the breakthrough for fiber optic transmission of optical signals over very long distances was achieved in the late 1980s.

Semiconductor laser amplifier (SOA)

Semiconductor optical amplifiers (SOA) are constructed like semiconductor lasers , but have an anti-reflective coating on the end faces at which the light emerges so that no undesirable resonance effects arise. Mostly they are operated with single mode fibers . Semiconductor laser amplifiers are inferior to EDFAs in terms of amplification, noise and polarization dependence, but there is a price advantage due to their ease of integration. In tunable laser ITLA , semiconductor amplifiers are integrated in the same housing in order to achieve a higher output power. In contrast to fiber amplifiers, the excitation in semiconductor amplifiers changes very quickly, so that there is mutual influence when several signals are amplified at the same time. This can lead to unwanted crosstalk, but it can also be used for fast optical switching or for optical frequency conversion.

Typical values:

  • Operating wavelength: mainly 1300 and 1500 nm (active medium: InGaAsP)
  • Amplification: up to 30 dB chip alone and approx. 20 dB including losses at the contact surfaces
  • Max. Optical output power: 5 dBm
  • Bandwidth: 25 nm

For high optical performance that is required in the scientific field, e.g. B. are required for laser cooling , Bose-Einstein condensation and laser spectroscopy , a semiconductor laser amplifier with a trapezoidal structure has prevailed. The trapezoidal section is required to reduce the power density at the exit facet.

Typical values:

  • Operating wavelength: from 633 to 1480 nm
  • Input power: 10 to 50 mW
  • optical output power: up to 3 watts

Raman amplifier

So-called Raman scattering is used for Raman amplifiers . Upon irradiation of an optical pump wave (high intensity ) in are silica fibers , the photons of the silica - molecules scattered. Some of the energy is transferred into phonons and the rest is scattered as photons of lower energy. This process initially takes place spontaneously, but it can also be stimulated by the signal wave to be amplified. One then speaks of stimulated Raman scattering .

Advantages over EDFA are:

  • high bandwidth: C and L band at the same time
  • Gain range adjustable via the pump wavelength
  • the amplification distributed over the entire fiber leads to a better signal-to-noise ratio

The principle was already demonstrated by Erich P. Ippen and Rogers H. Stolen in 1973. The advantage of signal transmission over long distances was already exploited in the 1980s (e.g. Linn F. Mollenauer at Bell Laboratories ), but was initially pushed into the background by the introduction of EDFAs and only experienced one in the course of the 1990s Boom.

See also

Web links

Individual evidence

  1. ↑ Volkmar Brückner: Elements of optical networks: Basics and practice of optical data transmission . 2nd Edition. Vieweg + Teubner, 2011, ISBN 3-8348-1034-7 , pp. 123 ff . ( limited preview in Google Book search).
  2. ↑ Payne, R. Mears, L. Reekie, IM Jauncey Low-noise Erbium-doped fiber amplifier at 1.54 μm , Electron. Lett., Vol. 23, 1987, pp. 1026-1028
  3. ^ E. Desurvire, P. Becker, J. Simpson High-gain erbium-doped traveling-wave fiber amplifier , Optics Letters, Vol. 12, 1987, pp. 888-890
  4. ^ Laser Diode Market . Hanel Photonics. Retrieved December 3, 2014.
  5. ↑ Mohammed N. Islam Raman amplifiers for telecommunications , IEEE Journal of selected topics in quantum electronics, Volume 8, No. 3, 2002