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External-cavity Diode Lasers

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Acronym: ECDL

Definition: non-monolithic diode lasers where the laser cavity (resonator) is completed with external optical elements

German: Diodenlaser mit externem Resonator

Category: lasers

How to cite the article; suggest additional literature

An external-cavity diode laser is a diode laser based on a laser diode chip which typically has one end anti-reflection coated, and the laser resonator is completed with, e.g., a collimating lens and an external mirror as shown in Figure 1. Another type of external-cavity laser uses a resonator based on an optical fiber rather than on free-space optics. Narrowband optical feedback can then come from a fiber Bragg grating.

The external laser resonator introduces various new features and options:

external-cavity diode laser

Figure 1: Simple setup of a diode laser with external cavity. The semiconductor chip is anti-reflection coated on one side, and the laser resonator extends to the output coupler mirror on the right-hand side.

Note that there are external-cavity semiconductor lasers which are not diode lasers: optically pumped vertical external-cavity surface-emitting lasers (VECSELs).

Tunable External-cavity Diode Lasers

Tunable external-cavity diode lasers (→ tunable lasers) usually use a diffraction grating as the wavelength-selective element in the external resonator. They are also called grating-stabilized diode lasers.

The common Littrow configuration (see Figure 2a) contains a collimating lens and a diffraction grating as the end mirror. The first-order diffracted beam provides optical feedback to the laser diode chip, which has an anti-reflection coating on the right-hand side. The emission wavelength can be tuned by rotating the diffraction grating. A disadvantage is that this also changes the direction of the output beam, which is inconvenient for many applications.

tunable external-cavity diode lasers

Figure 2: Tunable external-cavity diode lasers in Littrow and Littman–Metcalf configuration.

In the Littman–Metcalf configuration ([3], Figure 2b), the grating orientation is fixed, and an additional mirror is used to reflect the first-order beam back to the laser diode. The wavelength can be tuned by rotating that mirror. This configuration offers a fixed direction of the output beam, and also tends to exhibit a smaller linewidth, as the wavelength selectivity is stronger. (The wavelength-dependent diffraction occurs twice instead of once per resonator round trip.) A disadvantage is that the zero-order reflection of the beam reflected by the tuning mirror is lost, so that the output power is lower than that for a Littrow laser.

Competing types of tunable lasers are DBR laser diodes and small fiber lasers.

Mode-locked External-cavity Diode Lasers

In the context of mode locking (→ mode-locked diode lasers), external-cavity diode lasers have various interesting properties:

More details are found in the article on mode-locked diode lasers.

Mode-locked external-cavity diode lasers sometimes compete with mode-locked fiber lasers. They do not reach their potential for clean pulses and high output power, but are much more compact and cheaper to manufacturer.


Mode-locked ECDLs are mostly used in data transmitters for optical communications. Tunable devices find applications in areas such as absorption spectroscopy of trace gases.


[1]H. Edmonds and A. Smith, “Second-harmonic generation with the GaAs laser”, IEEE J. Quantum Electron. 6 (6), 356 (1970)
[2]M. G. Littman and H. J. Metcalf, “Spectrally narrow pulsed dye laser without beam expander”, Appl. Opt. 17 (14), 2224 (1978)
[3]K. Liu and M. G. Littman, “Novel geometry for single-mode scanning of tunable lasers”, Opt. Lett. 6 (3), 117 (1981)
[4]M. Fleming and A. Mooradian, “Spectral characteristics of external-cavity controlled semiconductor lasers”, IEEE J. Quantum Electron. 17 (1), 44 (1981)
[5]C. J. Hawthorn et al., “Littrow configuration tunable external cavity diode laser with fixed direction output beam”, Rev. Sci. Instrum. 72 (12), 4477 (2001)
[6]P. Zorabedian, “Tunable external-cavity semiconductor lasers”, in F. J. Duarte (ed.), Tunable Lasers, p. 349 (Academic Press, London, 1995)

(Suggest additional literature!)

See also: diode lasers, laser diodes, mode-locked diode lasers, semiconductor lasers, wavelength tuning, linewidth, mode-locked lasers, distributed Bragg reflector lasers, vertical external-cavity surface-emitting lasers
and other articles in the category lasers

In the RP Photonics Buyer's Guide, 17 suppliers for external-cavity diode lasers are listed.

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