Encyclopedia … combined with a great Buyer's Guide!

External-cavity Diode Lasers

Acronym: ECDL

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

More general term: diode lasers

German: Diodenlaser mit externem Resonator

Categories: optoelectronicsoptoelectronics, laser devices and laser physicslaser devices and laser physics


Cite the article using its DOI: https://doi.org/10.61835/te0

Get citation code: Endnote (RIS) BibTex plain textHTML

An external-cavity diode laser is a diode laser based on a laser diode chip integrated into a somewhat larger laser cavity that also contains other optical elements. The diode chip is typically antireflection coated at one end, and the laser resonator is completed with e.g. a collimating lens (or separate fast-axis and slow-axis beam collimators) 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 free-space optics. Narrowband optical feedback can then be provided by a fiber Bragg grating.

Possible Features

The external laser resonator introduces several 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.

External-cavity diode lasers can be fabricated with very compact setups. Depending on the additional optical elements required, it is often possible to make miniature lasers of this type.

Note that there are external-cavity semiconductor lasers that are not diode lasers: optically pumped vertical external-cavity surface-emitting lasers (VECSELs), which do not contain a p–n junction.

Tunable External-cavity Diode Lasers

Tunable external-cavity diode lasers (→ tunable lasers) typically use a diffraction grating as the wavelength-selective element in the external cavity. 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 a anti-reflection coating on the right 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. (Alternatively, one can rotate the assembly with the diode chip and lens.)

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

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 this mirror. This configuration provides a fixed output beam direction and also tends to have a narrower linewidth because the wavelength selectivity is greater. (Wavelength-dependent diffraction occurs twice instead of once per resonator revolution.) A disadvantage is that the zero-order reflection of the beam reflected by the tuning mirror is lost, so the output power is lower than that of 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 several interesting properties:

More details can be found in the article about mode-locked diode lasers.

Mode-locked external-cavity diode lasers sometimes compete with mode-locked fiber lasers. They do not share their potential for high output powers, but are much more compact and much cheaper to manufacture.


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

More to Learn

Encyclopedia articles:


The RP Photonics Buyer's Guide contains 21 suppliers for external-cavity diode lasers. Among them:


[1]H. Edmonds and A. Smith, “Second-harmonic generation with the GaAs laser”, IEEE J. Quantum Electron. 6 (6), 356 (1970); https://doi.org/10.1109/JQE.1970.1076460
[2]M. G. Littman and H. J. Metcalf, “Spectrally narrow pulsed dye laser without beam expander”, Appl. Opt. 17 (14), 2224 (1978); https://doi.org/10.1364/AO.17.002224
[3]K. Liu and M. G. Littman, “Novel geometry for single-mode scanning of tunable lasers”, Opt. Lett. 6 (3), 117 (1981); https://doi.org/10.1364/OL.6.000117
[4]M. Fleming and A. Mooradian, “Spectral characteristics of external-cavity controlled semiconductor lasers”, IEEE J. Quantum Electron. 17 (1), 44 (1981); https://doi.org/10.1109/JQE.1981.1070634
[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); https://doi.org/10.1063/1.1419217
[6]P. Zorabedian, “Tunable external-cavity semiconductor lasers”, in F. J. Duarte (ed.), Tunable Lasers, p. 349 (Academic Press, London, 1995)

(Suggest additional literature!)

Questions and Comments from Users


Could we consider ECDLs with Littrow or Littman as semiconductor lasers in regime 5 concerning the optical feedback?

The author's answer:

Yes, normally the optical feedback is relatively strong, so that you are in that regime.


So, does it mean that ECDLs are more immune to random back-reflections due to the fact that they operate in regime 5 of optical feedback?

The author's answer:

I am not sure about that. Having a laser resonator with relatively low output coupling does not necessarily imply low sensitivity to back-reflections. One would need to study the literature to assess that in detail.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Spam check:

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.


Share this with your network:

Follow our specific LinkedIn pages for more insights and updates: