External-cavity Diode Lasers
Author: the photonics expert Dr. Rüdiger Paschotta
Acronym: ECDL
Definition: non-monolithic diode lasers where the laser cavity (resonator) is completed with external optical elements
More general term: diode lasers
Categories: 
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:
- An optical bandpass filter can be used to determine the emission wavelength and to reduce the emission linewidth. diffraction grating or a volume Bragg grating (VBG, also called VHG = Volume Holographic Grating) can be used to obtain a fixed wavelength narrow-linewidth laser.
- Wavelength tuning is possible by incorporating an adjustable optical bandpass filter as a tuning element. Most often a diffraction grating is used. (See below for details.) Another possibility is to use a wavelength selector with a somewhat larger bandwidth and tune the emission wavelength within its range, e.g. by varying the drive current of the laser diode.
- Compared to a standard laser diode, the longer resonator increases the damping time of the intracavity light, allowing for lower phase noise and a smaller emission linewidth (in single-frequency operation). An intracavity filter such as a diffraction grating can further reduce the linewidth. Typical linewidths of external cavity diode lasers are below 1 MHz.
- The external cavity also adds important features for mode locking (see below).
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.)
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:
- Additional optical elements, such as a saturable absorber for passive mode locking or an optical filter, can be inserted into the laser resonator.
- The longer laser resonator allows for a lower pulse repetition rate (although usually still above 1 GHz), and also for tuning the repetition rate by changing the resonator length.
- Even for high repetition rates of tens of gigahertz, external-cavity lasers, then operated with harmonic mode locking, can be interesting because they exhibit lower laser noise, e.g. in the form of timing jitter.
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.
Applications
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:
Suppliers
The RP Photonics Buyer's Guide contains 20 suppliers for external-cavity diode lasers. Among them:
Bibliography
| [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) |
Questions and Comments from Users
2021-03-01
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.
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2021-02-15
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.