External-cavity Diode Lasers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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:

• diode lasers
• laser diodes
• mode-locked diode lasers
• semiconductor lasers
• wavelength tuning

Bibliography

(Suggest additional literature!)

Suppliers

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

Sacher Lasertechnik

The Micron laser is a mode-hop free tunable external cavity laser for output powers of up to 300 mW. Versions with emission wavelengths between 638 nm and 1700 nm are available.

TOPTICA Photonics

TOPTICA offers ECDLs with unique tuning capabilities. The CTL lasers tune up to 110 nm without any mode hop. Finding resonances of quantum dots or micro resonators as well as spectroscopy has never been easier. TOPTICA continues to provide the DL pro, which is now available at even more wavelengths and is well suited for laser cooling and many other AMO experiments.

eagleyard Photonics

Gain Chips are modified Fabry–Pérot laser diodes with an excellent AR coated output facet. While not self-lasing they are intended to be operated in External Cavity Diode Laser setups (ECDL), such as in Littman or Littrow configuration. With external feedback they reveal narrow single frequency operation in combination with superior tuning capabilities.

Alpes Lasers

Alpes Lasers offers mid-IR external cavity lasers with wavelengths spanning 4.3 to 14 μm. External cavity kits with integrated laser control electronics are also available. These are Fabry–Pérot lasers designed for maximum width of the gain profile.

DRS Daylight Solutions

The Stretto is a high precision tunable external-cavity diode laser, available for narrow linewidth emission from ultraviolet to infrared. Being designed for seamless OEM integration and efficient volume production scaling, Stretto is a go-to laser solution for industries requiring high precision and reliability.

RPMC Lasers

Serving North America, RPMC Lasers offers a wide selection of VBG based external-cavity diode lasers, available in wavelengths from ≈633 nm to ≈1070 nm, and from ≈10s of milliwatts up to ≈ 500 W output power, providing narrow linewidth, CW output. VBG based external-cavity diode lasers are used in many applications, including fiber and DPSS laser pumping, spectroscopy, Raman spectroscopy, CARS, differential absorption, and many more. These lasers are available in both fiber-coupled and free-space, and as both OEM components and modules as well as turnkey systems. Standard and custom options available. Let RPMC help you find the right laser today!

Questions and Comments from Users