External-cavity Diode Lasers
An external-cavity diode laser is a diode laser based on a laser diode chip which is integrated into a somewhat larger laser resonator, also containing other optical elements. The diode chip typically has one end anti-reflection coated, 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.
The external laser resonator introduces various new features and options:
- An optical bandpass filter can be used to determine the emission wavelength and reduce the emission linewidth. For example, one can use a diffraction grating or a volume Bragg grating (VBG, also called VHG = volume holographic grating) to obtain a fixed-wavelength narrow-linewidth laser.
- Wavelength tuning is possible by including some adjustable optical bandpass filter as tuning element. Most often, a diffraction grating is used for this purpose. (For details, see below.) 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.
- Comparing with a standard laser diode, the longer resonator increases the damping time of the intracavity light and thus allows for lower phase noise and a smaller emission linewidth (in single-frequency operation). An intracavity filter such as the diffraction grating can further reduce the linewidth. Typical linewidths of external-cavity diode lasers are below 1 MHz.
- The external resonator also adds important features for mode locking (see below).
External-cavity diode lasers can be made with very compact setups. Depending on the additional optical elements required, it is often possible to make miniature lasers of that type.
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. (One may alternatively rotate the assembly with the diode chip and the lens.)
In the Littman–Metcalf configuration (, 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. That 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.
Mode-locked External-cavity Diode Lasers
- Additional optical elements, such as a saturable absorber for passive mode locking or an optical filter, can be inserted in the laser resonator.
- The longer laser resonator allows for lower pulse repetition rates (although still usually 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 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 high output powers, but are much more compact and far cheaper to manufacture.
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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 categories optoelectronics, laser devices and laser physics