External-cavity Diode Lasers
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.
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.
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.)
Caption: Tunable external-cavity diode lasers in Littrow and Littman–Metcalf configurations.
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 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
- 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.
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|||P. Zorabedian, “Tunable external-cavity semiconductor lasers”, in F. J. Duarte (ed.), Tunable Lasers, p. 349 (Academic Press, London, 1995)|
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
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