External Cavity Diode Lasers | previous | next | feedback |
(Acronym: ECDL)
Definition: non-monolithic diode lasers where the laser cavity (resonator) is completed with external optical elements
An external cavity diode laser is a semiconductor laser based on a laser diode chip which typically has one end anti-reflection coated, and the laser resonator is completed with, e.g., a collimating lens 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 on free-space optics. Narrowband optical feedback can then come from a fiber Bragg grating.
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.
The external laser resonator introduces various new features and options:
- 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.
- Wavelength tuning is possible by including some adjustable optical filter as tuning element. Most often, a diffraction grating is used for this purpose. For details, see below.
- The external resonator also adds important features for mode locking (see below).
Note that there are external-cavity semiconductor lasers, which, however, are usually not diode lasers: vertical external-cavity surface-emitting lasers (VECSELs).
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.
Figure 2: Tunable external-cavity diode lasers in Littrow and Littman-Metcalf configuration
In the Littman-Metcalf configuration ([2], 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. This 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.
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 various interesting properties:
- 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.
Mode-locked external-cavity diode lasers sometimes compete with mode-locked fiber lasers. They do not reach their potential for clean pulses and high output power, but are much more compact and cheaper to manufacturer.
Applications
Mode-locked ECDLs are mostly used in data transmitters for optical communications. Tunable devices find applications in areas such as absorption spectroscopy of trace gases.
Bibliography
| [1] | M. G. Littman and H. J. Metcalf, "Spectrally narrow pulsed dye laser without beam expander", Appl. Opt. 17 (14), 2224 (1978) |
| [2] | K. Liu and M. G. Littman, "Novel geometry for single-mode scanning of tunable lasers", Opt. Lett. 6 (3), 117 (1981) |
| [3] | M. Fleming and A. Mooradian, "Spectral characteristics of external-cavity controlled semiconductor lasers", IEEE J. Quantum Electron. 17 (1), 44 (1981) |
| [4] | C. J. Hawthorn et al., "Littrow configuration tunable external cavity diode laser with fixed direction output beam", Rev. Sci. Instrum. 72 (12), 4477 (2001) |
See also: laser diodes, mode-locked diode lasers, semiconductor lasers, wavelength tuning, linewidth, mode-locked lasers, distributed Bragg reflector lasers, vertical external-cavity surface-emitting lasers
