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.

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 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.

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.

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 clean pulses and high output power, but are much more compact and cheaper to manufacturer.

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.

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[1]	H. Edmonds and A. Smith, “Second-harmonic generation with the GaAs laser”, IEEE J. Quantum Electron. 6 (6), 356 (1970), doi: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), doi: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), doi: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), doi: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), doi: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)

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