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Tunable Lasers

Definition: lasers the output wavelengths of which can be tuned

Alternative term: wavelength-tunable lasers

More specific term: wavelength-swept lasers

German: abstimmbare Laser

Category: laser devices and laser physics

Author: Dr. Rüdiger Paschotta

How to cite the article; suggest additional literature

URL: https://www.rp-photonics.com/tunable_lasers.html

A tunable laser (alternative spelling: tuneable laser) is a laser the emission wavelength of which can be tuned (i.e. adjusted) (→ wavelength tuning). That tuning is usually possible during operation, i.e., it does not only mean that a certain wavelength can be permanently set in the factory. Very wide tuning ranges (hundreds of nanometers) are achieved in some cases, while in other cases tuning is possible only over a fraction of a nanometer. Lasers are sometimes called wavelength agile or frequency agile when the tuning can be done with high speed.

Tunable lasers are usually operating in a continuous fashion with a small emission bandwidth, although some Q-switched and mode-locked lasers can also be wavelength-tuned.

The tuning characteristics can be of substantially different kinds:

There are single-frequency lasers, where the emission linewidth is very narrow, corresponding to a very well defined wavelength. Some of these lasers can be continuously tuned only over a small range of optical frequencies, while others can be tuned over a frequency range which is much larger than the free spectral range of the laser resonator. Only with relatively sophisticated technology, one can achieve tuning over a large range without mode hopping, i.e., a discontinuous evolution of the optical frequency.
Other lasers operate on multiple resonator modes simultaneously, so that their optical spectrum exhibits several or even many spectral lines. In such cases, wavelength tuning usually just means that the envelope of the optical spectrum can be shifted, but with no control of the individual line frequencies.

There are certain lasers which are optimized such that the output wavelength can be periodically and rapidly swept through a substantial range. They are called wavelength-swept lasers and discussed in a separate encyclopedia article. Some of those lasers are not suitable for arbitrary tuning, but only for the mentioned periodic mode.

Note that there are also other kinds of wavelength-tunable light sources such as optical parametric oscillators and sources based on supercontinuum generation. The latter are much more limited in terms of radiance and particularly spectral radiance, but can cover a very wide spectral range.

Widely Tunable Lasers

Some types of lasers offer particularly broad wavelength tuning ranges:

A few solid-state bulk lasers (see Figure 1), in particular titanium–sapphire lasers and Cr:ZnSe and Cr:ZnS lasers allow tuning over hundreds of nanometers in the near- and mid-infrared spectral region. (In general, transition-metal doped laser gain media offer larger tuning ranges than rare-earth-doped laser gain media, since the electrons involved in such media interact more strongly with the host lattice; see the article on vibronic lasers.) Output powers can be hundreds of milliwatts or even multiple watts.
Dye lasers also allow for broadband tunability. Different dyes can cover very broad wavelength ranges, e.g. throughout the visible region. There are narrow-linewidth dye laser systems (continuous-wave or pulsed) for use in laser spectroscopy, and also mode-locked dye lasers generating femtosecond pulses.
Some free electron lasers can cover enormously broad wavelength ranges, and often in extreme spectral regions.

tunable solid-state laser — Figure 1: Setup of a tunable solid-state bulk laser, realized e.g. with a Ti:sapphire laser crystal. The prism pair spatially disperses the different wavelength components, so that the movable slit can be used to shift the wavelength away from that of maximum gain.

Other types of lasers offer tuning ranges spanning a few nanometers to some tens of nanometers:

Rare-earth-doped fiber lasers, e.g. based on ytterbium, can often be tuned over tens of nanometers, sometimes even more than 100 nm. Most Raman fiber lasers also have the potential for wideband tuning.
Some rare-earth-doped laser crystals, often doped with ytterbium, also allow for substantial tuning ranges of bulk lasers. Examples are tungstates, vanadates, Yb:BOYS, and Yb:CALGO.
Color center lasers rely on broadband gain from certain lattice defects in a crystal, which can be generated e.g. with gamma irradiation. They are not widely used, however.
Most laser diodes can be tuned over a few nanometers by varying the junction temperature, but some special types such as external-cavity diode lasers and distributed Bragg reflector lasers can be tuned over 40 nm and more with additional means such as an intracavity diffraction grating.
Quantum cascade lasers are also broadly tunable mid-infrared laser sources.

Some fine tuning, often continuously without mode hops, is possible for other lasers:

Some compact solid-state bulk lasers such as nonplanar ring oscillators (NPROs, MISERs) allow continuous tuning within their free spectral range of several gigahertz. Tuning may be accomplished by applying stress to the laser crystal via a piezo, or by varying the crystal temperature.
Similar fine tuning is possible with some single-frequency laser diodes, e.g. by varying the drive current.

For wideband tuning in various spectral regions, optical parametric oscillators (OPOs) can be used. These are actually not lasers, but OPO sources are nevertheless sometimes included with the term tunable laser sources.

Applications of Tunable Lasers

Wavelength-tunable laser sources have many applications, some examples of which are:

In laser absorption spectroscopy, a wavelength-tunable laser with narrow optical bandwidth can be used for recording absorption spectra with very high frequency resolution. In a LIDAR system, a laser may be tuned to a wavelength which is specific to a certain substance to be monitored.
Various methods of laser cooling require a laser wavelength to be adjusted very precisely at or near some atomic resonance.
Tuning to atomic resonances is also used in laser isotope separation. The laser is then tuned to a particular isotope in order to ionize these atoms and subsequently deflect them with an electric field.
A tunable laser can be used for device characterization, e.g. of photonic integrated circuits.
In optical fiber communications with wavelength division multiplexing, a tunable laser can serve as a spare in the case that one of the fixed-wavelength lasers for the particular channels fails. Even though the cost for a tunable laser is higher, its use can be economical as a single spare laser can work on any transmission channel where it is needed. As the cost of a tunable lasers may not be much higher than for non-tunable ones, tunable lasers are sometimes even used throughout.
In optical frequency metrology, it is often necessary to stabilize the wavelength of a laser to a certain reference standard (e.g. a multipass gas cell or an optical reference cavity). This can be accomplished e.g. with an electronic feedback system, which automatically adjusts the laser wavelength.
Some interferometers and fiber-optic sensors profit from a wavelength-tunable laser source, e.g. if this makes it possible to remove an ambiguity or to avoid mechanical scanning of an optical path length.

Suppliers

The RP Photonics Buyer's Guide contains 96 suppliers for tunable lasers. Among them:

Stuttgart Instruments

The Stuttgart Instruments Alpha is an ultrafast and fully wavelength-tunable frequency conversion system in an ultra-compact and completely passively stable system based on revolutionary parametric oscillator design which guarantees outstanding stability, reproducibility and shot-noise limited performance.

The revolutionary design of Stuttgart Instruments Alpha, characterized by outstanding low noise and passive long-term stability, is based on the fiber-feedback optical parametric oscillator (FFOPO) technology and results in outstanding performance and high flexibility at the same time.

The Alpha covers a gap-free rapid tunable spectral range from 700 nm to 20 µm wavelengths, while maintaining high output power up to the Watt-level with femto- or picosecond pulses at several MHz pulse repetition rates. It provides multiple simultaneously tunable outputs with a selectable bandwidth from a few to 100 cm⁻¹. Shot-noise limited performance above 300 kHz, passive spectral stability (< 0.02 % rms) and wavelength-independent stable beam pointing (< 30 µrad) enable excellent sensitivity. In addition, each Alpha is equipped with a user-friendly ethernet and Wi-Fi interface and a matching graphical user interface (GUI) as well as easy to access API interfaces for e.g. LabView, Phyton, C++.

Typically, the Alpha is pumped by an ultra-low-noise Primus pump laser, which provides more than 8 W average output power at 1040 nm wavelength and 450 fs pulse duration at 42 MHz repetition rate. In addition, the Alpha can be operated with other pump lasers around 1 µm wavelength and enough power.

Due to our modular platform, the Alpha can be adapted and optimized for various applications and is particularly suited for spectroscopic applications requiring a robust and reliable tunable radiation with low noise.

Bibliography

[1]	J. J. Colles and C. R. Pidgeon, “Tunable lasers”, Rep. Prog. Phys. 38, 329 (1975), DOI:10.1088/0034-4885/38/3/001
[2]	C. V. Shank, “Physics of dye lasers”, Rev. Mod. Phys. 47, 649 (1975), DOI:10.1103/RevModPhys.47.649
[3]	J. R. Taylor, “Tunable solid state lasers”, J. Mod. Opt. 32 (12), 1450 (1985), DOI:10.1080/716099684
[4]	K. Kobayashi and I. Mito, “Single frequency and tunable laser diodes”, IEEE J. Lightwave Technol. 6 (11), 1623 (1988), DOI:10.1109/50.9978
[5]	P. F. Moulton, “Tunable solid-state lasers”, Proc. IEEE 80 (3), 348 (1992), DOI:10.1109/5.135352
[6]	E. Gulevich et al., “Current state and prospects for tunable titanium–sapphire lasers”, Proc. SPIE 2095, 102 (1994), DOI:10.1117/12.183081
[7]	C. Hönninger et al., “Efficient and tunable diode-pumped femtosecond Yb:glass lasers”, Opt. Lett. 23 (2), 126 (1998), DOI:10.1364/OL.23.000126
[8]	C. J. Chang-Hasnain, “Tunable VCSEL”, J. Sel. Top. Quantum Electron. 6 (6), 978 (2000), DOI:10.1109/2944.902146
[9]	C. Petridis et al., “Mode-hop-free tuning over 80 GHz of an extended cavity diode laser without antireflection coating”, Rev. Sci. Instrum. 72 (10), 3812 (2001), DOI:10.1063/1.1405783
[10]	L. A. Coldren et al., “Tunable semiconductor lasers: a tutorial”, J. Lightwave Technol. 22 (1), 193 (2004), DOI:10.1109/JLT.2003.822207
[11]	M. C. Y. Huang et al., “A nanoelectromechanical tunable laser”, Nature Photon. 2, 180 (2008), DOI:10.1038/nphoton.2008.3
[12]	F. Mollenauer, J. C. White, and C. R. Pollack, Tunable Lasers, Springer, Berlin (1993)
[13]	F. J. Duarte, Tunable Lasers Handbook, Academic Press, New York (1995)
[14]	M. C. Amann and J. Buus, Tunable Laser Diodes, Artech House Publishers, Norwood, MA (1998)

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