For some applications, it is required that the wavelength (and thus the optical frequency) of a laser beam can be tuned within some range. Whereas the article on tunable lasers discusses various types of lasers which have an adjustable output wavelength, this article explains several methods of wavelength tuning.
Tuning via the Gain Medium
The first method is to influence the laser gain medium in such a way that the wavelength of maximum gain is changed, and the output wavelength changes accordingly (Figure 1). This method is often applied to lasers with operation on multiple resonator modes, where the “center of gravity” of the optical spectrum may be tuned fairly continuously. The method can also be applied to single-frequency lasers, but then it usually leads to mode hopping rather than continuous tuning.
Laser diodes are often tuned via the temperature, e.g. by changing the drive current of a thermoelectric cooler on which the laser diode is mounted, or the drive current of the laser diode itself. Typically, laser diodes tune by ≈ +0.3 nm/K, and the total tuning range achieved in that way may be a few nanometers wide. The resonator mode frequencies are also affected by the temperature change, but they react less strongly than the gain spectrum. In the case of single-frequency operation, continuous tuning over a wider range may be achieved if appropriate measures are taken to appropriate tune the resonance frequencies and suppress mode hopping. For example, the resonator length may be tuned (see below) together with the drive current in the case of an external-cavity diode laser.
Tuning With an Intracavity Filter
The second method is to introduce a tunable optical filter into the laser resonator, which has a pronounced loss minimum at some adjustable wavelength (Figure 2). This makes it possible to influence the wavelength of maximum net gain, at which the laser is usually forced to operate. More precisely, the laser will usually operate on one or several resonator modes for which the inversion level of the gain medium required for lasing (i.e. for generating a gain which equals the resonator losses) is close to its minimum. In the steady state (for continuous-wave operation), light at the laser wavelength has zero round trip net gain, and all other wavelengths experience a negative net gain per resonator round trip (assuming homogeneous broadening of the gain spectrum). Note that even with an only slightly negative net gain, lasing can be completely suppressed.
This method of tuning is often applied to solid-state lasers. A wide wavelength tuning range of a laser requires a wide gain bandwidth of the gain medium. Some broadband gain media such as Ti:sapphire and Cr:ZnSe allow tuning over hundreds of nanometers. The tuning range obtained is usually the wavelength range in which sufficient net gain can be achieved. Its limits are often set by the points where the emission cross sections become too low or the resonator losses become too high. In some cases, the tuning range may be smaller because there is excited-state absorption, or because parasitic lasing at wavelengths with maximum laser gain can not be sufficiently suppressed. In some fiber lasers, the inversion level (and hence the gain at extreme wavelengths) is limited by amplified spontaneous emission near the wavelength of maximum gain.
Frequently used tuning elements in bulk laser resonators are:
- an etalon (Fabry–Pérot interferometer) or a birefringent tuner (Lyot filter) which can be rotated to adjust the wavelength of maximum transmission
- a prism pair in combination with a movable aperture
- a single prism in combination with and end mirror which can be tilted to adjust the wavelength for which the resonator is well aligned
- a volume Bragg grating used as a folding mirror with a variable angle of incidence
External-cavity diode lasers can also be tuned with an intracavity filter. A holographic diffraction grating can be used as an end mirror (Littrow configuration), which is rotated for tuning, or a fixed grating within the resonator combined with a movable end mirror (Littman configuration).
Tuning via the Resonator Length
A single-frequency laser can be tuned within approximately one free spectral range of its resonator by fine adjustment of the resonator length within a range of half a wavelength (for linear resonators) (Figure 4). The principle behind this is that the frequencies of the resonator modes are shifted. Attempts to tune further may cause the laser to mode hop to the next resonator mode, which then has the higher gain. A wider tuning range can be achieved if the wavelength of maximum gain is also tuned, or with an additional intracavity filter.
Relatively wideband mode-hop-free tunability can be obtained with very short laser cavities. This is used with, e.g., MEMS VCSELs, having a separate output coupling mirror the position of which can be tuned via thermal expansion, electrostatic forces, or a piezoelectric element.
Wavelength-tunable radiation can also be obtained with alternative techniques:
- Synchrotron radiation sources (wigglers and undulators, free electron lasers) allow wavelength tuning via the electron energy.
- Optical parametric oscillators are often tuned by affecting the phase-matching conditions.
- An optical parametric amplifier can be used for amplifying a variable part of a very broad spectrum, as obtained via supercontinuum generation. Alternatively, one may just use a supercontinuum source in conjunction with a tunable bandpass filter.
- The Raman self-frequency shift in an optical fiber can be exploited for wavelength tuning via the launched power of the pulses.
If a very fast periodic sweep of the optical frequency is required, an alternative method is available: short optical pulses are fed through a strongly dispersive element (e.g. a long fiber), so that the different wavelength components exit this element at different times.
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See also: wavelength, tunable lasers, distributed Bragg reflector lasers, wavelength-swept lasers, optical filters, Lyot filters, birefringent tuners, interferometers, The Photonics Spotlight 2009-12-08
and other articles in the categories laser devices and laser physics, methods