Applications facilitated by tunable lasers can be divided into two basic categories. The first are situations where one or more discrete wavelengths are not available from any single- or multiline fixed-wavelength laser. Second, the laser wavelength must be tuned continuously during the experiment or test, such as spectroscopy and pump-probe experiments.
There are many types of tunable lasers that can produce tunable continuous-wave (CW), nanosecond, picosecond and femtosecond output. Their output characteristics are governed by the laser medium used. A fundamental requirement of tunable lasers is their ability to emit over an extended, continuous wavelength range. Specialized optical elements are employed to select a specific wavelength or wavelength band from within this range. This article surveys currently available tunable lasers; their operation and the applications where these lasers are commonly used.
CW Ti:sapphire and dye lasers
Throughout the 1970s and 1980s, applications requiring tunable CW output utilized jet-stream dye lasers pumped by argon-ion lasers. While capable of very narrow linewidths, these ion-pumped-dye systems had a number of disadvantages and were cumbersome to operate. The titanium:sapphire laser emerged around 1987 and became a good alternative to dye lasers for many applications. In 1996, all-solid-state green pump lasers became available as reliable, low-noise alternatives to argon-ion pump lasers. The all-solid-state tunable system had arrived.
Today, high-power tunable CW lasers are available with both dye and Ti:sapphire laser gain media (tunable diode lasers have lower powers and pulse energies are not discussed in this article), and are configured in two basic resonator layouts: the standing-wave cavity and the ring laser cavity.
CW standing-wave lasers
Conceptually, the CW standing-wave laser is the simplest laser architecture. Consisting of a high reflector, gain medium and an output coupler, this laser can provide broad tunability using various laser gain media. In a standing-wave CW laser cavity, shown schematically in Figure 1, wavelength tuning is accomplished by the use of passive wavelength-stabilizing elements. The first of these elements is a multiplate birefringence or Lyot filter that is mounted within the cavity at Brewster’s angle to minimize reflection losses. This device modulates the spectral gain of the laser cavity by providing high transmission at a specific wavelength range within the spectral gain curve. The laser is forced to operate at that specific wavelength, which can be varied by rotating the birefringence filter (also referred to as the BiFi). The output of this type of laser has a linewidth of less than 40 GHz FWHM (<1.5 cm–1). By adding intracavity etalons, this linewidth can be narrowed further to less than 1 GHz.
Figure 1. In a CW Ti:sapphire laser, wavelength tuning is controlled by a birefringent filter.
CW ring lasers
The second major CW tunable cavity configuration type is the ring laser. Ring lasers have been used since the early 1980s to achieve tunable CW radiation from a single longitudinal cavity mode, with spectral bandwidths possible in the submegahertz region. Initially using a dye solution as the active medium, ring lasers are available today utilizing either dye or Ti:sapphire lasing media.
The main difference between a ring laser and a standing-wave laser is the fact that, in the ring laser, the ring structure allows us to cause the radiation circulating in the cavity to form a unidirectional traveling wave. This traveling wave, by its very nature, will not gain-deplete specific sections of the laser medium, and therefore the cavity can much more readily be forced into single-mode operation.
As shown in Figure 2, ring lasers use the same wavelength-selective elements that are used in tunable standing-wave lasers. Thus, ring lasers also employ a birefringence filter, a thin etalon and a thick etalon. These are the passive elements for stabilizing the cavity; their function is to select just one of the many longitudinal modes that are possible within the gain medium. The ring laser in Figure 2 is also equipped with a fast-piezo-driven thick etalon where a servo loop ensures that the etalon is accurately centered on the selected cavity mode.
Figure 2. Optical layout of a ring Ti:sapphire laser with external reference cell.
In the case of a passively stabilized ring laser, what determines the actual bandwidth of the single-frequency laser output? Although the finesse of an actively operating single-frequency ring laser will be quite high, the measured effective bandwidth from a ring laser will actually be determined by mechanical movements and vibrations over time.
To ensure a constant time-averaged cavity length, and thus a narrowband spectral output, two aspects of laser design are essential:
First, one must ensure that the laser is damped to minimize vibrational resonances and is built for low sensitivity to acoustic perturbations. The second design feature necessary to obtain very narrowband output is the use of state-of-the-art mechanisms for active cavity-length stabilization.
Effective passive stabilization of a ring laser can allow achievement of time-averaged spectral linewidths in the megahertz range. Further narrowing of the laser linewidth can now be achieved only by introducing active cavity-length elements into the ring cavity.
To achieve ultranarrow spectral bandwidths, a ring laser typically utilizes two types of elements to stabilize the length of the resonance cavity: mechanical piezo-driven mirrors for kilohertz response times and an electro-optical (E-O) modulator for the megahertz range.
In fact, in specialized laboratory setups, spectral bandwidths measured in hertz can actually be achieved. The key element in determining the ultimate spectral resolution of a ring laser is the external frequency reference cavity. For example, in Figure 2, there is an external reference cavity that is used to create an “absolute” reference signal necessary to laser cavity length. This external cell must be very effectively temperature-insulated from the environment, and it must be isolated mechanically and acoustically to preclude any movement or perturbation whatsoever. Ideally, the reference cell is well-separated, both mechanically and thermally, from the ring laser cavity itself to avoid any coupling between the two.
The Pound-Drever-Hall method is used to create the electro-optical feedback signal from the reference cavity to the cavity-length stabilization devices in the laser. An electro-optical modulator produces sidebands on a comparison beam from the laser output that is fed into the reference cavity. Electronic analysis of these sidebands generates an effective feedback signal for stabilizing the cavity.
A ring laser also can be used conveniently with an external cavity frequency doubler to produce tunable UV radiation from 205 to 500 nm.
Ultrafast Ti:sapphire lasers
A CW laser based on Ti:sapphire gain medium can be mode-locked – a condition where the laser resonator adopts a fixed-phase relationship between the longitudinal modes. This results in a stream of temporally short (picosecond or femtosecond) pulses. Mode locked lasers are often called ultrafast lasers and the applications they support, ultrafast spectroscopy. To achieve mode-locking the gain of the laser cavity is modulated at a time interval corresponding to the time necessary for the light pulse to travel one cavity round trip. When this condition is satisfied, the laser produces short pulses whose time duration is inversely correlated to the number of longitudinal modes. As a result, femtosecond lasers exhibit a broad output spectrum (>10 nm). Today, the majority of mode-locked lasers are based on Ti:sapphire, and mode-locking is achieved via the nonlinear Kerr effect where the high peak power generated by the short pulse induces a nonlinear self-focusing that serves to modulate the cavity gain profile. A disadvantage to this approach is that the stability of the laser suffers as the pulse duration becomes longer. In these cases, an acousto-optic modulator is used to stabilize the laser output. Regenerative mode-locking, where the AOM frequency is derived from the cavity repetition rate, has been used for more than 10 years to extend the capabilities of the Ti:sapphire laser and is considered the most reliable method for mode locking Ti:sapphire. The layout of this type of laser is shown in Figure 3. The pulse repetition rate of a mode-locked laser depends on the cavity length, with typical commercial lasers operating at around 80 MHz.
Figure 3. In a mode-locked Ti:sapphire laser, the center wavelength is tuned by moving a tuning slit, located between two dispersive prisms.
The biggest advance in this type of technology has been the development of one-box, computer-controlled, Ti:sapphire lasers. Here, the tunable Ti:sapphire oscillator and the solid-state pump laser are enclosed in a single, compact laser head. The significance of this development is the enablement of multiple applications. Historically, Ti:sapphire technology, due to its complexity, has been limited to users that are comfortable with laser systems. These systems require CW pump lasers to operate and the alignment of the Ti:sapphire cavity requires great care and precision. Automating the alignment process and built-in performance checks have made the technology accessible to less experienced users – biologists, chemists and even physicians. These applications require agile wavelength tuning and exceptional stability, both of which are facilitated using these one-box platforms.
Most recently, wavelength accessibility has been further increased using next generation one-box laser systems. These lasers offer an impressive 680 to 1300 nm tuning range in a compact, computer controlled package. Tunable ultrafast lasers are not only extremely useful for many applications but also extremely accessible to a wide range of users.
Ultrafast OPOs and OPAs
While the pulsed ultrafast output can be frequency doubled and even tripled, this nonetheless leaves spectral regions in the visible and IR that are not accessible with a Ti:sapphire laser and its harmonics. Users needing tunable ultrafast output within these regions utilize a mode-locked Ti:sapphire laser at a fixed wavelength to pump an optical parametric oscillator. For applications requiring higher pulse energy, the setup often consists of a mode-locked Ti:sapphire oscillator and Ti:sapphire amplifier pumping an OPA.
The principles of operation of an ultrafast optical parametric oscillator are virtually the same as for the nanosecond devices described later, with one important exception – timing. Inside the cavity of an ultrafast laser or OPO, the light consists of a short pulse that travels back and forth through the cavity. But unlike a conventional laser, the medium producing the tunable output cannot store gain; the OPO crystal can only emit light when the pump pulse is present. The successful operation of an ultrafast OPO requires the pulses from the pump source to arrive at the crystal at the same time as the idler and signal photons that are circulating around the OPO cavity. In other words, the fixed-wavelength Ti:sapphire laser and the ultrafast OPO must have the same pulse repetition frequency. This is called synchronous pumping and demands that the laser and OPO have exactly the same cavity round-trip time.
The layout of a typical ultrafast OPO is shown in Figure 4. Phase matching and cavity length are under automated control to select the desired wavelength and ensure the cavity round-trip time at that wavelength remains the same (80 MHz) as the Ti:sapphire pump laser. This type of OPO generates signal and idler outputs with a total wavelength coverage from 490 to 750 nm (signal output) and 930 nm to 2.5 µm (idler output) with pulse widths below 200 fs. When coupled with the tuning range of the Ti:sapphire fundamental (690 to 1040 nm), the system covers the wavelength range from 485 nm to 2.5 µm. Typical applications include soliton studies, time-resolved vibrational spectroscopy and ultrafast pump-probe experiments.
Figure 4. In a synchronously pumped OPO, the center wavelength is changed by adjusting the phase matching angle of the nonlinear crystal.
An OPA makes use of the same nonlinear optical process, but because the pump pulses have a higher peak power, an optical resonator is not required to achieve efficient wavelength conversion. A small part of the beam from an ultrafast amplifier is focused into a sapphire plate to generate a white-light continuum. This is used to seed the OPA crystals (usually BBO) pumped by the rest of the ultrafast amplifier beam and where it undergoes orders of magnitude amplification at the signal and idler wavelengths in a single pass. The center wavelength of the output is again controlled by the phase matching conditions of the crystal, and the spectral bandwidth is generally determined by the bandwidth of the pump and seed beams, or the acceptance bandwidth of the crystal.
This type of OPA can be operated in the femtosecond or picosecond domain with pulse energies as high as several millijoules per pulse. The output wavelength can be tuned from 1.1 to 2.6 µm. With harmonic conversion options, including difference frequency mixing, an OPA can cover the entire wavelength range from less than 200 nm in the UV to 20 µm in the far-infrared. The main applications for these systems are in sophisticated pump-probe experiments requiring multiple pulses at different wavelengths that are synchronized in time, such as time-resolved transient absorption.
Nanosecond pulsed dye lasers
The pulsed dye laser is one of the most conceptually simple tunable laser. In the most basic oscillator-only format, the medium is a single cuvette of fluorescent dye solution. This dye is optically pumped by a fixed-wavelength laser, such as a Q-switched Nd:YAG (at 532 or 355 nm). Depending on the repetition rate and the output power of the pump laser, the dye in the cuvette is circulated from a cooled reservoir to ensure fresh, relaxed dye for each pulse.
As shown in Figure 5, the dye cell sits in a cavity, which is defined by an output coupler and a diffraction grating (rather than the usual high-reflector rear mirror). The latter acts as a wavelength-selective mirror – only one wavelength is efficiently reflected back into the cavity. To maximize the effect of the grating and thereby minimize the wavelength output bandwidth, it is important that the beam cover a large area of the grating. Dye lasers generally utilize one of two designs. The Littmann design places the diffraction grating at a grazing incidence angle to the light beam, and a high reflector is used as the cavity end mirror. The Littrow design, on the other hand, uses a prism telescope to expand the beam across the grating, which also acts as the end mirror. In some lasers, these two methods are combined in a dual-grating arrangement to achieve linewidths as narrow as 0.03 cm–1.
Figure 5. In a pulsed dye laser, several optical schemes can be used or combined in the oscillator stage to ensure that the oscillator beam fills the diffraction grating.
The output beam from the oscillator is usually amplified in one or more dye cells. In practice, only 10 to 20 percent of the pump laser light is typically used to drive the oscillator; the remainder is used to pump the amplifier stage(s). The amplifier cells are usually single-pass amplifiers with no cavity optics; the beam passes through each amplifier one time on its way through the laser head.
Using different dyes and a Nd:YAG laser, which enables pumping at 532 or 355 nm, these lasers can deliver tunable output from 380 to 1500 nm with each dye typically tuning over a few tens of nanometers. With continual improvement in oscillator designs, spectral linewidths can be as low as 0.03 cm–1. Output energy can exceed 250 mJ.
The output pulse duration and repetition rate are dependent on the pump laser, with typical pulse rates from 10 to 100 Hz and typical pulse durations of about 10 ns. This short duration and high pulse energy mean the output can be efficiently shifted to other wavelength regions by nonlinear techniques such as frequency doubling and difference mixing, as well as Raman shifting. This extends output from the deep-UV to the mid-IR (up to 11 µm).
Typical uses for these lasers are pump-probe photochemistry, Thompson scattering experiments in high-energy physics and remote atmospheric sensing. A particularly interesting application is planar laser-induced fluorescence (PLIF), which is now used as a combustion diagnostic by manufacturers of furnaces and automotive engines.
Nanosecond OPOs and OPAs
The optical parametric oscillator (OPO) has emerged as a solid-state alternative to the tunable dye laser. An OPO relies on a nonlinear optical process termed parametric downconversion. As shown in Figure 6, an input photon is split to produce two photons of lower energy in an appropriate type of nonlinear crystal (e.g., LBO or BBO). These photons are referred to as signal and idler photons where the sum of these photons conserves the original photon energy. By definition, the signal has higher photon energy than the idler.
Figure 6. In parametric downconversion, a high-energy photon is converted into two lower-energy photons in a nonlinear optical crystal.
This process can only produce significant output when a so-called phase matching condition is met. The net result is that for a given wavelength, crystal temperature and incidence angle, there is only one signal (and idler) wavelength that can be generated. Changing the angle and/or temperature of the crystal thereby produces tunable output. Generating higher harmonics of signal or idler output, or difference frequency mixing the idler and signal beams, can provide even wider spectral coverage from a single OPO or optical parametric amplifier (OPA) – covering the near-UV through the mid-IR. A dye laser equipped with a difference frequency mixing unit followed by an OPA extends tunability further into the mid-IR. High-resolution IR spectra have been reported at 5.6 µm (formaldehyde) and 6.5 µm (acetylphenylalanine-O-methyl) using this type of a system.
Nanosecond OPOs are typically pumped by Nd:YAG lasers at the 1064-nm fundamental, 532-nm second harmonic or
355-nm third harmonic. In OPO systems that are pumped at the 355-nm third harmonic, there are two basic types of interactions – Type I and Type II. Both types provide tunability throughout the visible and out into the mid-IR to 2.5 µm.
In the most basic Type I interaction, the signal and idler have the same polarization. These OPOs characteristically have very high efficiencies and generate very smooth output beams. Their output linewidth at 500 nm is on the order of 15 cm–1.
In Type II interaction OPOs, the signal and idler are orthogonally polarized. This type of OPO is often referred to as the mid-band OPO. Here the OPO linewidth is much narrower, on the order of 2 cm–1. The narrow linewidths of mid-band OPOs enable efficient generation into the UV, delivering generation down to wavelengths as short as 206 nm. Other options involve mixing the various harmonics of the YAG laser with the signal of the OPO to further extend the energies and wavelengths accessible to the OPO system.
OPOs are ideal for applications requiring wide tunability and moderate linewidths. Nanosecond OPOs have replaced dye lasers in many applications that require tuning from the visible to 2.5 µm, including laser spectroscopy, laser-induced fluorescence, combustion studies, remote sensing and, more recently, photoacoustic imaging.