Many lasers allow the operator to tune or change the output wavelength in the UV to IR wavelength range as required. Following on from our previous discussion of how tunable lasers are implemented, this article will discuss in detail the types and applications of tunable lasers.
Applications using tunable lasers generally fall into two broad categories: those in which a single- or multi-line fixed-wavelength laser is unable to provide the desired discrete wavelength or wavelengths, and those in which the laser wavelength must be continuously tuned during experiments or tests, such as in spectroscopy and pump-probe experiments.
Many types of tunable lasers are capable of producing tunable continuous wave (CW), nanosecond, picosecond, or femtosecond pulse outputs. Their output characteristics are determined by the laser gain medium used.
A basic requirement for tunable lasers is that they are capable of emitting laser light over a wide range of wavelengths. Special optics can be used to select a specific wavelength or band of wavelengths from the emission band of the tunable laser.
There are a variety of gain materials capable of producing tunable lasers, the most common of which are organic dyes and titanium sapphire crystals (Ti:sapphire). In the case of these two gain materials, argon ion (Ar+) lasers or frequency-doubled neodymium ion (Nd3+) lasers are used as the pump source due to their efficient absorption of pump light at approximately 490 nm.
Dye molecules can be used to produce wavelengths in the ultraviolet to visible (UV-VIS) range. However, switching between many different dye molecules is required to achieve a wide tuning range, making the process quite cumbersome and complex. In contrast, solid-state lasers can achieve a wide tuning range using only one laser gain material (e.g., dielectric crystals), eliminating the need for frequent dye changes.
Currently, titanium sapphire has emerged as the primary tunable laser gain material, with a broad emission spectrum of 680 to 1100 nm that can be continuously tuned and an output that can be up-converted to the UV-VIS spectral range or down-converted to the IR spectral region. These properties enable a wide range of applications in chemistry and biology.
Tunable CW Standing Wave Lasers
Conceptually, the CW standing wave laser is the simplest laser architecture. It consists of a highly reflective mirror, gain medium, and an output coupler mirror (see Figure 1), which provides CW output using a variety of laser gain mediums. To achieve tunability, the gain medium needs to be selected to cover the target wavelength range.
Figure 1: Schematic of a titanium sapphire-based CW standing wave laser. A birefringent tuning filter is shown.
Many fluorescent dyes can be used to tune the laser wavelength to the desired range. The main advantage of dye lasers is the ability to cover a wide range of wavelengths in the UV-VIS band, but the disadvantage is that the use of a single dye/solvent provides only a narrow wavelength tuning capability. In contrast, solid-state titanium sapphire lasers have the advantage of providing a wide wavelength tuning range using a single gain medium, but have the disadvantage of only being able to operate in the near-infrared (NIR) band from 690 to 1100 nm.
For both gain media, wavelength tuning is achieved by passive wavelength stabilization elements. The first is the multi-plate birefringent filter or Lyot filter. This filter modulates the gain by providing high transmission at a specific wavelength, thus forcing the laser to operate at that wavelength.
Tuning is accomplished by rotating this birefringent filter. Although simple, the CW standing wave laser allows for multiple longitudinal laser modes. This produces a linewidth of about 40 GHz full-width half-height (<1.5 cm-1), which can be a limiting factor for some applications such as Raman spectroscopy. To achieve narrower linewidths, a ring configuration is required.
Tunable CW Ring Lasers
Since the early 1980s, ring lasers have been used to achieve tunable CW output via a single longitudinal mode with a spectral bandwidth in the kilohertz range. Similar to standing wave lasers, tunable ring lasers can use dyes and titanium sapphire as gain media. Dyes are capable of providing very narrow linewidths of <100 kHz, while titanium sapphire provides linewidths of <30 kHz. Dye lasers have a tuning range of 550 to 760 nm and titanium sapphire lasers have a tuning range of 680 to 1035 nm, and the outputs of both lasers can be frequency-doubled to the UV band.
According to Heisenberg's uncertainty principle, as the definition of energy becomes more precise, the pulse width that can be determined becomes less precise. For standing wave CW lasers, the cavity length defines the amount of energy allowed as a discrete longitudinal mode. When the cavity length is shorter, the number of longitudinal modes allowed increases, resulting in a wider, less defined output linewidth.
In the ring configuration, the laser cavity can be considered an infinitely long cavity and the energy can be precisely defined. Only a single longitudinal mode is present in the cavity. In order to achieve single-mode operating conditions, several optical elements are especially required (see Fig. 2).
Figure 2: Optical layout of a ring-shaped titanium sapphire laser with an external reference cavity.
First, a Faraday isolator is inserted into the cavity to ensure that the intracavity photons always follow the same path. An intracavity standardized fixture is used to further reduce the output linewidth. Unlike standing wave laser cavities, there are no end mirrors in the ring configuration. The photons circulate continuously within the laser cavity. Second, the cavity length must be stabilized to correct for any mechanical changes caused by environmental fluctuations such as heat or vibration.
To achieve ultra-narrow spectral bandwidths, the cavity can be stabilized using one of two methods: one method uses mechanical piezoelectric-driven mirrors to stabilize the cavity length with a response time in the kilohertz range, and the other method uses electro-optical (E-O) modulators to achieve response times in the megahertz range. Several specialized laboratory setups have shown that the spectral bandwidth can be measured in hertz. The key factor in determining the spectral resolution of the ring cavity is the external frequency reference cavity. As shown in Figure 2, a reference cavity is used to generate the signal needed to stabilize the laser cavity length. This external reference cavity must be isolated from environmental fluctuations caused by temperature, mechanical vibrations, and acoustic noise. The reference cavity should be well separated from the ring laser cavity itself to avoid unintentional coupling between the two. The reference signal is processed using the Pound-Drever-Hall method.
Mode-Locked Quasi-Continuum Lasers
For many applications, the precisely defined temporal characteristics of the laser output are more important than the precisely defined energy. In fact, achieving short optical pulses requires a cavity configuration in which many longitudinal modes resonate simultaneously. When these circulating longitudinal modes have a fixed phase relationship within the laser cavity, the laser is mode-locked. This will realize a single pulse oscillating within the cavity with a period defined by the laser cavity length.
Active mode-locking can be achieved using an acousto-optic modulator (AOM) or passive mode-locking through a Kerr lens. The former, which became more popular in the 1980s, utilizes the intracavity AOM as a transient shutter that opens and closes at half the frequency of the cavity length. Pulses of hundreds of picoseconds can be achieved using this method. In the last few decades, scientific applications have required improved temporal resolution and therefore shorter pulses.
Synchronously pumped dye lasers provide a viable method for tuning the center wavelength and shortening the optical pulse by an order of magnitude (to tens of picoseconds). To accomplish this, the dye laser cavity must have the same cavity length as the mode-locked pump laser. The pump and dye laser pulses meet at the gain medium to produce excited radiation from the dye molecules. The laser output is stabilized by adjusting the dye laser cavity length. Synchronized pumping configurations can also be used to drive optical parametric oscillators (OPOs) (discussed below).
The titanium sapphire mode-locked laser is an example of passive Kerr lens mode-locking (see Figure 3). In this approach, pulses are generated by gain modulation and the refractive index of titanium sapphire depends on the intensity.
In principle, as the pulse propagates through the gain medium, the peak intensity is higher in the presence of the pulse. This creates a passive lens that focuses the pulse beam more tightly and extracts the gain more efficiently until there is no gain to support the simultaneous resonance of the CW modes in the cavity. Mechanical perturbations to the cavity are used to induce intensity spikes to initiate mode locking. This approach allowed the titanium sapphire to produce pulses as short as 4 fs.
Figure 3: In a mode-locked titanium sapphire laser, the center wavelength is tuned by moving the tuning slit located between the two dispersive prisms.
It is worth noting that bandwidths of more than 300 nm can be combined into a single pulse. According to Heisenberg's uncertainty principle, shorter pulses require more longitudinal modes. Therefore, the laser cavity must have sufficient dispersion compensation from the cavity optics to maintain the phase relationship required for stable mode locking. As shown in Figure 3, compensating prisms are added to the cavity to ensure a constant phase relationship. Using this method, pulses as short as 20 fs can be obtained. In order to produce shorter pulses, higher order dispersion must also be compensated. This compensation is achieved using an optical chirp lens to maintain the phase relationship required for stable mode-locking.
Since chirped-lens mode-locking is most effective with shorter pulses (higher intensity), this method is primarily suited for generating femtosecond pulses. In the range of 100 fs~100 ps, a hybrid method called regenerative mode-locking can be used. This method uses intracavity AOM and the Kerr effect. the AOM drive frequency is derived from real-time measurements of the cavity repetition frequency, and its amplitude is dependent on pulse duration. As the desired pulse width increases and the Kerr effect decreases, the stabilized AOM amplitude increases to support mode locking. As a result, regenerative mode-locking can provide stable, tunable output over a wide range of 20 fs to 300 ps using a single laser system.
In the late 1990s, regenerative mode-locking enabled the first tunable, all-in-one computer-controlled titanium sapphire laser. This innovation made the technology more accessible to a wider range of researchers and applications. Advances in multiphoton imaging have been driven in large part by technological advances. Femtosecond laser pulses are now available to biologists, neuroscientists and physicians. A number of technological advances have been made over the years that have led to the general use of titanium sapphire lasers in bioimaging.
Ultrafast ytterbium lasers
Despite the broad utility of titanium sapphire lasers, some bioimaging experiments require longer wavelengths. Typical two-photon absorption processes are excited by photons at a wavelength of 900 nm. Because longer wavelengths mean less scattering, longer excitation wavelengths can more effectively drive biological experiments that require deeper imaging depths.
It is also critical to consider the wavelength of the subsequent fluorescent photons of the dye attached to the biological sample. The wavelength of such fluorescent photons is typically in the 450 to 550 nm band, which is more susceptible to scattering. Therefore, several fluorescent markers have been developed that progressively absorb infrared wavelengths. To meet this requirement, the industry has developed an all-in-one, computer-controlled, synchronously pumped OPO driven by a 1045 nm ytterbium laser with output wavelengths in the range of 680 to 1300 nm. For multiphoton imaging, this architecture offers a significantly higher performance alternative to titanium sapphire lasers.
Ultrafast Amplifiers
The above examples produce ultrafast pulses in the nano-Joule (nJ) energy range. However, many applications require higher energy tunable light sources. Since wavelength conversion is a nonlinear process, the efficiency depends on the energy available. For these applications, several techniques can be used to increase the energy and tunability of ultrafast lasers.
Amplification of ultrafast pulses can be divided into two main categories: multistage amplification and regenerative amplification. The former has the advantage that very high energies (100 mJ) can be achieved with very low input, but repeated passes through the amplification stage degrade the output beam quality. Therefore, regenerative amplification is the preferred method for generating pulse energies on the microJoule (µJ) or millijoule (mJ) scale.
In general, ultrafast pulse amplification is achieved by chirped-pulse amplification methods (see Fig. 4). The process starts with a mode-locked oscillator with femtosecond pulse duration, i.e., a seed laser. It is critical for the seed laser to have sufficient bandwidth so that the pulse duration can be stretched or chirped in time. Optical chirping occurs as a result of different colors of light, traveling through the optical material at different speeds. In general, red light travels faster than blue light. For example, a broadening grating introduces positively chirped red light before blue light to separate the wavelength components in time and space. Pulse broadening is necessary to reduce the strong peak power of millijoule-scale femtosecond pulses. After broadening, pulses of nearly 300 ps are directed to the second-stage regenerative laser cavity. The final step is to use a second grating to introduce a negative chirp and reconstruct the amplified pulse. The whole process is shown in Fig. 4.
Figure 4: Chirped pulse amplification
Today, most regenerative amplifiers use titanium sapphire, but other gain media (e.g. ytterbium) are becoming more and more popular. With both gain media, the amplifiers have a relatively narrow tunability, with titanium sapphire having a tuning range of about 780 to 820 nm, which limits their usefulness in spectroscopy applications. To overcome this limitation, several frequency conversion methods are available.
Harmonic frequency conversion, is the simplest way to tune the wavelength of an ultrafast oscillator or ultrafast amplifier system. In principle, the incident photons are upconverted to an integer multiple of the fundamental frequency. For titanium sapphire (fundamental tuning range 700~1000 nm), the tuning range of the second harmonic is 350~500 nm, the third harmonic is 233~333 nm, and the fourth harmonic is 175~250 nm. In practice, due to absorption by the harmonic crystals, the tuning of the fourth harmonic is limited to 200 nm. For applications that require a wavelength outside of this range, the Parameter For applications requiring wavelengths beyond this range, parameter conversion options are required.
Ultrafast OPO and OPA
Although the ultrafast pulse output can be multiplied or even tripled, the 700 to 1000 nm tuning range of the Titanium Sapphire leaves a wavelength gap in the UV-VIS and IR spectral regions. For experiments that require ultrafast pulses with wavelengths "in these 'blank' regions", down-conversion of the parameters is necessary. This method converts a single high-energy photon into two low-energy photons: a signal photon and an idler photon (see Figure 5).
Figure 5: Schematic of the parametric down-conversion.
The energy distribution between these two photons can be configured by the user. In a typical parametric configuration based on titanium sapphire, the incident photon at a wavelength of 800 nm, can be tuned continuously from about 1200 nm to 2600 nm.Since the down-parameter conversion is a nonlinear process, the conversion efficiency may become an issue. To overcome this limitation, an optical parametric oscillator (OPO) is used at the nanofocal energy level and an optical parametric amplifier (OPA) is used at the millifocal energy level.
In the OPO cavity, light consists of a short pulse that propagates back and forth through the cavity. However, unlike the dye laser configuration described above, the activation medium is a nonlinear crystal and does not store gain.The OPO crystal converts photons only in the presence of a pump pulse. Successful operation of an ultrafast OPO requires that the pulses from the pump source arrive at the crystal at the same time as the idle and signal photons circulating around the OPO cavity. In other words, a fixed-wavelength titanium sapphire laser and an ultrafast OPO must have exactly the same cavity length.
The layout of a typical ultrafast OPO is shown in Figure 6. Phase matching and cavity length automatically selects the desired wavelength and ensures that the intracavity round-trip time for that wavelength is kept at 80 MHz, which is the same as for a titanium sapphire pump laser. In this example, the OPO is driven by the second harmonic of the titanium sapphire pump laser. The resulting 400 nm beam produces signal and loiter outputs with a total wavelength coverage of 490 to 750 nm (signal output) and 930 nm to 2.5 µm (loiter output), with a pulse width of less than 200 fs. When combined with the titanium sapphire fundamental's tuning range of 690 to 1040 nm, the system covers a wavelength range of 485 nm to 2.5 µm. range. Typical applications include soliton studies, time-resolved vibrational spectroscopy and ultrafast pump-probe experiments.
Figure 6: In a synchronously pumped optical parametric oscillator (OPO), the center wavelength is varied by adjusting the phase-matching angle of the nonlinear crystal.
The OPA utilizes the same nonlinear optical process, but because the pump pulse has a higher peak power, an optical resonator is not required for efficient wavelength conversion. A small portion of the beam from the ultrafast amplifier is focused onto a sapphire plate to produce a white light continuum spectrum. The white light continuum spectrum is seeded into an OPA crystal (usually a barium borate crystal) and pumped with the rest of the ultrafast amplifier beam. A single pass of the beam through the OPA produces an order of magnitude amplified signal and stray light. The center wavelength of the output light is again controlled by the phase-matching conditions of the crystal, and the spectral bandwidth is usually determined by the bandwidth of the pump and seed beams or the received bandwidth of the crystal.
This OPA can operate in the femtosecond or picosecond range with energies of up to a few millijoules per pulse. At these energy levels, the resulting signal and idler light can be converted to their harmonics or by sum and/or difference frequency mixing.
OPAs pumped with millijoule pulse energies are capable of generating photons from the 190 nm deep ultraviolet to the far infrared spectral region. These devices facilitate many spectroscopic applications such as transient absorption spectroscopy, fluorescence upconversion, 2D infrared spectroscopy, and high harmonic generation.
Conclusion
Tunable lasers are now used in many important applications ranging from basic science research to laser manufacturing and life and health sciences. The range of technologies currently available is vast. Starting with simple CW tunable systems, their narrow linewidths can be used for high-resolution spectroscopy, molecular and atomic trapping, and quantum optics experiments, providing critical information to modern researchers.
More sophisticated ultrafast amplifier systems utilize high-energy, picosecond and femtosecond laser pulses to produce laser output in the UV to far-red bands. These ultrafast lasers are critical to understanding high-energy physics, high harmonics, and transient spectroscopy. The wide tuning range means that the same laser system can be used to study an infinite range of experiments in electronic and vibrational spectroscopy. Today's laser manufacturers offer one-stop-shop type solutions, providing laser outputs spanning more than 300 nm in the nanofocal energy range. More sophisticated systems span an impressively wide spectral range of 200 to 20,000 nm in the microfocus and millifocus energy ranges.
