Researchers from the Institute of Space Information Innovation of the Chinese Academy of Sciences (CAS) and the University of Chinese Academy of Sciences (UCAS) have created a compact solid-state nanosecond pulsed laser system that generates 193 nm coherent light at a repetition frequency of 6 kHz, which is expected to be used in the field of chip lithography in the future.
Specifically, the researchers have developed a Yb:YAG crystal amplifier that generates a 1030 nm laser that is split into two parts: one that generates a 258 nm laser through the fourth harmonic, and one that is used to pump a light parametric amplifier capable of generating a 1553 nm laser. Frequency mixing of these beams in the cascade crystal produces a 193 nm laser with an average power of 70 mW and a linewidth of less than 880 MHz.
By introducing a helical phase plate into the 1553 nm beam prior to frequency mixing, the researchers generated an orbital angular momentum beam.
To the researchers' knowledge, this is the first demonstration of a 193 nm orbital angular momentum beam from a solid-state laser.
Such a beam is valuable for exciting hybrid argon fluoride (ArF) excimer lasers and has potential applications in wafer processing and defect detection.
ArF is an excimer laser with a wavelength of 193 nm, which is in the deep ultraviolet band. In semiconductor manufacturing, ArF lasers are mainly used for high-resolution lithography.
It is also noted that the operating bandwidth of the system is less than 880 MHz, and its spectral purity performance is comparable to that of today's commercial systems. At the same time, the system occupies an optical platform of approximately 1200mm x 1800mm, and its footprint can be further reduced to meet the requirements of industrial applications.
The conversion process from a 1030 nm laser to a 193 nm laser is described as being very similar to previous work by researchers.
Specifically, a 1030 nm laser amplifier based on a 2mmx2mmx30mm Yb:YAG crystal pumped by a 100 W multimode laser diode (LD) at 969 nm is capable of delivering more than 14 W of 1030 nm pulsed laser light with a repetition frequency of 6 kHz and a pulse duration of 13.1 ns.
It is important to note that pumping - is a process that uses light to raise electrons from lower to higher energy levels in an atom or molecule.
In the study, the researchers were able to generate a 258 nm laser from a 1030 nm laser through successive second-harmonic generation and fourth-harmonic generation processes in lithium triborate crystals and lithium cesium hexaborate, respectively. 1030 nm lasers can also be used as a pumping source for two-stage optical parametric amplifiers in order to deliver a high-powered, pulsed 1553 nm laser.
Unlike the fiber-optic amplifier, the researchers used a laser source based on an optical parametric amplifier to generate the 1553 nm subwatt pulsed laser.
As a result of this modification, the system became more compact, and electronic controllers were no longer needed to synchronize the 1553 nm and 258 nm pulse trains in the sum-frequency generation, which could be accomplished using an optical delay line. (Note: Harmonic generation is a nonlinear optical process.)
The two-stage sum-frequency generation process, pumped by 1553 nm and 258 nm lasers, can generate 221 nm lasers and 193 nm lasers, respectively, through the use of a cascaded lithium triborate crystal.
For the 1553 nm pulsed laser source, it consists of two parts: a continuous-wave (CW) single-frequency distributed feedback laser diode acting as a seed source, and a two-stage optical parametric amplifier based on a periodically polarized lithium niobate crystal.
The single-frequency distributed feedback laser diode operates at 1553 nm and emits an average power of 12 mW. In the study, a 1030 nm pump laser was introduced into a 1mmx1mmx40mm periodically polarized lithium niobate crystal along with the seed laser to form the first stage of the optical parametric amplifier.
During this time, the amplified signal laser was filtered out of the output of the first stage of the optical parametric amplifier and the second stage of the optical parametric amplifier through a special optic, a dichroic mirror, accompanied by the residual pump laser and a 3-μm idler laser.
Subsequently, the researchers used a laser power probe to determine the power of the signal laser in order to distinguish the pulsed signal component from the continuous-wave seed laser.
Due to the low duty cycle of the pump laser and the weak power of the seed laser, the pumping threshold of the optical parametric amplifier was close to 600 mW. (Note: Duty cycle is the ratio of the time the signal is at a high level during a pulse cycle to the entire pulse cycle time, and is usually expressed as a percentage.)
With a pump laser at an average power of about 700 mW, the researchers obtained more than the pulse energy from the first stage of the optical parametric amplifier, corresponding to an average power of 48 mW.
The amplified pulse signal was then further amplified in the second stage of the optical parametric amplifier, where a maximum pump power of 3 W was obtained by using another 5mmx3mmx30mm periodically polarized lithium niobate crystal.
At the same time, the researchers kept the pump laser power density in the second stage of the optical parametric amplifier close to 30 MW/cm² to avoid photorefractive damage from the periodically polarized lithium niobate. (Note: Photorefractive damage is an undesirable optical effect that occurs when a photorefractive material is exposed to bright light.)
Image | Average power of the signal laser in the second stage of the optical parametric amplifier versus pump power (Source: Advanced Photonics Nexus)
With this, the researchers obtained a 700 mW signal laser at 1553 nm, corresponding to an efficiency of 23.3%.
This increase in efficiency suggests that the output power can be further improved as the pump power increases.
Image | Spectra of the seed source and signal laser from the first stage of the optical parametric amplifier and the second stage of the optical parametric amplifier (credit: Advanced Photonics Nexus)
The researchers found that the center wavelength of the amplified signal laser is the same as that of the seed laser, but the spectrum broadens slightly.
Although the parametric fluorescence noise may increase as the pump power increases, the signal-to-noise ratio remains close to 50 dB.
To accurately measure the linewidth evolution of the 1553 nm laser during the optical parametric amplification process, the researchers used a scanning interferometer with a resolution of about 1 MHz and a free spectral range of 1.5 GHz.
The initial linewidth of the continuous-wave laser broadens from 180 MHz to 370 MHz and 580 MHz during the first stage of the optical parametric amplifier and the second stage of the optical parametric amplifier, respectively.
Image | Researchers investigated the pulse duration of pump and signal lasers with an InGaAs photodetector (credit: Advanced Photonics Nexus).
Due to the parametric transition threshold of the optical parametric amplifier process, signal lasers have a steeper pulse front than pump lasers, and the duration is reduced from 13.1 ns to 9 ns.
Based on this, the researchers obtained an optical parametric amplifier-based 1553 nm pulsed laser with an average power of 700 mW and a pulse duration of 9 ns, which can be used as a pump source for generating 193 nm lasers.
To further extend the application of the 193 nm laser, the researchers have experimentally demonstrated for the first time a 1553 nm vortex beam, in which the fundamental Gaussian mode of the 1553 nm pulsed laser is converted into the Laguerre-Gaussian (LG) mode carrying the orbital angular momentum, by introducing a helical phase plate after the optical parametric amplifier. mode.
During this time, a 25 mm diameter spiral phase plate was mounted in a 25.4 mm diameter lens adapter.
Although the ends of the spiral phase plate were not coated with an anti-reflective coating, its transmission was greater than 90%.
The carried orbital angular momentum is then transferred to the 221 nm laser and 193 nm laser through a sum-frequency generation process.
To verify the generation of vortex beams, the researchers used a pyroelectric camera to record the beam profiles of a 1553 nm laser, a 221 nm laser and a 193 nm laser in different modes.
Prior to insertion of the helical phase plate, the 1553 nm laser, 221 nm laser, and 193 nm laser all exhibited Gaussian mode profiles. (Gaussian mode profile refers to a common beam pattern in which the light intensity distribution takes on the shape of a Gaussian function with specific profile characteristics.)
Upon insertion of the helical phase plate, the 1553 nm laser mode is converted and exhibits a circular intensity distribution trend that is characteristic of the Laguerre-Gaussian mode. (Note: The Laguerre-Gaussian mode is an important mode for laser beams.)
In determining its topological charge, the researchers found that the diffraction pattern of the Laguerre-Gaussian mode, the so-called Hermite-Gaussian (HG, Hermite-Gauss) mode, could be obtained by simply introducing a cylindrical lens. (Note: In optics, the Hermite-Gauss mode is an important beam pattern.)
To minimize the effect of the Gouy phase shift on the Hermite-Gauss mode, the 193 nm laser beam is initially focused by a calcium fluoride lens with a focal length of 200 mm. (Note: Gouy phase shift is a specific phase shift phenomenon associated with Gaussian beam propagation in optics.)
Since the cylindrical lens has a short focal length, it is placed near the focal point of the calcium fluoride lens.
The cylindrical lens converts the circular beam into two bright spots with a gap in the center, indicating the generation of a vortex beam with a topological charge of 1. This result is consistent with the 2π phase shift of the helical phase plate. (Note: the 2π phase shift implies that one wave completes a full cycle with respect to the other.)
Because of the significant difference in intensity distribution between the vortex beam and the Gaussian mode, the beam of the 258 nm laser has to be amplified to be able to cover the 1553 nm laser, ensuring better transfer of orbital angular momentum in the sum-frequency generator 1 and sum-frequency generator 2.
However, the weaker power density of the 258 nm laser compared to the full Gaussian mode experiments described above significantly reduced the conversion efficiency of the sum-frequency generation to the point where the researchers obtained only 30 mW of 221 nm laser and 3 mW of 193 nm laser.
According to the law of conservation of orbital angular momentum in nonlinear processes, the topological charge of the laser generated by sum-frequency generation is equal to the sum of the topological charges of the pump laser.
Therefore, the topological charge of the 1553 nm laser is 1, the topological charge of the 258 nm laser is 0 because it is in Gaussian mode, and the topological charge of the 221 nm laser is 1.
During this period, the diffraction pattern of the 193 nm vortex beam is split into three bright spots with two dark gaps in between, while the intensity distribution remains circular.
Compared with the basic vortex beam at 1553 nm, the vortex beam profiles of the 221 nm laser and the 193 nm laser are inevitably distorted during the sum-frequency generation process due to the phase mismatch and walk-off effects of the nonlinear crystal.
At the same time, the cascade structure increases the complexity of orbital angular momentum conversion and may even lead to mode degradation. (Mode degradation is a phenomenon in which the properties of specific modes originally present in an optical waveguide deteriorate or deviate from the ideal state.)
The researchers believe that it may be possible to improve the quality of the modes carrying orbital angular momentum by using shorter crystals, or by using a separate sum-frequency generation process.
Considering that the 1553 nm laser is pumped and amplified by the 1030 nm laser, the overall conversion efficiency from the 1030 nm laser to the 193 nm laser is about 0.55%. Therefore, despite the current low conversion efficiency, by increasing the pump power of the 1030 nm, the power of the 193 nm laser is expected to be in excess of hundreds of milliwatts and possibly even on the order of watts.
In addition, the use of nonlinear crystals with higher nonlinear coefficients will significantly improve the feasibility of achieving this goal.
At the same time, by inserting a helical phase plate, the Gaussian mode can be converted to a Laguerre-Gaussian mode, enabling the generation of a 1553 nm vortex beam carrying orbital angular momentum.
By changing the phase shift of the helical phase plate, the order of the topological charge can be easily changed. Previous studies have reported that beams carrying orbital angular momentum can be amplified in single-crystal fibers and nitrogen plasmas, suggesting that the 193 nm vortex beam can also be amplified in excimer lasers.
Based on this, researchers anticipate that the 193 nm laser could be used in a variety of new applications, utilizing its high power output and unique vortex beam characteristics.
