Researchers Demonstrate a Novel Design For 2 mm Soliton Lasers

By Pritam RoyReviewed by Susha Cheriyedath, M.Sc.Sep 2 2022

In a pre-proof article posted in Results in Physics, researchers demonstrated a novel design to construct 2 mm soliton lasers employing a bidirectional nonlinear polarization evolution Tm³⁺-doped mode-locked fiber laser oscillator.

Study: Soliton Lasers in a Bidirectional Nonlinear Polarization Evolution Tm³⁺-doped Fiber Oscillator. Image Credit: Titima Ongkantong/Shutterstock.com

Through independently adjusting the state of intracavity, polarization mode-locking operations were accomplished between clockwise and counter-clockwise mode-locked fiber laser oscillator signals. The maximum output powers were 332.5 and 367.8 mW, while the corresponding transfer efficiencies were 17.2% and 18.3%.

After observing hysteresis in the output powers and spectrum transformation between the two mode-locked fiber laser oscillator signals propagating in opposite directions, a thorough investigation was performed. The central wavelength of 2 mm soliton lasers could be continually changed between 1951.51 and 2015.89 nm due to the significant birefringence filtering effect. 

The study accomplished a 1992.55 nm central wavelength with a 633.02 MHz repetition rate and achieved steady high-order harmonic mode-locking (HML) functions. Additionally, the proposed 2 mm soliton lasers demonstrated exceptional performance, as evident from the intensity spectra stability analysis and the average output power variations.

Thus, owing to the successful demonstration of the novel 2 mm soliton lasers, the authors believe they can prove beneficial in various fields, including mid-infrared frequency metrology, molecular spectroscopy, ultrafast time-resolved measurements, etc.

Improving the Conventional 2 mm Soliton Lasers

The ultrafast 2 mm soliton lasers have received significant interest due to their inherent benefits of- (1) eye safety, (2) low-loss atmospheric transmission window, and (3) water-absorbing band. In addition, the 2 mm soliton lasers prove highly efficient in free space optical communications, transparent material micro-machining, and laser surgery. 

Holmium (Ho³⁺), thulium (Tm³⁺), and Ho³⁺/Tm³⁺ co-doped fibers are commonly used as the gain fiber in the cavity of the fiber lasers to produce 2 mm ultrashort pulses. However, Tm³⁺-doped fiber (TDF) displays some specific advantages over the other options. First, TDF has a gain bandwidth of 1700–2100 nm, enabling the direct production and amplification of ultrashort pulses of less than 100 fs in the cavity. Additionally, the central wavelength of the soliton fiber lasers can be flexibly adjusted and selected in a broad-spectrum range.

Through a variety of mode-locking approaches, primarily- (1) 2D saturable absorbers (SAs), (2) nonlinear polarization evolution (NPE), and (3) nonlinear fiber loop mirrors (NOLM), the traditional, dissipative, dispersion-managed, and self-similar 2 mm soliton lasers have previously been produced in TDF lasers. 

The nonlinear polarization evolution mode-locked technique is considered to be the most desirable method with some intrinsic advantages like a large modulation depth, a high damage threshold, and a rapid saturation absorption. A bidirectional nonlinear polarization evolution mode-locked fiber laser oscillator can be successfully used to produce dual-comb high-energy laser signals from opposing directions in a single cavity.

The ability to independently achieve mode-locking between mode-locked fiber laser oscillator signals propagating in the clockwise (CW) and counter-clockwise (CCW) directions is due to the flexibility of nonlinear polarization evolution mode-locked techniques. Adjusting cavity parameters like intracavity polarisation state, pumping rate, and cavity length help achieve the required flexibility to accomplish the mode-locking operations.

This paper experimentally explored the potential to produce 2 mm soliton lasers in a bidirectional nonlinear polarization evolution mode-locked TDF oscillator. Through precise control of the intracavity polarisation state, the mode-locking operation could be preferably switched in the 2 mm soliton lasers to lase either in the CCW or CW direction.

Research was conducted on the hysteresis in output power and spectrum transformation in signals traveling in opposite directions. Wideband wavelength adjustment of the mode-locked fiber laser oscillator was accomplished by appropriately adjusting the waveplates' position due to the strong birefringence filtering effect.

Finally, an inspection of the average output power variations, intensity spectra, and phase noise level was conducted to assess the stable performance of the 2 mm soliton lasers.

Designing the New Mode-Locked Fiber Laser

In contrast to a conventional nonlinear polarization evolution mode-locked fiber laser, the proposed mode-locked TDF laser did not require a polarization-insensitive isolator to provide bidirectional lasing in the cavity. The maximum output power was 10 W and was connected to TDF by a wavelength-division multiplexer (WDM) at 1560/2000 nm from a two-stage fiber amplifier at 1565.5 nm.

The suggested mode-locked TDF laser's long-term stability was evaluated in a lab setting at room temperature. Additionally, the intensity spectra were captured at five-minute intervals for 150 minutes, and the evolution of the spectral intensity data was reconstructed. Findings indicated that the output power increased from 144.80 mW to 160.62 mW before decreasing to 44.25 mW.

The phase noise spectra were investigated to correspond to various output powers to find the noise characteristics of the proposed bidirectional 2 mm wavelength mode-locked fiber laser. The bidirectional 2 mm wavelength mode-locked fiber laser exhibited a more significant phase noise level when compared to the phase noise spectra of the nonlinear polarization evolution mode-locked fiber laser doped with ytterbium.

Novel Technique Enhances the Traditional 2 mm Soliton Lasers

The paper demonstrated a unique technique to design improved 2 mm soliton lasers by applying a bidirectional nonlinear polarization evolution TDF mode-locked fiber laser oscillator.

The maximum output powers for the 2 mm mode-locking operations, which could operate in either CW or CCW directions, were 332.5 mW for CW and 367.8 mW for CCW, with efficiencies of 17.2% and 18.3%, respectively. The hysteresis effect of spectral transformation and output powers were observed for the mode-locking functions in both the CW and CCW orientations.

The significant birefringence filtering effect allowed the soliton laser's core wavelength to be constantly tuned between 1951.51 nm and 2015.89 nm. A 633.02 MHz repetition rate having a central wavelength of 1992.55 nm was achieved, and stable high-order HML operations were also accomplished.

The proposed fiber laser exhibited promising long-term stable and reliable output as measured by the average output power variations and the intensity spectra stability throughout sustained 150-minute operations.

The phase noise levels of the mode-locked fiber laser were characterized by independent functioning in both directions, which could be improved by adjusting ambient and laser system factors. The paper offered a viable method for producing a mode-locked fiber laser with 2 mm solitons, which can be used for various purposes like ultrafast time-resolved measurements, molecular spectroscopy, etc. 

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Reference

Feng, N., et al. (2022). 2 μm Soliton Lasers in a Bidirectional Nonlinear Polarization Evolution Tm³⁺-doped Fiber Oscillator. Results in Physics.
https://www.sciencedirect.com/science/article/pii/S2211379722005538

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