[Paper Review] Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator

Nathalie Nagl, Sebastian Gröbmeyer+4 more|arXiv (Cornell University)|May 30, 2019

Advanced Fiber Laser Technologies60 references56 citations

TL;DR

This paper presents the first directly diode-pumped, Kerr-lens mode-locked Cr:ZnSe laser oscillator, using a single 1650 nm InP laser diode to achieve over 500 mW average output power and 45 fs pulse durations—less than six optical cycles at 2.4 µm—representing a sixty-fold increase in peak power over prior diode-pumped systems and performance comparable to fiber-pumped oscillators.

ABSTRACT

Lasers based on Cr$^{2+}$-doped II-VI material, often known as the Ti:Sapphire of the mid-infrared, can directly provide few-cycle pulses with super-octave-spanning spectra, and serve as efficient drivers for generating broadband mid-infrared radiation. It is expected that the wider adoption of this technology benefits from more compact and cost-effective embodiments. Here, we report the first directly diode-pumped, Kerr-lens mode-locked Cr$^{2+}$-doped II-VI oscillator pumped by a single InP diode, providing average powers of over 500 mW and pulse durations of 45 fs - shorter than six optical cycles at 2.4 $\mu$m. These correspond to a sixty-fold increase in peak power compared to the previous diode-pumped record, and are at similar levels with respect to more mature fiber-pumped oscillators. The diode-pumped femtosecond oscillator presented here constitutes a key step towards a more accessible alternative to synchrotron-like infrared radiation, and is expected to accelerate research in laser spectroscopy and ultrafast infrared optics.

Motivation & Objective

• To develop a compact, cost-effective, and efficient alternative to fiber-pumped mid-infrared femtosecond lasers.
• To overcome the limitations of low-power, low-brightness diode pumping in Cr:ZnSe lasers by enabling direct diode pumping with high beam quality.
• To achieve ultrashort few-cycle pulses in the mid-infrared using Kerr-lens mode-locking with a diode-pumped gain medium.
• To demonstrate low-amplitude noise performance suitable for high-precision spectroscopy and frequency comb applications.

Proposed method

• A 5 mm-long polycrystalline Cr:ZnSe crystal was pumped by a single 1650 nm InP laser diode with up to 3.6 W output power.
• The diode beam was reshaped using aspheric and cylindrical lenses to achieve a focused spot of ~50 µm × 90 µm at the crystal.
• An asymmetric X-fold cavity with curved mirrors and a YAG plate was used to minimize astigmatism and manage cavity dispersion.
• Soft-aperture Kerr-lens mode-locking was initiated by mechanical mirror movement, enabling self-starting of ultrashort pulses.
• Group-delay dispersion (GDD) was compensated using a 3 mm sapphire plate at Brewster’s angle, and third-order dispersion (TOD) was corrected with custom-designed dispersive mirrors.
• Amplitude noise was measured via frequency-doubled pulse train using a low-noise current source, InGaAs photodetector, and RF spectrum analyzer with DC filtering.

Experimental results

Research questions

• RQ1 Can a directly diode-pumped Cr:ZnSe oscillator achieve femtosecond pulse durations comparable to fiber-pumped systems?
• RQ2 What is the maximum average output power and pulse duration achievable with direct diode pumping using a single-emitter laser diode?
• RQ3 How does the amplitude noise performance of a diode-pumped KLM Cr:ZnSe oscillator compare to established fiber-pumped systems?
• RQ4 Can dispersion management in a compact, monolithic cavity enable few-cycle pulses in the 2.4 µm range with high beam quality?
• RQ5 What is the impact of diode beam anisotropy and thermal loading on mode-locking stability and efficiency?

Key findings

• The oscillator achieved over 500 mW of average output power with 45 fs pulse duration, corresponding to less than six optical cycles at 2.4 µm.
• This represents a sixty-fold increase in peak power compared to the previous diode-pumped record and performance on par with more mature fiber-pumped oscillators.
• The integrated RMS relative intensity noise (RIN) was measured at < 0.068% across 20 Hz to 1 MHz bandwidth, indicating excellent low-noise performance.
• The system demonstrated stable operation with less than 0.12% RMS power fluctuation over 3 hours, confirming long-term stability.
• The use of a single-emitter diode enabled high beam quality and efficient mode-locking, overcoming limitations of diode bars.
• Dispersion management using a sapphire plate and custom dispersive mirrors enabled soliton mode-locking and pulse compression to the transform-limited regime.