Panigrahi, Adrija and Chowdhury, Sourav Das and Pal, Atasi and Pal, Debasis (2026) kHz-linewidth Ytterbium-doped Fiber Amplifier. In: International Symposium on Recent Trends in fiber Optics, Photonics, Materials & Devices, 18-20 June 2026, IIEST Shibpur, West Bengal. (Submitted)

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Abstract

A sub-MHz linewidth ytterbium-doped fiber amplifier operating at 1064 nm is reported, achieving 45 W output power under 976 nm pump excitation. The linewidth is measured to be approximately 87 kHz using an in-house developed delayed self-heterodyne interferometer. High-power narrow-linewidth lasers at 1μm have become increasingly important in precision photonics for their applications in coherent beam combining, high-resolution spectroscopy and gravitational wave detection (GWD). However, the simultaneous scaling of output power, maintaining kHz-level linewidth, is still a fundamental challenge, primarily limited by stimulated Brillouin scattering (SBS), noise transfer from the pump and amplifier stages, and modal instabilities. The master oscillator power amplifier (MOPA) configuration provides a reliable solution for power scaling maintaining the properties of the narrow linewidth seed laser. Liem et al. demonstrated 100 W output power with 70% efficiency by using a LMA ytterbium (Yb)-doped fiber with core/clad diameter of 28/400 μm and core NA of 0.06 [1]. Wellmann et al. reported a 200 W power scaling with 75% efficiency with PM Yb-doped fiber with core/cladding diameter of 25/250 μm [2]. More recently, Tao et al. achieved narrow linewidth of 85 kHz at 1 μm under tandem core-pumping at 1018 nm with output power of 250 W using LMA Yb-doped fiber with core/inner cladding diameter of 20/130 μm [3]. Despite these advances, there remains significant scope for designing high gain kHz-linewidth amplifier under direct cladding pump at 976 nm through introducing the pump induce thermal gradient. An all-fiber multistage narrow linewidth amplifier is developed at 1064 nm using Yb-doped gain fiber. The power amplifier uses a length of 4.1 m in-housed fabricated octagonal-shaped double clad Yb-doped fiber with core/inner clad dimeter of 20/130 μm and core NA: 0.08 to achieve slope efficiency of 66.8% as shown in Fig. 1. A gain of 6.9 dB is obtained for the power amplifier, constrained by the growth of backward-propagating power to ~15 mW, suggesting the onset of SBS. A three-port optical circulator is placed before the power amplifier stage to ensure isolation of the pre-amplifier stages and facilitate monitoring of backward-propagating power. A cladding pump stripper (CPS) is placed to separate out the unused pump power and output signal power. A delayed self-heterodyne interferometer has been designed by using Acousto-Optic Modulator (AOM) and 2.5 km delay fiber passive fiber to monitor the linewidth. The RF beat spectrum, shown in Fig. 2, is measured using an electronic spectrum analyzer (ESA) in conjunction with a low-bandwidth photodetector, providing an estimated 3 dB linewidth of 87 kHz. Achieving ~100 W output power while preserving kHz-level linewidth and spectral coherence is the next step of this work.

Item Type:	Conference or Workshop Item (Speech)
Subjects:	Electronics
Divisions:	Fiber Optics and Photonics
Depositing User:	Ms Upasana Sahu
Date Deposited:	23 Sep 2026 09:06
Last Modified:	23 Sep 2026 09:06
URI:	https://cgcri.csircentral.net/id/eprint/5824

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