Measurement of gain and saturation parameters of a single-mode Yb:silica fiber amplifier

doi:10.1016/j.yofte.2013.05.016

Optical Fiber Technology

Volume 19, Issue 5, October 2013, Pages 446-455

https://doi.org/10.1016/j.yofte.2013.05.016 Get rights and content

Highlights

•
A low power fiber master oscillator power amplifier is designed and fabricated.
•
Competition between ASE noises and input signal to the amplifier investigated.
•
We calculated the gain and saturation parameters of the fiber amplifier.
•
Change in pump power, doped fiber’s length and concentration affect the amplifier gain and saturation power.
•
Parasitic noises and their evolutions due to power change are discussed.

Abstract

A master oscillator-power amplifier (MOPA) has been fabricated and optimized for measuring the small-signal gain (γ₀) and saturation power (P_sat) of a continuous-wave (CW) single-mode (SM) double-clad (DC) Yb-doped (YD) silica fiber amplifier at 1082.5 nm. It was shown that the gain and saturation properties are strongly dependent on the pump power, dopant concentration and fiber length mainly due to the dominant effect of the overlapping factor.

Introduction

After the birth of DC doped fibers in the late 1980s [1], the fiber lasers’ efficiency was significantly enhanced after pumping via the inner clad rather than the active core. Then, the skew rays could be guided to pass the core when the cladding circular configuration was modified to the assymetric ones. The traditional rare-earth dopants include erbium (Er), neodymium (Nd) and ytterbium (Yb) that are widely used as gain media with various concentrations.

Er-doped silica fiber lasers have been extensively studied for their use as source in communication systems operating in the third communication window. The interest in Nd emitting at 1.06 μm, mainly arises from its efficient pump to signal power conversion. In the case of high pump powers, Nd³⁺ and Er³⁺ dopants suffer from the large excited state absorption (ESA) rate. Conversely, Yb³⁺ gains ²F_5/2–²F_7/2 transition as the adjacent excited state locates 10,000 cm⁻¹ far above. Therefore, the likelihood of ESA remarkably reduces to benefit Yb:silica respect to the other doped fibers [2]. Furthermore, YD silica fibers exhibit very broad absorption and emission bands ranging 800–1064 nm for the absorption and 970–1200 nm for the emission.

The simplicity of the level structure provides freedom from unwanted processes such as ESA, multiphonon nonradiative decay and the concentration quenching. These fiber lasers therefore offer a very efficient and convenient means for the wavelength conversion using a wide variety of pump lasers such as AlGaAs and InGaAs diodes. DC Yb:silica fiber amplifiers present the prospects for a number of interesting applications based on the broad amplification bandwidth and efficient performance as well as free competition processes encountered with other rare-earth dopants [3].

Fiber amplifiers are of great interest because of their high beam quality, great thermal control and high efficiency. The wide applications in medicine and communications [4] demand low power fiber lasers whereas for the industrial purposes mean power scales up to kW. The power scaling techniques include the pump enhancement, the coherent and incoherent beam combining and the MOPA. Strong pumping likely damages the laser components including the fiber gain media, the fiber Bragg gratings (FBGs) and the combiners while the nonlinear effects may degrade the beam quality too. The utilization of the beam combining methods require precise alignment mainly at the expense of coherence loss [5].

MOPA array benefits the excellent beam quality, coherence and compactness. The array usually consists of an oscillator with axial diode laser pumping through the clad. The seed signal, i.e., the output signal from oscillator, enters into the core of the amplifier to attain the proper condition. The signal becomes intense traversing through the amplifier and suppresses the amplified spontaneous emission (ASE) along the fiber. The spectra of the rare-earth ions and the nature of the host materials are essential to choose the lasing wavelengths. A number of cavity designs have been implemented for low loss coupling to transmission fibers, single longitudinal mode operation, Q-switching and mode-locking. With the development of low loss fibers and the availability of versatile laser diodes, there is a renewed interest in rare-earth doped glasses for DC fiber laser application. The fiber laser sources consist of the CW and pulsed oscillator–amplifier, mode-locked outputs, super-luminescent sources and second-order generators.

Tremendous efforts were made to develop the fiber amplifiers during the recent decade. The efficient operation of a YD silica fiber laser was reported at 980 nm [6]. CW tunable SM fiber laser from ²F_5/2–²F_7/2 transition of trivalent Yb³⁺ ions has been studied and the 975–1055 nm tunability range were achieved with different cavity configuration and various Yb³⁺ concentrations [7]. A significant progress in the fiber design demonstrates a step index SM fiber operation with large mode area (LMA) and small numerical aperture (NA) in the core [8].

In fact, the amplifier is characterized with a couple of significant coefficients i.e., small-signal gain and saturation power [9], [10], [11], [12], [13]. Here, a series of systematic experiments were carried out to measure the small-signal gain (γ₀) and saturation power (P_sat) in MOPA, particularly in low power (unsaturated) case. To our knowledge, there is no article available to explain the measurement of small-signal gain and saturation power in DC Yb:silica gain media. We have assembled and utilized a DC YD fiber MOPA array such that the spectrum analysis of the seed and amplifier signals, the competition between ASE and laser and the condition where ASE vanishes have been investigated.

Section snippets

Theory

In a MOPA, the pulse width, the beam divergence and the spectral width are primarily determined by the oscillator, whereas the pulse energy and the mean power are characterized by the amplifier. The oscillator–amplifier array achieves high laser energy having small divergence and narrow linewidth leading to the high brightness of the output beam.

The design of the amplifier takes into account the gain and the energy extraction, wavefront distortions, energy and power densities at the optical

Experimental setup

The experimental setup is schematically shown in Fig. 1. A master oscillator was made which consists of an Alfalight diode for pump with the central line at 976 nm, an LIEKKI 7×1 combiner, a IXFIBER high reflection (>99.9%) FBG at the central wavelength of 1082 nm, 10 m long LIEKKI Yb1200-6/125 DC silica fiber with core/inner-clad/outer-clad diameters of 6/125/250 μm having octagonal inner clad with NA of 0.15, a IXFIBER output coupler FBG with reflection of 10% and 30 cm long single-clad delivery

Result and discussion

After implementation of MOPA array, the real-time spectroscopy was sequentially performed using OSA. At first, the pump power is kept constant at 967 mW and the seed signal alters from 2.1 μW to 1.88 mW as shown in Fig. 2 by increasing the steady current of the oscillator pump from 450 mA to 650 mA accordingly. Noticeable ASE appears at the beginning, from 1070 to 1080 nm, however, the signal gradually grows up and the ASE is simultaneously suppressed such that finally vanishes at proper conditions

Conclusion

This work is a continuation of our previous study on the measurement of small-signal and saturation intensity of various gain media [9], [10], [11], [12], [13], [35]. Here, we have investigated the gain and saturation properties of DC Yb:silica amplifier as a function of the pump power, the concentration, and the fiber length. At first, a home-made CW SM YD DC fiber MOPA was assembled. Then, the conditions for vanishing ASE was determined using the real-time spectroscopic analysis of the MOPA

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View full text

Measurement of gain and saturation parameters of a single-mode Yb:silica fiber amplifier

Highlights

Abstract

Introduction

Section snippets

Theory

Experimental setup

Result and discussion

Conclusion

J. Non-Cryst. Solids

Opt. Laser Technol.

Opt. Laser Technol.

Opt. Commun.

Optik – Int. J. Light Electron Opt.

Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 μm region

IEEE J. Sel. Top. Quantum Electron.

Laser beam combining for high-power, high-radiance sources

IEEE J. Sel. Top. Quantum Electron.

Highly efficient 980 nm operation of a Yb3+-doped silica fibre laser

Electron. Lett.

Pressure dependence of the small-signal gain and saturation intensity of a copper vapor laser

Appl. Opt.

Temperature dependence of the amplifying parameters of a copper vapor laser

Laser Phys.

Measurement of the small-signal gain and saturation intensity of a XeF discharge laser

Appl. Opt.

Alternative Gaussian spot size polynomial for use with doped fiber amplifiers

IEEE Photon. Technol. Lett.

Theoretical and experimental study on transverse mode competition in a partial-coiled multimode fiber laser

Laser Phys.

Highly efficient 980 nm operation of a Yb³⁺-doped silica fibre laser