A new method for sampled fiber Bragg grating fabrication by use of both femtosecond laser and CO2 laser
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A new method for sampled fiber Bragg grating fabrication by use of both femtosecond laser and
CO2 laser
Abstract: A new method for sample fiber Bragg grating fabrication by use of both femtosecond laser and CO2 laser has been proposed and demonstrated. Such a method exhibits the advantages of high fabrication flexibility, and good thermal stability. The sampling period and duty cycle can be easily varied by changing the CO2 laser beam scanning pattern during operation. The gratings produced have potential applications in optical communications, fiber lasers, and optical fiber sensors.
1. Introduction
Sampled fiber Bragg grating (SFBG) has many attractive applications in optical communications [1,2], multi-wavelength fiber lasers [3,4], and optical fiber sensors [5–8]. It is usually fabricated by use of UV light irradiation on hydrogenated single mode fiber (SMF) and the amplitude or phase of its reflection spectrum is modulated by a sampling function. Such a modulation can be achieved by periodically triggering on and off the light exposure along the fiber [7,8], which needs an accurate control of the light exposure time, power and location for each subgrating, and the apolization is obtained by multiple exposures with different power levels for different subgrating sections [9]. Alternatively, a specially designed amplitude mask can be employed together with the phase mask during the grating inscription process however, owing to the fixed modulation envelope, different amplitude masks have to be used to produce different modulation profiles [5,10].
In this paper, we propose an SFBG fabrication method by use of both femtosecond laser and CO2 laser, which allows the modulation envelop to be flexibly changed according to the application requirement. Because of the multi-photon absorption [11] of the femtosecond laser inscription and the CO2 laser heating effect [12,13], SFBG can be created in SMF without hydrogen loading. The temperature test reveals that the SFBGs fabricated in this work have a good thermal stability up to 650 °C, superior to those fabricated in hydrogen loaded SMF by UV light without special treatment that begin to decay significantly at temperatures between 100 to 200 °C [14–16].
2. SFBG fabrication and theoretical background
2.1 SFBG fabrication method
A femtosecond laser and a CO2 laser are both used to fabricate the SFBG with flexible control of its reflection peaks and the peak spacing. The whole fabrication process is divided into two stages. First, a fiber Bragg grating (FBG) is inscribed in SMF using femtosecond laser pulses through a phase mask [17], which is initially demonstrated in [18]. The femtosecond laser pulses (120 fs, 1 mJ, 1 kHz repetition rate at λ = 800 nm) are produced by a Ti: Sapphire laser system (Spectra-Physics). The laser pulse energy could be adjusted as required by rotating a half wave-plate followed with a linear polarizer and the energy of ~0.6 mJ was used during the grating fabrication. The laser beam is broadened through a laser beam expander which is used to tune the laser spot size, and then focused by a cylindrical lens with a focal length of 60 mm before passing through a silica phase mask (Ibsen Photonics) and arriving at the fiber core. The position of the fiber is accurately controlled by a high-precision four-axis translation stage. By use of appropriate laser exposure condition, the FBG with predetermined length can be produced. The length of FBG can be varied from 4 to 12 mm by tuning the laser beam expander.
CO2 laser
Abstract: A new method for sample fiber Bragg grating fabrication by use of both femtosecond laser and CO2 laser has been proposed and demonstrated. Such a method exhibits the advantages of high fabrication flexibility, and good thermal stability. The sampling period and duty cycle can be easily varied by changing the CO2 laser beam scanning pattern during operation. The gratings produced have potential applications in optical communications, fiber lasers, and optical fiber sensors.
1. Introduction
Sampled fiber Bragg grating (SFBG) has many attractive applications in optical communications [1,2], multi-wavelength fiber lasers [3,4], and optical fiber sensors [5–8]. It is usually fabricated by use of UV light irradiation on hydrogenated single mode fiber (SMF) and the amplitude or phase of its reflection spectrum is modulated by a sampling function. Such a modulation can be achieved by periodically triggering on and off the light exposure along the fiber [7,8], which needs an accurate control of the light exposure time, power and location for each subgrating, and the apolization is obtained by multiple exposures with different power levels for different subgrating sections [9]. Alternatively, a specially designed amplitude mask can be employed together with the phase mask during the grating inscription process however, owing to the fixed modulation envelope, different amplitude masks have to be used to produce different modulation profiles [5,10].
In this paper, we propose an SFBG fabrication method by use of both femtosecond laser and CO2 laser, which allows the modulation envelop to be flexibly changed according to the application requirement. Because of the multi-photon absorption [11] of the femtosecond laser inscription and the CO2 laser heating effect [12,13], SFBG can be created in SMF without hydrogen loading. The temperature test reveals that the SFBGs fabricated in this work have a good thermal stability up to 650 °C, superior to those fabricated in hydrogen loaded SMF by UV light without special treatment that begin to decay significantly at temperatures between 100 to 200 °C [14–16].
2. SFBG fabrication and theoretical background
2.1 SFBG fabrication method
A femtosecond laser and a CO2 laser are both used to fabricate the SFBG with flexible control of its reflection peaks and the peak spacing. The whole fabrication process is divided into two stages. First, a fiber Bragg grating (FBG) is inscribed in SMF using femtosecond laser pulses through a phase mask [17], which is initially demonstrated in [18]. The femtosecond laser pulses (120 fs, 1 mJ, 1 kHz repetition rate at λ = 800 nm) are produced by a Ti: Sapphire laser system (Spectra-Physics). The laser pulse energy could be adjusted as required by rotating a half wave-plate followed with a linear polarizer and the energy of ~0.6 mJ was used during the grating fabrication. The laser beam is broadened through a laser beam expander which is used to tune the laser spot size, and then focused by a cylindrical lens with a focal length of 60 mm before passing through a silica phase mask (Ibsen Photonics) and arriving at the fiber core. The position of the fiber is accurately controlled by a high-precision four-axis translation stage. By use of appropriate laser exposure condition, the FBG with predetermined length can be produced. The length of FBG can be varied from 4 to 12 mm by tuning the laser beam expander.
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