Fabrication of FBGs

As discussed before in section 3.4.2, the fabrication of an FBG is done by changing the refractive indexδn_{ef f} of the optical fiber core locally and periodically for a certain length L. The fiber type of interest is usually a single mode fiber in which only one transversal mode is present.

The writing process is usually done with UV laser light, due to the reaction of the UV light with photosensitive elements in the fiber core such as Germanium (Ge). This chemical process changes the refractive index δn_{ef f} by a small factor, enough to induce a pertur-bation in the electrical permittivity ∆. The photosensitivity of the fiber core can be enhanced by implementing more Germanium (Ge) during the manufacturing process of the fiber [94]. The incident UV light forms a so called GeE’ color center through photoion-ization of a GeO defect in the fiber core. These resulting color centers are responsible for changes in the UV absorption spectrum of the glass whereas the refractive index change δn_{ef f} follows from the Kramers-Kronig relationship [94, 95]. The photo-induced refractive index changeδn_{ef f} is proportional to the Ge-dopant concentration. [67]

Another commonly used technique to increase the photosensitivity of a fiber is the so called hydrogen (H₂) loading under high pressure before the writing process. The high intense writing laser light causes the dissolved hydrogen to react in the glass with the Ge atoms, which results in a large permanent index change inside the fiber core [96, 97]. It has also been shown that the necessary laser intensity to write an FBG into the fiber is by the factor of seven lower in hydrogen loaded fibers than in unloaded fibers [98].

Amplitude Splitting Interferometer

In figure 3.40 a setup for writing FBGs in a fiber with an amplitude splitting inter-ferometer is shown [92]. The writing laser beam with its wavelengthλ_W is split up into

two beams with equal power. The wavelength of the laser light is usually located in the region in which the fiber is photosensitive, this is normally the case in the UV region.

Both beams interfere with each other at the fiber’s core location. The distance of the in-terference patterns with the maximum intensity (shown in blue bars) is the design grating pitch Λ and is given to [67]

Λ = λ_W

2 sinα (3.79)

where λW is the wavelength of the writing laser beam and α is the angle between the horizontal axis of the fiber and the single beam. By changing the angle α the grating pitch Λ is affected and so the design Bragg wavelength can be written into the fiber.

Laser Beam (λW)

BS

M

α M

Interference Pattern

Figure 3.40: Setup of the amplitude splitting interferometer for FBG writing, BS beam-splitter, M mirror. Drawing according description in [92].

Phase-Mask Technique

A better method in terms of spectral quality of the grating is the phase-mask writing technique. A diffraction grating (phase mask) is implemented between laser light and optical fiber, which spatially modulates the writing beam and generates hereby the nec-essary interferogram. Special attention must be paid for the design of the phase mask, due to the fact that the interference pattern is generated between the -1. order and the +1. order. Hence, the power in these orders must be maximized, whereas the power in the 0. order ideally is suppressed. [99, 100]

The period of the FBG grating is directly a function of the of the phase mask’s pitch [67]

Λ = ΛP

2 (3.80)

where ΛP is the pitch of the phase mask. A principal setup for the phase mask writing technique is illustrated in figure 3.41. The laser beams with the -1. order and +1. order create an interference pattern which is focused in the fiber core. The wavelength of the incident laser beam is located in the ultra-violet range (normally 244 nm generated by an Ar-Ion laser) for which the fiber shows a photosensitive behavior.

A tuning of the Bragg wavelength without changing the pitch period of the phase mask or the wavelength of the writing laser beam can be achieved by an angular orientation

Interference Pattern Laser Beam

Λp

Λ

0. Order -1. Order

1+

. O

rder

Optical Fiber

Phase Mask

Figure 3.41: Illustration of the FBG writing process using a phase mask and an UV writing laser. Drawing according description in [99, 100].

between phase mask and fiber [100] or by the combination of the amplitude splitting interferometer and the phase mask. In the latter case the phase mask acts as beam splitter and as Bragg wavelength reference. Such a combination allows the inscription of widely tunable FBGs with a tuning range of more than 800 nm [91].

Femto-Second Infrared (fs-IR) Pulse Technique

Another technology for writing FBGs came up with the commercial availability of femto-second IR mode-locked laser. For this technique, pulse durations in the range of 100 fs and pulse peak energies up to 2 mJ are necessary [101]. The mostly used laser system is a Ti:Sapphire laser with a central wavelength of 800 nm [102, 93, 103, 104].

The focus of the fs-IR pulse reaches in the fiber core the glass transition temperatureTG

and causes a refractive index change in the fiber due to induced dielectric breakdown.

This results in localized melting and glass material compaction, hence the density and so also the electrical permittivity are changed. This comes to the same result as for UV written gratings, whereas for UV grating a photo-chemical reaction is responsible for the density change. [93, 98, 105, 106]

A setup for writing FBGs with fs-IR pulses point-by-point is illustrated in figure 3.42.

The fs-IR pulses are focused into the fiber core with a microscope objective while the fiber is precisely moved along the fiber axis. The pulses and the focus position of the fiber must be exactly synchronized. The spectral and physical properties of such FBGs are similar to type II UV gratings [107]. The main challenge of this technique is the required sub-µm precision of the translation stage.

Other possibility is the use of a precise phase mask and the corresponding setup as presented in figure 3.41 with the only difference that a fs-IR laser source is used. With this solution the translation stage and the synchronization would be obvious. When the phase mask is used, special attention must be given to the broad optical spectrum of the fs-IR

Movement with a high precise translation stage

Λ

Optical Fiber

Microscope opitc Pulsed fs-IR laser

Figure 3.42: Setup for writing FBGs with a femto-second infrared (fs-IR) pulsed laser and a high precision translation stage [102].

pulses. When such a pulse passes through a phase mask, the pulse would be dispersed and its energy would be spread over a large area, resulting in lower peak intensity in the fiber. The pitch width of the grating must be readjusted to counteract the dispersive effect [108, 99]. It has also been shown that with the fs-IR technique it is possible to write both type-I and type-II FBGs into a fiber [101]. FBGs with a high refractive index change δn_{ef f} of 1.9·10⁻³ and a low polarization dependency can be produced with few light pulses [105]. It has been shown that with the fs-IR writing technology it is possible to form FBGs also in fluour (F)-doped, non Ge-doped and no hydrogen loaded fibers [2]

or in standard telecommunication SMF28 fibers [105].

In literature various setups for writing FBGs with fs-IR laser pulses are discussed. They can be summarized into four groups:

• Pure amplitude splitting interferometry: The setup as illustrated in figure 3.40 is used. This setup is theoretically possible but without a phase mask the spectral quality of the grating is poor, hence it has no practical relevance.

• Pure phase mask technique: For this process a setup as it is given in figure 3.41 is used. The phase mask is irradiated by the femto-second laser pulses and the interference pattern generated by the phase mask changes due to the high intensity the refractive index in the fiber core. This technique is often used and given in the references hereafter [109, 110, 106, 98, 101, 93, 103, 111, 104, 112].

• Combination of phase mask technique with the amplitude splitting interferometry:

By combining the amplitude splitting technique with the phase mask high quality gratings with adjustable Bragg wavelength can be written. The adjustment its done by changing the angle of the mirrors whereas the phase mask acts as beam splitter and wavelength reference. This technique is the most commonly used one and is also discussed in detail in the references herein [113, 105, 114, 115, 116, 117]

• Point-by-point-wise inscription: With this technique the grating structure is written into the fiber, as the name already says, point-by-point wise. The femto-second laser beam is concentrated into the fiber core with a microscope optic. The fiber core itself is brought into the correct position with a precise linear stage. The movement of the stage and the lasers pulse generation must be synchronized. In the following references this technique is presented in detail [118, 119, 102]

In figure 3.43 the spectral response of two FBG sensors written by UV laser light (black curve) and fs-IR laser pulses (right curve) are shown. The measurements were taken by the herein developed interrogator. The UV grating was made by the drawn tower grating technique [71]. The spectral quality is very good, only one small side lobe towards higher wavelengths can be observed. The parameters of this grating are summarized in table 3.4.

The fs-IR grating shown by the red curve in figure 3.43 is written by a 800nm mode-locked laser according the setup presented in figure 3.42. The grating structure is written point-by-point into the fiber with a high precision translation stage. The spectral quality is worser as for the UV written grating but for a type II grating very promising. Two side lobes can be seen on the spectral response, one smaller lobe towards lower wavelengths and one bigger lobe towards higher wavelengths. Despite of the high reflectivity of the fs-IR grating (about 90%), the side lobes were negligible small. The simulation in figure 3.33 shows that at higher reflections the side lobes increase dramatically. It was also observed in the measurement here that the fs-IR gratings do not exactly follow the coupled-mode theory model. This was also observed by other groups presented in [110, 105, 104].

Nevertheless, if high reflection values are needed which increases also the strength of the returned light signal, fs-IR written gratings are the agent of choice.