Recent Trends in Fiber Bragg Grating Technologies Seminar Report2010

Recent Trends in Fiber Bragg Grating Technologies Seminar Report2010

1. INTRODUCTION

Fiber Bragg grating is one of the key optical components, which are gaining increasing attention in different fields of optical technologies including optical fiber communication and sensing applications. The report entitled ‘Recent Trends in the Fiber Bragg Grating Based Technologies’ focuses the recent developments and advancements in the applications and fabrication methods of Fiber Bragg based optical systems. It also highlights the innovative technological developments and the limitations of fiber Bragg Gratings.A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of fiberthat reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength specific mirror. The ability to filter out a specific wavelength made it possible to use it as sharp wavelength filter which extensively found applications in the field of optical communication as multiplexers, demultiplexer and add-drop multiplexer etc [6-9]. The use of the fiber Bragg grating as the dispersion compensation element maderevolutionary developments in the field of fiber optic communication. [10].The Bragg wavelength as a function of grating pitch also made it possible to build transducers for precisely measuring many physical quantities like strain, temperature, acceleration etc [10-13].

2. BASIC THOERY OF FIBER BRAGG GRATING

A fiber Bragg grating is a periodic or aperiodic perturbation of the effective refractive index in the core of an optical fiber. Typically, the perturbation is approximately periodic over a certain length of e.g. a few millimeters or centimeters, and the period is of the order of hundreds of nanometers. This leads to the reflection of light (propagating along the fiber) in a narrow range of wavelengths, for which a Bragg condition is satisfied. This basically means that the wave number of the grating matches the difference of the wave numbers of the incident and reflected waves. (In other words, the complex amplitudes corresponding to reflected field contributions from different parts of the grating are all in phase so that can add up constructively; this is a kind of phase matching.) Other wavelengths are nearly not affected by the Bragg grating, except for some side lobes which frequently occur in the reflection spectrum (but can be suppressed by apodization). Around the Bragg wavelength, even a weak index modulation (with amplitude of e.g. 10-4) is sufficient to achieve nearly total reflection, if the grating is sufficiently long (e.g. a few millimeters).

The reflection bandwidth of a fiber grating, which is typically well below 1 nm, depends on both the length and the strength of the refractive index modulation. The narrowest bandwidth values, as are desirable e.g. for the construction of single-frequency fiber lasers or for certain optical filters, are obtained for long gratings with weak index modulation. Large bandwidths may be achieved with short and strong gratings, but also with aperiodic designs. As the wavelength of maximum reflectivity depends not only on the Bragg grating period but also on temperature and mechanical strain, Bragg gratings can be used in temperature and strain sensors. Transverse stress, as generated e.g. by squeezing a fiber grating between two flat plates, induces birefringence and thus polarization- dependent Bragg wavelengths.

Fig.1 Basic structure of Fiber Bragg Grating

Fig.2 Basic operation of the FBG

The fundamental principle behind the operation of a FBG is Fresnel reflection. Wherelight traveling between media of different refractive indices may both reflect and refractat the interface.The grating will typically have a sinusoidal refractive index variation over a definedlength. The reflected wavelength (λB), called the Bragg wavelength, is defined by therelationship,

………...... (1)

Where ‘n’ is the effective refractive index of the grating in the fiber core and Λ is the grating period. In this case n=(n2+n3)/3 , i.e. it is the average refractive index in the grating (see Fig. 1). The wavelength spacing between the first minimums (nulls), or the bandwidth (Δλ), is given by,

………………………………………. (2)

Where δn0 is the variation in the refractive index (n3 −n2), and η is the fraction of power in the core.

The peak reflection (PB (λ B)) is approximately given by

……………………… (3)

Where N is the number of periodic variations. The full equation for the reflected power PB (λ), is given by,

…………..(4)

Where,

………………… (5)

3. FBG FABRICATION METHODS

Fiber Bragg gratings are created by "inscribing" or "writing" the periodic variation of refractive index into the core of a special type of optical fiber using an intense ultraviolet (UV) source such as a UV laser. Two main processes are used: interference and masking. Which is best depends on the type of grating to be manufactured. A special germanium-doped silica fiber is used in the manufacture of fiber Bragg gratings. The germanium-doped fiber is photosensitive, in that the refractive index of the core changes with exposure to UV light, with the amount of the change a function of the intensity and duration of the exposure.

3.1 Interferometric Method

The first manufacturing method, specifically used for uniform gratings, is the use of two- beam interference. Here the UV laser is split into two beams which interfere with each other creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light that it is exposed to. This method allows for quick and easy changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser light.

Fig.3 Interferometric Method

The UV beam divided into two at a beam splitter and thenbrought together at a mutual angle of y, by reflections from two UV mirrors.This method allows the Bragg wavelength to be chosen independently of theUV wavelength as

………………………………… (6)

where λBragg is the Bragg reflection wavelength, neff is the effective mode indexin the fiber, nuv the refractive index of silica in the UV, λuvis the wavelengthof the writing radiation, and θis the mutual angle of the UV beams. The fiber is held at the intersection of the beams. This method was originallysuccessfully used to write gratings at visible wavelengths. The interferometer isideal for single-pulse

writing of short gratings, and great care has to be takenin the design of the optical mounts. Mechanical vibrations and the inherentlylong path lengths in air can cause the quality of the interferogram to change overa period of time, limiting its application to short exposures. For low-coherencesources, the path difference between the two interfering beams must be equalized;a simple method is to introduce a mirror blank in one arm to compensate for the path imbalance imposed by the beam splitter, as shown in Fig.Notethat in arriving at the fiber, the beam that is transmitted through the beamsplitter undergoes a 180rotation so that they have different spatial profiles. Thisis an important factor for spatially incoherent beams.

3.2 Photo mask Method

A photo mask having the intended grating features may also be used in the manufacture of fiber Bragg gratings. The photo mask is placed between the UV light source and the photosensitive fiber. The shadow of the photo mask then determines the grating structure based on the transmitted intensity of light striking the fiber. Photo masks are specifically used in the manufacture of chirped Fiber Bragg gratings, which cannot be manufactured using a n interference pattern.

Fig.4. Photo mask method

A major step toward easier inscription of fiber gratings was made possible bythe application of the phase mask as a component of the interferometer. Used intransmission, a phase mask is a relief grating etched in a silica plate. The significantfeatures of the phase mask are the grooves etched into a UV-transmittingsilica mask plate, with a carefully controlled mark-space ratio as well as etchdepth. The principle of

operation is based on the diffraction of an incidentUV beam into several orders, m =0, 1, 2 . . . . This is shown schematicallyin Fig5.

The incident and diffracted orders satisfy the general diffractionequation, with the period Λpm of the phase mask,

………………………………….(7)

Fig.5 Photo mask method

Where θm/2 is the angle of the diffracted order, λuv the wavelength, and θi theangle of the incident UV beam. In instances when the period of the grating liesbetween λuv and λuv/2, the incident wave is diffracted into only a single order(m =1) with the rest of the power remaining in the transmitted wave (m = 0).

3.3. Point-by-Point Write Method

A single UV laser beam may also be used to 'write' the grating into the fiber point-by- point. Here, the laser has a narrow beam that is equal to the grating period. This method is specifically applicable to the fabrication of long period fiber gratings. Point-by- point is also used in the fabrication of tilted gratings.

3.4 Production

Originally, the manufacture of the photosensitive optical fiber and the 'writing' of the fiber Bragg grating were done separately. Today, production lines typically draw the fiber from the preform and 'write' the grating, all in a single stage. As well as reducing associated costs and time, this also enables the mass production of fiber Bragg gratings. Mass production is in particular facilitating applications in smart structures.Utilizing large numbers (3000) of embedded fiber Bragg gratings along a single length of fiber.

4. GRATING STRUCTURE

The structure of the FBG can vary via the refractive index, or the grating period. Thegrating period can be uniform or graded, and either localized or distributed in asuperstructure. The refractive index has two primary characteristics, the refractive indexprofile, and the offset. Typically, the refractive index profile can be uniform or apodized,and the refractive index offset is positive or zero.

There are six common structures for FBGs;

1. Uniform positive-only index change,

2. Gaussian apodized,
3.Raised-cosine apodized,
4.Chirped ,

5. Discrete phase shift, and

6. Superstructure

Fig.6. Structure of the refractive index change in a uniform FBG (1), a chirped FBG (2), a tilted FBG (3), and a superstructure FBG (4).

Fig.7. Refractive index profile in the core of, 1) a uniform positive-only FBG, 2) aGaussian-apodized FBG, 3) a raised-cosine-apodized FBG with zero-dc change, and4) a discrete phase shift FBG

.

4.1 Apodized Gratings

There are basically two quantities that control the properties of the FBG. These are the grating length, Lg, given as

………………………………….(8)

and the grating strength, δn0 η. There are, however, three properties that need to becontrolled in a FBG. These are the reflectivity, the bandwidth, and the side-lobestrength. As shown above, the bandwidth depends on the grating strength, and not thegrating length. This means the grating strength can be used to set the bandwidth. Thegrating length, effectively, can then be used to set the peak reflectivity, whichdepends on both the grating strength and the grating length. The result of this is that theside-lobe strength cannot be controlled, and this simple optimization results insignificant side-lobes. A third quantity can be varied to help with side-lobe suppression.This is apodization of the refractive index change. The term apodization refers to thegrading of the refractive index to approach zero at the end of the grating. Apodizedgratings offer significant improvement in side-lobe suppression while maintainingreflectivity and a narrow bandwidth. The two functions typically used to apodized a FBGare Gaussian and raised-cosine.

4.1.1 Chirped fiber Bragg Grating

The refractive index profile of the grating may be modified to add other features, such as a linear variation in the grating period, called a chirp. The reflected wavelength changes with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the property of adding dispersion—namely; different wavelengths reflected from the grating will be subject to different delays. This property has been used in the development of phased-array antenna systems and polarization mode dispersion compensation, as well.

4.1.2. Tilted Fiber Bragg grating

In standard FBGs, the grading or variation of the refractive index is along the length of the fiber (the optical axis), and is typically uniform across the width of the fiber. In a tilted FBG (TFBG), the variation of the refractive index is at an angle to the optical axis. The angle of tilt in a TFBG has an effect on the reflected wavelength, and bandwidth.

4.1.3. Long period grating

Typically the grating period is the same size as the Bragg wavelength, as shown above. For a grating that reflects at 1500 nm, the grating period is 500 nm, using a refractive index of 1.5. Longer periods can be used to achieve much broader responses than are possible with a standard FBG. These gratings are called long-period fiber grating. They typically have grating periods on the order of 100 micrometers, to a millimeter, and are therefore much easier to manufacture

5. APPLICATIONS OF FIBER BRAGG GRATING

A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of fiberthat reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength specific mirror. The ability to filter out a specific wavelength made it possible to use it as sharp wavelength filter which extensively found applications in the field of optical communication as multiplexers, demultiplexer and add-drop multiplexer etc [6-9]. The use of the fiber Bragg grating as the dispersion compensation element made revolutionary developments in the field of fiber optic communication. .The Bragg wavelength as a function of grating pitch also made it possible to build transducers for precisely measuring many physical quantities like strain, temperature, acceleration etc.

6. APPLICATIONS IN OPTICAL COMMUNICATION

The wavelength selectivity of the FBG made it possible to find application extensively in the field of optical communication. It is widely used as the wavelength filter, Bandpass filter, and as Multiplexers/Demultiplexers. The main application of FBG in the field of Telecommunication is listed below

• Fiber Lasers
• Fiber Amplifiers
• Fiber Filters
• Dispersion Compensators
• Optical Fiber Phase Conjugator
• WDM

Multiplexers

Demultiplexers

6.1 Wavelength Filter

Fig.8 Wavelength filter

The figure shows the FBG wavelength filter. It is an essential component in the demultiplexer to filter out the wavelength encoded signal from a particular source. In the figure, a polychromatic optical signal is feeded in to a circulator. Circulator allows the signal to propagate only in one direction. The reflected signal from the FBG will be received through the other port of the circulator. The wavelength can be tuned by varying the grating pitch.

6.2 FBG based Demultiplexer

Fig 9. Demultiplexer using FBG

In the FBG based demultiplexer, there is a cascade of FBG filters tuned to different wavelengths. The polychromatic signal will be given to the input of the demultiplexer. The first filter will separate a wavelength specific to the Bragg grating associated with it. In the similar fashion all other filter will do the same job.

6.3 FBGbased Add-Drop multiplexer

Fig.10 Add-drop Multiplexer using FBG

The primary application of fiber Bragg gratings is in optical communications systems. They are specifically used as notch filters. They are also used in optical multiplexers and demultiplexer with an optical circulator, or Optical Add-Drop Multiplexer (OADM). Figure 5 shows 4 channels, depicted as 4 colors, impinging onto a FBG via an optical circulator. The FBG is set to reflect one of the channels, here channel 4. The signal is reflected back to the circulator where it is directed down and dropped out of the system. Since the channel has been dropped, another signal on that channel can be added at the same point in the network.

A demultiplexer can be achieved by cascading multiple drop sections of the OADM, where each drop element uses a FBG set to the wavelength to be demultiplexed. Conversely, a multiplexer can be achieved by cascading multiple add sections of the OADM. FBG demultiplexer and OADMs can also be tunable. In a tunable demultiplexer or OADM, the Bragg wavelength of the FBG can be tuned by strain applied by a piezoelectric transducer.

6.4 MACH-ZEHNDER-DWMMultiplexer / Demultiplexer

Fig 11. WDM Demux using Mach-Zehnder Configuration

Figure shows the FBG based WDM multiplexing and demultiplexing in a Mach-Zehnder configuration. In this the input multiwavelength signal is feeded to the Mach-Zehnder configuration. The first 3dB coupler splits the input light equally and allowed to pass through the two fiber arms. The FBG in the arm reflects back a particular wavelength for which it is tuned to. The returned and separated wavelength is collected from the other input port of the first 3dB coupler. All other wavelengths will be

passing through the arms unchanged. The demultiplexer consists of large number of this type of filters connected in cascade form.

Insertion of channel λk

Fig 12. WDM Mux using Mach-Zehnder Configuration

In the multiplexing technique, the technique of insertion of a particular wave length is shown as the above. In this the wavelength that is to be inserted will be feeded from the port 3 of the output coupler. This wavelength will be reflected from the FBG and added with the other wavelength signal to form the multiplexed signal.

6.5 Dispersion compensation using chirped grating

Fig.13 Dispersion compensation using FBG

The most important contribution of the FBG is that it can be used to reduce the dispersion in the optical fiber communication. The dispersion is the one of the effect that reduces the bandwidth of the optical