4. Experimental results of femtosecond transient FBGs

#### 4.1 Experimental setup

The experimental setup is standard for FBG inscription with the phase mask technique and is shown schematically below (Figure 1). A femtosecond laser (800 nm, 35 fs, 1 KHz) is focused on the fiber core, through a phase mask. The mask period is 2.14 μm, suitable for second-order Bragg gratings at 1550 nm. The fiber to be inscribed is connected to a probe signal source and an Optical Spectrum Analyzer (OSA) to monitor the FBG spectrum or to a fast photodiode (Thorlabs DETO8CFC) to monitor the dynamic effects. The signal source can be a broadband ASE source when characterize permanent FBG inscription or an amplified DFB laser when observing transient, dynamic effect. The probe laser mostly operated in CW mode providing 1 W output power and was operated in pulse mode for Kerr grating experiments.

#### 4.2 Transient Kerr grating

We tried to observe a transient Kerr grating with pulse energies below the inscription threshold in standard SMFs. In these experiments, we monitor the reflection from the grating with a photodiode. We found the permanent inscription threshold to be 160 μJ; thus, our pulse energy is limited below this value. For 100 μJ pulse energy, we expect grating index modulation of 8 � <sup>10</sup>�<sup>7</sup> , which will exist for 35 fs only. The expected reflection from such a grating is extremely weak; the

#### Figure 1.

Schematic of the optical setup.

coupling coefficient, calculated according to the theory outlined in Section 3 is <sup>κ</sup> <sup>8</sup> <sup>10</sup><sup>7</sup> <sup>1</sup> μm h i,four orders of magnitude lower than typical permanent gratings. Therefore, we drive our probe laser with 50 ns pulses at a 20 KHz pulse rate. In this mode, the laser outputs 1 KW peak power, tuned to the Bragg wavelength. The reflected efficiency expected for such index modulation is <sup>10</sup><sup>5</sup> :

applications. We can reduce the switching time to less than 2 μs at the cost of extinction ratio by setting the signal wavelength slightly away from the grating

Pulse measured out of a modulated permanent grating as a function of pulse energy.

While this is a very slow modulation time, it shows the natural response of the induced grating by femtosecond laser pulse at different pulse energies above the inscription threshold. Note that two different regimes are noticeable: for low pulse energies, a fast decay of the signal followed by a long μs tail. When increasing the pulse energy, the fast decay disappears. This indicates the formation of permanent index modification. We will show that the long μs tail can be cut off by performing

The ability of femtosecond lasers to modify the material refractive index of practically any optical material is a keystone in photonic device fabrication. However, when one wants to observe transient grating effects, it is a drawback, as it limits the applied pulse energy to energies below the multiphoton ionization threshold and permanent index change. It is known that the modified index has a limit, i.e., it can only grow to the order of 10<sup>3</sup> (positive change in silica fibers) when it reaches saturation. Reflection from a transient grating, however, depends

We have found that femtosecond photo pre-treatment can immunize a fiber up to a certain illumination intensity [94]. In fact, the reported multiphoton ionization and inscription threshold ( <sup>10</sup><sup>13</sup> <sup>W</sup>=cm2) can be raised so that permanent Bragg

Fiber immunizing can help avoid permanent index change and observation of transient FBG effects. After immunization, the fiber transient index change effects, such as heat or Kerr, are expected to be observed more clearly as the permanent

In order to immunize the fiber against femtosecond inscription, we remove the phase mask and inscribe it with a focal line pattern. The pulse energy in this

resonance.

Figure 2.

Femtosecond Transient Bragg Gratings DOI: http://dx.doi.org/10.5772/intechopen.84448

immunization.

31

4.4 Immunization to femtosecond inscription

on both the index modulation and the grating period.

index change is saturated and its effects are suppressed.

gratings first appear at a higher pulse energies.

Unfortunately, we could not detect any Kerr grating reflections with our detector or with a lock-in amplifier. Furthermore, we noticed an increase in the detector DC level, and a photodiode was able to measure a weak (nW) but slowly growing reflected power signal indicating permanent inscription. We repeated the experiment with 50 μJ pulse energy to find again permanent inscription.

The reflected power was extremely weak and could not be detected with a standard ASE source. The permanent inscription may be the result of tunneling ionization rather than multiphoton ionization, which is a much slower process that is expected for relative low intensities by the Keldysh theory [93]. Further investigation is required in order to produce Kerr grating, most likely in a different material with higher nonlinearity. In the following we will present different methods to observe transient gratings based on thermal effects in silica fibers.

#### 4.3 Permanent grating switching

In this configuration, we first fabricate a high-quality (>25 dB) grating and observed light transmitted through it, i.e., we measure the transmission loss of the grating rather than its reflections. To modulate the grating, we block half of the beam and illuminate only half of the permanent grating through the phase mask. Due to the induced heat of each pulse, the refractive index is elevated, causing half the grating to shift to a higher Bragg wavelength, leaving the other half intact. This opens a transmission gap in the grating, allowing a signal, at wavelength matching the grating Bragg wavelength to be detected by the photodiode. Essentially, we temporarily transform a uniform grating into a phase-shifted grating. Figure 2 shows the time profile of the transmitted signal through the shifted grating.

The switching mechanism is based on induced heat, as if the gratings were placed on a temperature-controlled controller. However, here the switching is done with an ultrafast laser that provides ultrafast rise time. As can be seen in Figure 2, the switching time here is about 8 μs, which makes it suitable for Q-switching

Figure 2. Pulse measured out of a modulated permanent grating as a function of pulse energy.

applications. We can reduce the switching time to less than 2 μs at the cost of extinction ratio by setting the signal wavelength slightly away from the grating resonance.

While this is a very slow modulation time, it shows the natural response of the induced grating by femtosecond laser pulse at different pulse energies above the inscription threshold. Note that two different regimes are noticeable: for low pulse energies, a fast decay of the signal followed by a long μs tail. When increasing the pulse energy, the fast decay disappears. This indicates the formation of permanent index modification. We will show that the long μs tail can be cut off by performing immunization.

#### 4.4 Immunization to femtosecond inscription

The ability of femtosecond lasers to modify the material refractive index of practically any optical material is a keystone in photonic device fabrication. However, when one wants to observe transient grating effects, it is a drawback, as it limits the applied pulse energy to energies below the multiphoton ionization threshold and permanent index change. It is known that the modified index has a limit, i.e., it can only grow to the order of 10<sup>3</sup> (positive change in silica fibers) when it reaches saturation. Reflection from a transient grating, however, depends on both the index modulation and the grating period.

We have found that femtosecond photo pre-treatment can immunize a fiber up to a certain illumination intensity [94]. In fact, the reported multiphoton ionization and inscription threshold ( <sup>10</sup><sup>13</sup> <sup>W</sup>=cm2) can be raised so that permanent Bragg gratings first appear at a higher pulse energies.

Fiber immunizing can help avoid permanent index change and observation of transient FBG effects. After immunization, the fiber transient index change effects, such as heat or Kerr, are expected to be observed more clearly as the permanent index change is saturated and its effects are suppressed.

In order to immunize the fiber against femtosecond inscription, we remove the phase mask and inscribe it with a focal line pattern. The pulse energy in this

coupling coefficient, calculated according to the theory outlined in Section 3 is

reflected efficiency expected for such index modulation is <sup>10</sup><sup>5</sup>

Fiber Optic Sensing - Principle, Measurement and Applications

ment with 50 μJ pulse energy to find again permanent inscription.

Therefore, we drive our probe laser with 50 ns pulses at a 20 KHz pulse rate. In this mode, the laser outputs 1 KW peak power, tuned to the Bragg wavelength. The

Unfortunately, we could not detect any Kerr grating reflections with our detector or with a lock-in amplifier. Furthermore, we noticed an increase in the detector DC level, and a photodiode was able to measure a weak (nW) but slowly growing reflected power signal indicating permanent inscription. We repeated the experi-

The reflected power was extremely weak and could not be detected with a standard ASE source. The permanent inscription may be the result of tunneling ionization rather than multiphoton ionization, which is a much slower process that is expected for relative low intensities by the Keldysh theory [93]. Further investigation is required in order to produce Kerr grating, most likely in a different material with higher nonlinearity. In the following we will present different methods to observe transient gratings based on thermal effects in silica fibers.

In this configuration, we first fabricate a high-quality (>25 dB) grating and observed light transmitted through it, i.e., we measure the transmission loss of the grating rather than its reflections. To modulate the grating, we block half of the beam and illuminate only half of the permanent grating through the phase mask. Due to the induced heat of each pulse, the refractive index is elevated, causing half the grating to shift to a higher Bragg wavelength, leaving the other half intact. This opens a transmission gap in the grating, allowing a signal, at wavelength matching the grating Bragg wavelength to be detected by the photodiode. Essentially, we temporarily transform a uniform grating into a phase-shifted grating. Figure 2 shows the time profile of the transmitted signal through the shifted grating. The switching mechanism is based on induced heat, as if the gratings were placed on a temperature-controlled controller. However, here the switching is done with an ultrafast laser that provides ultrafast rise time. As can be seen in Figure 2, the switching time here is about 8 μs, which makes it suitable for Q-switching

,four orders of magnitude lower than typical permanent gratings.

:

<sup>κ</sup> <sup>8</sup> <sup>10</sup><sup>7</sup> <sup>1</sup>

Figure 1.

30

μm h i

Schematic of the optical setup.

4.3 Permanent grating switching

#### Figure 3.

Transmission spectra of the inscribed permanent FBGs in untreated and treated fibers: untreated fiber, dashed blue lines; treated fibers, solid red lines. (a) 160 μJ pulse energy. (b) 180 μJ pulse energy.

pre-treatment was chosen to be slightly more than twice the average energy for FBG inscription, so that the peak intensity of the pre-treatment would be slightly higher than that of the FBG inscription.

The measured rise time is 2 ns, but we believe the actual rise time is significantly shorter, since our measurement was limited by our detection system (about the same rise time was also measured for our 35 fs laser source). For a 550 μJ illumination pulse, the reflected pulse duration is approximately 14.3 ns (measured between 1 and e points). An exponential fit is shown (light blue) in Figure 4a, with a time constant of �10 ns. For a 550-μJ illumination pulse, the measured reflected peak power is 0.38 mW, corresponding to a peak power reflectivity of 0.0435%. Assuming an effective grating length of 5 mm, and applying the theory for spatial-temporal gratings, we estimate an index change of <sup>Δ</sup><sup>n</sup> <sup>¼</sup> <sup>2</sup>:<sup>3</sup> � <sup>10</sup>�6. This is three orders of magnitude less than reported in the literature for permanent femtosecond inscribed gratings. However, it should be noted that tens of thousands of pulses are used to achieve the reported Δn for permanent inscription. The thermal grating, and the refractive index increase, decay time depends on the diffusion coefficient of the fiber and the grating period. In this case, the decay time expected to be 34 ns [96]. The reflected power from a temporal grating is proportional to

(a) Measured average time trace of the reflected pulses for different femtosecond illumination pulse energies. The inscribing pulse rate was 2 Hz. An exponential fit with parameters, a = 17.87, b = 2.06, c = 10.5, and d = 1.7, is shown for the highest reflectivity, indicating a thermal diffusion time of �10 ns. (b) The peak amplitude of the measured reflected signal as a function of pulse energy. The linear fit indicates a nonlinear

; thus, we expect from theory (Eq. (6)) a reflected signal decay time of 17 ns.

, which is a good indication that the grating is, indeed, based on

This is in good agreement with the experimental results, where differences may

Figure 4(b) shows the peak reflectivity as a function of applied pulse energy. A small increase in the inscribing pulse energy results in a higher induced transient refractive index change, leading to a significantly stronger reflected pulse. As suggested by the linear fit, the peak reflectivity has, indeed, a nonlinear growth that

With respect to Figure 2, and time scale reported with femtosecond induced index change [4], the immunization technique allows us to remove transient effects associated with material resolidification and access the previous phase of femtosec-

arise due to the presence of germanium in the fiber core.

relation between the pulse energy (intensity) and reflectivity.

Femtosecond Transient Bragg Gratings DOI: http://dx.doi.org/10.5772/intechopen.84448

ð Þ Δn 2

33

Figure 4.

corresponds to I

multiphoton absorption.

ond laser-matter interaction.

5

Figure 3 shows permanent gratings inscribed on a fresh fiber compared to a treated fibers. Before any pre-treatment, gratings with 25 and 30 dB transmission dips were inscribed, at pulse energies of 160 μJ (Figure 3a) and 180 μJ (Figure 3b), respectively. As is evident, when pre-treatment of the fibers is done at slightly more than double of the pulse energy, the results is a complete immunity for inscription at 160 μJ and in only 2 dB transmission loss at 180 μJ. For the latter, a 2.5-nm wavelength shift is observed, corresponding to an increase of <sup>2</sup> <sup>10</sup><sup>3</sup> of the average refractive index due to the pre-treatment. Thus, our treatment greatly reduces the ability to inscribe gratings. Pre-treatment of the fiber causes complete immunity or limited grating buildup at a considerably lower rate.

#### 4.5 Fast switching with transient thermal grating

Fiber immunization extremely limits the grating buildup. We characterized the transient grating reflections of an immunized fiber [95]. After completing the photo-treatment process on a standard SMF at a pulse energy of 1 mJ, the pump laser pulse rate was lowered to 2 Hz in order to reduce the average thermal effects, and reflected pulses were measured with our detector. Figure 4a shows the averaged time trace (100 pulses) of the reflected pulse for different femtosecond illumination pulse energies (all below half of the immunization pulse energy).

The reflected pulses (Figure 4a) have a very fast rise time followed by nanosecond decay. This is three orders of magnitude improvement compared to transient grating based on UV laser reported in Ref. [80, 81]. The observed transient increase in the reflectivity can mainly be attributed to local heating of the silica, due to nonlinear absorption, corresponding to a local increase in the refractive index and is followed by thermal diffusion that washes out the grating. The decay time is typical for thermal diffusion at these sizes and distances.

#### Figure 4.

pre-treatment was chosen to be slightly more than twice the average energy for FBG inscription, so that the peak intensity of the pre-treatment would be slightly higher

Transmission spectra of the inscribed permanent FBGs in untreated and treated fibers: untreated fiber, dashed

blue lines; treated fibers, solid red lines. (a) 160 μJ pulse energy. (b) 180 μJ pulse energy.

Fiber Optic Sensing - Principle, Measurement and Applications

Figure 3 shows permanent gratings inscribed on a fresh fiber compared to a treated fibers. Before any pre-treatment, gratings with 25 and 30 dB transmission dips were inscribed, at pulse energies of 160 μJ (Figure 3a) and 180 μJ (Figure 3b), respectively. As is evident, when pre-treatment of the fibers is done at slightly more than double of the pulse energy, the results is a complete immunity for inscription at 160 μJ and in only 2 dB transmission loss at 180 μJ. For the latter, a 2.5-nm wavelength shift is observed, corresponding to an increase of <sup>2</sup> <sup>10</sup><sup>3</sup> of the average refractive index due to the pre-treatment. Thus, our treatment greatly reduces the ability to inscribe gratings. Pre-treatment of the fiber causes complete

Fiber immunization extremely limits the grating buildup. We characterized the

The reflected pulses (Figure 4a) have a very fast rise time followed by nanosecond decay. This is three orders of magnitude improvement compared to transient grating based on UV laser reported in Ref. [80, 81]. The observed transient increase in the reflectivity can mainly be attributed to local heating of the silica, due to nonlinear absorption, corresponding to a local increase in the refractive index and is followed by thermal diffusion that washes out the grating. The decay time is typical

transient grating reflections of an immunized fiber [95]. After completing the photo-treatment process on a standard SMF at a pulse energy of 1 mJ, the pump laser pulse rate was lowered to 2 Hz in order to reduce the average thermal effects, and reflected pulses were measured with our detector. Figure 4a shows the averaged time trace (100 pulses) of the reflected pulse for different femtosecond illumination pulse energies (all below half of the immunization pulse energy).

immunity or limited grating buildup at a considerably lower rate.

4.5 Fast switching with transient thermal grating

for thermal diffusion at these sizes and distances.

than that of the FBG inscription.

Figure 3.

32

(a) Measured average time trace of the reflected pulses for different femtosecond illumination pulse energies. The inscribing pulse rate was 2 Hz. An exponential fit with parameters, a = 17.87, b = 2.06, c = 10.5, and d = 1.7, is shown for the highest reflectivity, indicating a thermal diffusion time of �10 ns. (b) The peak amplitude of the measured reflected signal as a function of pulse energy. The linear fit indicates a nonlinear relation between the pulse energy (intensity) and reflectivity.

The measured rise time is 2 ns, but we believe the actual rise time is significantly shorter, since our measurement was limited by our detection system (about the same rise time was also measured for our 35 fs laser source). For a 550 μJ illumination pulse, the reflected pulse duration is approximately 14.3 ns (measured between 1 and e points). An exponential fit is shown (light blue) in Figure 4a, with a time constant of �10 ns. For a 550-μJ illumination pulse, the measured reflected peak power is 0.38 mW, corresponding to a peak power reflectivity of 0.0435%.

Assuming an effective grating length of 5 mm, and applying the theory for spatial-temporal gratings, we estimate an index change of <sup>Δ</sup><sup>n</sup> <sup>¼</sup> <sup>2</sup>:<sup>3</sup> � <sup>10</sup>�6. This is three orders of magnitude less than reported in the literature for permanent femtosecond inscribed gratings. However, it should be noted that tens of thousands of pulses are used to achieve the reported Δn for permanent inscription. The thermal grating, and the refractive index increase, decay time depends on the diffusion coefficient of the fiber and the grating period. In this case, the decay time expected to be 34 ns [96]. The reflected power from a temporal grating is proportional to ð Þ Δn 2 ; thus, we expect from theory (Eq. (6)) a reflected signal decay time of 17 ns. This is in good agreement with the experimental results, where differences may arise due to the presence of germanium in the fiber core.

Figure 4(b) shows the peak reflectivity as a function of applied pulse energy. A small increase in the inscribing pulse energy results in a higher induced transient refractive index change, leading to a significantly stronger reflected pulse. As suggested by the linear fit, the peak reflectivity has, indeed, a nonlinear growth that corresponds to I 5 , which is a good indication that the grating is, indeed, based on multiphoton absorption.

With respect to Figure 2, and time scale reported with femtosecond induced index change [4], the immunization technique allows us to remove transient effects associated with material resolidification and access the previous phase of femtosecond laser-matter interaction.

We also note here that thermal grating diffusion time is highly dependent on grating period. The diffusion time is opposite to the square of the grating period; thus, working with first-order grating can reduce the time scale by a factor of four. The applicability to transient thermal grating for higher pulse rate and life time of such device is elaborated elsewhere [95].
