Fluorescence and doping profiles of Cr:YAG single crystal fiber

inner cladding diameter using Eq. (4.13 ) is 100.5 μm, which is in excellent agreement with the measured result. The EPMA measurement implied that the core of DCF still maintains the crystalline structure. Further proofs will be provided with HRTEM in the next section.

4.3 Fluorescence mapping and analysis

4.3.1 Fluorescence and doping profiles of Cr:YAG single crystal fiber

The fluorescence mappings of active ions were measured by using laser scanning confocal microscopy (LSCM), and compared with the EPMA measurements. Figure 4.13 show the comparison between LSCM and EPMA measurement results in the cross section of 920-μm Cr⁴⁺:YAG single crystal fiber. The fluorescence intensity was calibrated with the EPMA measurement to compare the Cr³⁺ and Cr⁴⁺ concentrations.

The Cr³⁺ ion sensitivity by LSCM measurement was determined to be 1.6×10¹⁷ cm^-3, which is equal to 4.4×10^-4 wt.% with a 0.3-sec/pixel measurement rate. In Fig. 4.13 (a), the Cr³⁺ fluorescence profile has a good agreement in the Cr2O3 doping distribution. However, the Cr⁴⁺ fluorescence is flatter than Cr2O3 profile in the cross section as shown in Fig. 4.13 (b). The of Cr⁴⁺ fluorescence intensity is 3 orders smaller than the Cr³⁺ fluorescence. The concentration of Cr⁴⁺ is less than 1% when compared to that of Cr³⁺. It is known that 4+ oxidation state of Cr ion in YAG is generated by Ca²⁺ ion with charge compensation. To explain the phenomenon of the distribution difference between of Cr³⁺ and Cr⁴⁺ fluorescence, the profile of the CaO dopant needs to be taken into account. The concentration of Cr⁴⁺ ion depends on both the concentrations of Cr2O3 and CaO. Although the concentration of Cr2O3 at the fiber center is lower than that at the edge, the concentration of CaO tends to accumulate near the fiber center, which results in the flatter distribution of Cr⁴⁺ fluorescence.

0 20 40 60 80 100

LSCM: Cr³⁺ fluorescence

0.00 LSCM: Cr⁴⁺ fluorescence

Percent diameter (%)

Fig. 4.13. LSCM-measurement profiles of (a) Cr³⁺ fluorescence and (b) Cr⁴⁺ fluorescence to EPMA line-scan measurements.

4.3.2 Fluorescence and doping profiles of Cr⁴⁺:YAG DCF

Figure 4.14 (a) shows the Cr2O3 and CaO doping concentration profiles of Cr⁴⁺:YAG DCF with EPMA measurement. The Cr2O3 distribution was uniform in both the core and inner cladding. The CaO tended to diffuse outward to the inner cladding, whereas it is contrary to its inward diffusion tendency in the single crystal YAG fiber. Comparing the doping concentrations between Cr⁴⁺:YAG-silica SCF and Cr⁴⁺:YAG DCF core region, there was almost no concentration decay in the co-drawing process. The Cr³⁺ and Cr⁴⁺ fluorescence profiles of Cr⁴⁺:YAG DCF were shown in the Fig. 4.14 (b). There are stronger Cr³⁺ and Cr⁴⁺ fluorescence intensities in the core region despite of almost the same Cr2O3 concentration in the inner cladding.

A possible reason is that the emission cross sections of Cr³⁺ and Cr⁴⁺ are smaller in the inner cladding than those in the core region.

-100 -75 -50 -25 0 25 50 75 100

Fig. 4.14. Measurements the Cr⁴⁺:YAG DCF, (a) doping concentration profiles of Cr2O3 and CaO by using EPMA method, and (b) fluorescence profiles of Cr³⁺ and Cr⁴⁺ by using the LSCM technique.

4.3.3 Emission and absorption spectra of Cr:YAG DCF in the inner cladding region In the core region of Cr:YAG DCF, the Cr³⁺ and Cr⁴⁺ absorption and emission spectra are the same as the single crystal fiber because the core structure maintains crystalline. The discussions below will be focused on the spectra of Cr ion within the inner cladding of Cr:YAG DCF. In Fig. 4.15 (a), the fluorescence spectrum of Cr³⁺

ions in the inner cladding of Cr:YAG DCF has a broadband emission from 650 to 950

nm with apparent peak around 700 nm. The emission bandwidth is broader than the spectrum of Cr³⁺ ions doped in YAG. Such a broad emission band can be attributed to Cr³⁺:γ-Al2O3. The emission spectrum was fitted into five Gaussian peaks to identify the Cr³⁺ emission levels in the inner cladding as shown in Fig. 4.15 (b). The five peaks are centered at 654.8, 699.9, 710.8, 755.1, and 853.8 nm with FWHM of 37.3, 16.7, 35.8, 99.8, and 110.9 nm, respectively. It can be deduced that Cr³⁺ associated with 654.8, 699.9, and 710.8 nm are located at the high-field sites (²E→⁴A2 transition), while Cr³⁺ associated with 755.1 and 853.8 nm are located at the low-field sites (⁴T2→⁴A2 transition) [4.16-4.18].

600 650 700 750 800 850 900 950

0.0

600 650 700 750 800 850 900 950

0.0

DCF inner cladding (x5 times)

(a) (b)

Fig. 4.15. (a) The Cr³⁺ fluorescence spectra of Cr:YAG DCF core and inner cladding. (b) Gaussian peaks fits to fluorescence spectrum at inner cladding.

1100 1200 1300 1400 1500 1600 1700

0.0

DCF inner cladding (x50 times)

1100 1200 1300 1400 1500 1600 1700 0.0

Fig. 4.16. (a) The Cr⁴⁺ fluorescence spectra of Cr⁴⁺ in core and inner cladding of Cr:YAG DCF. (b) Fitting Gaussian peaks to fluorescence spectrum at inner cladding.

In Fig. 4.16 (a), the emission peak for Cr⁴⁺ in the inner cladding is around 1210 nm and is shorter than the peak of Cr⁴⁺ in YAG. The decomposition of Cr⁴⁺ emission spectrum in the inner cladding is fitted by seven Gaussian peaks as shown in Fig. 4.16 (b). The peak wavelengths are at 1166.8, 1211.2, 1239.4, 1302.0, 1335.5, 1416.7, and 1432.1 nm with the corresponding FWHM of 31.3, 28.8, 90.8, 24.2, 72.8, 85.0, and 262.9 nm, respectively. The Gaussian peaks at 1166.8, 1211.2, and 1302.0 are narrower than that of other bands. We assigned that the Gaussian peaks 1, 2, and 4 are from Cr⁴⁺ located at high-field sites (¹E→³A2 transition), while the Gaussian peaks 3, 5, 6, and 7 are from the Cr⁴⁺ located at low-field sites (³T2→³A2 transition).

Table 4.4. Multi-peak Gaussian fittings of emission spectrum at DCF inner cladding [4.16-4.18].

Transition Crystal field

Cr³⁺ 1

In Fig. 4.17, it shows the absorption spectrum of Cr-doped glass fiber, i.e. the Cr:YAG was all diffused into silicate. The Gaussian peak fitting indicates that there are four absorption bands contributing to the broad absorption. The three apparent peaks at 452, 651, and 809 nm are also found in silica glasses [4.19-4.21] which are attributed to the Cr³⁺ ions with transitions of ⁴A2 to ⁴T1 and ⁴T2. The absorption peak at 938 nm with 437-nm bandwidth has been assigned to the transition from Cr⁴⁺ ion in alumina-silicate glass [4.22]. The details are listed in Table 4.5.

400 600 800 1000 1200 1400 1600

Fig. 4.17. Absorption spectrum of Cr-doped glass.

Table 4.5. Multi-peak Gaussian fittings of absorption spectrum at DCF inner cladding [4.19-4.22].

Peak (nm) Bandwidth (nm) Oxidation states Transition 452

To measure the emission spectra of Cr:YAG-silica SCF with pump wavelength from 750 to 1000 nm. As shown in Fig. 18, the emission spectrum has a red shift as increasing the pump wavelength [4.23]. It reveals the apparent peak around 1000 nm with pump wavelength of 750 and 800 nm. When pump wavelength is from 850 to 1000 nm, the emission peak shifts to 1150 nm and the broad emission band from 1250 to 1600 nm is generated. It is inferred that both Cr³⁺ and Cr⁴⁺ ions are coexistent in glass fiber. Figure 4.19 summarizes the pump wavelength dependence of emission peaks and bandwidths of Cr-doped-glass SCF. The emission peaks shifts from 1008 to 1144 nm, while the bandwidths increases from 268 to 406 nm when increase pump wavelength from 750 to 900 nm. In Table 4.4, the interesting broadband emission should be contributed by the Cr³⁺ at low-field sites and Cr⁴⁺ at both low- and high-field sites.

800 1000 1200 1400 1600

Fig. 4.18. Emission spectra of Cr-doped-glass SCF under pump wavelength from 750 to 1000 nm.

700 750 800 850 900 950 1000

900

Fig. 4.19. Pump wavelength dependence of emission peaks and bandwidths in the Cr-doped SCF.

900 1000 1100 1200 1300 1400 1500 1600 0.0

Fig. 4.20. The ASE output power of Cr-doped glass SCF with 900-nm pump. The inset is the fluorescence spectrum by 900-nm pump wavelength.

In Fig. 4.20, the ASE output power of the Cr-doped SCF was measured with a 900-nm pump. The inset curve is the corresponding spectrum by using a 950-nm long-wavelength-pass filter to against the pump power. In the experiment, as much as 12 μW of ASE power was obtained with around 700 mW absorbed pump power. To our knowledge, it is the first time to obtain the ASE output spectrum with more 400-nm broad emission band in Cr-doped glass fiber. It may be a candidate as a light source for high resolution OCT system.

4.4 Propagation loss analysis

For the case of the 10-μm core, which we have discussed in section 3.4.3, the core variation could be improved from 58% to 17% by using the sapphire tube in the growth system. It is more than a factor of 3 improvement. The variation of core diameter may still lead to a loss of propagating power because of the coupling of a low-order mode to a high-order mode. The tapered angle Ω derived from the core variation should meet the adiabatic criterion as follows [4.24-4.25]:

( )

_{. (4.14)}

2

2 1

π

β β

ρ

−

≤ Ω

where

ρ

is the core radius,

β

1 and

β

2 are the respective propagation constants of the modes before and after the tapering. For the 10-μm-core fiber, the adiabatic criterion requests that the tapering angle of the core must be within ±1.35-degree to avoid the mode leakage. Figures 4.21 (a) and (b) show the core diameter profile and tapering angle along the propagation axis measured by the LabVIEW vision program. All the measured core tapering angles of the fiber grown with sapphire tube meet the

±1.35-degree requirement.

Without mode leakage, the scattering loss from the random fluctuation of core diameter may become the dominant factor for light attenuation source [4.26-4.30].

The autocorrelation length of the core diameter variation as a function of position was found to be 1.7 mm, as shown in Fig. 4.21 (a). It is 2 orders of magnitude longer than the core diameter, and thus alleviates the scattering loss. Such a long length is expectable since the core variation due to the heating fluctuation power becomes slower, with the aid of the sapphire tube as a heat capacitor.

0 10 20 30 40 50 6

8 10 12 14

Core diameter (μm)

Fiber position (mm) (a)

0 10 20 30 40 50

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

Tapering angle (deg.)

Fiber position (mm) (b)

0 10 20 30 40 50

-0.5 0.0 0.5 1.0

autocorrelation

exponential curve fitting

Fiber position (mm)

Norm. correlation

(c)

Fig. 4.21. (a) Core diameter profile in the propagation axis. (b) Tapering angle of the 10-μm-core fiber fabricated with sapphire tube during LHPG growth. It meets the 1.35-degree adiabatic criterion. (c) Autocorrelation curve of the core diameter for a 10-μm-core DCF.

The measured fiber propagation losses with fibers of various core diameters are shown in Fig. 4.22. Each dot means one propagation loss measurement of a Cr⁴⁺:YAG DCF sample. The fiber losses range from 0.02 to 0.08 dB/cm. Compared with those fibers grown without sapphire tube, the propagation loss was improved from 0.6 dB/cm to 0.02 dB/cm for the 10-μm-core fiber.

8 10 12 14 16 18

0.00 0.02 0.04 0.06 0.08 0.10

Pr opagation los s ( dB/cm )

Core diameter (μm)

Fig. 4.22. Propagation losses of the Cr⁴⁺:YAG DCFs with various core diameters.

Chapter 5 Optical performance and discussion

Cr⁴⁺:YAG with a broadband emission spectrum that just covers the silica fiber transmission window has a potential to develop optical communication components, such as ASE light source, optical amplifier, and tunable laser. It can also be used as light source in biomedical OCT technology. In this chapter, we demonstrate and discuss the key factors to its optical performance. Cross sections of absorption, emission, and ESAs were also determined by experimental curve fitting. Further improvement is also discussed.

5.1 ASE light source

Based on the simulation in section 2.3, the core diameter is one of the important factors to ASE performance. Therefore, core-reduction processes were developed and evolved from the LHPG method to sapphire tube assisted CDLHPG technique. When using sapphire tube assisted CDLHPG to fabricate Cr⁴⁺:YAG DCF, core diameter around 10 μm can be obtained. In this section, the ASE output powers from fibers of different core diameters are measured in end-pump scheme.

In order to verify the effect of the core-diameter on the ASE output power, fibers with several core-diameters were prepared. The growth fibers were packaged with Pb-Sn alloy to improve the heat dissipation. The fiber endfaces were grinded and polished to obtain optical-quality input and output faces. A 1064-nm Yb-fiber laser with its wavelength at the peak of the Cr⁴⁺:YAG absorption spectrum was used to pump the Cr⁴⁺:YAG crystal fibers with 920, 100, 25, and 10-μm core diameters. We used the end pumping scheme for the ASE generation. The pump light was launched into the core of the crystal fiber. The corresponding fiber lengths were 1.5, 4.7, and 5.5 cm, respectively. Figure 5.1 shows the measured and simulated ASE powers as a function of the absorbed pump power. In the experiments, the ASE efficiency of the 25-μm-core fiber with the strongest optical confinement obtained the highest efficiency. The rising ASE efficiency as the core diameter decreasing has an agreement with our simulation before 25-μm core fiber. The roll-off in the φ = 25 μm curve is due to the short CDF length. The ASE power can be further improved by both

reducing the core diameter and increasing the fiber length. Compare the normalized spectra of Cr⁴⁺:YAG fluorescence and ASE output as shown in Fig. 5.2, the bandwidth is only slightly decreased from 277 nm to 265 nm. It should be noted that the output light has a large portion of spontaneous emission and a small portion of amplified spontaneous emission.

0 1 2 3 4

0 200 400 600 800 1000 1200 1400

φ = 920 μ m φ = 100 μ m

ASE p ower ( μ W)

Absorbed pump power (W) φ = 25 μ m

Dots: experiment Lines: simulation

Fig. 5.1. ASE powers versus pump powers for the samples with core diameters of 920, 100, and 25 μm. The dots and the lines represent the measured and simulated data.

Although we have made a series of breakthrough in fiber growth technique, such as the core reduction and glass clad process with CDLHPG, and the core-uniformity improvement with sapphire tube assisted in CDLHPG, and power feedback control of heating laser, the ASE efficiency is still low due to the small cross section ratio of ESA to GSA and pump ESA loss. The cladding pump scheme that was presented in section 2.3 should be adopted to suppress the pump ESA influence. The detailed discussion will be presented in section 5.4 and 5.5.

1200 1300 1400 1500 1600 0.0

0.2 0.4 0.6 0.8 1.0

Cr4+ :YAG spontaneous mission (a.u.)

Wavelength (nm) Bulk fluorescence measurement ASE measurement (φ=25 μm)

Fig. 5.2. The comparison of spectra between Cr⁴⁺:YAG fluorescence and ASE output.

5.2 Optical amplifier

The Cr⁴⁺:YAG DCF was used as the optical gain fiber for amplifier due to the smaller core diameter (10 μm) and lower propagation loss (0.02~0.07 dB/cm) than single crystal fiber. The small-core DCF sample is also suitable to combine with single mode fiber (SMF) since it has a good match in mode field diameter with that of SMF. This section presented the calculation of coupling efficiency between SMF and DCF. The insertion loss of SMF-DCF-SMF scheme was measured and compared with the theoretical values. The gain measurement was also shown in the last subsection.

5.2.1 Insertion loss

Reducing the insertion loss between Cr⁴⁺:YAG DCF and SMF is an important issue toward its practical utilizations in almost all applications. The refractive index profile of the Cr⁴⁺:YAG DCF was obtained by measuring the Fresnel reflection of the end face by LSCM with a 635-nm distributed feed-back laser. The 1.46 refractive index of the fused silica outer clad was used as the reference. As shown in Fig. 5.3, the measured refractive indices of the core, inner clad, and outer clad were 1.82, 1.66, and 1.46, respectively. The fiber is multimode since the refractive index difference between core and inner-clad is quite large.

0 40 80 120 160 200 240 280 320

Fig. 5.3. The refractive index profile of a 320-μm-diameter Cr⁴⁺:YAG DCF.

The mode coupling efficiency of the DCF fundamental mode to single mode fiber was simulated with superposition integral at 1064-nm pump wavelength, 1400-nm emission center wavelength, and 1550-nm communication wavelength. The simulation results are shown in Fig. 5.4, the core diameters for optimum mode coupling efficiency at 1064 nm, 1400 nm, and 1550 nm are 11.5 μm, 13.5 μm, and 14.5 μm, respectively. All the optimum mode coupling efficiencies exceed 96%.

Based on the simulation result, a 13-cm-long fiber in 13.5-μm core was chosen to measure the insertion loss from a single mode fiber to DCF and to another single mode fiber by butt coupling scheme. As shown in Fig. 5.5, the insertion loss was measured from 1260 nm to 1640 nm by using 4 sets of lasers as the light source. The measured insertion loss varies from 1.97 dB to 2.88 dB. The insertion loss includes the Fresnel losses, mode coupling loss, and propagation loss of the DCF in the scheme.

The definition of insertion loss and power propagation equation can be expressed as

(5.1)

where Pout and Pin are the input and output optical powers, respectively.

η

Air-DCF,

η

DCF-Air, and

η

Air-SMF are the Fresnel interface losses of air to DCF, DCF to air, and air to SMF, respectively.

η

mode is the mode coupling efficiency at input face from SMF to DCF. Based on Eqs. (5.1) and (5.2)s the insertion loss was estimated at various wavelengths, as shown in Fig. 5.5. It is clear conclude that the insertion loss is mainly from the Fresnel losses at the uncoated end faces.

5 10 15 20 25 30

40 50 60 70 80 90 100

1064 nm 1400 nm

Fundamental mode coupling efficiency (%)

Core diameter (μm)

1550 nm

SMF Cr⁴⁺:YAG DCF

Fig. 5.4. The simulation results of mode coupling efficiencies between the SMF28 and the Cr⁴⁺:YAG DCF using different signal wavelengths.

1250 1300 1350 1400 1450 1500 1550 1600 1650 0.0

0.5 1.0 1.5 2.0 2.5 3.0

SMF28

Insertion loss (dB)

Wavelength (nm) Experiment

Estimation

SMF28

Cr⁴⁺:YAG DCF

Fig. 5.5. The measured and simulated insertion losses. The inset shows the measurement scheme.

5.2.2 Gain measurement

The measurement configuration of optical amplifier is shown in Fig. 5.6. A Yb:fiber laser at the 1064-nm peak of Cr⁴⁺:YAG absorption spectrum was used as the pump light source. Considering the pump ESA loss discussed in section 2.3, the bi-direction pump scheme was chosen to alleviate the influence. A 1064-nm beam splitter divided the pump beam into two fibers at first. A broadband wavelength WDM coupler was used to combine the pump and signal lights into a SMF. Another pump beam was also connected with a broadband WDM coupler, then the SMF ends were connected to the two end faces of the Cr⁴⁺:YAG DCF by using butt-coupling method.

The signal light was entered from the left-side WDM coupler and went out from the right-side of the WDM coupler, and then transmitted into an optical spectral analyzer (OSA, HP 86142A) to measure the optical gain.

Fig. 5.6. Measurement configuration of amplifier gain.

In order to simplify the gain measurement, the concept of gross gain was adopted to directly obtain the optical gain in this system [5.1-5.2]. The definition of gross gain can be described as

(5.3)

The relation between net gain, gross gain, and insertion loss were expressed as

In this concept, the gross gain can be obtained from the OSA directly.

A 12-μm core-diameter Cr⁴⁺:YAG DCF with 7.8-cm fiber length was used to be the gain medium, since the core size was small and had a better mode coupling efficiency. A 1400-nm fiber laser at the peak of the Cr⁴⁺:YAG emission spectrum was used as input signal source. Shown as Fig. 5.7 (a), a gross gain of 1.9 dB was obtained in the bi-direction pump scheme. The gross gain as a function of pump power is shown in Fig. 5.7 (b). It saturated at around 1-W pump power due to the short fiber length.

(a)

(b)

Fig. 5.7. (a) The spectra of signal power densities in various pump powers in bi-direction pump scheme. (b) The gross gain as a function of pump power.

1398 1399 1400 1401 1402

-42

Fig. 5.8. Gain measurement scheme with double signal pass and bi-direction pump.

Based on the bi-direction pump scheme, the double pass method of signal was also measured to enhance the gross gain. As shown as Fig. 5.8, a tunable laser source entered from the right side SMF to be the light source of alignment due to the high refractive (HR) coating at 1400 nm. After finishing the alignment, the signal at 1400-nm wavelength from the left-side SMF was amplified, and then reflected the signal power with the HR coating and amplified again in the back-ward direction. The amplified signal power was also detected by the OSA. The measurement results were shown in the Fig. 5.9. The gross gain was improved to 3.2 dB and the pump saturation

Based on the bi-direction pump scheme, the double pass method of signal was also measured to enhance the gross gain. As shown as Fig. 5.8, a tunable laser source entered from the right side SMF to be the light source of alignment due to the high refractive (HR) coating at 1400 nm. After finishing the alignment, the signal at 1400-nm wavelength from the left-side SMF was amplified, and then reflected the signal power with the HR coating and amplified again in the back-ward direction. The amplified signal power was also detected by the OSA. The measurement results were shown in the Fig. 5.9. The gross gain was improved to 3.2 dB and the pump saturation

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