Frontiers in Guided Wave Optics and Optoelectronics Part 4 potx

The development of an optical gain medium and fiber amplifiers to cover the 1250~1650 nm region, which is the entire optical telecommunication windows of silica fiber, becomes an importa

6 Bismuth-doped Silica Fiber Amplifier

Young-Seok Seo and Yasushi Fujimoto

Korea Atomic Energy Research Institute

Korea

1 Introduction

To accommodate rapidly increasing quantities of information and communication, optical fiber transmission technology with high-speed high-capacity is demanded The development of an optical gain medium and fiber amplifiers to cover the 1250~1650 nm region, which is the entire optical telecommunication windows of silica fiber, becomes an important issue for ultra-wide broadband optical communication Bismuth-doped glasses exhibit a broadband luminescence in the near infrared region Thus, they are potential gain media for extending the spectral bandwidth of the current erbium-doped silica fiber amplifiers There are several reports on an infrared luminescence from bismuth-doped glasses such as germanate, phosphate, borate et al (Meng et al., 2005ab; Peng et al., 2005abc; Suzuki & Ohishi, 2006) According to their research, bismuth-doped glasses are therefore very promising for creating broadband amplifiers for fiber telecommunication lines and tunable or femto-second lasers

There are two useful wavelengths for optical communication One is the 1550 nm, doped fiber amplifiers (EDFAs) working wavelength, which has minimum losses EDFA developments within the third telecommunication window have contributed to the rapid growth of wavelength division multiplexing (WDM) transmission systems The L-band (1570~1605 nm) of EDFA can be used in WDM systems in conjunction with C-band (1530~1560 nm) However, efforts to use WDM techniques to exploit this capability have been hampered by nonlinear fiber effects such as four-wave mixing In addition, the amplification bandwidth of silica-based EDFA is as small as ~70 nm (Yamada et al., 1998) A broadband amplifier with a gain bandwidth of more than 70 nm was reported by the integration of EDFA with thulium-doped fiber amplifiers or fiber Raman amplifiers (FRAs) (Yamada et al., 1998)

erbium-The other useful wavelength (1300 nm) for optical communication is O-band, between 1260 and 1360 nm, which is the natural zero-dispersion region of silica glass fiber where the temporal distortion of transferred optical pulses can be minimized For example, praseodymium-doped fluoride fiber amplifiers (PDFFAs) and FRAs have been successfully used for 1300 nm amplification, but it also suffer from narrow bandwidth (~25 nm) in operating wavelength and low efficiency (Miyajima et al., 1991; Whitley, 1995) In addition,

a PDFFA normally made from fluoride glass, which is very brittle and cannot be fusion spliced to the silica glass fiber An alternative core fiber material and fiber amplifier for 1300

nm amplification is expected

High-average-power lasers are widely used for material processing, thin film fabrication with ablation, generation of extreme ultraviolet light sources for lithography, and various scientific research purposes As there are so many applications of high-power lasers, demand is growing rapidly for more powerful lasers; therefore, the requirements for thermally resistant laser media are becoming tougher Some of the absorption energy created in a laser medium by excitation with flash lamps or laser diodes is extracted as an output of laser energy, but most of the absorption energy generates heat accumulation in the medium This heat accumulation causes a thermal lensing effect and a thermal birefringence

in the medium; of course, the excess heat accumulation leads to destruction of the medium Therefore, laser media must be operated under strict conditions during excitation

Silica glass is one of the most attractive materials for high-peak-power or power lasers because it has favourable thermal and mechanical toughness, high optical transmittance from the ultraviolet to the infrared regions, and a low nonlinear refractive index compared to the other commercial laser glasses These properties are indispensable to the laser driver for inertial confinement fusion because phosphate laser glasses have such weak thermal shock toughness that they cannot work at a high repetition rate (Fujimoto et al., 1999)

high-average-Bismuth-doped silica glass (BiSG) is a new material that emits a broadband fluorescence peak at around 1250 nm with a bandwidth over 300 nm We suggested BiSG which can complement the problems of the other optical amplifiers (Fujimoto & Nakatsuka, 2001; 2003) It is important to understand the optical properties of the BiSG for technological applications such as optical fiber amplifier and fiber laser It is well known that bismuth ions contribute to increase the refractive index of the glass and give no specific absorption band

in the visible and near infrared region However, BiSG has three excitation bands at 500, 700 and 800 nm showing the luminescent bands at 750, 1120 and 1250 nm respectively Its 800

nm absorption band makes this material have a potential to be pumped by commercialized powerful laser diodes In addition, cw lasing has been obtained in the spectral region between 1150 and 1300 nm in a bismuth-doped aluminosilicate glass fiber (Dianov et al., 2005; Dvoyrin et al, 2006; 2007)

We previously reported a new infrared luminescent BiSG that had a possibility of being a high-power laser material and a possibility of being an optical fiber amplifier Also, we achieved optical amplification in a BiSG and bismuth-doped silica fiber(BiDF) at 1310 nm with 810-nm excitation BiSG has many attractive features, which make it suitable as a core fiber material of an optical fiber The near-infrared spectral regions with a wide luminescence in the range from 1000 ~ 1600 nm and a long lifetime of about 100~600 μs of luminescence make such a fiber promising for the development of lasers and amplifiers

In this chapter, we demonstrate an optical amplification at the 1260~1360 nm region band) in various BiSG and fiber The optical amplification was observed at five different wavelengths between 1260 and 1360 nm with 810 nm excitation The optical gain profile is similar to the fluorescence spectrum and the amplification bandwidth is greater than 75 nm

(O-in the 1300 nm region The laser diode pumped fiber amplifier at 1310 nm showed ga(O-in characteristics with a 5.0, 8.0-cm length BiDF and a wide-band tuned amplification through

an over 100-nm bandwidth We also report on a simultaneous amplification results for BiSG and fiber at two different wavelengths in the 1300 nm region Simultaneous amplification of

25 nm bandwidth in four different wavelength regions was obtained This technique can be useful for WDM optical amplifiers at the second telecommunications window

Bismuth-doped Silica Fiber Amplifier 107

2 Optical amplification in a BiSG

2.1 Sample preparation and experimental setup

The sample compositions (mol %) of BiSG are shown in Table 1 In the case of sample A,

bismuth-oxide (Bi2O3; 99.9 %), aluminum-oxide (Al2O3; 99.99 %) and silica powder (SiO2;

99.8 %) were mixed and kneaded in a mortar The mixture compound of Bi2O3: 1.0 mol %,

Al2O3: 7.0 mol %, and SiO2: 92 mol % was put in an alumina crucible and then melted at

1750 °C for 50 hour in the air atmosphere In the case of sample B, lithium oxide (Li2O) was

added and the mixture was put in an alumina crucible and then melted at 1700 °C for 30

hours The composition of the sample B was Bi2O3 (1.0 mol %), Al2O3 (7.0 mol %), SiO2 (92-x

mol %) and Li2O: (x = 1, 5, and 10 mol %) The samples were cut in the pieces (0.5 cm × 0.5

cm × 0.24 cm) and optically polished The color of the prepared samples was reddish brown

but transparent, with a transmittance of ~90 % at 1300 nm region

Bi2O3 Al2O3 SiO2 Li2O Sample

(mol %) (mol %) (mol %) (mol %)

A 1 7 92

1 7 91 1

B 1 7 87 5

Table 1 Glass composition of bismuth-doped silica glass and added in lithium oxide, Li2O

Fig 1 The experimental setup for the optical amplification in a bismuth-doped silica glass

The experimental setup for optical gain measurement is shown in Fig 1 The 810 nm

semiconductor laser (Unique Mode: UM2500-50-15) was used as an excitation source

Distributed feedback (DFB) semiconductor lasers (Mitsubishi Electric Corp.:

FU-436SDF-EW41Mxx) with five different wavelengths were used as probe beams The wavelengths of

the five DFB semiconductor lasers were 1272.5, 1297.6, 1307.5, 1322.8 and 1347.4 nm, and

these probe beams can be adjusted using a single laser driver The probe laser beams were

combined with the excitation beam using an optical coupler (OFR: 810/1310 nm), and then

focused into the multi-mode fiber (MMF) with an output focusing lens (focuser) The

combined beam was focused into the BiSG sample surface by the focuser and the probe beam was detected by an optical spectrum analyzer (Ando: AQ6317B) using a pigtail focuser The excitation beam is manually chopped to make the state of a probe beam

without excitation I 0 and a probe beam with excitation I The optical gain coefficient was defined as g = (1/l ) ln (I/I 0 ), where l is the length of the BiSG sample

The transmittance and the fluorescence spectra were measured using U-4100 spectrometer (Hitachi Ltd.) and SS-25 spectrometer (JASCO Corp.) with 150 W Xe lamp excitation The transmittance of the new sample in the region of 1000 to 2500 nm was 90 % at maximum, which is higher than previous result (Fujimoto & Nakatsuka, 2001) The fluorescence spectra

of BiSG samples with 800 nm excitation have a peak at ~1250 nm and the broad emission with a FWHM of ~300 nm The strong absorption at 300 nm is considered to be the absorption edge of the Bi2O3-containing glass system (Sugimoto et al, 1996) However, we could not find any luminescence derived from previously reported Bi3+ luminescence and the infrared emission was hardly observed from the sample without aluminum (Parke & Webb, 1973; Weber & Monchamp, 1973)

2.2 Gain characteristics of the bulk-type BiSG

Fig 2 Optical gain profiles of each sample at thickness 0.24cm

We have measured the gain characteristics of the BiSG samples over a useful part of the 1300

nm optical communication window The probe beam (1307.5 nm) and the excitation beam (810 nm) were focused onto the sample surface Between the two MMF pigtail focuser, we measured amplified signals in BiSG samples with the maximum launched pump power (1.0 W) at 810 nm Figure 2 shows the optical gain profile of two samples at 1307.5 nm The optical gain increases linearly with excitation power up to 1.0 W In the case of sample A, the maximum optical gain and the gain coefficient was 1.16 and 0.62 cm-1, respectively The absorbed energy was calculated as 42 % of excitation power, which is 0.42 W at 1.0 W excitation for a 0.24 cm thick sample If the excitation length such as fiber core material becomes longer, the absorption will increase then the gain will be larger than that is demonstrated in this experiment In the case of sample B, the maximum optical gain and the gain coefficient was 1.08 and 0.32 cm-1, respectively

Bismuth-doped Silica Fiber Amplifier 109 The spectral dependence of the optical gain is important if optical fiber amplifier is to be incorporated in broadband WDM system As shown in Fig 3, the measured spectral dependence of the optical gain shows a close resemblance to the fluorescence spectrum, as shown by the dotted line The optical gain was observed at five wavelengths, 1272.5, 1297.6, 1307.5, 1322.8 and 1347.4 nm The optical gain spreads widely between 1260 and 1360 nm A maximum optical gain and gain coefficient was 1.18 and 0.70 cm-1 respectively at the 1272.5

nm wavelength region, which is closed to the peak wavelength of the fluorescence spectrum Moreover, the broad fluorescence spectrum suggests that optical gain in excess of 1.09 (0.35 cm-1) should be available over a bandwidth as large as ~75nm (Seo et al., 2006a)

Fig 3 Optical gain as a function of different signal wavelength 1272.5, 1297.6, 1307.5, 1322.8 and 1347.4 nm Points and curve represents experimental measurements and fluorescence spectrum at 800 nm excitation, respectively

3 Optical amplification in a 6.5-cm-long BiSG

3.1 Sample preparation and experimental setup

The composition of this sample was Bi2O3 (1.0 mol %), Al2O3 (7.0 mol %), and SiO2 (92.0 mol

%) The BiSG can be drawn by pulling the melted material upward using an alumina bar The fabricated sample was cut in the same shape of 6.5-cm-long with an elliptical cross section (250

× 450 µm) The BiSG was optically polished at both ends to provide good optical transmission The experimental setup for optical amplification in a BiSG is shown in Fig 4 For the probe beam, a 1308 nm distributed feedback (DFB) semiconductor laser (Afonics: FXP0034, peak at

1308 nm) and four DFB semiconductor lasers (Mitsubishi Electric Corp.: EW41Mxx) with different wavelengths 1272.5, 1297.6, 1322.8, and 1347.4 nm respectively, were combined using a singlemode fiber (SMF) WDM coupler And the probe beams can be adjusted using a single driver The DFB semiconductor laser of 1308 nm was used as the anchor wavelength The probe laser beam was combined with the excitation beam in the optical coupler (OFR: 810/1310 nm) and then focused into the MMF with an output focusing lens (focuser) The combined beam was focused into the BiSG sample by the focuser, and the gained probe beam was detected by an optical spectrum analyzer (Ando: AQ6317B) using another pigtail focuser

FU-436SDF-Fig 4 Experimental setup for optical amplification in a 6.5-cm-long bismuth-doped silica glass

3.2 Gain characteristics of the rod-type BiSG

We measured the gain characteristics of the BiSG at 1300 nm optical communication window By pigtail focuser, the probe beam (1308 nm) and excitation beam (810 nm) were focused onto the sample surface The maximum optical gain was calculated to be 4.79 (6.80

dB) and therefore, gain coefficient g was 0.24 cm-1 The optical gain increases linearly with excitation power up to 360 mW The measured signal input/output power for BiSG was -8.3 dBm / -1.5 dBm (@ 1308 nm), exhibiting much lower power conversion efficiency The optical gain in the previous result with a bulk-type BiSG was 1.19 with excitation power of 2.0 W, though the sample thickness was 0.26 cm (Fujimoto & Nakatsuka, 2003) Because the optical gain coefficient of bulk sample was 0.13 cm-1 at 360 mW excitation power, the fiber shape clearly affects the gain increment due to beam mode matching between the pump and the probe If the excitation length is longer, absorption power will increase greatly, and then the gain will be larger than that demonstrated in this experiment

Between the two MMF pigtail focusers, we measured optical amplification in the BiSG with

a threshold of 360 mW launched pump power at 810 nm Figure 5 shows simultaneous amplification at two wavelengths near 1300 nm second telecommunication window The wavelength of the excitation beam was 810 nm, and the sample length was 6.5 cm The probe wavelengths to measure the simultaneous amplification in the BiSG sample were adjustable wavelengths (1272.5 nm, 1297.7 nm, 1322.8 nm and 1347.4 nm) and anchor wavelength (1308 nm) For the four cases of amplification experiments, the maximum optical gain at these wavelengths were 3.30 (5.19 dB), 3.18 (5.02 dB), 3.05 (4.84 dB) and 2.87 (4.58 dB) respectively and 3.12 (4.94 dB) at the anchor wavelength was obtained The optical gain shown in simultaneous measurements of two wavelengths suggests that it is possible

to realize WDM optical fiber amplifiers in O-band (1260~1360 nm) by using this gain material

The spectral dependence of optical gain is an important consideration if a fiber amplifier is

to be incorporated in a broadband WDM system As shown in Fig 6, the measured spectral dependence of the optical gain coefficient appears to closely resemble the fluorescence

Bismuth-doped Silica Fiber Amplifier 111

Fig 5 Simultaneously optical amplification properties at two wavelengths of 1300 nm range: (a) 1308 nm and 1272.5 nm; (b) 1308 nm and 1297.6 nm; (c) 1308 nm and 1322.8 nm; and (d) 1308 nm and 1347.4 nm

Fig 6 Optical gain coefficient as a function of different signal wavelengths: 1272.5, 1297.6, 1308.0, 1322.8 and 1347.4 nm Points and curve represent experimental measurements and fluorescence spectrum at 800 nm excitation, respectively

spectrum, as represented by a dotted line The optical gain spread widely from 1250 to 1360

nm The maximum optical gain was obtained at 1272.5 nm This wavelength is close to the peak wavelength of the fluorescence spectra Moreover, broad fluorescence spectrum suggests that optical gain coefficient exceeding 0.165 cm-1 should be available over a bandwidth as large as 75 nm (Seo et al., 2006b)

4 Optical amplification in a multi-mode BiDF

4.1 Sample preparation and experimental setup

BiDF sample was fabricated by modified rod-in-tube method The mixture compound of this sample was Bi2O3: 1.0 mol %, Al2O3: 7.0 mol %, and SiO2: 92 mol % Glass core was inserted into a tube of glass cladding to form a preform, which was drawn by heating in the drawing furnace The fiber with refractive index difference, Δn~0.017, was drawn from the perform The 8.0 cm long sample was optically polished at both ends to provide good optical transmission Its core diameter and outer diameter was 13 μm and 230 μm, respectively

Fig 7 Schematic diagram for optical gain measurement in bismuth-doped silica fiber The experimental setup for optical gain measurement in BiDF is shown in Fig 7 An 810-nm

CW laser diode was used as the pump source A 1308 nm distributed feedback (DFB) laser diode and two laser diodes with different wavelengths were used as probe beam The wavelengths of the two laser diodes were 1297 nm and 1323 nm, respectively, and these probe beams can be adjusted using a single driver The probe laser beam was combined with the excitation beam in the optical coupler (OFR 810/1310 nm), and focused on an end

of the fiber by an objective lens The amplified probe signal was observed with an optical spectrum analyzer, and an optical low path filter cut out the unused pump light

4.2 Gain characteristics of the multi-mode BiDF

Glass core was the optical amplification in an 8.0 cm long BiDF sample at single wavelength,

1308 nm, is shown in Fig 8 The maximum optical gain was calculated to be 3.76 (5.8 dB) and therefore, gain coefficient was 0.038 dB/mW The optical gain increased linearly with

Bismuth-doped Silica Fiber Amplifier 113

Fig 8 Optical gain profile of an 8.0 cm long bismuth-doped silica fiber

Fig 9 Simultaneous optical amplification properties at two wavelengths of 1300-nm range: (a) 1308 nm and 1297 nm; (b) 1308 nm and 1323 nm

excitation power up to 152 mW The measured signal input/output power for BiDF was -29 dBm / -23 dBm (@ 1308 nm), exhibiting much lower power conversion efficiency The optical gain in the previous result with a bulk-type BiSG was 1.16 with excitation power of 1.0 W, though the sample thickness was 0.24 cm Because the specific gain coefficient of the bulk sample was 0.62 cm-1/W, the fiber shape affects the gain increment due to beam mode matching between the pump and the probe

Figure 9 shows simultaneous amplification at two wavelengths near the 1300 nm region The signal wavelengths to measure the simultaneous amplification in the BiDF were adjustable wavelengths (1297 and 1323 nm) and anchor wavelength (1308 nm) For the two cases of amplification experiments, the maximum gain coefficients at the adjustable wavelengths (0.046 and 0.036 cm-1) and the anchor wavelength (0.036 and 0.034 cm-1) were obtained The optical gain shown in simultaneous measurements of two wavelengths suggests that it is possible to realize WDM optical fiber amplifiers in O-band (1260~1360 nm) Performance of fiber amplifier largely depends on the fiber specification Optical gain

in this experiment was much smaller than that for EDF or PDFFA The smaller core cross section gives promising potential to the practical gain performance The gain characteristics will be further improved by optimizing the fiber structure, such as a partially doped core structure and deformed shape of first clad layer for efficient pumping (Seo et al., 2007a)

5 Optical amplification in a single-mode BiDF

5.1 Sample preparation and experimental setup

Fig 10 Schematic diagram for the optical gain measurement of a bismuth-doped silica fiber BiDF with a refractive index difference, Δn~0.017, was drawn from the preform Its core diameter and outer diameter was 1.54 μm and 125 μm, respectively The experimental setup for the optical gain measurement in the BiDF is shown in Fig 10 An 810-nm cw laser diode was used as a pump source A 1310 nm distributed feedback (DFB) laser diode and four laser diodes with different wavelengths were used as a probe beam The wavelengths of the four DFB laser diodes were 1272, 1297, 1323 and 1347 nm, and these probe beams can be adjusted by using a single laser driver The probe laser beams were combined with an excitation beam using a WDM coupler (810/1310 nm) BiDF was fusion spliced (FSM-40PM, Fujikura) to output coupler with a SMF The combined beam goes through into the BiDF, and the gained probe beam was detected by an optical spectrum analyzer (Ando: AQ6317B)

5.2 Gain characteristics of the single-mode BiDF

The optical amplification in a 5.0-cm long BiDF sample at a single wavelength, 1310 nm, is shown in Fig 11 The maximum optical gain was calculated to be 9.6 dB and therefore, the gain coefficient was 0.442 cm-1 The optical gain increased linearly with an excitation power

up to 100 mW The measured signal input/output power for the BiDF was -30.0 dBm / -20.4 dBm (@ 1310 nm), exhibiting a much lower power conversion efficiency The optical gain in the previous result with a bulk-type BiSG was 1.16 with an excitation power of 1.0 W, though the sample thickness was 0.24 cm (Seo et al, 2005; 2006a) Because the specific gain coefficient of the bulk sample was 0.62 cm-1/W, the fiber shape clearly affects the gain increment due to a beam mode matching between the pump and the probe If the excitation

Bismuth-doped Silica Fiber Amplifier 115 length is longer, absorption power will increase greatly, and then the gain will be larger than that demonstrated in this experiment

Fig 11 Optical gain profile of the bismuth-doped silica fiber at a length of 5.0 cm

Figure 12 shows a simultaneous amplification at two wavelengths near the 1300 nm region The wavelength of the excitation beam was 810 nm, and the sample length was 5.0-cm The signal wavelengths to measure the simultaneous amplification in the BiDF were adjustable wavelengths (1297 nm and 1323 nm) and an anchor wavelength (1310 nm) For the two cases

of amplification experiments, the maximum optical gain at these wavelengths were 7.99 dB and 7.17 dB respectively and 8.69 dB at the anchor wavelength was obtained The optical gain shown from the simultaneous measurements of two wavelengths suggests that it is possible to realize WDM optical fiber amplifiers in the O-band (1260~1360 nm) One of the most fundamental parameters, the pump efficiency is defined as the net gain per unit pump power Net gain is obtained for over a 100-mW pump power The pump efficiency is 0.095 dB/mW (Seo et al., 2007b)

Fig 12 Simultaneous optical amplification properties at two wavelengths of the 1300-nm range: (a) 1310 nm and 1297 nm; (b) 1310 nm and 1323 nm

6 Conclusion

The BiSG can complement other optical fiber amplifiers by solving such problems as fusion splicing and wideband amplification Understanding the optical properties of BiSG is important due to its potential technological applications in optical fiber amplifiers and fiber lasers The spectroscopic properties of the investigated BiSG are different from those of the previously reported luminescence from Bi3+ ions ((Parke & Webb, 1973; Weber & Monchamp, 1973) The peaks of absorption and emission spectra of BiSG exist in the visible and infrared regions, respectively The absorption cross section of the 1S0→3P1 allowed transition for Bi3+ ions was estimated to be 2.4×10-17 cm2; that for bismuth ions of BiSG is 1.2×10-20 cm2 at best The Stokes shift between absorption and emission in a BiSG sample is much larger than rare earth ions Because of the large Stokes shift, population inversion can

be realized, and optical amplification occurs via the stimulated emission process

The origin of light emission in a BiSG, which is still unclear, is the valence electrons of the bismuth ions We are considering more important reasons why aluminum ions are needed

to generate BiSG luminescence Aluminum ions are needed to generate BiSG luminescence Aluminum is expected to have a special role in the formation of Bi luminescent center Therefore, discovering the aluminum status in BiSG, especially the aluminum coordination state, will help us understand the unknown luminescent center Aluminum coordination state can be investigated by using 27Al-NMR and XAFS Co-doping of Al and Bi is indispensable for the broadband infrared luminescence of BiSG The aluminum ion has to roles in BiSG: assisting the configuration of the peculiar luminescent center of Bi ion with some coupling effect, and increasing compatibility with the silica network (Fujimoto & Nakatsuka, 2006; Ohkura et al., 2007)

There are many attractive characteristics of BiSG for a laser application, such as, a long emission lifetime (over 600 μs) and a high absorption cross-section which enable an effective pumping and a short pulse generation It is important to measure the quantum yield, because the performance of a laser amplification and oscillation is very sensitive for the quantum yield The quantum yield is defined as the rate of the emitted photon number to the absorbed photon number The quantum efficiency of the BiSG was measured as 60-70 % (Fujimoto et al, 1999)

BiSG has many attractive features that make it suitable for use in an optical fiber amplifier First, because BiSG is a silica-based material, it can be transformed to an optical fiber and easily fusion connected to the silica fibers of a network system Second, the peak wavelength

is at 1250 nm, and the bandwidth is 300 nm of FWHM, which is five or six times wider than erbium-doped silica fibers Third, as the simultaneous amplifying signals near the 1300 nm region (~75 nm), BiSG fiber amplifier is considered a more effective solution for WDM broadband systems Fourth, a common commercial semiconductor laser (~808 nm) can be used for excitation of this medium As discussed above, BiSG is a promising candidate for the core fiber material of an optical amplifier at a natural zero-dispersion wavelength, 1300

nm, of silica glass fiber (Fujimoto & Nakatsuka, 2001; 2003)

In conclusion, we have demonstrated optical amplification in a bismuth-doped silica glass and BiDF at second telecommunication window The amplification was obtained at five different wavelength and the amplification bandwidth is greater than 75 nm (1272.4 ~ 1347.4 nm) in a 0.24 cm BiSG And we demonstrated simultaneous optical amplification at two wavelengths of the 1300 nm region in a BiSG Simultaneous amplification was obtained at four different wavelengths, and the amplification bandwidth was greater than 75 nm This

Bismuth-doped Silica Fiber Amplifier 117 technique can be useful for WDM optical amplifiers at 1300 nm second telecommunication windows Therefore, this gain medium is expected to be useful for applications in ultrawide broadband optical communication

Also we reported optical amplification in a BiDF The optical gain obtained in an long multi-mode BiDF at 1308 nm with 810-nm excitation and discussed simultaneous amplification at two wavelengths of the 1300 nm region And we have demonstrated on the optical amplification phenomenon in a 5.0-cm-long single-mode BiDF at 1310 nm and discussed a simultaneous amplification at two wavelengths of the 1300 nm region These spectroscopic characteristics and the amplification observed at the 1300 nm range have shown that such fibers are good candidates for cw and pulsed fiber lasers and fiber amplifiers for a spectral range of 1100~1400 nm

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7 Radio-over-Fibre Techniques and Performance

Roberto Llorente and Marta Beltrán

Universidad Politécnica de Valencia

Spain

1 Introduction

Radio-over-fibre (RoF) techniques have been subject of research during the last decades and find application in optical signal processing (photonic analogue-to-digital converters, photonic-microwave filters, arbitrary waveform generation), antenna array beamforming, millimetre-wave and THz generation systems, or photonic up- and down-converting links for applications such as broadband wireless access networks, electronic warfare and RADAR processing, imaging and spectroscopy or radio-astronomy (Seeds & Williams, 2006; Capmany & Novak, 2007) In these applications a radio signal typically in the millimetre-wave band is transmitted through optical fibre employing laser sources and electro-optical devices

The use of optical fibre links to distribute telecommunication standards is the more successful application of RoF technology, usually known as hybrid fibre-radio (HFR) networks (Jager & Stohr, 2001) HFR networks have been deployed in the last decade due to the increasing demand of high-bitrate communication services in today’s access network This demand is based on the steady market introduction of services requiring the transmission of massive data quantities, like high-definition movie distribution, on-line gaming and rich Internet experience by example (Merill Lynch, 2007)

The HFR concept applied to the enhancements of community antenna television (CATV) networks reflected in the so-called hybrid-fibre coax (HFC) network, in which a combination

of digital and analogue channels is distributed from a central location to many users distributed geographically (Darcie & Bodeep, 1990; Wilson et al., 1995) In HFC networks the last mile connection is provided through coaxial cable whilst in HFR networks the last mile connection is always a wireless link This is not a minor difference, as the wireless environment is much more hostile than cable imposing restrictive RoF link performance requirements in terms of linearity, noise and power handling capabilities, key parameters to guarantee a spurious free dynamic range (SFDR) for the whole link high enough to cope with geographical dispersion of users and complex modulation formats used by current wireless standards A simplified schematic of a HFR network is shown in Fig 1

RoF technology allows centralising the required RF signal processing functions in one shared location (Central Office, CO) and then to use optical fibre to distribute the RF signals

to the remote access units (RAU) This allows important cost savings as the RAUs can be simplified significantly, as they only need to perform optoelectronic conversion and filtering and amplification functions It is possible to use wavelength multiplexing techniques (WDM) in order to increase capacity and to implement advanced network features such as

dynamic allocation of resources This centralised and simplified RAU scheme allows lower cost system operation and maintenance, which are reflected into major system OPEX savings, especially in broadband wireless communication systems where a high density of RAUs is necessary

Core

Network

Optical Fibre Central

Office

WDM coupler

BS

Building

Wireless users

WiMAX, UWB, .

UWB WiMAX

Radio

Transmitter LD

Ext E/O

Radio Transmitter

Fig 1 Simplified schematic of a RoF system LD: Laser diode BS: Base Station RAU:

Remote Access Unit BPF: Band-pass filter Amp: Electrical amplifier

The CO and the RAU perform electro-optical (E/O) and opto-electronic (O/E) conversion of wireless signals respectively E/O conversion is achieved employing either directly modulated laser sources or external electro-optic modulators O/E conversion is done employing photodetectors or photoreceivers (Seeds & Williams, 2006) Regarding the RF transport, when the signal is transported directly at the frequency of operation there are benefits regarding cost, complexity and upgradeability, as there is no need for complex RF signal processing at the RAU involving up/down conversion or base-band mux/demux (Capmany & Novak, 2007; Jager & Stohr, 2001)

RoF techniques and complete transmission systems have been demonstrated for frequencies

up to 120 GHz (Hirata et al., 2003) As mentioned before, the most successful application of RoF technologies has been the transmission of wireless standards over optical fibre links in centralized architectures, also known as distributed antenna systems (DAS) for both indoor and outdoor applications The broad bandwidth of the optical fibre facilitates standard- independent multiservice operation for cellular systems, such as GSM (Owaga et al., 1992), UMTS (Persson et al., 2006), wireless LAN (WiFi 802.11 a/b/g/n) (Chia et al., 2003; Niiho et al., 2004; Nkansah et al., 2006) and also for emerging technologies WiMAX (Pfrommer et al., 2006) and Ultra-wideband (UWB) (Llorente et al., 2007) Available commercial systems however are typically limited to frequency ranges between 800-2500 MHz Demonstrations

of such DAS systems include their deployment to provide uniform wireless coverage in important sportive events such as the 2000 Olympic games and 2006 world cup (Rivas & Lopes, 1998; Cassini & Faccin, 2003) For indoor applications where picocell configurations

Radio-over-Fibre Techniques and Performance 121 are envisaged, advanced multi-function devices such as waveguide electro-absorption modulator (Wake et al., 1997) or polarization independent asymmetric Fabry-Perot modulators (Liu et al., 2003; Liu et al., 2007) are used as detector/modulator

Two key factors limiting the overall transmission performance in RoF systems are the optical source and the electro-optic modulation technique employed Regarding the laser source, at frequencies used for major wireless standards (GSM, WiFi 802.11 a/b/g, UMTS) and also WiMAX up to 5-6 GHz, directly modulated semiconductor lasers are preferred due

to lower cost (Qian et al., 2005) For higher frequencies, the required performances can be satisfied only by externally modulated transmitters Devices with bandwidth handling capabilities in excess of these required by near-term WIMAX deployments, in particular distributed feedback (DFB) lasers offering the required bandwidth and performances, exist commercially, but normally at a high cost taking into account the number of devices required for typical applications Recently, a lot of research efforts have been devoted to the development of low-cost/high-performance transmitters, for instance uncooled lasers (Ingham et al., 2003; Hartmann et al., 2003) or vertical-cavity surface-emitting lasers (VCSEL) (Persson et al., 2006; Chia et al., 2003) Probably, the most restrictive requirement for wireless services provision over RoF systems is the SFDR Nowadays SFDRs in excess of

100 dB·Hz2/3 have been demonstrated experimentally, providing enough dynamic range to

be employed in real applications (Seeds & Williams, 2006)

2 Ultra-wideband radio-over-fibre

2.1 Optical generation

The basic elements of RoF systems are broadband laser sources either employing direct or external modulation, a suitable transmission media such as multi-mode fibre (MMF), single-mode fibre (SMF) or plastic optical fibre (POF), and broadband photodetectors or photoreceivers (Seeds & Williams, 2006; Capmany & Novak, 2007; Dagli, 1999) The laser source and modulation method is the key element in the performance of RoF applications The generation of the optical signal to be transmitted in the RoF system is of special difficulty in the case of UWB signals UWB is a radio technology intended for cable replacement in home applications within a range of tens of meters (picocell range), with high-definition video and audio communications a potential application (Duan et al., 2006) UWB is also attractive in many other applications including medicine, sensor networks, etc UWB radio offers: High data rate capability (>1 Gbit/s), low radiated power spectral density (PSD) minimising the interference, low-cost equipment commercially available UWB is available in two main implementations: Multi-band orthogonal frequency-division multiplexing (MB-OFDM) and impulse radio The ECMA standard (ECMA-368, 2007) uses MB-OFDM in 528 MHz individual sub-bands, whilst the impulse-radio implementation employs short pulses (in the range of hundreds of picoseconds) modulated in amplitude, time, polarity or shape to fill a desired bandwidth MB-OFDM generally shows superior performance to the impulse-radio approach in terms of multi-path fading and intersymbol interference (ISI) tolerance, whilst impulse-radio is able to provide simultaneously communications, localization and ranging to a sub-centimetre resolution

Currently, UWB uses the unlicensed band from 3.1 to 10.6 GHz mainly for indoor communications (FCC 04-285, 2004; ECMA-368, 2007) and the 24 GHz band for vehicular short-range radar applications (SARA Group, 2009), with a bandwidth larger than 20% of the centre frequency or a 10-dB bandwidth of at least 500 MHz as in FCC regulation (FCC

04-285, 2004) or at least 50 MHz as in ETSI regulations (ETSI, 2008) and a maximum radiated PSD of -41.3 dBm/MHz to guarantee spectral coexistence with other wireless narrowband services complementary in terms of range and bitrate such as WiMAX Nevertheless, the whole UWB band 3.1-10.6 GHz is not available worldwide due to coexistence concerns (WiMedia, 2009) Outside the United States, available effective bandwidth is 1.5 GHz which only supports hundreds of Mbit/s However, the unlicensed 60 GHz band enables UWB multi-Gbit/s wireless communications worldwide, as shown in Fig 2, while challenges related to wireless channel and transceiver design have to be addressed (Daniels & Heath, 2007)

AustraliaCanadaUSA

575859

60

616263

646566

59.4

62.9

(*)EuropeJapan

Fig 2 International frequency allocations in the 60 GHz band (as of January 2009) (*) (ECC, 2009)

RoF distribution of UWB signals, termed UWB-over-fibre, has received great interest to extend the UWB range exploiting the advantages of the broad bandwidth, low loss, light weight, and immunity to electromagnetic interference offered by optical fibres

In this section, different techniques for generating impulse-radio UWB signals in the optical domain is reported, featuring frequencies ranging from baseband up to millimetre-wave bands, including 24 GHz and 60 GHz Some laser source characteristics are also discussed

2.1.1 Impulse-radio ultra-wideband baseband

For UWB-over-fibre systems, it is desirable to generate UWB signals directly in the optical domain, avoiding the use of additional E/O and O/E conversions and exploiting the advantages provided by optics such as broadband processing, light weight, small size, and immunity to electromagnetic interference Many techniques have been proposed to generate impulse-radio UWB signals in the 3.1-10.6 GHz band in the optical domain These techniques have mainly focused on generating Gaussian monocycle and doublet pulse shapes, which have been demonstrated to provide better bit error rate (BER) and multipath performance among different pulse types (Chen & Kiaei, 2002)

RoF distribution of UWB signals in the band from 3.1 to 10.6 GHz for high-definition audio/video broadcasting in optical access networks, e.g in fibre-to-the-home (FTTH) networks has been proposed (Llorente et al., 2008) The performance of both MB-OFDM and impulse-radio UWB implementations at 1.25 Gbit/s is experimentally analysed and compared for different SMF links, ranging from 25 km up to 60 km Both UWB implementations exhibit error-free operation (BER< 10-9) up to 50 km without dispersion compensation The impulse radio technology exhibits degraded performance compared

Radio-over-Fibre Techniques and Performance 123 with OFDM although other optimized impulse-radio generation and detection schemes could lead to different results OFDM-UWB degrades quickly with fibre length, due to the carrier suppression effect (Schmuck, 1995)

Several demonstrations on optical generation of impulse-radio UWB in the band from 3.1 to 10.6 GHz including fibre and/or wireless transmission have been reported achieving 65 cm wireless distance at 500 Mbps data employing on-off keying (OOK) amplitude modulation (Abtahi et al., 2008); 20 cm at 1.025 Gbit/s OOK-modulated data after up to 10 km of dispersion-compensated SMF (Hanawa et al., 2009); 5 cm at 1.6875 Gbit/s OOK-modulated data after 24 km of SMF (Pan & Yao, 2009a); at 1.625 Gbit/s data employing pulse-position modulation (PPM) after up to 200 m of SMF (Shams et al., 2009a), or 37 km of SMF with no wireless transmission (Shams et al., 2009b); and at 781.25 Mbit/s data employing binary phase-shift keying (BPSK) modulation after 30 km of SMF (Yu et al., 2009) In addition, techniques have been reported capable of pulse shape modulation (PSM) (Dong et al., 2009),

or reconfigurable for multiple modulation formats (Pan & Yao, 2009b)

Photonic generation of Gaussian monocycle pulses based on balanced photodetection of data Gaussian pulses has been proposed (Hanawa et al., 2007; Beltrán et al., 2008) Data Gaussian pulses are first generated by intensity modulation of an electrical data sequence with optical Gaussian pulses from a pulsed laser This technique is shown in Fig 3 Optical

BPD PD PD

Intensity Modulator

Fig 3 Gaussian monocycle pulse generation based on balanced photodetection ODL: Optical delay line BPD: Balanced photodetector PD: Photodetector

Frequency (GHz)Fig 4 Monocycle pulses generated employing the technique in Fig 3; (a) the temporal waveform and (b) its spectrum (resolution bandwidth: 30 kHz)

data pulses are split into two equal parts to drive the two inputs of the balanced photodetector Optical delay is employed to adjust the relative time delay between the two signals The pulse width of Gaussian pulses and the time-delay difference are adjusted so as

to generate the desired UWB bandwidth This approach has been experimentally demonstrated employing an actively mode-locked fibre laser and a Mach-Zehnder modulator (MZM) (Beltrán et al., 2008) To control the pulse width, a spool of standard SMF

is included after the MZM Fig 4 shows monocycles generated based on balanced photodetection exhibiting a UWB 10-dB bandwidth of 6 GHz at 1.25 Gbit/s

Gaussian monocycle pulses can also be generated based on differential photoreception of data Gaussian pulses (Beltrán et al., 2009b) targeting to reduce cost and complexity Fig 5 shows this technique Again, data Gaussian pulses are first generated by intensity modulation of an electrical data sequence with optical Gaussian pulses from a pulsed laser Optical data pulses are photodetected and amplified by an electrical amplifier providing complementary outputs The two outputs are combined after adjusting their relative time delay to generate monocycles The pulse width of Gaussian pulses and the time-delay difference are adjusted so as to generate the desired UWB bandwidth This approach has been experimentally demonstrated employing an actively mode-locked fibre laser and a

Pulsed

Laser

Monocycles Intensity

Modulator

Differential Photoreceiver

TIA PD

EDL

Fig 5 Gaussian monocycle pulse generation based on differential photoreceiver PD:

Photodetector TIA: Transimpedance amplifier EDL: Electrical Delay Line

Frequency (GHz)

Fig 6 Monocycle pulses generated employing the technique in Fig 5; (a) the temporal waveform and (b) its spectrum (resolution bandwidth: 300 kHz)