Broadband amplified spontaneous emission fibre

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Jan 10, 2005 - Amplified spontaneous emission and superfluorescence. The term 'superfluorescent fibre source' has been used to describe broadband.

journal of modern optics, 10 january 2005 vol. 52, no. 1, 109–118

Broadband amplified spontaneous emission fibre source near 2 mm using resonant in-band pumping YUEN H. TSANG, ASHRAF F. EL-SHERIF and TERENCE A. KING Laser Photonics Research Group, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK (Received 8 February 2004) Abstract. Broadband amplified spontaneous emission (ASE) at a peak wavelength of 1.85 mm from Tm:Ho-doped silica fibre sources is reported, using in-band pumping from a Yb:Er co-doped fibre laser operating at 1.61 mm. A maximum total output power from both ends of greater than 80 mW with slope efficiency >14% has been achieved. An ASE bandwidth at FWHM of near 70 nm was measured from the forward signal for the output power from 2–40 mW. The total fluorescence spectral width is >200 nm. This broadband ASE source has potential applications in optical metrology, fibre sensors, loss and dispersion tests on optical fibres, spectroscopy and medical imaging, including optical coherence tomography. A review of different pumping schemes in generating ASE around 1.9 mm in the Tm-doped fibre is given. Also the nature of three-level ASE and in-band pumping of Tm:Ho-doped silica fibre are also discussed. The results in this paper confirm that in-band pumped Tm-silica fibre is a route to achieve high power broadband output around 1.8 mm.


Introduction There are several diverse applications for broadband sources around 2 mm but such sources are not widely available at reasonable power levels. Broadband thermal sources such as tungsten halogen lamps have high divergence and low brightness. Light emitting diodes (LEDs) operating around 2 mm have relatively low power of a few milliwatts. Doped fibre sources have potential advantages of higher brightness, beam quality and intensity. In the last few years white light supercontinuum generation has been achieved by transmitting ultrashort picosecond or femtosecond pulses in photonic crystal fibres. The supercontinuum is derived from dispersive nonlinear processes of self-phase modulation, stimulated Raman scattering and four wave mixing [1, 2]. The high pump intensity is provided by pulses from a mode-locked laser which are propagated in the microstructured fibre to produce the supercontinuum typically over 400 to 1600 nm. Broadband sources have applications in, for example, optical metrology, fibre sensors, loss and dispersion tests on optical fibres, spectroscopy and medical imaging, including optical coherence tomography. In gas sensing many gases of interest have harmonic overtone or combination absorption bands in the nearinfrared (near-IR) region: for example, gaseous and liquid water, hydrogen dioxide, carbon dioxide, methane, arsine, fluorocarbon HFCs (replacement gases Journal of Modern Optics ISSN 0950–0340 print/ISSN 1362–3044 online # 2005 Taylor & Francis Ltd DOI: 10.1080/09500340410001703841

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for refrigerants) and nitrous oxide. The detection of carbon dioxide using a Tm:Ho-fibre fluorescence source has been demonstrated with high sensitivity [3]. Valuable characteristics of sources for optical coherence tomography (OCT) imaging are high average power, broad optical bandwidth, small focusable spot size, good spatial coherence and a high penetration depth in tissue. Although the tissue absorption increases in the wavelength range 1 to 3 mm, the Tm-fibre fluorescence source operating around 1.8 mm demonstrates a comparable imaging depth of penetration to a 1.3 mm superluminescent diode source [4]. This is due to the decrease in scattering with increasing wavelength, such that the net change in attenuation between 1.3 and 1.8 mm is small. In the fibre-optic gyroscope (FOG) it is necessary to have an incoherent, high beam quality, high-power and long-wavelength light source [5]. The requirements for a navigational grade FOG are a bandwidth of about 28 nm and a fibre-coupled power of about 100 mW and high wavelength stability [6], however broader bandwidths and higher powers are generally preferable. A broadband source with a peak wavelength of 1055 nm has been demonstrated with a power level of 485 mW and a bandwidth of 41 nm [7]. A low power 10 mW source with a peak wavelength near 1.5 mm produced a linewidth of 90 nm [8]. A pulsed continuum generation source with a 1.3 mm centre wavelength produced 2 nJ pulses at up to 75 MHz and an average power of 150 mW [9]. There is interest, from an application viewpoint, to extend these performances to longer wavelengths. A Tm-fibre fluorescence source has been successfully used as a test source for fibre Bragg gratings for near 2 mm in the evaluation of the fabrication of the gratings and their characterization [10]. There it was found that the fluorescence light from the fibre source was able to be readily coupled into the fibre Bragg grating by the use of a commercial fibre splicer. There are three pumping bands in a Tm-doped silica fibre useful for the excitation of fluorescence around 2 mm as shown in figure 1. One is on the 3 H6 ! 3 H5 transition, for which the absorption peak is around 1.2 mm. Since high power laser sources around 1.2 mm are not readily available, the common laser pumping sources for this band are the Yb fibre laser at about 1.09 mm [11] and the Nd:YAG laser at 1.064 mm [12] and at 1.319 mm [13]. These pumping schemes 1G


ESA 2 3F




ESA 1 3H 3

~1.09 µm

~1.6 µm ESA






~800 nm ~1.6 µm 1.8-2 µm




Tm Figure 1.

~1.47 µm ESA




480 nm



Partial energy level diagram of Ho3þ in silica, showing (a) sensitization from Tm3þ and (b) Tm3þ excitation transitions.

Broadband ASE fibre source near 2 m using resonant in-band pumping


have problems of strong excited state absorption (ESA) to the 3 F2, 3 and 1 G4 energy levels in Tm-silica or low pump power absorption. In the Tm-silica fibre laser, if laser action around 2 mm on the 3 F4 –3 H6 transition is not operating, the extent of ESA will be even stronger as the populations in the 3 F4 and 3 H4 levels increase and the pump intensity increases in the cavity. The evidence is that the transmitted 1.064 mm or 1.09 mm pump power increases proportionally with launched pump power at lower pumping rates and then gradually levels off for a Tm-doped fibre laser with no cavity mirrors (high loss cavity or non-lasing cavity) as the launched power is increased. The increase in absorption in the Tm-doped fibre is due to the increase of ESA in the fibre at higher pumping rates [11]. Such cavity design may also be used to produce amplified spontaneous emission (ASE) in a Tm fibre. Higher pumping power does not necessarily result in higher ASE output power and so far no ASE around 2 mm with a power above 1 mW when pumping this transition has been reported. A second pump band is for the transition 3 H6 ! 3 H4 , with an absorption peak around 800 nm. This is an attractive pump wavelength due to the availability of numerous solid-state sources operating in this region, such as the Ti:sapphire laser and AlGaAs diode lasers over 790–810 nm. It also has the beneficial crossrelaxation process (3 H4 ! 3 F4 , 3 H6 ! 3 F4 ), yielding two excited ions for one pump photon, and exhibits no significant ESA [5], allowing the slope efficiency to scale beyond the Stokes efficiency when operating continuous wave (CW) at 2 mm. 790 nm laser sources have been used to pump Tm-silica fibres to produce ASE broadband sources up to 7 mW power for 350 mW absorbed power and giving an ASE band of 80 nm FWHM central at 1.81 mm [4]. However, a lower Stokes limit 42% is one of the disadvantages of this 800 nm pumping scheme. A third pumping band is on the 3 H6 ! 3 F4 transition, the absorption peak of this pump band being around 1.6 mm. Compared to the two previous pumping schemes, the 1.6 mm in-band pumping offers the most efficient pump wavelength for generating fluorescence around 2 mm in Tm-doped silica fibres, since the Stokes limit of this pumping scheme is up to 86%. In-band pumping of the Tm-silica fibre laser has demonstrated a slope efficiency up to 71% [14] at low power, and even a 84% slope efficiency has been demonstrated [15] in the Tm-ZBLAN fibre laser. Previously, the forward superfluorescent emission of 2.3 mW from the 1.82 mm transition in a Tm-doped single mode fluoride fibre has been demonstrated with a low threshold of about 2 mW and a slope efficiency of about 15% [15] with respect to the launched pump power in the single pass cavity. It is expected the slope efficiency will be greater than 30% if power from both ends is counted. This performance has been extended in this paper; superfluorescent emission around 1.8 mm in a Tm-doped silica fibre using 1.61 mm in-band pumping has achieved 40 mW power for emission from one end of the fibre and a 3 dB linewidth of 70 nm.


Amplified spontaneous emission and superfluorescence The term ‘superfluorescent fibre source’ has been used to describe broadband emission from fibre lasers operating with small signal feedback, such that the device operates below the resonant threshold. In this case the output consists mainly of light resulting from the stimulated amplification of primary spontaneous photons (ASE) and the emission linewidth is narrowed by ASE. Where the line


Y. H. Tsang et al.

narrowing is significant and the power gain from ASE is also significant it is more appropriate to term this as an ASE source rather than a superfluorescent source. Care is required in the nomenclature related to this topic as there is an earlier and well-established phenomenon termed superfluorescence (SF), which occurs when a large population of atoms coherently prepared initially in a state of complete inversion undergoes relaxation by collective spontaneous decay [16–18]. For the purposes of this paper we will call this ‘true superfluorescence’. The collection of excited atoms acts as one large dipole and simultaneously emits coherent radiation within the coherence time of the medium. In order for true superfluorescence to be possible the excited ions must be in close proximity and they must be excited coherently. One of the defining characteristics of true SF is that the maximum intensity scales as the square of the atomic density [18]. Amplified spontaneous emission (ASE) occurs in a population inverted medium when spontaneous emission from a single atom is amplified through stimulated emission as it propagates. The signal sources from spontaneous emission are incoherent and there is no phase relation established among the radiating atoms [17]. There is an absence of an abrupt threshold between the spontaneous emission and ASE as the pump power increases, the output power increases nonlinearly and becomes mostly ASE [19]. The lack of an abrupt threshold for ASE is one of the features that distinguishes it from true superfluorescence. Another distinguishing feature is that for true superfluorescence, the length of the active material must be smaller than a critical cooperative length, while no such limitation exists for ASE or stimulated emission [20, 21]. In the broadband fibre source, some of the spontanous emission light is captured by the fibre core in both the forward direction (co-travelling with the pump) and the backward direction (against the pump). Below the laser threshold, but when the excitation of a rare earth-doped fibre is strong enough, energy stored in the population inversion is able to amplify the fluorescence guided along the fibre axis. This produces a relatively intense beam from both fibre ends. Such a source is temporally incoherent. The spontaneous emission is amplified through ASE in the fibre. The ASE process depends on many factors including the dopants and their concentration, pumping rates, optical gain, cavity losses and length of active media [21, 22]. The fluorescence spectrum is a result of spontaneous emission, which gives the broadest spectrum (typically FWHM > 150 nm for the 3 F4 ! 3 H6 transition) and it may also be multi-peaked. In a population-inverted medium, since spontaneous emission is in all directions, the fluorescence detected from the side of the fibre should be dominated by spontaneous emission as this emission passes through the least length of the medium. The output power of the fluorescence measured from the fibre ends of the Tm-doped fibre is usually much less than 1 mW. As the input and output powers increase, from the amplification processes mentioned above, a single fluorescence peak will be intensified and may become the only peak remaining in the spectrum [22] and the spectral width of the peak is also reduced. This is the so-called spectral narrowing effect [21, 22]. The probability of stimulated emission in an atom is proportional to the number of photons incident on the atom so that the amplification of photons at the peak of the fluorescence will be higher than for photons in the wings. Thus the bandwidth of the spectral emission reduces as the output power increases. The FWHM of a Tm-silica fibre source has been observed to decrease from 125 to 77 nm as the output power increased from

Broadband ASE fibre source near 2 m using resonant in-band pumping


about 1 mW to 1.2 mW [19], and for output powers greater than 0.7 mW, the FWHM approximately remains constant.


Experimental configuration and performance The experimental arrangement of the broadband Tm:Ho silica source is shown in figure 2, in which a Yb:Er-fibre laser operating at 1.61 mm is used to excite a Tm:Ho co-doped fibre. The Tm:Ho fibre was chosen in order to increase the emission intensity near 2 mm. The fibre used in this experiment was a singleclad Tm:Ho silica fibre with a core diameter of 8.5 mm, a numerical aperture (NA) of 0.21 and length 77 cm. The core glass was co-doped with 0.158 mol. % Tm2O3, 0.027 mol. % Ho2O3, 8.7 mol. % Al2O3, 1.4 mol. % GeO2, 3.4 mol. % P2O5 and 86.4 mol. % SiO2. The launch efficiency of the Yb:Er fibre laser output into the Tm:Ho-fibre, was found to be about 78%. The Ge filter separated out almost all of the 1.6 mm pump laser light and had a transmission of about 46% around 2 mm. The doped fibre is a high gain medium for laser oscillation, even when a fibre with a cavity made up of 4% Fresnel reflection at each end was pumped by the 1.6 mm source; consequently there is enough optical feedback to produce efficient laser action with 30% slope efficiency with respect to output from one end [23]. To suppress laser action in the cavity and enhance ASE, the fibre end facet reflectivity has to be reduced. To reduce the optical feedback a tilted glass slide was attached to the output end of the Tm fibre with an index-matching gel of n ¼ 1:466  0:0002 in between the slide and the fibre end. Then, the reflection around 2 mm depended on the light transmission in the gel and the tilted angle of the external face of the glass slide. The reflectivity from this end is then less than 4% from the Fresnel reflection. The transmission of the gel and glass slide is about 82% at 1.85 mm. The fluorescence spectral output is shown in figure 3 for a fluorescence emission power of 15 mW from one end of the fibre. The central fluorescence emission wavelength was around 1850 nm at all pumping powers. The ASE output power with respect to the launched pump power for forward emission from the distal end of the fibre is given in figure 4. The expected total output power from both ends is greater than 80 mW. The X-intercept in figure 4 is about 680 mW. The measured slope efficiency for the forward ASE was about 7.4% (and >14% when output from both ends is taken into account). For a shorter fibre length (59.5 cm) the slope efficiency for output from one end was reduced to 5.8%. In that case the reduction in the ASE source output power is due to the fibre

Coupling lenses

1.6 µm Yb:Er laser pump

Figure 2.

T m:Ho codoped fibre

Indexmatching gel

Glass slide


To monochromator

Amplified spontaneous emission source based on in-band pumping of a Tm:Ho co-doped fibre.

Y. H. Tsang et al.

114 35 Intensity (A.U)

30 25 20 15 10 5 0 1700 Figure 3.


1800 1850 1900 Wavelengths (nm)

1950 2000

Fluorescence spectrum of a Tm:Ho silica fibre at an output power of 15 mW. 45

ASE output power (mW)

40 35 30 25 20 15 10 5 0 0.6

Figure 4.


0.8 0.9 1 1.1 Launched pump power (W)



ASE output power of the Tm:Ho-silica fibre for variation of launched pump power.

length being too short to absorb all the pump power. The percentage absorption of the pump power in a fibre length of 77 cm is about 98%. So it is believed that the 77 cm fibre length is near to the optimum with respect to the forward output signals. However the shorter fibre length did not result in a broader emission bandwidth or significant change in the central wavelength, indicating that ASE remains dominant. The ASE spectrum has a maximum measured bandwidth of about 80 nm at an output power of 2.2 mW as shown in figure 5, and decreases to about 70 nm for output powers 5 mW. No emission peak was detected beyond 2 mm from the Tm:Ho fibre; this indicates that there is little fluorescence produced from Ho3þ ions in this fibre. A maximum slope efficiency of 65% has been determined for a Tm:Ho co-doped fibre when in-band pumped at 1.56 mm [23]. That is only slightly less than the previously reported 71% for a 1.57 mm pumped Tm-doped silica fibre laser [14]. The small slope efficiency reduction may be the result of energy transfer from Tm3þ to Ho3þ which reduces the population of the Tm3þ ions in the 3 F4 energy level. However, since the pump photon energy (6211 cm1, 1.61 mm), matches

ASE bandwidth (FWHM, nm)

Broadband ASE fibre source near 2 m using resonant in-band pumping 90 80 70 60 50 40 30 20 10 0

Figure 5.




15 20 25 30 35 ASE output power (mW)




Bandwidth (FWHM) of the ASE emission with output power.

the pump ESA energy separation of the 5 I7 –5 I5 (6000 cm1, 1.67 mm) transition in Ho3þ, this ESA process will increase pump photon losses and the Ho laser threshold and hence lasing on the Tm3þ 3 F4 to 3 H6 laser transition would reach threshold before the Ho3þ 5 I7 to 5 I8 laser transition [23]. It is believed that this process could also cause pump photon losses in the ASE source. A further reason for energy loss is the 8% transmission loss in this study through the tilted glass slide and index-matching gel. This can be minimized by anti-reflection (AR) coating the glass slide and using an index-matching gel with higher transmission at around 2 mm. Another technique to minimize the losses is to angle cleave the fibre end to suppress lasing. 4.

Discussion There is optimal length for the forward ASE output power in the three-level laser operation [24]. Some of these excited ions are stimulated downward by the forward signal itself. However, part of the excited ions decay spontaneously and are captured by the backward propagating mode and enhance the output of the backward signal. Of these backward travelling spontaneous photons, those which originate deeper in the negative gain region are less likely to reach the positive gain region and contribute to the backward output. Additionally, the forward signal power is larger closer to the optimum length. The largest contribution from this effect thus comes from the region just beyond the optimum length. As the fibre length is increased beyond the optimum length, decreasing contributions are expected from this effect per unit length added [24]. The backward output power grows and then saturates as the fibre length increases from the optimum length with respect to the forward signal. Also the backward ASE always experiences increasing pump power as it grows. For the above reasons, the forward ASE is always lower than the backward output power even for the optimal fibre length [5, 24]. For the optimal length, the signal gain at the distal fibre end is zero (the forward signal is saturated). Further increases in the fibre length will result in absorption of the forward signal (negative gain region). If the fibre length is shorter than optimal the forward output power drops from the maximum due to incomplete absorption of the pump. However, the additional fibre length could


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enhance the backward signal as the additional length could be excited by groundstate signal absorption in the three-level system. The slope efficiency with respect to the forward ASE could be half of the slope efficiency with respect to the backward ASE [24]. For the same reasons as for the output power, the forward signal exhibits higher threshold. In the source described here, the fluorescence from the pumped end of the fibre (backward propagating ASE) was not measured and hence serves as an energy loss; but it is expected that the total slope efficiency will be over 14%. The efficiency of the ASE emission may be improved by the use of a lower V-number fibre, to provide a single mode for the ASE wavelength range. This will ensure increased optimum overlap between the pump and ASE mode profiles and the dopant profile of the fibre, besides a small core would increase the field intensity and enhance the probability of ASE. The use of a low phonon energy host material (such as a heavy-metal oxide glass or fluoride glass) could give a longer lifetime for the upper laser level and a lower intrinsic loss [15, 19] and results in a lower threshold and better slope efficiency. The lifetime of the Tm 3 F4 emission level depends on the host material, doping technique and the chemical composition of the core glass. However, the measured 3 F4 level lifetimes in Tm-doped silica fibre range from 200 to 600 ms [25]. The efficiency of the ASE may be enhanced by using a Tm-doped ZBLAN fibre. The lifetime of the Tm 3 F4 level is relatively short in comparison with the lifetime of the Tm-doped ZBLAN glass, about 6 ms for the 3 F4 level [5] in a fluoride glass, which leads to sources with much lower thresholds as the population inversion for ASE can be achieved for even lower pumping rates. A lower operating threshold of about 2 mW and a higher operation slope efficiency of about 30%, for the ASE source, can be achieved in the Tm-doped ZBLAN fibre [15]. However, the ZBLAN fibre has the disadvantages of poor mechanical durability, lower glass transition temperature (257 C) and a melting point (450 C) which is substantially lower than for silica fibre [26]. The ZBLAN fibre can be readily melted when core pumped with a high power laser. The optimum pump wavelength of an in-band pumped Tm-silica laser operating on the 3 F4 –3 H6 transition has not been determined. It is expected that the optimum inversion occurs at the point where the difference between the absorption, a , and emission, e , cross-sections is a maximum [15]. It was found that the a =e ratio at 1.61 mm in Tm-silica is about 8.6, which is much lower than the a =e at 1.57 mm (23) as e increases substantially [27]. Consequently, stimulated emission at 1.61 mm in Tm-silica is possible to depopulate the upper level of 3 F4 and this leads to high threshold and also reduces the slope efficiency. But if a =e is high, this effect could be small. That means the performance could possibly be enhanced by using a shorter pump wavelength. The spectral properties of an ASE source are influenced by both homogeneous and inhomogeneous processes. Homogeneous mechanisms broaden the linewidth of the transitions between the Stark levels in the same manner for all ions. On the other hand inhomogeneous broadening leads to a change in the distribution of the Stark levels which differs from ion to ion depending on the ion’s physical site within the host. In the same vein, a different host environment, material or codopant, that surrounds the rare-earth ion will lead to a different distribution of the Stark levels. The fluorescence spectra of the Tm-doped fibre sources change with the composition of the host medium. For example, the Yb-doped ZBLAN fibre has a narrower emission spectrum and about 40 nm shift in its secondary

Broadband ASE fibre source near 2 m using resonant in-band pumping


emission peak in comparison with the Yb-doped silica fibre spectrum [28, 29] and the fluorescence spectrum is broader in Yb-doped germanosilicate glass [30]. The composition of the doped materials can also influence the central wavelength of their fluorescence spectra; for example, the emission peaks of Nd:Cs and Nd:Ge-doped silica superfluorescent fibre sources have a shift of 24 nm in their emission peaks [31]. The peak wavelength will shift to longer wavelengths as the fibre length is increased as a result of increasing self-absorption [19]. The fluorescence from Ho3þ ions is relatively strong with 1.064 mm pumping as ESA does not occur. When the same Tm:Ho co-doped fibre has been pumped with a Nd:YAG 1.064 mm laser it gave a spontaneous emission spectrum extending to beyond 2.3 mm [32], the spontaneous emission peaks were shifted from 2044 to 1920 nm as the fibre lengths were cut from 2.91 to 0.85 m because of reduced reabsorption. For long fibre lengths Ho3þ emission dominates and results in a fluorescence peak shifted to longer wavelength. A study of Nd3þ-doped fibre fluorescence and superfluorescence spectra as a function of the pump wavelength, the temperature and the absorbed power [21] indicated that the pumping wavelengths could also cause changes in the emission spectrum in some cases. These observations suggest that the bandwidth and shape of the fluorescence spectrum from the Tm:Ho co-doped fibre could be modified by using different pumping wavelength, fibre length, temperature, host or dopant.


Conclusions A broadband ASE fibre source has been sucessfully produced from a Tm:Ho co-doped silica fibre to give a combination of output power of >80 mW and a bandwidth of 70 nm. The Tm:Ho fibre source was excited by a Yb:Er fibre laser operating at 1.61 mm with a pump power of 1.2 W and had a slope efficiency of >14%. These results confirm that the in-band pumped Tm-doped silica fibre is a route to achieve high power broadband output around 1.8 mm. This combination of relatively high output power and bandwidth from the ASE source is achievable with efficiency and compactness and is valuable for various applications.

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