Photo darkening in ytterbium co-doped silica material Kent Erik Mattsson*a, Stig Nissen Knudsena, Benoit Cadierb, Thierry Robinb a Crystal Fibre A/S, Blokken 84, DK-3460 Birkerød, Denmark; b iX Fiber, Rue Paul Sabatier, 22300 Lannion, France ABSTRACT A model description of photo darkening based on 30 characterised fibres in an un-seeded amplifier setup is presented. Photo darkening of ytterbium / aluminium and/or phosphorous co-doped silica fibres is found to saturate following prolonged exposure to pump radiation. The photo darkening is associated with non-binding oxygen at surfaces of ytterbium / aluminium clusters. The dominant colour centre at near infrared wavelengths in MCVD material is a combination of 1.9 eV (FWHM of 0.62 eV) and 2.4 eV (FWHM of 0.85 eV) absorption dependent on average phonon energy of the glass material. Keywords: Photo darkening, double cladding fibre amplifiers, ytterbium co-doped silica material
1. INTRODUCTION Ytterbium doped double-clad fibre sources have become increasingly popular because of their high output beam quality and high output powers. The output powers of Yb-doped double-clad fibres have steadily increased over the past few years, rising to the kW level.1 This brings the fibre lasers and amplifiers in a position to become a commercial alternative to conventional types of high power lasers. High power devices are often realized with large-mode-area (LMA) fibres (i.e. large core diameter and low numerical aperture). LMA fibres are of interest because the larger core area decreases the intensity at a given power level which increases the threshold for undesirable nonlinear processes and for optical damage. In addition, the length of the doped fibre may be considerably shortened by increasing the core/cladding ratio and/or increasing the ytterbium (Yb) concentration, which results in higher pump absorption. Increased population inversion due to increased core/cladding ratio or increased ytterbium concentration results in increased photo darkening (PD) which potentially limits both the efficiency and the lifetime of Yb-doped fibre devices. PD is observed as formation of broadband absorption at visible and near infrared wavelengths over time. Reduction of this damage mechanism to a level where it no longer plays any role is needed to realise the commercial potential of pulsed and continuous wave ytterbium doped double-clad fibre sources.2 To achieve this goal a better understanding of the PD phenomenon is needed. It has been shown that the PD rate is correlated to the number density of ytterbium ions in the excited state and that the initial PD rate follows a power law which is approximately proportional to [Yb*]7, where [Yb*] is the number density of Yb ions in the excited state.3 Further, preliminary measurements indicate that the PD saturates following prolonged exposure to pump and signal radiation.3,4 In the present model study, a given fibre is shown to saturate at an excess absorption level which is dependent on the population inversion applied. In this work a model description of PD based on the characterisation of 30 fibres is given. This model describes both the initial PD rate and steady state saturation for PD of ytterbium co-doped silica material. It is found that the actual saturation levels depend upon inversion level. This again depends on the number of skew rays that are to be found especially in the double cladding fibres with a solid glass inner cladding. It is further found that the actual saturation levels depend upon the composition of the silica glass material. It is also speculated that the production method influences the actual spectral shape of the induced PD as well as the temporal behaviour. The model for photo darkened material is applied in combination with a set of traditional rate equations for amplifier / laser operation such that the influence of the dynamic loss introduced to the active material is handled directly during operation of the active fibre. This study covers CW operation of un-seeded amplifiers applied in accelerated PD tests pumped at 915 nm. *Send correspondence to
[email protected];
Fiber Lasers V: Technology, Systems, and Applications, edited by Jes Broeng, Clifford Headley, Proc. of SPIE Vol. 6873, 68731C, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.763117
Proc. of SPIE Vol. 6873 68731C-1 2008 SPIE Digital Library -- Subscriber Archive Copy
2. FIBRE DESIGN AND EXPERIMENTAL SETUP In this section the double clad fibre designs are described along with the un-seeded amplifier setup used for characterisation of PD. 2.1 Fibre design In this work a total number of 30 different double cladding fibres have been characterised. All fibres are produced by the stack and draw process. The fibres have either a micro-structured inner cladding (established by 120 – 210 tubes) or a solid inner cladding. The outer cladding in all fibres is established by 98 – 110 tubes surrounding the circular inner cladding. The core material in all fibres is fabricated by the modified chemical vapour deposition and solution doping technique. The bulk part of the fibres are constructed to have a single-mode core at the lasing wavelengths of ytterbium (1000 nm – 1100 nm), and to have a multi-mode inner cladding at the pump wavelength (915 nm) with a numeric aperture exceeding 0.58. All fibres with micro structured inner cladding have a depressed-index core with a refractive index lower than the silica glass used in the cladding whereas the solid inner cladding fibres have cores with a refractive index above the silica glass used in the cladding. A cross section of one of the double clad Yb-doped micro structured fibres used in this work is shown in figure 1.
OSA MO Yb fibre
Halogen lamp Taper fibre
7:1 combiner
Shutter
915nm pumps
Fig. 1. Cross section of a double –clad Yb doped fibre with micro-structured inner cladding
Fig. 2. Experimental set-up for measurement of PD
2.2 Experimental setup The set-up for characterisation of the fibre samples is shown in figure 2. The aim of the set-up is to provide information on the spectral shape of the PD induced colour centres at the visible and near infrared wavelengths and their temporal evolution. A 100W low voltage halogen lamp is used as probe light. The collimated light from the halogen lamp is coupled by a microscope objective into the large core, low NA end of a 4 m long taper fibre. The 400 µm core fibre end is tapered down to 150 µm, supporting a NA up to 0.65 at 950 nm, and this end is butt-joint coupled to the Yb doped fibre under test. Pump power is provided by three 915 nm multimode laser diode modules with 105/125 µm, 0.22 NA pigtails spliced onto a standard 7:1 multimode power combiner with a 125 µm, 0.45 NA common port. The common port is butt-joint coupled to the Yb doped fibre under test. In the present implementation of the set-up available pump power was limited to 11 W. Probe light is collected from one of the input ports of the power combiner connected to an optical spectrum analyzer (OSA). The OSA is capable of measuring from 350 nm to 1700 nm. The test fibre lengths in this study were typically 40-50 cm. To prevent lasing at high inversion levels, the test fibres were angle-cleaved at both ends. The measurement is started with the pumps turned off and performing a broadband reference scan with the OSA. Hereafter, the probe light is blocked by a shutter and the pumps are turned on. After some
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time, the pumps are turned off, the shutter is opened and a broadband OSA scan is performed. This is repeated 7 times during the first hour of the measurement and hereafter for every 30-60 min for a total test duration of at least 48 hours. The OSA is also used to measure the fluorescence spectrum from the test fibre while being pumped. With respect to transmitted probe light, the stability of the set-up is typically better than 0.2-0.3 dB over 48 hours.
3. THEORETICAL MODEL The model associates PD with non-binding oxygen at surfaces of ytterbium / aluminium clusters that forms in silica material co-doped with ytterbium and aluminium and/or phosphor. The non-binding oxygen originates from Yb3+ in substitute Si4+ sites. The model speculates that ytterbium and aluminium binds to each other through oxygen in octahedral complexes (clusters) and that non-binding oxygen of the surrounding tetrahedral network surrounds these octahedral complexes. The neighbour non-binding oxygen atoms are forced into positions much closer to each other than would be the case if they could find a network forming atom (such as silicon, ytterbium aluminium, phosphorous, etc.) to be covalently bond to. The oxygen to oxygen distance is speculated to be ≈160 pm in stead of the usual ≈250 pm oxygen to oxygen distance known from pure silicon dioxide. If this holds true the individual oxygen atoms will form vibrational states for the non-binding electrons that resemble the vibrational and rotational energy states of molecular oxygen. Subjecting the non-binding oxygen molecule to pump radiation will with a certain (low) probability (a) cause the “molecule” to absorb a pump photon and excite the triplet non-binding electron to an excited triplet non-binding electron state (“Rydberg state”). This is shown in figure 3 “a”. Subjecting the ytterbium to pump radiation, excess energy is radiated as phonons which with a certain probability (b) causes the excited triplet lone electron of the non-binding oxygen atom to shift into a singlet vibrational state (through change of momentum). This is shown in figure 3 “b”. The singlet state lone electron is with a certain probability (c) excited by an additional pump photon and transferred to a nearest neighbour non-binding oxygen atom as shown in figure 3 “c”. This creates a hole and a lone-electron pair and results in a coulomb field between the two oxygen atoms. An un-stable colour centre is hereby created which resembles the AX centres known from aluminium doped quartz material.5 These infrared radiation dominant colour centres hold average absorption at 2.2 eV – 2.4 eV for the A2 centre and 1.8 – 2.0 eV for the A1 centre with 0.82 – 0.87 eV and 0.6 – 0.7 eV in FWHM, respectively. C"
hv
/0
*
hvphoflofl/
/0
0
*
I
/0
Triplet state excitation
0.
hv
/0
Singlet state transition
Triplet to singlet state
_/ç•
/ o.
Unstable Colour center
Semi-stable Colour center
/0
/ /0 .
/0
Fig. 3. Colour centre creation by successive excitation of oxygen non-binding electrons
The unstable colour centre can, by the action of a phonon, be converted into a semi-stable centre. This requires that one electron of the lone electron pair is shifted to a nearest neighbour site. Hereby a semi-permanent coulomb field is established between the hole and the new lone electron pair. The change of position of the lone electron is most likely caused by a phonon produced when an excited colour centre relaxes stored energy into lattice vibrations – e.g. phonons. In summary, to activate the dominant colour centre absorption a lone electron has to be shifted to a neighbour lone electron site. This requires two pump photons in combination with a phonon. The action of further phonons can lead to
Proc. of SPIE Vol. 6873 68731C-3
bleaching of the colour centre as it may recombine the lone electron and hole. There is equal probability that the phonon shifts the lone electron further away from the hole hereby increasing the stability of the colour centre. The process of PD is in the model handled by a Markov chain statistical process where the chain length is a direct function of cluster size. The average cluster size is expected to increase with increasing ytterbium / aluminium concentration. This leads to a steady state distribution of lone electrons and steady state concentration of colour centres becomes a function of phonon production by ytterbium (population inversion) as well as material composition (average cluster size and composition). The colour centre creation rate (PD rate) is proportional to the triplet excitation (a) times ytterbium phonon transition from triplet to singlet state (b) times singlet to singlet transition (c). The creation rate is given by equation (1):
dN D (t ) dt
1 2
=
abc
τ
( N Sites − N D (t ))
(1)
Here ND(t) is the number of photo darkened sites at time t, and NSites is the total available number of non-binding oxygen sites. The lifetime τ is the average time that the electron exists in the singlet state. The bleaching of colour centres is performed by phonons produced by either ytterbium or the colour centre. In the following it is assumed that the highest phonon production is from the colour centres – with a probability rate (d).
dN B (t ) dt
=
1 2
d
N D (t ) (1 − PStable )
(2)
Here (1 – PStable) is the fraction of the colour centres that are unstable – i.e. the fraction of colour centres where the lone electron pair and hole is next to each other. The fraction of unstable or bleachable centres depends on the size of the cluster – i.e. the number of non-binding sites surrounding the cluster. The probability can be determined by the aid of a Markov chain process. This process is based on a finite number of states and known probabilities of moving between these. Under the assumption that change in state only occur at the action of one pump photon being captured by the colour centre, the stochastic transition matrix for a n site chain is given by the (n+1) x (n+1) matrix M. The steady state solution to the Markov chain is given by the steady state vector p. This steady state vector holds the property: M p = p Ù (M – I) p = 0, with I as a unity matrix. This leads with the preceding considerations to equation:
0 ... . ⎤ ⎡− ½ p ½ d 0 ⎢ ½ p − d ½d 0 ... 0 ⎥⎥ ⎢ ⎢ 0 ½d − d ½d ... 0 ⎥ (M − I ) p = ⎢ ⎥p= 0 0 ½d − d ... 0 ⎥ ⎢ 0 ⎢ . . 0 ½d ... ½d ⎥ ⎥ ⎢ . . 0 ... − ½d ⎥⎦ ⎢⎣ 0
(3)
In the transition matrix it is assumed that the probability of two pump photons and one phonon excitement of the electron is given by p = abc and the subsequent bleaching or strengthening by the phonon relaxation of the excited electron within the nonbinding oxygen complex with a probability d. The length of the Markov chain – i.e. size of the matrix is n. The solution to equation (3) is the steady state vector p which can be found as:
p =(
d2 , 2p
1 d, 2
1 d, 2
1 d, 2
1 d , ..., 2
1 d) 2
(4)
The number of states with at least one lone electron site between the lone electron pair and hole is hereby given in equation (5) as:
PStable
1 (n − 1) d 2 =1 − = 1 n d 2
Proc. of SPIE Vol. 6873 68731C-4
1 n
(5)
The system of the two equations (1) and (2) holds no analytical solution and is generally known as the predator-pray problem. It is, however, given that “steady state” is reached once the creation rate and bleaching rate becomes equal. This leads to the steady state photo darkened population given by equation (6):
N D (t SS ) =
1 dτ nabc
1 +
N Sites
nabc N Sites dτ
≈
(6)
Equation (6) indicates that the number of photo darkened sites at steady state is proportional to the product of the creation probability times the average size of the clusters and inverse proportional to the product of the beaching probability rate times the singlet excited state lifetime. The total number of available sites NSites is given by the number of clusters with two or more atoms which includes at least one ytterbium atom. This indicates that glass material with high concentration of cluster building atoms such as ytterbium and aluminium is anticipated to show higher number of photo darkened states than a material with a lower concentration. An estimate for the steady state time can be found by assuming that the number of photo darkened sites is small compared with the number of sites available. This leads by integration of equation (1) and insertion of the steady state population of equation (6) to the steady state time of equation (7):
t SS
=
2 abc
τ
+
(7)
d n
Equation (7) indicates that the steady state time is inversely proportional to the sum of the darkening rate and bleaching rate. I.e. a fibre that shows a low initial photo darkening rate will exhibit a prolonged period of photo darkening whereas a fibre that shows a high initial photo darkening rate will quickly find into steady state. The model above describes the situation with only one excited ytterbium atom inside a cluster giving raise to colour centre creation (PD). There will, however, be a number of clusters which contains more than one ytterbium atom and for these the average phonon production is expected to be higher as well as there is a possibility of co-operative upconversion photon emission. The higher phonon production can be handled through a small modification of the Markov chain matrix where the influence of additional ytterbium phonon production is taken into account. The steady state vector for this matrix becomes:
p =(
(b + d ) 2 , 2p
1 (b + d ) 2 , 2 d
1 (b + d ), 2
1 d, 2
1 d , ..., 2
1 d) 2
(8)
The number of states with at least one lone electron site between the lone electron pair and hole is for a cluster with more than one ytterbium atom hereby given in equation (9) as:
PStable +
=
b + (n − 1)d
(9)
2
b 2 + 5b + (n + 1)d d
Steady state is again reached when the darkening rate and bleaching rate equals each other. The steady state concentration of darkened sites then becomes:
N D (t SS ) =
1 +
1 d (1 − PStable + )τ abc
N Sites
Proc. of SPIE Vol. 6873 68731C-5
≈
abc (1 − PStable + ) N Sites dτ
(10)
The steady state time can again be found by assuming that the number of photo darkened sites is small compared with the number of sites available. This leads to the steady state time for clusters with more than one ytterbium atom:
t SS
=
2 abc
τ
(11)
+ d (1 − PStable + )
The double exponential (stretched) behaviour of photo darkening can hereby be explained as the photo darkening being performed by clusters with more than one ytterbium atom inside for the “fast” darkening rate and clusters with only one ytterbium being responsible for the “slow” darkening rate. The “fast population” saturates at the fraction of the total number of sites available given by equation (10), this is performed in the saturation time given by equation (11). Likewise the “slow population” follows equation (6) and equation (7). This aspect of the model can explain the observation that increased aluminium to ytterbium concentration tends to decrease the photo darkening rate. The fraction of clusters with more than one ytterbium is decreasing when increasing the total amount of aluminium relative to ytterbium, hereby decreasing the “fast population” fraction relative to the “slow population” fraction. Apart from the mechanism described above, for clusters with more than one ytterbium atom there is the possibility of cooperative up-conversion emission transferring the excited singlet lone electron to a silicon electron trap (E2- centre). The co-operative up-conversion can take place when two ytterbium atoms are connected to each other through an oxygen atom and both are excited by pump photons. In a co-operative emission process the two individual electrons of the two ytterbium atoms will recombine in one high energy photon in the 2.4 eV - 2.53 eV range (515 nm – 488 nm) which is to be observed as the green or bluish fluorescence from the fibre. The energy of the high energy photon is sufficient to promote a transition from the excited singlet lone electron (2.7 – 2.8 eV) to the silicon electron trap E2-. The E2- trap also known as Si-O vacancy with an electron trapped on the defect silicon from which the oxygen is missing. The E2- trap shows absorption near 5.3 eV – 5.5 eV (225 nm - 230 nm). The mathematics involved for the co-operative fluorescence induced photo darkening resembles the description above when considering clusters with only one ytterbium atom – however now replaced with the number of clusters including two or more ytterbium atoms. In the following it is assumed that only the first type of electron traps are at work – i.e. the calculations disregards the possibility of colour centre creation by the action of co-operative fluorescence. The numeric model divides the fibre into a large number of small volumes where the pump absorption is assumed to be uniform. The probabilities a, b, c and d can then be found from their respective rate equations. These rate equations resemble the well known rate equation for ytterbium which in the present case is directly proportional with b.6,7 The ytterbium absorption is assumed to be dominant for the dynamic behaviour of the active glass material. The absorption cross section for the triplet excitation rate a is assumed to be in the 10-26 cm2 – 10-29 cm2 range, for ytterbium it is in the 10-20 cm2 – 10-21 cm2 range, the singlet to singlet transition in the 10-16 cm2 – 10-18 cm2 range and finally for the colour centre in the 10-21 cm2 – 10-22 cm2 range. The triplet state lifetime is in the µs range, the ytterbium excited state lifetime is in the 0.6 – 0.8 ms range and the singlet lifetime is in the ns range.
A2
A3a
Fig. 4. Energy states of the color centre
The spectral response of PD is handled as a combination of four transitions in the ionized oxygen molecule. These four transitions are shown idealized in figure 4. Both ground level and excited state level are split in several states where the population is dependent on the actual phonon density of the material. The A1 centre is assumed to hold a mean energy of 1.9 eV (653 nm) with FWHM of 0.62 eV and the A2 centre holds a mean energy of 2.4 eV (517 nm) with an assumed
Proc. of SPIE Vol. 6873 68731C-6
FWHM of 0.85 eV. It is throughout the study assumed that the centres are invariant and only the relative composition change with the local environment of the cluster. The A3X centres are only of importance at wavelengths below 800 nm but influence the analysis of the measured spectral data.
4. EXPERIMENTAL RESULTS, MODELING AND DISCUSSION
0.45
7.8 W 5.3 W 3.2 W
0.40 0.35
Loss in pump core @ 605 nm (dB)
Loss in pump core @ 605 nm (dB)
The measured photo darkening of an ytterbium and aluminium co-doped fibre is shown in figure 5 for three different pump power levels which corresponds to 15 %, 13 % and 7 % population inversion respectively. The measured loss is for light coupled through the outer cladding of the double clad fibre. The photo darkening absorption can be observed to increase as a stretched exponential function.
Model Model Model
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
0.00
4.5 W
Model
-0.05 -0.10 -0.15 -0.20
0
10
20
Time (Hours)
30
40
50
60
Time (Hours)
Fig. 5 Photo darkening of ytterbium / aluminium core
Fig. 6. Photo darkening of ytterbium /phosphorous core
10
3
10
2
10
1
10
0
Core absorption (dB/m)
Core absorption (dB/m)
In figure 6 the measured photo darkening of an ytterbium and phosphorous co-doped fibre is shown for 10 % population inversion level. It is here to be observed that colour centres bleach with a resulting decrease in the pump core absorption. This can be handled by the model assuming that a part of the lone electrons are to be found as lone electron pairs initiated at the production stage of the glass material or similar if part of the lone electrons occupy E2- trap sites near silicon. It appears that phosphorous co-doped ytterbium glass material saturates after approximately 1 hour at low population inversion levels. In figure 7 and 8 the calculated core absorption as function of population inversion is shown after 1, 10, 100 and 1000 hours for the ytterbium / aluminium and ytterbium / phosphorous doped fibres, respectively.
1000 hours 100 hours 10 hours 1 hour 909
0
20
40
60
80
10
3
10
2
10
1
10
0
1000 hours 100 hours 10 hours 1 hour 750
0
40
60
80
Population inversion (%)
Population inversion (%) Fig. 7 Photo darkening of ytterbium / aluminium core
20
Fig. 8. Photo darkening of ytterbium /phosphorous core
Proc. of SPIE Vol. 6873 68731C-7
The initial PD, corresponding to the 1 hour curve, as function of population inversion level for the un-seeded amplifiers pumped at 915 nm of figure 7 follows a curve that covers 9 orders of magnitude. This curve resembles the findings of previous reports with regard to PD rate.3 A 7th order dependence on population inversion based on initial PD rate is, however, misleading as it conceal that a steady state absorption level eventually is reached for all population inversion levels as is demonstrated in figure 7 and figure 8. It is to be observed that PD of aluminium co-doped material saturates first for high population inversion levels whereas pre-PD of ytterbium / phosphorous material responds with a more complicated behaviour. Here the core absorption decreases with time for low population inversion levels and increases much less pronounced at high population inversion levels compared to the ytterbium and aluminium co-doped material. The saturated PD absorption at 605 nm for photo darkening of un-seeded amplifiers pumped at 915 nm divided by the 915 nm pump absorption in the pump cladding of the double cladding fibre is in figure 9 shown as function of population inversion level for all characterized fibres. The data from these fibres are analyzed in a manner where the individual parameters from each fibre is applied on a standardized fibre 16 µm core and 200 µm cladding diameter surrounded by an air-clad. The 200 µm cladding is micro structured to obtain highest possible pump scrambling. The “saturated” PD loss is assumed to be reached after 1000 hours of real time operation at the given population inversion level. The “standard fibre” is 50 cm long and assumed to operate at 22°C. It is from figure 9 to be observed that nearly all fibres follow the same slope after 1000 hours operation. The few exceptions are due to pre-photo darkened states that have not found equilibrium after 1000 hours. It is further to be observed that the PD loss slope appears to increase at high inversion levels. This behaviour is due to an excess production of phonons in the highly PD glass material. It is to be observed that the saturation level is a function of the population inversion level applied in the given application and that the co-doping influences the saturation level. The saturation level for un-seeded amplifiers is found to approximately follow a 2nd order dependence on the inversion level. The actual power law dependency is however a function of cluster size and the average number of ytterbium ions present within the individual clusters.
PD Loss / Pump abs
In figure 9 ytterbium and aluminium co-doped fibres are shown as solid lines, ytterbium and phosphorous co-doped fibres are shown as dotted lines and ytterbium, aluminium and phosphorous co-doped fibres are shown as dashed lines. It is to be observed that the phosphorous co-doped fibres saturate at a loss per pump absorption level that is about 10 times higher than ytterbium and aluminium co-doped fibres.
10
3
10
2
10
1
1000 hour 50 cm fiber 16 µm core 200 µm clad
PD loss @ 605 nm
Pump absorption @ 915 nm
10
100
Population inversion (%) Fig. 9. Saturated PD (605 nm) as function of population inversion level in 50 cm un-seeded fibre amplifier
Proc. of SPIE Vol. 6873 68731C-8
The ytterbium, aluminium and phosphorous co-doped fibres are found to be more critical in their process response as is also the case for phosphorous co-doped fibres. The presence of aluminium increases the pump absorption per ytterbium atom and hereby improves the PD loss per pump absorption. The core absorption at 1070 nm per pump absorption is shown in figure 10. The overall picture with a phosphorous codoping giving raise to 10 times higher saturated PD loss compared with aluminium co-doping is also found for the operational wavelength range. This suggests that phosphorous co-doped fibre material performs with a slope efficiency which is reduced by 10 % compared with an aluminium co-doped fibre material in steady state operation. Steady state can be reach fast for the phosphorous co-doped fibre compared with the aluminium co-doped fibre. The addition of aluminium to a phosphorous co-doped fibre will increase the pump absorption and hereby improve the PD loss to pump absorption ratio. A combination of aluminium and phosphorous co-doping can be applied to achieve good pump absorption in combination with a designed level of pre-PD resulting in an improved temporal behaviour of fibre lasers and amplifiers operated at medium to low population inversion levels while maintaining high slope efficiency.
1
PD Loss / Pump abs
10
0
10
10
-1
10
-2
1000 hour 50 cm fiber 16 µm core 200 µm clad
PD loss @ 1070 nm Pump abs. @ 915 nm
10
100
Population inversion (%) Fig. 10. Saturated PD (1070 nm) as function of population inversion level in 50 cm un-seeded fibre amplifier
Koponen et al.8 reported that the ratio of excess absorption between 633 nm and 1070 nm is 71. Taking the raw data from our measurements this ratio is shown in figure 11. Here it is to be observed that about half of our characterised fibres show this ratio. It is further to be observed that there appears not to be any influence of the co-doping on the obtained ratio. An analysis of the actual composition of AX colour centre lines reveals that part of the 633 nm absorption is due to the A1 and A2 centres that contributes with loss at the pump and signal wavelengths and part of the 633 nm absorption is due to A3a and A3b lines which only contribute with loss at wavelengths below ≈800 nm. Caution has to be taken not to conclude on the PD performance of a material based only on the 633 nm response as the ratio between the visible wavelength absorption and the near infrared absorption is not necessary constant 71. A composition analysis of A1 relative to A2 centres is shown in figure 12 for the 30 characterised fibres. The relative A1 to total A1 and A2 centres is calculated for the contribution at 1070 nm. From figure 11 and figure 12 it is to be observed that fibres with a total ratio between 633 nm and 1070 nm of 71 tend to contain a large fraction of A1 centres. The aluminium co-doped fibres can be made to perform with a relatively low contribution of A1 centres and there is a tendency that these fibres show lower PD loss / pump absorption. In Koponen et al.8 a ratio of 71 for all fibres indicates that material derived in a Direct Nano-particle Deposition (DND) process show a high content of A1 centres. The reason for this is not clear but could be due to either the DND process, the sintering process that follows the deposition process or co-doping not reported.
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633 nm to 1070 nm ratio
250
Al Al / P P 71 ratio
200 150 100 50 0
5
10
15
20
25
30
Fiber number Fig. 11. Ratio of 633 nm to 1070 nm excess absorption for 30 characterised fibres after 48 hours operation
1.0
A1 fraction
0.8 0.6 0.4 0.2 0.0
Al Al / P P
0
5
10
15
20
25
30
Fiber number Fig. 12. A1 centre contribution to infrared wavelength absorption Comparing aluminium co-doped fibres with phosphorous co-doped fibres it appears that phosphorous co-doped fibre material on average show higher fraction of A1 centres than “standard” aluminium co-doped fibre material. This behaviour is in agreement with the assumption that the colour centre ground state population in the “A1 centre ground state” increases with phonon density. It is expected that phosphorous co-doping of the glass material will increase the average phonon energy of the material which again promotes the “A1 centre ground state” of the colour centre.
Proc. of SPIE Vol. 6873 68731C-10
In general it is to be expected that the PD behaviour of ytterbium co-doped glass material can be influenced by the preform collapse process step and subsequent quenching as this step holds the highest process temperature and hereby greatly influences the level of defects produced in the glass material. The level of defects is believed to influence the average phonon energy of the glass material and hereby the A1 fraction of the glass material and the number of available non-binding oxygen sites near ytterbium.
5. CONCLUSION A model description of photo darkening based on 30 characterised fibres in an un-seeded amplifier setup is presented. Photo darkening of ytterbium and aluminium co-doped silica fibres is found to saturate following prolonged exposure to pump radiation. The saturation level is found to be a second order function of the induced ytterbium population inversion level of the material. The photo darkening is associated with non-binding oxygen at surfaces of ytterbium / aluminium clusters. Co-doping with phosphorous appears to introduce pre-photo darkening to the glass material. It is found that phosphorous co-doping on average gives raise to 10 times higher saturated PD loss per pump absorption compared with aluminium co-doping. This suggests that phosphorous co-doped fibre material performs with a slope efficiency which is reduced with 10 % compared with an aluminium co-doped fibre material once steady state operation is reached. Steady state is reached fast for phosphorous co-doped fibre material compared with aluminium co-doped material. The spectral absorption of PD colour centre at near infrared wavelengths can be described as a combination of A1 and A2 centre absorption. The actual fraction of A1 to A2 response in the near infrared wavelength range is determined by the glass material average phonon energy. This average phonon energy is again determined by co-doping of the glass material and is speculated to be influenced by the glass preform collapse and quenching process.
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