Optical glass, optical waveguide amplifier and optical...

Details Optical glass, optical waveguide amplifier and optical... Optical glass, optical waveguide amplifier and optical...

1999-04-12

2001-10-16

Spyrou, Cassandra (Department: 2872)

Optical waveguides

Having particular optical characteristic modifying chemical...

Of waveguide core

C385S144000, C385S123000, C372S006000

Reexamination Certificate

active

06304711

ABSTRACT:

This invention relates to optical glasses, optical waveguide amplifiers and optical waveguide lasers.
The possibility of using the
4
F
{fraction (3/2)}-
4
I
{fraction (13/2)}
transition in rneodymium doped waveguides as an efficient 1.3 &mgr;m optical amplifier has been the subject of many studies
1
. However there are two major drawbacks which have hindered its development.
Firstly the presence of signal excited state absorption (“ESA”, see
FIG. 1
) severely compromises the efficiency and operating wavelength of the device.
Secondly, the larger emission cross section for the 1050 nm transition leads to the generation of amplified spontaneous emission (ASE) at this wavelength, which in turn clamps the available gain around 1.3 &mgr;m to a relatively low value (about 5-6 dB). Both these phenomena are known in the literature
1,2,3
.
The ESA process is very dependent on the glass composition and a number of glass families have been studied in this regard (namely the silicates, fluorozirconates, fluoroberyllates and fluorophosphates)
1,2
. This study concluded that fluoroberyllate glasses were the optimum glass family for 1.3 &mgr;m amplifiers because of their very low ESA cross section at this wavelength. However, the toxicity of beryllium rules out this glass family for practical devices. The next best family was concluded to be the fluorophosphate glasses.
It is also known that the use of discrete or continuous 1050 nm ASE filters along the amplifier length allows for a significant increase in the gain at 1.3 &mgr;m. Some specific ASE filtering techniques for increasing the 1.3 &mgr;m gain in Nd
3+
-doped fibre amplifiers can be found in the literature. These include the use of wavelength division multiplexers (WDMs)
6
, mechanical gratings
7
, the use of rare earth co-dopants to absorb the 1050 nm ASE
8
and optimisation of the fibre design
9
.
This invention provides an optical fibre device formed at least in part of neodymium-doped fluoroaluminate optical glass having a refractive index, at a wavelength of 600 nm, of less than or equal to about 1.444.
This invention also provides an optical fibre amplifier operable at a peak signal wavelength of less than 1320 nm, the amplifier being formed it least in part of neodymium-doped fluoroaluminate optical glass.
Further respective aspects of the invention are defined in the appended claims.
The invention covers a range of fluoroaluminate glass compositions and, in various embodiments, their use as a Nd
3+
-doped optical waveguide amplifier or laser operating around 1.3 &mgr;m. Specifically it has been found that at least some of these compositions exhibit negligible ESA at the peak emission wavelength (1317 nm) allowing efficient amplification and lasing to occur at this wavelength.
The family of glass compositions of at least embodiments of the invention have the lowest values of Nd
3+
-1.3 &mgr;m ESA cross sections of any glass (with the exception of the fluoroberyllates which are dismissed on the grounds of their toxicity) and as such are considered the optimum glass host for an efficient optical fibre amplifier operating at wavelengths around 1317 nm. This is a shorter peak operating wavelength than that found in either fluorozirconate or fluorophosphate glasses both of which have lasing wavelengths longer than
1320
nm
4,5
. Indeed only fluoroaluminate glasses operate efficiently at wavelengths less than 1320 nm.
Thus, neodymium doped fluoroaluminate glass waveguides can be used as amplifiers and lasers within the second telecom window between 1300 and 1350 nm. Specifically fluoroaluminate glasses are preferred with compositions (mol %); (35-45)AlF
3
, (5-30)RF
2
, (5-25 mol %)MF where R=Mg, Sr, Ba, Ca and M=Na, K, Li, Rb.
Light alkali monovalent fluorides (MF) are an advantageously important component in fluoroaluminate glass compositions intended to obtain a shorter wavelength emission and gain at the 1300 nm transition of Nd
3+
. The inclusion of significant proportions of light MF components, such as LiF and NaF gives rise to a more ionic glass with a lower refractive index. The nephelauxetic effect associated with the low refractive index causes the emission to be blue-shifted to shorter wavelengths. High ionicity reduces the linestrength of 1300 nm ESA and shifts its peak wavelength away from the centre of the emission curve. Together, the blue-shifted emission and the reduced effects of ESA result in gain at shorter wavelengths, below 1320 nm.
This glass system shows a minimum excited state absorption at wavelengths around 1.3 &mgr;m when doped with Nd
3+
-ions. As a consequence these glass compositions are the only fluoride glass system (with the exception of the toxic fluoroberyllates) to operate efficiently as a waveguide amplifier or laser at wavelengths below 1320 nm.
The peak gain wavelength in these glasses is 1317 nm which corresponds to the peak emission wavelength thus indicating that negligible ESA occurs at this wavelength.
In order to achieve a high gain 1.3 &mgr;m amplifier or laser with these glasses some degree of 1050 nm ASE filtering is required. The filter response should ideally be that of a band blocking filter centred at 1050 nm with around 15 nm bandwidth (full width half maximum).
The neodymium-doped fluoroaluminate fibre amplifier can also offer reduced splicing loss to standard silica fibre, because the glass has a lower refractive index than silica. The refractive index is lower than in ZBLAN and fluorophosphate glasses. As a consequence the amplifier gain is expected to be higher than in other 1.3 &mgr;m neodymium-doped fibre amplifiers.
Embodiments of the invention can also provide two new techniques for 1050 nm ASE filtering in Nd
3+
-doped 1.3 &mgr;m waveguide amplifiers. The first involves the use of UV induced in-core gratings, which may be written directly into the waveguide core by UV light. This process is made possible by the addition of a photosensitizing agent (eg 1 mol % CeF
3
or tin fluoride) to the basic fluoroaluminate core glass composition.
The form of the grating may be either a blazed reflection grating or a long period grating coupling to discrete cladding modes. In either case the grating preferably acts as a band blocking filter, centred at 1050 nm and with a bandwidth of around 15 nm full width half maximum.
The complete amplifier may include many discrete filters or one continuous filter along the total amplifier length. For example, in-core gratings may be written directly into the waveguide core with UV light from, for example, an excimer laser. In order to photosensitise the glass to UV light the core glass is modified from the above compositions to incorporate small amounts (0.01-5 mol %) of cerium fluoride (CeF
3
) or tin fluoride.
The presence of Sn
3+
or Ce
3+
-ions greatly increases the photosensitivity of the core glass allowing the formation of periodic refractive index charges or gratings. Careful control of the grating parameters (period, blaze angle and depth of modulation) allows the formation of 1050 nm band-stop gratings with the correct bandwidth. A Nd
3+
-doped waveguide amplifier incorporating one or more of these filters exhibits substantially more gain at wavelengths around 1.3 &mgr;m than amplifiers with no filter.
A second method for ASE filtering in other embodiments is based on splicing Yb
3+
-doped fibres at intervals along the length of the Nd
3+
-doped fluoroaluminate fibre. These fibres have around 30 dB absorption at 1050 nm and negligible loss at the Nd
3+
-pump (800 nmn or 740 nm) and signal (1317 nm) wavelengths. The Yb
3+
-doped fibre may be silica, fluoroaluminate or any other fluoride fibre. The advantage in using fluoroaluminate fibre comes from the matching of the fibre refractive indices.
The plural alternate fibre sections act as discrete 1050 nm ASE filters and unlike Nd
3+
/Yb
3+
co-doped waveguides do not suffer from energy transfer between the two sets of ions. This greatly increases the device efficiency. (Clearly, by the use o

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