The sol itself was prepared in stages. A phosphosilicate (PSG)
sol was first formed by combining TEOS and P O solutions with H O and refluxing to hydrolyze on average two OR groups
per TEOS molecule ( ) using HCl as a catalyst. The PSG
sol was then stabilised to avoid gelation of the mixture. AlPSG,
ErAlPSG and YbErAlPSG sols were then obtained by successive
additions of IPA solutions of phosphorus pentaoxide and
aluminum, ytterbium and erbium salts, in ratios needed to obtain
the final composition.
Experimentally, an upper limit to the total concentration of
rare earth precursor was found, to avoid precipitation of the
sol. A compromise was therefore required between the target
Er concentration and the target Yb : Er ratio. In the demonstration
here, the core was doped with 10 mol% P O , 2.5 mol%
Al O , 0.25 mol% Er O , and 0.25 mol% Yb O , giving an asymmetric guide with an index step of and
an Yb : Er ratio of 1 : 1. Amplifiers were also fabricated with an
Yb : Er ratio of 3 : 1, but the Er concentration had to be reduced
to allow an increased Yb concentration, causing a reduction in
overall gain.
Rapid thermal processing was performed in O using an
AG Associates Heatpulse 610 rapid thermal annealer. Multiple
layers were deposited to yield an overall core thickness of 4 m
and the core was then consolidated at 1000 C in a conventional
furnace. A surface mask containing guide widths of 3 to 6 m
was then formed from Cr metal and hard baked resist. The
cores were then etched in a load-locked reactive ion etching
(RIE) system using a mixture of C F and He. The etching
was carried out at low (1 mtorr) pressure to assist removal
of involatile by-products, and hence avoid micromasking.
The etch rate of the sol-gel cores was around 300 min.
Examination of the cross section of the cores showed that a
small sidewall slope had arisen from the low-pressure etching.
The cladding was a 16- m-thick layer of borophosphosilicate
glass (BPSG) deposited by PECVD and then consolidated at
950 C for 6 h. The maximum overall process temperature was
therefore limited to 1000 C and the wafer was only exposed
to this temperature for a short time. As a result, no crystalline
phases appeared to be formed, as evidenced by the low scattering
of the guide at visible wavelengths. A transparent layer
with a uniform dispersion of Er ions was therefore obtained.
Characterization was performed using an optical spectrum
analyzer equipped with an internal 1550-nm edge-emitting
light-emitting diode (ELED) source (Agilent 86 142B).
Fig. 3 shows spontaneous emission spectra obtained from
5-cm-long EDWA with a 4- m-wide core, by pumping using
a 980-nm laser diode at different powers. Strong fluorescence
was observed, which did not completely saturate even at
powers as high as 200 mW. The fluorescence bandwidth was
19 nm [full-width at half-maximum (FWHM)]. Fig. 4 shows
single-mode fiber-device-fiber transmission measurements for the same device, obtained using the ELED source. The
background insertion losses are 1.0 dB and the Er
ions contribute an additional 7.6-dB loss (corresponding to 1.52 dB/cm) at 1535-nm wavelength. Almost complete inversion
is obtained at a pump power of 175 mW, giving an
apparent fiber-device-fiber gain of 5.75 dB and an internal gain
of 1.35 dB/cm.
Fig. 5(a) shows the variation of total signal power and spontaneous
emission with pump power, for the same device as before.
Clearly, the spontaneous emission is a small fraction of
the total. However, some correction is required to obtain accurate
gain figures. Fig. 5(b) shows the apparent fiber-device-fiber
gain obtained from Fig. 4 and the corrected gain after subtraction
of the spontaneous emission. Transparency is obtained at a
pump power of 25 mW and a corrected gain of 5.4 dB is
obtained at 175 mW.
In conclusion, a hybrid sol-gel/PECVD fabrication process
has been used to demonstrate a compact silica-on-silicon EDWA
with a useful fiber-device-fiber gain. The results obtained represent
both the highest external gain for a sol-gel EDWA and the
highest internal gain per centimeter to date for a silica-on-silicon
device. The high internal gain suggests that this approach may
be used to construct modest amplifiers suitable for Metro applications
in an in-line configuration, without the need for a folded
optical path. Further work is underway to optimize the guide design
and the active layer composition to reduce the pump power
requirements and tailor the gain spectrum.