Active
Waveguides
The photosensitivity
of silica fibers in the UV region of the spectrum was discovered
more than 20 years ago [1] and has proved to be a key point
in the advancement of guided wave devices. Using UV light
in a simple holographic setup [2] one can write gratings in
planar waveguides and optical fibers or can even write directly
waveguides in bulk glasses [3]. However, this method has inherent
limitations since many glasses are not sufficiently sensitive
to yield a large enough index of refraction change, and also
the UV photosensitivity range is very close to the absorption
edge of most glasses.
Mourou
and co-workers have proposed that femtosecond laser pulses
can be used to induce localized refractive index increase
in a wide variety of glasses. Thermally stable optical waveguides
were produced [4] in silicate, borosilicate, chalcogenide
and fluoride glasses and, also, more complex structures such
as a Y-junction splitter [5] and long period gratings [6]
have been reported.
We report
for the first time, to the best of our knowledge, an active
waveguide device directly written using near-IR femtosecond
laser pulses. The device is a waveguide amplifier in a Nd-doped
silicate glass.
Experimental
Details and Discussion:
The material
used in this study was a commercially available Nd-doped silicate
glass rod. From the measured absorption coefficient of the
glass we estimate the Nd doping level to be around 2 x 1020
ions/cm3.
For waveguide
fabrication, a Clark-MXR femtosecond workstation operating
at 775-nm was used. From throughput measurements of waveguides
of lengths varying between 2mm and 10mm we estimate the waveguide
propagation losses to be well below 0.5dB/cm. Gain measurements
were performed using an Argon-ion pump laser as a source at
514-nm and a signal at 1054-nm provided by a continuous-wave
laser. The data on the gain of the amplifier at a signal of
1054-nm is presented in Figure 1. (The gain was measured as
the ratio between the signal power with the pump turned on
and the signal power with the pump turned off.)
Figure 14-1 - Small signal gain versus launched pump power
Fluorescence
data indicate that the emission cross-section at 1054-nm is
only half as large as that at the 1062-nm the peak. Thus this
device should provide a peak unsaturated gain of about 3dB/cm
for launched pump power levels of about 140 mW.
References:
1. K.
O. Hill, Y. Fuji, D. J. Johnson, and B. S. Kawasaki, "Photosensitivity
in optical fiber waveguides: application to reflection filter
fabrication", Appl. Phys. Lett. 32, 647 (1978).
2. G. Meltz, W. W. Morey, and W. H. Glenn, "Formation
of Bragg gratings in optical fibers by transverse holographic
method", Opt. Lett. 14, 823 (1989).
3. M. Svalgaard and M. Kristensen, "Direct-writing of
planar waveguide devices using ultraviolet light", OSA
Tech. Dig. 17, BTuB2-1, 279 (1997).
4. K. Miura, J. Qiu, H. Inouye, and T. Mitsuyu, "Photowritten
optical waveguides in various glasses with ultrashort pulse
laser", Appl. Phys. Lett. 71, 3329 (1997).
5. D. Homoelle, S. Wielandy, A. Gaeta, N. F. Borrelli, and
C. Smith, "Infrared photosensitivity in silica glasses
exposed to femtosecond laser pulses", Opt. Lett. 24,
1311 (1999).
6. Y. Kondo, K. Nouchi, T. Mitsuyu, M. Watanabe, P.G. Kazansky,
and K. Hirao, "Fabrication of long-period fiber gratings
by focused irradiation of infrared femtosecond laser pulses",
Opt. Lett. 24, 646 (1999).
