Innovative Ultrafast Laser Solutions

Innovative Ultrafast Laser Solutions

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 14.1. (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.)

Waveguides

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).