Reducing the Optical Absorption of LPE Garnet Films at a Wacelength of approx. 1,000nm

1. INTRODUCTION

Currently, erbium-doped fiber amplifiers (EDFA) emploving 1,480nm laser diodes as a pumping light source are in practical use for the amplification of 1,550nm optical singles. These EDFAs incorporate optical isolators which function to forestall the return of veams to the pumping laser diode so as to stabilize the pumping opration. At tha same time, efforts are being made to develop an EDFA, incorporating a 980nm laser diode as a pumping light source for higher efficiency and lesser noise. Research is also underway to develop a praseodymium-doped fiber amplifier (PDFA) having a 1,017nm laser diode as a pumping light source.
All these amplifiers need optical isolators for a stable pumping operation. One problem is that optical absorption by iron garnet, shich serves as the Faraday rotators of optical isolators, peaks at a wavekength of around 900nm due to a crystal field transition from 6A1g to 4T1g of the Fe3+ at the octahedral sites. As a result, iron garnet has a large optical absorption coefficient a at atound 1,000nm, thereby posing a problem of a large optical absorption loss L when used as a 45Faraday rotator. Accordingly, the purpose of the present research was to develop an iron garnet with a small L value ataaround 1,000nm. To lower L, a must be decreased while F (Faraday rotation coefficient) must be increased.
The optical absorption pattern of Fe3+ in an iron garnet crystal, taht is cultivated by the flux method, is known to shift toward short wavekengths if the value of its crystal lattice constnat is increased. For this reason, it is predictable that an iron garnet with a large lattice constant will exhibit alow a at around 1,000nm.Further, a large lattice constant should enable substitution of a large amount of Bi, which has a large ion radius; thus, the value of F will be increased.
We grew a garnet single crystal with a large lattice constant by means of the Czochralski method. Then, employing this crystal as a substrate, we cultivated a Bi-substituted rare-earth iron garnet film (hereafter called "LPE film") by means of the LPE method in an attempt to obtain a low-a and high-F LPE film. This paper will introduce the techniques we applied to the cultivation of this seed crystal and iron garnet film, and the optical properties of the film at wavelengths of 980nm and 1,017nm.

2. SAMPLE PRODUCTON AND MEASUREMENT

Using a PbO-Bi203-B203 fluxm LPE films were grown on <111>-oriented substrates with their lattice constant ranging from 1.2496nm to 1.2653nm. Fifure 2 the relation between the absorption coefficient and wavelength of LPE films. From Figure 2, a rare-earth element was selected so that the lattice constant of LPE film would match the lattice constant of the substrate; then, LPE film composition was determined in such a way that LPE film would contain as much Bix as possible.
First, attempt was made to cultivate a LaO.9Bi2.1Fe5012 LPE film with a lattice constant of 1.2653nm, but no LPE film was obtained because the segregation coefficients of both Laand Bi are small. The compositions of LPe films obtained and the substrates used are listed in Table II. Sample D could no be obrained for the above-mentioned reason. Samples A and B had been added with a suitable amount of CaO or SiO2 to forestall optical absorption by Fe2+ and Fe4+. Fe4+ had been eliminated from Sample C by growing the LPE film in a nitrogen flow of 3 liters/min and, after the cultivation, subjecting the film to reduction treatment in a hydrogen flow of 2 liters/min under a 20mm Torr and 450 condition for 45 minutes.
Film composition was analyzed by EPMA. Measurement using the double-crystal X-ray diffraction technique indizated that the lattice constant differential the film and substrate was no more than 1.001nm for Samples A, B and C. Rach film's optical absorption spectrum was measured with a double-beam spectrophometer in an applied magnetic field of 5kOe. Optical absorption coefficient a and Faraday rotation coefficient F were measured using laser diodes of 971nm, 1,016nm and 1,029 wavelengths, and the calues of a and F at 980nm and 1,017nm were derived by means of interpolation.F was obtained under a 25 and 5ke magnetic field condition by means of the polarization modulation technique. Optical absorption loss L (dB) of the LPE films (as 45 rotators) were calculated by the following equation:

a (nm)

3.RESULTS AND DISCUSSION

Figure 3 shows the relation between peak optical absorption and lattice constant increased, peak optical absorption at around a 900nm wavelength shifted toward shorter wavelengths due an Fe3+ crystal filed transition from 6A1g to 4T1g, thus lowering the value of at around 1,000nm. Figure 4 shows the relation between the lattice constant and peak optical absorption wavelength &max due to an Fe3+ crystal field transition. In Sample C there was an overlap of opticak absorption by Nd3+ and that by Fe3+; therefore, &max was obtained by subtracting the optical absorption by Nd3+. Results indicated a liner realtionship, with ana inclination of -8.6nm/0.01nm. figure 5 shows the relation between optical absorption loss L and wavength. As the wavelength increased, L decreased.
Table III shows the optical properties of the LPE films at 980nm and 1,017nm. The largest value of F was obtained from Sample B as it constant the largest volime ofsubstituted Bi (107atoms/f.u.). Although Samples C and A contained a similar amount of substituted Bi, Sample C derived a far larger value of F tahn Sample A because the Nd contained in Sample C is known to contribute negatively to faraday rotation. At both 980nm and 1,017nm wavelengths, the lowest value of L was obtained from Sample C (a=1.2619nm), whose composition was Nd1.69Bi1.31Fe5012. Sample C's value of L was 2.6dB at 980nm, and 1.0dB at 1,017nm. Figure 6 shows the relation between L and teh wavekength in Sample C. L decreased as the wavelength increased. As for the wavelength of 1,047nm, which is receiving attention in connection with the new pumping optical source for PDFA, we obtained an L value of 0.5dB through extrapolation.

4.SUMMARY

To obtain substrates for growing Bi-substituted rare-earth iron garnet films, single crystals of (NdGd)3(ScFa)5012 with lattice constants of 1.2619nm and 1.2653nm were cultivated by means of the Czochralski method. Using these sibstrates, Bi-substituted rare-earth iron garnet films with lattice constans of 1.2496nm and 1.2619nm were grown by means of the LPE method, and the optical properties of the resultant LPE films were measured. It was found taht, as the lattice constant increased, the optical absorption pattern shifted toward short wavelengths due to an Fe3+ crystal filed transition. As a result the optical absorption value a decreased at a wavelength of around 1,000nm. The lowest value of L obrained from the LPE film of Nd1.69Bi1.3Fe5012 composition with a lattice constant of 1.2619nm--an L value of 2.6dB at 980nm, and 1.0dB at 1,017nm.