Internal Spatial Modes and Local Propagation Properties in Optical Waveguides Measured Using Near-Field Scanning Pptical
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the first observation of standing modes in a single mode waveguide, as well as the first determination of all components of the propagation vector [13]. EXPERIMENTAL SETUP AND DEVICE UNDER STUDY In Fig. 1, we show a schematic of the waveguide studied. It was fabricated using the traditional sputtering and optical lithographic methods [ 14,15]. The rectangular core region is composed of a compound Ta20 5 / SiO 2 glass of index of n=1.65. The cladding region is pure silica, SiO 2, n=1.44.
II
TF%
TMo
TE
Fig 2: BPM mode simulations of the allowed modes of the glass/silica rectangular waveguide.
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We have simulated the mode patterns for this waveguide that are displayed in cross-section in Fig. 2. From a linecut of the intensity of these simulations in the vertical or x-direction (Fig. 3), we can see that this waveguide is ideal an NSOM due into to the relatively largemeasurement, field penetration the air
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region. The field intensity at the air interface is about between 4% and 9% that of the ooo maximum in the core region.
The waveguides studied are designed
S(nm)
for operation at the telecommunication wavelengths around 1.55 gim. A schematic of the experimental configuration is displayed in Fgi. 4. A tunable external cavity laser (HP 8168F) was used as the light source, tunable from 1440nm to 1570nm. A conically lensed fiber is used to launch this light into the waveguide. Polarization control is performed using polarization paddles. Transmitted light at the exit facet is imaged onto both a CCD for optimization of Fig. 3 Vertical line cuts of mode simulations for the three modes guided by the device under test.
tunable
InGaAs
diode
detector
laser
Fig 4 Experimental setup used for characterizing internal optical fields inside guided-wave devices. Light from tunable laser is launched into the device under test, and the transmitted light is analyzed. The NSOM probe is scanned over the of the device, measuring both topography and sampling the evanescent field.
Ge detector
NSQM Sob'ective
lensed fiber
I A -surface
4
coupling and a Ge photodiode for quantitative measurements. A polarizer can be placed before these detectors, so that the desired output polarization can be chosen with the polarization paddles. The NSOM probe is scanned over the surface of the waveguide, routing the collected light to an InGaAs photodetector. The translation stage used to scan the tip is interferometrically calibrated to ± 0.3nm, and a shear-force tuning fork method [16] is usedduring surface scans to maintain a tip-surface separation of 10 nm. NSOM MEASUREMENTS The field at the waveguide surface will not be simply related to the internal modes if a large amount of scattering is present. Therefore, a vital initial measurement in this and similar studies is a comparison of the level of scattered light to that of evanescent field at the surface [13]. The measured optical intensity from NSOM scans in the vertical plane, perpendicular to the
750' Fig 5 (a) M
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