The thesis describes broadband supercontinuum (SC) generation in optical fibers for both the visible and mid-infrared regions of the spectrum, and their respective application to 3D imaging and stand-off reflection spectroscopy. Both SC sources leverage mature telecom technology, and are based on a common all-fiber integrated platform comprising a ~1.55 μm distributed feedback seed laser diode amplified to high peak powers in two stages of cladding pumped Erbium or Erbium-Ytterbium fiber amplifiers.
A visible SC extending from 0.45-1.20 μ m with 0.74 W of time-averaged power is demonstrated using a two step process. The output of the Er-Yb power amplifier is frequency doubled to ~0.78 μm using a periodically poled lithium niobate crystal, followed by non-linear spectral broadening in 2m of high non-linearity photonic crystal fiber. Numerical simulations based on solving the generalized non-linear Schrodinger equation are also presented to verify the underlying SC generation mechanisms and predict further improvements.
The above SC source is used in a Fourier domain line scan interferometer to measure the height and identify shape defects of ~300 μm high solder balls in a ball grid array. The 3D imaging system has an axial resolution of ~125 nm, transverse resolution of ~15 μm, and an angular measurement range between 20 to 60 degrees depending on the sample surface roughness.
The mid-infrared SC source is generated by pumping a 9m long ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber to obtain a spectrum spanning 0.8 – 4.3 μm with 3.9 W time-averaged power. The output power is linearly scalable with pump power, but requires optimization of the critical splices and thermal management of the gain fiber and pump diodes to ensure stable high power operation.
Finally, an application of the mid-IR SC is demonstrated by measuring the diffuse reflection spectra of solid samples at a stand-off distance of 5 m and 100 ms integration time. The samples can be distinguished using a correlation algorithm based on distinct spectral features in the reflection spectrum. Signal to noise ratio calculations show that the distance is limited by space constraints in our lab and can be extended to ~150 m.
VISIBLE SUPERCONTINUUM GENERATION IN PHOTONIC CRYSTAL FIBER
A block diagram of the all fiber integrated high power pump system at 1.55 μm is illustrated in Fig. 2.1. A 1553 nm DFB laser diode is driven by a pulse generator to produce 2 ns pulses at variable repetition rates and amplified using dual stage fiber amplifiers. The first stage consists of a 1 m long 4/125 μm Erbium doped fiber amplifier (EDFA) forward pumped by a 400 mW 980 nm single mode laser diode.
HIGH RESOLUTION LINE-SCAN 3D IMAGING SYSTEM FOR SOLDER BALL INSPECTION
Figure 3.2 illustrates the optical layout for the line scan interferometer. While the setup resembles a conventional point-scan FD-OCT system, the introduction of a cylindrical lens and replacement of a 1D array with a 2D CCD camera enables the measurement along an entire line instead of a single point with each camera image. The interferometer layout was modeled using ZEMAX software to provide the best imaging performance using standard off-the-shelf optics.
MID-INFRARED SUPERCONTINUUM GENERATION IN ZBLAN FIBER
The power-amp efficiency (power-amp average output power/input pump power) as a function of output peak power is shown in Fig. 4.3 and was measured both before and after optimization of the splice. In both cases, there is a decrease in efficiency with increasing peak power due to non-linear spectral broadening of the signal within the gain fiber itself.
STAND-OFF REFLECTION SPECTROSCOPY USING A MID-INFRARED SUPERCONTINUUM SOURCE
While the results of the previous section proved the feasibility of using the SC light source for transmission spectroscopy applications, a stand-off detection system must necessarily operate in the reflection mode. The experimental setup for a reflection spectroscopy based stand-off detection system is shown in Fig. 5.6. First, the diverging SC output from the ZBLAN fiber is collimated to a 1 cm diameter beam using a 25 mm focal length, 90 degrees off-axis, gold coated, parabolic mirror.
SUMMARY AND FUTURE WORK
A possible design of such an instrument is shown below in Fig. 6.2. In order to make the system compact, the visible SC source has been replaced with a superluminescent laser diode (SLD) source. The advantages and disadvantages of this source over the SC source have already been discussed previously in Section 3.5. The setup is in principle identical to the setup from Chapter 3 with mirror M2 and the sample positioned at the ends of the two arms of the interferometer.
Source: The University of Michigan
Author: Malay Kumar