Spatio-Temporal Shaping of Laser Pulses

As illustrated in Fig. 6.1 the spectrally shaped laser pulses are subse-quently shaped in their temporal and spatial properties, which will be discussed in this section.

From the model derived in Chapter 4: Interaction of Ultrashort Laser Pulses and Solar Cells it was concluded that the ultrashort laser pulses should be converted into continuous radiation for eliminating potential nonlinear effects in the measurements. In Chapter 5: Temporal Shap-ing of Ultrashort Laser Pulses and especially in Section 5.5 a fiber-based monolithic device was presented that enables such a transformation of the ultrashort pulses.⁸ It has also been demonstrated in Section 5.4.1 that the multimode fiber approach does not only yield temporal but also spatial

6In a further development stage of the prism monochromator, a step motor can be attached to the slit making a variation of the slit width even more convenient.

7An additional longpass filter ensures that undesired spectral components below 900 nm are blocked.

8As this device was discussed in detail beforehand, the temporal shaping is not further addressed in this section.

homogenization of the radiation which is an important property of so-lar cell characterization tools. In Chapter 8: EQE-Measurement of CPV Modulesone measurement approach is presented that exploits exactly this feature of multimode optical fibers.

However, the new Laser-DSR measurement setup aims at absolute SR-measurements for which illumination of the entire solar cell surface with uniformly distributed monochromatic radiation is beneficial. Outshining large area solar cells with typical edge lengths of approximately 15.6 cm in so-called overfilled measurements, hence requires a rather difficult 550x-magnification of the fiber output surface (assuming a 400 µmfiber core diameter). Furthermore, the concept of imaging the homogeneous fiber output onto the measurement plane in combination with the dual reference mode would require that all wavelengths are emitted by a single fiber resulting in additional absorption losses. The discussed difficulties going along with a direct imaging of the multimode fiber output motivate the development of a refined imaging setup that is discussed subsequently.

For combining the radiation from the different pulse-to-cw-converters (compare Fig. 6.1) they are placed close to the input facet of a mixing rod with hexagonal cross section. Besides spatially combining the radiation from the different fiber outputs, the hexagonal cross section of the mix-ing rod and the rather high NA input from the fiber converters (compare Section 5.5.2) yield a spatially homogeneous intensity distribution at the exit facet of the mixing rod. Thus, the exit facet of the mixing rod is kind of an equivalent to the fiber converter outputs regarding the spatial irra-diance uniformity, but is significantly larger in spatial dimension, thereby simplifying its imaging onto the measurement plane.

Starting from this general concept, optical simulations with the ray-tracing software OptiCAD^R have been conducted for optimizing the prop-erties of the optical components with respect to the size and uniformity of the illuminated area as well as its wavelength sensitivity. The final optical configuration composed of a hexagonal mixing rod, a lens, a flat tilted mirror and a parabolic mirror as well as simulated results of the optical setup are shown in Fig. 6.6. Fig. 6.6a illustrates the combination of different optical components to achieve a uniformly illuminated area of 18x18 cm². The close-up shows the mixing rod and the lens (the fibers,

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Figure 6.6: Layout and simulated results of the optical setup for il-luminating a 18x18 cm² area with spatially uniform irradiance dis-tribution. (a) Screenshot of optical setup implemented in ray-tracing software. Blue to red areas denote the illumination intensity from low to high. The green rays illustrate the beam paths. (b) Simulated non-uniformity over number of rays considered in simulation. (c) Sim-ulated intensity distribution of a 16x16 cm² area for 10 million rays resulting in a non-uniformity of approximately 3%.

not shown here, are placed at the left side of rod). Lens and steering mirror direct the radiation emitted from the rod onto a parabolic mirror.

The combination of lens (short focal length) and parabolic mirror (large focal length) result in a magnified image of the mixing rod output facet in the measurement plane (note the hexagonal intensity structure around the 18x18 cm² aperture). The short focal length of the lens ensures a higher magnification and avoidance of kaleidoscopic effects from the rod’s inner surfaces. Furthermore, as the lens merely re-directs or collects the emit-ted rays in order to ensure that they hit the parabolic mirror, the optical layout is rather insensitive to chromatic aberrations induced by the lens.

Fig. 6.6c shows the simulated centric 16x16 cm² intensity distribution for 10 million rays resulting in a non-uniformity of approximately 3%. For a selected 2x2 cm² area (typical reference solar cell area) a non-uniformity below 0.2% is achieved. The simulated non-uniformity strongly depends on the number of rays (see Fig. 6.6b), thus, less than 3% non-uniformity are expected for the final setup.⁹

In addition to the 18x18 cm²illumination setup shown in Fig. 6.6a, the new Laser-DSR setup can be conveniently equipped with further optical components for uniformly irradiating a 5x5 cm² area. This is especially advantageous when smaller devices are measured, as the monochromatic irradiance is increased by a factor of 13. For this, the fibers are plugged to a second hexagonal mixing rod and another combination of steering and parabolic mirror as well as aperture (see Fig. 6.7a, for convenience the other rod and lens - used for the large area setup - are not shown here).

The spatial intensity distributions obtained from (i) ray-tracing simu-lation and (ii) measurement shown in Fig. 6.7b demonstrate the high spa-tial uniformity being achieved with this optical setup (please note that the scale is identical to the scale in Fig. 6.6c). In fact, the best non-uniformity for a 2x2 cm² area is (i) 0.14% and (ii) 0.23% (for a 4x4 cm² area it is (i) 0.80% and (ii) 1.05%). Fig. 6.7b also indicates that the simulated in-tensity distribution resembles the measured one, especially regarding the tendency of a slight drop in intensity when approaching the edges of the measurement plane. Finally, Fig. 6.7c illustrates first results regarding

9At the time this thesis has been written the optical components for achieving a 18x18 cm²illuimination area have not been installed yet.

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Figure 6.7: Optical layout and intensity distribution for illuminating a 5x5 cm² area with a spatially uniform irradiance distribution. (a) Screenshot of optical setup implemented in ray-tracing software. Blue to red areas denote the illumination intensity from low to high. The green rays illustrate the beam paths. Steering and parabolic mirror as well as 5x5 cm² aperture are easily implemented into the large area setup (compare Fig. 6.6a). For convenience rod and lens of the large area setup are not shown here. (b) (i) Simulated and (ii) mea-sured intensity distribution in measurement plane at the same scale as Fig. 6.6c. Best non-uniformity for a 2x2 cm² area is (i) 0.14%

and (ii) 0.23%, for a 4x4 cm² area it is (i) 0.80% and (ii) 1.05%.

(c) Measured intensity distributions at (i) 700 nm and (ii) 1000 nm wavelength (please note the different scales as compared to (b)). Non-uniformities for a 2x2 cm² area at idential positions are (i) 0.24% and (ii) 0.23% as well as (i) 0.93% and (ii) 1.05% for a 4x4 cm² area.

the insensitivity of the setup to wavelength changes (here from 700 to 1000 nm, note that the scale is different as compared to Fig. 6.7b). Apart from the obvious similarity of both irradiance distributions, their non-uniformities at identical positions are very similar: for a 2x2 cm²area the non-uniformity for 700 nm equals 0.24% and for 1000 nm it equals 0.23%, for a 4x4 cm² area the respective non-uniformities are 0.93% (700 nm) and 1.05% (1000nm).

The results presented in this subsection demonstrate the excellent uni-formity achieved with the optical concept presented in Figs. 6.6a and 6.7a.

Moreover, the measurement setup can be switched from uniform illumi-nation of a larger (max. 18x18 cm², with an expected 3% non-uniformity over a 16x16 cm²) to a smaller area (max. 5x5 cm², with a 0.23% non-uniformity over a 2x2 cm²), thereby enhancing irradiance, hence, mea-surement signal by a factor of 13. During the completion of the large area setup, that was being conducted at the time this thesis has been written, a neat mechanical solution enabling a convenient change of the measurement setup has been developed. The entire setup conversion is accomplished by simply re-attaching the fibers to the second rod and sliding in a frame holding the parabolic and steering mirror as well as the aperture for the small area setup. Furthermore, first experimental results support the expected insensitivity of the chosen optical setup to wavelength variations. Not only that this feature reduces measurement uncertainties (being discussed in Section 6.4), the wavelength-insensitivity is as well of tremendous practical importance as it significantly reduces effort when calibrating the setup.