Commercially available supercontinuum light sources that cover most of the solar spectrum are well suited for instrumentation, where a well-collimated beam with wide spectral coverage is needed. Typically, the optical power is emitted from a single-mode photonic-crystal fiber and the output can either be collimated using a proprietary, permanently integrated, lens-based collimator or with a customer-provided, off-axis parabolic mirror. Here, we evaluate both approaches and conclude that, superior beam quality and collimation over the whole spectral range can be obtained with an off-axis parabolic mirror, however at the price of a more complex and bulky system requiring additional user alignment.
© 2014 Optical Society of America
Commercial supercontinuum light sources are becoming more widely used in instrumentation and measurement applications. They use intense pico- or femto-second laser pulses at high pulse repetition rate (i.e., ~1-100 MHz) coupled into a photonic-crystal fiber to produce a broadband optical continuum through highly nonlinear frequency conversion processes. The photonic-crystal fibers confine the light to small cross-sectional areas over large distances and have engineered dispersion to optimize the non-linear processes required for broadband generation [1,2]. The output spectra can cover most of the solar spectrum with a spectral range of 400 through 2400 nm with some sources only extending down to ~500 nm. However one should note that the shorter and longer wavelengths of the source’s spectral range typically have fairly low spectral power density. The optical output can have multi-watt average power and is emitted from a single-mode fiber with a circular and usually non-polarized beam pattern although some sources may use a birefringent photonic crystal fiber for polarized output . It can be collimated, yielding a laser-like beam if chromatic aberration can be reduced or avoided. Typical applications either use the full bandwidth of the beam or use switchable or tunable spectral filters to sequentially select specific wavelengths [4–6]. The two common collimation techniques used for the broad band sources are an achromat doublet that is designed to have the same focal point for two wavelengths or an off-axis parabolic mirror that is free from chromatic aberration .
This paper investigates the collimation of commercial supercontinuum light sources using a Fianium SC-400-2 light source with a proprietary, permanently integrated, achromatic lens-based collimator and a Fianium SC-400-6-06 light source with an unterminated optical output sealed behind a plate of flint glass that has been collimated with a custom collimator using an off-axis parabolic (OAP) mirror. Beam collimation and quality is investigated by determining spectrally resolved beam profiles at multiple distances from the fiber output.
Knowledge of beam collimation and quality are of the essence for acquiring a supercontinuum light source because the decision between an integrated lens-based collimator and an unterminated output is semi-permanent; the laser must be returned to the manufacturer to remove or add the lens collimator. In this paper, we describe measurements taken and compare the beam properties of a supercontinuum light source utilizing a factory lens-based collimator with that of a light source utilizing an OAP mirror-based collimator.
One of the supercontinuum sources used was a Fianium SC-400-2 that was supplied with a permanently integrated collimator. The collimator was an achromatic doublet lens with 6.25-mm diameter and a focal length of 10 mm. This source emits short pulses (< 10 ps) at a repetition rate of 20 MHz with a total output power of ~2 W. The second supercontinuum source was a Fianium SC-400-06 with an unterminated output that is sealed behind a flint-glass plate to prevent damage to the fiber. This source also emits short pulses (< 10 ps) but at a higher repetition rate of 60 MHz with a total output power of ~6 W. It was collimated using a 90-degree silver-coated OAP mirror with a diameter of 12.7 mm and an effective focal length of 15 mm. This is the OAP mirror used in the Thorlabs RC04 collimator housing. The OAP mirror was placed at 90° relative to the output of the fiber and iteratively adjusted until the beam was visually well collimated along a 2-m length beam path with minimal coma. Both sources were designed to have a spectral range from ~400 nm to greater than 2200 nm.
The supercontinuum source’s one-dimensional (i.e., horizontal) beam profile was characterized by scanning a 75-μm diameter pinhole across the beam and measuring the spectral power density transmitted through this pinhole. This was done by using a sphere assembly consisting of an integrating sphere with a pinhole as the entrance aperture and a detector fiber-coupled to the exit aperture. Integrating spheres are ideal for power measurements as they homogenize spatial, directional, and polarization distribution of the light entering through the entrance aperture and in addition reduce the power incident on the detector . The sphere assembly was scanned across the collimated beam by attaching it to a translation stage with 0.01-mm spatial resolution. An ASD FieldSpec® 3 Max spectroradiometer was used to obtain a measurement of the relative spectral power density exiting the pinhole by attaching its fiber input to the exit aperture of the integrating sphere. The spectroradiometer had a wavelength range from 350 nm to 2500 nm with a spectral resolution of 3 nm at the wavelength of 700 nm and 10 nm at wavelengths of 1400 nm and 2100 nm. The experimental setup is shown in Fig. 1. The sphere assembly was moved perpendicular across the beam in 0.1-mm increments, with a measurement of power spectral density, for wavelengths λ from 400 nm to 2100 nm, being performed for each increment. For wavelengths shorter than 400 nm and longer than 2100 nm the power density was too low to make a reasonable measurement. These scans were repeated at multiple distances from the collimator in order to characterize beam collimation at 50-nm steps across the wavelength range. A line drawing of the experimental setup is shown in Fig. 1.
For each full scan the radiance measured was plotted against pinhole position for each wavelength (Figs. 2 and 3). To retrieve the beam waists at the distances from the collimators shown in Tables 1 and 2, these experimental beam profiles were fitted with a Gaussian function using the Levenberg–Marquardt method asFigs. 2 and 3, respectively. Figure 2 shows the horizontal profile of the beam from the lens-based collimator at the wavelength of 1250 nm at multiple distances from the collimator. It can be seen that for distances less than 0.3 m from the collimator the horizontal profile is Gaussian. However, at a distance of ~0.3 m, the profile begins to show signs of deviation from Gaussian. At a distance of ~0.5 m, the profile shows very distinctive, symmetric spikes. For the beam from the lens-based collimator, these features were prevalent at all wavelengths and became more distinct and larger in magnitude with increasing distance from the collimator, that is in the far field. The sharpness of the spikes and the fact that they appear in the far field indicate that they were not side lobes due to beam truncation by the lens but that the collimation lens imaged irregularities at the fiber termination into the far field. As we have characterized only a single supercontinuum light source with lens-based collimator, we do not know if these spikes are due to an imperfection of the fiber termination of the specific light source used in our study, or if they are systemic to supercontinuum light sources with lens-based collimators.
The horizontal radiance profile of the beam from the OAP mirror-based collimator is shown in Fig. 3 for a wavelength of 1250 nm and multiple distances from the collimator. Unlike the beam from the lens-based collimator, the beam from the OAP mirror-based collimator did not exhibit the spikes and remained Gaussian in shape at all wavelengths and distances.
To evaluate the collimation of the beams from the lens-based and OAP mirror-based collimator, it was assumed that the fiber is single mode at all wavelengths and thus the beam quality factor M2 is approximately equal to 1. With this assumption, the measured beam waist (see Tables 1 and 2) was plotted as function of distance from the collimator and fitted with Eq. (2) using 1/σ2 weighing (Figs. 4(a) and 4 (b)).Tables 3 and 4. Also evaluated was the far field divergence angle calculated from the spot size using Eq. (3):Table 2 and Fig. 5 it can be seen that the beam from the OAP mirror-based collimator is fairly well collimated at all wavelengths with a beam divergence ranging from 0.14 to 0.7 mrad. However the beam from the lens-based collimator shows apparent chromatic aberration, with reasonable collimation in the visible at wavelengths of 500 and 750 nm, maximum beam divergence around 1400 nm, and becoming reasonably collimated again around 2100 nm. We infer from theseobservations that the chromatic focal shift for the achromat doublet is near zero around 550 nm and 2100 nm. The apparent chromatic aberration for the OAP mirror-based collimator shown in Fig. 4(b) and in Table 3 is likely caused by the imperfections in mirror alignment.
We have characterized beam quality and collimation of commercially available supercontinuum sources with a commercial lens-based collimator and an off-axis parabolic mirror-based collimator. Our results show that the lens-based collimator produces a beam with significant chromatic aberration especially in the near-infrared, further away from the reasonably well-collimated visible wavelengths. Symmetric spikes surrounding the Gaussian peak from the lens-based collimator were observed in the far field. We speculate that these spikes may be due to an imperfection of the fiber termination (imaged into the far field) of the specific laser used in our study. The OAP mirror-based collimator produced a Gaussian beam at all distances and wavelengths characterized with good and fairly uniform collimation at all wavelengths, largely devoid of chromatic aberration.
In conclusion, we recommend supercontinuum light sources with simple lens-based collimators if the user application requires a reasonably well-collimated beam in the visible spectrum. However, if the application requires good collimation and uniform Gaussian beams over the whole spectrum, the use of achromatic OAP mirror-based collimators is of the essence.
This material is based upon work supported by the National Science Foundation under Grant No. AGS-1040046, by NASA EPSCoR under Cooperative Agreement No. NNX10AR89A, and by NASA ROSES under Grant No. NNX11AB79G.
References and links
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