1.1

ALASCA

The ALASCA (Advanced Laser guide star Adaptive optics for Satellite Communication Assessments) project aims to develop a reliable optical communication uplink from an optical ground station (OGS) to communication satellites. The ALASCA main objective is the demonstration of a fully fledged, TRL6 operational facility that will enhance the performance of Optical Feeder Links (OFL) at the ESA Optical Ground Station in Tenerife, Spain based on Laser Guide Star Adaptive Optics technology. The project is based on the development of an upgrade of the hardware and software of the CaNaPy test facility jointly developed by INAF, Durham University and ESO.

The ALASCA laser propagation system (for both guidestar laser and infrared laser) will be designed following a versatile approach. A reconfigurable launch system will allow monostatic propagation through the entire 1-m aperture, and side launch using a smaller area of the OGS primary mirror.

The monostatic propagation of any laser through a telescope with a considerably large secondary mirror imposes an additional challenge for the system; the OGS secondary mirror would introduce significant central vignetting losses on the laser beams. Fortunately, an axicon-based laser shaping system can optimise the laser profile and convert the usual Gaussian into an annulus for efficient coupling to the OGS telescope.

1.2

The Axicon

Axicons are refractive optical elements with a flat front surface and a conical rear surface. The rays near the edge of the beam entering the axicon get located at the inside edge of the annular beam when exiting. Likewise, the rays at the centre of the incident beam get located around the edge of the annular beam when exiting.

The axicon was first introduced by John H. McLeod, 1954¹ as an optical element to generate zero-order Bessel beams, a beam comprised of rings equal in power to one another. The single axicon is illustrated in Figure 1(a). The output Bessel beam is defined by the outer diameter (Eq. 1) and the annulus thickness (Eq. 2), as a function of the axicon cone angle (α), the input diameter (d_in) and the distance to the image or to a second optical element (L) (Figure 1(b)).

Figure 1.

Schematic of the single axicon (a) and the axicon pair (b)

where L is the spacing between the two optical elements, n is the refractive index of the material, and α is the cone angle.

where d_in is the diameter of the input beam.

When the beam exits the first axicon and illuminates the second axicon (in opposite direction with respect to the first one), the rays become parallel and a collimated annular beam is generated as the output of the axicon pair. The annulus width remains constant within the propagation distance and it is just related to the diameter of the input beam, the axicon cone angle, and the spacing between the two elements.

Single axicons have been previously used in various applications like imaging with extended depth of field,² laser glass cutting,³ or fluorescence microscopy,⁴ among others. When combined with a second axicon element, double axicons can also be found in corneal surgery procedures⁵ and optical trapping systems.⁶

The CaNaPy system already integrates the axicon pair as a laser shaping system for the guidestar laser and the communications laser. The CaNaPy axicon subsystem receives the free-space propagated laser and produces a collimated annulus whose obscuration ratio is equivalent to the OGS obscuration ratio (35%). The annulus is relayed by the upstream optical elements until the telescope output.

2.1

Light Distribution Analysis

The axicon pair delivers a collimated zero-order Bessel beam defined by an output diameter of 13.5 mm and an obstruction ratio of 0.35 (between the diameters of obscured and illuminated areas). The obstruction ratio matches the OGS ratio between primary and secondary mirrors. Figure 3 shows the light distribution created by the axicon pair with an input gaussian beam of diameter 8.77 mm at the 2% intensity level.

Figure 3.

Light distribution of the annulus generated by the axicon pair from a collimated gaussian beam.

2.2

Wavefront Analysis

Zemax OpticStudio uses ray tracing to simulate the light propagation through the optical system. The wavefront calculation is based on a collection of rays travelling through the optical element to the image plane sampling the entire pupil. The axicon central area (also known as dead zone^*) generates a discontinuity in the ray tracing and thus, in the wavefront computation. In order to perform the wavefront error analysis, a negligible decentre of 0.1μm needed to be introduced to avoid the chief ray encountering this zone.

The wavefront error of an ideally aligned axicon (besides the trivial decentre) is shown in Figure 4 for the 1075-nm wavelength. No effects on the wavefront are visible at the output of the axicon pair.

Figure 4.

Wavefront map at the output of the axicon pair (wavelength = 1075 nm)

2.3

Thermal Behaviour

A thermal analysis has been performed over the operating range of -15°C to 20°C as per the expected operating temperature range. An aluminium housing has been considered the worst-case scenario (CTE = 23.5 x 10⁻⁶/°C) to perform the thermal study.

Results are shown in Figure 5. The maximum detected change as a result of the temperature variation in the axicon spacing is 24 microns which would only change the obscuration ratio by 4 x 10⁻⁴. Therefore, no athermalization of the final system is required.

Figure 5.

Thermal effects on the spacing between axicons (L) with Aluminium 6061 housing (CTE = 23.5 x 10⁻⁶/°C).