Optical interconnection elements summary

The three classes of optical element most suitable for the implementation of optical routing and fanout are Dammann-like gratings, Fresnel zone plates and microlenses. Each class has many different structures and so we make an optimum choice of one example from each and compare them in the next section. The criteria for choosing these examples are as follows:

(a) large array generation - higher parallelism and SBWP which gives improved processing power

(b) Ease of manufacture - can either be fabricated in-house or cheap to purchase (c) Design flexibility - to suit the system requirements

(d) High efficiency - in terms of power (e) Good uniformity

The choices which satisfy the above conditions in each class are described as follows:

(a) Dammann-like phase gratings

Binary separable Dammann gratings are chosen because these gratings can generate a large array of spots (201 x 201 experimentally), they are easy to design (1 00 1 x 1001 array

generator has been designed) and they are simple to fabricate. The efficiency is about 65% and the uniformity is about ± 8%.

(b) Fresnel zone plate arrays

Simple spatial multiplexing of Fresnel zone plates is the best alternative to a Dammann grating for diffractive array generation. The Fresnel zone plates can have multi-phase levels or can be blazed and an 8-level structure with 91% efficiency is the most appropriate. The

fabrication of an array of 245 x 245 Fresnel zone plates has been reported.

(c) Refractive microlenses

Finally, photoresist reflow microlenses are the best candidates out of the various types of refractive optical elements. This is due to the versatility in design and a simple and easily accessible fabrication technique. Arrays of almost any size can be fabricated by this process. The limitations are the flexibility of the shape of the lens (which is formed by surface tension) and the size of the wafer that can be processed unifomly.

Correlator system : P relim inary sy stem design 100 4.5.3 Comparison between the CGHs and microlenses

The three choices are now compared and contrasted in the following parameters to find out which is most suitable for our system:

(a) Design

The design of microlenses (both refractive and diffractive Fresnel zone plates) are simple although Fresnel zone plates require a small computer program to generate them. On the other hand, the computer generation of a Dammann grating is more complicated. First, the parameters such as array size, efficiency and uniformity of spots must be specified. Then, root-finding and optimisation algorithms are used to generate the desired grating. This involves more computational time and cost.

(b) Imaging properties

Both types of microlens behave like conventional lenses in terms of imaging. However, the Dammann grating is designed to generate an array of spots and in general, conventional imaging cannot be implemented by this element.

(c) Efficiency

Refractive microlenses have a collection efficiency of 100 % if there is no dead space between the lenses ignoring Fresnel reflections. In the case of the binary, separable, Dammann grating, the diffraction efficiency is only 65%. Higher efficiency is possible by making a multi-phase structure. An 8-level Fresnel zone plate has an experimental

efficiency of 91%. There is also reflection loss at the air-photoresist interface for these elements. Application of anti-reflection coating on both computer generated holograms is straight forward because of the fiat profiles. However, it is more difficult to deposit a uniform layer on the surface-relief microlenses.

(d) Focal length and system size

The photoresist microlenses cannot have very long focal lengths because of the inability of the surface tension to pull a thin layer into a spherical shape. On the other hand, the Fresnel zone plates are best designed to have longer focal lengths because short focal lengths would

require very fine structures in FZPs. Therefore, the system is more compact if a refractive microlens array is used. The Dammann grating requires external components in a system (two Fourier transform lenses), thus, making the system very bulky. The exceptional case is the Fresnel-Dammann grating where the FT lens is incorporated in the design.

(e) Spot size

Both types of microlens can give diffraction limited spot sizes if there is little fabrication error. The size of the spots generated by a Dammann grating is related to the overall size of the element assuming accurate fabrication and to the FT lenses’ numerical aperture .

(f) Aberrations

The photoresist microlenses are mainly used for on-axis applications and off-axis uses often bring about large aberrations due primarily to field curvature. This is partly due to the relatively short focal lengths (low f-nos) inherent in these microlenses. Fresnel CGHs, on the contrary, can be designed to compensate for aberrations. Aberrations in a Dammann system comes from the FT lenses and fabrication tolerances.

(g) Fabrication

The fabrication techniques for refractive microlenses and Dammann gratings are the standard photolithographic processes. There are certain problems with the fabrication of Dammaim gratings. It is difficult to make accurate structures in glass whereas structures fabricated in photoresist may suffer from variations in refractive index and poor durability. The fabrication of multi-level Fresnel zone plates requires either the alignment of the masks or direct electron beam writing on the substrate. In all cases, once the master is made, copies can be reproduced very cheaply by moulding and embossing.

In addition, photoresist microlenses can be used in both coherent and incoherent systems whereas computer generated holograms are designed for a specific wavelength only. Refractive microlenses have, in general, less dispersion and higher numeral aperture which means smaller spot size. Fresnel zone plates can have 100% fill factor but the size is limited by the minimum feature size. In addition, the FZPs only work as ideal optical elements over small bandwidths. In a Dammann grating, uniformity of spots is independent

Correlator system : P relim in ary system design 102 of the profile of the incident beam. The advantages and disadvantages of the three optical elements are summarised in table 4.1.

Optical elements Advantages Disadvantages

Microlenses

- Less Dispersion

- Higher Numerical apertures - Simple to fabricate - Can work in coherent and

incoherent systems - Compact

- Efficient (100% if no dead space)

- Mainly on axis

- Power lost through dead space (circular aperture)

Fresnel zone plates

- Can design for off-axis (elliptical) - Can design to compensate for

aberrations -100% fill factor

- Poor efficiency (binary) or complicated fabrication

- Size is limited by smallest feature size

- Works as an ideal optical element only over small bandwidth - Use efficiently in coherent system

Dammann-like gratings

- Good for array generation

- Uniformity of spots is independent of profile of the incident beam

- Require Fourier lenses - Need lengthy computation for

simulated annealing - In glass - difficult to make

accurate structures

- In photoresist - lacks of uniformity in refractive index and poor durability - Use in coherent system only

Table 4.1 Comparison between computer generated holograms and microlenses

4.5.4 Conclusions

We conclude that if someway can be formed to reduce the aberration of refractive microlenses for off-axis operation, they would make ideal interconnection elements for our system. In fact we have found a way to do this by lengthening the focal length sufficiently as desribed in chapter 7. However, it is worth noting that perhaps the most overriding

argument in favour of them was that our commercial sponsor wanted to gain knowledge of how to fabricate them for other in-house applications and they had not been sufficiently impressed after testing several Dammann gratings. Cost will be cheap if manufacture in- house.

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