Micro-optics

Author: the photonics expert Dr. Rüdiger Paschotta

Micro-optics (or microoptics) is the field of optics dealing with particularly small optical components. The small physical dimensions have various implications concerning fabrication techniques, usable optical materials, relevant physical effects, performance limitations and the practical handling. The article is meant to provide an overview on such aspects.

Fabrication of Micro-optical Components

Some miniature optical components are essentially fabricated in similar ways as traditional large optical elements. For example, various types of optical lenses, prisms, mirrors, beam splitters, apertures, diffusers, diffraction gratings etc. are made with rather small dimensions of e.g. 2 mm, and may then be considered as micro-optics. (There is no generally agreed dimension limit for this area.) A common application area is in the context of laser diodes, where small beam collimator lenses are required.

On the other hand, particularly optical elements with sub-millimeter dimensions are often made with totally different fabrication techniques. These largely have their origins in fields like microelectronics and optoelectronics, and often involve wafer-based manufacturing. Much of the technology came from the field of optical data transmission. For example, optoelectronic chips may contain microlasers and tiny photodetectors, which creates a need to condition incoming and outgoing optical radiation (light, often including infrared light). Concrete examples are beam collimators for lasers and tiny lenses to obtain an improved and more directional sensitivity of photodetectors, or efficient coupling to fibers.

A common technique for producing microlenses on optoelectronic chips is photoresist reflow. Here, one starts with the deposition of a photoresist material on a typically circular area with a tiny diameter of e.g. some tens of microns. Subsequently, the whole device is heated to a temperature which lets the photoresist melt. Due to the surface tension, it then acquires a well defined lens surface with an approximately spherical curvature.

Note that such a kind of lens is not a separately used optical component, but becomes the inseparable part of some larger devices. That is a typical aspect of micro-optics: most micro-optical elements are not fabricated and sold separately, but rather fabricated as parts of some larger optical microsystems, usually containing multiple components. These can be combined with micro-electromechanical systems (MEMS), leading to micro-optoelectromechanical systems (MOEMS or optical MEMS), which combine optical, electronic and mechanical aspects.

Photoresist reflow is only one of many possible fabrication techniques. There are also various replication techniques, which are partly also used in similar forms for much larger optical elements. Here, one first fabricates a master structure and then uses that for mass production of micro-optical components with techniques like injection molding, hot embossing or UV casting.

Another possibility is microcontact printing, also called soft lithography. It is based on the application of lithography to optically transparent materials. The phenomenon of surface tension can again be used to obtain smoothly curved surfaces.

Where particularly high flexibility is needed, direct laser writing technology can also provide interesting solutions.

Many of the applied techniques are particularly suitable for mass production of micro-optical components in the form of large one- or two-dimensional arrays. For example, there are microlens arrays used for the collimation of light from many small light emitters, or for Shack–Hartmann wavefront sensors.

In some cases, a single microlens is applied to some other part, for example to the end of an optical fiber, to a single laser emitter or to a single photodiode. That may be done, for example with specialized laser-based processes.

Optical Materials

Partly, micro-optics depends on traditional optical materials such as optical glasses and polymers. However, fabrication techniques as mentioned above often favor other materials, which better fit into that context. Some examples:

• Photoresist reflow could obviously not be applied to traditional hard optical materials.
• For infrared applications, one also often uses semiconductors, which are well transparent for photon energies below the band gap energy and can be processed with well established wafer-based lithography techniques.
• Some devices are based on transparent liquids, and one exploits them e.g. for realizing tunable fluidic microlenses.

The latter example demonstrates that some of these materials would hardly be usable for large optics.

Principles of Operation

Many micro-optical utilize exactly the same foundations of physical optics as are used in large optical components – for example refraction or reflection of light. Here, however, the small dimensions often introduce serious performance limitations via diffraction. That is particularly critical for imaging applications. Being a very basic, unavoidable effect, diffraction rather than optical aberrations due to non-ideal surface shapes then usually set the performance limits.

In other cases, however, diffraction is utilized as the basic principle of operation. For example, there are purely diffractive lenses based on relief structures which are reminiscent of Fresnel lenses, but are substantially different from those e.g. in terms of the intrinsically strongly wavelength-dependent focal length. That approach is not usually used in macro-optics.

See the article on diffractive optics for more details.

Micromirrors essentially work based on the same principles as large mirrors; they usually have a reflective coating which is either metallic or a dielectric multilayer structure. However, they are made with other techniques, such as lithography, and are often combined with certain types of miniature actuators, which allows one to fine-adjust their orientation during operation, or to switch their position. For example, some micromirror arrays are used for miniature displays.

Integration of Micro-optics with Actuators

As mentioned above, many micro-optical elements are combined with miniature actuators in order to obtain additional functionalities like switching or continuously adjusting optical functions. Although that principle is also used in large optics, e.g. in zoom objectives, it is particularly practical for micro-optics:

• The required range of movement is often very small. That allows for the use of very simple types of actuators.
• If necessary, the effective precision can be greatly improved with feedback techniques, which may also be implemented on the same chips.
• Due to the small sizes and masses and the small travel ranges, the movements can be extremely fast. That is essential for some of the applications (see below).
• In contrast to large movable optics, the cost can be very low because the required actuators can be fabricated in economical ways, with parallel processing of many of them.

Characterization of Micro-optics

For testing and the characterization of micro-optics, one needs to apply adapted techniques because the techniques from classical large optics are often not applicable. Often, such techniques need to be custom-developed in conjunction with the used manufacturing system. For example, one requires inspection devices which can subsequently investigate a large number of devices which are made together on a common wafer.

Typical Applications of Micro-optics

Micro-optics are used in a wide range of application fields like telecommunications, lighting, laser technology and biomedical devices. Some typical purposes of the used devices are briefly explained in the following.

Beam Collimation and Focusing

Microlenses may be used for collimating light from tiny light emitters, and microlens arrays are naturally suited for the application to arrays of emitters. For example, such light emitters can be part of diode bars (one-dimensional arrays of broad-area emitters), VCSEL arrays, or arrays of waveguides on photonic integrated circuits.

Similarly, incoming light can be focused with microlenses. That is used for Shack–Hartmann wavefront sensors, as already mentioned above as an application of microlens arrays.

Micro-optics may also be used for the tailored correction of the deviation from ideal parameters. For example, certain devices are used for correcting the “smile” of laser diode arrays (diode bars) in order to achieve the maximum radiance (brightness). More complex devices are used for fiber coupling of high-power laser diodes.

Displays

Very compact displays can be realized based on digital micromirror devices. A DMD chip may contain over a million micromirrors, each of which can be individually addressed by the connected electronics, and is responsible for one pixel of the projection image, or for one color component of one pixel. The control is binary, i.e., a certain image point can only be switched on or off, but effectively one can obtain gray values by rapid switching between on and off, too fast for the human eye to resolve.

Micro-optics may also be used for achieving uniform background illumination of liquid crystal displays.

Lighting

Various types of lighting, e.g. with light-emitting diodes (LEDs), can also profit from tailoring the spatial light emission by using micro-optical elements. The optimization of light extraction in white-light (and other) LEDs (which is one of the cornerstones for reaching a high luminous efficacy) may also be regarded as an application of micro-optics.

Data Multiplexing and Switching

In optical fiber communications, one is often dealing with a large number of data channels. For example, one may use wavelength division multiplexing, where different data channels correspond to optical signal with different center wavelength. On may then need to separate such channels emerging from a single optical fiber in order to extract some of them and inject others before sending the signals into another fiber. For such purposes, micro-optical devices have been developed. They may, for example, involve arrays of movable micromirrors, with one mirror for each channel. Such developments have played in important role in initiating and driving forward the field of micro-optics.

Tuning of Parameters

For tiny wavelength-tunable lasers, for example made as external-cavity diode lasers, one requires compact tuning elements, as can be realized with micro-optics containing actuators.

In other cases, one may need to tune parameters like an optical path length in an interferometer with a movable micromirror, or the transmission of light through a tiny aperture of variable size.

Liquid micro-optics, typically in the form of tunable lenses, are increasingly used in such areas.

Cost Structure

The fabrication, testing and characterization of micro-optics generally needs sophisticated machinery, which often needs to be tailored to the specific devices to be made. The cost for developing, building and operating such machinery is usually rather high.

On the other hand, the device throughput can be very high, so that micro-optical elements produced in large quantities can have a rather low cost per device. This has led, for example, to compact displays which are suitable for consumer products, an area with very high cost pressure. One may conclude that micro-optics are particularly suitable for mass products, less for small quantities. In the latter area, however, flexible laser-based fabrication techniques are developed, which may economically work with moderate production volumes.

More to Learn

Encyclopedia articles:

• microlens arrays
• diffractive optics