Applications (Optical Properties of Materials) Part 8

Optical Modulators and Switches

When discussing electronic devices in Section 8.7.12, we encountered a digital switch that is capable of turning the electric current on or off by applying a voltage to the gate of a MOSFET. An equivalent optical device is obtained by making use of the electro-optical waveguide (Fig. 13.56). In the present case, this device is biased initially just below the threshold, i.e., at a voltage which barely prevents the lowest-mode optical wave from passing. Then, by an additional voltage between metal and substrate, the EOW becomes transparent. In analogy to its electrical equivalent (Fig. 8.30), this device may be called an enhancement-type or normally-off electro-optical wave-guide. By varying the bias voltage periodically above the threshold, the EOW can serve as an effective modulator of light.

A depletion-type or normally-on EOW can also be built. This device exploits the Franz-Keldysh effect, i.e., the shift of the absorption edge to lower energies when an electric field is applied to a semiconductor (Fig. 13.57). The photon energy of the light is chosen to be slightly smaller than the band gap energy (dotted line in Fig. 13.57). Thus, the semiconductor is normally in the transparent mode. If, however, a large electric field (on the order of 105 V/cm) is applied to the device, then the band gap shifts to lower energies and the absorbance at that particular wavelength (photon energy) becomes several orders of magnitude larger, thus essentially blocking the light. (The Franz-Keldysh shift can be understood when inspecting Fig. 8.15, which shows a lowering of the conduction band and thus a reduction in the band gap energy when a reverse bias is applied to a semiconductor.)

Figure 13.57. Schematic representation of the Franz-Keldysh effect.

Finally, if a piezoelectric transducer imparts some pressure on a waveguide, the index of refraction changes. This photoelastic effect can also be utilized for modulation and switching.

Electro-optical modulators can be switched rapidly. The range of frequencies over which the devices can operate is quite wide.

Coupling and Device Integration

We now need to discuss some procedures for transferring optical waves (i.e., information) from one optical (or optoelectronic) device to the next. Of course, butting, i.e., the end-on attachment of two devices, is always an option, particularly if their cross-sectional areas are comparable in size. This technique is indeed frequently utilized for connecting optical fibers (used in long-distance transmission) to other components. A special fluid or layer which matches the indices of refraction is inserted between the two faces in order to reduce reflection losses. Optical alignment and permanent mechanical attachment are nontrivial tasks. They can be mastered, however. In those cases where no end faces are exposed for butting, a prism coupler may be used. This device transfers the light through a longitudinal surface. In order to achieve low-loss coupling, the index of refraction of the prism must be larger than that of the underlying materials. This is quite possible for glass fibers (n « 1.5) in conjunction with prisms made out of strontium titanate (n — 2.3) or rutile (n — 2.5), but is difficult for semiconductors (n « 3.6).

Phase coherent energy transfer between two parallel waveguides (or an optical fiber and a waveguide) can be achieved by optical tunneling (Fig. 13.58). For this to occur, the indices of refraction of the two waveguides must be larger than those of the adjacent substrates. Further, the width of the layer between the two waveguides must be small enough to allow the tails of the energy profiles to overlap.

Figure 13.58. Schematic representation of energy transfer between two waveguides (or a waveguide and an optical fiber) by optical tunneling.

The most elegant solution for efficient energy transfer is the monolithic integration of optical components on one chip. For example, a laser and a waveguide may be arranged in one building block, as schematically depicted in Fig. 13.59. Several points need to be observed however. First, the wavelength of the light emitted by the laser needs to be matched to a wavelength at which the absorption in the waveguide is minimal. Second, the end faces of the laser need to be properly coated (e.g., with SiO2) to provide adequate feedback for stimulated emission.

Another useful integrated structure involves a transverse photodiode that is coupled to a waveguide, see Fig. 13.60. As explained in Section 8.7.6, this photodiode is reverse biased.

Figure 13.59. Schematic representation of a monolithic laser/waveguide structure.

Figure 13.60. Schematic representation of a monolithic transverse photodiode/waveguide structure. A wide depletion layer (active region) is formed in the n-region by the reverse bias.

The electron-hole pairs are created in or near a long and wide depletion layer by photon absorption. The losses are minimized owing to the fact that the light does not have to penetrate the (inactive) p-region as in flat-plate photovoltaics. The quantum efficiency of the transverse photodiode can be considerably enhanced by increasing the length of the depletion layer.

All taken, the apparently difficult task of connecting optical fibers, waveguides, lasers, or photodetectors and their integration on one chip have progressed considerably in the last decade and have found wide application in a multitude of commercial devices.

Energy Losses

Optical devices lose energy through absorption, radiation, or light scattering, similarly as the electrical resistance causes energy losses in wires, etc. The optical loss is expressed by the attenuation (or absorbance), a, which was defined in Section 10.4. It is measured in cm—1 or, when multiplied by 4.3, in decibels per centimeter.

Scattering losses take place when the direction of the light is changed by multiple reflections on the "rough" surfaces in waveguides or glass fibers, or to a lesser extent by impurity elements and lattice defects.

Absorption losses occur when photons excite electrons from the valence band into the conduction band (interband transitions), as discussed in topic 12. They can be avoided by using light whose photon energy is smaller than the band gap energy. Free carrier absorption losses take place when electrons in the conduction band (or in shallow donor states) are raised to higher energies by intraband transitions. These losses are therefore restricted to semiconductor waveguides, etc., and essentially do not occur in dielectric materials. We know from (10.22) that the absorbance, a, is related to the imaginary part of the dielectric constant, e2, through

On the other hand, the free electron theory provides us with an expression for e2 (11.27), which is, for

where

(see (11.8)) and

(see (11.23)) and

(see (8.13)). Combining equations (13.26) through (13.30) yields

We note in (13.31) that the free carrier absorbance is a linear function of Nf and is inversely proportional to the mobility of the carriers. The absorbance is also a function of the square of the wavelength.

Radiation losses are, in essence, only significant for curved-channel waveguides, in which case photons are emitted into the surrounding media. A detailed calculation reveals that the radiation loss depends exponentially on the radius of the curvature. The minimal tolerable radius differs considerably in different materials and ranges between a few micrometers to a few centimeters. The energy loss is particularly large when the difference in the indices of refraction between the waveguide and the surrounding medium is small.

Photonics

A short note on the recently coined term "photonics" shall be added. Electronics deals with electrons and materials in which electrons propagate. Similarly photonics relates to photons and their interaction with photonic crystals. These crystals are materials that possess a periodicity of the dielectric constant so that they can affect the properties of photons in much the same way as electrons are affected by periodically arranged atoms, that is, by the lattice structure. However, photonic crystals need to be created artificially. The "lattice constant" of photonic crystals must be comparable to the wavelength of light, that is, the periodicity needs to be on the order of 500 nm. This requires high-resolution microlithography techniques, as known from semiconductor processing, involving X-rays or electron beams.

The solution of the Maxwell equations for this particular case (rather than the Schrodinger equation) leads to photonic band structures, Brillouin zones, and occasionally to band gaps quite similarly as known from electronics. Rather than displaying s- or p-bands, photonic band structures contain transverse magnetic (TM) or transverse electric (TE) modes. Doping can be accomplished by introducing point defects that affect the periodicity of the photonic crystal. This leads to localized photonic states within the gap similar to donor or acceptor states. Furthermore, a line defect acts like a waveguide and a planar defect behaves like a mirror. Photonic band structures are quite similar to phononic band structures (see topic 20.2) and, naturally, to electronic bands.

The research results of this field should be followed with considerable anticipation.

Optical Fibers

We have discussed in Sections 13.7, 13.8.10, 13.8.11, and 13.9 some fundamentals for the understanding of optical fibers. In the present section we summarize the information given before and supplement it with further details, in particular pertaining to materials for telecommunication. The crucial goal in telecommunications is to achieve a low attenuation of the transmitted signal. One of the methods to obtain this is by doping a silica fiber with germanium dioxide. This yields an energy loss of the light by only about 2 dB/km, which is considerable less than for copper cables. As a consequence, repeaters (amplifiers) can be distanced as far as 70-150 km (43-93 miles) from each other. Moreover, the erbium-doped fiber amplifier (Section 13.8.11) which utilizes a travelling-wave laser, involving stimulated emission, improves this distance by eliminating the transfers between a weak optical signal, to an electrical signal, and again to an enhanced optical signal. Optical fibers are not susceptible to electrical interference, wire tapping, cross talk between signals, laser-induced optical damage, and pick-up of environmental noise. Glass fibers are light in weight, and do not require much space. Fibers made of silica (doped or undoped) are therefore mainly used for long-distance, terrestrial transmissions of signals. On the other hand, fibers made of photonic-crystals (see Section 13.9.6) in which the light is guided by means of diffraction through a "lattice", entailing a periodic dielectric constant, have also been developed. They can carry a higher power than the fibers just discussed.

As shown in Fig. 13.30 commercial optical fibers have a minimum in energy loss around 1.31 and 1.55 mm. These IR "windows" are mainly used for communication purposes.

As already mentioned previously, each individual optical fiber is able to carry a large number of "channels" using different wavelengths, each of which can be modulated typically with about 40 Gb/s of information (multiplexing). This allows billions of simultaneous telephone calls.

An optical fiber consists of highly purified silica (doped or undoped), or a phosphosilicate core (about 8 to10 mm in diameter) which is surrounded by a borosilicate cladding of 125 mm in diameter, whereby the index of refraction of the core (nco = 1.48) is slightly larger than that of the cladding (ncl — 1.46). Because of this difference, the propagation of light within the core occurs under certain circumstances by internal (total) reflection. There exists a critical angle, aT, above which total reflection takes place, (see Footnote 10 in Section 13.9.1). Thus, the light has to impinge under a minimum angle, aT, onto the face of the core, called acceptance cone. The critical angle is determined by the ratio of the refractive indices between core and cladding (Footnote 10).

Irregular (rough) surfaces cause scattering of light and thus, some loss of light energy. The cladding is coated on the outside with a ~250 mm, tough, resin buffer which adds strength to the fiber. Finally a ~400 mm thick jacket serves as protection against mechanical abuse. Fibers are connected (spliced) to each other by arc-melting to fuse the ends together, or by special connectors. Both techniques yield some loss in energy (about 0.1 dB) and are by no means trivial tasks, compared to connecting two wires.

It should be added in closing that optical fibers are also used for medical applications (gastroscopes, endoscopes, minimally invasive surgery), for remote sensors, and for illumination purposes.

Optical Storage Devices

Optical techniques have been used for thousands of years to retrieve stored information. Examples are ancient papyrus scrolls or stone carvings. The topic you are presently reading likewise belongs in this category. It is of the random-access type.Other examples of optical storage devices are the conventional photographic movie film (with or without optical sound track) or the microfilm used in libraries. The latter are sequential storage media because all previous material has to be scanned before the information of interest can be accessed. They are also called read-only memories (ROM) because the information content cannot be changed by the user. All examples given so far are analog storage devices.

Another form of storage utilizes the optical disk, which has gained widespread popularity. (Specifically, 200 billion CDs (compact disks) have been sold worldwide in 2007, even though MP3 and other flash memories have cut into the CD market.) Here, the information is stored in digital form. The most common application, the just mentioned compact disk is a random-access, read-only memory (ROM) device. However, "write-once, read-many" (WORM) and erasable magneto-optical disks (Section 17.5) are also available. Further, rewritable CD-RW disks are on the market, see below. The main advantage of optical techniques is that the readout involves a noncontact process (in contrast to magnetic tape or mechanical systems). Thus, no wear is encountered. Moreover, all optical storage devices are of the non-volatile type, that is, the information is retained without maintaining a voltage.

Let us now discuss the optical compact disk. Here, the information is stored below a transparent, polymeric medium in the form of bumps, as shown in Fig. 13.61. The height of these bumps is one-quarter of a wavelength (1/4) of the probing light. Thus, the light which is reflected from the base of these bumps (called the "land") travels half a wavelength farther than the light reflected from the bumps. If a bump is encountered, the combined light reflected from bump and land is extinguished by destructive interference. No light may be interpreted as a zero in binary code, whereas full intensity of the reflected beam would then constitute a one. (Actually, the bumps and lands themselves do not immediately represent the zeros and ones. Instead, a change from bump to land or land to bump indicates a one, whereas no change constitutes a zero.) Sixteen ones and zeros represent one byte of data. For audio purposes, the initial analog signal is sampled at a frequency of 44.1 kHz (about twice the audible frequency) to digitize the information into a series of ones and zeros (similarly as known for computers, Section 8.7.12). Quantization of the signal into 16-digit binary numbers gives a scale of 216 or 65,536 different values. This information is transferred to a disk (see below) in the form of bumps and absences of bumps. For readout from the disk, the probing light is pulsed with the same frequency so that it is synchronized with the digitized storage content.

The spiral path on the useful area of a 120 mm diameter CD is 5.7 km long and contains 22,188 tracks spaced 1.6 mm apart. (As a comparison, 30 tracks can be accommodated on a human hair.) The spot diameter of the readout beam near the bumps is about 1.2 mm.

Figure 13.61. Schematic of a compact disk optical storage device. Readout mode. (Not drawn to scale.) The reflected beams in Fig. 13.61(b) are drawn under an angle for clarity. The land and bump areas covered by the probing light have to be of equal size in order that destructive interference can occur (see the hatched areas covered by the incident beam in Fig. 13.61(b)).

The information density on a CD is 800 kbits/mm2, i.e., a standard CD can hold about 7 x 109 bits. This number increases by a factor of ten when blue emitting lasers (1 — 405 nm, blue-ray format, see below) are used. The current playback time is about 80 minutes.

The manufacturing process of CDs requires an optically flat glass plate which has been covered with a light-sensitive layer (photoresist) about 1/4 in thickness. Then, a helium-neon laser whose intensity is modulated (pulsed) by the digitized information is directed onto this surface while the disk is rotated. Developing of the photoresist causes a hardening of the unexposed areas. Subsequent etching removes the exposed areas and thus creates pits in the photoresist. The pitted surface is then coated with silver (to facilitate electrical conduction) and then electroplated with nickel. The nickel mold thus created (or a copy of it) is used to transfer the pit structure to a transparent polymeric material by injection molding. The disk is subsequently coated with a reflective aluminum film and finally covered by a protective lacquer and a label.

The CD is read from the back side, i.e., the information is now contained in the form of bumps (see Fig. 13.61). In order to facilitate focusing onto a narrow spot, monochromatic light, as provided by a laser, is essential. At present, a GaAlAs heterojunction laser having a wavelength in air of 780 nm is utilized. The beam size at the surface of the disk is relatively large (0.7 mm in diameter) to minimize possible light obstruction by small dust particles. However, the beam converges as it traverses through the polymer disk to reach the reflecting surface that contains the information. Small scratches or fingerprints on the polymer surface are also tolerated quite well, but large scratches and blemishes make the CD useless. The aligning of the laser beam on the extremely narrow tracks is a nontrivial task, but it can be managed. It involves, actually, three light beams, obtained by dividing the impinging laser beam shown in Fig. 13.55(b) into three parts, utilizing a grating or a holographic element. One of these parts (the center one) is the above-described read beam. The other two are tracking beams which strike the inner and outer edges of the groove. The reflected signals from the tracking beams are subtracted from each other. A null signal indicates correct tracking while positive or negative signals cause the servo to move the read head to one or the other side. The tracking is accurate to about 0.1 mm.

The recordable compact disk (CD-R) contains a blank data spiral. During manufacturing, a photosensitive dye is applied before the metallization is laid down. The write laser of the CD recorder changes the color of the dye and thus, encodes the track with the digital data. This type of storage may undergo some degradation. Indeed, after a lifetime of 20 to 100 years (in some cases only 18 months, depending on the quality of the CD) the dye degrades, which is called "CD rot". A CD-R can be encoded only once.

The rewritable compact disk (CD-RW) utilizes the amorphous to crystalline transformation technique which we have discussed in detail at the end of Section 8.7.12. In short, a transformation between the two phases of chalcogenide glasses is caused by a writing laser beam which emits short (ns) pulses to the track. Some crystalline and amorphous chalcogenides have pronounced different indices of refraction and thus, differ in their reflectivity which can be utilized to distinguish between the ones and zeros. The estimated lifetime is considerably higher than for CD-Rs, (i.e. nominal 300 years).

The DVD-ROM (digital versatile disk or digital video disk-read only memory) and the DVD-RW (rewritable) work on the same phase transformation principal as just discussed. DVDs utilize a laser diode whose emission wavelength is shorter than for a CD, namely 650 nm. This allows a smaller width between bumps of 0.74 mm (compared to 1.6 mm for CDs, see Fig. 13.61) and thus, adds more storage capacity. A writing speed of 1x stores 1.35 MB/s. Recent models use writing speeds 18 or 20 times as fast. However, dual layer disks run at lower recording speeds. DVD-R and DVD + R have slightly different storage abilities, specifically 4.707 and 4.700 GB respectively in their single layer versions (and almost twice as much in their double layer rendition). Rewritable DVDs have a storage capacity of about 4.7 GB (single-sided, single layer), 8.5 GB (for single-sided double layer), and 9.4 GB (double sided, single layer) in contrast to the CD which stores up to 700 MB. Dual-layer disks employ a second film underneath the first one which is accessed by transmitting the laser light through the first, transparent layer.

The blue-ray disk (BD or BRD) utilizes a 405 nm light beam from a GaN laser and thus, allows focusing the beam to even smaller spots. As a consequence, almost 10 times more data can be encrypted than for a DVD. Specifically, a BD can store 25 GB on a single layer, 12 cm disk and 50 GB using double layer technology. Moreover, four-layer (100 GB) and even 16 data layers yielding 400 GB have been demonstrated with the goal to reach eventually a 1 TB blue-ray disk! Its main application is for high-definition videos and for video games. Since the data layer in blue ray disks is much closer to the surface than for DVDs, which makes the disks more vulnerable to scratches, several hard coating polymers have been developed and applied by different companies. The driving speed at 36 Mbit/s requires a writing time of 90 minutes on a single layer disk. This writing time can be reduced to only 9 minutes when the driving speed is increased by a factor of 10. Blue ray technology is, as of this writing, still in its development phase and there is no indication that it will substantially replace standard DVDs anytime soon mostly because of price and the fact that most users are satisfied with the present DVD technology. Still, sales of software on blue-ray disks amounted to 177 million pieces in 2009. A nuisance is industry’s implementation of regional codes for blue-ray (as well as for DVD) players in order to allow playing disks only in certain geographical areas of the world. (Third-party shops make alterations to players to overcome this problem.) Blue-ray disk recordable (BD-R) can be written once, whereas BD-RE can be erased and re-recorded several times.

A future technology is called holographic versatile disk which is predicted to hold eventually 3.9 TB of information.

The durability of the stored information on DVDs and similar disks is determined, among others, by the sealing method, the storage practice, and where it was manufactured. The predictions vary, as already outlined above. Some manufacturers forecast lifetimes between 2 and 15 years, whereas others claim lifetimes from 30 to 100 years and even longer. This compares to the lifetimes of ancient papyrus scrolls which still can be read after more than 2000 years.

An alternative to the above-described devices is the magneto-optical device, which employs a laser to read the data on the disk while the information is written by simultaneously exposing a small area on the disk to a strong laser pulse in addition to a magnetic field. This device will be further described in Section 17.5. As of this writing, 4.6 GB can be stored on a 5| inch (130 mm) magneto-optical disk. The data can be erased and rewritten many times.