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Photonic Integrated Circuits (PIC)

This is a continuation from the previous tutorial - optical amplifiers.


During the 1980s and early 1990s, there has been a significant number of developments in the technology of optical and electronic integration of semiconductor lasers and other related devices on a single chip.

These chips allow higher levels of functionality than that achieved with single devices. For example, lasers and electronic drive circuits have been integrated, serving as simple monolithic lightwave transmitters.

Similarly, optical detectors have been integrated with amplifier circuits based on field-effect transistors (FETs) or heterojunction bipolar transistors (HBTs). Such integrated devices serve as the front end of a receiver.

In addition to being a technological achievement in the area of photonics, optoelectronic integration is expected to bring down the system cost.

The name photonic integrated circuit (PIC) is generally used when all the integrated components are photonic devices, e.g., lasers, detectors, amplifiers, modulators, and couplers.

The name optoelectronic integrated circuit (OEIC) is used when the components are a combination of photonic and electronic devices.



Photonic Integrated Circuits

1. Arrays

The simplest of all photonic integrated circuits are one-dimensional arrays of lasers, LEDs. or photodetectors. These devices are fabricated exactly the same way as individual devices except the wafers are not scribed to make single-device chips but left in the form of a bar.

A schematic of a laser array is shown in Figure 12-1. Individual elements are isolated by etching V-shaped grooves between the emitters.


Figure 12-1.  Schematic and photograph of an independently addressable edge-emitting laser array consisting of covered-mesa buried-heterostructure (CMBH) lasers.


Arrays of lasers or LEDs can be used as sources for dense parallel optical interconnection, a technology finding application in the next generation of computing and switching systems.

A necessary requirement for the array technology is low power consumption and the absence of cross talk between the individual elements of the array. The former requirement arises from the limited heat-transfer capacity of the transmitter and receiver module under typical operating conditions and environment.

The power consumption problem is considerably reduced if low-threshold lasers are used. Such lasers (threshold current ~ 1 mA) have been fabricated both for the AlGaAs and the InGaAsP material systems. In both cases, the lasers have multiquantum-well (MQW) active regions, short cavity lengths and high-reflectivity facet coatings.

Laser and detector arrays have been fabricated such that they exhibit no cross talk up to data rates of 1.4 Gb/s.

LEDs and photodiodes have been packaged into 1x12 arrays using a 12-fiber bundle and Si V-grooves for alignment. The fiber bundles terminate in connectors which consist of Si V-grooves with a center-to-center spacing of 250 μm.

A schematic of an LED and a laser-array package is shown in Figure 12-2.


Figure 12-2. Schematic illustrations of (a) an LED and (b) an edge-emitting laser-array package used in a parallel optical interconnection. The package uses a 60- μm core 1x12 multimode fiber bundle which is connectorized to V-grooves in a block of Si. 


For two-dimensional emitter arrays, the surface-emitting laser technology is a natural extension of the surface-emitting LED-array technology and provides higher speed and power levels. Independently addressable two-dimensional arrays of surface-emitting lasers have been reported.


2. Integrated Laser Detector

Considerable attention has been paid to developing integrated laser-detector structures. In many cases, the purpose of the detector is to serve as a monitor for the output power of the laser. The output of the monitor can then be used to stabilize the power of the laser, if it drifts due to aging or temperature change, using a feedback circuit.

A typical integrated laser-detector structure is shown in Figure 12-3. The emitting region of the laser and the absorbing region of the photodiode are composed of the same material. The detector monitors the back-facet power of the laser. In this structure the laser has one cleaved and one etched facet.


Figure 12-3.  Integrated edge-emitting laser and a back-facet monitoring detector. The laser has one etched facet and another cleaved facet.


For DFB lasers, the power outputs of the two facets do not necessarily track. Hence the implicit assumption made in the structure of Figure 12-3, that the back-facet output is a good measure of the front-facet output, is not necessarily true for a DFB laser.

It is possible to fabricate an integrated DFB laser-detector which allows a front-facet monitor. In this structure (see Figure 12-4), a Y-junction waveguide is fabricated near the end of a DFB laser. One branch of the Y has a monitor photodiode and the second branch serves as the output. The laser output and the monitor photodiode characteristics of this device are shown in Figure 12-5.


Figure 12-4.  Integrated DFB laser-photodiode structure. The laser and the photodiode are coupled using a Y-branching waveguide. The photodiode monitors the output of the front facet.



Figure 12-5.  The laser output and the monitor photodiode output plotted as a function of laser current.


Photodiodes have also been integrated with a surface-emitting laser (SEL). The latter being planar lends itself more easily to integration. The device structure is shown in Figure 12-6.

All layers are grown by the MBE technique. The photodiode mesa is first etched and a self-aligned SEL is then fabricated using ion implantation for current confinement. Since a fraction of the emitted light is absorbed by the photodiode, the photocurrent is a good measure of the light output.


Figure 12-6.  Integrated surface-emitting laser and photodiode.



3. Integrated Laser Modulator

Most commercial lightwave systems use a directly modulated laser diode as the source of information transfer.

Under direct modulation, the 3-dB spectral width of a single-wavelength DFB laser is ~ 0.1 nm. This finite spectral width results in a dispersion penalty due to chromatic dispersion of the fiber and limits the transmission distance.

This problem can be partially solved if an external modulator is used to modulate the output of a continuously operating laser. For long-distance transmission, externally modulated lasers are needed. Such externally modulated sources use a CW DFB laser, the output of which is coupled to a LiNbO3 external modulator.

It is desirable to have both the laser and the modulator integrated on the same chip. Although several types of modulation schemes using III-V materials exist, the most common type is the electro-absorption modulator.

In this type of modulator, a change in absorption is produced by a change in electric field. The change in absorption at a given wavelength (close to the band gap) occurs due to the Franz-Keldysh effect, according to which the band gap decreases with increasing electric field.

The schematic of a DFB laser integrated with an electro-absorption modulator is shown in Figure 12-7. The fabrication of the device involves first the growth of the layers of the DFB laser on a planar wafer, then etching using a dielectric mask, followed by growth of the layers of the modulator.

Note that the band gap of the absorbing layer of the modulator is larger than that of the emitting layer of the laser. This is necessary so that the modulator is nonabsorbing at zero field. With increasing applied field, the band gap of the absorbing layer decreases which decreases the transmission through the modulator.


Figure 12-7.  Integrated DFB laser and electro-absorption modulator.



4. Integrated Laser Amplifier

Integration schemes in which the modulator is an amplifier as opposed to an absorber have been reported. Integrated DFB laser-amplifier structures also have very low spectral width under modulation. The schematic of such a structure is shown in Figure 12-8.

Both the DFB laser and the amplifier have an MQW active region. The laser section has a grating etched on the substrate for frequency-selective feedback. The Fe-doped InP between the laser section and the amplifier section provides good optical coupling and electrical isolation between the sections.

For low-chirp operation, the laser is modulated with a small current (~ 5 mA) and the amplifier is CW biased. It is the small modulation current that controls the chirp in this device. The amplifier is used to increase the output power to a level that is obtained with high modulation current.


Figure 12-8.  Integrated DFB laser and optical amplifier.


Another interesting PIC is a series of amplifiers connected in tandem with a laser. A schematic of such a PIC is shown in Figure 12-9. In this device, light is emitted normal to the surface of the grating.

The first grating-laser combination acts as an oscillator. A fraction of the light propagates along the waveguide and injects into the next amplifier and so on.

The device emits in a series of beams normal to the wafer. The emission is in a single wavelength and all the emitters are coherent with respect to each other. These array devices were discussed in the laser arrays tutorial.


Figure 12-9.  A number of DBR lasers and an amplifier connected in series.


Integration of lasers, amplifiers and DBR gratings of various types has been achieved. Some of these devices emit high power normal to the surface and others emit normal to a cleaved edge.



5. Heterodyne Receiver

Perhaps the most complicated PIC reported so far is a heterodyne receiver for coherent transmission. The essential elements of such a receiver are the local-oscillator laser, the optics for mixing local-oscillator light with incident signal, and the balanced receiver.

The receiver PIC fabricated by Koch et al. is shown in Figure 12-10. The device has a tunable DBR laser, which serves as the local oscillator; several branching waveguides, for mixing and splitting the signal; and two photodiodes, which serve as balanced receivers.


Figure 12-10.  Schematic of a heterodyne receiver photonic integrated circuit.



The next tutorial discusses about optoelectronic integrated circuits (OEICs).

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