It's a tall order. Nevertheless, we at Intel's Photonics Technology Lab have been working on these building blocks for several years. One of our latest achievements, announced last February, is the world's first continuous all-silicon laser, which is based on the Raman scattering effect. Named for the Indian physicist Chandrasekhara Venkata Raman, who first described it in 1928, this effect causes light to scatter in certain materials to produce longer or shorter wavelengths.
Raman scattering is used today, for example, to boost signals traveling through long stretches of glass fiber. It allows light energy to be transferred from a strong pump beam into a weaker data beam. Most long-distance telephone calls today benefit from Raman amplification.
Typically, a Raman amplifier requires kilometers of fiber to produce a useful amount of amplification, because glass exhibits very weak scattering. Silicon, though, has a crystal structure, so its Raman scattering is more than 10 000 times as strong as that of ordinary glass fiber. In other words, you could achieve the same amplification in a centimeter-square chip that you'd get in kilometers of glass fiber.
In fact, so intense is the light amplification in silicon that it sets the stage for creating a laser. To build a Raman laser in silicon, you first need to create a conduit, also known as a waveguide, for the light beam. This can be done using standard CMOS techniques to etch a ridge or channel into a silicon wafer. In any waveguide, some light is lost through imperfections, surface roughness, and absorption by the material. The trick, of course, is to ensure that the amplification provided by the Raman effect exceeds the loss in the waveguide.
In mid-2004, we discovered that increasing the pump power beyond a certain point failed to increase the Raman amplification and eventually even reduced it. The culprit was a process called two-photon absorption, which caused the silicon to absorb a fraction of the pump beam's photons and release free electrons. Almost immediately after we turned on the pump laser, a cloud of free electrons built up in the waveguide, absorbing some of the pump and signal beams and killing the amplification. The stronger the pump beam, the more electrons created and the more photons lost.
Intel researchers found a way to flush out the extra electrons by sandwiching the waveguide within a device called a PIN diode; PIN stands for p-typeintrinsicn-type. When a voltage is applied, the free electrons move toward the diode's positively charged side; the diode effectively acts like a vacuum and sweeps the free electrons from the path of the light. Using the PIN waveguide, we demonstrated continuous amplification of a stream of optical bits, more than doubling its original power.
Once we had the amplification, we created the silicon laser by coating the ends of the PIN waveguide with specially designed mirrors. We make these dielectric mirrors by carefully stacking alternating layers of nonconducting materials, so that the reflected light waves combine and intensify. They can also reflect light at certain wavelengths while allowing other wavelengths to pass through.
With the Raman amplifier between the two dielectric mirrors, we had the basic configuration needed for a laser. After all, laser stands for "light amplification by stimulated emission of radiation," and that's what was going on in our device. Photons that entered were multiplied in number by the Raman amplifier. Meanwhile, as the light waves bounced back and forth between the two mirrors, they stimulated the emission of yet more photons through Raman scattering.
The photons stimulated in this way were in phase with the others in the amplifier, so the beam generated was coherent. Once the round-trip gain of photons in the cavity exceeded the round-trip loss, we observed a steady beam of laser light exiting the silicon chip. We had built the first continuous silicon laser [see illustration, " Let There Be Light"].
Beyond building the light source and moving light through the chip, you need a way to modulate the light beam with data. The simplest option is switching the laser on and off, a technique called direct modulation. An alternative, called external modulation, is analogous to waving your hand in front of a flashlight beam: blocking the beam of light represents a logical 0; letting it pass represents a 1. The only difference is that in external modulation the beam is always on.
For data rates of 10 Gb/s or higher and traveling distances greater than tens of kilometers, this difference is critical. Each time a semiconductor laser is turned on, it "chirps." The initial surge of current through the laser changes its optical properties, causing an undesired shift in wavelength. A similar phenomenon occurs when you turn on a flashlight: the light changes quickly from orange to yellow to white as the bulb filament heats up.
If the chirped beam is sent through an optical fiber, the different wavelengths will travel at slightly different speeds, which warps data patterns. When there's a lot of data traveling quickly, this distortion causes errors in the data.
With an external modulator, by contrast, the laser beam remains stable, continuous, and chirp-free. The light enters the modulator, which shutters the beam rapidly to produce a data stream; even 10-Gb/s data can be sent up to about 100 km with no significant distortion. Fast modulators are typically made from lithium niobate, which has a strong electro-optic effect--that is, when an electric field is applied to it, it changes the speed at which light travels through the material.
You start by splitting the laser beam in two and then applying an electric field to one beam. If the speed changes enough to delay the beam by half of one wavelength, that beam will be out of phase with its mate. When the beams recombine, they will interfere with each other and cancel out [see illustration, " Encoding Photons With Data"].
If, on the other hand, no voltage is applied, the beams remain in phase, and they will add constructively when recombined. Encoding the beam with 1s and 0s, then, means making the beams interfere (0) or keeping them in phase (1).
A silicon-based modulator, as mentioned before, has the disadvantage of lacking this electro-optic effect. To get around this drawback, we devised a way to selectively inject charge carriers (electrons or holes) into the silicon waveguide as the light beam passes through. Because of a phenomenon known as the free carrier plasma dispersion effect, the accumulated charges change the silicon's refractive index and thus the speed at which light travels through it. The silicon modulator splits the beam in two, just like the lithium niobate modulator. However, instead of the electro-optic effect, it's the presence or absence of electrons and holes that determines the phases of the beams and whether they combine to produce a 1 or a 0.
The trick is to get those electrons and holes into and out of the beam's path fast enough to reach gigahertz data rates. Previous schemes injected the electrons and holes into the same region of the waveguide. When the power was turned off, the free electrons and holes faded away very slowly; the maximum speed was about 20 megahertz.
Intel's silicon modulator uses a transistorlike device rather than a diode both to inject and to remove the charges. Electrons and holes are inserted on opposite sides of an oxide layer at the heart of the waveguide, where the light is most intense. Rather than waiting for the charges to fade away, the transistor structure pulls them out as rapidly as they go in. To date, this silicon modulator has encoded data at speeds of up to 10 Gb/s--fast enough to rival conventional optical communications systems in use today.
Once the beam is flowing through the waveguide, photodetectors are used to collect the photons and convert them into electrical signals. They can also be used to monitor the optical beam's properties--power, wavelength, and so on--and feed this information back to the transmitter, so that it can optimize the beam.
Silicon absorbs visible light well, which is why it appears opaque to the naked eye. Infrared, however, passes through silicon without being absorbed, so photons at those wavelengths can be neither collected nor detected.
This problem can be overcome by adding germanium to the silicon waveguides. Germanium absorbs infrared radiation at longer wavelengths than does silicon. So using an alloy of silicon and germanium in part of the waveguide creates a region where infrared photons can be absorbed. Our research shows that silicon germanium can achieve fast and efficient infrared photodetection at 850 nanometers and 1310 nm, the communications wavelengths most commonly used in enterprise networks today.