Bell Labs Claims Laser First
It's called a "supercontinuum" laser, and it delivers a broad band of wavelengths simultaneously. This should not be confused with widely tunable lasers, which also offer a broad band of wavelengths, but only make one of them available at a time. Tunable lasers promise to reduce inventory costs by making it possible to use one type of laser instead of many different types throughout a DWDM system, whereas a single supercontinuum source could be used to replace all the lasers in a system.
And this idea isn't just pie in the sky. Last year, researchers from NTT Electronics Corp. (NEL) reported an experiment that used a single light source to supply all 106 channels in a DWDM system (see CLEO Report: Pick of the Papers).
However, NEL's light source was a complicated configuration involving a pulsed laser diode, an erbium-doped fiber amplifier (EDFA), and a piece of polarization-maintaining fiber. In the case of Bell Labs' laser, only the semiconductor laser diode is required, and it emits continuous – rather than pulsed – light.
Claire Gmachl and her co-workers report their results in today's issue of Nature (issue 415, p.883).
Gmachl has already made a bit of a name for herself for her work on the so-called "quantum cascade" laser. Invented in 1994 by Bell Labs researchers Federico Capasso (also an author on today's Nature paper) and Jerome Faist, quantum cascade (QC) lasers are designed in a completely different way from ordinary diode lasers. Indeed, the new laser unveiled today is a variation on a QC laser.
To understand how Gmachl's team made the new device, it's worth looking briefly at how an ordinary laser works. Ordinary lasers require both negatively-charged electrons and positively-charged "holes" (places in the crystal where electrons should be, but aren't). They attract each other, and when the electron pops into the hole, it gives up surplus energy as a photon of very specific energy. That energy determines its wavelength.
QC lasers work with electrons only. The semiconductor layers are designed in such a way that the electron can drop energy in a series of small amounts, giving up a photon each time it does so. "Consider it an electronic waterfall," says Bell Labs' description of a QC laser at http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl.html. The cascade of light generated in this way makes the QC laser much more powerful than existing lasers.
This waterfall effect is key to making the device broadband. Instead of creating all the small steps in the waterfall the same size, they can be made slightly different, so that each successive photon emitted has a slightly different wavelength. The laser reported today had 36 "steps" in the waterfall, each one created by growing thin layers of several different semiconductor materials.
The fact that all the different materials are grown on top of each other is significant because, with a bit of tweaking to the design of the layers, it is possible to create lasing at every wavelength in the band, not just at discrete wavelengths corresponding to the peak gain at each "step." This differentiates the device from so-called multiwavelength lasers, which are made by growing different materials side by side on a semiconductor wafer, and have wavelength spikes at the output.
But there is a catch. Since the electrons only make small energy jumps, the photons tend to have long wavelengths. So far, Bell Labs has only managed to make devices that emit light in the region 4 to 24 microns. (The device reported in Nature actually emitted all wavelengths in the range 6 to 8 microns.) It has been Bell Labs' stated intention for a quite a while to make a device that emits at the shorter telecom wavelengths of 1.3 and 1.55 microns, but so far no progress has been reported in this respect.
— Pauline Rigby, Senior Editor, Light Reading