8.2. WDM COMPONENTS 361
Figure 8.21: Schematic of a WDM laser made by integrating an AWG inside the laser cavity.
(After Ref. [137];©c1996 IEEE; reprinted with permission.)
waveguides and by tuning over a 1-nm range using a thin-film resistor [133]. In a differ-
ent approach, sampled gratings with different periods are used to tune the wavelengths
precisely of an array of DBR lasers [135]. The complexity of such devices makes it
difficult to integrate more than 16 lasers on the same chip. The vertical-cavity surface-
emitting laser (VCSEL) technology provides a unique approach to WDM transmitters
since it can be used to produce a two-dimensional array of lasers covering a wide wave-
length span at a relatively low cost [136]; it is well suited for LAN and data-transfer
applications.
A waveguide grating integrated within the laser cavity can provide lasing at several
wavelengths simultaneously. An AWG is often used for multiplexing the output of sev-
eral optical amplifiers or DBR lasers [137]–[139]. In a 1996 demonstration of the basic
idea, simultaneous operation at 18 wavelengths (spaced apart by 0.8 nm) was realized
using an intracavity AWG [137]. Figure 8.21 shows the laser design schematically.
Spontaneous emission of the amplifier located on the left side is demultiplexed into 18
spectral bands by the AWG through the technique of spectral slicing. The amplifier ar-
ray on the right side selectively amplifies the set of 18 wavelengths, resulting in a laser
emitting at all wavelengths simultaneously. A 16-wavelength transmitter with 50-GHz
channel spacing was built in 1998 by this technique [138]. In a different approach,
the AWG was not a part of the laser cavity but was used to multiplex the output of
10 DBR lasers, all produced on the same chip in an integrated fashion [139]. AWGs
fabricated with the silica-on-silicon technology can also be used although they cannot
be integrated on the InP substrate.
Fiber lasers can be designed to provide multiwavelength output and therefore act
as a CW WDM source [140]. A ring-cavity fiber laser containing a frequency shifter
(e.g., an acousto-optic device) and an optical filter with periodic transmission peaks
(such as a FP filter, a sampled grating, or an AWG) can provide its output at a comb
of frequencies set to coincide with the ITU grid. Up to 16 wavelengths have been
obtained in practical lasers although the power is not generally uniform across them.
A demultiplexer is still needed to separate the channels before data is imposed on them