Low-cost light source for optical fiber communication systems

Wavelength division multiplexing (WDM) is a technique used to increase the rate at which data is transmitted in optical fiber systems. It is often used in optical communications systems. In WDM systems, several wavelengths (channels) of light are simultaneously transmitted along a fiber. WDM systems are divided into different wavelength patterns: in dense WDM (DWDM) patterns the wavelengths are closely spaced (0.8nm apart) and require careful control to avoid overlap of neighboring channels (i.e., crosstalk). The other pattern, coarse WDM (CWDM), uses increased wavelength spacing and therefore requires less stringent controls on the output wavelength of the source lasers. The DWDM pattern is expensive, can be amplified, and is capable of higher data rate transmissions because many channels are included. For these reasons, DWDM systems are used for long-haul communications. CWDM is inexpensive because the channel spacing is wider and the channels generally used cannot be amplified. As a result CWDM is used for short-haul metropolitan networks. Importantly, the sources for CDWM systems are generally inexpensive as well.

The source for CWDM systems should be able to generate data streams at up to eight switchable wavelengths, with a wavelength spacing of 20nm between each channel, from a single output that is coupled into a fiber. Current solutions are based on using separate diode lasers routed to a single output using bulk optics.¹ This setup requires careful alignment of several optical elements mounted into a mechanically and thermally stable package, which can be very costly. A cheaper solution would be to integrate the lasers and switching elements onto a single chip with a single output waveguide. The challenge with this method is to develop devices on a single sample of semiconductor that operates at different wavelengths and to combine their output on the chip. Here, we describe a chip made from semiconductor laser material that produces data signals at four wavelengths from a single output.²

Figure 1. Chip with integrated devices. From the left are the four distributed feedback (DFB) laser diodes (LD) at different wavelengths (CH1–4), passive waveguides, a multimode interference (MMI) coupler, semiconductor optical amplifier (SOA), and the electro-absorption modulator (EAM).

The semiconductor lasers used in communications are made from III-V semiconductors with quantum well gain regions. Quantum wells are ultrathin layers of semiconductor that exhibit quantum effects, sandwiched between wider bandgap barriers. The bandgap of the wells can be increased by diffusing atoms between the wells and barriers, changing the composition of the wells.³ This process is known as quantum-well intermixing (QWI). We have developed a technique where we can cause QWI when the sample is annealed in locations we choose in the sample. Our post-growth process enables us to engineer the quantum wells in a spatially selective way via the sputtering of silica onto the sample with a composition that generates point defects in the semiconductor when annealed. During annealing, group III elements at the surface of the semiconductor move into the silica, creating point defects (group III vacancies) in the semiconductor. Once they are formed the point defects diffuse into the semiconductor and through the quantum wells to cause the intermixing. Essentially, we can achieve different bandgaps in different parts of the chip without having to etch and regrow the wafer. This flexibility enables us to change the bandgap in selected areas and have areas on the chip that can act as passive waveguides with low losses to route the signals from lasers to the output.

Figure 2. The spectrum of all four lasers operating simultaneously.

The first step in fabricating the chip is to perform the QWI in the areas where the passive waveguides are to be formed on a sample cut from a semiconductor laser wafer. After the QWI of the passive areas of the device, four distributed feedback (DFB) lasers, passive waveguides, a multimode interference coupler, a semiconductor optical amplifier (SOA), and an electro-absorption modulator (EAM) for the output were formed on the chip. All these devices were defined in a single electron-beam-lithography step, then reactive-ion etched to form the gratings for the lasers and ridge waveguide structures for the other devices. We used DFB lasers where the period of the gratings needed to provide the feedback for each laser is increased in 2nm increments so that each laser emits at a different wavelength (see Figure 1).

The lasing wavelengths of the four lasers were approximately 1529.8, 1542.8, 1554.4, and 1566.2nm each (see Figure 2). The wavelength spacing, which is determined by the design of the gratings, was 12nm. Our measurements show that the gain curve is sufficiently broad that the more conventional spacing of CWDM (i.e., 20nm) can be readily accommodated, bringing the device in line with international standards. The side mode suppression ratio for each channel is ∼43dB. Varying the SOA current was found to have a negligible effect on the performance of the lasers, so the SOA could be used to further boost the output power. A direct current extinction ratio of >12:5dB (with a reverse voltage applied on the EAM, V_EAM=−4V) was achieved for all four laser channels, which is acceptable, but further work on the device is needed. Indeed, we envisage our next-generation device will have a modulator at the output of each laser before the light from the lasers are combined into one waveguide.

The beam divergences were narrow and almost symmetric, and measured to be 21:2×25:1° (full width at half-maximum). We achieved the vertical divergence using a wafer design that produced a wide optical field in the vertical direction within the structure. A butt-coupling efficiency of ∼20% was achieved using a single-mode fiber, which is double that provided when using a conventional laser epilayer structure. The −1dB alignment tolerances in the horizontal, vertical, and optical axis have also been significantly relaxed. The relaxed tolerances on the alignment would make the packaging of the device easier and therefore cheaper.

In summary, there is a need for low-cost, robust sources for CWDM systems that are capable of emitting data streams at up to eight different wavelengths from a single output. We have demonstrated a chip with a single output waveguide that emits switchable output at four wavelengths. Our epilayer design improved the coupling efficiency of the output light into a single mode fiber. Our further work on these devices will include increasing the wavelength spacing between the channels and inserting modulators at each laser to modulate the channels individually.