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 fiber optic products systems.
In WDM system, a few wavelengths (channels) light propagation along the fiber at the same time. Wavelength division multiplexing (WDM) system can be divided into different wavelengths 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 patterns, coarse WDM (CWDM), use increase the wavelength spacing, so need less strict control output wavelength of the laser source.
The dense wavelength division multiplexing (DWDM) pattern is costly, can be amplified to higher data rate transmission for many channels are included. For these reasons, DWDM systems is used for long distance communication. CWDM is cheap because the channel spacing is widely and generally used channels can not enlarged. Results coarse wavelength division multiplexing is used for short metropolitan network. Importantly, the sources for CWDM systems are generally cheaper 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. The current solutions are based on using separate diode lasers routed to a single output using bulk optics. This setting requires careful alignment of several optical elements mounted into a mechanically and thermally stable package, which can be very expensive. A cheaper solution is to put the laser and the switching elements integrated on a chip and 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 describle a chip semiconductor laser materials, produce data signals at four wavelengths from a single output.
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. 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 growth process afterour engineer quantum well space in a selective way through sputtering silica to point defects on semiconductor when annealed samples composition. 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 the formation of the pointed defect spread to semiconductors and caused by quantum well mixed. In essentially, we can implement different properties in different parts of the chip without corrosion and regeneration of the chip. This flexibility allows us to change the band in selected areas and areas on the chip, can be used as a passive waveguide loss lower laser output signal routing.
Figure 2. The spectrum of all four lasers operating simultaneously.
The first step in the field of making chip is perform QWI passive waveleguides are formed in a sample cut from a semiconductor laser chip. After the QWI of the passive areas of the device, four DFB lasers, passive waveguides, a multimode interference coupler, a semiconductor optical amplifier, and an electro-absorption modulator for the output were formed on the chip. All of these devices is defined in a single electron-beam-lithography steps, and then reactive ion etching grating formed by laser and the ridge waveguide structure of other equipment. Use of distributed feedback laser grating cycle we need to provide feedback for each laser is to increase 2nm increment, each laser emit different wavelengths.
The lasing wavelengths of the four lasers were approximately 1529.8, 1542.8, 1554.4, and 1566.2nm each. The wavelength spacing, which is determined by design of the gratings, was 12nm. Our
measurements show that the gain curve is very broad, and more traditional spacing (CWDM. Namely 20nm) can be easily satisfied, make the equipment conform to international standards. Side mode suppression ratio for each channel is ∼43dB. Different SOA currents were found to have a little performance impact of the laser, so the SOA can be used to further improve the output power. A direct current extinction ratio of >12:5dB (with a reverse voltage applied on the EAM, VEAM=−4V) was achieved for all four laser channels, which is acceptable, but further work on the device is needed. In fact, we assume our next generation equipment will have a modulator output each laser in the laser light into a waveguide.
The beam divergences were narrow and almost symmetric, and measured to be 21:2 x 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 reached ∼20% using single mode fiber, it's double use ordinary laser epitaxial layer 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 an output waveguide, a four wavelength switchable output. Our epitaxial layer design to improve the coupling efficiency of the output of the light into a single mode optical fiber.
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