Device Development History
Optical Devices (4) - Progress in each Individual Device 2
The semiconductor laser diode (LD) emits light with a narrow spectral width plus high optical output power. When used as a high-coherence light source for interferometry, it can be difficult to specify each reflection point and noise may occur when there are many overlapping signals with wide-ranging interference. As a result, optical-sensing applications require low-coherence noise sources with a wide spectral width. Low-coherence light sources suppress oscillation by reducing facet reflectivity to achieve a wider spectral width and are called Super Luminescent Diodes (SLD). They are used in interferometry, and other applications.
As shown in Fig. (a) below, facet reflectivity can be suppressed by applying an anti-reflection (AR) coating formed by a dielectric film to both optical facets. Early development used a simple nitride- film AR coating and evolved with electron-beam evaporating and sputter film coating. However, since AR coating alone did not achieve sufficiently low reflectivity, the following countermeasures are used, including anti-reflection structures on the rear facet and formation of an angled waveguide active layer to the facets. The structural details are different for products using GaAs substrate in the 0.8-µm band and InP substrate in the 1-µm band. If there is large residual reflectivity, the spectrum has multiple modes as shown in Fig. 1 below, but it is changed to smooth shape with fewer spectral ripple as shown in Figs (2) and (3) below because the total facet reflectivity can be reduced by using AR coating and structures.
As with pumping LDs, the SLD active layer has also started using a bulk structure. Although the optical output power can be increased easily for bulk structures just by extending the active layer length, the wavelength bandwidth becomes correspondingly narrower. The current trend is to use a quantum-well structure formed by multiple extremely thin layers of different semiconductors to support low output power over a wide wavelength band with an adjustable balance between power and bandwidth. Introduction of quantum-well structures has resulted in better total performance. We have long experience in development of SLDs for use as light sources in measuring instruments manufactured by our group company. Our SLDs also have wide-ranging applications in the optical-sensing field, such as ophthalmic medical equipment and semiconductor wafer or optical film thickness inspection.
Various Reflectivity Reduction Structures
Spectral Change with Residual Facet Reflectivity
The Semiconductor Optical Amplifier (SOA) has the function of amplifying the input optical signal by reducing chip facet reflectivity as well as SLD. The optical signal must be input and output to and from both facets of the SOA chip and the module features optical fibers connected to both sides of the product module. In comparison to fiber amplifiers, SOA is much smaller, can be optically integrated, and has higher functionality. The following figure shows a typical example of an early SOA module from the 1990s and its current versus gain characteristics. Without driving current, the SOA active layer has large optical loss corresponding to the signal wavelength, but the loss drops rapidly and turns into gain as the driving current increases. Consequently, the SOA can operate as an optical switch with high extinction ratio by controlling driving current. In the first development stage, gain was suppressed intentionally to design a lossless or low-gain gate switch for optical-switching equipment. There are still application needs in tunable light source modules due to working in ns-order speeds.
Semiconductor optical devices essentially have a polarization dependence of gain. It is known that the gain polarization dependence can be suppressed by introducing strain in the active layer. In other words, this can be achieved by intentionally changing the size of the crystal lattice in the active layer and the substrate material. We have reported results of our research in international conference. Fortunately, we received enquiries from research institutions about this device, which was being used for research and development. SOA modules are longer than standard butterfly modules to incorporate polarization-independent isolators on both sides, as shown in the figure below. However, it was difficult to uniformly control the amount of strain in this device at that time. Moreover, we had yet to manufacture consistent products due to issues with yield and reproducibility, such as changes in characteristics resulting from strain occurring during manufacturing. Although future SOA development was suspended due to declining R&D interest, it restarted in the 2010s as a result of interest in SOA for optical transceivers. Current SOA development is making good use of early experience in SOA research with progress in cyrstal-growth technology advancing towards development of mass-produced devices with polarization-independent gain at low driving currents for widespread applications.
Early SOA Module
5. Gain Chip
The growth of the optical-communications market required a tunable-wavelength light source to expand Anritsu’s line of optical measuring instruments. Prior to this era, stabilized light sources output an optical signal at a fixed wavelength, but these instruments output laser light over a wide wavelength range. For example, when measuring the wavelength characteristics of optical devices such as filters, this instruments has a larger output per wavelength than a white-light source and support measurement over a wide dynamic range. Additionally, transmission characteristics can be measured at the required wavelength by stopping sweeping. The following figure shows the basic structure of a tunable-wavelength light source using a Littman-type external cavity in a commercialized product. The 0th-order reflected light from the diffraction grating is optical output 1 and the output from the other end of the gain chip is output 2. The lasing wavelength is determined by the wavelength selected by the diffraction grating, and it can be changed by the angle of the mirror. The one gain-chip facet has non-reflective, but the other has either high or low reflectivity depending on the configuration of the optical system. Although these devices are also sometime called SOA, our company calls them gain chips for laser-oscillator applications to distinguish them from SOA for amplification of input optical signals. The internal structure of the active layer is fundamentally the same as an LD, but the difference is the requirement for a wide-gain bandwidth. Consequently, various combinations of active-layer quantum-well numbers and chip lengths were tried and parameters were determined for high optical output over wide oscillation wavelengths. Although originally developed as devices for Anritsu measuring instruments, we now offer these tunable-wavelength light source modules for other companies and communications applications following discontinuation of Anritsu’s product. In addition to diffraction gratings for optical filters using external resonators, various configurations such as ring filters using silicon-waveguides, and liquid crystal filters are adopted by each company. Many customers for our gain chips evaluate them highly for fundamental performance and support.
Our device business was started originally with the intention of supplying key devices for Anritsu measuring instrument and we will soon be reaching our 50th business anniversary in several years. We are steadily developing unique optical devices despite our limited resources in comparison to superior-capitalized major manufacturers and will continue strengthening our integrated product line to further increase our customers’ satisfaction.