Device Development History
Optical Devices (3) - Progress in each Individual Device
This article introduces our company’s progress in developing key optical devices.
When first starting mass-production of optical devices, we used a liquid phase epitaxial (LPE) growth equipment to create semiconductor crystals as described in a previous article. The figure at the bottom left shows a cross-section of a typical LD manufactured at that time. This procedure is ideal for achieving high output power because the active InGaAsP (indium, gallium, arsenide, phosphide) layer could be buried in thick layers to present the world’s highest optical output level as reported in international conference.
In the late 1980s, direct amplification of 1.5-µm wavelength optical signals was reported by doping the optical-fiber core with the rare-earth erbium (Er). Since this direct amplification method uses a small and low-cost 1.48-µm semiconductor laser as the pumping light source, these LDs spread rapidly and played a key role in development of today’s high-speed, large-capacity communications networks. Our company became a major market player in the early 1990s with the release of a high-output 50-mW Pump-LD for optical fiber using our achievements in high-output LD technology; Pump-LDs subsequently developed into a main product line of our business.
The process for growing crystals changed in the 1990s following introduction of vapor-phase epitaxial growth equipment and the figure at the bottom right shows the major changes in the structure of modern Pump-LD designs. The new structure uses redesigned features, such as a semiconductor-type substrate, an etching shape of active layer, a cladding layer, and the length of LD chip.
To improve the output power, it is necessary to extend chip length and adjust the active layer thickness. The performance is greatly improved by using a quaternary InGaAsP cladding layer near InP composition under the active layer. Normally, the optical distribution is vertically symmetrical but with this structure, it is uniquely asymmetrical because the optical distribution is unbalanced towards the high refractive index substrate side. Expanding the active layer length and width in this manner plays a key role in increasing output power. The length of the LD chip at the first development stage was 300 µm but it has now reached several mm in modern designs. The result of research prototypes achieving chip output powers of more than 1 W was reported in the international conference. On the other hand, an increase in the optical output power can cause heat deformation of resin-fabricated parts in the optical receiver used by measuring instruments, with risks of burning holes in black paper and films as well as personal burn injury to misplaced fingers and hands! Our company continues this R&D to maintain world-class performance in our line of Pump-LD models.
Currently, our high-output products have achieved optical fiber outputs of 650 mW, or more than 10 times the first output levels. This has been achieved using a quantum-well structure fabricated from extreme thin semiconductor layers of just a few atoms due to progress in crystal epitaxial growth equipment and introduction of revolutionary structure designs.
Cross-Section of First LD Design
Cross-Section of Latest PumpLD Design
The general LDs, such as Pump-LD, oscillate with multiple longitudinal modes, but LDs used as light sources for optical communications must oscillate entirely at a single wavelength. Consequently, a distributed feedback laser diode (DFB-LD) with built-in grating for selecting a specific wavelength was developed. The grating is formed either as a periodic slit or as an etched pattern with fine corrugation; it is an optical element for creating an interference fringe using diffraction and interference corresponding to the period interval. Figure (a) below shows a schematic of an interference exposure equipment to fabricate the pattern. The beam from an argon laser is first expanded and then split into two paths before exposing the substrate to fabricate a striped pattern on the substrate. Figure (b) shows an example of the grating pattern fabricated on the substrate; the corrugation pitch depends on the target wavelength but is usually around quarter micron. Figure (c) shows the grating pattern after regrown semiconductor as the DFB-LD substrate. The pitch of the grating can be adjusted using the angle of the incident laser light to the substrate but requires very careful adjustment since the wavelength changes with slight position misalignment. It is an extremely delicate equipment that requires readjustment even after an earthquake. The size of wafers that can be processed depends on the power of the light source and the size of the magnified beam. It has the advantage of supporting pattern fabrication within an area in one shot but local changes in pitch can cause problems.
Fabrication of Grating by Interference Exposure Method
In recent years, the main trend has been towards electron-beam (EB) lithography methods instead of optical interference exposure methods, and our company has introduced this technology too. The EB lithography equipment can draw a fine pattern directly on the wafer using an electron beam from an electron gun. Since the pattern is drawn one line at a time, the processing time is long and at the early development stage, 3 days were required to process one 2-inch wafer in the early products. However, one merit is that LDs with slightly different lasing wavelength can be fabricated on one wafer, and the pitch of each grating can be changed within individual chip. The precision EB lithography equipment as well as interference exposure equipment is adversely affected by vibration but is also easily susceptible to magnetic fields. For example, even the presence of a nearby elevator risks adverse impact on the drawing results, so adequate attention must be paid to consideration about the equipment installation location. Moreover, power lines with large current generating magnetic fields distortion can cause problems with drawing accuracy.
The DFB-LD design is not used only as a simple light source for optical communications but is also integrated commonly with electro-absorption (EA) modulators. In addition, tunable-wavelength LDs supporting multiple wavelengths with one module are also becoming more common. On the other hand, gas-detection sensors require light sources with wavelengths matching the gas absorption line offering an application role for simple DFB-LDs with fixed wavelength and no requirement for high-speed modulation.
EB Lithography Equipment