Device Development History
This series of articles explains the history of product development by Anritsu Sensing & Devices Company.
The high-economic-growth period in Japan following the 1964 Tokyo Olympics saw a massive increase in communications demand. Nippon Telegraph and Telephone Public Corporation (former name of NTT) decided to build an optical-fiber transmission network, marking the start of the optical-communications era. As a result, our predecessor company started development of key devices, such as semiconductor lasers and high-speed hybrid ICs for use in measuring instruments to support optical networks. Many of today’s products from our Sensing & Devices Company inherit their development from this time. This series of articles explains the history of devices developed by our predecessor companies. This first article explains the opportunities and early stages in optical-device development.
Optical Devices (1) — Dawn of semiconductor laser development
Although optical fiber-based communications have many advantages, including low transmission loss, high speed, and large capacity, the glass optical-fiber core is easily broken. Locating breaks in long-distance optical fibers requires a measuring instrument called an optical time domain reflectometer (OTDR). Building such an instrument required a high-output semiconductor laser diode (LD) emitting light in the 1-µm wavelength band used by optical communications. The problem was that such products were not commonly available at that time.
Today, after audio CD players became commonplace in the 1980s, semiconductor laser devices such as LDs and light emitting diodes (LEDs) are in widespread use in offices and households, but back then, the word “laser” meant fixed lasers and gas lasers. Although these types of lasers had high output, the equipment was large and unsuitable for benchtop measuring instruments and the optical output did not match input to optical fiber. As a result, we started looking into future LD development. The Suematsu Laboratory at Tokyo Institute of Technology was an early pioneer in this field, and we had started fundamental research into LD development in the latter half of the 1960s based on their advice. The 1975-introduction of a crystal growth equipment enabled us to start real LD development and we finally successfully fabricated a 0.85-µm LD with continuous wave operation at room temperature on a gallium arsenide (GaAs) substrate. With technical guidance from the NTT Electrical Communications Laboratory, in 1979, we started research into a long-wavelength LD using an indium phosphide (InP) substrate.
After the 0.85-µm band OTDR release, our 1-µm band LD developed was finally used in the long-awaited communication-band OTDR in1984 (MW98A in above left figure).
The LD used in this OTDR is pictured in above right figure. The metal part with hole is a 6-mm width copper heatsink bonded to the LD chip on top; the chip itself was quite small at 0.3 (L) x 0.4 (W) x 0.1 (H) mm. Since the copper heatsink linear expansion coefficient is much larger than the LD InP substrate, a directly soldered bond suffered from reliability problems. Therefore, we used an industrial-diamond layer with an intermediate linear expansion coefficient and excellent thermal radiation characteristics between the LD chip and copper heatsink.
An OTDR analyzes the scattered and reflected optical signals propagating in an optical fiber to locate breaks in the fiber and measure loss, etc. The distance to faults and losses can be calculated from measuring time differences and levels between emitted and returning optical signals. Consequently, the light-source optical output must be a rectangular pulse rather than a continuous wave (CW). At that time, the fabrication process was still immature and although there were no problems with output of CW light, there were still issues with intensity fluctuation, resonating, and unstable reflection (see figure below) due to effects of the pulse drive and reflected light, making it necessary to inspect every optical waveform.
Most people’s image of laser light is light that is emitted in a straight line, rather like a laser pointer, but semiconductors emit light that is spread-out through free space because the emission area of LD is extremely small. Consequently, this light must be input to the 10-µm (one-thousandth of 1 mm) diameter core of the optical fiber for communications by using a light-collecting system composed of two lenses: a spherical lens, and graded-index (GRIN) lens. The latter is a cylindrical lens in which the index of refraction changes from the periphery to the central axis. Since the lens length controls the lens magnification, it has the advantages of being both small and easy to use. As shown in the figure below, these parts are mounted in sequence from close to the LD facet to the fiber end face, which requires micron-level alignment of the fiber axis. The mounting uses a two liquid adhesive requiring several hours to harden fully. Due to variations in hardening time and adhesive shrinkage, it was sometimes impossible to prevent axis misalignment which required uniformity modifications using an expert to adjust axis alignment considering movement as the adhesive hardened. The 1.55-µm OTDR was the world’s first measuring instrument to support single-mode fiber (SMF) and brought innovative quality to optical-fiber communications. Everyone involved in this development took pride that our LD with less than 1mm contributed such innovation.