Coherent light generators – Particular temperature control
Patent
1991-07-12
1993-04-20
Scott, Jr., Leon
Coherent light generators
Particular temperature control
372108, 372703, H01S 304
Patent
active
052048683
DESCRIPTION:
BRIEF SUMMARY
FIELD OF TECHNOLOGY
This invention concerns an optical isolator which uses temperature dependence to control the maximum isolation temperature.
BACKGROUND TECHNOLOGY
With the development of long-distance optic communications technology using LD (semiconductor lasers), optical isolators have been developed in order to prevent LD noise from reflected, returning light. With the proliferation of high density communications systems, the importance of these devices has grown considerably. The basic structure consists of a Faraday rotator composed of two polarizers and a garnet crystal, along with a permanent magnet to produce the Faraday effect by the magnetization of the Faraday rotator. For the polarizer, one could use a Rochon polarizing prism, polarized beam splitter, Grant-Thompson+prism, or polarized glass depending upon application. In order to achieve small size and a high magnetic field, a rare-earth permanent magnet is used. The Faraday rotator was the part which determined the characteristics of the optical isolator. There are currently two types of materials being used. One is where the FZ method is used to produce a bulk YIG (1/2Fe.sub.5 O.sub.12) single crystal and the other is where the liquid phase epitaxial (LPE) method is used in order to produce a BiRIG (rare earth bismuth-ison garnet) film on a garnet type of substrate. The Faraday rotation angle .theta..sub.f is proportional to the thickness of the crystals, and .theta..sub.f per unit of length is different from each material. In order to obtain an angle .theta..sub.f =45.degree. as required for an optical isolator, the YIG should be about 2 mm, and the BiRIG should be 200 to 500 .mu.m. In consideration of mass production and lowered costs, after the FZ method has been used to obtain the bulk YIG, in order to produce the required shape on the substrate using the LPE method without wasteful machining, it is possible to produce a large film 1/4 of the thickness but equivalent to the YIG in the Faraday rotator. This is of greet benefit for the economic proliferation of these elements. However, in u these YIG and LPE garnet crystals, there have some differences in optical characteristics which result in temperature or wavelength dependence in the Faraday rotation. FIG. shows the temperature dependence (a) and the wavelength dependence (b) of .theta..sub.f for a BiRIG. Depending on the materials, there are variations in the reverse slope, but generally, the .theta..sub.f corresponds to temperature and wavelength. Optical isolators are adjusted and assembled to have a maximum isolation at the wavelength and the ambient temperature which they are assembled. However, in temperature ranges from 0.degree. to 70.degree. C. where these devices will be practically used, near the temperatures at either end of this range (0.degree. C. and 70.degree. C.) the isolation characteristics tend to deteriorate. In YIG crystals, the Faraday rotation temperature dependence coefficient is generally K.sub.T =-0.04 deg/.degree. C. With the LPE method, in materials where there is essentially little absorption, it is K.sub.T =-0.04 to -0.07 deg/.degree. C. When the optical isolators are assembled at room temperature (about 23.degree. C.), they are adjusted so that .theta..sub.f =45.degree. at 23.degree. C. However, if we assume the temperature coefficient of an LPE garnet element to be -0.07 deg/.degree. C., then at the upper end of 23.degree. C. .+-.20.degree. C. (eg. 43.degree. C.), .theta..sub.f =45 -0.07.times.20=43.6.degree.. At the lower limit of 3.degree. C., this becomes 46.4.degree.. This greatly degrades the isolation due to the slippage from .theta..sub.f =45.degree.. In principle, isolation is -10 Log[sin.sup.2 (45-.theta..sub.f)], so in the previous example, at 3.degree. C. and 43.degree. C. it would be 32 dB. The graph in FIG. 2 (1) shows the temperature dependence of isolation for a typical optical isolator. The peak of isolation is at 23.degree. C., and it falls off below and above that temperature in a nearly symmetrical curve. In this case, when con
REFERENCES:
patent: 4367040 (1983-01-01), Goto
patent: 4762384 (1988-08-01), Hegarty et al.
patent: 4886332 (1989-12-01), Wolfe
patent: 4952014 (1990-08-01), Lieberman et al.
Konno Yoshihiro
Kume Hiroshi
Jr. Leon Scott
Namiki Precision Jewel Co. Ltd.
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