Patent Application
20120007005
The invention relates to opto-isolators. More particularly, the invention relates to an opto-isolator that utilizes a nontransparent hollow tube having a smooth inner surface with mirror-like qualities to optically couple optical signals between the transmitter and receiver modules of the opto-isolator.
An opto-isolator is a device that transfers a signal optically between two electrical circuits operating at different electrical potentials while, at the same time, electromagnetically isolating the circuits from each other. Opto-isolators also isolate one part of a system from electrical noise in another part of the system and protect circuits against damage from voltage surges. A transmitter module of the opto-isolator comprises an electrical-to-optical converter (EOC), such as a visible or infrared light emitting diode (LED), for example, that converts an electrical driver signal into an optical signal. A receiver module of the opto-isolator comprises an optical-to-electrical converter (OEC), such as a photodiode, for example, that converts the optical signal back into an electrical signal.
An optical waveguide optically couples the transmitter and receiver modules to each other to allow optical signals produced by the EOC of the transmitter module to be transmitted to the OEC of the receiver module. The optical waveguide is typically a length of optical fiber, but other optical waveguides are sometimes used for this purpose. For example, it is known to use an optically transmissive rod as the optical waveguide surrounded by a fluid having a refractive index that is different from the refractive index of the rod.
Although these types of optical waveguides generally work well at coupling optical signals between the transmitter and receiver modules while also providing electrical isolation, they are relatively costly to implement. A need exists for an opto-isolator that utilizes a less costly, yet optically efficient, optical waveguide for coupling optical signals between the transmitter and receiver modules of the opto-isolator while also providing sufficient electrical isolation.
The invention is directed to an opto-isolator and an opto-isolation method. The opto-isolator comprises a transmitter module housing, a transmitter module disposed within the transmitter module housing, a receiver module housing, a receiver module disposed within the receiver module housing, and a nontransparent hollow tube. The transmitter module has an EOC for converting an electrical drive signal into an optical signal. The nontransparent hollow tube has a first end and a second end and has inner and outer surfaces that define a wall of the hollow tube. The first end of the tube is mechanically coupled to the transmitter module housing. The inner surface of the tube is smooth and has mirror-like qualities. The optical signal produced by the EOC is optically coupled into the first end of the tube and propagates along the tube toward the second end of the tube via one or more reflections on the inner surface of the tube. The receiver module has an OEC for converting optical signals into electrical signals. At least a portion of the optical signal propagating along the tube toward the second end of the tube passes out of the second end of the tube and is optically coupled onto the OEC.
The method comprises the following. With an EOC, an electrical drive signal is converted into an optical signal. The optical signal is optically coupled into a first end of a nontransparent hollow tube. The nontransparent hollow tube has inner and outer surfaces that define a wall of the hollow tube. The inner surface of the tube is smooth and has mirror-like qualities. The optical signal propagates along the tube toward a second end of the tube via one or more reflections on the inner surface of the tube. The optical signal passing out of the second end of the nontransparent hollow tube is optically coupled onto an OEC. With the OEC, the optical signal coupled thereon is converted into an electrical signal.
These and other features and advantages of the invention will become apparent from the following description, drawings and claims.
In accordance with the invention, an opto-isolator is provided that utilizes a nontransparent hollow tube having a smooth inner surface as the optical waveguide for coupling optical signals between a transmitter module and a receiver module of the opto-isolator. The optical waveguide may have a variety of configurations, as will now be described with reference to
The transmitter module housing 2 is disposed on a first end 10a of the hollow tube 10 and the receiver module housing 3 is disposed on a second end 10b of the hollow tube 10. A transmitter module 4 is held within the transmitter module housing 2 by a first clamp 5. The transmitter module 4 includes an EOC 6, such as a laser diode or a light emitting diode (LED), which converts an electrical signal into an optical signal for transmission over the hollow tube 10. A receiver module 7 is held within the receiver module housing 3 by a second clamp 8. The receiver module 7 includes an OEC 9, such as a photodiode, that converts the optical signal transmitted by the transmitter module 4 over the hollow tube 10 into an electrical signal. Although clamps 5 and 8 are specifically mentioned above as mechanisms that are used to secure the modules 4 and 7 within their respective housings 2 and 3, any mechanism or device may be used for this purpose, such as glue or epoxy, in which case the clamps 5 and 8 may be eliminated.
Electrical leads 11 connect the transmitter module 4 to electrical driver circuitry 13 of the opto-isolator 1. The electrical driver circuitry 13 produces the electrical signals that drive the EOC 6. Electrical leads 12 connect the receiver module 7 to electrical receiver circuitry 14 of the opto-isolator 1. The electrical receiver circuitry 14 processes the electrical signals produced by the OEC 9. The distance between the electrical leads 11 and 12 is typically about 2 to 3 centimeters (cm). Thus, the hollow tube 10 has an end-to-end length that is less than about 3 cm. The opto-isolator 1 is particularly well suited for use in very short optical links in which high isolation in the kilovolt (kV) range is needed.
The hollow tube 10 has an inner surface 10c and an outer surface 10d that define the tube wall 10e. In accordance with this illustrative embodiment, the inner surface 10c has a constant diameter over the length of the tube 10 from the first end 10a to the second end 10b. The inner surface 10c of the tube 10 is very smooth and has a roughness that is generally not more than about 1/10λ, where λ is the wavelength of light being transmitted by the EOC 6 of the transmitter module 4. The inner surface 10c of the hollow tube 10 is made smooth through the use of a lapped and honed mold tool surface in this area that gives the inner surface 10c smooth, mirror-like qualities. In addition, the hollow tube 10 is nontransparent to prevent external light produced by sources external to the opto-isolator 1 from passing through the outer and inner surfaces 10d and 10c, respectively, and entering into the hollow tube 10. This feature ensures that only light produced by the EOC 6 will be received by the OEC 9.
In accordance with this illustrative embodiment, lenses 16 and 17 are integrated into the transmitter and receiver modules 4 and 7, respectively. The lenses 16 and 17 do not have to be integrated with the modules 4 and 7, respectively, but may be separate components that are mechanically coupled to the respective modules 4 and 7. Light produced by the EOC 6 is coupled by the lens 16 into the end 10a of the hollow tube 10. Some of this light, which is represented by dashed lines 21, passes through the tube 10 to the second end 10b of the tube 10 without being incident on the inner surface 10c of the tube 10. As the light 21 passes through the end 10b of the tube 10, it is coupled by the lens 17 onto the OEC 9. On the other hand, some of the light, which is represented by arrows 22, is incident on the inner surface 10c of the tube 10, as will now be described with reference to
Some of the light that is incident on the inner surface 10c of the tube 10 is not reflected, but rather, is refracted into the tube wall 10e. The amount of light 23 that is refracted into the tube wall 10e depends on the angle of incidence, α0. The angle of reflection, α0, is equal to the angle of incidence, α0. As the angle of incidence α0 decreases, more light 23 is refracted into the tube wall 10e. Conversely, as the angle of incidence α0 increases, less light 23 is refracted into the tube wall 10e and more of the light 22 is reflected by the inner surface 10c. The angle of refraction, αP, represents the angle of the light 23 relative to a normal 25. Because the tube wall 10e is nontransparent, most if not all of the refracted light 23 is absorbed by the tube wall 10e. The tube wall 10e can be made to be nontransparent in a number of ways, such as, for example, through the use of a pigment in the molded material.
The primary difference between the embodiments shown in
In the portion of the tube 210 where the diameter of the inner surface 210c is constant, the angles of incidence and reflection are equal, and the light is conducted through the tube 210 in the same manner in which light is conducted through the tube 10 shown in
The parabolically tapered inner surface 310c has decreased reflectivity near the end 310a of the tube 310, but increased reflectivity near the end 310b of the tube 310. The smaller diameter of the inner surface 310c near the end 310b causes more optical energy to be directed into the small area at the second end 310b of the tube 310 at which the lens 17 is located.
In the portion of the tube 410 where the diameter of the inner surface 410c is linearly tapered, the angles of reflection are greater than the angles of incidence. In other words, the angles of reflection are increased by the tilt angle of the inner surface 410c in the tapered portion of the tube 410 relative to the optical axis of the tube 410. This increase in the angle of reflection in the tapered portion causes the reflected light rays to be flattened out such that they are directed into the small area on the second end 410b of the tube 410 adjacent the lens 17. This causes more of the optical power produced by the EOC 6 to be coupled by the lens 17 onto the OEC 9, which can result in improved optical efficiency. In the portion of the tube 410 where the diameter of the inner surface 410c is constant, the angles of incidence and reflection are equal, and the light is conducted through the tube 410 in the same manner in which light is conducted through the tube 10 shown in
In the portion of the tube 510 where the diameter of the inner surface 510c is parabolically tapered, the angles of reflection are greater than the angles of incidence. In other words, the angles of reflection are increased by the tilt angle of the inner surface 510c in the tapered portion of the tube 510 relative to the optical axis of the tube 410. Because of the parabolic shape of the inner surface 510c in the tapered portion of the tube 510, the tilt angle gradually changes along the tapered portion as the distance from the first end 510a increases. The angle of reflection is increased by the tilt angle in the tapered portion, and thus the angle of reflection varies with variations in the tilt angle along the tapered portion. This change in the angle of reflection in the tapered portion causes the reflected light rays to be flattened out such that they are directed into the small area on the second end 510b of the tube 510 adjacent the lens 17. This causes more of the optical power produced by the EOC 6 to be coupled by the lens 17 onto the OEC 9, which can result in improved optical efficiency. In the portion of the tube 510 where the diameter of the inner surface 510c is constant, the angles of incidence and reflection are equal, and the light is conducted through the tube 510 in the same manner in which light is conducted through the tube 10 shown in
As indicated above with reference to FIGS. 1 and 3-7, the distance between the leads 11 and 12 is typically 2 to 3 cm, and the length of the hollow tube is slightly less than this distance. Thus, the overall length of the opto-isolator is relatively short. This distance, and thus the length of the opto-isolator, are chosen to provide an electrical separation between the transmitter and receiver modules that will work at isolating high transient voltages, e.g., transient voltages of about 12 kV and greater. A distance of about 22 millimeters (mm) will generally be sufficient to provide transient voltage isolation for 12 kV. The maximum distance is limited by the amount of optical power that needs to be received at the OEC 9 and by the bit rate, which may be 200 megabits per second (Mb/s), for example, and higher (e.g., up to 500 Mb/s for a LED due to the fact that the photodiode area is then very small). If a vertical cavity surface emitting laser diode (VCSEL) is used as the EOC 6, then the distance will be larger and the bit rate may be up to 10 Gb/s. Often, the distance and length limitations are based on the available space on a printed circuit board (PCB) or other mounting surface.
As indicated above, the hollow tube is typically made of a molded plastic material, and thus is very inexpensive to manufacture. This feature provides a significant cost savings over other types of opto-isolators that use optical fibers or rods surrounded by fluids as the optical waveguides. This cost savings is important, especially in cases where only very short optical links are needed and there is no need to use more expensive waveguides, such as optical fibers, which require expensive and time-consuming affixing and alignment processes. The opto-isolator of the invention provides an inexpensive and optically robust solution that is well suited for short optical links.
It should be noted that the invention has been described with reference to illustrative embodiments for the purpose of demonstrating the principles and concepts of the invention. The invention is not limited to these embodiments, as will be understood by those skilled in the art in view of the description provided herein. Other variations and modifications may be made to the embodiments described herein, as will be understood by those skilled in the art, and such modifications and variations are within the scope of the invention.
