The invention is directed towards semiconductor laser optical devices and more specifically, towards devices used in the monitoring and redirecting of light from a semiconductor laser light source.
Semiconductor lasers are used as components in many optical systems. A Vertical Cavity Surface Emitting Laser (VCSEL) is one type of semiconductor laser. A VCSEL is often the preferred light source in applications, since it can be manufactured using standard integrated circuit fabrication methods. However, the output power of a VCSEL changes with temperature and time as the VCSEL ages. To keep the output power steady, the VCSEL light output is monitored constantly. A portion of the VCSEL light output is typically diverted in a feedback loop onto a monitoring device, such as a photodiode. When the output power varies, the feedback circuitry adjusts the current in the VCSEL accordingly. A partially transmitting mirror, also known as a window-mirror, is commonly used to divert a portion of the VCSEL light output onto the photodiode.
To make a compact device and to reduce stray inductance, the circuitry that drives the VCSEL needs to be physically located near the VCSEL. If the photodiode is also located close to the VCSEL, the photodiode and driver circuitry can be combined into a single integrated circuit (IC) known as a monitor/driver IC. This arrangement is advantageous because it reduces the number of parts that need to be manufactured and assembled, saving both time and money. The distance between the light emitted from the VCSEL, and the light incident upon the photodiode, is indicative of how close the VCSEL and photodiode can be to each other.
A device frequently used in monitoring VCSEL light output is an Angled Window Can (AWC) 101, as shown in FIG. 1A. Ray tracings of the light paths are drawn in. For ease of explanation, reference will be made to these exemplary light rays rather than the entire beam itself. A window-mirror 103 is supported by an outer shell 104 and positioned at an angle to an optical axis 106 of a VCSEL 105. The window-mirror 103 transmits a portion of the light from the VCSEL 105 as transmitted light rays 107, and diverts a portion of the light as monitored light rays 109 onto a photodiode 111. The monitoring of the VCSEL 105 is most effective when all of the monitored light rays 109 are detected. However, the monitored light rays 109 are not focused and spread over a wide area, requiring a photodiode 111 with a large surface area to detect all of the monitored light rays 109. The larger the surface area needed, the more expensive will be the photodiode 111. Additionally, to optically couple a fiber-optic cable (not shown) to the transmitted light rays 107 of the AWC, the fiber-optic cable must be mounted perpendicular to the plane of the VCSEL 105 and photodiode 111. This configuration results in a bulky package awkward to assemble and use in optical systems.
FIG. 1B shows an optical monitor 113 made by Tyco Electronics, with simple light tracings included. Optical monitor 113 is used in transceivers such as Tyco Electronics"" product #1382345-1. Optical monitor 113 has an input lens 117 that collimates light from a VCSEL 116. A totally internally reflecting surface 115 reflects the collimated light towards a window-mirror 119. The window-mirror 119 partially transmits the light towards an output lens 123. The window-mirror 119 also partially reflects the light back at the totally internally reflecting surface 115, which reflects the light towards a monitor lens 121. The monitor lens 121 then focuses the light onto a photodiode 122. However, the light reflected by the window-mirror 119 reflects at a relatively large angle A125. Therefore, light path 127 onto the photodiode 122 is relatively far from light path 129 from the VCSEL 116xe2x80x94too far for the photodiode 122 to be integrated with the VCSEL driver into a single monitor/driver IC. Another drawback to the prior art optical monitor 113 is that it is not easily adaptable for parallel optics or Coarse Wavelength Division Multiplexing (CWDM). Furthermore, all of the optical components in this optical monitor 113 are surrounded by air. As the light travels through the air and the optical components, optical power is lost through Fresnel reflections. Fresnel reflections are caused when light travels between materials having different refractive indices.
Accordingly, there remains a need for an optical monitor that can monitor the light output from a VCSEL with a relatively small photodiode, to minimize the cost of the photodiode. Additionally, the optical monitor should have a low profile that keeps the VCSEL and photodiode on a plane parallel to the fiber-optic cable for easy assembly. Preferably, the distance between the light path from the VCSEL and the light path onto the photodiode is relatively small, which will allow the VCSEL driver and photodiode to be combined into a single monitor/driver IC. Fresnel reflections should also be kept to a minimum so as to minimize reflective optical power loss. Furthermore, the optical monitor should be easily adapted for use with parallel optics or CWDM.
In accordance with an aspect of the present invention, an optical device has input, output, and monitor lenses; a reflective surface; and a window-mirror. The device is preferably contained within an optically transmissive block made of moldable plastic, and the reflective surface is preferably a totally internally reflecting surface of the optically transmissive block. Incoming light from a light source is collimated by the input lens, reflected off of the totally internally reflecting surface, and then split by the window-mirror into partially reflected and partially transmitted light. The partially reflected light passes through the monitor lens to be focused onto a photodiode. Since the monitor lens can focus light onto a small area, a relatively small photodiode can be used, which results in a cost savings over the AWC. The partially transmitted light continues through the output lens to be focused onto a desired location. The internally reflecting surface and the window-mirror are angled such that the collimated incoming light and the partially reflected light have paths substantially parallel to each other. This allows the light source and the photodiode to be placed close together, and allows the photodiode to be integrated into a monitor/driver IC.
In a preferred embodiment, the window-mirror and lenses have substantially the same refractive index as the plastic to minimize reflective optical power loss from Fresnel reflections. The lenses can be molded from the same plastic as the optically transmissive block. Since the output light xe2x80x9cturnsxe2x80x9d and exits at an angle from the input light, this optical device shall hereinafter be called an optical turn. The output light turns at a right angle to the input light, which allows the light source and the photodiode to be assembled on a plane parallel to the fiber-optic cable. By keeping the light source and photodiode on a plane parallel to the fiber-optic cable, the light source, photodiode, and optical turn can be assembled into a package that is compact and has a low profile.
In an alternate embodiment, the optical turn is adapted for parallel optics and monitors a light array. An input lens array, monitor lens array, and output lens array are used in place of the input lens, monitor lens, and output lens.
In another embodiment, the optical turn monitors a light array and further includes a zigzag multiplexer. This embodiment of the optical turn is used to carry out CWDM, the combination of several light beams with different wavelengths into one light beam that can be transmitted as one signal.
Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.