Abstract:
Techniques for designing compact free-space optical device with all input/output ports on one side are disclosed. Instead of folding a fiber, a beam folding means is provided to turn a light beam to significantly reduce the size of the device. In one embodiment, there are a first collimator, a second collimator, and a beam folding means to turn a light beam from the first collimator back to the second collimator by two turns so that a first light path from the first collimator to the beam folding means and a second light path from the second collimator to the beam folding means are parallel. A substrate is provided to which the first and second collimators and filters are boned thereto.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is generally related to the area of optical communications. In particular, the present invention is related to optical wavelength multiplexing or add/drop devices and the method for making the same in compact size, where the size of the device is small enough to fit in small form-factor optical modules, such as C Form-factor Hot Pluggable 4 (CFP4) and Quad Small Form-Factor Pluggable 28 (QSFP-28) for client-side data communication. 
     2. The Background of Related Art 
     The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and perhaps more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements. 
     Optical add/drop and multiplexer/demultiplexer devices are those elements often used in optical systems and networks. For example, an exchanging of data signals involves the exchanging of matching wavelengths from two different sources within an optical network. In other words, an add/drop device can be advantageously used for the multi-channel signal for dropping a wavelength while simultaneously adding a channel with a matching wavelength at the same network node. Likewise, for transmission through a single fiber, a plurality of channel signals are combined via a multiplexer to be a multiplexed signal that is eventually separated or demultiplexed via a demultiplexer. 
     A typical prior art free-space WDM Demux, as disclosed in U.S. Pat. No. 8,537,468, is duplicated in  FIG. 1 . The device allows a multiple wavelength light traveling from a common port and to be separated into multiple narrow spectral bands respectively directed to individual channels. At each of the multiple channels, a dielectric thin film filter transmits a selected wavelength band to a designated channel port and reflect all other wavelengths.  FIG. 2  shows a typical optical transmission and reflection spectra of a single-channel band-pass filter that transmits λ i  and reflects wavelength from λ 1  to λ i−1  and λ i+1  to λ n . The reflected wavelength signals continue propagating to next channel ports, where an in-band signal is transmitted out and the out-band signals are reflected and continue propagating along the optical path. After multiple bounces in a zig-zag fashion as shown in  FIG. 1 , multiple channel signals are dropped. The device can also be used as a multiplexer (Mux) with the common port outputting all wavelengths multiplexed or added from the individual channels. The design has a number of advantages including low back-reflection, low loss and high reliability. 
     At the moment, 40 Gb/s or 100 Gb/s is achieved by using 4-channel WDM Mux/Demux that is widely adopted commercially in the CFP4 and QSFP28 packaging. To meet the exploring data consumption requirement and achieve higher data transmission data of 800 Gb/s, it is practical to double the number of channels from 4 to 8 or more. However, this change does not simply mean doubling the size of current 4-channel WDM. The design is, as matter of fact, very challenging. The most important issue that needs to be resolved is how to fit all the 8 channels Mux/Demux into the form factor of CFP4 or QSFP4 which has a very limited space inside. The CFP4 form factor is a CFP Multi-Source Agreement (MSA) defining an optical transceiver in the CFP4 form factor to support 40 Gbit/s and 100 Gbit/s interfaces for Ethernet, telecommunication and other applications. QSFP stands for Quad Small Form-factor Pluggable (QSFP) and is a compact hot-pluggable transceiver compact used for data communications applications. It interfaces networking hardware to a fiber optic cable. It is an industry format jointly developed and supported by many network component vendors, allowing data rates from 4×10 Gbit/s. The format specification is evolving to enable higher data rates. 
     The present application discloses the designs of 8-channel WDM devices that can fit into the form factor of CFP4 or QSFP4 
     SUMMARY OF THE INVENTION 
     This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions in this section as well as in the abstract and the title may be made to avoid obscuring the purpose of this section, the abstract and the title. Such simplifications or omissions are not intended to limit the scope of the present invention. 
     In general, the present invention pertains to compact multi-channel WDM devices for high-speed data communications. According to one aspect of the present invention, optical parts (e.g., filters and reflectors) are fairly evenly distributed on one side or both sides of a substrate to reduce the physical size of the packaging thereof. A reflecting mechanism (e.g., a prism or mirrors) is used to ensure all output ports are on end of the substrate to fit retroactively the form factor in compliance with an industrial standard. Depending on implementation, the industrial standard is generally related to a transceiver in a form factor of QSFP, QSFP28, CXP, XFP, CFP, CFP-2, or CFP-4. 
     According to another aspect of the present invention, optical filters are used as reflectors as well. Instead of using a separate reflector to bounce a light beam between the optical filters, an optical filter designated to transmit a signal optical filters at a designated wavelength is used to reflect the light beam carrying other wavelengths onto another optical filter, resulting in more space saving in packing the device. 
     According to one embodiment, the present invention is a WDM optical device, it comprises a port, a substrate with a first side and a second side, an optical turning part disposed on one end of the substrate, a first set of optical filters and a second set of optical filters disposed substantially in parallel on the first side of the substrate, wherein a light beam carrying N signals respectively at wavelengths λ 1 , λ 2 , . . . λ N  from the port impinges upon a first one of the optical filters in the first set, N is at least 8, the first one of the optical filters in the first set transmits through a first signal at a first wavelength λ 1  and reflects the light beam carrying other wavelengths onto a first one of the second set of optical filters, the first one of the second set of optical filters transmits through a second signal at a second wavelength λ 2  and reflects the light beam carrying other wavelengths onto a second one of the first set of optical filter, and wherein the first signal is coupled out via a printed circuit board disposed on another end of the substrate, and the second signal is turned by the optical turning part to the second side of the substrate and coupled out via the printed circuit board. In general, the WDM optical device is packaged in a form factor in compliance with an industry standard. 
     According to another embodiment, the present invention is a WDM optical device, it comprises a common port, a substrate with an upper side and a lower side, eight ports disposed on one end of the substrate, an optical turning part disposed on another end of the substrate, four optical filters in a first set and four optical filters in a second set disposed on the upper side of the substrate, wherein a light beam carrying eight signals respectively at wavelengths λ 1 , λ 2 , . . . λ 8  from the port impinges upon a first one of the four optical filters in the first set, the first one of the four optical filters in the first set transmits through a first signal at a first wavelength λ 1  and reflects the light beam carrying other wavelengths onto a first one of the four optical filters in the second set, the first one of the four optical filters in the second set transmits through a second signal at a second wavelength λ 2  and reflects the light beam carrying other wavelengths onto a second one of the four optical filters in the first set, and wherein the first signal is coupled out via one of the eight ports to a printed circuit board, and the second signal is turned by the optical turning part to the lower side of the substrate and coupled out via another one of the eight ports to the printed circuit board. 
     One of the objects, features, advantages of the present invention is to provide optical devices that are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost packaging, and easier manufacturing process. 
     Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  (prior art) duplicates a figure from U.S. Pat. No. 5,583,683; 
         FIG. 2  shows a typical optical transmission and reflection spectra of a single-channel band-pass filter that transmits λ i  and reflects wavelength from λ 1  to λ i−1  and λ i+1  to λ n ; 
         FIG. 3  shows an exemplary embodiment of a compact 8-channel WDM devices for high-speed data communications according to one embodiment of the present invention; 
         FIG. 4  shows another embodiment of the present invention in which a Mux/Demux device is rotated counterclockwise by 90° to fit a form factor; 
         FIG. 5  shows a laser bending technique achieved by using internal reflection in a 45° prism; 
         FIG. 6  shows a Mux/Demux with two layers to fully utilize the space on both sides of a substrate supporting the filters; 
         FIG. 7  shows a laser beam coming to a Com port is directed onto a filter that outputs a channel signal λ 1 , the filter reflects all others to a next filter, the next filter transmits a channel signal λ 2  that is impinged onto a prism (not shown); 
         FIG. 8  shows a side view of the device of  FIG. 6 ; 
         FIG. 9  shows some examples of an optical folding or turning part that includes a triangle prism, a pair of mirrors, two 45° prisms and a single prism; 
         FIG. 10  shows another design that includes eight filters only and has no mirrors; 
         FIG. 11  shows a prism being attached to one end of a substrate to fold 4 channels on the left to the right side; and 
         FIG. 12  shows another way by placing four left filters on the second side of the substrate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The detailed description of the present invention is presented largely in terms of procedures, steps, logic blocks, processing, or other symbolic representations that directly or indirectly resemble the operations of optical devices or systems that can be used in optical networks. These descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
     Referring now to the drawings, in which like numerals refer to like parts throughout the several views.  FIG. 3  shows an exemplary embodiment  300  of a compact 8-channel WDM devices for high-speed data communications according to one embodiment of the present invention. As shown in  FIG. 3 , an 8-channel WDM device includes 8 filters  302  and 7 mirrors  304  with a com port  306  and Demux channels  308  placed on two opposite sides. In another embodiment, an prism or two mirrors (not shown) may be added in the Com port to fold the beam in the Com port by 180°, all the channels and the Com port can be placed on the same side, which facilitates efficiently packing. The laser beams travel in the zig-zag fashion between filter and prism before they leave the device. 
     For Transmitter Optical Sub-Assembly (TOSA) applications, beams from 8 lasers with one in each channel after multiple reflections, merge into one beam in the Com port. A fiber receptacle or collimator plus a converging lens can be placed in the Com port to collect the merged beams into a single fiber. For Receiver Optical Sub-Assembly (ROSA) application, a laser beam with multiple wavelengths enters the device in the Com port. The multiple wavelengths are separated in free-space into different channels with one wavelength in each channel. A converging lens and photodiode (PD) is placed in each channel to detect the light with specified wavelength and transform into electronic signal with amplification. 
     In the 8-channel WDM device as shown in  FIG. 3 , the width of the device is about twice the one of the 4-channel WDM Mux/Demux device. As in the form factors like CFP4 and QSFP28, the width is very limited by the definition. Obviously it is very challenging to fit this design inside. To reduce the width of this design, it is natural to think to rotate this device.  FIG. 4  shows another embodiment  400  of the present invention. In this design, the Mux/Demux device  400  is rotated counterclockwise by 90° to fit the form factor. A reflective mirror  402  is added in the Com port to bend the laser in the Com port. This laser bending can also be achieved by using internal reflection in a  450  prism as shown in  FIG. 5 . By using total internal reflection, the polarization loss of beam (PDL) is very small, 0.02 dB or even less. As shown in  FIG. 4 , the beams in the Com port and 8 channels are perpendicular to each other. In the transceiver application, it is often needed that input and output are in the same direction. So a prism or mirrors can be added to bend all the 8-channel beams counterclockwise or clockwise by 90°. 
     In the design of  FIG. 4 , the overall size of the device is not decreased, which may leave no space for a PCB (printed circuit board) to be placed in the form factor, where the PCB is provided to support circuitry, such as photodetectors or laser diodes. To further reduce its size,  FIG. 6  shows a Mux/Demux  600  with two layers to fully utilize the space on both sides of a substrate  602  supporting the filters. A prism  604  is used to bend the reflected light beam to another layer. In reference to  FIG. 7 , a laser beam coming to the Com port  702  is directed onto a filter that outputs a channel signal λ 1 , the filter reflects all others to a next filter. The next filter transmits a channel signal λ 2  that is impinged onto the prism  604  (not shown in  FIG. 7 ). The prism  604  bends the channel signal λ 2  to output the same on the same side of outputting the channel signal λ 1 . At the same time, the next filter reflects the rest of the laser beam to a next filter to produce a channel signal λ 3 . The laser beams continues along the remaining filters. As a result, there are four channel signals λ 1 , λ 3 , λ 5  and λ 7  from the upper layer and four channel signals λ 2 , λ 4 , λ 6  and λ 8  from the lower layer. 
     The width or length of this design shown in  FIG. 6  or  FIG. 7  is roughly cut by half compared to the one shown in  FIG. 4  although the height is increased about 50%. A side view of the device  600  is shown in  FIG. 8 . A prism  802  is added on the side of the substrate  804  to fold the laser beam from a filter on a first layer onto a second layer after the laser beam goes through several times of zip-zig reflection and separated into 4 channels on the first layer. The beam folding is shown in the side view of  FIG. 8 . After reaching the second layer, the laser beam continues to follow the similar route of the first layer on the second layer and is separated into another 4 channels. The 4 channels in the first and second layers are located on two opposite sides. 
     Other than the triangle prism  802 , some others exemplary beam folding components are shown in  FIG. 9 , they are a pair of mirrors, two 45° prisms and a single prism. With two parts and active alignment, it is generally easier to get good parallelism of folding beam. 
     In the optical Mux/Demux with two layers, the filters and mirrors are located on two sides, which increases the height of the device substantially and also may create a challenge to fit it into the form factor due to the super sensitive locations of the mirrors and filters. To solve this issue,  FIG. 10  shows another design that includes eight filters only and has no mirrors. The eight channels are located on the two sides with 4 channels in each side. By using this layout, the width can be decreased without increasing the height. 
     In some application, it is needed all the 8 channels placed on one side. To meet this requirement,  FIG. 11  shows a prism  1101  being attached to one side of the substrate  1104  to fold the 4 channels on the left to the right side. It leaves enough space for the photodiodes (PD) or LD to be located on one single side, and easier for the PCB layout thereof. Another way to achieve this is to place the left 4 filters on the second side of the substrate as shown in  FIG. 12 . In this design, 4 filters are located on each side. 
     While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claim. Accordingly, the scope of the present invention is defined by the appended claims rather than the forgoing description of embodiments.