Abstract:
A new low-cost, highly reliable and compact optical parametric signal monitor is manufactured with simplified structure and improved configurations that require only lateral position adjustment for input/output beam alignment. A compact size is achieved by employing an integration of beam collimator, signal splitter, and wavelength selector. Thermal stability and reliable performance is achieved by applying multiple-layered optical reflection-transmission coating directly onto the end surface of a GRIN lens for tapping a small portion of the collimated beam onto the focus lens and the photo sensor. The insertion loss can be conveniently minimized by laterally shifting the relative position of a dual fiber ferrule and the GRIN lens without complicate angular adjustments. Optical parameters can be monitored with a simple attachment of a wavelength selective element between signal tapping unit and signal detector. A parametric optical signal monitor with compact size, high damage threshold, low manufacture cost and high performance stability is provided for convenient implementation in new and existing optical systems.

Description:
[0001]    This Formal Application claims a Priority Date of Mar. 15, 2002 benefited from a Provisional Application 60/364,983 filed on Mar. 15, 2002 and a Formal application Ser. No. 10/191,170 filed on Jul. 8, 2002 filed by the same Applicant of this Application. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to a device configuration and method for designing and manufacturing optical signal parametric monitors for optical fiber signal communication systems. More particularly, this invention relates to a simplified device configuration and method for manufacturing optical monitor to provide reliable performance.  
         BACKGROUND OF THE INVENTION  
         [0003]    A technical challenge is faced by those of ordinary skill in the art of designing and manufacturing optical devices to provide compact and reliable optical sensing and monitoring devices including wavelength selective optical parameter monitors with application versatility that can be conveniently implemented in different optical systems to carry out the self monitoring and diagnostic functions. Specifically, as the optical network communication systems are more broadly deployed to perform signal transmissions with higher bandwidths while various optical systems are becoming more complicate, the maintenance and trouble shooting of the optical systems become more difficult. For the purpose of reducing communication downtime and the costs required for system maintenance, diagnoses and repairs, there are ever increasing demands of “smart” systems provided with the self-monitoring, diagnostic and even self-correcting functions. A first step moving toward such systems is to introduce more sensing and monitoring devices in different optical system to carry out continuous sensing and detections of system for operation conditions. The sensor and monitors are implemented not only to the new systems but often are also required in the existing systems as well. Installation of sensors and monitors into existing systems often requires the use of smaller size sensors and monitors with broader range of operation versatility to fit into the existing configurations and operation conditions. Optical signal parameter monitor can provide crucial information of optical signals in optical fiber networks such as signal intensity within a defined channel, signal to noise ratio, bit-error rate, or even signal carrier wavelength, etc. with compactness, versatility and reliability.  
           [0004]    More particularly modern optical telecommunication systems are making progresses in becoming more intelligent and more reliable thus enabling the optical networks to function as the backbone of the information exchange system today and in the future. In view of these developments, it is highly desirable for various sensors to monitor optical parameters with improved measurement quality in situ to discover any form of system failure or performance degradation, and to quickly pin-point the root cause to set alarm or even, meanwhile, initialize the alternative backup system thereafter. Among all the parameters of the optical signals, total signal power in an optical fiber, signal intensity for each channel of WDM, optical signal-to-noise ratio, and bit-error-rate, etc. are the most essential parameters for measurement. In some optical networks, monitoring the supervisory channels is somewhat a guaranty for quality of a working system. However, the conventional technologies for optical sensing and monitoring devices as of now still cannot provide simple, cost-effective, compact, reliable and versatile monitoring devices to accurately measure these important operating parameters.  
           [0005]    For these reasons, there is still a need in the art of designing and manufacturing of wavelength-selective optical parametrical monitors with smaller size, more operational versatilities and higher reliability to overcome the difficulties discussed above. Specifically, an improved and simplified configuration and method for making smaller and reliable wavelength-selective optical parametrical monitors to accurately measure the signal intensity for each channel of WDM, optical signal-to-noise ratio, and bit-error-rate with reduced production cost are required.  
         SUMMARY OF THE PRESENT INVENTION  
         [0006]    It is therefore an object of the present invention to provide a new and improved simplified and reliable configuration and manufacturing method to provide compact and reliable detectors for measuring important optical parameters including channel specific and wavelength selective signals such that the above- mentioned difficulties and limitation can be resolved. This invention discloses a method to simplify the manufacture processes by forming the tapping layer as optical splitting/tapping layer directly on the GRIN lens without alignment and assembling processes. Furthermore, the new and improved configuration and manufacture method take advantage of the optical path deviations between incident/reflection beams through a GRIN lens when there is a small linear shift between the optical axes of the fiber pigtail and GRIN lens. This invention further discloses compact configurations for implementing compact and reliable signal detection and wavelength tunable signal monitoring devices. The parametric measuring detector and channel-specific and wavelength-selective signal monitors with reduced insertion loss can be achieved with new and improved alignment method implemented with a set of holding tubes to achieve minimum insertion loss between the incident and reflective beams.  
           [0007]    Specifically, it is an object of the present invention to provide a simple and reliable device to sample and monitor signals in an optical system. The optical signal monitor is implemented with a wavelength-selective filter-centered packaging design to achieve high environmental stability. Integration of the tapping and detection elements is implemented as part of the design to achieve a compact device configuration and to simplify the processes of device manufacture. The new and improved configuration also utilizes concurrent light collecting and focusing functions to improve the signal collection efficiency. The concurrent light collecting and focusing unit is packaged as part of a hermetically sealed sensor housing to provide flexibility of selecting sensors with various degrees of sensitivities and response time. The wavelength-selective optical parametric monitors disclosed in this invention are therefore useful not only for signal intensity detection but also for other broader ranges of applications such as data quality analyses. The new and improved device as disclosed in this invention thus provide several key advantages including but not limited to ultra compactness, broad versatility, long term device dependability and reliability, and design flexibility to fit into wide varieties of existing optical systems. The simplified configuration of the device as disclosed in this invention further streamline the manufacturing processes and reduce the production cost of the signal sensing devices.  
           [0008]    Briefly, in a preferred embodiment, the present invention discloses a new wavelength-selective optical parametric monitor. The wavelength-selective optical parametric monitor includes a collimator for collimating an incident light into a collimated beam. The wavelength-selective optical parametric monitor further includes a wavelength-selective beam splitter/tapping layer for transmitting a tapped portion of the collimated beam of a selected range of wavelengths through the splitter/tapping layer for detecting an optical parameter. The coated splitter/tapping layer on the collimator further reflects a remaining portion of the collimated beam back to another optical transmitting media for continuous optical transmission.  
           [0009]    This invention further discloses in a preferred embodiment, a method for monitoring a wavelength-selective optical parametric monitor. The method includes the steps of A) employing a collimating means for collimating an incident light into a collimated beam. And, step B) of coating a wavelength-selective splitter/tapping layer onto the collimating means for transmitting a tapped portion of the collimated beam of a selected range of wavelengths therethrough for measuring and monitoring a wavelength-selective optical parametric signal. In a preferred embodiment, the method further includes a step of receiving the incident optical beam through an input optical port of a dual fiber ferrule for projecting to the collimating means and receiving an output optical beam reflected from the splitter/tapping layer through an output optical port of the dual fiber ferrule. In another preferred embodiment, the method further includes a step of receiving the tapped portion of the collimated beam of the selected range of wavelengths from the beam splitting/tapping layer into an optical signal detecting means for detecting the wavelength-selective optical parametric signal relevant to the selected range of wavelengths.  
           [0010]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1A is a functional block diagram showing three major parts of an optical signal detection system of this invention;  
         [0012]    [0012]FIG. 1B is a cross sectional view of a schematic diagram showing the configuration of the signal splitter unit;  
         [0013]    [0013]FIG. 2 is a schematic diagram for showing vertical shift between two holding tubes for adjusting the relative position of the dual fiber ferrule and the GRIN lens to adjust the input and output beam projections;  
         [0014]    [0014]FIG. 3 is cross sectional view of two alternate shapes of holding tubes for housing and holding different sizes of focusing and sensing units;  
         [0015]    [0015]FIG. 4 is a perspective view of an optical parametric signal detector implemented with tunable filter of this invention; and  
         [0016]    [0016]FIG. 5 is a functional block diagram showing the data flow of signal processing for carrying out optical parametric detection and wavelength specific signal measurements of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Referring to FIG. 1A for a functional block diagram for showing three major units of a wavelength selective optical parametric detector of this invention. The wavelength selective optical parametric signal detector  100  receives an optical signal from an input optical fiber  106 . The input optical signal is processed by a signal splitter unit  101  for tapping a small portion of the input optical signal to project to a wavelength selector unit  102  for selecting a portion of the optical signal within a range of wavelengths for transmitting to a signal detector  103 . The signal detector unit  103  then carries out a wavelength-selected optical parametric signal detection to measure specific optical parameters for monitoring the operational conditions of an optical signal transmission system. The signal splitter unit  101  also reflects majority portion of the input signal back to an output fiber  107  for transmitting through a monitored optical signal transmission system (not shown).  
         [0018]    Referring to FIG. 1B for a preferred embodiment of wavelength-selective optical parametric signal monitor  100  of this invention. The improved wavelength-selective optical parametric signal monitor  100  includes a dual fiber ferrule  110  to receive an input beam from an input optical fiber  106  and projecting an output beam from an output optical fiber  107 . The input beam received from the input optical fiber  106  is projected into a beam collimator  120 , e.g., a GRIN lens  120  coated with a filter coating  125 . The filter coating  125  functions as a beam splitting-taping filter to sample a portion of the incident beam. The tapped portion of the signal is further selected by a wavelength selective filter  126  and detected by a photo detector through a focus lens  130  for measuring the intensity of the beam received form the input optical fiber  106 . The splitting-tapping filter  125  further reflect a major portion of the beam back through the GRIN lens  120  onto the output optical fiber  107  as an output optical beam.  
         [0019]    The splitting-tapping filter  125  is formed as a multiple-layered coating deposited onto the focusing surface of the GRIN lens  120  to maximize the thermal stability and to reduce the optical energy loss thus increasing the optical damage threshold. By coating the splitting-tapping filter  125  directly onto the GRIN lens  125  further simplifies the device structure and strengthens the mechanical integrity of the device and eliminates an alignment requirement between the GRIN lens  120  and a separate optical filter in the case a separate reflecting-tapping device is employed instead of the multi-layered coating filter. The ratio of the light splitting can be adjusted to accommodate different levels of photoelectric signal intensity and total optical signal intensity loss. Typically, the multi-layered filter coating is formed to generate a tapped signal in the range of 0.5% to 10% of the initial beam intensity.  
         [0020]    The wavelength selective filter  126  is for isolating the interested channel or channels from a WDM wavelength division multiplexing optical system. The wavelength selective filter  126  performs as a spectral selector to pick up a specific section to be analyzed while the rest is rejected. Several types of band pass filters could be deployed according to the nature of the signal in optical networks. The wavelength selective filter  126  may be an edge filter that is used for dividing the optical signal into two groups spectrally. Allowing all the signals that are carried on wavelengths either longer or shorter than a selected cut-off wavelength pass through the filter while the rest is rejected, one part of the signal can be monitored without the disturbance of the other. This configuration can be used for supervisory channel monitoring. The wavelength selective filter  126  may be a band-pass filter to select a certain range of wavelength that is of interests. In this case, a specific channel or channels could be isolated, monitored and analyzed. Based on the band pass filter parameters, this configuration can be used for channel monitoring in DWDM or CWDM systems. Wavelength selective filter  126  is attached, with Epoxy  128  as one embodiment shown, onto the splitting/tapping coating on the end surface of the GRIN lens  125 .  
         [0021]    The face-to-face end surfaces  101  and  111  of the dual fiber ferrule  110  and the GRIN lens  120  respectively are arranged as parallel surfaces with an incline angle θ where θ is eight degrees, i.e., 8°, to increase the return losses. Both of these end surfaces  101  and  111  are coated with anti-reflection multi-layered coating to reduce the optical losses in the transmission of the optical signal through these inclined surfaces. In order to adjust for minimizing the signal loss, the dual optical fiber ferrule  110  and the GRIN lens  120  are held in two holding tubes  105  and  124  respectively. These two holding tubes  105  and  124  have parallel interface to assure that there are only lateral linear position adjustments are made between the GRIN lens  120  and the dual fiber ferrule  110 . There is an air gap  121  between the dual fiber ferrule  105  and the GRIN lens  120  for focusing optimization. A very small gap  123  between these two holding tubes allows freedom of lateral movements between two holding tubes  105  and  124 . Held by these two holding tubes  105  and  124 , the relative position between the GRIN lens  120  and the dual fiber ferrule  110  is adjusted along two linear directions without relative angular adjustments thus significantly simplify the alignment processes. These two linear directions are along the optical axis, i.e., the X-axis and the Y-axis perpendicular to the optical axis, i.e., the Z-axis, to minimize the optical loss between the input port  106  and the output port  107 .  
         [0022]    [0022]FIG. 2 showed the alignment schematic. The relative movement between dual fiber ferrule  105  and GRIN lens  120  along the principle axis Z contributes to the focusing condition adjustments. Meanwhile, the relative lateral movements in the plane that contains X-axis and Y-axis change the angle between collimated incident beam and principle axis Z. Therefore, the relative position of incident beam and output beam can be adjusted by 3-dimention alignment without adjustment of tilt angle. In a preferred embodiment, the separation between the input and output optical fiber  106  and  107  is about 125 μm. The angle between the direction of the light beam emitted from the splitting-tapping filter coating  125  and the principle axis of the device along the Z-axis α is about 1.80.  
         [0023]    Referring to FIG. 1B again, for the purpose of increasing the structure integrity, a third holding tube  150  is holding the GRIN lens and the focus lens  140 . The holding tube contained also a photodiode  140  that has a hermetic sealing metal housing  160  to seal the holding tube and the photodiode  140  from the rear end of the holding tube  150 . The physical size of active area of the sensor determines the dark current and the capacitance and these functional parameters also respectively affect the signal to noise ratio (S/N) and the analog response bandwidth. In order to achieve higher S/N ratio and fast response, it is desirable to lower the capacitance and the dark current and in turn, that can be achieved by reducing the active area of the photodiode  140 . However, larger active area of the photodiode  140  can increase the overall response of the detector since more area will be illuminated. A balanced approach is to achieve adequate S/N ratio and response speed up to 2 GHz by adjusting the active area in the range from 50 μm to 100 μm in diameter. For a specific embodiment, the beam size of the light emitted from the beam splitting-tapping filter  125  is around 300 to 500 μm in diameter. In order to produce a compact wavelength-selective optical parametric signal-monitoring device, it is necessary to apply a focus lens  130  with a short focus length to reduce the size of the detector  140  and the optical path from the splitting-tapping filter  125  to the detector  140 . One specific configuration as shown in FIG. 1B uses a short focus lens  130  that is part of the hermetically sealed house  160 . A silica ball lens is implemented in a preferred embodiment to obtain low cost and good performance. To further reduce the space requirement for an even more compact wavelength-selective optical parametric signal monitor, a specially designed aspherical lens may also be employed.  
         [0024]    By using the holding tube  150  and the seal housing  160 , fixing the focus lens  130  and the photodiode  140  at fixed locations within the seal housing  160 , a self-aligned configuration is provided that greatly simplify the manufacture processes. The time and cost of manufacture is reduced and better structural integrity and product reliability is also achieved. As the tapped beam is focused onto the photodiode  140 , the photo energy is converted into electric energy by generating corresponding amount of photoelectric current that can be converted directly as light intensity measurement through two electrodes  145 . The photo-detector  140  can be further integrated by providing preamplifier onto the photo sensor chips to allow for even more flexibilities to process the tapped optical signal.  
         [0025]    Referring to FIG. 4 for a perspective view of an optical parametric signal detector  200  with a tunable wavelength selector that includes a signal splitter unit  210  for reflecting a majority portion of an input optical signal received from an input optical fiber  206  back to an output optical fiber  207 . A small portion of tapped signal is projected to a tunable filter  220  for transmitting a portion of the optical signal of selected range of wavelengths to an optical signal collecting and detecting unit  230  for carrying out a wavelength selective optical parametric signal measurement. Similar to band pass filter signal selection, a tunable filter  220  performs as a re-configurable channel monitor. With a fixed bandwidth that is tailored for the WDM system to be monitored, the center wavelength of the passing band can be adjusted (scanned) onto the points of interest. This configuration can be used as a channel monitor for signal intensities in each channel of the whole WDM system. Furthermore, by measuring both the signal intensity and noise level between the signal channels, a useful optical signal-to-noise ratio can be derived, which is vital to an optical network. Equipped with fast response photo sensors, signal quality can be monitored at bit-error-rate level.  
         [0026]    Referring to FIG. 5 for a functional block diagram showing an optical signal processing system  300  for receiving and processing the wavelength-selective optical parametric signals generated by the optical signal detector of this invention. The optical signals as measured by the optical detector  310 , e.g., detector  140  or  230  in FIGS. 1B and 4 respectively, are amplified by an electrical amplifier  320 . The amplified signals are then received by a data processor  330 . In the meantime, the data processor  330  also receives calibration data from a calibration unit  340  and signal criteria data from a criteria setting unit  350  to process the amplified optical signals to generate output data and to display information by using an information output device  360 .  
         [0027]    According to FIGS.  1  to  3  and above descriptions, this invention discloses a wavelength selective optical parametric signal monitor. The wavelength selective optical parametric signal monitor includes a collimating means  120  for collimating an incident light into a collimated beam. The parametric optical signal monitor further includes a wavelength selective beam splitter/tapping means for transmitting a tapped portion of the collimated beam of selected range of wavelengths therethrough for measuring and monitoring a wavelength selective optical parametric signal wherein the beam splitter/tapping means comprising a wavelength selective filter  126  together with a splitter/tapping layer  125  coated onto the collimating means. In a preferred embodiment, the wavelength selective optical parametric signal monitor further includes a dual fiber ferrule  110  having an input optical port  106  for receiving an incident optical beam to project to the collimating means  120 , the dual fiber ferrule further having an output optical port  107  for receiving an output optical beam reflected from the splitter/tapping means. In another preferred embodiment, the wavelength selective optical parametric signal monitor further includes an optical signal detecting means  140  for receiving the tapped portion of the wavelength-selective collimated beam from the wavelength-selective beam splitting/tapping layer for detecting an optical parameters relevant to the selected range of wavelengths. In another preferred embodiment, the optical signal detecting means further includes a focal lens  130  for focusing the tapped portion of the collimated beam onto a photodiode  140  for detecting an optical parameter. In another preferred embodiment, the wavelength selective optical parametric signal monitor further includes a first holding tube  105  for holding the dual fiber ferrule  110  and a second holding tube  124  for holding the collimating means  120  wherein the first holding tube and second holding tubes having a lateral position adjustment gap for minimizing an optical loss of the output optical beam reflected from the wavelength selective splitter/tapping means. In another preferred embodiment, the wavelength selective optical parametric signal monitor further includes a third holding tube  150  for holing the collimating means and the optical signal detecting means  140  for securely fixing a relative position therein. In another preferred embodiment, the wavelength selective optical parametric signal monitor further includes a seal housing  160  for sealing a rear opening of the third holding tube opposite the collimating means wherein the seal housing holds the focal lens  130  and the photodiode  140  at fixed positions for plugging and sealing the rear opening and placing the focal lens substantially at an optimal position relative to the wavelength-selective splitting/tapping layer. In another preferred embodiment, the focal lens is a silica ball lens. In another preferred embodiment, the focal lens is an aspherical lens. In a preferred embodiment, the collimating means is a GRIN lens having an inclined lens surface relative to a main optical axis of the GRIN lens and the inclined lens surface facing a-parallel ferrule surface of the dual fiber ferrule wherein the lens surface is disposed at a gap-distance from the parallel ferrule surface for achieving a focus optimization.  
         [0028]    This invention further discloses a method for monitoring a wavelength-selective optical parametric signal. The method includes the steps of A) Employing a collimating means for collimating an incident light into a collimated beam. And, step B) of coating a wavelength-selective splitter/tapping layer onto the collimating means for transmitting a wavelength-selective tapped portion of the collimated beam therethrough for measuring and monitoring an optical parameters relevant to the wavelength-selective optical signal. In a preferred embodiment, the method further includes a step of receiving the incident optical beam through an input optical port of a dual fiber ferrule for projecting to the collimating means and receiving an output optical beam reflected from the wavelength-selective splitter/tapping layer through an output optical port of the dual fiber ferrule. In another preferred embodiment, the method further includes a step of receiving the wavelength-selective tapped portion of the collimated beam from the beam splitting/tapping layer into an optical signal detecting means for detecting an optical parameters relevant to the wavelength-selective tapped optical signal.  
         [0029]    In a preferred embodiment, this invention further discloses a method of monitoring a wavelength selective optical parametric signal. The method includes the steps of A) employing a collimating and tapping means for tapping a wavelength-selective tapped portion of an optical signal to a focusing and detecting means for detecting the tapped portion of the optical signal. B) Employing a holding tube for securely holding and fixing the collimating and taping means at a fixed relative position from the focusing and detecting means. And, step C) employing a seal housing for sealing a rear opening of the holding tube opposite the collimating and tapping means and holding the focusing and detecting means in the seal housing for plugging and sealing the rear opening and placing at an optimal position relative to the collimating and tapping means.  
         [0030]    The holding tubes  105 ,  124  and  150  are securely attached to the dual fiber ferrule  110 , the GRIN lens  120  and the seal housing  160  respectively by applying thermally cured epoxy for long term reliable structure integrity and operation stability. The end surfaces between the holding tubes  105  and  124  are also securely attached together with properly applied adhesive material such as epoxy for fixing the relative position for minimizing the optical losses. The thermal stability and long term performance reliability of the optical monitor  100  is dependent on several structural and functional parameters. Specifically, these parameters are 1) the degree of fitness between the holding tubes and the dual fiber ferrule and the GRIN lens, 2) the length of the holding tubes  105  and  124 , 3) the quality of the end surfaces and the strength of mutual attachment of the holding tubes, 4) the uniformity of the interface between the holding tubes  105  and  124 , 5) the material of the holding tubes  105 ,  124  and  150 , 6) the types and the amount of the adhesive applied; 7) the pre-treatment of the end-surfaces to function as bonding surfaces; and 8) the settlement and curing processes of the adhesives for bonding the holding tubes thus bonding the device together. These interfaces and quality of attachment are greatly improved according to the configuration disclosed in this invention because the simple and linear interface configuration without curved or irregular shaped interface surfaces with proper freedom of movement for flexible position adjustment and conveniently application of adhesive materials on these interfaces.  
         [0031]    Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.