The increasing demand for voice, data and video in the access markets has driven the introduction of the Passive Optical Network (PON) architecture. Referring to
The ONT is used at the customer premises and is thus regarded as a consumer electronic instrument, and as such it is under mounting pressures for cost reduction. The most expensive component commonly included within the ONT is the Trx (transceiver). Generally, the ONT Trx consists of a Small Form Factor (SFF) Trx that is soldered onto the ONT board.
Accordingly, there is a need in the art for cost improvements in the production of various components of optical network terminals.
According to one aspect, the invention is directed to a method that may include accumulating data indicative of the variation of selected variables with temperature for a batch of sample optical components, over an operating temperature range; determining the values of the selected variables at a single temperature of at least one new optical component for installation within an optical sub-assembly; and estimating the values of the selected variables as a function of temperature over the operating temperature range for the at least one new optical component based on the accumulated data and the values determined at the single temperature.
Other aspects, features, advantages, etc. will become apparent to one skilled in the art when the description of the preferred embodiments of the invention herein is taken in conjunction with the accompanying drawings.
For the purposes of illustrating the various aspects of the invention, there are shown in the drawings forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one having ordinary skill in the art that the invention may be practiced without these specific details. In some instances, well-known features may be omitted or simplified so as not to obscure the present invention. Furthermore, reference in the specification to phrases such as “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of phrases such as “in one embodiment” or “in an embodiment” in various places in the specification do not necessarily all refer to the same embodiment.
An Optical Network Terminal (ONT) is a modem that is equivalent in function to that of a cable modem situated in a customer premises. The ONT converts data transported in the optical domain over fiber optic cable into an electronic form suitable for use by voice, video, and data equipment such as telephones, televisions and computers respectively. An ONT is thus considered a CPE (consumer premises equipment) device that demands a low selling price typical of that market.
Generally, the most expensive component of an ONT is the optical transceiver—the Trx. The Trx generally needs to be calibrated to deliver a set performance level throughout the operating temperature range, which may be as wide as −40 C (C=Celsius) to +85 C. Typically, this can be a very costly step during manufacturing as it requires calibrating the transceiver at several temperatures within the operating range.
Preferred embodiments of present invention are intended to produce cost savings in the volume manufacturing of ONT modems by doing one or more of the following.
1. Separating the transceiver (also referred to herein as a “trx”) into (a) a constituent Bidirectional Optical Sub-Assembly (BOSA); and (b) Peripheral Electronics (PE), and to place these components directly onto the ONT electronic board.
2. A method is presented to calibrate the resulting structure at a single point (preferably at room temperature) rather than at multiple calibration temperatures. This process simplification leads to lower priced devices and high volume throughput manufacturing when compared to current processes.
The following is a proposed cost-saving solution that in effect separates the SFF Trx into a Bidirectional Optical Sub-Assembly (BOSA) and the Peripheral Electronics (PE). A functional diagram of one possible implementation is shown in
For volume manufacturing of ONT modules, it is cost prohibitive to calibrate each and every ONT over the entire operating temperate range. Accordingly, the following is a proposed method for calibrating the BOSA using room temperature data in combination with statistically gathered data pertaining to the thermal characteristics of a given batch of Photodiodes and the Lasers that may be included in the BOSAs. In the following, we present the mathematical equations representing the behavior of ideal Lasers and APDs (Avalanche Photo-Diodes). However, in the general case, the microprocessor enables piece-wise continuous calculations that may be needed to account for non-ideal device behavior.
In a preferred embodiment of the present invention, recalibration of the lasers and/or APDs at different temperatures is unnecessary, since the combination of data from (a) room-temperature calibration of BOSA modules and (b) statistically measured parameters over the module operating temperature range, are sufficient to estimate calibration data over the entire range of operating temperatures.
A given Laser diode may be characterized by a light output versus forward bias-current scan as shown in
The extinction ratio, r, is defined as the ratio of the power in the “1” state to that in the “0” state and may be written as:
The light output, L, at any forward bias current, I, may be written as:
L=η(I−Ith) (3)
And the slope efficiency, η, may be written as:
Using equations (3) and (4), the average power may be written as:
Similarly, using equations (3) and (4), the extinction ratio may be written as:
Rearranging equations (5) and (6) and using the definition for r, we may write for IB and IMod:
In summary, to have the laser operate at a given average power, La, and a given extinction ratio, r, a scan of L vs. I is performed and the parameters η and Ith (threshold current) are determined. Subsequently, the required values of IB and IMod are calculated using equations (7) and (8) above, respectively.
The threshold current and the slope efficiency defined above are temperature dependent. As the ambient temperature is changed, from T to T′, the laser threshold current Ith changes approximately according the equation (9):
And the slope efficiency changes approximately given by:
In the above, α and T0 are constants that are preferably established at the time of calibration. According to the discussion above, once the values for the threshold current and slope efficiency are established at a calibration temperature, T, it is then generally possible to predict their values at any another temperature, T′, provided that the calibration constants α and T0 are established beforehand over the operating temperature range. The same equations (7) and (8) are subsequently used to predict the new required currents, IB and IMod.
A preferred embodiment of the present invention is directed to a room-temperature calibration of devices (lasers and/or APDs), during volume manufacturing, through the use of a priori measured parameters on a statistically representative sample of Lasers and APDs from the same batch of parts as those being manufactured and calibrated. The parameters measured have been shown to obey Gaussian distributions with an average value and a standard deviation with respect to the average. In order to estimate the errors in establishing the correct average power and extinction ratio we can write equations for the power at the “1” state and the “0” states using equations (3), (7) and (8):
The average power and extinction ratio may then be obtained from equations (1) and (2) and may be written as follows:
From equations (13) and (14) we can derive the uncertainty in setting the average power and extinction ratio:
The implications of equations (15) and (16) are that the uncertainty in setting the extinction ratio is zero and that the fractional uncertainty in setting the average power is equal to that of determining the slope efficiency from the a priori established relations. From a practical standpoint the conclusions that we may arrive from the above analysis are:
1) To ensure that the method presented above remains valid, the threshold current is preferably set above the expected value of the threshold current, at room temperature, which is provided by the manufacturer of the BOSA. The threshold temperature at temperatures other than room temperature may be calculated using equation (9). In one embodiment, to err on the side of safety, the threshold current may be deliberately set above the level that results from the combination of manufacturer specifications and the calculation from equation (9). In one embodiment, the offset (i.e. the amount by which the actual current gets increased over the theoretical current) may be between 0% and 10%. However, in other embodiments, the offset may be greater than 10%.
The magnitude of this overset (also referred to as an “offset”) is arbitrary since optical power is a function of the difference between the applied current and the threshold current. However, for practical purposes, the offset should be kept as small as possible so that the drive currents (bias and modulation) are kept as small as possible over the operating temperature range. If the drive currents are too high, the light output may become non-linear in relation to forward current and in some cases will lead to saturation effects. The threshold current calculated is thus intentionally offset on the high side by a safe margin.
2) The bias current applied to the device should be at a safe margin (also referred to as an offset) above the calculated threshold current. In one embodiment, the “safe margin” may be 0% and 20% of the threshold current. However, in other embodiments, the margin may be more than 20% of the threshold current. Under real world conditions, the magnitude of the margin (of the bias current over the threshold current) may vary among the various batches of parts. In practice, only the bias current is applied and the power derived will depend on the difference between the applied bias current and the real threshold current.
3) The parameters α and T0 are preferably chosen to lead to average power levels that are greater than the required minimum. This may be ensured by intentionally offsetting the average values in a predetermined direction by a number of standard deviations from the average. In one embodiment, the average values may be offset by one, two, or three standard deviations either upward or downward from the average (i.e. mean) value. In other embodiments, still greater offsets could be employed.
Explanations for the Laser Diode Statistical Batch Calibration Routine for Each Laser Diode from
The Laser diode statistical batch calibration routine is shown in
Each Laser diode from the statistically representative sample is preferably measured at n regular temperature intervals ΔT in an environmental chamber, from temperature TCh (chamber temperature)=TLow to TCh=THigh. In one embodiment, Tlow may be 0 C, and Thigh may be 85 C. However, Tlow and Thigh may be either lower or higher than the respective stated values.
Using Equations (9) and (10), the parameters α and T0 are determined for each laser diode for teach temperature value within the pertinent temperature range. Once data has been collected from the various sample devices, a “look-up table” is constructed consisting of the average values and standard deviations of α and T0 for the entire batch of sample laser diodes. These values, averages, and standard deviations, are then preferably used during the Laser operating cycle to determine Ith and η (the slope efficiency) at any temperature using a suitable interpolation algorithm.
The Laser diode room temperature calibration routine is shown in
Each Laser diode is scanned to determine the L-I (Power vs. current) curve for that diode. Then, the threshold current, Ith, and slope efficiency, η, for each laser diode may be determined from the laser L-I curve (i.e. the curve relating current to light power). The bias current, IB, and the modulation current, IMod, may then be set using equations (7) and (8). In actual practice, it may be possible to use the threshold current, Ith, and slope efficiency, η, as supplied by the BOSA vendor to calculate the bias current, IB, and the modulation current, IMod from (7) and (8).
The Laser diode operation routine from a cold start is shown in
Using the Back Facet Monitor current (IBFM) as feedback, we maintain the average Laser output power, La. Then, we again measure the laser diode temperature, T. We then use η from the look-up table and equation (8) to calculate IMod for all subsequent temperatures while the laser remains on.
Directing attention to
The APD gain, M, may be expressed by equation (17):
Where VB is the breakdown voltage, A is a constant, and V is the applied reverse bias voltage. From the above equation, it is clear that the bias voltage should be chosen a safe margin away from the breakdown voltage. The temperature dependence of the APD implies that the optimum gain and hence the optimum bias voltage are also temperature dependent. If the temperature changes from T to T′, the required optimum bias voltage, Vo′ is empirically known to follow the equation:
V
0(T′)=V0(T)+γ(T′−T) (18)
Where γ is a constant. As in the laser diode calibration discussion above, in a preferred embodiment of the present invention a room temperature calibration of APD devices is conducted using a priori established data from a statistically significant number of devices. In this case, the constant γ is determined from the data. In volume production, the APD is initially at temperature T and is biased at a safe margin below the breakdown voltage which coincides with the optimum voltage. In one embodiment, the APD may be biased at a level three volts below the breakdown voltage. However, in other embodiments, the bias voltage may be more or less than three volts below the breakdown voltage.
The look-up table in the microprocessor contains the values of the optimum voltage as a function of temperature as per equation (18), or a variant of it. The uncertainty in determining γ will in turn cause an uncertainty in determining the bias voltage which in turn will lead to an uncertainty in the receiver sensitivity. This uncertainty in the receiver sensitivity may be estimated as:
The APD statistical batch calibration routine is shown in
Using equation (18), the parameter γ is determined for each APD within each ΔT throughout the range.
Once all the data has been collected, a look-up table is constructed that includes the average value and standard deviations of γ for the entire batch of APD devices. These values, averages, and standard deviations can then be used during the Laser operating cycle to determine the optimum bias voltage, V, at any temperature using a suitable interpolation algorithm.
The APD room temperature calibration routine is shown in
In actual practice, it may be possible to use the optimum voltage as supplied by the BOSA vendor to calculate the actual bias voltage using the γ values from the look-up table and equation (18).
Explanations for the APD Operation Routine from a Cold Start
An embodiment of the APD operation routine from a cold start is shown in
In an embodiment, RAM 1106 and/or ROM 1108 may hold user data, system data, and/or programs. I/O adapter 1110 may connect storage devices, such as hard drive 1112, a CD-ROM (not shown), or other mass storage device to computing system 1100. Communications adapter 1122 may couple computing system 1100 to a local, wide-area, or global network 1124. User interface adapter 1116 may couple user input devices, such as keyboard 1126, scanner 1128 and/or pointing device 1114, to computing system 1100. Moreover, display adapter 1118 may be driven by CPU 1102 to control the display on display device 1120. CPU 1102 may be any general purpose CPU.
It is noted that the methods and apparatus described thus far and/or described later in this document may be achieved utilizing any of the known technologies, such as standard digital circuitry, analog circuitry, any of the known processors that are operable to execute software and/or firmware programs, programmable digital devices or systems, programmable array logic devices, or any combination of the above. One or more embodiments of the invention may also be embodied in a software program for storage in a suitable storage medium and execution by a processing unit.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/405,692, filed Oct. 22, 2010, [Attorney Docket No. 312-49], entitled “System and Method For Manufacturing Optical Network Terminal Components”, the entire disclosure of which is hereby incorporated by reference herein.
Number | Date | Country | |
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61405692 | Oct 2010 | US |