Patent Application
20090016725
This invention relates to a method and apparatus for compact multiplexing of optical carried data for logging tools used in oil and gas industry. More specifically, this invention relates to a wavelengths division multiplexer method and apparatus having a compact architecture for logging tools which obviates optical fiber bending within the logging tool.
Optical networks are commonly used in telecommunications for data transmission. A high transmission data rate is obtained by increasing the number of wavelengths propagating on the same fiber. Each wavelength is associated to a communication channel. Optical filter systems have been developed to selectively route different wavelengths along the network. These optical components allow adding or dropping wavelengths at each node along the fiber. Several filters can be combined in the same device to form an optical multiplexer (or demultiplexer).
Multiplexers based on wavelength division classically include a set of devices, which successively separate and isolate the different wavelengths. Several different technologies can be used such as arrayed waveguide grating and thin film filters. In the past thin film filters have been combined in series to form wavelengths based multiplexers. The principle of thin film multiplexing is based on backward reflection and an interaction of a light-wave on a plurality of thin film filters.
Each thin film filter outputs a signal that is centered on an associated wavelength whereas a reflected signal is input in the next filter. The filters are connected in series, one wavelength being extracted at each level and extended to “N” wavelengths. The number of filters required for a multiplexer is equal to “N,” the number of wavelengths to be separated.
A major constraint for previously known optical multiplexers is in the serial connection of sequential filters. At each level, a reflected port is connected to a successive filter input port. Consequently, the optical fiber must be bent by 180 degrees for connection to a successive filter input port. The radius of curvature of this half turn must be greater than the bend radius of the fiber. The maximum bend radius of a typical optical fiber (SMF 9/125, MMF 50/125) is approximately 25 mm. This bending radius leads to a significant size dimension, at least in one direction. In previously known multiplexers sizes larger than 100 mm in one dimension were not unusual.
Although this size may not be a limiting issue for many telecommunications applications, multiplexer size is a constraint for some applications such as borehole logging in the oil and gas industry. The physical restrictions in a borehole require a small tool diameter. A standard tool diameter size is approximately one and eleven sixteenths inches but even smaller diameters (below one inch) are desirable for some applications to pass borehole restrictions related to equipment for oil and gas production control (safety valves, production packers, down hole flow control and monitoring equipment).
A wide range of optical sensors have been developed, such as thermal, mechanical and electromagnetic. By operating at different wavelengths, data from the optical sensors can be multiplexed on the same fiber. This feature allows connecting only one fiber while performing measurements from several sensor sources.
Although classical multiplexer architecture has many benefits and has been useful in telecommunications applications, in the context of optical logging tools for oil and gas wells, conventional multiplexing architecture and size of previously known optical multiplexer systems is not well adapted for down hole sensing constraints and borehole geophysics applications. First, logging for oil and gas wells requires an extremely small dimension of the multiplexer due to the limited borehole diameter, especially in the case of production wells. Second, a small radial dimension of down hole logging tools does not permit bending of the optical fiber by an acceptable bend radius of the optical fiber.
The problems with multiplexing down hole data onto a single optical fiber discussed in the preceding are not intended to be exhaustive but rather are among many which demonstrate that optical, logging tool, multiplexing know in the past will admit to worthwhile improvement. In this, it would be desirable to provide compact optical multiplexing apparatus for multiplexing data from different sensors on the same fiber within the above logging tool constraints.
One embodiment of the invention comprises a method and apparatus for separating down hole discrete wavelengths carried down hole on an optical fiber for use in an optical well logging tool. In one specific example, a logging tool is equipped with three optical sensors performing borehole parameter measurements, such as temperature, pressure or borehole fluids characteristics. Each sensor measurement is attached to a given optical carrier wavelength. The multiplexer consists of a combination of three thin film filters, each with one input and two output ports. Each filter transmits a signal centered on a given wavelength, and reflects all complementary wavelengths. With the subject invention optical multiplexer architecture an input signal is fully multiplexed and each spectral component is isolated for use as a data carrier and return through the multiplexer back onto the optical fiber for transmission to the surface for data analysis.
Other aspects of the present invention will become apparent from the following detailed description of embodiments thereof taken in conjunction with the accompanying drawings wherein:
Referring now to the drawings and particularly to
On the surface the optical fiber is coupled to an opto-electronic recorder 112 for borehole data storage and processing. The logging tool 100 is equipped with a plurality of sensors and an optical multiplexer in accordance with the invention for assisting in transmission of the down hole data to the electronic recorder 112. Three typical borehole parameters that are measured comprise temperature, pressure, or borehole fluids characteristics.
Each sensor measurement is carried on a designated optical wavelength. These wavelengths are emitted simultaneously by the opto-electronic surface system 112 and propagate via a single optical fiber down to an optical multiplexer. The multiplexer ensures that an appropriate wavelength is dropped onto each sensor. Down hole data is then coupled with a specific optical carrier wavelength and reflected back through the multiplexer up to the surface for processing and storage as will be described below.
Optical multiplexers classically employ thin film filters to implement wavelength selection. One example of a thin film filter is disclosed in U.S. Pat. No. 4,373,782. The disclosure of this patent is incorporated by reference as though set forth at length.
In operation a bundled optical waveform 262 comprising wavelengths λ1 . . . λN is input via the optical fiber port 252 towards a first Graded Index (GRIN) lens 258. The optical filter 250 is positioned in series behind the first GRIN lens 258 and in front of a second GRIN lens 260. The optical filter 250 comprises an assembly of conventional thin films. The thin films are designed to pass wavelength components λ3 within a range corresponding to a preset central wavelength. The other wavelength components λ1, λ2, . . . XN, are reflected backward along the reflection port 256. The part of the signal whose energy is centered on the filter central wavelength λ3 passes through the thin films and is collimated by the second GRIN lens 260 into the transmission fiber 254. The complementary part of the signal 262 comprising wavelengths λ1, λ2, . . . -80 N is reflected backward by the filter 250 and passes back though the input GRIN lens 258. This lens performs collimation into the reflection port fiber 256. If a mirror 270 is placed at the end of the transmission port 254, the signal λ3 carried by the transmission port 254 is reflected back through the GRIN lens and thin filter set and the signal λ3 is propagated back onto the input fiber 252.
In order to provide a plurality of isolated wavelengths λ1, λ2, . . . λN the reflection port 256 can be bent 180 degrees and the above discussed architecture is repeated with a λ2 GRIN lens and thin filter set to isolate the λ2 wavelength. This process is repeated by bending the reflection port 180 degrees each time and selecting an appropriate thin filter set to isolate for multiplexing as many isolated wavelengths as desired. In other words, the filters are connected in series, one wavelength being extracted at each level, however, the optical fiber must be bent by 180 degrees for connection to a successive filter input port.
For an optical fiber the curvature radius of each half turn must be greater than the bend radius of the fiber. The maximum bend radius of conventional optical fiber (SMF 9/125, MMF 50/125) is approximately 25 mm. This bending leads to a significant dimension of optical multiplexers, at least in one direction. An optical multiplexer with a side longer than 100 mm is not unusual. In a telecommunications environment this size dimension may not be an issue, however, in a borehole logging environment where a standard diameter logging tool is 1 and 11/16 inches with even a one inch tool being desirable an optical multiplexer with the above 100 mm architecture is not acceptable.
In view of the foregoing, a new architecture which minimizes the overall size of the optical multiplexer and avoids any bending of the optical fiber would be highly desirable for a well logging environment.
The architecture comprises a combination of three (3) thin film filter sets 302, 304, and 306. Each filter set has one input port “I” plus two output ports namely a transmission port “T” and a reflection port “R” as discussed generally above. The first filter set 302 transmits a signal centered on λ1 to a transmission port 308 and diverts to the reflection port 310 complementary wavelengths λ2 and λ3.
The signal on the reflection port 310 comprising wavelengths λ1 and λ3 is then the input for the second filter set 304 that is centered on λ3. The wavelength λ2 is thus reflected by this filter set 304 and propagates towards a second channel 312. The second filter set 304 transmits wavelength λ3 to the third filter set 306. This filter set is selected to transmit only λ1, or alternatively only λ2, wavelengths and thus reflects λ3 optical wavelengths which propagate to a third channel 314. Alternatively, the third filter 306 could be replaced with an optical mirror with the same effect.
With this system architecture the input signal 300 has been fully multiplexed into three discrete channels without requiring a bend in the optical fiber. This architecture is of particular interest for a low number of wavelengths multiplexing as it permits a very compact assembly. Nevertheless, the principle can be extended to a large number of wavelengths. Complementary to these advantages, the three wavelengths multiplexer is implemented with only two types of thin film filters leading to worthwhile cost reduction.
The principle of the subject invention can be extended to N channels as shown in
With this system, the input signal 400 can be fully multiplexed as each spectral component has been isolated. In this connection the first multiplexer subunit 408 divides out discrete wavelengths λ1 and λ2 with all of the remaining wavelengths being transmitted to the next multiplexer subunit in series 410. This sub-unit divides out two more wavelengths λ3 and λ4 and the remaining wavelengths are transmitted on for further multiplexing until the final wavelengths λXN−1 and 2N are isolated by the final sub-unit 412. The total number of three component sub-units that will be needed to isolate or multiplex “N” optical wavelengths will by N/2.
In the context of borehole geophysics, the multiplexer system described can be advantageously implemented in a down hole logging tool as shown in
A borehole logging tool 500 is shown in this embodiment as being suspended from a conventional wireline 502 through a well casing 504 and production tubing to a geophysical production zone.
The logging tool 500 is equipped with three optical sensors 508, 510, and 512 for performing borehole parameter measurements, such as temperature, pressure, borehole fluid characteristics, or other measurements that are typically obtained in exploration and production of hydrocarbons. Each sensor measurement can be carried to the surface for processing and storage by a discrete wavelength of light λ1, λ2, or λ3. These wavelengths are initially emitted by an opto-electronic surface system 514 and propagate via a single fiber optic 516 carried along with the wireline 502 down to the logging tool 500
A three filter multiplexer 520 ensures that the appropriate optical wavelengths are dropped on each sensor. As discussed above, a first thin film filter 522 operably transmits wavelength λ1 onto sensor 512. The second thin film filter 524 transmits another wavelength λ3 and isolates and drops wavelength λ2 onto optical sensor 510. A third filter or mirror 526 then reflects the remaining third discrete wavelength λ3 onto optical sensor 508.
The separated wavelengths are then dropped onto individual data sensors 508, 510, 512, etc. where down hole data is attached and transmitted or reflected back through the multiplexer and onto the single fiber optic 516 for transmission of the data ladened wavelengths to the surface for demodulation and analysis. Each wavelength component λ1, λ2, and λ3 carrying optical sensor data is thus reflected or transmitted back through the multiplexer in the reverse direction and onto the single optical fiber up to the surface by for example a mirror placed at the termination of each optical sensor, according to the principle described in connection with
Although the multiplexer discussed in connection with
The multiplexer 520 is placed inside a logging tool for protection against a surrounding aggressive borehole environment. Due to its architecture, a small diameter is achievable for the tool allowing access through the well tubing. This architecture is of particular interest for a low number of wavelengths multiplexing as it leads to a very compact assembly. Nevertheless, the principle can be extended, as discussed, to a large number of wavelengths, as desired, without bending optical fiber within or associated with the multiplexer.
The various aspects of the invention were chosen and described in order to best explain principles of the invention and its practical applications. The preceding description is intended to enable those of skill in the art to best utilize the invention in various embodiments and aspects and with modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.
