Because of the relatively large distances fiberoptic transferring techniques are often preferred between control stations and equipment positioned close to oil wells and similar installations. In oil wells there are, however hostile environments with high temperatures rendering usual equipment useless. With temperatures in the range of 85°C to 200°C semiconductor components with small band gaps, such as components based on compound GaAs or hiP semiconductors, for example InGaAsP detectors and lasers, depending on the required wavelength ranges in the area typically around 1300nm and 1550nm, can in practice not be used. As is well known the 1300nm and 1550nm ranges are more suitable for long distance signals transfer at high rates (>lGb/s) in conventional optical fibres, and the technology and equipment, such as cables for this use is easily available. At the present optoelectronic elements having sufficient band gap for use in energy conversion between optical and electronic energy in high temperatures while having moderate thermal noise are typically based on silicon or AlGaAs, which limits the wavelengths being used to the 8-900nm range.

Shorter wavelengths undergo unacceptably high attenuation for communication lengths on the order of 1 km or more. Thus the transfer rate through the optical fibers being used for communicating between the temperate environment and the hot environment is limited If only passive measurements are to be performed one solution to this problem is to use optical sensors, such as the sensor described in US 5,032, 026. This is, however, impractical in most applications because of the limited supply of such components and the number of optical couplings necessary if a large number of sensors are to be used.

If control features are to be applied, such as for controlling electronic devices in the well providing feedback for electronic control, there presently are no practical systems providing means for optical signal transfers between the surface installations and the electronic instruments positioned in a hostile and hot environment.

The object of this invention is thus to provide an efficient method and system for optical communication between the electronic devices positioned in a hot environment and a temperate environment. The invention is characterised as described in the accompanying claims.

According to the invention two different concepts are thus used for transferring signals into the hostile environment (here called the"downlink") and from the hostile environment (here called the"uplink"). The invention provides a system having a capacity of >lkb/s with an bit error rate being < 10-6 on the downlink, and an uplink having a potential >lMb/s with a bit error rate being < 10-4 and tolerating hostile environments with temperatures <200°C. This also allows a link length of approximately 15 km.

The uplink generally will use the modulator in a near OOK (On-Off- Keying) type of modulation to maximise bandwidth. Downlink favourably uses a for of ASK (Amplitude Shift Keying) where the signal is modulated at a higher rate than the bit rate and effectively the modulation is turned on and off with the bit rate. This reduces the sensitivity to dark current variation and thermal related noise that is a strongly limiting factor even for a Silicon photo detector in the hot environment. The detector circuitry locks onto to the modulation signal.

This invention preferably utilises an optical modulator capable of modulating an optical signal as a response to electrical signals. The preferred embodiment of this modulator is based on modulation of the coupling of light transmitted from the temperate environment to the hostile environment and back to temperate environment, thus constituting a modulated reflection of the light. A system of this type is described in US 5. 898. 517. This solution does, however, not relate to a solution for communicating with the downhole equipment, but only transmitting signals from the hostile environment to the temperate environment.

A preferred modulator is based on the diffractive element described in US 5,311, 360. The modulator described in this patent does, however, not provide means for converting electrical signals into modulation of an optical signal. Also, the efficiency of this modulator is not sufficient to provide coupling into an optical fiber. It is an additional object of this invention to provide an optical modulator capable of modulating an incoming optical signal from an optical fiber by reflecting back into an

optical fiber as a response to an applied electrical signal. This modulator is of the type described in US 5,311, 360 including a partially reflective and partially transparent grating being positioned at a certain distance from a reflective surface, and further being characterised in that it comprises at least one optical fiber for transmitting the light to and/or from the modulator, in which the grating constitutes a pattern providing a diffractive lens and that the distance between the grating and the reflective surface is adapted to be modulated by the applied electrical signal.

The modulator thus provides a diffractive lens based on the same technology as described in international patent application PCT/N002/00422 (W003043377), with a fixed focal length but with efficiency depending on the distance between the grating and the reflective surface. Such a diffractive lens may be formed by litographic processes making reflective patterns in certain areas. The reflective pattern can be located on the surface of a transparent medium in the form of e. g. a thin metal (e. g. Gold) film on a glass substrate. Or the pattern may be formed by fingers or land between openings in a membrane structure (e. g. Silicon or Silicon Nitride on silicon) MEMS technology. In the case of a reflective pattern normally the second reflective surface that may be a uniform mirror will move. This structure is simpler to fabricate but may be more difficult to achieve high bandwidth particularly if not operated in vacuum. In the case of a membrane or fingers normally these will move relative to a stationary second reflector surface and have the advantage of lower mass that must be moved. An example of how such a grating may be produced is described in US 6.677, 783.

The invention will be described more in detail below with reference to the accompanying drawings, illustrating the invention by way of example.

Figure 1 illustrates a system according to the invention for transmitting signals to the hostile environment.

Figure 2a, 2b illustrates the downlink in the system in figure 1 for transmitting signals from the temperate environment to the hostile environment.

Figure 3 illustrates the uplink in the system in figure 1 for transmitting signals from the hostile environment to the temperate environment.

Figure 4 illustrates a preferred embodiment of the modulator according to the invention as illustrated in figure 2.

Figure 5 illustrates the principle of the modulator according to figure 4.

Figure 6 illustrates a multiplexed system according to the invention.

Figure 1 illustrates the asymmetrical system according to the invention schematically wherein the means for acquiring signals from the hostile environment H to the temperate environment T comprises a laser source 1 sending an optical signal through an optical fibre 6 to the down hole measuring or control device 2 including an optical modulator which reflects a modulated optical signal back through an optical fibre 7 to an interrogation unit 3 in the temperate environment T. In (offshore) oil exploration applications the temperate environment will typically be located at a surface installation or at a pod at the seabed for sub-sea installations while the hostile environment will be located in an oil well.

The optical system for acquiring signals from the hostile environment, hereafter called the"uplink"will operate at optical wavelengths in the ranges of 1300nm or 1550nm, this allowing for relatively large transfer rates through the optical fiber over long distances.

The system for transmitting signals to the hostile environment, hereafter called the"downlink", comprises a transmitter unit 4 transmitting signals through an optical fibre 8 to a receiver unit 5 in the hostile environment. As the availability detector means suitable for the high temperatures of the hostile environment is limited the optical signal may be transmitted at wavelengths in the range of 800-900nm, especially 850nm. This limits the transfer rates through the optical fibre, but allows for sufficient bandwidth to provide optical communication to the equipment in the hostile environment.

It should be noted that although three optical fibres are shown in the drawing a solution in which these three are constituted by one fiber is also possible by utilising optical couplers distributing signals to and from the parts of the system according to the invention. The means for obtaining this is considered to be obvious to a person known in the art and will not be discussed in any detail here.

In figure 2a the receiver unit 5 of the downlink is illustrated in more detail comprising a detector 11, preferably made from silicon or AlGaAs so as to withstand the temperatures, coupled to standard electronic circuitry 12 for transmitting electrical signals to the local equipment.

Figure 2b illustrates the uplink in more detail wherein the transmitter unit 1 emits an optical signal at a chosen wavelength through the fibre to the modulator unit 2 in the hostile environment. The modulator unit includes an optical modulator 20, which, as a response to an applied electrical signal 13 from instruments in the hostile environment provides a modulation of the transmitted signalt. The transmitted optical signal is modulated by an optical modulator 20 and the resulting modulated signal is transmitted back to in the temperate environment where a detector 14 with corresponding electronic circuitry receives and interprets the signal from the instruments in the hostile environment.

The emitted optical signal may be continuos or modulated. Modulation is particularly useful for some schemes of multiplexing. The positioning of the modulators can be optimised for time division multiplexing where a short optical pulse returned form one modulator will not overlap with the pulse from one of the other modulators ad thus can be discriminated at the detector. This enables multiplexed modulators to transmit at their maximum operation bandwidth while the total transmitting bandwidth is multiplied with the number of modulators used. However, the power budget suffers.

Figure 3 illustrates a preferred embodiment of the optical modulator 20 in figure 2b. The modulator is coupled to the optical fiber 6 transmitting optical signals from the emitter, wherein the optical signal propagates from the fiber end through a glass body 21. Opposite the glass body a diffractive element is provided which in the illustrated example is essentially constituted by i reflective grating 22 and a reflecting surface 24 being separated by a cavity. The dimensions of the cavity is chosen depending on the wavelengths of the light, e. g. having a minimum distance of % 2 wavelength being defined by a spacer 23.

As described in US 5,311, 360 the diffractive element 22,23, 24 will switch between two different modes, illustrated in the illustrations 26, 27 (figure 3) of the light intensities at the fiber ends. When the light reflected from the grating 22 and the reflector 24 is in phase the element will act essentially as a reflecting surface and the dominant reflection will be at the transmitting fiber end 6. This is illustrated in illustration 26 in which the intensity is largest symmetrically around the first fiber end 6.

When the phases of the light reflected form the grating 22 and the surface 24 are opposite there is no reflection back at the first fiber end 6, and the major reflection is positioned at a distance from the fiber centre depending on the grating as well as on the light wavelength. By positioning a second fiber end 7 at this position light reflected from the diffractive element may be coupled to the second optical fiber 7 and thus transmitted back to the temperate environment, as is illustrated in the second illustration 27.

Thus, by modulating the distance between the grating 22 and the reflecting surface 24 to coupling of light from the first to the second fiber end may be modulated.

The end stop or spacer 23 may be used to secure the surfaces at a chosen distance from each other, thus setting the modulator e. g. at the maximum or minimum coupling efficiency or at the maximum sensitivity.

As indicated in figure 3 the grating need not be a linear grating with equal distances between the reflecting and transparent parts. According to a preferred embodiment of the invention the grating is provided with a pattern constituting a phase lens so that the diffractive element, when the distance between the grating and the reflective surface is substantially different from d=, nA/2 where n is an integer constitutes a reflective phase lens focussing light received from the first fiber end 6 toward the second fiber end 7. This way the efficiency of the coupling is improved, being at its maximum when the distance is in the range of nkl2+X14 or alternatively the distance becomes very large (depending on design options).

Illustrations 26 and 27 illustrate how the state of the modulator is altered from a possible but weak coupling back to the first fiber end 6 when the state of the modulator is"O", i. e. acts as a simple mirror, to a strong, focussed coupling to the second fiber end when the state of the modulator is"I", i. e. when the modulator acts as a phase lens.

The distance between the grating 22 and the surface 24 is controlled by an actuator 25 which provides movements as a response to electrical signals, being and of any conventional type being suitable for use in high temperature environment, e. g. a piezoelectric element or electrostatic MEMS element. It is important that the actuator gives sufficiently fast response to make it possible to modulate the signal at sufficiently high rate, preferably having a rise time in the range of 200ns. Depending on the

application the actuator may provide an essentially continuos signal or an on/off switching of the signal coupled toward the temperate environment.

The reflective grating 22 may be produced in any conventional way and may be constituted by a dielectric or metallic layer on the glass body. The reflective surface may also be of any available type suitable for the high temperature environment.

Figure 4 illustrates an embodiment of the modulator according to the invention in which only one fiber is used, and in which the diffractive element is made as a phase lens being capable of focussing light more or less efficiently back to the fiber and thus modulating the light being transmitted back to the temperate environment.

Other embodiments of the modulator are described in the simultaneously file application No. 2003. 0435.

Figure 5a and 5b illustrates a slightly different embodiment of the invention wherein the diffractive element is constituted by an array of linear reflecting elements 30,31 which are adjustable relative to each other. Thus the reflecting elements 30,31 may be positioned in the same plane as in figure 5a, thus constituting a mirror, or different planes as in figure 5b, thus constituting a diffractive element similar to the one obtained in figure 3. As in figure 4 only one fiber is used, so that the modulation is performed as a function of the efficiency of the reflections back to the optical fiber.

MEMS or MOEMS (Micro Opto Electric Micro Structure) type modulators of other types may also be applied, depending on the rise time and other characteristics of the modulators under the specific conditions in each case. One possible solution is an electrically controlled Fabry-Perot based modulator coupled to the electronic instruments in the hostile environment so as to alter the distance between the mirros of the Fabry Perot.

Figure 6 illustrates a system comprising a number of modulators M1-M4 coupled to the same fiber through a star coupler. A simple multiplexer can use system margin to allow for few or single fiber transmission. 1 : 4 multiplexing uses approximately 8 dB for down-link and 16 dB for up-link and 1: 8 uses approximately 11 dB for down-link and 22 dB for up-link. This is probably acceptable up to 10 km link lengths. Using time division multiplexing up link transmission bandwidth can be maintained for each modulator at a slightly higher consumption of system margin.

Also, the downlink can utilise multiplexing, but then sharing the downlink bandwidth.

Although the invention here is primarily described in relation to signal transferring between hot environments and temperate environments other applications may be contemplated in which small band gaps may not be used, e. g. high radiation environments. Also, the present invention is described in relation to the silicon related 850nm wavelength range and the 1300nm or 1550nm ranges presently being optimal for transferring signals through optical fibers. Other ranges and combinations may, however, be applicable in other situations. If the modulator comprises parts made from silicon or another high band gap semi conducting material, said parts being positioned so as to receive parts of the optical signal, e. g. the reflector surface 24. The second detector may be embedded in this part of the modulator, thus receiving light from the same fiber that carries the first wavelength range without the use of any additional parts.

The wavelength ranges given here are not strict, as optoelectronic components are presently available ranging from below 1300nm to 1640nm, and other components may be expected in the future. The specific choices made when assembling the system will thus depend on the available technology, components already existing in a system in which the present invention is to be implemented and, as mentioned above, the conditions under which the components are to be used. If, for example, plastic optical fibers are required the preferred wavelength range may be approximately 670nm, and if the transfer rate is not important and the signal is to transmit from a high radiation environment to a hot environment 850nm may be used in the uplink as well.