WO2003026166A2

WO2003026166A2 - A method and system for free-space communication

Info

Publication number: WO2003026166A2
Application number: PCT/IL2002/000770
Authority: WO
Inventors: Pinchas Goldstein; Zvi Weinberger; Ehud Tirosh
Original assignee: Roqiya Networks Inc.
Priority date: 2001-09-17
Filing date: 2002-09-17
Publication date: 2003-03-27
Also published as: AU2002337493A1; WO2003026166A3

Abstract

Optical signals emanating from the tip of an optical communication fiber can be converted, using a non-linear crystal, to signals (carrier waves) whose wavelength fall in the range of far infra-red to RF. These longer wavelength converted signals maintain the modulation of the original data. Dense fog is inherently opaque to near infra-red waves used in fiber optics communications and relatively transparent to these longer wavelengths. The modulated long wave radiation can, therefore, serve to transmit data from a fiber tip through free space under foggy (and possibly other) prevailing weather conditions. The original optical wave may be recovered by reconverting the received long wave radiation in a heterodyning process taken place in a non-linear crystal with continuous wave laser radiation of an appropriate wave length. Consequently, fiber optics communications networks can be inter-connected seamlessly through a far infra-red/RF free space link.

Description

A METHOD AND SYSTEM FOR FREE-SPACE COMMUNICATION

FIELD OF THE INVENTION

This invention is generally in the field of Free Space Optics (FSO) or Free Space Communication techniques.

REFERENCES

1. Isaac I Kim, Bruce McArthur, and Eric Korevaar "Comparison of laser beam propagation at 785nm and 1550nm in fog and haze for optical wireless communications" p2 Optical Access Incorporated Web publication. http :/www. opticalaccess . com

2. H. Willebrand "Terrestrial Optical Communication Network of Integrated Fiber and Free-space Links Which Require No Electro-optical Conversion Between Links" US Patent 6,239,888 2001 column 5 3. PF Szajowsky, G Nykolak, JJ Auborn, HM Presby, GE Tourgy, D Romain "High power Amplifiers Enable 1550nm Terrestrial Free-Space Optical Links Operating @ WDM 2.5Gb/s Data Rates." Optical Wireless Communications II Proceedings of SPIE Volume 3850 1999

4. Art MacCarley "Advanced Image Sensing Methods for Traffic Surveillance and Detection" California PATH research Report UCB-ITS- PRR-99-11 p 16 1999

5. BR Strickland, MJ Lavan, E Woodbridge, V Chan "Effects of Fog on the Bit Error Rate of a Free-space Laser Communication system" Applied Optics 38 424-431 (1999) p428.

6. H Willebrand and M Achour "Hybrid Wireless Optical and Radio Frequency Communication Link" WO Patent 01/52450 7. GS Herman and NP Barnes "Method and Apparatus for Providing a Coherent Terahertz Source" US Patent 6,144,679 2000

8. Frits Zernike and John E. Midwinter Applied Non-Linear Optics, John Wiley 1973

6a- chapter 2 p43, 6b- chapter3, 6c-p45 6d-p68-69

9. 9. DTF Marple " Refractive Index of GaAS" Journal of Applied Physics 35 539 (1964)

10. JA Giordmaine and RC Miller "Optical Parametric Oscillation in the Visible

Spectrum" Applied Physics Letters " 9 298 (1966) 11. MH Chou J Hauden MA Arbore and MM Fejer "1.5 μm-band wavelength conversion based on difference frequency generation in LiNbθ3 waveguides with integrated coupling structures." Optics Letters 23 1004 (1998)

BACKGROUND OF THE INVENTION Fiber optical networks are rapidly replacing copper cables for high-bandwidth and reliable transmission of information over large distances. Optical communication using fibers have extremely large bandwidths (i.e. high transmission rate, typically tens of gigabits per second). The efficient utilization of fiber optics communication networks requires that all "end users" be connected to the fiber optic network.

US studies, however, indicate that less than 5% of US businesses are connected to the network although more than 75% are within one mile of the fiber backbone [1]. Over this "last mile", traditional copper cables are used for data transmission and the benefits of the wide bandwidths afforded by optical fibers are lost.

Deployment of fiber directly to all these end customers is costly and time consuming, as this requires the retrenching of urban streets and a license from the authorities. A proposed solution is to transmit the infra-red waves used in optical fiber communications directly over free space to a receiving optical fiber located at the end user's building [2] [3]. However, free space communication in the optical range may be adversely affected by prevailing weather conditions, and in particular, optical radiation is obstructed in dense fog conditions. For example, in a fog of O.lgm/m³ precipitated water droplets, the one-way attenuation is greater than 200 dB/km, while for the longer sub-millimeter waves, the attenuation is less than 10 dB/km, and for millimeter waves, less than 1 dB/km [4].

As a result of the high attenuation of laser radiation under dense fog conditions, the maximal required laser intensity in the optical range is well beyond practical capabilities [5], and even when available, it may be well beyond eye safety standards allowed for transmitted energy in air. A possible solution to cope with such optical range inherent limitations is to use longer waves (e.g., in the Radio Frequency range) which, as illustrated in the numerical example above, are less susceptible to atmospheric attenuation by fog and are not subject to any eye safety requirements, thus affording the reliable transmission of data through fog.

The use of longer wavelengths for free space communication under foggy weather conditions is known (see WO 01/52450 [6 j. The latter publication discloses an RF system that is used as a backup in atmospheric conditions (such as fog) which adversely affect transmission rate. This solution has several inherent shortcomings, including:

• It is expensive, since there is a need to duplicate the whole communication system, including the transmitter and the receiver.

• In accordance with publication [6], data generation and detection requires a conversion from optical to electrical and then from electrical to a RF wave. This is complex and expensive, and because of inherent limitations of the components involved in the specified conversions, the resulting bandwidth is limited to less than about lGbit/sec, thereby rendering the system infeasible in many communication applications. • The data rate limitation of existing RF systems further exacerbates when the original optical signal contains multiple carrier wavelengths, such as a DWDM

(Dense Wavelength Division Multiplexing) signal.

In this context, it is noteworthy that there are known in the art other publications (see. [7]) which disclose, generally, the generation of waves at terahertz frequencies by mixing near infrared waves. Accordingly, there is a need in the art for a system and method, which provides the seamless conversion of data from the optical to the RF domain.

There is a further need in the art to substantially reduce or eliminate the drawbacks of hitherto known solutions.

SUMMARY OF THE INVENTION

The invention provides for a method for communicating data modulated on an electromagnetic signal over free space, comprising the steps of a. transmitting a first electromagnetic signal having a first wavelength and a given data modulation; b. selectively switching between at least a first transmission mode and a second transmission mode; the first transmission mode includes the following steps (i) to (iii): i) converting the first signal to a second signal having a second wavelength, whilst substantially maintaining said data modulation, to generate said second signal; ii) propagating said second signal over said free space; iii) converting the second signal to a third signal having a third wavelength, whilst substantially maintaining said data modulation, to generate the third signal; the second transmission mode includes the following step (iv): iv) propagating said first signal over said free space.

The invention further provides for a method for communicating data modulated on an electromagnetic signal over free space, comprising the steps of

(a) transmitting a first electromagnetic signal having a first wavelength that falls in the range of substantially an infrared wavelength range and having a given data modulation;

(b) converting the first signal to a second signal having a second wavelength that falls in the range of substantially an infrared to RF wavelength range, whilst substantially maintaining said data modulation, to generate said second signal;

(c) propagating said second signal over said free space; and

(d) converting the second signal to a third signal having a third wavelength that falls in the range of substantially an infrared wavelength range, whilst substantially maintaining said data modulation, to generate the third signal. Still further, the invention further provides for a system for communicating data modulated on an electromagnetic signal over free space, comprising: a transmitter, transmitting a first electromagnetic signal having a first wavelength and a given data modulation; a switch for selectively switching between at least a first transmission mode and a second transmission mode; a first converter configured to operate in said first transmission mode and capable of converting the first signal to a second signal having a second wavelength, whilst substantially maintaining said data modulation, to generate said second signal; said converter is further configured to propagate said second signal over said free space; a second converter configured to operate in said first transmission mode and capable of converting the second signal to a third signal having a third wavelength, whilst substantially maintaining said data modulation, to generate the third signal; the transmitter is further configured to propagate said first signal over said free space in said second transmission mode.

Yet further, the invention provides for a device for detecting data modulated on a first signal at a wavelength range between substantially infrared to RF, comprising the steps of: a converter configured to convert the first signal received over free space to a second signal having a wavelength at the range of substantially infrared, while substantially maintaining signal modulation, to generate a second signal, and for transmitting said second signal for further processing.

The invention provides for a transmitter/receiver device comprising: a converter configured to convert a first transmitted signal having a wavelength at the range of substantially infrared to a second signal at a wavelength range between substantially infrared to RF, whilst substantially maintaining a data modulation of the first signal, to generate said second signal; said converter is further configured to propagate said second signal over a free space; the converter is further configured to convert a third signal received over the free space and having a wavelength at the range substantially infrared to RF, to a fourth signal having a wavelength at the range substantially infrared, while substantially maintaining a data modulation of the third signal, to generate a fourth signal, and for transmitting said fourth signal for further processing. a switch communicating with said converter for selectively disabling said converter to thereby propagate over the free space said first signal and transmit for further processing said third signal received over the free space.

Still further, the invention provides for a method for communicating data modulated on an electromagnetic signal over free space, comprising the steps of

(a) transmitting a first electromagnetic signal having a first wavelength and having a given data modulation;

(b) converting the first signal to a second signal having a second wavelength whilst substantially maintaining said data modulation, to generate said second signal;

(c) propagating said second signal over said free space; and (d) converting the second signal to a third signal having a third wavelength whilst substantially maintaining said data modulation, to generate the third signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail with reference to the following non-limiting embodiments which give a full description, features and advantages of the invention:

Fig. 1 illustrates a general architecture of a communication system, in accordance with one embodiment of the invention;

Fig. 2 shows the use of the system of Fig. 1 in an exemplary application; Fig. 3 illustrates a detailed system architecture in accordance with one embodiment of the invention;

Fig. 4 illustrates a detailed system architecture in accordance with another embodiment of the invention; Fig. 5 illustrates a detailed system architecture in accordance with yet another embodiment of the invention; and

Fig. 6 shows the use of the system of in another exemplary application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Attention is first directed to Fig. 1, illustrating a general architecture of a communication system (10), in accordance with one embodiment of the invention. As shown, a transmitter/receiver unit (11) transmits to a converter unit (13) a first electromagnetic signal (12) having a wavelength that typically, although not necessarily, falls in the near infrared range and having a given modulation. The converter converts the first signal to a second signal (14) having a wavelength that typically, although not necessarily, falls in the infrared to RF range whilst substantially maintaining the data modulation. The so-converted signal (optionally together with the first signal) is then transmitted through the free space (15) and is collected by a converter unit (16) which converts the second signal to a third signal (17) having a wavelength that typically, although not necessarily, falls in the near infrared range whilst substantially maintaining the data modulation. Note that the wavelengths of the first and the third signals are not necessarily the same. The third signal (and possibly also the first signal) is then received by a receiver (18) for further processing (of either or both of said received first and third signal), depending upon the specific application. It should be noted that the transmitted signal is composed of a carrier beam and modulated data (referred to herein for simplicity as signal). It should be further noted that the retention of data modulation during the conversion phases necessarily implies that there is no substantial reduction in the transmission rate between the transmission and receiving ends. As will be exemplified in further detail below, this modulation retention characteristic has significant bearings on many real-life applications which require the transmission of data in relatively high transmission rates. Reverting now to Fig. 1, similar to the procedure described above, an electromagnetic signal (19) transmitted by (18) is converted in unit (16), transmitted through the free space medium (15), converted by converter (13) (giving rise to signal 20) and received by transmitter/receiver unit (11). As explained above, the wavelengths of the signals (19 and 20) are not necessarily the same. Considering that units (11) and (18) both function as a transmitter and receiver they are, as a rule, identical, and this also applied to converter units (13) and (16).

By a preferred embodiment, the system of the invention further employs switching means configured to disable the operation of the converters thereby facilitating transmission of the first signal (12) through the free space and receipt thereof by receiver (18), substantially without altering the signal characteristics including the signal wavelength and modulation.

In accordance with this embodiment, the switching means is responsive to a switching criterion which may vary, depending upon the particular application.

In accordance with a specific embodiment (explained, e.g. with reference to Fig. 2 below), the switching criterion is responsive to weather condition data and more specifically prevailing foggy weather condition.

Those versed in the art will readily appreciate that the invention is not bound by the system architecture of Fig. 1. Thus for example, whereas, for simplicity, in the system of Fig. 1, only two transmitter/receiver units are shown. In other applications, the plurality of transmitter/receiver units may be employed.

The communication system as described in the various embodiments above has various advantages that include the following:

• It is a communication system which enables to transit data between at least two optical ends, through free space whilst substantially maintaining the data modulation of the original transmitted signal;

• The communication system transmits and receives signals that fall in the near infrared range and converts the signal (for the purpose of transmission through the free space) in a wavelength that falls in the range of infrared to RF. • The system employs an integral switching means which enable to override the conversion and consequently transmit the original signal to the receiving end without altering the signal's characteristics including the signal's wavelength and modulation. Such an integral switching means has an advantage in e.g. communication applications that should be operable under a wide prevailing variety of prevailing weather conditions. Thus, in a first transmission mode, the original signal (say, in the near infrared range) is transmitted through the free space. If adverse weather conditions such as fog, are detected, then in order to prevent a degradation in the transmission rate, the system is switched to a second transmission mode in which the near infrared transmitted signal is converted to, say, the RF range, whilst maintaining the signal modulation (and thereby the transmission rate), transmitted through the free space and thereafter, the signal is converted (or reconverted) at the other end to the near infrared range. Alternatively, the two modulated signals, the near IR and the RF, are transmitted and detected simultaneously. )

For a better understanding of the foregoing, attention is directed to Fig. 2 showing the use of the system of Fig. 1 in an exemplary application (30). As shown, a server 32 which is coupled to an optical fiber 31, transmits (and receives) data through the free space to (from) a multiple of clients 34-39. By this embodiment, each client employs a transmitter/receiver unit and its associated converter which operate as explained with reference to Fig. 1. It is recalled that in the so-called "last mile" region, the deployment of fiber infrastructure is avoided and an alternative transmission through free space is an applicable solution. In a typical (yet not exclusive) operational scenario, the optical communication system operates in a short wavelength range (e.g. near infrared), at relatively high transmission rates, which may be in the gigabit/sec range, and further the system is required to operate under a wide variety of weather conditions. In the embodiment of Fig.2, a switching means of the kind specified above is used such that in most prevailing weather conditions the signal modulated at the server side (32) is transmitted through the free space and is received by designated clients (say 35 and 35) without affecting the signal's characteristics (including wavelength and modulation). When adverse weather conditions such as fog are detected, the system is switched (either manually or automatically) to another transmission mode in which the transmitted signal is converted to, say an RF range (whilst maintaining the modulation) and it is thereafter transmitted through the free space and is reconverted before being received by the client. By following this approach, an operation in a wide variety of weather conditions is accomplished. It is recalled that in publication [6], the proposed system does not necessarily maintain modulation (i.e. degrading the transmission rate characteristics), therefore rendering it impractical for high data rate applications. In addition, the system according to the specified publication employs a separate RF backup module rendering it cumbersome and expensive.

Attention is now directed to Fig. 3 illustrating a detailed system architecture in accordance with one embodiment of the invention. As shown, the near infrared carrier beam λ_a 120 of the transmitted signal emanating from the fiber tip 110 is projected by a lens 130 to a non-linear crystal 140. Concurrently, a continuous wave laser beam λ_b 150 (constituting one form of a feeding signal) emitted by a laser 160 is also incident on the non-linear crystal 140. The two beams are combined by a spectrally selective dielectric beam combiner 170. Within the non-linear crystal 140, the two beams are heterodyned to give a difference frequency wave λ_c 180 that falls in the infrared to RF wavelength range. As shown, the first signal λ_ is converted to second signal λ_c using the converter that utilizes the feeding signal λ_b, beam combiner 170 and non-linear crystal 140.

This carrier beam, modulated with the original signal, is transmitted through the free space and collected by beam combiner 270 at the receiving end. A second non-linear crystal 240 receives the infrared to RF wavelength beam λc 180 and combines it, using a beam combiner 270, with a continuous wave laser beam λ_b 250 emitted from laser 260. The combined beams 300 enter the non-linear crystal and are mixed by the crystal to provide a heterodyned wave equal to the original carrier beam λ_a. The beam delivered from the non-linear crystal 240 also contains a portion of the continuous wave laser beam λ_b 290, and is filtered out by a beam splitter 280 giving rise to a modulated near infrared carrier beam λ_a 220 that is directed by a lens 230 to a fiber tip 210 that forms an end of the fiber optical network. Note that for simplicity Fig. 3 illustrates one direction transmission only. Those versed in the art will readily appreciate that the invention supports a bi-directional mode. Note that the invention is generally applicable for communication applications that require transmission rate of lOOMbit/sec and more, and is particularly useful for applications which involve relatively high transmission rates of 1 gigabit/sec or even a few gigabits per second and more.

It should be noted that in accordance with another aspect of the invention, that by a proper selection of wavelengths (see e.g. Fig 3), λ_a and λ_c can be composed, each being of more than one wavelength (e.g. λ_a and λ_a'; λ_c and λ_c'). In that case, data communication can be carried out in two or more independent channels, each of which being modulated independently. By doing that the total data rate may be increased. Turning now to Fig. 4, there is shown a detailed system architecture in accordance with another non-limiting embodiment of the invention. The architecture of Fig. 4 is generally similar to that described with reference to Fig. 3 except for the use of optical amplifier 190 that is adapted to amplify the carrier beam λa 120. In this case, λa will not be amplified by the λb CW laser 150. Attention is now drawn to Fig. 5, showing a detailed system architecture in accordance with yet another non-limiting embodiment of the invention. Fig. 5 concerns the case where the carrier beam 120 is sufficiently intense. Then, (as will be explained in greater detail below), λb can be generated within the crystal even without employing the CW laser and accordingly, λc can be generated within the crystal 140. Having described a detailed structure of the system according to some non-limiting preferred embodiments (with reference to Fig. 3 to 5), there follows now a description that provides the mathematical background for understanding how the λc carrier beam is generated from the combined λa carrier beam and the λb feeding signal, and how the modulation is maintained in accordance with a non-limiting embodiment of the invention.

Thus, the modulated carrier beam (λa) is described by:

A(t) cos ωd (la)

A(t) being the modulated signal superimposed on a sinusoidal carrier wave. The continuous wave feeding signal (λb) is given by:

B cos oobt ; B = constant (lb)

Within the non-linear crystal 240 there are waves proportional to the above two terms and also a term proportional to

[A cos ωd + B cos cod ]² = [A cos ωd ] ² + [B cos ωd ]² + 2AB cos ωd cos ωd. ( 1 c)

The quadratic terms on the right hand side of equation (lc) are equal to:

[A cos ωd ]² = A²[l+ cos 2ω_at]/2 (2a)

[B cos ω f = B²[l + cos 2ωtt]/2 (2b)

The remaining mixed term on the right hand side of equation (lc) yields:

2A(t)B cos cod cos ωd = A(t)B[cos

+ cos (ω_a+ωb)tj (2C)

From equations (1) and (2) it is clear that, within the non-linear crystal, waves of the following frequencies interact:

Cύa The signal carrier wave frequency (3 a) COb Continuous wave frequency (3b) 2ω_a Second harmonic of the carrier frequency (3 c)

2ωb Second harmonic of the continuous wave frequency (3d) _a+ωb Sum of signal and continuous wave frequencies (3e) b-ω_a: Difference of signal and continuous wave frequencies (3f) It is this difference frequency;

where c is the velocity of light, that is of interest for transmitting from the crystal through the free space.

As explained in the background, in order to get significant amplitudes over a typical range of transmission (i.e. decrease signal attenuation), the difference frequency signal should preferably be in the infrared to RF range.

In the above and following description, it is assumed that ωb>ω_a>ωc. The other case for which ω_a>ωb>ωc will be discussed later on. In terms of wave numbers, the relation between the wavelengths of interest can be expressed as:

A carrier wavelength of 1.550μm is commonly used in optical fiber communication. Hence, for this non-limiting example, the following values are chosen for the carrier and CW laser wavelengths: λa= 1.550 μm and λb = 1.548μm Equation (5) yields:

λc = 1220μm; or: ω_c = 0.25 terahertz (6)

which falls in the desired range.

In order to recover the difference frequency _c efficiently the following conditions must be met: 1. The crystal must be transparent to all three wavelengths of interest. 2. The crystal must have a high non-linear susceptibility.

3. For the efficient transfer of energy within the crystal from the short wavelength beam into the difference frequency beam, all three wavelengths must be phase matched, i.e., within the crystal the three wavelengths should be related as:

mωi, = n_cω_c + n_aω_a (7a) where n. are the refractive indices of the non-linear crystal to the three wavelengths in the direction of propagation, respectively [8a]. Alternatively, in terms of wavelengths: rib/kj_ = nJk_c + ria k_a (7b) Equations (4) and (7) are to be satisfied simultaneously. (Clearly a sufficient condition for satisfying both equations is when all n. are equal to each other. This is not, however, a necessary condition.)

The physical conditions for satisfying equations (4) and (7) simultaneously is discussed in the literature [8] and is the subject of a recent patent [7]. The substitution of these values into equations (4) and (7a) or, equivalently, equations (5) and (7b), shows that the equations are satisfied simultaneously to a high degree of accuracy. (Better than 4* 10^"6 with respect to the refractive index.)

In accordance with a preferred embodiment, temperature altering means are utilized in order to render the device operable in a selcted one out of a few possible wavelengths. Thus, a common wavelength region for fiber optics communication centers about the 1.550 μm wavelength. For the Sellmeir coefficients of GaAS reported in reference [6] then, at room temperatures, it is not possible to phase match the 1.550 μm carrier wavelength with a shorter CW wavelength and a resultant terahertz frequency. By altering the temperature of the crystal (140 in Fig. 3), the indices of refraction at shorter wavelengths change significantly, whereas the difference frequency change is relatively small [8d].

Extrapolating from the experimental data of Marple for the temperature dependence of GaAs [9], we find that phase matching is achieved at 265°C for the following wavelengths: λ(nm) n

1.548 3.4367931

1.550 3.436575

1200.0 3.605764

The substitution of these values into equations (4) and (7a) or, equivalently, equations (5) and (7b) shows that the equations are satisfied simultaneously to a high degree of accuracy. (Better than 4* 10^"6 with respect to the refractive index.) Temperature tuning of non-linear crystals has been described the literature [10].

The discussed above properties of the non-linear crystal guarantee that the only contribution to the converted signal is the term A(t)B COS (ωb-ω_a)t. Thus, the converted signal maintains the data modulation A(t) by definition.

There follows now a discussion for the specific embodiment concernning recovery of the original carrier frequency and its signal.

The original signal beam with frequency ω_a is recovered from the modulated terahertz beam with frequency ω_c by mixing with the continuous wave laser of frequency

in a non-linear crystal with properties analogous to the crystal used for generating the transmitter wavelength. The signal modulated frequency ω_a is recovered as the difference frequency: ω_a = β. j - ω_c (equation 4) This signal can then be transmitted to a fiber connection within the building to which the signal is sent by the terahertz frequency.

Examples for suitable non-linear crystals for accomplishing this conversion are CdTe ZnTe, GaP, GaAs, InAs and LiNbθ3

The discussion above is focused on the characteristics of the non-linear crystal which facilitate the generation of the converted λc carrier beam in accordance with a preferred embodiment of the invention. Note that by this embodiment, the converted signal propagates through the open space and is reconverted back to a signal of the same properties (wavelength and modulation) as the original signal by a similar technique and method as that used for converting the original signal in the first place.

There follows a discussion for The case of ωa>ωb>ω_c:

All the above arguments for the case where ωb >ω_a >ω_c also hold true for ω_a >ωb>ω_c, except that equation (5) is replaced by:

and equation (7b) by: na/λa = n_c/λc + n_b/λb (10)

This system is illustrated in Fig. 4.

If the carrier beam λa is sufficiently intense then λb and λc will be generated within the crystal, even when not stimulated by a CW beam. Thermal noise always prevailing in the crystal will provide the initial energy at wavelength λb that will be amplified by the carrier wave λa This system is illustrated in Figure 5. In all three cases illustrated in Figures 3-5, the photon of the shorter wavelength with frequency ω_s splits into two photons. One photon with the energy of the difference frequency ω_c and the other with the energy of the intermediate frequency G . Since the generation of the difference frequency and the regeneration of the carrier frequencies are reversible processes then, for a sufficiently long crystal equilibrium is reached and E(ft _.) → E(&).)+ E(cυ_c)„where E (ω) are the electric fields associated with the three frequencies. Since 60s ~ 6 » ω_cthen, if the initial energies of the short and intermediate beams are approximately equal, equilibrium is established before significant energy in the difference beam will be generated. The intensity of the shorter wavelength beam should therefore be at least an order of magnitude less than the intensity of the intermediate wavelength beam. In the generation of e.g. a modulated 1.597μm signal as a difference frequency of a CW beam of 0.784μm and a modulated signal at 1.539μm described in prior art [11], the intensity of the carrier beam λa was approximately 15 times greater than the intensity of the CW beam. It is recalled that by a preferred embodiment switching means are employed.

In one embodiment, and a explained in detail above, the switching means switches between a first transmission mode that utilizes the conversion and a second transmission mode that does not utilize the conversion. By a non-limiting embodiment blocking, the CW beam is enough for preventing the conversion, letting the original beam pass through the whole system without any altering its wavelength or modulation. This can be obtained e.g. by a shutter in the exit of the CW laser or in accordance with another example by turning the CW laser off. It should be noted that the invention is not bound by these exemplary switching means. Note that the invention is not bound to the specific example described with reference to Fig.2. Consider, for example, the following application: Antenna 330 receives an RF signal 320 (transmitted by a satellite 310) through the free space. The so received signal is converted to an optical signal, say, in the near infra-red range, and the so converted signal is transmitted through fibers 351, 342 and 433 to subscribers 341, 352 and 353, respectively.

In the method claims that follow, alphabetic characters and roman symbols used to designate claim steps are provided for convenience only and do not imply any particular order of performing the steps.

The present invention has been described with a certain degree of particularity but those versed in the art will readily appreciate that various alterations and modifications may be carried out without departing from the scope of the following claims:

Claims

1. A method for communicating data modulated on an electromagnetic signal over free space, comprising the steps of

a. transmitting a first electromagnetic signal having a first wavelength and a given data modulation;

b. selectively switching between at least a first transmission mode and a second transmission mode; the first transmission mode includes the following steps (i) to (iii):

i) converting the first signal to a second signal having a second wavelength, whilst substantially maintaining said data modulation, to generate said second signal; ii) propagating said second signal over said free space; iii) converting the second signal to a third signal having a third wavelength, whilst substantially maintaining said data modulation, to generate the third signal; the second transmission mode includes the following step (iv): iv) propagating said first signal over said free space.

2. A method according to Claim 1, in which the switching between the first transmission mode and the second transmission mode is determined according to a switching criterion that includes weather condition data.

3. The method according to Claim 2, in which the switching criterion stipulates that the first transmission mode is selected in response to foggy weather condition data and the second transmission mode is selected in response to any other prevailing weather condition data.

4. The method according to Claims 1 or 2, wherein said first wavelength and third wavelength fall in an infrared wavelength range.

5. The method according to any one of Claims 1 to 4, wherein said first wavelength and third wavelength being substantially the same.

6. The method according to any one of Claims 1 to 4, wherein said second wavelength falls in an infrared to RF wavelength range.

7. The method according to any one Claims 1 to 6, wherein said data modulation corresponds to a transmission rate exceeding 100 Mbits/sec.

8. The method according to any one of Claims 1 to 7, wherein either or both said steps (b(i)) and (b(iii)) further include amplifying said first signal or said second signal.

9. The method according to any one of Claims 1 to 8, in which said third signal is further transmitted on an optical fiber.

10. A method for communicating data modulated on an electromagnetic signal over free space, comprising the steps of

(c) propagating said second signal over said free space; and

(d) converting the second signal to a third signal having a third wavelength that falls in the range of substantially an infrared wavelength range, whilst substantially maintaining said data modulation, to generate the third signal.

11. The method according to Claim 10, further comprising the step of : selectively disabling said steps (b) to (d) and propagating said first signal over said free space.

12. The method according to any one of Claims 10 or 11, wherein said selectively disabling step is determined according to a switching criterion that includes weather condition data.

13. The method according to Claim 12, in which the switching criterion stipulates that steps (b) to (d) are selected in response to foggy weather condition data and are disabled in response to other prevailing weather condition data.

14. A system for communicating data modulated on an electromagnetic signal over free space, comprising: a transmitter, transmitting a first electromagnetic signal having a first wavelength and a given data modulation; a switch for selectively switching between at least a first transmission mode and a second transmission mode; a first converter configured to operate in said first transmission mode and capable of converting the first signal to a second signal having a second wavelength, whilst substantially maintaining said data modulation, to generate said second signal; said converter is further configured to propagate said second signal over said free space; a second converter configured to operate in said first transmission mode and capable of converting the second signal to a third signal having a third wavelength, whilst substantially maintaining said data modulation, to generate the third signal; the transmitter is further configured to propagate said first signal over said free space in said second transmission mode.

15. A system according to Claim 14, in which the switching between the first transmission mode and the second transmission mode is determined according to a switching criterion that includes weather condition data.

16. The system according to Claim 15, in which the switching criterion stipulates that the first transmission mode is selected in response to foggy weather condition data and the second transmission mode is selected in response to any other prevailing weather condition data.

17. The system according to any one of Claims 14 to 16, wherein said first wavelength and third wavelength fall in an infrared wavelength range.

18. The system according to any one of Claims 14 to 17, wherein said first wavelength and third wavelength being substantially the same.

19. The system according to Claim 17, wherein said second wavelength falls in an infrared to RF wavelength range.

20. The system according to any one of Claims 14 to 19, wherein said data modulation corresponds to a transmission rate exceeding 100 Mbits/sec.

21. The system according to any one of Claims 14 to 20, wherein said first converter further includes an amplifier for amplifying said first signal.

22. The system according to any one of claims 14 to 21, in which said third signal is further transmitted on through an optical fiber.

23. A device for detecting data modulated on a first signal at a wavelength range between substantially infrared to RF, comprising the steps of: a converter configured to convert the first signal received over free space to a second signal having a wavelength at the range of substantially infrared, while substantially maintaining signal modulation, to generate a second signal, and for transmitting said second signal for further processing.

24. A transmitter/receiver device comprising: a converter configured to convert a first transmitted signal having a wavelength at the range of substantially infrared to a second signal at a wavelength range between substantially infrared to RF, whilst substantially maintaining a data modulation of the first signal, to generate said second signal; said converter is further configured to propagate said second signal over a free space;

the converter is further configured to convert a third signal received over the free space and having a wavelength at the range substantially infrared to RF, to a fourth signal having a wavelength at the range substantially infrared, while substantially maintaining a data modulation of the third signal, to generate a fourth signal, and for transmitting said fourth signal for further processing. a switch communicating with said converter for selectively disabling said converter to thereby propagate over the free space said first signal and transmit for further processing said third signal received over the free space.

25. The device according to Claim 24, wherein said converter comprising a beam combiner receiving said first transmitted signal and combining it with a feeding signal having a feeding wavelength and having signal intensity different than the signal intensity of said first signal, so as to generate a first combined signal that is fed to a non-linear crystal configured to generate said second signal;

the beam combiner is further configured to receive said third signal received over free space and combining it with the feeding signal having a feeding wavelength and having a signal intensity different than the signal intensity of said third signal, so as to generate a second combined signal that is fed to the non-linear crystal configured to generate a signal that is fed to a beam splitter that filters out said fourth signal. said switch is configured to disable said converter by disabling said feeding signal.

26. The device according to Claim 23, wherein the converter comprising a beam combiner configured to receive the first signal received over the free space and combining it with the feeding signal having a feeding wavelength and having a signal intensity different than the signal intensity of said first signal, so as to generate a first combined signal that is fed to the non-linear crystal configured to generate a signal that is fed to a beam splitter that filters out the second signal.

27. The system according to Claim 14, wherein said first converter comprising a beam combiner receiving said first transmitted signal and combining it with a feeding signal having a feeding wavelength and having a signal intensity different than the signal intensity of said first signal, so as to generate a first combined signal that is fed to a non-linear crystal configured to generate said second signal;

Said second converter comprising a beam combiner configured to receive said second signal received over the free space and combining it with the feeding signal having a feeding wavelength and having a signal intensity different than the signal intensity of said second signal, so as to generate a second combined signal that is fed to the non-linear crystal configured to generate a signal that is fed to a beam splitter that filters out said third signal. said switch is configured to disable said converter by disabling said feeding signal.

28. The device according to Claim 25, wherein said first signal, second signal and feeding signal comply with the following algorithmic expression:(l/Eαm6fa ) = (1/Lamdat) - (1/Lamdal), where Lamdal is the wavelength of said first signal, Lamda2 is the wavelength of said second signal and Lamdat is the wavelength of said feeding signal; said third signal, fourth signal and feeding signal comply with the following algorithmic expressions: (\ILamda4) = (1 /Lamdat) - (1/Lamda3) and

(n^lLamda4) = (n/Lamdat) - (n_/Lamda3) where Lamda3 is the wavelength of said third signal, Lamda4 is the wavelength of said fourth signal, Lamdat is the wavelength of said feeding signal and n_4ι n_t; n₃ are the refractive indices respectively.

29. The device according to Claim 26, wherein said first signal, second signal and feeding signal comply with the following algorithmic expressions:

(\ILamda2) = (1 /Lamdat) - (1/Lamdal) and

(n₂/Lamda2) = (n/Lamdat) - (n Lamdal) where Lamdal is the wavelength of said first signal, Lamda2 is the wavelength of said second signal, Lamdat is the wavelength of said feeding signal and n_2; n_t are the refractive indices respectively.

30. The system according to Claim 27, wherein said first signal, second signal and feeding signal comply with the following algorithmic expressions:

(\ILamda2) = (1 /Lamdat) - (1/Lamdal) and (n₂/Lamda2) — (n/Lamdat) - (n/Lamdal), where Lamdal is the wavelength of said first signal, Lamda2 is the wavelength of said second signal, Lamdat is the wavelength of said feeding signal signal and n₂₎ n_t are the refractive indices respectively; said second signal, third signal and feeding signal comply with the following algorithmic expressions:

(\ILamda3) = (1 /Lamdat) - (1/Lamda2) and ( _i-ILamda3) = (n/Lamdat) - (n₂/Lamda2) where Lamda2 is the wavelength of said second signal, Lamda3 is the wavelength of said third signal, Lamdat is the wavelength of said feeding signal signal and n_3; n_t are the refractive indices respectively .

31. The device according to any one of Claims 24 or 25, wherein said converter is further associated with means including temperature altering means for altering the operating temperature of the crystal so as to render the device operatable in selected wavelengths for either or both said first or forth signal.

32. The device according to any ond of Claims 23 or 26, wherein said converter is further associated with means including temperature altering means for altering the operating temperature of the crystal so as to render the device operatable in selected wavelengths for first signal.

33. The device according to any one of Claims 14 or 27, wherein said first converter is further associated with means including temperature altering means for altering the operating temperature of the crystal so as to to render the device operatable in selected wavelengths for first signal; and said second converter is further associated with means including temperature altering means for altering the operating temperature of the crystal so as to to render the device operatable in selected wavelengths for third signal.

34. A method for communicating data modulated on an electromagnetic signal over free space, comprising the steps of

35. The method according to Claim 10, wherein said step (c) further comprising: propagating also said first signal over said free space.

36. The method according to Claim 34, wherein said step (c) further comprising: propagating also said first signal over said free space.