WO2005055436A2

WO2005055436A2 - Transceiver for optical transmission

Info

Publication number: WO2005055436A2
Application number: PCT/IL2004/001101
Authority: WO
Inventors: Ilan Haber; Rami Anolik; Tsvi Eitane
Original assignee: Rad-Op Ltd.
Priority date: 2003-12-03
Filing date: 2004-12-02
Publication date: 2005-06-16
Also published as: WO2005055436A3

Abstract

An optical transceiver (101, fig. 2), comprising a data interface (212, fig. 2) adapted to receive data signals (225, fig. 2), in electrical format, a laser diode (250, fig. 2) having an intrinsic rise time of at least 0.8 nanoseconds and a laser driver (204, fig. 2) adapted to drive the laser diode to generate a light beam carrying the data signals received by the data interface, for data signals carrying data at rates of at least 50 Mbps.

Description

TRANSCEIVER FOR OPTICAL TRANSMISSION RELATED APPLICATIONS The present application claims the benefit under 35 USC 119(e) of US provisional application serial number 60/526,837, filed December 3, 2003, the disclosure of which is incorporated herein by reference. FIELD OF THE INVENTION The present invention relates to free-space optical transmission of data. BACKGROUND OF THE INVENTION One method used for communicating data is transmission of modulated light beams. In many cases, the light beams are transmitted through optical fibers which provide a low interference medium for light beams, such that the light beams can be transmitted over large distances without being regenerated. The cost of laying optical fibers in dense metropolitan areas is generally high if not prohibitive. Therefore, systems for transmission of data over modulated light beams in free space (e.g., the atmosphere), between a transmitter and a receiver, have been suggested. While fibers are designed for transmission of light in infrared wavelengths, both visible and infrared wavelengths can be used in free space. U.S. patent 3,527,949 to Huth et al., the disclosure of which is incorporated herein by reference, describes a point to point visible light transmission system which uses rapid rise time pulse signals. U.S. patent publication _.2004/0126116 to Dogariu, the disclosure of which is incorporated herein by reference, describes use of partially coherent beams for free space optical transmissions. In the background, this publication mentions that it is known to use both -visible and infrared wavelength bands for free space transmission. PCT patent publication WO 99/52231 , the disclosure of which is incorporated herein by reference, describes a free space optical communication system, which uses the visible light wavelength range between 0.4-0.7 micrometers. U.S. patent 6,609,690 to Davis, the disclosure of which is incorporated herein by reference, describes an apparatus for mounting a free-space optical (FSO) transceiver to a window. The transceiver is said to be used with visible light, ultra-violet or infrared portions of the spectrum. Transmission distances achievable by light beams in free space are relatively limited. In order to maximize the transmission distances, e.g., to achieve transmission for a distance of several miles, high power, high accuracy transceivers are required. In addition, in- order to achieve high transmission bit rates, high speed transceivers are required. High speed transmission transceivers are therefore specifically designed with very low capacitance values and with accurate impedance matching. U.S. patent 5,371 ,755 to Murata et al., the disclosure of which is incorporated herein by reference, describes an optical transmitter that can communicate at a transmission speed exceeding a Gigabit per second (Gbps) with a low bit error rate. U.S. patent publication 2004/0190569 to Kang et al., the disclosure of which is incorporated herein by reference, describes an apparatus for compensating for characteristics of a laser diode, by varying the bias voltage of the laser diode, responsive to readings from a feedback photodiode. U.S. patent 6,603,554 to Eisenberg et al., the disclosure of which is incorporated herein by reference, describes a method of measuring the light attenuation of windows. The method is suggested for use in measuring the attenuation of a window in order to calibrate the amount of light transmitted from a FSO transceiver positioned behind the window. Another problem of optical free space transmission systems is alignment of transceivers that communicate with each other. U.S. patent 6,381,055 to Javitt et al., the disclosure of which is incorporated herein by reference, describes the use of a retro-reflector for alignment of free space transceivers. The abstract of Korean patent publication 10 2002 0019796 titled "Free Space Optical

Transmission Apparatus Having Optical System Arrangement Function Using Visible Ray", the disclosure of which is incorporated herein by reference, describes an arrangement optical module for a free space optical transmission system. U.S. patent publication 2004/0156638 to Graves et al., the disclosure of which is incorporated herein by reference, describes transceivers of a free space optical transmission system, which utilize telescopes for alignment. U.S. patent publication 2004/0022537 to Mecherle et al., the disclosure of which is incorporated herein by reference, describes an optical wireless transceiver having a sighting scope for initial alignment and an acrylic filter that limits transmission of visible light to prevent introduction of noise and heat to the transceiver. SUMMARY OF THE INVENTION An aspect of some embodiments of the present invention relates to using for high speed (i.e., above 50 MHz) free space optical transmission, at rates of above 50 Mbps (Mega bits per second), a laser diode having a rise time (RT) greater than 0.8 nanoseconds or even greater than 1.5 nanoseconds. Optionally, the laser diode has a high capacitance of above 5 picoFarad, 10 picoFarad or even above 30 picoFarad- The use of the relatively long rise time and/or high capacitance laser diodes is allowed by using an over driving current and/or by using a laser driver having a short rise time. In some embodiments of the invention, the high capacitance laser diodes are used for very high transmission speeds of at least 1 Gbps or even above 2.5 Gbps. Laser diodes of slow rise time and/or high capacitance are much cheaper than low capacitance transceivers. In accordance with embodiments of the present invention, the advantage of using long rise time and/or high capacitance laser diodes outweighs the disadvantages. An aspect of some embodiments of the present invention relates to using an edge emitter laser diode for free space transmission. Although VCSEL diodes are considered better suited for communication transmissions, the use of edge emitters in accordance with some embodiments of the present invention provides a cost effective transceiver solution. An aspect of some embodiments of the present invention relates to a free-space optical transceiver which has a sight for human aiming toward a remote transceiver. The sight does not include optical enlargement elements. The sight is useful for short range free space links and/or when the opposite end transceiver generates or reflects a visible light beam which is easily identified remotely. In some embodiments of the invention, the visible light beam used for alignment is the beam that is used for data transmission. An aspect of some embodiments of the present invention relates to a free-space optical transceiver which is adapted to display an indication of the reception power of the beam it transmits, as received by an opposite end transceiver. The display of the reception power is optionally performed based on encoded signals transmitted from the opposite end transceiver on a light beam it transmits. An aspect of some embodiments of the present invention relates to a small shield for blocking a return beam from a window. The shield blocks return light from the window from proceeding into a room in which a transceiver of the beam is located. In some embodiments of the invention, the shield has a small area suitable to block only a return light beam and its immediate surroundings, while most straight light paths from the meeting point of the transmitted beam with the window are open for light (as they do not carry return light of beam impinging on the window). In some embodiments of the invention, less than 20% or even 10% of the beam angles from the meeting point of the transmitted beam with the window are blocked by the shield. Alternatively, most of the angles from the beam-window meeting point are blocked by the shield, so as to allow easy alignment of the shield with the returned beam to be blocked-. In some embodiments of the invention, the shield is mounted directly on the window. Alternatively, the shield is mounted on a thin arm which allows adjustment of the position of the shield in the vicinity of the window. An aspect of some embodiments of the present invention relates to converting an elliptical cross-section light beam generated by a laser diode into a circular cross-section light beam by passing the light beam through an optical fiber, before transmitting the light beam through the atmosphere. The transmission through the atmosphere is over a distance of at least

10 or even 50 meters. An aspect of some embodiments of the present invention relates to adjusting the wavelength used in a free space optical (FSO) system according to the attenuation spectrum of a window in the path of the FSO system. Optionally, the wavelength adjustment is performed automatically. There is therefore provided in accordance with an exemplary embodiment of the invention, an optical transceiver, comprising a data interface adapted to receive data signals, in electrical format, a laser diode having an intrinsic rise time of at least 0.8 nanoseconds, a laser driver adapted to drive the laser diode to generate a light beam carrying the data signals received by the data interface, for data signals carrying data at rates of at least 50 Mbps. Optionally, the laser diode generates abeam of visible light. Optionally, the transceiver includes a lens adapted to lead the generated light beam generated by the laser diode into the atmosphere in a manner suitable for conveying data through the atmosphere over a distance of at least 10 meters. Optionally, the transceiver includes an optical fiber through which the light beam is passed from the laser diode to the lens. Optionally, the transceiver includes a sight for aiming at an opposite end transceiver, the sight not including optics. Optionally, the transceiver includes a screen on which light directed at the sight is aimed, for alignment. Optionally, the laser diode comprises an edge emitter laser diode. Optionally, the laser diode has a capacitance of at least 5 pico-Farad, at least 10 pico-Farad or even at least 30 pico-Farad. Optionally, the laser diode has a rise time of at least 1.5 ns or even at least 2.4 ns. Optionally, the laser driver is adapted to drive the laser diode with a signal in which the rise and fall periods cover less than 20% of the signal. Optionally, the laser, driver is adapted to drive the laser diode with a signal in which the rise and fall periods cover less than 10% of the signal. Optionally, the laser driver is adapted to drive the laser diode to generate a light beam carrying the data signals received by the data interface, for data signals carrying data at rates of at least 500 Mbps. Optionally, the transceiver includes a controller designed to receive from an opposite end transceiver an indication of a reception power level of the light beam generated by the laser diode. Optionally, the transceiver includes a controller designed to initiate transmission to an opposite end transceiver an indication of a reception power level of a light beam received from the opposite end transceiver. Optionally, the transceiver includes a heat regulator adapted to regulate the temperature of the laser diode. Optionally, the heat regulator comprises a thermo-electric cooler. Optionally, the transceiver includes a controller adapted to adjust a wavelength of the generated light beam responsive to an indication of the tiansmissivity of a window adjacent the transceiver. There is further provided in accordance with an exemplary embodiment of the invention, a method of transmitting data on a light beam, comprising receiving data signals at a rate of at least 50 Mbps in an electrical format; and driving a laser diode having an intrinsic rise time of at least 0.8 nanoseconds, to generate a light beam carrying the data signals. Optionally, the data signals have a rate of at least 500 Mbps. Optionally, the laser diode has a capacitance of at least 10 pico-Farad. Optionally, the method includes transmitting the light beam through a window into the atmosphere. Optionally, the method includes positioning near the window a shield adapted to block return light beams from the window. Optionally, the laser diode has a rise time of at least 2 nanoseconds. There is further provided in accordance with an exemplary embodiment of the invention, a method of aligning first and second transceivers of a free space transmission system, comprising transmitting a light beam from a first transceiver; and aiming a sighting apparatus of a second transceiver at the transmitted light beam of the first transceiver. Optionally, the sight does not include enlargement optics. BRIEF DESCRIPTION OF THE DRAWINGS Exemplary non-limiting embodiments of the invention will be described with reference to the following description of exemplary embodiments, in conjunction with the figures. Identical structures, elements or parts which appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear, and in which: Fig; 1 is a schematic illustration of an optical free-space transmission system, in accordance with an exemplary embodiment of the present invention; Fig. 2 is a schematic illustration of an optical transceiver, in accordance with an exemplary embodiment of the invention; Fig. 3 is a schematic graph of a single cycle of a driving signal generated by a laser driver, in accordance with an exemplary embodiment of the invention; Fig. 4 is a flowchart of acts performed in installing and aligning transceivers of a free space transmission system, in accordance with an exemplary embodiment of the invention; Fig. 5 is a schematic block diagram of a transceiver controller for exchanging power level indications, in accordance with an exemplary embodiment of the invention; Fig. 6 is a schematic illustration of a reflection blocker mounted on a window, in accordance with an exemplary embodiment of the invention; and Fig. 7 is a schematic illustration of a reflection blocker, in accordance with another exemplary embodiment of the invention. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Fig. 1 is a schematic illustration of an optical free-space transmission system 100, in accordance with an exemplary embodiment of the present invention. System 100 comprises a pair of optical data transceivers 101 (marked 101A and 101B), positioned in line of sight with each other. Transceivers 101 are optionally mounted behind windows 107 in adjacent or otherwise neighboring buildings 113. Transceivers 101 may also be used in other settings, for example for indoor transmission within large halls. Transceivers 101 are used to transmit data signals between computers 111 (marked 111A and 11 IB). Computers 111 may be switch routers, servers, work-stations, end-point computers or any other type of computer. Computers 111 may be stand alone computers or may aggregate data for transmission from one or more local and/or wide area networks. In operation, signals from computer 111A are transferred electronically (e.g., as Ethernet 100BaseTX signals) to transceiver 101A where they are modulated onto a light beam and transmitted as a transmitted light beam 115A, through free space. Light beam 115 A is received by transceiver 101B, as a received beam 117B. Transceiver 101B converts the signals it receives into electronic signals and transfers them to computer 11 IB. Similarly, signals from computer 11 IB are converted by transceiver 101B into a light beam 115B. Light beam 115B is received through free space by transceiver 101 A, as a received beam 117A. Transceiver 101 A converts the signals from received beam 117A into electronic signals, which are transferred to computer 111A. As many windows of buildings are planned to attenuate infrared light, light beams 115 are optionally of a visible light bandwidth, which is not greatly attenuated by generally utilized windows. Furthermore, the specific attributes of light beams 115 are optionally optimized for short distance transmission (e.g., up to 300 meters or even only up to 100 meters). Accordingly, beams 115 are optionally planned to have a relatively large divergence, e.g., of several milli- radians, such that there is no need to compensate for building sway. The divergence is optionally adjusted by adjusting the relative alignment of a laser diode of the transceiver and a corresponding collimation lens and/or by selection of the characteristics of the collimation lens. Alternatively, transceivers 101 are planned for large distance transmission of over 500 meters or even over a kilometer. Transceiver Transceiver 101 is optionally of a small size such that it does not block a large percentage of the window. In an exemplary embodiment of the invention, transceivers 101 cover less than 20x20 cm on the window. In some embodiments of the invention, transceivers 101 are light weight, for example weighing less than 2 Kilograms or even 1 kilogram (e.g., about 720 grams) in order to allow easy mounting on the window or near the window (e.g., on a dry wall, a window pane and or a window frame). In order to achieve such a light weight transceiver, light weight materials are used, such as plastic lenses. The mounting is optionally sufficiently rigid, such that transceivers 101 are kept in accurate alignment to an accuracy of milli-radians. Fig. 2 is a schematic illustration of transceiver 101, in accordance with an exemplary embodiment of the invention. Transceiver 101 includes a transmission lens 230 through which light is transmitted and a reception lens 228, through which light beams are received from a remote transceiver. A laser diode package 202 is driven by a laser driver 204 to generate a light beam 115, under control of a controller 212. Controller 212 optionally receives (e.g., from computer 111A) data for transmission, on an input line 225. In some embodiments of the invention,- a filter 210 and/or a short optical fiber 242 are positioned between laser diode package 202 and transmission lens 230, as explained hereinbelow. Transceiver 101 further includes a receiver 220, which converts light beams received through lens 228 into electrical signals. The electrical signals are passed on a data out line 227, for example to computer 111 A (Fig. 1). Laser diode package Laser diode package 202 optionally includes an edge emitter laser diode, which includes a laser diode 250 and a feedback photo diode 252. In some embodiments of the invention, laser diode package 202 comprises a standard laser package generally used in compact discs (CD), bar code readers and/or DVD readers. These laser diode packages are much cheaper than laser diode packages specifically designed for communications. In some embodiments of the invention, laser diode 250 has a relatively long rise time, which is measured as the time required by the laser diode to rise from 20% to 80% of a given voltage or current level, responsive to a step function of the given voltage or current level. Optionally, laser diode 250 has a rise time above 0.8 nanoseconds or even above 1.5 nanoseconds. In an exemplary embodiment of the invention, laser diode 250 has arise time above 2.4 nanoseconds (e.g., 2.5 nanoseconds). It is noted that it is generally considered necessary in the prior art to use a laser diode having a rise time up to 0.5 nanoseconds for laser communications at rates of above 50-100 Mbps. For transmission rates of 1 Gbps and greater, it was considered necessary in the prior art to use a laser diode with a rise time of 0.1-0.2 ns or shorter. In some embodiments of the invention, the capacitance of laser diode 250 is higher than heretofore considered acceptable for laser transmissions, such as greater than 10 pF (picoFarad), 20 pF or even greater than 40 pF. In an exemplary embodiment of the invention, the capacitance of laser diode 250 is greater than 50 pF. Due to the high capacitance of laser diode 250, the laser diode is generally used in the prior art with DC driving signals. As is now described, laser driver 204 is designed to compensate for the limitations of laser diode 250. Laser driver In order to allow use at higher bit rates, laser driver 204 optionally provides in each driving cycle, a driving current higher than the current normally used with transmission laser diodes, for at least a portion of the cycle. Optionally, the higher driving current is higher than the current required in order to bring the laser diode to its operational level, given a relatively long time. Fig. 3 is a schematic graph of a single cycle 284 of a driving signal of laser driver 204, in accordance with an exemplary embodiment of the invention. In a first section 285 of the cycle, a current higher than required in order to cause laser diode 202 to generate a light beam is induced by laser driver 204. In a second section 286, a current required to cause laser diode 202 to generate a light beam is used. The excess current of first section 285 loads the capacitance of laser diode 202 at a sufficiently fast rate. In an exemplary embodiment of the invention, first section 285 has a bias and modulation current of between about 20-30 milliamps, while a bias and modulation current of about 10 milliamps is used in second section 286 and is generally sufficient to have laser diode 202 emit light. The current level and/or the time span of first section 285 are optionally adjusted such that the laser diode 202 is brought at least close to its high level during the first section 285 of the cycle, but does not reach its maximal level, so that the excess current does not burn out diode 202. In an exemplary embodiment of the invention, the current level of first section 285 is at least 50% greater than the level of second section 286, optionally at least 80% greater. The time span of first section 285 is optionally greater than 10%, 20% or even 30% of the time span of the entire cycle 284. In some embodiments of the invention, the time span of first section 285 is smaller than 50% or even smaller than 40% of the time span of the entire cycle 284. In an exemplary embodiment of the invention, for a 1 Gigabit per second data transmission, cycle 284 has a time span of about 800 picoseconds (ps) and first section 285 is about 312 ps. For faster transmission rates, first section 285 may include a larger portion of cycle 284 or may even form the entire cycle, such that the entire cycle uses a high current level. Laser driver 204 optionally provides a driving current having a slew rate sufficiently high for the capacitance of laser diode 250. Optionally, laser driver 204 operates with a slew rate suitable for transmission at a rate of 622 Mbps in low capacitance laser diodes. In some embodiments of the invention, laser driver 204 is operated with a slew rate which is suitable for transmission of a rate of at least 3 times greater, or even five times greater, than the transmission rate of driver 204, on a laser diode having a capacitance smaller than 5 pF. For example, when laser diode 250 and driver 204 are operated at a transmission rate of 100 Mbps, laser driver 204 operates with a slew rate suitable for transmission at a rate of 622 Mbps in low capacitance (e.g., lower than 5 pF) laser diodes. In another example, when laser diode 250 and driver 204 are operated at a transmission rate of 1 Gbps, laser driver 204 optionally operates with a slew rate suitable for transmission at a rate of 2.5 Gbps in low capacitance (e.g., lower than 5 pF) laser diodes. In some embodiments of the invention, laser driver 204 generates a signal which has a rise and/or fall time of less than 500 pico-seconds, optionally less than 200 pico-seconds, for a transmission rate of less than 150 Mbps. In some embodiments of the invention, for transmission rates of less than 1.5 Gbps (e.g., lGbps), laser driver 204 has a rise and fall time of less than 100 pico-seconds. In some embodiments of the invention, the rise and fall times of laser driver 204 take up together less than 20%, 15% or even 10%o of the cycles of the laser driver 204. Optionally, laser driver 204 drives laser diode 250 with a relatively high peak power, for example of at least 12 milli- Watts. In some embodiments of the invention, laser driver 204 operates at a low efficiency working point. Although having low efficiency, this working point is preferred, due to the above mentioned advantages. In an exemplary embodiment of the invention, laser driver 204 comprises a CX02068 or CX02066 laser driver of the Mindspeed company. In some embodiments of the invention, laser diode 250 is kept in a desired working temperature (e.g., 25°) with a relatively high accuracy, as edge emitters are relatively sensitive to temperature changes. Optionally, the temperature of laser diode 250 is controlled using a thermo-electric cooler (TEC). Alternatively, the temperature of laser diode 250 is controlled using a fan and/or a heat sink. Visible light Laser diode 250 optionally generates light at a visible light bandwidth. As mentioned above, visible light is not strongly attenuated by windows 107. In addition, as explained in detail below, visible light allows easy alignment of transceivers 111 A and 11 IB relative to each other. Furthermore, laser diodes at visible light bandwidths are very cheap due to their use in CDs. In some embodiments of the invention, laser diode 250 generates light beams at one or more of the wavelengths 635, 650_^ or 670 nanometers (mil). Alternatively, laser diode 250 has an adjustable wavelength, for example, by changing the driving current of laser diode 250 and/or the temperature of the laser diode. The adjustment of the wavelength is optionally performed by the same mechanism that controls the temperature of laser driver 250. In an exemplary embodiment of the invention, the wavelength used is in the visible range, optionally between about 600-700 nanometers. In some embodiments of the invention, the actual wavelength used for the transmission beam is selected responsive to the properties of window 107. Optionally, transceiver 101 has a knob or other control which allows selection of the wavelength of the transmitted beam by a human operator. The light transmission characteristics of window 107 are determined, for example using the method described in above mentioned U.S. patent 6,603,554, and accordingly the human operator sets the wavelength of the transmitted beam. Alternatively, transceiver 101 tests the transmission of window 107 for various wavelengths and accordingly automatically selects a wavelength of transmitted beam 115. In an exemplary embodiment of the invention, after transceivers 101 are properly aligned, test beams are transmitted at a plurality of wavelengths. In the opposite end transceiver, the power level of the received beam is determined and an indication of the power level is transmitted back to the transmitting transceiver, for example encoded on a light beam as discussed below. The transmitting transceiver optionally selects to use the wavelength that achieved a highest power level or a highest signal to noise ratio. A driving wire 236 leading from laser driver 204 to laser diode 250 optionally has an impedance which matches the impedance of diode 250. The use of laser diode package 202 is advantageous, due to its low cost and easy availability. It is noted, however, that other generators of light beams could be used in transceivers 101, such as a visible-light vertical cavity surface emitting lasers (VCSEL). Internal optical fiber The light beams generated by edge emitters generally have an elliptical cross-section. In some embodiments of the invention, the elliptical beams are transferred through free space. Although elliptical beams have a larger transmission energy loss in free space data transmission systems than circular cross-section beams and/or involve more difficult alignment, the advantages of using an edge emitter (for example, over a VCSEL), as described throughout the present application, are considered to outweigh the disadvantages of the elliptical beams. In other embodiments of the invention, the elliptical light beam generated by laser diode 250 is passed through optical fiber 242 which converts the elliptical light beam into a circular cross- section light beam. One end of optical fiber 242 is positioned to receive the light beam generated by laser diode 250. A lens 266 optionally collimates the light into optical fiber 242. A second end of optical fiber 242 is positioned near a focus point of transmission lens 230 such that the light beam exits the fiber to be coUimated by lens 230 and transmitted therefrom in free space. Alternatively, any other apparatus is used to convert elliptical light beams into circular light beams. Filter In some embodiments of the invention, filter 210 is adapted to prevent sun light from damaging laser diode package 202 and/or from offsetting the feedback readings of feedback photo-diode 252. Filter 210 is optionally adapted to prevent passage of infrared radiation.

Alternatively or additionally, filter 210 is adapted to prevent passage of visible light not in the wavelength of transmitted beam 115. hi an exemplary embodiment of the invention, filter 210 is a band pass filter that only allows passage of light in a range of about 620-680 nm, within which range the opposite end transceiver transmits. Fig. 4 is a flowchart of acts performed in installing and aligning transceivers 101, in accordance with an exemplary embodiment of the invention. Installation points of the transceivers are selected, with an optical light path between them. The transceivers 101 are mounted (290) at their selected installation points. The transceivers 101 are optionally positioned indoors behind windows 107, so that dedicated roof space is not required for the transceivers and there is no need to construct transceivers 101 that endure harsh weather conditions. Transceivers 101 may be mounted directly on the window, on a wall near the window or on the ceiling. The mounting of the transceiver is performed using any method known in the art, for example, mounting on a wall bracket, window pane, a clamp-style window bracket or an adhesive backed-window bracket. After mounting of the transceivers 101, a coarse alignment procedure (292) is performed, in which the transceivers are sufficiently aligned so that they each receive the light beams transmitted from the other end transceiver. In some embodiments of the invention, the coarse alignment is performed using a sight 162 (Fig. 2) on each of transceivers 101, as described below. Thereafter, a fine alignment procedure (294) is performed, so as to maximize the power of the received beams. After the alignment is completed, return transmission beam shields are positioned (296) near one or more of transceivers 101, for example, as described below with reference to Figs. 6 or 7.

Sight The use of visible light to carry the data, makes the alignment of transceivers 101 much simpler, relative to alignment of transceivers that use infrared light, as will become evident. Optionally, one or more of transceivers 101 include sight 162, such as commonly used on rifles, for aiming the transceiver at the opposite end transceiver with which it is to communicate. Sight 162 is aligned with receiver 220 and laser package 202, so that once the sight is aligned with the opposite end transceiver, the transceivers are at least coarsely aligned. Optionally, sight 162 does not include any enlargement optics (e.g., lenses, mirrors), so that it is relatively cheap. Alternatively, for example when transceivers 101 are planned to be used for distances greater than 300 meters or even half a kilometer, sight 162 includes telescope optics. In some embodiments of the invention, sight 162 includes a simple hollow tube, with a crosshair target within the tube. Alternatively, the sight is formed of two rings separated from each other by between about 5-20 centimeters. In some embodiments of the invention, sight 162 includes a filter that only allows passage of wavelengths of the light beam to which sight 162 needs to be aligned. The use of the filter reduces the amount of background light that serves as noise, reaching the human eye. Alternatively or additionally to placing a filter at the proximal end of sight 162, the filter may be positioned at the distal end of sight 162 or maybe mounted directly on the viewing human's eyes, for example by wearing filtering eye glasses. The sight 162 is optionally permanently mounted on transceiver 101. In those embodiments in which the sight is relatively inexpensive (e.g., does not include enlargement optics), the permanent mounting of the sight has little, if any, affect on the cost of transceiver 101. In some embodiments of the invention, both transceivers have sights 162 mounted thereon. Alternatively, only one of transceivers 101 has a sight 162. Further alternatively, sight 162 includes a collecting lens at its distal end 199, which lens leads the light beam reaching the sight onto a screen, such as a translucent screen (e.g., a ground glass) at the proximal end of the sight. The screen optionally has a target at the center of the screen, and the human operator adjusts the position of transceiver 101, so that the light impinges on the target. Alternatively to placing the screen so that it displays light collected by sight 162, a light splitter is positioned between reception 228 and receiver -220. The light splitter directs a small portion of the received light to a screen with a target. In some embodiments of the invention, the light splitter is removed after alignment of transceivers 101, in order not to waste power of the data beam on the splitter. Alternatively to a beam splitter, a removable mirror, such as used in a reflex photographic camera, is used. These embodiments eliminate the need of the human operator looking directly into the received beam, which may involve discomfort. Instead, the operator views the light indirectly on the screen. Coarse alignment In some embodiments of the invention, during the coarse alignment both of transceivers 101 are operated to transmit visible light beams. A human operator at each of the transceivers directs the transceiver at the beam of the opposite end transceiver. The human operators aim sights 162 at the visible light beams emitted by the opposite end transceiver 101. In some embodiments of the invention, the human operators use telephone communications to instruct each other on the direction in which they should move their beams, if necessary. In some embodiments of the invention, the coarse alignment is performed in parallel for both transceivers 101. Alternatively or additionally, the coarse alignment is performed intermittently by the transceivers 101, until alignment is achieved. The coarse alignment optionally achieves an accuracy of between about 20-40 centimeters, depending on the distance between the transceivers 101 and the divergence of the transmitted beams. It is noted that the transmitted beam generally has a diameter of about 50 centimeters after traversing a distance of about 100 meters . Optionally, transceivers 101 are mounted rotatably on an arm which holds the transceiver in place. In some embodiments of the invention, alignment knobs are used to prevent the rotation of the transceiver on the arm. Alternatively, control dials are used to controllably move the transceiver relative to the arm or other base. The use of a visible light beam allows detection of the light by a human eye, without use of a CCD or other detector. In addition, the visible light beam allows detection from a relatively large distance without the need of optical enlargement and/or focusing apparatus (e.g., lenses, mirrors). In some embodiments of the invention, the alignment is performed at night when it is much easier to identify visible light beams, both on building exteriors and inside rooms. In some embodiments of the invention, the generated visible light beam is the same beam used for data transmission. Alternatively, transceivers 101 generate a separate alignment light beam substantially parallel the data light beam. In other embodiments of the invention, the light beam is generated by the transceiver performing the alignment and is returned by a retro- reflector on an opposite end transceiver 101. The retro-reflector may be permanently mounted on transceivers 101 or may be employed only for initial alignment. Retro-reflectors may be mounted on one of transceivers 101 or both of the transceivers. The retro-reflector is optionally designed to reflect incident light back along the path through which it hits the retro-reflector. The retro-reflector may comprise for example the 3M™ Scotchlite™ Diamond Grade™ VIP Reflective Sheeting. The retro-reflector allows the installer to see the reflected beam from the opposite terminal both during daytime and night hours. Using the retro-reflector allows one person installation. Fine alignment While the coarse alignment is directed at achieving reception of the beam from the opposite end transceiver, the fine alignment is optionally directed at maximizing the reception power of the beam. Receivers 220 of transceivers 101 optionally provide an RSSI output 265 which indicates the strength of the received beam. The RSSI output 265 is optionally provided to a LED array 280 (Fig. 2) which converts the value of the RSSI output into a human tangible display. Fine alignment dials (not shown) are optionally used to finely adjust the orientation of transceiver 101. Optionally, the fine alignment is performed by each of transceivers 101 separately, while the opposite end transceiver remains stationary. In some embodiments of the invention, the human operator of the transceiver 101 being aligned moves the transceiver in various directions. The other human operator tells the aligning operator, for example over the telephone, whether the movements increased or decreased the power of the received beam, based on the LED array 280 display. This process is continued until a maximal power is achieved. The fine alignment optionally achieves an accuracy of several centimeters. Alternatively to receiving the power indications from the opposite end operator, the power indications are transmitted encoded on the light beam used for the alignment, as is now described. The power readings of the opposite end transceiver are optionally displayed by the LED array 280 of the transceiver, which LED array normally displays the signal strength of the signals received by the local transceiver. Controller Fig. 5 is a schematic block diagram of controller 212, in accordance with an exemplary embodiment of the invention. Controller 212 serves to transmit the measured power level of a received light beam back to the opposite end transceiver 101 transmitting the light beam. In accordance with the embodiment of Fig. 4, three types of signals are transmitted between transceivers 101. The transmitted signals are either data signals 225, for example from computers 111, test patterns (335), for the other end transceiver 101 to estimate its received power, or an encoded signal (345) carrying an indication of the measured power. Controller 212 optionally receives data for transmission on input line 225. In a data transmission mode, the received data for optical transmission is transferred through an output line 309 to laser driver 204 for transmission. Transmission of test signal In a fine alignment stage, a button 238 is actuated by a human operator, located near the transceiver 101 or using a remote control. Button 238 causes a pattern coder 330 to generate a test pattern (335) or other test signal, which is passed through a multiplexer 305 to output line 309 instead of data signals 225. The selection of test pattern 335 by multiplexer 305 is optionally also actuated by button 238. The transmission of test pattern 335 optionally indicates to the opposite end transceiver to transmit the power readings on its transmitted beam. Alternatively or additionally, the test pattern allows the other end transceiver to determine the power level on a predetermined beam. Test pattern 335 is optionally encoded by a conditional diphase (CDP) coder to have a temporal frequency (or time signature) which allows simple differentiation from sporadic light from the environment, such as sun light and/or from data signals 225. In an exemplary embodiment of the invention, a frequency of 390 KHz or 780 KHz is used. Alternatively, the data input 225 is used for alignment and multiplexer 305 and pattern coder 330 are not required. The transceivers 101 optionally have a separate human operated control, for indicating when they are to transmit encoded signal 345, as test pattern 335 is not used in this alternative. When the test signal is received from the opposite end transceiver 101, controller 212 generates encoded signal 345, which carries the indication of the measured power. In some embodiments of the invention, encoded signal 345 comprises an encoding, e.g., a CDP encoding performed by a CDP coder 316, of analog RSSI readings 265, converted into digital format by an analog to digital converter 352. Transmission of encoded signals Signals optically received by receiver 220 are transferred on output line 227 (Fig. 2), in case they are data signals. In parallel, the received signals are provided to controller 212. In some embodiments of the invention, before being provided to controller 212, the received optical signal is passed through a filter 313, which removes data signals (225) so that they do not interfere with the operation of controller 212. For example, if the transmitted data is at a bit rate of at least 100 MHz, the encoded power signals (345) and the test patterns (335) maybe encoded at a low rate, for example less than 1 MHz. Accordingly, filter 313 optionally comprises a low pass filter. The signals received by controller 212 are optionally provided in parallel to a pattern decoder 311 and a CDP decoder 315. Pattern decoder 311 optionally determines whether a test pattern 335 is being received. If a test pattern 335 is being received, a multiplexer 307 selects the transmission of encoded signal 345, which provides the signal strength to the opposite end transceiver transmitting the test pattern 335. If a test pattern 335 is not received, e.g., an encoded signal 345 or data signals 225 are received, multiplexer 307. is set to transmit test pattern 335 or data signals 225. CDP decoder 315 optionally decodes the encoded signal 345 from the opposite end transceiver 101. In some embodiments of the invention, the decoded signal from CDP decoder 315 is converted into an analog signal, by a digital to analog converter 313. The analog signal indicating the signal strength is provided to a LED controller 328, which controls the display of LED array 280. The selection of whether LED array 280 displays the signal power readings of the local tiansceiver or the opposite end transceiver is optionally performed by a multiplexer 332 based on the state of button 238. In some embodiments of the invention, the number of LEDs in LED array 280 that are lit up indicate the signal strength. Alternatively, only one of the LEDs is lit up, and the position of the LED that is lit up is indicative of the signal strength, hi some embodiments of the invention, a LEDs filter 340, as is known in the art, is used to convert the instructions of LEDs control 328 into a desired display format. Blocking stray returned beams Light transmitted toward a window at an angle not perpendicular to the window, is partially returned. Therefore, if light beam 115 is not by chance perpendicular to the window, a portion of the transmitted beam 115 is returned into the room in which tiansceiver 101 is installed. While the percentage of light that is returned is small and is not dangerous, its presence in the room may be annoying to people in the room. Fig. 6 is a schematic illustration of a reflection blocker 400 mounted on a window 107, in accordance with an exemplary embodiment of the invention. When window 107 is at a relatively large angle relative to being perpendicular to beam 115 transmitted from transceiver 101, a portion 410 of beam 115 is retimed from the window. Reflection blocker 400 includes a tub 405 with opaque, preferably low reflection, walls that serve as a shield and prevent returned portion 410 from entering the room. Reflection blocker 400 optionally includes a window interface 420, which is attachable to window 107, for example using an adhesive or vacuum. A flexible joint 425 allows adjustment of the angle of tube 405 relative to interface 420 and hence to window 107. After installation of transceiver 101 and alignment with the opposite end transceiver, tube 405 is adjusted to allow beam 115 to pass completely through the tube, but to block reflected light portion 410. Tube 405 optionally has an inner diameter larger than a transmitting lens 230 and hence of transmitted beam 115. In an exemplary embodiment of the invention, tube 405 has a diameter greater than 25 millimeters or even greater than 35 mm, to allow passage of both the transmitted and received light beams. Alternatively or additionally, tube 405 has a diameter smaller than 75 or even 60 mm, so as to limit the angles not blocked by the tube. Tube 405 optionally has a length of between about 2-5 centimeters. In some embodiments of the invention, tube 405 blocks all beams that are deflected at an angle greater than 40%, 20%> or even 10%. Alternatively to a flexible joint 425, tube 405 comprises a flexible, optionally non- elastic, material, which is bent for alignment parallel to beam 115. Fig. 7 is a schematic illustration of a reflection blocker 450, in accordance with another exemplary embodiment of the invention. While blocker 400 is directed at blocking most of the angles in which light may return from a meeting point 433 of beam 115 and window 107, reflection blocker 450 blocks only a small percentage (e.g., less than 10% or even 5%) of the possible return angles from the meeting point of transmitted beam 115 with the window 107. Reflection blocker 450 is positioned at an angle at which light is reflected from window 107, i.e., the angle of reflection of the incident light beam 115. In some embodiments of the invention, reflection blocker.450 includes a window mounting interface 455, an adjustment arm 460 and an opaque light shield 465. Window mounting interface may attach to window 107 using any method known in the art, such as using an adhesive. Alternatively, reflection blocker 450 is not mounted on window 107, but rather on a nearby wall, on the ceiling and/or the floor. Further alternatively, reflection blocker 450 is mounted on transceiver 101. Optionally, reflection blocker 450 is light weight, weighing less than 500 grams, less than 200 grams or even less than 100 grams. Visible light in production The use of visible light for transmission is also advantageous for simpler alignment of receiver 220 and laser diode package 202 during production of transceivers 101. Instead of requiring dedicated alignment apparatus, the alignment may be performed based on the visible light beam transmitted from laser diode package 202. It will be appreciated that the above-described methods and apparatus may be varied in many ways, including changing sizes and materials used in forming the transceivers. For example, instead of LED array 280 any other display may be used, for example a digital display of numbers indicative of the power level. It should also be appreciated that the above described description of methods and apparatus are to be interpreted as including apparatus for carrying out the methods, and methods of using the apparatus. The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art. Furthermore, the terms "comprise," "include," "have" and their conjugates, shall mean, when used in the claims, "including but not necessarily limited to." It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the stiucture or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims.

Claims

1. An optical transceiver, comprising: a data interface adapted to receive data signals, in electrical format; a laser diode having an intrinsic rise time of at least 0.8 nanoseconds; and a laser driver adapted to drive the laser diode to generate a light beam carrying the data signals received by the data interface, for data signals carrying data at rates of at least 50 Mbps.

2. A transceiver according to claim 1, wherein the laser diode generates a beam of visible light.

3. A transceiver according to claim 1, comprising a lens adapted to lead the generated light beam generated by the laser diode into the atmosphere in a manner suitable for conveying data through the atmosphere over a distance of at least 10 meters.

4. A tiansceiver according to claim 3, comprising an optical fiber through which the light beam is passed from the laser diode to the lens.

5. A transceiver according to claim 3, comprising a sight for aiming at an opposite end transceiver, the sight not including optics.

^■ 6. A transceiver according to claim 5, comprising a screen on which light directed at the sight is aimed, for alignment.

7. A transceiver according to claim 1, wherein the laser diode comprises an edge emitter laser diode.

8. A transceiver according to claim 1, wherein the laser diode has a capacitance of at least 5 pico-Farad.

9. A transceiver according to claim 8, wherein the laser diode has a capacitance of at least 10 pico-Farad.

10. A transceiver according to claim 9, wherein the laser diode has a capacitance of at least 30 pico-Farad.

11. A transceiver according to claim 1, wherein the laser diode has a rise time of at least 1.5 ns.

12. A transceiver according to claim 11, wherein the laser diode has a rise time of at least 2.4 ns.

13. A transceiver according to claim 1, wherein the laser driver is adapted to drive the laser diode with a signal in which the rise and fall periods cover less than 20% of the signal.

14. A transceiver according to claim 13, wherein the laser driver is adapted to drive the laser diode with a signal in which the rise and fall periods cover less than 10% of the signal.

15. A transceiver according to claim 1, wherein the laser driver is adapted to drive the laser diode to generate a light beam carrying the data signals received by the data interface, for data signals carrying data at rates of at least 500 Mbps.

16. A transceiver according to claim 1, comprising a controller designed to receive from an opposite end transceiver an indication of a reception power level of the light beam generated by the laser diode.

17. A transceiver according to claim 1, comprising a controller designed to initiate transmission to an opposite end transceiver an indication of a reception power level of a light beam received from the opposite end transceiver.

18. A transceiver according to claim 1, comprising a heat regulator adapted to regulate the temperature of the laser diode.

19. A transceiver according to claim 1, wherein the heat regulator comprises a thermoelectric cooler.

20. A transceiver according to claim 1, comprising a controller adapted to adjust a wavelength of the generated light beam responsive to an indication of the ttansmissivity of a window adjacent the transceiver.

21. A method of transmitting data on a light beam, comprising: receiving data signals at a rate of at least 50 Mbps in an electrical format; and driving a laser diode having an intrinsic rise time of at least 0.8 nanoseconds, to generate a light beam carrying the data signals.

22. A method according to claim 21, wherein the data signals have a rate of at least 500 Mbps.

23. A method according to claim 21, wherein the laser diode has a capacitance of at least 10 pico-Farad. .

24. A method according to claim 21, comprising transmitting the light beam through a window into the atmosphere.

25. A method according to claim 24, comprising positioning near the window a shield adapted to block return light beams from the window.

26. A method according to claim 21, wherein the laser diode has a rise time of at least 2 nanoseconds.

27. A method of aligning first and second transceivers of a free space tiansmission system, comprising: transmitting a light beam from a first transceiver; and aiming a sighting apparatus of a second transceiver at the transmitted light beam of the first transceiver.

28. A method according to claim 27, wherein the sight does not include enlargement or focusing optics.