Patent ID: 12231169

DETAILED DESCRIPTION

Some implementations of the present disclosure will now be described more fully hereinafter with reference to the accompanying figures, in which some, but not all implementations of the disclosure are shown. Indeed, various implementations of the disclosure may be embodied in many different forms and should not be construed as limited to the implementations set forth herein; rather, these example implementations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like reference numerals refer to like elements throughout.

Unless specified otherwise or clear from context, references to first, second or the like should not be construed to imply a particular order. A feature described as being above another feature (unless specified otherwise or clear from context) may instead be below, and vice versa; and similarly, features described as being to the left of another feature else may instead be to the right, and vice versa. Also, while reference may be made herein to quantitative measures, values, geometric relationships or the like, unless otherwise stated, any one or more if not all of these may be absolute or approximate to account for acceptable variations that may occur, such as those due to engineering tolerances or the like.

As used herein, unless specified otherwise or clear from context, the “or” of a set of operands is the “inclusive or” and thereby true if and only if one or more of the operands is true, as opposed to the “exclusive or” which is false when all of the operands are true. Thus, for example, “[A] or [B]” is true if [A] is true, or if [B] is true, or if both [A] and [B] are true. Further, the articles “a” and “an” mean “one or more,” unless specified otherwise or clear from context to be directed to a singular form. Furthermore, it should be understood that unless otherwise specified, the terms “data,” “content,” “digital content,” “information,” and similar terms may be at times used interchangeably.

Laser eye safety. There are several physical mechanisms by which lasers can damage the human eyes. Very short, high intensity pulses can cause both denaturation of proteins and explosive boiling of water in the eye. Longer exposure times can lead to photochemical damage where light triggers chemicals reactions in the tissue.

The wavelength of the light determines the impact on the human eye. Visible light will typically induce a blink reaction and hence higher optical powers are considered eye safe. At longer wavelengths, the water inside the eye will absorb much of the light, preventing it from ever reaching the retina. As an example, at 1550 nm light the eye safe limit is set by the point where the light will burn the cornea, and has a value of ˜100 mW/cm2. In the case of a 50 mm diameter lens, the maximum launch power is ˜2W. In between is a range of wavelengths that can still pass through the water in the eye, but the retina response is much lower and so the beam will appear to the eye to be much dimmer than it actually is. In this range, the eye safe power limit is lower.

The intrinsic divergence of the light beam (typically referred to as alpha—α—and given in units of milliradians) also determines the impact on the eye. Any beam with an α<1.5 milliradians is considered collimated for eye safety, since the lens in the eye is can only focus to a minimum spot size of about 20 microns at the retina. Beams with α>1.5 milliradians will focus to a spot that is larger than 20 microns and can have higher intensities without causing eye damage. Note that the intrinsic beam divergence is different than the overall beam divergence. A laser exiting from a single mode fiber may cover a range of many 10's of degrees (the overall beam divergence), but the intrinsic divergence (α) is still less than 1.5 milliradians because the emission area set by the fiber is very small (5 to 10 microns).

FIG.1shows the MPE (maximum permissible exposure, American National Standard Z136.1-2014 page 104) for a collimated beam (α<1.5 mrad) for wavelengths ranging from 400 nm to 1400 nm and exposure durations from 0.1 second to ˜30,000 seconds (8 hours). The irradiance measured in Watts per cm2is plotted on the y-axis102against the exposure time on the x-axis100. Longer wavelengths generally have higher MPE's with the exception that 1400 nm is below 1315 nm. The MPE generally plateaus after 10 seconds.

If the beam comes from an extended source, such as a light bulb, then a is much larger than 1.5 milliradians and the eye does not focus the light to the minimum spot, but rather to a larger spot. This larger spot can have a larger total power without damaging the retina. Subsequently, the intensity in the beam incident on the eye can be higher.

FIG.2shows a plot of the eye safe optical intensity202(in mW/cm2) for the range of values of α 200 from 1.5 mrad up to 100 mrad. This plot is calculated at a wavelength of 850 nm and an exposure time of 10 seconds204and 60 seconds206. The upper level of 100 mrad)(˜6° is the point at which the total power on the retina becomes a concern.

Eye safety is used herein as defined by laser safety standards such as the American National Standards Institute (ANSI) Z136.1 standard (the ANSI Z136.1 standard), the International Electrotechnical Commission (IEC) 60825-1 standard (the IEC-60825-1 standard), and the like. Eye safety is set by class with different safety rules for each class. Current classes include Class 1, Class 1M, Class 2, Class 2M, Class 3R, Class 3B, and Class 4. Different example implementations will be eye safe for different classes. Many example implementations will be Class 1 or Class 1M. Some example implementations will be Class 2 or Class 2M. Some example implementations may be Class 3R, Class 3B, or Class 4.

Previous FSO systems. Most FSO communications systems have used either a fiber-coupled laser or a single element laser diode. These light sources have a natural angular range of emission, which is then collimated into a beam by using one or more lenses or mirrors. There is still a residual divergence that is set by the diameter of the emission area relative to the focal length of the collimated lens, or in the extreme case of a very small emission area, by the ratio of the spot size on the lens to the wavelength of light. This divergence can be quite small, less than 1 microradian in the case of a single mode fiber and a 50 millimeter collimation lens. For all of these systems, a is <1.5 milliradians and the minimum eye safety intensity applies.

As an example, at 850 nm, light passes through the water in the eye and so the limit is low, ˜2 mW/cm2for a collimated beam. The maximum power output for a 50 mm diameter lens is now limited to about 40 mW of total optical power. This total power in the beam limits the link distance of the FSO system.

To increase this limit, many FSO systems used 1310 nm wavelength lasers. In this case the light does not pass through the water in the eye and the eye safety threshold is set by the power level that will cause burns on the cornea. This is around 200 mW/cm2; now a 50 mm diameter lens can launch almost 4 W. However, as noted in previous applications, 850 nm is sometimes preferred since silicon based components can be used, which are currently much cheaper than materials such as InGaAs that are used for longer wavelength devices.

Advantage of DBFSO links. Example implementations of the present disclosure add a diffuser between the laser and the lens so that the beam nearly fills the lens and the intrinsic beam divergence is up to, but does not exceed the overall beam divergence. This increases the maximum launch power and hence the maximum distance and data rate that the link can achieve.

In previous disclosures and filings, a diverged beam system has been described where the source consists of a laser or laser array with one or more lenses for setting the system degree of divergence. If the lens(es) is set at a focal length from the laser, then the overall divergence of the beam is set by the size of the laser area relative to focal length of the lens, i.e., the minimum divergence is given by (laser array size)/(lens focal length).FIG.3Ashows this configuration. The laser array300has a finite width and the laser beam302diverges at the natural divergence angle of the laser or laser array300. The beam makes an angle σ304with the focusing lens306and then has a different divergence angle θ308with the normal. The finite extent of laser on the other side of the lens is given by α310.

In general, the system beam divergence can be increased by moving the lens closer to the laser. Even though the overall beam divergence will then increase, the localized (or intrinsic) divergence will still be set by the laser size divided by the distance to the lens. This holds until the lens gets close enough to the laser that it is limited by the laser beam divergence. At this point the overall divergence is set by the divergence of the laser itself, that is the lens starts to look like a window with little or no focusing power.

Many lasers and laser arrays have a divergence in the range of 20° to 40°, and in one implementation the divergence is between 24° and 34°. Each of the individual laser elements could be collimated by a lens, however since the elements are at different lateral points relative to the lens axis, each beam will be collimated in a different angular direction. The laser elements are also very close together, within tens of microns, so building an array of lenses where there is a lens for one or a few laser elements may be difficult.

Diverged beam without diffuser.FIG.3Aprovides an example implementation of the higher power allowed with a diverged beam system without a diffuser. In one case suppose the laser300is ˜1 mm across and the lens306is ˜40 mm away. The divergence for eye safety is then ˜25 milliradians, which increases the acceptable intensity from 2 mW/cm2to 34 mW/cm2(for 10 second exposure). The spot size at the lens is only 2.1 cm across, so the maximum system launch power is 118 mW.

Diverged beam with diffuser. Example implementations of the present disclosure increase the acceptable intensity by adding a diffuser between the laser(s) and the lens(es). The diffuser scatters the light over a controlled range of angles. This range of angles will always be larger than the beam divergence from the laser itself, otherwise the system would violate conservation of etendue and the 2nd law of thermodynamics. The spot illuminated on the diffuser by the laser becomes the effective area of the laser for the intrinsic beam divergence as the laser propagates through the rest of the transmitter and then system.

The area illuminated on the transmit lens is then set by the diffuser output angle and the distance between the diffuser and the transmit lens. The goal is to fill as much of the transmit lens as possible while still maintaining a large enough value for a. The longer the distance between the diffuser and the lens(es), the larger the beam size at the lens, but longer distances mean smaller values of α.

FIG.3Bshows an example implementation. The laser array312has a natural divergence and makes an angle σ322with the diffuser314. The beam from the diffuser316is at a different divergence angle and makes an angle σ2with respect to the lens324. The lens324changes the divergence of the beam again so that it makes an angle θ with respect to the normal. The area illuminated on the transmit lens324makes an angle α2326behind the lens324. This value of α is larger than inFIG.3Adue to the diffuser.

As an example implementation of the present disclosure, a 3 mm spot on the diffuser that is 40 mm from the lens gives a divergence of 75 milliradians and an acceptable intensity of 100 mW/cm2(at 10 second exposure). The beam size at the lens is nearly 5 cm, so the maximum system power is now 1963 mW. This is about a factor of 16 larger than the implementation without the diffuser shown inFIG.3A.

The laser source may be any of a number of laser sources, including, but not limited to, vertical-cavity surface-emitting lasers (VCSELs), VCSEL arrays, strip lasers, fiber lasers, or fiber coupled lasers with an additional amplification stage. One example of a fiber coupled laser may be a telecom laser in the C band that is connected to an EDFA (erbium doped fiber amplifier) and the fiber output of the EDFA is then connected to the transmitter. The size of the laser source may range from as small as 5 microns, for a visible wavelength fiber up to 10 millimeters, for a large VCSEL array. Some other examples include a 1 mm VCSEL array, a 9-micron core, single mode telecom fiber and a 100-micron multimode fiber. The laser wavelength may be any value that can be physically realized including UV, visible, near infrared, far infrared and others. Common wavelengths already available that may be used in these implementations include 800 nm, 850 nm, 905 nm, 1300 nm, the telecom C band, and the telecom L band.

For some example systems, diffusers from Brightview Technologies may be used. In one implementation, the spot product uses the C-HE40-PE-S-M-RA12 which increases the +/−12° divergence of the VCSEL array to about +/−27°. In this implementation the diffuser sits ˜7 mm from the laser. There is a limit to how close the diffuser can be. If it is too close to the laser, the small amount of absorption will heat the diffuser up and may melt it. Additionally, the further away the diffuser sits, the larger a can be because the spot size on the diffuser increase with increasing distance between the laser and the diffuser. The efficiency of this diffuser is quite high, ranging from 89% up to 97%. The degree of divergence can be engineered by selection of the particular diffuser. Brightview has standard products ranging from ˜5° up to 127° (FWHM) and custom diffusers can hit any target in that range.

Other diffusers may range from 1 degree up to 2 pi steradians.

Note that this scheme may only work in a highly diverged beam system, because the divergence introduced by the diffuser is well beyond the acceptable beam divergence for a collimated FSO system. That is, the system may have too large an angular range and may not have enough laser power to achieve any distance. This works in with diverged beam disclosures precisely because the beam is already diverged and the 25 to 200 mrad of divergence is within the design range of overall system divergence, which has ranged from 0.1° (1.75 mrad) to 20° (349 mrad) or more up a hemisphere or 2 pi steradians.

Note that the divergences (including laser emission, intrinsic, and overall) may have different values in different directions. This may be due to the design of the laser, for example strip lasers typically have a larger horizontal divergence as compared to their vertical divergence. It may be intentionally designed, for example it may be beneficial to have a transmitter with a larger horizontal divergence than vertical for use in broadcast configurations where one transmitter broadcasts to multiple receivers near the ground.

The intrinsic divergence and the overall divergence can be independently controlled with the requirement that the overall divergence is larger than the intrinsic divergence. The intrinsic divergence is set by the ratio of the spot size on the diffuser to the distance from the diffuser to the lens. The overall divergence can be changed by moving the lens from focus (where the overall divergence is equal to the intrinsic divergence) closer to the diffuser, thereby increasing the overall divergence. This will have an impact on the intrinsic divergence but it will be small compared to the change in overall divergence as the lens is moved.

Example implementations may also be used in a reflective configuration where the laser points at the front of the diffuser and laser light then reflects and scatters off of the diffuser to increase the divergence angle.FIG.4shows an example implementation of this configuration. The beam from the laser array400hits the diffuser404, and is reflected406before hitting the lens402. Again the final beam divergence may range from 0.1 degree up to 20 degrees up to 2 pi steradians.

A further advantage of example implementations of the present disclosure is that shorter focal length lenses can be used, particularly for the transmitter. This may decrease the overall system size. For example, to fill a 5 cm lens with a light diverging from the laser at +/−12° takes a focal length of ˜12 cm to nearly fill the lens. With a diffuser increasing that angle to +/−25°, the same spot size on the 5 cm lens may be achieved in ˜6 cm focal length, thus reducing the distance between the laser and the lens and potentially reducing the size of the system.

The diffuser may be a passive or active element. One example implementation uses a passive engineered diffuser from Brightview Technologies. Other passive diffusers can be used including ground glass and others. Active elements could be used for the diffusers in this description as well. These include elements that change some relevant optical property via a change in a voltage, a current, a mechanical strain or stress, or some other property. The active element(s) may be used to adjust the beam divergence based on various scenarios or feedback loops. In a fog scenario, for example, the intrinsic beam divergence may be decreased in order to increase the amount of light that detected by the receiver.

FIG.5Ashows an implementation. Here, the laser array500emits light that makes an angle σ504with the active diffuser502. The diffuser502may scatter light in any angle ranging from none506which makes an angle σ1508with the lens to a maximum beam510making an angle θ2512. Note that σ1508is equal to σ504.

In another scenario, a very wide divergence may be used for link acquisition and once the transceivers are pointed toward each other, the divergence may be decreased. Data rate may be changed with divergence, for example a very low data rate is used with a very wide divergence for link acquisition and then when the transceivers are aligned, the divergence can be decreased and the data rate increased. Receivers may use a similar effect to allow one detector to have a wide field of view with a very low data rate and then decrease the field of view and increase the data rate.

In some implementations, the diffuser may be physically moved relative to the source and the lens to change the overall beam divergence.FIG.5Bshows an implementation. Here, the laser array516emits light that hits a diffuser518in one of two positions (position 1520or position 2522). The diffracted rays from the diffuser in position 1520are shown by the dot-dashed line524. If the diffuser is further away from the source in position 2522, the rays are given by the solid line526. Each of these positions has an associated α with it as well. For position 1520α is given by α1530in the figure while for position 2522, α is given by α2532. Although this figure only depicts two positions, there are an infinite number of positions over a given range that the diffusers may occupy that will each fill a different extent of the lens and have a different associated α.

In some implementations, there may be both an active diffuser and a passive diffuser. If the active diffuser is closer to the laser (array) source, it can control the intrinsic divergence by controlling the spot size on the second, static, diffuser. The static diffuser and lens then control the overall divergence. This is shown inFIG.5C. A laser array534emits light that hits an active diffuser538. The active diffuser controls the intrinsic divergence of the light by dictating how much of the passive diffuser540, the beam fills. The passive diffuser540and lens536then control the overall divergence and thus the power density. As previously discussed, both of these quantities are important in eye safety. Alternately, the passive diffuser can be closer to the light source and the active diffuser can be after the passive diffuser, but still before the lens.

Note that anywhere a lens is described, the system may use one or more lenses or mirrors to achieve the desired effect. Lenses may be spherical, parabolic, aspheres, achromats, or other types.

Likewise, on the receive end there is an acceptance angle for the receiver. This is approximately set by the size of the detector divided by the focal length of the receive lens. The receive acceptance angle may take on any value from a microradian up to 2 pi steradians. In some implementation the acceptance angle may be larger than 1.7 mrad. In some implementations, it is within a few milliradians of the source divergence angle. In other cases, the receive acceptance angle is larger than the laser divergence angle; is may help with aligning the two endpoints of the link with each other. In some cases, the receive acceptance angle may be smaller than the transmitter divergence angle; this may increase the overall link distance or may insure that if a receiver is aligned with the transmitter on the other end of the link, then the transmitter on this end is also aligned with the receiver on the other end of the link.

As described above, the laser source may be any of a number of laser sources, including, but not limited to, VCSELs, VCSEL arrays, strip lasers, fiber lasers, or fiber coupled lasers with an additional amplification stage.FIG.6shows an example in which the laser source includes a fiber coupled laser600that is amplified by an EDFA602and then connected to the diverged beam optical transmitter by another fiber604.

FIG.7shows a diverged beam optical communications system700, according to some example implementations. The system includes a laser source702configured to emit a light beam, and one or more lenses704configured to collimate the light beam to an overall divergence of the light beam. The system includes a diffuser706configured to increase an intrinsic divergence of the light beam and to fill some portion of the one or more lenses such that the light beam is eye safe after the one or more lenses.

As also shown, the system700includes a receive lens708configured to focus the light beam onto a detector710. In some examples, the detector has an etendue sufficient to receive light from the receive lens with an acceptance angle larger than 1.7 mrad. In other examples, the detector has a etendue sufficient to receive light from the receive lens with an acceptance angle larger than 5 mrad. And in yet other examples, the detector has a etendue sufficient to receive the light beam from the receive lens with an acceptance angle larger than 17 mrad.

Many modifications and other implementations of the disclosure set forth herein will come to mind to one skilled in the art to which the disclosure pertains having the benefit of the teachings presented in the foregoing description and the associated figures. Therefore, it is to be understood that the disclosure is not to be limited to the specific implementations disclosed and that modifications and other implementations are intended to be included within the scope of the appended claims. Moreover, although the foregoing description and the associated figures describe example implementations in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative implementations without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.