The field of the present invention is in optical systems for illuminating areas and detecting objects in the areas. In particular the present invention may be applied to the field of LIDAR.

Optical sources can be used for measuring one or more external objects. This may simply be to detect the presence of the object or another value or property associated with the object such as its distance. Time-of-flight LIDAR is now widely used for ranging measurements in a variety of application including autonomous vehicles, handheld mobile devices and augmented reality applications. These typically use laser sources to generate a pulsed (<1 ns) light beam, that is subsequently detected and the time of flight distance information extracted. To be used in daylight, these lasers operate in atmospheric wavelength windows (near IR) where the intensity of background solar light radiation is low. These wavelength windows are relatively narrow in spectrum, so need stable laser wavelengths. Using multiple laser sources within these windows demands that the individual wavelengths be tightly spaced (~1 nm or less). Designing and manufacturing lasers with precise wavelengths is difficult and can be expensive. The nature of laser devices entails that temperature variations may undesirably alter the output wavelengths causing difficulties in the operation of LIDAR devices.

Summary

In a first aspect of the present invention there is provided an optical system for illuminating one or more external objects and detecting reflected illuminated light from the objects; the optical system comprising: a plurality of optical sources for outputting light to illuminate the external objects; the plurality of optical sources comprising: a first optical source for outputting light at a first peak wavelength towards a first area of the objects; a second optical source for outputting light at a second peak wavelength towards a second area of the objects, the first peak wavelength being different to the second peak wavelength; a plurality of optical receivers comprising: a first optical receiver; a second optical receiver; a first optical filter for: receiving reflected light from the first area; allowing said received reflected light of the first peak wavelength to be input to the first optical receiver; rejecting light of the second peak wavelength; a second optical filter for: receiving reflected light from the second area; allowing light of the second peak wavelength to be input to the second optical receiver; rejecting light of the first peak wavelength.

The first aspect may be adapted according to any feature or configuration described herein, including, but not limited to any one or more of the following.

The first area may be different to the second area.

The first and second optical filters may be athermalised.

The first area may overlap with the second area. The first and second optical sources may each respectively comprise: a laser comprising an optical gain section and an optical phase control section; an optical transmission filter configured to: receive light output from the laser; transmit light at the first wavelength; a partial reflector configured to: receive filtered light from the optical transmission filter; input filtered light back into the laser; output light to the first area.

The optical transmission filter of any of the said optical sources may be athermalised.

The optical system may be configured such that: the optical passband of the first optical filter is matched to the optical passband of the optical transmission filter of the first optical source; the optical passband of the second optical filter is matched to the optical passband of the optical transmission filter of the second optical source.

The optical system may comprise: a first module comprising the first optical source, the first optical filter, the first optical receiver; and, a second module adjacent to the first module; the second module comprising the second optical source, the second optical filter, the second optical receiver.

The optical system may be configured such that each of the plurality of optical sources: is located about a common plane; and, points in a different outward radial direction about the common plane. The optical system may be configured such that the modules are mounted on a common mount.

The optical system may further comprise an actuator for moving the mount.

The optical system may be configured such that the modules are arranged in a column.

The optical system may comprise an actuator for rotating the column about an axis parallel to the length of the column.

The optical system may be configured such that the modules are mounted on a common mount; the actuator configured to rotate the common mount about an axis running through the common mount.

In a second aspect of the present invention there is provided a LIDAR system comprising the optical system as described in the first aspect and optionally any one or more of the options described for the first aspect.

The LIDAR system may comprise an electronic controller for controlling the plurality of optical sources.

A vehicle may comprise: the LIDAR system as described in the second aspect; or the optical system as described in the first aspect.

In a third aspect of the present invention there is provided a LIDAR system for illuminating one or more external objects; the LIDAR system comprising: a plurality of optical sources for outputting light to illuminate the external objects; the plurality of optical sources comprising: a first optical source for outputting light at a first peak wavelength towards a first area of the objects; a second optical source for outputting light at a second peak wavelength towards a second area of the objects, the first peak wavelength being different to the second peak wavelength. The third aspect may be adapted according to any feature or configuration described herein, including, but not limited to any one or more of the optional features/configurations described for the first aspect and/or any one or more of the following.

The LIDAR system may be configured such that at least one of the first and second optical sources comprises: a laser comprising an optical gain section and an optical phase control section; an optical transmission filter configured to: receive light output from the laser; transmit light at the first wavelength; a partial reflector configured to: receive filtered light from the optical transmission filter; input filtered light back into the laser; output light to the first area.

The LIDAR system may be configured such that the optical transmission filter of any of the said optical sources is athermalised.

In a fourth aspect of the present invention there is provided a LIDAR system for detecting reflected light from one or more objects in a first area and second area; the LIDAR system comprising: a plurality of optical receivers comprising: a first optical receiver; a second optical receiver; a first optical filter for: receiving reflected light from the first area; allowing said received reflected light of a first peak wavelength to be input to the first optical receiver; rejecting light of a second peak wavelength; a second optical filter for: receiving reflected light from the second area; allowing light of the second peak wavelength to be input to the second optical receiver; rejecting light of the first peak wavelength. The fourth aspect may be adapted according to any feature or configuration described herein, including, but not limited to any one or more of the optional features/configurations described for the first aspect and/or any one or more of the following.

The LIDAR system may be configured such that the first and second optical filters are athermalised. The LIDAR system may be configured such that the first area and the second area are monitored areas that overlap.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figures 1a and 1 b show examples of an optical system;

Figure 2 show an example of an apparatus having a ring of optical sources and optical receivers; Figure 3 shows an example of an optical apparatus having a linear array of optical sources;

Figure 4 shows an example of an optical source suitable for use with the optical apparatus.

Detailed description

There is presented an optical system for illuminating one or more external objects and detecting reflected illuminated light from the objects. The optical system comprises a plurality of optical sources for outputting light to illuminate the external objects. The plurality of optical sources comprising: a first optical source for outputting light at a first peak wavelength towards a first area of the objects and a second optical source for outputting light at a second peak wavelength towards a second area of the objects. The first peak wavelength being different to the second peak wavelength. The optical system further comprises a plurality of optical receivers comprising a first optical receiver and a second optical receiver. The optical system further comprises a first optical filter for: receiving reflected light from the first area; allowing said received reflected light of the first peak wavelength to be input to the first optical receiver; rejecting light of the second peak wavelength. The optical system further comprises a second optical filter for: receiving reflected light from the second area; allowing light of the second peak wavelength to be input to the second optical receiver; rejecting light of the first peak wavelength.

The following description gives further optional details and provides examples of the above optical apparatus and advantages of the optical apparatus. Throughout the description the term ‘apparatus’ may be used instead of ‘system’. It is understood that the system or apparatus may be a single device or multiple devices that are spatially separated. Throughout the description, examples of LIDAR systems will be used to discuss the optical apparatus, however the principle can be used by any such optical system for illuminating one or more external objects using the features described above. Other examples include, for example, other range finder optical systems and 3D object mapping systems.

The term ‘optical receiver’ may also be referred to herein as ‘optical detector’, ‘detector’ or ‘receiver’. The term ‘optical source’ may also be referred to herein as ‘source’. The term ‘optical filter’ may also be referred to herein as ‘filter’.

It is envisaged that two or more optical sources may form part of the optical system. It is envisaged that two or more optical receivers may form part of the optical system. It is envisaged that two or more optical filters may form part of the optical system. In the optical system there may be a plurality of optical sources wherein each separate optical source is associated with a corresponding separate detector and filter. The optical source however is not necessarily limited to have a one to one correspondence between the number of detectors to numbers of sources and filter, for example, a single detector/filter could be set-up for receiving light of the same peak wavelength from a plurality of sources.

Figures 1a and 1 b show schematic examples of the optical system. These examples may be modified according to any configuration, adaptation or feature described herein. Furthermore, any of the features and configurations described for the examples in figures 1a and 1 b may be applied to any other example of optical apparatus described herein.

Figure 1a shows an example of the optical apparatus 2. In this example the apparatus 2 is a single device comprising a first optical source 4a and a second optical source 4b. These sources have different peak output wavelengths. The optical sources 4a/4b in this example are lasers. Each laser has illumination optics (not shown) which output the light from the optical apparatus in a dispersed manner such that a broad area of the local environment is illuminated. The illumination optics may comprise one or more lens systems and/or other optical components that, together, receive input light from one or more of the said optical sources 4a/4b and output light with a particular desired spatial distribution. Each source 4a/4b may have its own illumination optics. In some examples, optical sources 4a/4b may share illumination optics. The optical sources output light 10a, 10b in a spatial distribution towards a local environment 14 which may comprise one or more objects.

Light then reflects or scatters off the objects 14 and gets detected by detectors 6a and 6b by passing through optical filters 8a and 8b respectively. The filter 8a is designed to let through light, originating from source 4a, into the detector 6a but reject light from source 4b. The filter 8b is designed to let through light, originating from source 4b, into the detector 6b, but reject light from source 4a.

The detector/filter combination may accept (or monitor for) light within a particular spatial area in the environment 14, which is shown by monitoring areas 12a and 12b respectively. In the example of Figure 1a, the areas of the local environment 14 that are illuminated by the sources 4a/4b overlap with, but are not the same as, the area monitored by the associated detector/filters. However, it is envisaged that these areas may be the same or that a monitored area at least comprises all of the illumination area for the corresponding light source 4a/4b. Further discussion on illuminated and monitored areas overlapping are described elsewhere herein.

Figure 1 b shows another example of an optical apparatus 2 wherein like numerals represent like components with figure 1a. In this example, the components of the apparatus 2 are located remotely from each other in an overall system rather than being part of the same device. The apparatus 2 in this example is monitoring two sides of an object in an environment 14. The first source 4a is oriented to illuminate an area of the object 14 that overlaps with the monitoring area of the first detector/filter 6a/8a. The second optical source 4b, filter 6b and detector 8b measure light reflected off the object 14 from the other side in a similar manner.

The above two examples are intended to show some alternative variations of the optical apparatus 2, however these may be modified according to any suitable configuration or feature described herein.

The optical apparatus 2 therefore illuminates different areas or the same area with different wavelengths and uses detector-filter combinations, or assemblies, to detect the reflected light wherein each assembly is tuned to a particular wavelength.

This allows multiple areas of the local environment to be illuminated and analysed rather than a single area but without risking cross talk arising from unwanted scattered light originating from the first optical source entering the second detector. The different optical rejection characteristics (frequency responses) of the filters helps to minimize this cross talk. Where the same area is monitored, it allows for a dual measurement to be made of that area simultaneously, for redundancy purposes. The same area may be monitored by different source/filter/detector combinations but from different perspectives or angles. This may provide a more comprehensive measurement of objects in the said area, or even provide multi-dimensional information about the objects in the area. For example, two sources may be able to determine 3D data of an object in the area.

The apparatus may form at least part of a LIDAR system. This is now briefly described with reference to figure 1a., The optical sources in this example are pulsed and used to measure the distance to an object 14. The system bounces laser pulses off objects and measures how long the light from the laser source 4a takes to reflect from the object 14 to the corresponding detector 8a. Using the time of flight, the distance to the object is calculated by a processor or electronic controller in electronic communication with the LIDAR system, in real time. A plurality (preferably millions) of data points are used to generate a 3D point cloud, which is a complex “map” of the surroundings.

The LIDAR system, in this and any other examples described herein, may use wavelengths around any of, but not limited to: 890nm - 970nm, 900-950nm, 930-950nm; 1500 nm-1600nm. The full width half maximum (FWHM) of the filters 8a, 8b may be between any of (but not limited to) 1 nm-10nm, preferably 2nm-7nm. These wavelength ranges of operation may apply to the optical sources, the optical filters and the detectors.

Thus, for example, the optical source 4a may emit light with a peak wavelength at 1550nm. This radiation is detected by the detector 8a with filter 6a being a passband optical filter allowing through light at 1550nm. Correspondingly, the optical source 4b may emit light with a peak wavelength at 1555nm. This radiation is detected by the detector 8b with filter 6b being a passband optical filter allowing through light at 1555nm.

Illumination areas

The first area may be different to the second area. The different areas of the surrounding or local environment may therefore be monitored simultaneously with minimization of cross talk. The areas illuminated by at least two sources may overlap. Additionally, or alternatively the areas monitored by at least two of the detectors may overlap. At least one, or a plurality or each of the optical sources may illuminate an area that overlaps with a plurality of other areas illuminated by other optical sources of the apparatus.

For example, for optical sources disposed in a common plane but each having an optical output facing a different direction, the illuminated area for each source may overlap with two further areas illuminated by different optical sources of the apparatus, with one further area overlapping one side within the plane, the other further area overlapping the other side in the common plane. See for example figure 2. At least one, or a plurality or each of the optical detectors may illuminate an area that overlaps with a plurality of other areas monitored by other optical detectors of the apparatus. Therefore, similarly to the illuminated areas described above, for optical detectors disposed in a common plane but each having an optical input facing a different direction, the monitored area for each detector may overlap with two further areas monitored by different optical detectors of the apparatus, with one further area overlapping one side within the plane, the other further area overlapping the other side in the plane.

The optical system may be configured such that the first area overlaps with the second area.

When a LIDAR (or other optical) system described herein uses filters tuned to different wavelengths, as discussed above, the different detectors are able to filter out unwanted radiation from other sources of the apparatus as well as other optical noise from the environment. This allows the optical system to at least partially overlap the monitored areas and prevent blind spots.

Filters

The optical system may be configured such that the first and second optical filters are athermalised.

A problem with some existing sources, particularly LIDAR sources, is wavelength shift of the optical sources due to temperature variations in the local environment. Optical sources such as lasers are sensitive to temperature. When the temperature increases or decreases, different components in the source have different operating characteristics such as refractive index changes, which in turn affect the peak output wavelength. Optical filters that are not athermalised also suffer from the same problems in that temperature may affect the physical dimensions and refractive index of the material layers in the filter, resulting in a change in the frequency response of the filter. For example, if local heating occurs near the second detector and filter but not near the first detector and filter, the second filter may undesirably match its frequency response to that of the first filter, meaning that undesired light from the first source may be scattered into the second detector giving rise to crosstalk

Having the filters athermalised means that the optical system may operate in environments where temperature changes occur.

Sources

The optical sources used for the optical apparatus may be any optical sources but are preferably laser optical sources. Disclosed underneath is an example of an optical source that may be used for any of the optical sources described herein. An example of this optical source is shown in figure 4. This optical source is briefly described immediately underneath, with more details discussed elsewhere herein. At least one of the first and second optical sources may comprise:

I) a laser comprising an optical gain section and an optical phase control section;

II) an optical transmission filter configured to: a. receive light output from the laser; b. transmit light at the first wavelength;

III) a partial reflector configured to: a. receive filtered light from the optical transmission filter; b. input filtered light back into the laser; c. output light to the first area.

All of the optical sources of the optical system may have such a source. At least one source may be tuned to output a particular peak wavelength that is different to at least of the other sources in the system. Preferably each source is tuned to output a different peak wavelength to any of the other sources in the system.

The optical transmission filter of the optical source may be matched to the optical filter associated with the detector that monitors the area that the source illuminates. In other words, the two filters have at least one common passband peak wavelength or have a substantially similar passband.

Each optical source may be assembled onto a separate mount or package or the sources may be assembled onto a common mount or package. The sources may be cooled using one or more temperature controlling elements. The optical system may comprise an optical source assembly having all of the optical sources. The sources may be arranged with respect to each other in different ways. For example, the optical sources may be physically positioned and/or oriented in any particular way. Some non-limiting examples are given underneath of different positions and orientations that may be used for assembling the optical source assembly.

The sources may be located sequentially in a linear array. In this array the optical outputs may be aligned so that their optical outputs point away from the optical source assembly along a common plane. This example is akin to a set of traffic lights as depicted in figure 3.

In an alternative arrangement, each source in the linear array points outwardly in a different radial direction. This example is akin to a set of traffic light with each light facing a different radial direction.

In an alternative arrangement, each source is located about a common plane with each source pointing in a different radial direction about the common plane. This example is akin to a wheel with each source pointing outwardly from the wheel in a different direction as depicted in figure 2.

The optical system may be configured such that the optical transmission filter of any of the said optical sources is athermalised. Similarly, to the optical filter that is associated with the detector, the optical filter of the source may also be athermalised to ensure that the source consistently outputs the same peak wavelength within a degree of tolerance.

Any of the optical filters associated with the detector or the optical transmission filter of the source may have any of the following features. The filter may be comprised of one or more materials that are relatively insensitive to temperature variation. The optical filter may therefore comprise an athermalised optical thin film filter. The optical filter may comprise a substrate material having a wavelength shift with temperature below 2pm/K. This insensitivity may be temperature insensitivity with respect to refractive index and/or temperature sensitivity with respect to volume.

The filter may comprise a glass substrate that is coated with one or more successive layers of material to form a thin film interference filter. The glass substrate may compensate for the temperature dependence of the filter to provide an overall filter centre wavelength shift with temperature below 2pm/K.

A typical operating temperature range of the apparatus may be between -40 degrees C to 85 degrees. C. The filter may be designed to be substantially athermal compared to other components (such as the semiconductor waveguides) within this range.

The optical filter passband may be any suitable passband at normal incidence and example values of operation are discussed elsewhere herein.

The detector may be formed of any suitable material system that can detect the wavelength ranges output by the optical sources. This includes silicon based detectors for visible and near IR light, InP based detectors for near IR light and InAs, InSb for far IR light.

The optical system may be used in any suitable environment or mounted upon or used in any suitable apparatus, device or machine. For example, the optical apparatus may be mounted on or form part of a vehicle such as but not limited to any of: a land-based vehicle such as a car, lorry, van, train, a wheeled vehicle, a sea-based vehicle such as a boat or hovercraft, an air vehicle such as a plane, glider, drone or projectile.

The optical apparatus may be configured such that the optical output of at least one optical source and the optical input of at least one optical detector respectively illuminate and monitor a common area. The illuminated area may at least partially overlap the monitored area.

The elements of the optical system, for example the sources and the detectors, may be controlled by an electronic controller or electronic processing means. Examples of electronic processing means are described as follows. The processing means may comprise one or more processing devices. Any of the processing devices described herein may comprise one or more electronic devices. An electronic device can be, e.g., a computer, e.g., desktop computer, laptop computer, notebook computer, minicomputer, mainframe, multiprocessor system, network computer, e-reader, netbook computer, or tablet, on-board vehicle computer or controller. The electronic device can be a smartphone or other mobile electronic device. The electronic controller controls the said elements by transmitting one or more electronic signals. The electronic controller may also receive electronic signals from the element, for example the detector. The electronic controller may use the received electronic signals to determine the presence or proximity of an object in the monitored area of the detector. The controller may receive electronic signals from a plurality of the detectors, preferably each of the detectors, and determine the presence or proximity of one or more objects in a plurality of monitored areas.

Examples of the optical system

Any of the examples described below may output light in pulsed or continuous wave (CW) operation and use any suitable peak wavelengths discussed elsewhere herein. Furthermore, any of the examples described below may be adapted according to any configuration disclosed herein or be modified to remove features or include other features described in other examples disclosed herein. These examples show that different wavelengths can be used for vertical or horizontal LIDAR discrimination in the optical illumination. Each of these examples exemplifies a multi-wavelength LIDAR system having several laser-based sources of different wavelengths (and matching filters on the receivers) to allow wavelength dependent LIDAR ranging.

Figure 2 shows an example of a LIDAR optical system comprising a plurality of sources arranged in a loop, for example a ring. Each black square in this figure represent a module comprising a laser, a detector and an optical filter for receiving light from the illuminated environment and allowing filtered light to pass to the optical detector. The optical filter in each module is a thin film bandpass optical filter having a different peak bandpass wavelength to the other optical filters of the other modules.

The laser in each module, in this example, outputs light at a different wavelength to that of the other modules. The peak laser wavelength of each output module is matched (being identical or substantially similar) to the peak wavelength of the bandpass filter.

In the figure, the wavelengths emitted and detected by each module are respectively labelled l1 - l8. In this example it is assumed that, for each module, the area illuminated by the source is substantially similar to area monitored by the filter/detector, however in principle the areas may differ.

The arrangement of the sources is as follows. Each source is located about a common plane with each source pointing in a different outward radial direction about the common plane. The optical filter inputs of the modules are also pointing in a different outward radial direction about the common plane. Each of the modules may operate simultaneously to get a full 360-degree scan of the environment at least in the plane of the ring. At an outward radial position, within the local environment, from the optical system, the illumination areas of a module overlap, about the plane of the ring, with the adjacent modules in the ring. This entails that light emitted by one a module may be reflected or scattered into the filter of an adjacent module. The filter of that adjacent module has a frequency response that rejects this light wavelength. At the point of this overlap, the optical system has a full 360-degree scanning capability with no blind spots within the plane.

The whole system in Figure 2 may be static or it may move to scan other planes. For example, the optical system may move in a direction perpendicular to the plane of the ring. This may be accomplished by have a pole that the ring is mounted on wherein an actuator drives the ring along the length of the pole.

Figure 3 shows another example of a LIDAR optical system wherein the modules (similar to those of figure 2) are disposed in a column and are stacked with respect to each other. This may also be termed the array or linear array. Each module in the stack has illumination areas that point outwardly in the same direction from the stack along the line of the stack. The illumination area of each module has a central axis propagating outwardly from the module. These axes are labelled l1 - l7 in this figure corresponding to the seven modules in this example. The axes of the modules are parallel to each other; however, it is understood that slight deviations from parallel may occur. The whole stack is rotatable 16 about an axis running parallel to the length of the stack (parallel to the stacking direction). The rotation of the stack may be driven by an actuator such as a motor. This rotation allows scanning of the local environment by the stack 360 degrees around the stack. Rotary movement may be continuous or proceeded stepwise stopping at particular rotation position wherein measurements are taken. The scanning therefore covers a cylindrical external area.

The respective filter / detector and optical source for each module may be disposed in respective different position along the line of the stack or laterally alongside the source perpendicular to the kind of the stack. For example, the source for the module may reside above its corresponding filter/detector in the stack. As such the respective detector / filter assemblies may be interleaved with the sources along the line of the stack.

General considerations for figures 2 and 3.

The modules may be mounted on a common housing, mount or frame or otherwise rigidly held in position with respect to each other. The source and detector for any module maybe formed on a common platform or be mounted in a common housing.

Detector and filter may form part of the same device as the optical source and may be packaged in the same package, preferably a hermetically sealed package. Any package or device may include electronic circuitry to support its optical elements, for example electronic contacts that facilitate the delivery of electrical current to the components of the device; electronic drive circuitry, electronic controllers of processors.

Other arrangements of a plurality of modules may be envisaged including an arrangement where the optical apparatus is rotated about a plurality. The modules may only have the sources or the detector/filters; in these examples the system is made up of separate devices.

As such there is further presented, a LIDAR system for illuminating one or more external objects; the LIDAR system comprising:

I) a plurality of optical sources for outputting light to illuminate the external objects; the plurality of optical sources comprising: a) a first optical source for outputting light at a first peak wavelength towards a first area of the objects; b) a second optical source for outputting light at a second peak wavelength towards a second area of the objects, the first peak wavelength being different to the second peak wavelength.

The LIDAR optical source may be configured such that the first area overlaps the second area.

The LIDAR optical source may be configured such that first and second optical sources emit light simultaneously.

At least one, preferably both, of the first and second optical sources may comprise:

I) a laser comprising an optical gain section and an optical phase control section;

II) an optical transmission filter configured to: a. receive light output from the laser; b. transmit light at the first wavelength;

III) a partial reflector configured to: a. receive filtered light from the optical transmission filter; b. input filtered light back into the laser; c. output light to the first area.

The optical system may be configured such that the optical transmission filter of any of the said optical sources is athermalised.

As such there is further presented a LIDAR system for detecting reflected light from one or more objects in a first area and second area; the LIDAR system comprising:

I) a plurality of optical receivers comprising: c) a first optical receiver; d) a second optical receiver; II) a first optical filter for: receiving reflected light from the first area; allowing said received reflected light of a first peak wavelength to be input to the first optical receiver; rejecting light of a second peak wavelength;

III) a second optical filter for: receiving reflected light from the second area; allowing light of the second peak wavelength to be input to the second optical receiver; rejecting light of the first peak wavelength.

The LIDAR system may be configured such that the optical filters are athermalised.

The LIDAR system may be configured such that the first area and the second area are monitored areas that overlap.

Any of the above LIDAR system for detecting reflected light’ and the LIDAR system for illuminating one or more external objects may be adapted according to any configuration or remove or include any feature of other examples of optical systems described herein. The LIDAR system for detecting reflected light’ and the LIDAR system for illuminating one or more external objects’ may be combined to form an overall LIDAR system.

Using both source and detection systems allows for narrow optical filtering to be used with the LIDAR detector to discriminate against background light and increase the detection sensitivity. Increased sensitivity means more LIDAR range, or less optical source power required.

Example of an optical source for use in the optical system

Any of the features and configuration described in the following examples may be used with the optical systems and LIDAR system described elsewhere herein.

The optical sources 4a, 4b described herein may comprise an optical source 100 comprising a laser 120 and an optical filter 160 within a source cavity although other variations of this source may be used. The optical filter may be athermalised as described above.

It is advantageous, particularly for LIDAR systems, if the laser source has a well-defined wavelength and is stable over environmental changes (e.g. temperature). This allows optical filtering to be used at the receiver to discriminate against the background solar radiation and improve detection sensitivity. Today’s widely used LIDAR laser sources e.g. VCSELs are difficult to make at high yield at an exact wavelength. Other sources, e.g. Fabry Perot lasers, are broadband (>1 nm) and the wavelength changes significantly with temperature (~20nm over a 50°C temperature range). The same type of source described below may be used for a plurality of the sources for the optical system whereby each source has its internal filter adjusted to tune the source to the desired wavelength, for instance to a wavelength that matches that of the detector/filter assembly associated with that source.

The laser 120 in the source 100 may include an optical gain section 200 and an optical phase control section 220, however the optical source 100 may utilise a laser 120 without a multi-section design.

The optical source may further comprise an optical transmission filter 160 configured to receive and filter light output from an output facet 380 of the laser 120, a partial reflector 180 configured to receive filtered light from the filter 160 and to input filtered light back into the laser 120, for example by directing light back through the filter 160.

The optical source 100 may also comprise at least one base member 320 and a temperature control element 300. The temperature control element 300 may be in thermal connection with the at least one base member 320. The temperature control element 300 may be configured to receive one or more control signals for controlling the temperature of the at least one base member 320. At least the transmission filter and the partial reflector are mounted on a first of the at least one base member.

The temperature control element may be configured to control the temperature of the at least one base member. The mounting of the elements onto the at least one base member may be accomplished in any way such that a rigid attachment exists between the element and the base member.

In one example, to attach the temperature control element (TEC) 300 to the base member 320, a planar face 340 of the TEC 300 is placed in abutment with a planar face 360 of the base member 320, and the TEC 300 is mounted to the base member 320 via solder (not shown), or another appropriate form of attachment means, for example mechanical fixings such as screws, bolts or an adhesive.

The laser may be a Fabry Perot semiconductor laser operating at a central wavelength of 1550nm. However, the laser may take other forms. For example, the laser may alternatively be any semiconductor laser. The laser may operate in different wavelength regions in dependence on the intended application of the optical source as discussed elsewhere herein.

The partial reflector 180 may have a reflectivity in the range of 5 to 95%, and may be formed of thin film coated transparent substrate such as glass. It should be noted that the chosen material of the partial reflector may depend on the required reflectivity of the partial reflector and the laser wavelength. In an embodiment, the partial reflector is positioned at a distance of 3mm from an output facet of the laser. It will be understood that in other embodiments of the optical source, the partial reflector may be positioned closer to, or further from, the output facet of the laser. The laser and the external cavity reflector may form a further optical cavity for the optical source. Referring now to Figure 4, which illustrates an example of the optical source 100 that can be used as the optical sources 4a, 4b in the systems described elsewhere herein. There is shown a laser 120, lens 140, optical filter 160 and external cavity partial reflector 180 that are all mounted on a first base member 320. The laser 120 and the external cavity reflector 180 may form a further optical cavity for the optical source 100. It is also envisaged that other forms of assembly may be used including mounting the elements onto different sub-mounts and mounting those submounts into a common package.

An optional temperature control element 300, which in this case takes the form of thermo-electrical cooler (TEC) comprising a Peltier element controlled by a TEC controller, is coupled to the first base member 320 and is configured to control the temperature of the first base member 320. The Peltier element may be attached to, and in thermal contact with, the base member 320 whilst the TEC controller may be physically remote from the base member 320. The controller may be connected to the Peltier via wires or even a wireless connection wherein the Peltier element is provided with on board electronic control apparatus to receive electronic signals from the TEC controller and correspondingly provide a thermal output to the base member 320.

In other words, the packaging of the optical filter 160 and external cavity partial reflector 18 is thermally connected to the same thermal mount of the TEC cooled laser chip. In this way, variation in optical path length between components of the optical source 10, which is affected by temperature variation of the first base member 320, may be reduced through use of a single TEC 300 which may also be operable to control the laser temperature. It should be noted that whilst a TEC 300 forms the temperature control element 300 in this embodiment of the optical source, this is not essential. The temperature control element 300 could be formed of another suitable element, for example a heater such as a resistive heating element.

Note that whilst the laser 120, filter 160, and partial reflector 180 are mounted on the same base member in this embodiment, this should not be considered limiting. It is possible for more than one base member to be used in the optical source arrangement.

The first base member 320 comprises a glass substrate, although other substrates may be used. The glass substrate having a thermal expansion coefficient of close to zero. As an example, the substrate may be formed of the extremely low expansion glass ceramic such as lithium-aluminosilicate glass- ceramic material such as but not limited to Zerodur (RTM), which may have a coefficient of linear thermal expansion (CTE) of 0 ± 0.007 x 10-6/°K in the temperature range 0°C to 50°C. It is possible for other materials to be used for the first base member 32. For example, the first base member 32 may comprise a substrate having a different thermal expansion coefficient, e.g. Inovco (Super Invar (RTM)) which is a material best known in the area of mechanical engineering. Typically, Inovco is known to have a CTE of 0.55 x 10-6/°C in the temperature range 20°C to 100°C. It is preferable that the thermal expansion coefficient of the chosen substrate does not exceed 1 x 10- 6K, as this provides a degree of passive thermal compensation. That is to say, use of a material having a lower thermal expansion coefficient for the first base member 320 (or at least a part of the first base member 32) results in less expansion of said material for an identical temperature variation, and therefore less variation in optical path length of the optical source 100. Thus, the passive thermal compensation provided by the low thermal expansion coefficient material of the first base member 320 may work in conjunction with the active thermal control element 300 to control the variation in optical path length between components of the optical source 100.

In the example shown in Figure 4 the optical filter 160 and external cavity partial reflector 180 are copackaged on the same base member 320, i.e. the same thermal substrate, as the laser chip within a single hermetic package, so that the overall optical path length is controlled by a single TEC 30 that is in the hermetic package to control the laser chip temperature.

The components of the optical source 100 will now be described in further detail. As already noted, the laser 120 in the above-described embodiments comprise a Fabry-Perot (FP) semiconductor laser that may include an optical gain section 200 and an electrically isolated phase section 220. The sections are disposed between first and second optical reflectors 240, 260 that form a laser cavity. Another optical source cavity may exist between reflectors 240 and partial reflector 180 wherein light reflected from the partial reflector 18 feeds back into the laser 120. Use of a Fabry-Perot laser in the optical source 100 provides a cost-effective option. The electrically isolated phase section 220 enables the absolute frequency of the laser longitudinal modes to be changed by means of current injection or applied voltage to the phase section 220. The second optical reflector 260 is formed of a partial reflector having a reflectivity in the range of 5 to 95%, such that the second optical reflector 260 acts as an output coupler of the laser 12.

In operation, optical output from the laser 120 propagates through lens 140, where it is collimated before passing through the filter element 160 and to the external cavity partial optical reflector 180. In this way, the optical output from the laser 12 is filtered by filter element 160 before reaching the partial reflector 180. The partial reflector 180 reflects a portion of the incident filtered beam back towards the laser 120, through the filter 160, to form a filtered reflected beam. The reflected beam is reflected back along the optical axis of the optical source such that the reflected beam is coupled back into the semiconductor laser. It should be noted that the term ‘optical axis’ is well-known in the art and, as such, would be well understood by the skilled person. The optical axis of the optical source is defined as the axis that passes through the centre of each optical element of the source 100, and is aligned to the axis along which optical output from the laser 120 propagates, i.e. the optical axis of the laser.

The filter element 160 is a thin-film coated etalon bandpass design, whose centre frequency of transmission changes as a function of the filter angle with respect to the collimated optical beam. The etalon free-spectral range (FSR) is designed such that only a single order of the etalon filter 160 is transmissive over the required laser wavelength operation range. The absolute filter centre frequency, w1 , is determined by the angle of the filter 160 with respect to the laser beam, the filter angular tuning rate and the normal incidence wavelength of the filter 160. The filter 160 comprises a plane parallel substrate such that the collimated optical beam is mostly laterally displaced as the filter angle is changed. The filter element 160 passband may be designed to primarily pass one of the longitudinal modes of the laser 120, such that re-injection of the reflected beam results in a single particular longitudinal mode operation of the laser 120. The number of modes selected may be one or a plurality, preferably more than 5 longitudinal modes to form a Fabry-Perot laser with FWHM of approximately 1nm

The phase section 220 of the laser 120 allows a laser longitudinal mode to be fine-tuned in frequency to coincide with the filter frequency, w1 . The phase section 220 also thus allows optical phase tuning between the laser mode and the phase of the reflected signal.

The power monitor 280, which in this embodiment is in the form of a photodiode, is provided to receive optical output from the first optical reflector 240 of the laser 120. The power monitor 280 allows monitoring of the laser power with respect to the wavelength of operation of the laser 120.

The reflectivity of the partial reflector 18 is chosen to inject the optimum power back into the laser 120 to achieve stable laser operation. The partial reflector 18 design may incorporate wavelength selective coatings such that the reflection is only effective over a defined wavelength range. In this manner, the laser 12 is wavelength tuned to an absolute frequency by adjusting the filter angle and laser phase section until the required frequency is selected. The laser 120 can be directly modulated, via either the gain or phase section 200, 220, respectively, or both, in order to obtain a digitally modulated output signal from the laser source 120.

Once the optical source is set-up or assembled for use, or even during the assembly process itself, the optical source may be monitored to try and maintain a particular optical path length between at least the transmission filter and the partial reflector (hence monitoring and subsequently controlling at least part of the optical path length of the optical cavity of the source external to the laser 120).

This may be achieved by monitoring the temperature local to the optical source, for example on or within a package housing the optical source. The temperature monitoring may be measured by a temperature sensor that is separate to or incorporated within the abovementioned TEC. An example of a temperature sensor is a thermistor. The temperature sensor may be located upon the optical source or within or upon a package housing the optical source. The temperature sensor may send electrical signals corresponding to a measured temperature value or a change in temperature to a processing unit.

An optical power monitor such as the power monitor 280 described above, may also be used to monitor output power of the optical source and send electrical signals to a processing unit. A processing unit (not shown) may form part of the system comprising the optical source, 100. This may be the same or a different processor/processing unit as described for the optical system above.

The processing unit and any of the components required for the processing unit to electrically communication with the TEC; temperature sensor and optical power monitor, may form part of a package containing the optical source. The processing unit is set to receive electrical signals from at least any one or more of: the temperature sensor or optical power monitor; described above. The electrical connections, for example wires or any optional wireless transmitter/receiver apparatus; used to transport the electrical signals may also form part of an apparatus, or be separate to it. The processing unit may form part of a control loop that include the TEC computer. The TEC controller may be the processing unit or is in electronic communication with a separate processing unit. The processing unit may receive any of the electrical signals from the temperature sensor or power monitor, process the signals and subsequently send electrical control signals that cause the TEC to change the temperature of the base member. The temperature of the TEC (hence the base member it is heating/cooling), represented by the control signal, is set to compensate for temperature dependent optical path length effects in the optical source such as, but not limited to: temperature based refractive index changes in any of the abovementioned components of the optical source forming the source cavity; temperature based refractive index changes in the air between any two or more of the said components; temperature based physical expansion or contraction of components.

Changing the base member temperature causes expansion or contraction of the base member, which in turn lengthens or shortens the distance between the components mounted upon the base member, hence lengthens or shortens the optical length in that portion of the cavity of the optical source.

For example, if the temperature in the package housing the optical source increases, then this may increase the refractive index of the filter, lengthening the optical path. To compensate, the apparatus may have a control signal sent to the TEC to cool the base member, thus causing the contraction of the base member to shorten the optical path length between the mounted components; hence compensate for the change induced by the package temperature.

The variation in optical path length between the transmission filter and the partial reflector may therefore be held at a fixed value with changing environmental temperature. The variation in optical path length between an output facet of the laser and the transmission filter and/or the partial reflector may therefore be controlled to maximise the output optical power of the source.

The control signals may be to give effect to any of: increasing; decreasing or maintaining the TEC output temperature. The processor may perform an iterative correction/compensation of the optical path length by changing the temperature of the base member via the TEC, monitoring the output optical power, comparing the output optical power to a target power value, value range or power threshold; then outputting a further control signal based on the comparison. The generation of the control signal may include using any one of: a look-up table; a model; an algorithm; or any other data structure that can be used to compare a current measured temperature to a desired temperature of the base member or TEC. Data about the temperature dependence of any of: the materials and dimensions of each of the optical source components; the materials and dimensions of each of the base members, may be used to determine the required temperature change required in the base member.

The processing unit may be embodied in hardware and/or in software run on a computer system having a processor and associated memory. The computer system may form part of an apparatus further comprising the optical source.

The above control features may be applied to any of the examples of sources described herein.

The tuning of the laser mode frequency via the phase section 200 can compensate for the changes in laser mode frequency with small device temperature variations, allowing operation of the wavelength source over certain temperature ranges. However, for stable wavelength operation over wide external temperature ranges, the overall optical length variation with temperature between elements of the optical source 10, in particular between the laser 120 and the external partial reflector 180, must be reduced. This may be achieved, at least in part, by means of a temperature control element 300.