SYSTEM

TECHNICAL FIELD

The present disclosure generally relates to systems implementing coherent detection. More particularly, the present disclosure relates to systems utilizing depth sensing sensors.

BACKGROUND

Optical cohe ent. detection is a method of extracting information encoded as modulation of the phase and/or frequency of electromagnetic radiation in the wavelength band of visible or infrared light. The received light signal is compared with a brighter standard or reference light, often called a "local oscillator" (LO) beam, by analogy with a superhetrodyne recei e .

The comparison of the two light signals is typically accomplished by combining them in a photodiode detector. The two light frequencies may be similar enough that their difference or beat frequency produced by the detector is in the radio or microwave band that can be conveniently processed by electronic means .

Typical prior art block diagrams of systems implementing the above are presented in FIGs. 1 - 3.

The term "frequency-selective surface" (referred to herein as "FSS") denotes any repetitive surface designed to reflect, transmit or absorb elect omagnetic fields based on the frequency of the field. In this sense, an FSS is a type of an optical filter, in which the filtering is accomplished by virtue of the regular, periodic (usually metallic, but sometimes dielectric) pattern on the surface of the FSS. In some FSS designs, multiple unit cells are utilized to obtain the desired, optical effect, unavoidably, FSS has properties which vary with incidence angle and polarization as well.

FSS's can be modeled as large antenna arrays, where each element in the repetitive surface acts as an antenna unit cell in the antenna array. Each antenna unit cell is characterized by its mechanical properties (materials, dimensions, proximity to other unit cells, etc.) and its load impedance. The load impedance is most commonly either a short (zero ohms) or an open (infinity) . These values are useful when the FSS is designed to reflect the selected frequency. If absorption is desired, matched loads are used to absorb the received energy.

When optical sensors are arranged in tight and essentially repetitive arrays, they may be considered as an FSS where the detecting elements act as matched loads coupled to antenna elements, where a portion of the received energy is transferred to the load and is sensed, whereas the remaining energy is either absorbed as heat in the antenna elements or is reflected from the FSS. A portion of the reflected energy is shaped by the radiation pattern of the antenna array.

Following is a partial list of publications that relate to this subject:

US 20070132645 describes an integrated sub-millimeter and infrared reflectarray that includes a reflective surface, a dielectric layer disposed on the reflective surface, and a subwavelength element array and a subwavelength element array electromagnetically coupled to the reflective surface. The subwavelength element array includes (i) electrically conductive subwavelength elements on the dielectric layer, (ii) wherein the dielectric layer comprises a plurality of dielectric subwavelength elements, or (iii) the dielectric layer includes a plurality of embedded dielectric subwavelength elements.

US 5512901 discloses a compact radar system that includes a dielectric substrate having an upper and lower surface. A ground plane is formed on the upper surface of the dielectric substrate and includes a radiating slot formed therein. A radar transceiver is located below the dielectric substrate and generates transmit signals. A frequency selective surface spaced above the dielectric substrate includes a radiating aperture with a plurality of uniformly spaced holes. The frequency selective surface decreases flow of electromagnetic energy from the radiating slot in one direction towards the transceiver and increases the flow of electromagnetic radiation in an opposite direction away from the transceiver.

US 5208603 describes a frequency selective surface (FSS) for incorporation into the outer skin of an aircraft, for transmitting electromagnetic energy in a predetermined frequency band. The FSS includes three layers sandwiched together with a dielectric material. Arrays of apertures are formed in the two outer layers, which are conductive. The inner layer consists of patches of conductive material. The apertures and patches are in substantial alignment with one another. A dual-band FSS, having apertures and corresponding patches in two different sizes and spacing, can be used to transmit two separate frequency bands. US 5789750 discloses optical system architectures with improved spatial resolution in which the radiation useful for THz spectroscopy can be directionally coupled into and out of photoconductive structures such as dipole antennas. An optical system comprises a source for emitting radiation in a frequencies' range of from 100 GHz to 20 THz, a coupling lens structure for coupling radiation emitted by that source into free space, at least one collimating optical element for collimating received coupled radiation into a beam having a frequency independent diameter and no wavefront curvature, and a detector for detecting the beam collimated by the collimating optical element. In addition, the publication describes an optical system where a modified substrate lens structure is used and the collimating optical element is replaced by an optical element that focuses received coupled radiation onto a diffraction limited focal spot on or within the medium under investigation .

US 5164784 discloses a continuous wave Doppler LIDAR with an enhanced signal-to-noise ratio that greatly enhances its ability to determine relative fluid velocity. A laser source according to this disclosure produces coherent light that is split between a reference beam and a test beam by a beam splitter. Any particle in a fluid moving relative to the CW Doppler LIDAR system that passes through the target cell causes a Doppler shift in the frequency of the coherent light reflected from the particle and reverses the rotational direction of circular polarization of the reflected beam. The light reflected from the particle is combined with the reference beam, creating a difference signal incident on a photodetector .

In their article entitled "Plasmonic Holographic Imaging with V-Shaped Nanoantenna Array", published in Optics Express , Vol. 21, Issue 4, pp. 4348-4354 (2013), the authors Fei Zhou, Ye Liu, and Weiping Cai present a novel method of holographic imaging with nanoantenna array. In order to obtain the plasmonic holographic plate for a preset letter "NANO", the phase distribution of the hologram is firstly generated by the weighted Gerchberg-Saxton (GSW) algorithm, and then 16 kinds of V-shaped nanoantennas with different geometric parameters are designed to evenly cover the phase shift of 0 to 2n by finite- difference time-domain (FDTD) method. Through orienting these nanoantennas according to the phase distribution of the hologram, the plasmonic array hologram is obtained.

In order to obtain an optimal sensitivity in coherent detection systems, the signal power from both the local oscillator ("LO") and transmitter ("Tx") should be maximized. This typically may be achieved by dividing the available optical energy between the LO and the Tx, while this division is usually in equal portions .

Yet, due to the low energy conversion efficiency of most optical sensors (substantially less than 50%), the major part of the energy provided as LO is lost. This energy is either reflected from the sensor to strays in the system or is converted to heat.

The present invention seeks to solve the above drawbacks.

SUMMARY

The disclosure may be summarized by referring to the appended claims.

It is an object of the present disclosure to provide a device which has an improved performance by utilizing more efficiently available optical energy.

It is another object of the present invention to provide a method for utilizing FSS properties of an optical detector.

It is yet another object of the present disclosure to provide a method that eliminates essentially parallax between the transmit (Tx)and receive (Rx) optical paths.

It is still another object of the present disclosure to provide a device and a method for tight control of the illuminated field shape and light distribution, thereby improving light utilization.

Other objects of the present disclosure will become apparent from the following description. According to a first embodiment of the present disclosure, there is provided an optical arrangement for use in a coherent detection system, wherein the optical arrangement comprises a light beam generating source, an optical sensor (e.g. a detector) and an optical transmitter, and wherein the arrangement is characterized in that a) all, i.e. essentially all, of the optical energy generated by the light beam generating source is delivered to the optical sensor for use as a reference light signal, and b) at least part of the optical energy reflected from the optical sensor is delivered to the optical transmitter, for use in transmitting an optical (TX) signal .

It should be appreciated by those skilled in the art that although essentially all of the optical energy is delivered to the optical sensor, different techniques can be utilized to control the amount of optical energy that would be absorbed by the sensor. For example, by controlling the polarization of the incident bean relative to the polarization of the optical sensor, a measured amount of energy can be delivered to the sensor, allowing the remaining energy to be delivered to the optical transmitter. Such techniques are preferred particularly in cases where the total available power might be too high for the optical sensor.

The terms "reference light signal" and "LO signal" as used herein throughout the specification and claims is used to denote bright standard or reference light, that is used for comparing it with a weaker received light signal. The term "Local Oscillator" is used by analogy with superhetrodyne detection systems . The information carried by the received light is encoded as an amplitude, frequency and/or phase shift from the reference signal . The received signal and the reference signal may be introduced to a nonlinear signal-processing device (such as a photodiode) usually referred to as a mixing device (e.g. a multiplier or square law detector), to yield, an output signal.

The term "optical transmitter" as used herein through the specification and claims, is used to denote an arrangement comprising at least one optical element designed to collect optical energy reflected off the detector and shape it to illuminate a pre-defined field of view.

In accordance with another embodiment, the optical energy reflected from the optical sensor and received by the optical transmitter is emitted from the optical transmitter in a predefined pattern, in order to illuminate a pre-defined field of view .

According to another embodiment of the present disclosure, the optical sensor comprises a frequency selective surface (FSS) .

According to another embodiment, the FSS of the optical sensor is characterized in that its implementation ensures optimal absorption of optical energy of the reference light signal that matches incident beam power and phase profiles at the sensor surface.

in accordance with another embodiment, the FSS of the optical sensor is designed to enable reflecting optical energy therefrom towards the optical transmitter, in a pre-defined pattern, thereby enabling to optimize power delivery to the field of view. As a result of the principle of reversibility, optimization of power absorption from the received signal (RX) can be simultaneously achieved.

According to another embodiment, the light reflected from the FSS and received by the optical transmitter, is emitted therefrom to illuminate a pre-defined field of view (FOV) in a desired FOV shape. In accordance with another embodiment, the light is emitted from the optical transmitter in a non-uniform illumination intensity, in order to provide an increased optical energy to regions in the FOV where higher sensitivity is desirable, and/or less optical energy to regions in the FOV where less sensitivity may be tolerated.

According to another embodiment, the optical sensor comprises a plurality of nano rectifying antennas for detecting optical energy received thereat.

In accordance with another embodiment, the plurality of nano rectifying antennas are operative at near to far IR range of frequencies (e.g. at one or more wavelengths that are within 1 - 15 ym) .

By yet another embodiment, the optical sensor is a photodiode .

In accordance with another embodiment, the light reflected from the FSS forms a pre-defined pattern that meets at least one of the following requirements:

1. shape of a field of view that matches a shape of the optical sensor;

2. a non-uniform illumination at a target the field of view. For example, allowing more energy to be transmitted towards certain part(s) of the field of view, which is/are known to require higher sensitivity, and/or to allow transmitting less energy to certain parts that require lower sensitivity.

3. a field of view having a ^A blind spot' . For example, diverting optical energy in order to eliminate a potential loss due to a mirror shadow.

By yet another embodiment, the FSS of the optical sensor comprises an array of different FSS unit cells, where each of these unit cells has its own characteristic phase shift surface. According to still another embodiment the arrangement further comprises a beam shaping means configured to operate on the reference light signal, thereby enabling to match the light beam shape with that of the optical sensor shape. For example, if the optical sensor is an array in an essentially rectangular shape, the LO beam profile can be made rectangular, thereby ensuring that the available energy is delivered to the sensor.

In accordance with another embodiment of the disclosure, the illumination source is a gas laser.

By still another embodiment, the optical sensor comprises at least one array of sensing elements (e.g. array(s) of pixels ) .

In accordance with another aspect of the present invention there is provided a method for detecting an optical signal (e.g. a modulated optical signal) in a coherent detection system, wherein the method comprises the steps of:

generating a light beam;

directing all of the optical energy generated by a light source to an optical sensor, for use as a reference light signal thereat;

forwarding optical energy reflected from the optical sensor towards an optical transmitter;

emitting light received at the optical transmitter to illuminate a pre-defined field of view;

receiving a reflected signal from at least one target located within the pre-defined field of view; and

comparing the received signal with the reference light signal, thereby obtaining a coherent detection of the at least one target.

In accordance with a further embodiment, the optical sensor comprises a frequency selective surface (FSS) . According to another embodiment of this aspect, the light beam emitted from the optical transmitter is emitted in a predefined pattern.

By yet another embodiment, the FSS is designed to enable reflecting light therefrom towards the optical transmitter in a pre-defined pattern.

According to still another embodiment, the method further comprising a step of shaping the light beam to match its shape with that of the FSS of the optical sensor.

In accordance with another embodiment, the optical sensor comprises at least one array of sensing elements (e.g. array(s) of pixels ) .

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate several embodiments of the disclosure and, together with the description, serve to explain the principles of the embodiments disclosed herein.

FIGs . 1 - 3 demonstrate three different block diagrams of typical prior art coherent detection systems;

FIG. 4 presents a block diagram of a coherent system construed in accordance with an embodiment of the present disclosure; and FIG. 5 demonstrates another embodiment which illustrates a further aspect of the present invention's solution.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some of the specific details and values in the following detailed description refer to certain examples of the disclosure. However, this description is provided only by way of example and is not intended to limit the scope of the invention in any way. As will be appreciated by those skilled in the art, the claimed method and device may be implemented by using other methods that are known in the art per se. In addition, the described embodiments comprise different steps, not all of which are required in all embodiments of the invention. The scope of the invention can be summarized by referring to the appended claims .

As previously explained, in order to obtain optimal sensitivity in a coherent detection system, both the local oscillator (LO) and the Tx signals should be maximized. In prior art systems (as demonstrated in FIGs. 1-3) this is typically achieved by using a splitter for dividing the available optical energy generated by the light source between LO and Tx . In most cases, the optical energy is equally divided between the two.

However, due to the low energy conversion efficiency of most optical sensors («50%), the major part of the optical energy provided as LO is lost. This energy is either reflected from the sensor, straying in the system, or is converted to heat, which in turn further reduces the detection quality.

An optical arrangement 10 construed in accordance with an embodiment provided by the present disclosure, is schematically illustrated in a block diagram presented in FIG. 4. optical arrangement 10 is configured for use in an optical coherent detection system and is capable of transmitting information encoded as modulation of the phase and/or frequency (wavelength) of electromagnetic radiation in the wavelength band of visible or infrared light.

As may be seen in FIG. 4, the system exemplified herein comprises a light source 20 which generates a light beam that optionally may be modulated with chirp modulation. The term "symbol" is often used to denote the generated optical signal, characterized by its length, power, frequency and phase composition. The generated symbol is transmitted towards an FSS sensor 35 of optical sensor 30. In the embodiment illustrated in FIG. 2, the path of the transmitted light includes a mirror 40 that may be used to ensure that the light beam arrives perpendicularly to the FSS surface. The optical energy of the light beam that arrives at the FSS surface is combined with that of a received optical signal by the optical sensor mixer and the combined beam is forwarded towards ADC converter 50, optionally via an amplifier 60. The optical signal to be transmitted (the light beam generated by light source 20 which was chirp modulated thereat) , is forwarded to transceiver 70 which comprises combined Rx and Tx optics, and is transmitted therefrom.

FIG. 5 demonstrates another embodiment which provides a further view of the geometry associated with the present invention's solution. As described above, available optical energy 100 which is comprised within the optical beam generated by the light source, is conveyed to the FSS sensor 121 which, in the present example is printed on the surface of sensor die 120. This conveyance of optical energy is preferably done in a controlled manner in order to enable establishing a predetermined phase and spot size. In the present example, the desired phase and spot size are typical of a Gaussian beam waist created by lens 110 which is located along the optical path of the light beam. Similarly to the preceding example, mirror 130 may be added to ensure that the light beam is reflected from the mirror so that it arrives perpendicularly to the FSS surface.

Since according to the underlying principle of the present disclosure all of the available optical energy is first delivered to the sensor as LO, its magnitude would obviously be greater than that which can be achieved while following prior art configurations. Because the efficiency of the sensor's energy conversion is less than 50%, the major part of the optical energy will be reflected off its surface. Preferably, the surface of FSS sensor 121 is designed to reflect the optical power and consequently to illuminate the system field of view.

The energy reflected from objects located within the field of view, will be reflected back to sensor die 120. According to the present invention, the proposed system configuration may preferably be such that it ensures (by implementing a proper design) that the field of view and the illuminated field are essentially identical, thus eliminating losses that would otherwise occur due to the phenomenon known as parallax.

In a preferred embodiment, radiation pattern 140 is reflected from the surface of FSS sensor 121 in a way designed to match the aperture of lens 150. Lens 150 is designed so as to shape the transmitted beam to illuminate the field of view 160.

Preferably, FSS related techniques which are known in the art per se are used to shape the reflected beam, such techniques may be for example FSS, Plasmonic holography, Diffractive Optics, Metamaterials , and the like.

In an alternative option, the surface of FSS sensor 121 may be a Simple' specular mirror, thereby allowing beam expansion alone to determine the shape of the reflected beam. In yet another alternative, the surface of FSS sensor 121 can be made in a convex or a concave shape in order to achieve the same goal .

According to another embodiment of the present disclosure, the FSS surface is designed so that the light (radiation) reflected therefrom forms a pre-defined pattern, for example in order to meet specific implementation requirements. Few examples of such possible requirements are listed hereinbelow. 1. fields of view having different shapes, such as rectangular fields designed to match a rectangular sensor array.

2. non-uniform illumination, e.g. in order to allow more energy to be transmitted towards certain part(s) of the field of view, which is/are known to require higher sensitivity, and/or to allow transmitting less energy to certain parts that require lower sensitivity.

3. fields of view having a ^A blind spot' designed to divert optical energy away. For example, diverting the optical energy from mirror 130 of FIG. 5 in order to eliminate a potential loss that would otherwise occur due to the mirror shadow .

In order to obtain the desired radiation pattern which will be forwarded towards the target, the FSS may comprise an array of different FSS unit cells. In other words, a plurality of surface units will be used, where each of these surface units has its own characteristic phase shift surface.

In another embodiment of the present disclosure, the arrangement further comprises a beam shaping means which is operative on the LO signal before the latter is introduced to the detector die. This beam shaping operation enables matching the beam profile with the sensor array shape. For example, if the sensor array is in an essentially rectangular shape, the LO beam profile can be made rectangular to ensure that all of the available energy would indeed be delivered to the sensor. Another potential use may be for example, spreading the beam energy uniformly across the optical sensor die.

Such a shaping operation may be done by lenses (110 in FIG. 5) or by means of reflective arrays (FSS) techniques (e.g. by replacing mirror 130 with an array of reflective surfaces) .

FIG. 6 presents a flow chart demonstrating a non-limiting example of a method carried out in accordance with an embodiment of the present invention for detecting an optical signal in a coherent detection system.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.