Unpolarized light source on a chip

Abstract

An unpolarized light source (100) comprising, on a chip: a first light emitter unit (101) configured to output predominantly polarized light of a first preferred polarization; a second light emitter unit (102), coplanar with the first light emitter unit (101), configured to output predominantly polarized light of a second preferred polarization; wherein the first preferred polarization is coplanar with the second preferred polarization; a rotator (103) coupled to the second light emitter unit (102) and configured to rotate a polarization of the light output by the second light emitter unit; and a coupler (104) coupled to the first light emitter unit and to the rotator, and configured to couple light output by the first light emitter unit and light output by the rotator and to output the coupled light.

Description

[0001] Unpolarized light source on a chip

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to light sources. Particular embodiments relate to an unpolarized light source and a light detection system, and to a method of generating and combining light into unpolarized light.

[0004] BACKGROUND

[0005] The Polarization Extinction Ratio or PER measures the ratio of the optical power transmitted in the desired polarization state (e.g. horizontal or vertical) to the optical power transmitted in the orthogonal polarization state (e.g. vertical or horizontal). It is commonly expressed in decibels (dB) and is used to characterize the effectiveness of devices in maintaining a specific polarization state.

[0006] Unpolarized light sources, i.e. sources with PER close to 0 dB, are useful in various applications, such as Optical Coherence Tomography, OCT, and fiber sensing, in particular Fiber Bragg Grating, FBG, sensing. Thus far, only a selective amount of Superluminescent Diode, SLED, suppliers have been able to develop such sources, to a certain amount, after carefully developing and iterating a specialized layer stack for the optically active layer, including the build-in of strain into the layers to create equal spontaneous emission in the Transverse Electric, TE, and Transverse Magnetic, TM, modes of the waveguide. Workarounds have also been described to for instance compensate PER with the chip-fiber alignment to compensate differences in power output with a difference in coupling efficiency for the TE and TM mode exiting the chip.

[0007] SUMMARY

[0008] This workaround can be used with Polarization Maintaining (PM) fibers, for instance, but then the light would still be coherent, and after recombination it could coherently interfere and the PER could be lost.

[0009] On more generic integrated photonic or PIC platforms, the customization of the layer stack is not possible or compatible with the process, or is very costly. A typical PER of a Quantum Wells (QW) based layer stack is on the order of 10 dB. For use cases where polarization insensitivity is important, such as FBG monitoring, a solution is required to reduce PER, while still adhering to the PIC technology platforms.

[0010] One possible implementation is the use of a fiber-based depolarizer, e.g. a recirculating loop depolarizer, a Lyot depolarizer, an active polarization scrambler, etc., which scramble the light, placed after the chip. However this is not a fully integrated solution. The fiber-based depolarizer typically results in a power loss, and requires many loops of fibers, and considerable assembly time (splicing fiber loops together and predefined lengths), so it is not a scalable solution.

[0011] To enable further PIC integration, a solution is therefore required to define a PIC based solution.

[0012] Preferably, the solution should not be limited to only work for partial or small compensations.

[0013] Accordingly, there is provided in a first aspect of the present disclosure an unpolarized light source according to claim 1 , the unpolarized light source comprising, on a chip: a first light emitter unit configured to output predominantly polarized light characterized by a first preferred polarization; a second light emitter unit, coplanar with the first light emitter unit, configured to output predominantly polarized light characterized by a second preferred polarization; wherein the first preferred polarization is coplanar with the second preferred polarization; a rotator coupled to the second light emitter unit and configured to rotate a polarization of the light output by the second light emitter unit; and a coupler coupled to the first light emitter unit and to the rotator, and configured to couple light output by the first light emitter unit and light output by the rotator and to output the coupled light.

[0014] By introducing a rotation into the light output by the second light emitter unit, it can be achieved that the two light outputs will not (significantly) interfere with each other. By coupling, i.e. combining, the two light outputs, an unpolarized light output can be obtained. Thus, this allows to obtain a solution starting from a solid-state laser platform that intrinsically cannot provide such a source directly from its single emitter. In case there are two (or more) distinct light emitters (e.g. two SOAs, Semiconductor Optical Amplifiers), their light output is by default not coherent, which means that the skilled person will understand that the polarization of one of the light outputs can be rotated with respect to that of the other light output in order to combine them to obtain unpolarized light.

[0015] In case there is just one single light emitter (e.g. a single SOA), its light output is of course unitary, but after splitting this light output and then rotating one of the split light paths by exactly ±90 degrees (or a mathematical equivalent thereof), then it is possible to obtain an incoherent combination of the split light paths afterwards, as will be further explained with reference to Figure 2 below. For embodiments that feature just one single light emitter, it is to be understood that the first light emitter unit comprises the single light emitter as well as whatever elements are needed to result in a first, distinct light output, whereas the second light emitter unit comprises the same single light emitter as well as whatever elements are needed to result in a second, distinct light output that is distinct from the first light output.

[0016] Of course, several other elements than the single light emitter may also be shared by the two light emitter units for example, one or more waveguides may be shared, and of course an input port of a splitter may be shared by both light emitter units. The skilled person will therefore appreciate that the term light emitter unit is to be interpreted as a particular unit that is capable of outputting light, and that another light emitter unit may share structural elements of said particular unit.

[0017] Advantageously, this may avoid the need for a delay line.

[0018] In the context of the present disclosure, the notion of preferred polarization may be taken to refer to a preferred (and thus dominant) orientation of polarization in light, which means that said light is mostly polarized with said preferred polarization. Of course, this does not exclude that a certain amount of different polarization may exist in said light as well, but from a practical perspective, most or nearly all of said light would be polarized with the preferred polarization.

[0019] The notion of coherence (and more precisely, the lack of coherence) will now be explained more fully. An example of the light that could be emitted (i.e. output) by the emitter unit is the broadband light emitted by a SOA (Semiconductor Optical Amplifier), wherein broadband can be defined as light that contains a wide range of frequencies or wavelengths within a single optical signal. This can be achieved through the interaction of multiple frequencies within the gain medium of the SOA, resulting in the emission of photons with various energies. The resulting emitted light covers a wide spectral range, making it "broadband." Broadband light generated through stimulated emission in such a SOA tends to become temporally incoherent, after a well-defined time, namely the coherence time, which is the time over which a propagating wave may be considered coherent, meaning that its phase is, on average,

[0020] 1 A predictable. The coherence time is usually defined as r = — « —2- where A is the central wavelength of the source, Av and AA is the spectral width of the source in units of frequency and wavelength respectively, and c is the speed of light in vacuum. For FBG sensing, the coherence length is generally less than 0.05 nm bandwidth, which corresponds with a coherence time of 0.16 ns and corresponds to a coherence length (in air) of around 4.8 cm.

[0021] Temporal coherence refers to the stability of the phase relationship between different parts of a wave over time. In a temporally coherent source, like a laser, the emitted photons have a predictable and stable phase relationship, leading to well- defined interference patterns. However, in the case of broadband light generated through stimulated emission in a SOA, the emitted photons obtain varying phases and frequencies due to the broad spectrum of input photons that stimulated their emission, after the coherence time. These emitted photons thus do not maintain a consistent and predictable phase relationship over time, resulting in temporal incoherence after the coherence time. This means that the phases of the emitted photons are not well- correlated with each other, and they do not exhibit stable interference patterns.

[0022] In the context of the present disclosure, the term “incoherent” (or its synonym “non-coherent”) may be taken to refer to light sources or systems where there is no well-defined phase relationship between different parts of the wave, regardless of whether the lack of coherence is due to complete randomness or only partial correlation of phases. In other words, the term “incoherent” may thus be used to encompass both fully incoherent and low-coherent situations.

[0023] In various embodiments, light is predominantly polarized if said light has a polarization dependency characterized by a Polarization Extinction Ratio, PER, greater than 1 dB. In this context, the meaning of ‘predominantly’ is to explain that the intended embodiments fall in the realm of practical engineering and are differentiated from ‘strictly’ polarized light, which would have a polarization dependency characterized by a PER of infinity, which is only possible in hypothetical physics for perfect systems with no crosstalk across axes.

[0024] Conversely, in practice, light can be considered predominantly unpolarized if said light has a polarization dependency characterized by a PER between 0 and 1 dB, and, in hypothetical physics, ‘strictly’ unpolarized if said light would have a polarization dependency characterized by a PER of exactly 0 dB. It is noted that useful applications are possible by providing a sufficiently unpolarized light source, where the term ‘sufficiently’ may depend on the specific application, but can be expected to be any light source having a polarization dependency characterized by a PER of less than 3 dB, preferably less than 2 dB, more preferably less than 1 dB, most preferably less than 0.001 dB.

[0025] It is expected that the first and second light emitter units will in practice output light with a polarization dependency characterized by a PER of circa 10 dB, which evidently means that the light is predominantly polarized.

[0026] In various embodiments, the second light emitter unit is optically separated from the first light emitter unit.

[0027] In various embodiments, the second light emitter unit is separated from the first light emitter unit by a distance adapted to ensure that an evanescent field of the light output by the first light emitter unit does not interact with an evanescent field of the light output by the second light emitter unit.

[0028] In various embodiments, the rotator is configured to rotate the polarization of the light by such a rotation angle that the light output by the first emitter unit and the light output by the second light emitter unit are rotated by 90° or 270° with respect to each other.

[0029] In various embodiments, the rotator is configured for orthogonally rotating the polarization of light output by the second light emitter.

[0030] Moreover, there is provided in a second aspect of the present disclosure a light detection system according to claim 7, i.e. a light detection system comprising: an unpolarized light source according to any preceding claim; a light detector; and a splitter configured to pass unpolarized light output from the unpolarized light source into an optical fiber and configured to pass light reflected from the optical fiber into the light detector. Moreover, there is provided in a third aspect of the present disclosure a method according to claim 8, i.e. a method of generating and combining light into unpolarized light; the method comprising, on a chip: outputting predominantly polarized light characterized by a first preferred polarization, from a first light emitter unit; outputting predominantly polarized light characterized by a second preferred polarization, from a second light emitter unit, coplanar with the first light emitter unit; wherein the first preferred polarization is coplanar with the second preferred polarization; rotating a polarization of the light output by the second light emitter unit, using a rotator; and coupling light output by the first light emitter unit and light output by the rotator and outputting the coupled light.

[0031] In various embodiments, light is predominantly polarized if said light has a polarization dependency characterized by a Polarization Extinction Ratio, PER, greater than 1 dB.

[0032] In various embodiments, the method comprises keeping the second light emitter unit optically separate from the first light emitter unit.

[0033] In various embodiments, the rotator is configured to rotate the polarization of the light by such a rotation angle that the light output by the first emitter unit and the light output by the second light emitter unit are rotated by 90° or 270° with respect to each other.

[0034] In various embodiments, the rotator is configured for orthogonally rotating the polarization of light output by the second light emitter.

[0035] In various embodiments, the method comprises: passing unpolarized light output from the unpolarized light source into an optical fiber.

[0036] The embodiments described herein are provided for illustrative purposes and should not be construed as limiting the scope of the invention. It is to be understood that the invention encompasses other embodiments and variations that are within the scope of the appended claims. The invention is not restricted to the specific configurations, arrangements, and features described herein. The invention has wide applicability and should not be limited to the specific examples provided. The embodiments disclosed are merely exemplary, and the skilled person will appreciate that various modifications and alternative designs can be made without departing from the scope of the invention.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In the following description, a number of exemplary embodiments will be described in more detail, to help understanding, with reference to the appended drawings, in which:

[0039] Figure 1 schematically illustrates a first embodiment of the unpolarized light source according to the present disclosure;

[0040] Figure 2 schematically illustrates a second embodiment of the unpolarized light source according to the present disclosure;

[0041] Figure 3 schematically illustrates a third embodiment of the unpolarized light source according to the present disclosure;

[0042] Figure 4 schematically illustrates a fourth embodiment of the unpolarized light source according to the present disclosure; and

[0043] Figure 5 schematically illustrates a rotation of polarization.

[0044] DETAILED DESCRIPTION

[0045] Figure 1 schematically illustrates a first embodiment of the unpolarized light source 100 according to the present disclosure. The unpolarized light source 100 comprises, on a chip 130 (also known as a photonic integrated circuit or an integrated photonic platform): a first light emitter unit 101 configured to output predominantly polarized light characterized by a first preferred polarization; a second light emitter unit 102, coplanar with the first light emitter unit, configured to output predominantly polarized light characterized by a second preferred polarization; wherein the first preferred polarization is coplanar with the second preferred polarization, or, more strongly, wherein the first preferred polarization is identical to the second preferred polarization; a rotator 103 coupled to the second light emitter unit and configured to rotate a polarization of the light output by the second light emitter unit (preferably by ±90 degrees, or a mathematical equivalent thereof); and a coupler 104 coupled to the first light emitter unit and to the rotator, and configured to couple light output by the first light emitter unit and light output by the rotator and to output the coupled light.

[0046] The figure additionally shows further optional and more detailed elements, including a first termination 111 of the first light emitter unit 101 and a second termination 112 of the second light emitter unit 102, a first waveguide 121 configured for optically coupling the first light emitter unit 101 with the coupler 104, a second waveguide 122 configured for coupling the second light emitter unit 102 with the rotator

[0047] 103, and a third waveguide 123 configured for coupling the rotator 103 with the coupler

[0048] 104. Finally, the figure also shows a fourth waveguide 124 configured for outputting the coupled light stemming from the coupler 104.

[0049] Lastly, the figure additionally shows an optional monitoring platform integrated photodiode 131 , which may for example be used for separate or combined source calibration and monitoring.

[0050] The first termination 111 and / or the second termination 112 can be any of, but is / are not limited to: an antireflective output; an absorber to prevent back reflection (e.g. by using a platform-integrated photodiode in an electrically shorted connection); a mirror or reflector (for directional efficient coupling of all light outward to the right); a wavelength-selective reflector or grating (e.g. for cut-off spectral generation functionality.

[0051] The chip 130 may preferably be implemented as a solid-state active photonic integrated platform. In a particular embodiment, the chip 130 may be implemented as an InP layer stack, advantageously allowing the use of single wafer processed active / passive building blocks.

[0052] The coupler 104 may e.g. be an MMI splitter / combiner, or a similarly suitable element. The waveguides 121-123 may preferably be implemented as semiconductor materials, but it is not excluded that some or all of them may be implemented as fiber optic lines (as long as the dimensions can be respected to keep the unpolarized light source 100 on the chip 130). The waveguide 124 may also preferably be implemented as a semiconductor material, and may function as an outward coupling. Example implementations for waveguide 124 are as an edge-coupling facet, a spot-size converter (SSC), or a vertical grating coupler.

[0053] The first light emitter unit 101 and / or the second light emitter unit 102 may be implemented by an SOA, used for generation of a broadband spontaneous emission spectrum with typically a high PER of in a range of 10 to 20 dB. Preferably, distinct light emitter units may be used, but it is also possible to have an embodiment with an integrated light emitter unit, i.e. a same single SOA used as both the first and the second light emitter unit - such embodiments will be described in more detail with reference to Figure 2.

[0054] In a specific example embodiment, on-chip lasers can be used in orthogonal polarization state. The principle of combining two light sources with orthogonal polarization states can also be used for combining two lasers or other type of on-chip light sources. Lasers are typically highly polarized but can be combined in a similar way on chip.

[0055] Figure 2 schematically illustrates a second embodiment of the electronic device according to the present disclosure. As indicated above, the second embodiment is characterized by having an integrated SOA, i.e. a same single SOA used as both the first and the second light emitter unit.

[0056] The figure illustrates three variants of the second embodiment, namely variants (a), (b), and (c).

[0057] In all three variants (a)-(c), reference 201 (actually 201 A, 201 B, 201C, respectively) refers to a single light emitter unit, e.g. a single SOA. In all three variants (a)-(c), reference 203 (actually 203A, 203B1 and 203B2, and 203C, respectively) refers to rotators. In all three variants (a)-(c), reference 204 (actually 204A, 204B1 and 204B2, and 204C, respectively) refers to couplers, e.g. 2x1 (or 1x2) couplers.

[0058] For all three variants (a)-(c), it is noted that light having a particular polarization is not coherent with other light having an opposite polarization, so when light stemming from a single spontaneous emission source is split and one of the paths is rotated by exactly ±90 degrees (or a mathematical equivalent thereof) - for all wavelengths - then it is possible to obtain an incoherent combination of the split light paths afterwards. This principle can be exploited as explained for the following variants.

[0059] In variant (a), light can be output with a preferred polarization from light emitter unit 201A (in the direction from right to left in this exemplary figure) and subsequently coupled (in this case, split) by coupler 204A into two separate waveguides. In only one of those two separate waveguides, a rotator 203A is present, which is configured to rotate the light’s polarization by ±90 degrees (or a mathematical equivalent thereof). The two separate waveguides are both terminated by mirror terminations (shown with slanted striping), and thus the light will be reflected, i.e. retransmitted in the other direction (in the direction from left to right in this exemplary figure).

[0060] Preferably, the rotator 203A is bidirectional, which means that the light is subjected to a first rotation (e.g. by 45 degrees) on the first pass through the rotator 203A, and is then subjected after reflection (i.e. in the form of reflected light) to a second rotation (e.g. also by 45 degrees) on the second pass through the rotator 203A. Theoretically, if a rotator 203A could be built that is unidirectional (i.e. that only operates for light passing in one direction and not in the other direction), the reflected light would only be subjected to one rotation and would not be subjected to an opposite rotation - in this case, the rotation in one pass would be by ±90 degrees and there would be no rotation in the other pass. Therefore, in any case, the reflected light in the two separate waveguides can be coupled (in this case, combined) by coupler 204A and can then be output (via the light emitter unit 201A) as unpolarized light.

[0061] In variant (b), light can be output with a preferred polarization from light emitter unit 201 B (in the direction from left to right in this exemplary figure), and subsequently be coupled (in this case, split) by coupler 204B1 , which may be a 1x2 splitter, into two separate waveguides, each characterized by a sufficient path length to prevent interference due to coherence with the respective other waveguide. Subsequently, light in the first waveguide of those two separate waveguides may be subjected to a first rotator 203B1 , and, optionally, light in the second waveguide of those two separate waveguides may be subjected to an optional second rotator 203B2.

[0062] If only the first rotator 203B1 is used (i.e. if there is no second rotator 203B2), then the light’s polarization in the first waveguide should be rotated by ±90 degrees (or a mathematical equivalent thereof). If two rotators are used, their combined rotation effort should amount to ±90 degrees (or a mathematical equivalent thereof) with respect to each other. Subsequently, the light from the two separate waveguides may be coupled (in this case, combined) by coupler 204B2, which may be a 2x1 combiner. It will be understood from the present example that the term ‘coupler’ can be used for a combiner / splitter, and that combiners can be used as splitters (and vice versa) depending on the direction light is fed through the coupler.

[0063] In variant (c), light can be output with a preferred polarization from light emitter 201 C (in the direction from right to left in this exemplary figure), and subsequently be coupled (in this case, split) by coupler 204C, which may be a 1x2 / 2x1 splitter / combiner. The split light may be fed via two separate waveguide routes through rotator 203C, which may be configured to rotate the polarization of one of the two split light bundles by ±90 degrees (or a mathematical equivalent thereof) with respect to the polarization of the other of the two split light bundles. This may for example be implemented by only rotating the polarization of light by ±90 degrees (or a mathematical equivalent thereof) in one direction and not in the other, or alternatively by rotating the polarization of light in both directions relatively to each other by a total amount of ±90 degrees (or a mathematical equivalent thereof), e.g. by rotating the polarization of light by +45 degrees in one direction and by -45 degrees in the other direction.

[0064] Figure 3 schematically illustrates a third embodiment of the unpolarized light source according to the present disclosure. The example in this figure is identical to the example in Figure 1 (except as noted below) and therefore uses respectively corresponding reference numbers, but additionally illustrates a second rotator 340 arranged in the first waveguide 321. In this embodiment, the combined rotation effort by rotator 303 and rotator 340 on their respective light polarizations should amount to ±90 degrees (or a mathematical equivalent thereof) with respect to each other.

[0065] An additional advantage of controlling both polarizations: it is even possible to tune the degree of polarization by tuning the light sources separately.

[0066] Figure 4 schematically illustrates a fourth embodiment 400 of the unpolarized light source according to the present disclosure, which can be used to detect polarizations separately. The fourth embodiment 400 corresponds with the abovedescribed first embodiment 100 except as noted below, and therefore analogous reference numbers indicate analogous elements. Figure 4 shows that, when light sources (laser or broadband) 401 and 402 are both strongly polarized, they can be sent into a polarization-maintaining fiber, i.e. a PM-fiber, 460 independently from each other. Using this principle, it is possible to illuminate both polarization axes of the FBG separately, and detect them separately.

[0067] In more detail, the unpolarized light output into waveguide 424 may in this example be supplied to a circulator 440 such that the light may be further transmitted towards the PM-fiber 460 in which a plurality of FBGs 461 , 462, 463 (or other types of optical reflectors, i.e. Distributed Bragg Reflectors or DBRs) may be arranged. Light reflected from (one or more of) the plurality of FBGs 461 , 462, 463 may be switched via the circulator 440 to a detector 450.

[0068] The birefringence of the FBGs 461 , 462, 463 can contain information of certain parameters, such as for example pressure, or additionally or alternatively any other parameter that contains information linked to the birefringence, at or near the location of the specific FBG within the PM-fiber 460.

[0069] Figure 5 schematically illustrates a rotation of polarization. The figure shows how TE (transverse electric) and TM (transverse magnetic) polarization are orthogonal to each other, and how a preferred polarization can be rotated (in this example, from TE in part (a) to TM in part (b)). Furthermore, the figure shows in part (c) how a combination of predominantly TE and predominantly TM together can achieve an unpolarized light source.

[0070] As described above, various embodiments according to the present disclosure provide a desirable low-PER broadband light source, while being fully compatibility with generic InP PIC platforms which do not allow intrinsic PER control of active layers.

[0071] Moreover, they offer flexibility in operation, and allow for tuning of the polarization state (polarized or unpolarized), which may be useful in PM-OCT applications.

[0072] Other applications in which these embodiments may be useful include (but are not limited to): fiber sensing, including FBG type measurement, or other polarization multiplexing architectures; telecom; and gyroscopes.

[0073] As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. The systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non- obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation.

[0074] Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “obtaining” and “outputting” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by the skilled person.

[0075] It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals may have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the examples described herein. However, it will be understood by the skilled person that the examples described herein can be practiced without these specific details. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features. The description is not to be considered as limiting the scope of the examples described herein.

Claims

CLAIMS1. An unpolarized light source comprising, on a chip: a first light emitter unit configured to output predominantly polarized light characterized by a first preferred polarization; a second light emitter unit, coplanar with the first light emitter unit, configured to output predominantly polarized light characterized by a second preferred polarization; wherein the first preferred polarization is coplanar with the second preferred polarization; a rotator coupled to the second light emitter unit and configured to rotate a polarization of the light output by the second light emitter unit; and a coupler coupled to the first light emitter unit and to the rotator, and configured to couple light output by the first light emitter unit and light output by the rotator and to output the coupled light.

2. The unpolarized light source of claim 1 , wherein light is predominantly polarized if said light has a polarization dependency characterized by a Polarization Extinction Ratio, PER, greater than 1 dB.

3. The unpolarized light source of any preceding claim, wherein the second light emitter unit is optically separated from the first light emitter unit.

4. The unpolarized light source of any preceding claim, wherein the rotator is configured to rotate the polarization of the light by such a rotation angle that the light output by the first emitter unit and the light output by the second light emitter unit are rotated by 90° or 270° with respect to each other.

5. The light source of claim 4, wherein the rotator is configured for orthogonally rotating the polarization of light output by the second light emitter.

6. A light detection system comprising: an unpolarized light source according to any preceding claim; a light detector; and a splitter configured to pass unpolarized light output from the unpolarized light source into an optical fiber and configured to pass light reflected from the optical fiber into the light detector.

7. A method of generating and combining light into unpolarized light; the method comprising, on a chip: outputting predominantly polarized light characterized by a first preferred polarization, from a first light emitter unit; outputting predominantly polarized light characterized by a second preferred polarization, from a second light emitter unit, coplanar with the first light emitter unit; wherein the first preferred polarization is coplanar with the second preferred polarization; rotating a polarization of the light output by the second light emitter unit, using a rotator; and coupling light output by the first light emitter unit and light output by the rotator and outputting the coupled light.

8. The method of claim 7, wherein light is predominantly polarized if said light has a polarization dependency characterized by a Polarization Extinction Ratio, PER, greater than 1 dB.

9. The method of any one of claims 7-8, comprising keeping the second light emitter unit optically separate from the first light emitter unit.

10. The method of any one of claims 7-9, wherein the rotator is configured to rotate the polarization of the light by such a rotation angle that the light output by the first emitter unit and the light output by the second light emitter unit are rotated by 90° or 270° with respect to each other.

11. The method of claim 10, wherein the rotator is configured for orthogonally rotating the polarization of light output by the second light emitter.

12. The method of any one of claims 7-11 , comprising: passing unpolarized light output from the unpolarized light source into an optical fiber; and passing light reflected from the optical fiber into a light detector.

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