Optical power limiter

Abstract

An optical power limiter, a method of manufacturing an optical power limiter, an optical device or system including an optical power limiter, and a method of limiting optical power using the optical power limiter. The optical power limiter includes: a first optical mode modulation element configured to receive an input optical signal from a first waveguide; and an active medium coupled at a first end thereof to the first optical mode modulation element such that a mode modulated optical signal based on the input optical signal can enter the active medium, where a second end of the active medium is configured to couple the mode modulated optical signal into the second waveguide as an optical output signal; wherein the active medium has a thermo-optical coefficient, so that a mode modulation optical signal entering the active medium undergoes refractive index gradient change in the active medium due to an absorption effect; and a power of the optical output signal coupled into the second waveguide is limited to a maximum power value based on a degree of mode overlap of the mode modulated optical signal and the second waveguide.

Description

Technical Field

[0001] The present invention generally relates to an optical power limiter, and more particularly to a miniature optical power limiter with improved performance for quantum and classical optical communications. Background Art

[0002] Any reference to and / or discussion of the prior art in this specification should not in any way be taken as an admission that the prior art is common general knowledge or forms part of the common general knowledge in the field.

[0003] Various optical power limiter designs have been reported.

[0004] (1) PCT / SG2021 / 050403: An optical power limiter design that utilizes free-space optical transmission to introduce light into an active medium, where the power limitation is based on the thermo-optical defocusing effect in the active medium. When the input light beam has a relatively high optical power, the refractive index gradient caused by absorption in the active medium acts as a concave lens, defocusing the beam. In this way, the device can dynamically control the optical power transmitted through the aperture. Therefore, even if the input optical power increases, the final output optical power is limited. This design is bulky, heavy, and has high insertion loss.

[0005] (2) Fiber power limiter based on liquid core fiber [IEEE Photonics Letters, Vol. 24, pp. 297-299, (2011)]: A power limiting effect in liquid core fiber (LCOF) was proposed, where the absorption of the attenuation field by the thin absorption layer deposited on the LCOF cladding causes heat accumulation, thereby increasing the temperature of the fiber. Due to the difference in the thermo-optic coefficients of the core and cladding, the transmission efficiency of light in the fiber core is reduced, limiting the ultimate output power. Liquid core fiber has not yet been commercialized and has high manufacturing costs.

[0006] (3) Fiber Optic Power Limiter Based on Optical Adhesive [Applied Optics, Vol. 40, p. 6611, 2001]: This paper utilizes the thermo-optical effect of optical adhesive used to connect two fiber collimators to achieve output power limitation. Using optical adhesive to connect two fiber collimators is a technically demanding task, requiring consistent UV curing at all points of the adhesive and addressing issues such as precise collimator calibration due to adhesive shrinkage.

[0007] (4) Optical power limiter based on a photonic chip microring resonator [Scientific Reports, Vol. 4, p. 6676, (2014)]: A chip microring resonator has a specific operating frequency. When the input light is absorbed, causing the ring cavity temperature to rise, its operating wavelength shifts, thereby attenuating the input light intensity. Therefore, this power limiting effect only acts on the specific operating wavelength of the input signal, and has limited ability to limit the output power.

[0008] Embodiments of the present invention aim to address at least one of the above-mentioned problems. Summary of the Invention

[0009] According to a first aspect of the present invention, there is provided an optical power limiter, comprising:

[0010] a first optical mode modulation element configured to receive an input optical signal from the first waveguide; and

[0011] an active medium having a first end coupled to the first optical mode modulation element such that a mode-modulated optical signal based on the input optical signal can enter the active medium, wherein a second end of the active medium is configured to couple the mode-modulated optical signal into the second waveguide as an optical output signal;

[0012] wherein the active medium has a thermo-optic coefficient such that a mode-modulated optical signal entering the active medium experiences a refractive index gradient change within the active medium due to absorption; and

[0013] The power of the optical output signal coupled to the second waveguide is limited to a maximum power value based on the degree of mode overlap between the mode-modulated optical signal and the second waveguide.

[0014] According to a second aspect of the present invention, there is provided a method for manufacturing an optical power limiter, comprising:

[0015] configuring the first optical mode modulation element to receive an input optical signal from the first waveguide;

[0016] coupling the active medium to the first optical mode modulation element from a first end of the active medium, thereby enabling a mode-modulated optical signal based on the input optical signal to enter the active medium; and

[0017] configuring the second end of the active medium to couple the mode-modulated optical signal into the second waveguide as an optical output signal;

[0018] wherein the active medium has a thermo-optic coefficient such that a mode-modulated optical signal entering the active medium undergoes a refractive index gradient change within the active medium due to absorption; and

[0019] The power of the optical output signal coupled to the second waveguide is limited to a maximum power value based on the degree of mode overlap between the mode-modulated optical signal and the second waveguide.

[0020] According to a third aspect of the present invention, there is provided an optical device or system comprising the power limiter of the first aspect.

[0021] According to a fourth aspect of the present invention, there is provided a method of limiting optical power using the power limiter of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The embodiments of the present invention will be more readily understood and apparent to those skilled in the art through the following written description (for illustrative purposes only) in conjunction with the accompanying drawings, in which:

[0024] Figure 1 A schematic diagram of an optical power limiter according to an exemplary embodiment is shown.

[0025] Figure 2 A schematic diagram of an optical power limiter according to an exemplary embodiment is shown.

[0026] Figure 3 A schematic diagram of an optical power limiter according to an exemplary embodiment is shown.

[0027] Figure 4A Shown Figure 2 Experimental results for an optical power limiter are shown, where single-mode fibers are used as input and output waveguides and a GRIN lens is used as the input optical structure. The beam width varies while the dielectric length remains constant at 100 μm.

[0028] Figure 4B Shown Figure 2 Experimental results for an optical power limiter are shown, where single-mode fibers are used as input and output waveguides and a GRIN lens is used as the input optical structure. The beam width varies while the dielectric length remains constant at 200 μm.

[0029] Figure 4C Shows Figure 2 Experimental results for an optical power limiter are shown, where single-mode fibers are used as input and output waveguides and a GRIN lens is used as the input optical structure. The beam width varies while the dielectric length remains constant at 300 μm.

[0030] Figure 5 Shown Figure 3 Experimental results of an optical power limiter are shown, where single-mode fibers are used as input and output waveguides and GRIN lenses are used as input and output optical structures.

[0031] Figure 6A schematic diagram of a power limiter array for multi-channel applications is shown according to an example embodiment.

[0032] Figure 7 1 shows the responsivity results measured under PL protection, at a bias voltage of 2 V, and a continuous wave input optical power of 1 mW according to an example embodiment. The responsivity is measured after 1 MHz pulse illumination for 5 seconds.

[0033] Figure 8 Measured responsivity results are shown after 10 MHz pulse illumination for 5 s under PL protection at a bias voltage of 2 V and a continuous wave input optical power of 1 mW according to an example embodiment.

[0034] Figure 9 Measured responsivity results are shown for a 100 MHz illumination pulse for 5 s under PL protection at a bias voltage of 2 V and a continuous wave input optical power of 1 mW according to an example embodiment.

[0035] Figure 10 1 shows the responsivity change measured after 5 s of 1 MHz pulse illumination with a peak power of 14.7 W at a bias voltage of 2 V and a continuous wave input optical power of 1 mW without PL protection, according to an example embodiment.

[0036] Figure 11 A schematic diagram of an apparatus for accurately monitoring input optical power and performing feedback control in quantum cryptography to achieve 1) an ultra-low power limit threshold and 2) instantaneous optical power limit according to an example embodiment is shown.

[0037] Figure 12 A flow chart of a method for manufacturing an optical power limiter according to an example embodiment is shown. DETAILED DESCRIPTION

[0038] Embodiments of the present invention provide an optical power limiter. Embodiments of the present invention have various advantages, including but not limited to one or more of the following:

[0039] 1. Miniature size: The optical power limiter according to example embodiments is capable of achieving optical power limitation of micron-scale light beams and is suitable for waveguide implementations such as optical fibers and photonic integrated chips.

[0040] 2. Minimum insertion loss: The optical power limiter according to the exemplary embodiment optimizes optical mode coupling at low input power. Therefore, the insertion loss is minimized at low input power.

[0041] 3. Adjustable power limit threshold (ie, maximum output optical power): The optical power limiter according to example embodiments provides configurable system parameters to adjust the power limit threshold.

[0042] 4. Minimized signal distortion: The optical power limiter according to the exemplary embodiment only applies attenuation to the input optical signal, with minimal (if not negligible) impact on the intensity, phase, or polarization degrees of freedom of the input optical signal.

[0043] Industrial applications of the embodiments of the present invention include but are not limited to the following aspects:

[0044] 1. Optical communication

[0045] 2. Sensing

[0046] 3. Quantum Cryptography

[0047] The optical power limiter according to the example embodiments focuses on waveguide input and output, and processes a smaller input beam size than existing solutions (such as PCT / SG2021 / 050403). This is conducive to generating a higher power density in the active medium, thereby producing a stronger thermo-optical defocusing effect, ultimately shortening the transmission distance and reducing absorption loss. In addition, unlike the solution in PCT / SG2021 / 050403 that considers free-space light transmission, the embodiments of the present invention replace the aperture based on the mode overlap condition. By managing the mode overlap condition of the input and output light, the power limiting threshold and insertion loss relationship of the optical power limiter according to the example embodiments can be advantageously optimized to obtain optimal performance.

[0048] Figure 1 A first optical power limiter 100 according to an exemplary embodiment is shown, comprising input and output waveguides 102 and 104, tapered structures 106 and 108, and an active medium 110. In various exemplary embodiments, the waveguides 102 and 104 may be optical fibers, waveguides in a photonic integrated circuit (PIC), or the like. The tapered structures 106 and 108 are optical structures that can modify the optical modes of the input and output light from the waveguides 102 and 104. In various exemplary embodiments, examples of the tapered structures 106 and 108 may be core extensions in optical fibers, tapered structures, and large-core waveguide mode converters in PIC platforms. In the optical power limiter 100, beam parameters in the active medium 110 can be modified, and the degree of optical mode overlap of the output waveguide 104 can be managed. The active medium 110 is where the thermo-optical defocusing effect occurs.

[0049] Figure 2A second optical power limiter 200 is shown, according to an exemplary embodiment. The tapered portion is replaced by an optical structure 202 (for coupling to an input waveguide 203) that can alter the optical mode and beam parameters. In various exemplary embodiments, the optical structure 202 can be a lens, a gradient refractive index (GRIN) lens, a collimator, or waveguide versions of all of the aforementioned structures. The function of the optical structure 202 can be to focus light into the active medium 204, thereby providing an adjustable beam waist and focal length. In this way, adjustable system parameters (power limit threshold, insertion loss) can be achieved when coupling to the output waveguide 206.

[0050] Figure 3 A third optical power limiter 300 according to an example embodiment is shown. A first optical structure 301 is provided for coupling to an input waveguide 302, which can change the optical mode and beam parameters. A second optical structure 303 is provided on the output side for coupling to an output waveguide 304 to optimize the mode overlap between the optical mode after the active medium 306 and the output waveguide 304. In this way, at low input power, the mode mismatch (and insertion loss) is minimized. At high input power, due to the thermo-optical defocusing effect, the light beam in the active medium 306 diverges, resulting in significant mode mismatch in the light coupled to the output waveguide 304, thereby limiting the output optical power. In addition, different power limit thresholds can be obtained by adjusting the length of the active medium 306. In this way, an adjustable power limit threshold and minimal insertion loss can be achieved.

[0051] according to Figure 2 and Figure 3 In a non-limiting exemplary embodiment, single-mode optical fibers (SMF28, mode field diameter: 10.4 um @ 1550 nm) are used as input and output waveguides, GRIN lenses are used as input and output optical structures, and an optical adhesive with negative TOC is used as the active medium.

[0052] Based on this configuration, the optical power limiter 200 ( Figure 2 ) and optical power limiter 300( Figure 3 ) conducted experiments. Single-mode optical fiber is a commonly used waveguide in optical communications and quantum cryptography and is easily integrated into fiber optic systems. GRIN lenses are compact, with diameters and lengths of only ~mm. Optical adhesives are also widely used in optical systems. Both GRIN lenses and optical adhesives are cost-effective and readily available.

[0053] The experimental results of the optical power limiter 200 are as follows: Figures 4A-4CBy configuring the beam width (3um to 7.3um) and dielectric length (100um to 300um), an adjustable power limit threshold (10.53dBm (11.3mW) to 17.3dBm (53.7mW)) and insertion loss (3.1dB to 12.6dB) can be achieved.

[0054] The experimental results of the optical power limiter 300 are as follows: Figure 5 By varying the length of medium 306 (the distance between the two self-focusing lenses, from 6.5 mm to 15.8 mm), the power limit threshold can be adjusted between 13.36 dBm (21.7 mW) and 21.25 dBm (133.4 mW), and the insertion loss at low input power varies between 1.89 dB and 2.9 dB. In various exemplary embodiments, the insertion loss can be further reduced by improving mode matching and interface reflection.

[0055] As described above, it is proved that the embodiment of the present invention can achieve a miniaturized power limiting effect. Compared with the existing solutions (such as PCT / SG2021 / 050403, about 10 cm), it occupies a smaller space (about millimeter level), has extremely small insertion loss (about 1.89 dB), and has an adjustable power limiting threshold.

[0056] Optical power limiters according to example embodiments may be used in various industrial applications, such as quantum cryptography and optical communications:

[0057] In quantum cryptography, an optical power limiter according to an exemplary embodiment can be used as a countermeasure against Trojan horse attacks by limiting the energy of eavesdropping light. It can also serve as a potential protection measure for plug-and-play quantum key distribution against untrusted light sources, as well as a potential countermeasure against strong light illumination attacks (including laser damage attacks and detector blinding attacks) [PRX QUANTUM 2, 030304(2021)].

[0058] The embodiments of the present invention have the characteristics of low insertion loss and small footprint, and can be used as a universal component for protecting quantum cryptography systems. For example, the low insertion loss and compact size can achieve higher system integration, especially at the receiving end.

[0059] In the most widely deployed BB84 quantum key distribution (QKD) system, the receiver has been shown to be one of the most vulnerable components of the entire system. The detector can be manipulated by strong eavesdropping light [Nature Photonics, Vol. 4, p. 686 (2010); Reviews of Modern Physics, Vol. 92, No. 025002 (2020)]. The standard countermeasure to such attacks is to actively monitor the input optical power. However, studies have shown that monitoring equipment can also be vulnerable to laser damage attacks. In this case, calibrated system parameters can be altered, and the monitoring equipment can be damaged. How to address this issue remains an unresolved problem in the field [see Physical Review A, Vol. 94, No. 030302 (2016); Physical Review A, Vol. 91, No. 032326 (2015)]. Previous literature has shown that eavesdropping light powers exceeding 0.25 W are required to change device parameters. For example, silicon single-photon detectors were observed to exhibit changes in time parameters such as efficiency, dark count rate, and breakdown voltage under continuous light illumination of >0.25W [Physical Review Letters, Vol. 112, No. 070503 (2014)]. InGaAs PIN detectors used for monitoring showed changes in photosensitivity under continuous light input of >0.5W [Physical Review A, Vol. 94, No. 030302 (2016)]. In addition, attenuators, circulators, and isolators also showed parameter changes under continuous light input of >1W [Physical Review Applied, Vol. 13, p. 034017 (2020), arXiv preprint number 2201.06114]. Advantageously, the optical power limiter according to example embodiments can adjust the energy of the output light regardless of the intensity of the input light. Therefore, the optical power limiter according to example embodiments can provide excellent protection for calibrated components and equipment. Advantageously, this small insertion loss has minimal impact on the system signal-to-noise ratio, making the optical power limiter according to example embodiments suitable as a universal component for protection of both the transmitting end and the receiving end.

[0060] In optical communications, regulating optical power is also very important. The optical power limiter according to example embodiments can be used for power balancing in wavelength division multiplexing (WDM) systems, gain control of erbium-doped fiber amplifiers (EDFAs), receiver protection, etc.

[0061] In a WDM system, multiple wavelength channels arriving at a node may be transmitted via different optical paths and have different output powers. Before the combined signal enters the optical amplifier, the optical power of these channels needs to be equalized to maintain proper optical amplifier performance. This is typically achieved by actively monitoring and controlling the optical power [“MEMS Variable Optical Attenuator (VOA) for Dense Wavelength Division Multiplexing (DWDM) Applications.” Design, Test, Integration and Packaging of MEMS / MOEMS, 2002, Vol. 4755. International Society for Optical Engineering, 2002. “Micromechanical Electromagnetic Variable Optical Attenuator for Optical Power Equalization.” Journal of Micro / Nanolithography, MEMS and MOEMS 4.4 (2005): 041304]. The optical power limiter according to the example embodiment can provide automatic power control and minimize the insertion loss of the input signal. The optical power limiter according to the example embodiment has great potential to supplement or even replace power equalization technology in WDM systems.

[0062] Figure 6 A schematic diagram of a power limiter array 600 for multi-channel applications is shown according to an example embodiment.

[0063] Another problem in WDM networks is wavelength-dependent gain saturation of EDFAs. When the input optical power becomes a significant fraction of the pump power, it results in pump losses and reduced amplifier gain. Due to this effect, the loss or removal of one or more channels at the EDFA input can cause significant changes in the output power of the remaining channels [“Comparison of Gain Control Techniques for Stabilizing EDFAs in WDM Networks.” Proceedings of the Optical Fiber Communications Conference, OFC., IEEE, 1996, WIPO (PCT) WO2003014775A2]. Using an optical power limiter according to an exemplary embodiment, the input power of all channels can be limited to below a threshold, thereby preventing EDFA gain saturation.

[0064] Furthermore, the optical power limiter according to example embodiments has practical value for receiver protection. As previously mentioned, if a channel happens to have high-power optical input, receiver performance may be affected or even damaged. The optical power limiter according to example embodiments provides automatic power adjustment to prevent such damage and minimize optical signal loss under normal operating conditions.

[0065] The feasibility of using the optical power limiter in this embodiment for optical device protection has been verified through experiments. Figure 3 The optical power limiter 300 includes a pair of single-mode optical fibers and a GRIN lens. Four experimental configurations are shown in Table 1 below to test whether the optical power limiter of this embodiment can protect the photodetector from externally injected strong laser light.

[0066]

[0067] Table 1

[0068] Specifically, laser attack experiments were conducted using both continuous-wave (CW) and pulsed laser inputs. For CW laser input, the laser's optical power was gradually increased. The laser was then transmitted through an optical power limiter in an exemplary embodiment and injected into a fiber-coupled InGaAs photodiode. Attacks were performed with varying input optical powers (up to 1W), while maintaining the responsivity (or quantum efficiency) of the photodiode under test.

[0069] Experiments were also conducted with the optical power limiter removed. By increasing the continuous wave input optical power, irradiating the photodiode for 5 seconds, it was observed that the photodiode's responsivity began to decrease when the input optical power exceeded 300mW. This change in the photodiode is irreversible and will modify the system's calibration parameters.

[0070] In addition, experiments were conducted based on a series of pulsed laser inputs with high peak power to verify the effectiveness of the optical power limiter of the exemplary embodiment under these conditions. To this end, a two-stage erbium-doped fiber amplifier (EDFA) was used to amplify the pulsed laser signal generated by a 1550nm semiconductor laser. The pulse width (50-500ps) and repetition frequency (1MHz to 100MHz) of the laser pulses were varied to generate various laser pulse configurations with peak powers ranging from 2.9W to 14.7W. In these experiments, no significant changes in the photodiode responsivity were observed. Figures 7 to 9 The detailed measurement results of the responsivity at different input peak powers in this experiment are shown, indicating that the responsivity is generally stable.

[0071] For pulsed laser input, the experiment was also conducted without the optical power limiter in the device. By increasing the cumulative irradiation time and fixing the input pulse at 14.7W peak power and 1MHz repetition rate, it can be observed that the responsivity of the photodiode begins to drop sharply after 2s of irradiation. The experimental results are detailed as follows: Figure 10 shown.

[0072] The optical power limiter according to example embodiments can be a powerful solution for countering laser damage attacks in quantum cryptography, and we can expect to build more practical tools and methods based on it. For example, because the calibration parameters of the photodiode are reliable and cannot be modified by eavesdroppers when using the optical power limiter of example embodiments, people can accurately monitor the input optical power and perform feedback control, thereby achieving 1) an ultra-low power limit threshold and 2) instantaneous optical power limiting. Figure 11 A schematic diagram of an apparatus according to an example embodiment is shown.

[0073] Specifically, input optical signal 1100 first passes through power limiter (PL) 1102 and is then split by beam splitter (BS) 1104. A portion of the energy is transmitted to monitor photodiode (Mon PD) 1106, while the remaining energy is delayed and attenuated (by electronic variable optical attenuator (EVOA)) 1108) before being output to single-photon detector (SPD) or homodyne detector 1110. Because the optical components (monitor photodiode 1106 and beam splitter 1104) are protected by optical power limiter 1102, their calibration parameters are reliable, allowing the (average or instantaneous) input power of the optical signal to be monitored and the electronic variable optical attenuator 1108 to be actively controlled to achieve the desired output power.

[0074] Embodiments of the present invention may have one or more of the following features and associated benefits / advantages:

[0075]

[0076] Optical power limiters according to example embodiments may be used in applications such as protecting optical elements in optical communications and sensing systems, limiting information to eavesdroppers in quantum cryptography applications, and the like.

[0077] In one embodiment, an optical power limiter is provided, comprising: a first optical mode modulation element configured to receive an input optical signal from a first waveguide; and an active medium, a first end of the active medium coupled to the first optical mode modulation element so that a mode-modulated optical signal based on the input optical signal can enter the active medium, wherein a second end of the active medium is configured to couple the mode-modulated optical signal into a second waveguide as an optical output signal; wherein the active medium has a thermo-optic coefficient such that the mode-modulated optical signal entering the active medium experiences a refractive index gradient change in the active medium due to absorption; and wherein the power of the optical output signal coupled to the second waveguide is limited to a maximum power value based on a degree of mode overlap between the mode-modulated optical signal and the second waveguide.

[0078] The maximum power value may depend on the optical path length between the first end and the second end of the active medium.

[0079] The active medium can have a negative thermo-optic coefficient to diverge the light beam due to the refractive index gradient. Due to the refractive index gradient, the active medium can introduce a larger mode mismatch between the mode-modulated light signal and the second waveguide, while increasing the divergence of the mode-modulated light signal.

[0080] The first optical mode modulation element may include a core extension in an optical fiber as the first waveguide, a tapered structure, or a large-core waveguide mode converter in a photonic integrated circuit as the waveguide. The optical power limiter may include a second optical mode modulation element coupled between the second end of the active medium and the second waveguide to optimize the degree of mode overlap between the mode-modulated optical signal and the second waveguide until a maximum power value is achieved.

[0081] The first optical mode modulation element may be configured to focus the mode-modulated light signal in the active medium. The first optical mode modulation element may include one or more of the following: a microlens, a gradient index (GRIN) lens, a collimator, and a waveguide version of the microlens, gradient index (GRIN) lens, and collimator.

[0082] The optical power limiter may include a second optical mode modulation element coupled between the second end of the active medium and the second waveguide, configured to optimize a degree of mode overlap between the mode-modulated optical signal and the second waveguide until a maximum power value is achieved. The second optical mode modulation element may include one or more of the following: a microlens, a gradient-index (GRIN) lens, a collimator, and waveguide versions of the microlens, gradient-index (GRIN) lens, and collimator.

[0083] Figure 12 A method flow 1200 for fabricating an optical power limiter according to an exemplary embodiment is shown. In step 1202, a first optical mode modulation element is configured to receive an input optical signal from a first waveguide. In step 1204, an active medium is coupled from a first end of the active medium to the first optical mode modulation element, thereby allowing a mode-modulated optical signal based on the input optical signal to enter the active medium. In step 1206, a second end of the active medium is configured to couple the mode-modulated optical signal into a second waveguide as an optical output signal. The active medium has a thermo-optic coefficient such that the mode-modulated optical signal entering the active medium experiences a refractive index gradient change within the active medium due to absorption. Furthermore, the power of the optical output signal coupled to the second waveguide is limited to a maximum power value based on the degree of mode overlap between the mode-modulated optical signal and the second waveguide.

[0084] The maximum power value may depend on the optical path length between the first end and the second end of the active medium.

[0085] The active medium can have a negative thermo-optic coefficient to cause the light beam to diverge due to the refractive index gradient. Due to the refractive index gradient, the active medium can introduce a larger mode mismatch between the mode-modulated light signal and the second waveguide, while increasing the divergence of the mode-modulated light signal.

[0086] The first optical mode modulation element may include a core extension in an optical fiber as the first waveguide, a tapered structure, or a large-core waveguide mode converter in a photonic integrated circuit as the waveguide. The method may include coupling the second optical mode modulation element between the second end of the active medium and the second waveguide to optimize the degree of mode overlap between the mode-modulated optical signal and the second waveguide until a maximum power value is achieved.

[0087] The method may include configuring a first optical mode modulation element to focus a mode-modulated light signal into the active medium. The first optical mode modulation element may include one or more of the following group: a microlens, a gradient index (GRIN) lens, a collimator, and a waveguide version of the microlens, gradient index (GRIN) lens, and collimator. The method may include coupling a second optical mode modulation element between a second end of the active medium and a second waveguide to optimize a degree of mode overlap between the mode-modulated light signal and the second waveguide until a maximum power value is achieved. The second optical mode modulation element may include one or more of the following group: a microlens, a gradient index (GRIN) lens, a collimator, and a waveguide version of the microlens, gradient index (GRIN) lens, and collimator.

[0088] In one embodiment, an optical device or system is provided, comprising the power limiter of the above embodiment.

[0089] In one embodiment, a method for limiting optical power using the power limiter of the above embodiment is provided.

[0090] It will be understood by those skilled in the art that various variations and / or modifications may be made to the invention shown in the specific embodiments without departing from the spirit or scope of the invention as outlined herein. Therefore, these embodiments are to be considered in all respects as illustrative and not restrictive. Furthermore, the present invention encompasses any combination of features described with respect to the various embodiments (including in the Abstract), even if such features or combinations of features are not explicitly listed in the claims or the detailed description of the embodiments of the invention.

[0091] In general, in the following claims, the terms used should not be construed to limit the systems and methods to the specific embodiments disclosed in the specification and claims, but should be construed to include all processing systems that operate in accordance with the manner recited in the claims. Accordingly, these systems and methods are not limited by the present disclosure, but rather the scope of the systems and methods is to be determined entirely by the claims.

[0092] Throughout the specification and claims, unless the context clearly requires otherwise, the words “comprise,” “including,” and similar expressions should be construed in an inclusive sense, and not in an exclusive or exhaustive sense; that is, to mean “including but not limited to.” Words using the singular or plural number also include the plural or singular number, respectively. In addition, the words “herein,” “under,” “above,” “below,” and words of similar import refer to the entire application and not to any particular part of the application. When the word “or” is used in reference to a list of two or more items, the word encompasses all of the following: any item in the list, all of the items in the list, and any combination of the items in the list.

Claims

1. An optical power limiter, comprising: a first optical mode modulation element configured to receive an input optical signal from a first waveguide; as well as an active medium having a first end coupled to the first optical mode modulation element so that a mode-modulated optical signal based on the input optical signal can enter the active medium, wherein a second end of the active medium is configured to couple the mode-modulated optical signal to a second waveguide as an optical output signal; wherein the active medium has a thermo-optic coefficient, such that the mode-modulated optical signal entering the active medium undergoes a refractive index gradient change within the active medium due to absorption; and The power of the optical output signal coupled to the second waveguide is limited to a maximum power value based on the mode overlap between the mode modulated optical signal and the second waveguide.

2. The optical power limiter according to claim 1, wherein: The maximum power value depends on the optical path length between the first end and the second end of the active medium.

3. The optical power limiter according to claim 1 or 2, wherein: The active medium has a negative thermo-optic coefficient to diverge the light beam due to the refractive index gradient change.

4. The optical power limiter according to claim 3, wherein: Due to the refractive index gradient change, the active medium introduces a larger mode mismatch between the mode-modulated optical signal and the second waveguide, and simultaneously increases the divergence of the mode-modulated optical signal.

5. The optical power limiter according to any one of the preceding claims, wherein The first optical mode modulation element includes a core extension in an optical fiber as the first waveguide, a tapered structure, or a large-core waveguide mode converter in a photonic integrated circuit as the first waveguide.

6. The optical power limiter of claim 5 , comprising a second optical mode modulation element coupled between the second end of the active medium and the second waveguide to optimize a mode overlap between the mode-modulated optical signal and the second waveguide until the maximum power value is reached.

7. The optical power limiter according to any one of claims 1 to 4, wherein: The first optical mode modulation element is configured to focus the mode-modulated light signal in the active medium.

8. The optical power limiter according to any one of claims 7, wherein: The first optical mode modulation element includes one or more of the following group: a microlens, a gradient index (GRIN) lens, a collimator, and a waveguide version of the microlens, the gradient index (GRIN) lens, and the collimator.

9. The optical power limiter according to claim 7 or 8, comprising a second optical mode modulation element coupled between the second end of the active medium and the second waveguide to optimize the mode overlap between the mode-modulated optical signal and the second waveguide until the maximum power value is reached.

10. The optical power limiter according to claim 9, wherein: The second optical mode modulation element includes one or more of the following group: a microlens, a gradient index (GRIN) lens, a collimator, and a waveguide version of the microlens, the gradient index (GRIN) lens, and the collimator.

11. A method for manufacturing an optical power limiter, comprising: configuring the first optical mode modulation element to receive an input optical signal from the first waveguide; coupling an active medium from a first end of the active medium to the first optical mode modulation element, thereby enabling a mode-modulated optical signal based on the input optical signal to enter the active medium; and configuring the second end of the active medium so that the mode-modulated optical signal is coupled to a second waveguide as an optical output signal; wherein the active medium has a thermo-optic coefficient such that a mode-modulated optical signal entering the active medium undergoes a refractive index gradient change in the active medium due to absorption; and The power of the optical output signal coupled into the second waveguide is limited to a maximum power value based on the mode overlap between the mode-modulated optical signal and the second waveguide.

12. The method according to claim 11, wherein The maximum power value depends on the optical path length between the first end and the second end of the active medium.

13. The method according to claim 11 or 12, wherein: The active medium has a negative thermo-optic coefficient to diverge the light beam due to the refractive index gradient change.

14. The method according to claim 13, wherein Due to the refractive index gradient change, the active medium introduces a larger mode mismatch between the mode-modulated optical signal and the second waveguide, and simultaneously increases the divergence of the mode-modulated optical signal.

15. The method according to any one of claims 11 to 14, wherein The first optical mode modulation element includes a core extension in an optical fiber as the first waveguide, a tapered structure, or a large-core waveguide mode converter in a photonic integrated circuit as the waveguide.

16. The method of claim 15, comprising coupling a second optical mode modulation element between the second end of the active medium and the second waveguide to optimize mode overlap between the mode modulated optical signal and the second waveguide until the maximum power value is achieved.

17. The method according to any one of claims 11 to 14, wherein The method includes configuring the first optical mode-modulating element to focus the mode-modulated light signal in an active medium.

18. The method according to claim 17, wherein The first optical mode modulation element includes one or more of the following group: a microlens, a gradient index (GRIN) lens, a collimator, and a waveguide version of the microlens, the gradient index (GRIN) lens, and the collimator.

19. The method of claim 17 or 18, comprising coupling a second optical mode modulation element between the second end of the active medium and the second waveguide to optimize the degree of mode overlap between the mode modulated optical signal and the second waveguide until the maximum power value is reached.

20. The method according to claim 19, wherein The second optical mode changing element comprises one or more of the following group: a microlens, a gradient index (GRIN) lens, a collimator, and a waveguide version of a microlens, a gradient index (GRIN) lens, a collimator.

21. An optical device or system comprising the optical power limiter according to any one of claims 1 to 10.

22. A method of limiting optical power using the optical power limiter according to any one of claims 1 to 10.

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