FIELD

[001] The subject matter described herein relates to optical communications, and in particular, determining the wavelength of an optical source.

BACKGROUND

[002] Many electronic devices require data to be shared between devices. These devices include a wide variety of consumer and industrial products including mobile devices such as cell phones, handheld computing devices, laptops, and the like. As the features and capabilities available in mobile devices have increased, the need for higher throughput data connections has also increased. For example, sharing high-definition video with another device via a data connection requires a high-throughput data connection between the devices.

[003] Electronic devices may utilize optical communications between devices. The optical communications may include an optical transmitter at one device and an optical receiver at another device, or may include optical transceivers at both devices. An optical transmitter may include an optical source such as a laser, laser diode, light-emitting diode, or other source. The performance of some optical sources may drift over time and environmental conditions.

SUMMARY

[004] In one aspect there is an apparatus. The apparatus may include a first delay line passing a first portion of an optical signal to produce a first input signal. The apparatus may further include a second delay line passing a second portion of the optical signal to produce a second input signal. A combiner may combine the first input signal and the second input signal to produce a plurality of output signals. An analyzer may determine, from the plurality of output signals, at least a wavelength of the optical signal.

[005] In some variations, one or more of the features disclosed herein including the following features can optionally be included in any feasible combination. The first portion of the optical signal and the second portion of the optical signal may be produced by a one-by-two multimode interference coupler. The first portion of the optical signal and the second portion of the optical signal may be equal or nearly equal in intensity. The apparatus may further determine a linewidth of the optical signal. The combiner may include a multimode interference coupler. The multimode interference coupler may include a four-by-four multimode interference coupler. The combiner may include a ninety-degree hybrid coupler.

[006] The above -noted aspects and features may be implemented in systems, apparatuses, methods, and/or computer-readable media depending on the desired configuration. The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims. In some exemplary embodiments, one of more variations may be made as well as described in the detailed description below and/or as described in the following features.

DESCRIPTION OF DRAWINGS

[007] In the drawings,

[008] FIG. 1A depicts an example of a free-space optical transceiver system including a wavelength meter, in accordance with some example embodiments;

[009] FIG. IB depicts an example of a wavelength meter, in accordance with some example embodiments; [010] FIG. 2A depicts a schematic diagram of optical components in a wavelength meter, in accordance with some example embodiments;

[011] FIG. 2B depicts multimode interference couplers included in a wavelength meter, in accordance with some example embodiments;

[012] FIG. 3 depicts another example of a wavelength meter, in accordance with some example embodiments;

[013] FIG. 4A depicts another example of a wavelength meter that includes multimode interference couplers, in accordance with some example embodiments;

[014] FIG. 4B depicts an additional example of a wavelength meter that includes multimode interference couplers, in accordance with some example embodiments;

[015] FIG. 5 depicts an example of a process performed by a wavelength meter, in accordance with some example embodiments; and

[016] FIG. 6 depicts an example of an apparatus, in accordance with some example embodiments.

[017] Like labels are used to refer to the same or similar items in the drawings.

DETAILED DESCRIPTION

[018] Optical communications may be used by electronic devices such as cell phones, portable computers, gaming devices, and the like to share data with other mobile devices or fixed location devices. Optical communications may be implemented using a cable such as a fiber optic cable or without a cable such as free-space optical communications. In some situations, cables are inconvenient or prone to failure because they require connecting and disconnecting connectors that can break or become dirty interfering with their operation, and so on. In some situations, free-space optical communications is more robust and convenient. Free-space optical communications may be performed from an optical transmitter to an optical receiver through an open medium such as through air from a mobile device to another device.

[019] An optical transmitter may include an optical source and may also include other optical and electronic components. In some example embodiments, the wavelength of the light generated by an optical source at a transmitter may be determined by a wavelength meter. The wavelength meter may be included as part of an optical transmitter, or may be a separate apparatus. For example, a wavelength meter may determine the wavelength of an optical source in a wavelength multiplexed communication system. By measuring the wavelength of the optical source with the wavelength meter and adjusting the wavelength of the optical source, wavelength drift of the optical source may be avoided. In some example embodiments, the optical source is included in a piece of equipment that is not a transmitter.

[020] An optical receiver may include an optical detector and may also include other optical and electronic components. In some example embodiments, the optical wavelength of the light received at an optical receiver may be determined by a wavelength meter. The wavelength meter may be included as part of an optical receiver or may be a separate apparatus.

[021] Wavelength meters determine the wavelength of light incident to the meter. In some example embodiments, the incident light is narrow band, or the range of wavelengths in the incident light is small. In some example embodiments, wavelength meters may provide better accuracy than a spectrometer or grating monochromator. In some example embodiments, wavelength meters may be used to monitor the wavelength of a tunable laser, or a laser subject to wavelength drift.

[022] Wavelength meters consistent with the subject matter disclosed herein may determine the wavelength of incident light without the use of long-path techniques such as those used in spectrometers. In some example embodiments, a wavelength meter consistent with the subject matter disclosed herein may stabilize a laser being used to perform spectroscopy where the laser wavelength must be known accurately. In some example embodiments, a wavelength meter consistent with the subject matter disclosed herein may be used to determine the wavelength of a tunable laser used for cooling/trapping and/or differential-absorption LIDAR (optical/light radar). In some example embodiments, a wavelength meter consistent with the subject matter disclosed herein may be used in testing and manufacturing of dense wavelength-division-multiplexing (DWDM) equipment for optical communications. In some example embodiments, a wavelength meter consistent with the subject matter disclosed herein may be integrated with an optical source used for Quantum Key Distribution and/or sensing applications.

[023] FIG. 1 A depicts a mobile or fixed location apparatus in communication with another mobile or fixed apparatus. In some example embodiments, an optical transceiver may be incorporated into a wireless device or user equipment, such as a smart phone, or a cell phone, and/or any other radio. One or both of the mobile/fixed apparatus may include a wavelength meter incorporated into the apparatus.

[024] For example, mobile/fixed apparatus 110 may include a free-space optical transceiver 112 and wavelength meter 114. Mobile/fixed apparatus 110 may be communicating through optical channel 120 with mobile/fixed apparatus 130 which may also include free-space optical transceiver 112 and wavelength meter 114. Communication between apparatus 110 and apparatus 130 may be half-duplex or full-duplex.

[025] In some example embodiments, the wavelength meter may be included in a communications transceiver implementing a quantum key distribution. For example, digital communications between an apparatus such as mobile/fixed apparatus 110 and mobile/fixed apparatus 130 may include quantum key distribution (also referred to as quantum communications) and/or a modulated laser communications channel (also referred to as laser communications). For example, quantum communications may be implemented in optical transceiver 112. Laser-based communications may be implemented using a laser in a transmitter and a detector in a receiver. Moreover, data may be encoded with the keys generated by quantum communications to ensure secure communications. In some example embodiments, single photons may be transmitted and received. A wavelength meter may be used to determine the wavelength of a laser output or to determine the wavelength of a received signal. The wavelength meter may be included in any other type of optical communications transceiver as well.

[026] FIG. IB depicts a wavelength meter 140 that may be used as test equipment and/or as a diagnostic instrument. For example, wavelength meter 140 may be used in a laboratory or production environment to determine the wavelength of an optical source. The wavelength meter may have various inputs such as free-space input 142 and/or fiber optic input 148. Wavelength meter 140 may determine the wavelength of an optical input to free-space input 142 and/or fiber input 146.

[027] In some example embodiments, the wavelength meters of FIG. 1A and IB may measure an optical signal in quadrature where both quadrature signals may be measured at the same time or nearly the same time. In some example embodiments, the optical input signal may be split into two or more portions using one or more beam splitters or other beam splitting devices. In some example embodiments, each of the portions may be passed through a different length of waveguide or other material to cause a phase shift proportional to the length. For example, an optical signal may be measured in quadrature by measuring one portion of the split beam after having travelled a certain distance, and another portion after having travelled a longer distance.

[028] In some example embodiments, by making quadrature measurements on a signal split into two paths, each path having a different length, the wavelength of the optical signal can be measured over a wavelength range and the linewidth of the optical input can be determined. For example, an optical input signal may be split into two paths, where the two paths have different lengths and are both connected to the 90-degree hybrid. The 90- degree hybrid may produce four outputs that may be measured by four optical detectors, two optical detectors for each quadrature component. In some example embodiments, the measurements from the four optical detectors may allow for a determination of the wavelength of the optical signal and the linewidth of the optical signal. In some example embodiments, the wavelength range may increase exponentially with the number of different path lengths and the corresponding signals measured in quadrature.

[029] In some example embodiments, multiple 90-degree hybrid couplers may be used to measure complex spectra that may result in the determination of more information than a central wavelength and linewidth. In some example embodiments, if the spectrum is sparse in wavelength, the spectrum can be measured with a few random delays e.g., lengths) such as in a compressed sensing scheme. In some example embodiments, the use of short (e.g., a few microns to tens of microns), fixed delays may make the wavelength measurements robust against fabrication tolerances and environmental fluctuations such as temperature, stress, and so on. In some example embodiments, a predetermined accuracy of wavelength measurement may be determined without the need for calibration.

[030] FIG. 2A depicts a schematic diagram of some optical components in a wavelength meter, in accordance with some example embodiments. An optical input 210 may be split into two portions and sent through paths 220 and 230, each path having a different length. After travelling through paths 220 and 230, both potions may enter 90- degree hybrid 240. Each path may cause a different phase shift proportional to the length of the path/waveguide. 90-degree hybrid 240 combines the two phase shifted signals to produce outputs 242-248. Based on measurements by four photodiodes at 262-268, the wavelength, linewidth, and other parameters of the input signal 210 may be determined. A wavelength meter consistent with the subject matter herein is further detailed in the following paragraphs. [031] In some example embodiments, a wavelength meter may measure the wavelength, linewidth, and/or other aspects of an optical source. The optical source 210 to be measured may be divided equally with one portion passing through a reference arm such as reference arm 230 in FIG. 2A, and another portion passing through a delayed arm such as delayed arm 220 in FIG. 2 A. Delay arm 220 may be longer than reference arm 230. Outputs from the reference arm and from the delayed arm may be injected into a 90-degree hybrid such as 90-degree hybrid 240. The amplitudes of the outputs from 90-degree hybrid 240 may, in some example embodiments, be expressed as being proportional to X ₊ at 242, X_ and 244, Y ₊ at 246, and Y_ at 248 according to the following:

X+ o (r + s ^* e ^i<p ) Equation 1,

X. o (r - s ^* e ^i(p ) Equation 2,

Y+ oc (r + s ^* e ^{i((p +} f ^} ) Equation 3, and

Y_ oc (r - s ^* e ⁱ ( ^{(p +} ¾) Equation 4,

where r is the complex amplitude of the signal from the reference arm and s is the complex amplitude of the signal from the delayed arm (s is the complex conjugate of s). The value of φ in EQNs. 1-4 may depend on the implementation of the 90-degree hybrid (e.g. the waveguide material used and the corresponding phase shifts, any waveplates used, and/or whether multimode interferometers (MMI) are used), and the lengths of 220 and 230. The wavelength range and linewidth of the wavelength meter may depend on the difference in length between reference arm 230 and delayed arm 220. For at least this reason, φ, a phase offset that is common to the reference arm and delayed arm, can be set to zero in the following analysis without any loss in generality.

[032] In some example embodiments, the intensity of the outputs of 90-degree hybrid 240 may be measured by four photo detectors, one each at 242-248. The intensity values of X ₊ , X_, Y ₊ and Y_ may be used to calculate X = (X ₊ - X_), and Y = (Y ₊ - Y_). Photo detectors 242-248 may me photodiodes or any other type of optical sensor or detector. In some example embodiments, X and Y may be measured directly by balanced detectors that may produce:

where C is the responsivity of the detectors. In some example embodiments, balanced detectors may provide results with a reduced noise level. The value r may be chosen as a phase reference, and the value s may be represented as r multiplied by a phase shift according to ,where n is the mode effective index, is the

difference in length between the reference arm and the delayed arm, and λ is the central wavelength of the optical source.

[033] In some example embodiments, the following may express the relationship between X and Y:

Thus, Y/X may be independent of r (the amplitude of the reference source) and/or independent of C (the responsivity of the detectors).

[034] In some example embodiments, the range of measurable wavelengths may be related to the difference in lengths between the reference arm and the delayed arm. In some example embodiments, the difference in length between the reference arm and the delayed arm that will produce the desired wavelength range may be determined according to the following:

where N is an integer and may be determined modulo 2π, and atan2 is the arctangent function with values -π < atan2(X,Y) < π . The length difference between the length of the reference arm and the delayed arm, AL, may determine the wavelength range, Δλ, for the wavelength meter. This range may be chosen to include the wavelength range for the photo detectors, or the range of the optical source. Other values may also be used. For a central wavelength and range, λο ± Δλ/2, AL may be chosen to satisfy the following: Equation 9 A.

[035] The autocorrelation, Α(τ), of an optical signal for a delay , may be

determined where AL is the difference in path length between the reference arm and the delayed arm, and c is the speed of light. The autocorrelation may be determined as follows:

where S= X ₊ +X_ +Y ₊ +Y_, the sum of all the outputs, and represents

the convolution of

[036] The modulus of the autocorrelation may be represented as follows:

Equation 13

[037] For a Lorentzian linewidth, the modulus of the autocorrelation may decrease exponentially with the delay as follows: Equation 14.

[038] Based on which is determined by the measurements described above, the coherence time, may be determined and the linewidth may be determined (in frequency) as

[039] FIG. 2B depicts multimode interference couplers included in a wavelength meter, in accordance with some example embodiments. A multimode interference (MMI) coupler such as one-by-two hybrid coupler 250 may implement beam splitter or power divider to divide an optical input signal/source into equal or nearly equal portions such as 252 and 254. A multimode interference coupler, such as four-by-four MMI 260, may implement a 90-degree hybrid coupler such as 90-degree hybrid coupler 240.

[040] In some example embodiments, four-by-four MMI 260 accepts two input signals and produces four output signals. For example, one output from one-by-two MMI 250 passes through reference arm 230 (Li) to input 254 of four-by-four MMI 260 and the second output of one-by-two MMI 250 passes through delayed arm 220 (L ₂ ) to input 256 of four-by-four MMI 260. four-by-four MMI 260 may produce four outputs 262-268. Alternatively, inputs 255 and 257 may produce outputs 262-268. In some example embodiments, MMI 260 performs the function of 90-degree hybrid 240. Inputs 254 and 256 (or 255 and 257) correspond to the inputs to 90-degree hybrid 240 and output 262 corresponds to 242, 264 corresponds to 244, 266 corresponds to 246, and 268 corresponds to 248. Other implementations of 90-degree hybrids may also be possible.

[041] In some example embodiments, one end of reference arm 230 (Li) may connect to MMI 250 at 252, and the other end of the reference arm 230 (Li) may connect to four-by-four MMI 260 at 254. In some example embodiments, one end of the delayed arm 220 (L ₂ which may be longer than reference arm 230) may connect to MMI 250 at 253, and the other end of reference arm 220 (L ₂ ) may connect to four-by-four MMI 260 at 256.

[042] FIG. 3 depicts another example of a wavelength meter, in accordance with some example embodiments. In some example embodiments, a wavelength meter may include multiple multimode interferometers (MMI) to measure a wider range of wavelengths or measure a range of wavelengths more accurately. FIG. 3 also refers to FIGs. 1-2. [043] For example, one-by-two times N MMI 310 may split an input optical source 210 into 2N equal or nearly equal portions. The 2N portions may be connected to N 90-degree hybrids (e.g., four-by-four MMIs) through delays of various lengths. In some example embodiments, each 90-degree hybrid may be connected to 2 outputs of one-by-two times N MMI 310, one through a reference arm and another through a delayed arm, wherein the delayed arm for any 90-degree hybrid is longer in length that its corresponding reference arm. The reference arms to the N 90-degree hybrids may be of different lengths. For example, one-by-two times N MMI 310 may connect to 90-degree hybrids 240, 340, and 370 through delays of various lengths. One-by-two times N MMI 310 may connect to 90-degree hybrid 240 through reference arm 230 and delayed arm 220. One-by-two times N MMI 310 may connect to 90-degree hybrid 340 through reference arm 330 and delayed arm 320. One-by-two times N MMI may connect to 90-degree hybrid 370 through reference arm 360 and delayed arm 350.

[044] In some example embodiments, first interferometer 240 may determine a wavelength value, λο, within a range of wavelengths, Δλ ₀ , to within a wavelength accuracy of Δλι. Second interferometer 340 may have a wavelength range selected to be Δλι from the first interferometer 240, and due to the smaller wavelength range ( Δλι < Δλο), interferometer 340 may determine the wavelength λο with higher accuracy, Δλ ₂ ( Δλ ₂ < Δλι). The combination of the first interferometer 240 and second interferometer 340 may produce a wavelength measurement of the input source 210 wavelength , λο, to within the higher accuracy, Δλ ₂ , than may be determined by just the first interferometer. Thus, in some example embodiments, the wavelength range determined by the second interferometer is smaller in range but higher in accuracy. The corresponding path length difference between delayed arm 320 and reference arm 330 for the second interferometer 340 may be smaller than the path length difference between delayed arm 220 and reference arm 230 for the first interferometer 240 resulting in the smaller wavelength range, Δλ ₂ , of the second interferometer 340.

[045] In some example embodiments, a third 90-degree hybrid 370 may be included to provide a higher accuracy measurement of λο than can be achieved with the two 90-degree hybrids 240 and 340. For example, the difference in delayed arm 350 and reference arm 360 may be chosen to provide a wavelength range Δλ ₂ with a higher accuracy that can be provided by the second interferometer 340. By adding an appropriate number of 90-degree hybrids, any target accuracy may be met. In some example embodiments, additional 90-degree hybrids may extend the wavelength range of measurement.

[046] Each interferometer may provide an autocorrelation at the difference in delay between its corresponding reference arm and delayed arm. By using several interferometers, more information about the spectrum of the optical source may be determined. In some example embodiments, if an optical source has a spectrum that is sparse in frequency, a few random differences in delays of the multiple interferometers, and thus the related autocorrelation points, may support a compressed sensing scheme.

[047] FIG. 4A depicts an additional example of a wavelength meter that includes four-by-four multimode interference couplers, in accordance with some example embodiments. FIG. 4A depicts a four-way power splitter connected through four different delays to two 90-degree hybrids to provide eight outputs from which the wavelength and linewidth of an input source 210 may be determined. FIG. 4A also refers to FIGs. 1-3.

[048] FIG. 4A depicts a four- way power divider implemented as 1x4 MMI coupler

401. MMI coupler 401 splits input source 210 into four outputs of equal or nearly equal amplitudes. Two of the outputs of 1x4 MMI 401 may be connected to a 90-degree hybrid implemented as four-by-four MMI coupler 402. Reference arm Li in FIG. 4A may connect a first output of 1x4 MMI 401 to four-by-four MMI 402, and delayed arm L ₂ may connect a second output of 1x4 MMI 401 to four-by-four MMI 402. Reference arm L ₃ in FIG. 4A may connect a third output of 1x4 MMI 401 to four-by-four MMI 403, and delayed arm L ₄ may connect a fourth output of 1x4 MMI 401 to four-by-four MMI 403. The wavelength and linewidth may be determined from the eight outputs (Di-Dg) from MMIs 402 and 403.

[049] In some example embodiments, the lengths of L ₃ and L ₄ connected to four- by-four MMI 403 may be selected to enable determination of the wavelength, λο, and linewidth of input source 210 to within an accuracy, Αλ _1. In some example embodiments, the lengths of L ₁ and L ₂ four-by-four MMI 403 may be may be selected to determine the wavelength within the smaller wavelength range of Δλι to a higher accuracy, Δλ ₂ , as detailed in above with respect to FIG. 3. In some example embodiments, MMI 402 and MMI 403 may cover diverse sets of wavelength ranges.

[050] FIG. 4B depicts an additional example of a wavelength meter that includes 3x3 multimode interference couplers, in accordance with some example embodiments. FIG. 4B also refers to FIGs. 1-4A. FIG. 4B depicts a four- way power splitter connected through four different delays to two 120-degree hybrids implemented as 3x3 MMIs to provide six outputs (Di-D ₃ , D5-D7) from which the wavelength and linewidth of an input source 210 may be determined.

[051] In some example embodiments, the lengths of L ₃ and L ₄ connected to 3x3 MMI 404 may be selected to enable determination of the wavelength, λο, and linewidth of input source 210 to within an accuracy, Αλ ₁ . In some example embodiments, the lengths of L ₁ and L ₂ four-by-four MMI 405 may be may be selected to determine the wavelength of source 210 within the smaller wavelength range of Αλ ₁ to a higher accuracy, Δλ ₂ , as detailed in above with respect to FIG. 3. In some example embodiments, MMI 404 and MMI 405 may cover diverse sets of wavelength ranges. In some example embodiments, 120-degree hybrids may be more flexibly calibrated than 90-degree hybrids and may exhibit broader bandwidth than 90-degree hybrids. [052] Although 90-degree and 120-degree hybrid couplers are disclosed above, any other hybrid coupler may be used as well. For example, a 45-degree hybrid (implemented as an 8x8 MMI) or couplers with other phase relationships may be used as well. In some example embodiments, all the inputs of a chosen MMI may be used simultaneously. For example, the four inputs of a four-by-four MMI may be connected through a reference arm and three different delay arms to a four-way beamsplitter. Three phases may be determined from measurements of the four outputs of the four-by-four MMI.

[053] FIG. 5 depicts an example of a process performed by a wavelength meter, in accordance with some example embodiments. At 510, an optical source may be split into portions. At 520, each portion may be sent through a different length delay line resulting in a different phase shift for each portion. At 530, at least two of the phase shifted portions may be combined at a hybrid interferometer. At 540, the outputs of the hybrid interferometer may be measured. At 550, at least a wavelength of the input source may be determined from the outputs of the hybrid interferometer. FIG. 5 also refers to FIGs. 1-4.

[054] At 510, an optical source may be split into portions, in accordance with some example embodiments. For example, optical source 210 may be split into two portions by a beam splitter such as one -by-two multimode interference (MMI) coupler 250 in FIG. 2B. In another example, optical source 210 may be split into four portions by a beam splitter such as 1x4 multimode interference coupler 401 in FIGs. 4A/4B. In another example, optical source 210 may be split into 2N portions by a beam splitter such as one-by-two times N multimode interference coupler 310 in FIG. 3. Other beam splitters and/or multimode couplers may be used to split an optical source into portions as well.

[055] At 520, each portion is sent through a different length delay line resulting in a different phase shift for each portion, in accordance with some example embodiments. For example, optical source 210 may be split into two portions by a beam splitter such as one-by- two multimode interference coupler 250 in FIG. 2A. One portion may be passed through reference arm 230 to 90-degree hybrid 240 and a second portion may be passed through delay arm 220 to 90-degree hybrid 240 as shown in FIG. 2A. In another example shown in FIG 2B, a first portion may be passed through reference arm 230 to four-by-four MMI coupler 260 and a second portion may be passed through delay arm 220 to four-by-four MMI 260. In another example, the four portions from 1x4 multimode interference coupler 401 in FIG. 4A may be passed through lengths Li and L ₂ to four-by-four MMI 402, and through lengths L ₃ and L ₄ to four-by-four MMI 403. In another example, optical source 210 may be split into 2N portions by a beam splitter such as one-by-two times N multimode interference coupler 310 in FIG. 3. The 2N portions may be passed through 2N different delay lengths. Other lengths and types of couplers may be used as well.

[056] At 530, at least two of the phase shifted portions are combined at a hybrid interferometer, in accordance with some example embodiments. For example, in FIG. 2A the portion passed through reference arm 230 and the second portion be passed through delay arm 220 may be combined at 90-degree hybrid 240. In another example shown in FIG 2B, the first portion passed through reference arm and the second portion passed through delay arm 220 may be combined at four-by-four MMI coupler 260. In another example, the portions passed through lengths Li and L ₂ may be combined at four-by-four MMI 402, and the portions passed through lengths L ₃ and L ₄ may be combined at four-by-four MMI 403. In another example, the 2N portions passed through 2N different lengths may be combined in pairs by N 90-degree hybrids. Other numbers of delayed portions may be combined as well. Other interference combiners, interferometers and couplers may be used as well.

[057] At 540, the outputs of the hybrid interferometer may be measured, in accordance with some example embodiments. For example, the outputs 242-248 from 90-degree hybrid 240 may be measured by four photo detectors such as photodiodes, or the four outputs may be measured by two balanced detectors. In another example the outputs from four-by-four MMI

260 shown in FIG. 2B may be measured by four photo detectors or two balanced detectors. In another example, the eight outputs from four-by-four MMIs 402 and 403 may be measured by eight photo detectors or four balanced detectors, and/or the six outputs from 3x3 MMIs 404 and 406 may be measured by six photo detectors. In another example, the 4N outputs from the N 90-degree hybrids in FIG. 3 may be measured by 4N photo detectors or 2N balanced photo detectors. Each photo detector may measure an optical power of an optical signal at an output of a 90-degree hybrid or MMI. For example, a detector such as Di at 262 in FIG. 2B may measure an optical intensity, power level, or amplitude of an optical output such as an output from four-by-four MMI 260.

[058] At 550, at least a wavelength of the input source is determined from the outputs of the hybrid interferometer, in accordance with some example embodiments. For example, any combination of Equations 1-14 above may be used to determine the wavelength and/or linewidth of an optical source. Other calculations may also be performed to determine the characteristics of an optical source.

[059] FIG. 6 depicts an example of an apparatus, in accordance with some example embodiments. The apparatus 110 may comprise a user equipment, such as a cellular telephone, a smartphone, and/or any other radio including mobile and stationary radios.

[060] In some example embodiments, the apparatus 110 may include a free-space optical transceiver 112 and wavelength meter 114. Free-space optical transceiver 112 in 110 may couple to another free-space optical transceiver in another apparatus such as apparatus 130 in FIG. 1. In some example embodiments, apparatus 130 may be implemented in a manner similar to apparatus 110. In some example embodiments, wavelength meter 140 may be implemented in a manner similar to apparatus 110.

[061] In some example embodiments, apparatus 110 may establish communications to another apparatus, such as apparatus 130, using optical transceivers

112. Wavelength meter 114 at apparatus 110 may monitor or determine the wavelength of a transmitted optical signal from apparatus 110 and/or may determine the wavelength of a received optical signal from apparatus 130. Apparatus 140 may determine the wavelength of a free-space optical signal or a fiber-optic input signal.

[062] In some example embodiments, apparatus 110 may also include a radio communication link to a cellular network, or other wireless network. Apparatus 110 may send a message to a network node of the radio network indicating the capabilities of apparatus 110. For example, apparatus 110 may indicate to the network node that its capabilities include the capabilities of optical transceiver 112 and/or wavelength meter 114. The network node may enable the apparatus 110 to use its optical transceiver 112 and/or the network node may request that apparatus 110 forward information from the network node or from apparatus 110 to another apparatus such as apparatus 130. For example, the user of apparatus 110 may have information such as video or audio information that the user of apparatus 110 would like to send to apparatus 130.

[063] The apparatus 110 may include at least one antenna 12 in communication with a transmitter 14 and a receiver 16. Alternatively transmit and receive antennas may be separate.

[064] The apparatus 110 may also include a processor 20 configured to provide signals to and from the transmitter and receiver, respectively, and to control the functioning of the apparatus. Processor 20 may be configured to control the functioning of the transmitter and receiver by effecting control signaling via electrical leads to the transmitter and receiver. Likewise processor 20 may be configured to control other elements of apparatus 130 by effecting control signaling via electrical leads connecting processor 20 to the other elements, such as a display or a memory. The processor 20 may, for example, be embodied in a variety of ways including circuitry, at least one processing core, one or more microprocessors with accompanying digital signal processor(s), one or more processor(s) without an accompanying digital signal processor, one or more coprocessors, one or more multi-core processors, one or more controllers, processing circuitry, one or more computers, various other processing elements including integrated circuits (for example, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), and/or the like), or some combination thereof. Apparatus 110 may include a location processor and/or an interface to obtain location information, such as positioning and/or navigation information. Accordingly, although illustrated in FIG. 6 as a single processor, in some example embodiments the processor 20 may comprise a plurality of processors or processing cores.

[065] Signals sent and received by the processor 20 may include signaling information in accordance with an air interface standard of an applicable cellular system, and/or any number of different wireline or wireless networking techniques, comprising but not limited to Wi-Fi, wireless local access network (WLAN) techniques, such as, Institute of Electrical and Electronics Engineers (IEEE) 802.11, 802.16, and/or the like. In addition, these signals may include speech data, user generated data, user requested data, and/or the like.

[066] The apparatus 110 may be capable of operating with one or more air interface standards, communication protocols, modulation types, access types, and/or the like. For example, the apparatus 110 and/or a cellular modem therein may be capable of operating in accordance with various first generation (1G) communication protocols, second generation (2G or 2.5G) communication protocols, third-generation (3G) communication protocols, fourth-generation (4G) communication protocols, Internet

Protocol Multimedia Subsystem (IMS) communication protocols (for example, session initiation protocol (SIP) and/or the like. For example, the apparatus 110 may be capable of operating in accordance with 2G wireless communication protocols IS- 136, Time Division

Multiple Access TDMA, Global System for Mobile communications, GSM, IS-95, Code

Division Multiple Access, CDMA, and/or the like. In addition, for example, the apparatus 110 may be capable of operating in accordance with 2.5G wireless communication protocols General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), and/or the like. Further, for example, the apparatus 110 may be capable of operating in accordance with 3G wireless communication protocols, such as, Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (CDMA2000), Wideband Code Division Multiple Access (WCDMA), Time Division- Synchronous Code Division Multiple Access (TD-SCDMA), and/or the like. The apparatus 130 may be additionally capable of operating in accordance with 3.9G wireless communication protocols, such as, Long Term Evolution (LTE), Evolved Universal Terrestrial Radio Access Network (E-UTRAN), and/or the like. Additionally, for example, the apparatus 110 may be capable of operating in accordance with 4G wireless communication protocols, such as LTE Advanced and/or the like as well as similar wireless communication protocols that may be subsequently developed.

[067] It is understood that the processor 20 may include circuitry for implementing audio/video and logic functions of apparatus 110. For example, the processor 20 may comprise a digital signal processor device, a microprocessor device, an analog-to-digital converter, a digital-to-analog converter, and/or the like. Control and signal processing functions of the apparatus 110 may be allocated between these devices according to their respective capabilities. The processor 20 may additionally comprise an internal voice coder (VC) 20a, an internal data modem (DM) 20b, and/or the like. Further, the processor 20 may include functionality to operate one or more software programs, which may be stored in memory. In general, processor 20 and stored software instructions may be configured to cause apparatus 110 to perform actions. For example, processor 20 may be capable of operating a connectivity program, such as, a web browser. The connectivity program may allow the apparatus 110 to transmit and receive web content, such as location-based content, according to a protocol, such as, wireless application protocol, WAP, hypertext transfer protocol, HTTP, and/or the like.

[068] Apparatus 110 may also comprise a user interface including, for example, an earphone or speaker 24, a ringer 22, a microphone 26, a display 28, a user input interface, and/or the like, which may be operationally coupled to the processor 20. The display 28 may, as noted above, include a touch sensitive display, where a user may touch and/or gesture to make selections, enter values, and/or the like. The processor 20 may also include user interface circuitry configured to control at least some functions of one or more elements of the user interface, such as, the speaker 24, the ringer 22, the microphone 26, the display 28, and/or the like. The processor 20 and/or user interface circuitry comprising the processor 20 may be configured to control one or more functions of one or more elements of the user interface through computer program instructions, for example, software and/or firmware, stored on a memory accessible to the processor 20, for example, volatile memory 40, non- volatile memory 42, and/or the like. The apparatus 110 may include a battery for powering various circuits related to the mobile terminal, for example, a circuit to provide mechanical vibration as a detectable output. The user input interface may comprise devices allowing the apparatus 110 to receive data, such as, a keypad 30 (which can be a virtual keyboard presented on display 28 or an externally coupled keyboard) and/or other input devices.

[069] Moreover, the apparatus 110 may include a short-range radio frequency (RF) transceiver and/or interrogator 64, so data may be shared with and/or obtained from electronic devices in accordance with RF techniques. The apparatus 110 may include other short-range transceivers, such as an infrared (IR) transceiver 66, a Bluetooth (BT) transceiver 68 operating using Bluetooth wireless technology, a wireless universal serial bus (USB) transceiver 70, and/or the like. The Bluetooth transceiver 68 may be capable of operating according to low power or ultra-low power Bluetooth technology, for example, Wibree, radio standards. In this regard, the apparatus 110 and, in particular, the short-range transceiver may be capable of transmitting data to and/or receiving data from electronic devices within a proximity of the apparatus, such as within 10 meters. The apparatus 110 including the Wi-Fi or wireless local area networking modem may also be capable of transmitting and/or receiving data from electronic devices according to various wireless networking techniques, including 6LoWpan, Wi-Fi, Wi-Fi low power, WLAN techniques such as IEEE 802.11 techniques, IEEE 802.15 techniques, IEEE 802.16 techniques, and/or the like.

[070] The apparatus 110 may comprise memory, such as, a subscriber identity module (SIM) 38, a removable user identity module (R-UIM), and/or the like, which may store information elements related to a mobile subscriber. In addition to the SIM, the apparatus 110 may include other removable and/or fixed memory. The apparatus 110 may include volatile memory 40 and/or non-volatile memory 42. For example, volatile memory

40 may include Random Access Memory (RAM) including dynamic and/or static RAM, on-chip or off-chip cache memory, and/or the like. Non-volatile memory 42, which may be embedded and/or removable, may include, for example, read-only memory, flash memory, magnetic storage devices, for example, hard disks, floppy disk drives, magnetic tape, optical disc drives and/or media, non-volatile random access memory (NVRAM), and/or the like. Like volatile memory 40, non-volatile memory 42 may include a cache area for temporary storage of data. At least part of the volatile and/or non-volatile memory may be embedded in processor 20. The memories may store one or more software programs, instructions, pieces of information, data, and/or the like which may be used by the apparatus for performing functions of the user equipment/mobile terminal. The memories may comprise an identifier, such as an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus 110. The functions may include one or more of the operations disclosed herein with respect to free-space optical communications including the process flow of FIG. 5, and the like. The memories may comprise an identifier, such as, an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus 110. In the example embodiment, the processor 20 may be configured using computer code stored at memory 40 and/or 42 to provide the operations disclosed with respect to the process shown in FIG. 5 and the like.

[071] Some of the embodiments disclosed herein may be implemented in software, hardware, application logic, or a combination of software, hardware, and application logic. The software, application logic, and/or hardware may reside in memory 40, the control apparatus 20, or electronic components disclosed herein, for example. In some example embodiments, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a "computer-readable medium" may be any non-transitory media that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer or data processor circuitry, with examples depicted at FIGs. 1A, IB, 2A, 2B, 3, 4A, 4B, 5 and/or 6. A computer-readable medium may comprise a non-transitory computer-readable storage medium that may be any media that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer. Furthermore, some of the embodiments disclosed herein include computer programs configured to cause methods as disclosed herein (see, for example, the process of FIG. 5, and the like).

[072] The subject matter described herein may be embodied in systems, apparatus, methods, and/or articles depending on the desired configuration. For example, the systems, apparatus, methods, and/or articles described herein can be implemented using one or more of the following: electronic components such as transistors, inductors, capacitors, resistors, and the like, a processor executing program code, an application-specific integrated circuit (ASIC), a digital signal processor (DSP), an embedded processor, a field programmable gate array (FPGA), and/or combinations thereof. These various example embodiments may include implementations in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device. These computer programs (also known as programs, software, software applications, applications, components, program code, or code) include machine instructions for a programmable processor, and may be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the term "machine-readable medium" refers to any computer program product, computer-readable medium, computer-readable storage medium, apparatus and/or device (for example, magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions. Similarly, systems are also described herein that may include a processor and a memory coupled to the processor. The memory may include one or more programs that cause the processor to perform one or more of the operations described herein.

[073] Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is more accurate determination of the wavelength and/or linewidth of an optical source.

[074] Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations may be provided in addition to those set forth herein. Moreover, the example embodiments described above may be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flow depicted in the accompanying figures and/or described herein does not require the particular order shown, or sequential order, to achieve desirable results. Other embodiments may be within the scope of the following claims.